Synthesis and Characterization of a Series of (PP^RP)Co Complexes and Comparison of their Electronic Properties and...

The substituent on the central cyclic diamido phosphorus fragment of a tridentate bis-(phosphine)-pincer ligand framework (PP^RP) 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 (PP^RP)-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 (PP^RP)-CoI₂ compounds and calculated ν-(CO) stretching frequencies of hypothetical (PP^RP)-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^RP)-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.