The Co-CH(3) Bond in Imine/Oxime B(12) Models. Influence of the Orientation and Donor Properties of the trans Ligand As Assessed by FT-Raman Spectroscopy.
Near-IR FT-Raman spectroscopy was used to probe the properties of three types of methyl imine/oxime B(12) model compounds in CHCl(3) solution. These types differ in the nature of the 1,3-propanediyl chain and were selected to test the influence of electronic and steric effects on the Co-CH(3) stretching (nu(Co)(-)(CH)()3) frequency, a parameter related to Co-C bond strength. For the first type studied, [LCo((DO)(DOH)pn)CH(3)](0/+) ((DO)(DOH)pn = N(2),N(2)(')-propane-1,3-diylbis(2,3-butanedione 2-imine 3-oxime)), nu(Co)(-)(CH)()3 decreased from 505 to 455 cm(-)(1) with stronger electron-donating character of the trans axial ligand, L, in the order Cl(-), MeImd, Me(3)Bzm, 4-Me(2)Npy, py, 3,5-Me(2)PhS(-), PMe(3), and CD(3)(-). This series thus allows the first assessment of the effect of negative axial ligands on nu(Co)(-)(CH)()3; these ligands (L = Cl(-), 3,5-Me(2)PhS(-), CD(3)(-)) span the range of trans influence. The CH(3) bending (delta(CH3)) bands were observed at 1171, 1159, and 1150/1105 cm(-)(1), respectively. The decrease in C-H stretching frequencies (nu(CH)) of the axial methyl suggests that the C-H bond strength decreases in the order Cl(-) > 3,5-Me(2)PhS(-) > CD(3)(-). This result is consistent with the order of decreasing (13)C-(1)H NMR coupling constants obtained for the axial methyl group. The trend of lower nu(Co)(-)(CH)()3 and nu(CH) frequencies and lower axial methyl C-H coupling constant for stronger electron-donating trans axial ligands can be explained by changes in the electronic character of the Co-C bond. The symmetric CH(3)-Co-CH(3) mode (nu(CH)()3(-)(Co)(-)(CH)()3) for (CH(3))(2)Co((DO)(DOH)pn) was determined to be 456 cm(-)(1) (421 cm(-)(1) for (CD(3))(2)Co((DO)(DOH)pn). The L-Co-CH(3) bending mode (delta(L)(-)(Co)(-)(CH)()3) was detected for the first time for organocobalt B(12) models; this mode, which is important for force field calculations, occurs at 194 cm(-)(1) for ClCo((DO)(DOH)pn)CH(3) and at 186 cm(-)(1) for (CH(3))(2)Co((DO)(DOH)pn. The nu(Co)(-)(CH)()3 frequencies were all lower than those reported for the corresponding cobaloxime type LCo(DH)(2)CH(3) (DH = monoanion of dimethylglyoxime) models for planar N-donor L. This relationship is attributed to a steric effect of L in [LCo((DO)(DOH)pn)CH(3)](+). The puckered 1,3-propanediyl chain in [LCo((DO)(DOH)pn)CH(3)](+) forces the planar L ligands to adopt a different orientation compared to that in the cobaloxime models. The consequent steric interaction bends the equatorial ligand toward the methyl group (butterfly bending); this distortion leads to a longer Co-C bond. In a second imine/oxime type, a pyridyl ligand is connected to the 1,3-propanediyl chain and oriented so as to minimize butterfly bending. The nu(Co)(-)(CH)()3 frequency for this new lariat model was close to that of pyCo(DH)(2)CH(3). In a third type, a bulkier 2,2-dimethyl-1,3-propanediyl group replaces the 1,3-propanediyl chain. The nu(Co)(-)(CH)()3 bands for two complexes with L = Me(3)Bzm and py were 2-5 cm(-)(1) lower in frequency than those of the corresponding [L(Co((DO)(DOH)pn)CH(3)](+) complexes. The decrease in the axial nu(Co)(-)(CH)()3 frequencies is probably due to the steric effect of the equatorial ligand. Thus, the nu(Co)(-)(CH)()3 frequency can be useful for investigating both steric and electronic influences on the Co-C bond.
Hirota
Shun
S
Departments of Chemistry, Emory University, Atlanta, Georgia 30322, and Clark Atlanta University, Atlanta, Georgia 30314.
Polson
Suzette M.
SM
Puckett
James M.
JM
Moore
Scott J.
SJ
Mitchell
Mark B.
MB
Marzilli
Luigi G.
LG