Methyl B-12 models containing unsubstituted imidazole as an axial ligand investigated by structural and NMR spectroscopic methods. Evidence that mu-imidazolato-bridged dimers are formed by base addition to some analogues with macrocyclic equatorial ligands incorporating BF2

Imidazole (imH)-containing B-12 model complexes that could possibly be depr otonated at the imidazole NH to form the corresponding imidazolato (im) com plexes were investigated. Organomethyl complexes of cobaloxime and imine/ox ime (I/O) analogues with equatorial ligands possessing an O-BF2-O moiety in place of each deprotonatable O-H---O moiety of standard models were prepar ed. In some cases, NMR spectroscopy revealed formation of mu-imidazolato (m u-im) dimers in methanol-d(4) on addition of methoxide. The desired monomer ic imidazolato species was not formed at a characterizable level. We presen t the X-ray crystal structures of LCo(DBF2)(2)CH3 (where L = imH, 4-t-BuimH ) and [AsPh4][(mu-im)(Co(DBF2)(2)CH3)(2)] (DBF2 = the BF2-substituted monoa nion of dimethylglyoxime). The latter confirms the NMR solution studies and is the first structure of a B-12 model species bridged by an imidazolato m oiety. We have also structurally characterized the related I/O complexes, [ imHCo((DO)(DOBF2)pn)CH3]PF6 and [imHCo((DO)(DOH)pn)CH3]I. The former is the first reported I/O structure with a BF2 bridge. The structures presented h ere greatly increase the data available for B-12 model derivatives with imi dazole NH groups and nearly double the number of reported structures for th e LCo(DBF2)2CH3 type of B-12 models. As in all previous BF2-containing mode ls, the equatorial moieties adopt the extended chair conformation. For the LCo(DBF2)(2)CH3 (L = imH, 4-t-BuimH) compounds, this arrangement leads to a n L orientation that is unusual. Normally, the L plane bisects either the s ix-membered or the five-membered equatorial chelate rings (A and B orientat ions, respectively). The L orientation we find is intermediate between the typical A- and the B-type L orientations. Compared to that in A-type imHCo( DH2)(2)CH3 (DH = monoanion of dimethylglyoxime), the Co-N-L distance for im HCo(DBF2)(2)CH3 was longer. We attribute this lengthening to the intermedia te A-/B-type orientation. However, the distance shortens upon formation of the imidazolato dimer. The relationship of the mu-im ligand to both Co(DBF2 )(2)CH3 moieties is also intermediate A/B and hence unusual. Therefore, thi s shortening probably does not have a steric cause but results from the bet ter electron-donor ability of the imidazolato ligand. Measurements of (1)J( CH) of Co-CH3 are consistent with this conclusion.