Chemical Modeling of the Oxygen-Evolving Center in Plants. Synthesis, Structure, and Electronic and Redox Properties of a New Mixed Valence Mn−Oxo Cluster:  [Mn₂^III,IVO₂(bisimMe₂en)₂]³⁺ (bisimMe₂en = N,N‘-Dimethyl-N,N‘-bis(imidazol-4-ylmethyl)ethane-1,2-diamine). EPR Detection of an Imidazole Radical Induced by UV Irradiation at Low Temperature

Abstract

The compound [Mn 2 III,IV O 2 (bisimMe 2 en) 2 ](ClO 4 ) 3 ‚H 2 O (bisimMe 2 en ) N, N′-dimethyl-N,N′-bis(imidazol-4-ylmethyl)ethane-1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a ) 18.139-(8) Å, b ) 12.694(5) Å, c ) 17.694(8) Å, ) 107.5(6)°, V ) 3885(6) Å 3 , and Z ) 4. The cation [Mn 2 III,IV O 2 (bisimMe 2 en) 2 ] 3+ contains a di-manganese di-µ-oxo unit. The Mn-Mn axis is a C 2 axis. The Mnimidazole distances for Mn III or Mn IV are distinct: respectively, 2.208(9) and 2.004(8) Å. The ground state is a |S A d2,S B d 3 / 2 ,Sd 1 / 2 〉 state separated by 420 cm -1 from the S ) 3 / 2 state. The EPR spectrum has been simulated with |A 1x,y | ) 160.10 -4 cm -1 , |A 1z | ) 136.10 -4 cm -1 , |A 2x,y | ) 72.10 -4 cm -1 , |A 2z | ) 71.10 -4 cm -1 , g xy ) 1.997, g z ) 1.993. The ESEEM spectrum is reported. The 2 H nuclei corresponding to the exchangeable hydrogen atoms of imidazole groups in D 2 O have been detected at 2.4 MHz at 3480 G. In cyclic voltammetry, [Mn 2 III,IV O 2 (bisimMe 2en) 2 ] 3+ presents in oxidation two reversible waves at 1.04 and 1.40 V (all potentials versus NHE) in strong contrast with what has been always observed on other Mn 2 III,IV O 2 units, for which only one wave in oxidation ([III,IV] f [IV,IV]) is observed. EPR spectroelectrochemistry reveals the disappearance of the 16 line spectrum of the [III,IV] species on oxidation at 1.24 V. UV-vis spectroelectrochemistry at this potential confirms the formation of the [IV,-IV] dimer. The existence of two waves was related to the presence of slow equilibrium between two different [III,IV] forms. UV-irradiation of the starting [Mn 2 III,IV O 2 (bisimMe 2 en) 2 ] 3+ complex in an aqueous borate buffer at pH ) 10 at 77 K resulted in the formation of an EPR signal that is attributed to an imidazole radical coupled to the Mn III -Mn IV pair. This signal is unsplit and is understandable in a simple model of two spins S ) 1 / 2 in magnetic interaction. It has some analogy with that observed by Boussac et al. (Nature 1990, 347, 303-306) on the S 3 state of the Ca 2+ and Cl --depleted oxygen-evolving center, although in that case the signal was split. We relate this difference to small differences in the magnetic interaction of the two spins S ) 1 / 2 . ESEEM spectra of the irradiated [Mn 2 III,IV O 2 (bisimMe 2 en) 2 ] 3+ complex are reported for magnetic fields corresponding to the resonance of the radical or the metal cluster. These results suggest that the formed imidazole radical stays linked to the metal core despite the weak magnetic interaction detected. This rises the possibility that the S 3 radical in the natural system, is a direct ligand to Mn cluster.

New MnII Complexes with an N/O Coordination Sphere from TripodalN-Centered Ligands − Characterization from Solid State to Solution and Reaction with Superoxide in Non-Aqueous and Aqueous Media

“…This is consistent with the UV/Vis spectrum, as a possible assignment for the 300 and 360 nm bands is an OǞMn IV LMCT. [74,77,78] Scheme 2 provides a rationalization of these experimental observations. Scheme 2.…”

Synthesis, Structure, Electronic, Redox, and Magnetic Properties of a New Mixed-Valent Mn-Oxo Cluster: [Mn2III,IVO2(N,Nbispicen)2]3+ (N,Nbispicen =N,N-bis(2-pyridylmethyl)-1,2-diaminoethane)

“…The MnϪMn distance is 2.646(2) Å , which is comparable to the value found in dimanganese(III,IV)-di-µ-oxo complexes previously reported. [6] [12] An antibonding molecular orbital which is half occupied for the Mn(III) ion and empty for the Mn(IV) ion results fom the interaction of the d z 2 orbital with the σ orbitals of the axial pyridine N2 atom and the axial primary amino-group N4 atom. In previous studies it has been found that the average axial Mn(III) primary amine distance is 2.276 Å [9] 2 ] 3ϩ is 2.233 Å .…”

Fluorous Biphasic Catalysis: Complexation of 1,4,7‐[C8F17(CH2)3]3‐1,4,7‐Triazacyclononane with [M(C8F17(CH2)2CO2)2] (M = Mn, Co) To Provide Perfluoroheptane‐Soluble Catalysts for Alkane and Alkene Functionalization in the Presence of t‐BuOOH and O2

“…P2 and C 1 are both located above (2), Cl-P2-C2 108.32 (13). P2-C2-P3 119.9(2), C2-P3-C3 108.90 (14), C3-PI-Cl 103.8 (2). this plane; their positions in the ring are characterized by greatly different bond angles (P2-C2-P3: 119.9", Cl-Pl-C3: 103.8") and by distances between 178.9 and 181.2 pm, which are typical for P-C single bonds."…”