Theoretical study of the structure and stability of the dimers of heme analogues (MC34H32N4O4)2 and their ions (MC34H32N4O4) 2 + with 3d-metal atoms M

Abstract

The electronic and geometric structures and the energetic characteristics of a series of monomeric MC34H32N4O 4 0,+ and dimeric (MC34H32N4O4) 2 0,+ molecules, heme analogues and their positively charged ions with 3d-metal atoms M = Ti, V, Cr, and Mn, have been calculated by the density functional theory B3LYP method with the Gen−1 = 6−31G*(Fe) + 6−31G(C, H, N, O) and Gen−2 = 6−311+G*(Fe) + 6−31G*(C, H, N, O) basis sets. The computation results are compared with the analogous calculated data on the heme dimers (heme) 2 0,+ . Computations show that for the (MC34H32N4O4) 2 0,+ dimers, high-spin states are preferable. In these dimers, the rings are linked with each other by a pair of M-carbonyl bridges M⋯Ob=C(OH) and a pair of hydrogen bridges OHb⋯N. The calculated energies of dissociation D of the dimers into monomers point to a rather high stability of the dimers at the beginning of the 3d series (D ∼ 2.3–3.6 eV for M = Ti, V), which decreases rapidly as the atomic number of M increases (D ∼ 0.5 eV for M = Cr and ∼0.4 eV for (heme)2). The positive ions (MC34H32N4O4) 2 + are ∼0.8–1.0 eV are more stable to dissociation than their neutral congeners (MC34H32N4O4)2. The trends in the behavior of the energetic and structural characteristics of the dimers (distances R(M—N), displacements of M atoms from the porphyrin ring plane, parameters of the carbonyl and hydrogen bridges, character of ring distortions, etc.), as well as in the spin density distribution between the metal atoms and the rings in the monomers MC34H32N4O4 and dimers (MC34H32N4O4)2 caused by their ionization and going along the 3d series, are examined. In the mixed dimer (FeC34H32N4O4)(VC34H32N4O4), the rings are linked by only one strong carbonyl bridge V⋯Ob=C(OH), with some contribution made by the neighboring hydrogen bridge. The dissociation energy of this mixed dimer into monomers is close to a half of the dissociation energy of the “symmetric” dimer (VC34H32N4O4)2.