Stoichiometric and Catalytic Oxidation of BINAP by Dioxygen in a Rhodium(I) Complex

The square planar complex (BINAP)Rh(CO)Cl (BINAP = 2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl) was synthesized from [Rh(COD)Cl]2 (COD = 1,5 cyclooctadiene) and BINAP under a CO atmosphere. It reacts with oxygen to form the square planar (BINAP(O))Rh(CO)Cl in approximately 50% yield, along with molar equivalents of free BINAP dioxide (BINAP(O)2) and CO2. The oxygen atom of the BINAP(O) was shown crystallographically to be trans to the CO ligand. In the presence of excess BINAP under CO/O2 gas mixtures, the reaction is catalytic with a TOF of 0.16 h-1 at ambient temperature in chloroform. The kinetics of this transformation were investigated and conditions for optimum selectivity for BINAP(O) formation suggested. A stoichiometric mechanism is proposed that involves initial formation of an O2 adduct, followed by oxygen atom transfer to both phosphorus atoms of BINAP or to one phosphorus atom and the CO ligand trans to it. These processes might be concerted, or stepwise with intermediate RhO species. To the best of our knowledge, this chemistry represents the first example of a reaction exhibiting oxygen atom addition from O2 either to both phosphorus atoms in a bisphosphine ligand or to one phosphorus atom and a CO ligand in the same complex.