One electron reduction of a cobalt-sulfur cluster. Synthesis and molecular structure of [Co6(μ3-S)8(PEt3)6]·nthf

Experimental and theoretical evidence for low-lying excited states in [Cr6E8(PEt3)6] (E = S, Se, Te) cluster molecules

Three [Cr₆E₈(PEt₃)₆] cluster molecules with E = S, Se, and Te have been synthesized by reaction of stoichiometric mixtures of Cr(II) and Cr(III) metal salts with silylated chalcogen reagents E(SiMe₃)₂ (E = S, Se, Te) in the presence of L = PEt₃ = triethylphosphine. For the sulfide- and selenide-bridged clusters two crystallographic forms (trigonal R3̄ and triclinic P1̄), which differ in the presence of lattice solvent molecules, have been isolated. Structural data, optical spectra and quantum chemical calculations reveal the presence of low-lying excited states in [Cr₆E₈(PEt₃)₆] (E = S, Se), which would help in rationalizing the non-vanishing magnetic moments at 2 K revealed by DC magnetic measurements and EPR spectroscopy. These findings are partially in contrast to a previous report by Saito and co-workers (S. Kamiguchi, H. Imoto, T. Saito, Inorg. Chem., 1998, 37, 6852-6857.), who postulated an incorporated hydrogen atom as the source of paramagnetism at low temperatures for the trigonal forms of [Cr₆E₈(PEt₃)₆] (E = S, Se).

Tuning the electronic properties of hexanuclear cobalt sulfide superatoms via ligand substitution

The electronic properties of the Co₆S₈L₈ superatom can be tuned by changing its ligand composition while maintaining its electron count and closed shell.

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)_6–x(CO)_x (x = 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 spectrum to lower energy and increases its EA to 1.8 eV. Density functional theory calculations reveal that the increase of EA results from a monotonic lowering of the cluster highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Our work provides unique insights into the electronic structure and tunability of superatomic building blocks.

Cluster synthesis via ligand-arrested solid growth: triethylphosphine-capped fragments of binary metal chalcogenides

A new and potentially highly generalizable technique for synthesizing molecular fragments of binary solids is demonstrated through application to selected transition metal chalcogenides. Employing a metal atom reactor, the solids are evaporated with a tungsten heating boat, and the resulting vapor is co-condensed with triethylphosphine. Major cluster products identified from a survey of first-row transition metal sulfides include the known species Cr6S8(PEt3)6, Co6S8(PEt3)6, and Cu12S6(PEt3)8, as well as the unprecedented species Fe4S4(PBun3)4, Ni4S4(PEt3)8, and Cu6S4(PEt3)4. Reactions utilizing Cu2Se resulted in the much larger clusters Cu26Se13(PEt3)14 and Cu70Se35(PEt3)21. The core of the former has a Th-symmetry structure featuring a body-centered icosahedron of Se2- anions, while the latter adopts a triangular structure based on three hexagonal closest packed layers of Se2- anions. In both cases, the Cu+ cations occupy distorted tetrahedral or trigonal planar sites similar to those encountered in Cu2Se; however, emergence of the face-centered cubic anion lattice of the bulk solid is not yet apparent at these cluster sizes.