Sorting of Molecular Building Blocks from Solution to Surface.

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.

Overcoming artificial broadening in Gd(3+)-Gd(3+) distance distributions arising from dipolar pseudo-secular terms in DEER experiments.

By providing accurate distance measurements between spin labels site-specifically attached to bio-macromolecules, double electron-electron resonance (DEER) spectroscopy provides a unique tool to probe the structural and conformational changes in these molecules. Gd(3+)-tags present an important family of spin-labels for such purposes, as they feature high chemical stability and high sensitivity in high-field DEER measurements. The high sensitivity of the Gd(3+) ion is associated with its high spin (S = 7/2) and small zero field splitting (ZFS), resulting in a narrow spectral width of its central transition at high fields. However, under the conditions of short distances and exceptionally small ZFS, the weak coupling approximation, which is essential for straightforward DEER data analysis, becomes invalid and the pseudo-secular terms of the dipolar Hamiltonian can no longer be ignored. This work further explores the effects of pseudo-secular terms on Gd(3+)-Gd(3+) DEER measurements using a specifically designed ruler molecule; a rigid bis-Gd(3+)-DOTA model compound with an expected Gd(3+)-Gd(3+) distance of 2.35 nm and a very narrow central transition at the W-band (95 GHz). We show that the DEER dipolar modulations are damped under the standard W-band DEER measurement conditions with a frequency separation, Δν, of 100 MHz between the pump and observe pulses. Consequently, the DEER spectrum deviates considerably from the expected Pake pattern. We show that the Pake pattern and the associated dipolar modulations can be restored with the aid of a dual mode cavity by increasing Δν from 100 MHz to 1.09 GHz, allowing for a straightforward measurement of a Gd(3+)-Gd(3+) distance of 2.35 nm. The increase in Δν increases the contribution of the |-5/2〉→|-3/2〉 and |-7/2〉→|-5/2〉 transitions to the signal at the expense of the |-3/2 〉→|-1/2〉 transition, thus minimizing the effect of dipolar pseudo-secular terms and restoring the validity of the weak coupling approximation. We apply this approach to the A93C/N140C mutant of T4 lysozyme labeled with two different Gd(3+) tags that have narrow central transitions and show that even for a distance of 4 nm there is still a significant (about two-fold) broadening that is removed by increasing Δν to 636 MHz and 898 MHz.

Self-Assembled Organic Nanocrystals with Strong Nonlinear Optical Response.

Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.

Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths: innocent or non-innocent ligand?

A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy ((1) H, (13) C, (15) N, (31) P), IR spectroscopy, cyclic voltammetry, (57) Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)2 ] (5: R-PNN=tBu-PNN=6-[(di-tert-butylphosphino)methyl]-2,2'-bipyridine, 6: R-PNN=iPr-PNN=6-[(diisopropylphosphino)methyl]-2,2'-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2'-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)2 ] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO. The analogous Ph-substituted complex 7 was synthesized by a reaction of the free ligand with iron pentacarbonyl. The low-spin complexes 5-7 (S=0) are diamagnetic and have distorted trigonal bipyramidal structures in solution, whereas in the solid state the geometries around the iron are best described as distorted square pyramidal. Compared to other structurally characterized complexes with these PNN ligands, shortened interpyridine CC bonds of about 1.43 Šwere measured. A comparison with known examples, theoretically described as metal complexes bearing bipyridine π-radical anions (bpy(.) (-) ), suggests that the complexes can be described as Fe(I) complexes with one electron antiferromagnetically coupled to the ligand-based radical anions. However, computational studies, at the NEVPT2/CASSCF level of theory, reveal that the shortening of the CC bond is a result of extensive π-backbonding of the iron center into the antibonding orbital of the bpy unit. Hence, the description of the complexes as Fe(0) complexes with neutral bipyridine units is the favorable one.

On the Innocence of Bipyridine Ligands: How Well Do DFT Functionals Fare for These Challenging Spin Systems?

The electronic structures of a number of iron, cobalt, vanadium, and titanium complexes with the 2,2'-bipyridine (bpy) ligand were considered using the multireference CASSCF and NEVPT2 methods. Many of these systems have been studied in the past using B3LYP and were then found to contain the bpy ligand as a radical anion. For many of the cases, this is contradicted by our multireference calculations. While there are instances where the ligand is indeed a radical anion, in many cases it remains neutral and is involved in backbonding from the metal center. For those cases where CASSCF is too costly, a number of DFT functionals, including the newer double-hybrid functionals, were evaluated against the CASSCF data. It was found that nonhybrid functionals, especially those containing the kinetic energy density τ, were the best at predicting the electronic nature of the complexes. The τ-HCTH and HCTH functionals were the top performers, correctly predicting eleven out of eleven test cases and with the lowest mean unsigned errors (MUE, 7.6 and 7.8 kcal·mol(-1), respectively); the M06-L, N12, BLYP, PBE, and TPSS functionals also did well, while B3LYP had significant problems.

Authorizing multiple chemical passwords by a combinatorial molecular keypad lock.

A combinatorial fluorescent molecular sensor operates as a highly efficient molecular security system. The ability of a pattern-generating molecule to process diverse sets of chemical inputs, discriminate among their concentrations, and form multivalent and kinetically stable complexes is demonstrated as a powerful tool for processing a wide range of chemical "passwords" of different lengths. This system thus indicates the potential for obtaining unbreakable combination locks at the molecular scale.

Impact of long-range van der Waals forces on chiral recognition in a Cinchona alkaloid chiral selector system.

Singly-charged complexes of (8S,9R)-tert-butylcarbamoylquinine (tBuCQN), N-3,5-dinitrobenzoyl-(S,R)-leucine (DNB-S/R-leucine), and alkali metal counter ions (Li(+), Na(+), K(+)) were investigated by density-functional theory. It is shown that the cations prefer formation of an ionic pair with the carboxylate group of DNB-Leu over the formation of a cation-π interaction. The [tBuCQN·DNB-S/R-Leu·Na](+) complex is bound by a coulombic attraction, a hydrogen bond, a π-π interaction and van der Waals forces. The tBuCQN chiral selector preferentially complexes with the DNB-S-Leu enantiomer, because the favourable stereochemistry allows the stabilization of the complex by at least one binding mode more compared to the complex containing the DNB-R-Leu molecule. Weakening of the binding modes is observed using the lithium counter ion compared to the sodium one. The weakening is more pronounced in [tBuCQN·DNB-R-Leu·Li](+) than in [tBuCQN·DNB-S-Leu·Li](+). The exact opposite effect is observed using the potassium counter ion. Hence, the lithium counter ion enhances the enantioselectivity of tBuCQN while the potassium counter ion reduces the enantioselectivity of tBuCQN.

Interfacial halogen bonding probed using force spectroscopy.

Halogen bonding between complementary organic monolayers was directly observed in an organic environment using force spectroscopy. This non-covalent interaction is significantly affected by the nature of the organic media. We also demonstrated the effect of lateral packing interactions on the optical properties of the monolayers.

Intramolecular gas-phase reactions of synthetic nonheme oxoiron(IV) ions: proximity and spin-state reactivity rules.

The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV)(O)(L)(OTf)](+) (OTf = trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)Py(2)) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV)(O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV)(O)(L)](2+) cations. DFT calculations on [Fe(IV)(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ(CH)-σ*(z2) overlap, all the reactions still proceed preferentially on the quintet (S = 2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S = 2 reactions for both hexa- and pentacoordinate complexes.

Setting the environmental conditions for controlling gold nanoparticle assemblies.

Fundamental insights into the factors that control the properties and structure of gold nanoparticle (AuNP) based assemblies enable the design and construction of new materials. The dimensions (shape and size) and the optical properties of AuNP assemblies are affected by the electronic properties of the organic cross-linker and the nature of the AuNPs.

Formation of organoxenon dications in the reactions of xenon with dications derived from toluene.

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Trends in Aromatic Oxidation Reactions Catalyzed by Cytochrome P450 Enzymes: A Valence Bond Modeling.

The mixed density functional theory (DFT) and valence bond study described herein focuses on the activation of 17 benzene derivatives by the active species of Cytochrome P450, so-called Compound I (Cpd I), as well as by the methoxy radical, as a potentially simple model of Cpd I (Jones, J. P.; Mysinger, M.; Korzekwa, K. R. Drug Metab. Dispos. 2002, 30, 7-12). Valence bond modeling is employed to rationalize the P450 mechanism and its spin-state selectivity from first principles of electronic structure and to predict activation energies independently, using easily accessible properties of the reactants: the singlet-triplet excitation energies, the ionization potentials of the aromatics, and the electron affinity of Cpd I and/or of the methoxy radical. It is shown that the valence bond model rationalizes all the mechanistic aspects and predicts activation barriers (for 35 reactions) with reasonable accuracy compared to the DFT barriers with an average deviation of ±1.0 kcal·mol(-1) (for DFT barriers, see: Bathelt, C. M.; Ridder, L.; Mulholland, A. J.; Harvey, J. N. Org. Biomol. Chem. 2004, 2, 2998-3005). The valence bond modeling also reveals the mechanistic similarities between the P450 Cpd I and methoxy reactions and enables one to make predictions of barriers for reactions from other studies.

Comparative study of mono- and dinuclear complexes of late 3d-metal chlorides with N,N-dimethylformamide in the gas phase.

Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.

Reduction from copper(II) to copper(I) upon collisional activation of (pyridine)(2) CuCl(+).

Electrospray ionization of dilute aqueous solutions of copper(II) chloride-containing traces of pyridine (py) as well as ammonia permits the generation of the gaseous ions (py)(2) Cu(+) and (py)(2) CuCl(+) , of which the latter is a formal copper(II) compound, whereas the former contains copper(I). Collision-induced dissociation of the mass-selected ions in an ion-trap mass spectrometer (IT-MS) leads to a loss of pyridine from (py)(2) Cu(+) , whereas an expulsion of atomic chlorine largely prevails for (py)(2) CuCl(+) . Theoretical studies using density functional theory predict a bond dissociation energy (BDE) of BDE[(py)(2) Cu(+) -Cl] = 125 kJ mol(-1) , whereas the pyridine ligand is bound significantly stronger, i.e. BDE[(py)CuCl(+) -py] = 194 kJ mol(-1) and BDE[(py)Cu(+) -py] = 242 kJ mol(-1) . The results are discussed with regard to the influence of the solvation on the stability of the Cu(I) /Cu(II) redox couple.

Ligand effects on the [Cu(PhO)(PhOH)]+ redox active complex.

Under gas phase conditions, the [Cu(PhO)(PhOH)](+) complex is composed of copper(I), a phenoxy radical bound via the oxygen atom, and a phenol bound via the aromatic ring. Effects of additional ligand coordination on the molecular and electronic structure of the complex [Cu(PhO)(PhOH)](+) are investigated by mass spectrometric and quantum chemical means for [Cu(PhO)L](+) (L = H(2)O, CH(3)OH, tetrahydrofuran, NH(3), pyridine, imidazole, 1,2-dimethoxyethylene, N,N,N',N'-tetramethylethylenediamine, pyrrole, and thiophene) and [Cu(PhO)(PhOH)L(n)](+) (L = H(2)O, NH(3), and 4-methylimidazole) models. The nature and number of additional ligands critically influences the spin distribution in the complex, which is sensitively reflected by the phenoxy CO stretching mode.

Naphthol coupling monitored by infrared spectroscopy in the gas phase.

The reaction mechanism of copper(II)-mediated naphthol coupling in the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine) is studied using infrared multiphoton dissociation (IRMPD) spectroscopy and DFT calculations. It is shown that the coupling reaction proceeds in ad hoc formed binuclear clusters [(1-H)(2)Cu(2)Cl(TMEDA)(2)](+), where (1-H) is a deprotonated naphthol molecule (methyl ester of 3-hydroxy-2-naphthoic acid). The IRMPD spectra of the isolated cluster in the gas phase reveal that it contains two uncoupled naphtholate subunits and only the irradiation promotes the coupling reaction, which is thus observed as a genuine gas-phase reaction. The driving force for the C-C coupling is a keto-enol tautomerization of the initial coupling product, and the formation of the corresponding binol in the cluster is exothermic by 0.61 eV. In contrast, analogous C-O and O-O couplings are endothermic reactions.

Redox processes in the iron(III)/9,10-phenanthraquinone system.

With the use of the model complexes [(PQ)FeCl(CH(3)O)](+), [(phen)FeCl(CH(3)O)](+), and [(PQ)(phen)FeCl(CH(3)O)](+), where PQ is 9,10-phenanthraquinone and phen is 1,10-phenanthroline, the reactivity of phenanthraquinone in complexes with iron(III) is investigated. It is shown that 9,10-phenanthraquinone takes part in redox processes occurring at iron and thereby allows the oxidation of methanolate to formaldehyde. The oxidation is driven by the reduction of iron(III) to iron(II) and 9,10-phenanthraquinone to the semihydroquinone radical or semiquinolate, if the hydrogen atom is transferred from methanolate to chlorine rather than PQ. 1,10-Phenanthroline, on the other hand, acts as an innocent ligand, and the [(phen)FeCl(CH(3)O)](+) complex shows a typical two-state reactivity. The reactivity of [(PQ)(phen)FeCl(CH(3)O)](+) reveals that the hexacoordination of iron energetically facilitates the oxidation of methanolate, and therefore it is proposed that, in the presence of suitable reductants, the mixture of iron(III) and 9,10-phenanthraquinone can lead to the generation of the semihydroquinone radicals, species responsible for the toxicity of PQ. The fragmentation of [(PQ)(phen)FeCl(CH(3)O)](+) also demonstrates a strong binding of phen toward iron(III), which is a reason for using phen as an iron chelator in biochemistry. The structures and reactivities of the complexes are investigated by means of mass spectrometry, infrared multiphoton dissociation spectra, and density functional theory calculations.

Large effect of a small substitution: competition of dehydration with charge retention and coulomb explosion in gaseous [(bipy(R))Au(mu-O)2Au(bipy(R))]2+ dications.

Dinuclear gold(III) clusters with a rhombic Au(2)O(2) core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy(R)) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy(R))Au(mu-O)(2)Au(bipy(R))](2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(mu-O)(2)Au(bipy)](2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H(2)O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy(R) with R = CH(3), CH(CH(3))(2), CH(2)C(CH(3))(3), and 2,6-C(6)H(3)(CH(3))(2)) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au(2)O(2) core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.

The C-O stretch as an unprecedently large spectral marker for the electron transfer between Copper(II) and a phenolate anion.

An unprecedented red shift of more than 200 cm(-1) in the vibrational frequency of the C-O bond in the [Cu(PhO)Ln]+ complex (PhO = phenoxy), dependent on the number n of additional ligands L, is reported. Upon change of n from 1 to 2, the spin density is shifted from the aromatic ring to the oxygen and copper atoms, which is reflected in the bond order and thus vibrational frequency of the C-O bond.

The phenoxy/phenol/copper cation: a minimalistic model of bonding relations in active centers of mononuclear copper enzymes.

The "bare" complex [Cu(PhOH)(PhO)](+) with a phenol (PhOH) and a phenoxy (PhO) ligand bound to copper is studied both experimentally and computationally. The binding energies and structure of this complex are probed by mass spectrometry, infrared multi-photon dissociation, and DFT calculations. Further, the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+) are investigated for comparison. DFT calculations on the [Cu(PhOH)(PhO)](+) complex predict that a phenolate anion interacts with copper(II) preferentially through the oxygen atom, and the bonding is associated with electron transfer to the metal center resulting in location of the unpaired electron at the aromatic moiety. Neutral phenol, on the other hand, interacts with copper preferentially through the aromatic ring. The same arrangements are also found in the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+). The calculations further indicate that the bond strength between the copper atom and the oxygen atom of the phenoxy radical is weakened by the presence of neutral phenol from 2.6 eV in bare [Cu(PhO)](+) to 2.1 eV in [Cu(PhOH)(PhO)](+).