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.

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.

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).

In continuation of our studies on bond activation and catalysis by pincer complexes, based on metal−ligand cooperation, we present here a rare example of amine N−H activation by Rh(I) complexes. The novel dearomatized pincer complexes [(PNN*)RhL′] (PNN = 2-(CH 2 -P t Bu 2 )-6-(CH 2 -NEt 2 )C 5 H 3 N, PNN* = deprotonated PNN, L′ = N 2 (5), C 2 H 4 (6)) and [( i PrPNP*)RhL′] ( i PrPNP = 2,6-(CH 2 -P i Pr 2 ) 2 C 5 H 3 N, i PrPNP* = deprotonated i PrPNP, L′ = C 2 H 4 (7), cyclooctene (9)) were prepared and fully characterized by NMR and X-ray analysis. Complexes 5−7 and 9 undergo facile N−H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal center to form stable anilide complexes [(PNN)Rh(NHAr)] and [( i PrPNP)Rh(NHAr)] (Ar = C 6 H 5 , o-Br-C 6 H 4 , m-Cl-p-Cl-C 6 H 3 , p-NO 2 -C 6 H 4 ). Anilines possessing electron-withdrawing groups accelerate the N−H activation and yield more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the NH− protons of anilines prior to and after the N−H activation. The reverse N−H formation by metal−ligand cooperation from the anilide complexes was observed to give free anilines and dearomatized Rh(I) complexes upon addition of CO or PEt 3 . Deprotonation of complexes [(PNL)Rh(p-NO 2 -NH 2 C 6 H 4 )] (13, P = P t Bu 2 , L = NEt 2 ; 15, P = L = P i Pr 2 ) yields the dearomatized anionic complexes [(PNL*)Rh(p-NO 2 -NH 2 C 6 H 4 )]. An associative mechanism, involving N−H activation of an apically coordinated aniline in a pentacoordinated Rh(I) complex, is suggested.

Reaction of (PNP)Ir(COE)+PF6 - (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C−H activation takes place. No C−halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C−H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange−correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C−H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2 C-C intermediate, which is converted into an η2 C-H complex, both being important intermediates in the C−H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2 C-H complex leads to selective activation. The unobserved C−halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C−H activation process, but it experiences a higher activation barrier. The ortho C−H activation product is also thermodynamically more stable than the C−Cl oxidative addition complex.

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.

The anionic dearomatized complex [(PNP*)Rh I Cl]K (2; PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine, PNP* = deprotonated PNP) was prepared by reaction of the aromatic (PNP)Rh I Cl complex 1 with KN(SiMe 3 ) 2 in dry benzene. Spectroscopic characterization and DFT calculations confirm a nonaromatic square-planar structure of complex 2. Under an atmosphere of dry argon, 2 undergoes facile CÀH activation of benzene by cooperation between the metal center and the pincer ligand, with aromatization of the ligand, to form the complex (PNP)Rh I (C 6 H 5 ) (3a). This reaction is inhibited by dinitrogen, which reacts with 2 to form the complex (PNP*)Rh I (N 2 ) (4), indicating higher stabilization of the 14-electron (PNP*)Rh I species 5 by dinitrogen as compared with chloride. Similarly, treatment of 2 with CO results in KCl liberation to form the dearomatized (PNP*)Rh I CO (8). In a protic environment, the dearomatized complex 2 is quickly reprotonated to regenerate the aromatic starting complex 1. Upon treatment with MeI, 2 undergoes oxidative addition to form the nonaromatic (PNP*)Rh III (CH 3 )Cl (10), while the dearomatized ligand remains intact. Complex 2 undergoes facile activation of H 2 to form the monohydride (PNP)Rh I (H) (11a) and with D 2 to form (PNP)Rh I (D) (11b) with benzylic-D incorporation, via metalÀligand cooperation by aromatization of the ligand. The reactivity of 2 with H 2 is significantly higher than that of 4. ' INTRODUCTIONMetalÀligand cooperation plays an important role in homogeneous catalysis by metal complexes, 1À3 such as in the hydrogenation of polar bonds by ruthenium amido complexes. 1b,c Recently, we discovered a new mode of metalÀligand cooperation involving reversible dearomatization of pyridine-and acridine-based pincer complexes. 3À9 MetalÀligand cooperation via reversible dearomatization can play a key role in the catalytic OÀH bond activation of alcohols, 3 exemplified by the direct coupling of alcohols to form esters with the liberation of H 2 , 4a,b the hydrogenation of esters to alcohols under mild pressure, 4c the dehydrogenative coupling of alcohols with amines to produce amides 4d and polyamides, 4e,f the dehydrogenative amidation of esters with amines, 4g and the acylation of alcohols with esters. 4h These reactions are catalyzed by the dearomatized (PNN*)Ru II (H)(CO) complex (PNN = 2-((ditert-butylphosphino)methyl)-6-(diethylaminomethyl)pyridine; PNN* = deprotonated PNN). 4 Furthermore, this powerful complex promotes splitting of water to H 2 and O 2 in consecutive heatand light-induced steps. 5 Very recently, a bipyridine-based PNN analogue of this complex was shown to be effective in the direct hydrogenation of amides to form amines and alcohols. 4i The dearomatized (PNP*)Ru II (H)(CO) analogue (PNP = 2,6-bis-((di-tert-butylphosphino)methyl)pyridine; PNP* = deprotonated PNP) is effective in the NÀH bond activation of amines and ammonia, 6a the coupling of alcohols and amines to form imines and H 2 , 6b and the dehydrogenation of secondary alcohols to ketones....

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.