Cal Mol Research Articles

Stepwise Protonation and Electron-Transfer Reduction of a Primary Copper–Dioxygen Adduct

The protonation–reduction of a dioxygen adduct with [LCu(I)][B(C6F5)4], cupric superoxo complex [LCu(II)(O2(•–))]+ (1) (L = TMG3tren (1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine)) has been investigated. Trifluoroacetic acid (HOAcF) reversibly associates with the superoxo ligand in ([LCu(II)(O2(•–))]+) in a 1:1 adduct [LCu(II)(O2(•–))(HOAcF)](+) (2), as characterized by UV–visible, resonance Raman (rR), nuclear magnetic resonance (NMR), and X-ray absorption (XAS) spectroscopies, along with density functional theory (DFT) calculations. Chemical studies reveal that for the binding of HOAcF with 1 to give 2, Keq = 1.2 × 10(5) M(–1) (−130 °C) and ΔH° = −6.9(7) kcal/mol, ΔS° = −26(4) cal mol(–1) K(–1)). Vibrational (rR) data reveal a significant increase (29 cm(–1)) in vO–O (= 1149 cm(–1)) compared to that known for [LCu(II)(O2(•–))](+) (1). Along with results obtained from XAS and DFT calculations, hydrogen bonding of HOAcF to a superoxo O-atom in 2 is established. Results from NMR spectroscopy of 2 at −120 °C in 2-methyltetrahydrofuran are also consistent with 1/HOAcF = 1:1 formulation of 2 and with this complex possessing a triplet (S = 1) ground state electronic configuration, as previously determined for 1. The pre-equilibrium acid association to 1 is followed by outer-sphere electron-transfer reduction of 2 by decamethylferrocene (Me10Fc) or octamethylferrocene (Me8Fc), leading to the products H2O2, the corresponding ferrocenium salt, and [LCu(II)(OAcF)](+). Second-order rate constants for electron transfer (ket) were determined to be 1365 M(–1) s(–1) (Me10Fc) and 225 M(–1) s(–1) (Me8Fc) at −80 °C. The (bio)chemical relevance of the proton-triggered reduction of the metal-bound dioxygen-derived fragment is discussed.

THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bun, NEt2, and OBut and BDI = 2-[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene

The complexes (BDI)MgX(THF), where X = Bu(n), NEt2, and OBu(t), are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu(n), ΔH(#) (kcal mol(-1)) = 13.4 ± 0.4 and ΔS(#) (cal mol(-1) K(-1)) = 6.3 ± 1.6; X = NEt2, ΔH(#) (kcal mol(-1)) = 15.2 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 11.4 ± 2.3; X = OBu(t), ΔH(#) (kcal mol(-1)) = 16.4 ± 0.3 and ΔS(#) (cal mol(-1) K(-1)) = 9.5 ± 1.3. The apparent aryl group rotations involving the BDI ligands have, within experimental error, the same activation parameters as the THF dissociation, which suggests that the two are correlated involving a three coordinate reactive intermediate akin to what is well-known for related (BDI)ZnR compounds involving three-coordinate trigonal planar Zn(2+). At higher concentrations of THF for X = Bu(n) and OBu(t), but not for X = NEt2, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-d8, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the Ia mechanism to have activation parameters: ΔH(#) (kcal mol(-1)) = 5.4 ± 0.1 and ΔS(#) (cal mol(-1) K(-1)) = -20.9 ± 0.3 for X = Bu(n) and ΔH(#) (kcal mol(-1)) = 8.3 ± 0.8 and ΔS(#) (cal mol(-1) K(-1)) = -19.8 ± 3.0 for X = OBu(t). For the complex (BDI)MgBu(n)(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs more readily than for its THF analogue, as expected for the more sterically demanding Lewis base O-donor: ΔH(#) (kcal mol(-1)) = 12.8 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 8.6 ± 1.8. At very low temperatures in toluene-d8, restricted rotation about the Mg-O(THF) bond is observed for the complexes where X = NEt2 and OBu(t) but not for the complex where X = Bu(n). These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.

Pseudomonomolecular, Ionic sp2-Stereoinversion Mechanism of 1-Aryl-1-alkenyllithiums

The trans/cis stereoinversion of the trigonal carbanion centers C-α in a series of monomeric 2-(α-aryl-α-lithiomethylidene)-1,1,3,3-tetramethylindanes (known to be trisolvated at Li) is rapid on the NMR time scales (400 and 100.6 MHz) in THF solution. The far-reaching redistribution of electric charge in the ground-state molecules caused by lithiation (formal replacement of α-H by α-Li) is illustrated through NMR shifts, Δδ. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett ρ = +5.2), pointing to a mechanism that involves heterolysis of the C–Li bond via a solvent-separated ion pair (SSIP). This requires immobilization of only one additional (the fourth) THF molecule at Li+, which accounts for part of the apparent activation entropies of ca. −23 cal mol–1 K–1 and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp2-stereoinversion process is “catalyzed” by the solvent THF; its mechanism is monomo...

Mechanistic Aspects of Water Oxidation Catalyzed by Organometallic Iridium Complexes

AbstractThe reactions of three iridium water‐oxidation catalysts {[Cp*IrL1L2L3]Xn; 1: L1, L2 = 2,2‐bipyridine (bpy), L3 = Cl, n = 1, X = Cl; 2: L1, L2 = 2‐benzoylpyridine (bzpy), L3 = NO3; 3: L1 = L2 = L3 = H2O, n = 2, X = NO3; Cp* = pentamethylcyclopentadienyl} with cerium ammonium nitrate (CAN) and NaIO4 (sacrificial oxidants, SOs) have been studied by Clark electrode measurements (both in solution and in the gas phase), on‐line mass spectrometry, manometry and UV/Vis spectroscopy. Furthermore, cyclic voltammetry has been applied to evaluate the relative tendency of 1 and 2 to be oxidized. The turnover frequency (TOF) increases as the ratio (R) between the concentration of the SO and that of the catalyst increases. O2 production with CAN is observed in experiments with R = 20 for 1 and 3, whereas O2 becomes detectable with 2 only when R = 40. Catalyst 2 has the highest tendency to be oxidized to IrIV and forms a blue intermediate I characterized by a UV/Vis band at 574 nm. The formation of I occurs with the same velocity as that of the production of O2, which indicates that I is a species directly involved in the catalytic cycle. The disappearance of I, when O2 evolution is finished, is a second‐order process more than one order of magnitude slower than O2 production and is strongly accelerated by the presence of benzyl alcohol. This suggests that I is a molecular species that slowly undergoes disproportion when catalysis is over. Experiments in which multiple aliquots of SO (CAN) were added (R = 20 and 40) indicate that catalysts 1–3 can reinitiate the catalytic cycle once they have been kept in a dormant state for 0–9 min; the TOFs of the second and third additions are approximately equal and higher than that of the first addition. By combining manometry and on‐line mass spectrometry measurements, it was found that O2 evolution is parallel to the production of a small amount of CO2 owing to catalyst degradation. The TOFs of the experiments performed with NaIO4 as the SO are about 2–3 times lower than those with CAN, but the same reactivity order is found 3 > 2 > 1. The activation parameters were evaluated with NaIO4 for all catalysts and with CAN for 2 at 10–45 °C. ΔG# is practically the same in all situations (25–26 kcal mol–1), whereas ΔH# is appreciably lower for 2 (13.1 kcal mol–1 with CAN and 13.3 kcal mol–1 with NaIO4) than for 1 (16 kcal mol–1) and 3 (16.9 kcal mol–1). The lowest enthalpic cost with 2 is balanced by the highest entropic cost (–41 cal mol–1 K–1) that approaches that typical for an associative bimolecular process.

Development of Cyclic Propagation Kinetics for Modeling the Nitroxide‐mediated Radical Copolymerization of Styrene–Divinylbenzene

AbstractA kinetic model including the cyclic propagation (cyclization) is proposed for the nitroxide‐mediated radical copolymerization of styrene–divinylbenzene. The method involves a balance of sequences of units, which connect a radical center and a pendant double bond present in the same polymer chain. The rate constant for cyclization was considered a function of the sequence length. Good agreement between the model predictions and experimental data for solution and suspension controlled copolymerizations was found. The rate constant of cyclization for the smallest ring (3 monomeric units) was estimated to be 700 s−1 at 90 °C, and the activation energy was estimated to be 32 500 cal mol−1.

Single‐Cell Investigations of Silver Nanoparticle–Bacteria Interactions

Interaction of bacteria with citrate‐reduced silver nanoparticles (AgNPs) of size 25 nm ± 8.5 nm is studied using Raman spectroscopy in conjunction with plasmon resonance imaging of single bacterial cells. Distribution of isolated nanoparticles (NPs) inside Escherichia coli (ATCC 25922; E. coli) is observed by hyperspectral imaging (HSI) as a function of incubation time. Time‐dependent degradation of bacterial DNA upon incubation of AgNPs with E. coli is proven by Raman spectroscopic studies. While attachment of NPs is evident in HSI, molecular changes are evident from the surface‐enhanced Raman spectra of adsorbed DNA and its fragments. Distinct enhancement of DNA features is observed upon interaction of AgNPs and the number of such distinct features increases with incubation time, reaches a maximum, and decreases afterwards. This systematic interaction of DNA with the NPs system and its gradual chemical evolution is proven by investigating isolated plasmid DNA. A comparative Raman study with silver ions has shown that DNA features are observable only when bacteria are incubated with AgNPs. Energetics of interaction examined with microcalorimetry suggests the exothermicity of −1.547 × 1010 cal mol−1 for the NP–bacteria system. Specific interaction of AgNPs with exocyclic nitrogen present in the bases, adenine, guanine, and cytosine, leads to the changes in DNA.

The Archaeal Exosome: Identification and Quantification of Site-Specific Motions That Correlate with Cap and RNA Binding

The Archaeal Exosome: Identification and Quantification of Site-Specific Motions That Correlate with Cap and RNA Binding

Recombination of Allyl Radicals in the High Temperature Fall-Off Regime

The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900-1700 K, 10 ± 1, 29 ± 3, 57 ± 3, and 120 ± 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I → C3H5 + I and C3H5 + C3H5 → C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k(1a,124Torr) = (2.6 ± 0.8) × 10(55)T( -12.995) exp(-8426/T), k(1a,57Torr) = (1.7 ± 0.5) × 10(60)T( -14.49) exp(-9344/T), and k(1a,30Torr) = (7.5 ± 2.3) × 10(66)T( -15.935) exp(-10192/T) cm(3) mol(-1)s(-1). The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C6H10 ⇌ C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol(-1 )K(-1), which is significantly smaller than that group additivity predicts, but larger than other reference literature values.

Role of Equilibrium Associations on the Hydrogen Atom Transfer from the Triethylborane–Methanol Complex

The triethylborane-methanol system used in radical deoxygenation and dehalogenation processes has been investigated. Unambiguous evidence for the formation of a complex between triethylborane and methanol is provided. It was shown that the complexation process is exothermic (ΔH° ≈ -7.6 kcal mol(-1)) while being entropically disfavored (ΔS° ≈ -24 cal mol(-1) K(-1)). This study demonstrates that only very small quantities of complex (1-2%) are present in most of the reported conditions used in dehalogenation and deoxygenation processes. Recalculating the rate constant for the hydrogen transfer to a secondary alkyl radical with this concentration suggests a value in the 10(6) M(-1) s(-1) range for the complex itself, indicating a much more important activation of the O-H bond than previously thought. The importance of solvent effects is also highlighted. The formation of a larger amount of complex by the addition of methanol is accompanied by its deactivation via hydrogen bonding. These observations open new opportunitites for the future preparation of more effective hydrogen atom donors involving borane complexes.

Entropies of Organolithium Aggregation Based on Measured Microsolvation Numbers

The recent measurement (J. Am. Chem. Soc.2008, 130, 14179–14188) of the microsolvation numbers of monodentate, nonchelating ethereal donor ligands coordinating to the monomers and dimers of two sterically shielded ═C(aryl)–Li compounds permits the determination of well-founded dimerization enthalpies (ΔH0) and entropies (ΔS0) from properly formulated equilibrium constants, which must include the concentrations of the free donor ligands. The monomers are found to dimerize endothermically (ΔH0 > 0) in [D8]toluene solution in the presence of the donor tBuOMe or THF, but only slightly exothermically (ΔH0 = −0.5 kcal per mol of dimer) with the donor Et2O. The dimerization entropies ΔS0 (in cal mol–1 K–1) with the respective equivalents of released donor ligands are 7.2 and 11.0 (with 2 equiv of tBuOMe in the two cases), 6.1 (with 2 Et2O), and 34.1 (with 4 THF). It is shown that the improper omission of microsolvation from the equilibrium constant (a usual practice when the ligand numbers are not known) can lea...

Quartet formation of a guanine derivative with an isopropyl group: crystal structures of “naked” G-quartets and thermodynamics of G-quartet formation

The formation of guanine quartets with 9-isopropylguanine ((i)PG) is discussed in organic solvents. Crystal structures of the (i)PG quartets were determined by X-ray crystallography with template cations (Na(+) and Ca(2+)) and the structure without a template cation was also obtained by virtue of the stabilization by intermolecular hydrogen bonding with water molecules of crystallization. The difference in the quartet formation of (i)PG in the presence and absence of a template cation was clearly demonstrated by (1)H NMR measurements in CDCl(3)-CH(3)OH mixed solvents. The quartet formation is mainly governed by the enthalpy gain due to the electrostatic interaction between the O6 oxygen in (i)PG and the template cations in the presence of the cations rather than the intermolecular hydrogen bonding, while desolvation of (i)PG is the dominant factor for the formation in the absence of cations. In the presence of Na(+) and Ca(2+), ΔH and ΔS values in the formation of (i)PG-4-Na(+) and (i)PG-4-Ca(2+) complexes were determined to be ΔH = -8.4 kcal mol(-1) and ΔS = +50 cal mol(-1) K(-1) for Na(+) and ΔH = -12.9 kcal mol(-1) and ΔS = +34 cal mol(-1) K(-1) for Ca(2+) on the basis of van't Hoff plots attained from the results of temperature-dependent UV-Vis spectroscopic measurements.

Ruthenium catalyzed equilibrium ring-opening metathesis polymerization of cyclopentene

Polypentenamer was synthetized by equilibrium ring-opening metathesis polymerization (ROMP) using well-defined ruthenium catalyst systems. It was found that the equilibrium time is influenced by the catalyst loading or the catalyst activity, however as expected, the overall cyclopentene conversion is determined only by the applied reaction temperature. Equilibrium of the growing chain and monomer was observed and the activation enthalpy and entropy were determined as: ΔH = −5.6 kcal mol−1; ΔS = −18.5 cal mol−1 K−1. So far these values are the lowest which are reported for cyclopentene polymerization catalyst systems. This unique feature of the equilibrium polymerization opens a way for the synthesis of durable, environmentally friendly elastomers where tires can be not only synthetized but also readily recycled by the same transition metal catalyst system.

A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane–amine Lewis pairs

Calorimetry is used to measure the reaction enthalpies of hydrogen (H(2)) activation by 2,6-lutidine (Lut), 2,2,6,6-tetramethylpiperidine (TMP), N-methyl-2,2,6,6-tetramethylpiperidine (MeTMP), and tri-tert-butylphosphine (TBP) with tris(pentafluorophenyl)borane (BCF). At 6.6 bar H(2) the conversion of the Lewis acid Lewis base pair to the corresponding ionic pair in bromobenzene at 294 K was quantitative in under 60 min. Integration of the heat release from the reaction of the Frustrated Lewis Pair (FLP) with H(2) as a function of time yields a relative rate of hydrogenation in addition to the enthalpy of hydrogenation. The half-lives of hydrogenation range from 230 s with TMP, ΔH(H2) = -31.5(0.2) kcal mol(-1), to 1400 s with Lut, ΔH(H2) = -23.4(0.4) kcal mol(-1). The (11)B nuclear magnetic resonance (NMR) spectrum of B(C(6)F(5))(3) in bromobenzene exhibits three distinct traits depending on the sterics of the Lewis base; (1) in the presence of pyridine, only the dative bond adduct pyridine-B(C(6)F(5))(3) is observed; (2) in the presence of TMP and MeTMP, only the free B(C(6)F(5))(3) is observed; and (3) in the presence of Lut, both the free B(C(6)F(5))(3) and the Lut-B(C(6)F(5))(3) adduct appear in equilibrium. A measure of the change in K(eq) of Lut + B(C(6)F(5))(3) ⇔ Lut-B(C(6)F(5))(3) as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, ΔH = -17.9(1.0) kcal mol(-1) and a ΔS = -49.2(2.5) cal mol(-1) K, suggesting the Lut-B(C(6)F(5))(3) adduct is more stable in bromobenzene than in toluene.

Structure of Phosphorylated SF1 Bound to U2AF65 in an Essential Splicing Factor Complex

The essential splicing factors U2AF⁶⁵ and SF1 cooperatively bind consensus sequences at the 3' end of introns. Phosphorylation of SF1 on a highly conserved "SPSP" motif enhances its interaction with U2AF⁶⁵ and the pre-mRNA. Here, we reveal that phosphorylation induces essential conformational changes in SF1 and in the SF1/U2AF⁶⁵/3' splice site complex. Crystal structures of the phosphorylated (P)SF1 domain bound to the C-terminal domain of U2AF⁶⁵ at 2.29 Å resolution and of the unphosphorylated SF1 domain at 2.48 Å resolution demonstrate that phosphorylation induces a disorder-to-order transition within a previously unknown SF1/U2AF⁶⁵ interface. We find by small-angle X-ray scattering that the local folding of the SPSP motif transduces into global conformational changes in the nearly full-length (P)SF1/U2AF⁶⁵/3' splice site assembly. We further determine that SPSP phosphorylation and the SF1/U2AF⁶⁵ interface are essential in vivo. These results offer a structural prototype for phosphorylation-dependent control of pre-mRNA splicing factors.

Curcuminoids as antioxidants and theoretical study of stability of curcumin isomers in gaseous state

The antioxidant activity of three extracted curcuminods, namely, curcumin [(1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5dione] (C), demethoxycurcumin (DMC), and bisdemethoxycurcumin (BDMC), were studied with the DPPH, hydrogen peroxide and nitric oxide radical methods, and compared with the known antioxidant ascorbic acid. Structures for the extracted curcuminods are proposed on the basis of spectroscopic evidence. Curcumin molecule stability was studied in the gaseous state, and it has two isomers [the diketone form, curcumin(I), and the enol form, curcumin(II)] through the theoretical study by relying on the results of density functions theory (DFT). The results of each of the total energy and the high occupied molecular orbital (HOMO) to curcumin(I) are more stable than to curcumin(II). The increase of the amount of total energy is −0.01301141 a.u., or equivalent, −8,164.783 cal mol−1. The HOMO level is −0.35865 eV, also the thermodynamic values (the change in entropy ∆S and the change in enthalpy ∆H) of the isomerization conversion form curcumin(II) to curcumin(I) spontaneous and endothermic reaction, of ∆S and ∆H are −3.136 cal mol−1 K−1 and −0.673 kcal mol−1, respectively. The results showed that the wavelength for greatest absorption (λ max) of the enol form curcumin(II) is longer than curcumin(I). This is due to the formation of a new double bond which leads to the association distribution of the electronic density along the molecule in the enol form.

Binding Efficiency of Protein–Protein Complexes

We examine the relationship between binding affinity and interface size for reversible protein-protein interactions (PPIs), using cytokines from the tumor necrosis factor (TNF) superfamily and their receptors as a test case. Using surface plasmon resonance, we measured single-site binding affinities for binding of the large receptor TNFR1 to its ligands TNFα (K(D) = 1.4 ± 0.4 nM) and lymphotoxin-α (K(D) = 50 ± 10 nM), and also for binding of the small receptor Fn14 to TWEAK (K(D) = 70 ± 10 nM). We additionally assembled data for all other TNF-TNFR family complexes for which reliable single-site binding affinities have been reported. We used these values to calculate the binding efficiencies, defined as binding energy per square angstrom of surface area buried at the contact interface, for nine of these complexes for which cocrystal structures are available, and compared the results to those for a set of 144 protein-protein complexes with published affinities. The results show that the most efficient PPI complexes generate ~20 cal mol(-1) Å(-2) of binding energy. A minimal contact area of ~500 Å(2) is required for a stable complex, required to generate sufficient interaction energy to pay the entropic cost of colocalizing two proteins from 1 M solution. The most compact and efficient TNF-TNFR complex was the BAFF-BR3 complex, which achieved ~80% of the maximal achievable binding efficiency. Other small receptors also gave high binding efficiencies, while the larger receptors generated only 44-49% of this limit despite interacting primarily through just a single small domain. The results provide new insight into how much binding energy can be generated by a PPI interface of a given size, and establish a quantitative method for predicting how large a natural or engineered contact interface must be to achieve a given level of binding affinity.

Two-Step Binding of O2 to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V(N[(t)Bu]Ar)(3) (1) (Ar = 3,5-Me(2)C(6)H(3)) with O(2) was shown by stopped-flow kinetic studies to result in the rapid formation of (η(1)-O(2))V(N[(t)Bu]Ar)(3) (2) (ΔH(‡) = 3.3 ± 0.2 kcal/mol and ΔS(‡) = -22 ± 1 cal mol(-1) K(-1)), which subsequently isomerizes to (η(2)-O(2))V(N[(t)Bu]Ar)(3) (3) (ΔH(‡) = 10.3 ± 0.9 kcal/mol and ΔS(‡) = -6 ± 4 cal mol(-1) K(-1)). The enthalpy of binding of O(2) to form 3 is -75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of 3 and 1 to form 2 equiv of O≡V(N[(t)Bu]Ar)(3) (4) occurs by initial isomerization of 3 to 2. The results of computational studies of this rearrangement (ΔH = 4.2 kcal/mol; ΔH(‡) = 16 kcal/mol) are in accord with experimental data (ΔH = 4 ± 3 kcal/mol; ΔH(‡) = 14 ± 3 kcal/mol). With the aim of suppressing the formation of 4, the reaction of O(2) with 1 in the presence of (t)BuCN was studied. At -45 °C, the principal products of this reaction are 3 and (t)BuC(═O)N≡V(N[(t)Bu]Ar)(3) (5), in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.

Degenerate Molecular Shuttles with Flexible and Rigid Spacers

The preparation and dynamic behavior of degenerate rotaxane molecular shuttles are described in which a benzylic amide macrocycle moves back and forth between two naphthalimide-glycine units along a diphenylethyne spacer or an aliphatic spacer consisting of a C(9), C(12), or C(26) alkyl chain. Subtle differences in the (1)H NMR spectra of the rotaxanes can be related to the presence of conformers in which the macrocycle interacts simultaneously with both glycines, especially in the case of the C(9) spacer. The kinetic data of the shuttling behavior in the C(26) rotaxane were obtained from dynamic NMR spectroscopy. The Eyring activation parameters were found to be ΔH(‡) = 10 ± 1 kcal mol(-1), ΔS(‡) = -6.5 ± 2.0 cal mol(-1) K(-1), ΔG(‡)(298) = 11.9 ± 0.2 kcal mol(-1). For the systems with the shorter spacers, the shuttling rates were higher. Also in the diphenylethyne, rotaxane shuttling is rapid on the NMR time scale, indicating that the rigid unit does not impose a large barrier to the translocation of the macrocycle.

M–M Bond-Stretching Energy Landscapes for M2(dimen)42+ (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane) Complexes

Isomers of Ir(2)(dimen)(4)(2+) (dimen = 1,8-diisocyanomenthane) exhibit different Ir-Ir bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter Ir-Ir distance is favored at room temperature [K = ∼8; ΔH° = -0.8 kcal/mol; ΔS° = 1.44 cal mol(-1) K(-1)]. We report calculations that shed light on M(2)(dimen)(4)(2+) (M = Rh, Ir) structural differences: (1) metal-metal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A(2u)) distortion promotes twisting of the ligand backbone at short metal-metal separations. Calculated potential-energy surfaces reveal a double minimum for Ir(2)(dimen)(4)(2+) (∼4.1 Å Ir-Ir with 0° twist angle and ∼3.6 Å Ir-Ir with ±12° twist angle) but not for the rhodium analogue (∼4.5 Å Rh-Rh with no twisting). Because both the ligand strain and A(2u) distortional energy are virtually identical for the two complexes, the strength of the metal-metal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the Ir-Ir coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir(2)(dimen)(4)(2+) (R(e,Ir-Ir) = 2.87 Å; F(Ir-Ir) = 0.99 mdyn Å(-1)); (2) a single-minimum, anharmonic surface for the ground state of Rh(2)(dimen)(4)(2+) (R(e,Rh-Rh) = 3.23 Å; F(Rh-Rh) = 0.09 mdyn Å(-1)); (3) a double-minimum (along the Ir-Ir coordinate) surface for the ground state of Ir(2)(dimen)(4)(2+) (R(e,Ir-Ir) = 3.23 Å; F(Ir-Ir) = 0.16 mdyn Å(-1)).

Spectroscopic Evidence in Solid and Solution of a Discrete Copper(I) Tetrahedral Complex Dimer Supported by Supramolecular Interactions

The first study of the self-association of a copper(I) tetrahedral complex ([Cu(N-{4-nitrophenyl}pyridine-2-yl-methanimine)(PPh3)Br]) has been performed. The formation of a discrete dimer supported by supramolecular π–π stacking and C–H···Br interactions was established by X-ray diffraction techniques. 1D- and 2D NMR techniques were used for the structural characterization of this compound in solution. Unexpected 1H–1H NOE effects are coherent with the presence of a dimer in solution, whose structure replicates that found in the crystal. Dimerization constants obtained by VT-1H NMR spectroscopy allowed the determination ofthe thermodynamic parameters for the self-association process, namely ΔS = –0.67 ± 0.21 cal mol–1 K–1, ΔH = –2.00 ± 0.05 kcal mol–1 and ΔG(298 K) = –1.79 kcal mol–1. Consequently, this association process is enthalpy driven.