Bridging M-Cl Bonds with Ambiphilic Phosphine-Borane Ligands

A combined experimental/theoretical study provides insight into the bridging coordination of M-Cl bonds with the ambiphilic phosphine-borane ligands iPr(2)P-o(C6H4)-BR2 (PBCy2: R=Cy: PBMes(2): R=Mes). Reaction of [PdCl(allyl)(PBCy2)] (3) with HCl affords the related dinuclear complex [PdCl(R-Cl)(PBCy2)]2 (5). Subsequent cleavage of the chloride bridge by PPh3 leads to the heteroleptic mononuclear complex trans-[PdCl2 (PPh3)(PBCy2)] (6). The solid-state structures of complexes 5 and 6 substantiate the propensity of the PBCy2 ligand to bridge Pd-Cl bonds via P -> Pcl-Cl -> B interactions. DFT calculations carried out on both the model mononuclear complexes 3* and 6* reveal that in each system the energy of the linkage isomer with a Cl -> B interaction is very similar to that without. A comparison of the solution and solid-state B-11 NMR spectroscopic data for complexes 3 and 6 suggests the possible interconversion of the bridging and B-pendant forms in solution. Bridging coordination of the PBCy2 ligand across a Rh-Cl bond is observed in the solid-state structure of the related complex [RhCl(nbd)(PBCy2)] (7). Replacement of the Cy groups at boron by Mes substituents illustrates the role of steric factors on the participation of the Lewis acid upon coordination, no Cl -> B interaction being observed in the complex [PdCl(allyl)-(PBMes(2))] (8).