title: Metal coordination by sterically hindered heterocyclic ligands, including 2-vinylpyridine, assessed by investigation of cobaloximes.
abstract: 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-N(ax) bond is very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longer Co-N(ax) bond (2.193(3) A) 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-N(ax) 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 degrees 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 degrees C allowed us to assess qualitatively the relative binding ability of L as follows: 2,4-(CH3)2pyridine > 4-CH3quinoline approximately = quinoline approximately = 2-CH3pyridine > 2-CH3Opyridine > 2-CH3CH2pyridine > 2-CH2=CHpyridine. The broadness of the 1H NMR signals at 25 degrees 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-N(ax) 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.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Trieste, 34127 Trieste, Italy.
authors.forename: Patrizia
authors.initials: P
authors.lastname: Siega
authors.forename: Lucio
authors.initials: L
authors.lastname: Randaccio
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Marzilli
authors.forename: Luigi G
authors.initials: LG
authors.lastname: Marzilli
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000737:chemistry; D015394:Molecular Structure; D010091:Oximes / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*
chemical_list: D006571:Heterocyclic Compounds; D008024:Ligands; D008670:Metals; D010091:Oximes; D011725:Pyridines; D003035:Cobalt; C030953:2-vinylpyridine