Aromaticity of Benzene in the Facial Coordination Mode: A Structural and Theoretical Study

Abstract

The effects of facial coordination of benzene to a trinuclear transition-metal cluster have been studied by structure correlation and DFT calculational methods. Data taken from the X-ray crystal structures of twelve complexes [(eta-C(5)H(4)R")Co(3)(micro(3)-eta(2):eta(2):eta(2)-C(6)H(4)RR')] 1 b-1 m were analyzed by using standard statistical methods. The prototypal facial arene ligand is considerably expanded with respect to free benzene and shows a small but highly significant Kekulè distortion (d(CC)=1.42, 1.45 A). DFT MO calculations were carried out on the model complexes [(eta-C(5)H(5))M(3)(micro-eta(2):eta(2):eta(2)-C(6)H(6))] 1 a (M=Co), 2 (M=Rh), and 3 (M=Ir). Ring currents in the facial benzene and apical cyclopentadienyl ligands have been assessed by nucleus independent chemical shift (NICS) calculations. Compared to the free ligand (with the optimized D(6h) structure as well as with D(3h) and a C(3v) geometries similar to that in the prototypal facial arene), facial benzene exhibits somewhat reduced but still substantial cyclic electron delocalization (CED). The calculated order of CED is benzene approximately [(CO)(3)Cr(eta-C(6)H(6))] 4 > 1 > 2 > 3.

Computational Studies of Nucleophilic Attack and Protonation of Electron-Deficient Benzoheterocycle Triosmium Clusters

“…Although experiments have shed light on many aspects of these issues, one cannot resolve all of these problems at the atomic level by experiments alone. Computational approaches [especially hybrid density functional theory (DFT) methods such as B3LYP] have proven to be very useful in resolving such issues. − Recently, we have performed a series of relevant B3LYP studies on benzoheterocycle triosmium clusters. , This paper is a continuation of our previous studies on triosmium clusters and focuses on elucidation of the mechanisms of H - and H + addition to the electron-deficient triosmium clusters.…”

Computational Studies of the Energetics and Reaction Pathways of Molecular Transformations of Benzoheterocycles on Triosmium Clusters

“…Despite these extensive experimental studies, there remain many questions related to the mechanisms of molecular transformations on the trimetallic framework, as well as the general mechanisms of the reactions of organic molecules on polymetallic surfaces that would benefit from comprehensive computational investigations. In fact, computational studies on understanding the rearrangements of ligands bound to trimetallic clusters, − their electronic structure, ,− and spectroscopic properties ,, have already yielded useful insights as to the origin of the reactivity trends in this class of compounds. However, to date there have been no computational studies that address the basic energetics of these systems and the factors governing their reaction pathways, partially because of the existence of technical difficulties.…”

Experimental and Computational Studies of Nucleophilic Attack, Tautomerization, and Hydride Migration in Benzoheterocycle Triosmium Clusters

“…In light of the recent successes in using quantum-mechanical methods, in particular the density functional theory (DFT) method, for understanding the energetics and reaction dynamics of organometallic complexes, we have undertaken a computational study on the complexes referred to above. − The goals of these studies are (1) to identify the nature of the transition states for the observed tautomerizations and (2) to understand the factors contributing to the regioselective nature of nucleophilic attack on these species. The results of these studies are reported here, along with the experimental details of the reactions of 7 and 8 with H - /H + and the kinetics of tautomerization of the products of these reactions.…”