A hemilabile binucleating pincer ligand for self-assembly of coordination oligomers and polymers

Article abstract of DOI:10.1039/b716035f

The bis(PNP)-donor pincer ligand 1,4-C₆H₄{N(CH ₂CH₂PPh₂)₂}₂, 1, contains weakly basic nitrogen donor atoms because the lone pairs of electrons are conjugated to the bridging phenylene group, and this feature is used in the synthesis of oligomers and polymers. The complexes [Pd₂X ₂(μ-1)](OTf)₂, X = Cl, Br or OTf, contain the ligand 1 in bis(pincer) binding mode (μ-κ⁶-P₄N ₂), but [Pd₄Cl₆(μ_3-1) ₂]Cl₂ contains the ligand in an unusual unsymmetrical μ₃-κ⁵-P₄N binding mode. The bromide complex is suggested to exist as a polymer [{Pd₂Br ₄(μ₄-1)}_n] with the ligands 1 in μ₄-κ⁴-P₄ binding mode. The methylplatinum(ii) complexes [Pt₂Me₄(μ-1)] and [Pt ₂Me₂(μ-1)](O₂CCF₃)₂ contain the ligand 1 in μ-κ⁴-P₄ and μ-κ⁶-P₄N₂ bonding modes, while the silver(i) complex [Ag₂(O₂CCF₃)₂ (μ-1)] contains the ligand 1 in an intermediate bonding mode in which the nitrogen donors are very weakly coordinated. The complexes [Pd ₂(OTf)₂(μ-1)](OTf)₂ and [Ag ₂(O₂CCF₃)₂(μ-1)] react with 4,4′-bipyridine to give polymers [Pd₂(μ-bipy)(μ-1)](OTf) ₄ and [Ag₂(μ-bipy)(μ-1)](O₂CCF ₃)₂. The Royal Society of Chemistry.

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PAPER
www.rsc.org/dalton | Dalton Transactions
A hemilabile binucleating pincer ligand for self-assembly of coordination
oligomers and polymers†‡
Chang-Qiu Zhao, Michael C. Jennings and Richard J. Puddephatt*
Received 19th October 2007, Accepted 7th December 2007
First published as an Advance Article on the web 9th January 2008
DOI: 10.1039/b716035f
The bis(PNP)-donor pincer ligand 1,4-C6H4{N(CH2CH2PPh2)2}2, 1, contains weakly basic nitrogen
donor atoms because the lone pairs of electrons are conjugated to the bridging phenylene group, and
this feature is used in the synthesis of oligomers and polymers. The complexes [Pd2X2(l-1)](OTf)2, X =
Cl, Br or OTf, contain the ligand 1 in bis(pincer) binding mode (l-j6-P4N2), but [Pd4Cl6(l3–1)2]Cl2
contains the ligand in an unusual unsymmetrical l3-j5-P4N binding mode. The bromide complex is
suggested to exist as a polymer [{Pd2Br4(l4-1)}n] with the ligands 1 in l4-j4-P4 binding mode. The
methylplatinum(II) complexes [Pt2Me4(l-1)] and [Pt2Me2(l-1)](O2CCF3)2 contain the ligand 1 in l-j4-P4
and l-j6-P4N2 bonding modes, while the silver(I) complex [Ag2(O2CCF3)2 (l-1)] contains the ligand 1 in
an intermediate bonding mode in which the nitrogen donors are very weakly coordinated. The
complexes [Pd2(OTf)2(l-1)](OTf)2 and [Ag2(O2CCF3)2(l-1)] react with 4,4ꢀ-bipyridine to give polymers
[Pd2(l-bipy)(l-1)](OTf)4 and [Ag2(l-bipy)(l-1)](O2CCF3)2.
Introduction
Tridentate pincer ligands have proved to have many useful roles
in coordination chemistry, with applications in catalysis and
molecular materials.1,2 Within this class of ligand, the hemilabile
PNP-donor pincer ligands have been widely studied and the
presence of the weak nitrogen-donor centre in promoting some
catalytic reactions has been well established.1–3 The reversibility of
the N-donor group in ligands of the type RN(CH2CH2PPh2)2 is
illustrated by the complexes shown in Chart 1. Tridentate coor-
dination is illustrated by the cationic square planar palladium(II)
and platinum(II) complexes A–E, but the N-donor can be displaced
reversibly by ligands such as chloride to give the complexes such
as F, which contains an eight-membered chelate ring with a
cis-bidentate ligand, or G, which contains a sixteen-membered
macrocycle and in which the ligand bridges between palladium(II)
centres.4 The preference for formation of F (M = Pt) versus G (M =
Pd) appears to depend on the preference of the metal centre for cis
or trans stereochemistry, under conditions where the chelate effect
is not dominant. If the anions X− are weakly binding, such as
the triflate ion, the compounds of type E are formed exclusively.4f
Chart 1 Representative complexes with PNP pincer ligands.
When X = Cl, the complex which crystallizes may depend on the
substituent R in the ligand RN(CH2CH2PPh2)2, so that complex
the easy, reversible M–N bond cleavage offers a low energy route
A or D is formed if R = H or benzyl, but complex G is formed
for coordination of reagents and displacement of products in a
if R = C( O)-4-C6H4Me.4d,e Thus, it is known that the preference
=
catalytic cycle.1–4
for forming the cationic complexes A–E with tridentate ligands
or the neutral complexes F or G with bidentate ligands (Chart 1)
can be controlled by modifying the relative donor strength of the
nitrogen donor (by adjusting the electron donating or withdrawing
ability of the substituent R) and of the anion X−.4 In catalysis,
The hemilabile PNP-donor pincer ligands have not been
used to such a great extent in forming polymeric molecular
materials, although more rigidly binding pincer ligands have
proved to be exceptionally useful in this field.1,2 However, several
binucleating PNP-donor pincer ligands have been synthesized
and used in coordination chemistry. The palladium(II), plat-
inum(II) and copper(I) complexes H–J (Chart 1) are representative
examples.5 This paper reports a new bis(PNP)-donor ligand 1,4-
C6H4{N(CH2CH2PPh2)2}2 and a study of its use in forming
oligomeric and polymeric transition metal complexes.
Department of Chemistry, University of Western Ontario, London, Canada
N6A 5B7. E-mail: pudd@uwo.ca
† The HTML version of this article has been enhanced with colour images.
‡ CCDC reference numbers 664675–664677. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b716035f
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complex [Pd2Cl2(l-1)](OTf)2, 3a, which was characterized by a
single resonance in the d(31P) NMR spectrum at d(31P) = 32.3.
Complex 3a could also be formed by reaction of complex 2a with
triflic acid. Reaction of complex 3a with silver triflate occurred by
abstraction of the chloride ligands to give the triflate complex
[Pd2(OTf)2(l-1)](OTf)2, 4, which was characterized by a single
resonance in the d(31P) NMR spectrum at d(31P) = 43.6. These
reactions were reversible. Thus, treatment of complex 4 with two
or four equivalents of lithium chloride gave back the complexes
3a and 2a respectively, as monitored by 31P NMR spectroscopy.
The complexes 2a and 3a were characterized by structure
determinations. The structure of 3a is shown in Fig. 1. There
is a centre of symmetry at the middle of the C6H4 group, such that
the two square planar palladium(II) centres are equivalent. Each
palladium has stereochemistry similar to that in the mononuclear
complexes such as A (Chart 1).4h There are no close contacts
between palladium centres and the triflate anions. Compared to
complex A, the Pd–N distance in 3a is slightly longer [2.114(3)
Results and discussion
Synthesis of the ligand 1
The synthesis of the ligand 1,4-C6H4{N(CH2CH2PPh2)2}2 is
shown in Scheme 1. The precursor 1,4-C6H4{N(CH2CH2OH)2}2
was prepared by the literature method.6 It is reported to be
unstable so it was immediately converted to the tosylate derivative
1,4-C6H4{N(CH2CH2OTs)2}2 by reaction with tosyl chloride
in the presence of pyridine as base. Treatment of the tosylate
with lithium diphenylphosphide then gave the new ligand
1,4-C6H4{N(CH2CH2PPh2)2}2, 1. The ligand 1 was isolated as a
white solid and was characterized in the 31P NMR spectrum by a
singlet at d(31P) = −20.0, which is the typical range for ligands of
this type.4,5
˚
versus 2.065(5) A] while the Pd–Cl is slightly shorter [2.2866(9)
˚
versus 2.3082(2) A]. This difference can be rationalised in terms of
the C6H4 substituent in 3a making the nitrogen a weaker donor
than in A with a resulting decrease in the trans-influence.
Scheme 1 Synthesis of the ligand 1. Reagents are: (i) ClCH2CH2OH–
NaOH; (ii) tosyl chloride–pyridine; (iii) Ph2PLi.
Palladium chloride and triflate complexes
The reaction of ligand 1 with two equivalents of [PdCl2(COD)]
occurred cleanly to give a complex which was identified as
[Pd4Cl6(l3-1)2]Cl2, 2a (Scheme 2). In the 31P NMR spectrum,
complex 2a gave two equal intensity singlet resonances at d(31P) =
10.8 and 32.4, assigned to the phosphorus atoms in the central
macrocycle (compare to complex G in Chart 1 with average
d(31P) = 11.6)4e and the terminal units (compare to complexes
D and E in Chart 1 with d(31P) = 32.2 and 32.6 respectively)4b,e
respectively. The reaction of ligand 1 with an equimolar mixture
of PdCl2 and [Pd(acac)2] in the presence of triflic acid gave the
Fig. 1 Structure of the dicationic complex 3a. Selected bond pa-
rameters: Pd–N(31) 2.114(3); Pd–Cl 2.2866(9); Pd–P(2) 2.2968(9);
˚
Pd–P(1) 2.3227(10) A; N(31)–Pd–Cl 178.23(7); N(31)–Pd–P(2) 85.77(8);
Cl–Pd–P(2) 94.75(3); N(31)–Pd–P(1) 83.76(8); Cl–Pd–P(1) 95.89(3);
P(2)–Pd–P(1) 168.19(3)◦. Symmetry equivalent atoms: 1.5 − x, 1.5 − y,
4 − z.
The structure of the dicationic complex 2a is shown in Fig. 2(a).
There is an inversion centre in the centre of the molecule.
However, the central palladium(II) centres, Pd(2) and Pd(2A),
have trans-PdCl2P2 coordination. This part of the structure is
analogous to the structure of complex G (Chart 1), and the bond
parameters are also similar.4e An unusual feature is the presence of
a pair of dichloromethane molecules which bridge weakly between
the two PdCl2 units through secondary bonding interactions
of the type Pd(2)–Cl(4) · · · H(61D)CHCl–Cl(6D) · · · Pd(2A), with
˚
˚
Pd(2A) · · · Cl(6D) = 3.499 A and Cl(4) · · · H(61D) = 2.62 A, as
shown in Fig. 2(b). The terminal palladium(II) centres, Pd(1) and
Pd(1A) with PdClNP2 coordination, have similar stereochemistry
and bond parameters as in complex 3a as well as to A (Chart 1).
The Pd–N bond is slightly shorter in 2a than in 3a [2.103(7) versus
Scheme 2 Reagents are: (i) [PdCl2(COD)], − COD; (ii) PdCl2
+
˚
2.114(3) A] while the Pd–Cl bond is slightly longer [2.295(2) versus
2.2866(9) A]. A likely reason for these small differences is that
[Pd(acac)2] + HOTf; (iii) AgOTf, − AgCl; (iv) LiCl, − LiOTf; (v) HOTf, −
˚
HCl.
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Fig. 2 The structure of complex 2a: (a) structure of the dication;
(b) structure with phenyl groups omitted, showing the secondary bonding
to dichloromethane solvate molecules. Selected bond parameters: Pd(1)–
N(1) 2.103(7); Pd(1)–P(2) 2.293(3); Pd(1)–P(1) 2.293(3); Pd(1)–Cl(1)
2.295(2); Pd(2)–Cl(4) 2.295(2); Pd(2)–Cl(3) 2.298(2); Pd(2)–P(3A)
˚
2.310(2); Pd(2)–P(4) 2.313(2) A; N(1)–Pd(1)–P(2) 85.4(2); N(1)–Pd(1)–
P(1) 85.4(2); P(2)–Pd(1)–P(1) 165.2(1); N(1)–Pd(1)–Cl(1) 177.1(2);
P(2)–Pd(1)–Cl(1) 95.62(9); P(1)–Pd(1)–Cl(1) 94.1(1); Cl(4)–Pd(2)–Cl(3)
179.36(8); Cl(4)–Pd(2)–P(3A) 88.81(7); Cl(3)–Pd(2)–P(3A) 91.33(7);
Cl(4)–Pd(2)–P(4) 89.50(7); Cl(3)–Pd(2)–P(4) 90.56(7); P(3A)–Pd(2)–P(4)
161.21(7)◦. Symmetry equivalent atoms: A, −x, −y, −1 − z.
Scheme 3 Bromide complexes of palladium(II). Reagents: (i) Bu4NBr, −
Bu4NOTf.
the lone pair of the free nitrogen atom N(8) in 2a is conjugated
with the phenylene group7 and this leads to an increase in electron
density at N(1), thus increasing the donor power when compared
to the two nitrogen donors in 3a. We will discuss this effect in
greater depth below.
The structure of complex 2a illustrates very clearly the fine
balance in the competition between coordination of chloride
and the nitrogen atom of the pincer ligand to palladium(II) to
give either the trans-PdCl2P2(freeN) or the [PdClNP2]+Cl− unit.
Complex 2 contains two units of each type (Fig. 2). This
observation prompted a study of the corresponding bromide
complexes, assuming that the softer bromide would compete more
effectively than chloride for coordination to palladium(II).
36.8 for 3b]. The ESI-MS, recorded in the presence of formic
acid to aid ionization, gave the highest mass peak at m/z = 4373
+
corresponding to the ion [Pd6Br11(1)3 ]. We have not been able to
grow single crystals of 6 suited to structure determination, but the
spectroscopic data indicates an oligomeric or polymeric structure
as indicated for 6 in Scheme 3.
In another experiment, a solution of complex 4 in CD3CN
in an NMR tube was treated with successive one equivalent
amounts of n-Bu4NBr and the reaction was monitored by 31P
NMR spectroscopy. With one equivalent, the peak due to complex
4 at d(31P) = 44 [PdONP2] was replaced by two peaks assigned to
complex 5 (Scheme 3) at d(31P) = 37 [PdBrNP2] and 46 [PdONP2].
After addition of two equivalents, the spectrum contained only a
singlet resonance for complex 3b at d(31P) = 37 [PdBrNP2]. Note
that the chemical shifts of the 31P NMR resonances for complex
5 (d = 37 and 46) are similar to those of 4 (d = 44) and 3b
(d = 37), so the spectrum assigned to 5 could also be assigned
to a mixture of these three complexes. After addition of three
equivalents of bromide, there were again two resonances at d(31P) =
13 [PdBr2P2] and 37 [PdBrNP2], which are assigned to complex 2b
because the spectrum is similar to that of 2a. After four equivalents,
precipitation of 6 occurred but the remaining solution contained
only a singlet at d(31P) = 12 [PdBr2P2]. Similar steps were observed
in the addition of chloride to 4 except that the last addition gave no
precipitation or spectral change, indicating that 2a was unreactive
towards chloride addition. It is remarkable that the change from
chloride to bromide causes such a difference, but the chemistry
does show how the hemilabile nature of the nitrogen donor in 1 can
be used to make oligomeric or polymeric molecular compounds.
Palladium bromide complexes
The reaction of complex 4 with two equivalents of n-Bu4NBr
gave the complex [Pd2Br2(l-1)](OTf)2, 3b (Scheme 3), which was
characterized spectroscopically by comparison to the chloride
analog 3a. For example, the 31P NMR spectrum contained a singlet
at d(31P) = 36.8, in the range expected for a pincer complex [3a,
d(31P) = 32.3]. However, a new reaction occurred on reaction
of complex 4 with four equivalents of n-Bu4NBr in acetonitrile
solution. Most of the product precipitated as a yellow solid, which
analysed as Pd2Br4(l-1), 6. The formula is analogous to that
of complex 2a, but complex 6 is much less soluble than 2a and
has different spectroscopic features. Thus, the 31P NMR spectrum
contained only a single broad resonance at d(31P) = 12.3, in the
region found for the central unit in complex 2a [d(31P) = 10.8] and
no resonance in the region expected for pincer ligation [d(31P) =
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Synthesis of methylplatinum(II) complexes
The reaction of [Pt2Me4(l-SMe2)2] with ligand 1 occurred easily to
give the methylplatinum(II) complex [Pt2Me4(l-1)], 7 (Scheme 4).
In the 1H NMR spectrum, the methylplatinum resonance occurred
at d(1H) = 0.09 with coupling constant 2J(PtH) = 68 Hz and, in
the 31P NMR spectrum, there was a single resonance at d(31P) =
18.1 with coupling constant 1J(PtP) = 2280 Hz. These parameters
are characteristic for square planar platinum(II) complexes with
cis-PtMe2P2 coordination, so it is clear that the nitrogen atoms of
1 are not coordinated.8 Reaction of complex 7 with trifluoracetic
acid occurred to give methane and the pincer complex [Pt2Me2(l-
1)](CF3CO2)2, 8. Complex 8 gave a single resonance in the 31P
NMR spectrum at d(31P) = 31.5, in the range expected for a
Scheme 5 Synthesis if the disilver(I) complex 9. Reagent: (i) AgO2CCF3.
1
pincer complex,4 with a coupling constant J(PtP) = 3760 Hz,
much higher than in 7. The methylplatinum resonance for 8
appeared at d(1H) = 0.79, as a triplet [3J(PH) = 6 Hz] with
2
satellites due to coupling to 195Pt with J(PtH) = 80 Hz. The
increase in the coupling constant 2J(PtH) in complex 8 compared
to 7 is consistent with the trans-NPtMe stereochemistry. Reaction
of complex 8 with excess trifluoroacetic acid led to loss of the
methylplatinum resonances, but a mixture of platinum-containing
products was formed from which no pure compounds could be
isolated.
Fig. 3 Structure of the complex [Ag2(O2CCF3)2(l-1)], 9. Selected bond
˚
parameters: Ag–O(21) 2.268(2); Ag–P(2) 2.4149(8); Ag–P(1) 2.4496(8) A;
O(21)–Ag–P(2) 130.69(6); O(21)–Ag–P(1) 109.10(6); P(2)–Ag–P(1)
120.03(3)◦.
PMP angle is 120.03(3)◦ in 9 but always close to linear in 2a and
3a. The stereochemistry at silver(I) can be considered as trigonal
planar, with the monodentate trifluoroacetate ligand occupying
˚
the third coordination site. The Ag · · · N(3) distance of 2.97 A
is too long for a normal covalent bond but it could represent
a weak bonding interaction. For comparison the distances for
˚
2a are Pd(1)–N(1) = 2.104(7) A for the covalent bond and
Scheme
(ii) 2CF3CO2H, − 2CH4.
4
Synthesis of methylplatinum complexes: (i) −2SMe2;
˚
Pd(2) · · · N(8) = 5.15 A for the non-bonding interaction. The bond
˚
distances to the trifluoroacetate ligand, Ag–O(21) = 2.268(2) A
˚
and Ag · · · O(22) = 3.18 A, indicate an essentially monodentate
binding mode.
Synthesis and structure of a disilver(I) complex
Some bond parameters for the phenylenediamine units of the
complexes 2a, 3a and 9 are listed in Table 1, and illustrate
differences associated with the different bonding modes (note that
a common atom labelling system, different from the figures, is
used to facilitate comparisons). The simplest case is for complex
3a for which the bonding can be described as in I (Table 1). The
The direct reaction between silver trifluoroacetate and ligand
1 gave the complex [Ag2(O2CCF3)2(l-1)], 9. Complex 9 was
sparingly soluble in common organic solvents. It gave only a
broad single resonance in the 31P NMR spectrum at d(31P) = −2,
1
with no resolved coupling to silver. In the H NMR spectrum
the resonances for the CH2N and CH2P protons were also broad,
and the combined data indicate easy fluxionality of the complex
in solution. The low solubility did not allow a low temperature
NMR study. The structure was found to be 9 (Scheme 5) by X-ray
structure determination.
The structure of complex 9 is shown in Fig. 3. The two halves
of the molecule are related by an inversion centre. There do not
appear to be any related structures of silver(I) but complex J
(Chart 1) is a copper(I) analogue.5c The binding mode of the PNP
ligand 1 is different from complexes 2a and 3a in the sense that the
bond angles at the equivalent atoms N1 and N1a are close to
ꢀ
tetrahedral [sum of the three CNC angles, CN(1)C = 328(1)◦,
which is the expected value for an ideal tetrahedron], the distance
˚
N1–C1 = 1.482(4) A corresponds well to a single bond, and the
CC distances in the C6H4 unit are roughly equal. In this case the
lone pairs on the nitrogen donors of the ligand 1 are fully engaged
in r-bonding to palladium, so there is no nitrogen-aryl p-bonding.
Another extreme example is found in the unsymmetrical binding
mode of the ligand in complex 2a, which can be described as
close to III (Table 1). Thus, the sums of the three CNC angles,
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Table 1 Bond parameters for the para-phenylenediamine units in com-
plexes 2a, 3a and 9
Scheme 6 Synthesis of 4,4ꢀ-bipyridine bridged oligomers or polymers.
3a
9
2a
that the binding mode of 1 did not change on coordination of
the 4,4ꢀ-bipyridine ligand. For example, in the 31P NMR, 4 and
10 gave resonances at d(31P) = 43.6 [PdONP2] and 37.2 [PdN2P2],
and 9 and 11 gave resonances at d(31P) = −2.2 [AgOP2] and −3.2
[AgNP2], respectively. In principle, reaction of 10 or 11 with more
bipyridine ligand might convert the one dimensional polymer to a
two dimensional sheet material by displacement of the nitrogen-
donor of ligand 1. The compounds did react but it was not possible
to isolate pure materials.
N(1)–C(1)
N(1a)–C(1a)
C(1)–C(2)
C(1a)–C(2a)
C(1)–C(3)
C(1a)–C(3a)
C(2)–C(3a)
1.482(4)
1.482(4)
1.382(5)
1.382(5)
1.386(5)
1.386(5)
1.389(5)
1.389(5)
328(1)
1.433(3)
1.433(3)
1.384(4)
1.384(4)
1.383(4)
1.383(4)
1.392(4)
1.392(4)
342(1)
1.365(10)
1.482(10)
1.409(10)
1.380(11)
1.406(11)
1.383(11)
1.375(11)
1.380(11)
360(2)
C(2a)–C(3)
ꢀ
CN(1)C
CN(1a)C
ꢀ
328(1)
342(1)
327(2)
ꢀ
ꢀ
CN(1)C = 360(2)◦ and CN(1a)C = 327(2)◦ for 2a are close
Conclusions
to the ideal trigonal planar and tetrahedral stereochemistries
The bis(PNP)-donor pincer ligand 1,4-C6H4{N(CH2CH2PPh2)2}2,
1, contains weakly basic nitrogen donor atoms because the lone
pairs of electrons are conjugated to the bridging phenylene group.
For this reason, the ligand is more “hemilabile” than similar
ligands containing hydrogen or alkyl groups in place of the
1,4-C6H4 group,1–4 and this feature is particularly useful in the
synthesis of oligomeric and polymeric coordination compounds.
The ligand 1 can bind as a conventional bis(pincer) ligand in the
l2-j6-P4N2 binding mode, as in the complex cation [Pd2Cl2(l-
1)]2+ but it can also bind in the l2-j4-P4 and l4-j4-P4 binding
modes, as in the complexes [Pt2Me4(l-1)] and [{Pd2Br4(l4-1)}n],
respectively. In the silver complex [Ag2(O2CCF3)2 (l-1)], the ligand
binding mode is intermediate between the l2-j6-P4N2 and l2-
j4-P4 binding modes, with very weakly coordinated nitrogen
atoms. Finally, there is a symbiotic effect which can favour an
unsymmetrical l3-j5-P4N bonding mode for the ligand 1 and this
is found in the tetrapalladium complex [Pd4Cl6(l3-1)2]Cl2. Two
routes to polymeric complexes have been established, one relying
on conventional pincer ligand chemistry and the other relying on
the hemilabile nature of the ligand 1.
respectively. The distances C1a–N1a = 1.482(10) and C1–N1 =
˚
1.365(10) A are significantly different and closer to the expected
values for a single and double bond respectively (C–N 1.47 A,
˚
˚
=
C N 1.29 A). In addition, the distances C1–C2 and C1–C3 appear
longer than the other C–C distances in the C6H4 unit, though
the differences are comparable to the error limits (Table 1). It is
probably this communication between nitrogen donors through
the phenylene bridge of ligand 1 which favours the unsymmetrical
bridging mode, as found in 2a, when compared to other bis(pincer)
ligands such as those in complexes H–J (Chart 1). There is a
symbiotic effect because the N1-aryl p-bonding makes the lone
pair unavailable for coordination while increasing the electron
density at N1a and enhancing its donor ability, and coordination
of N1a draws electron density from the aryl group and enhances
the N1-aryl p-bonding. In complex 9 the sum of the three CNC
ꢀ
◦
˚
angles, CN(1)C = 342(1) and the distance C–N = 1.433(3) A
are intermediate values (Table 1). Therefore it seems that the lone
pair on nitrogen is weakly involved both in p-bonding to the C6H4
group and in coordination to silver(I), as illustrated by II (Table 1).
Synthesis of 4,4ꢀ-bipyridine bridged oligomers or polymers
Part of the research plan was to use the hemilabile nature of the
ligand 1 in forming polymers and network materials. This work
was partly successful but was limited by difficulty in crystallizing
and characterizing sparingly soluble products. The displacement
of the weakly bound oxygen-donor ligand in [Pd2(OTf)2(l-
1)](OTf)2, 4, or [Ag2(O2CCF3)2(l-1)], 9, by linear 4,4ꢀ-bipyridine
gavethecorrespondingpolymers[Pd2(l-4,4ꢀ-bipy)(l-1)](OTf)4, 10,
or [Ag2(l-4,4ꢀ-bipy)(l-1)] (O2CCF3)2, 11, respectively (Scheme 6).
The 1H and 31P NMR data for the coordinated ligand 1 indicated
Experimental
NMR spectra were recorded using a Varian Mercury 400 or
Inova 400 MHz NMR spectrometer. ESI-MS were recorded using
a Micromass LCT spectrometer in MeCN solution; MALDI-
TOF data were acquired using a Micromass TofSpec 2E mass
spectrometer in positive ion mode. The sensitive precursor 1,4-
C6H4{N(CH2CH2OH)2}2 was prepared by the literature method6
and it was used immediately for ligand synthesis.
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N,N,Nꢀ,Nꢀ-Tetrakis(2-diphenylphosphinoethyl)-1,
4-phenylenediamine, 1
then filtered to remove insoluble material, and the solvent was
evaporated under vacuum to give the product as a yellow solid. The
solid was purified by extraction into CH2Cl2 (5 mL), followed by
filtration to remove traces of insoluble material, concentration to
ca. 1 mL, and addition of ether (10 mL) to precipitate the product.
Yield: 350 mg, 99%. Anal. Calc. for C66H60F12N2O12P4Pd2S4: C,
44.89; H, 3.42; N, 1.59. Found: C, 44.54; H, 3.74; N, 1.35%. NMR
(CD3CN): d(1H) = 2.36 [m, 4H, 2J(HH) = 14 Hz, CHaHbN]; 3.23
[m, 4H, 2J(HH) = 14 Hz, CHaHbN]; 4.05 [m, 4H, CHaHbP]; 4.20
[m, 4H, CHaHbP]; 7.5–7.9 [m, 44H, Ph + C6H4]; d(31P) = 43.6 [s,
PdONP2].
To an ice-cold solution of 1,4-C6H4{N(CH2CH2OH)2}2 (1.42 g,
5 mmol) in dry pyridine (10 mL) was added 4-toluenesulfonyl
chloride (4.7 g, 25 mmol). The mixture was stirred at 0 ◦C for 16 h.
Cold ether (30 mL) was added to the mixture to precipitate the
product 1,4-C6H4{N(CH2CH2OTs)2}2 as a red solid, which was
separated by filtration, washed with ether and water, and dried
under vacuum. The yield was 3.9 g, 87%, and the product was
used in the next step without further purification.
To an ice-cold solution of diphenylphosphine (2 mL, 11.5 mmol)
in dry THF (20 mL), was added dropwise n-BuLi (1.60 M soluti◦on
in hexane, 7.2 mL, 11.5 mmol). The mixture was stirred at 0 C
for 1 h, then 1,4-C6H4{N(CH2CH2OTs)2}2 (2.06 g, 2.3 mmol) was
added. The mixture was stirred at 0 ◦C for 20 h, then hydrolysed
with water (50 mL), and the mixture was extracted with CH2Cl2
(3 × 50 mL). The organic layer was washed with water, dried over
MgSO4, filtered and the solvent evaporated to give the product
1,4-C6H4{N(CH2CH2PPh2)2}2, which was recrystallized from hot
acetone–methanol. The crystalline product was obtained as white
needles. Yield: 1.4 g, 64%. Anal. Calc. for C62H60N2P4: C, 77.81;
H, 6.32; N, 2.93. Found: C, 77.34; H, 6.01; N, 2.68%. NMR in
CDCl3: d(1H) = 3.20 [m, 8H, CH2]; 3.69 [m, 8H, CH2]; 6.90 [s, 4H,
C6H4]; 7.58–7.80 (m, 40H, Ph); d(31P) =. −20.0 [s, P].
[Pd2Br2(l-1)](OTf)2, 3b
A mixture of complex 4 (88 mg, 0.05 mmol) and n-Bu4NBr
(32.2 mg, 0.1 mmol) in MeCN (10 mL) was stirred for 1 h, then
the volume was reduced to ca. 3 mL and ether (10 mL) was added
to precipitate the product as a yellow solid, which was separated,
washed with methanol and ether, and dried under vacuum. Yield:
65 mg, 80%. Anal. Calc. for C64H60Br2F6N2O6P4Pd2S2: C, 47.22;
H, 3.72; N, 1.72. Found: C, 46.93; H, 3.88; N, 1.63%. NMR (d6-
acetone): d(1H) = 2.05 [m, 4H, 2J(HH) = 14 Hz, CHaHbN]; 3.15
[m, 4H, 2J(HH) = 14 Hz, CHaHbN]; 4.01 [m, 4H, CHaHbP]; 4.24
[m, 4H, CHaHbP]; 7.3–7.9 [m, 44H, Ph + C6H4]; d(31P) = 36.8 [s,
PdBrNP2].
[Pd4Cl6(l-1)2]Cl2, 2
[{Pd2Br4(l-1)}n], 6
PdCl2 (0.177 g, 1 mmol) was dissolved in hot MeCN (15 mL) with
stirring. The solution was cooled to room temperature, and ligand
1 (0.478 g, 0.5 mmol) was added. The mixture was stirred for 24 h,
the volume of solvent was reduced to ca. 5 mL, and the yellow
solid product was separated by filtration, washed with MeCN and
ether, and dried under vacuum. Yield: 0.56 g, 85%. Anal. Calc. for
C124H120Cl8N4P8Pd4: C, 56.77; H, 4.61; N, 2.14. Found: C, 56.35;
H, 4.33; N, 2.09%. NMR (d6-acetone–CF3COOH): d(1H) = 2.26
[m, 2H, 2J(HH) = 13 Hz, CHaHbN]; 2.60 [m, 4H, CH2N]; 2.92 [m,
2H, 2J(HH) = 13 Hz, CHaHbN]; 3.53 [m, 4H, CH2P]; 3.87 [m, 2H,
CHaHbP]; 4.06 [m, 2H, CHaHbP]; 7.28–7.92 [m, 44H, Ph + C6H4];
d(31P) = 10.8 [s, PdCl2P2]; 32.4 [s, PdClNP2].
To a solution of [Pd2(OTf)2(l-1)](OTf)2, 4 (44 mg, 0.025 mmol)
in CD3CN (0.5 mL) was added n-Bu4NBr (32.2 mg, 0.1 mmol).
The 31P NMR spectrum contained a single broad resonance at
d(31P) = 12.3. Most of the product precipitated as a yellow solid,
which was separated by filtration, washed with MeCN and ether,
and dried under vacuum. Yield: 26 mg, 70%. Anal. Calc. for
C62H60Br4N2P4Pd2: C, 50.00; H, 4.06; N, 1.88. Found: C, 50.14;
H, 4.05; N, 1.89%. NMR (CD3CN–CF3COOH): d(1H) = 2.55
[m, 4H, CH2N]; 3.50 [m, 4H, CH2P]; 7.28–7.92 [m, 44H, Ph +
C6H4]; d(31P) = 12.3 [s, PdBr2P2]. ESI-MS (MeCN–HCO2H):
m/z (cited for Br79) = 1405 [Pd2Br3(1)+]; 1485 [Pd2Br4(1)H+];
1669 [Pd3Br5(1)+]; 2889 [Pd4Br7(1)2 ]; 2969 [Pd4Br8(1)2H+]; 4373
+
+
[Pd6Br11(1)3 ].
[Pd2Cl2(l-1)](OTf)2, 3a
In another experiment the n-Bu4NBr was added to complex
4 in 1 equivalent amounts and the reaction was monitored by
31P NMR. 0 equiv., d(31P) = 44 [PdONP2]; 1 equiv., d(31P) = 37
[PdBrNP2] and 46 [PdONP2]; 2 equiv., d(31P) = 37 [PdBrNP2];
3 equiv., d(31P) = 13 [PdBr2P2] and 37 [PdBrNP2]; 4 equiv., d(31P) =
12 [PdBr2P2], 6. The solution remained homogeneous until the last
step, when yellow solid formed in the NMR tube.
A mixture of ligand 1 (96 mg, 0.1 mmol), HOTf (73% solution
in water, 60 mg, 0.3 mmol), [Pd(acac)2] (30.4 mg, 0.1 mmol) a◦nd
PdCl2 (18 mg, 0.1 mmol) in MeCN (4 mL) was heated at 80 C
for 2 h. The solution was cooled to room temperature and ether
(10 mL) was added to precipitate the product as a yellow solid.
Yield: 120 mg, 78%. Anal. Calc. for C64H60Cl2F6N2O6P4Pd2S2: C,
49.95; H, 3.93; N, 1.82. Found: C, 50.10; H, 4.06; N, 1.99%. NMR
2
(d6-acetone): d(1H) = 2.09 [m, 4H, J(HH) = 13 Hz, CHaHbN];
[Pt2Me4(l-1)], 7
3.11 [m, 4H, 2J(HH) = 13 Hz, CHaHbN]; 4.08 [m, 4H, CHaHbP];
4.19 [m, 4H, CHaHbP]; 7.4–7.95 [m, 44H, Ph + C6H4]; d(31P) =
32.3 [s, PdClNP2].
To a solution of [Pt2Me4(l-SMe2)2] (0.1 mmol) in acetone (5 mL)
was added a solution of ligand 1 (95.7 mg, 0.1 mmol) in acetone
(5 mL). The mixture was stirred for 3 h, then the solvent was
evaporated to give the product as a white solid, which was
recrystallized from acetone ether. Yield: 114 mg, 81%. Anal. Calc.
for C66H72N2P4Pt2: C, 56.33; H, 5.16; N, 1.99. Found: C, 56.10; H,
5.02; N, 1.82%. NMR (CD2Cl2): d(1H) = 0.09 [m, 6H, 2J(PtH) =
68 Hz, MePt]; 2.27 [m, 8H, CH2N]; 3.38 [m, 4H, 2J(PH) = 19 Hz,
[Pd2(OTf)2(l-1)](OTf)2, 4
To a suspension of [Pd4Cl6(l-1)2]Cl2, 2a, (260 mg, 0.1 mmol)
in CH2Cl2 (20 mL) was added a solution of AgOTf (230 mg,
0.9 mmol) in MeCN (10 mL). The mixture was stirred for 16 h,
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CH2P]; 6.25 [2, 4H, C6H4]; 7.0–7.3 [m, 40H, Ph]; d(31P) = 18.1 [s,
1J(PtP) = 2280 Hz, PtMe2P2].
[Ag2(l-4,4ꢀ-bipy)(l-1)] (O2CCF3)2, 11
To a solution of [Ag2(O2CCF3)2(l-1)], 9 (28 mg, 0.02 mmol) in
acetone (5 mL) was added 4,4ꢀ-bipyridine (3.2 mg, 0.02 mmol) in
acetone (2 mL). The mixture was stirred for 3 h, then the volume
of solvent was reduced to 2 mL and ether (10 mL) was added to
precipitate the product as a white solid. Yield: 25 mg, 80%. Anal.
Calc. for C76H68Ag2F6N4 O4P4: C, 58.70; H, 4.41; N, 3.60. Found:
C, 58.47; H, 4.26; N, 3.38%. NMR (d6-acetone): d(1H) = 2.35 [m,
8H, CH2N]; 3.45 [m, 8H, CH2P]; 7.2–8.5 [m, 52H, bipy + Ph +
C6H4]; d(31P) = −3.2 [br s, AgNP2].
[Pt2Me2(l-1)](CF3CO2)2, 8
To a solution of [Pt2Me4(l-1)], 7, (35 mg, 0.025 mmol) in CH2Cl2
(2 mL) was added CF3CO2H (3.8 lL, 0.05 mmol). The mixture
was stirred for 15 min, then ether (10 mL) was added to precipitate
the product as a white solid. Yield: 26 mg, 64%. NMR (CD2Cl2):
d(1H) = 0.79 [t, 6H, 3J(PH) = 6 Hz, 2J(PtH) = 80 Hz, MePt]; 1.88
2
2
[m, 4H, J(HH) = 14 Hz, CHaHbN]; 2.77 [m, 4H, J(HH) =
14 Hz, CHaHbN]; 3.29 [m, 4H, 2J(HH) = 13 Hz, 2J(PH) =
21 Hz, CHaHbP]; 4.32 [m, 4H, 2J(HH) = 13 Hz, 2J(PH) = 21 Hz,
CHaHbP]; 7.0–7.6 [m, 44H, Ph + C6H4]; d(31P) = 31.5 [s, 1J(PtP) =
3760 Hz, PtNMeP2].
X-Ray structure determinations
Crystals were mounted on glass fibres. Data were collected at 150 K
using a Nonius Kappa-CCD diffractometer with COLLECT
(Nonius B.V., 1998).9 The unit cell parameters were calculated
and refined from the full data set. Crystal cell refinement and data
reduction were carried out using DENZO (Nonius B.V., 1998).
The data were scaled using SCALEPACK (Nonius B.V., 1998).9
The SHELXTL-NT V6.1 suite of programs was used to solve
the structures by direct methods.9 Subsequent difference Fourier
syntheses allowed the remaining atoms to be located. The hydrogen
atom positions were calculated geometrically and were included
as riding on their respective carbon atoms. Details of the crystal
data and refinements are in Table 2.
A yellow block crystal of 2a·2CH2Cl2·2C2H4Cl2·6CHCl3 was
grown from a dichloromethane–dichloroethane–chloroform so-
lution by slow diffusion of pentane. The crystal was grown
with difficulty after many trials and it diffracted weakly. The
dichloroethane solvate molecule was disordered (60/40) and the
[Ag2(O2CCF3)2(l-1)], 9
To a solution of AgO2CCF3 (111 mg, 0.5 mmol) in acetone (5 mL)
was added ligand 1 (239 mg, 0.25 mmol). The mixture was stirred
for 3 h, then the volume of solvent was reduced to 3 mL and
ether (10 mL) was added to precipitate the product as a white
solid. Yield: 270 mg, 72%. Anal. Calc. for C66H60Ag2F6N2O4P4: C,
56.67; H, 4.32; N, 2.00. Found: C, 56.44; H, 4.09; N, 2.00%. NMR
(d6-acetone): d(1H) = 2.60 [m, 8H, CH2N]; 3.58 [m, 8H, CH2P];
7.2–7.8 [m, 44H, Ph + C6H4]; d(31P) = −2.2 [br s, AgOP2].
[Pd2(l-4,4ꢀ-bipy)(l-1)](OTf)4, 10
˚
To a solution of [Pd2(OTf)2(l-1)](OTf)2, 4 (35.2 mg, 0.02 mmol)
in MeCN (1 mL) was added 4,4ꢀ-bipyridine (3.2 mg, 0.02 mmol)
in MeCN (1 mL). The mixture was stirred for 30 min, then the
product was precipitated as a yellow solid by addition of ether
(5 mL). Yield: 21 mg, 55%. Anal. Calc. for C76H68F12N4O12P4Pd2S4:
C, 47.49; H, 3.57; N, 2.91. Found: C, 47.03; H, 3.37; N, 2.78%.
NMR (CD3CN): d(1H) = 2.04 [m, 4H, CHaHbN]; 2.91 [m, 4H,
CHaHbN]; 3.76 [m, 4H, CHaHbP]; 4.01 [m, 4H, CHaHbP]; 7.5–8.5
[m, 52H, bipy + Ph + C6H4]; d(31P) = 37.2 [s, PdN2P2].
C–Cl distances were fixed (1.72 A). A yellow block crystal of 3a
was grown from CHCl3–hexane. There were no unusual features
in the structure solution. A colourless block crystal of 9 was grown
from acetone solution by slow evaporation. There was disorder of
the fluorine atoms of the trifluoroacetate groups (80/20) and the
C–F bonds were restrained to be equal using the SADI command.
CCDC reference numbers 664675–664677.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b716035f
Table 2 Crystal data and refinement details
Complex
2a·2CH2Cl2·2C2H4Cl2·6CHCl3
3a
9
Formula
FW
C136H138Cl34N4P8Pd4
3707.16
150(2)
0.71073
Monoclinic
P21/c
15.9810(2)
12.3410(2)
41.0338(7)
99.264(1)
7987.2(2)
2
C64H60Cl2F6N2O6P4Pd2S2
1538.84
150(2)
0.71073
Monoclinic
C2/c
31.9519(5)
12.4261(2)
17.3210(3)
111.707(1)
6389.4(2)
4
C66H60Ag2F6N2O4P4
1398.78
150(2)
0.71073
Monoclinic
P21/c
12.3770(2)
11.8752(2)
21.1926(4)
100.225(1)
3065.41(9)
2
T/K
˚
k/A
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
d(calc.)/Mg m−3
l/mm−1
1.541
1.139
1.600
0.883
1.515
0.811
Data/restr./param.
R(int)
R1 [I > 2r(I)]
wR2 (all data)
13870/252/769
0.057
0.081
7334/0/349
0.049
0.041
6271/51/358
0.046
0.036
0.242
0.110
0.091
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Hii, Org. Biomol. Chem., 2004, 2, 301; (c) M. M. T. Khan, B. T. Khan
and S. Begum, J. Mol. Catal., 1988, 45, 305; (d) L.-C. Liang, J.-M. Lin
and W.-Y. Lee, Chem. Commun., 2005, 2462.
Acknowledgements
We thank the NSERC (Canada) for financial support. RJP thanks
the Government of Canada for a Canada Research Chair.
4 (a) M. M. T. Khan, V. V. S. Reddy and H. C. Bajaj, Inorg. Chim. Acta,
1987, 130, 163; (b) M. M. T. Khan, V. V. S. Reddy and H. C. Bajaj,
Polyhedron, 1987, 6, 921; (c) M. M. T. Khan and E. R. Rao, Polyhedron,
1987, 6, 1727; (d) L. Janosi, T. Kegl, L. Hajba, Z. Berente and L. Kollar,
Inorg. Chim. Acta, 2001, 316, 135; (e) K. K. Hii, M. Thornton-Pett,
A. Jutand and R. P. Tooze, Organometallics, 1999, 18, 1887; (f) T. E.
Muller, M. Berger, M. Grosche, E. Herdtweck and F. P. Schmidtchen,
Organometallics, 2001, 20, 4384; (g) M. I. Garcia-Seijo, A. Habtemariam,
D. Fernandez-Anca, S. Parsons and M. E. Garcia-Fernandez, Z. Anorg.
Allg. Chem., 2002, 628, 1075; (h) M. I. Garcia-Seijo, A. Habtemariam,
S. Parsons, R. O. Gould and M. E. Garcia-Fernandez, New J. Chem.,
2002, 26, 636.
5 (a) M. M. T. Khan, P. Paul and K. Venkatasubramanian, Polyhedron,
1991, 10, 1827; (b) M. M. T. Khan and B. Swamy, Inorg. Chem., 1987,
26, 178; (c) M. M. T. Khan, P. Paul, K. Venkatasubramanian and S.
Purohit, Inorg. Chim. Acta, 1991, 183, 229.
6 W. C. J. Ross, J. Chem. Soc., 1949, 183.
7 B. T. Kelly, J. M. Tanski, M. B. Anzovino and G. Parkin, J. Chem.
Crystal., 2005, 35, 969.
References
1 Reviews: (a) M. Albrecht and G. Van Koten, Angew. Chem., Int. Ed.,
2001, 40, 3750; (b) M. E. van der Boom and D. Milstein, Chem. Rev.,
2003, 103, 1759; (c) G. R. Newcome, Chem. Rev., 1993, 93, 2067; (d) J. T.
Singleton, Tetrahedron, 2003, 59, 1837; (e) L.-C. Liang, Coord. Chem.
Rev., 2006, 250, 1152.
2 Recent examples: (a) S. L. Jeon, D. M. Loveless, W. C. Yount and S. L.
Craig, Inorg. Chem., 2006, 45, 11060; (b) S. A. Kuklin, A. M. Sheloumov,
F. M. Dolgushin, M. G. Ezernitskaya, A. S. Peregudov, P. V. Petrovskii
and A. A. Koridze, Organometallics, 2006, 25, 5466; (c) O. V. Ozerov,
C. Y. Guo and B. M. Foxman, J. Organomet. Chem., 2006, 691, 4802;
(d) O. V. Ozerov, L. A. Watson, M. Pink and K. G. Caulton, J. Am.
Chem. Soc., 2007, 129, 6003; (e) A. J. Canty, T. Rodemann, B. W. Skelton
and A. H. White, Organometallics, 2006, 25, 3996; (f) F. E. Hahn, M. C.
Jahnke and T. Pape, Organometallics, 2006, 25, 5927; (g) W. W. Gerhardt,
A. J. Zucchero, C. R. South, U. H. F. Bunz and M. Weck, Chem.–Eur. J.,
2007, 13, 4467; (h) B. Shafaatian, A. Akbari, S. M. Nabavizadeh, F. W.
Heinemann and M. Rashidi, Dalton Trans., 2007, 4715.
8 (a) S. Jamali, M. Rashidi, M. C. Jennings and R. J. Puddephatt, Dalton
Trans., 2003, 2313; (b) M. Rashidi, M. C. Jennings and R. J. Puddephatt,
Organometallics, 2003, 22, 2612.
3 (a) C. Bianchini, L. Glendenning, M. Peruzzini, G. Purches, F. Zanobini,
E. Farnetti, M. Graziani and G. Nardin, Organometallics, 1997, 16, 4403;
(b) S. L. Parisel, N. D. Moorcroft, A. Jutand, D. J. Aldous and K. K.
9 (a) Computer programs: B. V. Nonius, COLLECT DENZO,
SCALEPACK, Nonius, B. V., Amsterdam, 1998; (b) G. M. Sheldrick,
SHELXTL-NT, Version 6.1, Madison, Wisconsin, 2000.
1250 | Dalton Trans., 2008, 1243–1250
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©