Abstract
Although electronically equivalent with the oxalate-bridge (Cp2Ti)2(µ-C2O4) (2) and the corresponding nitrogen-containing (Cp2Ti)2(µ-C2(NR)4) (3), the tetrathiolate-bridged (Cp2Ti)2(µ-C2S4) (1) (where Cp denotes η5-C5H5) exhibits sharply contrasting magnetic and electrochemical behavior at room temperature. 1 is diamagnetic and displays two quasi-reversible one-electron reductions, whereas 2 and 3 are paramagnetic (with two unpaired electrons) and only undergo single two-electron oxidations. The geometry of the TiX2C2X2Ti core in 1 (X = S) also differs markedly from those in 2 (X = O) and 3 (X = NR) in that its C2X4bridging ligand possesses a significantly shorter C–C bond (viz., 1.41 Å in 1 vs 1.523 (1 1) Å in 2 and 1.504 (6) Å in 3) and the folding angles (α) of the two TiX2planes out of the X2C2X2plane are much larger (viz., 46.5° (trans) in 1 vs 11.4° (cis) in 2 and 11.1° (trans) in 3). Molecular orbital calculations were carried out with the parameter-free Fenske-Hall model on the tetrathiolate-bridged 1, the oxalate-bridged 2, and their hypothetical ions. In order to assess the effects of change of geometry upon electronic structure, calculations were also performed on several alternative configurations of 1 and 2 (viz., C2vcis and C2htrans isomers of 2 with α = 11.4°; C2htrans isomers of 1 and 2 with α = 0.0°, 11.4°, and 46.5°). These calculations provide a quantitative electronic basis for the stereophysical differences arising from the formal replacement of the first-row oxygen atoms in the bridging C2X4ligand in 2 with less electronegative sulfur atoms in 1. A low-energy π-acceptor C2S4orbital (corresponding to the b3uLUMO for “free” C2S42-under symmetry) is the main contributor (60%) to the filled HOMO and is therefore responsible for the extensive electron delocalization from the metal centers in 1. The strong bonding interactions of this π-acceptor C2S4orbital with the antisymmetrical dititanium symmetry combination of the valence dx2-dz2Ti AOs produces a nondegenerate HOMO that accounts for the spin pairing of the unpaired electron on each Ti in 1. In contrast, the corresponding higher energy π-acceptor C2O42-orbital in 2 is energetically inaccessible such that the symmetrical and antisymmetrical dititanium combinations of the 3dx2-y2/3dz2Ti AOs form an accidentally degenerate set of half-occupied HOMOs. This MO representation is completely consistent with the valence-bond description in which 2 possesses an electron-localized configuration involving the interaction of a C2O42-dianion with two d1Ti(III) Cp2Ti+fragments. Atomic charge calculations obtained by the natural bond orbital method gave a significantly greater total negative charge for the C2S4ligand in 1 than for the C2O4ligand in 2. This negative charge difference is compatible with the C2S4ligand formally possessing a nonintegral oxidation state between a dianion (2-) and a tetraanion (4-); this result is also in agreement with the determined C–C distance (1.41 υ) in 1, which is intermediate between single-bond and double-bond distances. The geometry of the TiS2C2S2Ti core in 1 is strongly dependent on metal orbital overlap with the C2S4π orbitals (especially the HOMO); the large folding angles (α = 46.5°) represent an optimized balance of angular-dependent Ti-S σ- and π-type interactions. The near-planar geometry of the TiO2C2O2Ti core in 2 is due to the Ti–0 interactions being dominated by metal orbital overlap with the C2O4oxygen lone-pair orbitals (which favor α = 0.0°); the relatively weak Ti–O interactions involving the C2O4π orbitals presumably account for the small folding angles (α = 11.4°). Both the cis and trans isomers of 2 have virtually identical MO correlation diagrams and similar MO compositions; thus, the Fenske—Hall model does not account for the solid-state occurrence of the cis isomer. The different redox properties of 1 and 2 were analyzed from considerations of the HOMO/LUMO compositions. © 1991, American Chemical Society. All rights reserved.