First crystal structures of oxo-bridged [CrIIITaV] dinuclear complexes: spectroscopic, magnetic and theoretical investigations of the Cr–O–Ta core

Abstract

Heterodinuclear complexes [Cr(bpy)₂(H₂O)(μ-O)Ta(C₂O₄)₃]₂·3.5H₂O (1) and [H₂(terpy)][Cr(terpy)(C₂O₄)(μ-O)Ta(C₂O₄)₃]·0.5H₂C₂O₄·2.5H₂O (2) (bpy = 2,2′-bipyridine; terpy = 2,2′:6′,2′′-terpyridine) have been synthesised through a reaction of a tris(oxalate)oxotantalate(V) anion and a complex chromium(III) cation with corresponding aromatic N-donor ligands. These are the first examples of a Cr–O–Ta bridge in molecular compounds, and the first report of heterodinuclear complexes prepared from a tris(oxalate)oxotantalate(V) anion as a building block. Structural analysis of both compounds revealed that Ta^V in pentagonal bipyramid geometry is connected through a slightly bent μ-O bridge to Cr^III having a distorted octahedral environment. The geometry optimization performed with density functional theory (DFT) calculations gave very good agreement with the experimentally obtained structure of 1. Room temperature solid state UV-Vis spectra were recorded for both compounds and compared to those of starting reagents, in order to test for possible new electronic transitions attributed to the metal-to-metal charge transfer between Cr^III and Ta^V through the oxo-bridge. Infrared (IR) spectroscopy supported with DFT calculations was used to assign vibrational modes to all spectral features, especially those coming from the molecular μ-oxo bridge. Temperature dependence of magnetic properties of 1 and 2 were probed with magnetic susceptibility measurements on a SQUID magnetometer. Additionally, zero-field splitting parameters were determined from electron paramagnetic resonance (EPR) spectra of compound 1 and experimentally obtained values were confirmed by DFT calculations.