C-F Bond activation at Ni(0) and simple reactions of square planar Ni(II) fluoride complexes

Article abstract of DOI:10.1039/b510052f

The reaction of Ni(COD)₂ (COD = 1,5-cyclooctadiene) with triethylphosphine and pentafluoropyridine in hexane has been shown previously to yield trans-[NiF(2-C₅NF₄)(PEt₃)₂] (1a) with a preference for reaction at the 2-position of the heteroaromatic. The corresponding reaction with 2,3,5,6-tetrafluoropyridine was shown to yield trans-[NiF(2-C₅NF₃H)(PEt₃)₂] (1b). In this paper, we show that reaction of Ni(COD)₂ with triethylphosphine and pentafluoropyridine in THF yields a mixture of 1a and 1b. Competition reactions of Ni(COD)₂ with triethylphosphine in the presence of mixtures of heteroaromatics in hexane reveal a kinetic preference of k(pentafluoropyridine) : k(2,3,5,6-tetrafluoropyridine) = 5.4 : 1. Treatment of 1a and 1b with Me₃SiN₃ affords trans-[Ni(N ₃)(2-C₅NF₄)(PEt₃)₂] (2a) and trans-[Ni(N₃)(2-C₅NHF₃)(PEt ₃)₂] (2b), respectively. The complex trans-[Ni(NCO)(2- C₅NHF₃)(PEt₃)₂] (3b) is obtained on reaction of 1b with Me₃SiNCO and by photolysis of 2b under CO, while trans-[Ni(η¹-C≡CPh)(2-C₅NF₄)(PEt ₃)₂] (4a) is obtained by reaction of phenylacetylene with 1a. Addition of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C ₅NF₄)(PEt₃)₂] (5a X = CN, 6a X = I, 7a X = OAc), respectively. The PEt₃ groups of complex 1a are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) to produce [NiF(2-C₅F₄N)(dcpe)] (8a). Addition of dcpe to trans-[Ni(OTf)(2-C₅F₄N)(PEt₃)₂] (10a), however, yields the salt [Ni(2-C₅F₄N)(dcpe) (PEt₃)](OTf) (9a) by substitution of only one PEt₃ and displacement of the triflate ligand. The structures of 2b, 4a, 7a and 8a were determined by X-ray crystallography. The influence of different ancillary ligands on the bond lengths and angles of square-planar nickel structures with polyfluoropyridyl ligands is analysed. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510052f

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F U L L P A P E R
C–F Bond activation at Ni(0) and simple reactions of square planar
Ni(II) fluoride complexes
Suzanne Burling,a Paul I. P. Elliott,a Naser A. Jasim,a Richard J. Lindup,a Jennifer McKenna,a
Robin N. Perutz,*a Stephen J. Archibaldb and Adrian C. Whitwooda
a Department of Chemistry, University of York, Heslington, York, UK YO10 5DD.
E-mail: rnp1@york.ac.uk
b Department of Chemistry, University of Hull, Hull, UK HU6 7RX
Received 14th July 2005, Accepted 19th August 2005
First published as an Advance Article on the web 7th September 2005
The reaction of Ni(COD)2 (COD = 1,5-cyclooctadiene) with triethylphosphine and pentafluoropyridine in hexane
has been shown previously to yield trans-[NiF(2-C5NF4)(PEt3)2] (1a) with a preference for reaction at the 2-position
of the heteroaromatic. The corresponding reaction with 2,3,5,6-tetrafluoropyridine was shown to yield
trans-[NiF(2-C5NF3H)(PEt3)2] (1b). In this paper, we show that reaction of Ni(COD)2 with triethylphosphine and
pentafluoropyridine in THF yields a mixture of 1a and 1b. Competition reactions of Ni(COD)2 with
triethylphosphine in the presence of mixtures of heteroaromatics in hexane reveal a kinetic preference of
k(pentafluoropyridine) : k(2,3,5,6-tetrafluoropyridine) = 5.4 : 1. Treatment of 1a and 1b with Me3SiN3 affords
trans-[Ni(N3)(2-C5NF4)(PEt3)2] (2a) and trans-[Ni(N3)(2-C5NHF3)(PEt3)2] (2b), respectively. The complex
trans-[Ni(NCO)(2-C5NHF3)(PEt3)2] (3b) is obtained on reaction of 1b with Me3SiNCO and by photolysis of 2b under
1
CO, while trans-[Ni(g -C≡CPh)(2-C5NF4)(PEt3)2] (4a) is obtained by reaction of phenylacetylene with 1a. Addition
of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C5NF4)(PEt3)2] (5a X = CN, 6a X = I, 7a X = OAc),
respectively. The PEt3 groups of complex 1a are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane
(dcpe) to produce [NiF(2-C5F4N)(dcpe)] (8a). Addition of dcpe to trans-[Ni(OTf)(2-C5F4N)(PEt3)2] (10a), however,
yields the salt [Ni(2-C5F4N)(dcpe)(PEt3)](OTf) (9a) by substitution of only one PEt3 and displacement of the triflate
ligand. The structures of 2b, 4a, 7a and 8a were determined by X-ray crystallography. The influence of different
ancillary ligands on the bond lengths and angles of square-planar nickel structures with polyfluoropyridyl ligands is
analysed.
the 4-position of the ring only, yielding for palladium trans-
Introduction
Pd(F)(4-C5NF4)(PR3)2, while the platinum reaction results in a
Activation of the carbon–fluorine bond of fluoroaromatics by
rearrangement yielding trans-Pt(R)(4-C5NF4)(PR3)(PR2F).15
transition metal reagents is now a well-recognised process1–3
We address here the relative rates of reaction of Ni(COD)2 and
that has been exploited in catalysis and forms a systematic
PEt3 with different fluorinated heteroaromatics via competition
route for the synthesis of fluorinated organics.4,5 It also forms
reactions and the effect of solvent on the C–F activation reac-
an important route to metal fluoride complexes. There have
tion. The synthesis of further nickel(2-fluoropyridyl) derivatives
been striking recent advances in C–F activation of fluori-
is presented both as a prelude to further reductive elimination
nated alkenes by nickel, rhodium, ruthenium and zirconium
and to provide structural comparisons among complexes in
complexes;6 the reactivity of the zirconium complexes extends
this class. Among these, we selected the azide derivative for
to fluorinated alkanes and arenes.3,7 Our understanding of
investigation because of the reported photolability of the azide
the reactivity of fluoroalkyl derivatives has also expanded
ligand from cobalt and platinum complexes. For instance,
greatly through the work of Caulton8 and Hughes.9 The
[Co(NH3)5N3]Cl2 forms azide radicals upon photolysis16 while
C–F activation of pentafluoropyridine10 and other nitrogen-
[PtII(N3)2(PPh3)2] is reduced photochemically in 2-MeTHF to
containing fluoroaromatics11 by nickel is a rapid process studied
the azide-free product [Pt0(PPh3)2(2-MeTHF)n].17 We also in-
by Perutz and co-workers, with metallation occurring predomi-
vestigate the alternative possibility of replacing the PEt3 groups
nantly at the 2-position. The mechanism of C–F activation is
with a chelating phosphine, thus constraining the groups to be
thought to involve pre-coordination of the aromatic ring to
eliminated in a cis geometry.
the Ni(0) centre prior to intramolecular oxidative addition.12
The fluoride ligand can be replaced by various nucleophiles
including triflate, phenyl, phenoxide and methyl to produce
new (2-tetrafluoropyridyl)nickel derivatives.13 The subsequent
reductive elimination of such complexes leads to otherwise
inaccessible 2-substituted tetrafluoropyridines by C–C coupling
reactions, as was observed for generation of 2-C5NF4Me and
2-C5NF4(COMe) from trans-[Ni(Me)(2-C5F4N)(PEt3)2].13 How-
ever, reductive elimination succeeds only in a limited number
of cases. In a recent theoretical paper, we showed that C–F
oxidative addition at nickel bis(phosphine) was favoured relative
to platinum bis(phosphine) as a result of the greater strength of
the Ni–F bond.14 We have also reported substantial differences
in products of reaction of M(PR3)2 (M = Ni, Pd, Pt) with fluoro-
pyridines down the group. Reactions of pentafluoropyridine
with Pd(PR3)2 and Pt(PR3)2 (R = Cy, iPr) result in attack at
Results
1. Mechanistic studies of C–F oxidative addition
Effect of solvent on C–F oxidative addition. The syn-
theses of trans-[NiF(2-C5NF4)(PEt3)2] (1a) and trans-[NiF(2-
C5NF3H)(PEt3)2] (1b) by reaction of Ni(COD)2 with PEt3 and
pentafluoropyridine or 2,3,5,6-tetrafluoropyridine, respectively,
in hexane have been reported previously.10 It should be noted
that the reaction with pentafluoropyridine also gives up to 15%
of the 4-C5NF4 isomer and traces of the 3-C5NF4 isomers of
1a; these may be removed on recrystallisation. The reaction
with 2,3,5,6-tetrafluoropyridine gives ca. 1% of the 3-C5NF3H
isomer. We now report that the reaction of Ni(COD)2 with PEt3
and pentafluoropyridine in THF (in place of hexane) leads to
3 6 8 6
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Table 1 Reagent and product ratios for competition reactions with
Ni(COD)2, PEt3, pentafluoropyridine and 2,3,5,6-tetrafluoropyridine in
hexane
Entry
Ni(COD)2
C5NF5
C5NHF4
1a : 1b
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1.33
1.22
1.00
0.67
0.50
1.3
0.67
0.78
1.00
1.33
1.50
1.4
9.7
7.2
4.4
1.5
0.87a
5.5
Scheme 1
2.1
0.56
0.44
3.33
ca. 21
0.51a
Reactions of 1a and 1b
Synthesis of azido derivatives 2a and 2b. A solution
a The proportion of 1a is limited by availability of pentafluoropyridine
for these entries.
2
of 1a in hexane was reacted with Me3SiN3 producing trans-
[Ni(N3)(2-C5NF4)(PEt3)2] (2a) in 62% yield after 2 h. Similarly,
Me3SiN3 reacted with a solution of 1b to form trans-[Ni(N3)(2-
C5NHF3)(PEt3)2] (2b) in 71% yield. Both compounds were
a mixture of 1a and 1b, in a ratio of 1 : 0.57 in favour of 1a,
as indicated by integration of the 19F NMR spectrum. The
unexpected formation of 1b was also shown by the resonance at
d 6.24 in the 1H NMR spectrum, assigned to the aromatic ring
characterised by H, 31P and 19F NMR spectroscopy (Table 2).
1
The 31P NMR spectrum of complex 2a contains one resonance
only, a singlet at d 16.1, indicating that the two phosphorus nuclei
are equivalent and the complex adopts the trans geometry (see
Scheme 2). For the azidotrifluoropyridylnickel complex 2b, the
phosphorus peak appears at a lower chemical shift of d 15.7.
The 19F NMR spectrum of 2a contains four fluorine resonances:
an apparent triplet of doublets at d −85.3, an apparent triplet at
d −132.9, a multiplet at d −150.3 and a multiplet at d −172.6.
The 19F NMR spectrum of 2b contains three peaks: a triplet
of doublets at d −90.9, a doublet at d −111.5 and a multiplet
at d −150.6. The infrared bands observed at 2055 cm−1 and
2054 cm−1 for complexes 2a and 2b respectively are consistent
with azide stretching modes. The structure of 2b was also
determined by X-ray crystallography.
proton of 1b. The 31P{ H} NMR spectrum contains the doublet
1
resonances consistent with PEt3 coupled to fluorine for 1a and
1b. As a consequence of this unusual reactivity, it is essential to
avoid the use of THF for the synthesis of 1a (even when used
directly without isolation), otherwise trifluoropyridyl derivatives
are formed.†
Control reactions were performed in which PEt3 was reacted
with pentafluoropyridine alone in THF and hexane. The 1H and
19F NMR spectra showed that 2,3,5,6-tetrafluoropyridine was
formed in both solvent systems. We have observed this reaction
previously, but only in THF.18 It contrasts with the reaction with
primary and secondary phosphines which yields phosphorus-
substituted heterocycles.19
Crystals of 2b were obtained from hexane at −20 ◦C. An
ORTEP diagram of the crystal structure (Fig. 1a) confirms the
trans-square-planar geometry. Selected bond lengths and angles
are summarised in Table 3. Coordination of the trifluoropyridyl
ring to the nickel centre occurs at position 2, adjacent to the
ring nitrogen atom and the hydrogen is at the 4-position. The
angles between the adjacent ligands at nickel vary from 89.78(8)◦
to 90.55(7)◦. In previous studies, tetrafluoropyridyl complexes
have been disordered whereas complexes with a trifluoropyridyl
ligand have been ordered.3 In keeping with this observation,
disorder about the Ni(1)–C(1) bond is not observed. The
Competition reactions between heteroaromatics. The effect
of the degree of fluorination of the pyridyl ring on the rate of
C–F activation was studied by competition reactions between
pairs of heteroaromatics. In the first series (Table 1, entries 1–
5), Ni(COD)2 (1 equiv.) was treated with PEt3 (2.2 equiv.) to
which was added a solution in hexane containing pentaflu-
oropyridine and 2,3,5,6-tetrafluoropyridine totalling 2 equiv.
The proportions of trans-[NiF(2-C5NF4)(PEt3)2] (1a) and trans-
[NiF(2-C5NF3H)(PEt3)2] (1b) were determined by 19F NMR
spectroscopy. A plot of [1a]/[1b] versus [C5NF5]/[C5NHF4]
yielded a gradient of 5.4
0.4 (95% confidence limit) corre-
˚
Ni(1)–N(1) bond length is 1.923(2) A while the N(1)–N(2)
sponding to the kinetic preference ratio (Scheme 1). In a second
series of experiments (entries 6–8), the proportion of PEt3 was
held at 4 equiv and the excess of heteroaromatic was increased.
The results were consistent with the first series. The results of
competition reactions of this type with different reagent ratios
are listed in Table 1. A control with 1a + tetrafluoropyridine in
hexane showed no reaction.
˚
˚
and N(2)–N(3) bond distances are 1.185(3) A and 1.160(3) A
respectively. The azido ligand is close to linear with a N(1)–
N(2)–N(3) bond angle of 175.3(3)◦. The Ni(1)–N(1)–N(2) bond
angle of 132.3(2)◦ compares favourably with other metal azido
complexes; the complex [Co(N3)2(L)4] (L = 2-(p-pyridyl)-4,4,5,5-
tetramethylimidazoline-1-oxyl-3-oxide) contains two azido lig-
ands with a Co(1)–N(1)–N(2) bond angle of 128.91◦,20 while
the [Ru(tpy)(PPh3)2(N3)]+ cation21 has a Ru(1)–N(1)–N(2) bond
angle of 139.6(4)◦. A recent structure of an octahedral nickel(II)
† In addition to the resonances of 1a and 1b, the 19F spectrum contains
two minor resonances that appear as doublets at d −38.3 and −39.4 with
couplings of JFP = 643 and 631 Hz, respectively. These large coupling
constants are characteristic of PF bonds in fluorophosphoranes of type
PR3F2. The presence of two such compounds may be explained by partial
fluorination of the ethyl groups. We reported previously that similar
fluorophosphoranes are also formed in reactions of Ni(COD)2 with PEt3
and C6F6 in hexane. They can be observed in the pentafluoropyridine
reactions both when conducted in THF and in hexane, and increase
in significance with excess pentafluoropyridines. Similar fluorinated
phosphorane by-products were produced upon reaction of PEt3 with
pentafluoropyridine. The most prominent of these exhibited the follow-
ing NMR data (reaction and NMR spectroscopy in a THF/C6D6, 90 :
azido complex with terminal and bridging azides has a Ni–
22
˚
N(terminal) bond length of 2.057(3) A.
Photochemical reactivity of azide 2b under CO and formation
of isocyanato derivatives. A sample of 2b in hexane, under an
atmosphere of CO, was irradiated for 30 h (Hg arc, k > 290 nm).
A second sample of 2b in hexane, under a CO atmosphere, was
stirred without photolysis at room temperature. Both reactions
were monitored by IR spectroscopy and the same products
formed in each, although the photolytic reaction was more rapid
than the thermal one.
The IR spectra of the reaction showed that the azide stretching
mode of the starting material 2b at 2054 cm−1 decreases
in intensity and three new bands are formed at 2225, 2066
and 1992 cm−1. The 1H, 31P and 19F NMR spectra confirm
the presence of two products. The major product appears at
10, mixture):19F d −38.3 d, JPF = 648 Hz; 31P d −17.3 t, JPF
=
648 Hz. Notably, this species was also observed in the reaction of
pentafluoropyridine with triethylphosphine in hexane. It matches the
spectroscopic data for one of the fluorophosphoranes observed in the
reaction of Ni(COD)2 with PEt3 + C5NF5 in THF.
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
3 6 8 7

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Table 2 NMR Data for 2a–9a in C6D6 at 298 K (d (ppm) (J/Hz))a
1
Complex
1H
31P{ H}
16.1 (s)
15.7 (s)
15.8 (s)
20.7 (s)
19F
2a
2b
3b
4a
1.05 (m, 12H, CH2), 0.90 (t, 18H,
CH3)
−85.3 (td, J = 28.6, 15.2, 1F, F6), −132.9 (t, J =
28.6, 1F, F3), −150.3 (m, 1F, F4), −172.6 (m, 1F, F5)
−90.9 (td, J = 30.5, 5.7, 1F, F6), −111.5 (d, J = 32.4,
1F, F3), −150.6 (m, 1F, F5)
6.25 (m, 1H, CH), 1.10 (m, 12H,
CH2), 0.93 (m, 18H, CH3)
6.25 (m, 1H, CH), 1.05 (m, 12H,
CH2), 0.85 (m, 18H, CH3)
7.44 (d, J = 7, 2H, CH), 7.11 (m, 2H,
CH)b, 6.98 (t, J = 7, 1H, CH), 1.37
(m, 12H, CH2), 0.96 (m, 18H, CH3)
1.36 (m, 12H, CH2), 0.95(m, 18H,
CH3)
−91.0 (m, 1F, F6), −111.5 (d, J = 30.5, 1F, F3),
−150.7 (m, 1F, F5)
−85.7 (m, 1F, F6), −133.8 (m, 1F, F3), −151.9 (m, 1F,
F4), −173.6 (m, 1F, F5)
5a
6a
7a
8ac
21.6 (s)
15.6 (s)
−84.2 (td, J = 28.6, 15.3, 1F, F6), −132.9 (t, J =
28.6, 1F, F3), −149.9 (m, 1F, F4), −171.3 (m, 1F, F5)
−84.5 (td, J = 28.6, 15.2, 1F, F6), −131.9 (t, J =
28.6, 1F, F3), −149.4 (m, 1F, F4), −172.0 (m, 1F, F5)
−84.5 (td, J = 28.6, 15.3, 1F, F6), −131.0 (t, J =
26.7, 1F, F3), −149.9 (m, 1F, F4), −171.9 (m, 1F, F5)
−87.4 (td, J = 28.6, 13.4, 1F, F6), −132.3 (t, J =
28.6, 1F, F3), −150.1 (m, 1F, F4), −171.0 (m, 1F, F5),
−339.8 (br m, 1F, NiF)
1.36 (m, 12H, CH2), 0.90 (m, 18H,
CH3)
1.99 (s, 3H, CH3), 1.07 (m, 12H, CH2), 13.0 (s)
1.00 (m, 18H, CH3)
2.56 (m, 2H, CH2), 2.45 (m, 2H, CH2), 71.6 (dd, JPF = 99, JPP = 31,
1.94–0.76 (m, 44H, C6H11)
P2), 66.2 (m, P1)
9ac
2.38–0.72 (m, 63H, dcpe and PEt3)
72.4 (dd, J = 39, 27, P1), 67.5
(dd, J = 198, 27, P2), 7.6 (dd,
J = 198, 39, PEt3)
−76.7 (s, 3F, CF3), −83.0 (td, J = 28, 16, 1F, F6),
−128.8 (t, J = 28, 1F, F3), −147.2 (m, 1F, F4),
−168.4 (m, 1F, F5)
a F numbering as shown in Scheme 2. b Peak partly obscured by solvent. c P numbering as shown in Scheme 3.
Scheme 2
3 6 8 8
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5

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◦
˚
Table 3 Principal bond lengths (A) and angles ( ) of trans-[Ni(N3)(2-
C5F3HN)(PEt3)2] (2b)
Ni(1)–C(1)
Ni(1)–P(1)
Ni(1)–P(2)
1.881(2)
2.2053(9)
2.2003(9)
Ni(1)–N(1)
N(1)–N(2)
N(2)–N(3)
1.923(2)
1.185(3)
1.160(3)
N(1)–Ni(1)–C(1)
P(1)–Ni(1)–P(2)
C(1)–Ni(1)–P(1)
C(1)–Ni(1)–P(2)
N(1)–Ni(1)–P(1)
170.40(10)
174.35(3)
90.54(8)
90.55(7)
89.78(8)
N(1)–Ni(1)–P(2)
Ni(1)–N(1)–N(2)
N(1)–N(2)–N(3)
N(4)–C(1)–Ni(1)
90.08(8)
132.3(2)
175.3(3)
117.17(18)
The isocyanato derivative 3b was synthesised independently
to confirm its identity following a similar route to that for
complex 2b. Treatment of 1b with trimethylsilyl isocyanate,
afforded trans-[Ni(NCO)(2-C5NHF3)(PEt3)2] (3b) in 79% yield.
The product contains one strong band in the IR spectrum
at 2225 cm−1. The NMR data confirm that the isocyanato
derivative is very similar to the azido analogue, with resonances
occurring at almost identical chemical shifts to those of 2b qand
the photoproduct of 2b with CO (see Table 2). Thus the major
reaction product of 2b with CO is confirmed as 3b.
Reaction of 1a with phenylacetylene. The complex trans-
1
[Ni(g -C≡CPh)(2-C5NF4)(PEt3)2] (4a) was prepared in 26%
yield by treating a solution of 1a in hexane with phenylacetylene
in the presence of [Me4N]F (Table 2). The IR spectrum of the
product exhibited the characteristic stretching mode of the C≡C
bond of the phenylacetylenyl at 2090 cm−1, close to the value
of 2111 cm−1 for free PhC≡CH.12 The driving force behind the
synthesis of 4a was the formation of HF, which was then trapped
by [Me4N]F. It was anticipated that F− would help deprotonate
the alkyne and [Me4N]F would trap HF as [Me4N]+[FHF]−. A
control reaction performed in the absence of [Me4N]F failed to
produce the phenylacetylenyl complex.
The structure of 4a was determined by X-ray crystallography
following crystallisation from hexane at 4 ◦C. Selected bond
lengths and angles are summarised in Table 4. Compound 4a
exhibits a square-planar geometry with the tetrafluoropyridyl
ring, metalated at the 2-position, positioned trans to the
phenylacetylenyl ligand (Fig. 1b). The Ni–tetrafluoropyridyl
˚
(Ni–C(21)) distance is 1.919(10) A, but there is disorder about
the Ni–C(21) bond as has been observed in other complexes
of this type.3,6 There is also disorder in the methyl group of
C(12) which appears to occupy two sites. The angles between
the adjacent ligands at nickel vary from 87.1(3)◦ to 93.3(3)◦
˚
and the rms deviation from square planar geometry is 0.021 A.
The tetrafluoropyridyl ring is almost orthogonal both to the
coordination plane and to the phenyl ring of the ligand trans to
it with dihedral angles of 90.6◦ and 81.6◦ respectively. The Ni–
Fig. 1 ORTEP diagrams of molecular structure with 50% ellipsoids
of (a) 2b, (b) 4a, (c) 7a. Hydrogen atoms are omitted except for the
hydrogen on the trifluoropyridyl ring of 2b. Note that the structures of
4a and 7a exhibit disorder about the Ni–C bond (not illustrated) such
that N(1) exchanges places with one of the CF groups.
˚
C(8) distance to the phenylacetylenyl ligand is 1.866(12) A. The
˚
C≡C bond C(7)–C(8) is significantly shorter at 1.208(13) A than
all other CC bonds in the molecule. The Ni–C(8)–C(7)–C(6) four
atom fragment is almost linear with angles C(7)–C(8)–Ni and
C(6)–C(7)–C(8) of 177.2(10)◦ and 175.8(11)◦, respectively.
chemical shifts almost identical to the starting material 2b. The
31P NMR spectrum contains two singlets, the major resonance
at d 15.76 and a minor singlet at d 21.1. The 19F NMR spectrum
contains three major signals at d −89.6, −111.5 and −150.5
consistent with a coordinated trifluoropyridyl ring, and three
minor resonances at d −88.5, −121.9 and −132.1. The IR
spectrum also confirms that one of the products was very similar
to the azido complex 2b. The IR product peak at 2225 cm−1
falls in the range where isocyanates absorb suggesting that the
isocyanate derivative trans-[Ni(NCO)(2-C5NHF3)(PEt3)2] (3b)
has been formed. The minor product was not identified.‡,23
Salt metathesis reactions: KCN, KI, NaOAc. Reaction of
a suspension of KCN in THF with a hexane solution of 1a
formed in-situ resulted in the formation of trans-[Ni(CN)(2-
C5NF4)(PEt3)2] (5a) as yellow crystals in 43% yield (Table 2).
◦
1
˚
Table 4 Principal bond lengths (A) and angles ( ) of trans-[Ni(g -
C≡CPh)(2-C5NF4)(PEt3)2] (4a)
Ni(1)–C(8)
Ni(1)–C(21)
Ni(1)–P(1)
1.866(12)
1.919(10)
2.186(3)
Ni(1)–P(2)
C(8)–C(7)
C(6)–C(7)
2.190(3)
1.208(13)
1.437(14)
‡ The minor product may be the O-bonded isomer with a cyanato ligand
trans-[Ni(OCN)(2-C5NHF3)(PEt3)2]. There is very little information
about NCO/OCN linkage isomerism although they have been inves-
tigated theoretically, and there is one claim on the basis of vibrational
spectra.23
P(1)–Ni(1)–P(2)
C(21)–Ni(1)–C(8)
C(8)–Ni(1)–P(1)
C(8)–Ni(1)–P(2)
174.52(14)
177.6(5)
87.5(3)
C(21)–Ni(1)–P(1)
C(21)–Ni(1)–P(2)
C(7)–C(8)–Ni(1)
C(8)–C(7)–C(6)
93.3(3)
92.2(3)
177.2(10)
175.8(11)
87.1(3)
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
3 6 8 9

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The complex displayed the characteristic C≡N stretching mode
of the cyanide ligand in the infrared spectrum at 2107 cm−1.
The C≡N stretching frequency of 5a was considerably higher
than that of free CN− at 2080 cm−1 (in aqueous solution), in
keeping with the shift to higher frequency often observed for
CN− ligands.24 The synthesis of 2-cyanotetrafluoropyridine was
attempted by exposure to air but the complex was air stable and
failed to react.
In a method similar to that employed for the cyano derivative,
the iodo complex was prepared by addition of a hexane solution
of 1a (containing a trace of the 4-pyridyl isomer) to a suspension
of KI in THF resulting in the formation of trans-[NiI(2-
C5NF4)(PEt3)2] (6a) in 53% yield as red crystals (Table 2). The
31P and 19F NMR spectra confirmed the formation of 6a as the
2-pyridyl isomer by the four 19F resonances of equal intensity.
The 4-pyridyl isomer was formed in less than 5% yield.
The acetato derivative was prepared similarly: NaOAc was
added to a THF solution of 1a to yield trans-[Ni(OAc)(2-
C5NF4)(PEt3)2] (7a) after stirring overnight. The air-stable com-
=
plex 7a was obtained in 74% yield and exhibits a C O stretching
mode of the acetate ligand at 1622 cm−1 in the infrared spectrum
(ATR). This compares with 1560 cm−1 for NaOAc (ATR).
Complex 7a was crystallised by slow evaporation of hexane
at room temperature to afford orange blocks suitable for X-ray
diffraction. Selected bond lengths and angles are summarised
in Table 5. The crystal structure revealed that the acetato group
acts as a unidentate ligand. As expected the pentafluoropyridyl
ligand coordinates at the 2-position with disorder about the
Ni(1)–C(1) bond (Fig. 1c). The square-planar geometry of
the complex is confirmed with adjacent ligands having bond
angles which vary from 85.77(3)◦ to 93.80(3)◦. The acetate
ligand is separated from the metal centre by a Ni(1)–O(1) bond
Scheme 3
Complex 8a was crystallised from benzene at room tem-
perature as small yellow crystals. Selected bond lengths and
angles are summarised in Table 6. The dcpe complex crystallises
with two molecules in the asymmetric unit as 8a·21 C6H6.
4
Equivalent bond distances and bond angles around the metal
centre in the two complexes are essentially the same (Table 6).
As expected, the complex is square-planar with adjacent ligand
bond angles between 88.86(4)◦ to 93.20(6)◦ (values for molecule
1, Fig. 2). The Ni–F and Ni–C bond lengths are 1.8473(12)
˚
length of 1.9154(10) A. The O(1)–C(6) bond length of the
˚
acetate at 1.2908(19) A is significantly longer than the formal
˚
double bond C(6)–O(2) of 1.2275(19) A. The acetate ligand
and the pentafluoropyridyl ring are almost coplanar, and lie
approximately perpendicular to the P–Ni–P vector.
˚
˚
A and 1.917(2) A, respectively. The bite angle of the chelating
phosphine at 89.09(2)◦ is similar to the 88.82(5)◦ bite angle in
[NiCl2(dcpe)].26 The main differences between the two molecules
in the asymmetric unit, apart from inversion, lie in the out-of-
plane distances and the torsional angles. The nickel atom lies
Replacement of triethylphosphine by a bidentate ligand, syn-
thesis of [NiF(2-C5NF4)(dcpe)] (8a). The triethylphosphine
ligands of 1a were displaced by direct addition of dcpe [dcpe =
1,2-bis(dicyclohexylphosphino)ethane] to a benzene solution
of 1a, producing air-sensitive [NiF(2-C5NF4)(dcpe)] (8a) as a
yellow solid in 52% yield (Scheme 3). The isolated yield was
limited by the extreme solubility of 8a. The 31P NMR spectrum
clearly indicated the loss of triethylphosphine, with only two
signals evident, representing the coordinated diphosphine. The
phosphorus trans to fluoride appeared as a doublet of doublets
at d 71.6, with J(PFtrans) = 99 Hz and J(PP) = 31 Hz. The
phosphorus cis to fluoride appeared as a multiplet at d 66.2.
The 19F NMR spectrum contains the characteristic tetrafluo-
ropyridyl ligand signals at d −87.4, −132.3, −150.1 and −171.0.
The fluoride ligand appears at d −339.8 as a broad multiplet,
shifted significantly downfield compared to the trans analogue
1a which appears at d −371.4. Complex 8a may be compared to
Po¨rschke’s [NiF(C6F5)(tBu2PCH2CH2PtBu2)] which was made
˚
0.021(1) A out of the plane of the coordinating atoms P(1), P(2),
C(1) and F(1) for molecule 1 and 0.041(1) A for molecule 2. It
˚
˚
lies out of the plane of the pyridyl ring atoms by 0.276(1) A
˚
(molecule 1) and 0.174(1) A (molecule 2). The torsional angles
between the pyridyl ring and the Ni–F bond are 105.38(17)◦ and
−96.15(17)◦ for molecules 1 and 2, respectively.
2
by thermolysis of [Ni(g -C6F6)(tBu2PCH2CH2PtBu2)].25
◦
˚
Table 5 Principal bond lengths (A) and angles ( ) of trans-[Ni(OAc)(2-
C5NF4)(PEt3)2] (7a)
Ni(1)–C(1)
Ni(1)–O(1)
Ni(1)–P(1)
Ni(1)–P(2)
1.8753(14)
1.9154(10)
2.2106(4)
2.1983(4)
C(6)–O(1)
C(6)–O(2)
C(6)–C(7)
1.2908(19)
1.2275(19)
1.517(2)
P(1)–Ni(1)–P(2)
C(1)–Ni(1)–O(1)
C(1)–Ni(1)–P(1)
C(1)–Ni(1)–P(2)
O(1)–Ni(1)–P(1)
175.540(15)
177.53(5)
88.55(4)
91.83(4)
93.80(3)
O(1)–Ni(1)–P(2)
C(6)–O(1)–Ni(1)
O(1)–C(6)–C(7)
O(2)–C(6)–O(1)
85.77(3)
116.27(9)
114.33(13)
124.97(14)
Fig. 2 ORTEP diagram of molecular structure of 8a with 50%
ellipsoids. Molecule 1 is illustrated. Hydrogen atoms are omitted.
3 6 9 0
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5

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◦
˚
Table 6 Principal bond lengths (A) and angles ( ) of [NiF(2-C5NF4)(dcpe)] (8a)
Molecule 1
Molecule 2
Ni(1)–F(1)
Ni(1)–C(1)
Ni(1)–P(1)
Ni(1)–P(2)
1.8473(12)
1.917(2)
2.1970(6)
2.1361(6)
Ni(2)–F(6)
Ni(2)–C(32)
Ni(2)–P(3)
Ni(2)–P(4)
1.8498(12)
1.916(2)
2.1993(6)
2.1355(6)
C(1)–Ni(1)–P(1)
F(1)–Ni(1)–P(2)
F(1)–Ni(1)–C(1)
C(1)–Ni(1)–P(2)
F(1)–Ni(1)–P(1)
P(1)–Ni(1)–P(2)
175.08(7)
176.98(5)
89.01(7)
93.20(6)
88.86(4)
C(32)–Ni(2)–P(3)
F(6)–Ni(2)–P(4)
F(6)–Ni(2)–C(32)
C(32)–Ni(2)–P(4)
F(6)–Ni(2)–P(3)
P(3)–Ni(2)–P(4)
F(6)–Ni(2)–C(32)–N(2)
P(4)–Ni(2)–C(32)–N(2)
174.52(7)
178.04(4)
89.76(7)
92.04(6)
89.37(4)
89.09(2)
88.91(2)
F(1)–Ni(1)–C(1)–N(1)
P(2)–Ni(1)–C(1)–N(1)
−96.15(17)
105.38(17)
−73.86(16)
81.80(16)
The phosphine ligand substitution was also attempted on
the air-stable triflate complex trans-[Ni(OTf)(2-C5NF4)(PEt3)2]
(10a) in order to provide a simpler route to dcpe deriva-
tives. Substitution was successful but in this case only one
triethylphosphine ligand was displaced, together with the triflate
ligand, affording [Ni(2-C5F4N)(dcpe)(PEt3)][OTf] (9a) with the
triflate acting as the counterion (Scheme 3). The 31P NMR
spectrum contains three resonances of equal intensity (Table 1).
A doublet of doublets at d 7.6 is consistent with a coordinated
triethylphosphine ligand, with a trans coupling to a doublet
of doublets at d 67.5 of J(PP) = 198 Hz and a cis coupling
to a doublet of doublets at d 72.4 of J(PP) = 39 Hz. The
two resonances at low field are therefore attributed to the
coordinated dcpe ligand with a cis coupling to each other of
J(PP) = 27 Hz. The 19F NMR spectrum was also used to
characterise the product showing a singlet at d −76.7, assigned
as the CF3 of the triflate group, that was considerably sharper
and shifted slightly from its position in the starting complex 10a.
Complex 9a could be converted to 8a by reaction with an excess
of [Me4N]F.
nucleophilic attack of pentafluoropyridine at the 2-position
is with PHtBu2.19 Our relative rate data and the observed
regioselectivity are inconsistent with nucleophilic attack. It
might be thought that the preference we observe for the 2-
position of the ring is associated with the steric demands of
the Ni(PEt3)2. However, this argument is wholly inconsistent
with the preference for the 4-position shown by the far bulkier
Pd(PCy3)2 and Pt(PCy3)2 (see below). Our observations also
contrast with the reaction of pentafluoropyridine at the 4-
position with sterically demanding anionic complexes such
as [Rh(CO)2(PPh3)2]−.28 We, therefore, rule out nucleophilic
attack by nickel(0) and propose an alternative mechanism
(Scheme 4). This mechanism involves pre-coordination with
the fluorinated substrate, following the dissociation of COD
from Ni(COD)(PEt3)2 to produce [Ni(PEt3)2(C5NF5)] (A) as an
intermediate, followed by oxidative addition of the pyridine to
produce the new nickel–fluorine bond; the oxidative addition
must take place with a kinetic preference for attack at the 2-
position over the 4-position.
Discussion
The exact conditions under which the C–F activation of
fluorinated heteroaromatics is performed influences the nature
of the products formed. The reaction of Ni(COD)2 with PEt3
and pentafluoropyridine in THF afforded a mixture of 1a and 1b
whereas 1a was the nickel-containing product when the reaction
was performed in hexane. (Note that small amounts of the 3-
C5NF4 and 4-C5NF4 isomers of 1a are always detected prior to
recrystallisation, see above.) It was found that PEt3 reacts with
pentafluoropyridine in the absence of nickel complexes either in
THF or in hexane, to produce 2,3,5,6-tetrafluoropyridine. This
observation indicates that the reaction of PEt3 and pentafluo-
ropyridine to form 2,3,5,6-tetrafluoropyridine occurs prior to
C–F activation in THF. The failure to produce an appreciable
amount of 1b when the synthesis is performed in hexane suggests
that the reaction of pentafluoropyridine with the nickel complex
is very rapid and occurs in preference to the reaction with PEt3.
The competition reactions explore the effect on the rate of C–F
activation of the degree of fluorination of the pyridyl ring. They
indicate that the relative rates of reaction at room temperature
of pentafluoropyridine and 2,3,5,6-tetrafluoropyridine are 5.4 :
1. This gives immediate mechanistic insight into the C–F
activation process. The rates of nucleophilic attack should follow
the order pentafluoropyridine ꢀ 2,3,5,6-tetrafluoropyridine and
Scheme 4
The regioselectivity of oxidative addition for nickel contrasts
with that we reported recently for reaction of Pd(PR3)2 and
Pt(PR3)2 (R = cyclohexyl or isopropyl).15 In the palladium case
oxidative addition results exclusively in the 4-pyridyl isomer. For
platinum, activation also occurs at the 4-position but a major
rearrangement generates a P–F and Pt–C bond resulting in
[Pt(R)(4-C5NF4)(PR3)(PFR2)]. It is plausible that the reactions
with Pd(PR3)2 and Pt(PR3)2 take place by direct nucleophilic
attack at the 4-position without pre-coordination.
should be separated by several orders of magnitude.§,27
A
nucleophilic mechanism would also result in almost 100% attack
at the 4-position of pentafluoropyridine. The only example of
Two X-ray structures of complexes of pentafluoropyridine are
known, one N-coordinated [PtMe(Me2NCH2CH2NMe2)(g -N-
1
ꢁ
ꢁ
=
C5NF5)][BAr ]4 [Ar = 3,5-C6H3(CF3)2] and one C C coordi-
5
2
nated, [Rh(g -C5H5)(PMe3)(g -3,4-CC-C5NF5)].29,30 Molecular
orbital calculations show that the HOMO of pentafluoropyri-
dine is the 1a2 p-orbital which has a nodal plane through N and
C4 while the LUMO is the 3b1 p* orbital which has significant
§ The rate constant for attack by methoxide at −7.6 ◦C on C5NF5 is
ca. 5 × 104 higher than for attack on 2,3,5,6-C5NF4H. Relative rates for
the reaction of methoxide at the same temperature with 2,3,4,6-C5NF4H
and 2,3,4,5-C5NF4H are ca. 80 and 30 respectively.27
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
3 6 9 1

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electron density on all ring atoms. The N-based ‘lone pair’ is
found to lie 0.7 eV below the HOMO.31 If the interaction of the
NiP2 fragment with pentafluoropyridine is principally covalent
and dominated by HOMO–LUMO interactions, the transition-
at ambient temperature. All the complexes have bond angles
very close to the ideal square-planar geometry. In the trans
˚
complexes, the Ni–Caryl bond length varies from 1.851(3) A
˚
for the triflate to 1.881(2) A for the azido complex, leaving
=
metal complex is expected to be C C coordinated as indeed
aside the phenylacetylide complex that is less well determined.
The fluoride complexes exhibit Ni–Caryl bond lengths close to
the upper end of the range for oxygen ligands, suggesting that
the trans influence of fluoride is comparable to that of oxygen
ligands. The azido complex provides the only example with a
nitrogen ligand and its Ni–Caryl bond length exceeds that for any
of the complexes with oxygen or fluorine ligands.
found. The balance may evidently be tipped in favour of an N-
coordinated complex by a positive charge if electrostatic forces
become more important, i.e. the fragment becomes a harder
Lewis acid.
The syntheses of complexes 2a, 2b and 3b were achieved by
reacting the corresponding trimethylsilyl derivative with trans-
[NiF(2-C5F3NR)(PEt3)2] [(1a) R = F, (1b) R = H], prepared
in situ due to the air-sensitive nature of the Ni–F complexes.
Trimethylsilyl reagents were chosen for the strength of the bond
formed between the trimethylsilyl group and the fluorine atom
displaced from the precursor complex. The photolysis of trans-
[Ni(N3)(2-C5NHF3)(PEt3)2] (2b) in hexane, under an atmosphere
of CO, afforded the isocyanate complex trans-[Ni(NCO)(2-
C5NHF3)(PEt3)2] (3b), implying loss of N2 rather than N3. The
thermal reaction under the same conditions also gave 3b, which
was prepared independently with Me3SiNCO. The similarity of
the spectroscopic data of complexes 2b and 3b is consistent with
isoelectronic complexes. While trimethylsilyl derivatives were
employed in the initial stages of this study, we later showed
that simple salt displacement was sufficient to form iodide,
acetato and cyano complexes. The preference of the nickel
bis(phosphine) pyridyl system for these ligands is consistent with
simple hard/soft expectations.
The shortest Ni–X distances are found for the fluoride
˚
complexes with the minimum value at 1.8473(12) A for 8a.
The value for the phenylacetylide is slightly longer, but values
˚
above 1.9 A are found for X = N3 and X = OAc reaching
˚
a maximum for X = OTf at 1.957(2) A. Variation in Ni–
O bond lengths between the different complexes with oxygen
ligands is certainly significant and follows the order OTf >
OAc > OPh. If the fluorinated pyridyl or fluorinated aryl ligands
exert a much higher trans influence than phosphine like their
hydrocarbyl analogues, the Ni–F bond should be shorter in a
complex with cis phosphines than one with trans phosphines.
However, the difference in Ni–F bond length of 8a and 1b at
˚
0.009(2) A is barely significant and smaller than predicted by the
DFT calculations for the cis/trans pair trans-NiF(C6F5)(PH3)2
14
˚
˚
(1.79 A) and [NiF(C6F5)(H2PCH2CH2PH2)] (1.76 A).
Conclusions
The new nickel fluoropyridyl derivatives described here proved
stable to reductive elimination of 2-substituted fluoropyridines
at room temperature, as did others such as the phenyl derivative.
An alternative approach was taken to encourage elimination,
namely forcing the adoption of a cis geometry by introduc-
tion of a bidentate phosphine. Several bidentate phosphines
were investigated including bis(diphenylphosphino)methane
(dppm), 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1ꢁ-
bis(diphenylphosphino)ferrocene (dppf) but substitution was
only achieved using 1,2-bis(dicyclohexylphosphino)ethane
(dcpe). Bennett and Wenger32 reported the displacement
of triphenylphosphine from an analogous four-coordinate
nickel(0) complex [NiBr(2-Br-4,5-F2C6H2)(PPh3)2] by heating
with dcpe to produce [NiBr(2-Br-4,5-F2C6H2)(dcpe)]. This
method was adopted for the preparation of both complexes 8a
and 9a although heating was not required. While the displace-
ment of PEt3 from complex 1a produced the dcpe analogue
8a as expected, the addition of dcpe to trans-[Ni(OTf)(2-
C5NF4)(PEt3)2] (10a) resulted in the displacement of only one
PEt3 and the triflate group to produce the cationic complex
[Ni(2-C5F4N)(dcpe)(PEt3)](OTf) (9a).
The crystal structures reported here provide the first data
for trans-[NiX(ArF)P2] complexes with N or C ligands trans to
fluoroaryl and the first example of a cis-[NiF(ArF)P2] complex.
Table 7 shows the variation of four crystallographic parameters
for the tetrafluoropyridyl and trifluoropyridyl complexes studied
here together with those studied earlier. We believe that the
comments below are justified, although some caution is needed
in interpreting the data because several structures suffer from
disorder problems and the structure of 1b was determined
In this paper we have reported two advances in the understand-
ing of the mechanism of oxidative addition of polyfluorinated
pyridines to Ni(0). Firstly, we have revealed the existence of
the competing reaction between free triethylphosphine and
pentafluoropyridine yielding 2,3,5,6-tetrafluoropyridine. This
process is significant in THF as solvent but does not compete ef-
fectively with oxidative addition in hexane at room temperature.
Secondly, we have shown that there is a modest kinetic selectivity
for pentafluoropyridine over 2,3,5,6-tetrafluoropyridine of 5.4 :
1. When taken together with the regioselectivity of reaction,
these observations argue in favour of a mechanism involving
pre-coordination followed by oxidative addition.
We also report the synthesis of the nickel fluoropyridyl
complexes, trans-[NiX(C5F3NR)(PEt3)2] (where R = F, H and
X = N3, NCO, C≡CPh, CN, I, OAc), all exhibiting the
rare coordination of the pyridyl ring at the 2-position. Two
methodologies have been adopted, reaction of 1a or 1b with
trimethylsilyl derivatives and salt displacement reactions. In
the special case of the phenylacetylene reaction, [Me4N]F was
added to abstract fluoride from 1a. The X-ray structures of
1
trans-[Ni(N3)(2-C5NHF3)(PEt3)2] (2b), trans-[Ni(g -C≡CPh)(2-
C5NF4)(PEt3)2] (4a) and trans-[Ni(OAc)(2-C5NF4)(PEt3)2] (7a)
have been determined. The azido derivative 2b may be converted
to the isocyanato complex 3b by photolysis under CO. Sub-
stitution of the monophosphine ligands of complex 1a with
a chelating phosphine yielded the analogous nickel–fluoride
complex [NiF(2-C5NF4)(dcpe)] (8a), with the geometry now
fixed in the cis position, as confirmed by the X-ray crystal
structure. Surprisingly, a similar substitution reaction of the
Table 7 Comparative crystallographic parameters for tetrafluoropyridyl and trifluoropyridyl nickel complexes, trans-[NiX(ArF)P2]
P–Ni–P/◦
C–Ni–X/◦
˚
˚
Complex
X
T/K
r(Ni–CAr)/A
r(Ni–X)/A
1b10
F
N3
CCPh
OAc
F
OTf
OPh
296
150
150
115
120
220
220
1.869(4)
1.881(2)
1.919(10)
1.8753(14)
1.917(2)
1.851(3)
1.861(6)
1.856(2)
1.923(2)
1.866(12)
1.9154(10)
1.8473(12)
1.957(2)
176.70(4)
174.35(3)
174.52(14)
175.540(15)
89.09(2)
171.14 (4)
172.44(8)
176.58(12)
170.40(10)
177.6(5)
177.53(5)
89.01(7)
170.7 (1)
175.7(2)
2b
4a
7a
8a
10a13
trans-[Ni(OPh)(2-C5NF4)(PEt3)2]13
1.894(4)
3 6 9 2
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5

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trans-nickel triflate complex afforded the cationic complex [Ni(2-
C5F4N)(dcpe)(PEt3)](OTf) (9a). We will now undertake synthe-
sis of derivatives of the new chelated complexes with appropriate
nucleophiles, which should eliminate the desired substituted
fluoropyridine products more readily than the corresponding
trans-complexes.
trans-[Ni(N3)(2-C5NF4)(PEt3)2] (2a)
Ni(COD)2 (100 mg, 0.36 mmol) suspended in hexane (5 mL) was
treated with PEt3 (140 ll, 0.81 mmol) and pentafluoropyridine
(50 ll, 0.45 mmol). The solution was stirred for 2 h and
the volatiles were removed under vacuum. The residue was
redissolved in hexane and treated with trimethylsilyl azide
(50 ll, 0.38 mmol). The solution was stirred overnight and
the volatiles were removed under vacuum. The residue was
redissolved in hexane (5 mL). Crystals formed overnight at
−20 ◦C and were filtered off and dried under vacuum. Yield:
110 mg (0.23 mmol, 62%). Anal. Calcd for C17H30N4F4P2Ni: C,
41.92; H, 6.21; N, 11.50. Found: C, 42.16; H, 6.32; N, 10.84%. IR
(Nujol, cm−1): 2055 (mN3). MS (EI, m/z, relative intensity): 444
(13, [Ni(C5F4N)(PEt3)2]), 294 (8, [Ni(PEt3)2]+), 118 (51, [PEt3]+).
Experimental
General methods
Synthetic work was carried out on a Schlenk line or in an argon-
filled glovebox with oxygen levels below 10 ppm. All solvents
(AR grade) were dried over sodium benzophenone ketyl and dis-
tilled under argon before use. Benzene-d6 and THF-d8 (Apollo
Scientific Ltd) were dried by stirring over potassium and then
transferring under vacuum into NMR tubes fitted with Young’s
taps. Pentafluoropyridine and 2,3,5,6-tetrafluoropyridine were
obtained from Fluorochem Ltd and dried over molecular
trans-[Ni(N3)(2-C5NHF3)(PEt3)2] (2b)
Ni(COD)2 (100 mg, 0.36 mmol) suspended in hexane (5 mL)
was treated with PEt3 (140 ll, 0.81 mmol) and 2,3,5,6-
tetrafluoropyridine (50 ll, 0.50 mmol). The solution was stirred
for 2 h and the volatiles were removed under vacuum. The
residue was redissolved in hexane and treated with trimethylsilyl
azide (50 ll, 0.38 mmol). The solution was stirred overnight
and the volatiles were removed under vacuum. The residue
was redissolved in hexane (5 mL). Crystals formed overnight
at −20 ◦C and were filtered off and dried under vacuum. Yield:
120 mg (0.26 mmol, 71%). Anal. Calcd for C17H31N4F3P2Ni: C,
43.53; H, 6.66; N, 11.94. Found: C, 43.13; H, 6.93; N, 11.63%. IR
(Nujol, cm−1): 2054 (mN3). MS (EI, m/z, relative intensity): 426
(5, [Ni(C5F3HN)(PEt3)2]), 294 (6, [Ni(PEt3)2]+), 118 (46, [PEt3]+).
˚
sieves (4 A). 1,2-Bis(dicyclohexylphosphino)ethane (dcpe) was
obtained from Strem Chemicals Inc. All other chemicals were
obtained from Aldrich Chemical Company Ltd. Complexes 1a,
1b,10 and 10a,13 were prepared as described in the literature
though the amount of PEt3 was reduced to 2.2 equiv. [NMe4]F
was synthesised as described previously.33 Ni(COD)2 was either
purchased from Aldrich or prepared according to a literature
procedure by sodium reduction of NiCl2(py)4.34
The NMR spectra were recorded with a Bruker AMX 500
spectrometer. The 1H NMR chemical shifts were referenced to
1
residual C6D5H at d 7.15 or THF-d7 at d 1.8. The 13C{ H} spectra
were referenced to C6D6 at d 128.0 and THF at d 26.7. The 19
F
and the 31P{ H}NMR spectra were referenced to external CFCl3
and external H3PO4, respectively. Infrared spectra were recorded
on a Unicam Research Series FTIR spectrometer while mass
spectra were recorded on a VG Autospec mass spectrometer.
Elemental Microanalysis Ltd., Devon, carried out elemental
analysis. NMR data are listed in Table 2.
1
trans-[Ni(NCO)(2-C5NHF3)(PEt3)2] (3b)
Ni(COD)2 (100 mg, 0.36 mmol) suspended in hexane (5 mL)
was treated with PEt3 (140 ll, 0.81 mmol) and 2,3,5,6-
tetrafluoropyridine (50 ll, 0.50 mmol). The solution was stirred
for 2 h and the volatiles were removed under vacuum. The
residue was redissolved in hexane and treated with trimethylsilyl
isocyanate (50 ll, 0.37 mmol). The solution was stirred overnight
and the volatiles were removed under vacuum. The residue was
redissolved in hexane (5 mL). Crystals formed overnight at
−20 ◦C and were filtered off and dried under vacuum. Yield:
130 mg (0.28 mmol, 79%). Anal. Calcd for C18H31N2F3P2NiO:
C, 46.09; H, 6.66; N, 5.97. Found: C, 46.43; H, 6.72; N, 6.00%.
IR (Nujol, cm−1): 2225 (mNCO).
Photolysis (k > 290 nm) of samples was carried out in Pyrex
NMR tubes using a Philips HPK 125 W medium pressure
mercury lamp at room temperature equipped with a water
filter.
Synthesis of 1a and 1b in THF
The reaction of Ni(COD)2 with PEt3 and pentafluoropyridine
was carried out as in the literature but the reaction medium
was THF.10 The residue after pumping down was extracted with
hexane as previously.
trans-[Ni(g1-C≡CPh)(2-C5NF4)(PEt3)2] (4a)
Ni(COD)2 (100 mg, 0.36 mmol) suspended in hexane (5 mL)
was treated with PEt3 (140 ll, 0.95 mmol) and pentafluoropy-
ridine (50 ll, 0.45 mmol). The solution was stirred for 2 h and
the volatiles were removed under vacuum. The oily residue was
extracted with hexane (5 mL) and filtered through a cannula. The
solution was treated with phenylacetylene (40 ll, 0.36 mmol)
and tetramethylammonium fluoride (33 mg, 0.36 mmol) and
stirred for 48 h. The solution was then cooled to −78 ◦C to
precipitate the ammonium salts and the mixture was filtered
through a cannula. The solution was concentrated to 2 mL and
yellow crystalline needles formed at 4 ◦C, which were filtered
off and dried under vacuum. Yield: 50 mg (0.09 mmol, 26%).
Anal. Calcd for C25H35NF4P2Ni: C, 54.97; H, 6.46; N, 2.56.
Found: C, 55.18; H, 6.66; N, 3.21%. IR (Nujol, cm−1): 2090
(mC≡C). MS (EI, m/z, relative intensity): 545 (4, [M]+), 427 (2,
[Ni(PEt3)(C5F4N)(C≡CPh)]+), 307 (3), 294 (10, [Ni(PEt3)2]+),
268 (6, [Et3P–C5F4N)]+), 251 (34, [PhC≡C–C5F4N]+), 219 (100,
[Et3P–C≡CPh]+), 118 (23, [PEt3]+).
Competition reaction—pentafluoropyridine versus
2,3,5,6-tetrafluoropyridine
Entries 1–5 of Table 1. Ni(COD)2 (50 mg, 0.18 mmol)
suspended in hexane (2 mL) was treated with PEt3 (60 lL,
0.4 mmol) and was stirred to give a red solution. To this
was added a hexane solution (2 mL) containing 2 equiv. of
fluoropyridine mixture (Table 1). The mixture turned yellow and
was stirred for 2 h. The volatiles were removed under vacuum
and the residue dissolved in hexane (4 mL) and filtered through a
cannula. The solvent was removed from a portion of the filtrate
and the residue redissolved in C6D6 and transferred to an NMR
tube sealed with a Young’s tap. The product ratio was determined
by 19F NMR spectroscopy.
Entries 6–8 of Table 1. Ni(COD)2 (25 mg, 0.09 mmol),
suspended in hexane (2 mL), was treated with PEt3 (53 ll,
0.36 mmol). To this was added a solution containing penta-
fluoropyridine and 2,3,5,6-tetrafluoropyridine in hexane (2 mL)
at the concentrations specified in Table 1. The solution was
stirred for 1 h and the volatiles were removed under vacuum.
The residue was dissolved in C6D6 and analysed as above.
trans-[Ni(C≡N)(2-C5NF4)(PEt3)2] (5a)
Ni(COD)2 (100 mg, 0.36 mmol) suspended in hexane (5 mL) was
treated with PEt3 (140 ll, 0.95 mmol) and pentafluoropyridine
(50 ll, 0.45 mmol). The solution was stirred for 2 h. The solution
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
3 6 9 3

View Article Online
was then added to a suspension of KCN (28 mg, 0.43 mmol) in
THF (15 mL) and the mixture stirred overnight. The volatiles
were removed under vacuum, the residue redissolved in hexane
(5 mL) and the solution filtered through a cannula. Large yellow
crystals formed at −20 ◦C, which were filtered off and dried
under vacuum. Yield: 73 mg (0.16 mmol, 43%). Anal. Calcd for
C18H30N2F4P2Ni: C, 45.89; H, 6.42; N, 5.95. Found: C, 45.89; H,
6.62; N, 5.90%. IR (Nujol, cm−1): 2107 (mC≡N).
[Ni(2-C5F4N)(dcpe)(PEt3)](OTf) (9a)
A solution of dcpe (71 mg, 0.17 mmol) in benzene (2 mL) was
added to a solution of trans-[Ni(OTf)(2-C5NF4)(PEt3)2] (10a)
(100 mg, 0.17 mmol) in benzene (5 mL). The solution was stirred
for 2 handthe volatiles were removed under vacuum. Theresidue
was washed with diethyl ether (5 mL), filtered off and dried
under vacuum affording a microcrystalline yellow solid. Yield:
62 mg (0.07 mmol, 41%). Anal. Calcd for C38H63NF7P3O3SNi:
C, 50.79; H, 7.07; N, 1.56. Found: C, 50.18; H, 6.92; N, 1.30%.
trans-[NiI(2-C5NF4)(PEt3)2] (6a)
Complex 1a (80 mg, 0.17 mmol) was dissolved in hexane
(10 mL). This solution was then added to a suspension of KI
(34 mg, 0.21 mmol) in THF (5 mL) and the mixture darkened
after stirring for 2 h. The volatiles were removed under vacuum,
the residue extracted with hexane (5 mL) and the solution filtered
through a cannula. The volume of the solution was reduced and
the solution placed in the freezer. Red crystals formed which
were filtered off and dried. Yield 52 mg (0.09 mmol, 53%). Anal.
Calcd for C17H30NF4P2NiI: C, 35.7, H, 5.29, N, 2.45. Found: C,
36.16, H, 5.29, N, 2.47%.
X-Ray crystallography
Crystallographic data for 2b, 4a, 7a and 8a·21 C6H6 is shown in
4
Table 8.
Crystal structure of 2b. A crystal was mounted on a
glass fibre using perfluoropolyether oil and transferred to a
Sto¨e IPDS II imaging plate diffractometer equipped with an
Oxford Cryosystems 700 series cooling system. No absorption
correction was applied. The structure was solved by direct
methods (SHELXS 97)35 and refined by full-matrix least-squares
on F2 using SHELXL 97.36 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in idealized
positions. Structures were displayed with the aid of ORTEP for
Windows.37
trans-[Ni(OAc)(2-C5NF4)(PEt3)2] (7a)
Complex 1a (300 mg, 0.65 mmol) was dissolved in THF (20 mL)
and NaOAc (64 mg, 0.78 mmol) was added. The suspension was
stirred overnight and the volatiles were removed under vacuum
to yield a yellow solid. The product was extracted with hexane
(20 mL) and dried under vacuum to afford orange crystals of
Crystal structure of 4a. The crystal structure of 4a was
determined with a Rigaku AFC-6S 4-circle diffractometer with
cooling provided by an Oxford Cryosystems Cryostream. The
structure was solved, refined and displayed as for 2b. In complex
4a, the fluoropyridyl ligand exhibits disorder in the form of two
rotameric superpositions (differing by 180◦ rotation about the
C(21)–Ni bond). The relative importance of each rotamer was
refined yielding occupancies for C(22), F(1) and N(1) of 0.73 and
for C(22B), F(1B) and N(1B) of 0.27. These atoms were refined
isotropically. The methyl group also occupied two positions,
C(12) and C(12B); their occupancies were refined to yield values
of 0.71 and 0.29. Hydrogen atoms were placed with a riding
model.
◦
7a which may be recrystallised from hexane at −20 C. Yield:
242 mg (0.48 mmol, 74%). Anal. Calcd for C19H33NF4P2O2Ni:
C, 45.27; H, 6.60; N, 2.78. Found: C, 45.57; H, 6.65; N, 2.83%.
IR (ATR, cm−1): 1622 (mC=O). MS (FAB, m/z, relative intensity):
444 (31, [Ni(PEt3)2(C5F4N)]+), 268 (100, [Et3P–C5F4N)]+), 221
(15, [AcO–C5F4N]).
[NiF(2-C5NF4)(dcpe)] (8a)
A solution of dcpe (91 mg, 0.22 mmol) in benzene (2 mL)
was added to a solution of complex 1a (100 mg, 0.22 mmol)
in benzene (5 mL). The solution was stirred for 2 h and the
volatiles were removed under vacuum. The residue was washed
vigorously with hexane (2 × 5 mL) and filtered. The residue was
dried under vacuum affording a waxy yellow solid. The complex
was recrystallised from benzene at room temperature yielding
small yellow crystals. Yield: 75 mg (0.12 mmol, 52%). Anal.
Calcd for C31H48NF5P2Ni: C, 57.25; H, 7.44; N, 2.15. Found: C,
58.01; H, 7.52; N, 2.02%.
Crystal structure of 7a and 8a·21 C6H6. Crystal structures
4
of 7a and 8a were determined using a Bruker SMART Apex
X-ray diffractometer. The crystals were cooled, using an Ox-
ford Cryosystems Cryostream. Diffractometer control, data
collection and initial unit cell determination was performed
using SMART.38 Frame integration and unit-cell refinement
software was carried out with SAINT+.38 Absorption cor-
rections were applied by SADABS.38 Structures were solved,
Table 8 Crystallographic data for 2b, 4a, 7a and 8a·21 C6H6
4
2b
4a
7a
8a·2 1 C6H6
4
Formula
C17H31F3N4NiP2
469.11
9.1396(12)
12.7408(12)
19.819(3)
C25H35F4NNiP2
546.19
30.943(4)
30.943(4)
11.522(2)
C19H33F4NNiO2P2
504.11
10.7713(7)
13.1418(9)
16.9266(12)
2(C31H48F5NNiP2)·4 1 (C6H6)
2
M
a/A
1652.19
˚
˚
˚
14.2302(6)
14.8904(6)
21.0407(9)
87.9680(10)
77.0250(10)
76.7500(10)
4228.3(3)
120(2)
b/A
c/A
a /◦
b/◦
c /◦
97.383(11)
92.929(2)
3
˚
V/A
T/K
2288.7(5)
150(2)
P21/c
4
1.020
29439
9834
0.1246
R1 = 0.0450
wR2 = 0.0858
R1 = 0.1477
wR2 = 0.1082
11032(3)
150(2)
I41/a
16
0.859
4856
4856
0.000
R1 = 0.0617
wR2 = 0.1354
R1 = 0.3019
wR2 = 0.2219
2392.9(3)
115(2)
P21/n
4
0.990
16257
5489
0.0235
R1 = 0.0275
wR2 = 0.0694
R1 = 0.0327
wR2 = 0.0721
¯
Space group
Z
P1
2
l(Mo-Ka)/mm−1
Reflxn measd
Reflxn indep.
Rint
0.588
29431
19340
0.0278
R1 = 0.0426
wR2 = 0.1025
R1 = 0.0656
wR2 = 0.1151
Final R[I > 2r(I)]
Final R (all data)
3 6 9 4
D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5

View Article Online
refined and displayed as for 2b. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed using a
riding model and included in the refinement at calculated
positions.
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In complex 7a, the fluoropyridyl ligand exhibits disorder in
the form of two rotameric superpositions (differing by 180◦
rotation about the C(1)–Ni bond). The relative importance of
each rotamer has been refined using a second FVAR parameter
which determined the occupancy of F(1) and F(1A) and is
approximately 3 : 1. Each pair of C(2), N(1A) and C(2A),
N(1) is constrained to be coincident with identical anisotropic
displacement parameters. The sites of C(3)–C(5) and F(2)–F(5)
are assumed to be identical to those of the corresponding atoms
of the major rotamer and the occupancy is not refined for these
atoms (assumed to be unity). This disorder also manifests itself
in somewhat larger U values for these atoms.
CCDC reference numbers 278468, 280119, 278469 and
280118.
See http://dx.doi.org/10.1039/b510052f for crystallographic
data in CIF or other electronic format.
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Acknowledgements
18 T. Braun, R. N. Perutz and M. I. Sladek, Chem. Commun., 2001,
We acknowledge the support of EPSRC and the European
Commission (Marie Curie programme). We also thank Dr
P. Timmins and Prof. P. H. Walton for assistance with a crystal
structure and thank Professor Todd Marder (Durham) and Dr
John McGrady for advice.
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D a l t o n T r a n s . , 2 0 0 5 , 3 6 8 6 – 3 6 9 5
3 6 9 5