ChemInform Abstract: Spin Coupling in Metalloporphyrin π‐Cation Radicals.

Abstract

048ChemInform Abstract The magnetic interactions between metal and ligand spins in metalloporphyrin π-cation radicals (I) are investigated as a function of d orbital type for synthetically accessible compounds containing S = 1/2 metals from the first transition series. The preparation of (Ia)-(Id) from the corresponding neutral complex by oxidation with phenoxathiinylium hexachloroantimonate is described. Extensive studies are carried out on the presumably planar compound (Ib) with an overall S = 1 state. The ferromagnetic coupling of spins is rationalized as arising from the exchange interaction of unpaired electrons in orthogonal magnetic orbitals. By contrast, (Ia), crystallizing in the space group P1 with Z=2, has a ruffled porphyrin core in the solid state and is diamagnetic (S = 0). The antiferromagnetic coupling is rationalized in terms of overlap of the magnetic orbitals, i.e. bond formation with spin pairing in the bonding MO. Applications to analogous systems of other metals are discussed.

Interactions of Metal-Based and Ligand-Based Electronic Spins in Neutral Tripyrrindione π Dimers

“…This finding is consistent with the SQUID magnetometry data reported by Bröring and coworkers, 35 as well as with the expected value for a Cu(II) complex featuring a ferromagnetically coupled ligand-based radical ( S = 1, without significant population of the singlet state). 36, 40 In this complex, a metal-based unpaired electron formally resides in the d x2−y2 orbital of the Cu(II) center, which is orthogonal to the π orbitals of the tripyrrindione scaffold hosting the ligand-based electronic spin. This results in a ferromagnetic interaction between the unpaired electrons and in stabilization of the triplet state, as previously observed in Cu(II) complexes featuring planar radical ligands, 40–44 including porphyrin cation radicals, 36, 45 in which the magnetic orbitals are orthogonal.…”

“…Whereas π dimers of metalloporphyrin π-cation radicals in solution are well documented, 36–38 the tripyrrindione complexes offer the first opportunity to observe the dimerization of ligand-based radicals in neutral complexes of linear oligopyrroles. Unlike porphyrins and other tetrapyrrolic macrocycles, tridentate oligopyrroles allow access to the metal center in the plane of the ligand and are therefore more amenable to major modifications, a significant advantage for functional tailoring of properties.…”

“…36,40 In this complex, a metal-based unpaired electron formally resides in the d x 2 −y 2 orbital of the Cu(II) center, which is orthogonal to the π orbitals of the tripyrrindione scaffold hosting the ligand-based electronic spin. This results in a ferromagnetic interaction between the unpaired electrons and in stabilization of the triplet state, as previously observed in Cu(II) complexes featuring planar radical ligands, 40−44 including porphyrin cation radicals, 36,45 in which the magnetic orbitals are orthogonal.…”

Modulation of the molecular spintronic properties of adsorbed copper corroles

“…For all three complexes, the geometry of the singlet ground state is predicted to be significantly more saddled than the triplet states in a manner that is consistent with Kahn's concept of orthogonal magnetic orbitals 38 . The concept of orthogonal orbitals can be readily applied to rationalize the magnetic coupling of metal porphyrin π–cation radical complexes 39 40 . The theory is generally based on the symmetry of the orbitals on the metal and the ligand that contain unpaired electrons.…”