The M 2 Rh 2 bis(μ 3 -CO 2 ) complexes [Cp*(CO)(NO)Re(CO 2 )Rh(η 4 -cod)] 2 ( 1 ) and [Cp*(CO) 2 MCO 2 Rh(η 4 -cod)] 2, M = Fe ( 2 ) and Ru ( 3 ), were synthesized in moderate to high yields from treating [Rh(OH)(η 4 -cod)] 2 (cod = 1,5-cyclooctadiene) with the carbonyl salts Cp*(L)(CO) 2 M + (L = CO, M = Fe, Ru; L = NO, M = Re) and a base. [An alternative synthesis of 1 and its crystallographic structure determination has been reported.] Although details on using several bases in the synthesis of 2 are reported, the use of volatile EtMe 2 N in excess as the base is especially useful. IR and NMR spectral data are in accord with a M 2 Rh 2 bis(μ 3 -CO 2 ) core for 2 and 3: they retain two μ 3 -[η 1 -C(M):η 1 -O(Rh):η 1 -O‘(Rh‘)] carboxylate ligands that resemble those that were found for 1 and the Rh(I) carboxylates [(RCO 2 )Rh(diene)] 2 . Complexes 2 and 3 with their “open-book” structures are not fluxional at room temperature. Complex 2, its norbornadiene analogue, [Cp*(CO) 2 FeCO 2 Rh(η 4 -nbd)] 2 ( 4 ), and their 13 C-labeled derivatives also are accessible from reactions of Cp*(CO) 2 FeCO 2 K [or Cp*(CO) 2 Fe 13 CO 2 K] with [Rh(OSO 2 CF 3 )(η 4 -cod)] x . [Cp*(CO) 2 Fe( 13 CO 2 )Rh(η 4 -diene)] 2 [diene = cod ( 2 - 13 C ); nbd ( 4 - 13 C )] underwent carboxylate−carbonyl label shuttle to yield 1:1 mixtures of [Cp*(CO)( 13 CO)FeCO 2 Rh(diene)] 2 ( 2a - C 13 ) and 4a - C 13 . IR spectral assignments for the metallocarboxylate ν OCO absorptions are also presented.
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Tetrick et al. (1998) studied this question.