Perturbation of the Charge Density between Two Bridged Mo₂ Centers: The Remote Substituent Effects

A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo₂(ArNCHNAr)₃]₂­(μ-O₂CC₆H₄CO₂) (Ar = <i>p</i>-XC₆H₄, with X = OCH₃ (<b>1</b>), CH₃ (<b>2</b>), F (<b>3</b>), Cl (<b>4</b>), OCF₃ (<b>5</b>), and CF₃ (<b>6</b>)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the <i>para</i>-substituents (X) being about 8 Å away from the Mo₂ center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series <b>1</b> to <b>6</b>, the metal–metal bond distances (<i>d</i>_Mo–Mo) decrease slightly and continuously; the potential separations (Δ<i>E</i>_1/2) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λ_max) increase in order. More interestingly, the two types of methine protons, H_∥ on the horizontal and H_⊥ on the vertical ligands with respect to the plane defined by the Mo–Mo bond vectors and bridging ligand, display separate resonant signals δ_∥ and δ_⊥ in the NMR spectra. The displacements of the chemical shifts, Δδ_∥–⊥ = δ_∥ – δ_⊥, are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo₂] units function to fine-tune the metal–metal interactions crossing the bridging ligand. The experimental parameters, Δ<i>E</i>_1/2, λ_max, and Δδ_∥–⊥, which are linearly related with the Hammett constants (σ_X) of the X groups, can be used to probe the charge density on the two [Mo₂] units and the electronic delocalization between them