Article pubs.acs.org/IC
Synthesis, Characterization, and Photocatalytic H2‑Evolving Activity of a Family of [Co(N4Py)(X)]n+ Complexes in Aqueous Solution Warrick K. C. Lo,†,# Carmen E. Castillo,‡ Robin Gueret,‡ Jérôme Fortage,‡ Mateusz Rebarz,§ Michel Sliwa,§ Fabrice Thomas,‡ C. John McAdam,† Geoffrey B. Jameson,∥ David A. McMorran,† James D. Crowley,*,† Marie-Noel̈ le Collomb,*,‡ and Allan G. Blackman*,⊥ †
Department of Chemistry, University of Otago, P. O. Box 56, Dunedin 9054, New Zealand Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes, F-38000 Grenoble, France § Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 CNRS-Université Lille 1 Sciences et Technologies, 59655 Villeneuve d’Ascq Cedex, France ∥ Institute of Fundamental Sciences, Massey University, P. O. Box 11-222, Palmerston North 4442, New Zealand ⊥ School of Applied Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand ‡
S Supporting Information *
ABSTRACT: A series of [CoIII(N4Py)(X)](ClO4)n (X = Cl−, Br−, OH−, N3−, NCS−-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV−visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV−visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3]2+ as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [CoII(N4Py)(HA)]+ is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10−6 M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3]2+ by ascorbate followed by an electron transfer from [RuII(bpy)2(bpy•−)]+ to the [CoII(N4Py)(HA)]+ catalyst. The reduced catalyst then enters into the H2-evolution cycle.
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= Cl−, NO3−, CF3SO3−, n = 2: X = OH2, n = 3)30 prompts us to detail our studies of the synthesis, characterization, and reactivity studies of a number of [Co(N4Py)(X)]n+ complexes. The pentadentate tetrapyridyl N5 ligand N4Py (N4Py = 1,1di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine)31 has been widely used in the synthesis of iron,31−34 ruthenium,35 and manganese complexes,36−41 but few Co(III) complexes have been reported.30 The Ni(II) complexes [Ni(N4Py)(X)]2+ (X = OH2, NCMe) have also recently been reported to be efficient catalysts for the electrochemical production of H2 from water.42 Herein, we describe the synthesis and characterization of a series of [CoIII(N4Py)(X)](ClO4)n (X = Cl−, Br−, OH−, N3−, NCS−-κN, n = 2; X = OH2, NCMe, DMSO-κO, n = 3)
INTRODUCTION Increasing global energy demands and growing concerns over climate change due to consumption of fossil fuels have resulted in a considerable amount of research being directed into the development of alternative, renewable, and more sustainable energy sources.1,2 Photoinduced water-splitting into molecular hydrogen and oxygen catalyzed by complexes of non-noble, earth-abundant transition metal ions has emerged as a viable candidate for energy production in recent years.3−5 Of these earth-abundant metal ions, cobalt has shown particular promise in this respect, with a number of cobalt complexes having been reported to catalyze both the oxidation and reduction of water.5−15 Co(III) complexes of the pentadentate polypyridyl N5 ligands shown in Figure 1 have attracted attention as catalysts for these reactions.16−29 The recent report on photocatalytic water reduction mediated by the cobalt(III) complexes [Co(N4Py)(X)]n+ (X © XXXX American Chemical Society
Received: February 16, 2016
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DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Pentadentate polypyridyl N5 ligands.
[Co(N4Py)(OH)]2+ cation (Scheme 1) which was isolated as [Co(N4Py)(OH)](ClO4)2·2H2O following removal of a brown solid from the reaction mixture, removal of solvent, and recrystallization from hot water with added NaClO4. The red solid, obtained in 51% yield, was converted to the aqua complex [Co(N4Py)(OH2)](ClO4)3·H2O by addition of concentrated perchloric acid to an aqueous solution of [Co(N4Py)(OH)](ClO4)2·2H2O and recrystallization from hot water (Supporting Information, Scheme S1). Both [Co(N4Py)Cl](ClO4)2·H2O and [Co(N4Py)(OH2)](ClO 4 ) 3 ·H 2 O were used as starting materials for the preparation of other [CoIII(N4Py)(X)]n+ complexes. Thus, [Co(N4Py)(N3)](ClO4)2·H2O and [Co(N4Py)(NCS-κN)](ClO4)2 were prepared by anation of [Co(N4Py)Cl](ClO4)2· H2O, while [Co(N4Py)(NCMe)](ClO4)3·0.5MeCN and [Co(N4Py)(DMSO-κO)](ClO4)3·DMSO were prepared by solvolysis of [Co(N4Py)(OH2)](ClO4)3·H2O in excess hot acetonitrile or dimethyl sulfoxide, respectively (Scheme S1). The bromido complex [Co(N4Py)Br](ClO4)2 was prepared by reaction of [Co(N4Py)(NCMe)](ClO 4 ) 3 ·0.5MeCN and Bu4NBr in acetonitrile. IR, HRESI-MS, and NMR Spectral Data of [CoIII(N4Py)(X)]n+ Complexes. Peaks corresponding to the coordinated N4Py ligand (≈1600, 1450, and 770 cm−1) and perchlorate counterions (≈1070 and 620 cm−1) are observed in the IR spectra of all [CoIII(N4Py)(X)](ClO4)n complexes, while IR peaks characteristic of the monodentate ligand were also observed: azide stretches at 2024 and 1279 cm−1 for [Co(N4Py)(N3)](ClO4)2·H2O;47,48 a broad CN stretch, indicative of N-bound NCS−,49 at 2078 cm−1 for [Co(N4Py)(NCS-κN)](ClO4)2; a coordinated acetonitrile ligand nitrile stretch at 2326 cm−1 for [Co(N4Py)(NCMe)](ClO4)3· 0.5MeCN;50 and a coordinated dimethyl sulfoxide ligand S− O stretch at 953 cm−1 for [Co(N4Py)(DMSO-κO)](ClO4)3.51 HRESI-MS data for the [CoIII(N4Py)(X)](ClO4)n complexes were obtained from acetonitrile solution. Mass spectra of complexes where the monodentate ligand X is anionic (X = Cl−, OH−, N3−, NCS−, and Br−) contain peaks corresponding to cobalt(III) ([Co(N4Py)(X)] 2+ and [Co(N4Py)(X)(ClO4)]+) and cobalt(II) ([Co(N4Py)]2+ and [Co(N4Py)(X)]+) species (Supporting Information, Figures S20−S28 and
complexes. The electrochemical properties of these complexes have been investigated in MeCN, and the low-valent Co(II) and Co(I) species generated by bulk electrolyses spectroscopically characterized. These complexes have also been tested as catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid (H2A)/ascorbate (HA−), using [Ru(bpy)3]2+ as a photosensitizer. Results of these studies suggest that the active species for H2 generation derives from the cobalt(II) complex [Co(N4Py)(HA)]+ in all cases.
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RESULTS AND DISCUSSION Synthesis of [Co(N4Py)(X)]n+ Complexes. The [Co(N4Py)Cl]2+ cation was synthesized by air oxidation of a 1:1 MeOH/H2O solution of free base N4Py and [Co(OH2)6]Cl2 (Scheme 1).43 The crude product mixture, which also Scheme 1. Synthesis of [CoIII(N4Py)(X)]n+ from [Co(OH2)6]2+ and N4Py via Air Oxidationa
a Conditions: (i) 1 equiv of [Co(OH2)6]Cl2, 1:1 MeOH/H2O, air (O2), room temp, 1 day; (ii) 1 equiv of [Co(OH2)6](ClO4)2, 1:1 MeOH/H2O, air (O2), room temp, 1 day.
contained some [Co(N4Py)(OH2)]3+, was purified on a Dowex cation-exchange column by elution with HCl.44−46 The purple band comprising pure [Co(N4Py)Cl]2+ was collected and reduced to dryness and the crude product precipitated following dissolution in hot water and addition of NaClO4. The solid was recrystallized from hot water to give [Co(N4Py)Cl](ClO4)2·H2O as a purple solid in 53% yield. Use of [Co(OH2)6](ClO4)2 in the air oxidation reaction gave the B
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Inorganic Chemistry S33−S34), while those of [Co(N4Py)(NCMe)](ClO4)3 and [Co(N4Py)(DMSO-κO)](ClO4)3, which have a neutral X ligand, contain peaks corresponding to cobalt(III) ([Co(N4Py)(X)]3+ and [{Co(N4Py)(X)}(ClO4)2]+) and cobalt(II) ([Co(N4Py)]2+ and [{Co(N4Py)}(ClO4)]+) species (Supporting Information, Figures S29−S32). The mass spectrum of [Co(N4Py)(OH2)](ClO4)3 is identical to that of [Co(N4Py)(OH)](ClO4)2; peaks corresponding to [Co(N4Py)]2+, [Co(N4Py)(OH)]2+ (base peak), [Co(N4Py)(OH)]+, and [{Co(N4Py)(OH)}(ClO4)]+ are observed in both spectra. The 1H and 13C NMR spectra of [Co(N4Py)(X)](ClO4)n complexes in deuterated solvents (D2O, DMSO-d6, acetone-d6, and/or CD3 CN) show sharp and well-resolved peaks (Supporting Information, Figures S1−S18), consistent with diamagnetic, low-spin d6 Co(III) complexes. Coordination of N4Py to cobalt(III) results in downfield shifts of all proton signals of N4Py (Supporting Information, Figure S1). Additionally, the methylene protons of the N4Py ligand split into diastereotopic pairs (Hga and Hgb; see Scheme 1 for proton assignments) in the complexes, due to the restricted rotational freedom of the 2-picolyl arms and can appear as an AB quartet, or a singlet, depending on the complex, solvent, temperature, and/or ionic strength of the solution. For example, the methylene protons of [Co(N4Py)(OH)](ClO4)2 give rise to an AB quartet at 4.89 and 5.11 ppm (JAB = 18.6 Hz) in DMSOd6 whereas a singlet at 5.02 ppm is observed in D2O. Furthermore, the AB quartet observed in DMSO-d6 collapses to a singlet in the presence of increasing concentrations of NaCl electrolyte (Supporting Information, Figure S19). Assignments of all eight pyridyl proton signals were possible with the aid of two-dimensional 1H−1H gCOSY, TOCSY, and NOESY NMR experiments. For example, in the 1H−1H NOESY NMR spectrum, through-space interactions between the methylene protons Hga and Hgb and the methine proton Hf, and the methylene protons and the pyridyl proton Hi are observed (Supporting Information, Figure S4). The latter interaction confirms the assignment of Hi, which is the proton at the 3-position of the pyridyl ring of a 2-picolyl arm. X-ray Structural Characterization of [CoIII(N4Py)(X)]n+ Complexes. Crystals of [CoIII(N4Py)(X)](ClO4)n suitable for analysis by X-ray crystallography were obtained via the crystallization techniques outlined in the Experimental Section. Table S2 gives crystallographic information, and ORTEP52 diagrams of the [CoIII(N4Py)(X)]n+ cations are shown in Figure 2 and Figure S35 (Supporting Information). The crystal structure of the complex [Co(N4Py)Cl]Cl2·6H2O has recently been reported by Wang et al.,30 with the cation displaying a geometry very similar to that found in [Co(N4Py)Cl](ClO4)2· 2MeCN, the structure of which is reported below. In each of the [CoIII(N4Py)(X)]n+ cations the central Co(III) ion is bonded to all five N donor atoms of the N4Py ligand, with the remaining coordination site, which is trans to the aliphatic amine N atom of N4Py, being occupied by a monodentate ligand (X = Cl−, OH2, OH−, N3−, NCS−, NCMe, DMSO, or Br−). The coordination modes for the ambidentate thiocyanato (N-bound) and dimethyl sulfoxide (O-bound) ligands are consistent with IR data. The Co(III) ion displays a slightly distorted octahedral geometry in each case, with cis donor−cobalt−donor angles ranging from 82.6(1)° to 101.4(1)° and trans angles ranging from 169.2(2)° to 180.0(3)°. In all complexes, the Co(III) ion sits slightly above the mean plane defined by the four pyridyl N atoms of the N4Py ligand (cobalt-to-plane distances ranging
Figure 2. ORTEP diagrams of the (a) [Co(N4Py)(OH2)]3+ and (b) [Co(N4Py)(NCMe)]3+ cations. Only one of the two crystallographically independent [Co(N4Py)(NCMe)]3+ cations is shown. Thermal ellipsoids are drawn at the 50% probability level.
from 0.159 to0.176 Å). The Co−N bond lengths for pyridyl N range from 1.899(5) to 1.956(4) Å, significantly shorter than those found in the tetrapyridylcobalt(III) cation of the complex trans-[Co(py)4(Cl)2][Co(py)(Cl)3] (1.974(4)−1.983(6) Å; py = pyridine), attesting to the effect of the constraint present within the N4Py ligand.53 Co−N bond lengths involving the aliphatic amine N, which lies trans to the monodentate ligand in all cases, range from 1.893(5) to 1.938(6) Å (Table S1). There is no obvious trend in these bond lengths across the series of complexes. However, the effect of deprotonation of an aqua ligand is nicely demonstrated by comparison of the relevant bond lengths in the aqua and hydroxido complexes; the Co−Naliphatic bond length increases from 1.917(2) to 1.934(3) Å, while the Co−O bond length decreases from 1.935(2) to 1.876(2) Å on going from the aqua to the hydroxido complex, as would be expected on electrostatic grounds on replacing a neutral with an anionic ligand. Also of note are the bond lengths in the chlorido and bromido complexes; the sterically more demanding bromido ligand results in longer Co−Npyridyl and Co−Naliphatic bonds in the bromido complex, while bond angles remain relatively unaffected. Electrochemical and Spectroscopic Properties of [CoIII(N4Py)(X)]n+ and the Electrochemically Generated Low-Valent Cobalt(II) and Cobalt(I) Species in MeCN. Cobalt(III) Complexes. The UV−visible spectra of the [CoIII(N4Py)(X)]n+ complexes in acetonitrile display two C
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. UV−vis absorption spectra in MeCN of 1 mM solutions of [CoIII(N4Py)(X)]n+ ((a) 1 mm and (b) 1 cm path lengths) and after exhaustive electrolyses to give [CoII(N4Py)(X)]n+ ((c) 1 mm and (d) 1 cm path lengths).
the aqua derivative at λmax = 480 nm shifts to 454 nm in about 1 h after dissolution of the complex in dry MeCN, a wavelength corresponding to the NCMe derivative (Supporting Information, Figure S40); the exchange is slower in less dry solvent. The electrochemical properties of the N4Py ligand and the [Co(N4Py)(X)]n+ complexes have been investigated by cyclic voltammetry (CV) in MeCN. The cyclic voltammogram of the N4Py ligand shows two successive irreversible processes at Epa = +0.74 and +1.20 V versus Ag/0.01 M AgNO3, which are attributed to amine56 and pyridine57 oxidations, respectively (Supporting Information, Figure S41). The results from differential pulse voltammetry experiments suggest the process at +1.20 V is complex and likely involves oxidation of more than one pyridyl group of N4Py (data not shown). No ligand reduction process was observed within the potential range investigated (−2.3 to +1.4 V). The cyclic voltammogram of [Co(N4Py)(OH)]2+ exhibits an irreversible one-electron process at −0.76 V versus Ag/AgNO3 (not shown), while for all of the other cobalt complexes two successive reduction processes, assigned respectively to the CoIII/CoII and CoII/CoI, couples, are observed, as is usual for cobalt complexes containing amine ligands (Figures 4 and S42).19,30,58,59 No oxidation process was detected between 0.0 and +1.6 V, confirming that the monodentate anionic ligand (Cl−, Br−, NCS−, or N3−) remains coordinated to the cobalt(III) center in MeCN solution; irreversible oxidation processes corresponding to the electroactivity of these anions in this potential range should be observed if such anions are released into solution.60,61 For [Co(N4Py)(NCMe)]3+, the CoIII/CoII and CoII/CoI waves appear reversible and are respectively located at E1/2 = −0.04 V and E1/2 = −1.48 V versus Ag/AgNO3 (Figure 4a). As expected, very similar values are found for the aqua derivative (E1/2 = −0.03 V and E1/2 = −1.49 V, Figure S42).
bands at wavelengths of approximately 350 and 500 nm; in the case of the aqua, NCMe, and DMSO derivatives, the highenergy band appears as a shoulder on another absorption band at even higher energy (Figure 3 and Figures S36−S38, Supporting Information). These visible absorption bands are assigned to transitions from the 1A1g ground state to the upper 1 T1g and 1T2g states of [CoIII(N4Py)(X)]n+, as commonly found for d6 low-spin cobalt(III) complexes.54 Changing the monodentate ligand X (Br− → Cl− → OH− → OH2 → NCMe) causes a blue shift in the wavelength of the absorbance maximum (λmax = 542 → 520 → 490 → 480 → 454 nm) of the low-energy band, which is in agreement with the position of these ligands in the spectrochemical series.45 The molar absorptivities (ε) of the two observed absorption bands of [Co(N4Py)(N3)]2+ (7.23 × 104 M−1 cm−1 (334 nm), 877 M−1 cm−1 (496 nm)) and [Co(N4Py)(NCS)]2+ (1.87 × 104 M−1 cm−1 (360 nm), 1.14 × 104 M−1 cm−1 (504 nm)) are at least 4fold larger than those observed for the other [Co(N4Py)(X)]n+ complexes studied. These relatively large molar absorptivities mirror those of the respective pentaammine complexes ([Co(NH3)5(N3)]2+, 272 M−1 cm−1 (515 nm)55; [Co(NH3)5(NCS)]2+, 179 M−1 cm−1 (498 nm)49), both of which display significantly larger ε-values than the simple aqua and chlorido complexes. These bands presumably have some charge-transfer character, most probably from LMCT transitions into the metal eg orbitals. The UV−visible spectra of the [Co(N4Py)(X)]n+ compounds containing anionic monodentate ligands (X = Cl−, Br−, OH−, N3−, NCS−) in MeCN show no change over time, while those of the complexes with neutral DMSO and OH2 ligands suggest a slow exchange of the axial ligand with a solvent molecule, the rate of exchange depending on the amount of water in the MeCN solvent. For instance, the visible band of D
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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CoI state via π-back-bonding.15 Such interactions cannot be easily accommodated by the four single pyridine moieties of the N4Py ligand, and this leads to the observed very negative potential values of the CoII/CoI couple in these complexes. Electrochemical Generation of Cobalt(II) Species and UV− Vis and EPR Characterization. The stability of these complexes at the Co(II) oxidation state was confirmed by bulk electrolysis of solutions of the initial Co(III) compound in MeCN. For each complex, the electrolysis at a potential between −0.3 and −0.6 V consumes one electron per molecule of initial complex and leads to the quantitative formation of the Co(II) species, as evidenced by the resulting cyclic voltammograms (Figure 4a−e, red curves). The solutions of the Co(II) complexes have been characterized by UV−vis absorption (see Figure 3c,d, Figure 5, and Figures S43 and S44 for an evolution of the spectra over the course of the electrolysis) and EPR spectroscopies (Figure 6). The UV−vis absorption spectra of electrogenerated solutions of the Co(II) complexes show broad d−d visible bands, with λmax ranging from 466 to 512 nm (Figure 3c,d). The NCS− and N3− derivatives show additional intense bands at 345 and 357 nm, respectively. These differences provide strong evidence that the anionic monodentate ligands remain coordinated to the metal center of the Co(II) complexes in MeCN solution. This result agrees with the fact that substitution of the axial MeCN ligand by anions such as acetate and tosylate has been observed by Chang and co-workers62 for the structurally analogous [CoII(PY5)(NCCH3)]2+ complex in MeCN solution. In addition, the similarities between the spectra of the electrochemically generated Co(II) MeCN and aqua derivatives (Figure 3c,d) confirm that, at this oxidation state, the same species, namely, [CoII(N4Py)(NCMe)]2+, is present in solution in both cases, with the exchange of the aqua ligand by MeCN having already occurred in the [Co(N4Py)(OH2)]3+ complex (Figure S40). For the [Co(N4Py)(OH)]2+ derivative, a bulk electrolysis at −1.0 V led to the formation of about a half-equivalent of the Co(II) aqua derivative and other undefined species that are irreversibly oxidized around 0 V. These were not investigated further. The retention of the monodentate anionic ligands in the Co(II) complexes in solution was confirmed by X-band EPR spectroscopy. While the Co(III) complexes are diamagnetic, the Co(II) complexes [Co(N4Py)(X)]+ (X = N3−, NCS−, Cl−, and Br−) and [Co(N4Py)(X)]2+ (X = NCMe and OH2) are half-integer spin systems which can be characterized by EPR spectroscopy (Figure 6). At 13 K, the resonances are broad and distributed over a large spectral window (100−400 mT) for the [Co(N4Py)(X)]+ complexes (Figure 6b−e), consistent with the presence of a high-spin S = 3/2 Co(II) ion. It is known that the zero field splitting parameters are much larger than the Xband energy quantum for S = 3/2 Co(II) ions.63 Consequently, the two Kramer’s doublets are well-separated and only the transition within the lowest energy of these (e.g., ms = ±1/2) is observable. Simulation of the EPR spectra was thus performed by considering a fictitious S′ = 1/2 spin system, giving access to the effective g-values (g′) listed in Table 2. Significant differences in rhombicity (estimated from Δg′ = g′2 − g′3) were observed within the series, as expected for changes in the environment of the cobalt ion. The most rhombic spectrum corresponds to [Co(N4Py)(N3)]+, while the spectra of the halide complexes exhibit the smallest rhombicity. A welldefined hyperfine splitting is observed in the lowest field g′component of [Co(N4Py)Cl]+ (Figure 6b), as a result of the
Figure 4. Cyclic voltammograms of 1 mM solutions of (a) [Co(N4Py)(NCMe)]3+, (b) [Co(N4Py)Cl]2+, (c) [Co(N4Py)Br]2+, (d) [Co(N4Py)(N3)]2+, and (e) [Co(N4Py)(NCS)]2+ in MeCN. Initial solutions and the solutions after exhaustive reduction are shown as black and red traces, respectively. Supporting electrolyte, 0.1 M Bu4NClO4; glassy carbon electrode; scan rate = 100 mV s−1. Potentials are referenced to Ag/AgNO3 (10 mM).
These waves are shifted to more negative potentials for the compounds containing the anionic monodentate ligands (Cl−, Br−, N3−, and NCS−; Table 1), in accordance with the electrondonating properties of the anionic substituents compared to NCMe (Figure 4). The fact that the CoII/CoI wave is also shifted to more negative potentials suggests that the anionic ligand remains coordinated to the metal center in the Co(II) complexes. In addition, the loss of reversibility of this wave in this case is indicative of the release of the anionic ligand in the Co(I) complexes. Among the polypyridine cobalt catalysts in the literature, our series of complexes displays the most negative redox potentials for the CoII/CoI couple in MeCN solution; those exhibiting the least negative potentials include a bipyridine ligand in their coordination sphere that stabilizes the E
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Electrochemical Potentials (versus Ag/AgNO3) for [Co(N4Py)(X)]n+Complexes in MeCN (Supporting Electrolyte, 0.1 M Bu4NClO4) complex [Co(N4Py)(NCMe)] [Co(N4Py)(OH2)]3+ [Co(N4Py)(OH)]2+ [Co(N4Py)Cl]2+ [Co(N4Py)Br]2+ [Co(N4Py)(N3)]2+ [Co(N4Py)(NCS)]2+
3+
CoIII/CoII
CoII/CoI
E1/2 = −0.04 V (ΔEp = 85 mV) Epc = −0.06 V, Epa = +0.025 V E1/2 = −0.03 V (ΔEp = 100 mV) Epc = −0.07 V, Epa = +0.03 V Epc = −0.76 V E1/2 = −0.18 V (ΔEp = 120 mV) Epc = −0.24 V, Epa = −0.12 V (ΔEp = 120 mV) E1/2 = −0.09 V (ΔEp = 120 mV) Epc = − 0.15 V, Epa = −0.03 V E1/2 = −0.33 V (ΔEp = 180 mV) Epc = − 0.42 V, Epa = −0.24 V E1/2 = −0.19 V (ΔEp = 180 mV) Epc = − 0.28 V, Epa = −0.10 V
E1/2 = −1.48 V (ΔEp = 60 mV) Epc = −1.50 V, Epa = −1.45 V E1/2 = −1.49 V (ΔEp = 60 mV) Epc = −1.52 V, Epa = −1.46 V Epc = −1.82 V, Epa = −1.56 V Epc = − 1.72 V, Epa = −1.52 V Epc = − 1.80 V, Epa = −1.59 V Epc = − 1.66 V, Epa = −1.54 V
Figure 5. UV−vis absorption spectral changes of 1 mM solutions of [Co(N4Py)(NCMe)]3+ (left) and [Co(N4Py)Br]2+ (right) in MeCN during two exhaustive electrolyses at (a) −0.3 V and (b) −1.6 V versus Ag/AgNO3 for the acetonitrile derivative and at (c) −0.6 V and (d) −1.8 V for the bromido derivative (each electrolysis corresponds to the formation of Co(II) and Co(I) species, respectively). Supporting electrolyte, 0.1 M Bu4NClO4; optical path length, 1 mm.
symmetry axis with respect to the applied magnetic field. The eight-line pattern observed in the perpendicular region results from the hyperfine interaction with the cobalt nucleus. The corresponding hyperfine coupling constant (ACo,1 in Table 2) is close to that reported for the [Co(NCMe)6]2+ complex encapsulated into zeolite (7.2 mT).64 The g-anisotropy is somewhat larger, presumably because of the lower symmetry of [Co(N4Py)(NCMe)]2+, and it compares reasonably well with those reported for the [Co(CN)5]3− and [Co(NCMe)5]2+ complexes.64,65 The different spin state obtained for [Co(N4Py)(NCMe)]2+ (low spin) versus the Cl−, Br−, N3−, and NCS− derivatives (high spin) as well as the slightly different EPR signatures for the latter confirm that the monodentate
interaction of the electron spin with the nuclear spin (ICo = 7/ 2). In the other complexes, the line width is within the range of or higher than the hyperfine coupling constant, making the splitting barely observable. For the [Co(N4Py)(NCMe)]2+ complex, the situation is completely different. Sharp EPR signals are observed at 100 K in the region close to g = 2, revealing an unusual low-spin (S = 1/2) configuration for an octahedral cobalt(II) ion (Figure 6a).58 This behavior can be rationalized, within the present series, by the π-donating ability of the anionic ligands, which gives rise to smaller ligand fields than the NCMe ligand. The signal was found to be essentially axial, with g1 = g2 = g⊥ = 2.175 and g3 = g∥ = 1.980, where g⊥ and g∥ refer to the direction of the F
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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anionic ligands remain coordinated to the Co(II) complexes in MeCN solution. Electrochemical Generation of Cobalt(I) Species and UV− Vis Characterization. The Co(I) complexes where X = Cl−, OH2, N3−, NCS−, NCMe, and Br− were also tentatively generated by bulk electrolyses. These species exhibit low stability over time and decompose during the course of the electrolyses, in contrast to other cobalt complexes containing polypyridine28 or pyridine−bisimine ligands,58,66 whose Co(I) species, generated in organic solvents either by chemical or electrochemical reduction, display stabilities of several weeks under an inert atmosphere. In our case, although the Co(I) species have low stabilities in solution, their signatures have been clearly obtained in the early stages of the electrolysis for the acetonitrile, aqua, and bromido derivatives and compare well with those obtained with other polypyridinyl cobalt(I) complexes (Figures 5 and S43).28,67,68 The visible absorption spectra of the blue Co(I) solutions are mostly composed of two characteristic bands at approximately 450 nm and between 500 and 900 nm (shoulders at 610 and 760 nm) (Figures 5b,d and S44) which decrease in intensity after a couple of minutes. For the other derivatives, similar spectral features were observed (Supporting Information, Figures S43 and S44). It is significant that the absorption spectra of all of the Co(I) complexes show strong similarities. This indicates that the monodentate anionic ligand is most probably released into solution upon reduction of the Co(II) center to Co(I), resulting in the formation of a common species, assigned as [CoI(N4Py)]+, since cobalt complexes at this oxidation state generally prefer a tetra- or pentacoordinated geometry.54,66 UV−Vis Characterization of Cobalt Species in Buffered Aqueous Ascorbate (HA−)/Ascorbic Acid (H2A) Solution. Since the cobalt(III) N4Py complexes were used in photo-
Figure 6. X-band EPR spectra of acetonitrile solutions of cobalt(II) complexes (1 mM): (a) [Co(N4Py)(NCMe)]2+, (b) [Co(N4Py)Cl]+, (c) [Co(N4Py)Br]+, (d) [Co(N4Py)(NCS)]+, and (e) [Co(N4Py)(N3)]+. Microwave frequency, (a) 9.44 and (b−e) 9.63 GHz; power, (a) 5 and (b−e) 6.5 mW; modulation frequency, (a−e) 100 kHz; modulation amplitude, (a) 0.3 and (b−e) 0.4 mT; T, (a) 100 and (b− e) 13 K. Solid lines, experimental spectra; dotted lines, simulations using the parameters given in Table 2.
Table 2. EPR Parameters of the Cobalt(II) Complexes complex
S
g′1
g′2
g′3
ACo,1a
[Co(N4Py)(NCMe)]2+ [Co(N4Py)Cl]+ [Co(N4Py)Br]+ [Co(N4Py)(NCS)]+ [Co(N4Py)(N3)]+
1/2 3/2 3/2 3/2 3/2
2.175b 7.10 6.65 6.45 6.50
2.175b 2.61 3.05 3.25 2.80
1.980b 2.10 2.35 2.55 1.90
7.9c 8.5 7.9d 8.3 6.3d
a
In mT. bReal g-values for the (S = 1/2) spin system: g1, g2, and g3. cA1 = A2. dNot resolved in the spectrum: only an upper limit, estimated from the line width, is reported.
Figure 7. UV−vis absorption spectra (1 cm optical path length) of 1 mM aqueous solutions of (a) [Co(N4Py)(X)]n+ and (b) after addition of HA−/ H2A buffer (0.55 M/0.55 M) and (c, d) after addition of HA−/H2A buffer (0.05 M/0.05 M), pH = 4.0. G
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 3. Photocatalytic Activities of [CoIII(N4Py)(X)]n+ Catalysts toward Hydrogen Evolution, in Terms of TONCat and VH2 and Comparison with Previously Reported Catalysts under Similar Experimental Conditionsa catalyst blank [Co(OH2)6]Cl2 [Co(N4Py)(OH2)]3+ [Co(N4Py)(NCMe)]3+ [Co(N4Py)Cl]2+ [Co(N4Py)Br]2+ [Co(N4Py)(N3)]2+ [Co(N4Py)(NCS)]2+ [Co(N4Py)(OH2)]3+ [Co(N4Py)(OH2)]3+ [Co(N4Py)(OH2)]3+ blank [Co(N4Py)(OH2)]3+ [Co(N4Py)(OH2)]3+ (footnote d) [Co(N4Py)(OH2)]3+ (footnote e) [Co(N4Py)Cl]2+ [Co(N4Py)Cl]2+ (footnote d) [Co(N4Py)(N3)]2+ blank [Co(CR)Cl2]+ (ref 58) [Co{(DO)(DOH)pn}Br2] (ref 58) [Co(dmbpy)3]2+ (ref 58) [Rh(dmbpy)2Cl2]+ (ref 69)
[Cat], mol L−1 1 1 1 1 1 1 1 5 1 5
× × × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−6
5 5 5 5 5 5
× × × × × ×
10−5 10−5 10−5 10−5 10−5 10−5
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
[Ru], mol L−1 5 5 5 5 5 5 5 5 5 5 5 1 1 1 1 1 1 1 5 5 5 5 5
× × × × × × × × × × × × × × × × × × × × × × ×
ratio Ru/Cat
−4
10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4
5/1 5/1 5/1 5/1 5/1 5/1 10/1 50/1 100/1 2/1 2/1 2/1 2/1 2/1 2/1 5/1 5/1 5/1 5/1
NaHA/H2A mol L−1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1.1 1.1 1.1 1.1 1.1
TONCata (TONCat*)b
65 67 65 68 75 73 85 240 341
(59) (61) (59) (63) (70) (68) (72) (185) (233)
59 65 0 63 63 60
(56) (62)
828 12 15 240
(820) (4) (7) (236)
(60) (60) (57)
VH2a (VH2*)b, mL
irrad timec, h
0.066 0.037 0.783 (0.717) 0.819 (0.753) 0.791 (0.725) 0.835 (0.769) 0.919 (0.853) 0.900 (0.834) 0.508 (0.442) 0.292 (0.226) 0.209 (0.143) 0.017 0.363 (0.346) 0.395 (0.378) 0 0.385 (0.368) 0.388 (0.371) 0.369 (0.352) 0.100 10.15 (10.05) 0.150 (0.050) 0.180 (0.080) 2.940 (2.891)
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 21 21 21 21 21
a
Experiments were carried out at 25° C in water (5 mL) at pH 4.0 with the [Ru(bpy)3]2+ photosensitizer and a catalyst (Cat) at various concentrations, in the presence of NaHA and H2A under visible-light (λ = 400−700 nm) irradiation. TONCat and VH2 are respectively the maximum turnover number per catalyst exhibited by the system and the total volume of H2 produced by the system until gas evolution stopped. bTONCat* and VH2* are the corrected values of TONCat and VH2, respectively, obtained by subtracting the production of H2 stemming from [Ru(bpy)3]2+ without catalyst. cirrad time: time of irradiation after which H2 production stopped. dIn this experiment, 0.3 M NaCl was added. eIn this experiment, 0.3 M NaNO3 was added.
high-field region are exceedingly broad, we did not undertake simulations and we will limit our discussion to the global shape of the spectra. When total concentrations of HA−/H2A of 1.1 M are used, the low-field resonance is found to be remarkably similar for all of the compounds, with a slight difference observed in the case of [Co(N4Py)(NCS)]+ (Figure S47). This suggests that the nature of the cobalt(II) species, in particular the identity of the apical ligand, is similar for most of the compounds. Furthermore, the spectra in aqueous media differ significantly from those in acetonitrile solutions. It is therefore likely that the apical ligand identified in the crystal structure exchanges with ascorbate anion, generating the [CoII(N4Py)(HA)]+ complex in the 1.1 M aqueous HA−/H2A buffer. When the total concentration of HA−/H2A is decreased to 0.1 M, the global shape of the low-field resonance remains similar and is consistent with the presence of a similar [CoII(N4Py)(X)]n+ species. In addition, a low-intensity eight-line pattern, which resembles that of [CoII(N4Py)(NCMe)]2+, could be detected at around g = 2 for the starting Co(III) complexes containing MeCN, H2O, Cl−, and Br− as apical ligands. Such a pattern is typical for low-spin cobalt(II) species, suggesting that, in addition to [CoII(N4Py)(HA)]+, another species having a different spin state is present in solution at 0.1 M of HA−/H2A. Although the exact identity of this species is not known, it can be reasonably assigned as the hexacoordinate [CoII(N4Py)(OH2)]2+ complex (Figure S48). The Co(II) state of three [Co(N4Py)(X)]n+ complexes of the series (X = MeCN, H2O, and Cl) was also generated via an
catalytic reactions carried out in buffered aqueous solution at pH 4.0 (see below), we also conducted UV−vis experiments in aqueous media. While solutions of the Co(III) complexes in water give no evidence for the formation of [Co(N4Py)(OH2)]3+ over 20 min (Figure 7a), subsequent addition of ascorbate/ascorbic acid (HA−/H2A) buffer (1.1 M total concentration) resulted in an immediate color change due to reduction of Co(III) to Co(II) by HA− (Figure 7).58 The UV− vis spectra of the resulting solutions are similar in all cases, with a main visible band at approximately 480 nm (the absorption below 425 nm is due to the contribution of free ascorbate in solution) consistent with the formation of the same Co(II) complex, which was tentatively assigned as the hexacoordinate [CoII(N4Py)(HA)]+ in all cases (vide inf ra). The blue shift of the absorption band in comparison to acetonitrile solutions of the same compounds (except for [Co(N4Py)(NCMe)]3+) indicates that the apical ligand is exchanged in aqueous medium. When only 0.1 M total concentration of HA−/H2A was added, similar behavior was observed, except for the azido and thiocyanato complexes, for which the reduction of the cobalt center is much slowerafter 6 h, about 70 and 84% of the initial complexes are reduced, respectively (Figures S45 and S46). The X-band EPR spectra of the cobalt(III) complexes in aqueous ascorbate/ascorbic acid (HA−/H2A) buffer are shown in the Supporting Information, Figures S47 and S48. They are dominated by a resonance in the low-field region (g = 4−6), suggesting that high-spin cobalt(II) complexes with large ZFS parameters are formed. Because the associated patterns in the H
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry exhaustive electrolysis in water at −0.26 V versus Ag/AgCl (i.e., −0.6 V versus Ag/AgNO3), in the absence of ascorbate salt and along with 0.1 M NaNO3 as supporting electrolyte (pH 7.0). The three Co(II) species obtained electrochemically display essentially the same UV−visible and EPR signatures (Figures S49 and S50), which suggests that the same species are formed in each case. Similar to the situation found in the presence of ascorbate, high-spin and low-spin components are observed, with the latter predominating. The low-spin complex is assigned as [CoII(N4Py)(OH2)]2+ while the high-spin complex is presumably [CoII(N4Py)(ONO2)]+. These assignments are consistent with the positions of the OH2 and ONO2− ligands in the spectrochemical series. These electrochemical and spectroscopic data reveal unambiguously that the monodentate anionic ligand in the [Co(N4Py)Cl]2+, [Co(N4Py)Br]2+, [Co(N4Py)(N3)]2+, and [Co(N4Py)(NCS)]2+ complexes remains coordinated to the metal center in both the Co(III) and Co(II) oxidation states in MeCN solution. The low stability of the Co(I) complexes in MeCN solution precludes a definitive assessment of their geometric structure, but the similarity of the UV−vis spectra for several derivatives strongly suggest that the monodentate ligands are not retained in the coordination sphere. In aqueous solution, the monodentate ligands remain coordinated to the metal center at the Co(III) oxidation state, whereas reduction by ascorbate to give the Co(II) state results in an axial ligand exchange with the ascorbate anion to generate the complex [CoII(N4Py)(HA)]+. Photocatalytic Hydrogen Generation. The H2-evolving photocatalytic activities of the family of complexes [CoIII(N4Py)(X)]n+ (X = Cl−, NO3−, CF3SO3−, OH2) in water, using [Ru(bpy)3]2+ as the molecular photosensitizer (PS) in the presence of H2A as the sacrificial electron donor, was recently reported by Wang et al.30 The optimal experimental conditions for H2 production were found to be 1 × 10−4 M [Ru(bpy)3]2+, 5 × 10−5 M cobalt catalyst, and a total concentration of 0.1 M HA−/H2A. The catalytic activities of these complexes were found to be strongly dependent on the chemical nature of the apical monodentate ligand with the best activity observed for the chlorido derivative (TONCat = 13.5 versus 9.5 for the triflato and aqua complexes, and 2.5 for the nitrato complex). It was found that addition of NaCl (0.3 M) increased the TONCat by a factor of about 2 for the chlorido, triflato, and aqua derivatives whereas the nitrato compound remained almost inactive (TONCat = 2.8). This catalytic behavior was suggested by the authors to be consistent with nondissociation of the apical ligand during the catalytic process, and it was suggested that the formation of the Co(III) hydride species responsible for H2 release occurs subsequent to the protonation of one pyridine donor of the pentadentate ligand (N4Py) that decoordinates over the course of the catalysis. This study prompted us to investigate the photocatalytic activities of our series of cobalt complexes [CoIII(N4Py)(X)]n+ (X = NCMe, OH2, Cl−, Br−, N3−, and NCS−) which, except for the chlorido and aqua derivatives, differ in the nature of the apical ligand from those used by Wang et al.30 All six complexes were tested under visible-light irradiation (400−700 nm, 250 mW) at 298 K under both the optimized conditions of Wang et al. and our regular conditions,58,69−71 i.e., [Ru(bpy)3]2+ (5 × 10−4 M) and [CoIII(N4Py)(X)]n+ (1 × 10−4 M) in the presence of HA−/ H2A (total concentration of 1.1 M). The data obtained are summarized in Table 3. The catalytic activity of the [CoIII(N4Py)(OH)]2+ complex was not investigated since, at
pH 4.0, the aqua derivative is spontaneously generated by protonation of the hydroxido complex. As previously observed by Wang et al.,30 maximum activity was exhibited at pH 4.0. While control experiments at this pH in the absence of either [Ru(bpy)3]2+ or H2A/NaHA produced no appreciable amount of hydrogen, production of a small amount of hydrogen was detected in solutions containing [Ru(bpy)3]2+ and NaHA/H2A in the absence of a cobalt complex, in agreement with previous observations (Table 3).70,72 This amount of H2 was systematically subtracted from the total amount of H2 produced in order to calculate the corrected TONCat- and VH2-values (denoted TONCat* and VH2*, respectively) in all experiments (Table 3). The possibility of cobalt colloids, formed by complex decomposition, acting as the catalytically active species was ruled out by mercury poisoning experiments (Figure S51).58 The formation of such colloids was also excluded by Wang et al.30 by dynamic light scattering (DLS) and UV−visible absorption control experiments. In addition, if the molecular cobalt catalyst (i.e., [CoIII(N4Py)(X)]n+) is substituted by [Co(OH2)6]Cl2, a smaller amount of H2 relative to that produced by [Ru(bpy)3]2+/NaHA/H2A solutions is released, showing that there is no contribution of the simple cobalt salt to photocatalytic H2 production. Finally, the absence of an induction period in our photocatalytic experiments (see below) also indicates that colloid formation is not involved in the release of H2. A plot of hydrogen evolution versus irradiation time for all cobalt derivatives with [Ru(bpy) 3 ] 2+ (5 × 10 −4 M), [CoIII(N4Py)(X)]n+ (1 × 10−4 M), and HA−/H2A (1.1 M total concentration) is shown in Figure 8. All catalysts display a
Figure 8. Photocatalytic hydrogen production (TONCat, TONCat*, nH2, and VH2) as a function of time from a deaerated aqueous solution (5 mL) of NaHA (0.55 M) and H2A (0.55 M) at pH 4.0 under visiblelight irradiation (400−700 nm) in the presence of [Ru(bpy)3]2+ (5 × 10−4 M) and the appropriate cobalt catalyst (1 × 10−4 M).
similar moderate activity for hydrogen production, with TONCat*-values ranging from 59 to 70. The slight differences observed between the catalysts are within the margin of error for these experiments. Our results confirm that, regardless of the cobalt(III) complex initially used, the [CoII(N4Py)(HA)]+ species is present in solution after reduction of the [CoIII(N4Py)(X)]n+ complex by HA− (as shown above in Figure 7), and this complex is responsible for the catalysis in all cases. These results differ markedly from those of Wang et al.30 and challenge their proposed mechanism of H2 evolution with I
DOI: 10.1021/acs.inorgchem.6b00391 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry this photocatalytic system in which the cobalt catalyst retains its apical monodentate ligand (i.e., the X ligand) during the catalytic process. The hydrogen generation is initially rapid over the first 30 min and reaches a plateau after about 1 h of photolysis (Figure 8). The cessation of H2 evolution can be ascribed to the decomposition of both catalyst and photosensitizer, even though the cobalt complex seems to decompose faster than the Ru complex under these photocatalytic conditions. Indeed, the addition of further Ru photosensitizer to the 5 mL reaction mixture after 40 min of irradiation does not affect H2 production (Figure S52), which clearly indicates that the decomposition of catalyst is faster than that of the photosensitizer. The decomposition of the Ru complex was also confirmed by the UV−vis spectra of the photocatalytic solution at the beginning and the end of irradiation (Figure S53).26,70,73 Indeed, the final observed spectrum, with new visible bands at 467 and 350 nm, can be assigned to the [Ru(bpy)2(HA)]+ species which was proposed by Chang et al.74 to be the major photosensitizer decomposition product photogenerated under similar photocatalytic systems. A comparison of the H2-evolving activities of the [Co(N4Py)(X)]n+ catalysts with those of cobalt and rhodium complexes previously studied under the same experimental conditions in our group shows that the efficiency of [Co(N4Py)(X)]n+ is lower than those of [Co(CR)(Cl)2]+ (CR = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) and [Rh(dmbpy) 2 Cl 2 ] + (dmbpy = 4,4′-dimethyl-2,2′-bipyridine), but greater than those of [Co{(DO)(DOH)pn}Br2] ({(DO)(DOH)pn} = N2,N2′-propanediylbis(2,3-butanedione 2-imine 3-oxime) and [Co(dmbpy)3]2+ (Table 3).58,70 The low efficiencies of the [Co(N4Py)(X)]n+ complexes can be related to the much lower stability of the Co(I) species compared to those of [CoI(CR)(NCMe)]+ (several weeks)58 and [RhI(dmbpy)2]+ (half-life of approximately 40 min)75 as demonstrated in MeCN solution. Furthermore, the TONCat-values observed for the family of [Co(N4Py)(X)]n+ complexes are lower than those for other polypyridyl cobalt complexes, such as [Co(DPA-bpy)(OH 2 )] 3+ , 20 [Co(TPY−OH)Br] + , 76 [Co(CF 3 PY5Me)(OH2)]2+,21 and [Co(qpy)(OH2)2]2+,14 that exhibit TONCatvalues in the thousands. However, such systems use lower concentrations of catalysts (10−6 to 10−7 M) in combination with high PS/catalyst ratios (up to 500 for [Co(TPY− OH)Br]+). Under these conditions, even though TONCatvalues can be very high, the actual volume of H2 evolved is very small. The H2-evolving performance of the [Co(N4Py)(OH2)]3+ catalyst was also further assessed at lower catalyst concentration (
