Variable-temperature 1H NMR Studies Research Articles

Self-Assembled Molecular Gear: A 4:1 Complex of Rh(III)Cl Tetraarylporphyrin and Tetra(p-pyridyl)cavitand.

The components of a 4:1 mixture of Rh(III)Cl tetrakis(4-methylphenyl)porphyrin 1 and a bowl-shaped tetra(4-pyridyl)cavitand 4 self-assemble into a 4:1 complex 14•4 via Rh-pyridyl axial coordination bonds. The single-crystal X-ray diffraction analysis and variable-temperature (VT) (1)H NMR study of 14•4 indicated that 14•4 behaves as a quadruple interlocking gear with an inner space, wherein (i) four subunits-1 are gear wheels and four p-pyridyl groups in subunit-4 are axes of gear wheels, (ii) one subunit-1 and two adjacent subunits-1 interlock with one another cooperatively, and (iii) four subunits-1 in 14•4 rotate quickly at 298 K on the NMR time scale. Together, the extremely strong porphyrin-Rh-pyridyl axial coordination bond, the rigidity of the methylene-bridge cavitand as a scaffold of the pyridyl axes, and the cruciform arrangement of the interdigitating p-tolyl groups as the teeth moiety of the gear wheels in the assembling 14-unit make 14•4 function as a quadruple interlocking gear in solution. The gear function of 14•4 was also supported by the rotation behaviors of other 4:1 complexes: 24•4 and 34•4 obtained from Rh(III)Cl tetrakis[4-(4-methylphenyl)phenyl]porphyrin 2 or Rh(III)Cl tetrakis(3,5-dialkoxyphenyl)porphyrin 3 and 4 also served as quadruple interlocking gears, whereas 14•5 obtained from 1 and tetrakis[4-(4-pyridyl)phenyl]cavitand 5 did not behave as a gear. The results of activation parameters (ΔH(⧧), ΔS(⧧), and ΔG(⧧)) obtained from Eyring plots based on line-shape analysis of the VT (1)H NMR spectra of 14•4, 24•4, and 34•4 also support the interlocking rotation (geared coupled rotation) mechanism.

Bowl-to-Bowl Inversion Accelerated by the Introduction of Bulky 1,1,4,4-Tetramethylbutane-1,4-diyl Groups on Sumanene

A variable-temperature 1H NMR study revealed that the bowl-to-bowl inversion of sumanene is accelerated by the introduction of bulky 1,1,4,4-tetramethylbutane-1,4-diyl groups on the peripheral aromatic ring. This is likely to be induced by a bowl flattening, as shown in the DFT-optimized structures.

Synthesis and Characterization of 1,1′-Diphosphaplumbocenes: Oxidative Ligand Transfer Reactions with Divalent Thulium Complexes

The new 1,1′-diphosphaplumbocenes Pb(Dtp)2 (Dtp = 2,5-di-tert-butyl-3,4-dimethylphospholyl) and Pb(Dsp)2 (Dsp = 2,5-ditrimethylsilyl-3,4-dimethylphospholyl) have been synthesized and characterized by multinuclear NMR spectroscopy. 207Pb NMR spectra of both complexes, as well as for the known complex Pb(Htp)2 (Htp = 2,5-di-tert-butylphospholyl), show that the phospholyl ligands lead to an important downfield shift compared with that of cyclopentadienyl based plumbocenes. X-ray diffraction studies of the Pb(Dtp)2 complex revealed a structure with two slightly bent η5-bound ligands. This complex was employed in the oxidative ligand transfer reaction with bulky divalent thulium complexes. In the case of Tm(Dtp)2, the first example of a monomeric trivalent tris(phospholyl)lanthanide complex Tm(Dtp)3 was obtained. X-ray diffraction studies and low-temperature 1H NMR studies show the η1 coordination mode of the third Dtp ligand. In contrast, oxidation of Tm(Cpttt)2 (Cpttt is for tris-tert-butylcyclopentadienyl) led presumably to the formation of the ion pair complex [Tm(Cpttt)2][Dtp], in which no interaction between the free Dtp ligand and Tm was observed, as shown by variable temperature 1H and 31P NMR studies. The new trivalent complexes could be reduced back to the original divalent complexes with potassium graphite.

Unusual Recognition and Separation of Hydrated Metal Sulfates [M2(μ-SO4)2(H2O)n, M = ZnII, CdII, CoII, MnII] by a Ditopic Receptor

A ditopic receptor L1, having metal binding bis(2-picolyl) donor and anion binding urea group, is synthesized and explored toward metal sulfate recognition via formation of dinuclear assembly, (L1)2M2(SO4)2. Mass spectrometric analysis, (1)H-DOSY NMR, and crystal structure analysis reveal the existence of a dinuclear assembly of MSO4 with two units of L1. (1)H NMR study reveals significant downfield chemical shift of -NH protons of urea moiety of L1 selectively with metal sulfates (e.g., ZnSO4, CdSO4) due to second-sphere interactions of sulfate with the urea moiety. Variable-temperature (1)H NMR studies suggest the presence of intramolecular hydrogen bonding interaction toward metal sulfate recognition in solution state, whereas intermolecular H-bonding interactions are observed in solid state. In contrast, anions in their tetrabutylammonium salts fail to interact with the urea -NH probably due to poor acidity of the tertiary butyl urea group of L1. Metal sulfate binding selectivity in solution is further supported by isothermal titration calorimetric studies of L1 with different Zn salts in dimethyl sulfoxide (DMSO), where a binding affinity is observed for ZnSO4 (Ka = 1.23 × 10(6)), which is 30- to 50-fold higher than other Zn salts having other counteranions in DMSO. Sulfate salts of Cd(II)/Co(II) also exhibit binding constants in the order of ∼1 × 10(6) as in the case of ZnSO4. Positive role of the urea unit in the selectivity is confirmed by studying a model ligand L2, which is devoid of anion recognition urea unit. Structural characterization of four MSO4 [M = Zn(II), Cd(II), Co(II), Mn(II)] complexes of L1, that is, complex 1, [(L1)2(Zn)2(μ-SO4)2]; complex 2, [(L1)2(H2O)2(Cd)2(μ-SO4)2]; complex 3, [(L1)2(H2O)2(Co)2(μ-SO4)2]; and complex 4, [(L1)2(H2O)2(Mn)2(μ-SO4)2], reveal the formation of sulfate-bridged eight-membered crownlike binuclear complexes, similar to one of the concentration-dependent dimeric forms of MSO4 as observed in solid state. Finally, L1 is found to be highly efficient in removing ZnSO4 from both aqueous and semiaqueous medium as complex 1 in the presence of other competing Zn(II) salts via precipitation through crystallization. Powder X-ray diffraction analysis has also confirmed bulk purity of complex 1 obtained from the above competitive crystallization experiment.

Variable temperature 1H and 13C NMR study of restricted rotation in N,N-bis(2-hydroxyethyl)acetamide

N,N-bis(2-hydroxyethyl)acetamide shows restricted rotation about the amide bond in both 1H and 13C NMR spectra rendering the two hydroxyethyl groups non-equivalent. A variable temperature study in CD3SOCD3 allowed estimation of the free energy barrier to rotation as 75.6 ± 0.2 kJ mol−1. Previously published data in CDCl3 appears to be erroneous.

New cyclopentadienyl molybdenum polynuclear clusters from ring opening of Woollins’ reagent via phosphorus–selenium bond scission, phosphorus–phosphorus coupling and deselenation pathways

Cothermolysis of [(Ph)P(Se)(μ-Se)]2 or Woollins’ reagent (WR) with Cp2Mo2(CO)4 (2) in toluene at 110°C led to the isolation of three different metal clusters, [Cp2Mo2{(μ-Se)2(PPh(Se))}{(μ-Se)(PPh)3] (3), Cp4Mo4(CO)3(μ-Se)4 (4) and a pair of isomer complexes of Cp3Mo3(CO)4[Se3(PPh)2] (5a and 5b) in 17.5%, 3.1% and 4.9% yields, respectively. A similar reaction at 70°C gave only 5a and 5b in 2.0% and 4.9% yields, respectively. All complexes have been fully characterized via NMR, IR and ESI mass spectroscopy. Molecular structures of 3, 4 and 5a were confirmed by single crystal X-ray diffraction analyses. A variable temperature 1H NMR study of 4 was carried out. Finally, a free-radical addition pathway by selenation of WR was postulated.

Fluxional behaviour of phosphole and phosphine ligands on triosmium clusters

The unsaturated cluster [Os3(μ-H)2(CO)10] reacts with a variety of π-conjugated phosphole ligands: 2,5-bis(2-thienyl)-1-phenylphosphole, 2,5-bis(2-pyridyl)-1-phenylphosphole, and 1,2,5-triphenylphosphole, to give the monosubstituted [Os3(μ-H)(H)(CO)10(P)] and [Os3(μ-H)2(CO)9(P)] derivatives. Variable-temperature 1H NMR studies for the decacarbonyltriosmium complexes show that the hydride ligands undergo fluxional behavior and its decarbonylation gave the nonacarbonyl unsaturated species with equivalent hydrides. An X-ray crystallographic analysis of [Os3(μ-H)2(CO)9(P)] (2b) (P = 1,2,5-triphenylphosphole) is reported. From 2,5-bis(2-pyridyl)-1-phenylphosphole four isomers are obtained, the 1H NMR spectrum at low temperature indicates that the phosphole ligand may occupy pseudoaxial and equatorial sites at one of the osmium atoms bearing the bridging hydride. The reaction of [Os3(CO)10(CH3CN)2] with cyanoethyldiphenylphosphine affords mono- and bis(phosphine)-substituted clusters. The X-ray crystal structure of the monosubstituted [Os3(CO)11(η1-PC15H14N)] (3) derivative is discussed. The dynamic behavior observed for [Os3(CO)10(η1-PC15H14N)2] (4) is studied by variable-temperature 1H and 31P{1H} NMR spectroscopy. The latter studies show a mixture of two geometric isomers, which are in dynamic equilibrium at room temperature where the 1,2-trans–trans isomer (4a) is favored, whose molecular structure has been determined by X-ray crystallography.

Star Diblock Copolymer Concentration Dictates the Degree of Dispersion of Carbon Black Particles in Nonpolar Media: Bridging Flocculation versus Steric Stabilization

The solution behavior of a polystyrene–hydrogenated polyisoprene star diblock copolymer (Mn ∼ 384 K; 6 mol % polystyrene) is examined in nonpolar media. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated in n-dodecane at 25 °C: the apparent polystyrene content of 3.2 ± 0.2 mol % remains essentially constant on heating up to 100 °C. Physical adsorption of this star diblock copolymer onto carbon black particles is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. An isotherm is constructed for copolymer adsorption onto carbon black from n-dodecane at 20 °C using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Langmuir-type adsorption is observed with a maximum adsorbed amount, Γ, of ∼2.2 ± 0.1 mg m–2. In addition, thermogravimetric analysis is used to directly determine the amount of adsorbed cop...

Effects of the PEG molecular weight of a PEG-lipid and cholesterol on PEG chain flexibility on liposome surfaces

Abstract A variable-temperature 1H NMR study was performed to investigate the effects of the molecular weight of poly(ethylene glycol) (PEG) in PEG-lipids and cholesterol addition to the lipid bilayer on PEG chain flexibility at the liposomal surface. PEG-lipids, i.e., l -α-distearoylphosphatidylethanolamine (DSPE)-PEG, with PEG molecular weights of 750, 2000, and 5000 were modified to liposomes of ca. 100 nm. The 1H NMR peak of PEG in DSPE-PEG was overlapped by broad and sharp peaks, corresponding to rigid and flexible PEG components, respectively. When the PEG molecular weight was increased, the PEG peak became sharp, indicating that long-chain PEGs were more flexible on the liposome surface. The proportion of flexible components projecting into the water phase increased as the PEG chain length increased. Peak sharpening also occurred when the cholesterol content was increased from 0 to 30 mol%, demonstrating that cholesterol incorporation into the lipid bilayer enhanced the PEG chain flexibility. In addition, the PEG chain flexibility significantly increased when cholesterol was delocalized in the lipid bilayer at concentrations above 20 mol%. Lateral diffusion of the lipid with the presence of cholesterol in the lipid bilayer significantly affected the PEG chain flexibility.

Chiral Dopants Derived from Ephedrine/Pseudoephedrine: Structure and Medium Effects on the Helical Twisting Power

Chiral dopants were obtained by acylation of enantiomerically pure ephedrine and pseudoephedrine with promesogenic carbonyl reagents. The products have been investigated with respect to their chiral transfer ability on nematic host matrices characterized by extreme differences of the dielectric anisotropy. It has been found that the medium dependence of the helicity induction nearly disappears at reduced temperatures. Based on variable temperature 1H NMR studies on monoacylated homologues, the estimated coalescence temperatures and free activation enthalpies for the hindered rotation around C‒N bonds could be correlated with the helical twisting power. Measurements by dielectric spectroscopy reveal the correlation between the molar mass of substituents linked to the chiral building block and the dynamic glass transition of corresponding chiral dopants. Furthermore, the effect of intramolecular and intermolecular hydrogen bonds has been studied by ATR-FTIR spectroscopy.

Structures of tetrasilylmethane derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the gas phase, and their dynamic structures in solution.

The structures of the molecules (XMe2Si)2C(SiMe3)2, where X = H, Cl, Br, have been determined by gas electron diffraction (GED) using the SARACEN method of restraints, with all analogues existing in the gas phase as mixtures of C1- and C2-symmetric conformers. Variable temperature (1)H and (29)Si solution-phase NMR studies, as well as (13)C NMR and (1)H/(29)Si NMR shift correlation and (1)H NMR saturation transfer experiments for the chlorine and bromine analogues, are reported. At low temperatures in solution there appear to be two C1 conformers and two C2 conformers, agreeing with the isolated-molecule calculations used to guide the electron diffraction refinements. For (HMe2Si)2C(SiMe3)2 the calculations indicated six conformers close in energy, and these were modeled in the GED refinement.

Structures of Tetrasilylmethane Derivatives C(SiXMe2)4 (X=H, F, Cl, Br) in the Gas Phase and their Dynamic Structures in Solution

Abstract The structures of the molecules C(SiXMe2)4 (X=H, F, Cl, Br) have been determined by gas electron diffraction (GED). Ab initio calculations revealed nine potential minima for each species, with significant ranges of energies. For the H, F, Cl, and Br derivatives nine, seven, two, and two conformers were modelled, respectively, as they were quantum-chemically predicted to be present in measurable quantities. Variable-temperature 1H and 29Si solution-phase NMR studies and, where applicable, 13C NMR, 1H/29Si NMR shift-correlation, and 1H NMR saturation-transfer experiments are reported for C(SiXMe2)4 (X=H, Cl, Br, and also I). At low temperature in solution two conformers (one C1-symmetric and one C2-symmetric) are observed for each of C(SiXMe2)4 (X=Cl, Br, I), in agreement with the isolated molecule ab initio calculations carried out as part of this work for X=Cl, Br. C(SiHMe2)4 is present as a single C1-symmetric conformer in solution at the temperatures at which the NMR experiments were performed.

2]Rotaxane with multiple functional groups.

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2'-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π-π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide-alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature (1)H NMR study in DMSO-d6 of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.

Photophysical and electrochemical properties and temperature dependent geometrical isomerism in alkyl quinacridonediimines

Quinacridone diimines 1–10 were synthesized by the condensation of anilines with alkyl substituted quinacridones (QA). Photophysical and electrochemical properties of the compounds were investigated. Unconventional behavior of absorption spectra suggested a decrease in π_conjugation within the QA skeleton as well as lack of extended π_conjugation between the QA skeleton and the N-phenyl rings. A computational study of compounds 1–10, a variable temperature 1H NMR study of compounds 2, 7 and 10 (for instance), and single crystal X-ray analysis of 2, 3, 6, 7, 8 and 10 indicated that the anomalous behavior is due to the buckled, non-planar structure of the quinacridones. Moreover the molecules existed in two interconvertible geometric isomeric forms at different temperatures. Molecular orbital calculations were performed at B3LYP/6-31+G(d), B3PW91/6-31G(d) and PBEPBE/6-31G(d) levels of theory at B3PW91/6-31G(d) optimized structures for both isomers of all compounds (1–10); the results obtained are in close agreement with the experimentally determined values.

Spectroscopic, and electrochemical studies of [MCl2(η2-N,N-dpksc)] (M = Zn, Cd, Hg and dpksc = di-2-pyridylketone semicarbazone)

A series of group 12 metal chlorides of dpksc of the type [MCl2(η2-N,N-dpksc)] (M=Zn, Cd and Hg) were synthesized from the reactions between MCl2 and dpksc in refluxing CH3CN. Spectroscopic measurements performed on protophilic solutions of [MCl2(η2-N,N-dpksc)] divulged sensitivity of [MCl2(η2-N,N-dpksc)] to changes in their surroundings. Variable temperature 1H NMR studies revealed the amine protons to be more sensitive to temperature variations compared to the amide and aromatic protons. Electronic absorption spectral measurements disclosed acid–base inter-conversion between [MCl2(η2-N,N-dpksc)] and its conjugate base [MCl2(η2-N,N-dpksc-H)]−. In dmf, the acidity of [MCl2(η2-N,N-dpksc)] increases in the following order: [ZnCl2(η2-N,N-dpksc)]>[CdCl2(η2-N,N-dpksc)]>[HgCl2(η2-N,N-dpksc)]. The electrochemical properties of [MCl2(η2-N,N-dpksc)] are dominated by a series of irreversible redox transformations pointing to electrochemical decomposition of [MCl2(η2-N,N-dpksc)] upon electronic transfers.

Flexible dinucleating N,N,N-tridentate ligands based on a xanthene scaffold

A versatile route to flexible ditopic ligands based on a 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene scaffold, namely Williamson ether synthesis employing the 4,5-bis(chloromethyl) derivative of the xanthene and a phenol-substituted ligand donor, is demonstrated by the preparations of bis(terpyridine)xanthene L1 and the bis(di(2-pyridylmethyl)amine)xanthene L2 in good yields. The reaction of L1 with [Pd(PhCN)2Cl2] afforded the coordination complexes [L1(PdCl)2][PF6]2 (4[PF6]2) and [L1(PdCl)][PF6] (5[PF6]). The X-ray crystal structures of L1 and 5[PF6]·solvate were determined and reveal intra- and intermolecular offset face-to-face stacking between terpyridine rings and intramolecular C–H…π-phenyl ring interactions. Variable temperature 1H NMR studies show temperature-dependent chemical shift changes suggestive for face-to-face interactions between the terpyridines in 4[PF6]2 and of 5[PF6] being sustained in solution.

Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates

The new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)] (H2L3,5), has been prepared and the reactions of its dianion (Na2L3,5) with M3+ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M2(L3,5)3, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M2(L3,5)3 (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variable-temperature 1H NMR study of the Ga2(L3,5)3 helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H2L3,5) and the M2(L3,5)3 helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5–6 Å range in both the free ligand and the helicates.

2,4‐Dichlorophenoxyacetic Acid Derived Schiff Base and Its Lanthanide(III) Complexes: Synthesis, Characterization, Spectroscopic Studies, and Plant Growth Activity

2,4‐Dichlorophenoxyacetic acid derived Schiff base (HL) and its lanthanide [La(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Dy(III), Y(III)] complexes were synthesized and characterized by various spectroscopic (1H, 13C, DEPT and 2D HMQC NMR, FT‐IR, UV‐Vis, and mass) techniques and other analytical methods. HL exhibits “E” and “Z” isomerism and was confirmed by variable temperature 1H NMR studies. The spectral and analytical data reveals the bidentate coordination of HL to lanthanide(III) ion, through carboxylic acid group via deprotonation. Fluorescence spectrum of europium complex shows bands at 578, 592, and 612 nm assignable to , , and , respectively. Auxin activity of HL and lanthanum(III) complex on wheat seeds (Triticum durum) was measured at different concentrations. The percentage germination, root length, and shoot length were recorded. An enhancement in the plant growth activity of the ligand was observed on complexation and the best activity was observed at 10−6 M concentration.

Novel Ruthenium(II) Aluminate Anions: Building Blocks of Unique Cage Structures

New anionic ruthenium(II) aluminohydrides were synthesized from [Cp*RuCl]4, LiAlH4, and lithium alkyls. The reaction of [Cp*RuCl]4 with 8 equiv of LiAlH4 afforded the lithium salt [Li(DME)3][Cp*2Ru2(μ-H)(μ-AlH4)2] (1) in 65% isolated yield. Reactions of [Cp*RuCl]4, LiAlH4, and RLi (R = Me, Et, iPr, nBu) in THF yielded the new compounds [Li5(THF)2Cp*6Ru6Al4Me4H19] (2) and [Li2(THF)2Cp*RuAl2R2H7)]2 (3–5, R = Et, iPr, tBu). The compounds were characterized by single-crystal X-ray crystallography, multinuclear liquid and solid-state NMR spectroscopy, IR spectroscopy, and microanalysis. The molecular structures of 1–5 display novel anionic ruthenium(II) aluminate frameworks. In the case of 2–5, these anions associate into unusual cage structures via Li–H interactions in the solid state. Variable-temperature 1H and 7Li NMR studies on the fluxional complex 5 in deuterated toluene show that the cage structure of this compound is retained at low temperature in solution.

An Unprecedented Structural Interconversion in Solution of Aggregate Zinc(II) Salen Schiff-Base Complexes

This contribution explores the aggregation properties in solution of noncoordinating solvents of a series of amphiphilic Zn(salen) derivatives, through detailed (1)H NMR, DOSY NMR, and optical absorption spectroscopic studies. It is found that these aggregate species are involved in a unique structural interconversion between two defined dimers, A and B, driven by the concentration of water dissolved in chloroform. Dilute CHCl(3) solutions are characterized by the presence of dimeric species, A, in which both Zn(II) atoms of the Zn(salen) units mutually interact through a Zn···O axial coordination, likely adopting a square-base pyramidal structure. Investigations to higher concentrations indicate the existence of a new dimeric species, B, in equilibrium to that observed at lower concentrations, involving a coordination mode interconversion of an intermediate monomer presumably from a square-pyramidal to a trigonal bipyramidal structure. This behavior may be related to the nonconjugated, conformational flexible nature of the bridging diamine of the Schiff base, and is influenced by the solvent polarity. Variable-temperature (1)H NMR studies indicate the existence of a nonequivalent species B' in a fluxional equilibrium with species B.