Bis-bidentate Bridging Ligand Research Articles

Unraveling the acid-base characterization and solvent effects on the structural and electronic properties of a bis-bidentate bridging ligand.

Understanding the interactions and the solvent effects on the distribution of several species in equilibrium and how it can influence the 1H-NMR properties, spectroscopy (UV-vis absorption), and the acid-base equilibria can be especially challenging. This is the case of a bis-bidentate bridging ligand bis(2-pyridyl)-benzo-bis(imidazole), where the two pyridyl and four imidazolyl nitrogen atoms can be protonated in different ways, depending on the solvent, generating many isomeric/tautomeric species. Herein, we report a combined theoretical-experimental approach based on a sequential quantum mechanics/molecular mechanics procedure that was successfully applied to describe in detail the acid-base characterization and its effects on the electronic properties of such a molecule in solution. The calculated free-energies allowed the identification of the main species present in solution as a function of the solvent polarity, and its effects on the magnetic shielding of protons (1H-NMR chemical shifts), the UV-vis absorption spectra, and the acid-base equilibrium constants (pKas) in aqueous solution. Three acid-base equilibrium constants were experimentally/theoretically determined (pKa1 = 1.3/1.2, pKa2 = 2.1/2.2 and pKa5 = 10.1/11.3) involving mono-deprotonated and mono-protonated cis and trans species. Interestingly, other processes with pKa3 = 3.7 and pKa4 = 6.0 were also experimentally determined and assigned to the protonation and deprotonation of dimeric species. The dimerization of the most stable neutral species was investigated by Monte Carlo simulations and its electronic effects were considered for the elucidation of the UV-vis absorption bands, revealing transitions mainly with the charge-transfer characteristic and involving both the monomeric species and the dimeric species. The good matching of the theoretical and experimental results provides an atomistic insight into the solvent effects on the electronic properties of this bis-bidentate bridging ligand.

Photophysics of Cage/Guest Assemblies: Photoinduced Electron Transfer between a Coordination Cage Containing Osmium(II) Luminophores, and Electron-Deficient Bound Guests in the Central Cavity.

A heterometallic octanuclear coordination cage [Os4Zn4(Lnap)12]X16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (Lnap is a bis-bidentate bridging ligand containing two pyrazolyl-pyridine chelating units separated by a 1,5-naphthalenediyl spacer group). The {Os(NN)3}2+ units located at four of the eight vertices of the cube have a long-lived, phosphorescent 3MLCT excited state which is a stronger electron donor than [Ru(bipy)3]2+. The chloride form of Os•Zn is water-soluble and binds in its central cavity the hydrophobic electron-accepting organic guests 1,2,4,5-tetracyanobenzene, 1,4-naphthoquinone and 1-nitronaphthalene, with binding constants in the range 103-104 M-1, resulting in quenching of the phosphorescence arising from the Os(II) units. A crystal structure of an isostructural Co8 cage containing one molecule of 1,2,4,5-tetracyanobenzene as a guest inside the cavity has been determined. Ultrafast transient absorption measurements show formation of a charge-separated Os(III)/guest•- state arising from cage-to-guest photoinduced electron transfer; this state is formed within 13-21 ps, and decays on a time scale of ca. 200 ps. In the presence of a competing guest with a large binding constant (cycloundecanone) which displaces each electron-accepting quencher from the cage cavity, the charge-separated state is no longer observed. Further, a combination of mononuclear {Os(NN)3}2+ model complexes with the same electron-accepting species showed no evidence for formation of charge-separated Os(III)/guest•- states. These two control experiments indicate that the {Os(NN)3}2+ chromophores need to be assembled into the cage structure to bind the electron-accepting guests, and for PET to occur. These results help to pave the way for use of photoactive coordination cages as hosts for photoredox catalysis reactions on bound guests.

Designing a Dy2 Single-Molecule Magnet with Two Well-Differentiated Relaxation Processes by Using a Nonsymmetric Bis-bidentate Bipyrimidine- N-Oxide Ligand: A Comparison with Mononuclear Counterparts.

Herein we report a dinuclear [(μ-mbpymNO){(tmh)3Dy}2] (1) single-molecule magnet (SMM) showing two nonequivalent DyIII centers, which was rationally prepared from the reaction of Dy(tmh)3 moieties (tmh = 2,2,6,6-tetramethyl-3,5-heptanedionate) and the asymmetric bis-bidentate bridging ligand 4-methylbipyrimidine (mbpymNO). Depending on whether the DyIII ions coordinate to the N^O or N^N bidentate donor sets, the DyIII sites present a NO7 ( D2 d geometry) or N2O6 ( D4 d) coordination sphere. As a consequence, two different thermally activated magnetic relaxation processes are observed with anisotropy barriers of 47.8 and 54.7 K. Ab initio calculations confirm the existence of two different relaxation phenomena and allow one to assign the 47.8 and 54.7 K energy barriers to the Dy(N2O6) and Dy(NO7) sites, respectively. Two mononuclear complexes, [Dy(tta)3(mbpymNO)] (2) and [Dy(tmh)3(phenNO)] (3), have also been prepared for comparative purposes. In both cases, the DyIII center shows a NO7 coordination sphere and SMM behavior is observed with Ueff values of 71.5 K (2) and 120.7 K (3). In all three cases, ab initio calculations indicate that relaxation of the magnetization takes place mainly via the first excited-state Kramers doublet through Orbach, Raman, and thermally assisted quantum-tunnelling mechanisms. Pulse magnetization measurements reveal that the dinuclear and mononuclear complexes exhibit hysteresis loops with double- and single-step structures, respectively, thus supporting their SMM behavior.

A Heterobimetallic Anionic 3,6-Connected 2D Coordination Polymer Based on Nitranilate as Ligand.

In order to synthesize new coordination polymers with original architectures and interesting magnetic properties, we used the nitranilate ligand (C6O4(NO2)22− = C6N2O82−), derived from the dianionic ligand dhbq2− (2,5-dihydroxy-1,4-benzoquinone = H2C6O42−). The use of this bis-bidentate bridging ligand led to [(DAMS)2{FeNa(C6N2O8)3}·CH3CN]n (1) (DAMS+ = C16H17N2+ = 4-[4-(dimethylamino)-α-styryl]-1-methylpyridinium), a 2D heterometallic coordination polymer presenting an unprecedented structure for any anilato-based compound. This structural type is a 3,6-connected 2D coordination polymer derived from the well-known honeycomb hexagonal structure, where Fe(III) ions alternate with Na+ dimers (as Na2O12 units) in the vertices of the hexagons and with an additional [Fe(C6N2O8)3]3− anion located in the center of the hexagons connecting the three Na+ dimers. The magnetic properties of compound 1 show the presence of paramagnetic isolated high spin Fe(III) complexes with a zero field splitting, |D| = 8.5 cm−1.

Synthesis, crystal structure, chiroptical and ferroelectric properties of a multifunctional chiral silver(I) complex based on the chiral bis-bidentate bridging ligand

Using a chiral bis-bidentate isomer (–)-2,5-bis(4,5-pinene-2-pyridyl)pyrazine (LR) as bridging ligand to react with AgNO3 under solvothermal conditions, a chiral silver(I) complex formulated as [Ag(LR)·NO3]n (1) was obtained. Single crystal X-ray analysis revealed that 1 crystallizes in the non-centrosymmetric space group C2 with an infinite one-dimensional (1D) zigzag chain structure which is further linked together by C–H⋯π interactions to form a three-dimensional (3D) supramolecular framework. Each four-coordination Ag(I) atom is ligated by four N atoms from two LR ligands forming a distorted tetrahedral geometry. Circular dichroic (CD) spectrum confirmed its chiroptical activity. The measurements of second harmonic generation (SHG) and ferroelectric behavior showed that 1 displays nonlinear optical (NLO) activity and ferroelectricity. Furthermore, the investigation on luminescence property showed that 1 exhibits ligand center emission. All these indicate that 1 presents potential molecular-based multifunctional materials.

A tetrameric hetero-octanuclear cyclic helicate formed from a bridging ligand with two inequivalent binding sites

A bis-bidentate bridging ligand H2L with inequivalent hard and soft binding sites (catecholate and pyrazolyl-pyridine, respectively) reacts with a mixture of Ti(IV) and Zn(II) ions to afford an octanuclear heterometallic Ti4Zn4 cyclic helicate formed from four dinuclear {TiZn(μ-L)2} units connected in a ring via methoxide ions.

A family of tetrahedral coordination cages with different symmetries by assembly of bis-bidentate bridging ligands with first-row transition metal dications

A series of M4L6 tetrahedral cages, with a metal ion at vertex and a bis-bidentate bridging ligand spanning each edge, have been prepared and structurally characterised using three new ligands L1–L3. L1 contains two chelating pyrazolyl-pyridine termini connected to a 2,6-napthalene-diyl spacer by methylene groups: L2 and L3 contain chelating pyrazolyl-pyrazine termini connected to 1,8-naphthalene-diyl or 3,3-biphenyl centre units by methylene groups. The cages with L2 and L3 contain anions encapsulated in the central cavity. Although all three types of cage have the same basic tetrahedral structure, the cages display a range of molecular symmetries (S 4, T and C 3 for L1–L3, respectively) according to the combination of fac and mer tris-chelate metal sites in the complexes arising from the flexibility of the ligands.

A triple helix of double helicates: three hierarchical levels of self-assembly in a single structure

The bis-bidentate bridging ligand L reacts with Ag(I) ions to form a conventional dinuclear [Ag(2)L(2)](2+) double helicate; individual double helicate units assemble via Ag···Ag interactions into infinite chains, three of which wrap around a central spine of anions to give a triple helical braid, which is therefore an infinite triple helix composed of molecular double helicate subunits.

An octanuclear helical ‘molecular wheel’ from hierarchical assembly of four dinuclear Cu2 units in a mixed-ligand array

Reaction of Cu(II) salts with a combination of two different types of ligand affords an unusual ‘molecular wheel’, based on a cyclic array of four pyrazolate-bridged dinuclear Cu(II) units which are interconnected by bis-bidentate bridging ligands, in a hierarchical self-assembly process.

Luminescent triply-bridged dicopper(I) complex possessing 3,5-bis{6-(2,2′-dipyridyl)}pyrazole as a bis-chelating ligand

A new triply-bridged dicopper(I) complex, [Cu2(μ-dppm)2(μ-HL)](NO3)2 (1), has been prepared via successive treatment of cupric nitrate trihydrate with bis(diphenylphosphino)methane (dppm) and 3,5-bis{6-(2,2′-dipyridyl)}pyrazole (HL) in 2 : 4 : 1 molar ratio in dichloromethane. X-ray diffraction analysis of 1 reveals that the two Cu(I)s are in highly distorted tetrahedral environments with a distance of 4.2775(10) Å, triply bridged by two dppm ligands and one HL chelate as a bis-bidentate bridging ligand through two bipyridyl moieties. Intermolecular N···H–C hydrogen bonding and π···H–C interactions assemble the [Cu2(μ-dppm)2(μ-HL)]2+ cations into a 3-D supramolecular architecture with extended channels along the b-axis, filled with methanol and anions. Complex 1 shows weak low-energy absorptions at 350–425 nm, tentatively ascribed to Cu(I) to HL metal-to-ligand charge-transfer (MLCT) transition, probably mixed with some ILCT character inside HL. The emission is observed at ambient temperature for 1, both in solution and in the solid state, originating from the MLCT excited states.

Assembly of polyoxometalate-based inorganic–organic compounds from silver–Schiff base building blocks: synthesis, crystal structures and luminescent properties

Three polyoxometalate-based inorganic–organic hybrid compounds constructed from silver(I)-L (L = N,N′-bis-pyridin-2-ylmethylene-hydrazine) species and saturated Keggin type polyoxoanions, {[Ag3L3](PW12O40)(CH3CN)(DMF)2.5}n (1), [Ag4L3(CH3CN)4][Ag2L(CH3CN)3](PMo12O40)2·CH3CN (2) and [(SiMo12O40){Ag2L(CH3CN)3}2]·(CH3CN)2 (3), have been synthesized and structurally characterized by IR spectroscopy, UV spectroscopy, thermogravimetric (TG) analyses and complete single crystal structure analyses. The structure of 1 containing [PW12O40]3− anions as templates exhibits a one-dimensional chain structure constructed by the coordination of a silver center and bis-bidentate bridging ligands with the pyridyl rings adopting a transoid configuration. Compound 2 displays an AB-type arrangement supramolecular structure, in which A is a two-dimensional sheet formed by linking the adjacent [Ag4L3(CH3CN)4]4+ units through the cooperativity of intermolecular π–π interactions and C–H⋯π interactions, while B is composed of [PMo12O40]3− anions and binuclear [Ag2L(CH3CN)4]2+ cations. Compound 3 exhibits a new bisupporting Keggin-polyoxometalate structure in which the [SiMo12O40]4− core supports two [Ag2L(CH3CN)3]2+ fragments in two opposite positions. The luminescent properties of L and 1–3 in the solid state were investigated.

Molecular squares, cubes and chains from self-assembly of bis-bidentate bridging ligands with transition metal dications

The two new ligands L(fur) and L(th) consist of two chelating pyrazolyl-pyridine termini connected to furan-2,5-diyl or thiophene-2,5-diyl spacers via methylene groups. Reaction of these with a range of transition metal dications that prefer octahedral coordination affords a series of unusual structures which are all based on a 2M : 3L ratio. [M(8)(L(fur))(12)]X(16) (M = Co, Cu, X = BF(4); and M = Zn, X = ClO(4)) are octanuclear cubes with approximate D(4) symmetry in which two cyclic tetranuclear helicate M(4)L(4) units are connected by four additional 'pillar' ligands. In contrast [Ni(4)(L(fur))(6)](BF(4))(8) is a centrosymmetric molecular square consisting of two dinuclear Ni(2)L(2) units of opposite chirality that are connected by a pair of additional L(fur) ligands such that the four edges of the Ni(4) square are spanned by alternately two and one bridging ligands. [M(4)(L(th))(6)](BF(4))(8) (M = Co, Ni, Cu) are likewise molecular squares with similar structures to [Ni(4)(L(fur))(6)](BF(4))(8) with the significant difference that the two crosslinked double helicate M(2)L(2) units are now homochiral. The Cd(II) complexes both behave quite differently to the first-row metal complexes, with [Cd(L(fur))(BF(4))](BF(4)) being a simple mononuclear complex with a single ligand in which the furan oxygen atom is weakly interacting with the Cd(II) centre. In contrast, in {[Cd(2)(L(th))(3)](BF(4))(4)}(∞), where this quasi-pentadentate coordination mode of the ligand is not possible because thiophene is too poor an electron donor, the ligand reverts to bis-bidentate bridging coordination to afford a one-dimensional chain consisting of an infinite sequence of crosslinked, homochiral, Cd(2)(L(th))(2) double helicate units.

Structures and Dynamic Behavior of Large Polyhedral Coordination Cages: An Unusual Cage-to-Cage Interconversion

The bis-bidentate bridging ligand L {α,α'-bis[3-(2-pyridyl)pyrazol-1-yl]-1,4-dimethylbenzene}, which contains two chelating pyrazolyl-pyridine units connected to a 1,4-phenylene spacer via flexible methylene units, reacts with transition metal dications to form a range of polyhedral coordination cages based on a 2M:3 L ratio in which a metal ion occupies each vertex of a polyhedron, a bridging ligand lies along every edge, and all metal ions are octahedrally coordinated. Whereas the Ni(II) complex [Ni(8)L(12)](BF(4))(12)(SiF(6))(2) is an octanuclear cubic cage of a type we have seen before, the Cu(II), Zn(II), and Cd(II) complexes form new structural types. [Cu(6)L(9)](BF(4))(12) is an unusual example of a trigonal prismatic cage, and both Zn(II) and Cd(II) form unprecedented hexadecanuclear cages [M(16)L(24)]X(32)(X = ClO(4) or BF(4)) whose core is a skewed tetracapped truncated tetrahedron. Both Cu(6)L(9) and M(16)L(24) cages are based on a cyclic helical M(3)L(3) subunit that can be considered as a triangular "panel", with the cages being constructed by interconnection of these (homochiral) panels with additional bridging ligands in different ways. Whereas [Cu(6)L(9)](BF(4))(12) is stable in solution (by electrospray mass spectrometry, ES-MS) and is rapidly formed by combination of Cu(BF(4))(2) and L in the correct proportions in solution, the hexadecanuclear cage [Cd(16)L(24)](BF(4))(32) formed on crystallization slowly rearranges in solution over a period of several weeks to the trigonal prism [Cd(6)L(9)](BF(4))(12), which was unequivocally identified on the basis of its (1)H NMR spectrum. Similarly, combination of Cd(BF(4))(2) and L in a 2:3 ratio generates a mixture whose main component is the trigonal prism [Cd(6)L(9)](BF(4))(12). Thus the hexanuclear trigonal prism is the thermodynamic product arising from combination of Cd(II) and L in a 2:3 ratio in solution, and arises from both assembly of metal and ligand (minutes) and rearrangement of the Cd(16) cage (weeks); the large cage [Cd(16)L(24)](BF(4))(32) is present as a minor component of a mixture of species in solution but crystallizes preferentially.

Copper(I) Complex [Cu(ca2dapte)(NCS)] with the Flexible N2S2 Schiff-base Tridentate Ligand: Synthesis, Characterization and Crystal Structure

New copper(I) complex involving the flexible N2S2 Schiff-base ligand ca2dapte [Cu(ca2dapte)(NCS)], [ca2dapte = N,N′-bis-(cinnamaldehyde)-1,2-di(o-iminophenylthio)ethane], has been synthesized and structurally characterized by X-ray diffraction analysis. The coordination polyhedron around the Cu(I) center is best described as a distorted tetrahedron. The flexible N2S2 Schiff-base ligand ca2dapte acts as a tridentate ligand via two S atoms and one N atom, while the NCS- ligand is coordinated to the metal ion through its nitrogen atom. Crystal data: space group P-1, a = 10.7486(7), b = 12.3737(13), c = 12.7578(11) A, α = 89.385(7), β = 68.527(7), γ = 66.190(8)°; V = 1425.8(2) A3, Z = 2, R = 0.0417, wR 2 = 0.0744. The potentially tetradentate ligand ca2dapte have the ability to act as bis-bidentate bridging ligand to generate extended structures. In the new complex [Cu(ca2dapte)(NCS)], we show that this ligand is also able to act with a tridentate coordination mode.

Self-assembled half-sandwich Ir, Rh-based organometallic molecular boxes for reversible trapping of halocarbon molecules

Reactions of [Cp*MCl(mu-Cl)](2) (M = Ir or Rh) with 6,11-dihydroxy-5,12-naphthacenedione (H(2)DHNA) in the presence of base, gave the corresponding binuclear complexes [Cp*(2)M(2)(mu-DHNA)Cl(2)] (M = Ir (1a); M = Rh (1b)), respectively. Treatment of 1a or 1b with bidentate ligands (L) such as pyrazine, 4,4'-dipyridine (bpy), E-1,2-bis(4-pyridyl)ethene (bpe) or 2,5-bis(4-pyridyl)-1,3,5-oxadiazole (bpo) in the presence of AgOTf (OTf = CF(3)SO(3)) in CH(3)OH, gave the corresponding tetranuclear complexes, general formula [Cp*(4)M(4)(mu-DHNA)(2)(mu-L)(2)](OTf)(4) (3a, 4a, 5a, 6a: M = Ir; 3b, 4b, 5b, 6b: M = Rh), respectively. X-Ray analyses of 3a, 3b, 4a, 4b, 5a and 5b revealed that each of the half-sandwich metal centers was connected by pyridyl ligands and bis-bidentate bridging ligands to construct a rectangular cavity with different dimensions, and strong pi-pi interactions between independent molecules to form rectangular channels in the solid-state. Complexes 3a and 3b based on H(2)DHNA and pyrazine spacing ligands were found to exhibit selective and reversible small organic molecules adsorption properties. The example of C-H...Cl interactions served as a template by an interacted layer of monomeric complex for the creation of intercalated supramolecular arrays has been studied.

Cubes, Squares, and Books: A Simple Transition Metal/Bridging Ligand Combination Can Lead to a Surprising Range of Structural Types with the Same Metal/Ligand Proportions

Reaction of two structurally related bridging ligands L(26Py) and L(13Ph), in which two bidentate chelating pyrazolyl-pyridine units are connected to either a 2,6-pyridine-diyl or 1,3-benzene-diyl central group via methylene spacers, with first-row transition metal dications, results in a surprising variety of structures. The commonest is that of an octanuclear coordination cage [M(8)L(12)]X(16) [M = Co(II) or Zn(II); X = perchlorate or tetrafluoroborate] in which a metal ion is located at each of the eight vertices of an approximate cube, and one bis-bidentate bridging ligand spans each edge. The arrangement of fac and mer tris-chelate metal centers around the inversion center results in approximate (non-crystallographic) S(6) symmetry. Another structural type observed in the solid state is a hexanuclear complex [Co(6)(L(13Ph))(9)](ClO(4))(12) in which the six metal ions are in a rectangular array (two rows of three), folded about the central Co-Co vector like a partially open book, with each metal-metal edge containing one bridging ligand apart from the two outermost metal-metal edges which are spanned by a pair of bridging ligands in a double helical array. The final structural type we observed is a tetranuclear square [Ni(4)(L(26Py))(6)](BF(4))(8), with the four Ni-Ni edges spanned alternately by one and two bridging ligand such that it effectively consists of two dinuclear double helicates cross-linked by additional bridging ligands. A balance between the "cube" and "book" forms, which varied from compound to compound, was observed in solution in many cases by (1)H NMR and ES mass spectrometry studies.

Further investigations into tetrahedral M4L6cage complexes containing guest anions: new structures and NMR spectroscopic studies

A series of ligands LPh, Lnaph and Lanth, which contain two bidentate pyrazolyl–pyridine termini separated by an aromatic (1,2-phenyl, 2,3-naphthyl or 2,3-anthracenyl, respectively) spacer have been used to prepare tetrahedral cage complexes of the form [M4L6]Xn, in which a bis-bidentate bridging ligand spans each of the six edges of the M4 tetrahedron and one anion is bound in the central cavity. Several new examples have been structurally characterised, including an example with a new ligand (Lanth), the first example with a second-row transition metal ion [Cd(II)], and the first example of a cage containing a dianionic guest (hexafluorosilicate). The series of structurally similar Co(II) complexes [Co4L6(BF4)](BF4)7 (L = LPh, Lnaph and Lanth) have been examined in detail by NMR spectroscopy. The 1H NMR spectra are highly shifted between −110 and +90 ppm, but the spectra can be completely assigned by correlation of measured T1 relaxation times with distances of the protons in the complexes from the paramagnetic Co(II) centres. 1H DOSY measurements have been used to estimate diffusion constants which confirm the structural integrity of the cages in solution, and 19F DOSY measurements on the anions show that (i) the trapped [BF4]− anion diffuses at the same rate as the cage superstructure surrounding it, indicating that it is trapped inside the cage cavity; and (ii) the ‘free’ [BF4]− anions have diffusion rates consistent with substantial retardation due to ion-pairing with the 7+ complex cation.

Synthesis and coordination chemistry of new tetradentate N 2S 2 donor Schiff-base ligand ca 2-dapte: Mononuclear and dinuclear copper(I) complexes [Cu(ca 2dapte)]ClO 4 and [{Cu(PPh 3)(X)} 2(ca 2dapte)] (X = I and Br)

A mononuclear copper(I) complex, [Cu(ca 2dapte)]ClO 4 ( 1), and two dinuclear copper(I) complexes, [{Cu(PPh 3)(X)} 2(ca 2dapte)] (X = I ( 2) and Br ( 3)), of a new tetradentate N 2S 2 donor Schiff-base ligand ca 2dapte have been prepared (ca 2dapte = N, N′-bis( trans-cinnamaldehyde)-1,2-di(o-iminophenylthio)ethane). These compounds have been characterized by elemental analyses (CHN), FT-IR, UV–Vis and 1H NMR spectroscopy. The crystal structures of these copper(I) complexes have been determined by single-crystal X-ray diffraction. The coordination geometry around Cu(I) centers in these complexes is a distorted tetrahedron. The ca 2dapte is coordinated to Cu(I) as a tetradentate ligand in 1, while it acts as a bis-bidentate bridging ligand in 2 and 3.

Octanuclear Cubic Coordination Cages

Two new bis-bidentate bridging ligands have been prepared, L (naph) and L (anth), which contain two chelating pyrazolyl-pyridine units connected to an aromatic spacer (naphthalene-1,5-diyl and anthracene-9,10-diyl respectively) via methylene connectors. Each of these reacts with transition metal dications having a preference for octahedral coordination geometry to afford {M 8L 12} (16+) cages (for L (anth), M = Cu, Zn; for L (naph), M = Co, Ni, Cd) which have an approximately cubic arrangement of metal ions with a bridging ligand spanning each of the twelve edges, and a large central cavity containing a mixture of anions and/or solvent molecules. The cages based on L (anth) have two cyclic helical {M 4L 4} faces, of opposite chirality, connected by four additional L (anth) ligands as "pillars"; all metal centers have a meridional tris-chelate configuration. In contrast the cages based on L (naph) have (noncrystallographic) S 6 symmetry, with a diagonally opposite pair of corners having a facial tris-chelate configuration with the other six being meridional. An additional significant difference between the two types of structure is that the cubes containing L (anth) do not show significant interligand aromatic stacking interactions. However, in the cages based on L (naph), there are six five-membered stacks of aromatic ligand fragments around the periphery, each based on an alternating array of electron-rich (naphthyl) and electron-deficient (pyrazolyl-pyridine, coordinated to M (2+)) aromatic units. A consequence of this is that the cages {M 8(L (naph)) 12} (16+) retain their structural integrity in polar solvents, in contrast to the cages {M 8(L (anth)) 12} (16+) which dissociate in polar solvents. Consequently, the cages {M 8(L (naph)) 12} (16+) give NMR spectra in agreement with the symmetry observed in the solid state, and their fluorescence spectra (for M = Cd) display (in addition to the normal naphthalene-based pi-pi* fluorescence) a lower-energy exciplex-like emission feature associated with a naphthyl --> pyrazolyl-pyridine charge-transfer excited state arising from the pi-stacking between ligands around the cage periphery.

Dinuclear Amine–Thiophenolate Complexes Coligated by Ferrocenemonocarboxylate and 1,1′-Ferrocenedicarboxylate Anions: Preparation, Characterization and Structures of Trinuclear [LMII2(O2CC5H4FeCp)]+ and Pentanuclear [(LMII2)2(O2CC5H4)2Fe]2+ Complexes (M=Co, Ni, Zn)

A series of novel tri- and pentanuclear complexes composed of dinuclear LM(2) units (M=Co, Ni, Zn; L=24-membered macrocyclic hexaazadithiophenolate ligand) and ferrocenecarboxylate ([CpFeC(5)H(4)CO(2)](-)) or 1,1'-ferrocenedicarboxylate ([Fe(C(5)H(4)CO(2))(2)](2-)) groups is reported. The complexes [LM(II) (2)(O(2)CC(5)H(4)FeCp)](+) (M=Co (6), Ni (7), Zn (8)) and [(LM(II) (2))(2)(O(2)CC(5)H(4))(2)Fe](2+) (M=Co (9), Ni (10)) have been prepared by substitution reactions from labile [LM(II) (2)L'](+) precursors (L'=Cl, OAc) and the respective ferrocenecarboxylate anions in methanol. Mixed-valent [(LCo(II)Co(III))(2)(O(2)CC(5)H(4))(2)Fe](4+) (11) was prepared by oxidation of 9 with bromine. Complexes 7[BPh(4)], 8[BPh(4)], 9[BPh(4)](2), 10[BPh(4)](2), and 11[ClO(4)](4) have been characterized by X-ray crystallography; showing that the ferrocenyl carboxylates act as bidentate (7, 8) or bis-bidentate (9-11) bridging ligands towards one or two bioctahedral LM(2) subunits, respectively. The structures are retained in solution as indicated by NMR spectroscopic studies on the diamagnetic Zn(2)Fe complex 8[ClO(4)]. Electrochemical studies reveal significant anodic potential shifts for the oxidation potential of the ferrocenyl moieties upon complexation and the magnitude of the potential shift appears to correlate with the charge of the LM(2) subunits. This is qualitatively explained in terms of destabilizing electrostatic (Coulomb) interactions between the M(2+) ions of the LM(2) unit and the proximate ferrocenium fragment. An analysis of the temperature-dependent magnetic susceptibility data for 10[BPh(4)](2) shows the presence of weak ferromagnetic magnetic exchange interactions between the Ni(II) ions in the LNi(2) units. The exchange coupling across the ferrocenedicarboxylate bridge is negligible.