A-Frame Complexes of Dirhodium Bridged by Dicarbene and Diphosphine Ligands

The complexes [RhBr(COD)] 2 (μ-di-NHC), in which di-NHC represents the di-N-heterocyclic carbenes Me CC meth (1,1′-methylene-3,3′-dimethyldiimidazol-2,2′-diylidene), t Bu CC meth (1,1′-methylene-3,3′-di- tert -butyldiimidazol-2,2′-diylidene), and Me CC eth (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 t Bu CC eth 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- t Bu CC eth ) 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 Me CC eth and t Bu CC eth complexes with CO yields the corresponding dicarbene-bridged carbonyl products [RhX(CO) 2 ] 2 (μ-di-NHC) (X = Cl, Br), while reaction of the Me CC meth and t Bu CC meth complexes with CO instead yielded the products [Rh(CO) 2 (η 1:η 1 -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 (Ph 2 PCH 2 PPh 2 ) followed by CO yields the dicarbene- and dppm-bridged A-frame species [Rh 2 (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 Me CC eth complex yields [Rh 2 (CO) 2 (μ-OH)(μ- Me CC eth )(dppm)] +, in which the bridging hydroxide can be used to deprotonate a monoimidazolium iodide to give [Rh 2 Br(CO) 2 (IMe)(μ- Me CC eth )(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 (Ph 2 PCH 2 CH 2 PPh 2 ) did not give a dppe-bridged product and, instead, yielded [(Rh(CO)(dppe)) 2 (μ- Me CC eth )][CF 3 SO 3 ] 2 in which the dppe groups are chelating.

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