A crystalline phosphinyl radical cation

Article abstract of DOI:10.1021/ja1046846

One-electron oxidation of a readily available phosphaalkene derived from a cyclic (alkyl)(amino)carbene affords a phosphorus-centered radical cation that is indefinitely stable both in solution and in the solid state, allowing a single X-ray diffraction study to be performed. This species can be regarded as a phosphinyl radical bearing a cationic substituent or, alternatively, as a carbene-stabilized phospheniumyl radical (carbene-RP^+·).

Full text of DOI:10.1021/ja1046846

Published on Web 07/12/2010
A Crystalline Phosphinyl Radical Cation
Olivier Back,† Mehmet Ali Celik,‡ Gernot Frenking,‡ Mohand Melaimi,† Bruno Donnadieu,† and
Guy Bertrand*,†
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403, and Fachbereich Chemie, Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse,
35032 Marburg, Germany
Received May 29, 2010; E-mail: guy.bertrand@ucr.edu
Abstract: One-electron oxidation of a readily available phos-
phaalkene derived from a cyclic (alkyl)(amino)carbene affords a
phosphorus-centered radical cation that is indefinitely stable both
in solution and in the solid state, allowing a single X-ray diffraction
study to be performed. This species can be regarded as a
phosphinyl radical bearing a cationic substituent or, alternatively,
as a carbene-stabilized phospheniumyl radical (carbene-RP+•).
The involvement of a variety of phosphorus-centered radicals
in chemical reactions has been recognized for many decades.1 Some
of these paramagnetic species are persistent or even stable2 in
solution at room temperature. The gas-phase electron diffraction
Figure 1. Phosphorus radicals structurally characterized in the gas phase
(I) and in the solid state (II-VI)
structures of phosphinyl radicals Ia and Ib (Figure 1) have been
reported.3 However in the solid state, despite bulky substituents,
Scheme 1
compounds I dimerize, which readily explains why only a few
phosphorus radicals (II-VI)4,5 have been structurally characterized
by single-crystal X-ray diffraction studies. All these compounds
are resonance-stabilized radicals, and consequently, the calculated
spin density at a single phosphorus center is rather small (maximum
0.44e for VIb).4e Here we report the preparation of a phoshinyl
radical featuring a cationic substituent, for which the spin density
is mainly localized at a single phosphorus atom (0.67e). It is stable
at room temperature both in solution and in the solid state, allowing
a crystallographic study to be performed.
Recent reports have shown that although [carbene-(RO)2P(O) ·],6
(carbene-BR2 ·),7 and (carbene-MLn ·)6 adducts cannot be isolated,
they are significantly more stable than the corresponding free phos-
In the solid state, radical cation 2+• adopts a V-shaped geometry
phonyl, boryl, and metal radicals. Encouraged by the isolation of radical
with a N2-P1-C1 angle of 107.3° (Figure 2, right). The P1-C1
cations VI, each of which can be viewed as a P2+• fragment capped
(1.81 Å) and P1-N2 (1.68 Å) bond lengths in 2+• are significantly
by two carbenes,4e we envisioned that an RP+• unit could also be
longer and shorter, respectively, than the corresponding ones in 2
stabilized by a bulky and strong electron-donating carbene.8 Obviously,
(1.74 and 1.77 Å) (Figure 2, left). These two bond distances are at
such a carbene-phospheniumyl adduct can also be regarded as a
the lower ends of the ranges for P-C and P-N single bonds,
phosphinyl radical featuring a cationic substituent. To obtain the desired
radical cation, a synthetic strategy similar to that used for the
preparation of VI was chosen.4e Phosphaalkene 2 was prepared in two
steps by addition of (2,2,6,6-tetramethylpiperidino)phosphine dichlo-
ride9 to cyclic (alkyl)(amino)carbene VIIa10 followed by reduction
with excess magnesium (Scheme 1). The strong polarization of the
PC bond was indicated by the relatively high field 31P NMR signal
(136 ppm) and low field 13C NMR chemical shift (207 ppm). Not
surprisingly, because of the electron-rich P nucleus, a one-electron
oxidation readily occurred at 25 °C upon addition of Ph3C+B(C6F5)4-
to a benzene solution of 2. After workup, the radical cation 2+• was
isolated as a dark-brown powder in 38% yield (mp 94-96 °C), and
single crystals were grown by layering hexane on top of a fluoroben-
zene solution at 5 °C.11
Figure 2. Molecular views (with 50% thermal ellipsoids) of (left) 2 and
(right) 2+•. For clarity, the counterion [B(C6F5)4-] and hydrogen atoms have
been omitted. Selected bond lengths (Å) and angles (deg) in 2: P1-N2,
1.7655(11); P1-C1, 1.7376(14); N1-C1, 1.3805(16); N2-P1-C1, 108.90(6);
N1-C1-C4, 106.90(11). In 2+•: P1-N2, 1.6805(14); P1-C1, 1.8137(17);
N1-C1, 1.318(2); N2-P1-C1, 107.26(8); N1-C1-C4, 110.18(13).
† University of California, Riverside.
‡ Philipps-Universita¨t Marburg.
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10262 J. AM. CHEM. SOC. 2010, 132, 10262–10263
10.1021/ja1046846  2010 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. Financial support from the NSF (CHE-
0924410) and the DFG are gratefully acknowledged. We thank Dr.
D. Borchardt (UCR) for EPR assistance.
Supporting Information Available: Full experimental details,
including the electrochemical study, absolute energies, and optimized
geometries of 2+•, and X-ray crystallographic data for 2, 2+•, and 3 in
CIF format. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 3. EPR spectra of 2+• in (left) a fluorobenzene solution at 298 K
and (right) a frozen solution at 100 K.
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