A structural, solvent exchange and spin equilibrium study of (N,N-dimethylformamide)(R,S,R,S)-(1,4,8,11-tetramethyl-l,4,8,11- tetraazacyclotetradecane)nickel(II)

Abstract

The molecular structure of R,S,R,S-[Ni {(-N(Me)(CH₂)₂N(Me)(CH₂)_3-)₂} (OCHNMe₂)](SO₃CF₃)₂ has been determined in the solid state by X-ray diffraction methods. The geometry about nickel(11) is midway between square based pyramidal and trigonal bipyramidal with OCHNMe₂ occupying the axial or an equatorial site in these geometries respectively. The nickel(II) is displaced 0.29 Å from the least-squares tetraaza plane on the same side as the four methyl groups and OCHNMe₂. In OCHNMe₂ solution the formation of this five-coordinate high-spin species by coordination of OCHNMe₂ by its four-coordinate low-spin precursor is characterized by K(298.2 K) = 10 ± 2, ∆H = -12.8 ± 2.7 kJ mol^-1 and ∆S = -24.1 ± 9.O JK^-l mol^-1 as determined by spectrophotometry. ¹H (300 MHz) n.m.r. studies show that k_wx(298.2 K) = (1.6 ± 0.2) × 10⁷ s^-1, ∆H^‡ = 27.8 ± 0.4 kJ mol^-1 and ∆S^‡ = -13.O ± 2.0 JK^-1 mol^-1 characterize the exchange of OCHNMe₂ on R,S,R,S-[Ni {(-N(Me)(CH₂)₂N(Me)(CH₂)₃-)₂ }(OCHNMe₂)]²⁺.