Borabenzene derivatives. 30. Bis(1-methylboratabenzene) compounds of Germanium, Tin, and Lead. First structural characterization of facial bonding of a boratabenzene to a p-element and the structures of Pb(C5H5BMe)2 and its 2,2′-bipyridine adduct

Article abstract of DOI:10.1021/om990547q

Bis(1-methylboratabenzene)lead, Pb(C₅H₅BMe)₂ (1), was synthesized by the reaction of Li(C₅H₅BMe) with PbCl₂ as a low-melting (mp 60°C) thermochromic yellow crystalline solid (63%). The lighter homologues Sn(C₅H₅BMe)₂ (2) and Ge(C₅H₅BMe)₂ (3) were obtained from 1-methyl-2-(trimethylstannyl)-1,2-dihydroborinine, 2-(Me₃Sn)C₅H₅BMe, and SnCl₂ and GeI₂, respectively, as colorless liquids (70%). The plumbocene analogue 1 possesses a monomeric bent-sandwich structure with a bending angle of 135.2(3)°. Compound 1 forms addition compounds with nitrogen bases such as 1-TMEDA (6) and 1·bipy (7). Adduct 7 has a bent-sandwich structure with the 2,2′-bipyridine ligand in the pseudoequatorial plane; the bending angle is 139.1(5)°.

Full text of DOI:10.1021/om990547q

Organometallics 1999, 18, 4747-4752
4747
Bor a ben zen e Der iva tives. 30.
Bis(1-m eth ylbor a ta ben zen e) Com p ou n d s of Ger m a n iu m ,
Tin , a n d Lea d . F ir st Str u ctu r a l Ch a r a cter iza tion of
F a cia l Bon d in g of a Bor a ta ben zen e to a p -Elem en t a n d
th e Str u ctu r es of P b(C5H5BMe)2 a n d Its 2,2′-Bip yr id in e
Ad d u ct†,1
Gerhard E. Herberich,* Xiaolai Zheng, J o¨rg Rosenpla¨nter, and Ulli Englert
Institut fu¨r Anorganische Chemie, Technische Hochschule Aachen, D-52056 Aachen, Germany
Received J uly 14, 1999
Bis(1-methylboratabenzene)lead, Pb(C5H5BMe)2 (1), was synthesized by the reaction of
Li(C5H5BMe) with PbCl2 as a low-melting (mp 60 °C) thermochromic yellow crystalline solid
(63%). The lighter homologues Sn(C5H5BMe)2 (2) and Ge(C5H5BMe)2 (3) were obtained from
1-methyl-2-(trimethylstannyl)-1,2-dihydroborinine, 2-(Me3Sn)C5H5BMe, and SnCl2 and GeI2,
respectively, as colorless liquids (70%). The plumbocene analogue 1 possesses a monomeric
bent-sandwich structure with a bending angle of 135.2(3)°. Compound 1 forms addition
compounds with nitrogen bases such as 1‚TMEDA (6) and 1‚bipy (7). Adduct 7 has a bent-
sandwich structure with the 2,2′-bipyridine ligand in the pseudoequatorial plane; the bending
angle is 139.1(5)°.
In tr od u ction
the same bonding characteristics locally.9 Steric conges-
tion in highly substituted derivatives causes a decrease
of the interplanar angle between the two Cp ligands.
This effect culminates in decaphenylstannocene, Sn(C5-
Ph5)2, the first stannocene with coplanar ring ligands.10
Similarly, the plumbocene Pb(C5Me4SiMe2But)2 is the
first plumbocene with coplanar ring ligands.11
Boratabenzene ions are more akin to cyclopentadi-
enides than any other anion; they can form metal
compounds with metals of all parts of the periodic
table.12 We therefore set out to synthesize and study
compounds with group 14 elements. On one hand
boratabenzenes act as η1-bonded ligands in compounds
such as the 1,2-dihydroborinines 2-(Me3E)C5H5BMe
with E ) Si, Ge, Sn, Pb.13 On the other hand they can
form bent metallocenes with η6-bonded ring ligands such
as the new compounds M(C5H5BMe)2 1-3 (1, M ) Pb;
2, M ) Sn; 3, M ) Ge), which are the subject of this
paper.
The bis(cyclopentadienyl)element compounds GeCp2,2
SnCp2,3 and PbCp2 have a long history. The parent
4
compounds and their derivatives have been the subject
of several reviews5,6 and have found renewed interest
in recent years.7 In general, these compounds possess
bent-sandwich structures with a 14e valence electron
count. However, the lead compound PbCp2 exists as
three polymorphs. Two of these are polymers with one
side-on bonded η5-Cp ligand and one bridging µ,η5: η5-
Cp ligand.8,9 The third polymorph is a hexamer with
† Dedicated to Professor Fausto Calderazzo on the occasion of his
70th birthday.
(1) Part 29: Herberich, G. E.; Englert, U.; Fischer, A.; Ni, J .;
Schmitz, A. Organometallics, submitted.
(2) Scibelli, J . V.; Curtis, D. J . Am. Chem. Soc. 1973, 95, 924.
(3) Fischer, E. O.; Grubert, H. Z. Naturforsch. 1956, 11B, 423.
(4) Fischer, E. O.; Grubert, H. Z. Anorg. Allg. Chem. 1956, 286, 237.
(5) (a) J utzi, P.; Burford, N. Chem. Rev. 1999, 99, 969. (b) J utzi, P.;
Burford, N. In Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-
VCH: Weinheim, Germany, 1998; Vol. 1, pp 3-54. (c) J utzi, P. J .
Organomet. Chem. 1990, 400, 1. (d) J utzi, P. Adv. Organomet. Chem.
1986, 26, 217.
Resu lts a n d Discu ssion
(6) Germanium: Satge´, J .; Rivie`re, P.; Rivie`re-Baudet, M. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol.
2 (Davies, A. G., Volume Ed.), p 137. Tin: Davies, A. G. In Compre-
hensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 2
Syn th eses. All three bis(η6-1-methylboratabenzene)-
element compounds 1-3 can be obtained from a suitable
(9) Beswick, M. A.; Lopez-Casideo, C.; Paver, M. A.; Raithby, P. R.;
Russell, C. A.; Steiner, A.; Wright, D. S. J . Chem. Soc., Chem. Commun.
1997, 109.
(10) Heeg, M. J .; J aniak, C.; Zuckerman, J . J . J . Am. Chem. Soc.
1984, 106, 4259.
(Davies, A. G., Volume Ed.),
p 217. Lead: Harrison, P. G. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol.
2 (Davies, A. G., Volume Ed.), p 305.
(7) (a) Beswick, M. A.; Palmer, J . S.; Wright, D. S. Chem. Soc. Rev.
1998, 27, 225. (b) Beswick, M. A.; Gornitzka, H.; Ka¨rcher, J .; Mosquera,
M. E. G.; Palmer, J . S.; Raithby, P. R.; Russel, C. A.; Stahlke, D.;
Steiner, A.; Wright, D. S. Organometallics 1999, 18, 1148. (c) Evans,
W. J .; Clark, R. D.; Forrestal, K. J .; Ziller, J . W. Organometallics 1999,
18, 2401.
(8) (a) Overby, J . S.; Hanusa, T. P.; Young, V. G., J r. Inorg. Chem.
1998, 37, 163. (b) Panattoni, C.; Bombieri, G.; Croatto, U. Acta
Crystallogr. 1966, 21, 823.
(11) Constantine, S. P.; Hitchcock, P. B.; Lawless, G. A. Organome-
tallics 1996, 15, 3905.
(12) (a) Herberich, G. E. Boratabenzene Chemistry Revisited. In
Advances in Boron Chemistry; Siebert, W., Ed.; Special Publication
No. 201; The Royal Society of Chemistry: Cambridge, U.K., 1997; p
211. (b) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25,
199.
(13) Herberich, G. E.; Rosenpla¨nter, J .; Schmidt, B.; Englert, U.
Organometallics 1997, 16, 926.
10.1021/om990547q CCC: $18.00 © 1999 American Chemical Society
Publication on Web 10/20/1999

4748 Organometallics, Vol. 18, No. 23, 1999
Herberich et al.
Ta ble 1. P r oton Ch em ica l Sh ift Da ta for
Com p ou n d s 1-3 (500 MHz, CD2Cl2)
compound
4-H
3-/5-H
2-/6-H
Me
1
2
7.05
7.01
7.00
6.23
7.09
7.17
7.29
7.30
6.53
6.49
6.19
6.48
0.78
0.79
0.93
0.57
3
5a
a
A minimum amount of DME was added.
metal dihalide (PbCl2, SnCl2, GeI2) and the 1,2-dihy-
droborinine 2-(Me3Sn)C5H5BMe (4)13 using pentane or
1
hexane as solvents. The H NMR signals of the Me3Sn
F igu r e 1. PLATON plot21 of the molecule 1A. Displace-
ment ellipsoids are scaled to 30% probability.
groups of the starting material 4 and of the trimethyltin
halide formed can be used to monitor the progress of
the reaction. Distillative workup of the reaction mixture
gave 1 as a thermochromic, low-melting solid that is
pale yellow at -196 °C and yellow at ambient temper-
ature and gives a red liquid on melting. The lighter
homologues 2 and 3 are obtained as colorless liquids.
All three compounds are rather sensitive to air and
moisture.
tion,15 we can discuss qualitatively the main effects of
going from cyclopentadienide to boratabenzene. The
reduced symmetry and the lower donor property of the
boratabenzene ligand are expected to result in a de-
crease of the diamagnetic shielding (σdia) of the central
metal and a chemical shift to less negative values.
Furthermore the HOMO/LUMO separation of the ligand
is reduced, and this is expected to result in an increase
of the paramagnetic shielding contributions (σpara) and
a further chemical shift to less negative values.
2 2-(Me Sn)C H BMe
Sn(C H BMe)
+
5 5 2
SnCl2 +
f
3
5
5
4
2
2 Me3SnCl
Cr ysta l Str u ctu r e of 1. Crystals of the lead com-
pound 1 that were of acceptable quality could be
obtained by crystallization from pentane. The mono-
clinic elemental cell contains two independent mol-
ecules, 1A and 1B, the first one of a well-defined
molecular structure and the second one with severe
rotational disorder of one boratabenzene ligand. The
structure was solved in three steps. The first diffraction
data set was collected at -20 °C. It gave an acceptable
structure solution20 but with rather large displacement
ellipsoids for the disordered molecule 1B. A second data
set was then collected at -70 °C. On one hand this set
allowed the disorder to be resolved; on the other hand
it turned out to be of lower quality. In the third step
the disorder model so obtained was used to extract an
improved solution from the first data set, and this is
the solution documented here (Tables 2 and 3, Figures
1 and 2).
2 2-(Me Sn)C H BMe
Ge(C H BMe)
GeI2 +
f
+
2
3
5
5
5
5
4
3
2 Me3SnI
The corresponding bis(cyclopentadienyl)element com-
pounds are usually made more simply from the metal
dihalides and LiCp in THF. This method failed in the
present case. Although the formation of the compounds
1-3 could be observed initially by 1H NMR spectro-
scopy, the material turned insoluble after some minutes.
Later we found that the lead compound can be made
using LiC5H5BMe (5)14 in diethyl ether as the source of
the boratabenzene ring. This alternative synthesis is
simpler and gave much better yields.
2 LiC H BMe
Pb(C H BMe)
+ 2 LiCl
2
PbCl2 +
f
5
5
5
5
5
1
Both molecules 1A and 1B possess similar bent-
sandwich structures with approximately planar bo-
ratabenzene ligands and a bending angle of 135.2(3)°
for 1A. In the disordered molecule 1B the first ring (ring
C) is well-defined, while the second ring shows split
positions D and E with almost equal occupancies [54-
(2)/46(2)]. These rings D and E are coplanar, but E is
rotated against D by an angle of 20.2°.
There are some remarkably short intermolecular
contacts from the Pb atoms to ortho-carbon atoms of
neighboring molecules, especially Pb2-C21 [3.356(9) Å]
and Pb1-C45a [3.42(2) Å], and these involve, interest-
ingly, the rings B and D that have the longer intramo-
lecular Pb-ring distances. These interactions create a
tetrameric packing unit as shown in Figure 2.
The compounds 1-3 show simple mass spectra with
weak parent peaks (1-2%) and sequential loss of the
boratabenzene ligands; the most intense peak belongs
to the cation E(C5H5BMe)+ in all three cases. Proton
chemical shifts are collected in Table 1 together with
data for 5 for comparison. The chemical shift δ(207Pb)
of -2579 ppm for 1 is at rather high field,15 but the
shielding is less extreme than in PbCp2 [δ(207Pb) -5030
ppm]16 and PbCp* [δ(207Pb) -4390 ppm].16,17 The
2
chemical shift δ(119Sn) of -1535 ppm for 2 is again at
rather high field,18 and again this effect is less pro-
nounced than for SnCp2 [δ(119Sn) -2199 ppm],19a Sn-
(C5H4Me)2 [δ(119Sn) -2171.1 ppm],19b or SnCp*2 [δ(119Sn)
-2121 ppm].17,19c Following accepted lines of interpreta-
(14) Herberich, G. E.; Schmidt, B.; Englert, U. Organometallics
1995, 14, 471.
(15) Wrackmeyer, B.; Horchler, K. Annu. Rep. NMR Spectrosc. 1990,
22, 249.
(16) J aniak, C.; Schumann, H.; Stader, C.; Wrackmeyer, B.; Zuck-
erman, J . J . Chem. Ber. 1988, 121, 1745.
(17) For CP MAS spectra see: Wrackmeyer, B.; Sebald, A.; Merwin,
L. H. Magn. Reson. Chem. 1991, 29, 260.
(18) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1985, 16, 73.
(19) (a) Harris, R. K.; Kennedy, J . D.; McFarlane, W. In NMR and
the Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic Press:
London, U.K., 1978; p 342. (b) Bonny, A.; McMaster, A. D.; Stobart, S.
A. Inorg. Chem. 1978, 17, 935. (c) J utzi, P.; Hielscher, B. J . Organomet.
Chem. 1985, 291, C25.
(20) A preliminary account on the structure of 1 has been given in
ref 12a.

Borabenzene Derivatives
Organometallics, Vol. 18, No. 23, 1999 4749
underestimate the cell constants by about 2-3%.26 Our
calculations showed that both the individual plumb-
ocene molecules and the tetrameric unit may be re-
garded as van der Waals building blocks of a molecular
crystal. Therefore specific interactions are not needed
to explain the packing, but also, they are not excluded.
Further details concerning the computer modeling of
this and related compounds will be the subject of a
forthcoming study.
For molecule 1A the distances of the Pb atom to the
best ligand planes are 2.412 and 2.546 Å. For the
nondisordered ring ligands A-C the metal-ligand
bonding shows slip distortions27 of 0.14-0.28 Å, which,
quite remarkably, are essentially directed toward the
boron. This observation is in contrast to the situation
in d-metal complexes, where the metal always slips
toward carbon C-4 opposite the boron atom.12b The
distances Pb-C [2.865 Å (av), ranging from 2.712 to
3.037 Å] are larger on average than in monomeric
plumbocenes.7c The considerable variabilities of the
Pb-C distances, of the Pb-B distances, and of the slip
distortions indicate that the metal-ligand bond in 1 is
mechanically soft. It seems likely that the intermolecu-
lar interactions mentioned above contribute to this
structural variability.
F igu r e 2. SCHAKAL representation22 of the tetrameric
packing unit. The lead and the boron atoms are shaded,
and the weak intermolecular contacts are indicated by
dotted lines. The tetrameric unit is centrosymmetric with
two independent molecules. Ring E of molecule 1B is
omitted for clarity.
F or m a tion of Lew is Ba se Ad d u cts. The metal-
locenes PbCp2 and SnCp2 form weak adducts with
nitrogen Lewis bases such as TMEDA and the bipy
derivative 4,4′-Me2-2,2′-(C5H3N)2.28 We find that 1
displays the same type of reactivity. Reaction of 1 with
TMEDA readily gave a crystalline adduct Pb(C5H5-
BMe)2(TMEDA) (6) as yellow platelets. An attempted
structure determination failed because of severe disor-
der problems. We then turned to 2,2′-bipyridine as
Lewis base and readily obtained the orange crystalline
adduct Pb(C5H5BMe)2(bipy) (7).
Ta ble 2. Selected In ter a tom ic Dista n ces (Å) for 1
Pb1-C11
Pb1-C12
Pb1-C13
Pb1-C14
Pb1-C15
Pb1-B1
2.712(9)
2.811(9)
2.876(9)
2.821(9)
2.751(9)
2.809(10)
Pb1-C21
Pb1-C22
Pb1-C23
Pb1-C24
Pb1-C25
Pb1-B2
2.929(8)
3.037(8)
3.032(10)
2.887(10)
2.795(9)
2.858(11)
Pb2-C31
Pb2-C32
Pb2-C33
2.777(8)
2.887(9)
2.948(9)
Pb2-C34
Pb2-C35
Pb2-B3
2.851(9)
2.748(8)
2.804(9)
Cr ysta l Str u ctu r e of 7. Good crystals of compound
7 were obtained by crystallization from toluene. The
overall structure of 7 is that of a bent sandwich with a
somewhat distorted 2,2′-bipyridine ligand in the pseu-
doequatorial plane (Tables 4 and 5, Figure 3) and is
rather similar to that of PbCp2[4,4′-Me2-2,2′-(C5H3N)2]28b
(≡8).
Pb2-C41
Pb2-C42
Pb2-C43
Pb2-C44
Pb2-C45
Pb2-B4
2.81(2)
2.79(2)
2.89(2)
2.99(2)
3.01(2)
3.00(3)
Pb2-C51
Pb2-C52
Pb2-C53
Pb2-C54
Pb2-C55
Pb2-B5
2.73(2)
2.80(2)
2.94(2)
3.01(2)
2.94(2)
2.86(2)
Pb1‚‚‚C45a
3.42(2)
Pb2‚‚‚C21
3.356(9)
The boratabenzene ligands are essentially planar, and
the metallocene bending angle amounts to 139.1(5)° [cf.
Cp(centroid)-Pb-Cp(centroid) 139.7° for 8].28b The
distances of the Pb atom to the best ligand planes [2.588
and 2.597 Å] are markedly longer than for 1. This is
the expected consequence of the increase in coordination
number. What is more surprising are the Pb-N dis-
tances [2.666(8) and 2.607(7) Å], which are shorter than
for 8 [2.702(5) Å].28b Boratabenzenes (C5H5BR)- (R )
Me, Ph) have a lower charge density than cyclopenta-
dienide and, hence, are less basic29 and less nucleophilic
than Cp-.13 Thus we expect a larger positive charge
The choice of a van der Waals radius for lead is
somewhat ambiguous.23 Batsanov23a has recently pub-
lished values for a large number of elements, among
them 2.3 Å for tin and, in agreement with earlier data,
1.7 Å for carbon. On these grounds the above contacts
indicate weak intermolecular bonding. We decided to
use lattice energy minimizations24 based on the atom-
atom potential approach25 to judge the extent of bonding
within the tetrameric aggregate. In the absence of
specific intermolecular forces and for typical molecular
crystals these methods reproduce the packing arrange-
ment; at temperatures around 250 K they tend to
(21) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(22) Keller, E. SCHAKAL88-a FORTRAN Program for the Graphic
Representation of Molecular and Crystallographic Models; University
of Freiburg: Freiburg, Germany, 1988.
(23) (a) Batsanov, S. S. Izv. Akad. Nauk SSSR, Ser. Khim. 1995,
24. (b) Bondi, A. J . Phys. Chem. 1964, 68, 441.
(24) Schmidt, M. U.; Englert, U. J . Chem. Soc., Dalton Trans. 1996,
2077.
(25) Pertsin, A. J .; Kitaigorodsky, A. I. The Atom-Atom Potential
Method; Springer-Verlag: Berlin, Heidelberg, Germany, 1987.
(26) Wagner, T.; Wang, R.; Englert, U. Struct. Chem., submitted.
(27) The slip distortion is defined as the distance between the
projection of the Pb atom onto the C5B plane and the geometric center
of the C5B ring.
(28) (a) Armstrong, D. R.; Beswick, M. A.; Cromhout, N. L.; Harmer,
C. N.; Moncrieff, D.; Russell, C. A.; Raithby, P. R.; Steiner, A.;
Wheatley, A. E. H.; Wright, D. S. Organometallics 1998, 17, 3176. (b)
Beswick, M. A.; Cromhout, N. L.; Harmer, C. N.; Raithby, P. R.; Russel,
C. A.; Smith, J . S. B.; Steiner, A.; Wright, D. S. J . Chem. Soc., Chem.
Commun. 1996, 1977.

4750 Organometallics, Vol. 18, No. 23, 1999
Herberich et al.
Ta ble 3. P la n es a n d An gles for 1
(a) Molecule 1A
plane A ≡ (C11‚‚‚C15, B1) [with largest vertical displacement: 0.05(1) Å for B1]: distance Pb-plane A 2.412 Å
plane B ≡ (C21‚‚‚C25, B2) [with largest vertical displacement: 0.02(1) Å for B2]: distance Pb-plane B 2.546 Å
interplanar angle A,B: 44.8(3)°
rotational position A,B:a 64.7°
(b) Molecule 1B
plane C ≡ (C31‚‚‚C35, B3) [with largest vertical displacement: 0.04(1) Å for B3]: distance Pb-plane C 2.444 Å
plane D ≡ (C41‚‚‚C45, B4) [with largest vertical displacement: 0.04(2) Å for C41]: distance Pb-plane D 2.530 Å
plane E ≡ (C51‚‚‚C55, B5) [with largest vertical displacement: 0.02(2) Å for B5]: distance Pb-plane E 2.481 Å
interplanar angle C,D: 44.3(7)°
rotational position C,D:a 67.2°
interplanar angle C,E: 41.3(8)°
rotational position C,E:a 54.0°
a
The rotational position X,Y of ring Y relative to ring X is defined as the angle between the projections of the two B-C(Me) bond
vectors of ring X and of ring Y onto plane X.
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 7
apparatus and were uncorrected. Elemental analyses were
performed by Analytische Laboratorien, D-51779 Lindlar,
Germany.
Bond Distances (Å)
NMR spectra were recorded on a Varian Unity 500 (1H, 500
MHz; 13C, 125.7 MHz; 11B, 160.4 MHz; 119Sn, 186.5 MHz; 207Pb,
104.6 MHz), a Varian VXR 300 (1H, 300 MHz; 13C, 75.4 MHz),
and a PMR Firenze spectrometer (11B, 27.9 MHz). Chemical
shifts are given in ppm and are relative to TMS for 1H and
13C, to BF3.OEt2 for 11B, to SnMe4 for 119Sn, and to PbMe4 for
207Pb; in the case of 207Pb a 17% solution of PbEt4 in CD2Cl2
was used as external standard.32 Mass spectra were recorded
on a Finnigan MAT-95 at a nominal electron energy of 70 eV.
Pb-N1
Pb-C11
Pb-C12
Pb-C13
Pb-C14
Pb-C15
Pb-B1
2.666(8)
Pb-N2
Pb-C21
Pb-C22
Pb-C23
Pb-C24
Pb-C25
Pb-B2
2.607(7)
2.861(10)
2.944(10)
3.041(9)
3.034(10)
2.937(12)
2.921(12)
2.835(12)
2.819(11)
2.937(11)
3.112(11)
3.162(11)
3.040(13)
Bond Angles (deg)
61.0(2)
N1-Pb-N2
N1-C34-C35-N2
13(1)
Syn t h esis of Bis(1-m et h ylb or a t a b en zen e)lea d (1).
LiC5H5BMe (1.35 g, 13.8 mmol) in Et2O (20 mL) was added
dropwise to a suspension of PbCl2 (1.89 g, 6.80 mmol) in Et2O
(10 mL) at 0 °C. The temperature was allowed to rise to
ambient temperature, and stirring was continued for 6 h. A
yellow slurry formed. The solvent was completely removed
under vacuum, pentane (30 mL) was added to the residue, and
a precipitate of LiCl was filtered off. The filtrate was concen-
trated and cooled to -20 °C to give 1 as a yellow crystalline
solid. Concentrating the mother liquor and cooling afforded a
second crop of 1 (total yield 1.67 g, 63%): sensitive to heat,
air, and moisture, mp 60 °C, soluble in pentane, and very
soluble in toluene and CH2Cl2.
Da ta for 1: 1H NMR (500 MHz, CD2Cl2) δ 7.29 (dd, J )
10.1, 7.0 Hz, 3-/5-H), 7.00 (tt, J ) 7.0, 1.2 Hz, 4-H), 6.20 (dd,
J ) 10.1, 1.2 Hz, 2-/6-H), 0.92 (s, BMe); 13C NMR (126 MHz,
CD2Cl2) δ 137.6 (C-3,5; 207Pb satellites with 1J ) 78.7 Hz),
134.9 (br, C-2,6), 116.7 (C-4), 2.6 (br, BMe); 11B NMR (160
MHz, CD2Cl2, BF3‚OEt2 external) δ 36.1; 207Pb NMR (105 MHz,
CD2Cl2, PbEt4 external) δ -2584, relative to PbMe4;27 MS (EI)
m/z (Irel) 390 (>1, M+), 299 (100, M+ - C5H5BMe), 208 (30,
Pb+), 91 (40, C5H5BMe+). Anal. Calcd for C12H16B2Pb: C, 37.04;
H, 4.14. Found: C, 37.31; H, 4.19.
residing on the lead center of 1 as compared to that of
PbCp2 and, consequently, a stronger Pb-N bond for 7.
Con clu sion
This paper presents work on compounds of the p-block
metals with facial (or η6) bonding of boratabenzene
ligands. The structure determinations of 1 and 7
represent the first structural characterizations of this
bonding mode for p-block metals. We note that a facial
and bridging (or µ-η6:η6) bonding mode have been
assumed for thallium boratabenzenes Tl(C5H5BR) (R )
Me, Ph)30 by mere analogy with the bonding situation
in TlCp; we have never been able to grow crystals of
these thallium compounds for a crystal structure de-
termination. A third and more localized (or η1) bonding
mode is that found in the 1,2-dihydroborinines 2-(Me3E)-
C5H5BMe (E ) Si, Ge, Sn, Pb)13 and InMe(C5H5BMe)2,31
which are all fluxional with very low barriers to [1,3]-
sigmatropic automerization. Much more work will be
needed in the future to fully unfold these facets of
boratabenzene chemistry.
Syn th esis of Bis(1-m eth ylbor ataben zen e)tin (2). 1-Meth-
yl-2-(trimethylstannyl)-1,2-dihydroborinine13 (4) (2.4 g, 9.4
mmol) was added to a suspension of SnCl2 (2.0 g, 10.5 mmol)
in pentane. The reaction mixture was stirred overnight. Then
the excess SnCl2 was filtered off, and all volatiles, including
the Me3SnCl formed, were completely removed from the
filtrate under reduced pressure. The residue was then heated
to ca. 70 °C in a vacuum (10-6 bar), and 2 (2.2 g, 70%) was
collected by condensation as a very moisture-sensitive colorless
liquid.
Da ta for 2: 1H NMR (300 MHz, CD2Cl2) δ 7.18 (dd, J )
10.0, 7.0 Hz, 3-/5-H), 7.05 (tt, J ) 7.0, 1.3 Hz, 4-H), 6.51 (dd,
J ) 10.0, 1.3 Hz, 2-/6-H), 0.79 (s, BMe); 13C NMR (75 MHz,
CD2Cl2) δ 137.5 (C-3,5; 117/119Sn satellites with 1J ) 37.9 Hz),
134.0 (br, C-2,6), 116.8 (C-4), 3.0 (br, BMe); 11B NMR (28 MHz,
Exp er im en ta l Section
Gen er a l P r oced u r es. Reactions were carried out under
an atmosphere of dinitrogen by means of conventional Schlenk
techniques. Pentane and hexane were distilled from Na/K
alloy, toluene was distilled from sodium, and Et2O was distilled
from sodium benzophenone ketyl. Melting points were deter-
mined in sealed capillaries on a Bu¨chi 510 melting point
(29) (a) 1-Phenyl-1,4-dihydroborinine can be deprotonated by NaCp
in THF: Sandford, H. F., Ph.D. Thesis, University of Michigan, 1979.
(b) Gas-phase acidities: Sullivan, S. A.; Sandford, H.; Beauchamp, J .
L.; Ashe, A. J ., III. J . Am. Chem. Soc. 1978, 100, 3737.
(30) Herberich, G. E.; Becker, H. J .; Engelke, C. J . Organomet.
Chem. 1978, 153, 265.
(32) For PbEt4 (17% in CH2Cl2) δ(207Pb) +74.8(1) ppm: Kennnedy,
J . D.; McFarlane, W.; Pyne, G. S. J . Chem. Soc., Dalton Trans. 1977,
2332.
(31) Englert, U.; Herberich, G. E.; Rosenpla¨nter, J . Z. Anorg. Allg.
Chem. 1997, 623, 1098.

Borabenzene Derivatives
Organometallics, Vol. 18, No. 23, 1999 4751
Ta ble 5. P la n es a n d An gles for 7
plane A ≡ (C11‚‚‚C15, B1) [with largest vertical displacement: 0.03(1) Å for B1]: distance Pb-plane A 2.588 Å and
slip distortiona 0.21 Å
plane B ≡ (C21‚‚‚C25, B2) [with largest vertical displacement: 0.02(1) Å for B2]: distance Pb-plane B 2.597 Å
and slip distortiona 0.38 Å
plane C ≡ (Pb, N1, N2)
plane D ≡ (C30‚‚‚C34, N1) [with largest vertical displacement: 0.027(9) Å for C34]
plane E ≡ (C35‚‚‚C39, N2) [with largest vertical displacement: 0.016(9) Å for C35]
interplanar angle A,B: 40.9(5)°
interplanar angle A,C: 20.9(6)°
interplanar angle B,C: 20,8(8)°
centroid(A)-Pb-centroid(B): 135.9°
rotational position A:b 27.3°
rotational position B:b 136.0°
interplanar angle D,E: 15.4(10)°
a
b
For definition see ref 27. The rotational position is defined as the angle between the bisector of the two Pb-N bonds and the projection
of the B-C(Me) bond vector onto plane C.
Ta ble 6. Cr ysta l Da ta , Da ta Collection P a r a m eter s, a n d Con ver gen ce Resu lts for 1 a n d 7
1
7
formula
fw
C
12H16B2Pb
389.07
C22H24B2N2Pb
545.26
cryst syst
space group (no.)
a, Å
b, Å
monoclinic
P21/c (14)
orthorhombic
Pbca (61)
13.630(7)
16.855(6)
18.556(5)
14.032(1)
10.735(1)
18.479(3)
108.06(1)
2646.7(5)
13.956(1)
10.696(2)
18.391(6)
107.88(2)
2613(1)
c, Å
â, deg
cell volume, Å3
Z
4263(5)
8
8
F(000)
1440
2096
temperature, K
253
1.953
128.1
26.0
203
1.978
129.8
28.0
218
1.699
79.8
27.0
d
calcd, g cm-3
µ, cm-1
max, deg
θ
scan mode
ω
ω-2θ
ω-2θ
cryst dimens, mm3
no. of rflns
0.65 × 0.16 × 0.08
0.26 × 0.18 × 0.12
0.42 × 0.21 × 0.16
5018
25625
11769
no. of indep rflns
no. of indep obsd rflns
no. of variables
R,a obsd (all data)
3866
2872 (I > 2σ(I))
270
6285
2608 (I > 2σ(I))
270
4633
3229 (I > 1σ(I))
244
0.053
0.045
0.031 (0.060)
0.041 (0.13)
b
Rw
wR2,c obsd (all data)
GOF, obsd (all data)
max resid density, e Å-3
0.069 (0.074)
1.056 (0.969)
1.50
0.076 (0.092)
0.938 (0.713)
1.65
1.109
1.84
R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2; w ) 1/σ2(Fo). c wR2 ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2
.
a
b
2
2
Da ta for 3: 1H NMR (500 MHz, CD2Cl2) δ 7.09 (dd, J )
9.8, 7.0 Hz, 3-/5-H), 7.05 (tt, J ) 7.0, 1.5 Hz, 4-H), 6.53 (dd, J
) 9.8, 1.5 Hz, 2-/6-H), 0.78 (s, BMe); 13C NMR (75 MHz, CD2-
Cl2) δ 137.5 (C-3,5), 134.3 (br, C-2,6), 119.5 (C-4), 3.0 (br, BMe);
11B NMR (28 MHz, CD2Cl2, BF3‚OEt2 external) δ 41; MS (EI)
m/z (Irel) 256 (1, M+), 165 (100, M+ - C5H5BMe), 91 (25, C5H5-
BMe+). Because of the high sensitivity of 3, a satisfactory
elemental analysis was not obtained. Anal. Calcd for C12H16B2-
Ge: C, 56.64; H, 6.34. Found: C, 54.18; H, 6.50.
Syn th esis of th e TMEDA Ad d u ct of 1 (6). TMEDA (0.15
g, 1.29 mmol) was injected into a solution of 1 (0.48 g, 1.23
mmol) in toluene (10 mL). The mixture was stirred for 10 min
and was then filtered. The filtrate was concentrated to ca. 5
mL and stored at 4 °C to give 6 (0.43 g, 69%) as moisture-
sensitive yellow platelets, mp 124-128 °C (dec).
Da ta for 6: 1H NMR (500 MHz, CD2Cl2, 20 °C) δ 7.23 (dd,
J ) 9.5, 7.0 Hz, 3-/5-H), 6.94 (t, J ) 7.0 Hz, 4-H), 6.21 (d, J )
9.5 Hz, 2-/6-H), 0.87 (s, BMe); for TMEDA, 2.36 (s, 2 NCH2),
2.22 (s, 2 NMe2); cooling of solutions in CD2Cl2 or toluene-d6
resulted only in some line broadening. 13C NMR (126 MHz,
CD2Cl2): δ 137.2 (C-3,5), 134.8 (br, C-2,6), 115.7 (C-4), 2.4 (br,
BMe); for TMEDA, 57.9 (2 NCH2), 46.1 (2 NMe2). 11B NMR
(160 MHz, CD2Cl2, BF3‚OEt2 external): δ 36.1. Anal. Calcd
for C18H32B2N2Pb: C, 42.79; H, 6.38; N, 5.54. Found: C, 42.90;
H, 6.00; N, 5.83.
F igu r e 3. PLATON plot21 of the molecule 7. Displacement
ellipsoids are scaled to 30% probability.
CD2Cl2, BF3.OEt2 external) δ 38; 119Sn NMR (186 MHz, CD2-
Cl2, SnMe4 external) δ -1535; MS (EI) m/z (Irel) 302 (2, M+),
211 (100, M+ - C5H5BMe), 120 (25, Sn+), 91 (40, C5H5BMe+).
Anal. Calcd for C12H16B2Sn: C, 47.95; H, 5.36. Found: C,
47.54; H, 5.28.
Syn th esis of Bis(1-m eth ylbor a ta ben zen e)ger m a n iu m
(3). 1-Methyl-2-(trimethylstannyl)-1,2-dihydroborinine13 (4)
(1.4 g, 5.5 mmol) was added to a suspension of GeI2 (2.0 g, 6.1
mmol) in pentane (5 mL). Workup and condensation (ca. 40
°C, 10-6 bar) as described for 2 afforded 3 (1.0 g, 70%) as a
highly moisture-sensitive colorless liquid.

4752 Organometallics, Vol. 18, No. 23, 1999
Herberich et al.
Syn th esis of th e 2,2′-Bip yr id in e Ad d u ct of 1 (7). Bipy
(94 mg, 0.60 mmol) was added to a solution of 1 (0.23 g, 0.59
mmol) in toluene (8 mL). Workup as described for 6 gave 7
(0.24 g, 75%) as moisture-sensitive orange crystalline blocks,
mp 169-172 °C (dec).
within a standard deviation of 0.02 Å (refinement without
restraints gave an average difference of 0.05 Å for these bonds
and a largest difference of 0.15 Å between C43-C44 and C53-
C54). All other non-hydrogen atoms were refined with aniso-
tropic displacement parameters, and hydrogen atoms were
included as riding with fixed displacement parameters [C-H
) 0.98 Å, Uiso(H) ) 1.3 Ueq(C)] in the final full-matrix least-
squares refinement of the structure model.
An additional data set was collected on a second crystal at
lower temperature (cf. Table 6). The slightly smaller displace-
ment parameters [Uiso(-70 °C) ) 0.050 Å2 (av), Uiso(-20 °C)
) 0.066 Å2 (av) for the disordered non-hydrogen atoms] and
the higher resolution limit were of help in deconvoluting the
disorder. However, the overall quality of this second data set
was lower, as evidenced by the higher estimated standard
deviations and agreement factors.
Da ta for 7: 1H NMR (500 MHz, CD2Cl2) δ 7.13 (dd, J )
9.9, 6.8 Hz, 4H, 3-/5-H), 6.87 (t, J ) 6.8 Hz, 2H, 4-H), 6.08 (d,
J ) 9.9 Hz, 4H, 2-/6-H), 0.70 (s, 2 BMe); for bipy, 8.65 (ddd, J
) 4.9, 1.8, 0.9 Hz, 2H, 6-H), 8.26 (ddd, J ) 8.1, 1.2, 0.9 Hz,
2H, 3-H), 7.92 (ddd, J ) 8.1, 7.5, 1.8 Hz, 2H, 4-H), 7.47 (ddd,
J ) 7.5, 4.9, 1.2 Hz, 2H, 5-H); 13C NMR (126 MHz, CD2Cl2) δ
137.0 (C-3,5), 134.1 (br, C-2,6), 115.2 (C-4), 2.9 (br, BMe); for
bipy, 155.3 (C-2), 148.9 (C-6), 138.0 (C-4), 125.0 (C-5), 121.8
(C-3); 11B NMR (160 MHz, CD2Cl2, BF3‚OEt2 external) δ 36.0.
Anal. Calcd for C22H24B2N2Pb: C, 48.46; H, 4.44; N, 5.14.
Found: C, 48.67; H, 4.46; N, 5.37.
Cr ysta l Str u ctu r e Deter m in a tion s. The data collections
were performed on an ENRAF-Nonius CAD4 diffractometer
with Mo KR radiation (graphite monochromator, λ ) 0.7107
Å). Crystal data, data collection parameters, and convergence
results are given in Table 6. Empirical absorption corrections
on the basis of azimuthal scans33 were applied to the data sets
before merging symmetry-related reflections.
The structure of 7 was solved by conventional heavy atom
methods followed by Fourier difference syntheses and refined
on structure factors with the SDP program system.35 All non-
hydrogen atoms were refined with anisotropic displacement
parameters, and hydrogen atoms were included as riding with
fixed displacement parameters [C-H ) 0.98 Å, Uiso(H) ) 1.3
U
eq(C)] in the refinement of the structure model.
The structure of 1 was solved by conventional heavy atom
methods followed by Fourier difference syntheses and refined
on F2 with the SHELX-93 program.34 The unit cell contains
two independent molecules; one of the four boratabenzene
ligands suffers from rotational disorder. The disorder model
was established with the help of a second data set collected at
lower temperature (see below). Two almost equally occupied
orientations (ligand D, C41‚‚‚C46,B4; ligand E, C51‚‚‚C56,B5)
are coplanar within the limit of this structure determination
and are related to each other by an angle of ca. 20°. The boron
and carbon atoms of the disordered ligand were refined
isotropically, and seven restraints were applied to set the
corresponding bond distances of these atoms between two sites
Ack n ow led gm en t. This work was generously sup-
ported by the Deutsche Forschungsgemeinschaft and by
the Fonds der Chemischen Industrie.
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
distances and angles, anisotropic displacement parameters,
and positional coordinates for 1 and 7.36 This material is
available free of charge via the Internet at http://pubs.acs.org.
OM990547Q
(35) Frenz, B. A. ENRAF-Nonius SDP Version 5.0; Delft: The
Netherlands, 1989, and local programs.
(33) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(34) Sheldrick, G. M. SHELXS-93, Program for Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(36) Further details of the crystal structure determinations are
available from the Cambridge Crystallographic Data Center, on
quoting the depository numbers CCDC-133684 for 1 and CCDC-133685
for 7.