How do they know? Influencing the relative stereochemistry of the complex units of dinuclear triple-stranded helicate-type complexes

Dinuclear triple-stranded helicates or meso-helicates are formed by self-assembly of linear ligand strands with appropriate metal ions. Very often this self-assembly proceeds with high diastereoselectivity leading either to the helicate or the meso-helicate. The stereoselectivity of the self-assembly process can be influenced by different factors such as the rigidity of the ligand (-->helicate), the number of methylene units in the alkyl spacers (even number of CH2 units --> helicate; odd number --> meso-helicate), template effects, or chiral substituents.     

Endogenous arene hydroxylation promoted by copper(I) cluster helicates

A novel neutral triple-stranded hexanuclear copper(I) cluster helicate [Cu(I)(6)L(3)]·2CH(3)CN derived from a thiosemicarbazone ligand could be synthesized and crystallographically characterized. The MALDI mass spectrum of this complex suggests that the tetranuclear copper(I) cluster helicate [Cu(I)(4)L(2)] is also present in solution. These copper(I) cluster helicates are capable, in the presence of O(2), of hydroxylating the arene linker of their supporting ligand strands. The resulting dinuclear complex [Cu(II)(2)L'(OH)] is formed by two copper(II) centers, a new ligand arising from the hydroxylation reaction, and one hydroxide group. The magnetic investigation of this compound shows a strong antiferromagnetic coupling between the two Cu(II) centers. The kinetic studies for the hydroxylation process show values of ΔH(≠)=-70 kJ mol(-1), similar to those mediated by the tyrosinase enzymes.     

Supramolecular circular helicates formed by destabilisation of supramolecular dimers

The effect of changes in the angles at the connection points of linear/circular helicates is explored as a route to control the nuclearity and architecture of metallo-supramolecular arrays. This effect is probed by changing the geometry of the metal centre used to assemble bis-pyridylimine ligands that contain a 1,3-bis(aminomethyl) benzene spacer group. Tetrahedral metal ions favour linear dimers, whereas octahedral nickel(II) predominantly gives a triangular circular helicate. Five-coordinate copper(II) falls in the middle of these extremes and results in the formation of solvent-dependent mixtures of dimer and trimer. The trinuclear, triangular, circular helicate structures, which result from coordination to copper(II) and nickel(II), are structurally characterised by X-ray crystallography and reveal that the units can aggregate into hexagonal arrays that contain anion-filled tube-like channels in the solid state.     

Modeling the mononuclear, dinuclear, and trinuclear copper(I) reaction centers of copper proteins using pyridylalkylamine ligands connected to 1,3,5-triethylbenzene spacer

The structure and O2-reactivity of copper(I) complexes supported by novel ligands, Pye2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(2-(pyridin-2-yl)ethyl)-)aminomethyl)benzene), Pye3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(2-(pyridin-2-yl)ethyl))aminomethyl)benzene), MePym2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)benzene), and MePym3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)benzene) have been examined. The ligands are designed to construct mono-, di-, and trinuclear copper(I) complexes by connecting two or three pyridylalkylamine metal-binding sites to a 1,3,5-triethylbenzene spacer. Thus, the reaction of the ligands with [CuI(CH3CN)4]X (X = PF6, CF3SO3) or CuICl gave the expected mononuclear copper(I) complexes [CuI(Pye2)(CF3SO3)] (1) and [CuI(Pye3)](CF3SO3) (2), dinuclear copper(I) complex [CuI2(MePym2)(Cl)]CuICl2 (3), and trinuclear copper(I) complex [CuI3(MePym3)(CH3CN)3](CF3SO3)3 (4), the structures of which were determined by X-ray crystallographic analysis. The mononuclear copper(I) complexes, 1 and 2, exhibit a distorted three-coordinate T-shape structure and a trigonal planar structure, respectively, which are very close to the coordination geometry of the CuA site of PHM (peptidylglycine alpha-hydroxylating monooxygenase) and the CuB site of CcO (cytochrome c oxidase). Notably, 1 and 2 showed a significantly high oxidation potential (990 mV vs SCE), thus showing virtually no reactivity toward O2. On the other hand, the metal centers of the dinuclear and trinuclear copper(I) complexes, 3 and 4, exhibit a distorted trigonal planar geometry and a trigonal pyramidal geometry, respectively. In contrast to the mononuclear copper(I) complexes, these dinuclear and trinuclear copper(I) complexes reacted with O2 to induce an aromatic ligand hydroxylation reaction involving an NIH-shift of one of the ethyl substituents on the benzene spacer. The NIH-shift of the alkyl substituent on the aromatic ring is strong evidence of the electrophilic aromatic substitution mechanism, although the active oxygen intermediate could not be directly detected during the course of the reaction. The biological relevance of the copper(I) complexes is also discussed on the basis of structure and O2-reactivity.     

Metal-dependent assembly of a helical-[Co3L3] cluster versus a meso-[Cu2L2] cluster with O,N,N',O'-Schiff base ligand: structures and magnetic properties

Self-assembly of a tetradentate ligand, N, N'-bi(salicylidene)-2,6-pyridinediamine (H 2L), with Cu(II) or Co(II), affords a dinuclear [Cu 2L 2] complex ( 1) or a trinuclear [Co 3L 3] complex ( 2), which were characterized by the single crystal X-ray diffraction study. The coordination geometry of the Cu (II) centers in 1 is between square planar and tetrahedral, with the ligand adopting a cis-cis conformation to give a centrally symmetric structure, which can be regarded as a mesocate. However, the coordination geometry of Co (II) centers in 2 is distortedly tetrahedral, and the ligand adopts a cis-trans conformation. The whole complex of 2 is of a pseudo- C 3 symmatrical, torus-like structure, which can be regarded as a circular helicate. Both the mesocate and the helicate exhibit expanded supramolecular structures due to elaborate intercomplex pi-stacking interactions. These two complexes were also characterized by element analysis, IR spectra, and TGA. To verify the stability of 2, ESI-MS was carried out on both the crystal and the powdered samples. Variable temperature magnetic susceptibility measurements reveal that both 1 and 2 display antiferromagnetic properties. DFT calculations were carried out on 1 to verify the antiferromagnetic coupling between intracluster metal centers.     

A Bowl‐Shaped Circular Trinuclear Helicate Generated from a TiO4N2 Motif by a Multicomponent Self‐Assembly Approach

The synthesis of a bowl‐shaped trinuclear circular titanium‐based helicate is reported. The strategy allowing access to this neutral architecture is based on a multicomponent self‐assembly approach in which the ligands involved in the process are a bis‐biphenol strand and 2,2′‐ bipyrimidine. By reacting the bis‐biphenol ligand and 2,2′‐bipyrimidine with an equimolar amount [Ti(OiPr)₄], a bowl‐shaped architecture is obtained through the formation of 18 new coordination bonds. This aggregate built from three octahedral TiO₄N₂ nodes displays an unusually high stability in solution compared to related species. In addition, by modifying the stoichiometry of the initial components, two assemblies incorporating two titanium centers bridged by a 2,2′‐bipyrimidine ligand are obtained. The crystal structures of these species are reported.     

Synthesis and structural characterization of a novel mixed-valent Cu(II)Cu(I)Cu(II) triangular metallomacrocycle using an imine-based rigid ligand

A novel neutral mixed-valent Cu(I)Cu(II)(2) triangular metallomacrocycle [Cu(3)L(2)(HL)].3CH(3)OH.2H(2)O (1) was assembled by reaction of the tetradentate ligand bis(N-salicylidene-4,4'-diphenylamine), H(2)L, with a copper(II) salt. ESI-MS show peaks only corresponding to the triangular structural species, indicating the high stability of the trimer structure in solution. Magnetic study confirms that there are two Cu(II) ions and one Cu(I) ion in a discrete triangular molecule. The crystal structure of 1 reveals that the triangle is formed by three deprotonated ligands and three copper ions with a Cu-Cu separation of ca. 11.8 A. Each copper atom is coordinated by two oxygen atoms and two nitrogen atoms from two different bis-bidentate ligands in a heavily distorted tetrahedral geometry, while each ligand is bound to two metal ions in a bis-bidentate coordination mode and links the metal centers overlapping in an unprogressive manner. Strong intramolecular pi.pi interactions between the ligands are found to stabilize the constraint conformation of the triangle. Electrochemical study reveals that the mixed-valent Cu(I)Cu(II)(2) complex is the most stable state in solution condition, and the electrochemical communication between the copper ions might be explained on the basis of the through-bond interaction. UV-vis-NIR spectral measurement demonstrates the Robin-Day class II behavior of the mixed-valence compound with a weak copper-copper interaction.     

Solid-state self-assembly of polymeric double helicates leading to linear arrays of silver(I) ions and reversible strand/double helix interconversion in solution

Reaction of a bent py-hyz-pym-hyz-pym 1 and of a linear py-hyz-py-hyz-pym 3 (py=pyridine; pym=pyrimidine; hyz=hydrazone) ligand strands with silver(I) tetrafluoroborate in CH(3)NO(2) generates double-helical dinuclear 2 and trinuclear 4 complexes. These complexes form polymeric, highly ordered solid-state structures, with wirelike, linear continuous or discontinuous polycationic Ag(n) (+) arrays with Ag--Ag distances of 2.78 to 4.42 A. Ligand 5, an isomer of 1, is found to yield a [2x2] grid-type complex 6. Titration experiments reveal the formation of linear rack-type dinuclear species from 1 and 5. Acid-base modulated, reversible interconversion between strand 1 and double helicate 2 may be achieved by using tren as a competing complexing agent (tren=N(CH(2)CH(2)NH(2))(3)). Progressive addition of silver(I) ions to a 1:1 mixture of 1 and 5 leads to the preferential formation of the double helicate 2 over the grid complex 6, illustrating a process of self-organisation with selection of the correct ligand.     

New N,N,N-heteroscorpionates based on 2,2'-bis(pyrazolyl)ethanamine and its derivatives. Ligands designed for probing supramolecular interactions

The successful design and synthesis of the new bis(pyrazolyl)ethanamine ligand and its copper(I) triphenylphosphine complex is reported. The ligand coordinates to the copper(I) center in a fac tridentate fashion, through both the pyrazolyl rings and the nitrogen atom from the NH2 group. In the solid state, the compound is organized in a 2D noncovalent network by N-H...pi and C-H...pi interactions and hydrogen bonds. The analogous ligand with a benzyl group substituted on the amine forms a complex with the same copper(I) center that has a similar 2D supramolecular structure and, in addition, is organized by the benzyl synthon into a 3D architecture.     

Self-assembly and gelation behavior of tris(phenylisoxazolyl)benzenes

Low-molecular-mass organic gelators (LMOG), tris(phenylisoxazolyl)benzenes, were synthesized, and their self-assembling behavior was examined using (1)H NMR and UV-vis absorption spectroscopies. They turned into a gel in both nonpolar and highly polar solvents such as methylcyclohexane, ether, acetone, dimethylsulfoxide, etc. Field emission scanning electron microscopy (FESEM) observation of the xerogels of 1 and 3 possessing the saturated alkyl chains revealed that well-developed straight fibers were formed, whereas the unsaturated termini of the alkyl chains of 2 promoted the formation of both the right- and left-handed helical fibers. The self-association behavior of 1, 2, and 5 in solution were investigated using (1)H NMR and UV-vis spectroscopies. The flat aromatic compound 1 stacked in a columnar fashion along its C(3) axis via π-π stacking interactions. The assemblies were regulated by the peripheral alkyl substituents; the saturated alkyl groups facilitated the assemblies while terminal double bonds impeded the intermolecular association, and the branched substituents obviously interfered in the formation of the stacks, probably due to steric requirements. Theoretical calculations suggest that the three dipoles of the isoxazole groups adopt the circular array. The conformational search of the hexameric stacks of 4 using MacroModel V9.1 gave rise to two major conformers: one is nonhelical and the other is helical. Further detailed structural analysis of the assemblies of chiral 5 using circular dichroism (CD) measurements indicated that their assemblies adopt helical structures in solution. CD spectra and DFT calculations revealed that R-5 forms a left-handed supramolecular helicate. The coassembly of R- and S-5 displayed chiral amplification, since the chiral information from 5 was transferred to the supramolecular chirality of the helical assemblies of 1. A small amount of optically active 5 provided enough chiral stimulus to produce a remarkable chiral response and supramolecular helical structures of 1.     

Structures and redox reactivities of copper complexes of (2-pyridyl)alkylamine ligands. Effects of the alkyl linker chain length

Ligand effects on the structures and redox reactivities of copper complexes have been examined using (2-pyridyl)alkylamine derivatives as the supporting ligands, where particular attention has been focused on the effects of the alkyl linker chain length connecting the tertiary amine nitrogen atom and the pyridine nucleus: N[bond]CH(2)[bond]Py (Pym) vs N[bond]CH(2)CH(2)[bond]Py (Pye). X-ray crystallographic analysis of the copper(I) complex of tridentate ligand (Phe)L(Pym2) [N,N-di(2-pyridylmethyl)-2-phenylethylamine] (complex 1) has demonstrated that it possesses a trigonal pyramidal geometry in which a d[bond]pi interaction with an eta(1)-binding mode exists between the metal ion and one of the ortho carbons of the phenyl ring of the ligand side arm (phenethyl). The result shows sharp contrast to the d[bond]pi interaction with an eta(2)-binding mode existing in the copper(I) complex of (Phe)L(Pye2) [N,N-di[2-(2-pyridyl)ethyl]-2-phenethylamine] (complex 2). Such a d-pi interaction has been shown to affect the stability of the copper(I) complex in CH(2)Cl(2). Oxygenation of copper(I) complex 1 supported by (Phe)L(Pym2) produces a bis(mu-oxo)dicopper(III) complex, also being in sharp contrast to the case of the copper(I) complex 2 with ligand (Phe)L(Pye2), which preferentially affords a (micro-eta(2):eta(2)-peroxo)dicopper(II) complex in the reaction with O(2). Such an effect of the alkyl linker chain length of the metal binding site has also been found to operate in the RSSR (disulfide)/2RS(-) (thiolate) redox system. Namely, ligand (S2,R)L(Pym1) (di[2-[(alkyl)(2-pyridinylmethyl)amino]ethyl] disulfide) with the methylene linker group (Pym) induced the reductive disulfide bond cleavage in the reaction with copper(I) ion to give a bis(micro-thiolato)dicopper(II) complex, while the ligand with the ethylene linker group (Pye), (S2,Bn)L(Pye1) (di[2-[(benzyl)(2-(2-pyridinyl)ethyl)amino]ethyl] disulfide), gave a disulfide-dicopper(I) complex. These ligand effects in the Cu(2)[bond]O(2) and Cu(2)[bond]S(2) systems have been discussed by taking into account the difference in electron-donor ability of the pyridine nucleus between the Pym and Pye ligand systems.     

The 'Trinity' helix: synthesis and structural characterisation of a C3-symmetric tris-bidentate ligand and its coordination to Ag(I)

The synthesis and structural characterisation of a novel C3-symmetric tris-bidentate ligand, L, featuring a triphenylamine core appended by pyridylimine coordination sites is reported: 1H NMR compleximetric titration studies with Ag(I) and ESMS indicate the presence of [Ag3L2]3+ species in solution, consistent with the formation of a trinuclear double helicate complex: the Trinity helix.     

Formation of mono-, bi-, tri-, and tetranuclear Ag(I) complexes of C3-symmetric tripodal benzimidazole ligands

The C3-symmetric tripodal ligand tris(2-benzimidazolylmethyl)amine (ntb) and its alkyl-substituted derivatives tris(N-R-benzimidazol-2-ylmethyl)amine (R = methyl, Mentb; R = ethyl, Etntb; R = propyl, Prntb) react with various silver(I) salts to afford mononuclear [Ag(Prntb)(CF3SO3)].0.25H2O, 1, binuclear [Ag2(Mentb)2](CF3SO3)2.H2O, 2, trinuclear [Ag3(Etntb)2](ClO4)3.CH3OH, 3, and tetranuclear [Ag4(ntb)2(CH3CN)2(CF3CO2)2](CF3CO2)2.2H2O, 4. All four complexes have been characterized by elemental analyses, IR spectroscopy, and X-ray crystallography. The Ag(I) ion in 1 is coordinated to the three imine nitrogen atoms of the Prntb ligand and one oxygen atom of the trifluoromethanesulfonate anion in a distorted tetrahedral environment. Dinuclear 2 has C2 symmetry with each Ag(I) atom trigonally coordinated by two arms of one Mentb and one arm of another. Trinuclear 3 has C3 symmetry with a Ag3 regular triangle sandwiched between a pair of Etntb ligands such that one arm of each ligand is involved in linear coordination about an Ag(I) atom. In the tetranuclear complex 4, two linearly coordinated Ag(I) atoms lying on the molecular C2 axis are bridged by a pair of ntb ligands and the remaining pendant arm of each ntb ligand is attached to another Ag(I) atom whose tetrahedral coordination sphere is completed by an acetonitrile molecule and a chelating trifluoroacetate anion. Complexes 2 and 3 may be regarded as an aggregation of two tridentate ligands by a silver dimer and a trinuclear cluster with weak Ag...Ag interactions, respectively, while in 4 the aggregation of two tripodal ligands by four Ag(I) ions affords a multicomponent internal cavity. The packing modes of complexes 1-3 are dominated by weak supramolecular pi...pi and CH...pi interactions. Hexagonal or square channels are generated in 1 and 2, and a honeycomb layer structure is formed in 3 with solvate molecules and counteranions occupying the voids. The crystal structure of 4 consists of a three-dimensional network consolidated by NH...O and OH...O hydrogen bonds.     

Diastereoselective assembly of pentanuclear circular helicates

Reaction of a ligand which contains two N-donor and O-donor tridentate domains separated by a 1,3-phenylene spacer unit with Zn(2+) ions results in a pentanuclear circular helicate [Zn(5)(L)(5)](10+) and this structure persists in both the solid and solution state. The formation of this high nuclearity species is governed by unfavourable steric interactions between the phenyl units which destabilize the simple linear helicate. Incorporation of enantiopure units within the ligand strand controls the diastereoselectivity with up to 80% d.e.     

Copper(II/I) Complexes of a Hexakis(bipyridyl)cyclotriveratrylene Ligand: A Redox-Induced Conformational Switch

A series of copper(II) and copper(I) complexes have been synthesized with ligands combining 6-methyl-2,2'-bipyridines with cyclotriveratrylene (CTV) (1) and with catechol (2). The electrochemical, (1)H NMR, and mass spectrometry characterizations of these complexes are described and discussed. The six pendant bipyridines of ligand 1 allow for the formation of two trinuclear copper(I) complexes [(1)Cu(3)](BF(4))(3) differing only in the conformation "vic" or "int" adopted by the ligand to fit the tetrahedral cuprous ions. Similarly, 1 generates two trinuclear copper(II) complexes in which the conformation of the ligand fits the square planar geometry of cupric ions. In both the cuprous and cupric complexes, a conformational equilibrium exists. Ligand 2 bearing two methylbipyridines has proven to be a useful model of the coordinating sites of ligand 1. In this case, two homologous copper(I) complexes are obtained, [(2)Cu]BF(4) and [(2)(2)Cu(2)](BF(4))(2), modeling respectively two possible coordination conformations of ligand 1. With copper(II), ligand 2 yields only one complex [(2)Cu](CF(3)SO(3))(2), which allows for the unambiguous identification of the conformations observed for ligand 1 complexes. The different coordinating modes of ligand 1 in the complexes mentioned are in exchange but exhibit different physical properties, thus representing a new bistable system based on conformational isomerism which exhibits an electrochemical potential hysteresis. An equilibrium constant and thermodynamic data were obtained for this system by variable-temperature cyclic voltammetry. The influence of coordinating vs noncoordinating solvents was also studied.     

Quantitative evaluation of d-pi interaction in copper(I) complexes and control of copper(I)-dioxygen reactivity

Crystal structures of the copper(I) complexes 1(X), 2, and 3 of a series of tridentate ligands L1(X), L2, and L3, respectively (L1(X): p-substituted derivatives of N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X=H, Me, OMe, Cl, NO(2); L2: N,N-bis[2-(2-pyridyl)ethyl]-2-methyl-2-phenylethylamine; L3: N,N-bis[2-(2-pyridyl)ethyl]-2,2-diphenylethylamine) were solved to demonstrate that all the copper(I) complexes involve an eta(2) copper-arene interaction with the phenyl ring of the ligand sidearm. The Cu(I) ion in each complex has a distorted tetrahedral geometry consisting of the three nitrogen atoms (one tertiary amine nitrogen atom and two pyridine nitrogen atoms) and C(1)-C(2) of the phenyl ring of ligand sidearm, whereby the Cu-C distances of the copper-arene interaction significantly depend on the para substituents. The existence of the copper-arene interaction in a nonpolar organic solvent (CH(2)Cl(2)) was demonstrated by the observation of an intense MLCT band around 290 nm, and the magnitude of the interaction was evaluated by detailed analysis of the (1)H and (13)C NMR spectra and the redox potentials E(1/2) of the copper ion, as well as by means of the ligand-exchange reaction between the phenyl ring and CH(3)CN as an external ligand. The thermodynamic parameters DeltaH(o) and DeltaS(o) for the ligand-exchange reaction with CH(3)CN afforded a quantitative measure for the energy difference of the copper-arene interaction in the series of copper(I) complexes. Density functional studies indicated that the copper(I)-arene interaction mainly consists of the interaction between the d(z(2) ) orbital of Cu(I) and a pi orbital of the phenyl ring. The copper(I) complexes 1(X) reacted with O(2) at -80 degrees C in CH(2)Cl(2) to give the corresponding (micro-eta(2):eta(2)-peroxo)dicopper(II) complexes 4, the formation rates k(obs) of which were significantly retarded by stronger d-pi interaction, while complexes 2 and 3, which exhibit the strongest d-pi interaction showed significantly lower reactivity toward O(2) under the same experimental conditions. Thus, the d-pi interaction has been demonstrated for the first time to affect the copper(I)-dioxygen reactivity, and represents a new aspect of ligand effects in copper(I)-dioxygen chemistry.     

Prevalence of the alkyl/phenyl-folded conformation in benzylic compounds C6H5CH2-X-R (X=O,CH2, CO,S, SO,SO2): significance of the CH/pi interaction as evidenced by high-level ab initio MO calculations

Ab initio MO calculations were carried out to examine the conformational energies of various benzylic compounds C(6)H(5)CH(2)XR (X=O, CH(2), CO, S, SO, SO(2); R=CH(3), C(2)H(5), iC(3)H(7), tC(4)H(9)) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. Rotamers with R/Ph in gauche relationship are generally more stable than the R/Ph anti rotamers. In these stable geometries, the interatomic distance in the interaction of alpha- or beta-CH in the alkyl group and the ipso-carbon atom of the phenyl ring is short. The computational results are consistent with experimental data from supersonic molecular jet spectroscopy on 3-n-propyltoluene and NMR and crystallographic data on structurally related ketones, sulfoxides, and sulfones. In view of this, the alkyl/phenyl-congested conformation of these compounds has been suggested to be a general phenomenon, rather than an exception. The attractive CH/pi interaction has been suggested to be a dominant factor in determining the conformation of simple aralkyl compounds.     

DNA three-way junction with a dinuclear iron(II) supramolecular helicate at the center: a NMR structural study

A tetracationic supramolecular helicate, [Fe2L3]4+ (L = C25H20N4), with a triple-helical architecture is found to induce the formation of a three-way junction (3WJ) of deoxyribonucleotides with the helicate located in the center of the junction. NMR spectroscopic studies of the interaction between the M enantiomer of the helicate and two different oligonucleotides, [5'-d(TATGGTACCATA)]2 and [5'-d(CGTACG)]2, show that, in each case, the 2-fold symmetry of the helicate is lifted, while the 3-fold symmetry around the helicate axis is retained. The 1:3 helicate/DNA stoichiometry estimated from 1D NMR spectra supports a molecular model of a three-way junction composed of three strands. Three separate double-helical arms of the three-way junction are chemically identical giving rise to one set of proton resonances. The NOE contacts between the helicate and DNA unambiguously show that the helicate is fitted into the center of the three-way junction experiencing a hydrophobic 3-fold symmetric environment. Close stacking interactions between the ligand phenyl groups and the nucleotide bases are demonstrated through unusually large downfield shifts (1-2 ppm) of the phenyl protons. The unprecedented 3WJ arrangement observed in solution has also been found to exist in the crystal structure of the helicate adduct of [d(CGTACG)2] (Angew. Chem., Int. Ed. 2006, 45, 1227).     

Chiral distortion in a Mn(IV)(salen)(N3)2 derived from Jacobsen's catalyst as a possible conformation model for its enantioselective reactions

The Mn (IV)(salen)(N 3) 2 complex ( 3) from Jacobsen's catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting Mn (III)(salen)(N 3)(CH 3OH) complex ( 2) are determined in order to investigate the conformation of the high-valent Mn (IV)(salen) molecule in comparison with that of Mn (III)(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N 3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH 3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV-vis, electron paramagnetic resonance, (1)H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry.     

Boxes, helicates, and coordination polymers: a structural and magnetochemical investigation of the diverse coordination chemistry of simple pyridine-alcohol ligands

A structurally diverse array of polynuclear complexes has been identified and structurally characterized from the reaction of 6-methylpyridine-2-methanol (1) with a range of cobalt(II) salts under a variety of reaction conditions. A tetranuclear cubane, [Co4(1-H)4Cl4(H2O)3(CH3OH)], was isolated from the reaction of 1 with CoCl2.6H2O and NaOH in MeOH, and a tetranuclear double cubane, [Co4(1-H)6(NO3)2], was isolated from the reaction of 1 with Co(NO3)2.6H2O and NEt3 in MeOH. A bowl-shaped trinuclear complex, [Co3(1-H)3Cl3(dmso)], which features a triply bridging dmso ligand, assembled upon mixing 1 and CoCl2.6H2O in dmso. A 1-D coordination polymer, [Co(1)2(SO4)](infinity), where the sulfate ligands bridge "[Co(1)2]" units in a mu2:eta1 fashion to build up the polymer structure, was isolated from the reaction of 1 with CoSO4.7H2O. The reaction of the structurally related ligand 8-hydroxyquinaldine (2) with a mixture of CoCl2.6H2O and Co(OAc)2.4H2O lead to the formation of the tetranuclear double cubane, [Co4(2-H)6Cl2]. Temperature-dependent magnetic measurements have also been performed for these five complexes along with the hydrogen-bonded helicate [Co2(1)2(1-H)2]. The hydrogen bonds of the helicate mediate antiferromagnetic interactions between the cobalt(II) centers (J = -3.18(9) cm(-1), g = 2.25(2)). The sulfate bridging ligands of [Co(1)2(SO4)](infinity) are poor mediators of magnetic exchange. The Co(II) centers in the double-cubane complexes [Co4(1-H)6(NO3)2] and [Co4(2-H)6Cl2] are strongly antiferromagnetically coupled to each other at low temperature to give an S = 0 ground state. [Co4(1-H)4Cl4(H2O)3(MeOH)] exhibits rather complicated magnetic behavior; however, we did not observe any evidence for single-molecule magnetism as was seen for structurally related complexes.