Magnetic Interactions in Dinuclear Mn^IIIMn^IV Complexes Covalently Tethered to Organic Radicals: Spectroscopic Models for the S₂Y<i>_z</i>^• State of Photosystem II

A series of isostructural dimeric manganese complexes of the type [(Me4dtne)Mn2(μ-O)2(μ-R)]2+(X-)2 have been prepared and characterized. The dimanganese cores of these complexes are rigidly held together by the hexadentate ligand Me4dtne (Me4dtne = 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 = -O2BPh), 3 (R = -O2C-PROXYL), 4 (R = -O2C-TEMPO), and 5 (R = -O2BPhNIT) are reported here (HO2C-PROXYL = 3-carboxy-2,2,5,5-tetramethylpyrrolidin-1-yloxy; HO2C-TEMPO = 4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxy; and HO2BPhNIT = 2-(4-(dihydroxyboranyl)-phenyl)-4,4,5,5-tetramethyl-3-oxyimidazolidin-1-oxide). The structures of 1 (R = -OAc) and 6 (R = -O2CPhNIT) have been reported previously (HO2CPhNIT = 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 MnIIIMnIV core with an isolated magnetic ground state of S = 1/2. The exchange coupling between the manganese ions is strong throughout the series (J ≈ −130 ± 10 cm-1, H = −2JS1S2). The radical complexes 3, 4, and 6 exhibit, in addition, long-range exchange interaction (6.9, 7.7, and 8.8 Å, 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 (Jd,z = −120 × 10-4, −105 × 10-4, and −140 × 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.