Orbital Overlap and Antiferromagnetic Coupling in Substituted Tetraphenylporphinatomanganate(III) Tetracyanoethenide Based Magnets. The Importance of σ−dz2−pz Overlap

A correlation between the orientation of [TCNE]•- (TCNE = tetracyanoethylene) bound to a [MnIII(por)]+ (por = substituted meso-tetraphenylporphyrin) and the magnitude of magnetic coupling for a series of previously prepared [Mn(por)][TCNE]·2PhMe linear-chain molecule-based ferrimagnets has been identified. The tetraphenylporphyrin (H2TPP), tetrakis(4-chlorophenyl)porphyrin (H2TClPP), tetrakis(4-methoxyphenyl)porphyrin (H2TOMePP), tetrakis(2-fluorophenyl)porphyrin (H2TFPP), and tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin (H2TP‘P) ligands have all been utilized. Previous structural determinations as the ditoluene solvates indicate that the Mn−N distance varies ∼2.6%, while the Mn−(N−C)TCNE bond angle varies by as much as 25.2% from the mean values over this series of compounds. Hence the overlap between the [TCNE]•- π* SOMO and the MnIII SOMO d orbitals plays a significant role in controlling the magnetic properties. From molecular orbital overlap considerations, as determined from semiempirical INDO/SCF calculations, the expected dπ−π* (dπ = dxz, dyz) overlap is not as important as the σ-dz2−π* overlap between MnIII and the [TCNE]•-. Furthermore, the greater the deviation from 90° for the dihedral angle between the mean MnN4 [Mn(por)]+ plane and the [TCNE]•- mean plane increases the σ−dz2/pz overlap between MnIII and the [TCNE]•- leading to an increased intrachain coupling as the angle is decreased. An increase in the Tmin (the temperature at which the minima in the temperature dependence of the moment occurs) is observed as this angle becomes more acute, reflecting the stronger magnetic coupling. This suggests systems with smaller Mn(por)+/[TCNE]•- dihedral and Mn−(N−C)TCNE bond angles should have enhanced intrachain magnetic coupling, Jintra, leading to higher Tc's. Pressure may force such systems to have reduced angles and also lead to higher Tc's.