Manganese-chromium-cyanide clusters:: Molecular MnCr6(CN)18 and Mn3Cr6(CN)18 species and a related MnCr3(CN)9 chain compound

As part of an ongoing effort to design new single-molecule magnets, we are exploring synthetic routes to high-nuclearity metal-cyanide clusters. Here, we report the results of solution assembly reactions between [(Me(3)tacn)Cr(CN)(3)] (Me(3)tacn = N,N ' ,N " -trimethyl-1,4,7-triazacyclononane) and selected manganese(II) salts. Reaction with the perchlorate salt in the presence of AClO(4) (A = Na, K) gives A [(Me(3)tacn)(6)MnCr6(CN)(18)] (ClO4)(3), featuring a heptanuclear cluster in which six C(Me(3)tacn)Cr(CN)(3)] units surround a central Mn-II ion. The Mn coordination geometry closely approaches a trigonal prism, with triangular faces twisted away from a fully eclipsed position by an angle of 12.8 degrees and 11.3 degrees for A = Na and K, respectively. The magnetic behavior of both compounds indicates weak antiferromagnetic coupling between neighboring Mn-II and Cr-III centers (J = -3.0 and -3.1 cm(-1), respectively) to give an S = 13/2 ground state. Alternatively, addition of sodium tetraphenylborate to the reaction solution yields [(Me(3)tacn)(6)(H2O)(6)Mn3Cr6(CN)(18)] (BPh4)(6). 12H(2)O, in which attachment of two Mn-II ions to the preceding cluster generates a new species with two trigonal bipyramids sharing a common vertex. This compound displays magnetic behavior consistent with weak antiferromagnetic coupling and an S=3/2 ground state. Finally, a reaction employing manganese(II) triflate is found to produce the one-dimensional solid C(Me(3)tacn)(3)MnCr3(CN)(9)](CF3SO3)(2), exhibiting a closely related chain structure, and, again, weak antiferromagnetic coupling. (C) 2001 Academic Press.