ELECTRON-SPIN ECHO ENVELOPE MODULATION (ESEEM) SPECTROSCOPY OF COBALT(II) BIS(DIMETHYLGLYOXIMES) - EQUATORIAL CO-N COUPLING PARAMETERS

We present the first direct measurement of Co-N(equatorial) coupling p arameters, obtained from electron spin echo envelope modulation (ESEEM ) spectroscopy, of trans-bis (dimethylglyoximato)bis(methanol)- and -b is(pyridine)cobalt(II) species. ESEEM spectroscopy provides a means to detect changes in unpaired spin density in the equatorial plane. Simu lation of Fourier transformed ESEEM spectra of the low-spin cobalt(II) dimethylglyoxime complexes, collected in frozen methanol/toluene solu tion, reveal coupling parameters from four nearly magnetically equival ent; equatorially coordinated N-14 nuclei. Modulation of Co-N(equatori al) coupling is observed when the sigma-bonding strength of the axial ligands is increased from weakly coordinated methanol solvent to the m ore strongly coordinated bis(pyridine) complex. Nuclear hyperfine and quadrupole coupling parameters for the bis(methanol) and bis(pyridine) species are as follows: A(iso) = 2.1 +/- 0.1 MHz, e(2)qQ = 3.40 +/- 0 .05 MHz; and A(iso) = 1.17 +/- 0.05 MHz, e(2)qQ = 3.30 +/- 0.05 MHz, r espectively. Reduction in the magnitude of Co-N(equatorial) coupling f or the dimethylglyoxime species is consistent with a decrease in unpai red spin density on cobalt upon coordination of pyridine. Localization of the principle axis of the nuclear quadrupole interaction within th e cobalt(II) bis(dimethylglyoxime) plane (Euler angle beta = ca. 95 de grees) emphasizes the magnetic differences between dimethylglyoximes a nd porphine systems where the value for beta is consistently reported near 45 degrees (Magliozzo, R. S.;Peisach, J. Biochemistry, 1993, 33, 8446).