Regulating Access to Active Sites via Hydrogen Bonding and Cation-Dipole Interactions: A Dual Cofactor Approach to Switchable Catalysis.

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site.

A Comparison between Triphenylmethyl and Triphenylsilyl Spirobifluorenyl Hosts: Synthesis, Photophysics and Performance in Phosphorescent Organic Light-Emitting Diodes.

This study presents the synthesis and characterization of two spirobifluorenyl derivatives substituted with either triphenylmethyl () or triphenylsilyl () moieties for use as host materials in phosphorescent organic light-emitting diodes (PHOLED). Both molecules have similar high triplet energies and large energy gaps. Blue Ir(tpz) and green Ir(ppy) phosphorescent devices were fabricated using these materials as hosts.

Pincer-supported metal/main-group bonds as platforms for cooperative transformations.

Electron-rich late metals and electropositive main-group elements (metals and metalloids) can be combined to provide an ambiphilic façade for exploring metal-ligand cooperation, yet the instability of the metal/main-group bond frequently limits the study and application of such units. Incorporating main-group donors into pincer frameworks, where they are stabilized and held in proximity to the transition-metal partner, can allow discovery of new modes of reactivity and incorporation into catalytic processes. This Perspective summarizes common modes of cooperativity that have been demonstrated for pincer frameworks featuring metal/main-group bonds, highlighting similarities among boron, aluminium, and silicon donors and identifying directions for further development.

Crystal structures of phosphine-supported (η-cyclo-penta-dien-yl)molybdenum(II) propionyl complexes.

Three cyclo-penta-dienylmolybdenum(II) propionyl complexes featuring tri-aryl-phosphine ligands with different substituents, namely, dicarbon-yl(η-cyclo-penta-dien-yl)propion-yl(tri-phenyl-phosphane-κ)molybdenum(II), [Mo(CH)(CHO)(CHP)(CO)], (), dicarbon-yl(η-cyclo-penta-dien-yl)propion-yl[tris-(4-fluoro-phen-yl)phosphane-κ]molybdenum(II), [Mo(CH)(CHO)(CHFP)(CO)], (), and dicarbon-yl(η-cyclo-penta-dien-yl)propion-yl[tris-(4-meth-oxy-phen-yl)phosphane-κ]molybdenum(II) dichloromethane solvate, [Mo(CH)(CHO)(CHOP)(CO)]·CHCl, (), have been prepared from the corresponding ethyl complexes phosphine-induced migratory insertion. These complexes exhibit four-legged piano-stool geom-etries with mol-ecular structures quite similar to each other and to related acetyl complexes. The extended structures of the three complexes differ somewhat, with the substituent of the tri-aryl-phosphine of () (fluoro) or () (meth-oxy) engaging in non-classical C-H⋯F or C-H⋯O hydrogen-bonding inter-actions.

Bimetallic, Silylene-Mediated Multielectron Reductions of Carbon Dioxide and Ethylene.

A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two-electron transformations by one-electron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO to afford a bimetallic siloxane, featuring two Co centers, with liberation of CO; reaction of the silylene complex with ethylene yields a similar bimetallic product with an ethylene bridge. Experimental and computational studies suggest a plausible mechanism proceeding by [2+2] cycloaddition to the silylene complex, which is quite sensitive to the steric environment.

Crystal structures of -acetyl-dicarbon-yl(η-cyclo-penta-dien-yl)(1,3,5-tri-aza-7-phosphaadamantane)molybdenum(II) and -acetyl-di-carbon-yl(η-cyclo-penta-dien-yl)(3,7-diacetyl-1,3,7-tri-aza-5-phosphabi-cyclo-[3.3.1]nona-ne)molyb-den-um(II).

The title compounds, [Mo(CH)(COCH)(CHNP)(CO)], (1), and [Mo(CH)(COCH)(CHNOP)(CH)))(CO)], (2), have been prepared by phosphine-induced migratory insertion from [Mo(CH)(CO)(CH)]. The mol-ecular structures of these complexes are quite similar, exhibiting a four-legged piano-stool geometry with -disposed carbonyl ligands. The extended structures of complexes (1) and (2) differ substanti-ally.

Silylene-assisted hydride transfer to CO and CS at a [PSi]Ru pincer-type complex.

The synthesis and characterization of base-stabilized and base-free pincer-type bis(phosphine)/silylene [PSi]Ru complexes are reported. The base-free complex readily reduces CO and CSvia silylene-assisted hydride transfer, affording structurally distinct products with silicon-to-ruthenium formate and dithioformate bridges.

Examining the role of Rh/Si cooperation in alkene hydrogenation by a pincer-type [P2Si]Rh complex.

A bis(phosphine)/triflatosilyl pincer-type Rh(i) complex can reversibly store one equivalent of H2 across the Si-Rh bond upon triflate migration from silicon to rhodium. The triflatosilyl complex serves as an effective precatalyst for norbornene hydrogenation, but Si-OTf bond cleavage is not implicated in the major catalytic pathway. The combined findings suggest possible strategies for M/Si cooperation in catalytic processes.

Chalcogen extrusion from heteroallenes and carbon monoxide by a three-coordinate rh(i) disilylamide.

We report the reactions of several heteroallenes (carbon disulfide, carbonyl sulfide, and phenyl isocyanate) and carbon monoxide with a three-coordinate, bis(phosphine)-supported Rh(I) disilylamide (1). Carbon disulfide reacts with 1 to afford a silyltrithiocarbonate complex similar to an intermediate previously invoked in the deoxygenation of CO2 by 1, and prolonged heating affords a structurally unusual μ-κ(2)(S,S'):κ(2)(S,S')-trithiocarbonate dimer. Carbonyl sulfide reacts with 1 to afford a structurally unique Rh(SCNCS) metallacycle derived from two insertions of OCS and N-to-O silyl-group migrations.

Crystal structures of trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(di-methyl-phenyl-phosphane)molybdenum(II) and trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(ethyl-diphenyl-phosphane)molybdenum(II).

The title compounds, [Mo(C5H5)(COCH3)P(CH3)2(C6H5)(CO)2], (1), and [Mo(C5H5)(COCH3)P(C2H5)(C6H5)2)(CO)2], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C5H5)(CO)3(CH3)]. Both complex mol-ecules exhibit a four-legged piano-stool geometry with trans-disposed carbonyl ligands along with Mo-P bond lengths and C-Mo-P angles that reflect the relative steric pressure of the respective phosphine ligand. The structure of compound (1) exhibits a layered arrangement parallel to (100).

Control of emission colour with N-heterocyclic carbene (NHC) ligands in phosphorescent three-coordinate Cu(I) complexes.

A series of three phosphorescent mononuclear (NHC)-Cu(I) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.

Photophysical properties of cyclometalated Pt(II) complexes: counterintuitive blue shift in emission with an expanded ligand π system.

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors.

trans-Acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)[tris-(furan-2-yl)phosphane-κP]molybdenum(II).

The title compound, [Mo(C5H5)(C2H3O)(C12H9O3P)(CO)2], was prepared by reaction of [Mo(C5H5)(CO)3(CH3)] with tris-(furan-2-yl)phosphane. The Mo(II) atom exhibits a four-legged piano-stool coordination geometry with the acetyl and phosphine ligands trans to each other. The O atom of the acetyl ligand points down, away from the Cp ring.

Metal-ligand multiple bonds as frustrated Lewis pairs for C-H functionalization.

The concept of frustrated Lewis pairs (FLPs) has received considerable attention of late, and numerous reports have demonstrated the power of non- or weakly interacting Lewis acid-base pairs for the cooperative activation of small molecules. Although most studies have focused on the use of organic or main-group FLPs that utilize steric encumbrance to prevent adduct formation, a related strategy can be envisioned for both organic and inorganic complexes, in which "electronic frustration" engenders reactivity consistent with both nucleophilic (basic) and electrophilic (acidic) character. Here we propose that such a description is consistent with the behavior of many coordinatively unsaturated transition-metal species featuring metal-ligand multiple bonds, and we further demonstrate that the resultant reactivity may be a powerful tool for the functionalization of C-H and E-H bonds.

trans-Acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(methyl-diphenyl-phosphane)molybdenum(II).

The title compound, [Mo(C(5)H(5))(C(2)H(3)O)(C(13)H(13)P)(CO)(2)], was prepared by reaction of [Mo(CH(3))(C(5)H(5))(CO)(3)] with methyl-diphenyl-phosphane. The Mo(II) atom exhibits a four-legged piano-stool coordination geometry with the acetyl and phosphane ligands trans to each other. There are several inter-molecular C-H⋯O hydrogen-bonding inter-actions involving carbonyl and acetyl O atoms as acceptors.

Efficient singlet fission discovered in a disordered acene film.

Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission.

Cu4I4 clusters supported by P^N-type ligands: new structures with tunable emission colors.

A series of Cu(4)I(4) clusters (1-5) supported by two P^N-type ligands 2-[(diRphosphino)methyl]pyridine (1, R = phenyl; 2, R = cyclohexyl; 3, R = tert-butyl; 4, R = iso-propyl; 5, R = ethyl) have been synthesized. Single crystal X-ray analyses show that all five clusters adopt a rare "octahedral" geometry. The central core of the cluster consists of the copper atoms arranged in a parallelogram with μ(4)-iodides above and below the copper plane.

Symmetry-breaking intramolecular charge transfer in the excited state of meso-linked BODIPY dyads.

We report the synthesis and characterization of symmetric BODIPY dyads where the chromophores are attached at the meso position, using either a phenylene bridge or direct linkage. Both molecules undergo symmetry-breaking intramolecular charge transfer in the excited state, and the directly linked dyad serves as a visible-light-absorbing analogue of 9,9'-bianthryl.

Substituted 1,3-bis(imino)isoindole diols: a new class of proton transfer dyes.

A new class of excited-state intramolecular proton transfer (ESIPT) dyes based on a 1,3-bis(imino)isoindole diol motif has been prepared. These molecules exhibit orange emission (∼600 nm) with a large apparent Stokes shift (>6000 cm(-1)) and quantum efficiencies up to 45%. Selective modification of the substituents can be used to shift the equilibrium between the enol and keto forms of the molecule in both the ground and excited states.

A codeposition route to CuI-pyridine coordination complexes for organic light-emitting diodes.

We demonstrate a new approach for utilizing CuI coordination complexes as emissive layers in organic light-emitting diodes that involves in situ codeposition of CuI and 3,5-bis(carbazol-9-yl)pyridine (mCPy). With a simple three-layer device structure, pure green electroluminescence at 530 nm from a Cu(I) complex was observed. A maximum luminance and external quantum efficiency (EQE) of 9700 cd/m(2) and 4.

Singlet and triplet excitation management in a bichromophoric near-infrared-phosphorescent BODIPY-benzoporphyrin platinum complex.

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k(ST)((1)BDP→(1)Por) = 7.

Synthesis and characterization of phosphorescent three-coordinate Cu(I)-NHC complexes.

Cationic and neutral monomeric three-coordinate phosphorescent Cu(I) complexes were synthesized and characterized by XRD analysis, electrochemistry and photophysical studies in different environments. DFT calculations have aided the assignment of the electronic structure and excited state behavior of these complexes.

Efficient dipyrrin-centered phosphorescence at room temperature from bis-cyclometalated iridium(III) dipyrrinato complexes.

A series of seven dipyrrin-based bis-cyclometalated Ir(III) complexes have been synthesized and characterized. All complexes display a single, irreversible oxidation wave and at least one reversible reduction wave. The electrochemical properties were found to be dominated by dipyrrin centered processes.

Late metal carbene complexes generated by multiple C-H activations: examining the continuum of M=C bond reactivity.

Unactivated C(sp(3))-H bonds are ubiquitous in organic chemicals and hydrocarbon feedstocks. However, these resources remain largely untapped, and the development of efficient homogeneous methods for hydrocarbon functionalization by C-H activation is an attractive and unresolved challenge for synthetic chemists. Transition-metal catalysis offers an attractive possible means for achieving selective, catalytic C-H functionalization given the thermodynamically favorable nature of many desirable partial oxidation schemes and the propensity of transition-metal complexes to cleave C-H bonds.