Homoleptic nickel(0) phenyl isocyanide complexes

Article abstract of DOI:10.1515/znb-2004-0814

The tetrakis(phenyl isocyanide)nickel(0) complexes [Ni(CNR)₄](R = C₆H₅,1; R = C₆H₃-2,6-Me ₂, 2; R = C₆H₄-2-NO₂, 3) have been synthesized from [Ni(COD)₂

Full text of DOI:10.1515/znb-2004-0814

Homoleptic Nickel(0) Phenyl Isocyanide Complexes
F. Ekkehardt Hahna, Marco Mu¨ndera, and Roland Fro¨hlichb
a Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Corrensstraße 36, D-48149 Mu¨nster, Germany
b Organisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Corrensstraße 40, D-48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. F. E. Hahn. E-mail: fehahn@uni-muenster.de
Z. Naturforsch. 59b, 850 – 854 (2004); received April 30, 2004
The tetrakis(phenyl isocyanide)nickel(0) complexes [Ni(CNR)4](R = C6H5, 1; R = C6H3-2,6-Me2,
2; R = C6H4-2-NO2, 3) have been synthesized from [Ni(COD)2] and the corresponding phenyl iso-
cyanides in toluene. Complexes 1 – 3 were characterized by X-ray diffraction. All three complexes
contain a nickel atom which is coordinated in a slightly distorted tetrahedral fashion.
Key words: Phenyl Isocyanide Complexes, Nickel(0), Crystal Structures
Introduction
Carbene complexes with benzannulated N-hetero-
cyclic carbene ligands can be obtained from 2-
functionalized phenyl isocyanides. 2-(Trimethylsil-
oxy)phenyl isocyanide, for example, reacts after cleav-
age of the Si-O bond under formation of the NH,O-
stabilized carbene complex A [1], which can be N-
alkylated giving B (Scheme 1) [2]. This cyclization
reaction can also be used to generate complexes C
with NH,NH-stabilized carbene ligands [3]. Dialky-
lation of complexes of type C leads to complexes D
with NR,NR-stabilized benzannulated carbene ligands
(Scheme 1) [3]. Such carbene ligands are stable in
the free state and can also be generated in a metal
Scheme 2. Preparation of tetrakis(phenyl isocyanide)-
nickel(0) complexes.
template free reaction sequence from benzimidazol-2-
thiones [4].
We tried to prepare nickel(II) carbene complexes
by cyclization of coordinated 2-functionalized phenyl
isocyanides. However, nickel(II) is a catalyst for the
polymerization of isocyanides [5] and the prepara-
tion of nickel(II) isocyanide complexes is hampered
by this polymerization. We have therefore studied
the reaction of phenyl isocyanides with nickel(0) to
obtain homoleptic nickel(0) phenyl isocyanide com-
plexes. It was intended to oxidize these complexes
later to the nickel(II) derivatives, which should then
undergo the cyclization reaction to give the carbene
complexes. In this contribution we describe the reac-
tion of [Ni(COD)2] with three differently substituted
phenyl isocyanides and the molecular structures of
three homoleptic tetrakis(phenyl isocyanide)nickel(0)
complexes 1 – 3 (Scheme 2).
Scheme 1. Cyclization of 2-functionalized phenyl iso-
cyanides.
c
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F. E. Hahn et al. · Homoleptic Nickel(0) Phenyl Isocyanide Complexes
851
Fig. 1. Molecular structures of 1 and 3 (hydrogen atoms omit-
ted for clarity). Both molecules reside on special positions
∗
(site symmetry 4, and ∗∗ denotes symmetry equivalent po-
¯
˚
sitions) in the unit cell. Selected bond lengths [A] and an-
gles [◦] for 1 [3]: Ni-C1 1.828(3) [1.835(3)], C1-N1 1.162(4)
[1.167(3)], N1-C2 1.388(5) [1.388(3)]; C1-Ni-C1∗ 111.4(1) Fig. 2. Molecular structure of 2 (hydrogen atoms omitted
[114.40(15)], C1-Ni-C1∗∗ 106.6(2) [107.07(7)], Ni-C1-N1 for clarity). The molecule resides on a general position
˚
176.9(3) [174.3(2)], C1-N1-C2 168.4(4) [168.9(3)].
in the unit cell. Selected bond lengths [A] and angles
[◦]: Ni-C11 1.822(4), Ni-C21 1.839(3), Ni-C31 1.839(4),
Ni-C41 1.841(3), C11-N11 1.177(4), N11-C12 1.389(4),
C21-N21 1.161(4), N21-C22 1.388(4), C31-N31 1.173(4),
N31-C32 1.387(4), C41-N41 1.168(4), N41-C42 1.193(4);
C11-Ni-C21 109.45(14), C11-Ni-C31 101.69(14), C11-
Ni-C41 111.60(15), C21-Ni-C31 112.28(14), C21-Ni-C41
110.13(14), C31-Ni-C41 111.43(14), Ni-C11-N11 172.7(3),
C11-N11-C12 161.5(3), Ni-C21-N21 179.3(3), C21-
Results and Discussion
Homoleptic tetraisocyanide complexes of nickel(0)
complexes have been described as early as 1950
[6, 7]. Tetrakis(phenyl isocyanide)nickel(0) complexes
were prepared by substitution of the CO ligands
in [Ni(CO)4]. Nicolocene is reported to react with N21-C22 175.0(3), Ni-C31-N31 172.9(3), C31-N31-C32
169.9(3), Ni-C41-N41 175.9(3), C41-N41-C42 173.4(3).
phenyl isocyanide to give [Ni(CN-C6H5)4] [8] and
tetrakis(isocyanide)nickel(0) complexes were also ob-
isocyanide or 2-nitrophenyl isocyanide to give the
tained by disproportionation of K4[Ni2(CN)6] in the
tetrakis(phenyl isocyanide)nickel(0) complexes 1 – 3
in good (around 80%) yield (Scheme 2). All three com-
plexes are stable in air. Complexes 1 and 2 are yellow
presence of alkyl and aryl isocyanides [9]. Most
known tetrakis(aryl isocyanide)nickel(0) complexes
were characterized only by microanalytical data. The
solids which are soluble in almost all organic solvents
13C NMR spectrum of [Ni(CN-C6H5)4] has bee re-
with the exception of aliphatic hydrocarbons. Complex
ported [10]. IR spectra of some complexes have been
3 is a dark purple solid which is less soluble in chlori-
recorded showing one C≡N stretching mode slightly
nated organic solvents but is freely soluble in THF.
above 2000 cm−1, which is indicative of the expected
1H and 13C NMR spectra were recorded for 1 – 3.
tetrahedral coordination geometry [9, 11]. It has been
The chemical shifts for the isocyanide carbon atoms
noted that sometimes more than one C≡N stretching
fall in the narrow range of δ = 169.0 − 176.1 ppm
vibration was observed in the IR spectra of complexes
taking into account that the spectra were recorded in
[Ni(CNR)4] [9, 12]. It was proposed that M→C back-
three different solvents. The chemical shift for the iso-
cyanide carbon atom in 1 has been reported (15.1 MHz
in CHCl3) at δ = 165.6 ppm [10]. The substitution
pattern of the isocyanide ligand appears to have no in-
fluence on the chemical shift of the isocyanide carbon
atom.
bonding causes loss of linearity of the coordinated iso-
cyanide ligand and thus leads to a coordination ge-
ometry less symmetrical than Td for these complexes.
However, to date no structural data for any homolep-
tic tetrakis(isocyanide)nickel(0)complex have been re-
ported.
In our preparation of homoleptic nickel(0) iso-
IR spectra of 1 – 3 were measured in KBr. Two
cyanide complexes we have substituted the toxic C≡N absorptions (one vs around 2030 cm−1 and
[Ni(CO)4] or the other reported NiII starting materials one s around 2000 cm−1) were detected for each
by stable and easy to handle [Ni(COD)2]. [Ni(COD2] complex confirming earlier observations [9, 12] that
reacts with phenyl isocyanide, 2,6-dimethylphenyl the complexes possess less than ideal Td symme-
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F. E. Hahn et al. · Homoleptic Nickel(0) Phenyl Isocyanide Complexes
Table 1. Crystal and data col-
1
2
3
lection details for 1 – 3.
Crystal habit
Crystal size [mm]
Formula
yellow needle
0.50×0.07×0.7
yellow cube
0.20×0.15×0.10
C36H36N4Ni
583.40
11.537(1)
11.803(1)
12.549(1)
106.44(1)
106.70(1)
98.65(1)
black needle
0.15×0.05×0.05
C28H16N8NiO8
651.20
9.671(1)
9.671(1)
14.559(1)
90
C28H20N4Ni
f w [amu]
471.19
12.575(2)
12.575(2)
7.266(1)
90
˚
a [A]
˚
b [A]
˚
c [A]
α [deg]
β [deg]
90
90
90
90
γ [deg]
3
˚
V [A ]
1149.0(3)
1524.5(2)
1361.7(2)
P42/n (no. 86)
2
¯
¯
space group
P4 (no. 82)
P1 (no. 2)
Z
2
2
µ [mm−1
]
0.867
0.667
0.781
Abs. corr.
empirical
0.671 ≤ T ≤ 0.942
4.6 to 55.1
1316
empirical
0.878 ≤ T ≤ 0.936
3.6 to 52.5
6133
empirical
0.892 ≤ T ≤ 0.962
5.1 to 55.0
1566
2θ Range [deg]
Unique data
Obsvd. data (I > 2σ(I))
R(observed)[%]
R(all), [%]
1071
4308
1115
5.13, Rw = 9.09
6.90, Rw = 9.75
1.024
5.47, Rw = 10.17
9.51, Rw = 12.41
1.020
4.76, Rw = 11.12
7.57, Rw = 12.30
1.036
GOF
BASF
0.5
–
–
No. of variables
Res. el. dens. [e/A ]
75
378
102
3
˚
0.607/−0.356
0.354/−0.459
0.513/−0.635
try. Free 2-functionalized phenyl isocyanides exhibit from 161.5(3)◦ to 175.0(3)◦ are found for isocyanides
a C≡N stretching frequency around 2120 cm−1 [13]. coordinated to the same metal center. We therefore as-
The lower frequency absorptions observed in com- sume, that the non-linearity of the C≡N-C moieties in
plexes 1 – 3 indicate significant d → π ∗ backbond- 1 – 3 is not only determined by electronic effects but
ing to the isocyanide ligands as would be expected also by the packing of the molecules in the crystal.
for electron-rich nickel(0) centers. No significant dif-
ferences for the C≡N absorptions in 1 – 3 depend-
ing on the substituents of the isocyanide ligand were
observed. It should be noted that C≡N stretching
frequencies for 1 – 3 are observed around the value
found for [Ni(CO)4], and are much lower than ob-
served for the less electron-rich complexes of type
[Ni(X2)(CNR)2] [14].
The Ni-C bond lengths in 1 – 3 fall in the narrow
range of 1.822(4) to 1.841(3) A. These values are very
˚
similar compared to those of the only other homolep-
tic, albeit nickel(II) tetraisocyanide complex [Ni(CN-
C6H3-2,6-CH3)4]2+ [15]. As we have noticed with
alkyl isocyanides, enhanced backbonding from a metal
center to the isocyanide, which manifests itself by a re-
duction of the wave number for the C≡N vibration, is
not necessarily reflected in the M-CN bond length [16].
Backbonding from Ni0 to the isocyanide ligands in
1 – 3 is stronger than in the NiII phenyl isocyanide
complexes but significant differences in the M-CN
bond lengths are not observed. This holds also for the
C≡N bond lengths in 1 – 3, which are not significantly
longer than in NiII phenyl isocyanide complexes. The
bond parameters of the Ni(C≡N)4 core can be com-
pared to the molecular structure of [Ni(CO)4] [17],
where the Ni-C and C≡O separations are 1.817(2) and
Complexes 1 – 3 can be crystallized from toluene
and crystals suitable for X-ray diffraction studies were
obtained this way. The molecular structures of 1 and 3
are depicted in Fig. 1 and the molecular structure of 2
is shown in Fig. 2. The nickel atom in 1 and 3 resides
on a special position in the tetragonal unit cell (site
¯
symmetry 4) and the asymmetric unit contains only
1/4 of the complex. Complex 2 is located on a general
position in the unit cell. All three complexes show a
slightly distorted tetrahedral coordination environment
(range of C-Ni-C angles 101.69(14)◦ – 114.40(15)◦).
The unique C≡N-C angles in 1 and 3 deviate from
˚
1.127(3) A, respectively.
We have developed a new synthetic procedure for
linearity (C1-N1-C2 168.4(4)◦ for 1 and 168.9(3)◦ for the preparation of homoleptic Ni0 phenyl isocyanide
3). However, in 2 four different C≡N-C angles ranging complexes 1 – 3 starting from [Ni(COD)2]. The com-
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F. E. Hahn et al. · Homoleptic Nickel(0) Phenyl Isocyanide Complexes
853
plexes 1 – 3 are air stable and easy to handle com- 12 H, Ar-H), 2.39 (s, 24 H, CH3). – 13C NMR (50.3 MHz,
[D6]benzene, ppm): δ = 176.1 (CN-Ph), 134.3, 131.6, 126.9,
pounds, which will be useful starting materials for the
preparation of other Ni0 complexes.
˜
125.2 (Ar-C). IR (KBr): ν = 2029 (vs, CN), 1999 (s, CN). –
C36H36N4Ni (583.40): calcd. C 74.12, H 6.22, N 9.60; found
C 73.26, H 6.10, N 9.40.
Experimental Section
Analytical data for 3: Purple crystals (83% yield). –
1H NMR (200.1 MHz, CDCl3, ppm): δ = 8.0 (m, 4 H, Ar-
H), 7.7 (m, 8 H, Ar-H), 7.5 (m, 4 H, Ar-H). – 13C NMR
(50.3 MHz, [D8]THF, ppm): δ = 173.4 (br, CN-Ph), 134.1,
130.3, 127.2, 123.8 (Ar-C), only 4 of the expected 6 Ar-C
All experiments were carried out in an argon atmosphere
using standard Schlenk techniques. All solvents were dried
by standard methods and distilled prior to use. NMR spectra
were recorded on a Bruker AC 200 spectrometer. The iso-
cyanide ligands were prepared by dehydration of formamides
according to the procedure described by Ugi et al. [18].
Preparation of tetrakis(phenyl isocyanide)nickel(0) com-
plexes 1-3: An almost identical procedure was employed for
the preparation of 1 – 3. The synthesis of 1 is described as an
example. A sample of bis(cyclooctadiene)nickel(0) (100 mg,
0.37 mmol) was dissolved in 150 ml of toluene. Phenyl iso-
cyanide (200 mg, 1.94 mmol) dissolved in 10 ml of toluene
was added to this solution over a period of 1 h at room
temperature. Subsequently the reaction mixture was concen-
trated to 30 ml and cooled to −10 ◦C. After 24 h at −10 ◦C
yellow crystals of 1 were collected by filtration. Yield 142 mg
(30.1 mmol, 81%). Suitable crystals for the X-ray struc-
ture determination were obtained by recrystallization from
toluene at −10 ◦C. – 1H NMR (200.1 MHz, CD2Cl2, ppm):
δ = 7.32 (m, br, 20 H, Ar-H). – 13C NMR (50.3 MHz,
CD2Cl2, ppm): δ = 169.0 (CN-Ph), 130.1, 129.3, 126.2,
˜
resonances were detected. – IR (KBr): ν = 2037 (vs, CN),
2003 (s, CN). – C28H16N8NiO8 (651.20): calcd. C 51.65,
H 2.48, N 17.21; found C 52.67, H 2.75, N 16.96.
X-ray structure determination of 1-3 [19]: Data sets were
collected with a Nonius KappaCCD diffractometer, equipped
with a rotating anode generator Nonius FR591 at 198(2) K.
The data for 1 were refined as a racemic twin with a ra-
tio of 0.5 to 0.5. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atom were
placed on calculated positions and were refined as riding
atoms. The nickel atoms in 1 and 3 reside on special posi-
¯
tions (site symmetry 4) with 1/4 of the molecule per asym-
metric unit. Additional data collection and refinement de-
tails are listed in Table 1. Programs used: data collection
COLLECT [20], data reduction Denzo-SMN [21], absorp-
tion correction SORTAV [22], structure solution SHELXS-
97 [23], structure refinement SHELXL-97 [24], graphics
ORTEP [25].
˜
124.1 (Ar-C). IR (KBr): ν = 2031 (vs, CN), 1986 (s, CN). –
C28H20N4Ni (471.19): calcd. C 71.38, H 4.28, N 11.89;
found C 71.33, H 4.19, N 11.77.
Acknowledgment
Financial support of this work from the Deutsche
Forschungsgemeinschaft is gratefully acknowledged.
Analytical data for 2: Yellow crystals (89% yield). –
1H NMR (200.1 MHz, [D6]benzene, ppm): δ = 6.78 (m, br,
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