ORBITAL OVERLAP AND ANTIFERROMAGNETIC COUPLING IN SUBSTITUTED TETRAPHENYLPORPHINATOMANGANATE(II) TETRACYANOETHENIDE BASED MAGNETS - THE IMPORTANCE OF SIGMA-D(Z)2-P(Z) OVERLAP

A correlation between the orientation of [TCNE](.-)(TCNE = tetracyanoe thylene) bound to a [Mn-III(por)](+) (por = substituted meso-tetraphen ylporphyrin) and the magnitude of magnetic coupling for a series of pr eviously prepared [Mn(por)][TCNE]. 2PhMe linear-chain molecule-based f errimagnets has been identified. The tetraphenylporphyrin (H2TPP), tet rakis (4-chlorophenyl)porphyrin (H2TClPP), tetrakis(4-methoxyphenyl)po rphyrin (H2TOMePP), tetrakis (2-fluorophenyl)porphyrin (H2TFPP), and t rakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin (H2TP'P) ligands hav e all been utilized. previous structural determinations as the ditolue ne solvates indicate that the Mn-N distance varies similar to 2.6%, wh ile 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 t he [TCNE](.-) pi SOMO and the Mn-III SOMO d orbitals plays a signific ant role in controlling the magnetic properties. From molecular orbita l overlap considerations, as determined from semiempirical INDO/SCF ca lculations, the expected d(pi)-pi (d(pi) = d(pi), d(xz), d(yz)) overl ap is not as important as the sigma-d(z2)-pi overlap between Mn-III a nd the [TCNE](.-). Furthermore, the greater the deviation from 90 degr ees for the dihedral angle between the mean MnN4 [Mn(por)](+) plane an d the [TCNE](.-) mean plane increases the sigma-d(z2)/p(z) overlap bet ween Mn-III and the [TCNE](.-) leading to an increased intrachain coup ling as the angle is decreased. An increase in the T-min (the temperat ure at which the minima in the temperature dependence of the moment oc curs) is observed as this angle becomes more acute, reflecting the str onger magnetic coupling. This suggests systems with smaller Mn(por)(+) /[TCNE](.-) dihedral and Mn-(N-C)(TCNE) bond angles should have enhanc ed intrachain magnetic coupling, J(intra), lending to higher T-c's. Pr essure may force such systems to have reduced angles and also lead to higher T-c's.