PolyhedronVol. IO, No. 14, pp. 1721-1722, 1991

Printed in Great Britain

0277-5387/91 $3.OO+.OJl 0 1991 Pergamon Press plc

COMMUNICATION

ON THE FORMATION OF THE [RhH(SnC13),j3- COMPLEX ANION: DIRECT “‘Sn NMR EVIDENCE FOR THE FACILE

PROTONATION OF THE [Rh(SnCl,),]*- SPECIES EXTRACTED INTO CHLOROFORM

IRIS HALL and KLAUS R. KOCH*

Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7700, Cape Town, South Africa

(Received 19 April 1991; accepted 17 May 1991)

Abstract-The [Rh(SnC13),14- complex anion, extracted into chloroform containing a quaternary ammonium type liquid anion exchanger, is readily and reversibly protonated in the chloroform phase to yield the stereochemically rigid, [RhH(SnC13)S]3- complex.

The considerable current interest in transition metal-tin chemistry may be ascribed in part to the catalytic utility of these systems, the subject having been recently reviewed. I In particular, Saito and co- workers2T3 have reported the photo-enhanced cata- lytic dehydrogenation of propan-2-01 using a rho- dium-tin chloride system. Furthermore, the yellow solutions obtained when mixing RhC13 * 3H20, SnCl, * 2H20, propan-2-01 and 3 M hydrochloric acid, have been shown by means of l19Sn NMR to contain the [RhH(SnC1,)5]3- anion, 1.4 In these studies the formation of 1 has been assumed to proceed via the catalytic dehydrogenation of propan-2-01 to yield acetone. While the photo- enhanced dehydrogenation of propan-2-01 (in the absence of the hydrochloric acid) doubtlessly involves a hydride complex, 3 the nature and mech- anism of formation of such complexes has not been clearly elucidated.

that the yellow complex anion 1 is formed by direct protonation of the red-purple (La, = 470 nm) complex anion Rh(SnC13)54- (2).

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As part of our interest in the selective extrac- tion of the complex anions of the type [Rh(SnCl,), C1,_,13- (n = l-5) by solid polyurethane foams,5 we have used l19Sn NMR to examine the distri- bution of these complexes between dilute hydro- chloric acid and the quaternary ammonium liquid anion exchanger, tricaprylylmethylam- monium chloride (AQ-336, Aldrich) in chloroform. We here report convincing ‘19Sn NMR evidence

*Author to whom correspondence should be addressed.

When a suitably aged (35”C, 4 h) 0.5 M hydro- chloric acid solution containing RhC13 - 3H20 (7 mmol) and sufficient SnC12 * 2H20 (Sn/Rh mole ratio of 10: 1) is extracted with an equal volume of 10% (v/v) AQ-336 in CDC13, the red-purple rhodium-tin complex anions are quantitatively transferred into the organic phase. l19Sn NMR spectroscopy at 25°C (Fig. 1) shows that the pre- dominant species found to be extracted is 2 ; this complex is also known to predominate in the aque- ous phase under certain conditions.6 Addition of a small quantity of CF,COOH directly to the chloro- form extract results in a rapid colour change from red to yellow, while the ’ 19Sn NMR spectrum that obtains, is clearly ascribable to the [RhH(SnC13)S]3- complex [6(l19Sn) = - 14.1 ppm(eq), +78.7

ppm(ax), 1J(‘03Rh-“9Sn) = 594 Hz(eq), 528 Hz(ax), 2J(“7Sn-“9Sn) = 1973 Hz (cis, eq-eq), 25,334 Hz (tram, eq-eq), 1759 Hz (cis, ax-eq), *J( I 19Sn-“9Sn) = 1844 Hz (cis, ax-eq), 2J( ‘H- l19Sn) = 58 (cis, eq), 1115 Hz (tram, ax)]. An ident- ical spectrum may be obtained simply by increasing the hydrochloric acid concentration of the aqueous source phase to greater than 3 M, when the extrac- tion is accompanied by a gradual colour change from red to yellow. The l19Sn NMR spectra of extracts obtained from 5 M hydrochloric acid

1722 Communication

Fig. 1. ’ "Sn NMR spectra at 74.5 MHz of: (i) a CDCl,/AQ-336 extract obtained from 0.5 M hydro- chloric acid containing RhC13 and SnCl, (Sn/Rh = 10) and (ii) spectrum after addition to 1 drop of CF,COOH. [Also shown is the high-field satellite due to tran~

‘J(’ ’ 'Sn-' “Sn) coupling.]

solution containing tin chloride in an Sn/Rh mole ratio of 4, reveal the presence of a small amount of a second hydride complex anion assigned to be trans-[RhHC1(SnC1,)d]3P [6(’ “Sn) = - 92.5 ppm, ‘J(’ 03Rh-‘1gSn) = 666 Hz, ‘J(’ 17Sn--’ “Sn) = 2398 Hz, ‘J(‘H-“‘Sn) = 105 Hz], in addition to sub- stantial amounts of 1.

The partial reversibility of the proposed pro- tonation

in the AQ-336/CDCl, phase may be conveniently ascertained by means of a photometric titration. Addition of an appropriate amount of strong acid,

e.g. CF,COOH, to a chloroform extract containing 2, results in a progressive decrease of the absorb- ance at 470 nm with a concomitant increase in the absorbance below 350 nm. This trend may be reversed by the subsequent addition of the relatively basic N,N-dimethylformamide.

It is thus clear that the formation of 1 in solutions of propan-2-01 containing dilute hydrochloric acid results from the simple protonation of the penta- coordinate species 2 in the organic phase, and need not involve the deprotonation of propan-2-01 as has been tacitly assumed.3*4 Further support for this contention is obtained from the observation that the yellow complex anion 1 predominates in 4- methylpentan-Zone (MIBK) extracts obtained from aqueous source phases under a variety of acid concentrations and Sn/Rh ratios. All attempts to detect 2 in the MIBK phase proved fruitless. This observation can now be understood in view of the well established tendency for solvents such as MIBK to co-extract substantial amounts of hydro- chloric acid,7 which results in the rapid protonation of 2 on transfer into the MIBK phase. While the catalytic photo-enhanced dehydrogenation of pro- pan-Z01 most probably involves complex rhodium- chlorotin hydrido species, it is clear that the coor- dinatively saturated anion 1 is easily formed by facile protonation of 2 in organic solvents, even in the absence of alcohols such as propan-2-01.

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