New NiRh carbonyl clusters with unprecedented structural and electronic features: synthesis and X-ray structure of [NEt 4] 3[Ni 10Rh(CO) 19]·C 2H 5CN, [NMe 3CH 2Ph] 3[Ni 6Rh 3(CO) 17] and [NEt 4] 3[Ni 9Rh 3(CO) 22

Abstract

The reaction of the [Ni 6(CO) 12] 2− dianion with [Rh(COD)Cl] 2 (COD=cyclooctadiene) affords in sequence the new [Ni 10Rh(CO) 19] 3− ( 1) and [Ni 9Rh 3(CO) 22] 3− ( 2) trianionic clusters with good selectivity. The related [Ni 6Rh 3(CO) 17] 3− ( 3) trianionic cluster has been initially isolated among the degradation products of [Ni 9Rh 3(CO) 22] 3− ( 2) under carbon monoxide and has been more conveniently synthesized by condensation of [Rh(CO) 4] − with Ni(CO) 4 in excess. All these new bimetallic NiRh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and were characterized by elemental analysis, spectroscopy and X-ray diffraction studies. The structure of [Ni 10Rh(CO) 19] 3− ( 1) consists of a distorted and incomplete Rh-centered Ni 9(μ 9-Rh) icosahedral moiety, capped by the tenth Ni atom on one of the interlayer triangular faces. The [Ni 10Rh(CO) 19] 3− ( 1) trianion is moderately stable to oxidation and, as inferable from IR monitoring of the resulting reaction mixture, is irreversibly degraded upon exposure to an atmosphere of carbon monoxide to a mixture of Ni(CO) 4, [Rh(CO) 4] − and [Ni 5(CO) 12] 2−. The [Ni 9Rh 3(CO) 22] 3− ( 2) trianion displays a three layer metal frame identical to that previously found in the [H 4− x Ni 9Pt 3(CO) 21] x− ( x=2–4) clusters. However, it features two less valence electrons. The structure of [Ni 6Rh 3(CO) 17] 3− ( 3) is closely related to that of [Ni 9Rh 3(CO) 22] 3− ( 2) and may be formally derived from that of the latter by substitution of a Ni 3(CO) 3(μ-CO) 3 moiety with a face bridging carbonyl group. Notably, [Ni 6Rh 3(CO) 17] 3− ( 3) is isoelectronic but not isostructural with the known [Ni 6Ir 3(CO) 17] 3− congener. Both [Ni 6Rh 3(CO) 17] 3− ( 3) and [Ni 9Rh 3(CO) 22] 3− ( 2) are readily degraded by carbon monoxide at atmospheric pressure to colourless solutions containing Ni(CO) 4 and [Rh(CO) 4] − mixtures. Their reaction with [Rh(COD)Cl] 2 leads to the previously reported [Ni 6Rh 5(CO) 21] 3− trianion and other yet uncharacterised NiRh carbonyl clusters. All above NiRh clusters do not display protonation–deprotonation behaviour and chemically reversible redox propensity; electrochemical experiments failed in all cases to show the occurrence of electrochemically reversible redox changes.