Gas-Phase Thermolysis of a Guanidinate Precursor of Copper Studied by Matrix Isolation, Time-of-Flight Mass Spectrometry, and Computational Chemistry

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• 作者：Jason P. Coyle ; Paul A. Johnson ; Gino A. DiLabio ; Sen T. Barry ; Jens Mller
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：March 15, 2010
• 年：2010
• 卷：49
• 期：6
• 页码：2844-2850
• 全文大小：813K
• 年卷期：v.49,no.6(March 15, 2010)
• ISSN：1520-510X

文摘

The fragmentation of the copper(I) guanidinate [Me₂NC(NiPr)₂Cu]₂ (1) has been investigated with time-of-flight mass spectrometry (TOF MS), matrix-isolation FTIR spectroscopy (MI 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 MI 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 Me₂NC(=NiPr)(N=CMe₂) (3). In TOF MS experiment, 3 was clearly indentified by its molecular ion at 169.2 u. Whereas H₂p>+p> was detected, atomic Cu was not found in gas-phase thermolysis. In addition, the guanidine Me₂NC(NiPr)(NHiPr) (2) was detected as a minor component among the thermolysis products. MI 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 cmp>−1p>, an area where precursor 1 does not have any absorbances. The guanidine 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 cmp>−1p> appeared at 1687.9, 1668.9, 1635.1, and 1626.6 cmp>−1p> and were not caused by species 2. The oxidized guanidine 3 was synthesized for the first time and characterized by p>1p>H 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 iPr group, with cis-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 MI FTIR data showed that species 2 and 3 both eliminate HNMe₂ to give the carbodiimides iPrNCNiPr (CDI) and iPrNCN[C(=CH₂)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 carbodiimide deinsertion and β-hydrogen elimination as exergonic decomposition pathways (ΔG = −44.4 kcal/mol in both cases), but experiment showed β-hydrogen elimination to be the favorable route.