Sandwich-type silicotungstates: structure and magnetic properties of the dimeric polyoxoanions[[SiM2W9O34(H2O)]2]12- (M = Mn2+, Cu2+, Zn2+).

Abstract

The novel dimeric silicotungstates [[SiM2W9O34(H2O)]2]12- (M = Mn2+, Cu2+, Zn2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic measurements. X-ray single-crystal analyses were carried out on K4Na6Mn[[SiMn2W9O34(H2O)]2].33H2O (1), which crystallizes in the triclinic system, space group P1, with a = 12.2376(7) A, b = 13.6764(8) A, c = 15.6177(9) A, alpha = 70.2860(10) degrees, beta = 79.9150(10) degrees, gamma = 70.2760(10) degrees, and Z = 1; K3Na5[[SiCu2W9O34(H2O)]2].26H2O (2) crystallizes in the triclinic system, space group P1, with a = 11.4271(12) A, b = 12.5956(13) A, c = 15.3223(16) A, alpha = 80.456(2)degrees, beta = 76.383(2) degrees, gamma = 76.968(2) degrees, and Z = 1; K4Na6[[SiZn2W9O34(H2O)]2].34H2O (3) crystallizes also in the triclinic system, space group P1, with a = 12.2596(14) A, b = 13.2555(15) A, c = 16.2892(18) A, alpha = 96.431(2) degrees, beta = 100.944(2) degrees, gamma = 110.404(2) degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[SiW9O34]10- Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn) group leading to a sandwich-type structure. Magnetic measurements show that the central Mn4 unit in 1 exhibits antiferromagnetic (J = -1.77(5) cm(-1)) as well as weak ferromagnetic (J' = 0.08(2) cm(-1)) Mn-Mn exchange interactions. In 2 the Cu-Cu exchange interactions are antiferromagnetic (J = -0.10(2) cm(-1), J' = -0.29(2) cm(-1)).