Russian Chemical Bulletin, International Edition, Vol. 64, No. 12, pp. 2806—2810, December, 2015 2806 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2806—2810, December, 2015. 1066-5285/15/6412-2806 © 2015 Springer Science+Business Media, Inc. Hydrogen bonds, coordination isomerism, and catalytic dehydrogenation of alcohols with the bifunctional iridium pincer complex (HOCH 2 ) 2 (PC sp 3 P)IrHCl G. A. Silantyev, a E. M. Titova, a O. A. Filippov, a E. I. Gutsul, a D. Gelman, b and N. V. Belkova a a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 ul. Vavilova, 119991 Moscow, Russian Federation. Fax: +7 (499) 135 5085. E-mail: nataliabelk@ineos.ac.ru b Institute of Chemistry, The Hebrew University of Jerusalem, Edmond Safra Campus, 91904 Jerusalem, Israel. Fax: (+972) 2 658 5279 The reaction of iridium(III) hydride complex 1 based on the pincer dibenzobarrelene ligand (HOCH 2 ) 2(PC sp 3P) with pyridine proceeds stepwise to show different reactivities of the starting fac-isomers 1A and 1B. The kinetic product of the reaction is mer-complex 2 with a trans disposition of the pyridine and hydride ligands. Isomerization into the thermodynamic product 2´ containing pyridine in the cis-position with regard to hydride proceeds slowly. The estimation of activation parameters (ΔH ≠ and ΔS ≠ ) shows that the change in the geometry of fac-complexes upon coordination of pyridine occurs through an associative transition state, while isomeriza- tion of the mer-complexes is a dissociative process. The isomers of complex 1 and its pyridine- containing derivatives 2 and 2´ are shown to exhibit different reactivities in the formation of dihydrogen bond and the catalytic dehydrogenation of Pr i OH under model conditions. Key words: transition metal hydrides, hydrogen bonds, NMR spectroscopy, IR spectroscopy. The proton-hydride interactions play a key role in the chemistry of transition metal hydrides. For example, di- hydrogen bonding (MH ... HX) precedes the proton trans- fer and the formation of complex with molecular hydro- gen which subsequently can transform into dihydride or lose the hydrogen molecule. 1,2 The noncovalent interac- tions between hydride and proton were shown to play a role in the H/D exchange and some catalytic processes, for example, hydration of nitriles 3,4 and CO 2 hydrogena- tion. 5 Currently, it is generally admitted that the catalytic cycle of multiple-bond hydrogenation includes the steps of proton and hydride-ion transfer. In recent years, the so- called cooperative ligands are used increasingly often in the design of new efficient metal complex catalysts, where such ligands are involved in reversible transformations of the catalyst molecule during the catalytic process. 6—8 In these cases, the role of intra- and intermolecular interac- tions can be a key to understanding the mechanisms of catalytic processes. The bifunctional iridium(III) hydride complex 1 based on the pincer dibenzobarrelene ligand 1,8-bis(diphenyl- phosphanyl)-9,10-dihydro-9,10-ethanoanthrhacene-11,12- dimethanol (PC(sp 3 )P) is an efficient catalyst for de- hydrogenation of primary and secondary alcohols, the re- action rate increasing in the presence of bases, for example, Et 3 N, Cs 2 CO 3 , and K 3 PO 4 (see Ref. 8). The authors 8 assumed that the catalytic properties of 1 can be caused by the intramolecular H ... H interaction between the (—CH 2 OH) proton and the hydride ligand. Based on the experimental data and data from quantum chemical cal- culations, we have shown 9 the nonrigidity of complex 1 and the effect of different bases, such as Et 3 N, DMSO, and CH 3 CN, on its conformational mobility. In this com- munication, we present the results from the study of the reaction between complex 1 and pyridine and the reac- tions of complex 1 and its pyridine-containing derivatives with alcohols, which demonstrate different reactivities of isomers. Experimental Complex 1 was synthesized at the Hebrew University of Jerusalem according to the published procedure. 8 Pyridine- 15 N was prepared from 15 NH 4 Cl (Sigma—Aldrich). 10 IR spectra were measured on a Nicolet 6700 Fourier-transform spectrometer in CaF 2 cells using a Carl Zeiss Jena cryostat. 1 H NMR spectra