Enhanced photoreduction of water catalyzed by a cucurbit[8]uril-secured platinum dimer

A cucurbit[8]uril (CB[8])-secured platinum terpyridyl chloride dimer was used as a photosensitizer and hydrogen-evolving catalyst for the photoreduction of water. Volumes of produced hydrogen were up to 25 and 6 times larger than those obtained with the corresponding free and cucurbit[7]uril-bound platinum monomer, respectively, at equal Pt concentration. The thermodynamics of the proton-coupled electron transfer from the Pt(ii)–Pt(ii) dimer to the corresponding Pt(ii)–Pt(iii)–H hydride key intermediate, as quantified by density functional theory, suggest that CB[8] secures the Pt(ii)–Pt(ii) dimer in a particularly reactive conformation that promotes hydrogen formation.


Generalities
All reagents were purchased from chemical suppliers and used without further purification. Cucurbit [7]uril (CB [7]) and Cucurbit [8]uril (CB [8]) were prepared using known procedures. 1 Solvents were of analytical grade and either used as purchased or dried according to procedures described elsewhere. 2 Characterization by nuclear magnetic resonance spectroscopy (NMR) was carried out using a Bruker Ascend 500 MHz spectrometer. 1 H and 13 C NMR chemical shifts are reported in parts per million (ppm) and are referenced to TMS using the residual signal of the solvent as an internal reference. Coupling constants (J) are reported in hertz (Hz). Standard abbreviations used to indicate multiplicity are: s = singlet, d = doublet, dd = doublet of doublets, t = triplet. High resolution electrospray ionization mass spectrometry (HR-ESI-MS) was performed using a Thermo Fisher Scientific Q Exactive Plus hybrid quadrupole−Orbitrap mass spectrometer in positive mode. UV-Vis absorption spectra were recorded on an Agilent HP-8453 diodearray spectrophotometer. Wavelengths (λ) are reported in nanometers (nm) and molar absorption coefficients (ε) are reported in M -1 cm -1 . Computational work was carried out on the Owens cluster of the Ohio Supercomputer Center in Columbus, OH (23,392-core Dell Intel Xeon E5-2680 v4 machines). 4'-(ptolyl)-2,2':6',2''-terpyridine and chloro[4'-(p-tolyl)-2,2':6',2''-terpyridine]-platinum(II) chloride (1b) were prepared according to published procedures. 3

Photocatalytic H2 production
All photolysis experiment were carried out under buffered condition (pH 5, 0.10 M MES, 30 mM EDTA). Photolysis experiments were carried out at least in duplicates for each reaction condition. Each replicate had 6.0 mL of the solution in 23.0 mL vials equipped with PTFE/silicone septa caps. Samples were prepared in the dark and deoxygenated for 10 min using Ar prior to photolysis. Each solution was photolyzed using in-house built royal blue LEDs (λ irr = 447.5 ± 10 nm at fwhm) purchased from Luxeon Star LEDs (Quadica Developments, Inc., Lethbridge, Alberta, Canada). The output from each LED was 250 mW, and samples were placed 1 cm away from the light source. After photolyzing each solution, an aliquot (0.10 mL) was removed from the headspace using a Hamilton GASTIGHT syringe and injected into a Shimadzu GC-2014 gas chromatograph (GC; Ar carrier gas) with a packed ShinCarbon ST SilicoSmooth stainless steel column (2 m long × 1/8 in.o.d. × 2.0 mm i.d.; 80/100 mesh) and a Shimadzu TCD-2014 thermal conductivity detector. The GC conditions were as follows: injector temperature, 41 °C; column temperature, 30 °C; detector temperature, 150 °C; gas flow, 25 mL/min. The volume of injected H2 was determined using a calibration curve generated from known volumes of 100% H2.

Chemical actinometry
Chemical actinometry was carried out according to literature sources. 5,6 All sample preparations and photolysis experiments were performed in the dark and in triplicates. K3[Fe(C2O4)3] (6.0 mL, 0.15 M) in a 23 mL vial was photolyzed for 5 s with 447.5 ± 10 nm LED irradiation. An aliquot of the photolyzed solution (10 mL) was added to 0.1% buffered 1,10-phenanthroline (phen) solution (5.0 mL). The mixture was kept in the dark for 1 h. The process was repeated with irradiation times of 10, 15 and 30 s; a control experiment without irradiation was also carried out. Solutions were transferred into 10 mm cuvettes and their absorbances were measured using an Agilent Cary 8454 diode array UV−vis spectrophotometer (1 nm resolution, 0.5 s integration time).
Amounts of Fe 2+ ions n(Fe 2+ ) produced during photolysis were obtained as follows: where ΔA510 is the absorbance difference at 510 nm, l the path length (10 mm), ɛ510 the extinction coefficient of [Fe(phen)3] 2+ at 510 nm (11,100 M −1 ·cm −1 ), V1 the total volume of irradiated solution (6.0 mL), V2 the volume of the aliquot taken from the irradiated solution (10 mL) and V3 the volume that the aliquot is diluted into (5.0 mL).
The photon flux (qn,p) was calculated as follows: where Φ(λ) is the reported quantum yield of K3[Fe(C2O4)3] photodegradation at wavelength λ and t the irradiation time (s).

Mass spectrometry analysis during the photolysis of complex CB[8]·1b2
Photolysis prior to HR-ESI-MS analysis was carried out on a 0.50 mM solution of assembly CB [8]·1b2 in a 1.5 mM EDTA solution in LC-MS grade water. An aliquot (10 µL) was withdrawn from the solution at specific times, diluted 10 times with a 1:1 solution of water (LC-MS grade) and methanol (HPLC grade), and injected into the mass spectrometer.    Figure S4. (a) UV-Vis absorption spectra of complex 1b (in grey), binary assembly CB [7]·1b (in blue) and ternary assembly CB [8]·1b2 (in green); Pt concentration: 20 µM. (b) Extinction coefficients of complex 1b solutions at 384 nm as a function of concentration; fit with a dimerization model (solid black line) and 95% confidence interval (dashed lines). All spectra recorded in MES (0.10 M)/EDTA (30 mM) buffer.

Isothermal titration calorimetry
All isothermal titration calorimetry (ITC) experiments were carried out in MilliQ water at 25 ºC on a Malvern MicroCal ITC200 instrument. Due to the low solubility of complex 1b and CB [8], L-Cys-derived surrogate 1c was used as the titrant instead. The latter was set up in the injection syringe at concentrations ranging from 0.85 to 1.0 mM. Hosts (CB [7] and CB [8]) were in the sample cell at concentrations ranging from 0.050 to 0.10 mM. Experiments were carried out in triplicate. Each titration consisted of 20 injections with an injection spacing of 150 s. Raw data were analyzed (baseline correction, integration and fitting) with Affinimeter software.

X-ray crystallography
A specimen of C92H82Cl2N38O16Pt2, approximate dimensions 0.090 mm x 0.100 mm x 0.100 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured. The integration of the data using a triclinic unit cell yielded a total of 264585 reflections to a maximum θ angle of 27.00° (0.78 Å resolution), of which 47500 were independent (average redundancy 5.570, completeness = 90.6%, Rint = 5.81%, Rsig = 5.19%) and 39282 (82.70%) were greater than 2σ(F 2 ). The final cell constants of a = 17.9543(13) Å, b = 20.7821(16) Å, c = 33.976(3) Å, β = 82.949(4)°, volume = 12580.9(16) Å 3 , are based upon the refinement of the XYZ-centroids of reflections above 20 σ(I). The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.8000 and 0.8210. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group P -1, with Z = 4 for the formula unit, C92H82Cl2N38O16Pt2. The final anisotropic full-matrix least-squares refinement on F 2 with 2633 variables converged at R1 = 10.11%, for the observed data and wR2 = 25.54% for all data. The goodness-of-fit was 1.105. The largest peak in the final difference electron density synthesis was 3.504 e -/Å 3 and the largest hole was -4.534 e -/Å 3 with an RMS deviation of 0.203 e -/Å 3 . On the basis of the final model, the calculated density was 1.287 g/cm 3 and F(000), 4872 e -.