Luminescent Gold(I) Alkynyl Clusters Stabilized by Flexible Diphosphine Ligands

Treatment
of the homoleptic decanuclear compounds (AuC2R)10 with the cationic gold diphosphine complexes [Au2(PR′2-X-PR′2)2]2+ results
in high-yield formation of the new family of hexanuclear clusters
[Au6(C2R)4(PR′2-X-PR′2)2]2+ (PR′2-X-PR′2 = PPh2-(CH2)n-PPh2, n = 2 (1, R = diphenylmethanolyl), n = 3 (3, R = diphenylmethanolyl; 4, R =
1-cyclohexanolyl; 5, R = 2-borneolyl), 4 (6, R = 1-cyclohexanolyl); PR′2-X-PR′2 = PCy2-(CH2)2-PCy2 (2, R = diphenylmethanolyl); PR′2-X-PR′2 = 1,2-(PPh2-O)-C6H4 (7, R = diphenylmethanolyl); PR′2-X-PR′2 = (R,R)-DIOP (8, R = diphenylmethanolyl)). In the case of
PPh2-(CH2)4-PPh2 phosphine
and −C2C­(OH)­Ph2 alkynyl ligands an octanuclear
cluster of a different structural type, [Au8(C2C­(OH)­Ph2)6(PPh2-(CH2)4-PPh2)2]2+ (9), was obtained. Complexes 1–3, 7, and 9 were studied by X-ray crystallography.
NMR and ESI-MS spectroscopic investigations showed that all but two
(2 and 9) compounds are fluxional in solution
and demonstrate dissociative chemical equilibria between major and
a few minor forms. All of these complexes are intensely emissive in
the solid state at room temperature and demonstrate very high quantum
yields from 0.61 to 1.0 with weak influence of the alkynyl substituents
R′ and the diphosphine backbones on luminescence energies.
Two crystalline forms of the cluster 2 (P21/n and P21 space groups) exhibit unexpectedly contrasting yellow and sky blue
emission, maximized at 572 and 482 nm, respectively. Electronic structure
calculations with density functional methods demonstrate that the
transitions responsible for the highly effective phosphorescence are
dominated by contributions from the Au and π-alkynyl orbitals.