Rhodium and Iridium Complexes of Bulky Tertiary Phosphine Ligands. Searching for Isolable Cationic M^III Alkylidenes

Abstract

Cyclometalated chloride complexes of rhodium and iridium based on (η⁵-C₅Me₅)­M^III fragments that result from the metalation of the xylyl substituent of a coordinated PR₂(Xyl) phosphine (Xyl = 2,6-Me₂C₆H₃) have been prepared by reaction of the appropriate metal precursor with the corresponding phosphine. For iridium, the four complexes <b>1a</b>–<b>d</b>, derived from the phosphines PⁱPr₂(Xyl), PCy₂(Xyl), PMe₂(Xyl), and PPh₂(Xyl), respectively, have been prepared, whereas for rhodium only the complexes <b>2a</b>,<b>d</b>, derived from PⁱPr₂(Xyl) and PMe₂(Xyl), respectively, have been studied. Chloride abstraction from compounds <b>1</b> and <b>2</b> by NaBAr_F (BAr_F = B­(3,5-C₆H₃(CF₃)₂)₄) leads to either cationic dichloromethane adducts or to cationic hydride alkylidene structures resulting from α-H elimination. The rhodium complexes investigated yield only dichloromethane adducts. However, in the iridium system the less sterically demanding phosphines PMe₂(Xyl) and PPh₂(Xyl) also provide dichloromethane adducts as the only observable products, whereas for the bulkier PⁱPr₂(Xyl) and PCy₂(Xyl) ligands the hydride alkylidene formulation prevails. Nonetheless, variable-temperature NMR studies reveal that in solution each of these two structures exists in equilibrium with undetectable concentrations of the other by means of facile reversible α-H elimination and migratory insertion reactions. Reactivity studies on the cationic hydride alkylidene complexes of iridium are reported as well