Abstract:
DFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type LnM2Az, and M2Az2 (Az = azulene). The analysis of their electronic and molecular structures in relation to their electron counts allows a comprehensive rationalization of the bonding within this very large family of compounds.
A very rich coordination chemistry of azulene is apparent, even much richer than one could determine from the avalaible experimental data. The reason for this diversity comes in part from the marked dissymmetry of azulene, which is made up of two fused rings of very
different sizes. It comes also in large part from the very large electronic and structural flexibility of azulene (in contrast to its isomer naphthalene), which is able to adapt to the electronic demand of the metal(s). Any of the fused C5 and C7 rings of azulene can be
coordinated in various hapticities and symmetries, depending on the nature of the MLn moiety (or moieties) they are bonded to. This flexibility favors the possibility of existence of several isomers (sometimes enantiomers) of similar energy and of their interconversion in
solution, in particular through haptotropic shifts. The azulene asymmetry causes dinuclear complexes to exhibit very different coordination environments (sometimes different oxidation states). In some of them, M−M bonding is preferred over M−azulene bonding.
Most of the investigated complexes are expected to exhibit a rich fluxional behavior.
The density functional theory (DFT) calculations were carried out on Cu(DHA)2(DMSO)2 [1], Zn(DHA)2(DMSO)2 [2] and Cd(DHA)2(DMSO)2 [3], where their structural arrangement of which consists of a slightly distorted octahedron centred by a transition-metal with bidentate DHA ligands situated in equatorial positions and solvent molecules in axial positions. Results reveal the presence of one MO in the middle of large gap in [1], which allows its oxidation and its reduction. Thus two redox couples are envisageables.
The electronic structure of closed-shell anion [1]− is calculated and compared to that of the related [2] and [3] complexes, where substantial HOMO–LUMO gaps are computed.
A bonding analysis of these species shows the weakness of M–O (solvent) bonds compared to M–O (DHA) ones. The calculated vibrational data have been found in good agreement with experimental results. TD-DFT calculations rationalize the long-range electronic communication as a main characteristic of the DHA transition-metal species and as a key to improve MLCT and LMCT charge transfers.
