Unsymmetrical Dimer Research Articles

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4′-(1,3-difluoro-4-n-octyloxy) biphenyloxy or a 4′-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C7 central paraffinic spacer and a C8 alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGBC*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.

Synthesis of nucleoside dimers bridged on ribose with a butadiynyl group.

[reaction: see text] The nucleoside dimer linked by a butadiynediyl group at C-3'beta may serve as a building block for the preparation of backbone-modified oligonucleotides for DNA repair or mutation in functional genomics. We prepared this type of dimer by an Eglington or Sonogashira coupling reaction. The unsymmetrical dimer was synthesized by coupling the acetylene with the bromoacetylene. Only marginal cytotoxicity was detected for one of the dimers.

Monodispersive Unsymmetrical Tetramers Exhibiting a Columnar Phase

Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially through a flexible spacer (or multiple spacers) seem to be interesting materials from the points of view of both basic research and practical applications owing to their multifunctional characteristics and properties similar to those of polymers while retaining fluidity, viscosity etc. associated with those of low molar mass liquid crystals. The synthesis and evaluation of mesomorphic behavior of two novel monodispersive unsymmetrical tetramesogens have been presented here in detail. The molecular design of these tetramesogens incorporates the cholesteryl ester (as a chiral entity), biphenyl (as a supportive entity for mesomorphism), azobenzene (as a photoactive mesogen), and tolan (as a half-disc) segments interconnected through even-even-odd and odd-even-odd alkylene spacers. The target liquid crystalline tetramers have been realized by employing convenient synthetic strategies, which involves the preparation of two important unsymmetrical dimers and finally their condensation. The molecular structures of all the intermediates and unsymmetrical tetramers have been characterized by spectroscopic analyses. The mesomorphic property of both the unsymmetrical tetramers has been evaluated by optical microscopic, calorimetric and X-ray diffraction studies. Our study reveals that both oligomers show an identical mesophase. While X-ray studies suggest that the structure of the mesophase is similar to that of a rectangular columnar (Col r ) mesophase; a pseudoisotropic texture was seen under the microscope.

The synthesis of specifically metallated heterobimetallic dimeric porphyrins

A series of conjugated mixed metal heteroporphyrin dimers has been prepared using Wittig chemistry. They can be synthesized from a double Wittig reaction between porphyrin phosphonium salts and phthalaldehydes, or from stepwise Wittig reactions. This allows both symmetrical and unsymmetrical dimers to be prepared with complete control of porphyrin metallation.

Diphosphanylketenimines: new reagents for the synthesis of unique phosphorus heterocycles.

Two consecutive [3+2] cycloaddition reactions of the diphosphanylketenimine (PPh(2))(2)C[double bond]C[double bond]NPh (3), involving the phosphanyl groups, with two equivalents of the electron-poor alkynes dimethyl acetylenedicarboxylate or methyl acetylenecarboxylate give rise to the formation of the bicyclic 1 lambda(5),3 lambda(5)-diphospholes 5 a,b, which contain a phosphorane unit with five carbon substituents attached to the phosphorus center. Compound 3 undergoes cyclodimerization by crystallization, affording the unsymmetrical dimer 6, which is converted back to 3 by heating in toluene. Compound 6 can be oxidized stepwise on the three trivalent phosphorus atoms by treatment with H(2)O(2) affording 7, 9, and the transient species 10, which are transformed into their corresponding ketenimine monomers either spontaneously (10) or by heating in toluene (7, 9). In this way, the compound (O[double bond]PPh(2))(PPh(2))C[double bond]C[double bond]NPh (8) is quantitatively obtained. Compound 8 readily reacts with the alkynes MeO(2)CC[triple bond]CCO(2)Me and MeO(2)CC[triple bond]CH, and with phenyl isocyanate and ethyl isothiocyanate through regiospecific [3+2] cycloaddition processes furnishing several lambda(5)-phosphole and lambda(5)-azaphosphole derivatives. Finally, the reaction of 8 with N-methylpropargylamine yields the new 2,3-dihydro-1,4-lambda(5)-azaphosphinine 15 through a cycloaddition process involving two functional groups from each molecule.

New dimeric macrolide glycosides from the marine sponge Myriastra clavosa.

Clavosolides A-D (1-4), dimeric macrolides incorporating cyclopropyl, tetrahydropyranyl, and glycosidic ring systems, were isolated from the cytotoxic extract of a Philippines collection of the marine sponge Myriastra clavosa. The structures of the clavosolides, which occurred as only trace metabolites, were elucidated through extensive NMR spectroscopic analyses. Clavosolides A (1) and B (2) were recently reported metabolites from M. clavosa, while the unsymmetrical dimers clavosolides C (3) and D (4) had new structures.

Molecular structure of [In 2(μ,η 1-OR)(μ,η 2-OR)(η 2-OR) 3(η 1-OR)] R = C 2H 4NMe 2, a pincer ligand

Functional indium [In(OR) 3] m alkoxides (R = C 2H 4OMe, C 2H 4NMe 2) have been obtained by alcoholysis of In[N(SiMe 3) 2] 3 and characterised by elemental analysis, FT-IR and NMR spectroscopy. The 2-dimethylaminoethoxide was characterised by X-ray diffraction. It corresponds to the association of two InO 6 octahedra giving the unsymmetrical dimer [In 2(μ,η 1-OR)(μ,η 2-OR)(η 2-OR) 3(η 1-OR)] ( 2). 2 was used as a pincer ligand toward Cu(acac) 2.

Synthesis and evaluation of unsymmetrical bis(arylcarboxamides) designed as topoisomerase-Targeted anticancer drugs

Symmetrical dimers of lipophilic intercalating chromophores linked by cation-containing chains have recently been shown to have broad-spectrum in vivo anticancer activity. We report the preparation and evaluation of a series of both symmetric and unsymmetric dimers of a variety of intercalating chromophores of varied DNA binding strength, including naphthalimides, acridines, phenazines, oxanthrenes and 2-phenylquinolines. The unsymmetrical dimers were prepared by sequential coupling of the chromophores to linkers with selectively protected primary terminal amines to ensure high yields and unequivocal product. Protection of the internal (secondary) amines as BOC derivatives was used to ensure complete structural specificity, and was also an aid to the purification of these very polar compounds. The growth inhibitory abilities (as IC 50 values) of the compounds in a range of cell lines showed that the nature of the linker chain was important, and independent of the nature of the chromophore, with compounds containing the dicationic linker [–(CH 2) 2NH(CH 2) 2NH(CH 2) 2–] being on average 30-fold more potent than the corresponding compounds containing the monocationic linker [–(CH 2) 3NMe(CH 2) 3–]. However, the chromophores also play a role in determining biological activity, with the cytotoxicities of symmetric and unsymmetric dicationic dimers correlating with the overall DNA binding abilities of the chromophores.

Synthesis, structural characterization, photoluminescence and thermal properties of [(Ph3P)2Cu(μ-SeC{O}R)2Cu(PPh3)

A number of neutral unsymmetrical dimers [(Ph3P)2Cu(μ-SeC{O}R)2Cu(PPh3)] (R = C6H5 (1), C6H4-Me-4 (2), C6H4-OMe-4 (3), C6H4-Cl-4 (4), C6H4-F-4 (5) and CH3 (6)) were synthesized in moderate yield, and characterized by analytical and spectroscopic techniques including 31P NMR. Variable temperature 31P NMR experiments appear to indicate that the molecular structure is retained in solution. X-Ray structure determination of 2 and 6 confirmed the molecular geometry of the unsymmetrical dimer. In 2 the two 4-MeO-C6H4CO groups have anti stereochemistry with respect to the Cu2Se2 ring while MeCO groups have syn configuration in 6. A weak C–H⋯OC intramolecular hydrogen bond present between the two selenoacetato ligands appears to dictate the syn geometry in 6. This hydrogen bonding is also responsible for the non-planarity of the Cu2Se2 ring. These compounds are photoemissive in CH2Cl2 solution. Thermogravimetry and pyrolysis experiments in nitrogen atmosphere show that 1–6 are good single source precursors to copper selenide bulk materials. Preliminary experiments show that these compounds are good single source molecular precursors for cubic phase Cu2−xSe nanoparticles with an average size of 75 ± 15 nm.

Open and closed conformation of the E. coli purine nucleoside phosphorylase active center and implications for the catalytic mechanism

The crystal structure of the ternary complex of hexameric purine nucleoside phosphorylase (PNP) from Escherichia coli with formycin A derivatives and phosphate or sulphate ions is determined at 2.0 Å resolution. The hexamer is found as a trimer of unsymmetric dimers, which are formed by pairs of monomers with active sites in different conformations. The conformational difference stems from a flexible helix (H8: 214-236), which is continuous in one conformer, and segmented in the other. With the continuous helix, the entry into the active site pocket is wide open, and the ligands are bound only loosely (“open” or “loose binding” conformation). By segmentation of the helix (H8: 214-219 and H8′: 223-236, separated by a γ-turn), the entry into the active site is partially closed, the pocket is narrowed and the ligands are bound much more tightly (“closed” or “tight binding” conformation). Furthermore, the side-chain of Arg217 is carried by the moving helix into the active site. This residue, conserved in all homologous PNPs, plays an important role in the proposed catalytic mechanism. In this mechanism, substrate binding takes place in the open, and and the catalytic action occurs in the closed conformation. Catalytic action involves protonation of the purine base at position N7 by the side-chain of Asp204, which is initially in the acid form. The proton transfer is triggered by the Arg217 side-chain which is moved by the conformation change into hydrogen bond distance to Asp204. The mechanism explains the broad specificity of E. coli PNP, which allows 6-amino as well as 6-oxo-nucleosides as substrates. The observation of two kinds of binding sites is fully in line with solution experiments which independently observe strong and weak binding sites for phosphate as well as for the nucleoside inhibitor.

Molecular structure of bis(N-phenylsalicylideneiminato)aluminium-di(μ-isopropoxy)di(isopropoxo)aluminium(iii) and its reactions with alkoxyalkanols

A novel heterocyclic derivative of aluminium(III) [C6H4O{CHN(C6H5)}]2Al(μ-OPri)2Al(OPri)21 has been synthesized by the reaction of aluminium isopropoxide with N-phenylsalicylidene imine in 1 ∶ 1 molar ratio in refluxing anhydrous benzene. Reactions of 1 with alkoxyalkanols in 1 ∶ 1 and 1 ∶ 2 molar ratios in refluxing anhydrous benzene yielded binuclear complexes of the types, [C6H4O{CHN(C6H5)}]2Al(μ-OPri)2Al(OCH2CH2OR)(OPri) and [C6H4O{CHN(C6H5)}]2Al(μ-OPri)2Al(OCH2CH2OR)2 (where RCH3, C2H5 and C4H9n), respectively. All of these binuclear complexes have been characterized by elemental analysis, molecular weight measurements, and spectral studies. The FAB mass spectrum and 27Al NMR spectrum of 1 indicate that it is a discrete unsymmetrical dimer containing four- and six-coordinated aluminium(III) atoms as confirmed by its single crystal X-ray structure.

Synthesis and Crystal Structure of a Chloro-Bridged Dinuclear Germanium(IV) Heterocoordinate Complex with 5,14-Dihydro-6,8,15,17-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine

Abstract A novel chloro-bridged dinuclear germanium(IV) complex with 5,14-dihydro-6,8,15,17-tetramethyldibenzo[b,i]-[1,4,8,11]tetraazacyclotetradecine has been synthesized and its crystal structure shows an unsymmetrical heterocoordinate dimer which has a five-coordinate square pyramid and a six-coordinate octahedron.

Directional Effects of the Carbonyl Group on the Mesogenic Properties of Twin Troponoids

Transition temperatures of four twin types of troponoid liquid crystals, in which the direction of the tropone carbonyl groups and a kind of the spacer were different from each other, were discussed. Symmetrical twin dimers 1, in which two tropone carbonyl groups direct inside, had monotropic smectic C phases whereas another symmetrical twin dimers 3 were less mesogenic than 1. Unsymmetrical twin dimers 4 had smectic A phases. Compared their thermal stabilities, unsymmetrical 4 had the highest clearing point when the number of atoms of the inner spacer was identical.

ChemInform Abstract: Chlorin-Based Symmetrical and Unsymmetrical Dimers with Amide Linkages: Effect of the Substituents on Photodynamic and Photophysical Properties.

In this study, we report syntheses, in vivo biological activity, and photophysical properties of a series of chlorin-based symmetrical and unsymmetrical dimers with amide linkages. All compounds exhibited strong absorption maxima at wavelengths ranging between λmax 660 and 702 nm. Compared with the formylpyropheophorbide a dimer 7 and purpurin 18 dimer 9 containing electron-withdrawing substituents at peripheral positions, pyropheophorbide a dimer 6, 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a dimer 8, and unsymmetrical dimer 12 in which the chlorin e6 and 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a moieties are linked with amide bonds, produced high fluorescence yields. For all photosensitizers, energy transfer from the sensitizer triplet to the ground state of oxygen is irreversible with rate constants kTΣ ≈ 2 × 109 M−1 s−1, a value in the diffusion-limited rate range. This energy transfer resulted in relatively high singlet oxygen quantum yields (ΦΔ ≈ 0.50 for compounds 12 and 8; and ΦΔ ≈ 0.30 for compounds 6 and 7). Among these dimers, compound 9 with a six-membered anhydride ring system produced the lowest singlet oxygen quantum yield (ΦΔ 0.06). The in vivo PDT efficacy of these compounds was evaluated in DBA/2 mice bearing SMT/F tumors. Among all the dimers, the unsymmetrical dimer 12 was found to be most effective, but it was significantly less active than the related monomer 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a2.

A 3,4-benzo-1,2-germacyclobut-3-ene and related compounds. Palladium-catalyzed σ-metathesis, dehydrogenative coupling and hydrogermylation of the GeGe σ-bond

In the presence of catalytic amounts of Pd(PPh 3) 4, 3,4-benzo-1,1,2,2-tetraethyl-1,2-germacyclobut-3-ene ( 1) readily undergoes the reversible σ-bond metathesis to give a dimer, 1,2,5,6-dibenzo-3,4,7,8-tetragermacycloocta-1,5-diene ( 6) below 100°C. However, at 160°C in toluene containing catalytic amounts of Pd(PPh 3) 4, both 1 and 6 afford an unsymmetrical dimer, 1,2,4,5-dibenzo-3,6,7,8-tetragermacycloocta-1,4-diene ( 8) with two isomeric products. In the presence of Pd(PPh 3) 4, 1,2-bis(diethylgermyl)benzene ( 18) undergoes dehydrocouling to give 1, which is smoothly converted to 6 under these conditions. The metal-metal σ-bonds of 1 and its silicon analogue are highly susceptible to hydrogermylation but not to hydrosilylation in the presence of Pd(PPh 3) 4.

New Chiral Hosts Derived from Dimeric Tartaric Acid: Efficient Optical Resolution of Aliphatic Alcohols by Inclusion Complexation

The novel, chiral, host compounds 8 and 9 were derived from tartaric acid. Inclusion complexation with these host compounds permitted highly efficient resolution of some aliphatic alcohols (10−13). The symmetrical dimer host compound 8 is effective for optical resolution of alcohols 10, 12, and 13 by a combination of enantioselective inclusion complexation and distillation techniques. The unsymmetrical dimer host compound 9 is effective for optical resolution of cyanohydrin 11. The crystal structures of the inclusion complexes were analyzed by X-ray diffraction methods in order to elucidate the mechanism of the efficient chiral recognition in the inclusion crystals.

Antioxidant chemistry of green tea catechins. New oxidation products of (-)-epigallocatechin gallate and (-)-epigallocatechin from their reactions with peroxyl radicals.

The green tea catechins (-)-epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC) react with peroxyl radicals generated by thermolysis of the azo initiator 2,2'-azobis(2, 4-dimethylvaleronitrile) (AMVN) to produce several oxidation products. Structure elucidation of these products can provide insights into specific mechanisms of antioxidant reactions. We isolated and identified a previously unreported reaction product of EGCG and three reaction products of EGC. In the EGCG product, the B-ring was transformed into a ring-opened unsaturated dicarboxylic acid moiety. The EGC products include a seven-membered B-ring anhydride and a symmetrical EGC dimer, both analogues of previously described EGCG oxidation products. The third EGC product was an unsymmetrical dimer. In all identified products, changes occurred solely in the B-ring of EGCG or EGC. This confirmed our previous observation that the principal site of antioxidant reactions in EGCG and EGC is the trihydroxyphenyl B-ring, regardless of the presence of a 3-galloyl moiety. A stoichiometric factor n of 4.16 +/- 0.51 was measured for EGCG, whereas factors of 2.20 +/- 0.26 was found for EGC and 2.33 +/- 0.18 measured for methyl gallate. These values represent the net peroxyl radical trapping per catechin molecule by several competing reactions. EGCG and EGC oxidation involves addition of oxygen, which is not derived from water, but most likely from atmospheric oxygen via peroxyl radicals. Characteristic oxidation products may be useful markers for antioxidant actions in living systems.

Chemistry of thiocarboxylates: synthesis and structures of neutral copper(I) thiocarboxylates with triphenylphosphine.

The reactions of Na+ R[O]CS- (R = Me, Ph) with mixtures of CuCl and PPh3 in stoichiometric ratios yielded the compounds [Cu4(SC[O]Me)4(PPh3)4] (1), [Cu4(SC[O]Ph)4(PPh3)3] (2), [Cu2(SC[O]Me)2(PPh3)4] (3), [Cu(SC[O]Ph)(PPh3)2] (4), and [Cu2(SC[O]Ph)2(PPh3)3] (5) quantitatively. Compound 2 was also obtained from mixtures of CuCl, PPh3, and NaSC[O]Ph in the ratio 1:1:1. The analogous thioacetate compound similar to 2 and the thiobenzoate analogue of 1 could not be obtained. Attempts to prepare the unsymmetrical dimer of a thioacetate compound similar to 5 gave a mixture of 1 and 3. The structures of 1-4 have been determined by single-crystal X-ray diffraction methods. Crystal data for 1: triclinic space group Pl, a = 11.5844(3) A, b = 13.2459(3) A, c = 14.3433(3) A, alpha = 64.019(1) degrees, beta = 79.297(1) degrees, gamma = 69.426(1) degrees, V = 1850.98(7) A3, Z = 1, Dcalcd = 1.439 g.cm-3. Crystal data for 2.0.5CH2Cl2.H2O: triclinic space group P1, a = 12.4413(1) A, b = 15.5443(1) A, c = 20.4637(3) A, alpha = 94.974(1) degrees, beta = 95.976(1) degrees, gamma = 100.450(1) degrees, V = 3848.09(7) A3, Z = 2, Dcalcd = 1.416 g.cm-3. Single-crystal data for 3: monoclinic space group P2(1)/n, a = 15.2746(2) A, b = 23.2947(2) A, c = 19.0518(3) A, beta = 96.713(1) degrees, V = 6732.5(2) A3, Z = 4, Dcalcd = 1.309 g.cm-3. Crystal data for 4: triclinic space group P1, a = 10.2524(3) A, b = 12.9826(4) A, c = 14.5340(4) A, alpha = 87.723(1) degrees, beta = 75.322(1) degrees, gamma = 75.978(1) degrees, V = 1815.14(9) A3, Z = 2, Dcalcd = 1.327 g.cm-3. Compound 1, [mu 3-SC[O]Me-S)2(mu-SC[O]Me-S)2(CuPPh3)4], is a tetramer with a distorted stepladder structure in which two copper atoms are trigonally coordinated and the other two are tetrahedrally coordinated. Two bonding modes, namely, mu 3-S and mu 2-S, were observed for the Me[O]CS- anion. The structure of 2 may be described as a highly distorted cubanoid structure and formulated as [(mu 3-SC[O]Ph-S3)(mu 3-SC[O]Ph-S2,O)3(Cu)(CuPPh3)3]. In 2, three copper atoms have tetrahedral coordination geometry and one copper atom is trigonally coordinated. Unprecedented bonding modes, namely, mu 3-S, have been observed for the R[O]CS- anions, in 1 and 2 and mu 3-S2,O in 2. Compound 3, [(mu-SC[O]MeS)(mu-SC[O]Me-S,O)[Cu(PPh3)2]2] is a dimer with mu 2-S and mu 2-S,O bonding modes of Me[O]CS- ligands. Monomeric structure was found in 4 in which the copper atom has trigonal planar geometry with a very weak intramolecular interaction with O. Variable temperature 31P NMR studies in solution show the presence of various species in equilibria.

Chlorin-based symmetrical and unsymmetrical dimers with amide linkages: effect of the substituents on photodynamic and photophysical properties

In this study, we report syntheses, in vivo biological activity, and photophysical properties of a series of chlorin-based symmetrical and unsymmetrical dimers with amide linkages. All compounds exhibited strong absorption maxima at wavelengths ranging between λmax 660 and 702 nm. Compared with the formylpyropheophorbide a dimer 7 and purpurin 18 dimer 9 containing electron-withdrawing substituents at peripheral positions, pyropheophorbide a dimer 6, 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a dimer 8, and unsymmetrical dimer 12 in which the chlorin e6 and 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a moieties are linked with amide bonds, produced high fluorescence yields. For all photosensitizers, energy transfer from the sensitizer triplet to the ground state of oxygen is irreversible with rate constants kTΣ ≈ 2 × 109 M−1 s−1, a value in the diffusion-limited rate range. This energy transfer resulted in relatively high singlet oxygen quantum yields (ΦΔ ≈ 0.50 for compounds 12 and 8; and ΦΔ ≈ 0.30 for compounds 6 and 7). Among these dimers, compound 9 with a six-membered anhydride ring system produced the lowest singlet oxygen quantum yield (ΦΔ 0.06). The in vivo PDT efficacy of these compounds was evaluated in DBA/2 mice bearing SMT/F tumors. Among all the dimers, the unsymmetrical dimer 12 was found to be most effective, but it was significantly less active than the related monomer 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide a2.

Understanding the orientation and dynamic motion of planar heterocyclic N-donor ligands by exploiting the symmetry properties of mixed-ligand mu-oxorhenium(V) dinuclear complexes.

Factors influencing the orientation and dynamic motions of planar N-donor heterocyclic ligands (L) are of interest since such features have broad relevance in metallobiochemistry [Marzilli, L. G.; Marzilli, P. A.; Alessio, E. Pure Appl. Chem. 1998, 70, 961-968]. We found that mu-oxorhenium(V) dinuclear complexes [ReOCl2LsLt]-O-[ReOCl2LsLt] bearing either symmetrical (L = py = pyridine; 3,5-lut = 3,5-lutidine) or lopsided (L = Me3-Bzm = 1,5,6-trimethylbenzimidazole) cis L ligands are particularly useful for studying these factors. NMR data showed that terminal (Lt) and stacked (Ls) ligands were exchanged by approximately 180 degrees rotation about the Re-O-Re bond system. Such exchange occurred, however, between degenerate chiral conformers. Here we report a combined X-ray structural and solution NMR investigation of the AA + CC (racemic) and AC (meso) forms of two mixed-ligand mu-oxorhenium dimers that bear one lopsided and one symmetrical ligand on each Re atom, namely, Re2O3-Cl4(py)2(Me3Bzm)2 (1rac and 1meso) and Re2O3Cl4(3,5-lut)2(Me3Bzm)2 (2rac and 2meso). The presence of two different cis L ligands in 1 and 2 breaks the local symmetry at each Re atom, so that, in the racemic dimers, the exchange of terminal and stacked ligands leads to nondegenerate conformers. Overall, NMR data showed that the unsymmetrical dimers 1 and 2 undergo two dynamic processes contemporaneously, namely, 180 degrees rotation about the Re-N(py or 3,5-lut) bond and coupled rotation about the Re-O-Re/Re-N bonds. Both processes reach the slow exchange limit below -80 degrees C. Rotation of py in 1 occurs faster than that of 3,5-lut in 2; this difference is attributed to the higher steric demands of 3,5-lut compared to py. For both dimers NMR data provided compelling evidence of the preferred conformers in solution, including ligand orientations. The low-T solution structure of 1meso and 2meso is chiral, the same as that found in the solid state for 2meso, where the Me3Bzm on one Re atom is stacked with the 3,5-lut on the other Re atom. The remaining Me3Bzm and 3,5-lut, one on each Re atom, are both terminal. In solution the coupled Re-O-Re/Re-N rotations interconvert the two halves of each meso dimer to yield the same overall stable chiral conformation. For the racemic dimers, however, this process does not interconvert one enantiomer into the other, but instead interconverts two rotamers, R1 and R2, each of which is chiral. We found that, in the case of both 1rac and 2rac, the conformer with stacking symmetrical ligands (R1) is roughly 1 order of magnitude more stable than that with stacking Me3Bzm ligands (R2). Moreover, the solution conformation of R1 is the same as that found in the solid state of 1rac. Solution- and solid-state data indicate that the key interaction favoring the observed conformations is very likely the electrostatic attraction between the delta+ H2 atoms on the Me3Bzm ligands and the negative O and Cl groups in the core of the dimers. Finally, for both meso and racemic dimers we were also able to elucidate the preferred pathways of the coupled dynamic motions and establish that, very likely, the two halves of the dimers swing back and forth by approximately 130 degrees through the anti eclipsed form.