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

A series of Ru2II,III complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate (TiPB) of type trans-[Ru₂(TiPB) ₂(O₂CCH₃)₂X] [X = Cl (1), PF ₆ (2)] and [Ru₂(TiPB)₄X] [X = Cl (3), PF ₆ (4)] have been synthesised. The corresponding Ru2II,II complexes trans-[Ru₂(TiPB)₂(O₂CCH₃) ₂] (5) and [Ru₂(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 σ²π⁴δ²(δ¹π ¹)³ and σ²π⁴δ ²δ²π² 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 Ru2II,II complex 6 [2.2425(6) ] is the shortest observed for a diruthenium tetracarboxylate and, surprisingly, is 0.014 shorter than in the analogous Ru2II,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 [Ru₂(O₂CH)₄] and [Ru₂(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 δ¹ and π¹ orbitals.