Pillaring Role of 4,4′-Azobis(pyridine) in Substituted Malonate-Containing Manganese(II) Complexes: Syntheses, Crystal Structures, and Magnetic Properties

Six new manganese­(II) complexes of formulas [Mn2(Rmal)2(H2O)2(azpy)]n (1–3), [Mn­(Phmal)­(H2O)­(azpy)]n (4), [Mn2(Et2mal)2(H2O)4(azpy)2]n (5),
and [Mn­(Bzmal)­(H2O)3(azpy)] (6)
[azpy = 4,4′-azobispyridine (1–6), Rmal = methylmalonate (Memal) (1), dimethylmalonate
(Me2mal) (2), and butylmalonate (Butmal) (3), Phmal = phenylmalonate (4), Et2mal = diethylmalonate (5), and Bzmal = benzylmalonate
(6)] were synthesized and structurally characterized
by single-crystal X-ray diffraction. Complexes 1–3 are three-dimensional compounds whose structure consists
of corrugated layers of manganese­(II) linked through syn–anti carboxylate (Rmal) bridges, which are
pillared through the bis-monodentate azpy molecule. Complex 4 has a layered structure of manganese­(II) ions connected
by carboxylate (Phmal) bridges in the syn–anti coordination mode as in 1–3, the azpy group acting here as a terminally bound monodentate
ligand. The structure of 5 consists of Et2mal–Mn­(II) neutral chains linked through the azpy ligand,
giving rise to a complex three-dimensional network. Complex 6 is constituted by neutral [Mn­(Bzmal)­(H2O)3(azpy)] mononuclear units, which are interlinked through O–H···O,
O–H···N, and π–π type intermolecular
interactions to afford a three-dimensional supramolecular structure.
The topological analysis of these crystallographic structures shows
the occurrence of four different nets: a (3,4)-connected InS-type
(1–3), a binodal layered hcb (4), a uninodal CdS-type (5), and a (4,5)-connected
tcs topology (6). The magnetic properties of 1–5 were investigated in the 2.0–300 K
temperature range. Overall antiferromagnetic behavior occurs in 1–4 with susceptibility maxima in the
range 2.8–5.5 K, the exchange pathway being provided by the syn–anti carboxylate (substituted
malonate) bridge [manganese–manganese separation in the range
5.4365(3)–5.5274(1) Å]. Very weak antiferromagnetic interactions
are observed in 5 through the trans-bis-monodentate
Et2mal ligand, the intrachain manganese–manganese
separation being 7.328(3) Å. The much larger manganese–manganese
separation through the bis-monodentate azpy ligand in 1–5 (values greater than 13.5 Å) accounts
for the lack of any significant magnetic interaction though this extended
bridge.

本研究合成了6个新型二价锰配合物，其化学式分别为[Mn₂(Rmal)₂(H₂O)₂(azpy)]ₙ（1~3）、[Mn(Phmal)(H₂O)(azpy)]ₙ（4）、[Mn₂(Et₂mal)₂(H₂O)₄(azpy)₂]ₙ（5）以及[Mn(Bzmal)(H₂O)₃(azpy)]（6）；其中azpy为4,4'-偶氮双吡啶(4,4'-azobispyridine，1~6)，Rmal分别为甲基丙二酸酯(methylmalonate，Memal，1)、二甲基丙二酸酯(dimethylmalonate，Me₂mal，2)与丁基丙二酸酯(butylmalonate，Butmal，3)，Phmal为苯基丙二酸酯(phenylmalonate，4)，Et₂mal为二乙基丙二酸酯(diethylmalonate，5)，Bzmal为苄基丙二酸酯(benzylmalonate，6)。所有配合物均通过单晶X射线衍射完成合成与结构表征。
配合物1~3为三维化合物，其结构由经顺反（syn–anti）羧酸酯（Rmal）桥联的二价锰褶皱层构成，再由双单齿配体azpy作为柱撑基团拓展为三维网络。配合物4具有层状结构，二价锰离子通过与1~3一致的顺反配位模式羧酸酯（Phmal）桥联，而azpy在此充当端基单齿配体。配合物5的结构由Et₂mal-二价锰中性链经azpy配体相互连接，最终形成复杂的三维网络。配合物6由中性单核单元[Mn(Bzmal)(H₂O)₃(azpy)]构成，该单元通过O–H···O、O–H···N及π–π型分子间相互作用相互交联，形成三维超分子结构。
对上述晶体结构开展拓扑分析后发现，共存在四种不同的网络拓扑：1~3为(3,4)-连接的InS型网络，4为双节点层状hcb网络，5为单节点CdS型网络，6为(4,5)-连接的tcs拓扑结构。
针对配合物1~5，我们在2.0~300 K温度范围内对其磁性质进行了研究。配合物1~4整体表现出反铁磁行为，磁化率极大值出现在2.8~5.5 K区间，其磁交换路径由顺反羧酸酯（取代丙二酸酯）桥联提供，锰-锰间距范围为5.4365(3)~5.5274(1) Å。配合物5中通过反式双单齿Et₂mal配体存在极弱的反铁磁相互作用，链内锰-锰间距为7.328(3) Å。而1~5中通过双单齿azpy配体的锰-锰间距均大于13.5 Å，这一特征解释了该扩展桥联未产生显著磁相互作用的原因。