Synthesis and co-ordination chemistry of a novel multifunctional bis-phosphine containing a P-N-Si-N-P backbone

Article abstract of DOI:10.1039/b009722p

A novel multifunctionalised bidentate phosphine ligand containing the highly unusual P-N-Si-N-P backbone has been synthesized and its co-ordination chemistry studied. The ligand Ph₂PN(C₅H₄N-2)Si(Me)₂ N(C₅H₄N-2)PPh₂, which was characterised crystallographically, forms cis-P,P chelate complexes with platinum or palladium. The complexes MLCl₂ (M = Pt or Pd) readily hydrolyse under recrystallisation conditions to give chelate complexes [Pt(Ph₂PNHC₅H₄N-2-P,N)₂] Cl₂. The crystal structure of the platinum complex shows both P,N ligands to chelate to the metal, in contrast to the crystal structure reported previously [Pt(Ph₂PNHC₅H₄N-2-P,N)(Ph₂ PNHC₅H₄N-2-P)Cl]Cl. The dimethyl platinum complex PtLMe₂ is considerably more stable to hydrolysis, and it was therefore possible to characterise the six membered PtPNSiNP ring system by X-ray crystallography. In addition, evidence is also presented that the ligand and its complexes will react with other phosphorus electrophiles (with N-Si bond cleavage) to generate new compounds.

Full text of DOI:10.1039/b009722p

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Synthesis and co-ordination chemistry of a novel multifunctional
bis-phosphine containing a P–N–Si–N–P backbone
Stephen M. Aucott, Matthew L. Clarke, Alexandra M. Z. Slawin and J. Derek Woollins*
School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST.
E-mail: jdw3@st-andrews.ac.uk; fax: 44-1334-463384
Received 5th December 2000, Accepted 5th February 2001
First published as an Advance Article on the web 5th March 2001
A novel multifunctionalised bidentate phosphine ligand containing the highly unusual P–N–Si–N–P backbone has
been synthesized and its co-ordination chemistry studied. The ligand Ph2PN(C5H4N-2)Si(Me)2N(C5H4N-2)PPh2,
which was characterised crystallographically, forms cis-P,P chelate complexes with platinum or palladium. The
complexes MLCl2 (M = Pt or Pd) readily hydrolyse under recrystallisation conditions to give chelate complexes
[Pt(Ph2PNHC5H4N-2-P,N)2]Cl2. The crystal structure of the platinum complex shows both P,N ligands to chelate
to the metal, in contrast to the crystal structure reported previously [Pt(Ph2PNHC5H4N-2-P,N)(Ph2PNHC5H4N-2-
P)Cl]Cl. The dimethyl platinum complex PtLMe2 is considerably more stable to hydrolysis, and it was therefore
possible to characterise the six membered PtPNSiNP ring system by X-ray crystallography. In addition, evidence
is also presented that the ligand and its complexes will react with other phosphorus electrophiles (with N–Si bond
cleavage) to generate new compounds.
Introduction
Bidentate phosphine ligands are ubiquitous in co-ordination
chemistry and catalysis. Whereas most consist of a carbon
based backbone linking the two phosphine moieties, a smaller
group of ligands consist of phosphorus atoms linked by an
inorganic backbone containing main group and/or transition
metals. The co-ordination chemistry of bis(phosphino)amines 1
Fig. 1 Bidentate phosphines.
chemistry of Ph2PNHC5H4N-2 4 in considerable detail;9 it
shows a very strong preference for P,N chelation. Many com-
plexes have been isolated in which the ligand chelates a metal
under conditions in which many hemilabile ligands would
exhibit η1 co-ordination. In this paper we report on the prepar-
ation and reactivity of the multifunctionalised bidentate
phosphine 5 and its metal complexes.
and their oxidised derivatives has been studied extensively.1
These ligands exhibit a variety of co-ordination modes and con-
formations depending on the nature of the metal and reaction
conditions. They have also found applications in selective metal
extraction, as NMR shift reagents, and in catalysis.1d–f
Experimental
Bis(phosphino)hydrazine ligands of type 2 can be considered
the next member in the series of inorganic backboned ligands.2,3
Reports of ligands, 3, which contain three main group atoms
linking the phosphorus atoms are very scarce.4–7 These ligands
would form six membered inorganic heterocycles on co-
ordination to appropriate metal salts. We envisaged that vari-
ation of the heteroatom X could have considerable effects on
their steric, electronic and co-ordination properties. As these
ligands could provide new applications in both co-ordination
chemistry and catalysis, we have begun an investigation into the
co-ordination chemistry of this ligand class.
To the best of our knowledge, the only example of a diphos-
phine ligand containing a PNSiNP bridge is that reported by
Verkade and co-workers which contains a hyperco-ordinate
silicon atom (Fig. 1).4 We chose to prepare this type of
ligand by reaction of two equivalents of a deprotonated
phosphinoamine with dichlorodimethylsilane. The secondary
phosphinoamine 48,9 was chosen as starting material as the
presence of the pyridine moiety opens up the possibility of
bidentate bridging co-ordination modes for its silicon bridged
counterpart, 5. We have recently studied the co-ordination
General
All manipulations were carried out under an atmosphere
of nitrogen, unless stated otherwise. All solvents were either
freshly distilled from an appropriate drying agent (THF, Et2O,
DCM) or obtained as anhydrous grade. 1H and 31P NMR
spectra were recorded using a “Varian 2000” 300 MHz
spectrometer, IR spectra as KBr discs (prepared in air) on a
Perkin-Elmer PE1720 FTIR/RAMAN spectrometer. Pt(COD)-
Cl2, Pt(COD)Me2, and Ph2PNHC5H4N-2 4 were prepared by
literature methods.9,10 For simplicity the ligand bis[(diphenyl-
phosphino)(2-pyridyl)amino]dimethylsilane is referred to
PNSiNP throughout this section.
Preparations
Bis[(diphenylphosphino)(2-pyridyl)amino]dimethylsilane, or
PNSiNP, 5. A stirred solution of 2-(diphenylphosphino-
amino)pyridine (1.542 g, 5.54 mmol) in toluene (50 mL) was
cooled to Ϫ50 ЊC. Lithium diisopropylamide (2.8 mL of a 2 M
solution in heptanes–THF–benzene) was then added dropwise
over a couple of minutes. The mixture was then left for 3–4
972
J. Chem. Soc., Dalton Trans., 2001, 972–976
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b009722p

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hours to warm to room temperature. Dichlorodimethylsilane
(0.336 mL, 0.357 g, 2.77 mmol) was added in two portions 30
minutes apart, and left to stir at room temperature for 3–4
hours. The mixture was then left to settle for a short period of
time, prior to the filtering off precipitated lithium chloride
(using a filter stick). Removal of solvent under vacuum gave
a quantitative yield of bis[(diphenylphosphino)(2-pyridyl)-
amino]dimethylsilane, 5. The compound obtained in this way is
of 90% purity, and could be used in some of the complexation
reactions. Analytically pure, good quality crystals of 5 can be
obtained by layering a saturated toluene solution with hexane.
Leaving a solution containing 2.0 g of crude ligand at room
temperature in a sealed tube gave 182 mg of 5 as large colour-
less blocks. Found: C, 70.39; H, 5.66; N, 9.10. C36H34N4P2Si
requires C, 70.57; H, 5.59; N, 9.14%. νmax/cmϪ1 3052, 1587,
1566, 1480, 1463, 1427, 1294, 1265, 1182, 1155, 1091, 1050,
1027, 993, 934, 904, 815, 777, 742, 695, 515, 470 and 423.
δP(121.4 MHz; C6D6) 40.9 (s). δH(300 MHz; C6D6) 0.79 (6H, t, J
2), 6.44 (2H, d, J 8.5), 6.58 (2H, dd, coupling not resolved), 6.98
(2H, m), 7.26 (12H, m), 7.58 (8H, m) and 7.96 (2H, dd, J 2.0,
4.9 Hz).
(C᎐N) and 1567 (aromatic C᎐C). δ (121.4 MHz; CD Cl –C D )
᎐ ᎐
P 2 2 6 6
1
63.9 (s ϩ satellites, JP-Pt = 1986 Hz). δH(300 MHz; C6D6) 0.18
2
(m with platinum satellites, J = 72), 0.48 (6H, s), 6.55 (2H, d,
J 8.0), 6.75 (2H, dd, J 7.3, 4.8), 7.15–7.70 (22H, m) and 8.11
(2H, dd, J 1.9, 4.9). (ESϩ mass spectrum, cone voltage 90 V):
m/z 860.2051 [(M ϩ Na)ϩ requires 860.2043] and 822.1911
[(M Ϫ CH3)ϩ requires 822.1906].
2-(Ph2P)2NC5H4N, 11. Chlorodiphenylphosphine (0.033 mL,
0.040 g, 0.133 mmol) was added to a DCM (2mL) solution of
ligand 5 (112 mg, 0.183 mmol). After one hour the 31P NMR
spectrum was recorded. This confirmed that quantitative con-
version into ligand 11 had taken place. δP(121.4 MHz; CH2Cl2–
C6D6) 59.9 (s) (lit.11 59.9). Mass spectrum (EI): m/z 461 (Mϩ)
(lit.11 461).
X-Ray crystallography
Crystal structures were obtained at 298 K using a Bruker
SMART diffractometer with graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å). Intensity data were collected using
0.3 or 0.15Њ width ω steps accumulating area detector frames
spanning a hemisphere of reciprocal space for all structures. All
data were corrected for Lorentz, polarisation and long term
intensity fluctuations. Absorption effects were corrected on the
basis of multiple equivalent reflections. Structures were solved
by direct methods and refined by full-matrix least squares
against F2 (SHELXTL)12 for all data with I > 2σ(I). Crystal
data are summerised in Table 1.
Se(PNSiNP)Se, 6. To a Schlenk tube containing ligand com-
pound 5 (0.105 g, 0.171 mmol, 1 equivalent) and elemental
selenium (0.027 g, 3.43 mmol, 2 equivalents) was added toluene
(5 mL). The mixture was heated to 80 ЊC for one hour. 31P
NMR spectroscopy of this solution confirmed quantitative
conversion into the diselenide, 6. Removal of solvent and wash-
ing with Et2O gave the pure compound as a white powder.
Found: C, 56.11; H, 4.46; N, 7.21. C36H34N4P2Se2Si requires C,
56.24; H, 5.15; N, 6.40%. δP(121.4 MHz; C6D6) 47.6 (s ϩ satel-
lites, 1JP-Se = 778 Hz). 1H NMR identical to “free” ligand.
CCDC reference numbers 154169–154171.
See http://www.rsc.org/suppdata/dt/b0/b009722p/ for crystal-
lographic data in CIF or other electronic format.
Pt(PNSiNP)Cl2, 7. To a Schlenk tube containing ligand
compound 5 (0.099 g, 0.162 mmol, 1 equivalent) and Pt(COD)-
Cl2 (0.060 g, 0.162 mmol, 1 equivalent) was added DCM
(5 mL). The reaction was stirred for one hour. Removal of
solvent to near dryness and washing with Et2O (2 × 10 mL)
gave the pure compound as a white powder in essentially quan-
titative yield. Found: C, 49.28; H, 3.67; N, 6.73. C36H34Cl2-
N4P2SiPt requires C, 49.21; H, 3.90; N, 6.38%. νmax/cmϪ1 1596
Results and discussion
Ligand 5 can be prepared by deprotonation of 4 using BuLi
as base, followed by dropwise addition of Me2SiCl2. However,
the use of lithium diisopropylamide (LDA) as base results in
product of greater purity (Scheme 1). It is also important to
n
(C᎐N), 1575 (aromatic C᎐C), 318, 305 (cis-Pt–Cl). δ (121.4
᎐
᎐
P
1
MHz; CDCl3) 42.3 (s ϩ satellites, JP-Pt = 3902 Hz). δH(300
MHz; C6D6) 0.40 (6H, s), 6.43 (2H, d, J 8.0), 6.81 (2H, dd, 7.3,
J 4.8), 7.16 (2H, td, J 1.9, 5.8), 7.24–7.38 (12H, m), 7.88 (8H, m)
and 8.11 (2H, dd, J 1.9, 4.9 Hz). On recrystallisation this
compound hydrolysed to give good quality crystals of
Pt(Ph2PNHC5H4N-2)2Cl2 9: νmax/cmϪ1 3412, (NH), 1617 (co-
Scheme 1
ord. CN) and 1575 (aromatic C᎐C). δ (121.4 MHz; CDCl ) 51.4
᎐
P
3
(s, br ϩ satellites, 1JP-Pt = 3576 Hz).
allow sufficient reaction time after addition of Me2SiCl2, as
after 1–3 hours the product is contaminated with two other
phosphorus containing impurities which we assume to be
2-Ph2PN(Li)C5H4N-2 and Ph2P(C5H4N)NSi(Me)2Cl. Analytic-
ally pure crystals of 5 were obtained by recrystallisation
(toluene–hexane) under strictly anaerobic conditions.
Pd(PNSiNP)Cl2, 8. To a Schlenk tube containing ligand 5
(0.028 g, 0.046 mmol, 1 equivalent) and Pd(COD)Cl2 (0.060 g,
0.046 mmol, 1 equivalent) was added DCM (5 mL). Treatment
as above gave the pure compound as a white powder. Found: C,
55.26; H, 4.13; N, 7.54. C36H34Cl2N4P2PdSi requires C, 54.73;
The molecular structure of ligand 5 is shown in Fig. 2. The
structure confirms the exact identity and gives us inform-
ation regarding the P–N–Si–N–P chain, in which the pyridine
moieties adopt a staggered conformation with respect to each
other. The P–N bond lengths (1.724(2) and 1.7224(14) Å) are
fairly typical of a P–N single bond, although they are longer
than those found in Ph2PNHC5H4N-2 (1.705(3) Å). The nitro-
gen atoms, N(1) and N(2), are entirely planar (sum of angles
about N(1) and N(2) = 359.75 and 359.98Њ respectively) as is
commonplace in diphenylphosphinoamines. The C–N bonds
between the aromatic carbons of the pyridine group and phos-
phorus bound nitrogen atoms are 1.420(2) and 1.423(2) Å, and
are longer than those in Ph2PNHC5H4N-2 (1.329(4) Å). The
longer P–N and C–N bond lengths found in ligand 5 may reflect
the presence of the Me2Si group as the nitrogen lone pair will be
H, 4.34; N, 7.09%. νmax/cmϪ1 1597 (C᎐N) and 1573 (aromatic
᎐
C᎐C). δ (121.4 MHz; CDCl ) 64.44 (s). δ (300 MHz; C D )
᎐
P
3
H
6
6
0.40 (6H, s), 6.43 (2H, d, J 8.0), 6.81 (2H, dd, J 7.3, 4.8 Hz),
7.16–7.6 (20H, m) and 8.11 (2H, m).
Pt(PNSiNP)Me2, 10. To a Schlenk tube containing ligand 5
(0.289 g, 0.472 mmol, 1 equivalent) and Pt(COD)Me2 (0.149 g,
0.472 mmol, 1 equivalent) was added DCM (5 mL). Treatment
as above gave the pure compound as a white powder in near
quantitative yield. This compound stubbornly refused to yield
acceptable chemical analysis, probably due to traces of COD
being present. Recrystallisation by layering a DCM solution of
10 with hexane gives a few crystals. It has been characterised by
X-ray crystallography and spectroscopy. νmax/cmϪ1 1610, 1584
J. Chem. Soc., Dalton Trans., 2001, 972–976
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Table 1 Crystal data for compounds 5, 9 and 10
5
9
10
Empirical formula
M
C36H34N4P2Si
612.70
C35H31Cl5N4P2Pt
941.92
C28H40N4P2PtSi
837.86
Crystal system
Space group
Monoclinic
P21
Triclinic
P1
Monoclinic
P21/n
¯
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
U/Å
10.39310(10)
13.8309(2)
11.72530(10)
10.0529(2)
15.5961(6)
16.3639(3)
101.805(2)
105.754(2)
95.934(2)
2385.37(11)
2
12.5811(2)
18.3389(3)
15.4399(3)
105.9710(10)
90.8350(10)
1620.41(3)
2
0.203
3561.97(11)
4
4.096
Z
µ/mmϪ1
3.313
Reflections measured
Independent reflections
Final R1, wR2[I > 2σ(I)]
10133
4629
0.0237, 0.0582
14884
6838
0.0928, 0.2508
15304
5072
0.0259, 0.0517
Table 2 Selected bond lengths (Å) and angles (Њ) for compound 5
Ligand 5 reacts with the well known precursor Pt(COD)Cl2
to generate the dichloroplatinum() complex 7, which contains
an unusual PNSiNPPt heterocycle (Scheme 3). The phosphorus
P(1)–N(1)
1.724(2)
1.7754(14)
1.862(2)
1.420(2)
1.330(2)
P(1)–N(2)
1.7224(14)
1.769(2)
1.865(2)
1.423(2)
1.330(2)
Si(1)–N(1)
Si(1)–C(37)
N(1)–C(13)
C(13)–N(14)
Si(1)–N(2)
Si(1)–C(38)
N(2)–C(31)
C(31)–N(32)
N(2)–Si(1)–N(2)
P(1)–N(1)–Si(1)
C(13)–N(1)–P(1)
116.88(8)
117.48(8)
124.15(12)
C(37)–Si(1)–C(38)
P(2)–N(2)–Si(1)
C(31)–N(2)–P(2)
105.06(12)
119.48(9)
123.79(13)
Scheme 3
NMR spectrum shows a single line with platinum satellites
(δ 42.3, 1J{31P–195Pt} = 3902 Hz). This coupling constant is
fully consistent with a bidentate phosphine bound trans to
chloride ligands, although somewhat larger than that found
for Pt(dppe)Cl2 (1J{31P–195Pt} = 3620 Hz)14 or Pt(dppp)Cl2
(1J{31P–195Pt} = 3406 Hz).15 Addition of two equivalents
Pt(COD)Cl2 to the ligand does not generate the bimetallic
compound Cl2Pt(5)PtCl2 as we might have expected given the
strong preference that 4 shows for P,N chelation. Reaction with
Pd(COD)Cl2 gave the analogous palladium complex 8.
Fig. 2 The crystal structure of compound 5.
less available to strengthen the P–N and C–N bonds. It is
thought that silylated amines have much lower basicity than
their parent amines. Other selected bond lengths and angles are
found in Table 2.
Heating a toluene solution of ligand 5 with two equivalents
of elemental selenium gave the diselenide, 6 (Scheme 2). The
The position of ν(C᎐N) in the IR spectrum has been found
᎐
to be diagnostic in assigning bidentate or monodentate co-
ordination for ligand 4. We have previously found that on
pyridine co-ordination the position of ν(C᎐N) moves to higher
᎐
wavenumber by about 20–35 cmϪ1. The fact that the position of
ν(C᎐N) for 7 and 8 is in a fairly similar position to that of the
᎐
“free” ligand (1596 vs. 1597 cmϪ1) suggests that the pyridine
moieties are not co-ordinated. It is also well known that Pt–Cl
stretching frequencies can be observed at around 300 cmϪ1 in
the infrared spectrum of cis-dichloroplatinum complexes.
Complex 7 shows two sharp bands at 318 and 305 cmϪ1 in its IR
spectrum, which is also consistent with its structure (IR of
cis-Pt(dppp)Cl2 309, 288 cmϪ1).15
Attempts to obtain crystals of complex 7 suitable for X-ray
diffraction by recrystallisation (CH2Cl2–Et2O, slow diffusion)
yielded the hydrolysed product, Pt(Ph2PNHC5H4N-2)2Cl2, 9
(Scheme 4). Interestingly, the crystal structure of 9 shows
both P,N ligands chelating to the platinum centre, with the
two chloride ions located outside the co-ordination sphere.
We have previously prepared Pt(Ph2PNHC5H4N-2)2Cl2 by
more conventional means. The compound gave a single
broadened resonance in its phosphorus NMR spectrum (δ 51.4,
1J{31P-195Pt} = 3576 Hz). The structure we previously obtained
revealed one monodentate ligand and one chelating ligand to
Scheme 2
31P NMR spectrum shows the expected singlet with selenium
1
satellites. The magnitude of J{31P–77Se}(778 Hz) is typical of
a diphenylphosphinoamine selenide (1J{31P–77Se} for Ph2P(Se)-
NHP(Se)Ph2 = 793 Hz).13
974
J. Chem. Soc., Dalton Trans., 2001, 972–976

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Table 3 Selected bond lengths (Å) and angles (Њ) for complex 9
Table 4 Selected bond lengths (Å) and angles (Њ) for complex 10
Pt(1)–N(14)
Pt(1)–P(1)
P(1)–N(1)
2.126(11)
2.231(3)
1.690(8)
Pt(1)–N(32)
Pt(1)–P(2)
P(2)–N(2)
2.143(12)
2.233(4)
1.667(9)
Pt(1)–C(40)
Pt(1)–P(2)
P(1)–N(1)
N(1)–Si(1)
N(2)–C(31)
2.089(5)
2.2892(12)
1.709(4)
1.773(4)
1.422(6)
Pt(1)–C(39)
Pt(1)–P(1)
P(2)–N(2)
N(2)–Si(1)
N(1)–C(13)
2.101(5)
2.2961(12)
1.713(4)
1.785(4)
1.435(6)
N(14)–Pt(1)–N(32)
N(14)–Pt(1)–P(1)
99.5(4)
79.6(3)
P(1)–Pt(1)–P(2)
N(32)–Pt(1)–P(2)
101.50(12)
79.6(3)
C(40)–Pt(1)–C(39)
C(39)–Pt(1)–P(1)
P(1)–N(1)–Si(1)
C(13)–N(1)–P(1)
C(13)–N(1)–Si(1)
N(1)–Si(1)–N(2)
83.7(2)
91.2(2)
119.3(2)
122.0(3)
118.5(3)
107.7(2)
P(1)–Pt(1)–P(2)
C(40)–Pt(1)–P(2)
P(2)–N(2)–Si(1)
C(31)–N(2)–P(2)
C(31)–N(2)–Si(1)
C(37)–Si(1)–C(38)
95.24(4)
90.0(2)
118.6(2)
121.7(3)
118.0(3)
110.1(2)
Fig. 3 The crystal structure of complex 9.
Fig. 4 The crystal structure of complex 10.
moisture. We considered that the strong preference for P,N
chelation found in ligand 4 may be assisting the hydrolysis of
complexes of 5. It was hoped that if we removed the possibility
for P,N chelation a metal complex with considerably greater
stability might result. This would enable us structurally to char-
acterise a MPNSiNP ring system. We therefore decided to pre-
pare the dimethylplatinum complex of ligand 5. Addition of 5
to Pt(COD)Me2 results in the expected complex 10 (Scheme 5).
Scheme 4
be present. This did not tie up with the single phosphorus
environment indicated by NMR. The crystal structure we have
now obtained explains the broadened resonance in the NMR
spectra, as both bis(chelate) and mono(chelate) forms presum-
ably interconvert in solution. The IR spectrum of each of
the two sets of crystals is also consistent with the structures
obtained (mono-chelate, ν(C᎐N) 1620 and 1600; bis-chelate,
᎐
ν(C᎐N) 1617 cmϪ1). The platinum–chloride stretching vibra-
᎐
tions, which we observed in the IR spectrum of complex 7, are
not present for the crystals of 9, although they are replaced by a
weak, broad band at 329 cmϪ1
.
Scheme 5
The molecular structure of complex 9 (shown in Fig. 3)
shows the platinum centre to be square planar and confirms
that a bis-chelate species is formed. As has been found in other
crystal structures of Ph2PNHC5H4N-2, the chloride counter
ions form a hydrogen bond with the ligand NH functionality
(H1N–Cl(1) 2.39(3) Å, N(1)–Cl(1) 3.082(9) and H2N–Cl(2)
2.181(12), N(2)–Cl(2) 3.103(9) Å). As the refinement of this
structure was hampered by the presence of disordered solvent
molecules, and the possibility of the presence of small
amounts of a neutral deprotonated species,9 we have not
drawn accurate quantitative information from the bond
lengths and angles (Table 3). However, there are some general
features worth noting. The crystallographic bite angle of this
ligand is around 80Њ, and the P–N bond lengths (1.690(8),
1.667(9) Å) may be shorter than those observed in the “free”
ligand (1.705(3) Å). This has been observed in other chelate
structures of this ligand.
1
The size of JP-Pt in the phosphorus NMR spectrum (δ 63.9
ppm, J{31P–195Pt} = 1986 Hz) is typical of a bidentate phos-
1
phine bound trans to methyl ligands, and reflects the high trans
influence of the σ-bonded carbon ligands. This complex is
significantly less susceptible to hydrolysis compared to the
compounds described above. In fact, addition of 50% aqueous
THF to a DCM solution of 10 results in no decomposition even
after 24 hours. This suggests that chloride and/or the potential
for P,N chelation do play a part in the decomposition of
complexes 8 and 9.
Recystallisation of complex 10 from CH2Cl2–Et2O gave
crystals suitable for X-ray diffraction. The structure (Fig. 4 and
Table 4) clearly shows the Pt–P–N–Si–N–P ring system which
adopts a puckered ‘boat’ conformation with the silane moiety
located 1.96(1) Å from the Pt(1)–P(1)–P(2)–C(39)–C(40) plane.
The metal centre is essentially square planar, but with some
deviations from idealised geometry. The angle between the two
methyl ligands (C(40)–Pt(1)–C(39) 83.7(2)Њ) is contracted from
the ideal 90Њ. This is also observed in the related complex (also a
six membered platinacycle containing methyl ligands) cis-[(η6-
The palladium complex also decomposed to Pd(Ph2-
PNHC5H4N-2)2Cl2 under recrystallisation conditions. Hence
both ligand
5 and its dichloroplatinum and palladium
complexes are very susceptible to hydrolysis by atmospheric
J. Chem. Soc., Dalton Trans., 2001, 972–976
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Ph2P(CH2)3P(Ph)CH2Ph)Cr(CO)3)]PtMe2, in which the angle
between methyl ligands is 83.7Њ.16 This angle reflects the steric
demands of the diphosphine ligands. The angle between phos-
phine groups is larger than 90Њ (P(2)–Pt(1)–P(2) 95.24(4)Њ), but
similar to that found in the chromium complex (95.94(4)Њ). The
Pt–P bond lengths (2.2892(12) and 2.2961(12) Å) are typical of
a dimethylplatinum complex, and are longer than those found
in dichloroplatinum complexes (in cis-[PtCl(Ph2PNHC5H4N-2-
P,N)(Ph2PNHC5H4N-2-P)]Cl the Pt–P bond trans to chloride
is 2.219(2) Å). This reflects the stronger trans influence of the
methyl ligands. The methyl ligands found in complex 10 (Pt(1)–
C(39) 2.101(5), Pt(1)–C(40) 2.089(5)) are shorter than in the
related structures cis-[(η6-Ph2P(CH2)3P(Ph)CH2Ph)Cr(CO)3]-
PtMe2 (2.130(5) and 2.162(6) Å) and cis-[Pt(Ph2MeP)2Me2]
(2.122(6) and 2.119(5) Å),17 which may reflect that ligand 5 does
not exert a very strong trans influence. There is a slight contrac-
tion in P–N bond length on complexation [P(1)–N(1) 1.709(4),
P(2)–N(2) 1.713(4) compared to 1.724(2) and 1.7224(14) Å in
the “free” ligand], which suggests that some donation of charge
from the nitrogen lone pair through phosphorus may in fact be
occurring. The P–N bond lengths are shorter still in complexes
of Ph2PNHC5H4N-2 (1.686(6) and 1.689(7) Å in cis-[PtCl-
(Ph2PNHC5H4N-2-P,N)(Ph2PNHC5H4N-2-P)]Cl). The Si–N
bond lengths are very similar in both the co-ordinated and free
PNSiNP backbone. The angles about silicon have changed on
metallocycle formation. N(1)–Si(1)–N(2) has contracted from
116.88(8) to 107.7(2)Њ due to the constraints imposed by the six
membered ring.
We also made attempts at preparing metal complexes
of compound 5 from other well known starting materials such
as [RhCp*Cl2]2, [Rh(COD)Cl]2, Mo(c-C5H11N)2(CO)4, and
Pt(dppe)Cl2–AgClO4. In each case at least some (or complete)
hydrolysis of the N–Si bonds was found to occur. These reac-
tions are not described here for reasons of brevity. It was hoped
that we could use the labile nature of the Si–N bonds to our
advantage by treating complexes of 5 with electrophiles to
generate new and unusual complexes. To demonstrate the
viability of this approach we treated ligand 5 with Ph2PCl, and
found that the ligand reacts quantitatively at room temperature
to give the diphosphine 11 (Scheme 6).
ligand and shown it to act as a bidentate ligand for platinum
and palladium complexes. If the complexes contain chloride
ligands hydrolysis of the M–P–N–Si–N–P ring is facile with
resulting formation of a strong P, N five membered chelate ring.
It remains to be seen whether unfunctionalised P–N–Si–N–P
backboned ligands would exhibit greater stability. The M–P–
N–Si–N–P ring is considerably more stable in Pt(PNSiNP)-
Me2, and could be characterised crystallographically. This
structure suggests that this type of ligand may ideally chelate
to a metal with a larger valence angle than 90Њ, and that the
ligand may exert a slightly smaller trans labilising effect than
those of carbon backbone ligands. The reactivity of this type
of backbone suggests that these types of ligand are not
suitable stabilising ligands for catalysis and organometallic
chemistry, although the easily cleaved N–Si–N linkage may
well be exploitable in the synthesis of multimetallics and
co-ordination chemistry.
Acknowledgements
We are grateful to the EPRSC for financial support and
Johnson-Matthey for a loan of platinum metal salts. We also
wish to thank the Joint Research Equipment Initiative for
equipment grants.
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Scheme 6
Reaction of complex 10 with cis-Pt(Ph2PCl)2Cl2 did not
occur cleanly, but there is evidence that the desired bridged
product was detected in about 40% conversion. We suggest that
structure, cis-[Me2Pt{µ-Ph2PN(C5H4N-2)PPh2}2PtCl2], which
contains an eight membered PtPNPPtPNP ring, is the most
likely compound that fits the 31P NMR data observed (δP 65.2,
2J{31P–31P} = 17.8, 1J{195Pt–31P} = 1893; 50.9, 2J{31P–31P} =
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17.8, J{195Pt–31P} = 4645 Hz). We have made several further
1
14 Y. You, J. Chen, M. Cheng and Y. Wang, Inorg. Chem., 1991, 30,
unsuccessful attempts at this type of reaction, and reaction
conditions are clearly critical to the success of this type of
transformation. Further studies are in progress.
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Conclusion
We have prepared a novel PNSiNP backboned diphosphine
976
J. Chem. Soc., Dalton Trans., 2001, 972–976