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

Abstract

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 metal–phosphorus 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 [(η⁶-p-cymene)RuCl₂]₂ as starting materials. From a five-coordinate RuH(CCH₂) derivative, upon treatment with HBF₄, a cationic carbyneruthenium compound could be generated which is an active catalyst in olefin metathesis.