Proton exchange transition metal complexes

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

Pyridines in their transition metal complexes coordinate as )] (N), q (C,C), (N,C), or q ligands (Figured). By far the most common coordination mode is (N), in which the lone electron pair on the nitrogen donates to a Lewis acidic metal center (see Lewis Acids Bases). The M-N bond lies in the plane of the pyridine ring (Figure 6). Few reactivity studies of (N) pyridine ligands have been reported. One of potential interest for HDN is the base-catalyzed exchange of the pyridine protons for deuterium... 

Nagypal, I. and Fabian, I. (1982) NMR relaxation studies in solution of transition metal complexes. V. Proton exchange reactions in aqueous solutions of VO -oxalic acid, -malonic acid systems. Inorg. Chim. Acta, 61, 109-113. 

Benzeneselenol as a representative selenol is a colorless liquid of greater acidity than benzenethiol (p a = 5.9 (PhSeH) 6.5 (PhSH)). The bond energy of Se-H is 73 kcal/mol, is smaller than S-H (87 kcal/mol) [82]. These properties may contribute to the efficiency in the oxidative addition of selenols to low-valent transition metals, ligand-exchange reaction between high-valent transition metal complexes and selenols, and protonation process of carbon-metal bonds. Indeed, several transition metal complexes catalyze the highly selective hydrothiolation of alkynes and allenes. 

Transition metals also catalyze isotopic exchange reactions. Platinum is the most active catalyst for most heterocycles. The mechanism may involve metallation, addition, o--addition and ir-complex formation. a-Hydrogen exchange in pyridine is favored over 3- and 7-positions, particularly by a cobalt catalyst whereas platinum is much less selective. In isoquinoline both the 1- and 3-position protons are exchanged at almost the same rates with very little exchange at any other position. In 3-substituted pyridines exchange is preferred at the 6-position, the more so as the size of the 3-substituent increases (73AHC(15)140). 

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