Samantha E. Brown-Xu

Mo2 Paddlewheel Complexes Functionalized with a Single MLCT, S1 Infrared-Active Carboxylate Reporter Ligand: Preparation and Studies of Ground and Photoexcited States

From the reactions between Mo2(DAniF)3pivalate (DAniF = N,N-di(p-anisyl)formamidinate) and the carboxylic acids LH, the title compounds Mo2(DAniF)3L have been prepared and characterized: compounds I (L = O2CC≡CPh), II (L = O2CC4H2SC≡CH), and III (L = O2CC6H4-p-CN). The new compounds have been characterized in their ground states by spectroscopy ((1)H NMR, ultraviolet-visible absorption, near-infrared absorption, and steady state emission), cyclic voltammetry, and density functional theory calculations. The compounds show strong metal Mo2 to ligand L δ-π* transitions in their visible spectra. The nature of the S1 (1)MLCT and T1 states has been probed by time-resolved (femtosecond and nanosecond) transient absorption and infrared spectroscopy. The observed shifts of the C≡C and C≡N vibrational modes are found to be consistent with the negative charge being localized on the single L in the S1 states, while the T1 states are (3)Mo2 δδ*. The present results are compared to earlier studies of the photoexcited states of trans-Mo2(2,4,6-triisopropylbenzoate)2L2 compounds that have been assigned as either localized or delocalized.

Metal-metal quadruple bonds supported by 5-ethynylthiophene-2-carboxylato ligands: preparation, molecular and electronic structures, photoexcited state dynamics, and application as molecular synthons.

From the reaction between M2(T(i)PB)4 and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M2(T(i)PB)2(ThCCH)2, where M = Mo (I) or W (II) and T(i)PB = 2,4,6-triisopropyl benzoate, have been isolated and characterized by (1)H NMR, IR, MALDI-TOF MS, UV-vis, steady-state emission, transient absorption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo2δ orbital. The HOMO of both I and II is the M2δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the M2δ-to-ThCCH (1)MLCT transition. The S1 states for I and II are (1)MLCT. The T1 state is (3)MLCT for II, but (3)MoMoδδ* for I. The TRIR spectra of the ν(C≡C) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo2(T(i)PB)2(ThCCPh)2 (III) and trans-Mo2(T(i)PB)2(ThCCAuPPh3)2 (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.

Photophysical Studies of Metal to Ligand Charge Transfer Involving Quadruply Bonded Complexes of Molybdenum and Tungsten

Photoinduced metal-to-ligand charge transfer transitions afford numerous applications in terms of photon energy harvesting. The majority of metal complexes studied to date involve diamagnetic systems of d(6), d(8), and d(10) transition metals. These typically have very short-lived, ∼100 fs, singlet metal to ligand charge transfer ((1)MLCT) states that undergo intersystem crossing to triplet metal to ligand charge transfer ((3)MLCT) states that are longer lived and are responsible for much of the photophysical studies. In contrast, the metal-metal quadruply bonded complexes of molybdenum and tungsten supported by carboxylate, O2CR, and related amidinate ligands (RN)2C(R) have relatively long-lived (1)MLCT states arising from M2δ to Lπ* transitions. These have lifetimes in the range 1-20 ps prior to intersystem crossing to T1 states that may be (3)MLCT or (3)MMδδ* with lifetimes of 1-100 ns and 1-100 μs, respectively. The M2 quadruply bonded complexes take the form M2L4 or M2L4-nLn where n = 1-3. Thus, in their photoexcited MLCT states, these compounds pose the question of how the charge resides on the ligands. This Account reviews the current knowledge of how charge is positioned with time in S1 and T1 states with the aid of active IR reported groups located on the ligands, for example, C≡X multiple bonds (X = C, N, or O). Several examples of localized and delocalized charge distributions are noted along with kinetic barriers to the interconversion of MLCT and δδ* states. On the 50th anniversary of the recognition of the MM quadruple bond, these complexes are revealing some remarkable features in the study of the photophysical properties of metal-ligand charge transfer states.

MM quadruple bonds supported by cyanoacrylate ligands. Extending photon harvesting into the near infrared and studies of the MLCT states

The compounds trans-M2(TiPB)2(L)2 and trans-M2(TiPB)2(L′)2 have been prepared from the reactions between M2(TiPB)4 (TiPB = 2,4,6-triisopropylbenzoate, M = Mo or W) and LH or L′H (∼2 equiv.), respectively, where L = O2CC(CN)CH–C6H4–NPh2 and L′ = O2CC(CN)CH–C4H3S–C6H4–NPh2. These cyanoacrylate ligands promote intense M2δ to L or L′ π*-transitions that span the range 550–1100 nm. The two molybdenum complexes have been characterized by single crystal X-ray studies that reveal the extensive L–M2222–L or L′–M2222–L′ M2δ–ligand π-conjugation. The new compounds have been characterized by electronic structure calculations employing density functional theory (DFT) and time-dependent-DFT, cyclic voltammetry, electronic absorption and steady state emission spectroscopy and femtosecond (fs) and nanosecond (ns) time resolved transient absorption (TA) and fs time-resolved infrared spectroscopy (TRIR). The latter allows the determination of the S1 states as 1MLCT that are delocalized over both L and L′. For molybdenum the T1 states are 3MoMoδδ* whereas for tungsten they are 3MLCT.

Electronic Structure and Excited-State Dynamics of the Molecular Triads: trans-M2(TiPB)2[O2CC6H5-η6-Cr(CO)3]2, Where M = Mo or W, and TiPB = 2,4,6-triisopropylbenzoate

From the reactions between M(2)(T(i)PB)(4) and HO(2)CC(6)H(5)-η(6)-Cr(CO)(3) (2 equiv), the title compounds trans-M(2)(T(i)PB)(2)[O(2)CC(6)H(5)-η(6)-Cr(CO)(3)](2), where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate have been prepared and characterized. Compound I (M = Mo) was characterized by a single crystal X-ray structural determination which revealed a centrosymmetric MoMo quadruply bonded molecule. Compound I is red and the tungsten complex II is blue as a result of intense metal-to-ligand charge transfer (MLCT), which is principally M(2)δ to benzoate π* with some chromium t(2g) participation, according to calculations employing density functional theory. Compound I shows dual emission from S(1) and T(1) states that are assigned (1)MLCT and (3) MoMoδδ*, respectively. Both complexes have been studied by time-resolved infrared spectroscopy (TRIR) in the region of the carbonyl stretching frequency. Compound II displays a shift of ν(CO) to lower energy in both the (1)MLCT and (3)MLCT states in THF, while I in CH(2)Cl(2) shows ν(CO) bands shifted to both higher and lower energy. We attribute the shift to higher energy seen for I to a Cr t(2g) to benzoate π* transition which mixes with the Mo(2)δ to benzoate charge transfer upon excitation at 514 nm. In THF compound I undergoes a reversible photodissociation, potentially due to CO loss. Based on the TRIR of the carbonyl vibrations, it is proposed that the MLCT states are delocalized over both benzoate Cr(CO)(3) groups, as supported by calculations.

Molybdenum–molybdenum quadruple bonds supported by 9,10-anthraquinone carboxylate ligands. Molecular, electronic, ground state and unusual photoexcited state properties

The preparation of trans-Mo2(TiPB)2(O2C-2-AnQ)2, I, and Mo2(DAniF)3(O2C-2-AnQ), II, where, O2C-2-AnQ = 9,10-anthraquinone-2-carboxylate, TiPB = 2,4,6-triisopropylbenzoate and DAniF = N,N′-di-(p-anisyl)formamidinate are described together with electronic structure calculations employing density functional theory on model compounds. Compounds I and II have been further characterized by 1H NMR spectroscopy, MALDI-TOF mass spectrometry, electrochemical studies, UV-vis-NIR and steady state emission spectroscopy, time-resolved (fs and ns) transient absorption and infrared spectroscopy. Compound II has been structurally characterized by a single crystal X-ray crystallography study. Both compounds show metal-to-ligand charge transfer transitions, 1MLCT, corresponding to Mo2 δ to L π* (L = O2C-2-AnQ) and have S1 states with relatively long lifetimes (τI ∼ 8 ps; τII ∼ 2 ps). Femtosecond time-resolved IR studies, fs-TRIR indicate that for I, despite its relatively small HOMO–LUMO gap, and II the negative charge in the S1 state resides on one anthraquinone carboxylate ligand. For II, fs-TRIR spectroscopy detects the Mo23δδ* state with τ ∼ 100 ns despite the presence of a lower energy 3MLCT state. This, together with earlier findings indicates a significant kinetic barrier to the interconversion of MLCT and Mo2 δδ* states.

Concerning the ground state and S1 and T1 photoexcited states of the homoleptic quadruply bonded complexes Mo2(O2CC6H4-p-X)4, where X = C≡C-H or C≡N.

The preparation of the homoleptic MM quadruply bonded complexes Mo2(O2CC6H4-p-X)4, where X = C≡C-H (I) or C≡N (II), is reported along with the solution characterization data and electronic structure calculations employing density functional theory. The compounds are colored orange (I) and red (II) due to the metal-to-ligand charge transfer involving the HOMO, Mo2δ, and LUMO, which is a ligand-based π* combination. Studies of the S1 state, (1)MLCT, by femtosecond time-resolved infrared spectroscopy indicate that the negative charge is distributed principally over two trans ligands. The T1 states are (3)MoMoδδ* as determined by NIR emission and nanosecond transient absorption.

Coordination of N,N-chelated Re(CO)3Cl units across a Mo2 quadruple bond: synthesis, characterization, and photophysical properties of a Re-Mo2-Re triad and its component pieces.

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II and III, where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.

Modulating the M2δ-to-ligand charge transfer transition by the use of diarylboron substituents.

From the reactions between the quadruply bonded complexes M2(T(i)PB)4, where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate, and the carboxylic acids HOOC-C6H4-4-B(mesityl)2, LH (2 equivalents) the complexes trans-M2(T(i)PB)2L2 have been prepared. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, UV-Vis-NIR and steady-state emission spectroscopy, time-resolved transient absorption spectroscopy and cyclic voltammetry. These results are compared with the related properties of the benzoates, M2(T(i)PB)2(O2CPh)2 (prepared similarly) and with DFT calculations on model compounds where formate substitutes for T(i)PB. The new compounds M2(T(i)PB)2L2 are intensely colored in toluene or THF solutions: red (M = Mo) and green (M = W) and the introduction of the p-B(mesityl)2 group notably shifts these metal to ligand charge transfer transitions to lower energy in comparison to the benzoate complexes M2(T(i)PB)2(O2C-C6H5)2. Upon the addition of fluoride ions these intense absorptions are shifted to much higher energy in a reversible manner for M = Mo.

Photophysical properties of cis-Mo2 quadruply bonded complexes and observation of photoinduced electron transfer to titanium dioxide.

The compounds cis-Mo2(DAniF)2(L)2 have been prepared, where DAniF = (N,N)-p-dianisyl formamidinate and L = thienyl-2-carboxylate (Th), 2,2-bithienyl-5-carboxylate (BTh), and 2,2:5,5″-terthienyl-5-carboxylate (TTh). The compounds have been characterized by proton nuclear magnetic resonance ((1)H NMR), ultraviolet-visible (UV-vis) absorption and emission, differential pulse voltammetry, and time-resolved transient absorption and infrared (IR) spectroscopy. An X-ray crystal structure was obtained for the thienyl complex. The related salt [(n)Bu4N]2[Mo2(DAniF)2(TTh-CO2)2], where TTh-CO2 = 2,2:5,2″-terthienyl-5,5″-dicarboxylate, has also been prepared and employed in the attachment of the complex to TiO2 nanoparticles. The latter have been characterized by ground-state Fourier transform infrared spectroscopy (FTIR) and femtosecond time-resolved IR spectroscopy. The time-resolved data provide evidence for sub-picosecond charge injection from the Mo2 center to the semiconducting oxide particle.