Square-Antiprismatic Eight-Coordinate Complexes of Divalent First-Row Transition Metal Cations: A Density Functional Theory Exploration of the Electronic–Structural Landscape

Abstract

Density functional theory (in the form of the PW91, BP86, OLYP, and B3LYP exchange-correlation functionals) has been used to map out the low-energy states of a series of eight-coordinate square-antiprismatic (D2d) first-row transition metal complexes, involving Mn(II), Fe(II), Co(II), Ni(II), and Cu(II), along with a pair of tetradentate N4 ligands. Of the five complexes, the Mn(II) and Fe(II) complexes have been synthesized and characterized structurally and spectroscopically, whereas the other three are as yet unknown. Each N4 ligand consists of a pair of terminal imidazole units linked by an o-phenylenediimine unit. The imidazole units are the strongest ligands in these complexes and dictate the spatial disposition of the metal three-dimensional orbitals. Thus, the dx(2)-y(2) orbital, whose lobes point directly at the coordinating imidazole nitrogens, has the highest orbital energy among the five d orbitals, whereas the dxy orbital has the lowest orbital energy. In general, the following orbital ordering (in order of increasing orbital energy) was found to be operative: dxy < dxz = dyz ≤ dz(2) < dx(2)-y(2). The square-antiprism geometry does not lead to large energy gaps between the d orbitals, which leads to an S = 2 ground state for the Fe(II) complex. Nevertheless, the dxy orbital has significantly lower energy relative to that of the dxz and dyz orbitals. Accordingly, the ground state of the Fe(II) complex corresponds unambiguously to a dxy(2)dxz(1)dyz(1)dz(2)(1)dx(2)-y(2)(1) electronic configuration. Unsurprisingly, the Mn(II) complex has an S = 5/2 ground state and no low-energy d-d excited states within 1.0 eV of the ground state. The Co(II) complex, on the other hand, has both a low-lying S = 1/2 state and multiple low-energy S = 3/2 states. Very long metal-nitrogen bonds are predicted for the Ni(II) and Cu(II) complexes; these bonds may be too fragile to survive in solution or in the solid state, and the complexes may therefore not be isolable. Overall, the different exchange-correlation functionals provided a qualitatively consistent and plausible picture of the low-energy d-d excited states of the complexes.

Metalloporphyrin–Nitroxyl Interactions: The Low-Energy States of Reduced Manganese, Iron, and Cobalt Porphyrin Nitrosyls

“…37,38,40,41 Confoundingly, in other cases, OLYP yielded comparatively poor energetics. 43 We shall see that, in the present study, OLYP and B3LYP provide relatively similar descriptions of the various species and electronic states studied. 44 ■ RESULTS AND DISCUSSION a.…”

Tungsten Biscorroles: New Chiral Sandwich Compounds

“…Furthermore, both functionals exhibit ac lear preference for SAPr coordination for W[TPC] 2 and for the unsubstituted model complex W[corrole] 2 . [13] For each molecule, the calculations predicted two non-mirror-image isomers, in which the corrole ligandsa re rotated~45 8 and~135 8 relative to each other,w ith as light preference (< 0.1 eV)f or the 135 8 isomer, which also happens to be the one that has been experimentally observed. Each isomer corresponds to an overall molecular symmetry of C 2 ,w hich is ac hiral point group.…”

“…[11] It is in terms of their structures, however,t hat the tungsten biscorroles are unique:square-antiprismatic coordination at the metal centerc onfersa tb est C 2 symmetry on the molecules. [12,13] Thus, the molecules are chiral and, although we have not yet resolved the enantiomers, DFTc alculations strongly suggest that they should be configurationally stable, that is, not prone to spontaneous racemization. [14] An ew class of inherentlyc hiral chromophores thus appearst ob ea t hand, [15,16] with potential applicationsi ns uch diverse areas as chirals ensing and recognition, [17,18] chiral liquid crystals, [19,20] and nonlinearopticalmaterials.…”

“…Disappointingly,a side from indicatingt hat the complexes are diamagnetic, the 1 HNMR spectra (including variable temperature experiments)o ft he complexes provedu nenlightening,c onsisting largely of overlapping peaksi nt he aromatic region. DFT geometryo ptimizations on an unsubstituted tungsten biscorrole model compound indicated ac lear preference for square-antiprismatic (SAPr), [13] as opposed to square-prismatic (SPr), coordination. In otherw ords, the calculationsp redicted that the two corrolesa re rotated~45 8 or 135 8 relativet oe ach other.…”

Combined Mössbauer Spectral and Density Functional Study of an Eight-Coordinate Iron(II) Complex

“…Quite recently Conradie et al 10 have used density functional calculations with several different functionals to determine both the relative energies of the frontier orbitals and the optimized structures of 1 and 2. However, these authors did not report, for either the X-ray structure 2 of 2 or for its optimized structure, any calculated electric field gradients or s-electron densities at the iron(II) nucleus, two quantities that can be experimentally determined by Mossbauer spectroscopy.…”