Homo- and heteropolynuclear helicates with a '2 + 3 + 2'-dentate compartmental ligand

Article abstract of DOI:10.1039/b502423d

An octadentate ligand L has been prepared which contains a sequence of bidentate (pyrazolyl-pyridine), terdentate [bis(pyrazolyl)pyridine] and bidentate (pyrazolyl-pyridine) binding sites separated by p-xylyl spacers. This forms a range of double helical

Full text of DOI:10.1039/b502423d

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P A P E R
Homo- and heteropolynuclear helicates with a ‘2 þ 3 þ 2’-dentate
compartmental ligand
Stephen P. Argent,a Harry Adams,a Lindsay P. Harding,b T. Riis-Johannessen,c
John C. Jefferyc and Michael D. Ward*a
a Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.
E-mail: m.d.ward@sheffield.ac.uk; Fax: 0114 2229346; Tel: 0114 2229484
b Department of Chemical and Biological Sciences, University of Huddersfield, Huddersfield,
UK HD1 3DH
c School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
Received (in Durham, UK) 16th February 2005, Accepted 12th April 2005
First published as an Advance Article on the web 18th May 2005
An octadentate ligand L has been prepared which contains a sequence of bidentate (pyrazolyl-pyridine),
terdentate [bis(pyrazolyl)pyridine] and bidentate (pyrazolyl-pyridine) binding sites separated by p-xylyl
spacers. This forms a range of double helical complexes in which the two ligands define 4-, 6-, and
4-coordinate binding sites, and there is substantial p-stacking between overlapping parallel areas of the
ligands. In [Cu3L2][PF6]4 the sequence of oxidation states for the copper ions is þ1, þ2, þ1 with the Cu(I)
ions being four-coordinate at the terminal sites and Cu(II) being in the central six-coordinate site. In
[Cu3(OAc)2L2][PF6]4 all copper centres are in oxidation state þ2, with the terminal ions having an additional
monodentate acetate ligand giving them a five-coordinate geometry. The 4 þ 6 þ 4 arrangement of
coordination numbers means that reaction of L with a mixture of Fe(II) and Ag(I) results in high yield
formation of [Ag2FeL2][BF4]4 in which Ag(I) ions occupy the terminal 4-coordinate sites and Fe(II) occupies
the central pseudo-octahedral site. Reaction of L with Ag(I) produced a mixture of [Ag3L2][BF4]3 (major
product) and [Ag4L2][BF4]4 (minor product). In [Ag3L2][BF4]3 the central Ag(I) ion is, unusually, in a
pseudo-octahedral coordination environment from the two meridional, terdentate bis(pyrazolyl)pyridine
donors. In [Ag4L2][BF4]4 in contrast the central 6-coordinate cavity is occupied by two Ag(I) ions separated
by 2.85 A. The terdentate chelating bis(pyrazolyl)pyridine units at the centre of the helicate are now
substantially twisted such that each donates a bidentate pyrazolyl-pyridine to one Ag(I) centre and a
1
monodentate pyrazole unit to the other. In solution, H NMR and mass spectroscopic evidence indicates
that the fourth Ag(I) ion is lost and [Ag3L2][BF4]3 forms, unless a large excess of Ag(I) is present in which
case traces of [Ag4L2][BF4]4 can be detected by mass spectrometry.
expect to result in 4-, 6- and 4-coordinate sites in a double
Introduction
helical array. This has been exploited to prepare and struc-
turally characterise a range of helicates in which the presence of
different binding sites leads to the formation of heteronuclear
complexes, as well as some unexpected homonuclear com-
plexes. We note that Piguet and co-workers described hetero-
nuclear helicates with a related 2 þ 3 þ 2 compartmental ligand
L2 some years ago, although this paper concentrated on
solution rather than structural properties, providing a detailed
NMR analysis of the complexes and a discussion of the
thermodynamic and structural factors required for the heli-
cates to form.5
Complexes with double or triple helical structures (‘helicates’)
have been of interest in the field of coordination chemistry for
many years, and have elegantly demonstrated how the forma-
tion of architecturally complex systems is directed by the
interplay between simple parameters such as the stereoelec-
tronic preference of the metal ions and the disposition of the
binding sites in the ligand.1,2 In addition to the interest in their
structures and the mechanisms by which they assemble, they
can also provide a convenient way for assembling a collection
of different metal ions in heteronuclear arrays, as illustrated by
Piguet’s helicates containing both d-block and lanthanide
metal ions which are of interest for their inter-metallic photo-
induced energy-transfer.3 To assemble a selection of different
ions in such a helical array requires either ligands which
contain two distinct binding sites (e.g. bidentate and terden-
tate, which give 6- and 9-coordinate sites in triple helical
arrays),3 or the use of ligands wh0i0c0 h partition into inequivalent
domains, as in 2,20:60,200:600,20 0 0 :6 ,20 0 0 0 -quinquepyridine which
partitions into terdentate and bidentate domains, resulting in
6-coordinate and 4-coordinate sites in its double helicates
[Cu(II)/Cu(I) or Co(II)/Ag(I)].4
Results and discussion
Ligand synthesis
In this paper we describe the synthesis and coordination
chemistry of the ligand L (below) which contains a sequence of
bidentate, terdentate and bidentate binding sites, which we
The new ligand L was prepared according to the route in
Scheme 1. Reaction of 3-(2-pyridyl)pyrazole with excess
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 a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
904
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 4 – 9 1 1

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Scheme 1
1,4-bis(bromomethyl)benzene under phase-transfer conditions
gave intermediate A in 62% yield; it is necessary to use a
considerable excess of 1,2-bis(bromomethyl)benzene to prevent
substitution at both bromomethyl sites. Reaction of intermedi-
ate A, having one reactive bromomethyl site, with half an
equivalent of 2,6-bis(3-pyrazolyl)pyridine6 under the same
phase-transfer conditions, afforded the desired ligand L in
1
72% yield following chromatographic purification. H NMR
spectroscopy, electrospray mass spectrometry and elemental
analysis confirmed the identity of the ligand. The para phenyl-
ene linkage separating the binding sites was used to ensure that
each binding site had, for steric reasons, to bind a separate
metal ion. We accordingly prepared a series of complexes
which are sketched in Fig. 1; all were characterised on the
basis of electrospray mass spectra and elemental analyses, as
well as by X-ray crystallography.
Fig. 2 The complex cation of [Cu3L2][PF6]4 ꢀ 4MeCN with the ligands
shaded differently for clarity.
double helical arrangement of ligands about the three metal
ions. The central six-coordinate Cu(II) ion [Cu(2)] lies on a
twofold axis and has short, medium and long N–Cu–N axes
with Cu–N distances on these axes of 1.95, 2.16 and 2.35 A,
with the shortest axis involving the central pyridyl donors,
which is usual in terdentate chelates containing five-membered
chelating rings.7 The fact that one of the two axes involving the
pyrazolyl donors is substantially longer than the other is
consistent with the presence of Jahn–Teller distortion.8 The
four-coordinate Cu(I) ions are in irregular geometries that are a
long way removed from the D2d symmetry of ‘ideal’ bis-
chelates, with Cu(1)–N separations in the range 1.99–2.16 A
and an angle between the two CuNN planes of 711 (see Table
1). The Cu(1)ꢀ ꢀ ꢀCu(2) separations are 8.46 A.
Syntheses and structures of copper complexes
Reaction of [Cu(MeCN)4]PF6 with L in air afforded an orange
precipitate of [Cu3L2][PF6]4 [denoted Cu(121)] in which the
oxidation state distribution of þ1, þ2, þ1 for the Cu ions is
indicated by the presence of four counter ions and is consistent
with the expected stereoelectronic preferences of Cu(I) and
Cu(II) for four- and six-coordinate sites, respectively. The
crystal structure (Fig. 2) confirms this and also confirms the
A particularly striking feature of this structure is the series of
p-stacking interactions along the long axis of the helicate
between overlapping and essentially parallel aromatic units
(Fig. 3). From the left of the figure working to the right, these
domains comprise (i) the terminal pyridyl ring of one ligand
(ligand A, pale grey); (ii) a phenylene spacer group from the
alternate ligand (ligand B, dark grey); (iii) the central pyridyl
ring from ligand A; (iv) the second phenylene spacer from
ligand B; and (v) the terminal pyridyl ring of ligand A. The
average inter-planar separations along the stack are 3.4–3.5 A.
It will be apparent from this that the two ligands are in quite
different environments in the solid state, with the phenylene
groups of ligand B being interleaved between terminal and
central pyridyl rings of ligand A. In contrast, the phenylene
units of ligand A and the pyridyl rings of ligand B are not
involved in any significant intramolecular stacking interac-
tions. Aromatic stacking interactions between ligands in helical
structures is common, but is by no means essential for their
formation, as shown by examples of helicate structures in
which there is no significant inter-strand stacking.9
Reaction of L with copper(II) acetate in MeOH (2 : 3 ratio),
followed by precipitation of the complex as its hexafluoropho-
sphate salt, afforded [Cu3L2(OAc)2][PF6]4 [denoted Cu(222)] in
which all three sites are occupied by Cu(II) ions. The crystal
structure (Fig. 4) confirms the expected double helical arrange-
Fig. 1 Sketches of the structures of the complexes described in this
paper, and the nomenclature used.
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Table 1 Selected bond lengths (A) and angles (1) for [Cu3L2][PF6]4 ꢀ 4MeCN
Cu(1)–N(211)
Cu(1)–N(111)
Cu(1)–N(121)
Cu(1)–N(221)
1.987(4)
2.008(4)
2.140(4)
2.158(4)
Cu(2)–N(251)
Cu(2)–N(151)
Cu(2)–N(241)
Cu(2)–N(141)
1.951(5)
1.952(5)
2.157(4)
2.349(4)
N(211)–Cu(1)–N(111)
N(211)–Cu(1)–N(121)
N(111)–Cu(1)–N(121)
N(211)–Cu(1)–N(221)
N(111)–Cu(1)–N(221)
N(121)–Cu(1)–N(221)
N(241)–Cu(2)–N(141)
N(141A)–Cu(2)–N(141)
149.55(19)
121.43(17)
80.28(16)
79.58(17)
107.62(17)
122.98(15)
99.29(13)
154.46(19)
N(251)–Cu(2)–N(151)
N(251)–Cu(2)–N(241)
N(151)–Cu(2)–N(241)
N(241A)–Cu(2)–N(241)
N(241)–Cu(2)–N(141A)
N(251)–Cu(2)–N(141)
N(151)–Cu(2)–N(141)
180
78.26(11)
101.74(11)
156.5(2)
85.91(13)
102.77(10)
77.23(10)
Symmetry operation for generating equivalent atoms: ꢁx, y, ꢁz þ 0.5.
Fig. 3 Space-filling depiction of the complex cation of [Cu3L2][PF6]4 ꢀ
4MeCN, emphasising the aromatic p-stacking between the interpene-
trating ligands.
Fig. 4 The complex cation of [Cu3L2(OAc)2][PF6]4 ꢀ 2MeCN with the
ligands shaded differently for clarity.
a trigonal bipyramid, with the t parameter [on a scale of 0
(perfect square pyramid) to 1 (perfect trigonal bipyramid)]10
being 0.30; the axial ligand is N(121) which, in keeping with the
Jahn–Teller effect, has a longer bond to Cu(2) [2.282(5) A] than
the other four donors (average 2.01 A). The Cu(1)ꢀ ꢀ ꢀCu(2)
separations are 8.78 A. The conformation of the two ligands is
generally similar to what was observed in Cu(121), with the
phenylene spacers of one ligand (black) interleaved between the
pyridyl rings of the other (hollow bonds) to give a five-
component p-stack.
ment of ligands about the three metal ions. The central metal
ion Cu(1) is in an irregular 6-coordinate geometry and lies on a
C2 axis, with the same pattern of bond lengths as seen above—
viz. the shortest axis involves the Cu–N(pyridyl) bonds (ca.
1.98 A, see Table 2). In addition, one of the other two axes is
considerably elongated, with the bonds to N(241) and N(241A)
being 2.394(5) A, in contrast to the bonds to the alternate pair
of pyrazolyl donors N(141) and N(141A), which are 2.123(5)
A—clear evidence for a Jahn–Teller elongation along one
axis.8 The bite angles between the terminal pyrazolyl donors
of each terdentate unit are both o1601, reflecting the usual
steric limitations of such ligands which cannot stretch to give a
bite angle of 1801 between the termini.7 The Cu(II) centres at
the termini of the helicate, which are crystallographically
equivalent, are ligated by two bidentate pyrazolyl-pyridine
units, as well as a monodentate acetate ligand which allows
them to attain a five-coordinate geometry. The coordination
environment of these is closer to a square-based pyramid than
Synthesis and structure of a mixed-metal Ag2Fe complex
The successful isolation of
a Cu(I)–Cu(II)–Cu(I) system
[Cu(121)] suggested that a mixed-metal complex, containing
Ag(I) for the tetrahedral sites and Fe(II) for the octahedral sites,
should be possible. Reaction of L with a mixture of [Ag
(MeCN)4]BF4 and Fe(BF4)2 ꢀ 6H2O (2 : 2 : 1 ratio) in MeCN
Table 2 Selected bond lengths (A) and angles (1) for [Cu3L2(OAc)2][PF6]4 ꢀ 2MeCN
Cu(1)–N(151)
Cu(1)–N(251)
Cu(1)–N(141)
Cu(1)–N(241)
1.978(7)
1.987(7)
2.123(5)
2.394(5)
Cu(2)–O(11)
Cu(2)–N(211)
Cu(2)–N(111)
Cu(2)–N(221)
Cu(2)–N(121)
1.958(4)
2.002(5)
2.011(5)
2.068(5)
2.282(5)
N(151)–Cu(1)–N(251)
N(151)–Cu(1)–N(141)
N(251)–Cu(1)–N(141)
N(141A)–Cu(1)–N(141)
N(151)–Cu(1)–N(241)
N(251)–Cu(1)–N(241)
N(141A)–Cu(1)–N(241)
N(141)–Cu(1)–N(241)
N(241)–Cu(1)–N(241A)
180
O(11)–Cu(2)–N(211)
O(11)–Cu(2)–N(111)
N(211)–Cu(2)–N(111)
O(11)–Cu(2)–N(221)
N(211)–Cu(2)–N(221)
N(111)–Cu(2)–N(221)
O(11)–Cu(2)–N(121)
N(211)–Cu(2)–N(121)
N(111)–Cu(2)–N(121)
N(221)–Cu(2)–N(121)
92.6(2)
88.2(2)
78.12(14)
101.88(14)
156.2(3)
178.2(2)
160.05(19)
80.0(2)
103.88(14)
76.12(13)
84.28(18)
101.45(18)
152.2(3)
98.8(2)
93.57(18)
103.72(19)
77.83(19)
106.15(19)
Symmetry operation for generating equivalent atoms: ꢁx þ 1, y, ꢁz þ 1.5.
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Fig. 7 The complex cation of [Ag3L2][BF4]3 ꢀ CH3NO2 with the
ligands shaded differently for clarity.
Reaction of L with [Ag(MeCN)4][BF4] (2 : 3 molar ratio) in
MeCN afforded a clear solution from which a white solid
precipitated on addition of ether; mass spectrometry and
elemental analysis indicated this to be [Ag3L2][BF4]3 (denoted
Ag3). The crystal structure (Fig. 7) shows it to be a double
helicate similar to those described above, but with the central
Ag(I) ion in a six-coordinate N-donor environment. This is
rather unusual for Ag(I), which commonly forms complexes
with N-donor ligands having a coordination number of 4,
especially with bidentate chelating ligands such as diimines;
indeed the tendency of Ag(I) to bind two diimines in an
approximately mutually perpendicular arrangement has been
used as a design principle for a wide range of helicate, grid-like
and cylindrical complexes.11 The few examples of hexadentate
N-donor coordination around Ag(I)12,13 are often imposed by
a highly constrained ligand such as a macrocycle or a cryp-
tand.12 Surprisingly, no AgIN6 complexes arise from coordina-
tion of two meridional tris-chelates, and in fact Ag(I)
complexes with 2,20:60,200-terpyridine are usually mononuclear
four-coordinate complexes (with an additional solvent or
counter-anion completing the coordination)14 or have helical
structures in which the ligand bridges two metal ions and the
coordination number of the Ag(I) ions is low.15,16 In Ag3, the
average Ag–N bond length about the six-coordinate ion is
rather long, at 2.50 A (see Table 3), compared to average Ag–N
lengths of 2.34 A and 2.36 A about the four-coordinate ions
Ag(1) and Ag(3), respectively, which is to be expected on
electroneutrality grounds. The six Ag–N bonds all lie within
the relatively narrow range of 2.40–2.59 A such that none of
them can be discounted as being anomalously long; Ag(2) is
genuinely six-coordinate, presumably because of the limited
flexibility and high stability of the helical structure. We note
that a double helical complex of Ag(I) with a ligand containing
two terpyridyl binding domains still gave only five-coordina-
tion around the Ag(I) ions, as one of the terminal pyridyl units
was not coordinated and directed away from each Ag(I) ion.17
In Ag3 this is not possible because of the position of the
hexadentate cavity at the centre of the helical array; there is
less scope for one of the terdentate units to coordinate in-
completely without disrupting the rest of the structure. Con-
versely, binding of the two terminal Ag(I) ions in their ‘ideal’
four-coordinate geometry necessarily generates a hexadentate
cavity at the centre of the helicate.
Fig. 5 Room-temperature 1H NMR spectrum (300 MHz, CD3CN) of
Ag2Fe.
afforded in high yield a yellow solid whose ES mass spectrum
and elemental analysis were consistent with the formulation
1
[Ag2FeL2][BF4]4 (denoted Ag2Fe). The proton H NMR spec-
trum (Fig. 5) was highly paramagnetically shifted, with peaks
appearing in the range ca. ꢁ10 to þ65 ppm, indicating a high-
spin configuration for the Fe(II) centre. However the number of
signals is that expected for a complex in which both ligands are
equivalent and have two-fold symmetry, i.e. 16 peaks (assum-
ing that the CH2 protons are independent in the chiral envir-
onment, but the four protons on each phenyl ring split into two
sets of two equivalent protons).
The crystal structure (Fig. 6) shows the complex to be
essentially isostructural with Cu(121) apart from minor per-
turbations associated with the higher ionic radius of Ag(I),
which results in Ag–N distances in the range 2.26–2.44 A (see
Table 3). The angle between the two AgNN planes is 701. At
the Fe(II) centre the bonds to the central pyridine rings of the
terdentate chelates are again the shortest of the Fe–N bonds
(average 2.11 A), with the other two N–Fe–N axes being
longer, but the substantial elongation of one of these axes with
respect to the other that occurred with Cu(II) is not present.
The Agꢀ ꢀ ꢀFe separation is 8.50 A, almost identical to the
separation between adjacent Cu centres in Cu(121), and the
ligand conformations and p-stacking arrangements are essen-
tially identical.
Syntheses and structures of Ag(I) complexes
We next wanted to see what would happen if the ligand were
provided only with a metal ion that is not normally found in an
octahedral environment; specifically to see if an alternate self-
assembly arrangement could occur which would avoid a 2 : 2
helicate which necessitates a central octahedral binding site.
Accordingly we prepared complexes of L with Ag(I) alone, with
some interesting results.
The metal–metal separations are 8.25 A for Ag(1)–Ag(2) and
8.06 A for Ag(2)–Ag(3), which are slightly smaller than seen
for the other complexes, resulting in a more compact arrange-
ment of the helical superstructure. This compression is appar-
ent in the coordination geometry around Ag(2), where the two
terdentate chelating units are not mutually perpendicular, with
an angle of 751 between the two AgN3 mean planes. It is
noteworthy that whilst in the solid state the two ligands are
clearly inequivalent, this is not reflected in the solution 1H
NMR spectrum of the complex (see Experimental section)
which is consistent with four-fold symmetry, i.e. both ligands
Fig. 6 The complex cation of [Ag2FeL2][BF4]4 with the ligands shaded
differently for clarity.
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Table 3 Selected bond lengths (A) and angles (1) for [Ag2FeL2][BF4]4 and [Ag3L2][BF4]3 ꢀ CH3NO2
[Ag2FeL2][BF4]4
Ag(1)–N(11)
Ag(1)–N(91)
Ag(1)–N(101)
Ag(1)–N(21)
2.259(4)
2.304(4)
2.323(4)
2.440(4)
Fe(1)–N(61)
Fe(1)–N(51)
Fe(1)–N(41)
Fe(1)–N(71)
2.107(3)
2.118(6)
2.214(3)
2.269(4)
N(11)–Ag(1)–N(91)
N(11)–Ag(1)–N(101)
N(91)–Ag(1)–N(101)
N(11)–Ag(1)–N(21)
N(91)–Ag(1)–N(21)
N(101)–Ag(1)–N(21)
157.76(16)
128.82(16)
72.28(16)
71.98(15)
105.05(14)
120.44(13)
N(61)–Fe(1)–N(51)
N(61)–Fe(1)–N(41)
N(51)–Fe(1)–N(41)
N(41)–Fe(1)–N(41A)
N(61)–Fe(1)–N(71)
N(51)–Fe(1)–N(71)
N(41)–Fe(1)–N(71)
N(41)–Fe(1)–N(71A)
N(71)–Fe(1)–N(71A)
180.000(1)
105.51(11)
74.49(11)
149.0(2)
75.08(10)
104.92(10)
86.27(13)
101.70(13)
150.2(2)
Symmetry operation for generating equivalent atoms: ꢁx, y, ꢁz þ 0.5.
[Ag3L2][BF4]3 ꢀ CH3NO2
Ag(1)–N(211)
Ag(1)–N(111)
Ag(1)–N(121)
Ag(1)–N(221)
Ag(2)–N(251)
Ag(2)–N(141)
Ag(2)–N(151)
2.271(3)
2.289(3)
2.392(4)
2.412(3)
2.398(3)
2.416(3)
2.424(3)
Ag(2)–N(241)
Ag(2)–N(261)
Ag(2)–N(161)
Ag(3)–N(291)
Ag(3)–N(191)
Ag(3)–N(181)
Ag(3)–N(281)
2.560(3)
2.592(3)
2.594(3)
2.246(4)
2.293(4)
2.403(4)
2.493(4)
N(211)–Ag(1)–N(111)
N(211)–Ag(1)–N(121)
N(111)–Ag(1)–N(121)
N(211)–Ag(1)–N(221)
N(111)–Ag(1)–N(221)
N(121)–Ag(1)–N(221)
N(251)–Ag(2)–N(141)
N(251)–Ag(2)–N(151)
N(141)–Ag(2)–N(151)
N(251)–Ag(2)–N(241)
N(141)–Ag(2)–N(241)
N(151)–Ag(2)–N(241)
N(251)–Ag(2)–N(261)
N(141)–Ag(2)–N(261)
166.45(13)
120.51(12)
72.42(12)
71.63(12)
106.77(12)
118.71(11)
124.63(11)
165.79(11)
68.11(11)
67.79(11)
115.79(10)
101.88(11)
67.07(11)
85.24(11)
N(151)–Ag(2)–N(261)
N(241)–Ag(2)–N(261)
N(251)–Ag(2)–N(161)
N(141)–Ag(2)–N(161)
N(151)–Ag(2)–N(161)
N(241)–Ag(2)–N(161)
N(261)–Ag(2)–N(161)
N(291)–Ag(3)–N(191)
N(291)–Ag(3)–N(181)
N(191)–Ag(3)–N(181)
N(291)–Ag(3)–N(281)
N(191)–Ag(3)–N(281)
N(181)–Ag(3)–N(281)
123.75(11)
134.36(10)
101.62(11)
133.64(11)
65.58(11)
76.36(10)
119.21(10)
161.77(13)
125.94(13)
72.22(13)
71.63(13)
99.83(13)
125.67(12)
equivalent and each ligand having two-fold symmetry, indicat-
ing dynamic behaviour which interchanges the two ligand
environments quickly on the NMR timescale.
When growing crystals of Ag3 we occasionally noticed the
appearance of a small number of crystals with a distinctly
different habit and appearance from those of the bulk of the
product. These could be separated manually, and the elemental
analysis was clearly inconsistent Ag3 but suggested the for-
mulation [Ag4L2][BF4]4. An X-ray crystallographic investiga-
meters is appropriate. However, we obtained the same picture
from several independent crystals that were examined and are
happy that the gross structure as shown in Fig. 8 is correct. In
this complex the central six-coordinate cavity of the helical
structure is occupied by two Ag(I) ions, separated by 2.85 A,
each coordinated by three N atoms. The terdentate chelating
bis(pyrazolyl)pyridine units at the centre of the helicate are
now substantially twisted such that each donates a bidentate
pyrazolyl-pyridine to one Ag(I) centre and a monodentate
pyrazole unit to the other; this bridging mode of the bis(pyra-
zolyl)pyridine unit is similar to that shown by terpyridine in
some polynuclear complexes with Ag(I),15 and has been seen in
other dinuclear helical complexes of both Ag(I)16 and Cu(I)18 in
which a pair of metal ions is coordinated by two terdentate
ligands. The Ag–N(pyridyl) distances (E2.5 A) are clearly
much longer than the Ag–N(pyrazolyl) distances (E2.2 A),
so the coordination geometry around Ag(1) and Ag(2) can be
considered as approximately linear two-coordinate, with an
additional weaker interaction with a pyridyl donor. The ‘term-
inal’ Ag(I) ions Ag(3) and Ag(4) are in their usual irregular
four-coordinate geometry from two bidentate chelating units.
Attempts to characterize this species in solution were largely
unsuccessful. The 1H NMR spectrum and ES mass spectrum of
redissolved crystals of Ag4 were both identical to those ob-
tained for Ag3. The implication is that the cation [Ag4L2]41
exists only in the solid state, and loses one Ag1 ion to give
[Ag3L2]31 when dissolved in good donor solvents. To push
this equilibrium towards Ag4 we added a large excess of
tion confirmed formation of
a
tetranuclear helicate
[Ag4L2][BF4]4 (Ag4, Fig. 8). The small size and poor crystal-
linity of these crystals, and extensive disorder of anions and
solvent molecules, meant that the structure is of poor quality
(R1 ¼ 16%) and no detailed discussion of structural para-
Fig. 8 The complex cation of [Ag4L2][BF4]4 ꢀ 3MeCN with the ligands
shaded differently for clarity.
908
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 4 – 9 1 1

View Article Online
[Ag(MeCN)4][BF4] to a solution of redissolved crystals of Ag4
in MeCN, and under these conditions the ES mass spectrum
showed, in addition to several peaks arising from Ag3, a weak
peak at m/z 2101 which can be assigned to the tetranuclear
species [Ag4L2(BF4)3]1. It clearly exists to only a very small
extent in solution and its presence in the mix of crystalline
products suggests that it is largely a kinetic artefact of crystal-
lisation.
(OAc)2(PF6)3}1), 1004.5 ({Cu3L2(OAc)2(PF6)2}21). Found: C,
44.8; H, 3.4; N, 12.8%: [Cu3L2(OAc)2(PF6)4] ꢀ 4H2O requires
C, 45.2; H, 3.6; N, 12.9%. X-Ray quality crystals were grown
by diffusion of diethyl ether into a solution of the complex in
acetonitrile.
[Cu3L2][PF6]4
[Cu(MeCN)4]PF6 (39 mg, 0.11 mmol) was added to a stirred
suspension of L (50 mg, 0.07 mmol) in acetonitrile (3 cm3)
causing the ligand to dissolve. Addition of ether to the solution
precipitated the product as an orange crystalline solid which
was filtered off and dried in vacuo. Yield: 35 mg, 46%. ES MS:
m/z 1891.9 ({Cu3L2(PF6)2}1), 873.0 ({Cu3L2(PF6)}21), 533.8
({Cu3L2}31). Found: C, 47.0; H, 3.0; N, 13.8%: [Cu3L2(PF6)4]
requires C, 47.3; H, 3.2; N, 14.1%. X-Ray quality crystals were
grown by slow evaporation of a solution of the complex in
acetonitrile.
Experimental
General details
The following instruments were used for standard spectro-
scopic and analytical studies. For 1H NMR spectra, Bruker
AC 250 or Bruker AMX 400 spectrometers; FAB mass spectra,
a VG AutoSpec spectrometer; electrospray mass spectra, a
Waters LCT spectrometer (University of Sheffield) or a VG
Quattro II triple-quadrupole instrument (University of Hud-
dersfield); UV/Vis spectra, a Cary-50 spectrophotometer.
3-(2-Pyridyl)pyrazole19 and 2,6-bis(3-pyrazolyl)pyridine6
were prepared according to the published methods.
[Ag2FeL2][BF4]4
To a stirred suspension of L (350 mg, 0.50 mmol) in acetonitrile
(5 cm3) was added Fe(BF4)2 ꢀ 6H2O (84 mg, 0.25 mmol)
followed by [Ag(MeCN)4]BF4 (178 mg, 0.50 mmol); after
stirring, a yellow crystalline solid precipitated which was
filtered off and dried in vacuo. Yield: 474 mg, 93%. ES MS:
m/z 1942.9 ({Ag2FeL2(BF4)3}1). Found C, 49.0; H, 3.4; N,
14.6%: [Ag2FeL2(BF4)4] ꢀ 4H2O requires C, 49.1; H, 3.7; N,
14.7%. X-Ray quality crystals were grown by diffusion of
diethyl ether into a solution of the complex in nitromethane.
Preparation of ligand L
Intermediate A. (See Scheme 1.) A mixture of 1,4-bis(bro-
momethyl)benzene (5.20 g, 19.8 mmol), 3-(2-pyridyl)pyrazole
(1.43 g, 9.86 mmol), aqueous NaOH (10 M, 40 cm3), toluene
(130 cm3) and Bu4NOH (40% aqueous solution, 5 drops) was
stirred at room temperature for 1 h. The mixture was diluted
with H2O (100 cm3) before the organic layer was separated,
dried over MgSO4 and concentrated to a give a white solid.
Purification by column chromatography on alumina using
hexane–dichloromethane (1 : 4) as eluent gave the desired
product as a white crystalline solid. Yield: 2.01 g, 62%. EI
[Ag3L2][BF4]3
[Ag(MeCN)4]BF4 (76 mg, 0.21 mmol) was added to a stirred
suspension of L (100 mg, 0.14 mmol) in acetonitrile (3 cm3)
causing the ligand to dissolve. Addition of ether to the reaction
mixture precipitated the product as a white crystalline solid
which was filtered off and dried in vacuo. Yield: 122 mg, 87%.
ES MS: m/z 1908.0 ({Ag3L2(BF4)2}1), 578.3 ({Ag3L2}31). 1H
NMR (270 MHz, CD3CN) d 8.36 (4H, ddd, J ¼ 0.9, 1.7, 5.0;
pyridyl H6), 7.95–7.80 (10H, m, 6 ꢂ pyridyl H4, 4 ꢂ pyridyl
H3), 7.73 (4H, d, J ¼ 2.4, pyrazolyl), 7.66 (4H, d, J ¼ 7.36,
pyridyl H3), 7.51 (4H, d, J ¼ 2.1, pyrazolyl), 7.36 (4H, ddd, J ¼
1.7, 5.0, 7.2; pyridyl H5), 6.92 (4H, d, J ¼ 2.4; pyrazolyl), 6.84
(4H, d, J ¼ 2.4; pyrazolyl), 5.88 (4H, d, J ¼ 8.2; phenyl), 6.72
(4H, d, J ¼ 8.2; phenyl) 5.05–3.85 (16H, m; CH2). Found C,
50.7; H, 3.6; N, 15.2%: [Ag3L2(BF4)3] ꢀ 2H2O requires C, 50.8;
H, 3.7; N, 15.2%. X-Ray quality crystals were grown by
diffusion of diethyl ether into a solution of the complex in
nitromethane.
MS: m/z 327 (42%, M1), 248 (100%, {M ꢁ Br}1). H-NMR
1
(250 MHz, CDCl3): d 8.63 (1H, ddd, J ¼ 0.9, 1.8, 4.9; pyridyl
H6), 7.94 (1H, d, J ¼ 7.9; pyridyl H3), 7.71 (1H, pseudo-t;
pyridyl H4), 7.42 (1H, d, J ¼ 2.4; pyrazolyl H5), 7.41–7.34
(2H, m; phenyl), 7.26–7.17 (3H, m; 2 ꢂ phenyl, and pyridyl
H5), 6.92 (1H, d, J ¼ 2.4 Hz; pyrazolyl H4), 5.39 (2H, s; CH2-
pz), 4.47 (2H, s, CH2Br). Found: C, 57.5; H, 4.1; N, 12.4%:
C16H14N3Br ꢀ 0.5 H2O requires C, 56.9; H, 4.5; N, 12.5%.
Ligand L. A mixture of A (1.37 g, 4.16 mmol), 2,6-bis-
(3-pyrazolyl)pyridine (0.46 g, 2.18 mmol), aqueous NaOH
(10 M, 40 cm3), toluene (130 cm3) and Bu4NOH (40% aqueous
solution, 5 drops) was stirred at 60 1C for 2 h. After cooling the
mixture was diluted with H2O (100 cm3) before the organic
layer was separated, dried over MgSO4 and concentrated to a
give a white solid. Purification by column chromatography on
alumina using dichloromethane as eluent gave the desired
product as a white crystalline solid. Yield: 1.06 g, 72%. ES
A small number of crystals of [Ag4L2][BF4]4 could be
separated manually from the crystals of [Ag3L2][BF3]3 which
formed the bulk of the material. Found C, 46.4; H, 3.3; N,
13.5%; [Ag4L2(BF4)4] ꢀ 3H2O requires C, 46.0; H, 3.1; N,
13.7%.
MS: m/z 706 (100%, MH1). H-NMR (250 MHz, CDCl3): d
1
8.66 (2H, ddd, J ¼ 0.7, 1.6, 4.4; pyridyl H6), 7.97 (2H, d, J ¼
7.9; pyridyl H3), 7.91 (2H, d, J ¼ 7.9; pyridyl H3), 7.78–7.70
(3H, m; 3 ꢂ pyridyl H4), 7.43 (4H, d, J ¼ 2.4; pyrazolyl H5),
7.28–7.17 (10H, m; 2 ꢂ pyridyl H5 and 8 ꢂ phenyl), 7.07 (2H,
d, J ¼ 2.4; pyrazolyl H4), 6.94 (2H, d, J ¼ 2.4 Hz; pyrazolyl
H4), 5.41–5.38 (8 H, m; CH2). Found: C, 72.2; H, 4.9; N,
21.3%: C43H35N11 ꢀ 0.5H2O requires C, 72.3; H, 5.1; N, 21.6%.
X-Ray crystallography
For each complex a suitable crystal was coated with hydro-
carbon oil and attached to the tip of a glass fibre, which was
then transferred to a Bruker-AXS PROTEUM {for [Ag2-
FeL2][BF4]4 ꢀ 4MeCN} or SMART diffractometer (for all other
structures) under a stream of cold N2. Details of the crystal
parameters, data collection and refinement for each of the
structures are collected in Table 4. After data collection, in
each case an empirical absorption correction (SADABS),
based on symmetry-equivalent and repeated reflections, was
applied;20 the structures were then solved by conventional
direct methods and refined on all F2 data using the SHELX
suite of programs.21 Except where otherwise stated, non-
[Cu3L2(OAc)2][PF6]4
A mixture of L (100 mg, 0.14 mmol) and Cu(OAc)2 ꢀ 2H2O (37
mg, 0.17 mmol) in methanol (3 cm3) was stirred for 5 minutes
to give a clear solution, from which a precipitate appeared on
addition of aqueous KPF6. The pale green precipitate was
filtered off, rinsed with H2O and methanol before being dried
in vacuo. Yield: 75 mg, 47%. ES MS: m/z 2153.8 ({Cu3L2
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 4 – 9 1 1
909

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Table 4 Crystal, data collection and refinement details for the crystal structuresa
Complex
[Cu3L2(OAc)2][PF6]4 ꢀ
[Cu3L2][PF6]4 ꢀ
[Ag2FeL2][BF4]4
[Ag3L2][BF4]3 ꢀ
[Ag4L2][BF4]4 ꢀ
(ca. 3MeCN)
C94H82N26P4F24Cu3 C86H70N22B4F16Ag2Fe C87H73N23B3F12O2Ag3 C92H79N25B4F16Ag4
2MeCN
94H82N24O4P4F24Cu3
4(MeCN)
(MeNO2)
Formula
Formula weight
T/K
C
2382.34
150(2)
2346.36
150(2)
2030.47
100(2)
2056.72
150(2)
ca. 2313
150(2)
l/A
Crystal system
Space group
0.710 73
Monoclinic
C2/c
0.710 73
Monoclinic
C2/c
1.541 78
Monoclinic
C2/c
0.710 73
Monoclinic
P2(1)/c
0.710 73
Monoclinic
P2(1)/n
a/A
b/A
c/A
b/1
22.397(3)
15.0598(19)
30.411(4)
99.856(3)
10 106(2)
4
30.388(4)
14.590(2)
22.348(3)
98.943(3)
9788(2)
4
30.6701(18)
13.7810(8)
21.3348(13)
97.511(2)
8940.1(9)
4
28.943(5)
11.5611(19)
25.405(4)
100.848(3)
8349(2)
19.066(3)
28.822(4)
19.716(3)
115.093(2)
9812(2)
V/A3
Z
4
1.636
4
1.566
D
calc/Mg mꢁ3
1.566
0.797
1.592
0.820
1.509
5.553
m/mmꢁ1
0.790
0.50 ꢂ 0.40 ꢂ 0.20
67 846
0.876
0.25 ꢂ 0.20 ꢂ 0.10
82 310
Crystal size/mm
Reflections
collected
0.41 ꢂ 0.34 ꢂ 0.10
56 428
0.46 ꢂ 0.32 ꢂ 0.21 0.1 ꢂ 0.1 ꢂ 0.075
53 898
20 405
Independent
reflections
Data/restraints/
parameters
Final R indicesb:
R1, wR2
11 549
[R(int) ¼ 0.1498]
11 549/11/687
11 161 [R(int) ¼
0.0916]
11 161/8/684
7848
[Rint ¼ 0.0549]
7848/0/583
12 038
[R(int) ¼ 0.0464]
12 038/2/1172
22 151 [R(int) ¼
0.1060]
22 151/160/522
0.085, 0.269
0.071, 0.216
0.055, 0.135
0.038, 0.106
0.162, 0.514
a
All data sets were collected on a Bruker SMART diffractometer with Mo-Ka radiation, except for [Ag2FeL2][BF4]4 which was collected on a
b
Bruker PROTEUM diffractometer with Cu-Ka radiation. The value of R1 is based on selected data with I 4 2s(I); the value of wR2 is based on all
data.
hydrogen atoms were refined with anisotropic thermal para-
meters; hydrogen atoms were included in calculated positions
and refined with isotropic thermal parameters which were ca.
1.2 ꢂ (aromatic CH) or 1.5 ꢂ (Me) the equivalent isotropic
thermal parameters of their parent carbon atoms.
pond to three MeCN solvent molecules, which is what has been
assumed for the purposes of determining molecular weight,
density etc. in the final refinement. There are numerous resi-
dual electron-density peaks in the range 2–3 e Aꢁ3 which are
close to either anions or regions of disordered solvent. The
[Ag4L2]41 cation itself is reasonably well behaved, although
high thermal parameters for Ag(4) and some of the pyridyl and
pyrazolyl rings coordinated to it indicate unresolved disorder
in these also.
In [Cu3L2(OAc)2][PF6]4 ꢀ 2MeCN the central metal atom lies
on a C2 axis such that the asymmetric unit contains half of the
complex cation, two anions and one solvent molecule. One
hexafluorophosphate anion exhibits disorder with two of the F
atoms [F(11) and F(12)] disordered over two sites (site occu-
pancies 0.55/0.45); these F atoms were refined isotropically.
The MeCN molecule was also disordered, with C(22) and
N(23) split over two sites (occupancies 0.59/0.41); all three
heavy atoms in this molecule were refined isotropically.
[Cu3L2][PF6]4 ꢀ 4(MeCN) likewise has two-fold symmetry.
One of the two independent hexafluorophosphate anions
shows some disorder, with atoms F(22) and F(24) disordered
over two sites (occupancies 0.58/0.42); these atoms were refined
isotropically.
CCDC reference numbers 265432–265436.
See http://www.rsc.org/suppdata/nj/b5/b502423d/ for crys-
tallographic data in CIF or other electronic format.
Acknowledgements
We thank the EPSRC and the University of Sheffield for
financial support.
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[Ag4L2][BF4]4 ꢀ (ca. 3MeCN) has no internal symmetry, with
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911