Author Correction: Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

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Author Correction: Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

The original version of this Article contained errors in the symbols displayed in the eighteenth sentence of the third paragraph of the 'Determination of Hab and kET data for the Mo2 dimers' section of the Results, and the third sentence of the Discussion. This has been corrected in both the PDF and HTML versions of the Article.

Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

Thermal electron transfer through hydrogen bonds remains largely unexplored. Here we report the study of electron transfer through amide-amide hydrogen bonded interfaces in mixed-valence complexes with covalently bonded Mo2 units as the electron donor and acceptor. The rate constants for electron transfer through the dual hydrogen bonds across a distance of 12.5 Å are on the order of ∼ 1010 s-1, as determined by optical analysis based on Marcus-Hush theory and simulation of ν(NH) vibrational band broadening, with the electron transfer efficiencies comparable to that of π conjugated bridges. This work demonstrates that electron transfer across a hydrogen bond may proceed via the known proton-coupled pathway, as well as an overlooked proton-uncoupled pathway that does not involve proton transfer. A mechanistic switch between the two pathways can be achieved by manipulation of the strengths of electronic coupling and hydrogen bonding. The knowledge of the non-proton coupled pathway has shed light on charge and energy transport in biological systems.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product.

The catecholate groups in [{Pt(L)}3(µ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE1/2 = 160-170 mV. The monoradical [{Pt(dppb)}3(µ3-tctq˙)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(µ3-ctc˙)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency.

Photophysical and Cellular Imaging Studies of Brightly Luminescent Osmium(II) Pyridyltriazole Complexes.

The series of complexes [Os(bpy)3- n(pytz) n][PF6]2 (bpy = 2,2'-bipyridyl, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole, 1 n = 0, 2 n = 1, 3 n = 2, 4 n = 3) were prepared and characterized and are rare examples of luminescent 1,2,3-triazole-based osmium(II) complexes. For 3 we present an attractive and particularly mild preparative route via an osmium(II) η6-arene precursor circumventing the harsh conditions that are usually required. Because of the high spin-orbit coupling constant associated with the Os(II) center the absorption spectra of the complexes all display absorption bands of appreciable intensity in the range of 500-700 nm corresponding to spin-forbidden ground-state-to-3MLCT transitions (MLCT = metal-to-ligand charge transfer), which occur at significantly lower energies than the corresponding spin-allowed 1MLCT transitions. The homoleptic complex 4 is a bright emitter (λmaxem = 614 nm) with a relatively high quantum yield of emission of ∼40% in deoxygenated acetonitrile solutions at room temperature. Water-soluble chloride salts of 1-4 were also prepared, all of which remain emissive in aerated aqueous solutions at room temperature. The complexes were investigated for their potential as phosphorescent cellular imaging agents, whereby efficient excitation into the 3MLCT absorption bands at the red side of the visible range circumvents autofluorescence from biological specimens, which do not absorb in this region of the spectrum. Confocal microscopy reveals 4 to be readily taken up by cancer cell lines (HeLa and EJ) with apparent lysosomal and endosomal localization, while toxicity assays reveal that the compounds have low dark and light toxicity. These complexes therefore provide an excellent platform for the development of efficient luminescent cellular imaging agents with advantageous photophysical properties that enable excitation and emission in the biologically transparent region of the optical spectrum.

Dihydrogen phosphate-containing dinuclear double assemblies that demonstrate phosphate reactivity to the tetrafluoroborate anion.

The ligands L1 and L2 both form dinuclear assemblies with Cu(ii) and these react with dihydrogen phosphate so that the anion is incorporated within the assembly (e.g. [Cu2L2(H2PO4)]3+). However, in the presence of tetrafluoroborate anions the phosphate undergoes reaction with the anion forming [Cu3(L1)3(O3POBF3)]3+ and [Cu2(L2)2(O2P(OBF3)2)]+.

An iron-catalysed C-C bond-forming spirocyclization cascade providing sustainable access to new 3D heterocyclic frameworks.

Heterocyclic architectures offer powerful creative possibilities to a range of chemistry end-users. This is particularly true of heterocycles containing a high proportion of sp3-carbon atoms, which confer precise spatial definition upon chemical probes, drug substances, chiral monomers and the like. Nonetheless, simple catalytic routes to new heterocyclic cores are infrequently reported, and methods making use of biomass-accessible starting materials are also rare. Here, we demonstrate a new method allowing rapid entry to spirocyclic bis-heterocycles, in which inexpensive iron(III) catalysts mediate a highly stereoselective C-C bond-forming cyclization cascade reaction using (2-halo)aryl ethers and amines constructed using feedstock chemicals readily available from plant sources. Fe(acac)3 mediates the deiodinative cyclization of (2-halo)aryloxy furfuranyl ethers, followed by capture of the intermediate metal species by Grignard reagents, to deliver spirocycles containing two asymmetric centres. The reactions offer potential entry to key structural motifs present in bioactive natural products.

Mechanistic insight into proton-coupled mixed valency.

Stabilisation of the mixed-valence state in [Mo2(TiPB)3(HDOP)]2(+) (HTiPB = 2,4,6-triisopropylbenzoic acid, H2DOP = 3,6-dihydroxypyridazine) by electron transfer (ET) is related to the proton coordinate of the bridging ligands. Spectroelectrochemical studies suggest that ET is slower than 10(9) s(-1). The mechanism has been probed using DFT calculations, which show that proton transfer induces a larger dipole in the molecule resulting in ET.

Platinum(ii) complexes of mixed-valent radicals derived from cyclotricatechylene, a macrocyclic tris-dioxolene.

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(µ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4'-bis(tert-butyl)-2,2'-bipyridyl { t Bu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6- catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or t Bu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.

Hydrogen bonding and electron transfer between dimetal paddlewheel compounds containing pendant 2-pyridone functional groups.

The compounds M2(TiPB)3(HDON) (TiPB = 2,4,6-triisopropylbenzoic acid; H2DON = 2,7-dihdroxy-1,8-napthyridine; M = Mo (1a) or W (1b)) and Mo2(TiPB)2(O2CCH2Cl)(HDON) (1c) which contain a pendant 2-pyridone functional group have been prepared. These compounds are capable of forming self-complementary hydrogen bonds, resulting in the formation of "dimers of dimers" ([1a-c]2) in CH2Cl2 solutions. Electrochemical studies reveal two successive one-electron redox processes for [1a-c]2 in CH2Cl2 solutions that correspond to successive oxidations of the dimetal core, indicating stabilization of the mixed-valence state. Only small changes in the value of Kc are observed upon changing the ancillary ligand or metal, implying that proton coupled mixed valency is responsible for the stabilization. Dimethylsulfoxide (DMSO) disrupts the hydrogen bonding interactions in these compounds, and a single oxidation process is observed in DMSO which shifts to lower potential as the number of HDON ligands increases. Further substitution of carboxylate ligands with HDON leads to the formation of Mo2(TiPB)2(HDON)2 (2) and Mo2(HDON)4 (3), which adopt trans-1,1 and cis-2,2 regioisomers in the solid-state. (1)H NMR spectroscopy indicates that there are at least two regioisomers present in solution for both compounds. The lowest energy transition in the electronic absorption spectra of these compounds corresponds to a M2-δ → HDON-π* transition. The electrochemical, spectroscopic and structural results were rationalized with the aid of density functional theory (DFT) calculations.

Structural, spectroscopic and theoretical studies of diosmium(III,III) tetracarboxylates.

The preparation of Os2(TiPB)4Cl2 (1; TiPB = 2,4,6-triisopropylbenzoate) and Os2(TiPB)2(OAc)2Cl2 (2) by carboxylate exchange reactions with Os2(OAc)4Cl2 is reported. The structure of 1 has been determined by single-crystal X-ray studies, and shows a paddlewheel arrangement of the ligands about the triply bonded diosmium core. Both compounds have magnetic moments at room temperature that are consistent with the presence of two unpaired electrons, and their cyclic voltammograms show a single redox process corresponding to the Os2(5+/6+) redox couple. The electronic absorption spectra of 1 and 2 display an absorption at ~395 nm, corresponding to the π(Cl) →π*(Os2) LMCT transition, as well as numerous weaker absorptions at lower energy. Density functional theory (DFT) calculations on Os2(OAc)4Cl2 at different levels of theory (B3LYP and PBE0) have been used to probe the electronic structure of diosmium tetracarboxylates. The calculations show that these compounds have a σ(2)π(4)δ(2)δ*(1)π*(1) electronic configuration, and time-dependent DFT was used to help rationalize their optical properties.

Mixed valency in hydrogen bonded 'dimers of dimers'.

Dimolybdenum quadruply bonded compounds containing a pendant lactam functional group form self-complementary hydrogen bonded 'dimers of dimers' in the solid-state and CH(2)Cl(2) solutions. Electrochemical studies in CH(2)Cl(2) show two consecutive one-electron redox processes corresponding to oxidation of the Mo(2)(4+) cores. Spectroelectrochemical studies on the 'dimers of dimers' show no evidence of intervalence charge transfer bands in the mixed valence radical cations formed by one-electron oxidation, indicating that they are examples of proton-coupled mixed valency.

Tuning the electronic structure of Mo-Mo quadruple bonds by N for O for S substitution.

A series of quadruply bonded dimolybdenum compounds of form Mo(2)(EE'CC≡CPh)(4) (EE' = {NPh}(2), Mo(2)NN; {NPh}O, Mo(2)NO;{NPh}S, Mo(2)NS; OO, Mo(2)OO) have been synthesised by ligand exchange reactions of Mo(2)(O(2)CCH(3))(4) with the acid or alkali metal salt of {PhC≡CCEE'}(-). The compounds Mo(2)NO, Mo(2)NS and Mo(2)OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo(2)NO adopts a cis-2,2 arrangement of the ligands about the Mo(2)(4+) core, whereas Mo(2)NS adopts the trans-2,2 arrangement. The influence of heteroatom substitution on the electronic structure of the compounds was investigated using cyclic voltammetry and UV-Vis spectroscopy. Simple N for O for S substitution in the bridging ligands significantly alters the electronic structure, lowering the energy of the Mo(2)-δ HOMO and reducing the Mo(2)(4+/5+) oxidation potential by up to 0.9 V. A different trend is found in the optoelectronic properties, with the energy of the Mo(2)-δ-to-ligand-π* transition following the order Mo(2)OO > Mo(2)NO > Mo(2)NN > Mo(2)NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.

Oxalate bridged triangles incorporating Mo2(4+) units. Electronic structure and bonding.

The reactions between [Mo(2)L(2)(CH(3)CN)(6)][BF(4)](2) compounds and [Bu(n)(4)N](2)[O(2)CCO(2)] in CH(3)CN are shown to proceed under kinetic control to the formation of a mixture of molecular triangles and squares. The molecular triangles [L(2)Mo(2)(O(2)CCO(2))](3) I (L = DPhF, PhNCHNPh) and II (L = DAniF, p-MeO-C(6)H(4)NCHNC(6)H(4)-p-OMe) are the major products, and when 0.75 equivalents of [Bu(n)(4)N](2)[O(2)CCO(2)] is employed, they are formed to the exclusion of the square. The molecular structure of II is reported based on a single crystal X-ray determination. The molecular triangles do not enter into an equilibrium with their molecular square counterparts in CH(2)Cl(2), in contrast to their perfluoroterephthalate bridged counterparts. The compounds I and II are orange and have a strong electronic transition at lambda(max) approximately 460 nm assignable to metal-to-ligand charge transfer ((1)MLCT) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO(2))(2)Mo(2)(O(2)CCO(2))](3) and [(HNCHNH)(2)Mo(2)(O(2)CCO(2))](3) have been carried out and indicate the frontier occupied molecular orbitals are Mo(2) delta combinations e(4)a(2), and the lowest unoccupied are bridge pi* for the formamidinates and delta* for formates as ancillary ligands. Compounds I and II show quasi-reversible oxidation waves in their cyclic voltammograms and oxidation of II in 2-methyl-THF by reaction with AgPF(6) (1 equivalent) leads to a metal centered EPR signal, g approximately 1.95. The electronic absorption spectrum shows a low-energy broad band centered at 6418 cm(-1), which is assigned to an intervalence charge transfer (IVCT) band of a class III mixed valence ion.

Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands.

The reaction between M(2)(TiPB)(4) (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M(2)(TiPB)(2)(6-carboethoxy-2-azulenecarboxylate)(2). Compound (M = Mo) is blue and compound (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate that the two azulene carboxylate pi systems are coupled by their interactions with the M(2)delta orbitals. Their intense colors arise from M(2)delta to azulene pi* electronic transitions. While compound exhibits weak emission at approximately 900 nm, no emission has been detected for . Both and have been studied by fs and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of in the solid state confirmed the paddlewheel nature of its W(2)(O(2)C)(4) core and the trans orientation of the ligands.

Quadruply bonded dimetal units supported by 2,4,6-triisopropylbenzoates MM(TiPB)(4) (MM = Mo(2), MoW, and W(2)): preparation and photophysical properties.

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

Relationship between metal-metal bond length and internal rotation in diruthenium tetracarboxylate paddlewheel complexes.

The Ru-Ru bond length for Ru2II,III and Ru2II,II paddlewheel complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate was found to decrease despite a reduction in Ru-Ru bond order, due to increased internal rotation.

Dimolybdenum bis-2,4,6-triisopropyl-benzoate bis-4-isonicotinate: a redox active analogue of 4,4'-bipyridine with ambivalent properties.

The reaction between Mo2(TiPB)4 and 4-iso-nicotinic acid (2 equiv) in ethanol leads to the formation of trans-Mo2(TiPB)2(nic)2, I, where TiPB = 2,4,6-triisopropylbenzoate and nic = 4-isonicotinate. The molecular structures of I and I x 2DMSO were determined in the solid state by a single-crystal X-ray study, and its electronic structure was determined by DFT calculations on a model compound, where formate ligands were substituted for the bulky TiPB. The physicochemical properties of I are reported, and its potential as a redox active building block, a quasi-metalloorganic analogue of 4,4'-bipyridine, is described in the synthesis of molecular and solid-state assemblies. The molecular structure of I in the solid state consists of a 3-dimensional network in which each unit of Mo2(TiPB)2(nic)2 acts as a donor and acceptor via N to Mo coordination. In the structure of I x 2DMSO, the DMSO ligands coordinate axially with the Mo-Mo bond via oxygen. The reaction between I and Rh2(O2CMe)4 is shown to give a 1-D polymeric chain in the solid state: [{Rh2(O2CMe)4}{Mo2(TiPB)2(nic)2}] infinity, II. A similar structure was found for the product involving Rh2(O2CCMe3)4. Evidence is also reported for the formation of [(1,5-COD)MePt]2[mu-Mo2(TiPB)2(nic)2](PF6)2, III, and [(1,5-COD)Pt(mu-I)(PF6)2]n.

Concerning the molecular and electronic structure of a tungsten-tungsten quadruply bonded complex supported by two 6-carboethoxy-2-carboxylatoazulene ligands.

The preparation and molecular structure of a W2(4+)-quadruply bonded complex is reported wherein two mutually trans azulene-2-carboxylato ligands are shown to be strongly coupled by ligand pi-M2delta-ligand pi conjugation.

Studies of electronic coupling and mixed valency in metal-metal quadruply bonded complexes linked by dicarboxylate and closely related ligands.

Complexes of the form [(tBuCO2)3M2]2(mu-O2C-X-CO2), where M is Mo or W and X is a pi conjugated organic group, are ideally suited for studies of electronic coupling between the two redox centers via M2 delta-bridge pi conjugation. The complexes have intense metal-to-bridge charge-transfer transitions in the visible or near-IR region of the spectrum and exhibit thermo-, solvato- and electrochromic behavior. Chemical oxidation results in the formation of mixed-valence species that are particularly well-suited for the study of the class II/III border. The extent of electronic coupling is determined by a variety of spectroscopic techniques and, in particular, by EPR and electronic absorption spectroscopy. The latter provides a direct measure of the electronic coupling parameter Hab in pairs (Mo and W) of otherwise identical complexes. Similarly, the substitution within the bridge of the CO2 group by COS or RNCO allows an evaluation of the mechanism of the electronic coupling in closely related complexes. Electronic structure calculations employing density functional theory complement frontier molecular orbital theory in the interpretation of the physicochemical properties of these complexes.