Synthesis, Characterization, and Catalytic Properties of Iridium Pincer Complexes Containing NH Linkers

A series of tert-butyl-substituted pincer ligands based on 1,3-diaminobenzene and 3-aminophenol scaffolds, (PXCYP)-P-tBu4 (1e, X = Y = NH; 1f, X = NH; Y = O) and the corresponding iridium hydridochloro complexes ((PXCYP)-P-tBu4)IrHCl (2e, X = Y = NH; 2f, X = NH; Y = O) were prepared with moderate yields and high purity and were fully characterized by H-1 and P-31 NMR spectroscopy. Unsymmetrical hybrid pincer ligands (PNCOPtBu2)-P-R2 (1g, R = isopropyl; 1h, R = cyclohexyl) were prepared conveniently in high yield via a one-pot procedure by judiciously choosing reaction conditions and base; the corresponding iridium hydrido chloro complexes (PNCOPIrHCl)-P-iPr2-Ir-tBu2 (2g) and (PNCOPIrHCl)-P-cY2-Ir-tBu2 (2h) were synthesized by the reaction of [IrCl(COE)(2)](2) with ligands. X-ray crystallography reveals that these iridium pincer complexes adopt similar square-pyramidal geometries and exhibit strong intermolecular hydrogen bonding between the NH linker and chloride ions of the adjacent iridium complex in the solid state. H-1 NMR chemical shifts of tert-butyl based pincer ligated iridium hydrides move downfield when the electronegativity of the linker between the benzene backbone and phosphine moiety increases for 2a, 2e, 2f, and 2b. Accordingly the corresponding iridium pincer carbonyl complexes ((PXCYP)-P-tBu4)Ir(CO), 3a, 3e, 3f, and 3b show a blue shift in the CO stretching frequency. The activities of iridium complexes containing NH linkers were briefly examined for transfer dehydrogenation from cyclooctane to tert-butylethylene; ((PNCOPtBu2)-P-iPr2)IrHCl (2g) exhibits the highest activity among all tested iridium pincer complexes, including the most studied ((PCP)-P-tBu4)IrHCl (2a) and ((POCOP)-P-tBu4)IrHCl (2b). The enhanced catalytic activity could be related to combined electronic and steric effects of the NH/O hybrid linker and different alkyl groups at phosphorus. This new class of iridium pincer complexes could have great implications in catalytic transformation of polar compounds due to the strong hydrogen-bond-donating ability of the NH linker.