Review pubs.acs.org/Organometallics
Copper(I) Alkyne and Alkynide Complexes Heinrich Lang,* Alexander Jakob, and Bianca Milde Department of Inorganic Chemistry, Institute of Chemistry, Faculty of Natural Sciences, Chemnitz University of Technology, Straße der Nationen 62, 09111 Chemnitz, Germany ABSTRACT: The synthesis, structure, bonding motifs, reaction chemistry, and some potential applications of copper(I) alkyne and alkynide compounds as well as current trends in this field of chemistry are reported.
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two main parts. In the first section the synthesis, characterization and bonding of copper(I) alkynes and alkynides will be reported, while the second section discusses some potential applications of this family of compounds. This allows easy access for synthetic chemists into the area of copper(I) alkyne and alkynide chemistry but at the same time is also of interest for physicists and engineers. Please note that homoleptic alkynide complexes of copper(I) ranging from linear [Cu(C CR)2]− and trigonal-planar [Cu(CCR)3]2− anionic compounds to polymeric species [CuCCR]n will not be discussed in detail here, having been reviewed in 2011.29
INTRODUCTION The first organocopper(I) compound, cuprous ethyne-1,2-diyl [Cu2C2], was synthesized by Böttger as early as 1859 by passing “illuminating gas” (=acetylene gas) through an ammoniacal solution of [CuCl].1 The red solid thus obtained appeared to be highly explosive. Of somewhat greater thermal stability are alkynide copper(I) species of the type [CuCCR]n (R = singly bonded organic group, H).2,3 (Caution! Nevertheless, [CuCCR]n systems are still rather hazardous, especially when dry. For this reason, handling of these species must be done with extreme care, with small amounts of material, and never strike the dry material.) Following these pioneering works, a plethora of copper(I) alkynide and alkyne compounds appeared in the literature, mostly describing the synthesis, characterization, structure and bonding, and some applications, including catalytic transformations of alkynes, click chemistry (the azide−alkyne Huisgen cycloaddition reaction), gas-phase deposition processes, transmetalations, etc.4−9 The copper(I) alkyne and alkynide chemistry continued to excite researchers: i.e., monomeric or high-nuclearity copper(I) alkynide complexes possess excellent photoluminescent, nonlinear optic, and molecular conductive properties.10−13 In addition, copper(I) alkynides have attracted much attention because the alkynyl ligands provide a variety of transition-metal and main-groupelement bridging modes, allowing the preparation of oligo- and polynuclear complexes or clusters of diverse structures that is, for example, attributed to the high degree of electron delocalization by the alkynide group(s).14−19 Likewise, copper acetylides play an important role in the synthesis of wirelike polyynediyl units that bridge redox-active terminal groups.20−23 Very recently, copper(I) building blocks featuring π-bonded alkyne ligands were applied as gas-phase precursors (CVD) in the deposition of copper thin films and patterns for electronic devices.24−28 In summary, this contribution primarily focuses on the literature published after the year 2000 and is organized into © 2012 American Chemical Society
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SYNTHESIS AND CHARACTERIZATION OF COPPER(I) ALKYNIDES AND ALKYNES Copper(I) Alkynides. Though copper(I) acetylide [Cu2C2] (1) has been known since 1859 (vide supra), it is still of topical interest. In 2012 Matyás ̌ and co-workers reported about the sensitivity to friction of a selection of primary explosives, including 1.30 On the basis of the synthetic methodology, the shape and size of the copper(I) acetylide crystals, and the appropriate sensitivity measurements using a small BAM friction apparatus (BAM = aluminum magnesium boride, AlMgB14) it can be concluded that copper(I) acetylide is the most stable compound, as derived from the appropriate sensitivity curves within the series of tested substances (for example, lead azide and styphnate, tetrazene, dinol, triacetone, triperoxide, hexamethylenetriperoxide diamine, mercury fulminate, and heavy-metal acetylides).30 The respective sensitivity curves represent a unique set of data which is important, for example, for determining manipulation safety for primary explosives. However, please, note that these systems are still highly shock sensitive materials. Special Issue: Copper Organometallic Chemistry Received: July 6, 2012 Published: September 28, 2012 7661
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Scheme 1. Synthesis of 3a,32
a
Legend: (i) 1 equiv of nBuLi and 0.5 equiv of HCCSiMe3, thf, 0 °C; (ii) 0.45 equiv of NaCCH (18 wt % slurry in xylene oil), thf, 278 °C.
Scheme 2. Synthesis of 433
An oscillating acetylide C22− building block together with a flipping dppm ligand was discussed in rectangular [Cu4(μdppm)4(μ4-η1:η2-CC)]2+ (2; dppm = diphenylphosphinomethane), applying 31P and 1H NMR spectroscopy, respectively. It was found that the two independent fluxional processes possess similar activation energies. These findings were supported by DFT (B3LYP) calculations (DFT = density functional theory).31 The acetylide C22− unit can also act as a six-electron donor connector between copper centers in butterfly-shaped tetranuclear [Cu4(μ-PPh2pypz)4(μ4-η1:η2-C C)][ClO4]2 (3; PPh2pypz = 2-(diphenylphosphino-6-pyrazol1-yl)pyridine), which is accessible by the two synthetic procedures shown in Scheme 1.32 This molecule shows a blue-green luminescence (408, 430, 448 sh, and 475 sh nm). Ternary metal carbides of the general type M′[CuC2] (M′ = alkaline metal) were recently reviewed.29 These compounds can be found along the transition from ionic to metallic materials. The general synthetic methodology for these carbides is based on a high-temperature combination of the elements or the reaction of metal oxides with carbon. An access to monomeric and dimeric diorganocuprates using size-controlled bulky “Rind” (=EMind, Eind; MPhind, MsFluind) ligands is given in Scheme 2. The complex [(MPind)CuCCSiMe3][Li(thf)2] (4) is, as mentioned by the authors, the first isolable monomeric mixed diorganocuprate.33 Copper(I) alkynides [CuCCR((μ3-NH)3Ti3(η5-C5Me5)3(μ3-N))] (5) can be synthesized by treatment of [CuCl((μ3NH)3Ti3(η5-C5Me5)3(μ3-N))] with LiCCR (R = SiMe3, Ph).34
The phenyl derivative can also be prepared by the reaction of [(Ti(η5-C5Me5)(μ-NH))3(μ3-N)] with [CuCCPh]n. However, copper(I) alkynides 5 tend to decompose in solution to afford the copper(I) acetylide bridged double cubes 6 (for example, the Ph species is shown in Figure 1). Homoleptic [CuCCR]n (R = nPr,35,36 nBu,37 tBu,35,38 Ph,35,39 C6H4-4-Me, C10H7, CMe2(OMe),40 CMeCH2,41 SiMe3,42 CC[Re],43 CCCCCC[Re];43 [Re] = Re(η5C5Me5)(NO)(PPh3)) species have been synthesized (applying different synthetic methodologies, including transmetalation, metathesis, etc.) and structurally characterized (R = nPr, tBu, Ph).35,43−46 These copper alkynides were used as precursors for, e.g., alkynyl coupling and alkynyl transfer reactions as well as for the synthesis of metal-capped one-dimensional carbon allotropes. The d10 copper acetylide [CuCCtBu] crystallized as a benzene adduct, {[CuCCtBu]20·C6H6} (7), and its structure in the solid state reveals a Cu20 catenane moiety with different binding modes of the CCtBu ligands, as depicted in Figure 2.35 The solid-state structure of this cluster can be viewed as an interlocking construction of a somewhat distorted Cu8 building block with two puckered hexagonal Cu6 rings (Figure 2, bottom). Within these rings the CCtBu groups possess different coordination modes (μ,η1,1; μ-η1,2, ...) with copper−copper contacts between 2.498 and 3.482(1) Å.35 Nevertheless, the copper acetylide [CuCCnPr]n possesses a sheetlike polymeric structure consisting of discrete zigzag Cu4 subunits which are bridged by CuCCnPr connectors (coordination modes μ-η1,2, μ3-η1,1,2) (Figure 3).35 The Cu−Cu distances are 2.61(2) and 2.24(3) Å. In contrast, changing from 7662
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by reacting the respective copper alkynides [CuCCR]n with R′CC− or R′− reagents as reported in refs 36−41 and 49. A molecule in which [CuR′2]− (R′ = C6H4-2-CH2N(Me)CH2CH2NMe2)) and [Cu(CCR)2]− (R = C6H4-4-Me, C6H4-4-SiMe3) homocuprate moieties are present in a neutral mixed cuprate of type [Cu2Li2(CCR)2R′2] was recently described by van Koten and co-workers.50 This species is the first example containing two different homocuprates in one assembled structure. The molecular structure of this molecule was determined by single-crystal X-ray structure analysis, indicating that each part possesses a two-coordinated copper(I) center with a linear geometry, which are connected together via Li−Cipso contacts to two tetrahedral surrounded lithium ions. The coordination number 4 at Li+ is set up by the chelated o-diamine substituent (vide supra). In summary, cuprates [Cu2Li2(CCR)2R′2] are examples of ion pairing between two mixed homocuprate anions and two Li cations.50 Lithium dialkynyl cuprates [Cu(CCR)2]Li (R = Ph, nPr, tBu, SiMe3) undergo conjugate addition to activated 3-formylchromone to produce a series of 2-alkynylchroman-4-ones, which exhibit a marked instability to acid and, for example, partially isomerized to the respective 2-hydroxychroman-4-ones, while prolonged contact with silica gel effected total conversion.51 It was further observed that alkynyl transfer to chromones exhibits steric dependency. The 1,4-addition of [Cu(CCR)2]Li to 3cyanochromone proceeds anomalously to afford the appropriate eneynonitriles and 6H-bis[1]benzopyrano[2,3-b:3′, 4′-e]pyridine, respectively.51 Copper alkynides have, as stated earlier,35,36,39 oligomeric or polymeric structures. These aggregates can be broken down to smaller or even mononuclear species upon addition of Lewis bases such as phosphines, carbenes, etc. Examples are [CuC CnPr(P(NMe2)3)2]52 and [CuCCPh(PnBu3)3],53 respectively, of which the latter species is a reversible CO2 carrier. The copper phenylacetylide [CuCCPh(PPh3)] was recently applied to the formation of carbon chains on a series of ruthenium and osmium carbonyl clusters.54 As an example, the reaction of [CuCCPh(PPh3)] with [Ru3(μ-H)3(μ3CBr)(CO)9] afforded the cluster [Ru3(μ-H)3(μ3-CCCPh)(CO)9]. Trinuclear clusters featuring a [MCCPh] unit (M = Cu, Ag, Au) are chiral heterometallatetrahedranes of the type [Re2(AuPPh3)(MPPh3(μ-P(C6H11)2)(CO)7CCPh] (8).55 The respective copper alkynides could be prepared by consecutive synthetic methodologies, including the addition of LiCCPh to [Re2(AuPPh3)2(μ-P(C6H11)2)(CO)7Cl] to afford [Re2(AuPPh3)2(μ-P(C6H11)2)(CO)7CCPh], which undergoes a metathesis reaction in the presence of [CuCl(PPh3)], giving upon concomitant precipitation of [AgCl(PPh3] the title complex 8. The molecular structure of this cluster in the solid state was determined by single-crystal X-ray structure analysis, as shown in Figure 4.55 Other phosphine-stabilized copper phenylacetylide complexes are mononuclear [CuCCPh(C 5 H 3 N-2,6(CH2PtBu)2)] (9) and dimeric [Cu(μ-CCPh)(2-CH2PtBu6-Me-C5H3N)]2 (10), of which the latter species was characterized by a single-crystal X-ray structure determination.56 The geometry around copper in 10 is distorted tetrahedral. The bridging of the CCPh units induces a short Cu−Cu contact of 2.4417(4) Å. The copper(I) complexes with their cooperative diphosphinopyridine building blocks could be successfully applied in Cu(I) “click” catalysis, the [2 + 3] polar cycloaddition of azides and acetylenes. For example, the click reaction of phenylacetylene with benzyl azide
Figure 1. Simplified structure of [{Cu(μ-1κC1:2κC1-CCPh)Cu(μ4N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (6) in the solid state. The pentamethylcyclopentadienyl ligands, phenyl groups, hydrogen atoms, and C7H8 solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cu(1)−N = 2.120(3)−2.193(3), Cu(1)−C(4) = 1.945(4), Cu(1)···Cu(4) = 2.493(2), Cu(2)−C(1) = 1.959(5), Cu(2)−N = 2.128(4)−2.182(3), Cu(2)···Cu(3) = 2.513(2), Cu(3)−C(1) = 1.937(4), Cu(3)−N(3) = 1.858(3), Cu(4)−C(4) = 1.942(4), Cu(4)−N(8) = 1.865(3), C(1)−C(2) = 1.200(6), C(4)− C(5) = 1.224(6); N−Cu(1)−N = 87.8(1)−89.5(1), C(4)−Cu(1) −N = 102.5(2)−150.2(2), C(4)−Cu(1)−Cu(4) = 50.1(1), N−Cu(2)− N = 87.6(1)−88.6(1), C(1)−Cu(2)−N = 98.6(2)−136.7(2), C(1) −Cu(2)−Cu(3) = 49.5(1), C(1)−Cu(3)−Cu(2) = 50.2(1), C(1) −Cu(3)−N(3) = 150.8(2), N(3)−Cu(3)−Cu(2) = 158.9(1), C(4) −Cu(4)−Cu(1) = 50.2(1), C(4)−Cu(4)−N(8) = 149.1(2), N(8) −Cu(4)−Cu(1) = 160.3(1), Cu(2)−C(1)−Cu(3) = 80.4(2), Cu(2) −C(1)−C(2) = 146.2(4), Cu(3)−C(1)−C(2) = 132.3(4), C(1)− C(2)−Si(1)/C(3) = 175.4(6), Cu(1)−C(4)−Cu(4) = 79.8(2), Cu(1)−C(4)−C(5) = 149.5(4), Cu(4)−C(4)−C(5) = 130.7(4), C(4)−C(5)−Si(2)/C(6) = 175.8(5).34
an alkyl ligand to an aryl group as given in [CuCCPh]n produces a chain structure with extended copper−copper ladders with typical Cu−Cu separations of 2.49(4)−2.83(2) Å, as depicted in Figure 3 (right). Within this structure a crystallographic 21 screw axis is oriented parallel to the [CuCCPh]n polymeric chain direction, which bisects copper−copper distances in such a way that the phenylacetylides are positioned alternately up and down along the chain.35 Tetra- and pentavalent uranium acetylides can be prepared by the oxidation of [U(η5-C5Me5)2(NPh2)(thf)] and [U(η5C5Me5)2(N-2,6-iPr2-C6H3)(thf)] with the copper acetylide [CuCCPh]n. The respective [U(η5-C5Me5)2(NPh2)(C CPh)] and [U(η5-C5Me5)2(N-2,6-iPr2-C6H3)(CCPh)] complexes featuring UIV or UV ions have been characterized by NMR and UV−vis−near-IR spectroscopy, cyclic voltammetry, and single-crystal solid-state X-ray structure determination.47 The copper acetylide [CuCCnPr]n could successfully be applied to a copper(I)-mediated 1,2-metalate rearrangement to synthesize the phospholipase A2 inhibitor (+)-(4R)-manoalide, in which all 25 carbon atoms of the sesterterpenoid skeleton are constructed from 3-furaldehyde, trimethylalane, oxirane, CO, β-ionone, and propargyl bromide.48 On the basis of the copper alkynides [CuCCR]n, organocuprates ([Cu(CCR)2]−; R = Ph, nBu, ...) and ([Cu(CCR)(R′)]−) (R = see above; R′ = nBu, SnMe3, CHCHSnBu3, cC6H7-3-Me-6-CH2OTBDMS, ...) are accessible 7663
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Figure 2. Molecular structure of {[CuCCtBu]20·C6H6} (7) (top) and schematic drawings showing the cluster assembly of three interlocking [CuCCtBu] rings (bottom).35
produces in the presence of 1 mol % of 9 the appropriate fivemembered heterocyclic N-benzyl-4-phenyl-1,2,3-triazole in a yield of 40% (for further examples of azide−alkyne cycloaddition reactions see below and Applications).56 A further dinuclear copper alkynide is [Cu(μ-CCPh)(triphos)]2 (11; triphos = MeC(CH2PPh2)3) (Figure 5), which was synthesized by Bruce and co-workers by the addition of [CuCCPh]n to triphos.57 The molecular structure of 11 shows that the two copper(I) ions are unsymmetrically bridged by the two C CPh ligands, each attached through one C only with a Cu−C distance of 2.016(4) Å. The Cu−Cu distance is 2.4663(8) Å.
The triphos ligand binds only via two of the three CH2PPh2 arms to the copper atom with typical Cu−P bond distances of 2.281 and 2.273(1) Å, respectively.57 The luminescence properties of 11 were studied, where a discrepancy was found between the observed and calculated (assumed tetrahedral structure) energies.58 It is likely that the triplet state involves the second copper and/or alkynide center.57 Theoretical analysis of bonding and stereochemical trends in terminal and doubly alkynide bridged Cu(I)−Cu(I) dimers using DFT/B3LYP have been carried out, indicating that the acetylides in addition to being σ donors are competitive π 7664
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Figure 3. Molecular structures of [CuCCnPr]n (left) and [CuCCPh]n (right).35 Reprinted with permission. Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
donors and acceptors.59 It is most important to mention the σ donor capabilities of the alkynide ligands, which permit a short Cu−Cu separation. As the alkynides reorient, the Cu−Csp bonds weaken or strengthen alternately, although a major participation of the alkynide π system is not implied. Furthermore, it was found that distortion of the [Cu2C2] (1) assembly helps to reduce in part the inner electron repulsion.59 In addition to phosphines or phosphites, also unsaturated Nheterocyclic carbenes (=NHC)60 or saturated NHC ligands61 can be used as auxiliary ligands to prepare copper(I) alkynides. These species are of interest, since NHC copper(I) halides have been reported to be highly active catalysts for 1,2,3-triazole “click” reactions.62 The synthesis of unsaturated [CuC CPh(NHC)] (13) is possible by the reaction of [CuOAc(NHC)] (12) with LiCCPh (Scheme 3). Addition of azidodi-4-tolylmethane to this complex gave the appropriate mononuclear copper triazolide complex 14, which precipitates (Scheme 3). It was found that copper acetate 12 and copper triazolide 14 are excellent catalysts in click chemistry (vide supra, vide infra).61 [Cu(IPr)(OH)] (15; IPr = 1,3-bis(diisopropylphenyl)imidazole2-ylidene) is a versatile cuprous synthon, since it exhibits the capacity to activate diverse X−H bonds (X = C, N, P, O, S, Mo). This reactivity also includes the cleavage of Si−N and Si−C bonds. From the plethora of new unprecedented copper(I) complexes the copper(I) alkynide [1,3,5-((IPr)CuCC)3C6H3] (16) is given as an example. This species could be prepared by treatment of 15 with 1,3,5-(CCH)3C6H3.63 The monoalkynide complex [CuCCPh(IPr)] (17) is accessible by a straightforward synthesis methodology by the reaction of the copper gallyl [Cu(IPr)(Ga(DAB))] (DAB = (N(2,6-iPr2C6H3)CH)2) with
Figure 4. Molecular structure of 8. Selected bond lengths (Å) and angles (deg): Au1−Re1 = 2.874(1), Au1−Re2 = 2.841(1), Cu1−Re1 = 2.928(2), Cu1−Re2 = 2.856(2), Cu1−Au1 = 2.644(2), Re1−Re2 = 3.263(1); P1−Au1−Re1 = 143.6(1), P1−Au1−Re2 = 137.4(1), P2− Cu1−Re1 = 125.1(1), P2−Cu1−Re2 = 158.2(1).55
Figure 5. Molecule 11.57
Scheme 3. Synthesis of 13 and 1461
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phenylacetylene.64 The solid-state structure of 17 showed that it is a rare example of a monomeric copper(I) alkynide with a somewhat distorted linear geometry (C−Cu−C = 172.74(16)°) at copper(I).64 A further possibility to monomeric 17 (and a series of [CuX(IPr)] species; X = anilido, phenoxide, ethoxide, N-pyrrolyl) is given by the reaction of [CuR(IPr)] (R = Me, Et) with HCCPh. Upon addition of HCl to 17 complex [CuCl(IPr)] along with HCCPh was produced.65 Heterobimetallic copper(I)-based alkynyls and 1,3-diynyls can be prepared by several established synthetic procedures.66 For example, sequential reaction of [(P(4-Me-C6H4)3)AuC CCCH] with nBuLi and [CuCl(triphos)] gave [(triphos)CuCCCCAu(P(4-Me-C6H4)3)] (18), in which a gold(I) and copper(I) ion are connected via an all-carbon C4 chain.66 Such molecules have found use as intermediates in the preparation of longer chain systems by elimination of phosphine gold(I) halides.67 In 2011 Bruce and co-workers reported the synthesis of the heterobimetallic Cu−Ru complex [(Ph3P)CuCCCCRu(PPh3)2(η5-C5H5)] (19).68 The synthesis and electronic and photophysical properties of a series of trinuclear copper(I) acetylides of general composition [Cu3(μ-dppm)3(CCR)2] (20; dppm = diphenylphosphinomethane) carrying diverse alkynyl ligands with different electronic structures were recently described by several authors (Figure 6 and Table 1).69−75
Cu3P6 core is almost planar, with Cu2P2C rings adopting an envelope-like conformation with the C atom on the flap. The [Cu3(CCR)2] building block resembles a “five-center fourelectron” system with only minor contribution from the CC unit. The following bond distances are characteristic: Cu−Cu = 2.5−2.9, Cu−P = 2.2−2.3, and Cu−C = 2.0−2.4 Å (Table 1). The observation of copper−copper distances shorter than the sum of the van der Waals radii for Cu points to some weak metal−metal contacts, although short copper−copper interactions are also characteristic and are found in electrondeficient organocopper species.76,77 The trinuclear [Cu3(μ-dppm)3(CCR)2] clusters 20 possess high quantum yields of photoluminescence, and the energy and lifetime of the triplet state emission can be tuned to a large extent by the nature of the alkynide groups. These properties make these complexes suitable candidates for various applications: i.e., they can be used in optoelectronic devices and in time-resolved and gated luminescence detection (Table 1). The excited state is a metal-to-ligand charge transfer state. A complex featuring ferrocenyl termini, [Cu3(μ-dppm)3(C CFc)2] (Fc = Fe(η5-C5H4)(η5-C5H5)), exhibits reversible Fc oxidations (E1/2 = 114 ± 10 mV (first Fc oxidation), E1/2 = 224 ± 10 mV (second Fc oxidation), ΔEp = 60 mV, ΔE1/2 = 110 ± 10 mV, Kc = 77 ± 30).78 The weak IVCT band at 1250 nm resulting from the mixed-valence species was detected by UV− vis−near-IR spectroscopy. The stability of the mixed-valence compound arises from the reduction of electrostatic repulsion and statistical distribution.78 The molecule containing the benzo-15-crown-5 units (Table 1) allows sodium ion binding, which was studied in detail by electrospray ionization-mass spectrometry and emission spectroscopy, proving that this complex can be applied as a luminescence chemosensor for Na+ ion binding.79 Furthermore, molecular rods featuring the trinuclear Cu3(μdppm)3 building block as a terminal or linking unit in heterometallic alkynyl-based complexes are known.80,81 In 2002 Bruce and co-workers reported the synthesis and characterization of [Cu3(μ-dppm)3(μ3-I)(μ3-η1-CCC CAuCCCCH] (21) and [(Cu3(μ-dppm)3(μ3-I))2(μ3-η1CCCCAuCCCC][I] (22), respectively.80 Molecule 22 was prepared by the reaction of [Cu3(μ-dppm)3(μ3-I)2][I] with [Au(CCCCH) 2 ][ppn] (ppn = μ-nitrido-bis(triphenylphosphane)) in tetrahydrofuran at ambient temperature. The structure of the triangulo tricopper cluster 21 in the solid state was determined by single-crystal X-ray structure analysis, showing that this compound is closely related to many other species featuring this tricopper cluster unit attached to an η1-alkynide group (vide supra). The molecular solid-state structure of 21 also confirmed the C4AuC4H rod character of the respective organometallic species. The use of 1,3-diyne gold(I) systems also allowed the synthesis of a series of heteronuclear diyndiyls consisting organometallic and/or metal−organic building blocks, such as W(η5-C5H5)(CO)3, Au(PPh3), and Pt(dppe), respectively.80 The synthesis and luminescence behavior of mixed-metal rhenium(I)−copper(I) alkynide complexes of general composition [Cu 3(μ-LL) 3 (μ3-η 1-CCC 6H 2R 2-CC-4-Re(NN)(CO)3)2][X]2 (23) and [Cu3(μ-LL)3(μ3-η1-CCCCRe(NN)(CO)3)2][X]2 (24; LL = dppm, dppa, nPrPNP; NN = bpy, Me2bpy, tBu2bpy; R = H, Me; X = BF4, PF6) were reported in 2003 by Yam et al. (Scheme 4).81 All of these compounds display rich photoluminescence. Their low-energy emission was assigned to result from 3MLCT [dπ(Re) → π*(NN)] states.
Figure 6. Cluster [Cu3(μ-dppm)3(CCR)2] (20).69−75
Table 1. Synthesis of Clusters [Cu3(μ-dppm)3(CCR)2] (20)
Polynuclear complexes 20 are composed of trimetallic Cu3 cores which are bridged by three dppm ligands and additionally capped with two alkynide building blocks in a μ3-η1 fashion, as could be shown by single-crystal X-ray structure analysis. The 7666
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Scheme 4. Synthesis of 23a,81
a
M = Cu, Ag; P∧P = dppm, nPrPNP, dppa; N∩N = bpy, tBu2bpy, Me2bpy; R = H, Me; n = 0, 1.
Scheme 5. Synthesis of Cu(I)−Ag(I) Hexa-, Octa-, and Hexadecanuclear Alkynide Clusters 26−2883
η1-CCCCR′)(PR3)]4 (25; R = Ph, C6H4-4-Me, C6H4-4-F; R′ = Ph, C6H4-4-Me, C6H4-4-OMe).82 A crystal structure determination showed that 25 adopts a distorted-cuboidal geometry with the diynyl groups in an asymmetric μ3-η1 bridging fashion. The copper−copper distances are 2.5320(9)− 2.6943(13) Å and hence point to some irregular distortion within the Cu4 core. In addition, clusters 25 indicate some weak copper−copper interactions; however, such short distances are also commonly observed in electron-deficient organocopper complexes (vide supra).76,77 Spectroscopic (Raman, NMR, UV−vis) and photophysical (emission spectroscopy) properties of these compounds were determined. The origin of the low-energy emission was assigned as derived
The authors showed that the spectroscopic and emission properties of the rod-structured Re(I)−Cu(I) complexes 23 and 24 can be tuned by the variation of the diimine and the alkynide ligands, respectively. The solid-state structure of [Cu 3 (μ-dppm) 3 (μ 3 -η 1 -CCCCRe(Me 2 bpy)(CO) 3 ) 2 ][PF6]2 was determined. This complex consists of an isosceles triangle of copper atoms with a essentially linear (177.7(11)− 178.7(11)°) Re(Me2bpy)(CO)3C4 metallo moiety capping the Cu3 core in a μ3-η1 fashion. The bonding parameters are as expected for this type of molecule (vide supra).81 Very recently a series of copper(I) alkynide clusters featuring Cun cores (n = 4, 6, 8, 10−12, 14, 18, 20, and 26) was described.82−88 The smallest cluster within this series is [Cu(μ37667
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Scheme 6. Synthesis of Rhombic Dodecahedral Ag8M6 Cages 29 (M = Cu, Ag, Au) and 1,2,5-Azadiphospholium 3089
from a copper-centered 3d94s1 state, mixed with LMCT [diynyl → Cu4] as well as IL [π−π*(diynyl)] states.82 Likewise, a series of Cu(I)−Ag(I) hexa- (26), octa- (27), and hexadecanuclear (28) alkynide compounds have been prepared as depicted in Scheme 5. These clusters were synthesized by the reaction of polymeric [MCCC6H4-4-R]n (M = Cu, Ag; R = H, Me, OMe, NO2, C(O)Me) with [M′(μ-PPh2XPPh2)2(MeCN)2]2[ClO4]2 (M′ = Cu, Ag; X = NH, CH2).83 Depending on X and R, respectively, the clusters shown in Scheme 5 have been accessible. Hexanuclear 26 possesses a bicapped cubic core of composition Cu2Ag4C4, while octanuclear 27 exhibits either a waterwheellike structure set up by two Ag3Cu(CCPh)3 building blocks, which are connected by three PPh2CH2PPh2 bridges, or forms a dimer (28) containing waterwheel-structured Ag6Cu2(C CC6H4-4-C(O)Me)6 units through the formation of a Ag and a C(O)Me oxygen linkage. All clusters show an intense luminescence both in the solid state and in fluid solution; the emission is of phosphorescent nature. The emission state is derived from a 3LMCT (CCC6H4-4-R → Ag4Cu2/Ag6Cu2) transition, which is mixed with a metal cluster centered (d → s) excited state. Furthermore, it was found that the lowest lying excited state in clusters featuring electron-deficient CCC6H44-NO 2 or CCC 6 H 4 -4-C(O)Me units is most likely dominated by an intraligand 3[π − π*] character.83 In 2006 the synthesis of rhombic dodecahedral Ag8M6 (M = Cu, Ag, Au) cages (29) featuring ferrocenylethynyl units upon addition of [AgCCFc]n to [M2(Ph2PNHPPh2)2(MeCN)2]2+, as illustrated in Scheme 6, was reported.89 Within this reaction also the 1,2,5-azadiphospholium species [FcCCH(PPh2NPPh2)][BF4] (30) was formed by cyclic addition of FcCC to PPh2NHPPh2. Electrochemical studies indicated that intramolecular electronic communication between the Fc moieties is mediated by FcCCMCCFc (M = Cu, Ag) in the respective Ag8Cu6 core.89 Copper(I) alkynide clusters of the type [Cux+y(hfac)x(C CR)y] (x = 6, 8, 10; y = 4, 8) with Cu10−Cu18 cores have also been prepared.86 They were obtained from the reaction of cuprous oxide, hexafluoroacetylacetone, and the respective alkynes. The isolation of these species depends mainly on the differences between the nature of the alkyne applied and the solvents used, which is discussed in detail in ref 86. The solidstate structures of [Cu10(hfac)6(CCtBu)4(Et2O)], [Cu10(hfac)6(CC t Bu) 4 (Et 2 O)]/[Cu 10 (hfac) 6 (CC t Bu) 3 (CC n Pr)(Et2O)], and [Cu12(hfac)8(CCR)4] (R = nPr (with six thf
molecules), tBu, SiMe3) were reported. In 2002 two articles concerning the synthesis of a Cu18 and even Cu26 cluster were published by Higgs.87,88 The appropriate species [Cu18(hfac)10(CCnBu)8] (31) and [Cu26(hfac)12(CCR)14] (32) (R = n Bu, nC5H11, nC6H13) have broad structural similarities to other members of this family of compounds, possessing a Cu6/Cu6/Cu12 triangular double-layered copper structure assembled about interplanar linear Cu(CCR)2 units.87 The solid-state structure and the bonding parameters of the 32·nC5H11 derivative is shown as an example in Figure 7.
Figure 7. Molecular structure of 32·nC5H11 in the solid state. For clarity the Cu···Cu bonding, hydrogen and fluorine atoms, and the n C5H11 groups are omitted.87 For selected bond distances and bond angles see ref 87.
For example, the emission spectrum of 32·nC5H11 in nhexane possesses two fluorescent emissions, a high-intensity emission at 344 nm and a low-intensity vibronically structured emission at 366, 382, and 399 (sh) nm. In n-hexane solvent glass the excitation at 280 nm results in two emission bands, one at 350 nm and the other one at lower energy possessing a vibronic fine structure exhibiting maxima at 406, 419, 425, 433, and 448 nm and shoulders at 464 and 482 nm. Due to the fact that the latter event is absent at 293 K, it can be assigned to a phosphorescent emission, as confirmed by a lifetime of 170 ms.87 Two further high-nuclearity copper(I) alkynide clusters, 7668
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namely [Cu16(hfac)8(CCtBu)8] (33) and [Cu20(hfac)8(C CCH2Ph)12] (34), respectively, were prepared, which contain complicated CCtBu-bridged Cu(I) cores capped by peripheral hfac building blocks.90 In this report also the photoluminescent properties of 33 and 34 together with [Cu18(hfac)10(CCnBu)8] (31), [Cu26(hfac)11(CCnPr)15] (35), and monomeric [Cu(hfac)L] (36) (L = Me3SiC CSiMe3, HCCCH2OMe, cod) (for more information see below) was discussed, showing that the luminescence is derived from transitions within the delocalized Cu−hfac chelate ring.90 For the formation of the different cluster networks (under similar reaction conditions) the authors suggest that most likely the difference in the nature of the alkynes may be responsible. Increasing the alkynide to hfac ratio leads to larger clusters, as could be shown with straightchain alkynes, i.e. 31 cf. 35, while a bulkier alkynide (for example, CCtBu (33)) inhibits its ability to function as a bridging unit in the cluster core.90 The reaction of [M2(dppm)2(MeCN)2][X]2 (M = Cu, X = BF4; M = Ag, X = ClO4) with [Ru(CCCCM)(dppe)(η5C5Me5)] (M = Cu, Ag) gave the cationic mixed-metal diynyl clusters [M 6 (μ 3 -CCCC(Ru(dppe)(η 5 -C 5 Me 5 )) 4 (μdppm)2][X]2 (37) (Figure 8).84 Electrochemical studies
Copper(I) Alkynes. In this part the synthesis, characterization, properties, and some applications of a variety of alkyne copper(I) compounds are discussed. Quantum chemical calculations on the DFT level have been carried out in alkyne copper (as well as silver and gold) chemistry on complexes of the general type [M(η2-C2Hx)] (M = Cu, Ag, Au; x = 2, 4).92 Metal−ligand bonds of these species show more electronic than covalent character (40.6−44.2%). The covalent bonding results from the metal ← ligand σ donation (55.7−69.2%). The contribution of the metal → ligand π in-plane back-donation is much smaller (11%).92 In addition, the nature of the chemical bonding in 1:1 complexes formed between copper and HCCH have been investigated at the ROB3LYP/6-311+G(2d) level by the ELF topological approach, whereby the bonding is ruled by the nature of the organic ligand. Within the copper alkyne complex the cyclic C2v structure imposed by symmetry possesses two covalent M−C bonds; therefore, the multiplicity is given by the local core configuration.93 Studies on the agostic vs π-interaction in compounds of alkynyls and alkynylsilanes and -germanes of general type HCCH/XH3 (X = C, Si, Ge) with copper(I) ions in the gas phase have been carried out by the use of highlevel density functional theory methods, and the structures of the corresponding copper species were optimized at the B3LYP/6-311G(d,p) level. The final energies were found in single-point B3LYP/6-311+G(2df,2p) calculations. The agostic interactions between Cu+ and the hydrogen atoms of the XH3 building blocks play an important role. However, for the appropriate propyne derivative the interaction with the π system dominates and the appropriate π complex lies much lower in energy than in those species exhibiting agostic interactions with the methyl group.94 In addition, [Cu(η2HCCH)2]+ (38) and [Cu(η2-HCCH)(CO)]+ (39) mixtures in solid argon have been studied using infrared absorption spectroscopy and DFT calculations. Compound 38 is the only product which was predicted to have a 2B2u ground state with a planar D2h structure, while 39 possesses a 2B2 ground state with a planar C2v structure.95 DFT calculations (B3LYP) were carried out for the binding of transition-metal cations, including copper(I), to alkynyl/coronene/tribenzocyclyne π systems.96 A DFT investigation into the bonding in tetrahedral [Cu(μ3-CCH)L]4 clusters (L = NH3, PH3) was reported, showing that the organic ligands act in all cases as two-electron σ type ligands and that the covalent part of the Cu···Cu bonding depends mainly upon the a1 component of the orbital interaction between [Cu4L4]4+ and [(C2H)4]4‑ fragments, respectively.97 An extended family of copper(I) alkyne molecules are βdiketonate complexes of the type [Cu(β-diketonate)L] (36) with L = alkyne or alkyne-ene (Figure 9).98−104 Complexes of
Figure 8. Cluster 37 (M = Cu, Ag).84
showed that there is no interaction taking place between the terminal Ru entities. However, treatment of [M 2 (μdppm)2(MeCN)2][BF4]2 with [Ru(CCCCM′)(dppe)(η5-C5Me5)] (M, M′ = Cu, Ag) produced a mixture of Ag6−nCun assemblies, as could be demonstrated by ES mass spectrometry and crystallographic studies. The authors point out that probably an extensive disproportionation takes place within this reaction.84 Silica-supported bimetallic catalysts could be prepared by decomposing platinum−copper and platinum−gold clusters of structural type [Pt2M4(CCtBu)4] (M = Cu, Au).91 The catalytic activity of these species was evaluated with the toluene hydrogen reaction. It was found that the cluster-derived catalysts had apparent activation energies smaller than that of a traditionally prepared Pt−silica catalyst. Within this article, also trends in hydrogen chemisorption and toluene hydrogenation were discussed in terms of possible electronic, particle size, and ensemble size effects.91
Figure 9. General formula for alkyne and alkyne-ene copper(I) species 36 (for R, R′, R″, R‴, and R‴′ see Table 2).
this type, including [Cu(hafc)(η2-PhCCPh)], react rapidly with, for example, hydrogen sulfide gas, and quantitative 7669
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norbornadiene) were recently reported.116 These molecules are accessible by the reaction of stoichiometric amounts of [Cu2O] with oxalic acid and 2 equiv of the appropriate alkyne or alkene. Compounds 40 all decompose by an internal redox process to give copper(0) and carbon dioxide together with the free alkyne or alkene, which was verified by TG measurements. The structure of 40 in the solid state was determined, indicating that they exhibit the anticipated planar dinuclear structure with the oxalate in a μ-1,2,3,4 bridging mode and the alkyne/alkene as capping entities.116 In addition, dicopper oxalates [Cu2(C2O4)L4] (41; L = CNtBu, CNCMe2CH2tBu, CNC6H3Me2-2,6) could successfully be used as molecular Cu CVD precursors, which have, for example, been prepared from a copper(I) oxalate complex featuring an alkyne capping ligand, [Cu2(C2O4)(η2-Me3SiCCSiMe3)2] (40).117 Heating 41 to temperatures between 150 and 280 °C resulted in the formation of [CuCN] (with L= CNtBu, CNCMe2CH2tBu) or [CuO] (and then to [Cu2O]) (with L = CNC6H3Me22,6), which finally decompose to give elemental copper at higher temperatures.117 Furthermore, alkyne copper(I) carboxylates, β-diketonates, and formates can successfully be applied as Cu CVD and Cu spin-coating precursors for copper deposition.98,116,118,119 For example, the structure of {[Ti](μ-σ,η 2 -CCSiMe 3 ) 2 }CuO2CH·HO2CR was determined, showing a 3-fold-coordinated copper(I) ion set up by two π-bonded alkyne ligands and one oxygen atom. A formic acid building block is additionally hydrogen-bonded to the [CuO2CH] moiety.118 The decomposition behavior was studied by TG and TG-MS measurements. Instead of β-diketonate ligands also β-diketiminates can successfully be introduced in alkyne copper(I) chemistry.109,119,120 Within these molecules (Figure 10), the alkyne building blocks bis(trimethylsilyl)acetylene (42; BTMSA), 3hexyne (43), and diphenylacetylene (44) can be bonded to one copper(I) ion acting as a two-electron donor (42 and 43), or to two copper(I) ions acting as a four-electron donor (44). The role of hard and soft ancillary ligands possessing a similar structure (β-diketonate = t BuC(O)CHC(O)CH2CHMe2; β-diketiminate = Me2CHCH2NCMeCH CMeNH(CH2CHMe2)) in the determination of π backbonding interactions was studied. It has been shown that the copper(I) ion is a poor π back-bonding ion but that the degree of back-bonding is strongly influenced by the ancillary ligands (the β-diketiminate ligand is of greater basicity) in alkyne complexes. The degree of π back-bonding was indicated by a decrease of the Si−C−C bond angle of the coordinated BTMSA group, a decreased νCC stretching frequency, and a 13 C NMR downfield shift of the acetylenic carbon atoms.115
conversion is apparently enabled by the displacement of the ligands, resulting in the formation of porous Cu2S. It was found that the rates of reaction were faster than those of commonly used absorbents, and hence such compounds are discussed for gas cleanup and chemical analysis of hydrogen sulfide.105 In addition, copper(I) β-diketonate complexes 36 are useful in the chemical vapor deposition (=CVD) of pure copper layers or patterns on diverse substrates, since this process is considered as an alternative method for interconnect metalization in future generations of submicrometer integrated circuits, due to, for example, the low resistivity and superior electron migration resistance of copper.28,106−113 Typically, these molecules consist of a strongly bonded β-diketonate ligand and a second, weakly bonded two-electron-donor group and hence are monomeric 16-valence-electron species in which the Cu(I) center is tricoordinated by the chelating β-diketonate ligand and the datively bonded Lewis base L (Figure 9 and Table 2). Table 2. Selected Alkyne Copper(I) β-Diketonates compd R SiMe3 SiMe3 SiMe3 Et Et Et Et Et Et SiEt3 SiMe3 SiMe3 SiMe3 SiMe3 SiMe3 Me SiMe3
R′ C(Me)CH2 C(Me)CH2 C(Me)CH2 CHCH2 C(Me)CH2 (η2-CHCH2) Cu(hfac)a (η2-CHCH2) Cu(hfac)a C(Me)CH2 C(Me)CH2 C(Me)CH2 SiMe3 SiMe3 SiMe3 SiMe3 SiMe3 Me SiMe3
ref
R″ Me t Bu CF3 CF3 CF3 CF3
R″′
R″″
Me Bu CF3 CF3 CF3 CF3
H H H H H H
98 98 98 99 99−101,114 99
CF3
CF3
H
99
Me CF3 Me Me Me Me b c CF3 t Bu
CF3 CF3 Me CHMe2 t Bu n C6H13
H F H H H H
CF3 CH2CHMe2
H H
114 114 114 102, 103 103 103 103 103 104 115
t
a
hfac =1,1,1,5,5,5-hexafluoroacetylacetonate. b 1-(2′-methyl)cyclopropyl-1,3-butadionate. c1-cyclobutyl-1,3-butadionate.
In addition to [Cu(β-diketonate)L] complexes 36, also dicopper(I) oxalate compounds of the general composition [Cu2(C2O4)L2] (40) featuring as π-coordinating ligands L alkynes (RCCR′; R = R′ = SiMe3; R = SiMe3, R′ = tBu; R = R′ = Et) or alkenes (H2CCHR; R = SiMe2tBu, SiEt2Me;
Figure 10. Molecules 42−44.115,120,122 7670
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Scheme 7. [Cu(κ2S,N-SN-iPr)(η2-HCCR)]+ (47; R = C6H4-4-Me, C6H4-4-OMe): Synthesis and Alkyne Spinning125
Figure 11. Molecules 48 (left) and 49 (right).127
In 2009 Schaper reported the π back-bonding in dibenzyl-βdiketiminate copper diphenylalkyne compounds.120 The bonding between BTMSA and Cu(I) in organometallic complexes featuring either β-diketonate or β-diketiminate chelating ligands has been analyzed with the help of the ETS (extended transition state) energy decomposition scheme and the natural orbitals for chemical valence (NOCV) method.121 It was found that the Cu−BTMSA bond predominantly involves π back-donation (occupied Cu d → π* BTMSA). The π-bonding strength decreases with the subsequent replacement of O/N by P/S, while the changes in the substituents on the main chain of the β-diketonate/β-diketiminate ligands did not affect the π back-donation significantly. Furthermore, it was found that the β-diketonate species possess more stabilizing Cu−BTMSA bond energies, in spite of the reverse preference observed from the Cu−BTMSA back-bonding. For the analyzed species it was observed that the final trends in the total bonding energy value result mainly from Pauli repulsion, electrostatic, and distortion energy contributions.121 In 2009 a series of thermally stable monomeric isoleptic coinage-metal alkyne molecules was discussed.122 X-ray structure analysis showed that the M−C and M−N bond lengths vary in the order Cu > Au > Ag and that the C−CC bending is larger for Au followed by Cu and Ag, respectively. In addition, the Au derivatives showed the largest decrease in νCC, while the respective silver systems possessed the smallest change in comparison to that of the uncoordinated alkyne. From DFT calculations it can be seen that the M−alkyne bond energy varies in the order Ag < Cu < Au. The Au molecules have the longest CC, the largest C−CC angle deviation from linearity, and the smallest νCC, followed by Cu and Ag. In the triazapentadienyl coinage-metal alkynes the σ donation from alkyne → M dominates over the M → alkyne π back-donation.122 A thermally stable monoalkyne copper(I) organometallic complex supported by a fluorinated bis(pyrazolyl)borate could be synthesized by treatment of [H2B(3,5-(CF3)2pz)2]Cu(N CMe) (pz = pyrazolyl) with HCCPh.123 The solid-state structure of [Cu(H2B(3,5-(CF3)2pz)2)(η2-HCCPh)] (45) confirms the three-coordinated, trigonal-planar copper site, the bis(pyrazolyl)borate adopting the common boat conformation.
Complex 45 indicates minimal back-bonding from the d10 Cu(I) ion, as derived from ΔνCC (Δν = νfree − νcomplex) and the back-bending angle of the transition metal−alkyne complex; smaller ΔνCC values correspond to smaller bendback angles.123 Tris(pyrazolyl)borate copper(I) compounds can be applied as catalysts in cyclopropanation and aziridination reactions. Likewise, the use of the bulky hydrotris(3mesitylpyrazolyl)borate (=TpMs) allowed the isolation of stable [TpMsCu(η2-alkyne)] species (46; alkyne = HCCnBu, HC CPh, HCCCO2Et).124 The spectroscopic and structural features of 46 as well as relative reactivities have been studied, indicating a low π back-bonding from Cu to the alkyne. Furthermore, it could be shown that terminal alkyne copper complexes are more stable than copper(I) species featuring internal alkynes. Molecules 46 were used as catalysts in the alkyne cyclopropanation reaction with ethyl diazoacetate as the carbene source.124 It was found that under catalytic conditions metal centers with a low steric hindrance do not form alkyne adducts and display high catalytic activities. Tricoordinated copper(I) alkyne complexes bearing a bidentate soft/hard SN ligand of the type [Cu(κ2S,N-SN-iPr)(η2-HCCR)]+ (47; R = C6H4-4-Me, C6H4-4-OMe) were recently reported, and their mononuclear nature was supported by DFT calculations. Room-temperature NMR spectra of these compounds consist of rapid spinning of the alkyne around the axis connecting the copper(I) ion to the carbon−carbon triple bond on the NMR time scale (Scheme 7).125 The reaction of gold(I) alkynides [AuCCR(PPh3)] (R = Ph, 2-C5H4N) with [M(PPh3)2]+ cations (M = Cu, Ag) afforded the novel 48 and 49, respectively (Figure 11). These molecules are characterized by not only interactions of M with the carbon−carbon triple bond but also by the formation of strong M−Au interactions.126 The luminescent properties of 48 and 49 were studied, revealing intense room- and lowtemperature luminescence originating from 3IL(ππ*) located in the phosphine phenyl rings; however, a contribution from the alkynide ligand cannot be excluded. For the copper species solution excited states are not influenced by the alkynide substituents or the molecular structure of the appropriate complex. Nevertheless, in the solid state the assignment of the 7671
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hydrogen acceptors.131 Dinuclear 54 can be used as a starting material for the synthesis of a series of mononuclear alkyne copper(I) β-diketonates (36) (Figure 9), [Cu(β-diketonates)(η2-Me3SiCCSiMe3)] (β-diketonate = acetylacetonate, 1,1,1,5,5,5-hexafluoroacetylacetonate, 2,2,6,6-tetramethyl-3,5heptanedionate, 1,3-diphenyl-1,3-propanedionate, 2-methyl-4pyronate, troponolate).132 These molecules gave on further reaction with, for example, P(Me 2NCH 2 -2-C6 H 4 ) 3 the respective phosphino complexes [Cu(β-diketonate)(P(Me2NCH2-2-C6H4)3)2], which have been applied as CVD precursors for the deposition of copper thin films.132 Lang and co-workers also reported the synthesis and properties of heterometallic complexes of composition [Cu(μ-Cl)(η2(Ph3P)AuCCML n)] (55) (MLn = Fc, Au(PPh3)) in 2001.133 Nevertheless, it was found, when [CuBr] was used as the copper source, only [CuCC]n along with [AuBr(PPh3)] were formed, when [(Ph3P)AuCCAu(PPh3)] was reacted with [CuBr]. An explanation for the formation of [CuCC]n and [AuBr(PPh3)] is given by the assumption of the intermediate building of [(η2-(Ph3P)AuCCAu(PPh3))Cu(μ-Br)]2, which by an intramolecular ligand exchange finally produces copper(I) acetylide and triphenylphosphine gold(I) bromide.133 The structure of [Cu(μ-Cl)(η2-(Ph3P)AuC CFc)] in the solid state reveals a linear arrangement around gold(I) and a coplanar assembly of the Cu2Cl2 and the Au(I) alkynide entities.133 Cu2(μ-Br)2 four-membered rings are also responsible for the formation of coordination polymers, as evidenced by the reaction of [Hg(CCFc)2], which was accessible by treatment of [Hg(OAc)2] with HCCFc in a 1:2 molar ratio, with [CuBr].134 The π coordination of mercury bis(alkynides) to copper(I) and silver(I) is verified in, for example, [Hg((η1:η2-CCR)Cu(NCMe)2)2][PF6]2 and [Hg((η1:η2-CCR)ML)2][X]2, respectively (M = Cu, Ag; R = Ph, C6H4-4-Me, tBu; L = bpy, phen; X = PF6, BF4).135 Within these complexes, the group 11 metal atoms are π coordinated by the alkynide ligands. The coordination of copper(I) in [Hg((η1:η2-CCR)Cu(NCMe)2)2][PF6]2 is essentially trigonal planar and the molecules pack to form parallel sheets, though with marginal π−π overlap.135 The tetranuclear copper(I) alkyne [Cu(μ-Cl)(η2-HC CCMe2(OH))]4 (56) was found to be catalytically active in the isomerization of 2-methyl-3-butyn-2-ol into prenal in the form of [Ti(OR)4CuCl(R′CO2H)].136 The molecular structure of 56 is depicted in Figure 13. Molecule 56 consists of two [CuCl(C5H8O)]2 building blocks, forming an overall tetrameric arrangement of four Cu(I) ions positioned at the vertices of a somewhat distorted tetrahedron. The two metal centers of the dimeric entities are connected through a chloride bridge, which affords an eight-membered ring in a boat conformation. As is typical for other copper alkyne complexes, the copper atoms adopt an approximate trigonal-planar geometry in 56.136 Complexes featuring Au(I) alkynides η2 bonded to copper(I) or silver(I) ions have emerged as a new family of compounds which often exhibit intense photoluminescence (vide supra). 137−145,147 Emissions are modified by aurophilic (Au···Au) as well as Au···M (M = Cu, Ag) interactions between closed-shell d10 ions. Molecule structures possessing such contacts are shown in Figure 14. Six different types can be recognized: trinuclear Au2M (M = Cu, Ag) (structural type A molecule), tetranuclear Au2Cu2 (type B molecule), hexanuclear Au4Cu2 (type C molecule), pentanuclear Au3M2 (M = Cu, Ag; type D molecule), octanuclear Au6Cu2 (type E molecule), and dodecanuclear Au6Cu6 (type F molecule).81,147−151,153,156,162
origin is different, being proposed as an admixture of IL and MLCT for the dinuclear species.126 In 2010 the structures of {[CuCl2 (η 2 -HOCH2 CC CCH2OH)]2Ca}·4H2O (50) and [CuCl2(η2-HOCH2CC CCH 2OH)](C 7 H 5 N 2 H 2) (51) in the solid state were reported.127 Complex 50 consists of infinite bimetallic chains forming a three-dimensional framework through (O)H···Cl and C(O)−H···Cl hydrogen bonds, while 51 is set up by discrete [CuCl2(η2-HOCH2CCCCH2OH)]− anions, paired by edgeto-edge packing in the (100) direction and large C7H5N2H2+ (=BimH) cations with face-to-face packing. Characteristic for 50 and 51 is the trigonal environment around Cu(I) involving two chloride ligands and the π-coordinated alkyne unit. The same group also reported the solid structure of [CuCl2(η2HOCH2CCCCH2OH)]Na·2H2O, a compound which is built of discrete [CuCl2(η2-HOCH2CCCCH2OH)]− anionic stacks and polymeric [Na+(H2O)2]n cations among the stack, involving strong O−H···Cl and O−H···O hydrogen bonding.128 The zwitterionic copper(I) π complex [Cu 2 Br 3 (HC CCH2NH3)] was synthesized by the interaction of [CuBr] with [HCCCH2NH3]+ in aqueous solution at pH
