Gas-phase reactions of Tc+, Re+, Mo+ and Cu+ with alkenes

Organometallic complex ions, denoted as M+-L, where M = Tc, Re, Mo or Cu and L = CcHh, were produced by reacting laser-ablated M+ with alkenes (R = ethene, C2H4; propene, C3H6; cis-2-butene, C4H8; cyclohexene, C6H10 and 1,5-cyclooctadiene, C8H12). The compositions and abundances of the M+-L were determined by time-of-flight mass spectrometry. A primary goal was to elucidate gas-phase organotechnetium chemistry and differentiate the behaviors of Re+ and Mo+ from that of Tc+. The ion pairs, Tc+ + Re+ and Tc+ + Mo+, were coablated to provide direct comparisons of reactivities; these M+ were ablated from solid MOx in a copper matrix, which generated Cu+, whose reactivity was also evaluated. For Tc+, Re+ and Mo+, H-2-loss from the alkene was the primary reaction channel, and the M+-L yields and extent of multiple H-2-loss (i.e. the reactivity) generally paralleled the size of R. Primary products were: M+-C2H2 [R = C2H4]; M+-C3H4 [R = C3H6]; M+-C4H6 and M+-C4H4 [R = C4H8]; M+-C6H6 and M+-{C6H6}(2) [R = C6H10] and M+-C8H8 and M+-C8H6 [R = C8H12]. For cycloalkenes, C-C bond cleavage also occurred, e.g. M+ + C8H12 --> M+ -C6H6 + C2H6. The reactivity of Re+ was moderately greater than that of Tc+; thus for R = C6H10, the yield of Tc+-C6H6 and Re+-C6H6 were comparable but Tc+-C6H4 was minor relative to Re+-C6H4. Less coherent distinctions were observed between Tc+ and Mo+, although Tc+ was apparently slightly more reactive. Where ablated Mo+ were adequately abundant to measure Mof-L products, a greater reactivity, compared with naked Mf, was indicated; for R = C6H10 the product yields were: Tc+-C6H6/Tc+ approximate to 5% and TcO+-C6H6/TcO+ approximate to 40%. The especially facile reaction, Tc-2(+) + C4H8 -->Tc-2(+)-C4Hh + [(8 - h)/2]H-2 (h = 6 or 4), indicated greater reactivity for Tc-2(+) compared with Tc+ 'Closed-shell' (d(10)s(0)) Cu+ was relatively inert, generally producing primarily the adduct, Cu+-R (condensation was insignificant for other M+). However, Cu+ distinctively produced abundant Cu+-C4H6 (butadiene) from C6H10 and C8H12. The observed gas-phase organometallic chemistries of Cu+ and Mo+ are consistent with previous results by other techniques and with other organic substrates, and the reactivities of Tc+ and Re+ are in accord with general predictions. (C) 1998 Elsevier Science S.A. All rights reserved.