An Exceedingly Stable Diiron(II,III) Complex Ion

A record equilibrium constant of K(c)=10(19) was found for the comproportionation of the diiron(II,III) complex 1 in acetonitrile. In water the K(c) value is strongly diminished (10(7.9)), but still exceeds that of the Creutz-Taube ion. Intervalence charge transfer bands occur at 2520 nm (in CH(3)CN) and 2250 nm (in D(2)O), and all evidence points to a strongly coupled system with delocalized valences.

Exploration of mixed-valence chemistry: inventing new analogues of the Creutz-Taube ion.

Starting from the Creutz-Taube ion as the prototype of a molecule-bridged mixed-valent complex, a number of related systems are presented in which the metal, the co-ligands, the molecular bridge, the d electron configuration, the medium, the charge, the coordination mode, or the nuclearity have been modified. The new mixed-valent configurations, as obtained through various chemical approaches, display different stabilities and spectroscopic characteristics in relation to the Creutz-Taube ion; the analysis of these results serves to provide a better understanding of fundamental aspects of molecule-mediated metal-metal interaction.

Crucial role of Cu-S bonding for structural changes accompanying the reversible CuI/CuII transition in an unrestrained Cu(N[symbol: see text]S)2 coordination arrangement. An experimental and DFT study.

The structure of reversibly oxidizable [Cu(mmb)2](BF4) with 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) as bidentate N,S-donor ligand has been determined and compared with that of the copper(II) species [Cu(mmb)2(eta 1-ClO4)](ClO4). In the complex ions of the equilibrium [CuI(mmb)2](+) + ClO4- reversible e- + [CuII(mmb)2-(eta 1-ClO4)]+ the almost linear N-Cu-N backbone is invariant whereas the bonds to the thioether sulfur centers and especially the changing S-Cu-S angle (145.18(5) degrees for the CuII species, 109.33(3) degrees for the CuI form) reflect the metal oxidation state. In contrast to the perchlorate coordinating copper(II) species, [CuI(mmb)2](BF4) contains a cation with a very large vacant site at the metal center, resulting in elliptical channels within the crystal. DFT calculations on [CuI(mb)2]+, [CuII(mb)2]2+, and [CuII(mb)2(OClO3)]+ with mb = 2-methylthiomethyl-1H-benzimidazole confirm the essential role of the metal-sulfur bonds in responding to the reversible CuI/II electron transfer process, even in the absence of electronically stronger interacting thiolate sulfur centers or sophisticated oligodentate ligands.

Electron paramagnetic resonance and spectroscopic characteristics of electrogenerated mixed-valent systems [(eta 5-C5Me5)M(mu-L)M(eta 5-C5Me5)]+ (M = Rh, Ir; L = 2,5-diiminopyrazines) in relation to the radicals [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]+ and [(eta 5-C5Me5)M(mu-L)MCl(eta 5-C5Me5)]2+.

Electrochemical reduction of the dinuclear [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]2+ ions (M = Rh, Ir; L = 2,5-bis(1-phenyliminoethyl)pyrazine (bpip) and 2,5-bis[1-(2,6-dimethylphenyl)iminoethyl]pyrazine (bxip)) proceeds via the paramagnetic intermediates [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]+ (L = bpip) or [(eta 5-C5Me5)M(mu-L)MCl(eta 5-C5Me5)]2+ (L = bxip) and [(eta 5-C5Me5)M(mu-L)M(eta 5-C5Me5)]+. Whereas the first is clearly a radical species with a small g anisotropy, the chloride-free cations are distinguished by structured intervalence charge transfer (IVCT) bands in the near-infrared region and by rhombic electron paramagnetic resonance features between g = 1.9 and g = 2.3, which suggests considerable metal participation at the singly occupied MO. Alternatives for the d configuration assignment and for the role of the bisbidentate-conjugated bridging ligands will be discussed. The main difference between bpip and bxip systems is the destabilization of the chloride-containing forms through the bxip ligand for reasons of steric interference.

Metal vs ligand reduction in complexes of 1,3-dimethylalloxazine (DMA) with copper(I), ruthenium(II), and tungsten(VI). Crystal structures of (DMA)WO2Cl2 and (bis(1-methylimidazol-2-yl)ketone)WO2Cl2.

The complexes [(DMA)Cu(PPh3)2](BF4) (1) (DMA = 1,3-dimethylalloxazine), [(DMA)Ru(bpy)2](PF6)2 (2), and (DMA)WO2Cl2 (3) were obtained as O4-N5-chelated species, as evident from an X-ray crystal structure analysis for 3 and from spectroscopy (NMR, IR, and UV-vis spectroelectrochemistry) for 1 and 2. The tungsten(VI) center in 3 has its oxide ligands in a cis/equatorial position and the chloride ligands in a trans/axial position; it also exhibits a relatively short bond to O4 (2.232(3) A) and a very long bond to N5 (2.462(3) A). Comparison with the new structurally characterized compound (BIK)WO2Cl2 (4) (BIK = bis(1-methylimidazol-2-yl)ketone), which has W-N bonds of about 2.30 A, confirms the unusual length of the W-N bond in 3, probably caused by repulsion between one of the oxo ligands and the peri-hydrogen atom (H6) of DMA. One-electron reduction of the complexes occurs reversibly at room temperature in THF (1, 2) or at 198 K in CH2Cl2 (3). EPR spectroscopy reveals that this process is ligand-centered for 1 and 2 but metal-centered for 3. Density functional methods and ab initio methodology are used to illustrate the correspondence in spin distribution between the radical anion pi systems of alloxazine and isoalloxazine ("flavosemiquinone").

Dicopper(I) complexes with reduced states of 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine: crystal structures and spectroscopic properties of the free ligand, a radical species, and a complex of the 1,4-dihydro form.

The complexes [(mu-bmtz(*-))[Cu(PPh(3))(2)](2)](BF(4)) (1) and [(mu-H(2)bmtz)[Cu(PPh(3))(2)](2)](BF(4))(2) (2) (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine and H(2)bmtz = 1,4-dihydro-3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine) were obtained as stable materials that could be crystallized for structure determination. 1.2 CH(2)Cl(2): C(84)H(70)BCl(4)Cu(2)F(4)N(8)P(4); monoclinic, C2/c; a = 26.215(7) A, b = 22.122(6) A, c = 18.114(5) A, beta = 133.51(1) degrees; Z = 4. 2.CH(2)Cl(2): C(83)H(70)B(2)Cl(2)Cu(2)F(8)N(8)P(4); triclinic, P1; a = 10.948(2) A, b = 12.067(2) A, c = 30.287(6) A, alpha = 93.82(3) degrees, beta = 94.46(3) degrees, gamma = 101.60(3) degrees; Z = 2. Bmtz itself was also structurally characterized (C(10)H(6)N(8); monoclinic, P2(1)/c; a = 3.8234(8) A, b = 10.147(2) A, c = 13.195(3) A, beta = 94.92(3) degrees; Z = 2). Whereas the radical complex ion contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex of H(2)bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine ring.

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states.

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.