Synthesis and structural study of late transition metal complexes of N-[(diphenylphosphino)methyl]-2-pyridinamine and N-cyclohexyl-N-[(diphenylphosphino)methyl]-2-pyridinamine

Article abstract of DOI:10.1039/b109191n

The synthesis and structure of late transition metal complexes of N-[(diphenylphosphino)methyl]-2-pyridinamine and N-cyclohexyl-N-[(diphenylphosphino)methyl]-2-pyridinamine was investigated. The functionalization of pyridine-phosphines lead to enhancement

Full text of DOI:10.1039/b109191n

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Synthesis and structural study of late transition metal complexes
of N-[(diphenylphosphino)methyl]-2-pyridinamine and
N-cyclohexyl-N-[(diphenylphosphino)methyl]-2-pyridinamine
Hai-Bin Song,a Zheng-Zhi Zhangb and Thomas C. W. Mak*a
a Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong SAR, P. R. China. E-mail: tcwmak@cuhk.edu.hk
b State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin,
P. R. China
Received 9th October 2001, Accepted 23rd January 2002
First published as an Advance Article on the web 7th March 2002
The reaction of N-[(diphenylphosphino)methyl]-2-pyridinamine and N-cyclohexyl-N-[(diphenylphosphino)methyl]-
2-pyridinamine with palladium(), copper() and silver() salts and iron pentacarbonyl in different stoichiometric
ratios yielded a series of complexes that display various coordination modes, including a novel P,N(amine),-
N(pyridyl)-bridging mode in a dinuclear silver complex which is consolidated by the argentophilic interaction.
N-[(diphenylphosphino)methyl]-2-pyridinamine (L2)5b were
prepared recently according to Scheme 1. The possible coordin-
ation modes of this kind of ligands are shown in Scheme 2. The
Introduction
There has been continuing interest in the synthesis and
chemistry of hemilabile ligands in view of their wide use in
coordination, organometallic and catalyst chemistry.1 Pyridine-
phosphine ligands, which contain a soft Lewis base (the phos-
phorus atom) and a hard Lewis base (the nitrogen atom), have
attracted particular attention in recent years,2 as the electronic
differentiation of these donor atoms helps to stabilise various
types and oxidation states of metals. One prominent example
is 2-(diphenylphosphino)pyridine, Ph2Ppy, which displays
several ligation modes to transition metals: P-coordination,
N-coordination, P,N-chelating and P,N-bridging,3 generating
complexes that exhibit different spectroscopic properties,
reactivities and applications. The functionalisation of pyridine-
phosphines can lead to enhancement in catalytic reactivity.4 For
instance, higher activity and selectivity is exhibited by the
substituted ligand system I relative to Ph2Ppy in the palladium-
catalysed carbonylation of alkynes,4f–g and by II relative to
Ph2P(3-py) in the rhodium-catalysed hydroformylation of
1-octene.4h Furthermore, the introduction of methoxy groups in
the 2- and 6-positions in III facilitates the rhodium-catalysed
hydrogenation of olefins, whereas the parent pyridylphosphine
complexes are inactive.4j
Scheme 1 Reagents: (i) Ph2PCH2OH; (ii) H2SO4, C6H11OH; (iii)
Ph2PH, (CH2O)n.
Scheme 2 Possible coordination modes of phosphinopyridinamine
ligands.
P,N(pyridyl)-chelating coordination mode (d) of L1 and the
P,N(pyridyl)-bridging mode (h) of L2 have been substantiated
in previous works.5
Herein, we react L1 and L2 with iron pentacarbonyl and
palladium(), copper() and silver() salts in different stoichio-
metric ratios to form a series of complexes in which the ligands
adopt a variety of new coordination modes, namely the
The functionalised pyridinamine ligands N-[(diphenyl-
phosphino)methyl]-2-pyridinamine (L1)5a and N-cyclohexyl-
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DOI: 10.1039/b109191n

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P-coordinating (a) and P,N(pyridyl)-bridging (h) of L1 and
P,N(pyridyl)-chelating (d) modes, and a novel P,N(amine),-
N(pyridyl)-bridging mode (l) of L2, as determined from the
X-ray structures of these compounds.
[Ag(L1)2(2-Py-NH2)]ClO4, 7. 0.29 g (1 mmol) L1, 0.1 g (0.48
mmol) AgClO4 and 0.045 g (0.48 mmol) 2-aminopyridine were
added to 10 cm3 CH3CN. The mixture was stirred for 10 min,
then the solution was concentrated to 2 cm3 and diethyl ether
was diffused into it to give colorless crystals of 7ؒCH3CN
in 50% yield. Found: C, 55.20; H, 4.65; N, 9.60; calc. for
C41H40N6O4P2ClAg: C, 55.58; H, 4.55; N, 9.49%.
Experimental
General
Pd(L2)Cl2, 8. 0.13 g (0.34 mmol) L2 and 0.1 g (0.34 mmol)
Na2PdCl4 was mixed together in 10 cm3 CH2Cl2. The solution
was stirred for 20 min and then filtered to remove undissolved
material. The solution was concentrated in vacuo to ca. 1–2 cm3
and then EtOH diffused into the solution to afford yellow
crystals of 8. Found: C, 52.49; H, 4.97; N, 5.05; calc. for
C24H27N2PCl2Pd: C, 52.24; H, 4.93; N, 5.08%.
Unless otherwise stated, all reactions were performed under a
nitrogen atmosphere using Schlenk techniques. The solvents
were purified by standard methods. Infrared spectra were
recorded on a Nicolet Impact 420 FT-IR spectrometer from
KBr discs. Ligands L1 and L2 were prepared by literature
methods.5
CAUTION! While none of the perchlorate complexes proved
to be shock sensitive, appropriate precautions should always be
taken when working with them.
[Cu(L2)2]ClO4, 9. 0.1 g (3 mmol) [Cu(CH3CN)4]ClO4 was
added to a solution of 0.11 g (0.3 mmol) L2 in CH2Cl2 (10 cm3)
and the mixture was stirred until all the [Cu(CH3CN)4]ClO4
dissolved. Then, the clear solution was concentrated in vacuo
to ca. 2 cm3 and slow diffusion of diethyl ether into it gave
colorless single crystals of 9 suitable for X-ray analysis
in 85% yield. Found: C, 62.99; H, 5.97; N, 6.05; calc. for
C48H54N4O4P2ClCu: C, 63.22; H, 5.97; N, 6.15%.
Preparations
[Ph2P(O)CH2NH(C5H5N)]ClO4, 1. 0.08 g (0.27 mmol) L1
and 0.1 g (0.27 mmol) [Co(H2O)6](ClO4)2 were mixed in 10 cm3
CH3CN in air and the solution was stirred overnight. The
solution was concentrated and addition of diethyl ether gave
colorless crystals of 1 in 50% yield. Found: C, 52.83; H, 4.44; N,
6.63; calc. for C18H18N2O4PCl: C, 52.89; H, 4.44; N, 6.85%.
Ag(L2)ClO4, 10. 0.1 g (0.48 mmol) AgClO4 and 0.18 g
(0.48 mmol) L2 were mixed in 10 cm3 CH3CN–CH2Cl2 (1 : 1)
with stirring. After all the AgClO4 had dissolved, the solution
was concentrated in vacuo to ca. 2cm3 and diethyl ether diffused
into it to give needle-like microcrystals of 10 in 80% yield
(Found: C, 49.59; H, 4.75; N, 5.09; calc. for C24H27N2O4P2ClAg:
C, 49.55; H, 4.50; N, 4.82%), together with a small amount
of plate-like crystals of the solvate 11ؒCH2Cl2, [Ag2(µ-L2)2]-
[Ag(L2)2](ClO4)3ؒCH2Cl2.
trans-Fe(CO)3(L1)2, 2. This compound was prepared by
the published procedure for trans-Fe(CO)3(PR3)2 and single
crystals were recrystallised from CH2Cl2–CH3OH. Yield 60%.
6
Found: C, 64.51; H, 4.72; N, 7.83; calc. for C39H34N4O3P2Fe: C,
64.65; H, 4.73; N, 7.73%. IR ν(CO): 1880 cmϪ1
.
[Pd(L1)2](ClO4)2, 3. To a stirred solution of 0.2 g (0.68 mmol)
of L1 in 10 cm3 of CH2Cl2 was added 0.1 g (0.34 mmol) of
Na2PdCl4 at ambient temperature. After stirring the solution
for ca. 30 min, the precipitated NaCl was filtered off and to
the clear filtrate was added TlClO4 (0.21 g, 0.34 mmol). After
stirring the yellow solution for 30min, the white precipitate of
TlCl was filtered off and the filtrate concentrated in vacuo to ca.
2 cm3. Subsequent diethyl ether diffusion into the solution
afforded yellow crystals of 3 in 80% yield. Found: C, 48.65; H,
3.86; N, 6.29; calc. for C36H34N4O8P2Cl2Pd: C, 48.58; H, 3.85;
N, 6.30%.
X-Ray crystallography
For each of complexes 2, 3, 4ؒ2CH2Cl2, 5, 6, 9 and 11ؒCH2Cl2,
a selected single crystal was mounted on a Bruker SMART
1000 CCD diffractometer operating at 50 kV and 30 mA using
Mo-Kα radiation (0.71073 Å). Data collection and reduction
were performed using the SMART and SAINT software,7 with
frames of 0.3Њ oscillation in the θ range 1.5 < θ < 28Њ (for
11ؒCH2Cl2, θ < 26Њ). An empirical absorption correction
was applied using the SADABS program.8 The data for 1, 7ؒ
CH3CN and 8 were collected at 293 K in the variable ω-scan
mode on a Siemens R3m/V four-circle diffractometer using
Mo-Kα radiation (50 kV, 30 mA; 2θmax = 52Њ). An empirical
absorption correction was applied using ψ-scan data. The
structures were solved by direct methods and all non-hydrogen
atoms were subjected to anisotropic refinement by full-matrix
least squares on F 2 using the SHELXTL package.9 All
hydrogen atoms were generated geometrically (C–H bond
lengths fixed at 0.96 Å), assigned appropriate isotropic thermal
parameters and included in structure factor calculations.
Crystal structure data and refinement details are summarised
in Table 1.
[PdCl2(ꢀ-L1)]2, 4. 0.1 g (0.34 mmol) L1 and 0.1 g (0.34 mmol)
Na2PdCl4 were mixed in 10 cm3 CH2Cl2 and the solution was
stirred for 20 min. The undissolved material was filtered off and
the filtrate was concentrated in vacuo to ca. 2 cm3. Diffusion of
ethanol into this solution gave yellow crystals of 4ؒ2CH2Cl2 in
65% yield. Found: C, 46.12; H, 3.61; N, 5.80; calc. for
C18H17N2PCl2Pd: C, 46.04; H, 3,65; N, 5.97%.
[Cu(L1)2]ClO4, 5. 0.1 g (0.3 mmol) [Cu(CH3CN)4]ClO4 was
dissolved in 5 cm3 CH3CN and a solution of 0.19 g (0.65 mmol)
L1 in 5 cm3 CH2Cl2 was added with stirring. The reaction
mixture stirred for 20 min and concentrated in vacuo to ca. 2
cm3. Subsequent diethyl ether diffusion into the solution
afforded colorless crystals of 5 in 85% yield. Found: C, 59.67;
H, 4.54; N, 7.47; calc. for C36H34N4O4P2ClCu: C, 59.83; H, 4.58;
N, 7.50%.
CCDC reference numbers 171811–171820.
See http://www.rsc.org/suppdata/dt/b1/b109191n/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
[Ag(ꢀ-L1)(CH3CN)]n(ClO4)n, 6. 0.1 g (0.48 mmol) AgClO4
was dissolved in 5 cm3 CH3CN and a solution of 0.14 g (0.48
mmol) L1 in 5 cm3 CH3CN was added with stirring. After
stirring for 20 min, the solution was concentrated in vacuo to ca.
2cm3 and then diethyl ether added to give colorless crystals of
6 in 60% yield. Found: C, 44.13; H, 3.69; N, 7.57; calc. for
C20H20N3O4PClAg: C, 44.43; H, 3.73; N, 7.77%.
The reactions of N-[(diphenylphosphino)methyl]-2-pyridin-
amine, L1, and N-cyclohexyl-N-[(diphenylphosphino)methyl]-2-
pyridinamine, L2, with different metal salts are shown in
Scheme 3 and 4, respectively. When L1 is mixed with Co(ClO4)2ؒ
6H2O in air, the oxidation state of the phosphorus atom is
converted from III to V, and the pyridine ring is protonated to
form a perchlorate salt, namely compound 1. The structure of 1
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Scheme 3 Reactions of N-[(diphenylphosphino)methyl]-2-pyridinamine, L1. Reagents: (i) Co(ClO4)2ؒ6H2O; (ii) Fe(CO)5/KOH; (iii) Na2PdCl4/
TlClO4; (iv) Na2PdCl4; (v) [Cu(CH3CN)4]ClO4; (vi) AgClO4; (vii) AgClO4/(2-py)NH2.
Fig. 1 Perspective drawing (35% thermal ellipsoids) of the cationic
dimer in compound 1. Selected bond lengths (Å) and angles (Њ):
P(1)–O(1) 1.488(2), P(1)–C(1) 1.791(3), P(1)–C(7) 1.798(3), P(1)–C(13)
1.813; O(1)–P(1)–C(1) 113.0(1), O(1)–P(1)–C(7) 112.5(1), O(1)–P(1)–
C(13) 112.5(2), C(1)–P(1)–C(7) 108.7(2), C(1)–P(1)–C(13) 106.8(2),
C(7)–P(1)–C(13) 102.8(1).
Scheme 4 Reactions of N-cyclohexyl-N-[(diphenylphosphino)methyl]-
2-pyridinamine, L2. Reagents: (i) Na2PdCl4; (ii) [Cu(CH3CN)4]ClO4;
(iii) AgClO4.
spectra of both complexes have intense carbonyl absorptions at
1880 cmϪ1, indicating that the local symmetry at the Fe atom is
near D3h. In the crystal structure of 2, a crystallographic C2 axis
bisects the planar Fe(CO)3 unit of the molecule (Fig. 2). The
coordination environment about the iron atom is best described
as an FeP2C3 trigonal bipyramid with the phosphine ligands
occupying axial positions. The P–Fe–P angle of 179.44(3)Њ is
comparable to those in trans-Fe(CO)3(Ph2Ppym)2 [174.2(1)],11
trans-Fe(CO)3(Ph2Ppy)2 [177.1(1)]3d and trans-Fe(CO)3(Ph2-
PCH2mor)2 [178.24(3)Њ]10b The three C–Fe–C angles [119.5(2),
120.26(8) and 120.26(8)Њ] are very close to the ideal value of
120Њ. trans-Fe(CO)3(µ-L2)2 was successfully reacted with
[Cu(CH3CN)4]ClO4 to form a heterobimetallic seven-membered
as determined by X-ray crystallography (Fig. 1) confirms the
presence of a tertiary phosphine oxide with a P᎐O bond length
᎐
of 1.488(2) Å, which is consistent with appreciable double
bond character. A pair of cations is linked into a pseudo ten-
membered ring dimer through intermolecular P᎐O ؒ ؒ ؒ H–N
᎐
hydrogen bonding (O ؒ ؒ ؒ N 2.7, H ؒ ؒ ؒ O 1.956 Å; N–H ؒ ؒ ؒ O
164.7Њ). Other examples of P᎐O ؒ ؒ ؒ H–N hydrogen-bonded
᎐
dimers in Ph2P(O)CH2N(H)C5H3N(5-Cl)10a and [H{Ph2P(O)-
CH2(C4H8NO)}]2(Hg2I6)10b have been reported.
The reaction of L1 or L2 with KOH and Fe(CO)5 in refluxing
ethanol for 24 h gave yellow precipitates formulated as trans-
Fe(CO)3(L1)2, 2 and trans-Fe(CO)3(L2)2, respectively.5b The IR
macrocyclic complex with a Fe()
Cu() donor–acceptor
bond.5b However, the reaction of 2 with copper() salts did not
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Fig. 3 Perspective drawing (35% thermal ellipsoids) of the dication
in compound 3. Selected bond lengths (Å) and angles (Њ): Pd(1)–N(2)
2.131(3), Pd(1)–N(4) 2.175(3), Pd(1)–P(1) 2.231(1), Pd(1)–P(2)
2.235(1), N(2)–Pd(1)–N(4) 93.1(1); N(2)–Pd(1)–P(1) 83.83(8), N(4)–
Pd(1)–P(1) 161.05(9), N(2)–Pd(1)–P(2) 166.22(9), N(4)–Pd(1)–P(2)
89.09(8), P(1)–Pd(1)–P(2) 98.37(4).
Fig. 2 Perspective drawing (35% thermal ellipsoids) of the molecular
structure of 2. Selected bond lengths (Å) and angles (Њ): Fe(1)–C(1)
1.781(3), Fe(1)–C(2) 1.766(2), Fe(1)–P(1) 2.1958(6); C(2)–Fe(1)–C(1)
120.26(8), C(2A)–Fe(1)–C(2) 119.5(2), C(1)–Fe(1)–P(1) 89.72(2), C(2)–
Fe(1)–P(1) 87.38(7), C(2A)–Fe(1)–P(1) 92.90(7), P(1A)–Fe(1)–P(1)
179.44(3). Symmetry code: Ϫx, y, 1/2 Ϫ z.
afford an analogous stable binuclear complex. This seems to be
attributable to the greater bulk of L2 as compared to L1, since
the pyridine ring in L1 can rotate more freely than that in L2.
The reaction of L1 and Na2PdCl4 in a 2 : 1 molar ratio in
CH2Cl2 gave a yellow solution and a NaCl precipitate. When
the filtrate was treated with two equivalents of TlClO4 to
remove the chloride ligands, the complex [Pd(L1)2](ClO4)2, 3,
was formed. The dication has a near square-planar environ-
ment around the Pd center with two cis P,N-chelating ligands
(Fig. 3). The Pd–P and Pd–N distances are similar to those
found in palladium phosphine and pyridylphosphine com-
plexes. The planarity of the coordination geometry around the
palladium atom was accessed by fitting a least-squares plane to
the atoms Pd(1), P(1), P(2), N2 and N4, which showed a large
mean deviation of 0.25 Å. The plane defined by Pd and
the coordinated P atoms makes a dihedral angle of 21.9Њ with
the PdN2 mean plane.
Fig. 4 Perspective drawing (35% thermal ellipsoids) of the dinuclear
molecule of 4 in 4ؒ2CH2Cl2. Selected bond lengths (Å) and angles (Њ):
Pd(1)–N(1A) 2.116(4), Pd(1)–P(1) 2.252(1), Pd(1)–Cl(2) 2.300(1),
Pd(1)–Cl(1) 2.303(1); N(1A)–Pd(1)–P(1) 176.6(1), N(1A)–Pd(1)–Cl(2)
88.3(1), P(1)–Pd(1)–Cl(2) 88.26(5), N(1A)–Pd(1)–Cl(1) 90.2(1), P(1)–
Pd(1)–Cl(1) 93.29(5), Cl(2)–Pd(1)–Cl(1) 174.52(5). Symmetry code:
1 Ϫ x, 1 Ϫ y, 1 Ϫ z.
The reaction of L1 and Na2PdCl4 in a 1 : 1 molar ratio in
CH2Cl2 yielded a yellow solution from which Pd2Cl4(µ-L1)2ؒ
2CH2Cl2, 4ؒ2CH2Cl2, was isolated. Complex 4 consists of a
bimetallic twelve-membered ring located at an inversion center
(Fig. 4). The chloride ligands are in a trans arrangement about
each palladium atom, and a pair of bidentate L1 ligands in the
head-to-tail configuration bridge the two palladium atoms. The
Cl(1)–Pd(1)–Cl(2) [174.52(5)] and P–Pd–N [176.6(1)Њ] angles
are close to linear, and the Pd–P [2.252(1)], Pd–N [2.116(4)] and
Pd–Cl [2.300(1) and 2.303(1) Å] bond distances are quite
normal. The least-squares plane through the atoms Pd(1), Cl(1),
Cl(2), P(1) and N(1A) has a mean deviation of 0.05 Å. The
dihedral angle between the plane Pd(1)–Cl(1)–Cl(2)–P(1)–
N(1A) and the pyridine ring is 74.5Њ. The non-bonded
Pd ؒ ؒ ؒ Pd separation in this compound is 4.594 Å, which is
slightly shorter than that in twelve-membered ring dipalladium
complexes linked by a rigid phosphine ligand (4.710 Å).12
The complex [Cu(L1)2]ClO4, 5, was obtained from the
reaction of L1 with [Cu(CH3CN)4]ClO4 in a 2 : 1 molar ratio in
CH3CN–CH2Cl2. The structure of the cation is shown in Fig. 5.
The copper atom has a distorted tetrahedral geometry, in which
each L1 ligand chelates the metal center through its phosphorus
and pyridyl nitrogen atoms. The Cu–P [2.2282(6) and 2.2353(6)]
and Cu–N [2.113(2) and 2.171(2) Å] bond lengths fall within
the normal ranges for copper phosphine and pyridine com-
plexes. The bond angles at the copper atom vary from 94.58(7)
to 128.74(3)Њ.
The reaction of L1 with silver perchlorate in a 1 : 1 molar
ratio in CH3CN–CH2Cl2 leads to rapid formation of a color-
less solution, from which [Ag(CH3CN)(µ-L1)]n(ClO4)n, 6, was
isolated. The ligand L1 bridges consecutive metal centers via its
P,N(pyridyl)-donor sites. Each of the two independent silver
centers is coordinated by a P atom from one L1 ligand and
a pyridyl N atom from another, resulting in the formation of a
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Fig. 7 Perspective drawing (35% thermal ellipsoids) of the cation in
compound 7ؒCH3CN. Selected bond lengths (Å) and angles (Њ): Ag(1)–
N(6) 2.350(4), Ag(1)–P(1) 2.437(1), Ag(1)–P(2) 2.448(1); N(6)–Ag(1)–
P(1) 119.1(1), N(6)–Ag(1)–P(2) 112.9(1), P(1)–Ag(1)–P(2) 127.99(5).
Fig. 5 Perspective drawing (35% thermal ellipsoids) of the cation
in compound 5. Selected bond lengths (Å) and angles (Њ): Cu(1)–N(2)
2.171(2), Cu(1)–N(4) 2.113(2), Cu(1)–P(1) 2.2282(6), Cu(1)–P(2)
2.2353(6); N(4)–Cu(1)–N(2) 94.58(7), N(4)–Cu(1)–P(1) 120.45(5),
N(2)–Cu(1)–P(1) 96.37(5), N(4)–Cu(1)–P(2) 96.89(5), N(2)–Cu(1)–P(2)
115.61(5), P(1)–Cu(1)–P(2) 128.74(3).
Fig. 6 Structure of the polymeric chain [Ag2(CH3CN)2(µ-L1)2]n2nϩ in 6.
Selected bond lengths (Å) and angles (Њ): Ag(1)–N(1A) 2.185(7),
Ag(1)–P(2) 2.359(2), Ag(1)–N(5) 2.60(1), Ag(2)–N(3) 2.193(8),
Ag(2)–P(1) 2.369(2), Ag(2)–N(6) 2.55(1); N(1A)–Ag(1)–P(2) 166.0(2),
N(1A)–Ag(1)–N(5) 92.1(3), P(2)–Ag(1)–N(5) 101.7(2), N(3)–Ag(2)–
P(1) 159.1(2), N(3)–Ag(2)–N(6) 94.6(3), P(1)–Ag(2)–N(6) 105.4(3).
Symmetry code: x, y, z Ϫ 1.
cationic polymeric chain running in the direction of the c axis
(Fig. 6). The highly distorted trigonal planar coordination
environment about each Ag() atom is completed by a CH3CN
ligand, with a slightly longer Ag–N bond [2.60(1) and 2.55(1)
Å] that stabilises the resulting 16-electron configuration. The
three bond angles at Ag(1) and Ag(2) sum to 359.8 and 359.2Њ,
respectively. The non-bonded Ag ؒ ؒ ؒ Ag separations in the
polymeric chain are 5.827 and 6.019 Å, respectively. The crystal
structure of 6 consists of the packing of such chains and the
perchlorate ions.
Fig. 8 Perspective drawing (35% thermal ellipsoids) of the molecular
structure of compound 8. Selected bond lengths (Å) and angles (Њ):
Pd(1)–N(1) 2.105(4), Pd(1)–P(1) 2.192(1), Pd(1)–Cl(2) 2.290(1), Pd(1)–
Cl(1) 2.405(1); N(1)–Pd(1)–P(1) 92.5(1), N(1)–Pd(1)–Cl(2) 174.0(1),
P(1)–Pd(1)–Cl(2) 84.02(5), N(1)–Pd(1)–Cl(1) 92.5(1), P(1)–Pd(1)–Cl(1)
171.78(6), Cl(2)–Pd(1)–Cl(1) 91.60(5).
are similar to those seen in palladium phosphine and pyridyl-
phosphine complexes. The least-squares plane through the
atoms Pd(1), P(1), N(1), Cl(1) and Cl(2) shows a mean devi-
ation of 0.09 Å. The configuration at the tertiary amino
nitrogen atom is almost planar, as its deviation from the plane
of its three-bonded carbon atoms is 0.22 Å. The dihedral angle
of this plane and the pyridine ring is 27.4Њ, which indicates that
the lone pair electron of the tertiary amino nitrogen atom is
significantly conjugated with the pyridine ring.
The reaction of L1 with AgClO4 in a 2 : 1 molar ratio in the
presence of 2-aminopyridine afforded the mononuclear com-
plex [Ag(L1)2(2-Py–NH2)]ClO4, 7 (Fig. 7). The silver atom in 7 is
coordinated by two L1 ligands and one 2-aminopyridine, with
fairly normal Ag–P and Ag–N bond lengths. The three P(1)–
Ag(1)–N(6), P(2)–Ag(1)–N(6) and P(1)–Ag(1)–P(2) [119.1(1),
112.9(1) and 127.99(5)Њ] bond angles are not far from the ideal
value of 120Њ, which indicates trigonal planar coordination
geometry at the silver atom.
The reaction of N-cyclohexyl-N-[(diphenylphosphino)-
methyl]-2-pyridinamine, L2, with Na2PdCl4 in a 1 : 1 molar ratio
gave a yellow solution from which the complex PdCl2(L2), 8,
was isolated. The L2 ligand chelates the palladium atom
through its P and pyridyl N atoms, with two chloride ligands
arranged in cis positions (Fig. 8). The Pd–P and Pd–N distances
The compound [Cu(L2)2]ClO4, 9, was obtained from the
reaction of L2 with [Cu(CH3CN)4]ClO4 in a 2 : 1 molar ratio
in CH2Cl2. The structure of the cation is shown in Fig. 9. The
copper atom has a distorted tetrahedral geometry, in which
each L2 ligand chelates the metal center through its phosphorus
atom and nitrogen atom in the pyridine ring. The Cu–P
[2.2086(7) and 2.2279(7)] and Cu–N [2.107(2) and 2.181(2) Å]
bond lengths fall within the normal bond ranges for copper
phosphine and pyridine complexes. The bond angles at the
J. Chem. Soc., Dalton Trans., 2002, 1336–1343
1341

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phino)methane],13b 2.7–3.0 Å in polyhedral silver cages each
2Ϫ
encapsulating a C2 anion (C2@Agn, n = 6, 7, 8, 9)13d and
2.953(2)–2.986(2) Å in the silver alkynyl cage compounds
13e
᎐
[Ag (µ-C᎐C–tBu) X]Y (X = Cl, Br; Y = OH, BF ). These
᎐
14
12
4
distances are suggestive of significant argentophilic interaction
between the silver() centers,13 so this type of metallophilic
d10–d10 interaction also exists in the present dication, and the
coordination geometry at each silver atom can be described as
highly distorted tetrahedral (Fig. 10).
An ORTEP drawing of the cation [Ag(L2)2]ϩ in 11ؒCH2Cl2 is
shown in Fig. 11. A crystallographic C2 axis bisects the molecule
Fig. 9 Perspective drawing (35% thermal ellipsoids) of the cation in
compound 9. Selected bond lengths (Å) and angles (Њ): Cu(1)–N(1)
2.107(2), Cu(1)–N(3) 2.181(2), Cu(1)–P(2) 2.2086(7), Cu(1)–P(1)
2.2279(7); N(1)–Cu(1)–N(3) 99.18(9), N(1)–Cu(1)–P(2) 123.96(7),
N(3)–Cu(1)–P(2) 94.58(6), N(1)–Cu(1)–P(1) 95.04(6), N(3)–Cu(1)–P(1)
104.13(6), P(2)–Cu(1)–P(1) 133.34(3).
copper atom vary from 94.58(6) to 133.34(3)Њ. The configur-
ation at the tertiary amino nitrogen atom is nearly planar, with
its lone pair electrons fully conjugated with the pyridine ring.
The reaction of L2 with AgClO4 in a 1 : 1 molar ratio in
CH3CN–CH2Cl2 affords needle-like microcrystals of 10, of
stoichiometry Ag(L2)ClO4 by elemental analysis, plus a small
quantity of plate-like crystals, which were subsequently shown
by X-ray analysis to be a solvate with the structural formula
[Ag2(µ-L2)2][Ag(L2)2](ClO4)3ؒCH2Cl2, 11ؒCH2Cl2. The [Ag2(µ-
L2)2]2ϩ dication has crystallographic C2 symmetry with a pair of
P,N(pyridyl)-bidentate L2 ligands bridging the silver atoms in a
head-to-tail fashion (Fig. 10). Notably, the tertiary amino N
Fig. 11 Perspective drawing (35% thermal ellipsoids) of the cation
[Ag(L2)2]ϩ in compound 11ؒCH2Cl2. Selected bond lengths (Å) and
angles (Њ): Ag(2)–P(2) 2.388(2); P(2)–Ag(2)–P(2A) 164.8(2). Symmetry
code: 1 Ϫ x, y, 1 Ϫ z.
through the silver atom, which is coordinated by two equivalent
phosphine ligands with Ag–P = 2.388(2) Å and P–Ag–P =
164.8(2)Њ. The distance (2.647 Å) between the silver atom and
each pyridyl nitrogen atom is much longer than those in known
silver pyridine complexes, which means that only a feeble inter-
action exists between them.
Fig. 10 Perspective drawing (35% thermal ellipsoids) of the dinuclear
cation [Ag2(µ-L2)2]2ϩ in compound 11ؒCH2Cl2. Selected bond lengths
(Å) and angles (Њ): Ag(1)–N(1A) 2.252(9), Ag(1)–P(1) 2.372(3),
Ag(1)–N(2A) 2.603(8), Ag(1)–Ag(1A) 2.890(2); N(1A)–Ag(1)–P(1)
169.7(2), N(1A)–Ag(1)–N(2A) 54.6(3), P(1)–Ag(1)–N(2A) 122.7(2),
N(1A)–Ag(1)–Ag(1A) 100.9(2), P(1)–Ag(1)–Ag(1A) 88.60(7), N(2A)–
Ag(1)–Ag(1A) 87.9(2). Symmetry code: Ϫx ϩ 1, y, Ϫz.
Conclusion
Two functionalised amine ligands, N-[(diphenylphosphino)-
methyl]-2-pyridinamine (L1) and N-cyclohexyl-N-[(diphenyl-
phosphino)methyl]-2-pyridinamine (L2), have been reacted
with Fe(CO)5 and palladium(), copper() and silver() salts
in different molar ratios to generate a series of complexes
exhibiting a variety of new ligation modes: P-coordination (a),
P,N(pyridyl)-chelating (d) and P,N(pyridyl)-bridging (h) for
atom of each L2 is coordinated to a silver atom with Ag–N =
2.603(9) Å. This distance is longer than those in silver tertiary
amine complexes, but shorter than the sum of the van der
Waals’ radii of silver and nitrogen atoms. Furthermore, the
amino nitrogen atom is displaced by 0.39 Å from the plane of
its three-bonded carbon atoms, and the dihedral angle of this
plane and the pyridine ring (99.9Њ) is much larger than those in
complexes 8, 9 and the cation [Ag(L2)2]ϩ in 11ؒCH2Cl2. This
means that its lone pair electrons are no longer conjugated with
the pyridine ring, but involved in coordination with the silver
atom, although the binding is rather weak. The Ag–P and
Ag–N(pyridyl) bonds fall in the normal ranges for silver phos-
phine and pyridine complexes. In this dimeric cation, the
Ag ؒ ؒ ؒ Ag distance of 2.890(2) Å is comparable to twice the
metallic radius of silver (2.89 Å) and the corresponding
values of 2.934(2) and 2.946(2) Å in [Ag2(H2L)3]n2nϩ [H2L =
L1; P,N(pyridyl)-chelating (d) and
a novel P,N(amine),-
N(pyridyl)-bridging mode (l) for L2. Although there are thirteen
conceivable coordination modes for this kind of phosphino-
pyridinamine ligand, those that involve the weakly coordinating
and sterically hindered amino N atom seem unlikely to be
observed in metal complexes, and argentophilicity appears to
play a significant role in inducing the adoption of the tridentate
mode l in [Ag2(µ-L2)2]2ϩ
.
Acknowledgements
NЈ,NЈ-bis(salicylidene)-1,4-diaminobutane],13a
2.960(1) Å in [Ag2(µ-dcpm)2]ϩ [dcpm = bis(dicyclohexylphos-
2.936(1)–
This work is supported by Hong Kong Research Grants
Council Earmarked Grant CUHK 4022/98.
1342
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