Trinuclear coordinatively labile Cu(II) complex of 4,6-O-ethylidene-β- D-glucopyranosylamine derived Schiff base ligand and its reactivity towards primary alcohols and amines

Article abstract of DOI:10.1039/b413923b

A novel neutral trinuclear Cu(II) complex of a Schiff base ligand derived from D-glucose has been synthesised and structurally characterised, which exhibits excellent alcohol binding affinity and activates the C-Cl bond of chloroform in the presence of primary amine. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b413923b

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COMMUNICATION
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Trinuclear coordinatively labile Cu(II) complex of 4,6-O-ethylidene-b-
D-glucopyranosylamine derived Schiff base ligand and its reactivity
towards primary alcohols and amines{
Ajay K. Sah, Merii Kato and Tomoaki Tanase*
Received (in Cambridge, UK) 10th September 2004, Accepted 2nd November 2004
First published as an Advance Article on the web 8th December 2004
DOI: 10.1039/b413923b
dianion and the saccharide C2 hydroxo group bridges the copper
centres resulting in a Cu2O2 four membered rhombus.7,8 The
presence of a tertiary butyl group in the present case might have
restricted the formation of such a ring and thus resulted in a
coordinatively labile novel trinuclear complex suitable for ligand
exchange studies. The unique feature of complex 2a is that all the
saccharide hydroxyl groups are deprotonated and engaged in metal
ion binding. The saccharide C2 hydroxo group bridges the terminal
and central copper ions, while the C3 hydroxo group is bound to
the central copper resulting in a square planar geometry around it.
Each ligand acts as a trianionic species and occupies five
coordination sites around the metal centres to result in a neutral
trinuclear core. The terminal Cu(II) centres are expected to be
coordinatively unsaturated in complex 1 where phenolate, imine
nitrogen and saccharide C2 hydroxo group fulfil only three sites.
The fourth basal site is occupied by methanol in 2a, resulting in a
square planar geometry. Although acetonitrile is supposed to be a
good coordinating solvent for Cu(II), it is surprising to note that
even in the presence of excess acetonitrile, it is the trace amount of
methanol that occupies the fourth position. This selectivity of the
complex towards the alcohols may be attributed to the involvement
of alkoxy hydrogen in strong intramolecular hydrogen bonding
interaction with the saccharide C3 hydroxo group. Similarly the
complex was crystallised using ethanol, n-propanol and n-butanol
to result in X-ray suitable single crystals of [Cu3(L1)2(ROH)2]
(R 5 Et (2b), n-Pr (2c), n-Bu (2d)), where the fourth basal
coordination sites about the terminal copper is occupied by the
respective alcohol molecules.10 In all cases copper centres are in a
square planar geometry with an almost linear arrangement (Cu(1)–
Cu(2)–Cu(3), 166–172u). Linear trinuclear copper complexes are
rare in the literature and hence these complexes are important from
a magnetic point of view.11,12
A novel neutral trinuclear Cu(II) complex of a Schiff base
ligand derived from D-glucose has been synthesised and
structurally characterised, which exhibits excellent alcohol
binding affinity and activates the C–Cl bond of chloroform in
the presence of primary amine.
Copper is one of the most important transition metal ions in
biological systems, it plays an important role in transport and
activation of molecular oxygen.1–3 Due to its versatile nature, a lot
of work has been published on copper with a variety of ligands,
however, it is surprising to note that its chemistry with
carbohydrates, which is one of the chiral polyfunctional biomo-
lecules, has not been well documented. Only a countable number
of structurally characterised glucose derived Cu(II) complexes are
recorded in the literature.4–8 Hence, the lack of copper complexes
with saccharides has aroused our interest to explore its chemistry9
and in this direction, we are reporting here for the first time a
structurally characterised trinuclear complex of N-(3-tert-butyl-
2-hydroxybenzylidene)-4,6-O-ethylidene-b-D-glucopyranosylamine
(H3L1) and some of its reactivity towards alcohols and amines.
The ligand (H3L1) was synthesised in 84% yield by condensing
3-tert-butyl-2-hydroxybenzaldehyde with 4,6-O-ethylidene-b-D-
glucopyranosylamine in methanol (Scheme 1). Its reaction with
Cu(OAc)2?H2O (metal : ligand 5 3 : 2) in methanol afforded the
trinuclear complex, [Cu3(L1)2]?MeOH?H2O (1) in 78% yield.
Methanol vapour diffusion into the complex solution in CHCl3/
MeCN (1 : 1 mixed solvent) resulted in the formation of single
crystals of [Cu3(L1)2(MeOH)2] (2a) suitable for X-ray crystal-
lography (Fig. 1).{ All the previously reported copper complexes
with similar ligands are dinuclear where each ligand acts as a
{ Electronic supplementary information (ESI) available: Details of
synthesis, characterisation, structural data and ORTEP plots of 4 and 5.
See http://www.rsc.org/suppdata/cc/b4/b413923b/
When the complex was crystallised by diffusing the methanol
vapour into CHCl3/THF (1 : 1 mixed solvent) solution, THF
*tanase@cc.nara-wu.ac.jp
Scheme 1
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˚
Fig. 1 ORTEP view of 2a with atom labelling. Selected bond lengths (A) and angles (u): Cu(1)–O(1) 1.900(3), Cu(1)–O(2) 1.953(3), Cu(1)–O(13) 1.932(4),
Cu(1)–N(1) 1.941(4), Cu(2)–O(2) 1.941(3), Cu(2)–O(3) 1.913(3), Cu(2)–O(8) 1.934(3), Cu(2)–O(9) 1.911(3), Cu(3)–O(7) 1.887(3), Cu(3)–O(8) 1.941(3),
Cu(3)–O(14) 1.941(3), Cu(3)–N(2) 1.940(4), Cu(1)–Cu(2) 3.683(1), Cu(2)–Cu(3) 3.668(1), O(2)–Cu(1)–O(1) 176.0(1), O(8)–Cu(2)–O(2) 165.9(1), O(8)–
Cu(3)–O(7) 174.5(1), N(1)–Cu(1)–O(13) 171.6(2), O(3)–Cu(2)–O(2) 88.8(1), Cu(1)–Cu(2)–Cu(3) 172.41(3).
bound crystals of [Cu3(L1)2(MeOH)2(THF)2]?2THF (3) were
isolated (Fig. 2).§ In this case two molecules of THF are bound
to the terminal copper centres. The basal position about the
terminal copper is occupied by the methanol molecule and THF
occupy the axial positions, resulting in a square pyramidal
geometry around the terminal copper centres and square planar
˚
Fig. 2 ORTEP view of 3 with atom labelling, lattice THF molecules are removed for clarity. Selected bond lengths (A) and angles (u): Cu(1)–O(1)
1.900(7), Cu(1)–O(2) 1.956(7), Cu(1)–O(13) 1.979(9), Cu(1)–N(1) 1.96(1), Cu(1)–O(15) 2.394(7), Cu(2)–O(2) 1.923(8), Cu(2)–O(3) 1.907(6), Cu(2)–O(8)
1.932(6), Cu(2)–O(9) 1.924(7), Cu(3)–O(7) 1.888(7), Cu(3)–O(8) 1.944(6), Cu(3)–O(14) 1.944(8), Cu(3)–O(16) 2.466(7), Cu(3)–N(2) 1.959(9), Cu(1)–Cu(2)
3.673(3), Cu(2)–Cu(3) 3.648(3), O(2)–Cu(1)–O(1) 173.5(3), O(8)–Cu(2)–O(2) 163.0(3), O(8)–Cu(3)–O(7) 177.1(3), N(1)–Cu(1)–O(13) 171.1(4), O(3)–Cu(2)–
O(2) 88.5(3), Cu(1)–Cu(2)–Cu(3) 170.28(7).
676 | Chem. Commun., 2005, 675–677
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about the central one. Both the THF molecules are trans to each
Notes and references
other with respect to the molecular plane. All the equatorial bond
˚
lengths are in the range of 1.89–1.98 A for both the complexes and
˚
axial Cu–O distances are 2.394(7) and 2.466(7) A which are within
the range for similar geometry.13,14
{ Crystal data for 2a: C40H56Cu3N2O14, M 5 979.53, orthorhombic, a 5
3
˚
˚
6.4406(2), b 5 14.9072(5), c 5 42.412(1) A, U 5 4072.0(2) A , T 5 2120 uC,
space group P212121 (no. 19), Z 5 4, Dc 5 1.598 g cm23, m(Mo-Ka) 5
16.24 cm21, 22509 reflections measured, 13250 unique (Rint 5 0.052), R(I .
2.00s(I)) 5 0.040, Rw(I . 2.00s(I)) 5 0.050.
Such an affinity of complex 1 towards alcohols indicates
that primary amines could also be introduced into the tricopper
§ Crystal data for 3: C56H88Cu3N2O18, M 5 1267.95, triclinic, a 5 8.1487(1),
˚
b 5 12.2359(1), c 5 16.9972(3) A, a 5 72.113(8), b 5 78.973(9), c 5
3
˚
system, resulting in
a cyclic hydrogen bonding network
65.529(6)u, U 5 1463.92(3) A , T 5 2120 uC, space group P1 (no. 1), Z 5 1,
Dc 5 1.438 g cm23, m(Mo-Ka) 5 11.52 cm21, 7377 reflections measured,
5309 unique (Rint 50.014), R(I.2.00s(I)) 50.047, Rw(I.2.00s(I)) 50.056.
" Crystal data for 4: C24H32CuN2O2, M 5 444.07, monoclinic,
through the interaction of amino hydrogen donors with the
phenolate as well as the sugar C3 hydroxo groups. An attempt
along this line has been tried using aniline and p-toluidine,
however no suitable crystals have been isolated. Diffusion of
methylamine (40% in methanol) vapours into a chloroform
solution of complex 1 has resulted in interesting results. From
the reaction mixture, the crystals of brown colored Cu(II) complex,
[Cu(L2)2] (4, L2H 5 2-tert-butyl-6-[(methylimino)methyl]phenol),"
and the blue colored crystals of [Cu(NH2Me)5]Cl2 (5)I were
obtained and characterised by X-ray crystallography (see ESI).{
The formation of complex 5, strongly demonstrated the C–Cl
bond activation of the chloroform molecule. In order to avoid the
presence of any acid chloride in the chloroform, a number of
crystallisations were performed using freshly distilled CHCl3 as
well as NaOH treated CHCl3 and in all cases similar complexes
were isolated. The ESI-MS analysis of the mother liquor supported
the formation of N-methyl-4,6-O-ethylidene-D-glucopyranosyla-
mine revealing a single pot trans imination and trans amination
reactions. Generally C–Cl bonds of alkyl chloride are difficult to
activate due to intrinsically high bond energy and lower leaving
group ability,15 however, in our case, it has formed by
methylamine under mild conditions. Details of the solution study
and reactivity of the complex with other primary amines are in
progress.
˚
a 5 12.830(4), b 5 10.858(3), c 5 17.007(5) A, b 5 102.255(4)u,
3
˚
U 5 2315.3(11) A , T 5 2120 uC, space group P21/c (no. 14), Z 5 4,
Dc 5 1.274 g cm23, m(Mo-Ka) 5 9.64 cm21, 12529 reflections measured,
5096 unique (Rint 5 0.057), R(I . 2.00s(I)) 5 0.090, Rw(I .
2.00s(I)) 5 0.106.
I Crystal data for 5: C5H25CuN5Cl2, M 5 289.74, orthorhombic,
3
˚
˚
a 5 14.0065(10), b 5 11.6044(11), c 5 8.5136(5) A, U 5 1383.8(2) A , T 5
2120uC, space groupPnma(no. 62), Z54, Dc 51.391gcm23,m(Mo-Ka)5
19.38 cm21, 7810 reflections measured, 1023 unique (Rint 5 0.019), R(I .
2.00s(I)) 5 0.037, Rw(I . 2.00s(I)) 5 0.048.**
** CCDC 250717–250720. See http://www.rsc.org/suppdata/cc/b4/
b413923b/ for crystallographic data in .cif or other electronic format.
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A. K. Sah is grateful to Japan Society for the Promotion
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Ajay K. Sah, Merii Kato and Tomoaki Tanase*
Department of Chemistry, Faculty of Science, Nara Women’s
University, Kitauoya-higashi-machi, Nara 630-8285, Japan.
E-mail: tanase@cc.nara-wu.ac.jp; Fax: +81 742 203399;
Tel: +81 742 203399
´
´
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