Ni, Cu, Zn, Pd, Ag and Cd Tetraphenylporphyrin Ab Initio Thermochemistry: Enthalpy of Formation of ZnTPP Revisited

Groups 10–12 metalloporphyrins
have been recognized
for
their numerous properties essential for the development of new sensing
materials. In this work, accurate gas–phase enthalpies of formation,
ΔfHm0(g,298.15), are predicted for the series of
Ni, Cu, Zn, Pd, Ag, and Cd tetraphenylporphyrins (MTPPs) on the basis
of the reaction–based Feller–Peterson–Dixon approach
and high-level ab initio DLPNO–CCSD(T) calculations. Our recently
developed automatic generator of the balanced chemical reactions was
employed to reduce the bias of the theoretical ΔfHm0(g,298.15) toward a particular reaction. Theoretical ΔfHm0(g,298.15) for ZnTPP (227.0 ± 3.4 kcal
mol–1) does not support the previously reported
experimental value of 132 ± 2 kcal mol–1. The
origin of the discrepancy probably lies in the experimental solid–state
ΔfHm0(ZnTPP, cr,298.15) as it stems from our theoretical
evaluations of the ΔfHm0(cr,298.15) values
for the entire set of transition metal TPP complexes. The large discrepancy
between experiment and theory also holds when different DFT functionals
(ωB97M–V, PBE0–D4, and B3LYP–D4) paired
with quadruple-ζ quality basis sets are used for the theoretical
calculations. Experimental revisiting of the solid–state enthalpy
of formation of ZnTPP and analogue measurements for other transition
metal TPPs are needed to resolve the observed discrepancy. Based on
the predicted enthalpies of formation of MTPPs, the relative energies
of the metal–ligand bonding are evaluated and the trends are
compared to those for the complexes of the unsubstituted porphyrin
with the same set of metals derived in [Can. J. Chem., 2009, 87, 1063]. According to both
studies, Pd complexes exhibit the strongest bonding, while the Cd
species are the least stable metallocomplexes within the considered
series.