Synthesis, Electrochemistry, and Spectroscopic Characterization of Bis-dirhodium Complexes Linked by Axial Ligands

The synthesis and electrochemical and spectroscopic properties of bis-dirhodium complexes containing ap or dpf bridging ligands, (ap)₄Rh₂(C⋮C)₂Rh₂(ap)₄ (2) and (dpf)₄Rh₂(CNC₆H₄NC)Rh₂(dpf)₄ (4), were investigated (where ap and dpf are the 2-anilinopyridinate and N,N‘-diphenylformamidinate ions, respectively). The related “simple” dirhodium species, (ap)₄Rh₂(C⋮C)₂Si(CH₃)₃ (1) and (dpf)₄Rh₂(CNC₆H₅) (3), with the same set of bridging ligands were also synthesized and their properties compared to those of the analogous bis-dirhodium complexes. Compound 1 was obtained by mixing (ap)₄Rh₂Cl and Li(C⋮C)₂Si(CH₃)₃ in refluxing THF for 16 h under vacuum while compound 2 was prepared by a reaction between (ap)₄Rh₂(C⋮C)₂Li and (ap)₄Rh₂Cl under similar conditions. The reaction between (CF₃COO)₄Rh₂ and molten Hdpf under vacuum for 24 h leads to the generation of compound 3 with a yield of 65%. The red-orange compound 4 was obtained upon addition of 0.5 equiv of CNC₆H₄NC at room temperature to a CH₂Cl₂ solution containing (dpf)₄Rh₂ which was synthesized according to a method described previously in the literature. Compound 1 crystallizes in the triclinic space group P1̄, with a = 10.164(3) Å, b = 13.881(3) Å, c = 18.805(4) Å, α = 73.55(2)°, β = 77.89(2)°, γ = 84.85(2)°, and Z = 2. Crystals of 2 were not good enough to collect adequate data for X-ray analysis, but the identity of this compound was confirmed, along with its P1̄ space group. Crystals of 3 and 4 belong to the monoclinic, P2₁/c space group and the triclinic, P1̄ space group, respectively, with a = 13.5254(5) Å, b = 13.7387(4) Å, c = 27.2011(12) Å, β = 102.637(2)°, and Z = 4 for 3 and a = 13.866(8) Å, b = 14.756(7) Å, c = 15.008(6) Å, α = 79.91(3)°, β = 87.72(4)°, γ = 89.19(4)°, and Z = 1 for 4. Compound 1 exhibits a single reversible oxidation at E_1/2 = 0.66 V and a single reversible reduction at E_1/2 = −0.44 V vs SCE in THF, 0.2 M TBAP. Both processes involve a one-electron transfer. Compound 2 undergoes a reversible oxidation at E_1/2 = 0.60 V and two separate one-electron-transfer reductions at E_1/2 = −0.52 and −0.65 V in THF, 0.2 M TBAP. The oxidation involves two overlapped one-electron-transfer processes. Compounds 3 and 4 undergo two reversible oxidations in CH₂Cl₂, 0.1 M TBAP located at E_1/2 = 0.23 and 1.22 V (3) or 0.22 and 1.20 V (4). Each redox reaction of 3 involves a one-electron-transfer step while each redox reaction of 4 involves two overlapping one-electron transfers. Compound 2 shows interaction between the two dirhodium cores upon reduction, while 4 gives no evidence of electronic interaction between the two dirhodium units during either reduction or oxidation. An ESR signal with axial symmetry was obtained for the neutral compounds 1 and 2, and a similar spectrum was obtained for the singly oxidized products of compounds 3 and 4, thus suggesting the electronic configuration of (σ)²(π)⁴(δ)²(π*)⁴(δ*)¹ for the neutral compounds 1 and 2 as well as for the oxidized compounds 3 and 4. The four compounds were also characterized by FTIR and UV−visible spectroscopy as well as by mass spectrometry.