Torsionally Controlled Electronic Coupling in Mixed-Valence Oxodimolybdenum Nitrosyl Scorpionates - a DFT Study

Abstract

The density functional theory (DFT) method has been used to study the electronic communication in strongly interacting oxo-bridged di-{Mo(II,I)(NO(+))}(3+,2+) complexes stabilized by tris(3-methylpyrazol-1-yl)borate, [Tp(Me)](-) (dihydroxy 1' and its modified analogs), having fully localized valences on the two Mo centers (Class I), despite a short (ca. 3.8 A) Mo...Mo distance. Structural and electrochemical (separation between the redox potentials Delta(red/ox)E(1/2)) properties and IR spectra (in particular the nu(NO) frequencies) obtained from the B3LYP calculations for 1' are successfully related to experimental values. Strongly twisted geometry with the (O)N-Mo1...Mo2-N(O) angle close to 90 degrees (confirmed by DFT modeling performed for 1'(-1,0,+1) and X-ray diffraction study of [{Mo(NO)(Tp(Me2))(OH)}(2)(mu-O)](-) (1) presented herein) is a common, though so far not fully understood, structural feature of this class of mu-oxo species, in contrast to the closely related {Mo(V)(=O)}(3+) analogs. This study shows that the orthogonality of the local equatorial planes for the two Mo centers may be rationalized by the electronic structure, namely from the balance between the destabilizing repulsion of the Mo-based (d, pi(x)*)(b) electron pairs versus a favorable but relatively weak electron delocalization. Strongly repelling electron pairs avoid each other, which enforces the twisted geometries and blocks the electron delocalization. Steric hindrance (a nonbonding repulsion of the adjacent Tp(x) ligands and the weak hydrogen-bonding interactions, i.e., OH...ON, OH...OH, and C-H...O((NO/OH))) is shown not to be decisive since neither the removal of the inner 3-Me groups of [Tp(Me)](-) in complex 1' nor the substitution of OH groups by OCH(3) ligands did substantially influence the dihedral twist angle in the minimum energy structure. Yet the relative orientation of the {Mo(NO)}(2+,3+) cores along with the position of the bridging oxygen (significantly bent upon reduction) controls the prospective intramolecular through-bond electron transfer in the mixed valence form. Our DFT modeling demonstrates that a maximum delocalization (via a hole-transfer mechanism) of the unpaired electron in 1'(-), measured as a spin population on the nonreduced Mo2 center, is achieved for the structure with a torsional deflection of 23 degrees, at a cost of 16.5 kcal/mol. These results show that the electron exchange along the Mo-O-Mo array in the originally fully valence-trapped {17e:16e}(-) complexes may be controlled and can be thermally activated (e.g., using a high-boiling solvent or by irradiation at ca. 50-200 cm(-1)).

Twists and turns: Studies of the complexes and properties of bimetallic complexes featuring phenylene ethynylene and related bridging ligands

“…Similar conformational effects are now being more widely recognised in other systems [278,[289][290][291][292][293][294][295], influencing the distribution of electron spin density [296] and being noted to affect intramolecular energy and electron transfer pathways [297,298]. However, in the case of relatively electron rich ruthenium d 6 derivatives, even early computations highlighted appreciable structural reorganisation within the diethynyl bridging ligand resulting from oxidation of the closed shell precursors [279,280], and more recent descriptions have emphasised the ligand redox non-innocence of the bridging ligand in these cases [271,281].…”

Quantum-chemical insights into mixed-valence systems: within and beyond the Robin–Day scheme

“…Here quantum-chemical calculations may aid in the evaluation of the effects of such conformational motion, and a variety of studies along these lines has been performed. 239,261,[275][276][277] With respect to certain optoelectronic properties, it is in fact important to describe correctly not only the ground-state conformational energy surface but also that in crucial excited states. Shortcomings of TDDFT in case of charge-transfer-type excited states have been noted, 278,279 and more sophisticated methods 280 may have to be applied.…”

Autocatalytic cathodic dehalogenation triggered by dissociative electron transfer through a C–H⋯O hydrogen bond

“…[25][26][27] We used the B3LYP 28 hybrid functional because of its overall high accuracy and its good performance for predicting the structural and electronic properties of the {Mo(NO)Tp x } complexes we found previously. 29 Open-shell species were treated within the spinunrestricted scheme. Scalar relativistic effects were included in effective core potential (ECP) used for Mo.…”