A mononuclear copper(I) complex with N-[(4-methyl-2-pyrimidinyl)methylene]p-toluidine (1 center dot BF4) and a dinuclear copper(I) complex with N,N'-bis[(4-methyl-2-pyrimidinyl)methylene]-p-phenylenediamine (2 center dot(BF4)(2)) were synthesized as BF4- salts to evaluate the influence of the imine moiety on the pyrimidine ring rotation isomerism. 1 center dot BF4 existed in solution as a mixture of two isomers; 2 center dot(BF4)(2) was present as a mixture of three isomers. The redox potentials of the copper centers were changed by pyrimidine ring rotation. Comparison of 1(+) and 2(2+) indicated that increasing the steric congestion around the copper center increased the oil isomer ratio; the redox potentials of both the o- and i-isomers shifted in the positive direction, and the Cu-II/I redox reaction became slower.