The modular synthesis of rare earth-transition metal heterobimetallic complexes utilizing a redox-active ligand

Abstract

We report a robust and modular synthetic route to heterometallic rare earth-transition metal complexes. We have used the redox-active bridging ligand 1,10-phenathroline-5,6-dione (pd), which has selective N,N′ or O,O′ binding sites as the template for this synthetic route. The coordination complexes [Ln(hfac)₃(N,N’-pd)] (Ln = Y [1], Gd [2]; hfac = hexafluoroacetylacetonate) were synthesised in high yield. These complexes have been fully characterised using a range of spectroscopic techniques. Solid state molecular structures of 1 and 2 have been determined by X-ray crystallography and display different pd binding modes in coordinating and non-coordinating solvents. Complexes 1 and 2 are unusually highly coloured in coordinating solvents, for example the vis-NIR spectrum of 1 in acetonitrile displays an electronic transition centred at 587 nm with an extinction coefficient consistent with significant charge transfer. The reaction between 1 and 2 and VCp₂ or VCp^t₂ (Cp^t = tetramethylcyclopentadienyl) resulted in the isolation of the heterobimetallic complexes, [Ln(hfac)₃(N,N′-O,O′-pd)VCp₂] (Ln = Y [3], Gd [4]) or [Ln(hfac)₃(N,N′-O,O′-pd)VCp^t₂] (Ln = Y [5], Gd [6]). The solid state molecular structures of 3, 5 and 6 have been determined by X-ray crystallography. The spectroscopic data on 3–6 are consistent with oxidation of V(II) to V(IV) and reduction of pd to pd²⁻ in the heterobimetallic complexes. The spin-Hamiltonian parameters from low temperature X-band EPR spectroscopy of 3 and 5 describe a ²A₁ ground state, with a V(IV) centre. DFT calculations on 3 are in good agreement with experimental data and confirm the SOMO as the d_x²−y² orbital localised on vanadium.