Crowned Macrocycles Containing Two Pyrrolo[1,2-a] Indoles Created By Intramolecular Fusion

Heterocyclic fused-ring systems are of utmost importance because of their presence in many natural products with biological activity. Pyrroloindoles are tricyclic heterocycles that are present in various bioactive and medicinally valuable compounds. Herein, we report the synthesis of phenylene-bridged bis-pyrrolo[1,2-a]indole crowned macrocycles 1 – 3 in which the pyrrolo[1,2-a]indole moieties were formed via intramolecular fusion. The macrocycles were thoroughly characterized by 1D and 2D NMR, HRMS and X-ray crystallographic studies. The X-ray structure revealed that the two pyrrolo[1,2-a]indole moieties were parallel to each other, and one pyrrolo[1,2-a]indole unit was deviated by an angle of 9.54° while the other pyrrolo[1,2-a]indole unit was deviated by an angle of 12.0° from the mean plane defined by 28 core atoms. The macrocycles 1 – 3 absorb in the visible region and readily undergo oxidations because of their electron rich nature. The macrocycles 1 – 3 upon treatment with trifluoroacetic acid (TFA) generates the corresponding cation radicals 1 – 3 ^.+ which were stable in the open air for a week. The cation radicals 1 – 3 ^.+ absorb strongly in the NIR region and the experimental observations on crowned macrocycles 1 – 3 were corroborated by DFT and TD-DFT studies.     

Chemical reactions modulated by mechanical stress: extended Bell theory

A number of recent studies have shown that mechanical stress can significantly lower or raise the activation barrier of a chemical reaction. Within a common approximation due to Bell [Science 200, 618 (1978)], this barrier is linearly dependent on the applied force. A simple extension of Bell's theory that includes higher order corrections in the force predicts that the force-induced change in the activation energy will be given by -FΔR - ΔχF(2)∕2. Here, ΔR is the change of the distance between the atoms, at which the force F is applied, from the reactant to the transition state, and Δχ is the corresponding change in the mechanical compliance of the molecule. Application of this formula to the electrocyclic ring-opening of cis and trans 1,2-dimethylbenzocyclobutene shows that this extension of Bell's theory essentially recovers the force dependence of the barrier, while the original Bell formula exhibits significant errors. Because the extended Bell theory avoids explicit inclusion of the mechanical stress or strain in electronic structure calculations, it allows a computationally efficient characterization of the effect of mechanical forces on chemical processes. That is, the mechanical susceptibility of any reaction pathway is described in terms of two parameters, ΔR and Δχ, both readily computable at zero force.     

Ligand degradation and phosphorus scavenging in the reaction between 1,2-bis(diphenylphosphino)benzene (dppbz) and Ru6(μ6-C)(CO)17: Synthesis and X-ray structure of the edge-bridged square-pyramidal cluster HRu6(μ5-C)(μ3-P)(CO)14(dppbz)

Thermolysis or Me₃NO activation of the hexaruthenium cluster Ru₆(μ₆-C)(CO)₁₇ in the presence of the diphosphine ligand 1,2-bis(diphenylphosphino)benzene (dppbz) does not furnish the expected dppbz-substituted cluster Ru₆(μ₆-C)(CO)₁₅(dppbz) but rather HRu₆(μ₅-C)(μ₃-P)(CO)₁₄(dppbz), whose edge-bridged square-pyramidal structure has been established by X-ray crystallography. Accompanying the opening of the original closo Ru₆ polyhedron is the dephosphination of a second dppbz ligand through three rapid P-C bond cleavages, leading to the capture of the phosphorus atom as a face-capping phosphido ligand. This unprecedented reactivity between Ru₆(μ₆-C)(CO)₁₇ and the dppbz ligand is discussed relative to other diphosphine ligands.     

Processing, properties and some novel applications of magnetic nanoparticles

Magnetic nanoparticles have been prepared by various soft chemical methods including self-assembly. The bare or surface-modified particles find applications in areas such as hyperthermia treatment of cancer and magnetic field-assisted radioactive chemical separation. We present here some of the salient features of processing of nanostructured magnetic materials of different sizes and shapes, their properties and some possible applications. The materials studied included metals, metal-ceramic composites, and ferrites.     

Linker dependent bond rupture force measurements in single-molecule junctions

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.     

Dynamic behavior of the diphosphine ligand in H4Ru4(CO)10(dppe) revisited: kinetic data supporting a nondissociative isomerization of the dppe ligand

The kinetics for the bridge-to-chelate isomerization of the dppe ligand in H4Ru4(CO)10(dppe) have been investigated by UV-vis and NMR spectroscopies over the temperature range of 308-328 K. The isomerization of the ligand-bridged cluster 1,2-H4Ru4(CO)10(dppe) (1-br) was found to be reversible by 31P NMR spectroscopy, affording a K(eq) = 15.7 at 323 K in favor of the chelating dppe isomer 1-ch. The forward (k1) and reverse (k(-1)) first-order rate constants for the reaction have been measured in different solvents and in the presence of ligand-trapping agents (CO and PPh3). On the basis of the activation parameters and reaction rates that are unaffected by added CO and PPh3, a sequence involving the nondissociative migration of a phosphine moiety and two CO groups between basal ruthenium centers is proposed and discussed.     

Sonochemical stabilization of ultrafine colloidal biocompatible magnetite nanoparticles using amino acid,l-arginine,for possible bio applications

Materials obtained by the synergistic combination of nanotechnology and biomedicine are an important source of drug delivery and other health care related applications. The anchoring of amino acids onto the surface of nano-sized magnetite is one such example. Herein, we report on the binding of a semi-essential amino acid, l-arginine, onto the surface of nano magnetite, creating a stable aqueous suspension by an in situ one-step method using sonochemical synthesis. An ex situ two-step process was also attempted, but was soon discarded owing to the relative short duration of the suspension attributed to increase in particle size and lower extent of binding. The initial concentration of the amino acid was found to play an important role in controlling the particle size and also the binding motif. Lower concentrations of arginine were found to favor the formation of elongated tubular structures, while at higher concentrations, the elongated structures were less prominent and arginine was found to be adsorbed onto the surface of the magnetite. This surface-functionalized nanomagnetite with amino acids could become a promising vehicle for drug delivery.     

Chlorine-atom abstraction and cluster fragmentation in the reaction of H3Re3(CO)10(MeCN)2 with the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd): X-ray diffraction structure of fac-ClRe(CO)3(bpcd)

Treatment of the activated trirhenium cluster H₃Re₃(CO)₁₀(MeCN)₂ with the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) in CH₂Cl₂ does not afford the expected cluster product H₃Re₃(CO)₁₀(bpcd) but rather the mononuclear complex fac-ClRe(CO)₃(bpcd). The identity of fac-ClRe(CO)₃(bpcd) was determined in solution by IR and NMR (¹H and ³¹P) spectroscopies and the solid-state structure was established by X-ray diffraction analysis. fac-ClRe(CO)₃(bpcd) crystallizes in the triclinic space P-1, a = 9.958(2) Å, b = 11.991(3) Å, c = 13.676(3) Å, α = 73.230(4)°, β = 73.806(4)°, γ = 77.409(4)°, V = 1484.6(6) ų, Z = 2, and d _calc = 1.723 Mg/m³; R = 0.0367, R _w  = 0.0857 for 4253 reflections with I > 2σ(l).     

Thermolysis of the tetrahedrane cluster PhCCo3(CO)3(μ-CO)Cp2 with the unsaturated diphosphine ligands (Z)-Ph2PCH=CHPPh2 and 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd): Redox properties and molecular structure of PhCCo3(CO)(μ-CO)[(Z)-Ph2PCH=CHPPh2]Cp2

Thermolysis of the tricobalt cluster PhCCo₃(CO)₃(μ-CO)Cp₂ (1) with the diphosphine ligands (Z)-Ph₂PCH=CHPPh₂ and 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) has been examined and found to give the diphosphine-substituted clusters PhCCo₃(CO)[(Z)-Ph₂PCH=CHPPh₂](μ-CO)Cp₂ (2) and PhCCo₃(CO)(bpcd)(μ-CO)Cp₂ (3) in moderate yield. The new compounds 2 and 3 have been isolated and characterized in solution by IR and NMR (¹H and ³¹P) spectroscopies. VT ³¹P NMR data reveal that the chelating diphosphine ligand is fluxional in solution and exhibits a rocking motion between the axial and equatorial sites that renders both phosphorus moieties identical at ambient temperature. The molecular structure of PhCCo₃(CO)[(Z)-Ph₂PCH=CHPPh₂](μ-CO)Cp₂ (2) has been determined by X-ray crystallography. PhCCo₃(CO)[(Z)-Ph₂PCH=CHPPh₂](μ-CO)Cp₂ crystallizes, as the CH₂Cl₂ solvate, in the monoclinic space P2₁/n, a = 16.822(2) Å, b = 10.554(1) Å, c = 23.135(3) Å, β = 100.944(2)^°, V = 4032.4(8) ų, Z = 4, and d _calc = 1.537 Mg/m³; R = 0.0488, R _w = 0.0725 for 9431 reflections with I > 2σ(I). The solid-state structure of cluster 2 establishes the chelating nature of the ancillary (Z)-Ph₂PCH=CHPPh₂ ligand at the unique Co(CO)P₂ center via coordination at an equatorial and an axial site. The redox behavior of clusters 2 and 3 has been explored by cyclic voltammetry and chronocoulometry. Each cluster reveals the presence of two one-electron oxidations of common origin due to the oxidation of a Co–Co bonding orbital. Whereas cluster 2 does not exhibit an accessible reduction process in CH₂Cl₂, a ligand-based one-electron reduction was found for cluster 3 given its low-lying π* LUMO associated with the bpcd ligand. The electrochemical data for clusters 2 and 3 are discussed with respect to the reported redox chemistry for this genre of tricobalt cluster and the bpcd ligand.     

Diphosphine ligand substitution in H4Ru4(CO)12: X-ray diffraction structures and reactivity studies of the cluster compounds H4Ru4(CO)10(P–P) [where P–P–(Z)–Ph2PCH–CHPPh2, bpcd]

The tetraruthenium cluster H₄Ru₄(CO)₁₂ (1) has been studied for its reactivity with the unsaturated diphosphine ligands (Z)–Ph₂PCH–CHPPh₂ and 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) under thermal, near-UV photolysis, and Me₃NO-assisted activation. All three cluster activation methods promote loss of CO and furnish the anticipated substitution products H₄Ru₄(CO)₁₀[(Z)–Ph₂PCH–CHPPh₂] (2) and H₄Ru₄(CO)₁₀(bpcd) (3) that possess a chelating diphosphine ligand. Clusters 2 and 3 have been characterized in solution by IR and NMR spectroscopies, and these data are discussed with respect to the crystallographically determined structure for both new cluster compounds. The ³¹P NMR spectral data and the solid-state structures confirm the presence of a chelating diphosphine ligand in clusters 2 and 3. Cluster 2 crystallizes in the monoclinic space P2₁/c, a=11.768(6) ?, b=18.521(9) ?, c=20.48(1) ?, β=102.291(8)°, V=4361(4) A³, Z=4, and d _calc=1.726 Mg/m³; R=0.0225, R _w=0.0491 for 6798 reflections with I > 2σ(I). The four bridging hydrides were located in H₄Ru₄(CO)₁₀[(Z)–Ph₂PCH–CHPPh₂] and their adopted positions are discussed relative to the solution ¹H NMR spectrum. H₄Ru₄(CO)₁₀(bpcd) crystallizes in the orthorhombic space Pbca, a=19.072(3) ?, b=20.169(3) ?, c=22.774(3) ?, V=8760(2) A³, Z=8, and d _calc=1.870 Mg/m³; R=0.0428, R _w=0.0896 for 10296 reflections with I > 2σ(I). Sealed NMR tubes containing clusters 2 and 3 were found to be exceeding stable towards near-UV light and temperatures up to ca. 125 °C. The surprisingly robust behavior of 2 and 3 is contrasted with the related cluster Ru₃(CO)₁₀(bpcd) that undergoes fragmentation to the donor-acceptor compound Ru₂(CO)₆(bpcd) and the phosphido-bridged compound Ru₂(CO)₆(μ–PPh₂)[μ–C–C(PPh₂)C(O)CH₂C(O)] under mild conditions. The electrochemical properties of each substituted cluster have been investigated by cyclic voltammetry, and our findings are discussed with respect to the reported electrochemical data on the parent cluster H₄Ru₄(CO)₁₂.