Premium
Ligand and Substrate Effects in Gas‐Phase Reactions of NiX + /RH Couples (X=F, Cl, Br, I; R=CH 3 , C 2 H 5 , n C 3 H 7 , n C 4 H 9 )
Author(s) -
Schlangen Maria,
Schröder Detlef,
Schwarz Helmut
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700506
Subject(s) - chemistry , hydride , halide , alkane , molecule , ligand (biochemistry) , metal , hydrocarbon , inorganic chemistry , organic chemistry , biochemistry , receptor
Abstract The reactions of small saturated hydrocarbons by gaseous nickel cations NiX + (X=F, Cl, Br, I) are investigated by means of electrospray ionization mass spectrometry. The halide cations are obtained from solutions of the corresponding Ni II salts in water or methanol as solvents. NiF + is the only Ni II halide complex that brings about thermal activation of methane. The branching ratios of the observed reactions with C 2 H 6 , C 3 H 8 , and n C 4 H 10 are shifted systematically by changing the nature of both the ligand X and the substrate RH. In the elimination of HX (X=F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going from NiF + to NiI + . A reversed trend is observed in the losses of small closed‐shell neutral molecules, that is, H 2 , CH 4 and C 2 H 6 , which dominate the gas‐phase ion chemistry of NiI + /RH couples. Additionally, inner‐sphere electron‐transfer reactions take place for a few systems, that is, the delivery of hydride or methanide ions from the hydrocarbon to NiX + in the course of which the hydrocarbon is converted to a carbenium ion and the cationic metal complex gives rise to a neutral RNiX molecule (R=H, CH 3 ). This process gains importance with decreasing atomic number of the halides and with increasing the size of the alkane. Thus, it constitutes the major pathway in the reactions of NiF + with propane and n‐ butane, whereas it is not observed for any of the NiI + /RH couples investigated. Concerning the regioselectivity of the reactions with propane and n‐ butane, heterolytic cleavage of secondary carbonhydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies. For the NiF + /C 3 H 8 couple, the selectivity of the hydride transfer is as large as 360 in favor of the secondary positions. Though smaller, large preferences for the activation of secondary CH bonds are also operative in homolytic bond activation of RH (R= n C 3 H 7 , n C 4 H 9 ).