Photoinduced N2 loss as a route to long-lived organometallic alkane complexes: A time-resolved IR and NMR study

Abstract

Photolysis of CpRe(CO)2(N2) in cyclopentane or 2,2-dimethylbutane with a UV lamp via a quartz fibre inserted into the NMR probe allows generation of CpRe(CO)2(cyclopentane) and CpRe(CO)2(2,2-dimethylbutane). The latter is observed in three isomeric forms according to the site of co-ordination to the rhenium. The major isomer, CpRe(CO)2(2,2-dimethylbutane-η2-C1,H1), exhibits a 1H NMR resonance for the co-ordinated hydrogen at δ = −2.19 with 1JC–H = 118 Hz. The photochemistry of Cp‡Re(CO)2(N2) (Cp‡ = η5-1,2-C5H3(tBu)2) in alkane solution is also reported. Two new organometallic alkane complexes, Cp‡Re(CO)2(alkane) (alkane = cyclopentane, n-heptane) have been characterized by IR spectroscopy following irradiation of Cp‡Re(CO)2(N2) and their rate constants for reaction with CO have been determined. The reaction with cyclopentane has also been studied by NMR spectroscopy at 190 K with in situ laser irradiation at 355 nm. Cp‡Re(CO)2(c-C5H10) is shown to exhibit the characteristic features of an alkane complex in the NMR spectrum, viz. a large isotopic shift of the 1H resonance at δ = −2.44 upon partial deuteration of the alkane (Δδ = 1.77 ppm), a large 1JC–H (114 Hz) and a large negative 13C chemical shift (δ = −33.8). We find no evidence for CO loss or agostic interactions of the t-butyl groups under these conditions. Cp‡Re(CO)2(alkane) has a slightly shorter lifetime (ca. 5x) than CpRe(CO)2(alkane) for a given alkane. Photolysis of CpRe(CO)2(N2) to form the organometallic alkane complex occurs with a much higher yield than for CpRe(CO)3. Efficient photo-ejection of N2 from Cp‡Re(CO)2(N2) is observed upon either 266 or 355 nm laser irradiation. A dinitrogen precursor allows for the use of longer wavelength irradiation and the generation of a higher concentration of the alkane complex following each laser pulse.