Christopher B. Durr

Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)]2+ (1; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru(tpy)(Me2bpy)(py)]2+ (2; Me2bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru(tpy)(biq)(py)]2+ (3; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH3CN (λirr = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH3)]2+ (NN = Me2bpy, biq) complex with quantum yields, Φ500, of 0.16(1) and 0.033(1) for 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of 1, Φ500 < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)]2+ (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes 1–3 are stable in the dark in both CH3CN and H2O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me2bpy in 2 and biq in 3, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in 1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of 2 and 3 to undergo ligand exchange with λirr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.

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.

[MoO(S2)2L]1– (L = picolinate or pyrimidine-2-carboxylate) Complexes as MoSx-Inspired Electrocatalysts for Hydrogen Production in Aqueous Solution

Crystalline and amorphous molybdenum sulfide (Mo-S) catalysts are leaders as earth-abundant materials for electrocatalytic hydrogen production. The development of a molecular motif inspired by the Mo-S catalytic materials and their active sites is of interest, as molecular species possess a great degree of tunable electronic properties. Furthermore, these molecular mimics may be important for providing mechanistic insights toward the hydrogen evolution reaction (HER) with Mo-S electrocatalysts. Herein is presented two water-soluble Mo-S complexes based around the [MoO(S2)2L2]1- motif. We present 1H NMR spectra that reveal (NEt4)[MoO(S2)2picolinate] (Mo-pic) is stable in a d6-DMSO solution after heating at 100 °C, in air, revealing unprecedented thermal and aerobic stability of the homogeneous electrocatalyst. Both Mo-pic and (NEt4)[MoO(S2)2pyrimidine-2-carboxylate] (Mo-pym) are shown to be homogeneous electrocatalysts for the HER. The TOF of 27-34 s-1 and 42-48 s-1 for Mo-pic and Mo-pym and onset potentials of 240 mV and 175 mV for Mo-pic and Mo-pym, respectively, reveal these complexes as promising electrocatalysts for the HER.

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.

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation

From the reactions between Mo2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, the compounds Mo2(T(i)PB)2(Bzald)2, I; Mo2(T(i)PB)2(Avo)2, II; and Mo2(T(i)PB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S1 states having lifetimes ∼5-15 ps. The triplet states are Mo2δδ* with lifetimes in the microseconds. The spectroscopic properties of I and II are similar, whereas the planarity of the ligand in III greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.

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.

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.

On the Molecular Structure and Bonding in a Lithium Bismuth Porphyrin Complex: LiBi(TPP)2

A new BiLi porphyrin sandwich compound, LiBi(TPP)2 has been synthesized and characterized (TPP=tetraphenylporphyrin). The unique molecular structure of LiBi(TPP)2 is such that the Bi sits between the porphyrins and is directed towards the Li. This complex was shown to remain intact in solution by temperature-dependent 2D NMR spectroscopy. In order to investigate the potential interaction between these two metals, DFT calculations were used and showed a Bi 6s orbital polarized towards Li which could be indicative of a BiLi dative bond. This bond is remarkably short, 2.87 Å, and is among the shortest BiLi distances seen in a small molecule.