Cu14 Cluster with Partial Cu(0) Character: Difference in Electronic Structure from Isostructural Silver Analog

Abstract An atom‐precise Cu0‐containing copper cluster, Cu14(C2B10H10S2)6(CH3CN)8 (abbreviated as Cu14‐8CH3CN) is reported, which is synthesized via a simultaneous reduction strategy and fully characterized by single‐crystal X‐ray diffraction, ESI‐TOF‐MS, and X‐ray photoelectron spectroscopy. Cu14‐8CH3CN is the only copper cluster that has a virtually identical silver structural analog, i.e., Ag14(C2B10H10S2)6(CH3CN)8 (hereafter as Ag14‐8CH3CN). Nevertheless, density functional theory calculations reveal that the electronic structure of Cu14‐8CH3CN differs significantly from the superatom electronic configuration of Ag14‐8CH3CN. Moreover, Cu14‐8CH3CN shows room‐temperature luminescence and good electrocatalytic activities in the ethanol oxidation reaction and detection of H2O2. This pair of unprecedented analogous molecular nanoscale systems offer an ideal platform to investigate the fundamental differences between copper and silver in terms of catalytic activity and optical properties.

1. Experimental 1.1 Reagents. 1,2-dithiol-o-carborane were prepared by a literature method. [1] All other reagents and solvents used were of commercially available reagent grade and were used without any additional purification.

Apparatus and Characterization
UV-vis absorption spectra were recorded with a U-2000 spectrophotometer. The HRESI-TOF-MS spectrum of Cu 14 -8CH 3 CN was collected on a SolariX 9.4T ICR spectrometer. The HRESI-TOF-MS spectra of Cu 14 -8DMABN and the reaction solution were collected on an AB Sciex X500R Q-TOF spectrometer. 1 H and 11 B NMR spectra were recorded using Bruker AV300 spectrometer. Chemical shifts are expressed in parts per million (ppm) downfield from internal TMS. EDS measurement of Cu 14 -8CH 3 CN was carried out using Zeiss Sigma 500 system. X-ray photoelectron spectroscopy (XPS) measurements were performed with a VG Scientific ESCALAB 250 instrument equipped with a monochromatic Al Kα x-ray source (hν = 1486.8 eV). Samples were loaded into a custom built air-free sample holder, under a N 2 atmosphere. Prior to data collection, a baseline vacuum of 1.07 x 10 -9 mbar was achieved. For high resolution scans, a band pass energy of 20 eV was used. Binding energies were calibrated using the C 1s peak of adventitious carbon at 284.8 eV. The peak positions in the Cu 2p region and the LMM Auger emission were determined using the Casa XPS software package.
Powder X-ray diffraction (PXRD). PXRD data were collected at room temperature in air using an X' Pert PRO diffractometer (Cu Kα, λ = 1.54178 Å). In situ PXRD patterns were collected on samples immersed in the mother liquor on a Rigaku XtaLAB Pro diffractometer with Cu-Kα radiation.
Luminescence measurements. Luminescence spectra were recorded on a HORIBA FluoroLog-3 fluorescence spectrometer. Luminescence decay was measured on a HORIBA Scientific Fluorolog-3 spectrofluorometer equipped with a 355 nm laser operating in timecorrelated single photon counting mode (TCSPC) with a resolution time of 680 µs. The photoluminescent quantum efficiency in solution (1.6×10 -3 mol/L) was measured using an integrating sphere on a HORIBA Scientific Fluorolog-3 spectrofluorometer.
Single-crystal X-ray diffraction analysis (SCXRD). SCXRD measurements were performed on a Rigaku XtaLAB Pro diffractometer with Cu-Kα radiation (λ = 1.54184 Å) at 200 K for Cu 14 -8CH 3 CN and 150 K for Cu 14 -8DMABN. Data collection and reduction were performed using the program CrysAlisPro. [2] The intensities were corrected for absorption using an empirical method implemented in the SCALE3 ABSPACK scaling algorithm. The structures were solved with intrinsic phasing methods (SHELXT-2015) [3] for Cu 14 -8DMABN and direct methods (SHELXS-2015) [4] for Cu 14 -8CH 3 CN and were refined by full-matrix least squares on F 2 using OLEX2, [5] which utilizes the SHELXL-2015 module. [3] All non-hydrogen atoms were refined with anisotropic thermal parameters, and the hydrogen atoms were included at idealized positions. In Cu 14 -8CH 3 CN, the high symmetry, symmetry inconsistency (symmetry of Fm-3m space group is higher than that of the carborane moiety) and the site disorder of the 1,2-dithiolate-o-carborane made it difficult to solve the carborane structure in the correct icosahedron model. Fortunately, we succeeded in distinguishing two sets of halfoccupied 1,2-dithiolate-o-carborane as icosahedral carborane moieties. To tackle the connectivity problems for theoretical H addition to B atoms, a few "free" commands were used. There was a large solvent-accessible void volume in the crystals of Cu 14 -8CH 3 CN, which was occupied by highly disordered solvent molecules. No satisfactory disorder model could be found; therefore, the Solvent Mask program implemented in OLEX2 [5] was used to remove background residual electron densities. In Cu-Disulfide, one B atom in the carborane was refined with 0.5 occupancy to realize the coupling species with one closo carborane and one deboronated carborane. The B-H-B bridging H atom could not be localized based on Difference Fourier maps. The crystal structures are visualized using DIAMOND 3.2. [6] 1.3 Density functional theory (DFT) calculations. DFT and time-dependent density functional theory (TD-DFT) calculations were performed with Gaussian 16 [7] under the Perdew−Burke−Ernzerhof (PBE) functional. [8] All calculations were conducted using the Def2-SVP basis set for all atoms. [9] The single-crystal structure was chosen as the initial guess for ground-state optimization, and all reported stationary points were verified as true minima by the absence of negative eigenvalues in the vibrational frequency analysis. The calculated absorption spectra were obtained from GaussSum 2.1. [10] Hirshfeld population analysis was conducted by Multiwfn 3.4. [11]     The sharp peak at 18.74 ppm is in good accordance to the signal of H 3 BO 3 ; the signal from -1.94 to -18.93 ppm is assigned to the closo-carborane groups; the weak multiple signal around -34 ppm is tentatively attributed to the deboronated carborane units. [12] Figure S5. Comparison between 1 H NMR (CD 3 OD) spectra of Cu 14 -8CH 3 CN and 1,2-dithiol-o-carborane ligand. The signal for B-H is weak and unstructured due to the limit solubility of Cu 14 -8CH 3 CN. Reliable integration was not performed because of the peak overlap.

Electrocatalysis tests.
Ethanol electrooxidation reactions (EERs) evaluation. The EER activities of Cu 14 -8CH 3 CN were evaluated in a three-electrode cell with N 2 -saturated 0.1 M KOH solutions at room temperature. A glassy carbon (GC) electrode (5 mm in diameter, S = 0.1962 cm 2 ) was coated with Cu 14 -8CH 3 CN as follows: the Cu 14 -8CH 3 CN clusters (1.25 mg) and super P (1 mg) was ultrasonicated in a mixture of methanol (100 µL), dichloromethane (100 µL) and Nafion (30 µL) until a uniform ink was achieved. Then, 10 μL of the competent precatalyst ink was pipetted onto the GC electrode surface by using a micropipettor and dried at ambient temperature. After the solvent in 10 µL of slurry was completely evaporated, the known amount of solid sample was left on the surface of GCE to form a uniform precatalyst film. The Cu 14 -8CH 3 CN loading amount of GCE was 0.27 mg/cm 2 for ethanol electrocatalytic oxidation. This modified precatalyst film with Nafion cross-linking matrix on the GCE is very stable under the electrochemical test in electrolyte. The GC electrode coated with Cu 14 -8CH 3 CN was used as the working electrode, and an Ag/AgCl (KCl saturated) electrode and a platinum wire were used as the reference and counter electrode, respectively. Cyclic voltammograms of the electrocatalysts were detected at a scan rate of 50 mV s -1 .

Electrochemical Detection of Hydrogen Peroxide.
All measurements were carried out on a CHI 660E electrochemical workstation in a standard three-electrode cell at room temperature. An Ag/AgCl (KCl, saturated) electrode and Pt wire were used as the reference and counter electrode, respectively. A glassy carbon electrode coated with Cu 14 -8CH 3 CN cluster solution was used as the working electrode. For the electrochemical detection, a glassy carbon (GC) electrode was first polished with alumina slurries (0.05 μm) and then cleaned by successive sonication in dilute nitric acid solution, ultrapure water, and ethanol respectively. The catalyst solution was prepared by dispersing 0.5 mg Cu 14 -8CH 3 CN in 500 μL solution containing 250 μL of methanol, 230 μL of dichloromethane and 20 μL of 5 wt.% Nafion solution followed by ultrasonication for 5 min. 10 μL of catalyst solution was then dropcast onto the clean GC surface by using a micropipettor and the particle film dried at ambient temperature. The Cu 14 -8CH 3 CN loading amount of GCE was 0.14 mg/cm 2 for H 2 O 2 detection. This modified catalyst film with Nafion cross-linking matrix on the GCE is very stable under the electrochemical test in electrolyte.
ICP-MS (Inductively coupled plasma mass spectrometry) measurements were measured to detect the loss of Cu in electrolyte after the electrocatalytic test. As a result, no Cu component was detected in the electrolyte for ethanol electrooxidation reaction, and a negligible amount of Cu (0.05 ppm) appeared in the electrolyte for H 2 O 2 detection, indicating the fastness of catalyst on electrodes.