The Coordination Chemistry of Azulene: A Comprehensive DFT Investigation

Abstract

International audienceDFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type LnMAz, MAz2, 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

Bis(azulene) “submarine” metal dimer sandwich compounds (C10H8)2M2 (M = Ti, V, Cr, Mn, Fe, Co, Ni): Parallel and opposed orientations

“…The hexahapto fulvene bonding of this type of azulene ligand to one of the iron atoms leads to considerable distortion. Saillard and coworkers [13] discuss only the singlet (C 10 H 8 ) 2 Fe 2 structures Fe-S-O and Fe-S-P in their work rather than the triplet structures including Fe-T-P. structure Fe-T-P. Essentially, the same singlet (C 10 H 8 ) 2 Ni 2 structures Ni-S-O and Ni-S-P were studied by Saillard and coworkers, [13] who find Ni-S-P to lie 24.1 kcal/mol in energy above Ni-S-O as compared with our value of 14.3 kcal/mol.…”

“…The early transition metal (g 5 [13] using the Amsterdam density functional program and different basis sets. In the earlier work, Ti-S-O and V-T-O were found to be the lowest energy structures with Ti-S-P and V-T-P lying at higher energies of 23.6 and 6.9 kcal/mol, respectively, as compared with 19.7 and 4.1 kcal/mol, respectively, in our work using the B3LYP* functional.…”

“…Both the mononuclear azulene sandwiches (C 10 H 8 ) 2 M and binuclear azulene sandwiches (C 10 H 8 ) 2 M 2 have been discussed to a limited extent in a recent broad survey of azulene metal complexes using density functional theory (DFT). [13] The experimental chemistry of pentalene is more challenging than that of azulene because of the instability of free pentalene. However, dianions available from pentalenes are available as synthetic reagents for the preparation of pentalene metal sandwich complexes.…”

“…The previous theoretical study [13] examined only a single spin state for six of these metals, namely singlets for titanium, manganese, iron, cobalt, and nickel and triplets for vanadium. Our results are in essential agreement with the earlier results [13] for titanium, vanadium, and nickel.…”

“…However, for manganese, iron, and cobalt, as well as chromium not reported in the earlier work, we find the lowest energy structures to be triplet spin state structures rather than the singlet structures reported in the earlier work. [13] Theoretical Methods…”