Unexpected structural and electronic effects of internal rotation in diruthenium paddlewheel complexes containing bulky carboxylate ligands

Abstract

A series of Ru 2 II,III complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate (TiPB) of type trans-[Ru 2(TiPB) 2(O 2CCH 3) 2X] [X = Cl ( 1), PF 6 ( 2)] and [Ru 2(TiPB) 4X] [X = Cl ( 3), PF 6 ( 4)] have been synthesised. The corresponding Ru 2 II,II complexes trans-[Ru 2(TiPB) 2(O 2CCH 3) 2] ( 5) and [Ru 2(TiPB) 4] ( 6) were also isolated. Magnetic susceptibility measurements indicate that the diruthenium cores have the expected three ( 1– 4) or two ( 5 and 6) unpaired electrons consistent with σ 2π 4δ 2(δ ∗π ∗) 3 and σ 2π 4δ 2δ ∗2π ∗2 electronic configurations. Compounds 1– 4 and 6 were structurally characterised by X-ray crystallography, and show the expected paddlewheel arrangement of carboxylate ligands around the diruthenium core. The diruthenium cores of complexes 3, 4 and 6 are all distorted to minimise steric interactions between the bulky carboxylate ligands. The Ru–Ru bond length in the Ru 2 II,II complex 6 [2.2425(6) Å] is the shortest observed for a diruthenium tetracarboxylate and, surprisingly, is 0.014 Å shorter than in the analogous Ru 2 II,III complex 4, despite an increase in the formal Ru–Ru bond order from 2.0 ( 6) to 2.5 ( 4). This is rationalised in terms of the extent of internal rotation, or distortion, about the diruthenium core. This was supported by density functional theory calculations on the model complexes [Ru 2(O 2CH) 4] and [Ru 2(O 2CH) 4] +, that demonstrate the relationship between Ru–Ru bond length and internal rotation. Electrochemical and electronic absorption data were recorded for all complexes in solution. Comparison of the data for the ‘ bis– bis’ ( 1, 2 and 5) and tetra-substituted ( 3, 4 and 6) complexes indicates that the shortening of the Ru–Ru bond length results in a small increase in energy of the near-degenerate δ ∗ and π ∗ orbitals.