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)(4)Cl-2 (1) molecule in the following five crystalline solvates: 1 .0.85(C2H5)(2)O .0.15CH(2)Cl(2) (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 .1.75C(7)H(8).0.5C(6)H(14) (at 90, 110, 170, 298 K). For 1 .0.85(C2H5)(2)O 0.15CH(2)C(12) the molecule of 1 is almost symmetrical at 120 K (Co-Co distances of 2.3191(3) and 2.3304(3) Angstrom) and remains so at 296 K (2.2320(3) and 2.3667(4) Angstrom). For 1 .C4H8O the Co-3 chain is precisely symmetric at both 120 and 295 K though the Co-Co distances increase from 2.3111(4) to 2.3484(4) A 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 P2(1)/c, so that a symmetrical arrangement is no longer required and the two Co-Co distances then differ slightly (by 0.013 Angstrom). For 1 .C6H6 there is a phase change between 316 K (Pca2(1)) and 260 K (Pna2(1)). At all four temperatures, however, the molecule is almost symmetrical, with the two independent Co-Co distances never differing by more than 0.026 Angstrom. 1 .1.75C(7)H(8 .)0.5C(6)H(14) 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) Angstrom 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 Angstrom) at 90 K while in the second the difference decreases to only 0.006 Angstrom 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.