Photocatalytic CO2 Reduction with Manganese Complexes Bearing a

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Photocatalytic CO2 Reduction with Manganese Complexes Bearing a κ2‑PN Ligand: Breaking the α‑Diimine Hold on Group 7 Catalysts and Switching Selectivity Yasmeen Hameed, Bulat Gabidullin, and Darrin Richeson* Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research and Innovation University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada

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S Supporting Information *

sequently shown to be, in the presence of a Ru(bipy)32+ photosensitizer (PS) and an ED (TEOA), capable of photocatalytically reducing CO2 with selectivity for formic acid [89%; turnover frequency (TON) = 157].31 In all of these cases, a system consisting of a photosensitizer (PS) an electron donor (ED) and the catalyst (CAT) species is required. Success to date has been achieved with rather limited variation of the ligand architecture. In the case of group 7 complexes, while there has been some ligand variation with electroreduction catalysts,32,33 reported CO2 photoreduction catalysts have been exclusively limited to bidentate α-dimmine ligands.31,34−37 Ligand variation and discovery are a central challenge in catalysis, stimulating new catalyst design and revealing routes to new products and selectivities. Recent pertinent examples include the use of Ni complexes with mixed pyridine/carbene ligands25 or mixed pyridine/thioether ligands27 for selective visible-light-driven CO2 reduction catalysis. With the objectives of developing distinctive metal coordination geometries and novel reactivity, we now report the application of bidentate N-(diphenylphosphino)-2-aminopyridine ligands ({Ph2PNR}NC5H4; R = H, CH3) with MnI and ReI to yield complexes, competent in the photocatalytic reduction of CO2. Ligand selection was based on an interrogation of the role of the ligand on the catalyst performance. First, replacement of a py moiety with a more basic phosphine donor targeted modulation of the electron density on the catalytic metal center. Second, modification of the amino substituent probes the importance of this group on catalyst execution. Finally, this ligand frame lacks the π conjugation of the bipy scaffold, allowing interrogation of the criticality of this ligand feature. In fact, these complexes displayed not only uncommon selectivity for either CO or HCOOH, which contrasts with the current photocatalytic reduction literature, but a distinctive switch in this selectivity depending on the identity of the metal center. Direct reaction of Mn(CO)5Br with two different N(diphenylphosphino)-2-aminopyridines yielded the complexes [Mn{κ2-(Ph2P)NR(NC5H4)}(CO)3Br] [R = H (1), Me (2)], obtained in yields of 93% and 78%, respectively. 38,39 Confirmation of the identities of 1 and 2 was provided by matching the spectroscopic data with the literature and our independent single-crystal X-ray analyses (Figures S1 and S2). As represented schematically in Figure 1, these complexes

ABSTRACT: The fundamental challenge of reducing CO2 into more valuable energy-containing compounds depends on revealing new catalysts for this process. By removal of the long-standing limitation of α-diimine ligation, which is dominant in photocatalytic complexes in this area, new visible-light, CO2-reducing photocatalysts based on Mn and Re supported by κ2-PN phosphinoaminopyridine ligands were identified. These catalysts, [M{κ2-(Ph2P)NH(NC5H4)}(CO)3Br], displayed excellent product selectivity and, by a change of only the metal center, gave a dramatic product switch from CO with M = Mn to HCO2H with M = Re. The catalyst systems were explored with variation of the ligand, electron donor, solvent, and photosensitizer. The products were definitively traced using 13CO2 as a substrate. Both complexes quenched the excited-state photosensitizer Ru(bpy) 32+ *, suggesting oxidative quenching as a potential entry into the catalytic cycle.

T

he catalytic reduction of CO2, the end product of fuel combustion and cellular respiration, represents a fundamental challenge because of the inherent stability of this simple, pervasive compound. Success in this endeavor would represent a paradigm shift to viewing CO2 as a feedstock rather than as a waste product. The practical reduction of CO2 into chemically valuable products hinges on the identification of an appropriate energy source and catalysts that can convert energy input into chemical transformation.1−3 For eons, light has provided the limitless energy for plants to carry out selective and efficient CO2 reduction. Reports that [ReX(bipy)(CO)3] (bipy = 2,2′bipyridine and X = halide) complexes were selective photocatalysts for the reduction of CO2 to CO using triethanolamine (TEOA) as an electron donor (ED) demonstrated the potential of metal complexes as catalysts for this transformation and remain a benchmark more than 30 years later.4,5 While secondand third-row metal complexes have been shown to function as photocatalysts for CO2 reduction, it is widely recognized that earth-abundant, first-row transition-metal complexes must be the focus of designing economically viable, environmentally responsible, and sustainable catalysts.6−8 Recent photocatalysts for CO2 reduction have been built around Fe,9−12 Co,13−20 Ni,21−27 and Cu.28,29 Group 7 complexes have continued to hold a central position in these efforts after MnX(bipy)(CO)3 was demonstrated to electrocatalytically reduce CO230 and sub© XXXX American Chemical Society

Received: September 26, 2018

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DOI: 10.1021/acs.inorgchem.8b02719 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Table 1. Comparison of the Performance Parameters for Photocatalysts Mn(bpy)(CO)3Br (I),31 Mn(bpy)CO3CN (II),34 and Mn(phen)CO3Br (III)35 with Complexes 1, 2, and 4 CAT

H2 (μmol)

TONH2 (Φ, %)

Ia

1.6

14 (0.14)

14.2

1.6 (0.01)

IIb c

III 1d 2d 4d

Figure 1. Synthetic scheme for the preparation of phosphinoaminopyridine-PN-ligated Mn and Re complexes.

exhibited a fac-tricarbonyl coordination geometry that is supported by a bidentate PN ligand. The sixth coordination site of these pseudooctahedral compounds was completed by a bromo ligand. In complex 1, the Br ligand was exchanged with triflate by reaction with AgOTf to yield 3, which was fully characterized through NMR spectroscopy and single-crystal Xray analysis (Figure S3). The structural parameters for 1−3 are analogous, displaying similar Mn−Npy and Mn−P distances and Npy−Mn−P bite angles.38,39 Similarly, the analogous Re complex [Re{κ2(Ph2P)NH(NC5H4)}(CO)3Br] (4) was prepared, and spectroscopic and structural analyses (see the Supporting Information) were completely consistent with this recently reported complex.40 The photocatalytic reduction of CO2 was carried out in a glass reactor at room temperature using [Mn{κ2-(Ph2P)NH(NC5H4)}(CO)3Br] (1) combined with [Ru(bpy)3](PF6)2 (PS) and TEOA (ED) under a CO2 atmosphere in 4 mL of N,N-dimethylformamide (DMF). The reaction mixture was irradiated with 405 nm visible light from a light-emitting diode (LED; 1050 mW at 700 mA, 3.4 × 10−8 mol of photons/s). The gaseous product from this reaction was identified as CO and quantified by gas chromatography, thus confirming that, under these conditions, 1 was a visible-light photoredox catalyst for the selective production of CO with no other detected byproducts (Table S1 and Figure S4). The molar ratio of CO produced to catalyst 1 defined the TON for CO (TONCO), and with 0.1 mM 1 and 1 mM Ru(bpy)32+ the TONCO was 55 (Table S1). The time profile for CO production is shown in Figure S5 and demonstrated that 1 was well-behaved for over 96 h of irradiation with >99% selectivity for CO formation. These observations contrast with reported homogeneous α-diimine Mn-based CO2 reducing photocatalysts that display excellent selectivity for formic acid31,34,37 or an 80/20 ratio of CO/ formate (Table 1).35 Several control experiments established the need and roles of the different components of this catalyst system. Reactions run in the absence of 1 but otherwise identical conditions produced no CO. The complete photocatalytic reaction process was carried out under an N2 atmosphere and showed that no CO was generated, demonstrating that CO originated from CO2 rather than the decomposition of 1, the PS, or the ED. Variation of the ED was explored by employing TEOA, N-benzyl-1,4-dihydridonicotinamide (BNAH), triethylamine, or sodium ascorbate as the ED, and catalyst 1 displayed the highest activity for reduction of CO2 to CO with TEOA (Table S3). Furthermore, while both DMF and acetonitrile are capable of supporting the catalytic activity of 1, in general DMF gave superior catalyst activity. Definitive confirmation that CO arose from CO2 was

CO (μmol)

TONCO (Φ, %)

HCOOH (μmol)

2.4

12 (0.12)

31.4

3.2

7.1 (0.05)

254

64 55 (0.75) 28 (0.37)

200

800 22 11

137

TON (Φ, %) 149 (1.7) 127 (1.9) 16

343 (4.7)

PS = [Ru(bpy)3]2+; ED = BNAH; 4.3 × 10−8 mol of photons/s.31 PS = [Ru(dmb)3]2+; ED = BNAH; 2.51 × 10−7 mol of photons/s.34 c PS = Zn(tetraphenylporphyrin); ED = TEA; no photon flux was provided.35 dReaction conditions: 24 h, 0.1 mM CAT; PS = [Ru(bpy)3]2+ (1 mM); ED = TEOA, 3.4 × 10−8 mol of photons/s. a

b

provided by an isotope tracing experiment. Carrying out the photolysis reaction with 1, Ru(bpy)32+, and TEOA under a 13 CO2 atmosphere resulted in generation of 13CO (Figure S6), clear evidence that that 1 catalyzes CO2 reduction to CO selectively. Complexes 2 and 3 provided analogues of 1 with variation to the ligand environment. In the case of 2, where the N−H group has been replaced with N−Me, successful photocatalytic reduction was observed albeit with lower TONCO (Table S1). The time profile for catalyst 2 (Figure S5) reflects this lower activity compared to 1. Similarly, the replacement of bromide in 1 by triflate to yield 3 led to a reduction in the catalyst activity (Table S1). Encouraged by these results, we also attempted to employ an alternative, non-noble-metal PS.7 First-row transition-metal sensitizers have been explored for photocatalytic CO 2 reduction.29,41,35 We were attracted to the use of available organic PSs with complex 1. The use of p-terphenyl as a PS has been reported but required UV light (96 h of irradiation. Like 1, a variety of reaction parameters were B

DOI: 10.1021/acs.inorgchem.8b02719 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Interestingly, the cathodic scan in the cyclic voltammetry of 1 under N2 displayed two irreversible reductions at −1.72 and −2.20 V versus Fc/Fc+ (Figure S13). These features are reminiscent of the cyclic voltammograms of MnX(bipyR2)(CO)3 complexes.30,44 On the basis of these computational and experimental observations, the effect of the addition of bromide anion on the catalyst TON was measured for both 1 and 4 by the addition of NEt4Br to active systems (Table S6). In both cases, an increased TON was observed and supports a preliminary proposal of the retention of Br− when the catalyst receives the second electron needed produce either CO or HCO2H. The switch in the product selectivity observed for the reduction of CO2 by 1 compared to 4 indicated that the coordination between CO2 and the metal plays an important role in the cycle. In the case of 1, CO2 likely binds to the metal through the C center, and protonation of the O center leads to a metallocarboxylic acid. Subsequent protonation and cleavage of a C−O bond furnishes water and CO. Complex 4 appears to evolve into an O-bonded formate complex by protonation of the C center to ultimately yield formic acid as the reduction product. These results parallel the recently reported switching between CO and formate when the metal in identical ligand environments was changed from Co to Fe.20 The detailed sequencing of this catalytic process remains to be more precisely determined. Breaking with the historic hold of α-diimine ligands on the photocatalyst for CO2 reduction has yielded new Mn- and Rebased complexes with remarkable selectivities and good efficiency. We continue to explore ligand variation and probe the mechanistic features of these transformations.

explored with 4 (Tables S1, S4, and S5). The catalytic performance of 4 was quite sensitive to the identity of the ED (Table S5), with TEOA being superior. A 13C tracer experiment was carried out for the photolysis reaction with 4, Ru(bpy)32+, and TEOA under a 13CO2 atmosphere. High-resolution mass spectrometry analysis of the reaction headspace clearly documented the selective formation of H13COOH (Figure S7), conclusive evidence that the observed formic acid arose from CO2 reduction. They further highlight this unusual switch in product selectivity by replacing Mn with Re and significantly contrast with the reported selectivity of Re α-dimmine species which selectively yield CO as the product of CO2 photoreduction.6,37 Several experiments were explored in order to interrogate the mechanistic features for this photocatalytic activity. Quenching of the excited state PS* was examined and both complexes 1 and 4 were effective in quenching the PS* excited state and, in fact, are more efficient than either TEOA or TEA (Figures S8−S12). This observation suggests one possible entry into the catalytic cycle through the oxidative quenching proposed in Figure 2. Similar oxidative quenching of PS* by metal complexes has previously been reported.17−19,26

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02719. Experimental details, data for catalyst reactions, quenching, and computational data (PDF) Accession Codes

Figure 2. Proposed mechanism for the photocatalytic reduction of CO2 using [M{κ2(Ph2P)NH(NC5H4)}(CO)3Br] [M = Mn (1), Re (4)].

CCDC 1846829 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The nature of the reduced complex, [Mn{κ2-(Ph2P)NH(NC5H4)}(CO)3Br]− (1−), was examined through a density functional theory (DFT) computational investigation. Beginning with the optimized structure of 1 (Figures S14 and S15 and Table S7), the reduced species 1− was optimized (B3LYP, TZVP/DZVP basis set). During the optimization, a Br− anion dissociated to yield a distorted square-pyramidal complex, [Mn{κ2-(Ph2P)NH(NC5H4)}(CO)3] (A) (Figure S16), with the singly occupied molecular orbital (SOMO) electron density in this species localized in a molecular orbital that is a combination of dz2 and pyridyl π* in character (Figure S17 and Table S7). Similar species have been proposed and observed with the reduction of fac-MnX(bipyR2)(CO)3,30,44−46 and these commonly dimerize via the formation of a M−M bond to yield [Mn(κ2-bpyR2)(CO)3]2, which in some cases can be a component of the CO2 reduction catalysis cycle.34,45,46 The analogous DFT optimization for the single-electron reduction of 2 to yield 2−, revealed the SOMO electron density was distributed on the PN-ligand framework and not on the metal center (Figure S17).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.R.). ORCID

Darrin Richeson: 0000-0003-1503-2323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We are grateful for financial support of ths work provided by the Natural Sciences and Engineering Research Council of Canada and the Saudi Arabian Cultural Bureau in Ottawa, Ontario, Canada. C

DOI: 10.1021/acs.inorgchem.8b02719 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b02719 Inorg. Chem. XXXX, XXX, XXX−XXX