The Staudinger reaction of platinum(II)- and palladium(II)-coordinated 2-(azidomethyl)phenyl isocyanide. X-ray structure of trans-[PtCl{CN(H) C6H4-2CH2H(H)}(PPh3) 2][BF4]·CDCl3· H2O

Article abstract of DOI:10.1016/j.jorganchem.2003.10.029

2-(Azidomethyl)phenyl isocyanide, 2-(CH₂N₃) C₆H₄N≡C (AziNC), coordinates to some cationic Pt(II) and Pd(II) species to afford isocyanide complexes of the type trans-[MCl(AziNC)(PPh₃)₂][BF₄] (M=Pt, l; Pd, 2). AziNC is coordinated also in some neutral Pt(II) and Pd(II) species such as [MCl₂(AziNC)₂] (M=Pt, 3; Pd, 4) derived from the reactions of 2≡ of AziNC with [PtCl₂(COD)] and [PdCl₂(MeCN)₂], respectively. Complexes 1 and 2 react with 1≡ of PPh₃ affording the heterocyclic carbene complexes trans-[MCl{CN(H)} (PPh₃)₂][BF₄] (M=Pt, 5; Pd, 6). Complexes 3 and 4 react with 1≡ of PPh₃ displacing the isocyanide with the formation of the complexes cis-[MCl₂ (AziNC)(PPh₃)] (M=Pt, 7; Pd, 8. These latter ones react with 2 equiv. of PPh₃ affording as the final products the cationic carbene species trans-[MCl{CN(H)}(PPh₃)₂][Cl] (M=Pt, 9; Pd, 10). Complex 5 was also characterized by single crystal X-ray diffraction. The carbene complex is square-planar and the angle formed between the platinum square plane and the heterocyclic carbene ligand is 87.9(2)°. The C(1)-N(1) and C(1)-N(2) bond distances in the latter of 1.32(2) and 1.30(2) A?, respectively, are short for a single bond and indicate extensive π-bonding between the nitrogen atoms and the carbene carbon.

Full text of DOI:10.1016/j.jorganchem.2003.10.029

Journal of Organometallic Chemistry 689 (2004) 454–462
www.elsevier.com/locate/jorganchem
The Staudinger reaction of platinum(II)- and
palladium(II)-coordinated 2-(azidomethyl)phenyl isocyanide.
X-ray structure of trans-[PtCl{
(PPh3)2][BF4] ꢀ CDCl3 ꢀ H2O
Marino Basato , Franco Benetollo , Giacomo Facchin , Rino A. Michelin
(H)}
a
b
c
d,
*
,
d
d
d
Mirto Mozzon , Sandra Pugliese , Paolo Sgarbossa ,
d
d
Silvia Mazzega Sbovata , Augusto Tassan
a
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universitꢀa di Padova, Via F. Marzolo 1, 35131 Padua, Italy
ICIS – CNR, Corso Stati Uniti 4, 35127 Padua, Italy
b
c
Istituto di Scienze e Tecnologie Molecolari (ISTM – CNR), c/o Dipartimento di Processi Chimici dellÕIngegneria, Universitꢀa di Padova,
Via F. Marzolo 9, 35131 Padua, Italy
d
Dipartimento di Processi Chimici dellÕIngegneria, Universitꢀa di Padova, Via F. Marzolo 9, 35131 Padua, Italy
Abstract
2-(Azidomethyl)phenyl isocyanide, 2-(CH2N3)C6H4NBC (AziNC), coordinates to some cationic Pt(II) and Pd(II) species to
afford isocyanide complexes of the type trans-[MCl(AziNC)(PPh3)2][BF4] (M ¼ Pt, l; Pd, 2). AziNC is coordinated also in some
neutral Pt(II) and Pd(II) species such as [MCl2(AziNC)2] (M ¼ Pt, 3; Pd, 4) derived from the reactions of 2 equiv. of AziNC with
[PtCl2(COD)] and [PdCl2(MeCN)2], respectively. Complexes 1 and 2 react with 1 equiv. of PPh3 affording the heterocyclic carbene
complexes trans-[MCl{ (H)}(PPh3)2][BF4] (M ¼ Pt, 5; Pd, 6). Complexes 3 and 4 react with 1 equiv. of PPh3
displacing the isocyanide with the formation of the complexes cis-[MCl2(AziNC)(PPh3)] (M ¼ Pt, 7; Pd, 8). These latter ones react
with 2 equiv. of PPh3 affording as the final products the cationic carbene species trans-[MCl{ (H)}(PPh3)2][Cl]
(M ¼ Pt, 9; Pd, 10). Complex 5 was also characterized by single crystal X-ray diffraction. The carbene complex is square-planar and
the angle formed between the platinum square plane and the heterocyclic carbene ligand is 87.9(2)°. The C(1)–N(1) and C(1)–N(2)
ꢁ
bond distances in the latter of 1.32(2) and 1.30(2) A, respectively, are short for a single bond and indicate extensive p-bonding
between the nitrogen atoms and the carbene carbon.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Functionalized isocyanides; Transition metal complexes; N-heterocyclic carbene complexes
1. Introduction
trichloromethyl isocyanide, CCl3–NBC [1,5] in the
presence of amines or thiols, the hydrogen isocyanide,
H–NBC [3] in the presence of oxiranes or aziridines, and
the a-metalated isocyanides Y–ðCꢁÞH–NBC (Y ¼ CO2Et,
The chemistry of transition metal-coordinated func-
tional isocyanides, which are ligands that bear in the
hydrocarbon chain a nucleophile or a suitable precursor
of a nucleophilic center, has received a great interest in
the last years both from synthetic and reactivity points
of view [1–3]. Specific examples are, for instance: (i) the
hydroxyalkyl isocyanides HO–(CH2)n–NBC [4], the
SO2R (R ¼ p-tolyl), PPhþ3 ) [6] reported by Fehlhammer
and coworkers; (ii) the b-functional phenyl isocyanides
such as 2-hydroxyphenyl isocyanide, 2-(HO)C6H4–
NBC, and 2,6-dihydroxyphenyl isocyanide, 2,6-
(HO)2C6H4–NBC, and their derivatives, described by
Harm and coworkers [2] and (iii) c-functional isocya-
* Corresponding author. Tel.: +39-049-827-55-22; fax: +39-049-827-
55-25.
E-mail address: rino.michelin@unipd.it (R.A. Michelin).
nides of the type 2-(CH2Y)C6H4–NBC (Y ¼ Cl, I [1,7];
þ
CpM(CO)3 (M ¼ Cr, W) [8]; PR3 [1,9], OSiMe3, OH
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.10.029

M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
455
[10]; N3 [11]) reported by some of us. The organome-
tallic chemistry of these ligands has been addressed in
many cases [1,2] to the formation of N-heterocyclic
carbenes (NHC) through an intramolecular 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 (Eq. (1)).
Instrument Corporation (CH2C12 solution, KBr or
polyethylene (PE)) spectrophotometers; the wavenum-
1
bers (m) are given in cmꢁ1. H, 31P {1H} and 13C {1H}
~
NMR spectra were run at 298 K, unless otherwise sta-
ted, on a Bruker 200 AC spectrometer operating at
200.13, 81.015 and 50.32 MHz, respectively; d values in
ppm are relative to SiMe4 (1H and 13C) and H3PO4 85%
(31P). The elemental analyses were performed by the
Department of Analytical, Inorganic and Organome-
tallic Chemistry of the University of Padova. The
compounds 2-(azidomethyl)phenyl isocyanide (AziNC),
2-(CH2N3)C6H4NC, cis-[PtCl2(AziNC)2] (3) and cis-
[PdCl2(AziNC)(PPh3)] were synthesized as previously
reported [11a]. The complexes [PdCl2(MeCN)2] [9b],
[PdClðl-Cl)(PPh3)]2 [9b], [MCl2(PPh3)]2 (M ¼ Pt [14],
Pd [15]) and [PtCl2(COD)] (COD ¼ 1,5-cyclooctadiene)
[16] were prepared according to literature procedures.
ð1Þ
It must be noted that metal-NHC complexes [12]
have been receiving increased attention in the literature
since the aminocarbene ligands were found to be
promising alternative ligands to the commonly used
phosphines and phosphites and also for their favorable
application in homogeneous catalysis [12a,12f]. The
NHCs are, in fact, strong r-donor and weak p-acceptor
ligands and they afford a remarkable chemically and
thermally stable metal–carbon bond.
We wish to report herein the synthesis and charac-
terization of a novel type of NHC ligand, i.e. the qui-
nazolin-2-ylidene shown in Chart 1 coordinated to a
metal center, which is the result of the reactivity of the
functional isocyanide 2-(azidomethyl)phenyl isocyanide
(hereafter AziNC) [11] toward PPh3 – the so called
Staudinger reaction [13] – as it will be described in detail
in the following.
2.2. Synthesis of the complexes
2.2.1. Synthesis of trans-[PtCl(AziNC)(PPh3)2][BF4]
(1)
To a suspension of cis-[PtCl2(PPh3)2] (0.65 g, 0.82
mmol) in CH2C12 (50 ml) at room temperature was
added a 1.0 M acetone solution of AgBF4 (0.85 ml, 0.85
mmol). The reaction mixture was stirred for 30 min and
then the solid AgCl formed was filtered off. The solution
was treated with AziNC (0.14 g, 0.88 mmol). The green
solution was stirred for 2.5 h. An IR spectrum showed
the CBN absorption of the coordinated isocyanide at
2201 cmꢁ1. The solution was taken to dryness to give a
green solid. This latter was taken up with CH2C12 (30
ml), filtered through celite and to the resulting solution
Et2O was added to give a pale yellow solid which was
filtered off and dried under vacuum. Yield 0.41 g, 52.6%.
Anal. Calc. for C44H36BC1F4N4P2Pt ꢀ 1/2 CH2C12: C,
51.80; H, 3.61; N, 5.43. Found: C, 52.06; H, 3.69; N,
5.29%. IR (~m, CH2C12): 2201 (s, NBC); k(NBC) 1846 N/
m (from CH2C12 data); 2104 (s, N3); 345 (m, PtCl, PE
2. Experimental
2.1. General procedures and materials
All work was carried out with the exclusion of at-
mospheric oxygen under a dinitrogen atmosphere using
standard Schlenck techniques. Solvents were distilled
under dinitrogen prior to use; CH2Cl2 was distilled from
CaH2 or used as received (Baker analyzed reagent) for
the Staudinger reaction, diethyl ether and THF were
distilled from sodium benzophenone. IR spectra were
taken on an FT-IR AVATAR 320 (4000–400 cmꢁ1) or
an FT-IR Nexus (range 600–50 cmꢁ1) of the Nicolet
1
film). H NMR (d, CDC13): 7.74–7.45 (C6H4, m); 3.39
(CH2, s). 31P {1H} NMR (d, CDCl3): 18.80 (PPh3,
1JPt–P ¼ 2183 Hz, s).
2.2.2. Synthesis of trans-[PdCl(AziNC)(PPh3)2][BF4]
(2)
To a suspension of [PdCl2(PPh3)2] (0.42 g, 0.60
mmol) in CH2C12 (50 ml) at room temperature was
added a 1.0 M acetone solution of AgBF4 (0.65 ml, 0.65
mmol). The reaction mixture was stirred for 30 min and
then the solid AgCl formed was filtered off. The solution
was thermostated at )10 °C using a CHCl3/liquid N2
bath and then was treated with AziNC (0.10 g, 0.634
mmol). The green solution was stirred for 35 min. An IR
spectrum showed the disappearance of the CBN ab-
sorption of the free isocynaide and the simultaneous
formation of a band at 2204 cmꢁ1. The solution was
Chart 1.

456
M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
treated with Et2O (2 ꢂ 20 ml).The white solid was fil-
tered off and dried under vaccum. Yield 0.46 g, 94.3%.
Anal. Calc. for C44H36BClF4N4P2Pd ꢀ CH2Cl2: C, 54.25;
H, 3.84; N, 5.62. Found: C, 54.57; H, 3.63; N, 5.52%. IR
room temperature for 12 h and then was treated with
Et2O (40 ml). The yellow solid was filtered off, washed
with Et2O (2 ꢂ 10 ml) and dried under vacuum. Yield
0.26 g, 87.9%. Anal. Calc. for C44H38BClF4N2P2Pd: C,
59.70; H, 4.33; N, 3.16. Found: C, 59.45; H, 4.32; N,
3.08%. IR (~m, CH2Cl2): 3417, 3311 (s, NH). 1H NMR (d,
CDC13): 8.73 (NH, s); 7.82–6.38 (C6H4, m); 3.38 (CH2,
s). 31P {1H} NMR (d, CDCl3): 21.21 (PPh3, s).
~
(m, CH2Cl2): 2204 (s, NBC); k(NBC) 1851 N/m (from
CH2Cl2 data); 2105 (s, N3); 350 (m, PdCl, PE film). H
1
NMR (d, CDCl3): 7.79–7.47 (C6H4, m); 3.38 (CH2, s).
31P {1H} NMR (6, CDCl3): 24.07 (PPh3, s).
2.2.3. Synthesis of cis-[PdCl2(AziNC)2] (4)
2.3.3. Reaction of 3 with PPh3. Synthesis of cis-
[PtCl2(AziNC)(PPh3)] (7)
Complex cis-[PtCl2(AziNC)2] (3) was prepared as
previously reported [11a]. Complex 4 was prepared ac-
cording to the following procedure. To a suspension of
[PdCl2(MeCN)2] (0.88 g, 3.40 mmol) in Et2O (70 ml)
was added AziNC (1.13 g, 7.15 mmol) and the reaction
mixture was stirred for 2h. Then the solid was filtered
off and washed with Et2O (3 ꢂ 5 ml) and dried under
vacuum. Yield 1.41 g, 80.0%. Anal. Calc. for
C16H12Cl2N8Pd: C, 38.90; H, 2.43; N, 22.70. Found: C,
To a solution of 3 (0.40 g, 0.69 mmol) in CH2Cl2 (25
ml) was added PPh3 (0.19 g, 0.72 mmol). The solution
was stirred at room temperature for 1 h and then
treated with Et2O (20 ml) and n-hexane (20 ml). The
yellow solid that formed was filtered off, washed with
Et2O (2 ꢂ 20 ml) and dried under vacuum. Yield 0.46 g,
97.0%. Anal. Calc. for C26H21Cl2N4PPt: C, 45.50; H,
3.08; N, 8.16. Found: C, 45.88; H, 3.35; N, 8.51%. IR
~
38.49; H, 2.23; N, 22.18%. IR (m, CH2Cl2): 2230, 2213 (s,
~
(m, CH2Cl2): 2199 (s, NBC); k(NBC) 1842 N/m (from
CH2C12 data); 2104 (s, N3); 300, 350 (m, PtCl, PE
NBC); k(NBC) 1895, 1866 N/m (from CH2Cl2 data);
1
2105 (s, N3); 350, 339 (m, PdCl, PE film). H NMR (d,
1
film). H NMR (d, CDC13): 7.81–6.58 (C6H4, m); 4.37
CDC13): 7.67–7.44 (C6H4, m); 4.67 (CH2, s). 13C {1H}
(CH2, s). 31P {1H} NMR (d, CDC13): 9.16 (PPh3, 1JPt–P
3351, s).
NMR (d, CDCl3): 132.8–126.5 (C6H4, m); 51.4 (CH2, s).
2.3. Reactivity of the AziNC complexes with PPh3
2.3.4. Reaction of 4 with PPh3. Synthesis of cis-
[PdCl2(AziNC)(PPh3)] (8)
2.3.1. Reaction of 1 with PPh3. Synthesis of trans-
To a solution of 4 (0.40 g, 0.81 mmol) in CH2Cl2
(25 ml) was added PPh3 (0.22 g, 0.84 mmol). The so-
lution was stirred at room temperature for 12 h and
then treated with Et2O (20 ml) and n-hexane (20 ml).
The yellow solid that formed was filtered off, washed
with Et2O (3 ꢂ 10 ml) and dried under vacuum. Yield
0.46 g, 95.0%. Anal. Calc. for C26H21Cl2N4PPd: C,
52.24; H, 3.54; N, 9.73. Found: C, 51.80; H, 3.00; N,
[PtCl{
(H)}(PPh3)2][BF4] (5)
To a solution of 1 (0.30 g, 0.30 mmol) in CH2Cl2 (30
ml) was added PPh3 (0.09 g, 0.34 mmol). The pale
green solution turns immediately to yellow. An IR
spectrum showed the disappearance of the CBN and
the N3 absorptions at 2201 and 2104 cmꢁ1, respec-
tively, of the coordinated isocyanide. The solution
was stirred at room temperature for 48 h and then was
treated with Et2O (20 ml). The pale green solid was
filtered off, washed with Et2O (3 ꢂ 30 ml) and dried
under vacuum. Yield 0.20 g, 66.7%. Anal. Calc. for
C44H38BClF4 N2P2Pt ꢀ 1/2Et2O: C, 55.23; H, 3.83; N,
~
9.57%. IR (m, CH2Cl2): 2206 (s, NBC); k(NBC) 1854
N/m (from CH2Cl2 data); 2104 (s, N3); 339, 297 (m,
1
PdCl, PE film). H NMR (d, CDCl3): 7.83–6.64 (C6H4,
m); 4.40 (CH2, s). 31P {1H} NMR (d, CDCl3): 28.4
(PPh3, s). The spectroscopic data of complex 8 are
identical to that previously synthesized by a different
route [11].
~
2.80. Found: C, 55.25; H, 3.83; N, 2.48%. IR (m,
1
CH2C12): 3429, 3314 (s, NH). H NMR (d, CDC13):
8.45 (NH, s); 7.86–6.38 (C6H4, m); 3.37 (CH2, s). 31P
{1H} NMR (d, CDC13): 19.18 (PPh3, 1JPt–P ¼ 2615 Hz,
s). ESI-MS: m=z 887.2 ([M-BF4]þ; 851.1 ([887.2-Cl]þ;
589.2 ([851.1-PPh3]þ.
2.3.5. Reaction of 7 with PPh3. Synthesis of trans-
[PtCl{
(H)}(PPh3)2][Cl] (9)
To a yellow solution of 7 (0.29 g, 0.42 mmol) in
CH2Cl2 (25 ml) was added PPh3 (0.12 g, 0.47 mmol). An
IR spectrum showed the reduction of the CBN ab-
sorption at 2199 cmꢁ1 and of the N3 absorption at 2104
cmꢁ1 of the isocyanide, which were still present after
stirring at room temperature for 24 h. The addition of a
further equiv. of PPh3 (0.12 g, 0.47 mmol) and stirring
for an additional 24 h afforded the complete disap-
pearance of the CBN and N3 bands of the isocyanide.
2.3.2. Reaction of 2 with PPh3. Synthesis of trans-
[PdCl{
(H)}(PPh3)2][BF4] (6)
To a solution of 2 (0.30 g, 0.33 mmol) in CH2Cl2 (30
ml) was added solid PPh3 (0.10 g, 0.38 mmol). The
yellow solution turned immediately to orange. An IR
spectrum showed the disappearance of the CBN and the
N3 absorptions at 2204 cmꢁ1 and at 2104 cmꢁ1, re-
spectively, of the isocyanide. The solution was stirred at

M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
457
Table 1
Crystallographic data of compound 5
The solution was then treated with Et2O (30 ml) to give
a cream solid, which was filtered off, washed with Et2O
(3 ꢂ 10 ml) and dried under vacuum. Yield 0.20 g,
52.4%. Anal. Calc. for C44H38Cl2N2P2Pt ꢀ 1/2CH2Cl2: C,
55.38; H, 4.07; N, 2.90. Found: C, 55.60; H, 3.71; N,
3.36%. IR (~m, cmꢁ1): 3420 (w, NH, KBr), 309 (w, PtCl,
PE film). 1H NMR (d, CDCl3): 9.71, 9.13 (NH, s); 7.96–
6.37 (C6H4, m); 3.31 (CH2, s). 31P {1H} NMR (d,
Compound
5 ꢀ CDCl3 ꢀ H2O
C45H41BCl4F4N2OP2Pt
Chemical formula
Formula weight
Crystal system
Space group
1111.48
Monoclinic
P21/c
ꢁ
a/A
12.596(3)
10.931(3)
33.395(5)
93.39(3)
4590(2)
4
ꢁ
b/A
ꢁ
1
c/A
CDCl3): 19.17 (PPh3, JPt–P 2692, s).
b/°
V/A
3
ꢁ
Z
D
2.3.6. Reaction of 8 with PPh3. Synthesis of trans-
calc/g cmꢁ3
1.608
2200
[PdCl{
(H)}(PPh3)2][Cl] (10)
F ð000Þ
l(Mo–Ka)/mmꢁ1
Reflections collected
Reflections observed ½I P 2rðIÞꢃ
Final R1; wR2
3.41
8626
To a yellow solution of 8 (0.20 g, 0.33 mmol) in
CH2Cl2 (15 ml) was added PPh3 (0.19 g, 0.74 mmol).
After 30 min, an IR spectrum showed the reduction of
the CBN absorption at 2205 cmꢁ1 and of the N3 ab-
sorption at 2104 cmꢁ1 of the isocyanide. The reaction
mixture was stirred at room temperature for 24 h. After
this time an IR spectrum showed the complete disap-
pearance of the CBN and N3 bands of the coordinated
isocyanide. The solution was then treated with Et2O (30
ml) to give a cream solid, which was filtered off, washed
with Et2O (3 ꢂ 10 ml) and dried under vacuum. Yield
0.19 g, 71.0%. Anal. Calc. for C44H38Cl2N2P2]Pd ꢀ 1/
2CH2Cl2: C, 60.98; H, 4.48; N, 3.20. Found: C, 62.20;
7958
0.059, 0.134
2
P
P
P
P
2 1=2
R1 ¼ jjF0jꢁjFcjj= jF0j; wR2 ¼ ½ ½wðF02 ꢁFc2Þ ꢃ= wðF02Þ ꢃ
.
Table 2
Selected bond distances (A) and angles (°) for compound 5
ꢁ
Pt–Cl(1)
Pt–P(2)
2.359(3)
2.307(3)
1.820(9)
1.816(9)
1.819(9)
1.32(2)
Pt–P(1)
Pt–C(1)
P1–C(9)
2.320(3)
1.982(9)
1.803(8)
1.812(9)
1.830(9)
1.30(2)
P1–C(15)
P1–C(21)
P2–C(27)
P2–C(39)
N2–C(1)
H, 4.14; N, 2.84%. IR (m, cmꢁ1): 3457 (w, NH, KBr),
P2–C(33)
N1–C(1)
N1–C(2)
~
1
1525 (s, C@N, KBr), 318 (w, PdCl, PE film). H NMR
1.57(2)
89.5(1)
N2–C(4)
1.56(2)
86.7(1)
(d, CDCl3): not safely assigned (NH, s); 7.92–6.43
(C6H4, m); 3.40 (CH2, s). 31P {1H} NMR (d, CDCl3):
21.2 (PPh3,s).
Cl(1)–Pt–P(1)
Cl(1)–Pt–C(1)
P(1)–Pt–C(37)
Pt–C(1)–N(1)
N(1)–C(1)–N(2)
Cl(1)–Pt–P(2)
P(1)–Pt–P(2)
P(2)–Pt–C(1)
Pt–C(1)–N(2)
177.9(4)
91.7(3)
176.2(1)
92.1(3)
122.1(9)
120.7(8)
117.1(9)
2.4. X-ray measurements and structure determination of 5
Single crystals suitable for X-ray analysis were ob-
tained by slow precipitation from a CDCl3, solution in a
NMR tube. The intensities data of 5 ꢀ CDCl3 ꢀ H2O were
collected at room temperature using a Philips PW1100
single-crystal diffractometer (FEBO system) using a
graphite-monochromated (Mo–Ka) radiation, following
the standard procedures. There were no significant
fluctuations of intensities other than those expected
from Poisson statistics. All intensities were corrected for
Lorentz polarization and absorption [17]. The structure
was solved by heavy atom method [18]. Refinement was
carried out by full-matrix least-squares procedures
(based on F02) using anisotropic temperature factors for
all non-hydrogen atoms. The H-atoms were placed in
calculated positions with fixed, isotropic thermal pa-
rameters (1:2Uequiv of the parent carbon atom). The
calculations were performed with the SHELXL-97
program [19], implemented in the WinGX package [20],
drawings were produced using ORTEP3 [21]. Crystal-
lographic and experimental details of the structure are
summarized in Table 1, while Table 2 reports selected
bond lengths and angles.
3. Results and discussion
3.1. Synthesis of Pt(II)- and Pd(II)-AziNC complexes
The reactions sequence leading to the synthesis of
some cationic Pt(II)- and Pd(II)-AziNC complexes of
the type trans-[MCl(AziNC)(PPh3)2][BF4] (M ¼ Pt, 1;
Pd, 2) is reported in Eq. (2).
ð2Þ

458
M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
Complexes 1 and 2 are obtained in high yield from
the dichloro metal derivatives [MCl2(PPh3)2] by initial
treatment with 1 equiv. of AgBF4 in CH2Cl2–acetone
and then, after filtration of AgCl, with 1 equiv. of
AziNC. They are white air-stable solids which have
3.2. Reactivity of cationic AziNC complexes of the type
trans-[MCl(AziNC)(PPh3)2][BF4]
The reactions of the cationic complexes trans-
[MCl(AziNC)(PPh3)2][BF4] (M ¼ Pt, 1; Pd, 2) with 1
equiv. of PPh3 in a slight molar excess (ꢄ10%) were
carried out in CH2Cl2 at room temperature (Eq. (4)).
1
been characterized by elemental analysis, IR, H and
31P{1H} NMR (Section 2). The IR spectra (CH2Cl2
solution) show two strong absorptions around 2205
and 2105 cmꢁ1 corresponding to m(NBC) and m(N3),
~
~
~
respectively. The high and positive values of Dm ¼
m(NBC)coord ꢁ m(NBC)free of ꢄ80 cmꢁ1 observed for 1
~
and 2 reflect the electrophilicity of the isocyanide
~
carbon, which is therefore
a potentially reactive
center toward nucleophilic attack [22]. The IR spec-
tra (polyethylene film) of 1 and 2 show also the M–
Cl stretching at ꢄ350 cmꢁ1 as a medium intensity
band.
1
The H NMR spectra of 1 and 2 show the –CH2–
resonance at ꢄ3.40 ppm as a singlet. In the 31P{1H}
NMR spectrum of 1 the PPh3 ligands give rise to a
singlet at 18.8 ppm flanked by 195Pt satellites (1JPPt 2183
Hz), while the resonance of the phosphine ligands of 2 is
observed at 24.0 ppm. The presence of a singlet in the
31P{1H} NMR spectra supports the trans geometry of
both complexes.
Coordination of AziNC to neutral Pt(II) and Pd(II)
species has been also examined. While the synthesis of
cis-[PtCl2(AziNC)2] (3) was recently reported, the anal-
ogous Pd-complex cis-[PdCl2(AziNC)2] (4) was prepared
as illustrated in Eq. (3) starting from [PdCl2(MeCN)2]
and 2 equiv. of AziNC.
ð4Þ
They have been followed by IR spectroscopy by
monitoring the disappearance of the bands at ꢄ2200
and 2100 cmꢁ1 due to the coordinated isocyanide and
the azido groups. After 48 h (M ¼ Pt) and 18 h (M ¼ Pd)
such absorptions were no more detected and the com-
pounds 5 and 6 could be isolated as crystalline solids in
high yield. They were characterized by elemental anal-
ysis, ESI-MS, IR, H and 31P{1H} NMR (Section 2).
1
Complex 5 was also characterized by an X-ray diffrac-
1
tion analysis (see further). The H NMR spectra for 5
and 6 show the –CH2– resonance at 3.35 and 3.38 ppm
and a signal at 8.36 and 8.73 ppm due to one N–H group
of the heterocyclic carbene, respectively. In the 31P{1H}
NMR spectra of 5 and 6 the resonance of the phosphine
ligands is a singlet (flanked by 195Pt satellites in the case
of 5) confirming the trans geometry of the complexes.
A possible mechanism for the formation of the het-
erocyclic carbene ligands in 5 and 6 is described in
Scheme 1.
The initial step entails the formation of the isocya-
nide-phosphinimine intermediate I by attack of PPh3 to
the N3 group and liberation of N2 (Staudinger reaction),
followed by hydrolysis of the phosphinimine leading to
the formation of the isocyanide-amine species II. This
latter then undergoes intramolecular cycloaddition
yielding the final heterocyclic carbene product.
An alternative mechanism (Scheme 2) involves initial
intramolecular attack of the phosphinimine nitrogen of
intermediate I (which is now shown in the ylidic pre-
sentation in order to highlight the nucleophilicity of the
nitrogen atom) to the isocyanide carbon to give III,
which is then hydrolyzed to the final products.
ð3Þ
Complex 4 has been characterized by elemental analysis,
IR, 1H and 13C{1H} NMR (Section 2). In particular, the
IR spectra (CH2Cl2 solution) show two strong absorp-
tions at 2230 and 2213 cmꢁ1 due to the isocyanide
stretching and another strong band at 2105 cmꢁ1 cor-
~
~
responding m(N3). For this complex the observed Dm is
ꢄ90–110 cmꢁ1, thus confirming the high electrophilicity
of the isocyanide carbon atom.

M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
459
expected to be very reactive toward intramolecular
cyclization [1,10] and, as a consequence, not easily
detectable.
3.3. Description of the structure of trans-
(H)}(PPh3)2]þ [BF4]ꢁ (5)
[PtCl{
The asymmetric unit is composed of trans-
[PtCl{ cations
(H)}(PPh3)2]þ
(Fig. 1), [BF4]ꢁ anions and solvent molecules (CDCl3,
H2O) of crystallization.
The carbene complex shows the usual square-planar
geometry around platinum(II) with maximum deviation
ꢁ
from the C(1), Cl(1), P(1) and P(2) plane of )0.015 A.
However, the P(1)–Pt–P(2) (176.2(1)°) and the Cl(1)–
Pt–C(1) (177.9(3)°) angles deviate from linearity thus
indicating the presence of a slight distortion of the Pt
atom stereogeometry likely arising from the steric
Scheme 1.
In order to shed light on the reaction mechanisms
reported in Schemes 1 and 2, we followed the reaction of
complex 1 with 1 equiv. of PPh3 by monitoring the
31P{1H} NMR spectra of the reaction mixture in
CDC13 (previously treated with dry K2CO3) at room
and at low (253 K) temperature. In both experiments,
the spectra show the presence of signals due to the
1
starting complex 1 (18.8 ppm, JPPt 2183 Hz), the final
1
product 5 (19.1 ppm, JPPt 2615 Hz), free PPh3 ()5.0)
and phosphine oxide, Ph3P@O (29.5 ppm). Other signals
of low intensity were observed at 14.1 and 10.4 ppm.
The former can be attributed to the phosphinimine in-
termediate I (Scheme 1) by comparison with the values
reported for other isolated transition metal phosphini-
mine derivatives of the type [M(CO)5{CBNC6H4-2-
(CH2N@PPh3)}] (M ¼ W, Cr), which appear in the
range 14.6–14.9 ppm [11b]; however, the latter reso-
nance could not be safely assigned, although it indicates
the presence of another P-containing intermediate
(perhaps the cyclic phosphinimino complex III of
Scheme 2). Intramolecular attack of phosphinimines on
coordinated carbonyl [23] and isocyanide ligands [2]
have been already documented. There is no evidence of
the presence of the intermediate II, which however is
Fig. 1. ORTEP drawing of trans-[PtCl{
(PPh3)2]þ cation (5).
(H)}
Scheme 2.

460
M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
interaction between the carbene ligand and the two
bulky ancillary PPh3 ligands. The Pt–C(l) bond length of
equiv. of PPh3 to 7 and 8 in CH2Cl2 leads to the
slow formation of the cationic carbene species
ꢁ
1.982(9) A is in good agreement with those reported for
other square-planar carbene Pt(II) complexes [9b,15,24,
ꢁ
25], which generally occur in the range 1.82–2.01 A when
an halide is trans to the carbene ligand. Similarly, the
ꢁ
Pt–P bond distances of 2.320(3) and 2.307(3) A and the
ꢁ
Pt–Cl bond length of 2.359(3) A are within the expected
[MCl{
(H)}(PPh3)2]þ[Cl]ꢁ (M ¼ Pt,
9; Pd, 10) in fair to good yield. Their structure was es-
tablished by comparison of the IR, 1H and 31P{1H}
NMR data with those of the parent complexes 5 and 6
having BFꢁ4 as the counter ion.
values for these interactions in complexes of similar
structure [9b,15]. The heterocyclic carbene ligand is
nearly perpendicular to the co-ordination plane as evi-
denced by the value of the dihedral angle of 87.9(2)°.
The values of C(1)–N(1) and C(1)–N(2) bond lengths
4. Concluding remarks
In this study we have described the Staudinger reaction
of some Pt(II)- and Pd(II)-coordinated 2-(azidom-
ethyl)phenyl isocyanide (AziNC) complexes leading to a
new type of N-heterocyclic carbene i.e. the quinazolin-
2-ylidene ligand. The formation of this species has been
suggested to occur as a consequence of an initial nucleo-
philic attack of PPh3 to the azido group of the isocyanide
to give a phosphinimine intermediate, followed by its re-
arrangement to the NHC ligand as proposed in Schemes 1
and 2. This type of reactivity appears to be influenced by
the steric hindrance around the azido group of the coor-
dinated AziNC. In fact, it is observed that when AziNC is
bound to metal fragments ofthe type trans-{MCl(PPh3)2}
having two sterically hindered ancillary ligands, PPh3
attacks selectively the N3 moiety. On the other hand,
with less encumbered metal species such as those repre-
sented by the complexes cis-[MCl2(AziNC)2] or cis-[MCl2
(AziNC)(PPh3)] (M ¼ Pt, Pd), the phosphane ligand
preferentially adds to the electrophilic metal center.
In conclusion, taking also into account previous re-
sults [11b], the Staudinger reaction of transition metal-
coordinated AziNC is observed to proceed only when
one of the following requirements is fulfilled i.e. the
functional isocyanide is linked to coordinatively satu-
ꢁ
of 1.32(2) and 1.30(2) A, respectively, are short and
comparable to those found for the imidazolidin-2-
ylidene ligand in the platinum(II) complex cis-
[PtBr2{
(H)}(PPh3)] (1.34(1),
ꢁ
1.36(1) A) [24] and to the values quoted for other Pt(II)-
aminocarbene derivatives (Ccarbene–N bond distances
ꢁ
range 1.30–1.37 A) [25] and suggest extensive p-bonding
between the nitrogen atoms and the carbene carbon.
3.3.1. Reactivity of neutral AziNC complexes of the type
cis-[MCl2(AziNC)2]
The reactions of the complexes cis-[MCl2(AziNC)2]
(M ¼ Pt, 3; Pd, 4) with PPh3 are reported in Scheme 3.
When the di-isocyanide derivatives 3 and 4 are trea-
ted in CH2Cl2 at room temperature with 1 equiv. of
PPh3, the reactions lead to the selective displacement of
one coordinated isocyanide ligand with the quantitative
formation of the corresponding substitution complexes
cis-[MCl2(AziNC)(PPh3)] (M ¼ Pt, 7; Pd, 8). Complexes
7 and 8 were characterized by elemental analysis and
1
their IR, H and 31P NMR spectra (see Section 2). On
the other hand, addition at room temperature of 2
Scheme 3.

M. Basato et al. / Journal of Organometallic Chemistry 689 (2004) 454–462
461
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{M(CO)5} (M ¼ Cr, W) fragments or is bound to steri-
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other cases tested, the phosphane ligand attacks
the metal center rather than the azido group.
All this experimental evidence leads to a final con-
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5. Supplementary data
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Crystallographic data for the structure reported here
have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publications No.
CCDC 214527. Copies of the available material can be
obtained, free of charge from CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
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Acknowledgements
R.A.M. thanks MURST, the University of Padua
and C.N.R. for financial support.
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