Activation of Protic, Hydridic and Apolar E-H Bonds by a Boryl-Substituted GeII Cation.

The synthesis of a boryl-substituted germanium(II) cation, [Ge{B(NDippCH)2 }(IPrMe)]+ , (Dipp=2,6-diisopropylphenyl) featuring a supporting N-heterocyclic carbene (NHC) donor, has been explored through chloride abstraction from the corresponding (boryl)(NHC)GeCl precursor. Crystallographic studies in the solid state and UV/Vis spectra in fluorobenzene solution show that this species dimerizes under such conditions to give [(IPrMe){(HCNDipp)2 B}Ge=Ge{B(NDippCH)2 }(IPrMe)]2+ (IPrMe = 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene), which can be viewed as an imidazolium-functionalized digermene. The dimer is cleaved in the presence of donor solvents such as [D8 ]thf or [D5 ]pyridine, to give monomeric adducts of the type [Ge{B(NDippCH)2 }(IPrMe)(L)]+ . In the case of the thf adduct, the additional donor is shown to be sufficiently labile that it can act as a convenient in situ source of the monomeric complex [Ge{B(NDippCH)2 }(IPrMe)]+ for oxidative bond-activation chemistry. Thus, [Ge{B(NDippCH)2 }(IPrMe)(thf)]+ reacts with silanes and dihydrogen, leading to the formation of GeIV products, whereas the cleavage of the N-H bond in ammonia ultimately yields products containing C-H and B-N bonds. The facile reactivity observed in E-H bond activation is in line with the very small calculated HOMO-LUMO gap (132 kJ mol-1 ).

Univalent gallium complexes of simple and ansa-arene ligands: effects on the polymerization of isobutylene.

Using [Ga(C6 H5 F)2 ](+) [Al(OR(F))4 ](-) (1) (R(F) =C(CF3)3) as starting material, we isolated bis- and tris-η(6) -coordinated gallium(I) arene complex salts of p-xylene (1,4-Me2 C6 H4), hexamethylbenzene (C6 Me6 ), diphenylethane (PhC2 H4 Ph), and m-terphenyl (1,3-Ph2 C6 H4): [Ga(1,4-Me2 C6 H4 )2.5 ](+) (2(+)), [Ga(C6 Me6 )2 ](+) (3(+)), [Ga(PhC2 H4 Ph)](+) (4(+)) and [(C6 H5 F)Ga(µ-1,3-Ph2 C6 H4)2 Ga(C6 H5 F)](2+) (5(2+)). 4(+) is the first structurally characterized ansa-like bent sandwich chelate of univalent gallium and 5(2+) the first binuclear gallium(I) complex without a Ga-Ga bond. Beyond confirming the structural findings by multinuclear NMR spectroscopic investigations and density functional calculations (RI-BP86/SV(P) level), [Ga(PhC2 H4 Ph)](+) [Al(OR(F))4](-) (4) and [(C6 H5 F)Ga(µ-1,3-Ph2 C6 H4)2 Ga(C6 H5 F)](2+) {[Al(OR(F) )4] (-)}2 (5), featuring ansa-arene ligands, were tested as catalysts for the synthesis of highly reactive polyisobutylene (HR-PIB). In comparison to the recently published 1 and the [Ga(1,3,5-Me3 C6 H3)2](+) [Al(OR(F))4](-) salt (6) (1,3,5-Me3 C6 H3 =mesitylene), 4 and 5 gave slightly reduced reactivities. This allowed for favorably increased polymerization temperatures of up to +15 °C, while yielding HR-PIB with high contents of terminal olefinic double bonds (α-contents=84-93 %), low molecular weights (Mn =1000-3000 g mol(-1)) and good monomer conversions (up to 83 % in two hours). While the chelate complexes delivered more favorable results than 1 and 6, the reaction kinetics resembled and thus concurred with the recently proposed coordinative polymerization mechanism.

Univalent Gallium Complexes of Simple and ansa-Arene Ligands: Effects on the Polymerization of Isobutylene.

Using [Ga(C6 H5 F)2 ]+ [Al(ORF )4 ]- (1) (RF =C(CF3 )3 ) as starting material, we isolated bis- and tris-η6 -coordinated gallium(I) arene complex salts of p-xylene (1,4-Me2 C6 H4 ), hexamethylbenzene (C6 Me6 ), diphenylethane (PhC2 H4 Ph), and m-terphenyl (1,3-Ph2 C6 H4 ): [Ga(1,4-Me2 C6 H4 )2.5 ]+ (2+ ), [Ga(C6 Me6 )2 ]+ (3+ ), [Ga(PhC2 H4 Ph)]+ (4+ ) and [(C6 H5 F)Ga(µ-1,3-Ph2 C6 H4 )2 Ga(C6 H5 F)]2+ (52+ ). 4+ is the first structurally characterized ansa-like bent sandwich chelate of univalent gallium and 52+ the first binuclear gallium(I) complex without a GaGa bond. Beyond confirming the structural findings by multinuclear NMR spectroscopic investigations and density functional calculations (RI-BP86/SV(P) level), [Ga(PhC2 H4 Ph)]+ [Al(ORF )4 ]- (4) and [(C6 H5 F)Ga(µ-1,3-Ph2 C6 H4 )2 Ga(C6 H5 F)]2+ {[Al(ORF )4 ] - }2 (5), featuring ansa-arene ligands, were tested as catalysts for the synthesis of highly reactive polyisobutylene (HR-PIB). In comparison to the recently published 1 and the [Ga(1,3,5-Me3 C6 H3 )2 ]+ [Al(ORF )4 ]- salt (6) (1,3,5-Me3 C6 H3 =mesitylene), 4 and 5 gave slightly reduced reactivities. This allowed for favorably increased polymerization temperatures of up to +15 °C, while yielding HR-PIB with high contents of terminal olefinic double bonds (α-contents=84-93 %), low molecular weights (Mn =1000-3000 g mol-1 ) and good monomer conversions (up to 83 % in two hours). While the chelate complexes delivered more favorable results than 1 and 6, the reaction kinetics resembled and thus concurred with the recently proposed coordinative polymerization mechanism.

Experimental and quantum chemical studies of anionic analogues of N-heterocyclic carbenes.

A combination of quantum chemical and synthetic/crystallographic methods have been employed to probe electronic structure in two series of anionic ligands related to the well known N-heterocyclic carbene (NHC) class of donor. Analyses of (i) the respective frontier orbital energies/compositions for the 'free' ligands and the results of ETS-NOCV studies of the bonding in model group 11 complexes; and (ii) the structural metrics for (new) linear gold(i) compounds, have been used to probe the bonding in complexes of NHC ligands which incorporate a backbone-appended weakly-coordinating anion component (WCA-NHCs) and in systems featuring the isoelectronic (formally anionic) diazaborolyl ligand family. Key findings are that WCA-NHC ligands - in which the anionic component is attached to the ligand heterocycle via a methylene (CH2) spacer - offer electronic (and steric) properties which are largely unperturbed from their 'simple' NHC counterparts, while diazaborolyl donors (in which the negative charge is formally located at the boron donor atom) offer significantly stronger σ-donation and a very high trans influence.