Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne

Review pubs.acs.org/Organometallics

Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes Kyle D. J. Parker and Michael D. Fryzuk*

Organometallics 2015.34:2037-2047. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/11/19. For personal use only.

Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 ABSTRACT: This review focuses on niobium and tantalum organometallic complexes that feature a metal-bound alkyne (RCCR) unit. The contrasting MIII−alkyne/ MV−“alkenediyl” bonding motifs used to describe the metal−ligand interaction in these complexes are discussed. An overview of the spectroscopic and crystallographic structural metrics that are used to characterize alkyne complexes is also included. The various routes employed for the synthesis of tantalum and niobium alkyne complexes are summarized, and the reactivity of these complexes, including potential synthetic applications, is also addressed.

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INTRODUCTION Alkynes (RCCR) and alkenes (R2C=CR2) belong to a class of unsaturated hydrocarbon ligands that engage in bonding interactions with a wide range of transition metals.1−5 While in most cases an alkyne unit binds to a single metal center, some examples are known where an alkyne can bridge two metal centers; Scheme 1 depicts three classic examples of

Pauson−Khand reaction, which generates cyclopentenones from an alkyne, an alkene, and CO.8,9 In complex 3, alkyne moieties are coordinated to the iron center as both bridging and terminal ligands.10 Each of the late-transition-metal (groups 8−10) examples discussed above (1−3) was synthesized by the direct addition of an alkyne to an electron-rich, low- or zerovalent (i.e., PtII, Co0, Fe0) metal center. Similarly, some of the first examples11−14 of early-transition-metal (group 4 or 5) alkyne complexes employed a comparable synthetic methodology, as shown in Scheme 2; adding 1 equiv of alkyne to formally TiII metallocenes affords alkyne complexes such as 4 and 5.

Scheme 1

Scheme 2

However, a more general method to prepare early-metal alkyne complexes is via the reduction of an electron-poor, highvalent (typically d0) metal complex in the presence of an alkyne. Rosenthal and Burlakov,3,15 informed by initial studies

late-transition-metal alkyne complexes that illustrate these different binding modes. Complex 1, K[PtCl3(RCCR)], was synthesized6 via a substitution reaction with the well-known alkene complex Zeise’s salt,7 K[PtCl3(H2CCH2)], and illustrates the structural similarities between metal alkyne and metal alkene complexes. In both cases, the ligand binds “side-on” to the metal center through the C−C multiple bond. Complex 2, Co2(CO)6(RCCR), is used as a reagent in the © 2014 American Chemical Society

Special Issue: Mike Lappert Memorial Issue Received: October 13, 2014 Published: December 19, 2014 2037

dx.doi.org/10.1021/om5010385 | Organometallics 2015, 34, 2037−2047

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by Vol’pin and Shur,16,17 have developed an extensive body of work surrounding the chemistry of titanocene18 (6) and zirconocene19 (7) alkyne complexes (Scheme 3). In many Scheme 3 Figure 1. Resonance structures for metal−alkyne complexes representing the neutral alkyne (left) and “alkenediyl” (right) bonding motifs.

“metallacyclopropene” structure. These two scenarios are depicted in Figure 1. To determine which bonding model description in Figure 1 is more appropriate, elongation of the C−C bond as measured by X-ray crystallographic studies is a useful metric. For example, complexes that contain alkyne ligands consistent with the alkenediyl formulation feature C−C bond lengths ranging from approximately 1.28 to 1.35 Å, more typical of a CC double bond.29−34 Other metrical parameters, such as the angle formed between the C−C bond and the alkyne substituent R (θC−C−R), are useful indicators of the hybridization of the alkyne carbon atoms; bond angles that deviate significantly from 180° suggest sp2 hybridization, which is also indicative of an alkenediyl-type description. Vibrational spectroscopy can also be a useful tool for probing the strength of the metal−alkyne interaction.35 As with a crystallographically determined elongation of the C−C bond, a decrease in the alkyne stretching frequency (νC−C) also correlates with a longer C−C bond. In addition, 13C NMR experiments28,35 show that the chemical shifts of the quaternary carbons of an alkyne strongly bound to a metal center as a fourelectron donor generally give rise to resonances that are significantly downfield (δ >180); in contrast, alkyne ligands that interact with the metal center as a two-electron donor typically feature 13C NMR resonances in the δ 100−120 region. In most group 5 metal−alkyne complexes, the structural and spectroscopic data indicate that the MV−alkenediyl formalism is the more apt bonding description. Nevertheless, the term “metal alkyne” complex will be used throughout this review to refer to all Ta and Nb complexes that feature an alkyne ligand; this nomenclature is chosen for simplicity and brevity, despite (and acknowledging) the fact that the spectroscopic and crystallographic data usually point more toward the metallacyclopropene-type formulation. For the sake of consistency, all of the group 5 metal−alkyne interactions are shown using the MV alkenediyl structures.

cases, the reactivity of these complexes suggests that they can be regarded as synthons for “Cp2M” (i.e., formally MII, d2) metallocenes.3,15,20,21 This review deals exclusively with alkyne complexes of the group 5 metals, niobium and tantalum in particular, and includes a brief survey of the many synthetic routes to niobium and tantalum alkyne complexes, as well as an overview of common reactivity patterns for these complexes. In an effort to focus this material, we have tried to include those systems that can be categorized as MIII alkyne or MV alkenediyl derivatives (see below); however, there are a very small number of formally MI systems that lie outside of this arbitrary focus area and so are excluded.22−24 While the Lappert oeuvre does not specifically include alkyne complexes, the kinds of supporting ligands that are common to group 5 complexes, such as cyclopentadienyl and amido donors, owe much to the pioneering and sustained efforts from the Lappert group over many years.

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GENERAL CONSIDERATIONS The bond between an alkyne unit and a transition metal can be described using the standard Dewar−Chatt−Duncanson model,25−27 wherein the filled π orbitals from one of the C−C multiple bonds can form a σ-bonding interaction with an empty orbital on the metal center, along with synergistic π back-donation from filled metal d orbitals to empty π* orbitals of the alkyne. The C−C multiple bond of the alkyne perpendicular to the MCC plane can also engage in π bonding and δ back-bonding interactions. Alkynes are known to act as either two- or four-electron donors,28 depending on the electrophilicity of the metal center; the bond between a metal and an alkyne ligand can be described by two structural formalisms that reflect these extremes. In the case of group 5 (M = Ta, Nb) alkyne complexes, one possibility is that of a dative bond between a d2 MIII center and a neutral (two- or four-electron donating) alkyne ligand. Alternatively, the interaction can be viewed as the donation of four electrons via two formal covalent bonds between a d0 MV and an “alkenediyl” dianion, which results in a

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SYNTHESIS OF TANTALUM AND NIOBIUM ALKYNE COMPLEXES While there are a variety of methods for the preparation of NbV and TaV alkyne complexes, the common feature of all synthetic routes is the addition of an alkyne to a MIII precursor, although the means of generating this low-valent precursor vary. One of the first studies of group 5 alkyne complexes was undertaken by Cotton and co-workers, using the formally MIII starting material M2Cl6(THT)3 (8, M = Ta;34,36,37 9, M = Nb;38 THT = tetrahydrothiophene, C4H8S). Equimolar mixtures of the trivalent metal precursors and alkyne substrates react smoothly to generate dimeric MV complexes, the structures of which depend on the relative steric bulk of the alkyne (RCCR′). As shown in eq 1, most disubstituted alkynes such as diphenylacetylene (R = R′ = Ph) or tert-butylmethylacetylene 2038


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some representative examples from work reported by the groups of Etienne41−47 (17 and 18), Otero48 (19), Hill31 (20), and Fryzuk51 (21) are shown in Scheme 5. Scheme 5

(R = CMe3, R′ = Me) give rise to chloride-bridged bimetallic complexes [MCl2(L)(RCCR′)]2(μ-Cl)2 (10−12). In contrast, treating Ta2Cl6(THT)3 (8) with the bulkier ditert-butylacetylene (R, R′ = CMe3) affords the alkyne-bridged dimer [TaCl3(L)]2(μ-η2:η2-RCCR′) (13; eq 2).36 This

unusual bonding motif features a short Ta−Ta distance (2.677(1) Å), which the authors attribute to a metal−metal double bond between two TaIII centers. Nevertheless, the elongated C−C distance (1.351(21) Å) suggests that the alkyne moiety can be considered strongly bound as a two-electron donor to each metal center (i.e., an overall four-electron donor), similar to the case for complexes 10−12. In 1990, Pedersen and Roskamp reported33 the synthesis of a series of niobium alkyne trihalides, Nb(RCCR′)X3(DME) (15, where DME = 1,2-dimethoxyethane), mononuclear analogues of complexes 10−12. The synthetic route involves treatment of NbX3(DME) (14, X = Cl, Br) with 1 equiv of the desired alkyne (Scheme 4).39 Over a decade later, Oshiki and Scheme 4

Prior to the advent of these conveniently accessible Nb (15) and Ta (16) alkyne trihalide starting materials, the syntheses of most group 5 alkyne complexes typically involved the reduction of a high-valent MV or MIV organometallic precursor that already contained a stabilizing ancillary ligand, such as a cyclopentadienyl (Cp = C5H5) unit. Although there are some examples of MIII compounds that are stable enough to be isolated and characterized (e.g., 22, 28, 33; vide infra), in most cases the reduced MIII species was generated in situ and immediately treated with an alkyne substrate to afford the desired MV alkyne organometallic complex. The most common method for the reduction of a high-valent MV or MIV precursor cited in the literature is via the use of an external reducing agent, such as Zn, Mg, Al, or Na/Hg amalgam. One of the first examples of this route was the synthesis of a variety of monocyclopentadienyl Ta and Nb alkyne chloride complexes by Curtis and co-workers (Scheme 6).52−54 Treating a solution of Cp′MCl4 (Cp′ = C5H5, C5H4Me) with aluminum powder in the presence of the desired alkyne afforded Cp′M(RCCR)Cl2 (22). If the reduction was carried

co-workers reported32,40 the synthesis of the tantalum analogues of 15, Ta(RCCR′)Cl3(DME) (16) (Scheme 4), generated by the in situ reduction of TaCl5. The ease with which these compounds can be synthesized, as well as the variety of different alkyne substrates supported, make them ideal starting materials for group 5 alkyne complexes; indeed, there are numerous examples in the literature31,41−50 where 15 and 16 act as reagents for the installation of Nb(RC CR′) or Ta(RCCR′) units into a wide variety of ligand sets; 2039


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Scheme 6

Scheme 8

out in the presence of carbon monoxide (CO) instead of an alkyne substrate, the stable MIII dimer [Cp′M(CO)2Cl]2(μ-Cl)2 (23) could be isolated; the addition of alkyne to a solution of 22 also afforded 23.53 Gomez and co-workers also employed this method to prepare a Nb analogue, (CpSi)Nb(BTA)Cl2, from (CpSi)NbCl4 (CpSi = C5H4(SiMe3)).55 A variety of bis(cyclopentadienyl) alkyne complexes have also been synthesized via the reduction of the corresponding TaIV or NbIV dichloride complex with Na/Hg amalgam, as shown in Scheme 7; for example, Green and co-workers56 used

Although the majority of work in this field has focused on cyclopentadienyl-containing systems such as the ones discussed above, alkyne moieties have been installed via a similar reduction protocol in Ta and Nb complexes featuring a variety of ancillary ligand sets. For example, McConville and co-workers30 investigated the synthesis of Ta alkyne complexes featuring a pyridine-diamido pincer ligand, BDPP, as shown in Scheme 9. Scheme 9

Scheme 7

The reduction of [BDPP]TaCl3 (31) with Na/Hg in the presence of an excess of alkyne afforded the Ta alkyne chloride complex [BDPP]Ta(RCCR′)Cl (32). In addition, Wolczanski and co-workers reported61 the synthesis of the stable TaIII complex Ta(silox)3 (33, silox = tris(tert-butyl)siloxide) via the reduction of Ta(silox)3Cl2 with Na/Hg; as shown in Scheme 10, complex 33 readily combined with a variety of alkynes62,63 to form Ta(silox)3(RCCR) (34). Similarly, Wigley and co-workers have investigated the synthesis and reactivity of Ta alkyne complexes featuring 2,6diisopropylphenoxide (DIPP)64−67 and 2,6-diiisopropylphenylimide (DIPN)68 ligands; the steric bulk of these ligands plays a crucial role in determining the reactivity of these complexes with alkyne substrates. For example, the reduction (with Na/Hg) of the Ta dichlorides Ta(DIPP)3Cl2 (35) and Ta(DIPN)(DIPP)Cl2 (36), or the less sterically crowded Ta trichlorides Ta(DIPP)2Cl3 (37) and Ta(DIPN)Cl3 (38), in the presence of certain alkynes afforded the expected Ta(RCCR′) complexes (39−42), as shown in Figure 2. However, in some cases the addition of another 1 or 2 equiv of alkyne can lead to the formation of the butadienediyl tantalacycles (43), or Ta η6-arene products (44), depending on both the steric bulk at the Ta center and the size of the alkyne substituents. The reactivity

this methodology to prepare (CpiPr)2Ta(MeCCMe)Cl (24) from (CpiPr)2TaCl2 (CpiPr = C5H4iPr). Otero and Royo synthesized Cp2Nb(PhCCPh)Cl (25)57 and the ansa-niobocene alkyne complex (SiMe2(Cp)2)Nb(RCCR)Cl (26)58 in a similar fashion. Otero and Royo also showed that, in the case of (CpSi)2NbCl2 (27), the use of 1 equiv of reducing agent afforded the isolable NbIII monochloride (CpSi)2NbCl (28), as shown in Scheme 8.59 While the addition of 1 equiv of alkyne generated the desired alkyne complex (CpSi)2M(RCCR)Cl (29), treating 28 with 1 equiv of a diyne, such as 1,4-diphenylbuta-1,3-diyne, afforded the related complex Cp′2M(RCCCCR)Cl (30).60 2040


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Scheme 10

Scheme 11

paramagnetic NbIV complex bound to an alkenediyl (i.e., [PhCCH]2−) dianionic ligand.69 The reducing ability of the dinuclear TaIV diamidophosphine tetrahydride (48) with a variety of small-molecule substrates, including alkynes and dinitrogen, has been studied extensively.70 As shown in Scheme 12, 48 is formed via the reductive Scheme 12

Figure 2. Synthesis of Ta alkyne complexes 39−42 via the reduction of the corresponding Ta aryloxide or aryl imide chlorides. In some cases, these complexes go on to form Ta butadienediyl (43) or η6-arene (44) complexes when treated with additional equivalents of alkyne.

patterns of these Ta(DIPP) and Ta(DIPN) complexes will be addressed in more detail (vide infra). There are several examples from the Fryzuk group that demonstrate how Nb and Ta alkyne complexes can be formed via the displacement of a dinitrogen ligand from metal N2 complexes; this synthetic route is similar to the examples discussed above, in that the initial step involves the generation of a low-valent metal center via the use of an external reducing agent. For example, reduction of the niobium chloride [P2N2]NbCl (45) with KC8 in the presence of N2 afforded the dinuclear NbIV dinitrogen complex ([P2N2]Nb)2(μ-N2) (46; Scheme 11). The treatment of 46 with 2 equiv of phenylacetylene led to the extrusion of 1 equiv of N2 and the formation of 47, a mononuclear Nb alkyne complex. The magnetic and spectroscopic data indicate that complex 47 is also a

hydrogenation of the corresponding TaV trimethyl complex [PhP(CH2SiMe2NPh)2]TaMe3. Exposure of tetrahydride 48 to N2 gas under mild reaction conditions yielded the dinuclear TaV dihydride dinitrogen complex 49.71,72 The combination of 49 with various terminal alkynes led to the displacement of the N2 ligand and the formation of 50, a dinuclear Ta complex wherein the reduced alkyne unit is bridged between the two metal centers; 50 can also be accessed directly from 48, along with the loss of 1 equiv of H2.73 On the basis of the short Ta−C (∼2.05 Å) and long C−C (1.397(6) Å) bond distances, the 2041


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alkyne ligand in 50 is best described as a bis(μ-alkylidene) moiety, with each of the metal-bound carbon atoms engaged in a TaC double bond; this bis(alkylidene) formulation has also been proposed in the mononuclear complex [pyH][TaCl4(PhCCPh)(py)].74 Interestingly, when complex 50 was placed under reduced pressure, it went on to lose another 1 equiv of H2 to give the bis(μ-alkylidyne) complex 51. This stepwise reduction of the alkyne ligand from 50 to 51 and the resulting cleavage of the C−C triple bond are related to alkyne metathesis processes.75−78 In addition to the use of an external reducing agent, a second method for synthesizing Ta and Nb alkyne complexes involves a TaV or NbV precursor complex undergoing reductive elimination. For example, one of the earliest methods for the synthesis of group 5 alkyne complexes, initiated by Schwartz79−81 (52) and expanded upon by Herberich82,83 (53), proceeded via the thermally induced reductive elimination of H2 from the metallocene trihydride complexes Cp2MH3, in the presence of 1 equiv of dialkylacetylene (Scheme 13). Rothwell29 and

Scheme 14

Scheme 13 although the mechanism by which this would occur is unclear.86,87 Similar to the case for complexes 52−54, Schrock and co-workers reported88 the synthesis of a niobium alkyne chloride complex that is also believed to proceed via a reductive elimination step (Scheme 15). The hydrogenolysis of Scheme 15

co-workers have also reported the synthesis of a Ta alkyne complex via a similar H2 reductive elimination step: the addition of 3-hexyne to the Ta aryloxide dihydride complex Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2 resulted in the formation of Ta(OC6H3Ph2-2,6)2(3-hexyne)Cl(PMe3) (54; Scheme 13). In complexes 52−54, the addition of 1 equiv of alkyne led to the displacement of H2 from the metal center, rather than insertion into the M−H bond. However, further investigations by Herberich82 (and later Otero84,85) extended this work to include a variety of alkynes featuring both electron-donating and electron-withdrawing groups, and they found that the reactions of alkynes with group 5 metallocene trihydrides fall into two classes. Alkynes that featured electron-donating groups led to the displacement of H2, and the formation of metallocene alkyne hydride complexes (such as 52 and 53). In contrast, alkynes that contained electron-withdrawing substituents ultimately afforded the metallocene alkene hydride complexes (55 and 56; Scheme 14). Given the trans stereochemistry of the alkene moiety, the formation of these complexes was thought to occur via a stereoselective insertion step that results in a trans-alkenyl dihydride intermediate (57),

Cp*NbMe2Cl2 afforded the NbIII dimer [Cp*NbCl2]2 (59), likely via the loss of H2 from the putative NbV dihydride Cp*Nb(H)2Cl2 (58); treating complex 59 with 3-hexyne generated the mononuclear Nb alkyne complex Cp*Nb(3hexyne)Cl2 (60). In contrast to the Schwartz−Herberich or Rothwell systems (52−54), where the low-valent M III intermediate is generated in situ, complex 59 is stable enough to be isolated, similar to the case for complexes 28 (Otero59) and 33 (Wolczanski61). Tilley and co-workers reported89 the synthesis of a series of Ta analogues of 60 generated via a related reductive elimination route. Heating a benzene solution of Cp*Ta(TMS)Cl3 and the desired alkyne resulted in the formation of Cp*Ta(RCCR′)Cl2 (61) over the course of several days (eq 3). The reaction is believed to proceed via the reductive elimination of TMSCl, induced by alkyne coordination to the Ta center. 2042


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Scheme 17

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REACTIVITY OF TANTALUM AND NIOBIUM ALKYNE COMPLEXES Early investigations by Cotton and co-workers into the reactivity of the low-valent group 5 chlorides M2Cl6(THT)3 (M = Ta, 8; M = Nb, 9) with alkyne substrates revealed that, depending on the size of the substituents on the alkyne used, these complexes also functioned as effective polymerization and oligomerization catalysts.90 Whereas the addition of an excess of diphenylacetylene or di-tert-butylacetylene to solutions of 8 or 9 led to the formation of discrete metal alkyne complexes (10−12; eq 1), terminal alkynes (RCCH) and less bulky internal alkynes generated a mixture of cyclotrimers and poly(alkyne); the authors hypothesized that the formation of the cyclotrimer product proceeded via the insertion of an alkyne unit into the M−Calkyne bond of a M(RCCR′)Cl3 intermediate (similar to 15 or 16), as shown in Scheme 16.

Scheme 18

Scheme 16

This proposed mechanism is supported by the observation that Nb(RCCR′)Cl3(DME)33 (15) and Ta(RCCR′)Cl3(DME)91 (16) have also been shown to be effective catalysts for the cyclotrimerization of terminal alkynes. With regard to alkyne polymerization by a Ta alkyne complex, investigations by McConville and co-workers provide evidence for a possible mechanistic pathway.92 As shown in Scheme 17, treating the [BDPP]Ta(RCCR) acetylide complexes 62 with excess phenylacetylene afforded the metallacyclic product 64. The authors speculated that 64 was formed by the insertion of the coordinated alkyne unit into the Ta−CCPh bond (63), followed by the coordination and insertion of a phenylacetylene unit into the newly formed Ta−“alkynevinyl” bond. This hypothesis is similar to the wellknown Cossee−Arlman mechanism93,94 for the growth of polymer chains catalyzed by group 4 Ziegler−Natta type95−97 complexes. In addition to the simple group 5 alkyne halides studied by the Cotton group, the Ta aryloxide and arylimide chloride (35−38) or alkyne (39−42) complexes reported by Wigley and co-workers64−66,98,99 also engage in similar (although not catalytic) alkyne coupling and cyclotrimerization reactions, via the insertion of additional equivalents of alkyne into the Ta(RCCR) bond. Some representative examples of these

alkyne coupling reactions are shown in Scheme 18. For example, (DIPP)3Ta(PhCCPh) (39-Ph) reacted with 1 equiv of sterically small alkynes (such as 3-hexyne) to generate a Ta(butadienediyl) metallacyclic complex (65). In addition, the reduction of (DIPP)3TaCl2 (35), (DIPP)2TaCl3 (37), or (DIPN)TaCl3L2 (38), in the presence of excess amounts of certain alkynes, resulted in the formation of similar Ta(butadienediyl) complexes (66−68), a process which is believed to proceed by first forming the corresponding Ta alkyne complex (39, 40, or 42, respectively). Furthermore, some Ta(butadienediyl) complexes (69) underwent an additional insertion reaction with 1 equiv of alkyne or an organonitrile (RCN) to generate Ta(η6-arene) (70) or Ta(η2-pyridine) (71) complexes,100 as illustrated in Scheme 19. Hydrolysis of 70 or 71 afforded the organic C6R6 or NC5R5 2043


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Scheme 19

Scheme 21

product, making these Ta complexes potential reagents for the stoichiometric synthesis of substituted benzene and pyridine rings. In addition to alkynes and organonitriles, both Wigley67 (72) and Oshiki40 (73) have reported the synthesis of oxygencontaining tantallacycles similar to 65−68, formed via the insertion of ketones or aldehydes into the Ta(RCCR) bond (Scheme 20); Oshiki and co-workers40 have used this as means of stoichiometrically generating allylic alcohols. Scheme 20

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CONCLUSIONS AND FUTURE PROSPECTS For the majority of the synthetic applications reported for group 5 alkyne complexes, the bonding interaction between the metal and the alkyne ligand is best described by the high-valent “MV−alkenediyl” structural formalism (Figure 1). In this context, the alkyne ligand functions as an activated organic moiety that engages in further intra- or intermolecular reactivity with other small molecules; the formation of new C−C and C−N bonds in complexes 64−82 attests to the utility of this synthetic methodology. However, as was also discussed above, the ambivalent nature of the metal−alkyne interaction offers the possibility of envisioning the structure of these complexes in either the highvalent, MV, or low-valent, MIII, structural formalisms. Consequently, Nb and Ta alkyne complexes can function as potential MIII synthons, analogous to the group 4 “MII alkyne” complexes pioneered by Rosenthal and co-workers.3,15,20,21,106,107 Along these lines, recent examples from Cummins and coworkers49,50 illustrate how alkyne complexes can also be used as precursors for Nb or Ta diiodides. When they were treated with 1 equiv of iodine, the alkyne tris-amide complex 83 or 84 afforded the corresponding diiodide 85 or 86 and free alkyne (eq 4); this reaction can be envisioned as the formal oxidative addition of I2 to a MIII metal center to form the MV diiodide species. Complexes 83 and 84 were easily synthesized in high yield via a salt metathesis reaction between the lithium salt of the amide

The metallacyclic examples discussed above (65−73) were generated via the insertion of a substrate, such as an alkyne, nitrile, or ketone, into a metal−Calkyne bond. There is also an extensive body of related work that proceeds first via the insertion of a substrate, such as carbon monoxide,42,101−104 an organic isocyanide,51,55,101−105 or an alkyne51,83 into a M−Calkyl bond; the resulting iminoacyl, acyl, or vinyl moiety then undergoes a coupling reaction with the metal-bound alkyne ligand to afford a metallacyclic product. Similar transformations are also known to occur via insertion into M−H bonds as well. Scheme 21 depicts some representative examples of tantalacycle formation achieved via the coupling of a Ta alkyne ligand with various metal-bound functionalities, as reported by Curtis and Real105 (77), Etienne and Templeton42 (78), and some of our own work (80−82).51 2044


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ligand and the related alkyne trichloride reagents discussed earlier in this review, Nb(PhCCPh)Cl3(THF)2 (15, R = R′ = Ph, 2THF instead of DME39) and Ta(EtCCEt)Cl3(DME) (16, R = R′ = Et32,40). This route for accessing these synthetically versatile Nb or Ta halide functionalities is particularly attractive, since it obviates the use of more Lewis acidic starting materials such as NbCl5 and TaCl5, which are known to occasionally engage in unwanted side reactions with ancillary ligands.108−110 It is evident from the examples presented in this review that niobium and tantalum alkyne complexes are versatile reagents, with many potential synthetic applications. However, aside from the synthetic routes outlined in this section, and the few examples cited in Scheme 5 (complexes 17−21), Nb and Ta alkyne complexes such as M(RCCR)Cl3Ln (15, 16) remain underutilized as reagents in the organometallic and coordination chemistry literature. Given that these reagents are easy to prepare and support a variety of alkyne substrates, they should provide a convenient entry point for the installation of the synthetically versatile Nb or Ta alkyne moiety into a wide variety of anionic ligand sets. In addition, these alkyne compounds have been shown to serve as a convenient source for NbIII and TaIII synthons, which will be useful in future endeavors.

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Michael Fryzuk was born in Sarnia, Ontario, Canada, and received his B.Sc. and Ph.D. degrees from the University of Toronto. After a postdoctoral stint at Caltech, he moved to the University of British Columbia, where he is presently Professor and Head of the Chemistry department. His interests are in ligand design and small-molecule activation, particularly molecular nitrogen.

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ACKNOWLEDGMENTS M.D.F. thanks the NSERC of Canada for a Discovery Grant, and K.D.J.P. thanks the NSERC for Postgraduate Scholarships.

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DEDICATION Dedicated to the memory of Professor Michael F. Lappert. REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*M.D.F.: e-mail, [email protected]; fax, +1 604 822-8710; tel, +1 604 822-2471. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest. Biographies

Kyle Parker was born in Richmond Hill, Canada, and received his B.Sc. in chemistry from McMaster University. He competed his Ph.D. work under the supervision of Professor Fryzuk at the University of British Columbia in 2014, where he was an NSERC doctoral scholar. His research interests include tantalum alkyne complexes, early metal hydrides, and small-molecule activation. 2045


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Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne

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