Rational synthesis and characterization of dimolybdenum(II) compounds bearing ferrocenyl-containing ligands toward modulation of electronic coupling.

Three novel cis-to-trans-converted dimolybdenum(II) complexes, trans-[Mo2(O2C-Fc)2(DPPX)2][BF4]2 {2a-2c; DPPX = DPPA [N,N-bis(diphenylphosphino)amine], DPPM [1,1-bis(diphenylphosphino)methane], and DPPE [1,2-bis(diphenylphosphino)ethane], respectively}, were synthesized through the insertion of bulky diphosphine ligands, which force a permanent trans arrangement, as evidenced by X-ray crystallography and density functional theory calculations. All compounds were characterized by means of NMR, UV-vis, and IR spectroscopy as well as thermogravimetry-mass spectrometry measurements. Interestingly, uncommon UV-vis transitions and oxidation sequences were observed compared to previously reported ones. As verified by electrochemical measurements, all synthesized complexes show two separate one-electron-redox processes assigned to subsequent oxidations of the two redox-active ferrocenecarboxylate ligands, with a split of ca. 70 mV. This behavior reveals electronic interaction between the two equatorially trans-positioned ferrocenyl units. The presented work provides new insights into the rational synthesis of electronically coupled trans-coordinated Mo2 systems, paving the way toward the design of linear multicenter redox-active oligomers.