Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

Abstract

Metal-alkane binding energies have been calculated for [CpRe(CO) 2](alkane) and [(CO)2M(C5H4)C'C(C5H4)M(CO)2](alkane), where M ‫؍‬ Re or Mn. Calculated binding energies were found to increase with the number of metal-alkane interaction sites. In all cases examined, the manganese-alkane binding energies were predicted to be significantly lower than those for the analogous rhenium-alkane complexes. The metal (Mn or Re)-alkane interaction was predicted to be primarily one of charge transfer, both from the alkane to the metal complex (70 -80% of total charge transfer) and from the metal complex to the alkane (20 -30% of the total charge transfer).binding energy ͉ COH activation ͉ DFT calculations ͉ manganese C arbon-hydrogen (COH) bond activation reactions are important from a fundamental point of view, as well as in more practical terms to medicine, academia, and industry, as they are being used for the conversion of common, inexpensive alkanes into reactive (and physiologically active) molecules (1, 2). Many transition metal complexes are now known that can be used to activate COH bonds in alkanes (3). With low-valent metal complexes, the first bond-breaking step involves oxidative addition of an alkane to an unsaturated metal center (Fig. 1). The goal of subsequent steps is to convert the alkyl group R into an alcohol ROH or other potentially useful functionalized organic molecule.Many researchers today are focusing on the development of industrially practical organometallic oxidation catalysts (5-8), but there remain fundamental aspects of the COH activation process that are not fully understood. For instance, it is widely accepted that COH activation reactions proceed via alkane -complex intermediates (3) (Fig. 1, compound 3), and such species have recently been detected and studied in lowtemperature NMR experiments (9-11). Although complexes with intramolecular COH/metal (so-called agostic) interactions have been isolated and crystallographically characterized (3), in systems where strong evidence has been provided for intermolecular metal-alkane coordination in solution (9-11), isolation and solid state structural characterization have not yet been accomplished.† † Intermolecular metal-alkane complexes are therefore one of the most important targets in the study of COH activation; an understanding of the mechanisms of COH activation, which will allow researchers to better control and manipulate the outcome of the reactions, is crucial for the advancement of this area of chemistry.Quantum chemical methods have become useful and practical tools in study of TM complexes (14-17). Particularly, advancements in density functional theory (DFT) (16) and the use of effective core potentials (14, 15) have made qualitatively accurate predictions of the structures and chemistry of TM complexes possible at a reasonable computational cost (17). The goals of the present work are twofold: to make computational predictions of the relative stabilities of a selection of alkane -complexes, which will aid synthetic chemists in th...

The Electronics of CH Activation by Energy Decomposition Analysis: From Transition Metals to Main-Group Metals

“…Scheme 6 BP86 ALMO-EDA energies reported by Cobar et al [44] (kcal/mol) the BP86 ALMO-EDA results reported by Bergman, Head-Gordon, and coworkers for the alkane complex interaction energies (ΔE INT ) of methane and heptane with CpRe(CO) 2 . The total DFT interaction energies compare very well to experimental estimates.…”

“…Historically, the stability of metal-alkane complexes has been attributed to back-bonding orbital interactions, and this is thought to be critical for strong interactions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Bergman, Head-Gordon, and coworkers used the ALMO-EDA scheme to analyze the origin of alkane coordination to an experimentally relevant CpRe(CO) 2 complex [44]. As discussed earlier, the ALMO-EDA method is advantageous because it directly determines the relative amounts of forward-bonding and backbonding orbital interactions in the context of their quantitative contribution to the total interaction energy.…”

Experimental and Computational Studies on the Iridium Activation of Aliphatic and Aromatic CH Bonds of Alkyl Aryl Ethers and Related Molecules

“…1 H NMR (CDCl 3 , 258C): d = 14.32 (s, 1 H; Ir=CH); 7.35, 6.88, 6.69 (dd, dd, td, 3 JA C H T U N G T R E N N U N G (C,F) = 9 Hz; C-H ar ); 106.6, 106.3, 106.2 (CH pz ); 15. 4,15.3,12.7,12.6,12.5,11.7 (6 Me); elemental analysis calcd (%) for C 22 H 27 BFN 6 OIr: C 43.1, H 4.4, N 13.7; found: C 43.3, H 4.6, N 13.6.…”

“…4, 149.8, 143.7, 143.5, 142.9 (C qpz ); 131. 4,126.0,118.9,113.5 (CH ar 37 mmol) was dissolved in C 6 H 6 (8 mL) and p-fluoroanisole (0.05 mL, 0.44 mmol) was added. The solution was stirred at 60 8C for 14 h. After this period of time, the solvent was evaporated under vacuum.…”

“…[1,2] One of the main goals of these studies is the efficient functionalization of methane and other hydrocarbons, and although this has not yet been achieved, important progress has been made and mechanistic information about some key steps has been gained. [3] In view of the weakness of the electronic interaction between the metal and the C À H bond, [4] which makes functionalization of saturated hydrocarbons a difficult target, research has concentrated on unsaturated hydrocarbons and molecules like alcohols, aldehydes, ethers, amines, and so forth, since metal coordination to a p bond or to a heteroatom can facilitate the selective cleavage of adjoining CÀH bonds. [5] Much of our own work on CÀH bond activation has centered on ethers.…”