Reversible C−H Bond Activation of a Bifunctional Phosphine Bridging Ligand across Two Unbonded Metal Centers

Abstract

Reaction of [{Ir(µ-Cl)(cod)} 2 ] with the short-bite bifunctional N,P-donor ligand (1-benzyl-2-imidazolyl)diphenylphosphine (Ph 2 PBzIm) gives the yellow complex [IrCl(Ph 2 PBzIm)(cod)] (2). A further addition of [{Ir(µ-Cl)(cod)} 2 ] to 2 results in the reversible metalation of a phenyl ring across two unbonded iridium centers to give orange crystals of [IrCl(cod){µ-PPh(C 6 H 4 )-BzIm}IrHCl(cod)] (1). Complex 1 is in equilibrium with the mononuclear complex [IrCl(Ph 2 -PBzIm)(cod)] and the active species undergoing the sp 2 -C-H activation, [{IrCl(cod)} 2 (µ-Ph 2 PBzIm)], in solution. Abstraction of one chloride ligand from 1 with AgBF 4 produces the deinsertion of the C-H bond yielding the cationic complex [{Ir(cod)} 2 (µ-Ph 2 PBzIm)(µ-Cl)]-BF 4 , which regenerates 1 upon addition of a chloride-soluble salt. The cationic complex [{Ir-(cod)} 2 (µ-Ph 2 PBzIm)(µ-Cl)]BF 4 is inactive for the above-mentioned sp 2 -C-H bond activation and can be prepared alternatively from the reaction of 2 with [Ir(cod)(CH 3 CN) 2 ]BF 4 . A related binuclear sp 2 -C-H bond activation across two unbonded metals also occurs in the reaction of dppm with [{Ir(µ-Cl)(cod)} 2 ] in a 1:1 molar ratio. This reaction leads to a mixture in equilibrium of [{IrCl(cod)} 2 (µ-dppm)] and the hydride complex [IrCl(cod){µ-PPh(C 6 H 4 )CH 2 -PPh 2 )}IrHCl(cod)] in a 1.5:1 molar ratio, respectively, in dichloromethane at 20 °C. The structure of the mixed-valence complex 1 was solved by X-ray diffraction studies. Scheme 1 Scheme 2

Heteroatoms moving protons: Synthetic and mechanistic studies of bifunctional organometallic catalysis

“…Despite the attractiveness and novelty of the use of pyridylphosphines noted above, until our work on imidazolyl-and pyridylphosphines there appeared to be no other reports of catalysis using these species. Instead, hundreds of pyridylphosphine complexes [11,12] and fewer imidazoylphosphine complexes (examples, [13][14][15][16][17][18][19][20]) had been made for a variety of purposes other than catalysis. Here, hydrogen bonding or proton transfer in catalysis using heterocyclic phosphines and their complexes will be the focus.…”

Structures, Mechanisms, and Results in Bifunctional Catalysis and Related Species Involving Proton Transfer

“…Here we describe some fundamental studies of bifunctional catalysts and related complexes, with a focus on phosphines bearing an imidazolyl or pyridyl group and the roles the basic nitrogen substituent can play. Though many pyridyl [13,14] and fewer imidazoylphosphine complexes are known (examples [15][16][17][18][19][20][21][22]), as far as we are aware, relatively few reports of enhanced catalysis exist. In pioneering work at Shell in the 1990s [23,24], a pyrid-2-yl or 6-methyl-2-pyrid-yl phosphine led to very significant enhancements in rate and selectivity for Pd-catalyzed methoxycarbonylation of propyne.…”

“…Despite the benefits of the novel use of pyridylphosphines noted above, until our work on imidazolyl-and pyridylphosphines there appeared to be no other reports of catalysis using these species. Instead, hundreds of pyridylphosphine complexes [13,14] and fewer imidazoylphosphine complexes (examples, [15][16][17][18][19][20][21][22]) had been made for a variety of purposes other than catalysis. Here, hydrogen bonding or proton transfer in catalysis using heterocyclic phosphines and their complexes will be the focus.…”

Coordination Chemistry and Diphenylacetylene Hydrogenation Catalysis of Planar Chiral Ferrocenylphosphane‐Thioether Ligands with Cyclooctadieneiridium(I)

“…Precedents for this type of structural arrangement exist for P,N ligands. [66,67] As the equilibrium between the five-coordinate and the four-coordinate isomers of 2-R lies strongly in favor of the latter when R = tBu, the pendant StBu arm is available to coordinate the Ir center in the unsaturated [Ir-(COD)Cl] complex, which is in equilibrium with the dichloro-bridged dimer. It is rather remarkable, however, that this process is slower than the addition of the chloride anion to the same position.…”