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)(4)Rh-2(C=C)(2)Rh-2(ap)(4) (2) and (dpf)(4)Rh-2(CNC6H4NC)Rh-2(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)(4)Rh-2(C=C)(2)Si(CH3)(3) (1) and (dpf)(4)Rh-2(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)(4)Rh2Cl and Li(C=C)(2)Si(CH3)(3) in refluxing THF for 16 h under vacuum while compound 2 was prepared by a reaction between (ap)(4)Rh-2(C=C)(2)Li and (ap)(4)Rh2Cl under similar conditions. The reaction between (CF3COO)(4)Rh-2 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)(4)Rh-2 which was synthesized according to a method described previously in the literature. Compound 1 crystallizes in the triclinic space group P (1) over bar, with a 10.164(3) Angstrom, b = 13.881(3) Angstrom, c 18.805(4) Angstrom, alpha = 73.55(2)degrees, beta = 77.89(2)degrees, gamma = 84.85(2)degrees, 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 PI space group. Crystals of 3 and 4 belong to the monoclinic, P2(1)/c space group and the triclinic, P (1) over bar space group, respectively, with a = 13.5254(5) Angstrom, b = 13.7387(4) Angstrom, c = 27.2011(12) Angstrom, beta = 102.637(2)degrees, and Z = 4 for 3 and a 13.866(8) Angstrom, b = 14.756(7) Angstrom, c = 15.008(6) Angstrom, alpha = 79.91(3)degrees, beta = 87.72(4)degrees, gamma = 89.19(4)degrees, 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 (sigma)(2)(pi)(4)(delta)(2)(pi*)(4)(delta*)(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.