Structural, computational, and (59)Co NMR studies of primary and secondary amine complexes of Co(III) porphyrins.

Four novel low-spin bis(amine) Co(III) porphyrins Co(TPP)(BzNH(2))(2), 1, Co(TPP)(1-BuNH(2))(2), 2, Co(TPP)(PhCH(2)CH(2)NH(2))(2), 3, and Co(TPP)(1-MePipz)(2), 4, have been synthesized and characterized by low-temperature X-ray crystallography, IR, electronic, and NMR ((1)H, (13)C, and (59)Co) spectroscopy. The mean Co-N(p) distance for the four structures is 1.986(1) A. The Co-N(ax) distances for the 1 degrees amine derivatives average to 1.980(5) A; the axial bonds of the 2 degrees amine derivative are significantly longer, averaging 2.040(1) A. The porphyrin core conformation of 4 is significantly nonplanar (mixture of S(4)-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 degrees. The X-ray structures have been used with the coordinates for Co(TPP)(Pip)(2) (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 degrees amine) complexes are significantly more flexible than the bis(2 degrees amine) analogues. (59)Co NMR spectra have been acquired for a range of [Co(TPP)(amine)(2)]Cl derivatives as a function of temperature. The (59)Co chemical shifts increase linearly with increasing temperature due to population of thermally excited vibrational levels of the (1)A(1) ground state. Activation energies for molecular reorientation (tumbling) have been determined from an analysis of the (59)Co NMR line widths as a function of 1/T; lower barriers exist for the conformationally rigid 2 degrees amine derivatives (2.6-3.8 kJ mol(-1)). The (59)Co 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 sigma-donor strengths of the axial ligands: BzNH(2) > or = Cl(-) > 1-BuNH(2) > PhCH(2)CH(2)NH(2) > 1-Bu(2)NH > Et(2)NH. The (59)Co 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 (59)Co NMR spectra of Co(III) porphyrins.