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Our goal has been to make new complexes containing bidentate and tridentate carbene chelates for the catalytic hydrogenation of polar double bonds.  For hydrogenation of ketones and imines to be selective over other non polar double bonds, usually an ionic hydrogenation mechanism is required.  Ionic hydrogenation involves transfer of H^- and H⁺ from metal hydride and dihydrogen complexes, respectively.  Hence our short term goal has been to synthesize metal hydride complexes, preferably as isolatable complexes for full characterization, but potentially these complexes could be generated in situ.

Results

We began with the known ligand CH₂[Im^R]₂ where Im^R is 3-alkyl-imidazolyl.  Prior to our work, chelating carbene complexes of the form [CpFe(CO)(CH₂(Im^R)₂)]I had recently been reported with R = Me and iPr.[1]  We synthesized these complexes as well as the new t-butyl derivative, [CpFe(CO)(CH₂(Im^tBu)₂)]I, with the objective in mind of converting these complexes to hydride complexes.  However, despite several attempts with different hydride sources (Table 1), it seems that all attempts to substitute H^- for CO result in decomposition. 

Table 1

We also attempted to convert [CpFe(CO)(CH₂(Im^R)₂)]I to CpFeI(CH₂(Im^R)₂) by removal of the CO ligand through photolysis.  Our rationale was that an iodide ligand should be easier to replace with a hydride since there would be no backbonding as is the case for CO.  However, several attempts at photolysis all led to decomposition of the iron complex.  We also tried to remove the CO through chemical means since ONMe₃ is known in the literature as an O source that can convert CO ligands to CO₂ and allow for substitution reactions,[2] but addition of ONMe₃ to [CpFe(CO)(CH₂(Im^R)₂)]I failed to give any reaction.

Other efforts involved the attempted synthesis of CpFeCl(CH₂(Im^tBu)₂) by addition of the free carbene, (CH₂(Im^tBu)₂), generated in situ from nBuLi and the imidazolium precursor, to CpFeCl(dppe) (dppe = 1,2-bis(diphenylphosphino)ethane).  We postulated that the diphosphine could be replaced by the strongly donating carbene without any of the issues we encountered above.  However, this reaction showed no evidence of forming the desired product.  Another effort involved the use of the silver carbene complex, (CH₂(Im^tBu)₂)Ag•AgI₂, which we attempted to transmetallate with FeCl₂.  These efforts led to intractable mixtures of insoluble products and we have no evidence for the desired (CH₂(Im^tBu)₂)FeCl₂ complex.

At this point, we turned our efforts to ruthenium complexes since sometimes ruthenium hydrides are more stable than iron hydrides.  We modified the synthesis of CpRuH(dppe) to use a chelating bis(carbene) rather than a diphosphine; in the original synthesis Ru₃(CO)₁₂ is treated with CpH under reflux conditions to form CpRuH(CO)₂ and then dppe is added.  However in two attempts during which free carbene, (CH₂(Im^tBu)₂), generated in situ from nBuLi, was added in place of the phosphine, the only product isolated was the well known [CpRu(CO)₂]₂.

Future Directions

We are interested in using [(cymene)Ru(halide)(CH₂(Im^R)₂)]PF₆ complexes in the literature,[3] as well as their steric variants, to attempt to make [(cymene)Ru(H)(CH₂(Im^R)₂)]PF₆ by treatment of the above halide complex with hydride sources.  We are also interested in BH₂ linked chelating bis(carbene) complexes, and (cymene)Ru(halide)(BH₂(Im^R)₂) is a target which may allow for the synthesis of (cymene)Ru(H)(BH₂(Im^R)₂).   We are still interested in the other chelating bidentate and tridentate carbenes in our original proposal (for example through the synthesis of (HB(Im^R)₃)RuH or related complexes), but we believe we need to find a way around that the fact that very strong sigma donors like the chelating carbene ligands tends to make it difficult to remove a backbonding ligand like CO.  One possible solution may be to redesign the shape of our chelate to place the two carbene ligands trans to eachother.  Whittlesey and coworkers have seen some interesting hydride and dihydrogen complexes that contain two monodentate carbene ligands that are forced into a trans arrangement due to steric bulk.[4]  We would like to combine a trans arrangement of carbene ligands with the enhanced stability that chelates offer. 

Dissemination of Results

Postdoctoral fellow Mukesh Kumar gave an oral presentation on our efforts at the Middle Atlantic Seaboard Inorganic Symposium (MASIS) held on August 6, 2008 at the University of Delaware.