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

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posted on 2001-05-25, 00:00 authored by Orde Q. Munro, Sibongiseni C. Shabalala, Nicola J. Brown
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−Np distance for the four structures is 1.986(1) Å. The Co−Nax 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 D2d-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-33d 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.