X-RAY CRYSTALLOGRAPHIC STUDIES OF NICKEL(II) COMPLEXES OF TETRADENTATE [N2S2]2- LIGANDS .3. A STEPWISE DISTORTION FROM SQUARE-PLANAR TO PSEUDOTETRAHEDRAL GEOMETRIES

A series of neutral nickel(II) compounds of the tetradentate ligands N,N'-ethylene-, N,N'-trimethylene-, and N,N'-tetramethylenebis(methyl 2-amino-1-cyclopentenedithiocarboxylate) ((CH2)n bridge: Ni n = 2, I; Ni n = 3, II; Ni n = 4, III) were prepared by reaction of the ligands with nickel acetate in methanol. The ligands differ only by the number of methylene groups, n, bridging the imine nitrogens of each half of the ligand. Trends in the physical characteristics of the complexes in solution suggested distortion from planar geometries and population of a triplet spin state. Crystals of each complex suitable for X-ray diffraction studies have been examined on a Nicolet R3m/mu diffractometer, thus allowing an analysis of the relationships between structure and spectral properties in a closely related series of NiN2S2 complexes. NiC16H22N2S4.CH2Cl2 (I) crystallizes in the space group C2/c with a = 31.18 (1) angstrom, b = 7.529 (5) angstrom, c = 26.78 (1) angstrom, beta = 136.86 (3)-degrees, V = 4298.5 angstrom-3, and Z = 8. NiC17H24N2S4 (II) also crystallizes in the space group C2/c with a = 28.35 (1) angstrom, b = 9.015 (5) angstrom, c = 15.939 (7) angstrom, beta = 107.47 (4)-degrees, V = 3886.1 angstrom-3, and Z = 8. NiC18H26N2S4 (III) crystallizes in the space group P1BAR with a = 8.230 (3) angstrom, b = 9.869 (4) angstrom, c = 13.460 (4) angstrom, alpha = 105.87 (3)-degrees, beta = 95.04 (3)-degrees, gamma = 104.78 (3)-degrees, V = 1002.0 angstrom-3, and Z = 2. Block-diagonal least-square refinements of the trial structures result in discrepancy indices of R = 4.53%, 4.32%, and 5.29%, respectively, for 1849, 2874, and 3500 used reflections. The NiSS' and NiNN' planes intersect to form dihedral angles of 3.4, 18.9, and 38.6-degrees, for I, II, and III, respectively, displaying a systematic distortion from square-planar to pseudotetrahedral coordination geometry. Bond distances suggest thiolate-like S-Ni bonds and a large amount of electron delocalization in the pi-system of the six-membered chelate rings. As the dihedral angle increases, the S-Ni bond lengths decrease, while the N-Ni bond lengths increase. H-1 and C-13 solution NMR studies, as well as solid-state C-13 CP/MAS NMR studies, indicate significant contact shifts of resonances, resulting from singlet-triplet ground-state equilibrium and extensive electron delocalization of odd spin into the HOMO. The possible relevance to N2S2 ligand systems of biological importance and their coordinating geometries is presented.