Superphenylphosphines: Nanographene-Based Ligands That Control Coordination Geometry and Drive Supramolecular Assembly

Article abstract of DOI:10.1021/jacs.7b12251

Tertiary phosphines remain widely utilized in synthesis, most notably as supporting ligands in metal complexes. A series of triarylphosphines bearing one to three hexa-peri-hexabenzocoronene (HBC) substituents has been prepared by an efficient divergent route. These "superphenylphosphines", P{HBC(t-Bu)₅}_nPh_3-n (n = 1-3), form the palladium complexes PdCl₂L₂ and Pd₂Cl₄L₂ where the isomer distribution in solution is dependent on the number of HBC substituents. The crystalline structures of five complexes all show intramolecular π-stacking between HBC-phosphines to form a supramolecular bidentate-like ligand that distorts the metal coordination geometry. When n = 2 or 3, the additional HBC substituents engage in intermolecular π-stacking to assemble the complexes into continuous ribbons or sheets. The phosphines adopt HBC's characteristics including strong optical absorption, green emission, and redox activity.

Full text of DOI:10.1021/jacs.7b12251

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Article
Superphenylphosphines: Nanographene-based Ligands That
Control Coordination Geometry and Drive Supramolecular Assembly
Jordan Neale Smith, James M. Hook, and Nigel T. Lucas
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12251 • Publication Date (Web): 18 Dec 2017
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Journal of the American Chemical Society
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Superphenylphosphines: Nanographene-based Ligands That
Control Coordination Geometry and Drive Supramolecular Assembly
Jordan N. Smith,† James M. Hook‡ and Nigel T. Lucas*,†
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†MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry,
University of Otago, Union Place, Dunedin 9016, New Zealand
‡Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2052, Australia.
ABSTRACT: Tertiary phosphines remain widely utilized in synthesis, most notably as supporting ligands in metal com-
plexes. A series of triarylphosphines bearing one to three hexa-peri-hexabenzocoronene (HBC) substituents has been pre-
pared by an efficient divergent route. These “superphenylphosphines”, P{HBC(t-Bu)5}nPh3-n (n = 1-3), form the palladium
complexes Pd2Cl2L2 and Pd2Cl4L2 where the isomer distribution in solution is dependent on the number of HBC substitu-
ents. The crystalline structures of five complexes all show intramolecular π-stacking between HBC-phosphines to form a
supramolecular bidentate-like ligand that distorts the metal coordination geometry. When n = 2 or 3, the additional HBC
substituents engage in intermolecular π-stacking to assemble the complexes into continuous ribbons or sheets. The phos-
phines adopt HBC’s characteristics including strong optical absorption, green emission, and redox activity.
crystalline18-19 and liquid crystalline20-21 phases. Soluble
PAHs with long alkyl chains facilitate solution processing
INTRODUCTION
An important class of organophosphorus compounds
across all areas of chemical synthesis, phosphines remain
ubiquitous as ligands in coordination and organometallic
chemistry.1-3 The status of tertiary phosphines as excellent
supporting ligands in homogeneous catalysis is attributed
to the broad tunability of their electronic and steric char-
acteristics through modification of the three organic sub-
stituents.4 Beyond applications in transition metal cataly-
sis, phosphines and related structures have received at-
tention as soft donor ligands in multimetallic supramo-
lecular coordination assemblies,5 as stabilizing ligands for
metal clusters and nanoparticles,6 as bases in frustrated
Lewis pairs7 and as active materials in a range of opto-
electronic applications.8-9
of these carbon-rich materials into highly-ordered semi-
conducting films, driven by π-stacking aggregation.22-23
Herein we report a series of triarylphosphines (1-3) that
bear large PAH substituents capable of engaging in robust
π-stacked interactions (Chart 1). We have chosen hexa-
peri-hexabenzocoronene (HBC) as the extended aromatic
fragment to attach directly to the phosphorus donor
atom. HBCs have attracted attention because of their
electronic and optical properties, their high thermal and
chemical stability, as nano-sized graphene models and for
their propensity to reliably self-assemble through π-
stacking.23-26 As a Clar benzenoid PAH27 with seven
aromatic sextets, HBC can be regarded as a homologue of
benzene, which, along with its hexagonal symmetry and
readily functionalised peripheral sites, has earned it the
name “superbenzene”;26,28-30 accordingly, our HBC-
phosphines have been dubbed “superphenylphosphines”.
Chelating bidentate ligands typically produce a more
rigid coordination environment at metal centers, with the
‘bite angle’ of the donor atoms programmed by the lig-
and’s covalent backbone.4, 10 An alternative to convention-
al chelating ligands is the self-assembly of monodentate
ligands through non-covalent interactions.11-14 Aromatic
groups and interactions involving π-systems can critically
affect reactivity within the metal coordination sphere,15
however, π-π interactions are rarely utilized for directed
assembly of supramolecular chelate ligands, attributed to
the relative weakness of the interactions for small neutral
aromatic substituents.16 Although polarized C–H bonds
provide directionality through favourable offset face-to-
face (π-stacking) and edge-to-face motifs,16 multiple
interactions need to act in concert to direct
supramolecular order.17 For large polycyclic aromatic
hydrocarbons (PAHs), the dominant face-to-face
arrangement results in persistent columnar structures in
Chart 1. Structures of superphenylphosphines 1-3 and
related compounds.
R
R
R
R
R
R
R
R
R
R
R
R
X
P
X
X
P
P
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
1
R
2
R
3
R = t-Bu, X = -; a, X = O; b, X = BH3; c, X = Se
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A number of metal complexes with ligands bearing an coupling, and show a downfield shift across the series
HBC substituent have been reported,25, 31-38 along with N-
heteroaromatic analogues;39-41 in examples where a crystal
structure was obtained, the HBC groups play a leading
role in directing the supramolecular structure. Challenges
exist in synthesising HBC ligands, especially those with
multiple HBC substituents, as insolubility can result from
the extended π-stacking, thus limiting further application.
Our choice of alkyl substituent and synthetic approach
has allowed us to manage the issue of solubility, while
harnessing the robust interactions between HBCs to
influence both coordination geometry and bulk order of
the superphenylphosphine complexes.
(Figure 1). In contrast, the t-Bu groups adjacent to phos-
phorus show an additive upfield shift corresponding to
the shielding effect of the adjacent π-electron-rich HBC
surfaces. Although the solid state structure of 3a (vide
infra) shows a chiral propeller motif, NMR reveals that
the HBC(t-Bu)5 fragments have sufficient rotational free-
dom to racemize in solution with magnetic equivalence
about the 2-fold P-HBC axis. MALDI-TOF mass spectra
show a single peak envelope corresponding to the molec-
ular ion in each case (see Supporting Information).
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Scheme 1. Synthesis of phosphine oxides 1a–3a.
PdCl2(PPh3)2
CuI
R
P
O
R
Br
P
O
+
RESULTS AND DISCUSSION
NEt3/toluene
85 °C
12
Phosphine Synthesis. Mono-HBC-functionalized super-
phenylphosphine 1 was synthesized first, with an empha-
sis on finding methods that could be translated to higher
structures 2 and 3. Rather than introduce the HBC frag-
ment directly onto phosphorus, synthesis of the hexa-
phenylbenzene (HPB)-phosphine oxide 4a prior to oxida-
tive cyclodehydrogenation ensures adequate solubility
through this core coupling step (Scheme 1). Of several
approaches investigated, the highest yields of 4a were
achieved using a divergent route starting from phosphine
oxide 10 in a Sonogashira coupling, followed by Diels-
Alder cycloaddition reaction with 12. Cyclodehydrogena-
tion of HPB 4a to HBC 1a was achieved in 96% yield using
FeCl3/nitromethane, followed by extractive workup and
chromatography; pleasingly, the P=O moiety did not in-
terfere with this key step. The solubility of 1a in common
organic solvents is good (toluene, THF, CH2Cl2, CHCl3) to
excellent (CS2, 1,2-dichlorobenzene).
11
10
90%
O
R
R
Ph2O
250 °C
R = t-Bu
92%
R
R
8
O
P
O
P
R
R
R
FeCl3/MeNO2
CH2Cl2
R
R
R
96%
R
R
R
R
1a
4a
R
R
R
O
P
O
P
13
14
Extension of the methods developed for phosphine ox-
ide 1a to the synthesis of 2a and 3a was relatively straight-
forward (Scheme 1). Cycloaddition of the acetylenes 13
and 14 with 8 returned gram-scale quantities of 5a and 6a.
Following oxidative cyclodehydrogenation, 2a and 3a
were formed in excellent yields, especially noteworthy
considering these steps involve the formation of 12 and 18
new bonds, respectively. While 2a demonstrated similar
solubility to 1a, 3a was less soluble in chloroalkane sol-
vents (CH2Cl2, CHCl3) and could not be easily extracted
without the addition of CS2. The preferred method for
isolation of 3a was precipitation from the reaction mix-
ture with methanol.
R
R
8
8
91%
92%
Ph2O, 250 °C
Ph2O, 250 °C
R
R
R
R
R
R
R
R
R
R
R
R
O
P
O
P
R
R
R
R
R
R
R
R
R
R
R
R
R
5a
6a
1
All new compounds were fully characterized by H, 13C
and 31P NMR, and signals assigned with the use of 2-D
experiments as described in the Supporting Information.
The 31P NMR resonance frequencies of the phosphine ox-
ides 1a–3a show an increase across the series (δ1a = 32.8;
δ2a = 35.9; δ3a = 38.6 ppm), indicating significant changes
in the magnetic environment at phosphorus, and con-
sistent with the additive anisotropic de-shielding contri-
FeCl3/MeNO2
CH2Cl2, rt
FeCl3/MeNO2
CH2Cl2, rt
93%
88%
R
R
R
R
R
R
R
R
R
R
R
R
R
O
P
O
P
R
R
R
R
R
R
R
R
1
butions from the HBC cores. The H NMR spectra show
aromatic resonances attributed to core HBC protons,
along with distinct phenyl signals for 1a and 2a (Figure 1).
Furthermore, the HBC protons ortho to phosphorus ap-
pear as doublets (JHP = 13.5 Hz) that collapse with 31P de-
R
R
R
R = t-Bu
R
2a
3a
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Table 1. Reduction to phosphines 1-3, borane com-
plexes 1b-3b and their 31P chemical shifts.
1
2
3
4
Product
(yield, %)
31P δ
ppm
Reactant
Solvent
Conditionsa
5
6
7
8
1a
2a
3a
o-xylene
o-xylene
o-C6H4Cl2
a
a
a
1 (51)
2 (60)
3 (68)
−4.2
−4.1
−5.3
1a
2a
3a
o-xylene
o-xylene
o-C6H4Cl2
b
b
b
1b (94)
2b (90)
3b (92)
23.0
24.8
25.1
9
10
11
12
13
aReaction conditions: a) 1a–3a, SiH2Ph2 (33 eq.), o-xylene or
o-C6H4Cl2, 170 °C, 18 h. b) i. 1a–3a, SiH2Ph2 (33 eq.), o-xylene
or o-C6H4Cl2, 170 °C, 18 h. ii. BH3·SMe2 (10 eq.) rt, 4 h.
1
Figure 1. Cropped H NMR spectra (500 MHz, CDCl3) of
phosphine oxides 1a–3a, showing resonances arising from
protons ortho to phosphorus labelled with orange diamonds
(HBC) and blue diamonds (Ph). The t-Bu groups closest to
phosphorus are labelled with green diamonds.
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Complexation with Palladium(II) Chloride. Palladium(II)
chloride was chosen for complexation as it typically forms
thermodynamically driven, square-planar complexes of
the type PdCl2L2. It was of interest to probe the influence
of the HBC fragments on isomer distribution and other
structural variations across the phosphine series. Phos-
phine ligands 1–3 were each mixed with PdCl2 in THF in a
2:1 or 1:1 ratio, returning PdCl2L2 and Pd2Cl4L2 complexes,
respectively, in near quantitative yields (Scheme 3).
Phosphines 1–3 were prepared by the reduction of ox-
ides 1a–3a by LiAlH4-CeCl3 or diphenylsilane (Scheme 2).
The LiAlH4-CeCl3 method42 was complicated by the ap-
parent reduction of the P–HBC bond, as evidenced by
mass spectrometry and becoming increasingly prevalent
with the more highly-substituted homologues. The milder
43
reducing agent SiH2Ph2 showed no evidence of P−C
Scheme 3. Synthesis of palladium(II) complexes.
bond cleavage (Table 1). Phosphines 1-3 were less soluble
than the other phosphine adducts and could be isolated
as yellow powders by precipitation from the reaction mix-
ture with methanol. By an alternative method,44 phos-
phine-borane adducts 1b–3b were isolated and their en-
hanced stability and solubility allowed for easy solution
phase purification. The phosphine-borane complexes
were easily deprotected45 by heating MeOH/THF mix-
tures, followed by removal of the volatiles in vacuo. The
preferred approach is for the phosphines to be then react-
ed on in situ, with no evidence of phosphine oxide in the
products. The superphenylphosphines have been isolated
in practical quantities (>300 mg) following the efficient
five-step route outlined, thereby allowing their chemistry
to be further investigated on a range of fronts.
H
H
H
B
P
Ar
Ar
Ar
= Ph, HBC(t-Bu)5
Ar
1b-3b
THF/MeOH reflux
PdCl2
PdCl2
(1.0 equiv)
THF, rt
THF, rt
(0.5 equiv)
Ar
Ar
Ar
Ar
Ar
P
Cl
P
Cl
Cl
Cl
Ar
Ar
Pd
Ar
Ar
Ar
P
Ar
Ar
Pd
Pd
Ar
Ar
P
P
Cl
Cl
Cl
P
Cl
Cl
P
Cl Pd Cl
Pd
Pd
P
Ar
Ar
Cl
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
cis
trans
syn
anti
Scheme 2. Reductions to 1–3, conversion to phos-
phine-borane (1b–3b) and selenide (1c-3c) adducts.
PdCl2(1)2 (45:55) cis:trans
PdCl2(2)2 (0:100)
PdCl2(3)2 (0:100)
Pd2Cl4(1)2 (75:25) syn:anti
Pd2Cl4(2)2 (40:60)
Pd2Cl4(3)2 (30:70)
O
P
O
P
CeCl3, LiAlH4
THF
P
Isomers of PdCl2L2 were assigned based on distinct 31P
NMR resonances at ca. 35 ppm (cis) and ca. 20 ppm
(trans). Crude PdCl2(1)2 contained a mixture of cis (36.6
ppm) and trans (21.9 ppm) isomers, while X-ray diffrac-
tion of crystals of PdCl2(1)2 revealed the trans complex
(vide infra), with no evidence of the cis isomer after ex-
tensive crystal screening. Despite this observation, solu-
tion 31P NMR of recrystallized PdCl2(1)2 revealed a mixture
of isomers, similar in distribution to the crude material.
Solid-state NMR of the recrystallized product showed the
major phosphorus resonance at 15.2 ppm, consistent with
the trans isomer being the dominant component (see
Supporting Information). Thus, it appears that facile
isomerization in solution affords a mixture of isomers,
while the trans isomer is favored during crystallization,
Ar
Ar
Ar
Ar
Ar
Ar
+
+
HBC(t-Bu)5
Ar
Ar
Ar
inseparable mixtures
variable yields
1a-3a
1-3
1a-3a
SiPh2H2
o-xylene or
o-C6H4Cl2
Ar
= Ph, HBC(t-Bu)5
H
H
H
B
P
further
BH3·SMe2
THF
reactions
P
P
THF/MeOH
reflux
Ar
Ar
Ar
Ar
Ar
Ar
one pot
approach
Ar
Ar
Ar
1-3
1b-3b
90-94% (2 steps)
1-3
Se
P
Se
THF/MeOH
Ar
Ar
95-98%
reflux
Ar
1c-3c
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behavior with some precedent.46-47 1H NMR evidenced the
intermolecular π-interactions giving rise to extended 1-D
1
2
3
4
5
6
trans complexes of ligands 1 and 2 with a triplet arising
from protons ortho to phosphorus showing virtual cou-
pling across the square-planar bis-phosphine complexes.48
1H and 13C NMR spectra of PdCl2(3)2 exhibited significant
broadening of the phosphine ligand signals, particularly
for atoms near the congested coordination site.
ribbons running parallel throughout the structure. The
lateral offset between interacting pairs here (3.49–3.87 Å)
is typical of HBC dimers.
(b)
(a)
The 31P spectra of the chloro-bridged dimetallic Pd2Cl4L2
complexes each showed two resonances of similar fre-
quency (ca. 30–33 ppm), consistent with mixtures of syn
and anti isomers. With few exceptions, bis-monodentate
phosphine Pd2Cl4L2 complexes favor the anti configura-
tion, presumably due to steric demand in the coordina-
tion sphere, while bidentate phosphines favor the syn
isomer.49-51 That monodentate ligands 1–3 all give isomeric
mixtures indicates competing effects of the HBC substitu-
ents; intra-complex π–π interactions between HBC frag-
ments act to stabilize the syn isomer, while the steric bulk
has a destabilizing effect favoring the anti isomer. Assign-
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(c)
31
ing the P signals as low field (syn) and high field (anti)52
shows an increasing tendency towards the anti isomer
across the series Pd2Cl4(1)2–Pd2Cl4(3)2, consistent with
greater ligand steric demand. Interestingly, complex
Pd2Cl4(1)2—in which each ligand contains only a single
HBC unit—has a strong preference for the syn configura-
tion, showing that intra-complex π–π interactions are
suitably favorable despite the steric bulk. For all the com-
plexes, MALDI-TOF MS signals were assigned to fragment
ions, but no molecular ions were detected due to the in-
herent lability of Pd(II) phosphine complexes.
Crystallographic Studies. The solid state structure, in-
cluding effect of the HBC groups on molecular confor-
mation and packing, has been investigated using X-ray
crystallography for nine compounds including five com-
plexes. The structures all contain large solvent regions
that are often highly disordered (up 36% of the volume).
Figure 2. X-ray crystal ball and stick diagrams of (a) 1a dimer
pair, (b) 1b dimer pair [Symmetry code: (i) 1−x, 1−y, 1−z], and
(c) a packing diagram of 1a viewed down the solvent chan-
nels (ordered toluene shown in blue).
Crystals of the phosphine oxides 1a–3a all proved ame-
nable to X-ray diffraction. The structure of 1a revealed an
asymmetric unit containing eight independent phosphine
oxide molecules arranged into four dimers around a cen-
tral channel of ordered toluene and other disordered sol-
vent molecules (Figure 2). Each dimer is orientated per-
pendicularly to four neighbors, interacting through a se-
ries of edge-to-face motifs. The HBC fragments of each
dimer show the smallest lateral offset of all structures
presented here (0.47–0.72 Å) with a rotational offset of
the phosphorus moieties of ~90°. This arrangement of
intermolecular HBC dimers is unusual as the first exam-
ple where there is significant twisting (~30°) between the
‘nanographene’ planes (Figure 2a), known as turbostratic
stacking in bilayer graphene.53 The related borane 1b
(Figure 2b and Supporting Information) crystallizes in
centrosymmetric face-to-face dimers with a lateral trans-
lation of 3.63 Å and the more common Bernal (AB) bilayer
graphitic π-stacking.
(a)
(b)
Figure 3. (a) X-ray crystal ball and stick diagram of 2a dimer
pair, and (b) two 1-D ribbons. Solvent omitted for clarity.
Molecules of 3a adopt chiral, propeller structures with
pseudo-threefold rotational symmetry about the P=O axis
(Figure 4). One t-Bu group of each HBC sits over the face
of an adjacent HBC of the same molecule. The HBC
groups engage in intermolecular π-stacked dimers with
The two HBC fragments of phosphine oxide 2a are
linked through phosphorus with an angle of 84.1° between
the aromatic cores (Figure 3). Each HBC is involved in
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neighboring molecules of alternating orientation. Impres-
(a)
1
2
3
4
5
6
7
8
sively, this “up-down” arrangement is repeated at each
HBC in the structure, resulting in long range, 2-D layers; a
sixfold supramolecular motif of the layers is shown in
Figure 4b. The intermolecular interactions appear suita-
bly strong to distort the HBC components from the ideal-
ized planar conformation, with significant curvature and
twisting observed, up to 18.1° side-to-side (average 11.1°).
The interplanar distance between HBC dimers for 1a-3a
fall in the range 3.36 Å (close to that in graphite) to 3.51 Å.
Surprisingly, the C-P-Cav angle for the phosphine oxides
1a-3a does not show a large variance (105.4–105.5°).
9
(b)
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(a)
(c)
(b)
Figure 5. X-ray crystal ball and stick diagrams of (a) trans-
PdCl2(1)2, (b) trans-PdCl2(2)2, (c) trans-PdCl2(3)2. Solvent
omitted for clarity. ORTEP diagrams of the Pd coordination
sphere shown as an inset with P–Pd–P angles.
Figure 4. (a) X-ray crystal ball and stick, and space-filling
diagrams of 3a. (b) Six molecules of 3a showing intermolecu-
lar π-stacked motif with “up (gray)-down (blue)” orientations
(hydrogen atoms and solvent omitted for clarity).
The bis-phosphine supramolecular-chelate motif results
in a distortion of the P–Pd–P angles from the idealized
180° for trans-PdCl2L2 complexes. The P–Pd–P angle be-
comes increasingly strained with growing ligand size,
such that 158.10(5)° measured for PdCl2(3)2 (Figure 5c) is
the most acute recorded for any trans-palladium(II) com-
plex with two monodentate P-donor ligands.55 This trend
suggests that the coordination geometry in the solid state
is influenced not only by the pseudo-chelate motif, but
also solid-state intermolecular packing interactions be-
tween additional PAH substituents.
Having determined that offset face-to-face π-stacking
drives the assembly of the phosphine precursors, the role
HBC plays in the phosphine-metal complexes was investi-
gated next. Crystals grown from solutions of PdCl2(1)2
PdCl2(2)2, and PdCl2(3)2 were all revealed as the trans iso-
mer (Figures 5). The phosphine ligands adopt a confor-
mation such that a HBC substituent from each phosphine
engages in intramolecular π-stacking with a separation of
3.40–3.43 Å. The lateral offset between the HBCs is 3.54–
3.89 Å, similar to that of 2a–3a; however, the constraints
imposed by the metal coordination force a ~45° rotation
of the P-P axis relative to the plane of the PAHs and AB-
orthorhombic rather than Bernal graphitic stacking.54
The ability of the nanographene groups to direct su-
pramolecular order is also of interest. PdCl2(1)2—which
has no ‘free’ HBC groups—shows a head-to-head/tail-to-
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tail arrangement resulting in a lamellar arrangement of
Page 6 of 10
a bis-
the shortest Pd···Pd distance reported for
monodentate phosphine Pd2Cl4L2 complex,55 with only a
bidentate ligand complex showing greater distortion.49
1
2
3
4
5
6
7
8
PdCl2(1)2 molecules interspersed by layers of chloroform
solvent (Figure 6a). PdCl2(2)2 forms centrosymmetric di-
mers through π-interactions of one free HBC fragment
(Figure 6b) but steric hindrance at the second HBC pre-
cludes the formation of chains as observed for related
phosphine oxide 2a. In contrast, PdCl2(3)2 shows interdig-
itated ribbons due to intermolecular π-interactions be-
tween HBCs perpendicular to the plane of the intramo-
lecularly stacked dimer (Figure 6c). Across the series of
PdCl2(1-3)2, all the HBCs exhibit some curvature (average
8.1°), largest for a non-stacked HBC of PdCl2(3)2 (19.7° be-
tween peripheral rings).
The syn-configuration of Pd2Cl4(2)2 permits the
coordination plane to share the same axis as the
interacting HBCs (Figure 17b,c). This allows for strong
inter-HBC interactions without significantly distorting
the coordination sphere. The intra-complex HBC stacking
distance is 3.45 Å while the second HBC groups stack
9
intermolecularly (3.37 Å), giving
structure of 1-D ribbon motifs (Figure 7c).
a supramolecular
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(a)
(a)
(b)
(b)
(c)
(c)
Figure 7. X-ray crystal ball and stick diagrams of (a) anti-
Pd2Cl4(1)2, (b) syn-PdCl2(2)2 [Symmetry code: (i) −x, y, 1/2−z],
and (c) long range packing order in syn-PdCl2(2)2. Solvent
and hydrogen atoms omitted for clarity.
Figure 6. (a) X-ray crystal ball and stick diagram depicting
long range packing arrangement for trans-PdCl2(1)2 with
interlayer chloroform molecules. (b) A dimer of trans-
PdCl2(2)2. (c) Two chains of trans-PdCl2(3)2. Some solvent
and hydrogen atoms omitted for clarity.
The reliability of HBC(t-Bu)5 as a supramolecular
synthon contrasts with anthracene-based phosphines that
do not always adopt an π-stacked conformation.56 Alt-
hough compelling evidence for robust, inter-HBC interac-
tions was observed in the solid state, this structural rigidi-
ty was not replicated in solution. PdCl2(2)2 displayed only
The two metals of the Pd2Cl4L2 complexes provide a less
hindered coordination environment. Single crystals from
Pd2Cl4(1)2 and Pd2Cl4(2)2 reveal the anti and syn
complexes, respectively, consistent with the major
isomers in solution. Despite a P···P distance of 6.186(1) Å,
anti-Pd2Cl4(1)2 (Figure 7a) adopts a close inter-HBC
contact (3.40 Å) which induces significant curvature
across the interacting HBCs (up to 15.9° side-to-side) and
a reduction of the Pd···Pd distance to 2.9294(4) Å. This is
1
minor broadening for H NMR down to –50°C , indicating
that the π-π interactions remain dynamic down to this
temperature (see Supporting Information). The phos-
phine oxides 1a-3a also do not show significant concen-
tration effects by NMR, consistent with previous studies57
on HBC(t-Bu)6 (see Supporting Information).
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Having established that ligands 1-3 heavily influence co-
1.31 V, similar to HBC(t-Bu)6 (1.16 V).25 The oxidation po-
tential of phosphine oxide 1a was significantly higher than
that of 1, consistent with the oxide being a more electron-
withdrawing substituent. Across the phosphine oxide
series 1a–3a a quasi-reversible process in the range 1.22–
1.28 V is assigned as HBC centered since OPPh3 shows no
oxidation to 1.8o V. Reductions were not observed for any
of the compounds studied.
1
2
3
4
5
6
7
8
ordination geometry in palladium(II) complexes, the ste-
ric demand generated within the coordination sphere was
examined. Using the definition of Tolman,58 the cone an-
gle for phosphine 3 exceeds 250°, significantly greater
than other triaryl phosphines. However, the t-Bu groups
of HBC which define the cone angle are ca. 5 Å from the
metal center, too far to generate significant strain within
the coordination sphere. This is a similar situation to the
‘holey’ triethynylphosphines reported by Sawamura and
co-workers.59-60 To more realistically quantify ligand steric
bulk, percent buried volume (%Vbur) was determined us-
ing SambVca.61-62 Phosphine 3 gives a %Vbur of 30.7 %, not
dissimilar to that of PPh3 (29.6 %),63 indicating that the
peripheral HBC groups are too distant to impinge mean-
ingfully at the metal. %Vbur for the PdCl2L2 crystal struc-
tures, where the two interacting ligands were treated as a
single trans-chelating phosphine is 56.3–53.4%, decreas-
ing modestly across the ligand series 1–3; the second and
third HBC units of ligands 2 and 3 have little influence on
overall ligand steric demand. The trend towards decreas-
ing %Vbur across the series was attributed to a concomi-
tant decrease of the P–Pd–P bond angle, effectively ex-
truding more of the ligand from the coordination sphere.
Table 3. Cyclic voltammetry data for selected phos-
phines and their derivatives.
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Compound
HBC(t-Bu)6
E1pa
1.16
1.25
-
E1pc
1.07
-
E11/2
1.12
-
Δ
0.08
-
PPh3
OPPh3
-
-
-
P[HBC(t-Bu)5]Ph2 (1)
H3B·P[HBC(t-Bu)5]Ph2 (1b)
SeP[HBC(t-Bu)5]Ph2 (1c)
OP[HBC(t-Bu)5]Ph2 (1a)
OP[HBC(t-Bu)5]2Ph (2a)
OP[HBC(t-Bu)5]3 (3a)
1.11
-
-
-
1.30
1.14
1.31
1.32
1.37
1.17
-
1.23
-
0.13
-
1.12
1.17
1.19
1.22
1.25
1.28
0.18
0.16
0.17
Measured in 0.1 M Bu4NPF6 in CH2Cl2 at 100 mV s-1. Refer-
enced against the decamethylferrocene/decamethyl-
ferrocenium couple (–0.07 V vs SCE).
Electronic, Photophysical and Redox Properties. In pre-
liminary experiments to assess the activity of a supertri-
phenylphosphine complex, PdCl2(1)2 was utilized in the
Suzuki-Miyaura cross-coupling of simple aromatic sub-
strates (see Supporting Information). The complex
PdCl2(1)2 displayed catalytic activity comparable to
PdCl2(PPh3)2. To probe the electronic environment at
phosphorus in 1-3, J(31P–77Se) from phosphine-selenide
adducts64-65 (Scheme 2) was measured (Table 2). J(31P–
77Se) increased with increasing number of HBC substitu-
ents suggesting that the HBC fragments have a slightly
negative, additive effect on the electron donating ability
of the phosphorus donor atom. However, it is unclear if
this is a function of the electronics of HBC or an influence
of HBC steric bulk on the geometry at phosphorus. The
geometry measured crystallographically was deemed un-
reliable for correlation due to the major influence of in-
ter-HBC interactions on bond angles and distances.
Absorption and emission spectra for the HBC-
phosphines and their complexes showed spectral features
consistent with mono-functionalized, HBC(t-Bu)5R com-
pounds31 (see Supporting Information; representative
spectra Figure 8). Mono-HBC-compounds 1, 1a–1c have
absorption maxima in the range 362–364 nm, and profiles
that show the phosphorus has little influence on the HBC
chromophore. Emission maxima were similar across the
series with all compounds exhibiting strong green emis-
sion. Phosphine oxides showed only subtle red-shifting
across the series (362 (1a)–364 (3a) nm), however, molar
absorptivity displayed a dramatic, additive effect with
increasing number of HBC chromophores (Figure 8b),
also observed for the Pd complexes (Figure 8c).
(a)
Table 2. 31P chemical shifts and JPSe coupling con-
stants for phosphine-selenides.
Compound
31P δ (ppm)
37.6
JPSe (Hz)
731.0
1c
2c
38.6
734.0
(b)
(c)
3c
40.4
738.9
729.5
SePPh3
35.3
31P NMR spectra recorded at 162 MHz. Resonances exter-
nally referenced against PPh3 or 85% H3PO4 in D2O.
As HBC derivatives are often redox active,25 strongly ab-
sorbing and emissive,37 we investigated the influence of
the phosphorus moiety on these properties. Analysis of
cyclic voltammetric data (Table 3) across the series 1, 1a,
1b and 1c showed the first oxidation in the range 1.11–
Figure 8. (a) Absorption and emission spectra of 1a. (b) Ab-
sorption spectra of the series 1a–3a. (c) Absorption spectra of
the complexes PdCl2(1)2–PdCl2(3)2.
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CONCLUSION
1
REFERENCES
A series of triarylphosphines bearing 1–3 HBC units have
been synthesized in excellent yields (50-74% over ≤5
steps). The divergent route allows the preparation of the
soluble phosphine oxide precursors on a gram scale.
Mirroring the behaviour of simple HBC compounds, the
ligands are strong chromophores (absorption and green
emission) and redox active. To demonstrate the utility of
the superphenylphosphines as ligands, the complexes
PdCl2L2 and Pd2Cl4L2 were prepared and characterised in
solution and the solid state. The HBC substituents impart
constraints on the coordination geometry and
conformation of complexes bearing the phosphine
ligands. In all five crystal structures of the complexes,
robust intramolecular HBC···HBC π–stacking dominates
the ligand sphere, functioning as supramolecular
bidentate-like ligands. The strength of the intramolecular
interaction manifests through an unusually short Pd···Pd
separation in the complex Pd2Cl4(1)2 and record distortion
of the P–Pd–P angle in trans-PdCl2(3)2. The multi-HBC
ligands 2 and 3 drive long range supramolecular order to
porous solids, facilitated by the additional HBC
substituents. The functionalization chemistry of HBCs is
now relatively mature and presents an exciting
opportunity to further explore the coordination,
organometallic and supramolecular chemistry of
superphenylphosphines.
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ASSOCIATED CONTENT
Supporting Information. Synthesis procedures and charac-
terization
data;
NMR,
MALDI-TOF
MS,
UV-
visible/fluorescence spectra; cyclic voltammetry traces; catal-
ysis experiments; X-ray crystallography details and ORTEP
diagrams; %Vbur calculations and steric maps (PDF). Crystal-
lographic data for all reported structures (CIF).
AUTHOR INFORMATION
Corresponding Author
*E-mail: nlucas@chemistry.otago.ac.nz.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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Financial support was provided by the University of Otago
(Department of Chemistry, University of Otago Research
Grants scheme) and the MacDiarmid Institute for Advanced
Materials and Nanotechnology. J.N.S. thanks the University
of Otago for a Doctoral Scholarship.
ABBREVIATIONS
HBC, hexa-peri-hexabenzocoronene, hexabenzo[bc,ef,hi,kl,-
no,qr]coronene; HPB, hexaphenylbenzene.
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