Fusing Triphenylphosphine with Tetraphenylborate: Introducing
the 9-Phosphatriptycene-10-phenylborate (PTB) Anion
Marcus W. Drover
†
,
Koichi Nagata
†
, and
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California, 91125, United States
Abstract
In a fusion of two ubiquitous organometallic reagents, triphenylphosphine (PPh
3
) and
tetraphenylborate (BPh
4
−
), the 9-phosphatriptycene-10-phenylborate (PTB) anion has been
prepared for the first time. This borato species has been fully characterized by a suite of
spectroscopic methods, and initial reactivity studies introduce it as a competent ligand for
transition metals, including Co(II) and Fe(II).
Graphical abstract
Owing to low-cost, high availability, and ease of synthesis and handling, triphenylphosphine
(PPh
3
) and tetraphenylborate (BPh
4
−
) have evolved to become prevailing reagents in the
synthetic chemist’s toolbox. Congruent with interest in developing zwitterionic
organometallic complexes for catalysis,
1
including examples from our group featuring
P,B
-
and
N,B
-containing ligands,
2
we envisioned a tethering strategy between these two partners
in the form of a triptycene. Some years ago, we explored the use of conceptually related 3-
and 4-substituted acyclic diphenylphosphino(tetraphenyl)borates as electron-releasing
ligands for coordination to platinum-group metals, along with some preliminary C-C cross
coupling investigations (Chart 1A).
2a,b
It is noteworthy that a host of other phosphine-
containing ligands incorporating group 13 elements have also been reported.
3
Since the early report of parent 9-phosphatriptycene in 1974 by Bickelhaupt
et al.
(Chart
1B),
4
several research groups have reported related geometrically constrained compounds
Correspondence to: Jonas C. Peters.
†
These authors contributed equally.
Experimental details including
1
H,
31
P,
11
B, and
13
C NMR spectra for all complexes as well as crystallographic data for [
3
][NEt
4
],
[
3
][ASN], [
4
][ASN], [
5
][NEt
4
]
2
, [
7
][NEt
4
]
2
, [
8
][NEt
4
][Na(THF)
4
(NCMe)
2
], [
9
][NEt
4
], and [
10
][NEt
4
]
2
. CCDC
1824471
–
1824478
contains 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
.
HHS Public Access
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Chem Commun (Camb)
. Author manuscript; available in PMC 2019 July 12.
Published in final edited form as:
Chem Commun (Camb)
. 2018 July 12; 54(57): 7916–7919. doi:10.1039/c8cc04321c.
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containing group 14 or 15 elements, though the synthesis of anionic group 13 analogues has
not been achieved.
5
In this work, we present the formal joining of PPh
3
and BPh
4
−
in the
form of the 9-phosphatriptycene-10-phenylborate anion. To fully characterize this
phosphino(borate), several derivatives, including
cis
-dimethylplatinum(II) and
pentakis
(carbonyl)tungsten(0) PTB complexes, have been prepared. We also show that the
PTB anion can be employed for the synthesis of paramagnetic, Co(II) and Fe(II) complexes,
serving roles as both
σ
-donating ligand and halide abstracting equivalent.
The route used to prepare the 9-phosphatriptycene-10-phenylborate (PTB) anion [
3
]
−
is
shown in Scheme 1. Treatment of
tris
(
o
-bromophenyl)phosphine
5a
with
t
BuLi followed by
slow addition of PhBCl
2
at −78 °C and careful warming to room temperature furnishes
tetrakis
(tetrahydrofuran) lithium 9-phosphatriptycene-10-phenylborate, [
3
][Li(THF)
4
] as a
white solid in 64% yield after work-up. The countercation of [
3
]
−
is straightforwardly
tailored by treatment of a CH
3
OH solution of [
3
][Li(THF)
4
] with [Y]Br (Y = ASN; ASN =
5-azoniaspiro[4.4]nonane or NEt
4
) giving [
3
][ASN] or [
3
][NEt
4
] as white solids in 97 and
87% yield, respectively.
The
31
P{
1
H} NMR spectrum of [
3
][Li(THF)
4
] (CD
3
CN, 298 K) displays a singlet at
δ
=
− 43.7 ppm (d,
3
J
P-B
= 3.8 Hz), while
11
B{
1
H} NMR spectroscopy provides a sharp
resonance at
δ
= − 8.75 ppm (d,
3
J
P-B
= 3.8 Hz);
c.f.
δ
B
= − 6.7 ppm for Li[BPh
4
].
6
Notably,
this is a rare case where three-bond scalar coupling between phosphorus and boron (
I
= 3/2)
is experimentally observed. Analogous NMR spectroscopic parameters are observed for [
3
]
[ASN] and [
3
][NEt
4
] (see ESI) -all of which are significantly upfield-shifted compared to
PPh
3
(
δ
P
= −3.1 ppm), but in the same range as
1
(
δ
P
= −64.8 ppm) and
2
(
δ
P
= −44.4 ppm).
Additionally, the
7
Li{
1
H} NMR spectrum of [
3
][Li(THF)
4
]provides a signal at
δ
Li
= − 1.2
ppm.
Crystalline material of [
3
][Li(THF)
4
] was obtained from a saturated THF solution at −35 °C,
while crystals of [
3
][ASN] or [
3
][NEt
4
] could be grown from a saturated CH
3
OH solution at
−35 °C. The crystal structure of [
3
][ASN] is shown in Scheme 1 and features a constrained
phosphorus atom having an average <C-P-C bond angle of 96.5°
c.f.
106.3° for PPh
3
(Table
1) and an average <C-B-C bond angle of 104.4°. The <C-P-C bond angle for [
3
][ASN] is the
smallest reported among the known 9-phosphatriptycenes (<C-P-C = 98.0° for
1
and 99.3°
for
2
; Table 1).
Having prepared PTBs [
3
]
−
, we next assessed phosphine s-character and
σ
-donating ability
(Table 1 & Scheme 2). In agreement with incorporation of an insulated borate, the cyclic
voltammogram of [
3
][NEt
4
] shows an irreversible oxidative feature at 0.97 V that is
cathodically-shifted compared to related phosphatriptycene
1
and PPh
3
(Figure 1A).
Compound [
3
]
−
was further evaluated based on the following characteristics: a)
1
J
P,Se
= 746
Hz for the phosphine selenide, [
4
][ASN] (X-ray, Figure S30);
7
b)
1
J
Pt,C
= 597 Hz for the
Pt(II) complex,
cis
-[Pt(CH
3
)
2
(PTB)
2
][NEt
4
]
2
([
5
][NEt
4
]
2
; Figure 1B), and c)
ν
(CO) = 1943
cm
−1
for the W(0) complex, [W(CO)
5
(PTB)][Li(THF)
4
] ([
6
][Li(THF)
4
]). Despite having the
smallest <C-P-C bond angle of the series, these data point toward [
3
]
−
as having lower s-
character and similar
σ
-donating ability when compared with
1
or
2
. Of note, oxidation of
PTB to the phosphine selenide, [
4
][ASN], also results in increased pyramidalization at
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phosphorus (<(C-P-C) = 301.8° (
c.f.
289.4°)) and a decreased P(1)-B(1) distance of 2.911(5)
Å (
c.f.
3.020(2) Å) corresponding to a five-fold increase in
3
J
P-B
to 19.3 Hz.
Protonation of [
3
][NEt
4
] was also shown to be facile. Treatment of an MeCN solution of [
3
]
[NEt
4
] with [H-OEt
2
][BAr
F
4
] results in formation of the neutral zwitterion, PTB-H (
3
-
H
)
along with [NEt
4
][BAr
F
4
]. Protonation is evidenced by
31
P NMR spectroscopy that shows a
broad doublet at
δ
= −25.3 ppm (
1
J
P,H
= 512 Hz), which collapses to a singlet on
1
H
decoupling. Notably this value of
1
J
P,H
is close to that observed for [H-PPh
3
]
+
(
1
J
P,H
= 510
Hz) and lower than that observed for the [H-PH
3
]
+
cation (
1
J
P,H
= 548 Hz), which is near
ideally
sp
3
−
hybridized.
8
Next, we studied the coordination chemistry and spectroscopic features of [
3
]
−
with Co and
Fe. Combination of two equiv. of [
3
][NEt
4
] with CoBr
2
in CH
3
CN at ambient temperature
produced a dark turquoise solution (Scheme 3). Metalation of [
3
][NEt
4
] was confirmed by
1
H NMR spectroscopy, which displayed paramagnetically shifted resonances, while no
discernable signal was obtained by
31
P NMR spectroscopy (CD
3
CN, 298 K). In an effort to
provide material suitable for single crystal X-ray diffraction, cooling a CH
3
CN solution of
this reaction mixture to −35 °C resulted in a color change from turquoise to forest green,
while cooling a saturated THF solution caused no change in color, indicative of a solvent-
dependent coordination equilibrium. This process was later studied by variable temperature
UV-Visible spectroscopy (
vide infra
).
Ultimately, the identity of the constituent Co(II) species was obtained by layering the
aforementioned CH
3
CN solution with THF at −35 °C, which gave three different crystal
morphologies (ESI, Figure S26): i) thin blue sheets, ii) turquoise blocks, and iii) orange
blocks (Figure 2). Analysis by single crystal X-ray diffraction confirmed the identity of each
of these species to be: the
mono
(PTB) complex [Co(PTB)Br
3
][NEt
4
]
2
([
7
][NEt
4
]
2
) and
bis
(PTB) complexes [Co(PTB)
2
Br
2
][NEt
4
]
2
([
8
][NEt
4
]
2
) and [Co(PTB)
2
(NCCH
3
)
2
Br]
[NEt
4
] ([
9
][NEt
4
]). The Co(1)-P(1) bond length for [
7
][NEt
4
]
2
[2.358(4) Å] and [
8
][NEt
4
]
2
[2.386(4)/2.383(5) Å] agree with those reported for Co(PPh
3
)
2
Br
2
[2.349(2) Å],
9
while for
the Co(II) zwitterion, [
9
][NEt
4
], a shorter distance of 2.248(4) Å is noted. For the pseudo-
tetrahedral complex [
8
][NEt
4
], the angles, <Br(1)-Co(1)-Br(2) (117.54(9)°) and <P(1)-
Co(1)-P(2) (121.1(2)°) are also in line with those reported for Co(PPh
3
)
2
Br
2
(117.27(3)° and
115.88(2)°). Finally, for the trigonal bipyramidal complex [
9
][NEt
4
], the P(1)-Co(1)-P(2)
bond angle (177.58°) is close to 180°, while the sum of angles in the equatorial plane is
equal to 360°.
Probing the sample by optical spectroscopy allowed for the study of complex speciation in
CH
3
CN as a function of temperature. At 25 °C, the UV-Vis spectrum showed the presence of
several bands between 600 and 700 nm [617, 642, 668, 681, and 696 nm] (Figure 3A).
Related absorptions have been documented for the PPh
3
analogues, [Co(PPh
3
)Br
3
][NEt
4
]
and Co(PPh
3
)
2
Br
2
.
10
Cooling the sample to −40 °C, however, gradually revealed the
formation of a new species, exhibiting clear isosbestic behavior with growth of bands at 310
and 376 nm. We posit that these data result from a low-temperature metathesis equilibrium
of [
8
][NEt
4
]
2
(turquoise) to give the bis(acetonitrile) adduct [
9
][NEt
4
] (orange) and
[NEt
4
]Br; these species account for the forest green color observed at low temperature. This
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result highlights the ability of [
3
]
−
to undergo metathesis with an M-X (X = halide) bond,
providing an entryway into coordinatively unsaturated metal complexes that can react with
L-type donor ligands (
e.g.
, NCMe).
A related set of Fe(II) PTB complexes were also obtained with coordination being
substantiated by NMR spectroscopy (Scheme 3). Layering of a CH
3
CN solution with THF
and cooling to −35 °C gave colorless blocks – one of which was analyzed by X-ray
diffraction as [Fe(PTB)Br
3
][NEt
4
]
2
, [
10
][NEt
4
]
2
(X-ray, Figure S32). The Fe(1)-P(1) bond
length for [
10
][NEt
4
]
2
[2.4320(6) Å] is similar to that noted for [Fe(PPh
3
)Br
3
][HIPr] (HIPr
= 1,3-bis(2,6-diisopropylphenyl)imidazolium) [2.463(2) Å] and is otherwise comparable to
the Co derivative [
7
][NEt
4
].
11
The
57
Fe Mössbauer spectrum of a frozen CH
3
CN solution revealed the presence of three
Fe-containing species with parameters suggesting the presence of two four- and one higher-
coordinate Fe(II) centers (Figure 3B).
12
For the two major species, isomer shifts of
δ
= 0.87
and 0.79 mm/s, with quadrupole splittings of ΔE
Q
= 2.34 and 2.87 mm/s, respectively were
observed. These doublets are assigned to the four-coordinate species [Fe(PTB)Br
3
][NEt
4
]
2
(43%, [
10
][NEt
4
]
2
) and [Fe(PTB)
2
Br
2
][NEt
4
]
2
(47%, [
11
][NEt
4
]
2
). For comparison, values
of
δ
= 0.86 and 0.83 mm/s and ΔE
Q
= 2.10 and 2.71 mm/s were reported for
[Fe(quinoline)Br
3
][NEt
4
] and Fe(quinoline)
2
Br
2
.
13
A third species, accounting for the
remaining 10% of the overall fit, displayed an unusually large isomer shift,
δ
= 1.72 mm/s
with ΔE
Q
= 2.52 mm/s. We presume this species to be a higher-coordinate
Fe(PTB)
x
Br
y
(NCCH
3
)
z
complex, such as [Fe(PTB)
2
(NCCH
3
)
2
Br][NEt
4
], akin to [
9
][NEt
4
].
Given the low known isomer shift for [Fe(PEt
3
)
2
(NCCH
3
)
4
]
2+
(
δ
= 0.37 mm/s), it is unlikely
that this doublet results from the neutral zwitterion, Fe(PTB)
2
(NCCH
3
)
4
.
14
Inspired by the success of PPh
3
and BPh
4
−
as reagents of broad utility, we have provided
here the first synthesis of the 9-phosphatriptycene-10-phenylborate anion. The s-character
and
σ
-donating capability of this ligand have been assessed, and preliminary forays into
coordination chemistry with Co and Fe demonstrate that this anion can be used to advantage
as both a ligand and halide-abstracting agent. Future work will focus on the small-molecule
reactivity of these and other PTB-ligated transition metal complexes.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the NIH (GM070757), NSERC (Banting PDF award to MWD), and the Resnick
Sustainability Institute at Caltech (Postdoctoral award to MWD). KN thanks the Japan Society for the Promotion of
Science (JSPS) for an overseas postdoctoral fellowship. We thank Larry Henling and Dr. Mike Takase for assistance
with X-ray crystallography.
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Figure 1.
A)
The CV for [
3
]
−
with the oxidative feature highlighted (using an internal Fc/Fc
+
reference).
B
) Mercury depiction of the solid-state molecular structure for [
5
][NEt
4
]
2
(50%
ellipsoid probability, hydrogens and counterions omitted for clarity).
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Figure 2.
Mercury depiction of the solid-state molecular structure of the anions: [
7
]
2−
, [
8
]
2−
, and [
9
]
1−
(displacement ellipsoids are shown at the 50% probability, hydrogens and counterions
omitted for clarity).
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Figure 3.
A)
Variable temperature UV-Visible absorbance data collected on a mixture of [
7
]
2−
, [
8
]
2−
,
and [
9
]
1−
in CH
3
CN. Inset shows expansion of 550 to 750 nm window and the frozen
cuvette at −40 °C. Temperature increments are in 5 °C.
B)
Zero-field
57
Fe Mössbauer
spectra of a mixture of [
10
]
2−
(blue, 43%), [
11
]
2−
(green, 47%), and an unknown species
(yellow, 10%) as a frozen CH
3
CN solution collected at 80 K.
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Scheme 1.
Preparation of 9-phosphatriptycene-10-phenylborate and Mercury depiction of the solid-
state molecular structure of [
3
][ASN] (displacement ellipsoids are shown at the 50%
probability, hydrogens and ASN
+
counterion omitted for clarity). Selected bond lengths [Å]
and angles (°). P(1)-C(1) 1.827(2), P(1)-C(3) 1.839(2), P(1)-C(5) 1.837(2), B(1)-C(2)
1.649(2), B(1)-C(4) 1.655(2), B(1)-C(6) 1.647(2), B(1)-P(1) 3.020(2), <C-P(1)-C 96.5
(avg.), <C-B(1)-C 104.4 (avg.), <(C-P-C) 289.4.
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Scheme 2.
Assessing the s-character and
σ
-donating properties of [
3
]
−
.
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Scheme 3.
Generation of Co(II) and Fe(II) PTB complexes
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Chart 1.
Representative borate ligands
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Table 1
Characterizing the 9-phosphatriptycene-10-phenylborate (PTB) anion.
E
ox
/V
Lone pair
orbital
c
1
J
P,Se
(Hz)
Ar
3
P=Se
1
J
Pt,C
(Hz)
cis
-PtMe
2
L
2
ν
(CO) (cm
−1
)
W(CO)
5
L
<C-P-C (°)
by XRD
h
PPh
3
1.05
a
HOMO
736
d
616
d
1943
f
106.3
1
1.68
a
HMMO-5
828
d
--
1946
f
99.3
2
--
--
795
d
607
d
--
98.0
[
3
]
−
0.97
b
HMMO-4
746
e
597
e
1943
g
96.5
a
Irreversible peak potential
vs.
Ag/Ag
+
in CH
2
Cl
2
with 0.1 M [N
n
Bu
4
][ClO
4
].
b
Irreversible peak potential
vs.
Fc/Fc
+
in CH
2
Cl
2
with 0.1 M [N
n
Bu
4
][ClO
4
] for NEt
4
+
counterion.
c
Single-point calculations and geometry optimization were performed using DFT: B3LYP/6-311G(d) for P, and 6-31G(d) for all other atoms.
d
in CDCl
3
.
e
in MeCN-d
3
for ASN
+
counterion.
f
in hexane.
g
solid-state for Li(THF)
4
+
counterion.
h
Average of three bonds.
Note
: Data for PPh
3
and
1
are sourced from ref.
5c
; data for
2
is sourced from ref.
5a
.
Chem Commun (Camb)
. Author manuscript; available in PMC 2019 July 12.