Non-Prussian Blue Structures and Magnetic Ordering of Na2MnII[MnII(CN)6] and Na2MnII[MnII(CN)6]•2H2O

The aqueous reaction of Mn-II and NaCN leads to the isolation of the 3-D Prussian blue analogue (PBA) Na2Mn[Mn(CN)(6)]center dot 2H(2)O (1 center dot H2O), which under careful dehydration forms 1. 1 center dot H2O is monoclinic [P2(1)/n, a = 10.66744(32) angstrom, b = 7.60223(23) angstrom, c = 7.40713(22) angstrom, beta = 92.4379(28)degrees], while 1 is rhombohedral [R (3) over bar, a = 6.6166(2) angstrom, c = 19.2585(6) angstrom], and both structures are atypical for PBAs, which are typically face centered cubic. Most notably, the average angle Mn-N-C angles are 165.3(3)degrees and 142.4(4)degrees for 1 center dot H2O and 1, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high-spin Mn-II accommodating a reduced angle Mn-N-C to minimize void space. Both 1 and 1 center dot H2O magnetically order as ferrimagnets below their ordering temperature, T-c, of 58 and 30 K, respectively, as determined from the average of several independent methods. 1 and 1 center dot H2O are hard magnets with 5 K coercive fields of 15 300 and 850 Oe, and remnant magnetizations of 9075 and 102 emu.Oe/mol, respectively. These data along with previous T-c's reported for related materials reveal that T-c increases as the angle Mn-N-C deviates further from linearity. Hence, the bent cyanide bridges play a crucial role in the superexchange mechanism by increasing the coupling via shorter Mn-II center dot center dot center dot Mn-II separations, and perhaps an enhanced overlap.