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Preparation and Structure of Zinc(II) Complexes of Purine and 7-Azaindole William S. Sheldrick Gesellschaft für Biotechnologische Forschung mbH., Mascheroder Weg 1, D-3300 Braunschweig Z. Naturforsch. 37 b, 653-656 (1982); received November 16, 1981 Trichloro(purinium)zinc(II), Dichloro-bis-(7-azaindole)zinc(II), X-ray

Trichloro(purinium)zinc(II) (1) and dichloro-bis-(7-azaindole)zinc(II) (2) have been prepared from respectively 1:1 (in acid solution) and 2:1 aqueous solutions of the parent base and zinc(II) chloride. In the latter case the salt di-[7-azaindolium]zinc(II) tetra-chloride (3) is isolated under acid conditions. The structures of 1-3 have been established by X-ray structural analyses. Zinc coordinates the purine nitrogen N(7) in 1 and N(3) of 7-azaindole in 2. N(l) is protonated in 1. The bond length distribution of the cations 3 suggests a significant contribution from a resonance form in which N(9) carries the positive charge.

Introduction

Transition metal complexes of substituted pur-ines, both as individual bases and as components of nucleosides and nucleotides, have been the subject of intense synthetic and structural studies over the past decade [1, 2]. X-ray structural analyses have revealed two basic trends with regard to the site of metal coordination. When the purine base is sub-stituted at the N(9) position, as in nucleosides and nucleotides, the metal usually coordinates to N(7). An exception to this rule is observed for the trichloro(9-methyladenine)-zincate(II) ion, in which the zinc coordinates N( l ) [3]. Recently binding of N( l ) and 0(6), in addition to N(7), has been found for copper ions in the ternary complex [Cu4(5'-IMPH_1)2(o-phen)4(H20)42+] [N03-]2 [4], In unsubstituted purines, the metal usually coordi-nates that imidazole nitrogen which is protonated in the free neutral ligand, i.e. N(7) in theophylline and N(9) for all other naturally occurring purines. N(3) is often observed as a secondary site of co-ordination of bidentate bridging ligands. An ex-ception to the above rule is observed for trichlo-ro(adeninium)zinc(II) in which the metal coordinates to N(7) [5], This binding site allows the formation of an intramolecular Cl . . . H-N(6) bond.

In contrast to the above studies only relatively few metal complexes of purine itself have been isolated and characterised [6]. All four nitrogen atoms in purine are principle capable of acting as electronpair donors in donor-acceptor interactions with Lewis acids. Speca et al. who prepared 1:1

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complexes of the general type M(purine)Cl2 • wH20 considered these to be oligomeric with N(3) and N(9) as coordination sites of bridging purine moie-ties [6]. However, an X-ray analysis has demon-strated that N(7) is protonated in purine [7], so that on the basis of the trends discussed above, this would be expected to be the primary binding site rather than N(9). In the only metal complex of purine which has been crystallographically char-acterised, po^-di-//-chloro-tetrachloro(purinium)di-copper(II), N(3) was surprisingly found to be the only coordination site [9]. In order to provide further evidence for the preferred binding site and for the complexing ability of purine we have pre-pared complexes of purine and the analogous fused heterocyclic 7-azaindole with zinc(II) chloride. For the sake of ease of comparison, the purine number-ing scheme has also been used in the subsequent discussion for 7-azaindole. We present here the X-ray analyses of the title compounds 1-3.

Experimental

Large well-shaped crystals of trichloro(purinium)-zinc(II) (1) and di-[7-azaindolium]zinc(II)tetra-chloride (3) were obtained upon slow evaporation of respectively 1:1 and 2:1 mixtures of purine and 7-azaindole with zinc chloride in aqueous solution to which 1 N HCl was added to give a pH in the range 1-2. Their respective constitutions were established by the X-ray analyses.

Dichloro-bis-(7-azaindole)zinc(II) (2)

0.46 g of ZnCl2 in 5 ml of water were added to an aqueous solution (5 ml) of 0.8 g of 7-azaindole and the mixture heated to 60 °C for 20 min. It was then

654 W. S. Sheldrick • Zinc(II) Complexes of Purine and 7-Azaindole

Compound Stoichiometry Space group a (A) b ( A ) C ( A )

a (°) ß (°) y (°) z Mr D c (gem"3) Radiation fi (mm -1) 2 0 range (°) unique reflections collected Fo2 rejection criterion Refinement reflections R Rw

g

C5H5N4Cl3Zn P2x/c

8.896(1) 9.810(2)

11.554(2) 90

101.27(2) 90 4

292.8 1.97

MoKa 3.1 3 < 20 < 60°

2266 Fo2 < 2.0cr (Fo2) 1936

0.037 0.036 0.0002

Ci4H12N4Cl2Zn P2i/c

12.558(3) 9.410(4)

13.320(3) 90 98.46(3) 90 4

372.6 1.59

MoKa 1.8 3 < 20 < 60°

3609 Fo2 <2.0(T(F0 2) 2412

0.042 0.037 0.0002

2Jp7H7N2][ZnCl4] PI

8.956(4) 15.381(7) 7.195(3)

99.77(2) 105.70(4) 103.72(3)

2 445.5

1.65 MoKa

1.9 3 < 20 < 55°

4134 Fo2 <2.0a(Fo2) 2950

0.048 0.040 0.003

Table I. Crystal and refinement data.

allowed to cool and left to stand. Large prismatic crystals of 2 were precipitated within 24 h. These were filtered off, washed with water and air dried. Yield 80%. Microanalysis was carried out by Beller (Göttingen).

CiJhzNiChZn (362.6) Found C 45.17 H 3.20 N 15.20 Cl 18.83, Calcd C 45.14 H 3.25 N 15.04 Cl 19.03.

X-ray analyses of 1 -3 Crystal and refinement data are summarised in

Table I. Intensity data were collected on a Syn-tex P2i diffractometer (MoKa, graphite mono-chromator) in the 6-26 mode. Empirical absorption corrections based on azimuthal scan data were applied to all three data sets. The structures were solved with the aid of Patterson and difference syntheses. Hydrogen atoms were located in differ-ence syntheses and allowed to refine freely with individual isotropic temperature factors for 1 and 2 and a group isotropic temperature factor for 3. Anisotropic temperature factors were introduced

C11

Fig. 1. Molecular structure of 1.

for the non-hydrogen atoms. Weights were given by the expression w = k (a2(F0) + gF0 2) _ 1 . Crystallo-graphic calculations were performed with SHELX-76 (G. M. Sheldrick) and with local programs. Figs. 1-3 were drawn with RSPLOT (W. S. Sheldrick). The

655 W. S. Sheldrick • Zinc(II) Complexes of Purine and 7-Azaindole

Table II. Positional parameters and equivalent iso-tropic temperature factors (X 103).

x y z Ueq

Compound 1 Zn 0.2552(1 Cl(l) 0.4565(1 Cl(2) 0.0340(1 Cl(3) 0.2793(1 N(l) 0.3474(3 C(2) 0.2837(4 N(3) 0.2059(3 C(4) 0.1973(3 C(5) 0.2589(3 C(6) 0.3370(3 N(7) 0.2260(3 C(8) 0.1469(4 N(9) 0.1267(3 Compound 2 Zn 0.2596(1 Cl(l) 0.3174(1 Cl(2) 0.2790(1 C(l) —0.0690(4 C(2) 0.0398(3 N(3) 0.0989(2 C(4) 0.0463(3 C(5) —0.0649(3 C(6) —0.1230(3 C(7) —0.0865(4 C(8) 0.0055(4 N(9) 0.0881(2 C(l)' 0.3569(3 C(2)' 0.3224(3 N(3)' 0.3251(2 C(4)' 0.3640(3 C(5)' 0.4012(3 C(6)' 0.3962(3 C(7)' 0.4353(3 C(8)' 0.4189(3 N(9)' 0.3747(2 Compound 3 Zn —0.0985(1 Cl(l) 0.0566(1 Cl(2) —0.0723(1 Cl(3) —0.3571(1 Cl(4) —0.0159(1 C(l) 0.5205(5 C(2) 0.3559(5 N(3) 0.2734(4 C(4) 0.3532(4 C(5) 0.5261(5 C(6) 0.6077(5 C(7) 0.5659(5 C(8) 0.4247(6 N(9) 0.2953(4 C(l)' 0.3414(6 C(2)' 0.2717(6 N(3)' 0.2054(4 C(4)' 0.2063(5 C(5)' 0.2772(5 C(6)' 0.3437(6 C(7)' 0.2554(6 C(8)' 0.1784(6 N(9)' 0.1476(5

equivalent isotropic temperature factors Ue q in Table II were calculated according to the expression Ueq = 1/3 27i27jUiUjai*aj*äi • äj. Tables of aniso-tropic temperature factors hydrogen atom positional parameters and lists of observed and calculated structure factors may be obtained from the Fach-informationszentrum Energie Physik Mathematik GmbH., D-7514 Eggenstein-Leopoldshafen 2.

Discussion

The structural analysis of 1 demonstrates that zinc preferentially coordinates N(7) as would be expected on the basis of the fact that this nitrogen is protonated in the free base. N( l ) and N(9) are protonated in 1 thus lending the purine moiety a net positive charge. The zinc atom is tetrahedrally coordinated with a Zn-N distance of 2.054(3) A similar to that observed in zinc-adenine and zinc-guanine complexes. As has been observed in other metal-purine complexes [2] binding at N(7) in 1 has relatively little effect on the dimensions of the

Table III. Bond lengths (A) in 1-3.

Compound 1 Zn -Cl(l) 2.254(1 Zn -Cl(3) 2.226(1 N(l)-C(2) 1.348(5 C(2)-N(3) 1.313(5 C(4) -C(5) 1.393(4 C(5)-C(6) 1.362(4 N(7)-C(8) 1.317(5 Compound 2 Zn -Cl(l) 2.231(1 Zn -N(3) 2.063(2 C(l)-C(2) 1.386(6 C(l)-C(6) 1.375(7 C(2)-N(3) 1.343(5 N(3)-C(4) 1.337(5 C(4)-C(5) 1.420(5 C(4)-N(9) 1.352(5 C(5)-C(6) 1.371(6 C(5)-C(7) 1.431(7 C(7)-C(8) 1.337(7 C(8) -N(9) 1.389(6 Compound 3 Zn -Cl(l) 2.282(1 Zn -Cl(3) 2.255(1 C(l)-C(2) 1.356(6 C(l)-C(6) 1.376(6 C(2) -N(3) 1.341(7 N(3)-C(4) 1.334(5 C(4)-C(5) 1.425(5 C(4)-N(9) 1.329(6 C(5) -C(6) 1.357(7 C(5)-C(7) 1.442(6 C(7) -C(8) 1.342(8 C(8)-N(9) 1.379(5

0.7147(1) 0.5364 1) 28 1) 0.8546(1) 0.5956 1) 40 1) 0.8284(1) 0.5154 1) 43 1) 0.6120(1) 0.3692 1) 47 1) 0.2126(3) 0.6016 3) 33 1) 0.1539(3) 0.6859 3) 38 2) 0.2203(3) 0.7534 3) 36 1) 0.3540(3) 0.7307 3) 27 1) 0.4224(3) 0.6451 3) 24 1) 0.3467(3) 0.5774 3) 30 1) 0.5597(3) 0.6495 2) 31 1) 0.5710(3) 0.7343 3) 35 2) 0.4502(3) 0.7856 2) 34 1)

0.8815(1) 0.3171 1) 41 1) 0.6769(1) 0.3904 1) 47 1) 0.9123(1) 0.1561 1) 66 1) 1.0199(6) 0.2666 4) 78 3) 0.9970(4) 0.2654 3) 60 2) 0.9072(3) 0.3294 2) 43 2) 0.8430(4) 0.3971 3) 45 2) 0.8612(5) 0.4030 3) 61 3) 0.9519(6) 0.3354 4) 74 3) 0.7723(6) 0.4847 4) 83 3) 0.7072(6) 0.5221 3) 83 3) 0.7506(4) 0.4702 2) 57 2) 1.3005(4) 0.4114 3) 59 2) 1.1784(4) 0.3595 3) 49 2) 1.0484(3) 0.4021 2) 38 2) 1.0440(4) 0.5009 3) 38 2) 1.1634(4) 0.5614 3) 45 2) 1.2949(4) 0.5135 3) 58 2) 1.1108(5) 0.6609 3) 54 2) 0.9688(5) 0.6582 3) 51 2) 0.9270(3) 0.5613 2) 42 2)

0.2800(1) 0.2779 1) 40 1) 0.1957(1) 0.1780 2) 48 1) 0.4075(1) 0.1598 2) 56 1) 0.1871(1) 0.1695 2) 47 1) 0.3132(1) 0.6194 2) 49 1) 0.3999(3) 0.1362 " ) 52 2) 0.3701(3) 0.0780 7) 51 2) 0.4273(3) 0.1285 5) 46 1) 0.5129(3) 0.2411 6) 39 2) 0.5462(3) 0.3056 6) 42 2) 0.4878(3) 0.2510 7) 49 2) 0.6396(3) 0.4219 7) 53 2) 0.6560(3) 0.4188 " ) 59 2) 0.5788(3) 0.3109 6) 51 2) 0.9891(4) 0.2488 8) 59 2) 0.9044(3) 0.2708 7) 56 2) 0.8952(2) 0.4131 6) 48 2) 0.9709(3) 0.5395 6) 40 2) 1.0600(3) 0.5225 6) 45 2) 1.0682(3) 0.3727 7) 55 2) 1.1221(3) 0.6777 8) 62 2) 1.0700(4) 0.7763 8) 61 2) 0.9759(3) 0.6896 6) 52 2)

Zn -Cl(2) 2.234(1 Zn -N(7) 2.054(3 N(l ) -C(6) 1.345(4 N(3) -C(4) 1.336(4 C(4) -N(9) 1.358(4 C(5) -N(7) 1.382(4 N(9) -C(8) 1.353(4

Zn -Cl(2) 2.212(1 Zn -N(3) ' 2.038(3 C(l) '-C(2)' 1.377(5 C(l) '-C(6)' 1.379(6 C(2)'-N(3)' 1.347(5 N(3)'-C(4)' 1.335(4 C(4)'-C(5)' 1.421(5 C(4)'-N(9)' 1.359(4 C(5)'-C(6)' 1.389(5 C(5)'-C(7)' 1.420(5 C(7)'-C(8)' 1.351(6 C(8)'-N(9)' 1.385(4

Zn -Cl(2) 2.254(1 Zn -Cl(4) 2.293(1 C(l) '-C(2)' 1.360(8 C(l) '-C(6)' 1.373(8 C(2)'-N(3)' 1.326(7 N(3)'-C(4)' 1.347(6 C(4)'-C(5)' 1.409(6 C(4)'-N(9)' 1.322(7 C(5)'-C(6)' 1.375(8 C(5)'-C(7)' 1.429(7 C(7)'-C(8)' 1.348(8 C(8)'-N(9)' 1.403(7

656 W. S. Sheldrick • Zinc(II) Complexes of Purine and 7-Azaindole

purine moiety. As a result of the change in the site of protonation from N(7) to N(9) the angle

Table IV. Bond angles (°) in 1-3.

Compound 1 Cl(l)-Zn -Cl(2) Cl(2)-Zn -Cl(3) Cl(2)-Zn -N(7) C(2)-N(l)-C(6) C(2) -N(3)-C(4) N(3)-C(4) -N(9) C(4) -C(5)-C(6) C(6) -C(5) -N(7) C(5) -N(7)-Zn C(5) -N(7)-C(8) C(4) -N(9)-C(8) Compound 2 Cl(l)-Zn -Cl(2) Cl(2)-Zn -N(3) Cl(2)-Zn -N(3) ' C(2)-C(l)-C(6) C(l)-C(2)-N(3) C(2) -N(3)-Zn C(4) -N(3)-Zn C(2) -N(3)-C(4) N(3)-C(4)-C(5) N(9)-C(4) -N(3) C(5) -C(4)-N(9) C(4) -C(5) -C(6) C(4) -C(5)-C(7) C(6)-C(5)-C(7) C(l)-C(6)-C(5) C(5) -C(7)-C(8) C(7) -C(8)-N(9) C(4) -N(9)-C(8) Compound 3 Cl(l)-Zn -Cl(2) Cl(2)-Zn -Cl(3) Cl(2)-Zn -Cl(4) C(2) -C(l) -C(6) C(l) -C(2)-N(3) C(2) -N(3)-C(4) N(3)-C(4)-C(5) N(9)-C(4) -N(3) C(5)-C(4)-N(9) C(4) -C(5) -C(6) C(4) -C(5)-C(7) C(6)-C(5)-C(7) C(l)-C(6)-C(5) C(5)-C(7)-C(8) C(7) -C(8) -N(9) C(4) -N(9)-C(8)

111.1(1) 111.4(1) 102.8(1) 123.1(3) 112.8(3) 127.2(3) 117.4(3) 133.5(3) 130.1(2) 104.9(3) 106.9(3)

117.5(1) 108.1(1) 110.2(1) 121.2(4) 123.0(4) 118.6(3) 125.9(2) 115.5(3) 124.7(3) 126.8(3) 108.6(3) 118.2(4) 105.6(3) 136.2(4) 117.4(4) 107.5(4) 110.3(4) 108.0(3)

110.0(1) 112.6(1) 113.0(1) 121.7(5) 120.1(4) 120.1(3) 121.2(4) 129.4(4) 109.5(3) 118.1(4) 104.7(4) 137.2(4) 118.8(4) 106.8(3) 110.7(4) 108.3(4)

Cl(l)-Zn Cl(l)-Zn Cl(3)-Zn N(l ) -C(2) N(3) -C(4) C(5) -C(4) C(4) -C(5) N(l ) -C(6) C(8) -N(7) N(7) -C(8)

—Cl(3) -N(7) -N(7) -N(3) -C(5) -N(9) -N(7) -C(5) -Zn -N(9)

109.5(1) 116.7(1) 105.2(1) 124.2(3) 126.6(3) 106.2(3) 109.1(3) 115.8(3) 124.3(2) 112.9(3)

01(1)-Cl(l)-N(3)-0(2)'-0(1)'-C(2 y-C(4 )'-C(2)'-N(3)'-N(9)'-C (5)'-0(4)'-0(4)'-0(6)'-C(l)'-C(5)'-C(7)'-0(4)'-

01(1)-CL(L)-Cl(3) -C(2 ) '-C(l) ' -0(2)'-N(3)'-N(9)'-C(5)'-0(4)'-C(4 )'-0(6)'-0(1)'-0(5)'-C(7 )'-C(4 y-

Zn Zn Zn •0(1)' C(2)' -N(3)' -N(3)' -N(3)' •0(4)' •0(4)' -0(4)' •0(5)' 0(5)' "0(5)' 0(6)' -0(7)' -0(8)' -N(9)'

•N(3) 109.0(1 -N(3)' 110.0(1 -N(3)' 100.7(1 -C(6)' 120.2(4 -N(3)' 124.1(3 Zn Zn

-0(4) ' C(5)'

118.9(2 125.7(2 115.2(3 125.2(3

-N(3) ' 126.8(3 -N(9) ' 108.0(3 -C(6)' 117.0(3 0(7)'

-0(7) ' -0(5) ' -0(8) '

106.4(3 136.6(4 118.2(4 107.3(3

-N(9) ' 110.1(3 -C(8)' 108.2(3

Zn -Cl(3) Zn -Cl(4) Zn -01(4) C(l) '-C(6)' -C(2)'-N(3)' -N(3)'-C(4)' 0(4)' -0(5) ' •C(4)'N(3)' -0(4)'-N(9)' C(5)'-C(6)' -C(5)'-C(7)' -C(5)'-C(4)' -C(6)'-C(5)' -C(7)'-C(8)' -C(8)'-N(9)' -N(9)'-C(8)'

107.5(1 106.8(1 106.5(1 120.8(5 121.5(5 119.8(4 120.7(4 128.8(4 110.5(4 118.6(4 105.3(4 136.1(4 118.6(5 107.4(4 109.9(5 107.3(4

C(8)-N(9)-C(4) is widened from 104.2° in the parent molecule [7] to 106.9(3)° in 1. C(5)-N(7) and N(7)-C(8) display bond distances of 1.382(4) and 1.317(5) A which are comparable to C(4)-X(9) and C(8)-N(9) in purine [1.374 and 1.312 A]. The pro-tonation of N(l ) in 1 leads to a significant widening of the C(6)-N(l)-C(2) angle [123.1(3)° vs 118° in purine].

The structure of 1 suggests that N(7) and N(3) rather than N(9) and N(3) will be the coordination sites of bridging purine bases in oligoineric com-plexes. Indeed the former sites must be preferred on the basis of steric considerations alone. The latter coordination sites will, of course, be expected for bridging purine bases in dimeric complexes.

The analyses of 2 and 3 provide the first details of the molecular geometry of 7-azaindole, albeit in the complexed or protonated form. The facile pre-paration of 2 in aqueous solution suggests that the pyridine nitrogen N(3) in 7-azaindole is a much better electron donor than N(3) in purine. The zinc atom in 2 is tetrahedrally coordinated with Zn-N distances of 2.063(3) and 2.035(3) Ä similar to that in 1. Two strong intramolecular bonds 01(1) . . . H(9)-N(9) and 01(1) . . . H(9) ' -N(9) ' of length 3.29(1) and 3.28(1) Ä are formed in 2. In the salt 3 X(3) of the 7-azaindolium cations is pro-tonated. Bond distances in the two independent base moieties of respectively 2 and 3 are similar to one another. Protonation of N(3) leads to a much wider angle C(2)-N(3)-C(4) in 3 [120.0(2)°] in com-

parison to 2 [115.4(2)°]. Of particular significance is the shortening of the bond C(4)-N(9) on going from 2 to 3 from 1.356(4) to 1.326(4) Ä. This sug-gests that a resonance form B in which N(9) rather than N(3) carries the positive charge must play a significant role for the 7-azaindolium cation.

[1] D. J. Hodgson, Prog. Inorg. Chem. 23, 211 (1977). [2] V. Swaminathan and M. Sundaralingam, CRC

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Acta 390, 137 (1975). [4] R. W. Geliert, B. E. Fischer, and R. Bau, J. Am.

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Chem. Commun. 1970, 1668.

[6] A. N. Speca, C. M. Mikulski, F. J. Iaconianni, L. L. Pytlewski, and N. M. Karayannis, Inorg. Chim. Acta 46, 235 (1980).

[7] D. G. Watson, R. M. Sweet, and R, E. Marsh, Acta Crystallogr. 19, 573 (1965).

[8] W. S. Sheldrick, Acta Crystallogr. B 37, 945 (1981).