A T-shaped Ni[κ2-(CF2)4-] NHC complex: Unusual Csp3-F and M-CF bond functionalization reactions

Article abstract of DOI:10.1039/c5sc01886b

A T-shaped octafluoronickelacyclopentane-NHC complex is prepared and characterized. While the solid-state structure includes a weak isopropyl-CH₃ agostic interaction, the reactivity of this complex with Lewis- and Bronsted acids is clearly enhanced by its low coordination number. Reaction with Me₃SiOTf, for example, yielded a rare metal-heptafluorocyclobutyl complex whereas carboxylic acids gave substitution at the α-carbon and/or Ni-C^F bond protonolysis to afford thermally robust 4H-octafluorobutyl Ni complexes.

Full text of DOI:10.1039/c5sc01886b

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2
A T-shaped Ni[k -(CF ) –] NHC complex: unusual
2
4
F
Csp –F and M–C bond functionalization reactions†
3
Cite this: Chem. Sci., 2015, 6, 6392
Nicholas O. Andrella, Alexandre J. Sicard, Serge I. Gorelsky, Ilia Korobkov
and R. Tom Baker*
A T-shaped octafluoronickelacyclopentane–NHC complex is prepared and characterized. While the solid-
state structure includes a weak isopropyl-CH
3
agostic interaction, the reactivity of this complex with Lewis-
SiOTf, for
Received 25th May 2015
Accepted 27th July 2015
and Brønsted acids is clearly enhanced by its low coordination number. Reaction with Me
3
example, yielded a rare metal–heptafluorocyclobutyl complex whereas carboxylic acids gave substitution
DOI: 10.1039/c5sc01886b
F
at the a-carbon and/or Ni–C bond protonolysis to afford thermally robust 4H-octafluorobutyl Ni
www.rsc.org/chemicalscience
complexes.
F
excess isonitrile effected cleavage of the Ni–C bond (Scheme
Introduction
10
2b). With phosphite co-ligands, a remarkable hydrogenolysis
Fluorocarbons and their derivatives are valuable as refrigerants, reaction enables the selective catalytic hydrodimerization of
11
agrochemicals, unique solvents/surfactants and uo- TFE (Scheme 2c). As far as we know, this reaction is the only
ropharmaceuticals, with annual sales of the latter alone in the example of a peruoro-metallacyclopentane participating in a
1
12
billions of dollars. As the demand for uorinated chemicals catalytic cycle.
has increased, so too have synthetic methods for introducing
Our approach to metallacycle functionalization hinges on
2–4
13

uorine and uorocarbon groups. Despite recent advances, the reactivity of metal-activated Ca–F bonds wherein we
transition metal-mediated/-catalyzed routes are rare in replace a C–F bond by C–Nu vs. the current paradigm C–L (Nu ¼
comparison to the well-developed organometallic chemistry of nucleophile, L ¼ ligand). Using N-heterocyclic carbenes
5
14
hydrocarbons. The challenge rests in the stability of metal– (NHCs), we aimed to access low-coordinate PNCPs wherein the
F
6
peruoroalkyl (M–R ) bonds, relative to metal–alkyl bonds. M– strong M–CNHC bond may also prevent ligand migration to Ca.
C bonds are typically inert to processes such as insertion/alkyl There is considerable precedent for such an approach to low-
F
7
15
migration reactions, vital to metal-mediated catalytic cycles.
Moreover, C–F bonds are stronger than C–H bonds,
another obstacle to metal-based approaches.
coordinate metal complexes.
posing prepared a two-coordinate nickel–imido complex bearing the
exceptionally bulky IPr* ligand (analog of IPr with 2,6-bis(di-
Hillhouse and coworkers
1
a,b
We
complexes (PNCPs) as platforms for functionalized uorocar- phenyl). Similarly, Miyazaki and coworkers synthesized a T-
bons with an initial focus on fundamental stoichiometric shaped three-coordinate nickel(I) chloride species [Ni(IPr) Cl]
are
investigating
peruoronickelacyclopentane phenyl-methyl)phenyl groups instead of 2,6-diisopropyl-
16
2
1
7
reactions. PNCPs have been synthesized previously by reaction by treatment of two-coordinate [Ni(IPr)
] with aryl chlorides.
2
0
of tetrauoroethylene (TFE, CF
2
]CF
2
) with Ni complexes. The Also, Hartwig et al. synthesized a low-valent, three-coordinate
displacement of P ligands by bidentate ligands has also been palladium(II) norbornyl species [Pd(SIPr)(NHAr)(Nor)], which
8
reported (Scheme 1).
underwent facile C–N bond reductive elimination when
18
To date, reports concerning the reactivity of PNCPs are heated.
sparse: Burch and co-workers found that Lewis acidic BF3
effects uoride abstraction from Ca and phosphine migration uorometallacycles undergo facile Csp
to the activated carbon (Scheme 2a). Extending this reaction to cleavage as well as Ca-functionalization. We also demonstrate
In this report we show that low-coordinate NHC Ni per-
F
3
–F and M–C bond
9
9
the unsymmetrical P^S chelate, we showed that treatment with
Department of Chemistry and Centre for Catalysis Research and Innovation(CCRI),
University of Ottawa, 30 Marie Curie, Ottawa, ON K1N 6N5 Canada
†
Electronic supplementary information (ESI) available: Experimental details,
NMR spectra and X-ray crystallographic information. CCDC les 968465 (2),
68466 (3), 968467 (4a), 1028645 (5c) and 1412522 (3$H O); For ESI and
crystallographic data in CIF or other electronic format see DOI:
0.1039/c5sc01886b
9
2
Scheme 1 Synthesis of perfluoronickelacyclopentanes.
1
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spectrum of 3 in C
6
D
6
is consistent with ideal C2v symmetry at
room temperature, with only two distinct singlet resonances at
101.9 (Fa) and ꢀ138.6 ppm (Fb). While these resonances both
broaden signicantly at 213 K, it is apparent that the T-ip
ꢀ
20
interconversion encounters only a small energy barrier. To
conrm this, we carried out DFT calculations (at the B3LYP/
TZVP level with and without the empirical dispersion correction
21
of Grimme). Intriguingly, the calculations reveal two spin-
singlet structures with a very small energy difference (DG298 K
¼
ꢀ
1
0
.0–1.5 kcal mol ). The rst computed structure coincides well
with the observed solid-state structure of 3; the 3-center bond
order index between the Ni and the corresponding C–H bond of
0
.05 is much less than 8/27 (ꢁ0.3), the maximum possible value
for a 3-center 2-electron bond. As a result, the Mayer valence
0
Scheme
2
Previously reported reactivity of perfluoronickelacy- index for Ni in structure 3 is only 3.09. The second structure (3 )
3
clopentanes.
features a weak h interaction between the aryl group of the
NHC ligand at the 4th coordination site of the Ni atom (Fig. 1b).
Mayer bond orders for the corresponding three Ni–C interac-
the rst migration of a uoroalkyl to a reactive carbon center. tions are in the 0.02–0.05 range, with a total bond order of 0.09.
This is signicantly different from the reactivity previously This suggests a semi-bidentate binding mode for the class of
9
observed for phosphine Ni peruorometallacycle complexes.
NHC ligands possessing pendant aryl groups. From calculations
0
with the dispersion correction, structure 3 has the same Gibbs
0
free energy as 3. Without the dispersion correction, structure 3
Results and discussion
ꢀ1
is actually 1.5 kcal mol lower in energy than 3. The 3-coor-
1
0
dinate structure with trigonal coordination around Ni and
Starting from bis(phosphite) PNCPs (1a and b) we were able to
cleanly synthesize coordinatively-saturated or -unsaturated
nickel peruorometallacycles. Thus, 1a reacts smoothly with 1
equiv. of ItBu (ItBu ¼ 1,3-di-tert-butylimidazol-2-ylidene) to
afford the NHC/phosphite product 2 (Scheme 3, top; X-ray
symmetric binding of the C F ligand is a transition state with a
4
8
ǂ
ꢀ1
low energy (DG
¼ 2.1 kcal mol relative to 3). Thus, it is
298 K
3
clear that cleavage of the weak agostic and/or h -aryl bond is
facile and allows for rapid reorientation of the ligands around
0
19
the Ni center. Attempts to obtain evidence for structure 3 by low
structural characterization presented in ESI†). Signicantly,
the reaction between the larger SIPr ligand [SIPr ¼ 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene] and a nickel metalla-
cycle with sterically-demanding co-ligands (1b) results in
displacement of both phosphite ligands, yielding the pseudo-
three-coordinate/14e-Ni(II) metallacycle 3 (Scheme 3, bottom).
The molecular structure of complex 3, as determined by
single crystal X-ray diffraction, exhibits a T-shaped coordination
about the Ni and features a weak agostic interaction with the
temperature NMR were frustrated by dynamic processes asso-
ciated with the T-ip and hindered rotations about the M–C and
perhaps N–C bonds.
The HOMO of 3 (3 ¼ ꢀ6.01 eV; Fig. 2, le) is localized on the
Ni (87%), primarily from a dz
orbital contribution (71%).
2
Lower-lying orbitals display interactions between metal d , d
xz
yz
22
orbitals and the p-system of the aryl group. The LUMO (3 ¼
ꢀ
1.96 eV; Fig. 3, right) is an anti-bonding combination of the
F
metal dx2ꢀy2 orbital (total Ni character of 45%) with the p-donor
˚
isopropyl methyl group (Ni–C ¼ 2.757(1) A; compare Ni–C
˚
bond distance trans to the NHC (1.934(1) A) with that trans to
1
9
˚
the agostic interaction (1.875(1) A) (Fig. 1a). The F NMR
Fig. 1 (A) ORTEP representation of the molecular structure of 3.
Thermal-ellipsoid probabilities are set to 35% with hydrogen atoms
omitted for clarity. The Ni–C(1) distance is 1.854(2) A˚ . (B) Optimized
0
structure of low energy Ni–aryl isomer 3 ; Ni–Caryl distances are ¼
Scheme
3 Synthesis of NHC perfluoronickelacyclopentane 2.818, 3.329, 3.379, 4.166, 4.204, 4.543 A˚ . The Ni–C(17) distance is
complexes.
1.989 A˚ .
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was identied as a major product, suggesting formation of a Ni–
F thermolysis co-product.
The distorted square planar structure of complex 4a (Fig. 3)
features a bidentate triate ligand which can also likely access
1
19
the k -mode in solution as evidenced by the simple F NMR
27
spectrum and observed tendency to eliminate. The per-

uorocyclobutyl ring is nearly planar, the Ni–C bond is short
˚
[
1.890(2) A] due to the weak s-trans inuence ligand, and the
˚
Fig. 2 The HOMO (left) and LUMO (right) of 3. Isosurface values of Ca–F bond distance (1.384(3) A) is considerably longer than the
0
.04 au are used.
˚
other C–F bonds (average of 1.33 A).
The reactivity enhancement offered by low-coordinate 3 is
evidenced by the sluggish reaction of 4-coordinate complex 2
orbitals of the NHC and C F ligands. Thus, reactivity of the with TMSOTf to give a mixture of unidentied products. Indeed,
4
8
ꢂ
M–C bond is likely under orbital control and arises from an monitoring the reaction of 3 and TMSOTf at ꢀ25 C allowed for
interaction with the HOMO of 3. In contrast, C–F bond activa- the identication of a Ni–C F intermediate 5a apparently
4
7
1
9
tion is likely a combination of orbital and charge control with a containing a Ca–OTf linkage (triate CF F NMR resonance is
3
5
slant towards the latter as the hardness of the Lewis acid coupled to Ca–F: J ¼ 11 Hz). This is in contrast to previous
FF
23
9
increases.
suggestions of a metal uorocarbene intermediate (Scheme 5).
Initial studies on the C–F bond activation reactions of 3 are
Having established that the NHC ligand remains bound to
promising in the context of synthesizing functionalized uoro- the metal upon uoride-abstraction from 3, we shied our focus
carbons by metal-mediated approaches. Firstly, when 3 is to C–F bond functionalization using Brønsted acids. Treatment
2
8a
treated with the Lewis acid TMSOTf (TMS ¼ Me Si, OTf ¼ of 3 with triuoroacetic acid [TFA; pK ¼ 3.4 (DMSO)] gives HF
3
a
F
SO CF ), a-uoride-abstraction, accompanied by Ni–C bond and the more stable (vs. 5a) triuoroacetate-substituted metal-
cleavage and C –C bond formation, furnishes a rare per- lacycle 5b that could be characterized spectroscopically at room
3
3
F
F
24

uorocyclobutyl complex 4a (Scheme 4, 75% isolated yield).
temperature (Scheme 6). Nonetheless, accompanying formation
The driving force behind this transformation is likely related to of peruoro-cyclobutene, presumably formed via an analogous
the triate leaving group ability and the formation of a strong structure to 4a, led us to move to weaker Brønsted acids.
2
5
28b
C–C bond. Importantly, the NHC remains bound to the nickel Remarkably, reaction of 3 with acetic acid [pK ¼ 12 (DMSO)]
a
atom (i.e., does not migrate to Ca), potentially opening new (Scheme 6, bottom) yielded the stable ester metallacycle 5c (30%
pathways to functionalized uorocarbon derivatives. Upon isolated yield) as well as the Ni–C bond cleavage product 6a in a
F
ꢂ
28c
heating complex 4a (80 C in C D , 24 h), peruorocyclobutene 1 : 1 ratio. At a similar acidity [pKa ¼ 11 (DMSO)]
but
6
6
is produced, presumably via a b-uoride elimination mecha- increased steric bulk, 2,4,6-trimethyl-benzoic acid gave a 10 : 1
nism, although the metal-containing co-product(s) have not yet mixture favouring the ring cleavage product, 6b.
26
been identied. Interestingly, a single OTf containing product
The molecular structure of 5c features similar Ni–C bond
1
9
can be discerned by F NMR (ꢀ93.37 ppm) but a Ni–F signal distances (1.895(7) vs. 1.896(6) A˚ ) and a distorted square planar
1
could not be located. The H NMR shows that the NHC remains coordination (Fig. 4). The functionalized heptauoro-metal-
intact. Upon addition of PPh to the reaction mixture, PPh F
lacyclopentane ring is puckered with the smallest C–C bond
3
3 2
˚
distance being C(32)a–C(39)b [1.49(1) A]. The carbonyl oxygen
˚
completes the nickel coordination sphere (Ni–O1 ¼ 1.969(4) A).
1
9
The F NMR spectra of 5a–d are very similar and support our
original proposal for the low temperature intermediate 5a in the
Scheme 4 Synthesis and decomposition of 4a.
Fig. 3 ORTEP representation of the molecular structure of 4a with
thermal ellipsoid probabilities set to 30% and hydrogen atoms omitted
for clarity. The Ni–C(1) distance is 1.854(2) A˚ .
Scheme 5 Possible intermediates in the reaction of 3 with TMSOTf.
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F
Scheme 7 Proposed reaction pathways for C–F activation vs. Ni–R
protonolysis.
Fig. 4 ORTEP representation of the molecular structure of 5c with
thermal ellipsoid probabilities set to 30% and hydrogen atoms omitted
for clarity. The Ni–C(1) distance is 1.928(2) A˚ .
Conclusions
1
9
In summary, we have prepared the rst NHC–per-
reaction of 3 with TMSOTf. The Ca–F F chemical shis of the
functionalized carbon, (ꢀ117.2 and ꢀ119.2 ppm) can be
compared with those of the phosphonium analogs (ꢀ115.6 and

uorometallacyclopentane complexes and exploited the bulky
SIPr ligand to stabilize a pseudo-three-coordinate nickelacycle,
. Importantly, 3 undergoes Ca–F abstraction reactions without
3
ꢀ
117.8 ppm) shown in Scheme 2.
migration of the NHC ligand. Instead, we see an unprecedented
As expected, the ring-opened products 6a and b display
1
9
migration of the uoroalkyl to the reactive carbon center, giving
nearly identical F NMR chemical shi patterns with the Cg–
and Cd–F resonances distinguished by F–H coupling of 6 and 52
Hz, respectively. These unique complexes have been identied
as their potassium cation adducts using ESI-MS (747.2 g mol
and 851.4 g mol respectively) and are surprisingly inert to
F
rise to the novel peruorocyclobutyl complex via Ni–C bond
cleavage. More importantly, the low-coordinate nature of 3
allows for ring functionalization. Strong acids favour selective
Ca functionalization, but the resulting products are unstable
with respect to competing metallacycle ring contraction and
elimination of peruorocyclobutene. With less acidic reagents
stable ring-functionalized products are formed but a competing
ꢀ1
ꢀ1
ꢂ
thermolysis at 80 C in C
6
D
6
for 20 h.
Considering the importance of esters as synthons in organic
transformations this C–O bond-forming reaction 3 / 5 is very
appealing from the standpoint of synthesizing functionalized
uorocarbons. As such, understanding competing pathways for
M–C vs. C–F bond cleavage would be valuable. Viable reaction
pathways can be considered as proceeding via either 5- or 6-
membered transition states (Scheme 7). The selective HF
elimination observed for TFA, is eroded as Ni–C bond proto-
nolysis (orbital control) competes using acids of intermediate
29
F
Ni–C bond cleavage pathway comes into play and dominates
for bulkier carboxylic acids. These are the rst examples of
selective functionalization of a PNCP and synthesis of thermally
stable Ni–C F H complexes. These results are encouraging in
4 8
the context of developing metallacycle-based routes to func-
tionalized uorocarbons.
Ongoing work is focused on (a) expanding the scope of ring
functionalization substrates suitable for reactions with 3 and (b)
reductive (see Scheme 2, above) and oxidative approaches for
removing the functionalized uorocarbon fragments from the

30
31
acidity (e.g. pK
a
ꢁ11). With the bulkier trimethylbenzoic acid,
kinetic acidity factors in the tighter 5-membered ring transition
32
state could severely limit HF elimination.
metal. Preliminary results of the hydrogenolysis of compound 3
ꢂ
indicate enhanced reactivity towards H
2
(i.e., at 7 psig and 25 C)
11a
vs. reported 4-coordinate phosphite variants. However, loss of
selectivity is observed with the synthesis of two distinct prod-
ucts. Full details of these results will be published in due time.
33
Acknowledgements
We thank the NSERC and the Canada Research Chairs program
for generous nancial support and the University of Ottawa,
Foundation for Innovation and Ontario Ministry of Economic
Development and Innovation for essential infrastructure. N. O. A.
gratefully acknowledges support from the province of Ontario,
NSERC and the University of Ottawa (OGS and CGS-M/D).
Scheme 6 Reaction of 3 with trifluoroacetic, acetic and 2,4,6-trime-
thylbenzoic acids.
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