Complexes of 2,6-bis[N-(2′-pyridylmethyl)carbamyl]pyridme: Formation of mononuclear complexes, and self-assembly of double helical dinuclear and tetranuclear copper(II) and trinuclear nickel(II) complexes

Article abstract of DOI:10.1039/b414251a

The potentially pentadentate ligand 2,6-bis[N-(2′-pyridylmethyl) carbamyl]pyridine (H₂L¹), readily prepared from reaction of a diester of pyridine-2,6-dicarboxylic acid (H₂dipic) and 2-aminomethylpyridine (ampy), shows limited tendency to form 1: 1 M: L complexes with labile metal ions, although [CuL¹] and [NIL¹] were observed as minor species, the latter characterized by a crystal structure analysis. A mononuclear complex formed with inert Co(III) was characterized by a crystal structure as the neutral 1: 2 complex [Co(L¹)(HL ¹)] with two ligands acting as tridentate ligands, one coordinated by the central pyridine and its two flanking deprotonated amido groups, and the other by the central pyridine, one amido and one terminal pyridine group, with the remaining poorly coordinating protonated amide remaining unbound along with other terminal pyridine groups. Fe(III) is known to form a symmetrical 1: 2 complex, but that complex is anionic due to binding of all four deprotonated amido groups; the unsymmetrical neutral Co(III) complex converts into a symmetrical anionic species only on heating for hours in aqueous base in the presence of activated carbon. The most remarkable tendency of H ₂L¹, however, is towards the formation of robust double helical complexes: a dinuclear Cu(II) complex [Cu₂L₂ ¹] forms, as well as a trinuclear Ni(II) complex [Ni ₃(L¹)₂(OAc)₂(MeOH)₂]. Moreover, in the presence of added H₂dipic, the tetranuclear complex [Cu₄(L¹)₂(dipic)₂(OH ₂)₂] is obtained. All helical complexes have been characterized by X-ray crystal structure analyses, and all crystals feature a racemic mixture of left- and right-handed double helices stabilized by inter-ligand π-stacking (inter-ring distances of 3.2-3.8 A) of ligands which each span several metal ions. Using the chelating ligand pentane-2,4-dione (acac), each of the two pairs of adjacent monodentate ligands in [Ni ₃(L¹)₂(OAc)₂(OH₂) ₂] have been shown to be available for substitution without destroying the helical structure, to form [Ni₃(L¹) ₂(acac)₂], also characterized by a crystal structure.

Full text of DOI:10.1039/b414251a

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F U L L P A P E R
Complexes of 2,6-bis[N-(2ꢀ-pyridylmethyl)carbamyl]pyridine:
formation of mononuclear complexes, and self-assembly of double
helical dinuclear and tetranuclear copper(II) and trinuclear nickel(II)
complexes
Nathaniel W. Alcock,a Guy Clarkson,a Peter B. Glover,a Geoffrey A. Lawrance,*b
Peter Moore*a and Mery Napitupulub
a Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL
b Discipline of Chemistry, School of Environmental and Life Sciences, The University of
Newcastle, Callaghan N.S.W., 2308, Australia. E-mail: p.moore@warwick.ac.uk
Received 15th September 2004, Accepted 26th November 2004
First published as an Advance Article on the web 4th January 2005
The potentially pentadentate ligand 2,6-bis[N-(2ꢀ-pyridylmethyl)carbamyl]pyridine (H2L1), readily prepared from
reaction of a diester of pyridine-2,6-dicarboxylic acid (H2dipic) and 2-aminomethylpyridine (ampy), shows limited
tendency to form 1 : 1 M : L complexes with labile metal ions, although [CuL1] and [NiL1] were observed as minor
species, the latter characterized by a crystal structure analysis. A mononuclear complex formed with inert Co(III) was
characterized by a crystal structure as the neutral 1 : 2 complex [Co(L1)(HL1)] with two ligands acting as tridentate
ligands, one coordinated by the central pyridine and its two flanking deprotonated amido groups, and the other by
the central pyridine, one amido and one terminal pyridine group, with the remaining poorly coordinating protonated
amide remaining unbound along with other terminal pyridine groups. Fe(III) is known to form a symmetrical 1 : 2
complex, but that complex is anionic due to binding of all four deprotonated amido groups; the unsymmetrical
neutral Co(III) complex converts into a symmetrical anionic species only on heating for hours in aqueous base in the
presence of activated carbon. The most remarkable tendency of H2L1, however, is towards the formation of robust
double helical complexes: a dinuclear Cu(II) complex [Cu2L1 ] forms, as well as a trinuclear Ni(II) complex
2
[Ni3(L1)2(OAc)2(MeOH)2]. Moreover, in the presence of added H2dipic, the tetranuclear complex
[Cu4(L1)2(dipic)2(OH2)2] is obtained. All helical complexes have been characterized by X-ray crystal structure
analyses, and all crystals feature a racemic mixture of left- and right-handed double helices stabilized by inter-ligand
˚
p-stacking (inter-ring distances of 3.2–3.8 A) of ligands which each span several metal ions. Using the chelating
ligand pentane-2,4-dione (acac), each of the two pairs of adjacent monodentate ligands in [Ni3(L1)2(OAc)2(OH2)2]
have been shown to be available for substitution without destroying the helical structure, to form [Ni3(L1)2(acac)2],
also characterized by a crystal structure.
2,6-Bis[N-(2ꢀ-pyridylmethyl)carbamyl]pyridine (H2L1) under
Introduction
examination here has been described before, and displays
no unusual structural features in its crystal structure when
not coordinated,6,7 adopting the typical syn,syn conformation
resulting from intramolecular hydrogen bonding of the amide
NH-group with the central pyridine N-atom. The longer-chain
analogue (H2L2), with an additional methylene in each alkyl
chain, is also well known.8 Notably, a bisoxazoline pyridine
analogue has been observed to adopt a left-handed helical
superstructure in the solid state even in the absence of metal
ions,9 presaging helicity and chirality in complexes of this type
of ligand system. Chirality has also been introduced into related
ligands themselves, albeit in examples of lower denticity.10
The synthesis of ligands based on 2,6-disubstituted pyridine
has attracted a great deal of attention, with a broad array of
both acyclic and macrocyclic ligands reported.1 In particular,
diamides incorporating an –NH–CO–pyridine–CO–NH– core
based on pyridine-2,6-dicarboxylic acid (H2dipic) have been
examined, with interest in these compounds extending to
potential applications as diverse as azo dyes2 and telomerase
inhibitors.3 Whereas pyridine-2,6-dicarboxylic acid has been
used mainly in pursuit of the synthesis of low molecular
weight molecules, it has also been applied in the synthesis
of poly(amide esters),4 with both linear and cyclic structures
identified, including macrocycles with from 2 to 14 pyridine
groups in the ring. Macrocycles incorporating the pyridine-2,6-
diamide functionality are known,5 but will not be discussed here,
where the focus is on linear acyclic ligand systems.
2,6-Bis[N-(2ꢀ-pyridylmethyl)carbamyl]pyridine
molecule
(H2L1) is a potentially linear pentadentate ligand. Both
mononuclear and polynuclear metal complexes formed from a
family of potentially pentadentate ligands based on pyridine-
2,6-dicarboxylic acid have been described. The target molecule
(H2L1) has been reported to spontaneously form dicopper(II)
complexes which display double-stranded helicate behaviour,11
in contrast to the molecule with an additional methylene in each
arm (H2L2), which forms only monomeric copper(II) species.8
The Fe(III) complex of both H2L1 and H2L2 are monomeric,
forming mer-[FeL2]− ions with the central pyridine and both
deprotonated amides of each ligand bound and the terminal
pyridines unbound.12 A range of other related ligands featuring
different ‘arms’ have been reported, and form monomeric or
polymeric complexes.13–20
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D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
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The family of linear acyclic molecules with an –NH–CO–
pyridine–CO–NH– core have been shown to offer a range
of important applications. These include uses as reagents for
heavy metal selection and or removal from water,21–23,32 electro-
luminescent devices,24 tryptase inhibitors,25 artificial enzymes,26
catalysts for aldehyde hydrophosphonylation,27 and as bright-
ening agents for synthetic fibres.28 H2L1 is reportedly effi-
cient as an oxygenation catalyst in concert with cobalt(II)
acetate,29 and as an ionophore in the polymer membrane of a
copper(II)-selective electrode.30 The close analogue (H2L2), as the
[CoIII(L2)(OOR)] complex, is effective in the catalytic oxidation
of hydrocarbons,12,31 and H2L2 along with cobalt(II) acetate
catalyses the epoxidation of olefins and the selective oxygenation
of phenols to quinones.29
Although most examples of the coordination chemistry of
H2L1 and its analogue H2L2 reported to date have yielded
monomeric complexes with 1 : 1 or 1 : 2 stoichiometry, the
observation of the spontaneous formation of a 2 : 2 helical
dicopper(II) complex was a spur to our deeper investigation of
the chemistry of H2L1. We report the ability of H2L1 to form not
only monomer and helical dinuclear complexes, but also more
elaborate double helical trinuclear and tetranuclear complexes.
Synthetic, reactivity and structural details are presented herein.
recrystallised from benzene (2.5 g). Anal. C, 64.3; H, 5.1; N, 19.7.
C19H17N5O2·0.5H2O requires C, 64.0; H, 4.9; N, 19.7%. NMR:
dH (CDCl3) 4.74 (4H, d, CH2), 7.14 (2H, ddd, pyH), 7.32 (2H,
dd, pyH), 7.62 (2H, ddd, pyH), 7.96 (1H, t, pyH), 8.29 (2H, d,
pyH), 8.44 (2H, dd, pyH), 9.11 (2H, t, NH). dC (CDCl3) 44.5
(CH2), 122.1, 122.3 124.9, 136.8, 138.8, 149.2 (pyCH), 148.7,
+
=
156.9 (pyCtert), 163.6 (C O) ppm. EI MS: m/z 347 (M ), 213
(M+ − pyCH2NHCO), 149 (M+ − pyCH2NHCOpyCH2NH),
135 (pyCH2NHCO+), 107 (pyCH2NH+), 77 (py+). IR (cm−1):
1674, 1665 (CO), 1594, 1567, 1539, 1437, 1413 (NHCO), 1310,
1258 (CH), 1001, 765, 648 (py). UV-Vis spectrum (EtOH):
204 nm (e = 41800 dm3 mol−1 cm−1), 262 (16900).
(2,6-Bis[N-(2ꢀ-pyridinylmethyl)carbamido]pyridine)(2-[N-(2ꢀ-
pyridinylmethyl)carbamido]-6-[N-(2ꢀ-pyridinylmethyl)carbamyl]-
pyridine)cobalt(III) trihydrate, [Co(L1)(HL1)]·3H2O
To a solution of H2L1 (0.35 g, 0.001 mol) in MeOH (30 cm3)
was added with stirring a solution of Co(OAc)2·4H2O (0.25 g,
0.001 mol) in MeOH (20 cm3). The colour changed immediately
to a red–brown. After stirring at room temperature for 1 h, the
solution was evaporated to dryness with a rotary evaporator,
redissolved in EtOH (30 cm3), evaporated to dryness, and
redissolved in water (20 cm3). On slow evaporation, a small
amount of brown crystals formed, which were separated and air
dried (40 mg). Alternatively, chromatography of the reaction
mixture on SP-Sephadex C25 or Dowex 50W × 2 cation
exchange resins can be used to separate the major neutral free-
running band from those of minor ionic byproducts prior to
isolation in high yield as a brown powder by rotary evaporation
of the eluent to dryness. Anal. C, 57.6; H, 3.9; N, 17.9.
C38H32N10CoO4·3H2O requires C, 57.9; H, 4.1; N, 17.8%. NMR:
dH(D2O) 3.79 (2H, q, CH2), 4.00 (4H, br s, CH2 pendant), 5.47
(2H, d, CH2 pendant), 6.39 (2H, br. s, pyH), 6.87 (4H, m, pyH),
7.07 (2H, br s, pyH), 7.25 (6H, m, pyH), 7.53 (1H, t, pyH), 7.73
(1H, t, pyH), 7.80 (1H, t, pyH), 7.86 (1H, br. s, pyH), 8.36 (1H,
d, pyH), 8.41 (2H, d, pyH), 8.79 (1H, d, pyH). dC (D2O) 43.4,
43.9, 52.8, 62.7 (CH2), 121.2, 121.4, 121.6, 121.8, 122.7, 124.4,
124.9, 125.4, 125.5, 127.6, 128.1, 137.8, 137.9, 139.2, 139.4, 139.6
Experimental
Syntheses
The ligand precursors and metal salts were commercial samples
and were used in syntheses as received.
2,6-Bis(methoxycarbonyl)pyridine
To a mixture of 2,6-pyridinedicarboxylic acid (10 g, 46 mmol) in
methanol (200 cm3) was added 2,2-dimethoxypropane (80 cm3)
and conc. HCl (6.5 cm3). The mixture was refluxed for 4 h
whilst protected from the atmosphere by a CaCl2 guard tube,
and then the heating was turned off and the mixture stirred
overnight. A white solid that had formed was collected, washed
with a small volume of methanol then diethyl ether, and air
dried (8.0 g); it proved analytically pure as isolated. The solvent
was removed from the filtrate by rotary evaporation, and the
resulting brownish solid was collected, washed with diethyl ether,
and the off-white solid collected (4.0 g) (overall yield 97%).
Anal. C, 55.2; H, 4.6; N, 7.0. C9H9NO4 requires C, 55.4; H,
4.7; N, 7.2%. NMR: dH (CDCl3) 4.01 (6H, s, CH3O), 8.01 (1H, t,
pyH), 8.30 (2H, d, pyH). dC (CDCl3) 53.2 (CH3O), 128.0, 138.3
(pyCH), 145.2, 146.4, 147.7, 147.9, 154.7, 155.9 (pyCtert), 165.9,
−1
=
168.7, 204.0, 219.3 (C O) ppm. IR (cm ): 3546 (H2O), 3289
=
(NH), 3053, 2925 (CH), 1683, 1656 (C O), 1594, 1569, 1437
(NH), 1474, 1418, 1365 (amide), 1310, 1240 (CH), 1177, 1161,
996, 773, 742, 650 (py). UV-Vis spectrum (water): ∼350sh nm (e
∼ 920 dm3 mol−1 cm−1), 476 (158).
=
(pyCH), 148.0 (pyCtert), 165.0 (C O) ppm. EI MS: m/z 196
(M+), 165 (M+ − OMe), 137 (M+ − OCOMe), 105 (M+
−
Bis(2,6-bis[N-(2ꢀ-pyridinylmethyl)carbamido]pyridine)-
OCOMe − OMe), 77 (M+ − 2(OCOMe)). IR (cm−1): 1740
dicopper(II), [Cu2L1 ]
2
=
=
(C O), 1572, 1453, 1434 (in-plane C C), 1290, 1243 (C–O),
1165, 1145, 996 (py ring breathing), 949, 852, 813, 756 (out-of-
plane 2,6-disubstituted py).
To a solution of H2L1 (0.2 g, 0.58 mmol) in ethanol (8 cm3)
in a 50 cm3 round-bottomed flask was added with swirling a
solution of copper(II) acetate monohydrate (0.115 g, 0.58 mmol)
in ethanol (6 cm3) and water (2 cm3). The resulting blue
solution was brought briefly to boiling, then allowed to cool and
evaporated to dryness with a rotary evaporator. The blue–green
solid was redissolved in a small volume of ethanol (ca. 5 cm3)
and crystals grown in a diethyl ether atmosphere. Large dark
green crystals formed overnight, and were collected, washed
with a small amount of diethyl ether and air dried (0.15 g, 56%).
Further standing for several days yielded more green solid and
a small amount of a blue crystalline material. Anal. C, 55.2; H,
4.2; N, 14.9. C38H30N10Cu2O4·2EtOH·0.5H2O requires C, 54.9;
H, 4.7; N, 15.2%. FAB MS: m/z 881 (Cu3L1 ), 819 (Cu2L1 ),
2,6-Bis[N-(2ꢀ-pyridinylmethyl)carbamyl]pyridine, H2L1
A solution of (2-aminomethyl)pyridine (13.9 g, 65 mmol) in
toluene (40 cm3) was added rapidly to a solution of 2,6-
bis(methoxycarbonyl)pyridine (6.3 g, 32 mmol) in toluene
(120 cm3) in a 250 cm3 round-bottomed flask. The suspension
was then refluxed (oil bath) and stirred for 24 h, readily
dissolving during this process to a clear solution. The heat was
turned off and slow cooling allowed as the mixture was stirred
overnight. During this time, a white solid crystallized; this was
collected, washed with diethyl ether (ca. 30 cm3) and dried in a
vacuum dessicator (6.9 g, 61%); it proved analytically pure. A
second crop was obtained from the initial filtrate by removing
the solvent by rotary evaporation, redissolving the residue in
chloroform (100 cm3) and washing with water (3 × 50 cm3). The
chloroform solution was dried over anhydrous MgSO4, filtered,
and the solvent removed by rotary evaporation. The residue was
2
2
471 (Cu2L1), 410 (CuL1), 348 (L1). IR (cm−1): 3495 (OH2), 1590,
=
1560 (C O), 1481, 1424, 1383 (amide), 1278 (CH), 1148, 1088,
960, 760 (py) cm−1. UV-Vis spectrum (EtOH): 206 nm (e =
60000 dm3 mol−1 cm−1), 264 (28,600), 644 (160), 735sh (150).
The minor blue product appears to be the 1 : 1 complex; FAB
MS: m/z 410 (CuL1), 348 (L1).
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
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Diacetatodimethanolbis(2,6-bis[N-(2ꢀ-pyridinylmethyl)-
carbamido]pyridine)trinickel(II), [Ni3(L1)2(OOCCH3)2-
(HOCH3)2]
ether (2 × 20 cm3), redissolved in ethanol (10 cm3) and water
(1 cm3) and a solution of pyridine-2,6-dicarboxylic acid (24 mg,
0.145 mmol) in ethanol (10 cm3) added dropwise. The solution
was boiled briefly, allowed to stand overnight, then evaporated
to dryness, yielding a dark blue solid. The solid was washed
with acetone and dissolved in a minimum volume of water. Dark
blue crystals were grown in an acetone atmosphere (yield 40 mg,
85%). Anal. C, 45.5; H, 3.3; N, 12.1. C52H48Cu4N12O18 requires
To a solution of H2L1 (0.3 g, 0.86 mmol) in methanol (15 cm3)
in a 50 cm3 round-bottomed flask was added with swirling a
solution of nickel(II) acetate tetrahydrate (0.215 g, 0.86 mmol)
in methanol (15 cm3). The immediately formed orange solution
was rotary evaporated to dryness, washed with diethyl ether (2 ×
20 cm3), then was redissolved in a small volume of methanol
(ca. 8 cm3) and crystals grown in a diethyl ether/ethanol
atmosphere. Copious amounts of dark brown crystals formed
after two days, and were collected, washed with a small amount
of diethyl ether and air dried (0.22 g, 63%). Further standing for
several days yielded more dark brown solid and a small amount
of a red crystalline material. Anal. C, 48.1; H, 4.5; N, 12.1.
C44H44N10Ni3O10·3H2O requires C, 47.9; H, 4.5; N, 12.6%. FAB
MS: m/z 925 (Ni4L1 ), 867 (Ni3L1 ), 520 (Ni3L1), 404 (NiL1).
C, 45.2; H, 3.50; N, 12.2%. EI MS: m/z 1278 (Cu4L1 dipic2),
2
1216 (Cu3L1 dipic2), 1111 (Cu4L1 dipic), 1050 (Cu3L1 dipic), 943
2
2
2
(Cu4L1 ), 930 (Cu4L1dipic2), 881 (Cu3L1 ), 869 (Cu3L1dipic2),
2
2
819 (Cu2L1 ), 806 (Cu2L1dipic2), 764 (Cu4L1dipic), 757 (CuL1 ),
2
2
702 (Cu3L1dipic), 639 (Cu2L1dipic), 576 (CuL1dipic), 472
(Cu2L1), 410 (CuL1), 348 (L1 ). IR (cm−1): 3435 (H2O), 1629
2
−
=
(COO ), 1608, 1566 (amide C O), 1492, 1431, 1371, 1354
(amide), 1180, 1010, 865, 765 (py). UV-Vis spectrum (EtOH):
206 nm (e = 110000 dm3 mol−1 cm−1), 262 (39800), 646 (255),
714sh (270).
2
2
−1
−
=
IR (cm ): 3370 (OH2), 1606, 1573 (C O), 1560 (COO ), 1487,
1446, 1415, 1341 (amide), 1273 (CH), 1183, 1030, 850, 760 (py).
UV-Vis spectrum (EtOH): 204 nm (e = 60700 dm3 mol−1 cm−1),
264 (23500), 343 (7960), 908 (36), 1015 (120).
Spectroscopy
NMR spectra were recorded on solutions of complexes in CDCl3
or D2O using a Bruker DPX300 spectrometer, UV-visible spectra
were measured with a Hitachi 220A spectrophotometer, and FT-
IR spectra were recorded on Nicolet Avatar 370 spectrometer
fitted with a germanium golden gate accessory. FAB mass
spectra were recorded on a Kratos MS80 spectrometer.
(2,6-Bis[N-(2ꢀ-pyridinylmethyl)carbamido]pyridine)nickel(II),
[NiL1]
The second crop of crystals in the above synthesis included red
crystals of this monomeric material as a minor species (< 5%),
which were separated and air dried. Anal. C, 55.6; H, 3.6; N,
16.9. C19H15N5NiO2·0.5H2O requires C, 55.7; H, 3.9; N, 17.1%.
NMR: dH (CDCl3) 4.55 (4H, s, CH2), 7.14 (2H, ddd, pyH),
7.60 (2H, d, pyH), 7.61 (2H, d, pyH), 7.66 2H, ddd, pyH), 7.94
(1H, t, pyH), 8.29 (2H, d, pyH). FAB MS: m/z 462 (Ni2L1),
404 (NiL1), 348 (L1). IR (cm−1): 1643, 1614, 1601, 1564 (amide
X-Ray crystallographic data collection and processing
Data were collected with a Siemens SMART three-circle system
with CCD area detector.33 The crystal was held at 180(2) K
with an Oxford Cryosystem Cryostream Cooler. Absorption
correction was carried out by a semi-empirical method from
equivalents.34 No crystal decay was observed in any structure.
=
C O), 1483, 1473, 1420, 1387 (amide), 1180, 1015, 855, 760 (py).
UV-Vis spectrum (EtOH): 264 nm (e = 8390 dm3 mol−1 cm−1),
351 (3860), 908 (40), 1014 (90).
Bis(2,6-bis[N-(2ꢀ-pyridinylmethyl)carbamido]pyridine)bis-
(pentane-2,4-dionate)trinickel(II), [Ni3(L1)2(acac)2]
Structure analysis and refinement. For [Co(L1)(HL1)]·xH2O,
To a solution of H2L1 (0.1 g, 0.29 mmol) in methanol (10 cm3)
in a 50 cm3 round-bottomed flask was added with swirling a
solution of nickel(II) acetate tetrahydrate (0.108 g, 0.43 mmol)
in methanol (10 cm3). The resultant orange solution was rotary
evaporated to dryness, washed with diethyl ether (2 × 20 cm3),
and redissolved in a small volume of methanol (ca. 10 cm3).
Pentane-2,4-dione (29 mg, 0.29 mmol) was added, resulting in a
green–orange solution. After 10 min, the solvent was removed
by rotary evaporation and the solid redissolved in a minimum
volume of methanol and crystals grown in a diethyl ether–
ethanol atmosphere. Brown crystals formed after two days, and
were collected and air dried (0.10 g, 70%). A small amount of red
crystals of the monomer were also collected. Anal. C, 53.2; H,
4.5; N, 12.2. C48H44N10Ni3O8·2MeOH requires C, 53.2; H, 4.6;
N, 12.4%. FAB MS: m/z 925 (Ni4L1 ), 867 (Ni3L1 ), 520 (Ni3L1),
¯
there were no systematic absences; space group P1 was
chosen on the basis of intensity statistics. For [NiL1],
the merging R-values indicated clearly that it is mono-
clinic; systematic absences indicated space group P21/c. For
[Cu2(L1)2]·2EtOH·0.5H2O, there were no systematic absences,
1
¯
and space group P1 was chosen. For [Ni3(L )2(OAc)2(MeOH)2]
and [Ni3(L1)2(acac)2]·1.75MeOH·2H2O, systematic absences in-
dicated space groups Pbcn and P21cn, respectively; the crystals of
the latter are relatively weakly diffracting and include a region
of disordered solvent modeled as 4 methanol molecules (one
with 0.5 occupancy) and 11 water molecules (with 0.5 and 0.25
occupancy). For [Cu4(L1)2(dipic)2(OH2)2]·4H2O, space group
C2/c was chosen on the basis of intensity statistics.
The structures were solved by direct methods using
SHELXS35 with additional light atoms found by Fourier meth-
ods. Hydrogen atoms were added at calculated positions and
refined with isotropic displacement parameters using a riding
model; solvent H-atoms were not usually located, but the H-
atoms of the bound water molecules were observed on difference
syntheses and were included with their positional parameters
refined. Anisotropic displacement parameters were used for all
non-H atoms. Refinement used SHELXTL.36
2
2
1
−1
=
404 (NiL ). IR (cm ): 2965, 2937 (acac Me), 1594, 1571 (C O),
=
1513 (acac C O), 1486, 1458, 1400, 1340 (amide), 1286, 1183
(CH), 1155, 1110, 1076, 765 (py). UV-Vis spectrum (EtOH):
204 nm (e = 44200 dm3 mol−1 cm−1), 264 (22400), 304 (17900),
930 (45).
Diaquabis(2,6-bis[N-(2ꢀ-pyridinylmethyl)carbamido]pyridine)-
bis(pyridine-2,6-dicarboxylato)tetracopper(II) tetrahydrate,
[Cu4(L1)2(dipic)2(H2O)]·4H2O
Crystal data. Crystal data are collected in Table 1. Selected
bond lengths and angles are listed in Tables 2–4. The atomic
numbering scheme is included in Figs. 1–6.
CCDC reference numbers 246446 (2), 246447 (6), 246448 (1),
246449 (5), 246450 (3) and 246451 (4).
To a solution of H2L1 (0.05 g, 0.145 mmol) in ethanol (10 cm3)
and water (1 cm3) in a 50 cm3 round-bottomed flask was added
dropwise with stirring a solution of copper(II) acetate monohy-
drate (0.057 g, 0.29 mmol) in ethanol (10 cm3). The resultant
blue solution was stirred for 1 h then the solvent was removed
with a rotary evaporator. The blue solid was washed with diethyl
See http://www.rsc.org/suppdata/dt/b4/b414251a/ for cry-
stallographic data in CIF or other electronic format.
5 2 0
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7

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D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
5 2 1

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◦
˚
Table 2 Distances (A) and angles ( ) subtended around the metal ion in mononuclear complexes 1 and 3
[Co(L1)(HL1)] (1)
Co1
N111
N18
N28
N21
N117
N211
distance
1.862(4)
1.946(4)
1.878(4)
1.945(4)
1.962(4)
1.983(4)
N211
N117
N21
N28
176.59(18)
99.50(18)
N18
81.23(18)
100.43(17)
93.52(16)
82.87(17)
165.24(17)
88.61(16)
81.79(17)
162.99(17)
97.51(17)
91.44(16)
94.19(17)
90.74(16)
82.49(18)
[NiL1] (3)
Ni1
N12
N8
N1
N19
distance
1.834(2)
1.835(2)
1.930(2)
1.933(2)
N19
N1
166.52(10)
84.51(10)
N8
82.22(10)
82.59(10)
164.80(10)
110.69(10)
◦
˚
Table 3 Distances (A) and angles ( ) subtended around the metal in trinuclear nickel complexes 4 and 5
[Ni3(L1)2(OAc)2(MeOH)2] (4)
Ni1
distance
2.034(3)
2.048(3)
2.056(2)
2.147(3)
2.191(3)
2.197(3)
distance
1.9779(3)
1.977(3)
2.172(3)
2.172(3)
2.166(3)
2.166(3)
N8
80.40(11)
99.65(11)
171.20(10)
94.45(10)
90.92(10)
N19#
O021
O011
91.67(11)
88.35(11)
N22#
178.99(11)
N1
100.83(11)
80.18(11)
86.81(10)
167.56(11)
91.13(11)
87.86(11)
89.48(10)
N22#
O011
O021
N19#
N8
Ni2
N19
N19#
N8#
N8
78.65(11)
98.35(11)
N12#
175.76(17)
N12
N12#
N8
78.38(11)
104.62(11)
157.03(11)
92.27(10)
92.93(15)
104.62(11)
78.38(11)
92.27(10)
157.03(11)
98.35(11)
78.65(11)
91.62(15)
N8#
N19#
N19
[Ni3(L1)2(acac)2] (5)
Ni11
O136
O133
N11
N21
N18
distance
1.989(6)
2.018(6)
2.053(8)
2.078(7)
2.187(7)
2.224(7)
distance
1.964(7)
1.976(6)
2.157(7)
2.171(7)
2.189(6)
2.194(6)
distance
1.999(6)
2.007(6)
2.048(7)
2.063(7)
2.188(6)
2.211(7)
N28
N18
N21
N11
91.3(3)
89.6(3)
O133
93.2(3)
88.1(3)
172.3(3)
98.0(3)
79.6(3)
91.2(2)
171.4(3)
88.6(3)
80.3(3)
100.4(3)
87.9(3)
92.9(3)
177.5(3)
N28
Ni12
N112
N212
N18
N119
N28
N219
Ni13
O236
O233
N122
N222
N119
N219
N219
N28
N119
N18
78.9(3)
100.4(3)
N212
179.0(3)
100.2(3)
78.9(3)
91.5(2)
93.5(2)
157.3(2)
102.4(3)
78.4(3)
93.0(2)
90.9(2)
78.3(3)
102.3(3)
157.2(3)
N219
N119
N222
N122
92.0(3)
88.7(3)
O233
93.2(3)
88.5(3)
171.7(3)
99.4(3)
79.3(3)
92.6(2)
172.2(3)
86.8(2)
80.3(3)
100.9(2)
86.9(3)
92.6(3)
178.3(3)
attracted our attention and initiated a more extensive study of
the helicating properties of H2L1 in self-assembly reactions with
metal ions.
Results and discussion
The 2,6-disubstituted pyridine moiety is a classical component
of a range of ligand topologies. Previously, the potentially
pentadentate ligand H2L1 has been described and the crystal
structure of the free ligand was reported recently.6 Deprotona-
tion of the two amide groups was anticipated to yield a potent
dianionic ligand, and its mononuclear [Fe(L1)2]− and dinuclear
complex [Cu2(L1)2] were subsequently reported.11,12 Structural
characterization of the latter complex identified a capacity for
the ligand to form helical structures with the ligand spanning
across the two metal ions, and it was this potential which
Helicity known for the relatively simple [Cu2(L1)2] complex
has been elaborated here with the synthesis and observation of
helicity in trinuclear [Ni3(L1)2(OAc)2(OH2)2], [Ni3(L1)2(acac)2]
and tetranuclear [Cu4(L1)2(dipic)2(OH2)2] complexes. Also, ex-
amples of mononuclear complexation is extended with struc-
tural characterisation of [Co(L1)(HL1)] and [NiL1]. It is the
capacity of H2L1 to form polynuclear helical structures with at
least Cu(II), Ni(II) and, from preliminary evidence, Zn(II) that is
remarkable. All of the polynuclear complexes isolated feature
5 2 2
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7

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◦
˚
Table 4 Distances (A) and angles ( ) subtended around the metal in dinuclear (2) and tetranuclear (6) copper complexes
[Cu2(L1)2] (2)
Cu1
distance
1.907(2)
1.944(2)
2.068(2)
2.243(2)
2.265(2)
distance
1.907(3)
1.924(2)
2.088(3)
2.166(3)
2.421(3)
N412
N322
N41
80.78(9)
97.10(9)
N319
170.03(10)
N48
77.94(9)
103.85(8)
158.73(9)
81.35(8)
111.54(10)
78.42(9)
106.83(9)
N319
N41
N322
N412
Cu3
N38
N419
N31
N422
N312
N312
N422
N31
80.81(11)
98.06(10)
N419
171.03(12)
75.64(10)
103.92(10)
155.08(9)
79.87(9)
108.45(11)
80.11(10)
115.83(10)
[Cu4(L1)2(dipic)2(OH2)2] (6)
Cu1
distance
1.932(4)
1.980(4)
2.028(4)
2.044(4)
distance
1.908(4)
1.960(4)
2.035(4)
2.059(4)
2.322(4)
N119
N108
79.64(18)
104.53(18)
N122#
174.59(18)
N112
N122#
N108
N119
Cu2
79.70(18)
95.96(18)
159.23(17)
O100
O211
O201
80.91(17)
98.82(17)
N101
171.69(18)
N205
N101
O201
O211
O100
90.64(16)
97.67(17)
90.53(15)
91.51(15)
79.85(17)
99.95(16)
160.67(15)
Fig. 2 An ORTEP drawing of the helical [Cu2(L1)2] dinuclear complex
(2), showing atom numbering.
would appear to be a key to the formation of robust helical
complexes, although we note its presence in the lattice of the
mononuclear cobalt(III) complex also.
The major product of the reaction of the cobalt(II) acetate
and H2L1 in methanol in the presence of air is a neutral
dark brown diamagnetic complex. This crystallized in low yield
from a saturated aqueous solution of the reaction mixture,
although column chromatography on cationic SP-Sephadex
and Dowex resins confirmed it as the major species, readily
isolable upon rotary evaporation, with only traces of other ionic
complexes detected. Formation of a neutral complex requires
one of the four amides to remain protonated, and hence as
a poor presumably uncoordinated group, which would lead
to a dissymmetric complex, consistent with its complex NMR
spectrum. This behaviour differs from the complex formed with
Fig. 1 Views of the [Co(L1)(HL1)] complex (1): (a) an ORTEP
drawing of one of the two independent complex cations showing
atom numbering; (b) a view of part of the lattice showing p-stacking
interactions between complexes involving pendant pyridine groups.
a racemic mixture of left- and right-handed double helices;
notably, their structures (discussed below) are all stabilized by
˚
inter-ligand p-stacking (with inter-ring distances of 3.2–3.8 A)
of ligands which each span several metal ions. This p-stacking
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
5 2 3

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Fig. 6 An ORTEP drawing of the helical [Cu4(L1)2(dipic)2(OH2)2]
tetranuclear complex (6), showing atom numbering.
flanking deprotonated amido groups and the other by the central
pyridine, one deprotonated amido and one terminal pyridine
group, with the remaining protonated amide unbound along
with the other pyridine groups (Fig. 1(a)). The different modes
of coordination of the two ligands requires that the two central
pyridine groups lie in cis dispositions around the distorted cobalt
octahedron. For one ligand, the central pyridine is flanked by
two coordinated amido groups in a meridional arrangement,
with the tridentate component effectively planar. However, the
planes of the two pendant terminal pyridines are disposed almost
at right angles to the chelate plane (89.43 and 81.24◦), with one
involved in p-bonding with the coordinated terminal pyridine
of the other ligand, and the other involved in p-bonding to a
pyridine of an adjacent complex. The other tridentate unit is also
reasonably planar, with the O–C–N amide unit perpendicular to
the plane of the ligated component, but the terminal pyridine
attached to the free amide unit disposed roughly co-planar to the
coordinated fragment of the ligand and involved in p-bonding
interactions with other adjacent complexes (Fig. 1(b)). The Co–
N(py) distance to the central pyridine of the meridionally-bound
Fig. 3 An ORTEP drawing of the [NiL1] mononuclear complex (3),
showing atom numbering.
˚
–N–CO–py–CO–N– fragment of 1.862 and 1.855(4) A for Co1
and Co2, respectively, is substantially shorter than distances
Fig. 4 An ORTEP drawing of the helical [Ni3(L1)2(OAc)2(MeOH)2],
trinuclear complex (4), showing atom numbering.
˚
to other pyridine donors (1.983, 2.008(4) A for the bound
˚
terminal pyridine of the second ligand and 1.945, 1.931(4) A
for the central pyridine of that ligand for which the other
terminal pyridine is uncoordinated). This gives rise to the
tight ‘bite’ of this fragment, reflected in intraligand angles
such as N111–Co1–N18 of 81.23(18)◦ and N111–Co1–N117 of
81.79(17)◦. This mirrors the behaviour exhibited by the pyridine-
2,6-dicarboxylate ion when bound to metal ions.37 Of the three
bound amido groups, one (N28/N48) forms the central donor of
one tridentate meridionally-bound ligand. The Co–N distance
˚
˚
(1.878(4) A for Co1, 1.874(4) A for Co2) is consequently slightly
shorter than distances to the other two amido groups N18/N38
˚
˚
(1.946(4), 1.963(4) A) and N117/N317 (1.962(4), 1.976(4) A)
which act as equivalent terminal groups for the other tridentate
ligand. The angles around the Co involving N28/N48 likewise
are reduced compared with other angles. The complex is highly
hydrated, and the structure features twenty water molecules in
whole or partially occupied sites. The level of hydration, and the
apparent role of the water molecules in stabilizing the structure,
is somewhat reminiscent of biomolecules.
Fig. 5 An ORTEP drawing of the helical [Ni3(L1)2(acac)2] trinuclear
complex (5), showing atom numbering.
The other known mononuclear octahedral structure formed
by H2L1 is with Fe(III), which forms a low-spin mer-[Fe(L1)2]−
anion in which each ligand is bonded symmetrically by the
iron(III), where a monoanionic complex with all four amides
deprotonated and coordinated is the dominant species.12 The
characterization of the Co(III) complex was established by an
X-ray crystal structure; there are two independent complexes
in the unit cell, but these do not differ in any substantial
way. The structure confirmed the complex as the neutral 1 : 2
complex [Co(L1)(HL1)] with two ligands acting as tridentate
ligands, one coordinated by the central pyridine and its two
˚
central pyridine (Fe–N 1.881(3) A) and both amido nitrogens
˚
(Fe–N av. 1.955(3) A), the terminal pyridine groups being
pendants orientated in a propeller-like manner.12 The substan-
tially shorter distance to the central pyridine compared to the
flanking amido groups also is found for the one ligand in the
Co(III) complex of similar coordination mode. Very recently,
the symmetrical anionic mer-[Co(L1)2]− anion has been reported
5 2 4
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7

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and defined by an X-ray structure via a somewhat different route
from oxidation of a mixture CoCl2 and H2L1 in strongly basic
solution.38 This suggests that the unsymmetrical species isolated
here may be a kinetic rather than a thermodynamic product.
Subsequently, we have heated the unsymmetrical complex with
activated charcoal in water (2 h, 60 ◦C) to attempt to promote
the interconversion to the symmetrical anionic form. In neutral
aqueous solution, NMR spectroscopy indicates no detectable
rearrangement occurs even after several hours. However, in basic
solution (pH > 10) there is evidence of rearrangement, with
simplification of the NMR spectrum to that consistent with the
symmetrical monoanionic form observed, although this occurs
slowly and is not substantially complete until ≥5 h.
of range of efficient coordination. The central pyridine bond
˚
to the Ni atom is significantly shorter, at 1.834(2) A, than that
˚
of the coordinated terminal pyridine, at 1.930(2) A, reflecting
the demands of the rigid –N–CO–py–CO–N– unit. Further,
the amido group located between the two coordinated pyridine
˚
groups has a bond distance N8–Ni of 1.835(2) A, which is
significantly shorter than that to the other amido group N19–Ni
˚
of 1.933(2) A. The former distance reflects the constraints of
the two neighbouring five-membered rings of which it forms a
central part, as does the bent N1–Ni–N12 bond of 166.5(1)◦,
which shows almost as much distortion as the N8–Ni–N19
bond of 164.8(1)◦ in the central planar –N–CO–py–CO–N–
unit. Differences extend beyond the immediate coordination
sphere; for example, the distance from amido N8 to the adjacent
Reaction of copper(II) acetate with H2L1 in a 1 : 1 molar ratio
gave a green crystalline material as the major product, along
with a minor dark blue microcrystalline material. From FAB
˚
methylene C7 is 1.448(3) A compared to the slightly longer
˚
N19–C20 distance of 1.469(4) A where the terminal pyridine
mass spectrometry, the green solid was identified as a [Cu2L1 ]
in unbound. Despite the bond distance and angle distortions in
the square planar complex, distortions of the NiN4 donor set
from planarity is minimal. In the mean square plane defined by
the atoms Ni1, N1, N8, N12 and N19, deviations of each atom
from this plane are only 0.0144(9), −0.0204(10), 0.0184(13),
2
dimeric species, whereas the minor product was characterised
as a [CuL1] monomeric complex. Changes in colour when
dissolved in water (blue) and ethanol (green) suggest that
solution speciation may differ somewhat from the solid state
in some solvents. We have also observed that [Cu2(L1)2] converts
in solution in the presence of added pyridine-2,6-dicarboxylic
acid to the tetranuclear [Cu4(L1)2(dipic)2(OH2)2], which suggests
that dissociation and rearrangement reactions in solution are
reasonably facile, but that formation of helical complexes may
be thermodynamically preferred. Currently, we are investigating
speciation and determining formation constants in solution
separately.
˚
−0.0252(13) and 0.0127(10) A, respectively.
The major product of the reaction of H2L1 with nickel(II)
acetate was the trinickel complex [Ni3(L1)2(OAc)2(MeOH)2],
where a 6 : 6 : 6 coordination motif operates with each
nickel atom in a distorted octahedral environment (Fig. 4).
The most surprising feature revealed by the crystal structure
of this complex is that the amido groups are all involved in
bridging between nickel centers, via each deprotonated amido
group using both lone pairs of electrons on the nitrogen
donor. In simple nickel(II) polymers such as nickel oxide,
bridging oxides lead to antiferromagnetic properties at room
temperature;39 the effect the bridging amido groups on magnetic
properties in the present system is the target of current separate
investigations. The complex is soluble in both aqueous and
non-aqueous solvents. In CDCl3, the complex is paramagnetic
at room temperature, as shown by the 1H NMR spectrum,
which displayed a chemical shift range from −1 to +142 ppm.
The spectrum proved too complicated for facile analysis due
to overlapping resonances that were also sufficiently broad to
prohibit coupling experiments.
The crystal structure of the Cu2L12 dimer has been previously
reported,11 and the crystals isolated in this case proved to be
structurally identical to that reported. The dinuclear complex
contains two [L1]2− ligands coordinated to two copper(II) ions
in a 5 : 5 coordination motif (Fig. 2). Somewhat surprisingly,
the central pyridine distance to the copper atom is significantly
longer than that of the terminal pyridines even though the
former can be considered to occupy an equatorial site in a
significantly distorted square-based pyramidal geometry around
the copper.
Reaction of H2L1 with nickel(II) in solution yielded both a
minor red 1 : 1 Ni : L1 species and a dark brown major 3 : 2
Ni : L1 species. The latter complex was paramagnetic, as clear
from the observation of a complex wide-scan 1H NMR spectrum
spanning ca. 140 ppm, and octahedral coordination was implied.
The minor red complex proved to be diamagnetic, consistent
with square-planar coordination of the nickel(II) ion. The amide
Each of the two ligands in the trinickel complex binds to
all three metal ions. The central pyridine group of each ligand
occupies trans axial locations around the central nickel. Each
amido group binds in the plane of the metal ions in a bridging
mode to both the central nickel and different terminal nickel
atoms. The terminal pyridine then occupies an axial location
to the same metal ion as the adjacent amido group binds. This
leads to each ligand being wrapped as the separate arms of a
double helix around the central metal framework; symmetry in
the crystal is such that distances and angles in the two ligands
are close replicates. There is a right- and left-handed form of
the helix. The final two equatorial coordination sites on each
terminal nickel ion are occupied by an acetate and a methanol
group, completing their octahedra. Thus the central nickel lies
in a NiN6 environment of four amido and two axial trans
pyridine groups, whereas each terminal nickel lies in an NiN4O2
environment of two cis amido groups, two axial trans pyridine
groups, and an acetate and alcohol group in cis dispositions.
The central nickel is bound by two symmetry-related –N–CO–
1
hydrogen triplet present in the H NMR spectrum of the free
ligand at 9.10 ppm is absent in the complex, consistent with
deprotonation of both amides and their complexation to the
nickel ion; this is supported by collapse of the doublet of the
adjacent methylene found at 4.74 ppm in the free ligand to a
singlet. Four-coordination of nickel(II), with both amides and
the central pyridine bound, permits only one of the terminal
pyridine groups to bind, with the other being an unbound
1
pendant. The H NMR spectrum would be anticipated to be
unsymmetrical as a result; that this is not observed suggests rapid
chemical exchange between coordinated and uncoordinated
terminal pyridines is occurring.
The crystal structure of the complex confirmed the square-
planar nature of the coordination, and the presence of a pendant
terminal pyridine group (Fig. 3). The geometry of the ligand is
not optimal for forming square planar complexes. The small bite
angles within the planar –N–CO–py–CO–N– unit are reflected
in compressed N–Ni–N angles of 82.2(1) and 82.6(1)◦. The
terminal –N–CH2–py chelate ring is also somewhat compressed
at 84.5(1)◦, whereas the single angle not restrained by chelation is
significantly expanded, at 110.7(1)◦. That one terminal pyridine
remains uncoordinated is unusual, as nickel(II) complexes are
often five- or six-coordinate. The cause appears to relate to the
sp2 nature of the amide group holding the pyridine donor out
˚
py–CO–N– units, with the Ni2–N12(py) distance of 1.977(3) A
significantly shorter than the amido bonds Ni2–N8 of 2.172(3)
˚
˚
A and Ni2–N19 of 2.166(3) A. The central pyridine distance is
˚
even shorter than in the structure of the monomer (1.834(2) A).
However, the bridging amido groups are significantly elongated
compared with either type of amido group (1.835(2), 1.933(2)
˚
A) in the monomer structure. This presumably reflects both
the effect of bridging to two metal centers and the structural
demands of three adjacent octahedral nickel atoms. Whereas
the trans pyridine groups appear to exhibit little distortion from
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
5 2 5

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linearity (N12–Ni2–N12# 175.76(17)◦), the distorted octahe-
dron is revealed by the angles to groups in the plane (for example,
N12–Ni2–N8 78.65(11)◦ and N8–Ni2–N12# 98.35(11)◦). The
terminal nickel octahedra are crystallographically equivalent.
Different M–L distances to trans pyridines (2.034(3), 2.048(3)
˚
˚
A), cis amido groups (2.197(3), 2.191(3) A) and acetate (2.056(2)
˚
˚
A) and methanol (2.147(3) A) oxygens mean a distorted
octahedral environment, reflected also in some internal angles
(such as N1–Ni1–N19# 100.83(11)◦). Some weak p-stacking
˚
interactions (distances from 3.1–3.3 A) involving the two sets of
three pyridine rings on opposite sides of the plane containing
the nickel atoms may assist in defining the helical structure.
The terminal acetate and methanol ligands suggested them-
selves as a target for substitution reactions. In particular,
replacement of these with simple chelate ligands was anticipated
as a reasonable expectation. Attempts to perform substitution
reactions were pursued with chelating ligands which bind as
monoanionic species and thus would replicate the overall charge
in the displaced groups, namely pentane-2,4-dionate (acac),
dithiocarbamate ion and b-alaninate. The latter two led to
isolation of simple nickel complexes of dithiocarbamate and
alaninate; however, with acac, a trimer featuring substitution of
the monodentate ligands by the chelate, [Ni3(L1)2(acac)2], was
isolated. The structure was inferred from the FAB MS, and
confirmed by a crystal structure analysis (Fig. 5). Obviously,
there are close similarities with the structure of the precursor
trimer as the polydentate ligand is bound in an identical manner
in each, although the coordination of the symmetrical acac
chelate leads to some changes. As in the precursor, both left-
and right-handed helices exist in the crystal. Counter-intuitively,
the structure exhibits different environments for all three nickel
atoms.
The tetranuclear [Cu4(L1)2(dipic)2(OH2)2] was first detected
serendipitously, from the reaction of Cu(II) with a batch of
H2L1 contaminated with unreacted 2,6-bis(methoxycarbonyl)-
pyridine that undergoes metal catalysed hydrolysis of the diester.
Subsequently, direct reaction of equimolar amounts of H2L1
and pyridine-2,6-dicarboxylic acid with copper(II) acetate was
attempted, but did not yield the polymer, presumably because
the overall reaction in this case would produce eight moles of
acetic acid per cluster compared with four moles in the successful
reaction involving the diester. The greater acidity in the former
case lowering pH sufficiently to inhibit complexation. This was
solved by employing a two-stage reaction involving solvent
and concomitant evolved acid removal in the initial step, as
described in the Experimental section, leading to isolation of
the tetranuclear complex in high yield. Despite being a neutral
polymer, the double helix complex is surprisingly soluble in
water, perhaps related to the presence of terminal coordinated
water molecules in the complex. The structure of the helical
complex is shown in Fig. 6. The complex is inherently chiral, and
both right- and left-handed helices coexist in the crystal. At this
stage, no attempts to separate the forms has been undertaken.
Notably, a tetracopper(II) complex of H2L3 was also reported
recently to arise through serendipitous chemistry, although in
that case one of the three ligands present in the [Cu4(L3)2(L3–O)]
complex had the central pyridine oxidized to an N-oxide; details
of the structure have not appeared.40
The central nickel atom displays average Ni–N(py) (1.970(6)
˚
˚
A) and Ni–N(amido) (2.178(7) A) distances comparable to
average distances for the precursor structure (1.977(3) and
˚
2.169(3) A, respectively). For the two terminal Ni atoms, average
˚
˚
Ni–N(py) (2.060(7) A), Ni–N(amido) (2.202(7) A) and Ni–
˚
O (2.002 A) distances compare less closely with those in the
precursor (2.041(3), 2.194(3), and 2.102 A, respectively). Of
˚
course, this is a result of changing the type of O-donor ligands,
but there are secondary influences from introduction of a chelate
ligand which are reflected in distances and angles. The O–Ni–O
angle, for example, is opened out to 93.2(3)◦ in the acac chelate
but near that of a perfect octahedral angle at 89.48(10)◦ in the
acetato-methanol precursor.
It is notable that the 3 : 2 M : L coordination motif observed
with nickel(II) in the above two examples may be formed with
some other metal ions. With zinc(II), FAB MS spectrometry has
identified a species apparently of the same type. Unfortunately,
we have not as yet been able to crystallize this complex, and so
this observation still awaits confirmation. What is notable is that,
by providing additional types of ligands, the assembly of even
larger helical clusters is possible, as shown through isolation of
The copper(II) environments in [Cu4(L1)2(dipic)2(OH2)2] dis-
play a 5 : 4 : 4 : 5 coordination motif, with the outer
copper ions having a square pyramidal CuN2O3 geometry
including an axial terminal water molecule in each, whereas
the inner two have square planar CuN4 geometry. The two [L1]2−
ligands each span three copper ions. The central pyridine and
two adjacent amido nitrogens bind to an inner copper, with
each terminal pyridine subsequently coordinated to different
copper ions, one to a terminal copper and the other to the
second inner copper ion. Whereas the central amido–pyridine–
amido unit attached to the one copper is essentially planar, the
terminal pyridines are twisted via the tetrahedral methylene
groups to occupy a coordination site for another copper in
a different plane; Cu · · · Cu separation is defined in part by
the length of this pendant arm. Each terminal copper then
completes its coordination sphere with a tridentate pyridine-
2,6-dicarboxylate and a water molecule. The terminal copper
lies in a distorted five-coordinate environment, due partly to
Jahn–Teller distortion, which leads to the terminal water group
a Cu4L1 species also including other chelating ligands.
2
Recently, trinuclear M3L2 species were also reported as
copper(II) complexes of potentially pentadentate ligands
2,6-bis(pyridine-2-carboxamido)pyridine (H2L3) and 2,6-bis-
(pyrazine-2-carboxamido)pyridine (H2L4),40 of formula [Cu3-
(L)2(l2-OAc)2]. Here, the ligands have the amide functionality
‘reversed’ compared with the present system, as a result of
synthesis from 2,6-diaminopyridine rather than the diacid
analogue. The structures reveal the central pyridine of each
ligand binds to the central copper atom, with amide and terminal
pyridine sets bonding to terminal copper atoms, and acetate
groups bridging between a central and terminal copper atom.
Each copper lies in a distorted octahedral environment, with the
structure displaying a form of helicity via the ligands twisting
about the amide groups, although the methylene groups in
H2L1 allow greater ligand backbone flexibility in systems we
report.
5 2 6
D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7

View Article Online
˚
being relatively long at 2.322(4) A, and partly to the small
3 H. Bouchard and A. Hittinger, PCT Int. Appl., WO2002096903,
2002.
4 P. Chevallier, J.-C. Soutif, J.-C. Brosse and A. Brunelle, Rapid
Commun. Mass Spectrom., 2002, 16, 1476.
‘bite’ of the planar dipic dianion. Whereas Cu–O(carboxylate)
distances are relatively normal at 2.035(4) and 2.059(4) A,
the Cu–N distance of the central pyridine is significantly
contracted at 1.908(4) A compared with that of the pendant
˚
5 (a) F. Vo¨gtle, E. Weber, W. Wehner, R. Naetscher and J. Gruetze,
Chem. Ztg., 1974, 98, 562; (b) L. Fabbrizzi, T. A. Kaden, A. Perotti,
B. Seghi and L. Siegfried, Inorg. Chem., 1986, 25, 321.
6 J. Y. Qi, J. Chen, Q. Y. Yang, Z. Y. Zhou and A. S. C. Chan, Acta
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7 P. B. Glover, Masters thesis, University of Warwick, 2000.
8 F. A. Chavez, M. M. Olmstead and P. K. Mascharak, Inorg. Chem.,
1996, 35, 1410.
9 A. J. Preston, G. Fraenkel, A. Chow, J. C. Gallucci and J. R. Parquette,
J. Org. Chem., 2003, 68, 22.
10 G. Desimoni, G. Faita, M. Guala and C. Pratelli, Tetrahedron:
Asymmetry, 2002, 13, 1651.
11 D. S. Marlin, M. M. Olmstead and P. K. Mascharak, Inorg. Chim.
Acta, 2001, 323, 1.
12 D. S. Marlin, M. M. Olmstead and P. K. Mascharak, Inorg. Chem.,
1999, 38, 3258.
13 C. Stadler, J. Daub, J. Koehler, R. W. Saalfrank, V. Coropceanu, V.
Schuenemann, C. Ober, A. X. Trauwein, S. F. Parker, M. Poyraz,
T. Inomata and R. D. Cannon, J. Chem. Soc., Dalton Trans., 2001,
3373.
˚
˚
pyridine at 1.960(4) A. The O–Cu–N angles in the dipic ligand
are also significantly contracted at 80.91(17) and 70.85(17)◦
compared with O–Cu–N angles involving the terminal water
group [90.64(16) and 97.67(17)◦]. Of these angles, notably the
angle to the pendant is opened out, and the N–Cu–N angle
(171.69(18)◦) bent down towards the central copper, presumably
reflecting a compensation for this bridging pyridine pendant
between achieving square-based pyramidal geometry around the
external copper itself and adequate Cu · · · Cu separation. The
central square planar copper also shows considerable tetrahedral
distortion from planarity, with opposed N–Cu–N angles of
174.59(18) and 159.23(17)◦; the latter involves the terminal
pyridine from the second [L1]2− ligand, reflecting compensations
similar to those discussed for the terminal copper ions. The
˚
Cu–N distances range form 1.932(4) to 2.044(4) A, the shortest
distance associated with the central pyridine, resulting from
similar considerations to those applying with dipic. The two
sets of four pyridine rings on each side of the chain of metal ions
are p-stacked, with inter-ring distances in the range 3.2–3.8 A.
The terminal Cu1–central Cu2 internuclear distance is 3.552(9)
14 F. A. Chavez, M. M. Olmstead and P. K. Mascharak, Inorg. Chim.
Acta, 1998, 269, 269.
15 F. A. Chavez, M. M. Olmstead and P. K. Mascharak, Inorg. Chem.,
1997, 36, 6323.
˚
16 F. A. Chavez, M. M. Olmstead and P. K. Mascharak, J. Am. Chem.
Soc., 1998, 120, 9015.
17 F. A. Chavez, M. M. Olmstead and P. K. Mascharak, Inorg. Chim.
Acta, 2000, 297, 106.
18 E. K. van der Beuken, A. Meetsma, H. Kooijman, A. L. Spek and
B. L. Feringa, Inorg. Chim. Acta, 1997, 264, 171.
19 L. Zhao, C. J. Matthews, Z. Xu, L. K. Thompson and D. O. Miller,
Can. Mol. Cryst. Liq. Cryst. Sci. Technol. A: Mol. Cryst. Liq. Cryst.,
2002, 376, 389.
20 Z. Xu, L. K. Thompson and D. O. Miller, Polyhedron, 2002, 21, 1715.
21 D. A. Atwood, B. S. Howerton and M. Matlock, PCT Int. Appl.,
WO2002049967, 2002.
˚
A, whereas the internal central Cu1 · · · central Cu1 internuclear
˚
distance is 3.072(13) A.
The coordinated water groups in [Cu4(L1)2(dipic)2(OH2)2]
offer the potential for ligand substitution chemistry in the same
way that the unidentate ligands in [Ni3(L1)2(OAc)2(MeOH)2]
were clearly demonstrated above as sites for substitution. In
the present case, however, chelation cannot be introduced, as
the two water molecules lie in separate sites at opposite ends
of the molecule. Spectrophotometric titration of a solution
of the copper(II) tetramer with thiocyanate in aqueous solu-
tion displayed an absorbance change with an isosbestic point
maintained initially, consistent with sequential substitution of
the two water groups by thiocyanate. Unfortunately, we were
unable to crystallize the substituted product for structural char-
acterisation. However, the concept has been demonstrated for
the copper(II) polymer, complementing more detailed study of
the nickel(II) polymer. That selective substitution of unidentate
ligands can be achieved attests to the robustness of the helical
structures reported here.
22 M. M. Matlock, K. R. Henke and D. A. Atwood, J. Hazardous
Mater., 2002, 92, 129.
23 P. Chavallier, J.-C. Soutif, J.-C. Brosse and M. Grote, React. Funct.
Polym., 1999, 42, 129.
24 P. Matthias, M. Takahashi and K. Goto, Jpn. Kokai Tokkyo Koho,
JP11228692, 1999.
25 W. Bode, L. Moroder, P. J. B. Pereira, A. Bergner, R. Huber, C.
Sommerhoff, N. Schaschke, T. Bar, T. Martin, J. Stadlwieser, W.-R.
Ulrich, A. Dominik, U. Thibaut, D. Bundschuh, R. Beume and K.-J.
Goebel, PCT Int. Appl., WO9940083 and WO9940073, 1999.
26 M. Chavarot, S. Socquet, M. Kotera and J. Lhomme, Tetrahedron,
1997, 53, 13749.
Further examples demonstrating the facility of helicate
formation with polyamide–polypyridine ligands continue to
appear. Oligoamides derived from 2,6-diaminopyridine and 2,6-
pyridinedicarboxylic acid (H2n+2L5) have been reported recently
to form double helices with Cu(II) that are dinuclear when n =
27 M. C. Mitchell, A. Cawley and T. P. Lee, Tetrahedron Lett, 1995, 36,
287.
28 T. Maruyama, J. Koichi and K. Konishi, Kagaku Zasshi, 1962, 65,
1071.
29 T. Hirao, T. Moriuchi, T. Ishikawa, K. Nishimura, S. Mikami, Y.
Ohshiro and I. Isao, J. Mol. Catal. A: Chem., 1996, 113, 117.
30 Z. Brzozka, Analyst, 1988, 113, 891.
˚
1, and hexanuclear when n = 3. Cu · · · Cu distances are 3.74 A
˚
in the dinuclear species, and in the range 3.58–3.61 A for the
hexanuclear complex.41 The corresponding Cu · · · Cu distances
31 F. A. Chavez, C. V. Nguyen, M. M. Olmstead and P. K. Mascharak,
Inorg. Chem., 1996, 35, 6282.
1
˚
in [Cu4(L )2(dipic)2(OH2)2] are 3.05 and 3.55 A for central–
32 K. Hiratani and K. Taguchi, Bull. Chem. Soc. Jpn., 1990, 63,
central and central–terminal coppers, respectively.
3331.
33 SMART User’s Manual 1994, Siemens Industrial Automation Inc,
Madison, WI, USA.
34 J. Cosier and A. M. Glazer, J. Appl. Crystallogr., 1986, 19, 105.
35 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
36 G. M. Sheldrick, SHELXTL Ver. 5.1, 1997, Bruker Analytical X-ray
Systems, Madison, WI, USA.
37 N. W. Alcock, G. J. Clarkson, G. A. Lawrance and P. Moore,
Aust. J. Chem., 2004, 57, 565.
38 (a) A. N. Dwyer, C. A. Golden, J. R. Gomm, M. C. Grossel, P. N.
Horton, D. A. S. Merckel and R. A. Parker, XXIX International
Symposium on Macrocyclic Chemistry, Cairns, Australia, July 4–8,
2004, abstract MP17; (b) A. N. Dwyer, M. C. Grossel and P. N.
Horton, Supramol. Chem., 2004, 16, 405.
Acknowledgements
Support of the NMR, MS and X-ray facilities at Warwick,
and financial support for P. B. G. from the EPSRC (UK) is
gratefully acknowledged. G. A. L. thanks the Department of
Chemistry at the University of Warwick for access to facilities
during sabbatical leave.
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D a l t o n T r a n s . , 2 0 0 5 , 5 1 8 – 5 2 7
5 2 7