Article pubs.acs.org/JPCC
Role of Electronegative Substituents on the Bond Energies in the Grubbs Metathesis Catalysts for M = Fe, Ru, Os Monica Vasiliu, Anthony J. Arduengo, III, and David A. Dixon* Chemistry Department, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336, United States S Supporting Information *
ABSTRACT: Coupled cluster theory [CCSD(T)] with the aug-cc-pVDZ/aug-cc-pVDZ-PP basis sets is used to predict the thermodynamic properties of models of the Grubbs catalyst, H2ImM(PH3)(CRR′) and H2ImM(C2H4)(CRR′) for M = Fe, Ru, and Os and CRR′ = CH2, CHF, and CF2. The PH3 and C2H4, imidazolinium carbene (H2Im), and CRR′ bond dissociation energies (BDEs) are reported. Because of low metal-carbene BDEs, the M = Fe complexes are unlikely to form, so they will not be good catalysts for olefin metathesis. The metal−carbene BDE is an important component in metathesis catalyst design and correlates with the singlet− triplet splitting in the carbene. The two metallacyclobutane intermediates (cis and trans to the imidazolinium carbene) formed by reaction of the CRR′CRR′ with the 14-electron active species (H2ImM(CRR′)) (R and R′ either H or F) are investigated at the same level of theory. The metallacycles cis to the nitrogen heterocyclic carbene are lower in energy than the trans conformer with the exception of four M = Fe metallacycles in the gas phase and in CH2Cl2 solution at 298 K. The olefin π complex for the simplest CH2 ligand plus C2H4 reactant combination is more stable in the gas phase, but in CH2Cl2 solution at 298 K, the cis metallacycles are more stable than the π complexes or the trans metallacycles on the free energy scale as a result of the large dipole moments in the cis metallacycles. The results show that the best energy balance is achieved with M = Ru, a CH2 carbene substituent, and a C2H4 reactant. The energetics for the Grubbs catalysts are shown to differ from those of the Schrock catalysts.
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and has been observed experimentally.56,57 The olefin then coordinates to the metal center, forming an alkene π complex. Next, similar to the Schrock catalysts, a metallacyclobutane is formed by insertion of the coordinated ethylene. The final step is ring-opening, with regeneration of the catalyst and formation of the new olefin, as shown in Scheme I. A good example of the prior computational studies is that of Zhao and Truhlar,46 who studied the mechanisms in the Grubbs catalysts for M = Ru and benchmarked relative energies for a model of the Grubbs second-generation catalyst at the coupled cluster with single and double excitations and a perturbative correction for triple excitations (CCSD(T)) level. There are two generations of the Grubbs catalysts.58,59 The focus of the current work is on the highly active, secondgeneration Grubbs catalysts in which one of the PR3 ligands from the first-generation catalyst is substituted by a nitrogen heterocyclic carbene ligand, based on the Arduengo carbene structure.60,61 Although the Grubbs catalysts are ruthenium complexes, our interest is in modeling not only ruthenium, but also the two other Group 8B metals, iron and osmium, to determine if they offer any advantages. For our study, we use
INTRODUCTION Catalytic olefin metathesis enables a variety of organic reactions to occur, and the importance of the metathesis reaction in chemistry was recognized by the 2005 Nobel Prize in Chemistry.1−3 The Schrock2 and Grubbs3 metathesis catalysts complement each other in terms of their reactivity and properties. The Grubbs catalysts have proven to be tolerant toward functional groups, are air-stable, and react with olefins rather than with acids, alcohols, or water, whereas the Schrock catalyst displays higher activity.4 Because of their reactivity toward olefins, ruthenium-based Grubbs catalysts are widely used in organic synthesis to create new unsaturated compounds and polymers that are very difficult to synthesize using other routes. The metathesis mechanism using ruthenium carbene complexes (Grubbs) as catalysts has been widely studied experimentally3,5−22 and computationally.23−52 A recent focus is the synthesis of Z- and stereoselective ring-opening/crossmetathesis Ru-based catalysts from the Hoveyda group.53−55 It was initially believed that the first step is addition of the olefin with formation of an octahedral 18-electron complex, an associative mechanism. After further experimental investigation, the dissociative path was found to be favored, and the first step is phosphine dissociation with formation of a four-coordinate intermediate, which is considered to be the actual active species © XXXX American Chemical Society
Received: January 15, 2014 Revised: May 21, 2014
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dx.doi.org/10.1021/jp500472p | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Scheme I
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COMPUTATIONAL METHODS The geometries were optimized, and harmonic vibrational frequencies were calculated at the density functional theory (DFT)63 level with the hybrid B3LYP64,65 and generalized gradient approximation BP8666,67 exchange-correlation functionals. In the DFT optimization and frequency calculations, the aug-cc-pVDZ basis set68 for all first and second row atoms and the pseudopotential (PP)-based aug-cc-pVDZ-PP basis sets69−71 for the transition metals were used. The above combination of basis sets (aug-cc-pVDZ/aug-cc-pVDZ-PP) will be denoted as “aD”. The 10 electrons in the 1s2s2p orbitals of Fe, the 28 electrons in the 1s2s2p3s3p3d orbitals of Ru, and the 60 electrons in the 1s2s2p3s3p3d4s4p4d4f orbitals of Os are modeled by the pseudopotential. Single point CCSD(T)72−75 calculations were performed with the aD basis set at the optimized B3LYP geometries, where the electrons in the 1s orbitals for the first row elements (C, N, F), the (2s,2p) orbitals for the second row elements (Cl, P), the (3s,3p) orbitals for Fe, the (4s,4p) orbitals for Ru, and the (5s,5p) orbitals for Os were not correlated. Tight d functions were included for phosphorus and chlorine atoms.76 The open-shell CCSD(T) calculations were done with the R/ UCCSD(T) approach in which a restricted open-shell Hartree−Fock (ROHF) calculation was initially performed, and the spin constraint was then relaxed in the coupled cluster calculation.77−79 Even for these model Grubbs catalysts, it is too computationally expensive to nearly impossible to calculate the complete basis set (CBS) energetics because of the size of the molecules, the number of active electrons, and the presence of open shells. As a check, the calculated BDEs at the CCSD(T)/aD results from our similar studies of the model Schrock catalysts62 were found to be 2 kcal/mol more positive as compared with the CCSD(T)/CBS (complete basis set) results. The reaction energies calculated at the CCSD(T)/aD level for the Grubbs metallacycles are thus likely to be 2−3 kcal/mol more positive than the CCSD(T)/CBS values. Thus, the bond dissociation energies (BDEs) for the M−PH3, M− C2H4, M−H2Im, and M = CRR′ (CRR′ = CH2, CHF, CF2) and M = Fe, Ru, and Os were calculated from 1,
PH3 as the phosphine ligand on the metal and H’s as substituents on the nitrogens of the imidazolinium carbene (H2ImRu(PH3)(CRR′); the 2 chlorines were omitted in the formula for simplification, as shown in Scheme II. Scheme II
We are interested in the effect of electronegative ligands on the carbene center, so following our previous computational study of the Schrock catalysts,62 the reaction site carbenes on the metal are CRR′ = CH2, CHF, and CF2. The reactions of not only ethylene but the mono- to tetrafluoro-substituted ethylenes are also studied in the current work. In our previous work on the Schrock catalyst, it was shown that electronegative groups substituted on the carbenic carbon (CRR′) led to less stable Schrock complexes. The results showed that consideration of the singlet−triplet splitting in the CRR′ carbene in the initial catalyst as well as in the metal product formed by the retro [2 + 2] cycloaddition is a critical component in the design of an effective olefin metathesis catalyst in terms of the parent catalyst and the groups being transferred. The high-level correlated molecular orbital theory method CCSD(T) is used to study the thermodynamics of these models of the Grubbs catalyst. The PH3 and C2H4 bond dissociation energies (BDEs) were predicted for these Grubbs catalysts. The PH3 BDEs provide insight into the energy necessary for the first step of the metathesis mechanism, phosphine dissociation, and the C 2 H 4 BDEs provide information in terms of the energetics involving the formation of the ethylene π complex. The role of ligand electronegativity on the methylene carbene was investigated by studying the BDEs of the complexes when CRR′ = CH2, CHF, and CF2. The imidazolinium carbene (H2Im) BDEs were also calculated for these Grubbs catalysts. Although many of the mechanistic questions have been answered in terms of the first step with formation of the 14-electron active species,15,56 there are still questions in terms of subsequent steps, so the metallacyclobutane intermediates were also studied.
ΔH0K = ΔEaD + ΔEZPE
(1)
where the ΔEaD is the single point CCSD(T) energy and ΔEZPE is the zero point energy (ZPE) at the B3LYP/aD level. Single point free energies of solvation in CH2Cl2 at 298 K were calculated at the gas phase geometries using the self-consistent reaction field approach80 with the COSMO parameters80,81 as implemented in the Gaussian 03 program.82 For the COSMO (B3LYP/aD) calculations in Gaussian 03, the radii developed B
dx.doi.org/10.1021/jp500472p | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
by Klamt and co-workers were used to define the cavity.81 The dichloromethane solution Gibbs free energy (ΔG(CH2Cl2)) was calculated from eq 2, ΔG(CH 2Cl 2) = ΔGgas + ΔΔGsolv
(2)
where ΔGgas is the gas phase free energy and ΔΔGsolv is the CH2Cl2 solvation free energy. A dielectric constant of 8.93 corresponding to that of bulk CH2Cl2 was used in the COSMO calculations. The solvation energy is reported as the electrostatic energy (polarized solute − solvent) and the sum with the nonelectrostatic energies. An estimate of the potential for multireference character in the wave function can be obtained from the T1 diagnostic83 for the CCSD calculation (see Supporting Information). The values for the T1 diagnostics are small (
