Ligand and Substrate Effects in Gas-Phase Reactions of NiX+/RH Couples (X=F, Cl, Br, I; R=CH3, C2H5, nC3H7, nC4H9)

✍ Scribed by Maria Schlangen; Detlef Schröder; Helmut Schwarz

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 218 KB
• Volume: 13
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200700506

✦ Synopsis

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 carbonhydrogen 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 CH bonds are also operative in homolytic bond activation of RH (R=__n__C~3~H~7~, __n__C~4~H~9~).