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.

to unsaturated element centers, forming η 2 -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 η 2 -σ-complexes of lowoxidation-state transition metals have been isolated and thoroughly studied, those based on d 0 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 η 2 -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 backdonation 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.

A series of novel aluminium complexes containing cyclic β-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.