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Functional Tetrahedral Zinc Complexes.

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of them the C-PPh, unit has been homologated by the “insertion” of one and two CH, units, respectively.
Received: March 6, 1990 [Z 3833 IE]
German version: Angew. Chem. 102 (1990) 938
[l] H. J. Bestmann, R. Zimmermann in: Houben-Weylt Methoden der Organischen Chemie, Ed. E l , Thieme, Stuttgart 1982, p. 616.
[2] H. Schmidbaur. Anpew Chem. 95 (1983) 980; Anpew. Chem. I n f . Ed. End.
22 (1983) 907.
D. Heineke, H. Vahrenkamp, Organomefallics 9 (1990) 1697.
0.25 mmol of Ru,(CO),, and 0.5 mmol of salt-free Ph,P=CH,. 25 mL of
THF, -2O”C, warmup to room temperature in 20min, addition of
0.25 mmol of CH,OSO,CF, to remove anionic side products. Chromatography with hexane/CH,Cl, (1.1) over silica gel: first fraction, Ru,(CO),,;
second fraction, 1.1: yellow, m.p. 125°C (dec.). IR (CH,CI,). G = 2090 w,
2058 s, 2031 s, 1996 s cm-’. ‘H NMR (250 MHZ CDCI,): S = 7.4-7.7
(phenyl multiplet), 2.99 (d, J = 15.9 Hz, P-C-H), - 14.95 (s, Ru-H).
0.05 mmol of 1,20 mL of C,H,,, reflux for 5 h; workup by chromatogrdphy with hexane/CH,Cl, (3: 1) over silica gel yielded 2 in the first fraction.
2: yellow, m.p. 153 “C. IR (CH,CI,): D = 2096 m, 2062 s, 2031 vs, 1977 m,
1957 w, 1944 w cm-’. ‘H NMR: S = 7.4-7.8 (phenyl multiplet), - 17.73
(s, Ru-H).
A. J. Deeming, D. Nuel, N. I. Powell, C. Whittdker, J. Chem. Soc. Chem.
Commun. 1990. 68.
D. S . Bohle, H. Vahrenkamp, unpublished.
Crystal data for 1: triclinic, space group P i , a = 1662.816). b = 1014.8(4),
c = 985.2(4) pm, z = 67.69(2), @ = 88.46(2), y = 85.88(2)”, 2 = 2; 2510
reflections, R = 0.090. Crystal data for 2: triclinic, space group Pi ,
a = 1787.7(8), b = 1665.6(5), c = 1044.7(3)pm, a = 102.82(2), @ =
99.15(2), y = 93.00(2)”;2 = 4, 8389 reflections, R = 0.033. In both structures the hydride ligands could be located, but not refined. Further details
of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische
Information mbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-54524 the names of the authors, and the
journal citation.
A. G. Orpen, D. Pippard, G. M. Sheldrick, K. D. Rouse, ACIUCrystallogr.
B34 (1978) 2466.
B. F. G. Johnson, J. Lewis, T. I. Odidka, P. R. Raithby, J Organomel.
Chem. 216 (1981) C56.
See G. M. Sheidrick, J. P. Yesinowski, J Chem. Sac. Dalton Trans. 1975,
a) S . Ching, M. Sabat, D. F. Shriver, OrganometaNics8 (1989) 1047; b) H.
Schmidbaur, F. Scherbaum, B. Huber, G. Muller, Angew. Chem. 100
(1988) 441 ; Angew. Chem. Inr. Ed. Engl. 27 (1988) 419.
For the [Fea(CO)12(p4-C)]
system, cf. J. S . Bradley, Adv. Organomel Chem.
22 (1983) 1.
T. Albiez, H. Vahrenkamp, Angew. Chem. 99 (1987) 561; Angew. Chem.
I n f . Ed. Engl. 26 (1987) 572.
Functional Tetrahedral Zinc Complexes**
By Ralf Alsfasser, Anne K. Powell,
and Heinrich Vahrenkamp *
In the numerous zinc-containing enzymes, the zinc ion is
almost always coordinated in a tetrahedral fashion.[’] Three
[*J Prof. Dr. H. Vahrenkamp, DipLChem. R. Alsfasser, Dr. A. K. Powell
Institut fur Anorganische und Analytische Chemie der Universitat
Albertstrasse 21, D-7800 Freiburg (FRG)
[**I This work was supported by the Fonds der Chemischen Industrie and the
Deutsche Forschungsgemeinschaft.
Verlagsgesellschaft mbH. 0-6940 Wernherm, 1990
of the four coordination sites are occupied by peptide donor
groups, while the fourth, the “functional” site, is occupied by
the substrate of the enzyme (or, in the “resting state”, by a
water molecule). Because of the lability of zinc complexes
and the diverse coordination possibilities for zinc, it is unexpectedly difficult to obtain this simple coordination pattern,
L,ZnX, in model complexes.[‘] Thus, typical chelating L,
ligands such as triazacy~lononane,[~”~
tris(aminomethy1)methane,[3b1cis-triaminocyclohexane,[3‘~and even hydrotris( pyrazolyl)borate[3d1usually also afford octahedral 1 :2
complexes. Only a complicated and bulky L, ligand such as
tris(4,5-diisopropylimidazolyl)phosphane leads to a complex, [L,ZnCI]@, that models the structure and function of
the enzyme carbonic a n h y d r a ~ e . ~ ~ ]
To obtain novel L,ZnX complexes 1, we have now exploited the fact that the well-studied hydrotris(pyrazoly1)borate
ligands as their 3-phenyl or 3-tert-butyl-substituted derivatives form stable tetrahedral complexes L,MX, M =
Co,Ni,Cu,Zn.cS1 In complexes I, the hydrophobic pocket
formed around the fourth coordination site of the zinc by the
substituents of the pyrazole can accommodate a variety of
groups X. The utility of this concept is demonstrated by the
four compounds 1 a-1 c and 2, all of which have been characterized by X-ray structure analyses.[61
1. R = P h , t B u
Hydrotris(pheny1pyrazolyl)boratozinc nitrate (1 a) precipitates when methanolic solutions of Zn(NO,), . 6H,O
and the potassium salt of the ligand are combined (56%
yield after recrystallization from benzene). Complex 1 a is
first and foremost a further example of the well-investigated
class of hydrotris(pyrazoly1)boratozinc salts.[51Its structure
analysis clearly shows, however, that the coordination of the
nitrato ligand, as shown in the formula, is somewhere between monodentate and bidentate (Zn-0 195 and 247 pm).
Complex 1 a is thus a primitive analogue of the enzyme carbonic anhydrase in two respects: first, the pocket at zinc in
L,ZnX has room for both a monodentate X and a second,
small ligand (the substrate); second, the nitrato ligand is
coordinated in just the way discussed for the isoelectronic
hydrogen carbonate in carbonic anhydrase.14’
0570-0833/90j0808-0898$ 3 SO+ 2510
Angew. Chem. Int Ed Engl. 29 (1990) No. 8
Hydrotris( pheny1pyrazolyl)boratozinciomethane (1 b) is
formed in 90% yield as a practically analytically pure product when methylIithium (ca. 2 M, in ether, 10% excess) is
added dropwise to a solution of L,ZnC1 in benzene. After
concentration of the reaction solution to dryness, the product is separated from the lithium compounds by extraction
with toluene. Further such organozinc compounds are synthesized analogously. Together with the organozinc compounds of hydrotris(tert-butylpyrazo1yl)borate recently synthesized by Parkin et al.,"] they are the first monomeric
representatives of the class of compounds RZnX.[*] Although their bonding alone is intriguing (the Zn-C distance
in 1 b is 197 pm), their primary importance presumably lies in
their reactivity toward protic reagents HX. Treatment with
these reagents results in alkane elimination and formation of
the corresponding Zn-X compounds. In the simplest casefor example, reaction with HC1 or RCOOH-the resulting
compounds may be more easily obtained by other routes (cf.
also Ref. 171). As shown for 1 c in the following discussion,
however, this reaction can also lead to novel compounds, not
accessible via other routes.
Hydrotris( phenylpyrazoly1)boratozinc ethanethiolate (1 c)
is obtained practically analytically pure in 85 YOyield when
the tBuZn compound analogous to 1 b is allowed to react for
one week in a stirred solution containing a stoichiometric
amount of ethanethiot, folIowed by fiitration of the reaction
solution and concentration of the filtrate to dryness. The
easy preparation and manipulation of 1 c demonstrate especially clearly the advantages of the L, coligand chosen here,
since monomeric complexes of the type L,ZnSR were previously unknown. Zinc thiolates are usually oiigomeric,121and
only a few monomers of the type [Zn(SR),]2e[9a1 and
L,Zn(SR)2[9b1have been described. The importance of 1 c
(Zn-S distance 221 pm) lies in its structural relationship to
the numerous enzymes in which zinc is surrounded in a tetrahedral fashion by histidine-imidazole and cysteine-thiolate
coordination partners."'
[p-Hydroxobis{tris(tert-butylpyrazole)zinc]] trisperchlorate (2) is formed within a few minutes when the potassium
salt of the tert-butylpyrazolylborato ligand is allowed to react with Zn(CIO,), . 6H,O in methanol; after recrystallization from toluene, 2 can be isolated in 42% yield. The expected product was the cationic complex Id. Complex I d
may in fact be the precursor of 2 if the zinc increases the
hydrolytic activity of the coordinated water molecule in Id,
as it does in the enzymes containing the unit L,Zn-OH, in
the active site. If this zinc-catalyzed hydrolysis is directed
toward the complex itself in Id, the BH unit can be released
from the otherwise hydrolysis-stable pyrazolylborate as
H,BO, and a tris(pyrazo1e)zinc complex such as 2 can be
formed. Complex 2, more easily prepared from tert-butylpyrazole, KOH, and Zn(C10,), in methanol, shows Zn-0
distances of 190 and 192 pm and a Zn-0-Zn angle of 137".
It is the first isolable compound in which an L,Zn-OH unit
is present; such a unit has been discussed for the hydrolytic
function of carbonic anhydrase and carboxypeptidase.['*
Two typical hard ligands are bound to zinc in complexes
l a and 2 and two typical soft ligands in l b and l c . The
extreme cases are the coordination of an OH' ion and a
methyl group in comparable ligand environments. T k fact
that all four complexes are soluble in nonpolar media such as
toluene supports the assumption that, even in 1 a and 2, the
very polar Zn-X units are buried within a hydrophobic ligand environment. This should result in an unusual reactivity
of these Zn-X units.
Received: April 11, 1990 [Z 3916IEl
German version: Angew. Chem. 102 (1990) 939
[I] T. G. Spiro (Ed.): Zinc Enzymes, Wiley, New York 1983.
121 R. H. Prince in G. Wilkinson (Ed.): Comprehensive Coordinntion Chemisfry, Pergamon, Oxford 1987, Vol. 5, pp. 926-1045
[ 3 ] a) R. Yang, L. J. Zompa, Inorg. Chem. 15 (1976) 1499; b) A. Sabatini, A.
Vacca, J. Chem. SOC.Dalton Trans. 1980,519; J. Wirbser, H. Vahrenkamp,
unpublished; c) R. A. D Wentworth, Inorg. Chem. 7 (1968) 1030; d) S.
Trofimenko, J. Am. Chem SOC.89 (1967) 3170.
[4] R. S. Brown, N. 3. Curtis, J. Huguet, J. Am. Chem. Soc. 103 (1981) 6953.
[ S ] S. Trofimenko, J. Calabrese, J. S. Thompson, Inorg. Chem. 26 (1987) 1507.
[6] The complete crystallographic details will be reported in a forthcoming
paper. The most important compound described here, 2, forms orthorhombic crystals, space group P2,2,2,, u = 1015.9(8),b = 2275 4(5). c =
2544.7(6) pm,
= 1.32 gem-), MoK. radiation, p = 9.6 cm-', 3652 reflections with I > 3cr(I), R = 0.074.
[7] I. B. Gorrell, A. Looney, G. Parkin, J. Chem. Sor. Chem. Commun. t990,
IS] Cf. J. Boersma in G. Wilkinson, F. G. A. Stone, E. W. Abel (Eds.). Comprehensive Organometallic Chemistry, Pergamon, Oxford 1982, Vol. 2,
pp. 823-851.
[9] a) P. J. Blower, J. R. Dilworth, Coord. Chem. Rev 76 (1987) 121; b) D. T.
Corwin, S. A. Koch, Inorg. Chem. 27 (1988) 493, and references cited
[lo] Cf. P. Woolley, Nature (London) 258 (1975) 677; E. Kimura, T. Koike, K .
Toriumi, fnorg. Chem. 27 (1988) 3687.
A 1,2:2,1-Bis(2-silapropane-1,3-diyl)diborane(6):
Stabilization Product of Bis(trisyl)diborane(2)?
By The0 Mennekes, Peter Paetzold,* and Roland Boese*
Dedicated to Professor Edwin Hengge on the occasion
of his 60th birthday
In the course of our investigations of two-coordinate boron,"' we wanted to gain 1,2-bis(trisyl)diborane(2) (2) by a
kind of Wurtz synthesis from the known dichloro(trisy1)borane (l)[']and Na/K alloy. We hoped that the bulky "trisyl" group, (Me,Si),C, would kinetically stabilize 2. Instead,
however, the title compound 3 was obtained. Its constitution
was established from the NMR, MS, and IR spectra13]and
was confirmed by a n X-ray structure examination (Fig.
LMe3Si13C - B = B
- C(SiMe313
[*] Prof. Dr. P. Paetzold, T. Mennekes
Institut fiir Anorganische Chemie der Technischen Hochschule
Templergraben 55, D-5100 Aachen (FRG)
cation of 2.L- tert-butylpyrazole
Angev. Chem Inl. Ed. Engl. 29 (1990) No. 8
Dr. R. Boese
Institut fiir Anorganische Chemie der Universitat-Gesamthochschule
Universitatsstrasse 5 -7, D-4300 Essen (FRG)
Q VCH Verlagsgesellschaft mbH, 0-6940 Wemheim, 1990
0S70-0833/90~0808-0899$3.50f 2510
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complexes, function, zinc, tetrahedral
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