Gas-phase thermolysis of a guanidinate precursor of copper studied by matrix isolation, time-of-flight mass spectrometry, and computational chemistry
The fragmentation of the copper(i) guanidinate [Me2NC(N/Pr) 2Cu]2 (1) has been investigated with time-of-flight mass spectrometry (TOF MS), matrix-isolation FTIR spectroscopy (Ml FTIR spectroscopy), and density functional theory (DFT) calculations. Gas-phase thermolyses of 1 were preformed in the temperature range of 100-800 °C. TOF MS and Ml FTIR gave consistent results, showing that precursor 1 starts to fragment at oven temperatures above 150 °C, with a close to complete fragmentation at 260 °C. Precursor 1 thermally fragments to Cu( S), H2(g), and the oxidized guanidine Me 2NC(=N/Pr)(N=CMe2) (3). In TOF MS experiment, 3 was clearly indentifled by its molecular ion at 169.2 u. Whereas H2+ was detected, atomic Cu was not found in gas-phase thermolysis. In addition, the guanidine Me2NC(N/Pr)(NH/Pr) (2) was detected as a minor component among the thermolysis products. Ml thermolysis experiments with precursor 1 were performed, and species evolving from the thermolysis oven were trapped in solid argon at 20 K. These species were characterized by FTIR spectroscopy. The most indicative feature of the resulting spectra from thermolysis above 150 °C was a set of intense and structured peaks between 1600 and 1700 cm-1, an area where precursor 1 does not have any absorbances. The guanldine 2 was matrix-isolated, and a comparison of its FTIR spectrum with the spectra of the thermolysis of 1 indicated that species 2 was among the thermolysis products. However, the main IR bands In the range of 1600 and 1700 cm-1 appeared at 1687.9,1668.9,1635.1, and 1626.6 cm-1 and were not caused by species 2. The oxidized guanldine 3 was synthesized for the first time and characterized by1H NMR and FTIR spectroscopy. A comparison of an FTIR spectrum of matrix isolated 3 with spectra of the thermolysis of 1 revealed that the main IR bands in the range of 1600 and 1700 cm -1 are due to the presence of 3. The isomers exhibit the NMe2 group cis or trans to the /Pr group, with c/s-3 being significantly less stable than trans-3. At higher temperature secondary thermal fragments had been observed. For example at 700 °C, TOF MS and Ml FTIR data showed that species 2 and 3 both eliminate HNMe2 to give the carbodilmides /PrNCNPr (CDI) and /PrNCN[C(=CH 2)Me] (4), respectively. A DFT study of the decomposition of compound 1 was undertaken at the B3LYP/6-31+G(d,p) level of theory employing dispersion-correcting potentials (DCPs). The DFT study rationalized both carbodilmide delnsertion and β-hydrogen elimination as exergonic decomposition pathways (AG = -44.4 kcal/mol in both cases), but experiment showed β-hydrogen elimination to be the favorable route.
Coyle, J.P, Johnson, P.A. (Paul A.), Dilabio, G.A. (Gino A.), Barry, S.T, & Müller, J. (Jens). (2010). Gas-phase thermolysis of a guanidinate precursor of copper studied by matrix isolation, time-of-flight mass spectrometry, and computational chemistry. Inorganic Chemistry, 49(6), 2844–2850. doi:10.1021/ic902247w