Solid-state 25Mg NMR, X-ray crystallographic, and quantum mechanical study of bis(pyridine)-(5,10,15,20-tetraphenylporphyrinato)magnesium(II)

We report solid-state Mg-25 NMR, X-ray crystallographic, and quantum-mechanical calculation results for bis(pyridine)(5,10,15,20-tetraphenylporphyrinato)magnesium(II), Mg(TPP).Py-2. Mg(TPP).Py-2 crystallizes in the triclinic form, in the space group P (1) over bar. The unit cell parameters are: a = 9.6139(13) Angstrom, b = 11.0096(16) Angstrom, c = 11.8656(15) Angstrom; alpha = 102.063(3)degrees, beta = 103.785(3)degrees, gamma = 114.043(2)degrees; Z = 1. The Mg(II) ion is coordinated to four nitrogen atoms from the porphyrin ring and two nitrogen atoms from the axial pyridine ligands, forming a regular octahedron. The Mg-25 quadrupole coupling constant (C-Q) is 15.32 +/- 0.02 MHz, which represents the largest value so far observed for Mg-25 nuclei. The electric field gradient tensor at the Mg site is axially symmetric, eta(Q) = 0.00 +/- 0.05. The Mg-25 chemical shielding anisotropy is too small to be accurately determined. Quantum-mechanical calculations using a 6-31G(d) basis set reproduce reasonably well the observed Mg-25 NMR data for Mg(TPP).Py-2. The calculations also suggest that the span of the Mg-25 chemical shift tensor is less than 50 ppm. Using a theoretical approach, we also investigate the dependence of the Mg-25 quadrupole coupling constant on the Mg-N-ax bond distance. The calculation suggests that the Mg-25 quadrupole coupling constant for an Mg(II) ion at the center of a porphyrin ring without axial ligands is approximately 22 MHz, which may be treated as an upper limit of the Mg-25 quadrupole coupling constant for all Mg-porphy rin complexes.