Palladium(ii) and palladium(ii)-silver(i) complexes with N-heterocyclic carbene and zwitterionic thiolate mixed ligands: synthesis, structural characterization and catalytic properties

Article abstract of DOI:10.1039/c6dt04599e

The integration of a geometrically rigid Pd(ii), a coordinatively monotonous N-heterocyclic carbene 1,3-dimethylimidazoline-2-ylidene (IMe), and a flexible zwitterionic thiolate 4-(trimethylammonio)benzenethiolate (Tab) affords a class of Pd-IMe-Tab complexes with various nuclearities, namely, trans-[Pd(IMe)₂(Tab)₂](OTf)₂ (2, mononuclear), cis-[Pd(IMe)₂(Tab)₂](OTf)(Cl) (3a, mononuclear), cis-[Pd(IMe)₂(Tab)₂](PF₆)₂·MeCN (3b·MeCN, mononuclear), [Pd₂(IMe)₄(Tab)₂](PF₆)₄·2MeCN (4·2MeCN, dinuclear) and [Pd₄(IMe)₄(Tab)₆](OTf)₆(Cl)₂ (5, tetranuclear). Further presence of Ag(i) in the assembly provides a heterometallic octanuclear cluster of [Pd₄Ag₄(IMe)₈(Tab)₁₀](PF₆)₁₂ (6). Compounds 2-6 are formed by the reaction of trans-Pd(IMe)₂Cl₂ (1) with various additional reagents via different reaction pathways. These compounds are characterized by means of FT-IR, ¹H and ¹³C NMR, ESI-MS, elemental analysis and X-ray crystallography. Notably, the skeleton of compound 5 features a [Pd₄S₄] parallelogram wherein each of the four Pd(ii) centers bisects the edge defined by the S atoms. The main skeleton of compound 6 is an oval-shaped Pd₄Ag₄S₁₀ unit, featuring an edge-fused norbornane-like (Pd₂Ag₄S₆) framework appended by two additional PdS₂ motifs at the polar positions. Compounds 5 and 6 also feature Pd?Pd (5), Pd?Ag and Ag?Ag (6) interactions. Compound 5 as a representative example is highly effective at catalyzing Suzuki-Miyaura couplings in water, highlighting the potential of applying these types of homo- and heterometallic clusters as catalysts for organic transformations in environmentally benign media.

Full text of DOI:10.1039/c6dt04599e

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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DOI: 10.1039/C6DT04599E
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ARTICLE TYPE
Palladium(II) and palladium(II)ꢀsilver(I) complexes with Nꢀheterocyclic
carbene and zwitterionic thiolate mixed ligands: synthesis, structural
characterization and catalytic properties†
5
YuꢀTing Wang,a BinꢀBin Gao,a Fan Wang,a ShiꢀYuan Liu,a Hong Yu,*a WenꢀHua Zhang*a and JianꢀPing
Lang*ab
Received (in XXX, XXX) Xth XXXXXXXXX 201X, Accepted Xth XXXXXXXXX 201X
DOI: 10.1039/b000000x
Integration of geometrically rigid Pd(II), a coordinatively monotonous Nꢀheterocyclic carbene1,3ꢀ
10 dimethylimidazolineꢀ2ꢀylidene
(IMe),
and
a
flexible
zwitterionic
thiolate
4ꢀ
(trimethylammonio)benzenethiolate (Tab) affords a class of PdꢀIMeꢀTab complexes with various
nuclearities, namely, transꢀ[Pd(IMe)2(Tab)2](OTf)2 (2, mononuclear), cisꢀ[Pd(IMe)2(Tab)2](OTf)(Cl) (3a,
mononuclear),
cisꢀ[Pd(IMe)2(Tab)2](PF6)2·MeCN
(3b·MeCN,
mononuclear),
[Pd2(IMe)4(Tab)2](PF6)4·2MeCN (4·2MeCN, dinuclear) and [Pd4(IMe)4(Tab)6](OTf)6(Cl)2 (5,
15 tetranuclear). Further presence of Ag(I) in the assembly provides a heterometallic octanuclear cluster of
[Pd4Ag4(IMe)8(Tab)10](PF6)12 (6). Compounds 2−6 are formed by the reaction of transꢀPd(IMe)2Cl2 (1)
with various additional reagents via different reaction pathways. These compounds are characterized by
1
means of FTꢀIR, H and 13C NMR, ESIꢀMS, elemental analysis and Xꢀray crystallography. Notably, the
skeleton of compound 5 features a [Pd4S4] parallelogram wherein each of the four Pd(II) centers bisects
20 the edge defined by the S atoms. The main skeleton of compound 6 is an ovalꢀshaped Pd4Ag4S10 unit,
featuring an edgeꢀfused norbornaneꢀlike (Pd2Ag4S6) framework appended by two additional PdS2 motifs
at the polar positions. Compounds 5 and 6 also feature PdꢁꢁꢁPd (5), PdꢁꢁꢁAg and AgꢁꢁꢁAg (6) interactions.
Compound 5 as a representative example is highly effective in catalyzing SuzukiꢀMiyaura coupling in
water, highlighting the potential of applying this type of homoꢀ and heterometallic clusters as catalysts for
25 organic transformations in environmentally benign media.
in polar solvents such as H2O. In recent years, we have also
demonstrated that Tab is capable of supporting diverse clusters,
such as
[Ag14(ꢀ6ꢀS)(Tab)12(PPh3)8](PF6)12,7
[Bi5(Tab)18](PF6)7(NO3)8]·6MeCN,6b
and [M4(ꢀꢀ
50 Tab)6(Tab)4](PF6)8·S (M = Zn, S = DMF·4H2O; M = Cd, S =
DMF·5H2O).8 In the present study, by using transꢀPd(IMe)2Cl2
(1) as the precursor complex, its complexation with Tab in the
absence/presence of additional Ag(I) source resulted in a diverse
class of homoꢀ and heteroꢀnuclear molecular entities, including
55 mononuclear transꢀ[Pd(IMe)2(Tab)2](OTf)2 (2), dinuclear
Introduction
Transition metal NHC complexes have gained intense interest in
the last few decades as a superior class of catalysts for organic
transformations,1 as well as other applications in materials
30 science and pharmaceutical industry.2 These hot trends arise from
the facile preparation and derivation of the NHC precursors,
either on the skeletons or the pendant arms to tune their donation
strength or stereochemistry.3 Functional groups such as ꢀOR, ꢀNR,
ꢀPR and ꢀSR on the pendant arms serve to elevate structural
35 complexity to form diꢀ, triꢀand tetraꢀnuclear motifs.4 Further
structural complexity is however hindered by the preferable C, X
(X = O, N, P and S) chelation.5
[Pd2(IMe)4(Tab)2](PF6)4·2MeCN
[Pd4(IMe)4(Tab)6](OTf)6(Cl)2
(4·2MeCN),
(5) and
tetranuclear
octanuclear
[Pd4Ag4(IMe)8(Tab)10](PF6)12 (6). In addition, we also isolated
cisꢀ[Pd(IMe)2(Tab)2](OTf)(Cl) (3a) and cisꢀ
We herein report that, by the assembly of Pd(II) or Pd(II)/Ag(I)
with two separate functionalities, viz. a simple NHC (IMe) and a
40 zwitterionic thiolate ligand (Tab,6 Chart 1), we can readily
achieve increased structural diversity and complexity by taking
advantage of the flexible coordination patterns of S atom of the
Tab ligand. Tab is a nonꢀordous zwitterionic thiolate ligand
bearing a quaternary ammonium cationic center. The polar nature
45 of such a ligand endows stability of the coordination compounds
60 [Pd(IMe)2(Tab)2](PF6)2·MeCN (3b·MeCN) as byꢀproducts during
the isolation of 2 and 4·2MeCN. The Pd4 cluster of 5 as a
representative example is efficient in promoting SuzukiꢀMiyaura
coupling in water, highlighting the potential of using this class of
Sꢀbased
compounds for organic transformations in
65 environmentally benign solvents.
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DOI: 10.1039/C6DT04599E
55 3.87, N 9.27; found: C 30.23, H 4.07, N 9.26. IR (KBr disk):
3440 (s), 2985 (w), 2052 (w), 1692 (m), 1489 (w), 1384 (m),
1226 (w), 1128 (m), 1005 (w), 843 (s), 784 (w), 749 (w), 617 (w),
1
558 (m) cm−1. H NMR (400 MHz, DMSOꢀd6, ppm): δ 7.60 (dd,
J = 21.8, 8.8 Hz, 8H, CH in Tab), 7.26 (s, 8H, CH in IMe), 3.85
60 (s, 24H, CH3 in IMe), 3.51 (s, 18H, CH3 in Tab). 13C NMR (151
MHz, DMSOꢀd6, ppm): δ 158.57 (s, Ccarbene in IMe), 145.40 (s, Cꢀ
N in Ph of Tab), 138.08 (s, CH in Tab), 133.67 (s, CꢀS of Tab),
124.03 (s, CH in IMe), 120.31 (s, CH in Tab), 56.35 (s, CH3 in
Tab), 37.75 (s, CH3 in IMe).
Experimental
General procedures. TabHPF6 and transꢀPd(IMe)2Cl2 (1)10
were obtained through the literature methods. Tab was obtained
from the reaction of TabHPF6 with Et3N in MeCN followed by
filtration and dried in vacuo. All the solvents were preꢀdried over
activated molecular sieves. Other chemicals and reagents were
obtained from commercial sources and used as received.
Elemental analyses for C, H and N were performed on a Carloꢁ
9
5
65
Preparation of [Pd4(IMe)4(Tab)6](OTf)6(Cl)2 (5). Compound
5 was prepared as clear light yellow plate crystals in a manner
similar to that described for 2, except that the amount of Tab was
decreased to 0.15 mmol. Yield: 36 mg (52% based on Pd). Anal.
Calcd. for C80H110N14O18F18S12Cl2Pd4 (+5H2O): C 33.49, H 4.22,
10 Erba CHNOꢁS microanalyzer. IR spectra were recorded on a
Varian 1000 FTꢀIR spectrometer as KBr disks (4000−400 cm−1).
1H NMR and 13C NMR spectra were recorded at ambient
70 N 6.83; found: C 33.53, H 4.85, N 7.44. IR (KBr disk): 3441 (s),
3108 (w), 3028 (w), 2949 (w), 2081 (w), 1632 (m), 1490 (s),
1404 (w), 1384 (m), 1346 (w), 1278 (m), 1225 (m), 1162 (m),
1124 (w), 1085 (w), 1063 (w), 1031 (s), 1009 (w), 958 (w), 938
(w), 848 (m), 747 (m), 717 (w), 672 (m), 640 (s), 576 (w), 558
75 (w), 518 (m) cm−1.1H NMR (400 MHz, D2O, ppm): δ 8.74 (d, J =
8.8 Hz, 4H, CH in Tab), 8.08 (d, J = 8.8 Hz, 4H, CH in Tab),
7.75 (d, J = 8.9 Hz, 4H, CH in Tab), 7.58 (d, J = 8.9 Hz, 4H, CH
in Tab), 7.35 (d, J = 9.0 Hz, 4H, CH in Tab), 7.20 (d, J = 8.9 Hz,
4H, CH in Tab), 6.74 (d, J = 4.7 Hz, 8H, CH in IMe), 3.77 (d, J =
80 8.0 Hz, 36H, CH3 in Tab and IMe), 3.52 (s, 18H, CH3 in Tab),
3.35 (s, 18H, CH3 in Tab).13C NMR (75 MHz, D2O, ppm): δ
157.74 (s, Ccarbene in IMe), 145.91 (d, J = 19.3 Hz, CꢀN in Ph of
Tab), 145.32 (s, CꢀN in Ph of Tab), 140.34 (s, CꢀN in Ph of Tab),
136.18 (d, J = 9.0 Hz, CH in Tab), 133.87ꢀ133.21 (m, CꢀS of
85 Tab), 132.68 (d, J = 28.6 Hz, CꢀS of Tab), 123.88 (d, J = 68.4 Hz,
CH in IMe), 120.82 (s, CH in Tab), 119.92 (s, CH in Tab),
119.33 (s, CH in Tab), 57.21ꢀ55.88 (m, CH3 in Tab), 37.44 (d, J =
6.0 Hz, CH3 in IMe).
temperature on
a Varian UNITY plusꢀ600, 400 or 300
spectrometer. Electrospray ion mass spectra (ESIꢁMS) were
15 performed on an Agilent 1200/6200 mass spectrometer using
MeCN as the mobile phase.
Preparation of transꢀ[Pd(IMe)2(Tab)2](OTf)2 (2) and cisꢀ
[Pd(IMe)2(Tab)2](OTf)(Cl) (3a). Adding AgOTf (51 mg, 0.2
mmol) to a solution of Tab (50 mg, 0.3 mmol) in MeOH (3 mL)
20 resulted in a colourless solution. Mixing this colourless solution
with a CH2Cl2 (2 mL) solution containing 1 (37 mg, 0.1 mmol)
with stirring yielded a muddy yellow solution after 1 h which was
filtered. Carefully layering 15 mL of Et2O onto the filtrate
provided yellow block crystals of 2 in several days, accompanied
25 by
a
few
colourless
block
crystals
of
cisꢀ
[Pd(IMe)2(Tab)2](OTf)(Cl) (3a). The crystals were collected by
filtration and washed thoroughly with Et2O before drying in
vacuo. Yield: 10 mg (11% based on Pd). Anal. Calcd. for
C30H42N6O6F6S4Pd (+5H2O): C 35.28, H 5.13, N 8.23; found: C
30 35.25, H 5.09, N 8.13. IR (KBr disk): 3452 (m), 3161 (w), 3131
(w), 3103 (w), 3046 (w), 2940 (w), 1637 (m), 1572 (w), 1499
(w), 1480 (m), 1422 (w), 1402 (w), 1384 (m), 1354 (w), 1270 (s),
1226 (m), 1164 (m), 1150 (m), 1120 (m), 1088 (w), 1028 (s),
1006 (m), 961 (w), 944 (w), 839 (w), 748 (w), 724 (w), 692 (m),
Preparation of [Pd4Ag4(IMe)8(Tab)10](PF6)12 (6). The
90 synthetic route for 6 is the same as that for 4·2MeCN, except
[Ag(IMe)2][AgCl2] (24 mg, 0.05 mmol) in CH2Cl2 (2 mL) was
used as additional Ag source. Yield: 38 mg (30% based on Ag).
Anal. Calcd. for C130H194N26F72P12S10Ag4Pd4: C30.99, H 3.89, N
7.23; found: C 30.52, H 4.22, N 7.30. IR (KBr disk): 3442 (s),
95 2972 (w), 1629 (m), 1575 (w), 1490 (m), 1471 (w), 1441 (w),
1422 (w), 1385 (m), 1345 (w), 1313 (w), 1230 (m), 1201 (w),
1169 (w), 1125 (m), 1086 (w), 1011 (m), 959 (w), 938 (w), 843
35 636 (s), 572 (w), 518 (m) cm−1. H NMR (400 MHz, DMSOꢀd6,
1
ppm): δ 7.25 (d, J = 8.7 Hz, 4H, CH in Tab), 7.14 (d, J = 8.7 Hz,
4H, CH in Tab), 6.92 (s, 4H, CH in IMe), 3.85 (s, 12H, CH3 in
IMe), 3.42 (s, 18H, CH3 in Tab). 13C NMR (101 MHz, DMSOꢀd6,
ppm): δ175.08 (s, Ccarbene in IMe), 150.65 (s, CꢀN in Ph of Tab),
40 142.17 (s, CH in Tab), 133.80 (s, CꢀS of Tab), 122.21 (d, J = 10.6
Hz, CH in IMe), 118.17 (d, J = 5.9 Hz, CH in Tab), 56.25 (d, J =
18.7 Hz, CH3 in Tab), 36.97 (d, J = 18.9 Hz, CH3 in IMe).
Preparation of [Pd2(IMe)4(Tab)2](PF6)4·2MeCN (4·2MeCN)
and cisꢀ[Pd(IMe)2(Tab)2](PF6)2·MeCN (3b·MeCN). Treating a
45 suspension of TabHPF6 (94 mg, 0.3 mmol) in MeOH (3 mL) with
Et3N (1.25 mL) yielded a colourless clear solution. A separate
portion of clear solution of 1 (37 mg, 0.1 mmol) in CH2Cl2 (2
mL) was then introduced. Stirring the mixture for 1 h gave rise to
yellow precipitate. The precipitate was filtered off, reꢀdissolved
50 in MeCN (4 mL) and filtered again. Diffusing Et2O (15 mL)
slowly into the filtrate provided light yellow block crystals of
4·2MeCN after two weeks, accompanied by trace amount of cisꢀ
[Pd(IMe)2(Tab)2](PF6)2·MeCN (3b·MeCN). Yield: 26.5 mg (35%
based on Pd). Anal. Calcd. for C42H64N12F24P4S2Pd2: C 30.19, H
1
(s), 748 (m), 717 (w), 685 (m), 665 (w), 558 (m) cm−1. H NMR
(400 MHz, DMSOꢀd6, ppm): δ 7.67ꢀ7.29 (m, 40H, CH in Tab),
100 7.26 (s, 8H, CH in IMe), 6.94 (s, 8H, CH in IMe), 3.85 (s, 24H,
CH3 in IMe), 3.72 (s, 24H, CH3 in IMe), 3.48 (d, J = 18.0 Hz,
90H, CH3 in Tab). 13C NMR (151 MHz, DMSOꢀd6, ppm): δ
158.57 (s, Ccarbene in IMe), 145.41 (s, CꢀN in Ph of Tab), 138.07 (s,
CH in Tab), 133.85 (d, J = 54.7 Hz, CꢀS of Tab), 124.03 (s, CH
105 in IMe), 123.38 (s, CH in IMe), 120.31 (s, CH in Tab), 119.07 (d,
J = 37.1 Hz, CH in Tab), 56.40 (d, J = 15.6 Hz, CH3 in Tab),
37.75 (s, CH3 in IMe), 37.59 (s, CH3 in IMe).
Xꢀray data collection and structural determination.
Structure measurement of the six compounds were made on a
110 Bruker D8ꢀQuest (2, 3a, 3b·MeCN, 4·2MeCN and 6) and an
Agilent Xcalibur (5) CCD Xꢁray diffractometers by using
graphite monoꢀchromated Mo Kα (λ = 0.71073 Å) radiation. Each
2
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DOI: 10.1039/C6DT04599E
single crystal was mounted at the top of a glass fiber with grease 25 0.50 for its atoms. For 3a, 5 and 6, some spatially delocalized
in a stream of gaseous nitrogen. The cell parameter refinement
and the data reduction were performed on the program Bruker D8
Quest for 2, 3a, 3b·MeCN, 4·2MeCN, 6 and CrystAlisPro
(Agilent Technologies, Ver. 1.171.37.35, 2014) for 5, meanwhile
electron density (494 e for 3a, 100 e for 5 and 117 e for 6) in the
lattices were found but acceptable refinement results could not be
obtained for these electron densities. The solvent contributions
were then modeled using SQUEEZE in the Platon program
5
an absorption correction (multiꢁscan method) was applied.11 30 suite.13 As the NMR spectra for these compounds showed no
Direct methods and fullꢁmatrix leastꢁsquares techniques were
used to solve and refine (on F2) all the crystal structures with
SHELXTLꢁ2013 program.12 For 3a, one of the IMe ligands, OTf−
obvious presence of solvents, these electron densities were
therefore considered as water molecules. Thus the number of
water molecules in these compounds are 49, 10 and 12,
corresponding to 3a·6H2O (Z = 8), 5·10H2O (Z = 1) and 6·3H2O
10 and Cl− displayed positional disorder with the relative ratios of
0.46/0.54, 0.62/0.38 and 0.68/0.32 refined for the two 35 (Z = 4). The detailed crystallographic data and structure
components. For 5, oneꢀNMe3 involving N5 displayed positional
disorder with the relative ratios of 0.67/0.33 refined for the two
components. One OTf−was disordered with the relative ratios of
15 0.56/0.44 refined for the two components. The remaining Cl− was
refinement parameters of 2, 3a, 3b·MeCN, 4·2MeCN, 5 and 6
were summarized in Table 1.
Catalytic study. In a typical run, a mixture of aryl bromide
(1.0 mmol), phenylboronic acid (1.5 mmol), base (2 mmol),
also disordered over four sites with occupancy factors for these 40 precatalyst and H2O (2 mL) were added into a 25 mL Schlenk
components fixed at 0.40, 0.30, 0.20 and 0.10. For 6, one IMe
displayed positional disorder with the relative ratios of 0.46/0.54
refined for the two components. Two ꢀNMe3 groups displayed
tube. The mixture was stirred at 100 °C and cooled to ambient
temperature after the desired reaction time. This is followed by
extraction with CH2Cl2 (10 mL) for three times. The organic
layer was then dried over anhydrous Na2SO4 and removed in
20 rotational disorder with the relative ratios of 0.64/0.36 and
−
0.59/0.41 refined for the two components. One PF6 lies on a 45 vacuo. The crude product was purified by silica gel column
special position of higher symmetry than the molecule can
possess. It is treated as spatial disorder by applying PART−1 and
PART 0 in the .ins file with the site occupation factors changed to
chromatography with CH2Cl2 and petroleum ether as eluents. The
final pure product was dried in vacuo at 50°C for 12h.
Table 1 Crystal data and structure refinement parameters for 2, 3a, 3bꢁMeCN, 4ꢁ2MeCN, 5 and 6.
3a
Compounds 3bꢁMeCN 4ꢁ2MeCN
Empirical formula C30H42N6O6F6PdS4 C29H42N6O3F3ClPdS3 C30H45N7F12P2PdS2 C42H64N12F24P4Pd2S2C80H110N14O18F18Cl2Pd4S12 C130H194N26F72P12Ag4Pd4S10
2
5
6
Formula weight 931.33
817.71
monoclinic
C2/c
964.19
monoclinic
P21/n
1593.85
monoclinic
P21/c
2779.03
triclinic
Pꢀ1
5038.42
monoclinic
C2/c
Crystal system
monoclinic
Space group
a/Å
P21/c
16.4841(14)
10.5395(9)
11.6208(10)
90
30.6593(18)
8.5669(5)
33.864(2)
90
15.2582(12)
8.1923(6)
32.518(2)
90
10.4595(7)
12.4551(7)
25.8400(14)
90
12.6916(6)
15.6346(5)
17.4360(9)
101.276(4)
107.340(5)
97.781(4)
3168.7(3)
1.456
39.487(9)
18.813(4)
29.645(6)
90
b/Å
c/Å
α/°
β/°
108.772(2)
90
102.117(2)
90
92.608(2)
90
112.347(3)
90
115.292
90
γ/°
V/Å3
Dc/(g cm–3
1911.5(3)
1.618
8696.3(9)
1.249
4060.6(5)
1.577
3113.5(3)
1.700
19911(7)
1.681
)
Z
2
8
4
2
1
4
ꢀ (Mo–Kα)/mm–1 0.783
0.677
0.726
0.862
0.882
1.056
F(000) 952
3360
1960
1600
1404
10064
233883
24655
14991
1231
Total reflections 33954
Unique reflections4395
No observations 4051
109630
9951
78699
9329
75783
7129
32182
13957
8596
7571
6624
6175
No parameters
246
534
498
396
739
Rint
Ra
wRb
GOFc
0.0264
0.0206
0.0524
1.075
0.0446
0.0692
0.1815
1.183
0.0542
0.0446
0.0941
1.027
0.0433
0.0361
0.0806
1.087
0.0948
0.0948
0.2270
0.985
0.0961
0.0607
0.1389
1.067
2
2
2
50 aR1 = ||Fo|ꢀ|Fc||/|Fo|. bwR2= [w(Fo ꢀFc2)2/w(Fo )2]1/2. cGOF = {w((Fo ꢀFc2)2)/(nꢁp)]}1/2, where n = number of reflections and p = total numbers of parameters
refined.
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1/1.5equiv. Tab/2 equiv. AgOTf. Finally, when we reacted 1 with
3 equiv. of TabHPF6 and additional 0.5 equiv. of
Results and discussion
15
[Ag(IMe)2][AgCl2],14 we are able to isolate an octanuclear
heterometallic cluster of [Pd4Ag4(IMe)8(Tab)10](PF6)12 (6, 30%
yield) as the only product. We assume that for the formation of 2,
the sequestration of the transꢀlocated Cl− from 1 by Ag+ is likely
20 the driving force to promote Tab association with the Pd center.
The formation of 3a, 3b and 4 follows a multiple reaction
sequence wherein trans effect15 among IMe, Cl and Tab (IMe
>Cl > Tab) and the reaction equilibrium dominate (Scheme S1).
While the formation of 5 and 6 are collectively imposed by the
25 subtle interplay of Cl− precipitation, trans effect, as well as the
coordination flexibility of Tab (5 and 6) and Ag(I) ion (6).
Synthetic and spectral aspects. As depicted in Scheme 1,
treating the precursor compound 1 with three equiv. of Tab in the
presence of two equiv. of AgOTf as the Cl− scavenger resulted in
the formation of transꢀ[Pd(IMe)2(Tab)2](OTf)2 (2) as yellow
block crystals in 11% yield, accompanied by a few colourless
block crystals of cisꢀ[Pd(IMe)2(Tab)2](OTf)(Cl) (3a). Alternating
the Tab source as TabHPF6 in the absence of AgOTf afforded a
5
dinuclear
compound
[Pd2(IMe)4(Tab)2](PF6)4·2MeCN
10 (4·2MeCN) in 35% yield, with the coꢀexistence of trace amount
of cisꢀ[Pd(IMe)2(Tab)2](PF6)2·MeCN (3b·MeCN). In addition, a
tetranuclear homometallic cluster [Pd4(IMe)4(Tab)6](OTf)6(Cl)2
(5, 52% yield) was similarly prepared as the sole product from
Scheme 1 Reactions of 1 with Tab/TabHPF6 in the absence/presence of additional Ag(I) source.
singlets were found at 175.08 (2), 158.57 (4), 157.74 (5) and
158.57 (6) ppm. The relative downfield shift of 2 is due to its
50 trans configuration.16
30
Compounds 2−6 are stable under aerobic conditions.
Compounds 2, 3a and 5 are soluble in H2O, MeOH, EtOH, DMF
and DMSO while 3b, 4 and 6 are soluble in MeCN, DMF and
DMSO. The elemental analyses of the principal products 2, 4, 5
and 6 are in good agreement with their chemical formulae. In the
To further study the solution behaviours of 2, 4, 5 and 6, we
collected their ESIꢀMS data under positive mode in MeOH (2 and
5) or MeCN (4 and 6) solutions. The cationic fragments of 2 and
4 were identified as [Pd(IMe)2(Tab)2(OTf)]+ (m/z = 781.13, Fig.
55 S1a, ESI†) and [Pd2(IMe)4(Tab)2(PF6)]3+ (m/z = 359.07, Fig. S2a,
ESI†). In addition, other cationic species, such as
[Pd(IMe)(Tab)2(OH)·Et2O·MeOH·H2O]+,
35 IR spectra of 2−6, bands at around 2950 cm−1 and 1630 cm−1 are
ascribed as methyl CꢀH stretching vibrations and C=C absorption
in Tab or IMe. Bands around 1600 cm−1, 1500 cm−1, 1450 cm−1
and 1400 cm−1 are typical skeletal vibrations of aromatic rings.
Bands of CꢀN stretching vibrations and CꢀS weak absorption are
40 approximately at 1225 cm−1 and 1005 cm−1 in Tab. The SꢀO
asymmetric stretching vibrations as well as CꢀF stretching
vibrations for common OTf− anion in 2 and 5 are observed at
about 1163 cm−1 and 1030 cm−1. As for 4 and 6, bands at 843
cm−1 and 558 cm−1 are ascribed to the PꢀF characteristic
[Pd(IMe)2(Tab)(OH)·3MeOH]+ at m/z = 677.18, 578.07 (2, Figs.
S1b
and
S1c,
ESI†)
and
[Pd(IMe)(Tab)2]2+
[Pd2(IMe)3(Tab)2(PF6)3]+,
60 [Pd(IMe)2(Tab)2(PF6)]+,
and
[Pd(IMe)(Tab)(PF6)·2MeOH]+ at m/z = 1271.06, 777.16, 268.06,
578.10 (4, Figs. S2bꢀS2e, ESI†) were also found. Such
observations were however not made for 5 and 6 wherein only
dissociated cationic fragments were found. For 5, only two peaks
65 at m/z = 686.08 and 602.54 were found, corresponding to
[Pd2(IMe)2(Tab)4(OTf)2]2+ and [Pd2(IMe)2(Tab)3(OTf)2]2+ (5,
−
45 stretching vibrations of their common anion PF6 . Compounds 2–
6 are also characterized by 1H NMR and 13C NMR spectra (ESI†).
In the 13C NMR spectra of 2, 4, 5 and 6, the characteristic CIMe
4
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Figs. S3a and S3b, ESI†). In the case of 6, mononuclear
fragments such as [Pd(IMe)2(Tab)2]2+, [Pd(IMe)(Tab)·3MeOH]2+
and [Pd(IMe)(Tab)(PF6)·2MeOH]+ at m/z = 316.09, 232.56 and
578.10 (6, Figs. S4aꢀS4c, ESI†) were accordingly identified.
Further analysis of the solution upon the isolation of 5 and 6 also
indicated additional presence of PdꢀIMe, PdꢀIMeꢀTab and Agꢀ
IMe adducts, such as [Pd(IMe)(Tab)·4MeOH·2H2O]2+ (m/z =
266.57), [Ag(IMe)2·MeOH·4H2O]+ (m/z = 403.03) (after removal
of single crystals of 5, Figs. S3c−S3d, ESI†), and
5
10 [Pd(IMe)(OH)·2MeOH]+ (m/z = 283.16), [Ag(IMe)2·2H2O]+ (m/z
= 335.16), [Ag(IMe)·2H2O]+ (m/z = 239.23) (after removal of
single crystals of 6, Figs. S4d−S4f, ESI†).
(a)
15
Fig.
1
Xꢀray structures of (a) transꢀ[Pd(IMe)2(Tab)2]2+ of 2, (b)
[Pd2(IMe)4(ꢀꢀTab)2]4+ of 4, (c) cisꢀ[Pd(IMe)2(Tab)2]2+ of 3a, and (d) cisꢀ
[Pd(IMe)2(Tab)2]2+ of 3b. All the dissociated moieties and H atoms are
omitted. Colour codes: Pd (purple),S (yellow), N (blue), C (black).
50
(b)
Fig. 2 (a) The Xꢀray structure of [Pd4(IMe)4(Tab)6]8+ of 5, and (b) The
PdꢀS framework of 5. All the dissociated moieties and H atoms are
omitted. See Fig. 1 for colour codes. Symmetry code for (b): A –x, –y, 1 –
z.
20
Crystal structures of 2−6. As shown in Fig. 1a, the central Pd
in 2 adopts a squareꢀplanar coordination with the two IMe and
two Tab both in trans configurations. The average PdꢀCIMe
distance of 2.0205 Å (Table S1, ESI†) falls into the normal range
55
As illustrated in Fig. 2a, the structure of 5 can be readily
assumed as a pair of Pd dimers linked by a pair of Tab ligands.
Within the dimeric subꢀunit, the PdꢁꢁꢁPd separation of 3.1289(9)
Å exists. This distance is longer than that reported for the
25 and comparable with other PdꢀNHC complexes, such as
[Pd(IMePP)2][PF6]2 (IMePP 1ꢀmethylꢀ2ꢀ(pyridinꢀ2ꢀ
yl)imidazo[1,5ꢀa]pyridinꢀ4ꢀylium, 1.995(7) Å),17 (ꢀꢀCp)(ꢀꢀ
Cl)Pd2(IPr)2 (Cp cyclopentadienyl, IPr 1,3ꢀbis(2,6ꢀ
diisopropylphenyl)ꢀ1,3ꢀdihydroꢀ2Hꢀimidazolꢀ2ꢀylidene, 2.022(4)
30 Å)18 and [Pd(MIBIPhen)(Cl)(H3CCN)]PF6 (MIBIPhen = (3,3’ꢀ
(biphenylꢀ2,2’ꢀdiylbis(methylene))bis(1ꢀmethylꢀ1Hꢀimidazolꢀ3ꢀ
ium)bromide), 2.021(2) Å).19 In addition, the average PdꢀS
distance (2.3266 Å) is also comparable with those of
[Pd2(PPh3)2(Tab)2(ꢀꢀTab)2][PF6]4 (2.3212(15) Å),20 [(phen)2K(ꢀꢀ
35 phen)2K(phen)2][(4ꢀHC6F4S)2Pd(ꢀꢀSC6F4Hꢀ4)2Pd(SC6F4Hꢀ4)2]
=
60 Pd(I)ꢁꢁꢁPd(I) complex of [Pd2(CH3CN)6][SbF6]2 (2.4871(5) Å),23
but comparable with that in [Pd2(C4H6)2(PPh3)2][PF6]2 (3.1852(6)
Å).24 Such a PdꢁꢁꢁPd separation is notably shorter than the sum of
their van der Waals radii of 3.26 Å, indication of PdꢁꢁꢁPd
interaction. Within these dimeric units, each Pd(II) center is
65 associated by three ꢀꢀS atoms from three Tab ligands and one C
atom from IMe. The PdꢀꢀꢀS distances are in the range of 2.324(2)
Å to 2.388(2) Å (av. 2.351 Å). The PdꢀCIMe bond lengths
narrowly range from 1.995(11) Å to 2.023(10) Å (av. 2.009 Å)
(Table S1, ESI†). The PdꢀS framework of 5 can be considered as
70 a [Pd4S4] parallelogram wherein the four corners are defined by
four S atoms, with two additional S atoms projecting two
opposite directions with respect to the [Pd4S4] plane (Fig. 2b). All
the atoms within the [Pd4S4] plane are nearly coplanar, with the
maximum deviation of 0.3084 Å (S3 and S3A) and the minimum
75 deviation of 0.1720 Å (S2 and S2A) from the plane. The two
projecting S atoms are 1.4784 Å away from the plane. Within the
[Pd4S4] plane, the four Pd atoms form a rectangle with size of
3.734 × 3.129 Å2.
=
=
(phen
=
1,10ꢀphenanthroline, 2.333(3) Å)21 and cisꢀ
bis(diphenylphosphino)methane,
[Pd(dppm)(pymt)2] (dppm
=
pymt = pyrimidineꢀ2ꢀthiolate, 2.3267(9) Å).22 For 4, the two IMe
ligands on each Pd center are cis, imposing the two Tab ligands
40 to adopt a cisꢀanti configuration and bridge a pair of Pd(IMe)2
units to give a dimer (Fig. 1b). The PdꢀCIMe and PdꢀꢀꢀS distances
are 2.024 Å and 2.3776 Å on average, slightly longer than those
in 2 (Table S1). As for 3a and 3b, their structures are similar
except that their Tab ligands rotate to different degrees with
45 respect to the CꢀS bond (Figs. 1c and 1d).
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lengths are 2.3786 Å, 2.3620 Å and 2.012 Å on average. The
average AgꢀꢀꢀS and Agꢀꢀ3ꢀS distances are 2.5443 Å and 2.6230
40 Å (Table S1, ESI†).
Table 2 SuzukiꢀMiyaura coupling of aryl bromide with phenylboronic
acid.
Entrya
Preꢀ
catalyst
Base
Pd
loading
(mol%)
T
(°C)
Time
(h)
Yieldb
(%)
24
24
24
24
24
24
24
24
24
24
24
24
6
92
75
82
40
71
95
99
99
99
99
96
94
80
88
99
91
84
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2
Na2CO3
K2CO3
K3PO4
NaOAc
Cs2CO3
NaOH
KOH
0.01
0.01
100
100
100
100
100
100
100
100
100
100
100
90
1
2
0.01
3
0.01
4
(a)
0.01
5
0.01
6
0.01
7
KOH
0.005
0.001
0.0005
0.0004
0.0005
0.0005
0.005
0.01
8
KOH
9
KOH
10
11
12
13
14
15
16
17
KOH
KOH
KOH
100
100
100
100
100
6
KOH
5
(b)
Fig. 3 (a) The Xꢀray structure of [Pd4Ag4(IMe)8(Tab)10]12+of 6, and (b)
The PdꢀAgꢀS skeleton of 6. All the dissociated moieties and hydrogen
atoms are omitted. See Fig. 1 for colour codes except for Ag (cyan).
6
KOH
4
KOH
0.01
24
Na2CO3
0.01
a
Reaction conditions: 1 mmol of 4ꢀbromoacetophenone, 1.5 mmol of
phenylboronic acid, 2 mmol of base, 2.0 mL H2O, the reactions are
carried out under N2. bIsolated yield.
10
Compound 6 features an octanuclear [Pd4Ag4(IMe)8(Tab)10]12+
dodecaꢀcation (Fig. 3a). The identities of Ag and Pd centers are
distinguished by their coordination preferences (square planar for
Pd and trigonal/tetrahedral for Ag), as well as the charge counting.
The overall skeleton of 6 (Pd4Ag4S10, Fig. 3b) is ovalꢀshaped with
To evaluate the suitability of PdꢀNHCꢀTab compounds as
potential catalysts, we firstly selected waterꢀsoluble compound 5
45 as the precatalyst for the SuzukiꢀMiyaura crossꢀcoupling reaction
in water. We chose coupling reaction between 4ꢀ
bromoacetophenone (1.0 mmol) and phenylboronic acid (1.5
mmol) as the model reaction. The base, Pd loading, reaction
temperature and time were subsequently optimized. As shown in
50 Table 2, the optimal Pd loading is 0.0005 mol% with KOH as the
base (Table 2, entry 10), while the yield expectedly decreased
upon sequentially reducing the Pd loading (Table 2, entry 11).
It is apparent that the reaction temperature significantly
impacts the reaction rate, with refluxing condition being optimal
55 (100 °C). Using 0.0005 mol% Pd loading and 24 h as the reaction
time, the yield decreased from 99% to 94% when temperature
changed from 100 °C to 90 °C (Table 2, entries 10 and 12).
Further reducing the time to 6 h (0.0005 mol% Pd loading,
100 °C) leads to drastic yield reduction to 80% (Table 2, entry
60 13). The yield decrease caused by time cut can be compensated
by increasing the Pd loading. Thus, a high yield of 99% (Table 2,
entry 15) can be achieved with 0.01 mol% Pd loading at 100 °C
for 6 h. Since compound 2 is also soluble in H2O, its catalytic
activity is examined under the identical conditions (Na2CO3, 0.01
65 mol% Pd loading, 100 °C, 24 h). It turns out that 2 is less
effective than 5 and the yield decreased from 92% to 84% (Table
2, entry 17). The investigation of the catalytic property of other
15 an inversion center coinciding with the middle of the central
AgꢁꢁꢁAg bond. By detaching two PdS2 motifs at the polar
positions, we can easily assume the remaining part (Pd2Ag4S6) as
an edgeꢀfused norbornaneꢀlike framework. The two parts bearing
norbornane topology are fused through one of their overhead
20 bridges (AgꢁꢁꢁAg separation). The AgꢁꢁꢁAg separation of
3.0004(10) Å is significantly shorter than the sum of van der
Waals radii of 3.44 Å, indicating strong AgꢁꢁꢁAg interaction. It is
also notable that the PdꢁꢁꢁAg distance from the polar Pd atom to
the closest Ag features 3.2482(9) Å, shorter than the sum of their
25 van der Waals radii of 3.35 Å, also an indication of strong
PdꢁꢁꢁAg interaction. Each Pd atom adopts a squareꢀplanar
geometry and is associated with two cisꢀlocated IMe ligands and
two bridging Tab, in a cisꢀsyn fashion and contrasting that found
in the dimeric compound 4. The polar Pd center extends to the
30 Ag center via a pair of ꢀꢀTab, while the other type of Pd extends
to three Ag centers via one ꢀꢀTab and one ꢀ3ꢀTab. The three Ag
centers further cascade via one additional S and one AgꢁꢁꢁAg
interaction. Each Ag atom in turn employs a distorted (pseudo)
tetrahedron coordination mode. The Ag atoms associated with the
35 polar Pd atoms are bonded by three ꢀꢀTab and one ꢀ3ꢀTab while
the two central Ag atoms are coordinated by two ꢀꢀTab, one ꢀ3ꢀ
Tab and one Ag atom. The PdꢀꢀꢀS, Pdꢀꢀ3ꢀS and PdꢀCIMe bond
6
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compounds is either hindered by the low yield of the catalysts (3a
and 3b) or the solubility of the catalysts (4 and 6) in water.
to achieve functional molecular assemblies with heterometallic
45 synergy in catalysis. Our preliminary experiments demonstrated
that 5 showed excellent catalytic activity(a very low Pd loading
of 0.0005 mol%) towards SuzukiꢀMiyaura coupling of
phenylboronic acid with 4ꢀbromoacetophenone in water. In view
of the fascinating development in carbene chemistry, and their
50 interaction with diverse transition and main group metal ions, as
well as their potential in broad areas, our future work is to extend
the scope of both NHC and homoꢀ/heteroꢀmetal ions and to
unveil their structural complexity and their applications in
catalysis.
Table 3 SuzukiꢀMiyaura coupling of aryl bromide with phenylboronic
acid.
5
Yieldb (%)
Entrya
ArꢀBr
99
83
90
96
41
89
51
49
54
1
2
4ꢀbromoacetophenone
4ꢀbromobenzonitrile
4ꢀbromobenzaldehyde
4ꢀbromonitrobenzene
4ꢀbromofluorobenzene
βꢀbromonaphthalene
4ꢀbromoanisole
3
4
55 Acknowledgments
5
This work was financially supported by the National Natural
Science Foundation of China (21371126, 21401134, 21531006
and 21671143), the State Key Laboratory of Organometallic
Chemistry of Shanghai Institute of Organic Chemistry (2015kfꢀ
60 07), Science and Technology Department of Jiangsu Province
(BK20140307), Department of Education of Jiangsu Province
(14KJB150023) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
6c
7c
8d
9d
4ꢀbromotoluene
bromobenzene
a
Reaction conditions: 1.0 mmol of aryl bromide, 1.5 mmol of
phenylboronicacid, 2.0 mmol base, 2.0 mL H2O, 0.01 mol% Pd loading,
6h, 100°C under nitrogen.b Isolated yield.c0.05 mol% Pd loading. d0.1 mol%
Pd loading.
Notes and references
10
After acquiring the optimized reaction conditions, we
expanded the substrate scope from electronꢀpoor aryl bromides to
electronꢀrich ones. From Table 3, it can be concluded that
bromide substrates bearing electronꢀwithdrawing substituents
65 aCollege of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, People's Republic of China.
Eꢁmail: jplang@suda.edu.cn; whzhang@suda.edu.cn; Fax: 86ꢁ512ꢁ
65880089; Tel: +86ꢁ512ꢁ65882865
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
15 such as –COCH3, –CN, –CHO, –NO2 and –F are advantageous
for the reaction (Table 3, entries 1–5).25 Accordingly, the –F
group with the weakest electronꢀwithdrawing ability results in the
lowest yield (Table 3, entry 5). Including electronꢀdonating
groups like –OCH3 and –CH3 or–H in the bromide substrates are
20 unꢀfavoured (Table 3, entries 7–9). When the aryl changes from
phenyl to naphthyl, the yield also drops dramatically (Table 3,
entry 6). Our further analysis of the ESIꢀMS upon completion of
the catalysis (with 5 as the precatalyst) revealed that two
additional peaks at m/z = 387.06 and 427.04 exist, corresponding
25 to two cationic species [Pd(IMe)(Cl)·3MeOH·3H2O]+ and
[Pd(IMe)(OH)·2MeOH·8H2O]+, respectively. This observation
suggests that during the catalysis, the PdꢀC bond remains robust
and the precatalyst undergoes a PdꢀS bond dissociation to enter
the catalytic cycle.
70 of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
People's Republic of China
†Electronic Supplementary Information (ESI) available: Detailed ESIꢀMS
and NMR data for compounds 2, 4, 5 and 6, the catalytic data. CCDC
1505255–1505260 for compounds 2−6. For ESI and crystallographic data
75 in CIF or other electronic format see DOI:10.1039/b000000x/
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30 Conclusions
90
In this paper, the reactivity of NHCꢀligated Pd compound 1 as the
precursor towards Tab has been explored with interesting
structural outcome. Compounds 2–6 isolated herein range from
mononuclear to octanuclear, revealing the bridging flexibility of
35 the Tab ligand. The relative positions of NHC and Tab can be
readily switched from cis to trans depending on the reaction
parameters. In addition, the circumduction and rotation of the ꢀ
PhNMe3 moiety in Tab with respect to the CꢀS bond render much
flexibility of the system and facilitate the molecular packing.
40 Notably, additional heteroatom (Ag) can be easily included in the
structural framework to give a PdꢀAg heterometallic octanuclear
cluster of 6, highlighting the broad potential of such system in
hosting other useful heterometallic metals such as Ru, Ir and Au,
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DOI: 10.1039/C6DT04599E
Contents for Graphical Abstract
Combination of Pd(II), an N-heterocyclic carbene (NHC), and a thiolate (Tab) yields
several Pd-NHC-Tab complexes and an octanuclear Pd4Ag4-NHC-Tab cluster.
Figure for Graphical Abstract