Tripodal System Research Articles

A New Tripodal Ligand System with Steric and Electronic Modularity for Uranium Coordination Chemistry

The synthesis of a potentially redox active tripodal ligand containing a tris(aryloxide) functionalized mesitylene anchor, (((tBu)ArOH)(3)mes) (1), and its metalation with low-valent uranium to form [(((tBu)ArO)(3)mes)U] (1-U) is reported. The results from characterization by X-ray crystallography, spectroscopic studies, and computational analysis, as well as initial reactivity studies, support a +3 uranium oxidation state. Comparison to the previously synthesized complex, [(((tBu)ArO)(3)tacn)U] (2-U), featuring the redox-innocent triazacyclononane anchor reveals that changing the anchor from the flexible triazacyclononane to a rigid mesityl fragment increases the structural flexibility of the aryloxide substituents in complexes of 1. The synthesis and crystal structures of uranium(IV) amide complexes of 1-U and 2-U are discussed.

Self-Assembly of Bile Steroid Analogues: Molecules, Fibers, and Networks

Structural and rheological features of a series of molecular hydrogels formed by synthetic bile salt analogues have been scrutinized. Among seven gelators, two are neutral compounds, while the others are cationic systems among which one is a tripodal steroid derivative. Despite the fact that the chemical structures are closely related, the variety of physical characteristics is extremely large in the structures of the connected fibers (either plain cylinders or ribbons), in the dynamical modes for stress relaxation of the associated SAFINs, in the scaling laws of the shear elasticity (typical of either cellular solids or fractal floc-like assemblies), in the micron-scale texture and the distribution of ordered domains (spherulites, crystallites) embedded in a random mesh, in the type of nodal zones (either crystalline-like, fiber entanglements, or bundles), in the evolution of the distribution and morphology of fibers and nodes, and in the sensitivity to added salt. SANS appears to be a suitable technique to infer all geometrical parameters defining the fibers, their interaction modes, and the volume fraction of nodes in a SAFIN. The tripodal system is particularly singular in the series and exhibits viscosity overshoots at the startup of shear flows, an "umbrella-like" molecular packing mode involving three molecules per cross section of fiber, and scattering correlation peaks revealing the ordering and overlap of 1d self-assembled polyelectrolyte species.

Reversible photoisomerization of an azobenzene-functionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy

Sum-frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible, photoinduced trans/cis isomerization of an azobenzene-functionalized self-assembled monolayer (SAM) on a gold substrate. A C[triple bond]N marker group at the outer phenyl ring is used as a direct measure of the switching state. The azobenzene unit is connected to the surface by a tripodal linker system with an adamantane core, which results in both a sufficient decoupling of the functional azobenzene unit from the metallic substrate and a free volume to prevent steric hindrance, thus allowing the isomerization process. Optical excitation at 405 nm induces the trans-->cis isomerization, whereas light exposure at 470 nm leads to the back reaction. The effective cross sections for the reactions are sigma(eff)(cis) = 4 +/- 1 x 10(-18) cm(2) at 405 nm (trans-->cis) and sigma(eff)(trans) = 2.5 +/- 0.9 x 10(-19) cm(2) at 470 nm (cis-->trans). We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the azobenzene conjugate, analogous to the free molecules in solution.

Synthesis and properties of a novel tripodal bipyridyl ligand tb-carbinol and its Ru(II)–Re(I) trimetallic complexes: investigation of multimetallic artificial systems for photocatalytic CO2reduction

A novel tripodal ligand, tris[(4'-methyl-2,2'-bipyridin-4-yl)methyl]carbinol (tb-carbinol) and its homonuclear and heteronuclear Ru(II)-Re(I) complexes have been synthesized and characterized by NMR spectroscopy, elemental analysis, and mass spectroscopy. The spectroscopic, electrochemical and photocatalytic properties of the Ru(II)-Re(I) complexes have been investigated. In these supramolecular complexes with tb-carbinol as a bridging ligand, the intramolecular interaction among the terminal metal centers is very weak. In the cases of Ru(II) and Re(I) heteronuclear systems, when the Re(I) moieties are excited, the emission from the Re(I) moiety is efficiently quenched and the intensity of the emission from the Ru(II) moiety increases. The rate constant of energy transfer from Re(I) moieties to Ru(II) unit in RuRe(2) is 1.7 x 10(8) s(-1). From the point of view of the free energy change, the intramolecular electron transfer from the Ru(II) moiety to the Re(I) moiety could proceed smoothly in the ground state. Both of Ru(2)Re and RuRe(2) show excellent photocatalytic activities to the CO(2) reduction. RuRe(2) exhibits a turnover number of 190 for CO formation compared with 89 from the model complexes system after 16 h of irradiation (TN(CO) calculated based on Ru(II) moiety concentration). Ru(2)Re shows a higher turnover number than the model complexes system, 110 compared with 55 from the model system (TN(CO) calculated based on Re(I) moiety concentration). The bridging ligand of Ru(II)-Re(I) heteronuclear tripodal systems, tb-carbinol, plays an important role in converting radiant energy to chemical energy in the form of CO from CO(2). Enhancement of the photocatalytic response to light in the visible region has been achieved by fabricating supramolecular systems featuring covalently linked Ru(II) and Re(I) moieties.

Synthesis of an azobenzene-linker-conjugate with tetrahedrical shape

The synthesis of a tripodal linker system with an adamantane core unit and an azobenzene headgroup is reported for the preparation of photochromic SAMs on gold surfaces. For the final Sonogashira-coupling of the ethynylene-linker precursor 4 with the p-iodo substituted azobenzene 6 model studies were required to optimize the coupling step in the presence of a diazene functionality. The photochromic properties of the photoswitch-linker-conjugate 1 were investigated in solution, and compared to the behavior of the precursor 6.

Redox Active Cage for the Electrochemical Sensing of Anions

The tripodal system [1]3+ forms a 1:1 complex with CoII in which the metal is octahedrally coordinated by three bpy fragments. The [CoII(1)]5+ complex provides a cavity suitable for solvent or anion inclusion. X-ray diffraction studies on the crystalline complex salt of formula [CoII(1)...H2O]Cl(PF6)(4).2MeCN have shown that a water molecule is included in the cavity and the water oxygen atom receives six H-bonds from the C-H fragments of the three imidazolium subunits and of the three proximate pyridine rings, according to a slightly distorted trigonal prismatic geometry. Anion inclusion in an aqueous MeCN solution induces a distinct cathodic shift of the potential of the CoIII/CoII couple, whose magnitude decreases along the series: Cl->Br- approximately NCO->I- approximately NCS-, which reflects anion tendencies to receive H-bonds from the receptor. The variation of the water content in the MeCN solution (from 0 to 20%) induces a gradual change of the voltammetric response to anion titration: from two well distinguished peaks at a fixed potential to a single peak progressively shifted to a more cathodic potential. Such a behavior parallels the gradual decrease of the equilibrium constant for anion inclusion into the [CoII(1)]5+ receptor.

A Tripodal [2]Rotaxane on the Surface of Gold

Tripodal [2]rotaxane, 3, and the structurally related axle, 2, incorporating a viologen moiety, a crown ether, and three thiol anchoring groups have been synthesized. Analogous monopodal derivatives, 1, have also been prepared. Self-assembled monolayers of the above tripodal and monopodal systems on gold have been studied by cyclic voltammetry. It has been shown that a thiol anchoring group is required to attach the monopodal viologen 1 to the surface of gold and that the maximum surface coverage of 1 corresponds to 2.7 x 10(-10) mol.cm(-2). The adsorbed monopodal viologen 1 does not thread bis-p-phenylene-34-crown-10 ether, 6. However, the tripodal axle 2 adsorbed on the surface of gold threads the crown ether 6 to form a hetero [2]rotaxane. In the case of the tripodal axle 2, the surface coverage is 7 x 10(-11) mol.cm(-2), while for the tripodal [2]rotaxane 3 the surface coverage reaches 1.1 x 10(-10) mol.cm(-2).

A new tripodal ligand system based on the iminophosphorane functional group. Part 1: Synthesis and characterization

Two tripodal alcohols, viz., 1,1,1-tris(hydroxymethyl)ethane and α,α,α-tris(hydroxymethyl)toluene were converted by an efficient multi-step pathway involving azide formation into the corresponding tris(iminophosphorane) scaffolds bearing cyclopentyl (Cp) or phenyl groups on their phosphorus atoms, R–C(CH 2–N PR′ 3) 3 (R=Me or Ph, R′=Cp or Ph). The synthesis of some representative transition-metal complexes of Cu(I), Cu(II), Ni(II), and Pd(II) bearing these new tridentate ligands is also reported.

Homo‐ and Hetero‐Dinuclear Anion Complexes

The tripodal system 4, in which urea fragments are appended to the three terminal amine nitrogen atoms of a tris(2-aminoethyl)amine (tren) subunit, includes a Cu(II) ion and two anions X-, according to a cascade mechanism through three well defined stepwise equilibria in a DMSO solution. The first anion X- (halide, N3-, NCS-, NO2-, H2PO4-) seeks the Cu(II) centre coordinated by the tren moiety; the second anion X- interacts with the trisurea cavity, but this occurs only if the stronger H-bond acceptors, such as N3- and H2PO4-, are used. Binding of the second X- ion is favoured by the preorganising effect exerted by the metal and disfavoured by the steric and electrostatic repulsions between the anions. Under the appropriate conditions, heterodinuclear complexes of formula [Cu(II)(4)(Cl)(H2PO4)] can be obtained in solution, in which Cl- is bound to the metal centre and H2PO4- interacts with the trisurea compartment.

Toward Iron Sensors: Bioinspired Tripods Based on Fluorescent Phenol-oxazoline Coordination Sites

In the quest for fast throughput metal biosensors, it would be of interest to prepare fluorophoric ligands with surface-adhesive moieties. Biomimetic analogues to microbial siderophores possessing such ligands offer attractive model compounds and new opportunities to meet this challenge. The design, synthesis, and physicochemical characterization of biomimetic analogues of microbial siderophores from Paracoccus denitrificans and from the Vibrio genus are described. The (4S,5S)-2-(2-hydroxyphenyl)-5-methyl-4,5-dihydro-1,3-oxazole-4-carbonyl group (La), noted here as an HPO unit, was selected for its potential dual properties, serving as a selective iron(III) binder and simultaneously as a fluorophore. Three tripodal symmetric analogues cis-Lb, cis-Lc, and trans-Lc, which mainly differ in the length of the spacers between the central carbon anchor and the ligating sites, were synthesized. These ferric-carriers were built from a tetrahedral carbon as an anchor, symmetrically extended by three converging iron-binding chains, each bearing a terminal HPO. The fourth chain could contain a surface-adhesive function (Lc). A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, and electrochemistry was used to fully characterize the corresponding ferric complexes and to determine their stability. The quenching mechanism is consistent with an intramolecular static process and is more efficient for the analogue with longer arms. Detection limits in the low nanogram per milliliter range, comparable with the best chemosensors based on natural peptide siderophores, have been determined. These results clearly demonstrate that these tris(phenol-oxazoline) ligands in a tripodal arrangement firmly bind iron(III). Due to their fluorescent properties, the coordination event can be easily monitored, while the fourth arm is available for surface-adhesive moieties. The tripodal system is therefore an ideal candidate for integration with solid-state materials for the development of chip-based devices and analytical methodologies.

Synthesis and Properties of Cryptands with a Thioether Bridge and π‐Donors – Silver(I) and Copper(I)‐Complexes

AbstractThe syntheses of seven bridgehead diazabicycloalkanes are reported. The bridges are represented by either a thioether chain and two 3‐hexyne units (4 and 6–8), one selenoether chain and two 3‐hexyne units (5), or one dithioether chain and two 4,4′‐diethylbiphenyl units (9, 10). X‐ray investigations reveal for 4–10 the in/in conformation at the bridgehead nitrogen atoms. Reactions with CuI triflate yield the endohedral CuI complexes of 4–6. Endohedral complexes of 6, 7, and 10 were isolated with silver triflate. X‐ray investigations and spectroscopic studies on the metal complexes reveal interactions between the metal ion and the nitrogen‐, chalcogen‐, and π‐units. In addition to the bicyclic systems 4–10 we synthesized the tripodal systems 28–33 in a one‐pot reaction. In the cases of 28 and 33 we were able to isolate the corresponding AgI complexes. X‐ray studies show an interaction between the metal ion and the nitrogen atom and one double bond of each of the three adjacent phenyl rings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Porphyrins Bearing Mono or Tripodal Benzylphosphonic Acid Tethers for Attachment to Oxide Surfaces.

The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance, orientation, packing density) is essential for the design of a variety of molecular-based information storage devices. We describe the synthesis of a series of redox-active molecules wherein each molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins, a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared that is based on a tris[4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid group include derivatization of a bromoalk...

Porphyrins Bearing Mono or Tripodal Benzylphosphonic Acid Tethers for Attachment to Oxide Surfaces

The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance, orientation, packing density) is essential for the design of a variety of molecular-based information storage devices. We describe the synthesis of a series of redox-active molecules wherein each molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins, a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared that is based on a tris[4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid group include derivatization of a bromoalkyl porphyrin or use of a dimethyl or diethyl phosphonate substituted precursor in a porphyrin-forming reaction. The latter approach makes use of dipyrromethane building blocks bearing mono or tripodal dialkyl phosphonate groups. The zinc porphyrin-tripodal compound bearing benzylphosphonic acid legs tethered to a SiO(2) surface (grown on doped Si) was electrically well-behaved and exhibited characteristic porphyrin oxidation/reduction waves. Collectively, a variety of porphyrinic molecules can now be prepared with tethers of different length, composition, and structure (mono or tripodal) for studies of molecular-based information storage on oxide surfaces.

Copper Complexes of Nitrogen-Anchored Tripodal N-Heterocyclic Carbene Ligands

Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMEN(R) (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the methyl derivatized [H(3)TIMEN(Me)](PF(6))(3) imidazolium salt (H(3)5a) with silver oxide yields the silver complex [(TIMEN(Me))(2)Ag(3)](PF(6))(3) (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex [(TIMEN(Me))(2)Cu(3)](PF(6))(3) (10). Deprotonation of the tert-butyl and benzyl derivatives [H(3)TIMEN(t-Bu)](PF(6))(3) and [H(3)TIMEN(Bz)](PF(6))(3) yields the free tris-carbenes TIMEN(t-Bu) (5b) and TIMEN(Bz) (5c), which react readily with copper(I) salts to give mononuclear complexes [(TIMEN(t-Bu))Cu](PF(6)) (11b) and [(TIMEN(Bz))Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined by single-crystal X-ray diffraction. While the TIMEN(Me) ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMEN(t-Bu) and TIMEN(Bz) ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an additional, weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc(+), respectively, indicating sufficient electronic and structural flexibility of both TIMEN(R) ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMEN(Bz))Cu](OTf)(2) (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR spectroscopy, and SQUID magnetization measurements.

Electron-transfer kinetics of copper(II/I) tripodal ligand complexes.

The electron-transfer kinetics for each of three copper(II/I) tripodal ligand complexes reacting with multiple reducing and oxidizing counter reagents have been examined in aqueous solution at 25 degrees C, mu = 0.10 M. For all of the ligands studied, an amine nitrogen serves as the bridgehead atom. Two of the ligands (PMMEA and PEMEA) contain two thioether sulfurs and one pyridyl nitrogen as donor atoms on the appended legs while the third ligand (BPEMEA) has two pyridyl nitrogens and one thioether sulfur. Very limited kinetic studies were also conducted on two additional closely related tripodal ligand complexes. The results are compared to our previous kinetic study on a Cu(II/I) system involving a tripodal ligand (TMMEA) with thioether sulfur donor atoms on all three legs. In all systems, the Cu(II/I) electron self-exchange rate constants (k(11)) are surprisingly small, ranging approximately 0.03-50 M(-)(1) s(-)(1). The results are consistent with earlier studies reported by Yandell involving the reduction of Cu(II) complexes with four similar tripodal ligand systems, and it is concluded that the dominant reaction pathway involves a metastable Cu(II)L intermediate species (designated as pathway B). Since crystal structures suggest that the ligand reorganization accompanying electron transfer is relatively small compared to our earlier studies on macrocyclic ligand complexes of Cu(II/I), it is unclear why the k(11) values for the tripodal ligand systems are of such small magnitude.

The Use of Non-Covalent Interactions in the Assembly of Metal/Organic Supramolecular Arrays

The design and synthesis of molecules that can organize into specific supramolecular assemblies in the solid state is an area of considerable interest, since incorporation of specific structural components into a crystal lattice may lead to new materials with desirable chemical and physical properties. However, it is still difficult to reliably predict crystal structures especially those in non-centrosymmetric space groups. To develop compounds whose assembly is more controlled and predictable, we are investigating the formation of supramolecular assemblies from C 2 - and C 3 -symmetric molecular species. The C 2 -symmetric systems use a rigid template to control the orientation of appended groups. Using hydrogen bonds and weak metal-ligand interactions as the key forces to control molecular structure, a variety of helical forming compounds can be isolated. The assembly of these compounds was monitored in the crystalline phase and shown to yield a diverse group of lattice architectures. Molecular helices have produced i) microporous networks, ii) extended helices in the lattice and iii) chiral arrays that assemble enantioselectively during crystallization. The use of C 3 -symmetric compounds in supramolecular assembly is illustrated by a chiral tripodal system that forms non-centrosymmetric crystal lattices. A Ni(II)-F salt of this tripod has a lattice that is noteworthy for its six-fold symmetric open framework structure and the alignment of all the molecular Ni-F bonds along a crystallographic axis.

Areneruthenium(II) Complexes with Functionalized Phosphines Coordinating as Mono-, Bi- or Tridentate Ligands

Areneruthenium(II) compounds [Ru(p-cym)Cl2{κ-P-iPrP(CH2CH2OMe)2}], 3, and [Ru(arene)Cl2{κ-P-RP(CH2CO2Me)2}] 4 – 7 (arene=p-cym (=1-methyl-4-isopropylbenzene), mes (=1,3,5-trimethylbenzene); R=iPr, tBu) were prepared from the dimers [Ru(arene)Cl2]2 and the corresponding functionalized phosphine. Treatment of 6 and 7 with 1 equiv. of AgPF6 affords the monocationic complexes [Ru(mes)Cl{κ2-P,O-RP(CH2C(O)OMe)(CH2CO2Me)}]PF6, 10 and 11, while the related reaction of 5 – 7 with 2 equiv. of AgPF6 produces the dicationic compounds [Ru(p-cym){κ3-P,O,O-tBuP(CH2C(O)OMe)2}](PF6)2 (12) and [Ru(mes){κ3-P,O,O-RP(CH2C(O)OMe)2}](PF6)2, 13 and 14. Partial hydrolysis of one hexafluorophosphate anion of 12 – 14 leads to the formation of [Ru(arene){κ2-P,O-RP(CH2C(O)OMe)(CH2CO2Me)}(κ-O-O2PF2)]PF6, 15 – 17, of which 17 (arene=mes; R=tBu) has been characterized by X-ray crystallography. Compounds 13 and 14 react with 2 equiv. of KOtBu in tBuOH/toluene to give the unsymmetrical complexes [Ru(mes){κ3-P,C,O-RP(CHCO2Me)(CH=C(O)OMe)}], 18 and 19, containing both a five-membered phosphinoenolate and a three-membered phosphinomethanide ring. The molecular structure of compound 18 has been determined by X-ray structure analysis. The neutral bis(carboxylate)phosphanidoruthenium(II) complexes [Ru(arene){κ3-P,O,O-RP(CH2C(O)O)2}], 20 – 23 are obtained either by hydrolysis of 18 and 19, or by stepwise treatment of 4 and 5 with KOtBu and basic Al2O3. Novel tripodal chelating systems are generated via insertion reactions of 19 with PhNCO and PhNCS.

Areneruthenium(II) Complexes with Functionalized Phosphines Coordinating as Mono-, Bi- or Tridentate Ligands

Areneruthenium(II) compounds [Ru(p-cym)Cl2{κ-P-iPrP(CH2CH2OMe)2}], 3, and [Ru(arene)Cl2{κ-P-RP(CH2CO2Me)2}] 4 – 7 (arene=p-cym (=1-methyl-4-isopropylbenzene), mes (=1,3,5-trimethylbenzene); R=iPr, tBu) were prepared from the dimers [Ru(arene)Cl2]2 and the corresponding functionalized phosphine. Treatment of 6 and 7 with 1 equiv. of AgPF6 affords the monocationic complexes [Ru(mes)Cl{κ2-P,O-RP(CH2C(O)OMe)(CH2CO2Me)}]PF6, 10 and 11, while the related reaction of 5 – 7 with 2 equiv. of AgPF6 produces the dicationic compounds [Ru(p-cym){κ3-P,O,O-tBuP(CH2C(O)OMe)2}](PF6)2 (12) and [Ru(mes){κ3-P,O,O-RP(CH2C(O)OMe)2}](PF6)2, 13 and 14. Partial hydrolysis of one hexafluorophosphate anion of 12 – 14 leads to the formation of [Ru(arene){κ2-P,O-RP(CH2C(O)OMe)(CH2CO2Me)}(κ-O-O2PF2)]PF6, 15 – 17, of which 17 (arene=mes; R=tBu) has been characterized by X-ray crystallography. Compounds 13 and 14 react with 2 equiv. of KOtBu in tBuOH/toluene to give the unsymmetrical complexes [Ru(mes){κ3-P,C,O-RP(CHCO2Me)(CH=C(O)OMe)}], 18 and 19, containing both a five-membered phosphinoenolate and a three-membered phosphinomethanide ring. The molecular structure of compound 18 has been determined by X-ray structure analysis. The neutral bis(carboxylate)phosphanidoruthenium(II) complexes [Ru(arene){κ3-P,O,O-RP(CH2C(O)O)2}], 20 – 23 are obtained either by hydrolysis of 18 and 19, or by stepwise treatment of 4 and 5 with KOtBu and basic Al2O3. Novel tripodal chelating systems are generated via insertion reactions of 19 with PhNCO and PhNCS.

Electron-transfer kinetics of tris(2-(methylthioethyl))aminecopper(II/I). A tripodal ligand complex exhibiting virtual C3v symmetry.

The tripodal ligand TMMEA (tris(2-methylthioethyl)amine) forms a trigonal bipyramidal complex with copper(II) in which the bridgehead nitrogen occupies one axial site, a solvent molecule (or anion) occupies the opposite axial site, and the three thioether sulfurs occupy the three planar sites. Upon reduction to copper(I), the axial solvent molecule (or anion) dissociates to leave a trigonal pyramidal complex with shortened Cu-S bonds and an elongated Cu-N bond. Therefore, both oxidation states maintain virtual C3v symmetry similar to that found in the type 1 blue copper protein sites. The electron-transfer cross-reaction rate constants have been determined for the Cu(II/I)(TMMEA) system reacting with three reductants and three oxidants. The Marcus cross relation was then utilized to generate apparent values for the Cu(II/I) electron self-exchange rate constant (k(11)) from the kinetic data for each of the six reactions. The median value obtained from the three reduction reactions is log k(11(Red)) = -1.5 while the median from the three oxidation reactions is log k(11(Ox)) = +0.9. This difference of 2.4 orders of magnitude is consistent with the dual-pathway square scheme mechanism which we have previously proposed for electron transfer in Cu(II/I) complexes. For this tripodal ligand system, however, the pathway involving a metastable Cu(II)L intermediate (pathway B) appears to be preferred over the pathway involving a metastable Cu(I)L intermediate (pathway A), which is opposite to the trend we have previously observed for a number of systems involving macrocyclic and acyclic tetrathiaethers. Both pathways exhibit relatively sluggish electron-transfer kinetics which is attributed to the rupture/formation of the strongly bound inner-sphere water molecule and the accompanying solvent reorganization.

Synthesis of salicylamide and bipyridine containing ligands for iron(II) and iron(III) coordination

The synthesis and characterization of potential iron sequestering agents is reported. Both single-stranded ligands incorporating salicylamides for chelation of iron(III) and bipyridines for iron(II) as well as tripodal systems have been synthesized.