Electronic structure of bispidine iron(IV) oxo complexes

The electronic structure, based on DFT calculations, of a range of FeIV=O complexes with two tetra- (L1 and L2) and two isomeric pentadentate bispidine ligands (L3 and L4) is discussed with special emphasis on the relative stability of the two possible spin states (S = 1, triplet, intermediate-spin, and S = 2, quintet, high-spin; bispidines are very rigid diazaadamantane-derived 3,7-diazabicyclo[3.3.1]nonane ligands with two tertiary amine and two or three pyridine donors, leading to cis-octahedral [(X)(L)FeIV=O]2+ complexes, where X = NCCH3, OH2, OH-, and pyridine, and where X = pyridine is tethered to the bispidine backbone in L3, L4). The two main structural effects are a strong trans influence, exerted by the oxo group in both the triplet and the quintet spin states, and a Jahn-Teller-type distortion in the plane perpendicular to the oxo group in the quintet state. Due to the ligand architecture the two sites for substrate coordination in complexes with the tetradentate ligands L1 and L2 are electronically very different, and with the pentadentate ligands L3 and L4, a single isomer is enforced in each case. Because of the rigidity of the bispidine ligands and the orientation of the "Jahn-Teller axis", which is controlled by the sixth donor X, the Jahn-Teller-type distortion in the high-spin state of the two isomers is quite different. It is shown how this can be used as a design principle to tune the relative stability of the two spin states.     

Tuning the Properties of Copper(II) Complexes with Tetra‐ and Pentadentate Bispidine (=3,7‐Diazabicyclo[3.3.1]nonane) Ligands

The synthesis of a series of tetra‐ and pentadentate bispidine‐type ligands (bispidine=3,7‐diazabicyclo[3.3.1]nonane) – tetradentate ligands are donor‐substituted at C(2) and C(4), pentadentate ligands have an additional donor at N(3) or N(7), with pyridine, 2‐methylpyridine, or quinoline donor moieties – and of their Cu^II complexes are reported, together with single‐crystal structural analyses and solution studies (electrochemistry, electronic and EPR spectroscopy). Depending on the ligand geometry and on the co‐ligands (solvent or counter anion), there are various structural forms (pseudo‐Jahn–Teller elongation along all three molecular axes), and the structural data are correlated with the spectroscopic and electrochemical parameters.     

Copper-bispidine coordination chemistry: syntheses,structures, solution properties,and oxygenation reactivity

Copper(I) and copper(II) complexes of two mononucleating and four dinucleating tetradentate ligands with a bispidine backbone (2,4-substituted (2-pyridyl or 4-methyl-2-pyridyl) 3,7-diazabicyclo[3.3.1]nonanone) have been prepared and analyzed structurally, spectroscopically, and electrochemically. The structures of the copper chromophores are square pyramidal, except for two copper(I) compounds which are four-coordinate with one noncoordinated pyridine. The other copper(I) structures have the two pyridine donors, the co-ligand (NCCH(3)), and one of the tertiary amines (N3) in-plane with the copper center and the other amine (N7) coordinated axially (Cu-N3 > Cu-N7, approximately 2.25 A vs 2.20 A). The copper(II) compounds with pyridine donors have a similar structure, but the axial amine has a weaker bond to the copper(II) center (Cu-N3 < Cu-N7, approximately 2.03 A vs 2.30 A). The structures with methylated pyridine donors are also square pyramidal with the co-ligands (Cl(-) or NCCH(3)) in-plane. With NCCH(3) the same structural type as for the other copper(II) complexes is observed, and with the bulkier Cl(-) the co-ligand is trans to N7, leading to a square pyramidal structure with the pyridine donors rotated out of the basal plane and only a small difference between axial and in-plane amines (2.15, 2.12 A). These structural differences, enforced by the rigid bispidine backbone, lead to large variations in spectroscopic and electrochemical properties and reactivities. Oxygenation of the copper(I) complexes with pyridine-substituted bispidine ligands leads to relatively stable mu-peroxo-dicopper(II) complexes; with a preorganization of the dicopper chromophores, by linking the two donor sets, these peroxo compounds are stable at room temperature for up to 1 h. The stabilization of the peroxo complexes is to a large extent attributed to the square pyramidal coordination geometry with the substrate bound in the basal plane, a structural motif enforced by the rigid bispidine backbone. The stabilities and structural properties are also seen to correlate with the spectroscopic (UV-vis and Raman) and electrochemical properties.     

Epoxidation and 1,2-dihydroxylation of alkenes by a nonheme iron model system - DFT supports the mechanism proposed by experiment

The FeII complexes of two isomeric pentadentate bispidine ligands in the presence of H2O2 are catalytically active for the epoxidation and 1,2-dihydroxylation of cyclooctene (bispidine = 3,7-diazabicyclo[3.3.1]nonane; the two isomeric pentadentate bispidine ligands discussed here have two tertiary amine and three pyridine donors). The published spectroscopic and mechanistic data, which include an extensive set of 18O labeling experiments, suggest that the FeIV=O complex is the catalytically active species, which produces epoxide as well as cis- and trans-1,2-dihydroxylated products. Several observations from the published experimental study are addressed with hybrid density functional methods and, in general, the calculations support the proposed, for nonheme iron model systems novel mechanism, where the formation of a radical intermediate emerges from the reaction of the FeIV=O oxidant and cyclooctene. The calculations suggest that the S = 1 ground state of the FeIV=O complex reacts with cyclooctene in a stepwise reaction, leading to the formation of a carbon-based radical intermediate. This radical is captured by O2 from air to produce the majority of the epoxide products in an aerobic atmosphere. Under anaerobic conditions, the produced epoxide product is due to the cyclization of the radical intermediate. Several possible spin states (ST = 3, 2, 1, 0) of the radical intermediate are close in energy. As a result of the substantial energy barrier, calculated for the ST = 3 spin ground state, a spin-crossover during the cyclization step is assumed, and a possible two-state scenario is found, where the S = 2 state of the FeIV=O complex participates in the catalytic mechanism. The 1,2-dihydroxylation proceeds, as suggested by experiment, via an unprecedented pathway, where the radical intermediate is captured by a hydroxyl radical, the source of which is FeIII-OOH, and this reaction is barrierless. The calculations suggest that dihydroxylation can also occur by a direct oxidation pathway from FeIII-OOH. The strikingly different reactivities observed with the two isomeric bispidine FeII complexes are rationalized on the basis of structural and electronic differences.     

Modeling of the various minima on the potential energy surface of bispidine copper(II) complexes: a further test for ligand field molecular mechanics

Copper(II) complexes of bispidines (bispidine = tetra-, penta-, or hexadentate ligand, based on the 3,7-diazabicyclo[3.3.1]nonane backbone) display several isomeric forms. Depending on the substitution pattern of the bispidine and the type of coligands used, the structure elongates along one of the three potential Jahn-Teller axes. In an effort to develop a computational tool which can predict which isomer is observed, 23 bispidine-copper(II) complexes with 19 different ligands are analyzed theoretically by ligand field molecular mechanics (LFMM). With two exceptions, the lowest-energy LFMM structure and the experimental solid-state structure agree concerning the Jahn-Teller axis. However, in most cases and especially for six-coordinate complexes, LFMM predicts a second local minimum within a few kilojoules per mole. Although detailed analysis reveals that the current force field is too "stiff", reasonable quantitative reproduction of the structural data is achieved with Cu-L bond length root mean square (rms) deviations for nine complexes of 0.05 A or less and with 20 reproduced to a rms deviation of 0.1 A or less. Across all of the complexes, the Cu-amine and Cu-pyridyl bond length rms deviations are 0.07 and 0.12 A, respectively.     

Structure, bonding, and catecholase mechanism of copper bispidine complexes

Oxygen activation by copper(I) complexes with tetra- or pentadentate mono- or dinucleating bispidine ligands is known to lead to unusually stable end-on-[{(bispidine)Cu}(2)(O(2))](2+) complexes (bispidines are methyl-2,4-bis(2-pyridin-yl)-3,7-diazabicyclo-[3.3.1]-nonane-9-diol-1,5-dicarboxylates); catecholase activity of these dinuclear Cu(II/I) systems has been demonstrated experimentally, and the mechanism has been thoroughly analyzed. The present density functional theory (DFT) based study provides an analysis of the electronic structure and catalytic activity of [{(bispidine)Cu}(2)(O(2))](2+). As a result of the unique square pyramidal coordination geometry, the d(x(2)-y(2)) ground state leads to an unusual σ/π bonding pattern, responsible for the stability of the peroxo complex and the observed catecholase activity with a unique mechanistic pathway. The oxidation of catechol to ortho-quinone (one molecule per catalytic cycle and concomitant formation of one equivalent of H(2)O(2)) is shown to occur via an associative, stepwise pathway. The unusual stability of the end-on-peroxo-dicopper(II) complex and isomerization to copper(II) complexes with chelating catecholate ligands, which inhibit the catalytic cycle, are shown to be responsible for an only moderate catalytic activity.     

A combined experimental and computational study on the sulfoxidation by high-valent iron bispidine complexes

Iron-bispidine complexes are efficient catalysts for the oxidation of thioanisole to phenylmethylsulfoxide with iodosylbenzene as oxidant. With the tetradentate bispidine ligand L(1) (L(1) = 2,4-pyridyl-3,7-diazabicyclo[3.3.1]nonane)) the catalytic efficiency is smaller than with the pentadentate bispidine ligand L(2) (L(2) = 2,4-pyridyl-7-(pyridine-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane)). Based on the redox potentials (iron complexes with L(1) are stronger oxidants than with L(2)) and known efficiencies in catalytic olefin oxidation and C-H activation reactions, the expectations were different. A DFT-based analysis is used to explain the apparent contradiction, and this is based on differences in the electronic ground states of the ferryl complexes as well as in the oxygen transfer transition states.     

Coordination stability between metal/ligands interaction by modern spectroscopic studies: IR, electronic, EPR and cyclic voltammetry of cobalt(II) complexes with organic skeleton containing cyclic ligands

A comparative investigation of the interaction of two pyrrole-substituted, mixed oxygen and nitrogen donor, macrocycles ligands have been designed and their coordination interaction with cobalt(II) is studied. Cobalt(II) salts combine with a tetradentate and hexadentate macrocyclic nitrogen/oxygen donor ligands and formed novel cobalt(II) complexes which are characterized by elemental analysis, molar conductance, magnetic moments, mass, (1)H NMR, IR, electronic and EPR spectral studies. At the room temperature magnetic moment for cobalt(II) complexes lie in the range 4.70-5.01BM, which is higher than the spin-only value. All the complexes are high-spin type and have three unpaired electrons. Therefore, the electronic confutation and the splitting of the orbital will be t(2g)(5)eg(2). The electrochemical behaviour of the cobalt(II) complexes, the Co(III)/Co(II) couple are observed. Their positive potential indicates that metal in lower oxidation state is strongly bound to these ligands. The difference between the potential of the anodic peak and cathodic peak remains constant in all complexes. Also, the ratio between the cathodic peak current and square root of the scan rate is practically constant for the studied complexes.     

Syntheses, spectroscopic characterization and thermal behavior on novel binuclear transition metal complexes of hydrazones derived from 4,6-diacetylresorcinol and oxalyldihydrazine

4,6-Diacetylresorcinol (DAR) serves as precursor for the formation of different hydrazone ligands, which are di-, tetra- or hexa-basic with two symmetrical sets of O(2)N tridentate, O(2)N(2) tetradentate or O(4)N(2) hexadentate chelating sites. The condensation of 4,6-diacetylresorcinol (DAR) with oxalyldihydrazine (ODH), in the molar ratio 1:1 and 1:2, yields the corresponding hydrazone, H(6)L(a) and H(4)L(b), ligands, respectively. The structures of these ligands were elucidated by elemental analyses and IR, mass, (1)H NMR and UV-vis spectra. Reactions of the hydrazone ligands with cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), iron(III) and chromium(III) ions in 1:2 molar ratio afforded the corresponding transition metal complexes. A variety of binuclear transition metal complexes were obtained in its di-, tetra- or hexa-deprotonated forms. The structures of the newly prepared complexes were identified by elemental analyses and IR, UV-vis, mass, (1)H NMR and ESR spectra, as well as, magnetic susceptibility measurements and thermal gravimetric analysis (TGA). The bonding sites are the azomethine and CO oxygen atoms in either keto or enol forms and amino nitrogen atoms, and phenolic oxygen atoms. The metal complexes exhibit different geometrical structures such as tetrahedral and octahedral arrangements.     

Structural variation in transition-metal bispidine compounds

The experimentally determined molecular structures of 40 transition metal complexes with the tetradentate bispyridine-substituted bispidone ligand, 2,4-bis(2-pyridine)-3,7-diazabicyclo[3.3.1]nonane-9-one [M(bisp)XYZ]n+; M = CrIII, MnII, FeII, CoII, CuII, CuI, ZnII; X, Y, Z = mono- or bidentate co-ligands; penta-, hexa- or heptacoordinate complexes) are characterized in detail, supported by force-field and DFT calculations. While the bispidine ligand is very rigid (N3...N7 distance = 2.933 +/- 0.025 A), it tolerates a large range of metal-donor bond lengths (2.07 A < sigma(M-N)/4 < 2.35 A). Of particular interest is the ratio of the bond lengths between the metal center and the two tertiary amine donors (0.84 A < M-N3/M-N7 < 1.05 A) and the fact that, in terms of this ratio there seem to be two clusters with M-N3 < M-N7 and M-N3 > or = M-N7. Calculations indicate that the two structural types are close to degenerate, and the structural form therefore depends on the metal ion, the number and type of co-ligands, as well as structural variations of the bispidine ligand backbone. Tuning of the structures is of importance since the structurally differing complexes have very different stabilities and reactivities.     

Complexation to Fe(II), Ni(II), and Zn(II) of multidentate ligands resulting from condensation of 2-pyridinecarboxaldehyde with alpha,omega-triamines: selective imidazolidine/hexahydropyrimidine ring opening revisited

The condensation reaction between 2-pyridinecarboxaldehyde and diethylenetriamine, 3-[(2-aminoethyl)amino]propylamine, and 3,3'-iminobis(propylamine) in a 2:1 molar ratio yields ligands that may be isolated exclusively in the dissymmetric (cyclic) isomeric forms L(A), L(B)/L(B*), and L(C). The template effect of a metal center (Fe(II), Ni(II), and Zn(II)) results in the ring opening of L(C) including one hexahydropyrimidine ring and one (long) propylene bridge. The resulting symmetric bis-Schiff base isomeric form L(C') is stabilized through pentacoordination, yielding [Fe(II)L(C')(NCS)](NCS) (3), [Ni(II)L(C')(NCS)](NCS) (6), and [Zn(II)L(C')(NCS)](NCS) (9). The same metal centers are too bulky to exert a template effect on L(A) including one imidazolidine ring and one (short) ethylene bridge. L(A) acts as a tetradentate ligand yielding [Fe(II)L(A)(NCS)2] (1), [Ni(II)L(A)(NCS)2] (4), and [Zn(II)L(A)(NCS)2] (7). The template effect of the metal center is selective toward the ligand L(B)/L(B*) including a hexahydropyrimidine (imidazolidine) ring and the shorter ethylene (longer propylene) bridge. The Fe(II) cation is small enough to exert a template effect, resulting in the ring opening of L(B)/L(B*). The resulting bis-Schiff base L(B') is stabilized through pentacoordination, yielding [Fe(II)L(B')(NCS)](NCS) (2). Ni(II) is too bulky to promote the ring opening of L(B)/L(B*): L(B) acts as a tetradentate ligand, yielding [Ni(II)L(B)(NCS)2] (5) (the L(B*) isomer is totally converted to L(B)). The coordinative requirements and stereochemical preference of the bulkier Zn(II) cation allow neither the ring opening of L(B)/L(B*) nor the tetracoordination of L(B) or L(B*) but stabilize the novel tetradentate dissymmetric form L(B degrees) in [Zn(II)L(B degrees)(NCS)2].H2O (8) (L(B degrees) results from MeOH addition across the imine bond of L(B)). Density functional theory calculations performed for Ni(II) and Zn(II) complexes of the L(B)/L(B*)/L(B degrees) set of ligands allowed one to compare the relative stabilities of all possible isomers, showing that the most stable ones correspond to those experimentally obtained: isomerization, or methanol addition across the imine bond, of the tetradentate ligand depends on the relative stabilities of all possible isomeric complexes.     

Valence Tautomerism in Octahedral and Square‐Planar Phenoxyl–Nickel(II) Complexes: Are Imino Nitrogen Atoms Good Friends?

The two tetradentate ligands H(2)L and H(2)L(Me) afford the slightly distorted square-planar low-spin Ni(II) complexes 1 and 2, which comprise two coordinated phenolate groups. Complex 1 has been electrochemically oxidized into 1(+), which contains a coordinated phenoxyl radical, with a contribution from the nickel orbital. In the presence of pyridine, 1(+) is converted into 1(Py) (+), an octahedral phenolate nickel(III) complex with two pyridines axially coordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H(2)L(Me), in the presence of pyridine, and the hexadentate ligand H(2)L(Py) in CH(2)Cl(2) afford, respectively, the octahedral high-spin Ni(II) complexes 2(Py) and 3, which involve two equatorial phenolates and two axially coordinated pyridines. At 100 K, the one-electron-oxidized product 2(Py) (+) comprises a phenoxyl radical ferromagnetically coupled to the high-spin Ni(II) ion, with large zero-field splitting parameters, while 3(+) involves a phenoxyl radical antiferromagnetically coupled to the high-spin Ni(II) ion.     

The Trigonal Prism in Coordination Chemistry

Herein we analyze the accessibility of the trigonal‐prismatic geometry to metal complexes with different electron configurations, as well as the ability of several hexadentate ligands to favor that coordination polyhedron. Our study combines i) a structural database analysis of the occurrence of the prismatic geometry throughout the transition‐metal series, ii) a qualitative molecular orbital analysis of the distortions expected for a trigonal‐prismatic geometry, and iii) a computational study of complexes of several transition‐metal ions with different hexadentate ligands. Also the tendency of specific electron configurations to present a cis bond‐stretch Jahn–Teller distortion is analyzed.     

Structural and near-IR photophysical studies on ternary lanthanide complexes containing poly(pyrazolyl)borate and 1,3-diketonate ligands

The ligands tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (L1, potentially hexadentate) and bis[3-(2-pyridyl)pyrazol-1-yl]dihydroborate (L2, potentially tetradentate) have been used to prepare ternary lanthanide complexes in which the remaining ligands are dibenzoylmethane anions (dbm). [Eu(L1)(dbm)2] is eight-coordinate, with L1 acting only as a tetradentate chelate (with one potentially bidentate arm pendant) and two bidentate dbm ligands. [Nd(L1)(dbm)2] was also prepared but on recrystallization some of it rearranged to [Nd(L1)2][Nd(dbm)4], which contains a twelve-coordinate [Nd(L1)2]+ cation (two interleaved hexadentate podand ligands) and the eight-coordinate anion [Nd(dbm)4]- which, uniquely amongst eight-coordinate complexes having four diketonate ligands, has a square prismatic structure with near-perfect O8 cubic coordination. Formation of this sterically unfavourable geometry is assumed to arise from favourable packing with the pseudo-spherical cation. The isostructural series of complexes [Ln(L2)(dbm)2](Ln = Pr, Nd, Eu, Gd, Tb, Er, Yb) was also prepared and all members structurally characterised; again the metal ions are eight-coordinate, from one tetradentate ligand L2 and two bidentate dbm ligands. Photophysical studies on the complexes with Ln = Pr, Nd, Er, and Yb were carried out; all show the near-IR luminescence characteristic of these metal ions, with longer lifetimes in CD3OD than in CH3OH. For [Yb(L2)(dbm)2], two species with different luminescence lifetimes were observed in CH3OH solution, corresponding to species with zero or one coordinated solvent molecules, in slow exchange on the luminescence timescale. For [Nd(L2)(dbm)2] a single average solvation number of 0.7 was observed in MeOH. For [Pr(L2)(dbm)2] a range of emission lines in the visible and NIR regions was detected; time-resolved measurements show a particularly high susceptibility to quenching by solvent CH and OH oscillators.     

Manganese(II)-containing MRI contrast agent employing a neutral and non-macrocyclic ligand

The ligand N,N'-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn) coordinates divalent transition metal ions in either a pentadentate or hexadentate fashion. The coordination number correlates with the ionic radius of the metal ion, with larger cations being heptacoordinate as assessed by solid-state analysis. With Mn(II), the debpn ligand is hexadentate and remains bound to the oxophilic metal ion, even when dissolved in water. The ligand's incomplete coordination of the manganous ion allows water molecules to coordinate to the metal center. These two properties, coupled with the high paramagnetism associated with the S = 5/2 metal center, enable [Mn(debpn)(H(2)O)](ClO(4))(2) to serve as a stable and effective magnetic resonance imaging contrast agent despite the ligand's lack of both a macrocyclic component and an anionic charge.     

Synthesis and catalytic properties in olefin epoxidation of novel iron(II) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm

Three novel iron(II) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm were synthesized and characterized. Crystal structures of two of the complexes revealed high-spin iron(II) centers coordinated to the five ligand nitrogen atoms with no coordination of either the solvent molecules or anions, resulting in an unusual square-pyramidal geometry. Related tetradentate ligand CRH formed a low-spin iron(II) complex (meso form was structurally characterized) with a planar arrangement of the four nitrogen atoms from the macrocycle and two axial acetonitrile molecules. Similarly to the corresponding nickel and copper complexes of the pentadentate ligands, the protonation of the amino group on the ligand arm in iron(II) complexes was found to be reversible. Spectral changes and magnetic susceptibility measurements indicated that a change in the geometry and spin state of the metal center is associated with this acid-base process. In the presence of noncoordinating acids (e.g., triflic acid), these complexes, as well as their nonmethylated analogue, can efficiently catalyze the epoxidation of cyclooctene and 1-decene under mild conditions, using hydrogen peroxide as the oxidant. However, in the deprotonated form or in the presence of coordinating acids like HCl, no epoxidation occurs.     

Nickel(II) complexes with different chromospheres containing macrocyclic ligands: spectroscopic and electrochemical studies

The mixed donor tetradentate (L(1)=N(2)O(2)) and pentadentate (L(2)=N(2)O(2)S) ligands have been prepared by the interaction of 1,3-diaminopropane and thiodiglycolic acid with diamine. These ligands possess two dissimilar coordination sites. Different types of complexes were obtained which have different stoichiometry depending upon the type of ligands. Their structural investigation have been based on elemental analysis, magnetic moment and spectral (ultraviolet, infrared, (1)H NMR, (13)C NMR and mass spectroscopy methods). The Ni(II) complexes show magnetic moments corresponding to two unpaired electrons except [Ni(L(1))](NO(3))(2) which is diamagnetic. Ligand field parameters of these complexes were compared. N(2)O(2)S donor ligand complexes show higher values of ligand field parameters, which are used to detect their geometries. The redox properties and stability of the complexes toward oxidation waves explored by cyclic voltammetry are related to the electron-withdrawing or releasing ability of the substituents of macrocyclic ligands moiety. The Ni(II) complexes displayed Ni(II)/Ni(I) couples irreversible waves associated with Ni(III)/Ni(II) process.     

Synthesis and spectroscopic studies of mixed-ligand and polymeric dinuclear transition metal complexes with bis-acylhydrazone tetradentate ligands and 1,10-phenanthroline

Two types of dinuclear copper(II) and nickel(II) complexes with two tetradentate N2O2 donor ligands 1,4-bis(1-anthranoylhydrazonoethyl)benzene (L1), 1,4-bis(1-salicyloylhydrazonoethyl)benzene (L2) and N,N'-bidentate heterocyclic base [1,10-phenonthroline (phen)] have been synthesized and characterized by elemental analysis, infrared spectra, UV-vis electronic absorption spectra and magnetic susceptibility measurements. The reaction of metal(II) acetates with the solution containing ligand and 1,10-phenonthroline in methanol gives mixed-ligand dinuclear metal(II) complexes with general formula [M2L(phen)2]Cl2 (L=L1 or L2), whereas, the ligands react with metal(II) acetates to form polymeric dinuclear complexes with general formula [(M2L2)n] (L=L1 or L2). In the complexes, the ligands act as dianionic tetradentate and coordination takes place in the enol tautomeric form with the enolic oxygen and azomethine nitrogen atoms while the phenolic hydroxyl and amino groups of aroylhydrazone moiety do not participate in coordination. The effect of varying pH and solvent on the absorption behavior of both ligands and complexes has been investigated.     

An unusually flexible expanded hexaamine cage and its Cu(II) complexes: variable coordination modes and incomplete encapsulation

The bicyclic hexaamine "cage" ligand Me(8)tricosaneN(6) (1,5,5,9,13,13,20,20-octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) is capable of encapsulating octahedral metal ions, yet its expanded cavity allows the complexed metal to adopt a variety of geometries comprising either hexadentate or pentadentate coordination of the ligand. When complexed to Cu(II) the lability of the metal results in a dynamic equilibrium in solution between hexadentate- and pentadentate-coordinated complexes of Me(8)tricosaneN(6). Both [Cu(Me(8)tricosaneN(6))](ClO(4))(2) (6-coordinate) and [Cu(Me(8)tricosaneN(6))](S(2)O(6)) (5-coordinate) have been characterized structurally. In weak acid (pH 1) a singly protonated complex [Cu(HMe(8)tricosaneN(6))](3+) has been isolated that finds the ligand binding as a pentadentate with the uncoordinated amine being protonated. vis-NIR and electron paramagnetic resonance (EPR) spectroscopy show that the predominant solution structure of [Cu(Me(8)tricosaneN(6))](2+) at neutral pH comprises a five-coordinate, square pyramidal complex. Cyclic voltammetry of the square pyramidal [Cu(Me(8)tricosaneN(6))](2+) complex reveals a reversible Cu(II/I) couple. All of these structural, spectroscopic, and electrochemical features contrast with the smaller cavity and well studied "sarcophagine" (sar, 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) Cu(II) complexes which are invariably hexadentate coordinated in neutral solution and cannot stabilize a Cu(I) form.