Issue 1, 2003

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

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ions 1b, 2b, 4b, 5b in which the phosphorus-bound hydrogens (and one carbon-bound hydrogen in the case of 1b) have been replaced by NH2 groups: 2b (−39.6) ≫ 1b (0.0) 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

Supplementary files

Article information

Article type
Paper
Submitted
07 Aug 2002
Accepted
28 Oct 2002
First published
03 Dec 2002

Dalton Trans., 2003, 85-91

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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