Amidophosphine ligands as constructs for early-late hetero-bimetallic complexes; the formation of an anisotropic metallocage

Article abstract of DOI:10.1039/b105698k

A new amidophosphine ligand has been synthesised and has been shown to support the formation of an early-late mixed metal complex in which the metals are rigidly separated, so creating a metallocage with an anisotropic cavity.

Full text of DOI:10.1039/b105698k

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Amidophosphine ligands as constructs for early–late hetero-
bimetallic complexes; the formation of an anisotropic
metallocage†
Q. Folashade Mokuolu, Anthony G. Avent, Peter B. Hitchcock and Jason B. Love*
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton,
UK BN1 9QJ. E-mail: j.love@sussex.ac.uk
Received 28th June 2001, Accepted 9th August 2001
First published as an Advance Article on the web 20th August 2001
A new amidophosphine ligand has been synthesised and
has been shown to support the formation of an early–
late mixed metal complex in which the metals are rigidly
separated, so creating a metallocage with an anisotropic
cavity.
Early–Late HeteroBimetallic (ELHB) compounds have the
potential to promote chemical transformations that are not
possible with either metal alone.1 However, in the many
examples of such complexes, co-ordinative saturation of the
early metal and undesired bridging ligand interactions between
the metals often results in poor reactivity. We are interested in
the transformation of inert molecules such as N2, and so are
exploring the use of ELHB complexes to effect N–C and N–H
bond formation via the concomitant activation of the sub-
strates by the different metal moieties. In this context, nitrogen
has been shown to be hydrogenated to ammonia by mixing
W(N2) and Ru(η2-H2) complexes under an atmosphere of H2,2
plus mixtures of Ti(N2) and Pd0 species have promoted the
arylation of N2.3 Here, the activation of N2 by one metal species
and the second substrate (e.g. H2, Ar–X) by the other evidently
promotes the desired bond formation.
In order to generate ELHB complexes in which the potential
problems of early metal co-ordinative saturation and metal–
metal bridging interactions are circumvented, a new amido-
phosphine ligand that incorporates bulky amido substituents
and a m-substituted aryl spacer between the donor atoms has
been synthesised. The amido moiety should stabilise reactive,
early metal species (e.g. TiII, VII, MoIII) that are capable of N2
activation,4 and the rigid m-aryl ring should effectively separate
the two metals so that reaction between the activated substrates
can occur. Furthermore, amidophosphine ligands have been
little utilised in the formation of ELHB complexes.5 We report
here the generation of an unusual TiPt complex that is sup-
ported by a ditopic, amidophosphine ligand environment which
describes some of the above principles.
The synthesis of the new aminophosphine ligand (L) is
described in Scheme 1; the reaction of the known,6 primary
aminophosphine with ClSiMe3 in the presence of a strong base
generates L in moderate yield as a colourless, low melting
solid.‡ The in-situ lithiation of L and subsequent reaction
with TiCl4(THF)2 results in the clean formation of the air and
moisture sensitive, triply substituted, amidotitanium complex 1,
which was isolated as a yellow powder. All attempts to crystal-
lise 1 were unsuccessful, which is probably a consequence of the
motional freedom of the arylphosphine substituents. Complex
1 was found to react cleanly with Pt(norbornene)3 to yield
the novel, orange TiPt heterobimetallic complex 2. No other
products were observed when monitoring the reaction by
NMR spectroscopy at 25 ЊC. The low temperature X-ray crystal
structure of 2 was determined and is shown in Fig. 1.§ It is seen
Scheme 1 Synthesis of the TiPt complex 2. Reagents and conditions:
(i) ClSiMe3, DABCO, Et2O; (ii) BunLi, Et2O, 0 ЊC; (iii) TiCl4(THF)2,
Et2O, 0 ЊC; (iv) Pt(norbornene)3, PhMe, Ϫ78 ЊC.
that the three amidophosphine ligands provide a ditopic, C3-
symmetric co-ordination environment for the two metals; the
TiIV and Pt0 centres adopt tetrahedral and trigonal planar
geometries, respectively. Both the TiCl[N(SiMe3)Ar]3 and Pt-
(PPh2Ar)3 moieties have almost identical geometric parameters
to the monomeric species TiCl[N(SiMe3)2]3 and Pt(PPh3)3,8
7
which suggests that there is limited torsional strain within the
complex. The two parts of the molecule may therefore be
expected to act independently and so should show considerable
synthetic potential. Close contacts between the unique hydro-
gen H2 and that of an o-H of a phenyl ring (H ؒ ؒ ؒ H 2.895 Å)
and the Pt centre (H ؒ ؒ ؒ H 2.916 Å) are observed; similar
interactions are seen in the structure of Pt(PPh3)3.8 The two
metals are well separated by the aryl spacer (Ti–Pt 5.8 Å),
so forming a cavity capped at either ends by dissimilar metals.
This molecule can therefore be described as an anisotropic
metallocage.
The NMR data for 2 show that the solid state structure is
retained in solution.† The 31P{1H} [51.9 (s, JPtP 4468 Hz)] and
195Pt{1H} NMR spectra [Ϫ4606 ppm (q, JPtP 4468 Hz)] both
suggest equivalent phosphines. Close inspection of the crystal
structure of 2 shows that the phenyl rings of each PPh2 group
are different and orthogonal to each other; the N–SiMe3 groups
on the Ti centre are arranged in a staggered conformation to
the PPh2 groups. This arrangement is also observed in CD2Cl2
1
solution in both the H and 13C NMR spectra (300 MHz) of
3
2 at 25 ЊC; two ipso carbons [144.0 (1JPC 34.6, JPC 6.0 Hz)
3
and 139.6 ppm (1JPC 34.6, JPC 6.0 Hz)] and two sets of o-aryl
carbons [135.4 (m) and 134.3 ppm (m)], plus two sets of
o-aryl protons (7.37 and 6.78 ppm) were observed. These
1
assignments were corroborated by H–13C HETCOR experi-
ments. The close contact of H2 to the o-H50 observed in the
solid state structure is also evident in solution, as a strong,
positive NOE was recorded between these two protons, plus
a weaker interaction to another o-H was seen. Spin saturation
NMR experiments confirmed that no exchange between the
o-protons occurred at 25 ЊC, with exchange only becoming
apparent at temperatures approaching 100 ЊC (i.e. the phenyl
rings on each P become equivalent). In order to equilibrate the
phenyl rings, the N(SiMe3) groups must rotate past an eclipsed
conformation with the PPh2 groups, which is evidently a high
† Electronic supplementary information (ESI) available: full NMR
and mass spectroscopic data. See http://www.rsc.org/suppdata/dt/b1/
b105698k/
DOI: 10.1039/b105698k
J. Chem. Soc., Dalton Trans., 2001, 2551–2553
This journal is © The Royal Society of Chemistry 2001
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1
Fig. 2 1H NMR spectrum (500 MHz, selective H decoupled) of the
unique aryl H (bottom); simulated (top).
This is also the case in 2, where no reduction in the C–H2
coupling constant is observed (JCH 160 Hz). The observation
of this interaction in 2 is a result of the combination of the
proximity of H2 to the Pt centre and the rigidity of the cage
architecture.9
Platinum and palladium metallocryptands have been
shown to act as hosts for PbII and TlI ions in which the
guest ions are bound via metal–metal interactions; the filled
2
Pt/Pd dz can overlap significantly with empty guest pz
orbitals.10 The interaction of the unique hydrogen with the
platinum metal centre in 2 suggests that this molecule may be
able to act as a host to small molecules and ions; we are
currently probing the reactivity of the metallocage 2 towards
such species.
Fig. 1 The solid state structure of the TiPt complex 2. Selected bond
lengths (Å) and angles (Њ): Ti–N1 1.907(3), Ti–N2 1.900(4), Ti–N3
1.904(4), Ti–Cl 2.2565(14), Pt–P1 2.2696(12), Pt–P2 2.2631(12), Pt–P3
2.2677(12); P1–Pt–P2 120.09(4), P1–Pt–P3 117.95(4), P2–Pt–P3
121.35(4), N1–Ti–N2 109.84(15), N1–Ti–N3 109.15(15), N1–Ti–Cl
111.27(12), N2–Ti–N3 108.44(16), N2–Ti–Cl 109.68(11), N3–Ti–Cl
108.40(11).
Acknowledgements
We thank the Royal Society (University Research Fellowship,
J. B. L.) and the EPSRC (Q. F. M.) for generous funding.
Notes and references
energy process; a space filling model of 2 shows severe steric
interaction between the N(SiMe3) groups and the associated
bridging aryl rings. Furthermore, the unique aryl-proton on the
bridging phenyl group of L (H2) was found to be deshielded,
occurring at 8.7 ppm as a broad singlet (in C6D6); this reson-
ance correlated to a complex multiplet at 134.5 ppm in the
13C{1H} NMR spectrum. No such resonance was observed
for the Ti-only complex 1 and is therefore a consequence of
the formation of the metallocage 2. Selective decoupling of the
remaining bridging phenyl protons simplified the unique pro-
‡ Full NMR and mass spectroscopic data are available as ESI. Synthesis
of m-(Me3Si)HN(C6H4)PPh2 (L). To
a stirred solution of m-
H2N(C6H4)PPh2 (0.5 g, 1.80 mmol) and DABCO (0.30 g, 2.70 mmol) in
Et2O (15 mL) was added dropwise a solution of chlorotrimethylsilane
(0.29 g, 2.70 mmol) in Et2O (10 mL). After the addition was complete,
the reaction mixture was left to stir for 16 h at room temperature. The
reaction mixture was then filtered by cannula and the solvents were
removed under vacuum, yielding L as a pale yellow oil that solidified on
standing at room temperature (0.24 g, 44%). Found: C, 72.22; H, 6.87;
N, 4.13. C21H24NPSi requires: C, 72.17; H, 6.92; N, 4.01%.
ton into the multiplet shown in Fig. 2 (bottom), containing 31
P
Synthesis of [TiCl(L)3], 1. A solution of BunLi in hexanes (2.5 M,
1.55 mL) was added dropwise to a stirred solution of L (1.35 g, 3.868
mmol) in Et2O (30 mL) at 0 ЊC. The resultant orange solution was
stirred at room temperature for 0.5 h, cooled to 0 ЊC and added drop-
wise to a stirred slurry of TiCl4(THF)2 (0.43 g, 1.289 mmol) in Et2O
at 0 ЊC. The resultant orange slurry was stirred at room temperature for
3 h, and then the solvents were evaporated under vacuum. The brown
residues were extracted into hot (ca. 50 ЊC) hexane (2 × 30 mL), filtered
through Celite and the yellow filtrate evaporated to dryness under
vacuum yielding 1, (0.903 g, 62%) as a fluffy yellow solid. Found:
C, 66.76; H, 6.24; N, 3.58. C63H72ClN3P3Si3Ti requires: C, 66.90;
H, 6.37; N, 3.71%.
and 195Pt contributions only. Simulation of this resonance was
possible by assuming the P–P coupling was large (300 Hz)
[Fig. 2 (top)], and revealed coupling of the unique proton to
both phosphorus and platinum, with JPH 13.7 and JPtH 4.8 Hz.
Similar JPtH coupling constants and chemical shifts to high
frequency have been observed for platinum and palladium
complexes in which a hydrogen, generally from an associated
ligand, sits apically above the trigonal or square plane of
the metal.9 These interactions have been classified as weakly
agostic, as the JPtC coupling constant is relatively unchanged.
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Synthesis of [TiCl(L)3Pt], 2. To a stirred mixture of solid 1 (0.25 g,
0.222 mmol) and Pt(norbornene)3 (0.106 g, 0.222 mmol) was added
toluene (10 mL) at Ϫ78 ЊC. The resultant yellow solution was allowed
to warm to room temperature and stirred at this temperature for 16 h.
After this time, the solvents were evaporated to ca. 1 mL volume and
cooled to Ϫ20 ЊC, causing 2 to precipitate as a microcrystalline, orange
solid. A further crop of 2 could be isolated by the addition of pentane
to the toluene washings (total yield 0.201 g, 80%). Orange prisms suit-
able for X-ray diffraction were grown by the slow evaporation of a
C6D6 solution of 2 in the glove box. Found: C, 57.04; H, 5.19; N, 3.09.
C63H69ClN3P3PtSi3Ti requires: C, 57.16; H, 5.25, N, 3.17%.
§ Crystal data: [TiCl(L)3Pt]ؒ3.5(C6D6), orange prisms, 0.15 × 0.10 ×
0.10 mm, C63H69ClN3P3PtSi3Tiؒ3.5(C6D6), monoclinic, a = 19.7739(3),
b = 26.2764(6), c = 31.3507(6) Å, β = 103.886(1)Њ, U = 15813.4(5) Å3,
space group C2/c (no. 15), Z = 8, µ = 2.05 mmϪ1, F(000) = 6552, 46295
collected reflections, 13842 unique (Rint = 0.068). Data were collected at
173(2) K on a Kappa CCD diffractometer, λ = 0.71073 Å, θ = 3.72 to
25.01Њ, absorption correction applied using MULTISCAN, solved by
direct methods and refined using SHELXL-97.11 Of the three C6D6
molecules, one was disordered and refined as 50 : 50 occupancy; half
a molecule of C6D6 lies on a two-fold rotational axis. Final full-
matrix least-squares refinement on F 2 converged at R1 = 0.043 for 10181
reflections with I > 2σ(I ), wR2 = 0.087, S = 1.001 for all data and 895
parameters. CCDC reference number 167766. See http://www.rsc.org/
suppdata/dt/b1/b105698k/ for crystallographic data in CIF or other
electronic format.
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