posted on 2015-12-17, 04:32authored byAušra Jablonskytė, Lee R. Webster, Trevor
R. Simmons, Joseph A. Wright, Christopher J. Pickett
Protonation
at metal–metal bonds is of fundamental interest
in the context of the function of the active sites of hydrogenases
and nitrogenases. In diiron dithiolate complexes bearing carbonyl
and electron-donating ligands, the metal–metal bond is the
highest occupied molecular orbital (HOMO) with a “bent”
geometry. Here we show that the experimentally measured rates of protonation
(kH) of this bond and the energy of the
HOMO as measured by the oxidation potential of the complexes (E1/2ox) correlate in a linear free
energy relationship: ln kH = ((F(c – βE1/2ox))/(RT)), where c is a constant and β is the dimensionless Brønsted coefficient.
The value of β of 0.68 is indicative of a strong dependence
upon energy of the HOMO: measured rates of protonation vary over 6
orders of magnitude for a change in E1/2ox of ca. 0.55 V (ca. 11 orders of magnitude/V). This
relationship allows prediction of protonation rates of systems that
are either too fast to measure experimentally or that possess additional
protonation sites. It is further suggested that the nature of the
bridgehead in the dithiolate ligand can exert a stereoelectronic influence:
bulky substituents destabilize the HOMO, thereby increasing the rate
of protonation.