Carly R. Reed

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.

Photophysical Studies of trans-Bis(phenylethynyldiisopropylamidinato)bis(acetato)dimetal Complexes Involving MM Quadruple Bonds Where M = Mo or W

The title compounds trans-M(2)(O(2)CMe)(2)[C((i)PrN)(2)C≡C-Ph](2), I (M = Mo) and II (M = W), show electronic absorptions in the visible region of the spectrum assignable to (1)MLCT [M(2)δ to phenylethynylamidinate π*]. These compounds show dual emission from S(1) and T(1) states. For both I and II, S(1) is (1)MLCT, but for I the T(1) state is shown to be MMδδ* while for II T(1) is (3)MLCT. The lifetimes of the S(1) and T(1) states have been determined by femtosecond and nanosecond transient absorption spectroscopy: for I S(1) ∼ 20 ps and T(1) ∼ 100 μs and for II S(1) ∼ 6 ps and T(1) ∼ 5 μs. From solvent dependence of the absorption and emission spectra, we suggest that the S(1) states are localized on one amidinate ligand though the initial absorption is to a delocalized state.

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.

Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M[quadruple bond]M): a comparison of their chemical and photophysical properties.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO(2)H, and 2-selenophene carboxylic acid, SpCO(2)H in toluene, the new compounds trans-M(2)(T(i)PB)(2)(O(2)CFu)(2) (1a M = Mo, 2a M = W) and trans-M(2)(T(i)PB)(2)(O(2)CSp)(2) (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by (1)H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo(2)(T(i)PB)(2)(O(2)CSp)(2) (1b) has been characterized by single crystal X-ray crystallography. These data are compared with those previously reported for related 2-thiophene carboxylate derivatives: M(2)(T(i)PB)(2)(O(2)CTh)(2). The physico-chemical data correlate well with electronic structure calculations performed on model compounds. All compounds have detectible S(1) photoexcited states with lifetimes that vary from ∼5 ps to < 1 ps. The molybdenum compounds have T(1) states with microsecond lifetimes that are assigned as MMδδ* whereas the T(1) states for tungsten are (3)MLCT with lifetimes on the order of nanoseconds. In all cases, shorter lifetimes were seen in complexes containing heavier atoms.