Controlled C-H bond activation leads to orthometalation and ring-hydroxylation in Ni(II) and Pd(II) complexes of a common tridentate azophenyl-salicylaldimine ligand

Using a common tridentate azophenyl-appended salicylaldimine ligand in its deprotonated form (L-4)(-), two classical coordination complexes [Ni(L-4)(2)] (1) ((NiO2N2N)-O-II '(2) coordination) and [Pd(L-4)Cl]center dot CH2Cl2 (2) ((PdONN)-O-II ' Cl coordination), two cyclometalated complexes [M(L-4*)] (M = Ni 3 and Pd 4; (MONN)-O-II ' C coordination), and two azophenyl ring-hydroxylated complexes [M(L-4-O)] (M = Ni 5 and Pd 6; (MONN)-O-II ' O ' coordination), obtained due to selective oxidation of M-II-C bond, have been synthesized. Complex 1 is paramagnetic (S = 1) but all other complexes are uniformly diamagnetic (S = 0). For 1-6, absorption spectral and redox properties have been investigated, along with single-crystal structural analysis. The regiospecific aryl ring-hydroxylation of M-C bonds in 3 and 4 are achieved due to H2O2 and m-CPBA oxidation, respectively, affording 5 and 6. Coulometically-generated one-electron oxidation and one-electron reduction have been done on 1-6, and the nature of the resulting species has been probed by EPR and absorption spectral studies. While oxidation of 1 and 3 generate Ni(III) species, it is a resonance hybrid between Ni(III) double left right arrow Ni(II)-O(phenoxyl radical) species for 5. On the other hand, 2, 4, and 6 generate ligand radical species. Reduction of 1-6 generates ligand radical species uniformly. Density functional theory (DFT) calculations at the B3LYP level of theory have been done to extract information about the electronic structure of the complexes. Time-dependent (TD)-DFT calculations have been done to shed light on the origin of observed absorption spectra. Plausible mechanisms have been proposed for the observed orthometalation and ring-hydroxylation reactions.