Preparation, Hydrogen Bonds, and Catalytic Activity in Metal-Promoted Addition of Arylboronic Acids to Enones of a Rhodium Complex Containing an NHC Ligand with an Alcohol Function

Abstract

The complex RhCl(COD){3-benzyl-1-(2-hydroxy-2-phenylethyl)imidazol-2-ylidene} has been prepared by reaction of the dimer [Rh(μ-OMe)(COD)] 2 with 3-benzyl-1-(2-hydroxy-2-phenylethyl)-imidazolium chloride and characterized by X-ray diffraction analysis. The structure reveals that 75% of the molecules are associated through intermolecular O−H···Cl hydrogen bonds between the OH group of the NHC substituent of one molecule and the chloride ligand of the adjacent molecule. This complex catalyzes the addition of arylboronic acids to cyclic and acyclic enones in anhydrous toluene. The alcohol function of the substituent of the NHC ligand plays the role assigned to water in previous cases. ■ INTRODUCTIONThe chemistry of N-heterocyclic carbenes (NHCs) is a field of great current interest due to the transversal applications of the NHC complexes, including homogeneous catalysis, 1 antimicrobial and cytotoxic agents, 2 photoactive sites in luminescent materials for self-assembly into liquid crystalline materials and metallosupramolecular structures, and synthons for molecular switches and conducting polymeric materials. 3 Although several methods have been developed for the preparation of complexes containing these ligands, deprotonation of imidazolium salts is the most used synthetic pathway. 4 The vast majority of reported NHC ligands are substituted at nitrogens by alkyl or aryl groups. Imidazolium salts with carbonyl, 5 pyridyl, 6 pyrazolyl, 7 amine, 8 and phosphine 9 substituents are also known. They can be deprotonated and are thus suitable for the preparation of metal complexes.The catalytic transformations usually take place at the metal center. However, there is an increasing number of reactions involving both the metal center and an acidic hydrogen atom of the ligands, 10 which can further form hydrogen bonds. It has been found that processes involving passive diffusion depend primarily on the hydrogen-bonding capacity or polar surface area of a drug solute. 11 Intermolecular hydrogen bonds also play an important role in the design and engineering of architectures of many polymeric materials. Hydrogen bonding can affect the chain length, chain packing, rigidity, and molecular order. 12 In addition, the induction and stabilization of liquid crystallinity by hydrogen bonding has been shown. 13 As a consequence of these observations, the preparation of transition-metal complexes with N-heterocyclic carbenes bearing acidic substituents, such as alcohols, at the N center constitutes a special challenge.Alcohol-functionalized imidazolium salts are readily accessible by the nucleophilic opening of epoxides. 14 However, alcohol-functionalized imidazolylidene transition-metal complexes are scarce. 15 In order to justify this, it has been argued that the alcohol function is more acidic than the heterocycle. Thus, the single deprotonation leads to zwitterionic alcoholate imidazolium derivatives. 16 The hexahydride complex OsH 6 (P i Pr 3 ) 2 is basic enough to produce the deprotonation of imidazolium salts, generating famil...

Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC‐OH Ligand

“…The Rh I centre adopts a pseudo‐square‐planar geometry typical of four‐coordinate 16‐valence electron complexes. The NHC ring is oriented nearly perpendicular to the pseudo‐ D 4 h plane (dihedral angle 70.2(2)°), and the Rh−C NHC bond distance (2.042(4) Å) is in the range of reported values for related Rh−NHC complexes . The Rh−C cod bond distances in the trans position with respect to the NHC ligand (2.209(4) and 2.185(4) Å) are slightly elongated compared with those trans to the chloride (2.105(4) and 2.088(4) Å), which is in agreement with the strong σ donation of the NHC, also reflected by the increased olefinic C3=C4 (1.401(7) Å) bond distance compared with C7=C8 (1.379(6) Å).…”

“…The Rh−C cod bond distances in the trans position with respect to the NHC ligand (2.209(4) and 2.185(4) Å) are slightly elongated compared with those trans to the chloride (2.105(4) and 2.088(4) Å), which is in agreement with the strong σ donation of the NHC, also reflected by the increased olefinic C3=C4 (1.401(7) Å) bond distance compared with C7=C8 (1.379(6) Å). In the solid state, a clear intermolecular O−H⋅⋅⋅Cl hydrogen‐bonding interaction is observed between the free hydroxy pendant group of the bifunctional NHC‐OH ligand and the chloride ligand of an adjacent molecule, as reflected by the short H⋅⋅⋅Cl separation (2.35(5) Å) and the O−H⋅⋅⋅Cl angle (167(5)°), which is almost linear …”

“…One identified strategy for directinga nd stabilising early/late heterobimetallic assemblies is to take profit of bifunctional bridging ligands featuring two distinct coordination motifs: one hard, able to form strongb onds with oxophilic early TMs, and one soft, featuring as trong affinity for late TMs. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In most examples reported to date in the literature, the latemetald onor is ap hosphine-derivedm oiety.Encouraged by the flagshipr ole played by N-heterocyclic carbene (NHC) organometallicc omplexes in homogeneous catalysis, especially with late TMs, [21][22][23][24][25][26] we have identified alkoxy-functionalised NHC ligands [27][28][29][30] as promising alternatives.Q uite surprisingly,t ot he best of our knowledge,t hese ligandsh ave not been specifically used to stabilisee arly/late heterobimetallic assemblies so far. In as ingle precedent, Arnold and co-workersr eported that UL 4 (L = OCMe 2 CH 2 [1-C(NCHCHNiPr)]) reacts with molybdenuma nd tungsten carbonyls to yield new entities formulated as [UL 4 Mo(CO) 4 ]a nd [UL 4 W(CO) 5 ], respectively.Y et it is unclear how the metals are arranged in these compounds.…”

Mechanistic Studies on the Alkylation of Amines with Alcohols Catalyzed by a Bifunctional Iridium Complex

“…This is the first complex with an aliphatic alcohol functionalized bidentate ligand that has been successfully used in hydrogen-transfer reactions; the majority of complexes used in such reactions have relied on amine/amido 13, 14 and phenol/carbonyl moieties as the donor groups. 15−17 Importantly, to the best of our knowledge, the metal−ligand bifunctionality of such systems containing a tertiary alcohol 18 group has not yet been demonstrated.…”

Rhodium‐NHC Hybrid Silica Materials as Recyclable Catalysts for [2+2+2] Cycloaddition Reactions of Alkynes

“…Recently, Esteruelas and Yus25 have described an alternative method using the [Rh(μ‐OMe)(COD)] 2 complex in the presence of an imidazolium salt to form the corresponding Rh‐NHC species directly. The methoxide anion deprotonates the imidazolium ring releasing methanol from the metal center.…”