Temperature- and Solvent-Dependent Binding of Dihydrogen in Iridium Pincer Complexes

Mixtures of deuterium labeled complexes (p-XPOCOP)IrH2–xDx (1–6-d0–2) {POCOP = [C6H2-1,3-[OP(tBu)2]2; X = MeO (1), Me (2), H (3), F (4), C6F5 (5), and ArF = 3,5-(CF3)2-C6H3 (6)} have been generated by reaction of (p-XPOCOP)IrH2 complexes with HD gas in benzene followed by removal of the solvent under high vacuum. Spectroscopic analysis employing 1H and 2D NMR reveals significant temperature and solvent dependent isotopic shifts and HD coupling constants. Complexes 1–6-d1 in toluene and pentane between 296 K and 213 K exhibit coupling constants JHD of 3.8–9.0 Hz, suggesting the presence of an elongated H2 ligand, which is confirmed by T1(min) measurements of complexes 1, 3 and 6 in toluene-d8. In contrast, complex 6-d1 exhibits JHD = 0 Hz in CH2Cl2 or CDCl2F while isotopic shifts up to −4.05 ppm have been observed by lowering the temperature from 233 K to 133 K in CDCl2F. The large and temperature dependent isotope effects are attributed to non-statistical occupation of two different hydride environments. The experimental observations are interpreted in terms of a two component model involving rapid equilibration of solvated Ir(III) dihydride and Ir(I) dihydrogen structures

Electropholic Transition Metal Complexes: Catalysis of Isotope Exchange. April 1, 1992 - December 3, 1996. Final Report.

The central aim of this project is to exploit transition metal dihydrogen complexes to develop catalysts for isotope exchange reactions between hydrogen and substrates such as water. The authors have partially met this goal by the synthesis of novel cationic rhenium complexes of the form [Re(CO){sub 3}(PR{sub 3}){sub 2}(H{sub 2})]{sup +}. These complexes bind hydrogen somehwat more strongly than the neutral tungsten analogs but also activate the bound H{sub 2} to heterolytic cleavage. Thus rapid proton (deuteron) exchange between hydrogen and water can be achieved. An example of this reaction is the rapid formation of bound HD from the complex [Re(CO){sub 3}(PR{sub 3}){sub 2}(H{sub 2})]{sup +} and deuterium gas. Rapid incorporation of deuterium from D{sub 2}O has also been observed. In these systems, the competitive binding of water to the metal center is a drawback. The affinity of the Re center for water depends upon the nature of the phosphine ligands, with the presumably more electrophilic PPh{sub 3} complex binding water strongly and irreversibly, while the PCy{sub 3} complex binds water reversibly. These results have been published in J.Am.Chem.Soc 1994, 116, 4515 and J.Am.Chem.Soc 1997, 119, 4172