Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn4 V4 O17 (OAc)3 ](3-) . The compound is based on a [Mn4 O4 ](6+) cubane core, which catalyzes the homogeneous, visible-light-driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V4 O13 ](6-) polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)3 ](2+) and the oxidant persulfate, visible-light-driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s(-1) is observed. Electrochemical, mass-spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water-oxidation catalysts.

C–H activation by transition metal oxo complexes is a fundamental reaction in oxidative chemistry carried out by both biological and synthetic systems. This centrality has motivated efforts to understand the patterns and mechanisms of such reactivity. We have therefore thoroughly examined the C–H activation reactivity of the recently synthesized and characterized late transition metal oxo complex PhB ( t BuIm)3CoIIIO. Precise values for the pK a and BDFEO–H of the conjugates of this complex have been experimentally determined and provide insight into the observed reactivity. The activation parameters for the reaction between this complex and 9,10-dihydroanthracene have also been measured and compared to previous literature examples. Evaluation of the rates of reaction of PhB­( t BuIm)3CoIIIO with a variety of hydrogen atom donors demonstrates that the reactivity of this complex is dependent on the pK a of the substrate of interest rather than the BDEC–H. This observation runs counter to the commonly cited reactivity paradigm for many other transition metal oxo complexes. Experimental and computational analysis of C–H activation reactions by PhB­( t BuIm)3CoIIIO reveals that the transition state for these processes contains significant proton transfer character. Nevertheless, additional experiments strongly suggest that the reaction does not occur via a stepwise process, leading to the conclusion that C–H activation by this CoIII–oxo complex proceeds by a pK a-driven “asynchronous” concerted mechanism. This result supports a new pattern of reactivity that may be applicable to other systems and could result in alternative selectivity for C–H activation reactions mediated by transition metal oxo complexes.

Manganese oxides are a highly promising class of water-oxidation catalysts (WOCs), but the optimal MnOx formulation or polymorph is not clear from previous reports in the literature. A complication not limited to MnOx-based WOCs is that such catalysts are routinely evaluated by different methods, ranging from the use of a chemical oxidant such as Ce(4+), photoactive mediators such as [Ru(bpy)3](2+), or electrochemical techniques. Here, we report a systematic study of nine crystalline MnOx materials as WOCs and show that the identity of the "best" catalyst changes, depending on the oxidation method used to probe the catalytic activity.

Late transition metal oxo complexes with high d-electron counts have been implicated as intermediates in a wide variety of important catalytic reactions; however, their reactive nature has often significantly limited their study. While some examples of these species have been isolated and characterized, complexes with d-electron counts >4 are exceedingly rare. Here we report that use of a strongly donating tris(imidazol-2-ylidene)borate scaffold enables the isolation of two highly unusual Co-oxo complexes which have been thoroughly characterized by a suite of physical techniques including single crystal X-ray diffraction. These complexes display O atom and H atom transfer reactivity and demonstrate that terminal metal oxo complexes with six d-electrons can display strong metal-oxygen bonding and sufficient stability to enable their characterization. The unambiguous assignment of these complexes supports the viability of related species that are frequently invoked, but rarely observed, in the types of catalytic reactions mentioned above. The studies described here change our understanding of the reactivity and bonding in late transition metal oxo complexes and open the door to further study of the properties of this class of elusive and important intermediates.

Transition metal oxo species are key intermediates for the activation of strong C‒H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo...