Influence of coordinated ligands in a series of inorganic cobaloximes

The influence of the axial ligand on the Co–pyridine bond distance and on the extent of the pyridine dissociation was investigated in a series of cobaloximes featuring different inorganic ligands (X = Cl, N3, NO2) together with the corresponding metallorganic derivatives: ClCo(dmgH)2Py (1), N3Co(dmgH)2Py (2), NO2Co(dmgH)2Py (3), EtCo(dmgH)2Py (4), ClCo(dpgH)2Py (5) and EtCo(dpgH)2Py (6). The molecular structures of NO2Co(dmgH)2Py (3), EtCo(dmgH)2Py (4) and EtCo(dpgH)2Py (6) were determined by X-ray crystallography. The analysis of the bond distances in the X–Co–Py fragment showed an increment of about 0.1 Å in the Co–Py bond distance when passing from the chloride derivatives 1 or 5 to the ethyl derivatives 4 or 6. In the dmgH series, the Co–Py bond distance increases in the order 1 < 2 < 3 < 4. The dissociation reactions in which the pyridine ligand is displaced by the solvent was also explored via NMR. At 303 K, the lowest equilibrium constant was observed for 1 (Kdiss = 5.55 × 10−5 M) while the highest one (Kdiss = 1.37 × 10−3 M) was determined for 4. The corresponding standard free energy variations span over the range 3.97–5.90 kcal/mol. The topological properties of the electronic density at bond critical points in the X–Co–Py fragment revealed that the bonds between the cobalt atom and the axial ligands are predominantly ionic with slight covalent contribution.