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Aromatic Gallium Heterocycles Synthesis of the First Gallatabenzene.

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H Bock. T. Hauck, C. Nither. M. Rosch, M . Staufer. 0. Hiberlein. A n g r w
Chem. 1995. 107. 1439; Angew Cliem. Int. Ed. Engl. 1995,34, 1353: Seealso the
definition for "lipophllically wrapped polyion aggregates" given therein.
a ) S. Kulpe. 1. Seidel, E Herrmann, Crysf. Re.?. Trrhnol. 1984, f Y , 661; b) M. L.
.
1964.3. 1760; H. Richler. E. Fluck, H. Riffel. H. Hess. Z
Nielsen. I i i u ~ yCl7rm.
Anory. A & Chem. 1983,496.109, E. Herrmann, H. Nang, R. Dreyer. 2. Chem.
1979, l Y . 1x7, c ) O . Navratil, E. Herrmann, Collect. Czech. Chern. Commun.
1992.57.1655, and references therein; d ) see, for example, [Yb:{(H3C,0),0PN-PO(OC,,H,jl)~]: S. Kulpe, 1. Seidel. E. Herrmann, 2. Chem. 1981, 24,
333
The crystalline powder from reaction (a) was recrystallized from dry benzene at
room temperature under argon. Crystal structure determinatlon:
M , = 1666.2, crystal dimensions 0.66 x 0.52 x
(C,,H,,,NP,O,Na),(C,H,),,
0.40 mm. colorless prisms, temperature 200 1 K. a = 1398.4(1). h = 1758.5(1),
c =1788.9(l)pm, a =101.74(1j, 8=107.06(1). 7 =95.37(1)". V=4061.9(6).
10" pin". Z = 2, prillid = 1.362 g ~ m - ~p(MoK.)
.
=71.07 pm. triclinic, space
group Pi ( n o . 2 ) , Siemens P4 four-circle diffractometer, 14256 measured reflections within 3 1 2 0 1 50",13631 independent reflections ofwhich 10459 with
I > 20(/). structure solution by direct methods (SHELXS), 1043 parameters
refined on F(SHELXTL PC), R = 0.038, R, = 0.039. residual electron density
+ O 27:-0 3 2 e A ~ 3 S, =1.608. C, N, 0, P positions anisotropically refined.
hydrogens in ideal positions (d(C-H) = 96 pm) and refined isotropically using a
riding model with a temperature factor common to the phenyl rings. One molecule benzene is 3 3 % disordered; its main position was refined anisotropically
and minor positions refined isotropically as idealized hexagons. Further details
foi- the cryst;d structure determination can be requested from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), on
quoting the depository number CSD-58881.
l h e calculations were performed with the programs GAMESS (M. W. Schmidt.
K. K. Baldridge. J. A. Boatz, J. H. Jensen, S. Koseki. M. S. Gordon, K . A.
Nguyen. T. L Windus, S. T. Elbert. QCPE Bull. 1990.10,52) and MOPAC 6.00
(J J. P. Stewart. QCPE Programm 455, Indiana University, Bloomington) on
our work station IBM RISC 6000. The storage limit of 252 atoms required the
replacement of the phenyl groups by hydrogen. because the hexamer contains
374 centers a ) 'The a h initio calculations started from the structural data and
employed a "double zeta" basis set (S. Huzinaga. J. Andzelm, M. Klobukowski,
E Rad~io-Andzelin,Y . Sakai. H. Tatewaki, Guussian Bu.si) Sets for Moiecular
Orhitul C d d u f i n n , s ,Elsevier. Amsterdam, 1984). A potential energy plot yielded two preferred posirions for Na at the [O(HO),P-N'-P(OH),OJ- anion: in
the center of the chelate pincer with N a l . ' 0 distances of 233 and 234 pm, and
outside them with a distance of228 pm between Na3 and each oxygen atom. The
total enerzv
.. differences amounted to -603 ( N a l j and -462 kJmo1-I (Na3).
and the charge\ to + O 94 ( N a l ) and t 0 . 9 6 (Na3); b) based on the structural
data (Pig. I ) the following MNDO-enthalpies of formation (kJ mol-I) were
calculated: [(HO),OP-N-PO(OH),Na], for n = 1 -1358 and for n = 6 -9021;
for the differrncc AAHyYDo -873, for each monomer - 146 and for 6 Na'
+4515 and lor ( N a '),, + 7835; c) with the individual contributions of the fragmenty OIONalOZ(I. 030Na2040, and 050Na3060 of -1262, -1261. and
- 1251. respectively. a n enthalpy difference A A f l N D 0 of 8734 kJmo1-I for the
complexation (Na'), + hexamer and -4792 for 6 N a + + hexdmer (without
Coulomb repulsion is calculated, that is. an average stahihration of -200 for
each Na . O6 contact.
G. A. Petsko. Science 1985. 229, 23; J. Am. Chrm. Soc. 1986,
I
Is0 C. A. Hunter, J. Singh. J. Thornton. J. M o / . Biol. 1991, 218.
837, C A . Hunter, J. K. Sanders. J Am. Chmi. Soc. 1990, l f 2 , 5525; b) J.
Bernhtein. J. A. R. P. Sarma, A. Gavezzotti. Cheni. Phys. Lett. 1990, 174, 361;
c j P Hobza. H. L. Selzle. E. W. Schlag, J Am. Chem. Soc. 1994,116.3500. and
references therein.
Based on the miiiimum of the Morse curve at R, = 3.848 and Do = - 2.276 [4c]
variation yields a parameter a 2.7 for the optimum fit. Approximate distancedependent interdction energies (kJmol-') can thus be calculated. for 400 pm
- X OY. lor 500 pm - 0.85. and for 600 pm - 0.06.
a ) H. Bock, A John. C. Nither. Z. Havlas. Am. Cliem. Soc. 1995, 117. in
press. b) H.Bock. R. Dienelt. H. Schodel, Z. Havlds, J. Cliem. Suc. Cliem.
Conmmun. submitted: c) H. Bock, R. Beck, H. Schodel, 2. Havlas. unpublished
results; cf. R. Beck. Diplomarbeit, Universitat Frankfurt, 1995.
+
Aromatic Gallium Heterocycles: Synthesis of the
First Gallatabenzene""
A r t h u r J. Ashe HI,* Saleem Al-Ahmad, and Jeff W.
Kampf
Dedicated to Professor Herbert Schumann
on the occasion of his 60th birthday
Boratabenzene derivatives (1) have been intensely investigated for 25 years." -41 The high acidity of the precursor boracyclohexadienes (2),15]and the close resemblance of boratabenzene
metal complexes such as 3 with the corresponding cyclopentadienyl metal complexes[3.4, 61 convincingly demonstrate that 1 is
an anionic aromatic ring with six x electrons. Recent research
suggests that boron's heavier congener gallium may also be able
to form rt bonds with carbon;['*'] thus an exploration of the
chemistry of gallatabenzene should be of interest. We report
here on the first synthesis of an arylgallatabenzene (6) and structural data for its Mn(CO), complex (7).
M"(CO)3
2
I
3
In order to inhibit the Lewis acidity of the gallium atom
we chose the highly hindered arylgallatabenzene 6 (Ar =
tBu,C,H,) as our target. The precursor gallacyclohexadiene 5
may be prepared by an adaption of the method used by Jutzi
et al. to prepare silacycl~hexadiene.[~~
The reaction of (1Z,4Z)1,5-dilithio-l ,.l-pentadiene (4) with 2,4,6-tri(fert-butyl)phenylgallium dichloride['O1gives 5 as air- and moisture-sensitive
white crystals in 80% yield. The spectroscopic
properties are
.
completely consistent with the assigned structure. Compound 5
may be deprotonated with bases (tBuLi, Ph,CLi, lithium diisopropylamide, or fluorenyllithium) in T H F to afford yellow solutions of 6. Treatment of 5 with one equivalent of indenyllithium
in THF initially gives 6 and indene; however, on standing indenyllithium is reformed along with the conjugated gallacyclohexadiene 8. Quenching 6 with excess cyclopentadiene affords a
1 : 1 mixture of 5 and 8. Alternatively 5 may be completely converted to 8 by treatment with a trace of indenyilithium in THF.
~~
6
8
Thus, the acidity of 5 is bracketed between that of cyclopentadiene (pK, z 16)["] and that of indene (pK, z 21),["] while the
more stable 8 is less acidic than indene. Since 5 is considerably
[*I Prof. Dr. A. J. Ashe Ill. Dr. S. Al-Ahmad. Dr. J. W. Kampf
Department of Chemistry, The University of Michigan
Ann Arbor. MI 48109-1055 (USA)
Telefax: Int. code +(313) 747-4865
[**I This work was supported by the National Science Foundation.
A i i ~ r t i C%wn.
.
I m Ed. Ennl. 1995, 34. No. 12
c) VCH Verlags~e.sell.Fcha~tmbH, 0-69451 Weinhelm, 1995
Oj70-OX3319511212-1357 8 fO.OO+ 25iO
1357
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more acidic than acyclic 1,4-dienes (pK, = 31),[". 1 2 ] there is a
strong presumption that the gallium atom of 6 accepts n-electron density from the carbon atoms to form an aromatic ring
with six n electrons. However under similar conditions 2
(R = Me) is more acidic than cyclopentadiene.[lb.51 Thus the
driving force for aromatization must be smaller for gallatabenzene than for boratabenzene.
The 'H NMR spectrum of 6 in [DJTHF shows a characteristic first-order [AA'BB'C] pattern for the signals of the five gallatabenzene ring protons, while the signals in the ' 3C NMR
spectrum can be unambiguously assigned by using selective 'H13Cdecoupling. The 'H and I3C NMR chemical shift values of
1 (R = Me) and 6 are compared with those of the nonaromatic
cyclohexadienide 91i3]in Scheme 1. 3C NMR chemical shift
0 0
108 (6.18)
95 (4.95)
142 (7.50)
133 (7.28)
127 (6.47)
@I CH3
1
78 (3.7)
@k
6
123 (6.02)
0
c) ::::::'
H'
H
'
Table I . Spectroscopic data for 5-8
5 : ' H NMR (360 MHz. C,D,): 6 = 1.37 (s. p-rBu), 1.40 (s. o-tBu), 2.93 (m. CH,),
, CH(2.6)). 7.08 (dt, J = 1 4 , 3.7. CH(3.5)). 7.52 (s, m-CH);
6.73 (dt. 5 ~ 1 4 1.7.
13
C N M R (90.6 MHz, C,D,): 6 = 32.0 (p-rBu. CH,), 33.8 (o-tBu, CH,), 35.3 ( p 1Bu3C).36.2(CH,). 38.4(o-rBu, C), 120.9 (m-CH). 136.1 (ipso-c), 138,4(CH(2,6)),
calcd.:
149.3 (CH(3.5)). 150.3 (p-C), 157.7 (0-C); HRMS (EI): C,,H,,'%a:
380.1995; found: 380.2003
6: 'HNMR(360 MHz,[DJTHF):6 =1.42(s.p-rBu). 1.51 (s,o-/Bu).4,95(t,J= 9,
H4), 6.02 (d. J =12, H2.6). 7 30 (s. m-CH), 7.50 (dd, J = 12. 9, H3,S); "CNMR
(90 6 MHz, [D,]THF): 6 = 30.7 (p-tBu-C), 31.9 (o-rBu, CH,). 35.8 (p-rBu, CH,).
38.9 (o-tBu-C). 94.5 ('24). 119.5 (nl-CH). 123 (C2,6), 141.5 (C3.5). 145.0 (ipmC).
147.5 (p-C), 157.8 (0-C)
7 : ' H N M R ( ~ ~ ~ M H z , C =1.32(s.p-IBu).
~D~):~
1.39(s,o-rBu),2.82(d, J = 1 1 . 8 .
H2), 4.49 (t. J = 6.6. H4), 6.6 (dd. J-11.8, 6.6, H3). 7.47 (s, m-H); I3CNMR
(90.6 MHz, C,D,): 6 = 31.9 (p-rBu-Me). 34.4 (o-tBu-Me), p-rBu(C)), 37.8 (otBu(C)). 77.2 (C4), 77.5 (C2). 114.5 (C3), 120.8 (m-CH), 135.2 (ipso-C), 151.0
(p-C), 157.4 (0-C); IR (KBr): ;= 2011, 1937, 1914cm-'; HRMS (El):
C,,H,,h'GaMnO,:
calcd.: 51 8.1 144, found: 51 8.1 I47
8: 'H-NMR (360 MHz, [DJTHF): 6 = 1.32 (s, p-rBu), 1.43 (s, o-tBu), 1.69 (dd,
J=5.1.6.H6).5.91(m.H4).6.30(dt,J=5,2.5.H5).6.61(d.J=14.H2),7.01(dd,
5=14,6.3,H3),7.37(s,m-CH); 13C-NMR(90.6 MHz.[D,]THF):6 =157.7(0-C),
150.3 ( P - C ) ,134.6 ( ~ P s o - C145.7.
),
134.6, 120.0, 115.9 (C2-C5), 121.2 (m-CH), 38.7
(o-rBu(C)), 35.6(p-rBu(C)), 33.8 (o-tBu-CH,). 31.9(p-rBu-CH,), C6not observed;
HRMS (EI): C,,H,,h'Ga: calcd.: 380.1995. found: 380.1999
9
SC2l
Scheme 1. Comparison of the 13C NMR and the ' H NMR (in parentheses) chemical shift values for anions 1 (R = Me), 6, and 9 in [D,]THF
c22
values have been used extensively to evaluate n-charge densities
of car bani on^"^. 14] and hetero~arbanions.['~]
The spectrum of
cyclohexadienide (9) is typical for n-delocalized carbanions.[' 31
The signals for the negatively charged carbon atoms C1, C3, and
C5 occur approximately 50 ppm higher field than those for the
uncharged carbon atoms C2 and C4. The spectra of 1 (R = Me)
and 6 deviate from this pattern in that the upfield shifts of the
corresponding nuclei are much smaller, which is consistent with
diminished negative charge at these carbon centers due to n-electron donation to the respective heteroatoms. In the same manner
the 'H NMR chemical shifts of the ring protons of 1 (R = Me)
and 6 are shifted substantially downfield from the corresponding
signals for 9. This effect is consistent with ring current effects
and/or diminished negative charge at carbon.['] Thus the NMR
spectra suggest that gallatabenzene 6 has aromatic character.
In order to explore further the chemistry of 6 we have
prepared the Mn(CO), complex 7. The reaction of 6 with
[Mn(CO),(CH,CN),]PF, affords 7 in 46% yield as yellow air-
and moisture-sensitive crystals. An X-ray structural analysis
allowed determination of the molecular structure of 7 as illustrated in Figure 1.1161The structure of 7 shows that the gallatabenzene ring is q 5 coordinated through its planar pentadienyl moiety to the Mn(CO), group. The ring is folded so that the
C4-C5-C6-C7-CX and C4-Ga-CX planes intersect at 34.5". While
the five C-Mn bonds are of normal length (2.12-2.30
the Ga-Mn distance of 3.03 A is much longer than the 2.50 A
reported for a Ga-Mn bond of a related
The three
C-Ga bond lengths of the trigonally surrounded Ga atom are
not significantly different and fall in the expected range (1.941.96 A) for single bonds. The plane of the aryl substituents is
perpendicular to the C4-Ga-C8 plane. Thus the structure of 7
provides no evidence for Ga-C n bonding.
1358
C YCH V e r l r i g ~ ~ e s P l l . smhH,
~ h u ~ ~D-6Y451 Weinheini, 1995
cl8
C25
02
Fig. 1. Solid-state structure of 7 (ORTEP). Selected distances [A] and angles ["I.
Gal-C8 1.936(3). Gal-C4 1.942(4), Gal-C9 1.961(3), Gal-Mnl 3.0290(7), Mnl-C4
2.303(3), Mnl-CS 2.161(4). Mnl-C62.121(4). Mnl-C7 2.152(4), Mnl-C8 2.291(3).
C4-C5 1.396(5), C5-C6 1.416(5). C6-C7 1.434(6), C7-C8 1.387(5); C9-Gal-C4
132.75(15), C9-Gal-C8 133 78(14), C4-Gal-C8 93.5(2).
However, the electronic isolation of the Ga atom in 7 is a
probable consequence of steric effects. A displacement of the Ga
atom towards the ring plane, which is necessary to achieve v6
coordination, would involve severe steric
compression of the ortho rert-butyl group
(C24, C26) and the carbonyl (C303). In a
relevant related study Cowley et al. have
found that the planar 1-(2,4,6-tri-tertbutylphenyl)-2,3,4,5-tetramethylgalloledisCH3
torts on complexation to a CpCo group to
form 10 by folding the Ga atom away from
the metal.[81We intend to prepare derivatives of 7 with smaller substituents on galli10
um which may show q6 coordination.
cH3)7$;
&)
Experimental Procedure
5: A solution of 4, prepared from 5.25 mmol butyllithium in hexane (2.1 mL) and
(lZ,42)-1,5-diiodohexd-l,4-diene
(0.84 g, 2.63 mmol) in diethyl ether (30 mL), was
added dropwise with stirring to a solution ofArGaC1, (1.01 g, 2.63 mmol) in diethyl
ether (20 mL) a t -78'C. The reaction mixture was allowed to warm to 25°C over
90 min, after which the solvent was removed under vacuum. The residue was extracted with pentane (40 mL). The volume of the extracts was reduced under re-
n57o-#83.~~~Sl1212-$
i 3iO.OO+
5~
2510
Angrn. Cliem. In/. Ed. Engl. 1995, 34, No. i d
COMMUNICATIONS
duced pressure to 15 mL. Cooling to -78' C afforded 0.8 g (80%) of 5 as white
crystals: m.p. 127 'C.
6: To a solution of 5 (20 mg, 0.051 mmol) in [D8]THF (0.5 mL) at 78' C was
added a solution of teur-butyllithium (0.051 mmol) in pentane (30 pL). O n warming
to 25 "C the solution became yellow. The NMR spectrum was consistent with the
formation of 6. It showed only signals of 6. the solvent, and the conjugate acid of
the base. An identical spectrum was recorded for 6 prepared from 5 and fluorenyllithium. Compound 6 did not react with excess fluorene but was quenched to a 1 : 1
mixture of 5 and 8 by treatment with excess cyclopentadiene.
8: A solution of 5 (50 mg, 0.13 mmol) in THF (5 mL) was treated with a trace of
indenyllithium (3 mg) for 24 h. Removal of solvent left a waxy solid which was
recrystallized from pentane to give 42 mg (84%) of 8; m.p. 159°C.
7: A solution containing one equivalent of lithium diisopropylamide in diethyl ether
(10 mL) was added dropwise to a stirred solution of 5 (0.4 g, 1.05 mmol) in diethyl
ether (15mL) at -78°C. The reaction solution was allowed to warm to 25°C
affording a yellow solution, which was cooled to - 78 "C and added dropwise to a
suspension of [Mn(CO),(CH,CN),]PF6 (0.43 g31.05 mmol) in diethyl ether (10 mL)
at -78 "C. The reaction mixture was allowed to warm to 25°C and stirred for 3 h.
The solvent was removed under reduced pressure. and the residue was extracted
with pentane (40 mL). Reduction of the volume to 20 mL and cooling at - 20°C for
24 h afforded 0.25 g (46% yield) of yellow crystals of 7 ; m.p. 189 'C.
-
Received: December 13. 1994
Revised version: February 21, 1995 [Z 7544 IE]
German version: Angew. Chcm. 1995, f07, 1479--1481
Keywords: arene complexes . gallium compounds . heterocycles
manganese compounds
[ l ] a) G. E. Herberich, G. Greiss, H . F. Heil, Angevv. Chem. 1970.82. 838; A n g ~ w .
Cheni. Int. Ed. Engl. 1970, Y, 805: b) G. E. Herberich, B. Schmidt, U. Englert.
0rpnometullic.s 1995, 14, 471
[2] A. J. Ashe 111, P. Shu, J. A m . Chem. Soc. 1971, Y3. 1804.
13) G. E. Herberich. H. Ohst, Adv. Orgunomrt.Chmz. 1986. 25. 199.
[4] G. E. Herberich in Comprehensive Orgunomrtullic Chcnri.vfry. Val. f (Eds.:
G. Wilkinson, F. G . A . Stone, E. W. Abel), Pergamon, Oxford. 1982,
p. 381.
[5] S. A. Sullivan, H. Sandford, J. L. Beauchamp, A. J. Ashe Ill, J. A m . Chem.
Soc. 1978, 1/10, 3737; H . Sandford, Dissertation, University of Michigan,
1979.
[6] A. J. Ashe 111, E. Meyers, P. Shu. T. Von Lehmann, J. Bastide, J. A m . Clieni.
Soc. 1975, 97, 6865.
[7] A. H . Cowley, F. P. Gabbai', C. J. Carraho. L. M. Mokry, M. R. Bond. G .
Bertrand, A n g e ~ ,Chem.
.
1994,106, 584; Angew Chem. Int. Ed Engl. 1994,33,
578.
[8] A. H. Cowley, F. P. Gabbai', A. Decken, Angew. Clwm. 1994, 106, 1429;
Angew. Chem. I n f . Ed. Engl. 1994, 33, 1370.
[9] P. Jutzi, J. Baumglrtner, W. Schraut, J. Organornet. Chem. 1977. 132, 333.
[lo] M. A. Petrie, P. P. Power, H. V. Rasika Dias, K. Ruhlandt-Senge, K. M. Waggoner, R. J. Wehmschulte, 0rgunometuliic.s 1993,12, 1086; S. Schulz, S. Pusch,
E. Pohl, S. Dielkus, R. Herbst-Irmer, A. Meller, H . W. Roesky. h o g . Chem.
1993,32, 3343.
[ l l ] The pK:, values in ethers are not thermodynamic values but are reasonably
comparable with pK, values determined in cyclohexylamine or DMSO: A.
Streitweiser, Jr., E. Juaristl, L. L. Nebenzahl in Comprehemive Curbunion
Chemtsfrj, Purl A (Eds.: E. Buncel, T. Durst), Elsevier, Amsterdam, 1980.
p. 323.
[I21 F. G. Bordwell, G. E. Drucker, H . E. Fried. J. Org. Chrm. 1981,46,632;T. B.
McMahon, P. Kebarle, J An?. Chem. Soc. 1974, 96, 5940.
[13] G. A. Olah, G. Asensio, H . Mayr, P. von R. Schleyer. J Am. Chem. Soc. 1978,
100,4347.
[14] a) D. H . O'Brien in Comprehen.sivr Curbunion ChemOtry, Purf A (Eds., E.
Buncel, T. Durst), Elsevier, Amsterdam, 1980, pp. 271 -322; b) D. H . O'Brien,
A. J. Hart, C. R . Russell, J. Am. Chrm. Soc. 1975, 97, 4410; c) H. Spiesecke.
W. G. Schneider, Tetruhedron Left. 1961, 468.
[15] P. Jutzi, M. Meyer, H. V. Rasika Dias. P. P. Power, J. Am. Chcm. So?. 1990,
1t2,4841.
[16] 7: Monoclinic, space group C2/c (No. 15) with u =14.222(2), h =18.206(2),
c = 20.364(2) A,
/l=110.08(1)',
V = 4952(1) A',
Z =8
( P , , ~=
~~
1.393'gcrn-')), T = 1 7 8 K, (Mo,,, 7. = 0.71073 A. p =16.22cm-'; 4872
unique reflections were used in refinement (SHELXTL-93); R ( I t 2a(I)):
R, = 0.0352, wR, = 0.0638, GOP = 0.804 (all data); R , = 0.0715, ICR, =
0.0692. Further details of the crystal structure investigation are available on
request from the Director of the Cambridge Crystallography Data Centre, 12
Union Road, GB-Cambridge CB2lEZ (UK) on quoting the full journal citation.
[I71 M. R. Churchill, F. R. Scholer, lnoug. Chem. 1969.8, 1950.
I181 A. H . Cowley, A. Decken, C. A. Olazabal. N . C. Norman. Inorg. Chem. 1994.
33, 3435.
Anxew. Chem. I n f . Ed. Engl. 1995. 34, N o . t2
Metal 0 x 0 Cation Receptors: Multimode
Coordination to the Dioxoosmium(v1) Cation**
A. S. Borovik, Justin Du Bois, and
Kenneth N. Raymond*
The selective recognition of metal ions conventially has been
achieved by using a host with an appropriately sized spherical
cavity that matches the ionic radius of the metal guest.[',21In
contrast, the selective binding of nonspherical species often relies on combinations of weak and strong bonding interactions
-~'
between a molecular host and its nonspherical g ~ e s t . [ ~Successful binding requires proper spatial alignment of recognition
sites between the host and guest.
We are developing new molecular systems that utilize multimode coordination to bind nonspherical metal 0x0 cation^.[^-'^
The approach takes advantage of the chemical and morphological properties of the metal 0x0 cations by using ligands that can
simultaneously stabilize the highly oxidized metal center
through coordinate-covalent bonds, while interacting with the
0x0 moieties through intramolecular hydrogen bonds. To examine the requisite structural features for multimode binding to
metal 0x0 cations, we have synthesized a series of tetraamidato
ligands with appendages containing hydrogen bond donors. In
this report we present electrochemical evidence showing that
these ligands provide significant stabilization of the dioxoosmium(vr) cation. Furthermore, X-ray crystallographic results
show that the orientation of the appended hydrogen bond
donors can be varied and positioned toward the 0x0 moieties of
a bound osmyl cation to achieve multimode binding.
The multimode bonding approach has proven successful for
the selection and stabilization of metal - ligand adducts. Recent
examples of molecular systems that combine covalent and intramolecular hydrogen bonds include an Fe" porphyrin system
that stabilizes an 0, adduct,"'] uranyl Schiff base complexes
that recognize H,PO,,['
and a Pd" macrocyclic species that
selectively binds nucleobases.[' 21 The ligands described here
have been constructed with a binding pocket composed of two
N-acylated 1,2-diaminobenzene units fastened together with a
diethyl malonyl strap. The strongly basic nitrogen atoms of the
amidato ligands effectively stabilize highly oxidized metal centers, as has been shown by the elegant work of Collins
et a1.[13-'61The nitrogen atoms opposite the malonyl strap in
the H,-1 and H,-2 ligands used here (Scheme 1) are components
of urea groups whose function is twofold: the urea nitrogen
atoms attached to the aryl supports can be deprotonated to
complete the tetraamidato metal ion binding pocket, while the
tethered C(0)NHR moieties provide hydrogen bond donors to
the oxygen atoms of the 0x0 ligands of the osmyl cation. CPK
models suggest that hexadentate chelate formation with intramolecular hydrogen bonds is energetically favored through a
side-on approach of the appended ureas to the 0x0 oxygen
atoms. To test these design concepts the osmyl complexes of
H,-1 and H,-2 have been synthesized. These complexes have
been used to examine both the oxidative stability of a high
valent metal center in a tetraamidato pocket as well as the extent
of intramolecular hydrogen bonding between the urea moieties
[*] Prof. K. N . Raymond, Dr. A. S. Borovik, J. Du Bois
Department of Chemistry
University of California
Berkeley, CA 94720 (USA)
Telefax: Int. code (510)486-5283
[**I Stereognostic Coordination Chemistry, Part 4 -Part 3: [9]. This research was
supported by the United States National Science Foundation Grant no. CHE8919207. We thank Dr. Fred Hollander and Dr. Sonya J. Franklin for their help
in determining the solid-state structures.
(3 VCH ~ ~ r l u ~ . s ~ e s e / l mhH,
. ~ r h uD-6Y4S1
/~
Weinheim, I Y Y S
+
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1359
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