Alkylpyrrolidiniumtrialkoxysilyl iodides as organic iodide sources for dye-sensitised solar cells.

Two new alkylpyrrolidiniumtriethoxysilyl iodides have been developed as iodide sources for DSSCs; the compound with an undecyl spacer between the siloxane and the pyrrolidinium moieties furnished higher open circuit voltages than the propyl analogue and higher efficiencies at low light intensity.

Bis(acetylacetonato)ruthenium(II) complexes containing alkynyldiphenylphosphines. Formation and redox behaviour of [Ru(acac)2(Ph2PC triple bond CR)2] (R=H, Me, Ph) complexes and the binuclear complex cis-[{Ru(acac)2}2(micro-Ph2PC triple bond CPPh2)}2].

Two equivalents of Ph(2)PC triple bond CR (R=H, Me, Ph) react with thf solutions of cis-[Ru(acac)(2)(eta(2)-alkene)(2)] (acac=acetylacetonato; alkene=C(2)H(4), 1; C(8)H(14), 2) at room temperature to yield the orange, air-stable compounds trans-[Ru(acac)(2)(Ph(2)PC triple bond CR)(2)] (R=H, trans-3; Me=trans-4; Ph, trans-5) in isolated yields of 60-98%. In refluxing chlorobenzene, trans-4 and trans-5 are converted into the yellow, air-stable compounds cis-[Ru(acac)(2)(Ph(2)PC triple bond CR)(2)] (R=Me, cis-4; Ph, cis-5), isolated in yields of ca. 65%. From the reaction of two equivalents of Ph(2)PC triple bond CPPh(2) with a thf solution of 2 an almost insoluble orange solid is formed, which is believed to be trans-[Ru(acac)(2)(micro-Ph(2)PC triple bond CPPh(2))](n) (trans-6). In refluxing chlorobenzene, the latter forms the air-stable, yellow, binuclear compound cis-[{Ru(acac)(2)(micro-Ph(2)PC triple bond CPPh(2))}(2)] (cis-6). Electrochemical studies indicate that cis-4 and cis-5 are harder to oxidise by ca. 300 mV than the corresponding trans-isomers and harder to oxidise by 80-120 mV than cis-[Ru(acac)(2)L(2)] (L=PPh(3), PPh(2)Me). Electrochemical studies of cis-6 show two reversible Ru(II/III) oxidation processes separated by 300 mV, the estimated comproportionation constant (K(c)) for the equilibrium cis-6(2+) + cis6 <=> 2(cis-6(+)) being ca. 10(5). However, UV-Vis spectra of cis-6(+) and cis-6(2+), generated electrochemically at -50 degrees C, indicate that cis-6(+) is a Robin-Day Class II mixed-valence system. Addition of one equivalent of AgPF(6) to trans-3 and trans-4 forms the green air-stable complexes trans-3 x PF(6) and trans-4 x PF(6), respectively, almost quantitatively. The structures of trans-4, cis-4, trans-4 x PF(6) and cis-6 have been confirmed by X-ray crystallography.

Catalytic reduction of dinitrogen to ammonia at a single molybdenum center.

Since our discovery of the catalytic reduction of dinitrogen to ammonia at a single molybdenum center, we have embarked on a variety of studies designed to further understand this complex reaction cycle. These include studies of both individual reaction steps and of ligand variations. An important step in the reaction sequence is exchange of ammonia for dinitrogen in neutral molybdenum(III) compounds. We have found that this exchange reaction is first order in dinitrogen and relatively fast (complete in <1 h) at 1 atm of dinitrogen. Variations of the terphenyl substituents in the triamidoamine ligand demonstrate that the original ligand is not unique in its ability to yield successful catalysts. However, complexes that contain sterically less demanding ligands fail to catalyze formation of ammonia from dinitrogen; it is proposed as a consequence of a base-catalyzed decomposition of a diazenido (Mo-N=NH) intermediate.

Observation of 1MLCT and 3MLCT excited states in quadruply bonded Mo2 and W2 complexes.

The photophysical properties of the series of quadruply bonded M(2)(O(2)C-Ar)(4) [M = Mo, Ar = phenyl (ph), 1-naphthalene (1-nap), 2-naphthalene (2-nap), 9-anthracene (9-an), 1-pyrene (1-py), and 2-pyrene (2-py); M = W, Ar = ph, 2-nap] complexes were investigated. The lowest energy absorption of the complexes is attributed to a metal-to-ligand charge transfer (1)MLCT transition from the metal-based delta HOMO to the pi* O(2)C-Ar LUMO. The Mo(2)(O(2)C-Ar)(4) complexes exhibit weak short-lived emission (<10 ns) and a nonemissive, long-lived (40-76 mus) excited state detected by transient absorption spectroscopy. The short- and long-lived species are attributed to the (1)MLCT and (3)MLCT excited states, respectively, based on the large Stokes shift, vibronic progression in the low-temperature emission spectrum, and solvent dependence. Comparisons are made to the W(2)(O(2)C-Ar)(4) complexes, which are easier to oxidize and exhibit greater spin-orbit coupling than the Mo(2) systems. From the excited-state energy of the emissive (1)MLCT state and the electrochemical properties of the complexes, it is predicted that this excited state should be a powerful reducing agent. The crystal and molecular structure of Mo(2)(O(2)C-9-an)(4) is also reported together with electronic structure calculations employing density functional theory. To our knowledge, this is the first observation of MLCT excited states in quadruply bonded complexes. In addition, the photophysical properties of the present systems parallel those of organic aromatic molecules and may be viewed as metal-mediated organics. The introduction of the M(2) delta orbital in the complexes in conjugation with the organic pi-system of the ligands affords the opportunity to tune the excited-state energies and redox potentials.

New metal-organic polygons involving MM quadruple bonds: M8(O2CtBu)4(mu-SC4H2-3,4-{CO2}2)6 (M=Mo, W).

The reactions between M2(O2CtBu)4, where M=Mo or W, and thienyl-3,4-dicarboxylic acid (0.5-1.5 equiv) in toluene proceed via a series of detectable intermediates to the compounds M8(O2CtBu)4(mu-SC4H2-3,4-{CO2}2)6, which are isolated as air-sensitive yellow (M=Mo) or red (M=W) powders and show parent molecular ions in their mass spectra (MALDI). The structure of the molybdenum complex was determined by single-crystal X-ray crystallography and shown to contain an unusual M8 polygon involving four Mo2 quadruply bonded units linked via the agency of the six 3,4-thienylcarboxylate groups. The structure has crystallographically imposed S4 symmetry and may be described in terms of a highly distorted tetrahedron of Mo2 units or a bisphenoid in which two Mo2 units are linked by a thienyldicarboxylate such that intramolecular Mo2...O bonding is present, while the other thienylcarboxylate bridges merely serve to link these two [Mo2]...[Mo2] units together. The color of the compounds arises from intense M2 delta-to-thienyl pi transitions and, in THF, the complexes are redox-active and show four successive quasi-reversible oxidation waves. The [M8]+ radical cations, generated by one-electron oxidation with AgPF6, are shown to be valence-trapped (class II) by UV-vis-near-IR and electron paramagnetic resonance spectroscopy. These results are supported by the electronic structure calculations on model compounds M8(O2CH)4(mu-SC4H2-3,4-{CO}2)6 employing density functional theory that reveal only a small splitting of the M2 delta manifold via mixing with the 3,4-thienylcarboxylate pi system.

Thienyl carboxylate ligands bound to and bridging MM quadruple bonds, M = Mo or W: models for polythiophenes incorporating MM quadruple bonds.

A series of compounds of the form [M(2)L(4)] and [[((t)()BuCO(2))(3)M(2)](2)(mu-L')] have been made where M = Mo or W, L = a thienyl, bithienyl, or terthienyl carboxylate, and L' = a corresponding thienyl dicarboxylate. The electronic absorption spectra are reported and the electronic structures discussed. Intense metal-to-ligand charge transfer bands traverse the visible and near-IR regions of the electronic absorption spectrum. The compounds show reversible metal-based oxidations and quasireversible ligand-based reductions. The molecular structure of Mo(2)(O(2)C-2-Th)(4).2THF is reported, on the basis of a single crystal X-ray diffraction study. These compounds provide insight into the expected properties of related dimetalated polythiophenes incorporating MM quadruple bonds.

9,10-Anthracene dicarboxylate bridged complexes with M2 quadruply bonded dimetal units: [[M2(O2CtBu)3]2(mu-9,10-An(CO2)2)], where M = Mo or W.

From the reactions between [M2(O2CtBu)4] and 9,10-anthracenedicarboxylic acid in toluene, the dicarboxylate bridged complexes [[M2(O2CtBu)3]2(mu-9,10An(CO2)2)], have been obtained as microcrystalline yellow (M = Mo) and red (M = W) powders. The powders are soluble in THF forming intense red (M = Mo) and green (M = W) solutions. The electronic absorption spectra in 2-MeTHF have been recorded as a function of temperature (2-298 K) and show a small bathochromic shift on cooling. The electronic structures have been investigated by molecular orbital calculations employing density functional theory on the model compounds [(HCO2)3M2]2(mu-9,10-An(CO2)2) where the M4 unit is constrained to lie in a plane. These reveal a minimum energy, gas-phase structure wherein the plane of the anthracene is twisted by ca. 54 degrees with respect to its 9,10-carboxylate units for both Mo and W. The results of these calculations are correlated with the electronic absorption spectral data and the electrochemical measurements (CV and DPV) of the first and second oxidation waves. The EPR spectra of the radical cations formed by single-electron oxidation with [Cp2Fe](+)[PF6]- in a THF-CH2Cl2 solvent mixture show that the complexes are valence trapped at ambient temperature on the EPR timescale. These results are discussed in the light of recent studies of dicarboxylate-linked MM quadruple bonds.

Thienyl carboxylate ligands bound to M2 quadruple bonds involving molybdenum and tungsten. Models for dimetallated polythiophenes.

Thienyl-carboxylate and -dicarboxylate groups attached to dinuclear centers (M = Mo or W) having MM quadruple bonds show interesting electronic properties and provide insight into the probable nature of related dimetallated polythiophenes.