Manganese-Chromium-Cyanide Clusters: Molecular MnCr_6(CN)_(18) and Mn_3Cr_6(CN)_(18) Species and a Related MnCr_3(CN)_(9) Chain Compound

摘要

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_3tacn)Cr(CN)_3] (Me_3tacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) and selected manganese(II) salts. Reaction with the perchlorate salt in the presence of A ClO_4 (A = Na,K) gives A[Me_3tacn]_6MnCr_6(CN)_(18)](ClO_4)_3, featuring a heptanuclear cluster in which six [Me_3tacn]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 deg and 11.3 deg for A= Na and K, respectively. The magnetic behavior of both compounds indicates weak antiferromagntic 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_3tacn)_6(H_2O)_6Mn_3Cr_6(CN)_(18)](BPh_4)_6·12H_2O,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 [(Me_3tacn)_3MnCr_3(CN)_9](CF_3SO_3)_2,exhibiting a closely related chain structure, and, again, weak antiferromagnetic coupling.