A method for the synthesis of nickel(0) bis(carbene) complexes

Article abstract of DOI:10.1039/b714988c

A new method leading to Ni(NHC)₂ (NHC = IMes, IPrⁱ, SIPrⁱ, SIBu^t) complexes in moderate to good yields, involves the reaction of NHC (pre-formed or generated in situ) with Ni(CH ₃)₂(tmed), tmed = N,N′-tetramethylethylenediamine; in one case, the intermediate Ni[I(Me₂)Prⁱ] ₂(CH₃)₂, I(Me₂)Prⁱ = N,N′-diisopropyl-4,5-dimethylimidazol-2-ylidene, has been isolated and structurally characterised. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714988c

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A method for the synthesis of nickel(0) bis(carbene) complexes†
Andreas A. Danopoulos* and David Pugh
Received 28th September 2007, Accepted 30th October 2007
First published as an Advance Article on the web 6th November 2007
DOI: 10.1039/b714988c
A new method leading to Ni(NHC)2 (NHC = IMes, IPri,
SIPri, SIBut) complexes in moderate to good yields, involves
the reaction of NHC (pre-formed or generated in situ) with
Ni(CH3)2(tmed), tmed = N,Nꢀ-tetramethylethylenediamine;
in one case, the intermediate Ni[I(Me2)Pri]2(CH3)2, I(Me2)Pri =
N,Nꢀ-diisopropyl-4,5-dimethylimidazol-2-ylidene, has been
isolated and structurally characterised.
Reaction of Ni(tmed)(CH3)2 in THF with two equivalents of
SIPri, generated in situ from the corresponding imidazolinium
salt and KOBut, followed a similar course giving Ni(SIPri)2 as
a purple solid (ca. 65% yield).‡ X-Ray quality crystals were
obtained by cooling (5 ◦C) ether solutions.§ A diagram of the
molecule as determined by X-ray diffraction is shown in Fig. 1.
The structure comprises a linear nickel center [CNHC–Ni–CNHC
=
177.35(15)◦] with the two non-planar heterocyclic rings in a
staggered conformation (dihedral angle 47.9◦). The Ni–CNHC bond
One of the first homoleptic transition metal complexes prepared
by the reaction of the thermally stable 1,3-dimesitylimidazol-2-
ylidene, (IMes), with Ni(COD)2, COD = 1,5-cyclooctadiene, was
the remarkable Ni(IMes)2.1 The synthesis of Ni(IBut)2, IBut =
1,3-di-tert-butylimidazol-2-ylidene, was accomplished by metal
vapour synthesis,2 while the application of solution techniques
to the preparation of this complex presented difficulties arising
from incomplete substitution of the r-donor p-acceptor ligands in
Ni(COD)2 by IBut, or other side reactions.3 A series of catalytic
investigations described the in situ formation of Ni(NHC)2 by
substitution of COD in Ni(COD)2 by a preformed NHC or by
NHCs generated in situ (by the deprotonation of azolium salts).4
However, in many cases, the nature of the generated nickel NHC
complex was unclear. Furthermore, equilibrium mixtures with
Ni(COD)2 and Ni(IPri)2 have been suggested during the in situ
formation of Ni(IPri)2 from Ni(COD)2 and IPri, IPri = 1,3-
bis-(2,6-diisopropylphenyl)imidazol-2-ylidene, in THF.5 Finally,
interesting catalytic cyclisation and rearrangement reactions were
developed by combining Ni(COD)2 with SIPri, SIPri = 1,3-bis-
(2,6-diisopropylphenyl)imidazolin-2-ylidene, generated in situ.6
We have recently reported a general method for the synthesis
of Pd(NHC)2, NHC = 1,3-diarylimidazol(in)-2-ylidene, involving
the reaction of Pd(tmed)(CH3)2 with NHCs either pre-formed or
generated in situ.7 For NHCs with less bulky aromatic groups with-
out ortho-substitution, the initially formed Pd(NHC)2 underwent
further facile cyclometallation reactions at the aromatic ring to
thermally stable products.
˚
lengths [1.865(3)–1.886(3) A] fall in the range reported previously
for analogous Ni(0)(NHC) complexes.1,3 The stronger r-donation
by the SIPri is manifested by the downfield shift of the coordinated
CNHC in the 13C{ H}-NMR spectrum (d 211.2) in comparison to
1
the unsaturated analogue (d 193.8).
Fig. 1 ORTEP representation of the structure of Ni(SIPri)2 at 50%
probability level; H atoms are omitted for clarity. Selected bond lengths
(A) and angles ( ): C(13)–Ni(1) = 1.886(3); C(13)–N(1) = 1.357(4);
C(13)–N(2) = 1.374(4); C(40)–Ni(1) = 1.865(3); C(40)–N(4) = 1.375(4);
C(40)–N(3) = 1.381(4); C(40)–Ni(1)–C(13) = 177.35(15).
◦
˚
Herein, we wish to communicate preliminary results on the ex-
tension of this methodology to the synthesis of Ni(NHC)2 species.
The reaction of Ni(tmed)(CH3)2 with two equivalents of IPri
8
The method was also applied to the synthesis of alkyl substituted
NHCs. For example the reaction of two equivalents of 1,3-di(tert-
butylimidazolin-2-ylidene)9 with Ni(tmed)(CH3)2 led to the isola-
tion of Ni(SIBut)2 as brown crystals from petrol in moderate yield.
The structure of the molecule, determined crystallographically, is
shown in Fig. 2.
or IMes in THF underwent two distinct colour changes from
light yellow to dark yellow (−78 to −20 ◦C, 5 min) and dark
yellow to purple (room temperature, 16 h). After removal of the
THF in vacuo, Ni(IPri)2 and Ni(IMes)2 were obtained as purple
powders in excellent (ca. 80%) yields, respectively. The complexes
were characterised by analytical and spectroscopic methods. The
data for Ni(IMes)2 were identical to those previously reported.1
Ni(SIBut)2 is nearly linear [CNHC–Ni–CNHC = 174.5(11)◦] while
the non-planar saturated heterocyclic rings adopt a staggered
conformation (dihedral angle ca. 75◦). The Ni–CNHC bond length
Department of Chemistry, University of Southampton, Highfield, Southamp-
ton, UK SO17 1BJ. E-mail: ad1@soton.ac.uk; Fax: +44(0)2380596805
† CCDC reference numbers 662510–662512. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b714988c
i
˚
[1.874(8) A] was virtually equal to those observed in Ni(SIPr )2
reported above. The CNHC of the coordinated NHC appears at d
211.2.
30 | Dalton Trans., 2008, 30–31
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soluble organic products, methane and ethane. Attempts are under
way to isolate and fully characterise these reaction products.
In summary, the methodology described above provides a new
route to the synthesis of versatile Ni(NHC)2 complexes, and it
can be applied when substitution reactions on Ni(0) starting ma-
terials [e.g. Ni(COD)2, Ni(CO)4 etc.] by NHCs are problematical.
Interestingly, the alternative methyl imidazol(in)ium salt reductive
elimination, which has been dominant in many M–NHC methyl
complexes, is not observed in our system.11 The study of the
scope and details of this transformation by experimental and
computational methods as well as the use of the new complexes in
homogeneous catalysis is work in progress.
Notes and references
Fig. 2 ORTEP representation of the structure of Ni(IBut)2 at 50%
‡ Satisfactory elemental analyses were obtained for the new Ni(NHC)2
1
complexes. Spectroscopic data: For Ni(IPri)2, NMR (C6D6): H, d, 7.45,
˚
probability level; H atoms are omitted for clarity. Selected bond lengths (A)
=
7.35 (d and t, 12H, aromatic), 6.85 (s, 4H, CH CH backbone), 3.45
and angles (◦): C(5)–Ni(1) = 1.874(8); C(5)–N(1) = 1.444(15); C(5)–N(2) =
1
[septet, 8H, CH(CH3)2], 1.52 and 1.35 [two d, 48H, CH(CH3)2]; 13C{ H},
1.312(15); C(5)–Ni(1)–C(5ꢀ) = 174.5(11).
=
d, 193.8 (NCN), 146.2, 138.5, 129.9, 124.5 (aromatic), 121.1 (CH CH),
29.0 [CH(CH3)2], 25.3 and 23.9 [CH(CH3)2]. For Ni(SIPri)2, NMR (C6D6):
1H, d, 7.45, 7.35 (d and t, 12H, aromatic), 3.32 [sept, 8H, CH(CH3)2],
In an attempt to get a better insight into the reaction we reacted
Ni(CH3)2(tmed) with two equivalents of I(Me2)Pri, I(Me2)Pri =
1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene10 under the same
conditions as those described for Ni(SIBut)2. In this case we were
able to isolate Ni[I(Me2)Pri]2(CH3)2 in the form of yellow crystals
by stirring the reaction mixture at room temperature for 12 h. The
structure of this complex is shown in Fig. 3.
3.05 (s, 8H, CH2CH2), 1.42 and 1.32 [d, 48H, CH(CH3)2]; 13C{ H}, d,
1
211.15 (NCN), 147.4, 140.4, 124.5, 124.2 (aromatic), 54.1 (CH2CH2), 28.7
[CH(CH3)2] 25.3 and 25.1 [CH(CH3)2]. For Ni(SIBut)2, NMR (C6D6):
1H, d, 2.85 (s, 8H, CH2CH2 backbone), 2.05 [s, 36H, C(CH3)3]; 13C{ H},
1
d, 211.2 (NCN), 54.5 [C(CH3)3], 42.4 (CH2). For Ni[I(Me2)Pri]2(CH3)2,
NMR (C6D6): 1H, d, 6.2 [septet, 4H, CH(CH3)2], 1.68 (s, 12H, backbone
CH3), 1.60 and 1.05 [d, 24H, CH(CH3)2], −0.25 (s, 6H, Ni–CH3).
§ Crystal data. For Ni(SIPri)2: C54H76N4Ni; M = 839.90, monoclinic,
˚
˚
˚
P21/n, a◦= 12.8554(3) A, b = 27.1710(7) A, c = 13.9453(3) A, b =
3
˚
92.516(1) ; V = 4866.3(2) A , T = 120(2) K, Z = 4; 41 469 reflections
measured, 11 110 unique (Rint = 0.0682), which were used in all calcu-
lations. The final wR(F2) was 0.1786 (all data) and R = 0.0818 [I >
2r(I)]. For Ni(SIBut)2: C22H44N4Ni; M = 423.32, tetragonal, I41cd, a =
3
˚
˚
˚
b = 11.6779(6) A, c = 35.091(3) A; V = 4785.5(6) A , T = 120(2) K,
Z = 8; 26 139 reflections measured, 1408 unique (Rint = 0.1150), which
were used in all calculations. The final wR(F2) was 0.1837 (all data) and
R = 0.0863 [I > 2r(I)]. For Ni(NHC)2(CH3)2: C24H46N4Ni; M = 449.36,
˚
˚
˚
monoclinic, P21/c, a = 15.6502(2) A, b = 9.7001(1) A, c = 17.3931(2) A,
◦
3
˚
b = 104.387(1) ; V = 2557.61(5) A , T = 120(2) K, Z = 4; 41 469 reflections
measured, 5850 unique (Rint = 0.0441), which were used in all calculations.
The final wR(F2) was 0.0986 (all data) and R = 0.0480 [I > 2r(I)].
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Fig. 3 ORTEP representation of the structure of Ni[I(Me2)Pri]2(CH3)2
at 50% probability leve◦l; H atoms are omitted for clarity. Selected bond
˚
lengths (A) and angles ( ): C(4)–Ni(1) = 1.930(2); C(15)–Ni(1) = 1.910(2);
C(23)–Ni(1) = 1.966(2); C(24)–Ni(1) = 1.975(3); C(15)–Ni(1)–C(4) =
97.25(9); C(15)–Ni(1)–C(23) = 166.61(11); C(4)–Ni(1)–C(23) = 91.45(10);
C(15)–Ni(1)–C(24)
C(23)–Ni(1)–C(24) = 85.84(11).
=
88.12(10); C(4)–Ni(1)–C(24)
=
165.73(10);
The molecule adopted square planar cis geometry. The Ni–
CNHC and the Ni–CH3 bond lengths are in the range 1.910(2)–
˚
˚
1.930(2) A and 1.966(2)–1.975(3) A, respectively. The planes
of the heterocyclic rings form angles 61.47 and 73.3◦ with the
coordination plane.
Heating of C6D6 solutions of Ni[I(Me2)Pri]2(CH3)2 at 50 ◦C
results in the disappearance of the signals associated with the
starting material and the appearance of a new set of signals that can
be assigned to the Ni[I(Me2)Pri]2 together with minor amounts of
10 N. Kuhn and T. Kratz, Synthesis, 1993, 561.
11 D. C. Graham, K. J. Cavell and B. F. Yates, Dalton Trans., 2006, 1768.
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