Catalytic Proton Coupled Electron Transfer from Metal Hydrides to Titanocene Amides, Hydrazides and Imides: Determination of Thermodynamic Parameters Relevant to Nitrogen Fixation

The
hydrogenolysis of titanium–nitrogen bonds in a series
of bis­(cyclopentadienyl) titanium amides, hydrazides and imides by
proton coupled electron transfer (PCET) is described. Twelve different
N–H bond dissociation free energies (BDFEs) among the various
nitrogen-containing ligands were measured or calculated, and effects
of metal oxidation state and N-ligand substituent were determined.
Two metal hydride complexes, (η5-C5Me5)­(py-Ph)­Rh–H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η5-C5R5)­(CO)3Cr−H ([Cr]R-H, R= H, Me) were evaluated for formal H atom transfer reactivity
and were selected due to their relatively weak M–H bond strengths
yet ability to activate and cleave molecular hydrogen. Despite comparable
M–H BDFEs, disparate reactivity between the two compounds was
observed and was traced to the vastly different acidities of the M–H
bonds and overall redox potentials of the molecules. With [Rh]–H, catalytic syntheses of ammonia, silylamine and N,N-dimethylhydrazine have been accomplished from
the corresponding titanium­(IV) complex using H2 as the
stoichiometric H atom source. The data presented in this study provides
the thermochemical foundation for the synthesis of NH3 by
proton coupled electron transfer at a well-defined transition metal
center.