Gold(II) Porphyrins are Inherently Waved

Relativistic DFT (OLYP-D3/ZORA-STO-TZ2P) calculations predict low adiabatic ionization potentials for gold(II) porphyrins, from 4.60 eV for Au[TPP] (TPP = tetraphenylporphyrin) to 5.34 eV for Au[TPFPP] [TPFPP = tetrakis(pentafluorophenyl)porphyrin]. These values are over 1 eV lower than those calculated for analogous silver(II) porphyrins, reflecting much greater relativistic destabilization of the Au 5d orbitals relative to Ag 4d orbitals. The calculations reproduced the experimentally observed lateral distortion of the Au porphyrin core, ascribing the phenomenon to dx2-y2-dz2-6s mixing. The calculations also underscored a mild wave deformation as an intrinsic feature of the Au porphyrin core. Although previously noted, the origin of this unusual deformation has remained obscure. The topology of the HOMO and spin density profile appear to provide at least a partial explanation of the wave deformation: the electronegative Au(II) center appears to suck electron density from the porphyrin’s p-HOMO by triggering a wave deformation. The heretofore unsuspected orbital interaction results in significant spin populations on the porphyrin, leaving behind only about 20-25% of the spin density on the gold. In contrast, the analogous silver porphyrins exhibit strictly planar cores with approximate D4h local symmetry at the metal.