Light driven water oxidation by a single site cobalt salophen catalyst

Abstract

A salophen cobalt(II) complex enables water oxidation at neutral pH in photoactivated sacrificial cycles under visible light, thus confirming the high appeal of earth abundant single site catalysis for artificial photosynthesis.Inspired by the natural Mn 4 CaO x oxygen evolving centre in photosystem II, 1 remarkable efforts have been dedicated towards the development of multinuclear transition metal complexes enabling water oxidation for artificial photosynthesis. 2,3 Multimetallic catalysts could in principle distribute the oxidation equivalents necessary for water oxidation over several metal centres, thus lowering the energy barrier of the overall catalytic process. 3 However, the design of multi-metallic cores with oxygen evolving activity poses synthetic and stability hurdles. Noteworthily, single site metal complexes have been recently discovered, whose oxygen evolving activity offers a major opportunity to broaden the catalyst space within fundamental coordination chemistry. 4 Of particular interest are the earth-abundant cobalt complexes featuring polydentate organic ligands, such as corroles, polypyridines, porphyrins, and polyamines, which have been used under dark electrocatalysis conditions, 5 and in few cases also within photoactivated cycles. 6 Ligand diversity is expected to play a crucial role in tuning photocatalytic water oxidation, with the urgent quest to both optimize sequential photoinduced electron transfer and facilitate the dark-phase of the catalytic mechanism under a turnover regime.In this communication we present a cobalt(II) complex with a salophen ligand (N,N 0 -bis(salicylaldehyde)-1,2-phenylenediamine), ) as the sacrificial electron acceptor (Scheme 1). Combined UV-vis, dynamic light scattering (DLS) and Electron Paramagnetic Resonance (EPR) evidence identifies CoSlp as a competent oxygen evolving catalyst (OEC), enabling a two-fold photoinduced electron transfer in the ms time-frame. Our results confirm the ''privileged'' nature of the salophen ligand environment, readily available from simple condensation reactions, with wide applicability in different fields of catalysis, including bioinspired oxidations. CoSlp is obtained by direct reaction of cobalt acetate with the salophen ligand in methanol, followed by precipitation with diethyl ether and recrystallization.8 † Its solid state and solution identity was initially verified using FT-IR and UV-vis spectroscopy through comparison with literature data (Fig. S2 and S3, ESI †), 8 then confirmed using ESI-MS, where a base peak at m/z = 374 was observed, ascribed to the [CoSlpÁH] + ion (Fig. S4, ESI †). In the solid state, the cobalt ion in CoSlp displays a square planar geometry, 8c while in aqueous solution it extends the coordination sphere to square pyramidal, by ligation of a water molecule in the apical position. 8d Spectrophotometric titration yields a pK a = 6.40 for the aquo ligand (Fig. S5, ESI †), 9 which is therefore expected to be deprotonated at neutral pH, turning into a hydroxo moiety. Characterisation of CoSlp by...

Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer

“…40 2 water-oxidation catalysts have also been tested under photocatalytic conditions, usually using the sacrificial electron acceptor S2O8 2-and a molecular photosensitizer (PS) such as [Ru(bpy)3] 2+ or a derivative thereof. 6,7,23,28, Under such conditions, the TOF is generally much lower (< 1 s -1 ) than that obtained by CAN-driven chemical oxidation (> 300 s -1 ). 7,28, More importantly, the stability of photocatalytic systems is also lower (generally TON < 300, compared to >> 1000 for CAN-based systems), 7,28, which, independently of the nature of the catalyst, can be attributed to the decomposition of the photosensitizer.…”

“…6,7,23,28, Under such conditions, the TOF is generally much lower (< 1 s -1 ) than that obtained by CAN-driven chemical oxidation (> 300 s -1 ). 7,28, More importantly, the stability of photocatalytic systems is also lower (generally TON < 300, compared to >> 1000 for CAN-based systems), 7,28, which, independently of the nature of the catalyst, can be attributed to the decomposition of the photosensitizer. [64][65][66] The shift from chemical (CAN) to photocatalytic conditions brings more complexity to the wateroxidation reaction.…”

Earth-abundant metal complexes as catalysts for water oxidation; is it homogeneous or heterogeneous?

“…They checked the catalytic activity of the catalyst for photochemical water oxidation reaction using persulfate (S 2 O 8 2-) as the sacrificial electron acceptor and Ru(bpy) 3 2+ as the photosensitizer (PS) in neutral pH. 77 This complex was also able to catalyse the water oxidation to dioxygen in an electrochemical system. Under the catalytic reaction conditions, the quantum yield (ɸ O2 ) was found in the range of 0.048-0.079, which is equivalent to 9.6-15.8% efficiency in photon to O 2 conversion.…”

Cobalt complex containing pyrrol-2-yl-methanone as the light driven pre-catalyst to the oxidation of water

“…Structural characterization of the material indicates that, like PSII, the metal and oxygen atoms adopt a cubane arrangement [9]. Two representatives of homogeneous cobalt catalysts of WOC are the Co(II) pentapyridine complex reported by the Berlinguette group and the Co(II) porphyrin complex reported by the Sakai group [10][11][12]. Generally, homogeneous water oxidation to form dioxygen has to be driven by visible light, proceeding only in the presence of chemical oxidants.…”

“…The typical absorption of ligand at 323 nm shifts to 314 nm suggesting interaction of Co(II) with the conjugated system (3,5-dimethyl-1H-pyrrole-2-carbonyl portion, PPM section) in Co1. The bands at 524 nm, 569 nm and 637 nm are attributed to the 4 T 1g to 4 T 2g , 4 T 1g to 4 Characterization of Co1 by cyclic voltammetry in MeCN shows an irreversible peak at 0.98 V, assigned to the Co(II)/Co(III) redox potential [12]. In aqueous phosphate buffer at pH 8.5, the Co(II)/Co(III) redox potential shifted to 0.90 V (Fig.…”