Different orbital occupation by an added single electron in 1,10-phenanthroline and its 3,4,7,8-tetramethyl derivative. Evidence from electron paramagnetic resonance spectroscopy of the anion radicals and of their dimesitylplatinum(II) complexes. X-Ray molecular structure of dimesityl(1,10-phenanthroline)platinum(II)

Abstract

In contrast to the anion radicals of 1,10-phenanthroline (phen) or its 4,7-dimethyl derivative (dmphen) with their ²B₁ ground state, the 3,4,7,8-tetramethyl derivative tmphen˙^– has a ²A₂ ground state as evident from EPR/ENDOR measurements. The small π spin population at the potentially metal-coordinating nitrogen centres in the ²A₂ species as opposed to the ²B₁ state causes distinct spectroscopic effects in corresponding diorganoplatinum(II) complexes. The neutral compound (phen)PtMes₂, Mes = mesityl, has been structurally characterized as a square planar Pt^II complex with tightly protected axial sites: monoclinic, space group P2₁/c, Z= 4, a= 9.378(1), b= 15.791(2), c= 16.581(1)Å, β= 103.18(1)°. All complexes (N ⁁ N)PtMes₂, N ⁁ N = bpy (2,2′-bipyridine), phen, tmphen, dppz (dipyrido[3, 2-a : 2′,3′-c]phenazine) and pdo (1,10-phenanthroline-5,6-dione) are oxidized to EPR-silent platinum(III) species and reduced to anion radical complexes. The latter clearly contain Pt^II and heterocyclic anion radical ligands; however, they exhibit a trichotomous EPR behaviour. Whereas the radical complex [(phen)PtMes₂]˙^– shows a rather low g_iso-value of 1.983 and distinct g anistropy in frozen solution like the analogous complex of bpy˙^–, the formally related [(tmphen)PtMes₂]˙^– has an isotropic g-factor (2.0030) much closer to the free-electron value and a distinctly smaller g anisotropy. The phenanthroline-derived radical complexes [(pdo)PtMes₂]˙^– and [(dppz)PtMcs₂]˙^– exhibit virtually no contribution from the coordinated heavy metal to the singly occupied MO (b₁) which is mainly centred at the heteroatoms outside the phenanthroline π system.