Coinage metal complexes of NHC-stabilized silyliumylidene ions

Philipp Frisch and Shigeyoshi Inoue*
Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, 85748 Garching bei München, Germany. E-mail:

Received 27th September 2018 , Accepted 18th October 2018

First published on 18th October 2018

The first silyliumylidene ion coinage metal complexes are reported. Treatment of N-heterocyclic carbene (NHC) stabilized silyliumylidene ions (R = m-Ter, Tipp) with transition metal precursors (CuCl, AgOTf, (Me2S)AuCl) yields the first silyliumylidene coinage metal (M = Cu, Ag, Au) complexes, which have been fully characterized including X-ray diffraction analyses.

Silyliumylidene ions [RSi:]+ are a relatively new class of low-valent silicon compounds that feature a positive charge, a lone pair of electrons and two vacant orbitals on the silicon center.1 Their successful isolation generally requires steric and electronic stabilization of the Si(II) cation by utilization of bulky substituents as well as external donors (e.g. NHCs).2 A number of silyliumylidenes3 have been reported since Jutzi's seminal work on the pentamethylcyclopentadienyl silyliumylidene ion [Cp*Si:][B(C6F5)4] in 2004.4 So far, reactivity investigations remain limited to the activation of few small molecules and some specific subsequent reactions.3c,i,j,5

Due to the presence of a lone pair on the silicon atom – similarly to silylenes6 – silyliumylidene ions should also be able to act as ligands in transition metal complexes. However, it is important to note that the external donors required to stabilize the Si(II) cations generally reduce their π-acceptor ability by (partially) filling the vacant orbitals on the silicon center.

In contrast to silylenes, the coordination behavior of silyliumylidene ions has been barely explored. In 2013, we described bis(platinum) and bis(palladium) complexes I and II (Fig. 1), which can formally be regarded as base-stabilized silyliumylidene-phosphide complexes.7 In 2014, So et al. reported rhodium (III) and tungsten (IV) complexes of a base-stabilized Si(II) cation.8 Very recently, Filippou et al. also reported a molybdenum silylidyne complex synthesized from [Cp*Si:][B(C6F5)4].5e

image file: c8cc07754a-f1.tif
Fig. 1 Examples of (formal) silyliumylidene transition metal complexes (I–IV) as well as silylene coinage metal complexes (V–IX); DMAP = 4-dimethylaminopyridine; TMEDA = tetramethylethylenediamine.

While a variety of coinage metal compounds with silyl-based substituents have been reported over the last decades,9 group 11 complexes incorporating ligands with a low-valent silicon atom are quite rare. For example, Jutzi et al. described the AuCl complex of [(Cp*)2Si:], however, no crystal structure was reported.10 Dialkylsilylene complexes of copper (V) and silver (VI)11 and silacyclopropylidene complexes of copper and gold were structurally characterized.12 In recent years, Roesky's silylene [PhC(NtBu)2]SiX (X = Cl, N(SiMe3)2)13 has shown great potential in the isolation of various monomeric and dimeric silylene coinage metal complexes VII–IX.14 However, no coinage metal complexes with silyliumylidene ions as a ligand have been reported so far. Motivated by our recent reports on the interesting chemistry of NHC-stabilized silyliumylidene ions 13e,5a,b,15 we set out to utilize 1 as ligands in transition metal complexes. Herein we disclose the first silyliumylidene coinage metal complexes synthesized from stable NHC-stabilized Si(II) cations.

As shown in Scheme 1, the coinage metal complexes 2–4 can be conveniently synthesized by addition of simple metal precursors (CuCl, AgOTf, (Me2S)AuCl) to the silyliumylidene ions 1. Treatment of an acetonitrile solution of 1a or 1b with one equivalent of copper(I)chloride (Scheme 1) results in dissolution of the salt accompanied by rapid decolorization of the orange solution. A similar reactivity can be observed upon addition of (Me2S)AuCl. In the case of AgOTf, addition of the coinage metal salt leads to immediate precipitation of a colorless solid (AgCl) without any color change of the solution. This is due to an anion exchange of the chloride counter anion in 1 to a triflate anion. After stirring for 5 minutes, the colorless precipitate gradually dissolves and the solution changes color from orange to colorless. Complexes 2–4 can be isolated as air- and moisture-sensitive (in the case of 3a and 3b also light-sensitive) colorless solids in good to excellent yield (71–94%) by crystallization from a MeCN–Et2O mixture at −40 °C. The complexes are insoluble in benzene, hexane and Et2O and show good solubility in pyridine, difluorobenzene and acetonitrile.

image file: c8cc07754a-s1.tif
Scheme 1 Synthesis of NHC-stabilized silyliumylidene ion coinage metal complexes 2–4 (MX = CuCl (2), AgOTf (3), (Me2S)AuCl (4)); IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene.

The coordination of the coinage metal to the silyliumylidene ion is accompanied by a downfield shift of the 29Si NMR resonance (Table 1) from −68.8/−69.5 ppm (1a/1b)3e to −46.6/−48.8 (2a/2b, M = Cu), −44.1/−46.6 (3a/3b, M = Ag) and −34.6/−38.0 ppm (4a/4b, M = Au), however the downfield shift is significantly less pronounced than the one observed by So et al. upon coordination of their DMAP-stabilized silyliumylidene to rhodium and tungsten (−82.3 to +40.5 and +51.6 ppm, respectively).8 For comparison, the reported silylene coinage metal complexes show resonances in a relatively narrow range, e.g. 4.4 ppm (VIIa),14d 7.9 ppm (VIII)14a and 8.8 ppm (IX).14a It is noteworthy that the 29Si NMR of silver complex 3a shows two doublets at −44.1 ppm corresponding to the 109Ag and 107Ag isotopes with coupling constants of 1JSi109Ag = 408.1 Hz and 1JSi107Ag = 352.6 Hz. Similarly, complex 3b also shows two doublets at −46.6 ppm, with coupling constants of 1JSi109Ag = 410.1 Hz and 1JSi107Ag = 355.8 Hz. The 1H NMR spectra of the m-Ter substituted complexes 2a–4a are quite similar, each showing a single signal set for the m-terphenyl ligand with a broadened singlet for the mesityl CH-groups, which split into two separate singlets at −40 °C (cf. ESI, Fig. S5 for VT-NMR). The NHC wingtip CH3 groups split into two broad signals from one broad signal observed for the silyliumylidene ion 1a,3e which also coalesce into one signal at +60 °C. The NHC backbone methyl groups as well as the mesityl methyl groups give only one broad signal in total. At low temperature (−40 °C) as well as high temperature (+60 °C), this broad signal splits into multiple singlets corresponding to each unique methyl group. The 13C NMR spectra show resonances for the carbene carbon atoms and one signal set for the m-terphenyl ligand. Line broadening can also be observed. The 13C NMR spectra of 3 also exhibit a quartet at 122.1 ppm corresponding to the CF3SO3 anion (1JCF = 321.0 Hz).

Table 1 Comparison of the 29Si NMR and XRD data of silyliumylidene ions 13e and the coinage metal complexes 2–4
# R M 29Si NMR [ppm] Si1–M1 [Å] Si1–M1–Cl1 [°]
1a m-Ter −68.8
1b Tipp −69.5
2a m-Ter Cu −46.6 2.238(2) 169.3(1)
2b Tipp Cu −48.8
3a m-Ter Ag −44.1 2.379(1) 171.8(1)
3b Tipp Ag −46.6 2.398(1) 141.2(1)
4a m-Ter Au −34.6 2.281(1) 176.8(1)
4b Tipp Au −38.0

Interestingly, no signal broadening or splitting in the 1H NMR spectra of the Tipp substituted complexes 2b–4b can be observed. This corresponds to a less restricted rotation of the Tipp group and the coordinated NHCs due to the reduced steric bulk of the Tipp substituent compared to the m-Ter group. Compared to starting material 1b, the signals in the complexes appear slightly shifted downfield, except for the septet corresponding to the ortho iso-propyl groups, which appears shifted upfield. While we could observe gold complex 4b by 1H and 29Si NMR, we were unable to obtain satisfactory analytical data due to rapid decomposition (cf. ESI, Fig. S20–S25).

Single crystals suitable for XRD measurements of all three m-Ter substituted complexes 2a–4a were obtained by slow diffusion of Et2O into a concentrated acetonitrile solution at −40 °C. As expected, the solid-state structures of the complexes are very similar (Fig. 2). All three compounds are monomers in the solid state and feature a four-coordinate silicon(II) center with a distorted tetrahedral geometry. Going from copper to silver, an increase in the Si–M bond length (2.238(2) and 2.379(1) Å, respectively) is observed while going from Ag to Au the bond length decreases (2.379(1) to 2.281(1) Å, respectively). This is unsurprising since it has been previously shown that gold is indeed smaller than silver.16 Frenking et al. also reported that, among the coinage metals, silylenes form their strongest coordination complexes with gold,17 which could also explain a decrease in bond length. Furthermore, a similar trend can be observed in the coinage metal silylene complexes reported by Khan et al.14a The M–Si bond length in all three complexes lies in the same range as previously reported Si(II) coinage metal complexes (Cu: 2.17114e–2.28918 Å; Ag: 2.33714a–2.42514c Å; Au: 2.24614b–2.31814b Å). With increasing atomic number of the transition metal, the linearity of the M–Si1–Cl1 bond increases from 169.3(1)° to 171.8(1)° from Cu to Ag and finally to 176.8(1)° for the gold complex. The angle between the two coordinated NHCs (C25–Si1–C32) increases slightly from the silyliumylidene ion 1a (93.8(1)°)3e to 95.5(2)°, 95.8(2)° and 96.5(1)° for 2a, 3a and 4a, respectively.

image file: c8cc07754a-f2.tif
Fig. 2 Ellipsoid plot (50% probability level) of the molecular structures of complexes 2a (left), 3a (middle) and 4a (right). Hydrogen atoms and anions are omitted for clarity. Selected bond lengths [Å] and angles [°]: 2a: Si1–C1 1.916(3), Si1–C25 1.939(2), Si1–C32 1.938(2), Si1–Cu1 2.238(2), Cu1–Cl1 2.1477(6), Si1–Cu1–Cl1 169.3(1), C25–Si1–C32 95.5(2), C1–Si1–Cu1 119.4(2); 3a: Si1–C1 1.911(4), Si1–C25 1.936(3), Si1–C32 1.938(4), Si1–Ag1 2.379(1), Ag1–Cl1 2.366(1), Si1–Ag1–Cl1 171.8(1), C25–Si1–C32 95.8(1), C1–Si1–Ag1 114.8(1); 4a: Si1–C1 1.916(2), Si1–C25 1.942(2), Si1–C32 1.941(2), Si1–Au1 2.281(1), Au1–Cl1 2.354(1), Si1–Au1–Cl1 176.8(1), C25–Si1–C32 96.5(1), C1–Si1–Au1 119.0(1).

Single crystals suitable for XRD analysis for complex 3b were obtained by diffusion of Et2O into a concentrated solution of 3b in CH3CN[thin space (1/6-em)]:[thin space (1/6-em)]C7H8 (2[thin space (1/6-em)]:[thin space (1/6-em)]1) at −40 °C. Interestingly, the complex shows a dimeric structure in the solid state (Fig. 3). The two silyliumylidene ligands are bridged via a 4-membered Ag2Cl2 ring. The Si1–Ag1–Cl1 angle (141.2(1)°) is significantly smaller than in the monomeric complexes. The dimerization is most likely a result of the reduced steric bulk of the Tipp ligand. The Si–Ag bond length (2.398(1) Å) is slightly longer than in 3a, a possible consequence of the dimeric structure, but still well within the reported Si–Ag bond range. Due to their marked instability, we were not able to obtain single crystals of complex 2b and 4b, but judging from their comparable reactivity and similar 1H and 29Si NMR data, it is expected that their structures are analogous.

image file: c8cc07754a-f3.tif
Fig. 3 Ellipsoid plot (50% probability level) of the molecular structure of complex 3b. Hydrogen atoms and anions are omitted for clarity and the Tipp substituents are depicted as wireframes. Selected bond lengths [Å] and angles [°]: Si1–C1 1.919(2), Si1–C16 1.935(2), Si1–C23 1.931(2), Si1–Ag1 2.398(1), Ag1–Cl1 2.562(1), Si1–Ag1–Cl1 141.2(1), C16–Si1–C23 100.8(1), C1–Si1–Ag1 118.0(1).

While the m-terphenyl substituted compounds are stable indefinitely in the solid state and for weeks in acetonitrile solution at room temperature (under exclusion of light for 3a), slow decomposition occurs in C6H4F2 and pyridine. Decomposition also occurs slowly in acetonitrile upon heating at 90 °C, resulting in an unidentified product mixture. In contrast, the Tipp substituted complexes undergo full decomposition over a period of 2 to 4 hours (Scheme 2), which is accompanied by an NHC migration to the metal center. For example, the decomposition of an acetonitrile solution of gold complex 4b results in a linearly coordinated [(IMe4)2Au]Cl complex (5, cf. ESI for SC-XRD and NMR data) and two silicon-containing species with a significantly upfield shifted 29Si NMR resonance (40.5 and 64.0 ppm, respectively; cf. −38 ppm for 4b). Unfortunately, we were not able to unambiguously determine the composition of these silicon-containing species.

image file: c8cc07754a-s2.tif
Scheme 2 Decomposition of the Tipp-substituted Au complex 4b in solution to [Au(IMe4)2]Cl (5) and unidentified Si species.

In summary, we have reported the first coinage metal complexes 2–4 of an NHC-stabilized silyliumylidene ion. They were synthesized from the NHC-stabilized silyliumylidene ions 1 via addition of simple metal salts. The complexes bearing the sterically demanding m-terphenyl substituent show a monomeric structure in the solid state and are stable for weeks in acetonitrile solution. On the other hand, the complexes having the smaller Tipp substituent reveal a dimeric structure and a significant decrease in stability. Possible catalytic applications of the complexes as well as their reactivity are currently under investigation in our laboratory.

We are grateful to the WACKER Chemie AG and the European Research Council (SILION 637394) for continued financial support. We are also thankful to Dr A. Pöthig and Dr C. Jandl for advice pertaining to crystallography and L. Schiegerl, MSc for recording the ESI-MS spectra.

Conflicts of interest

There are no conflicts of interest.


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Electronic supplementary information (ESI) available: Experimental details and crystallographic data. CCDC 1870254 (2a), 1870256 (3a), 1870258 (3b), 1870255 (4a) and 1870257 (5). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc07754a

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