N-Heterocyclic Carbene Coinage Metal Complexes of the Germanium-Rich Metalloid Clusters [Ge9R3]− and [Ge9RI2]2− with R = Si(iPr)3 and RI = Si(TMS)3

We report on the synthesis of novel coinage metal NHC (N-heterocyclic carbene) compounds of the germanium-rich metalloid clusters [Ge9R3]− and [Ge9RI2]2− with R = Si(iPr)3 and RI = Si(TMS)3. NHCDippCu{η3-Ge9R3} with R = Si(iPr)3 (1) represents a less bulky silyl group-substituted derivative of the known analogous compounds with R = Si(iBu)3 or Si(TMS)3. The coordination of the [NHCDippCu]+ moiety to the cluster unit occurs via one triangular face of the tri-capped trigonal prismatic [Ge9] cluster. Furthermore, a series of novel Zintl cluster coinage metal NHC compounds of the type (NHCM)2{η3-Ge9RI2} (RI = Si(TMS)3 M = Cu, Ag and Au; NHC = NHCDipp or NHCMes) is presented. These novel compounds represent a new class of neutral dinuclear Zintl cluster coinage metal NHC compounds, which are obtained either by the stepwise reaction of a suspension of K12Ge17 with Si(TMS)3Cl and the coinage metal carbene complexes NHCMCl (M = Cu, Ag, Au), or via a homogenous reaction using the preformed bis-silylated cluster K2[Ge9(Si(TMS)3)2] and the corresponding NHCMCl (M = Cu, Ag, Au) complex. The molecular structures of NHCDippCu{η3-Ge9(Si(iPr)3)3} (1) and (NHCDippCu)2{η3-Ge9(Si(TMS)3)2} (2) were determined by single crystal X-ray diffraction methods. In 2, the coordination of the [NHCDippCu]+ moieties to the cluster unit takes place via both open triangular faces of the [Ge9] entity. Furthermore, all compounds were characterized by means of NMR spectroscopy (1H, 13C, 29Si) and ESI-MS.


Introduction
Polyatomic Zintl clusters [E 9 ] 4− or [E 4 ] 4− (E: tetrel element) can be extracted from the neat solids of their alkali metal salts with the compositions A 4 E 9 and A 12 E 17 (A = alkali metal, E = Si-Pb) and are then accessible for further reactions. The first synthesis of an organometallic Zintl cluster species, [Sn 9 Cr(CO) 3 ] 4− , was achieved in 1988 by treatment of the Zintl phase K 4 Sn 9 with Cr(CO) 3 Mes (Mes = mesitylene) [1]. Since then, a large number of transition metal compounds containing Zintl cluster ligands have been reported. In various studies, precursors, especially of the late transition metals (groups 10, 11 and 12), have successfully been treated with these anionic Zintl clusters [2,3]. However, the addition of early transition metal fragments to Zintl clusters has been recently achieved [4].
Reactions of Zintl clusters with coinage metal precursor complexes and Zn organyls revealed that the addition of a metal atom to the bare clusters can also initiate cluster growth under formation of larger intermetalloid clusters. For example the reaction of the Zintl phase K 4 Ge 9 with R 3 PCuCl (R = alkyl, aryl) at first leads to [(R 3 P)Cu(η 4 -Ge 9 )] 3− , but the subsequent substitution of the PR 3 group by a second [Ge 9 ] cluster unit results in the coinage metal-bridged dimeric species [(η 4 -Ge 9 )Cu(η 1 -Ge 9 )] 7− [5]. Furthermore, reactions of tetrel clusters with Ph 2 Zn yield compounds of the shape [E 9 ZnPh] 3− (E: Si-Pb) [6], and similar reactions with Cp 2 Zn afford heteroatomic closo-clusters [Ge 9 Zn] 2− , which, due to their electron donor and acceptor capabilities, allow for inter-cluster growth to form 1 ∞ [{Zn[(η 4 :η 1 -Ge 9 )]} 2− ] polymers [7]. In the case of the clusters of the heavier homologues Sn and Pb, the migration of Cu + into the polyhedral clusters under the loss of the original ligand sphere and formation of the endohedrally filled [Cu@E 9 ] 3− (E = Sn, Pb) clusters was observed for CuMes [8]. By contrast, the reaction of CuMes with the smaller tetrahedral Zintl clusters [E 4 ] 4− leads to the mesitylcopper complexes [(MesCu) 2 (η 3 -E 4 )] 4− (E = Si, Ge) [9,10]. Additionally, larger intermetalloids [Au 3 Ge 18 ] 5− and [Au 3 Ge 45 ] 4− were obtained in the reaction of bare [Ge 9 ] 4− clusters with (PPh 3 )AuCl [11,12]. In recent studies, we introduced N-heterocyclic carbene ligands  [13]. Thus, the obtained species can be regarded as low-temperature intermediates on the way to larger intermetalloid compounds. These products show that bare tetrel Zintl clusters can act as ligands in organometallic complexes, and that subsequent reactions of such species can lead to larger intermetalloids. However, in some cases, unforeseen oxidation reactions of the bare Zintl clusters can also occur, and it should be mentioned that, due to their highly negative charges, homogenous reactions of bare Zintl clusters usually require highly polar solvents (ethylendiamine, dmf, NH 3 (l)).
In continuation of our work, we examined the reactivity of coinage metal carbene compounds towards silylated Zintl clusters and studied the impact of a tris-silylated cluster ligand, bearing the sterically less bulky silyl group [Si( i Pr) 3 ] + on the structure of the resulting Cu-NHC complex. Furthermore, we investigated the reactivity of coinage metal carbene compounds towards solutions obtained from the silylation of the Zintl phase K 12 Ge 17 instead of the previously used phase K 4 Ge 9 . Finally, we studied the reactivity of NHCMCl (M = Cu, Ag, Au) complexes towards the bis-silylated cluster compound [Ge 9 {Si(TMS) 3 } 2 ] 2− .

Results and Discussion
In analogy to the synthesis of NHC Dipp Cu{η 3 -Ge 9 R 3 }, comprising the bulky silyl groups [Si( i Bu) 3 ] + or [Si(TMS) 3 ] + , we prepared the novel Zintl cluster coinage metal NHC complex with the less bulky silyl group [Si( i Pr) 3 ] + (1) by reacting the respective silylated cluster species [Ge 9 {Si( i Pr) 3 } 3 ] − with NHC Dipp CuCl in acetonitrile (Scheme 1). 1 H-NMR studies reveal a signal ratio of 3:1 for the signals assigned to the i Pr substituents (silyl groups) and the NHC Dipp ligand. Remarkably, the proton signals of the isopropyl groups (CH 3 and CH) have identical chemical shift values (pseudo-singlet), independent of the applied solvent (C 6 D 6 , thf-d 8 ), as also confirmed by 2D-NMR investigations (HSQC), in which the protons can be assigned to the carbon atom they are bound to (in the 13 C-NMR spectrum the signals of the CH 3 and the CH groups of the isopropyl units are split; Supporting Information; Figure   Recrystallization of 1 from toluene at −40 • C resulted in red crystals, suitable for single crystal X-ray diffraction. The purity of the sample was confirmed by elemental analysis of the red solid obtained by grinding the crystals. In 1, the [NHC Dipp Cu] + moiety is coordinated to the tri-capped trigonal prismatic [Ge 9 {Si( i Pr) 3 } 3 ] − cluster via one of the triangular faces in a η 3 -fashion with a mean Cu-Ge distance of 2.5328(9) Å ( Figure 2; bond lengths are summarized in Table 1).  Table 1 and the Supporting Information (Table SI 1 The Cu + center is linearly coordinated by the NHC and Zintl cluster ligands (177.7 • ; Table 1), and the Cu-C1 (C1: carbene carbon) bond length of 1.951(3) Å is in the range of previously reported data [24]. However, the orientation of the NHC Dipp ligand at Cu + with respect to the silyl substituents of the Zintl cluster in 1 differs from that in NHC Dipp Cu{η 3 -Ge 9 R 3 } with the bulkier ligands [Si( i Bu) 3 ] + or [Si(TMS) 3 ] + (Figure 3b,c). In 1, the NHC Dipp ligand shows a more ecliptic arrangement towards two of the [Si( i Pr) 3 ] + groups attached to the [Ge 9 ] unit (Figure 3a), whereas in the analogous compounds with the larger silyl groups one di-isopropyl (Dipp) wingtip of the NHC ligand shows a staggered arrangement towards two of silyl groups, and consequently the other wingtip is situated directly above the third silyl substituent (Figure 3b,c). As a result, the coordination of Cu + by NHC Dipp ligand and silylated cluster deviates less from linearity in 1, than in NHC Dipp Cu{η 3 -Ge 9 R 3 } with R = Si( i Bu) 3 or Si(TMS) 3 .  3 [19] and (c) R = Si(TMS) 3 [24].
In order to explore other sources for germanium clusters, we investigated the synthesis of silylated clusters by using the Zintl phase K 12 Ge 17 as cluster source. This phase contains [Ge 9 ] 4− clusters as well as tetrahedral [Ge 4 ] 4− units. Therefore, analogous reactions might lead to the extraction of other silylated cluster species. In a previous study, we were able to isolate [Si 4 ] 4− clusters as MesCu complexes from a starting material of the composition K 12 Si 17 , carrying out reactions in NH 3 (l) [9]. In the current study, we found that the heterogenous reaction of K 12 Ge 17 with 6 eq. of Si(TMS) 3 Cl in acetonitrile also leads to a deep red suspension upon stirring over night at room temperature, in analogy to the silylation reaction of K 4 Ge 9 [16]. Table 1. Selected distances and angles in compound 1 and its analogues NHC Dipp Cu{η 3 -Ge 9 R 3 } (R = Si( i Bu) 3 , Si(TMS) 3 ).

Distances [Å]
1 R = Si( i Bu) 3  After filtration, the addition of a solution of NHC Dipp CuCl in acetonitrile to this deep red filtrate immediately led to the formation of a brownish precipitate, which was isolated by filtering off the supernatant solution. The solid was then dissolved in toluene, and the mixture was filtered to remove KCl formed during the reaction. Subsequently, the solvent was removed in vacuo yielding the crude product as a brown solid. 1

H-NMR measurements indicated the attachment of [NHC Dipp
Cu] + to [Ge 9 ] by a significant divergence of the doublets assigned to the methyl groups of the diisopropylphenyl wingtips of the NHC ligand, which had previously been observed upon similar reactions [19,24]. Furthermore, the 1:1 signal ratio of the signals assigned to the protons of the hypersilyl groups and those of the NHC Dipp ligand, as well as the poor solubility of the product in acetonitrile and its good solubility in non-polar solvents such as thf and toluene suggested the presence of an uncharged species according to a composition of (NHC Dipp Cu) 2 {η 3 -Ge 9 (Si(TMS) 3 ) 2 } (2).
In ESI-MS examination of an acetonitrile solution of the product, signals were detected at m/z 1600. 8   Single crystals of 2 suitable for an X-ray diffraction structure determination were obtained by recrystallization from toluene at −40 • C. The analysis of the obtained diffraction data confirmed the assumptions derived from the NMR and ESI-MS experiments, and revealed the first dinuclear Zintl cluster coinage metal NHC complex (NHC Dipp Cu) 2 {η 3 -Ge 9 R I 2 } (R I = Si(TMS) 3 ) (2). The central [Ge 9 ] unit adopts the shape of a distorted, C 2v -symmetric, monocapped square antiprism, in which two opposite Ge atoms (Ge6 and Ge8) of the open square (Ge6-Ge7-Ge8-Ge9) carry the silyl groups.
The two [NHC Dipp Cu] + moieties coordinate in a η 3 -fashion to the two opposed trigonal faces of the [Ge 9 ] unit adjacent to the uncapped rectangle (Ge6-Ge7-Ge8-Ge9) and include the germanium atoms (Ge7 or Ge9), which do not bind to a silyl group (  Table 2) than that in 1 (Table 1). Furthermore, the two NHC Dipp moieties show a staggered orientation towards themselves and the silyl groups (Figure 5b). Both of these observations can be explained by interactions between the TMS groups of the hypersilyl ligands with the Dipp wingtips of the NHC ligands.  Table 2 and in the Supporting Information (Table SI 2).

General
All manipulations were performed under oxygen-free, dry conditions in an argon atmosphere using standard Schlenk or glove box techniques. Glassware was dried prior to use by heating it in vacuo. The solvents used were obtained from an MBraun Grubbs apparatus. All other commercially available chemicals were used without further purification. K 4 Ge 9 was prepared by fusion of stoichiometric amounts of the elements in stainless-steel tubes at 650 • C, and K 12 Ge 17 was synthesized by fusion of stoichiometric amounts of the elements in tantalum containers at 800 • C. 1,3-Bis(2,6diisopropylphenyl)imidazolium chloride, 1,3-dimesitylimidazolium chloride and the corresponding coinage metal halide complexes as well as K 2 [Ge 9 R I 2 ] (R I = Si(TMS) 3 ) and K[Ge 9 R 3 ] (R = Si( i Pr) 3 ) were synthesized according to modified literature procedures [14,16,[25][26][27][28]. All filtrations performed within this work were carried out using Whatman filter papers.

Single Crystal Structure Determination
The air-and moisture-sensitive crystals of 1 and 2 were transferred from the mother liquor into cooled perfluoroalkylether oil under a cold stream of N 2 gas. For diffraction data collection, the single crystals were fixed on a glass capillary and positioned in a 150 K (1) or 100 K (2) cold N 2 gas stream using the crystal cap system. Data collection was performed with a Bruker AXS D8 diffractometer (Mo-Kα radiation) (2) or a STOE StadiVari (Mo-Kα radiation) diffractometer equipped with a DECTRIS PILATUS 300K detector (1). Structures were solved by Direct Methods (SHELXS-2014) and refined by full-matrix least-squares calculations against F 2 (SHELXL-2014) [29]. The positions of the hydrogen atoms were calculated and refined using a riding model. Unless stated otherwise, all non-hydrogen atoms were treated with anisotropic displacement parameters. The supplementary crystallographic data for this paper have been deposited with the Cambridge Structural database and are available free of charge via www.ccdc.cam.ac.uk/data_request/cif. The crystallographic data for compounds 1 and 2 are summarized in Table 3. In compound 1, the electron density of a disordered toluene molecule was taken care of by the PLATON squeeze function [30]. Furthermore, some reflections were affected by the beamstop, and therefore they were excluded for refinement. In compound 2 one of the hypersilyl substituents is disordered and was refined on split positions. Furthermore, the electron density of a disordered toluene molecule was taken care of by the PLATON squeeze function [30].

NMR Spectroscopy
Sample preparation was performed in a glove box. NMR spectra were measured on a Bruker Avance Ultrashield 400 MHz spectrometer. The 1 H-and 13 C-NMR spectra were calibrated using the residual proton signal of the used deuterated solvents [31]. Chemical shifts are reported in parts per million (ppm) relative to TMS, with the residual solvent peak serving as internal reference. Abbreviations for signal multiplicities are: singlet (s), doublet (d), triplet (t), heptet (hept), multiplet (m). The evaluation of the spectra was carried out using the MestReNova program.

Electron Spray Ionization Mass Spectrometry (ESI-MS)
The sample preparation for the ESI-MS experiments was done in a glove box. ESI MS analyses were performed on a Bruker Daltronic HCT mass spectrometer (dry gas temperature: 300 • C; injection speed 240 µL/s), and the data evaluation was carried out using the Bruker Compass Data Analysis 4.0 SP 5 program (Bruker, Bremen, Germany). Spectra were plotted using OriginPro2016G (Origin Lab) and Excel 2016 (Microsoft).

Elemental Analyses (EA)
Elemental analyses were carried out in the micro-analytical laboratory of the Chemistry Department of Technische Universität München. Analyses of C, H, N were performed in a combustion analyzer (elementar vario EL, Bruker).

Syntheses
NHC Dipp Cu{η 3 -Ge 9 (Si( i Pr) 3 ) 3 } (1) K 4 Ge 9 (121 mg, 0.150 mmol) was treated with an acetonitrile solution (3 mL) of Si( i Pr) 3 Cl (87 mg, 0.450 mmol). A deep red reaction mixture was obtained after stirring at r.t. over night. The suspension was filtered to remove remaining solids, and a solution of NHC Dipp CuCl (73 mg, 0.150 mmol, 1 eq.) in acetonitrile (1.5 mL) was added, which led to the immediate formation of an orange precipitate. The supernatant solution (slightly orange) was filtered off, and the residue was washed with acetonitrile (2 mL). After removal of the solvent in vacuo, the solids were dissolved in toluene (1 mL) and filtered to remove KCl formed upon the reactions. The sample was stored in a freezer at −40 • C for crystallization yielding the pure product as red block-shaped crystals (116 mg, 50%), suitable for single crystal X-ray diffraction.   29.08 (s, CH iPr ), 25.15 (s, Me iPr ), 24.82 (s, Me iPr ), 20.97 (s, Me iPr(silyl) ), 15.76 (s, CH iPr(silyl) ). [*] Signal overlaps with the residual solvent signal of C 6 D 6 . Therefore, no multiplicity of the signal or a certain area for a multiplet could be determined. The integral of the signal is assumed to be 4, according to the 4 protons in meta-position at the phenyl ring causing this signal.
(NHC Dipp Cu) 2 {η 3 -Ge 9 (Si(TMS) 3 ) 2 } (2) Route A: K 12 Ge 17 (204.5 mg, 0.120 mmol) was treated with Si(TMS) 3 Cl (204.0 mg, 0.720 mmol) in a heterogeneous reaction in acetonitrile (2 mL) to form a deep red solution upon stirring at r.t. over night. The mixture was filtered, and a solution of NHC Dipp CuCl (117 mg, 0.240 mmol) in acetonitrile (2 mL) was added, instantly leading to the formation of a brown precipitate. The mixture was stirred for 15 min at r.t. to assure complete conversion of the reactants. After filtering off the supernatant solution (deep red) and removal of the solvent in vacuo, a brown solid was obtained. The solid was dissolved in toluene (3 mL) and filtered to remove KCl formed during the reaction. The toluene solution was concentrated to half of its original volume and placed in a freezer at −40 • C for crystallization, yielding red plate-shaped crystals of the product (74 mg, 30%), suitable for single crystal X-ray diffraction.

Route B:
A solution of K 2 [Ge 9 R 2 ] (R: Si(TMS) 3 ) (92 mg, 0.075 mmol) in acetonitrile (1 mL) was treated with an acetonitrile solution (1.5 mL) of NHC Dipp CuCl (73 mg, 0.150 mmol), instantly leading to the formation of a brownish precipitate. The mixture was stirred for 15 min at r.t. to assure complete conversion of the reactants. Subsequently, the supernatant solution (slightly red) was filtered off, and the residue was dried in vacuo. The residue was dissolved in toluene (2 mL) and filtered to remove KCl formed during the reaction, and then the solution was concentrated to half of its original volume. The sample was stored in a freezer at -40 • C for recrystallization, yielding red crystals (69 mg, 45%) of the product.  7.42 (m, 4H, CH Ph(p) ), 7.32 (d, 3 J HH = 7.8 Hz, 8H, CH Ph(m) ), 7.23 (s, 4H, CH Im ), 2.77 (hept, 3 J HH = 6.8 Hz, 8H, CH iPr ), 1.46 (d, 3 J HH = 6.9 Hz, 24H, Me iPr ), 1.09 (d, 3 J HH = 6.9 Hz, 24H, Me iPr ), 0.05 (s, 54H, Me TMS ).   (3 mL) and filtered to remove KCl formed during the reaction. The toluene solution was concentrated to half of its original volume and placed in a freezer at −40 • C for crystallization. However, crystals suitable for single crystal X-ray diffraction could not be obtained yet. The crude product was obtained as a brownish solid after removal of toluene (51 mg, 20%).

Route B:
A solution of K 2 [Ge 9 R 2 ] (R: Si(TMS) 3 ) (92 mg, 0.075 mmol) in acetonitrile (1 mL) was treated with an acetonitrile solution (1.5 mL) of NHC Dipp AgCl (80 mg, 0.150 mmol), instantly leading to the formation of a brownish precipitate. The mixture was stirred for 15 min at r.t. to assure complete conversion of the reactants. The supernatant solution (slightly red) was filtered off, and the residue was dried in vacuo. The residue was dissolved in toluene (2 mL) and filtered to remove KCl formed during the reaction, and the solution was concentrated to half of its original volume. The sample was stored in a freezer at −40 • C. However, crystals suitable for single crystal X-ray diffraction could not be obtained yet. The crude product was obtained as a brownish solid after removal of toluene (40 mg, 25%).
(NHC Dipp Au) 2 {η 3 -Ge 9 (Si(TMS) 3 ) 2 } (4) Route A: K 12 Ge 17 (204.5 mg, 0.120 mmol) was treated with Si(TMS) 3 Cl (204.0 mg, 0.720 mmol) in a heterogeneous reaction in acetonitrile (2 mL) to form a deep red solution upon stirring at r.t. over night. The mixture was filtered, and a solution of NHC Dipp AuCl (149 mg, 0.240 mmol) in acetonitrile (3 mL) was added, instantly leading to the formation of a brown precipitate. Filtering off the supernatant solution (deep red), and removal of the solvent in vacuo led to a brown solid which was dissolved in toluene (3 mL) and filtered to remove KCl formed during the reaction. The toluene solution was concentrated to half of its original volume and placed in a freezer at −40 • C for crystallization. However, crystals suitable for single crystal X-ray diffraction could not be obtained yet. The crude product was obtained as a brownish solid after removal of toluene (83 mg, 30%).

Route B:
A solution of K 2 [Ge 9 R 2 ] (R: Si(TMS) 3 ) (92 mg, 0.075 mmol) in acetonitrile (1 mL) was treated with an acetonitrile solution (2 mL) of NHC Dipp AuCl (93 mg, 0.150 mmol), instantly leading to the formation of a brownish precipitate. The mixture was stirred for 15 min at r.t. to assure complete conversion of the reactants. The supernatant solution (light red) was filtered off, and the residue was dried in vacuo. The residue was dissolved in toluene (2 mL) and filtered to remove KCl formed during the reaction, and the solution was concentrated to half of its original volume. The sample was stored in a freezer at −40 • C. However, crystals suitable for single crystal X-ray diffraction could not be obtained yet.
The crude product was obtained as a brownish solid after removal of toluene (52 mg, 30%).   A solution of K 2 [Ge 9 R 2 ] (R: Si(TMS) 3 ) (92 mg, 0.075 mmol) in acetonitrile (1 mL) was treated with an acetonitrile solution (2 mL) of NHC Mes CuCl (60.5 mg, 0.150 mmol), instantly leading to the formation of a brownish precipitate. The mixture was stirred for 15 min at r.t. to assure complete conversion of the reactants. The supernatant solution (light red) was filtered off, and the residue was dried in vacuo. The residue was dissolved in toluene (2 mL) and filtered to remove KCl formed during the reaction. The sample was stored in a freezer at −40 • C. However, crystals suitable for single crystal X-ray diffraction could not be obtained yet. The crude product was obtained as a brownish solid after removal of toluene (59 mg, 40%).   21.72 (s, CH Me(p) ), 18.69 (s, Me Me(o) ), 3.18 (s, Me TMS ).

Conclusions
Within this work, we studied the silylation reaction of K 12 Ge 17 and the reactivity of bis-and tris-silylated [Ge 9 ] clusters towards coinage metal carbene complexes NHC Dipp MCl (M: Cu, Ag, Au). The reaction of K 12 Ge 17 with 6 eq. of Si(TMS) 3 Cl yielded the bis-silylated cluster [Ge 9 {Si(TMS) 3 } 2 ] 2− as the main product in solution, contrasting the analogue reaction of K 4 Ge 9 , which exclusively results in the formation of [Ge 9 {Si(TMS) 3