The first examples of metal-mediated addition of a phosphorus imine to nitriles; the preparation and X-ray crystal structures of [PtCl 4-{NH=C(Et)N=PPh3}2] and [PtCl 2(EtCN){NH=C(Et)N=PPh3}]

Article abstract of DOI:10.1039/b502970h

Imino(triphenyl)phosphorane, Ph₃P=NH (1), reacts with nitrite complexes of Pt(IV) to generate hydrolytically sensitive [PtCl ₄{NH=C(R)N=PPh₃}₂] (R = Me 2a, Et 2b, Ph 2c), and with the Pt(II) complex [PtCl₂(EtCN)₂] to give [PtCl₂(EtCN){NH=C(Et)N=PPh₃}] (3) and [PtCl ₂{NH= C(Et)N=PPh₃}₂] (4); X-ray crystallography performed upon (2b) and (3) confirms the presence of an imine/nitrile addition ligand bound by the terminal nitrogen. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502970h

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C O M M U N I C A T I O N
The first examples of metal-mediated addition of a phosphorus imine
to nitriles; the preparation and X-ray crystal structures of [PtCl4-
=
=
=
=
{NH C(Et)N PPh3}2] and [PtCl2(EtCN){NH C(Et)N PPh3}]
Nadezhda A. Bokach,*a Vadim Yu. Kukushkin,a Paul F. Kelly,*b Matti Haukkac and
Armando J. L. Pombeirod
a St. Petersburg State University, 198904, Stary Petergof, Russian Federation.
E-mail: nadia-bokach@yandex.ru
b Department of Chemistry, Loughborough University, Leics, UK LE11 3TU.
E-mail: P.F.Kelly@lboro.ac.uk
c Department of Chemistry, University of Joensuu, Department of Chemistry, P.O. Box 111,
FIN-80101, Joensuu, Finland
d Centro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais,
1049-001, Lisbon, Portugal
Received 28th February 2005, Accepted 10th March 2005
First published as an Advance Article on the web 18th March 2005
=
Imino(triphenyl)phosphorane, Ph3P NH (1), reacts with
and to derivatised sulfimides, revealing a rich chemistry for the
resulting heterodiazadienes, both in ligated and free form.5
nitrile complexes of Pt(IV) to generate hydrolytically sensi-
=
=
=
tive [PtCl4{NH C(R)N PPh3}2] (R = Me 2a, Et 2b, Ph
The observations upon the reactivity of Ph2S NH in such
2c), and with the Pt(II) complex [PtCl2(EtCN)2] to give
circumstances raises the question of whether other examples
of heteroimines bearing atoms from Period 3 can participate
in this type of metal-mediated coupling. Though a range of
=
=
=
[PtCl2(EtCN){NH C(Et)N PPh3}] (3) and [PtCl2{NH
=
C(Et)N PPh3}2] (4); X-ray crystallography performed
upon (2b) and (3) confirms the presence of an imine/nitrile
addition ligand bound by the terminal nitrogen.
=
transition-metal complexes of the phosphorus imine Ph3P NH
1 (Fig. 1(b)) are known6 there appear to be no examples of
reactions in which the metal mediates addition of the incoming
imine to bound nitriles. In order to address this issue we have
investigated the reactivity of 1 towards [PtCln(RCN)2] (n = 2, 4).†
Reaction of 1 with [PtCl4(RCN)2] (R = Me, Et, Ph)
The ability of some metal centres to activate nitriles towards nu-
cleophilic attack is now well documented1 and constitutes an im-
portant synthetic route to a diverse range of nitrile/nucleophile
addition products, many of which can not be generated by metal-
free organic synthetic means. Within such metal-mediated reac-
tion systems, the most commonly employed strategy involves the
formation of a C–N bond. Recent examples include addition of
ammonia,2a amines2b and nitrogen heterocycles2c to the ligated
RCN unit, while metal-mediated nitrile–amine coupling has
been identified as a key intermediate step in the formation of
imidoylmidine(ato) complexes2d and phthalocyanines.2e
=
=
at low temperature readily generates [PtCl4{NH C(R)N
PPh3}2] (R = Me 2a, Et 2b, Ph 2c). The pale yellow products
are moisture sensitive; thus in the case of the most soluble of the
=
complexes, 2b, after 3 h in solution the formation of Ph3P O
can be observed by 31P NMR and an insoluble precipitate
5
=
of [PtCl4{NH C(Et)NH2}2] results. X-Ray crystallography
performed upon 2b confirms that addition of the phosphorus
imine to the original nitrile has taken place, generating the
=
=
HN C(Ph)N PPh3 ligand (Fig. 1(c)) (via formation of a C–
N bond and proton migration) which coordinates to the Pt
centre through the terminal nitrogen atom (Fig. 2).‡ The re-
sulting (triphenylphosphoranylidene)butan-2-imine ligands are
situated trans to each other (presumably dictated by steric
factors) and adopt a Z-configuration.
In contrast, examples of nitrile/heteroimine coupling are
much rarer; in one such case, part of the current author
team showed that N-bound monodentate ligands of the type
PhR C N–C(R) NH (R = Ph, OAlk3b) result from Pt(IV)
ꢀ
ꢀ
3a
=
=
ꢀ
=
mediated addition of PhR C NH to RCN. Examples wherein
the imine heteroatom originates from Period 3 are so far limited
=
to reactions of the sulfimide Ph2S NH (Fig. 1(a)). Kelly et al.
observed the formation of both monodentate and bidentate
=
=
ligands of type Ph2S NC(R) NH via Pt(II) mediated coupling
of the sulfimide to nitriles4 and subsequent joint work between
the two groups extended the latter reactions to Pt(IV) systems
Fig. 2 The crystal structure of 2b.
Reaction of 1 with the Pt(II) complex [PtCl2(EtCN)2] also
results in imine/nitrile coupling, though in this case the reaction
is slower than for Pt(IV) and the product varies with the solvent
=
system. In neat EtCN the result is [PtCl2(EtCN){NH C(Et)
=
=
Fig. 1 Structures of Ph2S NH (a) and Ph3P NH (b) together with
=
the new ligands formed in this study (c).
N PPh3}] (3), isolated in ca. 50% yield, while reaction in
1 3 5 4
D a l t o n T r a n s . , 2 0 0 5 , 1 3 5 4 – 1 3 5 6
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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a CH2Cl2–EtCN mixture generates a slightly larger yield of
K. and A. J. L. P. are grateful to the FCT and the POCTI
program (POCTI/QUI/43415/2001 project)(FEDER funded)
for financial support of their studies. We are grateful to Johnson
Matthey for loans of precious metals.
=
[PtCl2{NH C(Et)NPPh3}2] (4).† Both compounds are substan-
tially more stable towards water than (2) and X-ray crystallo-
graphy, performed upon complex (3), again confirms the pre-
sence of the imine/nitrile addition product as ligand (Fig. 3).‡
In this case it adopts an E-configuration and is located trans to
an unreacted nitrile group on the square-planar platinum.
Notes and references
† Synthesis of 2b: A solution of 1 (0.25 mmol) in CH2Cl2 (0.78 mL)
was added dropwise, with stirring, to a suspension of [PtCl4(EtCN)2]
(0.10 mmol) in EtCN (1 mL) cooled with liquid N2. After being allowed
to warm to ambient temperature the reaction mixture was stirred for a
further 20 min before the pale yellow product was obtained by filtration;
an additional yield of the product was obtained by addition of Et2O (ca.
0.3 mL) to the filtrate. Yield 60–70%. Anal. Calc. for C42H42N2Cl4P2Pt:
C, 50.36; H, 4.23; N, 5.59. Found: C, 49.89; H, 4.46; N, 5.36%. FAB+-
MS, m/z 1025 [M + Na]+, 1001 [M]+, 929 [M − 2HCl]+. IR spectrum in
KBr, selected bands, cm−1: 3425w br m(O–H), 3339m m(N–H), 1528s
m(C N and C C from Ar), 1115ms m(P N). 1H NMR in CDCl3,
d 7.73–7.50 (m, 15H, Ph), 5.78 (s + d, JPt–H 37.7 Hz, 1H, NH), 2.92
=
=
=
(m, 2H, CH2CH3) and 1.25 (t, 3H, CH2CH3). 31P{ H} NMR in CDCl3,
1
1
d 8.91 (s + d, JPt–P 41.6 Hz). 13C{ H} NMR was not measured because
of fast decomposition in the non-dried solvent. Compounds 2a and 2c
were prepared in similar manner, with full precipitation occurring during
the course of the reaction thanks to their lower solubility. Synthesis
of 3 and 4: A solution of 1 (0.25 mmol) in CH2Cl2 (0.78 mL) was
added dropwise, with stirring, to a solution of [PtCl2(EtCN)2] (37.5 mg,
0.10 mmol) in EtCN (1 mL) at ambient temperature. After 15 min the
resulting pale yellow precipitate was filtered off, washed with EtCN
(0.3 mL) and dried in air resulting in a ca. 50% yield of 3. An alternative
synthesis performed in identical fashion using EtCN–CH2Cl2 (1 mL;
1 : 2, v/v) as initial solvent for the Pt starting material resulted in a
lemon-yellow solution after 1 h of reaction. After reduction of volume
and addition of Et2O (2 mL) the precipitated yellow powder was filtered
off, washed with CH2Cl2–Et2O (1 mL; 1 : 1, v/v) and twice with
Et2O (1 mL) and dried in air to give a ca. 65% yield of 4. When the
reaction was performed in CH2Cl2 only, a broad mixture of products,
including both the addition and substitution products was obtained. 3:
Anal. Calc. for C24H26N3Cl2PtP: C, 44.17; H, 4.02; N, 6.44. Calc. for
C24H26N3Cl2PtP·H2O: C, 42.98; H, 4.21; N, 6.27. Found: C, 43.00; H,
4.11; N, 6.22%. FAB+-MS, m/z 598 [M + H]+, 562 [M − Cl]+, 525 [M −
2HCl]+. IR spectrum in KBr, selected bands, cm−1: 3435m br m(O–H),
Fig. 3 The crystal structure of 3.
The bond distances within the new ligands in 2b and 3
=
=
are broadly consistent with a localised N(1) C(1)–N(2) P(1)
bonding arrangement within the ligand backbone. The most
significant difference in bond lengths between the two structures
is associated with the metal-bound nitrogen atoms of the new
ligands. In the case of 3 Pt–N is significantly shorter than for 2b,
consistent with the nitrile possessing a weaker trans labilising
ability than the imine/nitrile adduct.
=
=
Free species of the type Ph3P NC(R) NH are known
(though no coordination chemistry has been reported), but
may only be prepared by direct reaction of imine with nitrile
when the latter is activated by a very electron-withdrawing R
group (such as CCl3 or CF3).7 Formation of other examples
uses more involved techniques, typically the reaction of N-
=
=
=
3281ms m(N–H), 1564ms m(C N and C C from Ar), 1112ms m(P N).
1H NMR in CDCl3, d 7.67–7.52 (m, 15H, Ph), 5.05 (s, br, 1H, NH), 3.02
(dq, 4JP–H 2.50 Hz, 2H, CH2) and 2.70 (q, 2H, CH2), 1.31 and 1.29 (two
1
t, 6H, two CH3). 13C{ H} NMR in CDCl3, d 173.14 (br, C N), 128.68
=
(p-Ph), 128.20 (d, 2JC–P 28.24 Hz) and 124.90 (d, 3JC–P 12.50 Hz) (o- and
m-Ph’s), 29.72 (d, 3JC–P 13.85) and 7.72 (Et), 8.30 and 5.39 (Et). 31P{ H}
1
NMR in CDCl3, d 8.53. 195Pt NMR in CDCl3, d 2158.8 (788 Hz). 4:
Anal. Calc. for C42H42N4Cl2PtP2: C, 54.24; H, 4.55; N, 6.03. Found: C,
54.15; H, 5.01; N, 5.84%. FAB+-MS, m/z 931 [M]+, 894 [M − HCl]+,
858 [M − 2HCl]+,. IR spectrum in KBr, selected bands, cm−1: 3425m
=
chlorobenzamidines (ArC( NCl)NH2) with PPh3 and base;
this of course requires prior preparation of the N-chlorobenz-
amidines.8 The resulting heterodiazadienes have become the
subject of some considerable interest thanks to their ability
to act as sources of Ar–C(N)N moieties during cyclisation
reactions. By way of example, Rossi et al. have shown that
=
=
br m(O–H), 3327ms m(N–H), 1541s m(C N and C C from Ar), 1113s
1
=
m(P N). H NMR in CDCl3, d 7.69–7.49 (m, 15H, Ph), 5.44 (s, br, 1H,
NH), 3.06 (dq, JP–H 2.34 Hz, 2H, CH2), 1.29 (t, 3H, CH3). 13C{ H}
4
1
=
NMR in CDCl3, d 176.31 (C N), 132.53 (m, p-Ph and m-Ph), 129.72
=
=
Ph3P NC(Ph) NH is an effective reagent for the one-pot
synthesis of dihydropyrimidines and pyrimidines via reaction
with both acyclic and cyclic a,b-unsaturated aldehydes.9
(o-Ph), 33.60 (d, 3JC–P 12.89) and 12.12 (Et). 31P{ H} NMR in CDCl3, d
1
6.24.195Pt NMR in CDCl3, d −1901.9 (455 Hz).
‡ The X-ray diffraction data were collected with a Nonius KappaCCD
˚
diffractometer using Mo-Ka radiation (k = 0.71073 A). The structures
The platinum-mediated reactions thus provide novel and very
facile routes to potential synthons–and platinum does appear
key to the success of the technique, as identical reactions
involving [PdCl2(EtCN)2] result simply in the substitution
were solved by direct (SHELXS) and refined by SHELXL program.
A multiscan absorption correction based on equivalent reflections
(XPREP in SHELXTL v. 6.14) was applied to all of the data. The
NH hydrogens were located from the difference Fourier map. Other
hydrogens were placed in idealized position and constrained to ride on
their parent atom. Crystal data for 2b: C24H26Cl2N3PPt, M = 653.44 g
=
product [PdCl2(HN PPh3)2]. Furthermore, the results of the
study confirm that heteroimine/nitrile coupling using imines
containing heavier (i.e. Period 3) atoms is not just limited to the
sulfimides and suggests that further work in this area will be
fruitful.
mol−1, monoclinic, space group C2/c (no. 15),◦a = 22.0217(3), b =
3
˚
˚
8.7854(5), c = 24.7704(10) A, b = 93.0890(10) , V = 4792.3(3) A ,
T = 120 K, Z = 8, Dc = 1.811 g cm−3, l(Mo-Ka) = 6.163 mm−1
,
14991 reflections collected , 4744 unique (Rint = 0.0582), The final
R1 was 0.0325 [I > 2r(I)] and wR2(F2) = 0.0625 [I > 2r(I)]. Crystal
data for 3: C42H42Cl4N4P2Pt, M = 1001.63 g mol−1, triclinic, space
Acknowledgements
¯
˚
3
group P1 (no. 2), a = 10.1210(2), b = 13.7987◦(4), c = 15.6217(4) A,
˚
a = 73.8900(10), b = 86.897(2), c = 89.625(2) , V = 2092.85(9) A ,
This work has been supported by the Russian Fund for Basic
Research (grants 03-03-32363 and 05-03-32140). N. A. B., M. H.
and V. Yu. K. would like to thank the Academy of Finland for
financial support. V. Yu. K. is very much obliged to ISSEP
for a Soros Professorship (2001–2004) and the Royal Society of
Chemistry for a Grant for International Authors. N. A. B., V. Yu.
T = 100 K, Z = 2, Dc = 1.589 g cm−3, l(Mo-Ka) = 3.719 mm−1
,
12933 reflections collected, 7363 unique (Rint = 0.0294), The final
R1 was 0.0301 [I > 2r(I)] and wR2(F2) = 0.0695 [I > 2r(I)].
CCDC reference numbers 264823 and 264824. See http://www.rsc.org/
suppdata/dt/b5/b502970h/ for crystallographic data in CIF or other
electronic format.
D a l t o n T r a n s . , 2 0 0 5 , 1 3 5 4 – 1 3 5 6
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