Influence of the Coordination of Donor Solvent Molecules to 1,4,7,10-Tetramethyl-1,4,7,10- tetraazacyclododecanenickel(II) and Analogous Nickel(II) Complexes on Their Steric Factors

Coordination equilibrium constants (K _NiS) of some donor solvent molecules to 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecanenickel(II) (Ni(Me₄12]aneN₄)]²⁺) were determined in nitrobenzene (a noncoordinating bulk solvent). The first (K _NiS1) and second stepwise coordination equilibrium constants (K _NiS2) for 1,4,7,10-tetraazacyclododecanenickel(II) (Ni(12]aneN₄)]²⁺), 1,4,8,11-tetraazac yclotetradecane- nickel(II) (Ni(14] aneN₄)]²⁺), 1,4,8,11-tetrathiacyclotetra-decanenickel(II) (Ni(14]aneS₄)]²⁺) were also reinvestigated. The K _NiS values for Ni(Me₄12]aneN₄)]²⁺ were compared to those of Ni(12]aneN₄)]²⁺, (1R,4S, 8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) (R,S,R,S-Ni(Me₄14]aneN₄)]²⁺), R,R,S,S-Ni(Me₄14]aneN₄)]²⁺, Ni(14]aneN₄)]²⁺, and Ni(14]aneS₄)]²⁺. Coordination of pyridine (Py), N,N,N′,N′-tetramethylurea (TMU), and N,N-dimethylacetamide (DMA) to Ni(Me₄12]aneN₄)]²⁺ was observed, although these donor solvent molecules did not coordinate to R,S,R,S-Ni(Me₄14]aneN₄)]²⁺. The K _NiS values for Py, TMU, and DMA are 7.9, 2.8, and 9.0 dm³⋅mol⁻¹, respectively. Some hydrogen-bonding waters were coordinated to R,S,R,S-Ni(Me₄14]aneN₄)]²⁺, but such waters did not coordinate to Ni(Me₄12] aneN₄)]²⁺. Also, the K _NiS2 values were larger than the corresponding K _NiS1 values for Ni(14]aneS₄)]²⁺. Furthermore, the K _NiS1 values for Ni(12]aneN₄)]²⁺ were the largest among these nickel(II) complex cations. The K _NiS, K _NiS1, and K _NiS2 values are discussed in terms of properties of the donor solvents and steric strains of these nickel(II) complex cations.