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 ¹J_C–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 ¹J_C–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.