Hydrogen bonds, coordination isomerism, and catalytic dehydrogenation of alcohols with the bifunctional iridium pincer complex $$^{{{\left( {HOC{H_2}} \right)}_2}}\left( {P{C_{s{p^3}}}P} \right)$$ ( H O C H 2 ) 2 ( P C s p 3 P ) IrHCl

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The reaction of iridium(iii) hydride complex 1 based on the pincer dibenzobarrelene ligand $$^{{{\left( {HOC{H_2}} \right)}_2}}\left( {P{C_{s{p^3}}}P} \right)$$ 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 isomerization 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 PriOH under model conditions.