Nathan J. Patmore

Silver phosphanes partnered with carborane monoanions: Synthesis, structures and use as highly active lewis acid catalysts in a hetero-Diels-Alder reaction

Four Lewis acidic silver phosphane complexes partnered with [1-closo-CB(11)H(12)](-) and [1-closo-CB(11)H(6)Br(6)](-) have been synthesised and studied by solution NMR and solid-state X-ray diffraction techniques. In the complex [Ag(PPh(3))(CB(11)H(12))] (1), the silver is coordinated with the carborane by two stronger 3c-2e B-H-Ag bonds, one weaker B-H-Ag interaction and a very weak Ag.C(arene) contact in the solid state. In solution, the carborane remains closely connected with the [Ag(PPh(3))](+) fragment, as evidenced by (11)B chemical shifts. Complex 2 [Ag(PPh(3))(2)(CB(11)H(12))](2) adopts a dimeric motif in the solid state, each carborane bridging two Ag centres. In solution at low temperature, two distinct complexes are observed that are suggested to be monomeric [Ag(PPh(3))(2)][CB(11)H(12)] and dimeric [Ag(PPh(3))(2)(CB(11)H(12))](2). With the more weakly coordinating anion [CB(11)H(6)Br(6)](-) and one phosphane, complex 3 [Ag(PPh(3))(CB(11)H(6)Br(6))] is isolated. Complex 4, [Ag(PPh(3))(2)(CB(11)H(6)Br(6))], has been characterised spectroscopically. All of the complexes have been assessed as Lewis acids in the hetero-Diels-Alder reaction of N-benzylideneaniline with Danishefskys diene. Exceptionally low catalyst loadings for this Lewis acid catalysed reaction are required (0.1 mol %) coupled with turnover frequencies of 4000 h(-1) (quantitative conversion to product after 15 minutes using 3 at room temperature). Moreover, the reaction does not occur in rigorously dry solvent as addition of a substoichiometric amount of water (50 mol %) is necessary for turnover of the catalyst. It is suggested that a Lewis assisted Brønsted acid is formed between the water and the silver. The effect of changing the counterion to [BF(4)](-), [OTf](-) and [ClO(4)](-) has also been studied. Significant decreases in reaction rate and final product yield are observed on changing the anion from [CB(11)H(6)Br(6)](-), thus demonstrating the utility of weakly coordinating carborane anions in organic synthesis.

[(PPh3)Ag(HCB11Me11)]: A Complex with Intermolecular Ag⋅⋅⋅H3C Interactions

Both in the solid state and in solution, the complex [(PPh3)Ag(HCB11Me11)] displays significant intermolecular Ag⋅⋅⋅H3C interactions. This complex serves as a model for the interaction between a d10 metal center and an alkane.

Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)4 (MM ) Mo2, MoW, and W2): Preparation and Photophysical Properties

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

Oxalate Bridged Triangles Incorporating Mo24+ Units. Electronic Structure and Bonding

The reactions between [Mo(2)L(2)(CH(3)CN)(6)][BF(4)](2) compounds and [Bu(n)(4)N](2)[O(2)CCO(2)] in CH(3)CN are shown to proceed under kinetic control to the formation of a mixture of molecular triangles and squares. The molecular triangles [L(2)Mo(2)(O(2)CCO(2))](3) I (L = DPhF, PhNCHNPh) and II (L = DAniF, p-MeO-C(6)H(4)NCHNC(6)H(4)-p-OMe) are the major products, and when 0.75 equivalents of [Bu(n)(4)N](2)[O(2)CCO(2)] is employed, they are formed to the exclusion of the square. The molecular structure of II is reported based on a single crystal X-ray determination. The molecular triangles do not enter into an equilibrium with their molecular square counterparts in CH(2)Cl(2), in contrast to their perfluoroterephthalate bridged counterparts. The compounds I and II are orange and have a strong electronic transition at lambda(max) approximately 460 nm assignable to metal-to-ligand charge transfer ((1)MLCT) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO(2))(2)Mo(2)(O(2)CCO(2))](3) and [(HNCHNH)(2)Mo(2)(O(2)CCO(2))](3) have been carried out and indicate the frontier occupied molecular orbitals are Mo(2) delta combinations e(4)a(2), and the lowest unoccupied are bridge pi* for the formamidinates and delta* for formates as ancillary ligands. Compounds I and II show quasi-reversible oxidation waves in their cyclic voltammograms and oxidation of II in 2-methyl-THF by reaction with AgPF(6) (1 equivalent) leads to a metal centered EPR signal, g approximately 1.95. The electronic absorption spectrum shows a low-energy broad band centered at 6418 cm(-1), which is assigned to an intervalence charge transfer (IVCT) band of a class III mixed valence ion.

Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands

The reaction between M(2)(TiPB)(4) (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M(2)(TiPB)(2)(6-carboethoxy-2-azulenecarboxylate)(2). Compound (M = Mo) is blue and compound (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate that the two azulene carboxylate pi systems are coupled by their interactions with the M(2)delta orbitals. Their intense colors arise from M(2)delta to azulene pi* electronic transitions. While compound exhibits weak emission at approximately 900 nm, no emission has been detected for . Both and have been studied by fs and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of in the solid state confirmed the paddlewheel nature of its W(2)(O(2)C)(4) core and the trans orientation of the ligands.

Electronic coupling in 1,4-(COS)2C6H4 linked MM quadruple bonds (M = Mo, W): the influence of S for O substitution

Electronic structure calculations employing density functional theory on the compounds [(HCO2)3M2]2(mu-X-C6H4-X) where M = Mo and W and -X = -CO2, -COS and -CS2 reveal that the successive substitution of oxygen by sulfur leads to enhanced electronic coupling as evidenced by the increased energy separation of the metal delta orbital combinations which comprise the HOMO and HOMO-1. This enhanced coupling arises principally from a lowering of the LUMO of the X-C6H4-X bridge which, in turn, increases mixing with the in-phase combination of the M2 delta orbitals. The compounds [(Bu(t)CO2)3M2]2(mu-SOC-C6H4-COS), where M = Mo and W, have been prepared from the reactions between M2(O2CBu(t))4 and the thiocarboxylic acid 1,4-(COSH)2C6H4 in toluene and the observed spectroscopic and electrochemical data indicate stronger electronic coupling of the M2 centers in comparison to the closely related terephthalate compounds.

Concerning the molecular and electronic structure of a tungsten-tungsten quadruply bonded complex supported by two 6-Carboethoxy-2-carboxylatoazulene ligands

The preparation and molecular structure of a W2(4+)-quadruply bonded complex is reported wherein two mutually trans azulene-2-carboxylato ligands are shown to be strongly coupled by ligand pi-M2delta-ligand pi conjugation.

2,5-Dianilinoterephthalate bridged MM quadruply bonded complexes of molybdenum and tungsten

From the reactions between 2,5-dianilinoterephthalic acid and M2(O2CBut)4 in toluene the dicarboxylate bridged complexes [(ButCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, (M=Mo) and (M=W) have been isolated. The compounds are air sensitive, sparingly soluble in aromatic hydrocarbons but appreciably soluble in tetrahydrofuran. Electronic structure calculations employing density functional theory on the model compounds [(HCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, indicate that the ground state structure contains a planar bridge and that for molybdenum the HOMO is a bridge based molecular orbital. However, the compounds show reversible oxidation waves (CV and DPV) that for both M=Mo and W are metal based oxidations. Furthermore, the cations + and + are shown to be valence trapped and fully delocalized respectively. The magnitude of the electronic coupling of the two M2 centers, Hab, can be estimated as 383 cm-1 for + and 1500 cm-1 for + based on the corresponding low energy IVCT or charge resonance bands.

Photophysical properties of metal complexes

This review summarises developments in the photophysical properties of metal complexes and their polynuclear assemblies reported in the literature for 2008.

Transition metal complexes of the weakly coordinating carborane anion [CB11H12]−: the first isolation and structural characterisation of an intermediate in a silver salt metathesis reaction

Reaction between CpMo(CO)3I and Ag[CB11H12] eventually affords CpMo(CO)3(η1-CB11H12), via the dimeric compound [CpMo(CO)3I·Ag{CB11H12}] 2 which represents the first structurally characterised intermediate in a silver salt metathesis reaction.