Unusual magnetic metal-cyanide cubes of ReII with alternating octahedral and tetrahedral corners.

Abstract

The design of cyanide-bridged transition-metal clusters is one of the leading topics in the field of molecular magnetism. Two of the main reasons for this high interest are 1) cyanide chemistry easily lends itself to a building-block approach and 2) the nature (ferromagnetic versus antiferromagnetic) of magnetic exchange interactions through a linear cyanide ligand is largely predictable. These attributes have inspired numerous research groups to pursue the synthesis of highspin, magnetically anisotropic metal cyanide molecules with the goal of engendering slow paramagnetic relaxation of the magnetization, a phenomenon that has been likened to the behavior of single-domain particles. This “superparamagnetic-like” magnetic behavior of molecules, commonly referred to as “single-molecule magnetism”, has been observed for paramagnetic clusters that exhibit a large spin ground state combined with an appreciable degree of anisotropy (i.e., a negative zero-field splitting parameter D). 10,11] Singlemolecule magnet (SMM) behavior was first noted over ten years ago for an {Mn12} cluster of the oxide family, [10,11] and many new examples of oxide-based SMMs have been prepared in the ensuing years. 12] Progress in this area has been only incremental, however, in terms of raising the blocking temperature of the magnetization. One of the main reasons for this situation is that it is difficult to control the sign and magnitude of D. This limitation to realizing high-temperature SMMs is one of the focal points of research in the field. One approach to increasing the magnetic anisotropy of paramagnetic clusters is to incorporate heavier transition elements, such as 5d metal ions, which exhibit strong spin– orbit coupling effects that can induce anisotropic magneticexchange interactions. In this vein, we have been investigating the use of the paramagnetic Re anion complex [Re(triphos)(CN)3] (triphos= 1,1,1-tris(diphenylphosphanylmethyl)ethane) as a building block for high nuclearity clusters with unusual magnetic properties. As a backdrop for these studies, we undertook a full investigation of the magnetic behavior and theoretical modeling of [Et4N][Re (triphos)(CN)3], which revealed that the compound exhibits an unusually strong temperature-independent paramagnetism due to spin–orbit coupling of the S= 1/2 ground state and low-lying excited states. Given these intriguing findings, we proceeded to explore reactions of [Re(triphos)(CN)3] with complementary building blocks including MCl2 reagents (M= 3d metal ion). The results of these studies with Fe and Co chlorides are reported herein. Single-crystal X-ray studies revealed the products to be distorted molecular cubes composed of both sixand fourcoordinate vertices (Figure 1). The triphos ligands act as