Metal Coordination by Sterically Hindered Heterocyclic Ligands, Including 2-Vinylpyridine, Assessed by Investigation of Cobaloximes

Structural and 1H NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reported
so far. We have isolated in noncoordinating solvents the complexes CH3Co(DH)2L (methylcobaloxime, where DH
= the monoanion of dimethylglyoxime) with L = sterically hindered N-donor ligands:  quinoline, 4-CH3quinoline,
2,4-(CH3)2pyridine, and 2-R-pyridine (R = CH3, OCH3, CH2CH3, CHCH2). We have found that the Co−Nax bond
is very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longer
Co−Nax bond (2.193(3) Å) than any reported for methylcobaloximes. The main cause of the long bonds is
unambiguously identified as the steric bulk of L by the fairly linear relationship found for Co−Nax distance vs CCA
(calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. The
linear relationship improves if L basicity (quantified by pKa) is taken into account. In anhydrous CDCl3 at 25 °C, all
complexes except the 2-aminopyridine adduct exhibit 1H NMR spectra consistent with partial dissociation of L to
form the methylcobaloxime dimer. 1H NMR experiments at −20 °C allowed us to assess qualitatively the relative
binding ability of L as follows:  2,4-(CH3)2pyridine > 4-CH3quinoline ≈ quinoline ≈ 2-CH3pyridine > 2-CH3Opyridine
> 2-CH3CH2pyridine > 2-CH2CHpyridine. The broadness of the 1H NMR signals at 25 °C suggests a similar
order for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecular
hydrogen bond between the NH2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridine
relative to the Co−Nax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bind
to the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.