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Crystal and Solution Structures of the Novel Complex of Sodium Ketimide with Two Ketimine Ligands [(tBu2C = NNa)4(HN = CtBu2)2] Stacking of Complexed and Uncomplexed (NNa)2 Rings to give a Highly Distorted (NNa)4 Cubane.

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ly resemble those of both the dinuclear carbonato complex 3
and the related acetatozinc complex [{q3-HBL,}Zn(q'OCOCH,)] (L = 3-tert-butylpyrazoly1).[' 'I Furthermore,
the similarity of the IR stretching frequencies of 2 (1675 and
1302 cm-') and 4 (1689 and 1297/1280 cm-') furnishes
good evidence for an analogous coordination mode.
The hydrogencarbonato complex 2, carbonato complex 3,
and methylcarbonato complex 4 are rapidly hydrolyzed by
water in organic solvents to the hydroxo complex 1. This
observation is particularly significant since facile displacement of bicarbonate and the regeneration of the hydroxozinc
species is a critical step in the proposed catalytic cycle for
carbonic anhydrase.
&f
Bu
2
1
5a: X = C I
4
5b: X = N 3
5C: X=CN
The pyrazolylboratozinc system in 2,3, and 4 models the
proposed action of carbonic anhydrase and also imitates the
inhibition of carbonic anhydrase activity that is observed in
the presence of small anions. Thus, methanol solutions of the
hydroxo complex 1 react rapidly with chloride, azide, and
cyanide salts, resulting in displacement of the hydroxo ligand
and formation of the neutral complexes 5a-c, which were
characterized by spectra and analyses. However, the most
well-known inhibitor of carbonic anhydrase, acetazolamide,["] did not react with 1.
These experiments indicate the close structural and chemical similarity that exists between the pyrazolylboratozinc
system and the carbonic anhydrase enzyme system. In particular, the zinc-bound hydroxo ligand in 1 is observed to be
both an efficient nucleophile and good leaving group, and
the hydrogencarbonato ligand in 2 is labile, thereby fulfilling
the prerequisites for catalytic turnover. The facile interconversions between these complexes provide the first functional model of carbonic anhydrase activity.
Received: August 19, 1991 [Z 4878 IE]
German version: Angen. Chem. 1992, 104. 57
Angew. Chem. Int. Ed. Engl. 31 11992) N o . 1
0
CAS Registry numbers:
1, 136379-29-8; 2, 137596-09-9; 3, 137596-10-2; 3 2(C6H,), 137596-15-7; 4,
137596-11-3; 513,137596-12-4; 5b, 137596-13-5: 5 c . 137596-14-6; CO,, 12438-9; pyrocarbonic acid dimethyl ester, 4525-33-1 ; carboanhydrase, 9001-03-0.
[I] B. L. Vallee in Zinc Emyrnes (Ed.: T. G. Spiro), Wiley, New York. 1983,
p. 1; Y. Pocker, S. Sarkanen, Adv. Enzvmol., 1978, 47, 149-274.
[2] For recent reviews see: a) Zinc Enzymes (Eds.: I. Bertini, C. Luchinat, W.
Maret, M. Zeppezauer), Birkhauser, Boston, 1986; b) S. Lindskog in [2a],
p. 307, c) E. A. Eriksson, T. A. Jones, A. Liljas in [2a], p. 317; d) A.
Galdes. B. L. Vallee in Zinc and its Role in Biology and Nutrition (Ed.: H.
Sigel), Dekker, New York, 1983, p. 1.
[3] For recent reviews see: J.-Y Liang, W. Lipscomb, Proc. Natl. Auxl. Sci.
U S A , 1990,87, 3675-3679; B. L. Vallee, D. S. Auld, ihid. 1990, 87. 220224. For recent theoretical studies see: 0. Jacob, R. Cardenas. 0. Tapia.
J. Am. Chem. Soc. 1990, 112, 8692-8705; M. Sola, A. Lledos, M. Duran,
J. Bertran, Inorg. Chem. 1991, 30, 2523-2527.
[4] See: R. S. Brown, J. Huguet, N. J. Curtis in [2d], p. 55; R. H. Prince in
Comprehensive Coordmution Chembtry Vol. 5, (Eds. : G. Wilkinson. R. D.
Gillard. J. A. McCleverty), Pergamon, Oxford, 1987. p. 925.
[5] E. Kimurd, T. Shiota, T. Koike, M. Shiro, M. Kodama, J. A m . Chem. Sue.
1990, 112, 5805-5811.
[6] R. Alsfasser, S. Trofimenko, A. Looney, G. Parkin, H. Vdhrenkamp, fnorg. Chem., 1991. 30. 4098-4100.
[7] Crystal data: 3 . 2(C6H6): monoclinic, C2/m, a = 20.475(5), b =
12.831(2), c = 17.360(4)A, fi = 132.78(1)", Z = 2, 1579 reflections, R =
0.042. 4: monoclinic, P 2 , / n , n = 10.698(2), h = 18.244(4). c =
15.657(3) A, fl = 90.08(3)", Z = 4, 3163 reflections, R = 0.045 Details of
the structure determinations will be given with the full publication.
[8] D. A. Palmer, R. van Eldik, Chem. Rev. 1983, 83, 651-731.
[9] See: N. Kitajima, K. Fujisawd, T. Koda, S. Hikichi, Y. Moro-oka. J. Chem.
Soc. Chem. Commun. 1990, 1357-1358; I. Murase, G. Vuckovic, M.
Koderd, H. Harada, N. Matsumoto, S. Kida, Inorg. Chem. 1991,30, 728733, and references cited in these papers.
[lo] M. Kato, T. Ito, Inorg. Chem. 1985, 24, 509-514.
[I I ] R. Han, I. B. Gorell. A. G. Looney, G. Parkin, J. Chem. Soc. Chem. Commun. 1991, 717-719.
[12] A. E. Eriksson, P. M. Kylsten, T. A. Jones, A. Liljas, Proteins 1988, 4.
283-293; J. E. Coleman, Annu. Reis. Phurmacol. 1975, 15,221-242; J. L.
Evelhoch, D. F. Bocian, J. L. Sudmeier, Biochemistr! 1981,20,4951-4954.
Crystal and Solution Structures of the Novel
Complex of Sodium Ketimide with Two Ketimine
Ligands [(tBu,C =NNa),(HN= CtBu,),] : Stacking
of Complexed and Uncomplexed ("a),
Rings
to give a Highly Distorted ("a),
Cubane**
By William Clegg, Murray MacGregor, Robert E. Muhey,*
and Paul A . O'Neil
Lithium ketimides (R1R2C=NLi), have proved to be valuable compounds in two respects. Firstly, they provide a convenient source of free ketimine anions (R'RZC=N)- for trL.isfer to other metal and metalloid centers in metathetical
reactions.['] Secondly, their crystal structures serve as models for the ring stacking in lithium structural chemistry.[21
The XLi compounds (X = e.g., alkyl, aryl, enolate, halide)
form small, single (XLi), ring units which maximize their
electrostatic (attractive) contacts in face-to-face stacking arrangements. Hexameric lithium ketimides form stacks of two
trimeric (R'R2C=NLi), rings.[31Prior to the work described
[*I
[**I
Dr. R. E. Mulvey, M. MacGregor
Department of Pure and Applied Chemistry
University of Strathclyde
GB-Gldsgow, G1 1XL (UK)
Dr. W. Clegg, P. A. O'Neil
Department of Chemistry
The University
GB-Newcastle upon Tyne, NE1 7RU (UK)
This work was supported by the U. K. Science and Engineering Research
Council and by the Royal Society. We thank Dr. P. R. Dennison (University of Strathclyde) for running the NMR spectra.
VCH Verlugsgesellschuft mhH. W-6940 Weinheim, 1992
0570-0833i92j0101-0093 $3.50+ ,2510
93
here, monometallic sodium ketimide ~hemistry'~]
was represented by a single report from 1934 on ammonia-contaminated (Ph,C=NNa), characterized by N, Na-microanalysis;['] this preempted the first lithium ketimide study[6]by
over thirty years. We investigated the metallation reaction of
the ketimine tBu,C=NH with nBuM (M = Li, Na) [Eq. (a)].
nBuM
+ rBu,C=NH
+ I/.
(fBu,C=NM),
+ nBuH
(a)
For M = Li the metallation proceeds as expected giving
crystalline (tBu,C=NLi), in nearly quantitative yield.['] Reaction mixtures having excess ketimine afford the same solid
product. In contrast, when M = Na, two products are detectable: a yellow amorphous material (the major product)
which is still under investigation[81and a few crystals of 1,
an unexpected complex of ketimide and nonmetalated
[(fBu,C=NNa),(HN=CtBu,),]
1
ketimine. We suspect the reaction stoichiometry was not exactly 1.O: 1.O, since the purity of the nBuNa reactant, freshly
prepared froin nBuLi and tBuONa,lgl though high, fell short
of 100%. Hence, a trace excess of ketimine is available to
complex two Na' cations in 1. On deliberately increasing the
nBuNa: tBu,C=NH ratio to 1.0: 1.5, consistent with the
metal: ligand ratio in 1 (4: 6), no amorphous material is produced and the yield of 1 (first batch, 46%) is greatly enhanced.
The crystal structure of 1 (Fig. I)[''] displays a cubanelike (imido-N-Na), core. This structure is made up of a stack
of complexed and uncomplexed ("a),
rings with tetra-
CI32-
/
Fig. 1. Crystal structure of 1 (without hydrogen atoms). Important distances
[A] and angles ['I: Na(1)-N(2) 2.449(6), Na(1)-N(3) 2.478(4), Na(2)-N(2)
2.462(4), Na(2)-N(3) 2.425(6), Na(3)-N(I) 2.351(4), Na(3)-N(4) 2.31 1(7),
Na(4)-N(1) 2.287(7), Na(4)-N(4) 2.275(4), Na(1)-N(4) 2.450(5), Na(2)-N(I)
2.476(5), Na(3)-N(2) 2.326(5), Na(4)-N(3) 2.291(5), Na(1)-N(5) 2.475(5),
Na(2)-N(6) 2.465(5), N a . . "a
3.021(4)-3.183(3), N(1)-C(1) 1.241(8), N(2)C(2) 1.253(8), N(3)-C(3) 1.255(8), N(4)-C(4) 1.254(7),N(S)-C(5) 1.266(7), N(6)C(6) 1.256(7); N(2)-Na(l)-N(3) 98.3(2), N(2)-Na(l)-N(4) 92.0(1), N(3)-Na(l)N(4) 91.2(1), N(Z)-Na(2)-N(3) 99.4121, N(2)-Na(2)-N(I) 93.6(1), N(3)-Na(2)N(1) 95.9(2). N(l)-Na(3)-N(4) 94.2(2), N(I)-Na(3)-N(2) 100.7(2), N(4)-Na(3)N(2) 104.2(2), N(l)-Na(4)-N(4) 96.9(2), N(I)-Na(4)-N(3) 105.3(2), N(4)-Na(4)N(3) 100.9(2), Na(3)-N(l)-Na(4) 82.1(2), Na(3)-N(l)-Na(2) 81.4(1), Na(4)N(l)-Na(2) 78.6(2), Na(3)-N(4)-Na(4) 83.2(2), Na(3)-N(4)-Na(l) 79.6(2),
Na(4)-N(4)-Na(l) X3.3(2), Na(l)-N(2)-Na(2) 80.8(2). Na(l)-N(2)-Na(3)
79.3(2), Na(2)-N(2)-Na(3) 82.2(1), Na(l)-N(3)-Na(2) S1.0(2), Na(1)-N(3)Nd(4) 82.3(1), Na(2)-N(3)-Na(4) 79.6(2), N(S)-Na(l)-N(2) 103.5, N(5)-Na(l)N(3) 157.6(2). N(S)-Na(l)-N(4) 91.412). N(6)-Na(Z)-N(2) 151.6(2), N(6)-Na(2)N(3) 107.8(2), N(6)-Na(2)-N(l) 91.8(2), N(I)-C(I)-C(I 1) 121.0(6), N(1)-C(1)C(12) 119.9(5). C(ll)-C(l)-C(l2) 119.1(5), N(2)-C(2)-C(21) 118.4(5), N(2)C(2)-C(22) 123.2(5), C(21)-C(Z)-C(22) 118.4(5), N(3)-C(3)-C(31) 121.5(5),
N(3)-C(3)-C(32) 120.1(4), C(31)-C(3)-C(32) 118.4(5), N(4)-C(4)-C(41) 120.7(7),
N(4)-C(4)-C(42) 119.5(5), C(41)-C(4)-C(42) 119.7(6), N(S)-C(S)-C(Sl) 121.3(6),
N(S)-C(S)-C(52) 115.9(5),C(Sl)-C(S)-C(52) 122.8(5). N(6)-C(6)-C(61) 116.3(4),
N(6)-C(6)-C(62) 120.9(5). C(61)-C(6)-C(62) 122.7(5).
94
0 VCH VerlugsgesellJchuft mbH,
W-6940 Weinhelm, 1992
(Na(1),(Na(2)) and tricoordinate (Na(3),Na(4)) sodium, respectively. To optimize stacking the ketimine nitrogens
(N(5),N(6)) lie approximately in the same plane as the complexed ring; the N(4)Na(l)N(5) and N(I)Na(2)N(6) bond angles are close to 90". Complexation weakens the intramolecular ring bonding, expanding the Na( I)N(2)Na(2)N(3) face
(perimeter 9.814 A; cf. 9.224 A for the uncomplexed
Na(3)N( l)Na(4)N(4) face). The deviation from an ideal cube
is most pronounced at the Na(4) corner resulting in the shortest N-Na edge lengths for those bonds with Na(4) (sum,
6.853 A, cf. 6.988 A for the related Na(3) corner). This
strong distortion brings Na(4) into close proximity with a
tBu hydrogen atom. Without independently refined hydrogen positions we cannot comment on the significance of this
feature except to note that assuming normal tBu C-H geometries, the calculated H . . . Na length would be remarkably short (2.186 &. Triply bridged by Na atoms, each imido
N center exhibits pseudotetrahedral coordination. The remaining N centers (N(5),N(6)) belonging to nonmetalated
ketimine molecules (i.e., retaining acidic N-H bonds) act as
monodentate, terminal Lewis bases to two of the four Na+
ions (Na(l),Na(2)). This is the first example of ketimine +
alkali-metal dative bonding. The structure most similar to 1
is the pinacolone-solvated sodium enolate of pinacolone, but
even here its (ONa), cubane core is solvated by four rather
than two molecules which could potentially be metalated." 'I
Evidence that 1 remains intact in [DJtoluene solution
over the wide temperature range examined (193-298 K)
comes from 'H NMR spectroscopic studies. The spectrum at
193 K shows the two ketimine (NH) protons downfield (6 =
9.09) and four distinct types of tBu resonances (6 = 1.39,
1.35, 1.03,0.81) with intensity ratios (36:36: 18:18) in agreement with the crystal structure. The first two tBu signals
which overlap can be assigned to ketimido ligands associated
with N(2),N(3) in the complexed ring (36H) and with
N(l),N(4) in the uncomplexed ring (36H). The remaining
signals are attributed to the two magnetically nonequivalent
tBu groups (18 H + 18 H) on each (identical) ketimine molecule and are a result of the rigid attachment of the C=N(S)H
and C=N(6)H units to the Na(l)N(2)Na(2)N(3) ring. The
spectrum at 298 K similarly shows four tBu resonances.
Thus both X-ray crystallographic studies and 'H NMR
spectroscopic studies confirm the presence of (ketimine)N-Na donor - acceptor contacts. This is an important distinction from the structural chemistry of organolithium
compounds, since Li' cations in (tBu,C=NLi), resist complexation by stronger donors.[']
Experimental Procedure
1: An equimolar amount of tBu,C=NH added to a chilled heptane suspension
of nBuNa under argon gave an insoluble yellow solid and a yellow solution. The
solution was refrigerated (-30°C) and large yellow crystals identified as 1
formed; 4 % yield of first batch (based on ketimine consumption). From the
reaction with the 1.0:l.S molar ratio a 46% yield of 1 was obtained.
M.p. = 91 -93°C; satisfactory C,H,N,Na-analyses. Compound 1 decomposes
in contact with moisture or oxygen hut is nonpyrophoric. Single crystals suitable for X-ray crystallography were sealed under argon in Lindemann capillary
tubes prior to data collection.
Received: July 26, 1991 [Z 4830 IE]
German version: Angew. Chem. 1991, 104, 74
CAS Registry numbers:
1. 137695-99-9; nBuNa, 3525-44-8; tBu,C=NH, 29097-52-7
[ I ] Such reports were usually published by Wade et al. For a detailed list of
examples see ref. 121, p. 168.
[Z] R. E. Mulvey, Chrrn. Sor. Rev. 1991, 20, 167. This review focuses on the
ring stacking of organonitrogen lithium compounds.
0570-0833/92j0l01-0094$3.50+.25/0
Angew. Chem. I n f . Ed. Engl. 31 (1992) No. 1
and NMR ('H, 13C, 31P)spectroscopy, and by X-ray structure analysis. The isopropyl groups are shown to be
nonequivalent in the 'H and 13C NMR spectra, which suggests restricted rotation of the R2N group about the PC-N
bond. The sp2-hybridized C atoms of the cyclic phosphorus
ligand are magnetically equivalent; one of the 'J(P, spz-C)
couplings is only 3.8 Hz. TWOsignals of equal intensity are
observed in the 31P NMR spectrum (6 =106.3 and 94.2),
and the 2J(P,P) is remarkably small (9.0 Hz). The frequencies of the CO valence bands are ca. 30cm-' higher than
those of the phosphanido-bridged carbonylnickel complexes
[(CO),Ni(q' -p2-PR,)Ni(CO)J
and are best compared
with the CO absorptions of a tricarbonylnickel-phosphaallene
The most important information on the unusual compound 2 is found in the crystal structure analysis.['] In accord
with the NMR and IR data, it shows that both [Ni(CO),]
fragments are bound to only one P atom (Fig. 1). In the heart
[3] D. R. Armstrong, D. Barr, R. Snaith, W. Clegg, R. E. Mulvey, K. Wade,
D. Reed. J. Chem. Sor. Dulton TranA. 1987, 1071.
[4] Two mixed lithium-sodium ketimides have been described: W. Clegg, R. E.
Mulvey. R. Sndith. G. E. Toogood, K. Wade, J. Chem. Soc. Chem. Commun. 1986, 1740; D. Barr. W. Clegg, R. E. Mulvey, R. Snaith, ibid. 1989,
57.
[5] G E. P. Smith, E W. Bergstrom. J, A m . Chem. Soc. 1934, 56, 2095.
(61 L.-H. Chan, E. G. Rochow, J. Orgunomel. Chem. 1967, 9, 231.
[7] Ether, pentamethyldiethylenetriamine(PMDETA), pyridine, tetramethylethylenediamine (TMEDA). and triethylamine fail to give solid adducts,
though a 1:1 adduct is formed with the exceptionally strong donor hexamethylphosphoramide (HMPA). D. Barr, R. Snaith, W. Clegg, R. E. Mulvey. K . Wade, J. Chenz. Soc. Dalton Trans. 1987, 2141.
[8] A strong IR band at 1600 c m - ' characteristic of a C=N bond and an 'H
NMR spectrum in [D,]toluene consisting of a singlet at S = 1.19 characteristic of a rBu group suggest this product is uncomplexed (rBu,C=NNa),.
M.p. 178- 180'C (decomp.).
[9] L. Lochmann. J. PospiSil, D. Lim, Tetruhedron Lelr. 1966. 257.
[lo] I M , = 935.5, triclinic, space group Pi,u = 13.072(5), b = 13.469(5). c =
20.613(8)
x =78.74(2), /3 =74.68(2), y = 64.67(2)", V = 3149.7
Z = 2, eCalcd
= 0.984gcm-'. F(OO0) = 1036, Mo,, radiation, R = 0.71073 A,
11 = 0.08 mm-'. The structure was determined by direct methods and refined from 4864 unique observed reflections measured at 240 K with a
with anisotropic thermal
Stoe-Siemens diffractometer (28,,, = 45
parameters and with isotropic H atoms in calculated positions: R = 0.103.
H, = 0.116. S =1.16 for 577 parameters. Further details of the crystal
structure investigation can be obtained from the Director of the Cambridge Crystallographic Data Centre, University Chemical Laboratory,
Lensfield Road, Cambridge, CB2 1EW (U. K.), on quoting the full journal
citation.
[ l l ] P. G. Willidrd, G. B. Carpenter, J1 A m . Chem. Soc. 1986, 108, 462.
'
A',
A,
O),
Unusual Coordination of the
1,3-DiphosphacyclobutadieneDerivative
(iF'r,NCP), to Two [Ni(CO),] Fragments**
[A]
Fig. 1. Molecular structure of 2 in the crystal; important bond lengths
and
angles ["I. P(I)-C(l) 1.850(4),P(l)-C(2) 1.855(3),P(1)-Ni(1) 2.271(1). P(1)-Ni(2)
2.271(1), P(2)-C(1) 1.784(3). P(2)-C(2) 1.783(4), C(1)-N(l) 1.304(4). C(2)-N(2)
1.308(4); C(l)-P(l)-C(2) 77.8(2), Ni(l)-P(l)-C(l) 107.2(1), Ni(2)-P(l)-C(l)
109.0(1), Ni(l)-P(l)-C(2) 114.7(1), Ni(2)-P(l)-C(2) 111.3(1), Ni(l)-P(l)-Ni(2)
125.8(1), C(l)-P(2)-C(2) 81.4(2), P(l)-C(l)-P(2) 100.4(2), P(l)-C(2)-P(Z)
100.3(2), N(1)-C(1)-P(1) 128.0(3), N(l)-C(l)-P(2) 131.6(3), N(2)-C(Z)-P(l)
128 2(3), N(2)-C(2)-P(2) 131.4(3), C(l)-N(l)-C(9) 121.2(3), C(9)-N(l)-C(IZ)
116.2(3), C(l)-N(l)-C(l2) 122.5(3), C(2)-N(2)-C(15) 122.2(3), C(2)-N(Z)-C(l8)
121.3(3), C(15)-N(2)-C(18) 116.4(3).
By Joseph Grobe,* Duc Le Van, Marianne Hegemann,
Bernt Krebs, and Mechtild Lage
The chemistry of the phosphaalkynes RC-P contains
many surprises, as the investigations of recent years have
shown.['' In particular, studies on reactivity of tertbutylphosphaethyne have uncovered many new aspects in
organophosphorus and complex chemistry.['] Yet phosphaalkyne derivatives with donor or acceptor substituents
have hardly received any a t t e n t i ~ n . ~ 'We
. ~ ] report here an
unexpected reaction of the only recently synthesized di(isopropy1)aminophosphaethyne iPr,NC=P (1)14]with tetracarbonylnickel.
On dropwise addition of 1 to a solution of [Ni(CO),] in
ether, CO evolution is observed, and complex 2, which contains two [Ni(CO),] groups on one of the phosphorus atoms
of the 1,3-diphosphacyclobutadienering, is formed as exclusive product (monitoring by IR during the reaction and by
NMR afterwards).
of the molecule all ten skeletal atoms of -[C,NCP],- lie in
one plane (average deviation: 0.059 A). This result indicates
an extensively delocalized electronic system. The distances
P 1-C 1 and P 1-C 2 are virtually the same and correspond
to the length of a P-C single bond (1.85 A). In contrast, the
practically identical P 2 - C l and P2-C2 distances (I ,784
and 1.783 A, respectively) result from a bond order greater
than 1. The participation of the lone pairs of the iPr,N substituents in the resonance formulas is confirmed by both the
planar environment and the marked shortening of the C 1N 1 and C2-N2 bonds (1.304 and 1.308 A, respectively).
The Ni 1-P 1 and Ni2-Pl distances are equal (2.271 A) and
20 "C
2 iPr,N-C-P + 2 [Ni(CO),]
[(CO),Ni{q'-p2-(iPr,NCP)2}Ni(CO),1 lie within the range expected for phosphanenickel com- 2co
plexes.
1
2
This interesting structure, and likewise the spectroscopic
data,
prove the presence of a delocalized K electron system.
The composition and constitution of the ruby-red crystals
The
bonding
relationships are approximately represented by
of compound 2 were established by elemental analysis, IR
-
[*I
Prof. Dr. J. Grobe, Dr. D. Le Van, DipLChem. M.Hegemann. Prof. Dr.
B. Krebs, M. Liige
Anorgdnisch-Chemisches Institut der Universitlt
Wilhelm-Klemm-Strasse 8, D-W-4400 Miinster (FRG)
[**I Reactive E=C(p-p)n Systems, Part 29. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Part 28: J. Grobe, D. Le Van, T. Grosspietsch, Z . Naturforsch. B, 1991,
46. 978-984.
Angew. Chem. Inl. Ed. Engl. 31 (1992) No. 1
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nna, crystals, two, complex, ctbu2, sodium, ketimide, stacking, tbu2c, ketimine, ring, cubana, complexes, ligand, uncomplexed, solutions, structure, give, distorted, novem, highly
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