Synthesis and Characterization of a Series of (PP ^R P)Co Complexes and Comparison of their Electronic Properties and Catalytic Activities

High Resolution Image Download MS PowerPoint Slide The substituent on the central cyclic diamido phosphorus fragment of a tridentate bis(phosphine)pincer ligand framework (PP R P) was varied systematically (R = NEt 2, N i Pr 2, OEt, O i Pr, OCH 2 CF 3, OCH(CF 3 ) 2, (−)-menthoxide, Me, CF 3 ) to tune the steric profiles as well as the σ-donor and π-acceptor properties of the pincer ligands. A series of (PP R P)CoI 2 compounds were synthesized and characterized using 1 H 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 (PP R P)CoI 2 compounds and calculated ν(CO) stretching frequencies of hypothetical (PP R P)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 (PP R P)CoI 2 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.