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Monoclinic and Triclinic Tetraisopropyl-p-phenylenediamine To what Extent do nN Interactions Determine Structures.

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servation of only a small calculated Pd-Pd overlap population.[2'l A structural indication for the existence of Pd-Pd
bonding interactions is the fact that the Pd(2) and Pd(3)
atoms are not centered in the tetrahedral B sites of the
Ta,NiS, parent structure, but instead are displaced from the
centers of these tetrahedral voids towards the two respective
neighboring Pd atoms. The Ta-Fa interactions are weak, as
expected from their large Fd-Ta distances (dTa-Ta:
The reasons for the preferential formation of the "filledin" Ta2NiQ, structure type with the tellurides are not yet
clear. Geometric conditions cannot be the only cause since
the respective radius relationships with the sulfides would
also allow an occupation of the tetrahedral sites within the
layers. A possible argument for the occurrence of the lattice
distortions associated with the incorporation of the interstitial atoms as well as for the strong tendency for formation of
the "filled-in" structure with the tellurides is the high polarizability of Te2- anions. Thus, lattice distortions should
preferentially occur with "soft" lattices.
Received: April 21. 1993 [Z6025IE]
1993, 105. 1795
German version: A I I ~ C ' ICli~m.
[ I ] a ) R. H. Friend. A. D. Yoffe, A d v . Phys. 1987.36. 1 : b) J. Rouxel. R. Brec.
Aiiml. Rim .Mlrtrr. Sci. 1986. 16. 137; c) F. Hulliger. Pli~sicsundChrmDirj
U{ .Mrrrrricilr wirh L ~ i s r w dSrrucrures. Voi. 5. S t r w t u r u l Chemisrr! of'Luy~ r - f i p cP/icr.srr (Ed.: F. Levy), Reidel. Dordrecht. 1976.
[2] G Betr. H. Tributsch, Prog. Solid S/crie Chc~n.1985. 16, 195.
[3] R. Chevrel. P. Gougeon. M. Potel, M. Sergent, J. Sulid Siurr Cliem. 1985,
[4] K. Yvon. C i i r r . Top. Muier. Scr. 1979. 3. 53.
[51 a ) J A. Wilson. F. J. DiSalvo. S. MahaJan. A d v . Plzu. 1975, 24. 117:
b) Elcc iruiiic PI uper.iir.s uf Inorgu17ic Qiiu.51 One-Diinenrionol Conipounrls,
Puri / uiu/2(Ed.: P. Monceau). Reidel. Dordrecht. 1985;~)Cr~stulClreriiisrri' a ~ Pruprriic,.s
of Muirriu/.s ii.ith Qirusi Oiic-D/n?emionril Srruciiirrs
(Ed.: J. Rouxel), Reidel. Dordrecht 1986.
[6] P. Bottcher. U. Kretschmann. Z. Anorg. A&. Chem. 1985. 52X. 145.
[7] a) W. A. Flomer. J W. Kolis. J A m . Clirm. Sue. 1988, 110. 3682; b) W.
Tremel. Inorg. Clirin. 1992, 31. 1030.
[8] R. Nesper. J. Curda. Z . Nutirrfurscli. 5 1987, 52. 557.
[9] M . E. Badding. F. J. DiSalvo. Inorg. Chen?. 1990. 29. 3952.
[lo] a ) Nb,Ge, ,Tee: L. Monconduit. M. Evain, F. Boucher, R. Brec. J. Rouxel,
Z . .4tior,q N l ~ y Chrm.
1992. 616, 177; bj Nb,SiTe,: J. Li, P. J. Carroll,
MUWI Rr\. 5uN. 1992. 27. 1073; c) Ta,ATe, (A = Si. Ge): H. Kleinke. V.
Ebstal'yev. W. Ti-emel. unpublished results: d ) H. Kleinke, W. Tremel. tinpublished results.
[l 11 W. Tremel. A n p ( w C I i ~ n i .1992. 104. 230; Angeii.. Clriwi. h i . Ed. Engl.
1992. 31. 217.
[12] Ta,Pd,Te,: Pure phase samples of thin needle-shaped, metallic lustrous
crystals (GI. 0.1 mm long) were prepared by reaction of stoichiometric
amounts of the respective elements with the addition of iodine as transport
agent (quartz ampule. 1 - 3 mgcm-3, V =7-10 cm3, transport from 850
+ 800 C ) In a typical synthesis of Ta,Pd,Te,,
a quartz ampule (ca.
12 mm diameter) was heated under vacuum. then charged with tantalum
powder (0.181 g. 1 mmol). palbadium powder (0.160 g. 1.5 mmol), tellurium powder (0.319 g. 2.5 mmol), and an appropriate amount of iodine (ca.
2 m g c m - > ampule volume). and finally flame-sealed. The ampule was
then slowly heated to SOOT in a two-zone furnace, after which time the
temperatures in the respective heating zones were raised to 800 and 850 C.
The polarity of rhe temperature gradient was reversed after one week.
Long.flat. metallic lustrous needles were formed on the cooler slde of the
ampule after an additional two weeks. Energy-dispersive X-ray microanalysis ( E D A X ) : Ta Pd!Te = 2.13'2.88!4.92.
[I31 X-r'iy structureanalysis: Space group P n m u ( n o . 62). 2 = 2, (1 = 13.989(3).
h = 3.713(1). ( ' = 18.630(4) A. Siemens P3. Mo,,, p = 42.44 m m - ' . crystal dimensions 0.002 x 0.006 x 0.08 mm. 0-20 scan. 20,,, = 65 . 1992 reflections. empirical absorption correction (XEMP). 1446 symmetry-independent reflections. R(R,) = 0.038(0.038). Further details of the crystal
structure investigation may be obtained from the Fachinformationszentruin Karlsruhe. Gesellschaft fur wissenschaftlich-technische Information
mbH. D-76344 Eggenstein-Leopoldshafen (FRG), on quoting the depositor) number CSD-57345. the names of the authors. and the journal citation.
[I41 S. A Sunshine. J. A. Ibers. lnorg. Chetn. 1985. 24. 3611
1151 w.Tremel, Angrv. C/wm. 1991, 103. 900; A n g i w . C/IEI71. Inr. Ed. Engl.
1991. 30. 840.
1161 Compounds having the compositions Nb,Pd, ,,Se,. Ta2Pd, "$e,, and
Nb,Pd,, .4Cu, ,?Se, have been described. but in these the coordination at
palladium is also either octahedral or square planar. a ) D. A. Keszler. J. A.
Ibers, M. Shang, J. Lu, J. Solid Start. CAW. 1985,57,68. b j P. J. Squattrito.
S. A. Sunshine. J. A. Ibers. ;bid. 1986, 64, 261; c j P. J. Squattrito. S. A.
Sunshine. J. A. Ibers. J Solid S f u f elonics 1986. 22.53.
[17] B. Krebs, G. Henkel, A~igeii.Clirm. 1991, 103. 785. A n g i w . Clwm. h i / . E d
Engl. 1991, 30, 769.
1181 C . A. Tolman, Chwn. RL'Y. 1977. 77. 313.
[19] a) D. Fenske. H. Fleischer. C. Persau, A n g w . Clirm. 1989, 101. 1740.
.4/i:eii. C/7rtn. /nf. Ed. Engl. 1989. 28, 1665: b) D. Fenske. C Persau, %.
Anorg. Ally. Cliem. 1991. 593. 61.
[20] H. Kleinke. W. Tremel. J Solid Srurr Clrem., submitted.
1211 A. Mar, J. A. Ibers. J. C / I ~ / SJIIX. . 1991. 639.
[22] Extended-Huckel(EH) approximation: R. Hoffmann, J C ' h ( v ~ .Pli.
1963. 39, 1397. H,, matrix elements: J. H. Ammeter. H.-B. Burgi, J.
Thibeault. R. Hoffmann, J Aiii. Chei?i. Soc. 1978, 1/10. 3686. Tightbinding approach: M.-H. Whangbo. R. Hoffmann. J. A m . C ' / i m i . So(..
1978. fl)O. 6093. M.-H. Whangbo. R. Hoffmann. R. B.Woodward. Pro<,.
R. So< London, Sw. A 1979, 366. 23. parameters for Ta: .I Li. R. Hoffmann, M. E. Badding. F. J. DiSalvo. I n o i g . C/io?/.1990,2Y. 3943. parameters for Ni and Te: J.-F. Halet. R. Hoffmann, W. Tremel. E. Liimatta. J. A.
Ibers. Cl7rn7. M a w . 1989, 1. 351. Special k points: R . Ramirez. M. C .
Bohm, h i . J. Quunitim C l w ? ~1986.
30. 391; ihid 1988. 34. 571
[23] Wheeler could show. based o n EH calculations. that the underlying Ma€,,
frame i n the structures of the [Pd,E,(PPh,),] clusters (E = As. Sb) will be
stabilized through the bonding of an additional "interstitial" (metal) atom.
[24] J. Neuhausen, R. Schlogl, R. K. Kremer, W. Tremel. unpublished results.
1251 Calculated Fa-Te and Pd-Te overlap populations (tight-binding C ~ I E U I U tions, EH approximation. parameters from ref. [23]): Ta -Te (4.137
- 0.017; Pd-Te
= 0.137; Pd-Te (3.000A) = 0.067: Pd Te
= 0.063. Additional values for Ta-Fa. Ta-Pd, and Pd-Pd
overlap populations: Ta-Ta = 0.051; Ta-Pd(l) = 0.095: Tit- Pd(2.3) =
0.136; Pd(2,3)--Pd(2.3) = 0.072.
Monoclinic and Triclinic Tetraisopropylp-phenylenediamine: To what Extent do nN/n
Interactions Determine Structures?**
By Hans Bock,* Ilka Gobel, Christian Nather,
Zdenek Havlas, Angelo Gavezzotti, and Giuseppe Filippini
Dedicared 10 Professor Robert Corriu
The structure of polymorphic modifications of the same
compound provide multifaceted information on weak interactions in and between organic molecules.['] In the following, we report on the serendipitously discovered monoclinic
and triclinic crystals of tetraisopropyl-p-phenylenediamine.r21which contain different conformers with and without nN/ninteractions between the nitrogen electron pairs nN
and the benzene n system (Fig. 1. center: dihedral angles
w(C,N-C,) = 28 or 74").[3341
The molecular structures (Fig. 1 top and center) demonstrate that in the conformer giving monoclinic crystals the
axes of the lone pairs on the nitrogen centers deviate from
those of the 71 system perpendicular to the six-membered ring
plane by only 28" and thus allow a tenfold stronger nN/n
delocalization [cos2 (28") = 0.781 relative to the triclinic one
[*] Prof. Dr. H . Bock. Dipl.-Chem. I. Gobel. Dip1.-Chem. C. Nither
Institut fur Anorganische Chemie der Universitit
Marie-CUrie-Strasse 11. D-60439 Frankfurt am Main ( F R G )
Dr. Z. Havlas
Institute of Organic Chemistry and Biochemistry of the
Czech Academy of Sciences
Flemingova Nam 2. CS-1160 Prag (Czech Republic)
Prof. Dr. A. Gavezzotti. Dr. G. Filippini
Institute of Physical Chemistry of the University
Via Golgi. 1-20133 Milano (Italy)
Structure of charge-perturbed and sterically overcrowded molecules as
well as interactions in crystals. Part 30. This research project has been
supported by the Deutsche Forschungsgeineinschaft. the state of Hesse.
and the Fonds der Chemischen 1ndustrie.-Part 29.. Ref. [4].
with dihedral angles of 74" [COS' (74") = 0.081. The latticeenforced effects of the diisopropylamino donor substituents
cause significant structural changes: the NC, pyramids are
flattened by 12" to 353', the NCri,, bonds are shortened by
4 pm to 141 pm, and the ips0 angles at the substituted centers
are reduced b y 3" to 1I 5..[5,
d = 1.071 g cm-3
d = 1.064 g ~ r n - ~
The molecular dynamics has been approached first for the
spatially not yet overcrowded tetramethyl derivative by
PM 3 enthalpy hypersurface calculations (Scheme 1). For an
Scheme 1.
assumed uncoupled rotation and inversion, rotation with a
barrier of only 16 kJmol-' should be favored and, therefore,
activated at room temperature (0:transition states). The
enthalpy surface for the tetramethyl derivative (Scheme 1)
reproduces all structural trends observed for the tetraisopropyl-p-phenylendiamine conformers (Fig. 1 top and cen-
Fig. 1. Top and center: molecular structures of
the tetrdisopropyl-p-phenylenediamine conformers exhibiting skeletal symmetry C , and C ,
in side and axial view (50% thermal ellipsoids)
[3]. Significant differences between the monoclinic and triclinic conformers (bond lengths
in pm and angles in "): dihedral angles
w(CZN-C,) 28/74, distances dcN 1411145.
angles &,Jring)
115/118, and angle sums
I(t CNC) 3531341. Bottom: lattice packing in
the polymorphic monoclinic (P2,/c, unit cell
with Z = 4 in .r direction) [3a] and triclinic modifications (Pi.unit cell with Z = 1 In .\- direction
and arrangement of the molecule around a crystallographic inversion center) [ 3b].
ter): the n,/n delocalization, which sets in with the decrease
of the dihedral angle w = 90" + O", flattens the NC, pyramids, shortens the NCringbonds, and reduces the ips0 ring
According to analogous PM 3 calculations, in tetraisopropyl-p-phenylendiamine a complete rotation of the bulky
[ (H,C),HC],N substituents around the NCri,, bonds is no
longer possible because of steric hindrance from the ortho
ring hydrogens (van der Waals radius rkdw z 120 pm16])
without deformation of the isopropyl groups, which can be
calculated on geometry optimization. An analysis of the
wave functions for the resulting conformer setf7' shows that
the maximum possible stabilization by nN/zdelocalization of
about - 15 kJmol-' at a dihedral angle of w = 0" is compensated on increased twisting of o to 90" by a fourfold H/H
repulsion of a total of up to 16 kJ mol - * (Scheme 2). This is
in accord with the almost identical enthalpies of formation
for the rather different monoclinic and triclinic crystalline
conformers of tetraisopropyl-p-phenylenediamine,AGM
= - 46 and -47 kJ mol- (Scheme 2), calculated based on
the experimental molecular structure data (Fig. 1).
The repulsive H/H interactions are in accord with the crystal structure data for both polymorphs (Fig. 1 top and center): In the monoclinic modification, the shortest intramolecular H ... H distances amount to only 200 and
221 pm (rest > 323 pm) in contrast to 254 and 296 pm
(rest > 359 pm) in the triclinic one. The almost identical en-
0570-0833/931/212-1756$ 10.00+ .25/0
Angrw. Cliem. Inr. Ed. EngI. 1993, 32, No. 12
thalpies of formation,
are in addition a n essential
precondition for the crystallization of polymorphic modifications, the lattice energies of which usually differ only by
< 20 kJ mol- 1 ,ll%
b. f l
able PM 3 enthalpies of formation (Scheme 2) suggests a rotamer mixture in the gas phase as well as in solution a t room
temperature. Possibly, therefore, the thermodynamically
based Oswald's step rule"
should be supplementedif surface effects could be excluded in the growth of the
monoclinic crystals by vacuum sublimation-with a kinetic
component: The monoclinic modification, which is more
stable according to both its density and the lattice energy
calculation, must have been formed by slow crystallization
from the gaseous rotamer mixture and the somewhat less
favored triclinic one possibly by a kinetically controlled
rapid growth from solution. This speculative assumption
further illustrates the subtle energy balance that enables
the isolation of two rather dissimilar conformers of tetraisopropyl-p-phenylenediamine by crystallization in different
local total energy minima.
n N / n interaction
Received: February 13th. 1993 [Z 5869 IE]
German version: Angew. Chem. 1993, fO5. 1823
[kJ rnol-'1
Scheme 1
The lattice energies of nondisordered polymorphic modifiCdtiOn5, according to an often successfully tested empirical
can be correlated with their crystallographically determined densities. Therefore, the monoclinic crystals grown
~ * ] possess a
by sublimation with e =1.071 g ~ m - ~ [ should
more negative lattice energy that the triclinic ones crystallized from petroleum ether solution @ = 1.064 gcm-3.[3b1In
the monoclinic crystal, the n,/x interacting conformers are
packed crosswise with shortest intermolecular H ... H distances of 242 pm (Fig. 1 bottom left), whereas in the triclinic
crystal the n,/o-planarized ones are stacked perpendicularly
to the aligned molecular long axes with shortest intermolecular H ... H distances of 240 pm (Fig. 1 bottom right). Because their difference in densities of only 0.7% borders on
the precision of measurement, in addition total lattice energies have been calculated by the atom/atom potential apa parameter
p r o ~ i m a t i o n [ ~ from
that recently has
been optimized for a large data base of organic molecular
crystals, by using either the crystallographically determined
torsion angles of all methyl groups or by imposing a constant
value of 60'.['01 The two resulting lattice (sublimation) energies for each of the modifications of tetraisopropyl-p-phenylenediamine are - 115.5 o r - 114.7 kJ mol- (monoclinic)
and - 108.8 o r - 109.4 kJ mol - ' (triclinic) and indicate that,
as inferred from the rather small density difference, the monoclinic modification should always be the thermodynamically more stable one, irrespective of the selected methyl group
torsion angles.
The monoclinic crystals have been grown by slow vacuum
sublimation of the triclinic melt over a period of about one
whereas the triclinic ones crystallize "in a flash"
(within a few minutes) from the saturated petroleum ether
solution. The band pattern at low ionization energies in the
photoelectron spectrumfz1and its agreement with the superposed PM 3 eigenvalue sequences for the different tetraisopropyl-p-phenylenediamine conformers exhibiting compar-
[I] Summaries a ) J. Bernstein, (Conformational polymorphism) in Orgunii.
Solid Store Chemistry, Srudies in Orgunic Chemistrs Vol. 32 (Ed.: G. R.
Desiraju). Elsevier. Amsterdam, 1987. pp. 471 -517 and 146 references
therein; b) G. R. Desiraju in Crj*stul Engineering (Muter. Sci. Monogr.
1989. 54, 285-301 ; c) J. D. Dunitz, in X-Roy Analysis und the Strucrrrre of
Orgunit Molecules, Cornell University Press, Ithaca, 1979, p. 321 ff. Further recommended reading: d) M. Egk J. D. Wallis, J. D. Dunitz. H e h .
Chim. Acra 1986. 69. 225 (seven conformers of l-dimethylamino-8-11tronaphthalene in three polymorphic modifications); e) I. Bar. J. Bernstein, Tetruhedron 1987,43, 1299 and references therein (poly- and isomorphic p-substituted benzylideneanilines); f ) J. Bernstein. J. A. R. P. Sarma.
A. Gavezzotti, Chem. Ptiys. Lert. 1990,174.361 (latticeenergycalculations
for monoclinic and orthorhombic modifications of benzene, naphthalene.
and anthracene); g) S. K. Burley, G. A. Petsko. Science 1985,229. 23 (stabilizing interactions between UnSdtUrdted rings in proteins).
121 1. Gobel, Ph.D. thesis. University of Frankfurt. 1993; contains all details
of the investigations: Tetraisopropyl-p-phenylenediaminewas prepared
by heating of p-phenylenediamine with a threefold excess of 2.2dimethoxypropane in acetonejacetic acid in presence of PtJAI,O, under a
pressure of 100 bar H, in an autoclave (A. Gayddsh, J. T. Arrigo, US-A
3234281. 1966; Chem. Ahsrr. 1966,64, 11 125h). Fractional distillation at
lo-' mbar yielded about 20% yield of a yellow oil from which at 277 K
a clorless solid with m.p. 308 K crystallized (correcl elemental analysis;
' H N M R (DCCI,, TMS) 6: 1.09-1.20 (d, 24H). 3.4-3.9 (m, 4H), m /
e = 2 7 6 ( M i ) , 261 (M' -CH,), 246 ( M i-2CH,), 233 (M' CH(CH,),). 218 (M' - CH3 - CH(CH3),. 203 ( M i - 2CH3 CH(CH,),). Crystal growth: 200 mg was dissolved in 5 mL petroleum
ether at 20 -C, the solvent evaporated at 20 'C and lo-' mbar until crystallization began, and the oily solution kept in a refrigerator at 277 K Under
petroleum ether the triclinic modification crystallized. After evaporation
almost to dryness, the monoclinic modification recrystallized at the glass
wall. but it can be better synthesized by sublimation of a melt of the
triclinic modification at lo-' mbar to a cooler glass wall. All crystalline
products werecharacterized by an X-ray determination oPthe unit cell; the
polymorphic modifications were unexpectedly discovered on selection of
crystals with an optimum reflection profile from different crystallizations.
Differential scanning calorimetry investigations of the polymorphic modifications by slow warming (1 Kjmin) of a melt at 110 K to 320 K did not
indicate any solid phase transitions. Investigations with photoelectron
spectroscopy in the gas phase 141 showed two broad ionization bands
between 6 and 9.4 eV; the peak of the first one begins at 6.6 eV, increases
slowly to the maximum at 7.83 eV and dros steeply to 8.1 eV; that of the
second one increases steeply from 8.2 eV to a double maximum at 8.64 and
8.92 eV and then drops slowly to about 9.6 eV. Band analysis based on
MNDO eigenvalue correlation suggests the assignmet (nN-x,)< ra. <
(n<-nN)and can he rationalized by assuming diiferent propotions of the
doubly twisted (dihedral angle w(C,N-C,) % 8 0 ,
-0.8eV). the
singly twisted (wl % 80 , <o24 10 , A/€,,2 % 2 eV), and the planarized
(01 % 1 0 . A/€,.
% 2.9 eV) conformers in a mixture.
131 Crystal structures C,,H,,N, (276.46). a ) Monoclinic modification: u =
1014.8(1), b=1236.9(1). t=1415.4(1)pm, {j =105.18(1)'. V =1714.62x
lO"pin'(150 K ) . 2 = 4 . ~ , , , , =1.071
gcm-'./i = 0.06mm-'.monoclinic. space group P2Jc (no. 14). Siemens AED-I1 four-cirlce diffractometer,
Mo,. radiation. 5988 measured reflections with 3' < 20 < 53 , of which
3502 are independent and 2619 with I > l o ( I ) , Rim,= 0.0238. Structure
solution by direct method and difference Fourier technique (SHELXTLPlus); R = 0.0474. R , = 0.0435 for 310 parameters, 11. = 1/(n2(F) +
0.0003 F'): G O F : 4.616, shiftierror < 0.001 ;extinction correction. residual electron density: +0.23!-0.21 e,/A3. All C and N atoms are anisotropically refined. all H atoms are unconditionally refined with isotropic temperature factors. b) Triclinic modification- u = 698.2(2). h = 746.3(2).
c = 970.5(4) pm, z =70.77(2), fi = 87.25(2), 7 = 65.35(2)', V = 431.58
x 106pm3(150K).2=l.@,,,,d = l . O 6 4 g ~ m - ~ . j=
r 0.06mm-'. triclinic.
space group Pi (no. 2). Siemens-AED four-circle diffractometer. Mo,,
radiation, 2550 measured reflections within 3 i28 < 55 , of which 1983
are independet and 1950 with I > 0.5 5 ( I ) . R,", = 0.0143. Structure solution with direct methods and difference Fourier technique (SHELXTLPlus); R = 0.0432. R, = 0.0451 for 156 parameters. II( = l / ( c 2 (f)
+ 0.00002 F'): G O F : 4.93. shiftkrror < 0.001 ; extinction corrections;
residual electron density: +0.27/-0.21 e,/A'. All C and N atoms anisotropically. all H atoms unconditioiially refined with isotropic temperature factors. The molecule is centered at a crystallographic inversion
center. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe. Gesellschaft fur
wissenschaftlich-technische Information mbH. D-76344 Eggenstein-Leopoldshafen ( F R G ) on quoting the depository number CSD-57768. the
names of the authors and the journal citation.
Additional investigations on conformers of p-phenylenediamine derivatives that like the benzylidenanilines [ l a . 31 exhibit two energetically Favored rotational degrees of freedom around both of the N-C,,,, bonds
concern inter aha N,N'-bis(trimethylsi1y)- and tetrakis(trimethy1si1y)phenylenediamines (H. Bock, J. Meuret, C. NHther. Terruhetlron Lcrr.
1993. 34. in press.). The tetrasubstituted molecule can be reduced in ether
solution to its blue radical anion(!)-in addition to the usual oxidation to
Wurster's Blue radical cation (F. Gerson. U. Krynitz. H. Bock. Aigeii..
Chen?. 1969. 81. 786, A n g e r . Cbm?. In,. Ed. Engl. 1969, 8. 767). This
surprising discovery can be rationalized from the different band patterns
at low ionization energies of PE spectra and their assignment by Koopman's correlation with eigenvalues from geometry-optimized AM1 calculations. by assuming that the disubstituted derivative must possess an
approximately planar skeleton SiN-C,H,-NSi, whereas the tetrasubstituted one should exhibit dihedral angles <u(Si,N -C6) = 90 . Single crystal
structures confirm the gas phase measurements: contrary to the situation
in the largely planar N.N'-substituted derivative, the steric overcrowding
by four bulky (H,C),Si substituents causes the H/H repulsion from the
orrho-ring hydrogens to exceed the stabilizing n,h interactions, which
results in a dihedral angle m(SizN-C6) = 83
Cf., for example, a) M. Kaftory in A c d k Orgrr/?onitr-ugenS/error/i I ? . 263;
b) A. Donienicano. A. Vaciago, Acru Crysrdogr. Sect. B 1979. 35. 1382;
A. Hoekstra. A. Vos, ;hid 1975, 1716. 1722.
H. Bock, K. Ruppert. C. NBther. Z. Havlas, H.-F. Herrmann. C. Arad. I.
Gobel. A. John. J. Meuret, S. Nick, A. Rauschenbach, W. Seitz, T. Vaupel.
B. Solouki. Aiigeri. Cbeiil. 1992. 104. 565-595; Angeir. Cl?ent. I n ! . Ed
Engl. 1992, 31, 550-581 and references therein.
The PM 3 potential energy surface calculations (J. J. P. Stewart. J. Coinpur.
Cheni.. 1989. IO. 209,221) have been performed based on the experimental
structural data 131 and partly geometry optimized (tetramethyl derivative)
using the program version MOPAC 6.0IQCPE no. 455 with a n IBM RISC
6000/320. Additional data (cf. Scheme 1): Inversion transition state
A f l M 3 = 1 0 4 k J m o l - ' , dc,,=147, dcr=138. 140pm. grCCC=121 :
conformer (w = 0")AHpM3=73 kJrno1-l. dcN=148. dcr =139, 140pm.
X C C C =121"; conformer ( i v -90") AHPM3= 87 kJmo1-I. clcN =149.
dcr = 139. 140 pm. K CCC = 120'. Estimate of the H ... H repulsion between isopropyl groups and H,,,,, (cf. Scheme 2): AAHP'' (kJmol-')/
(pm): 0/277, 11250. 2.5/222. 6/195, 9.5/170, 22/149. 36/
Cf.. for example. I. Bar. J. Bernstein. J. Pby.s. Chnn. 1984. 88, 243 and
references therein.
a) SummaryiSurvey: A. J. Pertsin. A. I. Kitaigorodsky. The A/on?-A/om
Porenriui Merhod, Springer. Berlin, 1987; b) G. Filippini. A. Gavezzotti.
Arru Crystuhgr. Swr. B 1993. 49. 868 (optimized parameter set for approximate atom/atom-potential calculations of lattice (sublimation) energies).
The H positions were readjusted by setting all CH bond lengths to 108 pm
and, partly, by inserting torsional angles of 6 0 for all methyl groups.
The potential is defined as an intermolecular one, and no corrections
for the intramolecular energy differences have been made. Additional
lattice vibration calculations yielded reasonable lattice vibration frequencies.
Cf. the summary N. N. Sirota. in Cry,?/. Res. Edinol. 1987, 22. 13431381.
aci-Nitrodiphenylmethane: A Hydrogen-Bonded
By Hans Bock*, Riidiger Dienelt, Holger SchGdel,
Zdenek Havlas, Eberhardt Herdtweck,
and Wolfgang A . Herrmann
Dedicated to Professor Helmut Schwarz
on the occasion of his 50th birthday
A recent report[*' offered a summary on the nitro $ acinitro tautomerism for prototype nitromethane as well as
predictions from highly correlated calculations, which largely reproduce the known experimental data sketched in
Scheme 1.
Scheme 1
The thermally forbidden", 31 H transfer has a prohibitively high barrier E', and a gas-phase equilibrium constant of
5 x lo1', which can be estimated from the difference in the
enthalpies of formation AAHf of 70 kJmol-', overwhelmingly favors the nitro tautomer.
The tautomerization
should predominantly shorten the CN bond length and close
the O N 0 angle. However, consideration of potential aggregation through hydrogen bonding, which was previously neglected, as well as a CSD search that revealed that the
compounds containing C-phenyl and C,C-diphenyl-0(t-buty1)dimethylsilyl groups are the only structurally characterized covalent aci-nitromethane derivatives prompted
further investigation^.'^^ We now report on the successful
crystal growth[41of the title compound, prepared from nitrodiphenylmethane (Scheme 2) and characterized by structure determination at both 163 and 238 Kr51(Fig. 1).
Scheme 2
The mi-nitrodiphenylmethane dimers are packed perfectly in the crystal lattice (Fig. 1 top). Obviously, this is favored
by the chair conformation of the H-bonded eight-membered
Prof. Dr. H. Bock, Dip].-Chem. R. Dienelt. Dip].-Chem. H. Schodel
Institut fur Anorganische Chemie der Universitdt
Marie-Curie-Strasse 1. D-60439 Frankfurt am Main (FRG)
Dr. Z. Havlas
Institute for Organic Chemistry and Biochemistry
of the Czech Academy of Sciences
Flemingova Nam 2, CS-16610 Prague (Czech Republic)
Prof. Dr. W. A. Herrmann, Dr. E. Herdtweck
Institut fur Anorganische Chemie der Technischen Universitit Munchen
Lichtenbergstrasse 4. D-85748 Garching (FRG)
Interactions in Molecular Crystals. Part 33. This research project has been
supported by the Deutsche Forschungsgemeinschaft, the State of Hesse,
the Fonds der Chemischen Industrie, and the A. Messer Foundation.
Part 32: Ref. [I].
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structure, interactions, monoclinic, determiners, extent, tetraisopropyl, phenylenediamines, triclinic
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