Sensitized Photocatalytic CO2 Reduction With Earth Abundant 3d Metal Complexes Possessing Dipicolyl-Triazacyclononane Derivatives

Complexes based on nitrogen and sulfur containing ligands involving 3d metal centers are known for the electrocatalytic reduction of CO2. However, photocatalytical activation has rarely been investigated. We herein present results on the light-driven CO2 reduction using either Ir(dFppy)3 [Ir, dFppy = 2-(4,6-difluorophenyl)pyridine] or [Cu(xant)(bcp)]+, (Cu, xant = xantphos, bcp = bathocuproine) as photosensitizer in combination with TEA (triethylamine) as sacrificial electron donor. The 3d metal catalysts have either dptacn (dipicolyl-triazacyclononane, L N3 ) or dpdatcn (dipicolyl-diazathiocyclononane, L N2S ) as ligand framework and Fe3+, Co3+ or Ni2+ as central metal ion. It turned out that the choice of ligand, metal center and solvent composition influences the selectivity for product formation, which means that the gaseous reduction products can be solely CO or H2 or a mixture of both. The ratio between these two products can be controlled by the right choice of reaction conditions. With using Cu as photosensitizer, we could introduce an intermolecular system that is based solely on 3d metal compounds being able to reduce CO2.

X-ray diffraction. The data collections were performed with a BRUKER D8 VENTURE area detector with Mo-Kα radiation (λ = 0.71073 Å). Multi-scan absorption corrections implemented in SADABS [1] were applied to the data. The structures were solved by intrinsic phasing method (SHELXT-2014) [2] and refined by full matrix least square procedures based on F 2 with all measured reflections (SHELXL-2018) [3] with anisotropic temperature factors for all non-hydrogen atoms. All hydrogen atoms were added geometrically and refined by using a riding model. Crystals for both complexes Fe-L N3 and Co-L N3 were obtained by slow vapor diffusion of Et2O to a high concentrated solution in MeCN. CCDC numbers 2096076 and 2096077 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. EPR spectra were recorded on a Bruker EMXplus X-band EPR spectrometer and a Bruker-ColdEdge-ER4112HV-CF10-H, Helium recirculating cryostat. The sample solution in the quartz EPR tube was frozen in liquid nitrogen and kept frozen until measured.

Photocatalysis.
A LOT-QuantumDesign GmbH 200 W Hg lamp was used for photocatalytic CO2 reduction experiments. The l>400 nm longpass filter used was sourced from LOT-QuantumDesign GmbH with the purpose of allowing only the light at greater wavelengths of the employed longpass filter to be transmitted. Each longpass filter is dielectrically coated and has an edge slope of 2%. The concentration dependent and time dependent measurements involving dry TEA (triethylamine) as the electron donor, 4.75 mL of a stock solution of the catalyst in dry DMF and 0.25 mL of dry TEA were added to a Schlenk vessel under inert conditions. Afterwards an equivalent amount of external photosensitizer stock solution was added under inert conditions. Schlenk vessels were employed that could host 5 mL of solution and a further 11 ml space for gas. In the case of water addition, a corrected headspace volume was used for the calculation of the TON. The solution was degassed thoroughly with CO2 for at least ten minutes and then the vessel sealed. Using a gas tight syringe, 250 µL of the gas phase was injected into the gas chromatograph (GC) and the TONCO or TONH2 were determined by taking the area of the peak corresponding to CO (with a retention time of 2.8-3.1 min -1 ) respectively H2 (with a retention time of 0.4-0.6 min -1 ) and corroboration of this value with the calibration curve to identify the TONCO or TONH2 value. The Shimadzu GC-2014 gas chromatograph was used for CO or H2 detection with a thermal conductivity detector and a Resteks ShinCarbon packed column ST 80/100 (2 m, 1/8" outer diameter, 2 mm inner diameter). The injector temperature was set to 200°C, the detector temperature set to 300°C and the gases were separated according to a temperature-time program on the column. CO and persistent gases such as argon and nitrogen were separated at the early stages of the measurement at an oven temperature of 40°C and after 20 minutes CO2 was eluted at the later stages. It was tried to determine the amount of formic acid which might have been formed, but no NMR method could be found. This is due to the combination of small amounts of formic acid being formed and the used solvent DMF having a signal close to the one of formic acid, hence overlapping that signal occasionally. Synthetic Details All reactions were performed under a dry Ar or N2 atmosphere using standard Schlenk techniques or by working in a glovebox. Starting materials and chemicals were obtained from commercial suppliers and used without further purification. The photosensitizer Ir(dFppy)3 was obtained from commercial supplier (Sigma-Aldrich), while the photosensitizer [Cu(xant)(bcp)] + was provided by the group of Dr. Michael Karnahl (TU Braunschweig, Germany). All solvents were dried and degassed according to standard methods or directly taken from MBraun solvent purification system (e.g. Et2O or MeCN). Dry DMF (99.8%) was purchased from Sigma-Aldrich and stored over molecular sieve (3 Å) prior to use. Thin-layer chromatography was performed using Merck TLC neutral aluminum oxide or silica gel 60 F254 sheets. For column chromatography, silica gel with a pore size of 60 Å from Acros Organics or neutral AlOx gel was used. The used solvent mixtures are given by volume fractions.

Synthesis of ligands
The synthesis of the ligands L N3 (4) and L N2S (9) are outlined in Scheme S1 and Scheme S2. Di-tosylethylenglykol and tri-tosyl-ethylentriamine were synthesized according to published procedures. [4] Scheme S1: Synthetic pathway of L N3 . Scheme S2: Synthetic pathway of L N2S .

Synthesis of 1,4-Di(picolyl)-1,4,7-trazacyclononane (4)
3 (1.42 g, 3.00 mmol) was combined with 10 mL of conc. H2SO4 and heated for 24 h at 150 °C. After cooling to 0 °C the black solution was basified to pH ~ 13 using saturated NaOH solution and extracted with vast amount of CHCl3 to yield 4a. The organic phases were combined, the solvent removed ( 1 H NMR spectrum of crude product in Figure S5) and the residue purified using gel chromatography on aluminum oxide (CH2Cl2-MeOH 98:2, third and fourth fraction). The combined fractions were dried over Mg2SO4 and the solvent was removed to yield 4 as a dark brown oil (0.76 g, 80%). [7] During the chromatographic work-up process 4a became protonated as indicated in Figure S4, where a NH signal at 10.5 ppm is observable. The proton source is currently not unambiguously identified; we believe that trace impurities in CH2Cl2 are the most likely origin.  84 g, 15.3 mmol, 1 eq.) was combined with NaOH (1.29 g, 32.1 mmol, 2.1 eq.) and dissolved in water (35 mL). TosylCl (6.13 g, 32.1 mmol, 2.1 eq.) was first dissolved in Et2O (30 mL) and then slowly dropped into the solution. The resulting mixture was vigorously stirred for 36 h at room temperature. Afterwards, the layers were separated and the aqueous layer extracted with CHCl3. The combined organic fractions were reduced to dryness. the residue was purified via silica gel chromatography (CH2Cl2-MeOH 9:1) yielding 6 as a bright yellow oil (6.00 g, 91%). [

Synthesis of 4,7-Bis-tosyl-1-thia-4,7-diazacyclononane (7)
1.88 g (78 mmol) of LiOH were dissolved in 75 ml water and combined with tetrabutylammonium bromide (460.7 mg, 1.42 mmol, 0.25 eq.) in 400 mL toluene and heated under reflux. To this solution di-tosyl-ethylene glycol (2.12 g, 5.72 mmol, 1 eq.) was added in small portions under heavy stirring. To this mixture 6 (2.45 g, 5.72 mmol, 1 eq.), dissolved in toluene (150 mL), was slowly added using a dropping funnel and afterwards stirred under reflux for 12 h. After cooling, the layers were separated and the organic phase was evaporated. The obtained residue was recrystallized in hot MeOH to yield 7 as a colourless solid (1.4 g, 54%). [9] 2.1.8 Synthesis of 1-Thia-4,7-diazacyclononane (8) 7 (2.70 g, 5.94 mmol) was dissolved in 50 mL of HBr in glacial acetic acid (33%) and stirred for 3 days. After completion, the mixture was concentrated by removing acetic acid as azeotropic mixture using toluene (4x20 mL). The crude product was dissolved in water and washed with CH2Cl2. The aqueous layer was concentrated, redissolved in distilled water and 10 eq. of 1 M NaOH were added. The aqueous phase was removed using a Dean-Stark-apparatus by using toluene, from which 8 was obtained as a pale yellow oil (160 mg, 18%) after reduction to dryness. [

1 H NMR spectrum of Co-L N3
Figure S11: 1 H NMR spectrum of Co-L N3 in MeCN-d 3 .