Characterization of a rhodium-sparteine complex, [((−)-sparteine)Rh(η4-COD)]+: Crystal structure and DNMR/DFT studies on ligand-rotation dynamics

Abstract

A cationic rhodium-sparteine complex, [((−)-sparteine)Rh(η⁴-COD)]⁺ (1⁺; COD = 1,5-cyclooctadiene) was obtained, isolated as its tetrafluoroborate salt (1BF₄), and characterized using X-ray crystallography and multinuclear (¹H, ¹³C) NMR spectroscopy. This is the first structurally characterized sparteine complex of rhodium. The Rh–N bonds are unusually long (2.214(3) and 2.242(3) Å), apparently due to steric repulsion between COD and sparteine. ¹H NMR exchange experiments (EXSY) demonstrate a dynamic process that results in an overall 180° rotation of the COD methine protons in solution (CD₂Cl₂) with a first-order rate constant of 460 s⁻¹ at the coalescence temperature (314 K) and interpolated rate constant of 150 s⁻¹ at 298 K. Temperature-dependent NMR studies yield ΔH^‡ = 13.0 ± 0.3 kcal mol⁻¹, ΔS^‡ = −5 ± 1 cal mol⁻¹ K⁻¹, such that ΔG^‡₂₉₈ = 14.3 ± 0.3 kcal mol⁻¹. DFT studies (B3LYP) indicate that the loosely bound (−)-sparteine ligand rotates through a pseudo-tetrahedral transition state where both ligands are rotated approximately 90° relative to each other. While both ligands remain bound (η⁴-COD, κ²-sparteine), bonding to sparteine is weakened much more than bonding to COD in the transition state. DFT computed ΔG^‡₂₉₈ and ΔS^‡ values (15.55 kcal mol⁻¹ and −2.67 cal mol⁻¹ K⁻¹, respectively) agree very well with the experimental values. Attempts to find alternative mechanisms involving partial dechelation of COD and (−)-sparteine yielded slightly higher barriers along with positive ΔS values for intermediate formation.