New insight into reactions of Ni(S2C2(CF 3)2)2 with simple alkenes: Alkene adduct versus dihydrodithiin product selectivity is controlled by [Ni(S2C 2(CF3)2

Article abstract of DOI:10.1021/ja063030w

The nickel bis(dithiolene) complex Ni(S₂C₂(CF₃)₂)₂ employs its sulfur centers in reactions with alkenes, and stable interligand S-bonded alkene adducts can be formed. The present study shows that the

Full text of DOI:10.1021/ja063030w

Published on Web 08/04/2006
New Insight into Reactions of Ni(S2C2(CF3)2)2 with Simple Alkenes: Alkene
Adduct versus Dihydrodithiin Product Selectivity Is Controlled by
[Ni(S2C2(CF3)2)2]- Anion
Daniel J. Harrison,† Neilson Nguyen,† Alan J. Lough,‡ and Ulrich Fekl*,†
Department of Chemical and Physical Sciences, UTM, UniVersity of Toronto, 3359 Mississauga Road N,
Mississauga, Ontario, Canada L5L 1C6, and X-ray Crystallography Lab, UniVersity of Toronto,
80 St. George Street, Toronto, Ontario, Canada M5S 3H6
Received May 1, 2006; E-mail: ufekl@utm.utoronto.ca
Scheme 1
Chemical technology heavily relies on alkenes (olefins) as feed-
stocks and synthetic intermediates, and their utilization often involves
transition metals.1 In most metal-mediated reactions, the alkene co-
ordinates to the metal in at least one step of the reaction sequence,
rendering such systems easily poisoned by impurities in crude
alkenes, which often bind more strongly to the metal than the
alkene. Metal complexes that bind alkenes through chelated ligands,
instead of at the metal center, should be more resistant to deactiva-
tion by contaminants, as emphasized by Wang and Stiefel in 2001.2
It was reported that neutral nickel bis(dithiolene) complexes (1R in
Scheme 1, R ) CN, CF3) form adducts with linear alkenes, such
as ethylene or 1-hexene, where the alkene binds through the ligand
S-atoms (Scheme 1a, structure 2R(alkene) was suggested), even in
the presence of H2S or CO. This unusual tolerance toward poisoning
led the authors to propose an olefin purification scheme in which
the dithiolene-bound alkene is released by electrochemical reduction
and 1R is regenerated by oxidation (Scheme 1b).2
Understanding the mechanism of alkene binding to nickel (bis)-
dithiolenes will be crucial for tuning the selectivity of such systems
and for developing novel applications, but key mechanistic questions
are unresolved. Orbital symmetry should prevent compounds 1R,
in their singlet ground state, from forming interligand alkene adducts
(Scheme 1a, 2R(alkene)) in a concerted, synfacial manner.3,4 In fact,
early structural proposals suggested intraligand binding (Scheme
1a, 3R(alkene)).5 Crystallographic characterization of an adduct of
1CF3 with norbornadiene,6 however, showed interligand binding, in
contrast to orbital symmetry predictions. To address the orbital
symmetry problem, a DFT contribution suggested a two-step
mechanism for alkene addition, in which the alkene approaches
the dithiolene complex in an antifacial fashion, giving an intermedi-
ate with tetrahedrally distorted geometry at nickel.7
kene reaction mixtures, such that adducts 2CF3(alkene) can be ob-
tained as dominant products, rather than DHD/MD products. We
also provide unambiguous evidence that ethylene, in the stable ad-
duct, binds to 1CF3 in a 1:1, interligand fashion (Scheme 1a, 2CF3
(ethylene)). Finally, the mechanistic implications of these results
are discussed.
Reactions between compound 1CF3 and alkenes (ethylene and 1-
hexene) were followed by NMR spectroscopy (19F and 1H). To our
knowledge, NMR methods have not been used previously to probe
the reactions of metal bis(dithiolenes) with simple, terminal alkenes.
Experiments were conducted under strictly water-free conditions,
since we found that trace water significantly affects the reactivity
in these systems (see below). Reacting compound 1CF3 (8.8 mM)
with ethylene (∼25 mM) for 2.5 h at ambient temperature in CDCl3
formed DHD(H) (Scheme 1a) in 70% yield.8,9 Upon overnight reac-
tion, conversion to DHD(H)/MD products was near-quantitative
(90%). DHD derivatives were identified by comparison of 19F and 1H
data with those of independently prepared samples.10 No adduct (2CF3
(ethylene)) was observed in the reaction between 1CF3 and ethylene.
We then investigated the effect of having anion [1CF3]
- present.
Compound 1CF3 quantitatively oxidizes up to 1 equiv of decam-
ethylferrocene (Fc*) to give the ion pair [Fc*]+[1CF3]-. When we
Reactions of simple alkenes (linear monoolefins) with 1R (R )
CF3, CN) were claimed to proceed selectively to give alkene
adducts, and competitive pathways leading to other products were
employed Fc* to reduce 22% of 1CF3 (originally 9.7 mM), the
-
resulting 1CF3/[1CF3
]
(78:22) mixture reacted (25 °C, 16 h, in
not mentioned.2 However, dithiolenes with aryl substituents (1aryl
,
CDCl3) with excess ethylene to afford 2CF3(ethylene) in 55% yield
and DHD(H) in 24% yield.8,9 Compound 2CF3(ethylene) was
e.g., aryl ) C6H4(p-F)) react with norbornadiene (NBD) to give,
in addition to 2aryl(NBD), a significant amount of dihydrodithiin
(DHD)/metal decomposition (MD) derivatives, likely via orbitally
allowed 3aryl(NBD) (see Scheme 1a).3c
In the present study, we reexamined the reactions of 1CF3 with
simple alkenes. We found that neutral 1CF3 reacts with ethylene or
1-hexene to give primarily DHD/MD products and little or no al-
kene adduct (Scheme 1a). Clearly, if 1CF3 is to be useful in olefin pur-
ification schemes,2 this lack of selectivity for stable alkene adducts
needs to be addressed. We report that the product distribution can
be impacted dramatically by adding monoanionic [1CF3]- to 1CF3/al-
1
identified by its AA′BB′ pattern in the H NMR spectrum and by
its two sets of fluorine signals (Figure 1).11 Photolysis of 2CF3
-
12
(ethylene) released the alkene, regenerating 1CF3
,
in accord with
the expected photochemistry13 of such adducts.
Thus, a clean, high-yielding synthesis of 2CF3(ethylene) requires
a considerable concentration of [1CF3]-. We prepared a 1:1 mixture
of 1CF3 and [Fc*]+[1CF3
]
- (both species ∼10 mM in CH2Cl2), and
reaction (50 °C, 1 h) with excess ethylene formed 2CF3(ethylene)
in 71% yield (isolated). X-ray quality crystals were grown under
ethylene atmosphere from benzene. The structure (Figure 1) clearly
shows interligand adduct (2CF3(ethylene)). While a crystal structure
of 2CF3(NBD) has been reported,6 the present structure is the first
example involving a simple, acyclic alkene. Notable is a bowl-
† Department of Chemical and Physical Sciences.
‡ X-ray Crystallography Lab.
9
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J. AM. CHEM. SOC. 2006, 128, 11026-11027
10.1021/ja063030w CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
-
of [1CF3
1
]
are produced in the reaction between NBD and neutral
Figure 1. 19F NMR spectrum (470 MHz, CDCl3, 343 K)11 of 2CF3(C2H4)
(left) and its X-ray crystal structure (right, 20% probability ellipsoids).
Distances and angles (Å, deg): Ni1-S1, 2.163(1); Ni1-S2, 2.159(1); Ni1-
S3, 2.165(1); Ni1-S4, 2.162(1); S1-C5, 1.842(3); S3-C6, 1.854(3); C1-
C2 ≈ C3-C4, 1.356(4); S1-Ni1-S2, 92.07(3); S3-Ni1-S4, 92.14(3).
CF3, as observed by Geiger.16 It is possible that [1CF3]- also reacts
directly with alkenes to give the intermediate21 [2CF3(alkene)]-
which is oxidized by 1CF3 to yield 2CF3(alkene) (Scheme 2b).
We conclude that 1CF3 reacts with simple alkenes to give, pref-
-
erentially, DHD and metal decomposition products, unless [1CF3
]
type distortion (centroidC1,C2-Ni1-centroidC3,C4 ) 22°), similar to
2
19.7°), leading to C1 molecular symmetry (Figure 1).
is present. Our results suggest that the mechanism of stable adduct
formation (2CF3(alkene)) is more complicated than previously thought
and probably involves charged intermediates. Interestingly, additives
such as water or organic sulfides favor larger adduct/DHD ratios,
very likely because these species are sufficiently reducing to
produce [1CF3]-.17,18 An electrochemical process, as originally
envisioned by Wang and Stiefel,2 might indeed be efficient for olefin
purification: if the charge-neutral metal complex is generated in
situ by oxidation from the monoanion, in the presence of the alkene,
a sufficient amount of monoanion will be present to influence the
reactivity toward interligand adducts. More detailed studies of this
potentially industrially useful reaction are in progress.
CF3(NBD).6 Also, the C2H4 bridge is twisted (torsion S1C5C6S3 )
1-Hexene also reacts with 1CF3 to give mainly decomposition
products in the absence of [1CF3]-. Reaction between 1CF3 (14 mM)
and 1-hexene (0.14 M) (25 °C, 1.9 h, in CDCl3), afforded some
(12% yield) interligand adduct, of type 2CF3(1-hexene), even if no
reductant was added, but DHD(nBu) was the main product (73%
yield).8,9,14 When we reduced 5.8% of 1CF3 (14 mM initially) with
Fc*, the ∼94:6 mixture of 1CF3/[Fc*]+[1CF3]- yielded, upon reaction
with 1-hexene, stable adducts (61% yield) and some DHD(nBu)
(27% yield) (25 °C, 2.9 h, in CDCl3).
The 1-hexene reactions gave much more complex NMR spectra
than the ethylene reactions, because of the C1 symmetry of the
resulting adducts and endo/exo isomerism. We observed three
different types of 1-hexene adducts, in the approximate ratio 1:2:
4, and propose that the two major species are interligand adducts
Acknowledgment. Funding by the Natural Science and Engi-
neering Research Council (NSERC) of Canada, the Canadian
Foundation for Innovation, the Ontario Research Fund (ORF) and
the University of Toronto (Connaught Foundation) is gratefully
acknowledged. We thank Mr. Aiman Alak for lab assistance.
Supporting Information Available: Experimental details (PDF)
and crystallographic information for 2CF3(ethylene) (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
2
CF3(1-hexene), as endo and exo isomers. The minor species might
be binuclear, with two nickel bis(dithiolene) units connected by an
alkyl bridge (Supporting Infoformation). Reduction with Na in THF-
d8 released 1-hexene from the equilibrium mixture of alkene
adducts, in agreement with reports2,6,15,16 about electrochemical
reversibility of alkene binding to sulfur centers. For a selectively
deuterated alkene, (E)-1-D-1-hexene, stereospecific binding was
observed, and photolysis released unchanged (E)-1-D-1-hexene.
In addition to Fc*, additives such as water, acetone, organic
sulfides, and trialkylphosphines also favor larger adduct/DHD ratios.
Water17 and acetone18 react with 1CF3 to produce [1CF3]-. Presum-
References
(1) Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 3rd ed.;
VCH: Weinheim, Germany, 1997.
(2) Wang, K.; Stiefel, E. I. Science 2001, 291, 106.
(3) (a) Baker, J. R.; Hermann, A.; Wing, R. M. J. Am. Chem. Soc. 1971, 93,
6486. (b) Herman, A.; Wing, R. M. J. Organomet. Chem. 1973, 63, 441.
(c) Kajitani, M.; Kohara, M.; Kitayama, T.; Akiyama, T.; Sugimori, A. J.
Phys. Org. Chem. 1989, 2, 131.
(4) Compounds 1R might possibly react in their triplet state, which was
computed (DFT, for 1Me) to be uphill by 60 kJ/mol in ground-state
geometry, as proposed in Szilagyi, R. K.; Lim, B. S.; Glaser, T.; Holm,
R. H.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc.
2003, 125, 9158.
ably, SR2 and PR3 also reduce 1CF3. If the adduct/DHD selectivity
reversal is due to [1CF3]-, then independently synthesized18 [1CF3
-
]
(5) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965, 87, 1483.
(6) Wing, R. M.; Tustin, G. W.; Okamura, W. H. J. Am. Chem. Soc. 1970,
92, 1935.
should induce the same effect. Indeed, 1CF3 (19 mM) reacted with
1-hexene (0.15 M) to give 2CF3(1-hexene) as the major product (2CF3
-
(1-hexene)/DHD(nBu) ≈ 3:1) when [NEt4]+[1CF3
]
- (0.9 mM) was
(7) Fan, Y.; Hall, M. B. J. Am. Chem. Soc. 2002, 124, 12076.
(8) Yields based on 1CF3, using product integration (1H/19F NMR) versus
internal standard (see Supp. Info.). For reactions where a fraction of 1CF3
was reduced with Fc*, yields are based on 1CF3 present after reduction.
(9) When DHD(R′) was formed, corresponding amounts of MD products were
visible as a manifold of 19F resonances, for ill-defined (Ni(S2C2(CF3)2))x.
(10) Krespan, C. G.; McKusick, B. C. J. Am. Chem. Soc. 1961, 83, 3438.
(11) Spectra are temperature-dependent (Supp. Info. (Figure S6)).
(12) Expectedly, some 1CF3 reacted with the released alkene to produce DHD-
(H) and associated decomposition products.
-
present. These observations confirm the importance of [1CF3
] in
determining product selectivity.
Full understanding of the reaction mechanism will require very
detailed kinetic studies, but preliminary kinetics,19 for 1-hexene,
-
provide some insight. In the simplest mechanistic scenario, [1CF3
]
catalyzes the production of 2CF3(alkene). If [1CF3]- accelerates one
(13) Kunkely, H.; Vogler, A. Inorg. Chim. Acta 2001, 319, 183.
of two parallel reactions, then kobs for decay of 1CF3 should increase
(14) Similarly, large DHD(nBu)/2CF3(1-hexene) product ratios were observed
-
-
with increasing [1CF3
]
concentration. However, we observed the
in CD2Cl2 and toluene-d8 when [1CF3
]
was not present.
(15) Goodman, J. T.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 5017.
(16) Geiger, W. E. Inorg. Chem. 2002, 41, 136.
opposite: While higher anion concentrations favor alkene adducts
-
over decomposition products, [1CF3
]
actually slows the decay of
(17) UV-vis data show reduction of 1CF3 by H2O (Supp. Info., S21 and S22).
(18) 1CF3 is reduced by ketones, nitriles, amides, amines, and DMSO: Davison,
A.; Edelstein, N.; Holm, R. H.; Maki, A. H. Inorg. Chem. 1963, 2, 1227.
(19) In CDCl3 or toluene-d8 in the presence of excess 1-hexene. In the absence
of [1CF3], decay of 1CF3 is pseudo-first-order, as reported in ref 2.
(20) While adducts of type 2CF3(alkene) are not easily oxidized by 1CF3 (ref
16), compounds 3CF3(alkene) are be expected to be more easily oxidized
since they contain an ene-1,2-dithiolate unit (Supp. Info. (S25)).
1CF3 (time traces in the Supporting Information). Therefore, if
[1CF3]- catalyzes formation of 2CF3(alkene), it must simultaneously
inhibit DHD production to account for the decreased rate. The path-
way leading to decomposition products may be suppressed as shown
in Scheme 2. In this proposal, the symmetry-allowed adduct 3CF3
-
(21) This proposal is novel, and transition states for this mechanism have not
(alkene) quickly degrades to DHD/MD products upon oxidation by
yet been computed. However, in the context of alkene release, the
20
-
-
thermodynamics of [2CN(ethylene)]- versus [1CN
]
+ ethylene were
1CF3
.
Higher [1CF3] concentrations shift the 3CF3(alkene)/[3CF3-
computed. (ref 7): ∆H ) 38 kJ/mol (not prohibitively uphill).
(alkene)]+ equilibrium to the left, slowing the decomposition path-
way (Scheme 2a). Such a proposal may explain why small amounts
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