The complexes [RhBr(COD)]2(μ-di-NHC), in which di-NHC represents the di-N-heterocyclic carbenes MeCCmeth (1,1′-methylene-3,3′-dimethyldiimidazol-2,2′-diylidene),
tBuCCmeth (1,1′-methylene-3,3′-di-tert-butyldiimidazol-2,2′-diylidene), and MeCCeth (1,2-ethylene-3,3′-dimethyldiimidazol-2,2′-diylidene), have been prepared from the reactions of the corresponding diimidazolium bromide salts with the acetate-bridged complex [Rh(μ-OAc)(COD)]2. The analogous complex containing the
tBuCCeth group (1,2-ethylene-3,3′-di-tert-butyldiimidazol-2,2′-diylidene) could not be prepared by this route, but the related chloro complex [RhCl(COD)]2(μ-
tBuCCeth) was prepared by prior double deprotonation of the corresponding diimidazolium salt and reaction of the resulting dicarbene with [RhCl(COD)]2. Reaction of the above MeCCeth and
tBuCCeth complexes with CO yields the corresponding dicarbene-bridged carbonyl products [RhX(CO)2]2(μ-di-NHC) (X = Cl, Br), while reaction of the MeCCmeth and
tBuCCmeth complexes with CO instead yielded the products [Rh(CO)2(η1:η1-di-NHC)]+, containing chelating dicarbenes. The X-ray structure determinations of several of the above dicarbene-bridged species have been carried out and show metal−metal separations of greater than 6.6 Å in all cases. Reaction of all of the above COD complexes with dppm (Ph2PCH2PPh2) followed by CO yields the dicarbene- and dppm-bridged A-frame species [Rh2(CO)2(μ-X)(μ-di-NHC)(dppm)]+ (X = Cl, Br), in which the larger bite of the dicarbenes, compared to that of dppm, induces strain within the complexes. Introduction of the bridging dppm and bromide groups lowers the Rh−Rh separation to about 3.3 Å. Replacement of the bridging bromide by hydroxide in the MeCCeth complex yields [Rh2(CO)2(μ-OH)(μ-MeCCeth)(dppm)]+, in which the bridging hydroxide can be used to deprotonate a monoimidazolium iodide to give [Rh2Br(CO)2(IMe)(μ-MeCCeth)(dppm)][I] (IMe = 1,3-dimethylimidazol-2-ylidene). In this species the bromide is on one metal while the monocarbene occupies the other, opening up the dicarbene bite to 5.18 Å and the metal−metal separation to 4.07 Å. Attempts to better match the larger bite of the dicarbenes by substituting dppm with dppe (Ph2PCH2CH2PPh2) did not give a dppe-bridged product and, instead, yielded [(Rh(CO)(dppe))2(μ-MeCCeth)][CF3SO3]2 in which the dppe groups are chelating.
