Metal(II) template assisted syntheses of two P-C-O-P chelating ligands incorporating a bulky 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosphaadamantyl cage

Article abstract of DOI:10.1016/j.inoche.2011.03.037

Facile condensation of one equiv. of the caged hydroxymethylphosphine Ad' PCH₂OH (Ad' = C₁₀H₁₆O₃) with ClPR₂ (R = Ph, ⁱPr), in the presence of one equiv. of MCl₂(cod) (M = Pt, Pd; cod = cycloocta-1,5-diene), afforded the five-membered chelate complexes cis-MCl₂(Ad'PCH₂OPR ₂) (1a, M = Pt, R = Ph; 1b, M = Pd, R = Ph; 2a, M = Pt, R = ⁱPr) in reasonable to excellent yields. The X-ray structures of 1a and 2a have been determined, the latter using diffraction data collected at a synchrotron source. The M - P - C - O - P metallacycle remains intact even after stirring in CD₃OD for 24 h at room temperature.

Full text of DOI:10.1016/j.inoche.2011.03.037

Inorganic Chemistry Communications 14 (2011) 940–943
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Metal(II) template assisted syntheses of two P―C―O―P chelating ligands
incorporating a bulky 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosphaadamantyl cage
b,⁎⁎
Gavin M. Brown a, Mark R.J. Elsegood a, Martin B. Smith a, , Kevin Blann
⁎
a
Department of Chemistry, Loughborough University, Loughborough, Leics, LE11 3TU, UK
b
Sasol Technology (Pty) Ltd, R&D Division, 1 Klasie Havenga Road, Sasolburg, 1947, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Facile condensation of one equiv. of the caged hydroxymethylphosphine Ad`PCH2OH (Ad`=C10H16O3) with
ClPR2 (R=Ph, iPr), in the presence of one equiv. of MCl2(cod) (M=Pt, Pd; cod=cycloocta-1,5-diene),
afforded the five-membered chelate complexes cis-MCl2(Ad`PCH2OPR2) (1a, M=Pt, R=Ph; 1b, M=Pd,
R=Ph; 2a, M=Pt, R=iPr) in reasonable to excellent yields. The X-ray structures of 1a and 2a have been
determined, the latter using diffraction data collected at a synchrotron source. The M―P―C―O―P
metallacycle remains intact even after stirring in CD3OD for 24 h at room temperature.
© 2011 Elsevier B.V. All rights reserved.
Received 18 January 2011
Accepted 14 March 2011
Available online 22 March 2011
Keywords:
Crystal structure
Ditertiary phosphine
Palladium
Platinum
Template synthesis
Ditertiary phosphines e.g. Ph2P(CH2)2PPh2 (often abbreviated
dppe) are an important class of ligand playing a major role in
coordination chemistry and catalysis. While symmetric diphosphines
are readily synthesised or indeed commercially available, their
nonsymmetric counterparts are often somewhat more challenging
to prepare [1,2]. We [3] recently described the synthesis and reactivity
of Ad`P(CH2)2PPh2 1, a bulky analogue of dppe by virtue of the 1,3,5,7-
tetramethyl-2,4,8-trioxa-6-phosphaadamantyl cage (Ad`). Moreover
we demonstrated a range of coordination capabilities of 1 dictated by
the sterically encumbered Ad` cage.
Mannich condensation reactions have been extensively used
to prepare new phosphorus ligands bearing P―C―N(H)R or
P―C―N(R)―C―P backbones [4–7]. This synthetic approach tolerates
various amines (primary, secondary and even NH3 [8]) which undergo
condensation with a suitable HOCH2PR2 (preformed or generated in
situ from the corresponding secondary phosphine and paraformal-
dehyde). Furthermore, work-up procedures are often trivial given the
only non-phosphorus based side product is water. An alternative
condensation strategy, involving a transition metal-assisted route, has
been used to access new precomplexed P―C―O―P bidentate ligands.
Whereas examples are known in the literature, albeit rare, for the non-
metal based coupling of two phosphorus species with P―C―O―P
formation [9–11], the use of a metal template reaction to directly access
the preformed complex is particularly attractive [12]. Previous studies
have shown that unusual P―C bond cleavage reactions at Rh [13], Pd
[14] or Pt [14] centres can proceed readily in complexes of P(CH2OH)3
affording unusual bidentate ligands with a P―C―O―P framework.
Herein, we demonstrate a simple approach to metal complexes
containing an Ad`P cage and different ―OPR2 groups (R=Ph, Pr).
Two examples have been characterised using single crystal X-ray
crystallography.
i
The synthesis of new P―C―O―P chelate complexes of PtII and PdII
(Scheme 1) was readily accomplished upon reaction of one equiv. of
Ad`PCH2OH [6], ClPR2 (R=Ph, iPr) and MCl2(cod) (M=Pt, Pd) [15] in
CH2Cl2 at ambient temperature [16]. It is plausible to assume
spontaneous elimination of HCl occurs [17], in the absence of base,
after initial cis coordination of the monodentate Ad`PCH2OH and ClPR2
ligands. Therefore under these conditions this transformation is
extremely facile. No spectroscopic monitoring was performed to
verify the existence of intermediate species such as A. Compounds 1a,
1b and 2a have been characterised by spectroscopic and analytical
methods [16]. 31P{1H} NMR spectroscopy (CDCl3) revealed two well
separated doublets at δ(Pphosphinite) 128.4 (1a), 158.6 (1b) and 174.1
(2a) and δ(Pphosphine) 51.1 (1a), 75.5 (1b) and 52.4 (2a)ppm
2
respectively with small JPP couplings (4.2−13 Hz) indicative of a
cis conformation. Significant downfield chemical shifts are consistent
with five-membered ring formation. In the 1H NMR spectra of 1a, 1b
and 2a the P―CH2―O―P hydrogens are diastereotopic, resonating
around 3–4 ppm.
Crystals suitable for X-ray crystallography were grown by slow
diffusion of diethyl ether into a CDCl3 solution (for 1a·2CHCl3) or
were obtained by slow diffusion of diethyl ether and hexanes into a
CH2Cl2 solution (for 2a). Due to small crystal size, and hence weak
diffraction, the crystallographic data for 2a were collected at a
synchrotron source. Each of the X-ray structures [18] of 1a (Fig. 1) and
⁎
Corresponding author. Tel.: +44 1509 222553; fax: +44 1509 223925.
⁎⁎ Corresponding author. Tel.: +27 16 960 5408; fax: +27 11 52 3190.
E-mail addresses: m.b.smith@lboro.ac.uk (M.B. Smith), kevin.blann@sasol.com
(K. Blann).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.03.037

G.M. Brown et al. / Inorganic Chemistry Communications 14 (2011) 940–943
941
Scheme 1. Preparation of complexes 1a, 1b and 2a.
2a (Fig. 2) reveals a square planar geometry with respect to the Pt(II)
metal centre with similar Pt―P bond lengths [range 2.201(3)–2.2179
(10)Å] and Pt―Cl [range 2.3516(10)–2.359(3)Å] to the known
complexes PtCl2{Ad`PCH2N(R)CH2PPh2} (R=C6H5, 4-CH3C6H4) [6].
Furthermore, the P―Pt―P bite angle is slightly contracted [85.18(4)°
for 1a; 85.89(11)° and 85.95(10)° for both independent molecules for
2a] from the ideal 90° and there is also an expected variation in Cl―Pt
(1)―P bond angles as a consequence of the different ―PPh2 [Cl(1 or
3)―Pt(1 or 3)―P(1) 90.04(4)° for 1a; 88.93(11)/89.62(10)° for 2a]
and ―Ad` cage [Cl(2 or 4)―Pt(1)―P(2 or 4) 96.26(4)° for 1a; 95.18
(11)/96.02(10)° for 2a] groups. The P(1 or 3)―O(4 or 8) distances
[1.619(3)Å for 1a; 1.640(7)/1.650(8)Å for 2a] are marginally less
than that observed by Faraone et al. [10] for a PdII mixed phosphine–
phosphite complex [1.590(4) Å]. The five-membered ring can be
described as a twisted envelope with the backbone C and O atoms
sitting above or below the plane formed by the platinum and two
phosphorus atoms (for 1a and 2a). The central carbon, C(11) and
oxygen, O(4), are found 0.14 Å below, and 0.39 Å above the Pt(1)/P
(1)/P(2) plane respectively for 1a. In 2a the central carbon atoms, C
(11) and C(28) lie 0.17 and 0.06 Å above the PtP2 plane, while oxygen
atoms O(4) and O(8) lie 0.18 and 0.29 Å below it. Thus the twist is
rather less pronounced in one of the molecules in 2a, demonstrating a
degree of conformational flexibility in these five-membered chelate
rings.
The choice of backbone connectivity between two ―PR2 groups
can significantly influence the properties of any resulting metal
complex. For phosphinoamines, bearing a single ―N(R)― spacer,
previous studies [24] have shown the susceptible nature of the P―N
bond to protic solvents under mild conditions. In order to establish the
(in)stability of our P―C―O―P metallacycles, CD3OD suspensions of
1a and 2a were stirred for approx. 24 h at ambient temperature.
Examination [31P{1H} NMR spectroscopy] of both the CD3OD solution,
and the recovered solid, showed no methanolysis had resulted,
consistent with the absence of formation of complexes such as MCl2
Fig. 1. Molecular structure of 1a·2CHCl3. All hydrogen atoms and solvent molecules of crystallisation have been omitted for clarity. Selected bond distances (Å) and angles (°):
Pt(1)―P(1) 2.2062(10), Pt(1)―P(2) 2.2179(10), Pt(1)―Cl(1) 2.3516(10), Pt(1)―Cl(2) 2.3580(10), P(1)―O(4) 1.619(3), O(4)―C(11) 1.436(4); P(1)―Pt(1)―P(2) 85.18(4),
P(1)―Pt(1)―Cl(1) 90.04(4), P(2)―Pt(1)―Cl(1) 174.92(4), P(1)―Pt(1)―Cl(2) 178.48(4), P(2)―Pt(1)―Cl(2) 96.26(4), Cl(1)―Pt(1)―Cl(2) 88.54(4), Pt(1)―P(1)―O(4) 109.54(11),
P(1)―O(4)―C(11) 116.2(2), O(4)―C(11)―P(2) 109.0(3).

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G.M. Brown et al. / Inorganic Chemistry Communications 14 (2011) 940–943
Fig. 2. Molecular structure of 2a. All hydrogen atoms have been omitted for clarity. Only one independent molecule is shown (parameters for the second molecule are given in
parentheses). Selected bond distances (Å) and angles (°): Pt(1)―P(1) 2.213(3) [2.201(3)], Pt(1)―P(2) 2.209(3) [2.209(3)], Pt(1)―Cl(1) 2.357(3) [2.359(3)], Pt(1)―Cl(2) 2.356(3)
[2.359(3)], P(1)―O(4) 1.640(7) [1.650(8)], O(4)―C(11) 1.431(13) [1.412(14)]; P(1)―Pt(1)―P(2) 85.89(11) [85.95(10)], P(1)―Pt(1)―Cl(1) 88.93(11) [89.62(10)], P(2)―Pt(1)―
Cl(1) 174.81(11) [172.80(10)], P(1)―Pt(1)―Cl(2) 177.28(11) [176.00(11)], P(2)―Pt(1)―Cl(2) 95.18(11) [96.02(10)], Cl(1)―Pt(1)―Cl(2) 90.01(11) [88.75(10)], Pt(1)―P(1)―O
(4) 109.3(3) [109.8(3)], P(1)―O(4)―C(11) 120.8(7) [120.4(6)], O(4)―C(11)―P(2) 110.1(7) [109.4(7)].
[11] S.O. Grim, W.L. Briggs, R.C. Barth, C.A. Tolman, J.P. Jesson, Inorg. Chem. 13 (1974)
(Ad`PCH2OD)(Ph2POCD3). This suggests, under these reaction condi-
tions, the M―P―C―O―P metallacycle is stable.
1095–1100.
[12] P. Bergamini, E. Costa, A.G. Orpen, P.G. Pringle, M.B. Smith, Organometallics 14
In conclusion, we have demonstrated a simple one-step approach to
nonsymmetric chelatingphosphorus ligands witha P―C―O―P skeletal
backbone. Further studies are currently in progress to probe the
variability of this reaction and to explore the coordination chemistry
and stability of these new M―P―C―O―P metallacycles. Recent studies
have shown that metal complexes of phosphines incorporating the
1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosphaadamantyl cage have found
important catalytic uses [25].
(1995) 3178–3187.
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[14] P.A.T. Hoye, P.G. Pringle, M.B. Smith, K. Worboys, J. Chem. Soc. Dalton Trans.
(1993) 269–274.
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[16] Synthesis of 1a: Under a N2 atmosphere, a solution of Ad`PCH2OH (0.080 g,
0.283 mmol) in dry degassed CH2Cl2 (5 mL) and a solution of ClPPh2 (0.063 g,
0.286 mmol) in dry degassed CH2Cl2 (5 mL) were added, simultaneously, to a
solution of PtCl2(cod) (0.106 g, 0.283 mmol) in dry degassed CH2Cl2 (20 mL). The
solution was stirred for 20 min, concentrated to ~2 mL by evaporation under
reduced pressure and diethyl ether (25 mL) added. The solid 1a was filtered and
dried in vacuo. Yield (0.102 g, 56%). Compounds 1b (91%) and 2a (56%) were
prepared in a similar manner using the appropriate MCl2(cod) (M = Pd, Pt) or
ClPiPr2 precursor. Selected data for 1a: 31P{1H} NMR (CDCl3, 162 MHz, 298 K):
128.4 ppm (1JPtP 3669 Hz), 51.1 ppm (1JPtP 3747 Hz), 2JPP 7.5 Hz. 1H NMR (CDCl3,
400 MHz, 298 K): 7.91−7.38 (arom. H, m, 10 H), 4.57 (CH2, ddd, 1H), 3.53 (CH2,
dt, 1H), 3.26 (CH2, dd, 1H) and 2.02−1.32 (CH2 and CH3, m, 15 H)ppm. FT−IR
(KBr disk): 321, 306 cm− 1 (PtCl). C23H28Cl2O4P2Pt·0.5CH2Cl2 requires C: 38.20,
H: 3.87. Found, C: 38.27, H: 3.87. Selected data for 1b: 31P{1H} NMR (CDCl3,
162 MHz, 298 K): 158.6 ppm, 75.5 ppm, 2JPP 13 Hz. 1H NMR (CDCl3, 400 MHz, 298
K): 7.96−7.40 (arom. H, m, 10 H), 4.70 (CH2, ddd, 1H), 3.65 (CH2, dt, 1H), 3.36
(CH2, dd, 1H) and 1.97−1.32 (CH2 and CH3, m, 15 H)ppm. FT−IR (KBr disk): 323,
293 cm− 1 (PdCl). C23H28Cl2O4P2Pt requires C: 45.46, H: 4.64. Found, C: 45.32,
H: 4.63. Selected data for 2a: 31P{1H} NMR (CDCl3, 162 MHz, 299 K): 174.1 ppm
Acknowledgements
We thank Sasol Technology (Pty) Ltd and Loughborough Univer-
sity for the award of a studentship (G.M.B.). We are grateful to
Johnson-Matthey for their loan of precious metals. We also thank STFC
for beam time at Daresbury Laboratory SRS and the scientific support
from Dr. T. J. Prior and Dr. J. E. Warren.
Appendix A. Supplementary material
CCDC 796617 and 796618 contain the supplementary crystallo-
graphic data for compounds 1a·2CHCl3 and 2a respectively. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
data_request/cif or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033 or
e-mail: deposit@ccdc.cam.ac.uk.
(1JPtP 3487 Hz), 52.4 ppm (1JPtP 3815 Hz), JPP 4.2 Hz. 1H NMR (CDCl3, 400 MHz,
2
299 K): 4.53 (CH2, ddd, 1H), 3.75 (CH2, d, 1H), 3.41 (CH2, dd, 1H), 2.74 (CH(CH3)2,
sept., 1H), 2.64 (CH(CH3)2, sept., 1H), 2.09−1.24 (CH2, CH3 and CH(CH3)2,
m, 27 H)ppm. FT−IR (KBr disk): 307, 291 cm− 1 (PtCl). C17H32Cl2O4P2Pt requires
C: 32.49, H: 5.13. Found, C: 32.73, H: 4.96.
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(c) A.W.G. Platt, P.G. Pringle, Inorg. Chim. Acta 160 (1989) 33–36.
[18] Intensity data for 1a·2CHCl3 were collected at 150(2) K on a Bruker SMART 1000
CCD diffractometer [19] using graphite-monochromated MoKα radiation,
λ=0.71073 Å. The structure was solved by Patterson synthesis for 1a using
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C
23H28Cl2O4P2Pt·2CHCl3: Mr =935.12, triclinic, space group P 1, a=8.7059(5),
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G.M. Brown et al. / Inorganic Chemistry Communications 14 (2011) 940–943
943
γ=115.446(3)°, V=2196.9(11) Å3, Z=4, μ(MoΚα)=6.795 mm− 1, 35158 data
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