Investigation of the Dynamic Solution Behavior of Chloro(diene)rhodium(I) Phosphine Complexes with a Pendant Unsaturated Heterocycle at Phosphorus (2-pyridyl, 2-imidazyl; diene = COD, NBD)

Reaction of chloro(diene)rhodium(I) dimer with 2-pyridyl- and 2-imidazylphosphines resulted in the formation of the four-coordinate, square-planar complexes chloro(diene)rhodium-κ¹-P-phosphine [diene = 1,5-cyclooctadiene (COD), bicyclo[2.2.1]hepta-2,5-diene (NBD)]. The nitrogen in the β-position to phosphorus provoked a fast exchange of the olefin sites in solution. Influences on this system were investigated through variation of the substituents on the phosphorus, the diene, the metal, and the heterocycle and the substitution pattern of the heterocycle. Additional isotopic labeling experiments were performed with a deuterium (1-d₁) or a trideuteromethyl group (1-Me-d₃) in the ortho-position to nitrogen in 1 and one deuterium in each of the olefinic positions of COD (1-COD-d₂). These findings resulted in an inverse kinetic isotope effect (KIE) in the case of 1-Me-d₃ and 1-COD-d₂. No KIE was found by complete line-shape analysis when the pyridyl-nitrogen was labeled with ¹⁵N. The temperature dependence of T₁ relaxation times for the para-proton in the pyridyl moiety was found to be a diagnostic criterion for an interaction of the pyridyl moiety with the rest of the complex, allowing the estimate of the strength of the attractive part of this interaction. No ground-state experimental proof (X-ray structure, scalar coupling constants in NMR spectroscopy) was found for this kind of interaction. DFT calculations support the nonexistence of ground-state complexes with Rh-coordinated pyridyl-nitrogen. Calculations on the pathway of the dynamic behavior in comparison to the experimental results exclude an interaction of the pyridyl-nitrogen with the COD double bonds and speak in favor of a temporary coordination of the pyridyl nitrogen to the metal center in a transition state. The calculated and measured kinetic data (including the KIEs) are in good agreement with this mechanism.