Tuning the redox profile of the 6,6'-biazulenic platform through functionalization along its molecular axis.

The E1/2 potential associated with reduction of the linearly-functionalized 6,6'-biazulenic scaffold is accurately correlated to the combined σp Hammett parameters of the substituents over >600 mV range. X-ray crystallographic analysis of the 2,2'-dichloro-substituted derivative revealed unexpectedly short C-Cl bond distances, along with other metric changes, suggesting a non-trivial cycloheptafulvalene-like structural contribution.

Valence-to-Core X-ray Emission Spectroscopy as a Probe of O-O Bond Activation in Cu2 O2 Complexes.

Valence-to-Core (VtC) X-ray emission spectroscopy (XES) was used to directly detect the presence of an O-O bond in a complex comprising the [CuII2 (µ-η2 :η2 -O2 )]2+ core relative to its isomer with a cleaved O-O bond having a [CuIII2 (µ-O)2 ]2+ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [CuII2 (µ-η2 :η2 -O2 )]2+ core corresponds to the copper stabilized in-plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O-O bond activation in µ-η2 :η2 -O2 species and highlight the potential of this method for time-resolved studies of reaction mechanisms.

Formally Copper(III)-Alkylperoxo Complexes as Models of Possible Intermediates in Monooxygenase Enzymes.

Reaction of [NBu4][LCuIIOH] with excess ROOH (R = cumyl or tBu) yielded [NBu4][LCuIIOOR], the reversible one-electron oxidation of which generated novel species with [CuOOR]2+ cores (formally CuIIIOOR), identified by spectroscopy and theory for the case R = cumyl. This species reacts with weak O-H bonds in TEMPO-H and 4-dimethylaminophenol (NMe2PhOH), the latter yielding LCu(OPhNMe2), which was also prepared independently. With the identification of [CuOOR]2+ complexes, the first precedent for this core in enzymes is provided, with implications for copper monooxygenase mechanisms.

Determination of the Cu(III)-OH Bond Distance by Resonance Raman Spectroscopy Using a Normalized Version of Badger's Rule.

The stretching frequency, ν(Cu-O), of the [CuOH]2+ core in the complexes LCuOH (L = N,N'-bis(2,6-diisopropyl-4-R-phenyl)pyridine-2,6-dicarboxamide, R = H or NO2, or N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide) was determined to be ∼630 cm-1 by resonance Raman spectroscopy and verified by isotopic labeling. In efforts to use Badger's rule to estimate the bond distance corresponding to ν(Cu-O), a modified version of the rule was developed through use of stretching frequencies normalized by dividing by the appropriate reduced masses. The modified version was found to yield excellent fits of normalized frequencies to bond distances for >250 data points from theory and experiment for a variety of M-X and X-X bond distances in the range ∼1.1-2.2 Å (root mean squared errors for the predicted bond distances of 0.03 Å). Using the resulting general equation, the Cu-O bond distance was predicted to be ∼1.80 Å for the reactive [CuOH]2+ core. Limitations of the equation and its use in predictions of distances in a variety of moieties for which structural information is not available were explored.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity.

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation.

Two new ligand sets, (pipMe)LH2 and (NO2)LH2 ((pipMe)L = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, (NO2)L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)-hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O-H bonds of the corresponding Cu(II)-OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)-hydroxide complexes from substrates with C-H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from -80 to -20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.

Probing Electronic Communications in Heterotrinuclear Fe-Ru-Fe Molecular Wires Formed by Ruthenium(II) Tetraphenylporphyrin and Isocyanoferrocene or 1,1'-Diisocyanoferrocene Ligands.

Two new heterotrinuclear Fe-Ru-Fe complexes of ruthenium(II) tetraphenylporphyrin axially coordinated with a pair of isocyanoferrocene ((FcNC)2RuTPP, 1) or 1,1'-diisocyanoferrocene (([C5H4NC]2Fe)2RuTPP, 2) ligands [Fc = ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2-) anion] were synthesized and characterized by UV-vis, magnetic circular dichroism, NMR, and FTIR spectroscopies as well as by electrospray ionization mass spectrometry and single-crystal X-ray diffraction. Isolation of insoluble polymeric {([C5H4NC]2Fe)RuTPP}n molecular wires (3) was also achieved for the first time. The redox properties of the new trinuclear complexes 1 and 2 were probed using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation methods and correlated to those of the bis(tert-butylisocyano)ruthenium(II) tetraphenylporphyrin reference compound, (t-BuNC)2RuTPP (4). In all cases, the first oxidation process was attributed to the reversible oxidation of the Ru(II) center. The second and third reversible oxidation processes in 1 are separated by ∼100 mV and were assigned to two single-electron Fe(II)/Fe(III) couples, suggesting a weak long-range iron-iron coupling in this complex. Electrochemical data acquired for 2 are complicated by the interaction between the axial η(1)-1,1'-diisocyanoferrocene ligand and the electrode surface as well as by axial ligand dissociation in solution. Spectroelectrochemical and chemical oxidation methods were used to elucidate the spectroscopic signatures of the [1](n+), [2](n+), and [4](n+) species in solution. DFT and time-dependent DFT calculations aided in correlating the spectroscopic and redox properties of complexes 1, 2, and 4 with their electronic structures.

Probing the electronic properties of a trinuclear molecular wire involving isocyanoferrocene and iron(II) phthalocyanine motifs.

A new trinuclear iron(II) complex involving two isocyanoferrocene ligands axially coordinated to iron(II) phthalocyanine, (FcNC)2FePc [Fc = ferrocenyl; Pc = phthalocyaninato(2-) anion], was isolated and characterized using a variety of spectroscopic methods as well as single-crystal X-ray diffraction. The redox behavior of the above molecular wire was investigated through electrochemical, spectroelectrochemical, and chemical oxidation approaches and compared to that of the bis(tert-butylisocyano)iron(II) phthalocyanine reference compound, (t-BuNC)2FePc. For both complexes, the first oxidation involves the phthalocyanine ligand and results in the formation of a red phthalocyanine cation-radical-centered [(RNC)2FePc](+) species, as evidenced by their UV-vis and electron paramagnetic resonance spectra. Despite the ~11.5 Šdistance between the isocyanoferrocene iron centers, the second and third oxidation potentials for (FcNC)2FePc are separated by ∼80 mV, which is indicative of a weak long-range metal-metal coupling in this system. Spectroscopic signatures of the mixed-valence [(FcNC)2FePc](2+) dication were obtained using spectroelectrochemical and chemical oxidation approaches. These experimentally assessed characteristics were also correlated with the electronic structure, redox properties, and spectroscopic signatures predicted by density functional theory (DFT) and time-dependent DFT analyses.

Diisocyanoarene-linked pentacarbonylvanadate(I-) ions as building blocks in a supramolecular charge-transfer framework assembled through noncovalent π-π and contact ion interactions.

Three synthetic routes to the unusual supramolecular complex ([Cp(2)Co](2)[{(OC)(5)V}(2)(µ-1,4-CNC(6)Me(4)NC)])(∞), which was crystallographically characterized, are presented. The dianion [{(OC)(5)V}(2)(µ-1,4-CNC(6)Me(4)NC)](2-) constitutes the first subvalent organometallics featuring a diisocyanoarene linker.

Linear 6,6'-biazulenyl framework featuring isocyanide termini: synthesis, structure, redox behavior, complexation, and self-assembly on Au(111).

The key step in accessing the title species (5), the first nonbenzenoid diisocyanobiaryl, involved an unexpected homocoupling of a 6-bromoazulene derivative. The reversible 2e(-) reduction of 5 was addressed electrochemically and computationally. The shifts in energies of the S(0)→S(1) and S(0)→S(2) transitions for a series of related 6,6'-biazulenyl derivatives correlate with the e(-)-donating/-withdrawing strength of their 2,2'-substituents but follow opposite trends. Species 5 adsorbs end-on (η(1)) to the Au(111) surface via one of its -NC groups to form a 2-nm-thick film. In addition, bimetallic coordination of 5's -NC termini can be readily achieved.