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Are -Diketone-Metal Complexes Aromatic.

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ZrCl4, Nb2C110. MoZC110 [81 exhibit a physically meaningful
trend.
The tetrahedral form of the ZrCld molecule in the gas phase
is already latent in the distorted octahedrons of the crystal
structure. Accordingly, the cleavage of the longest Zr-C1
bond with subsequent easy rehybridization at the Zr is assumed to be a plausible vaporization- and sublimation mechanism (cf. [91). ZrBr4 and HfCI4, and probably HfBr4, are isostructural with ( I ) .
(3), R
(4). R
(I), R = H
(2), R = CH3
Received: November 8, 1968
[ Z 922 IE]
German version: Angew. Chem. 81, 120 (1969)
=
H
=
CH3
(S), Bianthryl
~.
[*I Dr. 9. Krebs
Anorganisch-Chemisches Institut der Universitiit
34 Gottingen, Hospitalstr. 8-9 (Germany)
[l] H. Hansen, Z . physik. Chem., Part B 8 , 1 (1930).
[2] W. Klemm, Z. physik. Chem., Part B 12, 1 (1932); W. Fischer, Z. anorg. allg. Chem. 211, 321 (1933); cf. also V. M . GoldSchmidt, Fortschr. Mineralog., Kristaltogr. Petrogr. 15, 11, 73
(1931).
131 J . Weidlein, U. Miiller, and K . Dehnicke, Spectrochim. Acta,
Part A 24, 253 (1968).
141 L . Dahl and D . L. Wampler, Acta crystallogr. 15, 903 (1962).
151 V . P. Spiridonov, P . A. Akishin, and V . I. Tsirelnikov, 2.
strukturnoj Chim. 3, 329 (1962); M . Kimura, K . Kimura, M .
Aoki, and S . Shibatu, Bull. chem. SOC.Japan 29, 95 (1956).
161 D . H . Templeton and G. F. Carter, J. physic. Chem. 58, 940
(1954).
171 A . Zalkin and D. E. Sands, Acra crystallogr. 11, 615 (1958).
181 D . E. Sands and A . Zalkin, Acta crystallogr. 12, 723 (1959).
I91 B. Krebs, A. Miiller, and H. Beyer, Chem. Commun. 1968,
263.
Are P-Diketone-Metal Complexes Aromatic ? 111
By M . Kuhr and H. Mussol*l
The metal chelates of $-diketones have, for a long time, been
considered to exhibit a benzenoid resonance A t,A1 fZ1 and
as a consequence to possess an aromatic or quasiaromatic character (31. Numerous attempts to deduce such a
property, as in the case of benzene, from the shift of the
N M R signal for the protons bound in the plane of the chelate
ring have been differently interpreted 14-61.
I-\
(61,X
;t
o,x.o
o.xp
A
O&.’O
A’
oyo
C
B
A
o.x.o
C‘
The vibration spectra of isotopically substituted 2,4-pentanedionato complexes (acetylacetonates) of various metals indicate that a complete bond delocalization IS present in solution and in the crystalline state (A or B) but not in the enol
form of 2,4-pentanedione (X = H), as in C r;’ C1[1,71.
In order to determine unequivocally whether or not a cyclic
conjugation of the six x electrons continues over the metal
atom, we have examined derivatives of 2.4-pentanedione in
which protons are situated not in the plane but over the
chelate ring. A shift of the signal of such protons to higher
magnetic field in the N M R spectrum is a sure indication of
aromatic character (81.
From the chemical shift differences A 8 (cf. Table 1) of the
0- and p-methyl groups in the ‘H-NMR spectra of phenylmesitylene (f) (0.30 pprn). bimesityl (2) (0.45 ppm), and 9Angew. Chem. infernat. Edit. Vol. 8 (1969) / No. 2
H, K, Na, Li,
Be, Al, Pd, Co
(7). X =
H,K,
Li, Be,Pd
Table 1. Chemical shifts of a few protons of the 2,4-pentanedione
Me
O-CH3
Mesitylene
(1)
(2)
(61, X
7-
H
X - K
X-Na
X=Li
X = Be
X = Al
X = CoIII
X = Pd
Anthracene
(3)
(41
(5)
(71, X
=
H
X= K
A- A
=
X=Li
X = Be
X = Pd
CDCI,
CDCl3
CDCI,
CDCI3
DMSO
DMSO
DMSO
DMSO
CDC13
CDC13
CDCI,
CDCl3
CDCii
CDCI3
CDCll
CDCI3
CDCI,
DMSO
DMSO
DMSO
CDCI3
CDC13
2
1.86
1.88
2.09
2.06
2.03
2.08
2.08
2.16
2.15
2.13
2.07
-
YlPCH3
-
-
2.16
2.33
2.29
2.25
2.20
2.21
2.22
2.30
2.28
2.29
2.29
-
-
-
-7.10
5
-
-
-
-
-
-
I .68
2.46
-
-
-
-
0.00
0.30
0.45
0.20
0.19
0.17
0.13
0.14
0.14
7
7.32
7.10
7.89
7.50
7.41
8.03
7.99
7.98
7.55
8.02
8.20
7.94
7.60
7.48
8.16
8.11
8.06
0.13
0.16
0.22
0.00
-0.45
0.78
0.70
1.10
0.05
0.10
0.07
0.13
0.12
0.08
[a] Varian A 60, TMS as internal standard (r0.02 pprn).
[b] Centers of the A- and A- and D parts of the AzB2- and ABCDmultiplets.
mesitylanthracene ( 4 ) (0.78 ppm) and of the 1,s- and 4 5
hydrogen atoms of 9-phenylanthracene (3) (-0.45 ppm), ( 4 )
(0.70ppm), and 9.9’-bianthryl ( 5 ) (1.10 ppm) it can be concluded that a shift difference of ca. 0.5 ppm is to be expected
if a n aromatic ring is present beneath these protons. The
observed differences in these models (0.30-1.10 ppm) depend, on the one hand, on the somewhat different average
angles of twist of the two aromatic systems and, o n the other,
o n the fact that the 1,8-anthryl protons in (3), ( 4 ) , and ( 5 )
are fixed better over the other ring than are the protons of
the rapidly rotating CH3 groups in ( l ) , (21,and ( 4 ) , in wbich
only the mean value for the shift of favorable and unfavorable positions is indicated; finally, the electron density in the
phenylmesityl- and central anthryl-ring is naturally different.
By way of comparison, the analogous shift differences in 3rnesityl- and 3-anthryl-2,4-pentanedione
- (61,
X = H and (7).
X = H - are small. Since these differences in the metal complexes of ( 6 ) ( X = K , Na, Li, Be, Al. Pd, Co) and ( 7 ) (X= K, Li.
Be, Pd) of greatly varying stability d o not increase significantly with respect to the hydrogen compounds (X = H) and the
I47
free anions (X = K in DMSO), these chelate complexes do
not possess magnetic anisotropy typical of aromatic compounds and consequently are not aromatic in character.
Dulrrozzo came to the same conclusion via a different approach 191. Use of the term “quasiaromatic” [3,101 in connection with such systems is best avoided, since “non-aromatic”
is not generally understood to be included under this heading.
It would appear attractive to study, in this way, the bonding character of further metal complexes and other five- and
six-membered chelates and heterocycles (e.g. [11-131), since
the method used here is obviously less susceptible to polar
and other effects than the position of the signals of protons
in the plane of the ring 161.
According t o their N M R spectra, compounds (6) and (7)
(X = H) are present almost completely in the enolized form.
These compounds were obtained from iodomesitylene and 9bromoanthracene and from the chromium complex of 3bromo-2,4-pentanedione respectively, by heating in the presence of copper powder (4 h, 240OC) followed by acid hydrolysis: (61, X = H , m.p. 83 “C, yield 40%; ( 7 ) , X = H, m.p.
182 “C, yield 7 %. Although the Ullmann reaction is a well
known classical method for the coupling of aromatic rings,
it has been used here to provide evidence against the aromatic character of the metal complexes of (6) and (7).
Received: November 14, 1968
[Z 923 IE]
German version: Angew. Chem. 8 1 , 150 (1969)
[*I Dip].-Chem. M. Kuhr and Prof. Dr. H. Musso
Abteilung fur Chemie der Universitat Bochum und
Institut fur Organische Chemie der Universitat Marburg
355 Marburg, Bahnhofstrasse 7 (Germany)
[I] Part VI of Organometai Complexes - Part V: H. Junge and
H . Musso, Spectrochim. Acta A 24. 1219 (1968).
[21 y.Calvin and K. W. Wilson, J . Amer. chem. SOC.67, 2003
(1945); R. E. Martell and M . Calvin: Die Chemie der Metallchelatverbindungen. Verlag Chemle, Weinheim 1958, pp. 149,
157, 160
[3] J . P. Collmon. R. A. Moss, S. D. Goldby, and W. S. fiahanovsky, Chem. and Ind. 1960, 1213; J. P. Coliman and M. Yumada, J. org. Chemistry 28, 3017 (1963); J. P. Collman, Angew.
Chem. 77, 154 (1965); Angew. Chem. internat. Edit. 4, 132
(1965).
[4] J. P. Fackler, Progr. inorg. Chem. 7, 374 (1966).
[5] In favor of aromatic character: J . P. Callman, R. L. Marshall,
and W. L. Young, Chem. and Ind. 1962, 1380; R. E. Hester, ibid.
IY63, 1397; W. L . Young, Dissertation Abstr. 268, 1358 (1967).
[6] Against the aromatic character: R. H . Holm and F. A. Cofton, J . Amer. chem. SOC.80, 5658 (1958); J . A . S . Smith and E. J .
Wilkins, J. chem. SOC.(London) A 1966, 1749; R. C. Fay and
N. Serpone, J. Amer. chem. SOC.90,5701 (1968).
[7] H. Muss0 and H . Junge, Chem. Ber. IOI, 801 (1968).
[8] J. A. Pople, J. chem. Physics 24, 1111 (1956); C. E. Johnson
and F. A. Bovey, ibid. 29, 1012 (1958).
[9] E. Daltrozzo and K. Feldmann, Angew. Chem. 79, 153 (1967);
Angew. Chem. internat. Edit. 6, 182 (1967); E . Daltrozzo, private
communication.
1101 D. M . G. Lloyd and D. R. Marshall, Chem. and Ind. 1964,
1760.
[ l l ] H. C. Smitherman and L. N. Ferguson, Tetrahedron 24, 923
(1968).
1121 A. Trestian, H . Niculesco-Majiwska, I. Bully, A. Barabas,
and A . T. Balaban, Tetrahedron 24, 2499 (1968).
[13] E. Bayer, E. Breifmaier, and V . Schurig, Chem. Ber. 101,
1594 (1968).
CONFERENCE REPORTS
New Olefin Reactions
A Symposium was held at the University of Manchester
Institute of Science and Technology, June 25 and 26, 1968,
that dealt with some of the more important recent advances
and trends in olefin reactions.
G. Wilke (Mulheim, Germany) reviewed the factors which
influence the reactions of olefins with transition metals. Metalolefin complexes are in general stabilized by the presence of
basic ligands in the molecule, and by partial decoupling of
the olefin --orbitals, usually achieved by ring strain in the
case of cyclic olefins, or in mono-olefin complexes by rotation
of the olefin so that it is at an angle to the plane of the molecule. The point was also made that complexes of chelating
diolefins are more stable than the analogous mono-olefin
complexes.
Several factors influence the reactivity of the complexed
olefin, for example, charge transfer can occur from metal to
ligand or from ligand t o metal, and evidence was presented
that in the compound bis(bipyridy1)cyclooctadienenickel the
cyclooctadiene exists as a dianion. Hydrogen transfer reactions are also known in metal-olefin complexes, for example,
the compound bis(cyc1ooctadiene)cobalt hydride exists as an
equilibrium mixture of a a-enyl and x-ally1 structure.
R. P e f t i f (Austin, Texas, USA) reviewed the chemistry of
cyclobutadiene complexes of transition metals and demonstrated how these complexes may be used as a convenient
source of cyclobutadiene in the synthesis of organic compounds. The synthesis was reported of several para-bonded
benzene derivatives by oxidative cleavage of (CqH4)Fe(CO)3
in the presence of acetylenes. The preparation of pentacyclo[4.3.0.02S.O3.~.04~7]nonan-9-ol
( I ) from 5,5-diethoxycyclopentadiene and (C4H4)Fe(C0)3 was also described.
148
HG
m
/v?\
HO
(1)
(2)
Oxidative cleavage of (benzocyc1abutadiene)iron tricarbonyl
in the presence of Agf gives 4b,Sb.8~,8e-tetrahydrodibenzo[h,e]cyclopropa[g,h]pentalene (2). by a normally forbidden
(Woodward-Hoffmann rules) disrotatory ring opening of the
Diels-Alder adduct, which becomes allowed when the adduct
is complexed t o silver.
The lecture concluded with some observations on the structure of free cyclobutadiene. A study of some Diels-Alder
reactions of cyclobutadiene with dimethylmaleate and fumarate tentatively suggests that cyclobutadiene reacts in the
singlet state (as a rectangular diolefin).
J . K. Hutnblin (Sunbury-on-Thames, England) reported the
dimerization of propene to methylpentenes (mainly 4methyl-1-pentene) at 150 OC and about 100 atm with sodium
or potassium on pure graphite or potassium carbonate. This
reaction can be carried out o n a pilot-plant scale and under
the best conditions a yield of 92% of 4-methyl-1-pentene has
been achieved.
Allylic anions from propene or butenes will react with ethylene. In a typical reaction a 1:1 molar ratio of ethylene and
propene gives a 92% yield of n-pentenes with small amounts
of hexenes and heptenes. Similarly, reaction of ethylene with
1-butene gives 3-methyl-1-pentene and 2-hexene. The reactivity of the allylic anions towards ethylene decreases in the
order C3H5- > n-CdH7- > iso-CqH7-.
Angew. Chem. internat. Edit. Vol. 8 (1969) 1 No. 2
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