Copper-induced N-N bond cleavage results in an octanuclear expanded-core grid-like complex

Article abstract of DOI:10.1039/c2cc32018e

Reaction of copper(i) acetate and 4-amino-3,5-di(2-pyridyl)-1,2,4-triazole (adpt) in methanol under ambient conditions yields octanuclear [Cu ^II₈(dpt)₄(OH)₄(OAc)₈]; OAc = acetate anion, and dpt^- = anion of deaminated adpt, 3,5-di(2-pyridyl)-1,2,4-triazolate. However, reaction of copper(ii) acetate with dptH gives tetranuclear [Cu^II₄(dpt)₂(OH)(OMe) (OAc)₄]. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc32018e

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COMMUNICATION
Copper-induced N–N bond cleavage results in an octanuclear
expanded-core grid-like complexw
Nicholas G. White,a Jonathan A. Kitchen,a John A. Jouleb and Sally Brooker*a
Received 19th March 2012, Accepted 21st April 2012
DOI: 10.1039/c2cc32018e
Reaction of copper(I) acetate and 4-amino-3,5-di(2-pyridyl)-
1,2,4-triazole (adpt) in methanol under ambient conditions yields
octanuclear [CuII8(dpt)4(OH)4(OAc)8]; OAc = acetate anion,
and dptꢀ = anion of deaminated adpt, 3,5-di(2-pyridyl)-1,2,4-
triazolate. However, reaction of copper(II) acetate with dptH
gives tetranuclear [CuII4(dpt)2(OH)(OMe)(OAc)4].
blue-green crystals.z These were shown by X-ray crystallography
to be [CuII8(dpt)4(OH)4(OAc)8] (1), an octanuclear expanded-
core grid-like19,20 complex of the deaminated ligand, dptꢀ
(Fig. 2 and 3). Repeating the synthesis with the correct
stoichiometry for 1, i.e. 2 : 1 Cu : adpt, in either methanol or
acetonitrile, yielded a mixture of products. This mixture
included some blue-green crystals, which had the same unit
cell as 1; however, the bulk product did not give elemental
analysis data that were consistent with this formula.
The making and breaking of chemical bonds is the funda-
mental basis of the chemical sciences. Recently, interest has
focussed on N–N cleavage reactions, both as a convenient
synthetic route to primary amines,1 and to further under-
standing of nitrogenase enzymes, which transform N2 to
NH3 under ambient conditions.2,3 Notably, iron-sulfide4 and
iron-iminopyrimidine5 complexes have been used to cleave the
N–N bonds of azo compounds.
To probe the mechanism of the ligand deamination, the
reaction was then attempted using anhydrous copper(I) acetate.
The solubility of CuI(OAc) in acetonitrile was too low to allow
its use, so methanol was used instead. Stirring a 2 : 1 mixture of
CuI(OAc) : adpt open to the air, at RT, gave a homogeneous
sample of blue-green crystals of 1 (same unit cell as 1), in 56%
yield. Elemental analysis and IR spectroscopy were consistent
with the proposed formula.w Notably, this is by far the highest
nuclearity discrete complex seen for the dptꢀ/adpt/Rdpt family
of ligands (the previous largest being a tetranuclear complex).
The asymmetric unit (Fig. S1) of 1 contains one quarter
of the octanuclear complex, with the rest of the molecule
Previous studies have shown metal complexes of adpt
(Fig. 1) to have interesting magnetic properties, notably giving
spin crossover when complexed with iron(II) salts.6–13 However,
complexes of adpt almost exclusively give mononuclear or
binuclear systems (to the best of our knowledge, only two tetra-
nuclear, and two polymeric systems have been synthesised).6,14–16
In contrast, the non-aminated ligand, dptꢀ (Fig. 1) gives rise to
a rich variety of coordination architectures. Notably Chen and
co-workers have synthesised four distinct supramolecular
architectures having the formula [CuI(dptꢀ)]n (n = 4 or N).17
In an effort to synthesise interesting polymetallic complexes,
we are now looking at reacting adpt with metal acetate salts, as
the acetate ion can facilitate the formation of multi-nuclear
systems by bridging between metal centres.
¯
generated by a four-fold rotary inversion axis, 4, along the
c-axis (Fig. 2). Both unique copper ions have distorted square-
pyramidal N2O3 geometries (cis donor-Cu-donor angles =
80.1(3)–101.7(3)1, t = 0.24 and 0.15 for Cu1 and Cu2,
respectively), binding the deaminated dptꢀ ligand in the
bidentate pockets provided by nitrogen atoms from a pyridine
and triazolate ring (Cu1 by N1 and N2; Cu2B by N3 and N4).
Thus the triazolate ring provides a bridge between Cu1 and,
the symmetry generated, Cu2B. Within the asymmetric unit,
Cu1 and Cu2 are bridged by two 1,3-bridging acetate moieties
and a hydroxide group (Fig. S1). The Cu–X bond lengths vary
quite widely: the Cu–OH and one of the Cu–OAc bonds are
short (1.891(6)–1.972(8) A), while the other Cu–OAc bond is
much longer (2.233(7), 2.240(7) A) and the Cu–N bonds are
intermediate in length (2.026(9)–2.066(9) A). This means that
both acetate anions are quite asymmetric, with the equatorial
Cu–Oacetate bonds B0.27 A shorter than the axial Cu–Oacetate
Stirring a 1 : 2 mixture of CuII(OAc)2ꢁH2O : adpt18 at room
temperature (RT), open to the air in acetonitrile gave,
after vapour diffusion with diethyl ether, a small number of
a Department of Chemistry and MacDiarmid Institute for
Advanced Materials and Nanotechnology, University of Otago,
PO Box 56, Dunedin 9054, New Zealand.
E-mail: sbrooker@chemistry.otago.ac.nz;
Fax: +64 3 479 7906; Tel: +64 3 479 7919
b School of Chemistry, University of Manchester, Manchester,
UK M13 9PL
w Electronic supplementary information (ESI) available: [synthetic
details, diagram of hydrogen-bonding and of the asymmetric unit of
1, crystallographic details for 1 and 2ꢁMeOH, schemes showing
literature azo formation and decomposition mechanisms]. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc32018e
Fig. 1 Di-(2-pyridyl)-1,2,4-triazole ligands relevant to this work.
Chem. Commun., 2012, 48, 6229–6231 6229
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parallel ligand strands (triazole-pyridine mean planes intersect
at 15.01, with pyridine atoms 2.9–3.5 A out of the triazole
plane, and ring centroids separated by 4.06 A; Fig. 3).
Interestingly, when attempts to prepare the octanuclear
complex 1 were carried out using the correct ligand, i.e. the
already deaminated ligand (Hdpt18), with CuII(OAc)2ꢁH2O in
the same 1 : 2 ligand to metal ratio and same conditions (stir,
RT, methanol) the result was
a tetranuclear species
[CuII4(dptꢀ)2(OAc)4(OMe)(OH)] (2). Blue crystals and blue
powder formed upon the vapour diffusion of diethyl ether
into the methanolic reaction solution.z Microanalysis of
the bulk, dried, sample was consistent with the formula
2ꢁH2O. The single crystal X-ray structure of the blue crystals
of 2ꢁMeOH (Fig. 4) revealed that the asymmetric unit contains
half of the complex with the other half being generated by
inversion. Structurally the copper(II) centres are similar to
those in 1 in that they form dimers bridged by two acetate
moieties and have N2O3-donor environments, only instead of
distorted square-pyramidal geometries, in this complex the
copper(II) geometries are more trigonal bipyramidal (cis donor-
Cu-donor angles = 79.3(2)–113.8(2)1, t = 0.46 and 0.53 for
Cu1 and Cu2, respectively). Instead of the other (non-acetate)
bridging group being solely hydroxide (m-OH), in this struc-
ture it is either a hydroxide or a methoxide (this bridging site
contains a 0.5/0.5 OMe : OH disorder with a 0.5 occupancy
interstitial methanol present when the 0.5 occupancy hydro-
xide is bridging). The hydroxide group is the donor in an
intramolecular hydrogen bond to the 0.5 occupancy bridging
methoxide on the adjacent copper(II) dimer [dꢁ ꢁ ꢁa = 2.777(9) A,
od–Hꢁ ꢁ ꢁa = 1771] and the acceptor in an intermolecular
hydrogen bond from the 0.5 occupancy interstitial methanol
molecule [dꢁ ꢁ ꢁa = 2.729(12) A, od–Hꢁ ꢁ ꢁa = 1751].
Fig. 2 Perspective view of 1. Hydrogen atoms, and minor site of
disordered methyl group are omitted for clarity. Symmetry operations
used to generate equivalent atoms: (a) 3/2 ꢀ y, x, 12 ꢀ z; (b) y, 3/2 ꢀ x, 21 ꢀ z;
(c) 3/2 ꢀ x, 3/2 ꢀ y, z.
It is interesting that despite the apparent ease with which 1 can
be formed using adpt (that is it can be prepared, at least in some
amount from varied stoichiometries of copper(II) acetate in aceto-
nitrile or methanol, or in bulk from copper(I) acetate in methanol),
Fig. 3 Perspective view of of [CuII8(dpt)4(OAc)8(OH)4] 1 highlighting
one of the two channels present between parallel ligand strands.
Hydrogen atoms, except hydroxide protons, and minor site of
disordered methyl group are omitted for clarity. Symmetry operations
used to generate equivalent atoms: (a) 3/2 ꢀ y, x, 12 ꢀ z; (b) y, 3/2 ꢀ x, 21 ꢀ z;
(c) 3/2 ꢀ x, 3/2 ꢀ y, z.
bonds because the acetate ions bridge such that the oxygen
donor on one copper is axial whilst on the adjacent copper it is
equatorial. The centre of the octanuclear core is stabilised by
strong hydrogen-bonding between a hydroxide moiety and its
symmetry equivalent, such that each hydroxide oxygen acts as
both a hydrogen donor and acceptor (dꢁ ꢁ ꢁa = 2.689(9) A,
od–Hꢁ ꢁ ꢁa = 1691) (Fig. S2).w
Fig. 4 Perspective view of [CuII4(dpt)2(OAc)4(OH)(OMe)], the
complex in 2ꢁMeOH. Hydrogen atoms, and interstitial methanol
omitted for clarity. Half occupancy methoxide (O(20A)) is shown on
one dimer and half occupancy hydroxide (O(20)) shown on other
dimer (each dimer contains 0.5/0.5 OMe/OH) Symmetry operations
used to generate equivalent atoms: (a) ꢀx, 2 ꢀ y, 1 ꢀ z.
The four symmetry related dptꢀ ligand strands are in a
[2 ꢂ 2] grid-like arrangement, but enclose 4 pairs of metal ions,
not 4. This results in a close arrangement of each pair of
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Notes and references
z Crystal data: 1, C64H60N20O20Cu8, M = 1937.64, tetragonal, a =
15.2873(12), b = 15.2873(12), c = 14.960(2) A, V = 3496.1(7) A3,
T = 89(2)K, space group P42/n, Z = 2, 19 879 reflections measured,
3579 unique (Rint = 0.1053). Final wR2 = 0.2039 (all data) and R1 =
0.0672 (I 4 2s). 2ꢁMeOH, C34H36N10O11Cu4, M = 1014.89, ortho-
rhombic, a = 14.973(3), b = 12.682(2), c = 20.506(3) A, V =
3893.7(12) A3, T = 89(2)K, space group Pbca, Z = 4, 22 176
Fig. 5 Postulated mechanism for copper-induced deamination of
adpt.
reflections measured, 3984 unique (Rint = 0.1370). Final wR2 =
0.1674 (all data) and R1 = 0.0676 (I 4 2s). For further details:
CCDC 751993 and 751994.
it does not form at all when the ‘‘correct’’ ligand, Hdpt is used
instead of adpt. This suggests that some facet of the copper-
mediated N–N bond breakage is important in determining the
overall structure of the product.
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It has previously been shown that the adpt ligand can
deaminate in high temperature (130–160 1C) solvothermal
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an octanuclear complex, [CuII8(dmpzꢀ)8(OHꢀ)8] (dmpzꢀ
=
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obtain it as the bulk product when copper(I) is used.
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We speculate that a similar copper-mediated oxidative
dimerisation could occur in our system giving two molecules
of dpt linked by an azo group (Fig. 5). Formally, loss of
dinitrogen from this would generate two dpt radicals, which
can then be reduced by copper(I) to give the observed dptꢀ
complexes of copper(II). Decompositions of this type have
previously been observed for similar nitrogen-rich species
(Fig. S4 and S5).28,29 Regardless of the mechanism, the sum
of the chemical processes is an N–N bond cleavage reaction
that occurs rapidly and in good yield at ambient temperature
and pressure.
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In conclusion, when the deaminated ligand Hdpt is reacted
with copper(II) acetate, a tetranuclear copper(II) complex
forms. In contrast, when adpt is reacted with copper(I) or (II)
acetate salts, an octanuclear expanded-core grid-like copper(II)
complex of the deaminated ligand, dptꢀ, forms. The deamina-
tion occurs under ambient conditions and in good yield and
assembly of the octanuclear complex seems reasonably
tolerant to the initial oxidation state of the metal salt, as well
as the metal : ligand stoichiometry and choice of solvent.
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