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Dynamic Behavior of Intramolecularly Base-Stabilized Phosphatetrylenes. Insights into the Inversion Processes of Trigonal Pyramidal Geramanium(II) and Tin(II) Centers

Lookup NU author(s): Dr Keith Izod, Dr Ewan Clark, Emeritus Professor William McFarlane, Dr Ben Allen, Emeritus Professor Bill Clegg, Dr Ross Harrington

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Abstract

The reaction between SnCl2 and either 1 or 2 equiv of the lithium salt [{(Me3Si)(2)CH}(C6H4-2-CH2NMe2)P]Li gives the heteroleptic compound [{(Me3Si)(2)CH}(C6H4-2-CH2NMe2)P]SnCl (7) and the homoleptic, intramolecularly base-stabilized diphosphastannylene [{(Me3Si)(2)CH}(C6H4-2-CH2NMe2)P](2)Sn (8), respectively, in good yields. The solid state structure of 8 shows that the tin(II) center is three-coordinate, bound by the N and P atoms of a chelating phosphide ligand and the P atom of a second phosphide ligand. Both 7 and 8 are highly dynamic in solution. Variable-temperature NMR spectra suggest that compound 7 and its germanium analogue 5 are subject to two distinct dynamic processes in polar solvents, which are attributed to the formation of adducts between either 5 or 7 and the free phosphine {(Me3Si)(2)CH}(C6H4-2-CH2NMe2)PH (9) and interconversion between diastereomers of these adducts. Adduct formation is observed only in polar solvents and may be associated with the formation of weakly bound [[{((Me3Si)(2)CH}(C6H4-2-CH2NMe2)P]E(L)](+)center dot center dot center dot Cl- ion pairs in solution. The dynamic behavior of 8 has been studied by multielement and variable-temperature NMR experiments; at high temperatures there is rapid equilibrium between diastereomers, but at low temperatures a single diastereomer predominates and exchange between the chelating and terminal phosphide ligands is frozen out. DFT calculations on the model compound {(Me)(C6H4-2-CH2NMe2)P}SnCl (7a) suggest that epimerization occurs either through a vertex-inversion process at phosphorus [E-inv = 65.3 kJ mol(-1)] or an edge-inversion process at tin [E-inv = 141.0 kJ mol(-1)], of which the former is clearly favored. DFT calculations on the model complex {(Me)(C6H4-2-CH2NMe2)P}(2)Sn (8a) indicate that the lowest energy dynamic process involves exchange between the chelating and terminal phosphide ligands via a pseudotrigonal bipyramidal intermediate [E = -12.6 kJ mol(-1)]. Inversion at tin in 8a (via an unusual hybrid edge/vertex-inversion process) is calculated to have a barrier of 206.3 kJ mol(-1), whereas the barriers to vertex-inversion at phosphorus are 59.4 and 51.0 kJ mol(-1) for the chelating and terminal phosphorus atoms, respectively.


Publication metadata

Author(s): Izod K, Stewart J, Clark ER, McFarlane W, Allen B, Clegg W, Harrington RW

Publication type: Article

Publication status: Published

Journal: Organometallics

Year: 2009

Volume: 28

Issue: 12

Pages: 3327-3337

ISSN (print): 0276-7333

ISSN (electronic): 1520-6041

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/om8011757

DOI: 10.1021/om8011757


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