d → f energy transfer in a series of Ir(III)/Eu(III) dyads: energy-transfer mechanisms and white-light emission.

An extensive series of blue-luminescent iridium(III) complexes has been prepared containing two phenylpyridine-type ligands and one ligand containing two pyrazolylpyridine units, of which one is bound to Ir(III) and the second is pendant. Attachment of {Ln(hfac)(3)} (Ln = Eu, Gd; hfac = anion of 1,1,1,5,5,5,-hexafluoropentanedione) to the second coordination site affords Ir(III)/Ln(III) dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one Ir(III)/Eu(III) dyad reveals that in most cases the complexes can adopt a folded conformation involving aromatic π stacking between a phenylpyridine ligand and the bis(pyrazolylpyridine) ligand, but in one series, based on CF(3)-substituted phenylpyridine ligands coordinated to Ir(III), the steric bulk of the CF(3) group prevents this and a quite different and more open conformation arises. Quantum mechanical calculations well reproduce these two types of "folded" and "open" conformations. In the Ir(III)/Eu(III) dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the Ir(III)-based blue emission and the appearance of a sensitized red emission from Eu(III). Calculations based on consideration of spectroscopic overlap integrals rule out any significant contribution from Förster (dipole-dipole) energy transfer over the distances involved but indicate that Dexter-type (exchange) energy transfer is possible if there is a small electronic coupling that would arise, in part, through π stacking between components. In some cases, an initial photoinduced electron-transfer step could also contribute to Ir → Eu energy transfer, as shown by studies on isostructural iridium/gadolinium model complexes. A balance between the blue (Ir-based) and red (Eu-based) emission components can generate white light.

Structures, host-guest chemistry and mechanism of stepwise self-assembly of M4L6 tetrahedral cage complexes.

The ligand L(bip), containing two bidentate pyrazolyl-pyridine termini separated by a 3,3'-biphenyl spacer, has been used to prepare tetrahedral cage complexes of the form [M(4)(L(bip))(6)]X(8), in which a bridging ligand spans each of the six edges of the M(4) tetrahedron. Several new examples have been structurally characterized with a variety of metal cation and different anions in order to examine interactions between the cationic cage and various anions. Small anions such as BF(4)(-) and NO(3)(-) can occupy the central cavity where they are anchored by an array of CH···F or CH···O hydrogen-bonding interactions with the interior surface of the cage, but larger anions such as naphthyl-1-sulfonate or tetraphenylborate lie outside the cavity and interact with the external surface of the cage via CH···π interactions or CH···O hydrogen bonds. The cages with M = Co and M = Cd have been examined in detail by NMR spectroscopy. For [Co(4)(L(bip))(6)](BF(4))(8) the (1)H NMR spectrum is paramagnetically shifted over the range -85 to +110 ppm, but the spectrum has been completely assigned by correlation of measured T(1) relaxation times of each peak with Co···H distances. (19)F DOSY measurements on the anions show that at low temperature a [BF(4)](-) anion diffuses at a similar rate to the cage superstructure surrounding it, indicating that it is trapped inside the central cage cavity. Furthermore, the equilibrium step-by-step self-assembly of the cage superstructure has been elucidated by detailed modeling of spectroscopic titrations at multiple temperatures of an acetonitrile solution of L(bip) into an acetonitrile solution of Co(BF(4))(2). Six species have been identified: [Co(2)L(bip)](4+), [Co(2)(L(bip))(2)](4+), [Co(4)(L(bip))(6)](8+), [Co(4)(L(bip))(8)](8+), [Co(2)(L(bip))(5)](4+), and [Co(L(bip))(3)](2+). Overall the assembly of the cage is entropy, and not enthalpy, driven. Once assembled, the cages show remarkable kinetic inertness due to their mechanically entangled nature: scrambling of metal cations between the sites of pure Co(4) and Cd(4) cages to give a statistical mixture of Co(4), Co(3)Cd, Co(2)Cd(2), CoCd(3) and Cd(4) cages takes months in solution at room temperature.

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.

Iridium(III) luminophores as energy donors for sensitised emission from lanthanides in the visible and near-infrared regions.

Luminescent iridium(iii) complex units bearing pendant 2,2'-bipyridyl-type binding sites can be used to generate Ir/Ln dyads in which the Ir(iii) luminophore acts as an energy donor to the lanthanide by the Dexter mechanism, generating sensitised emission in the visible (from Eu) or near-infrared (Nd, Yb) regions.

Hierarchical self-assembly of heteronuclear co-ordination networks.

Two ligands L(1) and L(2) have been prepared which contain a chelating pyrazolyl-pyridine group with a pendant aromatic nitrile (in L(1), a benzonitrile; in L(2), a naphthonitrile). These ligands react with Ag(I) salts to give a range of infinite coordination networks or dimeric 'boxes' in which the pyrazolyl-pyridine chelates and the aromatic nitrile groups both participate in coordination to Ag(I) ions. In contrast, L(1) and L(2) form simple mononuclear tris-chelates [ML(3)](2+) with first-row transition metal dications (M = Co, Ni, Zn) in which the aromatic nitrile groups are pendant such that the complexes can be used as 'complex ligands'. The crystal structures of [M(L(2))(3)](BF(4))(2) are based on solely the mer tris-chelate geometry although in solution (1)H NMR spectroscopy reveals a mixture of both fac and mer isomers of the tris-chelates. Reaction of these with Ag(I) ions allows the interaction of the pendant nitrile groups with Ag(i) ions to generate coordination networks based on [ML(3)](2+) cations being crosslinked by Ag(I) ions. In these networks the [ML(3)](2+) cations have solely the fac geometry and lie on threefold rotation axes with all three pendant nitrile groups coordinated to Ag(I) ions which are three-coordinate. {[AgM(L(2))(3)][BF(4)](3)}(infinity) (M = Co, Ni) consist of two interpenetrated (10,3)a nets which have opposite chirality at the [M(L(2))(3)](2+) centres but are not strictly enantiomorphic as the two nets are not crystallographically equivalent. {[AgNi(L(1))(3)](BF(4))(3)}(infinity) in contrast contains two-dimensional sheets which have a (6,3) net structure of hexagonal rings of alternative Ni(II) and Ag(I) centres; although not interpenetrating, two such adjacent (and enantiomorphic) sheets interact with each other via numerous CH...pi interactions between aromatic ligands. Formation of these structures shows that the differential reactivity of the two binding sites in L(1) and L(2) (pyrazolyl-pyridine, and nitrile) can be used to generate mixed-metal coordination networks in a hierarchical, stepwise manner.

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.

Tri-, tetra- and octa-metallic vanadium(III) clusters from new, simple starting materials: interplay of exchange and anisotropy effects.

The crystal structure of the monomeric vanadium(III) species mer-[V(bipy)Cl(3)(MeCN)] (1; bipy = 2,2'-bipyridine) is reported. The solvothermal reaction of [V(bipy)Cl(3)(MeCN)]with Na(O(2)CPh) yields the T-shaped cluster [V(3)(O)Cl(3)(O(2)CPh)(2)(bipy)(2)(OEt)(2)], magnetic studies of which show strong intramolecular antiferromagnetic coupling giving a well isolated S = 1 ground state. Solvothermal treatment of 1 with triols yields a series of polymetallic clusters [V(4)Cl(6)(thme)(2)(bipy)(3)], [V(3)Cl(4)(Hcht)(2)(bipy)(2)]Cl and [V(8)(OH)(2)Cl(4)(cht)(4)(O(2)CPh)(6)(bipy)(2)], structurally related to previously reported {M(4)} centred triangles. Magnetic studies of this series reveal very weak intramolecular antiferromagnetic exchange and very strong local zero-field splitting effects.

Ferromagnetic Ni(II) discs.

A family of planar disc-like hexa-, octa- and decametallic Ni(II) complexes exhibit dominant ferromagnetic exchange. The deca- and octametallic clusters [Ni(II) (10)(tmp)(2)(N(3))(8)(acac)(6)(MeOH)(6)] (1, H(3)tmp=1,1,1-tris(hydroxymethyl)propane; acac=acetylacetonate) and [Ni(II) (8)(thme)(2)(O(2)CPh)(4)(Cl)(6)(MeCN)(6)(H(2)O)(2)] (2, H(3)thme=1,1,1-tris(hydroxymethyl)ethane) represent rare examples of Ni(II)-based single-molecule magnets, and [Ni(II) (10)] (1) possesses the largest barrier to magnetisation reversal of any Ni(II) single-molecule magnet to date.

Homonuclear and heteronuclear complexes of a four-armed octadentate ligand: synthetic control based on matching ligand denticity with metal ion coordination preferences.

The octadentate ligand 1,2,4,5-tetrakis-[3-(2-pyridyl)pyrazol-1-yl]benzene (L), with four bidentate arms radiating from a central phenyl ring, combines with 6-coordinate and 4-coordinate metal ions (as their tetrafluoroborate salts) in different ways according to the coordination number preferences of the metal ions. The four bidentate arms are not ideally matched to the requirement of octahedral metal ions, such that complexes with Cd(ii), such as tetranuclear [Cd(4)L(2)(micro-F)(2)(solv)(4)](BF(4))(6) (solv = MeOH or MeCN), and Co(ii)/Ni(ii), such as hexanuclear [Co(6)L(4)(micro-F)(2)][BF(4)](10), require monodentate ancillary ligands (solvent molecules or fluoride ions) to provide coordinative saturation. In contrast, a mixture of octahedral [M = Co(ii) or Ni(ii)] and tetrahedral [Ag(i)] metal ions reacts with L to afford the simpler trinuclear heterometallic complexes [Co(2)AgL(2)](BF(4))(5) in which the requirements of the metal ions (for 3 + 3 + 2 bidentate arms) are matched by two four-armed ligands. The maximum site occupancy principle can accordingly be used to direct self-assembly of heterometallic complexes.

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.

Mixed-ligand molecular paneling: dodecanuclear cuboctahedral coordination cages based on a combination of edge-bridging and face-capping ligands.

Reaction of a tris-bidentate ligand L(1) (which can cap one triangular face of a metal polyhedron), a bis-bidentate ligand L(2) (which can span one edge of a metal polyhedron), and a range of M(2+) ions (M = Co, Cu, Cd), which all have a preference for six coordination geometry, results in assembly of the mixed-ligand polyhedral cages [M12(mu(3)-L(1))4(mu-L(2))12](24+). When the components are combined in the correct proportions [M(2+):L(1):L(2) = 3:1:3] in MeNO2, this is the sole product. The array of 12 M(2+) cations has a cuboctahedral geometry, containing six square and eight triangular faces around a substantial central cavity; four of the eight M3 triangular faces (every alternate one) are capped by a ligand L(1), with the remaining four M3 faces having a bridging ligand L(2) along each edge in a cyclic helical array. Thus, four homochiral triangular {M3(L(2))3}(6+) helical units are connected by four additional L(1) ligands to give the mixed-ligand cuboctahedral array, a topology which could not be formed in any homoleptic complex of this type but requires the cooperation of two different types of ligand. The complex [Cd3(L(2))3(ClO4)4(MeCN)2(H2O)2](ClO4)2, a trinuclear triple helicate in which two sites at each Cd(II) are occupied by monodentate ligands (solvent or counterions), was also characterized and constitutes an incomplete fragment of the dodecanuclear cage comprising one triangular {M3(L(2))3}(6+) face which has not yet reacted with the ligands L(1). (1)H NMR and electrospray mass spectrometric studies show that the dodecanuclear cages remain intact in solution; the NMR studies show that the Cd 12 cage has four-fold (D2) symmetry, such that there are three independent Cd(II) environments, as confirmed by a (113)Cd NMR spectrum. These mixed-ligand cuboctahedral complexes reveal the potential of using combinations of face-capping and edge-bridging ligands to extend the range of accessible topologies of polyhedral coordination cages.

Synthesis, structural, and magnetic studies on a redox family of tetrametallic vanadium clusters: {VIV4}, {VIII2VIV2}, and {VIII4} butterfly complexes.

The synthesis, crystal structures, and magnetic properties are reported for a redox family of butterfly-type tetrametallic vanadium alkoxide clusters, namely [V2(VO)2(acac)4(RC{CH2O}3)2] (R=Me 1, Et 2, CH2OH 3), [V2(VO)2(acac)2(O2CPh)2(MeC{CH2O}3)2] (5), [(VO)4(MeOH)2(O2CPh)2({HOCH2}C{CH2O}3)2] (6), [V4Cl2(dbm)4(RC{CH2OH}3)2] (R=Me 7, Et 8, CH2OH 9), and [V4Cl2(dbm)4(MeO)6] (10). The cluster cores are {VIV4} (6), {VIII2VIV2} (1-5), and {VIII4} (7-10), with examples of both isomeric forms of the of the mixed-valence cores (either VIII or VIV ions forming the butterfly body). Magnetic studies reveal the clusters to be dominated by antiferromagnetic exchange interactions in each case. The magnetic exchange parameters are determined for representative examples of each core type. {VIV4} and {VIII4} have diamagnetic ground states. The two isomeric {VIII2VIV2} types are found to give rise to either an S=0 ground state with a number of low-lying excited states due to competing antiferromagnetic exchange interactions (VIII2 butterfly body) or to a well-isolated S=1 ground state (VIV2 butterfly body).

Highly reduced, polyoxo(alkoxo)vanadium(III/IV) clusters.

A family of high nuclearity oxo(alkoxo)vanadium clusters in unprecedentedly low oxidation states is reported, synthesised from simple vanadium diketonate precursors in alcohols under solvothermal conditions. Crystal structures of [V18(O)12(OH)2(H2O)4(EtO)22(O2CPh)6(acac)2] (1), [V16Na2(O)18(EtO)16(EtOH)2(O2CPh)6(HO2CPh)2]infinity (2), [V13(O)13(EtO)15(EtOH)(RCO2)3] in which R=adamantyl (3) or Ph3C (4), and [V11(O)12(EtO)13(EtOH)(Ph3CCO2)2(MePO3)] (5) are reported, revealing these to be {VIII 16VIV 2} (1), {VIII 9VIV 3VV} (3 and 4) and {VIII 3VIV 8} (5) clusters, while 2 consists of isolated {VIII 8VIV 8} clusters bridged into polymeric chains by {Na2(OEt)2} fragments. Solvothermal conditions are essential to the formation of these species, and the level of oxidation of the isolated clusters is in part controlled by the crystallisation time, with the lowest mean-oxidation-state species being isolated by direct crystallisation on controlled cooling of the reaction solutions.

A high-nuclearity, beyond "fully reduced" polyoxo(alkoxo)vanadium(III/IV) cage.

The solvothermal synthesis, crystal structure and preliminary magnetic studies are reported of the first high nuclearity V(III)-based polyoxo(alkoxo)vanadium cage, a V(III)16V(IV)2 complex.

Supertetrahedral decametallic Ni(II) clusters directed by micro(6)-tris-alkoxides.

We report the syntheses, structures and magnetic properties of two decametallic Ni(II) clusters with unprecedented supertetrahedral cores, stabilised by the (hitherto unobserved) micro(6)-coordination modes of the tris-alkoxides [MeC(CH(2)O)(3)](3-) and [C(6)H(9)O(3)](3-).