Solution-phase mechanistic study and solid-state structure of a tris(bipyridinium radical cation) inclusion complex

The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT(2(?+))) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY(?+)) guests has been investigated mechanistically and quantitatively. Two BIPY(?+) radical cations, methyl viologen (MV(?+)) and a dibutynyl derivative (V(?+)), were investigated as guests for the CBPQT(2(?+)) ring. Both guests form trisradical complexes, namely, CBPQT(2(?+))?MV(?+) and CBPQT(2(?+))?V(?+), respectively. The structural details of the CBPQT(2(?+))?MV(?+) complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV(?+) is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT(2(?+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT(2(?+))?MV(?+) and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT(2(?+))?MV(?+) complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 10(4) M(-1) and (7.9 ± 5.5) × 10(4) M(-1), respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k(f) and k(b) of (2.1 ± 0.3) × 10(6) M(-1) s(-1) and 250 ± 50 s(-1), respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT((2+)(?+))?V(?+) inclusion complex; in the case of the V(?+) guest, the rate of disassociation (k(b) = 10 ± 2 s(-1)) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT((2+)(?+)) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY(?+) radical cation and π-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical-radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY(?+) radical-radical interactions in solution as well as in the solid-state.     

A water chain with trimeric/hexameric clusters trapped in a POM/Ln compound

A new 3-D supramolecular compound based on a Keggin polyoxometalate and lanthanide (Ln) coordination cations [Ce(H₂O)₃L₃]₂[HSiMo₁₂O₄₀]₂[L] ? 7H₂O (1) (L = isonicotinic acid) has been synthesized and characterized by routine methods. Compound 1 has a water chain with trimeric and hexameric water clusters trapped in the network. The isonicotinic acid plays a key role in forming the 1-D water chains. Compound 1 shows intense photoluminescence at room temperature.     

Photoswitching of intramolecular magnetic interaction using diarylethene with oligothiophene pi-conjugated chain.

Diarylethenes having two nitronyl nitroxide radicals at both ends of the molecule with oligothiophene spacers were synthesized. The diradicals underwent photochromic reactions upon alternate irradiation with UV and visible light. ESR spectra of the diradicals reversibly changed with the photochromism. The magnetic interaction between spins through oligothiophene spacers was stronger than that through oligophenylene spacers, and the photochromic reactivity of the diradical diarylethenes with oligothiophene spacers was much reduced. The difference of the exchange interaction between open- and closed-ring form isomers was estimated to be more than 150-fold.     

Self-assembled monolayer of organic iodine on a Au surface for attachment of redox-active metal clusters

The attachment of a bifunctional iodo-organo-phosphinate compound to gold (Au) surfaces via chemisorption of the iodine atom is described and used to chelate a redox-active metal cluster via the phosphinate group. XPS, AFM, and electrochemical measurements show that (4-iodo-phenyl)phenyl phosphinic acid (IPPA) forms a tightly bound self-assembled monolayer (SAM) on Au surfaces. The surface coverage of an IPPA monolayer on Au was quantified by an electrochemical method and found to be 0.40 +/- 0.03 nmol/cm2, roughly corresponding to 0.4 monolayers. We show that the Au/IPPA SAM, but not the underivatized Au, adsorbs Mn4O4(Ph2PO2)6 from solution by a phosphinate exchange reaction to yield Au/IPPA/Mn4O4(Ph2PO2)5 SAM. The resulting SAM is firmly bound and not removed by sonication, as confirmed by manganese XPS (Mn 2p1/2) and by AFM. Electrochemistry confirms that Mn4O4(Ph2PO2)6 is anchored on the Au/IPPA surface and that redox chemistry can be mediated between the electrode and the surface-attached complex. Mn4O4(Ph2PO2)6 contains the reactive Mn4O46+ cubane core, a redox-active bioinspired catalyst.     

Periodic structure of spicular magnetic clusters in a magnetic liquid

The behavior of clusters formed by magnetic particles of magnetic liquid placed into a cylindrical capillary tube in magnetic field is described. Spicular clusters are formed from the sediment at the application of a magnetic field. They arrange themselves along the capillary repeating the direction of external magnetic field. Clusters distribute uniformly in the region of a magnetic field maximum. Such a state remains steady with respect to changes of the magnetic field gradient profile in definite limits. The structure of the uniformly ordered clusters is obtained experimentally. The capability of control of the structure period is shown. It is observed that increasing of magnetic field gradient up to the magnitude higher than a certain threshold value results in rearrangement of the clusters row into a multi-row hexagonal structure.     

Molecular structure of a cyclopropyl substituted vinyl cation

The experimentally determined molecular structure of vinyl cation 1 provides clear evidence for the occurrence of extended sigma-conjugation involving the cyclopropyl ring and the beta-C-Si bonds.     

Post-coordination of multinuclear transitional metal clusters to azulene-based polymers: a novel strategy for tuning properties in pi-conjugated polymers

Direct treatment of azulene-thiophene copolymers with Ru(3)(CO)(12) led to unique examples of coordination of a multinuclear transition metal cluster to conjugated polymers. The polymer-metal cluster formation could be confirmed by (1)H NMR spectroscopic studies, and the coordination mode could be deduced from the crystallographic structure of model compounds. The composition of ruthenium carbonyl cluster could be used as a governing factor to tune the electronic, optical, and morphological properties of the organometallic polymers. [structure: see text]     

Construction of extended networks with a trimeric pyrazole synthon

A H-bonded pyrazole trimeric motif drives the self-association of bifunctional tetramethyl-4,4'-bipyrazole, yielding highly interpenetrated 3D frameworks and allowing the engineering by design of a novel supramolecular isomer.     

Synthesis, characterization, and study of octanuclear iron-oxo clusters containing a redox-active Fe4O4-cubane core

A one-pot synthetic procedure yields the octanuclear Fe(III) complexes Fe(8)(micro(4-)O)(4)(micro-pz(*))(12)X(40, where X = Cl and pz(*) = pyrazolate anion (pz = C(3)H(3)N(2)-) (1), 4-Cl-pz (2), and 4-Me-pz (3) or X = Br and pz(*) = pz (4). The crystal structures of complexes 1-4, determined by X-ray diffraction, show an Fe(4)O(4)-cubane core encapsulated in a shell composed of four interwoven Fe(micro-pz(*))(3)X units. Complexes 1-4 have been characterized by 1H NMR, infrared, and Raman spectroscopies. M?ssbauer spectroscopic analysis distinguishes the cubane and outer Fe(III) centers by their different isomer shift and quadrupole splitting values. Electrochemical analyses by cyclic voltammetry show four consecutive, closely spaced, reversible reduction processes for each of the four complexes. Magnetic susceptibility studies, corroborated by density functional theory calculations, reveal weak antiferromagnetic coupling among the four cubane Fe centers and strong antiferromagnetic coupling between cubane and outer Fe atoms of 1. The structural similarity between the antiferromagnetic Fe(8)(micro(4-)O)(4) core of 1-4 and the antiferromagnetic units contained in the minerals ferrihydrite and maghemite is demonstrated by X-ray and M?ssbauer data.     

Structural,electronic and magnetic properties of small gold clusters with a copper impurity

A set of all-electron scalar relativistic calculations on Au _n Cu (n = 1–12) clusters has been performed using density functional theory with the generalized gradient approximation at PW91 level. The lowest energy geometries of Au _n Cu clusters may be considered as assemblies of triangular Au₃ moieties substituted with one Cu atom at the highest coordinated site. All these lowest energy geometries of the Au _n Cu clusters are slightly distorted but retain the planar structures of the Au _n+1 clusters due to the strong scalar relativistic effects. The Au–Cu bonds are stronger, and a few Au–Au bonds far from the Cu atom are weaker, than the corresponding Au–Au bonds in pure Au _n+1 clusters. After doping with a Cu atom, the thermodynamic stability and chemical reactivity are enhanced to some extent. The odd-numbered Au _n Cu clusters with even numbers of valence electrons are more stable than the neighboring even-numbered Au _n Cu clusters with odd numbers of valence electrons. Odd–even alternations of magnetic moments and electronic configurations for the Au _n Cu clusters can be observed clearly and may be understood in terms of the electron pairing effect.     

Reactivity of an aluminum hydride complex with a redox-active diimine ligand

Russian Chemical Bulletin - The reaction of the hydride [(dpp-bian)Al(H)Cl] (1) containing the 1,2-bis[(2,6-diisopropylphenyl) imino]acenaphthene (dpp-bian) radical anion with MeLi affords the...     

Structure and magnetic properties of iron-silicon clusters in a multiconfigurational calculation

Journal of Structural Chemistry - The structure and spin moments of iron-silicon clusters FeSi n are calculated by a multiconfigurational self-consistent field (MCSCF) method in order to refine the...     

Molecular solid solution of lanthanide-titanium-oxo clusters with enhanced photocatalytic hydrogen evolution

Molecular solid solutions of metal clusters containing different metal centers with well-defined structures can accurately regulate the HOMO-LUMO gap,but are rarely available.Herein,a series of colorless lanthanide-titanium-oxo clusters Ln₂Ti₄(μ₂-O)₂(μ₃-O)₄(Piv)₁₀(THF)₂(Ln₂Ti₄,Ln = Eu,Gd,Tb,and Ce,HPiv = pivalic acid) were synthesized by the reaction of pivalic acid with Ln(Ac)₃     

Bound electron states in clusters of inert atoms in a magnetic field

An external magnetic field is shown to stabilize negatively charged clusters of inert atoms. In an external magnetic field the critical number of atoms in a cluster, necessary to bound an excess electron to a neutral cluster, is less than that in the absence of a field. General conditions for creation of electron bound states in a cluster in a magnetic field are considered.     

Electronic structure of light-induced lophyl radical derived from a novel hexaarylbiimidazole with pi-conjugated chromophore

A novel photochromic hexaarylbiimidazole with a bithienyl group as an extended pi-conjugation unit was synthesized and the light-induced lophyl radical was found to be stabilized due to the delocalization of an unpaired electron, and to strongly absorb near-infrared light.     

Molecular and electronic structure of square-planar nickel II, nickel III and nickel III pi-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand

The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexes with the ligand o-phenylenebis(N'-methyloxamidate), [NiL]z (z = 2-, 1-, 0), have been evaluated by DFT and TDDFT calculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al., Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. The anionic species [NiL]2- and [NiL]- are genuine low-spin nickel II and nickel III complexes with diamagnetic singlet (S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel III complex presents shorter Ni-N(amidate) bond distances (1.85-1.90 A) than the parent nickel II complex (1.88-1.93 A) and characteristic LMCT bands in the NIR region (lambda max = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (lambda max = 346 and 349 nm). The neutral species [NiL] is a nickel III o-benzosemiquinonediimine pi-cation radical complex with a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentally different in orbital configuration. The singlet metal-radical ground state results from the antiferromagnetic coupling between the 3d(yz) orbital of the Ni III ion (S(M) = 1/2) and the pi(b) orbital of the benzosemiquinone-type radical ligand (S(L) = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal-radical excited state involves the ferromagnetic coupling between the Ni III 3d(zx) orbital and the benzosemiquinone-type pi(b) orbital, which are orthogonal to each other. The singlet and triplet states of the nickel III pi-cation radical complex possess characteristic quinoid-type short-long-short alternating sequence of C-C bonds in the benzene ring, as well as intense MLCT transitions in the VIS (lambda max = 664 nm) and NIR (lambda max = 884 nm) regions, respectively.     

Synthesis and structure of a nucleoside with pi-conjugated nitroxide spin label forming a one-dimensional ferromagnetic chain

Spin labeled 2'-deoxyuridine, in which a significant fraction of the spin density is delocalized from a nitroxide radical to the DNA base residue, was prepared as a crystalline solid, stable at ambient conditions. The crystal packing, which includes multiple hydrogen bonds, leads to one-dimensional chains of molecules with predominant intrachain ferromagnetic coupling and weaker interchain antiferromagnetic coupling.