Characterization of Heme Orientational Disorder in a Myoglobin Reconstituted with a Trifluoromethyl-Group-Substituted Heme Cofactor.

The orientation of a CF3-substituted heme in sperm whale myoglobin and L29F, H64L, L29F/H64Q, and H64Q variant proteins has been investigated using 19F NMR spectroscopy to elucidate structural factors responsible for the thermodynamic stability of the heme orientational disorder, i.e., the presence of two heme orientations differing by a 180° rotation about the 5-15 meso axis, with respect to the protein moiety. Crystal structure of the met-aquo form of the wild-type myoglobin reconstituted with 13,17-bis(2-carboxylatoethyl)-3,8-diethyl-2,12,18-trimethyl-7-trifluoromethylporphyrinatoiron(III), determined at resolution of 1.25 Å, revealed the presence of the heme orientational disorder. Alterations of the salt bridge between the heme 13-propionate and Arg45(CD3) side chains due to the mutations resulted in equilibrium constants of the heme orientational disorder ranging between 0.42 and 1.4. Thus, the heme orientational disorder is affected by the salt bridge associated with the heme 13-propionate side chain, confirming the importance of the salt bridge in the heme binding to the protein.

Thioacetyl-terminated ferrocene-anthraquinone conjugates: synthesis, photo- and electrochemical properties triggered by protonation-induced intramolecular electron transfer.

Two thioacetyl-terminated ferrocene-anthraquinone donor-acceptor molecules with different pi-electron conjugative units have been synthesized via a series of Stille and Sonagashira reactions. Their photochemical and electrochemical properties before and after addition of an organic acid are investigated, indicating that these complexes are sensitive to external perturbation of protonation, leading the structural change to an expansion of pi-conjugated system by cyclocondensation reaction and promoting intramolecular electron transfer from donor to acceptor. They would be good candidates for studies of novel SAMs, and the properties triggered by protonation-induced intramolecular electron transfer will make the SAMs be useful in designing new functional molecular devices.

Counterion-dependent valence tautomerization of ferrocenyl-conjugated pyrylium salts.

1-Ferrocenylethynylanthraquinone (1-FcAq), which is a donor (D)-acceptor (A) conjugated compound consisting of a ferrocene (Fc) acting as a donor, an anthraquinone (Aq) acting as an acceptor, and an ethynyl linker, undergoes a cyclocondensation reaction with strong organic acid, and forms 2-ferrocenyloxodihydrodibenzochromenylium salts ([1-FcPyl](+)X(-) where X = TFSI, TfO, PF(6), and BF(4)). [1-FcPyl](+) were also characterized as conjugated donor-acceptor compounds, and electrochemical properties, UV-vis absorption spectra, single-crystal X-ray analysis, and TD-DFT calculations have indicated that the LUMO level of [1-FcPyl](+) is lower than that of 1-FcAq because of the much larger pi-conjugated system in [1-FcPyl](+). Variable-temperature Mossbauer spectroscopy (12-300 K) showed that Fe(II) was dominant for the TFSI(-), PF(6)(-), and BF(4)(-) salts of [1-FcPyl](+); although the Fe(III) species was also observed at all temperature ranges, the molar ratio of Fe(III) species increased at higher temperatures in the TFSI(-) and PF(6)(-) salts. This finding indicates that valence tautomerization (VT) between 1-FcPyl(+) and 1-Fc(+)Pyl occurs in the solid state of the TFSI(-) and the PF(6)(-) salts, but not in the BF(4)(-) salt. Variable-temperature (3.5-310 K) IR spectroscopy showed that the frequencies of the skeletal vibration of the ferrocene moiety decreased with increasing temperature in the TFSI(-) and PF(6)(-) salts, indicating the development of a ferrocenium-like character. The precision of the bond lengths of the [1-FcPyl](+) moiety (0.003-0.004 A) determined by single-crystal X-ray analysis (113 and 273 K) is not sufficient to demonstrate the effect of the counterion on VT. The dihedral angle between the ferrocene and the pyrylium moieties in the BF(4)(-) salt (11.25(15) degrees) is larger than that in the TFSI(-) (6.63(12) degrees) and PF(6)(-) (9.55(15) degrees) salts. Furthermore, the planarity of the acceptor moiety (estimated from the dihedral angle between Ph1 and Ph2) is lower in the BF(4)(-) salt compared with that of other salts. These increased dihedral angles might cause a weaker D-A interaction and a destabilization of the acceptor moiety (i.e., raising a LUMO level), leading to lower stability of the Fe(III) (1-Fc(+)Pyl) species. Variable-temperature X-ray powder diffraction (VT XRPD, 100-300 K) revealed that the temperature dependence of the Fe-P distance in the PF(6)(-) salt was smaller than that of the Fe-B distance in the BF(4)(-) salt. Our interpretation of this phenomenon is that the molar ratio of the Fe(III) species is increased in the PF(6)(-) salt, and that the Coulombic force between the ferrocene moiety and PF(6)(-) anion increases, preventing an increase in the Fe-P distance. This indicates that the electrostatic interaction between the [1-FcPyl](+) moiety and the counteranion may affect the occurrence of VT.

Reversible photochromism of a ferrocenylazobenzene monolayer controllable by a single green light source.

A 3-ferrocenylazobenzene monolayer on an ITO electrode exhibits reversible azobenzene isomerization using a single green light source, assisted by electrochemical control of the ferrocene redox state.

Synthesis of heterometal cluster complexes by the reaction of cobaltadichalcogenolato complexes with groups 6 and 8 metal carbonyls.

Metalladichalcogenolate cluster complexes [{CpCo(S2C6H4)}2Mo(CO)2] (Cp = eta(5)-C5H5) (3), [{CpCo(S2C6H4)}2W(CO)2] (4), [CpCo(S2C6H4)Fe(CO)3] (5), [CpCo(S2C6H4)Ru(CO)2(P(t)Bu3)] (6), [{CpCo(Se2C6H4)}2Mo(CO)2] (7), and [{CpCo(Se2C6H4)}(Se2C6H4)W(CO)2] (8) were synthesized by the reaction of [CpCo(E2C6H4)] (E = S, Se) with [M(CO)3(py)3] (M = Mo, W), [Fe(CO)5], or [Ru(CO)3(P(t)Bu3)2], and their crystal structures and physical properties were investigated. In the series of trinuclear group 6 metal-Co complexes, 3, 4, and 7 have similar structures, but the W-Se complex, 8, eliminates one cobalt atom and one cyclopentadienyl group from the sulfur analogue, 4, and does not satisfy the 18-electron rule. 1H NMR observation suggested that the CoW dinuclear complex 8 was generated via a trinuclear Co2W complex, with a structure comparable to 7. The trinuclear cluster complexes, 3, 4, and 7, undergo quasi-reversible two-step one-electron reduction, indicating the formation of mixed-valence complexes Co(III)M(0)Co(II) (M = Mo, W). The thermodynamic stability of the mixed-valence state increases in the order 4 < 3 < 7. In the dinuclear group 8 metal-Co complexes, 5 and 6, the CpCo(S2C6H4) moiety and the metal carbonyl moiety act as a Lewis acid character and a base character, respectively, as determined by their spectrochemical and redox properties. Complex 5 undergoes reversible two-step one-electron reduction, and an electron paramagnetic resonance (EPR) study indicates the stepwise reduction process from Co(III)Fe(0) to form Co(III)Fe(-I) and Co(II)Fe(-I).

UV-Vis, NMR, and time-resolved spectroscopy analysis of photoisomerization behavior of three- and six-azobenzene-bound tris(bipyridine)cobalt complexes.

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties, namely, [Co(II)(dmAB)3](BF4)2 [dmAB = 4,4'-bis[3''-(4'''-tolylazo)phenyl]-2,2'-bipyridine], [Co(III)(dmAB)3](BF4)3, [Co(II)(mAB)3](BF4)2 [mAB = 4-[3' '-(4' ''-tolylazo)phenyl]-2,2'-bipyridine], and [Co(III)(dmAB)3](BF4)3, derived from the effect of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using UV-vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365 nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center. The cooperation among the photochemical structural changes of six azobenzene moieties in [Co(II)(dmAB)3](BF4)2 was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons indicated that each azobenzene moiety in [Co(II)(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability of 50% and without interactions among the azobenzene moieties.

Photon-, electron-, and proton-induced isomerization behavior of ferrocenylazobenzenes.

3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-ferrocenylazobenzene (5) and 3-ferrocenyl-4'-hydroxylazobenzene (11), were determined by X-ray diffraction analysis. 3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the Fe(III)/Fe(II) redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV light, exciting the pi-pi* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of 1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo pi* orbital) band excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization. Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light irradiation.

Photoluminescent dinuclear lanthanide complexes with tris(2-pyridyl)carbinol acting as a new tetradentate bridging ligand.

A tripodal ligand, tris(2-pyridyl)carbinol, affords a novel tetradentate coordination mode in homodinuclear lanthanide complexes, which exhibit remarkably short distances between metal ions. The strong luminescences of Eu(III) and Tb(III) complexes with the ligand demonstrate that the ligand has a suitable excited state for energy transfer from the ligand to the Eu(III) and Tb(III) centers, respectively.