COORDINATION OF WATER HYDROGEN-BONDED TO PYRIDINE-DERIVATIVES TO THE METHYL-1,4,8,11-TETRAAZACYCLOTETRADECANENICKEL(II) CATION IN NITROBENZENE

The axial coordination of pyridine, 2-methylpyridine, 4-methylpyridine and water (W) hydrogen-bonded to each donor solvent (S) to the methyl -1,4,8,11-tetraazacyclotetradecanenickel(II) cation ([Ni(tmc)]2+) in n itrobenzene has been investigated spectrophotometrically. Pyridine and its derivatives are hardly coordinated to [Ni(tmc)]2+ owing to steric hindrance, but the water which is hydrogen-bonded to the donor solven t as a 1: 1 complex (WS) can be easily coordinated to form the five co ordinated [Ni(tmc)(WS)]2+ cation with a coordination constant (K(MWS) in dm3 mol-1) of 6.5 for pyridine, 5.7 for 2-methylpyridine, and 11.5 for 4-methylpyridine. The smaller K(MWS) value for 2-methylpyridine is due to steric hindrance. It was found that the residual hydrogen atom of the coordinated water in WS can be hydrogen-bonded to further spec ies such as S and WS to form additional kinds of five-coordinated comp lexes, [Ni(tmc)(WS-S)]2+ and [Ni(tmc)(WS-WS)]2+. Coordination of the d oubly hydrogen-bonded water (WS-S) which is induced as a result is str onger for 4-methylpyridine than for pyridine. This reflects the substi tuent effect of 4-methyl group on the basicity. The basicity of pyridi ne and its derivatives has been discussed in connection with the elect rostatic/charge-transfer balance for hydrogen-bonding, compared with d imethyl sulfoxide.