Reactions of the heavier group 14 element alkyne analogues Ar′EEAr′ (Ar′ = C6H3-2,6(C 6H3-2,6-Pri2)2; E = Ge, Sn) with unsaturated molecules: Probing the character of the EE multiple bonds

Article abstract of DOI:10.1021/ja055372s

Reactions of the alkyne analogues Ar′EEAr′ (Ar′ = C ₆H₃-2,6(C₆H₃-2,6-Prⁱ ₂)₂; E = Ge (1); Sn (2)) with unsaturated molecules are described. Reaction of 1 and 2 with azobenzene afforded the new hydrazine derivatives Ar′E{(Ph)NN(Ph)}EAr′ (E = Ge (3); Sn (4)). Treatment of 1 with Me₃SiN₃ gave the cyclic singlet diradicaloid Ar′Ge{μ₂-(NSiMe₃)}₂GeAr′ (5), whereas 2 afforded the monoimide bridged Ar′Sn{μ₂- N(SiMe₃)}-SnAr′ (6). Reaction of 1 with t-BuNC or PhCN yielded the adduct Ar′GeGe(CNBuⁱ)Ar′ (7) or the ring compound Ar′Ge-N(Ph)CC(Ph)N-GeAr′ (8). In contrast, the tin compound 2 did not react with either t-BuNC or PhCN. Treatment of 1 with N ₂CH(SiMe₃) generated Ar′Ge{μ₂- CH(SiMe₃)}{μ₂:η²-N ₂CH(SiMe₃)}{μ₂-N₂-CH(SiMe ₃)}GeAr′ (9) which contains ligands in three different bridging modes and no Ge-Ge bonding. Reaction of 1 with an excess of N₂O gave a germanium peroxo species Ar′(HO)Ge(μ₂-O) (μ₂:η²-O₂)Ge-(OH)Ar′ (10) which features a Ge-O-Ge-O-O ring. Oxidation of 1 by tetracyanoethylene (TCNE) led to cleavage of the Ge-Ge bond and formation of a large multiring system of formula Ar′Ge³⁺{(TCNE)^2-}_3- {(GeAr′) ⁺}₃. The digermyne 1 also reacted with 1 equiv of Ph≡CPh to give the 1,2-digermacyclobutadiene 12, which has a Ge-C-C-Ge ring, and with Me₃SiC≡CH or PhC≡C-C≡CPh to activate a flanking C₆H₃-2,6-Prⁱ₂ ring and give the tricyclic products 13 and 14. The distannyne 2 did not react with these acetylenes. Overall, the experiments showed that 1 is highly reactive toward unsaturated molecules, whereas the corresponding tin congener 2 is much less reactive. A possible explanation of the reactivity differences in terms of the extent of the singlet diradical character of the Ge-Ge and Sn-Sn bonds is discussed.

Full text of DOI:10.1021/ja055372s

Published on Web 11/15/2005
Reactions of the Heavier Group 14 Element Alkyne Analogues
Ar′EEAr′ (Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2; E ) Ge, Sn) with
Unsaturated Molecules: Probing the Character of the EE
Multiple Bonds
Chunming Cui, Marilyn M. Olmstead, James C. Fettinger, Geoffrey H. Spikes, and
Philip P. Power*
Contribution from the Department of Chemistry, UniVersity of California DaVis,
One Shields AVenue, DaVis, California 95616
Received August 16, 2005; E-mail: pppower@ucdavis.edu
Abstract: Reactions of the alkyne analogues Ar′EEAr′ (Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2; E ) Ge (1); Sn (2))
with unsaturated molecules are described. Reaction of 1 and 2 with azobenzene afforded the new hydrazine
derivatives Ar′E{(Ph)NN(Ph)}EAr′ (E ) Ge (3); Sn (4)). Treatment of 1 with Me3SiN3 gave the cyclic singlet
diradicaloid Ar′Ge{µ2-(NSiMe3)}2GeAr′ (5), whereas 2 afforded the monoimide bridged Ar′Sn{µ2-N(SiMe3)}-
SnAr′ (6). Reaction of 1 with t-BuNC or PhCN yielded the adduct Ar′GeGe(CNBut)Ar′ (7) or the ring
compound Ar′GesN(Ph)CC(Ph)NsGeAr′ (8). In contrast, the tin compound 2 did not react with either t-BuNC
or PhCN. Treatment of 1 with N2CH(SiMe3) generated Ar′Ge{µ2-CH(SiMe3)}{µ2:η2-N2CH(SiMe3)}{µ2-N2-
CH(SiMe3)}GeAr′ (9) which contains ligands in three different bridging modes and no GesGe bonding.
Reaction of 1 with an excess of N2O gave a germanium peroxo species Ar′(HO)Ge(µ2-O)(µ2:η2-O2)Ge-
(OH)Ar′ (10) which features a GesOsGesOsO ring. Oxidation of 1 by tetracyanoethylene (TCNE) led to
cleavage of the GesGe bond and formation of a large multiring system of formula Ar′Ge3+{(TCNE)2-}3-
{(GeAr′)+}3. The digermyne 1 also reacted with 1 equiv of PhtCPh to give the 1,2-digermacyclobutadiene
12, which has a GesCsCsGe ring, and with Me3SiCtCH or PhCtCsCtCPh to activate a flanking C6H3-
2,6-Pri2 ring and give the tricyclic products 13 and 14. The “distannyne” 2 did not react with these acetylenes.
Overall, the experiments showed that 1 is highly reactive toward unsaturated molecules, whereas the
corresponding tin congener 2 is much less reactive. A possible explanation of the reactivity differences in
terms of the extent of the singlet diradical character of the GesGe and SnsSn bonds is discussed.
Introduction
This trend indicates increasing lone pair character and decreasing
E-E bond order upon descending the group, and this finding
Stable heavier group 14 element multiply bonded species have
attracted considerable attention over the last two decades because
of their unusual bonding and structures.1-4 Despite the large
number of theoretical calculations5 and attempts to synthesize
them,6 stable heavier alkyne congeners REER (E ) Si-Pb)
remained unknown until recently.7-11 The structures8-11 of these
compounds disclosed a trans-bent rather than linear geometry
in which the R-M-M angle decreases from silicon to lead.
is well correlated with computational results.5,12 Few reactions
of these compounds have been reported, however.13 It is highly
desirable to explore their chemical reactivity in order to under-
stand the bonding in these unique molecules. We have already
described the synthesis and structures of the alkyne analogues
of germanium9 and tin10 Ar′EEAr′ (Ar′ ) C6H3-2,6(C6H3-2,6-
Pri2)2; E ) Ge (1), Sn(2)) as well as their reduction with alkali
(6) For example: (a) Sekiguchi, A.; Zigler, S. S.; West, R.; Michl, J. J. Am.
Chem. Soc. 1986, 108, 4241. (b) Pietschnig, R.; West, R.; Powell, D. R.
Organometallics 2000, 19, 2724. (c) Wiberg, N.; Finger, C. M. M.; Polborn,
K. Angew. Chem., Int. Ed. 1993, 32, 1054. (d) Karni, M.; Apeloig, Y.
Schroeder, D.; Zummack, W., Rabezanna, R.; Schwartz, H. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 332.
(1) (a) Davidson, P. J.; Lappert, M. F. Chem. Commun., 1973, 317. (b)
Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. Chem.
Commun. 1976, 261.
(2) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(3) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R.
K. Chem. Commun. 1981, 191.
(7) Power, P. P. Chem. Commun. 2003, 2091.
(4) (a) Tokitoh, N.; Okazaki, R. In The chemistry of organic germanium tin
and lead compounds; Rapport, Z., Ed.; Wiley: Chichester, 2002; Vol. 2,
Chapter 13. (b) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39. (c)
Escudie, J.; Ranaivonjatovo, H. AdV. Organomet. Chem. 1999, 44, 113.
(d) Power, P. P. Chem. ReV. 1999, 99, 3463.
(5) (a) Karni, M.; Kapp, J.; Schleyer, P. v. R.; Apeloig, Y. In The Chemistry
of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley:
Chichester, 2002; Vol. 3, Chapter 1. (b) Ganzer, I.; Hartmann, M.; Frenking,
G. In The Chemistry of Organic Germanium Tin and Lead Compounds;
Rappoport, Z., Ed.; Wiley: Chichester, 2002; Vol. 2, Chapter 3.
(8) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524.
(9) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 1785.
(10) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 2002, 124, 5930.
(11) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755.
(12) Grev, R. S. AdV. Organomet. Chem. 1991, 33, 125. (b) Nagase, S.;
Kobayashi, K.; Takagi, N. J. Organomet. Chem. 2000, 611, 264. (c) Takagi,
N.; Nagase, S. Eur. J. Inorg. Chem. 2002, 2775.
(13) Stender, M.; Phillips, A. D.; Power, P. P. Chem. Commun. 2002, 1312.
9
17530
J. AM. CHEM. SOC. 2005, 127, 17530-17541
10.1021/ja055372s CCC: $30.25 © 2005 American Chemical Society

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
Scheme 1. Summary of Reactions Undergone by the Germanium and Tin Alkyne Analogues Ar′EEAr′ (M ) Ge, 1; Sn, 2; Ar′ )
C6H3-2,6(C6H3-2,6-Pri2)2) with Unsaturated Molecules
metals to give the reduced ions [Ar′EEAr′]- and [Ar′EEAr′]2-.14,15
The successful isolation of 1 and 2 in gram quantities has
allowed us to study their reaction chemistry with a variety of
species and has allowed insight into their electronic and
structural properties as well as a comparison of their reaction
patterns with the lighter alkynes. We have communicated the
initial result that Ar′GeGeAr′ (1) could be added to alkynes in
a [2+2] fashion to afford the first stable, formally antiaromatic
cyclobutadiene analogue incorporating heavier main group 14
{µ2-N(SiMe3)}2GeAr′,17 and have described briefly the high
reactivity of these compounds.18 We now present a more
extensive study of the reactivity of Ar′EEAr′ (E ) Ge, Sn)
toward a variety of unsaturated molecules (summarized in
Scheme 1) and discuss the implications of our results for the
MM bonding in these compounds.
Experimental Section
All operations were carried out by using modified Schlenk techniques
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. Ar′EEAr′ (M ) Ge,9 Sn10) were prepared as described in the
elements, i.e., 1,2-digerma-cyclobutadiene Ar′Ge{(Ph)CC(Ph)}-
GeAr′.16 In addition we reported that 1 reacted with N3SiMe3
to liberate N2 with formation of the diradicaloid, Ar′Ge-
1
literature. The H, 13C, 119Sn NMR spectroscopic data were recorded
1
on Varian INOVA 300 or 400 MHz spectrometers. The H and 13C
(14) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626.
(15) Singly and doubly reduced ionic species of formula [Ar*MMAr*]1-,2-
(M ) Ge or Sn; Ar* ) C6H3-2,6(C6H2-2,4,6-Pri3)2) having the related Ar*
substituents were discovered before the neutral Ar′EEAr′ compounds.
See: (a) Olmstead, M. M.; Simons, R. S.; Power, P. P. J. Am. Chem. Soc.
1997, 119, 11705. (b) Pu, L.; Senge, M. O.; Olmstead, M. M.; Power, P.
P. J. Am. Chem. Soc. 1998, 120, 12682.
NMR spectra were referenced to the deuterated solvent, and the 119Sn
NMR spectra were externally referenced to SnMe4/C6D6. UV-vis data
were recorded on a Hitachi-1200 spectrometer.
(17) Cui, C.; Brynda, M.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.
2004, 126, 6510.
(18) Power, P. P. Appl. Organomet. Chem. 2005, 19, 488.
(16) Cui, C.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2004, 126,
5062.
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17531

A R T I C L E S
Cui et al.
23.06 (CHMe2), 24.57 (CHMe2), 25.74 (CHMe2), 25.76 (CHMe2), 29.18
(CMe3), 30.86 (CMe3), 30.90 (CHMe2), 31.96 (CHMe2), 123.29, 123.86,
123.90, 125.19, 126.89, 129.06, 129.25, 129.54, 129.91, 141.49, 144.49,
145.83, 146.88, 147.19, 147.65, 148.24 (Ar-C). UV-vis (n-hexane):
λ ) 490 nm (ꢀ ) 3000). IR (KBr, Nujol): 2092 (m), CN stretch.
Ar′Ge{(Ph)NN(Ph)}GeAr′ (3) and Ar′Sn{(Ph)NN(Ph)}SnAr′ (4):
A mixture of Ar′GeGeAr′ (0.152 g, 0.160 mmol) and PhNNPh (0.029
g, 0.160 mmol) in n-hexane (10 mL) was stirred at room temperature
for 16 h. The solvent volume was reduced to ca. 3 mL under reduced
pressure. Storage at -20 °C for 2 days afforded orange crystals of 3
(0.140 g, 78%). Mp: 147 °C. 1H NMR (C6D6, 399.77 MHz) (all
broad): δ 1.04 (d, J ) 6.8 Hz, 48H, CHMe2), 1.29 (d, J ) 6.8 Hz,
24H, CHMe2), 2.71 (sept., J ) 6.8 Hz, 8H, CHMe2), 6.90 (d, J ) 6.8
Hz, 4H, m-C6H3), 6.97 (m, 4H, m-C6H5), 7.07 (d, J ) 8.0 Hz, 4H,
8H, m-Dipp), 7.19 (m, 2H, p-C6H3), 7.27 (m, 6H, p-Dipp and p-C6H5),
8.18 (d, J ) 8.2 Hz, 4H, o-C6H5). 13C{1H}NMR (C6D6, 100.59 MHz):
δ 24.90 (CHMe2), 25.39 (CHMe2), 30.93 (CHMe2), 123.23 (m-C6H5),
123.91 (m-Dipp), 126.62 (p-C6H3), 129.18 (p-C6H5), 129.36 (m-C6H3),
131.02 (i-C6H5), 139.60 (p-Dipp), 144.99 (i-Dipp), 146.61 (i-C6H3),
153.10 (o-C6H5), 158.09 (o-C6H3). UV-vis (n-hexane): λ ) 521 nm
(ꢀ ) 2800). In a similar manner, reaction of Ar′SnSnAr′ (0.17 g, 0.165
mmol) and azobenzene (0.030 g, 0.165 mmol) in toluene at room
temperature for 16 h, followed by recrystallization from n-hexane at
-20 °C, afforded orange crystals of 4 (0.128 g, 64%). Mp: 164 °C.
1H NMR (C6D6, 399.77 MHz): δ 0.96 (d, J ) 6.8 Hz, 6H, CHMe2),
1.01 (d, J ) 6.8 Hz, 12H, CHMe2), 1.07 (d, J ) 6.8 Hz, 6H, CHMe2),
1.11 (d, J ) 6.8 Hz, 6H, CHMe2), 1.30 (d, J ) 6.8 Hz, 6H, CHMe2),
1.36 (d, J ) 6.8 Hz, 6H, CHMe2), 1.46 (d, J ) 6.8 Hz, 6H, CHMe2),
2.83 (m, 4H, CHMe2), 3.37 (m, 4H, CHMe2), 6.54 (t, J ) 7.2 Hz, 2H,
p-Dipp), 6.72 (t, J ) 7.2 Hz, 2H, p-Dipp), 7.05 (t, J ) 7.2 Hz, 2H,
p-C6H3), 7.09 (m, 8H, m-Dipp), 7.22 (m, 10H, m-C6H3, m-C6H5 and
p-C6H5), 8.02 (d, J ) 7.2 Hz, 4H, o-C6H5). 13C NMR (C6D6, 100.59
MHz): δ 21.58 (CHMe2), 22.54 (CHMe2), 24.36 (CHMe2), 26.30
(CHMe2), 26.75 (CHMe2), 28.57 (CHMe2), 31.01 (CHMe2), 118.99,
123.06, 123.57, 124.24, 125.25, 128.69, 129.21, 129.62, 130.41, 130.81,
144.21, 145.62, 146.37, 147.37, 148.80, 153.25, 156.42, 158.08 (Ar-
C). 119Sn NMR (C6D6, 149.00 MHz): δ 463.9. UV-vis (n-hexane):
λ ) 486 nm (ꢀ ) 1700).
Ar′Ge{(Ph)CNNC(Ph)}GeAr′ (8): To a solution of 1 (0.124 g,
0.132 mmol) in n-hexane was added neat PhCN (0.028 g, 0.272 mmol).
The mixture was stirred at room temperature for 3 days. The solvent
was removed under reduced pressure, and the remaining red powder
was crystallized from diethyl ether at -20 °C to give orange crystals
1
of 8 (0.077 g, 51.6%). Mp: 167 °C (dec). H NMR (C6D6, 399.77
MHz): δ 0.96 (d, J ) 6.8 Hz, 24H, CHMe2), 1.23 (d, J ) 6.8 Hz,
24H, CHMe2), 2.75 (sept., J ) 6.8 Hz, 8H, CHMe2), 6.90 (m, 6H,
Ar-H), 7.10 (m, 12H, Ar-H), 7.32 (m, 6H, Ar-H), 8.12 (d, 4H, C6H5).
13C NMR (C6D6, 100.59 MHz): δ 21.78 (CHMe2), 25.36 (CHMe2),
32.06 (CHMe2), 122.90, 124.76, 125.87, 126.21, 130.26, 133.05, 136.42,
137.06, 138.02, 138.23, 148.67, 157.40, 169.38 (Ar-C). UV-vis (n-
hexane): λ ) 421 nm (ꢀ ) 4000).
Ar′Ge{µ2:η1-CH(SiMe3)}{µ2:η1-N2CH(SiMe3)}{µ2:η2-N2CH(Si-
Me3)}GeAr′ (9): To a solution of 1 (0.170 g, 0.181 mmol) in n-hexane
(10 mL) was added N2CHSiMe3 (0.6 mL, 1 M solution in n-hexane,
0.60 mmol). The mixture was stirred at room temperature for 2 d, and
the volume of the solution was reduced to ca. 3 mL under reduced
pressure. The solution was stored at ca. -20 °C for 1 week to give
1
yellow crystals of 9 (0.127 g, 56.0%). Mp: 118 °C (dec). H NMR
(C6D6, 399.77 MHz): δ -0.09 (s, 9H, SiMe3), -0.02 (s, 9H, SiMe3),
0.12 (s, 9H, SiMe3), (CHMe2), 1.17(d, J ) 6.8 Hz, 24H, CHMe2), 2.80
(sept., J ) 6.8 Hz, 8H, CHMe2), 6.90 (d, 4H, Ar-H), 7.10 (m, 8H,
Ar-H), 7.21 (m, 2H, Ar-H), 7.30 (m, 4H, Ar-H). 13C{1H} NMR
(C6D6, 100.59 MHz): δ 20.94 (CHMe2), 25.02 (CHMe2), 30.76
(CHMe2), 123.45, 124.09, 126.89, 130.16, 138.23, 143.40, 145.13,
147.09, 155.74 (Ar-C).
Ar′(HO)Ge(µ2-O)(µ2:η2-O2)Ge(OH)Ar′(10): A solution of 1 (0.170
g, 0.181 mmol) in toluene (5 mL) was exposed to an N2O atmosphere,
causing the original red solution to become yellow immediately. The
solution was stirred for 30 min, concentrated under reduced pressure,
and stored at -20 °C for 2 days to furnish colorless crystals of 10
Ar′Ge{µ2-N(SiMe3)}2GeAr′ (5) and Ar′Sn{µ2-N(SiMe3)}SnAr′
(6): To a solution of 1 (0.100 g, 0.106 mmol) in n-hexane (3 mL) was
added an excess of Me3SiN3 (0.073 g, 0.64 mmol). After the reaction
mixture was stirred at room temperature for 48 h, it was stored at 5 °C
for 2 d to afford dark violet crystals of 5 (0.104 g, 88%). Mp: 145 °C
(dec). 1H NMR (C7D8, 399.77 MHz): δ -0.30 (s, 18H, SiMe3), 0.91
(d, J ) 6.8 Hz, 48H, CHMe2), 1.18 (d, J ) 6.8 Hz, 24H, CHMe2),
2.75 (sept., J ) 6.8 Hz, 8H, CHMe2), 6.91 (m, 6H, Ar-H), 7.03 (m,
4H, m-C6H5), 7.10 (m, 12H, Ar-H). 13C NMR (d8-toluene, 100.59
MHz): δ 5.15 (SiMe3), 23.92 (CHMe2), 26.34 (CHMe2), 31.82
(CHMe2), 123.21, 125.90, 128.43, 130.12, 137.12, 137.56, 138.75,
152.98, 172.45 (Ar-C). UV-vis (n-hexane): λ ) 521 nm (ꢀ ) 6000).
In a similar manner, 2 (0.109 g, 0.106 mmol) in toluene (3 mL) was
reacted with an excess of Me3SiN3 (0.073 g, 0.64 mmol). The solution
was stirred at room temperature for 2 d and stored at ca. -20 °C for
1
(0.103 g, 57.5%). Mp: 127 °C (dec). H NMR (C6D6, 399.77 MHz):
δ 0.92 (d, J ) 6.8 Hz, 24H, CHMe2), 1.23 (d, J ) 6.8 Hz, 24H, CHMe2),
1.45 (s, 2H, OH), 2.91 (sept., J ) 6.8 Hz, 8H, CHMe2), 6.94 (d, 4H,
Ar-H), 7.08 (m, 8H, Ar-H), 7.24 (m, 2H, Ar-H), 7.32 (m, 4H, Ar-
H). 13C NMR (C6D6, 100.59 MHz): δ 20.72 (CHMe2), 24.87 (CHMe2),
30.26 (CHMe2), 122.98, 123.79, 126.05, 130.00, 136.83, 140.28, 145.92,
147.12, 154.47 (Ar-C). IR (KBr, Nujol): 3621 (s), O-H stretch.
Ar′Ge[(NC)2CC(CN)2]3(GeAr′)3 11. A mixture of 1 (0.170 g, 0.181
mmol) and tetracyanoethylene (0.035 g, 0.27 mmol) in toluene (10 mL)
was stirred at room temperature for 3 d. The solvent was removed under
reduced pressure, and the residue was dissolved in diethyl ether (15
mL). Storage at -20 °C for 3 d afforded pale yellow crystals of 11.
1
1 week to afford red crystals of 6 (0.075 g, 62.0%). Mp: 132 °C. H
1
NMR (C6D6, 399.77 MHz): δ -0.32 (s, 9H, SiMe3), 0.99 (d, J ) 6.8
Hz, 48H, CHMe2), 1.18 (d, J ) 6.8 Hz, 24H, CHMe2), 3.11 (sept., J )
6.8 Hz, 8H, CHMe2), 7.05 (m, 6H, Ar-H), 7.07 (m, 4H, Ar-H), 7.10
(m, 4H, Ar-H), 7.20 (m, 4H, Ar-H). 13C{1H} NMR (C6D6, 100.59
MHz): δ 3.26 (SiMe3), 21.74 (CHMe2), 24.20 (CHMe2), 29.18
(CHMe2), 121.32, 125.74, 127.56, 130.38, 138.71, 137.18, 139.14,
154.62, 174.35 (Ar-C). 119Sn NMR (C6D6, 149.00 MHz): δ 907.1.
UV-vis (n-hexane): λ ) 435 nm (ꢀ ) 7000).
Mp: 162 °C. H NMR (C6D6, 399.76 MHz): δ 0.86 (d, J ) 6.8 Hz,
12H, CHMe2), 0.98 (m, 12H, CHMe2), 1.08 (m, 12H, CHMe2), 1.13
(m, 18H, CHMe2), 1.25 (d, 12H, CHMe2), 1.33 (m, 18H, CHMe2), 1.45
(d, J ) 6.8 Hz, 12H, CHMe2), 2.38 (sept, 4H, J ) 6.6 Hz, CHMe2),
2.77 (sept, 4H, J ) 6.6 Hz, CHMe2), 2.97 (m, 8H, CHMe2), 6.94 (m,
6H, Ar-H), 7.03 (m, 3H, Ar-H), 7.11 (m, 6H, Ar-H), 7.12 (d, 3H,
Ar-H), 7.18-7.22 (m, 9H, Ar-H), 7.32-7.38 (m, 3H, Ar-H), 7.72
(m, 6H, Ar-H). 13C{1H} NMR (C6D6, 100.52 MHz): δ 22.69, 23.31,
23.43, 23.60, 25.17, 25.24, 25.51, 25.58, 26.08 (CHMe2), 30.90, 31.16,
31.22, 31.42 (CHMe2), 122.47, 122.89, 123.00, 123.88, 124.60, 125.80,
129.40, 129.61, 129.73, 131.42, 132.04, 134.70, 134.82, 138.93, 139.00,
145.74, 146.48, 146.73, 147.21, 147.33, 147.50, 147.89, 149.62, 150.44.
Ar′GeGe(CNBut)Ar′ (7): To a solution of 1 (0.162 g, 0.134 mmol)
in n-hexane (5 mL) was added neat t-BuNC (0.012 g, 0.14 mmol) at
room temperature. The solution was stirred for 20 h and stored at -20
°C for 1 week to give red dark crystals of 7 (0.065 g, 47%). Mp: 125
1
°C (dec). H NMR (C6D6, 399.77 MHz) (all broad): δ 0.88 (s, 9H,
Reaction with Alkynes; Synthesis of 12, 13, and 14. A mixture of
1 (1, 0.140 g, 0.149 mmol) and diphenylacetylene (0.053 g, 0.30 mmol)
in n-hexane (30 mL) was stirred at room temperature for 7 d. The
resulting deep red solution was concentrated and stored at 6 °C
t-Bu), 0.97 (m, 24H, CHMe2), 1.22 (m, 12H, CHMe2), 1.40 (m, J )
6.8 Hz, 12H, CHMe2), 2.94 (m, 8H, CHMe2), 6.94 (m, 8H, ArsH),
7.18 (m, 8H, ArsH). 13C NMR (C6D6, 100.59 MHz): δ 14.36 (CtN),
9
17532 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
Table 1. X-ray Crystallographic Data for Compounds 3, 4, 6, 7, 8, 9, 10, 11, 14
compd
3
‚n-hexane
4
‚
n-hexane
6
‚
0.5 n-hexane
7
‚
n-hexane
formula
fw
color, habit
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C78H98Ge2N2
1208.76
yellow, prism
orthorhombic
Pna21
20.743(4)
12.008(2)
27.095(5)
90
90
90
6749(2)
4
C78H98N2Sn2
1300.96
dark red, plate
monoclinic
P21
11.9584(19)
27.587(5)
20.622(3)
90
90.114(7)
90
6803(2)
4
C66H90NSiSn2
1162.86
orange, plate
monoclinic
I2/a
21.694(4)
18.392(4)
32.292(7)
90
96.47(3)
90
12803(4)
8
C71H96Ge2N
1108.67
red, needle
monoclinic
P21/c
14.585(3)
18.300(4)
24.129(5)
90
90.03(3)
90
6440(2)
4
Z
cryst dims, mm
0.40 × 0.28 × 0.28
0.52 × 0.46 × 0.22
0.12 × 0.10 × 0.02
0.34 × 0.34 × 0.12
d
calc, g/cm3
1.190
1.270
1.207
1.143
µ, mm-1
1.411
6496
4765
0.0328
0.777
40367
37791
0.0281
0.835
11720
7157
0.0577
0.972
14754
10483
0.0828
no. of reflns
no. of obsd reflns
R, obsd reflns
wR2, all
0.0779
0.0631
0.1736
0.1961
8
‚
0.5Et2O
0.65PhCN
compd
0.35Me(c-Pent)
9
‚
n-hexane
10
‚
0.5n-hexane
11
‚
(Et2O)2.25
14‚(Et2O)0.5
formula
fw
color, habit
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C
83.65H98.45Ge2N2.65
C78H118Ge2N4Si3
1341.21
yellow, plate
triclinic
C63H83Ge2O5
1065.47
colorless, block
monoclinic
P21/c
14.868(6)
18.065(7)
22.078(9)
90
104.633(9)
90
5737(4)Å
4
C
147H170.50Ge4N12O2.25
C96H104Ge2O
1418.97
yellow, rod
triclinic
1286.18
orange yellow, plate
triclinic
2431.81
pale yellow, block
monoclinic
P21/n
19.3869(9)
29.4879(13)
25.0943(11)
90
102.0570(10)
90
14029.4(11)
4
P1h
P1h
P1h
13.0890(17)
14.2078(17)
21.234(3)
93.795(3)
91.656(3)
115.630(3)
3545.2(8)
2
12.8515(9)
14.9117(11)
20.4565(15)
80.746(3)
80.117(3)
83.322(4)
3795.9(5)
2
13.7999(8)
15.1927(9)
20.3401(12)
87.3680(10)
79.4960(10)
66.6180(10)
3847.0(4)
2
Z
cryst dims, mm
0.40 × 0.24 × 0.10
0.30 × 0.23 × 0.15
0.12 × 0.09 × 0.05
0.20 × 0.15 × 0.14
0.46 × 0.19 × 0.15
d
calc, g/cm3
1.205
1.173
1.233
1.151
1.225
µ, mm-1
0.893
21151
15425
0.0468
0.882
17358
13971
0.0356
1.094
7505
3607
0.0907
0.902
24669
15875
0.0663
0.830
17678
12284
0.0425
no. of reflns
no. of obsd reflns
R, obsd reflns
wR2, all
0.1317
0.0937
0.2620
0.1814
0.1119
overnight to give dark red crystals of 12 (0.15 g, 78%). Mp: 178 °C.
1H NMR (C6D6, 300.08 MHz): δ 0.77 (br, 24H, CHMe2), 1.07 (br,
24H, CHMe2), 3.04 (sept, 8H, J ) 6.6 Hz, CHMe2), 6.65 (d, 4H, J )
2.5 Hz, Ar-H), 6.71 (d, 6H, J ) 2.5 Hz, Ar-H), 6.98 (d, 6H, J ) 7.5
Hz, Ar-H), 7.09-7.21 (m, 6H, Ar-H), 7.29 (d, 4H, J ) 6.9 Hz, Ar-
H). 13C{1H} NMR (C6D6, 100.52 MHz): δ 23.5 (CHMe2), 26.1
(CHMe2), 31.1 (CHMe2), 123.5, 126.4, 127.1, 129.1, 129.3, 130.8,
138.0, 140.2, 145.7, 146.8, 147.5, 159.2 (unsaturated carbon). UV-
vis (hexane): 375 nm (shoulder). The compound was 13 prepared in
a manner similar to 12, after work up yellow crystals of 13 were
obtained at -20 °C from n-hexane (0.12 g, 84%). Mp: 225 °C (dec).
1H NMR (C6D6, 300.08 MHz): δ -0.21 (s, 9H, SiMe3), -0.15 (s, 9H,
SiMe3), 0.24 (d, 6H, CHMe2), 0.90, 1.01, 1.19, 1.23-1.45 (m, 42H,
CHMe2), 1.68 (br, 1H, CHMe2), 2.03 (sept, 1H, J ) 6.7 Hz, CHMe2),
2.28 (sept, 1H, J ) 6.7 Hz, CHMe2), 2.82 (sept, 2H, J ) 6.7 Hz,
CHMe2), 3.05 (sept, 2H, J ) 6.7 Hz, CHMe2), 3.35 (sept, 1H, J ) 6.7
Hz, CHMe2), 5.38 (br s, 1H, R-CH), 5.76 (br, s, 1H, R-CH), 6.58 (d,
2H, J ) 7.5 Hz, Ar-H), 6.98-7.24 (m, 13H, Ar-H), 7.50 (d, 2H,
J ) 7.4 Hz, Ar-H). 13C{1H} NMR (C6D6, 75.5 MHz): δ -1.58
(SiMe3), -1.28 (SiMe3), 12.95 (CHMe2), 17.84 (CHMe2), 19.64
(CHMe2), 21.64 (CHMe2), 24.41 (CHMe2), 29.71 (CHMe2), 32.78
(CHMe2), 34.57 (CHMe2), 50.05 (R-C), 121.67, 122.74, 124.6, 126.08,
130.46, 131.06, 140.48, 145.14, 147.11, 159.21, 163.55 (unsaturated
carbon). The compound 14 was prepared in the same manner as that
described for 12. The crude product was recrystallized from diethyl
ether to give yellow crystals of 14. H NMR (C6D6, 300.08 MHz): δ
1
0.41, 0.55, 0.72, 0.83, 0.91, 0.96, 1.06, 1.08, 1.13, 1.29, 1.53, 1.92,
1.98, 2.21, 2.49, 2.85, 3.10, 3.22, 3.40, 3.90, 5.66, 5.91, 6.65, 6.84-
7.34, 9 (m). 13C{1H} NMR (C6D6, 100.52 MHz): δ 15.44, 21.00, 21.30,
21.85, 22.52, 23.06, 23.69, 24.07, 24.20, 24.31, 25.15, 25.24, 25.43,
25.72, 26.52, 27.70, 28.44, 30.05, 30.53, 30.62, 30.72, 31.09, 31.84,
38.48, 50.94, 65.76, 91.71, 93.85, 94.86, 95.07, 120.56, 122.26, 122.73,
123.19, 123.42, 124.23, 124.44, 125.95, 126.09, 126.23, 126.44, 126.50,
127.14, 128.65, 128.98, 129.79, 130.72, 131.34, 131.61, 135.43, 135.70,
137.28, 137.49, 138.58, 140.56, 140.99, 141.65, 144.89, 145.63, 146.18,
146.47, 146.72, 147.10, 147.23, 147.36, 147.42, 149.86, 150.46, 150.61,
166.17, 166.86.
X-ray Crystallographic 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 3
were collected at 130 K on a Siemens P4 diffractometer with use of
Cu KR (λ ) 1.4152 Å) radiation. Data for 4 (93 K), 6 (93 K), 7 (91
K), 8 (93 K), 9 (150 K), and 10 (90 K) were collected on a Bruker
SMART 1000, and data for 11 and 14 were collected at 90 K with a
Bruker SMART APEX II diffractometer with use of Mo KR (λ )
0.71073 Å) radiation and a CCD area detector. Empirical absorption
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17533

A R T I C L E S
Cui et al.
corrections were applied by using SADABS or XABS.19 The structures
were solved with use of either direct methods or the Patterson option
in SHELXS and refined by the full-matrix least-squares procedures in
SHELXL.20 All non-hydrogen atoms were refined anisotropically while
hydrogens were placed at calculated positions and included in the
refinement using a riding model. The structures of compound 6, 7,
and 10 exhibit somewhat poorer C-C bond precision and instances of
large thermal notion than the others. For 6 and 10, these problems arise
primarily from small crystal size. In the case of structure 10, few high
angle data were observed, and fewer than 50% of the independent data
below 2θ ) 45° have I > 2σ(I). Structure 7 was refined as a rotational
twin, with twin law [1 0 0 0 -1 0 0 0 -1]; twin parameter 0.471(2).
Thermal parameters were restrained by use of the SHELXLl ISOR
0.008 command; however, some anomalous thermal motion is still
evident in the peripheral Pri groups. Some details of data collection
and refinement are provided in Table 1. Further details are in the
Supporting Information.
Results
The reactions undergone by the germanium and tin alkyne
analogues 1 and 2 are shown in Scheme 1.
We first examined the reactions of 1 and 2 with azobenzene
which yielded the products 3 and 4. It is probable that the
reaction proceeds via a [2 + 2] addition pathway. A cyclic
product was not isolated, however (cf. reaction with PhCCPh
to afford 12 below).16 Furthermore, the addition of an excess
of the azobenzene did not induce further reaction, which is
consistent with the absence of a reactive E-E bond between
the heavier atoms in the molecules.
The molecular structures of 3 and 4 are shown in Figure 1
along with selected bond distances and angles. The structures,
which are similar to each other, show that cleavage of the Ge-
Ge and Sn-Sn bonds in 1 and 2 has been induced to afford
products based on a substituted hydrazine in which the nitrogen
atoms carry phenyl as well as Ar′Ge or Ar′Sn substituents. The
N-N bond lengths 1.453(5) Å (3) and 1.430(3) Å (avg 4) are
consistent with a single bond.21 The coordination of the nitrogens
is almost planar in each molecule with angular sums at nitrogen
(Σ°N) of 358.8(1)° (3) and 357.5(2)-358.5(2)° (4). The Ge-N
bond length (1.879(4) Å avg) is within the range found in other
germanium(II) amides (1.83-1.89 Å).22 The average Sn-N
distance in 4 (2.107(2) Å) is similar to those found in the two
coordinated tin species Sn{N(SiMe3)2}2 (Sn-N ) 2.09(1) Å)23
and Sn{N(SiMe3)2}{C6H3-2,6-(C6H2-2,4,6-Pri3)2} (Sn-N )
2.094(5) Å).24 The relative orientation of the nitrogen coordina-
tion planes in each molecule is indicated by the high torsion
angles for the arrays Ge1-N1-N2-Ge2 (59.6°) and Sn1-N1-
N2-Sn2 (57.2°).
Figure 1. Thermal ellipsoid plots 30% of 3 and 4. Hydrogen atoms and
C6H3-2,6-Pri2 groups (except ipso carbon atoms) are not shown. Selected
bond distances (Å) and angles (deg) for 3: Ge1-N1 1.882(4), Ge1-C1
2.025(5), Ge2-N2 1.875(4), Ge2-C31 2.039(5), N1-N2 1.453(3); Ge-
Ge 3.106(1); N1-Ge1-C1 106.14(18), N2-Ge2-C31 105.06(18), Ge1-
N1-N2 106.5, Ge2-N2-N1 107.2(3). For 4, the asymmetric unit contains
two almost identical molecules, one of which is shown: Sn1-N1 2.106-
(2), Sn1-C1 2.218(3), Sn2-N2 2.109(2), Sn2-C31 2.217(3), N1-N2
1.429(3); Sn1-Sn2 3.2257(6); C1-Sn1-N1 106.44(9), Sn1-N1-N2
105.34(15), N1-N2-Sn2 106.23(15), N2-Sn2-C31 103.65(9).
reported in a previous communication and is illustrated sche-
matically in Scheme 1.16 It consists of a planar Ge2N2 core in
which the geometry at nitrogen is also planar while that at
germanium is pyramidal, Σ°Ge ) 322.10(7)° with the Ar′
substituents arranged in a trans fashion across the core. The
Ge-N bond lengths, 1.863(2) and 1.874(2) Å, are within the
range found in other dimeric ring species (1.70-1.88 Å).25 In
sharp contrast, the reaction of 2 with N3SiMe3 produced the
new imide bridged species 6. It has the structure illustrated in
Figure 2.
It can be seen that a N(SiMe3) unit bridges two Ar′Sn
moieties. The two Sn-N distances (2.111(6) and 2.055(6) Å)
are slightly different and are similar to those found in 4 which
is consistent with single bonding. The geometry at the nitrogen
atom is almost planar (Σ° N ) 358.3(3)°). The Sn1-N1-Si1
(115.8(3)°) and Sn2-N1-Si1 (134.6(3)°) angles are very
dissimilar, however, and the shorter Sn2-N1 bond is associated
with the wider Sn2-N1-Si1 angle. Morever, the ipso carbon
(C31) bonded to Sn(2) lies 0.85 Å from the averaged Si1-
N1-Sn1-Sn2 plane, whereas the ipso carbon (C1) lies 1.60 Å
from this plane. The Sn-C (aryl) distances are slightly longer
than those observed in Sn(Mes*)2 (2.226(7) Å (Mes* ) 2,4,6-
Addition of an excess of N3SiMe3 to 1 and 2 resulted in the
formation of the diradicaloid Ar′Ge{µ2-N(SiMe3)}2GeAr′ (5)
and the monoimide bridged species Ar′Sn{µ2-N(SiMe3)}SnAr′
(6), respectively (Scheme 1). The structure of 5 has been
(19) SADABS an empirical absorption correction program from the SAINTPlus
NT, version 5.0 package; Bruker AXS: Madison, WI, 1998.
(20) SHELXTL, version 5.1; Bruker AXS: Madison, WI, 1998.
(21) The N-N bond length in (H3Si)2NN(SiH3)2 is 1.46(2) Å: Glidewell, C.;
Rankin, D. W. H.; Robiette, A. G.; Sheldrick, G. M. J. Chem. Soc. A.
1970, 318.
(22) See for example: (a) Schaefer, A.; Saak, W.; Weidenbruch, M. Z. Anorg.
Allg. Chem. 1998, 624, 1405. (b) Veith, M. Rammo, A. Z. Anorg. Allg.
Chem. 2001, 627, 662.
(23) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P., Thorne, A. J. Chem.
Commun. 1983, 639.
(24) Pu, L.; Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organometallics
1998, 17, 5602.
(25) Hitchcock, P. B.; Lappert, M. F.; Thorne, A. J. Chem. Commun. 1990,
1587. Ahlemann, J. T.; Roesky, H. W.; Murugavel, R.; Parsini, E.;
Noltemeyer, M.; Schmidt, H. G.; Mu¨ller, O.; Herbst-Irmer, R.; Markovskii,
L. N.; Shermolovich, Y. G. Chem. Ber. 1997, 130, 1133. Bartlett, R. A.;
Power, P. P. J. Am. Chem. Soc. 1990, 112, 3660.
9
17534 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
Figure 2. Thermal ellipsoid plot (30%) of 6. Hydrogen atoms and C6H3-
2,6-Pri2 groups (except ipso carbon atoms) are not shown. Selected bond
distances (Å) and angles (deg): Sn1-N1 2.111(6), Sn1-C1 2.267(7), Sn2-
N1 2.055(6), Sn2-C31 2.250(7), N1-Si1 1.732(6); N1-Sn1-C1 107.0-
(2), N1-Sn2-C31 108.1(2), Sn1-N1-Sn2 107.9, Sn1-N1-Si1 115.8(3),
Sn2-N1-Si1 134.6(3).
Figure 4. Thermal ellipsoid plot (30%) of 8. Hydrogen atoms and C6H3-
2,6-Pri2 rings (except ipso carbon atoms) are not shown. Selected bond
distances (Å) and angles (deg): Ge1-N1 1.8060(18), Ge1-Ge2 2.4963-
(4), Ge2-N2 1.8009(18), N2-C74 1.281(3), C74-C67 1.517(3), C67-
N1 1.280(3), Ge1-C1 1.982(2), Ge2-C31 1.980(2); N1-Ge1-Ge2
96.33(6), N1-Ge1-C1 108.39(8), C1-Ge1-Ge2 120.92(6), Ge1-Ge2-
N2 96.94(6), Ge1-Ge2-C31 119.68(6), N2-Ge2-C31 109.21(8), Ge1-
N1-C67 137.08(16), N1-C67-C74 119.28(19), C67-C74-N2 119.58-
(19), C74-N2-Ge2 136.35(15).
is retained in the reaction. The three-coordinate Ge1 atom has
a planar geometry (Σ°Ge ) 360.0(2)°), and the Ge2-Ge1-C1
bond angle (129.62(14)°) is very close to that found in the parent
digermyne Ar′GeGeAr′ (128.67(8)°).9 In contrast, the Ge1-
Ge2-C31 angle (102.77(15)°) at the base-free Ge2 is 25.9°
narrower than that in the digermyne Ar′GeGeAr′ suggesting the
lone pair at the Ge2 atom is now much more localized. The
Ge1-Ge2 distance (2.3432(9) Å) is 0.058 Å longer than that
in the Ar′GeGeAr′ precursor (2.285(5) Å) but remains consider-
ably shorter than a normal Ge-Ge single bond (ca. 2.44 Å),
indicating the retention of significant multiple-bonding character.
The Ge-Ge distance in 7 resembles that in the digermene,
Ar*(Me)GeGe(Me)Ar* (Ar* ) C6H3-2,6(C6H2-2,4,6-Pri3)2;
Ge-Ge ) 2.3173(3) Å).27 The Ge1-C1 distance is marginally
shorter (by 0.038 Å) than that of Ge2-C31 even though the
coordination number at Ge1 is higher (3 versus 2) than that of
the Ge2 atom. The Ge1-C61 donor-acceptor bond length
(1.957(5) Å) is slightly shorter than those of both Ge-C sigma
bonds and indicates very strong electron donation by the
isonitrile to the germanium atom.
Upon reaction of 1 with PhCN, compound 8 (Figure 4), which
has a unique quasi-aromatic six-membered ring, is formed. The
compound is effectively a pyrazine analogue which has incor-
porated two germanium atoms instead of carbons in its ring.
The ring is rather distorted from planarity, and the average
deviations (Å) of the ring atoms from the least squares plane
are N1(-0.054), Ge1(-0.053), Ge2(+0.05), N2(+0.060), C67-
(+0.20), and C74(+0.204). The geometry at each germanium
is quite pyramidal (Σ°Ge1 ) 325.64(7)°, Σ°Ge2 ) 325.83(7)°,
respectively). Moreover, the Ge1sGe2 bond length (2.4963(4)
Å) is slightly longer than the GesGe single bond length (2.44
Å) in elemental germanium.29a The GesN bond lengths (average
1.803(3) Å) correspond to a single bond. The N1sC67 (1.280-
Figure 3. Thermal ellipsoid plot of 7. Hydrogen atoms and C6H3-2,6-Pri2
groups (except ipso carbon atoms) are not shown. Selected bond distances
(Å) and angles (deg): Ge1-Ge2 2.3432(9), Ge1-C1 1.966(5), Ge1-C61
1.957(5), C61-N1 1.155(2), N1-C62 1.480(6), Ge2-C31 2.004(5); Ge2-
Ge1-C1 129.62(14), Ge1-Ge2-C31 102.77(15), Ge2-Ge1-C61 127.61-
(2), C1-Ge1-C61 112.7(2), Ge1-C61-N1 164.2(6), C61-N1-C62
168.3(11).
But3C6H2),26 Sn{N(SiMe3)2}{C6H3-2,6(C6H2-2,4,6-Pri3)2} (2.225-
(5) Å),24 and {Ar′Sn(µ2-Cl)}2 (2.225(6) Å).14,27
The “digermyne” 1 reacted smoothly with t-BuNC, PhCN,
and N2CHSiMe3 at ca. 25 °C to afford respectively the dark
red 1:1 isonitrile adduct 7 (Scheme 1), the cyclic product 8
which features a C2N2Ge2 six-membered ring, and the unusual
triply bridged species 9. In contrast, the tin analogue 2 reacts
with neither ButNC nor PhCN under the same conditions as
those for 2. It reacts slowly with N2CHSiMe3, but this leads to
1
complicated mixtures as indicated by the H NMR spectrum.
We have been unable to isolate any pure product from the
reaction under various conditions that involved different ratios
of 2 and N2CHSiMe3.
The X-ray structure of 7 (Figure 3) shows that the tert-butyl
isonitrile molecule is coordinated through carbon to one of the
germanium atoms. The coordination of the isonitrile occurs in
the molecular plane formed by the C1,Ge1,Ge2,C31 core of
Ar′GeGeAr′ to afford an almost planar C31,Ge2,Ge1,C1,C61
array. The geometry at the isonitrile carbon C61 deviates
somewhat from linearity (Ge1-C61-N1 ) 164.2(6)°), and the
C61-N1 distance, 1.155(20) Å, indicates that the CN triple bond
(26) Weidenbruch, M.; Schlaefke, J.; Scha¨fer, A.; Peters, K.; von Schnering,
H. G.; Marsmann, H. Angew. Chem., Int. Ed. 1994, 33, 1846.
(27) Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron 2001, 20,
551.
(28) Stender, M.; Pu, L.; Power, P. P. Organometallics 2001, 20, 1820.
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17535

A R T I C L E S
Cui et al.
Figure 6. Thermal ellipsoid plot (30%) of 10. Hydrogen atoms and C6H3-
2,6-Pri2 rings (except ipso carbon atoms) are not shown. Selected bond
distances (Å) and angles (deg): Ge1-O1 1.759(7), Ge1-O3 1.754(7),
Ge1-O4 1822(7), Ge1-C1 1.925(11), Ge2-O2 1.755(7), Ge2-O3 1.750-
(7), Ge2-O5 1.836(8), Ge2-C31 1.932(11), O4-O5 1.501(10), Ge1-Ge2
2.885(2), Ge1-O3-Ge2 110.8(4), Ge1-O4-O5 109.5(5), Ge2-O5-O4
1.04.8(5), O3-Ge1-O4 110.3(4), O3-Ge2-O5 110.7(4).
Figure 5. Thermal ellipsoid plot (30%) of 9. Hydrogen atoms and C6H3-
2,6-Pri2 rings (except ipso carbon atoms) are not shown. Selected bond
distances (Å) and angles (deg): Ge1-N1 1.9191(16), Ge1-N3 1.8749-
(17), Ge1-C1 1.9722(19), Ge1-C69 2.0022(18), Ge2-N1 1.9231(16),
Ge2-N4 2.0437(16), Ge2-C69 1.9720(18), Ge2-C31 1.9943(18); N1-
N2 1.394(2), N3-N4 1.341(2), N2-C61 1.274(3), N4-C65 1.314(3); N1-
Ge1-C1 128.55(7), N1-Ge1-N3 97.78(7), N1-Ge1-C69 84.29(7), C1-
Ge1-N3 116.88(8), C1-Ge1-C69 123.13(7), N3-Ge1-C69 98.34(8),
N1-Ge2-N4 90.80(7), N1-Ge2-C31 118.18(7), N1-Ge2-C69 85.01-
(7), N4-Ge2-C31 127.05(7), N4-Ge2-C69 91.29(7), C31-Ge2-C69
131.50(8), Ge1-N1-N2 118.10(12), Ge1-N1-Ge2 85.87(6), Ge2-N1-
N2 126.73(13), Ge1-N3-N4 106.13(12), Ge2-N4-N3 111.61(12), N3-
N4-C65 120.43(17), Ge2-N4-C65 127.94(15), Ge1-C69-Ge2 82.37(7),
Ge1-C69-Si3 116.38(10), Ge2-C69-Si3 124.22(10).
solution within five minutes. The germanium peroxide product
Ar′(OH)(µ2-O)Ge(µ2:η2-O2)Ge(OH)Ar′ (10) was obtained after
standard workup. Single-crystal X-ray analysis afforded the
structure shown in Figure 6.
It consists of a core Ge2O5 puckered ring in which the
germaniums are linked by oxo and peroxo bridges. The
germaniums are each bound to a terminal hydroxo and Ar′ group
disposed in a trans fashion with respect to each other across
the ring. The Ge-O bonds to the hydroxo and µ-oxo moieties
are similar (1.755(8) Å avg) but shorter than the Ge-O distances
to the peroxide oxygens (1.829(8) Å avg). The former values
resemble the average distance (1.78(4) Å) observed in more
than 400 structures which feature a Ge(µ-O)Ge unit.30 The latter
is consistent with the average distance (1.81(4) Å) in the three
known complexes in which two germaniums are bridged by
peroxide.31 The O-O bond length 1.50(1) Å is slightly longer
than the mean O-O bond length 1.45(2) Å observed in these
complexes. It is possible that the reaction proceeds through a
radical mechanism to form Ar′GeOOGeAr′ units, which react
with further equivalents of N2O with subsequent hydrogen atom
abstraction from the solvent to give the peroxy and oxy bridges
and the terminal hydroxy groups, respectively.
(3) Å) and N2sC74 (1.281(3) Å) bond lengths are consistent
with CdN double bonding,29b and the C67sC74 bond (1.517-
(3) Å) is obviously a single one.
The structure of 9 is shown in Figure 5. The most interesting
feature of this compound is that it possesses three different types
of bridging units that result from the reaction of 1 with
trimethylsilyldiazomethane. One molecule of N2CH(SiMe3)
eliminates N2 to form an alkylidene moiety that bridges two
germaniums. Two N2CHSiMe3 molecules are found to bridge
the two germaniums through the NN moiety in one case and
through the terminal N atom in the other. The germaniums are
four coordinate and have distorted tetrahedral geometry which
shows that the original Ge-Ge bond in 1 has been destroyed.
The bridging Ge-C(69) bond lengths indicate normal Ge-C
single bonds.29a The Ge1-N1 (1.919(2) Å) and Ge2-N1 (1.923-
(2) Å) bond lengths involving the µ2:η1 bridging N2CH(SiMe3)
ligand are slightly longer than those found in compounds 3 (avg
1.879(4) Å) and 5 (avg 1.868(6) Å). However, the bridging
distances involving the µ2;η2-N2CH(SiMe3) moiety Ge1-N3
(1.875(2) Å) and Ge2-N4 (2.044(2) Å) differ considerably,
possibly as a result of the greater coordination number of N4
in comparison to N3. The N1-N2 (1.394(2) Å) and N3-N4
(1.341(2) Å) distances indicate retention of multiple N-N bond
character.29b The geometry of the N1 atom is pyramidal
(Σ°N1 ) 330.70(10)°), whereas the N4 atom has a planar
geometry (Σ°N4 ) 359.98(15)°).
Compound 1 reacts with tetracyanoethylene in ether to give
the unexpected compound 11 as shown schematically in eq 1.
The reaction involves the reduction of six cyano groups from
three molecules of tetracyanoethylene to form a large fused ring
system 11 in which Ge(1) is polar covalently bound to three
cyano nitrogens each from separate TCNE moieties. A different
When 1 was exposed to an excess of N2O at normal pressure,
its red color disappeared immediately to give a pale yellow
(30) Cambridge Crystallographic Data Center, Cambridge, UK, 2001.
(31) (a) Masamune, S.; Batcheller, S. A.; Park, J.; Davis, W. M.; Yamashita,
O.; Ohta, Y.; Kabe, Y. J. Am. Chem. Soc. 1989, 111, 1888. (b) Kako, M.;
Akasaka, T.; Ando, W. Chem. Commun. 1992, 457.
(29) (a) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon; Oxford,
1984; p 1173. (b) Wells, A. F. Structural Inorganic Chemistry, 5th ed.;
Clarendon; Oxford, 1984; p 807.
9
17536 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
Figure 8. Thermal ellipsoid plot (50%) of 14. Hydrogen atoms and Dipp
and phenyl rings (except the Dipp ring involved in the addition reaction)
are not shown. Selected bond distances (Å) and angles (deg): Ge1-C1
1.981(2), Ge1-C51 2.021(2), Ge1-C62 1.996(2), Ge1-C82 1.956(2),
Ge2-C31 1.947(2), Ge2-C50 2.023(2), Ge2-C61 1.945(2), Ge2-C81
1.978(2), C49-C50 1.529(3), C50-C51 1.563(3), C51-C52 1.482(3),
C52-C53 1.326(3), C53-C54 1.452(3), C54-C49 1.355(3), C61-C62
1.364(3), C81-C82 1.361(3), Ge1-Ge2 3.089; C1-Ge1-C51 125.38(9),
C1-Ge1-C62 104.29(9), C1-Ge1-C82 118.49(10), C51-Ge1-C82
97.25(10), C51-Ge1-C62 102.91(9), C61-Ge2-C31 134.64(9), C61-
Ge2-C81 108.28(9), C31-Ge2-C81 110.50(9), C61-Ge2-C50 104.47,
C31-Ge2-C50 90.11(9), C81-Ge2-C50 101.39(9).
Figure 7. Thermal ellipsoid plot (30%) of 11. Hydrogen atoms and Ar′
rings (except ipso carbon atoms) are not shown. Selected bond distances
(Å) and angles (deg): Ge1-C1 1.917(4), Ge1-N121 1.853(4), Ge1-N131
1.847(4), Ge1-N141 1.859(4), Ge2-C31 2.013(5), Ge2-N124 2.027(4),
Ge2-N134 2.023(4), G3-C61 2.008(5), Ge3-N125 2.025(4), Ge3-N145
2.027, Ge4-C91 1.942(6), Ge4-N135 2.040(5), Ge4-N144 2.012(5),
N121-C121 1.202(6), C121-C122 1.354(7), C122-C123 1.466(7), C123-
C124 1.371(7), C124-N124 1.156(6), N134-C134 1.164(6), C134-C133
1.379(7), C133-C135 1.375(6), C135-N135 1.148(6), N144-C144 1.160-
(6), C144-C143 1.361(7), C143-C142 1.486(7), C142-C141 1.323(6),
C141-N141 1.205(5); N131-Ge1-N121 100.64(16), N131-Ge1-N141
102.41(16), N121-Ge1-N141 102.59, N131-Ge1-C1 114.89(17), N121-
Ge1-C1 121.15(17), N141-Ge1-C1 112.72, C31-Ge2-N134 96.80(17),
C31-Ge2-N124 99.95(17), N1343-Ge2-N124 86.57(16), C61-Ge3-
N125 96.88(17), C61-Ge3-N145 101.43(17), N125-Ge3-N145 86.80-
(15), C91-Ge4-N144 95.3(2), C91-Ge4-N135 103.0(3), N144-Ge4-
N135 87.2(3).
could be responsible for the activation of the CdC bond in the
flanking C6H3-2,6-Pri2 ring. To isolate the intermediate and
nitrogen from each TCNE is bound to a separate GeAr′ moiety,
and each of these GeAr′ units also behaves as an acceptor of
lone pair pair density from a nitrogen from a neighboring TCNE
unit. In this way, the fused ring structure in Figure 7 is generated.
The formal charges within the compound may be designated
by the formula Ar′Ge3+{(TCNE)2-}3{(GeAr′)+}3.
The structure of 11 is an unusual example of a large cage
system incorporating heavier group 14 elements. There are four
large fused rings, the largest of which is an 18-membered ring
formed by three germaniums (Ge2, Ge3, and Ge4), six nitrogens,
and nine carbon atoms. There are three 14-membered rings that
incorporate two germanium, four nitrogen, and eight carbon
atoms. Each 14-membered ring shares the four-coordinated,
tetravalent bridgehead Ge1 atom which is bonded to an Ar′
ligand and three imine nitrogen atoms. In the 18-membered ring,
the germanium atoms Ge2, Ge3, and Ge4 are three-coordinate
and linked by sNdCdCsCtN moieties. These germanium
atoms are divalent and have one GesC (Ar′) and one GesN
sigma bond, with the other coordination site being occupied by
a GesN dative bond from a nitrile moiety.
The reaction of 1 with PhCCPh and HCCSiMe3 has been
previously communicated by us.16 The reactions give either a
stable GesCsCsGe four-membered 1,2-digermacyclobutadi-
ene ring 12 (with PhCCPh) or further reaction (in the case of
HCCSiMe3) to give the intermediate II which activates a CdC
bond in one of the flanking Dipp rings to give 13. We proposed
that the intermediate II has 1,4-diradical character (eq 2) which
obtain further evidence to support this mechanism, we employed
the diacetylene PhCtCsCtCPh for the reaction. 1 takes up 2
equiv of PhCtCsCtCPh at room temperature and reacts with
one of the triple bonds in the PhCtCsCtCPh molecule in a
manner similar to Me3SiCCH followed by activation of one of
the C6H3-2,6-Pri2 rings in Ar′. The regioselectivity of this
reaction is the same as that observed in the reaction with
HCCSiMe3 and only isomer 14 was isolated.
The molecular structure of compound 14 is shown in Figure
8 along with selected bond distances and angles. The germanium
atoms have a distorted tetrahedral geometry and are connected
by three C-C units to form three six-membered rings. The
C50-C51 distance (1.563(3) Å) corresponds to a C-C single
bond. The Ge1-C51 and Ge2-C50 bond lengths (2.021(2) and
2.027(2) Å) are longer than those of the other Ge1-C bonds
because of the sp3 hybridized C51 atom.
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17537

A R T I C L E S
Cui et al.
Unlike 1, 2 did not react with the acetylenes PhCCCPh,
Ge-Ge double bond and there is a single lone pair that resonates
HCCSiMe3, or PhCCCCPh under the same conditions.
between positions at each germanium as shown by B and D.
Discussion
With the exception of the previously reported reaction of
Ar*GeGeAr* (Ar* ) C6H3-2,6(C6H2-2,4,6-Pri3)2) with 2,3-
dimethyl-1,3-butadiene, which afforded Ar*Ge(CH2C(Me)C-
(Me)CH2)CH2C(Me)d]2,13 the reactions undergone by Ar′EEAr′
(E ) Ge or Sn), as depicted in Scheme 1, represent the first
experimental studies of the reactivity of heavier group 14
element alkyne analogues with unsaturated molecules. The
reactions demonstrate the highly reactive nature of the GesGe
multiple bond and the significantly lower reactivity of the
corresponding tinstin interaction. The lower reactivity of the
tin species is counterintuitive on the basis of bond strengths
(SnsSn bonds are weaker than GesGe bonds), steric factors
(tin is larger than germanium and is thus less protected by the
same ligand), and the greater polarity of the SnsC bond versus
the GesC bond. Possible reasons for the greater reactivity of
the germanium species are discussed below.
The EE Bond. Previous studies have established the planar
trans-bent geometry of both 1 (Ge-Ge ) 2.285(5) Å; Ge-
Ge-C ) 128.67(8)°)9 and 2 (Sn-Sn ) 2.675(4) Å; Sn-Sn-C
) 125.24(7)°)10 as well as of the lead derivative Ar*PbPbAr*
which has a long Pb-Pb bond length of 3.1881(1) Å and a
Pb-Pb-C bending angle of 94.26(4)°.8 Recent work by
Sekiguchi and co-workers has disclosed the preparation and
structure of the related silicon species R′SiSiR′ (R′ ) SiPri{CH-
(SiMe3)2}2 which also possesses a planar, trans-bent structure
with Si-Si ) 2.0622(9) Å and a bending angle of 137.44(4)°.11
Thus, stable homonuclear alkyne analogues of all the heavier
group 14 elements have now been isolated and characterized.
The strong trans-bending observed in all these compounds shows
that the bond order is less than three and that nonbonding lone
pair character accumulates to an increasing degree at the group
14 element as the group is descended. The bending is thought
to be the result of the mixing of an in-plane π-orbital with a σ*
orbital whose energies are close enough to cause the interaction
of these orbitals in the heavier elements.7,12a The progressive
lowering of the bond order may also be represented crudely by
the sequence
It is also possible to represent the structure as the singlet
diradicaloid form E34 and this form is supported by calcula-
tions.35 The representation of the structure of 1 by the forms B,
D, or E suggests high reactivity. For example, in B and D the
Ge-Ge bond is depicted as dipolar, and it is a dipole that
includes a cationic Ge center that is two-coordinate. These
attributes are a prescription for high reactivity as is the singlet
diradical form represented by E which often (but not always)36
implies the existence of a relatively low-lying triplet state.
Previously available experimental data and theoretical analysis
of molecular energy levels13 generally support the applicability
of the simple bonding pictures illustrated by forms B-E. For
instance, forms B and D suggest that they should be reduced
readily. Cyclic voltammetry for 1 has shown that it has a quasi-
reversible reduction near 1.38 V vs SCE in THF. Moreover,
singly or doubly reduced salts of Ar′GeGeAr′ and the related
Ar*GeGeAr* (Ar* ) C6H3-2,6(C6H2-2,4,6-Pri3)2) can be readily
isolated and structurally characterized.14,15b Addition of one
electron results in only a small increase (0.048 Å) in the Ge-
Ge bond distance to 2.3331(4) Å and a narrowing of the Ge-
Ge-C bending angle from 128.67(8)° to 115.55(5)°.14 This is
consistent with the addition of the electron to an essentially
nonbonding orbital. Calculations on the MeGeGeMe model
species show that the LUMO is the nonbonding n+ or LP*
combination (using MO phraseology).33,35 Accordingly, the
addition of an electron to this orbital should result in a narrowing
of the Ge-Ge-C angle and a slight lengthening of the Ge-
Ge bond due to an increase in interelectronic repulsion. This is
what is found experimentally.14 For the tin compound, calcula-
tions on the model species MeSnSnMe,35 when its geometry is
restricted to that experimentally measured for Ar′SnSnAr′, show
that like its germanium counterpart the LUMO is the nonbonding
n+, LP* orbital. However, the addition of an electron to
Ar′SnSnAr′ in THF to give [Ar′SnSnAr′]- results in a signifi-
cantly greater lengthening of the SnSn bond from 2.6675(4) to
2.8081(9) Å.14 Furthermore, the reduction is accompanied by a
very large (>27°) decrease in the Sn-Sn-C bond angle from
125.24(7)° to 97.9(2)°.14 This unexpected behavior can be
explained by assuming that in solution the Ar′SnSnAr′ has a
structure which is more trans-bent than that experimentally
observed10 in the solid state. This assumption is based on
calculations35 on the MeSnSnMe model species which show
that its lowest energy structure features a more strongly bent
Sn-Sn-C angle of 100.0° and a much longer Sn-Sn distance
of 3.06 Å (due to the increased 5p character of the orbitals
comprising the Sn-Sn σ-bond) than those observed experi-
mentally (2.675(4) Å and 125.24(7)°) for Ar′SnSnAr′. In the
more bent, lower energy configuration, the tins possess more
where A corresponds to a triple bond in acetylene and its
derivatives and C represents the bonding in Ar*PbPbAr* in
which the Pb-Pb bond is essentially a single one and there is
a lone pair at each lead.8,32 The bonding is less clearly defined
for the Si, Ge, and Sn derivatives. However, it is believed that
the bonding in the germanium species is close to that represented
by B or D.7,33 In valence bond terms, there is an approximate
(34) Malcolm, N. O. J.; Gillespie, R. J.; Popelier, P. L. A. Dalton Trans. 2002,
3333.
(35) Jung, Y.; Brynda, M.; Power, P. P.; Head-Gordon, M., submitted.
(36) Jung, Y.; Head-Gordon, M. ChemPhysChem. 2003, 4, 522.
(32) Chen, Y.; Hartmann, M.; Diedenhofen, M.; Frenking, G. Angew. Chem.,
Int. Ed. 2001, 40, 2052.
(33) Allen, T. L.; Fink, W. H.; Power, P. P. Dalton Trans. 2000, 407.
9
17538 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
substantial lone pair character and the LUMO is an unfilled
π-orbital. In effect, the structure of the neutral Ar′SnSnAr′ in
solution resembles C (like the lead species Ar*PbPbAr*)8 rather
than B or D (like its germanium counterpart Ar′GeGeAr′).9 The
apparent readiness with which the geometry of the tin species
can be changed is consistent with the previous work of Nagase
and co-workers37 which showed that such a process requires
little energy (see also ref 35). The changed structure of
Ar′SnSnAr′ in solution is also consistent with the fact that it is
more readily reduced (-1.21V vs SCE in THF)14 than its
germanium counterpart, since the added electron enters what
is, in effect, a π-bonding orbital. The small energy differences
between the various geometric configurations in RSnSnR
compounds illustrate the finely balanced π-bonding and lone
pair tendency of the s-valence electrons.35,37,38
Reactivity of 1 and 2. The different electronic structures of
1 and 2, as indicated by the changes they undergo upon
reduction, are in harmony with the different chemical reactivities
shown by 1 and 2. It can be seen in Scheme 1 that 1 reacts at
room temperature with PhCtCPh, Me3SiCtCH, :CtNBut, or
PhCtN, whereas its tin analogue 2 remains unaffected. When
1 or 2 are reacted with N3SiMe3 two different products, 5 and
6, are obtained. The lower reactivity of the tin compound 2 is
reflected in the fact that, unlike 1, it does not react with a second
equivalent of N3SiMe3. Instead, the product 6, which features
one NSiMe3 group bridging two SnAr′ fragments, was isolated.
The reaction of 1 with N2CH(SiMe3) afforded the unusual triply
bridged product 9, and although 2 also reacts slowly with
N2CH(SiMe3), the products of the reaction are currently
unknown. Similarly, 1 reacts with N2O to produce the unusual
peroxo ring species 10. Compound 2 also reacts with N2O, but
in this case, the products appear to be mixtures of [Ar′Sn(µ-
OH)]2 and [Ar′Sn(µ-O)]2 to which no further addition of oxygen
occurs.39 The absence of further reaction with oxygen is another
manifestation of the lower reactivity of 2. With TCNE, 1
afforded the unusual fused ring product 11 whereas a similar
reaction with 2 has not afforded a crystalline product to date.
In summary, for the dozen or so reactions studied so far, the
lower reactivity of 2 is reflected in the fact that it either does
not react with the unsaturated substrates or it reacts more slowly
with them than 1.
apparently retained during the reactions of 1 with PhCtN or
PhCtCPh which give the cyclized derivatives 8 and 12.
However, in these products, the GesGe bonds, which form part
of four- and six-membered rings, are quite weak since they have
distances of 2.4708(9) and 2.4963(4) Å respectively which are
longer than the GesGe distance (2.44 Å) in elemental Ge.29a
Furthermore, the germaniums have trigonal pyramidal coordina-
tion with interligand angular sums of 325.64(8)° and 325.83-
(8)° in 8 and of 318.0(2)° and 317.3(2)° in 12. In an idealized
sense, the cyclizations transform the GesGe “triple” bond in 1
to a “double” bond in 8 and 12. In reality, however, the structural
data show that the transformation involves the conversion of
an approximate double bond in 1 to a weakened single bond in
8 and 12. The GesGe bond orders of less than one in 8 and 12
are also underscored by the pyramidal germanium geometries.
The lack of both planarity and significant delocalization in the
formally aromatic C2Ge2N2 ring in 8 is also notable. The Ges
Ge bond remains essentially undisrupted only in the case of
simple 1:1 isonitrile adduct 7.
Evidence for the Ionic and Diradical Resonance Struc-
tures. The structure of the isonitrile adduct 7 shows that the
isonitrile coordinates to germanium in the plane formed by the
core of the molecule. This is consistent with the n+ or LP*
character of the LUMO of 1. Furthermore, the Ge-Ge bond
undergoes moderate lengthening which is also in agreement with
the generally nonbonding character of the LUMO and with the
increase in coordination number at Ge(1). It is notable that the
Ge(1)-C(61) distance to the isonitrile carbon is essentially the
same as that to the ipso carbon (Ge(1)-C(1)) of the terphenyl
ligand. This strong binding of the isonitrile suggests that the
germanium is very electrophilic. However, it is the structural
changes that occur at the uncomplexed Ge(2) in the Ar′GeGeAr′
unit that offer deeper insight on the character of the GeGe bond
in 1. Most importantly, the bending angle at Ge(2) is narrowed
by over 25° in comparison to that in the precursor: 102.77-
(15)° in 7 versus 128.67(8)° in 1. Moreover, the Ge(2)-C(ipso)
bond length increases from 1.996(3) in 1 to 2.004(6) Å in 7.
These changes are consistent with a concentration of the lone
pair character at Ge(2) which produces the decrease in the
bending angle because of greater lone pair-bond pair repulsion.
In effect, the coordination of the isonitrile to 1 “freezes out”
one of the resonance forms (type B or D) in which the lone
pair is located on only one germanium atom. In other words,
the reaction of 1 with 7 provides strong support for the notion
that the Ge-Ge double bond-resonating lone pair models B and
D make a large contribution to the bonding in 1.
Another notable feature of the reactions described in Scheme
1 is that they result in either complete cleavage of the EE bond
or a weakening of the bond to the extent that it is weaker than
a single bond. For example, both 1 and 2 react with the
unsaturated nitrogen species PhNdNPh to give the products 3
and 4 which are isostructural. There are no GesGe or SnsSn
bonds in these products even though a cyclic Ge2N2 species is
sterically feasible (cf. cyclic structure of 12 below). It seems
likely that the more electronegative nitrogen substituents increase
the HOMO-LUMO energy separation which weakens possible
MM interactions between the divalent group 14 centers so that
no bonding is seen.40 No GesGe or SnsSn bonding is observed
in the products 5, 6, 9-11, 13, and 14. A GesGe bond is
The resonance forms represented by the dipolar structures B
and D result in a charge separation. However, the M-M bond
is homonuclear and is formally nonpolar which suggests that
the singlet diradical form E may be significant. In essence, the
trans-bending of a linear structure weakens the electron coupling
in one of the π-bonds. This possibility was first raised by
Popelier and coworkers34 and is supported by our computational
data where significant diradical character is calculated for the
germanium model species MeGeGeMe.35 For MeSnSnMe, the
calculated energy minimum has a more strongly bent geometry
(Me-Sn-Sn ) 100.0°) and much less diradical character than
its germanium analogue.35 These calculations on simple model
species thus offer a straightforward explanation for the differ-
ences in reactivity that is based on diradical character.
(37) Takagi, N.; Nagase, S. Organometallics 2001, 20, 5498.
(38) Power, P. P. Dalton Trans. 1998, 2939.
(39) Cui, C.; Spikes, G.; Power, P. P., unpublished work.
(40) An illustration of this phenomenon is provided by the effect of alkyl and
amide substitution as shown by the monomeric structure of [Sn-
{N(SiMe3)2}2]2 and the dimeric structure of [Sn{CH(SiMe3)2}2]1. The
22
-N(SiMe3)2 and -CH(SiMe3)2 ligands are isoelectronic and have a very
similar size, yet a monomeric structure is observed for the amide substituted
compound.
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17539

A R T I C L E S
Cui et al.
If, however, the experimentally observed bending angle
(125.24(7)°) and Sn-Sn bond length (2.6675(4) Å) in 2 is
assumed for MeSnSnMe, its diradical character is calculated35
to be similar to that of MeGeGeMe, which suggests that the
reactivity of 1 and 2 should also be similar. However, the
calculations also show that the tin compound has a quite flat
potential surface where the difference in energy between the
calculated and experimentally found trans-bent geometries is
less than ca. 5 kcal mol-1. In other words, changing the bending
angle and the SnSn distance in the tin species costs little energy.
This leads to the conclusion that the geometry (and electronic
structure) of 2 in solution may differ considerably from what it
is in the crystalline state where crystal forces may affect the
geometry. This is consistent with the previous results of
Nagase37 for germanium and tin compounds and those of
Frenking for lead derivatives.32 In addition, it is consistent with
the electrochemical behavior and the structure15 of the reduced
species [Ar′SnSnAr′]- as discussed above.
Experimental support for the diradical character of 1 comes
from several of its reactions. For example, in its reaction with
PhCN to produce 8, two nitriles are coupled to produce the NC-
(Ph)(Ph)CN moiety. In effect, a two-electron reductive coupling
has occurred which is accompanied by a change in the formal
oxidation states of the germaniums from +1 to +2. Both two-
electron and four-electron reductive coupling by transition metal
complexes to give diimino -1sNdC(R)sC(R)dNs1- (as
above) or enediimido 2-dNsC(R)dC(R)sNd2- products are
known, although the mechanism of such coupling has received
scant attention.41 Recent work by Cummins and co-workers,
however, has suggested that coupling of nitrile to give the
diimino product proceeds by initial one-electron reduction
followed by dimerization of the radical thus generated to afford
the CsC bonded species.42 Although no investigation of the
mechanism of the reaction of 1 with PhCN has been performed,
the previous work suggests that a one-electron reduction (by a
loosely coupled electron at each germanium in 1) of two
benzonitriles followed by CsC bond formation is a plausible
scheme for the generation of 8. The observation of three different
types of bridging ligand in 8 from the reaction of 1 with
N2CH(SiMe3), and the µ2-peroxo bridged structure for 10 are
also consistent with the involvement of radicals in these
reactions. The reaction of 1 with N2O to produce the peroxo
species 10 is notable because the known µ2:η2 peroxo derivatives
of Si, Ge, or Sn are derived from dioxygen in which the OsO
bond is already formed.31a,43 In contrast, the reaction of low
valent Si, Ge, or Sn with N2O affords µ-oxo products. The
observation of µ-OsO moieties derived from a source contain-
ing just one oxygen (N2O) provides evidence for a reductive
coupling reaction in which two N2O molecules each are reduced
by one electron with subsequent coupling to form the OsO
bond.
The diradicaloid character of this molecule is supported by its
structural and spectroscopic properties and by calculations as
previously described.17 In contrast, the reaction of N3SiMe3 with
2 gives the unique imido bridged species 6. Reaction of 2 with
a second N3SiMe3 ligand is not observed even with a large
excess of N3SiMe3. The reactions of 1 and 2 with N3SiMe3
provide a particularly striking illustration of the different
reactivity of the tin and germanium species. This is especially
true in light of the fact that a diradicaloid tin compound, ClSn-
{µ2-N(SiMe3)}2SnCl, related to 5, can be synthesized by a
different method.44 It seems probable that the generation of 5
and 6 proceeds through different pathways that have their origin
in the diradicaloid character of 1 although the details of either
pathway remain unclear.
The activation of one of the flanking aryl groups in products
13 and 14 (eq 8) is notable and is consistent with the presence
of a diradicaloid intermediate in the reaction. In this case,
treatment of 1 with less hindered alkynes such as Me3SiCtCH
or PhCtC-CtCPh produces the six-membered ring intermedi-
ates as shown by the structure:
Formally, this ring has six π-electrons, but instead of being
stable and aromatic, it may have considerable diradicaloid
character as illustrated above. It is probable that this form is
sufficiently important to induce activation of one of the flanking
C6H3-2,6-Pri2 rings and afford the products 13 or 14 in moderate
yield. The significance of diradicaloid structures in the reactivity
of several classes of main group compounds is beginning to be
recognized, and the behavior of 1 gives further support to the
widespread nature of this phenomenon.45 The regiospecificity
of the reactions that produce 13 or 14 is also notable. We believe
that this is an electronic effect of the SiMe3 or Ph substituents
which cause the closest germanium (in a putative intermediate
analogous to 12) to be more electrophlic (due to a flatter
geometry) and thus more reactive toward the more electron rich
carbon (carrying SiMe3 or Ph) of the second equivalent of
alkyne.
The reaction of 1 with TCNE was performed to investigate
the oxidation behavior of 1. Both 1 and 2 displayed irreversible
oxidation waves in their cyclic voltammograms,14 and it was
hoped that a 1:1 reaction of either 1 or 2 with TCNE would
provide oxidized products containing an [Ar′EEAr′]+ moiety.
For 1, the unusual complex 11 in which three TCNE molecules
oxidize two Ar′GeGeAr′ molecules to afford the fused ring
structure illustrated in Figure 7 was obtained. The formal
oxidiation state of +1 for the four germanium atoms in 2 equiv
of 1 become converted to one Ge4+ and three Ge2+ in 11. The
The involvement of diradicaloid structures in the germanium
derivatives obtained from 1 is also noteworthy. Thus, the
reaction of 1 with N3SiMe3 gives the Ge2N2 ring compound 5.
(41) Kukushkin, V. Y.; Pombeiro, A. J. L. Chem. ReV. 2002, 102, 1771. Cotton,
F. A.; Hall, W. T. J. Am. Chem. Soc. 1979, 101, 5094. Esjornson, D.;
Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1988, 27, 3066.
(42) Tsai, Y.-C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghui, M.
D.; Cummins, C. C. Organometalics 2003, 22, 2902. Mendiratta, A.;
Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova, E. V.; McDonough, J.
E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621.
(44) Cox, H.; Hitchcock, P. B.; Lappert, M. F. Pierssens, L. J.-M. Angew. Chem.,
Int. Ed. 2004, 43, 4500.
(45) Gru¨tzmacher, H.; Breher, F. Angew. Chem., Int. Ed. 2002, 41, 4006.
(43) Cardin, C. J.; Cardin, D. J.; Devereux, M. M.; Convery, M. A. Chem.
Commun. 1990, 1461.
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17540 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Character of EE (E
)
Ge, Sn) Multiple Bonds
A R T I C L E S
mechanism whereby 11 is formed is unknown, although it seems
probable that the reaction begins with a single electron transfer
from a molecule of 1 to a cyano group of TCNE. Transfer of a
further electron from the other germanium atom of 1 may then
occur to give the compound Ar′GeNCC(CN)(CN)CCNGeAr′
in which two of the four cyano groups from TCNE carry an
Ar′Ge substituent. This is the product that we expected from
the reaction. However, this species can apparently associate
through cyanide bridging (as the structure of 11 suggests) and
is also susceptible to oxidation by a further molecule of TCNE
to give the unusual product 11.
Finally, we draw attention to the fact that the lower reactivity
displayed by 2 offers little support for significant diradical
character. We have accounted for this behavior in terms of a
solution structure for 2 that has a more strongly bent geometry
and a longer Sn-Sn bond. In other words, it is a structure that
is closer to C than it is to B, D or E and it has increased lone
pair character at each tin. This proposal is supported by
previously published experimental work on the one-electron
reduction of 214,15 and theoretical calculations.35,37 Perhaps the
best illustration of the increased bending is provided by the lack
of reaction between 2 and CNBut. Even with the more open
structure that results from the larger size of the metal, the CNBut
cannot interact with the tins to afford a product analogous to 7
because of the increased lone pair density caused by the greater
bending in solution. The synthesis and characterization of
RSnSnR molecules that display more strongly bent conforma-
tions in the solid state than that seen in 2 are underway.
Conclusions
The experimental data for the reactions of 1 and 2 with a
variety of unsaturated molecules disclose a large difference in
their reactivity. The lower reactivity of 2 is contrary to what is
expected on the basis of bond energies and ionic or steric factors.
The high reactivity of 1 is due to its singlet diradical character,
a conclusion that is also supported by computational data.
Acknowledgment. We are grateful to the National Science
Foundation for financial support.
Supporting Information Available: X-ray data for com-
pounds 3, 4, 6, 7, 8, 9, 10, 11, and 14. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA055372S
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