A Phosphorus Lewis Super Acid: η5-Pentamethylcyclopentadienyl Phosphorus Dication

Article abstract of DOI:10.1016/j.chempr.2018.08.038

Abstraction of chloride from (Cp*PCl₂) with the silylium salt [Et₃Si][B(C₆F₅)₄] afforded [(η²-Cp*)PCl][B(C₆F₅)₄] (1), whereas further chloride abstraction u

Full text of DOI:10.1016/j.chempr.2018.08.038

Article
A Phosphorus Lewis Super Acid: h5-
Pentamethylcyclopentadienyl Phosphorus
Dication
Jiliang Zhou, Liu Leo Liu, Levy L.
Cao, Douglas W. Stephan
dstephan@chem.utoronto.ca
HIGHLIGHTS
h5-Pentamethylcyclopentadienyl
phosphorus dication
Unique phosphorus-centered
Lewis super acid reactivity
Intriguing hapticity shifts of the
cyclopentadienyl ring
An h5-pentamethylcyclopentadienyl phosphorus dication [(h5-Cp*)P]2+ has been
synthesized and characterized, representing an h5-cyclopentadienyl-bound
phosphorus species. The reactivity of the dication reveals its phosphorus-centered
Lewis super acidity and interesting hapticity shifts of the cyclopentadienyl ring.
Zhou et al., Chem 4, 1–10
November 8, 2018 Crown Copyright ª 2018
Published by Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.08.038

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Article
A Phosphorus Lewis Super Acid:
h5-Pentamethylcyclopentadienyl
Phosphorus Dication
Jiliang Zhou,1 Liu Leo Liu,1 Levy L. Cao,1 and Douglas W. Stephan1,2,
*
SUMMARY
The Bigger Picture
The last two decades have
witnessed the renaissance of
main-group chemistry. Of
particular interest are instances in
which main-group elements
behave analogously to transition
metals. The landmark work that
described metallocene
Abstraction of chloride from (Cp*PCl2) with the silylium salt [Et3Si][B(C6F5)4]
afforded [(h2-Cp*)PCl][B(C6F5)4] (1), whereas further chloride abstraction using
another equivalent [Et3Si][B(C6F5)4] was unsuccessful. The corresponding spe-
cies [(h2-Cp*)PF][B(C6F5)4] (2) was derived by fluoride abstraction from
(Cp*PF2). Treatment of (Cp*PF2) with 2 equiv of [Et3Si][B(C6F5)4] gave [(h5-Cp*)
P][B(C6F5)4]2 (3), representing an h5-Cp that is bound to phosphorus. 3 behaves
as a phosphorus-based Lewis super acid, abstracting fluoride from [SbF6]ꢀ anion
with concurrent h5-h2-Cp* rearrangement to give 2. Similarly, the extremely
high Lewis acidity and chlorophilicity of 3 is indicated by the equilibration of 3
with Et3SiCl to generate 1 and [Et3Si][B(C6F5)4]. The coordination of 2,20-bipyr-
idine (bipy) to the phosphorus of 3 induces the h5-h1-Cp* rearrangement, lead-
ing to a ‘‘slipped sandwich’’ compound [(h1-Cp*)P(bipy)][B(C6F5)4]2 (4). 3 also
reacts with Et3SiH via h5-s-Cp* rearrangement, affording [(s-Cp*)PH2SiEt3]
[B(C6F5)4] (5) and [Et3Si][B(C6F5)4].
complexes over 60 years ago has
continuously evolved and
influenced modern
organometallic chemistry and
catalysis. The present work
describes an
h5-pentamethylcyclopentadienyl
phosphorus dicationic species,
mimicking the common
INTRODUCTION
Since the landmark discovery of ferrocene by Kealy and Pauson,1 and subsequent
seminal work on metallocene complexes by Fischer and Wilkinson in the 1950s,2–4
systems containing p-bound ligands have permeated organometallic and inorganic
chemistry. Among the most prevalent organometallic ligands have been 6-electron
aromatic anionic cyclopentadienyl (Cp) groups, which typically adopt an h5-binding
mode. Indeed, a wide variety of transition-metal derivatives containing h5-Cp ligands
have been used in countless organic transformations and catalyses.5 Although efforts
to mimic transition-metal chemistry with main-group compounds have garnered
much attention in recent years,6–9 the use of h5-Cp ligands in p-block element com-
pounds remains relatively underexplored.10–12 Among the known examples, group
13 decamethylmetallocenium cations [(Cp*)2E]+ (E = B,13,14 Al,15 Ga16) have been re-
ported, in which only [(h5-Cp*)2Al]+ adopts an h5/h5 binding mode analogous to
ferrocene.17 In contrast to the discrete cations, the low valent neutral compounds
[(h5-Cp*)E]n (E = Al18, Ga19, In20, Tl21) (Figure 1A) form cluster structures in the solid
state. In addition, a terminal borylene complex containing an h5-Cp* ligand, namely
[(h5-Cp*)BFe(CO)4],22 was described by Cowley. In addition, examples of dicationic
[(h5-Cp*)B(L)]2+ species have been reported.23,24 For group 14 derivatives, the
coordination mode of
cyclopentadienyl ligands with
transition metals. This
h5-pentamethylcyclopentadienyl
phosphorus dication is shown to
exhibit unique phosphorus-
centered Lewis super acidity by
undergoing interesting hapticity
shifts of the cyclopentadienyl ring
upon reactions. Although main-
group-centered Lewis acids have
been extensively exploited in
organic transformations, catalysis,
and material chemistry, the
present work provides a unique
perspective on phosphorus-based
Lewis acids.
heavier elements form metallocenes(II) [(h5-Cp*)2E] (E = Si,25 Ge,26 Sn,26 Pb27
)
and half-naked E(II) cations of the form [(h5-Cp*)E]+ (E = Si,28 Ge,26 Sn,29 Pb30) (Fig-
ure 1B). Interestingly, a carbon dication [C6(CH3)6]2+, recently reported by Mali-
schewski and Seppelt,31 could be described as an h5-Cp*-bonded carbon dication
[(h5-Cp*)CCH3]2+ 32 Group 15 analogs of formulae [(Cp*)2Pn]+ (Pn = P,33 As,34
.
Chem 4, 1–10, November 8, 2018 Crown Copyright ª 2018 Published by Elsevier Inc.
1

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Figure 1. Cp*-Bonded Main-Group Species
(A–C) Representative examples of (A) group 13, (B) group 14, and (C) group 15 Cp*-bonded main-
group species.
(D) Present work.
Sb35,36) have been isolated (Figure 1C), and NMR data show fluxionality of the Cp*
ligands in solution37 consistent with rapid haptotropic shifts, and X-ray structure anal-
ysis revealed that the Cp* ligands are not ideally h5-bonded to the central Pn atoms.
For example, crystallographically nonequivalent C–P distances were described for
[(h2-Cp*)2P]+. Although a number of related group 15 cations [(h2-Cp*)PnX]+
(Pn = P, X = NMe2,38 tBu,39 NHtBu,40 Cl33; Pn = As, X = Cl33) have been studied,
the only dication of the form [(h5-Cp*)Pn]2+ was reported in the bismuth salt
(h5-Cp2Al) (h5-Cp*Bi) (m-AlI4) (AlI4)2$2C7H8.41
Strong Lewis acids based on main-group elements have gained increasing atten-
tion.42–45 Our group has a longstanding interest in phosphorus-based Lewis acids,
as well as their application in frustrated Lewis pair chemistry and Lewis acid catal-
ysis.46,47 This prompted us to envision the synthetic target [(h5-Cp*)P]2+. In this
article, we describe the synthesis of this target. In addition, we examine its reactivity,
demonstrating that this species features a half-naked dicationic phosphorus center
that exhibits unique phosphorus-based Lewis super acid reactivity proceeding with
changes in the hapticity of the Cp* ring.
1Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON M5S 3H6,
RESULTS AND DISCUSSION
Canada
We hypothesized that [Cp*P]2+ would feature an h5-structure and obtained prelim-
2Lead Contact
*Correspondence: dstephan@chem.utoronto.ca
inary confirmation of this proposal using density functional theory (DFT) calculations
at the M06-2X/Def2-SVP level of theory. No local minima featuring hn (n = 1–4)
https://doi.org/10.1016/j.chempr.2018.08.038
2
Chem 4, 1–10, November 8, 2018

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Figure 2. X-Ray Structures of 1‒3
POV-ray depictions of the cations of (A) 1, (B) 2, and (C) 3. Coloring is as follows: C, black; P, orange;
Cl, green; F, pink. All H atoms, solvent residue, and the [B(C6F5)4]ꢀ anions are omitted for clarity.
binding modes were identified in unrestricted geometry optimizations. Targeting
the synthesis of [Cp*P]2+, we initially attempted dechlorination of the Cp*-
substituted dichlorophosphine (Cp*PCl2) with an equimolar portion of silylium salt
[Et3Si][B(C6F5)4]. This reaction afforded the salt [(h2-Cp*)PCl][B(C6F5)4] (1) (Figures
S1–S5),33 the formulation of which was confirmed crystallographically (Table S1;
Figures 2A and S38A). Further chloride abstraction using an additional equivalent
[Et3Si][B(C6F5)4] was unsuccessful.
Noting the high fluorophilicity of silylium cations, the corresponding difluoro-
phosphine Cp*PF2 was prepared. Defluorination of Cp*PF2 with an equimolar
portion of [Et3Si][B(C6F5)4] afforded [(h2-Cp*)PF][B(C6F5)4] (2) (Figures S6–S10),
with distinctive doublet resonances at ꢀ73.9 ppm (1JPF = 1,211 Hz) and 17.2 ppm
(1JPF = 1,211 Hz) in the 31P{1H} NMR and 19F{1H} NMR spectra, respectively. The
formulation of 2 was further unambiguously confirmed by an X-ray diffraction study
(Table S1; Figures 2B and S38B).
Gratifyingly, double defluorination of Cp*PF2 with 2 equiv of [Et3Si][B(C6F5)4]
in toluene immediately produced a yellow precipitate 3. The formation of Et3SiF
was confirmed by 19F NMR spectroscopy of the crude mixture. After workup, 3
was isolated as a pale yellow solid in 97% yield (Scheme 1; Figures S11–S15). Com-
pound 3 was sparingly soluble in common organic solvents. Nevertheless, it dis-
solved in 1,2-difluorobenzene (DFB), allowing for spectroscopic characterization.
The 31P NMR spectrum of 3 revealed a very upfield sharp singlet at ꢀ401.6 ppm,
and the 19F NMR spectrum showed a set of signals attributable to the [B(C6F5)4]ꢀ
anion. Moreover, the single resonances in the 1H (C5Me5, 2.17 ppm) and
13C (C5Me5, 137.3 ppm; C5Me5, 8.8 ppm) NMR spectra imply the presence of a sym-
metrically bound Cp* ligand in the solution state (Figures S11 and S12). Single crys-
tals of 3 were obtained by layering pentane on a toluene/DFB solution of 3 at room
temperature, and the formulation of 3 was confirmed as [(h5-Cp*)P][B(C6F5)4]2 by
X-ray diffraction (Table S1; Figures 2C and S38C). Notably, the Cp* ligand is
h5-bonded to the central P atom, with C‒P bond lengths ranging from 2.063(6) to
Chem 4, 1–10, November 8, 2018
3

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Scheme 1. Synthesis of 1–3
˚
2.122(6) A, which are slightly longer than those reported for the cationic
48
˚
phosphapyramidane species [(Me3SiC)4P][B(C6F5)4] (1.934(2)–1.946(2) A). The
mean P–C–C angle in [(h5-Cp*)P]2+ is 70.0ꢁ. The Lewis acidity of the [(h5-Cp*)P]2+
dication is demonstrated by the weak coordination of the phosphorous with the
aromatic ring of a toluene molecular. The P‒C distances range from 2.79 to
6
˚
3.04 A, which are shorter than those in h -arene complexes of the (2,4,6-tri-tert-
49–55
˚
butylphe-nyl)iminophosphenium cation (2.87–3.38 A),
indicating the interac-
tion of all the ring carbons of the toluene molecular with the phosphorous. Thus,
the cation of 3 can be considered as a ‘‘bent sandwich’’ structure.
The lowest unoccupied molecular orbital + 1 (LUMO+1) (ꢀ9.89 eV) and LUMO
(ꢀ9.89 eV) of [(h5-Cp*)P]2+ are degenerate and primarily comprise the vacant p orbitals
of the P atom (Figures 3A and 3B). This result parallels that obtained from calculationson
[(Cp*)E]+ (E = Si, Ge, Sn) and Cp*E (E = Al, Ga, In)56, but with significantly lower energies
due to the additional cationic charge (Table S2). The highest occupied molecular orbital
(HOMO) (ꢀ18.90 eV) and HOMOꢀ1 (ꢀ18.90 eV) are also degenerate and involve the
bonding p orbitals of the P and ring C atoms (Figures 3D and 3E). This is in contrast
to the observations for the HOMOs of Cp*E (E = B, Al, Ga, In), which primarily involve
thelone pairof electronsat main-groupcenter.56 The lonepairof electronson theP cen-
ter is in HOMOꢀ2 (ꢀ19.82 eV) (Figure 3F), indicating a low predicted nucleophilicity for
[(h5-Cp*)P]2+. Natural bond orbital (NBO) analysis (M06-2X/Def2-TZVP//M06-2X/Def2-
SVP) demonstrates a highly positively charged phosphorus center (1.08 a.u.), whereas
the remaining positive charge is delocalized across the H atoms of the methyl groups.
This is also reflected by an electrostatic potential analysis (Figure 3C). The Wiberg
bond indices (WBIs) of C‒P bonds are small (0.49), consistent with h5-coordination of
the Cp* ligand to P. Collectively, these data suggest that P should exhibit significant
Lewis acidity, although being very weakly basic.
The strong Lewis acidity of 3 is reflected by its reaction with [NBu4][SbF6], giving 2
1
within seconds as indicated by 31P NMR (ꢀ73.5 ppm, JPF = 1,211 Hz) and 19F
NMR (16.1 ppm, 1JPF = 1,211 Hz) analyses (Figures S16–S19) (Scheme 2). The forma-
tion of 2 is concurrent with the formation of [NBu4][B(C6F5)4] and SbF5. Remarkably,
treatment of 3 with an equimolar portion of Et3SiCl in toluene-d8/DFB (1:1) (Figures
S20–S25) immediately led to an equilibrium mixture of 3, Et3SiCl, 1, and [Et3Si]
[B(C6F5)4], with a molar ratio for 3:1 of 100:76 (Figure S22). At ꢀ45ꢁC, the equilibrium
favors 3, with the molar ratio for 3:1 increasing to 100:55 (Figure S23). The genera-
tion of [Et3Si][B(C6F5)4] demonstrates that 3 has a stronger chlorophilicity but lower
fluorophilicity relative to [Et3Si]+. Whereas related h5- to h3- or h2-ring slippages
have been observed for base-[Cp*E]+ (E = Ge, Sn) adducts,57,58 the conversion of
3 to 1 or 2 represents has not yet been reported for phosphorus.
A DFB solution of 3 was treated with 2,2’-bipyridine (bipy) in a molar ratio of 1:1,
immediately resulting in the full consumption of the starting material and formation
4
Chem 4, 1–10, November 8, 2018

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
A
B
C
D
E
F
Figure 3. Frontier Molecular Orbitals
(A and B) LUMO+1 (A) and LUMO (B) for the cation of 3.
(C) Molecular electrostatic potential (MEP) map for the cation of 3. The blue areas indicate
attractive MEPs toward a negative point charge.
(D–F) HOMO (D), HOMO-1 (E), and HOMO-2 (F) for the cation of 3.
of a clear pink solution. A new phosphorus species 4 (Figures S26–S30) exhibits a
broad singlet at 33.6 ppm in the 31P NMR spectrum (Figure S28). Both 1H (C5Me5,
1.50 ppm) and 13C (C5Me5, 143.6 ppm; C5Me5, 10.9 ppm) NMR data (Figures S26
and S27) of the isolated 4 suggest that the Cp* ligand is in rapid dynamic circum-
ambulation. Single-crystal X-ray diffraction study revealed the formulation of 4 as
[(h1-Cp*)P(bipy)][B(C6F5)4]2 (Table S1; Figures 4A and S38D). In the solid state, the
cation of 4 exhibits a ‘‘slipped half sandwich’’ structure with an h1-Cp* ligand. The
˚
P‒C bond length is 1.870(11) A, whereas the P‒N bond lengths are 1.847(12) and
1.850(11) A. The Cp* ring is capped by the PN2 plane. Despite the h1-binding
˚
mode, the C‒C bond lengths of the ring are close to each other (average
˚
1.445 A). The P is almost vertical to the ring plane with angles of P-C1-C2 and
Chem 4, 1–10, November 8, 2018
5

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Scheme 2. Reactions of 3
P-C1-C5 of 92.0(7)ꢁ and 93.3(8)ꢁ, respectively. On the basis of an NBO analysis, the P
atom of the cation of 4 carries a large portion of the positive charge (1.23 a.u),
whereas both N atoms are negatively charged (ꢀ0.52 a.u. each). The WBIs of the
P‒C and two P‒N bonds are 0.78, 0.53, and 0.53, respectively. Thus, the electronic
structure of the cation of 4 is most suitably described as bipy-stabilized [(h1-Cp*)P]2+
.
Such bipy-stabilized phosphorus dications have very recently found applications in
catalytic C‒F bond reduction.59 4 is reminiscent of a few examples of bipy-stabilized
phosphorus cations reported by the Burford group.60,61
The mechanism of the formation of 4 was investigated via DFT calculations (SMD-
M06-2X/Def2-TZVP//M06-2X/Def2-SVP). The reaction begins with the approach of
the lone pair of electrons at N(1) of a bipy molecule toward the P atom of the cation
of 3 (Figure 5). This prompts an h5-h2 haptotropic rearrangement to form an interme-
diate IN1 (ꢀ16.7 kcal/mol) in a barrier-less process (Figure S37; Table S2). Subse-
quent rotation of the C‒C single bond in the bipy ligand and the ensuing coordina-
tion of N(2) to the P atom is concurrent with the h2-h1 rearrangement and proceeds
via TS1, with a low energy barrier of 2.8 kcal/mol, to produce the stable product
cation of 4 (ꢀ39.4 kcal/mol).
Figure 4. X-Ray Structures of 4 and 5
POV-ray depictions of the cations of (A) 4 and (B) 5. Coloring is as follows: C, black; P, orange;
N, blue; Si, pale-pink; H, white. All H atoms (except P‒H), solvent residue, and [B(C6F5)4]ꢀ anions, as
well as disordered parts and the cocrystallized cation [toluene$Et3Si]+ in 5, are omitted for clarity.
6
Chem 4, 1–10, November 8, 2018

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Figure 5. Free Energy (kcal/mol) Profile for the Formation of the Cation of 4
In the 3D structure of TS1, selected bond lengths are given in angstroms.
Compound 3 was also shown to react with excess Et3SiH in a toluene/DFB (1:1) so-
lution. The resulting 31P NMR spectrum revealed a triplet resonance at ꢀ72.3 ppm
(1JPH = 383 Hz), which collapses into a singlet upon proton decoupling, indicating
the presence of a PH2 fragment (Figures S33 and S34). Interestingly, co-crystals of
[(s-Cp*)PH2SiEt3][B(C6F5)4] 5 (Figures 4B and S38E) and [toluene$SiEt3][B(C6F5)4]
in a ratio of 1:1 were obtained by layering of pentane on the reaction mixture
at ꢀ35ꢁC (Figures S31–S36; Table S1). The bond lengths of P‒Si and P‒C were
˚
2.354(5) and 1.879(8) A, respectively. In the Cp* group, three C‒C single bonds
˚
(1.463(12), 1.466(13), and 1.516(11) A), and two C=C double bonds (1.335(9) and
1.348(9) A), were observed. These data, in addition to the three 1H NMR signals
˚
for the methyl groups (1.46, 1.39, and 0.82 ppm) and the corresponding 13C reso-
nances (18.8, 10.8, and 9.5 ppm), are consistent with a description of the binding
as a s-Cp* ligand (Figures S31 and S32).
A computational study of the mechanism of the formation of 5 reveals that the [SiEt3]+
was generated via barrier-less hydride abstraction from Et3SiH by the Lewis acidic
cation of 3, affording [(h2-Cp*)PH]+ IN2 (ꢀ28.3 kcal/mol) with a concurrent
h5-h2-Cp* rearrangement (Figure 6). In the presence of excess Et3SiH, IN2 activates
a second Et3SiH molecule to form IN3 (ꢀ19.7 kcal/mol), prompting h2-s-Cp* rear-
rangement. The ensuing cleavage of the Si‒H bond is facile via TS2
(ꢀ18.3 kcal/mol) with an activation energy of 10.0 kcal/mol (IN2/TS2), producing
the stable cation of 5 (ꢀ47.0 kcal/mol) (Table S2). Coordination of Et3Si-H to Lewis
acidic centers, such as a boraindene62 and Al(C6F5)3,63 has been previous reported;
however, the present result is a rare example in which such an interaction of Si‒H with
a main-group Lewis acid prompts its addition to a P atom to give P‒H and P‒Si bonds.
In summary, more than six decades after the discovery of the h5-cyclopentadienyl
derivatives of iron, this work uncovers an h5-Cp ligand on phosphorus with the isola-
tion of [(h5-Cp*)P][B(C6F5)4]2 3 via a defluorination strategy. The two degenerate
LUMOs at the P center prompt 3 to react as a P-based Lewis super acid, binding
toluene and reacting with Et3SiCl, [SbF6]ꢀ, bipy, and Et3SiH to give 1, 2, 4, and 5,
respectively. In the case of 1 and 2, the reactions prompt h2-Cp* binding, whereas
for 4 an h1 binding is seen. Notably, the reaction of 3 with silane effects hydride
Chem 4, 1–10, November 8, 2018
7

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
Figure 6. Free Energy (kcal/mol) Profile for the Formation of the Cation of 5
In the 3D structure of TS2, selected bond lengths are given in angstroms.
abstraction and addition of [R3Si]+ to P to give 5, concurrent with an h5-s rearrange-
ment of the binding of the Cp* ligand. The isolation of 3 illuminates a unique aspect
of P-based Lewis acids and super acid chemistry. The potential application of such
species in further reactivity and catalysis is the subject of current efforts in our
laboratory.
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in the Supplemental Information.
DATA AND SOFTWARE AVAILABILITY
Crystallographic data have been deposited in the Cambridge Crystallographic
Data Center under accession numbers CCDC: 1849893–1849897. These data can
be obtained free of charge from the Cambridge Crystallographic Data Center at
http://www.ccdc.cam.ac.uk/data_request/cif.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 38 fig-
ures, 3 tables, and 5 data files and can be found with this article online at https://doi.
org/10.1016/j.chempr.2018.08.038.
ACKNOWLEDGMENTS
D.W.S. is grateful for the financial support of the Natural Sciences and Engineering
Research Council of Canada, being awarded a Canada Research Chair, and the Ein-
stein Visiting Professor Fellowship from the Einstein Foundation. We thank Dr. Sau-
rabh S. Chitnis and Dr. Timothy C. Johnstone for valuable discussion.
AUTHOR CONTRIBUTIONS
J.Z. performed the synthetic studies. L.L.L. performed the computational experi-
ments. J.Z. and L.L.C. carried out single-crystal X-ray diffraction studies. D.W.S.
directed the project. D.W.S., L.L.L., and J.Z. wrote the manuscript with input from
all authors. All authors analyzed the results and commented on the manuscript.
8
Chem 4, 1–10, November 8, 2018

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: June 22, 2018
Revised: July 19, 2018
Accepted: August 29, 2018
Published: October 4, 2018
REFERENCES AND NOTES
1. Kealy, T.J., and Pauson, P.L. (1951). A new type
of organo-iron compound. Nature 168, 1039.
15. Dohmeier, C., Schnockel, H., Schneider, U.,
¨
27. Atwood, J.L., Hunter, W.E., Cowley, A.H.,
Jones, R.A., and Stewart, C.A. (1981). X-Ray
crystal structures of bis(cyclopentadienyl)tin
and bis(pentamethylcyclopentadienyl)lead.
J. Chem. Soc. Chem. Commun. 925–927.
Ahlrichs, R., and Robl, C. (1993).
Decamethylaluminocenium, a p-stabilized
R2Al+ cation. Angew. Chem. Int. Ed. 32, 1655–
1657.
2. Fischer, E.O., and Pfab, W. (1952). Zur
Kristallstruktur der Di-cyclopentadienyl-
verbindungen des zweiwertigen Eisens,
Kobalts und Nickels. Z. Naturforsch. B 7,
377–379.
16. Macdonald, C.L.B., Gorden, J.D., Voigt, A., and
Cowley, A.H. (2000). Synthesis and
characterization of the first example of a
gallocenium cation. J. Am. Chem. Soc. 122,
11725–11726.
28. Jutzi, P., Mix, A., Rummel, B., Schoeller, W.W.,
Neumann, B., and Stammler, H.-G. (2004). The
(Me5C5)Si+ cation: a stable derivative of HSi+.
Science 305, 849–851.
3. Pfab, W., and Fischer, E.O. (1953). Zur
Kristallstruktur der Di-cyclopentadienyl-
verbindungen des zweiwertigen Eisens,
Kobalts und Nickels. Z. Anorg. Allg. Chem. 274,
316–322.
29. Jutzi, P., Kohl, F., and Kruger, C. (1979).
¨
17. Macdonald, C.L., Gorden, J.D., Voigt, A.,
Filipponi, S., and Cowley, A.H. (2008). Group 13
decamethylmetallocenium cations. Dalton
Trans. 9, 1161–1176.
Synthesis and structure of the nido-cluster
(CH3)5C5Sn+. Angew. Chem. Int. Ed. 18, 59–60.
30. Jutzi, P., Dickbreder, R., and Noth, H. (1989).
¨
4. Wilkinson, G., Rosenblum, M., Whiting, M.C.,
and Woodward, R.B. (1952). The structure of
iron bis-cyclopentadienyl. J. Am. Chem. Soc.
74, 2125–2126.
Blei(II)-Verbindungen mit p-gebundenen
Pentamethylcyclopentadienylliganden –
Synthesen, Strukturen und
18. Dohmeier, C., Robl, C., Tacke, M., and
Schnockel, H. (1991). The tetrameric
¨
aluminum(I) compound [{Al(h5-C5Me5)}4].
Angew. Chem. Int. Ed. 30, 564–565.
Bindungsverhaltnisse. Chem. Ber 122, 865–870.
¨
5. Hartwig, J.F. (2010). Organotransition Metal
Chemistry: From Bonding to Catalysis
(University Science Books).
31. Malischewski, M., and Seppelt, K. (2017).
Crystal structure determination of the
pentagonal-pyramidal hexamethylbenzene
dication C6(CH3)62+. Angew. Chem. Int. Ed. 56,
368–370.
19. Loos, D., Baum, E., Ecker, A., Schnockel, H.,
¨
and Downs, A.J. (1997). Hexameric aggregates
in crystalline (pentamethylcyclopentadienyl)
gallium(I) at 200 K. Angew. Chem. Int. Ed. 36,
860–862.
6. Melaimi, M., Jazzar, R., Soleilhavoup, M., and
Bertrand, G. (2017). Cyclic(alkyl)(amino)carbenes
(CAACs): recent developments. Angew. Chem.
Int. Ed. 56, 10046.
32. Klein, J.E.M.N., Havenith, R.W.A., and Knizia,
G. (2018). The pentagonal-pyramidal
hexamethylbenzene dication: many shades of
coordination chemistry at carbon. Chemistry
24, 12340–12345.
20. Beachley, O.T., Churchill, M.R., Fettinger, J.C.,
Pazik, J.C., and Victoriano, L. (1986). Synthesis
and crystal and molecular structure of
7. Power, P.P. (2010). Main-group elements as
transition metals. Nature 463, 171–177.
In(C5Me5) – an apparent octahedral cluster.
J. Am. Chem. Soc. 108, 4666–4668.
8. Stephan, D.W. (2016). The broadening reach of
frustrated lewis pair chemistry. Science 354,
aaf7229.
33. Kraft, A., Beck, J., and Krossing, I. (2011). Facile
access to the pnictocenium ions [Cp*ECl]+
(E=P, As) and [(Cp*)2P]+: chloride ion affinity of
Al(ORF)3. Chemistry 17, 12975–12980.
21. Werner, H., Otto, H., and Kraus, H.J. (1986). Die
kristallstruktur von TlC5Me5. J. Organomet.
Chem. 315, C57–C60.
9. Wang, Y., and Robinson, G.H. (2014).
N-heterocyclic carbene-main-group chemistry:
a rapidly evolving field. Inorg. Chem. 53,
11815–11832.
22. Cowley, A.H., Lomelı, V., and Voigt, A. (1998).
´
34. Jutzi, P., Wippermann, T., Kruger, C., and
¨
Synthesis and characterization of a terminal
borylene (boranediyl) complex. J. Am. Chem.
Soc. 120, 6401–6402.
Kraus, H.J. (1983). Synthesis and structure of
metallocene cations of the Vb elements arsenic
and antimony. Angew. Chem. Int. Ed. 22, 250.
10. Budzelaar, P.H.M., Engelberts, J.J., and van
Lenthe, J.H. (2003). Trends in cyclopentadienyl ꢀ
main-group-metal bonding. Organometallics
22, 1562–1576.
23. Huang, J.S., Lee, W.H., Shen, C.T., Lin, Y.F., Liu, 35. Sitzmann, H., Ehleiter, Y., Wolmershauser, G.,
¨
¨
Y.H., Peng, S.M., and Chiu, C.W. (2016).
Cp*-substituted boron cations: the effect of
NHC, NHO, and CAAC ligands. Inorg. Chem.
55, 12427–12434.
Ecker, A., Uffing, C., and Schnockel, H. (1997).
¨
Zwei strukturell verschiedenartige
Stibocenium-Kationen. J. Organomet. Chem.
527, 209–213.
11. Jutzi, P. (2003). Strategies in the
cyclopentadienyl chemistry of p-block
elements. Pure Appl. Chem. 75, 483–494.
24. Shen, C.T., Liu, Y.H., Peng, S.M., and Chiu,
C.W. (2013). A di-substituted boron dication
and its hydride-induced transformation to an
NHC-stabilized borabenzene. Angew. Chem.
Int. Ed. 52, 13293–13297.
36. Wiacek, R.J., Jones, J.N., Macdonald, C.L.B.,
and Cowley, A.H. (2002). Structural
12. Jutzi, P., and Burford, N. (1999). Structurally
diverse p-cyclopentadienyl complexes of the
main group elements. Chem. Rev. 99, 969–990.
interrelationships between the
bis(pentamethylcyclopentadienyl)arsenic(III)
and antimony(III) cations and their precursor
chlorides. Can. J. Chem. 80, 1518–1523.
13. Jutzi, P., and Seufert, A. (1978). Synthese und
dynamisches verhalten des
25. Jutzi, P., Kanne, D., and Kruger, C. (1986).
¨
1-pentamethylcyclopentadienyl-2,3,4,5,6-
pentamethyl-2,3,4,5,6-pentacarba-nido-
hexaboran-kations. J. Organomet. Chem. 161,
C5–C7.
Decamethylsilicocene—synthesis and
structure. Angew. Chem. Int. Ed. 25, 164.
37. Jutzi, P. (1986). Fluxional.eta.1-cyclopentadienyl
compounds of main-group elements. Chem.
Rev. 86, 983–996.
26. Jutzi, P., Kohl, F., Hofmann, P., Kruger, C., and
¨
Tsay, Y.H. (1980).
38. Baxter, S.G., Cowley, A.H., and Mehrotra, S.K.
(1981). Pentamethylcyclopentadienyl-
14. Voigt, A., Filipponi, S., Macdonald, C.L.B.,
Gorden, J.D., and Cowley, A.H. (2000). The
structure of the decamethylborocenium cation:
the most tightly-squeezed metallocene?
Chem. Commun. 911–912.
Bis(pentamethylcyclopentadienyl)germanium
und -zinn sowie (Pentamethylcyclopentadienyl)
germanium- und -zinn-Kationen: Synthese,
Struktur und Bindungsverhaltnisse. Chem. Ber.
¨
113, 757–769.
substituted phosphorus and arsenic cations:
evidence for multihapto bonding between
group 5A elements and carbocyclic ligands.
J. Am. Chem. Soc. 103, 5572–5573.
Chem 4, 1–10, November 8, 2018
9

Please cite this article in press as: Zhou et al., A Phosphorus Lewis Super Acid: h5-Pentamethylcyclopentadienyl Phosphorus Dication, Chem
(2018), https://doi.org/10.1016/j.chempr.2018.08.038
39. Cowley, A.H., and Mehrotra, S.K. (1983). Ring
methyl to phosphorus hydrogen shifts in
pentamethylcyclopentadienyl-substituted
phosphorus cations: parallel between main-
group and transition-metal chemistry. J. Am.
Chem. Soc. 105, 2074–2075.
organofluorophosphonium salts:
hydrodefluorination by a saturated acceptor.
Science 341, 1374–1377.
an yldiide. Angew. Chem. Int. Ed. 54, 8542–
8546.
56. Macdonald, C.L.B., and Cowley, A.H. (1999).
A theoretical study of free and Fe(CO)4-
complexed borylenes (boranediyls) and
heavier congeners: the nature of the iron-
group 13 element bonding. J. Am. Chem. Soc.
121, 12113–12126.
48. Lee, V.Y., Sugasawa, H., Gapurenko, O.A.,
Minyaev, R.M., Minkin, V.I., Gornitzka, H., and
Sekiguchi, A. (2016). A cationic
40. Gudat, D., Nieger, M., and Niecke, E. (1989).
Synthesis, structure, and chemical reactivity of a
stable C5Me5-substituted phosphanylium ion:
(pentamethylcyclopentadienyl)(t-butylamino)
phosphanylium tetrachloroaluminate. J. Chem.
Soc. Dalton Trans. 693–700.
phosphapyramidane. Chem. Eur. J. 22, 17585–
17589.
49. Burford, N., Clyburne, J.A.C., Bakshi, P.K., and
Cameron, T.S. (1993). eta.6-Arene
57. Nguyen Trong, A., Elian, M., and Hoffmann, R.
(1978). Transits across a cyclopentadienyl:
organic and organometallic haptotropic shifts.
J. Am. Chem. Soc. 100, 110–116.
complexation to a phosphenium cation. J. Am.
Chem. Soc. 115, 8829–8830.
¨
41. Uffing, C., Ecker, A., Baum, E., and Schnockel,
¨
H. (1999). Eine kettenformige h5-Cp*Bi-
50. Burford, N., Clyburne, J.A.C., Bakshi, P.K., and
Cameron, T.S. (1995). pi.-coordination of
arenes to phosphorus: models of
¨
58. Kohl, F.X., Ewald, S., Peter, J., Carl, K., Gotthelf,
W., Peter, H., and Peter, S. (1984).
Verbindung mit 11fach koordiniertem Bismut.
Z. Anorg. Allg. Chem. 625, 1354–1356.
Azinkomplexe des
(Pentamethylcyclopentadienyl)germanium-
und -zinn-Kations. Chem. Ber 117, 1178–1193.
stable.pi.-complex intermediates in
electrophilic aromatic substitution.
Organometallics 14, 1578–1585.
42. Korte, L.A., Schwabedissen, J., Soffner, M.,
¨
Blomeyer, S., Reuter, C.G., Vishnevskiy, Y.V.,
Neumann, B., Stammler, H.G., and Mitzel, N.W.
(2017). Tris(perfluorotolyl)borane—a boron
lewis superacid. Angew. Chem. Int. Ed. 56,
8578–8582.
59. Chitnis, S.S., Krischer, F., and Stephan, D.W.
(2018). Catalytic hydrodefluorination of CꢀF
bonds by an air-stable PIII Lewis acid. Chem.
Eur. J. 24, 6543–6546.
51. Gudat, D., Haghverdi, A., Hupfer, H., and
Nieger, M. (2000). Stability and electrophilicity
of phosphorus analogues of arduengo
carbenes—an experimental and
43. Guillaume, C., Lauri, S., Lionel, M., Jean-
Franc¸ois, G., and Elisabet, D. (2017). Bond
strength and reactivity scales for Lewis
superacid adducts: a comparative study with
In(OTf)3 and Al(OTf)3. ChemPhysChem 18,
683–691.
computational study. Chem. Eur. J. 6, 3414–
3425.
60. Burford, N., Phillips, A.D., Spinney, H.A.,
Lumsden, M., Werner-Zwanziger, U., Ferguson,
M.J., and McDonald, R. (2005). Hypervalent,
low-coordinate phosphorus(III) centers in
complexes of the phosphadiazonium cation
with chelate ligands. J. Am. Chem. Soc. 127,
3921–3927.
52. Gudat, D., Schiffner, H.M., Nieger, M., Stalke,
D., Blake, A.J., Grondey, H., and Niecke, E.
(1992). Coordination isomerism in
pentamethylcyclopentadienyl-substituted
iminophosphanes: from classical structures to a
p-complexed iminophosphenium ion. J. Am.
Chem. Soc. 114, 8857–8862.
44. Kogel, J.F., Sorokin, D.A., Khvorost, A., Scott,
¨
M., Harms, K., Himmel, D., Krossing, I., and
Sundermeyer, J. (2018). The Lewis superacid
Al[N(C6F5)2]3 and its higher homolog Ga
[N(C6F5)2]3 – structural features, theoretical
investigation and reactions of a metal amide
with higher fluoride ion affinity than SbF5.
Chem. Sci. 9, 245–253.
61. Chitnis, S.S., Robertson, A.P.M., Burford, N.,
Patrick, B.O., McDonald, R., and Ferguson,
M.J. (2015). Bipyridine complexes of E3+ (E = P,
As, Sb, Bi): strong Lewis acids, sources of
E(OTf)3 and synthons for EI and EV cations.
Chem. Sci. 6, 6545–6555.
53. Niecke, E., Nieger, M., and Reichert, F. (1988).
Arylmino(halogeno)phosphanes
XP=NC6H2tBu3 (X = Cl, Br, I) and the
iminophosphenium tetrachloroaluminate
[PhNC6H2tBu3][AlCl4]: the first stable
compound with a PN triple bond. Angew.
Chem. Int. Ed. 27, 1715–1716.
62. Houghton, A.Y., Hurmalainen, J.,
45. Rezisha, M., Marcel, S., Claudia, L., and Lutz, G.
(2018). Bis(perchlorocatecholato)silane—a
neutral silicon Lewis super acid. Angew. Chem.
Int. Ed. 57, 1717–1720.
Mansikkamaki, A., Piers, W.E., and
¨
Tuononen, H.M. (2014). Direct observation of
a borane-silane complex involved in
frustrated Lewis-pair-mediated
54. Price, J.T., Lui, M., Jones, N.D., and Ragogna,
P.J. (2011). Group 15 pnictenium cations
supported by a conjugated bithiophene
backbone. Inorg. Chem. 50, 12810–12817.
hydrosilylations. Nat. Chem. 6, 983.
46. Bayne, J.M., and Stephan, D.W. (2016).
Phosphorus Lewis acids: emerging reactivity
and applications in catalysis. Chem. Soc. Rev.
45, 765–774.
63. Chen, J., and Chen, E.Y.X. (2015). Elusive
silane-alane complex [Si-H/Al]: isolation,
characterization, and multifaceted frustrated
Lewis pair type catalysis. Angew. Chem. Int. Ed.
54, 6842–6846.
55. Scherpf, T., Wirth, R., Molitor, S., Feichtner,
K.-S., and Gessner, V.H. (2015). Bridging the
gap between bisylides and methandiides:
isolation, reactivity, and electronic structure of
47. Caputo, C.B., Hounjet, L.J., Dobrovetsky, R.,
and Stephan, D.W. (2013). Lewis acidity of
10
Chem 4, 1–10, November 8, 2018