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Axial Ligation of Diazacyclooctanenickel and -Zinc Complexes.

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4 a : Compound 4 a was similarly prepared from AgNO, (0.375 mmol).
[(RhCp*Cl,],] (0.125 nimol). and l a (0.25 mmol) (yield: 50%). IR (KBr)
?[cm-'] = 1635 (NO); 940. 855 (Moo,).
O r
Received: August 4. 1992 [Z 5502 IE]
German version: AngrM. Chern. 1993. 105. 81
W 013
Fig. 1. Structure of the anion of 3b. The anion lies at a special position with
crystallographic m symmetry, the mirror plane passes through the M o l , N1,
0 1 , 010. 0 2 . 03. R h l . and 0100 atoms. Selected bond lengths [A1 and bond
angles ['I: Mol-Nl 1.77(3), N1-01 1.21(4), M o l - 0 1 0 2.14(2), Mol-032
2.01(2). Mo2-012 2.20(1). M o l - 0 1 3 1.94(2) Mo3-013 2.33(1). Mo2-022
1.66(2), Mo3-032 1.69(2), Mo2-021 1.75(2), Mo3-031 1.70(1), Rhl-021
2.11(2), Rhl-0100 2.13(2), Mol-N1-01 177.9(27), 010-Mol-N1 174.8(11),
M01-012-Mo2 108.2(7). Mo1-013-Mo3 106.1(6), 021-Rhl-0100 86.2(7).
021-Rhl-021' 82.6(8).
N1' P
[I] W. A. Herrmann. Conimenrs Inorg. Chem. 1988. 7. 73.
[2] F. Bottomley, L. Sutin. Mi,.Orgunun?rt Cliern. 1988. 28, 339.
131 V. W. Day, W. G. Klemperer, Srrmre 1985, 228, 533
[4] M. T. Pope. A. Muller. Angew. Cliem. 1991. 103, 5 6 ; Angen.. ('hem. Int. Ed.
Engl. 1991. 30. 34.
[5] A. R. Siedle, C. G . Markell, P. A. Lyon, K. 0. Hodgson, A. L. Roe, Inorg.
Chem. 1987. 26, 220.
161 For recent work, see: a ) D . K. Lyon, W. K. Miller, T. Novet, P. J. Domaille. E. Evitt, D. c . Johnson, R . G . Finke. J. An7. C h ~ m
Soc. 1991. l l s ,
7209; b) L. A. Combs-Walker. C. G. Hill, Inorg. Chem. 1991, 30, 4016;
c) C. Rong. M. T. Pope, J. Am. Chem. SOC.1992, 114. 2032.
[7]a) H. K. Chae. W. G . Klemperer, V. W. Day, Inorg. Client 1989,28. 1424:
6) Y. Hayashi. Y. Ozawa, K. Isobe, ibid. 1991, 30, 1025.
[8] a) C . J. Besecker. V. W. Day. W. G. Klemperer. M. R . Thompson. Inorg.
Ch(v~7.198527.44; b) C . J Besecker. V. W. Day, W. G . Klemperer. M. R .
Thompson. J Am. Clieni. Soc. 1984, 106, 4125; c) C. J. Besecker. W. G .
Klemperer, V. W. Day, ibid. 1982. 104, 6158; d) W. G . Klemperer. D. J.
Main, lnorg C/iem. 1990, ZY, 2345 and 2355.
[9] V. W. Day. M. F. Fredrich. M. R. Thompson, W, G. Klemperer, R. S.Liu.
W. Shum. A m C/7Cm. So?. 1981. 103. 3597.
[lo] R. G. Finke. M. W. Droege. J. Am. C h m . Soc. 1984, 106. 7274; R. G.
Finke. B. Rapko. P. J. Domaille. OrgunomefuNics1986, 5 , 175.
[ l l ] a) D. J. Edlung, R. J. Saxton, D. K. Lyon. R. G. Finke. OrgunometuNics
1988.7.1692:b) R. G. Finke, D. K. Lyon. K. Nomiya, S. Sur. N. Mizuno,
Inorg. C h i . 1990. 29, 1784.
[12] V. W Day. W G . Klemperer, A. Yagasaki, Cheni. Let!. 1990, 1267.
1131 a) Y Hayashi. K. Tonumi, K . Isobe, J. Am. Chcm. Soc. 1988. 110, 3666;
b) Y. Do. X.-Z. You. C. Zhang. Y. Ozawa, K. Isobe. ibid. 1991, 113. 5892.
[14] P. Gouzerh, Y. Jeannin, A. Proust, F. Robert, Angew. Chem. 1989, 101.
1377; Angew C'hein., I n / . Ed. Engl. 1989, 28, 1363.
[15] 3b: orthorhombic. Pmnn, M, =1208.16, 2 = 4. a = 10.969(5), b =
I3.227(5). c = 27.762(10)
V = 4028(3)
=1.99 gem-'.
1 < 8 < 2 3 , i.(MoK,)=0.71069Bi. p = l 9 . 4 0 c m - ' . R = 0 . 0 5 4 , R,. =
0.062. 2863 unique reflections collected; direct methods; 1232 reflections
( I 2 3a(/)) were used for the least-squares refinement; 242 refined
parameters. 4b . CH,CI,: orthorhombic, Prnnh, M , = 1482.94, 2 = 4.
LI =15.884(4). b =16.512(4). c = 17.650(4) A, V = 4629(2) A',
1.5 < 8 < 2 8 ,
i.(MoK,)=0.71069A. i1=29.99cm-',
R = 0.045, R, = 0.054. 5771 unique reflections collected; direct methods;
3164 reflections (it3 4 1 ) ) were used for least-squares refinement;
274 refined parameters. For both structures final refinement was carried
out with anisotropic thermal parameters for all atoms except the carbon
atoms in one disordered Cp* ring of4b. CH,CI,; hydrogen atoms were not
introduced. Further detaik 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-56624.
the names of the authors and the journal citation
Fig. 2. Structure of the anion of 4b. The anion lies at a special position with
crystallographic m symmetry, the mirror plane passes through the M o l , N1.
0 1 , 010, 0 2 , 0 3 , Rh2, Rh3, and Brl atoms. The Cp* ligand attached to Rh3
and bond angles ["I: Mol-N1 l.78(1),
is disordered. Selected bond lengths
N l - 0 1 1.16(2), M o l - 0 1 0 2.123(9), Mo2-021 1.753(8), Mo3-031 1.740(8),
Mo2-022 1.688(8), Mo3-032 1.683(8). Rh2-021 2.100(8), Rh3-031 2.118(8),
Rh2-Brl 2.539(2). Rh3-Brl 2.550(3), Mol-N1-01 178.7(19), 010-Mol-N1
177.5(7), 021-Rh2-Brl 91.4(2), 031-Rh3-Brl 92.4(2) 021-Rhl-021' 82.6(4).
031-Rh3-031' 81.4(2), Rh2-Brl-Rh3 125.70(8).
ported on polyoxomoiybdates where the organometallic
fragment is bound to the terminal oxygen atoms of the open
face of a lacunary polyoxoanion. They demonstrate the
unique properties of [MosOi,(OCH,),(NO)]3- as a ligand.
Experimental Procedure
3 a : AgNO, (0.084 g, 0.5 mmol) was added to a solution of [{RhCp*CI,),]
(0.078 9. 0.125 mmol) dissolved in CH,OH (10 mL). After the solution had
been stirred for approximately 30 min, AgCl was filtered off and washed with
methanol (5 mL); the filtrate was added to a solution of l a (0.34 g, 0.25 mmol)
in methanol (5 mL). The mixture was reflitxed for 7 h. during which time 3a was
deposited as an orange-brown solid. It was collected by filtration (yield: 35%).
IR(KBr): C[cm-'] = 1625 (NO); 935, 895. 855 ( M o o , ) .
1 16
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VCH Verlag.sgr~.~rNsr.b~~fr
m h H , W-6540 Wemhrun,15Y3
Axial Ligation of Diazacyclooctanenickel and
-Zinc Complexes**
By Dawn C. Goodman, Thawatchai Tuntulani,
Patrick J. Furmer, Marcetta Y: Darensboui-g,*
and Joseph H . Reibenspies
The 1,5-diazacyclooctane (daco) molecule has proven to
be a useful precursor for building ligands with various donor
atoms, including those of import to the biological chemistry
I*] Prof. Dr. M. Y. Darensbourg. D. C. Goodman, T. Tuntulani, P. J. Farmer.
Dr. J. H. Reibenspies
Department of Chemistry. Texas A & M University
College Station. TX 77843 (USA)
[**I This work was supported by the National Institutes of Health ( R 0 1
GM44865-01) and the National Science Foundation (CHE-8513273 for
purchase of the X-ray diffractometer and Crystallographic computing system; CHE-8912763 for the EPR spectrometer). We thank Prof. Paul Lindahl for helpful discussions.
10.00+ 2 5 0
Angew Chem. In!. Ed. EngI. 1993. 32, N o . 1
of nickel-N, S, and O.[' - 51 The coordination of a metal
center M results in the formation of two diazametallacyclohexane rings which share an N-M-N unit. A plethora of
X-ray crystal structures has established that the common
form of the two six-membered rings in metal-bound daco is
the boat/chair form (see 1, Equation (a)). Due to the steric
hindrance of the B-methylene CH, group in the boat/chair
form, axial ligation at the metal is blocked on one side, thus
accounting for examples of tetra- and pentacoordinate
Nil',[' 31 and pentacoordinate Co" and Co"' complexes.r41
This conformational preference is rationalized as avoiding
cross-ring methylene C-H interactions that might result
from the chair/chair form of the metalladiazacyclohexane
rings. Nevertheless, MM2 calculations that we[61and others
conducted suggest that, in the absence of C-H/metal agostic
interactions, the differences between the two conformers
represented in structure I are minor. In fact, the conformation lower in energy, the chair/chair conformation, is the one
typically not seen in the X-ray crystal structures thusfar reported.'*-".
l o ] Although disorder in the structure of 1,5diazacyclooctane-I ,5-diylbis(ethylthiolato)nickel(rr)
( l)I5'
may be indicative of a mixture of chair/chair and boat/chair
conformers, bona fide examples of exclusive chair/chair conformers are rare.[']
The methylation of 1 yields N,Nr-bis(3-thiabutyl)-1 ,S-diazacyclooctanenickel(I1) iodide (2),['] suggesting that the nucleophilic character of the nickel-bound thiolate sulfur
atoms may be engaged to extend the denticity of the N,S,
ligand by the introduction of additional donor atoms that
can bind to the Ni atom in axial positions. We report here on
the X-ray crystal structure of a Ni complex with a hexadentate N,S,O, ligand and that of a Zn complex with a pentadentate N,S,O ligand. These structures aptly illustrate that
the steric shielding of the metal center by the daco ligand
may be overcome when the metal atoms can attain a preferable coordination number and when ligands with strong
binding abilities are employed.
Blue, paramagnetic A-1 S-diazacyclooctane-1,5-diylbis
(3thiapentanoato)nickel(II) (3), analytically pure as the trihydrate, was obtained as illustrated in Equation (a) and crystallized in the tetragonal space group P4,2,2 (no. 96) o r P4,2,2
(no. 92).11 Analysis of structural refinement indicated that
-= boatrehair
,,*...,.=. chaidchair
3 ' 1 ,
P4,2,2 was the correct space group."'] Each chiral nickel
complex molecule in the asymmetric unit cell possesses a C ,
axis and is hydrogen-bonded through the distal oxygen of
the carboxyl group into a three-dimensional network of water molecules. This serpentine of hydrogen bonds through
the lattice has no influence on the hydrocarbon portion of
the hexadentate ligand and its directionality results from the
chirality of the space group of 3. As illustrated in Figure 1,
the Ni" resides in a slightly distorted octahedron whose
equatorial positions are taken up by N,S, units and the
apexes by 0, donor atoms. The Ni-S distance of 2.435(2) 8,
is within the usual range reported for six-coordinate Nil1SR, complexes (d = 2.416 A),[',] but more than 0.22 8,
Fig. 1. Crystal structure of 3 . 3H,O (thermal ellipsoids at 50% probability).
The atoms labeled a are related to the corresponding unlabeled afoins by a C ,
axis; hydrogen atoms on waters removed for clarity. Selected bond lengths [A]
and angles ["I: Ni(l)-S(l) 2.435(2), Ni-O(1) 2.067(3); Ni-N(l) 2.094(4). O(1)C(7) 1.261(6); C(7)-0(2) 1.254(6); S(1)-Ni-S(1a) Y8.5(1). N(1)-Ni-N(1a)
85.7(2); S(l)-Ni-N(l) 88.6(1), O(1)-Ni-O(1a) 170.6(2); O(I)-NI-S(I) 8O.Y(1),
O(l)-Ni-N(l) 96.7(1); C(3)-C(4)-C(5) 118.2(5); C(3)-N(l)-C(Sa) 112.5(4); NiS(l)-C(6) Y5.0(2).
longer in 3 as compared t o the tetradentate square-planar
complex 2 (d = 2.211(3), 2.204(3) A).['] The C-0 bonds
have essentially the same length, regardless of whether the 0
atom is bound directly to nickel o r distal to the metal center
and hydrogen-bonded to water molecules. The -CH,-CO
unit eclipses the Ni-S bond, creating fairly planar five-membered NiSC,O rings whose planes form a dihedral angle of
100 '; in other words, there is an opening of the S-Ni-S angle
by about 10" as compared to complexes 1 or 2, which show
regular 90" angles in the N,S,Ni plane.
The most striking feature of the structure of 3 is the uncommon chair/chair conformation of the diazametallacyclohexane rings. Some steric repulsion of the fi-CH, groups in
the folded-back daco ring is evident by the distortion of the
bond angles at C5 and C4 from tetrahedral; bond angles of
118.0(4)" and 118.2(5)" are obtained for Nla-C5-C4 and
C3-C4-C5, respectively.
Attempts to generate a pentacoordinate Ni" complex by
addition of only one equivalent of ICH,CO,Na to 1 led to a
green product, which on subsequent addition of a second
equivalent of ICH,CO,Na yielded blue 3. Elemental analysis of the green material showed it to be the sodium salt of the
complex anion iodo(5-sulfidoethyl-1,5-diazacyclooctyl)-3thiapentanoatonickelate(r1). This indicates that the axial
oxygen donor atom promotes hexacoordination to the Ni
center.[61In contrast, ICH,CO,Na did not react with the
dimeric zinc complex 4.
However, from the reaction of 4
with a fourfold excess of CICH,CO,H, low yields of 5 . H,O
were obtained [Eq. (b)].
mhH, W-6940 Wernherm, 1993
$ 10.00f 25/fi
The X-ray structure["] of 5 (Fig. 2) provides interesting
structural contrasts to that of compound 3. Notably, the
pound which is long-lived even at 22 "C. The simple features
of its EPR spectrum, measured on a frozen solution sample,
are consistent with a single EPR-active species with nearly
axial symmetry (Fig. 3 b). Spectral simulation (Fig. 3 b) reveals a smalI anisotropy in the g, feature; the best-fit values
are g,, = 2.190, g,, = 2.170, and g,, =
EPR features indicate that the unpaired electron resides in
the dzi metal orbital of an elongated octahedral d 7 metal
complex.['5. 1 6 ] Ho wever, in the absence of any hyperfine
interactions from donor atoms, assignment of the z axis and
structural inferences based on EPR spectra are equivocal.
Experimental Procedure
Preparation of 3 : Addition of two equivalents of ICH,CO,Na dissolved in
methanol to a purple solution of 1. in CH,CN at 60°C with stirring results in
a color change, first to brown. then gradually to a forest-green. and finally to
aquamarine-blue over the course of 4 h. Filtration and slow evaporation of
solvent in an open vessel gave blue, paramagnetic crysials of 3 (H,O), (H,O
from the hygroscopic Na acetate and from the air) in over 90% yield. Correct
C.H.N analysis. UV'VIS: i.,,,[nm] ( E ) = 600(49), 364(31): IR(KBr): i.
[cm-'1 =1593. 1385.
Fig. 2. Crystal structure of 5 . H,O (thermal ellipsoids at 50% probability).
Selected bond lengths [A] and angles
Zn-S(l) 2.295(3); Zn-S(2) 2.584(2);
Zn-N(I) 2.184(6): Zn-N(2) 2.189(7), Zn-O(1) 2.005(5) 0(1)-C(12) 1.279(9):
C(12)-012) 1.232(8). 0(2)-0(lw) 2.85(1): S(l)-Zn-S(Z) 95.6(1); N(l)-Zn-N(2)
83.X(2): S(I)-Zn-N(l) 88.5(2); S(Z)-Zn-N(2) 84.3(2); S(I)-Zn-N(2) 153.6(2);
S(2)-Zn-N(1) 161.0(2); O(l)-Zn-S(2) 82.6(1); S(l)-Zn-O(l) 112.1(2): C(3)-N(I)C(6) 1 11.2(6): C(3)-C(4)-C(5) I15.8(7). Zn-S(Z)-C(I I ) 93.5(2).
diazametallacyclohexane rings are in their usual chair/boat
configuration with tetrahedral bond angles at the C atoms.
The zinc ion is 0.431 A above the N,S, best plane, and shows
a distinct difference in distances to sulfur: d (ZnSthloether)
= 2.584(2) A, whereas d (Zn-Srhiolale)
= 2.295(3) A.
In contrast to 3, there is a difference in the lengths of the two
C-0 bonds; that directed toward the zinc atom is some
0.05 A shorter than that whose oxygen atom forms a hydrogen bond to the one water of crystallization. The NCH,CH,S
arms are eclipsed across the MN,S, plane in 4; in 3, they are
In contrast to 1 and 2 (as well as to other sulfidoalkyl- or
thioether/daco derivatives), which have electrochemically
accessible Ni"" couples,[51complex 3 shows no reduction to
- 2.0 V (cyclic voltammogram in acetonitrile referenced to
Ag "/AgCI). However, the oxidation at +0.99 V is reversible
(Fig. 3 a) and, as shown by the following EPR results, takes
place at the metal center.
+ t.0
+ 0.8
Fig. 3. a) Cydic voltammograin of 3 (2 mM) in CH,CN. 0 I M Bu,NPF,.
€, = 989 mV (Ag:AgCl reference), with A€A,c = 65 mV, ip:ip. = 0.91. h) Observed and calculated X-band EPR spectra for 3 (4 mM) in CH,OH bei 10 K
( Y = 9.419 GHz). Calculated y-values: 2.190. 2.170. 2.024.
Chemical oxidation of 3 with cerium ammonium nitrate
produced an air-sensitive, brick-red, paramagnetic com118
VCH mbH. W-6940 Wmheim, I993
Preparation of 5 - A solution of CICH,CO,H (70 mg, 0.67 mmol) in 5 mL of
anhydrous CH,OH was added to 4 (100 mg, 0.17 mmol) in a 50:SO mixture of
hot CH,OH!CH,CN and refluxed for 1 h. A small amount of white solid
precipitated upon cooling and stirring overnight. Recrystallization from pyridine!ether yielded colorless crystals in 15% yield. Correct C. H. N. analysis.
IR(KBr): ?[cm-'] =1633, 1344.
Oxidation of 3 for EPR sample: A 42 mg (91 mmol) sample o f 3 was dissolved
in 20 mL of anhydrous, degassed MeOH and stirred under N2. To this blue
solution was added with stirring one equivalent of cerium ammonium nitrate
dissolved in MeOH. A 500 pL brick-red sample for EPR study was withdrawn
and immediately frozen at -70 C. The red color of the frozen sample remains
indefinitely: however. in the course of 30 min at 22.C. the reaction mixture
changed from red back to the original blue. EPR data on frozen solutions were
obtained at 77 K on a Bruker ESP 300 spectrometer equipped with an Oxford
Instruments ER910 A cryostat. An NMR gaussmeter (Bruker ER035M) and
Hewlett Packard frequency counter (HP5352B) were used to calibrate the field
and microwave frequency, respectively.
Received: July 27, 1992 [Z 5482 IE]
German version: Angen. Chem. 1993, lU5, 72
[I] W. K . Musker. M. S. Hussain. fnorg. Chem. 1966, 5. 1416.
[?I J. C. A. Boeyens, C. C. Fox. R. D. Hancock, fnorg. Chim. Acia 1984, 87,
1 ; D. J. Royer. V. H. Schievelbein. A. R. Kalyanaraman. J. A. Bertrand.
ihid 1972, 6. 307.
[3] D. 0. Nielson. M. L. Larsen. R. D. Willett, J. I. Legg. J. Am. Chem. Sor.
1971.93.5079; D. F. Averill, J. I. Legg, D. L. Smith, fnorf. Chem. 1972. I f .
(41 W. E. Broderick, K . Kanamori, R. D. Willett, J. I. Legg, Inorg. Chem.
1991.30.3875; K. Kanamori, W. E. Broderick, R. F. Jordan, R. D. Willett,
J. 1. Legg. J A m . Chem. Sor. 1986, IUR, 1122.
[ S ] D. K. Mills, J. H. Reibenspies, M. Y. Darensbourg. Inorg. C/iem. 1990,29.
161 Complex 3 is the first reported example o f a complex with diazametallacyclohexane rings with a common N-M-N unit which is exclusively in the
chair!chair configuration. A second example of a hexacoordinate nickel
derivative prepared by alkylation of 1 with ICH,CH,OCH,CH,I has been
prepared recently: M. Y Darensbourg. 1. Font, D. K. Mills, M. Pala. J. H.
Reihenspies. lnorg. Chem , 1992. S t . 4965.
171 R. D. Hancock. M. P. Ngwenya, A. Evers. P. W. Wade, J. C. A. Boeyens.
S. M. Dobson. Inorg. Ckem. 1990, 29. 264.
[S] P. J. Farmer. T. Solouki. D . K. Mills. T. Soma. D. H. Russell, J. H. Reibenspies. M. Y. Darensbourg. J. Am. Cllem. Soc. 1992, lf4,4601
[9] D. K . Mills. Y.-M. Hsiao, P. J. Farmer. E. V. Atnip, J. H . Reibenspies,
M. Y Darensbourg. .lAm. Chem. Soc. 1991, f13, 1421.
[lo] T. Tuntulani, J. H. Reibenspies, P. J Farmer, M. Y. Darensbourg, Inorg.
Chem. 1992. 31. 3497.
[I I ] Crystal structure (296 K ) data for 3 . 3H,O: C,,H,,N,0,S2Ni.
A4 = 461.2, tetragonal crystals. space group P4,2,2, a = h = 8.0290(10).
c = 30.209(12) A,
v =194x A'. z = 4. pcdlrd=1.573 gem-'. {t =
1.242 mm- I ; 1450 reflections with F > 4a(F), R = 0.043 and R , = 0.044.
4 . H,O (193 K ) : C,,H,,N,O,S,Zn,
M = 373.8, monoclinic crystals,
space group P2,<, a = 12.479(4). /I = 9.322(3). c = 13.702(5) A. { j =
106.35(3). L'=1530AZ. Z = 4 . p L,,,E,=1.623gcm~3.
1941 reflections with f > 4a(F), R = 0.064, R, = 0.063. For both struc-
$ fU./)O+.25:0
Angeir.. C/icw. fnt. Ed. Eng1 1993, 32, No. 1
tiires. carbon-bound hydrogen atoms were idealized while hydrogens in
irater were located in a difference Fourier analysis and not refined. Further
details of the crystal structure investigations are available on request from
the Director of the Cambridge Crystallographic Data Centre, University
Chemical Laboratory. Lensfield Road. GB-Cambridge CB2 IEW, (UK).
on quoting the full journal citation.
[l?] A G. Orpen. L. Brammer, E H. Allen, 0 Kennard. D. G. Watson, R.
Tiiylor. J. <%ern.Soc. Dolron Puns. 1989. S.1.
[li] Spectral simulations were performed using the program XPOW for
S = 1 2 paramagnets.[14a] The latest update is described in ref. [14b].
[14] 21) R. L Bclford. M. J. Nilges. Cottipurer Siinulurion of' Pimcler Spwrru
E P K 5'1mposiimi. Zlst Rocky Mountain Conference, Denver. CO. August
1979: b) E. P. Duliba, Ph.D. Thesis, University of Illinois. 1983.
[15] S. A . Jacobs. D. W. Margcrum. h r g . CIiwri. 1984. 23. 1195.
[16] t; V. Lovecchio. E. S . Gore. D. H. Busch, J. A t t i . C'hcw. Snc. 1974. 96.
from isophthaloyl dichloride and 2-amino-6-methylpyridine) since this now has three primary conformations
(due to rotation about the phenyl-CO bond); syn-syn (see 2),
syn-anti, and anti-anti. In the syn-syn conformation the hydrogen-bonding cavity of 2 is appropriate for binding to urea
or barbiturate derivatives[131but would be too small to form
1 : I complexes with dicarboxylic acids. The carboxylic acid
groups on two different molecules could. however, bind to 2
A Self-Assembling, Hydrogen-Bonded Helix""
By Stcwn J. Geih, Cristina Vicent, Erkung Fan,
and Andrew D. Hamilton*
with one aminopyridine unit directed above and the other
below the plane of the binding cavity. Propagation of this
arrangement with a dicarboxylic acid would lead to an extended helical structure, as shown in 3.
In rncvioriam Margaret Etter
The design of molecular subunits that self-assemble into
defined structures in solution or in the solid state is an area
of intense current interest."] A key to controlling the shape
ofthe aggregate lies in manipulating the type and orientation
of the noncovalent interactions between the subunits.[*]The
strong and directional nature of hydrogen bonds has led to
their widespread use in seif-assembling systems. In the solid
state. rules have been delineated to allow the redsonable
prediction of hydrogen-bonding packing patterns in crystals.r31 This has led to a search for molecular components
that because of their hydrogen-bonding characteristics will
form persistent packing motifs in well-defined shapes or patterns. Recent results have demonstrated the formation of
two- and three-dimensional networks of di- and tetrapyridones:L"l extended sheet structures of ureylenedicarboxylic
acids;['] molecular tapes or (3 + 3) cyclic aggregates made
up of alternating melamine and barbiturate derivatives;16.
and variable molecular ribbons composed of hydrogenbonded acylaminopyridine and acid subunits.['] In this paper we report the design and structural properties of a molecule that in the presence of an aliphatic dicarboxylic acid of
the correct length will self-assemble into a helical arrangement of alternating components held together by a network
of hydrogen bonds.'"-"]
We have previously shown"'] that a receptor molecule
made up of two 2-amino-6-methylpyridine units separated
by a terephthalate spacer and in a syn orientation will form
strong complexes with dicarboxylic acids of appropriate
length with four hydrogen bonds as in 1 with adipic acid.
Increasing the length of the dicarboxylic acid beyond the
optimum leads to the anti conformation of the receptor molecule and formation of ribbon structures with bidentate hydrogen bonding between the two alternating components.["*
We were interested in investigating the recognition properties of the related isophthalate receptor molecule 2 (formed
Crystallization of a 1 : l mixture of 2 and pimelic acid
(heptanedioic acid) by slow diffusion of hexane into CHCI,
gave colorless triclinic needles. X-ray diffraction analysis[14. 1 5 1 showed a polymeric cocrystal composed of alternating units of 2 and pimelic acid linked by a network of
hydrogen bonds (Fig. 1). Compound 2 is not planar in the
[*] Prof' A. D. Hamilton. S. J. Geib. C. Vicenr. E. Fan
M;itci-i;ils Research Center and Department of Chemistry
Unixcrsiry of Pittsburgh
Pittsburgh. PA 15260 (USA)
This work was supported by the AFOSR (Univcrsity oFPittsburgh. Material\ Kcscarch Center) the Army Research Office and the National Science
Fouiidation. We are also grateful to the Spanish Ministerio de Educacion
y Ciciicia for a postdoctoral Fellowship to C. V
Fig. 1 . Section of the X-ray structure of the polymeric 1 :1 complex of 2 and
pimelic acid (stereoview).
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axial, ligation, diazacyclooctanenickel, complexes, zinc
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