Synthesis and cyclization reactions of platinum(II)-coordinated arsonium-substituted phenyl isocyanides, o-(I-R3As +-CH2)C6H4N≡C

Article abstract of DOI:10.1016/j.ica.2004.02.035

The arsonium-substituted isocyanides, o-(I^{- +}R ₃AsCH₂)C₆H₄NC (AsR ₃=AsPh₃, L₁; AsMePh₂, L₂; AsMe₂Ph, L₃), were prepared by reaction of o-(chloromethyl)phenyl isocyanide, o-(CH₂Cl)C₆H ₄NC, with a slight molar stoichiometric amount of the arsine in the presence of a 3-fold excess of NaI in acetone at room temperature. The isocyanides L₁-L₃ coordinate to some Pt(II) complexes such as trans-[PtX{o-(I^{- +}R₃AsCH₂)C ₆H₄NC}(PPh₃)₂] [BF₄] (AsR₃=AsPh₃, 1; AsMePh₂, 2; AsMe₂Ph, 3; X=Cl, I) and [PtX{o-(I^{- +}R₃AsCH ₂)C₆H₄NC}(Ph₂PCH=CHPPh₂)] [BF₄] (AsR₃=AsMePh₂, 4; X=Cl, I). Complexes 2-4 are converted in CH₂Cl₂ at room temperature in the presence of NEt₃ to the corresponding indolidin-2-ylidene derivatives trans-[PtX{(AsR₃)}(PPh₃)₂]BF₄] (AsR₃=AsPh₃, 5; AsMePh₂, 6; AsMe₂Ph, 7) and [PtX{(AsMePh₂)}(Ph₂PCH=CHPPh₂)][BF ₄] (8).

Full text of DOI:10.1016/j.ica.2004.02.035

Inorganica Chimica Acta 357 (2004) 3385–3389
www.elsevier.com/locate/ica
Note
Synthesis and cyclization reactions of platinum(II)-coordinated
arsonium-substituted phenyl isocyanides,
o-(IꢀR3Asþ–CH2)C6H4NBC
a
b,
b
Giacomo Facchin , Rino A. Michelin *, Mirto Mozzon ,
b
Paolo Sgarbossa , Augusto Tassan
b
a
Istituto di Scienze e Tecnologie Molecolari, ISTM-C.N.R., c/o Dipartimento di Processi Chimici dell’Ingegneria, Universitꢀa di Padova,
Via F. Marzolo 9, Padua 35131, Italy
b
Dipartimento di Processi Chimici dell’Ingegneria, Universitꢀa di Padova, Via F. Marzolo 9, Padua 35131, Italy
Received 30 November 2003; accepted 20 February 2004
Available online 7 May 2004
Abstract
The arsonium-substituted isocyanides, o-(Iꢀ þR3AsCH2)C6H4NBC (AsR3 ¼ AsPh3, L1; AsMePh2, L2; AsMe2Ph, L3), were
prepared by reaction of o-(chloromethyl)phenyl isocyanide, o-(CH2Cl)C6H4NBC, with a slight molar stoichiometric amount of the
arsine in the presence of a 3-fold excess of NaI in acetone at room temperature. The isocyanides L1–L3 coordinate to some Pt(II)
complexes such as trans-[PtX{o-(Iꢀ þR3AsCH2)C6H4NC}(PPh3)2] [BF4] (AsR3 ¼ AsPh3, 1; AsMePh2, 2; AsMe2Ph, 3; X ¼ Cl, I)
and [PtX{o-(Iꢀ þR3AsCH2)C6H4NC}(Ph2PCH@CHPPh2)] [BF4] (AsR3 ¼ AsMePh2, 4; X ¼ Cl, I). Complexes 2–4 are converted in
CH2Cl2 at room temperature in the presence of NEt3 to the corresponding indolidin-2-ylidene derivatives trans-
CN(H)–o–C H C
(AsMePh2)}(Ph2PCH@CHPPh2)][BF4] (8).
Ó 2004 Elsevier B.V. All rights reserved.
CN(H)–o–C H C
6 4
[PtX{
(AsR3)}(PPh3)2]BF4] (AsR3 ¼ AsPh3, 5; AsMePh2, 6; AsMe2Ph, 7) and [PtX{
6
4
Keywords: Functional isocyanides; Arsonium-substituted isocyanides; N-heterocyclic carbene complexes; Platinum(II) complexes
1. Introduction
NHC ligands upon metal coordination are illustrated in
Chart A. They include the hydroxyalkyl isocyanides I,
reported by Fehlhammer et al. [4], the b-functional
phenyl isocyanides II, investigated by Hahn and his
group [2] and the c-functional phenyl isocyanides III,
synthesized by some of us [1,5–12].
As an extension to these latter studies, we wish to
report herein the synthesis and the platinum(II)-pro-
moted cyclization reactions of some arsonium-substi-
tuted phenyl isocyanides shown in Chart B.
The organometallic chemistry of several functional-
ized isocyanides has been often addressed [1,2] to the
formation of N-heterocyclic carbenes (NHC), which
have been found to be promising alternative ligands to
the commonly used phosphines and phosphites and also
for their favorable application in homogeneous catalysis
[3]. The NHC ligands are obtained through an intra-
molecular 1,2-addition of the functional moiety, usually
a protic nucleophile such as an hydroxy, an amino or an
ylide group, across the CBN triple bond. Typical rep-
resentative examples of functional isocyanides leading to
2. Experimental
All reactions were carried out under dinitrogen using
standard Schlenk techniques. All solvents were dried by
conventional methods and distilled prior to use. Infrared
spectra in CH2Cl2 solution or nujol mulls were recorded
* Corresponding author. Tel.: +39-49-827-5522; fax: +39-49-827-
5525.
E-mail address: rino.michelin@unipd.it (R.A. Michelin).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.02.035

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G. Facchin et al. / Inorganica Chimica Acta 357 (2004) 3385–3389
2.3. Synthesis of the complexes trans-[Pt(L)X(PPh3)2]
[BF4] (L ¼ L1 (1); L2 (2), L3 (3); X ¼ Cl, I)
Complexes 1–3 were prepared by a similar procedure
which is described herein for 1. cis-[PtCl2(PPh3)2] (0.54 g,
0.69 mmol) and NaBF4 (0.30 g, 2.76 mmol) were dis-
solved in acetone (50 ml). The solution was then treated
dropwise with an acetone solution (30 ml) of L1 (0.38 g,
0.69 mmol) over a period of 45 min and stirred overnight.
The solvent was then removed under vacuum and the
residue treated with CH2Cl2. After filtration, the solution
was concentrated under reduced pressure to a small
volume (10 ml). Addition of Et2O gave a white-cream
precipitate, which was filtered off and then recrystallized
from CH2Cl2/Et2O. 1. Yield: 73%. IR (nujol): m(NBC)
2179 cmꢀ1. 1H NMR (d, CDCl3): 3.80 (s, CH2). 31P{1H}
NMR (d, CDCl3): 11.0 (s, PPh3, 1JP–Pt 2148 Hz), 19.3 (s,
PPh3, 1JP–Pt 2165 Hz). 2. Yield: 70%. IR (nujol): m(NBC)
2181 cmꢀ1. 1H NMR (d, DMSO-d6): 3.50 (s, CH2), 2.10
(s, CH3). 31P{1H} NMR (d, DMSO-d6): 19.1 (s, PPh3,
1JP–Pt 2210 Hz). 13C{1H} NMR (d, DMSO-d6): 5.20 (s,
CH3), 27.9 (s, CH2), 153.6 (s, NBC). 3. Yield: 88%. IR
Chart A.
Chart B.
on a Perkin–Elmer 983 or Nicolet FT-IR Avatar spec-
trometers. NMR spectra were run on a Bruker AC 200
(1H at 200.13 MHz, 31P at 81.01 MHz and 13C at 50.32
MHz), the chemical shifts (d in ppm) are referred to
Me4Si (1H and 13C) and external 85% H3PO4 (31P). The
1
1
1
(nujol): m(NBC) 2181 cmꢀ1, (CH2Cl2) 2188 cmꢀ1. H
spectra of all nuclei (except H) were H decoupled.
NMR (d, DMSO-d6): 3.50 (s, CH2), 2.10 (s, CH3).
31P{1H} NMR (d, DMSO-d6): 19.1 (s, PPh3, 1JP–Pt 2207
2.1. Starting materials
1
Hz), 11.1 (s, PPh3, JP–Pt 2174 Hz. 13C{1H} NMR (d,
DMSO-d6): 6.62 (s, CH3), 28.0 (s, CH2).
The arsines were Stream Chemicals products and
used as purchased. The compounds cis-[PtCl2(PPh3)2]
[13], [PtCl2(Ph2PCH@CHPPh2)] [14] and o-(cholorom-
ethyl)phenyl isocyanide [8a] were prepared according to
literature methods.
2.4. Synthesis of [PtCl(L2)(Ph2 PCH@CHPPh2)] (4)
This compound was prepared using a similar proce-
dure reported above for complex 1. [PtCl(L2)
(Ph2PCH@CHPPh2)] (0.37 g, 0.55 mmol), AgBF4 (0.24
g, 2.2 mmol) and L2 (0.27 g, 0.55 mmol). Yield: 81%. IR
2.2. Synthesis of arsonium-substituted phenyl isocyanides
o-(IꢀR3Asþ–CH2)C6H4NBC (AsR3 ¼ AsPh3 (L1), As-
MePh2 (L2), AsMe2Ph(L3))
1
(nujol): m(NBC) 2197 cmꢀ1. H NMR (d, CDCl3): 4.88
(s, CH2), 2.60 (s, CH3). 31P{1H} NMR (d, CDCl3): 50.1
(d, P trans to halide, 1JP–Pt 3586 Hz, 2JP–P 10.6 Hz), 53.2
(d, P trans to isocyanide, 1JP–Pt 3448 Hz, 2JP–P 10.6 Hz).
A typical procedure for the preparation of ligands
L1–L3 is reported herein for L1. Triphenylarsine (0.73 g,
2.4 mmol) and an excess of NaI (0.89 g, 6.0 mmol) were
added to an acetone solution of o-(ClCH2)C6H4NC (0.3
g, 2.0 mmol). The reaction mixture was stirred at room
temperature for 6 h and then taken to dryness. The
residue was dissolved in dichloromethane (40 ml) and
the resulting solution filtered off and concentrated to a
small volume (10 ml). Addition of Et2O (50 ml) gave a
white product, which was filtered off and recrystallized
from CH2Cl2/Et2O. L1. Yield: 38%. IR: (CH2Cl2),
2.5. Cyclization reactions of the Pt(II)-coordinated isocy-
anides. Synthesis of trans-[PtX{CN(H)–o–C6H4C(AsR3)}
(PPh3)2][BF4] (AsR3 ¼ AsPh3 (5), AsMePh2 (6),
AsMe2Ph
(7))
and
trans[PtX{CN(H)–o–C6H4C
(AsMePh2)}(Ph2PCH@CHPPh2)][BF4] (8), (X ¼ Cl, I)
These compounds were prepared by a standard pro-
cedure, which is described herein for complex 5. A so-
lution of 1 (0.67 g, 0.46 mmol) in acetone (20 ml) was
reacted with NEt3 (0.65 ml, 4.6 mmol) and stirred at
room temperature. The progress of the reaction was
monitored by following the decrease of the CBN ab-
sorption at ca. 2200 cmꢀ1. When the band was com-
pletely disappeared, the solution was taken to dryness
and the residue washed with several 10 ml portions of
H2O to remove the inorganic salts. The remaining solid
m(NBC) 2120 cmꢀ1; (nujol) 2118 cmꢀ1
CDCl3): 5.56 (s, CH2). L2. Yield: 89%. IR: (CH2Cl2),
m(NBC) 2120 cmꢀ1; (nujol) 2121 cmꢀ1 1H NMR (d,
.
1H NMR (d,
.
CDCl3): 5.14 (s, CH2), 2.85 (s, CH3). L3. Yield: 86%. IR:
1
(CH2Cl2), m(NBC) 2119 cmꢀ1; (nujol) 2119 cmꢀ1. H
NMR (d, CDCl3): 4.76 (s, CH2), 2.64 (s, CH3). 13C{1H}
NMR (d, DMSO-d6): 6.70 (s, CH3), 28.5 (s, CH2), 168.2
(s, NBC).

G. Facchin et al. / Inorganica Chimica Acta 357 (2004) 3385–3389
3387
~
was dissolved in CH2Cl2 (50 ml) and dried over anhy-
drous Na2SO4. The mixture was then filtered off, con-
centrated to a small volume and treated with Et2O (50
ml) to give a white precipitate. 5. Yield: 50%. IR (nujol):
agnostic spectral features (see Section 2) are the m(NBC)
wavenumber, which is observed around 2120 cmꢀ1
(CH2Cl2 solution or nujol mull), as also found for the
analogous
phosphonium-substituted
ligands,
o-
m(NH) 3356 cmꢀ1
.
1H NMR (d, DMSO-d6): 2.50 (s,
(BFꢀ4 R3Pþ–CH2)C6H4NBC [8a]. The methylene reso-
CH3), 11.4 (s, NH). 31P{1H} NMR (d, DMSO-d6): 11.6
nance of the R3Asþ–CH2– moiety shows up in the H
1
(s, PPh3, 1JP–Pt 2757 Hz), 16.3 (s, PPh3, 1JP–Pt 2815 Hz).
NMR spectra as a singlet in the range d 5.56–4.76, with
the highest and lowest downfield shifts for the AsPh3
and AsMe2Ph derivatives, respectively. The isocyanide
carbon resonance in the 13C{1H} NMR spectra was
detected only for L3, which was found in DMSO-d6 as a
broad singlet at 168.2 ppm, a value that matches those
observed for other isocyanide ligands such as o-
(Me3SiOCH2)C6H4NBC (d(NBC) 167.5, CDCl3 solu-
tion) [9], o-(HOCH2)C6H4NBC (d(NBC) 166.8, CDCl3
solution) [9], o-(N3CH2)C6H4NBC (d(NBC) 168.8,
CDCl3 solution) [10].
1
6. Yield: 50%. IR (nujol): m(NH) 3355 cmꢀ1. H NMR
(d, DMSO-d6): 2.50 (s, CH3), 11.2 (s, NH). 31P{1H}
1
NMR (d, DMSO-d6): 14.7 (s, PPh3, JP–Pt 2706 Hz),
18.8 (s, PPh3, 1JP–Pt 2769 Hz). 7. Yield: 63%. IR (nujol):
1
m(NH) 3345 cmꢀ1. H NMR (d, CDCl3): 2.10 (s, CH3),
11.2 (s, NH). 31P{1H} NMR (d, CDCl3): 16.4 (s, PPh3,
1JP–Pt 3011 Hz), 20.6 (s, PPh3, 1JP–Pt 3068 Hz). 13C{1H}
NMR (d, CDCl3): 7.8 (s, CH3). 8. Yield: 93%. IR (nu-
jol): m(NH) 3355 cmꢀ1. 1H NMR (d, DMSO-d6): 2.10 (s,
CH3), 11.7 (s, NH). 31P{1H} NMR (d, DMSO-d6): 49.0
1
2
(d, P trans to halide, JP–Pt 3632 Hz, JPP 8.8 Hz), 57.0
1
2
(d, P trans to carbene, JP–Pt 1955 Hz, JPP 8.8 Hz).
3.2. Platinum(II) complexes of L1–L3
13C{1H} NMR (d, DMSO-d6): 8.7 (s, CH3), 163.8 (dd,
2
2
C
carbene, JC–Pcis 7.6 Hz, JC–Ptrans 129.2 Hz).
The coordinating ability of the isocyanide ligand L1–
L3 has been tested in a series of reactions with some
Pt(II) metal complexes as illustrated in Scheme 1.
Complexes 1–3 are obtained in good yield (70–80%)
from the dichloro-platinum derivative cis-[PtCl2(PPh3)2]
by initial treatment with 1 equiv. of AgBF4 in CH2Cl2-
acetone and then, after filtration of AgCl, with 1 equiv.
of the required L1–L3 ligand. Compound 4 was obtained
in ca. 80% yield by a similar procedure starting from
[PtCl2(Ph2PCH@CHPPh2)] and the isocyanide L2. They
are white, air-stable solids, which have been character-
3. Results and discussion
3.1. Synthesis of the ligands
Eq. (1) reports the synthesis of the arsonium-substi-
tuted isocyanide ligands L1–L3. They are obtained in
moderate to high yield (ca. 38–89%) by reaction of the
known o-(chloromethyl)phenyl isocyanide [8a], o-
(CH2Cl)C6H4NBC, with a ca. 20% molar amount of the
arsine AsR3 (AsR3 ¼ AsPh3, AsMePh2, AsMe2Ph) in
the presence of a 3-fold excess of NaI in acetone at room
temperature for 6 h (Eq. (1)). The use of sodium iodide
is necessary since the arsines are not nucleophilic enough
to react directly with o-(CH2Cl)C6H4NBC to give the
corresponding arsonium chloride salts. As previously
reported [8a], NaI initially reacts with the chloromethyl-
isocyanide to afford the more reactive iodomethyl
analog, o-(CH2I)C6H4NBC, which then produces the
observed products.
1
ized by IR, H and 31P{1H} NMR (Section 2). The IR
spectra (nujol mull) show a strong absorption around
2180 cmꢀ1 corresponding to m(NBC), which is shifted to
~
lower wavenumbers of ca. 10 cmꢀ1 with respect to the
parent phosphonium-substituted isocyanide complexes
trans-[PtCl{o-BFꢀ4 R3Pþ–CH2}C6H4NBC)(PPh3)2][BF4]
(PR3 ¼ PPh3, PMePh2, PMe2Ph, PMe3) and of ca. 20
cmꢀ1 compared to the corresponding o-(chlorom-
ethyl)phenyl isocyanide derivatives trans-[PtCl(o-
ClCH2C6H4NBC)(PPh3)2][BF4] [8b]. The observed
decrease in the wavenumber of the NBC moiety was
explained [8b] with the presence of the bulkier o-R3Eþ–
CH2 (E ¼ P, As) group, which somewhat sterically hin-
ders the coordination of the isocyanide. Release of some
steric strain as in compound 4, where two trans-PPh3
ligands are replaced by a less encumbering diphosphine,
ꢀ1
~
shifts the m (NBC) to higher wavenumbers (2197 cm
nujol mull).
,
ð1Þ
~
~
~
The positive values of Dm ¼ m(NBC)coord ) m(NBC)free
of ca. 60–80 cmꢀ1 observed for 1–4 reflect the electro-
philicity of the isocyanide carbon, which is therefore a
potentially reactive center toward nucleophilic attack
[15].
Ligands L1–L3 are crystalline, pale cream, odorless
solids, which are stable in the solid state and in solution.
No decomposition is observed upon exposure to light
over a period of months. They are soluble in DMSO,
slightly soluble in chlorinated solvents and insoluble in
diethyl ether and other common organic solvents. Di-
1
The H NMR spectra of 1–3 show the –CH2– reso-
nance at ca. 3.50 ppm as a singlet, which is shifted to

3388
G. Facchin et al. / Inorganica Chimica Acta 357 (2004) 3385–3389
Scheme 1.
higher fields with respect to the free ligands L1–L3. Such
a shielding effect likely arises from the presence of the
aryl substituents in the metal-coordinated PPh3 ligands,
which are mutually trans and cis to the isocyanide li-
gands. Such effect appears to be less pronounced in
compound 4 (d(CH2) 4.88 ppm).
ing phosphonium-substituted complexes [8b], the pro-
posed mechanism for their formation entails, in the first
step, NEt3 attacks to the activated methylene group of
the –CH2AsRþ3 arsonium moiety to produce the highly
reactive ylide–isocyanide–metal intermediate I*, which
shows the ylidic function in the 1,2-zwitterionic struc-
ture. Subsequently, the ylidic carbanion of I* intramo-
lecularly adds to the adjacent coordinated isocyanide
giving the final 3-(arsonio)indolin-2-ylidene derivatives
5–8. The intermediate I* could not be isolated or even
detected by spectroscopic techniques, probably owing to
its enhanced reactivity toward the highly reactive metal-
coordinated isocyanide.
The 31P{1H} NMR spectra of 1 and 3 show two
singlets, flanked by 195Pt satellites, for the PPh3 ligands
likely due to the presence of a mixture of two complexes
arising from chloride-iodide exchange occurring in their
preparation (Scheme 1). The presence of a singlet reso-
nance for the PPh3 ligands in the 31P NMR spectra
supports the trans geometry of the complexes.
The ‘‘ylide-carbene’’ compounds 5–8 are stable in the
solid state and in solution. They are soluble in DMSO,
slightly soluble in chlorinated solvents and insoluble in
other common organic solvents. They have been char-
acterized by elemental analysis, IR, 1H and 31P{1H}
3.3. Cyclization reactions of platinum(II)-coordinated
L1–L3
Complexes 2–4 are found to react with a 10-fold ex-
cess of NEt3 in CH2Cl2 at room temperature to afford in
good yield the C-2 metal bonded indole derivatives 5–8
(Scheme 2). As previously reported for the correspond-
~
NMR (Section 2). The IR spectra (nujol mull) show m
1
(NH) around 3350 cmꢀ1, while in the H NMR spectra
the N–H resonance falls in the range 8–11 ppm. It is
Scheme 2.

G. Facchin et al. / Inorganica Chimica Acta 357 (2004) 3385–3389
3389
interesting to note that the 31C{1H} NMR spectrum of 8
shows the carbene carbon resonance at 163.8 ppm
(2JCPcis 7.6, JCPtrans 129.2 Hz), a value that is in
agreement with those reported for other platinum(II)
carbene systems [16].
(g) M. Regitz, Angew. Chem. Int. Ed. Engl. 35 (1996) 725.
[4] K. Bartel, W.P. Fehlhammer, Angew. Chem., Int. Ed. Engl. 13
(1974) 599.
2
[5] G. Facchin, P. Uguagliati, R.A. Michelin, Organometallics 3
(1984) 1818.
[6] G. Facchin, R. Campostrini, R.A. Michelin, J. Organomet. Chem.
294 (1985) C21.
[7] R.A. Michelin, G. Facchin, P. Uguagliati, Inorg. Chem. 23 (1984)
961.
Acknowledgements
[8] (a) R.A. Michelin, G. Facchin, D. Braga, P. Sabatino, Organo-
metallics 5 (1986) 2265;
R.A.M. thanks MIUR and the University of Padua
for financial support.
(b) R.A. Michelin, M. Mozzon, G. Facchin, D. Braga, P.
Sabatino, J. Chem. Soc., Dalton Trans. 1803 (1988);
(c) G. Facchin, M. Mozzon, R.A. Michelin, M.T.A. Ribeiro,
A.J.L. Pombeiro, J. Chem. Soc., Dalton Trans. 2827
(1992).
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