Coinage Metal Complexes of Germylene and Stannylene.

Group 11 metal (M = Cu, Ag, Au) complexes of heavier tetrylenes (namely, germylene and stannylene) have been studied for several years now. However, the field is mainly unexplored for their potential application either in homogeneous catalysis or in photophysical properties regardless of how the current reports allude to the rich and fascinating chemistry of group 14 elements. In this mini-review, we attempted to summarize the recent advances of heavier tetrylene-stabilized coinage metal complexes, which are majorly dominated by structure elucidation studies. This comprehensive summary intends to help researchers design and fine-tune the tetrylene ligand framework that can lead to coinage metal complexes for future applications as photoemitters in organic light-emitting diode fabrication and efficient catalysts in homogeneous catalysis.

Introduction

The carbene-stabilized coinage metal complexes have exhibited excellent catalytic and photophysical properties.[1] However, the coinage metal complexes of heavier congeners of carbenes, namely, silylene, germylene, and stannylene, are still in their infancy. They have been mainly used in small-molecule activation, coordination to low valent species, and, nowadays, in homogeneous catalysis.[2,3] Recently, we have summarized N-heterocyclic silylene coordinated coinage metal complexes and their potential application in homogeneous catalysis.[4] However, a compilation of the germylene and stannylene coordinated coinage metal complexes remains to be addressed. This motivated us to revisit their existing literature to compile their unique bonding scenario with coinage metals, which demand more investigation. We have covered the literature since 2010 and segregated the content based on germylene/stannylene supported copper(I), silver(I), and gold(I) complexes.

Tetrylene-Supported Coinage Metal Complexes

Germylene–Copper(I) Complexes

The behavior of tetrylenes as Lewis bases is characteristic of their coordination chemistry, enabling them to form various metal complexes. In 2012, Fulton and co-workers utilized the Lewis basicity of a germanium alkoxide [LGeOtBu (L = HC[C(Me)N-2,6-Pr2C6H3]2)] and prepared dimeric germanium(II)–copper(I) iodide adduct 1 (Chart ) with a four-membered bridged rhomboidal Cu2I2 core.[5] This was the first structurally isolated three-coordinate copper(I) complex possessing a Cu2I2 ring attached to the germylene center. Similarly, Zhao and co-workers also reported two different types of copper(I) complexes, 2 and 3 (Chart ), via the reaction of LGeMe (L = HC[C(Me)N-2,6-Pr2C6H3]2) and (CuC6F5)4.[6]2 displays a bridged (CuC6F5)2 unit, while 3 has a chain-like aggregation of (CuC6F5)4 with the Cu–Cu bond distance ranging from 2.3819(11) to 2.4919(8) Å. Due to these cuprophilic interactions (d10–d10), 2 and 3 exhibit photoluminescence properties. The photophysical properties of 2 and 3 were also studied in DCM at room temperature. The copper complex 2 exhibits green luminescence (λem = 497 nm) upon excitation at λex = 336 nm, whereas 3 shows a blue luminescence (λem = 458 nm) upon excitation at λex = 340 nm. Stahl and co-workers reported an N-heterocyclic germylene–copper(I) complex 4 consisting of a four-step ladder-type structure. The ladder steps of 4 comprise Ge–Cu and Cu–Cl bonds alternatively, where Ge and copper centers adopt terminal and bridging binding modes (Chart ).[7] The quick shuttling of germylenes (between the terminal and bridging copper(I) centers) makes them magnetically and chemically equivalent. An example of β-diketiminate ligand stabilized (phenylethynyl) germylene [LGe(C≡CPh)CuC6F5 (L = HC[C(Me)N-2,6-Pr2C6H3]2)] was utilized by Roesky and co-workers with 0.25 equiv of (CuC6F5)4, which led to the formation of LGe(C≡CPh)CuC6F5 (L = HC[C(Me)N-2,6-Pr2C6H3]2),[8] displaying an open coordination site at the Ge(II) center.

Chart 1

Germylene and Stannylene Supported Copper(I) and Silver(I) Complexes

Hence, the authors treated it further with another equivalent of (CuC6F5)4, which afforded complex 5.[8] Complex 5 features a germylene coordinated to the copper(I), where the copper possesses a Lewis acidic C6F5 group.

The chemistry of aminotroponiminato chloro germylene [(iBu)2ATIGeCl, (ATI = aminotroponiminate)] as a ligand was explored by Nagendran and co-workers in 2014.[9] They performed various reactions with CuI by varying the stoichiometry and solvents to afford complexes 6–8 (Chart ). In 6, the germylene center bears a tetrameric cubane of Cu4I4 with μ3 bridging of iodine with three copper atoms, while 7 displays a dimeric Cu2I2 core. In continuation with the previous report in 2015, Nagendran and co-workers investigated the reactivity of a spacer-arranged di(germylene)oxide [((iBu)2ATIGe)2O] with 2 equiv CuI for the synthesis of coinage metal complexes, reporting complex 9 with a Cu4I4 octahedral core and complex 10 with a trinuclear Cu3I3 core (Chart ).[10] Further addition of pyridine produced a structurally unique germylene- and pyridine-bound dimeric copper(I) complex, 11 (Chart ).

Cabeza and co-workers utilized a benzamidinate-germylene [PhC(NtBu)2GetBu] and prepared copper(I) complexes, 12 and 13, upon treatment with 1 equiv of CuCl and 0.5 equiv of [Cu(MeCN)4][BF4], respectively (Chart ). Complex 12 attained a pseudocubane-type Cu4(μ3-Cl)4 core, while 13 (Chart ) remained monomeric.[11] The authors also noted that benzamidinato tert-butyl germylene failed to stabilize any transition metal complexes, unlike silicon derivatives. Hence, the sterically demanding tert-butyl group plays an important role in stabilizing metal complexes. A half pincer-based germylene [(2-Et2NCH2–4,6-tBu2-C6H2)GeCl] was employed by Jambor and co-workers to treat with CuCl and CuI, which afforded the dimeric complexes 14 and 15 (Chart ). Both complexes display a four-membered Cu2X2 moiety comprising Cu(μ-X) (X = Cl, I) bridges.[12] They also isolated the mononuclear complexes, 16 and 17, via the reaction of 14 and 15 with 1 equiv of the corresponding germylene, respectively (Chart ).[13] Complexes 18 and 19 were prepared by reacting 17 with AgOAc and AgOTf, respectively.[13] Complexes 16–19 have a tricoordinate central Cu atom attached with two chlorides and one Ge(II) atom. Interestingly, 16 and 17 were found to be catalytically active for the ring-opening polymerization of l-lactide.[12] This study explores the brighter future of germylene-supported coinage metal complexes, a new mileage in homogeneous catalysis.

Germylene–Silver(I) Complexes

The examples of germylene supported silver(I) complexes are comparatively limited. Heterobimetallic complex 20 was also synthesized by reacting pentafluorophenyl silver with LGe(C≡CPh)CuC6F5 [L = HC[C(Me)N-2,6-Pr2C6H3]2], analogous to 5 (Chart ).[8] Another cationic complex [(LGeC≡CPh)2Ag]+[Ag(C6F5)2]− (21) (Chart ) was also formed with the addition of one more equivalent of pentafluorophenyl silver. For the silver compounds, 20 and 21, the persistence of Ge–Ag donor–acceptor bonding in solution is proved by the appearance of the blue luminescence band at 460 nm for 20 and green luminescence band at 526 nm for 21. Similarly, silver complexes 22 and 23, which are analogous to 12 and 13, were also reported (Chart ) by Cabeza et al.[11] Nagendran and co-workers also investigated the reaction of bisgermylene [((Bu)2ATIGe)2O] with silver iodide to afford 24 (Chart ), which contains a Ag4I4 octahedral core.[10] Recently, the nucleophilic cationic Ge(II) center was utilized by the group of Majumdar toward the coordination with coinage metals.[14] They isolated a biscoordinated cationic Ag(I) complex 25, which acquires a barbell-shaped structure (Chart ). For 25, the triflate moiety is weakly coordinated to the Ag center. In line with this, So and co-workers reported the synthesis of the bis(germyliumylidene)silver(I) complex, 26a (Chart ),[15] which upon treatment with an N-heterocyclic carbene, IMe [IMe = :C{N(Me)C(Me)}2], afforded a bis(germyliumylidene)silver(I) complex dication 26b. Germyliumylidene cations generally possess the ambiphilic nature of germylene and the high Lewis acidic character of the germyl cation.

Stannylene–Silver(I) Complexes

To our surprise, there are no reports of stannylene-supported copper(I) complexes after 2010, and the examples of silver(I) complexes are also minimal. As an ongoing investigation on heavier tetrylene’s coinage metal chemistry, we also performed a reaction of benzamidinato-stannylene [PhC(NtBu)2SnN(TMS)2] with AgSbF6, which led to the formation of cationic silver complex 27, displaying weak Ag(I)···F interactions (Chart ).[16] This weak noncovalent interaction indicates a lower σ-donation of stannylene than its lighter congeners. Jambor and co-workers utilized the intermolecularly stabilized NSnN ligand and explored the coordination with different silver(I) salts (Chart ).[17] Complexes 28 and 29 were isolated from the reaction of (2,6-(Me2NCH2)2C6H3)SnCl with AgOAc and Ag(acac), respectively. Both structures comprise a central cubane core of Ag4Cl4. Further, the reaction of the NSnN ligand with [(COD)Ag(Hfac)] resulted in a monomeric complex 30 (Chart ).[17] However, the reaction with [(THF)Ag{HB(3,5-(CF3)2Pz)3}] produced 31, where stannylene acts as a donor ligand only. In a very recent report by Majumdar and co-workers, intermolecularly (ene-amide) stabilized stannylene was used to coordinate with coinage metal salts, producing complexes 32 and 33 (Chart ). These complexes feature a mononuclear cationic silver(I) center, which is coordinated with two homoleptic stannylene molecules.[18]

Germylene–Gold(I) Complexes

NHC-stabilized gold(I) complexes are well-known for their catalytic application and photophysical properties due to Au···Au interactions. However, tetrylene-supported gold(I) complexes are still not well explored. To begin with, Saffon and co-workers demonstrated the coordination of phosphaalkenyl germylene [(NHC)Ge(CCl = PMes*)2; NHC = IPr2Me2 (1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene)] with gold(I) salts[19] and isolated 34 and 35 upon reaction with AuI and AuCl·Me2S, respectively. Both possess similarly Ge → Au bonded adducts displaying a near-linear geometry (Chart ). Following this, Cabeza and co-workers isolated 36 (Chart ) by reacting 2 equiv of [Ge(N(SiMe3)2)2] with AuCl(THT). Compound 36 is formed via the insertion of one of the Ge(II) centers into the Au–Cl bond, and the gold center remains linearly attached to both the germanium centers of the ligand,[20] where substitution of the THT ligand with Ge(N(SiMe3)2)2 at the gold center is evident. In 2013, Roesky and co-workers isolated a mononuclear dicoordinate gold(I) complex 37 (Chart ) from the reaction of (phenylethynyl) germylene and AuC6F5·SC4H8.[8] However, the reaction of LGe(C≡CPh)CuC6F5 with AuC6F5·SC4H8 yielded 38 (Chart ), a trimetallic Ge–Cu–Au compound with terminally bounded AuC6F5 at the Ge(II) center.

Chart 2

Germylene and Stannylene Stabilized Gold(I) Complexes

Contemporarily, Cabeza et al. employed [PhC(NBu)2GeBu] in the same context and prepared 39 (Chart ).[11]39 exhibits mononuclear nature, and the Au–Cl bond is linearly coordinated to the Ge(II) center. Similarly, digermylene oxide [((Bu)2ATIGe)2O] reaction with AuI afforded 40, which has a Au2I2 core with the Au–Au distance of 3.1562 Å, indicating the presence of an aurophillic interaction.[10] The emission spectrum of 40 displays four peaks of the same intensity at 486, 460, 431, and 407 nm (λex = 360 nm), suggesting d10–d10 interaction. In 2016, Marschner and his team used an acyclic NHC (IMe4) stabilized Ge(HMDS)2 and performed a reaction with AuCN, which led to the formation of complex 41 (Chart ).[21] Here, instead of the NHC unit, the tris(trimethylsilyl) unit moves to the gold center, and the cyanide group gets attached to the germanium atom. In continuation of the previous discussion, the gold(I) analogue of 25 was also prepared via the reaction of the Ge(II) dication and AuCl·Me2S and led to the isolation of a biscoordinated Au(I) complex 42.[14] In this case, the Ge–Au–Ge moiety is almost linear with two [LGe:]2+ units. Campos and co-workers used the GeCl2·dioxane complex and reacted it with cationic gold fragments [(PMe2ArDipp2)AuNTf2], generating germyl gold complexes. During halide abstraction with GaCl3 and NaBArF from germyl gold complexes,[22] complexes 43–46 (Chart ) were isolated. Subsequently, the electrophilicity of the Ge(II) center in 43–46 was tested by reacting them with 4-DMAP (4-dimethylaminopyridine), which resulted in 4-DMAP-coordinated complexes 47–50 (Chart ). Recently, we have isolated [PhC(NBu)2GeN(TMS)2]-stabilized cationic gold(I) complexes, where benzene (51) and toluene (52) are present in η2 binding fashion.[23] Complexes 51 and 52 were further used as efficient catalysts for glycosidation reaction. Moreover, 52 was found to be equally efficient like a silylene counterpart for the disaccharide formation. This study indicates that germylene-stabilized coinage metal complexes can be potential catalysts for various organic transformations.

Stannylene–Gold(I) Complexes

In contrast to silver and copper complexes, stannylene-supported gold complexes are comparatively more explored. As mentioned in the previous section, Cabeza and co-workers reported a similar reactivity with Sn(HMDS)2 and isolated 53, which is analogous to 36; only one THT group remains attached to one of the tin centers (Chart ).[20] Upon substituting the chloride ligand of 53 with LiBu and Li(HMDS), complexes 54 and 55 were obtained (Chart ). On the same note, Marschner and his team utilized the NHC adduct of disilylated acyclic stannylene and reacted it with AuCN to afford 56 (Chart ).[21] Complex 56 bears three tris(trimethylsilyl)silyl ligands attached to the tin and gold centers. Furthermore, Kira and co-workers reported adduct formation. A cyclic trigold anionic salt 57 (Chart ) was isolated using the same cyclic dialkyl stannylene as the ligand backbone and Et3PAuCl followed by reduction with KC8, where the Au3 ring presents a regular triangle with the average Au–Au bond lengths of 2.9343(4)Å.[24] Jambor and co-workers have also utilized the pincer-type N,C,N-coordinated stannylene as a ligand to synthesize the Sn(II)–Au(I) complex.[17] The reaction of LSnCl [L = 2,6-(Me2NCH2)2C6H3] with AuCl·SMe2 provided the desired adduct 58 (Chart ), displaying a linear Sn–Au–Cl bond and a weak aurophilic interaction of 3.0878(3) Å. Majumdar and co-workers recently reported the coordination toward Au(I) centers with pyridine-ene-amide stabilized stannylene.[18] They isolated the ionic distannylene–gold complex 59 by utilizing the above-mentioned ligand and 0.5 equiv of AuCl and trimethylsilyl trifluoromethanesulfonate, where the stannylene center remains coordinated to the Au cation from both sides, forming an Sn–Au–Sn linear geometry.

Summary and Future Outlook

The availability of a lone pair of electrons and a vacant p-orbital makes tetrylenes an attractive class of donor ligands for the transition metals, which is very clear from the significant contributions in this area in the past decade. Among transition metals, coinage metals have a very special place due to their interesting catalytic and photophysical properties. The N-heterocyclic carbenes have a tunable singlet–triplet energy gap that offers exclusive electrophilicity and could show faster radiative decay. Also, nowadays, carbene-metal-amides (CMAs) (metal = Cu, Ag, Au) are gaining popularity as they provide strong photoluminescence properties because of the extended donor–acceptor systems. These aspects are missing for the heavier tetrylenes, which could be a fascinating area of research. However, it requires a systematic study to design efficient photoluminescent complexes that can be utilized in OLED fabrication.

Similarly, homogeneous catalysis with heavier tetrylene-coordinated coinage metals is another gap in this area. Although many heavier tetrylene-stabilized coinage metal complexes have been isolated, only two reports prevail for their catalytic application. This mini-review mainly focuses on the coinage metal complexes of various germylenes and stannylenes and their associated structural properties. This discussion indicates that there is certainly much scope for tetrylene-ligated coinage metal complexes, majorly in the area of photophysical properties, as d10–d10 interactions are often responsible for attractive luminescent materials. We sincerely hope that due to their unique physical and chemical properties tetrylene coinage metal complexes will develop from academic curiosities to commercially important materials.