Tl(I) complexes of cymantrene-based tris(1-pyrazolyl)borates: Polymers and macrocycles

Article abstract of DOI:10.1039/b105692c

Cymantrene-based tris(1-pyrazolyl)borates have been synthesised from the reaction of CymBBr₂ and Cym'BBr₂ with pyrazole (Hpz) in the presence of triethylamine [Cym: cymantrenyl; Cym': (methyl)cymantrenyl]. The Tl(I) salts CymB(pz)₃Tl,2a, and Cym'B(pz)₃Tl,2b, have been structurally characterised by X-ray crystallography. Similar to the corresponding complex FcB(pz)₃Tl (Fc: ferrocenyl), CymB(pz)₃Tl features a polymeric structure with bridging [B(pz)3] fragments in the solid state. Cym'B(pz)₃Tl shows a related oligomeric structure. This time a macrocyclic tetramer rather than a linear chain is observed. To provide a ligand of high solubility in non-polar solvents, compound Cym'B(pz^4-R)₃Tl, [R = CH₂(C₆H₁₁)]2d, has been synthesised, which shows a reduced symmetry at the organometallic moiety, together with (cyclohexyl)methyl substituents in the 4-positions of all pyrazolyl rings.

Full text of DOI:10.1039/b105692c

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Tl(I) complexes of cymantrene-based tris(1-pyrazolyl)borates:
polymers and macrocycles
ShengLi Guo,a Jan W. Bats,b Michael Bolteb and Matthias Wagner*a
a Institut für Anorganische Chemie, J.W. Goethe-Universität Frankfurt, Marie-Curie-Str. 11,
D-60439 Frankfurt (Main), Germany. E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
b Institut für Organische Chemie, J.W. Goethe-Universität Frankfurt, Marie-Curie-Str. 11,
D-60439 Frankfurt (Main), Germany
Received 28th June 2001, Accepted 4th October 2001
First published as an Advance Article on the web 22nd November 2001
Cymantrene-based tris(1-pyrazolyl)borates have been synthesised from the reaction of CymBBr2 and CymЈBBr2 with
pyrazole (Hpz) in the presence of triethylamine [Cym: cymantrenyl; CymЈ: (methyl)cymantrenyl]. The Tl() salts
CymB(pz)3Tl, 2a, and CymЈB(pz)3Tl, 2b, have been structurally characterised by X-ray crystallography. Similar to
the corresponding complex FcB(pz)3Tl (Fc: ferrocenyl), CymB(pz)3Tl features a polymeric structure with bridging
[B(pz)3] fragments in the solid state. CymЈB(pz)3Tl shows a related oligomeric structure. This time a macrocyclic
tetramer rather than a linear chain is observed. To provide a ligand of high solubility in non-polar solvents,
compound CymЈB(pz4-R)3Tl, [R = CH2(C6H11)] 2d, has been synthesised, which shows a reduced symmetry at the
organometallic moiety, together with (cyclohexyl)methyl substituents in the 4-positions of all pyrazolyl rings.
Scorpionates now find applications in a wide range of
chemistry, from modelling the active site of metallo-enzymes,
Introduction
Tris(1-pyrazolyl)borates (“scorpionates”) A were invented by
through analytical chemistry and organic synthesis to catalysis
Trofimenko more than 30 years ago and are today well-
and materials science.2,3 Given this background, it is surprising
established as ligands in coordination chemistry (Fig. 1).1,2 In
to see that examples of redox-active4 or bifunctional5 scor-
pionate ligands are very rare. In order to fill this gap, we have
recently synthesised the ferrocene-based scorpionate ligands B
and C (Fig. 1).6 One aim is to use these species as building
blocks for the generation of oligonuclear aggregates and metal-
containing polymers.7–9 Numerous complexes of B have been
characterised by X-ray crystallography. The Tl() salt of B
provided the first example of a scorpionate complex featuring
a polymeric structure in the solid state.6 In the meantime, other
polymeric thallium scorpionates with bridging tris(1-pyrazolyl)-
borate units have been reported by Janiak et al.10 An important
factor influencing the solid state structure of thallium scor-
pionates appears to be the degree of steric crowding around the
boron centre.8,10 Ligand B in its tridentate coordination mode
suffers from unfavourable steric interactions between the bulky
ferrocenyl substituent and the hydrogen atoms in position 5 of
the pyrazolyl rings. This steric stress can be substantially
relieved if one of the three pyrazolyl rings is rotated by 90Њ
about the B–N axis, which in turn switches the tris(1-pyrazolyl)-
borate moiety from a tridentate to a bridging coordination
mode. However, it has to be taken into account that Tl–N
bonds in scorpionate complexes are rather flexible. In a discus-
sion of the respective solid state structures, crystal packing
forces must therefore not be neglected.
In this paper, the syntheses and structural characterisations
of scorpionate ligands bearing cyclopentadienyl manganese tri-
carbonyl (cymantrenyl: Cym) substituents at the boron centre
Fig. 1 Tris(1-pyrazolyl) borate A (substituents R1–R3 on two of the
three pyrazolyl rings are omitted for clarity) and its ferrocene-based
are described (Scheme 1).
derivatives B and C.
These species, which are conveniently accessible from bor-
ylated cymantrenes 1a and 1b,11 will offer convenient access
most complexes, the monoanionic molecules act as tripodal six-
electron donors. Derivatives can be generated by varying the
group R at boron or the substituents in the 3, 4 and 5 positions
of the pyrazolyl rings. This gives rise to an enormous number
of possible structural variations, permitting the design of scor-
pionate ligands with very specific steric and electronic features.
to kinetically stable, manganese containing heterooligometallic
complexes. Moreover, the steric characteristics of the [(C5H4)-
Mn(CO)3] unit are similar to those of the ferrocenyl substit-
uent. It will therefore be revealing to compare the solid state
structures of compounds 2 (Scheme 1) and the Tl() scorpionate
B (Fig. 1).
3572
J. Chem. Soc., Dalton Trans., 2001, 3572–3576
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105692c

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Table 1 Selected bond lengths [Å], angles [Њ] and torsion angles [Њ] of
2a and 2b. N(6a) indicates that the atom belongs to a neighbouring
molecule [i.e. Tl(1)–N(6a) is an intermolecular bond]
2a
2b
B–C(10)
B–N(1)
B–N(3)
1.602(6)
1.556(5)
1.566(5)
1.554(5)
2.654(3)
2.722(3)
2.720(3)
5.101(1)
1.650(13) [1.688(16)]b
1.515(12)
1.587(13)
1.538(12)
2.635(9)
2.582(9)
2.682(7)
4.793(1)
B–N(5)
Tl–N(2)
Tl–N(4)
Tl–N(6a)
Tl ؒ ؒ ؒ Tla
N(1)–B(1)–N(3)
N(1)–B(1)–N(5)
N(3)–B(1)–N(5)
C(10)–B(1)–N(1)
C(10)–B(1)–N(3)
C(10)–B(1)–N(5)
N(2)–Tl(1)–N(4)
N(2)–Tl(1)–N(6a)
N(4)–Tl(1)–N(6a)
110.0(3)
105.4(3)
107.8(3)
113.8(3)
106.3(3)
113.4(3)
70.4(1)
110.3(6)
110.0(7)
105.0(8)
123.3(8) [96.9(9)]b
97.6(7) [120.3(9)]b
108.7(7) [113.9(8)]b
68.7(3)
Scheme 1 (i) ϩ 3Hpz4-RЈ, ϩ 2NEt3 in toluene; (ii) ϩ TlOEt in toluene.
87.3(1)
83.2(1)
83.9(2)
81.6(2)
Results and discussion
Synthesis and spectroscopy
C(10)–B–N(1)–N(2)
C(10)–B–N(3)–N(4)
C(10)–B–N(5)–N(6)
N(3)–B–C(10)–C(11)
a 2a: Tl(1) ؒ ؒ ؒ Tl(1a); 2b: Tl ؒ ؒ ؒ TlB. b Values in brackets [ ] refer to the
disordered (MeC5H3)Mn(CO)3 unit.
174.8(3)
168.0(3)
Ϫ76.8(4)
Ϫ88.3(4)
173.6(8) [Ϫ174.8(9)]b
173.4(8) [Ϫ168.2(10)]b
96.7(9) [66.6(11)]b
Cymantrene was borylated with boron tribromide in hexane as
described previously by Siebert et al.11 The resulting dibromo-
boryl derivative 1a (Scheme 1) was obtained in moderate yield
after prolonged heating. The use of (methyl)cymantrene led to
shorter reaction times and improved yield (1b), which can be
attributed to an increased reactivity towards electrophilic sub-
stitution due to the electron-releasing effect of the methyl
group. Reaction of 1a and 1b with 3 equiv. of pyrazole and
2 equiv. of NEt3 gave the corresponding free acids; subsequent
deprotonation with TlOEt yielded the desired Tl() complexes
2a and 2b. In a similar way, the derivatives 2c and 2d have been
synthesised. Metal complexes of 2c are expected to provide
single crystals of particularly good quality, while 2d is highly
soluble, even in hexane.
Ϫ95.6(12) [Ϫ151.6(15)]b
The 11B NMR spectra of 2a–2d show resonances in the range
between Ϫ0.2 and 5.0 ppm, typical of tetracoordinate boron
nuclei.12 In the 1H and 13C NMR spectra of 2a, two resonances
are observed for the cyclopentadienyl ring, whereas the pyr-
azolyl fragments give rise to three signals. This points towards a
molecular structure of high average symmetry in solution. The
13C NMR resonance of the carbonyl ligands could not be
detected. However, the IR spectrum of 2a revealed three char-
acteristic bands for the CO stretching modes at ν(CO) = 2019,
1944 and 1922 cmϪ1. Similar signal patterns are found in the
NMR and IR spectra of all other Tl() scorpionates under
investigation here.
Fig. 2 Polymeric structure of 2a in the solid state.
The resonances of the pyrazolyl protons, which are broad
and poorly resolved at room temperature, sharpen up con-
siderably, when the sample is heated. This effect has therefore
to be attributed to a hindered rotation about the B–C bond, as
has already been reported previously for the related ferrocene-
based tris(1-pyrazolyl)borates.7 The barrier to rotation, how-
ever, appears to be smaller in the cymantrenyl scorpionates as
compared to their ferrocene analogues.
0.432(5)]. In both cases, each ligand binds to one Tl() ion in an
η2 fashion, while the third pyrazolyl ring is coordinated to
another (symmetry-related) Tl() ion (Figs. 4 and 5).
The main difference between the repeat units of 2a and 2b lies
in the relative orientation of the pyrazolyl ring [N(5) to C(9)]
with respect to the rest of the molecule, as evidenced by a tor-
sion angle C(10)–B–N(5)–N(6) of Ϫ76.8(4)Њ in 2a and 96.7(9)Њ
[66.6(11)Њ] in 2b. In both compounds, significant variations
between the individual Tl–N bond lengths are observed [2a:
Tl–N(2) = 2.654(3) Å, Tl–N(4) = 2.722(3) Å with ∆(Tl–N) =
0.068 Å; 2b: Tl–N(4) = 2.582(9) Å, Tl–N(6a) = 2.682(7) Å with
∆(Tl–N) = 0.100 Å]. The longest Tl–N distance [Tl–N(4)] in 2a
is found for the chelating fragment, while the longest bond in 2b
is established between the Tl() ion and the nitrogen atom of
the η1 coordinated pyrazole ring [Tl–N(6a)]. The Tl–N bonds
thus appear to be weak, and the corresponding potential energy
minima are probably rather shallow. This is in accord with the
1H and 13C NMR signal patterns observed for 2a and 2b (see
above), which suggest the scorpionate fragment to possess an
average C3 symmetry in solution. This can best be explained by
the assumption that any oligomeric structures are restricted to
Crystal structure determination
X-Ray quality crystals of 2a and 2b were grown from their
toluene solutions at Ϫ25 ЊC (Table 1).
The cymantrene derivative forms polymeric chains in the
solid state (Fig. 2), while cyclic tetramers are found in the case
of the (methyl)cymantrene complex (Fig. 3).
This provides an impressive example of the way in which
subtle changes at the periphery of a molecule can influence the
crystal packing. On the other side, the structural motifs exhib-
ited by the individual monomeric fragments of 2a and 2b are
very similar [Note: the (methyl)cymantrenyl substituent of 2b is
disordered over two positions; occupancy factors: 0.568(5) and
J. Chem. Soc., Dalton Trans., 2001, 3572–3576
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Fig. 5 Monomeric unit of 2b in the solid state. Atoms N6A and
TlB correspond to the symmetry operations ¾ ϩ y, ¼ Ϫ x, ¼ Ϫ z and
¼ Ϫ y, Ϫ¾ ϩ x, ¼ Ϫ z, respectively.
Fig. 3 Cyclic tetramer of 2b in the solid state.
to one Tl() ion in an η2 fashion, while the third pyrazolyl ring
is coordinated to another Tl() ion. The results of the X-ray
crystal structure determinations of 2a and 2b nicely support
our previous interpretation of the solid state structures of
various ferrocenyl scorpionates:8 Bulky substituents attached
to the boron centre apparently disfavour an η3 binding mode of
the scorpionate ligand, probably due to steric congestion. As
a consequence, oligomeric structures may be established in
complexes with weak metal–nitrogen bonds.
Even though the carbonyl ligands of cymantrene are easily
replaceable either thermally or photochemically by various
Lewis bases,13 the scorpionate complexes 2a–2d are stable at
ambient temperature and upon exposure to daylight. No self-
polymerisation with Mn–pyrazole adduct formation and CO
liberation is observed.
Experimental
General considerations
All reactions and manipulations of air-sensitive compounds
were carried out in dry, oxygen-free argon using standard
Schlenk apparatus. Toluene and hexane were freshly distilled
under N2 from potassium–benzophenone prior to use. NMR:
Bruker ACP 200, Bruker DPX 250, Bruker AMX 400 spectro-
meters. Abbreviations: s = singlet; d = doublet; t = triplet; vtr =
virtual triplet; mult = multiplet; n.r. = multiplet expected in the
1H NMR spectrum but not resolved; n.o. = signal not observed;
Cym = cymantrenyl, CymЈ = (methyl)cymantrenyl, Cp = cyclo-
pentadienyl, CpЈ = (methyl)cyclopentadienyl, Cyh = cyclohexyl,
pz = pyrazolyl. Elemental analyses were performed by the
microanalytical laboratory of the University of Frankfurt.
Complexes 1a and 1b,11 4-bromopyrazole14 and 4-(cyclo-
hexyl)methylpyrazole15 have been synthesised according to
literature procedures.
Fig. 4 Monomeric unit of 2a in the solid state. Displacement
ellipsoids are drawn at the 50% probability level. Atom N6A
corresponds to the symmetry operation Ϫx, y Ϫ ½, ½ Ϫ z.
the solid state, while monomeric thallium scorpionates are
formed when 2a and 2b are dissolved in benzene or DMSO.
Conclusion
Cymantrene-based tris(1-pyrazolyl)borates are readily access-
ible from the reaction of CymBBr2 and CymЈBBr2 with various
derivatives of pyrazole in the presence of triethylamine. Rather
peculiar structures have been found for their Tl() complexes in
the solid state. In the case of [CymB(pz)3Tl]∞ (2a, space group:
P21/c), polymeric chains are formed, while the crystal lattice of
[CymЈB(pz)3Tl]4, (2b, space group: I41/a), consists of macro-
cyclic tetramers. In spite of considerable differences in the crys-
tal packing, the individual monomeric units of 2a and 2b show
very similar structural motifs. In both cases, each ligand binds
Syntheses
Synthesis of 2a. A toluene solution (20 ml) of pyrazole
(1.00 g, 14.69 mmol) was added dropwise with stirring to a
toluene (30 ml) solution of 1a (1.83 g, 4.90 mmol) at Ϫ78 ЊC.
Afterwards, neat NEt3 (0.99 g, 9.78 mmol) was added, the reac-
tion mixture was allowed to warm to room temperature and
stirred overnight. Insoluble material was removed using a filter
cannula, the filtrate was cooled to 0 ЊC, treated with neat TlOEt
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J. Chem. Soc., Dalton Trans., 2001, 3572–3576

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Table 2 Crystallographic data for 2a and 2b [(MeC5H3)Mn(CO)3 disordered over two positions]
Compound
2a
2b
Formula
M
C17H13BMnN6O3Tlؒ0.5C7H8
665.52
C18H15BMnN6O3TlؒC7H8
725.61
Crystal system
Space group
Monoclinic
P21/c
Tetragonal
I41/a
a/Å
b/Å
c/Å
10.2126(8)
9.6558(5)
22.902(2)
94.674(5)
2250.9(3)
4
15.2801(10)
15.2801(10)
45.333(7)
90
10584.5(18)
16
β/Њ
V/Å3
Z
Dc/g cmϪ3
1.964
1.821
F(000)
1268
77.46
58.06
5600
65.97
63.22
µ(Mo-Kα)/cmϪ1
2θmax/Њ
Measured reflections
Unique reflections (Rint
Observed reflections [I > 2σ(I )]
Parameters refined
35811
5330 (0.0397)
4637
96081
8270 (0.0909)
4367
)
307
323
R1 [I > 2σ(I )]
0.0322
0.0574
1.149
0.99, Ϫ0.75
0.0873
0.1997
1.825
4.08, Ϫ2.49
wR2 [I > 2σ(I )]
GOOF on F 2
Largest diff. peak and hole/e ÅϪ3
(1.22 g, 4.89 mmol) and stirred at room temperature for 12 h,
whereupon a colourless precipitate gradually formed. The
crude product was collected on a frit, triturated with hexane
(2 × 15 ml) and recrystallised from toluene to yield analytically
pure 2a. Yield: 2.22 g (73%). IR (KBr, cmϪ1): ν(CO) 2019 (vs),
NEt3 (0.77 g, 7.60 mmol) and TlOEt (0.95 g, 3.80 mmol). Yield:
1.47 g (42%). IR (TlBr, cmϪ1): ν(CO) 2012 (vs), 1926 (vs), 1914
(shoulder). 11B NMR (128.3 MHz, C6D6): δ 1.1 (h = 450 Hz).
½
1H NMR (250 MHz, C6D6): δ 0.83–0.92, 1.04–1.18, 1.31–1.49
(3 × mult, 6H, 9H, 3H, Cyh-H), 1.54 (s, 3H, CH3), 1.61–1.74
(mult, 15H, Cyh-H), 2.32 (d, 6H, JHH = 6.9 Hz, CH2), 4.17,
4.54 (2 × vtr, 1H, 2H, CpЈ-H2,4,5), 7.24, 7.71 (2 × s, 2 × 3H,
pz-H3,5). 13C NMR (62.9 MHz, C6D6): δ 13.3 (CH3), 26.6
(Cyh-C3,5), 26.9 (Cyh-C4), 32.6 (CH2), 33.4 (Cyh-C2,6), 39.6
(Cyh-C1), 83.9, 88.5, 89.6 (CpЈ-C2,4,5), 118.4 (pz-C4), 134.8,
140.0 (pz-C3,5); n.o.: CpЈ-C1,3, CO. MS(ESI): m/z 717 (30)
[M Ϫ Tl]Ϫ. Calc. for C39H51BMnN6O3Tl [921.92]: C, 50.81; H,
5.57; N, 9.11. Found: C, 50.50; H, 5.29; N, 8.86%.
1944 (vs), 1922 (vs). 11B NMR (128.3 MHz, C6D6): δ Ϫ0.2 (h =
½
1
70 Hz). H NMR (400 MHz, C6D6): δ 4.16, 4.40 (2 × n.r., 2 ×
2H, Cp-H), 6.14 (n.r., 3H, pz-H4), 7.28, 7.69 (2 × n.r., 2 × 3H,
pz-H3,5). 13C NMR (100.5 MHz, C6D6): δ 84.4, 89.3 (Cp-C),
105.0 (pz-C4), 135.6, 139.7 (pz-C3,5); n.o.: Cp-C1, CO.
MS(CI): m/z 620 (12) [M]ϩ, 415 (45) [M Ϫ Tl]ϩ. Calc. for
C17H13BMnN6O3Tl [619.46]: C, 32.96; H, 2.12; N, 13.57.
Found: C, 32.91; H, 2.47; N, 13.94%.
Synthesis of 2b. 2b was prepared similarly to 2a from 1b
(0.98 g, 2.53 mmol), pyrazole (0.52 g, 7.64 mmol), NEt3 (0.51 g,
5.04 mmol) and TlOEt (0.63 g, 2.53 mmol). Yield: 1.22 g (76%).
IR (TlBr, cmϪ1): ν(CO) 2003 (vs), 1933 (vs), 1917 (vs). 11B NMR
Crystal structure determinations of 2a and 2b
X-Ray quality crystals were grown by storing a toluene solution
of 2a or 2b at Ϫ25 ЊC for one week. Crystal data and details of
the structure determinations are summarised in Table 2; plots
of the molecular structures of 2a and 2b are shown in Figs. 2
and 3. Compound 2a crystallises together with 0.5 equiv. of
toluene, compound 2b crystallises together with one equiv. of
toluene. The measurements were performed at 173 K (2a)
and 144 K (2b) using a SIEMENS SMART CCD diffract-
ometer with graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å). A pale yellow plate with dimensions 0.12 × 0.18 ×
0.38 mm was used for 2a, an orange block with dimensions
0.24 × 0.60 × 0.68 mm was used for 2b. Empirical absorption
corrections were made. The structures were determined by
direct methods using the program SHELXS16 (2a) and the pro-
gram SIR-9217 (2b). Hydrogen atoms were placed at calculated
positions and were not refined. All atoms of the [(MeC5H3)-
Mn(CO)3] fragment of 2b were refined with a split atom model,
since this entire group was found to be disordered over two
possible positions; the occupancy factor was 0.568(5) vs.
0.432(5). Distance constraints were used for the disordered
[(MeC5H3)Mn(CO)3] fragment and the toluene solvate groups.
CCDC reference numbers 165815 and 165816.
(128.3 MHz, [D6]DMSO): δ 0.2 (h = 70 Hz). 1H NMR
½
(250 MHz, C6D6): δ 1.50 (s, 3H, CH3), 4.09, 4.34 (vtr, mult, 1H,
2H, CpЈ-H2,4,5), 6.14 (vtr, 3H, JHH = 2.0 Hz, pz-H4), 7.28, 7.76
(2 × d, JHH = 1.4, 2.5 Hz, pz-H3,5). Significantly different chem-
ical shifts are observed in [D6]DMSO, e.g. δ 4.62, 4.86, 5.09 (3 ×
n.r., 3 × 1H, CpЈ-H2,4,5). 13C NMR (100.5 MHz, [D6]DMSO):
δ 12.9 (CH3), 78.9, 81.2, 89.5, 90.3 (CpЈ-C2,3,4,5), 102.2
(pz-C4), 132.6, 138.2 (pz-C3,5); n.o.: CpЈ-C1, CO. MS(ESI):
m/z 429 (100) [M Ϫ Tl]Ϫ. Calc. for C18H15BMnN6O3Tl [633.47]:
C, 34.13; H, 2.39; N, 13.27. Found: C, 34.44; H, 2.65; N,
13.47%.
Synthesis of 2c. 2c was prepared similarly to 2a from 1b
(1.30 g, 3.35 mmol), 4-bromopyrazole (1.48 g, 10.06 mmol),
NEt3 (0.68 g, 6.72 mmol) and TlOEt (0.84 g, 3.37 mmol). Yield:
1.93 g (66%). IR (TlBr, cmϪ1): ν(CO) 2016 (vs), 1933 (vs), 1917
(shoulder). 11B NMR (128.3 MHz, [D6]DMSO): δ 5.0 (h
=
½
250 Hz). 1H NMR (250 MHz, C6D6): δ 1.36 (s, 3H, CH3), 3.92,
3.98, 4.07 (3 × n.r., 3 × 1H, CpЈ-H2,4,5), 7.16, 7.68 (2 × s, 2 ×
3H, pz-H3,5). 13C NMR (62.9 MHz, [D6]DMSO): δ 13.5 (CH3),
82.1, 89.9, 90.5 (CpЈ-C2,4,5), 90.1 (pz-C4), 133.0, 139.4
(pz-C3,5); n.o.: CpЈ-C1,3, CO. MS(ESI): m/z 663 (100) [M Ϫ
Tl]Ϫ. Calc. for C18H12BBr3MnN6O3Tl [870.08]: C, 24.85; H,
1.38; N, 9.66. Found: C, 24.84; H, 1.67; N, 9.38%.
See http://www.rsc.org/suppdata/dt/b1/b105692c/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
Synthesis of 2d. 2d was prepared similar to 2a from 1b (1.47 g,
3.80 mmol), 4-(cyclohexyl)methylpyrazole (1.87 g, 11.40 mmol),
This research was supported by the Deutsche Forschungs-
gemeinschaft (DFG).
J. Chem. Soc., Dalton Trans., 2001, 3572–3576
3575

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10 C. Janiak, L. Braun and F. Girgsdies, J. Chem. Soc., Dalton Trans.,
1999, 3133.
References
1 S. Trofimenko, Chem. Rev., 1993, 93, 943.
11 T. Renk, W. Ruf and W. Siebert, J. Organomet. Chem., 1976, 120,
2 S. Trofimenko, Scorpionates–The Coordination Chemistry of
Polypyrazolylborate Ligands, Imperial College Press, London, 1999.
3 F. T. Edelmann, Angew. Chem., Int. Ed., 2001, 40, 1656.
4 K. Niedenzu, J. Serwatowski and S. Trofimenko, Inorg. Chem., 1991,
30, 524.
1.
12 H. Nöth and B. Wrackmeyer, Nuclear Magnetic Resonance
Spectroscopy of Boron Compounds, in NMR Basic Principles and
Progress, eds. P. Diehl, E. Fluck and R. Kosfeld, Springer, Berlin,
1978.
5 C. P. Brock, M. K. Das, R. P. Minton and K. Niedenzu, J. Am.
Chem. Soc., 1988, 110, 817.
6 F. Jäkle, K. Polborn and M. Wagner, Chem. Ber., 1996, 129, 603.
7 F. Fabrizi de Biani, F. Jäkle, M. Spiegler, M. Wagner and P. Zanello,
Inorg. Chem., 1997, 36, 2103.
8 E. Herdtweck, F. Peters, W. Scherer and M. Wagner, Polyhedron,
1998, 17, 1149.
9 S. L. Guo, F. Peters, F. Fabrizi de Biani, J. W. Bats, E. Herdtweck,
P. Zanello and M. Wagner, Inorg. Chem., 2001, 40, 4928.
13 W. Strohmeier, Angew. Chem., Int. Ed. Engl., 1964, 3, 730.
14 E. Buchner and M. Fritsch, Liebigs Ann. Chem., 1893, 273,
256.
15 B.-R. Tolf, J. Piechaczek, R. Dahlbom, H. Theorell, Å. Åkeson and
G. Lundquist, Acta Chem. Scand., Ser. B, 1979, 33, 483.
16 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
17 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C.
Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994, 27,
435.
3576
J. Chem. Soc., Dalton Trans., 2001, 3572–3576