title: Magnetic interactions in dinuclear Mn(III)Mn(IV) complexes covalently tethered to organic radicals: spectroscopic models for the S(2)Y(z)(*) state of photosystem II.
abstract: A series of isostructural dimeric manganese complexes of the type [(Me(4)dtne)Mn(2)(mu-O)(2)(mu-R)](2+)(X(-))(2) have been prepared and characterized. The dimanganese cores of these complexes are rigidly held together by the hexadentate ligand Me(4)dtne (Me(4)dtne = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane). Molecular structures for the entire series have been obtained by X-ray diffraction measurements, of which complexes 2 (R = (-)O(2)BPh), 3 (R = (-)O(2)C-PROXYL), 4 (R = (-)O(2)C-TEMPO), and 5 (R = (-)O(2)BPhNIT) are reported here (HO(2)C-PROXYL = 3-carboxy-2,2,5,5-tetramethylpyrrolidin-1-yloxy; HO(2)C-TEMPO = 4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxy; and HO(2)BPhNIT = 2-(4-(dihydroxyboranyl)-phenyl)-4,4,5,5-tetramethyl-3-oxyimidazolidin-1-oxide). The structures of 1 (R = (-)OAc) and 6 (R = (-)O(2)CPhNIT) have been reported previously (HO(2)CPhNIT = 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-3-oxyimidazolidin-1-oxide). All complexes exhibit several redox states, which have been investigated by electrochemistry. Complexes 1, 3, 4, and 6 contain a mixed-valent Mn(III)Mn(IV) core with an isolated magnetic ground state of S = 1/2. The exchange coupling between the manganese ions is strong throughout the series (J approximately -130 +/- 10 cm(-)(1), H = -2JS(1)S(2)). The radical complexes 3, 4, and 6 exhibit, in addition, long-range exchange interaction (6.9, 7.7, and 8.8 A, respectively) between the organic radical and the dimanganese core. The intramolecular anisotropic coupling was determined from cw-EPR line shape analyses at S-, X-, and Q-band frequencies and from the intensity of half-field signals detected in normal- and parallel-mode (J(d,)(z)() = -120 x 10(-)(4), -105 x 10(-)(4), and -140 x 10(-)(4) cm(-)(1), for 3, 4, and 6 respectively). Distance information was obtained for the dimanganese core and the organic radicals from these values by using a three-spin dipole model and local spin contributions for the manganese ions.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Dana S
authors.initials: DS
authors.lastname: Marlin
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Eberhard
authors.initials: E
authors.lastname: Bothe
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
mesh_terms: D016880:Anisotropy; D040761:Biomimetic Materials / Q000737:chemistry; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D008280:Magnetics; D008345:Manganese / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D005609:Free Radicals; D009942:Organometallic Compounds; D045332:Photosystem II Protein Complex; D008345:Manganese