Evidence for significant through-space and through-bond electronic coupling in the 1,4-diphenylcyclohexane-1,4-diyl radical cation gained by absorption spectroscopy and DFT calculations

Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.     

Model studies of the Cu(B) site of cytochrome c oxidase utilizing a Zn(II) complex containing an imidazole-phenol cross-linked ligand

Cytochrome c oxidase, the enzyme complex responsible for the four-electron reduction of O2 to H2O, contains an unusual histidine-tyrosine cross-link in its bimetallic heme a3-CuB active site. We have synthesised an unhindered, tripodal chelating ligand, BPAIP, containing the unusual ortho-imidazole-phenol linkage, which mimics the coordination environment of the CuB center. The ligand was used to investigate the physicochemical (pKa, oxidation potential) and coordination properties of the imidazole-phenol linkage when bound to a dication. Zn(II) coordination lowers the pKa of the phenol by 0.6 log units, and increases the potential of the phenolate/phenoxyl radical couple by approximately 50 mV. These results are consistent with inductive withdrawal of electron density from the phenolic ring. Spectroscopic data and theoretical calculations (DFT) were used to establish that the cationic complex [Zn(BPAIP)Br]+ has an axially distorted trigonal bipyramidal structure, with three coordinating nitrogen ligands (two pyridine and one imidazole) occupying the equatorial plane and the bromide and the tertiary amine nitrogen of the tripod in the axial positions. Interestingly, the Zn-Namine bonding interaction is weak or absent in [Zn(BPAIP)Br]+ and the complex gains stability in basic solutions, as indicated by 1H NMR spectroscopy. These observations are supported by theoretical calculations (DFT), which suggest that the electron-donating capacity of the equatorial imidazole ligand can be varied by modulation of the protonation and/or redox state of the cross-linked phenol. Deprotonation of the phenol makes the equatorial imidazole a stronger sigma-donor, resulting in an increased Zn-Nimd interaction and thereby leading to distortion of the axial ligand axis toward a more tetrahedral geometry.     

Synthesis, Characterization of Heterodinuclear Co-Cu Complex and Its Electrocatalytic Activity towards 02 Reduction： Implications for Cytochrome c Oxidase Active Site Modeling

A new dinudeating ligand consisting of a tetraphanylporphyrin derivative covalently linked with tris(2-benzimidazylmethyl)-amine and its homodinudear Co-Co and heterodinnelear Co-Cu complexes were synthesized and spectroscopically character-ized. The heterobimetallie cobalt-copper complex bearing three benzimidazole ligands for copper, as cytochrome c oxidase ac-tive site model, was applied to the surface of glassy carbon elec-trode to show electrocatalytie activity for O2 reduction in aque-ous solution at an addity level dose to physiological pH value.The kinetic parameters of this electrocatalytic process were ob-tained.     

Syntheses and structures of magnesium and zinc boraamidinates: EPR and DFT investigations of Li, Mg, Zn, B, and In complexes of the [PhB(NtBu)2].- anion radical

The first magnesium and zinc boraamidinate (bam) complexes have been synthesized via metathetical reactions between dilithio bams and Grignard reagents or MCl2 (M = Mg, Zn). The following new classes of bam complexes have been structurally characterized: heterobimetallic spirocycles {(L)mu-Li[PhB(mu-NtBu)2]}2M (6a,b, M = Mg, L = Et2O, THF; 6c, M = Zn, L = Et(2)O); bis(organomagnesium) complexes {[PhB(mu3-NtBu)2](MgtBu)2(mu3-Cl)Li(OEt2)3} (8) and {[PhB(mu3-NtBu)2](MgR)2(THF)2} (9a, R = iPr; 9b, R = Ph); mononuclear complex {[PhB(mu-NDipp)2]Mg(OEt2)2} (10). Oxidation of 6a or 6c with iodine produces persistent pink (16a, M = Mg) or purple (16b, M = Zn) neutral radicals {Lx-mu-Li[PhB(mu-NtBu)2]2M}. (L = solvent molecule), which are shown by EPR spectra supported by DFT calculations to be Cs-symmetric species with spin density localized on one of the bam ligands. In contrast, characterization of the intensely colored neutral radicals {[PhB(mu-NtBu)2]2M}. (5c, M = In, dark green; 5d, M = B, dark purple) reveals that the spin density is equally delocalized over all four nitrogen atoms in these D2d-symmetric spirocyclic systems. Oxidation of the dimeric dilithio complex {Li2[PhB(mu4-NtBu)2]}2 with iodine produces the monomeric neutral radical {[PhB(mu-NtBu)2]Li(OEt2)x}. (17), characterized by EPR spectra and DFT calculations. These findings establish that the bam anionic radical [PhB(NtBu)2].- can be stabilized by coordination to a variety of early main-group metal centers to give neutral radicals whose relative stabilities are compared and discussed.     

Metal-metal interactions in heterobimetallic d8-d10 complexes. Structures and spectroscopic investigation of [M'M' '(mu-dcpm)2(CN)2]+ (M' = Pt,Pd; M' ' = Cu,Ag, Au) and related complexes by UV-vis absorption and resonance Raman spectroscopy and ab initio calculations

X-ray structural and spectroscopic properties of a series of heterodinuclear d(8)-d(10) metal complexes [M'M' '(mu-dcpm)(2)(CN)(2)](+) containing d(8) Pt(II), Pd(II), or Ni(II) and d(10) Au(I), Ag(I), or Cu(I) ions with a dcpm bridging ligand have been studied (dcpm = bis(dicyclohexylphosphino)methane; M' = Pt, M' ' = Au 4, Ag 5, Cu, 6; M' ' = Au, M' = Pd 7, Ni 8). X-ray crystal analyses showed that the metal...metal distances in these heteronuclear metal complexes are shorter than the sum of van der Waals radii of the M' and M' ' atoms. The UV-vis absorption spectra of 4-6 display red-shifted intense absorption bands from the absorption spectra of the mononuclear trans-[Pt(phosphine)(2)(CN)(2)] and [M' '(phosphine)(2)](+) counterparts, attributable to metal-metal interactions. The resonance Raman spectra confirmed assignments of (1)[nd(sigma)-->(n + 1)p(sigma)] electronic transitions to the absorption bands at 317 and 331 nm in 4 and 6, respectively. The results of theoretical calculations at the MP2 level reveal an attractive interaction energy curve for the skewed [trans-Pt(PH(3))(2)(CN)(2)-Au(PH(3))(2)(+)] dimer. The interaction energy of Pt(II)-Au(I) was calculated to be ca. 0.45 ev.     

Novel copper(II)-dien-imidazole/imidazolate-bridged copper(II) complexes. Crystal structure of [Cu(dien)(Him)](ClO4)2 and of [(dien)Cu(mu-im)Cu(dien)](ClO4)3, a homobinuclear model for the copper(II) site of the CuZn-superoxide dismutase

The imidazolate-bridged binuclear copper(II)-copper(II) complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) and related mononuclear complexes [Cu(dien)(H(2)O)](ClO(4))(2), [Cu(dien)(Him)](ClO(4))(2) were synthesized with diethylenetriamine (dien) as capping ligand. The crystal structure of mononuclear [Cu(dien)(Him)](ClO(4))(2) and binuclear complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) have been determined by single crystal X-ray diffraction methods. The mononuclear complex [Cu(dien)(Him)](ClO(4))(2) crystallizes in the orthorhombic, Pca2(1) with a = 9.3420(9) A, b = 12.3750(9) A, c = 14.0830(9) A, beta = 90.000(7)(o) and Z = 4 and binuclear complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) crystallizes in the monoclinic space group P2(1)/a, with a = 15.017(7) A, b = 11.938(6) A, c = 15.386(6) A, beta = 110.30(4)(o) and Z = 4. The molecular structures show that copper(II) ions in an asymmetrically elongated octahedral coordination (type 4 + 1 + 1) and in binuclear complex Cu(1) atom has a asymmetrically elongated octahedral coordination (type type 4 + 1 + 1) and Cu(2) atom exhibits a square base pyramidal coordination (type 4 + 1). The bridging ligand (imidazolate ion, im) lies nearly on a straight line between two Cu(2+), which are separated by 5.812 A, slightly shorter than the value in copper-copper superoxide dismutase (Cu(2)-Cu(2)SOD). Magnetic measurements and electron spin resonance (ESR) spectroscopy of the binuclear complex have shown an antiferromagnetic exchange interaction. From pH-dependent cyclic voltametry (CV) and electronic spectroscopic studies the complex has been found to be stable over a wide pH range (7.75-12.50).