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How Can One Recognize a Triple Bond between Main Group Elements.

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HIGHLIGHTS
How Can One Recognize a Triple Bond between Main Group Elements?
Karl Wilhelm Klinkhammer*
The synthesis of compounds containing a double bond between heavy main group elements has challenged chemists since
the double-bond rule was established by Goubeau in 1957. According to this rule, no double bond, which is stable towards
polymerization, should exist between elements for which the
sum of their electronegativities exceeds a certain amount (approximately 5 ) .[ll Thus-with the exception of the combinations P/S, SjS, P/Cl, SjCl, and CI/Cl-a double bond is stable
only if at least one of the atoms is a second-row element.
The ingenuity of some chemists, however, canceled out this
rule with a trick called kinetic stabilization.[’] By using very
bulky substituents a few dozen compounds containing formal
double bonds between heavy electropositive elements were synt h e ~ i z e d . ~Compounds
~]
with corresponding triple bonds remained unknown. Some of the synthesized alkene homologues,
however, show unexpected differences to “ordinary” double
bonds: a) The four substituents no longer lie in a plane with the
double-bonded atoms (idealized D,, symmetry), but rather
show a typical trans-bent orientation (idealized C,, symmetry).
b) The double bond is only marginally shorter and, in a few
cases, even longer than the corresponding single bond.
c) Cleaving the double bond requires less energy than cleaving
the corresponding single bond; the compound and its carbenetype fragments are often in thermal equilibrium in solution.
um(1)-which, from a classical point of view, may be doublebonded species (R-E=E-R)-only
very long E-E distances
(> 350 pm) are observed. Moreover, only monomers are present
in
What are then the typical features which unambiguously define a multiple bond? Is a short E-E bond imperative? A recent
paper by Robinson et al. allowed us to ask this question again.
They reported a compound they call “the first gallyne”, that is,
the first species showing a triple bond between heavy main
group elernenkC6]The synthesis was accomplished by using an
extremely bulky m-terphenyl substituent. During the last years
substituents of this type had been frequently used, mainly by the
groups of Power and Robinson, to stabilize molecules with
atoms of unusually low c~ordination.[~.
71
“How short is a -Ga-Gatriple bond?” is the question
posed at the beginning of the paper on the synthesis and characterization of gallyne 1, a compound containing two-coordinate
1
Ar = 2,4,6-iPr,C6H2
R
\
/R
R/E=E\R
In accordance with Goubeau’s rule, these “abnormalities”
become more important for elements of low electronegativity.
Therefore, no (isolable) molecular compound is known to date
containing homonuclear double bonds between the heaviest elements of Group 14, 15, or 16. Molecules showing multiple
bonds between the more electropositive heavier elements of
Group 13 were unknown until very recently, when the research
groups of Uhl, Porschke, and Power reported the syntheses of
radical anions such as [Al,R,]- or [Ga,R,]- (bond order 3/2),
and Robinson et al. described the anionic cyclotrigallenes
[R3Ga3I2- (bond order 4/3)
These compounds display the
expected shortening of Al-A1 and Ga-Ga bond lengths compared to dialanes and digallanes, Al,H, and Ga,H,. However,
for dimeric cyclopentadienyl derivatives of indium(1) and thalli[*] Dr. K. W. Klinkhammer
Institut fur Anorganische Chemie der Universitat
Pfaffenwaldring 55, D-70569 Stuttgart (Germany)
Fax: Int code +(711)685-4241
e-mail: kw(ajanorg55.chemie.uni-stuttgart.de
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0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
gallium and an almost planar but nonlinear C-Ga-Ga-C framework.[*]This question possibly reflects a preliminary discussion
with Power, who thinks that a Ga-Ga distance of 2.32 A is too
long to be called a triple bond. In his opinion, the observed
C-Ga-Ga angles of 129-134” seem to tit better the alternative
description of a digallane
P
P
Angew. Chem. Int. Ed.
E n d . 1997, 36, No. 21
,Ga-Ga.
with one lone pair at each
/Ga5Ga
gallium atom (B) than a formulation as a triply bonded
R
R
system (A).[91How can one
A
B
verify the presence of a multiple bond? Classical criteria are short bond distances, high
bond energies, and large force constants as well as a characteristic geometry. Thus, C-C double bonds within alkenes are
always shorter than corresponding single bonds within alkanes,
much more energy is needed to cleave them, and both carbon
atoms exhibit trigonal-planar coordination. On the other hand,
the C-C triple bond is shorter and stronger than a double bond,
and both carbon atoms are linearly coordinated.
Theoretical analyses with ab initio methods show that the
elements from the second row of the periodic table have an
exceptional position, and the criteria mentioned above are not
imperative for multiple bonds between heavy main group elem e n t ~ . [‘1~ . This is caused by the different radial extension of
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HIGHLIGHTS
the s and p valence orbitals, which corresponds to a lower inclination to hybridize, and by markedly lower E-E bond energies.
Simply said: The amount of energy gained by the hypothetical
formation of a double bond from the corresponding carbene
homologues is diminished by the energy needed to promote the
fragments R,E from their singlet ground state to their triplet
valence state. The same holds for the formation of a triple bond
from carbyne- or borylene-homologous fragments.
For most of the carbene fragments R,C, the sum of the promotion energies is much lower than the C = C bond energy and
may even be zero in certain cases. In the case of the analogous
silylene, germylene, and stannylene fragments, however, the
promotion energy often amounts to more than half of the bond
energy of the E-E double bond.L31 In the case of lead, the
heaviest Group 14 element, as well as the corresponding EH
fragments of Group 13 (E = Al, Ga, In, Tl), promotion energies
are even higher, and, a t least for this simple model, dimerization
is totally inhibited. Lowering the energy of dimerization also
results in geometrical changes. The trigonal-planar and linear
coordination of the double o r triple bonded atoms is distorted.
The E-E bond is lengthened, and pyramidalization o r tilting of
the E-EH, and E-EH fragments, respectively, occurs.
That means that neither the bond length, nor the bond
strength o r the topology are reliable criteria for determining the
bond order of bonds between heavy main group elements. How
can one find a way out of this dilemma? If there are no reliable
experimental criteria for a multiple bond, we have to go back to
the original definition of a bond and analyze the mode of bonding with quantum mechanics: A covalent bond consists of an
electron pair which is localized on only two atoms, and a triple
bond is defined by three covalent bonds connecting two atoms.
Quantum mechanical calculations are nowadays mainly performed with the widespread LCAO (linear combination of
atomic orbitals) method, which leads to canonical molecular
orbitals, that is, delocalized orbitals with coefficients on all
atoms. By using the NBO (natural bond orbitals) analysis, however, one may easily transform the canonical orbitals into orbitals which are localized at one or two centers, corresponding
to lone pairs or bonds, respectively.[”. 12] The derived model is
virtually the same as the simple picture obtained using the octet
rule. In the cases of ethylene and acetylene, two and three bonds
between the carbon atoms, one 0 bond and one o r two x bonds,
are obtained. Applying this method to gallyne [Ga2Hz12-,
which differs from acetylene by a nonlinear trans-bent H-Ga-Ga
group, three bonds are also obtained: a) a CT bond, b) a n “ordinary“ x bond constructed from p orbitals perpendicular to the
molecular plane, and c) a “slipped” TC bond in which the maxima of electron density is shifted towards the gallium atoms
(Figure 1) .[’ 31 Closer inspection reveals that this view is equivalent to a model with two donor-acceptor bonds, which was
proposed as early as 1976 by Lappert et al. for describing bonding within the distannene [(Me,Si),CH],Sn=Sn[CH(SiMe,),],
(Figure 2).[14]In the present gallyne, the donor-acceptor bonds
are augmented by an additional x bond to yield a Ga-Ga triple
bond.[”]
A pure quantum mechanical definition of a multiple bond
which is not further verified by experimental data may o r may
not be accepted. However, the very existence of compounds
such as the gallyne remains exciting, because established, apparAngen. Chem. Inr. Ed. EngI. 1997, 36, No. 21
Figure 1. GaEGa bond orbitals derived from NBO analysis. The different signs of
the wave functions are represented by solid and dotted contours, a) cj bond,
b) x bond, c) “slipped” x bond.
Figure 2. Description of the bonding situation in Sn,R, (left) and [Ga,R,]’
according to the model of donor-acceptor bonds [14]
~
(right)
ently unique conceptions and models lead to ambiguous statements and, therefore, have to be modified. The principle of
kinetic stabilization supported by extremely bulky substituents
will hopefully yield further thrilling compounds.
German version: Angew. Chem. 1997. 109, 2414-2416
Keywords: a b initio calculations
elements . multiple bonds
-
bond theory
-
- subvalent compounds
main group
[l] J. Goubeau, Angew Chem. 1957,69, 3.
[2] From the point of view of physics, the term “kinetic stab~hzation”should not
be used because it intermingles kinetics and thermodynamics. The effects which
are described by this term, however, do consist of both a kinetic and a thermodynamic contribution in many cases: On the one hand. the oligomerization
reactions are slowed down, and, on the other, the oligomerization products are
thermodynamically destabilized.
[3] Review: M. Driess, H. Griitzmacher, Angeit.. Chem. 1996, 108, 900; Angen.
Chem. Int. Ed. Engl. 1996.35, 828
[4] W. Uhl, A. Vester, W. Kaim, J. Poppe, J. Orgonomet. Chcm. 1993, 454, 9 , C.
Pluta, K:R. Porschke, C. Kriiger, K. Hildenbrand, Angrw. Chrm. 1993, I05,
451; Angew. Chem. Inr. Ed. Engl. 1993. 32, 388; R. J. Wehmschulte, K. Ruhlandt-Senge, M. M. Olmstead, H. Hope, B. E. Sturgeon. P. P. Power. Inorg.
Chem. 1993, 32, 2983; X. He, R. A. Bartlett. M. M. Olmstead, K. RuhlandtSenge, B. E. Sturgeon, P. P. Power, Angew. Chem. 1993, 105, 761; Angew.
Chem. Inr. Ed Engl. 1993, 32, 717; X.-W. Li, W.T. Pennington, G. H
Robinson, J. Am. Chem. SOC.1995, I1 7,7578; X.-W. Li, Y. Xie, P. R. Schreiner,
K. D. Gripper, R. C. Crittendon, C . F. Campana, G. H. Robinson, H. F.
Schaefer 111, Organomerallics 1996, IS, 3798.
[S] H. Schumann. C. Janiak, J. Pickardt, U. Borner, Angr.n-.Chem. 1987, YY, 788;
A n p v . Chem. Inr. Ed. Engl. 1987,26,?89; H . Schumann. C- Jantak. F. Gorlitz,
J. Loebel, J Organomer. Chem. 1989,363, 243.
[6] J. Su, X:W Li, R. C. Crittendon, G. H. Robinson.1 Am. Chum. Soc. 1997.119,
5471.
[7) R. S. Simons, L. H. Pu, M. M. OlmStedd, P. P. Power, Orqonomerollic.~1997,
16, 1920: J. J. Ellison, P. P. Power, J Orgunornet. Chern. 1996, 526, 263;
J. J. Ellison, K. Ruhlandt-Senge. H. H. Hope, P. P. Power. Inorg. Chem. 1995,
34. 49.
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HIGHLIGHTS
[8] There are addltional, presumably ionic contacts (Ga..Na 308 pm (mean value)) to both sodium atoms, which are held in place by two chelating arene rings
and probably reduce the electrostatic repulsion between the negatively charged
Ga atoms.
[9] R. Dagani, Chem. Eng. News, June 10, 1997,9.
[lo] W. Kutzelnigg, Angew. Chem. 1984,96.262; Angew. Chem. Int. Ed. Engl. 1984.
23, 272.
[ l l ] J. E. Carpenter, F. Weinhold, J. Mol. Struct. 1988, 169, 41, and references
therein.
[12] The ELF (electron localizationfunction) is a further suitable tool for interpreting results of quantum chemical calculations in terms of such vivid concepts as
“lone pair” or “bond”: B. Silvi, A. Savin, Nature 1994,371,683; A. D. Becke,
E. Edgecombe, J Chem. Phys. 1990, 92,5397; A. Savin, A. D. Becke, J. Flad,
R. Nesper, H. G. von Schnering, Angew. Chem. 1991,103,421 ;Angew. Chem.
Int. Ed. Engl. 1991,30,409; A. Savin, R. Nesper, S . Wengert, T. F. Fassler, ibid.
1997,109,1892 and 1997,36,1808. This function, which measures the probability of finding two electrons of opposite spin in the same space, is in accordance with a triple bond within gallyne [Ga,H,]’-.
[13] I carried out the NBO analyses employing the NBO module implemented in
Gaussian 94: NBOVersion 3.1, E. D. Glendening, A. E. Reed, J. E. Carpenter,
F. Weinhold implemented in Gaussian 94, Revision D.l: M. J. Frisch, G. W.
Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R.
Cheeseman, T. Keith, G. A. Peterson, J. A. Montgomery, K. Raghavachan,
M. A. AI-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski,
B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y.Ayala, W.
Chen, M. W. Wong, J. L. Andres, E. S . Replogle, R. Gomperts, R. L. Martin,
D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon,
C Gonzalez, J. A. Pople, Gaussian, Pittsburgh (PA), 1995. The gallium atoms
were described by LANL2 pseudopotentlals. Basis sets of double-zeta quality
were used for hydrogen, carbon, and gallium.
[14] P. J. Davidson, D. H. Harris, M. F. Lappert, J. Chem. SOC.Dalton Trans. 1976,
2268; D. E. Goldberg, P. B. Hitchcock, M. F. Lappert, K. M. Thomas, A J.
Thorne, T. Fjeldberg, A. Haaland, J. Chem. SOC.Dalfon Trans. 1986, 2387.
Double or triple bond models with equivalent “banana bonds” instead of cs
and TI bonds are obtained by unitary transformations. An analogous relationship exists between the bonding model for gallyne [Ga,H,]’- derived here and
the model by Lappert of two donor-acceptor bonds (augmented by a further
TI bond)
1151 Quantum chemical investigations of the bonding wlthm homologues systems
of alkenes and alkines, E,H, and E,H,, yield analogous results. A survey is
given in ref. [3] as well as in K . Kobayashi, S . Nagase, Organometallics 1997,
16.2489, and references therein. The latter demonstrates that the introduction
of bulky ligands further stabilizes the trans-bent form of disilynes (R&),
which are isolelectronic to Robinson’s gallyne.
EVENTS
Announcing the
XXIII International Symposium
on Macrocyclic Chemistry
Macrocycles in Academe and Industry
June 7‘h- 12th, 1998
Turtle Bay, Hawai
Imaging, Chelation, Catalysis, Molecular Recognition, Sensing.
Poster Presentations Currently Being Accepted.
Site of symposium is the Turtel Bay Hilton on the North Shore of the Island of Oahu. Activities of interest
include golf, surfing, and tennis. An excursion to the Polynesian Center is planned. Organized by Jonathan
Sessler, Eric Anslyn, and Eiichi Kimura.
To acquire registration and poster contribution forms please write: Eric V. Anslyn, Department of Chemistry
and Biochemistry, The University of Texas at Austin, Austin TX 78712, or e-mail anslyn@ccwf.cc.utexas.edu.
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Angew. Chem. Int. Ed. Engl. 1997,36, NO. 21
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