Coordination of Water Hydrogen-Bonded to Pyridine Derivatives to the (1R,4S,8R,11S)-1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) Cation in Nitrobenzene

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Abstract
The axial coordination of pyridine, 2-methylpyridine, 4-methylpyridine and water (W) hydrogen-bonded to each donor solvent (S) to the (1R,4S,8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) cation ([Ni(tmc)]2+) in nitrobenzene 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 solvent as a 1 : 1 complex (WS) can be easily coordinated to form the five coordinated [Ni(tmc)(WS)]2+ cation with a coordination constant (KMWS in dm3 mol−1) of 6.5 for pyridine, 5.7 for 2-methylpyridine, and 11.5 for 4-methylpyridine. The smaller KMWS 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 species such as S and WS to form additional kinds of five-coordinated complexes, [Ni(tmc)(WS–S)]2+ and [Ni(tmc)(WS–WS)]2+. Coordination of the doubly hydrogen-bonded water (WS–S) which is induced as a result is stronger for 4-methylpyridine than for pyridine. This reflects the substituent effect of 4-methyl group on the basicity. The basicity of pyridine and its derivatives has been discussed in connection with the electrostatic/charge-transfer balance for hydrogen-bonding, compared with dimethyl sulfoxide.

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