Small Inorganic Rings in the 21st Century: From Fleeting

Review pubs.acs.org/CR

Small Inorganic Rings in the 21st Century: From Fleeting Intermediates to Novel Isolable Entities Gang He, Olena Shynkaruk, Melanie W. Lui, and Eric Rivard* Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2 3.5.3. Group 15/16 Element Heteroatomic Rings 3.6. Group 16 Element Rings Containing O, S, Se, and Te 3.6.1. Group 16 Element Homoatomic Rings 3.6.2. Group 16/Nitrogen Heteratomic Rings 3.6.3. Group 14/16 Element Heteratomic Rings Containing Peroxo (μ-O2) Units 3.6.4. Group 16 Element Heteroatomic Rings 3.7. Miscellaneous Inorganic Rings Containing Group 17 Elements 4. Conclusions and Future Prospects for the Field 5. Addendum Author Information Corresponding Author Notes Biographies Dedication Abbreviations References

CONTENTS 1. Introduction 2. Classification of Inorganic Rings: A Brief Comment on Ring Synthesis 3. Inorganic Rings Derived from Group 12−17 Elements 3.1. Group 12 Element Rings Containing Zn, Cd, and Hg 3.1.1. Group 12 Element Homoatomic Rings 3.1.2. Group 12/15 Element Heteroatomic Rings 3.1.3. Group 12/16 Element Heteroatomic Rings 3.1.4. Group 12/17 Element Heteroatomic Rings 3.2. Group 13 Rings Containing Boron 3.2.1. Homoatomic Boron-Containing Rings 3.2.2. Boron/Group 14 Element Heterocycles 3.2.3. Boron/Group 15 Element Heterocycles 3.2.4. Boron/Group 16 Element Heterocycles 3.3. Group 13 Element Rings Containing Al, Ga, In, and Tl 3.3.1. Homoatomic Three- and Four-Membered Rings Featuring Al, Ga, In, and Tl 3.3.2. Heteroatomic Group 13 Element/Nitrogen Rings 3.3.3. Inorganic Heterocycles Containing Heavier Group 13 and 15 Elements 3.3.4. Inorganic Heterocycles Containing Heavier Group 13 and 16 Elements 3.4. Group 14 Element Rings Containing Si, Ge, Sn, and Pb 3.4.1. Homoatomic Group 14 Element Heterocycles 3.4.2. Group 14 Element Heteroatomic Rings 3.4.3. Group 14/15 Element Heteroatomic Rings 3.4.4. Group 14/16 Element Heteroatomic Rings 3.5. Group 15 Element Rings Containing P, As, Sb, and Bi 3.5.1. Group 15 Element Homoatomic Rings 3.5.2. Group 15 Element Heteroatomic Rings

© 2014 American Chemical Society

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1. INTRODUCTION Cyclic structures represent a well-studied structural class due to the novel bonding modes and reactivity these units possess and the ubiquitous role of cyclic intermediates in a wide variety of chemical transformations. With the recent and rapid progression of modern inorganic chemistry, new cyclic molecular archetypes are continually being discovered which add to the chemical diversity that spurs growth in this area and often leads to opportunities to experimentally validate theoretical predictions. Given the vast nature of inorganic ring chemistry, this review will focus on rings with three or four constituent atoms containing exclusively non-carbon atoms as components and will be organized according to the chemical group in which the key element(s) in the ring structure belong. When multiple atom types are present, we will defer to classifying the heterocycle according to the element with the lowest group number; for example, electron-deficient B2N2 rings will be discussed as part of the section concerned with heterocycles containing group 13 elements. Within the context of our organizational strategy, this review will predominantly address inorganic rings containing elements from groups 12−17 inclusive with occasional highlights from the d-block (transition metals). For the sake of being concise, small inorganic rings

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In general, pathways A and B take advantage of the inherent reactivity of inorganic multiple bonds stemming from a combination of poor overlap between diffuse p-orbitals on heavier main group elements and/or the highly polar nature of many EE′ linkages (arising from large differences in electronegativity). For example, ketones R2CO feature strong C−O π-bonds and are stable with respect to dimerization (path B) into 1,3-dioxetanes [R2CO]2,5 while inorganic analogues such as germanones R2GeO readily oligomerize to form cyclic species [R2GeO]x (x ≥ 2).6 Notably, the formation of a stable monomeric germanone, Eind2GeO, 1 (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), was only reported in 2012 and necessitated the presence of sterically encumbered Eind groups at Ge to shield the Ge O double bond from undesired dimerization.7 Atom transfer (path A) to unsaturated inorganic species is a commonly employed synthetic strategy to access inorganic rings, and the synthesis of cyclothiadisilanes (3) from the reaction of elemental sulfur with aryl-substituted disilenes Ar2SiSiAr2 (Ar = aryl group; 2) is a nice illustration of this concept (eq 1).8

that exist as part of large atomic cluster motifs will not be covered. Moreover, this review will discuss molecular species that have appeared in the scientific literature from the year 2000 and onward, with some references to earlier historically important discoveries when appropriate. A major goal of this review is to show the versatile nature of these inorganic rings while giving possible inspiration for future research. As a prelude to some of the important applications associated with inorganic rings, it is salient to mention some key highlights in this introductory section.1 When small substituents are appended to inorganic heterocycles, the resulting species are often sufficiently volatile to enable their use as precursors to semiconducting materials via chemical vapor deposition (CVD).2 Moreover, the introduction of selenium to organic substrates is effectively accomplished using inorganic P2Se2 heterocycles such as Woollins’ reagent [PhP(Se)(μ-Se)2P(Se)Ph].3 In addition, cyclic inorganic species can be readily constructed with either intraring Lewis acidic or intraring Lewis basic sites, enabling the sequestration of anions or metals to occur to advance concepts in sensing and catalysis.4

2. CLASSIFICATION OF INORGANIC RINGS: A BRIEF COMMENT ON RING SYNTHESIS It is rational to classify inorganic rings by either the number of atoms in a ring (in this review three or four atoms) or by the element type. It is also instructional to consider rings within the context of how they are prepared/derived; when this is done, five common synthetic routes come immediately to mind (Scheme 1): atom transfer methodologies (A), bimolecular [2

Condensation processes (C) are advantageous from the standpoints that synthetically challenging species with inorganic multiple bonds are not required and the number of leaving groups (X and Y in Scheme 1) available to participate in ringforming reactions with nucleophilic partners is vast. Moreover, the generation of either volatile byproducts, such as HCl, or stable salts (e.g., LiCl or MgCl2) provides an added driving force. For example, the Chivers group has generated a series of novel boron-containing heterocycles by condensing readily accessible dianionic boraamidinates (e.g., [tBuNB(Ph)NtBu]2−) with various main group element halides (eq 2).9 Related

Scheme 1. Major Routes to Three- and Four-Membered Inorganic Ringsa

condensation routes are often employed in transition-metal and lanthanide chemistry and form the basis for catalyst design and subsequent metal-mediated small-molecule activation.10 Another major route to inorganic rings to be discussed in this review is the synthesis of cyclic structures via dative (or coordinative) bridging interactions. Halide substituents (X) readily participate in ring-forming bridging interactions due to the presence of multiple lone pairs at a single atom and a propensity to form dative X: → M bonds with electrondeficient elements (M); in a similar fashion, both amido (NR2) and alkoxide (OR) substituents can effectively form M(μNR2)M and M(μ-OR)M bridges, leading to cyclic structures.11 To briefly highlight this principle, two examples of LM−X complexes (M = Co2+ and Fe2+) featuring different multidentate ligand constructs (L) will be discussed. The chloro- and bromo-substituted Co(II) complexes [BP3]CoX (6) (X = Cl and Br; [BP3] = PhB(CH2PPh2)3−), first reported by the Peters group, adopt dimeric structures connected via central Co2X2 rings in the solid state; however, significant dissociation into

a

A and E refer to inorganic elements and X and Y to leaving groups or electron pair donors. R = functional group. M = alkali or alkaline-earth metal.

+ 2] cycloaddition (B), condensation (and salt elimination) (C), donation of electron pairs to form cyclic arrangements (D), cyclization via reductive coupling (E) (Scheme 1). This list is not exhaustive, and other ring-forming processes such as ring contraction and oxidation-based strategies (e.g., the formation of S42+ from the oxidation of S8) will be described. 7816

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Scheme 2. Role of Ligand and Halide Substituents in the Formation of M2X2 Rings

monomeric units transpires in solution (Scheme 2, left), thus demonstrating that the bridging Co−X−Co interactions in these complexes are quite weak.12 Furthermore, the iodo analogue [BP3]Co−I (7) adopts a monomeric structure both in solution and in the solid state; hence, the nature of the halide present has an important role in dictating if halide bridging and metallohalide ring formation (M2X2) will occur.13 The presence of sterically encumbered ligands tends to suppress metal− halide bridge formation, leading to monometallic LMX species, as shown convincingly by the Holland group during the development of their nacnac-supported iron complexes (nacnac = [HC(RCNDipp)2]−; R = Me or tBu; Dipp = 2,6-iPr2C6H3; see compounds 8 and 9 to the right of Scheme 2).14 The following sections of this review will cover the synthesis and properties of three- and four-membered rings containing elements from groups 12−17 (in sequential order). The nomenclature used in this review follows commonly encountered terminology in the literature; however, IUPAC has published a series of systematic rules for naming inorganic rings.15

Figure 1. Ligand-supported Hg3 rings.

involving Hg(II) or Hg(I) starting materials (e.g., [Hg2](OTf)2; OTf = O3SCF3−), and in one study, the dynamic nature of the appended bidentate diphosphine ligands in solution was investigated by 31P and 199Hg NMR spectroscopy.19e In related work, a planar Hg4 ring was formed as part of a fused Ir6Hg8 array in [(IrCp*(CO))6Hg8](F3CCO2)6 (Cp* = η5-C5Me5) (12) wherein long Hg−Hg intraring distances of 2.982(2)−3.0287(18) Å are present.20 3.1.2. Group 12/15 Element Heteroatomic Rings. Zn2Pn2 heterocycles (Pn = pnictogen or group 15 element) were initially synthesized for their potential use as volatile precursors to electrically novel materials by CVD;21 however, later it was noted that these species were also active as polymerization catalysts. In this regard, the dimeric Zn(II) species [MeZn(μ-N(SiMe3)2)]2 (13; Figure 2) was prepared as a volatile colorless solid and used as a precursor to cationic Zn complexes capable of polymerizing cyclohexene and εcaprolactone.22 A series of structurally related amidozinc(II) heterocycles was also prepared, including (Figure 2) [(PhCH2)Zn(μ-N(CH2Ph)2)]2 (14),23 [Ph2NZn(μ-NPh2)]2 (15),24 [(F5C6)Zn(μ-N(SiMe3)2)]2 (16),25 [Me3SiCH2Zn(μNHDipp)]2 (17),26 [EtZn(μ-NHMes)·THF]2 (Mes = 2,4,6Me3C6H2) (18),26 [Me3Si(Ph)NZn(μ-N(Ph)SiMe3)]2 (19),27 [RZn(μ-N(CH2CH2NMe2)2)2ZnR] (R = Cl, Et, NiBu2, and N(CH2CH2NMe2)2; 20−23),28 [R′Zn(CH2Pyr)]2 (R′ = Me and CH(SiMe3)2; Pyr = 2-pyridyl; 24 and 25),29 [R″Zn(μNHSiiPr3)]2 (R″ = Me and Et; 26 and 27), and the unusual adamantyl (Ad) amide [MeZn(THF)(μ-NHAd) 2 ZnMe(H2NAd)] (28).30 A nice illustration of how structural variety can exist among group 12 element compounds is found within a series of related bis(amido) complexes, whereby the Zn and Cd congeners [(( Me 2 SiCH 2 CH 2 SiMe 2 )N)M (μ-N(Me2SiCH2CH2SiMe2))]2 (M = Zn and Cd; 29 and 30) adopt dimeric structures, while the Hg analogue Hg(N(SiMe2CH2CH2SiMe2))2 (31) is monomeric.31 Monomeric

3. INORGANIC RINGS DERIVED FROM GROUP 12−17 ELEMENTS 3.1. Group 12 Element Rings Containing Zn, Cd, and Hg

The study of heterocyclic compounds based upon group 12 elements is dominated by zinc due, in part, to the drastically lower toxicity of zinc compounds in relation to their cadmium and mercury analogues. As we will see, Zn heterocycles have been shown to be active catalysts for lactide ring-opening polymerization (ROP),16 while novel luminescent properties have been noted for Cd-containing rings;17 the use of volatile M2E2 (M = Zn, Cd, and Hg; E = N, P, S, or Se) rings as precursors to bulk semiconductors has also been reported.18 This section, and those which follow, will be organized in a consistent manner going from homoatomic rings (e.g., Hg3) progressing toward heterocycles featuring elements arranged from left to right in the periodic table. 3.1.1. Group 12 Element Homoatomic Rings. A series of structurally interesting ligand-supported Hg3 rings were prepared by the Peringer group (Figure 1, compounds 10 and 11).19 Each of these complexes contains formal Hg34+ cores with an average oxidation state at Hg of +1.33 and crystallographically determined Hg−Hg bond lengths in the narrow range of 2.717(2)−2.8649(13) Å; for comparison, Hg− Hg bond lengths in the commonly encountered Hg(I) dimers [Hg−Hg]2+ are ca. 2.50 Å.19b The Hg3 rings in 10 and 11 were each derived from either partial reduction or disproportionation 7817

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Figure 2. Zn2N2 heterocycles selected from the recent literature.

Scheme 3. Hydride-Mediated Synthesis of Azido- and Alkoxy-Bridged Zn Heterocycles

Figure 3. Various E2Pn2 ring motifs (E = Zn and Hg; Pn = N and P) and thermal ellipsoid drawing (50%) of 44. Reprinted with permission from ref 38. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

diamond core Zn 2 N 2 motifs via Zn−N−Zn bridging interactions (33) (Figure 2).34 The Schulz group reported smooth hydride transfer between the Zn(II)−hydride complex [( Mes nacnac)Zn(μ-H)] 2 (Mesnacnac = [HC(MeCNMes)2]−) (34) and trimethylsilyl azide (Me3SiN3) and cyclohexene oxide to yield Zn2N2 and Zn2O2 rings (35 and 36), respectively; these reactions can be considered as skeletal substitution or formal ring atom replacement transformations wherein the H atoms are replaced by N and O atoms (Scheme 3).35

structural arrangements are more common for Hg relative to its lighter group 12 element counterparts Zn and Cd, and in the case of mercury(II) amides, the hard−soft mismatch between soft Hg and hard N centers weakens the intermolecular Hg−N interactions required for dimerization.27,32 An interesting Zn2N2 ring featuring hemilabile amidosilyl ether ligands was reported in 2011, [EtZn(μ-N(tolyl)SiMe2OtBu)]2 (tolyl = 4MeC6H4) (32; Figure 2).33 In a similar fashion, zinc(II) guanidinates RZnNC(NR2)2 show a tendency to form 7818

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Scheme 4. Copolymerization of CO2 and Cyclohexene Oxide with the Zinc Aryl Oxide Catalyst 49

Figure 4. Cadmium(II)-containing Cd2O2, Cd2S2, and Cd2Se2 heterocycles.

The successful ROP of D,L-lactide was accomplished with the Zn(II) catalyst 37 (Figure 3) to give polylactides of modest molecular weight (number average molecular weight, Mn, ca. 6000−12000 g mol−1) in a living fashion, as determined by a linear plot of Mn vs the extent of monomer conversion.36 Moreover, the Bunge group treated a series of guanidine complexes, 33 (Figure 2), with hindered alcohols to yield catalysts which convert rac-lactide into polylactides with a slight preference for isotactic stereochemistry.16d Given the everincreasing use of Zn species as ROP catalysts,37 it appears that dimeric complexes with central inorganic ring arrangements will remain actively studied in this regard. It has also been demonstrated that exposure of a mixture containing the dimeric zinc amide [(iBu2N)Zn(μ-NiBu2)2Zn(NiBu2)] (38) and hexylamine, hexNH2, to moist air for extended periods of time (ca. 5 days) at room temperature afforded the wide-bandgap semiconductor ZnO as a nanocrystalline material; the model species [(DippNH)Zn(μ-NHDipp)]2·HNiBu2 (39) was also structurally characterized in the same study.18e In pioneering studies by Buhro and co-workers, a series of metal(II) phosphido dimers, [M(P(SiMe3)2)2]2 (M = Zn, Cd, and Hg; 40−42), were prepared. The Cd analogue 41 was treated with methanol, leading to protolytic cleavage of the P− Si bonds and the generation of a putative “Cd(PH2)2” intermediate that later afforded Cd 3 P 2 upon thermal treatment.18a,b The alkyl-substituted analogues [Zn(PtBu2)2]2 (43) and [Hg(PtBu2)2]2 (44) (Figure 3) were later prepared via salt elimination (route C) between Li[PtBu2] and ZnI2 and HgCl2, respectively;38 interestingly, the Hg derivative 44 shows linear P−Hg−P angles (176.4(1)°) and long intermolecular Hg---P distances, suggesting the presence of weak contacts in the solid state (Figure 3).38,39

Complexation of a Lewis acidic Cr(CO)5 unit to terminal −PPh2 phosphide units in the quasi two-coordinate species Ph2P−Hg−PtBu2 results in dimer formation, [Cr(CO)5· Ph2PHg(μ-PtBu2)2HgPPh2·Cr(CO)5] (45), with short Hg−P single bonds throughout the Hg2P2 ring framework [2.447(3)− 2.660(3) Å].40 Structural versatility was observed for the Zn2P2 rings within potassium and sodium zincate salts of [MeZn(μPSitBu3)]22− with a planar Zn2P2 arrangement present in the K salt, while short Na---P contacts lead to buckling of the Zn2P2 unit into a butterfly motif in the Na analogue, 46 (Figure 3).41 Structural analogues to 46, [RZn(μ-P(SiMe3)2)]2 (R = iPr and CH2SiMe3; 47 and 48), were prepared from the condensation of diorganozinc reagents R2Zn with the pyrophoric reagent tris(trimethylsilyl)phosphine, P(SiMe3)3.42 3.1.3. Group 12/16 Element Heteroatomic Rings. The study of mixed element compounds exhibiting bonding between group 12 and 16 elements is buoyed by the discovery that hybrid nanoparticles, such as CdSe quantum dots, exhibit useful optoelectronic properties.43 Moreover, Darensbourg and co-workers reported that the bis(2,6-difluorophenoxy)zinc dimer [(2,6-F2C6H3)Zn(μ-O-2,6-F2C6H3)·THF]2, 49 (Scheme 4), catalyzes the copolymerization of CO2 with cyclohexene oxide at 80 °C and 55 bar to give high molecular weight poly(cyclohexenylene carbonate); in the absence of CO2, the efficient ROP of cyclohexene oxide was also noted.16c Replacement of the THF donors in 49 by PCy3 gave a Zn(II) heterocycle, 50 (Scheme 4), that was inactive as a polymerization catalyst; the related Cd analogue [(2,6-F2C6H3)Cd(μO-2,6-F2C6H3)·PCy3]2 (51) was also ineffective as a polymerization catalyst.16c,44 In a related study, the hindered Cd complexes [(2,6R2C6H3)OCd(μ-O-2,6-R2C6H3)]2 (R = Ph and tBu; 52 and 7819

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to II/VI semiconductors, such as HgSe, led to the preparation of the phosphine-capped selenolate dimer [(Ph3P)Hg(μSePh)SePh]2 (66) with a slightly distorted Hg2Se2 squareshaped core.51 Earlier studies by the Arnold group helped combat the instability that is generally noted for molecular M−Te complexes (M = Zn, Cd, and Hg) with the assistance of kinetic stabilization in the form of bulky (Me3Si)3Si− groups at Te.52 Specifically, a number of modestly stable species were prepared, including the centrosymmetric butterfly-shaped Zn 2 Te 2 heterocycle [(Me 3 Si) 3 SiTeZn(μ-TeSi(SiMe 3 ) 3 )] 2 (67); moreover, this species and its heavier Cd and Hg analogues (68 and 69) were prepared using two distinct condensation routes (Scheme 6).21,52,53 The Hg2Te2 heterocycle [(PhTe)Hg(μ-TePh)(PCy3)] (70) was recently prepared by sonication of a 1:1 mixture of Hg(TePh)2 and PCy3 in DMF; sonication is believed to increase the rate of dissolution of Hg(TePh)2, thus enabling efficient ring assembly to transpire.54 3.1.4. Group 12/17 Element Heteroatomic Rings. As mentioned in the Introduction, halide bridges M−X−M can often lead to the formation of inorganic rings, and subtle changes to the halide donor X, the acceptor (M), and/or the coordination environment at the halide-bound element (M) can readily disrupt these bridging interactions to yield monomeric element halide complexes. Although many M2X2 rings are known in the literature, we will focus on a few selected examples to reinforce some general concepts. Recently, a family of N-heterocyclic carbene (NHC)-bound group 12 halides, NHC·MX2 (M = Zn, Cd, and Hg; X = Cl, Br, and/or I; NHC = N-heterocyclic carbene), were independently prepared by various groups. Specifically, the Hg adducts IPr·HgCl2 (71) and IMes·HgCl2 (72) (IPr = [(HCNDipp)2C:]; IMes = [(HCNMes)2C:]) are both monomeric in the solid state,55 while the Zn congener [IMes·Zn(μ-Cl)Cl]2 (73) is a dimer with bridging Zn−Cl−Zn interactions; this observation indicates that mercury halide complexes are less prone to aggregate via halide bridging relative to zinc halides.56 Moreover, the propensity for halide bridging decreases as the halide becomes heavier as evidenced by the formation of the monomeric zinc(II) iodide adduct IPr·ZnI2 (74),57 while the related bromo analogue [IPr·Zn(μ-Br)Br]2 (75) is dimeric with a central Zn2Br2 ring.58 As an illustration of the lability of the halide bridges within many M2X2 rings, the centrosymmetric dimer [IMes·Zn(μ-Cl)Cl]2 (73) is readily cleaved by THF to form the THF-bound complex IMes·ZnCl2·THF (76).59 For comparison, the hydride analogue [IPr·ZnH(μ-H)]2 (77) can be prepared from ZnH2 and IPr in THF with no sign of THF coordination; thus, the bridging hydride−zinc interactions in 77 are more robust than the Zn−X linkages within their halide counterparts.60 As with element halides, the formation of centrosymmetric bridging M2H2 arrangements is common

53) were shown to bind epoxides to yield monomeric bisadducts [Cd(O-2,6-R2C6H3)2]·2(epoxide); however, no further polymerization was noted. These results indicate that Cd complexes are much less active as ROP catalysts relative to their Zn counterparts, and this effect is likely due to a reduction in Lewis acidity within the cadmium analogues.45 A rare structurally authenticated cadmium hydroxide molecular complex, ([(bmnpa)Cd(μ-OH)]2)(ClO4)2 (54) (Figure 4), was reported to fix CO2 in the form of a cadmium-bound carbonate.46 A comprehensive study involving various Cd2O2 heterocycles decorated with amido or aryl oxide groups at Cd (55−60; Scheme 5) and their use as precursors to CdS, CdSe, and CdTe Scheme 5. Preparation of CdE Nanomaterials (E = S, Se, and Te) via Cadmium(II) Aryloxide Heterocyclesa

a

LB = Lewis base.

nanomaterials was reported; this approach has an advantage of being modular in nature and affords pure nanomaterials of tunable optoelectronic properties.18c Moreover, the cadmium(II) thiolato dimer [(S2CNEt2)Cd(μ-SSi(OtBu)3)]2 (61) yields pure hexagonal CdS upon thermal treatment.18d,47 A series of luminescent Cd2S2 and Cd2Se2 complexes, [(L)2Cd(μ-EAr)]2(PF6)2 (62) (E = S and Se; Figure 4), were prepared with light emission (λem = 500−610 nm) noted in the solid state and in degassed MeCN solutions.17 A comproportionation reaction between Cd(SePh)2 and CdX2 (X = Br and I) in the presence of PCy3 cleanly generated the fourmembered Cd2Se2 heterocycles [XCd(μ-SePh)·PCy3]2 (63), while extended cluster motifs were obtained when excess Cd(SePh)2 was used as a reagent.48 Novel Hg2S2 ring motifs exist within the [(F5C6)2Hg(μ-SC6F5)2Hg(C6F5)2]2− dianions (64), derived via [SC6F5]− group transfer involving lanthanide thiolates (e.g., La(SC6F5)3) and Hg(SC6F5)2;49 a similar diamond core structure motif is present in the phosphonium salt (Ph4P)2[Hg2(SPh)6] (65).50 The search for new precursors

Scheme 6. Preparation of M2Te2 Heterocycles (M = Zn, Cd, and Hg) via Convergent Condensation Pathways

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Scheme 7. Atom Transfer to the Silaborene (tBu2SiMe)2Si B(TMP) (83)

within molecular hydrides throughout the periodic table, with the nondirectional nature of the hydrogen 1s orbitals supporting efficient orbital overlap to give M−H−M threecenter two-electron bonding. In this review we will only highlight the reactivity of selected hydride species which contain cyclic M2H2 bonding motifs.61 3.2. Group 13 Rings Containing Boron

The study of compounds containing boron remains a rich area of exploration,62 and accordingly, some areas in boron heterocycle chemistry that have attracted considerable attention since 2000 include the formation and reactivity of diradicaloid B2P2 rings by the Bertrand group (e.g., [R2P(μ-BtBu)]2 (78))63 and the renewed interest in amine−boranes (RNH2·BH3) and the cyclic and polymeric products that result from their dehydrogenation for applications in hydrogen storage.64 3.2.1. Homoatomic Boron-Containing Rings. The Siebert group uncovered unexpected structural versatility within extended boron assemblies when they investigated the reduction of amide-substituted diborane(4)s RB(Cl)−B(Cl)R (R = NiPr2 and TMP; TMP = 2,2,6,6-tetramethylpiperidino) with Na/K alloy.65 In the case of R = NiPr2, a blue-colored tetramer, cyclo-[(iPr2N)B]4 (79), with a butterfly-shaped B4 unit is obtained (Figure 5), while the yellow-colored TMP

TMEDA (TMEDA = Me2NCH2CH2NMe2) led to ringopening of 87 followed by tBu migration to give the iminoboranes [Li(TMEDA)]+[tBu2RBBNtBu]−; R = Me and Bu; 88 and 89) featuring bona fide B−N triple bonds.70 Furthermore, B−B bond scission in 87 can be instigated by the addition of unsaturated organic substrates such as ketones, imines, and substituted alkynes to give the ring-expanded heterocycles 91−93;71 the double insertion product 94 is obtained when 87 is combined with acetylene in the presence of F3B·OEt2 as a catalyst.71a Silylene insertion to give the B2NSi heterocycle 95 was also noted when a mixture of tBu2SiCl2 and Li metal was combined with 87.71a The oxidative addition of alkyldibromoboranes (Alk)BBr2 across the B−B bond in 87 was reported to give ring-opened products (96) which could later be reclosed under reducing conditions to generate boryl-substituted azadiboriridenes (97). Another interesting reaction transpired between CO and 87, leading to the tricyclic product 98 wherein each boron atom is linked by C and O atoms; upon heating 98, complete cleavage of the C−O bonds originally present in carbon monoxide occurs to yield 99 (Scheme 8). When 87 was combined with Me2S·BH2Cl, the ring-fused borane 100 was intercepted.72 A structurally novel BN2 heterocycle, [(Mes)BCl(μ-NMe2)NB(Mes)Cl] (101), was formed from the reaction of the stannyl-capped hydrazine Me2N−N(SnMe3)2 and 2 equiv of MesBCl2 (eq 3).73

Figure 5. Selected homoatomic boron rings reported recently in the literature.

analogue [(TMP)B]4 (80) adopts a tetrahedral B4 framework, illustrating the small energetic difference that exists between these isomeric forms; in both complexes, B−B bonds in the range of 1.695(6)−1.765(5) Å are present.65 The Me2Nappended hexamer B6(NMe2)6 (81) (Figure 5) was later reported by the same group and shown to contain a prominent B−B cross-ring interaction within the planar B4 cyclic array [1.633(2) Å]. This leads to a bifurcated B4 unit with two fused B3 rings;66 a highly distorted version of this structural motif was identified within B8F12 (82) wherein folding of the B4 unit into a butterfly geometry is accompanied by lengthening of one B− B bond within each cyclic B3 subunit to >1.95 Å.67 3.2.2. Boron/Group 14 Element Heterocycles. Atom transfer was explored with the unsaturated silaborene (tBu2SiMe)2SiB(TMP) (83) to give a series of BSiE (E = S and Se) (84 and 85) and BSiO2 (86) rings in high yield (Scheme 7).68 3.2.3. Boron/Group 15 Element Heterocycles. The availability of unsaturated boron-containing reagents (e.g., t BuBNtBu) has created many new opportunities for inorganic ring synthesis and subsequent ring expansion/bond activation transformations. For example, the Paetzold group explored the reactivity of the tri-tert-butylazadiboriridene heterocycle [tBuB(μ-NtBu)BtBu] (87) in great detail,69 and more recent explorations will be described here (Scheme 8). The B2N heterocycle in 87 can be considered to be aromatic due to its planar arrangement and 2π-electron count; however, this species is amenable to a variety of ring-opening/ring expansion processes as outlined in Scheme 8. Treatment of 87 with the lithium alkyls nBuLi and MeLi in the presence of

The use of dianionic boraamidinates [R′NB(R)NR′]2− as precursors to main group element containing MNBN rings (M = main group elements) was reviewed in detail in 2007.9b As shown in eq 2, salt elimination is generally employed as a synthetic strategy, and the electronic and steric parameters of the NBN chelates are readily modified by altering the side groups attached to the B and N atoms. In more recent work, spirocyclic boraaminidate systems such as E[PhB(μ-NtBu)2]2 (E = Ge and Sn; 102 and 103) were prepared by the Chivers group (Scheme 9), and oxidation of the Sn analogue 103 with SO2Cl2 affords a deep blue radical cation, [Sn(PhB(μNtBu)2)2]•+ (104); density functional theory (DFT) and solution electron paramagnetic resonance (EPR) studies are consistent with the singly occupied molecular orbital (SOMO) in 104 being located predominantly on the N atoms of one 7821

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Scheme 8. Diverse Reactivity of the Azadiboriridene [tBuB(μ-NtBu)BtBu] (87)

Scheme 9. Main Group Boraamidinate Rings and Radicals

NBN unit.74 This radical is unstable in solution above 0 °C, which is in stark contrast to the neutral Al and Ga spirocyclic radicals M[PhB(μ-NtBu)2]2]• (M = Al and Ga; 105 and 106), which are stable in the solid state at room temperature.75 Attempts to extend this synthetic strategy to yield a stable As radical, [As[PhB(μ-NtBu)2]2]•, led to the formation of a zwitterionic spirocycle wherein chloride transfer from SO2Cl2 to one boron atom transpired (107; Scheme 9).76 In an attempt to form a stable N-heterocyclic silylene featuring a NBN backbone, Cui and co-workers combined the hydrochloride Si(IV) precursor [PhB(μ-NDipp)2Si(H)Cl] (108) with the carbene ImtBu (ImtBu = [(HCNtBu)2C:]); a clean reaction transpired to afford the disilane product 109 from the formal insertion of the putative silylene :Si[PhB(μ-NDipp)2] into the Si−Cl bond of unreacted 108 (Scheme 9).77 Recently, the coordination of the dianionic boraguanidinate [(Dipp)NB(NiPr2)N(Dipp)]2− (as a dilithio salt) with various main group elements was explored.78

Literature reports since 2000 involving B2N2 four-membered rings have been dominated by the dimer of dimethylaminoborane, [H2BNMe2]2 (110), largely due to its formation during the dehydrogenative coupling of Me2NH·BH3 by transition-metal and main group element based catalysts.79 These studies represent models for the dehydrogenation of ammonia borane, H3N·BH3, which has been actively explored around the world as a possible source of hydrogen fuel.64a,c In 2001, the Manners group reported the synthesis of 110 via the elimination of H2 from Me2NH·BH3 at room temperature in the presence of [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) as a catalyst; due to the volatility of 110, this heterocycle can be readily purified by vacuum sublimation.80 A detailed investigation involving the dehydrocoupling of HNMe2·BH3 in the presence of cationic late-metal Rh−bisphosphine catalysts was reported by Weller, Hall, and co-workers, wherein they were able to intercept σcomplexes of the linear dimer H3B−NMe2−BH2−NMe2H and show that in the presence of added amine−borane Me2NH· BH3 the complex underwent further dehydrogenative coupling. 7822

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Figure 6. Selected B2N2 ring units within molecular and polymeric species.

Figure 7. Recently synthesized B2N2 heterocycles.

In addition, a Rh-based σ-complex of [H2BNMe2]2 was isolated (111; Figure 6).81 The metal-mediated dehydrogenative coupling of H3N·BH3 affords a more complex suite of products, including insoluble [NH2BH2]x oligomers, volatile borazine [HNBH]3, and polyborazylenes. In 2008, another key byproduct from this reaction was identified by the Autrey group (112; Figure 6) that contains a B2N2 ring with an appended −NH2BH3 group at one of the intraring boron centers.82 Aminoborane dimers related to 110 that were structurally authenticated by X-ray crystallography include [Cl2BNMe2]2 (113),83 [Br2BNMe2]2 (114),84 [H2BNMe(Ph)]2 (115),85 [(F 5 C 6 ) 2 BN(SiMe 3 ) 2 ] 2 (116), 86 and [Cl 2 BNH(SiMe 3 )] 2 (117).87 The efficient synthesis of monomeric and dimeric (e.g., [H2BNR2]2; R = alkyl group) aminoboranes was reported by treating the readily available amidoborohydrides Li[NR2BH3] with MeI, Me3SiCl, or benzyl chloride under ambient conditions.88 The synthesis of polymers containing inorganic elements as integral (backbone) components is, in general, quite challenging due to a lack of synthetic methods available.89 Chujo introduced a new synthetic strategy to this field when he reported the synthesis of poly(cyclodiborazane)s via hydroboration polymerization. For example, the arylborane MesBH2 can react with a wide range of aryl dicyanides NC−aryl−CN to give borylimines that link in dimeric arrangements containing B2N2 rings (Figure 6); in addition, many of the reported polymers (118) exhibit intense fluorescence in the visible region when irradiated with UV light.90 Furthermore, the preformed cyclodiborazane (119) featuring reactive aryl bromide residues can undergo Sonogashira coupling with aryldiiynes to afford the alkyne-spaced poly(cyclodiborazane)s 120.91 Substituted analogues of 119 were shown to display interesting two-photon absorption and electronic communication through the B2N2 cyclodiborazane cores; thus, these species are promising chromophores for imaging in biology.92

A series of novel diazadiboretidines, [(RO)B(μ-NR′)]2 (121), were prepared via the [2 + 2] dimerization of in situ generated iminoboranes [ROBNR′].93 The centrosymmetric azidoborane dimers [(F5C6)2B(μ-N3)]2 (122)94 and [(2,6F2C6H3)2B(μ-N3)]2 (123)95 were also prepared, and addition of an exogenous Lewis base or increasing the steric bulk of the groups at boron afforded monomeric azides such as [(F5C6)2B(N 3 )·py] (py = pyridine) and Mes* 2 BN 3 (Mes* = 2,4,6-tBu3C6H2). Structural characterization of the related complexes [(TMP)B(μ-NSiMe3)]2 (124),96 [Cl2B(μ-N CCl2)]2 (125),97 and [Br2B(μ-NPCl3)]2 (126)98 and the B2N2 heterocycles 127 and 128 was also reported (Figure 7). The field of phosphinoboranes (including many BP heterocycles) was reviewed in 1995 by Paine and Nöth,99 and thus, only more recent developments concerning P−B heterocycles will be discussed here. Well-defined π-complexes between the phosphinoboranes (F5C6)2BPR2 (R = Cy and tBu) and Pt centers was described, and the nature of Pt−B and Pt−P interactions in the resulting BPPt metallacycles was investigated by DFT.100 Treatment of the stable bisadduct IPr·P2·IPr (129) with excess THF·BH 3 yielded the donor-stabilized BP 2 heterocycle 130 as a B2H7− salt (eq 4); the bonding within the BP2 ring in 130 comprises orbital interactions of high pcharacter (82−93%) as is common for three-membered rings with acute internal bond angles.101

A new class of inorganic diradicals appeared in 2002 when Bertrand and co-workers reported the synthesis of the 7823

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Scheme 10. B2P2 Diradicals and Their Related Bicyclic Structural Isomers and Molecular Structure of 131a

a

Reprinted with permission from ref 63a. Copyright 2002 American Association for the Advancement of Science.

thermally stable yellow solid [tBuB(μ-PiPr2)]2 (131) from the reaction of the diborane(4) tBuB(Cl)−B(Cl)tBu with 2 equiv of Li[PiPr2].63a The B2P2 ring in 131 is planar, and the P−B bond lengths [1.892(2) Å, av] are in the range expected for single bonds (Scheme 10). Computational studies on the sterically uncongested parent analogue of 131 [H2P(μBH)2PH2] revealed that the singlet diradicaloid, with strong antiferromagnetic coupling of the two unpaired electrons, is ca. 17.2 kcal/mol lower in energy than the corresponding triplet form. In addition, this planar B2P2 diradicaloid exists only as a transition state in the inversion of the bicyclic isomer (cf. 133) and thus follows what is generally encountered within the carbon analogues.63a Therefore, the presence of sterically encumbered groups within 131 has a key role in facilitating the isolation of a stable planar P2B2 diradicaloid. Both diradicaloid (132a) and bicyclic (133) forms are obtained when the B2P2 units are linked by phenylene units in para and meta arrangements (Scheme 10); the tetraradical 132a is deep violet in color, while its bicyclic analogue 132b is colorless. In addition, heating the bis(diradical) 132a results in increased population of the bicyclic form 132b, and this process can be monitored by UV−vis and NMR spectroscopy.63b,c,102 The structurally modified heterocycle [iPr2P(μ-BtBu)2PPh2] (134) undergoes Ph migration from P to B upon irradiation at λ = 254 nm to yield the planar heterocycle 135.103 Scheme 10 outlines various bond activation processes that can be instigated with Bertrand’s diradical 131 under mild reaction conditions, leading to compounds 136−139.63d,e Various phosphinoboranes of the general form R2PB(C6F5)2 were evaluated for their abilities to activate H2 via frustrated Lewis pair (FLP) chemistry. The dimeric P2B2 heterocycles

[Et2PB(C6F5)2]2 (140) and [Ph2PB(C6F5)2]2 (141) did not react with hydrogen gas, while monomeric phosphinoboranes, such as Cy2PB(C6F5)2, readily cleave H2 in a heterolytic fashion to yield phosphine−borane adducts R2PH·BH(C6F5)2.104 The Miyoshi group prepared the metalized phosphinoborane dimer [(Cp(CO)2Fe)P(μ-BCl2)]2 (142).105 In addition, Nöth and coworkers published a study on planar [R(X)B(μ-PHR′)]2 rings (X = halogen)106 and synthesized the new B2P2 heterocycles [(Me3Si)3CB(μ-PtBu)]2 (143) and [(Me3Si)3CB(μ-PMes)]2 (144) with sterically encumbered groups positioned at the three-coordinate B and P centers.107 An interesting tBu abstraction reaction between [(TMP)B(μ-PtBu)]2 (145) and excess AlBr3 was discovered to give a cationic P2B2 bicycle (146; eq 5) with a transannular P−P single bond (2.315(2)

Å).108 The coordination of the bicyclodiboretane [(TMP)B(μP)]2 (147)109 (Figure 8) with various Lewis acids (e.g., 7824

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Figure 8. Selected boron−pnictogen and boron−chalcogen inorganic rings.

B(C6F5)3) was also described.110 The tendency of the parent group 13 hydrides HEEH and H2EPnH2 (E = B, Al, and Ga; Pn = N, P, and As) to undergo dimerization/oligomerization and the stability of B2Pn2 diradicals similar in structure to 131 were the subjects of theoretical investigations.111 The formation of boron heterocycles with heavier pnictogens (As, Sb, or Bi) is much more rare, and only a handful of such species have been reported in the literature. For example, the Cowley group prepared the diarsaboretidine [(TMP)B(μAsMes)]2 (148),112 while Power, Wells, and co-workers extended this series to include the four-coordinate B2As2 systems [PhB(Cl)(μ-AstBu2)]2 (149)113 and [I2B(μ-As(SiMe3)2)]2 (150).114 3.2.4. Boron/Group 16 Element Heterocycles. Boron shows a tendency to form six-membered boroxine B3O3 rings; however, when hindered groups are positioned at boron, fourmembered analogues, such as the 1,3-dioxa-2,4-diboretane [Mes*B(μ-O)]2 (151), can be isolated.115 More recently, the Braunschweig group trapped a formally dicationic B2O2 cyclic unit between two Pt(PCy3)2 fragments (153; eq 6).116 The

CuxIn1−xGaxSe2 (CIGS), necessitates the development of appropriate synthetic precursors that can be obtained in high purity.123 As a result, considerable attention has been given to developing volatile compounds for CVD technologies.2 3.3.1. Homoatomic Three- and Four-Membered Rings Featuring Al, Ga, In, and Tl. The synthesis of the dialane ((Me3Si)2CH)2Al−Al(CH(SiMe3)2)2 (158) by Uhl and coworkers in 1988 was a key breakthrough as this compound represented the first example of a stable molecule with an unsupported Al−Al bond.124 The Wiberg group later prepared the persilylated dialane (tBu3Si)2Al−Al(SitBu3)2 (159) and noted that the thermolysis of 159 at 100 °C in hydrocarbon solvent generated the tetrahedron-shaped Al(I) species [( t Bu 3 Si)Al] 4 (160) and the cyclotrialanyl radical [(tBu3Si)4Al3]• (161) as isolable products (eq 7).125 Com-

synthesis of 153 relied upon the formation of the BO complex trans-BrPt(BO)(PCy3)2 (152)117 followed by halide abstraction involving Krossing’s silver aluminate salt Ag[Al(OC(CF3)3)4].118 Compound 153 gave an expected 11B NMR resonance due to the presence of a three-coordinate boron environment (δ = 15 ppm), while X-ray crystallography confirmed the formation of a cyclic rhomboid-shaped B2O2 array with narrow intraring B−O−B bond angles of 81.2(5)° and 82.0(5)°. The corresponding B−O bond lengths in 153 [1.414(13) Å, av] indicate single bond character, while a short transannular B---B interaction [1.846(10) Å] suggests the possibility of additional B−B bonding.116 A structurally related B2O heterocycle, [((Me3Si)3CB)2(μ-O)] (154),119 (Figure 8) was shown to undergo ring expansion (cf. Scheme 8) with amides to yield six-membered BNBOCO rings.120 Small inorganic rings containing boron and either sulfur or selenium are more rare; however, the previously mentioned BSiS and BSiSe ring systems (Scheme 7; 84 and 85),68 and the TMP-substituted heterocycles [(TMP)B(μ-Ch)]2 (Ch = O, S, and Se; 155−157; Figure 8) are noteworthy.121

pound 161 is a dark green moisture- and air-sensitive paramagnetic solid that yields EPR spectral parameters (g value and hyperfine coupling constants) that were consistent with an electron in an Al−Al π-manifold. The novel cycloaluminene Na2[(AlArMes)3] (ArMes = 2,6(2,4,6-Me3C6H2)2C6H3) (162) was synthesized by Power and co-workers via the reduction of the aluminum iodide precursor ArMesAlI2 in diethyl ether with excess sodium metal (eq 8).126

X-ray crystallography revealed the presence of a triangle-shaped Al3 core in 162 capped above and below with Na cations (each Na+ cation forms additional contacts with peripheral aryl rings of the ArMes ligands). The average Al−Al distance was determined to be 2.5202(2) Å and lies within the range of Al−Al single bond lengths (2.50−2.52 Å) found for dialanes (R2Al−AlR2). Computational studies indicated some Al−Al πbonding character was present in 162 as an occupied orbital (HOMO − 2) consisting of an Al3-centered 3c−2e π-bond was

3.3. Group 13 Element Rings Containing Al, Ga, In, and Tl

The chemical behavior of the heavier group 13 elements, Al to Ga, often differs markedly from that of boron due to the prevalence of higher coordination numbers among the heavier triel elements and the increased stability of the lower (+1) oxidation state.122 In addition, the widespread incorporation of group 13 elements into optoelectronic devices, as exemplified by gallium arsenide (GaAs), indium tin oxide (ITO), and 7825

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aluminum amides, including four-membered cyclic species (Scheme 11), were reviewed in detail in 1999; thus, only more

present. However, the Na−Al interactions had considerable covalency; thus, one can also regard this species as an Al3Na2 molecular cluster. It should also be mentioned that 162 is isostructural with the Ga3 cyclic dianions M2[(ArMesGa)3] (M = Na or K; 163 (Figure 9) and 164) prepared earlier by the

Scheme 11. General Synthetic Route and Structures of [R2Al(μ-NR′2)]2 Heterocycles

Figure 9. Homoatomic three-membered Ga3 rings.

recent developments in this field will be described here.134 Table 1 outlines structurally characterized [R 2 AlNR′ 2 ]2 analogues reported in the literature, with salt or alkane elimination being the most common routes to these species (Scheme 11). By taking advantage of the Lewis acidic nature of many Al(III) compounds, Schulz and co-workers prepared the basestabilized iminoalane dimers [Me3N·HAl(μ-NDipp)]2 (204) and [Me2EtN·HAl(μ-NDipp)]2 (205) from the equimolar reaction between H3Al·NMe2R (R = Me and Et) and DippNH2, with extrusion of molecular hydrogen as a byproduct (Figure 10).147 Following a similar approach, the related heterocycles [DMAP·Cp*Al(μ-NSiEt3)]2 (206), [DMAP· Cp*Al(μ-NSi i Pr 3 )] 2 (207), and [DMAP·Cp*Al(μNSitBuMe2)]2 (208) were synthesized.154 Uhl and co-workers reported that hydroalumination of PhCN and Cl3CCN with di-tert-butylaluminum hydride or the corresponding diethyl analogue, Et2AlH, yielded the products [tBu2Al(μ-N C(Ph)H)]2 (209), [Et2Al(μ-NC(Ph)H)]2 (210), and [tBu2Al(μ-NC(CCl3)H)]2 (211), all of which form dimers possessing planar Al2N2 cores with two exocyclic CN double bonds.155 The Niecke group reported two dimeric phosphoraniminatoalanes, [R″2Al(μ-NP(NCy2)2H)]2 (R″ = Me and Et; 212 and 213), which were obtained by combining the aminophosphine (Cy2N)2PNH2 with the corresponding dialkylaluminum hydrides R2AlH (Figure 10).156 Structurally related compounds with bridging NPR3 groups, [Me2Al(μNPtBu3)]2 (214),157 [Cl2Al(μ-NPEt3)]2 (215),158 [H2Al(μNPR′3)]2 (R = iPr and tBu; 216 and 217), and [(TfO)HAl(μNPtBu3)]2 (218),159 were also reported. An interesting borazine-grafted Al2N2 ring, [(Me3Si)2AlN(H)B3(Me)2N3Me3]2 (220), was recently prepared by Paine and co-workers (Figure 10),160 and the thermally induced insertion of nitriles into the Al−Me bonds within [Me2Al(μ-NHDipp)]2 (219) was reported.161 Polycrystalline AlN was generated from the CVD of [Me2Al(μ-NHNMe2)]2 (221) on a silicon surface at 800 °C.162 In a similar fashion, films of hexagonal AlN and GaN were prepared from [Et2Al(μ-NHtBu)]2 (222) and [Et2Ga(μNHtBu)]2 (223).163 Treatment of the dimeric species [(Me2N)2Al(μ-NMe2)]2 (224) with ammonia, followed by heating to 700−1100 °C, afforded bulk AlN in high yield.164 [(Me2N)2Al(μ-NMe2)]2 (225) has also received attention from the synthetic organic community as this amide has been shown to be an active catalyst for transamination165 and is a precursor to guanidine-supported Al hydroamination catalysts.166 GaN quantum dots were prepared using a multistep procedure involving the reaction of [(Me2N)2Ga(μ-NMe2)]2 (226) with ammonia to give a product formulated as

Robinson group.127 The dark green radical (tBu3Si)4Ga3• (165; Figure 9) was generated as a product from the reaction of Na(THF)3[(tBu3Si)3Ga2]− with BrSitBu3 in pentane.128 The reduction of the hindered aryldichlorogallane, ArTripGaCl2 (ArTrip = 2,6-(2,4,6-iPr3C6H2)2C6H3) (166), with potassium metal gave the square-shaped Ga4 cluster K2[Ga4(ArTrip)2] (167) as a red-brown diamagnetic compound.129 A neutral Tl3 homoatomic ring (168) was obtained from the direct reaction of Li[ArXyl] (ArXyl = 2,6-(2,6-Me2C6H3)2C6H3) with TlCl in Et2O solvent (eq 9).130 The Tl−Tl bonds in 168

[3.2144(3)−3.3782(3) Å] are approximately 0.4 Å longer than typical Tl−Tl single bonds. When the sterically encumbered aryllithium reagent Li[ArDipp] (ArDipp = 2,6(2,6-iPr2C6H3)2C6H3) was combined with TlCl, a deep-redcolored dimer, ArDippTl-TlArDipp, was obtained in place of a cyclic oligomer, illustrating that steric effects play a major role in controlling the degree of aggregation within the thallium(I) aryl compounds formed. 3.3.2. Heteroatomic Group 13 Element/Nitrogen Rings. There are numerous examples of triel (group 13 element)/nitrogen heterocycles known in the literature, largely due to their ease of synthesis. In addition to the use of these species as precursors to functional bulk materials, the Chivers group demonstrated that E2N2 rings [E(NHtBu)2(μ-NHtBu)]2 (E = Al and Ga; 169 and 170) can be used as molecular building blocks upon deprotonation of the NH residues with RLi species, which renders the N centers sufficiently nucleophilic for subsequent nitrogen−element bond formation.131 Aluminum/nitrogen rings have received attention as part of the search for new routes to prepare aluminum nitride, AlN, a refractory material which is optically transparent, thermally conducting, and electrically insulating (each a useful property for optoelectronic applications).132 Due to a decrease in Al−N π-bond strength relative to B−N π-interaction strength, the isolation of monomeric aminoalanes R2AlNR′2 is a much more challenging venture. As a result aminoalanes tend to adopt dimeric or trimeric structures [R2AlNR′2]n (n = 2 and 3) containing Al2N2 and Al3N3 rings, respectively.133 Organo7826

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Table 1. Summary of [R2Al(μ-NR′2)]2 Heterocycles Known since 1999

a

compd no.

R

R′

171135 172135 173136 174137 175138 176138 177138,a 178139 179140,141 180140,141 181141 182141 183141 184141 185142 186142 187143 188144 189145 190145 191145 192145 193145 194146 195147 196148 197149 198149 199150 200150 201151 202152 203153

SiMe3 SiMe3 Cl and tBu Cl and Me Cl and Et Et Et Me and NHNHMe Me Me Me Me Me Me t Bu t Bu Me Me Me Et n Pr n Bu i Bu Me NHDipp and H Me t Bu CH2SiMe3 Et Et Me H H

Ph and H 2,6-iPr2C6H3 (Dipp) and H Me and Et SiMe2H and H SiMe2H and H SiMe2H SiMe3 NHMe and H 3,5-C6H3F2 and H 4-C6H4F and H 2-C6H4F and H 2,3,4,5-C6F4H and H 2,3,5,6-F4C6H and H C6F5 and H NHtBu and H H CH2Ph and H CH2Ph and iPr CH2Ph CH2Ph CH2Ph CH2Ph CH2Ph cyclopentyl and H Dipp and H C6F5 and nBu NHtBu and H SiMe3 and H 1-aminopyrrole and H 1-aminopiperidine and H 2-furylmethyl and SiMe3 SiMe2H i Pr

Al−N bond length (Å) 1.972(3), 1.986(5) 1.943(4), 1.962(4) 1.920(3)−1.924(3)

1.971(2), 1.989(2) 1.973(6), 1.993(6) 1.981(3)−1.993(3) 1.992(2), 1.995(2) 1.994(5)−2.019(5) 1.983(2)−2.009(2)

1.949(8), 1.961(7) 1.977(2), 2.031(2) 1.982(4) 1.988(3), 1.991(3) 1.984(3), 1.997(2) 1.943(3), 1.956(3) 1.956(3)−1.979(3) 1.993(3)−2.031(3) 1.977(2) 1.979(4), 1.976(3) 1.975(3)−1.985(4) 1.9673(8), 1.9641(8) 1.971(2), 2.025(2) 1.9584(18)−1.9652(17) 1.9636(8), 1.9679(8)

This compound has a dimeric structure in solution and is trimeric in the solid state.

obtained by heating [Cl2Ga(μ-NHSiMe3)]2 (227) to 500 °C.169 The gallium(III) dihydride [H2Ga(μ-NHtBu)]2 (228) formed upon the spontaneous self-dehydrocoupling of the primary amine−gallane adduct tBuNH2·GaH3 at room temperature;170 the structure of the related gallium−amide heterocycle [H2Ga(μ-NMe2)]2 (229) was determined in the gas phase.171 The centrosymmetric hydrazidogallium hydride dimer [H2Ga(μ-NHNMe2)]2 (230) can be obtained upon treatment of Me3N·GaH3 with H2N−NMe2, followed by subsequent heating of 230 to give GaN clusters.172,173 By increasing the steric bulk at Ga, an air-stable gallium hydride, [tBu(H)Ga(μNEt2)]2 (231), could be isolated.174 Oxidative addition of NH3 to the digallene Ar″GaGaAr″ (Ar″ = 2,6-Dipp2-4-(Me3Si)C6H2) yields the stable Ga(III) product [Ar′Ga(H)(μ-NH2)]2 (232) under mild conditions (Scheme 12).175 Redox chemistry was also noted between the digallene ArDippGaGaArDipp and ditolyldiazomethane to give the Ga2N2 heterocycle [ArDippGa(μ-NNCPh2)]2 (233) in place of the intended gallaalkene ArDippGaCPh2 (Scheme 12).176 Four-membered amidogallium heterocycles [R2 Ga(μNR′)2]2 have been actively investigated for approximately two decades due to their successful use as precursors to gallium nitride (GaN). In general, these rings are made by alkane elimination between organogallanes (R3Ga or R2GaH) and

Figure 10. Al2N2 ring assemblies derived from bridging amido and imino groups.

[Ga(NH)3/2]n which is converted into GaN quantum dots upon heating in trioctylamine at 360 °C in the presence of additional NH3;167,168 polycrystalline GaN films were also 7827

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Scheme 12. Oxidative Ga2N2 Ring Formation Involving Digallenes ArGaGaAr

by condensation (route C in Scheme 1),197 with a major driving force for ring formation being the production of volatile coproducts containing thermodynamically stable bonds (e.g., in H2, alkanes, silanes, or trialkylsilyl halides) or inorganic salts (generally LiCl and NaCl). A nice illustration of this concept is the convergent synthesis of [iBu2AlP(SiMe3)2]2 (287) by elimination of either Me3SiH or molecular hydrogen (Scheme 13); the latter reaction takes advantage of Al−Hδ−---δ+H−P protic−hydridic interactions to facilitate the extrusion of H2.198 An underlying theme of this review is that smaller substituents encourage dimerization/ring formation by enabling requisite orbital interactions to transpire without significant destabilizing steric interactions occurring between the side groups. For example, thermally induced butene elimination converts one of the peripheral tBu groups within the monomeric phosphinoalane tBu2Al−PtBu2 into an aluminum-bound hydride to yield a cis/trans mixture of the Al2P2 dimer [tBuAl(H)PtBu2]2 (288) (eq 11).199

amines as outlined in eq 10; Table 2 summarizes examples of these species that have been reported.

The thermally induced condensation reaction between Me2InCp and H2NtBu was shown to afford the dimeric indane [Me2In(μ-NHtBu)]2 (278) along with volatile cyclopentadiene, CpH, as a coproduct.186 Both the cis and trans isomers of the isopropyl amide derivative [Me2In(μ-NHiPr)]2 (279) were identified by X-ray crystallography.187 Dimeric and oligomeric diorganoindium azides with central In2N2 ring motifs, such as [Me2In(μ-N3)]2 (280) (Figure 11), were shown to give InN films via CVD;188 a novel dihaloazidoindane, [Cl2In(μN3)(THF)]2 (281; Figure 11), was structurally characterized by the group of Kouvetakis.189 InGaAsN heterostructures were grown by molecular beam epitaxy (MBE) using volatile [(Et2N)2In(μ-NEt2)]2 (282) as an In−N source, thus demonstrating the utility of using inorganic rings as precursors to solid-state materials of increased compositional complexity.190 The dimeric azaindatrane 283 (Figure 11) was prepared via aminolysis among Me3tren, N(CH2CH2NMeH)3, and [(Et2N)2In(μ-NEt2)]2 (282).191 Heterocycles containing both Tl and N are still quite rare, with the first well-defined system reported in the literature being [Tl(μ-N(SiMe3)2]2 (284) (Figure 12).192 Deprotonation of 1,3-bis(4-nitrophenyl)triazene followed by reaction of the resulting triazenide with TlNO 3 in water gave [Tl(ArNNNAr)] 2 (Ar = 4-O 2 NC 6 H 4 ) (285).193 A novel butterfly-shaped Tl2N2 ring was prepared in the form of the thallium(I) amide [1,8-{(Me3SiN)Tl}2C10H6] (286).194 With recent advances in ligand design,195 thallium-containing heterocycles should become more prevalent in the general literature; however, it is expected that this field will be dominated by Tl(I) species due to the redox instability of many Tl(III) compounds.196 3.3.3. Inorganic Heterocycles Containing Heavier Group 13 and 15 Elements. Cyclic group 13/15 element species with heavier inorganic elements are generally prepared

The parent phosphinoalane H2P−AlH2 can be intercepted with the assistance of dative interactions with Lewis acidic (LA) and Lewis basic (LB) groups at the P and Al centers, respectively. This concept, largely advanced by the group of Scheer,200,201 enables chemistry with such reactive main group hydrides to be investigated without the need for specialized equipment or conditions as oligomerization/side reactions are suppressed. The novel adduct Me3N·H2Al−PH2·W(CO)5 (289) can undergo stepwise dehydrogenation processes to give first the Lewis acid/base-stabilized [HAlPH]3 trimer (290; Scheme 14), which transforms into the bicyclic species 291 with further loss of H2 upon sonication in solution.202 Recently, the same group prepared the Al2P2 heterocycles [(CO)5W· HP−AlH·NEt3]2 (292) and [(CO) 5W·PhP−AlH·NMe3]2 7828

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Table 2. Summary of Structurally Characterized [R2GaNR′2]2 Heterocycles

compd no.

R

R′

235177 236177 237177 238178 239178 240178 241179 242179 243179 244179 245179 246179 247180 248181 249181 250173a 251173a 252173a 253173a 254172 255173b 256173b 257173b 258173b 259173b 260174 261174 262169 263169 264169 265169 266173c 267173c 268170 269182 270183 271171 272176 273184 274175 275185 276185 277174

N3 and Me2N N3 and Me2N N3 and Me2N PhMe2CCH2 PhMe2CCH2 PhMe2CCH2 Me Et i Pr Me Me Et Me Me Et Me and NMe2 Et and NMe2 n Bu and NMe2 Ph and NMe2 H Ph Ph Ph Ph Ph t Bu and H t Bu and Cl Cl Cl Cl Br H and NMe2 Ph and NHNMe2 H N(SiMe3)2 and Me t Bu H 2,6-Dipp2C6H3 (ArDipp) Me 2,6-Dipp2-4-(Me3Si)C6H2 and H Me Et t Bu, Cl

Me −CH2CH2NMe2 and Et 2,2,6,6-tetramethylpiperidino H n Pr and H Ph and H Me2N and H Me2N and H Me2N and H t BuNH and H Ph2N and H Ph2N and H 4-FC6H4 and H Et3Si and H Et3Si and H Me Me Me Me Me2N and H Me and H Et and H n Pr and H i Bu and H Ph and H Et Et Me3Si and H PhMe2Si and H t Bu and H Me3Si and H Me Me2N and H t Bu and H H t BuNH and H Me Ph2CN PyCH2 and H H PhNH and H PhNH and H Et

Ga−N bond length (Å) 1.992(3), 1.998(3)

2.037(3), 2.082(4)

2.026(2), 2.031(2)

2.025(4), 2.033(4) 2.016(3)−2.034(3)

1.991(5)−2.024(6)

2.025(3) 2.0205(11), 2.0218(11)

1.973(2), 1.977(2) 2.0099(15), 2.0230(15) 1.9935(18), 2.0244(18) 1.989(4), 1.992(3) 1.995(2), 2.004(2) 2.038(2) 1.862(3)−1.8991(3) 2.017(3) 1.986(2), 1.988(2) 2.019(4)−2.034(3) 2.025(2)−2.044(2) 1.971(2)−2.043(2)

(293) from the reaction of R3N·AlH3 (R = Et and Me) with H3P·W(CO)5 and PhPH2·W(CO)5, respectively; notably, two isomeric forms with cis- and trans-disposed amines were observed to be in equilibrium in solution.203 The Schulz group reported an elegant general route to group 13/15 element heterocycles via transmetalation between endocyclic aluminum centers and its heavier element congeners Ga, In, and Tl (for example, see eq 12).204 In this approach, aluminum−pnictogen rings [Me2Al(μ-E(SiMe3)2)]x (E = P, As, Sb, and Bi; x = 2 or 3; 294) are combined with a variety of

Figure 11. Selected bonding environments about In2N2 rings. 7829

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Figure 12. Tl2N2 rings known in the literature.

Scheme 13. Convergent Synthesis of [iBu2AlP(SiMe3)2]2 (287)

Figure 13. Aluminum−pnictogen-based four-membered rings.

sterically encumbered primary alane H2AlArMes in the bulk phase with liquid H2EPh (E = N, P, and As) to give the homologous series [ArMes(H)Al−E(H)Ph]2 (E = N, P, and As; 302−304).207 An interesting mixed pnictogen Al−P−Al−As heterocycle, [Et2Al(μ-P(SiMe3)2)AlEt2(μ-As(SiMe3)2)] (305) (Figure 13) was synthesized by equilibration of a 1:1 mixture of [Et2AlP(SiMe3)2]2 (306) and [Et2AlAs(SiMe3)2]2 (307) at 45 °C under ultrasonic conditions.208 The Schnöckel group described the green-colored triplet biradical [(tBu2P)Al(μ-PtBu2)]2 (308), which formally possesses unpaired spin density at each Al(II) site. This product was isolated from the reaction of metastable AlCl with Li[PtBu2] (Scheme 15); the formation of 308 is thought to proceed by in situ generation of “AlCl2” via disportionation of AlCl, which is intercepted by the di-tert-butyl phosphide anion.209 By repeating the reaction with AlBr as an aluminum source, while keeping the reaction mixture temperature below −20 °C, the yellow diamagnetic isomer (309) was also isolated. Attempts to directly confirm the radical character in 308 by EPR spectroscopy were thwarted by the apparent instability of this species in solution. The reaction of AlCl3 with Li2PnSiR3 yields either hexagonal prismatic (Pn = As; SiR3 = Si(CMe2iPr)Me) or ladder-shaped (Pn = P; SiR3 = SiiPr3) arrays derived from fused Al2Pn2 rings; treatment of these extended structures with Lewis bases afforded the butterfly-shaped cyclic species [Et3N·ClAl(μ-AsSi(CMe2iPr)Me2)]2 (310) and the planar Al2P2 heterocycle [py·ClAl(μ-PSiiPr3)]2 (311), respectively (Scheme 15).210 The synthesis of compounds with Al−Sb and Al−Bi bonds is a challenging field of main group element chemistry, with only a handful of ring structures known. However, the Schulz group

DMAP−trimethyltriel adducts, DMAP·MMe3 (M = Ga, In, and Tl), to yield group 13/15 heterocycles [Me2M(μ-E(SiMe3)2)]x (x = 2 or 3; 295) of controllable composition with DMAP· AlMe3 as a readily separated byproduct;204 this discovery should open up new CVD approaches to binary materials of tunable optoelectronic properties. Considering that phosphorus and arsenic exhibit overall similar reactivity, it is not surprising that the preparation of arsinoalane dimers [R2Al(μ-AsR′2)]2 follows protocols analogous to those of the Al2P2 systems. For example, Wells and coworkers reported a series of sterically encumbered alkylaluminum−phosphorus and −arsenic compounds, 205 [Br(Me3SiCH2)AlPn(SiMe3)2]2 (Pn = P and As; 296 and 297) and [(Me3SiCH2)2AlPn(SiMe3)2]2 (Pn = P and As; 298 and 299), via lithium halide elimination. Driess and co-workers combined the bulky primary silylarsanes iPr3Si−AsH2 and Me2(iPrMe2C)Si−AsH2 with H3Al·NMe3 to form the Me3Ncoordinated dimers [Me3N·HAl−As(SiiPr3)]2 (300) and [Me3N·HAl−As(SiMe2(CMe2iPr))]2 (301), respectively (Figure 13).206 The Power group investigated the reaction of the

Scheme 14. Dehydrogenation of a Donor−Acceptor-Stabilized H2Al−PH2 Unit Leading to Ring Structures

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Scheme 15. Structural Diversity within Cyclic Al2P2 Units

Table 3. E2Pn2 Heterocycles (E = Ga and In; Pn = P, As, and Sb) Reported since 2000 compd no.

E

R

Pn

R′

E−Pn bond length (Å)

318215 319216 320217 321218 322219 323220 324221 325222 326222 327223 328224 329225 330226 331227 332223 333228 334229

Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga In In In In In

CH2SiMe3 Cl and Fe(CO)2Cp H and C(SiMe3)3 Et Cl Me Et Et Et i Pr and Cl t Bu t Bu t Bu Et CH2Ph CH2tBu Mes

P P P P P P P As Sb As Sb Sb P P As P P

SiMe3 SiMe3 Ph H and Si(CMe2iPr)Me2 H and SitBu3 H and SitBu3 SiMe3 SiMe3 SiMe3 t Bu n Pr or iBu Et H and SiMe3 H and SiiPr3 t Bu i Pr Et

2.4843(11), 2.4931(11)

explored thermally induced dehydrosilylation (50 to 140 °C) between Sb(SiMe3)3 and the diorganoaluminum hydrides Et2AlH and iBu2AlH to give the highly moisture-sensitive heterocycles [Et 2 AlSb(SiMe 3 ) 2 ]2 (312) and [ i Bu 2 AlSb(SiMe3)2]2 (313), respectively;211 the cyclic species [tBu2AlE(SiMe3)2]2 (E = Sb and Bi; 314 and 315) and [Et2Al(μSbtBu2)]2 (316) were prepared using a method parallel to that mentioned above.212,213 Weidlein and co-workers reported a detailed study concerning the synthesis and structure of a wide range of group 13/15 hybrid rings of the general formula [Me2EPn(MMe3)2]2 or 3 (E = Al, Ga, and In; Pn = P and As; M = Si, Ge, and Sn; 317).214 Since many synthetic routes to Ga- and In-containing heterocycles with heavier pnictogen (group 15) elements are available, a large number of structurally authenticated species have been reported since 2000; Table 3 provides a summary of known [R2E(μ-PnR′2)]2 heterocycles (E = Ga and In; Pn = P, As, and Sb; 318−334). The thermolysis of inorganic heterocycles continues to be a viable route toward binary bulk materials and nanomaterials as demonstrated by the conversion of [nBu2E(μ-E′tBu2)]2 (E = Ga and In; E′ = P and As; 335 and 336) into EP and EAs materials upon heating to ca. 500 °C.230,231 Moreover, GaSb can be obtained from [tBu2Ga(μ-SbEt2)]2 (337) by CVD.232 An interesting phosphide-instigated dehydrohalogenation reaction was reported which converts a Ga2P2 heterocycle with four-coordinate Ga centers (338) into a planar Ga2P2 ring with three-coordinate gallium environments (339; eq 13).219

2.483(1), 2.4895(9) 2.383(1), 2.394(1) 2.447(4), 2.462(3) 2.4558(7) 2.5429(6), 2.5434(6) 2.7175(4), 2.7285(4) 2.500(1), 2.529(1) n Pr: 2.736(1), 2.738(1) 2.731(1)−2.735(1) 2.637(1), 2.683(1) 2.7121(5), 2.7164(4) 2.631(3)−2.664(2) 2.6300(12), 2.6364(9)

An In−P heterocycle similar in structure to 339, [Mes*In(μPSitBu3)]2 (340), was prepared by Westerhausen and coworkers.233

The phosphine-coordinated cyclodigalladistibine [(nPr2PPh)ClGa(μ-SbSiiPr3)]2 (341) has been structurally characterized by the von Hänisch group,234 as well as a Ga2As2 heterocycle (342) wherein the exo Ga−As bonds have partial π-bonding character by virtue of formal anionic charges at the terminal As atoms and the cofacial nature of the requisite Ga(p) and As(p) orbitals for multiple bonding (Figure 14).235

Figure 14. Representative Ga2As2 and Ga2Sb2 heterocycles. 7831

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3.3.4. Inorganic Heterocycles Containing Heavier Group 13 and 16 Elements. Organoaluminum compounds containing Al−O bonds have been intensively studied due to the discovery that methylalumoxane (MAO) is an extremely potent activator in the polymerization of ethylene and propylene by group 4 metallocenes.236 There have been several attempts to prepare discrete molecular organoalumoxanes [RAlO]x containing Al−O rings as structural models as the exact structure of MAO is not known. For example, the Barron group reported the dimeric dimethylaluminum alkoxide species [Me2Al(μ-OEPh3)]2 (E = C and Si; 343) by treating AlMe3 with the protic reagents HOCPh3 and HOSiPh3 (releasing volatile methane as a byproduct).237,238 The reactions of AlMe3 with 9-phenylfluorenol and 9-fluorenone give the fluorenecontaining compounds [Me2Al(9-R-fluoroxy)]2 (R = Ph and Me; 344).237 2,6-Diphenylphenol yields a similar Al2O2 ring, [Me2Al(μ-O(2,6-Ph2C6H3))]2 (345), in the presence of AlMe3.239 The Henderson group studied the reaction of Me3Al with a series of aromatic ketones to afford a mixture of dimethylaluminium enolates (346) or alkoxides (347), depending on whether enolization or alkylation transpired (Scheme 16).240 Similarly, [Ph2Al(μ-O(Trip)CCH2)]2 (348) is

Scheme 17. Survey of Al2O2 Heterocycles and the Synthesis of 357

the boryl oxide OBMes2 (Scheme 17); furthermore, equilibria between various ring structures could exist in solution, providing a pathway for incorporating less encumbered groups at the bridging positions.245 When less hindered functional groups are positioned at aluminum, five-coordinate environments can be isolated with cyclic structural motifs commonly encountered. For example, dialkylamino alcohols, R2N(CH2)nOH (n = 2 and 3) combine with group 13 metal alkyls to give a family of five corresponding amino alkoxides, 358−361, with dynamic structural behavior occasionally noted (Scheme 18).246

Scheme 16. Competitive Enolization and Alkylation Leading to Al2O2 Rings

Scheme 18. Al2O2 Heterocycles with Five-Coordinate Al Environments

formed from a related enolization/dimerization process between triisopropylphenyl methyl ketone and Ph3Al.241 The hydroxylamide-supported heterocycles [tBu2Al(μ-ONMe2)]2 (349) and [Me2Al(μ-ONiPr2)]2 (350) have been synthesized by protolytic cleavage of an Al−C bond in trialkylaluminum reagents with Me2NOH and iPr2NOH, respectively (Scheme 17).242 The structurally related peripherally fluorinated aluminum alkoxide dimer [(F5C6O)2Al(μ-OC6F5)]2 (351) was recently reported.243 When 1-methylcyclohexanol was allowed to react with mixtures of Li[AlH4] and AlCl3 in diethyl ether solvent, the haloalane rings [ClAl(H)(OCyMe)]2 (CyMe = 2-methylcyclohexyl) (352) and [Cl2AlO(CyMe)]2 (353) were obtained.244 The Serwatowski group also took advantage of smooth protonolysis between stable organohydroxides R2BOH and alkylaluminum reagents to construct the boryl oxidecapped aluminum oxide rings [R2Al(μ-OBMes2)]2 (R = Me and Et; 354 and 355) and [Me2Al(μ-O-9-BBN)]2 (356; 9-BBN = 9-borabicyclo[3.3.1]nonane; Scheme 17). Upon treatment of 354 with 2 equiv of tBuOH, alcoholysis of the Al−C bonds transpired wherein the initially bridging OBMes2 groups adopt nonbridging terminal ligation about Al in the final product [Me(Mes2BO)Al(μ-OtBu)]2 (357). This reaction can be rationalized on the basis of the increased electron density (and nucleophilicity) within the bridging OtBu group relative to

Extended structures bearing higher coordinate Al environments and four-membered Al2O2 arrays have also been reported in the literature. Specifically, the Kunicki group interacted various primary, secondary, and tertiary alcohols with MeAlCl2 to generate both tri- and tetrametallic aluminum clusters (363−370; Figure 15). In the case of tBuOH, a 1:1 condensation reaction transpired to give the alkoxide [tBuOAlCl2]x with an 27Al NMR signal (90 ppm) that suggested the presence of four-coordinate Al centers; this species decomposes over time to give an insoluble solid with accompanying gas evolution.247 7832

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generated from a mixture of R3Al with MesCH2SH.255 Related cyclic aluminum alkanethiolate complexes of the general form [(RS)2Al(μ-SR)]2 are also known.256,257 Heavier element cyclic organoaluminum chalcogenides (Al2Se2 and Al2Te2 rings) can be prepared using parallel condensation chemistry as noted for their O and S congeners. However, atom insertion is also a productive synthetic route to these rings as typified by the insertion of elemental Se into the Al−Me bond in (Me3Si)3CAlMe2·THF to yield the stable product [(Me3Si)3CAl(Me)(μ-SeMe)]2 (381; Figure 17).258 Similarly, tBu3Al reacts with the group 16 elements S, Se, and Te to yield the dimeric species [tBu2AlEtBu]2 (E = S, Se, and Te; 382−384).259 Furthermore, the Roesky group took advantage of chalcogen insertion chemistry with the aluminum(III) hydride [H2C(CH2NDipp)2]AlH·NMe3 in hot toluene to afford the aluminum chalcogenides [H2C(CH2NDipp)2Al(μE)]2 (E = S, Se, and Te; 385−387) and molecular H2 as a byproduct (Figure 17).260 Following a related approach, the Roesky group formed the poorly soluble Se and Te rings [{Dippnacnac}Al(μ-E)]2 (E = Se and Te; 388 and 389) from the reaction of the hydrido precursor [Dippnacnac]AlH2 with elemental Se or Te in the presence of catalytic phosphine (PMe3 and P(NMe2)3).261 The Raston group reported the reaction of Me3N·AlH3 with elemental selenium or tellurium to give [Me3N·AlH(μ-E)]2 (E = Se and Te; 390 and 391; Scheme 20).262 The selenide 390 further reacts with diorganodichalcogenides PhEEPh to convert the remaining Al−H linkages into thioether, selenoether, and telluroether residues, leading to the functionalized species [Me3N·Al(E′Ph)Se]2 (E′ = S, Se, and Te; 392−394).263 Al2Se2 and Al2Te2 rings containing bidentate ligands at aluminum (e.g., compounds 398 and 399 in Scheme 20) have been reported.261,264−266 Moya-Cabrera and co-workers prepared the novel chalcogen heterocycle 401 from the condensation of [(Mesnacnac)AlH]2(μ-O) (400) (Mesnacnac = [HC(MeCNMes)2]−) with metallic tellurium (eq 15).267

Figure 15. Structure of tri- and tetrametallic Al2O2 heterocycles.

The stable monomeric aluminum(III) hydroxy complex [(Dippnacnac)AlMe(OH)] (Dippnacnac = [HC(MeCNDipp)2]−) undergoes dehydrogenative coupling with a stoichiometric amount of H3Al·NMe3 to form the tetraalane [{DippnacnacAl(Me)](μ-O)(AlH2)]2 (371) supported by a planar Al2O2 ring;248 a hexanuclear aluminum complex, 372, was recently prepared by Ziemkowska et al. (Scheme 19).249 In addition to their use as activators for olefin polymerization (cf. MAO), extended polyaluminum complexes such as 373 (Figure 16) have been used as catalysts for the ring-opening polymerization of cyclohexene oxide with high polymer molecular weights reported (Mn values higher than 106 Da in toluene).250 The Brintzinger group synthesized N,N-dimethylaniline (DMA)-stabilized cations [Me2Al(μ-OSiR3)2AlMe· NMe2Ph]+ (R = OSiMe3 or Me/tBu; 374 and 375) and demonstrated that these species were suitable activators for ethene polymerization when partnered with Cp*2ZrCl2.251 The dimeric amino alkoxide-substituted complex [Me2Al(μ-OCMe2CH2NHPh)]2 (376) showed high catalytic activity in both the bulk and solution-phase ROP of L-lactide.252 Power and co-workers generated the low-coordinate organoaluminium sulfide [Mes*Al(μ-S)]2 (377) from the reaction of [Mes*AlH2]2 (Mes* = tBu3C6H2) with S(SiMe3)2 (eq 14). The

Lewis acidic nature of the Al centers in 377 was confirmed by the addition of 2 equiv of Me2SO to give the adduct [Mes*Al(OSMe2)(μ-S)]2 (378).253 The base-stabilized iodoaluminum sulfide [Et3N·Al(I)S]2 (379) was reported by the Schnö ckel group,254 while the sterically congested Al2S2 aluminum thiolate heterocycles (380; Figure 16) can be

Gallium and indium aryloxides and alkoxides are also known to adopt cyclic arrangements [R2M(μ-OR)]2 (M = Ga and In),

Scheme 19. Synthesis of Tetranuclear Aluminum Heterocycles

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Figure 16. Activators for ring-opening polymerization and structurally related Al2S2 heterocycles.

also synthesized a wide range of group 13 element−oxygen rings for CVD applications, including the following planar Ga2O2 heterocycles with five-coordinate Ga centers: [Et2Ga(μOR)] 2 (R = CH 2 CH 2 NMe 2 , CH(CH 3 )CH 2 NMe 2 , C(CH3)2CH2OMe, and CH(CH2NMe2)2; 405−408). Accordingly, these volatile precursors were used to make binary Ga2O3 thin films on glass by aerosol-assisted CVD.272 Figure 18 lists other five-coordinate alkoxygallium rings known in the literature (cf. 409,273 410,273 411,274 412,274 413,275 414,276 415,276 416,277 and 417−419278); interestingly, compound 412 can be used as an initiator for living rac-lactide polymerization, affording atactic polylactide (PLA) in the process.277 In the presence of nitrogen-based donors (e.g., 1,8diazabicyclo[5.4.0]undec-7-ene (DBU)), compound 416 (Figure 18) can catalyze the formation of diblock polylactides comprised of isotactically and heterotactically enriched blocks.279 The peroxy-substituted gallium−oxo heterocycle 420 was obtained by bubbling oxygen through a solution of t Bu2GaCH(SiMe3)2 (eq 16); compound 420 is quite thermally stable and resists decomposition in the solid state up to 132 °C.280

Figure 17. Selective Al2E2 (E = S, Se, and Te) rings from the literature.

Scheme 20. Cyclic Products Derived from Chalcogen Atom Transfer

with the most common synthetic routes to these species being the addition of alcohols to trivalent gallium or indium organometallic reagents [R3M or R2 M-NR′2] and salt elimination (Scheme 21). This field was reviewed by Carmalt and King in 2006.268

The Murugavel group reported the reaction of 1-(2hydroxyethyl)-3,5-dimethylpyrazole (HL) with AlCl3, GaCl3, and InBr3, which affords the dinuclear complexes [Al(μ-L)Cl2]2 (421), [Ga(μ-L)Cl2]2 (422), and [In(μ-L)Br2(H2O)]2 (THF)2 (423), respectively.281 Jain and co-workers prepared a series of air- and moisture-stable, photoemissive monoorganogallium and -indium complexes (424−435) derived from dianionic tridentate Schiff bases (Figure 19).282 Photoluminescence was noted at room temperature, and the quantum yields (1−7%) and emission wavelengths (460−680 nm) were mainly affected by the nature of the substituents attached to the aryl residues of the ligands instead of the group 13 element present. Thin films of In2O3 can be grown from mixtures of Me3In and donor-functionalized alcohols ROH (R = CH2CH2NMe2, CH(CH3)CH2NMe2, C(CH3)2CH2OMe, and CH2CH2OMe) under aerosol-assisted chemical vapor deposition (AACVD) at 550 °C; this process likely involves the formation of dimethylindium alkoxides [Me2In(μ-OR)]2 as intermediates.283 Mehrkhodavandi and co-workers demonstrated that the dimeric chiral indium salen complex 436 (Figure 20) is a highly active, isoselective catalyst for the ROP of racemic

Scheme 21. Commonly Employed Synthetic Routes to Ga2O2 and In2O2 Heterocycles

Wehmschulte and co-workers generated the terphenylsubstituted Ga2O2 heterocycle [ArMesGanBu(μ-OH)]2 (402) from the partial hydrolysis of ArMesGanBu2.269 Various complexes containing a formal cyclic [Ga2(μ-OH)2]4+ core are known.270 In the search for improved CVD precursors to binary materials such as Ga2O3, Mitzel and co-workers prepared the coordinatively labile complexes [tBu2Ga(μOCH2CH2NHtBu)]2 (403) and [tBu2Ga(μOCH2CH2CH2NHtBu)]2 (404).271 The Carmalt group have 7834

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Figure 18. Various Ga2O2 ring motifs.

Figure 19. Structures of photoemissive Ga2O2 and In2O2 heterocycles.

A benzoxaborole reagent was condensed with various group 13 metal trialkyls to form a series of group 13 metal benzoxaborolate dimers (437−439) with planar M2O2 cores (eq 17).286

Figure 20. Thermal ellipsoid drawing (50%) of the chiral ROP catalyst 436. H atoms and solvent molecules are not shown for clarity. Reprinted with permission from ref 284. Copyright 2013 Royal Society of Chemistry.

lactide.284,285 These complexes retain their dinuclear structures during lactide polymerization, leading to stable polymerylbridged states that resist chain termination, resulting in narrow polydispersities within the polymeric products.

Table 4 lists examples of small inorganic heterocycles [R2M(μ-ER′)]2 containing Ga and In in tandem with intraring-positioned group 16 elements (S, Se, and Te). The 7835

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Table 4. M2E2 Heterocycles [R2M(μ-ER′)]2 (M = Ga and In; E = S, Se, and Te) compd no.

M

R

E

R′

440287 441−444288 445, 446289

In Ga Ga

Me Me or Et HCNDipp

Se S O or Te

447290 448, 449291

Ga In

i

Pr2PNiPr2PTe (Me3Si)3C

Te Se or Te

Ph and Br

450−452292

In

(Me3Si)3C

S, Se, or Te

SPh, SePh, or TePh

453, 454293

Ga or In

ArDipp

O, S

455294 456, 457258 458, 459295

In Ga Ga or In

SCH2C(O)OMe and Me Me and C(SiMe3)3 Ph

S Se or Te S

CH2C(O)OMe Me SnCy3

460−463296

In

Mes

S

t

464297 465297 466297 467298 468299,300 469301

In Ga In In Ga In

CH2tBu CH2SiMe3 CH2tBu t BuS t Bu t Bu

Se Se S S Se S, Se

CH2tBu CH2SiMe3 Ph t Bu t Bu t Bu

470, 471302

Ga

t

BuS or iPrS

S

t

472−474303

Ga

(Me3Si)2C(Ph)C(Me3Si)N

S, Se, or Te

475304

Ga

Me

S

M−E bond length (Å)

Me SiPh3 or SiiPr3

Bu, tert-amyl, 2-tBuC6H4 or SiPh3

Bu or iPr

2-Ph2As−C6H4

Ga−O: 1.814(3)−1.905(3) Ga−Te: 2.6135(10)−2.6217(9) 2.5783(7)−2.653(1) In−Se: 2.6849(5)−2.7026(5) In−Te: 2.8819(5)−2.8935(4) In−S: 2.452(1)−2.626(1) In−Se: 2.7312(9)−2.7601(9) In−Te: 2.9298(1)−2.948(1) Ga−O: 1.826(3)−1.830(3) Ga−S: 2.2162(10)−2.2181(10) In−O: 2.0258(13) In−S: 2.4059(13)−2.4123(13) 2.549(1)−2.730(1) Ga−Te: 2.7434(8)−2.7537(7) Ga−S: 2.342(3) In−S: 2.551(2) t Bu: 2.615(5)−2.622(5) tert-amyl: 2.586(3)−2.598(4) SiPh3: 2.172(3)−2.698(1) 2.7053(14)−2.719(2) 2.523(2)−2.539(2) 2.618(2)−2.6318(14)

In−S: 2.594(2)−2.601(2) In−Se: 2.699(7)−2.704(4) i PrS: 2.3611(6)−2.3799(6) t BuS: 2.3614(9)−2.3961(10) Ga−Se: 2.369(1)−2.372(11) Ga−Te: 2.570(9)−2.576(9) 2.4075(6)−2.4372(6)

In(I) cluster In4[C(SiMe3)3]4 are cleaved in the presence of PhSeBr and PhTeBr, resulting in the oxidation of In to afford the In(III) dimers [(Me3Si)3CIn(μ-SePh)Br]2 (448) and [(Me3Si)3CIn(μ-TePh)Br]2 (449), respectively.291 In4[C(SiMe3)3]4 also reacts with diaryl dichalcogenides REER (E = S, Se, and Te; R = Ph or tolyl) to yield the corresponding In2E2 rings [(Me3Si)3CIn(ER)(μ-ER)]2 (450−452; Figure 21).292 The thermally stable group 13 dimetallenes DippArMMArDipp (M = Ga and In) combine with elemental sulfur to give the centrosymmetric dimers [DippArGa(μ-S)]2 (453) and [DippArIn(μ-S)]2 (454), respectively.293 Briand and co-workers synthesized a series of indium thiolate In2S2 rings (e.g., [Me(SCH2C(O)OMe)In(μ-SCH2C(O)OMe)]2 (455)) and showed that these species can polymerize bulk rac-lactide rapidly with high conversion albeit with broadened polydispersity indices (PDIs).294 Thallium compounds are generally less explored than their lighter congeners due to increased toxicity; however, their use as mild group transfer agents is known in transition-metal chemistry.305 An early report of a Tl2O2 heterocycle appeared from the Roesky group, where they successfully prepared the aryloxide thallium(I) complex [Tl(μ-O-2,4,6-(F3C)3C6H2)]2 (476; Figure 22) and showed that this volatile compound adopts a dimeric structure in the gas phase.306 The Tuck group prepared the stable dimer [Tl(μ-OC6H4C6H4OH)]2 (477) in high yield by treating thallium(I) acetate with 2,2′-biphenol.307

In2Se2 heterocycle [Me2In(μ-SeMe)]2 (440) was synthesized and used as a precursor to fabricate phase-pure, photoluminescent indium selenide thin films.287 In a related study, the Ga2S2 precursors [R2Ga(μ-SSiR′3)]2 (R = Me and Et; R′ = Ph and iPr) (441−444) were converted into pure gallium sulfide upon thermolysis.288 The Jones group has actively explored the reactivity of the anionic gallium(I) N-heterocyclic carbene analogue [(HCNDipp)2Ga:]−, including the reaction of this species with N2O or TePEt3 to give salts containing the dimeric, dianionic gallium(III) species, ([{HCNDipp}2Ga(μ-E)]2)2− (E = O and Te; 445 and 446; Figure 21).289 The tellurium-containing reagent Na[(iPr2PTe)2N] reacts cleanly with GaCl3 to produce the Ga2Te2 heterocycle [Ga(μTe){iPr2PNiPr2PTe}]2 (447) (Figure 21).290 Uhl and coworkers reported that the In−In bonds within the tetrameric

Figure 21. Selected group 13/15 element heterocycles. 7836

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Figure 22. Selected thallium−chalcogen rings.

analogues of cyclopropane (ER2)3 (E = Si, Ge, Sn, and Pb; termed cyclometallanes) was achieved in the 1980s by Masamune and co-workers.316 In general, reduction of halogenated precursors (e.g., X2ER2; path E in Scheme 1) is used to access these low oxidation state (EII) species as exemplified by the synthesis of the first cyclotrisilane, hexakis(2,6-dimethylphenyl)cyclotrisilane [Xyl2Si]3 (489), in 10% yield from the reduction of Xyl2SiCl2 with lithium naphthalenide.316a Following a similar overall protocol, the cyclotrigermane [Xyl 2 Ge] 3 (490) and cyclotristannane [Ar′′2Sn]3 (491) (Ar′′ = 2,6-Et2C6H3) analogues were prepared later by the same group (eq 18).316b,c

The tris(3-tert-butylpyrazolyl)methanesulfonate-bridged Tl(I) heterocycle (478) was also reported.308 The Power group reported a series of Tl2E2 rings, [Tl(μ-EArDipp)]2 (E = O and S; 479 and 480), from the reaction of the hindered phenol and thiophenols ArDippEH with TlCp (eliminating CpH as a byproduct).293 Analogous Tl2S2 and Tl2Se2 heterocycles supported by terphenyl ligands were reported by Niemeyer and co-workers (481 and 482),309 while the Briand group reported the cyclic Tl(III) species [Me2Tl(μ-SPh)]2 (483) and [Me2Tl(μ-SePh)]2 (484).310 A Tl2O2 heterocycle (485) with five-coordinate Tl environments was synthesized by Britton,311 while platinum(II)-containing Tl2O2 heterocycles [RPt{(PPh2O)2H}(PPh2O)Tl]2 (R = C6F5, 486; CCtBu, 487; CCPh, 488) with strong Tl−Pt interactions were reported.312 3.4. Group 14 Element Rings Containing Si, Ge, Sn, and Pb

The chemistry of the “inorganic” tetrel (group 14) elements is typified by the redox flexibility of Si, Ge, Sn, and Pb, wherein both +2 and +4 states can be attained.313 Another trend is the increasing reactivity of many ring systems as the tetrel element becomes heavier, and in the case of cyclotetrelanes [R2E]x, E−E bond scission and generation of singlet :ER2 monomers can occur in solution.314 A more recent addition to this field is the use of either intra- or intermolecular base stabilization to access new rings from reactive precursors, such as the base-stabilized silylenes IPr·SiX2 (IPr = [(HCNDipp)2C:]; X = Cl and Br).315 3.4.1. Homoatomic Group 14 Element Heterocycles. 3.4.1.1. Homoatomic Three-Membered Rings of Group 14 Elements. The isolation of the first stable heavier element

The photoinduced decomposition of 489 produces reactive equivalents of silylene Xyl2Si: and disilene Xyl2SiSiXyl2; the analogous Si3 ring [tBu2Si]3 (492) undergoes similar ring fragmentation.317 Given that the cyclotrimetallanes are intensely colored, E−E bond cleavage by photolysis (mediated by σ → σ* transitions) is a viable synthetic route to generate a wide array of transient silylenes, germylenes, and stannylenes (R2 Si:, R2Ge:, and R2Sn:).318−320,313b,321 It should be mentioned that a number of very informative reviews on the field of trimetallanes have been published by Weiden7837

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bruch,318,322,323 Masamune,324 and Driess and Grützmacher.325 As a result, we will focus on the most recent results in this field. A series of silyl-substituted trisilanes, [(R3Si)2Si]3 (R3Si = t BuMe2Si and iPr2MeSi; 493a and 493b), were prepared by reductive coupling of the corresponding 2,2-dibromosilanes (R3Si)2SiBr2 with lithium naphthalenide.326 The presence of the bulky SiR3 groups engendered the resulting rings with impressive stability to oxygen, water, and alcohols. The Marschner group synthesized the asymmetric cyclotrisilane [((Me3Si)2Si)2SiiPr2] (494; Figure 23) by combining the Mg2+

Scheme 23. Si3 Ring Formation from the Disilene 497 and Syntheses of the Hexasilabenzene and Hexaprismane Analogues 501 and 503

Figure 23. Structural representations of compounds 494 and 495.

salt of the 1,2-dianion [(Me3Si)2Si−Si(SiMe3)2]2− with 1 equiv of iPr2SiCl2 in Et2O.327 Of further note, cyclotrisilane 494 also possesses intraring Si−Si distances in the range of 2.3311(15)− 2.3439(17) Å, which represented the shortest trisilane Si−Si lengths known at the time. The dark brown cyclotrisilane radical anion [Si(NN)]3− [Si(NN) = Si{1,2(tBuCH2N)2C6H4}] was isolated as its sodium salt 495 (Figure 23); the EPR spectrum of 495 is consistent with the unpaired electron being delocalized over the Si3 unit.328 Complicated ligand redistribution leading to the isolation of the asymmetrical cyclotrisilane 496 (Scheme 22) transpired when the dihaloheptasilanes X2Si[SiMe(SiMe3)2]2 (X = Cl, Br) were treated with 2.2 equiv of KC8.329 Scheschkewitz and co-workers also obtained the cyclotrisilane [TripSi(Cl)SiPh2SiTrip2] (498) and its anionic counterpart (499; Scheme 23) via thermally or reductively induced isomerization/cyclization of the inorganic allyl chloride analogue, Trip2SiSi(Trip)Si(Cl)Ph2 (497).330 The rate of ring formation from 497 is accelerated in the presence of donor solvents, such as THF. The same group obtained the novel hexasilabenzene derivative [Trip6Si6] (501) by reducing the cyclotrisilane 500 with lithium naphthalenide (Scheme 23).331 Compound 501 is a green solid that can be exposed for short durations to air without appreciable decomposition, while DFT calculations show that the HOMO, HOMO − 2, and HOMO − 3 states contain transannular bonding contributions involving the low-coordinate silicon centers in the central Si4 unit.331 In addition, compound 500 can be thermally isomerized into the

1,2,3-isomer [TripSi(Cl)]3 (502), which later affords the hexaprismane 503 upon reduction with magnesium metal (Scheme 23).332 The heavier group 14 element cyclotrimetallanes [R2E]3 have been studied less than their silicon congeners; however, these species still offer interesting chemistry due to the presence of weakened intraring E−E interactions which can lead to tetrelylene-type, :ER2, reactivity. Following Masamune’s first synthesis of a cyclotrigermane, [Xyl2Ge]3 (490) (Xyl = 2,6Me2C6H3), in 1982,316b the structurally related cyclotrigermanes [Mes 2 Ge] 3 (504), 333,334 [ t Bu 2 Ge] 3 (505), 335 [(Me3Si)2Ge]3 (506),336 and cis- and trans-[(tBu3Si)GeCl]3 (507a and 507b)337 were prepared. The high degree of steric congestion about the Ge3 ring in 505 imparted by the peripheral tert-butyl groups leads to the presence of long Ge− Ge bonds of 2.563(1) Å (av). Photolysis of 507a/b in the presence of the trapping agent Et3SiH suggested the initial formation of a product mixture containing the germylene [( t Bu 3 Si)ClGe:], the digermene [( t Bu 3 Si)ClGeGeCl(SitBu3)], and the germylgermylene [(tBu3Si)Cl2Ge−Ge(SitBu3)]; the latter species is likely formed by a 1,2-chlorine migration involving the digermene.338 Cyclotristannanes [R2Sn]3 were the subject of a review in 1991.324 Sita and co-workers have investigated the thermolysis/ ring degradation of aryl-susbtituted cyclostannanes (e.g., [Ar(508) (Ar′′2Sn]3 (508) (Ar(508) (Ar′′ = 2,6-Et2C6H3)) to afford a number of structurally interesting Sn clusters [Ar′′Sn]x, including the dark blue-violet pentastanna[l.1.l]-

Scheme 22. Synthesis of the Cyclotrisilane 496 via Ligand Redistribution Followed by Silylene Insertion

7838

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Figure 24. Inorganic cyclopropene analogues and oxidative addition of a chemical equivalent of chlorine to give the cyclotrisilane 515.

propellane (509; Figure 24) and the disproportionation product Ar′′3Sn-SnAr′′3 (Figure 24).339−342 The cyclotristannane [Ar′′2Sn]3 (508) can be electrochemically reduced to give the radical stannylene anion [Ar′′2Sn]•−, while oxidation of 509 produces the radical cation of tristannane [Ar′′2Sn]3•+.343 The only known cyclotriplumbane, 510, was prepared by Weidenbruch and co-workers in 2003 from the reaction of 2 equiv of (2,4,6-triethylphenyl)magnesium bromide and lead(II) bromide in a THF/dioxane solvent mixture; dioxane was added to facilitate the separation of the MgBr2 byproduct in the form of insoluble MgBr2·dioxane (eq 19).344 Compound 510 is a

Scheme 24. Formation of the Cyclotetrasilene 516b

sodium metal; this red-orange solid contained a short internal SiSi double bond of 2.138(2) Å, while thermal stability to ca. 200 °C was noted.350 Later it was shown that 517 can undergo ring insertion with phenylacetylene to yield the fused heterocycle 518 (eq 20).351

soluble black solid that is thermally stable up to 80 °C, and Xray crystallography revealed Pb−Pb bond lengths [3.184(5) Å, av] that are longer than in the diplumbane Ph3Pb−PbPh3 [2.944(4) Å]. Despite the presence of weak Pb−Pb bonds in 510,207Pb NMR spectroscopy suggests that 510 retains a trimeric (cyclic) structure in solution. Three-membered inorganic cyclopropenes (R4E3 (511); E = Si, Ge, and/or Sn; Figure 24) represent a more recent class of homocycle and feature strained EE double bonds as part of the ring framework.345 The Sekiguchi group first isolated a stable member of this ring class (termed trimetallenes),346 and their active research pursuits in this field have been reviewed.345 The initially uncovered route to these species involved reaction of the anionic silyl and germyl species [tBu3Si]− and [tBu3Ge]− with GeCl2·dioxane to yield the cyclic trigermenes 512 and 513 via a disproportionation reaction, and these products were isolated via column chromatography under argon (Figure 24).346 The Kira group prepared a stable cyclotrisilene, 514, from the reduction of the trihalodisilane R3Si−SiBr2Cl (R = t Bu2MeSi) with potassium graphite (KC8) in THF. The Si− Si double bond in 514 is quite reactive and can be readily chlorinated with CCl4 to give the 1,2-addition product 515 (Figure 24).347 Irradiation of 514 at low temperature initiates a formal ring expansion transformation to yield the bicyclic tetrasilylene [(tBuMe2Si)2Si−Si(SiMe2tBu)]2 (516a), which isomerizes to the thermodynamically favored cyclotetrasilene (516b) upon warming the reaction mixture to 0 °C (Scheme 24).348,349 A structurally related cyclotrisilene, [(tBuMe2Si)4Si3] (517), was reported by the Sekiguchi group from the coreduction of (tBuMe2Si)2SiBr2 and (tBuMe2Si)SiBr3 with

Wiberg’s disilyne (tBu3Si)2MeSiSiSiSiMe(SitBu3)2 readily isomerizes in THF solvent to give the orange-colored cyclotrisilene (519),352 while thermally induced silyl group transfer (120 °C in C 6 D 6 ) from the trisilaallene (tBu2MeSi)2SiSiSi(SiMetBu2)2 also generates a trisilene ring, [(tBu2MeSi)4Si3], with tBu2MeSi groups at the periphery.353,354 The reactivity of the SiSi double bonds in trisilenes was probed via reaction with isonitriles (RNC),355 while the Cp*-appended trisilene 520 is sufficiently electrophilic to enable reversible coordination of an N-heterocyclic carbene (resulting in the formation of the adduct 521; Figure 25).356 It

Figure 25. Structure of 519 and the trisilene−NHC adduct 521.

was also shown that cyclotrisilene 520 is a viable alternate precursor for the formation of the hexasilabenzene cluster Trip6Si6 (501) via the lithium naphthalenide-initiated extrusion of Cp*Li.357 More recently, the same group reported the synthesis of a carbene−disilenylsilylene adduct, Trip2Si Si(Trip)Si(Trip)·ImMe2iPr2 (523) (ImMe2iPr2 = [(MeCNiPr)2C:]), from the ring-opening of the cyclotrisilene (522). As shown in Scheme 25, this process involves first the coordination of a carbene donor to the cyclotrisilene (522) to 7839

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Scheme 25. Synthesis of NHC−Disilenylsilylene Adduct 523

Scheme 26. Synthesis of Cyclotrimetallenes 528a and 528b

prepared by Wiberg and co-workers by reacting either Sn(OtBu)2 or Sn[N(SiMe3)2]2 with excess supersilylsodium [tBu3Si]Na (Scheme 27);365 a second cyclotristannene, Sn3[SiScheme 27. Synthesis of the First Cyclotristannene (531)

form 524, followed by substantial ring-opening to give 523 above ca. −30 °C; at higher temperatures carbene dissociation gives free ImMe2iPr2 and the cyclotrisilene 522 as determined by UV−vis and NMR spectroscopy.358 A general synthetic route to unsymmetrically substituted cyclotrigermenes 526 was uncovered by adding the germyl (R3Ge−) or silyl (R3Si−) anions to the cyclotrigermenium salts [(tBu3Si)Ge]3+[(3,5-(F3C)2C6H3)4B]− (525) (eq 21);359 more-

(SiMe3)3]4 (532), was formed along with the Sn10 metalloid cluster Sn10[Si(SiMe3)3]6 from the reaction of Li[Si(SiMe3)3] with metastable SnIBr; compound 532 forms black diamondshaped crystals, and a short SnSn double bond length of 2.575(4) Å was determined by X-ray crystallography.366 3.4.1.2. Homoatomic Four-Membered Rings of Group 14 Elements. The first examples of cyclosilanes [R2Si]x were prepared by Kipping and co-workers,367 with structural authentication later provided by the group of Gilman.368 The cyclotetrasilane octamethylcyclotetrasilane [Me2Si]4 (534) was formed as a product from the photolysis of dodecamethylcyclohexasilane in cyclohexane at 45 °C.369 Table 5 provides a summary of recently prepared cyclotetrasilanes [R2Si]4 and cyclotetragermanes [R2Ge]4 (535−555), with the predominant synthetic route being the Wurtz-type reduction of E(IV) dihalo precursors R2EX2 (E = Si and Ge; X = halogen) with alkali metals. An interesting series of chemical transformations leading to the formation of a silacyclobutadiene ligand, η4R4Si4 (R = SiMetBu2), was reported by the Sekiguchi group. Reduction of the bromo-functionalized cyclotetrasilane 545 with KC8 gave an unsaturated cyclotetrasilene, 556, which was later transformed in high yield to the silacyclobutadiene complex [(η4-R4Si4)Fe(CO)3] (557) upon treatment with excess Collman’s reagent, Na2[Fe(CO)4], in THF (Scheme 28).370 Recently, Iwamoto and co-workers obtained air-stable colorless crystals of the persila[1]staffane 559 by converting the cyclotetrasilane [(Me3Si)2SiSi(iBu)2]2 (547) into the dipotassium intermediate 558, followed by treatment with i Bu2SiCl2 (Scheme 29).371 The same group synthesized the silylenecyclotetrasilane 549 containing an exocyclic SiSi linkage and a nearly planar Si4 cyclotetrasilane ring by combining the halogenated cyclic precursor 548 with Cl2SiR2 (R = SiMe2tBu) in the presence of KC8 as a reducing agent (Scheme 29).372

over, addition of potassium halide salts to 525 yields the halidebound cyclotrigermenes 526f (eq 21).360 Less hindered tricyclogermenes have been shown to undergo both [2 + 2] and [4 + 2] cycloaddition reactions between the strained Ge Ge intraring π-bond and alkynes and dienes, respectively.361 Cyclotrigermenes and hybrid cyclodigermasilirenes (SiGe2 heterocycles) can be accessed via salt elimination/condensation processes. For example, treatment of the tetrachlorodigermane (tBu2MeSi)Cl2Ge−GeCl2(SiMetBu2) with the reactive 1,2dilthio species Li2Si(SiMetBu2)2 in toluene gave the SiGe2 ring [R2Si(GeR2)2] (527c; R = SiMetBu2); the same report featured chlorination of the Ge−Ge π-bond in 514 with CCl4 (Figure 24) and efficient ring expansion in CH2Cl2 to give CGe3 and CSiGe2 heterocycles (527; Scheme 26).362 Alkylsubstituted cyclotrimetallenes (528a and 528b) were obtained in moderate yields (55−66%) by treating 527a and 527b with 2 equiv of KC8. As shown in Scheme 26, this reaction proceeds via isolable butterfly-shaped bicyclo[1.1.0]butane derivatives that could be converted into the thermodynamically favored cyclotrimetallenes 528 upon heating.363 A recent addition to the field of Ge ring chemistry is the preparation of the cyclotrigermene [(tBu2MeSi)4Ge3] (529) by thermolysis of the pure digermene [(tBu2MeSi)2GeGe(SiMetBu2)2] at 170 °C for 1 h.364 Two examples of cyclotristannenes (R4Sn3) are known to date: the first stable cyclotristannene, [(tBu3Si)4Sn3] (531), was 7840

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Table 5. Summary of Recently Prepared Cyclotetrasilanes and Cyclotetragermanes

compd no. 373

535 536373 537374 538374 539375 540376 541376 542327 543377 544378 545379 546380 547371 548372 549372 550381 551336 552382 553383 554384 555384

element (E) Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Ge Ge Ge Ge Ge

R t

R′

Bu3Si, I Bu3Si, I Cl Br Me3Si t Bu, H t Bu, Br Me, SiMe3 Ph Me, Cl t Bu2MeSi, Br t BuMe2Si, Br i Bu Me, Br Me, (tBuMe2Si)2Si i Pr, Ph Cl, (Me3Si)3Ge Ph i Pr t Bu t Bu t

E−E bond length (Å)

t

Bu3Si, I Bu3Si, I Cl Br Me3Si t Bu t Bu Me, SiMe3 Me, Ph Me3Si t Bu2MeSi, Br t BuMe2Si Me3Si Me3Si Me3Si t Bu Cl, (Me3Si)3Ge Ph i Pr H, tBu Cl, tBu t

2.362(2), 2.413(1)

2.453(1), 2.454(1) 2.3630(13)−2.3940(13) 2.372(1)−2.380(1)

2.4234(7), 2.4269(7) 2.3786(5), 2.3851(5) 2.3625(8)−2.3773(8) 2.4404(8), 2.4576(8) 2.501, 2.505 2.465 2.4675(9)−2.4786(9) 2.478(1), 2.599(1) 2.508(1)−2.619(1)

intermediate as neither free germylene [:GeiPr2] nor the dimeric digermene [iPr2GeGeiPr2] could be detected in appreciable amounts, ruling out heterolytic Ge−Ge σ-bond cleavage pathways in this process.385 The crystal structures of octa-tert-butylcyclotetrastannane, [ t Bu 2 Sn] 4 (560), and octa-tert-amylcyclotetrastannane, [tAm2Sn]4 (561) (tAm = CMe2Et), were reported in 1986.386 Treatment of [tBu2Sn]4 (560) with I2 results in ring-opening of the cyclic Sn4 framework to afford the linear 1,4-diiodoocta-tertbutyltetrastannane, I−(SntBu2)4−I.387,368b The halosilyl-substituted cyclotetrastannane [(Me3Si)3SiSnCl]4 (562) with a folded Sn4 ring (18.9° deviation from planarity) was prepared from the equimolar reaction of SnCl2·dioxane with Li(THF)3[Si(SiMe3)3].388 Unsaturated cyclotetrasilenes, such as the first member of this series, [(tBuMe2Si)6Si4] (563), can be prepared in low yield from the reductive coupling of dihalosilanes R2SiX2 with the corresponding tetrahalodisilane RSiX2−SiX2R (eq 22).349

Scheme 28. Preparation of the Silacyclobutadiene Complex 557

The final entries in Table 5 represent a collection of structurally authenticated cyclotetragermanes [R2Ge]4 reported in the literature.336,382−384 The photochemistry of octaisopropylcyclotetragermane, [iPr2Ge]4 (553), was investigated recently by laser flash photolysis and accompanying trapping experiments. It was concluded from this study that the ringopening of [iPr2Ge]4 (553) occurs via a 1,4-biradical

Scheme 29. Synthesis of Silylenecyclotetrasilanes 549 and Persila[1]staffane 559

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Scheme 31. Synthesis of the Radical 568

The cyclotetrasilenes 565a and 565b can also be generated as intermediates in the photolysis of the tricyclic ladder oligosilanes (564a and 564b) and trapped by cycloaddition between the SiSi π-manifold and 2,3-dimethylbutadiene (Scheme 30).389 Using a similar procedure, the in situ generated cyclotetrasilene 565a combines with CS2 to form a transient S-heterocyclic carbene that attacks C60 to form the addition product 566 (Scheme 30).390 Cations, anions, and radicals of heavy group 14 element cyclobutadienes have been synthesized,391−393,379,394−399 and as an example, the reduction of the cyclotetrasilenylium cation 567 to form the heterocyclic radical 568393 is presented in Scheme 31.393 Sekiguchi and co-workers recently reviewed heavy analogues of the cyclobutadiene dianion [R4E4]2− (E = Si and Ge), including their coordination with transition metals.400,401 3.4.2. Group 14 Element Heteroatomic Rings. Mixed group 14 element cyclotrimetallanes (Table 6)345 have been less explored than their homoatomic analogues due to the challenges associated with selectively making intraring linkages between different tetrel elements. The first crystallographically studied member of this series, [Mes2Si(GeMes2)2] (569), was reported in 1991 by Baines and Cooke,402 followed later by the synthesis of the disilagermirane [(Me3Si)2Ge(Si(SiMe3)2)2] (570) by Heine and Stalke.403 The only reported disilastannirane, 577, was prepared as a thermally stable yellow solid by the Scheschkewitz group in 2009 from the reaction of the disilenide Trip2SiSi(Trip)Li with Me2SnCl2 (eq 23).404 The only known distannagermirane, [Mes2Ge(SnTrip2)2] (578) (Table 6), was reported by Escudié and co-workers in 1996.405

Table 6. Summary of Reported Mixed Group 14 Element Cyclotrimetallanes

The Sekiguchi group found a route to the first stable example of a mixed tetrel element cyclotrimetallene, [(tBu2MeSi)2Ge(Si(SiMetBu2)2)2] (583; Scheme 32). Compound 583 was obtained as red crystals from the coreduction of R2GeCl2 and RSiBr3 (R = SiMetBu2) with excess elemental sodium at room

compd no.

E

E′

R

569402 570336,403

Si Ge

Ge Si

Mes SiMe3

Mes SiMe3

571406

Ge

Si

Me3SiCH2

t

572407 573407 574408

Ge Ge Ge

Si Si Si

Ph Ph t Bu2MeSi

i

575408

Ge

Si

t

Bu2MeSi

576408,409

Ge

Si

t

Bu2MeSi

H, OC(Ph), t Bu2MeSi

577404

Sn

Si

Me

Trip, Cl

578405 579410

Ge Si

Sn Si, Ge

Mes Bu2MeSi

580410

Si

Si, Ge

t

Bu2MeSi

581411

Ge

Si

t

Bu2MeSi

Trip H, Li, t Bu2MeSi H, Me, t Bu2MeSi Cl, tBu2MeSi

582411

Si

Si, Ge

t

Bu2MeSi

Cl, tBu2MeSi

t

R′

BuCH2

Pr BuCH2 H, PhCH2O, t Bu2MeSi Me, I, t Bu2MeSi

E−E′ bond length (Å) Ge−Si: 2.391(1) Si−Si: 2.377(14) Ge−Si: 2.480(3), 2.458(4) Si−Si: 2.418(5)

t

Ge−Si: 2.4749(10), 2.4762(10) Si−Si: 2.3696(13) Ge−Si: 2.4524(5), 2.4901(5) Si−Si: 2.3383(6) Si−Sn: 2.5422(6), 2.6126(7) Si−Si: 2.3766(9)

Si−Ge: 2.467(1) Si−Si: 2.342(2)

temperature.412 This species is a red crystalline solid that is very air- and moisture-sensitive, and exposure of a solution of 583 to either UV light or heat leads to isomerization into the stable 2disilagermirane isomer (584) containing an internal SiGe

Scheme 30. Chemistry of the Transient Cyclotetrasilenes 565

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Scheme 32. Selected Reactivity of the Mixed Cyclotrimetallene 583

Scheme 33. Chlorination of Cyclotrimetallenes 583 and 584

double bond (Scheme 32).345 This disilagermirene 583 readily undergoes ring expansion/insertion with olefinic substrates,345,413,410 while the addition of protic and alkylating reagents (e.g., PhCH2OH and MeI) yields trans addition products (e.g., 574 and 575).408,410 Chlorination of the cyclotrimetallenes 583 and 584 with carbon tetrachloride affords the air-stable dihalocyclotrimetallane derivatives 581 and 582, which can be converted in a quantitative fashion into the starting heterocycles 581 and 582 after treatment with tBu3SiNa (Scheme 33).411 Four-membered inorganic rings with Si3Ge skeletons and saturated bonding environments at Si and Ge (585, 586) were initially prepared in 1991.414 Photolysis of 585 and 586 produced the corresponding cyclotrisilanes [iPr2Si]3 and [( t BuCH 2 ) 2 Si] 3 along with the hindered germylene (Me3SiCH2)2Ge:.415,416 Kira and co-workers synthesized the first example of a disiladigermacyclobutane, [iPr2Si−GeiPr2]2 (589), having a planar Si2Ge2 arrangement from the Wurtztype reductive coupling of tetraisopropyl-1,2-dichlorosilagermane, CliPr2Si−GeiPr2Cl, with sodium in toluene.417 The disiladistannacyclobutane [tBu2Sn−SiMe2−SiMe2−SntBu2] (590) was also generated as a byproduct from the magnesium-instigated cyclization of the stannyl-capped oligosilane, tBu2(Br)Sn−(SiMe2)2−Sn(Br)tBu (eq 24).418 Table 7 and Figure 26407 provide a summary of analogous inorganic heterocyclobutane compounds reported in the literature.

Table 7. Summary of Reported Si2EE′-Based Heterocyclobutanes

compd no.

E

E′

R

585414−416,419

Si

Ge

i

Pr

Me3SiCH2

586414−416

Si

Ge

t

BuCH2

Me3SiCH2

587407 588407 589417 590418 591420

Si Si Ge Sn Si

Ge Ge Ge Sn Sn

i

Pr BuCH2 i Pr t Bu, Me SiMe3, Me

Ph Ph i Pr t Bu Me

t

R′

E−E′ bond length (Å) Ge−Si: 2.452(1), 2.462(1) Si−Si: 2.380(1), 2.391(1) Ge−Si: 2.427(1), 2.461(1) Si−Si: 2.393(1)

Figure 26. Si2Sn2- and Ge2Sn2-based heterocyclobutanes.

Ring expansion transformations between the disilagermene 583 and GeCl2·dioxane in THF at room temperature gave the stable disiladigermene, Si2Ge2, heterocycle 595, whereby halide migration from GeCl2·dioxane to proximal silicon centers occurs in a regioselective manner (eq 25). Similar ring expansion occurred with SnCl2 to afford the disilagermastannetene [RGeSnR(SiClR)2] (R = SiMetBu2) (597),421 while PbCl2 7843

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heavy group 14 elements as part of a ring structure (e.g., [Me2Si(NtBu)2Ge:], 609).432 Reaction of the tetrachlorocyclodisilazane [Cl 2 Si(μNSitBu2Me)]2 (610) with Li[AlH4] yields the first cyclodisilazane (611; Table 8) with a SiH functionality.433 Selective replacement of silicon-bound chlorine groups in 610 by nucleophilic −NH2 substituents in the presence of excess ammonia occurs to give [Cl(H 2 N)Si(μ-NSi t Bu 2 Me)] 2 (612).434 Compound 614 (Table 8) is noteworthy as it contains reactive N−H and Si−F bonds as components of the same ring.436 The cyclodisilazanes 615 and 616 (Scheme 34) were synthesized by a novel condensation strategy starting from double deprotonation of hindered silylamines with 2 equiv of n BuLi, accompanied by a silyl shift to produce reactive 1,3dilithio silyl bis(amide)s [LiN(R)SiMe2N(R)Li] that are later combined with diorganosilicon dichlorides (such as Me2SiCl2) to afford the Si2N2 rings shown in Scheme 34. [Me2Si(μ-NSiMe3)]2 (615) was formed as a minor product (5% isolated yield after sublimation) from the reaction of GaCl3 with 1 equiv of Li[N(SiMe3)2]; the formation of 615 requires cleavage of a Si−C bond within the −N(SiMe3)2 group, providing a possible deactivation pathway when this bulky bis(silyl) amide is used as a ligand in main group element chemistry.437b In a similar fashion, compound 615 was also identified as a byproduct in the synthesis of lanthanide methylamidinate complexes TpMe2Ln[(RN)2CMe][N(SiMe3)2] (TpMe2 = tris(3,5-dimethylpyrazolyl)borate; Ln = Y and Er; R = Cy and iPr) (Scheme 35).437d In addition, thermolysis of Si2Cl6·TMEDA with Me3SiN3 followed by treatment with MeLi yields 615;437e it is believed that transient Cl2SiNSiMe3 is formed in this process, which is converted into the dimeric [Me2Si(μ-NSiMe3)]2 (615) upon Cl/Me group exchange. The cyclodisilazane [Me2Si(μ-NSiMe2Cl)]2 (631) formed unexpectedly in the GaCl3-assisted methyl/chlorine exchange reaction involving the amino(dichloro)arsine Cl2AsN(SiMe3)2 in dichloromethane (eq 28).440

chlorinates 583 to give the germadisilane ring [R2Ge(SiClR)2] (581) in place of the target Si2GePb heterocycle.422 The analogous four-membered Si3Sn cyclobutene 598 (Figure 27) was generated and spectroscopically identified in 2006.423

Figure 27. Si2Ge2- and Si3Sn-derived cyclobutenes.

3.4.3. Group 14/15 Element Heteroatomic Rings. 3.4.3.1. Group 14 Element/Nitrogen Inorganic Rings. Three-membered hybrid EE′N rings (E = Si, Ge, and Sn; E′ = Si, Ge, Sn, P, and As; 599−608; Figure 28) have been explored in depth over the past three decades, with structural characterization often provided by single-crystal X-ray diffraction studies.424−427 In addition, formal atom/group transfer between unsaturated disilenes (R2SiSiR2) and molecular azides has been shown to yield Si2N heterocycles via [2 + 1] cycloaddition.428 Conceptually related reactions between the digermene Trip2GeGeTrip2 and diazomethane, CH2N2, or (trimethylsilyl)diazomethane, HC(SiMe3)N2, give heterocycles 604 and 605, respectively (eq 26).429 One

commonly adopted synthetic strategy is to interact lowcoordinate silylene, germylene, or stannylene compounds (R2E:) with organoazides to yield nitrene insertion products as illustrated in eq 27.430,431

Stable heterocycles with terminal silanol groups (629 and 630) were generated from the controlled hydrolysis of the halosilyl-functionalized rings 627 and 628 (eq 29).439 A series of cyclodisilazanes, 633 and 634, were formed in a ring expansion reaction starting from strained SiN2 heterocycles (631 and 632; eq 30); X-ray analysis revealed the presence of a planar Si2N2 ring in 633 with short Si−N distances of 1.692(4) Å due to the presence of electron-withdrawing fluorine substituents at silicon.442

Examples of four-membered E2N2 rings (E = Si, Ge, Sn, and Pb) are plentiful, and Table 8 lists known cyclodisilazane compounds with Si2N2 ring motifs. It is salient to mention within the context of this area of study that the Veith group (among others) has devoted significant research effort toward preparing inorganic analogues of carbene featuring divalent

Figure 28. Three-membered rings with silicon/group 15 element combinations. 7844

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Table 8. Selected Examples of Cyclodisilazanes (Vi = −CHCH2)

compd no.

R1

R2

R3

R4

R5

R6

610433 611433 612434 613435 614436 615437 616437a 617437a 618437a 619437a 620437a 621437a 622437a 623437a 624437a 625438 626438 627439 628439 629439 630439 631440 632441 633442 634442 635443 636444

Cl H Cl Me F Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Ph t Bu Ph Me

Cl H NH2 Me Ph Me Me Me Me Me Me Me Me Me Me Me Me Ph Ph Ph Ph Me Me t Bu t Bu Ph Me

Cl H Cl Me t Bu Me Me Me Me H Me Me Me Me Me Me Me Me Me Me Me Me Me F i Pr Ph Me

Cl H NH2 Me t Bu Me Me Vi Me Cl Cl H Me Me or Cl Me Me Me Ph Ph Ph Ph Me Me F i Pr Ph Me

SitBu2Me SitBu2Me SitBu2Me C5H3MeN SitBuMe2 SiMe3 SiMe2H SiMe2H SiMe2H SiMe2H SiMe2H SiMe2H SiMe2H SiVi3 Ph SitBu2OSi(NiPr2)2F Si(tBu)2OSiMe3 SiMePhCl SiPh2Cl SiMePhOH SiPh2OH SiMe2Cl PhCNiPr SiPh2tBu SiHtBu2 SiPh2(OSiMe2)2Vi SitBu3

SitBu2Me SitBu2Me SitBu2Me C5H3MeN H SiMe3 SiMe2H SiMe2H SiMe2Vi or SiVi3 SiMe2H SiMe2H SiMe2H SiMe3 SiVi3 SiMe3 same as R5 Si(tBu)2OSiMe3 SiMePhCl SiPh2Cl SiMePhOH SiPh2OH SiMe2Cl PhCNiPr Ph H SiPh2(OSiMe2)2Vi SitBu3

Si−N bond length (Å) 1.728(4), 1.740(4) 1.729(2), 1.735(2) 2.336(1)

1.751(2)

1.742(2), 1.753(3) 1.730(3), 1.754(3) 1.744(2), 1.7444(19) 1.759(1), 1.760(1) 1.692(4)−1.791(4) 1.748(2), 1.749(2) 1.761(2)−1.778(2)

Scheme 34. Synthesis of the Cyclodisilazanes 615 and 616

dimerization of in situ generated silaimines R2SiNR′446 via a formal [2 + 2] cycloaddition process can also yield stable cyclodisilazanes with the general structure [R2Si(μ-NR′)]2.127

Cyclodisilazanes can also be incorporated into the backbone of polysiloxanes to improve the thermal stability of the resulting material relative to the unmodified polysiloxanes.445 The 7845

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Scheme 35. Formation of 615 as a Byproduct

Scheme 37. Synthesis of the Bis(silylene) Heterocycle [:Si(μNArTrip)2Si:] (642) and Thermal Ellipsoid Drawing (50%) of 642a

a

Hydrogen atoms and isopropyl groups are not shown in the thermal ellipsoid drawing. Reprinted with permission from ref 448. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

The diazasilaplumbetidine [Me2Si(μ-NtBu)2Pb:] (637) undergoes smooth transmetalation with PCl3 to give the diazasilaphosphetidine [Me2Si(μ-NtBu)2PCl] (638) and PbCl2 as an insoluble product (Scheme 36). Compound 638 is a progenitor to a wide array of ligand archetypes via both halide substitution and oxidation to afford P(V) centers with oxo or imido functionality.447 Building upon the use of donor-stabilized silicon dichloride, IPr·SiCl2, as an easy-to-handle source of SiCl2, the Roesky group devised an elegant synthesis of a Si2N2 heterocycle containing two intraring silylene Si(II) centers (Scheme 37).448 As outlined below, the stable silaimine adduct IPr·SiCl2 NArTrip was reduced using 2 equiv of KC8 to afford the dimeric silanitrile [:Si(μ-NArTrip)2Si:] (642) featuring a planar Si2N2 ring and average Si−N single bonds of 1.755(1) Å (Scheme 37). The bis(silylene) character in 642 was demonstrated by oxidative transfer of imine functionality at silicon to give the Si(IV) heterocycle [Me3SiNSi(μ-NArTrip)2SiNSiMe3] (643) with significantly shortened exocyclic SiN π-bonds [1.564(2) Å]. Ghadwal and co-workers recently explored the oxidation of 642 with both N2O and Me3NO, resulting in the stepwise

formation of the monosilylene [:Si(μ-NArTrip)2Si(OH)2] (644) and the bis(silanediol) [(HO)2Si(μ-NArTrip)2Si(OH)2] (645) (Scheme 38). The proposed mechanism of these transformations involves the transient formation of highly reactive terminal SiO units followed by attack by residual water to yield silanediol residues.449 The Roesky group also reported a reaction between the aluminum(I) heterocycle [(HCNDipp)2Al:] and tert-butyltrisazidosilane, tBuSi(N3)3, leading to the oxidation of each Al center (to Al(III)) accompanied by the formation of a Si2N2 ring in the final product, 646. This reaction is proposed to occur via azide group transfer from silicon to an electrophilic Al center within a transient alaimine (AlN) bonding environment, followed by dimerization of the resulting SiN π-bonds to yield a saturated Si2N2 ring (Scheme 39).450 Hybrid 1-aza-3-oxa-2,4-disilacyclobutanes (Si2NO rings) are known as stable entities.451 One particularly noteworthy route to these rings is the nBuLi-induced cyclization of tBu2SiNH2− O−SiF2Mes* to give the Si2NO heterocycle [tBu2Si(μ-NH)SiFMes*(μ-O)] (647; eq 31). In this process, selective

Scheme 36. Representative Reactivity of the Si2NP Heterocycle 638

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Scheme 38. Oxidative Transfer of Bis(hydroxy) Functionality to the Si(II) Heterocycle 642

Scheme 39. Si2N2 Ring Formation via the Dimerization of a Putative Silaimine (SiN) Linkage

Scheme 40. Synthesis of Amido-Bridged E(II) Heterocycles (E = Ge, Sn, and Pb)

adopting significantly pyramidalized geometries due to the presence of sterochemically active lone pairs.452 The dimeric tin(II) amido derivatives [(DippNH)Sn(μNHDipp)]2 (654) and [ClSn(μ-NHDipp)2Sn(NHDipp)] (655) have been prepared and show folded (butterfly-shaped) Sn2N2 ring arrangements; compound 654 is unstable in solution under ambient conditions and converts into the cubane-shaped Sn4N4 tetramer [SnNDipp]4 with release of H2 NDipp as a byproduct. 453 Dimeric metal(II) imido complexes [E(μ-NArTrip)]2 (E = Ge, Sn, and Pb; 656−658) were generated by the thermally induced (ca. 165 °C) solventfree reaction between the bis(trimethylsilyl amide) complexes E[N(SiMe3)2]2 and the hindered aniline H2NArTrip (eq 33);454

deprotonation of the −NH2 group (and generation of a nucleophilic amide) occurs in preference to nucleophilic attack of a nBu− anion on the polar Si−F bond.451a The spirocyclic complexes [tBuOSiMe2NR]2Sn: (R = tBu and tolyl; 648 and 649) were prepared according to eq 32 and characterized by Xray crystallography.451b

a related reaction between E[N(SiMe3)2]2 complexes and the primary phosphine H2PArDipp yields the isostructural E2P2 heterocycles [E(μ-PArTrip)]2 (E = Ge, Sn, and Pb; 659− 661).455 The dimeric amidotetrel heterocycles [ClGe(μ-NEt2)]2 (662) and [ClSn(μ-NMe2)]2 (663) were formed from the reaction of GeCl2·dioxane and SnCl2 with Et3GeNEt2 and [Sn(NMe)2]2, respectively. Of note, compound 662 has a butterfly-shaped Ge2N2 core, whereas the Sn2N2 ring in 663 is planar, with the presence of additional intermolecular interactions between the Sn and Cl centers in 663 likely giving rise to the observed structural difference;456 a related Sn(IV) heterocycle, [Me2SnCl(μ-NEt2)]2 (664), was reported and structurally characterized.457 A stable cyclodistannazane, [Me2Sn(μ-NDipp)]2 (665), was generated as a minor product upon fluorination of [MeAl(μNDipp)]3 with Me3SnF (eq 34).458 The halostannylene heterocycle [ClSn(μ-NHDipp)]2 (666) was obtained from

In 2005, Power and co-workers reported the first examples of divalent group 14 element monoamides containing the parent amido group, −NH2. The centrosymmetric E2N2 cyclic species [ArDippE(μ-NH2)]2 (E = Ge; 650) or [ArTripE(μ-NH2)]2 (E = Ge and Sn; 651 and 652) were obtained from the reaction of the requisite terphenyl element halide arylEIICl with excess liquid ammonia. The related lead(II) halide ArTripPbBr was combined with NH3(l) to give the ammine adduct ArTripPbBr· NH3, the first well-defined Pb(II) complex with ammonia. The target species [ArTripPb(μ-NH2)]2 (653) was later prepared from the direct salt elimination reaction between ArTripPbBr and LiNH2 (Scheme 40). Each of the E2N2 rings in the above species is planar, with each constituent group 14 element (E) 7847

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NtBu)2Sn:] (672) reacted with PhPCl2 to give a complicated reaction mixture containing the spectroscopically identifiable cyclic oligophosphines (PhP)4 and (PhP)5 resulting from a formal chlorophosphine dehalogenation reaction.465 A stable phosphinosilylcarbene, [Me2Si(μ-NtBu)2P]CSiMe3 (675), was prepared by Bertrand and co-workers via photoextrusion of dinitrogen from the phosphinodiazomethane precursor [Me2Si(μ-NtBu)2P]C(N2)SiMe3 (674) (Scheme 41). Compound 675 contains a planar SiN2P ring with a short exocyclic P−C bond [1.532(3)Å] indicative of phosphorus−carbon multiple bond character.466 In an attempt to generate stable N-heterocyclic silylenes under mild conditions, the Cui group treated the cyclic diaminohydrochlorosilane [Me2Si(μ-NDipp)2Si(H)Cl] (676) with the bulky N-heterocyclic carbene ImtBu. In both cases, successful HCl elimination occurred; however, the transient silylenes formed underwent insertion with 676 to afford the Si−Si-bonded product 677 (Scheme 42).77 The N-heterocyclic stannylene [Ph2Si(μ-NDipp)2Sn:] (678) was synthesized by reacting the corresponding dilithio diamide Li2[DippNSiPh2SiNDipp] with SnCl2.467 X-ray crystallography revealed the presence of a planar SiN2Sn ring in 678 with additional long-range Sn---aryl interactions between the tin atoms and the aryl rings of neighboring [Ph2Si(μ-NDipp)2Sn:] molecules. Transamination was reported between the Nheterocyclic stannylene [Me2Si(μ-NtBu)2Sn:] (672) and primary silylamines to give silyl-substituted tin−nitrogen heterocubanes (679) (eq 36); in this process 672 acts as a

salt metathesis between SnCl2 and Li[NHDipp], and this species combines with molecular oxygen to give an oxo-bridged Sn10 cluster.459 The stannacycle (667; Figure 29) containing

Figure 29. Representative interactions within both high- and lowvalent group 14 element/nitrogen heterocycles.

Sn(IV) centers in five-coordinate bonding environments was recently prepared, and the trans effect of the NMe2 donor group was evident in the lengthening of one set of Sn−N distances [2.1339(15) Å] in relation to the other Sn−N bonds [2.0428(15) Å].460 Boyle and co-workers examined a series of amido- and alkoxy-bridged tin(II) heterocycles (e.g., ([(RO)Sn(μ-NMe2)]2 and [(RO)Sn(μ-OR)2]2; 668 and 669; R = various alkyl and aryl groups) as precursors to tin oxide-based nanowires for use as anodes in lithium ion batteries.461 N-Heterocyclic carbenes represent a major ligand class in modern chemical synthesis.462 Not surprisingly, divalent heavier main group element congeners (silylenes, germylenes, stannylenes, and plumbylenes) have been explored in the context of coordination chemistry and to highlight the reactivity differences that exist between carbon and its inorganic tetrel element brethren.313b Mixed element amido heterocycles containing ENE′N ring motifs have been actively explored (E and E′ = Si, Ge, Sn, and Pb; see 670 in Figure 29). Not only can such species bind transition metals when a low-valent E center is present (via an E: → M coordinative interaction),463,464 oxidative addition can occur to generate E(IV) centers as illustrated by the reaction of [Me2Si(μ-NtBu)2Ge:] (609) with PhPCl2 to give the digermaphosphine (671) (eq 35). The previously reported stannylene derivative [Me2Si(μ-

soluble source of deliverable Sn(II).468 The coordination chemistry of [Me2Si(μ-NDipp)2Sn:] (680) with platinum is known and is typified by the formation of Sn: → Pt interactions.469 Reaction of the acyclic C,N-chelated stannylene L2Sn: (L = 2-(Me2NCH2)C6H4) with azobenzene yields the 1,2-cyclodiazadistannane [L2SnN(Ph)N(Ph)SnL2] (681). Compound 681 contains a planar four-membered Sn2N2 ring motif with an intraring N−N bond distance [1.440(4) Å] consistent with the presence of a single bond. Heating 681 to 70 °C in THF results in a clean ortho-metalation reaction to give the ring-fused product 682 with two formally Sn(IV) centers (Scheme 43).470 The Rivard group synthesized a series of monomeric germylene (683) and stannylene (684) SiN2E heterocycles formally derived from the coordination of the hindered dianionic ligands [iPr2Si(NR)2]2− (R = Dipp, SiPh3, and

Scheme 41. Synthesis of the Phosphinosilylcarbene 675

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Scheme 42. Transient Formation of an N-Heterocyclic Silylene Followed by Silylene Insertion

unique reactivity that is inherent to low-coordinate main group element complexes and sets the stage for future advances in main group element-based catalysis.473 Examples of cationic E2N2 heterocycles (691 and 692) are listed in Figure 30,474,475 while stable E2N2 (E = Si, Ge, and Sn;

Scheme 43. Oxidative Synthesis of the Sn2N2 Heterocycle 681 Starting from a Stannylene

Si(4-iPrC6H4)3) to Ge(II) and Sn(II) centers. The Ge(II) complexes were oxidized with both oxygen and sulfur atom transfer agents (Me3NO and S8, respectively) to yield dimerized species with planar central Ge2O2 and Ge2S2 ring structures that exit in approximately orthogonal arrangements to the flanking GeN2Si rings (685 and 686; eq 37).471 The Figure 30. Examples of cationic and biradical four-membered rings.

693−695) four-membered rings with non-Kekulé singlet biradical character were reported by the Sekiguchi, Power, and Lappert groups, respectively (Figure 30).476 Compounds 693−695 have planar centrosymmetric E2N2 rings (as determined by X-ray crystallography) and are EPR silent, as expected for diamagnetic molecules. A stable cyclic biradicaloid with a planar GeNGeO core (696; Figure 30) was synthesized by combining the digermyne ArDippGeGeArDipp with ONC6H42-Me; compound 696 is a purple-black solid that is thermally stable under N2 (mp 186 °C).477 Moreover, ArDippGeGeArDipp undergoes redox transformations with organoazides (RN3) to generate a wide range of products (697−699) depending on the nature of the azide-bound substituents present (see Scheme 44).478 The Sn2N2 heterocycle [ArMesC(O)OSn(μ-NSiMe3)]2 (700) was prepared by the Clyburne group (Figure 30), and X-ray crystallography showed an absence of transannular Sn···Sn interactions, consistent with a diradicaloid bonding arrangement being present.479 Mixed element diazaphosphasiletidine SiN2P rings (e.g., Scheme 36)480 have both hard and soft donors (N and P, respectively) built within the same ring framework, potentially facilitating a wide array of coordination modes.481 3.4.3.2. Heterocycles Containing Heavier Group 14/15 Element Combinations. The diphosphasilacyclopropane

Zheng group prepared a series of related N-heterocyclic metallylenes [Me2Si(μ-NDipp)2E:] (E = Ge, Sn, and Pb; 687a−c) and [Ph2Si(μ-NDipp)2E:] (E = Ge, Sn, and Pb; 688a−c) using either salt metathesis or transamination.472 The cyclic plumbylene [Me2Si(μ-NDipp)2Pb:] (687c) adopts a dimeric arrangement in the solid state with short intermolecular Pb···N interactions, while the diphenylsilane analogue [Ph2Si(μ-NDipp)2Pb:] (688c) is rigorously monomeric in the solid state.472 The activation of ammonia by the two-coordinate stannylene ArDipp2Sn leads to Sn−C bond scission and the formation of the centrosymmetric dimer [ArDippSn(μ-NH2)]2 (690) via the extrusion of the arene ArDippH (eq 38); this result highlights the

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Scheme 44. Reaction of the Hindered Digermyne ArDippGeGeArDipp with Molecular Azides

Figure 32. Distannadiphosphetane 707 and cyclic phosphinesupported low-valent main group chemistry.

silixon(II) hydride 710 underwent Si−H bond insertion with various alkyne substrates to yield base-stabilized silacyclopropylidenes.489 The cationic SiPN2 ring [(Me3Si)2Si(μ-NSiMe3)2P]+ (711) was isolated as a tetrachlorogallate salt, and treatment of this product with DMAP (4-(dimethylamino)pyridine) led to removal of DMAP·GaCl3 and halide transfer to give the P-chloro product (712) (eq 39).490 Structurally related GePN2 heterocycles with pyramidalized Ge(II) centers (713 and 714) have been prepared by Brusylovets, Mazières, and coworkers (Figure 33).491

[Trip2Si(μ-PH)P(SiFTrip2)] (701)482 and the phosphadisilacyclopropane [tBu(Trip)Si(μ-PH)Si(Trip)tBu] (702)483 were prepared by the Driess group in 1997 (Figure 31). Compound

Figure 31. Phosphine μ-PH-linked silicon heterocycles.

701 was obtained by heating the lithium phosphanide Trip2Si(F)−P[Li(THF)n]−PH−Si(F)Trip2, resulting in LiF elimination and cyclization.482 In 2000, Fritz and Scheer described the reactivity of Si/P heterocycles in a review.484 The four-membered Si2P2 cyclic species (703 and 704; Figure 31) were obtained as a mixture from the thermolysis of the lithiated precursor Mes*(F)2SiP(Li)SiMe3 and separated by fractional crystallization; a common feature of both products is C−H bond activation involving a Mes* group.485 The von Hänisch group explored the binding of the anionic Si2P2 ligand [P(μ-SiiPr2)2PH]− with various group 2 metals (705; Figure 31).486 A cyclic diphosphadigermylene, [((Me3Si)2CH)PGe(μ-PPh(CH(SiMe3)2)]2 (706), was obtained as an orange solid with a structurally authenticated butterfly-shaped Ge2P2 ring and pyramidalized Ge centers.487 As mentioned earlier, the Power group reported a series of E(II) (E = Ge, Sn, Pb) phosphinidene dimers, {E(μ-PArDipp)}2.455 The glass-assisted elimination of HF from the fluoro(phosphanyl)stannane Trip2Sn(F)PH2 affords the stable distannadiphosphetane [Trip2Sn(μ-PH)]2 (707) with Sn−P bond lengths in the range expected for single bonds (Figure 32).488 The Baceiredo group has used the SiPN2 heterocycle [Me2Si(μ-NtBu)2P] as a component of hindered phosphorus donors to support low-oxidation main group element chemistry. For example, the sila- and germacarbene analogues 708 and 709 were isolated as thermally labile solids, while the

Figure 33. P(V)-containing hybrid PNGeN rings 713 and 714 along with the PSi3 heterocycles 715 and 716.

A silicon-rich Si3P hybrid ring, [(iPr2Si)3PH] (715; Figure 33), was synthesized from the condensation reaction between the chlorosilane oligomer Cl−Si(iPr)2−Si(iPr)2−Si(iPr)2−Cl and 2 equiv of [Li(dme)PH2] (dme = dimethoxyethane).492 Furthermore, [(iPr2Si)3PH] (715) was able to coordinate W(CO)5 in the form of the P-bound adduct [(iPr2Si)3PH· W(CO)5] (716), while deprotonation of 715 with nBuLi gave the anionic heterocycle [(iPr2Si)3P]Li (717), which oxidatively couples in the presence of dibromoethane to yield the P−Pbonded product [(iPr2Si)3P]2 (718) with mutually orthogonal Si3P rings (Figure 33).493 7850

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Scheme 45. Selected Group 14−Arsenic-Containing Heterocycles

Scheme 46. Synthetic Routes Leading to Tetrel−Chalcogen Inorganic Rings and Molecular Structure of 731a

a

Reprinted from ref 502. Copyright 1997 American Chemical Society.

decomposes at ambient temperature, producing gaseous SiF4 and a mixture of CF2 insertion products. When the 1,2-bis(metallocenyl)disilenes Trip(R)SiSi(R)Trip (R = ferrocene or ruthenocene) are exposed to oxygen, the 1,3,2,4-dioxadisiletanes (725 and 726) are formed; in the presence of either elemental sulfur or selenium, stable Si2S and Si2Se rings (727−729) are obtained, with the metallocenyl groups adopting anti-positions on adjacent silicon centers (Scheme 46).500 The Tokitoh group also prepared perseleno rings of group 14 elements (SiSe2 and SnSe2; 730 and 731) by either reacting the in situ generated silylene Tbt(Dipp)Si: with elemental selenium (730)501 or from ring contraction (Se elimination) involving the tetraselenastannolane Tbt(Dltp)SnSe4 (Dltp = 2,6-(2-iPrC6H4)2C6H3) with 2 equiv of triphenylphosphine, PPh3 (Scheme 46).502 The Si2Te heterocycle [(tBu3Si)PhSi(μ-Te)SiPh(SitBu3)] (732) can be prepared from the formal oxidation of the hindered disilene (tBu3Si)PhSiSiPh(SitBu3) with tellurium.428 When (tBu3Si)PhSiSiPh(SitBu3) reacts with other chalcogens (S and Se), four-membered rings are formed with Si−S−S−Si and Si−Se−Se−Si atom connectivity.428 A Ge2Te ring, [Trip2Ge(μ-Te)2GeTrip2] (733), was obtained by

Three-membered rings consisting of group 14 elements and arsenic are rare, with [Trip2Si(μ-NNCPh2)As(SiiPr3)] (719)425 and [Trip2Si(μ-Te)As(SiiPr3)] (720) both prepared by the Driess group (Trip = 2,4,6-iPr3C6H2);494 interestingly, compound 720 was obtained via an atom transfer reaction between the disilaarsene Trip(tBu)SiAs(SiiPr3) and elemental tellurium (Scheme 45). Four-membered heterocycles with Si2As2,495 SiGeAs2,496 and Sn2As2497 cores were reported in the 1990s by various research teams. More recent examples of tetrel element/arsenic heterocycles include the diarsadistannetanes [tBu2Sn(μAsH)]2 (721) and [tBu2Sn(μ-AsEMe3)]2 (E = Si and Sn; 722 and 723; Scheme 45).498 3.4.4. Group 14/16 Element Heteroatomic Rings. 3.4.4.1. Three-Membered Group 14/16 Element Hybrid Rings. A major class of three- and four-membered heterocycles containing group 14 elements and oxygen are cyclic species with intraring peroxo (μ-O2) units. This structural type will be covered in detail later in section 3.6.3. Mitzel et al. prepared the SiON heterocycle [(F3C)F2SiONMe2] (724) from the reaction of Li[ONMe2] and F3C−SiF3.499 Compound 724 is a volatile liquid that 7851

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combining the tetragermabutadiene [Trip2GeGe(Trip) Ge(Trip)GeTrip2] with Et3PTe (eq 40);503 the analogous

Scheme 47. Selected Hydroxy-Bridged E2O2 Rings Including the Sn(II) Analogue 746

digermatellurane [Ar′2Ge(μ-Te)GeAr′2] (Ar′ = 2,6-Et2C6H3) (734) is also known.504 Moreover, the West group synthesized nearly isostructural Si2Se and Si2Te rings [Mes2Si(μ-Se)SiMes2] (735) and [Mes2Si(μ-Te)SiMes2] (736) using chalcogen atom transfer processes.505 The substituent-free dithiasilylene ring, SiS2 (737), was prepared and structurally characterized by gas-phase rotational spectroscopy.506 3.4.4.2. Four-Membered Group 14/16 Element Hybrid Rings. A wide variety of oxo-containing four-membered rings with tetrel (group 14) elements exist, and representative structural types include rings based on SiOSiN,451a,b Si3O,419 SiGeO2,507 and SnGeO2508 arrangements. The first structurally characterized Si3O rings appeared in 2002 (738; Figure 34) and

evidenced by the dinuclear stannylene [(tBuO)Sn(μ-OtBu)]2 (748).475 This species has a planar Sn2O2 ring with experimentally indistinguishable Sn−O distances [2.094(3) and 2.099(3) Å], and coordination chemistry involving the stereochemically active lone pairs at Sn has been reported (eq 41).514

Figure 34. Si3O and Si3N heterocycles.

were prepared by ring closure starting from organotrisilanes ClSiR2−SiR2−SiR2Cl in the presence of water as a nucleophile; the related amido-bridged Si3N rings (739) were prepared using lithium amides Li[NHR] as ring-forming agents.419 When −CH2tBu (Np = neopentyl) groups are appended to Si, the resulting Si3O rings are highly strained as evidenced by narrow Si−Si−Si intraring angles that approach ca. 68°. By virtue of having lone pairs at oxygen, hydroxide ligands readily participate in bridging interactions (μ-OH), often leading to small inorganic ring frameworks with M 2O 2 arrangements. For example, the hydroxy-bridged Sn(IV) heterocycles [R2Sn(μ-OH)(OTf)(H2O)]2 (R = nBu, tBu, and 2-phenylbutyl; 741−743; OTf− = CF3SO3−) can be synthesized by protonation of the oligomeric stannoxides [R2SnO]x with TfOH in the presence of water.509 X-ray crystallography confirmed the presence of Sn−OTf interactions in the solid state, and water/Lewis base exchange can transpire, leading to a family of tin hydroxide dimers with Sn2O2 ring motifs. The dimeric acetate analogue [nBu2Sn(μ-OAc)OTf]2 (744) was found to be a highly selective catalyst for the deacetylation of acetyl ethers.510,511 Scheme 47 contains recently reported Sn rings featuring ring-forming hydroxide groups (747),511,512 including the first reported examples of stable germanium(II) and tin(II) hydroxides, [ArDippE(μ-OH)2EArDipp] (E = Ge and Sn; 745 and 746) obtained from the oxidation of the tetrelynes ArDippEEArDipp with either N2O or TEMPO (tetramethylene Noxide).513 One possible mechanism for the formation of [ArDippE(μ-OH)2EArDipp] (E = Ge and Sn) involves the generation of an unstable radical, ArDippEO•, which abstracts hydrogen from the solvent (or residual moisture) to give the observed hydroxo-briged species. Not surprisingly, alkoxy or aryloxy units can also participate in ring-forming interactions as

The selective insertion of CO2 into a Sn−O bond in the dibutylstannane nBu2Sn(OiPr)2 (750) leads to formation of [nBu2(iPrO)Sn-OC(O)OiPr]2 (751).515 While compound 751 has a dimeric structure in the solid state with bridging isopropoxy and terminal isopropylcarbonato ligands, monomeric structures coexist in solution as determined by variabletemperature NMR studies. Related dimeric species with −OMe and −OAc units positioned between Sn centers have been reported.516,517 Diorganostannanes R2Sn(OMe)2 have been shown to catalyze the addition of methanol to CO2 to give organocarbonates. In this regard, Sn2O2 heterocycles have been implicated as possible intermediates (e.g., [MeOC(O)O−Sn(μOMe)2Sn−OC(O)OMe], 752) and structurally characterized.518 Examples of Pb,O-containing four-membered heterocycles are more rare, with the hydroxo-bridged Cr(CO)5-appended complex [Et4N]2[{PbCr2(CO)10}2(μ-OH)2][Et4N]2 (753)519 and the μ-OR-bridged dimers [{(CO)5Cr}2Pb(μ-OR)2Pb{Cr(CO)5}2]2− (R = Et, nPr, iPr, and allyl) (754) representing well-characterized examples.520 Interestingly, compound 753 fixes/activates CO2 to give cyclic products as shown in Scheme 48.519 Group 14 elements exhibit interesting chemical reactivity by virtue of having two commonly encountered oxidations states (+2 and +4). Accordingly, synthesis of the heterocycles [R2E(μ7852

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Scheme 48. CO2 Activation Instigated by a Pb2O2 Heterocycle

has a planar square-shaped geometry with the bulky aryl substituents (Tbt and Bbt) oriented in trans arrangements relative to each other (Scheme 50). Scheme 50. Synthesis of SiS2Pn Heterocycles (Pn = Sb and Bi)

Ch)]2 (E = Si, Ge, Sn, and Pb; Ch = S, Se, and Te) with the group 14 elements in a formal +4 oxidation state often follows two routes: (a) oxidative addition of a chalcogen atom to low oxidation state tetrelylenes R2E: or dimetallenes R2EER2; (b) ring closure starting from group 14 element precursors R2EX2 (X = leaving group) in the +4 oxidation state (Scheme 49).

Novel inorganic bicyclo[1.1.0]butane derivatives 765 were prepared by the Sekiguchi group via the chalcogen atominduced ring expansion of the cyclotrisilene 764 (Scheme 51).

Scheme 49. Commonly Used Synthetic Routes to [R2E(μCh)]2 Heterocycles (E = Si, Ge, Sn, and Pb; Ch = S, Se, and Te)

Scheme 51. Atom Insertion and Skeletal Isomerization/Ring Expansion Starting from a Stable Cyclotrisilene (764)

Subsequent photolysis of 765 converts these bicyclobutanes into their corresponding group 14/16 hybrid cyclobutenes 766 (Scheme 51).524 The sulfur analogue of 766 adopts a planar Si3S arrangement with an intraring SiSi double bond length of 2.1706(12) Å. The disilene trans-[(Me3Si)2N(η1-Cp*)SiSi(η1-Cp*)N(SiMe3)2] reacts smoothly with both elemental sulfur (S8) and nitrous oxide (N2O) to afford sulfido- and oxo-bridged heterocycles (Scheme 52). Interestingly, the sulfur analogue 767 exists exclusively as the trans isomer, while the corresponding oxo analogue exists as a mixture of cis and trans isomers (768 and 769).525 Examples of recently prepared cyclodigermadithianes [R2Ge(μ-S)]2 include species with R = Bbt and Br (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl)526 and [iPr2Si(μ-NR′)2Ge(μS)]2 (R′ = Dipp, SiPh3, and Si(4-iPrC6H4)3).471b The mixed Sn/Ge heterocycle [(tBu2MeSi)2Ge(μ-S)2SnTrip2] (770) (Scheme 52) was prepared from the direct oxidation of the germastannene [(tBu2MeSi)2GeSnTrip2] with sulfur.527 The preparation of extended cyclic frameworks containing Si2S2 and Ge2S2 rings (eg., 771) from the addition of S atom equivalents

The coupling of diorganodihalosilanes R2SiX2 with dilithiochalcogenides Li2Ch is a useful method (Scheme 49) to generate group 14/16 element heterocycles [R2Si(μ-Ch)]2 (R = Ph or SiMe3; Ch = S and Se; 757).521,522 In the case of the reaction of Ph2SiCl2 with Li2Ch, the formation of a trimeric Si3Ch3 cyclic arrangement, [Ph2Si(μ-Ch)]3 (758), occurs in high yield.521a In an elegant experiment conducted by the Leigh group, the reactive silylene Ph2Si: was generated by pholotolysis of the cyclic organosilane [Ph2Si(SiMe2CH2)2CH2] (759), and in the presence of propylene sulfide, S atom transfer occurred to yield the disiladithiarane [Ph2Si(μ-S)]2 (760) along with propylene and tetramethyldisilacyclopentane (761) as coproducts (eq 42).521b The Tokitoh group used a reactive silanedithiol synthon, Tbt(Mes)Si(SH)2, to prepare heterocyclic structures featuring pnictogen (Pn) centers, Tbt(Mes)Si(μ-S2)Pn(Bbt) (Pn = Sb and Bi; 762 and 763), by condensation of the in situ generated dianion [Tbt(Mes)Si(S)2]2− with the hindered substrates BbtPnBr 2 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl).523 The structurally characterized BiS2Si ring in 763 7853

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Scheme 52. Chalcogen Addition to Disilenes and Germylenes and Representative Products

(especially those with low oxidation states), the diphosphaalkenylstannylene adduct (783) underwent smooth oxidation with sulfur to yield a stable Sn2S2 heterocycle (784) with release of the carbene donor (ImMe2iPr2) from the coordination sphere of tin (eq 43).535 The structurally related siloxy-capped Sn2S2

to heterocyclic Si(II) and Ge(II) precursors has been reported and comprehensively reviewed by Roesky and co-workers.528,529 Boyle et al. also used the cyclic germanium(II) precursor [(Ph3SiS)Ge(μ-SSiPh3)]2 (772) to prepare GeS nanomaterials upon thermolysis.530 Saito and co-workers isolated the Sn2S2 ring [ArTrip(HS)Sn(μ-S)2Sn(SH)ArTrip] (773) in low yield as a coproduct from the reaction of ArTripSnCl3 with Na2S.531 Salt metathesis was also used to access the intramolecularly coordinated organotin(IV) sulfide dimer [{2,6-(Me2NCH2)2C6H3}(Ph)Sn(μ-S)]2 (774).532 Interestingly, the analogous Se and Te species form monomeric entities [{2,6-(Me2NCH2)2C6H3}(Ph)SnCh] (Ch = Se, Te) (775 and 776; Figure 35) with formal SnSe

heterocycle [(tBu3SiO)Sn(μ-S)]2 (785) was reported536 along with various Sn2Sn2 rings with peripheral metal-based substituents at tin.537 [E2S6]4− anions (e.g., 786 in Figure 36) featuring E2S2 central rings have been used as building blocks to construct a wide

Figure 35. Sn2Ch2 heterocycles with high coordination numbers at tin (Ch = S, Se, and Te).

and SnTe double bonds (although these bonds are better regarded as highly polarized Sn−Se and Sn−Te single bonds). A related centrosymmetric Sn2S2 heterocycle, [{2,6-[P(O)(OEt)2]2-4-tBuC6H2}Sn(μ-S)]2 (777), was prepared by the Jurkschat group.533 Compound 777 has a centrosymmetric dimeric structure and retains this arrangement in solution on the basis of osmometry. The chalcogenation of Sn[N(SiMe3)2]2 was reported in 1995 by the Lappert group, and the homologous product series [{(Me3Si)2N}2Sn(μ-Ch)]2 (Ch = S, Se, and Te; 778−780) was obtained in high yield; moreover, heterobimetallic ring systems were also synthesized by the same group (e.g., [{(Me3Si)2N}2Ge(μ-Ch)2Sn{N(SiMe3)2}2] (Ch = Se and Te; 781 and 782).534 Addition of chalcogen atoms (Ch = S and Te) to the C,N-chelated stannylene [2-(Me2NCH2)C6H4]2Sn: afforded Sn2S2 and Sn2Te2 heterocycles as determined by X-ray crystallographic analysis.470 In line with the increasing use of Nheterocyclic carbenes to stabilize main group element species

Figure 36. Selected tetrel−chalcogen element rings highlighting the compositional diversity in these systems.

range of extended structures and metal complexes.538 The group of Kanatzidis prepared multifunctional chalcogels (i.e., porous chalcogenide aerogels) with Fe4S4 cores each linked by isostructural [Sn2S6]4− anions and used the resulting assemblies as light-harvesting photoredox dyes for solar cell-driven catalysis.539 The analogous Se and Te tetraanions [E2Ch6]4− (E = Ge and Sn; Ch = Se and Te) show the same overall structural and binding modes as [Ge2S6]4−.540 Furthermore, Talapin and co-workers prepared [Sn2Ch6]4− (Ch = S, Se) 7854

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Scheme 53. Stable Diphosphenes and Dimerization of Unstable ArMesPPPh To Give the Corresponding Cyclotetraphosphine [ArMesPPh]2

asymmetric diphosphene, Ar Mes PPPh (Ar Mes = 2,6Mes2C6H3) by treating the diphosphine ArMesP(H)−P(Cl)Ph (800) with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) to induce dehydrohalogenation and P−P π-bond formation (Scheme 53). Instead, this approach afforded the cyclotetraphosphine [ArMesPPPh]2 (801) due to a lack of steric protection about the reactive PP double-bonded manifold in ArMesPPPh.558 Replacement of the Ph group with a larger Mes* substituent enabled the isolation of the stable monomeric diphosphene ArMesPPMes* (802).558 Tokitoh and co-workers reported the unexpected formation of the cyclotriphosphirane 805 from heating a reaction mixture (80 °C, THF) containing the asymmetric diphosphene (9Anth)PP(Bbt) (803) (9-Anth = anthracenyl) and Te PBu3. This process, outlined in Scheme 54, involves a ligand

anion-capped InP and InAs nanocrystals and demonstrated that these hybrid materials exhibit unusual conductivity properties as well as high field-effect electron mobility, making these materials of interest for optoelectronic applications.541 Reports of Te heterocycles with silicon and germanium centers are not commonplace. However, the Weidenbruch group has made some headway in this field, and accordingly, the heterocycles [tBu2Ge(μ-Te)]2 (787)542 and [tBu2Si(μCh)]2 (Ch = Se and Te; 788 and 789) were synthesized.543 More recently, the Lappert group obtained the cyclic distannane species [(ArNMe2)2Sn(μ-Ch)]2 (ArNMe2 = 2,6(Me2N)2C6H3; Ch = O, S, Se, and Te; 790−793).544 The von Hän isch group prepared the E 2 Te 2 heterocycles [(tBu2PhSiTe)E(μ-TeSiPhtBu2)]2 (E = Sn and Pb; 794 and 795) and [(Me3Si)3CPb(μ-TeSiPhtBu2)]2 (796) with the intention of using these species as precursors to thermoelectric SnTe and PbTe materials.545 A well-defined Pb2S2 four-membered heterocycle, [BrPb(μSArTrip)]2 (797), was generated as a side product from the reaction of 2 equiv of Li[SArTrip] with PbBr2 in an effort to prepare the lead(II) bis(thiolate) [ArTrip]2Pb:.546 Lastly, hybrid inorganic rings containing group 14 elements linked to transition metals via sulfido bridges (e.g., 798; Figure 36) have been reported by various groups.523,547

Scheme 54. Formal Ligand Scrambling To Yield the Asymmetric Cyclotriphosphirane 805

3.5. Group 15 Element Rings Containing P, As, Sb, and Bi

Cyclodipnictadiazanes [RPnIIINR′]2 (Pn = P, As, Sb, and Bi) represent a widely explored class of inorganic heterocycle.1 For example, these species have been used as molecular scaffolds for supramolecular chemistry,548 as ligands for main group549 and transition550,551 metals and can undergo novel ring-opening oligomerization processes.552,553 This section will focus on the recent developments in group 15 element (pnictogen) containing rings such as the versatile atom transfer chalcogenation of dipnictenes (RPnPnR) to yield three-membered Pn2Ch heterocycles [RPn(μ-Ch)PnR] (Ch = S, Se, or Te). 3.5.1. Group 15 Element Homoatomic Rings. Cyclic oligophosphines [RP]n have been extensively explored to extract conceptual parallels between the reactivity of phosphorus and that of carbon.554−556 Three-membered phosphorus-based homoatomic rings are less common than their four-membered counterparts as the formation of cyclotetraphosphanes [RP]4 often occurs from the self-dimerization ([2 + 2] cycloaddition) of unstable diphosphenes, RPPR.556 Symmetric diphosphenes, where identical bulky groups are bound to each phosphorus center, have been known since the landmark synthesis of Mes*PPMes* (Mes* = 2,4,6-tBu3C6H2) (799) in 1981 by Yoshifuji and co-workers.557 Protasiewicz and co-workers attempted to synthesize an

redistribution to form both the cyclotriphosphirane 805 and the diphosphene (Bbt)PP(Bbt) (804); notably, the production of 805 is only observed in the presence of Te PnBu3.559 In related studies by the Tokitoh group, a variety of kinetically stabilized dipnictenes (RPnPnR; Pn = P, Sb, and Bi) readily add chalcogen atoms (Ch = S, Se, and Te) across the PnPn double bonds to afford cyclic products (see section 3.5.3 for more details).560−562 In 2011, Mathey and co-workers reported mixtures of the cyclic phosphinidene oligomers (807 and 808) each coordinated by Cr(CO)5 units as decomposition products from the thermolysis of a 7-phenyl-7-phosphanorbornadiene, 806 (Scheme 55).563 The intended goal of this reaction was to extrude “PhP·Cr(CO)5” and form the known diphosphene complex 809.564 Three- and four-membered cyclopolyphosphinophosphonium cations encompassing formal monocationic endocyclic 7855

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Scheme 55. Phosphinidene Elimination and Oligomerization To Yield Cyclophosphine Adducts of Cr(CO)5

phosphorus centers were first reported by Burford and coworkers in 2005 (Scheme 56).565 The placement of sterically

phosphinophosphonium cations (e.g., [(PhP) 4 PPh 2 ] + , 812).566 As shown in Scheme 56, both alkylation and protonation of neutral cyclopolyphosphines with methyl trifluoromethanesulfonate (MeOTf) or trifluoromethanesulfonic acid (TfOH) can be used to synthesize phosphoniumcontaining rings.565,567 In addition, these cationic frameworks can be obtained by insertion of the in situ generated phosphenium cation, [Me2P]+, into the P−P bond of a cyclotriphosphine, [RP]3 (Scheme 56).565,567 The Burford group expanded upon their exploration of catena-phosphorus monocations to include the synthesis and characterization of the dicationic analogue (818) (Scheme 57).568,569 Compound 818 was obtained by combining the cyclodiphosphinophosphonium salt 816 with neat MeOTf; alkylation of a phosphorus center was accompanied by ring expansion to give a planar four-membered P4 ring (Scheme 57). Related cationic interpnictogen ring frameworks were also prepared by reacting cyclotetraphosphines [CyP]4 with pnictenium cations [Cy2Pn]+ (Pn = P, As, and Sb).570 In the case of phosphenium [Cy2P]+ cation addition, insertion/ring expansion occurred to give the cyclotetraphosphinophosphonium species 820.567 The heavier pnictenium cations combined with 813 to give stable four-membered rings (819) that can be viewed as a cyclotetraphosphine ligand, [RP]4, datively coordinated to [R2As]+ and [R2Sb]+ cations, respectively.570 Stephan and co-workers uncovered frustrated Lewis pair behavior between the hindered cyclophosphine [CyP]4 and B(C6F5)3, leading to the formation of the zwitterionic product 821 from the cooperative activation of phenylacetylene (eq 44).571

Scheme 56. Synthetic Routes to Triflate Salts of Cyclopolyphosphinophosphonium Cations

encumbered Cy and tBu alkyl groups at phosphorus was required to stabilize the smaller three- and four-membered rings (810−811) as analogous prior work with phenylsubstituted polyphosphines gave five-membered cyclotetraScheme 57. Dicationic P4 Ring Framework 818 and Addition of Pnictenium Residues to the Cyclotetraphosphine 813

7856

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Cyclotriarsine rings are rare, with the only examples in the literature prior to 2000 being [AstBu]3 (822)572 and [FcAs]3 (823)573 (Fc = ferrocenyl). More recently, Weber and coworkers generated the metalloalkyne-appended As3 ring 825 upon reacting the arsaalkene 824 with Au(CO)Cl (eq 45).574 This product is believed to form from cyclotrimerization of a transient arsanediyl [Tp*(CO)2MCAs] (M = Mo and W; Tp* = HB(3,5-Me2pz)3).

Scheme 59. Convergent Syntheses of [MesAs]4 (830)

Table 9. Recently Prepared Cyclotetrastibines [RSb]4 compd no. 831584 832585 833586 834587 835588 836583

R group

Sb−Sb bond length (Å)

Bu Mes Cp* (Me3Si)2CH 2-(Me2NCH2)C6H4 PMe3

2.814(2)−2.821(2) 2.853(1)−2.855(1) 2.836(1) 2.822(1)−2.878(1) 2.8474(5)−2.8605(4) 2.8411(6)−2.8607(5)

t

and PMe3 (eq 46).583 While 836 can be viewed as a [Sb4]4+ tetracation carrying four phosphine ligands, the structure was

The composition of salvarsan, a long-known remedy for syphilis, was investigated using electrospray ionization mass spectrometry (ESI-MS).575 The ESI-MS data suggested that salvarsan consists of cyclic species in solution with a preponderance of trimeric [AsR]3 and pentameric [AsR]5 rings (R = 3-amino-4-hydroxybenzene; 826 and 827), contrary to the earlier belief that polymeric species were present. Grobe et al. demonstrated that the cyclotetraarsane [F3C−As]4 (828) (and its pentameric counterpart [F3C−As]5) is a source of transient bis(trifluoromethyl)diarsene, F3CAsAsCF3, which could be intercepted as a side-on π-complex with a Pd(0) center (829; Scheme 58).576The tetraarylcyclotetraarsine [MesAs]4 (830) can be prepared in a Wurtz coupling reaction between MesAsCl2 and excess magnesium powder in THF577 and from the efficient zirconium-mediated catalytic dehydrocoupling of mesitylarsane, MesAsH2 (Scheme 59). The Breunig group has explored the synthesis and reactivity of antimony and bismuth homocycles (Sbx and Bix; x ≥ 4) in considerable detail and has written comprehensive reviews of this field.578−582 Table 9 lists recently prepared cyclostibanes, [RSb]4 (R = alkyl or aryl group), with a common synthetic principle being the use of sterically hindered substituents and/ or exogenous Lewis basic donors at antimony to stabilize the resulting ring structures. A novel planar polycationic Sb4 ring with pendant PMe3 groups, [Sb(PMe3)]4(OTf)4 (836), was prepared from the reaction of SbF3 with a mixture of Me3SiOTf

described as a predominantly neutral cyclotetrastibine core with considerable positive charge on the flanking phosphorus centers (i.e., to give phosphonium Sb−PMe3 bonding environments) (Figure 37). Homocyclic bismuth three- and four-membered rings were first conclusively identified in 1998 by Breunig and coworkers.589 In this study, reduction of the disilylmethylsubstituted bismuth precursor (Me3Si)2CHBiCl2 with magnesium afforded solutions of the trimer [(Me3Si)2CHBi]3 (837) and tetramer [(Me3Si)2CHBi]4 (838) (Scheme 60); compounds 837 and 838 exist in equilibrium in solution, and cooling leads to the isolation of the thermally unstable tetramer 838 as a black solid.589 Table 10 summarizes related bismuth homocycles along with a listing of the crystallographically determined Bi−Bi single bonds; in each case, very little deviation in bond length was noted [2.970(5)−3.038(2)

Scheme 58. Cyclic Homoarsenic Structures and Delivery of Reactive Diarsene Equivalents

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Table 10. Recently Reported Homonuclear Bismuth Systems [RBi]n

Figure 37. Thermal ellipsoid drawing (50%) of 836. Hydrogen atoms and noninteracting portions of the anions are not shown. Some structural details are given in Table 9.583 Reprinted with permission from ref 583. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

compd no.

R group

839a (n = 3)589 839b (n = 4)589 840 (n = 4)591 841 (n = 4)592 842a (n = 3)593 842b (n = 5)593 843a (n = 3)594 843b (n = 5)594 844a (n = 3)595 844b (n = 4)595

(Me3Si)2CH (Me3Si)2CH (Me3Si)3Si (Me3C)3Si Me3SiCH2 Me3SiCH2 Me3CCH2 Me3CCH2 2-(Me2NCH2)C6H4 2-(Me2NCH2)C6H4

Bi−Bi bond length (Å) 2.970(5)−3.044(2) 3.0134(6)−3.0302(5) 3.013(1)−3.038(2)

3.0095(16)−3.0221(16)

Scheme 61. Reactivity of [(Me3SiCH2)Bi]n Rings with Transition-Metal Carbonyl Compounds

1,578,580

Å]. A general observation with these systems is that trimer−tetramer equilibria exist in solution, with occasional formation of pentameric species observed; trimeric bismacycles [RBi]3 become more prevalent upon dilution and heating, while higher Bi4 and Bi5 cyclic oligomers are favored at lower temperatures.590 Breunig and co-workers showed that small bismuth ring systems can react with transition-metal carbonyl compounds, such as THF·W(CO) 5,593 Cp*Mn(CO)2 (THF),581 and Fe2(CO)9,581,596 leading to ring-cleaved products as outlined in Scheme 61. For example, a 1:1 equilibrium mixture of [(Me3SiCH2)Bi] 3 (842a) and [(Me3SiCH2)Bi]5 (842b) combines with THF·W(CO)5 to form a cis-dibismuthene (845) that is coordinated in “side-on” fashion by two bridging W(CO)5 units in a bicyclic Bi2W2 butterfly structure.593 Moreover, the dibismuth complex Bi2[Mn(CO)2Cp*] (846) is produced when a mixture of 842a/b and Cp*Mn(CO)2(THF) is irradiated with UV light, while the same bismuth rings combine with Fe2(CO)9 to give the Bi2Fe2 heterocycle [RBiFe(CO)5]2 (847) as a red solid.591 The bismuth−iron cubane [nBu4N]4[(CO)4Fe(μ3-Bi)]4 (848) was synthesized by reacting [(Me3Si)3SiBi]4 (840) with 8 equiv of Na2[Fe(CO)4], followed by the addition of an excess of nBu4NCl in THF (eq 47).597 3.5.2. Group 15 Element Heteroatomic Rings. Azadiphosphiridines (three-membered rings with PPN cores) and diazaphosphiridines (PNN heterocycles) have not been actively studied since 2000; however, it is still salient to briefly mention the previous pioneering work in this field by Niecke and others. The azadiphosphiridines [(Me3Si)2NP(μ-NSiMe3)P(NR2)] (R = SiMe3 and iPr; 849a and 849b; Figure 38) were prepared in 1981. While the persilylated analogue 849a exhibited considerable thermal stability, the less hindered heterocycle 849b underwent [2 + 1] cycloreversion above 50 °C to liberate the aminophosphine iPr2NPNSiMe3 and polymeric products.598 The metalated azadiphosphiridine [(CO)5Cr·PPh(μ-NPh)PPh·Cr(CO)5)] (850) was obtained from the [2 + 1] cycloaddition (nitrene insertion) of PhN3 with

the PP manifold within the diphosphene complex PhP PPh·[Cr(CO)5]3.598e Structurally related azadiphosphiranimines can be formed from the thermally induced isomerization of P2N2 heterocycles, as outlined in Scheme 62;599 this process

Scheme 60. Homocyclic Bi3 and Bi4 Rings

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Figure 39. Examples of stable diazaphosphiridines and diazaphosphiridine oxides.

Figure 38. Selected azadiphosphiridines.

Scheme 62. P2N2 to P2N Ring Conversion via Transient Iminophosphines and the Cyclic Phosphonium Salt 852

reported in the literature, and this topic has been reviewed by both Chivers4a and Stahl.549 In particular, cyclodiphosphadiazanes [RPNR′]2 (860) have received the most attention given the greater accumulated chemical knowledge about phosphorus in relation to its heavier group 15 element congeners. Early work and the application of P2N2 rings as macrocyclic building blocks and their use as coordinating anionic or neutral ligands have been discussed in several detailed reviews.4a,550,603 Since the most recent literature survey by Chivers in 2007,4a Burford and co-workers have established an efficient method to synthesize mono- and dicationic cycloazaphosphenium rings (862 and 863) from the readily available cyclodichlorophosphadiazanes [ArNPCl]2 (Ar = Mes* and Xyl; 861; Scheme 64).604 Each of these syntheses involved halogen/OTf Scheme 64. Synthesis of Cycloazaphosphenium Mono- and Dications

is believed to involve the generation of transient iminophosphines followed by their [2 + 1] cyclocondensation to form P2N heterocycles. The reverse of this process (P2N to P2N2 ring expansion) can be initiated in the presence of Lewis acids such as MBr2 (M = Mg and Zn) and triflic acid, HOTf; in the latter case, the cyclic phosphonium salt [tBu(tBuNH)P(μNtBu)2PtBu]OTf (852) (Scheme 62) can be isolated en route to the ultimate formation of [tBuP(μ-NtBu)2PtBu] (853).599c Another particular noteworthy transformation is the ring expansion of [Me3SiNP(tBu)(μ-NSiMe3)PtBu] (854) at 140 °C to give the only reported examples of P3N heterocycles (855 and 856; Scheme 63).599e The diazaphosphiridine 3-oxide PN2 heterocycle [tBu(O)P(μ-NtBu)2] (857) was reported, and ring-opening chemistry with protic reagents such as methanol was described.600 The analogous Et3C-substituted heterocycle [Et3C(O)P(μ-NtBu)2] (858) was synthesized by an efficient addition reaction between the monomeric iminophosphine Et3CPNtBu with the nitroso compound tBuNO, followed by oxo transfer from nitrogen to phosphorus.601 Diazaphosphiridines with P(III) centers (e.g., compounds 859a and 859b) were also reported by Niecke and co-workers (Figure 39).602 Four-membered Pn2N2 heterocycles with alternating pnictogen (Pn = P, As, Sb, and Bi) and nitrogen centers have been

exchange at a phosphorus center, followed by displacement of the OTf− group with stoichiometric amounts of Lewis base, such as DMAP or alkylated phosphines (e.g., Me3P) to afford the cationic heterocycles 862 and 863. The P−N bond lengths within the known monocationic heterocycles 862 lie in the narrow range of 1.696(2)−1.702(2) Å, while substantial elongation of the P−N interactions to 1.735(3)−1.730(3) Å was found in the dicationic analogues 863 (presumably due to increased Coulombic repulsion between the positively charged P centers).604 In comparison, the triflato-substituted cyclodiphosphadiazanes [(TfO)PNAr]2

Scheme 63. Ring Expansion of the Azadiphosphiranimine 854

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(Mes*NAsOTf) adopt monomeric structures in solution, while the Sb analogue [(TfO)Sb(μ-NMes*)]2 (874) remains exclusively as a dimer.553 The deep purple As2N2 diradicaloid [As(μ-NArMes)2As] (875) was prepared directly from the reduction of [ArMesNAsCl]2 (867) with either excess Mg metal or [Cp2TiCl]2 (Scheme 66).610 Compound 875 is extremely

(Ar = Mes* and Xyl; 861) have P−N bond lengths of 1.701(2)−1.704(2) Å; depending on the donor present, either cis (with DMAP) or trans (with PMe3) arrangements are found about the P2N2 rings in 863. The formation of the cationic species is noteworthy given that the role of the phosphorus atom has switched from its traditional Lewis basic nature to Lewis acidic character. Balakrishna et al. reported the watersoluble P2N2 heterocycle (865; eq 48), which was used as a

Scheme 66. Synthesis and Reactivity of the Biradicaloid [As(μ-NArMes)2As] (875)

cationic ligand to form gold(I) complexes for in vitro antitumor studies.605 There are many examples in the literature of P2N2 heterocycles coordinating transition metals and other main group elements, and this field has been surveyed.606,4b The chemistry of the heavier pnictogen analogues of cyclodipnictadiazanes [RPnNR′]2 (Pn = As, Sb, and Bi) has been somewhat slow to develop due to the weaker and more reactive nature of heavier element−nitrogen bonds in comparison with P−N linkages.4a Despite these challenges, the Schulz group recently reported the kinetically stabilized cyclic As2N2 cation [As(μ-NArMes)2AsCl]+ in the form of a GaCl4− salt, 866.607 Scheme 65 outlines the synthesis of 866, which begins with the preparation of the neutral cyclodihalodiarsadiazane [ArMesNAsCl]2 (867) by treating the monomeric aminodichloroarsine ArN(H)AsCl2 with DBU to induce HCl elimination. The resulting terphenyl-substituted As2N2 heterocycle [ArMesNAsCl]2 (867) underwent halide abstraction with GaCl3 to give the cyclic binary As/N cation [As(μ-NArMes)2AsCl]+ as a GaCl4− salt (866) featuring a dicoordinated As center as part of the ring backbone. Chloride/ azide exchange was also observed when 867 was combined with an excess of Me3SiN3, leading to the formation of the azidosubstituted cation [As(μ-NArMes)2As(N3)]+ (868). The same research team reported the cationic antimony and bismuth heterocycles [Sb(μ-NAr Mes ) 2 SbCl] + (869) and [Bi(μNArMes)2BiX]+ (X = Cl and I; 870 and 871) using similar synthetic approaches.608 By adding steric bulk about a cyclodipnictadiazane, [RPnIIINR′]x, the formation of a monomeric iminopnictine, RPnNR′, can be achieved.609 As an illustration of this concept, placing hindered Mes* groups at nitrogen yields the monomeric species Mes*NPX (X = Cl and Br) in solution, while the less encumbered Xyl and Dipp analogues result in the generation of dimeric diphosphadiazanes [R″NPX]2 (R″ = Xyl and Dipp; 872 and 873); the latter compounds can participate in ring expansion to form the trimeric species [R″NPX]3 upon sequential treatment of the dimers with GaCl3 and DMAP.609a The P(III) species Mes*NPOTf and its As congener

reactive toward moisture and oxygen; however, this species is thermally stable to 245 °C in the solid state under N2. This biradicaloid rapidly interacts with CS2, CCl4, S8, and elemental Se to give a variety of oxidized As(III) products with reactivity confined to the radical As centers (Scheme 66). This study follows a prior report by the Schulz group concerning the synthesis of the P−N biradicaloid [P(μ-NArMes)2P] (879), while the analogous (Me3Si)3Si-substituted P2N2 diradicaloid [P(μ-NSi(SiMe3)3)2P] (880) was intercepted as the bis(trimethylsilyl)acetylene addition product 881 (eq 49).611

Several compounds featuring Sb2N2 rings have been reported in the literature with discrete molecular units [R2N−Sb(μNR)2Sb−NR2], dianionic synthons [RNSb(μ-NR)2SbNR]2−, and Sb2N2 rings linked into larger macrocyclic arrangements commonly observed.612,613 This field was thoroughly reviewed in 2007, and thus, only more recent developments in this area (and likewise for the accompanying section on Bi 2 N 2 chemistry) will be presented here.4a Schulz and co-workers prepared the kinetically stabilized antimony(III) azido ring [(N3)Sb(μ-NMes*)2Sb(N3)] (882). The resulting compound

Scheme 65. Generation of the As2N2 Cyclic Cation in 866

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was stable to 190 °C and did not exhibit any shock sensitivity, in contrast to what is normally observed in group 15 element− azide chemistry.614 Homoleptic inorganic polyazides, E(N3)x (E = main group element), are of interest from a fundamental standpoint due to their very high heats of formation and explosive nature, which places them on the cusp of stability.615 As a result of the Lewis acidic nature of Sb(III) and Sb(V) sites, their respective polyazides can adopt Sb(μ-N3)Sb bridges, leading to centrosymmetric Sb2N2 ring formation. A nice illustration of this structural motif is Sb(N3)3, which forms extended structures containing Sb2N2 heterocycles.616 In a similar fashion, phosphoraniminato ligands (NPR3)− can also form E(μ-NPR3)E bridges and support E2N2 ring formation; this area of research has been reviewed in depth by Dehnicke.617 Burford and co-workers have prepared a homologous series of Pn2N2 heterocycles of the general form [(DippNH)Pn(μNDipp)2Pn(NHDipp)] (Pn = P, As, Sb, and Bi; 883a−d),618 while the Chivers group examined the reactivity of the heavier pnictogen dihalides PhECl2 (E = As, Sb, and Bi) with LiNHtBu, including the preparation of the diazadistibane ring [PhSb(μNtBu)2SbPh] (884).619,620 The Sb centers in this class of heterocycles are pyramidalized due to the presence of a lone pair, and accordingly, either trans or cis arrangements of the Sbbound substituents can arise. In one study, the trans and cis isomers of [(DippNH)Sb(μ-NtBu)2Sb(NHDipp)] (885a and 885b; Scheme 67) could be separated by fractional

The first structurally authenticated Bi2N2 heterocycle [(DippNH)Bi(μ-NDipp)2Bi(NHDipp)] (883d) was reported by Roesky and co-workers, and this compound contains a planar Bi2N2 core and trans-disposed NHDipp groups.623 The reaction of BiCl3 with 3 equiv of KN(SiMe3)2 resulted in the generation of a cyclic species, [(Me3Si)2N−Bi(μ-NSiMe3)2Bi− N(SiMe3)2] (888), as an unexpected byproduct, in addition to target trisamidobismane Bi[N(SiMe3)2]3.624 Lastly, Lewis base appended bismuth azide complexes were prepared which contained cyclic Bi2N2 units resulting from either weak intramolecular Bi−N3---Bi bridges (as in 889) or strong intramolecular interactions as found in [bipy(N3)2Bi(μ-N3)2Bi(N3)2bipy] (890) (Figure 40).625,615a

Scheme 67. Reactivity of the trans and cis Isomers of [(DippNH)Sb(μ-NtBu)2Sb(NHDipp)] with nBuLi

Figure 40. Selected Bi2N2 heterocycles.

The terphenyl-substituted heterocycle [ClBi(μNArMes)2BiCl] (891)626 can be generated as a red crystalline solid from the Sn/Bi transmetalation reaction outlined in Scheme 68; treatment of 891 with 1 equiv of GaCl3 afforded the stable dibismadiazenium salt [ClBi(μ-NArMes)2Bi]GaCl4 (892).608a Stahl and von Hänisch prepared the planar binary element P2As2 heterocycle [(Me3Si)2CHAs(μ-P(SiPh2tBu))]2 (894) from the 1:1 reaction of Li2P(SiPh2tBu) and the hindered dichloroarsine (Me3Si)2CHAsCl2 (eq 50).627

3.5.3. Group 15/16 Element Heteroatomic Rings. Recently, dioxadiphosphetane complexes containing stable P2O2 rings (895) were obtained as products from the deoxygenation of isocyanates628 and carbon dioxide629 by electrophilic terminal phosphinidene complexes (Scheme 69). The first structural authentication of a well-defined As2O2 ring was reported in 2000 when single-crystal X-ray crystallography was performed on a sample of AsOCl3 at −120 °C, revealing the presence of a centrosymmetric dimer, [Cl3As(μ-O)]2 (896).630 Another As2O2 heterocycle was formed during an attempt to alkylate OPF3 with [MeOSO]AsF6; in place of obtaining a [MeOPF3]+ cation, the methoxy-bridged arsane [F4As(μ-OMe)]2 (897) was isolated (Figure 41).631 The dimeric triphenylantimony(V) oxide [Ph3Sb(μ-O)]2 (898)

crystallization, and differing reaction profiles with the base, n BuLi, were observed: the trans isomer (885a) gave a mixed element Li2Sb2N4 cubane-shaped product (886) when reacted with nBuLi, while the cis isomer (885b) participated in a formal Sb extrusion reaction, leading to the aggregated species 887 (Scheme 67).621 A family of structurally related centrosymmetric Sb2N2 heterocycles, [XSb(μ-NMes*)]2 (X = OTf, F, Cl, Br, and I), were prepared. As expected, the X−Sb−N angles in these species decrease in the order I > Br > Cl > F [101.59(4)− 91.31(7)°] in accordance with an increase in p-character in bonds involving more electronegative elements.622 7861

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Scheme 68. Preparation of [ClBi(μ-NArMes)2BiCl] (891) and Subsequent Halide Abstraction with GaCl3

Scheme 69. Synthesis of 895 from the PhosphinideneMediated Deoxygenation of Isocyanates and Carbon Dioxide

Figure 42. Lawesson’s and Woollins’ reagents.

(906) and were able to coordinate a metal carbonyl unit to the Sb2S2 ring in 907 (eq 51).588

was obtained by the oxidation of Ph3Sb with hydrogen peroxide in acetone.632 In general, stibonic acids [RSb(O)(OH)2]x form aggregated species that are insoluble in most common organic solvents, and thus adequate structural characterization of these species is lacking. However, in 2008, the soluble arylstibonic acid [ArMesSb(μ-O)(OH)2]2 (899) was obtained by the Beckmann group by placing a sterically encumbered terphenyl substituent at antimony.633 Tokitoh and co-workers prepared the stable Sb2O2 and Bi2O2 heterocycles (900 and 901; Figure 38) by exposing the corresponding distibines and dibismuthenes (TbtSbSbTbt and TbtBiBiTbt) to air.634 The related higher oxidation state Sb(V)− and Bi(V)−oxo dimers [(2-(MeO)C6H3)3E(μ-O)]2 (E = Sb and Bi; 902 and 903; Figure 41) are known, and the Bi analogue 903 was shown to convert primary and secondary alcohols into aldehydes and ketones under mild conditions.635 Group 15 elements can form stable four-membered rings with sulfur and selenium, and these compounds have attracted attention from the standpoint of organic synthesis as readily (and commercially) available atom transfer agents, as typified by Lawesson’s, [MeOC6H4P(S)(μ-S)]2 (904), and Woollins’, [PhP(Se)(μ-Se)]2 (905), reagents (Figure 42).3 For example, these P2S2 and P2Se2 heterocycles can effectively generate thiocarbonyl and selenocarbonyl functionalities, respectively, from carbonyl groups.3 Silvestru and co-workers prepared the cyclodithiodistibane [RSb(μ-S)]2 (R = 2-(Me2NCH2)C6H4)

Atom transfer is a dominant synthetic strategy for constructing group 15 element rings with the heavier chalcogens S, Se, and Te. For example, a series of stable three-membered heterocycles, [ArPn(μ-Ch)PnAr] (Pn = P, Sb, and Bi; Ar = Bbt and Tbt), can be prepared by chalcogen element transfer (route A) to dipnictenes RPnPnR.636 An early example of this synthetic approach appeared when the thiodiphosphirane ring [Mes*P(μ-S)PMes*] (909) was obtained from the reaction of Mes*PPMes* with elemental sulfur, with the formation of the monosulfide Mes*P(S) PMes* (908) as an intermediate (Scheme 70).562 Tokitoh and co-workers used a similar approach starting with the asymmetrically substituted diphosphene [(9-Anth)PP(Bbt)] (803) to give the three-membered rings [(9-Anth)P(μCh)2P(Bbt)] (Ch = S and Se; 910 and 911);637 when 803 was combined with synthetic equivalents of Te, no discernible reaction occurred. Chalcogen atom transfer to unsaturated distibenes (RSb SbR) and dibismuthenes (RBiBiR) has been successfully conducted by the Tokitoh group to yield stable Sb2Se, Sb2Te, Bi2Se, and Bi2Te rings.636,638,560 While the sulfurization and selenization of RPPR with elemental sulfur or elemental

Figure 41. Various pnictogen−oxo heterocycles. 7862

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Scheme 70. Oxidative Synthesis of the Mixed Element Pnictogen−Chalcogen Heterocycle 909

Scheme 72. Synthesis of CF3-Substituted Phosphorus− Tellurium Heterocycles

selenium cleanly afforded the three-membered P2S and P2Se chalcogen-containing rings,561,562 the transfer of chalcogens to heavier group 15 dipnictenes (e.g., RBiBiR) is not straightforward. Reactions of distibenes and dibsmuthenes with elemental sulfur often yield four-, five-, or six-membered polysulfide rings (Scheme 71).560 The reaction of dibismu-

tellurium dissolved in oleum, H2SO4·xSO3, to give brightly colored solutions. Subsequent detailed investigations revealed that these reaction mixtures contained cyclic homopolyatomic cations, such as S82+;641 however, the true nature of the species responsible for the colors observed is still under investigation.642 Of particular interest within the context of this review is that square-shaped inorganic cations, E42+ (E = S, Se, and Te), can be formed via the controlled oxidation of chalcogens with high-valent fluorides such as AsF5 and SbF5; in the case of E = S, catalytic Br2 has to be added to facilitate the oxidation of sulfur to yield S4[AsF6]2 (920; eq 52).643 Diatomic S2 gas was identified in volcano plumes of Io (a moon of Jupiter), and the red color of the volcanic emissions was postulated to arise from cyclic S3 and S4.644

Scheme 71. Chalcogenation of Distibenes and Dibismuthenes

The chemistry of the homopolyatomic cations E42+ (E = S, Se, and Te) is still ripe for exploration; however, a series of very nice studies by the Passmore group has led to important discoveries in main group radical chemistry. For example, a 1:1 mixture of S42+ and S82+ (as either the AsF6− or SbF6− salts) reacts quantitatively with unsaturated substrates to yield cycloaddition products that are formally derived from a “S3•+” unit (Scheme 73).645 When S42+/S82+ mixtures are allowed to thenes with elemental selenium in the presence of Et3N gives the selenadibismirane, but considerably longer reaction times are required relative to those for diphosphenes.634 In 2012, Dostál et al. isolated E2Ch2 rings (E = Sb and Bi; Ch = S and Se) with the aid of a NCO chelating pincer-type ligand (Scheme 71).639 Karaghiosoff and co-workers reported a series of phosphorus−tellurium heterocycles with electron-withdrawing CF3 groups at phosphorus.640 For example, the three-membered P2Te heterocycle [(F3C)P(μ-Te)P(CF3)] (916) was obtained as a coproduct with larger mixed element P/Te cyclic species from the reaction of F3CPCl2 with (Me3Si)2Te, while the P3Te ring [(F3C)P(μ-Te)(μ-PCF3)P(CF3)] (918) was generated by combining Na2Te with F3CPCl2 in THF at −78 °C (Scheme 72); each of the reported species was identified on the basis of NMR spectroscopy.640

Scheme 73. Synthesis of Trithiazolium, CNSSS, 7π Radical Cations from Sulfur Homopolyatomic Cations

interact with nitriles, RCN, in liquid SO2 solvent, 7π [RCNSSS]•+ trithiazolium radicals are generated. Following a similar strategy, cyanogen, (CN)2, interacts with the sulfur cations to afford stable main group diradicals [(SSSNC)(CNSSS)]2+[PnF6]2 (Pn = As and Sb; 921 and 922). Compounds 921 and 922 are rare examples of main group diradicals which retain their radical character in the solid state.646 If strong ferromagnetic coupling could be instigated between adjacent CNSSS•+ rings in the solid state, then these materials could be promising non-metal-based magnets. The tetrameric compound [PhSeBr]4 (923) was structurally authenticated and shown to be linked into square Se4 units with

3.6. Group 16 Element Rings Containing O, S, Se, and Te

3.6.1. Group 16 Element Homoatomic Rings. Research in this area indirectly began in the 18th and 19th centuries when it was noticed that elemental sulfur, selenium, and 7863

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Se(μ-NtBu)2SeO] (932).661 A dipalladium complex of Se2N2, [Ph4P]2[Br3Pd(Se2N2)PdBr3] (933), was prepared from the reaction between Se(NSO)2 and [Ph4P][Pd2Br6].662 TeCl4 reacts with 4 equiv of LiNHtBu to yield two cyclic products: the six-membered Te3N3 ring [tBuNTe]3 and the dimeric tellurium(IV) diimide [tBuNTe(μ-NtBu)]2 (934) (Scheme 75).663 Compound 934 adopts a cis arrangement of the flanking imido NtBu groups in the solid state; however, coordination of this species with coinage metals, such as Cu(I) centers, leads to cis to trans isomerization, indicating that a high degree of structural flexibility is present about this heterocycle.664 The exocyclic imido nitrogen centers in 934 are sufficiently basic to bind B(C6F5)3 to generate the 1:1 adduct (F5C6)3B· [tBuNTe(μ-NtBu)]2 (935), while in the presence of THF these reagents participate in FLP chemistry, leading to the ringopening of tetrahydrofuran and the formation of 936 (Scheme 75).665 When 934 was combined with the hydroborane adduct H2O·B(C6F5)3, the stepwise hydrolysis of the terminal imido groups in 934 transpired to give the borane-complexed tellurium(IV) oxos [(F 5 C 6 ) 3 B·OTe(μ-N t Bu) 2 TeN t Bu] (937) and [(F5C6)3B·OTe(μ-NtBu)2TeO]2 (938) each containing planar Te2N2 rings as central structural motifs.666 3.6.3. Group 14/16 Element Heteratomic Rings Containing Peroxo (μ-O2) Units. The ring-forming ability of chalcogen atoms is well-known, and synthetic chemistry has benefitted greatly from the use of strained epoxides as reagents to generate value-added products. These studies are predicated by the need to efficiently prepare epoxides, and metal-mediated oxidation of olefinic substrates has become an integral component of modern chemistry. With respect to the focus of this current review, it is salient to mention that many of these transformations involve three-membered metalloperoxide rings as intermediates (MO2; M = transition metal), and this everexpanding field has been reviewed in significant depth recently.667 Examples of stable peroxide rings featuring main group elements (if one does not include carbon) are much more rare and will be covered in more detail below. Stable NHC adducts of dioxasiliranes (939 and 940) were prepared by the Driess group following the protocol outlined in Scheme 76; the intraring O−O distances in 939 and 940 [1.547(3) and 1.510(3) Å, respectively] were slightly longer than the O−O bond lengths found in diorganodioxiranes (ca. 1.50 Å).668 When a solution of 940 in toluene was allowed to stand, crystals of the ring-cleaved product 941 were isolated (Scheme 76).668 The 1,2-digermadioxetane [Ar″2Ge(μ-O2)GeAr″] (942) (Ar″ = 2,6-Et2C6H3) was formed initially from the reaction of the digermene Ar″2GeGeAr″2 with molecular dioxygen; photolysis of 942 led to its structural rearrangement into the 1,3-digermadioxetane isomer [Ar″Ge(μ-O)]2 (943) (Scheme 77).669 The 1,2-stannagermadioxetane [Trip2Sn(μO2)GeTrip2] (944) was later prepared from oxygen transfer to the stannagermene Trip2SnGeTrip2.508 Tin(IV) peroxides were synthesized from the reaction of Sn(II) precursors with O2, and the presence of three-membered SnO2 rings was inferred by IR spectroscopy.670 The metallogermylene complexes [(R3P)2MGe(N(SiMe3)2)2] (M = Pd or Pt; R = Et or Ph) each reacted smoothly with O2 to give 1,2-addition products, leading to MGeO2 heterocycles (945; Figure 43); as in the case of 942, irradiation with UV light produced cyclic 1,3-dioxirane isomers.671 A hypervalent cyclic peroxide featuring a hexacoordinate P(V) center was characterized as the potassium crown ether

long Se−Se bonds in the range of 3.004(2)−3.0512(2) Å, while the single bond Se−Se distance in PhSe−SePh is 2.29(1) Å; accordingly, the weak Se−Se interactions in 923 are cleaved upon addition of phosphines to give monomeric products of the general form R3P·PhSeBr.647 The related tetrameric phenyliodotelluride [PhTeI]4 (924) was also shown to adopt square Te ring motifs in the solid state where each Te4 unit in 924 is linked into an extended array via intermolecular I---I interactions.648 The Wachter group also reported a stable metal carbonyl tetraadduct, [Te(Cr(CO)5)]4 (925), which contains a formal neutral allotrope of tellurium in the form of a Te4 ring (Scheme 74).649 Scheme 74. Selected Chalcogen-Based Heterocycles

3.6.2. Group 16/Nitrogen Heteratomic Rings. Perhaps the most chemically intriguing species in this area is the explosive heterocycle disulfur dinitride, S2N2 (926).650,651 Not only is this species of fundamental interest as an example of an inorganic 6π aromatic ring,652 it slowly transforms under ambient temperature (or faster at 75 °C) into highly conductive (SN)x via ring-opening polymerization.653,654 Moreover, S2N2 has been recently polymerized in the zeolite matrix Na-ZSM5,655 and exposure of S2N2 vapor to fingerprints induces polymerization to (SN)x on contact, leading to the ability to visualize fingerprints on a wide variety of surfaces.656 S2N2 is prepared by carefully heating S4N4 (also an explosive binary sulfur−nitride) in the presence of silver wool as a catalyst under low pressures (eq 53). The reduction of S2N2 was investigated

by a combined cyclic voltammetry (CV) and EPR study, which revealed that this species is initially reduced to give the dimeric radical anion [S4N4]•−, followed by decomposition into [S3N3]−.657 The formation of group 6 phosphoraniminato complexes of S2N2 (e.g., [(μ-S2N2)MCl4(NPPh3)2] (M = Mo and W; 927 and 928) was reported by Dehnicke and coworkers.658 The stable thiadiaziridine [O2S(NtBu)2] (929a) (Scheme 74) participates in Cu(I)-catalyzed ring expansion with olefinic substrates to yield stable C2NSN rings (929b) with scission of the N−N bond in 929.659 The resulting diamido C2NSN heterocycles are related to known biologically active species and can be readily converted into chiral diamine ligands for metalmediated asymmetric catalysis. The Chivers group has studied selenium(IV) imido species in considerable detail and were able to obtain X-ray crystal structures of the following SeSN2 and Se2N2 heterocycles: [tBuNSe(μ-NtBu)2SO2] (930),660 [tBuNSe(μ-NtBu)2SeO] (931) (Scheme 74),660 and [O 7864

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Scheme 75. Representative Reactions Involving [tBuNTe(μ-NtBu)]2 (934) and Molecular Structure of 938a

a

Reprinted from ref 666. Copyright 2005 American Chemical Society.

Scheme 76. Carbene-Supported Dioxasilaranes and Subsequent SiO2 Ring Cleavage

from phosphorus-based d-orbitals in PO2 ring formation.674 Three-membered thiadioxiranes, R2SO2, have been identified as possible intermediates in the oxidation of diorganosulfides by singlet oxygen.675 Cage structures containing dioxo Sb−O−O− Sb linkages and Sb2O2 rings were prepared by oxidizing triarylstibines, Ar3Sb, with hydrogen peroxide in the presence of oximes.676 3.6.4. Group 16 Element Heteroatomic Rings. In a nice illustration of how an inorganic element can dictate molecular structure, bis(pentafluorophenyl)tellurium oxide, [(F5C6)2Te(μ-O)]2 (948), is a centrosymmetric dimer with a planar Te2O2 ring, while its selenium congener is a weakly associated hexamer, [(F5C6)2SeO]6 (949), with long Se---O and Se---Se bonds; both species were formed via the oxidation of the Se(II) and Te(II) precursors, Ch(C6F5)2 (Ch = Se and Te), with m-

Scheme 77. Preparation of 1,2- and 1,3-Digermadioxiranes

salt 946 by X-ray crystallography.672 The direct observation of the phosphadioxirane 947 (Figure 43) at −80 °C was possible by 17O and 31P NMR spectroscopy; this species was prepared by the interaction of tris(o-methoxyphenyl)phosphine with singlet dioxygen (1O2).673 The bonding in phosphadioxiranes, R3PO2, was examined by DFT using a combination of natural resonance theory (NRT) and natural bond orbital (NBO) analysis; it was shown that intraring three-center four-electron bonding was a major contributor with negligible participation

Figure 43. Main group element-containing dioxirane heterocycles. 7865

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chloroperbenzoic acid in chloroform.677 Structurally related tellurium oxide heterocycles characterized in the literature include a polymorph of 948,678 [(2,4,6-Ph3C6H2)Te(μ-O)X]2 (X = Br and I; 950 and 951),679 the siloxy-capped tellurium(VI) oxide [(Me3SiO)4Te(μ-O)]2 (952),680 [(2,6(MeO)2C6H3)Te(μ-O)OMe]2 (953),681 salts of the dianion [F3Te(μ-O)TeF3]2− (954),682 and the ureatotellurium oxide [(tBuNC(O)NtBu)Te(μ-O)]2 (955).683

acyclic Cl3+ cations.684 Attempts to prepare the Cl2+ cation by oxidation of elemental chlorine with the powerful oxidant IrF6 afforded the deep blue cyclic homopolyatomic cation Cl4+ as an IrF6− salt (958; Scheme 78).685 A rhomboid-shaped Cl4 ring is found in 958 with short intraring Cl−Cl bonds of 1.941(3) Å, accompanied by a set of elongated Cl−Cl interactions at 2.936(7) Å derived from π*−π* interactions (similar to those found in Cl2O2+). Warming 958 above −78 °C resulted in the formation of Cl3+ salts, highlighting the thermodynamic instability of the Cl4+ rings. The cyclic iodine dication, [I4]2+, was first prepared in 1983 by the groups of Gillespie and Passmore via oxidation of I2 with AsF5 in liquid SO2.686 Since this report, [I4]2+ salts with SbF6− and Sb3F14− anions have been made, and structural characterization has revealed geometric features similar to those of Cl4+; namely, a set of long I---I intraring bonds (ca. 3.28 Å) are present along with correspondingly shorter I−I “covalent” bonds (ca. 2.58 Å).687 The homoaromatic character within the I42+ homopolyatomic cation was also examined recently by computational methods.688

3.7. Miscellaneous Inorganic Rings Containing Group 17 Elements

The vast majority of inorganic rings contain halides in bridging environments M−X−M, with each M2X2 ring derived from the formal dimerization of two terminal M−X linkages. However, there are some noteworthy examples of small inorganic rings containing either entirely group 17 elements and/or mixed systems derived from halogens bonding to other electronegative atoms such as oxygen. This field is relatively less explored in terms of the number of publications as a result of the challenges associated with working in the highly oxidizing environments required (e.g., to generate electrophilic reagents such as O2+). The group of Seppelt reported the synthesis of two groundbreaking cyclic cations, Cl2O2+ and Cl4+.684,685 Specifically, the synthesis of [Cl2O2]SbF6 (956) and its Sb2F11− salt, [Cl2O2]Sb2F11 (957), was accomplished by combining the respective dioxygenyl cation, O2+, with elemental chlorine in anhydrous HF (Scheme 78). Both Cl2O2+ cations were isolated

4. CONCLUSIONS AND FUTURE PROSPECTS FOR THE FIELD As can be seen from this review, the number of elements that can be incorporated within cyclic arrangements is vast with concomitantly varied reactivity profiles. Although the main focus of our survey was on homo- and heteroatomic rings featuring elements from groups 12−17, inorganic ring chemistry featuring d-block elements is equally versatile and has spurred advances in the field of catalysis (e.g., olefin polymerization); in fact, an entire treatise on this subject would be a logical complement to the present review. In terms of summarizing potential future research directions for inorganic rings, one can envision a few particularly exciting avenues of study. Specifically, the increasing role of cooperative interactions between main group elements (or hybrid p- and dblock systems) will likely adopt increasingly important roles in advancing small-molecule activation.689 This has been elegantly demonstrated through the emergence of the FLP concept,690 where one could take advantage of heterocyclic structures to effectively place the cooperative Lewis acidic and basic sites in close proximity for substrate activation. Moreover, the use of Lewis basic N-heterocyclic carbenes (and related carbon-based donors) has enabled reactive species, such as the low-oxidation silicon species SiCl2, to be used in coordinated form as precursors to new inorganic rings. With the entire palette of the periodic table now essentially available to inorganic chemists, the study of inorganic rings should remain a fertile domain for synthetic creativity, while possibly affording new and efficient routes to predesigned functional materials of increasing dimensional order and sophistication.

Scheme 78. Synthesis of the Cyclic Cl2O2+ and Cl4+ Cations

as violet-colored salts and shown by single-crystal X-ray crystallography to adopt trapezoidal Cl2O2+ units with long intramolecular Cl−O bonds (e.g., 2.425(13) Å, av, for 956), indicative of weak Cl−O bonding interactions resulting from π*−π* orbital overlap between the Cl2 and O2+ units (Figure 44). The [Cl2O2]+ salts 956 and 957 each contain one unpaired electron, as evidence by EPR and magnetic susceptibility measurements, and warming these materials to room temperature led to elimination of O2 and the generation of stable

5. ADDENDUM Given the continual advances being made in the field of inorganic rings, we would like to briefly mention research activities that appeared in the literature after our original manuscript was completed. New members of the amidozinc ring system [R′Zn(μ-NHR)]2 (R′ = Me, Et, or N(SiMe3)2; R = SiPh3 or Si(NMe2)3; 959) were reported by Johnson and coworkers.691 The phosphine-triggered insertion of CO into the FeB2 heterocyclic unit in [(CO)3Fe(μ-BDur)2] (Dur = 2,3,5,6Me4C6H) (960; Scheme 79) yields the metallacycle [(Cy3P)-

Figure 44. Molecular structure of the [Cl2O2]+ cation in 956 with bond lengths in picometers. Reprinted from ref 684. Copyright 1999 American Chemical Society. 7866

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Scheme 79. Recent Developments in Inorganic Rings

phazanes [ClP(μ-NR)2PCl] (973) can be readily accomplished by either thermally induced or base-induced silyl halide elimination from the aminophosphines Cl2PN(SiMe3)R.705 The use of cyclophosphine (RP)4 arrays decorated with phosphorus-bound chiral auxiliaries as ligands for asymmetric catalysis has been effectively demonstrated recently.706 The cyclic dication [MeSe]42+ was prepared as its BF4− salt ([Me4Se4](BF4)2; 974) from the oxidation of MeSeSeMe with a XeFe2/F3B·OEt2 mixture.707 The Woollins group reported the synthesis and structural characterization (by X-ray crystallography) of a wide range of P(III)−Te heterocycles, including three- and four-membered rings, from the reaction of various dichlorophosphines (such as Mes*OPCl2 and AdPCl2) with Na2Te2.708

(CO)2Fe(μ-CO)(μ-BDur)2] (961); double CO insertion, and FeC2B2 ring formation, occurs when excess phosphine is combined with the amido-substituted FeB2 ring [(CO)3Fe(μBDur)(μ-BN(SiMe3)2)] (962).692 In a study involving the depolymerization of polyaminoboranes [RNHBH2]n (R = Me and H) in the presence of Lewis bases, it was noted that the B2N2 heterocycle [Me2NBH2]2 (110) undergoes a ring cleavage reaction with the N-heterocyclic carbene IPr to give the metastable product IPr·BH2NMe2.693 In addition, lanthanum hydride complexes were shown to induce the catalytic dehydrocoupling of Me2NH·BH3 at 60 °C in THF to yield 110 and minor quantities of (Me2N)2BH as a byproduct; moreover, stoichiometric reactions led to the interception of La complexes bearing encapsulated H2BNMe2 residues.694 The insertion of acetonitrile into the B−P bond in the borafluorene complex [(FlB)PtBu2] (FlB = 9-borafluorenyl) affords the cyclodiboradiazane [FlB(μ-NC(PtBu2)Me]2 (963) as a mixture of cis/trans isomers (Scheme 79).695 Donor-stabilized parent borenium cations [BH2]+ could be isolated as the dimeric species [LB·BH(μ-H)2BH·LB]2+ (964; LB = Lewis base) with cyclic B2H2 cores.696 The cyclic aluminum hydrazide [tBu2Al(μNAd)NCy] (965) is capable of activating CO2 via FLP chemistry that is mediated by Al−N bond scission.697 The first hydrido amido alane compounds were synthesized as centrosymmetric dimers by employing hindered terphenyl ligands at aluminum, [Ar(H)Al(μ-NH2)]2 (966) (Ar = terphenyl ligands); these species were prepared by dehydrogenative coupling between ammonia and the requisite alanes [ArAlH2]2.698 A nearly planar Si4 heterocycle, [{(tBu2N)2CPh}Si(μ-SiN(SiMe3)Dipp)2Si{PhC(NtBu2)2}] (967), that exhibits significant π-electron delocalization throughout the central Si4 unit was recently reported;699 following this report, the So group also synthesized structurally related Ge4 and Sn2Ge2 rings.700 The first stable complexes of a disilyne, [(Cy3P)M(RsSi SiRs)] (M = Pd and Pt; 968 and 969; Rs = C(SiMe3)2CH2tBu), were prepared by Ishida, Iwamoto, and co-workers.701 Addition of Ni(COD)2 (COD = 1,5-cyclooctadiene) to the stable Si(II) adduct ImMe4·Si(H)SitBu3 generated the novel NiSi2 heterocycle [(ImMe4)2Ni(μ-Si(SitBu3)H)2] (970) as a yellow solid.702 The cyclotrisilene [Trip4Si3] (522) undergoes a surprisingly mild reaction with CO to yield a ring-expanded Si3C unit that dimerizes to afford the macrocycle 972 (Scheme 79).703 GaCl3 adducts of cyclodiphosphazenes [R2PN·GaCl3]2 (R = alkyl or aryl groups; 973) were reported in a joint study by Manners and Schulz.704 The synthesis of P-chlorocyclodiphos-

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 780-492-4255. Notes

The authors declare no competing financial interest. Biographies

Gang He was born in Hanzhong, China, in 1982. He received his B.Sc. degree at Shaanxi Normal University in 2005 and his Ph.D. degree in the area of conjugated polymer-based sensors under the supervision of Professor Yu Fang at the same university in 2011. He is currently a postdoctoral fellow in the group of Professor Eric Rivard at the University of Alberta. His current research focuses on the design and synthesis of conjugated oligomers and polymers containing group 16 elements (S, Se, and Te) for solar cell applications. 7867

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Eric Rivard was born in Gander, Newfoundland, Canada, in 1978. He completed his B.Sc. (Honors) degree at the University of New Brunswick in 1995 and later obtained a Ph.D. degree at the University of Toronto under the supervision of Professor Ian Manners FRS (Fellow of the Royal Society) in 2004. He then conducted NSERCsponsored postdoctoral work with Professors Jonas Peters (Caltech) and Philip Power FRS (University of California, Davis), followed by a research stay at Monash University with Professor Cameron Jones. In 2008 he began his independent career as an Assistant Professor at the University of Alberta, and has been recently promoted to the rank of Associate Professor. He has been the recipient of an Alberta InnovatesTechnology Futures New Faculty Award, a Petro-Canada Young Innovator Award, and various undergraduate teaching awards. His current research interests involve main group molecular and polymer chemistry, including the stabilization of low-oxidation main group hydrides for use as precursors to nanomaterials and the development of new conjugated polymer systems via zirconium-mediated syntheses for optoelectronic applications. He has published around 60 papers thus far.

Olena Shynkaruk was born in 1990 in Kyiv, Ukraine. She received her B.Sc. degree in inorganic chemistry from Taras Shevchenko National University of Kyiv in 2011. She then joined Professor Eric Rivard’s research group at the University of Alberta, and her current Ph.D. research is focused on the synthesis of polymers containing conjugated spirocyclic units for use in photovoltaic and polymer light-emitting devices.

DEDICATION This review is dedicated to the memory of Jean Rivard. ABBREVIATIONS AACVD aerosol-assisted chemical vapor deposition Ad 1-adamantyl Alk alkyl group t Am tert-amyl (CMe2Et) Ar aryl Ar′ 2,6-Dipp2-4-(Me3Si)C6H2 Ar″ 2,6-Et2C6H3 ArDipp 2,6-(2,6-iPr2C6H3)2C6H3 ArMes 2,6-(2,4,6-Me3C6H2)2C6H3 ArTrip 2,6-(2,4,6-iPr3C6H2)2C6H3 ArXyl 2,6-(2,6-Me2C6H3)2C6H3 Bbt 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl bpmna N,N-bis[2-(methylthio)ethyl]-N-[(6-(neopentylamino)-2-pyridyl)methyl]amine bipy bipyridine t Bubipy 4,4′-di-tert-butyl-2,2′-bipyridine cat catecholate (1,2-O2C6H4) COD 1,5-cyclooctadiene Cp cyclopentadienyl (η5-C5H5) Cp* pentamethylcyclopentadienyl (η5-C5Me5) CV cyclic voltammetry CVD chemical vapor deposition Cy cyclohexyl DBU 1,8-diazabicyclo[5.4.0]undec-7-ene Dipp diisopropylphenyl (2,6-iPr2C6H3) Dipp nacnac [(HC(MeCNDipp)2]− DFT density functional theory DMA N,N-dimethylaniline DMAP 4-(dimethylamino)pyridine Dmp dimethylphenyl (2,6-Me2C6H3) Dur duryl (2,3,5,6-Me4C6H) Eind 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl EPR electron paramagnetic resonance FlB 9-borafluorenyl Fc ferrocenyl FLP frustrated Lewis pair hex hexyl (−(CH2)5CH3)

Melanie W. Lui hails from Toronto, Ontario, Canada. She received her B.Sc. degree from Western University in 2012. During her undergraduate studies, she had the privilege of working with both Professors Paul J. Ragogna and Mark S. Workentin at Western University and conducted Natural Sciences and Engineering Research Council of Canada Undergraduate Student Research Award (NSERC USRA) research with Professor George K. H. Shimizu at the University of Calgary. She is currently pursuing her Ph.D. studies as an NSERC postgraduate scholar with Professor Eric Rivard at the University of Alberta, where she is investigating aspects of ligand design as they pertain to low oxidation state main group element chemistry.

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Chemical Reviews ImtBu IMes IPr LA LB MAO Me3-tren Mes Mes nacnac Mes* NBO NHC NN NRT Np OTf phen py Pyr ROP Tbt TMEDA TMP tolyl Tp* Trip Vi Xyl

Review

[(HCNtBu)2C:] [(HCNMes)2C:] [(HCNDipp)2C:] Lewis acid Lewis base methylalumoxane [MeAlO]x N(CH2CH2NHMe)3 mesityl (2,4,6-Me3C6H2) [HC(MeCNMes)2]− supermesityl (2,4,6-tBu3C6H2) natural bonding orbital N-heterocyclic carbene 1,2-(tBuCH2N)2C6H4 natural resonance theory neopentyl (CH2tBu) triflate anion (F3CSO3−) phenanthroline pyridine 2-pyridyl ring-opening polymerization 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl tetramethylethylenediamine (Me2NCH2CH2NMe2) 2,2,6,6-tetramethylpiperidino 4-MeC6H4 HB(3,5-Me2pz)3 (3,5-Me2pz = NC(Me)C(H)C(Me)N) triisopropylphenyl (2,4,6-iPr3C6H2) vinyl (−CHCH2) xylyl (2,6-Me2C6H3)

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