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Issue 1, 2003
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Exploration of isomeric bis(phosphanyl)-substituted carbenium ions: identification and synthesis of the most stable isomer

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Abstract

Isomeric bis(phosphanyl) carbenium ions have been investigated by theoretical [MP2/6-31G(d,p) level of theory] and experimental means. On the potential energy surface (PES) of phosphorus cations of the formula [P2CH5]+, five stable isomers are found. Their structures follow the stability order (ΔErel/kcal mol−1): P-protonated diphosphiranium, [(H2P)(HP)CH2]+ 1a (0.0) > C-phosphonio phosphaalkene, [H3P–HC[double bond, length as m-dash]PH]+ 2a (4.5) ≫ bis(phosphanyl) carbenium, [CH(PH2)2]+ 3a (18.5) > C-phosphanyl methylenephosphonium, [H2P–CH[double bond, length as m-dash]PH2]+ 4a (22.6) ≫ diphosphirenium, 5a (47.1). The stability order is very different for the family of the four related stable [P2CH(NH2)4]+ ions 1b, 2b, 4b, 5b in which the phosphorus-bound hydrogens (and one carbon-bound hydrogen in the case of 1b) have been replaced by NH2 groups: 2b (−39.6) ≫ 1b (0.0) > 5b (2.2) ≥ 4b (3.7). None of these ions has a structure comparable to the classical amidinium ion [HC(NH2)2]+. While all amino carbenium ions have a positively charged carbon atom, all phosphorus analogues carry a negative charge on carbon. In some, even the p(π)-population exceeds unity suggesting that these should be considered as cationic methanides. Experimentally, a derivative of the diphosphiranium ion 1a could be prepared by methylation of 1,2-bis-(2,4,6-tri-tert-butylphenyl)diphosphirane with MeO3SCF3, and this new compound 8 was characterized by NMR methods. Although the most stable member in this family, it could only be observed under stable ion conditions (T < −40 °C, five-fold excess of MeO3SCF3) and decomposes at higher temperatures.

Graphical abstract: Exploration of isomeric bis(phosphanyl)-substituted carbenium ions: identification and synthesis of the most stable isomer

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Publication details

The article was received on 07 Aug 2002, accepted on 28 Oct 2002 and first published on 03 Dec 2002


Article type: Paper
DOI: 10.1039/B207749C
Dalton Trans., 2003, 85-91

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    Exploration of isomeric bis(phosphanyl)-substituted carbenium ions: identification and synthesis of the most stable isomer

    S. Loss, C. Widauer, H. Rüegger, U. Fleischer, C. M. Marchand, H. Grützmacher and G. Frenking, Dalton Trans., 2003, 85
    DOI: 10.1039/B207749C

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