Accessing the coordination chemistry of phosphorus(I) zwitterions

Article abstract of DOI:10.1002/anie.201205744

Triphosphenium cations (1) have a rich history dating back to pioneering work in 1982 from Schmidpeter et al.[1] This work has since served as the cornerstone for the synthesis of related derivatives featuring various substitutions and ring sizes.[2] Three canonical structures (1a-c; Scheme 1) can be considered for these compounds, where the phosphanide-like structure 1a is the most curious and tantalizing, as the two non-bonding pairs of electrons make these species candidates as ideal ligands for a wide variety of Lewis acids. Despite this intuitive application, there remains an obvious absence of examples of these electron-rich compounds playing this role.

Full text of DOI:10.1002/anie.201205744

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Angewandte
Communications
DOI: 10.1002/anie.201205744
Phosphanide Ligands
Accessing the Coordination Chemistry of Phosphorus(I) Zwitterions**
Jonathan W. Dube, Charles L. B. Macdonald, and Paul J. Ragogna*
In memory of Adam “MCA” Yauch
Triphosphenium cations (1) have a rich history dating back to
pioneering work in 1982 from Schmidpeter et al.[1] This work
has since served as the cornerstone for the synthesis of related
derivatives featuring various substitutions and ring sizes.[2]
Three canonical structures (1a–c; Scheme 1) can be consid-
in detail and confirms the presence of two “lone pairs” on the
two-coordinate phosphorus atom. The HOMO typically
constitutes the p-type “lone pair”, while the s-type “lone
pair” is primarily attributed to a somewhat more stable
occupied orbital.[2b,3] However, even if the complication of
a reactive anion is removed (2, 3),[4] further rationale for the
poor donor ability of these cations are provided by the
computational work. The non-bonding electrons in the
frontier orbitals participate in significant p-backbonding
(negative hyperconjugation) with the flanking phosphorus
centers, thus they become too stabilized to participate in
Lewis basic chemistry. The positive charge on the cation may
further contribute to the relative inertness of the non-bonding
electrons. This behavior is in stark contrast to the analogous
neutral carbon(0) compounds, the carbodiphosphoranes (4),
which were first developed in 1961[5] and have seen wide-
spread use as a ligand for transition metals (4AuCl).[6]
Recently, Bertrand et al. have demonstrated substantial
donor ability from carbodicarbenes,[7] while in related work
the coordination chemistry of the two-coordinate carbon in
electron rich allenes and heterocumulenes, was extensively
investigated by Alcarazo et al.[8] In these cases, the magnitude
of any p-backbonding is diminished as the p-acidity of the
flanking NHC groups are substantially less than that of the
ligating phosphanes.[9] The particularly electron-rich nature of
the two-coordinate carbon atom in both Bertrandꢀs and
Alcarazoꢀs systems is emphasized by their ability to bind
either one or two Lewis acids simultaneously.[8,10] Electroni-
cally similar “inverse carbodiphosphoranes” (that is, C-P-C
vs. P-C-P) have been reported by Stalke et al.[11] with the
subsequent chemical and electronic studies showing behavior
consistent with a phosphanide Lewis structure. The two-
coordinate phosphorus atom in such systems can act as a four-
electron donor to two supported metal fragments (Cs, Mn),
and also to two unsupported {W(CO)5} fragments (5).[12]
Elegant charge-density studies have been performed and
reveal two distinct valence-shell charge concentrations
(VSCC) in the non-bonding region, which are consistent
with two “lone pairs” of electrons on the phosphorus
atom.[12,13] This parallels the calculated bonding environment
for the triphosphenium systems (1a), which themselves have
no experimental evidence for the equivalent Lewis structure.
Nevertheless, some basic/nucleophilic reactivity has been
demonstrated with derivatives of 1 using strong electrophiles
Scheme 1. Representations of triphosphenium cations and the isova-
lent carbodiphosphoranes.
ered for these compounds, where the phosphanide-like
structure 1a is the most curious and tantalizing, as the two
non-bonding pairs of electrons make these species candidates
as ideal ligands for a wide variety of Lewis acids. Despite this
intuitive application, there remains an obvious absence of
examples of these electron-rich compounds playing this role.
This dearth can be explained in part by the presence of an ion
pair, wherein the counteranion is often more reactive than the
phosphorus(I) center, thus inhibiting the study of any donor
chemistry the central phosphorus atom might exhibit. The
electronic structure for derivatives of 1 has been investigated
[*] J. W. Dube, Prof. P. J. Ragogna
Department of Chemistry and The Centre for Advanced Materials
and Biomaterials Research (CAMBR), Western University
1151 Richmond Street, London, Ontario, N6A 5B7 (Canada)
E-mail: pragogna@uwo.ca
Homepage: http://publish.uwo.ca/~pragogna/
Prof. C. L. B. Macdonald
Department of Chemistry and Biochemistry
The University of Windsor
401 Sunset Ave, Windsor, Ontario, N9B 3P4 (Canada)
(H+, CH3 ),[14] but coordination to neutral Lewis acids or
+
transition metals has remained less extensively explored.[15]
One particularly noteworthy example of the unique possibil-
ities of such compounds was reported by Driess and co-
workers, who showed that the phosphanide complexes can
[**] The authors are grateful for financial support from NSERC, CFI,
ORF, Western University and The University of Windsor.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205744.
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function as sources of “free” Pn(I) for planar four-coordinate
phosphonium and arsonium salts.[16]
To expand and exploit the donor chemistry of triphos-
phenium species in a manner analogous to the well-estab-
lished carbon(0) chemistry (carbodiphosphoranes and carbo-
dicarbenes), we rationalized that two modifications to deriv-
atives of 1 might prove fruitful: 1) the counteranion and
cationic charge should be eliminated; and 2) a more electron-
rich supporting ligand may attenuate the backbonding
component within the system, and instead promote electron
donation from both of the “lone pairs” on the central
phosphorus atom. A convenient solution that addresses
both of these aims is to employ a zwitterionic approach,
which is used extensively by the d-block chemists to promote
greater solubility for their catalytic systems.[17] In particular,
the bis(phosphino)borate class of ligands developed by Peters
et al.[18] are ideal candidates to address the deficiencies of
triphosphenium salts because they carry a remote anionic
charge but still allow one to exploit the well-established and
convenient P!P coordination and redox chemistry used to
generate such species.[19] The resulting phosphorus(I) zwitter-
ion was anticipated to have greater solubility, increased
electron density and thus much better donor properties
relative to analogues 1–3. In this context, we report the
synthesis and comprehensive characterization of new zwit-
terionic P(I) centers and their ability to act as neutral
phosphanide ligand in binding not only one, but two {AuCl}
fragments. Computational investigations provide insights into
the electronic structures of these compounds and pave the
way for the comprehensive understanding of this new ligand
set and how it can be further modified for wider application.
These results represent, for the first time, the structural
authentication of a complex in which both lone pairs on the
phosphorus(I) atom simultaneously bind two metal centers,
therefore acting as a novel neutral phosphanide-like four-
electron m-type ligand.[20]
The 1:1:3 stoichiometric addition of the bis(phosphino)-
borate [Li(tmeda)2][(Ph2PCH2)2BPh2] (6)[21] to PBr3 and
cyclohexene results in the facile formation of a yellow
solution and white precipitate. Analysis of the reaction
mixture by 31P{1H} NMR spectroscopy reveals a doublet and
triplet (dP = 32 and dP = ꢀ220, respectively; 1JP-P = 414 Hz;
Supporting Information, Figure S17), which is consistent with
the quantitative formation of a triphosphenium compound.
The volatile components were removed in vacuo and the
product was extracted into a 80:20 pentane/dichloromethane
mixture, which upon concentration and standing at ꢀ308C
provides colorless crystals. Single crystal X-ray diffraction
studies revealed the solid state structure to be the zwitterionic
phosphorus(I) species, 7, isolated in 75% yield (Scheme 2).
As anticipated by the zwitterionic nature of the compound,
compound 7 was readily soluble in non-polar solvents such as
diethyl ether, benzene, and large portions of pentane.
Scheme 2. Synthesis of the phosphorus(I) zwitterion 7 and its {AuCl}
complex 8. Reagents and conditions (% yield): a) THF, RT, 10 min
(75%); b) [AuCl(SMe2)] (1 equiv), CH2Cl2, RT, 10 min (64%).
phosphines is also evident in the fact that 2P will readily
undergo ligand exchange stronger electron donors (for
example PMe3; NHC),[22] while 7 shows no reaction with
these strong Lewis bases.
Upon confirming the identity and structure of 7, we then
sought to explore its coordination chemistry. Treatment of 7
with one or two equivalents of [AuCl(SMe2)] results in
a significant shift in the 31P{1H} NMR spectrum (dP = 30 and
dP = ꢀ110; 1JP-P = 314 Hz; Supporting Information, Fig-
ure S17) consistent with the binding of the central phosphorus
1
to an electrophilic center. The H NMR spectrum showed
a slight downfield shift of the methylene protons (DdH = 0.11).
Single crystals were grown from the vapor diffusion of
a dichloromethane/hexane solution into toluene and were
suitable for X-ray diffraction, which confirmed the product to
be the triphosphenium zwitterion bound to one {AuCl} Lewis
acid by the central phosphorus atom (compound 8), isolated
in 64% yield (Scheme 2).
While the geometry about phosphorus clearly suggests the
presence of a second “lone pair” of electrons, further addition
of [AuCl(SMe2)] to 8 did not result in the formation of the
diaurinated species. To further understand the reluctance of
the second “lone pair” of electrons to simultaneously bind to
a second metal center, DFT calculations were conducted on
a series of models of compound 7 and related species. The
model complexes reproduce the geometrical features of the
experimental structures quite accurately and attest to the
validity of the method used; extensive results are presented in
the Supporting Information and only the most pertinent
insights are described herein.
NBO and Molden analyses confirm the presence of two
“lone pairs” on the di-coordinate phosphorus atoms in all of
the model compounds. The zwitterionic model complexes 9R
(R = H, Me, Ph; Scheme 3) are all predicted to be consid-
erably more reactive electron donors than the corresponding
cationic triphosphenium models 10R. The energies of the
frontier MOs, NBO “lone pair” orbitals, proton affinities, and
the larger negative charge on the central phosphorus atom are
all consistent with this assessment and the trend in reaction
energies for the complexation of {AuCl} by the model ligands
support that conclusion. Within each group, P-methyl sub-
stituents are predicted to generate more reactive donors than
It should be noted that 7 can also be prepared by simple
ligand-exchange reaction of 2P with the bis(phosphino)bo-
rate ligand, 6. The quantitative formation of 7 is observed in
the 31P{1H} NMR spectrum within 5 min in conjunction with
the presence of free dppe (dP = ꢀ12). The increased donor
strength of the bis(phosphino)borate compared to neutral
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Scheme 4. Synthesis of the phosphorus(I) zwitterion 11 and the
diaurinated complex 12. Reagents and conditions (% yield): a) PBr3
(1 equiv), cyclohexene (6 equiv), Et2O, ꢀ788C, 16 h (68%); b) [AuCl-
(SMe2)] (2 equiv), CH2Cl2, RT, 10 min (74%).
Scheme 3. Structures of the model complexes used for the calcula-
tions.
the P-phenyl ligands. The reaction energies for the complex-
ation of {AuCl} by the model ligands are found to be very
exothermic (ꢀ213 to ꢀ238 kJmolꢀ1 in every instance) and the
coordination of a second {AuCl} fragment is exothermic by
a somewhat smaller amount (ca. ꢀ170 kJmolꢀ1). Conse-
quently, the formation of the diaurinated complex is clearly
favorable. These reaction energies are comparable to the
ꢀ235 kJmolꢀ1 calculated for the complexation of {AuCl} and
PMe3 at the same level of theory. In stark contrast, the
complexation energies calculated for the cationic variants 10R
are considerably smaller (ca. ꢀ150 kJmolꢀ1 for attachment of
a single {AuCl} fragment).
In light of the predicted favorability of the ligation of
a second {AuCl} fragment for the zwitterionic model com-
plexes, we surmised that the reason for the contrasting
experimental observation is almost certainly ascribable to the
steric bulk of the phenyl substituents. In fact, examination of
the structural features of the optimized model of 9H bound to
two {AuCl} fragments reveals that replacement of the H
atoms on boron with Ph groups would result in a sterically
impossible structure. Likewise, a space-filling representation
of the X-ray structure of 8 also shows the intrusion of a Ph
group into the region in which a second {AuCl} fragment
would be bound.
The solid-state structures are shown in Figure 1. The most
noteworthy feature of the free ligands is the absence of
a halide counterion, thus verifying the zwitterionic nature of 7
and 11.[23] The metrical parameters of 7 reveal an average P P
ꢀ
bond length of 2.135 ꢁ, which is slightly longer than other
triphosphenium cations (2.11–2.13 ꢁ), consistent with slightly
mitigated p-backbonding. The P2-P1-P3 angle is 95.70(3)8,
comparable to the literature precedent for six-membered
cyclic triphosphenium cations (94–978). While unligated 7
exists in a twist-boat conformation, upon coordination to
{AuCl} the ring adopts an almost boat-like conformation in
the solid state. The phosphorus(I) center in compound 8 is
trigonal-pyramidal (ꢀangles = 3078), which is consistent with an
AX3E VSEPR geometry, confirming the presence of an
Given the results of the computational work we sought to
mitigate some of the steric bulk on the bis(phosphino)borate
ligand by substituting the P(aryl) groups with P(alkyl) groups. The
bis(diisopropyl)phosphino analogue has already been pre-
pared by Peters and Thomas[21] and this ligand reacts cleanly
with PBr3 and excess cyclohexene to give the zwitterionic
phosphorus(I) species 11. The 31P{1H} NMR spectrum reveals
the characteristic doublet and triplet (dP = 57.1 and dP =
ꢀ268.3, respectively; 1JP-P = 420 Hz; Supporting Information,
Figure S17) with the latter being shifted significantly upfield
(DdP = 48) when compared to 7. Single crystals, grown from
a saturated Et2O solution at ꢀ308C, confirm the identity of
the product and the solid-state structure reveals similar
metrical parameters to 7.
The reaction of 11 with two stoichiometric equivalents of
[AuCl(SMe2)] results in a drastic downfield shift of the central
phosphorus atom in the 31P{1H} NMR spectrum (dP = ꢀ81.7;
Supporting Information, Figure S17). There is also a dramatic
difference in the P–P coupling constant (1JP-P = 153 Hz cf. 1JP-
P = 314 Hz in 8) consistent with a significant decrease in the
Figure 1. The solid-state structures of 7 (top left), 8 (top right), 11
(bottom left), and 12 (bottom right). Ellipsoids are set to 50%
probability and hydrogen atoms and solvent molecules have been
removed for clarity. For 12, the one disordered {AuCl} fragment is
removed for clarity. Selected bond lengths [ꢀ] and angles [8] for 7: P1–
P2 2.1371(9), P1–P3 2.1328(9); P2-P1-P3 95.70(3); 8: P1–Au1
2.2512(17), Au1–Cl1 2.3170(15), P1–P2 2.174(2), P1–P3 2.200(2); P1-
Au1-Cl1 177.76(6), P2-P1-P3 94.59(9); 11: P1–P2 2.1349(9), P1–P3
2.1341(9); P2-P1-P3 96.50(4); 12: P1–Au1 2.257(3), P1–Au1A 2.255(3),
P1–Au2 2.249(3), Au1–Cl1 2.316(6), Au1A–Cl1A 2.310(5), Au2–Cl2
2.297(3), P1–P2 2.215(3), P1–P3 2.216(4), Au1–Au2 3.332(5); P1-Au1-
Cl1 171.41(18), P1-Au1-Cl1A 172.0(2), P1-Au2-Cl2 167.61(10), P2-P1-P3
100.55(14).
ꢀ
P P bond strength and diminished p-backbonding from the
phosphorus(I) lone pairs of electrons. Single crystals were
grown from a CH2Cl2 solution layered with pentane and
confirm the identity of the product to be the triphosphenium
zwitterion with the central phosphorus being simultaneously
ligated by two {AuCl} fragments (Compound 12; Scheme 4).
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additional non-bonding pair of electrons on phosphorus. The
Keywords: coordination chemistry · gold · P ligands ·
triphosphenium ions · zwitterionic compounds
.
ꢀ
metrical parameters reveal longer P P bond lengths as
compared to 7 (2.174(2), 2.200(2) ꢁ; compare with av.
ꢀ
2.135 ꢁ) and a smaller P2-P1-P3 angle of 94.59(9)8. The P
Au bond length is 2.2512(17) ꢁ, which is comparable to the
average phosphine–gold(I) bonds (av. 2.22 ꢁ), while the P-
Au-Cl angle is nearly linear at 177.76(6)8.[24] Therefore, the
solid-state structure of 8 shows 7 acting as a two-electron
donor to {AuCl} with parameters consistent with resonance
form 1a. For the dinuclear gold species 12, the phosphorus(I)
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ꢀ
center has a distorted-tetrahedral AX4 geometry, while the P
P bond lengths are 2.215(3) and 2.216(4) ꢁ. These are
somewhat longer than those in compound 11 (2.1341(9) and
2.1349(9) ꢁ), but remain consistent with single bonds as well
31
1
ꢀ
as the P{ H} NMR data. The P Au bonds are nearly
identical at 2.249(3) and 2.257(3) ꢁ, showing the equal
donor ability of the each of the lone pairs of electrons on
the central phosphorus. The P2-P1-P3 angle has expanded to
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nated
phosphido
complex
[Mes*P(AuPPh3)3][BF4]
(3.1546(3) ꢁ).[26]
In conclusion, a new type of zwitterionic phosphorus(I)
compound has been isolated in good yields and fully
characterized. These species show high solubility in solvents
that are typically unsuitable for their ionic relatives. The
derivative with phenyl substituents on the flanking phospho-
rus atoms is able to form a stable, isolable complex with
{AuCl}. Unfortunately, the steric bulk in the ligand backbone
prevents a second coordination. Simple modification of the
organic groups on the backbone phosphorus centers to
isopropyl adjusts the nature of the phosphorus(I) center to
allow access to both “lone pairs” of electrons. This is
represented in the first example of simultaneous ligation of
two Lewis acids (metal centers) by a triphosphenium based
complex; a feat not achieved until now, in spite of significant
previous investigations of such compounds. The electronic
structure calculations suggest that the use of the anionic
bis(phosphino)borate ligand provides access to new struc-
tures and coordination chemistry about the two-coordinate
phosphorus atom that cannot be observed with the neutral
phosphine ligands (PPh3, dppe, dppp). This discovery opens
a door to the use of these compounds as a new class of
sterically demanding neutral phosphorus-based ligands that
are strong sigma donors that cannot behave as p-acceptors.
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Received: July 18, 2012
Revised: August 27, 2012
Published online: November 12, 2012
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ꢀ
{AuCl} fragments. In this compound there is a slight Au Cipso
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These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
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