57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

During this time period one postdoctoral scholar was directly funded and a second postdoctoral scholar was closely associated with all of the work reported here. This second postdoctoral scholar will be fully funded during the second year of the grant period. The first year has impacted both postdoctoral scholars and the PI by providing the opportunity to broaden their computational simulation expertise. For example, during year one the postdoctoral scholars and PI were able to expand their expertise of electronic structure calculations to include valence bond-type calculations as well as ab initio direct dynamics simulations.

Computational work during this first year focused on: 1) Thermodynamic stability of metal alkyl and metal heteroatom bonds. 2) The nature of C-H and H-H bond activation transition state structures. 3) Comparison of aliphatic and aromatic Ni mediated C-H activation.

1. Thermodynamic stability of metal alkyl and metal heteroatom bonds

The stability of metal alkyl (M-C) and metal heteroatom (M-X) bonds plays a critical role in the thermodynamics and the kinetics of C-H bond activation reactions. We have carried out density functional and correlated ab initio calculations that directly compare and analyze bonding interactions in late-transition-metal M-C and M-X bonds with Pt, Ru, Rh, and Ir metal centers. This study follows up on our initial study that suggested d_pi-p_pi repulsion in C-H activation transition states is not as important as ground-state effects, such as the cis-ligand effect. What evolved from this study was a clear model that orbital charge transfer stabilization determines bonding trends along the alkyl to heteroatom ligand series (CH₃, NH₂, OH, F). This study also ruled out the controlling feature of Pauli repulsion, which includes contributions from d_pi-ppi repulsion.

2. The nature of C-H and H-H bond activation transition state structures.

To understand the chemical factors that control transition-state geometries we investigated dihydrogen activation transition states as a model for hydrocarbon C-H bond activation. This was done because H₂ provides a sigma-type bond without complication by a tetrahedral sp³ carbon center or effects from pi-type interactions. We used absolutely localized molecular orbital energy decomposition calculations to analyze dihydrogen activation transition states and reaction pathways for transition metal complexes with d⁰, d⁶, d⁸, and d¹⁰ orbital occupation with a diverse range of metal ligands. We discovered that for transition states, similar to dihydrogen sigma complexes, a continuum exists of activated H-H bond lengths that can be classified as “dihydrogen” (0.8-1.0Å), “stretched or elongated” (1.0-1.2Å), and “compressed dihydride” (1.2-1.6Å). Our calculations also revealed, quantitatively for the first time, that the transition-structure geometry depends on back-bonding orbital interactions and not forward-bonding orbital interactions. This is true regardless of the mechanism or whether the metal ligand complex acts as an electrophile, ambiphile, or nucleophile towards dihydrogen.

3. Comparison of aliphatic and aromatic Ni mediated C-H activation

We have also used this time period to begin to understand the difference between aliphatic and aromatic C-H activation reactions. We have used density functional theory to investigate the the kinetics and mechanism of the C–H activation/metalation step involved in the formation of PCP- and POCOP-type pincer complexes of nickel. Experimental competition studies showed that metalation is more facile for aromatic ligands 1,3-(i-Pr₂PE)₂C₆H₄ vs their aliphatic counterparts 1,3-(i-Pr₂PECH₂)₂CH₂ (sp² C–H > sp³ C–H; E = O, CH₂) with NiBr₂. Our density functional calculations showed that PCP and POCOP ligands react with NiBr₂ to generate nonmetalated, phosphine-coordinated, intermediates first and then C-H activation proceeds by two-steps involving ionic dissociation of the bromide to give a tight ion pair intermediate followed by bromide-assisted deprotonation of the C–H bond. We found that the aromatic C-H bonds activate/metalate more readily than their aliphatic analogues for multiple reasons, including the higher ground state energy of the nonmetalated intermediates formed with aromatic ligands, the stronger C_sp²–Ni bond formed via metalation, and the more stabilized anionic charge on the C atom being metalated.