Xantphos-Capped Pd(II) and Pt(II) Macrocycles of Aryldithiolates: Structural Variation and Catalysis in C-C Coupling Reaction

Article abstract of DOI:10.1021/acs.inorgchem.8b02726

The self-assembly of Xantphos-capped M(OTf)₂ (M = cis-[M′(Xantphos)]²⁺ M′ = Pd, Pt) with bridging ligands 1,4-benezenedithiol or 4,4′-biphenyldithiol has been investigated. The reactions have yielded complexes [M{S(C₆H₄)_nSH}]₂(OTf)₂ (I) and [M₂{S(C₆H₄)_nS}]₂(OTf)₄ (II) (n = 1 or 2). The equilibrium between I and II has been established in platinum complexes for n = 2, whereas the analogous Pd complex exclusively exist as II. These results are different from our previously reported dppe or triethyl phosphine-capped complexes which showed only type II. The same reaction with 1,3-benezenedithiol lead to the complex [M₂(SC₆H₄SSC₆H₄S)](OTf)₂ (III), containing a S-S bond between two thiolate ligands, formed via a complex of type I in solution. Characterization of the complexes was accomplished by NMR spectroscopy, UV-vis spectroscopy and mass spectrometry, and X-ray crystallography. Density functional calculations were performed to estimate the relative stability of three types of complexes. The palladium complexes are excellent catalysts in Suzuki C-C cross coupling reactions under mild conditions, and can be reused eight times without losing significant yield. The activity of the Pd catalysts derived from three dithiol ligand follows opposite trend of the stability as III > II > I. The comparative catalytic activity of the tetranuclear Pd complexes (II) of bis-phosphines of varied bite angles, including the structurally characterized [Pd₂(dppf)₂(SC₁₂H₈S)]₂(OTf)₄ has also been demonstrated.

Full text of DOI:10.1021/acs.inorgchem.8b02726

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Xantphos-Capped Pd(II) and Pt(II) Macrocycles of Aryldithiolates:
Structural Variation and Catalysis in C−C Coupling Reaction
Pravin A. Mane,† Sandip Dey,*,†,∥ Arup Kumar Pathak,‡,∥ Mukesh Kumar,§
and Nattamai Bhuvanesh⊥
‡
§
†Chemistry Division, Theoretical Chemistry Section, and Radiation Biology and Health Science Division, Bhabha Atomic
Research Centre, Mumbai-400085, India
∥Homi Bhabha National Institute, Training School Complex, Mumbai-400094, India
⊥Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States
S
* Supporting Information
ABSTRACT: The self-assembly of Xantphos-capped
M(OTf)2 (M = cis-[M′(Xantphos)]2+; M′ = Pd, Pt) with
bridging ligands 1,4-benezenedithiol or 4,4′-biphenyldithiol
has been investigated. The reactions have yielded complexes
[ M { S ( C 6 H 4 ) n S H } ] 2 ( O T f ) 2 ( I ) a n d [ M 2 { S -
(C6H4)nS}]2(OTf)4 (II) (n = 1 or 2). The equilibrium
between I and II has been established in platinum complexes
for n = 2, whereas the analogous Pd complex exclusively exist
as II. These results are different from our previously reported
dppe or triethyl phosphine-capped complexes which showed
only type II. The same reaction with 1,3-benezenedithiol lead
to the complex [M2(SC6H4SSC6H4S)](OTf)2 (III), contain-
ing a S−S bond between two thiolate ligands, formed via a
complex of type I in solution. Characterization of the complexes was accomplished by NMR spectroscopy, UV−vis
spectroscopy and mass spectrometry, and X-ray crystallography. Density functional calculations were performed to estimate the
relative stability of three types of complexes. The palladium complexes are excellent catalysts in Suzuki C−C cross coupling
reactions under mild conditions, and can be reused eight times without losing significant yield. The activity of the Pd catalysts
derived from three dithiol ligand follows opposite trend of the stability as III > II > I. The comparative catalytic activity of the
tetranuclear Pd complexes (II) of bis-phosphines of varied bite angles, including the structurally characterized
[Pd2(dppf)2(SC12H8S)]2(OTf)4 has also been demonstrated.
structures were generated containing metal−pyridyl bond. A
few such Pt complexes have been recently explored as
functional materials to generate chiral metallogels,11 stabilizer
for G-quadruplexes,12 and antitumor agent.13 The self-
assembled Pd6 compound from triimidazole or oxadiazole
ligand has been established as catalysts in Diels−Alder14 or
Suzuki coupling reactions.15
INTRODUCTION
■
The development of new metallo-supramolecular complexes as
functional materials has been of tremendous interest in the
past decade. Some of these coordination complexes isolated as
discrete molecules have potential uses in drug delivery,1
enzyme mimics,2 molecular sensing,3 guest storage,4 and
homogeneous catalysis.5−7 The nature of applications depend
upon the metal ions and organic ligands used, capable of
showing noncovalent interactions, as well as the rigidity, size of
cavity, charge and solubility of the supramolecular complexes.
Among the transition metals, Pd(II) and Pt(II) ions have been
widely exploited in the construction of metallo-supramolecular
assembly owing to their square-planar geometry and higher
stability over the main group metals, which generally show
unpredictable geometry. The final structure obviously depends
on the bonding mode of building blocks, thermodynamic
stability, entropy change, steric factor and many more.8
Employing a large number of bis-pyridyl ligands of varying
spacer group, by incorporating organic group,1−6,8 or
heteroatom in the ligand backbone,9,10 a library of fascinating
However, the thiolate ligand (RS−), capable of forming rigid
structures owing to stronger metal−sulfur bond, is under
explored in the construction of metallo-supramolecular
complexes. It may be due to the tendency of the ligand to
form sparingly soluble oligomer or polymer rather than the
discrete molecules. We have recently reported the supra-
molecular assemblies of Pd(II) and Pt(II) complexes using
mercaptobenzoic acid16 via intermolecular H-bonding and aryl
dithiolate ligands7 via coordinate bond. The macrocyclic
palladium complexes, [Pd2(dppe)2{S(C6H4)nS}]2(OTf)4 and
Received: September 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02726
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
which was recrystallized from dichloromethane−ether mixture to
yield yellow crystals of 2a (305 mg, 78%; mp 145 °C (dec)). Anal.
Calcd for C41H32F6O7P2PdS2·(CH2Cl2)0.5(H2O)3.5: C, 45.78; H, 3.70;
[Pd2(dppe)2(SCH2C6H4CH2S)]4(OTf)8 (n = 1, 2; dppe = 1,2-
bis(diphenylphosphino)ethane) showed excellent catalytic
activity in Suzuki7 and Heck17 C−C coupling reactions.
Encouraged by these results we decided to investigate the
effect of wide-bite-angle phosphine, Xantphos (9,9-dimethyl-
4,5-bis(diphenylphosphino)xanthenes), as capping ligand on
the structures as well as the catalytic activity of the resulted
complexes derived from the same dithiol ligands. The capping
ligand strongly influences the nuclearity and structures of the
self-assembled products particularly in solution.18,19 It is well-
established that the large bite angle of diphosphine has positive
influence on the catalytic activity of Pd complexes in various
C−C,20 C−N,21,22 and C−O23 bond forming reactions. The
wide bite angle and flexible coordination environment of
Xantphos24 are factors responsible for making it highly efficient
ligand among other large bite angle diphosphines such as
DPEphos (bis(2-diphenylphosphinophenyl)ether), dppf (1,1′-
bis(diphenylphosphino)ferrocene), or Binap (2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl) in catalysis reac-
tions.25,26
1
S, 5.89. Found: C, 45.28; H, 3.49; S, 5.69%. H NMR (300 MHz,
CDCl3): δ 1.95 (s, 6H, CH3), 7.18−7.25 (m, 10H, m-H of PPh2 +
CHCHCH), 7.33−7.42 (m, 14H, o-H/p-H of PPh2 + CPCHCH),
3
7.91 (d, JHH = 7.8 Hz, 2H, CHCHCC), 31P{1H} NMR (121 MHz,
CDCl3): δ 26.6 (s). UV/vis (acetone): λmax (ε in M−1 cm−1)
329(9515) nm.
2b, Pt(Xantphos)(OTf)2. Prepared in a manner similar to that for
2a, using Pt(Xantphos)Cl2 (432 mg, 0.51 mmol) and AgOTf (394
mg, 1.53 mmol) to give colorless solid which was recrystallized from
dichloromethane−hexane mixture to yield colorless crystals of 2b
(498 mg, 91%; mp 172 °C (dec)). Anal. Calcd for C41H32F6O7P2PtS2·
(H2O)2.: C, 44.45; H, 3.28; S, 5.79. Found: C, 44.71; H, 3.36; S,
5.66%. 1H NMR (300 MHz, CDCl3): δ 1.94 (s, 6H, CH3), 7.17−7.22
(m, 10H, m-H of PPh2 + CHCHCH), 7.32−7.39 (m, 14H, o-H/ p-H
3
of PPh2 + CPCHCH), 7.87 (d, JHH = 7.8 Hz, 2H, CHCHCC).
1
31P{1H} NMR (121 MHz, CDCl3): δ −4.4 (s, JPt−P = 4132 Hz).
UV/vis (acetone): λmax (ε in M−1 cm−1) 327(1266) nm.
3a, [Pd(Xantphos)(1,4-SC6 H4 SH)]2 (OTf)2 (I) and
[Pd2(Xantphos)2(1,4-SC6H4S)]2(OTf)4 (II). To an acetone solution
of (5 mL) of 1a (5.4 mg, 0.038 mmol), acetone solution (5 mL) of 2a
(75.2 mg, 0.076 mmol) was added. The wine-red solution was stirred
for 4 h. The solvent was evaporated in vacuuo; the red residue was
washed with diethyl ether and extracted with acetone (3 × 5 mL).
Diethyl ether (10 mL) was added to yield red colored solid of 3a
(48.1 mg, 0.024 mmol, 65%; mp >230 °C (dec)). Anal. Calcd for 3a-
I·(H2O)2 C92H74F6O8P4Pd2S6·(H2O)2: C, 55.64; H, 3.96; S, 9.67%.
Calcd for 3a-II·(H2O)4 C172H136F12O16P8Pd4S8·(H2O)4: C, 56.00; H,
3.93; S, 6.95%. Found: C, 55.11; H, 3.75; S, 9.98%. Calcd for 3a-I: Pd,
We have investigated the reactions of [M′(Xantphos)-
(OTf)2] and arylenedithiols of varying position of thiol groups
as bridging ligands (Scheme 1). Interestingly, the 31P NMR
Scheme 1. Aryldithiol and Diphosphine Ligands with Bite
a
Angles in Degree
1
10.91%. Calcd for 3a-II: Pd, 11.77%. Found: Pd, 12.00%. H NMR
(300 MHz, acetone-d6): δ 1.50 (s, 6H, CH3), 1.97 (s, 6H, CH3), 5.60
(br s, 4H, C6H4), 6.88 (br s, 12H, p-H of PPh2 + CHCHCH), 7.39
(m, 20H, m-H of PPh2 + CPCHCH), 7.66 (br s, 16H, o-H of PPh2),
7.92 (br s, 4H, CHCHCC), 1H NMR (500 MHz, CD3OD): δ 1.50 (s,
6H, CH3), 1.97 (s, 6H, CH3), 5.53 (br s, 8H, C6H4), 6.84 (br s, 12H,
p-H of PPh2 + CHCHCH), 7.29 (m, 20H, m-H of PPh2
+
CPCHCH), 7.59 (br s, 16H, o-H of PPh2), 7.90 (br s, 4H,
CHCHCC). 31P{1H} NMR (121 MHz, acetone-d6): δ 4.3 (br, s), 7.9
(br, s) in ∼1:1 ratio. 31P{1H} NMR (243 MHz, CD3OD): δ 5.1 (br,
s), 12.8 (br, s) in ∼1:1 ratio. Electrospray ionization mass
spectrometry (ESI-MS; ion, relative intensity): m/z 1168.1
([Pd3(Xantphos)3(SC6H4SH)2 + 2H]2+, 2%), 1165.1 ([3a-II + H −
(3OTf + Pd(Xantphos) + 2C6H4)]2+, 1%), 825.1 ([3a-I − (2OTf +
Pd(Xantphos)(SC6H4SH))]+, 7%), 759.1 ([Pd(Xantphos)S + 2H +
a
Flexibility range in parentheses.
spectrum of the resulted Pd and Pt complexes showed two
distinct signals indicating the presence of two species in
solution. One of the dithiolate complex, [M′2(Xantphos)2{S-
(C6H4)nS}]2(OTf)4, is similar to the dppe analogue7 which
was earlier isolated as a single product, whereas the other is
identified as dinuclear complex containing free thiol group,
[M′(Xantphos){S(C6H4)nSH}]2(OTf)2. The same reaction
with 1,3-benezenedithiol lead to the complex
[M′2(Xantphos)2(SC6H4SSC6H4S)](OTf)2 containing a S−S
bond. The performance of the catalytic activity of the isolated
Pd complexes was evaluated in Suzuki C−C coupling reactions
in details. Moreover, a new complex of another large bite angle
diphosphine dppf was also prepared. Under optimized reaction
conditions, i.e., reflux in methanol, the comparative activity of
the tetranuclear complexes [Pd2(P∩P)2(SC12H8S)]2(OTf)4 of
varied bite angles and flexibility range27,28 of P∩P = Xantphos,
dppf, and dppe has been evaluated (Scheme 1).
C H 3 C N ] + , 1 0 % ) , 7 5 4 . 1 ( [ 3 a - I I
−
( 4 O T f
+
Pd2(Xantphos)2(SC6H4S))]2+, 5%), 710.0 ([Pd(Xantphos)(SC6H4)2
+ CH3CN − 3C6H5]+, 100%), 685.1 ([Pd(Xantphos) + H]+, 7%).
UV/vis (acetone): λmax (ε in M−1 cm−1) 350(28123), 397(sh, 15883),
503(17707) nm.
3b, [Pd2(Xantphos)2(4,4′-SC12H8S)]2(OTf)4 (II). Prepared in a
manner similar to that for 3a, using 2a (75.1 mg, 0.076 mmol) and 1b
(8.3 mg, 0.038 mmol) to yield the title complex as red solid (50.3 mg,
0.013 mmol, 70%; mp > 230 °C (dec)). Anal. Calcd for 3b
C184H144F12O16P8Pd4S8: C, 58.63; H, 3.85; S, 6.81. Found: C, 58.60;
1
H, 3.82; S, 6.94%. H NMR (300 MHz, acetone-d6): δ 1.62 (s, 6H,
CH3), (another peak due to the methyl group merged with the
solvent peak at δ 2.05 ppm), 5.89 (d, 3JHH = 7.8 Hz, 4H, o-H, C12H8),
6.32 (d, 3JHH = 7.8 Hz, 4H, m-H, C12H8), 7.09 (br s, 4H, CHCHCH),
7.22 (br s, 8H, p-H of PPh2), 7.30−7.49 (m, 20H, m-H of PPh2 +
CPCHCH), 7.53 (t, 3JHH = 6.9 Hz, 16H, o-H of PPh2), 7.97 (d, 3JHH
= 7.8 Hz, 4H, CHCHCC). 1H NMR (500 MHz, CD3OD): δ 1.56 (s,
3
6H, CH3), 2.01 (s, 6H, CH3), 5.84 (d, JHH = 8.0 Hz, 4H, o-H,
3
EXPERIMENTAL SECTION
C12H8), 6.22 (d, JHH = 8.0 Hz, 4H, m-H, C12H8), 7.01 (br s, 4H,
■
CHCHCH), 7.14 (br s, 8H, p-H of PPh2), 7.31−7.44 (m, 20H, m-H
2a, Pd(Xantphos)(OTf)2. To a dichloromethane solution of (25
mL) of Pd(Xantphos)Cl2 (301 mg, 0.39 mmol), solid AgOTf (307
mg, 1.19 mmol) was added. The reaction mixture was stirred for 16 h
with exclusion of light; the solvent was filtered and concentrated to 10
mL. Diethyl ether (25 mL) was added to precipitate yellow solid
of PPh2 + CPCHCH), 7.53 (t, 3JHH = 7.5 Hz, 16H, o-H of PPh2), 7.89
3
(d, JHH = 7.5 Hz, 4H, CHCHCC). 13C{1H} NMR (201 MHz,
1
acetone-d6): δ 23.4 (s, CH3), 38.7 (s, CCH3), 117.6 (d, JPC = 45.1
Hz, CP of C15H12O), 127.4 (s, o-C of C12H8), 127.9 (s, CH of
B
DOI: 10.1021/acs.inorgchem.8b02726
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Article
C15H12O), 130.3 (s, m-C of C12H8), 130.5 (s, CH of C15H12O), 130.8
(s, m-C of C6H5), 131.8 (s, CS of C12H8), 132.9 (d, JPC = 71.6 Hz,
similar to that for 4a, using 2b (48.1 mg, 0.045 mmol) and 1b (4.9
mg, 0.022 mmol) and recrystallized from acetone−hexane mixture to
yield yellow crystals of 4b (36.1 mg, 0.008 mmol, 78%; mp > 230 °C
(dec)). Anal. Calcd for 4b-II C184H144F12O16P8Pt4S8: C, 53.59; H,
1
CP of C6H5), 134.2 (s, o-/p-C of C6H5), 134.7 (s, CH of C15H12O),
137.6 (s, CC of C12H8), 139.0 (s, CCC(CH3) of C15H12O), 157.4 (s,
CO of C15H12O). 19F{1H} NMR (376 MHz, acetone-d6): δ −78.7
(s). 31P{1H} NMR (121 MHz, acetone-d6): δ 9.1 (s). ESI-MS (ion,
relative intensity): m/z 901.1 ([3b-II − (4OTf + Pd2(Xantphos)2)]2+,
13%), 832.1 ([Pd(Xantphos)S2 + 2CH3CN + 2H]+, 7%), 816.6
([Pd2(Xantphos)2S(OTf) + 2CH3CN + 2H]+2, 9%), 719.1 ([Pd-
(Xantphos)(SC12H8) + 2CH3CN − 3C6H5]+, 22%), 710.0 ([Pd-
(Xantphos)(SC6H4)2 + CH3CN − 3C6H5]+, 100%), 395.0
([Pd2(Xantphos)]2+, 24%). UV/vis (acetone): λmax (ε in M−1 cm−1)
344(41063), 395(sh, 27590), 518(28950) nm.
1
3.52; S, 6.22. Found: C, 53.87; H, 4.08; S, 5.64%. H NMR (300
MHz, acetone-d6): δ 1.58 (s, 6H, CH3), another peak correspondence
to the methyl group merged with the solvent peak at δ 2.05 ppm, 5.77
(br s, 4H, o-H, C12H8), 6.28 (br s, 4H, m-H, C12H8), 7.12 (br s, 4H,
CHCHCH), 7.24 (br s, 8H, p-H of PPh2), 7.34−7.54 (m, 36H, o-H/
m-H of PPh2 + CPCHCH), 7.98 (d, 3JHH = 7.2 Hz, 4H, CHCHCC).
1H NMR (500 MHz, CD3OD): δ 1.53 (s, 6H, CH3), 2.01 (S, 6H,
CH3), 5.81 (br s, 4H, o-H, C12H8), 6.26 (br s, 4H, m-H, C12H8), 6.97
(br s, 4H, CHCHCH), 7.11 (br s, 8H, p-H of PPh2), 7.27−7.60 (m,
36H, o-/m-H of PPh2 + CPCHCH), 7.89 (br s, 4H, CHCHCC).
13C{1H} NMR (201 MHz, acetone-d6): δ 22.9 (s, CH3), 39.1 (s,
3c, [Pd(Xantphos)(1,3-SC6H4SH)]2(OTf)2 (I′) and
[Pd2(Xantphos)2(1,3-SC6H4SSC6H4S)](OTf)2 (III). Prepared in a
manner similar to that for 3a, using 2a (106.4 mg, 0.108 mmol) and
1c (15.5 mg, 0.109 mmol) to give a red colored solid [(68.5 mg, 0.035
1
CCH3), 116.2 (d, JPC = 57.5 Hz, CP of C15H12O), 127.4 (s, o-C of
C12H8), 130.1 (s, m-C of C12H8), 130.5 (s, CH of C15H12O), 130.7 (s,
mmol, 65%; mp
> 230 °C (dec)). Anal. Calcd for
1
m-C of C6H5), 131.6 (br s, CS of C12H8), 133.1 (d, JPC = 77.0 Hz,
C92H70F6O8P4Pd2S6: C, 56.65; H, 3.82; S, 9.86. Found: C, 56.11;
CP of C6H5), 134.2 (s, CH of C15H12O), 134.5 (s, o-C of C6H5),
138.4 (br s, CC of C12H8), 138.9 (s, CCC(CH3) of C15H12O), 158.1
(s, CO of C15H12O), the peaks due to one CH group of C15H12O and
p-C of C6H5 are merged with the peaks at δ 130.1 and 134.5 ppm,
respectively. 31P{1H} NMR (121 MHz, acetone-d6): δ −0.89 (br s,
1JPt−P = 3380 Hz). ESI-MS (ion, relative intensity): m/z 1911.3 ([4b-
1
H, 3.59; S, 8.83%. H NMR (600 MHz, acetone-d6): δ 1.83 (br s,
CH3), 1.88 (s, CH3), 6.38−8.22 (m, C15H12O + PPh2 + C6H4).
31P{1H} NMR (243 MHz, acetone-d6): δ 15.9 (s), 18.5 (br s), 27.0
(s)], which on recrystallization from acetone-hexane mixture at −5 °C
to yield orange crystals of 3c (34.8 mg, 0.017 mmol, 33%; mp > 200
°C (dec)). Anal. Calcd for C92H72F6O8P4Pd2S6: C, 56.70; H, 3.72; S,
9.87. Found: C, 56.46; H, 3.70; S, 9.57%. 1H NMR (600 MHz,
II − 2OTf]2+, 9%), 1375.7 ([4b-II − (4OTf + Pt(Xantphos))]2+,
1%), 1224.5 ([4b-II − 3OTf]3+, 100%), 881.2 ([4b-II − 4OTf]4+,
88%). UV/vis (acetone): λmax (ε in M−1 cm−1) 328(33457), 341(sh,
27427), 411(32383) nm.
acetone-d6): δ 1.43 (s, 6H, CH3), 1.68 (s, 6H, CH3), 5.19 (d, 3JHH
=
7.8 Hz, 2H, C6H4), 6.31 (br s, 2H, C6H4), 6.78 (t, 3JHH = 7.8 Hz, 2H,
C6H4), 6.87 (br m, 20H, m-H of PPh2 + C6H4 + CHCHCH), 7.02
(br, 4H, p-H of PPh2), 7.09 (m, 4H, p-H of PPh2), 7.24 (d, 3JHH = 7.8
4c, [Pt(Xantphos)(1,3-SC6 H4 SH)]2(OTf)2 (I′) and
[Pt2(Xantphos)2(1,3-SC6H4SSC6H4S)](OTf)2 (III). Prepared in a
manner similar to that for 4a, using 2b (61.2 mg, 0.057 mmol) and
1c (8.1 mg, 0.056 mmol) and recrystallized from acetone−hexane
mixture to yield yellow powder [(44.1 mg, 0.020 mmol, 73%; mp 221
°C (dec)). Anal. Calcd for C46H37F3O4P2PtS3: C, 51.93; H, 3.51; S,
9.04. Found: C, 51.35; H, 3.32; S, 8.75%. 1H NMR (600 MHz,
Hz, 2H, CHCHCH), 7.30−8.25 (br m, 24H, o-H of PPh2
+
C15H12O). 31P{1H} NMR (243 MHz, acetone-d6): δ 22.5 (s). UV/vis
(acetone): λmax (ε in M−1 cm−1) 353(20159), 430(14269), 461(sh,
10458) nm.
3d, [Pd(Xantphos)(1,4-SCH2C6H4CH2SH)]2(OTf)2 and
[Pd2(Xantphos)2(1,4-SCH2C6H4CH2S)]m(OTf)2m. Prepared in a
manner similar to that for 3a, using 2a (70.9 mg, 0.072 mmol) and
1d (6.1 mg, 0.035 mmol) to yield the title complex as orange solid
(42.5 mg, 0.005 mmol, 62%; mp 202 °C). Anal. Calcd for
[Pd(Xantphos)(SCH2C6H4CH2SH)]2(OTf)2 C96H82F6O8P4Pd2S6:
acetone-d6): δ 1.85 (s, 6H, CH3), 2.20 (s, 6H, CH3), 3.99 (d, 3JHH
=
=
7.8 Hz, 2H, C6H4), 5.10 (t, 3JHH = 7.8 Hz, 2H, C6H4), 5.28 (d, 3JHH
3
4
9.6 Hz, 2H, C6H4), 6.47, 7.06 (dt, JHH (t) = 7.8 Hz, JPH (d) = 2.4
Hz, 8H, p-H of PPh2), 6.88−7.06, 7.57−7.82 (m, 32H, m-/o-H of
3
PPh2), 7.14, 8.04 (each t, JHH = 8.4 Hz, 4H, CHCHCH), 8.15 (d,
1
C, 57.46; H, 4.12; S, 9.59. Found: C, 57.96; H, 4.23; S, 8.94%. H
3JHH = 8.4 Hz, 2H, CPCHCH, another peak for the same group has
NMR (300 MHz, acetone-d6): δ 1.63 (br s, 6H, CH3), (another peak
due to the methyl group merged with the solvent peak at δ 2.05 ppm),
3.28 (s, 8H, SCH2), 6.45−8.45 (br m, 60H, Xantphos + C6H4).
31P{1H} NMR (121 MHz, acetone-d6): δ 10.8 (br s), 12.9 (br s) in
been merged with peak of Ph), 8.24, 8.26 (each d, J = 9.0 Hz, 4H,
2
CHCHCC). 31P{1H} NMR (243 MHz, acetone-d6): δ 7.5 (d, JPP
=
1
2
1
24 Hz, JPt−P = 3057 Hz), 18.1 (d, JPP = 24 Hz, JPt−P = 2902 Hz)].
The yellow solid redissolved in minimum quantity of acetone, a few
drops of hexane were added to yield a reddish solid with few crystals
of 3c (21.8 mg, 0.010 mmol, 36%; mp > 250 °C (dec)). Anal. Calcd
for C92H74F6O8P4Pt2S6: C, 51.93; H, 3.51; S, 9.04. Found: C, 51.23;
∼1:1 ratio.
4a, [Pt(Xantphos)(1,4-SC6 H4 SH)]2 (OTf)2 (I) and
[Pt2(Xantphos)2(1,4-SC6H4S)]2(OTf)4 (II). An acetone solution (5
mL) of 2b (100.1 mg, 0.093 mmol) was added to a methanolic
solution (5 mL) of 1a (6.6 mg, 0.046 mmol) with stirring which
continued for 24 h. The solvent was evaporated in vacuuo; the red
residue was washed with hexane and extracted with acetone (3 × 5
mL). A few drops of hexane were added to yield yellow crystal of 4a
(71.3 mg, 0.033 mmol, 72%; mp > 230 °C (dec)). Anal. Calcd for 4a-
I C92H74F6O8P4Pt2S6: C, 51.93; H, 3.51; S, 9.04. Calcd for 4a-II
C172H136F12O16P8Pt4S8: C, 52.02; H, 3.45; S, 6.46. Found: C, 52.02;
1
H, 3.48; S, 8.73%. H NMR (600 MHz, acetone-d6): δ 1.31 (s, 6H,
CH3), (another peak due to the methyl group merged with the
solvent peak at δ 2.05 ppm), 4.74 (br s, 2H, C6H4), 5.10 (br s, 2H,
C6H4), 6.54 (br s, 2H, C6H4), 6.65−7.63 (m, 40H, PPh2), 7.77 (t,
3JHH = 7.8 Hz, 4H, CHCHCH), 7.94 (br s, 4H, CPCHCH), 8.09 (br,
4H, CHCHCC). 31P{1H} NMR (243 MHz, acetone-d6): δ 12.2 (br s,
1
1JPt−P = 2956 Hz), 14.9 (br s, JPt−P = 3049 Hz), two small broad
1
peaks were observed at δ 17.7 ppm (ca. 7%) and 23.8 ppm (ca. 12%).
UV/vis (acetone): λmax (ε in M−1 cm−1) 329(18816), 373(12663)
nm.
H, 3.65; S, 8.85%. H NMR (500 MHz, acetone-d6): δ 1.52 (s, 6H,
CH3), 2.00 (s, 6H, CH3), 5.30 (br s, 4H, C6H4), 5.98 (br s, 4H,
C6H4), 6.20−8.25 (br m, 48H, Ph + C15H12O), 8.91 (br, 4H,
1
C15H12O). 31P{1H} NMR (121 MHz, acetone-d6): δ −1.46 (s, JPt−P
5, [Pd2(dppf)2(4,4′-SC12H8S)]2(OTf)4 (II). To a methanol solution
of (5 mL) of 1b (7.9 mg, 0.036 mmol) was added a dichloromethane
solution (5 mL) of Pd(dppf)(OTf)2 (69.5 mg, 0.072 mmol). The
wine-red solution was stirred for 4 h. The solvent was evaporated in
vacuuo; the red residue was washed with diethyl ether and extracted
with acetone (3 × 5 mL). Diethyl ether (1 mL) was added to yield
red crystal of the title complex (63.6 mg, 0.017 mmol, 95%; mp > 230
°C (dec)). Anal. Calcd for C164H128F12O12P8Pd4S8Fe4: C, 53.64; H,
1
= 3505 Hz), −8.52 (s, JPt−P = 3365 Hz). ESI-MS (ion, relative
intensity): m/z 1299.7 ([4a-II − (4OTf + Pt(Xantphos))]2+, 8%),
1255.7 ([4a-II + 4CH3CN + H − (Xantphos + 4C6H5 + SC6H4S)]2+,
2%), 1173.8 ([4a-II − 3OTf]3+, 100%), 1132.8 ([4a-II − (2OTf + Pt
+ C6H5)]3+, 15%), 914.1 ([4a-I − (2OTf + [Pt(Xantphos)-
(SC6H4SH))]+, 2%), 843.1 ([Pt2(Xantphos)2(SC6H4S)]2+, 20%).
UV/vis (acetone): λmax (ε in M−1 cm−1) 332(22172), 366(sh,
15763), 447(5577) nm.
1
3.51; S, 6.99. Found: C, 52.95; H, 3.29; S, 7.27%. H NMR (500
MHz, acetone-d6): δ 4.43 (s, 8H, Hβ-ferr), 4.77 (s, 8H, Hβ-ferr), 4.85
4b, [Pt(Xantphos)(4,4′-SC12H8SH)]2(OTf)2 (I) and
[Pt2(Xantphos)2(4,4′-SC12H8S)]2(OTf)4 (II). Prepared in a manner
(s, 16H, Hα-ferr), 6.18 (br s, 8H, o-H, C12H8), 6.49 (d, 3JHH = 8.5 Hz,
C
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Scheme 2. Syntheses of Palladium and Platinum Complexes with Arylenedithiols
8H, m-H, C12H8), 7.62 (br s, 32H, m-H of Ph), 7.74 (br s, 16H, p-H
of Ph), 7.80−7.99 (m, 32H, o-H of PPh2). 31P{1H} NMR (243 MHz,
acetone-d6): δ 34.1 (s).
Procedure for Catalyst Recycling. Similar to the above
procedure, the reactants 4-bromoacetophenone (1.0 mmol), PhB-
(OH)2 (1.3 mmol), and catalyst 3b (0.4 mol %) were used. Keeping
other reaction conditions as above, the mixture was stirred for 6 h and
processed analogously. After the reaction, hexane (3 × 15 mL) was
added to the product mixture resulting in the formation of two layers.
The upper layer of hexane containing the product was removed to
give 99% yield. Catalyst 3b, contained in the lower aqueous layer, was
then employed for another reaction; accordingly, a proportional
amount of reactants were further added. After 6 h, the first reaction
cycle resulted in 98% yield; in repeating the process, the second and
third reaction cycles yielded 97 and 96% of the product.
Density Functional Calculations. Full geometry optimizations
of all the Pd and Pt complexes are carried out using BP86 density
functional. BP86 is a generalized gradient approximation functional
that is formed by the combination of combines Becke’s 1988
exchange functional with perdew’s 1986 correlation functional.29,30
Pseudo-Newton−Raphson based algorithm has been employed for
Suzuki−Miyaura Cross-Coupling Reaction. An oven-dried
flask was charged with aryl halide (1 mmol), aryl boronic acid (1.3
mmol), palladium complex 3a (0.1 mmol, equivalent to 0.4 mol % of
Pd), methanol (3.0 mL), and aqueous K2CO3 (2 mmol, 1 mL) and
placed on an oil bath at 70 °C under a nitrogen atmosphere; the
reaction mixture was stirred until maximum conversion of aryl halide
to product occurred. The reaction mixture was cooled to room
temperature, was diluted with water (10 mL) and neutralized by
dropwise addition of dilute HCl (aqueous). The mixture was
extracted with hexane (3 × 15 mL), washed with water (2 × 10
mL) followed by brine solution (2 × 10 mL), and dried over
anhydrous Na2SO4. The solvent of the extract was removed with
rotary evaporator, and the resulting residue was analyzed by 1H NMR
spectroscopy.
D
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PEt3 analogue [Pt2(PEt3)4{S(C6H4)nS}]2(OTf)4 (1JPt−P
=
geometry search. Gaussian type double split valence basis functions,
namely, 6-31G(d,p), are used for S, O, P, C, and H atoms, and 3-21G
basis set for Pd is used for the calculations. SARC-ZORA basis set is
used for Pt. The basis sets for Pt and Pd are obtained from Extensible
Computational Environment Basis Set database (PNNL). The solvent
water is modeled by employing conductor-like screening model
(COSMO).31 All electronic structure calculations are carried out
applying the GAMESS suites of ab initio program systems on a
LINUX cluster platform.32 Visualizations of molecular geometry and
orbitals are carried out by the MOLDEN and MOLEKEL program
systems.33,34
2907−2911 Hz).7 The present NMR data suggest the
possibility of two types of complexes in solution. The single
crystal X-ray diffraction analysis of 4a confirmed the dinuclear
structure [Pt(Xantphos)(SC6H4SH)]2(OTf)2 containing the
“Pt2S2” bridge by two thiolates with two free thiols designated
as I in Scheme 2.
As expected, complex 3b showed a sharp peak at δ 9.1 ppm
in 31P{1H} NMR spectrum (Figure S8). The 1H NMR
spectrum displayed two sharp singlets at δ 1.56 and 2.00 ppm
for the methyl groups, two doublets for the ortho and meta
hydrogens of the biphenylene group and other expected peaks
in the aromatic region (Figure S6). The 31P{1H} NMR
spectrum of the platinum complex 4b showed a broad singlet
RESULTS AND DISCUSSION
■
Syntheses and NMR Spectroscopy. The complexes
Pd(Xantphos)(OTf)2 (2a) and Pt(Xantphos)(OTf)2 (2b)
were isolated in 78 and 91% yield, respectively from the
reaction of Pd(Xantphos)Cl2 and Pt(Xantphos)Cl2 with excess
of AgOTf (OTf = CF3SO3). The crystalline Pd (yellow) and Pt
(colorless) complexes are highly soluble in chlorinated
solvents, methanol, acetone, etc., and were characterized by
different analytical techniques. The two methyl groups showed
a sharp singlet deshielded with respect to their chloride
complexes. The 31P{1H} NMR spectrum showed one singlet
with Pt satellites (1JPt−P = 4132 Hz) for 2b. The X-ray
structure of 2a shows two coordinated water molecules with
triflate as counteranions.
1
with Pt satellites of JPt−P about 3380 Hz (Figure S10). The
1
pattern of H and 13C{1H} NMR spectra of 4b are similar to
those of 3b; however, the peaks are broadened, for example,
the ortho and meta hydrogens or carbons of the biphenylene
group showed two broad singlets (Figures S11−S13). The
relative integration and elemental analysis indicates the
composition [M2(SC12H8S)]m(OTf)2m for 3b and 4b. Single-
crystal X-ray analyses of 4b confirmed the tetranuclear
structure [Pt2(Xantphos)2(SC12H8S)]2(OTf)4. It is note-
worthy the analogous reactions of 1a and 1b with M(OTf)2
(M = [Pt(PEt3)2]2+ or [Pd(dppe)]2+) resulted in the single
p r od uc t s a s t et r a n uc le a r c o mp l ex e s [M2 { S -
(C6H4)nS}]2(OTf)4.7 The possibility of two types of
complexes in 3a and 4a at room temperature, as well as the
broadening of the 31P NMR peaks of 4b, persuaded us to do
low-temperature NMR experiments for the latter. The 31P{1H}
NMR spectrum of 4b was recorded in methanol-d4 at three
different temperatures (Figure 2). At room temperature, it
The reactions of bis triflate complexes 2a and 2b with
arylene dithiols 1a and 1b in 2:1 ratio, immediately resulted in
dark chocolate colored palladium complexes 3a and 3b and
yellow colored platinum complexes 4a and 4b (Scheme 2).
These new Xantphos-capped Pd(II) and Pt(II) complexes of
dithiolate ligands were characterized by elemental analysis,
NMR, UV/vis spectroscopy and mass spectrometry which are
outlined in the Experimental Section. The 31P{1H} NMR
spectrum of phenylene-1,4-dithiolate of palladium complex 3a
displayed two slightly broad peaks at δ 4.3 and 7.9 ppm. The
analogous platinum complex 4a also showed two compara-
tively sharp peaks at δ −8.5 and −1.5 ppm (Figure 1). The
1JPt−P values of 3365 and 3494 Hz are significantly reduced
1
relative to the JPt−P value of 2b due to the stronger trans
influencing thiolate groups but are higher than those of the
Figure 2. Variable-temperature 31P{1H} NMR spectra (202 MHz,
CD3OD) of 4b.
seems the line broadening (full width at half-maximum, fwhm
1
∼ 531 Hz) is slightly more with approximate JPt−P value of
3316 Hz as compared to the spectrum recorded in acetone-d6
(fwhm ∼ 398 Hz). At 0 °C the main peak was extremely
broadened and merged with the Pt satellites. After further
lowering the temperature to −30 °C, it split into two sharp
peaks at δ −7.1 and 0.5 ppm with Pt satellites; the respective
1JPt−P values are 3228 and 3488 Hz. These spectral features
have similarity with the 31P NMR spectrum of 4a which was
recorded at room temperature. These results indicate that the
two complexes, I and II, are in equilibrium which is fast at
Figure 1. 31P{1H} NMR spectra (121 MHz, acetone-d6) of 3a
(above) and 4a (below).
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Figure 3. ESI mass-spectrum of 4b. The insets show the experimentally obtained isotope patterns of the fragments. The found and calcd values are
for the most abundant peak of the ion.
room temperature in the case of the biphenyl system. The
sharp 31P NMR peak for 3b at room temperature may be
attributed to the faster ligand-exchange processes in case of Pd
complex as compared to that of Pt analogue. The wide bite
angle and flexible behavior of the presently used ligand
Xantphos, inducing a dynamic coordination environment
around the crowded Pd(II) or Pt(II) centers, may be
responsible for the fluxional nature of the resulted complexes.35
To confirm the other peak for methyl group, which is often
Experimental Section. Overall, the peaks are slightly more
broadened in methanol-d4 as compared to those in acetone-d6.
Moreover, the integration ratio of the peaks corresponding to
the aromatic part of Xantphos group and dithiolate group are
calculated as 1:1 for 3a in methanol-d4 and 1:1 for 4a in
acetone-d6 which indicate the presence of I in solution. The
thermal stability has been assessed by thermogravimetric (TG)
analyses under argon atmosphere. The TG curve of 3b shows
the decomposition starts after 320 °C (Figure S32) which
implies the solid compound is highly stable. However,
palladium complexes 3a and 3b are found to be sensitive in
chloroform. The 31P{1H} NMR spectrum of 3b in CDCl3
1
merged with the solvent peak of acetone-d6, the H NMR
spectra of 3a, 3b, and 4b were recorded in methanol-d4
(Figures S4, S7, and S12), which are outlined in the
F
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solution, when monitored for 3 days, showed an additional
peak at δ 22.4 ppm with the peak at δ 8.0 ppm corresponding
to 3b in ∼3:2 ratio; the former peak has been attributed to
Pd(Xantphos)Cl2.
analogy, two types of complexes [Pd(Xantphos)-
(SCH2C6H4CH2SH)]2(OTf)2 and [Pd2(Xantphos)2-
(SCH2C6H4CH2S)]m(OTf)2m have been tentatively assigned
which was not proceeded further (Scheme 2). Palladium
complex 5 of bisphosphine dppf, containing a wide bite angle
(106°) but smaller than that of Xantphos, has been prepared
with 1b as another catalyst for comparative study. The NMR
spectrum and single crystal structure of 5 confirmed the
tetranuclear structure [Pd2(dppf)2(SC12H8S)]2(OTf)4 of type
II, both in solution and solid state.
Mass Spectrometry. The mass spectra of the complexes
were obtained by soft ionization ESI technique. The ESI mass
spectra clearly showed the molecular ion peaks minus the
counterions and confirm the formation of the complexes
(Figures 3 and S25−S27). The fragmented ions are observed
with +1 to +4 charge states, the peaks are isotopically resolved
which agree well with their theoretical distributions (Tables S3
and S4). For 3a, the relevant peaks were observed at m/z
1165.1, 825.1, and 754.1 corresponding to [3a-II + H −
(3OTf + Pd(Xantphos) + 2C6H4)]2+, [3a-I − (2OTf +
Pd(Xantphos)(SC6H4SH))]+, and [3a-II − (4OTf +
Pd2(Xantphos)2(SC6H4S))]2+. A similar mass spectrum was
observed for the analogous biphenylene dithiolate complex of
palladium 3b; the peaks at m/z 901.1 and 816.6 have been
attributed to [3b-II − (4OTf + Pd2(Xantphos)2)]2+ and
[Pd2(Xantphos)2S(OTf) + 2CH3CN + 2H]2+ suggesting the
tetranuclear structure [Pd2(Xantphos)2(SC12H8S)]2(OTf)4.
However, no peak corresponding to binuclear structure was
detected in case of 3b.
The mass analyses of platinum complexes 4a and 4b show
lesser fragmentation than palladium complexes. The prominent
peaks were observed for 4a at m/z 1299.7 ([4a-II − (4OTf +
Pt(Xantphos))]2+), 1173.8 ([4a-II − 3OTf]3+), and 914.1
([4a-I − (2OTf + Pt(Xantphos)(SC6H4SH))]+). Similarly, for
analogous dithiolate complex 4b, the peaks at m/z 1911.3
([4b-II − 2OTf]2+), 1224.5 ([4b-II − 3OTf]3+), and 881.2
([4b-II − 4OTf]4+) confirm the formation of tetranuclear
complex II. The mass analysis of both 3a and 4a strongly
support the presence of II as another complex by a large
number of fragmented ions, as indicated by NMR spectro-
scopic data. The other complex, I, is confirmed by the X-ray
structure of 4a.
UV/Vis Absorption Spectra. The Pd complexes are dark
maroon colored, and the Pt complexes are yellow colored
crystalline solids. The absorption spectrum in acetone solution
show a broad but strong band in the visible region, which is
responsible for the color of the complexes (Figure S29−S30).
Palladium complexes 3a, 3b, and 3c display a long wavelength
band at 503, 518, and 461 nm, respectively, whereas analogous
platinum complexes 4a, 4b, and 4c display the absorption in
higher energies at about 447, 411, and 373 nm. It is to be
noted that the similar band is absent in the case of starting
precursors 2a or 2b. This long wavelength band has been
assigned to S and phenyl ring orbitals based HOMO to
phosphine and metal orbital based LUMO charge transfer
transition in the case of analogous complexes of dppe/Pd and
PEt3/Pt systems,7 which has been analyzed by DFT
calculations (see below). However, the bands of the present
Xantphos complexes are bathochromically shifted in compar-
ison to the dppe or PEt3 analogues. The complexes
[Pd2(dppe)2(SC12H8S)]2(OTf)4 and [Pt2(PEt3)4(SC12H8S)]2-
(OTf)4, similar to 3b and 4b, showed absorption maxima at
447 and 364 nm, respectively.7 The molar extinction
Next, we sought to change the relative position of two thiol
groups to see the effect of the ligand on the products and
structures; accordingly, the same reactions were investigated
with 1,3-benezenedithiol. The addition of 2a and 1c in 1:1
ratio initially gave a red colored product which shows three
peaks at δ 15.9, 18.5, and 27.0 ppm in 31P{1H} NMR
spectrum, the middle peak is broad whereas the later is highly
sharp. In acetone-hexane mixture, this product yielded orange
colored crystals, which displays a single resonance at δ 22.5
ppm, and the X-ray structure confirmed the dinuclear complex
[Pd2(Xantphos)2(SC6H4SSC6H4S)](OTf)2 (3c-III) contain-
ing a S−S bond between two thiolate ligands. The formation of
these products was also monitored in an NMR tube. The
mixing of 2a and 1c in acetone-d6 within 1 h showed a peak at
δ 15.9 ppm, which is attributed to the more likely anti
conformer of 3c-I′ [Pd(Xantphos)(SC6H4SH)]2(OTf)2
(Scheme 2); however, after 1 day other prominent peaks
appeared at δ 15.9, 18.5, 22.5, 27.0, and 47.0 ppm (Figures
S15−S17). The broad peak at δ 18.5 ppm is assigned to syn
conformer of 3c-I′ which is converted to 3c-III, whereas the
sharp peaks at δ 27.0 and 47.0 ppm are assigned to dioxide of
Xantphos36 and its Pd-ligated complex.
Similarly, the reaction of platinum complex 2b and 1c gives a
yellow solid in solution which is transformed to a red product
in a day. The 31P{1H} NMR spectrum of the yellow product
1
exhibit two sharp doublets at δ 7.5 and 18.1 ppm with JPt−P
values 3057 and 2902 Hz, respectively (Figure S20). Although
the structure could not be predicted, the elemental analysis
suggest the composition of [Pt(Xantphos)-
2
(SC6H4SH)]n(OTf)n; the JPP value of about 24 Hz indicates
two magnetically inequivalent 31P nuclei. The 31P{1H} NMR
spectrum of the red product (Figure S21) display mainly two
1
broad singlets at δ 12.2 and 14.9 ppm with JPt−P values 2956
and 3049 Hz, respectively, which have been assigned as syn and
anti conformer of 4c-I′, [Pt(Xantphos)(SC6H4SH)]2(OTf)2
(Scheme 2). The X-ray structure of the crystals found with the
red product is established as the analogous structure of
palladium complex [Pt2(Xantphos)2(SC6H4SSC6H4S)](OTf)2
(4c-III). It may be noted in the case of I of 3a, 3b, and 4b that
a flat square-planar M′2S2 ring with two thiolate groups
adopting an anti configuration,37 has been assigned in solution
that is confirmed by solid-state structure. The DFT
calculations were also performed to check the feasibility of
the analogous tetranuclear structure obtained from phenylene-
1,4-dithiolate and biphenylene-1,4-dithiolate ligands. However,
the calculations showed that the similar complex
[M′2(Xantphos)2(1,3-SC6H4S)]2(OTf)4 (II′) with phenyl-
ene-1,3-dithiolate and the possibility of another neutral
complex [M′2(Xantphos)2(SC6H4S)2] (IV) are inherently
unstable, probably due to the overcrowding of ligands (Scheme
S1). The reported platinum complex [Pt2(terpyridine)2(1,3-
SC6H4S)](PF6)2 exist as linear dimeric complex.38
Another bridging ligand 1,4-benzenedimethanethiol was also
taken up to check the above results, and to compare the
catalytic activity. The 31P{1H} NMR spectrum of the prepared
palladium complex 3d, shows two broad peaks at δ 10.8 and
12.9 ppm, which suggest also two types of complexes (Figure
S22). The dppe analogue has been reported as octanuclear
structure with Pd:dithiolate ratio is 2:1.7 In the present case, by
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coefficient value of the order ∼104 M−1 cm−1 suggest a
symmetry allowed transition.
Crystal Structures. Molecular structures of [Pt-
(Xantphos)(SC6H4SH)]2(OTf)2 (4a-I), [Pt2(Xantphos)2-
(SC1 2 H8 S)]2 (OTf)4 (4b-II), [Pd2 (Xantphos)2 -
(SC6H4SSC6H4S)](OTf)2 (3c-III), [Pt2(Xantphos)2-
( S C 6 H 4 S S C 6 H 4 S ) ] ( O T f ) 2 ( 4 c - I I I ) , a n d
[Pd2(dppf)2(SC12H8S)]2(OTf)4 (5) were established by single
crystal X-ray analyses. All structures are cationic, as the triflate
ions are placed at large, nonbonding distance from palladium
or platinum centers. In case of 4a-I, 4b-II, 4c-III and 5 the
triflate ions were found highly disordered and was initially
modeled using partial occupancies with multiple conforma-
tions. However, no satisfactory model could be obtained,
therefore the triflate ions were removed from the model and
the refinement was carried out using solvent mask utility as
implemented in Olex2.39 ORTEP drawing with atomic
numbering schemes are shown in Figures 4−7 and S33 while
selected interatomic parameters are given in Table 1.
F i g u r e 6 . S i n g l e - c r y s t a l X - r a y s t r u c t u r e o f
[Pd2(Xantphos)2(SC6H4SSC6H4S)](OTf)2 (3c-III) ellipsoids drawn
at 25% probability. The triflate ions, the phenyl groups of Xantphos
and hydrogen atoms are omitted for clarity.
distorted square planar whereas the [Pt2(μ-SAr)2] core is
planar rhombus shaped. The two phosphorus atoms stay above
and below the Pt2S2 plane by 0.6 and 0.8 Å. The backbone of
the xanthene moiety is as usual nonplanar. One side of the
coordination plane of Pt center is crowded by the three phenyl
rings of both the ligands. The π···π stacking distance between
the phenyl ring of thiolate group and the nearest phenyl ring of
the xanthene group is 3.7 Å. The intermolecular S···S and S···H
distances found as 3.54 and 4.14 Å.
The tetranuclear structures of [M′2(P∩P)2(SC12H8S)]2-
(OTf)4 (M′/P∩P = Pt/Xantphos, 4b-II; Pd/dppf, 5) resemble
the structure of Pd/dppe analogue.7 The macrocyclic
structures consist of two 4,4′-biphenylenedithiolate ligands
which bridge two “M′2(P∩P)2” units at head and tail positions.
The resulting four-membered nonplanar “M′2S2” rings at both
the ends capped by two Xantphos or dppf ligands look over
crowded compared to the Pd/dppe analogue. The dihedral
angle between two S−Pt−S planes is 48.1° in 4b-II, whereas it
is 44.1° between S−Pd−S planes in 5. The length of the
rectangle as earlier defined by edge-to-edge S···S distance is
10.6 Å, and the height defined by the S···S distance in a same
M′2S2 unit is 3.1 Å in both the structures. The π···π stacking
distance between the two opposite centroids of benzene rings
Figure 4. Single-crystal X-ray structure of [Pt(Xantphos)-
(SC6H4SH)]2(OTf)2 (4a-I) ellipsoids drawn at 25% probability.
The triflate ions were masked, the phenyl groups of Xantphos and
hydrogen atoms except for thiol groups are omitted for clarity.
The cationic part of complex 4a-I is a centro-symmetric
binuclear molecule, in which two Pt atoms are held together by
bridging thiolato group of the ligand with the free thiol groups
adopting an anti configuration. The geometry around Pt(II) is
Figure 5. Single-crystal X-ray structure of [Pt2(Xantphos)2(SC12H8S)]2(OTf)4 (4b-II) ellipsoids drawn at 25% probability. The triflate ions were
masked, the phenyl groups of Xantphos and hydrogen atoms are omitted for clarity.
H
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favors the S−S bonding in a suitable position. The S−S bond
distance of 2.07 Å in both the structures match well with S−S
bond distances in Pt complexes such as in [PtS4(dppe)] (avg
2.04 Å),41 [Pt2(4,4′-py2S2)2(PEt3)2](NO3)2 (2.03 Å).9 The
π···π stacking distance found between the phenyl ring of
thiolate group and the nearest phenyl ring of the xanthene
backbone is 3.6 Å. Unlike in 4a-I, the M′2S2 rings are
nonplanar; the calculated dihedral angles for 3c-III and 4c-III
are 23.4 and 21.6°. However, these hinge distortions are
comparatively less than that in 4b-II or 5.
A slight variation in Pt−S bond distances was noticed in all
the three structures: 2.37−2.39 Å for for 4a-I, 2.36−2.37 Å for
4b-II, and 2.37−2.39 Å for 4c-III. These distances match well
with the Pt−S bond distances in analogous complex [Pt2(μ-o-
SC6H4(CF3))2(dppe)2](OTf)2 (2.38 Å)42 but are longer than
those in in [Pt2(dppf)2(C8H8NS)](BF4) (2.34 Å)43 and
[Pt2(terpyridine)2(1,3-SC6H4S)]2(PF6)2 (2.31 Å).38 The bite
angles (101−102°) for 4a-I and 4b-II agree well with those of
c o m p l e x e s [ P t ( X a n t p h os ) C l 2 ] 4 4 ( 1 0 1 ° ) a n d
[Pt3(Xantphos)2{(SeCH2)2C(CH2OH)2}]Cl2]44 (101°) but
are smaller than that of the allyl complex [Pt(η3-allyl)-
(Xantphos)](OTf) (107°),45 whereas the bite angles 98−99°
for 3c-III are shorter than the usual values for a wide bite angle
(>101°) in Xantphos complexes. Although the exact reason is
not clear, the P···P distances (3.50 Å) are found to be shorter
in 4c-III in comparison to those in 4a-I (3.56 Å) and 4b-
II(3.59 Å). In the case of Pd(0) complexes, a correlation has
been noticed among P···P separation, Pd−P distances, and P−
Pd−P bite angles.46 The average Pt−P bond distances of about
2.30−2.32 Å for the three complexes match well with the
reported Pt−P bond distances of Xantphos complexes
Pt(II).44,45
The structure of [Pd(Xantphos)(OH2)2](OTf)2·
(H2O)1.5(CH2Cl2)0.5 {2a·(H2O)3.5(CH2Cl2)0.5} shows two
water molecules coordinated to palladium center, whereas
the triflate anions remain separated in the lattice with
electrostatic interactions with complex cations. The solvent
molecules water and dichloromethane were also found in the
lattice (Figure S34). The coordinated water molecules and O-
atoms of triflate anions are involved in hydrogen bonding. The
Pd−P bond length (∼2.24 Å) matches well with the Pd−P
distances in complexes of large bite angle phosphines
F i g u r e 7 . S i n g l e - c r y s t a l X - r a y s t r u c t u r e o f
[Pd2(dppf)2(SC12H8S)]2(OTf)4 (5) ellipsoids drawn at 25% proba-
bility. The triflate ions were masked, the phenyl groups of dppf and
hydrogen atoms are omitted for clarity.
of dithiolate ligand are ∼3.5 and 3.4 Å, respectively, for
platinum and palladium structures. Similarly, for another π···π
stacking the closest distance between one phenyl ring attached
to P atom and phenyl ring of Xanthene backbone is 4.0 Å in
4b-II.
The dinuclear structures of 3c-III and 4c-III can be
envisioned as the modified form of the syn conformer of 4a-I
after bonding between two free SH groups of two different
ligands containing one thiolate and another thiol form
positioned at 1,3-position in the benzene ring. It forms a
rigid but unique unit “PtSC6H4SSC6H4SPt” having a S−S
bond. It is noteworthy that the similar reactions of three
−
dithiols 1a−1c with {[(PPh3P)Au]3O}+BF4 resulted only in
Au(I) thiolates without any such S−S bond in the
complexes.40 This is probably due to the square planar
geometry of M′(II) in addition to the bridging mode of
thiolate ligand and tetrahedral geometry around “S” atom
Table 1. Selected Interatomic Distances [Å] and Angles [°] of 4a-I, 4b-II, 3c-III·(CH3COCH3)2, 4c-III, and 5
4a-I
4b-II
4c-III
3c-III
5
M = Pt
M = Pd
M1−S1
2.3871(17)
2.3720(16)
2.364(2)
2.368(2)
2.355(2)
2.366(3)
2.318(3)
2.322(2)
2.325(3)
2.311(3)
82.34(9)
93.65(9)
163.42(10)
101.00(10)
82.08(10)
101.90(10)
2.371(3)
2.395(3)
2.379(3)
2.382(3)
2.294(3)
2.293(3)
2.313(3)
2.291(3)
83.00(10)
85.56(11)
164.21(12)
97.93(11)
83.12(10)
99.07(11)
2.414(3)
2.371(3)
2.389(3)
2.382(3)
2.324(3)
2.309(4)
2.315(4)
2.360(4)
84.55(11)
95.79(12)
163.38(12)
98.04(13)
84.87(11)
98.68(13)
2.3660(19)
2.384(2)
a
M1−S1 /S2
M2−S2
a
M2−S2 /S1
M1−P1
M1−P2
M2−P3
M2−P4
S1−M1−S1 /S2
S1−M1−P1
S1−M1−P2
P1−M1−P2
S2−M2−S2 /S1
P3−M2−P4
2.3005(17)
2.3044(18)
2.340(2)
2.326(2)
a
81.91(6)
85.06(6)
158.47(7)
101.22(7)
83.14(8)
86.66(8)
173.78(8)
99.54(9)
a
a
Symmetry related atom.
I
DOI: 10.1021/acs.inorgchem.8b02726
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Inorganic Chemistry
Article
Table 2. Energy Parameters of Complexes Calculated Applying BP86 DFT Functional
Pd complex
Pt complex
4b-II
3a-I
3a-II
3b-II
3c-III
4a-I
4a-II
4c-III
HOMO−LUMO gap (eV)
1.39
1.42
1.43
1.73
1.65
1.68
1.68
2.04
stability (kcal/mol)
−435
−870
−872
−1199
−538
−1068
−1069
−1298
a
Table 3. Reaction between Aryl Halide and Aryl Boronic Acid
b
entry
ArBr
complex
time (h)
mol % of “Pd”
yield (%)
TON
TOF (h−1
)
c
1
2
3
4
5
6
7
8
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
2-OHCC6H4Br
4-CH3COOC6H4Br
4-O2NC6H4Br
4-FC6H4Br
4-CNC6H4Br
4-CNC6H4Br
4-OHCC6H4Br
4-OHCC6H4Br
4-OHCC6H4Br
4-CH3OC6H4Br
4-CH3OC6H4Br
4-OHCC6H4Cl
4-OHCC6H4Cl
1-C10H7Br
PdCl2
6
6
6
6
6
10
10
10
10
10
10
10
6
0.4
0.4
0.4
0.4
12
54
70
25
96
90
83
70
67
78
70
92
97
98
80
94
68
95
79
70
44
98
29
47
94
80
78
68
86
30
135
175
63
240
2250
2075
1750
1675
19500
17500
2300
243
5
23
29
11
40
225
208
175
168
1950
1750
230
41
Pd(OAc)2
PdCl2(PPh3)2
PdCl2(Xantphos)
d
d
3a
3a
3b
3c
3d
3a
3b
3a
3a
3a
3a
3b
3d
3a
3c
3d
3a
3a
3b
3a
3a
3a
3b
3d
3a
0.4
0.04
0.04
0.04
0.04
0.004
0.004
0.04
0.4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
6
6
0.4
0.4
245
200
41
33
10
10
10
10
10
6
0.04
0.04
0.04
0.04
0.04
0.4
0.4
2
2
0.4
2350
1700
2375
1975
1750
110
245
15
24
235
235
170
238
198
175
18
41
1
0.8
39
33
e
f
6
16
28
6
6
6
g
2-SC4H3Br
2-SC4H3Br
2-SC4H3Br
4-NC9H6Br
0.4
0.4
0.4
0.4
200
195
170
215
33
28
36
6
6
a
b
1
Conditions: Aryl halide (1.0 mmol), phenyl boronic acid (1.3 mmol), K2CO3 (2 mmol) in H2O, methanol (3 mL). Determined by H NMR
c
d
e
f
g
spectroscopy. CH3CN/MeOH. CHCl3/MeOH. TBAB (1 mmol) used. TBAB (5 mmol) used. TBAB (3 mmol) used.
[Pd(P∩P)(OH2)2](OTf)2 (P∩P: dppf (2.26 Å),47 dppn
((diphenylphosphino)binaphthyl, 2.24 Å)48 (Table S5). The
bite angle ∼101° agrees with the bite angles for other
complexes such as [Pd(η3-benzyl)(Xantphos)](OTf)25 (107°)
and [Pd(η3-allyl)(Xantphos)](OTf)9 (107°).
DFT Calculations. Geometry optimizations of the
structures were carried out by applying BP86 functional with
mixed atomic basis functions for the metal and other atoms.
This functional performed well for the Pd/Pt based
complexes.7 The final optimized structures of Pd and Pt
complexes resemble the available X-ray structures; the selected
bond distances and angles are summarized in Table S6. The
calculated distances of Pt−S and Pt−P bonds are ∼0.13 Å
longer than the experimental data of the complexes 4a-I, 4b-II,
4c-III, and 3c-III. The deviation in bond distance may be
attributed to the unavailability of larger basis set for the metal.
The calculated S−Pt−S and P−Pt−P angles match very closely
with the X-ray data.
The calculations of the frontier orbitals, namely, the highest
occupied molecular orbitals (HOMO) and the lowest
unoccupied molecular orbitals (LUMO) of all the complexes
are also carried out and are plotted in Figures S35 and S36. It
is observed that the HOMO is composed of orbitals mainly
from phenylene group of the aryl dithiolate ligands with some
additional contributions from S 3p, P 3p, and Pd 4d or Pt 5d
orbitals (Table S7), whereas the LUMO is formed as more
mixed orbitals spread over the system with significant
contributions from metal and phenylene orbitals. Thus, the
long wavelength band is attributed to the electronic transition,
from phenylene based HOMO to a LUMO, composed of
mixed orbitals. The calculated values of HOMO−LUMO gap
for I and II are close to each other, i.e., 1.39−1.42 eV for Pd
complexes and 1.65−1.68 eV for Pt complexes (Table 2).
Similarly, changing from phenylene to biphenylene system
there is hardly any change of energy, such as 1.42 and 1.43 eV
for 3a-II and 3b-II and 1.68 eV for 4a-II and 4b-II. This is in
agreement with the slight change in the low energy absorption
J
DOI: 10.1021/acs.inorgchem.8b02726
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Inorganic Chemistry
Article
a
Table 4. Effect of Catalyst Loading on Suzuki Reaction
b
entry
mol % of “Pd”
time (h)
% yield
TON
970
9500
91000
TOF (h−1
)
1
2
3
4
5
6
7
0.1
0.01
6
6
6
10
10
10
10
97
95
91
94
92
76
51
162
1583
15167
9400
92000
760000
5100000
0.001
0.001
0.0001
0.00001
0.000001
94000
920000
7600000
51000000
a
b
Conditions: 4-Bromoacetophenone (1.0 mmol), phenyl boronic acid (1.3 mmol), K2CO3 (2 mmol) in H2O, methanol (3 mL). Determined by
1H NMR spectroscopy.
band, i.e., 503 and 518 nm for 3a and 3b, respectively.
However, replacing the metal center Pd by Pt, the HOMO−
LUMO gap increases by 0.25 eV as calculated on going from
complex 3a to 4a or from 3b to 4b. This change qualitatively
agrees with the hypsochromic shift of the long wavelength
band, as the difference in energy of 0.31 eV of the experimental
absorption maximum calculated for 3a (503 nm, ≈2.46 eV)
and 4a (447 nm, ≈2.77 eV). It is clear from Table S7 that
there is almost no change in energy of HOMO for 3a-II and
4a-II; however, a slight destabilization is observed when their
LUMO energy levels are considered. The LUMO, composed
of large contribution from metal orbital, which interact with S
and P orbitals. In the case of Pt complexes, the 5d orbitals of
higher energy and larger size can strongly interact, hence more
destabilization, as compared to 4d orbitals in Pd complexes.
The HOMO−LUMO gap further widen up to ≈0.3 eV in the
complexes of 1,3-phenylenedithiolate. The absorption maxima,
461 nm for 3c and 373 nm for 4c, are hypsochromically
shifted, as compared to the absorption in complexes of 1,4-
phenylenedithiolate (3a or 4a) and 4,4′-biphenylenedithiolate
(3b or 4b).
Interestingly, the relative stability significantly differs from
one structure to another or Pd to Pt complexes. The stability as
well as the HOMO−LUMO energy gap follows the order III >
II > I, irrespective of metal center. Macrocyclic tetranuclear
complexes II are more stable than the “M′2S2” bridged
dinuclear complexes I, at ca. −435 kcal/mol for Pd complex 3a
and −530 kcal/mol for Pt complex 4a, whereas II of
phenylenedithiolate and biphenylenedithiolate systems are
almost equally stable in the cases of both Pd (3a, 3b) and
Pt (4a, 4b) complexes (Table 2). An extra stability gained by
the comparatively rigid structures by forming S−S bond in the
complexes 3c-III (−1199 kcal/mol) and 4c-III(−1298 kcal/
mol). Considering I and I′ have comparable stability, the large
difference in stability between I′ and III, facilitate their
transformation via the anti-to-syn conversion of I′, presuming
the syn configuration is slightly less preferable than anti
configuration. The varied stability of I, II, and III can influence
the catalytic activity of the Pd complexes.
of the unactivated 4-bromotoluene with phenylboronic acid
were chosen as the model reaction which was initially
optimized with respect to solvent, base, and temperature. As
the present Pd complexes are poorly soluble in dioxane which
is also carcinogenic in nature,49 the other solvents such as
DMA, DMF, and methanol were used for screening.
Surprisingly, the refluxing condition in methanol at 78 °C
gave better results than in DMA and DMF at higher
temperatures, 95 and 104 °C, respectively, which is attributed
to methanol being a more efficient reductant, as compared to
DMA or DMF. There might be better coordination in case of
methanol to the palladium center in the transition state.50
When using methanol as solvent in Suzuki reactions, activities
increased dramatically for palladacycles containing thioether
group.51 The inorganic bases K2CO3, Cs2CO3, KOH, and
Na2CO3 afforded higher yields than the organic bases
Bu4NOH, Et3N. K2CO3 was chosen as base for the
investigations. As expected, the yield of the product increased
with the temperature. The variation of time showed 93% of the
coupling product within 2 h of reaction time, which leveled off
at 6 h. Under the optimized reaction condition, the traditional
Pd salts showed lower catalytic activity (Table 3, entries 1−4)
in comparison with those of the present complexes.
The effect of varying the arylbromides was investigated
under the optimized reaction condition with phenylboronic
acid, and results are presented in Table 3. The activated
(entries 12−20) and nonactivated (entries 5−7) arylbromides
resulted in very good quantitative yields (80−98%) of the
biaryls by using 0.4−0.04 mol % Pd. A variety of functional
groups such as aldehyde, acetyl, nitro, cyano, and so on are
tolerated. The deactivated aryl bromide, 4-bromoanisole gave a
somewhat lower conversion; however, with the addition of 1
equiv of TBAB, the yield became excellent (entries 21−22).
We also investigated the electronically activated aryl chloride;
the best yield 47% was obtained for the coupling of 4-
chlorobenzaldehyde, in the presence of 3 mol % TBAB (entry
24). Moreover, the reaction with heteroaryl bromides and
naphthyl bromide also led to the formation of the desired
products in moderate to excellent yields (68−94%, entries 25−
29).
Suzuki Cross-Coupling Reactions. The catalytic appli-
cations of the palladium complexes were investigated in the
Suzuki cross-coupling reactions of aromatic substrates. There
are only few reports, employing Pd(II) thiolates as catalysts in
Suzuki or C−C cross-coupling reactions, and to our knowl-
edge, no reports are available using the Pd complexes of
Xantphos and thiolate ligands. The catalytic activities of Pd
complexes 3a, 3b, 3c, and 3d were evaluated in the coupling
reactions of phenylboronic acid and aryl halides. The reactions
The performance of low catalyst loading was tested in the
coupling reactions of both activated and nonactivated
arylbromides. For 4-bromoacetophenone, although the yields
are gradually decreased from 97 to 51%, with lowering of
concentration of catalyst 3b from 0.1 to 0.000001 mol % of Pd,
the resulted turnover numbers (TON) become very high up to
5 × 107 (Table 4). A similar trend was found in the case of 4-
bromotoluene: The yields 78 and 70% were obtained when at
K
DOI: 10.1021/acs.inorgchem.8b02726
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 5. Reusability Experiment
cycle
% yield
0
99
1
98
2
97
3
96
4
94
5
93
6
91
7
89
8
85
b
a
Conditions: 4-Bromoacetophenone (1.0 mmol), phenyl boronic acid (1.3 mmol), K2CO3 (2 mmol) in H2O, methanol (3 mL), 0.4 mol % of Pd
b
1
used. Determined by H NMR spectroscopy.
loadings of Pd of 3a and 3b were decreased to 0.004 mol %,
and the corresponding TONs are 2 × 104, indicating both the
catalysts are active (Table 3). Moreover, the feasibility of the
repeated use of the Pd catalyst was examined. In Table 5, the
results of the coupling reaction of 4-bromoacetophenone and
phenylboronic acid by 3b for eight consecutive cycles are
summarized. After completion of each round, hexane was
added to extract the organic part having the product; then, to
the remaining aqueous layer containing the catalyst, the
required amounts of reactants were freshly added for the next
round of reaction. The yields were remained very high, from 99
to 85%, even after 8 successive cycles, which suggests the
catalyst is still very active.
All the Pd complexes show good catalytic activities under
mild conditions as revealed in Table 3. However, the fine
comparison of the yields (entries 6−9; 18−20; and 26−28)
shows that the catalytic activities follow the order 3a > 3b > 3c
> 3d. The trend may be explained based on the relative
stability of the structures with the order III > II > I. The DFT
calculations show the reasonable differences among them as
calculated between 3b-II and 3a-I is 437 kcal/mol and for 3c-
III and 3b-II is 327 kcal/mol (Table 2). The proportions of I
and II in phenylene-1,4-dithiolate complex 3a are almost equal
in solution as indicated by 31P NMR spectrum, whereas in the
case of 4,4′-biphenyldithiolate complex 3b, only complex II
exclusively exists in solution. Thus, the contribution of
comparatively less stable I makes complex 3a more catalytically
active or vice versa for 3c. A similar phenomena was observed
in the case of dppe analogue.7 The catalytic activities of the
present complexes are comparable with the reported Pd
supramolecular and macrocyclic complexes under mild
conditions and low loading (Table S8).
To evaluate the effect of varied bite angles of bis-phosphine
on the catalytic activity, dppe and dppf were chosen for
comparison with Xantphos. The ligand 4,4′-biphenylenedi-
thiolate was selected as it can form tetranuclear complex-
[Pd2(P∩P)2(SC12H8S)]2(OTf)4 as single product in solution.
In the present reaction condition, i.e., reflux in methanol,
among the complexes of three phosphines, Xantphos derivative
gave the highest yield (Table S9). The Pd complexes of
Xantphos, dppf, and dppe, encompassing bite angles of 104.6,
99.6, and 84.7°, respectively, yielded 70, 51, and 42% of
product using 0.004 mol % of Pd as catalyst. The high activity
of Xantphos complex points to its flexible coordination
behavior, which is absent in dppe and dppf, apart from the
large bite angle.35
Though it is difficult to identify the active catalyst,
preliminary investigations were carried out to learn the nature
of the Pd complex after the catalysis reaction. The UV−vis
spectrum of the compound separated after the zero cycle of
catalysis reaction of 3b showed a long wavelength absorption
band at 408 nm (Figure S31), whereas a similar band observed
at 518 nm for 3b. The ESI mass spectrum of the same brown
colored compound displayed the prominent peaks at m/z
1395.2 and 1551.2 corresponding to [Pd(Xantphos)-
(SC12H8S)(SC6H4)(OTf)2 + 4CH3CN − C6H5]+ and
[Pd2(Xantphos)2(S(C6H4)S) + CH3CN + 2H]+. These results
and the poor solubility suggest the polymeric nature of the
compound, which is composed of dithiolate and Xantphos
ligated ionic species of Pd , formed after catalysis reaction.
Interestingly, the powder X-ray diffraction (PXRD) pattern of
the similar compound obtained after six catalysis cycles (Figure
S37c), showed two broad peaks at 2θ ∼ 40.1 and 44.6°, which
may be attributed to the Pd-nanoparticles (PdNPs) (Powder
Diffraction File no. 46−1043, Joint Committee on Powder
Diffraction Standards, 1993), whereas the largest peak at 2θ ∼
22.2° is possibly due to the bulk species present in the
compound. The size of the PdNPs calculated using the
Debye−Scherrer formula is ∼3 nm. The PXRD pattern of the
similar compound obtained after zero cycles also showed the
highly broadened peak at 2θ ∼ 40.1° (Figure S37b). Perhaps
the reducing atmosphere by the presence of solvent methanol,
phosphine, and thiol ligands favor the formation of PdNPs.52
A
large number of Pd-containing metallocycles showed excellent
catalytic efficiency in C−C coupling reactions, which have
been established now due to the formation of active
intermediate PdNPs as catalyst.53,54 Others have reported
that Pd clusters may also be generated with PdNPs, including
the Pd complex, can interconvert to make the active catalytic
system for the successful coupling of wide range of substrates
as presently observed.55,56 However, more investigations are
required to establish the true nature of present catalyst.
CONCLUSIONS
■
In summary, we have successfully isolated Xantphos-blocked
Pd(II) and Pt(II) complexes with bridging dithiol ligands by
self-assembly. The reactions with 1,4-benezenedithiol or 4,4′-
biphenyldithiol resulted in dinuclear complex I and tetranu-
clear complex II. The equilibrium between I and II has been
established in platinum complexes of 4,4′-biphenyldithiolate,
which lies at the side of II. In striking contrast, when we
previously used the smaller bite angle dppe or triethyl
phosphines as capping ligand, the analogous complexes were
exclusively isolated as II. Steric crowding coupled with flexible
coordination around the metal center is the primary reason to
adopt such structures. By varying the positions of two thiol
groups, employing 1,3-benezenedithiol, the same reactions lead
to dinuclear complex III, containing a S−S bond between two
thiolate ligands. The steric reason does not allow to form or
stabilize to tetranuclear complex II′ of 1,3-benezenedithiolate.
Moreover, the palladium complexes show excellent activity
in Suzuki C−C cross coupling reactions under mild conditions.
The catalytic systems can couple a wide range of aryl halides,
giving high yields and TON and can be reused 8 times without
losing significant yield. The HOMO−LUMO gap and stability
increase in the order III > II > I, which reflects in their
comparative catalytic activity. We have demonstrated the
comparative catalytic activity of tetranuclear Pd complexes II
L
DOI: 10.1021/acs.inorgchem.8b02726
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Inorganic Chemistry
Article
(2) Wiester, M. J.; Ulmann, P. A.; Mirkin, C. A. Enzyme Mimics
based upon Supramolecular Coordination Chemistry. Angew. Chem.,
Int. Ed. 2011, 50, 114−137.
of bis-phosphines of varied bite angles, Xantphos, dppf, and
dppe. The PdNPs with dithiolate and Xantphos ligated Pd, as
poorly soluble bulk compound has been identified, after the
catalysis reactions of the palladium complexes. Overall, the
present Pd complexes showed improved catalytic activity than
the dppe analogues. The positive charge and stronger metal−
sulfur bond of the present type of complexes can make them
capable as potential functional materials for other solution-
based applications by modification of suitable capping ligand.
(3) Ghosh, S.; Mukherjee, P. S. Self-Assembly of a Nanoscopic
Prism via a New Organometallic Pt3 Acceptor and Its Fluorescent
Detection of Nitroaromatics. Organometallics 2008, 27, 316−319.
(4) Cook, T. R.; Zheng, Y. R.; Stang, P. J. Metal−Organic
Frameworks and Self-Assembled Supramolecular Coordination
Complexes: Comparing and Contrasting the Design, Synthesis, and
Functionality of Metal−Organic Materials. Chem. Rev. 2013, 113,
734−777.
(5) Hastings, C. J.; Pluth, M. D.; Bergman, R. G.; Raymond, K. N.
Enzymelike Catalysis of the Nazarov Cyclization by Supramolecular
Encapsulation. J. Am. Chem. Soc. 2010, 132, 6938−6940.
(6) Murase, T.; Horiuchi, S.; Fujita, M. Naphthalene Diels−Alder in
a Self-Assembled Molecular Flask. J. Am. Chem. Soc. 2010, 132,
2866−2867.
(7) Vivekananda, K. V.; Dey, S.; Maity, D. K.; Bhuvanesh, N.; Jain,
V. K. Supramolecular Macrocyclic Pd(II) and Pt(II) Squares and
Rectangles with Aryldithiolate Ligands and their Excellent Catalytic
Activity in Suzuki C−C Coupling Reaction. Inorg. Chem. 2015, 54,
10153−10162.
(8) Chand, D. K.; Biradha, K.; Kawano, M.; Sakamoto, S.;
Yamaguchi, K.; Fujita, M. Dynamic Self-Assembly of an M3L6
Molecular Triangle and an M4L8 Tetrahedron from Naked PdII
Ions and Bis(3-pyridyl)-Substituted Arenes. Chem. - Asian J. 2006,
1−2, 82−90.
(9) Tabellion, F. M.; Seidel, S.; Arif, A. M.; Stang, P. J. Discrete
Supramolecular Architecture vs Crystal Engineering: The Rational
Design of a Platinum-Based Bimetallic Assembly with a Chairlike
Structure and Its Infinite, Copper Analogue. J. Am. Chem. Soc. 2001,
123, 7740−7741.
(10) Dey, S.; Vivekananda, K. V.; Bhuvanesh, N. Supramolecular Pt
and Pd Complexes of 4,4’-Dipyridylditelluride/diselenide Ligands via
Self-Assembly. Eur. J. Inorg. Chem. 2018, 2018, 3579−3586.
(11) Sun, Y.; Li, S.; Zhou, Z.; Saha, M. L.; Datta, S.; Zhang, M.; Yan,
X.; Tian, D.; Wang, H.; Wang, L.; Li, X.; Liu, M.; Li, H.; Stang, P. J.
Alanine-Based Chiral Metallogels via Supramolecular Coordination
Complex Platforms: Metallogelation Induced Chirality Transfer. J.
Am. Chem. Soc. 2018, 140, 3257−3263.
(12) Kieltyka, R.; Englebienne, P.; Fakhoury, J.; Autexier, C.;
Moitessier, N.; Sleiman, H. F. A Platinum Supramolecular Square as
an Effective G-Quadruplex Binder and Telomerase Inhibitor. J. Am.
Chem. Soc. 2008, 130, 10040−10041.
(13) Grishagin, I. V.; Pollock, J. B.; Kushal, S.; Cook, T. R.; Stang, P.
J.; Olenyuk, B. Z. In vivo anticancer activity of rhomboidal Pt(II)
metallacycles. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 18448−18453.
(14) Samanta, D.; Mukherjee, S.; Patil, Y. P.; Mukherjee, P. S. Self-
Assembled Pd6 Open Cage with Triimidazole Walls and the Use of Its
Confined Nanospace for Catalytic Knoevenagel- and Diels−Alder
Reactions in Aqueous Medium. Chem. - Eur. J. 2012, 18, 12322−
12329.
(15) Zhao, C.-W.; Ma, J.-P.; Liu, Q.-K.; Yu, Y.; Wang, P.; Li, Y.-A.;
Wang, K.; Dong, Y.-B. A self-assembled Pd6L8 nanoball for Suzuki−
Miyaura coupling reactions in both homogeneous and heterogeneous
formats. Green Chem. 2013, 15, 3150−3154.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publication Web site at The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.inorgchem.8b02726.
Details of general procedures of the experiments,
crystallographic and structure refinement data tables of
all complexes, tables of calculated and experimental ion
peaks of the complexes, bond distances and angles of 2a,
percentage composition of frontier molecular orbitals of
3a-II and 4a-II, effect of bite angle of phosphine on
catalysis of [Pd2(P∩P)2(SC12H8S)]2(OTf)4, 1H and
31P{1H} NMR spectra of all the complexes, molecular
structure of 2a, 19F{1H} NMR spectrum of 3b, VT H
1
NMR spectrum of 4b, ESI-mass spectra of 3a, 3b, and
4a, UV−vis spectra of 3b, 4b, and compound after
catalytic reactions of 3b, thermogravimetric curve of 3b,
X-ray structures of 2a and 4c-III, plots of HOMO and
LUMO of 3a-II and 4a-II, PXRD pattern of 3b before
and after catalysis reaction, H NMR spectra of all the
biaryls as coupling products (PDF)
1
Accession Codes
CCDC 1865095−1865100 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: 91 22 25592589. E-mail: dsandip@barc.gov.in.
ORCID
Sandip Dey: 0000-0001-9198-8808
Arup Kumar Pathak: 0000-0002-5857-0132
Notes
The authors declare no competing financial interest.
(16) Vivekananda, K. V.; Dey, S.; Wadawale, A.; Bhuvanesh, N.; Jain,
V. K. Supramolecular 3-/4-Mercaptobenzoic Acid Complexes of
Palladium(II) and Platinum(II) Stabilized by Hydrogen Bonding. Eur.
J. Inorg. Chem. 2014, 2014, 2153−2161.
(17) Mane, P. A.; Dey, S.; Vivekananda, K. V. Macrocyclic Pd(II)
dithiolate complexes as catalysts in Heck reactions. Tetrahedron Lett.
2017, 58, 25−29.
(18) Fujita, M.; Sasaki, O.; Mitsuhashi, T.; Fujita, T.; Yazaki, J.;
Yamaguchi, K.; Ogura, K. On the structure of transition-metal-linked
molecular squares. Chem. Commun. 1996, 1535−1536.
(19) Zhang, L.; Niu, Y.-H.; Jen, A. K.-Y.; Lin, W. A Highly
Electroluminescent Molecular Square. Chem. Commun. 2005, 1002−
1004.
ACKNOWLEDGMENTS
■
P.M. is thankful to the Department of Atomic Energy-Mumbai
University Collaborative Scheme for providing senior research
fellowship. We are grateful to the staff of Fuel Chemistry
Division of BARC for providing the mass spectra.
REFERENCES
■
(1) Cook, T. R.; Vajpayee, V.; Lee, M. H.; Stang, P. J.; Chi, K.-W.
Biomedical and Biochemical Applications of Self-Assembled Metal-
lacycles and Metallacages. Acc. Chem. Res. 2013, 46, 2464−2474.
M
DOI: 10.1021/acs.inorgchem.8b02726
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(20) Liron, F.; Fosse, C.; Pernolet, A.; Roulland, E. Suzuki−Miyaura
Cross-Coupling of 1,1-Dichloro-1-alkenes with 9-Alkyl-9-BBN. J. Org.
Chem. 2007, 72, 2220−2223.
(21) Van Haaren, R. J.; Goubitz, K.; Fraanje, J.; Van Strijdonck, G.
P. F.; Oevering, H.; Coussens, B.; Reek, J. N. H.; Kamer, P. C. J.; Van
Leeuwen, P. W. N. M. X-ray Study of the Effect of the Bite Angle of
Chelating Ligands on the Geometry of Palladium(allyl) Complexes:
Implications for the Regioselectivity in the Allylic Alkylation. Inorg.
Chem. 2001, 40, 3363−3372.
(22) Wagaw, S.; Yang, B. H.; Buchwald, S. L. A Palladium-Catalyzed
Method for the Preparation of Indoles via the Fischer Indole
Synthesis. J. Am. Chem. Soc. 1999, 121, 10251−10263.
(23) Gao, G.-Y.; Colvin, A. J.; Chen, Y.; Zhang, X. P. Versatile
Synthesis of meso-Aryloxy- and Alkoxy-Substituted Porphyrins via
Palladium-Catalyzed C−O Cross-Coupling Reactions. Org. Lett.
2003, 5, 3261−3264.
(24) Van der Veen, L. A.; Keeven, P. H.; Schoemaker, G. C.; Reek, J.
N. H.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Lutz, M.; Spek, A.
L. Origin of the Bite Angle Effect on Rhodium Diphosphine
Catalyzed Hydroformylation. Organometallics 2000, 19, 872−883.
(25) Johns, A. M.; Utsunomiya, M.; Incarvito, C. D.; Hartwig, J. F. A
Highly Active Palladium Catalyst for Intermolecular Hydroamination.
Factors that Control Reactivity and Additions of Functionalized
Anilines to Dienes and Vinylarene. J. Am. Chem. Soc. 2006, 128,
1828−1839.
(26) Yu, H.; Zhang, G.; Huang, H. Palladium-Catalyzed
Dearomative Cyclocarbonylation by C−N Bond Activation. Angew.
Chem., Int. Ed. 2015, 54, 10912−10916.
(27) Dierkes, P.; van Leeuwen, P. W. N. M. The bite angle makes
the difference: a practical ligand parameter for diphosphine ligands. J.
Chem. Soc., Dalton Trans. 1999, 1519−1529.
(28) Gun, K.-S.; Hor, T. S. A. 1.1′-Bis(diphenylphosphino)ferrocene
- Coordination Chemistry, Organic Syntheses and Catalysis. In
Ferrocenes: Homogeneous Catalysis, Organic Synthesis, Materials Science,
Togni, A., Hayashi, T., Ed.; Wiley-VCH: New York, 1995; Chapter 1,
p 31.
(29) Becke, A. D. Density-functional exchange-energy approxima-
tion with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt.
Phys. 1988, 38, 3098−3100.
(30) Perdew, J. P. Density-functional approximation for the
correlation energy of the inhomogeneous electron gas. Phys. Rev. B:
Condens. Matter Mater. Phys. 1986, 33, 8822−8824.
(31) Su, P.; Li, H. Energy decomposition analysis of covalent bonds
and intermolecular interactions. J. Chem. Phys. 2009, 131, 014102.
(32) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K.
A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General
atomic and molecular electronic structure system. J. Comput. Chem.
1993, 14, 1347−1363.
(33) Schaftenaar, G.; Noordik, J. H. Molden: a pre- and post-
processing program for molecular and electronic structures. J.
Comput.-Aided Mol. Des. 2000, 14, 123−134.
(34) Portmann, S.; Luthi, H. MOLEKEL: An Interactive Molecular
Graphics Tool. Chimia 2000, 54, 766−770.
(38) Tang, R. P-L.; Wong, K. M-C.; Zhu, N.; Yam, V.W.-W.
Luminescence, electrochemistry and host−guest properties of
dinuclear platinum(II) terpyridyl complexes of sulfur-containing
bridging ligands. Dalton Trans. 2009, 3911−3922.
(39) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.
K.; Puschmann, H. OLEX2A Complete Structure Solution, Refine-
ment and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341.
(40) Ehlich, H.; Schier, A.; Schmidbaur, H. Aurophilicity-Based
One-Dimensional Arrays of Gold(I) Phenylene-1,3- and −1,4-
dithiolates. Inorg. Chem. 2002, 41, 3721−3727.
(41) Briant, C. E.; Calhorda, M. J.; Hor, T. S. A.; Howells, N. D.;
Mingos, D. M. P. Chemistry of platinum sulphido-complexes. Part 3.
Crystal and molecular structure of tetrasulphido[1,2-bis-
(diphenylphosphino)ethane]platinum(II) and a study of its bonding
and reactions. J. Chem. Soc., Dalton Trans. 1983, 1325−1330.
(42) Rivera, G.; Bernes, S.; Torrens, H. Mono- and bimetallic
platinum(II) and palladium(II) complexes containing fluorinated
benzothiolates. Polyhedron 2007, 26, 4276−4286.
(43) Smith, V. C. M.; Aplin, R. T.; Brown, J. M.; Hursthouse, M. B.;
Karalulov, A. I.; Malik, K. M. A.; Cooley, N. A. Scope of the C-S
Insertion Reaction of Thiazolium Salts with Platinum(0) Diphosphine
Complexe. J. Am. Chem. Soc. 1994, 116, 5180−5189.
̈
(44) Niksch, T.; Gorls, H.; Friedrich, M.; Oilunkaniemi, R.; Laitinen,
R.; Weigand, W. The Extension of the Solid-Angle Concept to
Bidentate Ligands. Eur. J. Inorg. Chem. 2010, 2010, 74−94.
(45) Ohshima, T.; Miyamoto, Y.; Ipposhi, J.; Nakahara, Y.;
Utsunomiya, M.; Mashima, K. Platinum-Catalyzed Direct Amination
of Allylic Alcohols under Mild Conditions: Ligand and Microwave
Effects, Substrate Scope, and Mechanistic Study. J. Am. Chem. Soc.
2009, 131, 14317−14328.
(46) Kranenburg, M.; Delis, J. G. P.; Kamer, P. C. J.; van Leeuwen,
P. W. N. M.; Vrieze, K.; Veldman, N.; Spek, A. L.; Goubitz, K.;
Fraanje, J. Palladium(0)−tetracyanoethylene complexes of diphos-
phines and a dipyridine with large bite angles, and their crystal
structures. J. Chem. Soc., Dalton Trans. 1997, 1839−1849.
(47) Ferrer, M.; Mounir, M.; Rossell, O.; Ruiz, E.; Maestro, M. A.
Equilibria between Metallosupramolecular Squares and Triangles with
the New Rigid Linker 1,4-Bis(4-pyridyl)tetrafluorobenzene. Exper-
imental and Theoretical Study of the Structural Dependence of NMR
Data. Inorg. Chem. 2003, 42, 5890.
(48) Phua, P. H.; White, J. P.; de Vries, J. G.; Hii, K. K. Enabling
Ligand Screening for Palladium-Catalysed Enantioselective Aza-
Michael Addition Reactions. Adv. Synth. Catal. 2006, 348, 587.
(49) Magano, J.; Dunetz, J. R. Large-Scale Applications of Transition
Metal-Catalyzed Couplings for the Synthesis of Pharmaceuticals.
Chem. Rev. 2011, 111, 2177−2250.
(50) Proutiere, F.; Schoenebeck, F. Solvent Effect on Palladium
Catalyzed Cross Coupling Reactions and Implications on the Active
Catalytic Species. Angew. Chem., Int. Ed. 2011, 50, 8192−8195.
(51) Wang, W.-C.; Peng, K.-F.; Chen, M. − T.; Chen, C.−T.
Palladacycles bearing tridentate CNS-type benzamidinate ligands as
catalysts for cross-coupling reactions. Dalton Trans. 2012, 41, 3022−
3029.
(52) Lu, C. -H; Chang, F.-C. Polyhedral Oligomeric Silsesquioxane-
Encapsulating Amorphous Palladium Nanoclusters as Catalysts for
Heck Reactions. ACS Catal. 2011, 1, 481−488.
(35) Martinelli, J. R.; Watson, D. A.; Freckmann, D. M. M.; Barder,
T. E.; Buchwald, S. L. Palladium-Catalyzed Carbonylation Reactions
of Aryl Bromides at Atmospheric Pressure: A General System Based
on Xantphos. J. Org. Chem. 2008, 73, 7102−7107.
(36) Ji, Y.; Plata, R. E.; Regens, C. S.; Hay, M.; Schmidt, M.; Razler,
T.; Qiu, Y.; Geng, P.; Hsiao, Y.; Rosner, T.; Eastgate, M. D.;
Blackmond, D. G. Mono-Oxidation of Bidentate Bis-phosphines in
Catalyst Activation: Kinetic and Mechanistic Studies of a Pd/
Xantphos-Catalyzed C−H Functionalization. J. Am. Chem. Soc. 2015,
137, 13272−13281.
(53) Louie, J.; Hartwig, J. F. A Route to Pd° from PdII Metallacycles
in Animation and Cross Coupling Chemistry. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2359−2361.
(54) Astruc, D. Palladium Nanoparticles as Efficient Green
Homogeneous and Heterogeneous Carbon−Carbon Coupling
Precatalysts: A Unifying View. Inorg. Chem. 2007, 46, 1884−1894.
(55) Ananikov, V. P.; Beletskaya, I. P. Toward the Ideal Catalyst:
From Atomic Centers to a “Cocktail” of Catalysts. Organometallics
2012, 31, 1595−1604.
(56) Eremin, D. B.; Ananikov, V. P. Understanding active species in
catalytic transformations: From molecular catalysis to nanoparticles,
leaching, “Cocktails” of catalysts and dynamic systems. Coord. Chem.
Rev. 2017, 346, 2−19.
́
(37) Capdevila, M.; Clegg, W.; Gonzalez-Duarte, P.; Jarid, A.;
́
Lledos, A. Hinge Distortion in Platinum(II) Dimers with a Pt2S2 Ring.
An ab Initio Molecular Orbital Study. Inorg. Chem. 1996, 35, 490−
497.
N
DOI: 10.1021/acs.inorgchem.8b02726
Inorg. Chem. XXXX, XXX, XXX−XXX