Ligand-Templated Four-Metal Chains Dimerize into a Unique [CuII8] Cluster

Abstract

Within the context of what has been termed coordination supramolecular chemistry, remarkable progress has been made in the design and preparation of multidentate ligands capable of participating in complicated molecular structures upon complexation, in predetermined manners, with transition metals. [1] This approach is starting to take hold in the area of molecular magnetism. Thus, in recent years, a few important reports have appeared describing the formation of new magnetic transition metal clusters through the template action of specifically designed multinucleating ligands. [2] We have reported the synthesis of an O-pentaden-tate donor possessing a phenol and two b-diketone units (H 3 L), which was conceived to direct the formation of chains of closely spaced transition metals. [3] Initial efforts into the investigation of its coordination properties involved reactions with Mn II ions, and led to the preparation of a trinuclear complex with formula [Mn 3 (HL) 3 ] which displayed an asym-metric array of metals. [4] A significant step forward in the exploration of the templating capability of this new ligand is reported here with the fascinating methoxide-mediated aggregation of two ligand-templated [Cu 4 ] chains into an entirely new octanuclear Cu II cluster held together by a fully deprotonated form of H 3 L. The title compound was prepared by treating a methanolic solution of Cu(NO 3) 2 ¥ 3 H 2 O with a solution containing 0.125 equivalents of H 3 L and 1.8 equivalents of NtBu 4 OH in MeOH. The green powder that precipitated was recrystallized from MeOH/Et 2 O to yield green crystals of [{Cu 4 (L)(OMe) 4-(NO 3)} 2 ] (1) [5] in an overall yield of 21 %. The organic salt is introduced into the reaction mixture as the base necessary to deprotonate H 3 L and also as the acceptor of the protons from methanol to give the MeO À ions that become incorporated into the complex. The fact that the complex binds methoxide instead of hydroxide ions despite the larger Br˘nsted basicity of the former can be explained by the stability of the final product combined with the large excess of methanol present in the system. The latter point is supported by the observation that, upon prolonged exposure to air, complex 1 changes color from green to brown, presumably following a process of complete hydrolysis of the methoxide ligands, as suggested by elemental analysis. [6] The structure of 1 [7] (Figure 1) shows a unique [Cu II 8 ] cluster formed by the dimerization of two ligand-held four-metal atom rows. Both metallic chains are bound to each other through eight methoxide ligands: six in a triply bridging Figure 1. POV-Ray representation of [{Cu 4 (L)(OMe) 4 (NO 3)} 2 ] (1), where the axial bonds around the Cu II ions are highlighted by striped lines. manner [8] and two in a m-OMe À fashion. The coordination around the Cu centers is completed by nitrate ions, which connect, in an extremely unusual h 3 :m 3-NO 3 À mode, [9] the [Cu 8 ] units into one-dimensional polymeric chains of clusters parallel to the a-axis (Figure 2). In...

A Self‐Assembled Tetracopper Triple‐Stranded Helicate: Towards the Controlled Synthesis of Finite One‐Dimensional Magnetic Chains

“…The structure13 of the cation in 1 (Figure 1) consists of three ligand strands that twist around a pseudolinear array of four six‐coordinate Cu II centers (Cu1‐Cu2‐Cu3 172.0°; Cu2‐Cu3‐Cu4 171.6°) that defines the helical axis. The bending of the Cu 4 chain is slight in comparison to the only other known tetranuclear copper( II ) chain compound where a marked curvature is observed 14. The long external dimension of the helicate is approximately 18 Å and the internal Cu1Cu4 distance is 11.149(1) Å, with adjacent copper–copper separations of Cu1‐Cu2 3.743(1), Cu2‐Cu3 3.743(1), and Cu3‐Cu4 3.730(1) Å.…”