Synthesis and characterization of the monomeric sterically encumbered diaryls E{C6H3-2,6-(C6H3-2,6-Pr i2)2}2 (E = Ge, Sn, or Pb)

Article abstract of DOI:10.1002/zaac.200500532

The reaction of two equivalents of LiC₆H₃-2,6-(C ₆H₃-2,6-Prⁱ₂)₂ with GeCl₂·dioxane, SnCl₂ or PbBr₂ in a diethyl ether solution resulted in the isolation of the monomeric σ-bonded diaryl tetrylene series E {C₆H₃-2,6-(C₆H ₃-2.6-Prⁱ₂)₂}₂ (E = Ge (1), Sn (2), or Pb(3)). Compounds 1-3 are highly sterically congested blue crystalline solids, which possess V-shaped structures and wide interligand bond angles. The solid state structures of 1-3 were determined by single-crystal X-ray methods while their solution structures were investigated by UV spectroscopy and in the cases of 2 and 3, respectively, by ¹¹⁹Sn and ²⁰⁷Pb NMR spectroscopy. The series 1-3 constitutes the most sterically crowded examples of σ-bonded diorgano group 14 derivatives yet isolated and, in contrast to previously reported:ER₂ species, the C-E-C angles increase with increasing atomic number.

Full text of DOI:10.1002/zaac.200500532

DOI: 10.1002/zaac.200500532
Synthesis and Characterization of the Monomeric Sterically Encumbered
Diaryls E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ؍ Ge, Sn, or Pb)
Geoffrey H. Spikes, Yang Peng, James C. Fettinger, and Philip P. Power*
Davis, CA / USA, University of California Davis, Department of Chemistry
Received December 22nd, 2005.
Dedicated to Professor Hansgeorg Schnöckel on the Occasion of his 65th Birthday
Abstract. The reaction of two equivalents of LiC6H3-2,6-(C6H3-2,6-
Pri2)2 with GeCl2·dioxane, SnCl2 or PbBr2 in a diethyl ether solu-
tion resulted in the isolation of the monomeric σ-bonded diaryl
tetrylene series E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ϭ Ge (1), Sn (2),
or Pb(3)). Compounds 1-3 are highly sterically congested blue cry-
stalline solids, which possess V-shaped structures and wide interli-
gand bond angles. The solid state structures of 1-3 were determined
by single-crystal X-ray methods while their solution structures were
investigated by UV spectroscopy and in the cases of 2 and 3, respec-
tively, by 119Sn and 207Pb NMR spectroscopy. The series 1-3 consti-
tutes the most sterically crowded examples of σ-bonded diorgano
group 14 derivatives yet isolated and, in contrast to previously re-
ported :ER2 species, the C-E-C angles increase with increasing
atomic number.
Keywords: Germanium; Tin; Lead; Tetrylenes; Steric effects
Introduction
thought to be stabilized in solution by intramolecular F...E
interactions [14Ϫ16] while dimethylamine substituted aryl
species are stabilized by a secondary metal-amine interac-
tion in the solid state [17, 18]. However, the use of bulky
terphenyls, which act as unidentate ligands without lone
pairs on the ligating atom, has been shown to avoid these
secondary interactions [19]. Our group has previously em-
ployed the very bulky terphenyl ligand ArЈ (ArЈ ϭ C6H3-
2,6-(C6H3-2,6-Pri2)2) to stabilise the synthesis and structural
characterization of the alkyne analogues of germanium [20,
21] and tin [22] ArЈEEArЈ (E ϭ Ge, Sn) and the alkene
analogue ArЈ(H)GeGe(H)ArЈ [23, 24]. Here we report the
synthesis and characterization of the heavier tetrylene series
E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ϭ Ge (1), Sn (2), or
Pb(3)).
The development of sterically encumbering ligands to sta-
bilize heavier low-valent group 14 element alkylidene, al-
kene and alkyne analogues has been a major theme in main
group organometallic chemistry over the past three decades.
The first series of stable σ-bonded metallane diyls, the di-
alkyls E{CH(SiMe3)2}2 (E ϭ Ge, Sn or Pb), were reported
by Lappert and co-workers beginning in 1973 [1Ϫ4]. The
germanium [2], tin [1] and lead [5] derivatives were dimeric,
with weak E-E interactions in the solid state, but their
chemistry proved to be consistent with their formulation as
bent singlet :ER2 species in solution [6]. A small increase
in steric congestion due to further silyl substitution, as in
Ge{CH(SiMe3)2}C(SiMe3)3, or structural rigidity as in the
cyclic species ETC(SiMe3)2(CH2)2CU(SiMe3)2(E ϭ Ge, Sn) led
to the isolation and structural characterisation of mono-
mers in the crystalline phase [7Ϫ9].
Experimental Section
All operations were carried out by using modified Schlenk tech-
niques under an atmosphere of dry argon or nitrogen. Solvents
were dried over an alumina column and degassed prior to use. The
chemicals used in this study were purchased from Aldrich or Acros
and used as received. GeCl2·dioxane [3] and ArЈLi (ArЈ ϭ C6H3-
2,6(C6H3-2,6-Pri2)2) [25] were prepared as described in the litera-
ture. 1H, 13C, 119Sn, and 207Pb NMR spectroscopic data were re-
corded on Varian INOVA 400 MHz and 300 MHz spectrometers.
1H and 13C NMR spectra were referenced to the deuterated
solvent, 119Sn NMR externally to SnMe4/C6D6 and 207Pb NMR
externally to PbMe4/C6D6. UV-vis data were recorded on a
Hitachi-1200 spectrometer.
Similar complexes, stabilized by bulky diaryl ligands have
demonstrated a tendency for further metal-ligand interac-
tions. The well characterized GeMes*2 (Mes* ϭ C6H2-
2,4,6-But3) species [10, 11] undergoes a C-H insertion reac-
tion below room temperature while the equivalent tin spec-
ies SnMes*2 has been shown to rearrange to the arylalkyl-
stannylene species SnMes*(CH2CMe2C6H2-2,5-But2) in
solution [12, 13]. Trifluoromethyl aryl derivatives are
* Prof. Philip P. Power
Department of Chemistry, University of California Davis
1 Shields Avenue
Davis, CA 95616 / USA
Preparation of :Ge(ArЈ)2, (1)
To a slurry of GeCl2·dioxane(0.70 g, 3.00 mmol) in Et2O (20 mL)
E-mail: pppower@ucdavis.edu
was added a solution of ArЈLi (3.00 g, 7.43 mmol) (ArЈ ϭ C6H3-
Z. Anorg. Allg. Chem. 2006, 632, 1005Ϫ1010
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1005

G. H. Spikes, Y. Peng, J. C. Fettinger, P. P. Power
2,6-(C6H3-2,6-Pri2)2 in Et2O (100 mL) at ambient temperature. The
resultant green solution was stirred at room temperature for 24 h.
Removal of the solvent in vacuo followed by hexane extraction,
concentration to incipient crystallisation and storage at ca. Ϫ18 °C
yielded 1 as air and moisture sensitive blue crystals, (yield 0.91 g,
1.05 mmol, 35 %). Mp: 159-161 °C.
1H NMR (C7D8, 300.08 MHz, 90 °C): δ 0.76 (m, 24H), 0.97 (m, 24H,
CHMe2), 2.96 (m,. 8H , CHMe2), 6.74 (m, 4H, Ar-H), 6.9-7.18 (m, 14H, Ar-
H). 13C NMR (C7D8, 75.45 MHz, 90 °C): δ 24.2 (CHMe2), 26.2 (CHMe2),
31.5 (CHMe2), 124.1, 127.2, 127.5, 133.7, 137.3, 139.7, 145.4, 175.0 (unsatu-
rated carbon atom). UV-Vis (n-hexane): 608 nm (ε ϭ 1320).
Table 1 Selected X-ray Crystallographic Data for Compounds 1,
2 and 3.
compound
formula
fw
:Ge(ArЈ)2, (1)
C60H74Ge.0.5C6H14 C60H74Sn.C7H8 C60H74Pb
910.81
:Sn(ArЈ)2, (2)
:Pb(ArЈ)2, (3)
1006.02
blue cube
orthorhombic
Fddd
25.1923(7)
26.6369(7)
32.6326(8)
90
1002.38
blue block
monoclinic
P21/c
19.7729(19)
11.6855(11)
21.704(2)
90
96.847(2)
90
4979.2(8)
4
color, habit
cryst syst
space group
blue block
triclinic
¯
P1
˚
a /A
11.3595(5)
12.7280(5)
19.5446(8)
82.655(1)
87.907(1)
69.292(1)
2621.46(19)
2
˚
b /A
˚
c /A
Ͱ /deg
β /deg
γ /deg
90
90
Preparation of :Sn(ArЈ)2, (2)
3
˚
V /A
21897.9(1)
16
Z
In a similar manner SnCl2 (0.57 g, 3.00 mmol) was reacted with
ArЈLi (3.00 g, 7.43 mmol) to yield 2 as a blue, air and moisture
sensitive, powder, yield 1.70 g, 1.86 mmol, 62 %). Mp: 128 °C (de-
comp.).
cryst dims, mm 0.48 x 0.24 x 0.09 0.35 x 0.28 x 0.12 0.25 x 0.09 x 0.07
dcalc, /(g/cm3)
µ /mmϪ1
no. of reflns
1.099
0.619
34331
1.221
0.506
45805
6305
1.337
3.425
35651
9262
no. of obsd reflns 10903
1H NMR (C6D6, 400.08 MHz, 25 °C): 0.93 (m, 12H, CHMe2), 1.03 (d, 12H,
J ϭ 6.9 Hz, CHMe2), 1.13 (m, 18H, CHMe2), 1.37 (d, 6H, J ϭ 6.9 Hz,
CHMe2), 2.87 (sept, 4H, J ϭ 6.6 Hz, CHMe2), 3.09 (m, 4H, CHMe2), 6.97-
7.32 (m, 18H, Ar-H). 13C NMR (C6D6, 75.45 MHz, 25 °C): δ 24.4 (CHMe2),
24.5 (CHMe2), 24.9 (CHMe2), 25.9 (CHMe2), 30.2 (CHMe2), 30.8 (CHMe2),
31.8 (CHMe2), 121.1, 121.5, 122.9, 123.0, 127.6, 128.2, 130.0, 138.3, 139.7,
145.5, 145.7, 146.1, 147.1, 197.9, 199.2 (unsaturated carbon). 119Sn {1H}
NMR (C6D6, 149.00 MHz, 25 °C): δ 2235. UV-Vis (n-hexane): 600 nm (ε ϭ
1430).
R, obsd reflns
wR2, all
0.0496
0.1695
0.0310
0.0865
0.0572
0.1438
Further details are in the Supporting Information: Crystallographic
data for the structures have been deposited with the Cambridge
Crystallographic Data Centre. CCDC 293514Ϫ293516. Copies of
the data can be obtained free of charge on application to the Direc-
tor, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax:
int.code ϩ(1223)336-033; e-mail: deposit@ccdc.cam.ac.uk).
Blue crystals of 2 suitable for X-ray crystallography were obtained
by dissolving the blue powder in toluene (10 mL) reducing the vol-
ume to about 4 ml, and cooling in a freezer for 1 day at ca. Ϫ18 °C.
Results and Discussion
Preparation of :Pb(ArЈ)2, (3)
The compounds E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ϭ Ge
(1), Sn (2), or Pb(3)) were obtained in moderate yields of
35 %, 62 % and 58 %, respectively, by the reaction of two
equivalents LiC6H3-2,6-(C6H3-2,6-Pri2)2 with the corre-
sponding metal dihalide in diethyl ether. Removal of the
solvent in vacuo followed by extraction with n-hexane pro-
vided samples of 1Ϫ3 for spectroscopic analysis. X-ray
quality blue crystals of 1 and 3 were obtained by recrystalli-
zation from n-hexane and of 2 by recrystallization from
toluene. Crystals of 1 possess considerable thermal stability
with a melting point of 159Ϫ161 °C while those of 2 and 3
decompose in the solid state at 128 °C and 98 °C, respec-
tively.
In a similar manner PbBr2 (1.10 g, 3.00 mmol) with ArЈLi (3.00 g,
7.43 mmol) yielded 3 as, air and moisture sensitive, blue crystals,
(yield 1.74 g, 1.74 mmol, 58 %). Mp: 90 °C (decomp.).
1H NMR (C6D6, 400.08 MHz, 25 °C): 0.91 (d, 6H, J ϭ 6.9 Hz, CHMe2),
1.01 (d, 6H, J ϭ 6.9 Hz, CHMe2), 1.09 (d, 6H, J ϭ 6.9 Hz, CHMe2), 1.18
(d, 12H, J ϭ 6.9 Hz, CHMe2), 1.25 (d, 12H, J ϭ 6.9 Hz, CHMe2), 1.36 (d,
6H, J ϭ 6.9 Hz, CHMe2), 2.76 (sept, 4H, J ϭ 6.6 Hz, CHMe2), 2.90 (sept,
2H, J ϭ 6.6 Hz, CHMe2), 3.01 (sept, 2H, J ϭ 6.6 Hz, CHMe2), 7.12-7.27
(m, 14H, Ar-H), 7.71 (d, 4H, Ar-H). 13C NMR (C6D6, 100.52 MHz, 25 °C):
δ
24.4 (CHMe2), 24.5 (CHMe2), 24.6 (CHMe2), 24.7 (CHMe2) 24.8
(CHMe2), 25.7 (CHMe2), 30.1 (CHMe2), 30.8 (CHMe2), 31.2 (CHMe2),
122.6, 122.9, 123.0, 124.6, 126.3, 128.6, 139.7, 141.1, 143.3, 146.2, 146.9,
147.0, 147.1 (unsaturated carbon atom). 207Pb NMR {1H} (C6D6,
62.77 MHz, 25 °C): δ 9430. UV-Vis (n-hexane): 586 nm (ε ϭ 1490).
The products 1Ϫ3 were characterized by 1H and 13C
NMR spectroscopy. At ambient temperature (25 °C) 1 dem-
onstrated a highly complicated 1H NMR spectrum with
broad peaks corresponding to a number of inequivalent iso-
propyl environments with restricted rotation indicative of a
highly congested steric environment. At elevated tempera-
tures (90 °C) in solutions of d8-toluene the broad peaks
sharpened and two regions could be observed correspond-
ing to the isopropyl CHMe2 protons however no coupling
X-ray Crystalllographic Studies
The crystals were removed from the Schlenk tube under a rapid
flow of argon and immediately submerged in hydrocarbon oil. A
suitable crystal was selected, mounted on a glass fiber attached to
a copper pin, and rapidly placed in the cold stream of N2 of the
diffractometer for data collection. Data for 1, 2 and 3 were col-
lected on a Bruker SMART 1000 with use of MoKα (λ ϭ
˚
0.71073 A) radiation and a CCD area detector. Empirical absorp-
tion corrections were applied using SADABS [26]. The structures
were solved with use of either direct methods or the Patterson op-
tion in SHELXS and refined by the full-matrix least-squares pro-
cedures in SHELXL [27]. Non-hydrogen atoms were refined aniso-
tropically while hydrogens were placed at calculated positions and
included in the refinement using a riding model. Some details of
data collection and refinement are provided given in Table 1.
1
information could be resolved. The H NMR spectra of 2
and 3 at ambient temperature did not show the same degree
of inequivalency. However in both cases three isopropyl en-
vironments indicated by three isopropyl hydrogen and six
overlapping diastereotopic methyl resonances were dis-
cernable in the ratio 2:2:4 indicating some restricted ro-
1006
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 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 1005Ϫ1010

Monomeric Sterically Encumbered Diaryls E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ϭ Ge, Sn, or Pb)
Table 2 UV-Vis data and C-Ge-C angles /° for selected compounds with two-coordinated divalent germanium atoms.
λmax /nm
C-Ge-C /°
Ref.
Ge{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (1)
Ge(C6H2-2,4,6-Pri3)(C6H2-2,4,6-(CH(SiMe3)2)3)
Ge{C6H3-2,6-Mes2}2
608
580
578
112.8
unknown
114.4
this work
[31]
[19]
G
T
eC(SiMe3)2(CH2)2C
U
(SiMe3)2
450
430
91
108
[8]
[10]
GeMes*2
Ge{CH(SiMe3)2}2
Ge{C6H2-2,4,6-(CF3)3}2
414
374
107 (gas phase)
100.0
[1, 3]
[32]
Ge(C6H3-2,6-(1-naphthyl)2)2
Ge{CH(SiMe3)2}{C(SiMe3)3}
Ge{C6H3-2,6-(NMe2)2}2
Orange
Red/Orange
Orange
102.7
111.3
105.1
[29]
[7]
[17, 18]
Table 3 119Sn chemical shifts, UV-Vis data and C-Sn-C angles /° for selected compounds
With divalent two-coordinate tin atoms.
λmax /nm
119Sn /ppm
C-Sn-C /°
Ref.
Sn{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (2)
Sn(C6H2-2,4,6-Pri3)(C6H2-2,4,6-(CH(SiMe3)2)3)
Sn{C6H3-2,6-Mes2}2
600
561
553
546
495
484
476
345
Red
Red
Red
2235
2208
1971
2299
2328
2323
980
723
442
1323
1426
117.6
unknown
114.7
117.6
97 (gas phase)
86.7
103.6
98.3
105.6
this work
[33]
[19, 34]
[35]
[1, 36]
[9]
[12]
[14]
[17]
S
T
nC(SiMe3)2SiMe2CH2SiMe2C
Sn{CH(SiMe3)2}2
nC(SiMe3)2(CH2)2C
SnMes*2
U
(SiMe3)2
ST
U(SiMe3)2
Sn{C6H2-2,4,6-(CF3)3}2
Sn{C6H3-2,6-(NMe2)2}2
Sn(C6H2-2,4,6-Pri3)C(N2)SiPri3
99.2
94.4
[37]
[37]
S
T
nC6H2-2,4,-Pri2-6-C(Me)CH2C
U
SiPri3
tation of the flanking phenyl rings in solution.Similar in-
equivalence was observed in the corresponding 13C NMR
spectra. This is in marked contrast to the previously iso-
lated series E{C6H3-2,6-Mes2}2 (E ϭ Ge, Sn, or Pb, Mes ϭ
C6H2-2,4,6-Me3) where in all cases only one ortho-methyl
environment was observed and no dynamic character was
observed in the range Ϫ90 to 80 °C [19].
gether with a corresponding decrease in λmax [28]. Compari-
son of the above maxima with those of the less crowded
series E{C6H3-2,6-Mes2}2 [19] (E ϭ Ge, Sn, or Pb) (λmax ϭ
578, 553 and 526 nm) demonstrates the expected higher val-
ues for λmax caused by the increased size of ArЈ in compari-
son to C6H3-2,6-Mes2 (Tables 2, 3 and 4). Wider C-E-C
bond angles are therefore expected for 1, 2 and 3 (see be-
low). The recently synthesized and structurally charac-
terized diarylgermylenes Ge(triph)2 (triph ϭ C6H2-2,4,6-
Ph3) and Ge(bisnap)2 (bisnap ϭ C6H3-2,6-(1-naphthyl)2),
which were reported as ‘strain-free’ germylenes with little
steric congestion, were not analysed by UV-vis spectroscopy
but were reported as orange solids which suggests a much
lower λmax and consequently a higher HOMO-LUMO gap
[29].
In addition, compounds 1Ϫ3 were studied by UV-vis
spectroscopy. Absorption maxima were observed at 608,
600 and 586 nm, respectively. Heavier tetrylenes :ER2 (E ϭ
Ge, Sn, Pb) exist in a singlet ground state with the lone pair
on E in a mainly s-type orbital and an empty formally non-
bonding p-orbital. λmax corresponds to the singlet-triplet
gap, which can be correlated within limits to the solution
C-E-C bond angle [28]. In cases of extreme sterical conges-
tion at the metal atom, widening of the C-E-C bond angle
with a corresponding lengthening of the E-C bonds is ob-
served. This lowers the energy of the p-orbital and raises
the energy of the n-orbital thereby decreasing the HOMO-
LUMO gap [19]. However other factors including the elec-
tronic properties of the ligand and the presence of second-
ary element-ligand interactions clearly play a role [28]. For
example, the correlation between λmax and the interligand
angle does not provide a full explanation for a blue-shift
observed for GeMes*2 (λmax ϭ 430 nm) [11] in comparison
to SnMes*2 (λmax ϭ 476 nm) [12] suggesting that metal p-
orbital / π or π* interactions from geometrically distorted
aryl groups are possible. Nonetheless, an increasing singlet-
triplet gap is observed in the sequence Ge < Sn < Pb to-
The 119Sn NMR chemical shift of 2 (2235 ppm) is in the
range previously observed for monomeric species 2328-
1971 ppm (Table 3) but is shifted down-field in comparison
to SnMes*2 (980 ppm) [12] and the trifluoromethyl aryl de-
rivative Sn{C6H2-2,4,6-(CF3)3}2 (723 ppm) [14]. This can be
rationalized from the general expectation of low shielding
due to the low coordination environment at the tin atoms
with the latter examples stabilized by further metal-ligand
interactions in solution. It is not possible however to ration-
alize the 119Sn NMR chemical shifts on the basis of the σ-
electron donor characteristics of the ligands since the shift
values tend to be dominated by the paramagnetic contri-
butions [30]. A wider interligand angle is expected to in-
crease such contributions and this is consistent with the
Z. Anorg. Allg. Chem. 2006, 1005Ϫ1010
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1007

G. H. Spikes, Y. Peng, J. C. Fettinger, P. P. Power
Table 4 207Pb NMR chemical shifts, UV-Vis data and C-Pb-C angles /° for selected compounds with two-coordinate divalent lead atoms.
λmax /nm
207Pb /ppm
C-Pb-C /°
Ref.
P
T
bC(SiMe3)2SiMe2CH2SiMe2C
U
(SiMe3)2
610
610
586
10050
9751
9430
117.1
116.3
121.5
[38]
[39]
this work
Pb{(C6H2-2,4,6-(CH(SiMe3)2)3)}2
Pb{C6H3-2,6-(C6H3-2,6-Pri2)2}2(3)
Pb(C6H2-2,4,6-(CH2SiMe3)3)
(C6H2-2,4,6-(CH(SiMe3)2)3)
560
556
550
531
526
520
462
470
470
466
460
406
Purple
Purple
Yellow
8873
8858
8888
8884
8844
8275
7275
7853
8738
7420
6657
5067
9112
8888
4878
unknown
97.6
unknown
unknown
114.5
102.5
94.5
100.5
91.8
101.4
95.6
[39]
[41]
[39]
[39]
[19, 40]
[41]
[41]
[40]
[41]
[40]
[40]
[5]
Pb{C6H3-2,6-(C6H2-2,4,6-Pri3)2}CH2C6H4-4-Pri
Pb(C6H2-2,4,6-Pri3)(C6H2-2,4,6-(CH(SiMe3)2)3)
Pb{CH(SiMe3)2}(C6H2-2,4,6-(CH(SiMe3)2)3)
Pb{C6H3-2,6-Mes2}2
Pb{C6H3-2,6-(C6H2-2,6-Pri2)2}But
Pb{C6H3-2,6-(C6H2-2,6-Pri2)2}C6H4-4-But
Pb{C6H3-2,6-(C6H2-2,4,6-Pri3)2}But
Pb{C6H3-2,6-(C6H3-2,6-Pri2)2}Me
Pb{C6H3-2,6-(C6H2-2,4,6-Pri3)2}Me
Pb{C6H3-2,6-(C6H2-2,4,6-Pri3)2}Ph
PbMes*(CH2CMe2C6H2-2,5-But)2
Pb{CH(SiMe3)2}2
94.8
unknown
unknown
94.5
[42]
[39]
[16]
Pb(C6H2-2,4,6-Pri3)(C6H2-2,4,6-(CH(SiMe3)2)3)
Pb{C6H2-2,4,6-(CF3)3}2
266 ppm downfield chemical shift of 2 (C-Sn-C ϭ 117.6°)
in comparison to Sn{C6H3-2,6-Mes2}2 (C-Sn-C ϭ 114.7°)
[19]. Similarly the 207Pb NMR chemical shift of 3
(9430 ppm) is in the range previously observed (10050-
7420 ppm) (Table 4) but downfield of Pb{C6H2-2,4,6-
(CF3)3}2 (4878 ppm) [16]. In addition the chemical shift of
3 is almost 600 ppm further downfield than that of
Pb{C6H3-2,6-Mes2}2 which is consistent with the wider C-
Pb-C angle (121.5° vs 114.5°) [19].
X-ray Crystal Structures
Crystals of 1-3 were studied by X-ray diffraction and ther-
mal ellipsoid plots are shown in Figures 1, 2 and 3, respec-
tively. They exist as V-shaped, discrete monomers with the
˚
closest E-E (E ϭ Ge, Sn, Pb) distances being 10.245 A,
˚
˚
11.648 A and 9.777 A, respectively. The E-C bond lengths
˚
˚
˚
are 2.033(2) A and 2.048(2) A for 1, 2.255(2) A for 2 and
2.379(9) A and 2.390(8) A for 3 with any further metal-li-
gand interactions being longer than 3.0 A. These values are
˚
˚
˚
Figure 1 Thermal ellipsoid of 1 with 50 % probability. Hydrogen
˚
similar to those seen in previously reported σ-bonded di-
valent organometallic species although for 3 a lengthening
atoms are not shown. Selected bond distances /A and angles /°:
Ge(1)-C(1) 2.033(2), Ge(1)-C(31) 2.048(2), C(1)-C(6) 1.415(3), C(1)-C(2)
1.419(3), C(31)-C(32) 1.428(3), C(31)-C(36) 1.432(3); C(1)-Ge(1)-C(31)
112.77(9), C(32)-C(31)-C(36) 116.4(2), C(32)-C(31)-Ge(1) 139.4(2), C(36)-
C(31)-Ge(1) 102.7(2), C(6)-C(1)-C(2) 118.0(2), C(6)-C(1)-Ge(1) 125.5(2),
C(2)-C(1)-Ge(1) 113.8(2).
˚
of the Pb-C bonds of 0.03-0.04 A is observed in comparison
˚
to Pb{C6H3-2,6-Mes2}2 (2.334(12) A). However the C-E-C
bond angles vary from 112.77(9)° for Ge to 121.5(3)° for
Pb whereas for E{C6H3-2,6-Mes2}2 (E ϭ Ge, Sn, Pb) they
were essentially invariant having values of 114.4(2)°,
114.7(2)° and 114.5(6)°, respectively [19]. The large C-Ge-
C angles in 1 and Ge{C6H3-2,6-Mes2}2 may also be com-
pared with those experimentally observed in Ge(C6H3-2,6-
(1-naphthyl)2)2 (102.72(9)°) and calculated for GePh2
(101.6°) [29]. In the solid state the crowded nature of
Ge{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (1) leads to significant dis-
tortions in the terphenyl substituents. The flanking rings of
one terphenyl substituent are tilted by the angles 15.89° and
8.68° from the C-C vectors (i.e. C(32)-C(37) and C(36)-
C(49)) through which they are attached to the central phe-
nyl ring and the central phenyl ring is tilted 27.6° from the
Ge(1)-C(31) vector (Fig. 4). In contrast for 2 the flanking
rings of the terphenyl substituents are tilted by 8.30° and
1.35° and the central phenyl ring is only tilted by 14.1° from
the Sn(1)-C(1) vector whilst for 3 the terphenyl sustituents
are tilted by 9.77Ϫ4.94° and the central phenyl rings are
tilted by 14.07° and 14.54°. For 2 and 3 the C-E-C (E ϭ Sn,
Pb) bond angles are wider than those in previously reported
species which is consistent with the steric bulk of ArЈ. The
highly red shifted UV-vis absorption maxima observed for
1008
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 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Monomeric Sterically Encumbered Diaryls E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E ϭ Ge, Sn, or Pb)
Figure 4 Thermal ellipsoid plot with 50 % probability illustrating
the distortions of one of the terphenyl substituents in 1.
rently known C-E-C (E ϭ Ge, Sn or Pb) angles for :ER2
species in the case of tin and lead and the second widest in
the case of germanium. Examination of their UV-Vis spec-
tra show that they display the longest wavelength n Ϫ p,
HOMO-LUMO, transitions of all known diorgano deriva-
tives of GeII, SnII and PbII. The red shift to > 600 nm in
the case of the germanium and tin derivatives 1 and 2 led
us to speculate that the C-E-C angles may be even wider in
solution than they are in the solid state. This is consistent
with the experimentally determined angular sequence C-Pb-
C (121.5°) > C-Sn-C (117.6°)> C-Ge-C (112.8°) found in
the solid state which is opposite to that normally expected
on steric grounds (cf. C-E-C in E{C6H2-2,4,6-(CF3)3}2, E ϭ
Ge (100.0°) [32], Sn (98.3°) [14], Pb (94.5°) [16].
Figure 2 Thermal ellipsoid of 2 with 50 % probability. Hydrogen
atoms are not shown. Selected bond distances /A and angles /°:
Sn(1)-C(1) 2.255(2), C(1)-C(2) 1.412(2), C(1)-C(6) 1.421(2); C(1)Ј-Sn(1)-C(1)
117.56(8), C(2)-C(1)-C(6) 117.3(2), C(2)-C(1)-Sn(1) 125.30(12), C(6)-C(1)-
Sn(1) 114.72(11).
˚
Acknowledgements. We would like to acknowledge the National
Science Foundation for funding this work and Dr. A. F. Richards
and M. Jarosh for their help in solving the crystal structure of 2.
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