The aqueous reaction of Mn(II) and NaCN leads to the isolation of the 3-D Prussian blue analogue (PBA) Na(2)Mn[Mn(CN)(6)]·2H(2)O (1·H(2)O), which under careful dehydration forms 1. 1·H(2)O is monoclinic [P2(1)/n, a = 10.66744(32) Å, b = 7.60223(23) Å, c = 7.40713(22) Å, β = 92.4379(28)°], while 1 is rhombohedral [R ̅3, a = 6.6166(2) Å, c = 19.2585(6) Å], and both structures are atypical for PBAs, which are typically face centered cubic. Most notably, the average ∠Mn-N-C angles are 165.3(3)° and 142.4(4)° for 1·H(2)O and 1, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high-spin Mn(II) accommodating a reduced ∠Mn-N-C to minimize void space. Both 1 and 1·H(2)O 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·H(2)O 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 ∠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)···Mn(II) separations, and perhaps an enhanced overlap.
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Kareis et al. (2011) studied this question.