Axially chiral racemic half-sandwich nickel(ii) complexes by ring-closing metathesis

Article abstract of DOI:10.1039/C6DT04811K

A remarkable nickelacycle has been synthesised via olefin metathesis of the α,ω-diene complex [Ni(η⁵-C₅H₄R)(Br)(NHC)] (R = C(CH₃)₂CH₂CH═CH₂, NHC = 1-(6-hexenyl)-3-(2,4,6-trimethylphenyl)-imidazol-2-ylidene). Single-crystal X-ray analysis reveals a helical shape of the molecule and stretching of some interatomic distances in the Ni(ii) coordination sphere. The Cp-NHC tethered complex shows catalytic activity resembling that of the parent complexes.

Full text of DOI:10.1039/C6DT04811K

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Axially Chiral Racemic Half-Sandwich Nickel(II) Complex by Ring-
Closing Metathesis
Włodzimierz Buchowicz,a* Łukasz Banach,a Radosław Kamiński,b Piotr Buchalskia
lReceived 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
remarkable nickelacycle has been synthesised via olefin
metathesis of the α,ω-diene complex [Ni(η5-C5H4R)(Br)(NHC)] (R =
C(CH3)2CH2CH=CH2, NHC = 1-(6-hexenyl)-3-(2,4,6-trimethylphenyl)-
imidazol-2-ylidene). Single-crystal X-ray analysis reveals a helical
shape of the molecule and stretching of some interatomic
distances in the Ni(II) coordination sphere. The Cp-NHC tethered
complex shows catalytic activity resembling that of the parent
complexes.
Asymmetric catalysis with chiral derivatives of the ubiquitous
cyclopentadienyl (Cp) ligand remains an undeveloped area of
research.1 The diamagnetic complexes [Ni(Cp)(X)(NHC)] (X = Cl,
Br, I, alkyl; NHC = N-heterocyclic carbene), initially discovered
by Abernethy et al,.2 have found numerous applications as pre-
catalysts3 or robust substrates in synthesis of novel species.4
We have presumed that a convenient synthesis of a Cp-NHC
tethered nickel complex of this type could open up access to
novel axially chiral molecular frameworks with applications in
catalysis, chiral recognition, or separation.
Scheme 1. (a) General synthetic approaches to Cp-NHC complexes: (i)
double deprotonation, then MX2 (ii) suitable metathesis catalyst, this
work; (b) Examples of Ni(II) complexes prepared by route (i).6,7
Based on the precedence of complexes prepared from
bidentate pro-ligands that are 6- or 7-membered metalacycles
(Scheme 1b), we expected that complex
1
would readily
was
Transition-metal complexes bearing chelating Cp-NHC ligands
are usually prepared from bidentate pro-ligands and suitable
metal sources (Scheme 1, path i).5 Despite the considerable
variety of the reported so far complexes [Ni(Cp)(X)(NHC)], to
the best of our knowledge, few examples of closely related
indenyl-NHC Ni(II) complexes6 and C5Me4-NHC congeners7
have been synthesised by this route. We hypothesized that a
Cp-NHC tether could be readily formed via ring-closing
metathesis (RCM)8,9 in bis-alkenyl complexes (Scheme 1, path
ii). Herein, we explore this novel route to axially chiral half-
sandwich Ni(II) complexes.
undergo RCM to form the 10-membered ring. Thus,
treated with 6%mol of [Ru(=CHPh)Cl2(PCy3)(SIMes)]10 under
1
dilute conditions ([Ni] = 0.01 M) in refluxing toluene or CH2Cl2
(Scheme 2). Surprisingly, RCM in complex
1
proved to be
could be
ineffective.11 The presence of the expected product
3
determined only by means of mass spectrometry ([M]+ at m/z
496 (58Ni, 79Br)).
Plausible RCM substrates, the α,ω-diene complexes
1 and 2,
were prepared according to the standard procedure2 from the
appropriate imidazolium salts and 1,1'-bis-(alkenyl)nickelocene
(Scheme S1).
Scheme 2. RCM in complexes
6%mol, toluene or CH2Cl2, reflux.
1
and
2
: (i) [Ru(=CHPh)Cl2(PCy3)(SIMes)],
Fortunately, RCM in complex
2
catalysed by [Ru(=CHPh)Cl2
(PCy3)(SIMes)] proceeded smoothly in hot toluene to yield the
12-membered nickelacycle
4. Formation of the Cp-NHC tether
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renders all proton and carbon signals non-equivalent in the bond is possible, in
NMR spectra of
4
it is hindered due to the presence of the
4
, e.g. the Cp protons are represented by four macrocyclic ring. This makes the molecules of 4 axially chiral,
multiplets at δ 4.96, 4.86, 4.61, and 4.02 ppm. Moreover, which has been confirmed in solution by NMR (Figure 3).
1
detailed inspection of the H and 13C NMR spectra reveals the We have found a small fraction of the Z isomer (ca. 2-3%) in
presence of two isomeric products. This finding is most clearly the crystal of 4, which is consistent with the NMR spectra. The
evidenced by two Ni-Ccarbene resonances at δ 165.3 ppm olefin C10 and C11 atoms of the Z isomer are quite well visible
(major) and 165.8 ppm (minor). We identify these two on the residual density plot (Figure S2). Even though the full
products as, respectively, E and Z isomers of the C=C double refinement of both isomers was not possible, it appears that
bond formed in the metathesis reaction (see below). The two the geometry of the C=C double bond has no significant effect
isomers were obtained in a ca. 10:3 ratio.12
The X-ray diffraction studies allowed to derive the crystal The coordination geometry at the nickel centre is comparable
structures of complexes and . Attempts to obtain a in both cases (Figure 2). The most significant differences occur
sufficient quality crystal structure of complex were for the NHC ligand, which is oriented in the solid state in
moderately successful (Figure S1). Nevertheless, the complex in the opposite direction than in , as evidenced by
qualitative confirmation of the molecular structure of shows the Br1-Ni1-C16-N2 torsion angles of –112.6(2)° and 114.5 (2)°
its general similarity to that of . Consequently, a direct for and , respectively. This suggests that must adopt the
comparison of and allows to gain structural insights into less favourable conformation to facilitate the intramolecular
on the overall shape of the molecule.13
1
4
2
1
4
2
1
1
4
1
1
4
the differences and similarities of complexes before and after metathesis.
the intramolecular metathesis reaction.
(a)
Fig. 2. Overlay of 1 (red, labels in Italics) and 4 (green, labels
underlined).
Formation of the macrocyclic ring also induces considerable
distortions in the overall ligand arrangement. The large ring
tends to push slightly the Cp and NHC ligands away. This is
most clearly demonstrated by comparing the C16-Ni1-Cpcg
(b)
angles (131.89(8)° for
centre of gravity). Moreover, the average Ni1-CCp bond lengths
are approximately 0.02 Å larger for (2.157 Å on average)
than for (2.140 Å on average) with the Ni1-C5 bond being the
most significantly elongated (2.203 (2) Å for compared to
2.252(3) Å for ). The same trend is observed for the Ni1-C16
bonds (1.877(2) Å in and 1.884(3) Å in ).
In order to probe chirality of complex in solution, its NMR
spectra in the presence of an NMR chiral chemical shift
reagent ((R)-(-)-1-(9-anthryl)-2,2,2-trifluoroethanol)14
) were
1 and 132.14(8)° for 4; Cpcg = Cp ring
4
1
1
4
1
4
4
Fig. 1. Molecular structures of complexes 1 (a) and 4 (E isomer) (b).
Note that in 4 the C10-C11 (1.341(5) Å) bond is the double bond with E
(5
configuration. Atomic thermal motion is represented as ellipsoids (50%
probability level), and some hydrogen atoms are removed for clarity.
recorded (Figure 3). Doubling of the signals due to the
formation of two diastereoisomers was clearly observed for
the -NCH2 multiplet at δ 3.43 ppm, singlet of the o-CH3 group
at δ 2.74 ppm (E isomer), and singlet of the C5H4C(CH3)2 group
δ 1.26 ppm. This observation suggests that the chiral structure
Compounds
1
and
ꢀ
1 space groups, respectively. In both cases only one
4 crystallise in the monoclinic P21/c and
triclinic P
molecule is present in the asymmetric unit (Figure 1). As both
crystals are centrosymmetric, two enantiomers of each
molecule are present in the analysed crystal structures.
of
that the bromine ligand in
interaction with the polar reagent.
4
is also stable in this solution.15 Moreover, we conclude
is the preferred site of its
4
However, whereas in complex
1 rotation along the Ni1-C16
2 | J. Name., 2012, 00, 1-3
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Lupa, L. B. Jerzykiewicz, A. Makal, K. Woźniak, Eur. J. Inorg.
Chem., 2010, 648; f) A. M. Oertel, V. Ritleng, M. J. Chetcuti,
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Henrion, L. Brelot, C. Darcel, M. J. Chetcuti, J.-B. Sortais, V.
Ritleng, Adv. Synth. Catal., 2012, 354, 2619; h) A. R. Martin,
Y. Makida, S. Meiries, A. M. Z. Slawin, S. P. Nolan,
Organometallics, 2013, 32, 6265; i) Ł. Banach, P. A. Guńka, D.
Górska, M. Podlewska, J. Zachara, W. Buchowicz, Eur. J.
Inorg. Chem., 2015, 5677; j) A. Włodarska, A. Kozioł, M.
Dranka, J. Jurkowski, A. Pietrzykowski, J. Mol. Catal. A:
Chem., 2014, 395, 481; k) Ł. Banach, P. A. Guńka, W.
Buchowicz, Dalton Trans., 2016, 45, 8688.
Catalytic activity of racemic
4 was tested in three C-C bond
formation reactions (Table S1). In the case of Suzuki coupling,
the activity and selectivity to the desired cross-coupling
product was comparable to those of parent [Ni(Cp)(X)(NHC)]
complexes (entry No. 1). Polymerization of styrene in the
presence of
4 and methylalumoxane (MAO) yielded the
expected atactic poly-styrene (entries No. 2 and 3). Complex
4
with methyl methacrylate and MAO showed moderate activity
at 50°C (entry No. 4) and almost no activity at 20°C (entry No.
5).
4
Selected referrences: a) C. Radloff, F. E. Hahn, T. Pape, R.
Fröhlich, Dalton Trans., 2009, 7215; b) A. M. Oertel, J.
Freudenreich, J. Gein. V. Ritleng, L. F. Veiros, M. J. Chetcuti,
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Buchowicz, Ł. Banach, J. Conder, P. A. Guńka, D. Kubicki, P.
Buchalski, Dalton. Trans., 2014, 43, 5847; e) S. Pelties, D.
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Jubilee International Conference on Organometallic
Chemistry Celebratory Book, ed. A. J. L. Pombeiro, J. Wiley &
Sons, 2014, 10, 133–143.
In summary, we have shown that a Cp-NHC tether can be
readily formed via olefin metathesis in the Ni(II) coordination
sphere. The length of the alkenyl substituents, as well as the
dynamics of the system in solution, are both the key factors
determining the propensity of the intramolecular reaction. The
helical shape of
4 opens up prospects for its applications in
asymmetric catalysis after resolution of enantiomers.
W.B., Ł.B. and P.B. would like to thank the National Science
5
Centre
for
financial
support
(grant
DEC-
2011/01/B/ST5/06297).
6
7
8
H.-M. Sun, D.-M. Hu, Y.-S. Wang, Q. Shen, Y. Zhang, J.
Organomet. Chem. 2007, 692, 903.
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L. Postigo, R. Lopes, B. Royo, Dalton Trans., 2014, 43, 853.
a) E. B. Bauer, J. A. Gladysz, in: Handbook of Metathesis, ed.
R. H. Grubbs, Wiley-VCH, Weinheim, Germany, 2003; Vol. 2;
2.11, 403–431; b) A. Pietrzykowski, W. Buchowicz, in:
Advances in Organometallic Chemistry and Catalysis: The
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Pombeiro, J. Wiley & Sons, 2014, 12, 157–170; c) T. Fiedler, J.
A. Gladysz, in: Olefin Metathesis: Theory and Practice, ed. K.
Grela, John Wiley & Sons, 2014, 9, pp 311–328.
9
Selected references: a) M. Ogasawara, W.Y. Wu, S. Arae, K.
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Am. Chem. Soc., 2016, 138, 7649.
Fig. 3. (a) Selected sections of 1H NMR (500 MHz, C6D6) spectrum of
complex (E and Z isomers), signals assigned to the -NCH2 proton, o-
CH3 group and C5H4C(CH3)2 group (blue bottom line); (b) the same
4
sample after addition of ca. equimolar amount of 5 (brown top line).
Notes and references
10 M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett., 1999, 1,
953.
1
2
3
C. G. Newton, D. Kossler, N. Cramer, J. Am. Chem. Soc., 2016,
138, 3935, and references therein.
C. D. Abernethy, A. H. Cowley, R. A. Jones, J. Organomet.
Chem., 2000, 596, 3.
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S. Pasynkiewicz, A. Pęcherzewska, A. Pietrzykowski, J. Mol.
Catal. A: Chem., 2006, 257, 118; c) D. A. Malyshev, N. M.
Scott, N. Marion, E. D. Stevens, V. P. Ananikov, I. P.
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11 [Ru(=CHPh)Cl2(PCy3)2] also did not afford efficient RCM in
complex . Some related α,ω-diene complexes, e.g. [Ni(C5H4
1
CH2CH=CH2)(Br)(allyl-IMes)], [Ni(C5H4 CH2CH=CH2)(Cl) (allyl-
IMes)], [Ni(C5H4(CH2)2CH=CH2)(Br)(CH2=CH(CH2)2IMes)] (see
Figure S3) did not undergo RCM under conditions similar to
those for 2.
12 According to integration of doublets of the imidazole protons
at δ 6.32 ppm (J = 1.9 Hz, E) and δ 6.30 ppm (J = 1.9 Hz, Z).
13 A similar trend was observed for diansa-metallocenes: W.
Buchowicz, A. Furmańczyk, J. Zachara, M. Majchrzak, Dalton
Trans., 2014, 41, 9296-9271.
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14 S. Braun, H.-O. Kalinowski, S. Berger, 100 and More Basic
NMR Experiments: A Practical Course, VCH, Weinheim, 1996.
15 As noted by the Referee, taking into account the chirality and
E/Z isomers of the C=C bond, four stereoisomers of
formed in this reaction (Δ,Z; Δ,E; Λ,Z; Λ,E). Doubling of
signals for the Z isomers in the presence of was clearly
observed for the NCH2- signal (see Figure S4). Preliminary
attempts to hydrogenate the C=C bond in (H2/PtO2,
toluene, room temp., 48 h) resulted in incomplete
conversion of
4 are
5
4
4.
4 | J. Name., 2012, 00, 1-3
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