Synthesis and Characterization of a Series of (PPRP)Co Complexes and Comparison of their Electronic Properties and Catalytic Activities.

The substituent on the central cyclic diamido phosphorus fragment of a tridentate bis-(phosphine)-pincer ligand framework (PPRP) was varied systematically (R = NEt2, N i Pr2, OEt, O i Pr, OCH2CF3, OCH-(CF3)2, (-)-menthoxide, Me, CF3) to tune the steric profiles as well as the σ-donor and π-acceptor properties of the pincer ligands. A series of (PPRP)-CoI2 compounds were synthesized and characterized using 1H NMR and EPR spectroscopy and single crystal X-ray diffraction. The differences in electron density at the cobalt centers imparted by the varying ligand substituents were evaluated using cyclic voltammetry of the (PPRP)-CoI2 compounds and calculated ν-(CO) stretching frequencies of hypothetical (PPRP)-Co-(CO)-(H) compounds. In an attempt to deconvolute variations in σ-donor and π-acceptor properties, the 1 J P-Se coupling constants of a series of phosphine selenide compounds were used to evaluate basicity as a direct measure of σ-donor strength. The catalytic activities of the (PPRP)-CoI2 compounds for the hydroboration of styrene and α-methylstyrene at room temperature were evaluated quantitatively through kinetic studies to determine the rate constant (k obs ) for each precatalyst. In general, it was found that compounds featuring a less electron-rich cobalt center were more active alkene hydroboration catalysts, with steric properties playing a more prominent role with the bulkier 1,1'-disubstituted alkene substrate.