Tetradentate selenium ligand as a building block for homodinuclear complexes of Pd(II) and Ru(II) having seven membered rings or bis-pincer coordination mode: High catalytic activity of Pd-complexes for Heck reaction

Article abstract of DOI:10.1039/c0dt00561d

1,2,4,5-Tetrakis(phenyselenomethyl)benzene (L) has been synthesized by reaction of in situ generated PhSe^- with 1,2,4,5- tetrakis(bromomethyl)benzene in N₂ atmosphere. Its first bimetallic complexes and a bis-pincer complex having compositions [(η³- C₃H₅)₂Pd₂(L)][ClO₄] ₂ (1) [Pd₂(C₅H₅N) ₂(L)][BF₄]₂ (2) and [(η⁶-C ₆H₆)₂Ru₂(L)Cl₂][PF ₆]₂ (3) have been synthesized by reacting L with [Pd(η³-C₃H₅)Cl]₂, [Pd(CH ₃CN)₄][BF₄]₂ and [(η⁶-C₆H₆)₂RuCl ₂]₂ respectively. The structures of ligand L and its all three complexes have been determined by X-ray crystallography. In 1 and 3, ligand L forms with two organometallic species seven membered chelate rings whereas in 2 it ligates in a bis-pincer coordination mode. The geometry around Pd in 1 or 2 is close to square planar whereas in 3, Ru has pseudo-octahedral half sandwich "Piano-Stool" geometry. The Pd-Se bond distances are in the ranges 2.4004(9)-2.4627(14) A and follow the order 1 > 2, whereas Ru-Se bond lengths are between 2.4945(16) and 2.5157(17) A. The 1 and 2 have been found efficient catalysts for Heck reaction of aryl halides with styrene and methyl acrylate. The 2 is superior to 1. The TON and TOF values (per Pd) are up to ～ 47500 and ～2639 h^-1 respectively. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00561d

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Tetradentate selenium ligand as a building block for homodinuclear complexes
of Pd(II) and Ru(II) having seven membered rings or bis-pincer coordination
mode: high catalytic activity of Pd-complexes for Heck reaction†
Dipanwita Das, Pradhumn Singh, Monika Singh and Ajai Kumar Singh*
Received 28th May 2010, Accepted 8th September 2010
DOI: 10.1039/c0dt00561d
1,2,4,5-Tetrakis(phenyselenomethyl)benzene (L) has been synthesized by reaction of in situ generated
PhSe- with 1,2,4,5-tetrakis(bromomethyl)benzene in N2 atmosphere. Its first bimetallic complexes and a
3
bis-pincer complex having compositions [(h -C3H5)2Pd2(L)][ClO4]2 (1) [Pd2(C5H5N)2(L)][BF4]2 (2) and
6
3
[(h -C6H6)2Ru2(L)Cl2][PF6]2 (3) have been synthesized by reacting L with [Pd(h -C3H5)Cl]2,
6
[Pd(CH3CN)4][BF4]2 and [(h -C6H6)2RuCl2]2 respectively. The structures of ligand L and its all three
complexes have been determined by X-ray crystallography. In 1 and 3, ligand L forms with two
organometallic species seven membered chelate rings whereas in 2 it ligates in a bis-pincer coordination
mode. The geometry around Pd in 1 or 2 is close to square planar whereas in 3, Ru has
pseudo-octahedral half sandwich “Piano–Stool” geometry. The Pd–Se bond distances are in the ranges
˚
2.4004(9)–2.4627(14) A and follow the order 1 > 2, whereas Ru–Se bond lengths are between
˚
2.4945(16) and 2.5157(17) A. The 1 and 2 have been found efficient catalysts for Heck reaction of aryl
halides with styrene and methyl acrylate. The 2 is superior to 1. The TON and TOF values (per Pd) are
up to ~ 47500 and ~2639 h-1 respectively.
selective formation of the branched allylic products in the absence
Introduction
of directing groups, has been catalyzed by a Se–C–Se pincer-Pd
complex,17 which has also been used in the catalytic system for
single pot synthesis of homoallyl alcohols.5 There is one report in
which Heck reaction18a has been catalyzed by Se–C–Se pincer-Pd
complex. The outstanding activity of Se–C–Se pincer in selective
synthesis18b of organometallic compounds has also been reported.
The pincer ligand of Se–N–Se type was recently reported by our
group12 and its Pd-complex was found to show high catalytic
activity for Heck coupling. There are few reports on S–C–S type
bis-pincer ligands. They have been used as building blocks for
supramolecular arrays19 and polymers.20,21 Only two reports22,23
on the bis-pincer ligands of Se–C–Se type are in our knowledge.
In one22 analogues of present ligand L having different organic
groups in the place of Ph are reported, which have been found
to show propensity for trapping Hg(II). The other one describes
structurally characterized 1 : 1 complex of Mo(CO)4 unit with
methyl analogue of L.23
Metal complexes of a variety of pincer ligands having N, P, S and Se
donors in side arms and one of B, C, N and P as central donor atom
have received attention in the recent past1–12 mainly due to their ap-
plications in catalyst designing. The interest in selenium containing
pincer ligands appears to be as strong as in others. Selenylation of
propargyl-, allyl-, benzyl-, and benzoyl halides under mild reaction
conditions using trimethylstannylphenylselenide as selenylating
agent has been catalyzed13 with a complex of palladium(II) with
pincer ligand of Se–C–Se type. The Pd(II) complex of a similar type
of pincer ligand14 is involved in an efficient route to functionalized
allylboronic acids and potassium trifluoro(allyl)borates. One-pot
efficient synthesis of stereo-defined a-amino acids and homoallyl
alcohols was designed by integration of the Pd(II)-pincer ligand
S–C–S / Se–C–Se complex catalyzed borylation of allyl alcohols
in the Petasis borono-Mannich reaction and in allylation of
aldehydes and ketones2 respectively. A palladium-pincer Se–C–
Se ligand complex catalyzes direct boronation of allyl alcohols
with diboronic acid under mild conditions.15 Highly regio- and
stereoselective coupling of allyl alcohols with aldehydes has been
achieved using 5 mol % of Se–C–Se pincer-Pd complex catalyst
and p-toluenesulfonic acid in the presence of diboronic acid.16
Coupling of allylboronic acids with iodobenzenes, which results in
Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi,
110016, India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;
Fax: +91-011-26581102; Tel: +91-011-26591379
† Electronic supplementary information (ESI) available: Tables of addi-
tional crystal data and refinement parameters (Table S1), bond lengths and
angles (Table S2), IR data, Figures of non-covalent interactions with bond
lengths (Figs S1–S5), 77Se NMR spectra (Table S6–S9). CCDC reference
numbers 763938 (L), 765901 (1), 763939 (2) and 770943 (3). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00561d
No bimetallic complex (metal: ligand ratio 2 : 1) of a Se–C–
Se type bis-pincer ligand has been structurally characterized so
far. It was therefore thought worthwhile to synthesize L and
such complexes and structurally characterize them. The reac-
3
6
tions of L with [Pd(h -C3H5)Cl]2, [Pd(CH3CN)4][BF4]2 and [(h -
C6H6)2RuCl2]2 result in two type of species. With (CH3CN)4Pd(II),
3
it behaves as a Se–C–Se type bis-pincer ligand, whereas with (h -
6
C3H5)Pd(II) and (h -C6H6)Ru(II) it forms a pair of seven membered
10876 | Dalton Trans., 2010, 39, 10876–10882
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chelate rings. The two Pd(II) complexes show high catalytic
activities for Heck reaction. The results of these investigations
are presented in this paper.
(0.2 mmol), both dissolved in ~10 mL of methanol, were mixed and
stirred for ~2 h at room temperature. The resulting light orange
coloured solution was filtered and the solvent was evaporated
off under reduced pressure on a rotary evaporator to get a pale
orange solid 1. Its single crystals were grown from a 1 : 1 : 1
mixture of dichloromethane, acetonitrile and methanol. Yield
(0.098 g, 79%). mp 135 ◦C. (Found: C, 38.13; H, 2.97%. calc.
for C40H40Pd2Se4·2ClO4: C, 38.15; H, 3.93%); dH(300.13 MHz;
DMSO-d6; Me4Si) 4.33 (8 H, br s, C5-H + 4 H, br s, C allyl-Hsyn),
5.81-5.84 (2 H, m, C allyl-Hcentral), 7.48-7.62 (14 H, m, C1-H +
C2-H + C3-H + C7-H), the signal of C allyl-Hanti merged with
DMSO-d6. signal; dC(75.47 MHz; DMSO-d6; Me4Si) 34.0 (C5),
73.4 (C terminal allyl), 119.6 (C central allyl), 128.8 (C1), 130.2
(C2), 130.4 (C3), 133.1 (C7), 133.8 (C4), 135.5 (C6), one C terminal
allyl peak merged with that of DMSO-d6; dSe(57.24 MHz; CDCl3;
Me2Se) 394.5.
Experimental
Materials and instruments
Diphenyldiselenide,
and tetrakis(acetonitrile)
1,2,4,5-tetrakis(bromomethyl)benzene
palladium(II) tetrafluoroborate
procured from Sigma–Aldrich (USA) were used as received.
3
6
m-Dichlorobis(h -allyl)palladium(II)
and
di-m-chlorobis(h -
benzene) dichlororuthenium(II) were prepared according to
literature methods.24,25 All the solvents were dried and distilled
before use, by well known standard procedures. The common
reagents and chemicals available commercially in the country
were used. The C, H and N analyses were carried out with a
1
1
Perkin–Elmer 2400 Series II C, H, N analyzer. The H, 13C{ H}
Synthesis of [Pd2(C5H5N)2(L)][BF4]2 (2)
1
and77Se{ H}NMR spectra were recorded on a Bruker Spectrospin
[Pd(CH3CN)4][BF4]2 (0.088 g, 0.2 mmol) and L (0.075 g,
0.1 mmol), each dissolved in 10 mL of acetonitrile, were mixed and
the mixture refluxed for ~3 h. Thereafter the mixture was stirred
with pyridine (~0.2 mmol) at room temperature for 0.5 h. The
resulting orange solution was filtered and solvent from the filtrate
was evaporated off under reduced pressure on a rotary evaporator
until its volume reduced to ~5 ml. Diethyl- ether (~15 mL) was
mixed with the concentrate resulting in pale orange solid 2. Its
single crystals were grown by slow diffusion of diethyl ether into
its acetonitrile solution. Yield (0.110 g, 85%). mp 168 ◦C. (Found:
C, 40.43; H, 2.62; N 2.12%. calcd. for C44H38N2Pd2Se4·2BF4: C,
40.45; H, 2.65; N, 2.16%); dH(300.13 MHz; CD3CN; Me4Si) 3.4–
3.6 (8 H, m, C benzylic-H), 7.46–7.98 (20 H, m, C aryl-H), 8.52–
8.81 (10 H, m, C pyridine-H); dC(75.47 MHz; CD3CN; Me4Si)
21.5 (C5), 127.4, 130.1, 130.2, 132.7, 147.7, 151.1 (C aryl), 126.4,
134.0, 152.0 (C pyridine); dSe(57.24 MHz; CD3CN; Me2Se) 396.4.
DPX–300 NMR spectrometer at 300.13, 75.47 and 57.24 MHz
respectively. IR spectra in the range 4000-400 cm-1 were recorded
on a Nicolet Prote´ge 460 FT–IR spectrometer as KBr pellets. For
single crystal structure data were collected with a Bruker AXS
˚
SMART Apex CCD diffractometer using Mo-Ka (0.71073 A)
radiation at 273(2) K. The software SADABS26 was used for
absorption correction (if needed) and SHELXTL for space group,
structure determination and refinements.27 Hydrogen atoms were
included in idealized positions with isotropic thermal parameters
set at 1.2 times that of the carbon atom to which they are attached
in all cases. The melting points determined in open capillary are
reported as such.
Synthesis of ligand L
Diphenyldiselenide (0.64 g, 2 mmol) dissolved in 30 mL of EtOH
was stirred under nitrogen atmosphere and sodium borohydride
(0.152 g, 4 mmol) dissolved in 5 mL of aqueous NaOH (5%) was
added to it dropwise till it became colorless due to the forma-
tion of PhSeNa. 1,2,4,5-Tetrakis(bromomethyl)benzene (0.449 g,
1 mmol) dissolved in 15 mL mixture (1 : 1) of ethanol and THF
was mixed with the colorless solution with constant stirring and
the mixture stirred further for 3 h. It was poured into cold
water (30 mL). The ligand L was extracted with chloroform
(4 ¥ 25 mL) from aqueous layer. The extract was washed with
water (3 ¥ 40 mL) and dried over anhydrous sodium sulfate. The
solvent was evaporated off under reduced pressure on a rotary
evaporator to get a pale yellow solid, which on recrystallization
from chloroform–hexane (1 : 1), resulted in pale yellow single
crystals of L. Yield (0.53 g, 70%). mp 83 ◦C. (Found C, 53.95;
H, 3.92; calc. for C34H30Se4: C, 53.98; H, 3.88%); dH(300.13 MHz;
CDCl3; Me4Si) 4.02 (8 H, s, C5-H), 6.69 (2 H, s, C7-H), 7.22-
7.28 (12 H, m, C1-H + C2-H), 7.38 (8 H, d, JH–H 7.5 Hz, C3-H);
dC(75.47 MHz; CDCl3; Me4Si) 29.1 (C5), 127.5 (C1), 129.0 (C2),
130.2 (C4), 132.7 (C7), 133.8 (C3), 135.3 (C6); dSe(57.24 MHz;
CDCl3; Me2Se) 361.2.
Synthesis of [(g6-C6H6)2Ru2(L)Cl2][PF6]2 (3)
6
[h -(C6H6)2RuCl2]2 (0.05 g, 0.1 mmol) dissolved in 10 mL of
dry methanol was treated with the solution of L (0.075 g,
0.1 mmol) made in 10 mL of dry methanol with vigorous
stirring. The reaction mixture was stirred further for 10 h at
room temperature. Its volume was reduced to ~5 mL on a rotary
evaporator and ammonium hexafluorophosphate (0.2 mmol) was
added to get an orange precipitate of 3. The precipitate was
filtered and washed with cold methanol. The single crystals of
3 were grown from chloroform–acetonitrile mixture (1 : 1). Yield
(0.10 g, 70%). mp 153 ◦C. (Found C, 37.89; H, 2.75%; calc.
for C46H42Cl2Ru2Se4·2PF6: C, 37.87; H, 2.77%); dH(300.13 MHz;
CD3CN; Me4Si) 4.03 (8 H,s, C5-H), 5.61 (12 H, s, C–Ru–Ar–H),
6.70 (2 H, s, C7-H), 7.19–7.32 (12 H, m, C1-H + C2-H), 7.38 (8 H,
d, JH–H 7.5 Hz, C3-H); dC(75.47 MHz; CD3CN; Me4Si) 29.6 (C5),
88.4 (CRu–Ar), 127.5 (C1), 129.0 (C2), 130.2 (C4), 132.7 (C7),
133.8 (C3), 135.3 (C6); dSe(57.24 MHz; CD3CN; Me2Se): (d, ppm)
392.0.
Synthesis of [(g3-C3H5)2Pd2(L)][ClO4]2 (1)
General procedure for catalytic Heck reaction
3
The solution of [Pd(h -C3H5)Cl]2 (0.0366 g, 0.1 mmol) made
A mixture of styrene–methyl acrylate (2 mmol), aryl bromide
in 25 ml of CH2Cl2, L (0.075 g, 0.1 mmol) and NaClO4·H2O
(1 mmol), n-butylamine (0.146 g, 2.0 mmol), DMA (~ 4 mL)
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Table 1 Catalytic performancea of 1 and 2 in Heck reactions of aryl
bromide with styrene
Results and discussion
The synthesis of L and its complexes 1–3 are summarized in
Scheme 1. The ligand L was synthesized by reacting NaSe-
(generated in situ by reaction of NaBH4 with diphenyldiselenide in
ethanol) with 1,2,4,5-tetrakis(bromomethyl)benzene (Yield 70%).
The complexes 1 (Yield 79%) and 3 (Yield 70%) were synthesized
Catalyst 1
Catalyst 2
Ar–X
% Yield TON TOF/h-1 % Yield TON TOF/h-1
30
25
26
75
82
82
15000 833
12500 694
13000 722
37500 2083
41000 2278
41000 2278
43
31
36
90
92
90
21500 1194
15500 861
18000 1000
45000 2500
46000 2555
45000 2500
3
6
by reacting [Pd(h -C3H5)Cl]2 and [h -(C6H6)2RuCl2]2 at room
temperature in CH2Cl2 and methanol with L in 1 : 1 molar ratio,
respectively. On refluxing L with [Pd(CH3CN)4][BF4]2 in 1 : 2 molar
ratio in acetonitrile, followed by reaction with pyridine at room
temperature resulted in complex 2 in 85% yield. The reaction of L
with [Na2PdCl4] or [Pd(CH3CN)2Cl2] resulted in a highly insoluble
non crystalline orange solid, which probably did not have Pd–C
bond. Therefore [Pd(CH3CN)4][BF4]2 was chosen as a precursor
of Pd(II), as its reaction with L gives orange coloured solution
instantaneously. This is in agreement with the general observation
that if Pd is not bonded to halogen, formation of Pd–C bond is
facile.28 The ligand L has good solubility in all common organic
solvents viz., CHCl3, CH2Cl2, CH3OH and CH3CN. The complex
1 is moderately soluble in common organic solvents CHCl3,
CH2Cl2, CH3OH and CH3CN but has good solubility in DMF and
DMSO. The complexes 2 and 3 show good solubility in CH3CN
but are moderately soluble in CHCl3, CH2Cl2 and CH3OH. The
pale yellow solid L is stable and can be stored at room temperature
for several months. The solution of complex 1 in DMSO shows
signs of decomposition after 24 h, whereas the solutions of
complexes 2 and 3 made in CH3CN are stable for several days.
a Yield, TON and TOF are per Pd.
Table 2 Catalytic performancea of 1 and 2 for Heck reactions of aryl
bromide with methyl acrylate
Catalyst 1
Catalyst 2
Ar–X
% Yield TON TOF/h-1 % Yield TON TOF/h-1
Crystal structures
35
28
28
79
85
82
17500 972
14000 778
14000 778
39500 2194
42500 2361
41000 2278
45
36
38
92
94
95
22500 1250
18000 1000
19000 1055
46000 2555
47000 2611
47500 2639
Single crystal structure of L, 1, 2 and 3 have been determined by
X-ray crystallography The selected crystal data and refinement
parameters are given29 in the section “Notes and Reference” and
their more details in ESI (Table S1†). The L exhibits two type of
bonding modes. In complexes 1 and 3 seven membered chelate
rings are formed whereas in 2 it coordinates as a bis-pincer ligand.
The ORTEP diagram of L with selected bond lengths and angles
is shown in Fig. 1. More bond angles and distances are given
in ESI (see Table S2†). The Se–C(aryl) is somewhat shorter than
Se–C(alkyl). The two Pd–Se bond length of cation of 1 (ORTEP
diagram with selected bond lengths and angles is given in Fig. 2),
˚
2.4482(16)–2.4627(14) A are longer than the values 2.3543(16)–
˚
2.3669(11) A reported for Pd(II)–complexes of tridentate selenated
Schiff bases.30–32 The elongation of Pd–Se bond distance in cation
of 1 may be attributed partly due to large size of chelate ring.
The ORTEP diagram of cation of 2 is shown in Fig. 3 and in
this complex L ligates with each Pd in a bis-pincer mode. The
geometry around each Pd is slightly distorted square planar. The
a Yield, TON and TOF are per Pd.
and complex 1 or 2 (10-4 M, 100mL (in DMA), ~0.001 mol %) was
stirred for 18 h at 100–110 ◦C on an oil bath. It was cooled to
room temperature, treated with chloroform (40 mL) and filtered.
The chloroform extract was washed with acidified (HCl) water,
dried over anhydrous Na2SO4 and its solvent was evaporated on
a rotary evaporator to obtain the product which was purified by
column chromatography on silica gel using chloroform–hexane
mixture (1 : 1). It was characterized by proton and carbon-13
NMR spectra. The % yields, TON and TOF values (per Pd) are
given in Tables 1 and 2.
Pd–Se bond distances in cation of 2, 2.4004(9) and 2.4083(9) A
˚
are shorter than those of cation of 1. In complexes in which
two Se donor atoms are trans to each other this distance is
33–36
˚
generally around 2.4 A.
Some examples followed by their Pd–
˚
Se bond lengths (A) are: Pd–complex of Se–C–Se pincer ligand,
2.430(2);18 [PdCl2{(C4H3S)SeCH3}2], 2.439(2);33 Pd(II) complex of
1,6-[CH3SeCH2CH2–NH–C( O)]2C5H3N, 2.425(1), 2.422(1);34
(Me3SiCH2SeMe)2PdC12, 2.429;35 and trans-dichlorobis(phenyl-
2,4,6-trimethylbenzyl-selenido)palladium(II), 2.4299(6).36 How-
ever, Pd–Se bond distances in cation of 2 are shorter than all
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Scheme 1 Synthesis of L and complexes 1–3.
37
˚
Pd–Se bond length is further shorter (2.381(2) A). The Pd–Se
bond distances of complex 1 which has two Se cis to each other
are longer in comparison to the distances of this cis complex. The
non-covalent interactions (hydrogen bonds), O ◊ ◊ ◊ H and H ◊ ◊ ◊ F in
crystals of 1 and 2 have been noticed. The crystal packing factors
are among the reasons of their origin (See Figs. S2–S4† in ESI).
˚
The Pd–N bond length of cation of complex 2, 2.147(6) A, is some
˚
what longer than the value ~ 2.0 A reported for Pd-complexes of
tridentate selenated Schiff bases.30–32
The ORTEP diagram of cation of 3 is given in Fig. 4 with
selected bond lengths and angles. Further details are given in ESI
(Table S2†). The L ligates with two ruthenium atoms forming
two seven membered chelate rings. Each Ru in cation of 3
has pseudo-octahedral half sandwich “Piano–Stool” geometry.
6
The h -benzene ring occupies one face of octahedron and Cl
and two Se the other one. The half sandwich Ru-complex
of selenoether ligands has been scantly investigated. However,
6
6
Ru(II)(h -benzene)- and Ru(II)(h -p-cymene)-complexes of sele-
nated pyrrolidine derivative have been reported by our research
group recently in which Ru is coordinated through Se and N
forming a five membered chelate ring.38 The 3 is another example
of such Ru-selenoether complex. The Ru–C distances (2.162(11)–
Fig. 1 ORTEP diagram of L with ellipsoids at the 30% probability
level. Hydrogen atoms have been omitted for clarity. Bond lengths
˚
(A): Se(1)–C(5) 1.907(9), Se(1)–C(4) 1.971(9), Se(2)–C(12) 1.939(8),
Se(2)–C(11) 1.973(10); Bond angles (◦): C(5)–Se(1)–C(4) 103.5(4),
C(12)–Se(2)–C(11) 98.6(4). Symmetry transformation used to generate
equivalent atoms: 1-x, -y, -z.
˚
2.210(10) A) in the cation of 3 are normal and consistent with
the earlier reports.38–40 In the crystal of 3 also, weak interactions
(Figs.S5† in ESI) have been noticed. The Ru–Se bond lengths
these distances. In Pd complex of (3H-1,4,5,7-tetrahydro-2,6-
benzodiselenonine) which has Se donor atoms cis to each other,
˚
of the cation of 3 (2.4945(16) and 2.5157(17) A) fall within the
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Fig. 4 ORTEP diagram of the cation of 3 with ellipsoids at the
30% probability level. Hydrogen atoms and the PF6 anions have
Fig. 2 ORTEP diagram of the cation of 1 with ellipsoids at the 30%
probability level. Hydrogen atoms and the ClO4 anions have been
-
-
˚
˚
been omitted for clarity. Bond lengths (A): Ru–Se(1) 2.4945(16),
omitted for clarity. Bond lengths (A): Pd(2)–C 2.108(14)–2.155(12),
Pd(2)–Se(3) 2.4567(15), Pd(2)–Se(4) 2.4627(14); Bond angles (◦):
Ru–Se(2) 2.5157(17), Ru(1)–Cl(1) 2.394(4), Ru–C 2.162(11)–2.210(10);
Bond angles (◦): Se(1)–Ru(1)–Se(2) 91.75(5), C(7)–Se(1)–Ru(1) 112.6(3),
C(11)–Se(2)–Ru(1) 109.9(3). Symmetry transformation used to generate
equivalent atoms: -x, y, 0.5-z.
C(35)–Se(3)–C(34) 98.5(5), C(24)–Se(4)–C(30) 97.5(4), C–Pd(2)–C
36.1(6)–67.0(6),
Se(3)–Pd(2)–Se(4)
109.46(5)
C(21)–Pd(2)–Se(3)
91.7(5), C(22)–Pd(2)–Se(3) 127.3(7), C(23)–Pd(2)–Se(3) 157.1(5),
C(21)–Pd(2)–Se(4) 157.9(5), C(22)–Pd(2)–Se(4) 122.7(7), C(23)–Pd(2)–
Se(4) 91.1(4).. Symmetry transformation used to generate equivalent
atoms: (i) 1-x, 2-y, 2-z; (ii) -x, 1-y, 1-z.
[RuCp(CO)(C CPh)(m-Se)ZrCp2] 2.494(1) A,43 is closer to those
˚
5
5
of cation of 3. In [Ru(h -C5Me5)(m2-SeR)3Ru(h -C5Me5)]Cl (R =
Tolyl) Ru–Se bond distances are in the range 2.446(4)–2.466(4) A
and shorter than those of cation of 3, because RSe- is expected to
44
˚
be bonded strongly in comparison to a selenoether. In a diselenide
bridged complex [Ru(MeCp)(PPh3)]2(m-Se2)2(Otf)2, Ru–Se bond
45
˚
distances are 2.518(1) and 2.556(1) A, somewhat longer than
˚
those of cation of 3. The Ru–Cl bond distance of 2.394(4) A
in the cation of 3 is slightly shorter as compared to earlier
reports on half sandwich Ru(II) complexes of selenated and
46
˚
tellurated benzotriazoles (2.416(2) and 2.412(3) A, respectively)
but consistent with those reported38 for similar Ru(II) complexes of
˚
thio- and seleno-pyrrolidine derivatives, 2.3815(17) and 2.406(2) A
respectively.
NMR spectra
1
The signal in 77Se{ H}NMR spectrum of L appearing at 361.2 ppm
shifts to higher frequency by ~32–35 ppm on formation of
complexes 1–3 and does not split indicating that all four Se donor
sites of coordinated L are identical in solution. This concurs
with the molecular structures (see above) determined by X-ray
crystallography on their single crystals. Thus in solution of each
of 1 and 3 two seven membered chelate rings are equivalent and in 2
Fig. 3 ORTEP diagram of the cation of 2 with ellipsoids at the
30% probability level. Hydrogen atoms and the BF4 anions have been
-
˚
omitted for clarity. Bond lengths (A): Pd(1)–C(9) 2.008(7), Pd(1)–N(1)
2.147(6), Pd(1)–Se(2) 2.4004(9), Pd(1)–Se(1) 2.4083(9); Bond angles (◦):
C(9)–Pd(1)–N(1) 178.0(2), C(9)–Pd(1)–Se(2) 86.07(18), N(1)–Pd(1)–Se(2)
92.19(16), C(9)–Pd(1)–Se(1) 85.99(18), N(1)–Pd(1)–Se(1) 95.74(16),
Se(2)–Pd(1)–Se(1) 172.06(3). Symmetry transformation used to generate
equivalent atoms: 0.5-x, 0.5-y, -z.
1
there are two equivalent pincer coordination. In the 13C{ H}NMR
spectrum of complex 1 signal of CH2(Se) appears at a higher
frequency (4.9 ppm) relative to that of free L whereas it is shifted
to lower frequency (7.6 ppm) on the formation of 2. However, in
˚
range 2.4756(10)–2.5240(9) A reported for Ru–Se bond lengths in
1
clusters [Ru3(m3-Se)(CO)7(m3-CO)(m-dppm)] and [Ru3(m3-Se)(m3-
S)(CO)7(m-dppm)].41 The half sandwich complexes of Ru(II) with
selenated pyrrolidine derivative have Ru–Se bond distance38 in the
13C{ H}NMR spectrum of 3 the CH2(Se) signal shifts to a higher
frequency marginally (0.5 ppm) relative to that of free ligand. The
signal of Ar–C bonded to Se (C4) appears at higher frequency
1
˚
range 2.4770(5)–2.4918(9) A consistent with those of cation of 3.
(3.6 ppm) relative to that of free L in 13C{ H}NMR spectrum of 1.
2
2
For Ru(IV) complex [RuCp*{h -Se2P(i-Pr)2{h -SeP(i-Pr)2}][PF6]
However, in the case of complex 3 no shift was noticed in the signal
the Ru–Se bond lengths42 are reported in the range 2.538(2)–
of C4, which could not be identified unequivocally in the spectrum
1
˚
2.590(2) A, longer than those of cation of 3 due to steric
of 2. In 13C{ H} NMR spectrum of 1 the signals of allyl group
crowding. The Ru–Se bond distance found in bimetallic species
appearing at 73.4 and 119.6 ppm (Cterminal and Ccentral respectively)
10880 | Dalton Trans., 2010, 39, 10876–10882
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are at higher frequencies by 10.5 and 8.4 ppm respectively, relative
complexes of pincer ligands for Heck reaction.65 The two soft
donor selenium atoms may be further facilitating the formation
of Pd(0) resulting in good catalytic activity of 1 and 2 both. In
comparison to phosphine or N-heterocyclic nucleophilic carbenes
(NHCs) based catalysts 1 and 2 have the additional advantage of
moisture and air insensitivity.
3
to those of [Pd(h -C3H5)Cl]2. The signal of another Cterminal of allyl
1
was found merged with the signal of DMSO-d6. In H NMR
spectrum of 1 the signal of Hsyn of allyl group was found merged
with SeCH2 signal at 4.33 ppm. The signal of Hanti of allyl group
also merged with DMSO-d6 signal. The signal of Hcentral of allyl
group appearing at 5.82 ppm in the spectrum of 1 is at a higher
frequency (0.3 ppm) in comparison to the corresponding signal
Conclusions
3
of [Pd(h -C3H5)Cl]2, These observations imply the dispersion of
electron density towards Pd in 1. In comparison to complex of
1,2,4,5-Tetrakis(phenyselenomethyl)benzene (L) coordinates as a
bis-pincer ligand with Pd(II) in 2 whereas in 1 and 3 it forms
two seven membered chelate rings with Pd as well as Ru. Such
behaviour is shown for the first time for a tetradentate ligand
of type L. The Pd–Se bond lengths are in the ranges 2.4004(9)–
3
Pd(h -C3H5) with (Se, N) the dispersion in our case of (Se, Se)
ligand is lower,47 probably due to difference in electronegativity
1
between nitrogen and selenium. In H NMR spectrum of 3 the
6
signal (singlet) of h -benzene appears at 5.61 ppm, shifted to lower
6
frequency by 0.3 ppm with respect to that of [h -(C6H6)2RuCl2]2,
˚
2.4627(14) A and follow the order 1 > 2. The range of Ru–Se
due substitution of Cl with Se, which has relatively lower
˚
bond length is 2.4945(16)–2.5157(17) A. The geometry around
electronegativity. Similar observations are reported earlier.38,39,48,49
Pd in 1 and 2 both is close to square planar whereas Ru in
cation of 3 has pseudo-octahedral half sandwich “Piano–Stool”
geometry. Heck reaction between aryl bromides and styrene or
methyl acrylate can be efficiently catalyzed homogeneously with 1
and 2 both. The TON (per Pd) and TOF (per Pd) values are up
to ~ 47500 and ~2639 h-1 respectively. The catalytic efficiency of
2 is somewhat better than that of 1. In comparison to phosphine
or N-heterocyclic nucleophilic carbenes (NHCs) based catalysts 1
and 2 have the advantage of moisture and air insensitivity.
1
The signal of benzene ring in 13C{ H} NMR spectrum of 3 at
88.4 ppm is at a lower frequency (2.2 ppm) in comparison to that
6
of [h -(C6H6)2RuCl2]2,25 implying the presence of somewhat more
electron density on the benzene ring in 3 in comparison to that of
6
[h -(C6H6)2RuCl2]2, as corroborated with its 1H NMR spectrum.
Applications as catalyst in Heck reaction
The Heck reaction is a powerful tool for C–C bond formation in
organic synthesis.50–53 Outstanding catalyst systems developed to
date include those that employ sterically bulky and electron-rich
phosphine ligands54–56 or phosphine mimics such as N-heterocyclic
nucleophilic carbenes (NHCs).57,58 Phosphorus-, nitrogen-, and
sulfur-containing palladacycles have also emerged recently as
powerful catalysts for this reaction.59–64 The strong donating
properties of organoselenides can make their Pd(II) complex good
catalyst system.18 This has motivated us to explore 1 and 2 as
catalyst for Heck reaction at concentration 0.001 mol % using
n-butyl amine as base (eqn (1)).
Acknowledgements
Authors thank Council of Scientific & Industrial Research, New
Delhi (India) for project no 01(2113)07/EMR-II. The partial
financial assistance given by DST (India) to establish single crystal
X-ray diffraction facility at IIT Delhi, New Delhi (India) under
its FIST programme is gratefully acknowledged. D.D. and P.S.
thank University Grants Commission (India) for the award of
Junior/Senior Research Fellowship.
Notes and References
(1)
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All exploratory reactions were carried out for 18 h. The results
summarized in Table 1 reveal that complexes 1 and 2 both show
very good catalytic activity for Heck reaction. The 2 having L
coordinated in a bis-pincer mode is somewhat more efficient than
1. TON and TOF values depend on reaction conditions (solvent,
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of Se–C–Se mono-pincer ligand,18a give values higher than this.
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The Pd(0) based pathway for catalytic activity of 1 and 2 for Heck
C–C coupling appears to be most plausible particularly in case
of 2, as it has been reported that release of highly active Pd(0)
in tiny amounts is responsible for high catalytic activity of Pd(II)
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 10876–10882 | 10881
©

View Article Online
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94.325(9), V = 1476.4(10) A , Z = 2, rc = 1.697 g cm , F(000) = 740.0,
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were independent (Rint = 0.0904). The structure was refined to final
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˚
and residual electron density max./min. = 0.891 and -0.861 e A .
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˚
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7000.
˚
e A . 2: C44H38N2Pd2Se4·2BF4, Mr = 1297.02, monoclinic space group
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˚
C2/c, a = 22.765(4), b = 10.926(2), c = 18.967(4) A, a = g = 90.00, b =
3
-3
˚
102.809(3), V = 4600.3(15) A , Z = 4, rc = 1.873 g cm , F(000) = 2504.0,
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-3
˚
and residual electron density max./min. = 1.220 and -0.676 e A . 3:
C46H42Cl2Ru2Se4·2PF6, Mr = 1473.62, monoclinic space group C2/c,
˚
a = 28.964(8), b = 11.270(3), c = 19.155(6) A, a = g = 90.00, b =
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˚
electron density max./min. = 0.848 and -0.682 e A .
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10882 | Dalton Trans., 2010, 39, 10876–10882
This journal is
The Royal Society of Chemistry 2010
©