Angewandte
Communications
Chemie
Phosphanoxyl Complexes
Strong Evidence of a Phosphanoxyl Complex: Formation, Bonding, and
Reactivity of Ligated Phosphorus Analogues of Nitroxides
Tobias Heurich, Vitaly Nesterov, Gregor Schnakenburg, Zheng-Wang Qu,* Stefan Grimme,
Dedicated to Professor Masaaki Yoshifuji on the occasion of his 75th birthday
Abstract: Facile access to [W(CO)5(Ph2P-OTEMP)] is used to
initiate a study on the generation, properties, and reactions of
transient phosphanoxyl complexes [MLn(R2PO)], the first
example of which could be trapped via heterocoupling with the
trityl radical. It is also demonstrated that the phosphorus
nitroxyl complex acts as radical initiator in the polymerization
Scheme 1. Oxygen-bound main-group-element derivatives of TEMPO
(I), related phosphorus derivatives (II; R=organic substituent,
Z=electron pair, or MLn for a transition-metal complex), and radical
À
À
of styrene. The quest for P O versus O N bond homolysis, as
well as the initial steps of the polymerization were studied by
DFT methods.
À
À
species (III, IV) derived from P O and O N bond cleavages in II.
O
ver the last decades, stable nitroxides[1] (or nitroxyls or
aminoxyls), such as TEMPO (2,2,6,6-tetramethyl piperidin-1-
oxyl) and O-bound main-group element derivatives
stable P-OTEMP phosphanes II (Z = electron pair) are
unknown.[6] But there are reports on transient derivatives
formed at low temperature. In one case, the intermediate P-
OTEMP phosphane generated different radicals via homo-
I
(Scheme 1) have received increasing attention in molecular
and polymer synthesis. For example, the thermally induced
reverse homolysis of activated alkoxyamines I (E = C) has
recently been recognized as a facile process for the clean
generation of C-centered radicals and led to various funda-
mental radical reactions based on carbon radical/nitroxide
pairs[2] and/or was used in living radical polymerizations,[3]
making use of the persistent radical effect.[2,4] Despite this
long-standing research interest and numerous reports on
main-group element derivatives of TEMPO I[5] (Scheme 1),
À
lytic O N bond cleavage leading then subsequently to silyl
migration to give an O-silyl phosphinate derivative.[7] We
have recently shown that a PIII derivative of II (R = Ph, Z =
lone pair), obtained at low temperatures by the reaction of
diphenylphosphane with TEMPO, yielded the corresponding
PV derivative (Z = O) via homolytic O N bond cleavage as
À
the key reaction step.[8] Transition-metal complexes II (Z =
MLn) were mentioned only briefly, as a stable AuI[9] or
a thermally unstable W0 complex.[10] It was proposed that
decomposition of O-phosphano hydroxylamines of the gen-
[*] T. Heurich, Dr. G. Schnakenburg, Prof. Dr. R. Streubel
Institut fꢀr Anorganische Chemie
[6,11]
À
eral formula R2NOPR’2
proceeds via homolytic O N
bond cleavage[11a] to give rearrangement products R2NP-
(O)R’2. Therefore, our aim was to develop suitable transition-
metal complexes having PIII-II as a ligand to enable the study
Rheinische Friedrich-Wilhelms-Universitꢁt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: r.streubel@uni-bonn.de
Dr. V. Nesterov
Institut fꢀr Siliciumchemie
Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85748 Garching (Germany)
À
À
of homolytic P O or O N bond cleavage. In the long term,
the hitherto unknown stable kP-phosphanoxyl complexes IV
(Z = MLn), somehow related to functional phosphanyl com-
plexes III (Z = MLn),[10,12] could thus become accessible.
Herein, facile methods to synthesize a representative
phosphane metal complex II bearing the O-bound TEMPO
substituent are reported. A trapping reaction revealed that II
is a precursor for a transient phosphanoxyl complex IVas well
as a radical initiator for the polymerization of styrene. State-
of-the-art DFT calculations provide insight into reaction
mechanisms and molecular properties, such as spin density
distributions of various open-shell species.
Dr. Z.-W. Qu, Prof. Dr. S. Grimme
Mulliken Center for Theoretical Chemistry
Rheinische Friedrich-Wilhelms-Universitꢁt Bonn
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: qu@thch.uni-bonn.de
K. Hazin, Prof. Dr. D. P. Gates
Chemistry Department, University of British Columbia
2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada)
Priv.-Doz. Dr. M. Engeser
Kekulꢂ-Institut fꢀr Organische Chemie und Biochemie
Rheinische Friedrich-Wilhelms-Universitꢁt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Starting from pentacarbonyl tungsten(0) complexes 1 and
3, the P-nitroxyl substituted phosphane tungsten complex 4
was obtained via two different routes (i, ii): (i) used a lithium/
chlorine exchange of chlorophosphane complex 1[13] with
tBuLi to generate the lithium phosphanido tungsten complex
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!