Mapping Bridge Conformational Effects on Electronic Coupling in Mo2-Mo2 Mixed-Valence Systems

Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H4 )]+ have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH2 Cl2 )) on ([{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H3 -n-OMe)]+ (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the β-HOSO and β-LUSO, whilst the relative phases of the β-HOSO and β-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules.

Multiple d-d bonds between early transition metals in TM2Lin (TM = Sc, Ti) superatomic molecule clusters

The synthesis and application of compounds with Cr-Cr and V-V d-d quintuple bonds (σ, 2π, 2δ) have led to new thinking about whether d-d multiple bonds also exist between early transition metals such as Sc-Sc and Ti-Ti. In this study, by extensive unbiased global search at the density functional theory level, the low-energy structures of 26e and 30e TM2Lin clusters were obtained. Based on the super valence bond (SVB) theory, the prolate double-core structure of TM2Lin clusters was regarded as a superatomic molecule, of which each half was regarded as an open-shell superatom, and the electronic shell-closure was realized by forming multiple bonds between superatoms. Then, the quintuple super bonds (2δ, 2π, σ) of the Li18Ti2, Li20Sc2, [Li17V2]+, [Li17Ti2]- clusters and the triple super bonds (2π, σ) of the Li24Sc2 and Li24Y2 clusters were confirmed via chemical-bonding analysis. This way of forming multiple bonds between early transition metals through superatomic bonding has promoted the experimental synthesis and application of early transition metal multiple bond compounds.

Mediation of Electron Transfer by Quadrupolar Interactions: The Constitutional, Electronic, and Energetic Complementarities in Supramolecular Chemistry

Studies of intermolecular interactions enhance our knowledge of chemistry across molecular and supramolecular levels. Here, we show that host-guest quadrupolar interaction has a profound influence on the molecular system. With covalently bonded dimolybdenum complex units as the electron donor (D) and acceptor (A) and a thienylene group (C₄H₂S) as the bridge (B), the mixed-valence D-B-A complexes are shaped with clefts in the middle of the molecule. Interestingly, in aromatic solvents, the D-A electronic coupling constants (H_ab) and electron transfer rates (k_et) are dramatically reduced. Theoretical computations indicate that an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C₄H₂S bridge evokes a charge redistribution, which increases the HOMO-LUMO energy gap, intervening in the through-bond electron transfer. These results demonstrate that a supramolecular system is unified underlying the characteristics of the assembled molecules through constitutional, electronic, and energetic complementarities.

Electronic Coupling and Electron Transfer between Two Mo2 Units through meta- and para-Phenylene Bridges

A series of three Mo₂ dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo₂ centers is dramatically weakened; consequently, the ET rates (k_et ) are lowered by two to three orders of magnitude. In the para series, the EC parameters (H_ab ) and ET rates (k_et ) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, H_ab =300-400 cm^-1 and k_et ≈10⁹  s^-1 . These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.

Substituent Effects on the Electrochemistry and Electronic Coupling of Terphenyl-Bridged Cyclometalated Ruthenium-Amine Conjugated Complexes

Six terphenyl-bridged cyclometalated ruthenium-amine conjugated complexes 4(PF6)-9(PF6) were synthesized and studied. Three different substituents, methoxy, methyl, and chloro, were used to vary the electronic nature of the amine unit, and two terminal ligands 2,2':6',2″-terpyridine (tpy) and trimethyl-4,4',4″-tricarboxylate-2,2':6',2″-terpyridine (Me3tctpy) were used to tune the electronic nature of the ruthenium component. All complexes, except 7(PF6) with the methoxy substituent and Me3tctpy ligand, display two well-separated redox waves in the potential range of +0.5 to +1.1 V versus Ag/AgCl. The regular electrochemical changes of these complexes help to establish the oxidation order of ruthenium and amine and hence of the direction of the electron transfer in odd-electron state. The degree of electronic coupling was estimated by analyzing the donor-to-acceptor charge transfer band in the near-infrared region obtained by oxidative spectroelectrochemical measurements. Electron paramagnetic resonance analyses and density functional theory calculations were performed on the one-electron oxidized forms to obtain information on the spin distribution of these complexes.