Theoretical study of first-row transition metal porphyrins and their carbonyl complexes |
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Authors: | O P Charkin A V Makarov N M Klimenko |
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Institution: | (1) Institute of Problems of Chemical Physics, Russian Academy of Sciences, Institutskii pr. 18, Chernogolovka, Moscow oblast, 142432, Russia;(2) Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571, Russia |
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Abstract: | The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C20H12N4) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN?, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G* basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation) of “active” $ d_{x^2 - y^2 } The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row
transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C20H12N4) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN−, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G*
basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown
for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and
V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in
energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of
binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate
ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation)
of “active” , and , d
xz
, and d
yz
AO in bonding of M to the P ring and axial ligands, but also the fraction of the total bond energy consumed for the preparation
(promotion) of those “valence states” of the M(P) molecules that are realized in M(P)CO and M(P)(CO)(AB) complexes. For the
series of compounds Fe(P)(CO)2 − Fe(P)(CO)(CS) − Fe(P)(CS)2 − Fe(P)(CO)(CN−) in the singlet, triplet, and ionized states, the trans influence of axial ligands in low-spin metalloporphyrins is shown
to follow the same qualitative scheme as is typical of octahedral transition metal complexes: in mixed-ligand complexes (as
compared to the symmetric ones), the stronger bond becomes shorter and even stronger, while the weaker bond becomes longer
and even weaker. It is assumed that the same scheme will persist for more complicated low-spin six-coordinate metalloporphyrins
in the states with the vacant AO and occupied d
xz
and d
xz
AOs involved in bonding with both axial ligands with the filled shell.
Original Russian Text ? O.P. Charkin, A.V. Makarov, and N.M. Klimenko, 2008, published in Zhurnal Neorganicheskoi Khimii,
2008, Vol. 53, No. 5, pp. 781–794. |
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