Ambidentate Ligand Reactivity with the Rhenium(I) Compounds [BrRe(CO)4]2 and cis‐BrRe(CO)4L: A Kinetic and Mechanistic Study

Abstract

The reaction of the halo‐bridged dimer [BrRe(CO)4]2 with the ambidentate donor 2‐(diphenylphosphanyl)pyridine (PN) has been investigated. The substitution reaction is rapid at room temperature and regioselective for phosphorus coordination to yield exclusively cis‐BrRe(CO)4(κP‐PN) (1). Thermolysis of 1 furnishes the PN‐chelated product fac‐BrRe(CO)3(κP,N‐PN) (2) and CO. The kinetics for the conversion of 1 → 2 + CO have been measured by UV/Vis spectroscopy in toluene over the temperature range of 323–343 K. On the basis of the activation parameters [ΔH‡ = 28.0(0.9) kcal/mol and ΔS‡ = 31(3) eu], a dissociative process is supported, and this was corroborated by electronic structure calculations. The regioselectivity in the ligand‐substitution reaction involving [BrRe(CO)4]2 and the tridentate donor 6‐(diphenylphosphanyl)‐2‐formylpyridine (PON) was also studied, and consistent with the PN donor, only the κP‐product, cis‐BrRe(CO)4(κP‐PON) (3) is formed. The reaction of the dimer [BrRe(CO)4]2 with 2‐(diphenylphosphanyl)pyridine (PN) has been computationally modeled by DFT calculations. The energetics for the creation of an unsaturated intermediate through the cleavage of one of the bridging bromide ligands, followed by the addition of the donor to the unsaturated rhenium center versus the direct attack of the pnictogen donor on the dimer have been evaluated. The latter process is computed as the preferred route for dimer activation, with an attack of the pyridyl moiety slightly favored compared to the phosphine moiety. The computed pnictogen preference agrees with the kinetic data published by Zingales and coworkers. Concerning the reaction of the PN ligand with [BrRe(CO)4]2, we predict the initial formation of the κN‐isomer of 1 as the kinetic product of substitution, which in turn undergoes a rapid isomerization to furnish the thermodynamically more stable κP‐isomer through a reversible ligand dissociation process. The intramolecular linkage isomerization of κN‐1 to κP‐1 in 1 was also investigated by DFT, and the concerted process was found to lie 6.5 kcal/mol above the two‐step process involving ligand dissociation.