New Constrained-Geometry C2-Symmetric Di-N-heterocyclic Carbene Ligands and Their Mono- and Dinuclear Rhodium(I) Complexes: Design, Synthesis, and Structural Analysis

A new C-2-symmetric di-N-heterocyclic carbene (di-NHC) ligand is synthesized, and its coordination behavior with Rh(I) salts is examined. A comprehensive list is presented describing the synthesis of all chiral di-NHC ligands reported to date, including pertinent catalytic and structural features. The historical perspective indicates few structural archetypes have been investigated and more structural alternatives are needed. Accordingly, the synthesis of the diimidazolium salts [DEA-MI](I)(2) (4) and [DEA-MBI](I)(2) (8) (where DEA = 9,10-dihydro-11,12-ethanoanthracene; MI = methylimidazolium, and MBI = methylbenzimidazolium) are presented. When 8 is treated with 2 equiv of KN(SiMe3)(2), the strained enetetramine [DEA-MBY] (9) is obtained and is characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The highly strained mononuclear complex [(DEA-MBY)Rh(COD)]I (10-COD) is obtained when 8 is treated with KN(SiMe3)(2) followed by half an equivalent of [Rh(COD)CI](2). A H-1 NMR spectrum of 10-COD revealed an unusual downfield signal at 9.26 ppm, assigned to an aliphatic bridge proton, and provides a clue as to the relative orientation of the anthracene ligand to the metal center. Ultimately, a single-crystal X-ray diffraction experiment assisted in determining the absolute solid-state structure of 10-COD. Although an X-ray crystal structure was not obtained for the imidazole derivative [DEA-MY][Rh(COD)]I (11-COD), a similar structure is implied by the occurrence of a downfield signal for the bridge proton at 8.24 ppm. In addition, the dinuclear complexes [mu-DEA-MY][Rh(NBD)I](2) (12-NBD) and [mu-DEA-MBY][Rh(COD)CI](2) (13-COD) are obtained. A structural comparison of the ligand precursors 2 and 7 center dot (HCl)(2) and the dinuclear species 12-NBD and 13-COD is presented.