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Bis(5-2 3 5-tricarbahexaboranyl)nickel a Nickelocene Analogue.

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Bis(q5-2,3,5-tricarbahexaboranyl)nickel,
a Nickelocene Analogue**
By Joachim Zwecker, Hans Pritzkow, UIrich Zenneck. and
Walter SieberP
The oligodecker complexes 1 (n = 1-8) with terminal
2,3,5-tricarbahexaboranyl ligands are formed in an as yet
unclear way from the bis(2,3-dihydro-l,3,4,5-tetraalkyl1,3diboro1e)nickel sandwich compounds 2.“’ Decisive for the
carbaboranyl formation and the stacking reaction is the
reactive axial CH-bond at the pentacoordinated C2 atom
of the ligands 3a, b in 2.[’l
Bis(qs-2,3,5-tricarbahexaboranyl)nickel 4, the parent
member of the class of compounds 1 (for n=O), is not observed in the conversion of 2.13)We now report the synthesis and characterization of the paramagnetic complex 4a
as the first bis(tricarbahexaborany1)metal complex analogous to the metallocenes.
I
CY
C13
Fig. I . Crystal structure of 4a. Selected bond lengths (A](mean standard deNi-B4(6) 2.198, 2.204; Ni-C2(3) 2.227, 2.240; Ni-CS 2.150.
viation 0.003
Bl-B4(6) 1.802, 1.808; BI-C2(3) 1.711, 1.708: BI-C5 1.679; B4(6)-C3(2) 1.559,
1.551: B4(6)-CS 1.555, 1.566; C2-C3 1.484.
A):
Ni
Y
-p.
Ni
HH
I
1
2
3 R1
R2
a C H
CH3
b CH3
CHg
2 5
(preReaction of excess 3a with tri~(ethene)nickel[~”l
pared from nickel/toluene cocondensate and ethene[4b])at
low temperature in toluene leads to the formation of crystalline 4a in 12.5% yield. The tricarbahexaboranyl(a11yl)nickel complex 5a[’”I is formed at the same time and
can be isolated by distillation. Since this 18e-complex is
not converted into 4a under the reaction conditions, 4a
must be formed from another precursor of the reaction of
3a with [Ni(CzH4)3J.We assume that the replacement of
two ethene ligands in [Ni(C2H4),] by 3a leads to a labile 2,3dihydro-l,3-diborolyl(ethene)hydridonickel complex.1sb1
By uptake of a H,CB group from 3a, a reactive ethene(hydrido)-qs-tricarbahexaboranylnickel complex may then
form as educt of 4a.
The structure proposed for 4a is consistent with the analytical and mass spectrometric data and has been confirmed by an X-ray structure analysis (Fig. I)[”: In the
crystal, the Ni atom is located at an inversion center; the
molecule is therefore centrosymmetric. The structure of the
carbaboranyl ligand does not differ from that in the triple
decker 1, n =
The Ni-C and Ni-B distances are similar
to those found for ni~kelocene~”and the bis(dicarbo11ide)nickel dianion.I8l The distance of the nickel atom from
the ring plane is 1.77 A (maximum deviation of the ring
atoms from the plane kO.03 A).
Both the cyclovoltammetrically determined electrochemical behavior of 4a (Fig. 2a) and the ESR spectrum of the
19e-cation 4
(Fig. 2b) reveal the relationship between
4a and nickelocene. Magnetic measurements on 4a in the
region of 120-300 K indicate Curie- Weiss behavior and
give perr=
3.06 +- O.2pB.[’I ] For nickelocene perf
= 2.89 +0.15pB.1’z1
Thus, 4a likewise occurs as triplet molecule.
The results prove that the nido-2,3,5-tricarbahexaborate
C3B3R5HQand the cyclopentadienide ion are isolobal.
I
..
1
0
.~
..
~
1
E [VI-
~~
[*I
[*‘I
Prof. Dr. W. Siebert, Dr. J. Zwecker, Dr. H. Pritzkow, Dr. U. Zehneck
Anorganisch-chemisches Institut der Universitat
Im Neuenheimer Feld 270, D-6900 Heidelberg 1 (FRG)
q’-Carbaboranylmetal Complexes, Part 2. This work was supported by
the Deutsche Forschungsgemeinschaft, the Stiftung Volkswagenwerk,
the Fonds der Chemischen Industrie, and BASF AG.-Part I : [I].
Angewr Chem. inr. Ed. Engl. 25 (19861 No. 12
Fig. 2. a) Cyclovoltammogram of 4a in CH2C12/(C4HY)4N@Pe
at Pt wire
electrodes vs. SCE; bottom: range 1.7 to -2.0 V, u = 200 mV/s: top: range
f0.9 to - 1.1 V, u=200 mV/s: and P=polarogram at platinum rotating disc
electrode (RDE): E ( O , + ) = -0.1 1 V (reversible), E , ( + , 2 + ) = 1.5 V (irreversible), ED(- ,0) = 2.0 V (irreversible) 191. b) X-band ESR spectrum of 4a
(in
closed ESR cell generated with [(C5H5)2FelBF,) in CH2Cl2 at T = - 160°C.
Standard (ST): Li@TCNQaQ,gsT=2.0025. g1-2.14, g2=2.03, g , = 1.98: at
room temperature (g) is not determinable [lo].
0 VCH VerlaqsqesdlsehaJi mbH. 0-6940 Weinheim. 1986
+
0570-0833/86/1212-1099 $ 02.50/0
1099
Procedure
A solution of 3a (0.9 g, 6.1 mmol) in toluene (40 mL) was treated at -50°C
with 4 m L of a solution of tris(ethene)nickel in toluene (2.0 mmol Ni(C2H4)?;
titer determined by reaction with P(OCH,), and 1,5-cyclooctadiene). The
stirred mixture was allowed to warm to room temperature in 30 min and the
solvent was then removed under vacuum. At 45OC/IO-' torr, light green 4a
sublimed, which is colorless in thin layers (75 mg, 0.19 mmol, 12.5%). MS:
m / z 403 ( M e , 100"/o), correct isotopic distribution, hardly any fragmentat~on
Correct C,H analysis.
( A s = 12.1 ppm)."] Because 7 combines marked annulene
behavior with the porphyrin structure, we refer to it as
"[26]p0rphyrin."~'~
HE
I
Received: July 22, 1986:
supplemented: September 1, 1986 [Z 1868 IE]
German version: Angew. Chem. 98 (1986) 1129
[I] Th. Kuhlmann, H. Pritzkow, U. Zenneck, W. Siebert, Angew. Chem. 96
(1984) 994; Angew. Chem. hi.Ed. Engl. 23 (1984) 965.
[2] W. Siebert, Angew. Chem. 97(1985)924; Angew. Chem. I n t . Ed. Engl. 24
(1985) 943.
[3] Th. Kuhlmann, Dissertation. Universitat Heidelberg 1985.
141 a) K. Fischer, K. Jonas, G. Wilke, Angew. Chem. 85 (1973) 620; Angew
Chem. Int. E d . Engl. 12 (1973) 565; b) R. M. Atkins, R. Mackenzie, P. L.
Timms, T. W. Turney, J. Chem. Sor. Chem. Commun. 1975. 764.
[5] a) W. Herter, Dusertarron, Universitat Heidelberg 1984. b) The reaction
with (CSH5)C~(C2H4)2
corresponding to the postulated oxidative addition of 3a to the (C2H,)Ni fragment leads to labile cyclopentadienyl(2.3dihydro- 1,3-diborolyl)hydridocobalt complexes: K. Stumpf, W. Siebert,
R. Koster. G. Seidel, 2. Naturforsch. 8.in press.
16) 4a: Space group P2,/n, a=9.074(1), b=13.056(2), c= 10.166(2)
/?=95.23(1)', V = 1199.4
Z = 2 , R=0.048 for 2812 observed reflections (Sloe two-circle diffractometer, MoK,. radiation). Further details of
the crystal structure investigation are available on request from the
Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-75 14
Eggenstein-Leopoldshafen2 (FRG), o n quoting the depository number
CSD-52080, the names of the authors, and the full citation of the journal.
[7] P. Seiler, J. Dunitz, Acra CrysraNogr. Sect. 836 (1980) 2255.
191 R. M. Wing, J. Am. Chem. Sor. 92 (1970) 1187.
191 For data for nickelocene under comparable conditions see: J. L. Robbins, N. Edelstein, B. Spencer, J. C. Smart, J. Am. Chem. SOC.104 (1982)
1882.
[lo] The ESR parameters of the two particles differ, mainly because of the
Jahn-Teller activity of the nickelocene cation: M. V. Rajasekharan, R.
Bncher, E. Deiss, L. Zoller, A. K. Salzer, E. Moser, J. Weber, J. H. Ammeter, J. Am. Chem. SOC.10s (1983) 7516.
1111 U. Zenneck, L. Vasquez, unpublished.
[I21 H. P. Fritz, K.-E. Schwarzhans, J . Organomer. Chem. I (1963/64) 208; P.
Prins, J. D. van Voorst, C. J. Schinkel, Chem. Phys. Lett. I(1967) 54.
A',
A,
i
Ha
5
10
-6
0
-5
-10
Fig. I . 300-MHz ' H ~ h M Kspectrum 01 the [26]porphqrine 7b ~n CIX'la.
The motivation for the synthesis of 7 was the ring-current effect observed in the 'H-NMR spectrum of the
N,N',N",N"'-tetramethylporphyrin 214.51 (Scheme I),
which, despite the steric hindrance and nonplanar structure, exceeds that of the [18]annulene 1 . Since the steric
hindrance in the ring center is reduced for the ring-enlarged [26]porphyrin 7 , an even larger ring-current effect
was expected to be favored. On the other hand, according
to several assumptions, a decrease in the ring-current effects was expected for larger annu1enes.l"
1
Synthesis of a Fourfold Enlarged Porphyrin
with an Extremely Large, Diamagnetic
Ring-Current Effect**
Scheme I . 'H-NMR signals (d balues) l'or inner and outer protons ot' the
[I8]annulene 1 (in [Dx]THF) 171 and of the N.N',N".N"'-tetramethylporphyrine 2 (in CD,CN) [5].
By Martin Gosmann and Burchard Franck'
Porphyrins have an aromatic n-electron system of the
[ 18]annulene type."] Like the annulenes, they exhibit a dia-
magnetic ring current in the 'H-NMR spectrum, which
serves as a qualitative criterion for aromatic character.[*'
By cyclotetramerization of the pyrrylpropenol 3a, we have
now achieved the first synthesis of a fourfold enlarged porphyrin, 7 , having a [26]annulene structure.
The deep red-violet compound 7 is characterized by the
largest ring-current effect found so far for a cyclic conjugated n-electron system (Fig. 1). The shift difference (A6)
of the resonances for the inner (6= - 11.64) and outer ring
protons (6= 13.67) is.25.3 ppm! This shift difference is
twice as large as that found for the [18]annulene 1
+
["I Prof. Dr. B. Franck, DipLChem. M. Gosmann
Organisch-chemisches lnstitut der Universitat
Orleansring 23, D-4400 Munster (FKG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen lndustrie.
1100
0 VCH Verlagsgecellsrhaf, mbH. 0-6940 Weinheim. 1986
The N-methylpyrrylpropenol 3a was selected as the key
building block for the synthesis of the [26]porphyrin 7
(Scheme 2). It is known that 2-(aminomethyl)- and 2-(hydroxymethy1)pyrroles of type 3b can be condensed, in analogy to the route of porphyrin biosynthesis, by acid-catalyzed cyclotetramerization to form porphyrin~gens.'~.~'
We
were especially interested in whether the remarkable selectivity of this reaction (no oligomers, hardly any polymers)
would also hold for the vinylogous compound 3a.
For the synthesis of the very reactive compound 3a, 3,4diethylpyrrole 51y1was converted into 4a by a vinylogous
Vilsmeier formylation with 3-(dimethylamino)acrolein.
Compound 4a was N-methylated to 4b, which, in turn,
was reduced with NaBH4 in situ to 3a. Condensation of 3a
gave the colorless, non-cycloconjugated [26]porphyrinogen
6"' in 27% yield. Compound 6 was dehydrogenated with
Br, to give the bisquaternary [26]porphyrin 7a, chromatographically purified (silica gel, CH,CN/trifluoroacetic
#S70-0833/86/1212-11#~ $ 02.50/0
Angew Chem. Inr. Ed. Engl. 25 (1986) No. 12
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nickell, bis, tricarbahexaboranyl, nickelocene, analogues
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