Chiral coordination polymers based on thallium(I) complexes of new bis- and tris(pyrazolyl)borate ligands with externally-directed 4-pyridyl groups

Article abstract of DOI:10.1039/b306659b

The new ligands dihydro-bis[3-(4-pyridyl)pyrazol-1-yl]borate (Bp ^4py) and hydro-tris[3-(4-pyridyl)pyrazol-1-yl]borate (Tp ^4py) are derivatives of the well known bis- and tris(pyrazolyl) borate cores, bearing 4-pyridyl substituents attached to the pyrazolyl C ³ positions; crystal structures of the Tl(I) complexes show that both are infinite one-dimensional polymeric chains, in which the Tl(I) centre within each bis-pyrazolyl or tris-pyrazolyl cavity is also connected to a pendant 4-pyridyl residue from an adjacent complex unit to build up the chain.

Full text of DOI:10.1039/b306659b

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Chiral coordination polymers based on thallium(I) complexes of
new bis- and tris(pyrazolyl)borate ligands with externally-directed
4-pyridyl groups
Graham M. Davies, John C. Jeffery and Michael D. Ward*
L e t t e r
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
Received (in London, UK) 11th June 2003, Accepted 11th September 2003
First published as an Advance Article on the web 3rd October 2003
The new ligands dihydro-bis[3-(4-pyridyl)pyrazol-1-yl]borate
(Bp4py) and hydro-tris[3-(4-pyridyl)pyrazol-1-yl]borate (Tp4py
Results and discussion
)
The key intermediate for formation of [Bp4py ꢀ and [Tp4py ꢀ
] ] is
are derivatives of the well known bis- and tris(pyrazolyl)borate
cores, bearing 4-pyridyl substituents attached to the pyrazolyl
C3 positions; crystal structures of the Tl(I) complexes show that
both are infinite one-dimensional polymeric chains, in which the
Tl(I) centre within each bis-pyrazolyl or tris-pyrazolyl cavity is
also connected to a pendant 4-pyridyl residue from an adjacent
complex unit to build up the chain.
3-(4-pyridyl)pyrazole, which was prepared from 4-acetylpyri-
dine in the usual two-step procedure (Scheme 1) for conversion
of acetyl groups to pyrazole groups.5–7 Reaction of 4-acetyl-
pyridine with KBH4 in a melt was then used to prepare the
ligands; as reported by Trofimenko, control of stoichiometry
and temperature allows selective formation of the bis- or
tris-(pyrazolyl)borate in moderate yields.1 Thus, reaction of
KBH4 with 3-(4-pyridyl)pyrazole in a 1 : 2.5 ratio at 175–
180ꢁC for 30 minutes afforded KBp4py after workup; use of
3.3 equivalents of 3-(4-pyridyl)pyrazole at 230ꢁC for 3.5 hours
afforded KTp4py. For the purposes of purification and easier
characterisation, these were converted to their Tl(I) salts
(which were more crystalline) by reaction with one equivalent
of Tl(I) acetate in MeOH. The resulting white solid in each case
was filtered off, dried, and crystallised from CH2Cl2–MeOH
(for TlTp4py) or DMF–ether (for TlBp4py). Analytical and
mass spectrometric data for both complexes are in agreement
with the formulations; the crystal structures are shown in Figs.
1 and 2.
Introduction
Since their introduction by Trofimenko, poly(pyrazolyl)borate
ligands have proven to be extremely popular. Initial develop-
ment of the ligands concentrated on controlling their steric
properties using substituents at the C3 position of each pyrazo-
lyl ring, in order to provide a protecting environment around a
metal ion.1 More recently, a new generation of poly(pyrazo-
lyl)borate derivatives has been developed in which additional
coordinating groups have been attached to the pyrazolyl rings.
The structure of TlBp4py (Fig. 1) shows that the complex is a
coordination polymer, with one pendant 4-pyridyl group on
each TlBp4py monomer unit being used to complete the coor-
dination environment about the adjacent Tl(I) centre, resulting
in helical one-dimensional chains. This behaviour is what
we hoped to achieve by using externally-directed 4-pyridyl
Prominent among these is the ligand [Tp2py ꢀ
in which the
]
attachment of a 2-pyridyl substituent to each of the three pyr-
azolyl rings at the C3 position makes each ‘arm’ of the ligand a
bidentate chelate.2 This completely changes the coordination
behaviour, as the ligand is now hexadentate and shows beha-
viour such as encapsulation of lanthanide(III) or actinide(III)
ions in 12-coordinate species [M(Tp2py)2]+,3 and triply-brid-
ging coordination modes in tetrahedral cage complexes such
as [Mn4(Tp2py)4]4+ 4
. Other coordinating substituents which
have been appended to the pyrazolyl C3 positions of poly(pyr-
azolyl)borates in a similar vein have included bipyridyl,5
thioanisyl,6 pyrazinyl,7 and carboxamide8 units.
In this communication we report the synthesis and structural
characterisation of the ligands [Bp4py ꢀ and [Tp4py]ꢀ, in which
]
4-pyridyl units (instead of 2-pyridyl) are used as the pyrazolyl
C3 substituents. Again this changes the character of the
ligands. The central cavity now reverts to the bidentate or ter-
dentate character of the simple bis- and tris(pyrazolyl)borates
respectively, with the modification that the externally-directed
4-pyridyl groups cannot chelate to the central metal ion but
instead are ‘exodentate’ and capable of binding to additional
metal centres to form, in principle, multinuclear assemblies;
or, if they are used to complete the coordination sphere around
a metal ion in a different complex unit, coordination polymers
will result. An exactly similar principle has been used by
Champness and co-workers to prepare structurally elaborate
multinuclear assemblies,9 and we note that the 3-pyridyl sub-
stituted Tp analogue, described recently by Vahrenkamp,
formed coordination polymers with K+.10
Scheme 1 Synthesis of the ligands [Bp4py ꢀ and [Tp4py]ꢀ. Reagents
and conditions: (i) N,N-Dimethylformamide–dimethyl acetal, reflux;
(ii) hydrazine hydrate, EtOH, reflux.
]
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New J. Chem., 2003, 27, 1550–1553
DOI: 10.1039/b306659b
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Fig. 1 Two views of the crystal structure of TlBp4py: (a) two mono-
mer units (shown with 40% thermal ellipsoids) illustrating the labelling
scheme; (b) a view showing how several monomer units form a helical
Fig. 2 Two views of the crystal structure of TlTp4py: (a) the mono-
mer unit (shown with 40% thermal ellipsoids) illustrating the labelling
scheme; (b) a view showing how several monomer units form a helical
˚
chain. Selected bond distances (A) and angles (deg): Tl(1)–N(102)
2.641(6), Tl(1)–N(113) 2.751(7), Tl(1)–N(109A) 2.678(7), N(102)–
Tl(1)–N(109A) 77.4(2), N(102)–Tl(1)–N(113) 84.5(2), N(109A)–Tl(1)–
N(113) 82.1(2), N(101)–B(1)–N(112) 110.5(6).
˚
chain. Selected bond distances (A) and angles (deg): Tl(1)–N(122)
2.577(8), Tl(1)–N(142) 2.609(9), Tl(1)–N(102) 2.791(7), Tl(1)–N(131A)
2.978(8), N(122)–Tl(1)–N(142) 77.9(3), N(122)–Tl(1)–N(102) 72.3(2),
N(142)–Tl(1)–N(102) 67.3(2), N(131 A)–Tl(1)–N(102) 131.7(3),
N(131 A)–Tl(1)–N(122) 71.3(2), N(131 A)–Tl(1)–N(142) 74.9(2),
N(141)–B(1)–N(101) 111.4(10), N(141)–B(1)–N(121) 109.5(8), N(101)–
B(1)–N(121) 107.9(9).
substituents on the ligand core. Each Tl(I) centre is in a pyra-
midal three-coordinate environment, arising from the two
chelating pyrazolyl donors from one ligand and a 4-pyridyl
donor from an adjacent complex unit. The pyramidal geo-
metry implies that the lone pair is stereochemically active and
occupying the vacant site which would therefore complete an
approximately tetrahedral array of electron pairs around the
metal.11,12 The N–Tl–N angles are however considerably com-
pressed compared to the tetrahedral ideal, a consequence of
the length of the Tl–N bonds. In each ligand the pyridyl groups
are slightly twisted out of the plane of the attached pyrazolyl
ring, with the torsion angles being 4ꢁ and 26ꢁ for the pyridyl
rings containing N(12) and N(109) respectively.
an adjacent TlTp4py unit to form a rather long Tlꢂ ꢂ ꢂN bond
˚
˚
[2.978 A, compared to an average value of 2.66 A for the three
Tlꢂ ꢂ ꢂN(pyrazolyl) bonds]. The longest of the Tlꢂ ꢂ ꢂN(pyrazolyl)
˚
interactions [2.791 A, to N(102)] is the one that is approxi-
mately trans to the additional pyridyl ligand; the insertion of
this pyridyl ring between two of the arms [involving the pyr-
azolyl donors N(122) and N(142)] results in the angle N(122)–
Tl(1)–N(142) being the largest of the three bite angles at 77.9ꢁ.
It is noteworthy that both crystals have chiral space groups
[P2(1) for TlBp4py, Pc for TlTp4py] despite being composed of
achiral components. Crystalline materials in chiral space
groups have been of interest recently for their second-order
non-linear optical properties.14,15 Accordingly these new com-
plexes are likely to be of interest in this respect, in addition to
the possibilities available for assembly of polynuclear com-
plexes by attachment of additional metal ions at the peripheral
4-pyridyl sites of the monomer units.13
The structure of TlTp4py is shown in Fig. 2; again, a chiral
one-dimensional polymeric chain results by virtue of a brid-
ging interaction between a pendant 4-pyridyl donor on one
complex unit and the Tl(I) centre of another. The monomer
unit looks structurally similar to the complex TlTptol, in which
the Tl(I) centre is coordinated by the three pyrazolyl donors in
a pyramidal geometry with a stereochemically active lone pair
in the apical position, and the three aromatic substituents (p-
tolyl groups) arrayed in a cone-like conformation protecting
the Tl(I) centre.11 Replacement of tolyl substituents by 4-pyri-
dyl units does not affect this basic structure, but the diverging
cone-like array of 4-pyridyl units is reminiscent of tris(4-pyri-
dyl)-functionalised bowl-shaped ligands prepared recently by
Fujita and co-workers which were used as the basis for assem-
bling capsule-like structures.13 The pyridyl substituents are in
each case twisted away from the mean plane of the attached
pyrazolyl ring, with torsion angles of 29, 35 and 27ꢁ for the
pyridyl rings containing N(151), N(131) and N(111) respec-
tively. A packing diagram of TlTp4py reveals an additional
bridging interaction which results in formation of a one-
dimensional coordination polymer; the pyridyl donor N(131)
on one complex unit is directed between the ligating arms of
Experimental
Preparations
3-(4-Pyridyl)pyrazole. This was prepared in two steps from
4-acetylpyridine as shown in Scheme 1. In the first step, a mix-
ture of 4-acetylpyridine (24.2 g, 0.2 mol) and N,N-dimethylfor-
mamide–dimethyl acetal (40 cm3) was heated to reflux for 2 h.
After concentration in vacuo, recrystallisation of the orange
residue from CHCl3–hexane afforded an orange/yellow pow-
der (intermediate A) in 58% yield. Chemical ionization MS:
1
m/z 177 (M + H)+. H NMR (CDCl3): d 8.64 (2H, d, J 5.9;
New J. Chem., 2003, 27, 1550–1553
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pyridyl H2/H6), 7.79 (1H, d, J 12.5; alkene CH), 7.63 (2H, d, J
5.9; pyridyl H3/H5), 5.60 (1H, d, J 12.5 Hz; alkene CH), 3.12
(3H, s, Me), 2.89 (3H, s, Me). Found: C, 64.5; H, 6.8; N, 15.2.
Required for [C10H12N2Oꢂ0.5H2O]: C, 64.8; H, 7.0; N, 15.1%.
A mixture of A (20.3 g, 0.115 mmol), ethanol (30 cm3) and
hydrazine hydrate (30 cm3) was then heated to 60ꢁC with stir-
ring for 30 min. After addition to cold water (130 cm3) and
overnight refrigeration, the resulting off-white precipitate was
filtered off, washed with copious amounts of water, and dried
to give 3-(4-pyridyl)pyrazole in 77% yield. Although characteri-
sation data were satisfactory at this stage, the material could
be further purified by vigorous washing with toluene (ultra-
sound cleaning bath) followed by recrystallisation from water.
once more. The reaction mixture was refrigerated (5ꢁC) for
several hours before filtration of the precipitate which was
washed with ether to give TlTp4py as a white powder in 51%
yield. Colourless crystals, suitable for X-ray diffraction studies,
were grown from slow evaporation of a CH2Cl2–MeOH
solution of the complex. FAB-MS: m/z 650 [M + H]+; 505
[MH+ ꢀ pyridylpyrazole unit]. Found: C, 43.2; H, 2.9; N,
18.8. Required for [C24H19N9BTl]: C, 43.2; H, 3.2; N, 18.9%.
IR: nB–H 2441 cmꢀ1
.
X-Ray crystallography
1
Data for TlBp4py, C16H14BN6Tl: monoclinic, space group P21 ,
EI-MS: 145 (M+). H NMR (CDCl3): d 11.68 (1H, br s, NH),
ꢁ
˚
a ¼ 10.454(2), b ¼ 6.3740(13), c ¼ 12.338(3) A, b ¼ 98.96(3) ,
8.65 (2H, d, J 6.2, pyridyl H2/H6), 7.72 (2H, d, J 5.9, pyridyl
H3/H5), 7.68 (1H, d, J 2.6; pyrazolyl), 6.76 (1H, d, J 2.2 Hz;
pyrazolyl). Found: C, 66.3; H, 5.1; N, 29.2. Required for
C8H7N3 : C, 66.2; H, 4.9; N, 29.0%.
U ¼ 812.1(3) A , Z ¼ 2, Mr ¼ 505.51 g molꢀ1, rcalc ¼ 2.067 g
3
˚
cmꢀ3, m ¼ 9.954 mmꢀ1 (Mo-Ka radiation). Refinement on
F2 of 3693 independent reflections (Rint ¼ 0.0455; 2ymax ¼ 55ꢁ)
converged at R1 ¼ 0.0370 [based on selected data with
F ꢃ 4s(F)], wR2 ¼ 0.0785 (for all data), using 225 parameters.
Flack parameter: ꢀ0.020(14).
KBp4py. 3-(4-Pyridyl)pyrazole (1.44 g, 9.9 mmol) and KBH4
(0.215 g, 4 mmol) were ground together thoroughly in a pestle
and mortar and heated for 30 min in a Schlenk tube at 175–
180ꢁC under N2 . The mixture was stirred throughout,
although this was hindered once melting occurred at 150ꢁC.
After cooling to rt, toluene (30 cm3) was added and the mix-
ture was agitated in an ultrasound cleaning bath for 20 min.
The suspension was filtered, and the solid was washed with
hot toluene and hot hexane and then dried gave KBp4py as a
white powder in 80% yield. Negative-ion electrospray MS:
Data for TlTp4py, C24H19BN9Tl: monoclinic, space group
˚
Pc, a ¼ 8.7477(9), b ¼ 16.5928(17), c ¼ 8.3841(9) A, bꢀ¼1
3
ꢁ
˚
96.281(2) , U ¼ 1209.6(2) A , Z ¼ 2, Mr ¼ 648.66 g mol
Refinement on F2 of 5428 independent reflections (Rint
,
rcalc ¼ 1.781 g cmꢀ3, m ¼ 6.708 mmꢀ1 (Mo-Ka radiation).
¼
0.0636; 2ymax ¼ 55ꢁ) converged at R1 ¼ 0.0337 [based on
selected data with F ꢃ 4s(F)], wR2 ¼ 0.0838 (for all data),
using 316 parameters. Flack parameter: 0.031(10).
1
In both cases a suitable crystal was mounted on a Bruker
APEX diffractometer at 100 K and the data collection covered
a full sphere of reciprocal space. After integration data were
corrected for Lorentz and polarisation effects, and for absorp-
tion effects using an empirical method (SADABS).16 Data
reduction, structure solution and refinement were performed
with the SHELX suite of programs.17
m/z 301 [C16H14N6B]ꢀ. H NMR (MeOD): d 8.43 (4H, d, J
6.2; pyridyl H2/H6), 7.82 (4H, d, J 6.6; pyridyl H3/H5), 7.61
(2H, d, J 2.2; pyrazolyl), 6.64 (2H, d, J 1.9; pyrazolyl). 11B
NMR (MeOD): d ꢀ7.59. Found: C, 53.9; H, 4.9; N, 23.7.
Required for [C16H14N6BKꢂH2O]: C, 53.6; H, 4.5; N, 23.5%.
IR: nB–H 2380, 2263 cmꢀ1
.
Crystallographic data for the two structural analyses have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC reference nos. 212496 and 212497. See
http://www.rsc.org/suppdata/nj/b3/b306659b/ for crystallo-
graphic data in .cif or other electronic format.
TlBp4py. Methanolic solutions of thallium(I) acetate (0.18
mmol) and KBp4py (0.18 mmol), both in 5 cm3 MeOH, were
combined and stirred at rt for 40 min, during which time
a white precipitate formed. The solution was concentrated
in vacuo and refrigerated for several hours before being filtered
off and washed with MeOH to give a white powder. Recrystal-
lisation by diffusion of Et2O vapour into a solution of the com-
plex in DMF afforded colourless crystals suitable for X-ray
diffraction studies. These were filtered off and washed with
Et2O to give pure TlBp4py in 42% yield. FAB-MS: m/z 507
[M+] 358 [M+ ꢀ pyridylpyrazole unit]. Found: C, 37.8; H,
2.5; N, 16.5. Required for [C16H14N6BTl]: C, 38.0; H, 2.8; N,
Acknowledgements
We thank the EPSRC (UK) for financial support.
16.6%. IR: nB–H 2412, 2389, 2264 cmꢀ1
.
References
KTp4py. 3-(4-Pyridyl)pyrazole (0.96 g, 6.6 mmol) and KBH4
(0.108 g, 2 mmol) were ground together thoroughly in a pestle
and mortar, and heated under N2 in a Schlenk tube, with stir-
ring, at 230ꢁC for 3.5 h. After cooling to rt, 30 cm3 of toluene
were added and the mixture was agitated in an ultrasound
cleaning bath for 20 min. The suspension was filtered, and
the solid was washed with hot toluene and hot hexane and then
dried gave KTp4py as a white powder in 25% yield. Negative-
ion ES MS: m/z 444 [C24H19N9B]ꢀ. 1H NMR (MeOD): d
8.50 (6H, d, J 6.2, pyridyl H2/H6), 7.88 (6H, d, J 6.2, pyridyl
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N, 25.0. Required for [C24H19N9BKꢂH2O]: C, 57.5; H, 4.2;
N, 25.1%. IR: nB–H 2418.
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