Dinuclear CuII–CuII and CuI–CuII Complexes of a Compartmental Ligand – Syntheses, Structures, Magnetic, and Catalytic Studies

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Abstract

Dinuclear CuII–CuII (1) and CuI–CuII (2) complexes were derived from a new N4O2 donor compartmental ligand (H2L) by changing the nature of the Cu precursors used. Single-crystal X-ray diffraction studies revealed that the Cu1 site in 1 has octahedral geometry and is in the inner compartment of the ligand axially coordinated by two ClO4 anions, whereas the outer Cu2 ion has square-pyramidal geometry. In 2, there are two copper dinuclear complexes (A and B) in the asymmetric unit; the inner core is occupied by Cu1/Cu3 in the +2 oxidation state in a square-planar geometry. The Cu2/Cu4 ion occupies the outer sites and has distorted tetrahedral geometry with a +1 oxidation state. Complexes 1 and 2 were obtained simply by changing Cu(ClO4)2 in 1 to Cu(ClO4)2 and Cu(bipy)(NO3)2 in 2; the bipyridyl (bipy) ligand induces the reduction of CuII to CuI, which is trapped in the CuI–CuII dinuclear product. The oxidation states of the metal ions were ascertained from charge-balance considerations as well as from bond valence sum (BVS) calculations. No signature of intervalence charge transfer (IVCT) was observed by spectroscopy (UV/Vis/NIR) as well as from the low-temperature magnetic studies. This might be because of the presence of two copper centers in two different geometries with a wide separation between them (ca. 3.27 Å). For 1, the best fit of the χT vs. T data to the dimer model gives J/kB = –262(1) K and gav = 2.05(5), which indicates that there is a strong antiferromagnetic coupling between the two Cu atoms. In 2, the CuI center is diamagnetic and, thus, the remaining S = 1/2 CuII magnetic center follows a Curie law with g = 2.06(5). Under homogeneous conditions, both complexes showed catalytic epoxidation of cyclooctene, styrene, and norbornene to the corresponding epoxides with high selectivities and turnover numbers (TONs), which seem to be slightly better than the reported values for CuII Schiff base complexes.

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