Increased basicity and hydrogen-bonding donor ability of water by hydrogen bonding to dimethyl sulphoxide and its coordination to 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) in nitrobenzene

Abstract

The axial coordination of the four-coordinated 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) cation ([Ni(tmc)]²⁺) by water which is hydrogen-bonded to dimethyl sulphoxide (DMSO) has been investigated spectrophotometrically in nitrobenzene (NB) containing < 0.05 mole fraction DMSO at 25 °C. Although water in NB is only weakly coordinated to the cation, water which is hydrogen-bonded to DMSO in NB can be easily coordinated to form the five-coordinated [Ni(tmc)(WS)]²⁺ cation with a coordination constant of 24.0 dm³ mol^–1, where W is water, S is DMSO, and WS is the 1:1 water–DMSO species H—O—H⋯S. (—O— is the coordinating oxygen atom and the formation constant for WS was determined by using NMR methods.) The residual hydrogen atom of the coordinated water in WS can be hydrogen-bonded to further species, including additional DMSO and either WS or NB-solvated water, to form two additional kinds of five-coordinated complexes: [Ni(tmc)(WS–S)]²⁺ and either [Ni(tmc)(WS—WS)]²⁺ or [Ni(tmc)(WS—W)]²⁺, where WS–S is S⋯H—O—H⋯S, WS—WS is S⋯H—O—H⋯OH—H⋯S, and WS—W is S⋯H—O—H⋯OH—H. The corresponding equilibrium constants for the hydrogen bonding of water were 1.5 dm³ mol^–1 for WS, 12.3 for [Ni(tmc)(WS–S)]²⁺, 8.6 for [Ni(tmc)(WS—WS)]²⁺ and 1.3 for [Ni(tmc)(WS—W)]²⁺. The increased hydrogen-bonding donor ability and basicity are discussed in terms of polarization of the O—H bond through hydrogen bonding and coordination.