The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations. The ligands feature dialkyl phosphine units (R = (i)Pr or (t)Bu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M-Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the Rh[double bond, length as m-dash]C bond to give an η(2) ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.

No abstract

An electron-rich PCsp2P ligand, incorporating N,N-dimethylamino groups para to the anchoring carbene donor of the ligand, was prepared and coordinated to iridium, producing the iridium carbene chloride 2. This species undergoes facile reaction with N2O to afford an iridaepoxide complex, 3, in which an oxygen atom has been transferred to the Ir=C bond. The rate of this reaction is significantly faster than that observed for the less electron rich, unsubstituted ligand. However, further reaction of 3 involving cleavage of one of the ligand C–C bonds was observed, producing the bis-phosphine chorido complex 4. This process was accelerated by the presence of H2. Heating 4 under H2 resulted in hydrogenolysis of the ortho-metalated phosphine ligand to give a hydrido complex (5) and decarbonylation of the acyl phosphine ligand to give, finally, Vaska’s complex analog 6. All compounds were fully characterized, and the sequence represents the dismantling of the PCsp2P ligand framework.