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 25Mg NMR, X-ray crystallographic, and quantum-mechanical calculation results for bis(pyridine)(5,10,15,20-tetraphenylporphyrinato)magnesium(II), Mg(TPP)·Py2. Mg(TPP)·Py2 crystallizes in the triclinic form, in the space group P1. The unit cell parameters are: a = 9.6139(13) Å, b = 11.0096(16) Å, c = 11.8656(15) Å; α = 102.063(3)°, β = 103.785(3)°, γ = 114.043(2)°; 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 25Mg quadrupole coupling constant (CQ) is 15.32 ± 0.02 MHz, which represents the largest value so far observed for 25Mg nuclei. The electric field gradient tensor at the Mg site is axially symmetric, ηQ = 0.00 ± 0.05. The 25Mg 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 25Mg NMR data for Mg(TPP)·Py2. The calculations also suggest that the span of the 25Mg chemical shift tensor is less than 50 ppm. Using a theoretical approach, we also investigate the dependence of the 25Mg quadrupole coupling constant on the Mg—Nax bond distance. The calculation suggests that the 25Mg 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 25Mg quadrupole coupling constant for all Mg–porphyrin complexes.