O2 activation by metal-ligand cooperation with Ir(I) PNP pincer complexes.

A unique mode of molecular oxygen activation, involving metal-ligand cooperation, is described. Ir pincer complexes [((t)BuPNP)Ir(R)] (R = C6H5 (1), CH2COCH3 (2)) react with O2 to form the dearomatized hydroxo complexes [((t)BuPNP*)Ir(R)(OH)] ((t)BuPNP* = deprotonated (t)BuPNP ligand), in a process which utilizes both O-atoms. Experimental evidence, including NMR, EPR, and mass analyses, indicates a binuclear mechanism involving an O-atom transfer by a peroxo intermediate.

Cationic, neutral, and anionic PNP Pd(II) and Pt(II) complexes: dearomatization by deprotonation and double-deprotonation of pincer systems.

A series of cationic, neutral, and anionic Pd(II) and Pt(II) PNP (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) complexes were synthesized. The neutral, dearomatized complexes [(PNP*)MX] (PNP* = deprotonated PNP; M = Pd, Pt; X = Cl, Me) were prepared by deprotonation of the PNP methylene group of the corresponding cationic complexes [(PNP)MX][Cl] with 1 equiv of base (KN(SiMe(3))(2) or (t)BuOK), while the anionic complexes [(PNP**)MX](-)Y(+) (PNP** = double-deprotonated PNP; Y = Li, K) were prepared by deprotonation of the two methylene groups of the corresponding cationic complexes with either 2 equiv of KN(SiMe(3))(2) or an excess of MeLi. While the reaction of [(PNP)PtCl][Cl] with an excess of MeLi led only to the anionic complex without chloride substitution, reaction of [(PNP)PdCl][Cl] with an excess of MeLi led to the methylated anionic complex [(PNP**)PdMe](-)Li(+). NMR studies, X-ray structures, and density functional theory (DFT) calculations reveal that the neutral complexes have a "broken" aromatic system with alternating single and double bonds, and the deprotonated arm is bound to the ring by an exocyclic C=C double bond. The anionic complexes are best described as a pi system comprising the ring carbons conjugated with the exocyclic double bonds of the deprotonated "arms". The neutral complexes are reversibly protonated to their cationic analogues by water or methanol. The thermodynamic parameters DeltaH, DeltaS, and DeltaG for the reversible protonation of the neutral complexes by methanol were obtained.

Metal-ligand cooperation in the trans addition of dihydrogen to a pincer Ir(I) complex: a DFT study.

DFT calculations on the hydrogenation of a (PNP)Ir(I) complex, to give the trans--rather then the cis--dihydride isomer, show that the reaction proceeds via a deprotonation/protonation of the ligand arm with concomitant dearomatization/aromatization of the pyridine core. Thus, the actual H(2) activation step occurs by an Ir(III) complex and not by the Ir(I) starting complex, as supported by experimental observations. This ligand participation allows for products that would otherwise be inaccessible. In addition, trace amounts of water, which are likely to be present in the solvent, facilitate proton transfer reaction steps.

Pyridine-based SNS-iridium and -rhodium sulfide complexes, including d8-d8 metal-metal interactions in the solid state.

The cationic, pincer-type complexes [(SNS)Ir(COE)][BF4] (1) and [(SNS)Rh(COE)][BF4] (2) (SNS = 2,6-bis(t-butylthiomethy1)pyridine; COE = cyclooctene) complexes were prepared, and their structure and reactivity were studied. They are fluxional at room temperature as a result of "arm" hemilability, which can be frozen at low temperatures. Reaction of complex 1 with H2 resulted in a dimeric dihydride complex [(SNS)Ir(H2)]2[BF4]2 (3) in which the sulfur atoms bridge between two metal centers. The Rh complex 2 did not react with H2. Both the carbonyl complexes [(SNS)Ir(CO)][BF4] (5) and [(SNS)Rh(CO)][BF4] (6) show differences in the IR stretching frequencies in solution vs. solid states, which are a result of uncommon metal-metal interactions between square planar d8 systems in the solid state. Complexes 1, 3, 5 and 6 were structurally characterized by X-ray crystallography. A network of hydrogen bonds involving the BF4(-) counter anion and hydrogen atoms of complex 5 was observed.

Mononuclear Rh(II) PNP-type complexes. Structure and reactivity.

The Rh(II) mononuclear complexes [(PNPtBu)RhCl][BF4] (2), [(PNPtBu)Rh(OC(O)CF3)][OC(O)CF3] (4), and [(PNPtBu)Rh(acetone)][BF4]2 (6) were synthesized by oxidation of the corresponding Rh(I) analogs with silver salts. On the other hand, treatment of (PNPtBu)RhCl with AgOC(O)CF3 led only to chloride abstraction, with no oxidation. 2 and 6 were characterized by X-ray diffraction, EPR, cyclic voltammetry, and dipole moment measurements. 2 and 6 react with NO gas to give the diamagnetic complexes [(PNPtBu)Rh(NO)Cl][BF4] (7) and [(PNPtBu)Rh(NO)(acetone)][BF4]2 (8) respectively. 6 is reduced to Rh(I) in the presence of phosphines, CO, or isonitriles to give the Rh(I) complexes [(PNPtBu)Rh(PR3)][BF4] (11, 12) (R = Et, Ph), [(PNPtBu)Rh(CO)][BF4] (13) and [(PNPtBu)Rh(L)][BF4] (15, 16) (L = tert-butyl isonitrile or 2,6-dimethylphenyl isonitrile), respectively. On the other hand, 2 disproportionates to Rh(I) and Rh(III) complexes in the presence of acetonitrile, isonitriles, or CO. 2 is also reduced by triethylphosphine and water to Rh(I) complexes [(PNPtBu)RhCl] (1) and [(PNPtBu)Rh(PEt3)][BF4] (11). When triphenylphosphine and water are used, the reduced Rh(I) complex reacts with a proton, which is formed in the redox reaction, to give a Rh(III) complex with a coordinated BF4, [(PNPtBu)Rh(Cl)(H)(BF4)] (9).

Metal-ligand cooperation in C-H and H2 activation by an electron-rich PNP Ir(I) system: facile ligand dearomatization-aromatization as key steps.

Unusual reactions are reported, in which the aromatic PNP ligand (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) acts in concert with the metal in the activation of H2 and benzene, via facile aromatization/dearomatization processes of the ligand. A new, dearomatized electron-rich (PNP*)Ir(I) complex 2 (PNP* = deprotonated PNP) activates benzene to form the aromatic (PNP)Ir(I)Ph 4, which upon treatment with CO undergoes a surprising oxidation process to form (PNP*)Ir(III)(H)CO 6, involving proton migration from the ligand "arm" to the metal, with concomitant dearomatization. 4 undergoes stereoselective activation of H2 to exclusively form the trans-dihydride 7, rather than the expected cis-dihydride complex. Our evidence, including D-labeling, suggests the possibility that the Ir(I)-Ph complex is transformed to the dearomatized Ir(III)(Ph)(H) (independently prepared at low temperature), which may be the actual intermediate undergoing H2 activation.

Selective ortho C-h activation of haloarenes by an Ir(I) system.

The cationic PNP-Ir(I)(cyclooctene) complex 1 (PNP = 2,6-bis-(di-tert-butyl phosphino methyl)pyridine) reacts with benzene at 25 degrees C to quantitatively yield the crystallographically characterized, square pyramidal, iridium phenyl hydride complex cis-(PNP)Ir(Ph)(H), 2, in which the hydride is trans to the vacant coordination site. The cationic complex 2 is stable to heating at 100 degrees C, in sharp contrast to the previously reported unstable neutral, isoelectronic (PCP)Ir(H)(Ph) (PCP = eta(3)-2,6-((t)()Bu(2)PCH(2))(2)C(6)H(3)). Heating of 2 at 50 degrees C with other arenes results in arene exchange. Complex 1 activates C-H bonds of chloro- and bromobenzene with no C-halide oxidative addition being observed. Selective ortho C-H activation takes place, the process being directed by halogen coordination and being thermodynamically and kinetically favorable. The meta- and para-C-H activation products are formed at a slower rate than the ortho isomer and are converted to it. NMR data and an X-ray crystallographic study of the ortho-activated chlorobenzene complex, which was obtained as the only product upon heating of 1 with chlorobenzene at 60 degrees C, show that the chloro substituent is coordinated to the metal center.