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Tetrakis(dimethylamino)ethylene Dication Two CC Singly Bonded and Almost Perpendicularly Twisted ((H3C)2N)2C Carbenium Ions.

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Tetrakis(dimethy1amino)ethylene Dication:
Two CC Singly Bonded and Almost Perpendicularly
Twisted ((H3C),N),Co Carbenium Ions
n
By Hans Bock,* Klaus Ruppert, Kurf Merzweiler,
Dieter Fenske,* and Helmut Goesmann
Dedicated to Professor Edgar Heilhronner
41
The most simple hydrocarbon with a single n-bond,
H,C=CH,, changes its structure on uptake of the n --t n*
excitation energy by twisting of the two molecular halves.[']
Twofold adiabatic ionization requiring considerably higher
energy, should produce a dication with a CC single bond
between perpendicular H,C@ subunits.['] To substantiate
this prediction, we allowed the fourfold (H,C),N-donorsubstituted and, therefore, more easily oxidized 13] tetrakisand
(dimethy1amino)ethylene to react with C1, and BrZ13.41
determined the single crystal structures[51of the resulting
dichloride and dibromide salts (Fig. l), which are stabilized
by improved charge delocalization.[61
Accordingly, the twofold oxidation of the electron-rich
tetrakis(dimethy1amino)ethylene 1 proceeds via tremendous
structural changes (cf. Table 1 and Fig. 1). The two
Fig. 1. Structure of the tetrakis(dimethylam1no)ethylene dication in its dichloride and dihrornide salts IS]. Most important distances [pm] and angles ["I for
the dibromide (i1.5 pm, i1") and (in brackets) for the dichloride (k0.3 pm,
20.2"). which contains a twofold crystallographic axis: C1-C2 151.2 (152.4),
C1-Nl 133.1 (132.0). C b N 2 131.9 (131.5), C2-N3 131.2 (131.5), C2-N4 129.7
(132.0); Nl-CLN2 125 (128.5). N3-C2-N4 127.2 (128.5), Cll-N1-Cl2 113.8
(113.4), C21-NZ-C22 115.7 (113.1), C31-N3-C32 115.1 (113.l), C41-N4-N42
112.3(113.4); NlCl-C2N3 67.2(75.8), C2Cl-NlC11 23.0(17.1),C2Cl-N2C21
18.3 (l8.5), ClC2-N3C31 24.4 (18.5), ClC2-N4C41 26.3 (17.1).
Owing to their higher effective nuclear charge, the pyramidally coordinated amino nitrogen centers in the neutral
molecule are 'negativated' by the neighboring methyl
groups, whereas the electron density at the trigonal planar
Table 1. Calculated and experimentally determined characteristic structure
parameters of tetrakis(dimethy1amino)ethylene and its dication.
IM'@1 f
CIe [51
M
MNDO [2]
M2e
MNDO [2I
Xe
14
136
147
105 [a]
0 [a]
62
155
136
120 [a]
29
76
152
131 [b]
120
18 [hl
=
W
2
Bre [5]
~~~~
o(N2C-CN,) ["I
W e ) Ipml
4 C N ) Ipml
G N C ) I"]
ca( C-NC2) ["I
67
151
131 [h]
120
23 P I
[a] Not optimized. [h] Average value.
Scheme 1. MNDO charge distributions for tetrakis(dimethy1amino)ethene and
its dication.
((H3C),N),Ce molecular halves rotate relative to each other
by up to 76" and the CC double bond lengthens to a CC
single bond. Simultaneously, the CN single bonds are shortened by 16 pm (!) and the now planar (H3C),N6e groups are
twisted to different extents out of the two CCN, planes. The
energetically enforced two electron removal leads -according to geometry-optimized MNDO calculations, the results
of which are supported by satisfactory agreement with the
measured data-to the changes in charge distribution presented in Scheme 1.
[*] Prof. Dr. H. Bock, Dip].-Chern. K. Ruppert, Dr. K. Merzweiler
Institute of Inorganic Chemistry, University Frankfurt
Niederurseler Hang, D-6000 Frankfurt am Main 50 (FRG)
[**I
Prof. Dr. D. Fenske, Dr. H. Goesmann
Institute of Inorganic Chemistry, University Karlsruhe
Engesserstrasse, D-7500 Karlsruhe (FRG)
Structures of Perturbed 7[ Systems, Part 2, and Electron Transfer and Ion
Pairing, Part 13. This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Land Hesen.Part 1: H. Bock, H.-F. Herrmann, D. Fenske, H. Goesmann, Angew.
Chem. 100 (1988) 1125; Angew. Chem. Inf. Ed. EngI. 27 (1988) 1067;
part 12: H. Bock, H.-F. Herrmann, J. Am. Chem. Soc. 111 (1989) 7622.
1684
0 VCH
@rlagsgeselischafi mbH. 0-6940 Weinheim, 1989
coordinated carbon n centers remains balanced. In the dication on the other hand, 213 of the added positive charges are
located at the threefold-coordinated carbon centers of the
twisted CC single bond and 4/3 in the eight peripheral methyl
groups. Thus, despite an almost constant negative charge on
the amino nitrogens each (H3C),N substituent, as predicted
by the MNDO valence electron densities (Scheme l), carries
a partial positive charge of about 1/3, i.e. the total charge
distribution in the dication is also dominated by the high
effective nuclear charge of the nitrogen centers. The Coulomb repulsion between the positive charges, delocalized in
each of the (R,N),C@ molecular halves, gives rise to both
their twisting relative to each other as well as the charging of
their "alkyl envelopes".
The structures of the dications (Fig. 1, Table 1) are in accord with numerous other measured
The 'H-NMR
spectrum,r3a1in contrast to that of the neutral molecule,
shows two signals for the methyl group,"] a result which
confirms the fixation, also in solution, of the R,N substituents due to drastically shortened R,N-C bonds caused
0570-0833j89jl212-16X48 02.50j0
Angew. Chem. Ini. Ed. Engl. 28 (1989) No. 12
by the x charge delocalization. The vibrational spectrum13a1
exhibits two slightly split CCNe bands at 1667 and 1672 cm-',
which corroborate a distorted (N,C-CN,)25 skeleton lacking an inversion center. A comparison of the UV excitation
energies at 36630 and 48800cm-' with that of the tetramethylformamidinium ion ((H,C),N),CH@ at 44640 cm-'
reveals the continued existence of n interaction between both
molecular halves, and supports, also in solution, a twist
angle of 0 < w(N,C-CN,) < 90", which reflects the counteracting effects of x conjugation and sterically inflicted rotational distortion.
An extensive literature search concerning analogous structural changes in other ethylene dications provided a hint of
twisting (w (SFC-C5S,) z 60")
in the tetrathiafulvalene
dication, and for a dihedral angle o(CfC-C@C,) = 41" in
that of tetra@-methoxyphenyI)ethylene, which is bent at the
C = C bond.IEblIn addition, it is pointed out that the twoelectron oxidation of 1,2,4,5-tetrakis(dimethylamino)benzene generates a twisted x-conformation of the six-membered ring containing two [Me,NC-CNMe,15 subunits,
each of which show CC bond distances of 153 pm and dihedral angles of w(NC-CN)@ = 49".[8c1
Summarizing, the- according to molecular state argumentsf9]-inevitable structural changes accompanying differences in energy, and especially differences in the number
of valence electrons of a given molecule, thus predominantly
lead, in the simplest alkene, H,C = CH,, and its derivatives,
X,C = CX,, to an elongated CC bond, shortened CX bonds
and a twisting of the two molecular halves relative to each
other.
Received: May 19, 1989 [Z 33481
German version: Angew. Chem. 102 (1989) 1715
[I] A summary of spectroscopic data and ofquantum chemical results concerning the ?i + x* excitation of ethylene ( v > 40000 cm-' = 5 eV) into the
singlet state A('B,,) and its presumed D,,structure exhibiting a dihedral
angle w = 90" between both molecular halves is given by M. B. Robin in:
Higher Excited States of Polyutomic Molecules, Vol. 2, 3, Academic Press,
New York 1975, 1985, pp: 2-22, 213-227, respectively; cf. also the references cited therein.
121 K. Ruppert, Diplomarbeit, Universitat Frankfurt 1989. Approximate geometry-optimized MNDO calculations predict for [H,C-CH,I2@ a total
energy difference of A E % 27 eV, a dihedral angle w = 90" and a bond
distanced,, = 146 ppm. Ah initio SCF calculations (cf. e.g. K. Lammertsma, M. Barzaghi, G. A. Olah, J. A. Pople, A. J. Kos, P. von R. Schleyer,
1 A m . Chem. Soc. 105 (1983) 5252 or G. Frenking, W. Koch, H. Schwarz.
J. Comput. Chem. 7 (1986) 406) confirm the semiempirical result. The
unknown structure of the 38-atom neutral molecule of the composition
((H,C),N),C =C(N(CH,),), is MNDO-optimized based on the assumption of pyramidal (H,C),N groups, and the resulting satisfactory correlation with measurement data can be illustrated by the calculated first vertical
ionization energy of 7.0 eV (exp. 5.95 eV [3b]). For the dication, planar N,C
subunits with freely rotating R,N groups are restrictively assumed and
reasonable agreement with the slightly differing crystal structure data of the
dibromide ( 2 H,O, orthorhombic) [5] and the dichloride (2 H,O, monoclinic C) [S] is achieved (cf. Fable 1).
131 Cf. a) redox investigations and preparation of salts: N. Wiberg, J. W. Buchler, Chem. Ber. 96 (1963) 3223; summary: N. Wiberg, Angew. Chem. 80
(1968) 809; Angew. Chem. Int. Ed. Engl. 7 (1968) 766. b) PE spectrum: B.
Cetinkaya, G. H. King, S. S. Krishnamurthy, M. F. Lappert, J. B. Pedley,J.
Chem. Soc. Chem. Commun. 1971,1370.c) Half-wave oxidation potential in
aprotic (c,,. < 1 ppm) dimethylformamide solution (H. Bock, D. Jaculi,
Angcw. Chem. 96 (1984) 298; Angew. Chem. In$. Ed. Engt. 23 (1984) 305):
the cyclovoltammogram exhibits an unstructured (two-electron) wave between E,. = -0.63 V and E,. = -0.52 V.
[4] Cf. [2]. Dichloride salt: 3 g (15 mmol) tetrakis(dimethylamino)ethylene
were 'titrated' at - 10 "C under Ar with an oxygen-free CI,/HCCI, solution
until the orange color vanished. After solvent evaporation at room temperature in vacuo, the residue was dissolved in a minimum of CH,OH and the
salt precipitated by pouring the solution dropwise into 200 ml of cold diethyl ether: yield 4.0g (97%), decomp. 243-247"C, 'H NMR (CD,OD/
TMS): 6 = 3.41 (s, 12H), 3.77 (s, 12H); IR cf. [3a]. The growth of a single
cryslal was achieved within six weeks in a two-limbed trap by slow difusion
Angew. Chem. Int. Ed. Engl. 28 (1989) No. 12
0 VCH
of n-hexane into the CH,OH solution. Dibromide salt: After oxidation with
0.4 mL of Br, in 30 mL of carefully degassed HCCI, at 0 "C, 2.3 g (95%) of
a hygroscopic powder precipitate from the orange-colored solution at room
temperature. Decomposition at 280°C; 'H NMR identical to that of the
dichloride; elemental analysis: C 33.06 (33.33), H 6.87 (6.67), N 15.29
(15.56). Growth ofsinglecrystal within about 24 h from 5 mL ofa C,H,OH
solution of 1 g of salt by superposing a layer of 3 mL ofn-hexane (non-compliance of the concentrations quoted yields only powders).
Crystal structure analysis: [C,,H,,NJeBCIF . 2 H,O, space group C2/c.
Z = 4, lattice constants (211 K): (I = 1524.3(8), h = 763.7(4), c =
1758.7(9)pm, /l
= 125.25"(3), V = 1672 x lo6 pm3, ~(Mo,,) = 3.38 cm-'.
Siemens AED-2 diffractometer, 3" < 2 0 < 52", 1853 reflections, of which
1350 are independent with I > 3u(I). Direct methods, C1, N, C and 0
positions anisotropically, detected H positions isotropically refined; R =
0.039, R, = 0.039. The dication contains a twofold axis, crystallographically enforced. Angle sums around C1, C2, N l , N2, N3, N4 each 360".
Contact distances CI-N 359-416 pm, 0 - N > 400 pm, C1-0 323 pm.
Additional
structure
parameters:
H,C-N
147.5- 148.3 pm.
[C,,H,,N,]eBBr~ . 2 H,O: space group Pcrb. Z = 8, lattice constants
(218 K): a = 928.3(9), b = 2583.8(30), c = 1474.0(15)pm. V = 3535 x
lo6 pm'. ~(Mo,,) = 44.81 cm-'. Siemens AED-2 diffractometer, 3" <
2 0 < 48", 6829 reflexes, of which 1718 are independent with I > 2 a(I).
Direct methods, Br, N, C and 0 positions anisotropically, detected H positions isotropically refined; R = 0.085, R, = 0.084. Angle sums around C1,
C2, NI, N2, N3, N4 each 360". Most important contact distances Br-N
362-386 pm, 0-N > 400 pm, Br-O 328-334 pm. Additional structural
parameters: H,C-N 145.7- 148.2 pm.-Details for both crystal structure
determinations are available on request from the Fachinformationszentrum
Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH,
D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository
number CSD-54165, the names of the authors, and the literature citation.
After submission of our manuscript, we were informed that the crystal
structure of l-(PFF), has been determined. In close agreement with our
results for the dichloride and dibromide salts [4], a CC bond drstance of
151 pm and a dihedral angle of 72" were found. C. Krieger, K. Elbl-Weiser,
H. A. Staab, unpublished. We thank Professor H . A . Stnnb. Max-Planck
Institut, Heidelberg, for communicating these unpublished data (cf. also
[W).
The signal pattern in the ESR spectrum of the radical cation
[((H,C),N),C - C(N(CH3)z)]'e, generated by one-electron oxidation, also
exhibits different 'H couplings of 0.328 mT and 0.284 mT (K. Kuwata,
D. H. Geske, J. Am. Chem. Sor. 86 (1964) 2101; J. M. Fritsch, H. Weingarten, J. D. Wilson, hid. 92 (1970) 4038).
a) The structure investigation announced for the tetrathiafulvalene dication
(B. A. Scott, S. J. La Placa, J. B. Torrance, B. Silverman, B. Welber, J. Am.
Chem. Soc. 99 (1977) 6631) could not be traced. b) The tetra@methoxypheny1)ethylene dication (cf. G . A. Olah, P. von R. Schleyer in
Curbonium Ions, Vol. 5 , Wiley Interscience, New York 1976, p. 2445, and
references cited therein), however, possesses a shorter CC distance of
143 pm and angle sums around the ethylene carbon atoms of only 350". c)
K. Elbl, C . Krieger, H. A. Staab, Angew. Chem. 98 (1986) 1024; Angew.
Chem. In!. Ed. Engl. 2S (1986) 1023.
Cf. H. Bock, Angew. Chem. 89 (1977) 631; Angew. Chem. Int. Ed. Engl. 16
(1977) 613, and references cited therein.
Tetraphenylethylenedisodium :
The Band Structure of the CC Singly Bonded,
Twisted (C,H,),Ce Carbanions **
B y Hans Bock,* Klaus Ruppert, and Dieter Fenske*
Dedicated to Professor Jack D.Dunitz
Increased mastering of intricate reaction conditions permits "the preparative exploration of ever more shallow potential troughs"Iz1-for instance, in the synthesis of air-,
[*I Prof. Dr. H. Bock, Dip].-Chem. K. Ruppert
Institut fur Anorganische Chemie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main 50 (FRG)
Prof. Dr. D. Fenske
Institut fur Anorganische Chemie der Universitat
Engesserstrasse, D-7500 Karlsruhe (FRG)
[**I Structures of Perturbed K Systems, Part 3, and Electron Transfer and Ion
Pairing, Part 14. The work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Land Hessen.
Parts2 and 13, respectively: [la].
Verlugsgesellschufi mbH, 0-6940 Weinheim, 1989
0570-0833j89ji212-1685 $02.50j0
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two, almost, dimethylamino, ions, tetrakis, dication, h3c, bonded, carbenium, ethylene, single, twisted, perpendicular
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