(I) Zinc complexes as synthetic analogues for carbonic anhydrase and as catalysts for H₂ production and CO₂ functionalization . . .

The multidentate alkyl ligand, [Tptm] ([Tptm] = tris(2-pyridylthio)methyl), provides an organometallic counterpart to the more common tripodal ligands, [Tp] ([Tp] = tris(pyrazolyl)hydroborato) and [Tm] ([Tm] = tris(2-mercaptoimidazolyl) hydroborato). A wide range of [Tptm] zinc complexes have been synthesized, enabling a diverse range of both stoichiometric and catalytic chemical transformations including the production of H₂ and the functionalization of CO₂. The [Tptm] ligand has been used to isolate the first mononuclear alkyl zinc hydride complex, [κ³-Tptm]ZnH. The hydride complex may be easily synthesized on a multigram scale via reaction of the trimethylsiloxide complex, [κ⁴-Tptm]ZnOSiMe₃, with PhSiH₃. The hydride complex, [κ³-Tptm]ZnH, provides access to a variety of other [Tptm]ZnX derivatives. For example, [κ³-Tptm]ZnH reacts with (i) R₃SiOH (R = Me, Ph) to give [κ⁴-Tptm]ZnOSiR₃, (ii) Me₃SiX (X = Cl, Br, I) to give [κ⁴-Tptm]ZnX and (iii) CO2 to give the formate complex, [κ⁴-Tptm]ZnO2CH. [κ³-Tptm]ZnH is hydrolyzed to give the dimeric hydroxide complex, {[κ³-Tptm]Zn(μ–OH)}₂, which when treated with CO₂, results in the bicarbonate complex, [κ⁴-Tptm]ZnOCO₂H. The halide complexes, [κ⁴-Tptm]ZnX (X = Cl, Br, I), can be used to synthesize the fluoride complex, [κ⁴-Tptm]ZnF, via treatment with tetrabutylammonium fluoride (TBAF). The bis(trimethylsilyl)amide complex, [κ³-Tptm]ZnN(SiMe₃)₂, which has been prepared directly via the reaction of [Tptm]H with [ZnN(SiMe₃)₂]₂, reacts with CO₂ to give the isocyanate complex, [κ⁴-Tptm]ZnNCO. The formation of the isocyanate complex results from a multistep sequence in which the initial step is insertion of CO₂ into the Zn-N(SiMe₃)₂ bond to give the carbamato derivative, [Tptm]Zn[O2CN(SiMe₃)₂], followed by rearrangement to [κ⁴-Tptm]ZnOSiMe₃ with the expulsion of Me₃SiNCO, which further reacts to give [κ⁴-Tptm]ZnNCO. An important discovery is that the rate of the final metathesis step, to give [κ⁴-Tptm]ZnNCO, is enhanced by CO₂. Specifically, insertion of CO₂ into the Zn-O bond of [κ⁴-Tptm]ZnOSiMe₃ gives the carbonate complex [κ⁴-Tptm]Zn[O₂COSiMe₃], which is more susceptible towards metathesis than is the siloxide derivative. The [Tptm] ligand has also been effective for other metals, such as magnesium and nickel. While [Tptm] complexes of magnesium exhibit chemistry that is similar to that of zinc, the linear nickel nitrosyl complex, [κ³-Tptm]NiNO, shows diverse reactivity involving its nitrosyl ligand. For example, oxygenation of [κ³-Tptm]NiNO is reversible. The reaction of [κ³-Tptm]NiNO with air gives the paramagnetic nitrite complex, [κ⁴-Tptm]Ni[κ²-O₂N], the latter which may be deoxygenated via reaction with trimethylphosphine. Additionally, the tetradentate alkyl ligand, tris(1-methyl-imidazol-2- ylthio)methyl, [TitmMe], has been studied as a comparison to the [Tptm] system. The bis(trimethylsilyl)amide complex, [κ³-TitmMe]ZnN(SiMe₃)₂ has been synthesized, and it also reacts with CO₂ to give the isocyanate complex, [κ⁴-TitmMe]ZnNCO. The hydroxide complexes, [TpBut,Me]ZnOH ([TpBut,Me] = tris(3-t-butyl-5- methylpyrazolyl)hydroborato), and {[κ³-Tptm]Zn(μ–OH)}₂, were used to model transformations with CO2 that are of relevance to the mechanism of action of carbonic anhydrase. Low temperature ¹H and ¹³C NMR spectroscopic studies on solutions of the hydroxide complex, [Tpᴮᵘᵗᴹᵉ]ZnOH, in the presence of 1 atmosphere of CO₂ have allowed for the identification of the bicarbonate complex, [Tpᴮᵘᵗᴹᵉ]ZnOCO₂H. In the presence of less than 1 atmosphere of CO₂, both [Tpᴮᵘᵗᴹᵉ]ZnOH and [Tpᴮᵘᵗᴹᵉ]ZnOCO₂H may be observed in equilibrium, thereby allowing for the measurement of the equilibrium constant for insertion of CO₂ into the Zn–OH bond. At 217 K, the equilibrium constant is 6 ± 2 x 10³ M⁻¹, corresponding to a value of ΔG = –3.8 ± 0.2 kcal mol⁻¹. In addition to the solution-state spectroscopic studies, [Tpᴮᵘᵗᴹᵉ]ZnOCO₂H and [κ⁴-Tptm]ZnOCO₂H have been structurally characterized by X-ray diffraction, thereby providing the first examples of structurally characterized terminal zinc bicarbonate complexes. The bicarbonate complexes afford important metrical data of importance to the critical bicarbonate intermediate of the mechanism of action of carbonic anhydrase. The dimeric hydroxide complex, {[κ³-Tptm]Zn(μ–OH)}₂, is sufficiently reactive towards CO₂ that it is able to abstract CO₂ directly from air to form the bridging carbonate complex, [Tptm]Zn(μ-CO₃)Zn[Tptm]. Both the bicarbonate and carbonate complexes are reduced by silanes to give the formate derivative, [κ⁴-Tptm]ZnO₂CH, a transformation that is significant for the functionalization of CO₂. The alkyl zinc hydride complex, [κ³-Tptm]ZnH, has also proven to be an effective and robust catalyst for a variety of transformations including (i) the rapid generation of hydrogen on demand, (ii) the hydrosilylation of aldehydes and ketones producing siloxanes and (iii) the functionalization of CO₂ to produce a useful formylating agent, (EtO)₃SiO₂CH. The trimethylsiloxide complex, [κ⁴-Tptm]ZnOSiMe₃, may also be used as an effective precatalyst for these reactions. For example, in the [κ⁴-Tptm]ZnOSiMe₃ catalyzed hydrolysis and methanolysis of PhSiH₃, three equivalents of H₂ are released, with the methanolysis reaction achieving 105 turnovers and turnover frequencies surpassing 106 h-1. Additionally, [κ³-Bptm*]ZnO₂CH (Bptm* = bis(2-pyridylthio)(p-tolylthio)methyl) has been synthesized using the tridentate [Bptm*] ligand, which has only two chelating pyridyl arms, forbidding a κ⁴-coordination. It serves as a room temperature catalyst for the hydrosilylation of CO₂, resulting in more rapid CO₂ functionalization compared to the [Tptm] system. [κ³--Tptm]ZnH and [κ³--Bptm*]ZnO₂CH provide the first two examples of zinc complexes that catalyze the hydrosilylation of CO₂. These results provide evidence that, in suitable ligand environments, inexpensive and abundant nontransition metals can perform reactions that are typically catalyzed by precious metal-containing compounds. The use of Li[Me₃SiNR] in order to generate an isocyanide complex from its carbonyl precursor provides a novel, convenient synthetic method that circumvents the use of the free isocyanide as a reagent. Metal isocyanide compounds are most commonly synthesized using the free isocyanide. By contrast, the reaction of transition metal carbonyl compounds, LnMCO, with Li[Me₃SiNR] yields the corresponding isocyanide derivative, LnMCNR. This reaction is driven by the cleavage of a weak silicon-nitrogen bond with concomitant formation of a stronger silicon-oxygen bond. Both sterically hindered and enantiopure isocyanide complexes have been synthesized. Thimerosal, [(Arᶜᵒ²)SHgEt]Na, an organomercurial utilized since the 1930s as a topical antiseptic, and more recently as a vaccine preservative, previously was not structurally characterized. Therefore, the molecular structures have been determined for thimerosal, its protonated derivative, (Arᶜᵒ²ᴴ)SHgEt, and its mercurated derivative, [(Arᶜᵒ²ᴴᵍᴱᵗ)SHgEt]₂, using single crystal X-ray diffraction. ¹H NMR spectroscopic studies indicate that the appearance of the ¹⁹⁹Hg mercury satellites of the ethyl groups is highly dependent on the magnetic field and the viscosity of the solvent; this observation is attributed to relaxation caused by chemical shift anisotropy. The relative signs of the Hg-H coupling constants (i.e. ²JHg-H and ³JHg-H) have been determined by virtue of the fact that the inner pair of satellites appears as a singlet at 400 MHz. Reactivity studies involving (Arᶜᵒ²ᴴ)SHgEt provide evidence that the Hg-C bond is kinetically stable with respect to protolytic cleavage. Finally, a series of known dithiol compounds have been synthesized for use as mercury chelating agents

Bespoke Photoreductants: Tungsten Arylisocyanides

Modular syntheses of oligoarylisocyanide ligands that are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have been developed; tungsten complexes incorporating these oligoarylisocyanide ligands exhibit intense metal-to-ligand charge-transfer visible absorptions that are red-shifted and more intense than those of the parent W(CNdipp)_6 complex. Additionally, these W(CNAr)_6 complexes have enhanced excited-state properties, including longer lifetimes and very high quantum yields. The decay kinetics of electronically excited W(CNAr)_6 complexes (*W(CNAr)_6) show solvent dependences; faster decay is observed in higher dielectric solvents. *W(CNAr)_6 lifetimes are temperature dependent, suggestive of a strong coupling nonradiative decay mechanism that promotes repopulation of the ground state. Notably, *W(CNAr)_6 complexes are exceptionally strong reductants: [W(CNAr)_6]+/*W(CNAr)_6 potentials are more negative than −2.7 V vs [Cp_2Fe]^+/Cp_2Fe

Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)_3]^(2+), is 7 GM. All measurements were performed using a nanosecond-pulsed laser system

Electronic Excited States of Tungsten(0) Arylisocyanides

W(CNAryl)_6 complexes containing 2,6-diisopropylphenyl isocyanide (CNdipp) are powerful photoreductants with strongly emissive long-lived excited states. These properties are enhanced upon appending another aryl ring, e.g., W(CNdippPh^(OMe)_2)_6; CNdippPh^(OMe)_2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide (Sattler et al. J. Am. Chem. Soc. 2015, 137, 1198−1205). Electronic transitions and low-lying excited states of these complexes were investigated by time-dependent density functional theory (TDDFT); the lowest triplet state was characterized by time-resolved infrared spectroscopy (TRIR) supported by density functional theory (DFT). The intense absorption band of W(CNdipp)_6 at 460 nm and that of W(CNdippPh^(OMe)_2)_6 at 500 nm originate from transitions of mixed ππ*(C≡N–C)/MLCT(W → Aryl) character, whereby W is depopulated by ca. 0.4 e– and the electron-density changes are predominantly localized along two equatorial molecular axes. The red shift and intensity rise on going from W(CNdipp)_6 to W(CNdippPh^(OMe)_2)_6 are attributable to more extensive delocalization of the MLCT component. The complexes also exhibit absorptions in the 300–320 nm region, owing to W → C≡N MLCT transitions. Electronic absorptions in the spectrum of W(CNXy)_6 (Xy = 2,6-dimethylphenyl), a complex with orthogonal aryl orientation, have similar characteristics, although shifted to higher energies. The relaxed lowest W(CNAryl)_6 triplet state combines ππ* excitation of a trans pair of C≡N–C moieties with MLCT (0.21 e–) and ligand-to-ligand charge transfer (LLCT, 0.24–0.27 e–) from the other four CNAryl ligands to the axial aryl and, less, to C≡N groups; the spin density is localized along a single Aryl–N≡C–W–C≡N–Aryl axis. Delocalization of excited electron density on outer aryl rings in W(CNdippPh^(OMe)_2)_6 likely promotes photoinduced electron-transfer reactions to acceptor molecules. TRIR spectra show an intense broad bleach due to ν(C≡N), a prominent transient upshifted by 60–65 cm^(–1), and a weak down-shifted feature due to antisymmetric C≡N stretch along the axis of high spin density. The TRIR spectral pattern remains unchanged on the femtosecond-nanosecond time scale, indicating that intersystem crossing and electron-density localization are ultrafast (<100 fs)

Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W(CNXy)_6 and W(CNIph)_6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400–550 nm). MLCT emission (λ_max ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)_6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)_6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bun4N][PF_6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6]+ and [anth]^•–. ET from *W to benzophenone and cobalticenium also is observed in [Bun4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6]^(+)/*W couple is −2.8 V vs Cp_(2)Fe^(+/0), establishing W(CNIph)_6 as one of the most powerful photoreductants that has been generated with visible light

Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surface

Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2′-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2′-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry

Comparative analysis of glutaredoxin domains from bacterial opportunistic pathogens

NMR structures of the glutaredoxin (GLXR) domains from Br. melitensis and Ba. henselae have been determined as part of the SSGCID initiative. Comparison of the domains with known structures reveals overall structural similarity between these proteins and previously determined E. coli GLXR structures, with minor changes associated with the position of helix 1 and with regions that diverge from similar structures found in the closest related human homolog