Synthesis and characterization of the non-Kekule, singlet biradicaloid Ar′Ge(μ-NSiMe3)2GeAr′ (Ar′ = 2,6-Dipp2C6H3, Dipp = 2,6-i-Pr2C6H3)

Article abstract of DOI:10.1021/ja0492182

Reaction of Ar′GeGeAr′ (1) with an excess of Me₃SiN₃ gives the non-Kekule, biradicaloid Ar′Ge(μ-NSiMe₃)₂GeAr′ (3, Ar′ = 2,6-Dipp₂C₆H₃, Dipp = 2,6-i-Pr₂C₆H₃) which has a planar Ge₂N₂Si₂ array and pyramidal geometry at the germaniums. DFT calculations for the model MeGe(μ-NSiH₃)₂GeMe indicate no Ge-Ge bonding and a singlet ground state. The calculated energy difference between the optimized singlet and triplet states is 17.51 kcal/mol. Copyright

Full text of DOI:10.1021/ja0492182

Published on Web 05/08/2004
Synthesis and Characterization of the Non-Kekule´, Singlet Biradicaloid
Ar′Ge(µ-NSiMe3)2GeAr′ (Ar′ ) 2,6-Dipp2C6H3, Dipp ) 2,6-i-Pr2C6H3)
Chunming Cui, Marcin Brynda, Marilyn M. Olmstead, and Philip P. Power*
Department of Chemistry, UniVersity of California DaVis, One Shields AVenue, DaVis, California 95616
Received February 12, 2004; E-mail: pppower@ucdavis.edu
Scheme 1
Biradicals are thought to play a crucial role in bond breaking
and formation.1 Typical organic biradicals such as trimethylene-
methyl (I), cyclobutane-1,3-diyl (II), and cyclopetane-1,3-diyl (III)
are short-lived species. Upon modification of the substituents at
the central atoms, a few biradicals can be observed spectropically.2
However, in 1995 Niecke and co-workers reported the 1,3-
diphosphacyclobutane-2,4-diyl Mes*P(µ-CCl)2PMes* (Mes* )
found in other dimeric germanium imide species (1.70-1.88 Å).11
2,4,6-t-Bu3C6H2) (IV), which is a stable compound with carbon-
centered singlet biradical character.3 Following this finding, several
The Ge-Ge separation (2.755 Å) is about 0.3 Å longer than a
further biradicals with C2P2 framework were also reported.4 More
normal Ge-Ge single bond (average 2.44 Å),12 but it is comparable
recently, Bertrand and co-workers have described a different class
to those found in the cyclic dimers (R′GeNR′′)2 (R′ ) 2,4,6-
2
of boron-centered singlet biradicaloids, for example i-Pr2P(µ-BBu-
Me3C6H2, R′′ ) NCC12H8; R′2 ) MeNCH2CH2NMe, R′′ ) NSi-
t)2PiPr2 (V), and have examined some of their reactions.5
(t-Bu)3) and (GeNR)2 (R ) Mes*, 2,4,6-(CF3)3C6H2) (2.66-2.86
Å) in which there is no Ge-Ge bonding.11 The long Ge-Ge
separation is consistent with the biradical character of 3. Nonethe-
less, 3 displays no EPR signal at 77-300 K. It has normal 1H and
13C NMR signals which also indicate that it has a singlet ground
state.
DFT calculations performed on the model compound MeGe(µ-
NSiH3)2GeMe, where the ligand Ar′ was replaced with a smaller
For heavier Group 14 elements, Sita and Kinoshita have reported
the pentastanna[1.1.1]propellane Sn5(C6H3-2,6-Et2)6 and related
derivatives, which possess singlet biradical character.6 In addition,
the germanium moiety in the Zintl phase Ba3Ge4 has biradical
characteristics.7 We have recently reported the synthesis and
structures of the germanium and tin alkyne analogues Ar′MMAr′
(M ) Ge, 1; M ) Sn, 2)8 and are currently investigating their
reaction chemistry with a variety of unsaturated small molecules
including CO, H2, alkynes, isonitriles, nitriles, and azides and so
on. We now report that the reaction of 1 with the azide Me3SiN3
leads to the formation of the new singlet biradicaloid, the
germanium-centered Ar′Ge(µ-NSiMe3)2GeAr′ (Ar′ ) 2,6-Dipp2C6H3,
Dipp ) 2,6-i-Pr2C6H3).
methyl group and SiMe3 with SiH3, predict geometrical features
that are similar to those found in the X-ray structure of 3.13
Inspection of the frontier Kohn-Sham orbitals (Figure 2) shows
that the HOMO corresponds mainly to a nonbonding combination
centered on germanium atoms with a minor component at the
nitrogen centers. This HOMO orbital also has a weak Ge-C
component. The HOMO-1 and HOMO-2 orbitals correspond to out
of phase and in phase combinations of nitrogen p orbitals with minor
ligand components. The calculated energy differences between the
orbitals are HOMO-LUMO, ∆E ) 57.97; HOMO-HOMO-1, ∆E
) 30.44; HOMO-1-HOMO-2 ∆E ) 16.53 kcal/mol. The energy
difference between the optimized singlet and triplet state of MeGe-
(µ-NSiH3)2GeMe with use of the spin-corrected energy gap method
proposed by Yamaguchi and co-workers14 is 17.51 kcal/mol, which
is very similar to the 17.2 kcal/mol calculated for V.5a
The reaction of 1 with an excess of Me3SiN3 in n-hexane at ca.
25 °C yielded, after workup, dark violet, almost black crystals of
Ar′Ge(µ-NSiMe3)2GeAr′ (3, Scheme 1).9 Compound 3 is extremely
air and moisture sensitive and rapidly changes to a white powder
once exposed to the atmosphere. The dark violet color disappears
when it is heated to 145 °C in a sealed capillary tube. Crystals of
3 could be stored under an inert atmosphere, but its solutions in
benzene, toluene, and cyclohexane become pale yellow after 2 days.
The isolation of products of these reactions are currently under
1
investigation. Compound 3 has been characterized by H and 13C
NMR, IR, UV spectroscopy, and single-crystal X-ray analysis.
The structure of 3 has a crystallographically required center of
symmetry with a perfectly planar Ge2N2 core (Figure 1).10 The
geometry at nitrogen is trigonal-planar (sum of interligand angles
) 359.97(8)°) and that of germanium is pyramidal (sum of
interligand angles ) 322.10(7)°). The two Ar′ rings are arranged
in a trans fashion across the four-membered Ge2N2 ring. The Ge-N
bond lengths (1.8626(16) and 1.8741(16) Å) are within the range
Figure 1. Thermal ellipsoid of 3 with 30% probability. Hydrogen atoms
and Dipp rings (except ipso carbon atoms) are not shown. Selected bond
distances (Å) and angles (deg) for 3: Ge1-N1 1.8626(16), Ge1-N1*
1.8741(16), Ge1-C1 2.0413(18), Ge1-Ge1* 2.7550(4); N1-Ge1-N1*
85.00(7), Ge1-N1-Ge1* 95.00(7), N1-Ge1-C1 123.28(7), N1*-Ge1-
C1 113.82(7), Si1-N1-Ge1 135.35(9), Si1-N1-Ge1* 129.62(9).
9
6510
J. AM. CHEM. SOC. 2004, 126, 6510-6511
10.1021/ja0492182 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Amii, H.; Vranicar, L.; Gornitzka, H.; Bourissou, D.; Bertrand, G. J. Am.
Chem. Soc. 2004, 126, 1344.
(6) (a) Sita, L. R.; Kinoshita, I. J. Am. Chem. Soc. 1992, 114, 7024. (b) Sita,
L. R.; Kinoshita, I. J. Am. Chem. Soc. 1991, 113, 5070. (c) Sita, L. R.;
Kinoshita, I. J. Am. Chem. Soc. 1990, 112, 8839.
(7) Zurcher, F.; Nesper, R. Angew. Chem., Int. Ed. 1998, 37, 3314.
(8) (a) Philips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2002, 124, 5930. (b) Stender, M.; Philips, A. D.; Wright, R.
J.; Power, P. P. Angew. Chem., Int. Ed. 2002, 41, 1785.
(9) All manipulations were carried out under anaerobic and anhydrous
conditions. 3: 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 days to afford dark violet crystals of 3 (0.104 g, 88%). Mp:
145 °C (dec). 1H NMR (d8-toluene, 399.77 MHz): δ -0.30 (s, 18H,
SiMe3), 0.91 (d, 24H, CHMe2), 1.18 (d, 24H, CHMe2), 2.75 (sept, 8H,
CHMe2), 6.91 (m, 6H, Ar-H), 7.10 (m, 12H, Ar-H). 13C NMR (d8-toluene,
100.52 MHz): δ 5.15 (SiMe3), 23.92 (CHMe2), 26.34 (CHMe2), 31.82
(CHMe2), 123.2, 125.9, 129.0, 131.2, 137.2, 139.1, 154.6, 174.4 (Ar-
C). IR (KBr, Nujol): 1928 (w), 1586 (w), 1571 (m), 1552 (w), 1421 (w),
1340 (w), 1318 (w), 1245 (s), 1226 (w), 1178 (w), 1160 (m), 1125 (w),
1070 (w), 1055 (m), 955 (m), 915 (s), 834 (s), 816 (m), 790 (m), 755 (s),
741 (s). UV-vis (n-hexane): λmax ) 521 nm (ꢀ ) 5600).
(10) Crystal data for 3 at 91(2) K with Mo KR (λ ) 0.710 73 Å): monoclinic,
space group C2/c, a ) 23.9746(16), b ) 11.6981(7), and c ) 25.186(2)
Å, â ) 107.243(4)°, R1 ) 0.0385 for 7873 observed reflections (I >
2σ(I)), wR2 ) 0.1056 (all data).
(11) (a) Hitchcock, P. B.; Lappert, M. F.; Thorne, A. J., J. Chem. Soc., Chem.
Commun. 1990, 1587. (b) Ahlemann, J. T.; Roesky, H. W.; Murugavel,
R.; Parisini, E.; Noltemeyer, M.; Schmidt, H. G.; Muller, O.; Herbst Irmer,
R.; Markovskii, L. N.; Shermolovich, Y. G. Chem. Ber. 1997, 130, 1113.
(c) Veith, M.; Rammo, A. Z. Anorg. Allg. Chem. 2001, 627, 662.
(12) (a) Mackay, K. M. The Chemistry of Organic Germanium, Tin, and Lead
Compounds; Patai, S., Ed.; Wiley, Chichester, 1995; Chapter 2. (b) Baines,
K. M.; Stibbs, W. G. AdV. Organomet. Chem 1996, 39. 275.
Figure 2. Representations of the frontier Kohn-Sham orbitals of the
MeGe(µ-NSiH3)2GeMe from DFT calculations.13
The UV-vis spectrum of 3 in n-hexane shows a strong
absorption maximum at λ ) 521 nm (ꢀ ) 5600), which is red-
shifted compared to those of IV (478 nm)3 and V (446 nm).5a This
corresponds to an energy difference of 54.88 kcal/mol, which is
close to the calculated HOMO-LUMO gap (57.97 kcal/mol) for
MeGe(µ-NSiH3)2GeMe.
In summary, the reaction of 1 with the azide Me3SiN3 afforded
a new non-Kekule´ molecule, 3. Compound 3 has Ge-centered
biradical character as indicated by the intense color, the Ge-Ge
separation, and its high reactivity toward solvents.15 The DFT
calculations support no bonding interaction between the two
germanium atoms as well as a singlet ground state. The extent of
the biradical character of 3, as judged by occupancy numbers for
bonding and nonbonding orbitals associated with the two radical
sites, is not currently available, but the similarities of the calculated
singlet-triplet energies for V and 3 suggest their similar oc-
cupancy.16
(13) The geometry optimizations were performed in gaseous phase using DFT
theory with hybrid B3LYP functional. The molecular structure of MeGe-
(µ-NSiH3)2GeMe was first optimized with Los Alamos LanL2DZ basis
set using an effective core potential (ECP) approximation; a subsequent
optimization of the geometry was performed with 6-31g* basis set using
unrestricted calculations with broken symmetry (BS) technique. All the
calculations were performed with the Gaussian 03 package13b and the
representations of the molecular structures and molecular orbitals were
generated with the MOLEKEL program.13b The optimized geometrical
parameters (bond distances (Å) and angles (deg), geometry optimized for
a singlet state) are almost identical with those found in the crystal structure
of 3: Ge1-N1 1.866, Ge1-N1* 1.868, Ge1-C1 1.980, Ge1-Ge1* 2.735;
N1-Ge1-N1* 85.8, Ge1-N1-Ge1* 94.2, N1-Ge1-C1 108.6, Si1-
N1-Ge1 133.0, Si1-N1-Ge1* 132.8. The only exceptions are the angles
between the N-Ge and Ge-C bonds, which are slightly less opened in
the optimized model structure (∆ ) 14.7°). This is most probably related
to the more important sterical constraints imposed by the bulky Dipp
ligand. (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr. T.; Kudin,
K. N. Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyata, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene. M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi,
R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.;
Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Ciolowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision A.1. Gaussian, Inc.: Pittsburgh, PA, 2003. (b) Flukiger, P.; Luthi,
H. P.; Portmann, S.; Weber, J. MOLEKEL 4.3; Swiss Center for Scientific
Computing: Manno (Switzerland), 2000-2002.
Acknowledgment. We thank the National Science Foundation
for support of this work. The work of M. Brynda in Davis was
supported by Swiss National Science Foundation Grant 8220-
067593. We are also grateful to Professor G. Bertrand and M. F.
Lappert17 for useful discussions.
Supporting Information Available: The X-ray data (cif) for 3.
This material is available free of charge via Internet at http://
pubs.acs.org.
References
(1) (a) Grutzmacher, H.; Breher, F. Angew. Chem., Int. Ed. 2002, 41, 4006.
(b) Borden, W. T. Diradicals; Wiley-Interscience: New York, 1998; p
708. (c) Jain, R.; Sponsler, M. B.; Coms, F. D.; Dougherty, D. A. J. Am.
Chem. Soc. 1988, 110, 1356. (d) Nguyen, K. A.; Gordon, M. C.; Boatz,
J. A. J. Am. Chem. Soc. 1994, 116, 9241.
(2) (a) Abe, M.; Adam, W.; Nau, W. M. J. Am. Chem. Soc. 1998, 120, 11304.
(b) Abe, M.; Adam, W.; Heidenfelder, T.; Nau, W. M.; Zhang, X. J. Am.
Chem. Soc. 2000, 122, 2019. (c) Abe, M.; Adam, W.; Hara, M.; Hattori,
M.; Majima, T.; Nojima, M.; Tachibana, K.; Tojo, S. J. Am. Chem. Soc.
2002, 124, 6540.
(3) Niecke, E.; Fuchs, A.; Baumeister, F.; Nieger, M.; Schoeller, W. W.
Angew. Chem., Int. Ed. Engl. 1995, 34, 555.
(4) (a) Baumgartner, T.; Gudat, D.; Nieger, M.; Niecke, E.; Schiffer, T. J. J.
Am. Chem. Soc. 1999, 121, 5953. (b) Niecke, E.; Fuchs, A.; Nieger, M.
Angew. Chem., Int. Ed. 1999, 38, 3028. (c) Niecke, E.; Fuchs, A.; Nieger,
M.; Schmidt, O.; Schoeller, W. W. Angew. Chem., Int. Ed. 1999, 38, 3031.
(d) Niecke, E.; Fuchs, A.; Schmidt, O.; Nieger, M. Phosphorus, Sulfur
Silicon Relat. Elem. 1999, 146, 41. (e) Sugiyama, H.; Ito, S.; Yoshifuji,
M. Angew. Chem., Int. Ed. 2003, 42, 3802.
(14) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys. Lett.
1998, 149, 537.
(15) Compound 3 also reacts directly with H2 in solution at room temperature
and pressure to give a product that has been identified tentatively as
Ar′(H)Ge(µ-NSiMe3)2Ge(H)Ar′. Details of the reactivity of 3 will be
reported subsequently.
(16) The calculated occupancy for V is ca. 0.17, see: Jung, Y.; Head-Gordon,
M. ChemPhysChem 2003, 4, 522.
(5) (a) Scheschkewitz, D.; Amii, H.; Gornitzka, H.; Schoeller, W. W.;
Bourissou, D.; Bertrand, G. Science 2002, 295, 1880. (b) Schoeller, W.
W.; Rozhenko, A.; Bourissou, D.; Bertrand, G. Chem. Eur. J. 2003, 9,
3611. (c) Scheschkewitz, D.; Amii, H.; Gornitzka, H.; Schoeller, W. W.;
Bourissou, D.; Bertrand, G. Angew. Chem., Int. Ed. 2004, 43, 385. (d)
(17) Professor M. F. Lappert has informed us that he and co-workers have
synthesized a biradicaloid Sn2N2 species with different substtuents and
via a route different than that in Scheme 1.
JA0492182
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