Tuning the metal - metal bonds in the linear tricobalt compound Co₃(dpa)₄Cl₂: Bond-stretch and spin-state isomers

Sixteen crystal structures have been determined for the Co₃(dpa)₄Cl₂ (1) molecule in the following five crystalline solvates: 1·0.85(C₂H₅) ₂O·0.15CH₂Cl₂ (at 120, 213, 296 K); 1·C₄H₈O (at 120, 295 K); 1·C₆H₆ (at 170, 213, 260, 316 K); 1·C₆H₁₂ (at 120, 213, 295 K); and 1·1.75C₇H₈·0.5C₆H₁₄ (at 90, 110, 170, 298 K). For 1·0.85-(C₂H₅)₂ O·0.15CH₂Cl₂ the molecule of 1 is almost symmetrical at 120 K (Co-Co distances of 2.3191(3) and 2.3304(3) Å) and remains so at 296 K (2.2320(3) and 2.3667(4) Å). For 1·C₄H₈O the Co₃ chain is precisely symmetric at both 120 and 295 K though the Co-Co distances increase from 2.3111(4) to 2.3484(4) Å as the temperature rises. Compound 1·C₆H₆ is isomorphous with 1·C₄H₈O at 213 and 295 K and has rigorously symmetrical molecules at these two temperatures. Between 213 and 120 K the space group changes from Pccn to P2₁/c, so that a symmetrical arrangement is no longer required and the two Co-Co distances then differ slightly (by 0.013 Å). For 1·C₆H₆ there is a phase change between 316 K (Pca2₁) and 260 K (Pna2₁). At all four temperatures, however, the molecule is almost symmetrical, with the two independent Co-Co distances never differing by more than 0.026 Å. 1·1.75C₇H₈·0.5C₆H₁₄ contains, at all temperatures between 90 and 298 K, two crystallographically independent molecules, each of which is distinctly unsymmetrical at 298 K (Co-Co distances of 2.312(2) and 2.442(2) Å for one and 2.310(2) and 2.471(2) for the other). In the first of these the distances converge to a much smaller separation (0.056 Å) at 90 K while in the second the difference decreases to only 0.006 Å at 90 K. Magnetic susceptibility measurements from 1.8 to 350 K indicate in each case that a gradual spin crossover, from a doublet to a quartet state, occurs over this temperature range.