Synthesis, characterization, and catalytic application of mononuclear and dendritic cationic CuI iminopyridine-ligated complexes in aryl iodide hydroxylation

Abstract

A series of mononuclear (C1–C4) and dendritic G1 (DC1–DC4) and G2 (DC5–DC8) cationic CuI iminopyridine complexes of the general formula [Cu{(R-C5H3N)CH=N(nPr)-κ2- N,N}2][BF4] (C1: R = 6-Me; C2: R = H; C3: R = 6-Br; C4: R = C4H3) and [DAB-Gx-PPI-(Cu{(R-C5H3N)-κ2-N,N}y)z][BF4]n [DAB: 1,4- diaminobutane; PPI: poly(propyleneimine); for G1: x = 1, y = 4, z = 2, n = 2; for G2: x = 2, y = 8, z = 4, n = 4; DC1/DC5: R = 6- Me; DC2/DC6: R = H; DC3/DC7: R = 6-Br; DC4/DC8: R = C4H3] have been prepared and characterized by a range of spectroscopic and analytical techniques. The mononuclear and dendritic complexes were found to be active catalysts for the hydroxylation of 4-iodotoluene to p-cresol in DMSO/H2O mixtures. A positive dendritic effect on catalytic activity was observed. Furthermore, our catalyst system was found to be active for the hydroxylation of 4-iodotoluene in neat water. The active catalyst could be recycled twice before catalyst deactivation was observed. HRTEM analysis revealed that catalyst deactivation arose as a result of metal agglomeration. A series of poisoning experiments provided evidence for the mediation of hydroxylation by a homogeneous active species for both classes of pre-catalysts, and a radical-trapping experiment in combination with our experimental observations provided evidence that the reaction proceeds through a similar mechanism to that reported for Cucatalyzed halide exchange