Modular approach to tridentate N,O,N′ ligands using pyrazolylborate chemistry

Article abstract of DOI:10.1002/chem.200600142

Two anionic tridentate N,O,N′ chelators, [pz(Ph)B(μ-pz)(μ-O) B(Ph)pz]^- (3^-) and [pz^ph(Ph)B(μ-pz)(μ-O) B(Ph)pz^ph]^- (4^-), as well as the corresponding complexes [Fe(3)(py)Cl], [Fe(3)Cl_2<

Full text of DOI:10.1002/chem.200600142

FULL PAPER
Chem. Eur. J. 2006, 12, 4735 – 4742
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4735

DOI: 10.1002/chem.200600142
Modular Approach to Tridentate N,O,N’ Ligands Using Pyrazolylborate
Chemistry
Susanne Bieller, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*[a]
Abstract: Two anionic tridentate
N,O,N’ chelators, [pz(Ph)B(m-pz)(m-
O)B(Ph)pz]À (3À) and [pzPh(Ph)B
(m-
(m-O)B(Ph)pzPh]À (4À), as well as
corresponding complexes
[Fe(3)Cl2] and
[Cu(3)Cl], have been synthesised and
structurally characterised by X-ray
crystallography (pz: pyrazolyl, pzPh: 3-
phenylpyrazolyl, py: pyridine). Since
our synthesis approach takes advantage
of the highly modular pyrazolylborate
chemistry, inexpensive and relatively
resistant N,O,N’ ligands of varying
steric demand are readily accessible.
The complexes [Fe(3)(py)Cl] and
[Fe(3)Cl2] possess a distorted trigonal-
bipyramidal configuration withthe pyr-
azolyl rings occupying equatorial posi-
tions and the oxygen donor being lo-
cated at an apical position. The com-
plex [Cu(3)Cl] crystallises as chloro-
bridged dimers featuring CuII ions with
ligand environments that are inter-
mediate between a square-planar and a
trigonal-bipyramidal geometry.
G
ACHTREUNG
AHCTREUNG
pz)
the
ACHTREUNG
[Fe(3)(py)Cl],
Keywords: boron · chelates · ligand
design · N,O ligands · pyrazolyl-
AHCTREbUNG orates
Introduction
steric properties of the ligand framework can be fine-tuned
almost at will.
In the field of homogeneous catalysis, tridentate ligands are
widely used in order to create tailor-made molecular envi-
ronments for the catalytically active metal ion and thereby
to influence the (stereo)selectivity of the catalysed reaction
in the desired way. Two fundamentally different classes of
tridentate chelators have to be distinguished: trans- and cis-
coordinating ligands. Among the former, pincer-type mole-
Given this background, it is our goal to exploit the advan-
tages of pyrazolylborate chemistry also for the preparation
ACHTREUNG
cules A,[1–3] 2,6-bis(imino)pyridines B,[4–8] as well as chiral
pyridine-2,6-bis(oxazoline) ligands (Pybox, C)[9] turned out
to provide versatile platforms for the generation of many
and diverse derivatives that can be specifically designed for
numerous catalytic applications (Figure 1). The development
of cis-coordinating ligands was greatly advanced by the dis-
covery of poly(pyrazol-1-yl)borate (“scorpionate”) donors
D[10,11] 40 years ago (Figure 1). Since then, scorpionate li-
gands have enjoyed tremendous success, because they are
readily accessible by means of a highly modular assembly.
The scorpionate ligands allow for extensive variation of the
substituents R1–R3, by which both the electronic and the
[a] S. Bieller, Dr. M. Bolte, Dr. H.-W. Lerner, Prof. Dr. M. Wagner
Institut für Anorganische Chemie
Figure 1. Tridentate meridionally coordinating pincer (A), 2,6-bis-
Johann Wolfgang Goethe-Universität Frankfurt am Main
Max-von-Laue-Strasse 7, 60438 Frankfurt (Germany)
Fax : (+49)69-798-29260
ACHTRE(UGN imino)pyridine (B), and pyridine-2,6-bis(oxazoline) (C) ligands; triden-
tate facially coordinating tris(pyrazol-1-yl)borate ligands (D; substituents
R1–R3 on two pyrazolyl rings not shown); proposed tridentate meridio-
nally coordinating bis(pyrazol-1-yl)borate ligands (E).
E-mail: matthias.wagner@chemie.uni-frankfurt.de
4736
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Chem. Eur. J. 2006, 12, 4735 – 4742

Pyrazolylborate Chemistry
FULL PAPER
of molecules withthe potential to be trans-chelating ligands.
We have selected compounds of the general structure E
(Figure 1) as promising target molecules for which many of
the established design principles in scorpionate chemistry
are expected to be still valid. Appropriate choice of substitu-
ents R1 thus offers the possibility to adjust the steric
demand of E, whereas substituents R2 can be employed to
influence its electronic properties. To meet the specific re-
quirements of the coordinated complex fragment, the
system may further be optimised by varying the central
donor site X (e.g., X=O, S, NH).
THF mixture at reflux temperature leads to the clean as-
sembly of ligand 3À through a transamination reaction. In
contrast, according to a study conducted by Niedenzu
et al.,[12] 2 reacts with2 equivalents of Hpz in the absence of
Lipz to form the bridged pyrazabole PhB
(Ph)O]BPh as the major product besides substantial quanti-
ties of the 1,3-diboroxane derivative [pz(Ph)B(m-
ACHRTUNEG(m-pz)2ACHTREUNG[m-OB-
AHCTREUNG
ACHTREUNG
pz)2B(Ph)]2O. In the latter compound, two pyrazabole moi-
eties are linked by an oxygen atom. NMR spectroscopy on
the crude product 3À excludes the formation of cis-/trans-iso-
mers in which the two pyrazolyl substituents would be locat-
ed either at the same side or at opposite sides of the [B
ACHTREUNG(m-
pz)(m-O)] plane. Our current working hypothesis is that the
AHCTREUNG
Results and Discussion
observed cis-selectivity of the reaction (cf. X-ray crystallog-
raphy) may be due to a template effect of the Li+ counter-
ion. To prove the versatility of our synthesis approach, we
have next prepared ligand 4À bearing phenyl substituents at
the 3-positions of its terminal pyrazolyl groups. First,
1 equivalent of Lipz was added to 2 in toluene/THF, the
mixture was then refluxed for 8 h to form the likely inter-
To restrict the conformational flexibility of the ligand back-
bone, we decided to introduce a rigid pyrazolide bridge be-
tween the two boron atoms of E as a substitute for two dan-
gling R side chains. Moreover, an oxygen atom was chosen
as third donor X to make the ligand as resistant as possible
to hydrolysis. These considerations led to the selection of
the monoanionic compound 3À (Scheme 1) as promising ini-
mediate Li
ACHTRE[UNG Me2N(Ph)BACHERT(UGN m-pz)ACHTRE(UNG m-O)B(Ph)NMe2], which was
Ph
subsequently treated with2 equivalents of Hpz
to give 4À
in decent yield (Scheme 1, R=Ph). The successful synthesis
of 4À is important for perspective applications of our ligand
system in homogeneous catalysis, because steric protection
of the active site is often a decisive factor in catalyst design.
For example, Brookhart[4] and Gibson[5] have shown that the
performance as ethylene polymerisation catalysts of 2,6-bis-
AHCTREUNG
(imino)pyridine-based FeII and CoII complexes crucially de-
pends on the presence of bulky aryl substituents R at the
imino nitrogen atoms (cf. B, Figure 1). The FeII, FeIII and
CuII complexes of ligand Li-3 are readily accessible from
THF upon addition of anhydrous FeCl2, FeCl3 and CuCl2,
respectively (cf. 5–7, respectively; Scheme 2). In the case of
Scheme 1. Synthesis of the tridentate N,O,N’ ligands Li-3 and Li-4. i) H2O
(0.5 equiv), NEt3 (1 equiv); toluene, RT; ii) 1. Lipz (1 equiv), 2. HpzR
(2 equiv); toluene/THF, reflux temperature.
tial system for the development of trans-chelating scorpi-
ACHTREUNG
Scheme 2. Synthesis of the metal complexes 5–7. i) 5: FeCl2 (1 equiv);
THF/pyridine, RT; 6: FeCl3 (1 equiv); THF, RT; 7: CuCl2 (1 equiv); THF,
out to be a proper starting material for the preparation of
3À (Scheme 1). Following the published procedure,[12] 2 can
be obtained in 54% yield from the reaction of B,B’,B’’-tri-
phenylboroxin, [-B(Ph)O-]3, and bis(dimethylamino)phenyl-
RT.
the FeII complex 5, it was necessary to add a small amount
of pyridine to obtain a well-defined single-crystalline prod-
uct.
borane, PhBACHTREUNG(NMe2)2. However, we preferred to synthesise 2
by hydrolysis of bromo(dimethylamino)phenylborane,
PhB(Br)NMe2,[13,14] with0.5 equivalents of water in the pres-
ence of triethylamine (yield: 82%, cf. Experimental Sec-
tion). Treatment of 2 with1 equivalent of lithium pyrazolide
(Lipz) and 2 equivalents of pyrazole (Hpz) in a toluene/
NMR spectroscopy: The 11B NMR spectra of ligands Li-3
and Li-4 are characterised by one single resonance at d=
5.4 ppm testifying[15,16] to the presence of four-coordinate
Chem. Eur. J. 2006, 12, 4735 – 4742
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4737

M. Wagner et al.
1
boron nuclei. In its H NMR spectrum, Li-3 gives rise to two
sets of pyrazolyl signals at d=6.27, 7.55, 7.59 ppm (intensity
ratio 2:2:2) and d=6.58, 7.86 ppm (intensity ratio 1:2).
These data are in accord with a molecular framework con-
taining one symmetrically coordinated and two magnetically
equivalent but unsymmetrically coordinated pyrazolyl rings.
This interpretation is further supported by the 13C NMR
spectrum of Li-3. However, it is difficult to decide at this
stage whether Li-3 possesses cis- or trans configuration. As
to be expected, the NMR parameters of Li-3 and Li-4 are
similar, but for the fact that Li-4 shows only two resonances
for the terminal pyrazolyl fragments and that a second set
1
of phenyl resonances can clearly be identified both in the H
and 13C NMR spectra. As a result of their paramagnetic
nature, meaningful NMR spectra of the complexes 5–7
could not be acquired.
X-ray crystallography: All the compounds under investiga-
tion were structurally characterised by X-ray crystallogra-
phy. Selected crystal data and structure refinement details
are compiled in Table 1. The lithium salt Li-3 crystallises
from THF/hexane with one THF molecule coordinated to
the Li+ ion (Li
ACHTREUNG(thf)-3, triclinic space group P1; Figure 2).
¯
Figure 2. ORTEP plot (50% probability ellipsoids) of Li
omitted for clarity. Selected bond lengths [Š], bond angles [8] and torsion
ACHTRE(UNG thf)-3; H atoms
À
À
À
À
angles [8]: Li1 N12 2.023(4), Li1 N22 2.034(4), Li1 O1 1.940(4), Li1
À
À
À
À
O61 1.888(4), B1 N11 1.566(3), B1 N51 1.585(3), B1 O1 1.448(2), B2
À
À
N21 1.573(3), B2 N52 1.584(3), B2 O1 1.445(2); N12-Li1-N22 115.9(2),
N12-Li1-O1 88.8(2), N22-Li1-O1 88.3(2), O1-Li1-O61 123.8(2), B1-O1-
B2 115.9(2), N51-B1-O1 101.7(2), N52-B2-O1 101.6(2), O1-B1-C31
116.1(2), O1-B2-C41 116.6(2); O1-B1-N11-N12 15.5(2), O1-B2-N21-N22
À11.4(2), B1-N11-N12-Li1 À11.6(2), B2-N21-N22-Li1 1.6(2).
The X-ray crystal structure analysis of LiACHTRE(UNG thf)-3 confirms the
proposed structure of the ligand framework, with one bridg-
ing and two terminal pyrazolyl groups. Most importantly,
the structure analysis also shows that the latter are located
in a mutual cis-arrangement thereby enabling 3À to coordi-
nate the Li+ ion in a tridentate fashion through two nitro-
gen atoms and one oxygen donor. The ligand sphere of the
Li+ centre is distorted from a tetrahedral geometry in the
direction of a square-planar environment withthe N-Li1-O1
bond angles being contracted to about 908 while the N12-
Li1-N22 and the O1-Li1-O61 angles are expanded to
115.9(2)8 and 123.8(2)8, respectively. The dihedral angle be-
tween the two pyrazolyl rings amounts to 109.78. Despite of
the supporting chelate effect and the electrostatic attraction,
À
À
the Li O contact to 3 is 0.052 Š longer than the bond to
Table 1. Crystal data and structure refinement details for Li
ACHRTEUNG(thf)-3, LiAHCTRE(UGN thf)-4, 5, 6 and [(7)2].
Li(thf)-3 Li(thf)-4
A
G
5
6
[(7)2]
formula
Mr
C25H27B2LiN6O2
472.09
C37H35B2LiN6O2·0.5C6H14
667.36
C26H24B2ClFeN7O·C4H8O
635.55
C21H19B2Cl2FeN6O
519.79
C42H38B4Cl2Cu2N12O2
984.06
colour, shape
T [K]
colourless, block
173(2)
colourless, block
173(2)
colourless, block
173(2)
dark yellow, block
173(2)
blue, needle
173(2)
crystal system
space group
a [Š]
b [Š]
c [Š]
triclinic
P1
triclinic
P1
monoclinic
P21
10.5983(9)
8.9755(4)
16.6678(12)
90
101.508(6)
90
1553.65(19)
2
monoclinic
P21/c
8.9100(4)
14.0959(4)
18.8314(9)
90
96.752(4)
90
2348.72(17)
4
monoclinic
P21/c
17.1292(13)
16.2248(10)
17.1173(13)
90
110.275(6)
90
4462.4(6)
4
¯
¯
9.5835(10)
9.8411(10)
14.4380(15)
74.836(8)
75.329(8)
74.406(8)
1241.1(2)
2
9.8802(5)
14.1338(7)
14.3611(7)
78.950(4)
72.460(4)
76.886(4)
1845.86(16)
2
a [8]
b [8]
g [8]
V [Š3]
Z
1calcd [gcmÀ3
]
1.263
1.201
1.359
1.470
1.465
F
A
496
706
660
1060
2008
]
0.081
0.074
0.611
0.896
1.125
crystal size [mm]
reflns collected
0.52”0.48”0.46
22472
0.39”0.32”0.25
50552
0.17”0.15”0.11
19796
0.27”0.24”0.22
50938
0.26”0.07”0.05
58911
independent reflns (Rint
)
4647 (0.0511)
4647/0/325
1.079
8113 (0.0326)
8113/0/461
1.059
5754 (0.0831)
5754/1/388
1.041
4411 (0.0376)
4411/0/298
1.057
8404 (0.1740)
8404/6/577
1.170
data/restraints/parameters
GOOF on F2
R1/wR2 [I>2s(I)]
R1/wR2 (all data)
largest diff. peak/hole [eŠ
0.0555/0.1505
0.0674/0.1571
0.618/À0.641
0.0531/0.1427
0.0558/0.1450
0.418/À0.354
0.0533/0.0904
0.0691/0.0950
0.246/À0.385
0.0274/0.0653
0.0307/0.0667
0.320/À0.344
0.1347/0.3070
0.1662/0.3219
3.254/À2.019
À3
]
4738
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Chem. Eur. J. 2006, 12, 4735 – 4742

Pyrazolylborate Chemistry
FULL PAPER
À
À
the THF ligand (Li1 O1=1.940(4) Š, Li1 O61=
À
1.888(4) Š). The B N bond lengths lie in the usually ob-
served range for pyrazaboles and poly(pyrazol-1-yl)bo-
rates.[17] The B O bonds are also unexceptional. Most of the
À
corresponding bond angles are close to the ideal value of
1098 expected for sp3-hybridised boron atoms. The largest
deviations are found for the two O-B-C angles (mean
values: 116.48) and the (m-pz)N-B-O angles, which, as a
result of their incorporation into the five-membered ring,
are rather acute (N51-B1-O1=101.7(2)8, N52-B2-O1=
101.6(2)8). X-ray crystallography on Li-4, which crystallises
¯
as a THF adduct LiACHTRE(UNG thf)-4 (triclinic space group P1;
Figure 3), provides conclusive evidence that the unsubstitut-
Figure 4. ORTEP plot (50% probability ellipsoids) of 5; H atoms omitted
for clarity. Selected bond lengths [Š], bond angles [8] and torsion angles
À
À
À
À
[8]: Fe1 N12 2.082(3), Fe1 N22 2.084(4), Fe1 N61 2.180(4), Fe1 O1
À
2.179(3), Fe1 Cl1 2.292(1); N12-Fe1-N22 117.9(2), N12-Fe1-N61 93.7(1),
N12-Fe1-O1 78.4(1), N12-Fe1-Cl1 119.7(1), N22-Fe1-N61 89.3(1), N22-
Fe1-O1 78.5(1), N22-Fe1-Cl1 120.4(1), N61-Fe1-O1 159.9(1), N61-Fe1-
Cl1 101.1(1), O1-Fe1-Cl1 98.9(1); O1-B1-N11-N12 5.7(5), O1-B2-N21-
N22 À3.7(5), B1-N11-N12-Fe(1) 2.8(5), B2-N21-N22-Fe1 À3.7(4).
coordinate complexes, the parameter t=(qÀf)/608[18] pro-
vides a quantitative measure of whether the ligand sphere
more closely approaches a square-pyramidal (t=0) or a
trigonal-bipyramidal geometry (t=1; q, f are the two larg-
est bond angles and q>f). In the case of 5, we have q=
N61-Fe1-O1=159.9(1)8 and f=N22-Fe1-Cl1=120.4(1)8,
suchthat t possesses a value of 0.66. The coordination ge-
ometry about the FeII ion is thus best described as distorted
trigonal bipyramidal with the pyridine molecule and the
oxygen atom occupying the apical positions. All angles be-
tween equatorial ligands are close to 1208 (N12-Fe1-N22=
Figure 3. ORTEP plot (50% probability ellipsoids) of Li
omitted for clarity. Selected bond lengths [Š], bond angles [8] and torsion
ACHTRE(UNG thf)-4; H atoms
À
À
À
À
angles [8]: Li1 N12 2.035(3), Li1 N22 2.051(2), Li1 O1 1.942(2), Li1
À
À
À
À
O61 1.920(2), B1 N11 1.578(2), B1 N31 1.592(2), B1 O1 1.455(2), B2
À
À
N21 1.579(2), B2 N32 1.587(2), B2 O1 1.458(2); N12-Li1-N22 114.9(1),
N12-Li1-O1 90.1(1), N22-Li1-O1 88.7(1), O1-Li1-O61 122.3(1), B1-O1-
B2 115.8(1); O1-B1-N11-N12 14.7(2), O1-B2-N21-N22 À1.7(2), B1-N11-
N12-Li1 À13.5(1), B2-N21-N22-Li1 À10.0(1).
117.9(2)8,
N12-Fe1-Cl1=119.7(1)8,
N22-Fe1-Cl1=
120.4(1)8). The two pyrazolyl rings include a dihedral angle
of 129.48. As to be expected for geometric and electrostatic
À
ed pyrazolyl ring indeed occupies the B,B’-bridging position,
while the 3-phenylpyrazolyl moieties act as the dangling
substituents. Evidently, no pyrazolyl scrambling has occurred
during ligand synthesis. As desired, the terminal pyrazolyl
donors are again found in a mutual cis-arrangement (dihe-
reasons, the mean Fe N(pz) bond lengthis smaller by
À
0.097 Š than the Fe1 N61=2.180(4) Š contact. According-
À
ly, the Fe N(pz) bonds of 5 are also shorter than those of
the related complex [Fe(L)Cl2][19] bearing a neutral ligand
L=2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine
(5:
mean
À
dral angle: 110.18). The key structural parameters of Li
G
value=2.083 Š; [Fe(L)Cl2]: mean value=2.158 Š). The Fe
À
4 are essentially the same as in the case of Li
A
Cl bond lengths in 5 (Fe1 Cl1=2.292(1) Š) and [Fe(L)Cl2]
À
À
the notable exception that the Li1 O61 bond in Li
A
(Fe Cl=2.304(2), 2.308(1) Š), however, are almost identical
elongated by 0.032 Š to a value of 1.920(2) Š. This finding
indicates that, similar to the chemistry of poly(pyrazol-1-yl)-
borate ligands, attachment of bulky phenyl substituents to
the terminal pyrazolyl rings leads to a measurable steric
shielding of the coordinated metal ion.
within experimental error. Complex 5 crystallises together
with1 equivalent of THF, the oxygen atom of whichforms a
short contact to one of the pyridine ortho-H atoms
((THF)O···HACHTRE(UGN pyridine)=2.662 Š).
A similar ligand arrangement as in 5 is observed in the
FeIII complex 6 (monoclinic space group P21/c; Figure 5),
apart from the fact that a chloro ligand is substituted for the
pyridine donor in the apical position (O1-Fe1-Cl2=
The FeII complex 5 (monoclinic, P21; Figure 4) contains a
five-coordinate iron centre that binds to a chloro ligand, a
pyridine molecule and the tridentate chelator 3À. For five-
Chem. Eur. J. 2006, 12, 4735 – 4742
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4739

M. Wagner et al.
amidal configuration. Steel et al. have recently published a
CuII complex [Cu(L’)Cl2] (L’=8-(2-pyridylmethoxy)quino-
line) that bears a tridentate N,O,N’ ligand withsp 2-hybri-
dised nitrogen atoms and, moreover, establishes a distorted
square-pyramidal geometry in the crystal lattice (t=0.27).[20]
A comparison of [(7)2] with[Cu(L ’)Cl2] reveals somewhat
À
shorter Cu N bond lengths in the former (mean value=
1.962 Š) than in the latter compound (mean value=
À
1.992 Š). Differences in Cu Cleq bond lengths are small be-
À
À
tween bothcompounds ([( 7)2]: Cu1 Cl2=2.294(3) Š, Cu2
À
Cl1=2.292(3) Š; [Cu(L’)Cl2]: Cu Cleq =2.244(1) Š), where-
as a large discrepancy is observed for the Cu O contacts
À
À
À
([(7)2]:
Cu1 O1=2.035(8) Š,
Cu2 O2=2.015(7) Š;
À
[Cu(L’)Cl2]: Cu O=2.170(1) Š). As an explanation, one
has again to bear in mind that [(7)2] contains anionic ligands,
whereas L’ is uncharged. Moreover, the oxygen atoms of
[(7)2] are bonded to two electropositive four-coordinate
boron atoms; this probably leads to a pronounced accumula-
tion of negative partial charge and in turn to a contraction
Figure 5. ORTEP plot (50% probability ellipsoids) of 6; H atoms omitted
for clarity. Selected bond lengths [Š], bond angles [8] and torsion angles
À
of the Cu O bond.
À
À
À
À
[8]: Fe1 N12 2.027(1), Fe1 N22 2.030(1), Fe1 O1 2.119(1), Fe1 Cl1
À
2.219(1), Fe1 Cl2 2.239(1); N12-Fe1-N22 125.2(1), N12-Fe1-O1 78.5(1),
N12-Fe1-Cl1 118.2(1), N12-Fe1-Cl2 93.3(1), N22-Fe1-O1 78.9(1), N22-
Fe1-Cl1 112.8(1), N22-Fe1-Cl2 93.1(1), O1-Fe1-Cl1 94.5(1), O1-Fe1-Cl2
161.8(1), Cl1-Fe1-Cl(2) 103.7(1); O1-B1-N11-N12 2.7(2), O1-B2-N21-N22
À0.4(2), B1-N11-N12-Fe1 À5.2(2), B2-N21-N22-Fe1 7.8(2).
Conclusion
Tris(pyrazol-1-yl)borate (“scorpionate”) ligands are a well-
established class of cis-coordinating tridentate chelators.
Their highly modular assembly makes a multitude of differ-
161.8(1)8) and that the N12-
Fe1-N22 angle is expanded to
125.2(1)8 as compared to only
117.9(2)8 in 5 (6: t=0.61; dihe-
dral angle between the pyrazol-
yl rings=139.88). Due to the
trivalent oxidation state of the
À
central iron atom, all Fe X
bond lengths are shorter in 6
than the corresponding bond
lengths in 5 (X=N, O, Cl).
The CuII complex 7 crystalli-
ses in the monoclinic space
group P21/c and forms chloro-
bridged dimers [(7)2] in th
solid state (Figure 6). The indi-
vidual monomeric units possess
pronouncedly planarised struc-
tures withbond angles N12-
Cu1-N22=150.2(4)8 and N62-
Cu2-N72=147.4(4)8. The corre-
sponding dihedral angles be-
tween the pyrazolyl rings are
153.38 and 140.68. Most impor-
tantly, both halves of the dimer
molecule have a t value of 0.46
indicating the ligand environ-
e
Figure 6. ORTEP plot (50% probability ellipsoids) of [(7)2]; H atoms omitted for clarity. Selected bond
À
À
lengths [Š], atom···atom distances [Š], bond angles [8] and torsion angles [8]: Cu1 N12 1.960(11), Cu1 N22
À
À
À
À
À
1.977(9), Cu1 O1 2.035(8), Cu1 Cl1 2.637(3), Cu1 Cl2 2.294(3), Cu2 N62 1.972(10), Cu2 N72 1.940(11),
À
À
À
Cu2 O2 2.015(7), Cu2 Cl1 2.292(3), Cu2 Cl2 2.767(4), Cu1···Cu2 3.545(2); N12-Cu1-N22 150.2(4), N12-Cu1-
O1 83.3(3), N22-Cu1-O1 84.4(4), N12-Cu1-Cl1 107.7(3), N12-Cu1-Cl2 95.3(3), N22-Cu1-Cl1 99.6(3), N22-Cu1-
Cl2 96.1(3), O1-Cu1-Cl1 90.8(2), O1-Cu1-Cl2 177.8(3), N62-Cu2-N72 147.4(4), N62-Cu2-O2 84.6(4), N72-Cu2-
O2 83.8(4), N62-Cu2-Cl1 98.3(3), N62-Cu2-Cl2 97.9(3), N72-Cu2-Cl1 95.6(3), N72-Cu2-Cl2 112.0(3), O2-Cu2-
Cl1 175.0(3), O2-Cu2-Cl2 87.6(2), Cl1-Cu1-Cl2 91.2(1), Cl1-Cu2-Cl2 88.0(1), Cu1-Cl1-Cu2 91.7(1), Cu1-Cl2-
Cu2 88.4(1); O1-B1-N21-N22 3.1(15), O1-B2-N11-N12 À11.1(15), O2-B3-N61-N62 0.3(14), O2-B4-N71-N72
À1.1(15), B1-N21-N22-Cu1 4.6(14), B2-N11-N12-Cu1 2.8(14), B3-N61-N62-Cu2 7.7(13), B4-N71-N72-Cu2
II
ment of eachCu ion in [(7)2]
to be closer to a square-pyrami-
dal rather than a trigonal bipyr- À3.2(13).
4740
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Chem. Eur. J. 2006, 12, 4735 – 4742

Pyrazolylborate Chemistry
FULL PAPER
Synthesis of 2: Water (63 mL, 0.063 g, 3.50 mmol) and triethylamine
(0.713 g, 7.05 mmol) were added to a solution of 1 (1.484 g, 7.00 mmol) in
toluene (20 mL) at RT withstirring whereupon a colourless precipitate
formed. The resulting slurry was stirred for 8 h, the precipitate collected
on a frit (G4) and the filtrate evaporated in vacuo. The off-white, slightly
oily product obtained can be used without further purification. Yield:
0.802 g (2.86 mmol, 82%); 11B NMR (128.4 MHz, CDCl3): d=29.8 ppm
(h1/2 =270 Hz); 1H NMR (250.1 MHz, CDCl3): d=2.60, 2.72 (2s, 2”6H;
CH3), 7.08–7.14 ppm (m, 10H; PhH); 13C NMR (62.9 MHz, CDCl3): d=
ent tailor-made derivatives readily available. It is our aim to
exploit the advantages of pyrazolylborate chemistry for the
development of N,O,N’ tridentate and trans-chelating li-
gands. To this end, we prepared two anionic tridentate
N,O,N’ ligands, [pz(Ph)B
[pzPh(Ph)B (m-O)B(Ph)pzPh]À (4À), and investigated
(m-pz)ACHTREUNG
(m-O)B(Ph)pz]À (3À) and
ACHRTUNEG(m-pz)ACHTREUGN
G
their coordination behaviour towards FeII, FeIII and CuII cen-
tres. Depending from the nature of the coordinated complex
fragment, 3À turned out to achieve (pz)N-M-N(pz) bite
À
36.4, 38.8 (CH3), 127.1 (PhCp), 127.4 (PhCm), 132.5 ppm (PhCo), n.o. (C
B).
angles between 117.9(2)8 and 150.2(4)8 (pz: pyrazolyl; pzPh
:
Synthesis of LiACHTER(UNG thf)-3: Lithium pyrazolide (0.847 g, 11.45 mmol) and pyr-
azole (1.556 g, 22.86 mmol) in THF (10 mL) were added to a solution of
2 (3.199 g, 11.43 mmol) in toluene (20 mL) withstirring. The resulting
solution was heated to reflux temperature for 8 h, the solvent was re-
moved in vacuo and the resulting colourless solid residue washed with
3-phenylpyrazolyl). In contrast, the corresponding bite
angles in tris(pyrazol-1-yl)borate complexes of iron and
copper are found within an interval of 808–1048[17] and 808–
1038,[17] respectively (no restrictions to oxidation state and
coordination number of the metal centres were applied). A
further flattening of the ligand framework of 3À is most
likely to be incompatible with
hexane (10 mL). X-ray quality crystals of LiACHTREUNG
ing a solution of the crude product in THF with hexane. Yield of LiACHTREUNG
3: 3.443 g (7.29 mmol, 64%); 11B NMR (128.4 MHz, CD3CN): d=
5.4 ppm (h1/2 =160 Hz); 1H NMR (400.1 MHz, CD3CN): d=6.27 (vt, 3J-
ACHRTEUNG CAHTRE(UGN H,H)=2.2 Hz, 1H; m-pzH-4),
(H,H)=1.9 Hz, 2H; pzH-4), 6.58 (t, 3J
the simultaneous presence of 1)
7.08–7.11 (m, 6H; PhHm, Ph H), 7.14–7.17 (m, 4H; PhHo), 7.55 (d, 3J-
p
an sp3 oxygen donor, 2) sp3
3
(H,H)=1.7 Hz, 2H; pzH-3 or 5), 7.59 (d, J
(H,H)=2.2 Hz, 2H; pzH-5 or
(H,H)=2.2 Hz, 2H; m-pzH-3,5); 13C NMR (100.6 MHz,
3), 7.86 ppm (d, JACHTREUNG
3
boron atoms and 3) the pyra-
CD3CN): d=106.4 (pzC-4), 110.8 (m-pzC-4), 127.4 (PhCp), 127.9 (PhCm),
131.1 (m-pzC-3,5), 132.5 (PhCo), 132.9 (pzC-5 or 3), 139.1 ppm (pzC-3 or
zolyl brace (cf. Figure 7). Com-
parable and truly trans-coordi-
nating ligands are either confor-
À
5),
n.o.
(C B);
elemental
analysis
calcd
(%)
for
C21H19B2LiN6O·C4H8O·0.5H2O (481.10): C 62.41, H 5.87, N 17.46; found:
C 62.19, H 5.67, N 17.25.
Figure 7. Structural peculiari-
mationally more flexible (e.g.,
ties of ligand 3À preventing a
[(ArNCH2CH2)2O]2À
; Ar=2,6-
Synthesis of LiACHTER(UNG thf)-4: Neat lithium pyrazolide (0.225 g, 3.04 mmol) was
fully planar tridentate coordi-
nation mode.
Me2C6H3, 2,6-iPr2C6H3)[21] or
added to a solution of 2 (0.850 g, 3.04 mmol) in toluene (20 mL) and
THF (5 mL) with stirring. After the resulting solution had been heated
to reflux temperature for 8 h, it was allowed to cool to RT for the addi-
tion of 3-phenylpyrazole (0.877 g, 6.08 mmol). The mixture was refluxed
for further 8 h, all volatiles were removed in vacuo and the resulting col-
ourless solid residue was washed with hexane (10 mL). X-ray quality
feature exclusively sp2-hybrid-
AHCTREUNG
andiyl-2,2’-bis(4-phenyloxazoline)[22] and 2,6-bis(pyrazol-1-
yl)pyridine[19,23]). Interestingly, the [(tBuN-o-C6H4)2O]2À
ligand, which is an analogue of [(ArNCH2CH2)2O]2À witha
more rigid backbone, was found to adopt a coordination
mode similar to 3À rather than to [(ArNCH2CH2)2O]2À.[24] In
summary, ligands 3À and 4À represent the first pincer-type
N,O,N’ ligands that are based on pyrazolylborate chemistry.
They are easy to derivatise and exhibit a flexible coordina-
tion mode that makes them useful in many areas of coordi-
nation chemistry.
crystals of Li
uct in THF withhexane. Yield of Li
11B NMR (128.4 MHz, CD3CN): d=5.4 ppm (h1/2 =180 Hz); 1H NMR
(250.1 MHz, CD3CN): d=6.61 (d, 3J
(H,H)=2.3 Hz, 2H; PhpzH-4), 6.65
(t, 3J
(H,H)=2.3 Hz, 1H; m-pzH-4), 7.14–7.18 (m, 6H; PhHm, Ph H),
7.23–7.27 (m, 10H; PhHo, PhpzHm, PhpzHp), 7.68–7.72 (m, 4H; PhpzHo),
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
p
7.70 (d, 3J(H,H)=2.2 Hz, 2H; PhpzH-5), 7.98 ppm (d, 3J
ACTHERNGU ACHTRE(UGN H,H)=2.3 Hz,
2H; m-pzH-3,5); 13C NMR (100.6 MHz, CD3CN): d=104.6 (PhpzC-4),
111.3 (m-pzC-4), 126.7 (PhpzCo), 127.7 (PhCp), 128.2 (PhCm), 128.4
(PhpzCp), 129.7 (PhpzCm), 131.6 (m-pzC-3,5), 132.5 (PhCo), 134.7, 134.8
À
(PhpzC-3,5), 152.0 ppm (PhpzCi), n.o. (C B); elemental analysis calcd
(%) for C33H27B2LiN6O·C4H8O·0.5C6H14 (667.36): C 71.99, H 6.34, N
12.59; found: C 71.17, H 6.24, N 12.65.
Synthesis of 5: The ligand LiACTHRE(UNG thf)-3 (0.195 g, 0.41 mmol) was dissolved in
a mixture of THF (15 mL) and pyridine (2 mL). Neat FeCl2 (0.053 g,
0.41 mmol) was added at RT. After the reaction mixture had been stirred
for 10 h, the resulting yellow solution was evaporated and the yellow resi-
due recrystallised from THF/hexane (1:3). Yield of crystalline 5: 0.123 g
(0.22 mmol, 54%); LR-MS (MALDI+): m/z (%): 563 (37) [M]+; elemen-
tal analysis calcd (%) for C26H24B2ClFeN7O·C4H8O (635.55): C 56.70, H
5.08, N 15.42; found: C 56.25, H 5.04, N 15.86.
Experimental Section
General considerations: All reactions and manipulations of air-sensitive
compounds were carried out in dry, oxygen-free nitrogen using standard
Schlenk glassware. Solvents were freshly distilled under argon from Na/
benzophenone (toluene, THF), Na/Pb alloy (hexane, heptane) or stored
over 4 Š molecular sieves prior to use (CH2Cl2, CD3CN). NMR: Bruker
Avance 400, Bruker AM 250, Bruker DPX 250 spectrometers. 11B NMR
spectra are reported relative to external BF3·Et2O. Unless stated other-
wise, all NMR spectra were run at ambient temperature. Abbreviations:
s=singlet; d=doublet; t=triplet; vt=virtual triplet; dd=doublet of dou-
blets; br=broad; n.r.=multiplet expected in the 1H NMR spectrum but
not resolved; n.o.=signal not observed; Me=methyl; Ph=phenyl; py=
pyridine, pz=pyrazolide, THF=tetrahydrofuran. Elemental analyses
were performed by the microanalytical laboratory of the University of
Frankfurt and by Quantitative Technologies Inc. (QTI). Compound 1 can
be synthesised according to a literature procedure.[13]
Synthesis of 6: Neat FeCl3 (0.069 g, 0.42 mmol) was added at RT to a sol-
ution of LiACHTREU(NG thf)-3 (0.200 g, 0.42 mmol) in THF (20 mL). After stirring for
10 h, the red solution was evaporated to dryness in vacuo. The resulting
red residue was extracted into CH2Cl2 (10 mL). Layering of the solution
of the residue in CH2Cl2 withhexane gave red crystals suitable for an X-
ray crystal structure analysis. Yield of crystalline 6: 0.154 g (0.30 mmol,
71%); elemental analysis calcd (%) for C21H19B2Cl2FeN6O (519.79): C
48.52, H 3.68, N 16.16; found: C 48.46, H 3.58, N 15.96.
Synthesis of 7: Neat anhydrous CuCl2 (0.042 g, 0.31 mmol) was added at
RT to Li
ACHTRE(UNG thf)-3 (0.145 g, 0.31 mmol) in THF (12 mL). During the first
5 min of stirring, the colour of the reaction mixture changed from blue to
Chem. Eur. J. 2006, 12, 4735 – 4742
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4741

M. Wagner et al.
green. After 10 h, the solution was evaporated and the resulting green
oily residue extracted into CH2Cl2 (10 mL). Layering of the solution of
the residue in CH2Cl2 withheptane gave turquoise crystals of [( 7)2] suita-
ble for an X-ray crystal structure analysis. Yield of crystalline 7: 0.116 g
(0.24 mmol, 77%); elemental analysis calcd (%) for C21H19B2ClCuN6O
(492.03): C 51.26, H 3.89, N 17.08; found: C 51.20, H 3.93, N 17.18.
[7] S. Park, Y. Han, S. K. Kim, J. Lee, H. K. Kim, Y. Do, J. Organomet.
Chem. 2004, 689, 4263–4276.
[8] G. J. P. Britovsek, J. England, S. K. Spitzmesser, A. J. P. White, D. J.
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[9] G. Desimoni, G. Faita, P. Quadrelli, Chem. Rev. 2003, 103, 3119–
3154.
[10] S. Trofimenko, Chem. Rev. 1993, 93, 943–980.
Crystal structure determinations of LiACHTRE(UNG thf)-3, LiACHRTE(UGN thf)-4, 5, 6 and [(7)2}:
[11] S. Trofimenko, Scorpionates—The Coordination Chemistry of Poly-
pyrazolylborate Ligands, Imperial College Press, London, 1999.
[12] J. Bielawski, K. Niedenzu, Inorg. Chem. 1986, 25, 1771–1774.
[13] P. A. Barfield, M. F. Lappert, J. Lee, J. Chem. Soc. A 1968, 554–559.
[14] C. Brown, R. H. Cragg, T. J. Miller, D. O. Smith, J. Organomet.
Chem. 1983, 244, 209–215.
Data collections were performed on a Stoe-IPDS-II two-circle diffrac-
tometer with graphite-monochromated MoKa radiation (l=0.71073 Š).
[25]
Empirical absorption corrections withthe MULABS option
in the pro-
gram PLATON[26] were performed. The structures were solved by direct
methods[27] and refined withfull-matrix least-squares on F2 using the pro-
gram SHELXL97.[28] Hydrogen atoms were placed on ideal positions and
[15] “Nuclear Magnetic Resonance Spectroscopy of Boron Compounds”:
H. Nçth, B. Wrackmeyer in NMR Basic Principles and Progress
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[17] F. H. Allen, Acta Crystallogr. Sect. B 2002, 58, 380–388 (Cambridge
Crystallographic Database, CSD Version 5.27, November 2005).
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refined withfixed isotropic displacement parameters using
a riding
model. Ligand Li(thf)-4 crystallises together with half a molecule of
ACHTREUNG
hexane in the asymmetric unit. The crystal lattice of 5 contains one
equivalent of THF solvate molecules; for 5 the absolute structure was de-
termined: Flack x parameter=0.08(2). CCDC-294679 (Li
ACHTRE(UNG thf)-3), CCDC-
294680 (Li(thf)-4), CCDC-294681 (5), CCDC-294682 (6) and CCDC-
ACHTREUNG
294683 ([(7)2]) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] F. Calderazzo, U. Englert, C. Hu, F. Marchetti, G. Pampaloni, V.
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[21] R. R. Schrock, F. Schattenmann, M. Aizenberg, W. M. Davis, Chem.
Commun. 1998, 199–200.
Acknowledgement
The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG)
and the Fonds der Chemischen Industrie (FCI) for financial support.
[22] S. Kanemasa, Y. Oderaotoshi, S. Sakaguchi, H. Yamamoto, J.
Tanaka, E. Wada, D. P. Curran, J. Am. Chem. Soc. 1998, 120, 3074–
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[23] D. L. Jameson, K. A. Goldsby, J. Org. Chem. 1990, 55, 4992–4994.
[24] R. Baumann, W. M. Davis, R. R. Schrock, J. Am. Chem. Soc. 1997,
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Received: February 2, 2006
Published online: May 19, 2006
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Chem. Eur. J. 2006, 12, 4735 – 4742