Redox Reactions of a Stable Dialkylphosphinyl Radical

Article abstract of DOI:10.1021/acs.organomet.5b00323

A stable dialkylphosphinyl radical, 2,2,5,5-tatrakis(trimethylsilyl)-1-phosphacyclopentan-1-yl (R^H₂P^?), showed both irreversible one-electron oxidation and reduction peaks at -0.24 and -2.29 V vs ferrocene/ferrocenium couple. One-electron reduction of R^H₂P^? with KC₈ in the presence of 18-crown-6 (18-c-6) or [2.2.2]cryptand (crypt-222) gave the corresponding phosphides [K(18-c-6)]⁺[R^H₂P]^- and [K(crypt-222)]⁺[R^H₂P]^-. Whereas [K(18-c-6)]⁺[R^H₂P]^- exists as a contact ion pair, [K(crypt-222)]⁺[R^H₂P]^- exists as a solvent-separated ion pair in the solid state. Reaction of R^H₂P^? with AgOTf afforded an unexpected product, a silver(I) phosphaalkene complex. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00323

Communication
pubs.acs.org/Organometallics
Redox Reactions of a Stable Dialkylphosphinyl Radical
Fumiya Hirakawa, Hitomi Ichikawa, Shintaro Ishida,* and Takeaki Iwamoto*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: A stable dialkylphosphinyl radical, 2,2,5,5-tatrakis-
(trimethylsilyl)-1-phosphacyclopentan-1-yl (RH P•), showed both
2
irreversible one-electron oxidation and reduction peaks at −0.24 and
−2.29 V vs ferrocene/ferrocenium couple. One-electron reduction of
RH P• with KC8 in the presence of 18-crown-6 (18-c-6) or
2
[2.2.2]cryptand (crypt-222) gave the corresponding phosphides
[K(18-c-6)]+[RH P]− and [K(crypt-222)]+[RH P]−. Whereas [K(18-
2
2
c-6)]+[RH P]− exists as a contact ion pair, [K(crypt-222)]+[RH P]−
2
2
exists as a solvent-separated ion pair in the solid state. Reaction of
RH P• with AgOTf afforded an unexpected product, a silver(I)
2
phosphaalkene complex.
wo-coordinate phosphorus-centered radicals (R2P•, phos-
(Epa = +0.76 V). The low oxidation and reduction potentials of
1 can be explained by the unpaired electron lying on the 3p
orbital of the phosphorus atom of 1.5a
Reduction of phosphinyl radical 1 with KC8 in the presence
of 18-c-6 in THF at room temperature for 3 h gave yellow
crystals of the potassium phosphide [K(18-c-6)]+1− in 68%
yield (Scheme 2). A similar reaction in the presence of crypt-
T
phinyl radicals) are one of the representative reactive
intermediates in phosphorus chemistry.1−3 Since phosphinyl
radicals are expected to afford the corresponding phosphides
R2P− and phospheniums R2P+ by redox reactions and these
three species still have lone pairs, they can act as redox-active
(noninnocent) ligands for transition metals as well as aminyl
radicals.4 Several dicoordinate stable phosphinyl radicals have
been studied extensively in recent years; however, the redox
behavior of phosphinyl radicals is still unclear. Previously we
synthesized the stable and spin-localized dialkylphosphinyl
a
Scheme 2. Reactions of 1
radical 1 (RH P•), as shown in Scheme 1.5 We report herein the
2
Scheme 1. Phosphinyl Radical and Related Compounds
a
Abbreviations: 18-c-6, 18-crown-6; crypt-222, [2.2.2]cryptand.
222 afforded [K(crypt-222)]+1− as orange crystals in 86% yield.
The structures of both [K(18-c-6)]+1−and [K(crypt-222)]+1−
were determined by NMR spectroscopy and X-ray diffraction
(XRD) analysis. Both of them are protonated gradually to give
redox properties of phosphinyl radical 1: the electrochemical
behavior of 1 using cyclic voltammetry and chemical redox
reactions of 1 involving formation of phosphide anions [K(18-
c-6)]+1− and [K(crypt-222)]+1− (18-c-6 = 18-crown-6, crypt-
222 = [2.2.2]cryptand) by reduction with KC8 and the
unexpected formation of the phosphaalkene silver(I) complex
2 by the reaction with AgOTf as an oxidant.
RH PH in THF.
2
The molecular structure of [K(18-c-6)]+1− is a contact ion
pair structure with monomeric form (Figure 1a), while
[K(crypt-222)]+1− exists as a solvent-separated ion pair in
the solid state (Figure S17 in the Supporting Information).6
The shortest distance between the potassium and phosphorus
atoms in [K(18-c-6)]+1− is 3.3079(11) Å, which is in the range
The cyclic voltammogram of radical 1 in THF showed
irreversible oxidation and reduction waves at −0.24 and −2.29
V (vs Fc/Fc+ couple; Fc = ferrocene) (Figure S14 in the
Supporting Information). The reduction potential of 1 (−2.29
V) is more cathodic than that of the structurally similar
phosphaalkene 3 (−3.01 V), while the oxidation potential of 1
Received: April 17, 2015
Published: May 26, 2015
is considerably lower than that of the dialkylphosphine RH PH
2
© 2015 American Chemical Society
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DOI: 10.1021/acs.organomet.5b00323
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Communication
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, a table, and CIF and XYZ files giving details of the
preparations and spectral and XRD data for new compounds
and computational data for computed structures. The
Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00323.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.I.: sishida@m.tohoku.ac.jp.
*E-mail for T.I.: iwamoto@m.tohoku.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by MEXT KAKENHI Grant Number
24109004 (T.I.) (Grant-in-Aid for Scientific Research on
Innovative Areas “Stimuli-responsive Chemical Species”),
JSPS KAKENHI Grant Numbers 25708004 (S.I.) and
25620020 (S.I.), a Grant-in-Aid for JSPS Fellows (F.H.), and
the Cooperative Research Program of the “Network Joint
Research Center for Materials and Devices”. We thank an
anonymous reviewer for helpful suggestions.
Figure 1. ORTEP drawings of (a) [K(18-c-6)]+1− and (b) 2. Thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
REFERENCES
■
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2
Reaction of 1 with 1.5 equiv of AgOTf in C6H6 at room
temperature afforded the phosphaalkene silver(I) complex 2
and Me3SiOTf in 92% and 90% yields with the precipitation of
silver, instead of the expected phosphenium cation RH P+.
2
Reaction of 1 with 1 equiv of AgOTf gave 2 and unreacted 1.
The molecular structure of 2 determined by XRD analysis is
shown in Figure 1b. The silver atom adopts a planar three-
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2
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observed electrochemical redox behavior, reduction of 1 with
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(9) Theoretical calculations predict that formation of 4 from an ion
pair of phosphenium cation RH2P+ and TfO− and subsequent
elimination of trimethylsilyl triflate from 4 to provide 3 are exothermic.
For details, see the Supporting Information.
(10) Reaction of 1 with [Ag(benzene)3]+[B(C6F5)4]− gave
phosphaalkene 3 in 33% yield, unidentified compounds, and a black
precipitate that was presumably elemental silver. No 31P resonances
assignable to the phosphenium cation RH2P+ were observed in low-
field region (350−750 ppm).
(11) Details of reactions of 3 with AgOTf and of RH2PCl with AgOTf
are described in the Supporting Information.
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