Reactions of Cyclometalated Oxazoline Half-Sandwich Complexes of Iridium and Ruthenium with Alkynes and CO

The ligand 4,4-dimethyl-2-oxazolinylbenzene is easily cyclometalated by [IrCl2Cp*]2 or [RuCl(MeCN)2(p-cymene)]PF6 in the presence of sodium acetate. In the case of iridium the resultant complex dissolves in acetonitrile in the presence of KPF6 to give an acetonitrile-coordinated cationic complex. The analogous complex is formed directly in the ruthenium cyclometalation reaction. These labile cationic complexes undergo insertion reactions with internal and terminal alkynes. Internal alkynes give only monoinsertion products, whereas terminal alkynes give mono- or di-insertion products. The cations will also react with CO, but no insertion occurs in this case

Mechanistic Study of Acetate-Assisted C−H Activation of 2-Substituted Pyridines with [MCl₂Cp*]₂ (M = Rh, Ir) and [RuCl₂(<i>p</i>-cymene)]₂

Reactions of 2-substituted pyridines HL with [MCl2Cp*]2 (M = Ir, Rh) and [RuCl2(p-cymene)]2 have been carried out in the presence and absence of sodium acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(L1)(ring)] 1a−c (M = Rh, Ir, ring = Cp*; M = Ru, ring = p-cymene). However, in the case of 2-acetylpyridine (HL2) sp3 CH activation occurs cleanly with rhodium to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the reactions with iridium and ruthenium give unseparable mixtures of products. The N,C cyclometalated products [MCl(L2)(ring)] 2a−c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium enolates of 2-acetylpyridine. Notably, in the absence of acetate, [RhCl2Cp*]2 shows no reaction with 2-acetylpyridine, whereas [IrCl2Cp*]2 and [RuCl2(p-cymene)]2 react to form equilibrium mixtures of the starting materials and N,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O chelate complexes [MCl(HL2)(ring)][PF6] 4a,c,d (M = Ir, ring = Cp*; M = Ru, ring = p-cymene, mesitylene) can be isolated. These are not intermediates en route to the N,C cyclometalated products. These results suggest that for CH activation to occur under these mild conditions acetate must coordinate to the metal prior to coordination of the ligand

Mechanistic Study of Acetate-Assisted C−H Activation of 2-Substituted Pyridines with [MCl₂Cp*]₂ (M = Rh, Ir) and [RuCl₂(<i>p</i>-cymene)]₂

Reactions of 2-substituted pyridines HL with [MCl2Cp*]2 (M = Ir, Rh) and [RuCl2(p-cymene)]2 have been carried out in the presence and absence of sodium acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(L1)(ring)] 1a−c (M = Rh, Ir, ring = Cp*; M = Ru, ring = p-cymene). However, in the case of 2-acetylpyridine (HL2) sp3 CH activation occurs cleanly with rhodium to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the reactions with iridium and ruthenium give unseparable mixtures of products. The N,C cyclometalated products [MCl(L2)(ring)] 2a−c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium enolates of 2-acetylpyridine. Notably, in the absence of acetate, [RhCl2Cp*]2 shows no reaction with 2-acetylpyridine, whereas [IrCl2Cp*]2 and [RuCl2(p-cymene)]2 react to form equilibrium mixtures of the starting materials and N,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O chelate complexes [MCl(HL2)(ring)][PF6] 4a,c,d (M = Ir, ring = Cp*; M = Ru, ring = p-cymene, mesitylene) can be isolated. These are not intermediates en route to the N,C cyclometalated products. These results suggest that for CH activation to occur under these mild conditions acetate must coordinate to the metal prior to coordination of the ligand

Electrophilic C−H Activation at {Cp*Ir}: Ancillary-Ligand Control of the Mechanism of C−H Activation

Density functional calculations on the low-temperature cyclometalation of dimethylbenzylamine with [IrCl2Cp*]2/NaOAc have characterized a novel electrophilic activation pathway for C−H bond activation. C−H activation occurs from [Ir(DMBA-H)(κ2-OAc)Cp*]+, and OAc plays a central role in determining the barrier for reaction. Dissociation of the proximal OAc arm sets up a facile intramolecular deprotonation via a geometrically convenient six-membered transition state. Dissociation of the distal OAc arm, however, leads to a higher energy four-membered (σ-bond metathesis) transition state, while oxidative addition is even higher in energy. For this Ir3+ system, these three mechanisms appear to lie within a continuum in which the participation of the metal center and an H-accepting ancillary ligand are inversely related. The ability of the ancillary ligand to act as a proton acceptor is the key factor in determining which mechanism pertains

N−H versus C−H Activation of a Pyrrole Imine at {Cp*Ir}: A Computational and Experimental Study

Reaction of a pyrrole imine with [IrCl2Cp*]2/NaOAc leads to N−H activation in preference to C−H activation at the pyrrole; however, with the N-methylated ligand C−H activation occurs. Density functional calculations show that N−H bond activation is both kinetically and thermodynamically preferred to C−H activation. Both reactions occur with relatively low energy barriers by an electrophilic agostic interaction with the metal with simultaneous intramolecular hydrogen bonding with acetate leading to deprotonation via a six-membered transition state

N−H versus C−H Activation of a Pyrrole Imine at {Cp*Ir}: A Computational and Experimental Study

Reaction of a pyrrole imine with [IrCl2Cp*]2/NaOAc leads to N−H activation in preference to C−H activation at the pyrrole; however, with the N-methylated ligand C−H activation occurs. Density functional calculations show that N−H bond activation is both kinetically and thermodynamically preferred to C−H activation. Both reactions occur with relatively low energy barriers by an electrophilic agostic interaction with the metal with simultaneous intramolecular hydrogen bonding with acetate leading to deprotonation via a six-membered transition state