Low-dimensional materials continue to remain of interest throughout the world-wide community owing to the range of properties displayed. Among highly tunable analogues are those derived from cyanometalates as they continue to offer a rich landscape for the systematic study of self-assembly processes, their structures, and magnetic properties within a given structural archetype. Poly(pyrazolyl)borate building blocks such as [(Tp R )Fe III -LS (CN) 3 ] À are attractive precursors for magnetic chain construction owing to their ability to exhibit first-order orbital contributions to their S = ½ spin ground states (2.4 g 2.9) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. These welldefined molecule-based complexes generally undergo selective self-assembly reactions to afford products whose nuclearity, numbers and orientation of formed M(l-CN)M 0 linkages, and properties are strongly correlated with the steric demand of their ancillary ligands [1][2][3][19][20][21][22]. These versatile, magnetically anisotropic, and highly tunable pyrazolylborate cyanometalates may be incorporated into a variety of discrete polynuclear complexes and chains that display single-molecule [1][2][3][4][5][6][7][8][9][10][11][12][13][14]16,17,[23][24][25] and single-chain magnetic [25][26][27][28][29], solvent-assisted rearrangement and gas sorption [30,31], and photomagnetic [28,29,[32][33][34][35][36][37][38][39] behavior.
The first cyanometalate to display thermo-and photochromic behavior, K 0.2 Co 1. 4 [Fe(CN) 6 ]Á6.9H 2 O, was originally described by Hashimoto in 1996 [40]. The color and magnetism changes seen are a direct result of electron transfer where diamagnetic {Fe II LS (l-CN)Co III LS } linkages are reversibly converted into paramagnetic {Fe III LS (l-CN)Co II HS } ones at ca. 250 K [40][41][42][43][44][45]. Subsequent reports indicate that a variety of bistable cyanide-bridged complexes [32][33][34][35][36][37][38][39], chains [28,29], thin films and core-shell materials [46][47][48][49][50][51], may also be engineered to mimic behavior seen for the three-dimensional lattice.
A second structurally related solid, RbMn[Fe(CN) 6 ]ÁH 2 O, is also known to exhibit optical and magnetic bistability that is reminiscent of the better known Fe/Co Prussian blues [52][53][54][55][56][57][58]. In these analogues valence tautomeric {Fe II LS (l-CN)Mn III HS } units are reversibly converted into {Fe III LS (l-CN)Mn II HS } ones with increasing temperature or upon light exposure. The structural changes that accompany electron transfer also induces a tetragonal to cubic phase transformation, as conversion of Jahn-Teller distorted Mn III HS to Mn II HS ions occurs [52][53][54][55][56][57][58]. Surprisingly, while numerous complexes are known to mimic optical and magnetic bistability seen in Fe/Co Prussian blue analogues, no molecule-based complexes or chains containing bistable Fe/Mn pairs are currently known [5,8,[59][60][61][62][63][64][65][66][67]. As part of a continuing effort to better understand how to manipulate the optical and magnetic properties of low-dimensional materials, our attention turned towards the preparation of tunable one-low-dimensional {FeMn} n networks that may undergo electron transfer. The results of these efforts are reported below.
, 848 (s), 806 (vs), 791 (s), 781 (s), 763 (s), 729 (s), 661 (w), 640 (w), 624 (s), 489 (w), 458 (w), 422 (m). UV-vis (Nujol/KBr): k max /nm 455 (m, sh), 525 (s, sh), 624 (vs), 643 (vs), 750 (m).
Infrared spectra were recorded as Nujol mulls between KBr plates on a Thermo-Fisher Nicolet Impact 6700 FTIR instrument in the 400-4000 cm À1 region. Variable temperature infrared data were collected using a liquid nitrogen cooled Janis ST-100 cryostat equipped with a LakeShore 331 temperature controller operating between 80 and 350 K. Variable temperature electronic spectra were obtained as Nujol mulls between Mylar films on a Janis EXOL cryostat equipped with a LakeShore 335 temperature controller (20-300 K range), CTI SC Cryo helium compressor and CTI M22 cold head, Ocean Optics Flame-S-UV-VIS-ES spectrophotometer, and a DH-2000-BAL balanced deuterium tungsten light source (200 to 850 nm range).
Magnetic measurements on a microcrystalline sample of 1 (22.2 mg) were collected on a Quantum Design MPMS-XL 7 magnetometer operating between 2 and 300 K and applied magnetic fields ranging between À7 0 7 T. The magnetic data were corrected for the sample holder and diamagnetic contributions were estimated using Pascal's constants [68]. Elemental analyses were performed by Robertson Microlit Laboratories. Magnetic data figures were generated using Origin 2019b (www.originlab.com).
Single crystal structural data for 1 were collected at 100(2) K on a Bruker Apex-II CCD diffractometer using graphite-collimated MoK a (k = 0.71073 Å) radiation. All crystals were mounted in Paratone-N oil on nylon loops. The structures were solved by direct methods (SHELXS97) [69,70] and completed by difference Fourier methods (SHELXL-2016) [69,70]. Refinement was performed against F 2 by weighted full-matrix least-squares (SHELXL-2016) [69,70] and empirical absorption corrections (SADABS) [71] were applied. Hydrogen atoms for 1 were found in difference maps and subsequently placed at calculated positions using suitable riding models with isotropic displacement parameters derived from their carrier atoms. Non-hydrogen atoms were refined with anisotropic displacement parameters. Atomic scattering factors were taken from the International Tables for Crystallography Vol. C. 82 [72]. All figures were generated using CrystalMaker X Ò (Crys-talMaker Software Ltd, www.crystalmaker.com).
Treatment of a 1:1 ratio of tricyanoferrate(III) and Mn(phen) 2 2+ complexes in methanol readily affords the one-dimensional chain, {[(pzTpFe III (CN
, where phen = 9,10-phenanthroline and pzTp = tetra(pyrazolyl)borate.
The infrared spectrum of 1 displays two strong m CN cyanide stretching absorptions that are shifted to higher energies relative to the one seen for [NEt 4 ][(pzTp)Fe III (CN) 3 ]ÁH 2 O (2119 cm À1 ) [7,8]. These high energy absorptions [2152 and 2144 cm À1 ] are in the range expected for compounds containing Fe III LS (l-CN)Mn II linkages, while a third one [2122 cm À1 ] is ascribed to a terminal Fe-CN unit [1,3,5,7,8,14,[59][60][61][62][63][64][65][66][67]. These cyano stretches resemble those seen in the infrared spectra of RbMn[Fe(CN) 6 ]ÁH 2 O [2155 and 2164 cm À1 ] for temperatures above ca. 250 K [52][53][54][55][56][57][58]. However at lower temperatures, the Fe/Mn Prussian blue analogue displays lower energy absorptions belonging to Fe II (l-CN)Mn III units [2086, 2091, 2110, and 2130 cm À1 ], in addition to a single higher energy one [2202 cm À1 ] ascribed to putative Fe III (l-CN)Mn III units [52][53][54][55]. Unfortunately, no changes in the infrared spectra of 1 were seen between 80 and 350 K in the absence or presence of white light (5 h), indicating that the Fe III /Mn II pairs are the preferred valence tautomeric state. Likewise, solid state variable-temperature UV-vis spectra collected for 1 as Nujol mulls or between Mylar films show intense and broad absorptions seen at ca. 624 and 643 nm that are reminiscent of those in RbMn[Fe(CN) 6 ]ÁH 2 O [52][53][54][55][56][57][58]. However, these bands remain invariant with respect to changes in temperature or white light exposure, confirming that 1 does not exhibit thermoor photochromism.
Compound 1 crystallizes in the monoclinic P2 1 /n space group as a cationic one-dimensional sesquihydride chain (Table 1). The asymmetric unit contains a [(pzTp)Fe III (CN) 3 ] À anion that is linked via a bridging cyanide to an adjacent [Mn II (phen) 2 ] 2+ fragment, leaving a single terminal cyanide per iron center. A single charge-balancing and disordered perchlorate anion and fractionally occupied lattice water are also found in the interchain region of its structure. The terminal cyanide Fe1-C distance [Fe1-C14, 1.931(2) Å] is smaller than the bridging ones [Fe1-C13, 1.913(2); Fe1-C15, 1.909(2) Å] while the Fe-CN units are slightly bent, ranging between 173.4(2) to 178.4(2) Å for Fe1-C15-N11 and Fe1-C14-N10, respectively (Table 2). The [Mn(phen) 2 (l-NC) 2 ] fragment adopts a distorted MnN 6 coordination environment due to extensive steric interactions between phen and pzTp ligands [Fig. 1 We note that all but one of the hydrogen atoms were found in the difference maps of 1. The fractionally occupied lattice water (O1S) was refined to be ca. ¼ per {FeMn} n repeat unit and is in close proximity to several ancillary ligands, the terminal cyanide (N10), and disordered perchlorate anion atoms (O4). The close contacts between terminal cyanides and lattice water [N10Á Á ÁO1S, 2.754 2) Å] are also seen. We were unable to find any electron density ascribable to a hydrogen atom along the O4Á Á ÁH-O1S vector in the structure owing to the disordered nature of the charge balancing perchlorates and their geometric relationship to the O1S-H-N10 interaction. Assuming these close contacts are consequences of crystal packing, rather than bona fide hydrogen bonds, we elected to exclude this perchlorate O4/O4 0 Á Á ÁHÁ Á ÁO1S(H 2 O) contact from the structural description. Lastly, we note that these attributes are also found in the structure of one-dimensional {[Fe III (bpb)(CN) 2 ][Mn II (L 1 )][ClO 4 ]} n -Á 1 / 2 H 2 O, where bpb 2À = 1,2-bis(pyridine-2-carboxamido)benzenate and L 1 = 3,6-diazaoctane-1,8-diamine [64].
At 300 K, the vT product for 1 is near the expected value for a 1:1 ratio (5.06 cm 3
Selected bond distances (Å) and angles (°) for 1. (S = 1 / 2 ; 2.4 g 2.8) and Mn II HS (S = 5 / 2 ; g ~2) ions, if each site contributes ca. 0.68 and 4.4 cm 3 K mol À1 , respectively. However, the experimental vT product is actually lower [4.79 cm 3 K mol À1 ], suggesting that spin-orbit contributions from the Fe III ions are largely quenched. Curie-Weiss fits of v À1 vs T data between 30 and 300 K gave a Weiss constant (h) of À2.7(2) K [for C = 4.79(2) cm 3 K mol À1 ] which is somewhat larger than that reported [h = À1.73 K] for chains such as {[Fe III (bpb)(CN) 2 ][Mn II (L 1 )][ClO 4 ]} n Á 1 / 2 H 2 O, where bpb 2À = 1,2-bis(pyridine-2-carboxamido)benzenate and L 1 = 3,6-diazaoctane-1,8-diamine [64].
The temperature dependence of the vT vs T data at an applied magnetic field of 1000 Oe clearly shows that compound 1 undergoes antiferromagnetic exchange between the iron and manganese sites [Fig. 3]. This behavior is consistent with both orbital symmetry and Goodenough-Kanamori arguments [78]. With decreasing temperatures, the vT product gradually approaches a minimum value [4.41 cm 3 K mol À1 ] at 12 K, confirming local antiferromagnetic interactions are present [Fig. 3]. At lower temperatures, the vT values again increase towards a maximum of 8.00 cm K mol À1 at 2.5 K. This behavior is reminiscent of that reported for several one dimensional {Fe III LS Mn II HS } n S = 2 chains [59][60][61][62][63][64][65][66][67]. The field dependence of the magnetization was measured at 2 and 5 K and the M vs H data clearly shows the chains are easily saturated and approach values of 4.80 l B at 2 K. The experimental value is close to the expected one [4.90 l B ] assuming that g iso ~2.2 for an antiferromagnetically coupled {Fe III Mn II } n chain [Inset of Fig. 3].
The v vs T data were initially modeled following methods described by Ni and Jiang for one-dimensional {Fe III Mn II } n chains [64]. Assuming that an isotropic exchange Hamiltonian, H = -2J [S 1 ÁS 2 ], adequately describes the magnetic exchange interactions between the low spin Fe III LS and high spin Mn II HS ions, the susceptibility of the {Fe III Mn II } repeat unit may be described via the following equations:
where u = coth(J 2 S d (S d + 1)/k B T) À k B T/J 2 S d (S d + 1). In the present case, g iso (average g value for a Fe III Mn II dimer) and J 1 were found to be 2.06(2) and À0.92(1) cm À1 , which is consistent with modest antiferromagnetic cyanide-mediated superexchange within the significantly bent Fe III (l-CN)Mn II units [59][60][61][62][63][64][65][66][67]. Additional intra-and interchain exchange interactions (J 2 and zJ) proved to be exceptionally small and were deemed to be physically meaningless. This behavior is reminiscent of Julve's 4,2-ribbons, {Fe III (dmbpy)(CN) 4 ] 2 -Mn II (H 2 O) 2 } n , where dmbpy = 4,4 0 -dimethyl-2,2 0 -bipyridine [65]. However, in contrast to 1 and Julve's chain, MAGPACK [79,80] [67]. Overall, we conclude that the deduced magnetic parameters fall within the typical ranges reported for a variety of one-dimensional Fe III /Mn II networks [59][60][61][62][63][64][65][66][67].
In summary, the preparation, structure, and magnetic properties of a cyanide-bridged one-dimensional {Fe III Mn II } n chain are described. Owing to the presence of a highly bent Fe(l-CN)Mn linkages inefficient superexchange is found between the magnetic orbitals. The apparent absence of electron transfer in 1 is attributed to the highly distorted structure of the chains which provide an inadequate ligand field for oxidation of the manganese and concomitant reduction of the iron sites within the Fe III LS (l-CN)Mn II HS units.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.