Photoswitchable Hydride Transfer from Iridium to 1‑Methylnicotinamide Rationalized by Thermochemical Cycles

Visible light-triggered hydride transfer
from [Cp*Ir­(bpy)­(H)]+ (1) to organic acids
and 1-methyl­nicotin­amide
(MNA+) is reported (Cp* = penta­methyl­cyclo­penta­dienyl;
bpy = 2,2′-bipyridine). A new thermo­chemical cycle for
determining excited-state hydride donor ability (hydricity) predicted
that 1 would be an incredibly potent photo­hydride
in aceto­nitrile. Photo­triggered H2 release
was indeed observed from 1 in the presence of various
organic acids, providing experimental evidence for an increase in
hydricity of at least 18 kcal/mol in the excited state. The rate and
product selectivity of hydride transfer to MNA+ are photo­switchable:
1,4-dihydro-1-methyl­nicotin­amide forms slowly in the dark
but rapidly under illumination, and photolysis can also produce doubly
reduced 1,4,5,6-tetrahydro-1-methyl­nicotin­amide.