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 � 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 kwx(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+.