Transition metal dihydrogen complexes: isotope effects on reactivity and structure

Abstract

Complexation of dihydrogen to transition metal centers was discovered by Kubas and coworkers in 1984. The notion that the simplest molecule in chemistry can act as a ligand to form relatively stable transition metal complexes has led to a paradigm shift in coordination chemistry. Crucial to the exploration of this intriguing new chemistry has been the use of isotope substitution. Several aspects of the coordination chemistry of dihydrogen have revealed fascinating isotope effects on reactivity, spectroscopy and in some cases structure. Complexation of HD has been used to diagnose bond distances from measurements of J HD and isotope effects for D 2 versus H 2 binding have been evaluated. Examples of quantum mechanical exchange coupling in H 2 complexes have been described. These effects disappear when one of the H atoms is replaced by D. In molecules with bound hydrogen adjacent to a hydride ligand, non-statistical occupancy of hydrogen versus hydride sites by deuterons has been observed. In some cases, isotope-dependent structures have been established by the study of HD, HT and DT complexes using NMR spectroscopy.

Iridium-Catalysed ortho-Directed Deuterium Labelling of Aromatic Esters—An Experimental and Theoretical Study on Directing Group Chemoselectivity

“…In one particularly active branch of such labelling research, hydrogen isotope exchange (HIE) is employed to deliver either deuterium or radioactive tritium to pharmaceutical drug candidates in one synthetic step. As well as circumventing the requirement for isotopically-enriched starting materials in preparing tritiated drug candidates [1,5], HIE can also provide analogous deuterated compounds for use as internal standards for mass spectrometry [11,12], for kinetic isotope studies [13,14], and for the alteration of reaction pathways in total synthesis [15].…”

Mild Redox Complementation Enables H2 Activation by [FeFe]-Hydrogenase Models

“…Reactions in which H 2 cleavage is known to be rate determining typically exhibit normal kinetic isotope effect (eg k H / k D ~ 2.0). 9d Although few reports describe kinetic isotope effects for H 2 binding, 13 an inverse kinetic isotope effect has been observed for H 2 binding to Ir(H) 2 Cl(PBu t 2 Me) 2 . 14 The inverse isotope effect measured for our reaction is inconsistent with rate-determining heterolytic cleavage of H 2 .…”

Meta‐substituent effects on organoiridium‐catalyzed ortho‐hydrogen isotope exchange

“…Initial coordination of substrates' directing atoms to iridium (4-5 in Figure 1) is common to both Paths a and b, and the decoordination (labeled as 5a,b-labeled as 4a,b) is very unlikely to be the ratedetermining step as it is rapid and reversible. The ligand isomerization step (6a,6b-7a,7b) is also unlikely to be ratedetermining, as such rearrangements have been shown 16 to have low activation energies (o5 kcal/mol). Our second set of experiments shows that the metallacycle forms (6a,b and 7a,b) are thermodynamically less stable than the ring-opened forms (5).…”