Structural and Spectroscopic Characterization of Reaction Intermediates Involved in a Dinuclear Co–Hbpp Water Oxidation Catalyst

Abstract

An end-on superoxido complex with formula {[Co III (OH 2 )(trpy)][Co III (OO·)(trpy)](µ-bpp)} 4+ , 3 4+ , (bpp -is bis-2-pyridyl-3,5-pyrazolate; trpy is 2,2';6':2"-terpyridine) has been characterized by resonance Raman, electron paramagnetic resonance and x-ray absorption spectroscopies. These results together with on-line mass spectrometry experiments using 17 . Density Functional Theory calculations agree and complement the experimental data, and offer a complete description of the transition states and intermediates involved in the catalytic cycle.Oxygen activation by first-row transition metal complexes in low oxidation states has been a very active field of research for the last two decades.1 A plethora of transition metal peroxido and superoxido complexes in different coordination modes have been prepared and characterized with spectroscopic techniques and even via single-crystal X-ray diffraction in selected instances.2 The reverse reaction, the oxidation of water to molecular oxygen assisted by first rowtransition metal complexes is a field that has emerged recently and the proper characterization of the potential peroxido and/or superoxido reaction intermediates is practically nonexistent. 3 The characterization of such intermediates is hampered by the lability of the metal-ligand bonds that can undergo substitution by water solvent molecules and by the relatively low temperature range at which the reaction can be operated. In sharp contrast, the inverse reaction i.e. the oxygen activation can be carried out in organic solvents and at very low temperatures. Additionally, for the water oxidation reaction, in a number of cases, a competing and/or preferential ligand oxidation occurs 4 which prevents extraction of reliable and meaningful information. In previous work, we have reported the synthesis and X-ray structure of the dinu-3+ hereafter, (trpy is 2,2';6':2"-terpyridine; bpp -is the bis-2-pyridyl-3,5-pyrazolate) that behaves as powerful catalyst for the 4e -reduction of dioxygen to water. 5 The key structures are depicted in Scheme 1. Further, we have electrochemically characterized the properties of 1 3+ and have shown by voltammetric and potentiometric techniques its capacity to act as a catalyst for the 4e -oxidation of water to dioxygen. ).

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”

Observation of O–O Bond Forming Step of Molecular Co4O4 Cubane Catalyst for Water Oxidation by Rapid-Scan FT-IR Spectroscopy

“…Specifically, assignment to a Co=O moiety, which for the partially labelled intermediate would only give rise to bands at 833 and 804 cm -1 is ruled out. In the absence of a Co4O4 cubane structure, the18 O frequency shift for the fully labeled CoOOCo μ-O2 moiety is 44 cm -1 40. The smaller, 30 cm -1 shift of the cubane peroxido intermediate observed hereimplies that the O-O stretch mode is coupled with Co-16 O cubane stretch modes thereby diminishing the isotope shift of the normal mode absorbing at 834 cm -1 (as mentioned above, oxygens of the Co4O4 cubane core of the16 O parent compound were shown not to exchange with water).17,18 18 O isotope shifts of normal modes that are smaller than the ones expected for pure O-O bond modes are due to coupling with16 O bond vibrations of similar frequency, which is an established phenomenon, e.g.…”

Armed by Asp? C-terminal carboxylate in a Dap-branched peptide and consequences in the binding of CuII and electrocatalytic water oxidation

“…Several Mn, [15][16][17][18] Fe, [19][20][21][22] Co, 23,24 Ni 25, 26 and Cu 27-41 containing complexes have been reported to promote the OER. While the homogeneous nature of catalysis has been clearly demonstrated in a number of cases, other complexes underwent changes (ligand loss, exchange and/or decomposition) and served as precursors to in situ deposited, stable electrocatalytic thin layers with characteristic activity.…”