This critical review presents a comprehensive study of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) towards construction and synthesis of metal–organic frameworks (MOFs). We describe the geometries of 131 SBUs, their connectivity and composition. This contribution presents a comprehensive list of the wide variety of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) in the construction and synthesis of metal–organic frameworks. The SBUs discussed here were obtained from a search of molecules and extended structures archived in the Cambridge Structure Database (CSD, version 5.28, January 2007) which included only crystals containing metal carboxylate linkages (241 references).

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Secondary building units, nets and bonding in the chemistry of metal–organic frameworks

White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

White Organic Light‐Emitting Devices for Solid‐State Lighting

ion step follows the decarboxylation, which is consistent with the deuterium isotopic effects observed for thymine 7-hydroxylase which indicate that an irreversible step (or steps) occurs prior to the C-H bond breaking.395 It has also been shown for prolyl 4-hydroxylase that a substrate-derived radical is generated in the reaction, which is consistent with a rebound mechanism.437 It is important to point out that no oxygen intermediate (i.e., bridged superoxo or oxo-ferryl) has been observed for any R-KGdependent enzyme. This warrants future theoretical and experimental study. A detailed molecular mechanism has been proposed for IPNS based on spectroscopic and crystallographic studies.422 Resting IPNS/FeII is also 6C and thus relatively stable toward dioxygen. Substrate ACV binds directly to FeII IPNS through its thiolate group, providing an open coordination position at the FeII. O2 can then react to form an FeIII-superoxo intermediate. This intermediate is suggested422 to perform the first hydrogen-atom abstraction step and close the â-lactam ring, resulting in the formation of the first water molecule and generating an FeIVdO-II intermediate, which completes the second ringclosure process by hydrogen-atom abstraction forming a thiazolidine ring. Previously proposed mechanisms of ACCO involved direct binding of cosubstrate ascorbate to the iron before O2 as part of the oxygen activation process.438,439 The EPR and ESEEM studies of the NO complex of ACCO suggested a quite different molecular mechanism for ACCO.435 An FeIII-superoxo intermediate is proposed. Whether it is preceded by a 6C f 5C process with substrate binding is presently under study.440 This intermediate is thought to initiate a radical process by single hydrogen-atom abstraction or electron-coupled proton transfer (PT)ion or electron-coupled proton transfer (PT) from the bound amino group. The resulting substrate radical may undergo spontaneous conversion into products. The role of cosubstrate ascorbate is proposed to reduce the toxic peroxo byproduct to water. Alternatively, the two-electron reduction of FeIIIsuperoxo by the cosubstrate ascorbate could result in an FeIVdO-II intermediate which initiates the radical reaction.435 4. Rieske-Type Dioxygenases Biochemical Characterization. The Rieske ironsulfur center is a two iron-two sulfur cluster ([2Fe2S]) which has a 2His (on one iron), 2Cys (on the other iron) coordination environment, instead of the 4Cys present in plant ferredoxins. It plays a key role in the electron transport pathway in membranebound cytochrome complexes as well as in some dioxygenases.441 The latter are mainly comprised of two protein components: a reductase containing flavin and a ferredoxin [2Fe-2S], and a terminal oxygenase containing a Rieske [2Fe-2S] cluster and a non-heme iron active site.442 Except for the recently reported alkene monooxygenase that has a binuclear iron site in its terminal oxygenase,10 most of the Rieske-type oxygenases have a mononuclear iron site, which is believed to be the site of dioxygen activation and substrate oxygenation.442,443 The majority of the Rieske-type mononuclear non-heme oxygenases form a family of enzymes which are aromatic-ring-hydroxylating dioxygenases. These catalyze the regioand stereospecific cis-dihydroxylation of an aromatic ring using dioxygen and NAD(P)H (Table 1). Examples include benzene dioxygenase (BDO, EC 1.14.12.3),444 phthalate dioxygenase (PDO, EC 1.14.12.7),445 toluene dioxygenase (EC 1.14.12.11),446 and naphthalene 1,2-dioxygenase (NDO, EC 1.14.12.12),447 which initiate the aerobic degradation of aromatic compounds in the soil bacteria and are targets for bioengineering in bioremediation. This step is the first step in the pathway that ultimately leads to ring cleavage by the intraand extradiol dioxygenases (sections II.B.2 and II.C.1).443 Besides these bacterial dioxygenases, other Rieske-type mononuclear non-heme oxygenases include anthranilate 1,2-dioxygenase (EC 1.14.12.1),448 which deaminates and decarboxylates the substrate to produce catechol; chlorophenylacetate 3,4-dioxygenase (EC 1.14.2.13),449 which converts substrate to catechol with chloride elimination; and 4-methoxybenzoate O-demethylase (putidamonooxin),450 which catalyzes the conversion of 4-methoxybenzoic acid to 4-hydroxybenzoic acid and formaldehyde. The reductase component is usually a monomer (MW ) 12-15 kDa) and utilizes flavin to mediate ET from the two-electron donor NAD(P)H to the oneelectron acceptor [2Fe-2S] cluster and is specific to each terminal oxygenase; other electron donors do not support efficient oxygenation.442 The crystal structure of phthalate dioxygenase reductase is available.451 The terminal oxygenases are large protein aggregates (MW ) 150-200 kDa) containing either multiples of R subunits (BDO R2, PDO R4) or an equimolar combination of R and â subunits (toluene dioxygenase R2â2, NDO R3â3). The R subunits contain a Rieske [2Fe-2S] cluster and a catalytic non-heme FeII center. â subunits do not seem to be involved in the catalytic function (vide infra). Kinetics. Steady-state kinetic studies coupled with various rapid reaction studies of the partial reactions of PDO allowed Ballou et al. to propose a kinetic scheme (Scheme 15).443 On the basis of steady state 278 Chemical Reviews, 2000, Vol. 100, No. 1 Solomon et al.

Geometric and Electronic Structure/Function Correlations in Non-Heme Iron Enzymes

The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C∧NPt(O∧O),where C∧N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2‘-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O∧O is a β-diketonato ligand. Reaction of K2PtCl4 with a HC∧N ligand precursor forms the chloride-bridged dimer, C∧NPt(μ-Cl)2PtC∧N, which is cleaved with β-diketones such as acetyl acetone (acacH) and dipivaloylmethane (dpmH) to give the corresponding monomeric C∧NPt(O∧O) complex. The thpyPt(dpm) (thpy = 2-(2‘-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 A, consistent with moderate π−π interactions and no evident Pt−Pt interactions. Most of the C∧NPt(O∧O) complexes...

Synthesis and characterization of phosphorescent cyclometalated platinum complexes.

Hydrogen Production by Molecular Photocatalysis

The electronic spectrum of [Pt(tpy)Cl]^+ (tpy = 2,2':6',2"-terpyridine) is influenced dramatically by intermolecular stacking interactions in solution and in the solid state. The crystal structure of [Pt(tpy)Cl]ClO_4 (monoclinic, P2_1/c (No. 14); a = 7.085(2), b = 17.064(5), c = 26.905(8) A; β = 90.0(1) °; Z = 8) consists of discrete Pt_2 units (Pt-Pt = 3.269(1) A) arranged along an infinite tpy-π stack (spacing ~ 3.35 A). Variable-temperature and
concentration studies of the absorption and emission spectra of [Pt(tpy)Cl]^+ suggest that similar metal-metal
and ligand-ligand interactions persist in the solution phase. The high concentration, low-temperature emission
spectrum (5:5:1 ethanol:methanol:DMF) reveals a 740-nm band indicative of M-M oligomerization, a 650-nm band attributable to tpy π-π interactions, and a 470-nm band characteristic of mononuclear [Pt(tpy)Cl]^+ π-π*
emission. Concentration-dependent absorption spectra were fit to a "two-dimer" model, yielding equilibrium
constants for the formation of Pt-Pt-, and tpy-tpy-bound dimers of 1.3(1) x 10^3 and 1.0(1) x 10^3 M^(-1), respectively, in 0.1 M aqueous NaCl. The low temperature solid-state luminescence of [Pt(tpy)Cl]^+ is assigned to a ^3(MMLCT) (MMLCT = metal-metal-to-ligand charge transfer) transition. The energy of this band is highly dependent on the counterion (PF_6^-, ClO_4^- , C1^-, CF_3SO_3^-), in line with the different colors of these various salts. In contrast, the room-temperature solid-state emission spectra are more difficult to interpret. While the red perchlorate salt exhibits a relatively narrow emission band at 725 nm (red-shifted from the 77-K maximum at 695 nm), consistent
with a 3(MMLCT) transition, the orange (Cl^-, ClO_4^-, CF_3SO_3^-) and yellow (PF6^-) salts have extremely broad
room-temperature emission bands that all appear at nearly the same energy (λ_(max) ~ 640 nm). We assign this
luminescence to an eximeric intraligand transition resulting from π- π interactions and propose that the temperature dependent emissions from the orange and yellow solid materials originate from multiple electronic states.

Electronic Spectroscopy of Chloro(terpyridine)platinum(II)

The effect of linear chain structure on the electronic structure of pt(II) diimine complexes

Two types of emission behavior for Pt(II) complexes containing alpha-diimine ligands have been observed in dilute solution. If the complex also has weak field ligands such as chloride, ligand field (d-d) excited states become the lowest energy excited states. If only strong field ligands are present, a diimine 3(pi-pi/asterisk/) state becomes the lowest. In none of the cases studied did metal-to-ligand charge transfer excited state lie lowest.

Electronic spectra and photophysics of platinum(II) complexes with alpha-diimine ligands - Solid-state effects. I - Monomers and ligand pi dimers

Electronic spectra and photophysics of platinum(II) complexes with .alpha.-diimine ligands solid-state effects. 2. Metal-metal interaction in double salts and linear chains

Emission properties have been studied for a series of compounds of the formula (L_2)PtC1_2, where L_2 is N,N,N',N'-tetramethylethylenediamine, 2,2'-bipyridine (bpy), 4,4'-Me_2bpy, 5,5'-Me_2bpy, 4,4'-(t-Bu)_2bpy, 3,3'-(CH_3OCO)_2bpy, and 1,10-phenanthroline, and also for the compound Pt(bpy)I_2. Most of them exhibit orange to red luminescence from a triplet ligand-field (^3LF) state, both as solids and in glassy solution. These emissions are very broad (fwhm 2300-3400 cm^(-1) at 10 K) and structureless and are strongly Stokes-shifted from absorption. The two exceptions are the solid "red" form of Pt(bpy)Cl_2, which exhibits a relatively narrow (fwhm 1050 cm^(-1) at 10 K), vibronically structured (Δν ~ 1500 cm^-1)) red emission, and Pt(3,3'-(CH_3OCO)_2bpy)Cl_2, which exhibits a broad (fwhm 2500 cm^(-1) at 10 K) but structured (Δν ~1300 cm^(-1)) orange emission. Both of these emissions are assigned to triplet metal-to-ligand charge-transfer (^3MLCT) excited states. For the former compound, a linear-chain structure has destabilized a dσ*(d_(z^2)) level, yielding a dσ* → π*(bpy) state as the lowest energy excited state, while for the latter, the strongly electron-withdrawing substituents have stabilized a bpy π* level, yielding a dσ* → π*(bpy) state as the lowest energy excited state. The relative energies of the various types of excited states, including ligand 3_(ππ*) states, are discussed in detail. The crystal structures of Pt(5,5'-Me_2bpy)Cl_2 (monoclinic Cc, Z = 4, a = 13.413(7) A, b = 9.063(4) A, c = 12.261(9) A, 0 = 121.71(6)') and Pt(3,3'-(CH_3OCO)_2bpy)Cl_2 (triclinic P1, Z = 2, a = 7.288(2) A, b = 9.932(3) A, c = 11.881(5) A, α = 98.04(3)°, β = 103.56(3)°, γ = 106.54(3)°) are reported.

Vincent M. Miskowski

Papers

Electronic Spectroscopy of Chloro(terpyridine)platinum(II)

The effect of linear chain structure on the electronic structure of pt(II) diimine complexes

Electronic spectra and photophysics of platinum(II) complexes with alpha-diimine ligands - Solid-state effects. I - Monomers and ligand pi dimers

Electronic spectra and photophysics of platinum(II) complexes with .alpha.-diimine ligands solid-state effects. 2. Metal-metal interaction in double salts and linear chains

Electronic spectra and photophysics of platinum(II) complexes with α-diimine ligands. Mixed complexes with halide ligands