Structural characterization of the picket fence (TpivPP) porphyrins Co(TpivPP), Co(TpivPP)(NO₂)(1-MeIm), and Co(TpivPP)(NO₂)(1,2-Me₂Im)

The compounds Co(TpivPP) (1), Co(TpivPP)(NO₂)(1-MeIm) (2), and Co(TpivPP)(NO₂)(1,2-Me₂Im) (3) have been synthesized (TpivPP = meso-tetrakis(α,α,α,α-o-pivalamidophenyl)porphyrinato dianion), and their structures have been determined with single-crystal X-ray diffraction methods. 1: a = 17.578(1) Å, b = 17.596(1) Å, c = 20.639(1) Å, β = 115.03(1)°, P2₁/c, Z = 4, T = -120 °C. 2: a = 18.522(4) Å, b = 18.942(4) Å, c = 18.177(4) Å, β = 90.68(3)°, C2/c, Z = 4, T = -70 °C. 3: a = 18.998(4) Å, b = 19.187(4) Å, c = 18.000(4) Å, β = 90.96(3)°, C2/c, Z = 4, T = -120 °C. Compounds 2 and 3 have crystallographically imposed 2-fold axes. In 2 and 3, which represent R-state (relaxed) and T-state (tense) models, respectively, for hemoglobin, the NO₂ ligand is bound on the 'picket' side to the Co atom, and either 1-MeIm (for 2) or 1,2-Me₂Im (for 3) is bound to the Co atom at the sixth coordination site on the sterically unhindered side of the molecule. The average deviations of atoms from the 24-atom porphyrin core are 0.031, 0.129, and 0.117 Å for 1, 2, and 3, respectively. The Co atom is -0.043(1) Å out of the mean 24-atom porphyrin plane toward the 1-MeIm ligand in 2 and -0.089(1) Å out of the plane toward the 1,2-Me₂Im ligand in 3. The bonds of both axial ligands in the R-state model 2, 1.898(4) Å for Co-N(O₂) and 1.995(4) Å for Co-N(base), are shorter than the corresponding bonds in the T-state model 3, 1.917(4) Å for Co-N(O₂) and 2.091(4) Å for Co-N(base).