Synthesis, structure, luminescence, and theoretical studies of tetranuclear gold clusters with phosphinocarborane ligands

Treatment of the tetranuclear gold cluster Au4((PPh2)2C2B9H10)2(AsPh3)2] (1), which contains the nido-carborane-diphosphine 7,8-(PPh2)2C2B9H10]-, with various tertiary phosphines leads to derivatives Au4((PPh2)2C2B9H10)2-(PR3)2] (PR3 = PPh3 (2), P(4-MeC6H4)3 (3), P(4-OMeC6H4)3 (4)). The X-ray crystal structure of complex 4 shows a tetrahedral framework of gold atoms, two of which are chelated by the diphosphine, and two are coordinated to one monophosphine ligand each. These compounds are very stable and are obtained in high yield. MP2 calculations suggest that the two types of chemically nonequivalent gold atoms can be formally assigned as Au(I) (those attached to the arsines or phosphines) and Au(0) (those bonded to the anionic diphosphine) and emphasize the role of correlation in the gold-gold interactions. The compounds are luminescent. The emission is assigned to a gold-centered spin-forbidden transition; the assignment of the oxidation state of the gold centers on this basis leads to results coincident with those obtained by theoretical calculations.