Effect of Axial Ligands on the Oxidation State, Structure, and Electronic Configuration of Diruthenium Complexes. Synthesis and Characterization of Ru2(dpf)4Cl, Ru2(dpf)4(C⋮CC6H5), Ru2(dpf)4(C⋮CC6H5)2, and Ru2(dpf)4(CN)2 (dpf = N,N‘-Diphenylformamidinate)

The effect of axial ligands on the structure, oxidation state, and electronic configuration of two diruthenium(II,III) and two diruthenium(III,III) complexes is reported. The investigated compounds are represented by Ru 2 (dpf) 4 Cl ( 1 ), Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) ( 2 ), Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) 2 ( 3 ), and Ru 2 (dpf) 4 (CN) 2 ( 4 ), where dpf is the N, N ‘-diphenylformamidinate ion. The chloro and mono(phenylacetylide) complexes, which contain a Ru 2 5+ core, are paramagnetic and contain three unpaired electrons, while the bis(phenylacetylide) and bis(cyano) species, which contain a Ru 2 6+ core, are diamagnetic. Compound 1, Ru 2 (dpf) 4 Cl, was prepared by heating Ru 2 (CH 3 COO) 4 Cl and excess molten N, N ‘-diphenylformamidine (Hdpf) for 10 h at 130 °C under an argon atmosphere. Compound 2, Ru 2 (dpf) 4 (C⋮CC 6 H 5 ), was prepared by reaction of purified Ru 2 (dpf) 4 Cl with LiC⋮CC 6 H 5 (in a 1:5 ratio) in THF at room temperature (2 h). Compound 3, Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) 2, was also obtained from Ru 2 (dpf) 4 Cl by using excess LiC⋮CC 6 H 5 . A related diruthenium(III,III) complex, Ru 2 (dpf) 4 (CN) 2 ( 4 ), was also synthesized in order to determine whether the observed octahedral distortion in 3 was due to electronic or steric effects. Ru 2 (dpf) 4 (CN) 2 was prepared by reaction of Ru 2 (dpf) 4 Cl and a large excess of NaCN in THF at room temperature (10 h). All four compounds were isolated and characterized by single-crystal X-ray diffraction, 1 H NMR, 13 C NMR, IR, and UV−visible spectroscopy, and mass spectrometry, as well as by electrochemistry. Compound 1 crystallizes in the tetragonal space group P 4/ ncc, with a = 14.837(2) Å, c = 21.599(5) Å, and Z = 4, while compound 2 belongs to the monoclinic space group I 2/ c, with a = 18.590(5) Å, b = 30.913(12) Å, c = 17.360(4) Å, β = 93.61(2)°, and Z = 8. The magnetic moments of the two diruthenium(II,III) species, 1 and 2, are 3.89 and 3.87 μ B, respectively, in the solid state at 297 K, consistent with three unpaired electrons and an electronic configuration of (σ) 2 (π) 4 (δ) 2 (δ*π*) 3 . Compounds 3 and 4 are diamagnetic in their neutral form but paramagnetic upon controlled-potential reduction by one electron. The two reduced Ru II Ru III species, [Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) 2 ] - and [Ru 2 (dpf) 4 (CN) 2 ] -, were characterized by ESR spectroscopy after electrochemical generation and display a rhombic signal whose shape is consistent with the presence of only a single unpaired electron. Compound 2, which contains three unpaired electrons, gives a bis adduct Ru 2 (dpf) 4 (C⋮CC 6 H 5 )(CNCH 2 C 6 H 5 ) upon addition of excess CNCH 2 C 6 H 5 to the solution of Ru 2 (dpf) 4 (C⋮CC 6 H 5 ). The bis adduct species has only one single unpaired electron and shows an ESR spectrum similar to that of electroreduced [Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) 2 ] - and [Ru 2 (dpf) 4 (CN) 2 ] - . Crystals of 3 and 4 belong to the monoclinic space groups C 2/ c and P 2 1, with a = 35.322(7) Å, b = 15.163(3) Å, c = 31.820(9) Å, β = 93.59(2)°, and Z = 12 (compound 3 ) and with a = 16.184(4) Å, b = 15.784(3) Å, c = 12.896(3) Å, β = 112.29 (2)°, and Z = 2 (compound 4 ). The Ru−Ru bond distances of 1 and 2 are in the expected range for diruthenium(II,III) complexes, i.e., 2.339(1) Å for compound 1 and 2.400(1) Å for compound 2 . Compounds 3 and 4, however, have Ru−Ru bond distances which are much longer than anticipated for diruthenium(III,III) complexes with the same type of structure. The measured values are 2.556(1) Å for 3 and 2.539(1) Å for 4 . All four compounds can undergo conversions between Ru 2 6+, Ru 2 5+, and Ru 2 4+ while compound 3, Ru 2 (dpf) 4 (C⋮CC 6 H 5 ) 2, also undergoes a Ru 2 7+ /Ru 2 6+ redox process. Values of E 1/2 range between +0.54 and −0.61 V for the Ru 2 6+ /Ru 2 5+ process and between −0.64 and −1.54 V for the Ru 2 5+ /Ru 2 4+ one.