Supramolecular polymers based on dative boron-nitrogen bonds

Heteroditopic monomers containing an arylboronate ester and a dialkyl-4-aminopyridine group aggregate via dative boron-nitrogen bonds to give main chain supramolecular polymers. The degree of polymerization can be tuned by changing the electronic and steric properties of the boronate ester.     

A stable binuclear complex containing PdHPd bonds

The preparations of the binuclear hydrido-bridged cations [(terdentate ligand)Pd(μ-H)Pd(terdentate ligand)]⁺ from [(terdentate ligand)Pd(acetone)]⁺ and NaO₂CH and [(terdentate ligand)Pd(μ-H)Pt(terdentate ligand)]⁺ from [(terdentate ligand)Pd(acetone)]⁺ and [(terdentate ligand)PtH] (terdentate ligand = 2,6-(Ph₂PCH₂)₂C₆H₃) are reported. The preparation of the cation [(terdentate ligand)Pt(μ-H)Pt(terdentate ligand)]⁺ is also reported.     

Synthesis,structure and properties of an imidazolate bridged copper,zinc binuclear complex

A new imidazolate bridged Cu~(2+),Zn~(2+)binuclear complex[(dtma) CulmZn (dtma)]ClO_4·2.5H_2O taken as active site model for Cu,Zn-SOD has been synthesized and its crystal structuredetermined.All the bond lengths, bond angles and the distance between Cu and Zn atoms in Cu-Im-Zn core of the model complex are close to those in Cu,Zn-SOD.ESR parameters of the modelcomplex as a function of pH show that the imidazolate bridge is stable in pH range 10—12,and isbroken on Zn side at pH～9.With decreasing pH,the imidazole is released at pH～4 and the dtmaligand dissociates from the Cu containing fragment at pH～2.4.     

Selective anion binding by a cofacial binuclear zinc complex of a Schiff-base pyrrole macrocycle

The synthesis of the new cofacial binuclear zinc complex [Zn(2)(L)] of a Schiff-base pyrrole macrocycle is reported. It was discovered that the binuclear microenvironment between the two metals of [Zn(2)(L)] is suited for the encapsulation of anions, leading to the formation of [K(THF)(6)][Zn(2)(μ-Cl)(L)]·2THF and [Bu(n)(4)N][Zn(2)(μ-OH)(L)] which were characterized by X-ray crystallography. Unusually obtuse Zn-X-Zn angles (X = Cl: 150.54(9)° and OH: 157.4(3)°) illustrate the weak character of these interactions and the importance of the cleft preorganization to stabilize the host. In the absence of added anion, aggregation of [Zn(2)(L)] was inferred and investigated by successive dilutions and by the addition of coordinating solvents to [Zn(2)(L)] solutions using NMR spectroscopy as well as isothermal microcalorimetry (ITC). On anion addition, evidence for deaggregation of [Zn(2)(L)], combined with the formation of the 1:1 host-guest complex, was observed by NMR spectroscopy and ITC titrations. Furthermore, [Zn(2)(L)] binds to Cl(-) selectively in THF as deduced from the ITC analyses, while other halides induce only deaggregation. These conclusions were reinforced by density functional theory (DFT) calculations, which indicated that the binding energies of OH(-) and Cl(-) were significantly greater than for the other halides.     

Octahapto cyclooctatetraene rings and metal-metal multiple bonds in binuclear niobium carbonyl chemistry

Theoretical studies on (C₈H₈)₂Nb₂(CO)_n (n = 6, 5, 4, 3, 2, 1) predict structures mainly with octahapto and tetrahapto C₈H₈ rings. In all cases, the lowest energy singlet spin state structures lie below the corresponding lowest energy triplet spin state structures. Thus the lowest energy (C₈H₈)₂Nb₂(CO)₄ structure has two η⁸-C₈H₈ rings and an unbridged Nb-Nb single bond of length ∼3.15 Å. The lowest energy (C₈H₈)₂Nb₂(CO)₂ structure has two η⁸-C₈H₈ rings but a doubly bridged NbNb triple bond of length ∼2.64 Å. The lowest energy structure of (C₈H₈)₂Nb₂(CO)₃ also has a formal NbNb triple bond of similar length (2.66 Å) but with only one of the rings fully coordinated as an octahapto η⁸-C₈H₈ ligand. The other C₈H₈ ring in this tricarbonyl has “slipped” to form a hexahapto η⁶-C₈H₈ ligand. The lowest energy structure of the monocarbonyl (C₈H₈)₂Nb₂(CO) again has two octahapto η⁸-C₈H₈ rings and an extremely short NbNb distance of 2.45 Å, suggesting a formal quadruple bond. The lowest energy structures for the carbonyl-richer species (C₈H₈)₂Nb₂(CO)_n (n = 6, 5) have one η⁸-C₈H₈ and one η⁴-C₈H₈ ring (n = 5) and two η⁴-C₈H₈ rings (n = 6). The qualitatively assigned Nb-Nb bond orders are consistent with the Wiberg bond indices obtained from the Weinhold natural bond orbital analysis. Comparison of the (C₈H₈)₂Nb₂(CO)_n (n = 6, 5, 4, 3, 2, 1) derivatives with the isovalent (C₇H₇)₂Mo₂(CO)_n is made.     

Molecular structure of imidazolate bridged binuclear zinc complex and its single crystal ESR spectra doped with bridged Cu—Zn complex

The X-ray crystal structure of [(dtma)ZnImZn(dtma)]ClO_4·2.5H_2O (Hdtma=4-Diethyl-enetriamineacetic acid) was determined.The crystal is of orthorhombic,space group Pbcn with a-14.104(5),b=14.897(5),c=25.384(9),and Z=8.The least-square refinement of the structureleads to conventional R factor of 0.066.The magnetic properties of [(dtma)CulmZn(dtma)]CIO_4·2.5H_2Owere investigated.From the single crystal ESR spectra of Zn—Im—Zn dimer doped withCu—Im—Zn complex,the anisotropic g and A tensors and electronic spin-density of the Cu—Zncomplex are obtained and the bonding nature of Cu is discussed.     

Bistability of an iron-cobalt binuclear complex

Bistability of the four cis/trans isomers of the proposed iron-cobalt binuclear complex [(CO)₂(benzoate-)Fe^II/III(-terephthalate-)Co^III/II(-benzoate)(CO)₂]¹⁺, arising from the Fe^II/III ↔ Co^III/II intramolecular charge transfer (IMCT) is investigated computationally at (TD)DFT-B3LYP/LanL2DZ level of theory. Energies, geometries, atomic charges, and the UV-Vis spectra are considered in this investigation. Results approve IMCT bistability of all cis/trans isomers by locating two stable states with distinctly different structures and charge distributions (Fe^II-Co^III and Fe^III-Co^II oxidation states). Also, well-defined first-order saddle points between these two IMCT states are found and characterized using QST2/QST3 method. Based on the analysis of the calculated charge distributions and the 0.35-1.66 eV activation (barrier) energies of the Fe^II-Co^III ↔ Fe^III-Co^II IMCT reactions, it can be predicted that electric field or NIR radiation may be used to switch between the two IMCT states of this bistable binuclear complex. It is also found that the cis/trans isomerization has significant effects on the energetics of this IMCT reaction, and that the trans-Fe^II/III-trans-Co^III/II isomer is the best candidate for prospective switching application due to having the least energy dissipation and the largest charge transfer.     

Synthesis,crystal structure,and DNA binding of a new oxalato-bridged binuclear zinc(II) complex

A new binuclear zinc(II) complex bridged by μ-oxalate, and end-capped with 2,2′-bipyridine (bpy), [Zn₂(ox)(bpy)₄](ClO₄)₂ · H₂O, has been synthesized and characterized by elemental analyses, molar conductance, IR, and electronic spectra and single-crystal X-ray diffraction. The single-crystal X-ray analysis reveals that the [Zn₂(ox)(bpy)₄]²⁺ cation has two zinc(II) centers bridged by a planar bis(bidentate) oxalate group with Zn···Zn distance of 5.482(3) Å; each zinc(II) is in a distorted octahedral environment. The crystal structure is stabilized by non-classical C–H···O hydrogen bonds and π–π stacking interactions to form a 3-D supramolecular structure. The interaction of the complex with calf-thymus DNA (CT-DNA) was explored by using electronic and fluorescence spectra and viscosity measurements. The results reveal that the complex intercalates with CT-DNA with intrinsic binding constant of 4.1 × 10⁴ M^?1.     

Study of the dative bond in 2-aminoethoxydiphenyl borate at various levels of theory: another poor performance of the B3LYP method for B-N dative bonds

Aminoethoxydiphenyl borate (2-APB), 1, is a potent inhibitor of store-operated calcium entry channels (SOCCs). Other SOCC inhibitors are being investigated as promising pharmacological agents for a variety of conditions. Though toxic, 2-APB could be useful in the development of additional inhibitors, but its preferred binding structure must first be determined. Thus, we performed ab initio calculations to study the conformers and the strength of the dative bond of 2-APB. As a first step, we performed a series of computations at various levels of theory. We obtained vastly different dissociation energies for the dative bond depending on whether we used MP2 or B3LYP (7-10 kcal/mol different). This discrepancy has previously been observed for other B-N dative bonds by Gilbert, who found that the MP2 values were in much better agreement with experimental values (Gilbert, T. M. J. Phys. Chem. A 2004, 108, 2550-2554). Since we lacked experimental data for comparison, we performed CCSD(T) calculations and found them to have similar results to those from MP2. Thus, we conclude that MP2 is more accurate for 2-APB. The dissociation free energy at the MP2 level is 7 kcal/mol and indicates that the dative bond conformer will be the predominant structure in the gas phase. The dissociation energy is comparatively low due to the electron donation from the oxygen atom to the boron atom and due to the ring strain in the dative bond conformer.     

Photoinduced electron transfer via nonbuttressed metal-metal bonds. The photochemical study of binuclear complexes with platinum-thallium bonds

The photochemistry of binuclear metal-metal bonded complexes [(NC) 5Pt-Tl(solv) x ] (solv is water or dimethylsulfoxide) has been studied in aqueous and dimethylsulfoxide solutions. Both stationary and nanosecond laser flash photolysis have been carried out on the species. The metal-metal bonded complexes have been photolyzed by irradiation into the corresponding intense MMCT absorption bands. Photoexcitation results in the cleavage of the platinum-thallium bond and the formation of a solvated thallous ion and a cyano complex of platinum(IV), [Pt(CN) 5(solv)] (-), in both cases. The species have been characterized by multinuclear NMR and optical spectroscopy. The products of the photoreaction indicate a complementary two-electron transfer occurring between platinum and thallium ions in the binuclear Pt-Tl species. Quantum yield values for the photodecomposition of the species have been determined. The intermediates of the photoinduced metal-to-metal electron transfer have been detected and characterized by optical spectroscopy. The kinetics of transient formation and decomposition have been studied, and mechanisms of the photoactivated redox reaction have been suggested.     

Cationic five-coordinate Pt(II) complexes as donors in the formation of Pt-->Ag dative bonds

A unique trinuclear cluster containing two short Pt-->Ag dative bonds was prepared and characterized crystallographically. Two monocationic orthometalated Pt(II) complexes containing 2-phenylpyridine and the tridentate ligand 9S3 (1,4,7-trithiacyclononane) exhibit axial sulfur interactions with Pt as well as short dative bonds. The Pt-->Ag dative bonding results in a dramatic shortening of the axial Pt-S distance in each Pt complex cation, relative to the mononuclear Pt complex. Evidence for Pt-Ag interactions in solution is also presented.     

Unsaturated binuclear cyclopentadienylrhenium carbonyl derivatives: metal-metal multiple bonds and agostic hydrogen atoms

The cyclopentadienylrhenium carbonyls Cp 2Re 2(CO) n (Cp = eta (5)-C 5H 5; n = 5, 4, 3, 2) have been studied by density functional theory. The global minima for the Cp 2Re 2(CO) n ( n = 5, 4, 3, 2) derivatives are predicted to be the singly bridged structure Cp 2Re 2(CO) 4(mu-CO) with a formal Re-Re single bond; the doubly semibridged structure Cp 2Re 2(CO) 4 with a formal ReRe double bond; the triply bridged structure Cp 2Re 2(mu-CO) 3 with a formal ReRe triple bond; and the doubly bridged structure Cp 2Re 2(mu-CO) 2, respectively. The first three of these predicted structures have been realized experimentally in the stable compounds (eta (5)-C 5H 5) 2Re 2(CO) 4(mu-CO), (eta (5)-Me 5C 5) 2Re 2(CO) 4 and (eta (5)-Me 5C 5) 2Re 2(mu-CO) 3. In addition, structures of the type Cp 2Re-Re(CO) n with both rings bonded only to one metal and unknown in manganese chemistry are also found for rhenium but at energies significantly above the global minima. The unsaturated Cp 2Re-Re(CO) n structures ( n = 4, 3, 2) have agostic Cp hydrogen atoms forming C-H-Re bridges to the unsaturated Re(CO) n group with a Re-H distance as short as 2.04 A.     

Structural transformations in dinuclear zinc complexes involving Zn-Zn bonds

Reduction of Zn(2)(mu-eta(2)-Me(2)Si(NDipp)(2))(2) with 4 equiv. of KC(8) resulted in a dramatic structural transformation into [(eta(2)-Me(2)Si(NDipp)(2))ZnZn(eta(2)-Me(2)Si(NDipp)(2))](2-) featuring a Zn-Zn bond instead of [Zn(2)(micro-eta(2)-Me(2)Si(NDipp)(2))(2)](2-); the mechanism of the observed structural transformations arising from the Zn-Zn bond formation involving the intermediate of [Zn(2)(mu-eta(2)-Me(2)Si(NDipp)(2))(2)](-) was elucidated by elaborate computations.     

Synthesis and structure of a binuclear ruthenium 4-chlorobenzamidato complex

The compound Ru₂Cl(4-Cl-C₆H₄CONH)₄ was prepared by reaction of Ru₂Cl(O₂CCH₃)₄ with 4-Cl-C₆H₄CONH₂ at 180°C. Crystals of the composition Ru₂Cl(4-Cl-C₆H₄CONH)₄CH₃OH were obtained by slow diffusion of CH₃OH containing Et₄NCl into a Me₂SO solution of the compound. The structure of the crystalline product, which loses solvent of crystallization on removal from the mother liquor, was solved by X-ray crystallography by mounting a single crystal in a capillary containing the mother liquor. The crystals belong to the space group P1? (triclinic crystal system) with a = 12.731(3) Å, b = 14.389(3) Å, c = 12.604(3) Å, α = 103.41(2)°, β = 106.43(2)°, γ = 64.90(2)°, V = 1988.6(8) ų and Z = 2. There are two half ruthenium dimers linked by a Cl atom and an uncoordinated solvent CH₃OH molecule per asymmetric unit. The ruthenium dimers lie on two centers of inversion at 0, 0, 0 and 1/2, 0, 0. The chloride ions bridge dinuclear cations in the crystal, forming infinite zigzag chains. The average Ru-Ru distance is 2.296[1] Å and each ruthenium atom has a RuClN₂O₂ coordination sphere where the average Ru′-Ru-Cl angle is virtually linear (175.68[6]°). The metal oxidation states in the complex are + 2 and + 3, giving an average value of + 2.5. The arrangement of four bridging 4-Cl-benzamidato ligands is of the 2 : 2 type. The average Ru-N, Ru-O, Ru-Cl distances and Ru(1)-Cl(1)-Ru(2) angle are 2.036[6] Å, 2.044[5] Å, 2.583[2] Å and 117.26(8)°, respectively. The IR spectrum of the compound shows two N-H stretches at 3380 and 3340 cm^?1. The electronic spectrum of the compound in Me₂SO exhibits bands at 558 nm (ε = 340 M^?1 cm^?1), 425 nm (1000) and 320 nm (22,700).     

Optical properties of highly planar diketopyrrolopyrrole derivatives fixed by coordinate bonds

Diketopyrrolopyrrole (DPP) derivatives bearing 2-pyridyl groups were synthesized, which could serve as anionic N,N′-chelate ligands consisting of two nitrogen atoms of the lactam moieties and the pyridyl groups. Coordination of the DPP ligand to boron and platinum yielded the corresponding DPP complexes. Their crystal structures exhibited coplanar structures between the DPP core and the 2-pyridyl groups, which indicated extended π-conjugation. The maximum absorption wavelength of the DPP complexes was longer than that of the corresponding DPP ligands. The choice of the introduced element affects the optical properties of the DPP complexes; the platinum complex has a longer absorption wavelength than the boron complex. The theoretical calculation revealed that these absorptions can be attributed to the π–π* transition. The introduction of the coordinating elements predominantly lowers the band gap. In addition, the substituent on the 2-pyridyl group can also modulate the energy levels.     

Synthesis and structures of new binuclear zinc alkyl,aryl and aryloxo complexes

The synthesis of a series of binuclear zinc complexes with Cl, N and O bridges is reported. The reaction of EtZnCl with B(C₆F₅)₃ in the presence of hexamethylbenzene affords the arene complex [Zn(μ-Cl)(C₆F₅)(η-C₆Me₆)]₂ in which the C₆Me₆ ligand may be regarded as η³-bonded. The comproportionation of Zn[N(SiMe₃)₂]₂ with ZnBu^t₂ or Zn(C₆F₅)₂ · toluene gave [Bu^tZn{μ-N(SiMe₃)₂}]₂ and [C₆F₅Zn{μ-N(SiMe₃)₂}]₂, respectively, with three-coordinate zinc. The reaction of ZnEt₂ with C₆F₅OH in the presence of pyridine gave [EtZn(μ-OC₆F₅)(py)]₂, while ZnMe₂ and C₆F₅OH followed by recrystallisation from THF gave [Zn(OC₆F₅)(μ-OC₆F₅)(THF)₂]₂ with five-coordinate zinc in a trigonal-bipyramidal geometry. The structures of these compounds have been determined.