Intramolecular heterolytic dihydrogen cleavage by a bifunctional frustrated pyrazolylborane Lewis pair

The reaction of bis(pentafluorophenyl)borane, HB(C(6)F(5))(2), with 3,5-di-tert-butyl-1H-pyrazole (3) affords the zwitterionic pyrazolium-borate trans-5 and, after dehydrogenation by use of the frustrated carbene-borane Lewis pair 1/B(C(6)F(5))(3), the bifunctional pyrazolylborane 6, which is able to cleave dihydrogen heterolytically with the formation of a mixture of cis-5 and trans-5.     

Catalytic hydrogenation of polar organic functionalities based on Ru-mediated heterolytic dihydrogen cleavage

This article highlights Ru complexes, which effect catalytic hydrogenation of polar organic functionalities containing C-O bonds other than aldehydes or ketones. The unique ability of Ru complexes to undergo heterolytic dihydrogen cleavage seems to play a key role in these catalyses.     

Rapid intramolecular heterolytic dihydrogen activation by a four-membered heterocyclic phosphane-borane adduct

A four-membered cyclic intramolecular phosphane-borane adduct activates dihydrogen to yield the respective ethylene-bridged zwitterionic phosphonium-hydridoborate system, which reduces benzaldehyde.     

Facile activation of H-H and Si-H bonds by boranes

The borane B(C(6)F(5))(3) is a precatalyst for H/Dexchange between H(2) and deuterium-labeled silanes (D(3)SiPh, D(2)SiMePh, DSiMe(2)Ph, DSiEt(3)). Experimental and DFT studies reveal that B(C(6)F(5))(3) itself cannot activate dihydrogen but converts to HB(C(6)F(5))(2) under the action of hydrosilane. The latter species easily activates H-H and Si-H bonds by a σ-bond metathesis mechanism, which was further confirmed by the reactions of BD(3)·THF with H(2).     

Hydride complexes of ruthenium derived from the heterolytic activation of dihydrogen by amidophosphine complexes

The reaction of 〚NPNH〛Ru(η³:η²-cyclooctadienyl) (1) (where 〚NPNH〛 = {PhNHSiMe₂CH₂P(Ph)CH₂SiMe₂NPh}), an organometallic mono-amide complex of ruthenium(II), with hydrogen gas (1–4 atm) generates three ruthenium hydride species: 〚NPNH〛RuH (2), 〚NPNH₂〛RuH₂(C₇H₈) (3) and 〚NPNH₂〛RuH₂ (4). All of these complexes result from hydrogenation of the cyclooctadienyl group; complexes 3 and 4 also undergo conversion of the amido linkage into a ruthenium hydride and an amine. Complexes 2 and 3 have been characterized both in solution by NMR spectroscopy and in the solid state by X-ray Diffraction and Infrared Spectroscopy. While 4 was fully characterized in solution by NMR spectroscopy, attempts to recrystallize this material yielded 2; the reaction of 2 with H₂ does not produce 4. The starting complex 1 acts as a catalyst precursor for the hydrogenation of imines such as benzylidene aniline; however, none of the isolated hydride species 2, 3 or 4 were active as catalyst precursors.     

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.     

Heterolytic cleavage of dihydrogen promoted by sulfido-bridged tungsten-ruthenium dinuclear complexes

A series of sulfido-bridged tungsten-ruthenium dinuclear complexes Cp*W(mu-S)(3)RuX(PPh(3))(2) (4a; X = Cl, 4b; X = H), Cp*W(O)(mu-S)(2)RuX(PPh(3))(2) (5a; X = Cl, 5b; X = H), and Cp*W(NPh)(mu-S)(2)RuX(PPh(3))(2) (6a; X = Cl, 6b; X = H) have been synthesized by the reactions of (PPh(4))[Cp*W(S)(3)] (1), (PPh(4))[Cp*W(O)(S)(2)] (2), and (PPh(4))[Cp*W(NPh)(S)(2)] (3), with RuClX(PPh(3))(3) (X = Cl, H). The heterolytic cleavage of H(2) was found to proceed at room temperature upon treating 5a and 6a with NaBAr(F)(4) (Ar(F) = 3, 5-C(6)H(3)(CF(3))(2)) under atmospheric pressure of H(2), which gave rise to [Cp*W(OH)(mu-S)(2)RuH(PPh(3))(2)](BAr(F)(4)) (7a) and [Cp*W(NHPh)(mu-S)(2)RuH(PPh(3))(2)](BAr(F)(4)) (8), respectively. When Cp*W(O)(mu-S)(2)Ru(PPh(3))(2)H (5b) was treated with a Br?nstead acid, [H(OEt(2))(2)](BAr(F)(4)) or HOTf, protonation occurred exclusively at the terminal oxide to give [Cp*W(OH)(mu-S)(2)RuH(PPh(3))(2)](X) (7a; X = BAr(F)(4), 7b; X = OTf), while the hydride remained intact. The analogous reaction of Cp+W(mu-S)(3)Ru(PPh(3))(2)H (4b) led to immediate evolution of H(2). Selective deprotonation of the hydroxyl group of 7a or 7b was induced by NEt(3) and 4b, generating Cp*W(O)(mu-S)(2)Ru(PPh(3))(2)H (5b). Evolution of H(2) was also observed for the reactions of 7a or 7b with CH(3)CN to give [Cp*W(O)(mu-S)(2)Ru(CH(3)CN)(PPh(3))(2)](X) (11a; X = BAr(F)(4), 11b; X = OTf). We examined the H/D exchange reactions of 4b, 5b, and 7a with D(2) and CH(3)OD, and found that facile H/D scrambling over the W-OH and Ru-H sites occurred for 7a. Based on these experimental results, the mechanism of the heterolytic H(2) activation and the reverse H(2) evolution reactions are discussed.     

Facile activation of dihydrogen by an unsaturated heavier main group compound

The germanium alkyne analogue Ar'GeGeAr' (1, Ar' = C6H3-2,6(C6H3-2,6-Pri2)2) reacts with 1, 2, or 3 equiv of dihydrogen at room temperature, and at 1 atm pressure, to afford a mixture of the products Ar'HGeGeHAr' (2), Ar'H2GeGeH2Ar' (3), or Ar'GeH3 (4). The relative amounts of each product are governed by the number of equivalents of hydrogen used. A mechanism for the initial step in the reaction is proposed. The appearance of 4 among the reaction products was accounted for in terms of either its dissociation to monomers or isomerization to the bridged Ar'Ge(mu-H)2GeAr'. The reactions were monitored by 1H NMR spectroscopy. The products 2, 3, and 4 were characterized by X-ray crystallography, and 4 was synthesized independently by the reduction of Ar'Ge(OMe)3. These reactions represent the first direct addition of hydrogen to a closed shell unsaturated main group compound under ambient conditions.     

Selective heterolytic P-P bond cleavage of white phosphorus by a frustrated carbene-borane Lewis pair

Frustrated carbene-borane Lewis pairs are able to affect the selective cleavage of one of the six P-P bonds in white phosphorus (P(4)) to afford an adduct, in which an abnormal carbene of the imidazolium-4-yl type and B(C(6)F(5))(3) are bound in a trans,trans fashion to a butterfly-like bicyclo[1.1.0]tetraphosphabutane moiety.     

Facile conversion of alcohols into esters and dihydrogen catalyzed by new ruthenium complexes

An efficient, environmentally benign method for the preparation of esters from alcohols under mild, neutral conditions without the need for carboxylic acid derivatives and condensing agents was developed. Catalyst design, based on new Ru(II) hydrido carbonyl complexes incorporating electron-rich PNP and PNN ligands has resulted in the novel complex (I) which is an outstanding catalyst for the dehydrogenation of primary alcohols to esters and H(2) under neutral conditions.     

Facile Ru-H2 heterolytic activation and intramolecular proton transfer assisted by basic N-centers in the ligands

The use of the phosphine PPh2py instead of PPh3 in complexes of the type [Cp*RuH(P)2] enormously alters the kinetic control of the proton-transfer reactions over this compound and its chemical behavior. The reaction at low temperature of [Cp*RuH(PPh2py)2], 2, with HBF4 gives as products the classical dihydride trans-[Cp*RuH2(PPh2py)2](BF4), 3 (1 equiv of HBF4) or the dihydrogen-bonded complex [Cp*RuHH(PPh2pyH)(PPh2py)](BF4)2, 4 (2 equiv of HBF4). These complexes exhibit very accessible intramolecular processes of proton transfer, and finally, a slow release of H2 takes place at room temperature. Derivatives 2 and 3 are active catalysts for the deuterium labeling of H2 using methanol-d4 as an isotopic source. This demonstrates that the release of hydrogen is reversible, that the heterolytic activation of H2 is an easy process, and that acid species participate in the intramolecular proton-transfer processes. These observations are supported by reaction-coordinate calculations at the DFT/B3LYP level that show the existence of a low-energy reaction path that easily transforms the classical trans dihydride complex into the nonclassical cis dihydrogen compound in a reversible way, through the involvement of hydrogen- and dihydrogen-bonded intermediates and the essential participation of the pyridine centers. The different energy minima of this reaction profile are very accessible through low-energy transition states, all of which have been located.     

A new strategy for neurochemical photodelivery: metal-ligand heterolytic cleavage

A new strategy to build caged-compounds is presented. The approach is based on heterolytic photocleavage of a metal-ligand bond in a coordination compound. A ruthenium polypyridine complex, containing the neurocompound 4-amino pyridine (4AP) is used as the core of the phototrigger. The biomolecule is released by irradiation with visible light (>480 nm). The liberated 4AP promotes the activation of a leech neuron by means of blocking its K+ channels. The syntesis, characterization, and the inherent advantages of this method are discussed.     

Copper-mediated cleavage of disulfides by tertiary phosphines: a new route to As-S anions

The synthesis of [(PhAs)2(micro-S2)(micro-S)] and its reactions to form novel Cu(I) complexes are reported, together with a new cleavage reaction of disulfides by Cu(I) thiolates and tertiary phosphines.     

Reaction of coordinated phosphines: VI. Divalent palladium promoted cleavage of carbon—phosphorus bonds in tertiary phosphines

The important role of divalent palladium in the cleavage of carbon—phosphorus bond of tertiary phosphines is revealed by the study of the phenylation in the Pd(OAc)₂Ph₃P-styrene system under various conditions; reaction atmosphere, ratio of Ph₃P/Pd(OAc)₂, and addition of ethanol or Cu^II(OAc)₂ · H₂O.     

Complexes of tertiary phosphines with iron(II) and dinitrogen,dihydrogen, and other small molecules

The treatment of [FeCl₂(diphosphine)₂] with sodium borohydride in ethanol produces the hydrides [FeH₃(diphosphine)₂]⁺ in high yield. These complexes react with a variety of small molecules including N₂ to give further complexes. CO₂ and CS₂ insert into the iron hydride bond to yield formato- and dithioformato-complexes, respectively, and carboxymethyl acetylene and phenyl acetylene yield a cyclic alkenyl complex and an acetylide, respectively.     

A "push-pull" mechanism for heterolytic o-o bond cleavage in hydroperoxo manganese porphyrins

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H(2)O(2) to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H(2)O(2). The visible spectrum of a compound 0 type intermediate, assigned as Mn(III)(OH)(OOH)TDMImP, can be directly observed under saturating H(2)O(2) conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (ΔH(≠) = 4.2 ± 0.2 kcal mol(-1)) and a large, negative activation entropy (ΔS(≠) = -36 ± 1 cal mol(-1) K(-1)). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s(-1). These observations indicate that the O-O bond in Mn(III)(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) (-)OH is partially deprotonated ("push"), while the terminal oxygen in (-)OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.