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

Abstract

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.

Carbene-anchored/pendent-imidazolium species as precursors to di-N-heterocyclic carbene-bridged mixed-metal complexes

“…The 1 H NMR spectral parameters for the series of complexes (2c and 2d) are closely comparable to those described for 2b 46 and also to those of previously reported pendent complexes involving other metals. 18,31,67,81-89 Complexes 2b-2d show typical resonances for the coordinated COD ligands (between d 1.7 and d 5.7) as given in the Experimental section.…”

“…Most often, single deprotonation of the diimidazolium salts is carried out using complexes containing basic ligands such as acetate or methoxide groups, which serve to deprotonate these salts. This is the method used with some success in this report, and in previous reports by us 46 and others. 18,31,46,81-84 ,86-88, [90][91][92][93] In a previous study we had shown that the acetate-bridged complex, [Rh(m-OAc)(COD)] 2 , was effective in the deprotonation of a number of diimidazolium salts generating the corresponding series of di-NHC-bridged dirhodium products.…”

“…The strain within complex 16b, resulting from diphosphine chelation to a square-planar centre, is evident in the geometry within the depm group as well as in the angles about Pd. Within the diphosphine the P(1)-P(2) separation (2.6907(11) A ˚) is much smaller than is usually observed in an unstrained depm moiety, where values near 3.05 A ˚are more typical, 46 and is accompanied by an acute P(1)-C(20)-P(2) angle (94.71(15) • ) which is significantly less than the idealized 109.5 • . Accompanying this strain, the P(1)-Pd-P(2) angle (72.29(3) • ) is also compressed from the idealized 90 • .…”

“…18,31,46,81-84 ,86-88, [90][91][92][93] In a previous study we had shown that the acetate-bridged complex, [Rh(m-OAc)(COD)] 2 , was effective in the deprotonation of a number of diimidazolium salts generating the corresponding series of di-NHC-bridged dirhodium products. 46 In one case the mononuclear carbeneanchored/pendent-imidazolium complex [RhBr(COD)( tBu C(H)h 1 -C meth )][Br] (2b, Scheme 1) was generated and found to be a probable intermediate in the formation of the di-NHC-bridged target. Although our initial attempts to prepare the other members of this series, namely, […”

“…63 (c) Mixed-metal, di-NHC-bridged heterobimetallic complexes. In our initial study, 46 we were able to prepare a binuclear dicarbene-bridged complex from a mononuclear, carbene-anchored/pendent-imidazolium precursor by deprotonating the pendent acidic imidazolium hydrogen of 2b using one-halfequivalent of [Rh(m-OAc)(COD)] 2 yielding the dirhodium target. Attempts to extend this strategy to generate mixed-metal complexes of rhodium using 2b-2d as precursors and using the complexes [Ir(m-OAc)(COD)] 2 , or [Pd(OAc) 2 ], containing the basic acetate ligands, all failed.…”