Synthesis, Characterization, and Catalytic Studies of Dinuclear d8 Metal-Phosphazane Complexes

Article abstract of DOI:10.1021/acs.inorgchem.8b00787

Herein we report the synthesis and characterization of two diphosphazane (P-N-P) ligands, along with their corresponding novel palladium and platinum complexes. Compounds were characterized by FTIR and NMR spectroscopy, single crystal X-ray diffraction (S

Full text of DOI:10.1021/acs.inorgchem.8b00787

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis, Characterization, and Catalytic Studies of Dinuclear d8
Metal−Phosphazane Complexes
Matthew R. Crawley, Alan E. Friedman, and Timothy R. Cook*
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
S
* Supporting Information
ABSTRACT: Herein we report the synthesis and characterization of two
diphosphazane (P−N−P) ligands, along with their corresponding novel palladium
and platinum complexes. Compounds were characterized by FTIR and NMR
spectroscopy, single crystal X-ray diffraction (SC-XRD), and powder X-ray
diffraction (PXRD)), as well as mass spectrometry. The Pd(II) complex of the p-
phenyldiphosphazane was shown to effectively and efficiently catalyze Suzuki-
Miyaura cross-coupling reactions between a variety of substrates. Yields were as
high as 96%, with reaction times as short as 15 min at room temperature and open
to air. No additional supporting ligands, such as triphenylphosphine, were needed.
The work reported here expands the use of phosphazane ligands to support
catalytic centers and provides an understanding of phosphazane metal−ligand
bonding interactions (specifically diphosphazanes).
Whereas the chemistry of bidentate P−C−P ligands has been
well-studied by many groups including those of Shaw,9−30
Puddephatt,31−33 Kubiak,34−55 and White,56−58 phosphazane
ligands have received far less attention. From a synthetic
standpoint, phosphazanes offer a higher degree of tunability
owing to the wide availability of primary amines that are used in
their formation. The amine building blocks enable both steric
and electronic tuning depending on the specific R groups that
are selected. Phosphazanes are significantly more cost-effective
to synthesize; they are not formed from expensive or
commercially unavailable bis-halogenated alkyl or benzyl
precursors like their P−C−P analogues.
1. INTRODUCTION
Phosphazane compounds are an underexplored class of
phosphorus−nitrogen compounds of interest to organic and
inorganic chemists, the latter drawn to the interesting transition
metal complexes thereof. Interest in phosphorusnitrogen
bonding prompted early explorations of phosphazanes and
related species.1 Since then, bis(diphenylphosphino)amine
(dppa) emerged as one of the most popular and well-studied
phosphazanes. First reported in 1965 by Clemens et al., it was
used to study the mechanism of chloroamination reactions.
Initial thrusts into the coordination behavior of phosphazane
ligands focused on the nature of the electronic properties of the
P−N bond along with its influence on the interaction of the
donor phosphorus atom with a metal center.2−4 Natural
comparisons were made between phosphazane cores and their
corresponding bisphosphine analogues, for example dppa
versus bis(diphenylphosphino)methane (dppm). A suite of
Mo(CO)4(L−L) (L−L = bidentate P−N−P or P−C−P, or two
PR3 ligands) complexes provided a platform to investigate the
σ-donating and π-accepting abilities of the phosphorus-based
ligand library. Findings revealed that changing the bridging
atom from carbon to nitrogen did not have profound effects on
the carbonyl stretching mode, and therefore the nature of the
metal−phosphorus bond.4 Although both P−N−P and P−C−
P ligands possess small bite angles, the former will not always
form the same architectures as the latter. For example, Mague
et al. found that so-called dinuclear “A-frame” complexes
formed when P−C−P ligands were used. In contrast, a series of
mononuclear rhodium and iridium complexes were isolated
when phosphazanes were employed.5 That said, it is possible to
isolate A-frame type complexes bearing phosphazane ligands
that are structurally similar to their corresponding phosphine
analogues.6−8
Scheme 1 illustrates a typical bis(diphenyl)phosphazane
ligand synthesis. Preparations are mild and circumvent the use
Scheme 1. Typical Bis(diphenyl)phosphazane Ligand
Synthesis
of aggressive organometallic reagents, strong bases, and the
need for low temperature reaction conditions.59 Figure 1
illustrates a substantial cost difference between the synthesis of
a simple bis-P−N−P (1) versus its bis-P−C−P counterpart.
Summing the reagent costs associated with treating α,α,α′,α′-
tetrabromo-p-xylene with lithium diphenylphosphide reveals an
Received: March 24, 2018
© XXXX American Chemical Society
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completion the solution was allowed to stand for 5 min at room
temperature. Compounds 1 and 2 crystallized out of solution
and were collected by simple filtration. The crystals were
washed with acetonitrile and dried in vacuo. The filtrate was
allowed to evaporate slowly overnight in air to afford a second
crop of crystals. The combined yields approached 91% with no
1
further purification necessary based on both H and 31P{1H}
NMR. Ligands 1 and 2 are readily soluble in halogenated
solvents and THF, insoluble in alcohols and paraffins, and
sparingly soluble in aromatic solvents.
Complexes 3−5 were synthesized through a simple ligand
exchange reaction in which the 1,5-cyclooctadiene (COD)
ligand was replaced by the bidentate P−N−P moiety of a given
diphosphazane ligand (Scheme 3). Complexes 4 and 5
Figure 1. Comparing the cost of synthesis of a P−N−P ligand in
comparison to the analogous P−C−P ligand.
∼48-fold greater expense relative to synthesis of 1 reported
herein.60
Scheme 3. Synthesis of Dinuclear Complexes 3−5
The central nitrogen atom of a phosphazane ligand typically
deviates only slightly from an idealized sp2 geometry. Caution is
necessary when strong nucleophilic reagents (i.e., strong bases
and organometallic reagents) and/or electron-withdrawing R-
groups, especially those on the nitrogen atom, are used. The
P−N bond is susceptible to nucleophilic attack, resulting in
bond cleavage that will ultimately reduce yields of a target
phosphazane and necessitate further separation techniques.1 To
the same end, electron withdrawing R-groups can labilize the
P−N bond, promoting isomerization to an N−P−P spe-
cies.61,62 As such, any reaction involving the outlined reagents
should be performed prior to the P−N bond formation.
In this work, we report the modified synthesis and
characterization of N1,N1,N4,N4-tetrakis(diphenylphosphino)-
benzene-1,3-diamine (m-tdpb, 1) and N1,N1,N4,N4-tetrakis-
(diphenylphosphino)benzene-1,4-diamine (p-tdpb, 2), as well
as their corresponding dinuclear metal complexes: (PtCl2)2(m-
tdpb) (3), (PtCl2)2(p-tdpb) (4), and (PdCl2)2(p-tdpb) (5).
Group 10 metals in conjunction with phosphorus-based ligands
are classic cross-coupling catalysts. Additionally, palladium
phosphazane complexes have been shown to catalyze Suzuki-
Miyaura cross-coupling reactions.63 As such, we demonstrate
that complex 5 is capable of effectively catalyzing Suzuki-
Miyaura coupling reactions with short reaction times, in air, at
room temperature, even when the substrate is an aryl chloride
in place of the more reactive aryl bromide typically employed in
cross-coupling reactions.
precipitated from CH2Cl2 during the synthesis as a white and
light yellow crystalline solids, respectively. Complex 3 remained
in solution and was isolated as a colorless crystalline solid by
slow evaporation of the reaction solvent. Complexes 3-5 were
characterized by proton NMR, 3 and 4 with 31P{1H} NMR
(due to poor solubility, a 31P{1H} of 5 was not acquired), along
with ESI-mass spectrometry, and single crystal (3 and 4) and
powder (4 and 5) X-ray diffraction studies.
All three reported metal complexes were insoluble or
sparingly soluble in most common laboratory solvents with
the following exceptions: complex 3 was initially soluble in
CH2Cl2 during its synthesis. Once isolated from the reaction
mixture its solubility in CH2Cl2 dropped significantly. This
marked difference in solubility is typically due to solvent
molecules cocrystallizing with a given complex, which alters the
solubility of the resulting material. Solvents of crystallization
were observed in diffraction studies, consistent with this
explanation. To resolvate 3 1,1,2,2-tetrachloroethane-d2 was
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization. Ligands 1 and 2
were synthesized according to a modified literature procedure
(Scheme 2).64 We have improved upon the isolation method to
provide excellent yields and purity in one step that avoids
dissolution into diethyl ether, a solvent in which 1 and 2 are
only sparingly soluble. The triethylammonium chloride was
filtered from the crude reaction mixture and the filtrate added
to a beaker filled with acetonitrile (4x the volume of the
filtrate). During addition, the acetonitrile was stirred and upon
1
used with sonication, enabling characterization by H NMR.
Furthermore, 3 showed solubility in hot DMSO. Complexes 4
and 5 show an even lower degree of solubility, NMR
characterization was only possible through solvation in
1,1,2,2-tetrachloroethane-d2 (4) or hot DMSO-d6 (5). In
terms of air and water stability, the complexes are stable in
air and water on the time scale of days and weeks at room
temperature. In refluxing DMF, complex 5 undergoes a P−N
isomerization resulting in decomposition of the dinuclear
species.
Scheme 2. Synthesis of Phosphazane Ligands 1 and 2
1
The H NMR of all three complexes show the expected
splitting patterns, integrations, and chemical shifts consistent
with dinuclear Pt2 and Pd2 species. The spectrum of complex 3
contained a singlet, a doublet, and a triplet in the 6−7 ppm
range, representative of the four protons on the central
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Figure 2. Oakridge Thermal Ellipsoid Plot (ORTEP) of compounds 2−4 drawn at the 50% probability level, hydrogen atoms and solvent molecules
omitted for clarity. In the case of complex 4, Z′ = 0.5 and half the formula unit is symmetry generated by an inversion symmetry element, indicated
by the primed labels.
phenylene moiety along with a series of three multiplets in the
7.3 to 7.7 ppm range that were assigned to the protons of the
phenyl groups on phosphorus atoms. The 31P{1H} NMR
on the P−N−C asymmetric stretching region (1110 to 930
cm−1). Symmetry analysis predicts four IR active P−N stretches
for 1, but only two for 2. In the spectra of 1, there were four
major peaks at 1091, 982, 930, and ∼929 cm−1, with a strong
shoulder on the peak at 930 cm−1 but only two major peaks
(1091 and 923 cm−1) in the spectrum of 2. The spectra of 4
and 5 are similar since these species differ only in their metal
ions and are otherwise expected to be largely isostructural.
Since complex 5 was difficult to characterize by other methods
due to its poor solubility, confirming that it had a similar IR to
that of 4, which was readily characterized, supports the
formation of the Pd2 species (Figure S9).
2.2. X-ray Diffraction Studies. Single crystals of 2 were
grown by layering pentanes onto a saturated solution of the
ligand in CH2Cl2. After standing at 0 °C for 1 day, colorless
clear blocks were obtained. Complex 3 was crystallized by slow
solvent evaporation of the reaction mixture filtrate (CH2Cl2),
producing large clear and colorless block-like crystals that were
prone to twinning. Crystals of 3 desolvated and fractured
rapidly upon removal from the mother liquor, regardless of the
solvents used during crystallization. Nonetheless, a tractable
data set was obtained by minimizing exposure times to limit the
duration of the diffraction experiment. Complex 4 was
dissolved in 1,1,2,2-tetrachloroethane and the solvent was
slowly evaporated over the course of 3 weeks at room
temperature without agitation, resulting in the formation of
small block-like crystals suitable for X-ray diffraction.
All structures were solved and refined using the Olex2
software package.69 The structure of 2 was solved using
SHELXS70 via Direct Methods. Complex 4 was solved using
SHELXS70 using the Patterson Method. Complex 3 was solved
using SHELXT71 with Intrinsic Phasing. All structures were
then refined using SHELXL using a least-squares minimiza-
tion.72 Ligand 2 refined quite well and did not require the use
of any constraints or restrains in the refinement. Complex 4
contained four 1,1,2,2-tetrachloroethane units which required
the use of SADI commands to restrain bond lengths to values
which are chemically equivalent to similar lengths. The rigid
bond restraint DELU was used to restrain anisotropic
displacement parameters of bonded atoms. SIMU commands
were also applied to the misbehaved solvent molecules. DFIX
restraints were used to restrain the bond lengths and ISOR
restraints were used to allow for isotropic refinement of
anisotropic atom. Additionally, a SADI restraint was used on
one of the phenyl rings of the phosphazane ligand to give more
homogeneous C−C bond lengths (it is chemically reasonable
to expect that the C−C bond in a phenyl ring should be
approximately the same). Several CH2Cl2 molecules were
spectrum featured a singlet at 23.09 ppm with platinum
195
satellites (J31
P− Pt
= 3320 Hz), which is fully consistent with
cis phosphazane coordination to a square planar platinum ion.
The magnitude of the coupling constant gives insight into the
195
P‑ Pt
coordination environment about each platinum atom, a J31
∼ 3300 Hz indicates cis coordinated phosphorus donors,
whereas J
∼ 2300 Hz indicates trans coordination.65 In
31P−195Pt
the case of complex 4, the 1H NMR contained a singlet at 6.21
ppm, assigned to the four phenylene protons, indicative of the
higher symmetry of 4 as compared to 3. Also present were two
multiplets, a result of the protons of the phenyl rings. Due to
poor solubility, the platinum satellites were not observed in the
31P{1H} spectrum; however, a singlet was observed at 21.33
ppm, a similar shift to that of 3, further supporting that the
desired complex was successfully synthesized. Due to its poor
1
solubility, even in hot DMSO, only a H NMR was acquirable
for 5, which contained similar splitting pattern and shifts to that
of 4.
ESI-mass spectrometry further corroborates that the desired
complexes and ligands were successfully synthesized.66,67
Compounds 2−5 were sonicated in CH2Cl2 and filtered prior
to MS analysis (a small amount of acetonitrile was found to
improve the solubility of 5 slightly). The mass spectrum of 2
(Figure S10) showed peaks corresponding to [2 + H]+ along
+ +
with [2 − PPh2+ + 2H+]+ and [2 + PPh2 ] , all of which are in
good agreement with simulated spectra for each species and are
reasonable fragments of the parent compound (Figure S11).
Complexes 3 and 4 show similar mass spectra (Figures S12 and
S14, respectively); this is to be expected since they are isomers.
Both show a peak corresponding to the parent complex with a
Li+ cation [M + Li+]+, matching simulated isotopic distribution
and mass to charge ratio (Figure S13). In the spectrum of
complex 5, a peak consistent with the loss of a chloride ligand
and addition of acetonitrile was observed (Figure S15), in good
agreement with the theoretical spectrum (Figure S16). A
similar peak without an additional acetonitrile was also
observed at an m/z of 1162.98 (predicted 1162.96).
The infrared spectra of all compounds presented were
acquired. Compounds 1−3 showed a weak band around 3040−
3050 cm−1, assigned to aromatic C−H stretches. These peaks
were less intense in compounds 4 and 5. All five compounds
had a series of three to four peaks in the range of 1586−1432
cm−1, attributed to aromatic ring stretching.68 The higher
degree of symmetry present in compound 2 (idealized D2h)
compared to compound 1 (idealized C2v) was apparent based
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Table 1. Select Bond Lengths and Angles from 2, 3, and 4
2
3
4
P1−N1 (Å)
P2−N1 (Å)
P3−N2 (Å)
P4−N2 (Å)
−
1.729(1)
1.726(1)
1.726(1)
1.725(1)
P1−N1 (Å)
P2−N1 (Å)
P3−N2 (Å)
P4−N2 (Å)
P1−Pt1−P2 (deg)
P3−Pt2−P4 (deg)
1.71(1)
1.71(1)
1.70(1)
1.72(1)
73.0(1)
73.0(1)
P1−N1 (Å)
P2−N1 (Å)
−
−
P1−Pt1−P2 (deg)
−
1.708(6)
1.704(6)
73.02(8)
−
located and modeled, their occupancies allowed to freely refine
in the crystal structure of 3; however, this did not account for
all the residual electron density and required the use of a
solvent mask, applied with Olex2. The electron density was not
localized around the desired molecule, but rather in the void
space between, suggesting disordered solvent. Again SIMU and
DELU restraints were applied when necessary and reasonable.
Two on the phenyl ring of the phosphazane ligands were
disordered; however, the disorder was modeled and SADI
restrains were used to maintain P−C bond homogeneity.
We are the first to report the solid state crystal structures of 2
(CCDC number: 1565704), 3 (CCDC number: 1565706), and
4 (CCDC number: 1565705). Each model refined well against
the empirical data. The model of 4 shows a considerable
amount of residual electron density near Pt1, this is not
unexpected given the size and amount of electron density
associated with a platinum.
Figure 2 illustrates the formula units of each compound. The
thermal ellipsoids of 2 are considerably larger than those of 3
and 4, despite the same probability level, a result of the room
temperature data collection; however, this does not detract
from the structure. Solvent molecules of crystallization are
found in both 3 and 4, 1.44 CH2Cl2 and 8 1,1,2,2-
tetrachloroethane units, respectively. A noninteger number of
CH2Cl2 was found in the case of complex 3 as a result of
permitting the occupancy to freely refine.
Bond lengths are all within expected tolerances. Bonds of
special interest are the P−N bonds along with the bite angle
(defined as the PMP angle). Bond lengths and angles of
interest have been compiled in Table 1. The free ligand
possessed a P−N bond length of ∼1.73 Å in contrast to the
platinum(II) complex, in which it was ∼1.71 Å. Moreover, the
bite angles of 3 and 4 were shown to be ∼73.0°. To ensure that
the experimentally determined bond lengths and bite angles
make chemical sense, a survey of the CCDC was performed
using ConQuest (version 1.19) and the results analyzed using
Mercury (version 3.9).73,74 A search for complexes containing
the P−N−P motif chelating to any transition metal identified
284 structures. Of these matches, the average P−N bond length
was found to be 1.70(2) Å and the average bite angle was
70(3)°.
Scheme 4. Resonance Structures Which Aid to Explain the
Observed Planarity and P−N Bond Contraction
resonance localizes additional negative charge on the
phosphorus atoms, further shielding the nuclei and causing
an upfield shift with respect to the free ligand.
The poor solubility of 5 prevented the growth of high quality
crystals suitable for single crystal X-ray diffraction. Although a
single crystal was obtained after refluxing 5 in DMF for 3 h in
air and allowing the resulting solution to stand for several
weeks, it was revealed to be hydrolysis decomposition products
Pd(PPh2OH)2Cl2 and Pd[(PPh2)2N(C6H4NH2)]Cl2, which
cocrystallized with nonmerohedral twinning. No hydrolysis
had been previously observed for these complexes and thus it
appears only to occur at elevated temperatures, such as those
reached in refluxing DMF. Examination of the ESI-MS of 5 that
was not subjected to refluxing DMF conditions did not show
any evidence of the two hydrolysis products, and thus, the bulk
material was stable.
In order to gain insight into the atomic arrangement and
molecular packing of 5 in the solid state, powder X-ray
diffraction (PXRD) data was acquired (see Figure 3). A
comparison of the powder patterns of 4 and 5 revealed a
striking similarity in peak position, suggesting that the two
materials have similar symmetries; however, the intensities of
each peak varied due to the compositional differences (i.e., Pd
vs Pt). As with the similarities in IR, these results support that 5
is isostructural to 4. Using the FOX software package (Version
An examination of the bond lengths in Table 1 reveals an
interesting result of complexation: the P−N bond lengths
decrease when complexed with platinum. Additionally, the four-
membered Pt−P−N−P ring is almost perfectly planar in both 3
and 4. This observation in conjunction with the P−N bond
length contraction is evidence that the nitrogen atom lone pair
is delocalized, leading to the formation of a conjugated ring
system (see Scheme 4). Further support is offered by the work
of Eady et al., who also observed a near perfectly planar iron-
phosphazane ring structure.75 The 31P{1H} NMR also support
the delocalization as both 3 and 4 showed upfield shifts in their
phosphorus resonances relative to the free ligand. The
Figure 3. PXRD pattern for complexes 5 (top) and 4 (bottom).
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Table 2. Substrate Scope for Catalytic Activity Screening of 5
a
b
d
Catalyst loading of 1 mol % used. Catalyst loading of 0.6 mol % used. Turnover number.
1.9.7-#1313), a Le Bail fit of the pattern of 5 was performed
using the space group (P21/n) and starting unit cell dimensions
of 4 determined from its SC-XRD parameters (Figure S19).
After allowing a free fit to these parameters, the pattern fit quite
well with a final Rwp of 14.6% and a goodness of fit (GooF) of
2.293. The unit cell converged to 15.6234 × 19.8425 × 17.0690
Å3 and β of 114.911° (calculated volume 4799.28 Å3). This is
consistent with Z = 4 (reasonable for P21/n) based on the
approximation that all nonhydrogen atoms occupy ∼18 Å3,
further supporting that the Le Bail fit was able to fit an
appropriate unit cell and that the desired material was
synthesized.
2.3. Suzuki-Miyaura Coupling Screening. Complex 5
was hypothesized to catalyze Suzuki-Miyuara cross-coupling
reactions given the propensity for palladium-based complexes
to effect these transformations.76 We found that 5 is able to
efficiently catalyze the aforementioned cross-coupling reaction
with yields spanning 22% up to 96% under mild, aerobic
conditions in 1:1 DMF/H2O. The catalyst did not require the
addition of a supporting ligand such triphenylphosphine, often
used in cross-coupling reactions to stabilize Pd(0) species.
Eliminating the need for supporting ligands improves atom
economy and reduces the amount of waste produced. Table 2
illustrates the substrate scope of the reaction.
The performance of 5 exceeds that of other bidentate
bis(diphenylphosphino)methane-type catalyst based on reac-
tion time, the use of milder conditions, and high yields.77
Furthermore, the reported catalyst shows enhanced tolerance
toward aerobic conditions when compared to other phospha-
zane-based catalyst used for cross-coupling catalysis which have,
to date, only been reported under inert conditions.63
All product mixtures, regardless of the aryl halide used, also
contained biphenyl and triphenylboroxine. The biphenyl can be
attributed to the homocoupling of the phenylboronic acid. The
homocoupling of boronic acids has been studied in great detail
and it has been found that the reaction, under an O2 containing
atmosphere, proceeds through an η2-O2 peroxo complex, as
reported by Adamo et al.78 As for the triphenylboroxine, it is a
product of the dehydration of phenylboronic acid. The
presence of both compounds is not unexpected given that 1.5
equiv of phenylboronic acid with respect to aryl halide was
present in solution and thus there would be half an equivalent
remaining after the heterocoupling.
3. EXPERIMENTAL SECTION
3.1. General Information. Unless otherwise noted, all manipu-
lations were carried out under an inert dinitrogen atmosphere either in
a Vigor glovebox or using standard Schlenk techniques. All reagents
were used as received unless otherwise indicated. Solvents were
purified using a Grubbs style solvent system purchased from Pure
Process Technology. 1H NMR spectra and were acquired on a Varian
300 or 400 MHz spectrometer; 31P{1H} spectra were acquired on a
300 MHz NMR operating at 121 MHz. Chemical shift (δ) are
reported in parts per million (ppm) and referenced against the residual
A range of halides with varying steric bulk and electronic
properties were employed to study the generality of the catalyst
as well as its tolerance to various functionalities. Excellent yields
(96%) with short reaction times (15 min) were achieved when
coupling phenylboronic acid with bromobenzene. Colorless
crystals of biphenyl were observed almost immediately and the
reaction was quenched after 15 min by extracting the reaction
mixture with diethyl ether. Conversion was confirmed by both
1H NMR and GC-MS for all samples and yields were
1
protio-solvent peak in H experiments and 85% H3PO4 in 31P{1H}
experiments. For mass spectrometry samples were infused directly
using an ESI (positive mode) linear ion trap (LTQ)-Orbitrap XL mass
spectrometer (Thermo Fisher Scientific). For gas chromatography−
mass spectrometry analysis an HP 5890 Series II GC in tandem with
an HP 5972 Series Mass Selective Detector was used. FTIR spectra
were acquired on a PerkinElmer 1760 FTIR spectrometer. Powder X-
ray diffraction data was acquired on a Rigaku Ultima IV diffractometer
1
determined via quantitative H NMR. A standard of 1,1,2,2-
tetrachloroethane was used, with a chemical shift of 6.00, it did
not interfere nor overlap with any of the peaks of any of the
substrates or products studied.
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equipped with a monochromated copper X-ray source (Cu Kα 1.5406
Å).
3 was 89% (0.14942 g, 0.10853 mmol). 1H NMR (1,1,2,2-
tetrachloroethane-d2, 300 MHz) δ 7.63−7.60 (8H, m, Ph), 7.54−
7.49 (16H, m, Ph), 7.35−7.31 (16H, m, Ph), 6.86 (1H, t, J = 8.0 Hz),
6.30 (2H, d, J = 8.0 Hz), 6.22 (s, 1H). 31P{1H} (CH2Cl2, 121 MHz) δ
23.09 (JPt−P = 3300 Hz). MS (ESI): m/z 1382.13 (calcd for
C54H44Cl4N2P4Pt2Li [M + Li+]+, 1382.06). FT-IR (neat, cm−1)
3046, 1579, 1475, 1432, 1247, 1179, 1149, 1091, 1025, 982, 930, 854,
771, 742, 691, 671, 515, 500, 492, 480, 463, 442, 424.
3.6. Synthesis of (PtCl2)2(p-tdpb) 4. Complex 4 was synthesized
in a manner analogous to that of 3 (Pt(COD)Cl2, 104.27 mg, 0.27868
mmol; 2, 104.43 mg, 0.12361 mmol), the deviations being the use of 2
in place of 1 as the phosphazane ligand and 4 crashed out of solution
during the overnight stirring. In this case, the white solid was collected
via vacuum filtration, washed with hexanes, and dried in vacuo. Yield of
4 was 86% (0.14722 g, 0.10693 mmol). 1H NMR (1,1,2,2-
tetrachloroethane-d2, 300 MHz) δ 7.75−7.42 (40H, m, Ph), 6.21
(4H, s). 31P{1H} (1,1,2,2-tetrachloroethane-d2, 121 MHz) δ 21.33. MS
(ESI): m/z 1382.13 (calcd for C54H44Cl4N2P4Pt2Li [M + Li+]+,
1382.06). FT-IR (neat, cm−1) 1586, 1501, 1480, 1435, 1312, 1247,
1184, 1139, 1107, 998, 958, 887, 818, 748, 723, 700, 687, 617, 557,
556, 539, 519, 498, 489, 460, 440.
3.2. X-ray Crystallography. Single-crystal X-ray diffraction of 2
and 3 was performed on a Bruker D8 Venture diffractometer equipped
with a fixed-chi goniometer, an Oxford cryostat, a molybdenum X-ray
source (Mo Kα 0.7107 Å) with a graphite monochromator, and a
Photon-100 area detector. The data set for 4 was collected at Cornell
University by Dr. Samantha MacMillan using a Bruker X8
diffractometer equipped with an APEX-II CCD detector, a
molybdenum X-ray tube (Mo Kα 0.7107 Å), and Oxford cryostat.
Crystals of 2 and 3 were mounted on MiTeGen MicroLoops with a
drop of N-Paratone oil. Data for 2 was collected at room temperature,
while 3 was held at 100 K and 4 at 223 K for the entirety of data
collection. A series of φ and ω scans were used to cover reciprocal
space. For a complete outline of all the crystallographic metrics see
Table S1.
3.3. Synthesis of N1,N1,N4,N4-tetrakis(diphenylphosphino)-
benzene-1,3-diamine (m-tdpb) 1. Compound 1 was synthesized
using a slightly modified protocol found in the literature.79 A three-
neck flask was equipped with a gas inlet adapter, a pressure equalized
addition funnel, a Teflon-coated magnetic stir bar, and a rubber
septum. m-Phenylenediamine (2.930 g, 27.09 mmol) was added to the
pressure equalized addition funnel, sealed with a rubber septum, and
the apparatus put under an inert atmosphere. Via cannula, 200 mL of
THF was added to the addition funnel and 150 mL to the three-neck
flask. Freshly distilled chlorodiphenylphosphine (21.0 mL, 25.8 g, 116
mmol) and triethylamine (17.0 mL, 12.3 g, 122 mmol) were added to
the three-neck flask by cannula and stirred to form a colorless, clear
solution (note: if triethylamine has not been thoroughly dried and
degassed a white precipitate will form immediately). The three-neck
flask and its contents were cooled to 0 °C, the m-phenylenediamine
solution was then slowly added dropwise. After completion of
addition, the reaction mixture was a thick white suspension
(NEt3HCl), the reaction mixture was warmed to room temperature,
and the mixture was stirred for an additional 2 h. In air the white
suspension was filtered through a frit and the filtrate was added to 600
mL of acetonitrile and stirred for 45 min, followed by standing for 15
min. The white crystalline solid was collected by filtration. The filtrate
solvent removed by rotary evaporation, and the residue redissolved in
100 mL of THF and added to 300 mL acetonitrile for afford a second
crop of crystals. Yield of 1 was 90.4% (20.699 g, 24.500 mmol) as a
3.7. Synthesis of (PdCl2)2(p-tdpb) 5. Compound 5 was
synthesized in a manner analogous to that of 4, the only deviation
being that Pd(COD)Cl2 was used in place of Pt(COD)Cl2
(Pd(COD)Cl2, 100.01 mg, 0.35029 mmol; 2, 140.81 mg, 0.16667
mmol). The yield of 5 was 82% (0.16410 g, 0.13681 mmol). 1H NMR
(DMSO-d6, 300 MHz) δ 7.90−7.27 (m, Ph), 6.39 (4H, s). MS (ESI):
m/z 1204.01 (calcd for C56H47Cl3N3P4Pd2 [M + ACN-Cl−]+,
1203.99). FT-IR (neat, cm−1) 1504, 1479, 1435, 1312, 1233, 1140,
1103, 999, 954, 894, 820, 746, 722, 686, 565, 548, 511, 495, 482, 433.
3.8. General Procedure for Suzuki-Coupling Reaction. A
scintillation vial, in air, was charge with 0.50 mmol arylhalide, 0.75
mmol arylboronic acid, 0.0025 mmol (unless otherwise noted, see
Table 2) 5, 1.0 mmol K2CO3, 2.0 mL of distilled water, and 2.0 mL of
DMF. The reaction mixture was then stirred at room temperature for
the indicated amount of time. The reaction mixture was then added to
15 mL of brine and extracted with diethyl ether (3 × 15 mL). The
organic layer was dried over magnesium sulfate, filtered, and the
volatiles were removed by rotary evaporation. The residue was then
1
dissolved in CDCl3 and subjected to H NMR and GC-MS analysis.
Yields were determined by NMR using an internal standard (1,1,2,2-
tetrachloroethane).
77
1
white crystalline solid. The H and 31P{1H} match literature values.
1H NMR (CDCl3, 300 MHz): δ 7.31−7.15 (43H (overlaps with
residual CHCl3), m, Ph), 6.66 (1H, s), 6.49 (1H, t, J = 6.0 Hz), 6.26
(2H, d, J = 6.0 Hz). 31P{1H} NMR (CDCl3, 121 MHz) δ 67.49. FT-IR
(neat, cm−1) 3046, 1579, 1475, 1431, 1247, 1179, 1149, 1091, 1024,
981, 930, 854, 771, 742, 691, 670, 515, 500, 492, 480, 4623, 442, 424.
3.4. Synthesis of N1,N1,N4,N4-tetrakis(diphenylphosphino)-
benzene-1,4-diamine (p-tdpb) 2. Synthesis and isolation of 2
follows the same procedure as 1. p-Phenylenediamine (4.400 g, 40.688
mmol), triethylamine (25.0 mL, 18.1 g, 179 mmol), chlorodiphenyl-
phosphine (34.0 mL, 41.8 g, 189 mmol), and 400 mL of THF total. X-
ray quality crystals were grown by layering pentanes onto a saturation
solution of 2 in CH2Cl2. Yield of 2 was 79% (27.096 g, 32.072 mmol).
1H NMR (CDCl3, 300 MHz) δ 7.35−7.10 (45H (overlaps with
4. CONCLUSIONS
The modified synthesis of two phosphazane ligands has been
developed wherein purification consists of a single crystal-
lization followed by direct isolation. Two isomeric ligands were
used to form Pd2 and Pt2 dinuclear complexes via their
straightforward combination with M(COD)Cl2 (M = Pt(II) or
Pd(II)). The ligands and resulting complexes were fully
characterized by a suite of techniques, including FTIR and
1
NMR (both H and 31P{1H}, for all except 5, which was too
insoluble to 31P{1H}), ESI-mass spectrometry, SC-XRD (2, 3,
and 4), and PXRD (4 and 5). Although the Pd2 complex was
insoluble to an extent that hindered some characterization
methods, comparisons of the IR and powder X-ray diffraction
patterns of the Pt2 and Pd2 complexes strongly supports the
isostructural relationship between the two species. The activity
of 5 toward Suzuki-Miyaura couplings was investigated. Yields
ranged from 22% to 96%, depending on the substrates used.
Regardless of substrate, all coupling reactions proceeded at
room temperature, in air, in a water/DMF solvent mixture,
without the addition any supporting ligands. This work
contributes to the fields of cross-coupling catalysis as well as
the growing field of phosphazane ligands and their applications.
Furthermore, the modularity of the phosphazane platform
residual CHCl3), m, Ph), 6.00 (4H, s). 31P{1H} (CDCl3, 121 MHz) δ
+
70.23. MS (ESI): m/z 845.27 (calcd for C54H45N2P4 [M + H]+,
845.25). FT-IR (neat, cm−1) 3042, 1585, 1516, 1493, 1478, 1430,
1310, 1203, 1184, 1091, 1069, 1026, 1016, 996, 923, 901, 793, 751,
739, 694, 617, 564, 519, 500, 477, 461, 425.
3.5. Synthesis of (PtCl2)2(m-tdpb) 3. Pt(COD)Cl2 (102.46 mg,
0.27384 mmol) and 1 (102.50 mg, 0.12132 mmol) were weighed in
air, and the solids moved to dinitrogen atmosphere glovebox. Each was
dissolved in 2 mL of CH2Cl2 and the solution of 1 was added dropwise
to the solution of Pt(COD)Cl2. The faint yellow solution was then
stirred overnight at room temperature. Overnight, the reaction mixture
became colorless, it was then removed from the glovebox and allowed
to slowly evaporate which afforded large, clear, colorless block-like
crystals suitable for X-ray diffraction studies. The crystals were then
ground to a powder, washed with hexanes, and dried in vacuo. Yield of
F
DOI: 10.1021/acs.inorgchem.8b00787
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(8) Cook, T. R.; Surendranath, Y.; Nocera, D. G. Chlorine
Photoelimination from a Diplatinum Core: Circumventing the Back
Reaction. J. Am. Chem. Soc. 2009, 131, 28−29.
opens the door to a rich field of chemistry and allows for
enhanced tunability for targeted properties.
(9) Langrick, C. R.; McEwan, D. M.; Pringle, P. G.; Shaw, B. L.
Bimetallic Systems. Part 2. Synthesis and Interconversion of
Monodentate- and Bridging-Bis(diphenylphosphino)methane Plati-
num, Diplatinum, and Mercury-Platinum Acetylides. J. Chem. Soc.,
Dalton Trans. 1983, 2487−2493.
(10) Pringle, P. G.; Shaw, B. L. Bimetallic systems. Part 1. Synthesis
and Reactions of the Heterobimetallic Complex [ClPd([small micro]-
Ph2PCH2PPh2)2PtCl]. J. Chem. Soc., Dalton Trans. 1983, 889−896.
(11) Blagg, A.; Hutton, A. T.; Pringle, P. G.; Shaw, B. L. Bimetallic
Systems. Part 6. Chromium(0)-, Molybdenum(0)-, or Tungsten(0)-
Platinum(II) Acetylide Complexes Containing Bridging
Ph2PCH2PPh2, Including Their Efficient Formation from Platinum-
Silver Complexes by Transmetallation. Crystal Structure of [(p-
MeC6H4C≡C)-Pt(μ-C≡CC6H4Me-p)(μ-Ph2PCH2PPh2)2W(CO)3]. J.
Chem. Soc., Dalton Trans. 1984, 1815−1822.
(12) Cooper, G. R.; Hutton, A. T.; Langrick, C. R.; McEwan, D. M.;
Pringle, P. G.; Shaw, B. L. Bimetallic Systems. Part 4. Synthesis and
Characterisation of Mixed Copper(I)-, Silver(I)-, or Gold(I)-Platinum-
(II) Acetylide Complexes Containing Bridging Ph2PCH2PPh2. J.
Chem. Soc., Dalton Trans. 1984, 855−862.
(13) Langrick, C. R.; Pringle, P. G.; Shaw, B. L. Bimetallic Systems.
Part 5. Isonitrile-Platinum(II)- or -Palladium(II)-Bis-
(diphenylphosphino)methane Complexes Including Heterobimetallics
with Silver(I), Gold(I), or Rhodium(I). J. Chem. Soc., Dalton Trans.
1984, 1233−1238.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00787.
Crystallographic details, NMR, FT-IR, and MS (PDF).
Accession Codes
CCDC 1565704−1565706 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: trcook@buffalo.edu.
ORCID
Matthew R. Crawley: 0000-0002-2555-9543
Alan E. Friedman: 0000-0002-4764-8168
Timothy R. Cook: 0000-0002-7668-8089
(14) Pringle, P. G.; Shaw, B. L. Bimetallic Systems. Part 3. Complexes
Containing the Ligand Bis(diphenylphosphino)methane Bridging
from the Platinacycle PtCH2CH2CH2CH2 to Another Metal. J.
Chem. Soc., Dalton Trans. 1984, 849−853.
(15) Carr, S. W.; Shaw, B. L.; Thornton-Pett, M. Bimetallic Systems.
Part 13. Platinum-Manganese Carbonyl Complexes Containing
Bridging Ph2PCH2PPh2(dppm) Ligands: Crystal Structure of
[(OC)3Mn(μ-dppm)2PtH(Br)]BF4. J. Chem. Soc., Dalton Trans.
1985, 2131−2137.
Author Contributions
M.R.C. designed and carried out the work and prepared the
manuscript. A.E.F. collected ESI-MS characterization data.
T.R.C. designed the work and prepared the manuscript. All
authors have given approval to the final version of the
manuscript.
Notes
(16) Hassan, F. S. M.; Markham, D. P.; Pringle, P. G.; Shaw, B. L.
Bimetallic Systems. Part 7. Platinum and Palladium Dicyanides
Containing Terminal or Bridging Ph2PCH2PPh2 and Heterobimetal-
lics with Silver, Gold, Mercury, Rhodium, Iridium, or Molybdenum. J.
Chem. Soc., Dalton Trans. 1985, 279−283.
(17) Hutton, A. T.; Langrick, C. R.; McEwan, D. M.; Pringle, P. G.;
Shaw, B. L. Bimetallic Systems. Part 12. Mixed Rhodium(I)-
Platinum(II) Acetylide Complexes Containing Bridging
Ph2PCH2PPh2. Crystal Structures of [(MeC≡C)Pt(μ-dppm)2(σ,η-
C≡CMe)Rh(CO)]PF6 and of [ClPt(μ-dppm)2(σ,η-C≡CMe)Rh-
(CO)]PF6. J. Chem. Soc., Dalton Trans. 1985, 2121−2130.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank SUNY Fredonia for use of their
diffractometer. M.R.C. thanks the ACA Summer Course on
Chemical Crystallography, especially Dr. Robert Papoular for
helpful conversation regarding powder X-ray diffraction. We
thank Cressa R. P. Fulong for acquiring the PXRD data and Dr.
Sam MacMillan for collecting the data set for 4. T.R.C. thanks
the University at Buffalo and State University of New York
Research Foundation for startup support funding.
(18) Hutton, A. T.; Pringle, P. G.; Shaw, B. L. Bimetallic Systems.
Part 11. Heterobimetallic and Unsymmetrical Diplatinum Complexes
from cis-[PtR2(dppm-P)2](dppm Å Ph2PCH2PPh2; R = Me, 1-
naphthyl, or C6H4Me-o): Crystal Structure of [(C6H4Me-o)2Pt(μ-
dppm)2PtMe2]. J. Chem. Soc., Dalton Trans. 1985, 1677−1682.
(19) Iggo, J. A.; Shaw, B. L. Bimetallic Systems. Part 9. The Synthesis
of and Nuclear Magnetic Resonance Studies on 10-Membered Ring
Complexes of Type [(OC)4M1(μ-Ph2PCH2CH2PPh2)2M2(CO)4](M1,
M2= Cr, Mo, or W). J. Chem. Soc., Dalton Trans. 1985, 1009−1013.
(20) Langrick, C. R.; Pringle, P. G.; Shaw, B. L. Bimetallic Systems.
Part 10. Synthesis of Complexes of Type [(RC≡C)Pt(μ-dppm)2Pt-
(C≡CR)](dppm = Ph2PCH2PPh2, R = Ph or p-tolyl) and their
Corresponding ’A frames’ [(RC≡C)Pt(μ-dppm)2(μ-H)Pt(C≡CR)]Cl
or [(RC≡C)Pt(μ-dppm)2(μ-X)Pt(C≡CR)] with X = CS2 or
MeOOCC≡CCOOMe. J. Chem. Soc., Dalton Trans. 1985, 1015−1017.
(21) Langrick, C. R.; Shaw, B. L. Bimetallic Systems. Part 8.
Heterobimetallic Di-isonitrile or Isonitrile-Carbonyl Complexes of
Rhodium or Iridium bridged to Silver or Gold by Ph2PCH2PPh2. J.
Chem. Soc., Dalton Trans. 1985, 511−516.
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