Facile formation of stable tris(imido)phosphate trianions as their tri- and hexanuclear Pd(II) complexes in protic solvents

Article abstract of DOI:10.1021/ic400686e

Employing Pd(OAc)₂, a facile deprotonation route to access the highly basic tris(alkylimido)phosphate trianions, [(RN)₃PO] ^3- (R = ^tBu, ^cHex, or ⁱPr), analogous to the orthophosphate (PO₄^3-) ion in polar and in protic solvents has been achieved. Trinuclear and prismatic Pd(II) clusters of these imido trianions having the formula {Pd₃[(NR) ₃PO](OAc)₃}_n (n = 1 or 2) or as mixed-bridged clusters of the type {Pd₃[(NⁱPr)₃PO](OAc) ₂(OR′)}₂ (R′ = Me or H) were isolated exclusively in all these reactions in which the trianionic species acts as a tripodal chelating ligand to the trinuclear Pd₃ unit. Reactivity studies aiming at the Pd(II) atoms in these clusters with nucleophilic reagents, such as primary amines (R″NH₂), have led to a new trimeric cluster with the formula {Pd₃[(NR)₃PO](OAc) ₃(R″NH₂)₃} in which the tripodal coordination of the Pd-N_imido moieties remained unaffected, exemplifying the robustness of the Pd₃ unit in all these clusters. We have also shown the catalytic activity of these Pd(II) complexes in Mizoroki-Heck type coupling reactions in the presence of Cu(OAc)₂.

Full text of DOI:10.1021/ic400686e

Article
pubs.acs.org/IC
Facile Formation of Stable Tris(imido)phosphate Trianions as Their
Tri- and Hexanuclear Pd(II) Complexes in Protic Solvents
Arvind K. Gupta, S. Arun Dixith Reddy, and Ramamoorthy Boomishankar*
Department of Chemistry, Mendeleev Block, Indian Institute of Science Education and Research (IISER), Pune, Dr. Homi Bhabha
Road, Pune 411008, India
S
* Supporting Information
ABSTRACT: Employing Pd(OAc)2, a facile deprotonation route to access the
highly basic tris(alkylimido)phosphate trianions, [(RN)3PO]3− (R = tBu, cHex, or
iPr), analogous to the orthophosphate (PO43−) ion in polar and in protic solvents
has been achieved. Trinuclear and prismatic Pd(II) clusters of these imido
trianions having the formula {Pd3[(NR)3PO](OAc)3}n (n = 1 or 2) or as mixed-
bridged clusters of the type {Pd3[(NiPr)3PO](OAc)2(OR′)}2 (R′ = Me or H)
were isolated exclusively in all these reactions in which the trianionic species acts
as a tripodal chelating ligand to the trinuclear Pd3 unit. Reactivity studies aiming
at the Pd(II) atoms in these clusters with nucleophilic reagents, such as primary
amines (R″NH2), have led to a new trimeric cluster with the formula
{Pd3[(NR)3PO](OAc)3(R″NH2)3} in which the tripodal coordination of the
Pd−Nimido moieties remained unaffected, exemplifying the robustness of the Pd3
unit in all these clusters. We have also shown the catalytic activity of these Pd(II)
complexes in Mizoroki−Heck type coupling reactions in the presence of Cu(OAc)2.
starting from [(NHR)3PO] and Pd(OAc)2 in polar solvents
such as CH3OH, CH3CN, (CH3)2SO, etc. These highly basic
anions were isolated as their corresponding tri- and hexanuclear
Pd(II) clusters of the type {Pd3[(NR)3PO](OAc)3}n (5, R =
INTRODUCTION
■
Imido analogues of common phosphorus oxo anions have been
a topic of immense research interest in main group chemistry
and are employed as ligands in coordination chemistry.1 Metal
complexes of several P(V)-imido anions were utilized as
catalytic systems for olefin oligomerization and polymerization,
ring-opening polymerization of lactides, hydroamination, trans-
fer hydrogenation, etc.2 Current synthetic procedures to access
these imido-P(V) anions involve the use of highly reactive
metal reagents in reaction with a phosphonium salt3 like
[(NHPh)4P]Cl, phosphoramides4 such as [(RNH)3PE] (E =
NSiMe3, O, S, or Se), or preformed iminophosphoranes.5
However, because of the highly reactive nature of the metal
precursors employed in the reactions as well as the presence of
residual metal−alkyl/aryl/halide/silylamide bonds in these
complexes, their stabilities have largely been limited to
anhydrous aprotic and nonpolar solvents. It has also been
noticed that use of Lewis acidic transition metal ions such as
Cu2+ ions in these deprotonation reactions has led to the
cleavage of P−N bonds instead of the N-H deprotonation.6
Hence, we started looking at using salts of soft transition metal
ions as a source of a base to generate the P(V)-bound
polyimido species in a polar medium, which would allow us to
explore the potential of these species for broader synthetic
applications. Spurred by the remarkable utility of palladium and
its compounds in various organic bond-activation reactions,7 we
employed the salts of Pd(II) ions in deprotonation reactions
with tris(alkylamido)phosphate ligands, [(NHR)3PO]. Herein,
we report on the facile syntheses of the fully deprotonated
tris(imido)phosphate trianions of the type [(RN)3PO]3−
i
tBu, n = 1; 6, R = CHex, n = 2; 7, R = Pr, n = 2) or as mixed-
bridged clusters of the type {Pd3[(NiPr)3PO](OAc)2(OR′)}2
(7a, R′ = Me; 7b, R′ = H) depending upon the R and R′
groups and the reaction solvents (Scheme 1). All these
complexes contain one or two triangular Pd3 motifs in which
the Pd(II) atoms are bound to three chelating Nimido moieties.
Further reactions at the Pd(II) atoms in the hexameric Pd6
clusters with nucleophilic reagents such as primary amines
(R″NH2) lead to a trimeric species with the formula
t
{Pd3[(NR)3PO](OAc)3(R″NH2)3} (8, R = Bu, R″ = CHex;
9, R = CHex, R″ = CHex; 10, R = iPr, R″ = CHex) obtained via
the symmetrical cleavage of the hexamer. This suggests that the
three Pd−Nimido moieties attached to the Pd3 subunit exhibit a
remarkable stability and remain unaffected during the cleavage
reaction. To the best of our knowledge, the Pd(II) complexes
reported here are the first examples of metal complexes isolated
in protic solvents that feature the highly basic imido-phosphate
trianions [(RN)3PO]3− as ligands. Although the imido anions
analogous to H2PO4− and HPO42− ions can be generated using
certain reactive Ag(I) salts from the P(V) moieties having fairly
acidic N-H groups, the imido trianions corresponding to the
orthophosphate (PO43−) ion remain elusive in all these Ag(I)-
mediated reactions.8 In addition, we have explored the
Received: March 20, 2013
© XXXX American Chemical Society
A
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Article
3498, 3197, 1638, 1464, 1427, 1384, 1168, 1121, 933, 897, 794, 542.
ESI(+): 222.17 (M)+, 199.08 (M − (C3H7NH))+. Anal. Calcd for
C9H24N3OP: C, 48.85; H, 10.93; N, 18.99. Found: C, 49.14; H, 10.76;
N, 18.74.
Scheme 1. Synthetic Routes To Access the Tri- and
Hexanuclear Pd(II) Clusters of the Tris(alkylimido)
Phosphate Trianions
5. To a solution of [OP(NHtBu)3], 1-H3 (20 mg, 0.08 mmol), in
methanol was added palladium acetate Pd(OAc)2 (51 mg, 0.24 mmol)
in acetonitrile. The resulting mixture was stirred for 1 h and kept for
crystallization. Orange blocklike crystals were obtained after 5 days.
1
Yield: 90% (52 mg, based on P). mp: 205−208 °C. H NMR (400
MHz, {(CD3)2SO}): δ 1.20 (s, 27H, CH3), 2.04 (s, 9H, CH3COO).
13C NMR (100 MHz, {(CD3)2SO}): δ 27.63, 32.20, 52.88, 179.19. 31
P
NMR (161 MHz, {(CD3)2SO}): δ 69.04. FT-IR data in KBr pellet
(cm−1): 3454, 1728, 1561, 1413, 1275, 1175, 1041, 976, 752, 634.
ESI(+): 778 (M + Na)+, 698 (M − C4H9)+. Anal. Calcd for
C18H36N3O7PPd3: C, 28.57; H, 4.80; N, 5.55. Found: C, 28.60; H,
4.60; N, 5.38.
6.H2O. To a solution of [OP(NHcHex)3], 2-H3 (20 mg, 0.06
mmol), in methanol was added palladium acetate Pd(OAc)2 (40 mg,
0.18 mmol) in methanol. The resulting mixture was stirred for 1 h and
kept for crystallization. Dark orange prismatic crystals were obtained
1
after 10 days. Yield: 70% (69 mg, based on P). mp: 180−182 °C. H
NMR (400 MHz, {(CD3)2SO}): δ 1.08 (m, 24H, CH2), 1.60 (m, 12H,
CH), 1.75 (m, 12H, CH2), 2.25 (s, 18H, CH3COO), 2.74 (m, 24H,
CH2). 13C NMR (100 MHz, {(CD3)2SO}): δ 113.23, 117.12, 139.15,
148.56, 154.24. 31P NMR (161 MHz, {(CD3)2SO}): δ 73.03. FT-IR
data in KBr pellet (cm−1): 3332, 1649, 1451, 1414, 1113, 1018, 647.
ESI(+): 1694 (M + Na)+, 836 (M/2)+. Anal. Calcd for
C48H84N6O14P2Pd6: C, 34.53; H, 5.07; N, 5.03. Found: C, 33.87; H,
5.04; N, 4.88.
preliminary catalytic activity of these Pd(II) complexes in the
Mizoroki−Heck (M−H) type coupling reaction of phenyl-
boronic acid with alkenes.
7.2H2O. A mixture of [OP(NHiPr)3], 3-H3 (20 mg, 0.09 mmol),
and palladium acetate Pd(OAc)2 (60 mg, 0.27 mmol) in THF was
stirred for 1 h and kept for crystallization. Platelike orange-colored
crystals were obtained after 10 days. Yield: 75% (98 mg, based on P).
mp: 198−200 °C. 1H NMR (400 MHz, {(CD2)4O}): δ 1.19 (d, 36H,
CH3), 2.68 (septet, 6H, CH), 2.47 (s, 18H, CH3COO). 13C NMR
(100 MHz, {(CD2)4O}): δ 22.53, 28.84, 52.20, 183.33. 31P NMR (161
MHz, {(CD2)4O}): δ 78.87. FT-IR data in KBr pellet (cm−1): 3338,
1611, 1552, 1425, 1261, 1140, 1019, 692. MALDI-TOF/TOF: 1423
(M)+, 1365 (M − C3H7NH)+. Anal. Calcd for C30H60N6O14P2Pd6: C,
25.21; H, 4.23; N, 5.88. Found: C, 25.35; H, 4.24; N, 5.77.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations involving phosphorus halides
were performed under dry nitrogen atmosphere in standard Schlenk
glassware. Solvents were dried over potassium (thf, hexane) or sodium
(toluene). The primary amines were purchased from Merck or from
Aldrich and used as received. POCl3 was purchased locally and was
distilled prior to use. The ligands 1-H3 and 2-H3 were synthesized
using literature methods.9 NMR spectra were recorded on a Jeol 400
MHz spectrometer (1H NMR 400.13 MHz, 13C{1H} NMR 100.62
MHz, 31P{1H} NMR 161.97 MHz) or on a Bruker 500 MHz
spectrometer (1H NMR 500.00 MHz, 13C{1H} NMR 125.725 MHz,
31P{1H} NMR 202.404 MHz) at room temperature using SiMe4 (1H,
13C) and 85% H3PO4 (31P). The two-dimensional diffusion-ordered
NMR spectroscopy (2D-DOSY) experiments were performed on a
Bruker 500 MHz NMR with standard pulse program ledbpgp2S using
bipolar gradient pulse pair and two spoiling gradients.10a The gradient
strength was changed from 2 to 95% with a linear type of ramp. Other
parameters, such as the diffusion time (Δ = 40−50 ms), sine-shaped
pulse length (δ = 1.1−2.1 ms), and relaxation delay (D1 = 4−8 s),
were employed as well.10b The electrospray ionization (ESI) mass
spectra in the positive ion mode were recorded in a Waters-SYNAPT-
G2 high-resolution spectrometer. The matrix-assisted laser desorption
ionization time-of-flight (MALDI-TOF) spectra were obtained on an
Applied Biosystems MALDI-TOF/TOF spectrometer. Fourier trans-
form infrared (FT-IR) spectra were taken on a Perkin-Elmer
spectrophotometer with samples prepared as KBr pellets. Melting
points were obtained using an Electro thermal melting point apparatus
and were uncorrected.
7a. A mixture of [OP(NHiPr)3], 3-H3 (20 mg, 0.09 mmol), and
palladium acetate Pd(OAc)2 (60 mg, 0.27 mmol) in methanol was
stirred for 1 h and kept for crystallization. Orange blocklike crystals
were obtained in 12 h. Yield: 85% (105 mg, based on P). mp: 180−
182 °C. 1H NMR (400 MHz, CD3OD): δ 1.12 (d, 36H, CH3), 2.49 (s,
12H, CH3COO), 2.69 (septet, 6H, CH), 3.12 (s, 6H, OCH3). 13C
NMR (100 MHz, CD3OD): δ 24.59, 25.29, 42.87, 53.74, 173.88. 31P
NMR (161 MHz, CD3OD): δ 73.60. FT-IR data in KBr pellet (cm−1):
3335, 1603, 1550, 1420, 1257, 1140, 1019, 722, 690. MALDI-TOF/
TOF: 1369 (M)+. Anal. Calcd for C28H60N6O12P2Pd6: C, 24.49; H,
4.40; N, 6.12. Found: C, 24.08; H, 4.15; N, 5.94.
7b.2DMSO. A mixture of [OP(NHiPr)3], 3-H3 (20 mg, 0.09 mmol),
and palladium acetate Pd(OAc)2 (60 mg, 0.27 mmol) in dimethyl
sulfoxide (DMSO) was stirred for 1 h and kept for crystallization.
Orange blocklike crystals were obtained after 5 days. Yield: 75% (101
mg, based on P). mp: 220−222 °C. 1H NMR (400 MHz,
{(CD3)2SO}): δ 1.10 (d, 36H, CH3), 2.68 (s, 12H, CH3COO), 2.74
(septet, 6H, CH), 2.84 (s, 2H, OH). 13C NMR (100 MHz,
{(CD3)2SO}): δ 25.67, 25.85, 50.03, 177.14. 31P NMR (161 MHz,
{(CD3)2SO}): δ 73.67. FT-IR data in KBr pellet (cm−1): 3677, 1646,
1532, 1426, 1255, 1143, 1025, 696. MALDI-TOF/TOF: 1423 (M +
DMSO)+, 671 (M/2)+. Anal. Calcd for C30H68N6O14P2S2Pd6: C,
24.00; H, 4.57; N, 5.60. Found: C, 24.83; H, 4.11; N, 5.88.
Synthesis. 3-H3. POCl3 (5 mL, 8.23 g, 0.0536 mol) was added
dropwise to an excess of isopropyl amine (23.12 mL, 16.69 g, 0.282
mol) in toluene (250 mL) at 0 °C. The resulting mixture was slowly
brought to room temperature and stirred for 4 h. The precipitated
isopropylammonium chloride was removed by filtration. The filtrate
was reduced to 80 mL, and hexane (50 mL) was added to it. The
mixture was kept at −15 °C for 2 days to yield a white impure solid.
The impure solid was purified by sublimation at 120 °C/1 × 10−3
8. To a solution of 5 (30 mg, 0.04 mmol) in methanol was added
cyclohexylamine (12.0 mg, 14 μL, 0.12 mmol). The resulting mixture
was stirred for 30 min and kept for crystallization. Pale yellow-colored
solid was obtained upon slow evaporation. Yield: 80% (72 mg, based
1
Torr. Yield: 75% (8.90 g, based on P). mp: 122−125 °C. H NMR
1
(400 MHz, CDCl3): δ 1.14 (s, 18H, CH3), 2.15 (br, 3H, NH), 3.35
(septet, 3H, CH). 13C NMR (100 MHz, CDCl3): δ 26.00, 43.25. 31P
NMR (161 MHz, CDCl3): δ 12.84. FT-IR data in KBr pellet (cm−1):
on P). mp: 143−145 °C. H NMR (400 MHz, CD3OD): δ 1.28 (m,
12H, CH2), 1.33 (s, 27H, CH3), 1.65 (m, 3H, CH), 1.88 (m, 6H,
CH2), 2.08 (br, 6H, NH2), 2.63 (s, 9H, CH3COO), 2.74 (m, 12H,
B
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CH2). 13C NMR (100 MHz, CD3OD): δ 25.08, 25.79, 27.66, 32.50,
38.02, 40.86, 52.86, 179.35. 31P NMR (161 MHz, CD3OD): δ 72.05.
FT-IR data in KBr pellet (cm−1): 3477, 1718, 1619, 1568, 1408, 1283,
1145, 1032, 968, 748, 625. MALDI-TOF/TOF: 1077 (M + Na)+.
Anal. Calcd for C36H75N6O7PPd3: C, 41.01; H, 7.17; N, 7.97. Found:
C, 41.03; H, 7.83; N, 7.22.
Å) for 5, 6.H2O, 7.2H2O, 7a, 7b.2DMSO, 9.2H2O, and 10 and on a
Bruker Smart Apex II diffractometer at 273 K for 3-H3 (Table S1,
Supporting Information). Structures were refined by full-matrix least-
squares against F2 using all data (SHELX97).11 All non-hydrogen
atoms were refined aniostropically if not stated otherwise. Hydrogen
atoms were constrained in geometric positions to their parent atoms.
Crystals of 7.2H2O and 7a were weakly diffracting at higher angles;
hence, a 2θ = 50° cutoff was applied. Two of the three cyclohexyl
groups in the asymmetric unit of 6.H2O were disordered. The
cyclohexyl moiety of the primary amino group in the asymmetric unit
of 9 was similarly disordered. Atom positions of the disordered C
atoms were isotropically refined over two positions using similar
distances and similar U-restraints. Five C atoms and one O atom in
7.2H2O had slightly bad ellipsoids and hence were refined with partial
isotropic parameters. The solvated DMSO molecule in the asymmetric
unit of 7b was disordered and is freely refined over two positions using
similar distances and similar U-restraints. The water molecules in
6.H2O and 7.2H2O were disordered as well and isotropically refined.
9.2H2O. Method A: To a solution of 6.H2O (30 mg, 0.017 mmol) in
methanol was added cyclohexylamine (11 mg, 13 μL, 0.10 mmol). The
resulting mixture was stirred for 30 min and kept for crystallization.
Platelike pale yellow-colored crystals were obtained after 7 days. Yield:
80% (32 mg, based on P). Method B: To a mixture of
[PO(NHcHex)3], 2-H3 (20 mg, 0.06 mmol), and palladium acetate
Pd(OAc)2 (40 mg, 0.18 mmol) in methanol was added cyclohexyl-
amine (17 mg, 20 μL, 0.18 mmol). The resulting mixture was stirred
for 30 min. Platelike pale yellow-colored crystals were obtained after
10 days. Yield: 80% (56 mg, based on P). mp: 198−200 °C. 1H NMR
(400 MHz, {(CD3)2SO}): δ 1.08 (m, 24H, CH), 1.53 (m, 6H, CH),
1.73 (m, 24H, CH2), 2.49 (s, 9H, CH3COO), 2.19 (m, 6H, NH2), 2.74
(m, 18H, CH2). 13C NMR (100 MHz, {(CD3)2SO}): δ 25.19, 25.66,
31.21, 34.49, 36.15, 40.94, 50.02, 177.18. 31P NMR (161 MHz,
{(CD3)2SO}): δ 73.50. FT-IR data in KBr pellet (cm−1): 3463, 1629,
1448, 1281, 1103, 894, 845, 712. ESI(+): 1132 (M)+, 955 (M −
3(OAc))+. Anal. Calcd for C42H81N6O7PPd3: C, 44.55; H, 7.21; N,
7.42. Found: C, 44.18; H, 7.97; N, 6.97.
RESULTS AND DISCUSSION
■
Synthesis and Spectra. The phosphoric triamide ligands,
t
i
c
[(NHR)3PO] (1-H3, R = Bu; 2-H3, R = Pr; 3-H3, R = Hex),
were prepared by a slightly modified literature procedure.4d,9
The reactivity of these ligands was further tested with
Pd(OAc)2 (4) in an appropriate polar solvent at room
temperature with the view of obtaining various imido-P(V)
derivatives. Crystals of {Pd3[(NtBu)3PO](OAc)3} (5) and
{Pd3[(NcHex)3PO](OAc)3}2 (6) were obtained from the
respective reaction of 1-H3 and 2-H3 with Pd(OAc)2 in
MeOH within 10 days. On the other hand, it has been observed
that reaction of 3-H3 under similar conditions in MeOH gave
{Pd3[(NiPr)3PO](OAc)2(OMe)}2 (7a) as a major compound.
When the solvent system is changed to (CH3)2SO, crystals of
{Pd3[(NiPr)3PO](OAc)2(OH)}2.2(CH3)2SO (7b.2DMSO)
were obtained. Formation of these mixed-bridged clusters 7a
and 7b was attributed to the dynamic nature of bridging OAc
groups in [Pd3(OAc)6] (4) where it exists as a mixture of 4 and
[Pd3(OMe)2(OAc)4] (4a) in MeOH, as observed by diffusion-
10. Method A: To a solution of 7 (30 mg, 0.02 mmol) in methanol
was added cyclohexylamine (12 mg, 14 μL, 0.12 mmol). The resulting
mixture was stirred for 30 min and kept for crystallization. Platelike
pale yellow-colored crystals were obtained after 5 days. Yield: 70% (28
mg, based on P). Note: 10 can also be obtained from a similar reaction
of 7a with cyclohexylamine. Method B: To a mixture of [OP(NHiPr)3],
3-H3 (20 mg, 0.09 mmol), and palladium acetate Pd(OAc)2 (60 mg,
0.27 mmol) in methanol was added cyclohexylamine (27 mg, 30 μL,
0.27 mmol), the mixture was stirred for 30 min and kept for
crystallization. Platelike pale yellow-colored crystals were obtained
1
after 5 days. Yield: 80% (73 mg, based on P). mp: 215−217 °C. H
NMR (400 MHz, CD3OD): δ 1.13 (d, 18H, CH3), 1.29 (septet, 3H,
CH), 1.51 (m, 3H, CH), 1.71 (m, 12H, CH2), 2.41 (m, 18H, CH2),
2.63 (s, 9H, CH3COO), 3.12 (br, 6H, NH2). 13C NMR (100 MHz,
CD3OD): δ 22.39, 25.81, 29.32, 33.84, 34.67, 35.16, 52.91, 179.15. 31
P
NMR (161 MHz, CD3OD): δ 73.67. FT-IR data in KBr pellet (cm−1):
3465, 1640, 1574, 1496, 1395, 1196, 1037, 849. ESI(+): 1035 (M +
Na)+, 856 (M − 3(OAc) + Na)+. Anal. Calcd for C33H69N6O7PPd3: C,
39.16; H, 6.87; N, 8.30. Found: C, 40.34; H, 7.00; N, 8.08.
12
1
ordered H NMR spectroscopy (1H-DOSY). Similarly, in a
solvent containing a large excess of water (such as DMSO), it
exists as a mixture of 4 and [Pd3(OH)2(OAc)4] (4b). Hence, it
is evident that formation of 7a and 7b is mostly due to the
reaction of 3-H3 with 4a and 4b, respectively. Finally, crystals of
the all acetate-bridged species {Pd3[(NiPr)3PO](OAc)3}2 (7)
were exclusively obtained from the corresponding reaction in
tetrahydrofuran. The 31P NMR spectra of the reaction mixture
of 3-H3 and Pd(OAc)2 (4) after 12 h gave three signals at δ
77.7 (minor), 72.5 (major), and 65.8 (minor) in the cluster
region, indicating that there are three species in solution having
the anionic species 3. The peaks at δ 77.7 and 72.5 indicate the
presence of two hexameric clusters, while the minor peak at δ
65.8 is due to a trimeric species, suggesting that the formation
of the hexameric cluster proceeds via a trimeric species similar
to 5 (Figure S1, Supporting Information). The 1H-DOSY
measurements of the reaction mixture of 3-H3 and 4 in CD3OD
after 15 h at the acetate region show the presence of at least
two major species in solution (Figure S2, Supporting
Information). The first one, at a diffusion rate of log D =
−8.98, shows the presence of an all acetate species (mostly 7),
and the second one, at log D = −9.33, corresponds to 7a
General Procedure for the M−H Type Coupling Reaction of
Phenylboronic Acid with Alkenes. In a typical reaction, a screw-
capped pressure tube (15 mL) was charged with phenylboronic acid
(1.2 mmol), 3 mol % of the Pd(II) catalyst (5/6/7a), and 2.0 equiv of
Cu(OAc)2.H2O. To this mixture was added the alkene precursor in
dimethylformamide solvent (7.5 mL), and the resulting mixture was
purged under a flow of N2 for 15 min and then refluxed at 100 °C for 4
h. The resultant dark brownish-green-colored mixture was filtered in a
thick pad of Celite, and the filtrate was then extracted with ethyl
acetate. The separated ethyl acetate layer was then dried and
evaporated to get a crude mixture of the product which was purified
by column chromatography using ethyl acetate/hexane as eluents. All
these reactions proceeded smoothly to afford the corresponding
phenylated products in reasonable yields (Table 1).
Crystallography. Reflections were collected on a Bruker Smart
Apex Duo diffractometer at 100 K using Mo Kα radiation (λ = 0.71073
Table 1. M−H Type Reaction of PhB(OH)2 with Alkenes
% yield of the product
starting material coupled product catalyst 5 catalyst 6 catalyst 7a
1
11a
11b
11c
11d
12a
12b
12c
12d
90
90
92
95
90
92
90
95
95
90
95
98
(similar to 4a which gives more than one OAc peak in H-
DOSY).12 On the other hand, the reaction mixture of 1-H3 and
4 and that of 2-H3 and 4 in methanol after 12 h show a single
peak in the 31P NMR spectra in the cluster region, indicating
C
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the formation of only 5 and 6 in their respective reactions. The
absence of the mixed-bridged species in these reactions might
be due to (a) the faster reactivity of 4 with the ligands 1-H3 and
2-H3 and (b) the stronger steric hindrance exhibited by the
1d). The crystal structure of 7.2H2O was obtained in the
monoclinic space group C2/c featuring the complete molecule
in its asymmetric unit (Figure S14, Supporting Information).
The molecular cores in 6.H2O and 7.2H2O consist of a
hexameric Pd6 cluster in a prismatic geometry sandwiched
between two tris(imido)phosphate ligands (2 or 3). The
prismatic Pd6 cluster can be viewed as a pair of planar Pd3
subunits (of the type found in 5 as well as in 4) that are stacked
upon each other in a nearly eclipsed manner. Each imido-
phosphate ligand acts as a hexadentate cisoidal ligand and is
bonded in a chelating fashion to the triangular Pd3 unit. These
individual Pd3 units are further connected by six carboxylate
ligands (Harris notation: 2.1.1)14 completing the hexameric
structure. A closer look at these structures reveals that these six
carboxylate ligands are interacting in a set of two and connect a
pair of Pd(II) atoms from the adjacent triangular units. Thus,
these prismatic Pd6 cluster assemblies contain three such Pd2
pairs, and each of them is held together by two bridging
carboxylate ligands.
c
t
bulky Hex and Bu substituents for smaller bridging (methoxy
or hydroxy) groups. The 31P NMR spectra of all the isolated
compounds gave a single peak in the range between δ 78.9 and
69.0 (Figure S3, Supporting Information). Further, the
structure of all these clusters in solution has been confirmed
by ESI(+) or MALDI-TOF mass spectra showing the isotopic
distribution of peaks centered at the values corresponding to
their [M+] or [M + Na]+ ions along with other prominent
fragment ion peaks (Figures S4−S12, Supporting Information).
Crystal Structures. The trinuclear species 5 crystallized in
the orthorhombic space group Cmc2(1) (Table S2, Supporting
Information). The molecular core consists of one fully
deprotonated ligand 1, three Pd(II) atoms, and three charge-
balancing acetate ions (Figure 1a,b). The imido ligand is
The mixed-bridged cluster 7a crystallized in the monoclinic
space group P2(1)/c featuring the whole molecule in the
asymmetric unit. The molecule 7b.2DMSO crystallized in the
monoclinic space group C2/c and contains one-half of the
molecule and one solvated DMSO molecule in the asymmetric
unit. The core structures of 7a and 7b resemble closely that of
7 and contain two μ2-bridging methoxy and hydroxy groups,
respectively, in place of two of the six acetate groups (Figure
2a,c). Two of the three Pd2 pairs present in the prismatic
Figure 1. Molecular structure (a and b) and the triangular view of the
Pd(II) atoms (c) in 5. Molecular structure (d) and the prismatic
assembly of the Pd(II) atoms (e) in 6.
hexadentate and is involved in bridging the three equilaterally
arranged Pd(II) atoms in a chelating fashion. Each Pd(II) atom
is located at a square planar coordination of two N-sites from 1
and two O-sites from the three bridging acetate units. It is
interesting to compare the structure of 5 with the solid-state
structure of the precursor Pd(OAc)2.13 In the solid state,
Pd(OAc)2 exists in the form of a trimer as Pd3(OAc)6 (4) with
a central triangular Pd3 unit sandwiched between three bridging
carboxylate units on either side of it. In contrast, the molecular
structure of 5 consists of three chelating N-sites of the
trianionic ligand 1 on one side of the Pd3 plane. The three
acetate units on the other side of it remained intact and help to
restore the charge balance in the molecule. Each imido nitrogen
is bonded to two Pd(II) atoms at almost equal distances
ranging from 2.052(3) to 2.061(3) Å, indicating the trianionic
charge is equally delocalized on the three N-sites. Similarly, the
Pd−O distances (av 2.053(3) Å) found in 5 are almost equal,
pointing to the delocalized nature of the carboxylate charges.
The molecules 6 and 7 are iso-structural and crystallized as
6.H2O and 7.2H2O, respectively. The molecular structure of
6.H2O was solved in the orthorhombic space group C222(1)
with one-half of the molecule in the asymmetric unit (Figure
Figure 2. Molecular structures of 7a (a) and 7b (c) showing their two
Pd3 subunits stacked upon each other in an eclipsed manner. Prismatic
view of the Pd(II) atoms in 7a (b) and 7b (d).
cluster of 7a and 7b possess the mixed-bridged environment
with one acetate moiety and one methoxy/hydroxy unit each,
while the third pair contains only acetate bridges (Figure S15,
Supporting Information). The two hydroxyl groups present in
7b are hydrogen-bonded to two solvated disordered molecules
of DMSO (Figure S16, Supporting Information). The Pd−N
and Pd−O distances found in 6.H2O, 7.2H2O, 7a, and
7b.2DMSO are similar and comparable to those found in 5
(Table S2, Supporting Information).
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It is interesting to compare the Pd−Pd distances found in
these trimeric and hexameric assemblies. The two unique Pd−
Pd distances of 2.796(0) and 2.813(1) Å, found in the trimeric
assembly of 5 (Figure 1c), are considerably shorter than those
found in the crystal structures of Pd3(OAc)6 (4, av 3.151(13)
Å).13 But in the hexameric assemblies of 6, 7, 7a, and 7b, the
Pd−Pd distances found within the triangular segment are
slightly longer than those found in 4 owing to the open
bridging coordination of the acetate groups between the two
Pd3 subunits. A further look at the interplanar Pd−Pd distances
found along the three rims of the prismatic assemblies reveals
large deviations for the mixed-bridged clusters 7a and 7b in
comparison with the all acetate-bridged clusters 6 and 7. Thus,
in 7a and 7b, two of the three rim distances are relatively much
shorter (7a, 3.018(2) Å (average); 7b, 2.961(2) Å) than the
third (7a, 3.672(3) Å; 7b, 3.762(2) Å), as the shorter ones are
found at the Pd2 pairs containing mixed (OAc and OR′)
bridging groups (Figure 2b,d). All three interplanar Pd−Pd
distances found in 6 (av 3.445(1) Å) and 7 (av 3.439(1) Å) are
very similar to each other (Figure 1e and Figure S14). As a
consequence, the mean plane deviation between the two
triangular Pd3 subunits in 7a and 7b is larger at 14 and 15°,
respectively, while those found in 6 and 7 deviated minimally
by an angle of 1°.
A Cambridge Structural Database (CSD) search on the
structures of palladium-containing hexameric complexes
indicates that the prismatic clusters observed in our reactions
are rather unprecedented. Although a handful of octahedral
structures are reported,15 there are very few examples known
for the prismatic arrangement of the six Pd(II) ions.16 Hence,
we presume that the key requirements for obtaining such
prismatic Pd(II) clusters are the intact Pd3 unit in its local
structure and ligand sets which minimize the steric crowding
between the two Pd3 subunits in the eclipsed conformation.
Even though crystalline Pd(OAc)2 exists in the form of a
discrete trimeric Pd3 unit, most ligand displacement reactions
tend to destroy this arrangement. In this regard, the tripodal
nature of the ligand moieties in 2 and 3 proves to be
compatible, sterically and electronically, with providing a
structurally rigid triangular motif by tethering the Pd3 unit
together from one side of it. In addition, the acetate groups are
well-suited at the bridging positions as the steric requirements
for the two terminal imido-phosphate substituents are minimal
in the prismatic arrangement.
Figure 3. Syntheses of the new trinuclear clusters 8, 9, and 10 (left);
molecular structure of the trinuclear cluster 10 (right).
Pd(II) species to a mononuclear complex has been observed
previously,17 formation of 8, 9, and 10 in the present instance is
attributed to the rigid tripodal chelation of the imido-phosphate
which provides the necessary stability to the Pd3 units. In
addition, formation of 9/10 has been observed in a direct
reaction involving 2-H3/3-H3, Pd(OAc)2, and cyclohexylamine.
The preferential deprotonation of 2-H3/3-H3 over cyclohexyl-
amine confirms that the trianionic species 2/3 provides a vital
stability to keep triangular Pd3 motifs intact. The 31P NMR
spectra of the compounds 8 (δ 72.05), 9 (δ 73.50), and 10 (δ
73.67) in CD3OD gave a single peak in the cluster region.
Furthermore, from the mass spectral data, their existence in
solution has been confirmed. The single-crystal X-ray analysis
of 9 and 10 reveals that these clusters are iso-structural in the
solid state. The asymmetric unit of 9.2H2O contains one-third
of the molecule as it was crystallized in the trigonal space group
P3-c1 (Figure S17, Supporting Information). The molecular
structure of 10 was solved in the monoclinic space group
P2(1)/n, in which the asymmetric unit contains the whole
molecule (Figure 3, right). As observed before, one side of the
Pd3 plane in 9 and 10 was found with the tripodally chelating
imido ligand, while the other side of it contains three hanging
cyclohexyl amines and three acetate moieties. In addition, the
H-bonding interactions between the acetate anions and the
amino protons result in the formation of a two-dimensional
hexagonal network for 9 and a one-dimensional zigzag chain for
10 (Figures S17 and S18 and Table S3, Supporting
Information).
Reactivity Studies of 5, 6, and 7. In order to test the
robustness of tripodally chelated (Pd−Nimido)3 moieties in 5, 6,
and 7, we studied the reactivity of these complexes in the
presence of a primary amine which acts as a nucleophilic
reagent to the Pd(II) ions. Thus, the reaction of 5, 6, and 7
with stoichiometric equivalents of cyclohexylamine in MeOH
proceeds spontaneously as observed by a visual color change in
solution (from orange to yellow) from which yellow-colored
crystalline solids of 8, 9, and 10, respectively, were obtained
(Figure 3). Single-crystal X-ray analysis of the crystals of 9 and
10 indicates the presence of a new trinuclear species of the
Application in Catalytic Reactions. Inspired by their rigid
Pd−Nimido tripodal motifs, we were curious to see whether the
Pd(II) atoms in these clusters are still active for catalytic
reactions. As these imido motifs are strongly bound to the
Pd(II) atoms, we thought a catalytic reaction that utilizes the
Pd(II) ions as an active catalyst would be more suitable to
examine (Figure S20, Supporting Information). Hence, we
employed these complexes in the Mizoroki−Heck type
reactions in which the Pd(II) species catalyzes the coupling
reaction of phenylboronic acid with alkenes.18,19
Thus, the treatment of phenylboronic acid (1.2 equiv) with
n-butyl acrylate (11a) in the presence of the trinuclear complex
5 (3 mol %) and Cu(OAc)2 (2 equiv) in DMF at 100 °C for 4
h leads to the corresponding phenylated trans-alkene derivative
(12a) in 90% isolated yield (Scheme 2). The reaction was also
tested with the hexameric complexes 6 and 7a under the same
conditions which gave the alkene product 12a in 90 and 95%
yields, respectively. Similarly, we have tried the coupling
reaction with a few more alkenes, such as methyl acrylate
c
formula {Pd3[(NR)3PO](OAc)3(cHexNH2)3} (9, R = Hex;
i
t
10, R = Pr), while a similar structure for 8 (R = Bu) was
confirmed by a combined 31P NMR and mass spectral analysis.
These new trinuclear assemblies were obtained via a
nucleophilic attack on the Pd(II) ions by the amino groups,
and in fact, 9 and 10 were obtained from a symmetrical
cleavage of the hexanuclear assemblies in 6 and 7, respectively.
Although such an amine-triggered cleavage of a dinuclear
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Scheme 2. M−H Type Reaction of PhB(OH)2 with Various
Alkenes
ACKNOWLEDGMENTS
■
We thank Dr. M. Jeganmohan for valuable discussions and Dr.
T. S. Mahesh for NMR experiments. This work was supported
by IISER, Pune and the Department of Science and
Technology, India through Grant SR/FT/CS-014/2008 (R.B.).
DEDICATION
■
This paper is dedicated to Prof. K. N. Ganesh on the occasion
of his 60th birthday.
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* Supporting Information
X-ray crystallographic data for 3-H3, 5, 6.H2O, 7.2H2O, 7a,
7b.2DMSO, 9.2H2O, and 10 in CIF format. NMR and mass
spectra for all the compounds, additional figures, tables of bond
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Corresponding Author
*E-mail: boomi@iiserpune.ac.in.
Notes
The authors declare no competing financial interest.
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