Trifunctional pNHC, Imine, Pyridine Pincer-Type Iridium(III) Complexes: Synthetic, Structural, and Reactivity Studies Fan He, † Andreas A. Danopoulos,* ,†,‡ and Pierre Braunstein* ,† † Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite ́ de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France ‡ Universite ́ de Strasbourg Institute for Advanced Study (USIAS), 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France * S Supporting Information ABSTRACT: Iridium(III) complexes with a trifunctional pincer ligand containing protic N-heterocyclic carbene (pNHC), pyridine, and imine donor groups were obtained in two sequential steps: (i) protonation of 2-(1-(2,6-diisopropylphenyl- imino)ethyl)-6-(1-imidazolyl)pyridine (L CH ; the superscript specifies the position of the tautomerizable H atom in the imidazole ring) with HBF 4 ·Et 2 O to give the imidazolium salt [HL CH ] + [BF 4 ] - (protonation always occurs at the imidazole N atom) and (ii) metalation of the latter with [Ir(cod)(μ-Cl)] 2 to give the hydrido pincer complex [Ir(H)(Cl)(NCMe){L NH - κ 3 N imine ,N Py ,C NHC }] + [BF 4 ] - (3 + [BF 4 ] - ). Substitution of MeCN in 3 + [BF 4 ] - by treatment with triisopropylphosphine gave the analogue [Ir(H)(Cl)P(i-Pr) 3 {L NH -κ 3 N imine ,N Py ,C NHC }] + [BF 4 ] - (4 + [BF 4 ] - ). Chloride abstraction from 3 + [BF 4 ] - by AgBF 4 gave [Ir(H)(NCMe) 2 {L NH -κ 3 N imine ,N Py ,C NHC }] 2+ [BF 4 - ] 2 (5 2+ [BF 4 - ] 2 ). The centrosymmetric dinuclear Ir(III) complex [Ir(H)(NCMe){μ-(L CH -H)-κ 3 N imine ,N Py ,C2,κN3}] 2 2+ [B- (C 6 F 5 ) 3 F - ] 2 (6 2+ [B(C 6 F 5 ) 3 F - ] 2 ) was obtained after deprotonation of 5 2+ [BF 4 - ] 2 with KO-t-Bu, followed by addition of B(C 6 F 5 ) 3 . It contains two Ir pincer moieties, each with a N imine ,N Py ,C2 donor set, which are connected by the Ir-N bonds involving the imidazolide rings, leading to a μ-C,N bridging mode for the latter. Remarkably, all of the donor atoms in the tetradentate bridging chelating ligands are chemically different. The molecular structures of 3 + [BF 4 ] - ·CH 2 Cl 2 , 4 + [BF 4 ] - ·CH 2 Cl 2 , 5 2+ [BF 4 - ] 2 ·2CH 2 Cl 2 , and 6 2+ [B(C 6 F 5 ) 3 F - ] 2 ·4CH 2 Cl 2 have been determined by X-ray diffraction. ■ INTRODUCTION As tunable and strong σ-donor ligands, N-heterocyclic carbenes (NHCs) have triggered fast-growing interest in organometallic chemistry in the last two decades. 1 A considerable number of NHC metal complexes have been produced where, in most cases, both N sites carry a substituent R (R = alkyl, aryl in I) that allows fine tuning of the steric and electronic properties of the NHC ligand. In contrast, protic NHC (pNHC; R = alkyl, aryl, H in II) metal complexes are less common. Owing to the presence of the acidic NH, pNHCs are not stable as free ligands, in contrast to their corresponding imidazole tautomers; pNHCs cannot generally be obtained by simple deprotonation of the corresponding imidazolium salts. 2 Thus, pNHC complexes have been comparatively less investigated and their synthesis can occasionally be challenging; their coordination chemistry has been recently reviewed. 3 Conversely, the NH moiety in a pNHC complex can act as a reactive site of potential relevance to bifunctional catalysis 4 and substrate recognition. 3a,c,5 Furthermore, metal complexes with pincer-type ligands are attracting continuing interest, largely driven by their unique photophysical and chemical properties, the latter in particular with respect to catalytic transformations. 6 Interestingly, among the limited number of pNHC metal complexes reported, the only ones of the pincer type are depicted in Scheme 1 and some of them possess interesting catalytic properties, rendering this family of complexes attractive synthetic targets for further developments. 7 In a pNHC-based pincer-type donor system, the carbene donor occupies a terminal (wingtip) rather than the central (bridge- head) position, in contrast to the many known NHC-based pincer ligands. 6,8 The Ru(II) complex with pincer-type pNHC ligand and heterotopic wingtips reported by Kuwata and Ikariya (A in Scheme 1) has two different NH groups: one on the pNHC and the other on a pyrazole. Treatment of A with 1 equiv of base led to the deprotonation of the pyrazole, indicating that the NH of the pNHC is less acidic. 7a The Ru(II) complex with Received: November 6, 2015 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.5b00926 Organometallics XXXX, XXX, XXX-XXX