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.

Oxidative coupling of silylated nonagermanide clusters.

Polyhedral main group element clusters of tetrel elements are discussed as suitable building units to form atom-precise nano-structures. Herein we report the oxidative coupling of two [Ge9{Si(TMS)3}2]2- clusters (TMS = trimethylsilyl) resulting in the dimeric cluster [Ge9{Si(TMS)3}2]22-. The dimer is structurally characterized as the [NHCiPrCu]+ adduct {NHCiPrCu[Ge9{Si(TMS)3}2]}2 [NHCiPr = 1,3-di(isopropyl)imidazolylidine]. The linkage of two molecular [Ge9{Si(TMS)3}2]2- anions under formation of an exo Ge-Ge bond occurs in the presence of Cy2BCl (Cy = cyclohexyl) and is mediated by trace amounts of oxygen as indicated by the isolation of the by-product Cy2B-O-BCy2.

Filled trivacant icosahedra as building fragments in 17-atom endohedral germanides [TM2@Ge17]n- (TM = Co, Ni).

The syntheses and the characterization of two 17-atom endohedral Ge clusters, [Co2@Ge17]6- (1a) and [Ni2@Ge17]4- (2a), are reported. The anions 1a and 2a, which close the gap between the known 16- and 18-atom Ge clusters, are investigated by single crystal X-ray diffraction and by quantum chemical calculations. The structures mark a new example on the pathway for cluster growth towards larger clusters with icosahedral symmetry. Furthermore, the [Co@Ge10]3- anion (3a) is obtained from liquid ammonia.

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).

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.

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.