Rigid NON- and NSN-ligand complexes of tetravalent and trivalent uranium: comparison of U–OAr2 and U–SAr2 bonding

Abstract

A rigid NSN-donor proligand, 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylthioxanthene (H₂[TXA₂], 1) was prepared by palladium-catalyzed coupling of 2,6-diisopropylaniline with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylthioxanthene. Deprotonation of 1 using ⁿBuLi provided Li₂(DME)₂[TXA₂] (2), and subsequent reaction with UCl₄ afforded [Li(DME)₃][(TXA₂)UCl₃] (4). The analogous NON-donor ligated complex [(XA₂)UCl₃K(DME)₃] [3; XA₂ = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene] was prepared by the reaction of K₂(DME)_x[XA₂] with UCl₄. A cyclic voltammogram (CV) of 3 in THF/[NBu₄][B(C₆F₅)₄] at 200 mV s⁻¹ showed an irreversible reduction to uranium(III) at E_pc = −2.46 V versus FeCp₂^0/+1, followed by a product wave at E_1/2 = −1.83 V. Complex 4 also underwent irreversible reduction to uranium(III) [E_pc = −2.56 V], resulting in an irreversible product peak at E_pa = −1.83 V. One-electron reduction of complexes 3 and 4 using K(naphthalenide) under an argon atmosphere in DME yielded 6-coordinate [(XA₂)UCl(DME)] (5) and the thermally unstable 7-coordinate [(TXA₂)U(DME)Cl₂Li(DME)₂] (6), respectively. The U–S distances in 4 and 6 are uncommonly short, the C–S–U angles are unusually acute, and the thioxanthene backbone of the TXA₂ ligand is significantly bent. By contrast, the xanthene backbone in XA₂ complexes 3 and 5 is planar. However, κ³-coordination and an approximately meridional arrangement of the ancillary ligand donor atoms is maintained in all complexes. DFT and Atoms in Molecules (AIM) calculations were carried out on 3, 4, 5, 6, [(XA₂)UCl₃]⁻ (3B), [(TXA₂)UCl₂(DME)]⁻ (6B) and [(TXA₂)UCl(DME)] (6C) to probe the extent of covalency in U–SAr₂ bonding relative to U–OAr₂ bonding.