A Computational Study of Ethylene C−H Bond Activation by [Cp*Ir(PR3)]

Abstract

It has previously been demonstrated that both [(C5Me5)Ir(PMe3)(CH=CH2)H] and [(C5Me5)Ir(PMe3)(H2C=CH2)] are formed when [(C5Me5)Ir(PMe3)] is thermolytically generated in the presence of ethylene. At higher temperatures, the vinyl hydride is converted to the  2 -ethylene adduct. Density functional theory has now been used to investigate this reaction, using the B3LYP functional, two types of basis sets (LanL2DZ and TZV*) and two models of the [( C5R5)Ir(PR3)] species (R = H and CH3). The study consists of full optimizations of local minima, first order saddle points, and minimum energy crossing points (MECP). The experimental results are best accounted for by considering both singlet and triplet spin surfaces. The relative energies of singlet [(C5R5)Ir(PR3)(CH3)H], [(C5R5)Ir(PR3)(CH=CH2)H], and [(C5R5)Ir(PR3)(H2C=CH2)] are in good agreement with experiment, as is the calculated barrier for the conversion from the vinyl hydride to the  2 -alkene complex. However, the singlet surface alone fails to explain the experimentally observed product ratio, or the intermediate inferred from experimental isotope effect studies. Locating the MECP between singlet and triplet surfaces indicates that the thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5R5)Ir(PR3)]. The weak van der Waals adduct of triplet [(C5R5)Ir(PR3)] and ethylene is proposed to be the key intermediate in the overall reaction.The interchanging of the available ethylene CH bonds in this triplet -complex accounts for the observed kinetic isotope effects, and partitioning between alkene -complexation and CH bond activation may also occur from this common intermediate. The possible role of steric factors and molecular dynamics are also discussed.

Orbital Splitting and Pairing Energy in Open‐Shell Organometallics: A Study of Two Families of 16‐Electron Complexes [Cp2M] (M = Cr, Mo, W) and [CpM(PH3)] (M = Co, Rh, Ir)

“…For the Rh and Ir systems, the triplet was reported to be more stable by 8.17 and 20.1 kcal mol –1 at the MP2/LANL1DZ level,18 and by 13.2 and 16.9 kcal mol –1 at the B3LYP/LANL2DZ level,19 respectively. However, another report on the Ir system gives a relative stability for the triplet of 6.4 or 8.4 kcal mol –1 by using B3LYP and the LANL2DZ or TZV basis sets 14. In all reported cases, except one,26 the stability of the triplet state relative to the singlet on going from the 4d to the 5d metal decreases for the Cp 2 M system and increases for the CpM(PH 3 ) system.…”

Theoretical Investigation of the Low-Energy States of CpMoCl(PMe3)2 and Their Role in the Spin-Forbidden Addition of N2 and CO

“…5 This general area, also termed "multiple state reactivity" or "spin crossover reactivity", is quite distinct from the more widely appreciated field of photochemical reactivity since no photoncoupled excitation or decay processes are involved. It has been shown to be relevant in gas-phase reactions of highly unsaturated metal-containing fragments, [6][7][8][9] in biochemically relevant processes, [10][11][12][13] in C-H activation processes by organometallic complexes, 14,15 and in olefin polymerization catalysis. [16][17][18][19][20] Unsaturated organometallic complexes (e.g.…”

Open shell organometallics: a general analysis of their electronic structure and reactivity

“…Schematic drawing of the competitive coordinative and oxidative additions of ethylene to CpIr(PH 3 ). Adapted from [68].…”

“…19(b). Incidentally, the reactive intermediate is generated by reductive elimination of cyclohexane from the diamagnetic cyclohexyl hydride, thus this is another example of a spin accelerated reaction, though according to the calculations the MECP leading to the triplet intermediate is not significantly stabilized relative to the singlet dissociation asymptote [68].…”

“…1:2 ratio could not find a proper rationalization when the consideration were limited to the spin singlet PES [67]. It is only possible to rationalize this result upon addressing the process as a spin crossover reaction [68]. By using the simplified CpIr(PH 3 ) model, which has a spin triplet ground state, it was found that a common triplet intermediate, the r-complex CpIr(PH 3 )(g 2 -H-C 2 H 3 ), leads to two TSs of similar energy, MECP(1) and SP(1), see Fig.…”