Using "click" chemistry to access a new class of tripodal P 3-ligand containing PC bonds

Article abstract of DOI:10.1039/c002940h

High yielding "click" reactions between phosphaalkynes and organo-triazides have afforded examples of tris(triazaphosphole)s, the utility of which as a new class of tripodal P₃-ligand has been demonstrated with the preparation of an unusual diplatinum complex. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002940h

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Using “click” chemistry to access a new class of tripodal P3-ligand containing
=
P C bonds†
Sam L. Choong, Cameron Jones* and Andreas Stasch
Received 11th February 2010, Accepted 31st March 2010
First published as an Advance Article on the web 8th April 2010
DOI: 10.1039/c002940h
4,5
=
High yielding “click” reactions between phosphaalkynes
and organo-triazides have afforded examples of tris(triaza-
phosphole)s, the utility of which as a new class of tripodal
P3-ligand has been demonstrated with the preparation of an
unusual diplatinum complex.
RP CR2, are known to have these electronic attributes, which
has led to their P-coordinated transition metal complexes finding
widespread use in homogeneous catalysis.4 Here we report the near
quantitative “click” synthesis and structural characterisation of
several tris(triazaphosphole) compounds, which can be considered
as phosphalkenic analogues of 1 and 2. The potential of one of
these as a tridentate P-donor ligand is demonstrated with the
preparation of two platinum complexes.
Tripodal ligand systems have been extensively used in the for-
mation of complexes involving metals from across the periodic
table.1 The enhanced chelate effect they have, relative to mono-
and bidentate ligands, allows them to robustly ligate metal centres,
and often permits the stabilisation and isolation of unusual
complex types and reactive intermediates. Consequently, metal
complexes of tripodal ligands have found numerous applications
in, for example, catalysis, enzyme mimicry and small molecule
activation.1 Given their importance, the development of facile,
high yielding synthetic routes to new tripodal ligand systems with
variable steric and electronic properties has been an ongoing theme
in inorganic and organometallic coordination chemistry. Perhaps
most relevant to this study are neutral, sterically bulky tripodal
tris(N-heterocyclic carbene) ligands, e.g. 1 and 2, the coordination
chemistry of which is being developed by Meyer et al.2 The utility
of d-block metal complexes of such ligands for the activation of
biologically relevant small molecules is currently emerging.2
≡
It is well known that phosphaalkynes, P CR, readily undergo
uncatalysed 1,3-dipolar cycloaddition reactions with alkyl or aryl
azides, R¢N3, to give 1,2,3,4-triazaphospholes, R¢N3PCR, with
complete regioselectivity, and in close to quantitative yields.6
Such reactions are closely related to copper-catalysed Huisgen
alkyne-azide cycloadditions, and like those reactions fulfil all
the requirements of “click” chemistry.7 It seemed to us that
phosphaalkyne/azide “click” reactions could be utilised to rapidly
prepare multi-dentate P-donor triazaphosphole ligands in high
yields. To the best of our knowledge the coordination chemistry
of these heterocycles has never been explored.8 As a proof of
concept, the reactions of the sterically hindered and unhindered
t
≡
phosphaalkynes, P CR (R = Bu or Me), with two triazides were
explored (Scheme 1).9 All reactions were carried out at ambient
temperature and were complete within two hours, affording
excellent yields of the tris(triazaphosphole) compounds, 3–5. The
very low solubility of the product from the reaction between
≡
N(CH2CH2N3)3 and P CMe prevented its characterisation.
It would be of considerable interest to develop tripodal ligand
systems that have very similar steric profiles to tris(N-heterocyclic
carbene)s, but different electronic properties. N-heterocyclic car-
benes (NHCs) are well known to be very strong s-donors, but they
are generally poor p-acceptor ligands.3 If the NHC fragments
of 1 and 2 could be replaced by weaker s-donating/stronger
p-accepting moieties, the coordinative properties of the resul-
tant ligands would complement those of the tris(NHC)s, and
could allow access to metal complexes with attractive catalytic
characteristics. Compounds containing phosphaalkene fragments,
Scheme 1 Reagents and conditions: i, 1/3 C6Me3(CH2N3)3-1,3,5; ii, 1/3
N(CH2CH2N3)3.
Compounds 3–5 are thermally very stable and solid samples of 3
are indefinitely stable in the air. In contrast, 4 and 5 slowly oxidise
under aerobic conditions. The most diagnostic spectroscopic
1
information for the compounds comes from their 31P{ H} NMR
spectra. Each exhibits a singlet resonance (3: d 177.0 ppm; 4:
d 172.4 ppm; 5: d 173.2 ppm) which is ca. 230 ppm downfield
from the signal for the phosphaalkyne starting material,5 and
is suggestive of the compounds possessing phosphaalkene frag-
ments. Indeed, very similar chemical shifts have previously been
reported for a variety of 1,2,3,4-triazaphospholes.6 The fact that
School of Chemistry, Monash University, Melbourne, PO Box 23, Victoria,
3800, Australia. E-mail: cameron.jones@sci.monash.edu.au
† Electronic supplementary information (ESI) available: Full synthetic and
spectroscopic details for 3–5 and 7. CCDC reference numbers 766028–
766030. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c002940h
5774 | Dalton Trans., 2010, 39, 5774–5776
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only one resonance is seen in the spectrum of each compound
indicates chemical equivalence of their heterocycles in solution.
X-Ray crystallographic experiments were carried out on 3
and 4 and their molecular structures are depicted in Fig. 1.‡
These represent the first structural characterisations of 1,2,3,4-
triazaphospholes and reveal significant differences between the
compounds. Compound 3 is effectively isostructural with 1 in that
it displays an asymmetric arrangement of its three heterocycles
with respect to the mesitylene linker. This is at odds with the NMR
spectroscopic data for the compound and implies that in solution
there is free rotation of the heterocycles about the methylene
carbon centres. In contrast, the heterocycles of the less hindered
compound, 4, all lie on the same side of the mesitylene plane,
and their P-lone pairs are all directed inwards. As a result, the
compound appears “pre-organised” to act as a tridentate ligand.
The bond lengths within the 1,2,3,4-triazaphosphole fragments of
both compounds show that there is a significant degree of aro-
matic delocalisation within the planar six p-electron heterocyclic
d 180.0 ppm with 195Pt satellites having a 1JPtP coupling constant
of 6070 Hz. This value is large and is not indicative of weak Pt–P
bonding in 6. Indeed, the coupling constant is considerably greater
than expected for related phosphine complexes, e.g. [Pt0(PAr3)3].13
Upon warming the reaction mixture to 20 ◦C, it rapidly took on
1
a deep red colour and its 31P{ H} NMR spectrum showed the
development of several new products (at the expense of 6), one
of which was later identified by X-ray crystallography (vide infra)
to be the unusual bimetallic species, 7. Because of the apparent
instability of 6 in solution at ambient temperature, efforts to isolate
and fully characterise the material were not successful. Similarly,
no identifiable products could be isolated from the reactions of 4
or 5 with [Pt(norbornene)3].
As compound 7 is thermally stable, it was rationally syn-
thesised in a 74% yield by treating 3 with two equivalents of
[Pt(norbornene)3]. Likewise, treating an in situ generated solution
of 6 with one equivalent of [Pt(norbornene)3] led to a comparable
yield of 7, suggesting that 6 is an intermediate in its formation.
1
The 31P{ H} NMR spectrum of the deep red compound exhibits
˚
systems. Of most note are their P–C distances (ca. 1.72 A) which
˚
lie between the normal values for localised double (ca. 1.66 A) and
a doublet resonance for its two chemically equivalent P-centres
(d 138.8 ppm, 2JPP = 82 Hz) that has two sets of platinum satellites
with similar 1JPtP couplings (2936 Hz and 2902 Hz). Both of these
couplings are considerably lower than that in 6, which is in line
with the bridging ligating mode of the P-centres. In contrast, the
platinum satellites about the triplet signal corresponding to the
5
˚
single (ca. 1.87 A) bonded interactions. Previous computational
studies have predicted significant aromaticity within 1,2,3,4-
triazaphospholes.10
unique P-centre (d 179.9 ppm), display JPtP couplings (1JPtP
=
2
8310 Hz; JPtP = 106 Hz) which differ by nearly two orders of
magnitude. The one bond coupling is significantly greater than
that for 6, as might be expected, given the apparent weakness of
the interactions of the other two P-centres with the P3-coordinated
Pt centre. All of the aforementioned JPtP couplings are reflected in
the 195Pt NMR spectrum of 7, which displays two signals at high
field (Pt(1): d -4702 ppm; Pt(2): d -5165 ppm). The observation of
195Pt satellites flanking each of these signals allowed the assignment
of the 1JPtPt coupling constant for the complex as 3342 Hz. While it
is known that it is difficult to correlate 1JPtPt couplings with Pt–Pt
bond distances and strengths,14 the magnitude of the coupling in
7 appears consistent with a strong interaction between the metal
centres.
Fig. 1 Molecular structures of (a) 3 and (b) 4 (25% thermal ellipsoids;
◦
˚
hydrogen atoms omitted). Selected bond lengths (A) and angles ( ) for
4: P(1)–N(1) 1.698(2), P(1)–C(5) 1.723(3), N(1)–N(2) 1.340(3), N(2)–N(3)
1.311(3), N(3)–C(5) 1.353(3); N(1)–P(1)–C(5) 86.30(12). Symmetry oper-
ations: ¢ -y + 1, x - y + 1, z; ¢¢ -x + y, -x + 1, z.
An X-ray crystallographic study was carried out on 7 and its
molecular structure is depicted in Fig. 2. This shows it to be a
bimetallic species with one platinum centre, Pt(1), coordinated
by the three P-centres of the ligand, while the other, Pt(2), is
In order to demonstrate the ligating properties of 3, it was
reacted with one equivalent of [Pt(norbornene)3] at -80 ◦C in
an attempt to prepare the tridentate platinum(0) complex 6
(Scheme 2). It was thought that the very large “coordination
cavity” expected for this ligand (cf. 111) would lead to weakly
ligated Pt centres, thus making the generated complex amenable
2
ligated by the two bridging P-centres and has an h -interaction
with a molecule of norbornene. The terminal Pt(1)–P(1) distance
˚
˚
(2.183(3) A) is at the short end of the known range (2.114–2.514 A,
15
1
˚
mean 2.285 A ), which is in line with the very large JPtPt coupling
associated with it. In contrast, the four Pt–Pbridging distances are
at the higher end of the known range. There is no significant
Pt–arene interaction in the compound (Pt(1)–arene centroid:
1
to use in homogeneous catalysis.12 The 31P{ H} NMR spectrum
◦
of the pale yellow reaction mixture at 0 C exhibited a singlet at
˚
3.125 A). If the platinum centres of 7 are considered as having
closed shell 5d10 electronic configurations, it is difficult to reconcile
˚
the short Pt–Pt distance (2.6218(8) A) in the compound, which is
in the normal range for PtI–PtI bonds.15,16 That said, unsupported,
albeit longer, Pt0–Pt0 interactions have been crystallographically
˚
characterised (e.g. 2.765(1) A in [(DBPP)PtPt(DBPP)] (DBPP =
(But2PCH2)2CH2)17), and are thought to arise from mixing of filled
Pt d-orbitals with low lying empty p-orbitals.18 Moreover, Pt–Pt
interactions of a similar length to that in 7 are common in formally
Scheme
Pt(norbornene)3.
2
Reagents and conditions: i, Pt(norbornene)3; ii,
2
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of The American Chemical Society Petroleum Research Fund.
The EPSRC Mass Spectrometry Service, Swansea, is also
thanked.
Notes and references
‡ Crystal data for 3: C27H42N9P3, M = 585.61, orthorhombic, space group
3
˚
˚
˚
˚
Pbca, a = 19.543(4) A, b = 10.510(2) A, c = 30.197(6) A, V = 6202(2) A ,
Z = 8, Dc = 1.254 g cm-3, F(000) = 2496, m(Mo-Ka) = 0.225 mm-1,
123(2) K, 6767 unique reflections [R(int) 0.0199], R (on F) 0.0375, wR (on
F2) 0.1076 (all data); 4: C18H24N9P3, M = 459.37, hexagonal, space group
3
¯
˚
˚
˚
R3, a = b = 17.456(3) A, c = 12.415(3) A, V = 3276.2(9) A , Z = 6, Dc =
1.397 g cm-3, F(000) = 1440, m(Mo-Ka) = 0.298 mm-1, 123(2) K, 1586
unique reflections [R(int) 0.0548], R (on F) 0.0492, wR (on F2) 0.1311 (all
data); 7·(toluene): C41H58N9P3Pt2, M = 1160.05, monoclinic, space group
Fig. 2 Molecular structure of 7 (25% thermal ellipsoids; hydrogen
◦
˚
˚
˚
◦
P21/n, a = 16.300(3) A, b = 10.424(2) A, c = 28.869(6) A, b = 92.59(3) ,
˚
atoms omitted). Selected bond lengths (A) and angles ( ): Pt(1)–P(1)
3
-3
˚
V = 4900.5(17) A , Z = 4, Dc = 1.572 g cm , F(000) = 2272, m(Mo-Ka) =
5.837 mm-1, 123(2) K, 8596 unique reflections [R(int) 0.0934], R (on F)
0.0653, wR (on F2) 0.1800 (all data).
2.183(3), Pt(1)–P(2) 2.358(4), Pt(1)–P(3) 2.359(3), Pt(1)–Pt(2) 2.6218(8),
P(1)–N(1) 1.654(12), P(1)–C(13) 1.710(13), N(1)–N(2) 1.349(14),
N(2)–N(3) 1.328(15), N(3)–C(13) 1.345(17), Pt(2)–P(2) 2.302(4),
Pt(2)–P(3) 2.304(4), P(2)–C(18) 1.702(14), P(2)–N(4) 1.706(11), N(4)–N(5)
1.376(15), N(5)–N(6) 1.291(16), N(6)–C(18) 1.336(16); P(1)–Pt(1)–P(2)
126.01(13), P(1)–Pt(1)–P(3) 124.65(13), P(2)–Pt(1)–P(3) 108.94(12),
Pt(2)–P(2)–Pt(1) 68.47(10), Pt(2)–P(3)–Pt(1) 68.42(10), N(1)–P(1)–C(13)
88.0(6), C(18)–P(2)–N(4) 87.2(6), N(7)–P(3)–C(23) 85.4(7).
1 See for example (a) S. Trofimenko, Scorpionates, The Coordination
Chemistry of Polypyrazolylborate Ligands, Imperial College Press,
London, 1999; (b) R. R. Schrock, Acc. Chem. Res., 1997, 30, 9; (c) T. A.
Betley and J. C. Peters, Inorg. Chem., 2003, 42, 5074; (d) H. A. Mayer
and W. C. Kaska, Chem. Rev., 1994, 94, 1239.
2 (a) K. Meyer and S. C. Bart, Adv. Inorg. Chem., 2008, 60, 1; (b) C.
Vogel, F. W. Heinemann, J. Sutter, C. Anthon and K. Meyer, Angew.
Chem., Int. Ed., 2008, 47, 2681.
3 (a) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47,
3122; (b) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290.
4 P. Le Floch, Coord. Chem. Rev., 2006, 250, 627.
5 K. B. Dillon, F. Mathey and J. F. Nixon, in Phosphorus: The Carbon
Copy, Wiley, Chichester, 1998.
6 W. Ro¨sch, T. Facklam and M. Regitz, Tetrahedron, 1987, 43, 3247.
7 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed.,
2001, 40, 2004.
8 N. B. Several, examples of P-coordinated 1,2,4,3-triazaphosphole
complexes have been reported. See for example J. G. Kraaijkamp, G.
van Koten, K. Vrieze, D. M. Grove, E. A. Klop, A. L. Spek and A.
Schmidpeter, J. Organomet. Chem., 1983, 256, 375.
platinum(0) triangular clusters, [{(L)Pt(m-L)}3] (L = CO, isonitrile
etc.).15 Another very unusual structural feature of 7 is the fact that
two of its triazaphosphole ligands bridge the Pt–Pt interaction. As
a result, the P-centres of these ligands have distorted tetrahedral
geometries, as opposed to the trigonal planar geometry of P(1).
Despite these differences, the intra-ring geometries of the three
effectively planar heterocycles are similar to each other, and to
that in the free ligand, 3. At this stage we cannot be certain what
the nature of the bonding within the Pt2(m-P) fragments of 7 is,
but it is of note that this bridging mode is comparable to that
exhibited by the phosphabenzene ligand, PC5Ph3-2,4,6 (TPPB),
in the palladium(0) trimer, [{(Et3P)Pd(m-TPPB)}3].19 In that case,
computational studies suggested that both s- and low lying p-
orbitals at phosphorus are involved in bridging its short Pd–Pd
bonds.
In summary, high yield “click” chemistry has been used to
prepare several examples of tris(triazaphosphole)s which are
spatially similar to previously reported tris(NHC) ligands. The
utility of these as a new class of tripodal P3-ligand has been
demonstrated with the preparation of an unusual diplatinum
complex. Studies are under way to systematically investigate the
curious electronic and ligating properties of tris(triazaphosphole)s.
9 N. B. Several, bis- and tris(triazaphosphole) substituted cyanopyridine
compounds have been reported. See for example S. V. Chapyshev, U.
Bergstrasser and M. Regitz, Izv. Akad. Nauk, Ser. Khim., 1996, 252.
10 L. Nyula´szi, T. Veszpre´mi, J. Re´ffy, B. Burkhardt and M. Regitz, J. Am.
Chem. Soc., 1992, 114, 9080.
11 H. Nakai, Y. Tang, P. Gantzel and K. Meyer, Chem. Commun., 2003,
24.
12 M. L. Clarke, Polyhedron, 2001, 20, 151.
13 C. J. Cobley and P. G. Pringle, Inorg. Chim. Acta, 1997, 265, 107.
14 J. Autschbach, C. D. Inga and T. Ziegler, J. Am. Chem. Soc., 2003, 125,
1028.
15 As determined from a survey of the Cambridge Crystallographic
Database, February, 2010.
16 P. Leoni, G. Chiaradonna, M. Pasquali and F. Marchetti, Inorg. Chem.,
1999, 38, 253.
17 T. Yoshida, T. Yamagata, T. H. Tulip, J. A. Ibers and S. Otsuka, J. Am.
Chem. Soc., 1978, 100, 2063.
18 (a) A. Dedieu and R. Hoffmann, J. Am. Chem. Soc., 1978, 100, 2074;
(b) P. Pyykko¨, Chem. Rev., 1997, 97, 597.
19 M. T. Reetz, E. Bohres, R. Goddard, M. C. Holthausen and W. Thiel,
Chem.–Eur. J., 1999, 5, 2101.
Acknowledgements
We gratefully acknowledge financial support from the Australian
Research Council (fellowships for CJ and AS), and the donors
5776 | Dalton Trans., 2010, 39, 5774–5776
This journal is
The Royal Society of Chemistry 2010
©