Divergent Electronic Structures of Isoelectronic Metalloclusters: Tungsten(II) Halides and Rhenium(III) Chalcogenide Halides

Abstract

Same but different: DFT calculations on hexanuclear tungsten(II) halide clusters [W(6)X(8)X'(6)](2-) (X, X'=Cl, Br, I) indicate a breakdown in the isoelectronic analogy between themselves and the isostructural rhenium(III) chalcogenide clusters [Re(6)S(8)X(6)](4-) (see figure).The hexanuclear tungsten(II) halide clusters and the sulfido-halide clusters of rhenium(III) are subsets of a broad system of 24-electron metal-metal bonded assemblies that share a common structure. Tungsten(II) halide clusters and rhenium(III) sulfide clusters luminesce from triplet excited states upon ultraviolet or visible excitation; emission from both cluster series has been extensively characterized elsewhere. Reported here are density-functional theory studies of the nine permutations of [W(6)X(8)X'(6)](2-) (X, X'=Cl, Br, I). Ground-state properties including geometries, harmonic vibrational frequencies, and orbital energy-level diagrams, have been calculated. Comparison is made to the sulfide clusters of rhenium(III), of which [Re(6)S(8)Cl(6)](4-) is representative. [W(6)X(8)X'(6)](2-) and [Re(6)S(8)Cl(6)](4-) possess disparate electronic structures owing to the greater covalency of the metal-sulfur bond and hence of the [Re(6)S(8)](2+) core. Low-lying virtual orbitals are raised in energy in [Re(6)S(8)Cl(6)](4-) with the result that the LUMO+7 (or LUMO+8 in some cases) of tungsten(II) halide clusters is the LUMO of [Re(6)S(8)Cl(6)](4-) species. An inversion of the HOMO and HOMO-1 between the two cluster series also occurs. Time-dependent density-functional calculations using asymptotically correct functionals do not recapture the experimentally observed periodic trend in [W(6)X(14)](2-) luminescence (E(em) increasing in the order [W(6)Cl(14)](2-) < [W(6)Br(14)](2-) < [W(6)I(14)](2-)), predicting instead that emission energies decrease with incorporation of the heavier halides. This circumstance is either a gross failure of the time-dependent formalism of DFT or it indicates extensive multistate emission in [W(6)X(8)X'(6)](2-) clusters. The inapplicability of isoelectronic analogies between clusters of Group 6 and Group 7 is emphasized.

Preparation and Luminescence of Cluster Compounds [W6Br8L6]2– with L = CF3COO and C7H7SO3

“…[10] Interestingly, this trend behaves oppositional to the (calculated) [27] [10] which is in line with our [W 6 X 8 L 6 ] 2-compounds.…”

“…[8][9][10]27] Generally, photoexcitation by UV or visible radiation induces an S (ground state) to S n transition, followed by intersystem crossing resulting in the population of triplet states T n and subsequent emission from these triplet states, a process called phosphorescence. However, the photoluminescence is quenched in the presence of O 2 (in solution).…”

“…W(CO) 6 ] (0.11 mmol) in 4 mL DCM in a Schlenk tube, AgL (0.66 mmol) was added and stirred in the dark for 16 h [Equation (4]. The resulting suspension of solid AgBr in a yellow solution was filtered off and the solvent was removed under vacuum yielding a yellow powder.…”

“…Except for very few exceptions, compounds of this type contain 24 electrons in metal-metal bonding cluster states. The electronic structure of [M 6 X 14 ] 2-clusters appears sufficiently well understood, [3][4][5] and photophysical studies have been carried out since the late 1980s. [6][7][8][9][10][11] Detailed accounts on the binary tungsten bromides W 6 Br 12 , W 6 Br 14 , W 6 Br 16 , and W 6 Br 18 have been published by Siepmann [1] and von Schnering [2] in 1968, followed by extensive work on tungsten halide clusters by Hogue and McCarley in 1970.…”

Combined theoretical and time-resolved photoluminescence investigations of [Mo6Bri8Bra6]2−metal cluster units: evidence of dual emission

“…18,33 The calculated emission energies they obtained are higher than the experimental ones. They also mentioned that multistate luminescence can explain theoretical/experimental discrepancies.…”