Stable intramolecularly base-stabilized germylene- and stannylene-borane adducts: M[C6H3(NMe2)2-2,6] 2BH3 (M = Ge, Sn)

Article abstract of DOI:10.1021/om980388v

The synthesis and characterization of two novel and thermally surprisingly stable main-group-element Lewis acid (BH₃) adducts (3 and 4) of a monomeric germylene (1) and of a diarylstannylene (2) is reported. X-ray crystal structures of the adducts MAr₂(BH₃) reveal a four-coordinated germanium (3) but five-coordinated tin (4), with regard to the C,N-potentially bidentate chelating ligand ^-C₆H₃(NMe₂)₂-1,2 and BH₃, respectively.

Full text of DOI:10.1021/om980388v

3838
Organometallics 1998, 17, 3838-3840
Sta ble In tr a m olecu la r ly Ba se-Sta bilized Ger m ylen e- a n d
Sta n n ylen e-Bor a n e Ad d u cts: M[C6H3(NMe2)2-2,6]2BH3
(M ) Ge, Sn )
Christian Drost, Peter B. Hitchcock, and Michael F. Lappert*
The Chemistry Laboratory, School of Chemistry, Physics and Environmental Sciences,
University of Sussex, Brighton BN1 9QJ , U.K.
Received May 18, 1998
Summary: The synthesis and characterization of two
novel and thermally surprisingly stable main-group-
element Lewis acid (BH3) adducts (3 and 4) of a
monomeric germylene (1) and of a diarylstannylene (2)
is reported. X-ray crystal structures of the adducts
MAr2(BH3) reveal a four-coordinated germanium (3) but
five-coordinated tin (4), with regard to the C,N-poten-
tially bidentate chelating ligand -C6H3(NMe2)2-1,2 and
BH3, respectively.
There is current interest in the base behavior of
monomeric bivalent group 14 metal compounds MX2
with reference to main-group-element Lewis acids M′X′n.
Established compounds are (i) the X-ray-characterized
Arduengo-type carbene-M′X′n adducts I (M′X′n ) BH3
or BF3 (R ) Me; R′ ) Me, Et, or Pri),1 AlH3 (R ) H, R′
) C6H2Me3-2,4,6),2 M′Me3 (M′ ) Al or Ga, R ) H, R′ )
But),3 or SiCl4, SiCl2Me2, SiCl2Ph2, SnCl2Ph2 or SnCl2
(R ) Me; R′ ) Me, Et or Pri)]4 and (ii) the labile silylene
adduct II (t1/2 ca. 30 days at 20 °C in PhMe), which
readily isomerized into the insertion product III.5
A
single labile (but X-ray-authenticated) diorganotin(II)-
Lewis acid (SnCl2) adduct (IV) has been obtained from
its acid/base precursors but underwent metathesis in
tetrahydrofuran (THF) at 25 °C, yielding the organotin-
7a
(II) chloride V.6 An early report of Sn(η5-C5H5)2:BF3
proved to have misformulated the compound; it is now
established to be the Sn-B-bond-free salt [Sn(η5-C5H5)2-
(µ-η5-C5H5)Sn(THF)][BF4].7b The cubanes [M(µ3-NBut)]4
formed the 1:2 AlCl3 adducts VI, which slowly decom-
posed in solution; X-ray data for the tin compound VIb
were provided.8 Not only nucleophilic carbenes9a but
also silylenes,9b germylenes,10 stannylenes,10 and plum-
bylenes MX210 are well-known as ligands in a transition-
metal (M′) context, but generally M′ has the possibility
of engaging in synergic dfpπ back-bonding, as in
[Cr(CO)5{Sn[CH(SiMe3)2]2}].11
We now report the first main-group-element Lewis
acid (BH3; the archetype) adducts of a monomeric
intramolecularly base-stabilized germylene and of a
diarylstannylene. Treatment of the recently described12
yellow diarylgermylene GeX2 (1) or -stannylene SnX2
(2) (Xj ) VII) with a solution of BH3(THF) in diethyl
ether and then successive removal of volatiles in vacuo
and recrystallization from Et2O afforded colorless (3) or
amber (4) crystals of X2M:BH3 (M ) Ge, 3; M ) Sn, 4)13
in good yields (Scheme 1). Each of the compounds 3 and
4 gave satisfactory microanalytical as well as NMR (1H,
11B, 13C{1H}, and (for 4) 119Sn{1H}), IR, and EI-MS
spectra.14 Compound 3 was thermally stable both in
the solid state and in solution, but a solution of the tin
analogue 4 slowly deposited tin.
(1) Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J .; Boese, R.;
Maulitz, A. H. Chem. Ber. 1993, 126, 2041.
(2) Arduengo, A. J .; Dias, H. V. R.; Calabrese, J . C.; Davidson, F. J .
Am. Chem. Soc. 1992, 114, 9724.
(3) Li, X.-W.; Su, J .; Robinson, G. H. Chem. Commun. 1996, 2683.
(4) Kuhn, N.; Kratz, T.; Bla¨ser, D.; Boese, R. Chem. Ber. 1995, 128,
245.
(5) Metzler, N.; Denk, M. Chem. Commun. 1996, 2657.
(6) Leung, W.-P.; Kwok, W.-H.; Xue, F.; Mak, T. C. W. J . Am. Chem.
Soc. 1997, 119, 1145.
(7) (a) Harrison, P. G.; Zuckerman, J . J . J . Am. Chem. Soc. 1970,
92, 2577. (b) Dory, T. S.; Zuckerman, J . J .; Barnes, C. L. J . Organomet.
Chem. 1985, 281, C1.
(8) Veith, M.; Frank, W. Angew. Chem., Int. Ed. Engl. 1985, 24, 223.
(9) (a)Lappert, M. F. J . Organomet. Chem. 1988, 358, 185. (b)
Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
2162.
(10) Lappert, M. F.; Rowe, R. S. Coord. Chem. Rev. 1990, 100, 267.
(11) Cotton, J . D.; Davidson, P. J .; Lappert, M. F. J . Chem. Soc.,
Dalton Trans. 1976, 2275.
(12) Drost, C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J .-M.
Chem. Commun. 1997, 1141.
S0276-7333(98)00388-4 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/13/1998

Communications
Organometallics, Vol. 17, No. 18, 1998 3839
stannylene (2) are isostructural, both having the metal
in a four-coordinate environment (one M-C and one
M-N bond from each ligand VII),12,16 the adducts differ
in that 3 has four-coordinate germanium (two Ge-C,
one Ge-N, and one Ge-B bond), while 4 has five-
coordinate Sn (two Sn-C, two Sn-N, and one Sn-B
bond). Thus, each aryl ligand VII is bound in a C,N-
bidentate chelating fashion in 1,12 2,16 and the cen-
trosymmetric 4; however, in 3 only one of the ligands
VII adopts this mode, the other being merely C-bonded.
Selected comparative geometric parameters for 1-4 are
given in Table 1; further details for 116 will appear in
the full paper. Compounds 3 and 4 have no close
analogue in Ge-B or Sn-B chemistry; the M-B bond
lengths for the two former species (Table 1) may be
compared with the Ge-B length (2.165(10) Å) in VIII17
and the Sn-B lengths in cis-IXa (2.305-2.323(7) Å) and
trans-IXb (2.286-2.277(17) Å),18 respectively. The
germanium complex 3 may also be compared with salt
F igu r e 1. Crystal structure and labeling scheme of 4.
Selected bond lengths and angles are in Table 1; addition-
ally, C(1)-Sn-N(1) ) 61.18(10)°, B-Sn-N(1) ) 112.86-
(6)°, C(2)-C(1)-Sn ) 100.4(2)°, and C(6)-C(1)-Sn )
141.0(2) °.
+
X,19 noting that BH3 and CH3 are isoelectronic.
F igu r e 2. Crystal structure and labeling scheme of 3.
Selected bond lengths and angles are in Table 1.
Sch em e 1. Syn th esis of th e Dia r ylm eta llen e-BH3
Ad d u cts 3 a n d 4 (X- ) VII)
The nonbonding nitrogen atom of each -C6H3(NMe2)2-
2,6 ligand VII is remote from the metal center, e.g.,
3.815(2) Å in 4; cf. 3.394(5) and 3.783(5) Å in the free
stannylene 2.12 The M-C and M-C′ bonds are slightly
shorter in the borane adducts 3 and 4 than in the free
bases 1 and 2, consistent with the former being dipolar,
X2M+--BH3. Such shortening is even more pronounced
The crystal structures of the borane adducts are
illustrated in Figures 1 (4) and 2 (3).15 Whereas the
crystalline parent base-stabilized germylene (1) and
(15) Crystal data are as follows: 3: C20H33BGeN4, Mr ) 412.9,
orthorhombic, space group Pbca (No. 61), a ) 10.256(2) Å, b ) 16.289-
(13) Preparation of 3 and 4: a BH3(THF) solution (2.2 mL of a 1.0
mol L-1 solution in THF; 2.25 mmol for 1, 2.5 mL for 2) was added to
a stirred yellow solution of the germylene 1 (1 g, 2.51 mmol) or
stannylene 2 (1 g, 2.25 mmol)9 in Et2O (100 mL) at ca. 25 °C. The
solution slowly became decolorized (for 3) or slightly darkened (for 4)
and was stirred overnight. Volatiles were removed in vacuo to yield
solid residues, which were extracted with Et2O. Filtration to remove
a slight cloudiness, concentration of the filtrate in vacuo, and cooling
to -30 °C afforded colorless crystals of 3 (0.9 g, 95%) or amber-colored
crystals of 4 (0.85 g, 82%).
(14) Selected data for 3 and 4 are as follows. NMR spectra at 298 K
in C6D6 (1H, 13C) or PhMe + C6D6: 1H NMR, 250.0 MHz; 13C{1H} NMR,
62.86 MHz;11B NMR, 80.21 MHz;119Sn{1H} NMR, 186.36 MHz. 3:
colorless; mp 145-147 °C; 1H NMR δ 2.61 (s, 24 H, NMe2), 6.50 (d, 4
H, H-3/5, 3J (1H-1H) ) 7.9 Hz), 7.16 (t, 2 H, H-4, 3J (1H-1H) ) 7.9 Hz);
13C{1H} NMR δ 44.5 (NC2), 110.8, 129.4, 134.8, 156.8 (aromatic C);
11B NMR δ -35.0 (q) (1J (11B-1H) ) 73 Hz). EI-MS (70 eV) parent ion
at m/z 414 (7% intensity of the most abundant ion); IR (Nujol) 2235,
2315, 2351, 2389 cm-1. 4: amber; mp ca. 90 °C dec; 1H NMR δ 2.62 (s,
24 H, NMe2), 6.49 (d, 4 H, H-3/5, 3J (1H-1H) ) 7.9 Hz), 7.16 (t, 2 H,
H-4, 3J (1H-1H) ) 7.9 Hz); 11B NMR δ -34.3 (broad); 119Sn{1H} NMR
δ 328. EI-MS (70 eV) parent ion at m/z 460 (1% intensity of the most
abundant ion); IR (Nujol) 2333 cm-1 (br).
(5) Å, c ) 26.016(10) Å, V ) 4346(2) Å3, F(000) ) 1744; Z ) 8, Fcalcd
)
1.26 g/cm-3, µ(Mo KR) ) 14.2 cm-1, specimen 0.3 × 0.1 × 0.1 mm,
3810 reflections collected for 2 < θ < 25°, 3810 independent reflections,
R1 ) 0.075 for 1930 reflections with I > 2σ(I), wR2 ) 0.183 (for all
data), S ) 1.007. 4: C20H33BN4Sn, Mr ) 459.0, monoclinic, space group
C2/c (no. 15), a ) 13.154(5) Å, b ) 11.373(7) Å, c ) 14.832(5) Å, â 98.21-
(3)°, V ) 2196(2) Å3, F(000) ) 944; Z ) 4, Fcalcd ) 1.39 g/cm-3, µ(Mo
KR) ) 11.7 cm-1, specimen 0.3 × 0.3 × 0.25 mm, 2022 reflections
collected for 2 < θ < 25°, 1941 independent reflections, R1 ) 0.0257
for 1849 reflections with I > 2σ(I), wR2 ) 0.0682 (for all data), S )
1.141. For both 3 and 4: data at T ) 173(2) K, Enraf-Nonius CAD-4
diffractometer, absorption correction, structural solution by direct
methods, full-matrix least-squares refinement on F2 using SHELXL-
93 with non-H atoms anisotropic.
(16) Drost, C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J .-M.
unpublished work. Pierssens, L. J .-M. D.Phil. Thesis, University of
Sussex, 1997.
(17) Mayer, E. P.; No¨th, H.; Rattay, W.; Wietelmann, U. Chem. Ber.
1992, 125, 401.
(18) Frankhauser, P.; Pritzkow, H.; Siebert, W. Z. Naturforsch. 1994,
49B, 250.
(19) Schmidt, H.; Keitemeyer, S.; Neumann, B.; Stammler, H.-G.;
Schoeller, W. W.; J utzi, P. Organometallics 1998, 17, 2149.

3840 Organometallics, Vol. 17, No. 18, 1998
Communications
Ta ble 1. Selected Geom etr ic P a r a m eter s for Cr ysta llin e 1-4
compd
param
116
212
3
4a
M-C, M-C′/Å
M-N, M-N′/Å
M-B/Å
C-M-C′/deg
C-M-B, C′-M-B/deg
∑/degb
2.014(8), 2.024(7)
2.394(6), 2.722(6)
2.212(5), 2.216(5)
2.607(5), 2.669(5)
1.962(8), 1.959(8)
2.110(6)
2.041(11)
112.7(3)
125.7(3), 117.2(4)
355.6
2.170(3), 2.170(3)
2.456(2), 2.456(2)
2.262(5)
118.98(13)
120.51(7), 120.51(7)
360
105.1(3)
105.6(2)
a
b
Centrosymmetric. Sum of the angles C-M-C′, C-M-B, and C′-M-B.
for the M-N and M-N′ bonds. The atom M is the
centroid of a near-equilateral (more closely for 4 than
other Lewis-base-borane adducts, e.g. Me2O-BH3 (δ
2.5), Me3N-BH3 (δ -8.3), Me2S-BH3 (δ -20.1), or
Me3P-BH3 (δ -36.8).20
3) CBC′ triangle; cf. ∑M in Table 1; the C-M-C′ angles
are wider than in the parent base 1 or 2. The dihedral
angles in 4 between the SnCBC′ plane and each plane
containing Sn and the C6N2 skeleton of a ligand VII is
78.04(8)°. The five-coordinate Sn atom may be de-
scribed as being in a trigonal-bipyramidal environment
with the N and N′ atoms occupying trans-apical sites.
To enable optimal M-N bonding to be achieved, both
C-M bonds in 4 are tilted toward the coordinating N
atoms (C(6)-C(1)-Sn ) 141.0(2)°, C(2)-C(1)-Sn )
100.4(2)°), but only one in 3 (114.6(6), 96.1(5)°) and
hence are strongly (4) or less strongly (3) divergent from
sp2 values; this is illustrated in Figure 2 for 3.
The germylene 1 and stannylene 2 have been shown
to form quite strong bonds to BH3, as is also evident
from the fact that their preparation involved displace-
ment of THF from BH3(THF). The choice of the ligand
VII in MX2 (Xj ) -C6H3(NMe2)2-2,6, VII) was crucial,
since we found that under conditions similar to those
used for the synthesis of 3 and 4 (Scheme 1), Sn[CH-
(SiMe3)2]2 was unreactive toward BH3(THF), while Sn-
[N(SiMe3)2]2 was slowly reduced to elemental tin. The
results here presented demonstrate that the germylene
1, stannylene 2, and ligand VII have a significant
potential use as new versatile ligands.
1
At 298 K the H and 13C{1H} NMR solution spectra
Ack n ow led gm en t. This work was supported by the
European Commission (category 30 fellowship for C.D.)
and the EPSRC.
of 3 and 4 each showed singlet NMe2 signals,14 indicat-
ing that (as in 1 and 212) there is a rapid exchange
involving 2-Me2N-M T 6-Me2N-M groups. The 119Sn-
{1H} signal at δ 328 in 4 was at lower frequency than
in the free base 2 (δ 442).12 The 11B signals showed
well-resolved proton coupling for 3 but not for 4, per-
haps because of 14N coupling. The 11B chemical shifts
of δ -35 (3) or δ -34 (4) are similar to those in I (M′X′n
) BH3; δ ( -35)1 or in the ylides Me3PCH2BH3 and I
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, data collection, and solution and refinement parameters,
atomic coordinates, bond distances and angles, anisotropic
displacement coefficients, and hydrogen atom coordinates for
3 and 4 (13 pages). Ordering information is given on any
current masthead page.
1
(M′Xn ) CH2BH3)1 as are the J (11B-1H) values of 73
OM980388V
Hz in 3 and 86 Hz in I (M′X′n ) BH3).1 They may
also be compared with the δ[11B] values of a variety of
(20) Wrackmeyer, B. Ann. Reports NMR Spectroscopy, 1988, 20, 61.