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Aromatic Boron Heterocycles The Generation of 1 H-Borepin and the Structure of Tricarbonyl(1-phenylborepin)molybdenum.

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[11] D. D. Heinrich, K. Folting, J. C. Huffman, J. G. Reynolds, G. Christou,
Fig. 3 . ORTEP representation of complex 3 (ellipsoids at the 50% probability
level). Selected distances [A] and angles ["I: V-S2 2.547(1), V-N8 2.124(2), V-N9
2.252(2); S2-V-S2' 156.05(4), S2-V-NS' 97.24(6), S2-V-N8 66.85(8), S2-V-N9
97.63(6), S2-V-N9 100.45(6), N8-V-N8' 99.89(13), N8-V-N9 90.65(9); N8-VN 9 164.27(8), N9-V-NY 81.64(12).
compound 2 in CH,Cl, yields a white precipitate [Na(pyt)
according to IR spectroscopy]. Efforts are currently underway to characterize this material, since mononuclear
"[V(pyt),]" may be unstable with respect to aggregation.
Complexes 1 and 2 were studied by electron ionization
mass spectrometry to determine gas-phase fragmentation
pathways and probe possible C-S bond cleavage reactions.
These transformations may serve as a models for some of the
high-energy reactions occurring during hydrodemetallation/
hydrodesulfurization of petroleum. Under normal ionization conditions, both compounds show one or more fragmentation pathways involving C-S bond cleavage, as
previously detected in [V(StBu),], in which the C-S bonds
are more easily cleaved.[' For example, [VO(C,H,NS),]+
( m / z 287; 100% intensity) gives fragment ions assigned to
[VO(C,H,NS)(C,H,N)]+ ( m / z 255; 4%), pO(C,H,N),]+
(m/z 223; 38 YO),
and [VO(C,H,NS)S]+ (m/z 208,209; 10%)
which appear to form through a one- or two-step process
involving C-S bond cleavage. The ion [V(C,H,NS),]+ responds similarly under the same conditions. Further details
of both the mass spectrometric investigation and the reactivity studies will be reported in due course.
fnorg. Chem. 1991,30, 300.
[12] For recent examples, see: a) A. J. Deeming, M. Karim, P. A. Bates, M. B.
Hursthouse, Polyhedron 1988, 7, 1401 ; b) K. Umakoshi, A. Ichimura, I.
Kinoshita, S. Ooi, Inorg. Chem. 1990, 29, 4005; c) J. H. Yamamoto, W.
Yoshida, C. M. Jensen, Inorg. Chem. 1991,30,1353 and references therein.
[13] A green-blue solution of [VOCl,(thf),] in TH F turns dark brown on addition of Na(pyt). The solvent is removed under vacuum, the residue extracted with CH,CI,, and the resulting solution filtered and placed in a
freezer to allow the product to crystallize. Suitable crystals for the X-ray
crystallographic study were obtained by filtering the initial reaction mixture through a tine frit and layering the solution with hexanes. Crystals so
obtained lose solvent rapidly when removed from the mother liquor.
746.76 g
[14] Crystal data for 1 2 THF .1/3 C,H,,: C,,H,,N,,O,S,V,,
mol-', hexagonal, RJ, a = b = 30.888(21), c = 8.960(6) A, V =
7402.84 A', 2 = 9. A total of 1310 unique reflections with f > 2.330(1)
were relined to values of R(R,) of 0.0613 (0.0563).
1151 To our knowledge, only one complex with this ligation mode has been
reported: M. V. Castano, A. Macias, A. Castineiras, A. S . Gonzalez, E. M.
Martinez, J. S . Casas, S. Sordo, W.Hiller, E. E. Castellano, J. Chem. Soc.,
Dalton Trans. 1990, 1001.
[16] Addition ofNa(pyt) to a red solution of [VCl,(thf),] yields a black solution
which slowly turns purple and finally turns blood-red. After overnight
stirring, the solution was filtered and placed in a freezer to allow the
product to crystallize.
[17] Crystal data for 2: C,,H,,N,O,S,NaV,
658.76 gmol-', triclinic, Pi,
a = 10.442(2), b = 16.476(4), c = 9.465(2) A, a = 100.73(4) ',
109.20(4)", y = 87.86(3)", V =1510.33 A3, Z = 2. A total of 3391 unique
reflections with I > 2.33o(f) were relined to values of R(RJ of 0.0412
[18] a ) N . Vuletic, C. Djordjevic, J. Chem. Soc., Dalton Trans. 1973, 1137;
b) R. E. Drew, F. W. B. Einstein, fnorg. Chem. 1973, 12, 829; c) R. A.
Levenson, R. J. G. Dominguez, M. A. Willis, F. R. Young 111, ibid. 1974,
13, 2761; d) D. Begin, F. W. B. Einstein, J. Field, ibid. 1975, 14, 1785.
1191 J. J. H. Edema, W. Stauthamer, F. Bolhuis, S . Gambarotta, W J. J. Smeets,
A. L. Spek, Inorg. Chem. 1990, 29, 1302.
1201 A mixture of [VCl,(tmeda),] and two equivalents of Na(pyt) in CH,CI, or
THF immediately turns a deep red-purple color. After several hours of
stirring, the mixture is filtered and the solution is layered with hexanes to
allow the product to crystallize.
387.45 gmol-', orthorhombic, 822'2,
[21] Crystal data for 3: C,,H,,N,S,V,
a = 14.338(5),b = 17.755(7),c =7.255(2) A, V =1846.86 A', Z = 4. A total of 1192 unique reflections with f > 2.330(1) were refined to values of
R(R,) of 0.0220 (0.0261).
. Schmidt, G. Henkel, B. Krebs, Proc. Inr. Conf. Coord. Chem. 1990,28,
[22] W
1-9. This conference abstract includes the structure of (PPh,)[V@yt),].
I231 All structures were solved by direct methods and refined by full-matrix
least-squares. Further details of the crystal structure investigations may be
obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur
wissenschaftlich-technische Information mbH, D-W-7514 EggensteinLeopoldshafen 2 (FRG), on quoting the depository number CSD-56417,
the names of the authors, and the journal citation.
Received: April 8,1992 [Z 5287 IE]
German version: Angew. Chem. 1992, 104, 1275
CAS Registry numbers:
1, 142839-69-8; 2, 142865-80-3; 3, 142865-18-9; [VOCl,(thf)2], 29666-18-0;
[VCl,(thf),], 19559-06-9; [VCl,(tmeda),], 122116-24-9.
[I] T. F. Yen in The Role of Trace Metals in Petroleum, Ann Arbor Science,
Ann Arbor, Michigan, 1975, Chapter 1.
[2] P. Psundararaman, Anal. Chem. 1986,57,2204.
[3] J. G. Reynolds, E. J. Gallegos, R. H. Fish, J. J. Komlenic, Energy Fuels
1987, 1, 36.
[4] a) B. G. Silbernagel, R. R. Mohan, G. H. Singhal, ACS Symp. Ser. 1984,
248. 91; b) B. G. Silbernagel, J. C o l d 1979, 56, 315; c) S . Asaoka, N.
Nakata, C. Takeuchi, ACS Symp. Ser. 1987,344,275; d) M. Rose-Brussin,
D. Moranta, Appl. Catal. 1984, 11, 85; e)P. C. H. Mitchell, C. E. Scott,
J.-P. Bonnelle, J. G. Grimblot, J. Chem. Soc., Faraday Trans. 1 1985, 81,
[5] a) J. K. Money, J. C. Huffman, G. Christou, fnorg. Chem. 1985,24,3297;
b) J. K. Money, K. Folting, J. C. Huffman, D. Collison, J. Temperly, F. E.
Mabbs, G. Christou, ibid. 1986, 25, 4583.
[6] J. K. Money, J. R. Nicholson, J. C. Huffman, G. Christou, Inorg. Chem.
1986. 25, 4072.
[7] J. K. Money, J. C. Huffman, G. Christou, J. Am. Chem. Soc. 1987, 109,
[S] J. K. Money, K. Folting, J. C. Huffman, G. Christou, fnorg. Chem. 1987,
[Y] J. K . Money, J. C. Huffman, G. Christou, Inorg. Chem. 1988, 27, 507.
[lo] G. Christou, D. D. Heinrich, J. K. Money, J. R. Rambo, J. C. Huffman,K.
Folting. Polyhedron 1989,8, 1723.
Angew. Chem. Int. Ed. Engl. 1992, 31, No. 9
Aromatic Boron Heterocycles: The Generation
of 1H-Borepin and the Structure of
By Arthur J. Ashe IZZ,* Jefs W Kampf,
Yasuhiro Nakadaira,* and Jennifer M . Pace
Dedicated to Professor William von E. Doering
on the occasion of his 75th Birthday
1H-Borepin (1) is of great interest because it is isoelectronic with tropylium (2).['. Prior work over thirty years has
[*] Prof. Dr. A. J. Ashe 111, Dr. J. W. Kampf, J. M. Pace
Department of Chemistry
The University of Michigan
Ann Arbor, MI 48109-1055 (USA)
Prof. Dr. Y Nakadaira
Department of Chemistry, The University of Electro-Communications
Chofu, Tokyo 182 (Japan)
[**I This work was supported by the Research Corporation and the donors of
the Petroleum Research Fund, administered by the American Chemical
VeriagsgesellschafzmbH, W-6940 Weinheim, 1992
produced a number of imaginative syntheses of highly substituted bore pin^,'^ -'] while the minimally substituted
1-methylborepin (3a) has only recently been synthesized."']
We report here on the first synthesis of the parent heterocycle
1 and on structural data which show that I-substituted borepins can serve as ~ 7 ' aromatic ligands for transition metals.
3a R = CH3
3b R = CeH,
Borepins are most conveniently prepared by an exchange reaction of the appropriate stannepin with boron
Thus, the reaction of 1,l -dibutylstannepin
(4)I' '1 with phenylboron dibromide in benzene affords 1phenylborepin (3b) in 30% yield. The reaction of 4 with
excess BCI, in refluxing liquid butane affords l-chloroborepin (5) as a volatile liquid (67 % yield), which is easily purified by pot-to-pot distillation. Reaction of 5 with CH,OH in
CH,Cl, gives I-methoxyborepin (6) in 73% yield. These
borepins are air sensitive and 5 is particularly sensitive, but
they can be easily handled using Schlenk techniques. The
NMR data for these compounds is summarized in Table 1.
When an excess of Bu,SnH (50 mg) is added to a frozen
solution of 10 mg of 5 in 500 pL of C,D, in an NMR tube
at - 78 "C and then warmed to 25 "C, the 'H NMR spectrum
of the reaction mixture shows that the signals of 5 decrease
over 15min and new signals simultaneously appear. The
spectrum after the reaction is complete is shown in FigWe assign this spectrum to Iff-borepin as discussed
ure 1
below. Since 1H-borepin is extremely moisture, oxygen, and
heat sensitive, it has not been isolated. However, in a reaction characteristic of secondary b o r a n e ~ ~ 1
' ~reacts
methanol to afford the methoxy derivative 6.
The proof of the existence of 1H-borepin rests largely on
the analysis of the 'H, I'B, and 13CNMR spectra (Table 1).
The 'HNMR spectra of the four borepins 1, 3b, 5, and 6
show virtually identical patterns of peaks, since the observed
Fig. 1. The low-field region of the 300 MHz 'H N M R spectrum of ltl-borepin
in C,D,. The signals due to excess Bu,SnH and Bu,SnCI are not seen. The
insert shows an expansion of the signal at 6 = 6.89arising from the y protons.
Signals for unidentified oxidation products are marked with an x.
ring H-H coupling constants are all nearly
signals of the L,? protons are very broad doublets without tine
structure because of strong quadrupolar coupling to "B,
while the signals of the c( protons are also somewhat broadened due to a weaker coupling. Only the signals for the
remote y protons are sharp. For 1 the signal of the proton
bound to the boron atom was not observed presumably due
to the signal width.['51 However, the I'B NMR spectrum
shows a broad doublet ( J = 99 Hz), which collapses to a
singlet on 'H broad-band irradiation, indicating B-H coupling.
The I3C NMR spectra of borepins 1,3b, and 5 show that
the a and p carbon atoms are almost identically deshielded
while the signals of the y carbon atoms are in the normal
region for aromatic and oletinic carbon atoms. The STO-3G
MO calculations on 1 give p-n orbital populations of 0.94,
0.94, and 0.98 electrons for the a, p, and y carbon atoms,
Since the I3C NMR shifts are sensitive to
changes in K electron density, the observed shifts are consistent with the calculations.
The low-field 'H NMR shifts of the ring protons of substituted borepins have been taken as evidence of a diamagnetic
ring current.[4.'-lo] The low-field signals of 1, particularly
the signals of the a protons, also fit this pattern. Analysis of
the 'HNMR spectrum of 1 permits a discussion of its conformation. The Karplus relationship between the magnitude
and the torsion angle
of the vicinal coupling constant 3JH3,H4
H3-C3-C4-H4 applies to cycloheptatriene and heterocycloheptatrienes.[I6- 'I The observed value of 33H3,H4
of 8.3 Hz
is very close to that calculated from the torsion angle of 0"
which is required for the planar conformation predicted by
Table 1. 'H, "B, and "C NMR shifts of the new borepin derivatives and the borepin complex 7 (C,D,, 25 "C,coupling constants in Hz).
(d, 5(2,3) = 12.4)
7.60 (br d)
(m. J(3,4)= 8.3)
(br d, .
= 99)
152 (br)
(d, 3(2,3) = 12.9)
7.62 (br d)
(m, 5(3,4) = 8.0)
149.5 (br)
(d, 5(2,3) = 12.4)
7.25 (br d)
(m, J(3,4) = 8.3)
150 (br)
(d, 5(2,3) =13.1)
7.30(br d)
(m, J(3,4) = 8.4)
140 (br)
(d, 5(2,3) =11.6)
5.31 (br d)
(m, J(3,4) =7.8)
98.1 (br)
112.1 [a]
96.8 [a]
[a] Assignment uncertain.
Veriugsgeselischufi mbH, W-6940 Weinheim. i992
Angew. Chem. Int. Ed. Engl. 1992,31,N o . 9
MO calculations.[' 8 , Thus, the data suggest that borepin
is a planar aromatic compound.
In order to explore further the aromaticity of borepins, we
have prepared the Mo(CO), complex 7 by the reaction of 3b
with tricarbonyltris(pyridine)molybdenum and BF, . OEt,.
The NMR signals of the orange, air-stable complex 7 are
shifted markedly upfield in comparison to those of the free
ligand 3b. Therefore the borepin ring is q7-coordinated to
MO.['~'Since prior structural data are available only for
it was of interest
complex 8 with the fused benzene
to obtain a crystal structure for 7.['01
The structure of 7 (Fig. 2) shows a nearly planar borepin
ring which is q7-coordinated to the Mo(CO), moiety. The
B-Mo distance (2.523(3) A) is somewhat larger than the CMo distances of 2.33-2.42 A; the boron atom is slightly
displaced (0.04 A) from the plane of the ring. This is consistent with the larger size of the boron atom and conforms to
a pattern shown by other heterocyclic boron ligands.[2'1The
uncoordinated phenyl ring is canted by 28" relative to the
boron ring.
under reduced pressure, and the residue was subjected to pot-to-pot distillation
(120 T/O.OS Torr) giving 1.35 g of product. Analysis by ' H NMR indicated
that the product was approximately 10% I-phenylborepin (30% yield), and the
remainder was largely dibutyltin dibromide. This mixture can be used in subsequent reactions. Careful redistillation gave 70mg (15%) of 3b (b.p. 95"C/
0.05Torr). - El-MS: m / z (rel. intensity): 166 (45, M' for C,,H,,"B),
5 : To a gently refluxing solution of BCI, (1.0 g, 8.5 mmol) in 10 mL of butane
at 0 "C, 4 (1.0 g, 2.4 mmol) was added dropwise with stirring. On addition, the
rate of reflux increased and the color of the reaction mixture turned greenbrown as a brown precipitate formed. The mixture was allowed to stir as the
butane and excess BCI, were removed by distillation. The reaction mixture was
allowed to warm to 25 "C and finally heated to 50 "C. The brown residue was
then subject to pot-to-pot distillation (5OoC/0.5 Torr) giving 200 mg (67%) of
product which formed colorless crystals at - 78 "C. EI-MS: m / z (rel. intensity):
126 (17, M ' , for C,H,"B "CI) 123(100). High-resolution El-MS: calcd for
C,H,' ' B 3 W : 123.0173; found : 123.0171.
6 : A solution ofmethanol (55 mg, 1.72 mmol) in 1.0 mL of CH,CI, was added
to 5 (200 mg, 1.61 mmol) at - 78 "C. The mixture was allowed to warm to 25 "C
with stirring for 15 min. The solvent and excess methanol were removed under
vacuum leaving an oil which was distilled pot-to-pot (50 "C. 1 Torr) to give
140mg (73%) of 6. El-MS: m / z (rel. intensity): 120 (33, M' for C,H,"BO),
119(100). High-resolution EI-MS: calcd. for C,H,"BO: 119.0668; found:
7:A solution of 1.35 g of the mixture of 3b and dibutyltin dibromide (0.8 mmol
of 3b) above in 5 m L of Et,O was added to a suspension of
[(C,H,N),Mo(CO),] (0.38 g, 0.9 mmol) in 5 mL of Et,O. The BF, ' OEt,
(0.4 mL, 2.8 mmol) was added dropwise with stirring over 5 min. The reaction
mixture gradually darkened to a golden brown. After 2 h the solvent was removed under vacuum, and the residue was extracted with hexane (2 x 25 mL).
The yellow-brown extracts were washed with 10 mL H,O and dried over anhydrous Na,SO,. Removal of solvent left a thick yellow oil (0.54 g) which solidified on cooling. 'H N M R showed the material to be about 20% product.
Recrystallation from pentane gave 10 mg bright orange crystals, m.p. 7576 "C.
Received: April 8,1992 [Z 5290 IE]
German version: Angew Chem. 1992, 104, 1261
Fig. 2. Solid-state structure of 7 (ORTEP). Selected distances [A]: MOB
2.523(3), MoC4 2.400(3), MoC5 2.346(3), MoC6 2.327(3), MoC7 2.348(3),
MoC8 2.380(3), MoC9 2.416(3), BC4 1.525(4), BC9 1.532(4), BClO 1.578(4),
C4C5 1.390, C5C6 1.421(4), C6C7 1.408(5), C7C8 1.423(4), C8C9 1.391(4).
The C-C bonding distances of the borepin ring (1.391.42 A) are typical of those found for complexed monocyclic
ligands containing boron. The small range of bond lengths
(deviation 0.04 A) indicates that the n: bonds are not very
localized. Interestingly, this range is identical to that shown
by the C-C bond lengths of the-phenyl ring (1.36- 1.40 A).
The intraring B-C bond length (1.53 A) is significantly
shorter than the exocyclic B-C,H, distance of 1.58 A, indicating B-C n bonding in the borepin ring. These structural
data clearly demonstrate that borepin can serve as an aromatic ligand.
3b: Phenylboron dibromide (0.84 g, 3.4 mmol) in 5 mL of benzene was added
dropwise with stirring to 4 (1.0 g, 2.4 mmol) in 5 mL of benzene at 25°C. The
reaction mixture turned green on addition. After 5 min the solvent was removed
Angen. Chem. Int. Ed. Engl. 1992, 31, No. 9
CAS Registry numbers:
1, 291-62-3; 3b, 142868-01-7; 4, 114245-35-1; 5, 142868-02-8; 6, 142868-03-9;
7,142868-00-6; C,H,BBr,, 4151-77-3; BCI,, 10294-34-5; Bu,SnBr,, 996-08-7;
[(C,H,N),MO(CO),], 15279-79-5.
111 M. b. Vol'pin, Russ. Chem. Rev. (Engl. Transl.) 1960, 29, 129.
[2] a) J. M . Schulman, R. L. Disch, M . L. Sabio, J. Am. Chem. SOC.1982,104,
3785; ibid 1984, 106, 7696; b) J. M. Schulman, R. L. Discb, M. L. Sabio,
Tetrahedron Lett. 1983, 24, 1863; c) N. L. Allinger, J. H. Siefert, J. Am.
Chem. Soc. 1975,97,152.
[3] E. E. van Tamelen, G. Brieger, K. G. Untch, Tetrahedron Lett. 1960, No.
8, 14.
[4] a) A. J. Leusink, W. Drenth, J. G. Noltes, G. J. M. van der Kerk, Tetrahedron Lett. 1967, 1263; b) G. Axelrad, D. Halpern, Chem. Commun. 1971,
291; c) A. J. Ashe 111, J. W Kampf, C. M. Kausch, H. Konishi, M. 0.
Kristen, J. Kroker, Organometallics 1990, 9, 2944.
[5] A. T Jeffries 111, S. Gronowitz, Chem. Scr. 1973, 4, 183.
[6] a) J. J. Eisch, J. E. Galle, J. Am. Chem. SOC.1975, 97, 4436; b) However,
see: A. Geisberger, Ph.D. Thesis, Universitat Giessen, 1990.
[7] W Schacht, D. Kaufmann, Angew. Chem. 1987,99,682; Angew. Chem. Int.
Ed. Engl. 1987, 26, 665.
[8] a) A. J. Ashe 111, F. J. Drone, J. Am. Chem. SOC.1987,109,1879; ibzd. 1988,
110, 6599; b) A. J. Ashe 111, F. J. Drone, C. M. Kausch, J. Kroker, S. M.
Al-Taweel, Pure Appl. Chem. 1990, 513.
[9] Y Sugihara, T. Yagi, I. Murata, A. Imamura, J. Am. Chem. Soc. 1992, 114,
[lo] Y Nakadaira, R. Sato, H. Sakurai, Chem. Lett. 1987, 1451.
[ l l ] R. Sato, Ph.D. Thesis, Tohoku University, 1984; Y Nakadaira, R. Sato,
H . Sakurai, 6th Int. Symp. Novel Arom. Comp. Osaka, 1989, A-47.
[12] The same result is obtained with nBu,SnH, or tBu,SnH,.
[13] H. C. Brown, E. Negishi, J. Am. Chem. SOC.1971,93, 6682.
[14] The observed spectra can be simulated with the PANIC program (Bruker).
The following 'H-IH coupling constants were obtained for 1:
5(2,3) =12.4, J(2,4) =1.0, J(2,5) = 0.7, J(2,6) = 0, 5(2,7) = 3.2, 5(3,4) =
8.3, J(3,5) =1.0, J(3,6) = J(3,7) = 0, J(4,5) 4 1 . 1 Hz.
[15] The borale complex [CpCoC,Ph,BH] displays similar behavior: F. E.
Hong, C. W. Eigenbrot, T. P. Fehlner, J. A m Chem. Soc. 1989, 111, 949.
[16] Y. Nakadaira, R. Sato, H. Sakurai, Organometallics 1991, 10, 435.
[17] M. Karplus, J. Am. Chem. SOC.1963,8S, 2870.
[IS] H. Gunther, M. Gorlitz, H. Meizenheimer, Org. Magn. Reson. 1974, 6,
Verlagsgesellschafi mbH, W-6940 Weinheim. 1992
[19] G. E. Herberich, E. Bauer, J. Hengesbach, U. Kolle, G. Huttner, H.
Lorenz, Chem. Ber. 1977, ff0,760.
[20] 7: Monoclinic, space group P2Jn (No. 14) with u = 12.406(4), b =
6.733(2), c = 16.390(4)A, = 95.11(2)", V = 1363.6(5)A3,and 2 = 4
(e.,.,, =1.685 gcm-'), p(MoK.) = 93.18 cm-', 3997 unique reflections,
3777 with F, 2 0.6u(F) were used in refinement; R = 0.0466,
R , = 0.0452; GOF = 1.22. Further details of the crystal structure investigation are available on request from the Director of the Cambridge Crystallographic Data Centre, University Chemical Laboratory, Lensfield
Road, GB-Cambridge CB2lEW (UK), on quoting the full journal citation.
[21] a) G. E. Herberich, W. Boveleth, B. Hessner, D. P. J. Koffer, M. Negele, R.
Saive, 1 Organomel. Chem. 1986,308, 153; b) W. Siebert, M. Bochmann,
J. Edwin, C. Kriiger, Y-H. H. Tsay, Z . Nururforsch. B1978,33,1410; c) G.
Huttner, W. Gartzke, Chem. Ber. 1974. 107,3786; d) G. E. Herberich, B.
Hessner, S. Beswetherick, J. A. K. Howard, P. Woodward, J. Orgunomet.
Chem. 1980,192,421; e) G. E. Herberich, B. Hessner, M. Negele, J. A. K.
Howard, J. Orgunomet. Chem. 1987, 336, 29.
cyclopentadienide l[3b1
reacted with C,H; and solvated
nickel(@ bromide to give the nickelocene 2 (two pairs of
enantiomers syn-2alb and anti-Za/b in the ratio 5: l).15]Deprotonation of 2 yielded the green anion 3,15] a key compound for further reaction with transition metal ions. The
title compound 4C51was obtained after addition of solvated
chromium(i1) chloride to a solution of 3 in T H E 4 is an
air-sensitive compound isolated as mossy green crystals of
4 . toluene from toluene and solvent-free olive-green glittering platelets from hexane (42 % yield).
Stepwise Stacking of Three
Paramagnetic Metallocene Units:
[CpNiCp(SiMq), CpCrCp(SiMe,), CpNiCp]* *
By Pierre Bergerat, Janet Blumel, Monika Fritz,
Johann Hiermeier, Peter Hudeczek, Olivier Kahn,
and Frank H . Kohler*
Cr C12
Molecular materials which are ferro-, ferri, or antiferromagnetic as a result of interactions between spin-carrying
metal centers are currently subjects of intensive research.[''
A particularly rich chemistry is derived from chelate compounds in which like and-more promisingly-unlike metals
are assembled in close proximity.["I Other spin sources are
organometallic compounds. Thus, the reactions of polyalkylmetallocenes with tetracyanoethylene and related electron acceptors yield the salts [CpZM]' [K anion]- which
crystallize as stacks and exhibit spontaneous magnetization
at low temperature when the metal is iron, manganese, or
chromium.['] In these materials the stacks are linear, that is,
vertical with respect to the plane of the K ligands, and'
their stability is determined by ionic interactions. This
organometalEc/organic concept implies that different metal
centers cannot be introduced in one stack by using simple
sandwich molecules.
We wished to explore an alternative concept concerning
pure organometallic compounds with stacking orientations
other than vertical, with covalent bonding between the
metallocenes, and with two different paramagnetic metal
centers. We therefore synthesized biscyclopentadienyl ligands that may serve to link different metallocenes in a stereochemically well-defined manner.[31After testing less reactive
diamagnetic synthetic building blocks and their complexation behavior14]we report here on the title compound 4, the
prototype of a heteronuclear compound containing three
paramagnetic metallocene units.
The synthesis (Scheme 1) is based on the stepwise metalation of two doubly bridged cyclopentadiene~.[~]
Thus the
The cyclic voltammogram of 4 (Fig. 1) shows five electron
transfers leading to a pentacation. An interaction between
the two terminal nickelocenes is disclosed by splittings of 65
and 190 mVf61associated with the potentials for the oxidation to the nickelocene monocations (near 1 V) and dications
(near 2 V), respectively. Not surprisingly for highly charged
Fig. 1. Cyclic voltammogram of 4 in propionitrile, 8 x
molL- ', -2O"C,
scan rate 200 mVs-', potential E vs internal Cp,Co/Cp,Co+. Electron transfers from left to right: El,, (AE,) 340(65), 980(80), 1045(80), 1850(170),
2040(105) mV; assignment see text.
Prof. Dr. F. H. Kohler, Dr. J. Bliimel, Dr. M. Fritz, Dr. J. Hiermeier,
DipLChem. P. Hudeczek
Anorganisch-chemisches Institut der Technischen Universitat Miinchen
Lichtenbergstrasse 4, D-W-8046 Garching (FRG)
Prof. 0 . Kahn,'*' P. Bergerat
Universite de Paris Sud
Laboratoire de Chimie Inorganique, CNRS URA 420
F-91405Orsay (France)
['I Wilhelm Manchot Research Professor of the Technische Universitat
Miinchen 1991.
[**I This work was supported by the Pinguin-Stiftung, the Deutsche
Forschungsgemeinschaft, and the Fonds der Chemischen Industrie.
Verlagsgesellschuft mbH. W-6940 Weinheim, 1992
species, the electron transfers leading to the tetra- and pentacation are chemically not reversible. The central chromocene
is oxidized more easily (J!?,,~= 340 mV) in accord with what
is known from the parent metall~cenes.['~In addition, a
broad, unresolved wave with E, = - 1140 mV indicates the
irreversible reduction to 43-.
From the paramagnetic compound 4 we have obtained
13C NMR signals in the range from 6 = - 400 to + 1600
(Fig. 2). Their number and their approximate areas are in
0570-0833/Y2/090Y-1258$3.50+ .25/0
Angew. Chem. In[. Ed. Engl. 1992, 3f, No. 9
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phenylborepin, structure, borepin, generation, heterocyclic, molybdenum, aromatic, boron, tricarbonyl
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