Structural Characterization of a Boron(III) η2-σ-Silane-Complex.

Coordination of H-E (E = H, C, Si, etc.) σ-bonds to unsaturated element centers, forming η2-σ-complexes, is a crucial step in H-E bond activations which are involved in numerous chemical transformations. Structural characterization of σ-complexes has provided invaluable insight into the σ-bond activation process. While numerous η2-σ-complexes of low-oxidation-state transition metals have been isolated and thoroughly studied, those based on d0 metals or high-oxidation-state main group elements still remain elusive, despite their suggested role as key intermediates in σ-bond metathesis. Here we report the synthesis of a highly electrophilic o-carboranyl-substituted hydroborenium and its η2-σ-(H-Si)-complex, the latter of which was characterized by X-ray analysis. Unlike previous known σ-(H-Si)-complexes in which back-donation contributes significantly to their stability, our computational studies revealed that no back-donation takes place between the borenium and silane moieties, and the key factors for the stability of this silane-borenium complex are (1) the strong interaction between the empty p orbital of the boron center and the bonding σ orbital of H-Si and (2) steric accessibility of the boron center due to the presence of a hydrogen substituent.

Living ring-opening homo- and copolymerisation of ε-caprolactone and L-lactide by cyclic ß-ketiminato aluminium complexes.

A series of novel aluminium complexes containing cyclic ß-ketiminato ligands of type Me2Al{O-[(ArN=CHC4H4(C6H4))]} (3a, Ar = 2,6-(i)Pr2C6H3; 3b, Ar = C6H5; 3c, Ar = C6F5) have been prepared in high yields. These complexes were identified by (1)H, (13)C NMR spectroscopy and elemental analysis. X-ray structural analyses for 3a-c revealed that these complexes have a distorted tetrahedral geometry around Al, and both bond distances and bond angles were considerably influenced by the ligand structure. These complexes were tested as catalyst precursors for ring-opening polymerisation of ε-caprolactone (ε-CL) and L-lactide (L-LA) in the presence of 2-propanol as an initiator. Complex 3a could polymerize ε-CL in a controlled manner with high efficiency. Based on the living characteristics, the preparation of well-defined block copolymers PCL-b-PLLA via sequential addition of monomers was performed by 3a. Note that complex 3c exhibited rather high catalytic activity for the ROP of L-LA with narrow molecular weight distribution. The monomer conversion reached completion only in 4 h when the L-LA/Al molar ratio was 100 at 80 °C. PLLA-b-PCL copolymers were thus easily produced by 3c.

Carbonylation reactions of intramolecular vicinal frustrated phosphane/borane Lewis pairs.

The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 4 adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound 5. The intramolecular FLP 7 contains an exo-3-B(C6F5)2 Lewis acid and an endo-2-PMes2 Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at -35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound 8. In contrast, FLP 7 is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl 9 without involvement of the adjacent phosphanyl moiety. The free FLP 7 was generated in the gas phase from its FLPH2 product 10. A DFT study has shown that the phosphonium hydrido borate zwitterion 10 is formed exergonically in solution but tends to lose H2 when brought into the gas phase.