Halide ligand protonation

RT, the protonation equilibrium constant is 203 M for RhCKCOCAsPhj) it is 75 M. Similar K values are found for Ir and Rh complexes with phosphine and halide ligands. Finally, ruthenocene is half-protonated in H SO with an acidity function, Hg, of —5.7, whereas half-protonation of ferrocene requires an Hg of —7.7. 

The higher kinetic acidity of H2 complexes requires that the reverse reaction, protonation of a metal hydride, occur at H rather than at M, for which there is ample evidence. Actually protonation at the hydride is misleading because it is really the M-H bond that is protonated to form M-j/2-H2, as pointed out in a review by Kuhlman that addresses site selectivity of protonation of hydride-halide complexes, MH(X).64 Formal protonation of a hydride ligand would give M-ff -H2, which is not known to be stable. Proton transfer to halide ligands is quite rare because an add with a lower than the coordinated HX produced is necessary. One example is protonation of trens-PtHX(P Bu3)2 with triflic acid, which gives trans-[Pffl( f2-H2)(P Bu3)2][OTf] for X = H and an unstable spedes claimed to be... 

The neutral complexes can be converted into the cationic Ind complexes of the type [NiInd(PR 3)L] (197 L = PR 3, PR 3, MeCN, CNBu ) by halide abstraction, protonation of organic ligands, or simple displacement of the triflate moiety. This strategy has been employed for the preparation of an extensive series of cationic complexes, including the monometallie species [Ni(2-Me-Ind)(dippe)]BPh4, 198, " and the bimetallic Ni complexes bearing... 

The title compound is commonly used as a base in transition metal-catalyzed reactions. Because these reactions involve multiple elementary steps, KOAc may act as a base in many different ways, and the precise mechanism of deprotonation may not always be known. A study on the influence of base on the palladium-black-catalyzed methoxycarbonylation of aryl iodides identified potassium acetate as a suitable base. This method has been widely adopted, and both sodium and potassium acetate are commonly used (e.g., eq 29). Alternatively, the acetate base may directly remove a proton from a metal center. For example, the Heck reaction (e.g., eq 30) generates a palladium(II) complex that bears hydride and halide ligands, and generation of the catalytically active palladium(0) corr5>lex requires deprotonation with stoichiometric base (eq 31). 

Overall, these early studies of carboxylate-assisted intramolecular C-H activation established the key features of these AML A/CMD processes, where an electron-deficient metal center works in concert with a pendant carboxylate base to promote C-H activation. This is most evident when an agostic intermediate is involved and such species also rationalize how these systems can also perform C(sp )-H bond activation. Whether C-H activation is achieved as a one- or two-step process appears rather system dependent. Alternative mechanisms, for example, proton transfer onto a halide ligand, oxidative addition, or AMLA-4 processes involving proton transfer onto the inner (Pd-bound) oxygen of the carboxylate were all ruled out. Likewise, no evidence for S Ar processes had been reported. Subsequent work was set against this background and considered the various other parameters that may affect the C-H bond activation process. 

Polymeric amines can be proton acceptors, acyl transfer agents, or ligands for metal ions. The 2- and 4-isomers of poly(vinylpyridine) (11) and (12) and the weakly basic ion exchange resins, p-dimethylaminomethylated PS (2) and poly(2-dimethylaminoethyl acrylate), are commercial. The ion exchange resins are catalysts for aldol condensations, Knoevenagel condensations, Perkin reactions, cyanohydrin formation and redistributions of chlorosilanes. " The poly(vinylpyridine)s have been used in stoichiometric amounts for preparation of esters from acid chlorides and alcohols, and for preparation of trimethylsilyl ethers and trimethylsilylamines from chlorotrimethylsilane and alcohols or amines. Polymer-suppored DBU (l,8-diazabicyclo[5.4.0]undec-7-ene) (52) in stoichiometric amounts promotes dehydrohalogenation of alkyl bromides and esterification of carboxylic acids with alkyl halides. The protonated tertiary amine resins are converted to free base form by treatment with aqueous sodium hydroxide. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

The configurational stability of triorganotin halides is considerably enhanced by the presence of an amine ligand that can coordinate intramolecularly with the tin atom. This was demonstrated by analysis of the 111 NMR spectrum of the stannyl bromide 47 depicted in Figure 626. Below 30 °C, both theN-methyl and the benzylic protons are diastereotopic ... 

Studies on the effect of pH on peroxidase catalysis, or the heme-linked ionization, have provided much information on peroxidase catalysis and the active site structure. Heme-linked ionization has been observed in kinetic, electrochemical, absorption spectroscopic, proton balance, and Raman spectroscopic studies. Kinetic studies show that compound I formation is base-catalyzed (72). The pKa values are in the range of 3 to 6. The reactions of compounds I and II with substrates are also pH-dependent with pKa values in a similar range (72). Ligand binding (e.g. CO, O2 or halide ions) to ferrous and ferric peroxidases is also pH-dependent. A wide range of pKa values has been reported (72). The redox potentials of Fe3+/Fe2+ couples for peroxidases measured so far are all affected by pH. The pKa values are between 6 and 8, indicative of an imidazole group of a histidine residue (6, 31-33),... 

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