Alkylations of n4-(4-pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole: First observation of room-temperature rearrangement of an N4- substituted triazole to the N1 analogue

Article abstract of DOI:10.1002/asia.200900485

Attempts to use alkylation to introduce a positive charge at the nitrogen atom of the 4-pyridyl ring in the bis(bidentate) triazole ligand N ⁴-(4-pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole (pydpt) were made to ascertain what effect a strongly electron-withdrawing group would have on the magnetic properties of any subsequent iron(II) complexes. Alkylation of pydpt under relatively mild conditions led in some cases to unexpected rearrangement products. Specifically, when benzyl bromide is used as the alkylating agent, and the reaction is carried out in refluxing acetonitrile, the N⁴ substituent moves to the N¹ position. However, when the same reaction is performed in dichloromethane at room temperature, the rearrangement does not occur and the desired product containing an alkylated N⁴ substituent is obtained. Heating a pure sample of N⁴-Bzpydpt·Br to reflux in MeCN resulted in clean conversion to N¹-Bzpydpt·Br. This is consistent with N⁴-Bzpydpt·Br being the kinetic product whereas N¹-Bzpydpt·Br is the thermodynamic product. When methyl iodide is used as the alkylating agent, the N⁴ to N ¹ rearrangement occurs even at room temperature, and at reflux pydpt is doubly alkylated. The observation of the lowest reported temperatures for an N⁴ to N¹ rearrangement is due to this particular rearrangement involving nucleophilic aromatic substitution: a possible mechanism for this transformation is suggested.

Full text of DOI:10.1002/asia.200900485

FULL PAPERS
DOI: 10.1002/asia.200900485
Alkylations of N4-(4-Pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole: First
Observation of Room-Temperature Rearrangement of an N4-Substituted
Triazole to the N1 Analogue
Jonathan A. Kitchen,[a, b] David S. Larsen,[a] and Sally Brooker*[a, b]
Abstract: Attempts to use alkylation to
introduce a positive charge at the nitro-
gen atom of the 4-pyridyl ring in the
bis(bidentate) triazole ligand N4-(4-pyr-
idyl)-3,5-di(2-pyridyl)-1,2,4-triazole
(pydpt) were made to ascertain what
effect a strongly electron-withdrawing
group would have on the magnetic
properties of any subsequent iron(II)
complexes. Alkylation of pydpt under
relatively mild conditions led in some
cases to unexpected rearrangement
products. Specifically, when benzyl bro-
mide is used as the alkylating agent,
and the reaction is carried out in re-
fluxing acetonitrile, the N4 substituent
moves to the N1 position. However,
when the same reaction is performed
in dichloromethane at room tempera-
ture, the rearrangement does not occur
and the desired product containing an
alkylated N4 substituent is obtained.
This is consistent with N4-Bzpydpt·Br
being the kinetic product whereas N1-
Bzpydpt·Br is the thermodynamic
product. When methyl iodide is used as
the alkylating agent, the N4 to N1 rear-
rangement occurs even at room tem-
perature, and at reflux pydpt is doubly
alkylated. The observation of the
lowest reported temperatures for an N4
to N1 rearrangement is due to this par-
ticular rearrangement involving nucleo-
philic aromatic substitution: a possible
mechanism for this transformation is
suggested.
Heating
a
pure sample of N4-
Bzpydpt·Br to reflux in MeCN resulted
in clean conversion to N1-Bzpydpt·Br.
Keywords: alkylation
·
iron
·
N ligands · structure elucidation ·
triazoles
Introduction
should alter the nature of the triazole ring and hence
change the ligand-field strength of the resulting ligand. To
Our primary interest in N4-substituted 3,5-di(2-pyridyl)-
1,2,4-triazoles stems from the observation of spin-crossover
(SCO) transitions in iron(II) complexes when these ligands
are incorporated into the coordination sphere.[1–3] Amongst
other possible variations, the general route to N4-substituted
3,5-di(2-pyridyl)-1,2,4-triazoles (Rdpt) developed in this
group provides access to Rdpt with almost any substituent
at the N4 position.[4] We have used a range of such Rdpt li-
gands to prepare families of iron(II) complexes
(Figure 1).[3,5] The electron-donating and -withdrawing abili-
ty of the N4 substituent is an obvious feature to vary as this
[a] Dr. J. A. Kitchen, Assoc. Prof. D. S. Larsen, Prof. S. Brooker
Department of Chemistry
University of Otago
PO Box 56, Dunedin 9054 (New Zealand)
Fax : (+64)3-479-7906
E-mail: sbrooker@chemistry.otago.ac.nz
[b] Dr. J. A. Kitchen, Prof. S. Brooker
Figure 1. N4-substituted-3,5-di(2-pyridyl)-4H-1,2,4-triazole ligands (Rdpt)
used in this and previous studies. The ligand in the box is pydpt, which
was employed in this study.
MacDiarmid Institute for Advanced Materials and Nanotechnology
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900485.
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Chem. Asian J. 2010, 5, 910 – 918

date, the N4 substitutents employed in Rdpt ligands from
which iron(II) complexes have been prepared have included
amino, pyrrolyl, phenyl, tolyl, 4-pyridyl, 3,5-dichlorophenyl,
isobutyl, and methyl (Figure 1).[1–3,5] Although there are
both electron-donating and -withdrawing substituents in this
selection, we have observed that the differences in the
iron(II) complexes of these Rdpt ligands are not always as
substantial as were hoped for. What is missing from this
family is a highly electron-withdrawing N4 substituent. One
way of producing a very electron-withdrawing substituent is
to introduce a positive charge on the N4 substituent. Ligands
with a positive charge have been utilized by other research
groups to prepare iron(II) complexes. Of most interest here
is the work initiated by Constable and co-workers, in which
terpyridine ligands with a pendant 4-pyridyl group were al-
kylated with a variety of alkylating agents, and the resulting
positively charged ligands were successfully complexed.[6–8]
Since our pydpt ligand contains a 4-pyridyl group as the N4
substituent, in principle an analogous alkylation would be a
convenient route to an Rdpt ligand with the desired positive
charge. In turn, this should allow us to investigate how such
a strongly electron-withdrawing group affects the magnetic
behavior of the subsequently prepared iron(II) complexes.
Scheme 1. Observed products of alkylations of pydpt with 10 equivalents
of benzyl bromide; subsequent conversion of N4-Bzpydpt·Br into N1-
Bzpydpt·Br.
room temperature in the hope of avoiding the rearrange-
ment. These Rdpt ligands are highly soluble in chlorinated
solvents, so the reaction was attempted in dichloromethane
(DCM; Scheme 1). An excess (10 equivalents) of benzyl
bromide was added to a solution of pydpt in DCM to afford
a white precipitate. Elemental analysis of the white powder
after drying in vacuo gave data consistent with the pure ben-
zylated product Bzpydpt·Br (80% yield). The 1H NMR
spectrum showed only one set of signals for the two 2-pyrid-
yl rings; thus, the substituent had remained at the
N4 position and the desired product N4-Bzpydpt·Br was ob-
tained. When this room-temperature reaction is carried out
in MeCN rather than DCM the same outcome is observed,
but the reaction is slower and the desired product does not
precipitate directly from the reaction solution, so the
workup is less convenient. The solubility of the N4-substitut-
ed variant differs from that of the N1-substituted variant.
This difference was particularly noticeable during the con-
version of the bromide salt into the tetrafluoroborate salt, as
the N4-substituted product precipitated immediately (in
64% yield; see below for X-ray structure determination;
Figure 4) and did not require cooling to 48C as was required
for the N1-substituted product.
Results and Discussion
Preparation of Compounds
The alkylating agent chosen for the initial experiments was
benzyl bromide. A similar procedure to that employed by
Constable and co-workers to alkylate terpyridine ligands
was followed:[7] pydpt was dissolved in acetonitrile, 10 equiv-
alents of benzyl bromide was added, and the solution was
heated at reflux for 3 hours. When the solution was cooled
to room temperature, a white solid precipitated. Filtering
and drying in vacuo resulted in a sample that gave micro-
analytical data that is consistent with that expected for the
desired benzylated ligand with a bromide counter anion, N4-
(4-benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole bromide
(N4-Bzpydpt·Br), in 39% yield. However, the reduced sym-
metry evident from the 1H NMR spectrum indicated that
the N4 substituent had shifted to the N1 position, giving N1-
(4-benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole bromide
(N1-Bzpydpt·Br), in which the two 2-pyridyl rings are not
equivalent (Scheme 1).
The reaction product was crystallized as the BF4 salt by
allowing an aqueous solution of it and excess aqueous am-
monium tetrafluoroborate to stand at room temperature.
Single-crystal X-ray structure determination of the resulting
large colorless blocks confirmed them to be N1-Bzpydpt·BF4
(Figure 3, see below). Subsequent cooling of the aqueous so-
lution to 48C gave a bulk, analytically pure, crystalline
sample in an overall yield of 99%.
Heating N4-Bzpydpt·Br, obtained from the room-tempera-
ture alkylation in DCM, to reflux in acetonitrile yields the
rearranged product N1-Bzpydpt·Br. This clean conversion
indicates that N4-Bzpydpt·Br is the kinetic product, whereas
N1-Bzpydpt·Br is the thermodynamic product of monoalky-
lation of pydpt.
Such rearrangements in 1,2,4-triazoles are not unprece-
dented; there are reports of similar rearrangements in which
the N4 substituents used were either alkyl or allyl.[9,10] How-
ever, in all of the systems studied to date the reactions re-
Although this rearrangement was interesting, the N4-sub-
stituted compound N4-Bzpydpt·Br was still required for
complexation studies. As reaction at 818C (b.p. of MeCN)
gave N1-Bzpydpt·Br, the reaction was instead carried out at
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FULL PAPERS
quired high temperatures, typically above 3008C, and most
were carried out in sealed glass tubes. The lowest tempera-
ture reported prior to the present study is 1508C (and not in
a sealed tube) for 4-phenacyl-1,2,4-triazole.[10] In that case
the accepted mechanism involves two SN2 type reactions
(Scheme 2). The first reaction involves the formation of the
our case R is a highly electron-withdrawing, positively
charged, benzylated (and hence activated towards nucleo-
philic substitution) 4-pyridyl ring. This highly polarized N4
substituent will be more susceptible to nucleophilic attack at
C4, thus significantly lowering the energy barrier towards re-
arrangement and leading to the low rearrangement tempera-
ture we observed (refluxing MeCN, 818C). Consistent with
this view, heating the unalkylated, R=4-pyridyl, starting
material pydpt at reflux in MeCN, or even at 1508C in 1-
methyl-2-pyrrolidinone, gave no rearranged product. Un-
changed starting material was recovered in both cases along
with partial decomposition products in the case of the
1508C reaction. Thus, it is the benzylation of the 4-pyridyl
substituent that activates the system towards the observed
N4 to N1 rearrangement.
Scheme 2. The accepted mechanism of N4 to N1 rearrangement in 4-sub-
Two plausible mechanisms, both of which involve nucleo-
philic attack at the C4 position of the 4-pyridyl ring, are pro-
posed for the rearrangement (Schemes 3 and 4). The first is
similar to the previous triazole rearrangement reactions in
that the N1 nitrogen atom of a second triazole species acts
as the nucleophile; therefore, the rearrangement might pro-
ceed as shown in Scheme 3.
This mechanism would involve a relatively bulky nucleo-
phile that would generate a sterically demanding intermedi-
ate I (Scheme 3). Alternatively, the nucleophile might be a
bromide anion, for which the reaction would proceed in a
similar fashion (Scheme 4), only this time the intermediate
II would be less sterically cumbersome. Such mechanisms
are not unprecedented; in 2001, Castagnoli Jr and co-work-
ers proposed a similar mechanism for the rearrangement of
some indazolyl derivatives.[11]
stituted-1,2,4-triazoles.
intermediate ion pair (triazolium/triazolate) in which a
second N substituent attaches to the N1 position of what be-
comes a triazolium cation, and the resulting triazolate anion
has lost all N substituents. The second nucleophilic attack
occurs between the resulting ions whereby the N1 atom of
the triazolate anion attacks the N4 substituent on the triazo-
lium cation to give the N1-substituted 1,2,4-triazole products.
In our system, the mechanism, rather than being SN2 like,
would most likely be an aromatic nucleophilic substitution
reaction (Schemes 3 and 4). Instead of R=alkyl or allyl, in
Scheme 4. Proposed mechanism for the N4 to N1 rearrangement if the nu-
cleophile is a bromide ion.
Unlike the first postulated mechanism, the second is
anion-dependent, so we carried out an initial test of this de-
pendency. As stated above, when N4-Bzpydpt·Br is heated at
reflux in MeCN, the result is complete conversion into the
thermodynamic product N1-Bzpydpt·Br. To test whether a
Scheme 3. Postulated mechanism for the observed N4 to N1 rearrange-
ment (in which the nucleophile is a second triazole species) when alkyla-
tion is carried out with a) R=benzyl in refluxing acetonitrile and b) R=
methyl at room temperature.
second triazole species (Scheme 3) or
a bromide ion
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Alkylations of N4-(4-Pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole
(Scheme 4) acts as the nucleophile, we prepared N4-
Bzpydpt·BPh4 because the tetraphenylborate anion cannot
act as a nucleophile. It was isolated in 88% yield by the ad-
dition of an aqueous solution of NaBPh4 to an aqueous solu-
tion of N4-Bzpydpt·Br followed by filtration of N4-
Bzpydpt·BPh4, which formed as a fine white precipitate im-
mediately on mixing. Trace amounts of bromide (0.5%)
present in this sample were removed through recrystalliza-
tion by vapor diffusion of diethyl ether into an acetone solu-
tion of the sample, as the second mechanism would be cata-
lytic in bromide. The result was a white crystalline sample in
which no bromide was detectable (<0.3%). Heating N4-
Bzpydpt·BPh4 (25 mg) at reflux in MeCN (10 mL) for 3 h
(the same time and volume of MeCN as was used for the re-
arrangement of N4-Bzpydpt·Br to N1-Bzpydpt·Br) resulted
in the formation of a yellow solid that was subsequently
identified by 1H NMR spectroscopy as the rearrangement
product N1-Bzpydpt·BPh4. Although we cannot rule out the
mechanism shown in Scheme 4, as undetectable traces of
bromide could still be present, this result lends support to
our suggestion that the mechanism shown in Scheme 3 is the
one that is operational in this case.
In contrast to this result, the reaction using benzyl bro-
mide (to prepare N4-Bzpydpt·Br) did not result in rear-
rangement at room temperature in DCM. To ascertain
whether the iodide anion facilitated the observed methyla-
tion and rearrangement of pydpt to N1-Mepydpt·I in DCM
at room temperature, the benzylation of pydpt with benzyl
bromide in the presence of 10 mol% nBu4NI was attempted
at room temperature in DCM. The alkylation proceeded
smoothly to give N4-Bzpydpt+ (as it did in the absence of
IÀ). No trace of N1-Bzpydpt+ was observed. This result indi-
cates that the rearrangement is not anion-dependent and
hence that the mechanism shown in Scheme 4 is not opera-
tional in this case.
Remarkably, when the methylation of pydpt was carried
out in refluxing MeCN, and the intense orange reaction so-
lution was subjected to vapor diffusion of diethyl ether, the
product was identified as the doubly alkylated N1-2Mepydpt·2I
and was obtained in 60% yield. While the 4-pyridyl nitrogen
atom is the most susceptible to alkylation, there are multiple
sites for the second alkylation to occur. The first alkylation
and rearrangement results in the two 2-pyridyl groups no
longer having equivalent chemical environments; therefore,
either of these groups can be alkylated, and two of the
triazole nitrogen atoms are also in principle available for
alkylation. The site of the second alkylation is clearly re-
vealed by the X-ray structure determination (Figure 6, see
below).
Following on from this intriguing study into the benzylat-
ed derivatives, we decided to attempt the alkylations with
methyl iodide. The less sterically bulky methyl group, and
more reactive iodide, were chosen to ascertain whether the
resulting, less bulky, ligand would more readily bind iron(II)
(see below).
The methyl iodide alkylation reactions gave quite differ-
ent results to the benzyl bromide reactions. Using the same
protocols as above (DCM at room temperature and MeCN
at reflux), we again obtained alkylated products (Scheme 5).
However, even when pydpt was stirred at room temperature
in DCM with excess MeI (10 equivalents), the rearranged
product N1-Mepydpt·I was isolated in good yield (69%) and
X-ray Crystallography
Single platelike crystals of pydpt were grown by the slow
evaporation of a solution in dichloromethane. The X-ray
crystal structure determination is included here to provide
comparisons to the alkylated derivatives. The compound
crystallized in the orthorhombic space group Pbca and con-
tains one molecule in the asymmetric unit (Figure 2). Analy-
1
characterized by H NMR spectroscopy, microanalysis, mass
spectrometry, and X-ray crystallography (Figure 5, see
below).
Figure 2. Perspective view of pydpt. Hydrogen atoms omitted for clarity.
sis of the 2-pyridyltriazole mean-plane angles reveals a rela-
tively planar structure [mean-plane angle range=6.5(3)–
21.9(5)8], but the 4-pyridyl moiety is closer to a right-angled
orientation with respect to the central triazole ring [mean-
plane angle=72.3(6)8].
Scheme 5. Observed products of alkylations of pydpt with 10 equivalents
of methyl iodide.
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Single crystals of N1-Bzpydpt·BF4 were grown by the slow
evaporation of an aqueous solution, and the X-ray crystal
structure was determined. The compound crystallized in the
¯
triclinic space group P1 and contained one molecule in the
asymmetric unit (Figure 3). The connectivity of the com-
Figure 4. Perspective view of N4-Bzpydpt·BF4. Only one of the two inde-
pendent molecules within the asymmetric unit is shown. Hydrogen atoms
omitted for clarity.
Figure 3. Perspective view of the asymmetric unit of N1-Bzpydpt·BF4. Hy-
drogen atoms other than those on the interstitial water molecule omitted
for clarity.
bound, is twisted by only 418 relative to the attached
triazole. Both of the BF4 counterions are involved in
À
anion···p interactions with the triazole rings of the cationic
À
components, and one of the BF4 counterions has
twirl
disorder
[F(23)···centroid(triazoleN(2))=3.123(2) ꢁ,
and F(7)···cen-
pound is in full agreement with the deduction made on the
F(4)···centroid(triazoleN(22))=3.293(7) ꢁ,
1
basis of the H NMR data; that is, the benzylated 4-pyridyl
troid(triazoleN(22))=3.065(7) ꢁ]. The F···triazole interaction
clearly is not strong enough to constrain the BF4 , as one of
the disordered fluorine atoms is involved in the aforemen-
tioned interactions [F(4) and F(7)].
À
group is located on the N1 atom rather than the desired
N4 position. There is an interstitial water molecule that
forms two hydrogen bonds to tetrafluoroborate counteran-
ions [O(1)···F(4)=2.756(4) ꢁ, O(1)–H(1X)···F(4)=1318, and
O(1)···F(3)i =3.003(4) ꢁ, O(1)–H(1Y)···F(3)i =1328], and a
strong interaction occurs between the positively charged
benzylated 4-pyridyl ring and the tetrafluoroborate counter-
Single crystals of N1-Mepydpt·I were obtained as colorless
rods by the vapor diffusion of tert-butylmethyl ether into a
solution of N1-Mepydpt·I in DMSO. The compound crystal-
lized in the chiral, orthorhombic space group P212121, which
contained one molecule in the asymmetric unit (Figure 5).
The crystal structure confirmed the connectivity predicted
from the NMR studies with the methylated 4-pyridyl group
located at the N1 position. The crystal structure contains no
solvent or disorder. The iodide counteranion has short con-
tacts to the positively charged 4-pyridyl nitrogen atoms on
anion
[F(1)···N(6)=2.939(1) ꢁ
and
F(1)···centroid=
2.956(1) ꢁ]. The dihedral angles formed between the mean
planes of the 2-pyridyl rings and the triazole ring are 218
and 128 for the rings of N(1) and N(4), respectively, so this
portion of the compound is relatively planar. However, the
N1-4-pyridyl ring is significantly twisted relative to the tria-
zole ring (418), albeit not as much as the N4-4-pyridyl was in
the starting material pydpt (728).
Single crystals of N4-Bzpydpt·BF4 were obtained by re-
crystallization from hot water/methanol (10:1), and the X-
ray crystal structure was determined (Figure 4). The asym-
metric unit contained two independent molecules of the de-
sired product N4-Bzpydpt·BF4 and no molecules of solva-
tion. The connectivity of the two independent molecules is
1
consistent with that predicted from the H NMR spectrum,
since the benzylated 4-pyridyl substituent is connected at
the N4 nitrogen atom. The di(2-pyridyl)triazole portion of
these two molecules is relatively flat [range of mean plane
angles=1.43–13.318], while the mean planes of the benzylat-
ed 4-pyridyl ring and the attached triazole ring are at near
right angles (808 and 898). The rings are much less coplanar
than in N1-Bzpydpt·BF4, in which the substituent, albeit N1-
Figure 5. Perspective view of the asymmetric unit of N1-Mepydpt·I. Hy-
drogen atoms omitted for clarity.
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Alkylations of N4-(4-Pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole
adjacent molecules in the unit cell [I(1)···N(6)=3.636(6) ꢁ
and I(1)···N(6)’=3.735(6) ꢁ], thus facilitating the linking of
molecules. The angles between pyridyl and triazole rings are
similar to those seen in N1-Bzpydpt·BF4 with a mean-plane
angle range of 8.49(4)–24.09(3). The pyridinium ring is
closer to a right-angled orientation to the triazole ring with
a mean-plane angle of 67.41(2), similar to that of the start-
ing material pydpt.
Single orange platelike crystals of N1-2Mepydpt·2I were
grown by diffusion of diethyl ether vapor into the acetoni-
trile reaction solution. The compound crystallized in the
monoclinic space group P21/n containing one molecule in
the asymmetric unit. The compound is clearly doubly alky-
lated with the second methylation having occurred at one of
the two 2-pyridyl rings (Figure 6). The mean-plane angles
Figure 7. a,b) Potential binding modes of N1-Rpydpt·X. c) The observed
binding mode of the analogous alkylated terpyridyl ligand.[6,7]
two 2-pyridyl nitrogen atoms and the N4 triazole nitrogen
atom (Figure 7b). This second binding mode bears some re-
semblance to that seen for similarly alkylated terpyridyl
moieties (Figure 7c) complexed with iron(II),[6,7] however
this binding mode is anticipated to be less favorable than it
is for terpy, since rather than having a central six-membered
heterocycle, it involves a five-membered heterocycle.[13]
Initially, the complexations were carried out in the same
way as for the dinuclear bis and mononuclear tris FeII com-
plexes of Rdpt reported earlier.[3,5] However, when excess
iron(II) tetrafluoroborate was added to either N1-
Bzpydpt·BF4 or N4-Bzpydpt·BF4 in acetonitrile, unlike in the
case of the other Rdpt ligand reactions with iron(II), there
was no color change, and vapor diffusion of diethyl ether
into the solution or evaporation of the solvent yielded only
white precipitates or, in the case of N1-Bzpydpt·BF4, color-
less crystals, which were found to uncomplexed ligands. This
observation indicates that in introducing a highly electron-
withdrawing positive charge to the N1 or N4 substituent
there has been such a dramatic reduction in the electron-do-
nating ability of the nitrogen atoms that typically bind to
the metal ions that they no longer coordinate readily. Be-
cause the ligand reactivity was so severely reduced, we at-
tempted the complexation reaction at reflux in acetonitrile
with N1-Bzpydpt·BF4 (but not N4-Bzpydpt·BF4 as it would
likely rearrange). Despite the elevated temperature, there
was no observed color change (this is not required, as a
high-spin complex could be white/colorless; however, there
is usually some small change in the color of the reaction so-
lution upon complexation), and the slow evaporation of the
acetonitrile reaction solution gave large colorless blocks
coated in brown oil. The large blocks were again found to
be the noncoordinated ligand N1-Bzpydpt·BF4. The identity
of the brown oil could not be established. Similarly, we have
been unable to isolate any iron(II) complexes to date with
the less bulky methyl groups attached to the ligand in N1-
Mepydpt·I. We cannot rule out that such complexes can be
formed, but it is clear that, totally unlike the alkylated ter-
pyridines, the alkylated Rdpt compounds are at best reluc-
tant ligands for FeII.
Figure 6. Perspective view of the asymmetric unit of doubly alkylated N1-
2Mepydpt·2I. Hydrogen atoms omitted for clarity.
formed between the triazole and pendant 2-pyridyl rings
[28.8(2)8 to 38.5(2)8] are further from planar than in the pre-
vious structures. However, with an angle of just 29.9(3)8 the
methylated 4-pyridyl ring makes the smallest angle to the
triazole ring of all four of the structures presented here.
Again, there are short contacts between the positively
charged, alkylated, nitrogen atoms and the iodide counter-
anions [N(1)···I(1)=3.636(9) ꢁ and N(6)···I(2)=3.811(9) ꢁ].
Complexation Studies
Having successfully attached the highly electron-withdraw-
ing, positively charged substituent to the triazole ring at
both the N4 (N4-Bzpydpt·BF4) and N1 (N1-Bzpydpt·BF4) po-
sitions, we attempted to prepare iron(II) complexes of these
benzylated ligands. Unlike typical Rdpt ligands,[2,12] there
are two very different binding modes possible for N1-
Bzpydpt·BF4 (Figure 7). In one mode, a bidentate binding
pocket is formed by an N2 atom of the triazole and the ni-
trogen atom of a 2-pyridyl ring (Figure 7a), and a second
potential bidentate binding pocket is formed “out the back”
by an N4 atom of the triazole and the nitrogen atom of the
other 2-pyridyl ring. The second, quite different binding
mode involves a terdentate binding pocket, formed by the
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Conclusions
Experimental Section
General: The ligand 4-(4-pyridyl)-3,5-di(2-pyridyl)-4H-1,2,4-triazole
(pydpt) was prepared as described earlier.[4] Fe
(BF4)2·6H2O, benzyl bro-
Alkylation of pydpt utilizing excess benzyl bromide or
methyl iodide in either refluxing acetonitrile or at room
temperature in dichloromethane gave some unexpected al-
kylated products. The expected product N4-Bzpydpt·Br was
obtained from a reaction using excess benzyl bromide in
DCM at room temperature. However, in refluxing acetoni-
trile, the N4 to N1 rearrangement product N1-Bzpydpt·Br re-
sulted. This rearrangement occurred at the lowest tempera-
ture seen until now: the previous minimum was 1508C.
Heating a pure sample of N4-Bzpydpt·Br to reflux in
MeCN resulted in clean conversion into N1-Bzpydpt·Br.
This result is consistent with N1-Bzpydpt·Br being the ther-
modynamic product and N4-Bzpydpt·Br being the kinetic
product.
Unlike in previously reported systems, the rearrangement
in our system, which features an alkylated 4-pyridyl substitu-
ent, is believed to proceed by means of nucleophilic aromat-
ic substitution rather than by SN2 like reactions. Hence, the
positively charged aromatic substituent facilitates the aro-
matic nucleophilic substitution reaction, leading to the lower
rearrangement temperature observed. A test reaction using
a bulky nonnucleophilic anion in place of bromide (N4-
Bzpydpt·BPh4) still gave the rearrangement product N1-
Bzpydpt·BPh4, indicating that it is probably the triazole spe-
cies that acts as the nucleophile (not the bromide anion, al-
though this cannot be ruled out as only catalytic/trace
amounts of bromide are required). However, a full investi-
gation of the details of the mechanism of this rearrangement
is beyond the scope of the present study. Rather it is antici-
pated that this report will stimulate such studies by appro-
priate organic mechanistic experts.
AHCTUNGTRENNUNG
mide, and methyl iodide (stabilized) were purchased from commercial
sources and used as received. All solvents were laboratory reagent grade
except for methanol and acetonitrile. Methanol was dried by freshly dis-
tilling over magnesium and iodine before use. Acetonitrile was dried by
freshly distilling over calcium hydride before use. Elemental analyses
were carried out by the Campbell Microanalytical Laboratory at the Uni-
versity of Otago. Infrared spectra were recorded over the range 4000–
400 cmÀ1 with a Perkin–Elmer Spectrum NBX FT-IR spectrophotometer
as a potassium bromide pellet or a nujol mull. ESI mass spectra were re-
corded on a Bruker MicrOTOFQ spectrometer by Mr Ian Stewart. 1H
and 13C NMR spectra were recorded on either a Varian INOVA-500, a
Varian INOVA-300, a Varian 400-MR, or a Varian 500 MHz VNMRS
spectrometer at 258C. X-ray data (Table S1, Supporting Information)
were collected on a Bruker APEX II area detector diffractometer at the
University of Otago using graphite-monochromated MoKa radiation (l=
0.71073 ꢁ). The data were corrected for Lorentz and polarization effects,
and semi-empirical absorption corrections (SCALE) were applied. The
structures were solved by direct methods (SHELXS-97) and refined
against all F2 data (SHELXL-97).[14] Hydrogen atoms were inserted at
calculated positions and rode on the atoms to which they were attached
and all non-hydrogen atoms were made anisotropic. The SQUEEZE rou-
tine in PLATON was required to remove a poorly behaved 0.5 occupan-
cy methanol molecule and a 0.5 occupancy water molecule in N1--2Me-
pydpt·2I.[15] CCDC 748521 (pydpt), CCDC 748522 (N4-Bzpydpt·BF4),
CCDC 748523 (N1-Bzpydpt·BF4), CCDC 748524 (N1--Mepydpt·I), and
CCDC 748525 (N1--2Mepydpt·2I) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
N1-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole bromide (N1-
Bzpydpt·Br): To a solution of pydpt (100 mg, 0.33 mmol) in acetonitrile
(50 mL), benzyl bromide (280 mg, 0.4 mL, 1.65 mmol) was added. The re-
sulting colorless solution was heated at reflux for 3 h, cooled to room
temperature, and filtered to give N1-Bzpydpt·Br as a white powder
(60 mg, 39%). Anal. calcd. for C24H19N6Br·1= H2O (480.35 gmolÀ1): C
2
60.01, H 4.20, N 17.50, Br 16.63; found: C 59.99, H 4.23, N 17.53, Br
16.59%. 1H NMR (400 MHz, [D6]DMSO, reference DMSO at d=
2.50 ppm): dH =5.90 (s, 2H, H18), 7.51–7.41 (m, 4H, H21, H22, H23 and
H11), 7.58–7.53 (m, 2H, H20 and H24), 7.62 (ddd, J=7.9, 5.0, 1.1 Hz, 1H,
H2), 8.01 (td, J=7.8, 1.8 Hz, 1H, H3), 8.12 (td, J=7.8, 1.7 Hz, 1H, H10),
8.24 (m, 2H, H4 and H9), 8.33 (d, J=7.1 Hz, 2H, H14 and H17), 8.47 (d,
J=4.7 Hz, 1H, H1), 8.74 (d, J=4.6 Hz, 1H, H12), 9.30 ppm (d, J=7.2 Hz,
2H, H15 and H16). 13C NMR (101 MHz, [D6]DMSO, reference DMSO at
d=39.52 ppm): dC =162.4 (C6), 155.7 (C7), 151.1 (C13), 150.5 (C12), 149.5
(C1), 148.3 (C5), 146.6 (C15/16), 146.1 (C8), 138.7 (C10), 137.9 (C3), 134.5
(C19), 129.9–129.4 (C20–24), 126.8 (C2), 125.7 (C11), 125.3(C9), 123.5 (C14/17),
122.9(C4), 63.5 ppm (C18). IR (KBr): n˜ =3425, 3109, 3030, 1632, 1586,
1522, 1511, 1496, 1468, 1447, 1423, 1390, 1353, 1280, 1250, 1215, 1154,
1087, 1044, 1008, 996, 827, 803, 795, 765, 742, 725, 701, 623, 608, 592,
Using methyl iodide in place of benzyl bromide, we were
able to further reduce the rearrangement temperature to
room temperature to form N1-Mepydpt·I in DCM. In con-
trast, alkylation but no rearrangement was observed when
pydpt was treated with benzyl bromide at room temperature
in DCM. Hence, we repeated the benzylation reaction in
the presence of 10 mol% iodide. Again, only N4-Bzpydpt+
was obtained, indicating that the rearrangement is not
anion-dependent.
Finally, in carrying out the methylation reaction in reflux-
ing MeCN, we observed another unexpected product, the
doubly alkylated dication, which was obtained as the iodide
salt N1-2Mepydpt·2I.
In the course of this study, we were able to isolate the de-
sired product N4-Bzpydpt·X with a benzyl group attached to
the 4-pyridyl nitrogen atom, but the presence of the associ-
ated highly electron-withdrawing positive charge appeared
to render the ligand unreactive as no iron(II) complexes
have been obtained to date. While not yet giving the hoped
for results with FeII, this study does show just how much the
choice of N4 substituent can influence the properties of the
resulting ligands and has also revealed the lowest N4 to N1
rearrangement temperatures seen to date.
479 cmÀ1
.
N1-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole tetrafluoroborate
(N1-Bzpydpt·BF4): To a colorless solution of N2-(4-benzylpyridinium)-3,5-
di(2-pyridyl)-1,2,4-triazole bromide (N1-Bzpydpt·Br) (50 mg, 0.1 mmol)
in water (5 mL) was added an aqueous solution (5 mL) of ammonium tet-
rafluoroborate (50 mg, 0.4 mmol). The solution was allowed to stand at
room temperature to produce a few large, blocklike, crystals, which were
removed for X-ray structure determination. The remainder of the solu-
tion was placed in the refrigerator (48C) overnight to yield 50 mg (99%)
of N1-Bzpydpt·BF4 as
a colorless crystalline solid. Anal. calcd. for
C24H19N6BF4 (478.25 gmolÀ1): C 60.27, H 4.00, N 17.57; found: C 60.33, H
4.06, N 17.66%. 1H NMR (300 MHz, [D6]DMSO, reference DMSO at
d=2.50 ppm): dH =5.90 (s, 2H, H18), 7.46–7.50 (m, 4H, H21, H23, H2 and
H11), 7.55–7.60 (m, 2H, H20 and H24), 7.65 (d, 1H, H22), 8.04 (td, 1H, H3),
8.15 (td, 1H, H10), 8.24 (d, 2H, H9), 8.28 (d, 1H, H4), 8.35 (d, 2H, H14
916
www.chemasianj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 910 – 918

Alkylations of N4-(4-Pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole
and H17), 8.49 (dd, 1H, H1), 8.77 (dd, 1H, H12), 9.28 ppm (d, 2H, H15 and
H16).
the yellow color become more intense. The darker yellow solution was
then stirred for a further 48 h, over which time a bright yellow precipitate
formed (95 mg, 69%). Anal. calcd. for C18H15N6I (442.26 gmolÀ1): C
48.88, H 3.42, N 19.00, I 28.69; found: C 48.79, H 3.54, N 19.05, I
29.02%. 1H NMR (400 MHz, [D6]DMSO, reference DMSO at d=
2.50 ppm): dH =4.39 (s,3H, H18), 7.56 (ddd, J=7.6, 4.7, 0.9 Hz, 1H, H2),
7.62 (ddd, J=7.6, 4.8, 0.9 Hz, 1H, H11), 8.02 (td, J=7.7, 1.7 Hz, 1H, H3),
8.13 (td, J=7.7, 1.6 Hz, 1H, H10), 8.31–8.24 (m, 4H, H14,H17,H4,H9), 8.49
(d, J=4.8 Hz, 1H, H12), 8.74 (d, J=4.7 Hz, 1H, H1), 9.06 ppm (d, J=
7.1 Hz, 2H, H15 and H16). 13C NMR (101 MHz, [D6]DMSO, reference
N4-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole bromide (N4-
Bzpydpt·Br): To a solution of pydpt (50 mg, 0.16 mmol) in dichlorome-
thane (20 mL) was added benzyl bromide (140 mg, 0.2 mL, 0.825 mmol).
The resulting colorless solution was stirred for 3 h, over which time a
white precipitate formed (60 mg, 80%). Anal. calcd. for C24H19N6Br·H2O
(489.37 gmolÀ1): C 58.90, H 4.33, N 17.17, Br 16.33; found: C 58.82, H
4.61, N 16.92, Br 16.11%. 1H NMR (300 MHz, [D6]DMSO, reference
DMSO at d=2.50 ppm): dH =5.97 (s, 2H, H18), 7.43–7.58 (m, 7H, H20–24
,
ACTHNUTRGNEUNG
DMSO at d=39.52 ppm): dC =162.4 (C6), 155.5 (C7), 150.6(C13), 150.5
H2 and H11), 8.02 (td, 2H, H3 and H10), 8.15 (dd, 2H, H4 and H9), 8.30
(dd, 2H, H1 and H12), 8.41 (d, 2H, H14 and H17), 9.32 ppm (d, 2H, H15
and H16). 13C NMR (100 MHz, [D6]DMSO, reference DMSO at d=
39.52 ppm): dC =155.7 (C6/7), 153.7 (C13), 149.5 (C5/8), 148.8 (C1/12), 146.5
(C15/16), 138.0 (C4/9), 129.8 (BzC), 129.4 (BzC), 128.5 (C14/17), 125.5 (C2/11),
125.3 (BzC), 124.2 (C3/10), 63.5 ppm (C18). IR (KBr): n˜ =3419, 3038, 2979,
2918, 1636, 1595, 1585, 1526, 1508, 1460, 1447, 1432, 1204, 1175, 1163,
1151, 1087, 1046, 995, 989, 873, 844, 792, 744, 723, 714, 686, 637, 620, 606,
(C1), 149.5 (C12), 148.3(C5), 147.4 (C15/16), 146.1 (C8), 138.7 (C10), 137.9
(C3), 126.7 (C11), 125.7 (C2), 125.3 (C9), 122.9 (C4 and C14/17), 48.3 ppm
(C18). IR (KBr): n˜ =3004, 1630, 1583, 1518, 1458, 1431, 1281, 1247, 1205,
1186, 1170, 1094, 995, 841, 795, 740, 719, 628, 599 cmÀ1. ESIMS (pos.): m/
z 315.132 [N1-Mepydpt]+.
N1-(4-Methylpyridinium)-3-(2-methylpyridinium)-5-(2-pyridyl)-1,2,4-tria-
zole diiodide (N1--2Mepydpt·2I): To
a solution of pydpt (105 mg,
475 cmÀ1
.
0.35 mmol) in acetonitrile (20 mL) was added methyl iodide (0.2 mL, sta-
bilized MeI, 3.2 mmol). The resulting clear yellow solution was heated at
reflux for 1 h, during which time the clear yellow solution become intense
orange. The orange solution was heated with stirring for a further 6 h,
over which time a small amount of bright yellow precipitate formed. The
reaction mixture was cooled to À48C and the resulting precipitate of N1--
2Mepydpt·2I was filtered (92 mg, 60%). Anal. calcd. for C19H18N6I2
(584.20 gmolÀ1): C 39.06, H 3.11, N 14.39; found: C 38.90, H 3.19, N
14.25%. 1H NMR (400 MHz, [D6]DMSO, reference DMSO at d=
2.50 ppm): dH =4.44 (s, 3H, H19), 4.74 (s, 3H, H18), 7.69 (ddd, J=7.7, 4.8,
1.1 Hz, 1H, H2), 8.18 (td, J=7.8, 1.7 Hz, 1H, H3), 8.36–8.27 (m, 2H, H4
and H11), 8.42 (d, J=8.2 Hz, 2H, H14 and H17), 8.53 (ddd, J=4.8, 1.6,
0.9 Hz, 1H, H1), 8.83–8.79 (m, 2H, H9 and H10) 9.16 (d, J=8.2 Hz, 2H,
H15 and H16), 9.28 ppm (d, 1H, H12). 13C NMR (101 MHz, [D6]DMSO,
reference DMSO at d=39.52 ppm): dC =156.0, 155.5, 150.1, 149.7, 149.3,
147.8, 146.5, 145.2, 143.3, 138.9, 129.8, 129.0, 127.3, 125.6, 123.5, 49.6,
48.6 ppm. IR (KBr): n˜ =3039, 1640, 1625, 1579, 1517, 1467, 1444, 1397,
1382, 1358, 1289, 1219, 1180, 1143, 1086, 1045, 1010, 855, 814, 779, 751,
729, 720, 696, 686, 609, 535, 514, 435, 406 cmÀ1. ESIMS (pos.): m/
N4-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole tetrafluoroborate
(N4-Bzpydpt·BF4): To a colorless solution of N4-(4-benzylpyridinium)-3,5-
di(2-pyridyl)-1,2,4-triazole bromide (N4-Bzpydpt·Br) (50 mg, 0.1 mmol)
in water (5 mL) was added an aqueous solution (5 mL) of ammonium tet-
rafluoroborate (50 mg, 0.4 mmol). A white crystalline solid (35 mg, 69%)
was formed immediately. Anal. calcd. for C24H19N6BF4 (478.25 gmolÀ1): C
60.27, H 4.00, N 17.57; found: C 60.47, H 4.02, N 17.80%. (300 MHz,
[D6]DMSO, reference DMSO at d=2.50 ppm): dH =5.96 (s, 2H, H18),
7.43–7.55 (m, 7H, H20–24, H2 and H11), 8.02 (td, 2H, H3 and H10), 8.11 (d,
2H, H4 and H9), 8.30 (d, 2H, H1 and H12), 8.41 (d, 2H, H14 and H17),
9.31 ppm (d, 2H, H15 and H16). Recrystallizing the white crystalline solid
from a hot solvent mixture of water/methanol (10:1) gave single crystals
suitable for X-ray diffraction studies.
N4-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole
tetraphenylbo-
rate (N4-Bzpydpt·BPh4): To a colorless solution of N4-(4-benzylpyridini-
um)-3,5-di(2-pyridyl)-1,2,4-triazole bromide (N4-Bzpydpt·Br) (40 mg,
0.08 mmol) in water (10 mL) was added an aqueous solution (5 mL) of
ammonium tetraphenylborate (55 mg, 1.6 mmol). Immediately, a very
fine white solid was formed (50 mg, 88%). Anal. calcd. for C24H19N6B-
z 165.077 [N1--2Mepydpt]2+
.
ACHTUNGTRENNUNG
N
DMSO at d=2.50 ppm): dH =5.96 (s, 2H, H18), 6.78 (t, 4H, BPh4-H), 6.91
(t, 8H, BPh4-H), 7.16–7.19 (m, 8H, BPh4-H), 7.44–7.55 (m, 7H, H20–24, H2
and H11), 8.03 (td, 2H, H3 and H10), 8.11 (d, 2H, H4 and H9), 8.29 (d, 2H,
H1 and H12), 8.40 (d, 2H, H14 and H17), 9.30 ppm (d, 2H, H15 and H16). To
remove the trace amount of bromine present, a small sample of the fine
white solid (25 mg) was dissolved in acetone and subjected to vapor dif-
fusion of diethyl ether. The result was a fine white crystalline solid
(10 mg) in which there was no detectable amount of bromine (<0.3%).
1H NMR data were consistent with those of N4-Bzpydpt·BPh4.
Acknowledgements
This work was supported by the University of Otago and the Marsden
Fund. We thank the Tertiary Education Commission (New Zealand) for
the award of a Bright Futures Top Achiever Doctoral scholarship to
J.A.K., the MacDiarmid Institute for Advanced Materials and Nanotech-
nology (J.A.K. and S.B.), and the referees for helpful comments.
N1-(4-Benzylpyridinium)-3,5-di(2-pyridyl)-1,2,4-triazole
tetraphenylbo-
rate (N1-Bzpydpt·BPh4): A colorless solution of the fine white solid of
N4-Bzpydpt·BPh4 (25 mg, not recrystallized) in 10 mL MeCN was heated
at reflux for 3 h. Removal of solvent in vacuo resulted in a pale yellow
oil, which became solid upon drying in vacuo. 1H NMR (400 MHz,
[D6]DMSO, reference DMSO at d=2.50 ppm): dH =5.90 (s, 2H, H18),
6.78 (t, 4H, BPh4-H), 6.89 (t, 8H, BPh4-H), 7.16–7.19 (m, 8H, BPh4-H),
7.46–7.49 (m, 4H, H21, H23, H2 and H11), 7.55–7.60 (m, 2H, H20 and H24),
7.66 (d, 1H, H22), 8.04 (td, 1H, H3), 8.15 (td, 1H, H10), 8.23 (d, 2H, H9),
8.29 (d, 1H, H4), 8.35 (d, 2H, H14 and H17), 8.49 (dd, 1H, H1), 8.77 (dd,
1H, H12), 9.28 ppm (d, 2H, H15 and H16). When 5 mg of the fine white
crystals from the acetone/diethyl ether recrystallization of N4-
Bzpydpt·BPh4 was heated at reflux in 5 mL MeCN and the solvent was
removed, the result was similar: a pale yellow solid was obtained which
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Received: September 22, 2009
Revised: December 17, 2009
Published online: March 12, 2010
918
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 910 – 918