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Chemical Science Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Tuning the electronic properties of hexanuclear cobalt sulfide superatoms via ligand substitution† Cite this: Chem. Sci.,2019,10,1760 a b a c All publication charges for this article Gaoxiang Liu, Andrew Pinkard, Sandra M. Ciborowski, Vikas Chauhan, have been paid for by the Royal Society Zhaoguo Zhu,a Alexander P. Aydt,b Shiv N. Khanna, *c Xavier Roy *b of Chemistry and Kit H. Bowen*a Molecular clusters are attractive superatomic building blocks for creating materials with tailored properties due to their unique combination of atomic precision, tunability and functionality. The ligands passivating these superatomic clusters offer an exciting opportunity to control their electronic properties while preserving their closed shells and electron counts, which is not achievable in conventional atoms. Here we demonstrate this concept by measuring the anion photoelectron spectra of a series of hexanuclear cobalt sulfide superatomic clusters with different ratios of electron-donating and electron-withdrawing ligands, Co6S8(PEt3)6Àx(CO)x (x ¼ 0–3). We find that Co6S8(PEt3)6 has a low electron affinity (EA) of 1.1 eV, and that the successive replacement of PEt3 ligands with CO gradually shifts its electronic Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Received 29th August 2018 spectrum to lower energy and increases its EA to 1.8 eV. Density functional theory calculations reveal Accepted 1st December 2018 that the increase of EA results from a monotonic lowering of the cluster highest occupied and lowest DOI: 10.1039/c8sc03862g unoccupied molecular orbitals (HOMO and LUMO). Our work provides unique insights into the rsc.li/chemical-science electronic structure and tunability of superatomic building blocks. Introduction that the characteristics of the building blocks can be tuned pre- assembly without changing the total electron count of the ¼ Electron affinity and ionization energy are fundamental prop- superatom. Using the molecular cluster Co6Te8L6 (L passiv- This article is licensed under a erties of the elements. Together they govern the interactions ating ligand) as a model system, Khanna et al. recently pre- and bonding of close-contacting atoms and control the collec- dicted theoretically that changing L from PEt3 to CO would tive properties of solids. Because they are intrinsic to each increase its electron affinity, in effect transforming the cluster Open Access Article. Published on 03 December 2018. Downloaded 10/1/2021 9:48:40 PM. element, however, the electron affinity and ionization energy of from a superatomic alkali metal toward a superatomic 33,34 a given atom cannot be altered. This presents an immense halogen. The potential to alter the donor/acceptor property synthetic challenge for the design of tunable materials as of these superatomic clusters sheds light on assembling substituting atoms oen leads to entirely new structures, superatomic materials with ne-tuned properties. Experimen- interactions and collective behaviors. tally demonstrating this concept by measuring the electron By analogy to “atomic” building blocks, certain clusters can affinity and/or ionization energy of metal chalcogenide clusters be used as “superatomic” building blocks for the assembly of is challenging, however, because it requires bringing the novel materials.1–22 Within this context, the family of metal charged clusters into the gas phase without damaging the chalcogenide molecular clusters has recently received renewed inorganic core or dissociating the ligands. attention for the creation of functional solids with tunable In this work, we use anion photoelectron spectroscopy to properties, including ferromagnetism, electrical conductivity, probe the electron affinity (EA) and electronic structure of tunable optical gaps and thermal switching.1,7,23–32 One of the a series of cobalt sulde clusters, whose ligand shells con- ff key advantages of this approach over traditional atomic solids is sist of di ering combinations of PEt3 and CO. The clusters Co6S8(PEt3)6Àx(CO)x are synthesized from the parent compound Co6S8(PEt3)6 by ligand substitution with CO. These clusters are aDepartment of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, then brought into the gas phase, where electrons are attached to USA. E-mail: [email protected] form anions, using a unique infrared desorption/photoelectron- bDepartment of Chemistry, Columbia University, New York, New York 10027, USA. E-mail: [email protected] emission/supersonic helium expansion source. Mass spec- cDepartment of Physics, Virginia Commonwealth University, 701 W. Grace St., trometry con rms the existence of carbonylated products with x À À Richmond, Virginia 23284, USA. E-mail: [email protected] up to 3 (i.e. Co6S8(PEt3)5(CO) ,Co6S8(PEt3)4(CO)2 , and Co6- † À Electronic supplementary information (ESI) available: Calculated energies, S8(PEt3)3(CO)3 ). We nd that the electron affinity and vertical one-electron energy levels, and Cartesian coordinates of neutral and anionic detachment energy increase with the number of CO ligands, Co6S8(PEt3)6Àx(CO)x clusters. See DOI: 10.1039/c8sc03862g 1760 | Chem. Sci.,2019,10,1760–1766 This journal is © The Royal Society of Chemistry 2019 View Article Online Edge Article Chemical Science demonstrating the electronic spectral tunability of this family of electron energy analysis (resolution 35 meV at 1 eV EKE). The superatoms. These results are further examples of the supera- third harmonic (355 nm, 3.49 eV per photon) of a Nd:YAG was tom concept, and an important step towards designing mate- used to photodetach electrons from the cluster anion of rials from programmable building blocks. interest. Photoelectron spectra were calibrated against the well- À known atomic lines of the copper anion, Cu . À To make the parent anions Co6S8(PEt3)6Àx(CO)x in the gas Methods phase, a specialized infrared desorption/laser photoemission (IR/ Synthesis PE) supersonic helium expansion source is employed.37 Briey, Dicobalt octacarbonyl and sulfur were purchased from Strem a low power IR pulse (1064 nm) from a Nd:YAG laser hits Chemicals. Triethylphosphine was purchased from Alfa Aesar. a translating graphite bar thinly coated with the Co6S8(PEt3)6Àx- All other reagents and solvents were purchased from Sigma (CO)x sample. Because the graphite absorbs most of the energy, Aldrich. Dry and deoxygenated solvents were prepared by a localized thermal shock lasting a few nanoseconds propels the elution through a dual-column solvent system. Unless other- clusters into the gas phase. Almost simultaneously, a high power wise stated, all reactions and sample preparations were carried pulse of 532 nm light from a second Nd:YAG laser strikes a close- out under inert atmosphere using standard Schlenk techniques by photoemitter (Hf wire), creating a shower of electrons that attach to the evaporated neutral clusters. Also, almost simulta- or in a N2- lled glovebox. While a multi-step synthesis of Co6- 35 neously, a plume of ultrahigh purity helium gas rapidly expands S8(PEt3)6 has been previously reported, we have developed an alternative one-pot approach detailed below. from a pulsed valve located slightly upstream, cooling the nascent anions and directing them into the mass spectrometer, where they Co6S8(PEt3)6. Sulfur (1.16 g, 36.25 mmol) was suspended in areanalyzed.BecausetheCo6S8(PEt3)6Àx(CO)x clusters are slightly 30 mL of toluene in a 200 mL Schlenk ask under N2 atmo- sphere. In two separate asks, Co (CO) (4.12 g, 12.05 mmol) sensitive to air and moisture, they are coated onto the graphite bar 2 8 and PEt (4.27 g, 36.14 mmol) were dissolved in 20 mL of in a N2- lled glove box. The graphite bar is then enclosed inside 3 “ ” Creative Commons Attribution-NonCommercial 3.0 Unported Licence. an air-tight suitcase container, which is only opened under high toluene. The Co2(CO)8 solution was added to the S suspension, vacuum aer being transferred to the vacuum chamber. followed by quick addition of the PEt3 solution. The reaction mixture was re uxed under N2 for 2 days. The reaction mixture was then opened to air, and hot ltered through Celite. The Theoretical calculations ltrate was cooled to room temperature and le to stand for First-principles electronic structure calculations on the anion and 3 h. Black crystals formed during that period; the resulting neutral forms of Co6S8(PEt3)6Àx(CO)x (x ¼ 0–3) clusters were suspension was ltered through a ne frit and the solid was carried out within density functional theory formalism. The ADF washed with toluene, and diethyl ether. The dark, black crystals set of codes were used to perform the calculations where a PBE0 were collected, dried in vacuo, and stored under N2. Yield: 2.2 g This article is licensed under a hybrid functional comprised of the PBE generalized gradient (42%). MS-MALDI m/z+ calculated 1317.92; found, 1317.95. functional and 25% Hartree–Fock exchange, as proposed by Co6S8(PEt3)6Àx(CO)x. Safety note: CO is a toxic gas; this Ernzerhof et al., is used to incorporate exchange and correlation reaction can only be performed in a well-ventilated fumehood. 38–40 effects. The atomic wave functions are expressed in terms of Open Access Article. Published on 03 December 2018. Downloaded 10/1/2021 9:48:40 PM. Sulfur (0.15 g, 4.68 mmol) and Co S (PEt ) (1.00 g, 0.76 mmol) 6 8 3 6 Slater-type orbitals (STO) located at the atomic sites and the were combined in 75 mL of toluene in a 200 mL Schlenk ask. cluster wave functions are constructed from a linear combination An external bubbler was attached to the system, and the mixture 41 of these atomic orbitals. ATZ2Pbasissetandalargefrozen was sparged with CO for 30 min. The mixture was heated to electron core was used. The zero-order regular approximation 100 C and stirred under a CO atmosphere for 16 h.
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