Synthesis and reactivities of new NCN-type pincer complexes of nickel

Article abstract of DOI:10.1021/om5010176

This report describes the preparation, characterization, and reactivities of a new family of Ni(II) complexes based on the pincer-type ligands R-NCN^pz (κ^N,κ^C,κ^N-1,3-bis(pyrazole),5-R-C₆H₂; R = H, OMe). Ullman coupling of pyrazole with 1,3-diiodobenzene or 1,3-dibromo-5-methoxybenzene gave the compounds R-NC(H)N^pz, which were refluxed in xylene with (i-PrCN)NiBr₂ and NEt₃ to give the complexes (NCN^pz)NiBr (1) and (MeO-NCN^pz)NiBr (2) via C-H nickelation. The aryloxide derivative (NCN^pz)Ni(OAr) (3; Ar = 2,6-t-Bu₂-4-Me-OC₆H₂) was prepared by treating the bromo precursor 1 with NaOAr, whereas the analogous reaction with NaOEt gave instead the protonated or ethoxy-functionalized ligand NC(H)N^pz (major) and NC(OEt)N^pz (minor). The new complexes 1-3 were fully characterized, including solid-state structures. Attempted oxidation of these complexes failed to give the target trivalent derivatives, leading instead to intractable, deeply colored Ni-containing solids, various ligand-derived side products, or an unusual I₂ adduct. For example, treatment of 1 or 2 with N-bromosuccinimide gave sparingly soluble dark solids that appear to be paramagnetic, whereas 3 reacted with Br₂ to give short-lived dark intermediates that decomposed over seconds to give ArOH. Reaction of the bromo complexes with aqueous H₂O₂ (30%) gave the functionalized ligands Br-NC(OH)N^pz and NC(OH)N^pz (from 1) or MeO-NC(OH)N^pz (from 2), whereas 1 reacted with I₂ to generate (NCN^pz)NiBr·I₂, an iodine adduct displaying weak Br-I interactions. Heating 1 in EtOH in air generated NC(OEt)N^pz. This ligand derivatization could be extended to other alcohols (MeOH, i-PrOH, CF₃CH₂OH) and amines (morpholine, cyclohexylamine, aniline). Possible mechanistic scenarios for the observed oxidative, Ni-promoted C_ipso-X bond formation reactions are discussed in the context of relevant literature precedents.

Full text of DOI:10.1021/om5010176

Article
pubs.acs.org/Organometallics
Synthesis and Reactivities of New NCN-Type Pincer Complexes of
Nickel
Jean-Philippe Cloutier, Boris Vabre, Berline Moungang-Soume, and Davit Zargarian*
́
́ ́ ́ ́ ́
Departement de chimie, Universite de Montreal, Montreal, Quebec, Canada H3C 3J7
*
S Supporting Information
ABSTRACT: This report describes the preparation, characterization, and reactivities of a new family of Ni(II) complexes based
pz
N C N′
on the pincer-type ligands R-NCN (κ ,κ ,κ -1,3-bis(pyrazole),5-R-C H ; R = H, OMe). Ullman coupling of pyrazole with 1,3-
6
2
pz
diiodobenzene or 1,3-dibromo-5-methoxybenzene gave the compounds R-NC(H)N , which were refluxed in xylene with (i-
PrCN)NiBr and NEt to give the complexes (NCN )NiBr (1) and (MeO-NCN )NiBr (2) via C−H nickelation. The aryloxide
derivative (NCN )Ni(OAr) (3; Ar = 2,6-t-Bu -4-Me-OC H ) was prepared by treating the bromo precursor 1 with NaOAr,
whereas the analogous reaction with NaOEt gave instead the protonated or ethoxy-functionalized ligand NC(H)N (major) and
NC(OEt)N (minor). The new complexes 1−3 were fully characterized, including solid-state structures. Attempted oxidation of
these complexes failed to give the target trivalent derivatives, leading instead to intractable, deeply colored Ni-containing solids,
pz
pz
2
3
pz
2
6
2
pz
pz
various ligand-derived side products, or an unusual I adduct. For example, treatment of 1 or 2 with N-bromosuccinimide gave
2
sparingly soluble dark solids that appear to be paramagnetic, whereas 3 reacted with Br to give short-lived dark intermediates
2
that decomposed over seconds to give ArOH. Reaction of the bromo complexes with aqueous H O (30%) gave the
2
2
pz
pz
pz
functionalized ligands Br-NC(OH)N and NC(OH)N (from 1) or MeO-NC(OH)N (from 2), whereas 1 reacted with I to
generate (NCN )NiBr·I , an iodine adduct displaying weak Br−I interactions. Heating 1 in EtOH in air generated NC(OEt)N .
This ligand derivatization could be extended to other alcohols (MeOH, i-PrOH, CF CH OH) and amines (morpholine,
2
pz
pz
2
3
2
cyclohexylamine, aniline). Possible mechanistic scenarios for the observed oxidative, Ni-promoted Cipso−X bond formation
reactions are discussed in the context of relevant literature precedents.
INTRODUCTION
including the C−H activation step involved in the anaerobic
■
8
oxidation of methane by methanotrophic archaea.
Transition-metal complexes featuring NCN-type pincer ligands
have proven to be highly versatile in diverse applications,
including catalysis and materials. NCN ligands have also
played a prominent role in the context of organonickel
chemistry. For instance, a variety of mononuclear, divalent
species (NCN)NiX and [(NCN)NiL] are known to promote
Heck type cross-coupling and Michael additions, whereas
polynuclear dendrimers composed of (NCN)Ni structural units
have displayed important materials properties. Another
important characteristic of NCN ligands is their capacity to
stabilize trivalent nickel species that catalyze atom transfer
radical additions and polymerizations. Isolable and stable
II
van Koten’s group introduced the (NCN)Ni and (NCN)-
III
9
1
Ni families of complexes in the early 1980s, and most of the
major developments in this area, including the elaboration of
synthetic routes for the preparation of a large variety of these
complexes, have been driven by this group. The two main
synthetic strategies employed for this purpose involve (a)
oxidative addition of halogenated NCN ligands, NC(X)N (X =
Br, I), to zerovalent nickel precursors and (b) transmetalation
of a lithiated ligand, NC(Li)N, first to Au(I) and then to
+
2
3
4
9
b,10
Ni(II).
While these synthetic routes generally lead to good
5
yields of the target (NCN)NiX complexes, they involve
manipulation of air- and moisture-sensitive reagents (e.g.,
III
complexes such as (NCN)NiX , featuring Ni −C bonds, are
valuable models for difficult to isolate trivalent species that play
2
6
important roles in hydrocarbon oxidation, in various coupling
Received: October 6, 2014
Published: December 23, 2014
7
reactions, and in many bioinorganic and enzymatic reactions,
©
2014 American Chemical Society
133
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Scheme 1. Reactivities of NC(H)N - and NC(H)N -Type Ligands with Ni(II) Salts
Article
im
pz
Scheme 2. Synthetic Routes to Ligands a and b and Complexes 1 and 2
RLi, Ni(COD) ) and require the use of elaborate (halo-
The present report describes the preparation of (R-
2
pz
genated) preligands and relatively expensive gold precursors. In
contrast, a number of pincer-Ni compounds can be prepared
more conveniently through direct C−H nickelation of the
NCN )NiBr (R = H (1), MeO (2)) via direct C−H
nickelation. Spectroscopic and solid-state characterization of
pz
1, 2, and the aryloxide derivative (NCN )Ni(OAr) (Ar = 2,6-t-
1
1
12
13
ligand by simple Ni(II) salts (e.g., PCP, POCOP, POCN,
Bu , 4-Me-C H ; 3) has allowed us to compare the main
2 6 2
1
4
structural features of these complexes to those of previously
reported NCN-Ni complexes. Attempts to oxidize these new
complexes did not lead to the target Ni(III) derivatives,
revealing instead unexpected and uncommon reactivities that
PIMCOP ). Given many of the unique characteristics of
NCN-Ni systems, it would be desirable to develop simple
synthetic routes to this family of complexes involving a direct
C−H nickelation strategy. Just such an approach appeared in
pz
allow functionalization of NCN ligands by fairly facile C−O
2
011: the group of Gong and Song reported that 1,3-
and C−N bond formation reactions, as described below.
phenylene-based NC(H)N ligands featuring imidazole donor
im
moieties react directly with NiCl2 to give (NCN )NiCl
(
Scheme 1), the first NCN-type complex of nickel prepared
RESULTS AND DISCUSSION
■
15
by C−H nickelation.
The synthetic methodology used for the preparation of the 1,3-
bis(pyrazolyl)arene ligands a and b and their nickelation is
shown in Scheme 2. Ligand a is a known substance that can be
prepared using a Cu-catalyzed Ulmann coupling of pyrazole
16
Our longstanding interest in organonickel chemistry and
17
pincer-type Ni complexes, as well as the great promise of
NCN ligands for developing the chemistry of Ni(III)
complexes, inspired us to explore the efficient synthesis of
new families of NCN-Ni complexes via C−H nickelation. An
initial effort in this direction was unsuccessful: 1,3-bis(3,5-
21
with dihalobenzenes. A modified version of this coupling
methodology allowed the preparation of the new ligand 1,3-
bis(pyrazolyl)-5-methoxybenzene (b) from 1,3-dibromo-5-
methoxybenzene, which was, in turn, obtained from methyl-
ation of commercially available 3,5-dibromophenol using MeI
dimethylpyrazolyl)propane reacted with NiBr (EtOH/acetone,
2
reflux, 24 h) or (THF) NiBr (benzene or toluene, room
2
2
2
0
temperature, 30 min) to give the blue tetrahedral complex
and K CO . Treating a or b with (i-PrCN)NiBr in refluxing
2
3
2
N
N′
pz
xylene over 2 days gave the target complexes 1 or 2 in 50% and
4% yields, respectively (Scheme 2). Both complexes are stable
(
κ ,κ -NC (H)N )NiBr , which could not be induced to
sp3 2
2
undergo the required C−H nickelation to yield the target
pincer compound (κ ,κ ,κ -NC N )NiBr (Scheme 1).
This finding prompted us to modify our approach in two
respects. First, we focused on examining the reactivities of
NCN ligands based on a 1,3-phenylene backbone, which is
usually more prone to undergo the requisite C−H nickelation.
Second, we selected the starting materials (MeCN) NiBr
in air and are quite robust thermally: 1 can be heated to 250 °C
N
C
N′
pz
18
sp3
without decomposition, and 2 starts decomposing at ∼190 °C.
1
21
The H NMR spectrum of a matches reported data, and b
1
displayed the expected H resonances, namely three doublets
pz
and two triplets for the aromatic protons, plus a singlet for
1
OCH . The H NMR spectra of 1 and 2 display four
3
19
2
n
resonances for the eight aromatic protons that are pairwise
2
0
and (i-PrCN)NiBr2, which have proven to be effective Ni
precursors for C−H nickelation of a variety of pincer ligands.
equivalent, thanks to the C axis bisecting these structures and
2
overlapping C4, C1, Ni, and Br atoms; they also display a triplet
1
34
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Article
Figure 1. Molecular structures of 1 (full view, top left; side view, bottom) and 2 (top right; only one of the two molecules present in the asymmetric
unit is shown). Thermal ellipsoids are shown at the 50% probability level, and H atoms are omitted for clarity. Selected bond distances (Å) and
angles (deg) are as follows. 1: Ni1−Br1 = 2.3680(9); Ni−N1 = 1.908(3); Ni−C1 = 1.825(5); C1−Ni1−Br1 = 180.000(1); N1a−Ni1−N1 =
162.07(17). 2: Ni1−Br1 = 2.3846(7); Ni1−N1 = 1.902(3); Ni1−N3 = 1.898(3); Ni1−C1 = 1.822(4); N3−Ni1−N1 = 162.00(13); C1−Ni1−Br1 =
177.05(11).
Chart 1. Relative Ease of Oxidation for (ECE)NiBr Complexes
for H4 in 1 and a singlet for OCH in 2. The following features
details of data collection. The Ni center in both compounds
adopts a square-planar geometry, and the solid-state structures
are virtually flat, as can be inferred from the near-coplanarity
(angle <1°) of the two fused five-membered nickelacycles
defining the (NCN)Ni moiety. The smaller than ideal N−Ni−
N angles of 162° observed in 1 and 2 are due to the small bite
angle of the NCN ligand, a phenomenon observed in other
NCN-Ni systems, including van Koten’s phebox- or imine-
3
of these spectra most readily confirmed the nickelation of a and
b. First, the spectra of 1 and 2 did not show a signal due to H2,
the proton ortho to both pyrazole moieties. Second, the signal
for H7 shifted downfield by ca. 0.10 ppm in 1 and 0.15 ppm in
2, whereas the remaining protons in both complexes shifted
upfield, by about 0.7 ppm for H3 and 0.06−0.1 ppm for all
others. Lastly, multiplicities of the methoxy and pyrazole
protons remained unchanged, but H4 changed from a multiplet
to a triplet and H3 changed from a multiplet to a doublet in 1
22
based (NCN)NiBr systems. The Ni−Br bond distances in the
latter complexes are somewhat shorter relative to those of 1 and
2 (2.36 and 2.34 Å vs 2.37 and 2.38 Å, respectively), whereas
less consistent differences were noted in the Ni−C and Ni−N
bond distances of these complexes (Ni−C, 1.83 and 1.82 Å vs
1.85 and 1.82 Å, respectively; Ni−N, 1.91 and 1.90 Å vs 1.94
1
3
1
and from a doublet to a singlet in 2. The C{ H} NMR spectra
of the complexes displayed eight singlets, five for the pairwise-
equivalent carbon nuclei off the C axis and three for ipso-C, p-
2
C, and OCH3.
22
Single crystals suitable for X-ray analysis were obtained by
slow recrystallization of 1 and 2 from warm THF; Figure 1
shows the solid-state structures of these complexes, and Table
S1 (in the Supporting Information) gives the crystal data and
and 1.91 Å, respectively).
Cyclic Voltammetry Measurements and Chemical
Oxidation Attempts on Complexes 1 and 2. As mentioned
earlier, some of the main characteristics of the tertiary amine
1
35
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Article
Figure 2. Molecular structure of 1·I (side view, left; plane view, right; one-dimensional polymer, bottom). Thermal ellipsoids are shown at the 50%
2
probability level, and H atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ni1−Br1= 2.3744(5); Ni−N1 = 1.905(3); Ni−
N3= 1.901(3); Ni−C1 = 1.827(3); I1−I1a = 2.7233(4); I2−I2b = 2.7322(4); Br1−I1 = 3.2406(4); Br1−I2 = 3.2110(6); C1−Ni1−Br1 =
1
=
78.23(10); N1−Ni1−N3 = 163.11(12); Ni1−Br1−I1 = 115.60; Ni1−Br1−I2 = 114.21; Br1−I1−I1a = 177.12; Br1−I2−I2a = 169.64; I1−Br1−I2
92.36.
based (NCNNR2)NiX complexes pioneered by van Koten were
their highly electron-rich character and strong tendency to form
stable trivalent derivatives. With the new NCN complexes 1
and 2 in hand, we have investigated their redox properties and
tested their reactivity toward chemical oxidants, as described
below.
donating character of the pyrazole moieties in our complexes.
Comparing these oxidation potentials to the corresponding
values measured for closely related ECE-Ni pincer com-
pz
1
4,17,25,26
plexes
allows us to arrange these compounds in terms
of their ease of oxidation, summarized in Chart 1.
It is noteworthy that the methoxy-substituted system in 2
appears to be less susceptible to oxidation relative to 1; this is in
contrast to our previous observation that (p-MeO-POCOP)-
Cyclic voltammetry measurements (Figure S22 in the
Supporting Information) showed that the redox profiles of 1
and 2 are quite different from those of their tertiary amine
II
NiBr is more susceptible (by ca. 190 mV) to undergo a Ni /
II
III
III
based counterparts: 1 showed a quasi-reversible Ni /Ni redox
couple at 452 mV, whereas 2 showed an irreversible oxidation
Ni oxidation relative to its unsubstituted analogue, whereas
(p-MeOOC-POCOP)NiBr is less susceptible (by ca. 130
2
3
26
at a higher potential (E = 545 mV). Both of these values are
mV). We speculate that the observed higher oxidation
ox
NMe2
much higher than the E° value for van Koten’s (NCN
NiBr, which can be rationalized by the poorer electron
)-
potential of 2 relative to 1 may be due to a less efficient
conjugation in this molecule brought about by the slight planar
1
/2
2
4
1
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Article
distortion that is presumably caused by the methoxy substituent
In its solid state, each 1·I exhibits Br−I interactions with half
2
(
Figure 1).
of two I molecules, thus creating a one-dimensional zigzag
2
pz
Even though the new (R-NCN )NiBr complexes 1 and 2
chain parrallel to the c axis. Interlocking of four of these chains
pz
appeared to be less electron rich than most (ECE)NiX systems
results in π stacking of the (NCN )NiBr units parallel to the c
that are known to give isolable trivalent species (e.g.,
axis with a 3.3 Å distance. Although most structural features of
NR2
pz
(
NCN )NiX, (PC P)NiBr, and (POC OP)NiBr), we
sp3 sp3
the (NCN )NiBr unit are largely unperturbed in the solid-state
were encouraged that they appeared to be more electron rich
than the related (POCN)NiBr compounds, which are known to
structure of 1·I , close inspection of some parameters reveals
2
slight but meaningful differences in comparison to those of 1.
13
stabilize trivalent species. Therefore, we proceeded to test the
For instance, 1·I displays a somewhat longer Ni−Br bond
2
pz
capacity of R-NCN ligands to stabilize trivalent compounds
distance and a very slightly larger N1−Ni−N3 bite angle
relative to 1 (2.3744(5) vs 2.3680(9) Å; ca. 163 vs 162°), as
well as a slightly longer I−I bond distance relative to that of free
by treating 1 and 2 with oxidants that had proven effective for
generation of trivalent Ni species from some of the pincer-type
29
Ni(II) complexes studied by our group (e.g., Br , N-
I (average 2.727(4) vs 2.715(6) Å). These observations are
2
2
12b,13a,25,27
bromosuccinimide (NBS), or CuBr2).
In all cases,
consistent with the partial transfer of electron density from Br
we observed the same color changes as noted previously during
the oxidation attempts on other (ECE)NiX systems: the
reaction mixture turned dark immediately and a deeply colored
solid precipitated out of the solution.
to I . Similar observations are often encountered in transitionn-
metal halides engaged in noncovalent-type halogen bonds.
2
30,31
Although there are many examples of such transition-metal−
3
2
halogen−halogen interactions in the literature, to our
These sparingly soluble compounds appeared to be very
knowledge 1·I is only the second complex to display Ni−
2
33
similar to the trivalent species previously isolated by our
X−halogen bonding. On the other hand, halogen−halogen
interactions have been observed in the closely related PCP-Pd
and NCN-M complexes (M = Pd, Pt) shown in Chart 2.
1
2b,13a,25,27,28
group.
For instance, dissolving the solids in
question in pyridine or acetonitrile gave deep purple samples
that proved NMR silent, consistent with the anticipated
paramagnetism of the trivalent derivatives; these solutions
also displayed UV−vis spectral patterns similar to those of fully
Chart 2. Other Pincer Complexes Featuring Halogen
Bonding
characterized octahedral (NCNNMe2)NiBr (L). A crude
9c
2
sample obtained from the attempted oxidation of 2 with NBS
registered a magnetic susceptibility of 2.96, which is much
higher than the value anticipated for a single unpaired electron.
The observation of higher than expected magnetic moment for
the paramagnetic samples obtained by oxidation of complex 1
can be rationalized by invoking a scenario in which these
samples exist in both low- and high-spin states with one and
three unpaired electrons, respectively, which would result in an
intermediate value for the magnetic moment corresponding to
the bulk sample. Unfortunately, all attempts to obtain
analytically pure samples and grow crystals suitable for X-ray
analysis were unsuccessful, circumventing an unambiguous
identification of these products.
Reactivity of 1 with I . An unexpected result was obtained
2
when we treated complex 1 with iodine in anticipation of
oxidizing the Ni center (eq 1). Addition of 1 equiv of I to a
A very close analogue of 1·I is the complex (NCN)PdI·I
2
2
2
34
THF solution of 1 generated a deep orange mixture from which
an orange-brown solid precipitated over 20 min. In contrast to
the dark solids obtained from the oxidation attempts noted
reported by van Koten. The solid-state structure of this
compound shows two iodine molecules interacting with the
Pd−I moiety and forming nearly linear outer Pd−I−I−I angles
(172−176°) and virtually perpendicular inner I −(PdI)−I
above, the orange-brown solid obtained from the I reaction
2
2
2
was soluble in CD Cl and CDCl and displayed well-resolved
angles. In this compound, the I−I distance for the I molecules
2
2
3
2
NMR spectra, attesting to its diamagnetic character. Curiously,
is ca. 2.760(1) Å, and the average PdI−I distance is 3.280 Å,
2
1
the H NMR spectrum of this orange solid was virtually
which is nearly 17% shorter than the sum of van der Waals
(vdW) radii for two iodide atoms (3.96 Å).
identical with the spectral pattern of the starting material, yet
the sample was still orange-brown (as opposed to yellow for 1).
Fortunately, recrystallization (from CH Cl ) gave single crystals
Wendt et al. have also reported X−I interactions between
(PCP)PdX (X = Cl, Br, I) and one iodine or two electron-poor
2
2
35
that allowed us to identify this product as the iodine adduct 1·I2
shown in Figure 2.
hydrocarbyl iodides. These authors have argued that the
strong trans influence of the central aryl moiety in these
compounds elevates the energy of the halide lone pairs, thus
promoting their noncovalent interactions with I or RI. The
2
Br−I distance in (PCP)PdBr·I is nearly 24% shorter than the
2
2
35
sum of vdW radii for Br and I atoms (3.83 Å); by comparison,
we observe ∼16% compression in 1·I .
2
BAM
Of the Pt complexes shown in Chart 2, (NCN )Pt(Me)2I·
I (BAM = bis(7-azaindol-1-yl)methyl) shows I−I interactions
2
2
3
6
similar to those in the cases discussed above, whereas in
NMe2
(
NCN
)PtI·I the noncovalent interaction is between the
2
1
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Article
37
Pt-based HOMO and I2. Finally, both types of interactions,
38
PtI−I and Pt−I , are observed in (NCN)PtI(I ) . These
2
2
2 2
latter compounds have been proposed by van Koten et al. as
models for the initial stage of dihalide oxidative addition to
Pt(II).
Reactivities of 1 and 2 with Hydrogen Peroxide.
Following the unsuccessful oxidation attempts described above,
we sought a different chemical oxidant for the preparation of
trivalent derivatives. The absence in 1 and 2 of donor moieties
such as phosphines and phosphinites that are susceptible to
oxidation opens up the possibility of using a stronger oxidant
such as hydrogen peroxide for the purpose of generating
trivalent species. Thus, as a final attempt to oxidize our (R-
pz
NCN )NiBr complexes, we tested their reactivities with this
oxidant, as described below.
Addition of an excess of H O (30% solution in water) to a
pz
2
2
Figure 3. Molecular structure of Br-NCN -OH. Thermal ellipsoids
are shown at the 50% probability level, and H atoms are omitted for
clarity. See the Supporting Information for the CIF file giving
complete structural parameters and Table S2 for selected bond
distances.
CH Cl solution of 1 brought about an immediate color change
2
2
from yellow to deep purple. The intense color of the reaction
reaction mixture turned gradually to pale orange (ca. 12 h at
room temperature), indicating that the in situ generated
product has limited thermal stability. All attempts at arresting
this deep purple species before it decomposed proved
unsuccessful, giving intractable dark solids that have remained
unidentified. Thus, we turned our attention to isolating and
identifying the product(s) of decomposition in the hopes of
finding clues to the nature of the oxidation process. For this
purpose, the pale orange final reaction mixture of an oxidation
reaction using H O was further diluted with water and
the pyrazole moieties has rotated 180°, placing the imine
nitrogen away from the OH moiety.
Complex 2 reacted in a similar manner to give the
pz
hydroxylated product, MeO-NC(OH)N , but no bromination
product was observed. It is worth noting that the yellow to
deep purple color change observed for 1 was not detected in
the reaction of 2, suggesting that this phenomenon is related to
bromination of the p-C−H moiety; the latter presumably
involves aromatic intermediates featuring unpaired electrons.
Two tests were conducted to probe the nature of this reaction.
First, we repeated the treatment of 1 with H O in the presence
of 1.2 equiv of TEMPO; this experiment yielded only
NC(OH)N , implying that free radicals are likely involved in
the formation of Br-NC(OH)N . Second, it was established
that neither of the above functionalization products was
observed when a THF mixture of 1 and NaOH was refluxed
for 18 h; the only tractable product of this experiment was the
protonated ligand NC(H)N . We conclude that an in situ
generated Ni−OH derivative is insufficient for this function-
alization reaction, the oxidizing character of H O being an
2
2
extracted with CH Cl , followed by column chromatography of
2
2
the organic layer on silica. This workup procedure afforded two
products arising from the oxidative degradation of 1, namely
pz
pz
NC(OH)N (32%) and Br-NC(OH)N (7%), as shown in eq
. Control experiments confirmed that the Ni center plays a
2
2
2
crucial role in this reaction: none of the latter species was
pz
pz
detected when NC(H)N (a) was treated with H O or NBS
2
2
pz
in the absence of 1.
The functionalized derivatives of ligand a have been
characterized by NMR and MS analyses (electrospray
1
pz
ionization source). The H NMR of NC(OH)N is consistent
pz
with its C symmetry and displays a triplet and a doublet for
2
v
3
the mutually coupled protons of the central ring ( J = 8 Hz),
HH
2
2
1
pz
whereas the H NMR of Br-NC(OH)N displays only a singlet
for the symmetry-related protons of the central ring. Perhaps
the most interesting feature in these spectra is the downfield
essential requirement.
The oxidatively induced functionalization of the NCNpz
ligand in 1 noted above, i.e., hydroxylation at the ipso position
and bromination at the para position, are reminiscent of
analogous reactivities reported in the literature. For instance,
pz
signal due to the acidic OH proton, at 12.32 ppm for NCN -
OH and 12.43 ppm for Br-NC(OH)N . Isolation of a suitable
pz
II
single crystal for the latter substance has allowed us to examine
its solid-state structure by X-ray diffraction studies. The
molecular structure thus elucidated (Figure 3) shows the O−
H−N interaction responsible for the downfield NMR signals;
the observed O−H, N−H, and N−O distances are 0.827, 1.78,
and 2.54 Å, respectively. It is also interesting to note that one of
Hillhouse has demonstrated that L Ni (R)(X) (X = OR′,
2
NR′ ) can be induced to undergo reductive elimination of R-X
2
in the presence of external oxidants that can generate reactive
+
39
Ni(III) intermediates, including O , I , and (AcCp) Fe .
A
2
2
2
very recent report has also shown that a pincerlike complex of
Ni(II) can promote bromination of aromatic substrates in the
1
38
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Organometallics
Article
Scheme 3. Reactivities of 1 with [OR]−
4
8,49
presence of strong oxidants such as PhI(OAc) and (C F )I-
tion.
The analogous reaction with KO(t-Bu) did allow the
2
6 5
4
0
(
OCOCF ) .
observation of a more persistent intermediate, but the detected
species decomposed to intractable materials before it could be
identified with confidence.
3
2
On the other hand, van Koten’s group has reported a number
of studies documenting the CuX -induced (X = Cl, Br) para
2
chlorination and para bromination of NCN ligands in the
That the reactions observed above with alkoxide salts are, in
fact, even more complex than they appear was revealed by the
following observations. First, when the reaction with NaOEt
was conducted under rigorously oxygen-free conditions, we
obtained ligand a only, the ethoxy-functionalized product not
being detected at all; the analogous reaction with KO(t-Bu) led
to a complex mixture from which no tractable compound could
be isolated or identified. Second, simply refluxing an ethanol
solution of 1 in air (without any base or source of ethoxide) led
to exclusive formation of the C−O coupling product,
2
+
trivalent pincer complexes [(NCN)Ru(tpy)] (NCN = 2,6-
bis[(dimethylamino)methyl]phenyl, 2,6-bis(2-pyridyl)phenyl;
41
tpy = 2,2′:6′,2″-terpyridine). Mechanistic probes of these
halogenations have established that a first-order oxidation of
the divalent Ru center precedes the halogenation. Moreover,
depending on reaction conditions, the oxidized species can
undergo a C−C bond formation that leads to a dimerization of
42
the trivalent complex. Similar reactivities, including dimeriza-
tion, have been observed with the PCP analogue of these
4
3,44
complexes.
These observations imply that the spin density
pz
NC(OEt)N ; the same reactivity was observed with complex
. Although these observations do not provide a clear
2
+
in the paramagnetic species [(pincer)Ru(tpy)] generated by
the one-electron oxidation of the pincer complexes is partially
localized on the para position of the phenylene ligand. We
2
mechanistic picture for this reactivity, they have encouraged
us to seek a novel methodology for functionalization of the
pyrazole-based NCN ligands under study; the results of these
studies are presented in the next section.
The aryloxide derivative 3 was isolated as a bright orange
powder that proved to be much more soluble in standard
solvents relative to its precursor 1. This new derivative was
found to be highly stable at higher temperatures, decom-
pz
speculate that a similar phenomenon is at play in our NCN -Ni
systems, which can explain the observed bromination reaction,
the irreversible electrochemical oxidation, and also the unstable
products of chemical oxidation.
Preparation of (NCN)Ni(OR) and Oxidation Attempts.
Previous studies have shown that the nature of X ligands in
pincer-type complexes (EXE)NiX has a dramatic influence on
the oxidation potential of these species. For instance, the
1
position of solid-state samples starting above 220 °C. The H
and 13C{ H} NMR spectra of 3 were in accordance with the
proposed structure of this complex. For instance, we observed
five aromatic signals each integrating for two protons, indicating
1
oxidation half-cell potential values, E , are much larger for
O
45
(
POCN)NiBr than for their amido derivatives, and the same
trend has been observed with (POC OP)NiX (OR < Br) and
sp3
pz
4
6
the pairwise equivalence of the symmetry-related NCN and
BHT aryl protons. In addition, the signal due to the p-H on the
central aryl moiety of NCN is a triplet, and there is only one
signal for the 18 equivalent protons of the t-Bu moieties,
implying a low energy barrier for the rotation of BHT about the
Ni−O and C−O axes. Finally, conversion of the bromo
precursor to the BHT derivative caused upfield shifts for all
NCN protons by 0.5−1.28 ppm.
The solid-state structure of 3 was confirmed by X-ray
diffraction studies conducted on single crystals obtained from
(
PCP)NiX (X = NH , OH, CN). In light of the generally
2
greater thermal stability of Ni-OR derivatives, we elected to
pz
pz
prepare (NCN )Ni(OR) and attempt their oxidation, as
shown in Scheme 3 and described below.
Complex 1 reacted very differently with different sources of
−
OR (Scheme 3): treatment with the in situ deprotonated BHT
(
butylated hydroxytoluene, 2,6-(t-Bu) -4-Me-C H OH) gave
2 6 2
pz
the anticipated simple Ni-OAr derivative 3, whereas NaOEt
gave instead the protio ligand a as the major product, as well as
pz
the ethoxide-functionalized compound NC(OEt)N . Although
recrystallization using Et O at room temperature (Figure 4).
no intermediate species was detected during the latter
reactions, we speculate that the observed side products are
2
The geometry around the Ni center in this complex is distorted
square planar (N−Ni−N = 163°), and most structural
parameters are similar to the corresponding values in 1 and
2. The BHT ring is nearly perpendicular to the coordination
plane (88°). The Ni−O distance in 3 is similar to the
derived from decomposition of the unstable intermediate
pz
(
NCN )Ni(OEt). For instance, β-H elimination of the latter
would give acetaldehyde and (NCN)Ni(H), the latter giving a
47
by reductive elimination. On the other hand, direct reductive
pz
elimination from (NCN )Ni(OEt) would generate the ethoxy-
corresponding distance in (POC OP)Ni(OMes) (ca. 1.88 Å
vs 1.89 Å; Mes = 2,4,6-(Me) C H ).
sp3
pz
50
functionalized product NC(OEt)N by C−O bond forma-
3
6
2
1
39
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Organometallics
Article
inspired us to explore the scope of this unusual reaction, as
described below (eq 3 in Table 1); the preliminary studies
undertaken to date were conducted with complex 1 only.
Refluxing 1 with 99% EtOH for 24 h while exposing the
pz
mixture to ambient air led to the formation of NC(OEt)N in
ca. 77% yield, whereas using 95% EtOH gave the same yield but
the ethoxylation reaction proceeded much more quickly (Table
1
, entry 1). The analogous reactions with MeOH and i-PrOH
were more sluggish, but in the presence of added water (5% by
volume) we obtained 65 and 70% yields, respectively (entries 2
and 3). Small and variable quantities of the protonated ligand a
were also detected in the reactions with MeOH. Interestingly,
alcohols featuring more acidic O−H moieties resulted in either
no reaction at all (PhOH and PhCH OH) or a much lower
2
yield (CF CH OH, entry 4); for the latter reaction, addition of
3
2
5
% water had no impact on the yield or reaction time.
To our delight, the functionalization could be extended to
amines to give C−N bond formation products, albeit in
generally lower yields. For instance, the secondary amine
morpholine gave 41% yield (Table 1, entry 5) and the primary
amines cyclohexylamine and aniline gave 10% and 32% yields,
respectively (entries 5 and 6), whereas p-fluorobenzylamine and
ethanolamine did not react at all. As was the case with the
alcoholysis reaction using CF CH OH, addition of water had
Figure 4. Molecular view of 3: only one of the three molecules in the
unit cell is shown here. Thermal ellipsoids are shown at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): Ni1−O1 = 1.8827(14); Ni1−N4 =
1
.8990(17); Ni1−N1 = 1.9143(17); Ni1−C1 = 1.822(2); N1−Ni1−
N14 = 162.64(7); C1−Ni1−O1 = 173.37(8); Ni1−O1−C13 =
3
2
129.68(12).
no impact on the aminolysis of 1.
The C−O and C−N bond formation reactions observed with
Cyclic voltammetry measurements carried out on 3 (Figure
complex 1 are somewhat analogous to a recently reported Pd-
S22 in the Supporting Information) confirmed the much lower
oxidation potential of this aryloxide derivative relative to its
bromo precursor (−73 vs 452 mV); indeed, 3 undergoes a one-
electron irreversible oxidation at a potential lower than that of
ferrocene. Observation of such a facile electrochemical
oxidation seemed to bode well for the prospects of preparing
trivalent derivatives by chemical oxidation, but treatment of 3
with Br , I , or H O failed to yield the target products. For
51
catalyzed amination of alkenes in the presence of O2, and
even more so to the stoichiometric intra- or intermolecular C−
N bond formation involving a dicationic Cu(III) species
52
featuring a tetradentate macrocyclic ligand. Heating the latter
in the absence of a “nucleophilic” substrate leads to the
intramolecularly N-functionalized ligand arising via an apparent
trans C−N reductive elimination reaction between the aryl and
the tertiary amine moieties present in the coordination sphere
of the Cu center (Scheme 5). The product of intermolecular N-
functionalization is obtained at room temperature in the
presence of weakly nucleophilic amide-type substrates possess-
2
2
2
2
instance, reaction with 0.5 equiv of Br led to generation of 1
2
and free BHT, whereas the reaction with excess H O gave the
2
2
pz
hydroxylated ligand, NC(OH)N , and protonated BHT
Scheme 4). Evidently, the lower oxidation potential
(
ing a fairly acidic N−H bond (pK < 17; e.g., pyridone,
a
phthalimide, sulfanamide, etc.), whereas less acidic substrates
give a mixture of the two products. Evidently, the
intermolecular C−N bond formation reactions promoted by
this Cu(III) system require acidic substrates, in contrast to the
reactivity of complex 1 toward alcohols or amines.
Scheme 4. Attempted Oxidation of 3
As stated earlier, the alcoholysis and aminolysis reactions
pz
promoted by (NCN )NiBr (Table 1) appear to require aerobic
conditions: conducting these reactions using degassed sub-
strates and under a nitrogen purge gave protonated ligand and
only traces of the functionalized products. It occurred to us that
other mild oxidants might also promote this reaction, and to
test this idea we treated the iodine adduct 1·I with MeOD in
2
an NMR tube; the sample was kept at room temperature and
1
under a nitrogen atmosphere. H NMR analysis confirmed the
pz
complete conversion to NC(OMe)N over 5 min, implying a
dramatic acceleration of the methanolysis reaction, which
otherwise requires refluxing over several hours under aerobic
conditions.
engendered by the presence of the aryloxide moiety does not
translate into a stable and isolable trivalent derivative, implying
an inherent aspect of NCN ligands that renders such
pz
CONCLUSION
pentacoordinate, 17-electron derivatives inaccessible.
■
pz
Functionalization of NCN via Alcoholysis and
This study has led to the elaboration of a convenient, C−H
nickelation-based synthetic route for the preparation of the first
members of a new family of pyrazole-based NCN complexes.
Access to complexes 1 and 2 has allowed us to evaluate their
Aminolysis of 1. As mentioned earlier, 1 and 2 react with
pz
EtOH to yield ethoxide-functionalized derivatives of the NCN
and MeO-NCNpz ligands, respectively. These observations
1
40
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Organometallics
Article
Table 1. Functionalization of NCNpza
a
Reaction conditions: complex 1 (0.072 mmol), XH (10 mL), and 1-bromo-3,5-dimethoxybenzene (internal standard) were stirred at 70 °C for the
pz
required time. In all cases, yields were determined by GC/MS using a previously prepared calibration curve based on NC(OEt)N . In some cases,
the product was isolated following a standard chromatographic workup.
Scheme 5. Cu(III)-Promoted C−N Bond Formation
promoted alcoholysis and aminolysis of 1,3-bis(pyrazolyl)-
benzene. Although the scope of these reactions remains rather
narrow so far, and even though they require stoichiometric
quantities of Ni(II), we are hopeful that with a better
understanding of the complex pathways involved in these
reactions it might be possible to elaborate more efficient,
practical, and catalytic methodologies for functionalizing a wide
range of these aromatic substrates. Future studies will be
designed to attain these objectives.
EXPERIMENTAL SECTION
■
Materials and Methods. Unless otherwise indicated, all
manipulations were carried out using standard Schlenk and glovebox
techniques under a dry nitrogen atmosphere. Where necessary, the
solvents were dried to water contents of less than 10 ppm (determined
using a Mettler-Toledo C20 coulometric Karl Fischer titrator) by
passage through activated aluminum oxide columns (MBraun SPS)
and storage over 4 Å molecular sieves. The following starting materials
were purchased from commercial sources and used without further
purification: 1,3-diiodobenzene, 3,5-dibromophenol, pyrazole, methyl
iodide, CuI, N,N-dimethylglycine, Ni powder, Br , I , i-PrCN, 2,6-t-
oxidation potentials and assess their propensity toward
oxidation of the nickel center. The results presented in this
report do not bode well for the prospects of isolating stable
high-valent species based on pyrazole donor moieties; on the
other hand, the chemical oxidation attempts undertaken in the
course of this work have revealed potentially useful C−O and
C−N bond formation reactions that facilitate oxidative, Ni-
2
2
1
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Organometallics
Article
Bu -4-Me-C H OH (BHT), t-BuOK, N-bromosuccinimide, and
(57 mg, 33%). Recrystallization from warm CH Cl gave orange
2
6
2
2
2
morpholine (Aldrich); NEt3 and H O2 (Acros); c-C H NH
crystals after standing overnight. Gradual, spontaneous sublimation of
iodine circumvented an accurate elemental analysis of this compound.
The 1H NMR and UV−vis spectra of this adduct were virtually
identical with the corresponding spectra of 1 (see Figures S1 and S31
in the Supporting Information).
2
6
11
2
(
American Chemicals). NiBr (NCiPr) was synthesized according to
2
20
a literature procedure.
The NMR spectra were recorded on the following spectrometers:
1
1
Bruker AV400rg ( H at 400 MHz) and Bruker ARX400 ( H at 400
MHz and 13C{ H} at 100.56 MHz). Chemical shift values are reported
1
N
C
N′
{
κ ,κ ,κ -2,6-(pyrazole) -C H }Ni(BHT) (3). A dry Schlenk flask
1
2 6 3
in ppm (δ) and referenced internally to the residual solvent signals ( H
purged with N was charged with 1 (0.200 g, 0.575 mmol), BHT
and 13C: 7.26 and 77.16 ppm for CDCl ; 7.16 and 128.06 ppm for
2
3
(
0.127 g, 0,575 mmol), and THF (30 mL). NaH (0.022 g, 0.96 mmol)
C D ). Coupling constants are reported in Hz. The elemental analyses
6
6
was then added slowly to this mixture against a flow of N . The final
2
́
were performed by the Laboratoire d’Analyze Elem
ment de chimie, Universite de Montreal.
5-Methoxy-1,3-phenylene)bis(1H-pyrazole) (b). A dried
́ ́
entaire, Departe-
mixture was stirred at room temperature for 20 h. The resulting orange
solution was evaporated under vacuum and the yellow residue
́
́
(
extracted with dry Et O (5 × 10 mL). Evaporation to dryness gave a
2
Schlenk flask was charged with 3,5-dibromo-1-methoxybenzene (1.13
1
bright orange powder (0.280 g, 78% yield). H NMR (C D ): δ 7.39
6
6
g, 4.25 mmol), pyrazole (0.868 g, 12.8 mmol), K CO (3.52 g, 25.5
2
3
(
6
2H, s, H10), 6.88 (2H, d, JHH = 2.1, H5), 6.67 (1H, t, JHH = 7.8, H4),
mmol), CuI (0.032 mg, 0.170 mmol), and N,N-dimethylglycine (0.029
.57 (2H, d, JHH = 2.4, H7), 6.05 (2H, d, JHH = 7.8, H3), 5.47 (2H, t,
g, 0.286 mmol). The system was purged three times with N , and 15
13
2
JHH = 2.4, H6), 2.63 (3H, s, H12), 2.19 (18H, s, H14). C NMR
mL of DMSO was added. This mixture was then heated at 130 °C for
(
1
C D ): δ 167.6 (1C, s, C8), 143.97 (2C, s, C2), 141.18 (2C, s, C5),
40.64 (2C, s, C9), 139.88 (1C, s, C1), 126.30 (2C, s, C10), 124.77
6 6
96 h and then cooled to room temperature. Water (30 mL) and ethyl
acetate (30 mL) were then added to the final mixture, and the
resulting mixture was partitioned. The organic layer was extracted with
ethyl acetate (3 × 15 mL) and washed with brine. The ethyl acetate
fractions were combined and evaporated next, and the residue was
purified by column chromatography (silica gel; eluent 20/80 to 40/60
mixtures of ethyl acetate and hexanes). Evaporation of the combined
(
1C, s, C4), 124.36 (2C, s, C7), 121.82 (1C, s, C11), 107.23 (2C, s,
C6), 106.91 (2C, s, C3), 36.22 (2C, s, C13), 32.37 (6C, s, C14), 22.09
(
1
1C, s, C12). Anal. Calcd for C H N NiO (487.26): C, 66.55; N,
1.50; H, 6.62. Found: C, 66.16; N, 11.30; H, 6.58.
27 32 4
4
-Bromo-2,6-bis(1H-pyrazol-1-yl)phenol and 2,6-Bis(1H-pyr-
azol-1-yl)phenol. A Schlenk flask was charged with 1 (0,050 g, 0,144
1
fractions gave the final product as an off-white solid (0.171 g, 46%). H
mmol), CH Cl (30 mL), and H O (1 mL of 30% solution in water,
2
2
2
2
NMR (CDCl , 400 MHz), δ 7.99 (2H, d, J = 2.5, H5), 7.74 (2H, d,
3
HH
9.79 mmol), and the mixture was stirred for 18 h. The initially yellow
mixture changed to deep purple over 20 min, but this color faded
JHH = 1.6, H7), 7.64 (1H, t, JHH = 1.9, H1), 7.22 (2H, d, J = 1.9,
HH
H3), 6.49 (2H, t, JHH = 1.9, H6), 3.93 (3H, s, H8). 13C NMR
gradually such that the final mixture was yellowish. Addition of H O
2
(
CDCl ): δ 161.42 (1C, s, C4), 142.01 (2C, s, C2), 141.47 (2C, s,
3
(20 mL) followed by extraction with CH Cl (3 × 20 mL) and
2
2
C7), 127.19 (2C, s, C5), 108.13 (2C, s, C6), 102.98 (2C, s, C3),
02.23 (1C, s, C1), 56.02 (1C, s, C8). MS: m/z 240; the mass
spectrum is shown in the Supporting Information (Figure S23).
evaporation of the combined organic layers gave an oily residue which
was purified by column chromatography (silica; eluent CHCl3).
Evaporation of the first and second fractions gave the minor and major
components of the product mixture, respectively, as white powders (6
mg, 7%; 22 mg, 32%).
1
N
C
N′
{
κ ,κ ,κ -2,6-(pyrazole) -C H }NiBr (1). An oven-dried Schlenk
2 6 3
was taken inside the glovebox and charged with NiBr (NC(i-Pr))
2
(
1.64 g, 5.71 mmol), ligand a (1.00 g, 4.76 mmol), and NEt (0.927
Characterization Data for 4-Bromo-2,6-bis(1H-pyrazol-1-yl)-
3
1
phenol. H NMR (CDCl ): δ 12.43 (1H, s, H8), 8.21 (2H, d, J
mL, 0.673 g, 6.67 mmol). The reaction vessel was then taken out of
3
HH
=
2.4 H5), 7.75 (2H, d, H7), 7.71 (2H, s, H3), 6.52 (2H, t, J = 2.1,
the glovebox and 10 mL of m-xylene added against a flow of N . The
HH
2
13
H6). C NMR (CDCl ): δ 140.91 (1C, s, C1), 140.10 (2C, s, C7),
resulting mixture was refluxed for 36 h, cooled to room temperature,
and evaporated under reduced pressure. The resulting solid residue
was stirred with distilled water (3 × 20 mL, in air) and filtered on a
Buchner funnel, the solid residue being washed consecutively with 3 ×
3
1
29.75 (2C, s, C5), 129.09 (2C, s, C2), 121.84 (2C, s, C3), 111.05
+
(
1C, s, C4), 107.37 (2C, s, C6). LRMS: expected mass (M + H )
05.0, 307.0; found (M + H ) 305.1, 307.1.
+
3
Characterization Data for 2,6-Bis(1H-pyrazol-1-yl)phenol. 1H
2
0 mL each of cold MeOH and hexanes. The yellow powder thus
NMR (CDCl ): δ 12.32 (1H, s, H8), 8.20 (2H, d, J = 2.3 H5), 7.75
obtained was then extracted with THF, concentrated under reduced
pressure, and allowed to crystallize to give the target complex as
orange crystals (0.720 g, 44%). H NMR (CDCl ): δ 7.95 (2H, d, J
3
HH
(
6
1
2H, s, H7), 7.55 (2H, d, J = 8.1, H3), 7.01 (1H, t, JHH = 8.1, H4),
HH
.51 (2H, t, JHH = 2.0, H6). 1 C NMR (CDCl ): δ 142.07 (1C, s, C1),
3
1
3
3
HH
39.72 (2C, s, C7), 129.62 (2C, s, C5), 128.38 (2C, s, C2), 119.47
=
6
(
2.2, H5), 7.85 (2H, d, JHH = 2.7, H7), 7.18 (1H, t, JHH = 7.8, H4),
.85 (2H, d, JHH = 7.8, H3), 6.35 (2H, t, JHH = 2.4, H6). 13C NMR
DMSO-d ): δ 144.07 (2C, s, C5), 142.74 (2C, s, C2), 137.34 (1C, s,
(
2C, s, C3), 119.42 (1C, s, C4), 106.91 (2C, s, C6). LRMS: expected
+
+
mass (M + H ) 227.1, found (M + H ) 227.2.
2-Methoxy-1,3-phenylene)bis(1H-pyrazole). A round-bottom
6
(
C1), 128.58 (2C, d, C7), 126.87 (1C, s, C4), 108.66 (2C, s, C6),
08.57 (2C, s, C3). Anal. Calcd for C H BrN Ni (347.82): C, 41.44;
flask was charged with 1 (0.040 g, 0.116 mmol), MeOH (30 mL), and
distilled water (1.5 mL), and the mixture was stirred at reflux for 24 h.
The resulting pale yellow mixture was cooled to room temperature and
evaporated. The residue was extracted with diethyl ether (3 × 20 mL)
and evaporated, and the solid mixture was purified by column
chromatography (silica; eluent 20/80 ethyl acetate/hexane) to give
both the protonated ligand and the functionalized ligand (13 mg, 48%
1
12
9
4
N, 16.11; H, 2.61. Found: C, 41.41; N, 16.16; H, 2.53.
N
C
N′
{
κ ,κ ,κ -2,6-bis(pyrazole),4-MeO-C H }NiBr (2). The above
6 2
procedure was followed using NiBr (NC(i-Pr)) (1.06 g, 3.68 mmol),
2
ligand b (0.632 g, 2.63 mmol), and NEt (0.513 mL, 0.373 g, 3.69
3
mmol) in xylene (10 mL). The target complex 2 was obtained as
1
orange crystals (0.235 mg, 24%). H NMR (CDCl ): δ 7.89 (2H, d,
1
3
yield). H NMR (CDCl , 400 MHz): δ 8.10 (2H, d, J = 2.4, H5),
3
HH
JHH = 2.0, H5), 7.81 (2H, d, JHH = 2.7, H7), 6.46 (2H, s, H3), 6.34
7
.75 (2H, d, JHH = 1.6, H7), 7.69 (2H, d, JHH = 8.0, H3), 7.32 (1H, t,
(
2H, t, JHH = 2.4, H6), 3.87 (3H, s, H8). 13C NMR (CDCl ): δ 159.82
13
3
JHH = 8.0, H4), 6.49 (2H, t, JHH = 2.0, H6), 3.28 (3H, s, H8).
C
(
1C, s, C4), 144.50 (2C, s, C5), 142.68 (2C, s, C2), 127.92 (1C, s,
NMR (CDCl ): δ 144.85 (1C, s, C1), 140.98 (2C, s, C7), 134.97 (2C,
3
C1), 125.59 (2C, s, C7), 107.85 (2C, s, C6), 94.76 (2C, s, C3), 56.19
s, C2), 131.28 (2C, s, C5), 125.09 (1C, s, C4), 124.18 (2C, s, C3),
(
1
1C, s, C8). Anal. Calcd for C H BrN Ni (377.85): C, 41.32; N,
4.83; H, 2.93. Found: C, 41.23; N, 14.78; H, 2.86.
12 9 4
1
07.40 (2C, s, C6), 60.94 (1C, s, C8). GC/MS: m/z 240.
2-Ethoxy-1,3-phenylene)bis(1H-pyrazole). A round-bottom
(
N
C
N′
{
κ ,κ ,κ -2,6-(pyrazole) -C H }NiBr·I , (1·I ). A CH Cl solution
2
flask was charged with 1 (0.150 g, 0.431 mmol) and 95% EtOH (40
mL), and the mixture was stirred for 5 h at reflux. The resulting pale
yellow solution was cooled to room temperature and then evaporated.
The solid residue was then purified by column chromatography on
silica with acetone as eluent. The combined fractions were evaporated
2
6
3
2
2
2
2
of I (0.073 g, 0.288 mmol, in 10 mL) was added dropwise to a THF
solution of 1 (0.100 g, 0.288 mmol, in 30 mL). The mixture turned
immediately from yellow to deep orange, and an orange powder
precipitated after 20 min. The final mixture was filtered after an
additional 2 h of stirring to give the crude product as an orange solid
1
to give the desired product as an oil (47 mg, 68% yield). H NMR
1
42
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Article
CDCl ): δ 8.13 (2H, d, J = 2.4, H5), 7.75 (2H, d, JHH = 1.6, H7),
.69 (2H, d, JHH = 8.1, H3), 7.32 (1H, t, JHH = 8.1, H4), 6.49 (2H, t,
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3
HH
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3
1,1′-(2-Isopropoxy-1,3-phenylene)bis(1H-pyrazole). The pro-
1
3
HH
HH
6.1, H9). 13C NMR (CDCl ): δ 142.6 (1C, s, C1), 140.88 (2C, s,
3
C7), 136.0 (2C, s, C2), 125.0 (2C, s, C5), 124.97 (1C, s, C4), 124.28
2C, s, C3), 106.9 (2C, s, C6), 29.86 (1C, s, C8), 21.58 (2C, s, C9).
GC/MS: m/z 268.
-(2,6-Bis(1H-pyrazol-1-yl)phenyl)morpholine. The procedure
used above for the preparation of 1,1′-(2-ethoxy-1,3-phenylene)bis-
1H-pyrazole) was repeated with 1 (34 mg, 0.098 mmol) and
morpholine (20 mL) to give an off-white solid after evaporation (8 mg,
(
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(
1
2
9%). H NMR (CDCl ): δ 7.75 (2H, d, J = 1.8, H5), 7.73 (2H, d,
3 HH
Chim. Pays-Bas 1994, 113, 267. (e) Granel, C.; Dubois, Ph.; Jer
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́ ̂
ome,
JHH = 1.8, H7), 7.41 (2H, d, JHH = 7.8, H3), 7.24 (1H, t, JHH = 7.5,
́
H4), 6.47 (2H, t, JHH = 1.9, H6), 3.38 (4H, t, J = 4.6, H8), 2.51
HH
(
(
1
4H, t, JHH = 4.6, H9). 13C NMR (CDCl ): δ 142.7 (1C, s, C1),
40.81 (2C, s, C7), 138.2 (2C, s, C2), 131.80 (2C, s, C5), 129.0 (1C,
3
(
s, C4), 124.34 (2C, s, C3), 106.83 (2C, s, C6), 67.13 (2C, s, C8),
Dalton Trans. 2012, 41, 7915. (b) Zhang, C.-P.; Wang, H.; Klein, A.;
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50.06 (2C, s, C9). GC/MS: m/z 295.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving NMR, UV−vis, and GC/
MS spectra, cyclic voltammetry traces, tables of crystal data and
collection/refinement parameters, procedures for data collec-
tion and structure resolution, and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org. Complete details of the X-ray analyses reported
herein have also been deposited at The Cambridge Crystallo-
(
(
Science 1997, 278, 1457. (e) Evans, D. J. Coord. Chem. Rev. 2005, 249,
582. (f) Yang, N.; Reiher, M.; Wang, M.; Harmer, J.; Duin, E. C. J.
1
Am. Chem. Soc. 2007, 129, 11028. (g) Dey, M.; Telser, J.; Kunz, R. C.;
Lees, N. S.; Ragsdale, S. W.; Hoffman, B. M. J. Am. Chem. Soc. 2007,
graphic Data Centre (CCDC 1027168 (1), 1027169 (2),
1
29, 11030. (h) Ragsdale, S. W. J. Biol. Chem. 2009, 284, 18571.
pz
1
027126 (1·I ), 1027124 (3), 1027125 (Br-NC(OH)N )).
2
(i) Scheller, S.; Goenrich, M.; Mayr, S.; Thauer, R. K.; Jaun, B. Angew.
Chem., Int. Ed. 2010, 49, 8112. (j) Scheller, S.; Goenrich, M.; Boecher,
R.; Thauer, R. K.; Jaun, B. Nature 2010, 465, 606.
These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif, by e-mailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, U.K. (fax +44
(9) (a) Grove, D. M.; van Koten, G.; Zoet, R. J. Am. Chem. Soc. 1983,
1
05, 1379. (b) Grove, D. M.; van Koten, G.; Ubbels, H. J. C.; Zoet, R.
Organometallics 1984, 3, 1003. (c) Grove, D. M.; van Koten, G.; Mul,
W. P.; van der Zeijden, A. A. H.; Terheijden, J. Organometallics 1986,
1223 336033).
5
(
, 322.
10) (a) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; Van Koten, G.
AUTHOR INFORMATION
Corresponding Author
E-mail for D.Z.: zargarian.davit@umontreal.ca.
■
J. Am. Chem. Soc. 2000, 122, 11822. (b) Contel, M.; Stol, M.; Casado,
M. A.; van Klink, G. P. M.; Ellis, D. D.; Spek, A. L.; van Koten, G.
Organometallics 2002, 21, 4556.
*
Notes
(
11) (a) Moutlon, C. J.; Shaw, B. L. Dalton Trans. 1976, 1020.
The authors declare no competing financial interest.
(
(
b) Rimml, H.; Venanzi, L. M. J. Organomet. Chem. 1983, 259, C6.
c) Rimml, H.; Venanzi, L. M. J. Organomet. Chem. 1984, 260, C52.
ACKNOWLEDGMENTS
The authors are grateful to the NSERC of Canada for a
Discovery Grant to D.Z., Universite
Centre in Green Chemistry and Catalysis for graduate
fellowships to J.-P.C., Dr. Michel Simard and Prof. F. Hein
Schaper for their valuable assistance with crystallography, Dr.
Christine Lepetit for discussions regarding theoretical treat-
ments of halogen bonds, and Prof. O. Wendt (Lund University)
for valuable discussions on the formation of 1·I2.
■
(d) Kennedy, A. R.; Cross, R. J.; Muir, K. W. Inorg. Chim. Acta 1995,
231, 195. (e) Campora, J.; Palma, P.; del Rıo, D.; Alvarez, E.
Organometallics 2004, 23, 1652. (f) van der Boom, M. E.; Liou, S.-Y.;
Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2004,
́
́
́
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l (FESP) and
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57, 4015. (g) Groux, L. F.; Belanger-Gariepy, F.; Zargarian, D. Can. J.
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2+
3+
(
23) All potentials are referenced to the Fe /Fe redox couple for
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(
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NMe2
240 mV and the E°red value for (NCN
)NiBr is 50 mV, both
2
values being measured in acetone and referenced to Ag/AgCl. Under
+
these conditions, the E°1/2 potential for Fc/Fc is given as 630 mV,
NMe2
which means that (NCN
)NiBr is much more susceptible to
pz
oxidation relative to (R-NCN )NiBr (by nearly 1 V). See: Grove, D.
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(
4
(
3
(
2
(
(
1
(
(45) Spasyuk, D. M.; Zargarian, D. Inorg. Chem. 2010, 49, 6203.
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(
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2
5
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́
(
1
(
́
(
7
(
1
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