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

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posted on 02.09.2014, 00:00 by Tao Cheng, Miao Meng, Hao Lei, Chun Y. Liu
A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo2(ArNCHNAr)3]2­(μ-O2CC6H4CO2) (Ar = p-XC6H4, with X = OCH3 (1), CH3 (2), F (3), Cl (4), OCF3 (5), and CF3 (6)), 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 para-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 1 to 6, the metal–metal bond distances (dMo–Mo) decrease slightly and continuously; the potential separations (ΔE1/2) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λmax) 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, ΔE1/2, λmax, and Δδ∥–⊥, which are linearly related with the Hammett constants (σX) of the X groups, can be used to probe the charge density on the two [Mo2] units and the electronic delocalization between them.