Detection of σ-alkane complexes of manganese by NMR and IR spectroscopy in solution: (η5-C5H5)Mn(CO)2(ethane) and (η5-C5H5)Mn(CO)2(isopentane).

Abstract

Irradiation of CpMn(CO)3 in liquid ethane at 135 K at 355 nm yields a photoproduct that exhibits ν(CO) bands in the IR spectrum shifted to low wavenumber with respect to CpMn(CO)3 that are indicative of a Mn(i) dicarbonyl. Parallel experiments employing in situ irradiation within an NMR probe (133 K, 355 nm photolysis) reveal the 1H NMR signals of this product and confirm its formulation as the σ-ethane complex CpMn(CO)2(η2-C1-H-ethane). The resonance of its coordinated C-H group is observed at δ -5.84 and decays with lifetime of ca. 360 s. Analogous photolysis experiments in isopentane solution with IR detection produce CpMn(CO)2(η2-C-H-isopentane) with similar IR bands to those of CpMn(CO)2(η2-C-H-ethane). 1H NMR spectra of the same species were obtained by irradiation of CpMn(CO)3 in a 60 : 40 mixture of propane and isopentane; three isomers of CpMn(CO)2(η2-C-H-isopentane) were detected with coordination of manganese at the two inequivalent methyl positions and at the methylene group, respectively. The lifetimes of these isomers are ca. 380 ± 20 s at 135 K and do not vary significantly from each other. These σ-complexes of manganese are far more reactive than those of related CpRe(CO)2(alkane) complexes which are stable in solution at 170-180 K. The room temperature lifetimes of CpMn(CO)2(η2-C-H-ethane) and CpMn(CO)2(η2-C-H-isopentane), as determined by TRIR spectroscopy, are 2.0 ± 0.1 and 28 ± 1 μs, respectively.