- Redox Potential Tuning of Dimolybdenum Systems through Systematic Substitution by Guanidinate Ligands
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We report the synthesis and characterization of a series of dimolybdenum paddlewheel complexes of the type Mo2(DAniF)4-n(hpp)n (n = 1-3), where DAniF is the anion of N,N′-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the electronic structure of these tetragonal paddlewheel dimolybdenum compounds was studied upon systematic substitution of formamidinate ligands by the more basic guanidinates. Mo-Mo distances in the paddlewheel structures decreased upon guanidinate ligand substitution, and were found to be 2.0844(6) and 2.0784(6), for Mo2(DAniF)3(hpp) (1) and trans-Mo2(DAniF)2(hpp)2 (2), respectively. Electrochemical studies show that the half-wave potential of the Mo25+/Mo24+ couple shifts cathodically upon ancillary ligand substitution ranging from -0.286 V for the tetraformamidinate complex to -1.795 V for the tetraguanidinate analogue and with redox potentials of -0.75, -1.07, and -1.14 V for 1, 2, and 3 (Mo2(DAniF)(hpp)3), respectively. The presence of a second redox event assigned to the Mo26+/Mo25+ couple was not observed until two guanidinate ligands were introduced. Raman spectroscopy shows that the v(M-M) stretch gets systematically strengthened upon formamidinate ligand substitution by the guanidinate ligand hpp. The induced delta bond destabilization by the basic hpp ligand was measured using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO-LUMO energy gap was supported by the red shift in the UV-vis spectra of the compounds: 412, 442, and 450 nm for 1, 2, and 3, respectively.
- Metta, Nathalie,Metta-Magana, Alejandro J.,Rodríguez-López, Nancy,Villagrán, Dino
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- Modifying Electronic Communication in Dimolybdenum Units by Linkage Isomers of Bridged Oxamidate Dianions
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Reactions of Mo2(O2CCH3)(DAniF) 3, DAniF = N, N′-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr]2-, Ar = C6H5 and p-anisyl, give pairs of isomeric compounds where the [Mo2] units are bridged by the oxamidate anions. For the α isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the β isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the ΔE1/2 for the oxidation of each of the Mo2 units is significantly better for the β isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the β isomers but not the α isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the α isomers and the relative energy differences between the α and β isomers.
- Cotton, F. Albert,Liu, Chun Y.,Murillo, Carlos A.,Villagran, Dino,Wang, Xiaoping
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p. 13564 - 13575
(2007/10/03)
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