Mechanistic Study of Oxy Insertion into Nickel–Carbon Bonds with Nitrous Oxide

Abstract

Transition-metal-mediated oxy insertion into metal–carbon bonds is useful for the development of catalytic cycles for selective hydrocarbon oxidation. However, there are few bona fide examples of net oxy insertion with transition-metal complexes. An extremely rare example of a 3d metal mediating oxy insertion into metal–carbon bonds is a series of NiII alkyl complexes reacting with nitrous oxide (N2O) reported by Hillhouse and co-workers; however, the mechanism was never fully elucidated. A computational study has been performed on bipyridyl nickel metallacycles that form nickel alkoxides upon reaction with N2O to attain insight into future catalyst design for oxygen atom transfer reactions. Two possible mechanisms are explored. Of the two pathways, the computations suggest that the preferred mechanism proceeds through a Ni–oxyl intermediate followed by alkyl migration of the nickel–carbon bond to form an alkoxide. Oxyl formation was found to be the rate-determining step, with a free energy barrier of 29.4 kcal/mol for bpyNiII(cyclo-(CH2)4). Complexes that contain sp2-hybridized molecules at the β-carbon site within the metallacycle ring do not undergo oxy insertion due to elevated barriers. While exploring insertion with another oxidant, namely pyridine N-oxide, we found that N2O is critical for net oxy insertion with this complex due to the substantial thermodynamic advantage of N2 expulsion. Reaction with pyridine N-oxide necessitated expulsion of a “worse” leaving group, resulting in much higher barriers (ΔG ⧧ = 49.7 kcal/mol) for the oxyl formation step.

PtII and RhIII Hydrocarbyl Complexes Bearing Coordinated Oxygen Atom Delivery Reagents

“…[35] The reaction of [(bpy)Ni(cyclo-CH 2 ) 4 ] with N 2 O, which affords a cyclic Ni-alkoxido, was found to proceed through a nickel-oxido intermediate. Perhaps, the proclivity of the Ni II complex to adopt a high-spin oxyl (O -) configuration provides a reactive but energetically accessible intermediate that is a precursor for the oxy-insertion into the Ni-alkyl bond.…”

“…[28,29] Oxygen atom insertion into Pd-aryl bonds has been reported, [30,31] and Hillhouse and coworkers have reported examples of Ni-alkyl-to-Ni-alkoxido transformations using N 2 O. [32][33][34][35] Abu-Omar and Espenson reported that the reaction of methyltrioxorhenium (MTO) with oxidants generates methanol, [36] and recently, Periana, Goddard, and co-workers reported a proposed mechanism for this reaction that proceeds by oxidant coordination and subsequent methyl migration to the coordinated oxygen. [37,38] The reaction has also been extended to an analogous Re-aryl system, O 3 ReAr (Ar = mesityl, 2,6-xylyl), to produce the corresponding phenol, and a flavincatalyzed variant with MTO has been reported.…”

Computational Hammett analysis of redox based oxy-insertion by Pt(ii) complexes

“…However, when this reaction was probed computationally, the reaction was proposed to go through a Ni III −oxyl intermediate followed by alkyl migration onto the oxyl (O˙−) ligand. 3 Oxygen atom transfer to palladium-carbon bonds have also been reported for several Pd II systems. These reactions were proposed to form Pd IV -oxo complexes prior to aryl migration, which forms the aryloxide product.…”

“…2), suggested an important conceptual connection between the two transition states, which were linked in a structural sense to the identity of the external oxidant for d 6 and d 8 complexes. 3 It was proposed that oxidants that afford "better" leaving groups (Y) resulted in more "oxo-like" transition states and "worse" leaving groups yielded more "BV-like" transition states, Fig. 3.…”

Oxy-functionalization of Group 9 and 10 transition metal methyl ligands: use of pyridine-based hemi-labile ligands

“…48 The large calculated ΔG ‡ for the O atom transfer suggests an unfavorable oxy-insertion for this and related Ni complexes when using OPy as oxidant. Previous computational research on Ni II -R complexes suggests that more potent oxidants such as N 2 O are needed for oxyinsertion into Ni-carbon bonds, 49 which is supported by experiment. [50][51][52][53] Note in the cases modeled here that methyl migration to form methoxy complex has a lower barrier than the initial O atom transfer to make the Ni oxo complex.…”