The thermodynamic and structural factors that influence the redox properties of an extensive set of tungsten–alkylidyne complexes (W(CR)L4X) are analyzed by combining synthesis, electrochemistry, and computational modeling based on free energy calculations of oxidation potentials at the density functional theory level. The observed linear correlations among oxidation potentials, HOMO energies, and gas-phase ionization energies are found to be consistent with the approximately constant solvation free energy differences between reduced and oxidized species over the complete set. The W–X bond length, trans to the alkylidyne ligand, is found to be a good descriptor of the positioning of the key frontier orbitals that regulate the redox properties of the complexes.Keywords: density functional theory; descriptor; photoredox chromophores; redox potentials; tungsten−alkylidyne complex;

Monolayers of five-coordinate gallium octaethylporphyrin complexes (Ga(OEP)X; X = Cl, Br, I, O3SCF3, CCPh) on highly oriented pyrolytic graphite were studied at the solid–liquid (1-phenyloctane) interface using scanning tunneling microscopy (STM) to probe the dependence of their properties on the nature of the axial X ligand. Density functional theory calculations of the gas-phase structures of the free molecules reveal that the gallium atom is positioned above the plane of the porphyrin macrocycle, with this pyramidal distortion increasing in magnitude according to X = O3SCF3 (displacement = 0.35 Å) < Cl, Br, I (∼0.47 Å) < CCPh (0.54 Å). All compounds exhibit pseudohexagonal close-packed structures in which the porphyrin is oriented coplanar with the surface and the axial ligand is oriented perpendicular to it, and with unit-cell parameters that are within experimental error of each other (a, b = 1.34 (3)–1.39 (2) nm, Γ = 66 (2)–68 (1)°). In contrast to these close similarities, the stabilities of the monolayers are sensitive to the nature of the axial ligand: the monolayers of Ga(OEP)(O3SCF3) and Ga(OEP)(CCPh) exhibit damage during the STM experiment upon repeated scanning and upon toggling the sign of the bias potential, but monolayers of Ga(OEP)Cl, Ga(OEP)Br, and Ga(OEP)I do not. A second important ligand-influenced property is that Ga(OEP)I forms bilayer structures, whereas the other Ga(OEP)X compounds form monolayers exclusively under identical conditions. The top layer of the Ga(OEP)I bilayer is oriented with the iodo ligand directed away from the surface, like the bottom layer, but the molecules pack in a square, lower-density geometry. The comparatively large polarizability of the iodo ligand is suggested to be important in stabilizing the bilayer structure.

The Ni(I) hydrogen oxidation catalyst [Ni(PCy2NtBu2)2]+ (1+; PCy2NtBu2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane) has been studied using a combination of electron paramagnetic resonance (EPR) techniques (X-, Q-, and D-band, electron–nuclear double resonance, hyperfine sublevel correlation spectroscopy), X-ray crystallography, and density functional theory (DFT) calculations. Crystallographic and DFT studies indicate that the molecular structure of 1+ is highly symmetrical. EPR spectroscopy has allowed determination of the electronic g tensor and the spin density distribution on the ligands, and revealed that the Ni(I) center does not interact strongly with the potentially coordinating solvents acetonitrile and butyronitrile. The EPR spectra and magnetic parameters of 1+ are found to be distinctly different from those for the related compound [Ni(PPh2NPh2)2]+ (4+). One significant contributor to these differences is that the molecular structure of 4+ is unsymmetrical, unlike that of 1+. DFT calculations on derivatives in which the R and R′ groups are systematically varied have allowed elucidation of structure/substituent relationships and their corresponding influence on the magnetic resonance parameters.

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n−1Ph](dppe)2Cl (n = 1–5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

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.

The d1 tungsten–alkylidyne radical [W(CPh)(dppe)2Cl]+ reacts with H2 to give the d0 hydride [W(CPh)(H)(dppe)2Cl]+, which on deprotonation yields the d2 photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.

The electrochemistry and electronic structures of over 30 tungsten–alkylidyne compounds of the form W(CR)LnL′4–nX (R = H, But, Ph, p-C6H4CCH, p-C6H4CCSiPri3; X = F, Cl, Br, I, OTf, Bun, CN, OSiMe3, OPh; L/L′ = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)3, CO, CNBut, py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal–ligand interactions. The first oxidation potentials are found to span a range of ∼2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal dxy orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the dxy orbital, being much more sensitive to the nature of the equatorial L ligands (π symmetry, with respect to dxy, ΔE1/2 ≅ 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to dxy, ΔE1/2 < 0.3 V/L). The oxidation potential is linearly correlated with the calculated dxy orbital energy (slope ≅ 1, R2 = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.

The photophysical properties of self-assembled zinc–porphyrin/tungsten–alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S1 excited state sensitizes the tungsten–alkylidyne 3[dπ*] state. The luminescent metalloligand W(≡CC6H4CCpy)(dppe)2Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTPClP (TPClP = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S1 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 S1 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 (ϕEnT ≈ 80%), thus strongly enhancing light harvesting for the tungsten–alkylidyne compound. The mechanism and rates of the net S1→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.

Picosecond transient-absorption spectra are reported for the S1 excited state of zinc tetraphenylporphyrin (ZnTPP) and related compounds in the near-infrared region (λ ≤ 1400 nm). The spectra exhibit a prominent absorption band (ɛ ≌ 4400 M−1 cm−1 for ZnTPP) between 1200 and 1300 nm whose position is sensitive to solvent (toluene and THF), zinc coordination number (ZnTPP(py-4-CCPh)), the metal center (GaTPP(Cl)), and the positions of the porphyrin substituents (zinc octaethylporphyrin, ZnOEP). The transient-absorption profile between 850 and 1200 nm is also sensitive to these variations. The near-infrared bands for the TPP compounds are assigned to transitions from the S1 state to gerade-symmetry dark states based on their correspondence to relative energies predicted by previously reported time-dependent density functional theory calculations and their constant energy gap relative to the S2 state. The characteristics of the prominent near-infrared transient-absorption band make it well suited for selectively probing photophysical and photoredox processes of the S1 state because it does not overlap with absorption bands of the T1 excited state or of [ZnTPP]+ or [ZnTPP]−, in contrast to the features in the typically probed visible-wavelength region.Graphical abstractHighlights► Near-IR transient absorption spectra were measured for ZnTPP and related compounds. ► The S1 state of ZnTPP and related compounds have a diagnostic TA band near 1250 nm. ► The near-IR TA data place gerade symmetry dark states 0.1–0.2 eV above the S2 state. ► The near-IR band allows for the properties of the S1 state to be selectively probed.

The molecular structure of the tungsten–benzylidyne complex trans-W(≡CPh)(dppe)2Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (dxy)2 ground state and luminescent triplet (dxy)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, (dxy)1-configured 1+, and (dxy)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)Ln (E = O, N) compounds with analogous (dxy)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.

Complexes of the form XL4W≡C—C≡WL4X (L = 1/2 dmpe, 1/2 depe, P(OMe)3; X = Cl, OTf) have been synthesized from (ButO)3WCCW(OBut)3 in two steps via Cl3(dme)WCCW(dme)Cl3, which undergoes facile four-electron reduction in the presence of L. The compounds possess formal d2−d2 electron configurations. The molecular structures of Cl(dmpe)2WCCW(dmpe)2Cl and Cl{P(OMe)3}4WCCW{P(OMe)3}4Cl were determined by X-ray crystallography; bond distances within the backbone are consistent with a W≡C—C≡W canonical structure. Density-functional-theory calculations on Cl(dmpe)2WCCW(dmpe)2Cl and the model compound Cl(PH3)4WCCW(PH3)4Cl, and on their monometallic analogs W(CH)(dmpe)2Cl and W(CH)(PH3)4Cl, indicate that the WCCW backbone is strongly π-conjugated; this is supported by the observation of low-energy π → π* transitions for the compounds. The calculations predict that δ symmetry dxy-derived orbitals should be (or lie near) the highest occupied molecular orbital. Consistent with this prediction, the electronic spectra of the compounds exhibit a band attributable to dxy → π* transition(s), as the lowest-energy feature and electrochemical studies demonstrate that they undergo sequential one-electron oxidations to produce (dxy)2−(dxy)1 and (dxy)1−(dxy)1 congeners. Due to the δ symmetry of the redox orbitals, the oxidized congeners maintain the W≡C—C≡W canonical structure of the parent d2−d2 compounds. The first and second oxidation potentials of Cl(dmpe)2WCCW(dmpe)2Cl are separated by ≤0.4 V, corresponding to Kcom = 104−106; the interaction between redox orbitals is largely electrostatic in origin and not the result of significant direct δ orbital overlap. The reaction between Cl(dmpe)2WCCW(dmpe)2Cl and HCl (2 equiv) produces the d0−d0 dihydride ion [Cl(H)(dmpe)2WCCW(dmpe)2(H)Cl]2+, which is formulated as maintaining the W≡C—C≡W backbone on the basis of its X-ray crystal structure and NMR spectra. This family of WCCW derivatives expands the relatively small class of M≡C—C≡M compounds and is distinctive among them because their ancillary ligands should allow incorporation of the L4WCCWL4 unit into interior positions of metalloyne oligomers and polymers.

The new zinc porphyrin/tungsten alkylidyne dyad Zn(TPP)−C≡CC6H4C≡W(dppe)2Cl (1) possesses novel photophysical properties that arise from a tunable excited-state triplet−triplet equilibrium between the porphyrin and tungsten alkylidyne units. Dyad 1 exhibits 3(dxy ← π*(WCR)) phosphorescence with a lifetime that is 20 times longer than that of the parent chromophore W(CC6H4CCPh)(dppe)2Cl (2). The triplet−triplet equilibrium can be tuned by the addition of ligands to the Zn center, resulting in phosphorescence lifetimes for 1(L) that are up to 1300 times longer than that of 2. The “lifetime reservoir” effect exhibited by 1(L) is approximately 1 order of magnitude larger than previously reported examples of the phenomenon.

The electronic-absorption and -emission spectroscopy and photophysical properties of metal–alkylidyne (–carbyne) complexes are reviewed. Emission has been observed in fluid solution at room temperature from compounds of a variety of different metals (Mo, W, Re, Os) and electron configurations (d0, d1, d2). The emissive excited states are of the types dxy → π*, π → π*, π → dxy, and MLCT. This compositional and electronic diversity enables the luminescence properties of metal–alkylidyne complexes to be broadly tuned.

The d1 tungsten–alkylidyne radical [W(CPh)(dppe)2Cl]+ reacts with H2 to give the d0 hydride [W(CPh)(H)(dppe)2Cl]+, which on deprotonation yields the d2 photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.