Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

Patrick Herr,Oliver S Wenger

doi:10.3390/inorganics8020014

Abstract

Diisocyanide ligands with a m-terphenyl backbone provide access to Mo0 complexes exhibiting the same type of metal-to-ligand charge transfer (MLCT) luminescence as the well-known class of isoelectronic RuII polypyridines. The luminescence quantum yields and lifetimes of the homoleptic tris(diisocyanide) Mo0 complexes depend strongly on whether methyl- or tert-butyl substituents are placed in α-position to the isocyanide groups. The bulkier tert-butyl substituents lead to a molecular structure in which the three individual diisocyanides ligated to one Mo0 center are interlocked more strongly into one another than the ligands with the sterically less demanding methyl substituents. This rigidification limits the distortion of the complex in the emissive excited-state, causing a decrease of the nonradiative relaxation rate by one order of magnitude. Compared to RuII polypyridines, the molecular distortions in the luminescent 3MLCT state relative to the electronic ground state seem to be smaller in the Mo0 complexes, presumably due to delocalization of the MLCT-excited electron over greater portions of the ligands. Temperature-dependent studies indicate that thermally activated nonradiative relaxation via metal-centered excited states is more significant in these homoleptic Mo0 tris(diisocyanide) complexes than in [Ru(2,2′-bipyridine)3]2+.

Highlights

Hexacarbonyl complexes of Cr0, Mo0, and W0 are prototypical coordination compounds obeying the 18-electron rule with a low-spin d6 valence electron configuration
Temperature-dependent studies indicate that thermally activated nonradiative relaxation via metal-centered excited states is more significant in these homoleptic Mo0 tris(diisocyanide) complexes than in [Ru(2,20 -bipyridine)3 ]2+
The isocyanides are less π-accepting than CO, yet the ligand field remains very strong, and all 6 d-electrons are paired in the t2g -set of d-orbitals, which represent the HOMO in octahedral symmetry

Summary

Introduction

Hexacarbonyl complexes of Cr0 , Mo0 , and W0 are prototypical coordination compounds obeying the 18-electron rule with a low-spin d6 valence electron configuration. Isocyanides (CNR) are formally isoelectronic with CO, and it is unsurprising that hexakis(isocyanide) complexes of the abovementioned d6 metals as well as some heteroleptic complexes comprising both CO and CNR ligands have long been known [1,2,3,4,5,6,7]. The isocyanides are less π-accepting than CO, yet the ligand field remains very strong (even in the Cr0 complexes), and all 6 d-electrons are paired in the t2g -set of d-orbitals, which represent the HOMO in octahedral symmetry. There is some π-conjugation between the C≡N group and the aryl π-system, and antibonding ligand-based orbitals become the LUMO in hexakis(arylisocyanide) complexes of Cr0 , Mo0 , and. The resulting electronic structure with a metal-based HOMO and a ligand-centered LUMO is closely related to that encountered for isoelectronic RuII and OsII polypyridine complexes. In analogy to this well-known class of precious metal-based complexes, arylisocyanide complexes of

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Results

Conclusion