Catalytic Transfer Hydrogenation by a Trivalent Phosphorus Compound: Phosphorus‐Ligand Cooperation Pathway or PIII/PV Redox Pathway?

Main‐group‐element catalysts are a desirable alternative to transition‐metal catalysts because of natural abundance and cost. However, the examples are very limited. Catalytic cycles involving a redox process and E‐ligand cooperation (E=main‐group element), which are often found in catalytic cycles of transition‐metal catalysts, have not been reported. Herein theoretical investigations of a catalytic hydrogenation of azobenzene with ammonia–borane using a trivalent phosphorus compound, which was experimentally proposed to occur through P^III/P^V redox processes via an unusual pentavalent dihydridophosphorane, were performed. DFT and ONIOM(CCSD(T):MP2) calculations disclosed that this catalytic reaction occurs through a P‐O cooperation mechanism, which resembles the metal‐ligand cooperation mechanism of transition‐metal catalysts.     

Ligand‐Enabled Catalytic C?H Arylation of Aliphatic Amines by a Four‐Membered‐Ring Cyclopalladation Pathway

A palladium‐catalyzed C H arylation of aliphatic amines with arylboronic esters is described, proceeding through a four‐membered‐ring cyclopalladation pathway. Crucial to the successful outcome of this reaction is the action of an amino‐acid‐derived ligand. A range of hindered secondary amines and arylboronic esters are compatible with this process and the products of the arylation can be advanced to complex polycyclic molecules by sequential C H activation reactions.     

Lanthanide‐to‐Lanthanide Energy‐Transfer Processes Operating in Discrete Polynuclear Complexes: Can Trivalent Europium Be Used as a Local Structural Probe?

This work, based on the synthesis and analysis of chemical compounds, describes a kinetic approach for identifying intramolecular intermetallic energy‐transfer processes operating in discrete polynuclear lanthanide complexes, with a special emphasis on europium‐containing entities. When all coordination sites are identical in a (supra)molecular complex, only heterometallic communications are experimentally accessible and a Tb→Eu energy transfer could be evidenced in [TbEu( L5 )(hfac)₆] (hfac=hexafluoroacetylacetonate), in which the intermetallic separation amounts to 12.6 Å. In the presence of different coordination sites, as found in the trinuclear complex [Eu₃( L2 )(hfac)₉], homometallic communication can be induced by selective laser excitation and monitored with the help of high‐resolution emission spectroscopy. The narrow and non‐degenerated character of the Eu(⁵D₀?⁷F₀) transition excludes significant spectral overlap between donor and acceptor europium cations. Intramolecular energy‐transfer processes in discrete polynuclear europium complexes are therefore limited to short distances, in agreement with the Fermi golden rule and with the kinetic data collected for [Eu₃( L2 )(hfac)₉] in the solid state and in solution. Consequently, trivalent europium can be considered as a valuable local structural probe in discrete polynuclear complexes displaying intermetallic separation in the sub‐nanometric domain, a useful property for probing lanthanido‐polymers.     

Dynamic Equilibria in Solvent‐Mediated Anion,Cation and Ligand Exchange in Transition‐Metal Coordination Polymers: Solid‐State Transfer or Recrystallisation?

The solution properties of a series of transition‐metal–ligand coordination polymers [ML(X)_n]_∞ [M=Ag^I, Zn^II, Hg^II and Cd^II; L=4,4′‐bipyridine (4,4′‐bipy), pyrazine (pyz), 3,4′‐bipyridine (3,4′‐bipy), 4‐(10‐(pyridin‐4‐yl)anthracen‐9‐yl)pyridine (anbp); X=NO₃^?, CH₃COO^?, CF₃SO₃^?, Cl^?, BF₄^?; n=1 or 2] in the presence of competing anions, metal cations and ligands have been investigated systematically. Providing that the solubility of the starting complex is sufficiently high, all the components of the coordination polymer, namely the anion, the cation and the ligand, can be exchanged on contact with a solution phase of a competing component. The solubility of coordination polymers is a key factor in the analysis of their reactivity and this solubility depends strongly on the physical properties of the solvent and on its ability to bind metal cations constituting the backbone of the coordination polymer. The degree of reversibility of these solvent‐induced anion‐exchange transformations is determined by the ratio of the solubility product constants for the starting and resultant complexes, which in turn depend upon the choice of solvent and the temperature. The extent of anion exchange is controlled effectively by the ratio of the concentrations of incoming ions to outgoing ions in the liquid phase and the solvation of various constituent components comprising the coordination polymer. These observations can be rationalised in terms of a dynamic equilibrium of ion exchange reactions coupled with Ostwald ripening of crystalline products. The single‐crystal X‐ray structures of [Ag(pyz)ClO₄]_∞ ( 1 ), {[Ag(4,4′‐bipy)(CF₃SO₃)] ? CH₃CN}_∞ ( 2 ), {[Ag(4,4′‐bipy)(CH₃CN)]ClO₄ ? 0.5 CH₃CN}_∞ ( 3 ), metal‐free anbp ( 4 ), [Ag(anbp)NO₃(H₂O)]_∞ ( 5 ), {[Cd(4,4′‐bipy)₂(H₂O)₂](NO₃)₂ ? 4 H₂O}_∞ ( 6 ) and {[Zn(4,4′‐bipy)SO₄(H₂O)₃] ? 2 H₂O}_∞ ( 7 ) are reported.     

l‐Cysteinium semioxalate: a new monoclinic polymorph or a hydrate?

The title compound, C₃H₈NO₂S⁺·C₂HO₄⁻, (I), crystallizes in the monoclinic C2 space group and is a new form (possibly a hydrate) of l ‐cysteinium semioxalate with a stoichiometric cation–anion ratio of 1:1. In contrast to the previously known orthorhombic form of l ‐cysteinium semioxalate, (I) has a layered structure resembling those of monoclinic l ‐cysteine, as well as of dl ‐cysteine and its oxalates. The conformations of the cysteinium cation and the oxalate anion in (I) differ substantially from those in the orthorhombic form. The structure of (I) has voids with a size sufficient to incorporate water molecules. The residual density, however, suggests that if water is in fact present in the voids, it is strongly disordered and its amount does not exceed 0.3 molecules per void. The difference in conformation of the cysteinium cations in (I) and in the orthorhombic form is similar to that in dl ‐cysteine under ambient conditions and in dl ‐cysteine under high pressure or at low temperature.     

Distinct Stepwise Reduction of a Nickel?Nickel‐Bonded Compound Containing an α‐Diimine Ligand: From Perpendicular to Coaxial Structures

A nickel? nickel‐bonded complex, [{Ni(μ‐L^.?)}₂] ( 1 ; L=[(2,6‐iPr₂C₆H₃)NC(Me)]₂), was synthesized from reduction of the LNiBr₂ precursor by sodium metal. Further controllable reduction of 1 with 1.0, 2.0 and 3.0 equiv of Na, respectively, afforded the singly, doubly, and triply reduced compounds [Na(DME)₃] ? [{Ni(μ‐L^.?)}₂] ( 2 ; DME=1,2‐dimethoxyethane), [Na(Et₂O)]Na[(L^.?)Ni? NiL^2?] ( 3 ), and [Na(Et₂O)]₂Na[L^2?Ni? NiL^2?] ( 4 ). Here L represents the neutral ligand, L^.? denotes its radical monoanion, and L^2? is the dianion. All of the four compounds feature a short Ni? Ni bond from 2.2957(6) to 2.4649(8) Å. Interestingly, they display two different structures: the perpendicular ( 1 and 2 ) and the coaxial ( 3 and 4 ) structure, in which the metal? metal bond axis is perpendicular to or collinear with the axes of the α‐diimine ligands, respectively. The electronic structures, Ni? Ni bonding nature, and energetic comparisons of the two structure types were investigated by DFT computations.     

Decomposition of 2‐Mercaptoethyl O‐Ester: SN2 Displacement or Acyl Transfer? A Theoretical Study

The mechanism for the decomposition of 2‐mercaptoethyl O‐ester was theoretically investigated. The mechanism that 2‐mercaptoethyl O‐ester undergoes an S_N2 displacement of the O atom by the S atom on α‐C is much favored over the mechanism of N‐to‐S acyl transfer. The length of the alcohol moiety has large effects on the decomposition efficiency of thiol‐substituted alkyl O‐esters. The reactivities of these esters are controlled by distortion energies. Only 2‐mercaptoethyl O‐ester can undergo the decomposition at room temperature due to the low distortion energy to achieve the transition state geometry. If the thiol group of 2‐mercaptoethyl O‐ester is replaced by an amino group, the N‐to‐N acyl transfer mechanism is more favored than the S_N2 displacement mechanism.     

Protonation of a Biologically Relevant CuII μ‐Thiolate Complex: Ligand Dissociation or Formation of a Protonated CuI Disulfide Species?

The proton‐induced electron‐transfer reaction of a Cu^II μ‐thiolate complex to a Cu^I‐containing species has been investigated, both experimentally and computationally. The Cu^II μ‐thiolate complex [Cu^II₂( L^MeS )₂]²⁺ is isolated with the new pyridyl‐containing ligand L^MeSSL^Me , which can form both Cu^II thiolate and Cu^I disulfide complexes, depending on the solvent. Both the Cu^II and the Cu^I complexes show reactivity upon addition of protons. The multivalent tetranuclear complex [Cu^I₂Cu^II₂( LS )₂(CH₃CN)₆]⁴⁺ crystallizes after addition of two equivalents of strong acid to a solution containing the μ‐thiolate complex [Cu^II₂( LS )₂]²⁺ and is further analyzed in solution. This study shows that, upon addition of protons to the Cu^II thiolate compound, the ligand dissociates from the copper centers, in contrast to an earlier report describing redox isomerization to a Cu^I disulfide species that is protonated at the pyridyl moieties. Computational studies of the protonated Cu^II μ‐thiolate and Cu^I disulfide species with LSSL show that already upon addition of two equivalents of protons, ligand dissociation forming [Cu^I(CH₃CN)₄]⁺ and protonated ligand is energetically favored over conversion to a protonated Cu^I disulfide complex.     

Molecular Tethering or Aggregation: Is the Existence of Charge‐Transfer Bands Indicative of the Formation of Blue‐Box/Tetrathiafulvalene Inclusion Complexes?

The interaction between tetrathiafulvalene and tetracation cyclobis(paraquat‐p‐phenylene) fragments—the key elements of many rotaxane systems—was investigated theoretically by using ab‐initio second‐order perturbation methods. In addition to the inclusion complex observed in the solid state, a thermodynamically stable “exterior” complex was identified. Calculation of the UV/Vis spectra for the inclusion and the exterior complexes indicated that the charge‐transfer band that is often used to predict the formation of the inclusion complexes in solution is, in reality, due to the exterior mode of complexation. These results suggest that UV/Vis spectroscopy is not a reliable method for assigning the complexation modes in TTF:BB⁴⁺ rotaxanes and related systems.     

Directional Preference of DNA‐Mediated Electron Transfer in Gold‐Tethered DNA Duplexes: Is DNA a Molecular Rectifier?

Electrical properties of self‐assembling DNA nanostructures underlie the paradigm of nanoscale bioelectronics, and as such require clear understanding. DNA‐mediated electron transfer (ET) from a gold electrode to DNA‐bound Methylene Blue (MB) shows directional preference, and it is sequence‐specific. During the electrocatalytic reduction of [Fe(CN)₆]^3? catalyzed by DNA‐bound MB, the ET rate constant for DNA‐mediated reduction of MB reaches (1.32±0.2)10³ and (7.09±0.4)10³ s^?1 for (dGdC)₂₀ and (dAdT)₂₅ duplexes. The backward oxidation process is less efficient, making the DNA duplex a molecular rectifier. Lower rates of ET via (dGdC)₂₀ agree well with its disturbed π‐stacked sub‐molecular structure. Such direction‐ and sequence‐specific ET may be implicated in DNA oxidative damage and repair, and be relevant to other polarized surfaces, such as cell membranes and biomolecular interfaces.     

Energy Transfer by a Hopping Mechanism in Dinuclear IrIII/RuII Complexes: A Molecular Wire?

The synthesis and electrochemical and photophysical properties of a series of heterodinuclear ruthenium-iridium complexes linked by a modular para-phenylene bridge [Ir-ph(n)-Ru]3+ (Ir=Ir(ppyFF)2bpy, Ru=Ru(bpy)3, ppyFF=2-(2,4-difluorophenyl)pyridine), bpy=2,2'-bipyridine, ph=phenylene, n=2, 3, 4, 5) are reported. The use of a high-energy iridium complex, which can act as an energy donor when coupled to the lower energy ruthenium-based component, allows the investigation of photoinduced energy transfer from the excited iridium-centre to the ruthenium fragment (energy acceptor). The rate constants of the energy-transfer processes are determined by time-resolved emission and sub-picosecond transient absorption spectroscopy. Interestingly, there is almost no decrease in transfer efficiency or rates as the length between the two chromophores (number of spacers) is increased. This "molecular wire" behavior indicates the dominance of the incoherent hopping mechanism, allowing a very fast energy transfer over long distances (with n = 5 the metal-to-metal distance is estimated to be 32.5 Angstrom). This is the first case in which such behavior is observed for metal complexes, and could lead to new development in molecular electronics.     

Oligoene‐Based π‐Helicenes or Dispiranes? Winding up Oligoyne Chains by a Multiple Carbopalladation/Stille/(Electrocyclization) Cascade

A domino approach consisting of up to five consecutive steps to access either highly substituted dispiranes or π‐helicenes from oligoyne chains is reported. The domino sequence consists of several carbopalladation reactions, a Stille cross‐coupling to obtain the helicenes, and, depending on the steric demands of the helicene, a final 6π‐electrocyclization to afford the dispiranes. Formally, the latter transformation contravenes the Woodward–Hoffmann rules, as revealed by X‐ray crystallography of the dispirane. Additionally, the racemization barrier of the (Z,Z,Z)‐triene‐based helicene has been determined by a kinetic analysis and compared with results from density functional theory calculations. Characteristic points on the reaction coordinate were further analyzed according to their relaxed force constants (compliance constants).     

C?F Activation at Rhodium Boryl Complexes: Formation of 2‐Fluoroalkyl‐1,3,2‐Dioxaborolanes by Catalytic Functionalization of Hexafluoropropene

Fluorinated building blocks by C? F bond cleavage : Catalytic C? F activation reactions that give novel dioxaborolanes have been developed (see scheme). The reactions proceed at room temperature, and catalytic intermediates are presumably rhodium hydride and boryl species.