4‑Bromopyridine-Induced Chirality in Magnetic M^II-[Nb^IV(CN)₈]^4– (M = Zn, Mn, Ni) Coordination Networks

The introduction of 4-bromopyridine (4-Brpy) to a self-assembled M^II-[Nb^IV(CN)₈] (M = 3d metal ion) coordination system results in the formation of three-dimensional cyanido-bridged networks, {[M^II(4-Brpy)₄]₂­[Nb^IV(CN)₈]}­·<i>n</i>H₂O (M = Zn, <i>n</i> = 1, <b>1</b>; M = Mn, <i>n</i> = 0.5, <b>2</b>; M = Ni, <i>n</i> = 2, <b>3</b>). All these compounds are coordination frameworks composed of octahedral [M^II(4-Brpy)₄­(μ-NC)₂] complexes bonded to square antiprismatic [Nb^IV(CN)₈]^4– ions bearing four bridging and four terminal cyanides. <b>1</b> and <b>2</b> crystallize in the chiral <i>I</i>4₁22 space group as the mixture of two enantiomorphic forms, named <b>1</b>(<b>+</b>)/<b>1</b>(<b>−</b>) and <b>2</b>(<b>+</b>)/<b>2</b>(<b>−</b>), respectively. The chirality is here induced by the spatial arrangement of nonchiral but sterically expanded 4-Brpy ligands positioned around M^II 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, <b>3</b> crystallizes in the nonchiral <i>Fddd</i> 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 Ni^II and results in the overall nonchiral character of the network. <b>1</b> is a paramagnet due to paramagnetic Nb^IV centers separated by diamagnetic Zn^II. <b>2</b> is a ferrimagnet below a critical temperature, <i>T</i>_c of 28 K, which is due to the CN^–-mediated antiferromagnetic coupling within Mn–NC–Nb linkages. <b>3</b> reveals a ferromagnetic type of Ni^II–Nb^IV interaction leading to a ferromagnetic ordering below <i>T</i>_c of 16 K, and a hysteresis loop with a coercive field of 1400 Oe at 2 K. Thus, <b>1</b> is a chiral paramagnet, <b>3</b> is a nonchiral ferromagnet, and <b>2</b> 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