Bis(2,4,6-tri-tert-butylphenyl)germylene reinvestigated: Crystal structure, Lewis acid catalyzed C-H insertion, and oxidation to an unstable germanone

Article abstract of DOI:10.1021/om950558t

Bis(2,4,6-tri-tert-butylphenyl)germylene (1), which had been reported to undergo an insertion of the germanium atom into a C-H bond of an o-tert-butyl group at room temperature, was found to be quite stable. Such a C-H insertion does occur in solution, forming germaindane 2, but only in the presence of a Lewis acid such as the starting material GeCl₂. 1 can be stored unchanged at -30 °C for months; at 20 °C it decomposes within several weeks under release of 1,3,5-tri-tert-butylbenzene. Compound 1 was characterized by X-ray diffraction methods. The X-ray structure of 1 shows that the two aryl ligands have very different geometries: one aryl group is greatly distorted to give a "boat" form and the Ge atom is remote from any of the ring planes, while the second aryl group is only slightly deformed and the Ge atom lies in the ring plane. UV/vis spectra exhibit an intense absorption at 430 nm in solution and at 405 nm in the solid state, which represents an enormous hypsochromic shift compared to other germylenes with bulky aryl ligands. Oxidation of 1 with trimethylamine N-oxide leads to a germanone, which rearranges very rapidly by a C-H insertion process yielding the germaindanol 3.

Full text of DOI:10.1021/om950558t

Organometallics 1996, 15, 741-746
741
Bis(2,4,6-tr i-ter t-bu tylp h en yl)ger m ylen e Rein vestiga ted :
Cr ysta l Str u ctu r e, Lew is Acid Ca ta lyzed C-H In ser tion ,
a n d Oxid a tion to a n Un sta ble Ger m a n on e
Peter J utzi,* Holger Schmidt, Beate Neumann, and Hans-Georg Stammler
Fakulta¨t fu¨r Chemie, Universita¨t Bielefeld, Universita¨tsstrasse, D-33615 Bielefeld, FRG
Received J uly 21, 1995X
Bis(2,4,6-tri-tert-butylphenyl)germylene (1), which had been reported to undergo an
insertion of the germanium atom into a C-H bond of an o-tert-butyl group at room
temperature, was found to be quite stable. Such a C-H insertion does occur in solution,
forming germaindane 2, but only in the presence of a Lewis acid such as the starting material
GeCl2. 1 can be stored unchanged at -30 °C for months; at 20 °C it decomposes within
several weeks under release of 1,3,5-tri-tert-butylbenzene. Compound 1 was characterized
by X-ray diffraction methods. The X-ray structure of 1 shows that the two aryl ligands
have very different geometries: one aryl group is greatly distorted to give a “boat” form and
the Ge atom is remote from any of the ring planes, while the second aryl group is only slightly
deformed and the Ge atom lies in the ring plane. UV/vis spectra exhibit an intense absorption
at 430 nm in solution and at 405 nm in the solid state, which represents an enormous
hypsochromic shift compared to other germylenes with bulky aryl ligands. Oxidation of 1
with trimethylamine N-oxide leads to a germanone, which rearranges very rapidly by a C-H
insertion process yielding the germaindanol 3.
In tr od u ction
tert-butylphenyl ()Mes*),9 ((dimethylamino)ethyl)tet-
ramethylcyclopentadienyl10).
The synthesis of divalent germanium compounds with
organic ligands (germylenes) has now been a goal of
organometallic research for two decades. Species with
small, nonfunctionalized ligands aggregate quite rapidly
to form oligo- and polymeric products.1 Polymerization
can be prevented by kinetic and/or thermodynamic
stabilization.
Kinetic stabilization is also a very important factor
in the examples given above. There are only a few
germylenes which are stablilized merely through steric
effects: Lappert’s famous germylene, Ge(CH(SiMe3)2)2,
is monomeric in solution11 and in the gas phase,12 but
X-ray crystallography has shown that even the bulky
CH(SiMe3)2 ligands are not able to prevent dimerization
in the solid state.13 We found, however, that the
exchange of one ligand by the even bulkier C(SiMe3)3
group leads to a germylene which is also monomeric in
the solid state.14
Thermodynamic stabilization is achieved by the use
of n- or π-donor ligands, which bring about a reduction
of the electron deficiency by donating electrons to the
vacant p-orbital of the Ge center. The following classes
of donor-stabilized germylenes have been characterized
Considering the kinetic effects of aryl groups, the
stabilization seems to depend on the steric requirements
of these groups. Germylenes with less bulky aryl groups
2
structurally: (i) sandwich compounds of the type GeCp2
(Cp ) (substituted) cyclopentadienyl); (ii) half-sandwich
compounds of the type Cp*GeR (Cp* ) pentamethylcy-
clopentadienyl, R ) Cl,2c alkyl, aryl, amino2h,3 ) and of
the type Cp*Ge+ X-;2h,4 (iii) nitrogen-, phosphorus-, and
oxygen-substituted germylenes of the type GeX2 (X )
NR2,5 PR2,6 OR7 (R ) alkyl, aryl)); (iv) germylenes of
the type RGeCl (R ) (NtBu)SiMe2(NHtBu),8 2,4,6-tri-
(5) (a) Lappert, M. F.; Slade, M. J .; Atwood, J . L.; Zaworotko, M. M.
J . Chem. Soc., Chem. Commun. 1980, 621. (b) Pfeiffer, J .; Maringgele,
W.; Noltemeyer, M.; Meller, A. Chem. Ber. 1989, 122, 245. (c) Chorley,
R. W.; Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P.; Power, P. P.;
Olmstead, M. M. Inorg. Chim. Acta 1992, 198-200, 203. (d) Herrmann,
W. A.; Denk, M.; Behm, J .; Scherer, W.; Klingan, F.-R.; Bock, H.;
Solouki, B.; Wagner, M. Angew. Chem. 1992, 104, 1498. (e) Meller, A.;
Ossig, G.; Maringgele, W.; Noltemeyer, M.; Stalke, D.; Herbst-Irmer,
R.; Freitag, S.; Sheldrick, G. M. Z. Naturforsch. 1992, 47b, 162. (f)
Braunschweig, H.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J .-
M. Angew. Chem. 1994, 106, 1243.
X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Neumann, W. P. Chem. Rev. 1991, 91, 311.
(2) (a) Almlo¨f, J .; Fernholt, L.; Faegri, K., J r.; Haaland, A.; Schilling,
B. E. R.; Seip, R.; Taugbol, K. Acta Chem. Scand. 1983, A37, 131. (b)
Grenz, M.; Hahn, E.; du Mont, W.-W.; Pickardt, J . Angew. Chem. 1984,
96, 69. (c) Fernholt, L.; Haaland, A.; J utzi, P.; Kohl, F.-X.; Seip, R.
Acta Chem. Scand. 1984, A38, 211. (d) Schumann, H.; J aniak, C.;
Hahn, E.; Loebel, J .; Zuckerman, J . J . Angew. Chem. 1985, 97, 765.
(e) Schumann, H.; J aniak, C.; Hahn, E.; Kolax, C.; Loebel, J .; Rausch,
M. D.; Zuckerman, J . J .; Heeg, M. J . Chem. Ber. 1986, 119, 2656. (f)
J utzi, P.; Schlu¨ter, E.; Hursthouse, M. B.; Arif, A. M.; Short, R. L. J .
Organomet. Chem. 1986, 299, 285. (g) Cowley, A. H.; Mardones, M.
A.; Avendano, S.; Roman, E.; Manriquez, J . M.; Carrano, C. J .
Polyhedron 1993, 12, 125. (h) J utzi, P. Adv. Organomet. Chem. 1986,
26, 217 (review).
(6) Driess, M.; J anoschek, R.; Pritzkow, H.; Rell, S.; Winkler, U.
Angew. Chem. 1995, 107, 1746.
(7) (a) Cetinkaya, B.; Gu¨mru¨kcu¨, I.; Lappert, M. F.; Atwood, J . L.;
Rogers, R. D.; Zaworotko, M. J . J . Am. Chem. Soc. 1980, 102, 2088.
(b) Fjeldberg, T.; Hitchcock, P. B.; Lappert, M. F.; Smith, S. J .; Thorne,
A. J J . Chem. Soc., Chem. Commun. 1985, 939.
(8) Veith, M.; Hobein, P.; Ro¨sler, R. Z. Naturforsch. 1989, 44b, 1067.
(9) J utzi, P.; Leue, C. Organometallics 1994, 13, 2898.
(10) J utzi, P.; Schmidt, H.; Stammler, H. G.; Neumann, B. J .
Organomet. Chem. 1995, 499, 7.
(11) Davidson, P. J .; Harris, D. H.; Lappert, M. F. J . Chem. Soc.,
Dalton Trans. 1976, 2268.
(12) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.;
Thorne, A. J . J . Chem. Soc., Dalton Trans. 1986, 1551.
(13) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J .; Thorne, A. J . J .
Chem. Soc., Chem. Commun. 1984, 480.
(14) J utzi, P.; Becker, A.; Stammler, H. G.; Neumann, B. Organo-
metallics 1991, 10, 1647.
(3) (a) J utzi, P.; Hampel, B.; Hursthouse, M. B.; Howes, A. J .
Organometallics 1986, 5, 1944. (b) J utzi, P.; Becker, A.; Leue, C.;
Stammler, H. G.; Neumann, B.; Hursthouse, M. B.; Karaulov, A.
Organometallics 1991, 10, 3838.
(4) J utzi, P. J . Organomet. Chem. 1990, 400, 1 (review).
0276-7333/96/2315-0741$12.00/0 © 1996 American Chemical Society

742 Organometallics, Vol. 15, No. 2, 1996
J utzi et al.
Sch em e 1. Rea ction s of 1
have been reported in the literature as transient spe-
cies.15 du Mont et al. synthesized the more bulky Mes*2-
Ge (1) first in 1987 and characterized the compound by
IR, NMR, and MS methods.16 They reported that at
room temperature the compound decomposes by inser-
tion of the germanium atom into a C-H bond of an
o-tert-butyl group, leading to germaindane 2. They
further noted that the reaction of 1 with sulfur leads to
an unstable germathione which then rearranges by a
similar C-H insertion. Mochida, Udagawa, et al.
proved the monomeric character of 1 in solution by
EXAFS.17 Only recently Okazaki et al. synthesized
another bulky diarylgermylene GeTb(Tip) (Tb ) 2,4,6-
tris(bis(trimethylsilyl)methyl)phenyl, Tip ) 2,4,6-tri-
isopropylphenyl), which was characterized by a UV/vis
spectrum in solution although it has not yet been
isolated.18
During our research with Mes*-containing germ-
ylenes,3b,9 we found that the stability of 1 is much
greater than previously assumed, so that we are now
able to report the first fully characterized diarylger-
mylene which is stable under ordinary conditions.
of the acid or the polarity of the solvent could not be
found.19 Stoichiometric amounts of GeCl2(dioxane) lead
to an instantanous C-H insertion; the formation of
Resu lts
The title compound 1 was prepared from GeCl2-
(dioxane) and Mes*Li and isolated as orange-red crys-
tals in good yields. These can be stored unchanged at
a temperature of -30 °C for months. At room temper-
ature their color slowly changes; after some days the
crystals have turned dark, but 1H NMR spectra show
no sign of decomposition. Within several weeks 1
releases Mes*H and forms oligomers of so far unknown
structure; seperation and identification of the decom-
position products will be the subject of further investi-
gation. The rate of decomposition increases at higher
temperatures and is quite fast above the melting point,
but C-H insertion does not occur. After some days at
ambient temperature in various solvents, 1 decomposes,
releasing Mes*H; no other products could be traced by
1H and 13C NMR methods. At higher temperatures, this
reaction accelerates, but there is no sign of a C-H
insertion process. When exposed to air, 1 decomposes
within some hours. It is very soluble in common
aliphatic and aromatic hydrocarbons and in ether
solvents but reacts rapidly with chlorinated solvents.
1H and 13C NMR spectra give signals corresponding to
four equivalent o-tert-butyl groups even at -80 °C. UV/
vis spectra of 1 dissolved in hexane or THF show an
intense absorption maximum at 430 nm; a spectrum of
solid 1 shows a maximum at 405 nm.
9
Mes*GeCl via ligand exchange is not observed. It is
therefore very important to avoid an excess of GeCl2
during the synthesis of 1 and to extract rapidly the
product with hexane. We believe that du Mont’s results,
which he interpreted in terms of a thermally initiated
insertion, were in fact due to an insertion process
catalyzed by the presence of GeCl2(dioxane) in the
reaction mixture.
The (2π + 4π) cycloaddition with conjugated dienes
can be used to trap unstable germylenes and also to
form derivatives of stable compounds such as GeTb-
(Tip).18 In contrast to other germylenes, compound 1
does not react with 2,3-dimethylbutadiene even when
the diene is used as a solvent and the solution is stirred
for several hours.
Oxidation of 1 with trimethylamine N-oxide yields
germaindanol 3 (see Scheme 1). We have shown that
compound 2 is not an intermediate in this process, since
2 does not react at all with Me3NO.
Crystals suitable for an X-ray crystallographic analy-
sis could be prepared from a solution of 1 in hexane at
-30 °C. Figure 1 shows the crystal structure of 1 as
an ORTEP plot, and Figure 2 gives a different view in
which the methyl groups are omitted. Tables 1-3 give
selected bond lengths and angles, atomic coordinates,
and crystallographic data.
After addition of catalytic amounts (10 mol %) of a
Lewis acid to a stirred solution of 1, the mixture quickly
loses its color. 1H and 13C NMR spectra show the
exclusive formation of the C-H insertion product 2 (see
Scheme 1). The rate of the C-H insertion process
depends on the Lewis acid and the solvent used; the
time for a complete reaction varies from some seconds
to several hours. A general trend regarding the strength
There are no structural data for other diarylger-
mylenes, and so it is not possible to make precise
(19) The reaction was carried out in three different solvents (hexane,
toluene, THF) with five different Lewis acids (GeCl2(dioxane), GeCl4,
AlCl3, BBr3, TiCl4). All combinations of solvent and acid except two
gave the same product. In hexane and toluene, TiCl4 reacted with 1
under formation of unsoluble decomposition products which might be
the result of a redox process. A suspension of TiCl4‚2THF in hexane
remained unchanged but also showed no catalytic activity. To examine
the influence of the solvent we used GeCl4 and BBr3 (which are both
soluble in common solvents) as catalysts under identical conditions.
Small amounts of GeCl4 in a solution of 1 in THF lead to a complete
decolorization within a few seconds. The same process took some
minutes when carried out in toluene and several hours when performed
in hexane. BBr3 generally worked much slower: the fastest reaction
was achieved in toluene, and there was found no difference between
hexane and THF as a solvent.
(15) Ando, W.; Itoh, H.; Tsumuraya, T. Organometallics 1989, 8,
2759.
(16) Lange, L.; Meyer, B.; du Mont, W.-W. J . Organomet. Chem.
1987, 329, C17.
(17) Mochida, K.; Fujii, A.; Tsuchiya, N.; Tohji, K.; Udagawa, Y.
Organometallics 1987, 6, 1811.
(18) Tokitoh, N.; Manmaru, K.; Okazaki, R. Organometallics 1994,
13, 167.

Bis(2,4,6-tri-tert-butylphenyl)germylene
Organometallics, Vol. 15, No. 2, 1996 743
Ta ble 2. Atom ic Coor d in a tes (×104) a n d Isotr op ic
Disp la cem en t P a r a m eter s U(iso) or U(eq)a
(Å2 × 103) for 1
x
y
z
U(eq)
Ge(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
1341(1)
1522(4)
362(4)
3240(1)
2036(3)
2136(3)
2241(4)
2189(3)
1851(4)
1729(4)
2043(4)
3219(4)
1526(4)
1225(4)
2467(4)
1965(5)
3765(4)
1999(5)
1136(4)
1850(5)
851(4)
1388(1)
873(1)
526(1)
42(2)
-131(2)
195(1)
689(1)
657(2)
620(2)
1158(2)
294(2)
-659(2)
-997(2)
-714(2)
-812(2)
993(2)
1023(2)
1499(2)
743(2)
1684(1)
1453(1)
1674(2)
2131(2)
2365(2)
2158(1)
962(2)
29(1)
23(1)
24(1)
28(1)
27(1)
29(1)
25(1)
29(1)
38(1)
36(1)
45(1)
36(1)
68(2)
76(2)
69(2)
30(1)
49(1)
36(1)
54(2)
25(1)
25(1)
31(1)
30(1)
30(1)
27(1)
29(1)
46(1)
50(1)
48(1)
36(1)
54(2)
63(2)
53(2)
30(1)
41(1)
41(1)
35(1)
470(4)
1633(4)
2706(4)
2665(4)
-1020(4)
-1716(4)
-1044(4)
-1814(4)
1729(5)
577(5)
C(9)
F igu r e 1. Crystal structure of 1 (ORTEP plot with 50%
probability).
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(27)
C(28)
C(29)
C(30)
C(31)
C(32)
C(33)
C(34)
C(35)
C(36)
1697(6)
2954(5)
3861(4)
5099(4)
3623(4)
4068(5)
3157(4)
3882(4)
5128(4)
5673(4)
4917(4)
3674(4)
3440(4)
2045(4)
4322(5)
3457(5)
7067(4)
8024(4)
7151(5)
7451(5)
2957(4)
1651(4)
2649(4)
3794(4)
-8(4)
3757(4)
4545(4)
4850(4)
4477(4)
3763(4)
3399(4)
5159(4)
4878(4)
4874(4)
6462(4)
4843(4)
4344(5)
6146(4)
4411(5)
2703(4)
2179(4)
3527(4)
1704(4)
733(2)
596(2)
1047(2)
2348(2)
2042(2)
2353(2)
2867(2)
2502(1)
2276(2)
2897(1)
2742(1)
F igu r e 2. Crystal structure of 1 (methyl groups omitted
for clarity).
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 1
Ge-C(1)
2.053(4)
1.436(5)
1.380(5)
1.405(5)
1.549(5)
1.562(5)
1.397(5)
1.386(5)
1.421(5)
1.548(6)
Ge-C(19)
2.041(4)
1.392(5)
1.390(5)
1.422(5)
1.539(6)
1.416(5)
1.394(5)
1.403(5)
1.562(5)
1.548(5)
C(1)-C(2)
C(3)-C(4)
C(5)-C(6)
C(2)-C(7)
C(6)-C(15)
C(20)-C(21)
C(22)-C(23)
C(19)-C(24)
C(22)-C(29)
C(2)-C(3)
C(4)-C(5)
C(1)-C(6)
a
U(eq) is defined as one-third of the trace of the orthogonalized
C(4)-C(11)
C(19)-C(20)
C(21)-C(22)
C(23)-C(24)
C(20)-C(25)
C(24)-C(33)
Uij tensor.
C(5), and C(6) (angle R + â in Figure 3). It is also bent
by 32.2° out of the plane defined by the ring atoms C(1),
C(2), and C(6) (angle R in Figure 3). In ring 1 the o-tert-
butyl groups are bent away from the Ge atom, so that
the angles C(1)-C(2)-C(7) and C(1)-C(6)-C(15) in-
crease from 120° to about 124°. Ring 2 (containing
C(19)-C(36)) is nearly planar, and the Ge atom lies
almost in the ring plane. The o-tert-butyl groups move
further away from the Ge atom (indicated by the angles
C(19)-C(20)-C(25) (127.0(4)°) and C(19)-C(24)-C(33)
(126.4(4)°) but are closer to the ring plane than in
ring 1.
The torsional angle between the average ring planes
(least-squares method) is 43.2°, and the angles between
the ring planes and the C(1)-Ge-C(19) plane are 50.1°
(ring 1) and 75.3° (ring 2). An overall view of these
distortions is given by Figure 2.
C(1)-Ge-C(19)
C(6)-C(1)-Ge
108.0(2) C(2)-C(1)-Ge
127.4(3) C(2)-C(1)-C(6)
118.9(4) C(2)-C(3)-C(4)
116.3(4)
122.4(4) C(1)-C(6)-C(5)
123.4(4) C(3)-C(2)-C(7)
123.9(4) C(5)-C(6)-C(15)
122.5(3) C(24)-C(19)-Ge
105.1(3)
116.1(4)
123.6(4)
C(1)-C(2)-C(3)
C(3)-C(4)-C(5)
C(4)-C(5)-C(6)
C(1)-C(2)-C(7)
C(1)-C(6)-C(15)
C(20)-C(19)-Ge
119.6(4)
117.5(4)
116.2(3)
119.1(3)
C(20)-C(19)-C(24) 118.1(4) C(19)-C(20)-C(21) 119.5(4)
C(20)-C(21)-C(22) 123.1(4) C(21)-C(22)-C(23) 116.6(4)
C(22)-C(23)-C(24) 123.2(4) C(19)-C(24)-C(23) 119.3(4)
C(19)-C(20)-C(25) 127.0(4) C(21)-C(20)-C(25) 113.6(4)
C(19)-C(24)-C(33) 126.4(4) C(23)-C(24)-C(33) 114.1(3)
comparisons. It is interesting however that the lengths
of the Ge-C bonds (2.041(4), 2.053(4) Å) in 1 lie in the
same range as those in the dialkylgermylene (Me3-
Si)3CGeCH(SiMe3)214 (2.012, 2.067 Å); the same applies
for the C-Ge-C angles (108.0(2) and 111.3°, respec-
tively). The closest Ge-Ge distance in the solid is 8.64
Å, which far exceeds any binding distance.
A striking feature of the structure is the very different
conformation of the two Mes* ligands: Ring 1 (contain-
ing C(1)-C(18)) is strongly distorted into a “boat”
conformation20 with the ends tipped through about 8
and 15° (see Figure 3 and Table 4). The Ge atom is well
out of the plane defined by the ring atoms C(2), C(3),
Very close distances between the Ge atom and two of
the methyl carbon atoms are found: Ge-C(26) (2.84 Å)
and Ge-C(34) (2.77 Å). These short distances are signi-
ficant, since the sum of the van der Waals radii of a
methyl group and a germanium atom is approximately
4 Å.21-23
(20) A similar deformation of a Mes* ligand has been found in
several sterically overcrowded molecules: (a) Yoshifuji, M.;Shima, I.;
Inamoto, N.; Hirotsu, K.; Higuchi, T. Angew. Chem. 1980, 92, 405. (b)
Yoshifuji, M.; Inamoto, N.; Hirotsu, K.; Higushi, T. J . Chem. Soc.,
Chem. Commun. 1985, 1109. (c) Weber, L.; Reizig, K.; Bungardt, D.;
Boese, R. Chem. Ber. 1987, 120, 1421.

744 Organometallics, Vol. 15, No. 2, 1996
J utzi et al.
Ta ble 3. X-r a y Cr ysta llogr a p h ic Da ta for 1
formula
fw
C36H58Ge
563.41
size
cryst syst
space group
0.3 × 0.3 × 0.3 mm3
monoclinic
P21/n
a
b
c
â
V
Z
Dcalc
10.447(3) Å
11.732(4) Å
28.150(7) Å
99.11(2)°
F igu r e 3. Deformation angles of the supermesityl ligands.
3407(2) Å3
4
Ta ble 4. Defor m a tion An gles (d eg) of th e
Su p er m esityl Liga n d s
1.099 g/cm3
Siemens P21
-100 °C
Mo KR (λ ) 0.710 73 Å)
graphite
3° < 2θ < 55°
8146
7739
ring 1
ring 2
ring 1
ring 2
diffractometer
T
radiation
monochromator
range
no. of reflns collcd
no. of indpdt reflns
Rint
R
â
γ
32.2
15.1
11.9,13.9
4.9
5.1
4.5,6.0
δ
ꢀ
7.8
1.0
0.5
1.2
the combination of a short C-El bond and a small
C-El-C angle: The C-El bonds in 1 (2.04, 2.05 Å) are
among the shortest found in this class of compounds
(Mes*2Fe, 2.05 Å; Mes*2Hg, 2.08 Å; Mes*2Mg, 2.11, 2.12
Å; Mes*2Mn, 2.11 Å, Mes*2Sn, 2.26, 2.27 Å), while the
C-El-C angle (108.0°) is the second smallest (Mes*2-
Fe, 158.9°; Mes*2Mg, 158.3, 158.0°; Mes*2Mn, 159.7°;
Mes*2Hg, 173.4°; Mes*2Sn, 103.6°). It appears that the
unequal deformation of the two Mes* rings in 1 makes
it possible to avoid too close contact between nonbonded
atoms without a great expansion of the bonding angle
or a pronounced lengthening of the bonds.28
0.068
scan type
abs corr
programs
ω
ψ-scans
Siemens SHELXTL
plus/SHELXL-93
direct methods
full-matrix least-squares
on F2
solution
refinement
H atoms
params
riding model
352
µ
0.919 mm-1
1224
F(000)
goodness-of-fit
RF (basing on 4333 reflns
with I > 2σ (I))
1.373
0.080
It is noteworthy that the asymmetric nature of
compound 1 is not reflected in its solution NMR
spectra: Even at -80 °C, there is no evidence for a ring
inequivalence between the Mes* groups.
2
RF (basing on 4333 reflns
0.098
with I > 2σ (I))
largest diff peak
0.6 e Å-3
(II) Lew is Acid Ca ta lyzed C-H In ser tion . This
reaction, shown in Scheme 1, is unique in germylene
chemistry. Alkyl C-H bonds are normally stable
toward germylenes, even toward such reactive species
as Me2Ge.1 Our proposal for the mechanism for C-H
insertion in 1 attributes the activation of the germylene
to the inductive effect of the Lewis acid, when it
interacts with the lone pair on Ge. A donor-acceptor
complex is formed,29 and the electron density at the Ge
atom is decreased. The acceptor ability of the vacant
Ge orbital is thereby increased. Because of the very
close approach of Ge to a methyl group of an ortho-tert-
butyl group (see discussion of the solid-state structure),
an electrophilic attack by Ge on a C-H bond is possible.
The system then rearranges to form a Ge-C and a
Ge-H bond. At the same time the Lewis acid is
released.
Discu ssion
We have found that two Mes* ligands can kinetically
stabilize the Ge(II) compound bis(2,4,6-tri-tert-butyl-
phenyl)germylene (1). When these two bulky substit-
uents are attached to the germanium atom, the com-
pound is not able to dimerize or polymerize. Further-
more, there are certain structural pecularities which
influence both the spectroscopic properties and the
chemical behaviour. In the following sections we will
discuss the specific features which distinguish 1 from
other known germylenes.
(I) Str u ctu r a l Effects. Compared with other com-
pounds of the type Mes*2El (El ) Fe, Mg, Mn, Sn, Hg),
1 shows a much greater stereochemical difference
between the two Mes* rings. The Mes* groups in Mes*2-
Fe,24,25 Mes*2Mg,25 Mes*2Mn,25 and Mes*2Sn26 are es-
sentially equivalent; those in Mes*2Hg27 show relatively
small differences. Obviously, the effect is related with
More generally, we have not found any other example
of a Lewis acid catalyzed C-H insertion which simply
leads to a rearranged product.30
(III) La ck of Lew is Acid ity. Germylenes normally
interact with Lewis bases to form adducts. With matrix-
isolated unstable germylenes, complex formation has
(21) Pauling, L. The Chemical Bond; Cornell University Press:
Ithaca, NY, 1967.
(22) Emsley, J . The Elements, 2nd ed.; Clarendon Press: Oxford,
U.K., 1991.
(23) (a) The van der Waals radius of a methyl group is about 2.0
Å.21 A van der Waals radius for divalent Ge is difficult to define, but
the covalent radius is 1.22 Å 22 and van der Waals radii are normally
about 0.8 Å longer than covalent radii.21 (b) A “nonbonded” radius of
1.58 Å has been found for Ge in: Glidewell, C. Inorg. Chim. Acta 1979,
36, 135.
(24) Mu¨ller, H.; Seidel, W.; Go¨rls, H. Angew. Chem. 1995, 107, 386.
(25) Wehmschulte, R. J .; Power, P. P. Organometallics 1995, 14,
3264.
(26) Weidenbruch, M.; Schlaefke, J .; Scha¨fer, A.; Peters, K.; von
Schnering, H. G.; Marsmann, H. Angew. Chem. 1994, 106, 1938.
Additional structural data were kindly provided by the authors.
(27) Huffman, J . C.; Nugent, W. A.; Kochi, J . K. Inorg.Chem. 1980,
19, 2749.
(28) The difference between Mes*2Fe and Mes*2Hg might be due to
significantly differing covalent radii of the central atoms (Fe, 1.17 Å;
Hg, 1.44 Å).22
(29) Stable adducts between germylenes and 16 valence-electron
transition metal fragments have been isolated: (a) Marks, T. J . J . Am.
Chem. Soc. 1971, 93, 7090. (b) Marks, T. C.; Newman, A. R. J . Am.
Chem. Soc. 1973, 95, 769. (c) J utzi, P.; Steiner, W. Chem. Ber. 1976,
109, 3473. (d) J utzi, P.; Steiner, W. Angew. Chem. 1977, 89, 675. (e)
Lappert, M. F., Miles, S. J .; Power, P. P.; Carty, A. J .; Taylor, N. J . J .
Chem. Soc., Chem. Commun. 1977, 458. (f) J utzi, P.; Steiner, W.; Ko¨nig,
E.; Huttner, G.; Frank, A.; Schubert, U. Chem. Ber. 1978, 111, 606.
(g) du Mont, W.-W.; Lange, L.; Pohl, S.; Saak, W. Organometallics 1990,
9, 1395. (h) Veith, M.; Becker, S.; Huch, V. Angew. Chem. 1990, 102,
186. (i) See ref 18.

Bis(2,4,6-tri-tert-butylphenyl)germylene
Organometallics, Vol. 15, No. 2, 1996 745
Ta ble 5. UV/vis Ma xim a of
Dior ga n oger m ylen es15,18
germylene
λ
max/nm
germylene
λmax/nm
Me2Ge
Et2Ge
420a
440a
440a
466a
508a
Mes2Ge
Tip2Ge
Tb(Tip)Ge
Mes*2Ge
550a
558a
MePhGe
Ph2Ge
580b
405c/430b
tBuMesGe
Matrix-isolated. Solution in hexane. c Solid state. Mes )
2,4,6-trimethylphenyl, Tip ) 2,4,6-triisopropylphenyl, Tb ) 2,4,6-
tris(bis(trimethylsilyl)methyl)phenyl, and Mes* ) 2,4,6-tri-tert-
butylphenyl.
a
b
d
F igu r e 4. Configurational changes in diarylgermylenes
induced by bulky tert-butyl substituents.
We believe this effect is due to the configurational
change observed in compound 1. As discussed earlier,
this change removes the coplanarity between aryl
groups and the C(aryl)-Ge-C(aryl) plane and also
allows the bond angle at Ge to decrease (see Figure 4).
In terms of the previous discussion, this both raises the
energy of the p-orbital and lowers the energy of the
n-orbital, hence increasing the HOMO-LUMO gap. A
similar argument has been deployed to account for the
spectral changes in a series of (trimethylsily)(aryl)-
silylenes.33,34
been demonstrated using UV/vis spectra,15 while stable
germylenes with intramolecular base coordination have
been characterized by X-ray methods.8,10 Even the
sterically hindered germylene GeTb(Tip) forms a weak
adduct with the Lewis base THF, as shown by the color
change between solutions in hexane and THF.18 In
contrast, germylene 1 shows no Lewis-acidity: there is
no change in the UV/vis absorption maximum when the
solvent is similarly changed.
This lack of reactivity is presumably a result of the
strong steric shielding around Ge. The same explana-
tion may accound for the lack of reaction with 2,3-
dimethylbutadiene.
(IV) Blu e-Sh ifted Visible Sp ectr u m a n d Str u c-
tu r a l Con sequ en ces. The UV/vis spectrum of 1 shows
a dramatic hypsochromic shift of the λmax value when
compared with other sterically overcrowded diarylger-
mylenes. This is illustrated clearly by the data in Table
5. On the basis of previous spectroscopic studies and
theoretical treatments for silylenes and germylenes, we
propose the following explanation.15,31-33 The value of
As 1 and Me2Ge exhibit nearly the same value for
λ
max, the C-Ge-C angle should also be similar. How-
ever, calculations on Me2Ge have proposed an angle of
97°,12 which is much smaller than the respective angle
in 1 (108°). It has to remain the subject of further
discussion if additional effects leading to a stabilization
of the lone pair or a destabilization of the p-orbital are
responsible for the observed extreme blue shift.
(V) Oxid a tion P r ocess. Oxidation of 1 does not lead
to the expected germanone but to the germaindanol 3.
It seems likely that the initial product is a germanone
since similar intermediates have been found in the
oxidation of other germylenes.35-37 But unlike these
other germanones, Mes*2GeO does not form dimers,36
oligomers, or polymers:35 Instead insertion of the GeO
unit into a C-H bond yields 3, which can be isolated as
an air-stable solid. A comparable insertion process was
observed by du Mont et al. when 1 reacted with
elementary sulfur to produce a germaindanethiol. The
rearrangement is so fast that the germanone can neither
be isolated as such nor trapped; obviously the insertion
process results from the close Ge-methyl distance as
discussed earlier.38 Despite its great steric demand, the
Mes* group does not seem to be an appropriate ligand
to stablilize double-bonded germanium species.
λmax depends on the energy difference between the lone-
pair orbital (n, HOMO) and the vacant p-orbital (p,
LUMO) on germanium. This energy difference depends
on two factors. First, when the bond angle at Ge
increases as a result of steric interactions, the energy
of the lone-pair orbital also increases. Second, when
aryl groups are present, the energy of the p-orbital is
decreased by interaction with the aryl π* orbital; for this
to occur, however, the aromatic system must be coplanar
with the adjacent C(aryl)-Ge-C(aryl) plane. When
both these factors are present as in GeTb(Tip), the
HOMO-LUMO gap is especially small and λmax is
correspondingly large.
In the case of 1, a λmax value similar to that of GeTb-
(Tip) (580 nm)18 might be expected; two Mes* ligands
should produce a similar increase in the C-Ge-C angle
as the combination of a smaller (Tip) and a larger (Tb)
ligand. However, our experimental values of λmax ) 430
nm (solution) and 405 nm (solid state) are far smaller
and almost identical with that reported for Me2Ge (420
nm). This corresponds to a blue shift of at least 150
nm.
Exp er im en ta l Section
All manipulations were carried out using standard Schlenk
techniques under an atmosphere of dried, oxygen-free argon.
Solvents were dried and purified by standard methods.
Spectra were recorded on following instruments: 1H-NMR,
Bruker AM 300, 300.1 MHz, CHCl3 or C6D5H as internal
(34) Bott, S. G.; Marshall, P.; Wagenseller, P. E.; Wang, Y.; Conlin,
R. T. J . Organomet. Chem. 1995, 499, 11.
(30) The C-H insertion of a cationic center has been observed in
some decomposition reactions: (a) Barclay, L. R. C.; MacDonald, M.
C. Tetrahedron Lett. 1968, 7, 881. (b) Barclay, L. R. C.; Sonawane, H.
R.; MacDonald, M. C. Can. J . Chem. 1972, 50, 281. (c) Cowley, A. H.;
Kilduff, J . E.; Norman, N. C., Pakulski, M. J . Am. Chem. Soc. 1983,
105, 4845. (d) Okazaki, R.; Ishida, T.; Inamoto, N. J . Chem. Soc., Chem.
Commun. 1988, 40.
(31) Apeloig, Y.; Karni, M.; West, R.; Welsh, K. J . Am. Chem. Soc.
1994, 116, 9719.
(32) Apeloig, Y.; Karni, M. J . Chem. Soc., Chem. Commun. 1985,
1048.
(35) (a) Barrau, J .; Bouchaut, M.; Lavayssie`re, H.; Doisse, G.; Satge´,
J . Helv. Chim. Acta 1979, 62, 152. (b) Barrau, J .; Boichaut, M.; Castel,
A.; Cazes, A.; Doisse, G.; Lavayssie`re, H.; Rivie`re, P.; Satge´, J . Syn.
React. Inorg. Met.-Org. Chem. 1979, 9, 273. (c) Rivie`re, P.; Satge´, J .;
Castel, A.; Cazes, A. J . Organomet. Chem. 1979, 177, 171.
(36) Veith, M.; Becker, S.; Huch, V. Angew. Chem. 1989, 101, 1287.
(37) The successful trapping of a diaryl-germanone has been re-
ported in: Matsumoto, T.; Kishikawa, K.; Tokitoh, N.; Okazaki, R.
Phosphorus Sulfur 1994, 93-94, 177.
(38) A similar reaction has been reported for intermediate PO
units: Cadogan, F. J . G.; Cowley, A. H.; Gosney, I.; Pakulski, M.;
Yaslak, S. J . Chem. Soc., Chem. Commun. 1983, 1408.
(33) Kira, M.; Maruyama, T.; Sakurai, H. Chem. Lett. 1993, 1345.

746 Organometallics, Vol. 15, No. 2, 1996
J utzi et al.
standard; 13C-NMR, Bruker AM 300, 75.5 MHz; mass spectra,
Autospec, Sison’s instruments, where peak groups of Ge-
containing fragments are specified by their maxima; solution
UV/vis, Shimadzu UV-160 A; solid-state UV/vis, Beckman Acta
MIV; FT-IR, Mattson Polaris, Bruker IFS-66. Elemental
analyses were carried out by the analytical laboratory of the
Universita¨t Bielefeld. Melting points are uncorrected.
Bis(2,4,6-tr i-ter t-bu tylp h en yl)ger m ylen e (1). Within 5
min 20 mL of a 1.6 M solution of BuLi in hexane was added
to a stirred solution of 10.0 g of Mes*Br (30.7 mmol) in 30 mL
of THF at -60 °C . After the mixture was stirred for 1 h at
the same temperature, a solution of 3.20 g of GeCl2(dioxane)
(13.8 mmol) in 10 mL of THF was added, the solution stirred
for further 15 min, and the solvent removed in vacuo. The
residue was extracted with hexane. Cooling of the deep red
liquid yielded 4.09 g (7.26 mmol) of 1 as analytically pure
orange crystals (52.6% referring to GeCl2(dioxane)).
875 s, 800 s, 746 s, 741 s, 685 m, 646 m, 575 m, 569 m, 487 m.
MS [CI] [m/z (Irel)]: 563 (29.0%, M+ - H), 319 (97.9%,
Mes*Ge+), 231 (26.3%, [Mes* - CH2]+), 57 (100%, C4H9+).
The reaction was further carried out with GeCl2(dioxane),
AlCl3, BBr3, and GeCl4 as catalysts in hexane, THF, and
toluene; the rate of the reaction varied over a wide range.
Rea ction of 1 w ith Tr im eth yla m in e N-Oxid e. A solu-
tion of 0.36 g of 1 (0.64 mmol) in 5 mL of THF was slowly
added to a suspension of 48 mg of Me3NO (0.64 mmol) in 5
mL of THF, cooled to -60 °C. Within 10 h the mixture was
allowed to warm to ambient temperature. The solvent and
Me3N were removed in vacuo, and the residue was dissolved
in hexane. Cooling the solution to -30 °C after filtration
yielded 0.22 g of 3 as a colorless solid (0.38 mmol, 59%).
A
reaction in the presence of 2,3-dimethylbutadiene or 2,2-
dimethylbutan-3-one as a trapping reagent gave the same
product.
1H NMR (300.1 MHz, CDCl3): δ ) 0.72 (s, 1H, -OH, gives
H/D exchange in CD3OD/THF), 0.84, 0.93 (2 s, 9 H, t-Bu), 1.25
(s, 3 H, Me), 1.28, 1.29, 1.50 (3 s, 9 H, t-Bu), 1.56 (s, 3 H, Me),
1H NMR (300.1 MHz, C6D6): δ ) 1.31 (s, 36 H, o-t-Bu), 1.34
(s, 18 H, p-t-Bu), 7.39 (s, 4 H, ring H). 13C-NMR (75.5 MHz,
C6D6): δ ) 31.5 (p-(C(CH3)3), 34.2 (o-C(CH3)3), 34.7 (p-C(CH3)3),
39.2 (o-C(CH3)3), 122.4 (m-ring C), 150.2 (p-ring C), 158.3 (o-
2
2
1.60 (d, J HH ) 14.3 Hz, 1 H, CH2), 1.74 (d, J HH ) 14.3 Hz, 1
ring C), 162.1 (ipso-ring C). UV/vis (hexane or THF): λmax
)
)
4
4
H, CH2), 7.18 (d, J HH ) 2.0 Hz, 1 H, ring H), 7.23 (d, J HH
)
430 nm (ꢀ ) 520 L‚mol-1‚cm-1). UV/vis (solid state): λmax
1.7 Hz, 1 H, ring H), 7.32 (d, 4J HH ) 1.7 Hz, 1 H, ring H), 7.33
405 nm. IR (KBr, cm-1): 2962 s, 2903 m, 2865 m, 1593 m,
1577 m, 1476 s, 1464 s, 1390 s, 1360 s, 1243 s, 1209 m, 1197
m, 1120 m, 898 m, 874 s, 743 s, 648 s, 564 m, 478 m. MS [CI]
[m/z (Irel)]: 563 (11.6%, M+ - H), 319 (21.2%, Mes*Ge+), 246
(38.9%, Mes*H+), 231 (100%, [Mes* - CH2]+). Mp: 61 °C.
Anal. Calcd for C36H58Ge: C, 76.74; H, 10.38. Found: C,
76.48; H, 10.65.
(d, J HH ) 2.0 Hz, 1 H, ring H). 1H NMR (300.1 MHz, C6D6):
4
δ ) 0.76 (s, 1 H, -OH), 1.05, 1.06, 1.30 (3 s, 9 H, t-Bu), 1.33
(s, 3 H, Me), 1.34 (s, 9 H, t-Bu), 1.68 (s, 3 H, Me), 1.71 (d, 2J HH
2
) 14.4 Hz, 1 H, CH2), 1.75 (s, 9 H, t-Bu), 1.88 (d, J HH ) 14.4
4
Hz, 1 H, CH2), 7.36 (d, J HH ) 1.9 Hz, 1 H, ring H), 7.44 (d,
4J HH ) 1.6 Hz, 1 H, ring H), 7.53 (d, J HH ) 1.6 Hz, 1 H, ring
4
H), 7.58 (d, 4J HH ) 1.9 Hz, 1 H, ring H). 13C NMR (75.5 MHz,
CDCl3): δ ) 31.2, 31.4 (C(CH3)3), 31.6 (-(CH3)2C-Ge), 32.1
(C(CH3)3), 32.6 (-CH2-), 32.9, 33.7 (C(CH3)3), 34.4, 34.8, 37.4,
38.7 (quart C), 39.1 (-(CH3)2C-Ge), 39.3, 42.0 (quart C), 119.1,
122.3, 122.9, 123.7 (tert ring C), 131.7, 134.3, 149.6, 152.5,
153.8, 157.4. 159.0, 159.2 (quart ring C). IR (KBr, cm-1): 3409
s, 2961 s, 2908 m, 2866 m, 1592 s, 1535 m, 1479 m, 1464 s,
1393 s, 1362 s, 1237 m, 1214 m, 1138 m, 904 m, 880 s, 738 s,
693 m, 663 m, 646 m, 623 s, 580 m, 569 m, 489 m, 421 m, 382
m. MS [CI] [m/z (Irel)]: 579 (7.1%, M+ - H), 563 (21.8%, M+
- OH), 335 (100%, M+ - Mes*), 57 (79.2%, C4H9+). Mp: 175
°C. Anal. Calcd for C38H58GeO: C, 74.62; H, 10.09. Found:
C, 74.56; H, 9.94.
Rea ction of 1 w ith Lew is Acid s (Exa m p le).19 To a
stirred solution of 0.20 g of 1 (0.36 mmol) in 2 mL of THF was
added a solution of 6 mg of TiCl4 (0.03 mmol) in 0.25 mL of
THF. The solution lost its color within 10-15 s. After removal
of the volatiles the colorless residue was examined by NMR,
IR, and MS methods. These indicate the exclusive formation
of germaindane 2 (NMR, IR, and MS data are in accord with
the literature16).
1H NMR (300.1 MHz, CDCl3): δ ) 0.77, 0.89 (2 s, 9 H, t-Bu),
1.14, (s, 3 H, Me), 1.27, 1.28, 1.53 (3 s, 9 H, t-Bu), 1.59 (s, 3 H,
2
3
Me), 1.61 (dd, J HH ) 14.0 Hz, J HH ) 2.4 Hz, 1 H, CH2), 1.69
2
3
(dd, J HH ) 14.0 Hz, J HH ) 5.2 Hz, 1 H, CH2), 5.87 (m, 1H,
4
4
Ge-H), 7.14 (d, J HH ) 2.1 Hz, 1 H, ring-H), 7.20 (d, J HH
)
1.7 Hz, 1 H, ring H), 7.23 (d, 4J HH ) 2.1 Hz, 1 H, ring H), 7.30
(d, J HH ) 1.7 Hz, 1 H, ring H). 1H NMR (300.1 MHz, C6D6):
4
Ack n ow led gm en t. The financial support of the
“Deutsche Forschungsgemeinschaft” and the “Fonds der
Chemischen Industrie” as well as a gift of germanium
from Chemetall GmbH, Frankfurt/Main, Germany, is
gratefully acknowledged.
δ ) 1.01, 1.05 (2 s, 9 H, t-Bu), 1.27 (s, 3 H, Me), 1.33, 1.34,
2
1.65 (3 s, 9 H, t-Bu), 1.75 (s, 3 H, Me), 1.78 (dd, J HH ) 14.0
3
2
3
Hz, J HH ) 2.4 Hz, 1 H, CH2), 1.89 (dd, J HH ) 14.0 Hz, J HH
4
) 5.2 Hz, 1 H, CH2), 6.24 (m, 1H, Ge-H), 7.39 (d, J HH ) 1.7
4
Hz, 1 H, ring H), 7.41 (d, J HH ) 2.1 Hz, 1 H, ring H), 7.48 (d,
4J HH ) 1.7 Hz, 1 H, ring H), 7.50 (d, J HH ) 2.1 Hz, 1 H, ring
4
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data and structure refinement, measurement, and program
parameters, hydrogen coordinates and isotropic displacement
parameters, bond lengths and angles, and anisotropic dis-
placement parameters (6 pages). Ordering information is
given on any current masthead page.
H); 13C NMR (75.5 MHz, C6D6): δ ) 29.7 (-CH2-), 31.4, 31.6
(C(CH3)3), 31.9 (-(CH3)2C-Ge), 32.5, 33.6, 34.0 (C(CH3)3), 34.6,
34.9, 37.6, 39.0 (quart C), 39.1 (-(CH3)2C-Ge), 39.2, 43.5
(quart C), 118.8, 122.5, 123.4, 123.7 (tert ring C), 131.2, 133.6,
149.8, 152.2, 154.8, 157.9, 159.5, 160.4 (quart ring C). IR (KBr,
cm-1): 2958 s, 2903 m, 2866 m, 2086 s, 1588 s, 1532 m, 1478
m, 1464 s, 1392 s, 1361 s, 1239 m, 1212 m, 1197 m, 1138 m,
OM950558T