Light-Initiated C–H Activation via Net Hydrogen Atom Transfer to a Molybdenum(VI) Dioxo

Abstract

MoO2Cl2(bpy- t Bu) (1) is shown to be a potent one-electron oxidant upon irradiation with 365 nm light in various solvents, while being a weak two-electron oxidant in the dark. Complex 1 is characterized to activate various types of C–H bonds photochemically, including allylic and benzylic positions as well as alkanes and aldehydes. In all of these oxidations, 1 ultimately forms a bimetallic Mo(V)/Mo(V) species with a μ-oxo ligand (2). Depending on the substrate, the major organic product is identified as either an oxygenated or a C–C coupled (homodimerized) compound along with a minor chlorinated species. The product selectivity is proposed to be dependent upon the relative values between the bond dissociation enthalpy (BDE) of a potentially new C–OH bond within the product versus the BDE of a Mo–OH motif within a Mo(V)O(OH) intermediate. Based on this, we can estimate the BDE for Mo–OH to be 83–93 kcal/mol. Mechanistic studies suggest that the C–H activation occurs via a net hydrogen atom transfer (HAT) from 1* occurring either asynchronously or via a stepwise electron–proton transfer (ET–PT) process. Complex 2 is further demonstrated to reform dioxo 1 in the presence of chemical oxidants.

The Role of Anionic Ligands on Photoreactivity in Mo(VI) Dioxo Complexes of the Form MoO2X2(NN)**

“…The region greater than 250 nm in compounds [ 1 ]–[ 5 ] shows a combination of LMCT and ligand to ligand charge transfer (LLCT) bands that extend near to or into the visible range (Figure 3). It was shown that the lower energy band in complex [ 2 ] is LMCT pby that was used for photoreactions with substrates [1] . Complex [ 3 ] also shows a similar band to [ 2 ] with a slight red‐shift.…”

“…The details of the starting materials and suppliers and further purifications are provided in the Supporting Information. Compounds MoO 2 Cl 2 (bpy‐ t Bu) and MoO 2 Cl 2 (DMSO) 2 were synthesized as previously reported [1] …”

“…We have previously proposed that the first excited state of MoO 2 Cl 2 (bpy‐ t Bu) photochemically activates C−H bonds upon a ligand to metal charge transfer (LMCT) from the bipyridyl ligand to a Mo‐based orbital. Increased electron density on the MoO 2 core and the transient radical cation character of the bipyrdyl ligand in the excited state is proposed to mediate an asynchronous stepwise hydrogen atom transfer, with the bipyridyl ligand acting as the electron acceptor and the Mo−O accepting the proton [1] . With such a dearth of examples of metal oxos capable of photochemical C−H activation and functionalization, systematic ligand variation studies to interrogate the critical structural properties that underpin photoreactivity are also rare.…”

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The photoreactivity of d 0 metal dioxo complexes in activating CÀ H bonds has been recently studied. [1][2][3] We have previously reported that MoO 2 Cl 2 (bpy-t Bu) is an effective platform for light initiated CÀ H activation with unique product selectivity for the overall functionalization. [1] Herein we expand on these studies and report the synthesis and photoreactivity of several new Mo(VI) dioxo complexes with the general formula MoO 2 (X) 2 (NN); where X=F À , Cl À , Br À , CH 3 À , PhO À , t BuO À and NN = 2,2'-bipyridine (bpy) or 4,4'-tert-butyl-2,2'bipyridine (bpy-t Bu).

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“…[1][2][3] We have previously reported that MoO 2 Cl 2 (bpy-t Bu) is an effective platform for light initiated CÀ H activation with unique product selectivity for the overall functionalization. [1] Herein we expand on these studies and report the synthesis and photoreactivity of several new Mo(VI) dioxo complexes with the general formula MoO 2 (X) 2 (NN); where X=F À , Cl À , Br À , CH 3 À , PhO À , t BuO À and NN = 2,2'-bipyridine (bpy) or 4,4'-tert-butyl-2,2'bipyridine (bpy-t Bu). Among these compounds, MoO 2 Cl 2 (bpy-t Bu) and MoO 2 Br 2 (bpy-t Bu) are able to participate in bimolecular photoreactivity with several substrates containing CÀ H bonds of various types such as allyls, benzyls, aldehydes (RCHO) and alkanes.…”