Structural, Computational, and ⁵⁹Co NMR Studies of Primary and Secondary Amine Complexes of Co(III) Porphyrins

Four novel low-spin bis(amine) Co(III) porphyrins [Co(TPP)(BzNH₂)₂](SbF₆), 1, [Co(TPP)(1-BuNH₂)₂](SbF₆), 2, [Co(TPP)(PhCH₂CH₂NH₂)₂](SbF₆), 3, and [Co(TPP)(1-MePipz)₂](SbF₆), 4, have been synthesized and characterized by low-temperature X-ray crystallography, IR, electronic, and NMR (¹H, ¹³C, and ⁵⁹Co) spectroscopy. The mean Co−N_p distance for the four structures is 1.986(1) Å. The Co−N_ax distances for the 1° amine derivatives average to 1.980(5) Å; the axial bonds of the 2° amine derivative are significantly longer, averaging 2.040(1) Å. The porphyrin core conformation of 4 is significantly nonplanar (mixture of S₄-ruf and D_2d-sad distortions) due to a staggered arrangement of the axial ligands over the porphyrin core and meso-phenyl group orientations < 90°. The X-ray structures have been used with the coordinates for [Co(TPP)(Pip)₂](NO₃) (Scheidt et al. J. Am. Chem. Soc. 1973, 95, 8289−8294.) to parametrize a molecular mechanics (MM) force field for bis(amine) complexes of Co(III) porphyrins. The calculations show that two types of crystal packing interactions (van der Waals and hydrogen bonding) largely control the crystallographically observed conformations. Gas phase conformational energy surfaces have been computed for these complexes by dihedral angle driving methods and augmented with population distributions calculated by MD simulations at 298 K; the calculations demonstrate that the bis(1° amine) complexes are significantly more flexible than the bis(2° amine) analogues. ⁵⁹Co NMR spectra have been acquired for a range of [Co(TPP)(amine)₂]Cl derivatives as a function of temperature. The ⁵⁹Co chemical shifts increase linearly with increasing temperature due to population of thermally excited vibrational levels of the ¹A₁ ground state. Activation energies for molecular reorientation (tumbling) have been determined from an analysis of the ⁵⁹Co NMR line widths as a function of 1/T; lower barriers exist for the conformationally rigid 2° amine derivatives (2.6−3.8 kJ mol^-1). The ⁵⁹Co chemical shifts vary linearly with the DFT-calculated radial expectation values 〈r^-3〉_3d for the Co(III) ion. The correlation leads to the following order for the σ-donor strengths of the axial ligands:  BzNH₂ ≥ Cl^- > 1-BuNH₂ > PhCH₂CH₂NH₂ > 1-Bu₂NH > Et₂NH. The ⁵⁹Co NMR line widths are proportional to the square of the DFT-calculated valence electric field gradient at the Co nucleus. Importantly, this is the first computational rationalization of the ⁵⁹Co NMR spectra of Co(III) porphyrins.