Selective Ortho C-H Activation of Haloarenes by an Ir(I) System Eyal Ben-Ari, † Mark Gandelman, † Haim Rozenberg, ‡ Linda J. W. Shimon, ‡ and David Milstein* ,† Departments of Organic Chemistry and Chemical SerVices, The Weizmann Institute of Science, RehoVot 76100, Israel Received September 1, 2002; E-mail: david.milstein@weizmann.ac.il The selective activation of strong C-H bonds in the presence of substitutionally reactiVe groups, such as halo-substituents, is important for synthetic applications. Recent examples of C-H activation in haloarenes by soluble transition metal complexes were reported. 1 Because of steric reasons, activation of the less hindered meta or para C-H bonds in haloarenes was observed. Activation of the ortho C-H bond is desirable since it may lead to the functionalization of haloarenes in the most sterically hindered position. 2 Here we report on an electron-rich cationic Ir(I) system which undergoes facile arene C-H bond activation, leading with benzene to a crystallographically characterized, unsaturated, stable Ir(ΙΙΙ) hydridophenyl complex. Remarkably, this system exhibits unprecedented regioselectiVity toward ortho-C-H bond activation in chloro- and bromobenzene, utilizing the halogen as a directing group. We have recently reported electron-rich, neutral iridium com- plexes of 2,6-bis-(di-tert-butyl phosphino methyl)pyridine (PNP). 3 When the cationic PNP-Ir(I) complex 1 4 was dissolved in benzene, quantitative C-H activation took place, yielding the iridium phen- yl hydride complex 2 within 48 h at 25 °C or 1 h at 60 °C (Scheme 1). 31 P{ 1 H} NMR of 2 in CD 2 Cl 2 4 exhibits a doublet at 54 ppm and 1 H NMR reveals a highfield shifted triplet at -44 ppm, characteristic of a hydride trans to a vacant coordination site, 5 indicating a square pyramidal geometry. The X-ray structure of 2 4 reveals that the iridium atom is located in the center of a slightly distorted square pyramid with the hydride positioned trans to the vacant site (Figure 1). Complex 2 does not decompose upon heating to 100 °C for 24 h in the solid state or in a benzene solution, representing a rare example of a thermally stable coordinatively unsaturated M(III) d 6 hydridoaryl complex. Unsaturated hydridoaryl complexes were postulated as key intermediates in catalytic processes such as hydroarylation of alkenes 2 and decarbonylation of aldehydes 6 although not observed in either case. Interestingly, Goldman reported recently the neutral, isoelectronic, thermally unstable five- coordinate (PCP)Ir-hydridophenyl complex 7 (PCP ) η 3 -2,6-( t Bu 2 - PCH 2 ) 2 C 6 H 3 ) which was characterized in solution at low temper- ature. Werner reported the thermally stable Ir(P i Pr 3 ) 2 (H)(Ph)(Cl), 8 which is the only crystallographically characterized unsaturated Ir(H)(R) (R ) alkyl, aryl) complex. This trigonal bipyramidal complex is stabilized by strong π-donation of the chloride ligand to an empty d-orbital of the metal. Complex 2 lacks such stabilization. We believe that it is the first crystallographically characterized square pyramidal iridium hydrido-aryl (or -alkyl) complex. 9 Square pyramidal d 6 PCP complexes are common. 10 Although stable, complex 2 exhibits arene exchange reactivity. When a slurry of 2 in C 6 D 6 was heated at 50 °C, disappearance of the hydride and phenyl signals was observed in the 1 H NMR, while the 31 P NMR remained unchanged. Formation of (PNP)Ir(D)(C 6 D 5 ) was confirmed by 2 H NMR. As shown recently by Goldman, such an intermolecular arene exchange in the unstable PCP-based Ir(H)(Ph) system takes place rapidly at low temperature and follows a dissociative mechanism. 7 It is likely that the arene exchange in the case of 2 requires heating to overcome the barrier of C-H reductive elimination. 2 and (PCP)Ir(H)(Ph) represent a rare case of a differently charged, isoelectronic couple of the same metal that undergoes C-H reductive elimination/oxidative addition. A comparison of the CO absorption frequency of (PCP)Ir(H)(Ph)CO (1973 cm -1 ) 7 and that of 2-CO (2005 cm -1 ;H trans to CO in both cases) 4 indicates that the electron density at the metal center in the cationic Ir(III) complex is lower, as expected. The reasons for the higher stability of 2, which seems counterintuitive, are being explored. 11 Complex 1 activates C-H bonds of aryl halides with not even traces of C-halide oxidative addition products, even with bromoben- zene (vide infra). When 1 was heated in fluorobenzene at 50 °C, three Ir-hydridoaryl complexes were formed and characterized by † Department of Organic Chemistry. ‡ Department of Chemical Services. Figure 1. ORTEP drawings of 2 (left) and 4a. Hydrogen atoms (except hydride) and hexafluorophosphate counterion were omitted for clarity. Selected bond lengths (Å) and angles (deg): 2: Ir(1)-N(5) 2.142(5), Ir(1)- C(1) 2.043(7), Ir(1)-H(1) 1.66(7), C(1)-Ir(1)-H(1) 88, N(5)-Ir(1)-H(1) 94. 4a: Ir(1)-C(21) 2.045, Ir(1)-Cl(1) 2.816, C(26)-Cl(1) 1.78, Ir(1)- Cl(1)-C(26) 72, Cl(1)-C(26)-C(21) 63. Scheme 1 Published on Web 03/27/2003 4714 9 J. AM. CHEM. SOC. 2003, 125, 4714-4715 10.1021/ja028362p CCC: $25.00 © 2003 American Chemical Society