Rational Synthesis and Characterization of Dimolybdenum(II) Compounds Bearing Ferrocenyl-Containing Ligands toward Modulation of Electronic Coupling

Abstract

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.

Filling a Gap: Electrochemical Property Comparison of the Completed Compound Series [Mo2(DArF)n(O2C-Fc)4–n] (DArF = N,N′-Diarylformamidinate; O2C-Fc = Ferrocenecarboxylate)

“…2a,b both crystallize in a triclinic space group, illustrating their lack of symmetry in comparison to the corresponding bissubstituted complexes. 15 The packing pattern (Figures S1 and S2 16 illustrating the nonlinearity of the two trans-arranged Fc− CO 2…”

“…In summary, the ΔE 16 This illustrates that the electronic donating ability is proportional to ligand basicity in this complex series and decreases in the order DAniF − > DTfmpF − > Fc−CO 2 − ≫ CH 3 CN.…”

“…In theory, six configurations of Mo 2 complexes coordinated by two different bridging ligands are feasible (Figure 1). There are numerous examples of the Tetra, 41 Bis, 15,16,19,40 and Non 43,48 (o-DMophF = N,N′-bis(2-methoxyphenyl)formamidinate) 54 has been reported to date.…”

Synthesis and Electrochemical Properties of cis- and trans-[Mo2(O2C-Fc)2(DArF)2] (O2C-Fc = Ferrocenecarboxylate; DArF = N,N′-Diarylformamidinate)

“…A fluxional behavior of the two Fc units of cis-2a/2b in solution is assumed because only one set of proton signals can be observed, while the Fc arrangement found in the X-ray crystal structures could result in multiple resonances. 15 Although electronically different DArF − ligands are coordinated, the signals of the Cp ring protons are rather close, indicating minor electronic impact on the Fc units. Both the electron-donating capability of DArF − and its influence on the Fc unit will be further discussed as part of electrochemical investigations.…”

“…14 Electrochemical Investigations. In order to shed light on the completely different electrochemical behavior of Mo 2 complexes bearing formamidinate ligands as described here as well as in Cotton's publications 24,27,50,51 and diphosphines as mentioned in a previous study, 15 under the three examination conditions T1, T2, and T3 were intended to give a detailed interpretation with regard to the anionic (DArF − ) and neutral (DPPE) ligand. From Figure 6, it is evident that a similar redox sequence to Cotton's system was obtained for all products, with [Mo 2 ] 4+ preferentially oxidized before Fc.…”