Ammonia-stabilized superoxo-rhodium species

Polyhedron Vol. I I, No. 21, pp. 27574758, Printed in Great Britain

1992 0

AMMONIA-STABILIZED

SUPEROXO-RHODIUM

IAN J. ELLISON

0277-5387/92 $5.00 + .oO 1992 Pergamon Press Ltd

SPECIES’

and R. D. GILLARD*

School of Chemistry and Applied Chemistry, University of Wales, Cardiff, P.O. Box 912, Cardiff CFl 3TB, U.K.

J. P. MAHER

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 lTS, U.K. (Received 30 June 1992 ; accepted 13 July 1992)

Abstract-The ESR spectrum of S. M. Jorgensen’s product, from the oxidation by hypochlorite on chloropentamminerhodium(II1) chloride, shows it to be a superoxide.

The superoxodirhodium(II1, III) moiety, [Rh-( pstabilized within a framework of 02)-Rh15+, nitrogenous ligands, is well known. 2The first example characterized was in [{Rh(py),Cl} *(P-O,)] (ClO&. Other N-heterocyclic ligands, such as 2,2’bipyridyl, have been used,3 and complexes that contain coordinated oxygen-based ligands4 have been studied. To date, most well-defined superoxide

complexes of rhodium involve pyridine as the principal ligand. The corresponding ions involving ammonia (or the closely similar 1,Zdiaminoethane) ligands were described later’ as new compounds, and structural studies were made on them. These were chiefly tetraamines. The preparations gave rather low yields, by exhaustive substitution of hydrogen peroxide into

J :

: I 2.175

I

I 2.107

I

I

I

2.043

I 1.983

I

I

I

1.927

Fig. 1. ESR spectrum of the frozen solution (77 K) resulting from the action of sodium hypochlorite solution on [Rh(NH3)sCl]C12 with heat. * Author to whom correspondence should be addressed. 2757

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I. J. ELLISON et al.

pre-formed ammines of rhodium(II1) ; the syntheses of some of these precursors5v6 are complicated and time-consuming. We point out that such ammonia-stabilized superoxodirhodium centres had been formed in other ways using relatively simple procedures. First, Goldfarb and Kevan’ incorporated chloropentamminerhodium(II1) chloride, from its aqueous solution into a zeolite, and heated it in the presence of dioxygen, giving a pammagnetic centre: this should certainly be reformulated as involving a superoxo-ammine of rhodium(II1). Further, in 1883 S. M. Jorgensen studied’ the properties of [Rh(NH,),C1]C12, which can be formed readily from rhodium trichloride. 9He heated the solid with hypochlorite solution and obtained an olive-green solid and a green solution, which is spectroscopically similar to species observed in the preparation of Claus’ blue. ‘O We have shown” that oxidations in aqueous solution using hypochlorite on rhodium(II1) species often lead to superoxo complexes of rhodium(II1). Jorgensen’s observation is no exception. The ESR spectrum of the frozen solution of his material is shown in Fig. 1. The g values (gz = 2.08, gY= 2.04, gX= 2.02) demonstrate” very clearly the presence of the superoxo ligand on rhodium. We are developing syntheses of superoxo-rhodium species based on this very convenient route. It is increasingly clear that the action of any oxidant on aqueous solutions containing rhodium(II1) with nitrogens (and indeed oxygenated) ligands is likely to lead to superoxide complexes rather than to higher oxidation states of rhodium itself. The many formulations of substances precipitated from aqueous solutions of rhodium(II1) on treatment with oxidants, containing rhodium(IV) or even higher states in general, refer actually to super-

oxides. It is possible” to make rhodium(IV) in oxide matrices, but only under extreme conditions. thank SERC and the Universities of Bristol and Bath for the purchase of the Bruker 300E ESR spectrometer on which the spectra were recorded. Acknowledgements-We

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