Synthetic control and empirical prediction of redox potentials for Co4O4 cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation.

Chemical Science View Article Online Open Access Article. Published on 07 April 2017. Downloaded on 31/08/2017 10:18:18. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2017, 8, 4274 View Journal | View Issue Synthetic control and empirical prediction of redox potentials for Co4O4 cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation† Andy I. Nguyen, ab Jianing Wang,a Daniel S. Levine, and T. Don Tilley*ab a Micah S. Ziegler ab The oxo-cobalt cubane unit [Co4O4] is of interest as a homogeneous oxygen-evolution reaction (OER) catalyst, and as a functional mimic of heterogeneous cobalt oxide OER catalysts. The synthesis of several new cubanes allows evaluation of redox potentials for the [Co4O4] cluster, which are highly sensitive to 4+ III IV 5+ and the ligand environment and span a remarkable range of 1.42 V. The [CoIII 4 O4] /[Co3 Co O4] Received 10th February 2017 Accepted 3rd April 2017 IV 5+ III IV 6+ redox potentials are reliably predicted by the pKas of the ligands. [CoIII 3 Co O4] /[Co2 Co2 O4] Hydrogen bonding is also shown to significantly raise the redox potentials, by
500 mV. The potential- pKa correlation is used to evaluate the feasibility of various proposed OER catalytic intermediates, DOI: 10.1039/c7sc00627f including high-valent Co-oxo species. The synthetic methods and structure–reactivity relationships rsc.li/chemical-science developed by these studies should better guide the design of new cubane-based OER catalysts. Introduction Research on catalysts for the oxygen evolution reaction (OER), motivated by the goal of creating an articial photosynthesis system, has generated a number of hypotheses that challenge the traditional limits of transition-metal oxidation states and bonding. Thus, catalytic OER cycles oen invoke unusually high and rare oxidation states for the metal centers, which are bound to reactive terminal oxo ligands.1–9 In particular, for intensely studied cobalt-based OER catalysts, a CoIV-oxo intermediate is commonly invoked.3,6,9 However, the CoIV oxidation state is quite rare, and ligand eld considerations seem to suggest a high instability for a terminal oxo ligand bound to CoIV.10 Recently, several experiments have conrmed the presence of CoIV species during water oxidation catalysis, but evidence for a discreet, terminal CoIV oxo species remains indirect.3,9,11,12 Remarkably, many density functional theory (DFT) calculations of cobalt-catalyzed OER suggest an even more unconventional species, a CoV-oxo intermediate that is sometimes described as a CoIV-oxyl radical.4,5 Experimental evidence for this type of intermediate is provided by kinetic analysis of OER mediated by a molecular Co4O4 cubane, which is a rare example of a Department of Chemistry, University of California, Berkeley, California 94720-1460, USA. E-mail: b Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA † Electronic supplementary information (ESI) available. CCDC 1532026–1532032. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc00627f 4274 | Chem. Sci., 2017, 8, 4274–4284 a structural and functional model for the active site of OER catalysis.6a For the oxo cubane Co4(m-O)4(OAc)4(py)4 a singly IV oxidized state, formally CoIII 3 Co , is rmly established by 3,6a isolation and spectroscopy. The doubly oxidized state, IV formally CoIII Co , is observed at highly positive potentials, by 2 2 cyclic voltammetry.13 While the kinetic analysis points to involvement of the doubly oxidized state in the oxygen evolution mechanism, the nature of intermediates associated with this oxidation state remains largely unknown. IV For evaluation of possible CoIII (or alternatively 2 Co2 III V Co3 Co ) intermediates, it would be quite useful to establish reliable methods for predicting the redox potentials associated with particular coordination environments. For example, the Co4O4 cubane Co4(m-O)4(OAc)4(py)4 has been shown to form the hydroxide complex [Co4O4(OAc)3(OH)2(py)4] by displacement of acetate, and kinetic studies on OER indicate that this species is oxidized to high-valent species.6 The resulting, transient intermediates have not been directly observed; therefore, alternative, indirect experimental methods for evaluation of candidate structures are useful. As shown here, a strategy for estimation of CoIV/CoV redox potentials for transitory intermediates is based on extrapolation of linear free-energy relationships (LFERs). This analysis requires a large and diverse set of related cubanes with various ligand sets and redox potentials, to provide a useful LFER from which redox properties can be condently predicted. In this report, we demonstrate that the Co4O4 core is readily and precisely manipulated to tune its chemical and electronic properties over an unprecedented range. While the parent cluster Co4O4(OAc)4(py)4 has been well-studied, its controlled This journal is © The Royal Society of Chemistry 2017 View Article Online Open Access Article. Published on 07 April 2017. Downloaded on 31/08/2017 10:18:18. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Edge Article structural modication has never before been demonstrated; such clusters are generally prepared by “self-assembly” methods rather than by rational syntheses.14 Synthetic methods were used to obtain electron-rich or electron poor cubanes, cubanes with mixed-carboxylate ligand sets, and cubanes possessing secondary-sphere hydrogen-bond donors. It is noteworthy that this synthetic methodology allows introduction of secondary-sphere hydrogen bonding into the cubane structure since the role of hydrogen bonding in electron transfer and water oxidation (especially in the OEC of photosystem II) has been an important topic for many years.1b–h Analysis of a substantial library of oxo cubanes provides empirical linear correlations between ligand pKa values and redox potentials for singly and doubly oxidized species, over a range of 1.42 V. This analysis also quanties the effect of hydrogen bonding on redox properties in this cubane system. These relationships offer a useful predictive tool for evaluating potential intermediates in water-splitting mechanisms. They should also provide important guidance in catalyst design studies for OER. Results and discussion Synthesis of new cobalt oxo cubanes The synthesis of Co4O4 cubanes has previously been accomplished by the “self-assembly” route.14b While this method is simple to Chemical Science execute, its harshly oxidizing conditions and less predictable nature prevents the targeted synthesis of many structurally diverse cubane clusters. Notably, the “self-assembly” route only leads to symmetrically ligated cubanes, and has only been demonstrated with ligands that are not very el (...truncated)