Michael D. Hopkins

Ground-State and Excited-State Structures of Tungsten–Benzylidyne Complexes

The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

FRET Sensitization of Tungsten–Alkylidyne Complexes by Zinc Porphyrins in Self-Assembled Dyads

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.

Effects of cation-anion interactions on the structures and photophysical properties of anionic d0 tungsten-benzylidyne complexes

Abstract The structures and photophysical properties of anionic tungsten–benzylidyne complexes of the type [Na(L)][W(CPh)(OBut)4] ([Na(L)]1; L=blank, 15-crown-5, crypt-2,2,2) are strongly dependent on the nature of the [Na(L)]+ ion. The structures of the 1− ions are qualitatively similar, consisting of square-pyramidal tungsten centers with short WC bonds, but differ as a function of cation in the extent of their Na–OBut interactions, the geometries of their OBut ligands, and their WO bond distances. The 1− ions exhibit long-lived luminescence (τ>1 μs) in fluid solution and the solid state at room temperature. The emission lifetimes and energies are cation dependent even in polar solvents, indicating the presence of [Na(L)]+/1− ion pairs in solution for some L. The emissive state is a spin triplet of either [π(WCPh)]1[dxy]1 or [π(WCPh)]1[π*(WCPh)]1 configuration.

Chemical bonding in methylidyne complexes: neutron diffraction study on a single crystal of BrWCH(dmpe-d12)2

Carbyne complexes have been extensively studied but only a few of them contain the MC–H moiety (M = transition metal). In order to contribute to the debate about the linearity of this fragment, two neutron diffraction experiments on single crystal have been performed at 100 and 293 K on the trans-W(CH)(dmpe)2Br (dmpe = 1,2-bis(dimethylphosphino)ethane) (1). The results point, without ambiguity, to the linearity of the fragment. The specific behaviour of the dmpe ligand is modelled and interpreted.

Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore

Significance In natural photosynthesis, the reducing equivalents for the reduction of CO2 are derived from photochemical water splitting. In homogeneous artificial-photosynthetic systems for CO2 reduction, the source of the reducing equivalents is generally a nonrenewable sacrificial electron donor. These sacrificial reagents aid proof-of-concept experiments by suppressing unproductive electron-transfer reactions but negate the solar-energy-storing potential for the system. Here, we describe an integrated homogeneous system in which a zinc porphyrin photosensitizes CO2 reduction and H2 oxidation catalysts, allowing the reducing equivalents for CO2 photoreduction to be derived from renewable H2. This system is thermodynamically competent to photochemically drive the energy-storing reverse water–gas shift reaction. Energy-storing artificial-photosynthetic systems for CO2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water–gas shift reaction (CO2 + H2 → CO + H2O), where the reducing equivalents are provided by renewable H2. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H2 oxidation catalysts of the form [CpRCr(CO)3]–, and CO2 reduction catalysts of the type Re(bpy-4,4′-R2)(CO)3Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ∼15 μs, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.

Molecular building blocks for magnetic spin chains

The paramagnetic di(metalloethynyl)benzene ion [1,4-C_6H_4{CW(depe)_2Cl}_2]^(2+) was synthesized from diamagnetic 1,4-C_6H_4{CW(depe)_2Cl}_2 (depe = 1,2-bis(diethylphosphino)ethane). Systematic measurements of magnetic susceptibility for both crystalline and powder-formed compounds indicate a predominant super-exchange coupling between the magnetic tungsten centres. We provide a quantitative description of the observed susceptibility using a decoupled Heisenberg dimer model, and find that all the complexes exhibit a robust antiferromagnetic coupling between spins, J~38 K. We note their potential use as building blocks for one-dimensional spin chains—with or without disorder—and describe possible synthetic routes to these architectures.