Synthesis, Structures and Properties of Half-Sandwich Nickel(II) Complexes with Backbone-Modified NHC Ligands

Article abstract of DOI:10.1002/ejic.201500993

A series of seven nickel complexes [Ni(η⁵-C₅H₅)(X)(NHC)] (X = Cl or Br; NHC = N-heterocyclic carbene) containing backbone-modified NHC ligands was prepared by treatment of nickelocene with the corresponding NHC·HX salts or, in the case of complex 1h [Ni(η⁵-C₅H₅)(Cl)(IMes^O)] (IMes^O = 4-oxo-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), by deprotection of the corresponding NHC ligand in complex 1g [Ni(η⁵-C₅H₅)(Cl)(IMes^OPiv)] [IMes^OPiv = 4-(pivaloyloxy)-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Single-crystal X-ray structures were determined for complexes 1d [Ni(η⁵-C₅H₅)(Cl)(Bn₂-bimy)] (Bn₂-bimy = 1,3-dibenzylbenzimidazolin-2-ylidene), 1g and 1h. The coordination spheres around the nickel atoms resemble those of d⁸ NiL₄ square-planar complexes, with Ni-C(Cp) bond length differentiation and C-C bond alternation in Cp ligands being taken into account. Desymmetrization of the five-membered heterocyclic ring and a tautomeric equilibrium were observed for complex 1h. All new complexes are efficient precatalysts in three C-C bond-forming reactions (polymerization of styrene, polymerization of methyl methacrylate, Suzuki-Miyaura cross-coupling). Complexes [Ni(η⁵-C₅H₅)(X)(NHC)] featuring backbone-modified NHC ligands were synthesized. Their solid-state structures, solution dynamics, keto-enol equilibria and catalytic activity in C-C bond-forming reactions (polymerization of styrene, polymerization of methyl methacrylate, Suzuki-Miyaura cross-coupling) were investigated.

Full text of DOI:10.1002/ejic.201500993

FULL PAPER
DOI:10.1002/ejic.201500993
Synthesis, Structures and Properties of Half-Sandwich
Nickel(II) Complexes with Backbone-Modified NHC
Ligands
Łukasz Banach,[a] Piotr A. Gun´ka,[a] Dominika Górska,[a]
Maja Podlewska,[a] Janusz Zachara,[a] and
Włodzimierz Buchowicz*[a]
Keywords: Nickel / N-Heterocyclic carbenes / C–C coupling / Polymerization / Protecting groups
A series of seven nickel complexes [Ni(η5-C5H5)(X)(NHC)]
(X = Cl or Br; NHC = N-heterocyclic carbene) containing
backbone-modified NHC ligands was prepared by treatment
of nickelocene with the corresponding NHC·HX salts or, in
the case of complex 1h [Ni(η5-C5H5)(Cl)(IMesO)] (IMesO = 4-
oxo-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), by
deprotection of the corresponding NHC ligand in complex
1g [Ni(η5-C5H5)(Cl)(IMesOPiv)] [IMesOPiv = 4-(pivaloyloxy)-
imidazolin-2-ylidene), 1g and 1h. The coordination spheres
around the nickel atoms resemble those of d8 NiL4 square-
planar complexes, with Ni–C(Cp) bond length differentiation
and C–C bond alternation in Cp ligands being taken into ac-
count. Desymmetrization of the five-membered heterocyclic
ring and a tautomeric equilibrium were observed for complex
1h. All new complexes are efficient precatalysts in three C–
C bond-forming reactions (polymerization of styrene, poly-
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene].
Single- merization of methyl methacrylate, Suzuki–Miyaura cross-
crystal X-ray structures were determined for complexes 1d
[Ni(η5-C5H5)(Cl)(Bn2-bimy)] (Bn2-bimy = 1,3-dibenzylbenz-
coupling).
et al., has proved to be versatile and has resulted in a con-
siderable number of different NHC complexes of the gene-
ral formula [Ni(η5-C5H4R)(X)(NHC)] (1, R = H or alkyl;
X = Cl, Br or I).[6,7] These complexes serve as precatalysts
in a remarkably broad range of transformations, such as
amination of aromatic halides,[6a,6i] polymerization of styr-
ene,[6b,6f] polymerization of methyl methacrylate,[8] Suzuki–
Miyaura cross-coupling,[6g,9,10] polymerization of phenyl-
acetylene,[6f] regioselective hydrothiolation of alkynes,[11]
hydrosilylation of aldehydes, ketones and imines,[12] and α-
arylation of acyclic ketones.[13]
Our aim is to optimize the catalytic activity of [Ni(η5-
C5H4R)(X)(NHC)] complexes through suitable modifica-
tions of the ligands. Previously, significant efforts were de-
voted to modifications of N-substituents in these com-
plexes.[6] More recently, we have explored exchange of the
chloride in [Ni(η5-C5H4R)(Cl)(NHC)] for a nitrile or an an-
ionic (trifluoroacetate or nitrate) ligand.[14] In this report
we examine synthesis, structures and properties of a number
of complexes containing backbone-modified NHC ligands.
The electron-donating properties of NHC ligands
(Scheme 1) are systematically varied, from ligands that are
significantly stronger σ-donors that the standard IMes li-
gand (e.g. IMes2Me) to those that are weak donors (e.g.,
IMesO). For comparison, the available experimentally mea-
sured or calculated νCO frequencies quantifying the overall
electronic strength of each studied ligand are shown in
Scheme 1.
Introduction
Over recent years N-heterocyclic carbenes (NHCs) have
found vast areas of application in organometallic chemistry,
catalysis and other areas of chemistry.[1] Their electronic
and steric properties, as well as strong binding to metal
centres, made them excellent ligands.[2] An interesting fea-
ture of NHC ligands is that their electronic and steric prop-
erties are influenced to a great extent by different structural
features. Whereas spatial properties of NHCs depend gen-
erally on the nature of groups connected to the nitrogen
atoms, their electronic properties are influenced mostly by
substitution at the 4- and 5-positions of the imidazole ring
and/or introduction of heteroatoms into the heterocycle.[3]
In the field of transition-metal NHC complexes, particu-
lar attention is paid to nickel compounds, due to their cata-
lytic activity in a number of organic reactions.[4] The first
NHC cyclopentadienyl nickel(II) complex
–
[Ni(η5-
C5H5)(Cl)(IMes)] [1a, IMes = 1,3-bis(2,4,6-trimethylphen-
yl)imidazol-2-ylidene] – was synthesized in a reaction be-
tween nickelocene and 1,3-dimesitylimidazolium chloride in
THF at reflux.[5] This procedure, developed by Abernethy
[a] Warsaw University of Technology, Faculty of Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland
E-mail: wbuch@ch.pw.edu.pl
http://zkichm.ch.pw.edu.pl
Supporting information and ORCID(s) from the author(s) for
this article are available on the WWW under http://dx.doi.org/
10.1002/ejic.201500993.
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Scheme 3. The synthesis of complexes 1g and 1h. Reaction condi-
tions: a THF, reflux, b Et3N, then pivaloyl chloride (ref.[3f]), c THF,
room temp., d NH3(aq.)/MeOH, room temp (Mes = 2,4,6-trimeth-
ylphenyl, Dipp = 2,6-diisopropylphenyl, Piv = trimethylacetyl).
Scheme 1. Ligands explored in this study (Mes = 2,4,6-trimeth-
ylphenyl, Bn = benzyl). The reported average experimentally mea-
sured or calculated νCO frequencies in [Ir(Cl)(NHC)(CO)2] com-
plexes (in cm–1) are shown: [a] experimentally measured value,[15]
[b] experimentally measured value,[3b] [c] converted from experi-
mentally measured νCO frequencies in the [Rh(Cl)(NHC)(CO)2]
complex[3f] by use of the equation developed by Wolf and Plenio,[16] gent,[21] giving much better results than HCl(aq.) or K2CO3/
[d] converted from calculated TEP (Tolman electronic parameter,
MeOH, which caused decomposition of the substrate.
this work) value by use of the reversed equation published by Glor-
In the case of the diamidocarbene IMes2O, which is one
ius and co-workers,[15] [e] experimentally measured value for the
of the poorest NHC donors reported to date,[18] heating of
its precursor (2-chloro-1,3-dimesitylimidazolidine-4,5-di-
one) with nickelocene in THF at reflux resulted in ill-de-
fined, brownish solids. NMR spectra of these materials did
not feature any signals assignable to a Ni-bound cyclo-
pentadienyl ligand.[19]
5,6-dimethyl derivative,[17] and [f] experimentally measured val-
ue;[18a] complex [Ni(η5-C5H5)(Cl)(IMes2O)] could not be obtained.
The calculated TEP values for each ligand are listed in Table S5 in
the Supporting Information.
Results and Discussion
Synthesis
Characterization
Complexes 1b–1f were obtained as red solids in yields
varying from 41% to 87% by use of the straightforward
procedure originally published by Abernethy et al.,[5] with
minor modifications (Scheme 2). However, attempts to use
The NMR spectra of complexes 1b–1h were conven-
tionally recorded in CDCl3 at ambient temperature. These
spectra consist of signals originating from the cyclopen-
tadienyl (Cp) moiety and the appropriate NHC ligand. In
all cases the Cp protons are represented by sharp singlets
appearing either from 4.52 ppm to 4.63 ppm for the imid-
azole-based NHC complexes 1b, 1c, 1g and 1h, or from
5.10 ppm to 5.15 ppm for the benzimidazole- and 1,2,4-tri-
azole-derived NHC complexes 1d–1f. Whereas the presence
of signals for the Cp protons in the range from 4.5 ppm to
4.8 ppm is common for N,NЈ-diaryl- or mixed N,NЈ-alkyl-
aryl-nickel NHC complexes of this type,[6] its occurrence
downfield from 5 ppm is usual for N,NЈ-dialkyl ana-
logues.[6d,6g,6h,22] Our results confirm that the chemical
shifts of Cp protons are influenced by N-substituents rather
than by the structure of the heterocycle.
Upon coordination to the nickel centre, signals for all
C(sp2)-bonded protons from the NHC ligand core are
shifted upfield in comparison with their precursors (e.g., Δδ
= –1.38 ppm for the CH proton in the 1,2,4-triazole hetero-
cycle in 1f). This could be a result of reduced ring current
caused by transfer of electron density to the metal centre.
The 1H NMR spectra of 1d and 1e each feature two intri-
guing, well-resolved (separated by ca. 0.7 ppm) doublets
(1d: δ = 6.92 ppm and 6.22 ppm, J = 16.2 Hz; 1e: δ =
6.89 ppm and 6.14 ppm, J = 16.0 Hz). Because in both
cases Δν/J Ͼ 10 (17.3 for 1d and 18.8 for 1e) those signals
represent “the roof effect” (i.e., the inner peaks of two
4-hydroxy-1,3-dimesitylimidazolium chloride (IMesOH
·
HCl)[3c,3f] or its 1,3-bis(2,6-diisopropylphenyl) analogue
IPrOH·HCl[3c,3f] in this procedure were unsuccessful
(Scheme 3, path a). In both cases light blue paramagnetic
solids were obtained; they could not be fully identified, due
to uninformative NMR spectra and absence of signals char-
acteristic of organonickel species in the EI MS spectra.[19]
Therefore, we decided to protect the OH group in
IMesOH·HCl. Thus, IMesOH·HCl was deprotonated with
Et3N by the published procedure[3f] and subsequently
treated with pivaloyl chloride to give the desired ester
IMesOPiv·HCl.[20] Gratifyingly, IMesOPiv·HCl appeared to
be very reactive towards nickelocene, providing complex 1g
within 2 h at room temperature in 84% yield (Scheme 3).
The target complex 1h was then obtained from 1g upon
hydrolysis of the ester protecting group in the NHC ligand.
In this reaction NH3(aq.)/MeOH was the most suitable rea-
Scheme 2. Synthesis of complexes 1b–1f. Reaction conditions:
THF, reflux, 2.5–12 h.
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neighbouring doublets are higher than the external peaks).
Analysis of the spectra for 1d and 1e leads to the conclusion
that these doublets represent diastereotopic methylene pro-
tons from the benzyl groups. Non-equivalence of these
hydrogen atoms bonded to the same carbon atom originates
from the restricted rotation of the Ni–Ccarbene or N–CBn
bonds. Similar dynamic behaviour was observed for some
dialkyl or mixed alkyl-aryl nickel NHC complexes.[6c,6f,10,23]
The nature of such restricted rotation has been attributed
mainly to steric factors related to the bulkiness of halogen
or Cp ligands, although electronic factors cannot be com-
pletely excluded.[6d,6f] Restricted rotation of the Ni–Ccarbene
and N–CR bond could be also caused by a CH···XNi inter-
action, as observed in the solid state for [Ni(η5-
C5H5)(Cl)(NHC)] [NHC = 1-cyclohexyl-3-(2,4,6-trimeth-
ylphenyl)-4,5-dihydroimidazol-2-ylidene].[6f] Another expla-
nation for such a phenomenon could be anagostic CH···Ni
interactions reported for the square-planar nickel com-
plexes [Ni(Br)2(NHC)2] (NHC = 1,3-diisopropylbenzimid-
azolin-2-ylidene).[24] Although the occurrence of large
downfield shifts for the benzyl methylene protons upon co-
ordination (e.g., Δδ = –0.45 ppm and –1.15 ppm for 1d) sug-
gests possible anagostic interactions in solution, no such
short contacts are observed in the solid state for 1d (vide
infra). If it is assumed that the NiII centres are saturated in
complexes [Ni(η5-C5H5)(X)(NHC)], we do not at this point
have a satisfactory explanation for this interesting behav-
iour.
Scheme 4. Deuterium exchange in 1h via the enol tautomer (R =
2,4,6-trimethylphenyl).
1
Moderate heating has negligible effect on the H NMR
spectra of complex 1h (e.g., the -CH2- singlet shifts from
4.30 ppm in CDCl3 at ambient temperature to 4.28 ppm at
50 °C). However, the chemical shift of the heterocycle
-CH2- singlet varies considerably with the solvent polarity:
from 3.41 ppm in C6D6 to 4.61 ppm in (CD3)2CO. This
could be a result of the tautomeric equilibrium in polar
solvents that is fast on the NMR timescale. In order to con-
firm the presence of the elusive enol form of 1h, H/D ex-
change experiments were performed in (CD3)2CO/D2O.
The -CH2- singlet at δ = 4.67 ppm disappeared completely
upon treatment of 1h with an excess of D2O overnight at
room temperature. Moreover, the corresponding -CH2- res-
onance at δ = 54.27 ppm in the 13C{1H} NMR spectrum
turned into a broad band due to splitting with D atoms
(Scheme 4, see also Figure S2 in the Supporting Infor-
mation).
Interestingly, during these studies on 1h we noticed that
the sharp -CH2- singlet at δ = 4.67 ppm broadened upon
cooling and split at –50 °C into two broad resonances
centred at δ = 4.83 and 4.77 ppm. At the same time, the
two signals of non-equivalent meta aromatic protons at δ =
7.14 and 7.11 ppm also significantly broadened (Figure S3
in the Supporting Information). These observations could
be explained in terms of a dynamic process that becomes
slow on the NMR timescale at low temperature (vide su-
pra), most likely the hindered rotation of the N-mesityl sub-
stituents.
Similar spectroscopic features as for 1d and 1e are ob-
served for the 1,2,4-triazole-derived complex 1f. However,
in 1f all four methylene protons are inequivalent, due to the
restricted rotation and desymmetrization of the NHC core
owing to the presence of a nitrogen atom at the 4-position
of the heterocycle. The Ph-CH2- signals appear as unre-
solved and partially overlapping multiplets from 5.9 ppm to
6.3 ppm at room temperature.
The unsymmetrical nature of other ligands substituted
at the NHC backbone (i.e., IMesMe, IMesOPiv and IMesO)
In the 13C{1H} NMR spectra of complexes 1b–1h the Cp
singlets are present in a narrow range from 92.1 to
93.8 ppm. The carbene carbon chemical shifts in complexes
1b–1h varied from 161.7 ppm for 1b to 220.1 ppm for 1h.
Although these complexes feature the most- and the least-
electron-donating carbene ligands, respectively, no corre-
lation of Ccarbene chemical shifts with the electronic proper-
ties of ligands is observed within the series of complexes
1a–1h (Figure S4 in the Supporting Information).
1
demonstrates itself in the H NMR spectra through non-
equivalence of the methyl groups from the mesityl substitu-
ents. The methyl groups appear as three singlets, with a
6:6:6 intensity ratio in the cases of 1c and 1h, or as four
singlets with a 3:3:6:6 ratio in that of 1g. Thus, only in the
latter case do the para-methyl groups resonate as two sepa-
rate singlets.
In the case of 1h a keto–enol tautomeric equilibrium
could be expected (Scheme 4). Nonetheless, only two sing-
lets appear in the diagnostic part of the 1H NMR spectrum,
at δ = 4.30 ppm and at δ = 4.63 ppm with a 2:5 intensity
ratio. We assign these to the heterocycle -CH2- and Cp pro-
tons, respectively. Moreover, no signal that could be as-
X-ray Studies
¯
Compound 1d crystallizes in the triclinic 1 space group
signed to a single C(sp2)-bonded proton was present in this (no. 2) whereas compound 1g crystallizes in the P21/n space
spectrum. Thus we conclude that only the keto form of 1h group (no. 14) of the monoclinic crystal system. Both of the
is present in CDCl3 at room temperature, similarly to what crystal structures contain two independent molecules in the
was observed for [Rh(Cl)(COD)(IMesO)][3f] in CD2Cl2 and asymmetric unit. However, the conformational differences
for [Ir(Cl)(NHC)(CO)2] in CDCl3 when related NHC li- in the substituents in the NHC ligands do not exert signifi-
gands derived from 4-hydroxy-5-alkyl- or 5-arylimidazoles cant influence either on the nickel coordination sphere or
were used.[25]
on the imidazolium ring geometry, so only one molecule
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from the asymmetric unit is presented in Figure 1 and Fig- of the benzimidazole plane as the chloride anion, whereas
ure 2.
in the other molecule the phenyl rings are on the opposite
side of the plane. The C18–C17–C16–N1 and C11–C10–
C9–N2 torsion angles, describing the orientation of the
phenyl rings within the latter structure, amount to 2.96(16)°
and 24.35(14)°, respectively, and the analogous C48–C47–
C46–N31 and C41–C40–C39–N32 torsion angles in the for-
mer one are 52.18(14)° and –71.85(14)°, respectively.
The conformational differences between independent
molecules of complex 1g are twofold. Firstly, the NHC li-
gand is oriented differently with respect to the Ni–Cl bonds,
as evidenced by the values of the torsion angles Cl1–Ni1–
C6–N1 and Cl41–Ni41–C46–N41 of 58.87(12)° and
–60.17(12)°, respectively. Secondly, the orientations of the
pivaloyl group with respect to the imidazolium ring of the
NHC are different, as can be seen from the values of the
C27–O1–C7–C8 and C67–O41–C47–C48 torsion angles,
which equal –91.5(2)° and 91.8(2)°, respectively. Compound
1h crystallizes in the orthorhombic Pbca space group
(no. 61) and there is one molecule in the asymmetric unit
(Figure 3). The C7–O1 and C7–C8 bond lengths of 1.197(3)
and 1.507(3) Å, respectively, indicate unequivocally that the
NHC ligand exists in the keto form in this complex.
Figure 1. Molecular structure of complex 1d. See the Supporting
Information for the other molecule from the asymmetric unit.
Thermal ellipsoids are drawn at the 50% level of probability and
hydrogen atoms are omitted for clarity. Selected interatomic dis-
tances and angles: Ni1···CpCg 1.7646(8) Å [Ni1···Cpplane
1.7630(3) Å], Ni1–Cl1 2.2005(4) Å, Ni1–C6 1.8700(13) Å, N1–C6–
N2 105.85(11)°, C6–N1 1.3606(17) Å, C6–N2 1.3600(17) Å, N2–C8
1.3945(17) Å, N1–C7 1.3931(18) Å, C7–C8 1.393(2) Å.
Figure 2. Molecular structure of complex 1g. See the Supporting
Information for the other molecule from the asymmetric unit.
Thermal ellipsoids are drawn at the 50% level of probability and
hydrogen atoms are omitted for clarity. Selected interatomic dis-
tances and angles: Ni1···CpCg 1.7727(11) Å [Ni1···Cpplane
1.7708(3) Å], Ni1–Cl1 2.1880(6) Å, Ni1–C6 1.888(2) Å, N1–C6–
N2 103.65(18)°, C6–N1 1.361(3) Å, C6–N2 1.362(3) Å, N2–C7
1.388(3) Å, N1–C8 1.396(3) Å, C7–C8 1.339(3) Å.
Figure 3. Molecular structure of complex 1h. Thermal ellipsoids
are drawn at the 50% level of probability and hydrogen atoms are
omitted for clarity. Selected interatomic distances and angles:
Ni1···CpCg 1.7720(13) Å [Ni1···Cpplane 1.7698(6) Å], Ni1–Cl1
2.1829(7) Å, Ni1–C6 1.872(2) Å, N1–C6–N2 107.09(19)°, C6–N1
1.386(3) Å, C6–N2 1.328(3) Å, N2–C8 1.469(3) Å, N1–C7
1.409(3) Å, C7–C81.507(3) Å, C7–O11.197(3) Å.
In one of the molecules of complex 1d the phenyl rings
Interestingly, the amidocarbene NHC ligand in 1h fea-
from both benzyl substituents are located on the same side tures significant elongation of the N2–C8 and C6–N1
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bonds and shortening of the C6–N2 bond with respect to Table 1. Polymerization of styrene with [Ni(η5-C5H5)(X)(NHC)]
(1a–1h)/MAO.[a]
the analogous bonds in complexes 1d and 1g (see Figure 1,
Figure 2 and Figure 3 captions for numerical values). This
indicates localization of electrons in the π orbitals of C6–
N2 bond in complex 1h, whereas in molecules 1d and 1g
they are delocalized over N1, C6 and N2 atoms. Conse-
quently, the mesomeric formula A depicted on Scheme 5 is
[c]
[c]
Entry Complex
Yield[b] Mn
Mw/Mn
dominant in the imidazolium ring electronic structure in
compound 1h.
1
2
3
4
5
6
7
[Ni(η5-C5H5)(Cl)(IMes)] (1a)[d]
98
11100
8000
8000
7400
430/18000
6000
1.8
3.0
2.9
3.0
[Ni(η5-C5H5)(Cl)(IMes2Me)] (1b) 94
[Ni(η5-C5H5)(Cl)(IMesMe)] (1c)
96
[Ni(η5-C5H5)(Cl)(Bn2-bimy)] (1d) 90
[Ni(η5-C5H5)(Br)(1,2,4TBn)] (1f)
82
[Ni(η5-C5H5)(Cl)(IMesOPiv)] (1g) 94
[Ni(η5-C5H5)(Cl)(IMesO)] (1h)
96
2.0/1.9 [e]
4.0
[e]
7600
2.8
[a] Reaction conditions: styrene/Ni 15000:1, Al/Ni 300:1, toluene,
30 min at room temp., then 3 h at 50 °C. [b] Isolated yield. [c] De-
termined by GPC. [d] Data from ref.[6f] [e] Bimodal distribution.
Scheme 5. The mesomeric structures of the NHC ligand in complex
1h.
Similarly to those in in compounds 1d and 1g, the Ni–
CCp bond lengths in 1h show a pronounced trans effect of
the NHC ligand. The Ni–CCp bond length with carbon ly-
ing cis to the NHC ligand (trans to the chloride ligand) is
ca. 0.1 Å shorter than the bond lengths to carbons on the
opposite side of Cp (see Table S2 in the Supporting Infor-
mation). The slippage of Cp rings is very small in all com-
pounds and does not exceed 0.095 Å. The C–C bond
lengths in the Cp rings are varied in an analogous way as
in the Cp* complexes of Ni with tertiary phosphine ligands
[Ni(X)(Cp*)(PEt3)]:[26] that is, the two bonds approximately
parallel to the CNHC–Ni–Cl plane are shorter than the
others, resulting in the so-called “diene distortion” (see
Table S3 in the Supporting Information). Detailed analysis
of the electronic structure presented by Holland et al.[26]
and references cited therein indicates that the coordination
sphere of the nickel atom resembles the square-planar ge-
ometry characteristic of d8 NiL4 complexes. The overlap of
Ni d and Cp π orbitals results in the observed C–C bond
length alternation in the non-slipped Cp ring.
The GPC measurements indicate that low-molecular-
weight polystyrene (PS, Mn from 6000 to 8000 Da, in com-
parison with the target Mn of 1560 kDa) with rather broad
Mw/Mn ratios were obtained. In the case of complex 1f low-
molecular-weight oligomers were also present. The 13C{1H}
NMR spectra of PS were consistent with atactic microstruc-
ture in all cases.
Because the steric demands of N-mesityl- and N-benzyl-
substituted NHCs are different,[27] the effects of the elec-
tronic properties of the NHC ligands on this catalytic pro-
cess could be compared in the series of N,NЈ-dimesityl com-
plexes 1b, 1c, 1a, 1g and 1h: that is, in the order of decreas-
ing donor strength of ligands (IMes2Me Ͼ IMesMe Ͼ IMes
≈ IMesOPiv Ͼ IMesO). Because all of them provide the same
yields of polystyrene, these results suggest that factors other
than the electronics of the NHCs define the overall effi-
ciency of this catalytic system.[28]
Methyl Methacrylate Polymerization
We have recently described methyl methacrylate (MMA)
polymerization in the presence of a series of [Ni(η5-
C5R5)(X)(NHC)] complexes together with MAO.[8] Our fin-
dings, supported by literature precedents, allowed us to sug-
gest a coordinative-anionic mechanism in which the mono-
mer molecule is activated through coordination to the Ni
centre.[29] The highest level of conversion was achieved with
Catalysis
Styrene Polymerization
We reported in 2006 that complexes [Ni(η5- complex [Ni(η5-C5H5)(Cl)(SIPr)], bearing the bulky N,NЈ-
C5H4R)(X)(NHC)] bearing the standard NHC ligands initi- 2,6-diisopropylphenyl NHC ligand (Table 2, Entry 1). In
ate polymerization of styrene in the presence of an excess the present work, we have found that MMA polymerization
of methylaluminoxane (MAO).[6b] A significant effect of the in the presence of complexes 1/MAO can be preferably car-
steric bulk of NHC and Cp ligands on the catalytic activity ried out at room temperature. The results of MMA polyme-
was concluded from those experiments. The results of styr- rization under the optimized conditions are shown in
ene polymerization with all the new complexes are listed Table 2. Initially, complex [Ni(η5-C5H5)(Cl)(SIPr)] was
in Table 1. All studied complexes polymerize styrene with tested (Entry 2), and provided PMMA [poly(methyl meth-
comparable yields (90–98%), with the exception of the tri- acrylate)] with 47% yield and slightly higher content of rr
azole-derived complex 1f (Entry 5), which provided a some- triad than in previous runs at 50 °C. Then, complexes 1a–
what lower yield (82%). Under more challenging conditions 1h were tested and gave up to 61% yields of PMMA (1g,
(i.e., at Al/Ni ratio 100:1) the reference complex 1a per- Entry 9). The same syndiotactic-rich polymer microstruc-
formed considerably better (95% yield) than 1c (67% yield). ture was obtained for all studied complexes.
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Table 2. Polymerization of methyl methacrylate in the presence of Cross-Coupling Reactions
[Ni(η5-C5H5)(X)(NHC)] (1a–1h)/MAO.[a]
Both neutral complexes [Ni(η5-C5R5)(X)(NHC)] and cat-
ionic complexes [Ni(η5-C5R5)(RCN)(NHC)]+A– are ef-
ficient precatalysts for Suzuki–Miyaura cross-coupling be-
tween aryl halides and phenylboronic acid.[6g,9,14] The back-
bone-modified complexes 1b–1h were tested under standard
conditions in toluene with K3PO4 as a base (Table 3). All of
the new complexes catalyze formation of the cross-coupling
product from 4Ј-bromoacetophenone and phenylboronic
acid with yields from 52% to 78%, with excellent selectivity.
Moreover, complex 1g was significantly less efficient when
4Ј-chloroacetophenone was used as a substrate (33% yield,
Entry 8). Surprisingly, phenylboronic acid pinacol ester
(Entry 10), which has often been used as a substrate in Su-
zuki cross-couplings, did not react under these conditions.
Previously, a considerable number of tested complexes
[Ni(η5-C5R5)(X)(NHC)] and their ionic congeners have
given similar levels of conversion in Suzuki–Miyaura cross-
couplings. Therefore, it has been suggested that complexes
1 are probably reduced in situ to provide catalytically active
Ni0 complexes.[6g,9] Our findings could be explained con-
sistently with this proposal because the electronic and steric
properties of NHC ligands tested in this study do not
significantly influence the outcome of these cross-coupling
reactions.
Entry Complex
Yield[b]
Triad fractions[c]
rr:mr:mm
1
2
3
4
5
6
7
8
9
[Ni(η5-C5H5)(Cl)(SIPr)][d]
34
47
34
44
55
58
24
36
61
67:25:8
74:20:6
72:21:7
75:19:6
75:20:5
72:22:6
73:20:7
71:22:7
73:21:6
[Ni(η5-C5H5)(Cl)(SIPr)]
[Ni(η5-C5H5)(Cl)(IMes)] (1a)
[Ni(η5-C5H5)(Cl)(IMes2Me)] (1b)
[Ni(η5-C5H5)(Cl)(IMesMe)] (1c)
[Ni(η5-C5H5)(Cl)(Bn2-bimy)] (1d)
[Ni(η5-C5H5)(Br)(Bn2-bimy)] (1e)
[Ni(η5-C5H5)(Br)(1,2,4TBn)] (1f)
[Ni(η5-C5H5)(Cl)(IMesOPiv)] (1g)
[a] Reaction conditions: MMA/Ni 1000:1, Al/Ni 100:1, toluene,
30 min at room temp., then 20 h at room temp. [b] Isolated yield.
[c] Determined by H NMR. [d] Data from ref.[8] (30 min at room
1
temp., then 3 h at 50 °C, MMA/Ni 1000:1, Al/Ni 100:1); SIPr =
1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene.
Determination of the molecular weights of these poly-
mers by GPC proved to be troublesome owing to unex-
plained back pressure increases. However, measurements of
representative samples indicate that these polymers are of
complex, multimodal composition with two major frac-
tions: one with Mn ca. 100 kDa and the second one with
Mn Ͼ 1000 kDa. These data were further verified with dy-
namic light scattering (DLS) measurements that show the
presence of two fractions corresponding to sizes of roughly
5 nm and 25 nm.
In preliminary tests, we have also found that complexes
[Ni(η5-C5H5)(X)(NHC)] are active in Kumada–Corriu cou-
Because ligand modification had no effect on the micro-
structure of the polymers, we conclude that Ni species do
not participate in the propagation steps. Collectively, these
findings provide further evidence for an anionic MMA po-
lymerization mechanism.
Scheme 6. Kumada–Corriu coupling catalyzed by complex 1b.
Table 3. Suzuki cross-coupling between phenylboronic acid and 4Ј-haloacetophenones.[a]
Entry
Complex
X
Conversion[b] [%]
Selectivity A:B[b]
[c]
1
2
3
4
5
6
7
8
9
[Ni(η5-C5H5)(Cl)(IMes)] (1a)
Br
Br
Br
Br
Br
Br
Br
Cl
Br
Br
68
72
78
74
52
73
73
33
67
13
B not detected
[Ni(η5-C5H5)(Cl)(IMes2Me)] (1b)
[Ni(η5-C5H5)(Cl)(IMesMe)] (1c)
[Ni(η5-C5H5)(Cl)(Bn2-bimy)] (1d)
[Ni(η5-C5H5)(Br)(Bn2-bimy)] (1e)
[Ni(η5-C5H5)(Br)(1,2,4TBn)] (1f)
[Ni(η5-C5H5)(Cl)(IMesOPiv)] (1g)
[Ni(η5-C5H5)(Cl)(IMesOPiv)] (1g)
[Ni(η5-C5H5)(Cl)(IMesO)] (1h)
98:2
99:1
99:1
99:1
98:2
99:1
B not detected
99:1
[d]
10
[Ni(η5-C5H5)(Cl)(IMes2Me)] (1b)
99:1
[a] Reaction conditions: 3 mol-% [Ni(η5-C5H5)(X)(NHC)] in toluene, K3PO4 (2.6 equiv.), 90 °C, 1 h. [b] Determined by GC. [c] Data from
ref.[9] (reaction time 30 min). [d] Run with phenylboronic acid dimethyl ester; phenylboronic acid pinacol ester did not react.
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ene and filtered. The desired complexes were obtained by crystalli-
zation from the filtrate.
pling. For example, the reaction between phenylmagnesium
bromide and meta-bromotoluene in the presence of 3 mol-
% of complex 1b (90 min, room temp.) gives a 92% yield of
the cross-coupling product (Scheme 6).
[Ni(η5-C5H5)(Cl)(IMes2Me)] (1b): This complex was obtained from
nickelocene (388.0 mg, 1.053 mmol) and IMes2Me·HCl (198.0 mg,
1.053 mmol) in THF (10.0 mL) with use of a reaction time of 3 h,
1
yield 56%, red solid (289.3 mg, 0.589 mmol). H NMR (400 MHz,
CDCl3): δ = 7.12 (s, 4 H, m-ArH), 4.52 (s, 5 H, C5H5), 2.44 (s, 6
H, p-ArCH3), 2.08 (s, 12 H, o-ArCH3), 1.86 (s, 6 H, ImCH3) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ = 161.7 (NCN), 138.9 (ArC),
136.2 (ArC), 135.3 (ArC), 129.4 (ArC), 128.0 (ImC), 92.1 (C5H5), 21.5
(p-ArCH3), 18.6 (o-ArCH3), 9.6 (ImCH3) ppm. EI MS: m/z (58Ni,
35Cl, %) = 490 (39) [M]+, 455 (31) [M – Cl]+, 425 (16) [M – Cp]+,
331 (100) [IMes2Me – H]+. C28H33ClN2Ni·H2O (509.8): calcd. C
65.98, H 6.92, N 5.50; found C 66.45, H 6.72, N 5.69.
[Ni(η5-C5H5)(Cl)(IMesMe)] (1c): This complex was obtained from
nickelocene (260.0 mg, 1.38 mmol) and IMesMe·HCl (480.0 mg,
1.35 mmol) in THF at reflux (13.0 mL) with use of a reaction time
of 2.5 h, yield 47%, red solid (307.0 mg, 0.647 mmol). 1H NMR
(400 MHz, CDCl3): δ = 7.13 (s, 2 H, m-ArH), 7.10 (s, 2 H, m-ArH),
6.82 (s, 1 H, ImCH), 4.54 (s, 5 H, C5H5), 2.44 (br. s, 6 H, p-ArCH3),
2.16 (s, 6 H, o-ArCH3), 2.11 (s, 6 H, o-ArCH3), 1.92 (s, 3 H,
ImCH3) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 165.0 (NCN),
139.2 (ArC), 139.0 (ArC), 137.0 (ArC), 136.4 (ArC), 136.0 (ArC),
134.7 (ArC), 132.6 (ImC), 129.4 (ArC), 129.3 (ArC), 121.6 (ImC), 92.1
(C5H5), 21.5 (p-ArCH3), 21.4 (p-ArCH3), 18.6 (o-ArCH3), 18.5 (o-
ArCH3), 10.1 (ImCH3) ppm. EI MS: m/z (58Ni, 35Cl, %) =
476 (10) [M]+, 411 (1) [M – Cp]+, 317 (100) [IMesM – H]+.
C27H31ClN2Ni·0.5H2O (486.7): calcd. C 66.63, H 6.63, N 5.76;
found C 66.68, H 6.53, N 5.73.
Conclusions
A number of backbone-functionalized NHC precursors
have been successfully applied in the direct synthesis of
complexes [Ni(η5-C5H5)(X)(NHC)]. In the case of 4-
hydroxy-substituted NHC precursor IMesOH·HCl we have
developed a protection/deprotection strategy to obtain the
desired 4-oxo complex 1h. Crystallographic data for com-
plexes 1d, 1g and 1h show that the backbone substituents
influence the structure of the heterocycle ring but do not
significantly modify the overall geometry of the Ni coordi-
nation sphere. All new complexes are catalytically active in
three C–C bond-forming reactions: polymerization of styr-
ene (TON up to 14400), polymerization of methyl meth-
acrylate (TON up to 610) and Suzuki–Miyaura cross-cou-
pling (TON up to 24). However, the effect of variable prop-
erties of the NHC ligands on the reactivity of the studied
complexes is not pronounced.
Experimental Section
[Ni(η5-C5H5)(Cl)(Bn2-bimy)] (1d): This complex was obtained from
nickelocene (170.0 mg, 0.901 mmol) and Bn2-bimy·HCl (300 mg,
0.897 mmol) in THF at reflux (8.5 mL) with use of a reaction time
of 12 h, yield 41%, red solid (167.1 mg, 0.365 mmol). 1H NMR
(400 MHz, CDCl3): δ = 7.37 (m, 6 H, C6H5), 7.27 (m, 4 H, C6H5),
7.10 (m, 4 H, bimyC), 6.92 (d, J = 16.2 Hz, 2 H, PhCH2), 6.22 (d,
J = 16.2 Hz, 2 H, PhCH2), 5.10 (s, 5 H, C5H5) ppm. 13C{1H} NMR
(101 MHz, CDCl3): δ = 179.9 (NCN), 136.0 (bimyC), 135.4 (C6H5),
129.0 (C6H5), 127.9 (C6H5), 126.6 (C6H5), 122.9 (bimyCH), 110.7
General: All manipulations (except polymer separation and purifi-
cation and workup of the Suzuki cross-coupling reactions) were
performed under inert atmosphere (argon) with use of Schlenk
techniques. Solvents were purified by conventional methods.[30]
Styrene (ReagentPlus®, Aldrich) was distilled from CaH2 under re-
duced pressure and passed through a column with neutral Al2O3.
Methyl methacrylate (Aldrich, 99%) was distilled from CaH2
under reduced pressure. Ligand precursors IMes2Me·HCl,[31]
IMesMe·HCl,[31] 1,2,4TBn·HBr,[32] IMesOH·HCl, IPrOH·HCl and
IMesOPiv·HCl,[3f] Bn2-bimy·HCl,[33] Bn2-bimy·HBr[34] and 2-chloro-
1,3-dimesitylimidazolidine-4,5-dione[18a] were prepared by litera-
ture procedures. Other reagents were purchased from commercial
suppliers and used without further purification.
(
bimyCH), 92.3 (C5H5), 53. 6 (PhCH2) ppm. EI MS: m/z (58Ni, 35Cl,
%) = 456 (7) [M]+, 91 (100) [PhCH2]+. C26H23ClN2Ni·0.5H2O:
calcd. C 66.92, H 5.18, N 6.00; found C 67.08, H 5.25, N 5.95.
[Ni(η5-C5H5)(Br)(Bn2-bimy)] (1e): This complex was obtained from
nickelocene (500.0 mg, 2.645 mmol) and Bn2-bimy·HBr (940 mg,
2.478 mmol) in THF at reflux (25 mL) with use of a reaction time
of 10 h, yield 29%, red solid (360 mg, 0.717 mmol). 1H NMR
(400 MHz, CDCl3): δ = 7.38–7.33 (m, 6 H, C6H5), 7.25–7.23 (m, 4
H, C6H5), 7.08 (s, 4 H, bimyC), 6.89 (d, J = 16 Hz, 2 H, PhCH2),
6.14 (d, J = 16 Hz, 2 H, PhCH2), 5.15 (s, 5 H, C5H5) ppm. 13C{1H}
NMR (101 MHz, CDCl3): δ = 181.5 (NCN), 136.0 (bimyC), 135.5
(C6H5), 129.0 (C6H5), 127.9 (C6H5), 126.5 (C6H5), 122.8 (bimyCH),
110.7 (bimyCH), 92.32 (C5H5), 53.80 (PhCH2) ppm. EI MS: m/z
(58Ni, 79Br, %) = 500 (4) [M]+ , 435 (1) [M – Cp]+, 91 (100)
[PhCH2]+. C26H23BrN2Ni·0.5C7H8 (548.2): calcd. C 64.64, H 4.96,
N 5.11; found C 64.21, H 5.00, N 5.61.
NMR spectra were recorded at ambient temperature with a Mer-
1
cury-400BB spectrometer operating at 400 MHz for H NMR and
at 101 MHz for 13C NMR spectroscopy. The average molecular
weights of PS and PMMA were measured with a LabAlliance li-
quid chromatograph equipped with a Jordi Gel DVB Mixed Bed
column (250 mmϫ10 m) with CH2Cl2 as the mobile phase at 30 °C
and calibrated with the appropriate standard (PS or PMMA). Dy-
namic light scattering (DLS) experiments with PMMA were per-
formed with a Malvern ZetaSizer Nano instrument in CH2Cl2.
Levels of conversion and selectivity of Suzuki reactions were deter-
mined with an Agilent Technologies 7820 GC System equipped
with a FID detector and an Agilent 19091J-413 column. Tetradec-
ane was used as an internal standard.
[Ni(η5-C5H5)(Br)(1,2,4TBn)] (1f): This complex was obtained from
nickelocene (220.0 mg, 1.16 mmol) and 1,2,4TBn·HBr (410.0 mg,
1.24 mmol) in THF at reflux (12.0 mL) with use of a reaction time
of 12 h, yield 87%, red solid (460.0 mg, 1.01 mmol). 1H NMR
(400 MHz, CDCl3): δ = 7.85 (s, 1 H, TriazCH), 7.41–7.32 (m, 10 H,
C6H5), 6.15–6.04 (b, 4 H, PhCH2), 5.15 (s, 5 H, C5H5) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ = 170.8 (NCN), 143.58
Synthesis of Complexes: Complexes 1b–1g were each prepared from
nickelocene and the appropriate imidazolium salt in THF by the
published method[5] with minor modifications. Briefly, a THF solu-
tion of nickelocene was added to NHC·HX. The resulting mixture
was heated at reflux for the specified time (or vigorously stirred at
ambient temperature in the case of 1g). The solvent was then re-
moved under vacuum, and the solid residue was treated with tolu-
(
TriazCH), 135.65 (C6H5), 134.82 (C6H5), 129.44 (C6H5), 128.97
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(C6H5), 128.40 (C6H5), 128.21 (C6H5), 128.10 (C6H5), 92.46
(C5H5), 57.13 (PhCH2), 53.60 (PhCH2) ppm. EI MS: m/z (58Ni,
CCDC-1039261 (for 1d), -1039262 (for 1g) and -1039263 (for 1h)
contain the supplementary crystallographic data for this paper.
79Br, %) = 451 (14) [M]+, 386 (5) [M – Cp]+, 307 (25) [M – These data can be obtained free of charge from The Cam-
Cp Br]+, 202 (6) [M – NHC]+, 91 (100) [PhCH2]+. C21H20BrN3Ni
(453.02): calcd. C 55.68, H 4.45, N 9.28; found C 55.76, H 4.64, N
8.93.
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Catalysis
[Ni(η5-C5H5)(Cl)(IMesOPiv)] (1g): This complex was obtained from
nickelocene (347.0 mg, 1.839 mmol) and IMesOPiv·HCl (793.2 mg,
1.801 mmol) in THF (17.0 mL) with use of a reaction time of 2 h
at ambient temperature, yield 84%, red solid (852. 9 mg,
1.514 mmol). 1H NMR (400 MHz, CDCl3): δ = 7.11 (s, 2 H, m-
ArH), 7.08 (s, 2 H, m-ArH), 6.99 (s, 1 H, ImCH), 4.56 (s, 5 H,
C5H5), 2.43 (s, 3 H, p-ArCH3), 2.41 (s, 3 H, p-ArCH3), 2.23 (s, 6
H, o-ArCH3), 2.15 (s, 6 H, o-ArCH3), 1.02 [s, 9 H, C(CH3)3] ppm.
13C{1H} NMR (101 MHz, CDCl3): δ = 173.2 (CO2), 163.0 (NCN),
139.9 (ImCH), 139.5 (ArC), 139.3 (ArC), 137.0 (ArC), 136.7 (ArC),
136.2 (ArC), 132.5 (ArC), 129.3 (ArC), 129.2 (ArC), 111.3 (ImC–O),
92.3 (C5H5), 39.2 [C(CH3)3], 26.6 [C(CH3)3], 21.5 (p-ArCH3), 21.4
(p-ArCH3), 18.6 (o-ArCH3), 18.5 (o-ArCH3) ppm. EI MS: m/z
(58Ni, 35Cl, %) = 562 (81) [M]+, 497 (7) [M – Cp]+, 469 (100) [M –
NiCl]+. C31H37ClN2NiO2 (563.81): calcd. C 66.04, H 6.61, N 4.97;
found C 66.09, H 6.54, N 4.98.
General Procedure for Styrene Polymerization: Reactions were run
in duplicate as described previously.[6e] In a typical experiment, a
solution of MAO in toluene (1.80 mL, 2.70 mmol) was added to a
solution of complex 1a (4.2 mg, 0.0090 mmol) in toluene (0.45 mL)
and the resulting solution was stirred at room temp. for 30 min.
Styrene (neat, 15.5 mL, 0.135 mol) was added, and the reaction
mixture was placed in an oil bath maintained at 50 °C with vigor-
ous stirring for 3 h. After cooling to room temp. the reaction mix-
ture was poured into methanol (ca. 200 mL). The precipitated poly-
mer was filtered off, washed with methanol and dried under high
vacuum, yield 11.9 g (85%).
General Procedure for Methyl Methacrylate Polymerization: A solu-
tion of MAO in toluene (10wt.-%, Aldrich, 0.80 mL) was added to
a solution of [Ni(η5-C5H5)(Cl)(IMes2Me)] (1b, 6.4 mg, 13.0 μmol,
Al/Ni 100) in toluene (10.0 mL). The colour of the reaction mixture
changed immediately from red to dark brown and white fumes ap-
peared. After the system had been stirred for 30 min at room tem-
perature, methyl methacrylate was added (neat, 1.40 mL,
13.0 mmol). The resulting mixture was stirred vigorously for 20 h
at room temperature. Next, methanol (ca. 200 mL) was added to
the reaction mixture. The resulting precipitate was isolated by fil-
tration, washed with methanol and dried in vacuo. This crude prod-
uct was redissolved in CH2Cl2 (40 mL) and stirred overnight with
hydrochloric acid (10%, ca. 40 mL). After phase separation, the
organic solution was concentrated in vacuo and poured into meth-
anol (ca. 200 mL). The resulting white precipitate was isolated by
filtration, washed with methanol and dried in vacuo, yield 0.59 g,
44%.
[Ni(η5-C5H5)(Cl)(IMesO)] (1h): Complex [Ni(η5-C5H5)(Cl)-
(IMesOPiv)] (1g, 215.5 mg, 0.383 mmol) was dissolved in methanol
(9.5 mL) and the resulting solution was placed in an acetone/dry
ice cooling bath. Aqueous ammonia solution was added (1.1 mL,
14.72 mmol NH3), which was followed by an immediate change of
colour from red to orange-yellow. After 5 min the cooling bath was
removed and stirring was continued at room temperature for
10 min. Solvents were evaporated in vacuo and the remaining crude
product was dissolved in toluene and filtered through Celite. The
filtrate was concentrated in vacuo and applied to silica gel on a
chromatography column. Complex 1h was eluted with diethyl ether/
hexane [gradient from 1:4 to 1:1 (v/v)], yield 63%, red solid
(115.0 mg, 0.240 mmol). 1H NMR (400 MHz, CDCl3): δ = 7.11 (s,
2 H, m-ArH), 7.10 (s, 2 H, m-ArH), 4.63 (s, 5 H, C5H5), 4.31 (s, 2
H, CH2), 2.40 (s, 6 H, p-ArCH3), 2.35 (s, 6 H, o-ArCH3), 2.30 (s,
6 H, o-ArCH3) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 220.1
(NCN), 170.7 (C=O), 139.9 (ArC), 136.5 (ArC), 135.7 (ArC), 131.5
General Procedure for Suzuki Cross-Coupling: These reactions were
carried out similarly to those described previously.[14] 4Ј-Bromo-
acetophenone (55.1 mg, 0.277 mmol) and phenylboronic acid
(44 mg, 0.361 mmol) were dissolved in toluene (0.80 mL). K3PO4
(153 mg, 0.722 mmol) and tetradecane (internal standard, 7.0 μL)
were then added, followed by complex 1h (4.0 mg, 8.4 μmol). The
reaction was maintained at 90 °C for 1 h with stirring. The reaction
was quenched with cold water, and the reaction mixture was then
diluted with diethyl ether, washed with water and dried with anhy-
drous Na2SO4. The substrate conversion and selectivity were deter-
mined by GC.
(
ArC), 130.0 (ArC), 129.7 (ArC), 129.2 (ArC), 128.3 (ArC), 93.8
(C5H5), 54.5 (CH2), 21.5 (p-ArCH3), 21.4 (p-ArCH3), 18.9 (o-
ArCH3), 18.6 (o-ArCH3) ppm. EI MS: m/z (58Ni, 35Cl, %) = 478
(64) [M]+, 159 (100) [CpNiCl + H]+. C26H29ClN2NiO (479.69):
calcd. C 65.10, H 6.09, N 5.84; found C 65.21, H 6.12, N 5.81.
X-ray Diffraction Studies: Single crystals of 1d suitable for X-ray
measurements were obtained from hexanes/toluene at 4 °C. Single
crystals of 1g were obtained from toluene/hexanes at 4 °C. Single
crystals of 1h were grown from toluene/n-heptane at –20 °C. Dif-
fraction data were collected with an Agilent κ-CCD Gemini A Ul-
tra diffractometer and use of graphite-monochromated Mo-Kα ra-
diation at 100 K for compounds 1d and 1h and at 120(1) K for 1g.
Cell refinement and data collection, as well as data reduction and
analysis, were performed with the CrysAlisPRO software.[35] The
structures were solved by direct methods and subsequent Fourier-
difference synthesis with ShelXS and refined by full-matrix least-
squares against F2 with ShelXL within the Olex2 program
suite.[36,37] All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were introduced at calculated positions and re-
fined as riding atoms with isotropic displacement parameters re-
lated to that of the parent atoms. Data analysis was carried out by
use of Olex2 and Mercury.[38,39] Crystal data and structure refine-
ment parameters are given in Table S1 in the Supporting Infor-
mation.
Computational Details: The TEP parameters for ligands were ob-
tained by scaling the νCO(A1) calculated for [Ni(NHC)(CO)3] com-
plexes.[3d] Geometry optimizations and frequency calculations for
[Ni(NHC)(CO)3] were carried out by use of the mPW1PW91[40]
functional in the Gaussian suite of programs[41] with the 6–
311+G(2d) basis set for nickel and 6–311+G(d,p) for all other
atoms.[42] All optimized geometries were verified to have real har-
monic frequencies.
Acknowledgments
The authors thank the National Science Centre, Poland for finan-
cial support of this work (grant number DEC-2011/01/B/ST5/
06297). Financial support was also provided by Warsaw University
of Technology (P. A. G. and J. Z.). The authors thank Dr. M. Kosz-
ytkowska-Stawin´ska for helpful discussion on the synthesis and de-
Eur. J. Inorg. Chem. 2015, 5677–5686
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Martin, Y. Makida, S. Meiries, A. M. Z. Slawin, S. P. Nolan,
Organometallics 2013, 32, 6265–6270.
protection of the pivaloyl esters, and Dr. I. Wielgus for DLS mea-
surements and data interpretation. Calculations were carried out at
the Interdisciplinary Centre for Mathematical and Computational
Modelling (grant number G59-0).
[7] In a recent paper Wu et al. reported that these complexes can
be also obtained from monomeric NiI complexes [Ni(η5-
C5H5)(NHC)] by chemical oxidation followed by treatment
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Received: October 5, 2015
Published Online: November 9, 2015
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim