Anomalous Non-Prussian Blue Structures and Magnetic Ordering of K2MnII[MnII(CN)6] and Rb2MnII [MnII(CN)6]
The reaction of Mn{sup II} and KCN in aqueous and non-aqueous media leads to the isolation of three-dimensional (3-D) Prussian blue analogues, K{sub 2}Mn[Mn(CN){sub 6}] (1a-d, 1e, respectively). Use of RbCN forms Rb{sub 2}Mn[Mn(CN){sub 6}] (2). 1 and 2 are isomorphic {l_brace}monoclinic, P2{sub 1}/n: 1 [a = 10.1786(1) {angstrom}, b = 7.4124(1) {angstrom}, c = 6.9758(1) {angstrom}, {beta} = 90.206(1){sup o}]; 2 [a = 10.4101(1) {angstrom}, b = 7.4492(1) {angstrom}, c = 7.2132(1) {angstrom}, {beta} = 90.072(1){sup o}]{r_brace}, with a small monoclinic distortion from the face centered cubic (fcc) structure that is typical of Prussian blue structured materials that was previously reported for K{sub 2}Mn[Mn(CN){sub 6}]. Most notably the average Mn-N-C angles are 148.8{sup o} and 153.3{sup o} for 1 and 2, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high spin Mn{sup II} accommodating a reduced M-CN-M{prime} angle and minimizing void space. Compounds 1a,b have a sharp, strong V{sub OH} band at 3628 cm{sup -1}, while 1e lacks a V{sub OH} absorption. The V{sub OH} absorption in 1a,b is attributed to surface water, as use of D{sub 2}O shifts the V{sub OH} absorption to 2677 cm{sup -1}, and that 1a-e are isostructural. Also, fcc Prussian blue-structured Cs{sub 2}Mn[Mn(CN){sub 6}] (3) has been structurally [Fm{sub 3}m: a = 10.6061(1) {angstrom}] and magnetically characterized. The magnetic ordering temperature, T{sub c}, increases as K{sup +} (41 K) > Rb{sup +} (34.6 K) > Cs{sup +} (21 K) for A{sub 2}Mn[Mn(CN){sub 6}] in accord with the increasing deviation for linearity of the Mn-N-C linkages [148.8 (K{sup +}) > 153.3 (Rb{sup +}) > 180{sup o} (Cs{sup +})], decreasing Mn(II){hor_ellipsis}Mn(II) separations [5.09 (K{sup +}) < 5.19 (Rb{sup +}) < 5.30 {angstrom} (Cs{sup +})], and decreasing size of the cation (increasing electrostatic interactions). Hence, the bent cyanide bridges play a crucial role in the superexchange mechanism by increasing the coupling via shorter Mn(II){hor_ellipsis}Mn(II) separations, and perhaps enhanced overlap. In addition, the temperature dependent magnetic behavior of K{sub 4}[Mn{sup II}(CN){sub 6}] {center_dot} 3H{sub 2}O is reported.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - OFFICE OF SCIENCE
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1019909
- Report Number(s):
- BNL-95755-2011-JA; INOCAJ; TRN: US201115%%545
- Journal Information:
- Inorganic Chemistry, Vol. 49, Issue 4; ISSN 0020-1669
- Country of Publication:
- United States
- Language:
- English
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