Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters: tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues.

Reaction of [Fe3(CO)9(μ3-Te)2] (1) with the corresponding phosphine has been used to prepare the phosphine-substituted tellurium-capped triiron clusters [Fe3(CO)9(μ3-Te)2(PPh3)] (2), [Fe3(CO)8(μ3-Te)2(PPh3)] (3) and [Fe3(CO)7(μ3-Te)2(μ-R2PXPR2)] (X = CH2, R = Ph (4), Cy (5); X = NPri, R = Ph (6)). The directly related cluster [Fe3(CO)7(μ3-CO)(μ3-Te)(μ-dppm)] (7) was isolated from the reaction of [Fe3(CO)10(μ-Ph2PCH2PPh2)] with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltammetry, and selected complexes have been tested as proton reduction catalysts.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical.

Negative ion photoelectron (NIPE) spectra, with 193, 266, 300, and 355 nm photons, of the radical anion of 1,8-naphthoquinone (1,8-NQ) have been obtained at 20 K. The electron affinity of 1,8-NQ is determined from the first resolved peak in the NIPE spectrum to be 2.965 ± 0.

Hydrogenase biomimics containing redox-active ligands: Fe(CO)(μ-edt)(κ-bpcd) with electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) as a potential [Fe-S] surrogate.

[FeFe]-hydrogenases contain strongly electronically coupled diiron [2Fe]H and tetrairon [Fe4-S4]H clusters, and thus much recent effort has focused on the chemistry of diiron-dithiolate biomimics with appended redox-active ligands. Here we report on the synthesis and electrocatalytic activity of Fe2(CO)4(μ-edt)(κ2-bpcd) (2) in which the electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) acts as a surrogate of the [Fe4-S4]H sub-cluster. The complex is prepared in low yield but has been fully characterised, including a crystallographic study which shows that the diphosphine adopts a basal-apical coordination geometry in the solid state.

Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet.

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8-NQ), to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a A ground state, with three other electronic states- B , B , and B -within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8-NQ will find that 1,8-NQ is a diradical with a singlet ground state.

A new diphosphine-carbonyl complex of ruthenium: an efficient precursor for C-C and C-N bond coupling catalysis.

Reaction of 1,2-bis(diphenylphosphino)benzene (dppbz) with [{Ru(CO)2Cl2}n] affords [Ru(dppbz)(CO)2Cl2], where the two carbonyls are mutually cis and the two chlorides are trans. The molecular structure of [Ru(dppbz)(CO)2Cl2], has been determined by X-ray crystallography, and the stability of the different available stereoisomers has been computationally evaluated. [Ru(dppbz)(CO)2Cl2] has been found to serve as an excellent pre-catalyst for catalytic Suzuki-type C-C coupling and Buchwald-type C-N coupling reactions.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of the Relative Energies of the Low-Lying Electronic States of 2,7-Naphthoquinone.

Cryogenic negative ion photoelectron (NIPE) spectra of the radical anion of 2,7-naphthoquinone (NQ) have been taken at 20 K, using 193, 240, 266, 300, and 355 nm lasers for electron detachment. The electron affinity of the NQ diradical is determined from the first resolved peak in the NIPE spectrum to be 2.880 ± 0.

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3(.).

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3(-) ion. The adiabatic electron detachment energy of HCPN3(-) (electron affinity of HCPN3(•)) was measured to be 3.555 ± 0.

Negative ion photoelectron spectroscopy of PN: electron affinity and electronic structures of PN˙.

We report here a negative ion photoelectron spectroscopy (NIPES) and study of the recently synthesized planar aromatic inorganic ion PN, to investigate the electronic structures of PN and its neutral PN˙ radical. The adiabatic detachment energy of PN (electron affinity of PN˙) was determined to be 3.765 ± 0.

Negative ion photoelectron spectroscopy confirms the prediction that carbon trioxide (CO) has a singlet ground state.

The CO radical anion (CO˙) has been formed by electrospraying carbonate dianion (CO) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO˙ shows that, unlike the isoelectronic trimethylenemethane [C(CH)], carbon trioxide (CO) has a singlet ground state. From the NIPE spectrum, the electron affinity of singlet CO was, for the first time, directly determined to be EA = 4.

Theoretical Analysis of the Fragmentation of (CO)5: A Symmetry-Allowed Highly Exothermic Reaction that Follows a Stepwise Pathway.

B3LYP and CCSD(T) calculations, using an aug-cc-pVTZ basis set, have been carried out on the fragmentation of 1,2,3,4,5-cyclopentanepentone, (CO)(5), to five molecules of CO. Although this reaction is calculated to be highly exothermic and is allowed to be concerted by the Woodward-Hoffmann rules, our calculations find that the D(5h) energy maximum is a multidimensional hilltop on the potential energy surface. This D(5h) hilltop is 16-20 kcal/mol higher in energy than a C(2) transition structure for the endothermic cleavage of (CO)(5) to (CO)(4) + CO and 11-15 kcal/mol higher than a C(s) transition structure for the loss of two CO molecules.

Computational exploration of the mechanism of copper-catalyzed aromatic C-H bond amination of benzene via a nitrene insertion approach.

The mechanism of aromatic C-H amination of benzene via a nitrene insertion approach catalyzed by the Tp(Br3)Cu(NCMe) complex was computationally investigated. The results of computational studies show that addition of the nitrene moiety of the Tp(Br3)Cu-nitrene intermediate to benzene, and therefore, to form an aziridine intermediate, is more favorable than the nitrene moiety induced hydrogen atom abstraction from a sp(2) C-H bond of benzene. Subsequently, the cleavage of a C-N bond of the aziridine intermediate followed by an H-atom transfer step might occur, due to the driving force of the rearomatization, to afford the desired aromatic C-H amination product.

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions.

Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State.

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB(•-)) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus violates Hund's rule. The NIPE spectrum of TOTMB(•-) gives a value of -ΔEST = 3.5 ± 0.

The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)3(•-)--a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3(•-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3.

Computational and ¹³C investigations of the diazadienes and oxazadienes formed via the rearrangement of methylenecyclopropyl hydrazones and oximes.

Computational and further experimental investigations of the previously reported diazadienes, obtained via the rearrangement of methylenecyclopropyl hydrazone 1 are reported. Calculations at the CCSD(T)/cc-pVTZ//B3LYP/6-31G(d) level of theory indicate that the initially reported product 3 would, if formed, undergo rapid electrocyclic ring opening and, hence, would be unstable under the reaction conditions. Based on this computational prediction, further analysis of the (13)C NMR spectrum, previously attributed to 3, led to the revision of structure 3 to that of its N-tosylaminopyrrole constitutional isomer 11.

Heavy-atom tunneling in the ring opening of a strained cyclopropene at very low temperatures.

The highly strained 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 1 is metastable, and rearranges to 4-oxacyclohexa-2,5-dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K.

The negative ion photoelectron spectrum of meta-benzoquinone radical anion (MBQ˙⁻): a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.

Why is (SiO)(4) calculated to be tetrahedral, whereas (CO)(4) is square planar? A molecular orbital analysis.

Qualitative molecular orbital (MO) theory predicts that square-planar tetrasilacyclobutanetetraone D4h-(SiO)4 should, like D4h-(CO)4, have a triplet ground state, and the results of the (U)CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df) calculations, reported here, confirm this expectation. Calculations at the same level of theory find that square-planar tetrasilacyclobutanetetrathione D4h-(SiS)4 also has a triplet ground state. However, these ab initio calculations predict that (SiO)4 and (SiS)4 both have a singlet state of much lower energy, with a tetrahedral (Td) equilibrium geometry and six, electron-deficient, Si-Si bonds.

Calculations on tunneling in the reactions of noradamantyl carbenes.

Noradamantylchlorocarbene has been found experimentally to undergo ring expansion to 2-chloroadamantene at cryogenic temperatures. The rate constant, calculated with inclusion of small-curvature tunneling, is within a factor of 2 of the rate constant measured at 9 K in a nitrogen matrix. Our calculations predict that noradamantylfluorocarbene will not be found to rearrange under these conditions.

How to make the σ0π2 singlet the ground state of carbenes.

Successful strategies have previously been developed to stabilize the σ(2)π(0) singlet states of carbenes, relative to σ(1)π(1) triplet states. However, little or no attention has been paid to the stabilization of the σ(0)π(2) singlet states. We present two simple strategies to stabilize the σ(0)π(2) singlet states of carbenes, relative to both the σ(2)π(0) singlet and σ(1)π(1) triplet states.

The ground state of (CS)4 is different from that of (CO)4: an experimental test of a computational prediction by negative ion photoelectron spectroscopy.

Cyclobutane-1,2,3,4-tetrathione, (CS)4, has recently been calculated to have a singlet ground state, (1)A1g, in which the highest b2g σ MO is doubly occupied and the lowest a2u π MO is empty. Thus, (CS)4 is predicted to have a different ground state than its lighter congener, (CO)4, which has a triplet ground state, (3)B1u, in which these two MOs are each singly occupied. Here, we report the results of a negative ion photoelectron spectroscopy (NIPES) study of the radical anion (CS)4(•-), designed to test the prediction that (CS)4 has a singlet ground state.

Like (CO)4, Do (CS)4 and (CSe)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetraone, (CO)4, was computationally predicted and, subsequently, experimentally confirmed to have a triplet ground state, in which a b2g σ MO and an a2u π MO were each singly occupied. In contrast, the (U)CCSD(T) calculations reported herein found that cyclobutane-1,2,3,4-tetrathione, (CS)4, and cyclobutane-1,2,3,4-tetraselenone, (CSe)4, both had singlet ground states, in which the b2g σ MO was doubly occupied and the a2u π MO was empty. Our calculations showed that both the longer C=X distances and smaller coefficients on the carbon atoms in the b2g and a2u MOs of (CS)4 and (CSe)4 contributed to the difference between the ground states of these two molecules and the ground state of (CO)4.

Negative ion photoelectron spectroscopy confirms the prediction that (CO)5 and (CO)6 each has a singlet ground state.

Cyclobutane-1,2,3,4-tetraone has been both predicted and found to have a triplet ground state, in which a b2g σ molecular orbital (MO) and an a2u π MO are each singly occupied. In contrast, (CO)5 and (CO)6 have each been predicted to have a singlet ground state. These predictions have been tested by generating the (CO)5(•-) and (CO)6(•-) radical anions in the gas phase, using electrospray vaporization of solutions of, respectively, the croconate (CO)5(2-) and rhodizonate (CO)6(2-) dianions.

Synchronized aromaticity as an enthalpic driving force for the aromatic Cope rearrangement.

We report herein experimental and theoretical evidence for an aromatic Cope rearrangement. Along with several successful examples, our data include the first isolation and full characterization of the putative intermediate that is formed immediately after the initial [3,3] sigmatropic rearrangement. Calculations at the B3LYP/6-31G(d) level of theory predict reaction energy barriers in the range 22-23 kcal/mol for the [3,3]-rearrangement consistent with the exceptionally mild reaction conditions for these reactions.

Molecular orbitals of the oxocarbons (CO)n, n = 2-6. Why does (CO)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetrone has been both predicted and found to have a triplet ground state, in which a b(2g) σ MO and an a(2u) π MO are each singly occupied. The nearly identical energies of these two orbitals of (CO)(4) can be attributed to the fact that both of these MOs are formed from a bonding combination of C-O π* orbitals in four CO molecules. The intrinsically stronger bonding between neighboring carbons in the b(2g) σ MO compared to the a(2u) π MO is balanced by the fact that the non-nearest-neighbor, C-C interactions in (CO)(4) are antibonding in b(2g), but bonding in a(2u).