Probing Charge-Transfer Processes in a Covalently Linked [Ge9 ]-Cluster Imine Dyad.

C60 donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge9 ] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge9 ] cluster linked to an extended π-electron system. [Ge9 {Si(TMS)3 }2 {CH3 C=N}-DAB(II)Dipp ]- (1- ) is formed upon the reaction of [Ge9 {Si(TMS)3 }2 ]2- with bromo-diazaborole DAB(II)Dipp -Br in CH3 CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1- yields the deep green, zwitterionic cluster [Ge9 {Si(TMS)3 }2 {CH3 C=N(H)}-DAB(II)Dipp ] (1-H) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.

FLP-type nitrile activation and cyclic ether ring-opening by halo-borane nonagermanide-cluster Lewis acid-base pairs.

Even though homoatomic nine-atom germanium clusters are known for two decades, their chemical properties are still rarely investigated. We now discovered that Zintl ion main group-element clusters possess a reactive lone pair of electrons, and we show a new pathway to bind ligands with functional groups to the [Ge9] cluster core through Ge-C bond formation. We report on the reactivity of [Ge9{Si(TMS)3}2]2- (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge9{Si(TMS)3}2]2- and DAB o-tol-Br (DAB = 1,3,2-diazaborolidine; o-tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge9{Si(TMS)3}2DAB o-tol]- (1a) with direct Ge-B interactions, or in [Ge9{Si(TMS)3}2(CH2)4O-DAB o-tol]- (2a) featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DABR-Br [R: o-xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)Dipp-Br and acyclic ( i Pr2N)2BBr without Ge-B bond formation as shown for the structural characterization of the ring-opened products of thf (3, 4) and trimethylene oxide (5). In contrast to thf, the activation of CH3CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge9{Si(TMS)3}2CH3C[double bond, length as m-dash]N-DABMes]- (6a). Within the presented compounds, 3 and 4 show an unusual substitution pattern of the three ligands at the [Ge9] core in the solid state. The [Ge9] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge9] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid.

Boranyl-Functionalized [Ge9 ] Clusters: Providing the Idea of Intramolecular Ge/B Frustrated Lewis Pairs.

The unique three-dimensional structure of spherical, homoatomic nine-atom germanium clusters opens various possibilities for the spatial arrangement of functional groups. Ligands comprising lone pairs have recently been introduced in the cluster sphere, and we now report the addition of a boranyl group to the cluster featuring a Ge-B exo-cluster bond. The reaction of the twofold-silylated cluster [Ge9 {Si(TMS)3 }2 ]2- (TMS=trimethylsilyl) with 2-chloro-1,3,2-diazaborolidines DABR -Cl leads to the first boranyl-functionalized [Ge9 ] clusters [Ge9 {Si(TMS)3 }2 DABR ]- (R=methyl (1 a), iso-propyl (2 a), ortho-tolyl (3 a)). The anions 2 a and 3 a were structurally characterized as [NHCDipp Cu]+ complexes (NHCDipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine) through single crystal X-ray structure determination. Quantum-chemical calculations manifest the frustrated Lewis pair (FLP) character of the boranyl-functionalized cluster [Ge9 {Si(TMS)3 }2 BCy2 ]- (4 a).

Cluster Expansion versus Complex Formation: Coinage Metal Coordination to Silylated [Ge9] Cages.

Deltahedral nine-atom tetrel element Zintl clusters are promising building blocks for the straightforward solution-based synthesis of intermetalloid clusters through the reaction with organometallic compounds. Herein we report on novel coordination sites of metal-N-heterocyclic carbene (NHC) complexes to [Ge9] clusters and unexpected cluster isomerization. We present the synthesis of a series of coinage metal-NHC complexes of silylated [Ge9] clusters [NHCiPrCu(η4-Ge9{Si(TMS)3}3)] (1; TMS = trimethylsilyl) and [NHCRM(η4-Ge9{Si(TMS)3}2)]- (2a, M = Cu, R = iPr; 3a, M = Cu, R = Mes; 4a, M = Cu, R = Dipp; 5a, M = Ag, R = Dipp; 6a, M = Au, R = Dipp), in which the coinage metals coordinate to open rectangular cluster faces and act as additional cluster vertex atoms. Besides representing promising intermediates on the way to larger intermetalloid clusters, the formation of compound 1 shows that Cu-NHC fragments also coordinate to the open-square Ge faces of the tris-silylated [Ge9] clusters, contrasting the typical interactions with triangular faces of tris-silylated [Ge9] clusters. In compounds 3a and 4a bearing bulky NHC moieties, an unusual silyl group substitution pattern is observed in contrast to 2a, which corresponds to the silyl group arrangement of other metal complexes of bis-silylated [Ge9] clusters. In this context, potential silyl group migration mechanisms are discussed.

Early-Transition-Metal Complexes of Functionalized Nonagermanide Clusters: Synthesis and Characterization of [Cp2(MeCN)Ti(η1-Ge9{Si(TMS)3}3)] and K3[Cp2Ti(η1-Ge9{Si(TMS)3}2)2].

Anionic clusters with a homoatomic [Ge9] core are species with Ge atoms in low oxidation states. In this study their reaction with early-transition-metal complexes was investigated. We report the syntheses of Ti(III) complexes of silylated [Ge9] Zintl clusters, which are rare examples of molecular complexes with Ge-Ti interactions. The neutral species [Cp2(MeCN)Ti(η1-Ge9{Si(TMS)3}3)] (1) was obtained by the reaction of K[Ge9{Si(TMS)3}3] with [Cp2TiCl]2 in toluene as dark orange crystals, whereas the reaction of K2[Ge9{Si(TMS)3}2] with [Cp2TiCl]2 in thf and subsequent recrystallization from toluene resulted in dark green crystals of K3[Cp2Ti(η1-Ge9{Si(TMS)3}2)2] (2). Compound 2 contains the trianion [Cp2Ti(η1-Ge9{Si(TMS)3}2)2]3- (2a), which can be described either as a [Cp2Ti]+-bridged [Ge9{Si(TMS)3}2]2- dimer or as a heavily distorted tetrahedral Ti(III) complex bearing two germanide cluster ligands. Both compounds display donor-acceptor interactions between single Ge vertex atoms of the [Ge9] clusters and Ti(III). Compounds 1 and 2 were characterized by single-crystal X-ray diffraction, EPR spectroscopy, and elemental analysis.

Enhancing the Variability of [Ge9 ] Cluster Chemistry through Phosphine Functionalization.

The synthetic approach towards molecules that contain Ge atoms with oxidation state 0, and which are exclusively connected to other Ge atoms, is explored by using anionic clusters extracted from binary solids. Besides providing a novel variable method for the introduction of alkenyl moieties to [Ge9 ] cluster compounds, this work expands the spectrum of mixed-functionalized [Ge9 ] cluster anions, which are suitable for the straightforward synthesis of zwitterionic compounds upon coordination to metal cations. In detail, the synthesis of a series of mixed-functionalized [Ge9 ] clusters is reported, including [Ge9 {Si(TMS)3 }3 PRRI ] (R=tBu, RI =(CH2 )3 CH=CH2 ; 2) and [Ge9 {Si(TMS)3 }2 PRRI ]- (R and RI : alkyl, alkenyl, aryl, aminoalkyl; 3 a to 11 a, TMS: (trimethyl)silyl). In 2 and 3 a, pentenyl functionalization of the [Ge9 ] clusters was achieved by reaction of the novel chlorophosphine tBu{(CH2 )3 CH=CH2 }PCl (1) with silylated [Ge9 ] clusters. Furthermore, the reactivity of the cluster anions 3 a to 11 a towards NHCDipp MCl (NHCDipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine; M=Cu, Ag) showed a dependency on the steric demand of the phosphine either zwitterions (3-MNHCDipp to 7-MNHCDipp ) featuring P-M interactions are formed, or Ge-M coordination (8-MNHCDipp to 11-MNHCDipp ) occurs. For M=Ag, the formation of zwitterionic complexes was unequivocally proven by NMR investigations showing 1 J(31 P-107 Ag/109 Ag) spin-spin coupling.

Synthesis and Reactivity of Multiple Phosphine-Functionalized Nonagermanide Clusters.

Herein we report on the synthesis of the first multiple phosphine-substituted nonagermanide clusters in a one-step reaction from K4 Ge9 . Their reactions towards various transition metal complexes show a large variety of reactive sites. The novel threefold phosphine-functionalized [Ge9 ] clusters [Ge9 {PRRI }3 ]- {R: Ni Pr2 ; RI : Ni Pr2 (1 a) or t Bu (2 a)} are obtained by reaction of pristine [Ge9 ]4- clusters with the respective chlorophosphines. Subsequent reactions with NHCDipp CuCl yield neutral compounds (NHCDipp Cu)[Ge9 {P(Ni Pr2 )2 }3 ] (3) and (NHCDipp Cu)[Ge9 {P(Ni Pr2 )t Bu}3 ] (4), respectively. The reaction of neutral compound 3 with Cr(CO)5 (thf) yields the first uncharged fivefold substituted [Ge9 ] cluster (NHCDipp Cu)2 [Ge9 {P(Ni Pr2 )2 }2 Cr(CO)5 ] (5) via a ligand exchange reaction at the [Ge9 ] cluster core. Compounds 3 and 5 are characterized by single crystal structure determination.

On the Variable Reactivity of Phosphine-Functionalized [Ge9 ] Clusters: Zintl Cluster-Substituted Phosphines or Phosphine-Substituted Zintl Clusters.

The reaction of [(Ge9 {Si(TMS)3 }2 PtBu2 )]- with NHCMes CuCl yields [(Ge9 {Si(TMS)3 }2 )(tBu2 P)]Cu(NHCMes ) (1), which is a new derivative of the recently reported monomeric zwitterionic tetrel cluster compounds [(Ge9 {Si(TMS)3 }2 )(tBu2 P)]M(NHCDipp ) (M: Cu, Ag, Au). By contrast, the reaction of the same anion [(Ge9 {Si(TMS)3 }2 PtBu2 )]- with the more labile copper phosphine complex Cy3 PCuCl leads to the formation of [Ge9 {Si(TMS)3 }2 {(tBu)2 PCu}2 Ge9 {Si(TMS)3 }2 ] (2), which is a neutral dimeric twofold-bridged [Ge9 ] cluster compound, with the exo-bonded phosphine substituent being involved in the cluster bridging. In case of the presence of sterically more demanding phosphines in [Ge9 {Si(TMS)3 }2 PR2 ]- [R: Mes (3) and NiPr2 (4)], reactions with NHCDipp CuCl yielded the complexes NHCDipp Cu[η3 -Ge9 {Si(TMS)3 }2 (PR2 )] [R: Mes (5) and NiPr2 (6)], comprising exclusively Cu-Ge bonds. Compounds 5 and 6 show varying reactivity in dependence of the identity of the phosphine group and represent the first examples of fourfold-substituted [Ge9 ] clusters with three different ligands bound to the [Ge9 ] cluster core. All compounds were characterized by 1 H, 13 C, 31 P, and 29 Si NMR spectroscopy. Additionally, compounds 3 and 4 were analyzed by ESI-MS, and the structures of compounds 1, 2, and 5 were characterized by single-crystal X-ray diffraction.

Low oxidation state silicon clusters - synthesis and structure of [NHCDippCu(η4-Si9)]3.

The reaction of NHCDippCuCl with the silicide phases A12Si17 (A: K, K/Rb, Rb) in NH3(l) yields [NHCDippCu(η4-Si9)]3- (1) as only the third example of a substituted [Si9] cluster. The corresponding salts A3[A(2.2.2-crypt)]3[NHCDippCu(η4-Si9)]2·26NH3 (A: K (1a), K/Rb (1b), Rb (1c)) crystallize isostructurally in the space group P1[combining macron] and have been characterized by single crystal structure determination. ESI-MS and NMR experiments reveal that the anion [NHCDippCu(η4-Si9)]3- can also be transferred to pyridine or acetonitrile solutions at low temperature. However, at room temperature dissociation under the release of the NHC ligand occurs. Variation of the NHC ligand in the [Cu-NHC]+-substituted silicide clusters is possible, whereas the reactions of K12Si17 with the corresponding NHCDippMCl (M: Ag, Au) complexes result in a partial oxidation of the silicide clusters, without attachment of [M-NHC]+, yielding [Si9]3- species.

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{η³Ge9R3} 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{η³Ge9RI2} (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{η³Ge9(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.

Derivatization of Phosphine Ligands with Bulky Deltahedral Zintl Clusters-Synthesis of Charge Neutral Zwitterionic Tetrel Cluster Compounds [(Ge9{Si(TMS)3}2)tBu2P]M(NHCDipp) (M: Cu, Ag, Au).

Reactions of silylated clusters [Ge9{Si(TMS)3}3]- or [Ge9{Si(TMS)3}2]2- with dialkylhalophosphines R2PCl (Cy, iPr, tBu) at ambient temperature yield the first tetrel Zintl cluster compounds bearing phosphine moieties. Varying reactivity of the dialkylhalophosphines toward the silylated clusters is observed depending on the bulkiness of the phosphine's alkyl substituents and on the number of hypersilyl groups at the tetrel cluster. Reactions between phosphines with small cyclohexyl- (Cy) or isopropyl- (iPr) groups and the tris-silylated cluster [Ge9{Si(TMS)3}3]- yield the novel neutral cluster compounds [Ge9{Si(TMS)3}3PR2] (R: Cy (1), iPr (2)) with discrete Ge-P exo bonds. By contrast, the bulkier phosphine tBu2PCl does not react with [Ge9{Si(TMS)3}3]- due to steric crowding. However, the reaction with the bis-silylated cluster [Ge9{Si(TMS)3}2]2- yields the novel cluster compound [Ge9{Si(TMS)3}2PtBu2]- (3). Subsequent reactions of compound 3 with NHCDippMCl (M: Cu, Ag, Au) yield the charge neutral zwitterionic compounds [(Ge9{Si(TMS)3}2)tBu2P]M(NHCDipp) (M: Cu, Ag, Au) (4-6), in which compound 3 acts as a phosphine ligand bearing a bulky tetrel Zintl cluster moiety. Compounds 4-6 also represent the first uncharged examples for 3-fold substituted tetrel Zintl clusters.