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Towards Spontaneous Heterolysis of the Homonuclear PP Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides
Author(s) -
Benkő Zoltán,
Burck Sebastian,
Gudat Dietrich,
Hofmann Martin,
Lissner Falk,
Nyulászi László,
Zenneck Ulrich
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902931
Subject(s) - chemistry , covalent bond , crystallography , metathesis , molecule , ionic bonding , homonuclear molecule , bond length , orthorhombic crystal system , phosphole , stereochemistry , crystal structure , ion , organic chemistry , polymerization , polymer
Computational studies on a series of polyphospholyl‐substituted N‐heterocyclic phosphines (CH) 2 (NR) 2 PP n (CH) 5− n (R=Me, n =1–5) disclosed that increasing formal replacement of CH units in the phosphole ring by phosphorus atoms is associated with an increase in PP distances and charge separation, and a decrease in covalent bond orders. Altogether, these trends imply that the CH versus P substitution enhances ionic PP bond polarization in these compounds. Experimental verification of this hypothesis was obtained for the triphospholyl diazaphospholenes (CR) 2 (NR′) 2 PP 3 (C t Bu) 2 ( 8 a : R=H, R′= t Bu; 8 b : R=Me, R′=Mesityl [Mes]), which were prepared through metathesis reactions from suitable precursors and identified by solution and solid‐state NMR data and a single‐crystal X‐ray diffraction study of 8 a . Analysis of J PP coupling patterns suggested that both species are characterized by the absence of a strong covalent PP bond connecting both rings. This interpretation was confirmed by the finding of a unique PP distance of 2.79 Å for crystalline 8 a , and further supported by computational studies, which led to the conclusion that both species are better described as diazaphospholenium‐triphospholide contact ion pairs rather than covalent molecules. Variable‐temperature (VT) NMR spectra of 8 b showed a collapse of J PP couplings between atoms in different rings, which indicates scrambling of the diazaphospholenium and triphospholide units between different molecules in solution, and further substantiates the proposed view on the molecular structure.