A C3-symmetric chiral hexadentate podand ligand based on a tris(pyrazolyl)borate core

Article abstract of DOI:10.1039/b102295b

A hexadentate podand ligand having C₃ symmetry has been prepared, consisting of a tris(pyrazolyl)borate core bearing pinene-functionalised 2-pyridyl units attached to the C³ position of each pyrazolyl ring such that each arm is a chiral bidentate chelate; complexes with Tl(I) and Tb(III) have been prepared and structurally characterised.

Full text of DOI:10.1039/b102295b

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A C3-symmetric chiral hexadentate podand ligand based on a
tris(pyrazolyl)borate core
Graham R. Motson,a Olimpia Mamula,b John C. Jeffery,a Jon A. McCleverty,*a
Michael D. Ward*a and Alex von Zelewskyb
a School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS.
E-mail: mike.ward@bristol.ac.uk
b Institute of Inorganic Chemistry, University of Fribourg, Pérolles, CH-1700 Fribourg,
Switzerland
Received 12th March 2001, Accepted 2nd April 2001
First published as an Advance Article on the web 12th April 2001
A hexadentate podand ligand having C3 symmetry has been
prepared, consisting of a tris(pyrazolyl)borate core bearing
pinene-functionalised 2-pyridyl units attached to the C3
position of each pyrazolyl ring such that each arm is a
chiral bidentate chelate; complexes with Tl(I) and Tb(III)
have been prepared and structurally characterised.
Chiral C3-symmetric ligands are of considerable interest for use
in asymmetric catalysis and chiral recognition.1 Amongst these,
chiral derivatives of the well-known tris(pyrazolyl)borates2
have been studied principally by Tolman and co-workers, who
have investigated the synthesis and coordination behaviour of
several chiral tris(pyrazolyl)borate (generically, Tp) deriv-
atives3,4 and the use of their complexes in asymmetric
catalysis.5,6 Also related to these are a few examples of chiral
tris(pyrazolyl)phosphine oxides5,7 and
a chiral tetrakis-
(pyrazolyl)borate.8 All of the known chiral Tp ligands are
potentially terdentate, with a chiral unit such as camphoryl or
menthyl fused to two sites of the pyrazolyl ring, such that the
coordination behaviour of the ligand is basically that of a
conventional but sterically hindered Tp ligand.4
Scheme 1 Preparation of [TpPy*]Ϫ. (i) N,N-Dimethylformamide–
dimethylacetal, reflux for 6 h; (ii) hydrazine hydrate, ethanol, 60 ЊC, 30
minutes; (iii) KBH4, 4-methylanisole, reflux, 24 h.
We describe here the first example of a C3-symmetric
hexadentate tripodal ligand tris[3-{2-(pinene[4,5]pyridyl)}-
pyrazolyl]hydroborate (hereafter abbreviated as [TpPy*]Ϫ) based
on a tris(pyrazolyl)borate core. The achiral hexadentate parent
ligand [TpPy]Ϫ has been extensively studied by us,9–12 and the
addition of 2-pyridyl substituents to the C3 positions of the
pyrazolyl rings, such that each arm of the ligand is now a bi-
dentate chelate, results in completely different coordination
behaviour from the simpler terdentate Tp ligands.2 [TpPy]Ϫ can
accommodate a single metal ion such as a lanthanide() ion9
or thallium()11 in its hexadentate cavity, or can act as a tri-
nucleating bridging ligand, with each bidentate arm coordin-
ated to a different metal ion to afford tetrahedral cages such as
[Mn4(TpPy)4]4ϩ in interesting self-assembly processes.10
The new ligand [TpPy* Ϫ mimics the donor set of [TpPy]Ϫ but
]
is chiral by virtue of the pinene units fused to the C4 and C5
positions of each pyridyl ring. The synthesis is summarised in
Scheme 1 and follows the same basic route as that for [TpPy]Ϫ.†
The key intermediate is the chiral 2-acetylpyridine derivative
(Ϫ)-2-acetyl{pinene[4,5]pyridine} A, which is prepared in
several steps starting from 2,3-butanedione: protection of one
acetyl group as its oxime allows the other to be converted to a
pyridine ring by a Kröhnke-type reaction with (Ϫ)-myrtenal in
the presence of ammonium acetate. This synthesis is summar-
ised in Scheme 2; full experimental details will be described
elsewhere.13 Conversion of the acetyl group of A to a pyrazole is
straightforward,9 and the pyrazole B was then converted to
[TpPy* Ϫ by reaction with KBH4 in 4-methylanisole at reflux,14
]
followed by isolation of the ligand as its Tl() complex to give
[Tl(TpPy*)].† The formulation of the complex was confirmed by
1H and 11B NMR spectroscopy, a FAB mass spectrum, and a
crystal structure.
Scheme
2
Preparation of (Ϫ)-2-acetyl{pinene[4,5]pyridine} (A).
(i) Br2, 0 ЊC; (ii) NH2OHؒHCl, water, Na2CO3, 0 ЊC; (iii) pyridine, Et2O,
rt; (iv) (Ϫ)-myrtenal, NH4OAc, dmf, 80 ЊC; (v) aq. HCl, reflux.
DOI: 10.1039/b102295b
J. Chem. Soc., Dalton Trans., 2001, 1389–1391
This journal is © The Royal Society of Chemistry 2001
1389

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Fig. 1 Crystal structure of one of the independent complex molecules
of [Tl(TpPy*)]. Selected bond distances (Å) (the corresponding values
for the alternate independent molecule are given in brackets): Tl(1)–
N(12), 2.719(3) [Tl(2)–N(42), 2.721(3)]; Tl(1) ؒ ؒ ؒ N(21), 3.101(3)
[Tl(2) ؒ ؒ ؒ N(51), 3.131(3)].
Fig. 3 Crystal structure of the complex molecule of [Tb(TpPy*)-
(NO3)2]ؒ(CH2Cl2)3. Selected bond distances (Å): Tb(1)–N(72), 2.477(3);
Tb(1)–O(3), 2.491(3); Tb(1)–N(12), 2.491(3); Tb(1)–O(7), 2.506(3);
Tb(1)–O(2), 2.519(3); Tb(1)–N(42), 2.523(4); Tb(1)–O(6), 2.571(3);
Tb(1)–N(81), 2.642(3); Tb(1)–N(21), 2.643(4); Tb(1)–N(51), 2.735(4);
B(1)–N(41), 1.539(7); B(1)–N(11), 1.543(7); B(1)–N(71), 1.551(6).
rings has, as we wanted, preserved the basic coordination
behaviour of the podand ligand. Attachment of substituents at
the pyridyl C6 position would bring the chiral groups closer to
the metal centre, but at the expense of a substantial degree of
steric crowding which would completely change the coordin-
ation behaviour.18 Obvious avenues for investigation with the
lanthanide complexes include (i) the possibility of chiral
quenching with the different enantiomers of [Ru(bipy)3]2ϩ
{*Tb()→Ru photoinduced energy transfer19 and *Ru→
Eu() photoinduced electron-transfer20 with [Ru(bipy)3]2ϩ are
both known}, and (ii) their use as Lewis-acid catalysts for
e.g. Diels–Alder reactions,21 since dissociation of the nitrate
ions in a polar solvent9 will allow direct access of substrates to
the primary coordination sphere of the metal in a C3-symmetric
environment. It will also be of interest to investigate the use of
[TpPy* Ϫ in the diastereoselective assembly22 of tetrahedral
]
cages (cf. [Mn4(TpPy)4]4ϩ) which are inherently chiral because of
the presence of four tris-chelate metal centres which all have the
same configuration.10
We thank the EPSRC for financial support. M. D. W. is the
Royal Society of Chemistry Sir Edward Frankland Fellow for
2000/2001.
Fig. 2 Alternative view of [Tl(TpPy*)] looking down the threefold
Tl ؒ ؒ ؒ B axis.
The crystal structure of [Tl(TpPy*)] is shown in Figs. 1 and 2.‡
It is similar to that of the related non-chiral complex [Tl-
(TpPy)],11 with the Tl() centre being coordinated principally by
the three pyrazolyl N donors in a pyramidal arrangement [Tl–
N(pyrazolyl) distances, 2.719(3) in one independent molecule
and 2.721(3) Å in the other], and the fourth coordination site
vacant. Fig. 2 is a view down the Tl ؒ ؒ ؒ B axis which emphasises
the C3 symmetry. The Tl ؒ ؒ ؒ N(pyridyl) distances are too long
for these interactions to be considered as full bonds [3.101(3)
in one independent molecule and 3.131(3) Å in the other],
nevertheless the presence of a weak interaction is shown by
the fact that the pyridyl rings are oriented such that the lone
pairs of the N atoms are pointing at the metal centre. We
have observed similar weak Tl ؒ ؒ ؒ N interactions in related
complexes,15 and we note that the unusual structural properties
of Tl()/Tp complexes in general continue to be a subject of
current interest.16
Notes and references
† Preparation of [Tl(TpPy*)]: (Ϫ)-2-Acetyl{pinene[4,5]pyridine} (A)
(ref. 13) was converted to the corresponding pyrazole (B) by the two-
step procedure we have described before (refs. 9 and 18), and was fully
characterised by NMR and mass spectrometry as well as X-ray crystal-
lography; full details will be reported in due course. A mixture of B
(2.09 g, 8.75 mmol) and KBH4 (0.14 g, 2.5 mmol) dissolved in
4-methylanisole was heated to reflux for 24 h under N2, and the solvent
was then distilled off in vacuo. The remaining brown oil was dissolved
in thf (10 cm3), and to this was added a solution of TlNO3 (1 equiv. per
hexadentate ligand) in water (10 cm3). The resulting solution was
extracted with several portions of CH2Cl2 which were combined, dried
over MgSO4, and evaporated to dryness to give a brown oil which, on
trituration with diethyl ether and treatment with ultrasound, crystal-
lised to an off-white solid of [Tl(TpPy*)] (30% yield). Found: C, 57.5;
H, 4.8; N, 13.9. C45H49BN9Tl requires C, 58.1; H, 5.3; N, 13.5%. X-Ray
quality crystals were grown by slow evaporation from CDCl3 in an
Reaction of [Tl(TpPy*)] with Tb() nitrate in MeOH afforded
[Tb(TpPy*)(NO3)2], whose identity was confirmed by its FAB
mass spectrum.† The crystal structure (Fig. 3) is likewise similar
to that of the achiral complexes [Ln(TpPy)(NO3)2] (Ln = Eu, Er,
Pr),9 with a 10-coordinate metal ion arising from hexadentate
coordination of the podand ligand and two bidentate nitrates.
The presence of two nitrate ligands breaks the threefold sym-
metry that would otherwise be imposed by the ligand. Several
lanthanide() complexes with substituted Tp derivatives have
been reported by others recently.17
1
NMR tube. H-NMR (270 MHz, CDCl3): δ 8.13 (1H, s; pyridyl H6),
7.73 (1H, d, J = 1.6; pyrazolyl H5), 7.47 (1 H, s; pyridyl H3), 6.59 (1 H,
d, J = 1.5; pyrazolyl H4), 2.96 (2H, d, J = 2.4; pinene Ha), 2.80 (1H, t,
J = 5.5; pinene Hc), 2.67 (1H, m; pinene Hd), 2.27 (1 H, m; pinene Hb),
1.39 (3H, s; pinene Hf), 1.19 (1 H, d, J = 9.5 Hz; pinene He), 0.61 (3H, s;
pinene Hg) (for the labelling scheme used for the pinene fragment, see
Scheme 1). 11B-NMR: (300 MHz, CDCl3) δ Ϫ3.51 (s) [this is character-
istic of the BH unit of a tris(pyrazolyl)borate: there was no sign of a
resonance corresponding to a bis(pyrazolyl)borate at between Ϫ8 and
Ϫ10 ppm, or a tetrakis(pyrazolyl)borate at between ϩ1 and ϩ2 ppm].
FAB-MS: m/z 694 [100%, Mϩ Ϫ one bidentate arm (B)].
Together these structures confirm that attachment of the
chiral pinene units to the C4 and C5 positions of the pyridyl
1390
J. Chem. Soc., Dalton Trans., 2001, 1389–1391

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7 C. J. Tokar, P. B. Kettler and W. B. Tolman, Organometallics, 1992,
11, 2737.
Preparation of [Tb(TpPy*)(NO3)2]: [Tl(TpPy*)] (0.050 g, 0.05 mmol)
and Tb(NO3)3ؒ5H2O (0.024 g, 0.05 mmol) were dissolved in MeOH
(10 cm3) and the solution was stirred for 24 hours. The solution was
then reduced in vacuo to ca. 2 cm3 and refrigerated. The resultant
precipitate was collected by filtration, washed with H2O, and dried
in vacuo to give an off-white solid of [Tb(TpPy*)(NO3)2] (67% yield).
Found: C, 52.4; H, 5.0; N, 13.3. C45H49BN11O6Tb requires C, 53.5;
H, 4.9; N, 15.3%. Low C and N values are common for lanthanide
tris(pyrazolyl)borate complexes (see ref. 9). X-Ray quality crystals were
grown from CH2Cl2–hexane. FAB-MS: m/z 948 [100%, Mϩ Ϫ NO3].
Satisfactory analytical data were obtained for the new complexes.
‡ Crystal data for [Tl(TPPy*)]: C45H49BN9Tl, M = 931.11, rhombohedral
(hexagonal setting), space group R3, a = b = 20.3970(18), c = 17.347(3)
Å, U = 6250.1(12) Å3, Z = 6, Dc = 1.484 Mg mϪ3, µ(Mo-Kα) = 3.920
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mmϪ1
,
F(000) = 2808, T = 173 K, 6361 independent reflections
(Rint = 0.0345) with 2θ < 55Њ. Refinement of 358 parameters converged
at final R1 [for selected data with I > 2σ(I)] = 0.0261, wR2 (all
data) = 0.0502. The Flack parameter was Ϫ0.008(5). The asymmetric
unit contains two independent units which each constitute one-third of
a complex molecule, i.e. an entire bidentate ligand ’arm’ with one-third
of the associated B and Tl atoms which lie on C3 axes. There are accord-
ingly two crystallographically independent but very similar complex
molecules in the crystal, each lying on a C3 axis.
Crystal data for [Tb(TpPy*)(NO3)2]ؒ(CH2Cl2)3: C48H55BCl6N11O6Tb,
M = 1264.46, orthorhombic, space group P212121, a = 12.6126(17),
b = 17.623(2), c = 24.975(4) Å, U = 5551.3(13) Å3, Z = 4, Dc = 1.513 Mg
mϪ3
, , F(000) = 2560, T = 173 K, 12704
µ(Mo-Kα) = 1.619 mmϪ1
independent reflections (Rint = 0.0475) with 2θ < 55Њ. Refinement of 677
parameters converged at final R1 [for selected data with
I > 2σ(I)] = 0.0357, wR2 (all data) = 0.1155. The Flack parameter was
0.001(7).
X-Ray measurements were made using a Bruker SMART CCD area-
detector diffractometer; structure solution and refinement was with the
SHELXTL program system.23 CCDC reference numbers xxx. See http://
www.rsc.org/suppdata/dt/b1/b102295b/ for crystallographic data in
CIF or other electronic format.
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J. Chem. Soc., Dalton Trans., 2001, 1389–1391
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