Synthesis, Proton NMR Spectroscopy, and Structural Characterization of Binuclear Ruthenium Porphyrin Dimers

The binuclear Ru(II) porphyrin dimers (Ru(OEP))₂and (Ru(TPP))₂(OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato; TPP = 5,10,15,20-tetraphenylporphyrinato) have been synthesized by the vacuum pyrolysis of the mononuclear bis(pyridine) complexes Ru(OEP)(py)₂and Ru(TPP)(py)₂(py = pyridine). A detailed analysis of the paramagnetic shifts observed in the ¹H NMR spectra is presented, and a new model for referencing dipolar shifts in a dimeric porphyrin structure type is described. The calculated contact shifts exhibit π symmetry and indicate that the dominant mode of spin transfer into the porphyrin ring is derived from P 3e(π) → Ru charge transfer. In addition, the crystal structure of the OEP dimer is reported. (Ru(OEP))₂crystallizes as the dipentane solvate in space group C_2h ⁶-C2/c with Z = 8 in a unit cell of dimensions a = 27.989 (11) Å, b = 27.255 (12) Å, c = 17.737 (8) Å, and β = 102.50 (2)°. The final agreement indices, based on 767 variables and 9806 unique intensities collected at -103 °C on an automatic diffractometer, are R(F²) = 0.066 and R_w(F²) = 0.124. The conventional R index on F for 7304 reflections having F²> 3σ (F_o ²) is 0.042. The distance between the Ru atoms is 2.408 (1) Å and the mean Ru-N distance is 2.050 (5) Å. Each Ru atom is situated 0.30 Å out of a plane defined by four coordinating N atoms in the direction of the other Ru atom. The two porphyrin macrocycles are twisted 23.8 (1)° with respect to each other. Both porphyrinato cores have domed-type distortions with average deviations from the least-squares planes of 0.07 and 0.10 Å and maximum deviations of 0.19 and 0.24Å, respectively. The X-ray and NMR results are consistent with a qualitative MO diagram that suggests a formal Ru-Ru bond order of 2.