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

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• 作者：Takuro Ohno ; Szymon Chorazy ; Kenta Imoto ; Shin-ichi Ohkoshi
• 刊名：Crystal Growth & Design
• 出版年：2016
• 出版时间：July 6, 2016
• 年：2016
• 卷：16
• 期：7
• 页码：4119-4128
• 全文大小：615K
• 年卷期：0
• ISSN：1528-7505

文摘

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]}·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 [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. 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 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, 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 Ni^II and results in the overall nonchiral character of the network. 1 is a paramagnet due to paramagnetic Nb^IV centers separated by diamagnetic Zn^II. 2 is a ferrimagnet below a critical temperature, T_c of 28 K, which is due to the CN^–-mediated antiferromagnetic coupling within Mn–NC–Nb linkages. 3 reveals a ferromagnetic type of Ni^II–Nb^IV interaction leading to a ferromagnetic ordering below T_c 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.