New bimetallic NiRh carbonyl clusters: synthesis and X-ray structure of the trigonal antiprismatic [NiRh₅(CO)₁₄]³⁻ and body-centered cubic [NiRh₁₄(CO)₂₈]⁴⁻ cluster anions

Abstract

The reaction of the [Ni₆(CO)₁₂]²⁻ dianion with [Rh(COD)Cl]₂ (COD=cyclooctadiene) in excess affords a complicate mixture of bimetallic NiRh clusters, from which the new [NiRh₅(CO)₁₄]³⁻ and [NiRh₁₄(CO)₂₈]⁴⁻ have been isolated and characterised. The [NiRh₁₄(CO)₂₈]⁴⁻ tetraanionic cluster has been also obtained from reacting [Ni₂Rh₁₂(CO)₂₅]⁴⁻ and [NiRh₁₃(CO)₂₅]⁵⁻ with [Rh(CO)₂Cl]₂, whereas [NiRh₅(CO)₁₄]³⁻ has been more conveniently obtained by reduction with alkali metals or hydroxides of [NiRh₅(CO)₁₅]⁻. All these new bimetallic NiRh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and were characterised by elemental analysis, spectroscopy and X ray diffraction studies. The structure of the diamagnetic [NiRh₅(CO)₁₄]³⁻ trianion is identical to that of the homometallic [Co₆(CO)₁₄]⁴⁻ tetraanion, the unique Ni atom being disordered over the 6 equiv. trigonal antiprismatic sites. The [NiRh₅(CO)₁₄]³⁻ trianion is slowly and irreversibly degraded to a mixture of Ni(CO)₄, [Rh(CO)₄]⁻ and [Rh₇(CO)₁₆]³⁻ upon exposure to an atmosphere of carbon monoxide. The [NiRh₁₄(CO)₂₈]⁴⁻ tetranion displays a metal frame consisting in a hexacapped cube of rhodium atoms centered by the unique nickel atom. This metal frame has previously been found in the homometallic [Rh₁₅(CO)₃₀]³⁻ cluster. However, at difference from the latter, [NiRh₁₄(CO)₂₈]⁴⁻ features two less valence electrons and displays a shrinked cubic moiety. [NiRh₁₄(CO)₂₈]⁴⁻ is slowly degraded by carbon monoxide and halide ions to give [NiRh₁₃(CO)₂₅]⁵⁻ and other yet uncharacterised NiRh carbonyl clusters. All above NiRh clusters do not display protonation behaviour.

Introduction

Several bimetallic NiRh carbonyl clusters have been isolated and structurally characterised, e.g. [NiRh₆(CO)₁₆]²⁻, [1] [Ni₆Rh₅(CO)₂₁]³⁻, [2], [3] [Ni₁₀Rh(CO)₁₉]³⁻, [3] [Ni₉Rh₃(CO)₂₂]³⁻ [3] [Ni₆Rh₃(CO)₁₇]³⁻, [3] [NiRh₁₃(CO)₂₅]⁵⁻, [4] [Ni₂Rh₁₂(CO)₂₅]⁴⁻, [4] and [Ni₅Rh₉(CO)₂₅]ⁿ⁻ (n=2, 3) [4]. The tetradecanuclear [NiRh₁₃(CO)₂₅]⁵⁻ [Ni₂Rh₁₂(CO)₂₅]⁴⁻ and [Ni₅Rh₉(CO)₂₅]ⁿ⁻ (n=2, 3), albeit isostructural, are not isoelectronic with the homometallic [Rh₁₄(CO)₂₅]⁴⁻, [5] and feature 182–184 cluster valence electrons, rather than 180. Likely, this is a consequence of nickel, rather than rhodium centring of the pentacapped cubic metal frame, as well as progressive substitution of rhodium with nickel side-caps. Furthermore, nickel centring triggers redox aptitude in [NiRh₁₃(CO)₂₅]⁵⁻ and [Ni₂Rh₁₂(CO)₂₅]⁴⁻, and multivalence in [Ni₅Rh₉(CO)₂₅]ⁿ⁻ (n=2, 3) [4].

It was, therefore, of interest to try to synthesise a pentadecanuclear bimetallic NiRh cluster to verify whether a mixed hcp–bcc or a regular bcc geometry would result, to be respectively, compared with the known homometallic [Rh₁₅(CO)₂₇]³⁻ [6] and [Rh₁₅(CO)₃₀]³⁻ [7] species. We report here the synthesis of [NiRh₁₄(CO)₂₈]⁴⁻ and [NiRh₅(CO)₁₄]³⁻. These two new bimetallic NiRh clusters have been initially isolated from the usual reaction of synthesis of most NiRh bimetallic clusters [2], [3], [4]. Their structural characterisation by single crystal X ray diffraction studies readily prompted the set up of tailored syntheses.