Characterization and evaluation of pyrone and tropolone chelators for use in metalloprotein inhibitors

The tetrahedral zinc and cobalt complexes [(Tp^Ph,Me)ZnOH] (Tp^Ph,Me = hydrotris(3,5-phenylmethylpyrazolyl)borate) and [(Tp^Ph,Me)CoCl] were combined with 3-hydroxy-2H-pyran-2-one (3,2-pyrone), 3-hydroxy-4H-pyran-4-one (3,4-pyrone), and tropolone to form the corresponding [(Tp^Ph,Me)M(L)] complexes (L = bidentate ligand, M = Zn²⁺, Co²⁺). X-ray crystal structures of these complexes were obtained to determine the mode of binding for each chelator and the coordination geometry of each complex. The complexes [(Tp^Ph,Me)M(3,2-pyrone)] (M = Zn²⁺, Co²⁺) are the first structurally characterized metal complexes with this chelator. These complexes with the various chelators show that the cobalt(II) complexes are generally isostructural with their zinc(II) counterparts. In addition to structural characterization, inhibition data for each ligand against two different zinc(II) metalloproteins, matrix metalloproteinase-3 (MMP-3) and anthrax lethal factor (LF), were obtained. Examination of these chelators in the MMP-3 active site demonstrates the possible mode of inhibition.     

Heteroleptic lanthanide terphenyl/tris(pyrazolyl)borate compounds of ytterbium

The synthesis and structural characterization of the two novel unsolvated heteroleptic ytterbium compounds DanipYb(Tp^Me,Me)Cl (1) and DanipYb(Tp^Me,Me)CH₂SiMe₃ (2) by simple salt metathesis reaction is reported [Danip = 2,6-di(o-anisol)phenyl); Tp^Me,Me = hydrotris(3,5-dimethyl-pyrazolyl)borate]. In the molecular structure of 2 a flexible bonding mode of the donor-functionalized terphenylic ligand is observed.     

Redox and catalytic properties of alkoxides based on tris(3,5-dimethylpyrazolyl)borato tungsten nitrosyls

Previously we reported on the catalytic properties of species based on the {Mo(NO)(Tp^Me2)O₂} moiety in the cathodic reduction of chloroform. Here, we have performed cyclic voltammetry and spectroscopic studies of the tungsten bis-alkoxide [W(NO)(Tp^Me2)(OEt)₂], a novel chelate [W(NO)(Tp^Me2)O(CH₂)₄O], and a mono-alkoxide [W(NO)(Tp^Me2)Cl(OEt)] [Tp^Me2 = hydrotris(3,5-dimethylpyrazol-1-yl)borate]. All these complexes efficiently catalyse the cathodic reduction of chloroform which proceeds even at ca. −1.77 V versus Fc⁺/Fc in the presence of the chloro(ethoxy) complex. The chelate complex exhibits a quasi-reversible one-electron reduction at a potential 180 mV more anodic than its bis(ethoxy) counterpart. The UV-Vis spectrum of the former complex shows a red-shifted band (by 70 nm) in the visible region when compared with the latter.     

Molybdenum doubly bridged and chelate nitrosyl complexes incorporating saturated n-alkanediolate ligands

The redox-active doubly bridged species [{Mo(NO)(Tp^Me2)Q}₂] [Tp^Me2 = tris(3,5-dimethylpyrazol-1-yl)hydroborate, Q = O(CH₂)_nO, n = 3, 5, or OCH₂(CF₂)_n−2CH₂O, n = 5, 6], and a chelate complex [Mo(NO)-(Tp^Me2)O(CH₂)₅O] were prepared and characterised by elemental and mass analyses, ¹H NMR and IR spectroscopy. The bimetallic species with C₃, C₅, and C₅(F) bridges exhibit two well-resolved reduction processes in their cyclic voltammograms (ΔE_1/2 values of 290, 170, and 170 mV, respectively). These results indicate that the presence of the second bridge increases the extent of electrochemical interactions (by ca. 90-130 mV) in comparison with their singly bridged analogues. All non-fluorinated and the chelate species were catalytically active in cathodic reduction of chloroform.     

A series of oxo-vanadium(IV) complexes containing mixed ligands of poly(pyrazolyl)borate and organic carboxylic acid: Synthesis, structural characterization and primary study of bromination reaction activities

A series of oxo-vanadium(IV) complexes: Tp^∗VO(pzH^∗)(CH₃COO) (1), Tp^∗VO(pzH^∗)(CCl₃COO) (2), Tp^∗VO(pzH^∗)(C₆H₅COO) (3), Tp^∗VO(pzH^∗)(m-NO₂-C₆H₄COO)·CH₃CN (4) and [Tp^∗VO(pzH^∗)(H₂O)]⁺[m-NO₂-C₆H₄-SO₃]⁻·CH₃OH (5) (Tp^∗ = hydrotris(3,5-dimethylpyrazolyl)borate; pzH^∗ = 3,5-dimethylpyrazole) are synthesized in methanol solution under physiological conditions. They are characterized by elemental analysis, IR, UV-Vis and X-ray crystallography. Structural analyses show that the vanadium atoms in complexes 1-5 are all in a distorted-octahedral environment with the N₄O₂ donor set, and intra- or inter-hydrogen bonding linkages have been also observed in each complex. Bromination reaction activity of the complexes has been evaluated by the method with phenol red as organic substrate in the presence of H₂O₂, Br⁻ and phosphate buffer, indicating that they can be considered as potential functional model vanadium-dependent haloperoxidases. In addition, thermal analysis and quantum chemistry calculations were also performed and discussed in detail.     

1,2-Borotropic shifts and B-N bond cleavage reactions in molybdenum hydrotris(3-isopropylpyrazolyl)borate chemistry: Mixed-valence MoMo2 and pyrazole-rich oxo-Mo complexes

Red-black [Tp^iPr∗Mo^VO]₂(μ-O)(μ-Mo^VIO₄) (1, Tp^iPr∗ = hydrobis(3-isopropylpyrazolyl)(5-isopropylpyrazolyl)borate) has been isolated as a by-product in the synthesis of NEt₄[Tp^iPrMo(CO)₃] (Tp^iPr = hydrotris(3-isopropylpyrazolyl)borate) and characterized by spectroscopic and X-ray crystallographic techniques. The trinuclear, mixed-valence complex contains two distorted octahedral anti-Tp^iPr∗Mo^VO centers bridged by bent oxo (Mo-O-Mo av. 158.7°) and tetrahedral κO,κO′-molybdate ligands. The complex contains a six-membered, non-planar Mo₃(μ-O)₃ core and two 1,2-borotropically-shifted Tp^iPr∗ ligands (with the shifted pyrazolyl trans to Mo^V=O). Aerial decomposition of solid NEt₄[Tp^iPrMo(CO)₃] produces sky-blue, diamagnetic Tp^iPrMoO(ⁱPrpz)(ⁱPrpzH) (2, ⁱPrpz^- = 3-isopropylpyrazolate, ⁱPrpzH = 3-isopropyl-2H-pyrazole). Molecules of 2 feature a tridentate fac-Tp^iPr ligand and mutually cis terminal oxo (MoO = 1.665(2) Å) and monodentate ⁱPrpz⁻ and ⁱPrpzH ligands. The latter are formed by B-N bond cleavage of Tp^iPr. The complex can also be synthesized by reacting NEt₄[Tp^iPrMo(CO)₃] with excess 3-isopropylpyrazole and dioxygen at 100 °C. Cleavage of the B-N bond(s) of Tp^iPr was also observed in the formation of Tp^iPrMoO(SPh)(ⁱPrpzH) (3) as a by-product in the synthesis of Tp^iPrMoO₂(SPh). In the monohydrate, 3 exhibits a distorted octahedral geometry defined by a tridentate fac-Tp^iPr ligand and mutually cis terminal oxo (MoO = 1.676(3) Å) and monodentate SPh⁻ and ⁱPrpzH ligands. The pyrazole β-NH group is observed to participate in a hydrogen-bond to the lattice water molecule. The complex can be synthesized in high yield by reducing Tp^iPrMoO₂(SPh) by HSPh or PPh₃ in the presence of excess 3-isopropylpyrazole.     

Magnesium (II) poly(pyrazolyl)borate derivatives - Synthesis, spectral and structural studies

A series of magnesium complexes of general formula [Mg(Tp^x)₂] (Tp^x = Tp, Tp^∗, Tp^∗Cl, pzTp) and [Mg(Tp^x)X] (X = Cl, Tp^x = Tp^tBu or pz⁰Tp^p-Tol; X = acetate, Tp^x = Tp^tBu) were synthesised from magnesium chloride or acetate and M(Tp^x) (M = K, Na or Tl) in dichloromethane or alcoholic solution. These compounds are air-stable solids, sparingly soluble in most organic solvents; they have been characterized by elemental analysis, IR, ¹H and ¹³C NMR spectra and, in selected cases, also by conductivity and molecular weight measurements. Single crystal X-ray diffraction studies of [Mg(Tp)₂], [Mg(Tp*)₂] and [Mg(Tp*^Cl)₂] show unsolvated neutral bis(tripod ligand)magnesium(II) molecules with six-coordinate magnesium atoms (〈Mg-N〉 2.167(6), 2.19(2), 2.205(4) Å).     

Cyanoscorpionates: Co(II), Mn(II), and Ni(II) complexes coordinated only through the cyano group

New complexes have been synthesized of scorpionate ligands with cyano substituents in the 4-positions of the pyrazoles and tert-butyl substituents in the 3-positions of the pyrazoles. Reaction of Co²⁺, Mn²⁺, and Ni(cyclam)²⁺ (cyclam = 1,4,8,11-tetraazacyclotetradecane) with Tp^t-Bu,4CN in a 1:2 ratio produced new octahedral metal complexes of the form (Tp^t-Bu,4CN)₂ML₄ (L₄ = (H₂O)₄, (H₂O)₂(MeOH)₂, or cyclam). Unlike the sandwich complexes previously isolated with Tp^Ph,4CN, the crystal structures showed none of the pyrazole nitrogen atoms coordinated to the metal. Rather, the metal is coordinated to one CN nitrogen atom from each ligand, with two Tp anions coordinated trans to each other around the metal center. This leaves the Tp pyrazole nitrogen atoms open for another metal to coordinate, which could to lead to heterometallic complexes, new coordination polymers, as well as the framework for supramolecular complexes.     

Synthesis and derivatization of halflanthanidocene aryl(alk)oxide complexes

The reaction of halflanthanidocene aryloxides Cp^R′Ln(OAr^tBu,R)₂ (Ln = Y, La, Lu; Cp^R′ = C₅Me₅, C₄Me₄H; R = H, Me) and halflanthanidocene alkoxides [(C₅Me₅)Ln(OCH₂CMe₃)₂]₂ (Ln = Y, Lu) with trimethylaluminum (TMA) was investigated. Monomeric Cp^R′Ln(OAr^tBu,R)₂, derived from the ortho-tBu-substituted OC₆H₂tBu₂-2,6-R-4 (R = H, Me) ligands, form mono(tetramethylaluminate) complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) for the smaller lanthanide metal centers yttrium and lutetium. Such an [aryloxide] → [aluminate] ligand exchange was not observed at the larger lanthanum metal center. The mobility of the tetramethylaluminate ligands of complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) (Ln = Y, Lu) was examined by variable-temperature (VT) ¹H NMR spectroscopy, revealing two signals for bridging and terminal methyl groups at lower temperatures. The treatment of complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) with donor solvent d₈-THF gave Cp^R′Ln(OAr^tBu,R)(Me)(d₈-THF)₂ (Ln = Y, Lu) with terminal methyl groups, according to a donor-induced aluminate cleavage reaction. Dimeric [(C₅Me₅)Ln(OCH₂CMe₃)₂]₂ (Ln = Y, Lu) was synthesized from (C₅Me₅)Ln(NiPr₂)₂(THF) and reacted with two equivalents of TMA per Ln center to yield monomeric bis(TMA) adduct complexes (C₅Me₅)Ln(OCH₂CMe₃)₂(AlMe₃)₂(Ln = Y, Lu). VT NMR spectroscopic studies confirmed a high mobility of the Ln(μ-OCH₂CMe₃)(μ-Me)AlMe₂ moieties at an ambient temperature. Both bis(TMA) adduct complexes were characterized by X-ray structure analysis.     

Syntheses, crystal structures, and characterization of heteronuclear complexes based on a versatile ligand with both acetylacetonate and bis(2-pyridyl) units

A new type of multidentate ligand with both acetylacetonate and bis(2-pyridyl) units on the 1,3-dithiole moiety, 3-[2-(dipyridin-2-yl-methylene)-5-methylsulfanyl-[1,3]dithiol-4-ylsulfanyl]-pentane-2, 4-dione (L), has been prepared. Through reactions of the ligand with Re(CO)₅X (X = Cl, Br), new rhenium(I) tricarbonyl complexes ClRe(CO)₃(L) (2) and BrRe(CO)₃(L) (3), have been obtained. With the use of 2 or 3 as the precursors, the further reactions with (Tp^Ph2)Co(OAc)(Hpz^Ph2) (Tp^Ph2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate); Hpz^Ph2 = 3,5-diphenyl-pyrazole) or M(OAc)₂(M = Mn, Zn), afford four new heteronuclear complexes: ClRe(CO)₃(L)Co(Tp^Ph2) (4), BrRe(CO)₃(L)Co(Tp^Ph2) (5), [ClRe(CO)₃(L)]₂Mn(CH₃OH)₂ (6) and [ClRe(CO)₃(L)]₂Zn(CH₃OH)₂ (7), respectively. Crystal structures of complexes 2 and 4-7 have been determined by X-ray diffraction. Their absorption spectra, photoluminescence and magnetic properties have been studied.     

Isovalent and mixed-valent molybdenum complexes containing Mo(μ-E)(μ-S2)Mo (E = O, S) core units

The reactions of Tp^iPrMoO(SR)(NCMe) (Tp^iPr = hydrotris(3-isopropylpyrazolyl)borate) with propylene sulfide in toluene result in the formation of the diamagnetic, isovalent Mo(V) complex, [Tp^iPrMo^VO]₂(μ-S)(μ-S₂). This complex and its previously reported μ-oxo analog, [Tp^iPrMo^VO]₂(μ-O)(μ-S₂), react with cobaltocene to produce one-electron-reduced, mixed-valent complexes, [CoCp₂][{Tp^iPrMo^IV,VO}₂(μ-E)(μ-S₂)] (E = S or O, respectively). All complexes have been isolated and characterized by microanalysis, mass spectrometry, IR and ¹H NMR or EPR spectroscopies, and X-ray crystallography. Neutral [Tp^iPrMo^VO]₂(μ-S)(μ-S₂) exhibits a pseudo-C₂ symmetric structure, with distorted octahedral anti oxo-Mo(IV) centers coordinated by Tp^iPr and linked by μ-sulfido and μ-disulfido ligands. A similar structure is adopted by the anion in mixed-valent [CoCp₂][{Tp^iPrMo^IV,VO}₂(μ-S)(μ-S₂)]; this compound adopts a hexagonal, supramolecular structure with columns of tight ion-pairs with interactions, interconnected through weaker contacts to three neighboring columns. The structure contains large interstitial voids filled with lattice solvent molecules. EPR investigation of the mixed-valent complexes gave rise to unusually broad signals with no evident hyperfine splitting. The synthesis and characterization of a number of cis-dioxo-Mo(VI) precursors are also reported.     

Zinc(II) complexes based on sterically hindered hydrotris(pyrazolyl)borate ligands: Synthesis, reactivity and solid-state structures

The coordination chemistry and reactivity of zinc(II) complexes supported by monoanionic hydrotris(pyrazolyl)borate ligands substituted by 3,3,3-mesityl groups (Tp^Ms) and 3,3,5-mesityl groups (Tp^Ms∗) have been investigated. Salt metathesis of ZnCl₂, ZnEt₂, and Zn(OAc)₂ with Tl[Tp^Ms] or Tl[Tp^Ms∗] cleanly afforded the corresponding compounds Tp^MsZnCl (1), Tp^MsZnEt (2), Tp^Ms∗ZnEt (3), and Tp^MsZnOAc (5). Compound 3 slowly disproportionates in benzene solution to afford the bis(ligand) complex (κ²-Tp^Ms∗)₂Zn (4). Acetate complex 5 as well as Tp^MsZnOCOPh (6) and [Tp^Ms∗ZnOAc]₂ (7) were alternatively prepared by acidolysis of the parent ethyl complexes (2, 3) with the corresponding carboxylic acid. No reaction was observed between 2 and 3 and alcohols (ROH; R = Et, iPr, Bn), while salt metathesis reactions of ZnEt(OR) with Tl[Tp^Ms] led to 2 instead of the desired zinc-alkoxide complex. Compounds 1-7 were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, as well as by X-ray diffraction studies for 1, 2, 4, 5 and 7. The former compounds adopt a monomeric structure in the solid state while [Tp^Ms∗ZnOAc]₂ (7) exists as an anti-syn bridged acetate dimer. Complex 4 is four-coordinated, featuring a rare bidentate coordination mode of the Tp^Ms∗ ligands. The results are rationalized in terms of the variable steric constraint around the zinc atom provided by the Tp^Ms and Tp^Ms∗ ligands.     

Synthesis and structure of heteroleptic ytterbium (II) tetrahydroborate complexes

The synthesis and characterization of (Tp^tBu,Me)Yb(BH₄)(THF)_n (n = 0, 3; n = 1, 4) complexes are reported. The compounds represent rare examples of lanthanide (II) tetrahydroborate complexes. The X-ray crystal structure of complex 4 has been determined and it shows a monomeric, formally seven coordinate ytterbium center, bearing one κ³ bonded Tp^tBu,Me ligand, a tetrahydroborate ligand and a coordinated THF molecule. The tetrahydroborate ligand binds in a κ³ fashion, via three bridging hydrogen atoms. IR spectroscopy data are consistent with the solid-state structure and the corresponding BD₄ analog of 4 shows the expected IR isotope shifts. The ¹H NMR spectra of 3 and 4 shows one set of resonances each for the BH₄ and the pyrazolylborate ligands indicating dynamic solution behavior. For complex 3, although X-ray quality crystals could not be obtained, the IR and NMR data are consistent with its formulation as the solvent-free analog of complex 4 with κ³-bonded BH₄ ligand.     

Syntheses and crystal structures of the first iridium complexes with m- and p-terphenyl (tp). {[Ir2(p-tp)(cod)2](BF4)2 · 2CH2Cl2}3 and [Ir(m-tp)(η-C5Me5)](BF4)2

Novel two iridium terphenyl complexes were prepared and their structures were characterized crystallographically. The reaction of [Ir(cod)₂]BF₄ with p-terphenyl (p-tp) in CH₂Cl₂ was carried out to afford dinuclear Ir(I) complex {[Ir₂(p-tp)(cod)₂](BF₄)₂ · 2CH₂Cl₂}₃ (cod=1,5-cyclooctadiene) (1 · 2CH₂Cl₂), whereas the reaction of the intermediate [Ir(η⁵-C₅Me₅)(Me₂CO)₃]³⁺ in Me₂CO with m-terphenyl (m-tp) was done to provide mononuclear Ir(III) complex [Ir(m-tp)(η⁵-C₅Me₅)](BF₄)₂ (2). In complex 1 · 2CH₂Cl₂, two Ir atoms are η⁶-coordinated to both sides of terminal benzene rings from the upper and lower sides in the p-tp ligand, while one Ir atom is η⁶-coordinated to one side of the terminal benzene ring in the m-tp ligand in complex 2. Each crystal structure describes the first coordination mode found in metal complexes with the m- and p-tp ligands.     

Rhodium and iridium olefin complexes with bulky cyclopentadienyl and hydrotris(pyrazolyl)borate ligands

The synthesis of a series of rhodium and iridium complexes bearing bulky cyclopentadienyl or hydro(trispyrazolyl)borate ligands is described. The rhodium cyclopentadienyl and hydrotris(pyrazolyl)borate diene compounds, [(η⁵-C₅Me₄Bu^t)Rh(η⁴-2,3-MeRC₄H₄] (R = H, 1; Me, 2) and Tp^MsRh(η⁴-2,3-MeRC₄H₄) (R = H, 3; Me, 4; Tp^Ms is hydrotris(3-mesitylpyrazol-1-yl)borate), respectively, have been prepared from the corresponding Rh(I) diene precursors and Zn(C₅Me₄Bu^t)₂ (for 1 and 2), or TlTp^Ms (for 3 and 4), as effective C₅Me₄Bu^t or Tp^Ms transfer reagents. In contrast with these results, attempts to obtain a bis(ethylene) derivative of the Tp^tolIr(I) unit (Tp^tol stands for hydrotris(3-p-tolylpyrazol-1-yl)borate) have provided instead the Ir(III) complex [(κ⁴-N,N′,N″,C-Tp^tol)-Ir(C₂H₅)(C₂H₄)] (5), whose formation requires C-H bond activation of a molecule of ethylene and of one of the Tp^tolp-tolyl substituents. In refluxing toluene 5 experiences metalation of a second p-tolyl substituent to give [(κ⁵-N,N′,N″,C,C′-Tp^tol)-Ir(C₂H₄)] (6), which features unusual κ⁵-Tp^tol coordination. The latter compound reacts with carbon monoxide to yield the corresponding carbonyl, 7.     

N,N-dimethylcarbamato complexes of zinc

By reaction of Zn₄O(O₂CNMe₂)₆ (1) with [NH₂Me₂][O₂CNMe₂] in toluene as medium, the homoleptic zinc compound [Zn(O₂CNMe₂)₂] (2) was obtained, which reverted back to the tetranuclear μ-oxo derivative by controlled hydrolysis. The reaction of ZnO in MeCN with an excess of [NH₂Me₂][O₂CNMe₂] in concentrated solution produced high yields of [Zn(O₂CNMe₂)₂] (2) or [NH₂Me₂][Zn₂(O₂CNMe₂)₅] · xMeCN, 3 · xMeCN, x = 1 or 2, depending on the experimental conditions.     

[S2], [S3], and [N3] coordination modes of hydrotris(thioxotriazolyl)borato. Zinc, nickel, cobalt, and bismuth complexes

The ligand hydrotris(1,4-dihydro-3-methyl-4-phenyl-5-thioxo-1,2,4-triazolyl)borato (Tr^Ph,Me) was synthetized as natrium salt and the complexes [Zn(Tr^Ph,Me)₂] · 7.5H₂O · 1.5CH₃CN (2a), [Zn(Tr^Ph,Me)₂] · 8DMF (2b), [Co(Tr^Ph,Me)₂] · 8DMF (3a), [Ni(Tr^Ph,Me)₂] · H₂O · 6DMSO (4a), [Bi(Tr^Ph,Me)₂]NO₃ (5), have been isolated and structurally characterized by X-ray diffraction. In the zinc derivatives the ligand adopts different denticity and coordination modes, η² and [S₂] for 2a and η³ and [N₃] for 2b, depending on the crystallization solvent, giving rise to tetrahedral and octahedral geometry, respectively. In the octahedral cobalt and nickel complexes the ligand is η³ and [N₃] coordinated whereas in the bismuth complex the η³ and [S₃] coordination is exhibited.     

Synthesis, characterisation and crystal structure of hydroxylamido-κN,O(iodo)[tris(3,5-dimethylpyrazol-1-yl)borato]nitrosylmolybdenum(II)

The unusual 18e seven-coordinate Mo(II) complex [Mo(NO)(H₂NO-κ²N,O)(Tp^Me2)I] (1; [Tp^Me2]⁻ is hydrotris(3,5-dimethylpyrazol-1-yl)borate) has been synthesised and characterised by IR, ¹H NMR and ESI-MS spectroscopies and by a single crystal X-ray diffraction study. The complex has a distorted pentagonal bipyramid structure with equatorial κ²-NH₂O⁻ ligand (d_N-O = 1.387 Å, d_Mo-N and d_Mo-O equal 2.049 and 2.092 Å, respectively). In the solid state 1 exists as a dimer (the point group C_i) due to the formation of two NH?O hydrogen bonds (d_N-H?O = 2.064 Å) between the adjacent NH₂O⁻ ligands, whilst in solution at/or above RT it resolves itself giving a monomer, which readily isomerises to more thermodynamically stable diastereoisomer.     

Design, Syntheses, and Characterization of a Sterically Encumbered Dioxo Molybdenum (VI) Core

Dioxo-Mo^VI complexes of general formula Tp^∗MoO₂(p-SC₆H₄Dn) (6a-6c) (where Tp^∗ = hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Dn = dendritic unit) have been synthesized and characterized by spectroscopy and mass spectrometry. ¹H NMR spectra of the metal complexes indicate that the C_s local symmetry about the metal core does not change by the incorporation of dendritic functionality at the thiophenolato ring. Electrochemical data show a ∼20 mV change in the redox potential in the complexes with dendritic ligands suggesting a very small perturbation in the redox orbital, which is also supported by small changes in the electronic spectra. The peak-to peak separation (ΔE_p) increases from 125 mV in 6(a) to 240 mV in 6(c), suggesting sluggish electron transfer in molecules with larger dendritic ligands.     

Syntheses and structures of four- and five-coordinate bio-related zinc complexes containing dithiolate-diamine ligands

Two new zinc complexes, namely, [{Zn(N₂H₂S₂)}₂] (3) [N₂H₂S₂²⁻ = N,N-bis(2-mercaptophenyl)ethylendiamine (2−)] and [Zn(N₂Me₂S₂)] (4) [N₂Me₂S₂²⁻ = N,N′-dimethyl-N,N′-bis(2-mercaptophenyl)ethylendiamine) (2−)] have been synthesized and structurally characterized by X-ray structure analyses. The structure of 3 consists of a bis(μ-thiolato) binuclear unit, in which each zinc center was found to reside in an N₂S₃ array between square-pyramidal and trigonal-bipyramidal environment. The two zinc centers are bridged by one of the two thiolates of an [N₂S₂] ligand. In the crystal packing, the neighboring binuclear units interact with each other by H-bonding interaction, which extends the binuclear unit into a 3D network. In contrast to 3 complex 4 is mononuclear, where each zinc center now was found to reside in an N₂S₂ distorted tetrahedral environment with a large S-Zn-S bite angle. The relevance of these compounds in biological systems is discussed. Unlike 3, the formation of hydrogen bridges in 4 is no longer possible and instead the molecular packing is determined by π-stacking between the phenyl rings.