Angewandte
Chemie
DOI: 10.1002/anie.201201262
Cationic Phosphorus
[P9]+[Al(ORF)4]À, the Salt of a Homopolyatomic Phosphorus Cation**
Tobias Kçchner, Tobias A. Engesser, Harald Scherer, Dietmar A. Plattner, Alberto Steffani, and
Ingo Krossing*
Dedicated to Professor Dieter Naumann on the Occasion of his 70th Birthday
Since its first discovery by Hennig Brand in 1669, elemental
phosphorus has continued to fascinate scientists all over the
world.[1–4] Metastable white phosphorus is currently produced
on a million-ton scale per year, and its compounds have wide-
ranging applications in basic and materials chemistry as well
as the life sciences. Currently six modifications of elemental
phosphorus are known; the last three were discovered as late
as 2004 and 2005.[5,6] Pure phosphorus anions [Pn]xÀ are known
in hundreds of structural variations since 1955[7] and cover
almost all combinations of n and x.[8] They reversibly
intercalate lithium ions, for example, as electrode materials
for lithium-ion batteries.[9] Elemental and anionic phosphorus
has a strong tendency to catenate and forms isolated
molecular, 1D, 2D, and 3D structures in the solid state.
Despite this structural diversity of neutral or anionic (poly)-
phase knowledge and the absence in condensed phases by
using the pseudo gasphase conditions provided by the very
weakly coordinating [Al(ORF)4]À anion (RF = C(CF3)3).[17]
The optimized synthesis of the first pure phosphorus
cation [P9]+[Al(ORF)4]À proceeds by oxidation of excess P4
(> 2.5 equivalents) in CH2Cl2 solution with [NO]+[Al-
(ORF)4]À [18] and intermediate formation of the known
yellow species [P4NO]+[Al(ORF)4]À [19] according to Fig-
ure 1.Previous gas-phase collision-induced dissociation
(CID) experiments suggested that the NO moiety is excluded
from the [P4NO]+ cluster as PNO, as [P3]+ was formed by
direct exclusion of PNO upon collisional activation. With
separation of monomeric (SiO2-analogous) PNO, the reaction
to [P9]+ is slightly endergonic according to our conservative
estimate. However, starting with the dimer (PNO)2 this
reaction is already highly exergonic (Figure 1c, Supporting
Information). As expected from the calculated frontier
orbitals (Supporting Information; Ref. [19]) and the yellow
color of the intermediate, the progress of the reaction is
facilitated by broad band UV/Vis irradiation that additionally
transforms most of the excess P4 to insoluble red phosphorus,
destroys the intermediate [P4NO]+, but, according to repeated
investigation by NMR spectroscopy, leaves the [P9]+ cation
intact. Without irradiation we were never able to prepare
pure [P9]+ by this route. After irradiation and filtration from
insoluble materials, a clear yellow-orange solution is obtained
that only shows signals in the NMR spectrum for the intact
anion and the [P9]+ cation together with a small amount of
white phosphorus (< 10 mol%). In agreement with earlier
calculations,[15,16] [P9]+ is a D2d-symmetric Zintl cation that
contains three symmetry-independent atom groups A, B, and
C (Figure 1a). As the atoms in each subgroup A and C are
chemically equivalent but magnetically inequivalent, the
NMR spectrum is of higher order (Figure 1b).
À
phosphorus compounds and the relative strengths of a P P
single bond (about 200 kJmolÀ1), currently no homopolya-
tomic phosphorus cation is known in the condensed phase.
However, some examples of cationic phosphorus-containing
clusters, such as [P5X2]+ (X = Cl,[10] Br, I[11]), the series
[P5Ph2]+, [P6Ph4]+, and [P7Ph6]+,[12] or the recently reported
[P4R2]2+ (R = PPh3, AsPh3) have been reported.[13] In the gas
phase, mass spectrometric (MS) investigations[14] and quan-
tum-chemical calculations[15,16] suggest that the diamagnetic
cluster cations [Pn]+ with uneven values of n are more stable
than the paramagnetic even-value radical cations. Smaller
[Pn]+ cations with n = 5 and 7 should be electron-deficient
Wade clusters, while the larger clusters starting with n = 9 are
expected to have electron-precise Zintl-type structures with
four-coordinate, formally positively charged phosphonium
atoms. Herein, we close this gap between the extensive gas-
[*] Dr. T. Kçchner, Dipl.-Chem. T. A. Engesser, Dr. H. Scherer,
Prof. Dr. I. Krossing
To gain deeper insight into the 31P NMR signals and the
spin system of the [P9]+ cation, the structure was optimized at
the PBE0/aug-cc-pVTZ level. The different contributions to
the nuclear spin–spin coupling were calculated as a single
point with PBE0 theory using the aug-cc-pVTZ-J basis set
that was especially designed for such purposes.[20] By this
analysis, we assigned the spin system as A2A’2BC2C’2. The
calculated spin–spin coupling constants served as a very
reasonable starting point for the iteration[21] of the exper-
imental coupling constants. Together with the over 160
observed lines in the measured spectrum, the iteration
converged to an adjusted RMS of 0.1887, confirming the
assigned spin system and being in good agreement with the
calculated coupling constants (Table 1).
Institut fꢀr Anorganische und Analytische Chemie, Freiburger
Materialforschungszentrum (FMF)
and
Freiburg Institute for Advanced Studies (FRIAS),
Section Soft Matter Science
Albert-Ludwigs-Universitꢁt Freiburg (Germany)
E-mail: krossing@uni-freiburg.de
Prof. Dr. D. A. Plattner, M. Sc. A. Steffani
Institut fꢀr Organische Chemie und Biochemie
Albert-Ludwigs-Universitꢁt Freiburg (Germany)
[**] This work was supported by the DFG, the ERC, the Freiburg Institute
for Advanced Studies (FRIAS) in the section Soft Matter Science, the
Freiburger Materialforschungszentrum (FMF), and the FCI.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 6529 –6531
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6529
Koechner, Tobias
