A carbone-stabilized two-coordinate phosphorus(III)-centered dication

Article abstract of DOI:10.1002/anie.201209223

A coordinatively unsaturated dicationic phosphorus compound has been synthesized and characterized (see structure; P pink, N blue, C gray). DFT calculations revealed that the polycation should show superior Lewis acid and σ-donor properties to the corresponding bis(dimethylamino)phosphenium cations. Preliminary investigations with PMe₃ confirmed unusual reactivity patterns for the dication.

Full text of DOI:10.1002/anie.201209223

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DOI: 10.1002/anie.201209223
Phosphenium Dications
A Carbone-Stabilized Two-Coordinate Phosphorus(III)-Centered
Dication**
´
Madelyn Qin Yi Tay, Yunpeng Lu, Rakesh Ganguly, and Dragoslav Vidovic*
In recent years, the synthesis and structural elucidation of
numerous mononuclear main-group polycations hase been
achieved.[1–5] Electron-rich substituents, such as N-heterocy-
clic carbenes (NHCs),[4a,b] polyethers,[3c,d] pyridines,[2a,b] and
diimines,[5a,b,d] were used to stabilize these highly reactive
compounds. A majority of these polycations are either
of the target dicationic compounds IV in the condensed phase
would probably require both substituents R and L to be good
p-donors.
The use of a dialkylamino group as the R substituent was
already predetermined,[7c] but we needed to identify a suitable
neutral ligand for L. It was then recognized that carbodi-
phosphoranes (VI), also known as carbones, fit the criteria for
L, as computational and experimental studies showed that VI
contains two lone electron pairs with s and p symmetry that
are available for bonding.[8] Additionally, the stability of an
electron-deficient dihydridoborenium cation was attributed
to p-donor capabilities of the carbone ligand.[9] Therefore, we
set out to explore the synthesis of the target dicationic species
using carbobis(triphenyl)phosphorane (1) as L.
[2a]
coordinatively saturated (for example, [B(NC5H5)4][Br]3
[2d]
and [{CH(CMe)2(NC6F5)2}Al {N(CH2CH2NMe2)3}][OTf]2
)
or resemble classical Lewis bases ([(NHC)3Ge][OTf2][3e]),
rendering them inactive as Lewis acids. This certainly holds
true with respect to the recently reported NHC-stabilized
dicationic (I) and tricationic PIII-centered compounds (II;
Scheme 1).[4a,b] Even though dication I (R = Cl; L = NHC)
The synthesis of the dication is shown in Scheme 2. First,
1 was added to a benzene solution containing excess
iPr2NPCl2. Chloride displacement and the formation of [2+]
Scheme 1. General structures of three- (I, II and III) and two-coordi-
nate (IV and V) phosphorus compounds, and that of carbodiphosphor-
anes (VI).
Scheme 2. Key reagents/conditions: a) iPr2NPCl2 (excess)ꢀ, benzene for
X=Clꢀ, addition of AlCl3 (1.0 equiv), CH2Cl2 for X=AlCl4
;
was subjected to chloride substitution[4b] and trication II (L =
NHC) showed exciting coordination chemistry,[4a] they still
resemble Lewis basic neutral phosphines (III). Therefore, our
goal was to establish a ligand system that would stabilize
b) 1.0 equiv of AlCl3 for X=AlCl4ꢀ and 2.0 equiv of AlCl3 if X=Clꢀ in
CH2Cl2.
a
two-coordinate PIII-centered dication (IV) containing
[Cl] was elucidated from ES-MS and 31P{1H} NMR experi-
ments. The ES-MS experiment showed a peak at m/z 702.2339
(calcd for 2+: 702.2370) with the correct isotopic pattern
predicted for 2+ (see the Supporting Information). The room-
temperature 31P{1H} NMR spectrum showed two second-
order signals at dP = 25.9 (d) and 133.4 ppm (t), which were
resolved into three doublets of doublets at ꢀ908C, consistent
with the carbone-for-chloride exchange and the formation of
carbone-stabilized phosphenium cation 2+.
a formally vacant P3p orbital and potentially exhibiting
reactivity associated with Lewis acids.[6] It is evident that the
most stable two-coordinate PIII-centered monocations (V)
that have been reported, also known as phosphenium cations,
employed at least one, if not both, substituents R with strong
p-donating capabilities, such as an amino group (NR’2; R’ =
alkyl, aryl).[7] It was subsequently proposed that the synthesis
The unambiguous identity of 2+, as a tetrachloroaluminate
ꢀ
´
[*] M. Q. Y. Tay, Dr. Y. Lu, Dr. R. Ganguly, Dr. D. Vidovic
(AlCl4 ) salt, was confirmed by single-crystal X-ray diffrac-
tion;[13b] one of the two independent cationic molecules is
SPMS-CBC, Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: dvidovic@ntu.edu.sg
ꢀ
shown in Figure 1. The average P N (1.656(5) ꢀ) bond
ꢀ
length, which is comparable to the average P N (1.663(4) ꢀ)
[**] We thank the A*STAR (Grant ID: 1220703062) for funding. We
would also like to thank Ee Ling Goh (NTU) and Ian Riddlestone
(University of Oxford) for the help with NMR and ESMS,
respectively.
bond length observed for [(iPr2N)2P(DBN)]+ (DBN = 1,5-
diazabicyclo[4.3.0]-non-5-ene),[10] is consistent with the estab-
lished bond order of 1.5 for similar systems.[11] The average
ꢀ
Pcentral Ccarbone bond distance of 1.814(6) ꢀ is slightly shorter
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209223.
ꢀ
than the average P C bond distance (1.838(6) ꢀ) observed
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3.585(9) ꢀ, respectively, for the two independent molecules),
are at the extreme end of the sum of van der Waalꢁs radii for P
and Cl (3.55 ꢀ).[13a] As similar close contacts have been
observed for analogous phosphenium salts,[14] it is generally
accepted that these contacts have little or no effect on the
structural features of the cation(s) and the anion(s)[14a] unless
hydrogen bonding is involved.[15]
To gain more insight into the structural/electronic features
of dication 32+, a series of density function theory (DFT)
calculations were performed (see the Supporting Informa-
tion). The optimized structural parameters for 32+, including
the C2NPCP2 fragment, are in good agreement with the
experimental values. The delocalized nature of the HOMO
Figure 1. Molecular view of 2+ and 32+ (ellipsoids set at 50% proba-
bility). For clarity, only one of the two asymmetric units is shown for
both structures, and the counterions [AlCl4ꢀ] together with hydrogen
atoms have been omitted. Selected bond lengths [ꢀ] and angles [8]
(values in the parenthesis are of the second asymmetric unit): 2+: P1–
C1, 1.812(5) (1.815(6)), P1–N1, 1.660(4) (1.652(5)), P1–Cl1, 2.173(2)
(2.178(2)); N1-P1-C1, 111.6(2) (110.8(2)), N1-P1-Cl1, 106.58(18)
(107.06(18)), C1-P1-Cl1, 100.84(18) (100.9(2)). 32+: P1–C1, 1.741(6)
(1.79(7)), P1–N1, 1.623(6) (1.622(6)); N1-P1-C1, 118.1(3) (117.3(3)).
orbital for 32+ (Figure 2b) along the N P C fragment seems
ꢀ ꢀ
for carbene-stabilized mono- and dicationic three-coordinate
PIII-centered compounds, but is within the values expected for
a P C single bond.[4b] Pyramidal geometry for 2+ is confirmed
ꢀ
by the value of 318.9(2)8 for the sum of bond angles around
Figure 2. Selected molecular orbitals for 32+. a) LUMO, b) HOMO.
the Pcentral
.
The formation of dication 32+ from [2+][Cl] was achieved
by halide abstraction using 2.0 equiv of AlCl3 (Scheme 2). The
most indicative piece of evidence for the heterolytic P Cl
to be very different from the HOMO for [P(NMe2)2]+, which
is similar to filled N2p atomic orbitals.[11] The molecular
orbital that resembles the lone pair located on the central P
atom for 32+ (HOMOꢀ12,[16] ꢀ12.737 eV; see the Supporting
Information) is quite destabilized with respect to the analo-
gous orbital for [P(NMe2)2]+ (ꢀ13.518 eV), hinting at 32+
being a better s-donor than the phosphenium cation.
ꢀ
bond cleavage in 2+ and the formation of dication 32+
originated from the appearance of a signal shifted downfield
at dP = 355.7 ppm, which is within the range of 200–500 ppm
established for the known two-coordinate phosphenium
cations.[7b–e] Furthermore, depleted electron density at the
dication was also evident from the 13C NMR spectrum, as the
Ccarbone signal (dC = 67.8 ppm) was found at about 50 ppm
shifted downfield with the respect to the same signal for 2+
(dC = 19.5 ppm). This observation also suggested an increase
in electron donation from the carbone lone pair with p
symmetry to the newly created and formally vacant P3p
orbital.
Interestingly, the most bonding p-orbital, HOMOꢀ14[16]
(ꢀ13.810 eV; see the Supporting Information) for 32+ is more
stabilized than the same orbital for [P(NMe2)2]+ (ꢀ13.450 eV)
ꢀ
ꢀ
implying a more effective p interaction along the N2p P3p
C2pp fragment for 32+ with respect to the analogous N2p
ꢀ
+ [11]
ꢀ
P3p N2p fragment for [P(NMe2)2] .
Nonetheless, the
Indeed, the molecular structure of 32+ (one of the two
asymmetric units is shown in Figure 1)[13b] exhibits planarity
along the C2NPCP2 fragment, placing not only the amino but
also the carbone p symmetry lone pair in an ideal position to
LUMO (p*, Figure 2a) detected at ꢀ7.638 eV for 32+ is still
more stabilized relative to the LUMO (ꢀ6.910 eV) for
[P(NMe2)2]+, which is presumably due to a greater positive
charge at the former compound. Thus, this particular obser-
vation indicates that 32+ should also be a better Lewis acid (eꢀ
acceptor) than [P(NMe2)2]+.
!
interact with the formally vacant P3p orbital. This N!P
C
p-interaction is manifested by shortening of the average
Pcentral N (1.623(6) ꢀ for 3 vs. 1.656(5) for 2+) and Pcentral
Ccarbone bonds (1.745(7) ꢀ for 32+ vs. 1.813(6) ꢀ for 2+) with
the respect to the precursor. Taking into account that 1) the
Lewis acidic properties of the dication were explored by
the addition of either 1 or 2 equiv of PMe3 into a dichloro-
methane solution containing [32+][AlCl4]2 (see the Supporting
Information for details). It was immediately evident that
a simple Lewis acid–base adduct formation did not occur, but
a more complex reaction mixture was developing. Variable-
temperature 31P NMR studies showed the formation of not
only 32+·PMe3 (dP = 40.7 ppm) adduct but also 2+ precursor,
suggesting the existence of a dynamic equilibrium system
(Scheme 3). On the other hand, the reaction between [32+]-
[AlCl4]2 and DMAP (dimethylaminopyridine) resulted in the
2+
ꢀ
ꢀ
ꢀ
accepted description of the P N bond is somewhere between
2+
ꢀ
a single and double bond, and 2) the average P C bond for 3
ꢀ
is very similar to the P C bond (1.7376(14) ꢀ) observed for
a phosphaalkene,[12] it is, therefore, reasonable to assume that
the NPC fragment for 32+ is allene-like, providing the
necessary thermodynamic stabilization for the dication.
At this point it is worth noting that the interion
interactions, manifested by P···Cl contacts (3.527(8) and
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Scheme 3. Proposed equilibria.
ˇ
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formation of [2+][AlCl4] and DMAP·AlCl3, an observation
which is consistent with the HSAB concept regarding PMe3
and DMAP.[7b]
To eliminate counterion interference with respect to the
dication reactivity with DMAP, [32+][BArf4]2 (Arf = 3,5-
(CF3)2-C6H3) was synthesized (see the Supporting Informa-
tion). Indeed, introducing 1.0 equiv of DMAP in a solution
containing [32+][BArf4]2 resulted in the formation of the target
adduct [32+·DMAP] (dP = 131.2 ppm).
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Therefore, the unprecedented equilibrium system
(Scheme 3), especially the equilibrium involving a heterolytic
[17]
ꢀ
P P bond formation/cleavage, for phosphenium chemistry
revealed that PMe3 has a higher preference to interact with
AlCl4 than with the cation but that the preference can be
ꢀ
altered with a decrease in temperature. Lastly, the presence of
a (partially) vacant P3p orbital on 32+ and, hence, the Lewis
acidic nature of this dication was confirmed with the observed
formation of 32+·PMe3 and 32+·DMAP.
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In conclusion, we have succeeded in utilizing the unique
electron-donor properties of 1 to stabilize the very first
example of coordinatively unsaturated PIII-centered dication
32+. Electronic (based on DFT) and reactivity properties
(based on the reactions with PMe3 and DMAP) promise an
exciting chemistry for this polycation. In fact, we hope that 32+
could be to phosphenium chemistry as cyclic (alkyl) (amino)
carbenes (CAACs) are to carbene chemistry.[18] Reactivity
investigations of this interesting polycationic molecule, espe-
cially involving transition-metal chemistry, are ongoing and
will be reported in due course.
Received: November 17, 2012
Revised: January 9, 2012
Published online: January 31, 2013
[9] B. Inꢃs, M. Patil, J. Carreras, R. Goddard, W. Thiel, M. Alcarazo,
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[12] O. Back, M. A. Celik, G. Frenking, M. Melaimi, B. Donnadieu,
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[13] a) The Cambridge Crystallographic Data Centre (CCDC),
http://www.ccdc.cam.ac.uk/products/csd/radii/table.php4
(accessed October 2012); b) CCDC 905325 and 907050 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Keywords: carbodiphosphoranes · Lewis acids ·
ligand synthesis · phosphenium dications · phosphorus
.
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[15] F. Reiß, A. Schulz, A. Villinger, Eur. J. Inorg. Chem. 2012, 261 –
271.
[17] To our knowledge, equilibrium (a) is the very first heterolytic
ꢀ
formation/cleavage of a P P bond without any intermolecular
(S. Burck, D. Gudat, D. Nieger, Angew. Chem. 2004, 116, 4905 –
4908; Angew. Chem. Int. Ed. 2004, 43, 4801 – 4804) or intra-
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2009, 694, 229 – 236).
[16] Even though the most p-bonding (HOMOꢀ14) and the lone-
pair (HOMOꢀ12) molecular orbitals seem to be buried among
other MOs, their relative energies are comparable with the
similar orbitals observed for [P(NMe2)2]+. In fact, there is a fair
number of occupied MOs that are not only comparable in terms
of shape and energy but are located on the Ph groups of the
carbone ligand (see the Supporting Information).
[18] V. Lavallo, Y. Canac, C. Prꢆsang, B. Donnadieu, G. Bertrand,
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