Cyclopalladation of dimesityl selenide: Synthesis, reactivity, structural characterization, isolation of an intermediate complex with C-H...Pd intra-molecular interaction and computational studies

Article abstract of DOI:10.1039/c3dt50935d

The reaction of dimesityl selenide (Mes₂Se) with either PdCl₂(PhCN)₂ in toluene or PdCl₂ in toluene-acetonitrile yields a chloro-bridged binuclear palladium complex, [Pd₂Cl₂(μ-Cl)₂(Mes₂Se) ₂] (1), whereas with Na₂PdCl₄ in refluxing ethanol, a cyclometallated palladium complex, [Pd₂(μ-Cl) ₂{MesSeC₆H₂(Me₂)CH₂} ₂] (2) is afforded. 2 can also be obtained when 1 is refluxed in ethanol. On treatment with Pb(Epy)₂ in dichloromethane, 2 afforded the Epy-bridged binuclear complexes, [Pd₂(μ-Epy) ₂{MesSeC₆H₂(Me₂)CH₂} ₂] (3; E = S (3a) or Se (3b)). Treatment of 2 with PPh₃ yields a bridge-cleaved monomeric complex, [PdCl{MesSeC₆H ₂(Me₂)CH₂}(PPh₃)]. The molecular structures of 1-3 were established by X-ray diffraction analyses. All the complexes are dimeric, with the palladium atoms acquiring a distorted square planar configuration. There are intra-molecular C-H...Pd interactions (d_M-H: 2.75 A and 3) bond leading to metallation. The optimized geometry of 1 obtained using the DFT(B3LYP) computational method identified a C-H...Pd contact distance of 2.78 A. There are two independent molecules of 2 in the unit cell, which differ slightly in bond lengths, bond angles and torsion angles. The mechanism of formation of the dimeric species 2 is examined using DFT (B3LYP) computations. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt50935d

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Cyclopalladation of dimesityl selenide: synthesis,
reactivity, structural characterization, isolation of an
intermediate complex with C–H⋯Pd intra-molecular
interaction and computational studies†
Cite this: Dalton Trans., 2013, 42, 10828
Siddhartha Kolay,a Amey Wadawale,a Dasarathi Das,a Hemanta K. Kisan,b
Raghavan B. Sunoj*b and Vimal K. Jain*a
The reaction of dimesityl selenide (Mes2Se) with either PdCl2(PhCN)2 in toluene or PdCl2 in toluene–
acetonitrile yields a chloro-bridged binuclear palladium complex, [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1), whereas
with Na2PdCl4 in refluxing ethanol, a cyclometallated palladium complex, [Pd2(µ-Cl)2{MesSeC6H2(Me2)-
CH2}2] (2) is afforded. 2 can also be obtained when 1 is refluxed in ethanol. On treatment with Pb(Epy)2
in dichloromethane, 2 afforded the Epy-bridged binuclear complexes, [Pd2(µ-Epy)2{MesSeC6H2(Me2)-
CH2}2] (3; E = S (3a) or Se (3b)). Treatment of 2 with PPh3 yields a bridge-cleaved monomeric complex,
[PdCl{MesSeC6H2(Me2)CH2}(PPh3)]. The molecular structures of 1–3 were established by X-ray diffraction
analyses. All the complexes are dimeric, with the palladium atoms acquiring a distorted square planar
configuration. There are intra-molecular C–H⋯Pd interactions (dM–H: 2.75 Å and <C–H⋯Pd: 111.23°) in 1
which facilitate the activation of the C–H (sp3) bond leading to metallation. The optimized geometry of 1
obtained using the DFT(B3LYP) computational method identified a C–H⋯Pd contact distance of 2.78 Å.
There are two independent molecules of 2 in the unit cell, which differ slightly in bond lengths, bond
angles and torsion angles. The mechanism of formation of the dimeric species 2 is examined using DFT
(B3LYP) computations.
Received 9th April 2013,
Accepted 15th May 2013
DOI: 10.1039/c3dt50935d
www.rsc.org/dalton
A plethora of organic compounds containing N, P, As, S
donor atoms undergo cyclopalladation (Scheme 1) so as to
Introduction
Since the early study of Cope and Siekman,1 cyclometallated give, in general, a four-electron C-anionic donor (C∩Y) ligand.
palladium complexes (or palladacycles) have been of consider- Although several mechanisms have been proposed for the
able interest and have emerged as one of the most important cyclopalladation reaction, it is generally believed that initially,
families of organometallic compounds.2 There are several a ligand is coordinated to the palladium atom and in this
obvious reasons for this sustained interest in these complexes intermediate species, the C–H bond is activated only when it is
which can be attributed to their outstanding applications as within the metal coordination plane.4 Such intermediates are
catalysts (e.g., [Pd2(µ-OAc)2{tol2PC6H4CH2-o}2] by Herrmann seldom isolated, although their existence can be inferred. For
and coworkers3) in organic synthesis,3–6 the fact that they are instance, the metallation of Me2ECH2Ph (E = N, P, As, Sb)
metallomesogenic7 as well as their intriguing photophysical shows a pronounced dependence on the size of E. While
properties,8 in materials science9,10 and their rich reaction dimethylbenzylamine and dimethyl benzylphosphine are
chemistry.2,11
readily cyclopalladated,12,13 the dimethylbenzylarsine deriva-
tives, PdX2(Me2AsCH2Ph)2, (X = Cl, Br, I) do not metallate in
refluxing 2-ethoxy ethanol both in the presence or absence of a
base.14
aChemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
bDepartment of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai
400076, India. E-mail: jainvk@barc.gov.in; Fax: +91 22 2550 5151;
Tel: +91-22-2559 5095
†Electronic supplementary information (ESI) available. CCDC 901678–901680
and 930643 for [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1), [Pd2(µ-Cl)2{MesSeC6H2(Me2)CH2}2]
(2), [Pd2(µ-Spy)2{MesSeC6H2(Me2)CH2}2] (3a) and [Pd2(µ-Sepy)2{MesSeC6H2(Me2)-
CH2}2] (3b) respectively. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt50935d
Scheme 1
10828 | Dalton Trans., 2013, 42, 10828–10837
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There is extensive literature on cyclopalladated complexes using the Fourier technique.22 The non-hydrogen atoms were
derived from Group 15 donor (N, P, As) ligands. Among the refined anisotropically and fitted with hydrogen atoms in
Group 16 donor ligands, only sulfur compounds have been their calculated positions. All the calculations were performed
investigated2,4 and have also been used in C–C coupling reac- using
a
crystal structure crystallographic software
tions as catalysts.15 Cyclopalladation employing organo- package.23,24 The molecular structures were drawn using
selenium ligands is rarely reported, [Pd2(µ-OAc)2(C6H4SeBut)2] ORTEP.25
being the only complex described so far16 where palladation
Synthesis
takes place through aromatic C–H bond activation. The lack of
studies on the cyclometallation of organoselenium ligands
[Pd2Cl2(µ-Cl)2(Mes2Se)2] (1). (i) PdCl2(PhCN)2 (233 mg,
could possibly be due to the formation of either simple coordi- 0.61 mmol) was added to a toluene solution (25 ml) of Mes2Se
nation complexes, [PdX2(SeR2)2] or the cleavage of the Se–C (206 mg, 0.65 mmol) and the reaction mixture was stirred at
bond under cyclometallation reaction conditions. For example, room temperature for 4 h. The solution was dried under
in an attempt to cyclometallate di-o-tolyl selenide with palla- vacuum and the residue was washed thoroughly with hexane,
dium acetate, Singh and coworkers isolated [Pd(OAc)(Setol-o)]4 diethyl ether and toluene and extracted with dichloromethane,
formed by the cleavage of the Se–C bond.17 It is worth which was evaporated under vacuum to give
a brown
noting that palladium complexes with seleno ether ligands residue. The latter was recrystallized from dichloromethane
show better catalytic activity than the corresponding thio containing a few drops of hexane as red crystals (182 mg, 60%
ligands.18
yield, m.p.: 193 °C (dec)). Analysis Calcd for C36H44Cl4Pd2Se2:
With the above perspective, we have examined the reactivity C, 43.71; H, 4.48; Found: C, 43.42; H, 4.44%. 1H NMR
of dimesityl selenide and isolated not only the cyclopalladated (500 MHz, CDCl3) δ: 2.22, 2.24, 2.49, 2.65 (s, ratio 4 : 2 : 1 : 6,
complex, but also the intermediate complex with a C–H⋯Pd Me), 6.77, 6.83, 6.85, (s, C6H2); 77Se {1H} NMR (CDCl3) δ:
intra-molecular interaction. We have also investigated the reac- 322 ppm.
tivity of the cyclometallated complex. The results of this work
are reported herein.
(ii) PdCl2 (250 mg, 1.41 mmol) was added to a toluene–
acetonitrile (5 : 25 cm−3
solution of Mes2Se (447 mg,
)
1.41 mmol) and the whole mixture was refluxed at 80 °C for
4 h, whereupon a red precipitate formed which was filtered
and washed with hexane, diethyl ether and toluene. The brown
solid was extracted with dichloromethane. The solvent was
evaporated under reduced pressure to give the title complex
(yield: 540 mg, 77%). The NMR data were consistent with the
above preparation.
[Pd2(µ-Cl)2{MesSeC6H2(Me2)CH2}2] (2). Mes2Se (480 mg,
1.51 mmol) was added to an ethanolic (25 ml) solution of
Na2PdCl4 (434 mg, 1.47 mmol) and the reactants were refluxed
for 4 h. The solvent was evaporated under reduced pressure
and the residue was washed thoroughly with hexane and
diethyl ether and then extracted with toluene and dried. The
complex was recrystallized from acetone containing a few
drops of diethyl ether as rod-like orange crystals (415 mg, 61%
yield; m.p.: 165 °C (dec)). Analysis Calcd for C36H42Cl2Pd2Se2:
C, 47.19; H, 4.62. Found: C, 47.50; H, 4.31%. 1H NMR
(500 MHz, CDCl3) δ: 1.91 (s, 4-Me, non-metallated), 2.24, 2.26
(each s, 4- and 6-Me, metallated), 2.40 (br s, 2,6-Me, non-metal-
lated), 3.37 (AB pattern, CH2, ΔγAB 46 Hz, JAB = 12.6 Hz), 6.75,
6.78 (each s, C6H2 metallated), 6.85 (s, C6H2 non-metallated)
ppm. 77Se {1H} NMR (CDCl3) δ: 408 ppm.
Experimental section
General
The solvents were dried and distilled under a nitrogen atmos-
phere prior to use according to a literature method.19 All the
reactions were carried out in a Schlenk flask under a nitrogen
atmosphere. Bis(2,4,6-trimethylphenyl)selenide, (Mes)2Se,20
was prepared according to the reported method and the other
reagents were obtained from commercial sources and were
used without further purification. Elemental analyses were
carried out on a Carlo-Erba EA-1110 CHN-S instrument.
Melting points were determined in capillary tubes and are
uncorrected. 1H NMR spectra were recorded on a Bruker
Avance 500 and Avance II-300 NMR spectrometers operating at
500 and 300 MHz, respectively while 77Se{1H} NMR spectra
were recorded on a Bruker Avance II-300 NMR spectrometer
operating at 57.24 MHz. The chemical shifts are relative to an
internal chloroform peak (δ 7.26 for 1H) and external Me2Se
for 77Se{1H} (secondary reference: Ph2Se2 in CDCl3,
463 ppm).
δ
[Pd2(µ-Spy)2{MesSeC6H2(Me2)CH2}2] (3a). Pb(Spy)2 (48 mg,
0.11 mmol) was added to a dichloromethane solution of
X-ray crystallography
Single crystal X-ray data on [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1), [Pd2(µ- [Pd(µ-Cl){MesSeC6H2(Me2)CH2}]2 (100 mg, 0.11 mmol) and the
Cl)2{MesSeC6H2(Me2)CH2}2] (2), [Pd2(µ-Spy)2{MesSeC6H2(Me2)- contents were stirred overnight at room temperature. The
CH2}2] (3a) and [Pd2(µ-Sepy)2{MesSeC6H2(Me2)CH2}2] (3b) were supernatant was passed through Celite and the filtrate was
collected on a SuperNova or Bruker APEX-II CCD diffracto- evaporated under reduced pressure to give a red residue. The
meter. The crystallographic data together with the data col- residue was washed with hexane and diethyl ether and extracted
lection and refinement details are given in Table 1. All the with acetone. The solution was concentrated to 1 cm−3 and on
data were corrected for Lorentz and polarization effects. The the addition of a few drops of diethyl ether gave yellow crystals
structures were solved by direct methods21 and expanded (98 mg, 84% yield; m.p.: 176 °C). Analysis Calcd for
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Table 1 Crystallographic and structural determination data for [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1), [Pd2(µ-Cl)2{MesSeC6H2(Me2)CH2}2] (2), [Pd2(µ-Spy)2{MesSeC6H2(Me2)-
CH2}2] (3a) and [Pd2(µ-Sepy)2{MesSeC6H2(Me2)CH2}2] (3b)
[Pd2(µ-Cl)2{MesSeC6-
H2(Me2)CH2}2]
[Pd2(µ-Spy)2{MesSeC6-
H2(Me2)CH2}2]
[Pd2(µ-Sepy)2{MesSeC6-
H2(Me2)CH2}2]
Complex
[Pd2Cl2(µ-Cl)2(Mes2Se)2]
36H44Cl4Pd2Se2
989.24
0.12 × 0.05 × 0.02
100(2)
SuperNova
Cu-Kα (1.54184 Å)
Triclinic
ˉ
P1
8.5654(5)
11.1973(8)
11.5045(6)
110.442(6)
97.754(5)
109.719(6)
932.46(12)
1.762
Empirical formula
Formula weight
Crystal size (mm3)
Temperature(K)
Diffractometer
Radiation
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
C36H42Cl2Pd2Se2
916.32
0.03 × 0.03 × 0.01
100(2)
SuperNova
Cu-Kα (1.54184 Å)
Monoclinic
P21/c
15.1293(5)
15.0902(5)
15.1702(4)
90.00
C46H50N2Pd2S2Se2
1065.72
0.25 × 0.15 × 0.15
298(2)
Bruker APEX-II CCD
Mo-Kα (0.71073 Å)
Triclinic
ˉ
P1
8.735(5)
10.919(5)
23.272(5)
98.092(5)
93.574(5)
96.018(5)
2178.7(17)
1.625
C46H50N2Pd2Se4
1159.52
0.20 × 0.15 × 0.10
298(2)
Bruker APEX-II CCD
Mo-Kα (0.71073 Å)
Monoclinic
C2/C
24.997(5)
9.941(5)
18.149(5)
90.00
97.287(3)
90.00
3435.45(18)
1.772
97.153(5)
90.00
4475(3)
1.721
γ (°)
Volume (Å3)
ρcalcd (Mg m−3
)
Z
1
4
2
4
μ (mm−1)/F(000)
θ range (°)
12.829/488
4.2772–66.4884
−10 ≤ h ≤ 10
−13 ≤ k ≤ 13
−13 ≤ l ≤ 13
13 045/3294
3294/0/205
0.0291/0.0631
0.0407/0.0676
1.021
12.475/1808
4.1459–66.4852
−18 ≤ h ≤ 17
−17 ≤ k ≤ 17
−18 ≤ l ≤ 12
10 027/4265
5990/0/389
0.0446/0.0895
0.0723/0.1016
0.983
2.626/1064
2.78–26.48
−10 ≤ h ≤ 10
−13 ≤ k ≤ 13
−27 ≤ l ≤ 29
33 744/8908
8908/0/498
0.0277/0.0701
0.0393/0.0796
1.111
4.090/2272
2.64–26.76
−32 ≤ h ≤ 33
−13 ≤ k ≤ 12
−24 ≤ l ≤ 23
5558/3544
5558/0/249
0.0336/0.0759
0.0804/0.1013
1.029
Index range
Reflections collected/unique
Data/restraints/parameters
Final R1, ωR2 indices
R1, ωR2 (all data)
Goodness of fit on F2
C46H50N2Pd2S2Se2: C, 51.84; H, 4.73; N, 2.63; S, 6.02. Found: C, m.p.: >200 °C). Analysis Calcd for C36H36ClPdPSe: C, 60.00; H,
1
1
51.30; H, 4.72; N, 2.02; S, 5.78%. H NMR (300 MHz, acetone- 5.04. Found: C, 59.71; H, 5.32%. H NMR (300 MHz, CDCl3) δ:
d6) δ: 1.93 (s, 4-Me, non-metallated), 2.22 (s, 4- and 6-Me, 1.95 (s, 4-Me, non-metallated), 2.17, 2.24 (each s, 4- and 6-Me,
metallated), 2.43 (s, 2,6-Me, non-metallated), 3.46 (AX pattern, metallated), 2.69 (br, CH2), 6.48, 6.70 (each s, 3-,5-H, metal-
CH2, ΔγAX 135 Hz, JAX = 14 Hz), 6.75 (s, C6H2 metallated), 6.88 lated), 6.88 (s, 3-,5-H, non-metallated), 7.41–7.44 (br, Ph),
(s, C6H2 non-metallated), 6.10 (t, CH-4, C5H4N), 6.81 (t, CH-5, 7.72–7.78 (br, Ph) ppm. 31P{1H} (300 MHz, CDCl3) δ: 31.7 ppm.
C5H4N), 7.13 (d, 7.8 Hz, CH-3, C5H4N), 7.74 (d, 5 Hz, CH-6, 77Se{1H} NMR (CDCl3) δ: 393 (d, 2J(Se–P) = 167 Hz) ppm.
C5H4N) ppm. 77Se {1H} NMR (CDCl3) δ: 406 ppm.
Computational methods
[Pd2(µ-Sepy)2{MesSeC6H2(Me2)CH2}2]
(3b). Pb(Sepy)2
(142 mg, 0.027 mmol) was added to a dichloromethane solu- The reactants, intermediates and transition states were fully
tion of [Pd(µ-Cl){MesSeC6H2(Me2)CH2}]2 (250 mg, 0.027 mmol) optimized in the gas phase at the B3LYP level of theory26
and the contents were stirred for 6 h. The solution was dried using the Gaussian09 suite of quantum chemical programs.27
and the complex was washed with hexane (5 ml × 2). The The hybrid density functional B3LYP is used in conjunction
residue was extracted with toluene and re-crystallized as pale with the LANL2DZ basis set28 for palladium and the remain-
yellow crystals from an acetone–hexane mixture (175 mg, 55% ing elements were treated with Pople’s 6-31G** basis set. All
yield; m.p.: 155 °C). Analysis Calcd for C46H50N2Pd2Se4: C, the stationary points were characterized by frequency calcu-
1
47.65; H, 4.35; N, 2.42. Found: C, 48.37; H, 4.63; N, 2.32 . H lations to authenticate the nature of the transition states
NMR (300 MHz, CDCl3) δ: 0.80–2.33 (m), 2.69–4.01 (m), 6.12 (TSs) as having one and only one imaginary frequency repre-
(d, 7.2 Hz), 6.45 (t, 6.2 Hz), 6.66–6.87 (br, m), 7.38 (d, 4.8 Hz), senting the desired reaction coordinate. Other minima were
7.59 (t, 7.6 Hz), 8.02 (d, 7.8 Hz) ppm. 77Se {1H} NMR (CDCl3) δ: characterized by a Hessian index of zero. The Intrinsic Reac-
341, 378, 380, 408.
tion Coordinate (IRC) calculations were performed at the
[PdCl{MesSeC6H2(Me2)CH2}PPh3]. PPh3
(60
mg, B3LYP level of theory to examine whether the TS on the
0.023 mmol) was added to a dichloromethane solution of energy profiles connect to the desired minima.29 The conti-
[Pd2(µ-Cl2){MesSeC6H2(Me2)CH2}2] (102 mg, 0.011 mmol) and nuum solvation effects were included through an SMD sol-
the contents were stirred for 10 min. The reaction mixture was vation model.30 All the stationary points were subjected to
dried in vacuo and the residue was washed with hexane and single-point energy calculations at the SMD(EtOH)/B3LYP/
extracted with toluene. The toluene extract was dried under 6-31G** level of theory. The Gibbs free energies and enthal-
vacuum to yield a cream colored solid (114 mg, 71% yield; pies for all the stationary points in the condensed phase were
10830 | Dalton Trans., 2013, 42, 10828–10837
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obtained by adding the corresponding ZPVE and thermal 2, on treatment with PPh3, results in bridge-cleavage to give a
energies obtained in the gas phase computations to the mononuclear product [PdCl{MesSeC6H2(Me2)CH2}(PPh3)].
1
single point energies.
The H NMR spectra showed the expected resonances. The
spectrum of 2 exhibited resonances attributable to the metal-
lated and non-metallated mesityl group. The metallated CH2
protons are anisotropic and appear as a distinct AB pattern at
a significant downfield of 3.37 ppm with ΔγAB = 46 Hz, and JAB
= 12.6 Hz due to the strong trans influence of the metallated
Results and discussion
The reaction of dimesityl selenide (Mes2Se) with either carbon. In 3a, the above mentioned metallated CH2 showed a
PdCl2(PhCN)2 in toluene or PdCl2 in a toluene–acetonitrile distinct AX pattern further downfield at 3.46 ppm with a larger
mixture yields a chloro-bridged binuclear palladium complex, chemical shift difference (ΔγAX = 135 Hz) and coupling con-
[Pd2Cl2(µ-Cl)2(Mes2Se)2] (1) in a fairly good yield. When the stant ( JAX = 14 Hz) in the 1H NMR spectra. The 77Se NMR
reaction with Na2PdCl4 in refluxing ethanol was carried out, a spectra of these complexes displayed a single resonance which
cyclometallated
palladium
complex,
[Pd2(µ-Cl)2{Mes- is deshielded with respect to the free ligand (δ 234 ppm). The
SeC6H2(Me2)CH2}2] (2) was isolated (as shown in Scheme 2). In 77Se NMR resonances for 2 (408 ppm) and 3a (406 ppm) are
some preparations, 2 in the former and 1 in the latter reactions considerably deshielded compared to 1 (322 ppm). Such a
also formed in very poor yields (<5%) and could be separated large deshielding could be due to the presence of a strong
conveniently on the basis of their solubility. 2 can also be trans influencing group (CH2) cis to the selenium in the metal-
obtained when 1 was refluxed in ethanol. The density func- lated complexes (2 and 3a) rather than the non-metallated
tional theory computations suggest that the formation of 1 derivative (1), which has a weak trans influencing ligand (Cl).
and 2 are exoergic, respectively, by −61.2 and −63.3 kcal The 1H NMR spectrum of 3b was quite complex due to the
mol−1. 2, on treatment with Pb(Epy)2 in dichloromethane, existence of other isomeric species in solution which were also
afforded the Epy-bridged binuclear complexes, [Pd2(µ-Epy)2- corroborated by the 77Se NMR spectrum, which displayed reso-
{MesSeC6H2(Me2)CH2}2] (3; E = S (3a) or Se (3b)). The thiolato- nances attributable to two isomeric forms (sym-cis and
bridged derivative is exoergic by −84.3 kcal mol−1 31
.
Complex sym-trans). However, only one isomeric form could be crystallized
Scheme 2
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Fig. 1 ORTEP drawing of [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1), the ellipsoids are drawn
with 25% probability.
from
solution.
The
cyclopalladated
complexes,
[Pd2(Me2NCH2C6H4)2{μ-SC5H3(R)N}2] (R = H or Me),32,33 are
known to adopt different isomeric forms and in solution, a
dynamic equilibrium may exist between them.33
Fig. 3 ORTEP drawing of [Pd2(µ-Spy)2{MesSeC6H2(Me2)CH2}2] (3a), the ellip-
soids are drawn with 25% probability.
The molecular structures of 1, 2, 3a and 3b, established by
X-ray diffraction analyses, are shown in Fig. 1–4. Selected inter-
atomic parameters are summarized in Tables 2–5. All the com-
plexes are dimeric, with the palladium atoms acquiring a dis-
torted square planar configuration. The Pd–Se distances are
well within the range reported in palladium selenolate com-
plexes, such as [PdCl(SeCH2CH2NMe2)]3,34 [Pd(OAc)-
(SeCH2CH2CH2NMe2)]235 and [Pd(OAc)(SeC6H4Me-2)]4.17
Complex 1, a centrosymmetric dimer, comprises a chloro-
bridged “Pd2(µ-Cl)2” core. The coordinated seleno ether
ligands are mutually trans. The Pd–Cl(bridging) distances are
longer than the terminal Pd–Cl distance. The Pd–Clbridging
trans to the seleno ether ligand is marginally longer than the
one trans to the terminal chloride ligand. The C1–Se1–C10
angle is 104.08°. The mesityl ring lies ∼7° above the Pd2Cl2
plane so as to result in a intra-molecular C–H⋯Pd interaction.
The Pd⋯H7C distance and Pd⋯H7C–C7 angle are 2.75 Å and
111.23° respectively. The bond distance and angle indicate
that this interaction is agostic in nature. The Pd⋯H7C and
Cl2⋯H7B distances are smaller than the sum of their van der
Waals radii, 2.75 vs. 2.83 Å and 2.86 vs. 2.95 Å, respectively.
These interactions support the involvement of the metal
center in C–H (sp3) bond activation leading to metallation.
The above proposed mechanism of agostic interaction-driven
Fig. 2 ORTEP drawing of [Pd2(µ-Cl)2{MesSeC6H2(Me2)CH2}2] (2) and relative orientation of the rings, the ellipsoids are drawn with 50% probability.
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Table 3 Selected bond lengths (Å) and angles (°) of [Pd2(µ-Cl)2{Mes-
SeC6H2(Me2)CH2}2] (2). The theoretical values are obtained at the B3LYP/
6-31G**, LANL2DZ(Pd) level of theory
Molecule a
Experimental Theoretical Molecule b
Pd1–Se1
Pd1–C7
Pd1–Cl1
Pd1–Cl1i
Se1–C1
2.3438(8)
2.024(7)
2.4786(17)
2.3739(16)
1.912(7)
1.948(6)
3.472
2.423
2.042
2.564
2.445
1.938
1.955
3.559
Pd2–Se2
Pd2–C25
Pd2–Cl2
Pd2–Cl2i
Se2–C19
Se2–C28
Pd2–Pd2i
2.3342(9)
2.039(7)
2.4833(17)
2.3713(17)
1.925(7)
1.946(7)
3.440
Se1–C10
Pd1–Pd1i
Se1–Pd1–Cl1 93.02(4)
Se1–Pd1–Cl1i 177.92(5)
95.18
174.91
86.49
178.08
89.85
88.53
118.17
97.37
110.52
90.52
Se2–Pd2–Cl2 92.72(5)
Se2–Pd2–Cl2i 176.84(5)
Se2–Pd2–C25 87.0(2)
C25–Pd2–Cl2 179.3(2)
C25–Pd2–Cl2i 90.5(2)
Cl2–Pd2–Cl2i 89.78(6)
Pd2–C25–C20 117.4(5)
Pd2–Se2–C19 98.6(2)
Pd2–Se2–C28 107.62(19)
Pd2–Cl2–Pd2i 90.22(6)
Se1–Pd1–C7
C7–Pd1–Cl1
87.22(19)
179.7(2)
C7–Pd1–Cl1i 91.1(2)
Cl1–Pd1–Cl1i 88.67(6)
Pd1–C7–C2
Pd1–Se1–C1
117.0(5)
98.5(2)
Pd1–Se1–C10 107.49(19)
Pd1–Cl1–Pd1i 91.33(6)
of the mesityl ring comes into the central Pd2Cl2 plane to
satisfy the square planar geometry around palladium, while
the non-metallated mesityl rings remain nearly perpendicular
(86.17° for molecule a and 83.86° for molecule b) to the planar
metallated PdCCCSe rings. The two five-membered planar
metallated PdCCCSe rings are co-planar with the four-mem-
bered rectangular Pd2Cl2 ring. The coordination around each
palladium atom is defined by two Cl atoms and a C and Se
Fig. 4 ORTEP drawing of [Pd2(µ-Sepy)2{MesSeC6H2(Me2)CH2}2] (3b); ellipsoids
are drawn with 25% probability.
benzylic C–H bond activation is also supported by theoretical atom of the metallated seleno ether. The two bridging Pd–Cl
calculations, as discussed letter. The optimized structural distances are distinctly different. The one trans to the metal-
parameters, as obtained through the DFT(B3LYP) compu- lated carbon atom is longer than the one trans to the selenium,
tations, are in good general agreement with the experimental owing to the strong trans influence of the CH2 group. This is
(X-ray) values.
in good agreement with the reported values (e.g. [Pd2(µ-
There are two independent half molecules of 2 in the unit Cl)2(py-C6H4)2]; Pd–Cl = 2.426(1), 2.349(2) Å.36 The Pd–C
cell, which differ slightly in bond lengths, bond angles and and Pd–Se18 distances conform with the reported values (e.g.,
torsion angles. Only one of these is shown in Fig. 2. The mole- [PdCl{OC6H4C(Ph) = NCH2CH2SePh}], Pd–Se = 2.3575(6) Å).18
cule adopts a sym-trans configuration. In contrast to molecule The Pd⋯Pd separation of ∼3.45 Å is within the range observed
1, where both the mesityl rings of the seleno ether are nearly in chloro-bridged cyclometallated palladium complexes
perpendicular to the central Pd2Cl2 plane, in molecule 2, one (∼3.5 Å).2
8,36
Table 2 Selected bond lengths (Å) and angles (°) of [Pd2Cl2(µ-Cl)2(Mes2Se)2] (1). The theoretical values are obtained at the B3LYP/6-31G**, LANL2DZ(Pd) level of
theory
Experimental
Theoretical
Experimental
Theoretical
Se1–Pd1
Pd1–Cl1i
Pd1–Cl2
2.3837(5)
2.3307(10)
2.2810(10)
2.464
2.444
2.328
Pd1–Cl1
C1–Se1
C10–Se1
2.3504(9)
1.947(4)
1.945(4)
3.452
176.37(3)
116.04(11)
103.38(11)
104.08(16)
95.04(3)
2.439
1.951
1.958
3.551
177.29
112.84
114.44
104.84
93.37
Pd⋯Pd
Cl1–Pd1–Se1
Cl1i–Pd1–Se1
Cl2–Pd1–Se1
Cl1i–Pd1–Cl1
Cl2–Pd1–Cl1
172.57(3)
97.51(3)
86.04(3)
84.96(3)
91.42(3)
172.71
88.23
94.32
86.08
91.28
Cl2–Pd1–Cl1i
C1–Se1–Pd1
C10–Se1–Pd1
C10–Se1–C1
Pd1–Cl1–Pd1i
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Table 4 Selected bond lengths (Å) and angles (°) of [Pd2(µ-Spy)2{MesSeC6H2(Me2)CH2}2] (3a). The theoretical values are obtained at the B3LYP/6-31G**, LANL2DZ-
(Pd) level of theory
Experimental
Theoretical
Experimental
Theoretical
Pd1–Se1
Pd1–C6
Pd1–S1
Pd1–N2
Se1–C8
Se1–C15
2.4005(12)
2.037(3)
2.3136(13)
2.179(2)
1.924(3)
1.950(3)
2.477
2.061
2.399
2.242
1.937
1.955
Pd2–Se2
Pd2–C29
Pd2–S2
Pd2–N1
Se2–C31
Se2–C38
Pd1–Pd2
Se2–Pd2–S2
Se2–Pd2–C29
Se2–Pd2–N1
C29–Pd2–N1
C29–Pd2–S2
S2–Pd2–N1
Pd2–Se2–C31
Pd2–Se2–C38
C31–Se2–C38
Se1–Pd1–Pd2–S2
Se1–Pd1–Pd2–Se2
2.4184(6)
2.025(3)
2.3218(9)
2.171(2)
1.919(3)
1.949(3)
2.9288(9)
172.12(2)
85.05(8)
96.85(6)
176.47(10)
87.70(8)
90.55(7)
97.09(9)
118.57(8)
103.39(12)
64.56(3)
2.476
2.062
2.398
2.242
1.936
1.955
3.033
172.19
85.66
94.95
173.13
86.53
92.82
96.92
110.97
105.47
60.84
−111.49
Se1–Pd1–S1
Se1–Pd1–C6
Se1–Pd1–N2
C6–Pd1–N2
C6–Pd1–S1
S1–Pd1–N2
Pd1–Se1–C8
Pd1–Se1–C15
C8–Se1–C15
S1–Pd1–Pd2–S2
S1–Pd1–Pd2–Se2
172.71(2)
85.94(8)
96.86(6)
174.19(11)
86.89(9)
90.40(6)
97.89(10)
114.57(8)
102.35(12)
−122.78(3)
64.91(2)
172.19
85.66
94.94
173.15
86.53
92.82
96.94
110.92
105.49
−126.80
60.85
−107.749(19)
Table 5 Selected bond lengths (Å) and angles (°) of [Pd2(µ-Sepy)2{MesSeC6H2(Me2)CH2}2] (3b). The theoretical values are obtained at the B3LYP/6-31G**,
LANL2DZ(Pd) level of theory
Experimental
Theoretical
Experimental
Theoretical
C6–Pd1
2.045(4)
2.154(3)
2.068
2.241
3.078
2.491
C1–Se1
Pd1i–Se1
C15–Se2
C8–Se2
1.900(5)
2.4197(8)
1.953(4)
1.934(4)
1.904
2.496
1.954
1.942
N1–Pd1
Pd1i–Pd1
3.1429(8)
2.4197(8)
2.4156(8)
171.26(16)
110.18(14)
87.92(12)
84.15(13)
78.396(16)
104.033(16)
172.059(19)
102.04(13)
−142.25
Se1i–Pd1
Se2–Pd1
2.507
C6–Pd1–N1
C6–Pd1–Pd1i
C6–Pd1–Se1i
C6–Pd1–Se2
Se1i–Pd1–Pd1i
Se2–Pd1–Pd1i
Se2–Pd1–Se1i
C1–Se1–Pd1i
Se1–Pd1i–Pd1–Se1i
Se2–Pd1–Pd1i–Se2i
176.05
101.26
87.78
83.43
81.56
101.42
171.11
103.29
−130.96
−114.54
C15–Se2–Pd1
C8–Se2–Pd1
C1–N1–Pd1
C5–N1–Pd1
N1–Pd1–Pd1i
N1–Pd1–Se1i
N1–Pd1–Se2
106.71(11)
95.84(12)
124.9(3)
115.4(3)
78.53(9)
94.77(9)
93.13(9)
108.01
95.26
125.85
114.72
88.98
92.73
95.93
Se1i–Pd1P–d1i–Se2i
Se1–Pd1i–Pd1–Se2
45.50
45.50
56.96
57.52
−126.74
The two palladium atoms in 3a and 3b are held together by distances are in accord with those reported in [Pd2(µ-Spy)2-
bridging pyridyl chalcogenolate groups (Epy), forming an (Me2NCH2C6H4-C,N)2],32
[Pd3(µ-Spy)2(Me2NCH2C6H4-C,N)3]-
eight-membered ring which adopts a distorted twist boat con- [BF4]33 and [Pd2(µ-SCNCH2CH2NMe)2(Bzq)2].8 The Pd–Se dis-
formation. The chalcogen atoms of the Epy ligands are trans to tances can be compared with [Pd2(µ-SePh)2(C10H6NMe2-C,
the Se atoms of the metallated ligand resulting in an anti con- N)2]11 and [Pd2(µ-SePh)2(Me2NCH2C6H4-C,N)2].32 The Pd⋯Pd
figuration, which is generally observed for Pd2(µ-Spy)2L4 type distance in 3a (2.9288(9) Å) is within the generally acceptable
complexes.8,37,38 The configuration around the palladium in range of intra-molecular Pd⋯Pd interactions (<3.00 Å), while
both the complexes is distorted square planar but the mutual in the corresponding selenolate complex (3b) it is much
orientation of the square planes are different for 3a and 3b. longer (3.143 Å). In complex 3a, one of the mesityl rings on
The angle between the square planes S1Pd1Se1N2 and both the selenium ligands are parallel to each other and are
S2Pd2Se2N1 is 20.97° in 3a and between Se1Pd1iN1iSe2i and separated by 3.587 Å, which may be attributed to π-stacking,
Se2Pd1Se1iN1 is 39.02° in 3b, resulting in different orien- and are positioned on the opposite side of the pyridine thio-
tations of the metallated mesityl ring in the two complexes. late rings. In 3b, one mesityl group on each of the Se(Mes)2
The metallated mesityl rings of 3a display fairly good intra- ligands is also almost but not exactly parallel to the Sepy
molecular π–π stacking with an angle of 2.04° between the rings.
planes and average stacking separation of 3.587 Å. This type of
Density functional theory computations have been carried
interaction is absent in 3b. The Pd–S, Pd–N and Pd–C out to gain additional insight on the mechanism for the
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Scheme 3 Mechanistic pathways for the formation of 2 [Pd2(µ-Cl)2{MesSeC6H2(Me2)CH2}2] through aryl C–H bond activation.
formation of compound 2. While several pathways could be energies are provided in Fig. 5. The first key step involves the
envisaged as likely under the experimental conditions, we have conversion of trans-[PdCl2(Mes2Se)2] (10) to the chloro-bridged
presented those which are computed to exhibit lower energies. dimer 1. One of the methyl C–H bonds that remains closest to
All the higher energy possibilities are described in the ESI the palladium is activated in such a way that it combines with
(Fig. S5–S8, Schemes S1–S4†). The most likely mechanistic the chloride ligand to form HCl. The expulsion of HCl would
scenarios are summarized in Scheme 3 and the computed furnish a monocyclopalladated intermediate 4. The transition
states for the formation of HCl from both palladium atoms are
located. These transition states TS(1–4) and TS(4–2), as shown
in Fig. 6, are of very similar energies. It can further be noticed
from the computed energy profile that the C–H activation with
Fig. 6 Optimized geometries of the important transition states (B3LYP/
Fig. 5 Gibbs free energy profile (in kcal mol−1) for the formation of [Pd2-
(µ-Cl)2{MesSeC6H2(Me2)CH2}2] (2) from PdCl2 and Mes2Se obtained at the
SMD(EtOH)/B3LYP/6-31G**//B3LYP/6-31G** level of theory.
6-31G**, LANL2DZ(Pd) level of theory) involved in the formation of palladacycles
4 and 2. Distances are in Å. Only select hydrogen atoms are shown. Atom
colors: C, black; H, ivory; O, red; Cl, yellow; Pd, green; Se, pink.
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the assistance of methanol, designated as pathways B and C, is 19 D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purifi-
of higher energy than the unassisted process.
cation of Laboratory Chemicals, Pergamon, Oxford, 2nd edn,
1980.
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Rigaku, Rigaku/MSC 2000–2005, 9009 New Trials Dr., The
Wood, TX 77381, USA.
Conclusion
Cyclopalladation of an organoselenium ligand, dimesityl sele-
nide, is accomplished in mild reaction conditions. The poss-
ible intra-molecularly coordinated Pd⋯HC intermediate is
isolated. The cyclopalladated complex serves as a useful pre-
cursor for the synthesis of other derivative containing metal-
lated selenium ligands. The computed energy profile shows
that the methanol assisted C–H activation is of higher energy
than the unassisted process.
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P. W. Betteridge, CAMERON – A Molecular Graphics Package,
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Acknowledgements
We thank Dr S. K. Sarkar for his encouragement of this work
and Prof. U. P. Singh, IIT Roorkee for his help in recording
some of the single crystal XRD data. IIT Bombay computer
center is acknowledged for the computing time.
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