Bis(germavinylidene) [(Me3SiN=PPh2)2C=Ge→Ge=C- (Ph2P=NSiMe3)] and 1,3-Dimetallacyclobutanes [M{(μ2-C(Ph2P=NSiMe3)2} ]2 (M = Sn, Pb)

Article abstract of DOI:10.1002/1521-3773(20010702)40:13<2501::AID-ANIE2501>3.0.CO;2-Q

Low-valent metallavinylidenes :M = C are scarce owing to the low stability of such species. Compound 1, prepared from [CH(Ph₂P=NSiMe₃)₂Li(thf)] and GeCl₂·dioxane, represents the first stable bis(germavinylidene)

Full text of DOI:10.1002/1521-3773(20010702)40:13<2501::AID-ANIE2501>3.0.CO;2-Q

COMMUNICATIONS
Ph2
P
Bis(germavinylidene)
Me3Si
SiMe3
ˆ
ˆ
ˆ
[(Me3SiN PPh2)2C Ge !Ge C-
N
N
SiMe3
Ph2P
HC
Ph2P
N
N
Me3Si
C
ˆ
(Ph2P NSiMe3)] and 1,3-Dimetallacyclo-
P
1/2 GeCl2 · dioxane
N
2
Ph2
Li
THF
Ge
Ge
ˆ
butanes [M{m -C(Ph2P NSiMe3)2}]2
Et2O
Ph2P
(M ˆ Sn, Pb)**
C
2
Wing-Por Leung,* Zhong-Xia Wang,
Hung-Wing Li, and Thomas C. W. Mak
SiMe3
PPh2
Me3Si
1
N
PbCl2
Et2O
Compounds containing a double bond between a
nBuLi THF
ˆ
Group 14 element and carbon (>M C < ; M ˆ Si, Ge,
SiMe3
Sn) have attracted much attention in the past 15 years,
and have been the focus of several reviews.[1] By
contrast, the low-valent, analogous metallavinylidenes
SiMe3
N
Ph2P
Ph2P
PPh2
N
M
Ph2P
N
PPh2
N
M{N(SiMe3)2}2
toluene
C
:
ˆ
C
( M C < ) are scarce. Nevertheless, spectroscopic and
M
theoretical studies of the transient silavinylidene
SiMe3
Me3Si
PPh2
N
N
:
:
ˆ
ˆ
[ Si CH2] and germavinylidene [ Ge CH2] have been
carried out.[2±5] The low stability of these metallavinyl-
idenes could be due to the reduced steric crowding at the
low-coordinate metal center, which more readily leads
to oligomerization. We report here the synthesis and
structures of the first example of a stable bis(germavin-
ylidene) as well as two novel, low-valent 1,3-dimetalla-
Me3Si
Me3Si
M = Pb (3), Sn (4)
Scheme 1.
2
4
[9]
ˆ
cyclobutanes; the latter are believed to form by dimerization
of the intermediate metallavinylidenes
and [(AlMe2)2{m -C(Ph2P NSiMe3)2-k C,C'N,N'}]
result
from double deprotonation of the P CH2 P backbone.
2
ˆ
ˆ
The monolithium salt [CH(Ph2P NSiMe3)2Li(thf)] (1) was
The dimetallacyclobutanes [M{m -C(Ph2P NSiMe3)2}]2
(M ˆ Pb (3), Sn (4)) can also be prepared from the reaction
used as the ligand transfer reagent. It was prepared by the
ˆ
metalation reaction of bis(iminophosphorano)methane,
of M{N(SiMe3)2}2 with CH2(Ph2P NSiMe3)2. Similar reactions
ˆ
CH2(Ph2P NSiMe3)2, with nBuLi in THF. This lithium salt
have been reported by Cavell and co-workers in the synthesis
of hafnium and samarium complexes containing M C
bonds.[10, 11] It is suggested that 3 and 4 result from the head-
had been reported by Stephan and Ong,[6] but its structure was
not well-established as X-ray quality crystals were not
obtained. We have now determined the X-ray structure of 1.
The NMR spectral data of 1 are slightly different from that of
the unsolvated compound reported previously.[6]
ˆ
to-tail cyclodimerization of metallavinylidene intermediates
:
ˆ
ˆ
[ M C(Ph2P NSiMe3)2] (M ˆ Pb, Sn).
Compounds 1 ± 4 have been characterized by elemental
analysis, NMR spectroscopy, and X-ray crystallography.
The X-ray structure data for 1 and 4 will be reported
elsewhere.
The reaction of GeCl2 ´ dioxane with two equivalents of 1
afforded bis(germavinylidene) 2 (Scheme 1). Unexpectedly 2
The molecular structure of 2 is shown in Figure 1.[12] It
ˆ
ˆ
ˆ
ˆ
[(Me3SiN PPh2)2C Ge !Ge C(Ph2P NSiMe3)2]
2
:
ˆ
ˆ
comprises two germavinylidenes [ Ge C(Ph2P NSiMe3)2]
bonded together in a head-to-head manner. The molecule is
asymmetrical with two different germanium environments:
Ge(1) is bonded to the methanediide carbon atom C(1') and
Ge(2), and subtended at an angle of 89.3(3)8. The four-
coordinate Ge(2) is bonded to Ge(1), C(2'), and two imino
nitrogen atoms from the germavinylidenes. Each germavinyl-
idene has an additional uncoordinated imino group. The
Ge Ge bond distance of 2.483(1) Š is longer than those
reported for compounds containing a Ge Ge double bond
(2.213 ± 2.347 Š).[13] Therefore, the Ge Ge bonding in 2 is
more appropriately described as a donor± acceptor interac-
tion, similar to that in the tin(ii) ± tin(ii) complexes
[RN2 Sn !SnCl2] (RN ˆ CH(SiMe3)C6H6N-8) and the asym-
metric distannene 5.[14, 15] In 2, the four-coordinate Ge(2)
behaves as the donor and the two-coordinate Ge(1) behaves
as a Lewis acid center. The Ge C bond in 2 (1.905(8) and
1.908(7) Š) are much shorter than the Ge C single bonds in
some GeII ± alkyl and GeII ± aryl compounds (2.012 ±
2.135 Š).[16] The structure of 2 also shows trans linkage of
was formed upon further deprotonation of 1. Compound 1
acts both as a ligand transfer reagent and as a base for
dehydrochlorination. The by-product in the reaction is
presumably the neutral bis(iminophosphorano)methane.
The analogous reaction of
1
with PbCl2 afforded
2
ˆ
the 1,3-plumbacyclobutane [Pb{m -C(Ph2P NSiMe3)2}]2 (3).
Metal compounds derived from monolithium salts by double
deprotonation of the ligands have been reported.
For example, [Pd(m-Cl)2Pt{C(PPh2)2}n],[7] [Pt2{C(PH2P S)2}-
ˆ
(MeOcod)2]
(MeOcod ˆ 8-methoxycyclooct-4-ene-1-yl),[8]
[*] Prof. Dr. W.-P. Leung, Dr. Z.-X. Wang, H.-W. Li, Prof. T. C. W. Mak
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (China)
Fax : (‡852)26035057
E-mail: kevinleung@cuhk.edu.hk
[**] We thank the Hong Kong Research Grants Council for supporting this
work.
ˆ
ˆ
the C Ge Ge C skeleton (C(1')-Ge(1)-Ge(2) 89.3(3)8,
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stand for 5 d, compound 2 was obtained as red-
orange crystals (0.24 g, 42%), m.p. 110 ± 1118C.
Elemental analysis calcd (%) for C62H76N4Ge2P4-
Si4: C 59.16, H 6.09, N 4.45; found: C 58.95, H 6.30,
N 4.47; 1H NMR (300 MHz, C6D6, 258C, TMS):
d ˆ 0.08 (s, 36H, SiMe3), 7.00 ± 7.08 (m, 24H, Ph),
7.73 ± 7.80 (m, 16H, Ph); 13C{1H} NMR
(75.47 MHz, C6D6, 258C, TMS): d ˆ 3.53 (SiMe3),
128.10, 130.47, 131.95, 137.02 (Ph); 31P{1H} NMR
(161.92 MHz, C6D6, 258C, 80% H3PO4): d ˆ 34.59.
ˆ
3 (method 1): A mixture of CH2(Ph2P NSiMe3)2
(1.28 g, 2.29 mmol) and Pb{N(SiMe3)2}2 (1.23 g,
2.33 mmol) in toluene (25 mL) was stirred at room
temperature for 40 h. Solvent was removed in
vacuo and the residue was dissolved in Et2O. After
filtration, the filtrate was concentrated to about
5 mL. After the mixture was allowed to stand for
3 d, yellow crystals of 3 (1.40 g, 79.8%) were
obtained, m.p. 234 ± 2378C. Elemental analysis
calcd (%) for C62H76N4P4Pb2Si4: C 48.74, H 5.01,
N 3.67; found: C 48.80, H 5.08, N 3.74; 1H NMR
(300 MHz, C6D6, 258C, TMS): d ˆ 0.43 (s, 36H,
SiMe3), 6.90 ± 7.70 (m, 40H, Ph); 13C{1H} NMR
(75.47 MHz, C6D6, 258C, TMS): d ˆ 4.18 (SiMe3),
128.24, 128.50, 130.47, 131.14, 131.94, 133.15 (Ph);
31P{1H} NMR (161.92 MHz, C6D6, 258C, 80%
H3PO4): d ˆ 5.46.
Figure 1. ORTEP drawing of 2; hydrogen atoms are omitted for clarity. Selected bond distances [Š]
and angles [8]: Ge(1) Ge(2) 2.4827(13), Ge(1) C(1') 1.908(7), Ge(2) C(2') 1.905(8), Ge(2) N(1)
1.971(6), Ge(2) N(3) 1.974(6), P(1) C(1') 1.755(8), P(2) C(1') 1.754(8), P(3) C(2') 1.702(9),
P(4) C(2') 1.746(8), P(1) N(1) 1.623(7), P(2) N(2) 1.535(8), P(3) N(3) 1.655(7), P(4) N(4)
1.552(8); C(1')-Ge(1)-Ge(2) 89.3(3), C(2')-Ge(2)-N(1) 120.1(3), C(2')-Ge(2)-N(3) 80.4(3), N(1)-
Ge(2)-N(3) 108.4(3), C(2')-Ge(2)-Ge(1) 128.2(3), N(1)-Ge(2)-Ge(1) 100.5(2), N(3)-Ge(2)-Ge(1)
117.5(2), P(1)-C(1')-P(2) 121.0(4), P(3)-C(2')-P(4) 125.7(5).
3 (method 2): A cooled solution of 1 (0.77 g,
1.21 mmol) in Et2O (20 mL) was added to PbCl2
(0.17 g, 0.61 mmol) with stirring. The mixture was
warmed to RT and stirred for 18 h. After filtration
and concentration of the filtrate, 3 obtained as yellow crystals (0.39 g,
83.6%).
C(2')-Ge(2)-Ge(1) 128.2(3)8). The bis(germavinylidene) skel-
eton is twisted along the Ge Ge axis at an angle of 43.98.
ˆ
4:
A
mixture of CH2(Ph2P NSiMe3)2 (0.27 g, 0.49 mmol) and
Sn{N(SiMe3)2}2 (0.44 g, 1.00 mmol) in toluene (15 mL) was stirred at room
temperature for 42 h. After removal of the solvent in vacuo, the residue was
extracted with Et2O and filtered. Hexane (20 mL) was added to the filtrate,
and the solution concentrated to about 2 mL. After the mixture was
allowed to stand for 3 d, yellow crystals of complex 4 (0.23 g, 70.2%) were
obtained, m.p. 210 ± 2118C. 1H NMR (300 MHz, C6D6, 258C, TMS):
[{1-{N(tBu)C(SiMe3)C(H)}-2-{N(tBu)(SiMe3)CC(H)}-C6H4}Sn !
Sn{1,2-{N(tBu)(SiMe3)CC(H)}2C6H4}]
5
1
The H and 13C NMR spectra of 2 showed only one set of
SiMe3 signals, which does not correspond to the X-ray
structure. This may be due to fluxional coordination of the
imino nitorogen atoms at the germanium centers in solution.
The molecular structure of 1,3-dimetallacyclobutane 3 is
shown in Figure 2. It consists of two metal atoms bridged by
two methanediide carbon atoms, forming a 1,3-M2C2 four-
membered ring. The two imino nitrogen atoms of the ligand
coordinate to the tetrahedral metal center to form MCPN
four-membered rings. These MCPN rings together with the
M2C2 ring, as the base, form a structure framework similar to
an ªopen boxº. The geometry around each of the metal
centers is square pyramidal.
The average Sn C distance of 2.376 Š in 4 is longer than
ˆ
that in the stannene [{(Me3Si)2CH}2Sn C{(BtBu)2C(SiMe3)2}]
(2.025(4) Š).[16] Similarly the Pb C bond in 3 (2.477 Š) is
longer than that in the lead(ii) alkyl compound [Pb{CH(Si-
Me3)(C9H6N-8)}2] (2.336(3) Š).[17] The metal ± metal distances
in dimetallacyclobutanes 3 and 4 are too long to consider the
presence of bonding interactions.
Figure 2. ORTEP drawing of 3; hydrogen atoms are omitted for clarity.
Selected bond distances [Š] and angles [8]: Pb(1) C(2') 2.452(7),
Pb(1) C(1') 2.493(6), Pb(1) N(2) 2.619(5), Pb(1) N(3) 2.708(6),
Pb(2) C(1') 2.451(7), Pb(2) C(2') 2.510(7), Pb(2) N(4) 2.572(5),
Pb(2) N(1) 2.689(6), P(1) N(1) 1.573(6), P(2) N(2) 1.604(5), P(3) N(3)
1.581(6), P(4) N(4) 1.595(6); C(2')-Pb(1)-C(1') 91.6(2), C(1')-Pb(2)-C(2')
91.2(2), C(1')-Pb(1)-N(2) 63.2(2), C(2')-Pb(1)-N(2) 108.6(2), C(2')-Pb(1)-
N(3) 61.3(2), C(1')-Pb(1)-N(3) 109.7(2), N(2)-Pb(1)-N(3) 168.45(18), C(1')-
Pb(2)-N(4) 106.2(2), C(2')-Pb(2)-N(4) 64.21(19), C(1')-Pb(2)-N(1) 62.4(2),
C(2')-Pb(2)-N(1) 113.1(2), N(4)-Pb(2)-N(1) 168.53(19).
Experimental Section
2: A solution of 1 (1.16 g, 1.82 mmol) in Et2O (25 mL) was added to a
cooled (ca. 908C) suspension of GeCl2 ´ dioxane (0.21 g, 0.91 mmol) in
Et2O with stirring. The mixture was stirred at room temperature for 18 h
and then filtered. Hexane (20 mL) was added to the filtrate, and the
solution concentrated to about 5 mL. After the mixture was allowed to
2502
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d ˆ 0.04 (s, 36H, SiMe3), 6.92 ± 7.11 (m, 24H, Ph), 7.40 ± 7.74 (m, 16H, Ph);
13C{1H} NMR (75.47 MHz, C6D6, 258C, TMS): d ˆ 3.65 (SiMe3), 130.09,
132.87, 139.25, 140.44 (Ph); 31P{1H} NMR (161.92 MHz, C6D6, 258C, 80%
H3PO4): d ˆ 12.34.
A Synthetic Pore-Mediated Transmembrane
Transport of Glutamic Acid**
Â
Jorge Sanchez-Quesada, Hui Sun Kim, and
M. Reza Ghadiri*
Received: January 18, 2001
Revised: March 22, 2001 [Z16444]
Synthetic constructs that are specifically designed to allow
transport of hydrophilic substances across cell membranes are
important research tools with possible applications in gene
and antisense therapy, metabolite regulation, and drug
delivery.[1] However, surprisingly the design of synthetic
transmembrane pores from first principles has remained
largely unexplored.[2, 3] Here we describe the design and
functional characterization of a self-assembling transmem-
brane peptide nanotube channel that is capable of highly
efficient transport of l-glutamic acid.
Appropriately designed cyclic peptides with an even
number of hydrophobic a-amino acids with alternating d
and l configurations have been previously shown to self-
assemble through directed hydrogen-bonding networks into
antiparallel b-sheet tubular structures in lipid bilayers forming
active ion channels.[4] One attractive feature of the self-
assembling peptide nanotube class of transmembrane supra-
molecular structures is that pore size can be tuned by the
choice[5] and the number of a-amino acids employed in the
cyclic peptide subunit design. In this context we have shown
previously that cyclic octapeptides size-selectively transport
small ions,[4, 6] whereas decapeptides can also transport small
molecules such as glucose.[3]
The present system is based on eight- and ten-residue cyclic
peptides 1[4] and 2,[3] respectively, which form transmembrane
channels (Scheme 1). Inspection of space-filling models
derived from X-ray structural analogues of peptide nanotubes
suggests that a completely dehydrated glutamate ion in an
extended conformation would barely fit inside a channel
formed by the cyclic octapeptide 1 (7 Š van der Waals
internal diameter) but could be easily accommodated in a
transmembrane pore derived from cyclic decapeptide 2 (10 Š
van der Waals internal diameter).[3] We therefore sought to
examine the utility of self-assembling peptide nanotubes for
size-selective transmembrane transport of glutamate ions.
Previous attenuated total reflection/Fourier transform
infrared (ATR-FTIR) spectroscopic studies of the peptide 1
in lipid multibilayers indicated that the self-assembled trans-
membrane channel adopts an orientation of 7 Æ 18 relative to
the average plane of membrane.[7] In analogous studies for
decapeptide 2, the peptide assembly in the lipid bilayer
displayed backbone amide bands indicative of the expected
[1] a) J. Escudie, H. Ranaivonjatovo, Adv. Organomet. Chem. 1999, 44,
113; b) M. Driess, H. Grützmacher, Angew. Chem. 1996, 108, 900;
Angew. Chem. Int. Ed. Engl. 1996, 35, 828.
[2] H. Leclercq, I. Dubois, J. Mol. Spectrosc. 1979, 76, 39.
[3] M. Izuha, S. Yamamoto, D. Saito, J. Chem. Phys. 1996, 105, 4923.
[4] W. H. Harper, E. A. Ferrall, R. K. Hilliard, S. M. Stogner, R. S. Grev,
D. J. Clouthier, J. Am. Chem. Soc. 1997, 119, 8361.
[5] S. M. Stogner, R. S. Grev, J. Chem. Phy. 1998, 108, 5458.
[6] C. M. Ong, D. W. Stephan, J. Am. Chem. Soc. 1999, 121, 2939.
[7] S. I. Al-Resayes, P. B. Hitchcock, J. F. Nixon, J. Chem. Soc. Chem.
Commun. 1986, 1710.
[8] J. Browning, K. R. Dixon, R. W. Hilts, Organometallics 1989, 8, 552.
[9] A. Kasani, R. McDonald, M. Ferguson, R. G. Cavell, Organometallics
1999, 18, 4241.
[10] R. P. Kamalesh Babu, R. McDonald, R. G. Cavell, Chem. Commun.
2000, 481.
[11] A. Kasani, M. Ferguson, R. G. Cavell, J. Am. Chem. Soc. 2000, 122,
726.
[12] Crystal data for 2 (C64H81Ge2N4O0.5P4Si4): M ˆ 1295.75, crystal size
0.60  0.45  0.25 mm, a ˆ 21.030(4), b ˆ 16.453(3), c ˆ 19.919(4) Š,
a ˆ 90, b ˆ 91.62(3), g ˆ 908, Vˆ 6890(2) Š3, 1calcd ˆ 1.249 gcm 3, m ˆ
1.075 mm 1, Z ˆ 4, monoclinic, space group P21/c, l ˆ 0.71073 Š, Tˆ
293(2) K, 2qmax ˆ 488, 11179 measured reflections, 10823 independent
and 5511 observed reflections (I > 2s(I)), 697 refined parameters,
R1 ˆ 0.0725, wR2 ˆ 0.1843, largest diff. peak and hole 0.3833 and
3
0.696 eŠ
.
Crystal data for 3 (C62H76N4P4Pb2Si4): M ˆ 152.89,
crystal size 0.78  0.19  0.17 mm, a ˆ 10.4752(14), b ˆ 44.827(6), c ˆ
14.068(2) Š, a ˆ 90, b ˆ 100.796(3), g ˆ 908, Vˆ 6489.0(15) Š3, 1calcd
ˆ
1.564 gcm 3, m ˆ 5.395 mm 1, Z ˆ 4, monoclinic, space group P21/n,
l ˆ 0.71073 Š, Tˆ 293(2) K, f/w scans, 2qmax ˆ 56.228, 45654 meas-
ured reflections, 15684 independent and 8598 observed reflections
(I > 2s(I)), 686 refined parameters, R1 ˆ 0.0468, wR2 ˆ 0.0804, largest
diff. peak and hole 1.356 and 1.686 eŠ 3. The crystal data were
measured on an IP Rigaku diffractometer (for 2) or a Brüker SMART
CCD area detector (for 3). The structures were solved by direct
methods using SHELXS-97 and refined using SHELXL-97. Crystallo-
graphic data (excluding structure factors) for the structures reported
in this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC-160423
and 160424. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
[13] K. M. Baines, W. G. Stibbs, Adv. Organomet. Chem. 1996, 39, 275.
[14] W.-P. Leung, W.-H. Kwok, F. Xue, T. C. W. Mak, J. Am. Chem. Soc.
1997, 119, 1145.
[15] W.-P. Leung, H. Cheng, R.-B. Huang, Q.-C. Yang, T. C. W. Mak,
Chem. Commun. 2000, 451.
[16] H. J. Meyer, G. Baum, W. Massa, S. Berger, A. Berndt, Angew. Chem.
1987, 99, 559; Angew. Chem. Int. Ed. Engl. 1987, 26, 546.
[17] W.-P. Leung, W.-H. Kwok, L.-H. Weng, L. T. C. Law, Z.-Y. Zhou,
T. C. W. Mak, J. Chem. Soc. Dalton Trans. 1997, 4301.
Â
[*] Prof. M. R. Ghadiri, Dr. J. Sanchez-Quesada, Dr. H. Sun Kim
Departments of Chemistry and Molecular Biology, and
The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax : (‡1)858-784-2798
E-mail: ghadiri@scripps.edu
[**] This work is supported by a grant from the National Institutes of
Health (Grant: GM52190).
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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