Synthesis and X-ray structure of the MnIICl2 and MnIIIF2 complexes of N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. High-field electron paramagnetic resonance and density functional theory studies of the MnIII complex. Evidence for a low-lying spin triplet state.

Two manganese complexes, (py2(NMe)2)MnIICl2 (1) and [(py2(NMe)2)MnIIIF2]+ (2), are here described with the macrocyclic ligand py2(NMe)2 (py2(NMe)2 = N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane). For both, the crystal structure is reported. The UV-visible spectrum of 2 exhibits a very broad near-infrared (NIR) band corresponding to the transition between the two e(g)-type orbitals split by the Jahn-Teller effect. A negative D value of ca. -4 cm(-1) was estimated by high-field and high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy, which was consistent with symmetry considerations. Density functional theory (DFT) calculations on 2 support the 5B1 electronic ground state predicted from the X-ray structure. Moreover, to explain the large value of the D parameter, a spin triplet first excited spin state was postulated to occur at low energy. This was confirmed by the DFT calculations.

Structures of Fe(II) Complexes with N,N,N'-Tris(2-pyridylmethyl)ethane-1,2-diamine Type Ligands. Bleomycin-like DNA Cleavage and Enhancement by an Alkylammonium Substituent on the N' Atom of the Ligand.

The complexes [L(5)Fe(II)Cl]BPh(4) and [L(5)Fe(II)(H(2)O)](BPh(4))(2) (L(5) = N,N,N'-tris(2-pyridylmethyl)-N'-methyl-ethane-1,2-diamine) have been isolated. Bernal et al. (Bernal, J.; et al. J. Chem. Soc., Dalton Trans. 1995, 3667-3675) have prepared this ligand and the corresponding complex [L(5)Fe(II)Cl]PF(6). We obtained the structural data of [L(5)Fe(II)Cl]BPh(4) by X-ray diffraction. It crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 17.645(7) Å, b = 16.077(6) Å, c = 13.934(5) Å, V = 3953(3) Å(3), and Z = 4. It presents Fe(II)-N bond lengths close to 2.2 Å, typical of high-spin Fe(II). In solution the [L(5)Fe(II)(H(2)O)](BPh(4))(2) complex showed a dependence of spin state upon the nature of the solvent. It was high spin in acetone and changed to low spin in acetonitrile. This was detected by UV-vis spectroscopy and by (1)H NMR. Bernal et al. (ibidem) showed that these complexes in the presence of an excess of H(2)O(2) give a purple species, very likely the [L(5)Fe(III)(OOH)](2+) derivative, with spectroscopic signatures analogous to those of "activated bleomycin". The formation of [L(5)Fe(III)(OOH)](2+) is confirmed here by electrospray ionization mass spectrometry. We found that a L(5)/Fe system gave single-strand breaks on plasmid DNA in the presence of either a reducing agent (ascorbate) and air or oxidants (H(2)O(2), KHSO(5), MMPP) at 0.1 &mgr;M concentration. The methyl group in L(5) was substituted by a (CH(2))(5)N(CH(3))(3)(+) group in order to get higher affinity with DNA. The corresponding ligand L(5)(+) was used to prepare the complexes [L(5)(+)Fe(II)Cl]Y(2) (Y = BPh(4)(-), PF(6)(-), ClO(4)(-)) and [L(5)(+)Fe(II)Br](PF(6))(2). The crystal structure of [L(5)(+)Fe(II)Cl](ClO(4))(2) was solved. It crystallizes in the monoclinic space group P2(1)/a with a = 14.691(2) Å, b = 13.545(2) Å, c = 17.430(2) Å, beta = 93.43(1) degrees, V = 3462(1) Å(3), and Z = 4. The Fe(II)-ligand distances are similar to those of [L(5)Fe(II)Cl]BPh(4). At the relatively low concentration of 0.01 &mgr;M, [L(5)(+)Fe(II)Br](2+) promoted DNA breaks. The reaction was not inhibited by hydroxyl radical scavengers. The reaction might involve a nondiffusible oxygen reactive species, either a coordinated hydroperoxide or a high-valent metal-oxo entity.

A New Manganese Dinuclear Complex with Phenolate Ligands and a Single Unsupported Oxo Bridge. Storage of Two Positive Charges within Less than 500 mV. Relevance to Photosynthesis.

The compound [Mn(III)(2)OL(2)](ClO(4))(2).2.23CHCl(3).0.65CH(2)Cl(2) where L(-) is the monoanionic N,N-bis(2-pyridylmethyl)-N'-salicyliden-1,2-diaminoethane ligand, has been synthesized. The complex dication [Mn(III)(2)OL(2)](2+) contains a linear Mn(III)-O-Mn(III) unit with a Mn-Mn distance of 3.516 Å. The pentadentate ligand L(-) wraps around the Mn(III) ion. Electrochemically, it is possible to prepare the one electron oxidized trication [Mn(2)OL(2)](3+) which crystallizes as [Mn(2)OL(2)](ClO(4))(2.37)(PF(6))(0.63).1.5CH(3)CN. The complex trication [Mn(2)OL(2)](3+) contains a Mn(III)-O-Mn(IV) unit with a Mn-Mn distance of 3.524 Å and a Mn-O-Mn angle of 178.7(2) degrees. The contraction of the coordination sphere around the Mn(IV) is clearly observed. The [Mn(2)OL(2)](2+) dication possesses a S = 0 electronic ground state with J = -216 cm(-)(1) (H = -JS(1)().S(2)()), whereas the [Mn(2)OL(2)](3+) trication shows a S = (1)/(2) ground state with J = -353 cm(-)(1). The UV-visible spectrum of [Mn(2)OL(2)](3+) exhibits an intense absorption band (epsilon = 3040 M(-)(1) cm(-)(1)) centered at 570 nm assigned to a phenolate --> Mn(IV) charge-transfer transition. The potentials of the redox couples determined by cyclic voltammetry are E degrees ([Mn(2)OL(2)](3+)/[Mn(2)OL(2)](2+)) = 0.54 V/saturated calomel electrode (SCE) and E degrees ([Mn(2)OL(2)](4+)/[Mn(2)OL(2)](3+)) = 0.99 V/SCE. Upon oxidation at 1.3 V/SCE, the band at 570 nm shifts to 710 nm (epsilon = 2500 M(-)(1) cm(-)(1)) and a well-defined band appears at 400 nm which suggests the formation of a phenoxyl radical. The [Mn(2)OL(2)](3+)( )()complex exhibits a 18-line X-band electron paramagnetic resonance (EPR) spectrum which has been simulated with rhombic tensors |A(1)(x)()| = 160 x 10(-)(4) cm(-)(1); |A(1)(y)()| = 130 x 10(-)(4) cm(-)(1); |A(1)(z)()| = 91 x 10(-)(4) cm(-)(1); |A(2)(x)()| = 62 x 10(-)(4) cm(-)(1); |A(2)(y)()| = 59 x 10(-)(4) cm(-)(1); |A(2)(z)()| = 62 x 10(-)(4) cm(-)(1) and g(x)() = 2.006; g(y)() = 1.997; g(z)() = 1.982. This EPR spectrum( )()shows that the 16-line paradigm related to a large antiferromagnetic exchange coupling and a low anisotropy can be overcome by a large rhombic anisotropy. Molecular orbital calculations relate this rhombicity to the nature of the orbital describing the extra electron on Mn(III). This orbital has a majority but not pure d(z)()2 contribution (with the z axis perpendicular to the Mn-Mn axis). Low-temperature resonance Raman spectroscopy on an acetonitrile solution of [Mn(2)OL(2)](4+) prepared at -35 degrees C indicated the formation of a phenoxyl radical. This suggests that the ligand was oxidized rather than the Mn(III)Mn(IV) pair to Mn(IV)Mn(IV), which illustrates the difficulty to store a second positive charge in a short range of potential in a manganese mono-&mgr;-oxo pair. The relevance of these results to the study of the photosynthetic oxygen evolving complex is discussed.

Selective Metal-Assisted Oxidative Cleavage of a C-N Bond: Synthesis and Characterization of the Mononuclear Iron(III) [Fe(BPG)Cl(2)] Complex and Its Two [Fe(BPA)Cl(3)] and [Fe(BPE)Cl(3)] Derivatives.

A new example of a site-selective metal-assisted oxidative cleavage of a C-N bond is reported. This phenomenon occurs in the nitrogen-centered tetradentate tripodal BPG ligand (BPG = (bis(2-pyridylmethyl)amino)acetate), which combines one carboxylate and two pyridines as pendant groups, when coordinated to iron(III). The corresponding iron(III) [Fe(BPG)Cl(2)] complex (1) is transformed to the iron(III) [Fe(BPA)Cl(3)] complex (2) with the tridentate BPA ligand (BPA = bis(2-pyridylmethyl)amine) retaining only two pyridine pendant groups, while the initial BPG carboxylate group is transformed to glyoxylic acid. The [Fe(BPA)Cl(3)] complex (2) has been fully characterized as well as another, [Fe(BPE)Cl(3)] (3), formed with the tridentate BPE ligand (BPE = methyl (bis(2-pyridylmethyl)amino)acetate) which, in addition to two pyridines, presents an ester group. The crystal structures of these two complexes have been resolved. The asymmetric unit of complex 2 has been characterized by two mononuclear neutral molecules linked by two hydrogen bonds. (Crystal data for 2: orthorhombic, Pna2(1), a = 15.603(8) Å, b = 8.485(4) Å, c = 22.752(11) Å, alpha = beta = gamma = 90 degrees, V = 3012(3) Å(3), Z = 8, R = 0.0582 [2813 reflections with I > 2sigma(I)], and R(w) = 0.1430.) Complex 3 is symmetric with the iron center, the amine nitrogen, one choride and the three atoms of the carboxylate group in special positions in the mirror plane. (Crystal data for 3: orthorhombic, Pmn2(1), a = 10.418(4) Å, b = 12.952(5) Å, c = 7.353(3) Å, alpha = beta = gamma = 90 degrees, V = 992.2(7) Å(3), Z = 2, R = 0.0500 [513 reflections with I > 2sigma(I)], and R(w) = 0.1231.) Redox potentials corroborate the structural features (E degrees = -38 mV for 1, -19 mV for 2, and +145 mV for 3 vs SCE in acetonitrile). Preliminary studies of the [Fe(BPG)Cl(2)] complex have shown that it reacts in its reduced form with dioxygen.