The coordination chemistry of two symmetric double‐armed oxadiazole‐bridged organic ligands with copper salts

Two new symmetric double‐armed oxadiazole‐bridged ligands, 4‐methyl‐{5‐[5‐methyl‐2‐(pyridin‐3‐ylcarbonyloxy)phenyl]‐1,3,4‐oxadiazol‐2‐yl}phenyl pyridine‐3‐carboxylate (L1) and 4‐methyl‐{5‐[5‐methyl‐2‐(pyridin‐4‐ylcarbonyloxy)phenyl]‐1,3,4‐oxadiazol‐2‐yl}phenyl pyridine‐4‐carboxylate (L2), were prepared by the reaction of 2,5‐bis(2‐hydroxy‐5‐methylphenyl)‐1,3,4‐oxadiazole with nicotinoyl chloride and isonicotinoyl chloride, respectively. Ligand L1 can be used as an organic clip to bind Cu^II cations and generate a molecular complex, bis(4‐methyl‐{5‐[5‐methyl‐2‐(pyridin‐3‐ylcarbonyloxy)phenyl]‐1,3,4‐oxadiazol‐2‐yl}phenyl pyridine‐3‐carboxylate)bis(perchlorato)copper(II), [Cu(ClO₄)₂(C₂₈H₂₀N₄O₅)₂], (I). In compound (I), the Cu^II cation is located on an inversion centre and is hexacoordinated in a distorted octahedral geometry, with the pyridine N atoms of two L1 ligands in the equatorial positions and two weakly coordinating perchlorate counter‐ions in the axial positions. The two arms of the L1 ligands bend inward and converge at the Cu^II coordination point to give rise to a spirometallocycle. Ligand L2 binds Cu^I cations to generate a supramolecule, diacetonitriledi‐μ₃‐iodido‐di‐μ₂‐iodido‐bis(4‐methyl‐{5‐[5‐methyl‐2‐(pyridin‐4‐ylcarbonyloxy)phenyl]‐1,3,4‐oxadiazol‐2‐yl}phenyl pyridine‐4‐carboxylate)tetracopper(I), [Cu₄I₄(CH₃CN)₂(C₂₈H₂₀N₄O₅)₂], (II). The asymmetric unit of (II) indicates that it contains two Cu^I atoms, one L2 ligand, one acetonitrile ligand and two iodide ligands. Both of the Cu^I atoms are four‐coordinated in an approximately tetrahedral environment. The molecule is centrosymmetric and the four I atoms and four Cu^I atoms form a rope‐ladder‐type [Cu₄I₄] unit. Discrete units are linked into one‐dimensional chains through π–π interactions.     

Opioid ligands with mixed properties from substituted enantiomeric N-phenethyl-5-phenylmorphans. Synthesis of a micro-agonist delta-antagonist and delta-inverse agonists

Enantiomeric N-phenethyl-m-hydroxyphenylmorphans with various substituents in the ortho, meta or para positions of the aromatic ring in the phenethylamine side-chain (chloro, hydroxy, methoxy, nitro, methyl), as well as a pyridylethyl and a indolylethyl moiety on the nitrogen atom, were synthesized and their binding affinity to the mu-, delta-, and kappa-opioid receptors was examined. The higher affinity ligands were further examined in the [(35)S]GTPgammaS assay to study their function and efficacy. 3-((1R,5S)-(-)-2-(4-Nitrophenethyl)-2-aza-bicyclo[3.3.1]nonan-5-yl)phenol ((-)-) was found to be a mu-agonist and delta-antagonist in that functional assay and was about 50 fold more potent than morphine in vivo. 3-((1R,5S)-(-)-2-(4-Chlorophenethyl)-2-aza-bicyclo[3.3.1]nonan-5-yl)phenol ((-)-) and several other ligands displayed inverse agonist activity at the delta-opioid receptor. The absolute configuration of all of the reported compounds was established by chemical conversion of (-)- to 1R,5S-(-)-.HBr.     

Konformative Beweglichkeit Flexibler Ringsysteme–XI Untersuchungen mit Hilfe der Protonenresonanzspektroskopie Ringinversion bei Methyl- und Alkoxylcyclohexanen

The ring inversion of cyclohexane derivatives with one, two and three pairs of geminal methyl and alkoxyl groups has been studied by investigations of the temperature dependence of the NMR signals. The rate of the inversion depends on the number and the relative positions of the ligand pairs as well as on their size. A single pair of geminal ligands has only a very slight effect on the inversion rate. Two ligand pairs hinder the inversion when they are in the 1 and 4 positions relative to one another, but facilitate it when they are in the 1 and 3 positions. In both cases the effects are smaller with methoxyl groups than with methyl groups. Three ligand pairs significantly increase the inversion rate when they are in the 1, 3 and 5 positions, but reduce it when they are in the 1, 2 and 4 positions. The effect of 1,3 substituents is attributed to an increase in the energy of the ground state of the molecules whereas the effect of 1,4 arranged substituents is attributed to an increase in the energy of the transition state of the chair inversion.     

Polymethylated DOTA ligands. 1. Synthesis of rigidified ligands and studies on the effects of alkyl substitution on acid-base properties and conformational mobility

This work describes the synthesis and the conformational properties of new polymethylated macrocyclic ligands of potential interest for magnetic resonance imaging. M4cyclen, (2S,5S,8S,11S)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecane, was obtained by cyclotetramerization of (2S)-1-benzyl-2-methylaziridine followed by catalytic hydrogenation. The ligands M4DOTA, [(2S,5S,8S,11S)-4,7,10-tris-carboxymethyl-2,5,8,11-tetramethyl- 1,4,7,10-tetraazacyclododecan-1-yl]acetic acid, and M4DOTMA, (R)-2-[(2S,5S,8S,11S)-4,7,10-tris-((R)-1-carboxyethyl)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid, were prepared by carboxyalkylation of M4cyclen in the presence of Na(2)CO(3). The triacetic ligand M4DO3A, [(2S,5S,8S,11S)-4,7-bis-carboxymethyl-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]acetic acid, was obtained in good yields without traces of M4DOTA if NaHCO(3) was the acid scavenger when adding the carboxylic arms. In the same conditions, cyclen yielded M4DOTA in 82% yield. The difference between the reactivity of cyclen and M4cyclen is assigned to the high basicity of the substituted tetraamine as estimated by NMR titration. The one- and two-dimensional (1)H and (13)C NMR spectra of M4DOTA and M4DOTMA in the H(4)L or H(6)L(2+) forms are interpreted as arising from a slow exchange between two elongated geometries in which the methyl substituents are in one of the two possible equatorial-like positions, either close to or away from the carboxylic arms. The axial-like positions are sterically too crowded and cannot be occupied by the methyl groups. An elongated conformation is also adopted by DOTMA, (R)-2-[4,7,10-tris-((R)-carboxyethyl)-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid, in the H(6)L(2+) form. The rigidification of the polymethylated ligands allows a detailed NMR analysis that cannot be carried out on the parent unsubstituted ligand DOTA.     

Remote Impacts of Methyl Substituents on the Guest‐Binding Ability of Self‐Assembled Cages

We synthesized self‐assembled coordination cages in which 1,10‐phenanthroline derivatives serve as capping ligands. Substituents at the 2,9‐positions of the phenanthroline ligand covered the outside of the cage but had an impact on the guest binding inside the cage. Introduction of methyl groups at the 2,9‐positions allowed the cage to accommodate tetraphenylsilane. Bulky mesityl groups overhanging the cage framework significantly shrunk the cage cavity through π–π interactions with the aromatic panels of the cage. The p‐methyl group of the mesityl substituent was a determinant of the restricted motion of 4,4′‐dimethoxybenzil inside the cage at high temperature. Thus, the presence or absence of one methyl group, which is far from the guest‐binding site, makes a significant difference in the guest species and motions inside the cage.     

Polymethylated DOTA ligands. 2. Synthesis of rigidified lanthanide chelates and studies on the effect of alkyl substitution on conformational mobility and relaxivity

M4DOTA, [(2S,5S,8S,11S)-4,7,10-tris-carboxymethyl-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]acetic acid (2e), and M4DOTMA, (R)-2-[(2S,5S,8S,11S)-4,7,10-tris-((R)-1-carboxyethyl)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid (3e), are derivatives of ligand DOTA (1e) that form sterically crowded lanthanide chelates. M4DOTMA forms highly symmetric and totally rigid single Y(3+) and Yb(3+) species in which the ring substituents occupy corner positions in a square antiprismatic arrangement as shown by molecular mechanics calculations and by a quantitative interpretation of the relative magnitudes of the paramagnetic (1)H NMR shifts of dipolar origin. The NMR spectrum of YbM4DOTMA(-) displays two intense methyl peaks outside the 0-10 ppm range whose shift difference is strongly temperature dependent. YbM4DOTMA(-) (3d) could be a useful probe in magnetic resonance thermometric imaging. With only four methyl substituents on the tetraaza ring, M4DOTA forms three Yb(3+) species in solution. The methyl substituents prevent the inversion of configuration of the ethylenic groups but not of the acetate arms. Although the methyl groups are likely to preferably occupy ring corner positions, the dipolar equations do not allow one to distinguish with certainty between the two available corner (equatorial) orientations. Reliably applying the dipolar equations is less obvious than usually assumed. A single methyl substituent as in ligand MDOTA (5e) suffices to rigidify the tetraaza cycle but not the acetate arms. Racemic YbMDOTA(-) (5d) is present in solution as four totally asymmetric topomers with the methyl groups occupying either one of the two equatorial positions. A complete assignment of the solution structures on the basis of the dipolar equations is again uncertain. The nuclear magnetic relaxation dispersion curves of the Gd(3+) chelates of all the methylated DOTA ligands including DOTMA, (R)-2-[4,7,10-tris-((R)-carboxyethyl)- 1,4,7,10- tetraazacyclododecan-1-yl]propionic acid, are very similar, and intermolecular conformational processes appear to have no influence on the relaxivity of these small complexes for which the relaxation T(1) is mainly determined by the rotational correlation time (tau(r)). The hydration number of the Tb(3+) chelates measured by fluorescence decreases from DOTMA to M4DOTMA presumably because steric crowding leads to an increase of the metal-water distance.     

Effect of Positions of Methyl Substituents in the Complexation Kinetics of Macrocyclic Tetraamines with Copper(II) in Strongly Basic Aqueous Media

The complexation kinetics of the reaction of copper(II) with isomeric tetraamine macrocyclic ligands, C-rac-5, 7, 7, 12, 12, 14-hexamethyl-l, 4, 8, 11-tetraazacyclotetradecane (tet c), C-meso-5, 7, 7, 12, 12, 14-hexamethyl-l, 4, 8, 11-tetraazacyclotetradecane (tet d), and C-meso-5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane (tet a) in strongly basic aqueous media have been examined at 25.0 ± 0.1°C by means of the stopped-flow technique. The variation in the values of the resulting rate constants indicates that the positions of the methyl substituents play a significant role in these reactions. These reactions exhibit associative character and second-bond formation is proposed as the rate-determining step.     

Rhenium tricarbonyl core complexes of thymidine and uridine derivatives

Thymidine and uridine were modified at the C2' and C5' ribose positions to form amine analogues of the nucleosides (1 and 4). Direct amination with NaBH(OAc)3 in DCE with the appropriate aldehydes yielded 1-{5-[(bis(pyridin-2-ylmethyl)amino)methyl]-4-hydroxytetrahydrofuran-2-yl}-5-methyl-1H-pyrimidine-2,4-dione (L1), 1-{5-[(bis(quinolin-2-ylmethyl)amino)methyl]-4-hydroxytetrahydrofuran-2-yl}-5-methyl-1H-pyrimidine-2,4-dione (L2), and 1-[3-(bis(pyridin-2-ylmethyl)amino)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl]-1H-pyrimidine-2,4-dione (L5), while standard coupling procedures of 1 and 4 with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (2) and 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid (3) in the presence of HOBT-EDCI in DMF provided a second novel series of bifunctional chelators: 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid [(3-hydroxy-5-(5-methyl-4-oxo-3,4-dihydro-2H-pyrimidin-1-yl)tetrahydrofuran-2-yl)methyl] amide (L3), 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid [(3-hydroxy-5-(5-methyl-4-oxo-3,4-dihydro-2H-pyrimidin-1-yl)tetrahydrofuran-2-yl)methyl] amide (L4), 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid [2-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-3-yl] amide (L6), and 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid [2-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-3-yl] amide (L7). The rhenium tricarbonyl complexes of L1-L4, L6, and L7, [Re(CO)3(LX)]Br (X=1-4, 6, 7: compounds 5-10, respectively), have been prepared by reacting the appropriate ligand with [NEt4][Re(CO)3Br3] in methanol. The ligands and their rhenium complexes were obtained in good yields and characterized by common spectroscopic techniques including 1D and 2D NMR, HRMS, IR, cyclic voltammetry, UV, and luminescence spectroscopy and X-ray crystallography. The crystal structure of complex 6.0.5NaPF6 displays a facial geometry of the carbonyl ligands. The nitrogen donors of the tridentate ligand complete the distorted octahedral spheres of the complex. Crystal data: monoclinic, C2, a = 24.618(3) A, b = 11.4787(11) A, c = 15.5902(15) A, beta = 112.422(4) degrees , Z = 4, D(calc) = 1.562 g/cm3.     

Neutron Diffraction Study of [Nd(AlMe4)3]⋅0.5 Al2Me6 at 100 K: The First Detailed Look at a Bridging Methyl Group with a Trigonal-Bipyramidal Carbon Atom

Weak agostic Nd⋅⋅⋅H interactions and Nd−C bonds are involved in the bonding of the bridging methyl groups in the title compound (see sketch on the right): Two of the three H atoms of the methyl group are directed at the Nd center. The C atoms have distorted trigonal-bipyramidal geometry with the Nd atom and one of the H atoms (H_A) as axial ligands, and the Al atom and the other two H atoms (H_B and H_C) in equatorial positions. The Al₂Me₆ “solvate” molecule is disordered.     

Synthesis of an arabinogalactan-type octa- and two isomeric nonasaccharides. Suitable tuning of protecting groups

An arabinogalactan-type double-branched octa- and two isomeric nonasaccharides were synthesized using the (2-naphthyl)methyl (NAP) and the acid sensitive but base stable (methoxydimethyl)methyl (MIP) protecting groups. The β-(1→6)-linked hexagalactan skeleton was synthesized having a benzyl and a (2-naphthyl)methyl (NAP) group at positions 2 of the second and the penultimate galactopyranosyl units, and this made possible sequential introduction of α-l-arabinofuranosyl or α-l-arabinofuranosyl-(1→5)-α-l-arabinofuranosyl side chains.     

Thermodynamics of binding of 2-methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine to the major urinary protein

In the present study we examine the thermodynamics of binding of two related pyrazine-derived ligands to the major urinary protein, MUP-I, using a combination of isothermal titration calorimetry (ITC), X-ray crystallography, and NMR backbone (15)N and methyl side-chain (2)H relaxation measurements. Global thermodynamics data derived from ITC indicate that binding is driven by favorable enthalpic contributions, rather than the classical entropy-driven hydrophobic effect. Unfavorable entropic contributions from the protein backbone and side-chain residues in the vicinity of the binding pocket are partially offset by favorable entropic contributions at adjacent positions, suggesting a "conformational relay" mechanism whereby increased rigidity of residues on ligand binding are accompanied by increased conformational freedom of side chains in adjacent positions. The principal driving force governing ligand affinity and specificity can be attributed to solvent-driven enthalpic effects from desolvation of the protein binding pocket.     

Bile acids LXIX. Selective K-Selectride reduction of 3,7-diketo steroids

The K-Selectride reduction at low temperature (-45°C) of 7-oxo-5α-holestan-3β-yl acetate and methyl 7-oxo-3α-hydroxy-5(β-cholanoate resulted in almost quantitative yield of the 7α-alcohol in the 5α-compound but only moderate yield of the 5β-analog. The simultaneous reduction of two carbonyl groups in the 3 and 7 positions afforded good to excellent yields of the diaxial diol in planar steroids (methyl 3,7-dioxo-5α-cholanoate, 3,7-dioxo-5α-cholestane and methyl 3,7-dioxo-5α-cholestan-27-oate) and only 14% of 3α,7α-(OH)₂ from methyl 3,7-dioxo-5β-cholanoate.     

Organometallic supramolecular chemistry with monosaccharides: triethylammonium mu-chloro-bis[chloro(eta5-cyclopentadienyl)-(methyl 4,6-o-benzylidene-beta-D-glucopyranosidato-1kappaO2,1:2kappaO3) zirconate

The reaction of [CpZrCl3(thf)2] with methyl 4,6-O-benzylidene-beta-D-glucopyranoside (beta-MeBGH2, 1) in the presence of Et3N results in the formation of the zirconate complex [Et3NH] [(CpZrCl)2(mu-Cl) (mu-(beta-MeBG)]2] (2). X-ray structure analyses were performed from the ligand precursor beta-MeBGH2 1 as well as from 2. Compound 1 crystallizes in the monoclinic chiral space group P2(1). The molecules show a flat arrangement including the benzylidene protecting group, and are packed in columns. The columns are held together in pairs by the formation of hydrogen bonds between the hydroxy functions in positions 2 and 3. Compound 2 crystallizes in the orthorhombic space group P2(1)2(1)2(1). The beta-MeBG ligands are chelating the Zr atoms through the oxygen atoms in positions 2 and 3 of the glucopyranosidato ligand revealing a 1-zircona-2,5-dioxolane moiety each; the oxygen atom in position 3 is linked to both of the Zr atoms. Additionally one chloro ligand is bridging the two Zr centers. Two terminally bound chloro ligands stick out from the two Zr atoms into a chiral U-shaped cavity constructed by the two beta-MeBG ligands. The cavity incorporates the tertiary ammonium cation [Et3NH]+ which is bound to one of the terminal chloro ligands through a hydrogen bond. The inclusion of the [Et3NH]+ cation in the U-shaped cavity, even in solution, is demonstrated by NMR spectroscopic data.     

Nickel(II)-phenoxyl radical complexes: structure-radical stability relationship

Nickel(II) complexes of N3O-donor tripodal ligands, 2,4-di-tert-butyl-6-[([bis(2-pyridyl)methyl]amino)methyl]phenol (HtbuL), 2,4-di-tert-butyl-6-[([(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)amino)methyl]phenol (HtbuLMepy), and 2,4-di-tert-butyl-6-[([bis(6-methyl-2-pyridyl)methyl]amino)methyl]phenol (HtbuL(Mepy)2), were prepared, and [Ni(tbuL)Cl(H2O)] (1), [Ni(tbuLMepy)Cl] (2), and [Ni(tbuL(Mepy)2)Cl] (3) were structurally characterized by the X-ray diffraction method. Complexes 1 and 3 have a mononuclear structure with a coordinated phenolate moiety, while 2 has a dinuclear structure bridged by two chloride ions. The geometry of the Ni(II) center was found to be octahedral for 1 and 2 and 5-coordinate trigonal bipyramidal for 3. Complexes 1-3 exhibited similar absorption spectra in CH3CN, indicating that they all have a mononuclear structure in solution. They were converted to the phenoxyl radicals upon oxidation with Ce(IV), giving a phenoxyl radical pi-pi transition band at 394-407 nm. ESR spectra at low temperature and resonance Raman spectra established that the radical species has a Ni(II)-phenoxyl radical bond. The cyclic voltammograms showed a quasi-reversible redox wave at E1/2=0.46-0.56 V (vs Ag/AgCl) corresponding to the formation of the phenoxyl radical, which displayed a first-order decay with a half-life of 45 min at room temperature for 1 and 26 and 5.9 min at -20 degrees C for 2 and 3, respectively. The radical stability increased with the donor ability of the N ligands.     

Methylation of the sulfolane ring at the 2 and 2, 5 positions

The possibility of alkylating sulfolane and its derivatives is investigated. A new method of introducing a methyl group at positions 2 and 2, 5 in the sulfolane ring, using sulfolanylmagnesium bromide is put forward.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

Isolation and structural characterization of anionic and neutral compounds resulting from the oxidative addition of HI or CH3I to [IrI2(CO)2](-)

The active iridium species in the methanol carbonylation reaction has been crystallized as the [PPN][IrI(2)(CO)(2)] complex and the X-ray structure solved, showing a cis-geometry and a square planar environment. Hydriodic acid reacts very quickly with this compound to provide [PPN][IrHI(3)(CO)(2)], the X-ray crystal structure of which has been determined. The two CO ligands remain in mutual cis-position in a pseudooctahedral environment. The same cis-arrangement has been observed from the X-ray structure for [PPN][IrI(3)(CH(3))(CO)(2)] resulting from the slower oxidative addition of CH(3)I to [PPN][IrI(2)(CO)(2)]. By iodide abstraction with InI(3), the anionic methyl complex gave rise to the dimeric neutral complex [Ir(2)(mu-I)(2)I(2)(CH(3))(2)(CO)(4)]. An X-ray structure showed that the methyl ligands are in the equatorial positions of the two octahedrons sharing an edge, formed by the two bridging iodide ligands. All these four complexes have been fully characterized by mass spectrometry, (1)H and (13)C NMR, and infrared both in solution and in the solid state. When necessary, the (13)CO- or (13)CH(3)-enriched complexes have been prepared and analyzed.     

Pyrazolyl methyls prescribe the electronic properties of iron(II) tetra(pyrazolyl)lutidine chloride complexes

A series of iron(II) chloride complexes of pentadentate ligands related to α,α,α',α'-tetra(pyrazolyl)-2,6-lutidine, pz(4)lut, has been prepared to evaluate whether pyrazolyl substitution has any systematic impact on the electronic properties of the complexes. For this purpose, the new tetrakis(3,4,5-trimethylpyrazolyl)lutidine ligand, pz**(4)lut, was prepared via a CoCl(2)-catalyzed rearrangement reaction. The equimolar combination of ligand and FeCl(2) in methanol gives the appropriate 1:1 complexes [FeCl(pz(R)(4)lut)]Cl that are each isolated in the solid state as a hygroscopic solvate. In solution, the iron(II) complexes have been fully characterized by several spectroscopic methods and cyclic voltammetry. In the solid state, the complexes have been characterized by X-ray diffraction, and, in some cases, by M?ssbauer spectroscopy. The M?ssbauer studies show that the complexes remain high spin to 4 K and exclude spin-state changes as the cause of the surprising solid-state thermochromic properties of the complexes. Non-intuitive results of spectroscopic and structural studies showed that methyl substitution at the 3- and 5- positions of the pyrazolyl rings reduces the ligand field strength through steric effects whereas methyl substitution at the 4-position of the pyrazolyl rings increases the ligand field strength through inductive effects.     

Excited State Properties and Energy Transfer within Dipyrrin‐Based Binuclear Iridium/Platinum Dyads: The Effect of ortho‐Methylation on the Spacer

Luminescent cyclometalated iridium complexes based on pyridyl appended dipyrrin ligands were prepared and characterized both in the solid state and in solution. The functionalization of the peripheral pyridyl moiety causes dramatic changes on the emission properties of both mono‐ and hetero‐ binuclear complexes. A detailed photophysical investigation of the two mononuclear derivatives of the [(Ppy)₂Ir(dpm‐py)] family (Ppy=2‐phenylpyridine, dpm‐py=5‐(4‐pyridyl)dipyrrin) was carried out. Introduction of methyl groups at the 3 and 5 positions on the pyridyl unit diminishes the non‐radiative rate constant by locking the peripheral pyridyl group orthogonally to the dipyrrinato plane. Thus, they limit the rotational degree of freedom, as well as the charge‐transfer character of the excited state. The coordination of these two complexes to a cyclometalated [(dppy)Pt] fragment (dppy=2,6‐diphenylpyridine) led to the formation of binuclear species in which the iridium and platinum complexes behave as acceptors and donors, respectively. In these heterobinuclear compounds, the methyl groups do not influence the energy transfer efficiency, which is estimated to be above 90 %. However, they do limit the charge‐transfer character of the acceptor’s excited state, as well as its rotational degree of freedom, thus avoiding the detrimental effect upon the photophysical performance.     

A new pyridine-based 12-membered macrocycle functionalised with different fluorescent subunits; coordination chemistry towards Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II)

The coordination chemistry of the new pyridine-based, N2S2-donating 12-membered macrocycle 2,8-dithia-5-aza-2,6-pyridinophane (L1) towards Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II) has been investigated both in aqueous solution and in the solid state. The protonation constants for L1 and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those of ligand L2, which contains a N-aminopropyl side arm. The measured values show that Hg(II) in water has the highest affinity for both ligands followed by Cu(II), Cd(II), Pb(II), and Zn(II). For each metal ion considered, 1:1 complexes with L1 have also been isolated in the solid state, those of Cu(II) and Zn(II) having also been characterised by X-ray crystallography. In both complexes L1 adopts a folded conformation and the coordination environments around the two metal centres are very similar: four positions of a distorted octahedral coordination sphere are occupied by the donor atoms of the macrocyclic ligand, and the two mutually cis-positions unoccupied by L1 accommodate monodentate NO3- ligands. The macrocycle L1 has then been functionalised with different fluorogenic subunits. In particular, the N-dansylamidopropyl (L3), N-(9-anthracenyl)methyl (L4), and N-(8-hydroxy-2-quinolinyl)methyl (L5) pendant arm derivatives of L1 have been synthesised and their optical response to the above mentioned metal ions investigated in MeCN/H2O (4:1 v/v) solutions.