The ambivalent behaviour of aryl-functionalized phosphines: coordination, hemilability and beyond

Aryl-functionalized phosphines of the general composition R2P(CH2)n(aryl) with bulky substituents R at the phosphorus atom react with olefin-rhodium(I) and -iridium(I) compounds to give complexes in which the phosphine behaves either as 2-electron or (2 + 6)-electron donor ligand. The aryl moiety is weakly coordinated and can be replaced by CO, acetonitrile or H2 without breaking the metalphosphorus bond. In some cases, the interaction of the phosphines with the rhodium or iridium centre leads to an insertion of the metal into one of the C-H bonds of the aryl group to afford a six-membered chelate ring system. This cyclometalation reaction not only proceeds under mild conditions but is also completely reversible as shown by addition reactions and labeling experiments. With the must bulky phosphine tBu2P(CH2)2C6H3-2,6-Me2, the isolation and structural characterization of a cis-configurated rhodium dicarbonyl cis-[RhCl(PR3)(CO)2] and of an unprecedented dinuclear alkylrhodium(III) compound built up by two 14-electron [RhCl2(alkyl)(PR3)] units has been achieved. Ruthenium(II) complexes with tBu2P(CH2)2C6H5 and tBu2P(CH2)2OC6H5 as ligands have been prepared from either RuCl3·3H2O or [( (eta)^6-p-cymene)RuCl2]2 as starting materials. From a five-coordinate RuH(=C=CH2) derivative, upon treatment with HBF4, a cationic carbyneruthenium compound could be generated which is an active catalyst in olefin metathesis.