Bitopic bis- and tris(1-pyrazolyl)borate ligands: Syntheses and structural characterization

Article abstract of DOI:10.1021/om049954e

Starting from 1,4- and 1,3-diborylated benzene derivatives, bitopic bis(1-pyrazolyl)borates (K₂[1,4-tBuBpz₂)₂C ₆H₄]; (Li,K)₂[1,3-(tBuBpz₂) ₂C₆H₄]) and tris(1-pyrazolyl)borates (K ₂[1,4-(Bpz₃)₂C₆H₄]) have been synthesized (pz = pyrazolyl). X-ray single-crystal structure analyses revealed K₂[1,3-(tBuBpZ₂)₂C₆H ₄] to establish a polymeric structure in the solid state. In contrast, the unsymmetrically substituted hydrolysis product Li ₂[1,3-(tBuBpz₂)(tBuB(OH)pz)C₆H₄] forms discrete dimers in which four lithium cations are encapsulated by their organic ligands. As a representative of bitopic 1,2-diboryl benzene-based scorpionates, the 9,10-dihydro-9,10-diboraanthracene derivative K ₂[(Bpz₂)₂(C₆H₄) ₂] is described, which bears two pyrazolyl substituents at each of its boron atoms and stands out due to its stereorigidity.

Full text of DOI:10.1021/om049954e

Organometallics 2004, 23, 2107-2113
2107
Bitop ic Bis- a n d Tr is(1-p yr a zolyl)bor a te Liga n d s:
Syn th eses a n d Str u ctu r a l Ch a r a cter iza tion
Susanne Bieller,† Fan Zhang,† Michael Bolte,† J an W. Bats,‡
Hans-Wolfram Lerner,† and Matthias Wagner*,†
Institut fu¨r Anorganische Chemie, J .W. Goethe-Universita¨t Frankfurt, Marie-Curie-Strasse 11,
D-60439 Frankfurt (Main), Germany, and Institut fu¨r Organische Chemie,
J .W. Goethe-Universita¨t Frankfurt, Marie-Curie-Strasse 11,
D-60439 Frankfurt (Main), Germany
Received J anuary 16, 2004
Starting from 1,4- and 1,3-diborylated benzene derivatives, bitopic bis(1-pyrazolyl)borates
(K2[1,4-(tBuBpz2)2C6H4]; (Li,K)2[1,3-(tBuBpz2)2C6H4]) and tris(1-pyrazolyl)borates (K2[1,4-
(Bpz3)2C6H4]) have been synthesized (pz ) pyrazolyl). X-ray single-crystal structure analyses
revealed K2[1,3-(tBuBpz2)2C6H4] to establish a polymeric structure in the solid state. In
contrast, the unsymmetrically substituted hydrolysis product Li2[1,3-(tBuBpz2)(tBuB(OH)-
pz)C6H4] forms discrete dimers in which four lithium cations are encapsulated by their organic
ligands. As a representative of bitopic 1,2-diboryl benzene-based scorpionates, the 9,10-
dihydro-9,10-diboraanthracene derivative K2[(Bpz2)2(C6H4)2] is described, which bears two
pyrazolyl substituents at each of its boron atoms and stands out due to its stereorigidity.
In tr od u ction
Poly(1-pyrazolyl)borates (“scorpionates”) are well es-
tablished, versatile, and easily accessible ligands in
coordination chemistry, which have found widespread
applications ranging from analytical chemistry to ho-
mogeneous catalysis and materials science.1 Considering
the fact that increasing interest exists in the develop-
ment of ligands able to bind more than one transition
metal ion, surprisingly few examples of oligotopic scor-
pionates are known to date. Depending on their specific
geometries, either these species might serve as rigid
linkers in the synthesis of coordination polymer net-
works or they could be used to bring about cooperative
effects between different transition metal ions by forcing
them into close proximity.
The first example, A (Figure 1), of a discorpionate
ligand was reported by Niedenzu et al. in 1988.2 Our
F igu r e 1. Bitopic scorpionate ligands A, B, and C.
group has synthesized the ferrocene-based tris(1-pyra-
zolyl)borate B,3 which was successfully employed for the
generation of heterotrimetallic complexes.4,5 Reger et al.
recently published the related bis(1-pyrazolyl)methane
system C and several silver(I) coordination polymers
thereof.6
The purpose of this paper is to describe a convenient
route from 1,4-, 1,3-, and 1,2-diborylated benzene de-
rivatives to bitopic homo- and heteroscorpionates with
potential applications both in materials science and in
homogeneous catalysis.
* To whom correspondence should be addressed. E-mail:
matthias.wagner@chemie.uni-frankfurt.de.
Resu lts a n d Discu ssion
† Institut fu¨r Anorganische Chemie.
Selection of Liga n d System s. In most cases, tris-
(1-pyrazolyl)borates act as tripodal six-electron donors
toward transition metal ions. Thus, if we consider
dinuclear metal complexes of homoscorpionate ligands
featuring two [Bpz3]- substituents (pz ) pyrazolyl) at
‡ Institut fu¨r Organische Chemie.
(1) Trofimenko, S. Scorpionates-The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999.
(2) Brock, C. P.; Das, M. K.; Minton, R. P.; Niedenzu, K. J . Am.
Chem. Soc. 1988, 110, 817-822.
(3) J a¨kle, F.; Polborn, K.; Wagner, M. Chem. Ber. 1996, 129, 603-
606.
(4) Fabrizi de Biani, F.; J a¨kle, F.; Spiegler, M.; Wagner, M.; Zanello,
P. Inorg. Chem. 1997, 36, 2103-2111.
(5) Guo, S. L.; Peters, F.; Fabrizi de Biani, F.; Bats, J . W.;
Herdtweck, E.; Zanello, P.; Wagner, M. Inorg. Chem. 2001, 40, 4928-
4936.
(6) Reger, D. L.; Brown, K. J .; Gardinier, J . R.; Smith, M. D.
Organometallics 2003, 22, 4973-4983.
the same benzene ring (e.g., [1,4-(Bpz3)2C6H4]2-, [3]2-
;
Scheme 1), both angles metal‚‚‚B-C(ipso) will normally
be close to 180°. The two metal ions are therefore not
only sterically shielded but always kept at the same
distance from each other. Given this background, the
conformationally rigid bitopic homoscorpionate ligand
10.1021/om049954e CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/23/2004

2108 Organometallics, Vol. 23, No. 9, 2004
Bieller et al.
Sch em e 2. Syn th esis of K2[9]a
Sch em e 1. Syn th eses of th e Bitop ic Bis- a n d
Tr is(1-p yr a zolyl)bor a te Liga n d s K2[3], K2[4a ],
M2[4b] (M ) Li+, K+), a n d Li2[5]a
a
(i) 6 + exc BBr3, toluene, r.t.; 7 + 2 equiv of Me3SiNMe2,
toluene, -30 °C to r.t. (ii) 8 + 2 equiv of Kpz/2 equiv of Hpz,
toluene, reflux.
plexes with different average metal‚‚‚metal distances,
we have synthesized both the 1,4-isomer [4a ]2- and the
1,3-isomer [4b]2-. tert-Butyl groups were chosen as the
innocent spectator substituents to give the boron atoms
maximum protection from nucleophilic attack and to
increase the ligands’ solubility in less polar solvents.
Simultaneous introduction of [RBpz2]- moieties (R ) pz,
tBu) into the positions 1 and 2 of a benzene ring would
result in severe steric congestion. Overcrowding in hypo-
thetical [1,2-(RBpz2)2C6H4]2- ligands can, however, be
avoided when the two substituents R are replaced by a
second bridging 1,2-phenylene bridge as in the 9,10-
dihydro-9,10-diboraanthracene-based system K2[(Bpz2)2-
(C6H4)2], K2[9] (Scheme 2).
Syn th eses a n d Sp ectr oscop y: K2[3], M2[4a ] (M )
K+), and M2[4b] (M ) Li+, K+) are obtained from readily
available 1,4-bis(dibromoboryl)benzene, 1a ,7 and 1,3-bis-
(dibromoboryl)benzene, 1b,7 respectively (Scheme 1).
8
For the synthesis of K2[3], 1,4-[(Me2N)2B]2C6H4 is
prepared from 1a and HNMe2 and then treated with 2
equiv of Kpz and 4 equiv of Hpz (pz ) pyrazolyl) in
toluene solution. Since the introduction of tert-butyl
substituents at the boron centers via direct reaction of
1a and 1b with tBuLi suffers from side product forma-
tion, both dibromoboranes are first transformed into the
corresponding aminochloroborane derivatives 1,4-[(Me2-
N)ClB]2C6H4 and 1,3-[(Me2N)ClB]2C6H4.9 Subsequent
addition of 2 equiv of tBuLi generates 2a and 2b.
Further reaction of 2a and 2b with 2 equiv of Mpz (M:
Li+, K+) and 2 equiv of Hpz in toluene at reflux
temperature gives K2[4a ], Li2[4b], and K2[4b] in excel-
lent yields. While the potassium salts proved to be
rather stable toward hydrolysis, stirring of a THF
a
(i) + 10 equiv of HNMe2, toluene, -78 to +80 °C; + 2 equiv
of BCl3, toluene/heptane, -78 °C to r.t.; + 2 equiv of tBuLi,
toluene, -78 °C to r.t. (ii) 1a + 10 equiv of HNMe2, toluene,
-78 to +80 °C; + 2 equiv of Kpz/4 equiv of Hpz, toluene, reflux.
(iii) + 2 equiv of Mpz/2 equiv of Hpz, toluene, reflux. (iv) +H2O,
THF, r.t.
[3]2- appears to be a well-suited building block for the
generation of strictly one-dimensional, rodlike coordina-
tion polymer chains. The related heteroscorpionate
ligands, however, in which dipodal four-electron donors
[RBpz2]- are connected via a C6H4 backbone (e.g., [1,4-
(RBpz2)2C6H4]2-, [1,3-(RBpz2)2C6H4]2-, [4a ,b]2-, R )
tBu; Scheme 1) will give rise to metal complexes with
metal‚‚‚B-C(ipso) angles of about 90°. As a conse-
quence, these aggregates can easily adopt conformations
that bring both metal atoms in sufficiently close prox-
imity to facilitate their simultaneous action on the same
substrate molecule. To give access to dinuclear com-
(7) Kaufmann, D. Chem. Ber. 1987, 120, 901-905.
(8) Wasgindt, M.; Klemm, E. Synth. Commun. 1999, 29 (1), 103-
110.
(9) 1,4-[(Me2N)BrB]2C6H4 and 1,3-[(Me2N)BrB]2C6H4, which are
directly accessible from 1a /1b and 2 equiv of Me3SiNMe2 in toluene,
may also be employed for the synthesis of 2a and 2b.

Bitopic Bis- and Tris(1-pyrazolyl)borate Ligands
Organometallics, Vol. 23, No. 9, 2004 2109
solution of Li2[4b] in an open vessel for several days
resulted in the complete breakdown of all boron-
nitrogen bonds (NMR spectroscopical control). In con-
trast, single crystals of Li2[1,3-(tBuBpz2)(tBuB(OH)pz)-
C6H4], Li2[5], which represents one of the earliest
hydrolysis products, could be isolated in about 80% yield
after Li2[4b] in THF had been exposed to air without
stirring (Scheme 1). Thus, careful hydrolysis of Li2[4b]
offers a convenient way to generate the unsymmetrically
substituted bitopic ligand Li2[5]. The 9,10-dihydro-9,-
10-diboraanthracene scaffold underlying K2[9] (Scheme
2) is accessible from 1,2-bis(trimethylsilyl)benzene, 6,
and 3 equiv of BBr3. Treatment of the resulting bromo
derivative 710 with Me3SiNMe2 leads to the aminobo-
rane 8, which readily reacts with 2 equiv of Kpz and 2
equiv of Hpz in toluene to give the discorpionate ligand
K2[9] in about 70% yield.
F igu r e 2. Structure of K2[4b] in the crystal. Selected bond
lengths (Å), atom‚‚‚atom distances (Å), bond angles (deg),
and torsion angles (deg); H atoms omitted for clarity:
K(1)-O(41) ) 2.676(2), K(1)-N(12) ) 3.018(2), K(1)-N(12)i
) 2.884(2), K(1)-N(22) ) 2.859(2), K(1)-N(22)i ) 2.987-
(3), K(1)-C(32) ) 3.061(3), K(1)-C(33) ) 3.060(3), K(1)‚‚‚
K(1)i ) 3.426(1), B(1)-N(11) ) 1.588(4), B(1)-N(21) )
1.583(4), B(1)-C(1) ) 1.657(4), B(1)-C(32) ) 1.634(4);
N(12)-K(1)-N(22) ) 62.6(1), N(12)i-K(1)-N(22)i ) 62.7-
(1), N(12)-K(1)-N(12)i ) 109.1(1), N(22)-K(1)-N(22)i )
108.3(1), N(12)-K(1)-N(22)i ) 75.3(1), N(22)-K(1)-N(12)i
) 79.4(1), K(1)-N(12)-K(1)i ) 70.9(1), K(1)-N(22)-K(1)i
) 71.7(1); C(1)-B(1)-C(32)-C(33) ) 99.0(3), B(1)-N(11)-
N(12)-K(1) ) 15.0(3), B(1)i-N(11)i-N(12)i-K(1) ) 70.8-
(3), B(1)-N(21)-N(22)-K(1) ) -6.4(3), B(1)i-N(21)i-
The 11B NMR spectrum of K2[3] reveals one signal
at 1.0 ppm, thereby testifying to the presence of
magnetically equivalent, tetracoordinated boron at-
oms.11 In the 1H and 13C NMR spectra, a singlet
resonance at δ(1Η) ) 6.93 and a signal at δ(13C) ) 130.9
have to be assigned to the 1,4-disubstituted benzene
ring. The pyrazolyl groups give rise to three doublets
of doublets at δ(1Η) ) 6.00, 7.15, and 7.39. The relative
integral values of all four proton signals (4:6:6:6) are in
accord with the assumption that K2[3] contains two tris-
(1-pyrazolyl)borate functionalities per molecule. The
NMR spectra of K2[4a ] resemble those of K2[3] apart
from the fact that an additional signal appears at δ-
(1H) ) 0.90 [δ(13C) ) 31.3], which is due to the two tert-
butyl groups. Moreover, the integral ratios of the proton
resonances indicate each boron atom of K2[4a ] to bear
only two (rather than three) pyrazolyl substituents. The
only important difference between the NMR spectra of
K2[4a ] and K2[4b] comes from the C6H4 fragment, since
three resonances, characteristic of a 1,3-disubstituted
1
N(22)i-K(1) ) -78.6(3). i: symmetry transformation /2-
1
x, /2-y, 1-z.
result of partial hydrolysis, one boron atom of Li2[5]
bears a pyrazolyl ring and a tert-butyl- and a hydroxy
group and thus has a chiral configuration. The other
boron atom is still substituted by two pyrazolyl rings,
which exhibit slightly different proton shift values due
to the presence of a chiral center in the meta position
of the C6H4 bridge. The conclusion that Li2[5] possesses
two different boryl groups is supported by the observa-
tion of two singlets for the tert-butyl substituents
[δ(1H) ) 0.66, 0.67] and two multiplets for the pro-
tons in positions 4 and 6 of the central benzene ring
[δ(1H) ) 7.07, 7.17]. All general features outlined for
1
benzene ring, are visible both in the H NMR spectrum
[δ ) 6.91 (tr, 1H), 7.11 (d, 2H), 7.25 (s, 1H)] and in the
13C NMR spectrum of K2[4b] (the signal of the ipso-13C
atoms is broadened beyond detection, which has to be
attributed to the quadrupolar relaxation of the adjacent
boron nucleus11). Exchange of K+ for Li+ has no signifi-
cant effect on the NMR parameters of [4b]2-. Two
1
the H NMR spectrum of Li2[5] are also reflected by its
13C NMR spectrum, which therefore does not merit
further discussion. The most revealing NMR data of K2-
[9] are its 11B NMR shift value of 0.0 ppm, the presence
of two doublets of doublets at δ(1H) ) 6.77 and 6.87,
indicative of 1,2-disubstituted benzene moieties, and an
integral ratio of C6H4 to pyrazolyl proton signals of 2:3,
which is in accord with the molecular structure sug-
gested in Scheme 2.
partially overlapping resonances are observed in the 11
B
NMR spectrum of Li2[5] (δ ) 1.6, 2.5). In addition, two
sets of pyrazolyl proton signals can be identified. A
comparison of the respective integral values with the
integrals of the C6H4 proton signals leads to the conclu-
sion that the first set of resonances at δ(1H) ) 6.00
(virtual tr), 7.25 (d), and 7.63 (d) represents one pyra-
zolyl ring. The second set features a pattern of five
signals at δ(1H) ) 5.89, 5.90 (2 × virtual tr, 2 × 1H),
6.96, 6.98 (2 × d, 2 × 1H), and 7.42 (d, 2H) assignable
to two pyrazolyl moieties which are placed in a very
similarsbut not quite the sameschemical environment.
Considering the fact that Li2[5] was formed upon
exposure of Li2[4b] to air and moisture, these NMR
features can be explained in the following way: As a
Cr ysta l Str u ctu r e Deter m in a tion s
K2[4b] crystallizes from THF/Et2O (1:2) in the mono-
clinic space group C2/c (Figure 2). Crystal data and
structure refinement details for K2[4b] are compiled in
Table 1. The molecule consists of a central benzene ring
bearing two [tBuBpz2]- substituents in its 1 and 3
positions. The two symmetry-related tBu groups adopt
a trans configuration with respect to the plane of the
six-membered ring [torsion angle C(1)-B(1)-C(32)-
C(33) ) 99.0(3)°]. The bond angles around boron fall in
the interval between 106.1(2)° [N(11)-B(1)-N(21)] and
112.3(2)° [C(1)-B(1)-N(21)]. Deviations from the value
(10) For syntheses of the corresponding chloro and iodo derivatives,
see: Eisch, J . J .; Kotowicz, B. W. Eur J . Inorg. Chem. 1998, 761-769
and Siebert, W.; Schmidt, M.; Gast, E. J . Organomet. Chem. 1969, 20,
29-33, respectively.
(11) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds. In NMR Basic Principles and Progress;
Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, 1978.

2110 Organometallics, Vol. 23, No. 9, 2004
Bieller et al.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t Deta ils for K2[4b] a n d {Li2[5]}2
K2[4b]
{Li2[5]}2
formula
fw
C
26H34B2K2N8 × 2 C4H8O
C46H64B4Li4N12O2 × 2 C4H8O × C6H14
702.64
1118.47
color, shape
temp (K)
radiation
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
colorless, block
173(2)
Mo KR, 0.71073 Å
monoclinic
C2/c
19.652(3)
18.294(2)
11.9262(17)
90
120.552(10)
90
3692.4(9)
4
1.264
1496
0.299
colorless, rod
154(2)
Mo KR, 0.71073 Å
monoclinic
P21/c
9.2660(16)
18.810(5)
37.439(7)
90
93.209(13)
90
6515(2)
4
1.140
2408
Z
Dcalcd (g cm-3
F(000)
)
µ (mm-1
)
0.071
cryst size (mm)
0.23 × 0.22 × 0.12
0.16 × 0.18 × 0.70
no. of rflns collected
no. of indep rflns (Rint
18 391
72 476
)
3536 (0.0854)
3536/0/218
0.814
17 335 (0.0710)
17 335/0/766
1.092
no. of data/restraints/params
GOOF on F2
R1, wR2 (I > 2σ(I))
0.0451, 0.0812
0.1003, 0.0907
0.354, -0.312
0.1121, 0.1813
0.1926, 0.2093
0.460, -0.465
R1, wR2 (all data)
largest diff peak and hole (e Å-3
)
of 109.3° expected for an ideal tetrahedron are due to
the steric requirements of the tBu substituent. Each
potassium ion is coordinated by four pyrazolyl rings of
two different [4b]2- ligands, thereby creating a poly-
meric structure in the solid state. All K-N bond lengths
[K(1)-N(12) ) 3.018(2) Å, K(1)-N(22) ) 2.859(2) Å;
K(1)-N(12)i ) 2.884(2) Å, K(1)-N(22)i ) 2.987(3) Å; i:
same side of the bridging benzene ring with torsion
angles (H3C)3C-B-C(ipso)-C(ortho) ranging from 63.8-
(4)° to 80.3(3)°. The cations Li(1) and Li(4) are each
coordinated by a chelating [tBuBpz2]- moiety, giving
rise to obtuse N-Li-N angles of 96.7(2)° and 98.6(3)°,
respectively. In contrast, acute N-K-N angles of 62.6-
(1)° [N(12)-K(1)-N(22)] and 62.7(1)° [N(12)i-K(1)-
N(22)i] are found in K2[4b], which has to be attributed
to the larger ionic radius of K+ compared to Li+. Li(2)
and Li(3) bind to [tBuB(OH)pz]- groups in an η2 fashion.
Most interestingly, these two lithium cations are also
chelated by two pyrazolyl rings belonging to different
boron atoms [i.e., Li(2) to N(10), N(12); Li(3) to N(4),
N(6)]. As to be expected, the Li-N bond lengths to
terminal pyrazolyl rings are shorter [Li(1)-N(8) )
1.997(6) Å, Li(4)-N(2) ) 2.012(6) Å] and the Li-N-
N-B torsion angles are smaller [B(3)-N(7)-N(8)-Li-
(1) ) 6.9(4)°, B(1)-N(1)-N(2)-Li(4) ) 10.1(4)°] than the
corresponding bonds and torsion angles at bridging
pyrazolyl ligands [e.g., Li(1)-N(10) ) 2.129(6) Å, Li-
(4)-N(4) ) 2.205(6) Å; B(3)-N(9)-N(10)-Li(1) ) -46.8-
(3)°, B(1)-N(3)-N(4)-Li(4) ) -56.1(3)°]. Along the
pathways N(8)-Li(1)-N(10)-Li(2)-N(12)-Li(3) and
N(2)-Li(4)-N(4)-Li(3)-N(6)-Li(2), alternating shorter
and longer Li-N bonds are observed, the shorter bonds
generally being associated with smaller B-N-N-Li
torsion angles. Each of the two outer Li+ cations, Li(1)
and Li(4), is bonded to two oxygen (BOH, THF) and two
pyrazolyl nitrogen atoms in a distorted tetrahedral
geometry. The two inner Li+ cations, Li(2) and Li(3),
both have a coordination number of five and bind to one
hydroxy group (BOH), three pyrazolyl nitrogen atoms,
and the central benzene ring [Li(2)-C(35) ) 2.494(6)
Å; Li(3)-C(12) ) 2.387(6) Å]. Li(1) and Li(2), as well as
Li(3) and Li(4), are doubly bridged by a pyrazolyl ring
and a BOH ligand with Li+‚‚‚Li+ distances of 2.594(7)
and 2.649(8) Å, respectively. Li(2) and Li(3) are con-
nected at a distance of 2.714(7) Å by two µ2-pyrazolyl
ligands. The BOH hydroxyl groups show an intramo-
lecular O-H‚‚‚π(benzene) contact with H‚‚‚Cg distances
1
1
symmetry transformation /2-x, /2-y, 1-z] as well as
the corresponding N-K-N bond angles [N(12)-K(1)-
N(22) ) 62.6(1)°, N(12)i-K(1)-N(22)i ) 62.7(1)°] are
rather similar. In contrast, substantial differences are
observed in the torsion angles B(1)-N(11)-N(12)-K(1)
) 15.0(3)° and B(1)-N(21)-N(22)-K(1) ) -6.4(3)° on
one hand, compared to B(1)i-N(11)i-N(12)i-K(1) )
70.8(3)° and B(1)i-N(21)i-N(22)i-K(1) ) -78.6(3)° on
the other. It may therefore be concluded that the
interaction of K(1)+ with the first chelating [tBuBpz2]-
moiety occurs via filled sp2-type orbitals on the nitrogen
atoms N(12) and N(22), whereas the second chelating
[tBuBpz2]- ligand binds mainly via p-type orbitals on
N(12)i and N(22)i. The coordination sphere of each
potassium cation is completed by one THF molecule
[K(1)-O(41) ) 2.676(2) Å] and the C6H4 ring [two short
contacts: K(1)-C(32) ) 3.061(3) Å, K(1)-C(33) ) 3.060-
(3) Å], which results in an overall coordination number
of six. The potassium ions in polymeric K2[4b] are
grouped into dinuclear aggregates [interatomic dis-
tances K(1)‚‚‚K(1)i ) 3.426(1) Å] held together by four
bridging pyrazolyl rings with angles K(1)-N(12)-K(1)i
) 70.9(1)° and K(1)-N(22)-K(1)i ) 71.7(1)°. K2[4b]
forms parallel chains running along the face-diagonal
of the ac-plane.
Li2[5] features two different coordination sites, a
[tBuBpz2]- and a [tBuB(OH)pz]- group, in the bitopic
ligand molecule (Figure 3). Contrary to polymeric K2-
[4b], Li2[5] forms discrete dimers {Li2[5]}2 of ap-
proximate C2 symmetry in the solid state (monoclinic
space group P21/c; Table 1), which are composed of two
[5a ]2- anions connected by four lithium atoms. More-
over, the two tBu substituents are now placed at the

Bitopic Bis- and Tris(1-pyrazolyl)borate Ligands
Organometallics, Vol. 23, No. 9, 2004 2111
but also via pyrazolyl rings belonging to the two
different boron atoms of the ligand molecule. Thus, [5]2-
apparently possesses a higher degree of structural
flexibility than we had initially expected. Our findings
clearly demonstrate the promising potential of our
ligand system for the development of coordination
network solids on one hand and for the stabilization of
oligometallic cluster cores on the other.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All reactions and manipulations
of air-sensitive compounds were carried out in dry, oxygen-
free argon using standard Schlenk ware. Solvents were freshly
distilled under argon from Na/benzophenone prior to use.
NMR: Bruker AMX 250, AMX 400, Bruker DPX 250. 11B NMR
spectra were reported relative to external BF3‚Et2O. Abbrevia-
tions: s ) singlet, d ) doublet, dd ) doublet of doublets, tr )
triplet, vtr ) virtual triplet, mult ) multiplet, br ) broad,
n.r. ) multiplet expected in the 1H NMR spectrum but not
resolved, n.o. ) signal not observed, pz ) pyrazolide. Elemen-
tal analyses were performed by the microanalytical laboratory
of the University of Frankfurt. Compound 6 was synthesized
according to a literature procedure.12
Syn th esis of 2a . Step 1. First, 1a was transformed into
1,4-[(Me2N)2B]2C6H4 following a literature procedure.8 A solu-
tion of BCl3 in heptane (1 M, 5.20 mL, 5.20 mmol) was added
dropwise with stirring to 1,4-[(Me2N)2B]2C6H4 (0.70 g, 2.56
mmol) in toluene (15 mL) at -78 °C. The clear solution was
allowed to warm to room temperature and stirred for 2 h. All
volatiles were removed in vacuo to leave behind a colorless
solid residue, which was used for the subsequent reaction
without further purification. Yield: 0.62 g (94%). 11B NMR
(128.4 MHz, C6D6): 36.8 (h1/2 ) 250 Hz). 1H NMR (250.1 MHz,
C6D6): 2.43, 2.69 (2 × s, 2 × 6H, NCH3), 7.56 (s, 4H, C6H4).
13C NMR (62.9 MHz, C6D6): 39.8, 40.3 (NCH3), 132.4 (C6H4),
n.o. (CB).
F igu r e 3. Structure of {Li2[5]}2 in the crystal. Selected
bond lengths (Å), atom‚‚‚atom distances (Å), bond angles
(deg), and torsion angles (deg); H atoms omitted for
clarity: Li(1)-O(1) ) 1.987(6), Li(1)-O(3) ) 1.987(6), Li-
(2)-O(1) ) 1.994(5), Li(3)-O(2) ) 2.051(6), Li(4)-O(2) )
1.977(6), Li(4)-O(4) ) 1.980(6), Li(1)-N(8) ) 1.997(6), Li-
(1)-N(10) ) 2.129(6), Li(2)-N(6) ) 2.106(6), Li(2)-N(10)
) 2.086(6), Li(2)-N(12) ) 2.283(6), Li(3)-N(4) ) 2.049-
(6), Li(3)-N(6) ) 2.481(6), Li(3)-N(12) ) 2.080(6), Li(4)-
N(2) ) 2.012(6), Li(4)-N(4) ) 2.205(6), Li(2)-C(35) )
2.494(6), Li(3)-C(12) ) 2.387(6), Li(1)‚‚‚Li(2) ) 2.594(7),
Li(2)‚‚‚Li(3) ) 2.714(7), Li(3)‚‚‚Li(4) ) 2.649(8), B(1)-N(1)
) 1.593(5), B(1)-N(3) ) 1.588(5), B(2)-N(5) ) 1.590(4),
B(2)-O(1) ) 1.521(4), B(3)-N(7) ) 1.588(4), B(3)-N(9) )
1.595(4), B(4)-N(11) ) 1.593(4), B(4)-O(2) ) 1.530(4);
N(8)-Li(1)-N(10) ) 96.7(2), N(6)-Li(2)-O(1) ) 80.7(2),
N(10)-Li(2)-N(12) ) 113.2(2), N(12)-Li(3)-O(2) ) 81.8-
(2), N(4)-Li(3)-N(6) ) 111.1(2), N(2)-Li(4)-N(4) ) 98.6-
(3), Li(1)-N(10)-Li(2) ) 76.0(2), Li(1)-O(1)-Li(2) ) 81.3-
(2), Li(2)-N(6)-Li(3) ) 72.0(2), Li(2)-N(12)-Li(3) ) 76.8(2),
Li(3)-N(4)-Li(4) ) 76.9(2), Li(3)-O(2)-Li(4) ) 82.2(2);
C(7)-B(1)-C(11)-C(12) ) 78.0(4), C(20)-B(2)-C(13)-
C(14) ) 63.8(4), C(30)-B(3)-C(34)-C(35) ) 80.3(3), C(43)-
B(4)-C(36)-C(37) ) 74.9(4), B(1)-N(1)-N(2)-Li(4) )
10.1(4), B(1)-N(3)-N(4)-Li(4) ) -56.1(3), B(1)-N(3)-
N(4)-Li(3) ) 28.3(5), B(2)-N(5)-N(6)-Li(2) ) 26.7(3),
B(2)-N(5)-N(6)-Li(3) ) -51.6(3), B(3)-N(7)-N(8)-Li(1)
) 6.9(4), B(3)-N(9)-N(10)-Li(1) ) -46.8(3), B(3)-N(9)-
N(10)-Li(2) ) 42.4(4), B(4)-N(11)-N(12)-Li(2) ) -53.6-
(3), B(4)-N(11)-N(12)-Li(3) ) 31.2(3).
Step 2. tBuLi (1.6 M, 0.94 mL, 1.50 mmol) in pentane was
added dropwise with stirring to a solution of 1,4-[(Me2N)-
ClB]2C6H4 (0.19 g, 0.74 mmol) in toluene (30 mL) at -78 °C.
The reaction mixture was slowly allowed to warm to room
temperature and stirred for 3 h, whereupon
a colorless
precipitate formed. After filtration, the solvent was removed
from the filtrate in vacuo to give 2a as colorless microcrystal-
line solid. Yield: 0.20 g (90%). 11B NMR (128.4 MHz, C6D6):
45.6 (h1/2 ) 450 Hz). 1H NMR (250.1 MHz, C6D6): 1.25 (s, 18H,
CH3), 2.46, 2.68 (2 × s, 2 × 6H, NCH3), 7.21 (s, 4H, C6H4). 13
C
NMR (62.9 MHz, C6D6): 30.4 (CH3), 40.3, 43.8 (NCH3), 128.7
(C6H4), n.o. (CB). 2a is extremely moisture sensitive; a decent
elemental analysis was not obtained.
of 2.49 and 2.65 Å and O-H‚‚‚Cg angles of 131° and
127° (Cg is the centroid of a benzene ring).
Syn th esis of 2b. Step 1. HNMe2 (12.20 g, 270.60 mmol)
was condensed at -78 °C into a solution of 1b (11.20 g, 26.84
mmol) in toluene (100 mL). The resulting mixture was slowly
warmed to room temperature, heated at 80 °C for 2 h, and
cooled to room temperature again. After filtration, the solvent
was removed from the filtrate in vacuo and the solid crude
product distilled under reduced pressure to give colorless
crystals of 1,3-[(Me2N)2B]2C6H4. Yield: 6.34 g (86%). 11B NMR
(128.4 MHz, C6D6): 33.4 (h1/2 ) 240 Hz). 1H NMR (400.0 MHz,
C6D6): 2.65 (s, 24H, NCH3), 7.41 (mult, 1H, H-5), 7.50 (mult,
2H, H-4,6), 7.69 (br, 1H, H-2). 13C NMR (62.9 MHz, C6D6): 40.7
(NCH3), 128.3 (CH-5), 133.3 (CH-4,6), 139.5 (CH-2), n.o. (CB).
Step 2. A solution of BCl3 in heptane (1M, 26.00 mL, 26.00
mmol) was added dropwise with stirring to 1,3-[(Me2N)2B]2C6H4
(3.57 g, 13.03 mmol) in toluene (40 mL) at -78 °C. The clear
solution was allowed to warm to room temperature and stirred
for 3 h. All volatiles were removed in vacuo and the oily residue
Con clu sion
Bitopic bis- and tris(1-pyrazolyl)borate ligands have
been synthesized from 1,4-, 1,3-, and 1,2-diborylated
benzene derivatives. X-ray single-crystal structure analy-
ses revealed K2[1,3-(tBuBpz2)2C6H4], K2[4b], and the
unsymmetrically substituted compound Li2[1,3-(tBuBpz2)-
(tBuB(OH)pz)C6H4], Li2[5], to form complex supramo-
lecular aggregates in the solid state. K2[4b] has its two
tert-butyl substituents in a transoid conformation with
respect to the central C6H4 ring and thus establishes a
polymeric structure {K2[4b]}n. In contrast, a cisoid
conformation is adopted by the tert-butyl groups of Li2-
[5], giving rise to a dimeric structure {Li2[5]}2 in which
four lithium cations are encapsulated by their organic
ligands. [5]2- is capable of chelating lithium cations not
only via pyrazolyl rings attached to the same boron atom
(12) Kitamura, T.; Todaka, M.; Fujiwara, Y. Org. Synth. 2002, 78,
104-112.

2112 Organometallics, Vol. 23, No. 9, 2004
Bieller et al.
of 1,3-[(Me2N)ClB]2C6H4 distilled under reduced pressure.
Syn th esis of K2[4b]. The preparation was carried out
similar to the synthesis of K2[4a ] using 0.42 g (1.40 mmol) of
2b, 0.30 g (2.83 mmol) of neat Kpz, and 0.19 g (2.79 mmol) of
neat Hpz in 40 mL of toluene. Yield: 0.71 g (91%). X-ray
quality crystals were grown from THF/Et2O (1:2) at -20 °C.
Yield: 3.12 g (93%). 11B NMR (128.4 MHz, C6D6): 36.7 (h1/2
)
1
240 Hz). H NMR (250.1 MHz, CDCl3): 2.85, 2.98 (2 × s, 2 ×
6H, NCH3), 7.32 (tr, 1H, 3J HH ) 7.4 Hz, H-5), 7.50 (d, 2H, 3J HH
) 7.4 Hz, H-4,6), 7.67 (s, 1H, H-2). 13C NMR (62.9 MHz,
C6D6): 39.8, 40.3 (NCH3), 127.2 (CH-5), 134.0 (CH-4,6), 137.7
(CH-2), n.o. (CB).
1
11B NMR (128.4 MHz, d8-THF): 0.8 (h1/2 ) 290 Hz). H NMR
(250.1 MHz, d8-THF): 0.87 (s, 18H, CH3), 5.97 (vtr, 4H,
3
3J HH ) 2.0 Hz, pzH-4), 6.91 (tr, 1H, J HH ) 7.3 Hz, H-5), 7.11
Step 3. tBuLi (1.6 M, 3.90 mL, 6.24 mmol) in pentane was
added dropwise with stirring to a solution of 1,3-[(Me2N)-
ClB]2C6H4 (0.77 g, 3.00 mmol) in toluene (30 mL) at -78 °C.
The reaction mixture was allowed to warm to room temper-
ature and stirred for 10 h, whereupon a colorless precipitate
formed. After filtration, the solvent was removed from the
filtrate in vacuo and the oily crude product distilled under
reduced pressure to give 2b as a colorless viscous liquid.
3
(d, 2H, J HH ) 7.3 Hz, H-4,6), 7.25 (s, 1H, H-2), 7.29 (d, 8H,
3J HH ) 2.0 Hz, pzH-3,5). 13C NMR (62.9 MHz, d8-THF): 31.3
(CH3), 102.5 (pzC-4), 125.2 (C-5), 132.3 (C-4,6), 134.3, 137.6
(pzC-3,5), 139.9 (C-2), n.o. (CB). Anal. Calcd for C26H34B2K2N8
(558.43) × 2 C4H8O (72.11): C, 58.12; H, 7.17; N, 15.95. Found:
C, 57.82; H, 7.15; N, 15.65.
Syn th esis of Li2[5]. X-ray quality crystals of {Li2[5]}2 ×
2 THF × 1 hexane were obtained after a solution of Li2[4b] ×
0.5 hexane (0.82 g, 1.52 mmol) in THF had been exposed to
air for a prolonged time without stirring. Yield: 0.67 g (79%).
The crystals contain hexane, which was previously used in the
workup of Li2[4b] (see above). 11B NMR (128.4 MHz, d8-THF,
330 K): 1.6, 2.5 (both resonances are overlapping; reliable
Yield: 0.81 g (90%). 11B NMR (128.4 MHz, C6D6): 45.4 (h1/2
)
330 Hz). 1H NMR (250.1 MHz, CDCl3): 0.92 (s, 18H, CH3),
2.52, 2.97 (2 × s, 2 × 6H, NCH3), 6.82 (s, 1H, H-2), 6.90 (d,
3
3
2H, J HH ) 7.4 Hz, H-4,6), 7.14 (tr, 1H, J HH) 7.4 Hz, H-5).
13C NMR (62.9 MHz, CDCl3): 30.1 (CCH3), 40.5, 43.7 (NCH3),
125.9 (CH-5), 126.5 (CH-4,6), 129.9 (CH-2), n.o. (CB). 2b is
extremely moisture sensitive; a decent elemental analysis was
not obtained.
h
1/2 values could not be determined). 1H NMR (250.1 MHz, d8-
THF): 0.66, 0.67 (2 × s, 2 × 9H, CH3), 5.89, 5.90 (2 × vtr, 2 ×
3
3
Syn th esis of K2[3]. First, 1a was transformed into 1,4-
[(Me2N)2B]2C6H4 following a literature procedure.8 To a stirred
solution of 1,4-[(Me2N)2B]2C6H4 (1.00 g, 3.65 mmol) in toluene
(15 mL) was added at ambient temperature 0.80 g (7.54 mmol)
of neat Kpz and 0.99 g (14.54 mmol) of neat Hpz. After the
resulting suspension had been heated to reflux for 24 h, the
insolubles were collected on a frit, washed with toluene (2 ×
5 mL) and pentane (5 mL), and dried in vacuo. Yield: 1.75 g
(83%). 11B NMR (128.4 MHz, d6-DMSO): 1.0 (h1/2 ) 450 Hz).
1H NMR (250.1 MHz, d6-DMSO): 6.00 (dd, 6H, 3J HH ) 2.1 Hz,
1H, J HH ) 2.1 Hz, pzH-4), 6.00 (vtr, 1H, J HH ) 1.9 Hz, pzH-
3
4*), 6.85 (vtr, 1H, J HH ) 7.4 Hz, H-5), 6.96, 6.98 (2 × d, 2 ×
3
1H, J HH ) 2.1 Hz, pzH-3 or 5), 7.07, 7.17 (2 × mult, 2 × 1H,
3
H-4,6), 7.25 (d, 1H, J HH ) 1.9 Hz, pzH-3* or 5*), 7.42 (d, 2H,
3J HH ) 2.1 Hz, pzH-5 or 3), 7.63 (d, 1H, 3J HH ) 1.9 Hz, pzH-5*
or 3*), 7.79 (s, 1H, H-2). 13C NMR (62.9 MHz, d8-THF): 29.8,
30.5 (CH3), 101.8 (pzC-4), 103.0 (pzC-4*), 125.1 (C-5), 130.0,
132.8 (C-4,6), 133.2, 135.5 (pzC-3*,5*), 137.5, 138.0 (pzC-3,5),
140.7 (C-2). Note: chemical shift values marked with (*) are
assigned to the pyrazolyl ring of the Bpz(tBu)OH moiety. Anal.
Calcd for C23H32B2Li2N6O (444.04) × 1 C4H8O (72.11) × 0.5
C6H14 (86.18): C, 64.43; H, 8.47; N, 15.03. Found: C, 64.78; H,
8.53; N, 15.37.
3
1.5 Hz, pzH-4), 6.93 (s, 4H, C6H4), 7.15 (dd, 6H, J HH ) 2.1
4
3
Hz, J HH ) 0.6 Hz, pzH-3 or 5), 7.39 (dd, 6H, J HH ) 1.5 Hz,
4J HH ) 0.6 Hz, pzH-5 or 3). 13C NMR (62.9 MHz, d6-DMSO):
101.5 (pzC-4), 130.9 (C6H4), 132.3, 137.3 (pzC-3,5), n.o. (CB).
Anal. Calcd for C24H22B2K2N12 (578.34): C, 49.84; H, 3.83; N,
29.06. Found: C, 49.59; H, 3.75; N, 28.77.
Syn th esis of 7. To a solution of 6 (2.42 g, 10.89 mmol) in
10 mL of toluene was added BBr3 (7.64 g, 30.50 mmol) with
stirring at ambient temperature. After the mixture had been
refluxed overnight, all volatiles were removed in vacuo and
the solid residue was washed with pentane (5 mL) and
recrystallized from toluene (10 mL). Yield: 1.05 g (58%). 11B
NMR (128.4 MHz, C6D6): 41.8 (h1/2 ) 250 Hz). 1H NMR (250.1
Syn th esis of K2[4a ]. To a stirred solution of 2a (0.22 g,
0.73 mmol) in toluene (15 mL) was added at ambient temper-
ature 0.16 g (1.51 mmol) of neat Kpz and 0.10 g (1.46 mmol)
of neat Hpz. After the resulting suspension had been heated
to reflux for 24 h, the insolubles were collected on a frit,
washed with toluene (2 × 5 mL) and pentane (5 mL), and dried
in vacuo. Yield: 0.35 g (86%). 11B NMR (128.4 MHz, CD3CN):
2.5 (h1/2 ) 210 Hz). 1H NMR (250.1 MHz, CD3CN): 0.90 (s,
3
MHz, C6D6): 7.27, 7.67 (2 × dd, 2 × 4H, J HH ) 5.4 Hz,
4J HH ) 3.3 Hz, C6H4). 13C NMR (100.6 MHz, C6D6): 131.3,
131.7 (C6H4), n.o. (CB). 7 is extremely moisture sensitive; a
decent elemental analysis was not obtained.
3
18H, CH3), 6.00 (vtr, 4H, J HH ) 1.7 Hz, pzH-4), 6.94 (s, 4H,
Syn th esis of 8. A solution of Me3SiNMe2 (0.74 g, 6.30
mmol) in toluene (5 mL) was added dropwise with stirring to
7 (1.05 g, 3.15 mmol) in toluene (10 mL) at -30 °C. The
resulting colorless solution was slowly allowed to warm to room
temperature and stirred overnight. After all volatiles had been
removed in vacuo, the colorless solid residue was used for the
subsequent reaction without further purification. Yield: 0.77
g (93%). 11B NMR (128.4 MHz, C6D6): 32.5 (h1/2 ) 230 Hz). 1H
NMR (250.1 MHz, C6D6): 2.84 (s, 12H, NCH3), 7.24, 7.61 (2 ×
3
C6H4), 7.22, 7.34 (d, n.r., 2 × 4H, J HH ) 1.7 Hz, pzH-3,5). 13C
NMR (62.9 MHz, CD3CN): 31.3 (CH3), 102.4 (pzC-4), 133.4
(C6H4), 135.0, 137.9 (pzC-3,5), n.o. (CB). Anal. Calcd for
C
26H34B2K2N8 (558.43): C, 55.92; H, 6.14; N, 20.07. Found: C,
55.76; H, 6.46; N, 19.79.
Syn th esis of Li2[4b]. To a stirred solution of 2b (0.81 g,
2.70 mmol) in toluene (60 mL) was added at ambient temper-
ature 0.41 g (5.54 mmol) of neat Lipz and 0.39 g (5.73 mmol)
of neat Hpz. After the resulting suspension had been heated
to reflux for 24 h, the solvent was evaporated and the solid
residue dried in vacuo at 60-80 °C for 2 h to remove unreacted
pyrazole. Li2[4b] was recrystallized by layering its toluene
solution with hexane. Yield: 1.17 g (88%). 11B NMR (128.4
3
4
dd, 2 × 4H, J HH ) 5.4 Hz, J HH ) 3.2 Hz, C6H4). 13C NMR
(62.9 MHz, C6D6): 41.3 (NCH3), 126.6, 131.7 (C6H4), n.o. (CB).
8 is moisture sensitive; a decent elemental analysis was not
obtained.
1
Syn th esis of K2[9]. The preparation was carried out similar
to the synthesis of K2[4a ] using 0.77 g (2.93 mmol) of 8, 0.63
g (5.93 mmol) of neat Kpz, and 0.40 g (5.88 mmol) of neat Hpz
in 15 mL of toluene. Yield: 1.03 g (68%). 11B NMR (128.4 MHz,
CD3CN): 0.0 (h1/2 ) 230 Hz). 1H NMR (250.1 MHz, d6-
MHz, d8-THF): 1.2 (h1/2 ) 380 Hz). H NMR (250.1 MHz, d8-
3
THF): 0.58 (s, 18H, CH3), 5.94 (vtr, 4H, J HH ) 1.8 Hz, pzH-
3
3
4), 6.94 (tr, 1H, J HH ) 7.5 Hz, H-5), 7.08 (d, 4H, J HH ) 1.8
3
Hz, pzH-3 or 5), 7.23 (d, 2H, J HH ) 7.5 Hz, H-4,6), 7.45 (d,
4H, J HH ) 1.8 Hz, pzH-5 or 3), 7.92 (s, 1H, H-2). 13C NMR
3
3
DMSO): 5.79 (dd, 4H, J HH ) 2.2 Hz, 1.6 Hz, pzH-4), 6.77,
(62.9 MHz, d8-THF): 30.4 (CH3), 102.0 (pzC-4), 125.4 (C-5),
133.7 (C-4,6), 137.5, 137.9 (pzC-3,5), 144.6 (C-2), n.o. (CB).
Anal. Calcd for C26H34B2Li2N8 (494.12): C, 63.20; H, 6.94; N,
22.68. Found: C, 63.55; H, 6.99; N, 22.28.
3
4
6.87 (2 × dd, 2 × 4H, J HH ) 5.5 Hz, J HH ) 3.3 Hz, C6H4),
6.92, 7.17 (2 × d, 2 × 4H, 3J HH ) 2.2 Hz, 1.6 Hz, pzH-3,5). 13C
NMR (62.9 MHz, d6-DMSO): 100.7 (pzC-4), 123.1, 132.4

Bitopic Bis- and Tris(1-pyrazolyl)borate Ligands
Organometallics, Vol. 23, No. 9, 2004 2113
(C6H4), 133.2, 137.3 (pzC-3,5). Anal. Calcd for C24H20B2K2N8
(520.30): C, 55.40; H, 3.87; N 21.54. Found: C, 55.06; H, 3.99;
N, 21.27.
structure was determined by direct methods using the program
SHELXS.15 All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The H atoms at the hydroxyl
groups were taken from a difference Fourier synthesis and
were refined with individual isotropic thermal parameters. All
other H atoms were geometrically positioned and treated as
riding atoms. The structure was refined on F2 values using
the program SHELXL-97.16 {Li2[5]}2 crystallizes together with
1 equiv of hexane, which is disordered. The C atoms of the
THF groups also possess rather large displacement param-
eters.
Cr ysta l Str u ctu r e Deter m in a tion s of K2[4b] a n d {Li2-
[5]}2. K2[4b]. Data collection was performed on a Stoe-IPDS-
II two-circle diffractometer with graphite-monochromated Mo
KR radiation. An empirical absorption correction with the
MULABS option13 in the program PLATON14 gave a correction
factor between 0.935 and 0.965. Equivalent reflections were
averaged [R(I)int ) 0.085]. The structure was solved by direct
methods15 and refined with full-matrix least-squares on F2
using the program SHELXL-97.16 Hydrogen atoms were placed
on ideal positions and refined with fixed isotropic displacement
parameters using a riding model.
CCDC reference numbers: 228437 (K2[4b]), 228438 ({Li2-
[5]}2).
{Li2[5]}2. Data collection was performed on a SIEMENS
SMART diffractometer with graphite-monochromated Mo KR
radiation. Repeatedly measured reflections remained stable.
An empirical absorption correction with the program SAD-
ABS17 gave a correction factor between 0.951 and 1.000.
Equivalent reflections were averaged [R(I)int ) 0.071]. The
Ack n ow led gm en t. This research was generously
supported by the Deutsche Forschungsgemeinschaft
(DFG).
Su p p or tin g In for m a tion Ava ila ble: This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Blessing, R. H. Acta Crystallogr. Sect. A 1995, 51, 33-38.
(14) Spek, A. L. Acta Crystallogr. Sect. A 1990, 46, C34.
(15) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467-473.
(16) Sheldrick, G. M. SHELXL-97, A Program for the Refinement
of Crystal Structures; Unversita¨t Go¨ttingen, 1997.
OM049954E
(17) Sheldrick, G. M. SADABS; Unversita¨t Go¨ttingen, 2000.