NMR and EPR Spectroscopic and Structural Studies of Low-Spin, (d_xz,d_y_z)⁴(d_x_y)¹ Ground State Fe(III) Bis-<i>tert</i>-Butylisocyanide Complexes of Dodecasubstituted Porphyrins^†

The bis-(1,1-dimethylethylisocyanide) (tert-butylisocyanide) complexes of three iron porphyrinates (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, OETPP; 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin, OMTPP; and 2,3,7,8,12,13,17,18-tetra-β,β‘-tetramethylene-5,10,15,20-tetraphenylporphyrin, TC6TPP) have been prepared and studied by EPR and 1H NMR spectroscopy. From EPR and NMR spectroscopic results it has been found that the ground states of the bis-(t-BuNC) complexes of OETPP, OMTPP, and TC6TPP are represented mainly (99.1−99.4%) as (dxz,dyz)4(dxy)1 electron configurations, with an excited state lying 700 cm-1 to higher energy for the OMTPP complex, and probably at lower and higher energies, respectively, for the OETPP and TC6TPP complexes. In the 1H NMR spectra the (dxz,dyz)4(dxy)1 electron configurations of all three complexes are indicated by the large and positive meso-phenyl-H shift differences, δm−δo and δm−δp, and close to the diamagnetic shifts of groups (CH3 or CH2) directly attached to the β-carbons. However, in comparison to meso-only substituted porphyrinates such as [FeTPP(t-BuNC)2]ClO4, the meso-phenyl shift differences are much smaller, especially for the OETPP complex. 2D NOESY spectra show that the flexibility of the porphyrin core decreases with increasing nonplanar distortion in the order TC6TPP > OMTPP > OETPP and in the same order the stability of the binding to t-BuNC ligands decreases. In addition, the structures of two crystalline forms of [FeOMTPP(t-BuNC)2]ClO4 have been determined by X-ray crystallography. Both structures showed purely saddled porphyrin cores and somewhat off-axis binding of the isocyanide ligands. To our knowledge, this is the first example of a porphyrin complex with a purely saddled conformation that adopts the (dxz,dyz)4(dxy)1 ground state. All structurally-characterized complexes of this electron configuration reported previously are ruffled. Therefore, we conclude that a ruffled geometry stabilizes the (dxz,dyz)4(dxy)1 ground state, but is not necessary for its existence.