4-Bromopyridine-Induced Chirality in Magnetic MII-[NbIV(CN)8]4- (M = Zn, Mn, Ni) Coordination Networks.

The introduction of 4-bromopyridine (4-Brpy) to a self-assembled MII-[NbIV(CN)8] (M = 3d metal ion) coordination system results in the formation of three-dimensional cyanido-bridged networks, {[MII(4-Brpy)4]2[NbIV(CN)8]}·nH2O (M = Zn, n = 1, 1; M = Mn, n = 0.5, 2; M = Ni, n = 2, 3). All these compounds are coordination frameworks composed of octahedral [MII(4-Brpy)4(µ-NC)2] complexes bonded to square antiprismatic [NbIV(CN)8]4- ions bearing four bridging and four terminal cyanides. 1 and 2 crystallize in the chiral I4122 space group as the mixture of two enantiomorphic forms, named 1(+)/1(-) and 2(+)/2(-), respectively. The chirality is here induced by the spatial arrangement of nonchiral but sterically expanded 4-Brpy ligands positioned around MII centers in the distorted square geometry, which gives two distinguishable types of coordination helices, A and B, along a 4-fold screw axis. The (+) forms contain left handed helices A, and right handed helices B, while the opposite helicity is presented in the (-) enantiomers. On the contrary, 3 crystallizes in the nonchiral Fddd space group and creates only one type of helix. Half of them are right handed, and the second half are left handed, which originates from the ideally symmetrical arrangement of 4-Brpy around NiII and results in the overall nonchiral character of the network. 1 is a paramagnet due to paramagnetic NbIV centers separated by diamagnetic ZnII. 2 is a ferrimagnet below a critical temperature, Tc of 28 K, which is due to the CN--mediated antiferromagnetic coupling within Mn-NC-Nb linkages. 3 reveals a ferromagnetic type of NiII-NbIV interaction leading to a ferromagnetic ordering below Tc of 16 K, and a hysteresis loop with a coercive field of 1400 Oe at 2 K. Thus, 1 is a chiral paramagnet, 3 is a nonchiral ferromagnet, and 2 combines both of these functionalities, being a rare example of a chiral molecule-based magnet whose chirality is induced by the noninnocent 4-Brpy ligands. [ABSTRACT FROM AUTHOR]