Synthesis, Structures, Isomerism, and Dynamic Behavior of Metalated Derivatives of Methyl-Substituted Pyrroles in Triruthenium Clusters

Abstract

Direct thermal reaction of [Ru3(CO)12] with 1-methylpyrrole leads to the cluster [Ru3(μ-H)(μ3,η3-C4H3NMe)(CO)9] (1), in which metalation has occurred at the 3-position of the ring. In solution, two isomers, 1a,b, detected by 1H NMR at low temperature, are in dynamic equilibrium ([1a]/[1b] = 1.8 at 260 K in CDCl3), and coalesced signals are observed above room temperature. Crystals of 1 deposited from the mixed isomer solution were shown by single-crystal X-ray structure determination to be those of isomer 1a. The ligand μ3,η3-C4H3NMe in cluster 1a is coordinated in a previously unidentified way to the Ru3 triangle through σ-Ru−C bonds to two metal atoms and through an η2-interaction to the third. A zwitterionic description with positive charge largely located at the NMe group and a negative charge delocalized on the metal atoms is presented. The dynamic equilibrium between 1a and 1b involves a hydride shift between Ru−Ru edges and a 1,2-double bond shift within the μ3-ligand. Clusters [Ru3(μ-H){μ3,η3-C4H(Me)2NMe}(CO)9] (2) and [Ru3(μ-H){μ3,η3-C4H(Me)2NH}(CO)9] (3) obtained from 1,2,5-trimethylpyrrole and 2,5-dimethylpyrrole in refluxing THF adopt crystal structures corresponding to that of 1a but, unlike 1, exist in only one isomeric form in solution at low temperatures. However, different products are obtained from [Ru3(CO)12] and 2,5-dimethylpyrrole in refluxing toluene: [Ru3(μ-H){μ3,η3-CHC4H2(Me)N}(CO)9] (4, X-ray structure reported) and [Ru3(μ-H){μ3,η2-CHC4H2(Me)N}(μ-CO)(CO)9] (5), our proposed structure of which is based on spectroscopic evidence. Changing the solvent for the reaction has resulted in cleavage on the N−H bond to give N-bonded ligands, in activation of a methyl group, and in loss of H2. Thermal treatment of 1a,b in refluxing cyclohexane leads to a third isomer, [Ru3(μ-H)2(μ3,η2-C4H2NMe)(CO)9] (6), completely analogous to a known, structurally characterized Os complex. In more polar solvents, 6 reverts to a mixture of 1a and 1b.