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The straightforward synthesis of redox-active arylazothioformamide (ATF) ligands allows for electronic diversity as to measure the weak-binding interactions of transition metal salts in supramolecu...

This report describes the implementation of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye into the ligand framework of a borane. The redox-active nature of the BODIPY dye is utilized to generate a family of molecular boranes that are capable of exhibiting tunable Lewis acidities through BODIPY-based redox events.

Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2 NPh2 )(15 NH3 )][BArF4 ], (PtBu2 NPh2 =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; ArF =3,5-(CF3 )2 C6 H3 ), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N-H bonds and the formation of an N≡N bond was achieved by coupling H+ and e- transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6-tri-tert-butylphenoxyl radical (t Bu3 ArO. ) as the H atom acceptor. Employing an excess of t Bu3 ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15 N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond-forming step in the catalytic cycle.

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙−), and doubly reduced benzoquinone ([Q]2−) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol−1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid–Q/[Q]˙−/[Q]2− adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol−1, and the capacity to transfer a hydride by about 30 kcal mol−1.

The synthesis and characterization of three new organic hydrazines containing BODIPY dyes is described. The respective aminomethyl complexes were also synthesized to aid in the assignment of the physical properties that were hydrazine-based vs. BODIPY-based. Incorporation of a BODIPY dye into an organic hydrazine introduced a reduction event (average value of −1.70 V vs. Cp(2)Fe/Cp(2)Fe(+)). Although two irreversible oxidation events were observed, it was unclear whether the oxidation events arose from BODIPY-based or amine/hydrazine-based oxidations. The respective BODIPY-appended hydrazine complexes exhibited excited state lifetimes on the order of 2–6 ns, suggesting the presence of a singlet excited state. The excited state lifetimes of the BODIPY-appended hydrazine complexes were about a factor of ten greater than the respective aminomethyl complexes. Computational analysis showed that by appending a BODIPY dye to a hydrazine fragment the hydrazine fragment becomes more susceptible to transfer H(2) equivalents as protons and hydrides as opposed to H-atoms, which occurs with common organic hydrazines. Computational analysis also revealed that the BODIPY-based redox events can be used to manipulate the mechanism for H(2) transfer from the BODIPY-appended hydrazine, where a BODIPY-based reduction favors H-atom transfer and a BODIPY-based oxidation favors proton transfer followed by hydride transfer.

Fluorescent dyes have been widely utilized as chemical sensors and in photodynamic therapy, but exploitation of their redox-active nature in chemical reactions has remained mostly unexplored. This report describes the isolation of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based radical. The redox-active nature of the BODIPY compound can be utilized in combination with a guanidine center, the basicity of which can be manipulated by greater than 14 pKa units, to promote the conversion of protons and electrons into H-atoms for transfer to substrate molecules.

Redox-active ligands lead to ambiguity in often clearly defined oxidation states of both the metal centre and the ligand. The arylazothioformamide (ATF) ligand class represents a redox-active ligand with three possible redox states (neutral, singly reduced, and doubly reduced). ATF-metal interactions result in strong colorimetric transitions allowing for the use of ATFs in metal detection and/or separations. While previous reports have discussed dissolution of zerovalent metals, the resulting oxidation states of coordination complexes have proved difficult to interpret through X-ray crystallographic analysis alone. This report describes the X-ray crystallographic analysis combined with computational modelling of the ATF ligand and metal complexes to deconvolute the metal and ligand oxidation state of metal-ATF complexes. Metal(ATF)2 complexes that originated from zerovalent metals were found to exist as dicationic metal centers containing two singly reduced ATF ligands. When employing Cu(I) salts instead of Cu(0) to generate copper-ATF complexes, the resulting complexes remained Cu(I) and the ATF ligand remained "innocent", existing in its neutral state. Although the use of CuX (where X = Br or I) or [Cu(NCMe)4]Y (where Y = BF4 or PF6) generated species of the type: [(ATF)Cu(μ-X)]2 and [Cu(ATF)2]Y, respectively, the ATF ligand remained in its neutral state for each species type.

The synthesis and characterization of one or two BODIPY fragments appended to four new chiral and one new achiral diamines is described. All of the examined BODIPY-appended diamines exhibit a quasireversible-irreversible reduction, with two reductions (separated by about 100 mV) observed in the case of diamines containing two BODIPY molecules. Only the BODIPY-appended ortho-phenylenediamines did not fluoresce under UV light. Computational analysis showed that the absence of fluorescence of the BODIPY-appended ortho-phenylenediamines is likely due to intramolecular quenching of the excited state electron within the phenylenediamine ligand. Computational analysis also showed that the incorporation of a BODIPY molecule greatly reduces the basicity of the amine center, by about 10–14 pKa units. The BODIPY moiety was found to be more electron withdrawing than a tosyl and a pentafluorophenyl group, suggesting why excess metals are needed in heavy metal sensor applications (heteroatom-appended BODIPYs = poor ligands). An improved procedure for the scalable synthesis (greater than three grams) of 8-methanethio-BODIPY, a common starting material for the generation of heteroatom-appended BODIPY molecules, is also described.

This report describes the implementation of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye into the ligand framework of a Rh-based catalyst. The redox-active nature of the BODIPY dye is utilized to generate a catalyst that is capable of exhibiting redox-switchable catalytic behavior for the hydroboration of alkenes through a BODIPY-based reduction.

We present a strong correlation of the Ni(salophen) structure and properties measured in single-molecule vs bulk quantities and in ultra high vacuum vs solution phase. Under a scanning tunneling microscope (STM), Ni(salophen) forms a self-assembled monolayer (SAM) on Au(111) at 23 °C with molecular structure identical to that of the X-ray crystallographic measurement. The HOMO and LUMO levels are determined using elastic tunneling spectroscopy at the single-molecule level with confirmation by monolayer-quantity ultraviolet photoelectron spectroscopy (UPS) and by cyclic voltammetry (CV) measurements. The STM-determined HOMO-LUMO gap of 3.28 eV and (HOMO-1)-HOMO gap of 0.36 eV form a new foundation for the selection of hybrid functionals with a simple basis set to be effective in accurately calculating single-molecule Ni(salophen) frontier MO levels. Our results suggest that microscopy-based experiments on a surface, along with free-molecule gas-phase calculations, can provide useful insights into the physical properties of metal(salen) complexes, especially when such direct measurements are not available in solution.

This special web collection of Dalton Transactions focuses on the inorganic chemistry of the p-block elements, as a tribute to the 150th anniversary of the development of the periodic table.

The reduction of carbon dioxide (CO(2)) is of interest to the chemical industry, as many synthetic materials can be derived from CO(2). To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO(2) to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride’s ability to insert CO(2) into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO(2) insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO adduct of (t)Bu(3)P and B(C(6)F(5))(3) resulted in converting the sequestered CO(2) molecule to CO. Reduction of the CO(2) adduct of (t)Bu(3)P and B(C(6)F(5))(3) with both electrons and protons resulted in only proton reduction.

Redox-active ligands lead to ambiguity in often clearly defined oxidation states of both the metal centre and the ligand. The arylazothioformamide (ATF) ligand class represents a redox-active ligand with three possible redox states (neutral, singly reduced, and doubly reduced). ATF-metal interactions result in strong colorimetric transitions allowing for the use of ATFs in metal detection and/or separations. While previous reports have discussed dissolution of zerovalent metals, the resulting oxidation states of coordination complexes have proved difficult to interpret through X-ray crystallographic analysis alone. This report describes the X-ray crystallographic analysis combined with computational modelling of the ATF ligand and metal complexes to deconvolute the metal and ligand oxidation state of metal-ATF complexes. Metal(ATF)2 complexes that originated from zerovalent metals were found to exist as dicationic metal centers containing two singly reduced ATF ligands. When employing Cu(I) salts instead of Cu(0) to generate copper-ATF complexes, the resulting complexes remained Cu(I) and the ATF ligand remained “innocent”, existing in its neutral state. Although the use of CuX (where X = Br or I) or [Cu(NCMe)4]Y (where Y = BF4 or PF6) generated species of the type: [(ATF)Cu(μ-X)]2 and [Cu(ATF)2]Y, respectively, the ATF ligand remained in its neutral state for each species type.

Electron bifurcation, or the coupling of exergonic and endergonic oxidation-reduction reactions, was discovered by Peter Mitchell and provides an elegant mechanism to rationalize and understand the logic that underpins the Q cycle of the respiratory chain. Thought to be a unique reaction of respiratory complex III for nearly 40 years, about a decade ago Wolfgang Buckel and Rudolf Thauer discovered that flavin-based electron bifurcation is also an important component of anaerobic microbial metabolism. Their discovery spawned a surge of research activity, providing a basis to understand flavin-based bifurcation, forging fundamental parallels with Mitchell's Q cycle and leading to the proposal of metal-based bifurcating enzymes. New insights into the mechanism of electron bifurcation provide a foundation to establish the unifying principles and essential elements of this fascinating biochemical phenomenon.

Hydride transfer promoted by the coordination of a substrate molecule to a Lewis acid is a critical step in many catalytic transformations. This computational study investigates the nature of the interaction between a polar substrate molecule and a Lewis acid by examining the influence of Lewis acid strength on the ability to reduce (transfer a hydride to) the coordinated substrate molecule. To investigate this interaction, the coordination of 10 probe substrates to seven Lewis acids was analyzed. Coordination of the probe substrate molecules to a Lewis acid resulted in a more favorable reduction of the substrate molecule by 20-70 kcal mol-1. Further examination of the coordination of the substrate molecules to Lewis acids of varying Lewis acid strengths resulted in a direct linear correlation between the ability of the Lewis acid-substrate adduct to accept a hydride and the Lewis acid strength. The linear correlations also revealed that between 44 and 70% of the Lewis acidity of the Lewis acids translated to the Lewis acid-substrate adducts. From the results obtained in this study, the minimum Lewis acid strength needed to activate the substrates for the reduction with [BH4]- and the implications of employing a Lewis acid to promote the reduction of an unsaturated polar substrate in catalytic reactions are also described.

Interest in reductions with main group hydrides has been reinvigorated with the discovery of frustrated Lewis pairs. Computational analysis showed that the borohydride of the commonly used Lewis acid B(C6F5)3 was determined to be 15 kcal/mol less reducing than borohydride ([BH4]−), 22 kcal/mol less reducing than aluminum hydride ([AlH4]−), and 41 kcal/mol less reducing than superhydride ([HBEt3]−). In addition to [HB(C6F5)3]−, a hydride donor ability scale with an estimated error of ∼3 kcal/mol includes 132 main group hydrides with gradually changing reducing capabilities spanning 160 kcal/mol. The scale includes representatives from organosilanes, organogermanes, organostannanes, borohydrides, boranes, aluminum hydrides, NADH analogues, and CH hydride donors. The large variety of reducing agents and the wide span of the scale (ranging from 0.5 to 160 kcal/mol in acetonitrile) make the scale a useful tool for the future design of metal-based or main group reducing agents.

The steric bulk of reducing agents has been widely employed in the generation of chiral centers and selective reductions of organic compounds, but reports addressing the quantification of the steric bulk surrounding the main group hydride have been virtually non-existent. To address the limited amount of steric bulk information of main group reducing agents, the cone angles of 84 main group hydrides related to the chemistry of frustrated Lewis pairs are reported. Of the 84 main group hydrides, the steric bulk of two carbon-based hydrides, four aluminum hydrides, 11 borane-Lewis base adducts derived from borenium cations, nine organosilanes, two organostannanes, one organogermane, and 55 borohydrides are investigated. In addition to the main group hydride complexes reported, the cone angle of four highly active metal-hydride complexes, used in the asymmetric reduction of ketones, are determined and compared with chiral main group hydrides. Steric bulk analysis suggests that chiral hydride complexes exhibiting cone angles >165° achieve highly enantioselective reductions (>90 % ee).

This manuscript describes a combination of DFT calculations and experiments to assess the reduction of borazines (B–N heterocycles) by η6-coordination to Cr(CO)3 or [Mn(CO)3]+ fragments. The energy requirements for borazine reduction are established as well as the extent to which coordination of borazine to a transition metal influences hydride affinity, basicity, and subsequent reduction steps at the coordinated borazine molecule. Borazine binding to M(CO)3 fragments decreases the thermodynamic hydricity by >30 kcal mol−1, allowing it to easily accept a hydride. These hydricity criteria were used to guide the selection of appropriate reagents for borazine dearomatization. Reduction was achieved with an H2-derived hydride source, and importantly, a pathway which proceeds through a single electron reduction and H-atom transfer reaction, mediated by anthraquinone was uncovered. The latter transformation was also carried out electrochemically, at relatively positive potentials by comparison to all prior reports, thus establishing an important proof of concept for any future electrochemical BN bond reduction.

In this computational study, the thermodynamics of hydrogen, hydride, and proton transfer from 22 phosphonium-borohydride intramolecular and intermolecular frustrated Lewis pairs (FLPs) to eight probe substrates was investigated. The purpose of this study was to gain insight into the thermodynamics of H2 transfer with intramolecular phosphonium-borohydrides; to determine whether intramolecular or intermolecular FLPs are preferred in FLP-catalyzed hydrogenation reactions. Comparison of the computed thermodynamic values showed that by connecting a borohydride and phosphonium center through a linker, H2 loss from the respective intramolecular phosphonium-borohydride became less favorable by about five and seven kcal mol−1 in acetonitrile and toluene, respectively. Connecting the borohydride and phosphonium centers also resulted in both hydride and proton loss becoming less favorable, on average, by about 10.0 kcal mol−1 and about 4.6 pKa units, respectively. Analysis of hydrogen, proton, and hydride transfer to eight probe substrates showed that initial proton transfer is 49 and 20 kcal mol−1 more favorable than the initial hydride transfer in the reduction of nitrogen-containing and oxygen-containing unsaturated substrates, respectively. These results suggest that proton transfer, followed by hydride transfer occurs in the reduction of imines, ketones, aldehydes, and enamines. From the thermodynamic analysis of proton and hydride transfer, an intramolecular phosphonium-borohydride was the desired catalyst for the reduction of imines and enamines, while an intermolecular phosphonium-borohydride was the favored catalyst for the reduction of ketones and aldehydes.

Proton transfer promoted by the coordination of protogenic Lewis bases to a Lewis acid is a critical step in catalytic transformations. Although the acidification of water upon coordination to a Lewis acid has been known for decades, no attempts have been made to correlate the Bronsted acidity of the coordinated water molecule with Lewis acid strength. To probe this effect, the pKa's (estimated error of 1.3 pKa units) in acetonitrile of ten protogenic Lewis bases coordinated to seven Lewis acids containing Lewis acidities varying 70 kcal mol−1, were computed. To quantify Lewis acid strength, the ability to transfer a hydride (hydride donor ability) from the respective main group hydride was used. Coordination of a Lewis acid to water increased the acidity of the bound water molecule between 20 and 50 pKa units. A linear correlation exhibiting a 2.6 pKa unit change of the Lewis acid–water adduct per ten kcal mol−1 change in hydride donor ability of the respective main group hydride was obtained. For the ten protogenic Lewis bases studied, the coordinated protogenic Lewis bases were acidified between 10 and 50 pKa units. On average, a ten kcal mol−1 change in hydride donor ability of the respective main group hydride resulted in about a 2.8 pKa unit change in the Bronsted acidity of the Lewis acid–Lewis base adducts. Since attempts to computationally investigate the pKa of main group dihydrogen complexes were unsuccessful, experimental determination of the first reported pKa of a main group dihydrogen complex is described. The pKa of H2-B(C6F5)3 was determined to be 5.8 ± 0.2 in acetonitrile.

Protonation of an iron C–H activated complex containing pendant amines in the presence of N2 generated a cis-(H)FeII–N2 complex. Addition of acid protonates the pendant amines. Reduction of the protonated complex results in N2 loss and H2 formation, followed by N2 binding. The origin of H2 formation in this Fe system is compared to proposed mechanisms for H2 loss and N2 coordination in the E4 state of nitrogenase.

The addition of acids to ferrous dinitrogen complexes [FeX(N2)(P(Et)N(Me)P(Et))(dmpm)](+) (X = H, Cl, or Br; P(Et)N(Me)P(Et) = Et2PCH2N(Me)CH2PEt2; and dmpm = Me2PCH2PMe2) gives protonation at the pendent amine of the diphosphine ligand rather than at the dinitrogen ligand. This protonation increased the νN2 band of the complex by 25 cm(-1) and shifted the Fe(II/I) couple by 0.33 V to a more positive potential. A similar IR shift and a slightly smaller shift of the Fe(II/I) couple (0.23 V) was observed for the related carbonyl complex [FeH(CO)(P(Et)N(Me)P(Et))(dmpm)](+). [FeH(P(Et)N(Me)P(Et))(dmpm)](+) was found to bind N2 about three times more strongly than NH3. Computational analysis showed that coordination of N2 to Fe(II) centers increases the basicity of N2 (vs free N2) by 13 and 20 pKa units for the trans halides and hydrides, respectively. Although the iron center increases the basicity of the bound N2 ligand, the coordinated N2 is not sufficiently basic to be protonated. In the case of ferrous dinitrogen complexes containing a pendent methylamine, the amine site was determined to be the most basic site by 30 pKa units compared to the N2 ligand. The chemical reduction of these ferrous dinitrogen complexes was performed in an attempt to increase the basicity of the N2 ligand enough to promote proton transfer from the pendent amine to the N2 ligand. Instead of isolating a reduced Fe(0)-N2 complex, the reduction resulted in isolation and characterization of HFe(Et2PC(H)N(Me)CH2PEt2)(P(Et)N(Me)P(Et)), the product of oxidative addition of the methylene C-H bond of the P(Et)N(Me)P(Et) ligand to Fe.

The activation of amines by B(C6F5)3 is shown to effect the catalytic racemization of a chiral amine. Extending this strategy to a bimolecular process, catalytic transfer hydrogenation of imines, enamines, and N-heterocycles is demonstrated using iPr2NH as the source of hydrogen and a catalytic amount of the Lewis acid B(C6F5)3.

The addition of H3PO4 to Cp*Ir(TsDPEN-H), where TsDPEN = H2NCHPhCHPhNTs–, is a simple method to obtain a water-soluble hydrogenation catalyst capable of reducing aromatic ketones to their corresponding alcohols in aqueous solutions. Key to the reactivity is the low affinity of the coordinatively unsaturated [Cp*Ir(TsDPEN)]+ for H2PO4–. Catalyst degradation proceeds via the protonation of the tosylamido ligand, as was established by the crystallographic characterization of the tosylamine complex [Cp*Ir(NCMe)(HTsDPEN)]2+.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Super model: Diiron dithiolates exist in nature in [FeFe]-hydrogenase, for the purpose of making or oxidizing molecular hydrogen. Evidence for how H2 interacts with diiron complexes has been realized using model diiron dithiolato carbonyl complexes. The pathway for this interaction has been elucidated computationally as well as synthetically, by using silanes in place of H2.

The coordination tendencies of the unsaturated 16e Lewis acid [Cp*Ir(TsDPEN)]+ ([1H]+), where TsDPEN is H2NCHPhCHPhNTs−, are surveyed, together with parallel studies on analogous complexes such TsDACH (TsDACH = H2NC6H10NTs−) and Tsen (Tsen = H2NC2H4NTs−) derivatives as well as Rh analogues. Crystallographic analyses of the adducts of [Cp*IrL(TsDPEN)]+, where L = NCMe, NH3, PPh3, and CO, and [Cp*Ir(CO)(R,R-TsDACH)]+ are described. In the TsDPEN system, the Lewis base adducts contain an absolute configuration that is opposite that for the TsDPEN ligand and feature equatorial phenyl groups. In the case of [Cp*Ir(CO)(R,R-TsDACH)]+, both R and S metal centers cocrystallize. Isomerization of the R to the S metal center was first order in [Cp*(R-Ir)(CO)(R,R-TsDACH)]+ with minimal solvent effects. The pKa of the amine of the Lewis base adducts correlated linearly with the pKa of the free ligand in MeCN and the pKa of the amine (H2NCHPhCHPhNTs) of the Lewis base adduct in MeCN. Amines with pKa < 16 (MeCN scale), i...

The synthesis of a series of P(Et)P(NRR(')) (P(Et)P(NRR(')) = Et2PCH2CH2P(CH2NRR')2, R = H, R' = Ph or 2,4-difluorophenyl; R = R' = Ph or (i)Pr) diphosphine ligands containing mono- and disubstituted pendant amine groups and the preparation of their corresponding molybdenum bis(dinitrogen) complexes trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of triflic acid (HOTf) to trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes in tetrahydrofuran at -40 °C show that the electronic and steric properties of the R and R' groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N2 ligand, or a combination of these sites. For example, complexes containing monoaryl-substituted pendant amines are protonated at Mo and the pendant amine site to generate mono- and dicationic Mo-H species. Protonation of the complex containing less basic diphenyl-substituted pendant amines exclusively generates a monocationic hydrazido (Mo(NNH2)) product, indicating preferential protonation of an N2 ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo(NNH2) product that is additionally protonated at a pendant amine site. In addition, trans-Mo(N2)2(PMePh2)2(depe) (depe = Et2PCH2CH2PEt2) was synthesized to serve as a counterpart lacking pendant amines. Treatment of this complex with HOTf generated a monocationic Mo(NNH2) product. Protonolysis experiments conducted on several complexes in this study afforded trace amounts of NH4(+). Computational analysis of trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes provides further insight into the proton affinity values of the metal center, N2 ligand, and pendant amine sites to rationalize differences in their reactivity profiles.

[84029-37-8] C20H32Cl2Ir2 (MW 727.81) InChI = 1S/2C10H15.2ClH.2Ir.2H/c2*1-6-7(2)9(4)10(5)8(6)3;;;;;;/h2*1-5H3;2*1H;;;;/q;;;;2*+1;;/p-2 InChIKey = UJBRNSQETRDVQC-UHFFFAOYSA-L (reagent used for oxidation, hydrogenation, hydrodesulfurization, and arylation reactions) Alternate Names: [Cp*IrCl(μ-H)]2, [Cp*IrHCl]2, where Cp* = η5-C5Me5. Physical Data: mp 210 °C (dec.).1 Solubility: sol toluene, benzene, aromatics, alcohols insol hexanes, slightly sol ethers. Form Supplied in: dark blue solid that is not currently commercially available. Analysis of Reagent Purity: 1H NMR1, 2 and IR1. Preparative Methods: prepared from a slurry of [Cp*IrCl2]2 in benzene, isopropanol, and triethylamine at room temperature for 24 h in 95% yield.1 Also, can be prepared from [Cp*IrCl2]2 with KBH4 in isopropanol for 12 h in 90% yield.2 A deuterated derivative can be prepared from a slurry of [Cp*IrCl2]2 with triethylamine under D2 for 36 h (80% yield).1 Purification: recrystallized from addition of hexanes to isopropanol solutions or from addition of light petroleum to benzene solutions. Common impurities are [Cp*IrCl2]2, [Cp*IrCl]2(μ-Cl) (μ-H), and [Cp*Ir]2 (μ-H)3+. Handling, Storage, and Precautions: blue solid exhibiting moderate air stability. Solids are preferentially stored under an inert atmosphere. Detailed toxicological information is unknown. [Cp*IrCl(μ-H)]2 reacts with dienes in polar solvents (ethanol or acetone) resulting in η1-methylallyl complexes.1 [Cp*IrCl(μ-H)]2 can also isomerize olefins (i.e., 4-methylcyclohexene and 1-methylcyclohexene).1

Hydrogenation of the N-bound phenyl rings of amines, imines, and aziridine is achieved in the presence of H(2) and B(C(6)F(5))(3), affording the corresponding N-cyclohexylammonium hydridoborate salts.

The hydrogenation of chiral imines in the presence of tris(pentafluorophenyl)borane as catalyst is studied.

The paper surveys the binding of anions to the unsaturated 16e Lewis acid [Cp*Ir(TsDPEN)](+) ([1H](+)), where TsDPEN is racemic H(2)NCHPhCHPhNTs(-). The derivatives Cp*IrX(TsDPEN) were characterized crystallographically for X(-) = CN(-), Me(C═NH)S(-), NO(2)(-), 2-pyridonate, and 0.5 MoS(4)(2-). [(1H)(2)(μ-CN)](+) forms from [1H](+) and 1H(CN). Aside from 2-pyridone, amides generally add reversibly and bind to Ir through N. Thioacetamide binds irreversibly through sulfur. Compounds of the type Cp*IrX(TsDPEN) generally form diastereoselectively, although diastereomeric products were observed for the strong ligands (X = CN(-), H(-) (introduced via BH(4)(-)), or Me(C═NH)S(-)). Related experiments on the reaction (p-cymene)Ru(TsDPEN-H) + BH(4)(-) gave two diastereomers of (p-cymene)RuH(TsDPEN), the known hydrogenation catalyst and a second isomer that hydrogenated acetophenone more slowly. These experiment provide new insights into the enantioselectivity of these catalysts. Diastereomerization in all cases was first order in metal with modest solvent effects. The diphenyl groups are generally diequatorial for the stable diastereomers. For the 2-pyridonate adduct, axial phenyl groups are stabilized in the solid state by puckering of the IrN(2)C(2) ring induced by intramolecular hydrogen-bonding. Crystallographic analysis of [Cp*Ir(TsDPEN)](2)(MoS(4)) revealed a unique example of a κ(1),κ(1)-tetrathiometallate ligand. Cp*Ir(SC(NH)Me)TsDPEN) is the first example of a κ(1)-S-thioamidato complex.

In 2006, our group reported the first metal-free systems that reversibly activate hydrogen. This finding was extended to the discovery of “frustrated Lewis pair” (FLP) catalysts for hydrogenation. It is this catalysis that is the focal point of this article. The development and applications of such FLP hydrogenation catalysts are reviewed, and some previously unpublished data are reported. The scope of the substrates is expanded. Optimal conditions and functional group tolerance are considered and applied to targets of potential commercial significance. Recent developments in asymmetric FLP hydrogenations are also reviewed. The future of FLP hydrogenation catalysts is considered.

Reductions of chiral ketimines effected under H(2) by catalytic amounts of B(C(6)F(5))(3) result in moderate to excellent diastereoselectivities. In the case of camphor and menthone derived imines, the reductions proceeded with greater than 95% diastereoselectivity.

The series of o-benzylphosphino-boranes, o-(R(2)B)C(6)H(4)CH(2)PtBu(2) (R = Cl 3, Ph 4, Cy 6, C(6)F(5) 7, Mes 8) and o-(BBN)C(6)H(4)CH(2)PtBu(2) (5), were synthesized from reactions of the respective chloroboranes with the lithiated benzyl-phosphine. In an analogous fashion, the α-methylbenzyl(N,N-dimethyl)amine-boranes o-(R(2)B)C(6)H(4)CH(Me)NMe(2) (R = Cl 10, Ph 11, Cy 12, C(6)F(5) 13, Mes 14) were prepared. While these species were inactive in the catalytic hydrogenation of tBuN═CHPh, compounds 7, 8, and 14 did react with H(2) at elevated temperatures (100 °C), resulting in the elimination of HC(6)F(5) and mesitylene, respectively. In the latter case, the species o-((Mes)HB)C(6)H(4)CH(Me)NMe(2) 15 was isolated. 14 was also shown to react with H(2)O to give the species o-((Mes)(HO)B)C(6)H(4)CH(Me)NMe(2) 16 with the loss of mesitylene. The structure of these compounds and the nature of these reactions were also probed spectroscopically, crystallographically, and computationally. The energies for the products of hydrogenation, the phosphonium and ammonium hydridoborates, were computed. In all cases, these products were endothermic with respect to the precursor phosphine-boranes and amine-boranes and H(2). The barriers to H(2) activation were found to be in the range of 24-38 kcal/mol. These theoretical studies also demonstrate that the steric bulk around the boron center dramatically affects the activation barrier for H(2) activation, while the Lewis acidity of the borane has the largest effect on the stabilization of the resulting onium-borohydride. In the case of the elimination reactions, the driving forces appear to be the loss of arene byproduct and formation of a strong donor-acceptor bond.

This study examines the acid-catalyzed hydrogenation of ketones by amido-amine chelates of Ru and Ir, focusing on the hydrogen activation step. Addition of H(2) to the catalyst Cp*Ir(TsDPEN-H) (1, TsDPEN = racemic H(2)NCHPhCHPhNTs(-)) is more favorable than for corresponding (cymene)Ru derivatives. Depending on the acid, the rate of the proton-catalyzed addition of H(2) to 1 varies over 3 orders of magnitude even for strong acids. Acids protonate the NH center in the five-coordinate diamides to give the amido-amine, e.g., [Cp*Ir(TsDPEN)](+) ([1H](+)). The rate of proton-catalyzed hydrogenation of 1 was found to be first order in both H(2) and in [1H](+) for X(-) = BF(4)(-), OTf(-), ClO(4)(-), NO(3)(-). For X(-) = ClO(4)(-) and BAr(F)(4)(-) (BAr(F)(4)(-) = B(C(6)H(3)-3,5-(CF(3))(2))(4)(-)), the rate showed an additional dependence on [1]. The hydrogenation of 1 is proposed to occur via the dihydrogen complex ([1H(H(2))](+)) followed by proton transfer to 1, either directly (third-order pathway) or via anion-assisted proton transfer (second-order pathway). The pK(a) (H-H bond) of [1H(H(2))](+) is predicted to be 13.88 +/- 0.37 (MeCN solution) whereas the pK(a) (N-H bond) of [1H](+) is about 21.6. The rate of hydrogenation of 1 was fastest for acids about 3 orders of magnitude (pK(a) approximately 10) more acidic than [1H(H(2))](+), but slower for stronger acids. Although the affinity of H(2) for [Cp*Ir(TsDPEN)](+) is orders of magnitude lower than for 1 (298 K), the cationic complex adds H(2) far faster. Similar trends are seen for (cymene)Ru(TsDPEN-H) (2) and its derivatives. The affinity of H(2) for 2 was found to be 3x less than for 1.

Condensation of Fe2(SH)2(CO)6, acetaldehyde, and (NH4)2CO3 affords the methyl-substituted azadithiolate Fe2[(SCHMe)2NH](CO) 6 (1). The complex exists mainly ∼95%) as the meso diastereomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d,l isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two 13C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO)3 groups. The pKa value of the amine in 1 is 7.89 (all pKa's determined in MeCN solution), which is similar to a redetermined value for Fe2[(SCH2)2NH](CO)6 (2, pK a = 7.98) and only slightly less basic than the tertiary amine Fe2[(SCH2)2NMe](CO)6 (pKa = 8.14). Substitution of 1 with PMe3 proceeded via the intermediacy of two isomers of Fe2[(SCHMe)2NH](CO)5(PMe 3), affording Fe2[(SCHMe)2NH](CO) 4(PMe3)2 (3). 31P NMR spectra confirm that the two PMe3 ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pKa value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe2[SCHMe)2NH](CO)5(PPh3) as a single regioisomer. © 2008 American Chemical Society.

Organometallic complexes containing non-innocent ligands of the type Cp*Ir(tBAFPh)(1), where H2tBAFPh is 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, were found to activate H2 in a redox-switchable manner. The 16e- complex 1 was inert with respect to H2, CO, as well as conventional basic substrates until oxidation. Oxidation of 16-electron 1 with 1 equiv of Ag+ resulted in ligand-centered oxidation affording salts of [1]+, which were characterized by crystallographically, EPR, and elemental analyses. [1]+ was reduced to 1 in the presence of H2 and the sterically hindered base, 2,6-(tBu)2C5H3N, via a pathway that is first-order in both metal and dihydrogen. Compound [1]+ forms adducts with MeCN, which inhibits catalysis. The catalytic oxidation of H2 was established by electrochemical methods to be associated with the monocation.

Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (1H(H)) with O2 generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O2]), resulting in an overall rate constant of 0.024 +/- 0.013 M(-2) s(-1). Isotopic labeling revealed that the rate of the reaction of 1H(H) + O2 was strongly affected by deuteration at the hydride position (k(HH2)/k(DH2) = 6.0 +/- 1.3) but insensitive to deuteration of the amine (k(HH2)/k(HD2) = 1.2 +/- 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated DeltaH = 82.2 kJ/mol, DeltaS = 13.2 J/mol K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl2 solution under 0.30 atm of H2 and 0.13 atm of O2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BAr(F)4 (BAr(F)4- = B(C6H3-3,5-(CF3)2)4-). The formation of water from H2 was verified by 2H NMR for the reaction of D2 + O2. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O2 with RNH2BH3 (R = H, t-Bu), resulting in the formation of water and H2. The deactivation of the catalyst 1 in its reactions with O2 was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O2.

Oxidation of Cp*Ir((rac-TsDPEN)H (DPEN = H2NCHPhCHPhNTs) with Cp2FePF6 or Ph3CPF6 in MeCN solution generates [Cp*Ir(TsDPEN)(NCMe)]PF6 ([1H(NCMe)]PF6) together with H2 and Ph3CH, respectively. Labeling studies revealed that the Ir-H was abstracted. The formation of a transient electrophilic species is implicated by the formation of a cyclometalated derivative. The labile species [1H(NCMe)]+ was also obtained by protonation of the diamido derivative Cp*Ir(TsDPEN-H) (1) in MeCN solution (BArF4- = B(C6H3-3,5-(CF3)2)4-). The unsaturated, "naked" cation [1H]BArF4 can be prepared by protonation of 1 with H(OEt2)2BArF4 in CH2Cl2 solution or by thermal elimination of MeCN from [1H(NCMe)]+. Crystallographic analysis confirms the structure of this 16e cation in [1H]BArF4. The formally unsaturated species 1 and [1H]BArF4 have strongly contrasting Lewis acidities, with the cation binding PPh3, CO, and NH3. 1 does not measurably bind these same ligands. [1H]BArF4 is reactive toward H2, at least in the absence of inhibiting donor ligands such as MeCN. [1H]BArF4 (CH2Cl2 solutions) catalyzes the addition of H2 to 1 by proton transfer from an apparent dihydrogen complex. This work demonstrates that the protonation activates the Lewis acidity of unsaturated Ir(III) amides, giving rise to novel organometallic Lewis acids.