Inorganica Chimica Acta 288 (1999) 206–209

A new carbonate bridged dinuclear zinc complex with tripodalamine ligands

Jens Dietrich, Frank W. Heinemann, Antje Schrodt, Siegfried Schindler *Institute for Inorganic Chemistry, Uni6ersity of Erlangen-Nurnberg, Egerlandstraße 1, 91058 Erlangen, Germany

Received 12 November 1998; accepted 18 February 1999

Abstract

A zinc complex with the tripodal ligand bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine (pmap) reacts with carbon dioxide toform a new dinuclear carbonate bridged zinc complex, [m-CO3{Zn(pmap)}2](ClO4)2. This complex was structurally characterizedand shows a binding mode for carbonate, which so far has not been observed for zinc carbonate complexes. © 1999 ElsevierScience S.A. All rights reserved.

Keywords: Crystal structures; Zinc complexes; Tripodal amine complexes; Carbonate complexes

1. Introduction

The carbonic anhydrases are zinc containing metal-loenzymes that catalyze the reversible hydration ofcarbon dioxide in a very efficient way [1]. Many investi-gators have studied carbonic anhydrase itself, and inaddition different research groups have been involvedin the synthesis and characterization of model com-plexes for the enzyme [2–7]. Therefore, several zinccarbonato complexes with different binding modes ofthe carbonate ligand have been structurally character-ized (Fig. 1(a–c)), [5,6,8–14].

Macrocyclic as well as non-macrocyclic amines havebeen used as ligands to complex the zinc ion to modelthe active site of carbonic anhydrase [3,4,7,15]. Some ofthese ligands were also employed successfully to modelcopper proteins and furthermore, these copper com-plexes could be used as model complexes for carbonicanhydrase. The tripodal ligand tmpa (= tris[(2-pyridyl)methyl]amine) is a versatile ligand and the X-ray structure of the zinc tmpa carbonate compoundshowed a trinuclear complex with three zinc units coor-dinated to oxygen atoms of the carbonate (binding

mode Fig. 1(a)) [10]. In contrast, the analogous coppertmpa carbonate compound crystallizes differently,forming a dinuclear complex with a carbonate bindingmode as shown in Fig. 1(b) (Cu instead of Zn) [11].Interestingly, an additional binding mode (shown inFig. 1(d)) was observed in several copper carbonatecomplexes [15–19]. Here, each of the copper ions bindsto one oxygen atom of the carbonate and furthermore,they share the third oxygen. So far, this binding modehas not been observed for zinc carbonate complexes.

2. Experimental

2.1. Materials and methods

Reagents and solvents used were of commerciallyavailable reagent grade quality. 1H NMR spectra wererecorded on a Bruker ESP-300E 300-MHz spectrome-ter.

2.1.1. Bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine(pmap)

The ligand was synthesized similar to a proceduredescribed for a different ligand [20]. An excess offreshly distilled 2-vinylpyridine (58 g, 0.55 mol) wasadded to a solution of 2-(aminomethyl)pyridine (11.9 g,

* Corresponding author. Tel.: +49-9131-8528 383; fax: +49-9131-852 7387.

E-mail address: schindls@anorganik.chemie.uni-erlangen.de (S.Schindler)

0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 0 2 0 -1693 (99 )00093 -6

J. Dietrich et al. / Inorganica Chimica Acta 288 (1999) 206–209 207

Fig. 1. Binding modes in carbonate bridged zinc complexes.

Table 1Summary of crystal data, details of data collection, structure solutionand refinement for [m-CO3{Zn(pmap)}2](ClO4)2·7H2O

Empirical formula C41H58Cl2N8O18Zn2

1152.59Formula weight (g mol−1)Crystal system monoclinicSpace group P21/n (No. 14)Z 4Unit cell parameters

a (A) 13.753(3)b (A) 19.423(4)

19.480(4)c (A)b (°) 96.82(3)

V (A3) 5167(2)Dcalc. (g cm−3) 1.482Crystal size (mm) 0.60×0.50×0.40m (mm−1) 1.109u range for data collection (°) 1.8–27.0Index ranges −175h517,

−15k524,−15l524

Reflections collected 13 025Independent reflections 11288 (Rint=0.0389)Reflections observed 3628

[Fo]4s(Fo)]Parameters refined 772Goodness-of-fit on F2 0.751R1/wR2 0.0543/0.1384Largest difference peak, hole 0.569, −0.403

(e A−3)

0.11 mol) in 150 ml of methanol. Acetic acid (16.5 g,0.27 mol) was added as catalyst and the reaction mix-ture was heated under nitrogen for 4 days. Themethanol was removed by rotary evaporation and theresulting oil was washed with aqueous NaOH (150 ml,15%) and extracted with dichloromethane. The organiclayer was dried over Na2SO4, the solvent removed byrotary evaporation and the brown residue was carefullydistilled under high-vacuum. There is no definite boilingpoint and if the temperature gets too high the viscousyellow oil becomes dark orange. The product was ob-tained in 55% yield. Small amounts of impurities (2-vinylpyridine can form as a decomposition productduring the distillation) were eliminated by chromato-graphy on silica gel with methanol. Alternatively, it isalso possible to avoid the high vacuum distillation andto purify the product immediately by chromatography.1H NMR (CDCl3): d 8.49–7.02 (m, 12H, aromatic py),3.89 (s, 2H, –CCH2), 2.98 (m, 8H, –CCH2–CCH2).

2.1.2. [m-CO3{Zn(pmap)}2](ClO4)2

Zinc perchlorate·6H2O (0.97 g, 2.6 mmol) and pmap(0.83 g, 2.6 mmol), were mixed in 50 ml methanol,heated to 60°C and filtered. Triethylamine (1 ml) wasadded and while the yellow solution was slightlywarmed, carbon dioxide was bubbled for 3 h throughthe solution. A small amount of precipitate was filteredoff and the remaining solution (ca. 25 ml) was kept inthe refrigerator overnight. White needles formed, werefiltered off and dried under vacuum (yield: 0.49 g, 55%).Anal. Calc. for C41H44N8Cl2O11Zn2·2H2O: C, 46.35; H,4.55; N, 10.55. Found: C, 46.40; H, 4.88; N, 10.34%.

The white crystalline solid obtained was recrystallizedfrom methanol together with a small amount of triethyl-amine while bubbling carbon dioxide through the solu-tion. Slow evaporation led to the formation of crystalssuitable for X-ray analysis.

2.2. X-ray data collection and structure refinementdetails

X-ray measurements were performed at room tem-perature on a Nicolet R3m/V diffractometer usinggraphite-monochromated Mo Ka radiation (l=

0.71073 A). Data have been corrected for Lorentz andpolarization effects. Absorption effects have been cor-rected using Psi-scans on 16 reflections (6.652u537.8°, Tmin=0.1398, Tmax=0.1679). Crystal data andinformation on data collection are given in Table 1.The structure was solved by direct methods, full-matrixleast-squares-refinement was carried out on F2 values(SHELXTL 5.03 [21]). All non-H atoms were refined withanisotropic displacement parameters. Hydrogen atompositions were taken from a difference Fourier synthe-ses. Their positional parameters have been refined usinga fixed common isotropic displacement parameter. Thehydrogen atoms of the water solvate molecules werefixed on their positions with a common isotropic dis-placement parameter.

3. Results and discussion

If the ligand tmpa is coordinated to a metal ion, eachof its pyridine arms forms a five-membered chelate ring.Increasing the arm length one by one leads to theligand series shown in Fig. 2.

While pmea and tepa are well known, so far onlypreliminary studies were performed with copper com-plexes of the ligand pmap but no other metal com-pounds of this interesting ligand are known [22].Therefore, we chose this ligand to study the effect of

J. Dietrich et al. / Inorganica Chimica Acta 288 (1999) 206–209208

Fig. 2. Tripodal ligand series derived from tmpa by increasing thelength of the pyridine arms.

Table 2Selected bond lengths (A) and bond angles (°) for [m-CO3(Zn-(pmap))2](ClO4)2·7H2O

Bond lengths (A)Zn(1)–O(11) 2.106(4)2.079(3) Zn(1)–N(31)

Zn(1)–N(21) 2.132(4)Zn(1)–N(11) 2.112(4)Zn(1)–N(1) 2.366(4)Zn(1)–O(13)2.180(4)

Zn(2)–N(41) 2.127(5)2.067(4)Zn(2)–O(12)Zn(2)–N(51) 2.130(5)Zn(2)–N(61) 2.130(5)

2.357(4)2.181(5) Zn(2)–O(13)Zn(2)–N(2)

Bond angles (°)O(11)–Zn(1)–N(31) 95.9(2) O(11)–Zn(1)–N(11) 96.1(2)

91.6(2)N(31)–Zn(1)–N(11) 97.5(2) O(11)–Zn(1)–N(21)161.3(2)N(31)–Zn(1)–N(21) 98.6(2) N(11)–Zn(1)–N(21)

98.1(2)N(31)–Zn(1)–N(1)O(11)–Zn(1)–N(1) 165.6(2)89.3(2)N(21)–Zn(1)–N(1)N(11)–Zn(1)–N(1) 79.1(2)

N(31)–Zn(1)–O(13) 155.3(2)O(11)–Zn(1)–O(13) 59.4(1)N(21)–Zn(1)–O(13) 85.2(2)N(11)–Zn(1)–O(13) 84.1(2)

106.4(2) O(12)–Zn(2)–O(13)N(1)–Zn(1)–O(13) 60.1(1)

increasing chelate ring size in zinc carbonate complexes.In contrast to tmpa in metal complexes, pmap forms ifcoordinated one five-membered chelate ring and two six-membered chelate rings. The ligand pmap was easilysynthesized from 2-(aminomethyl)pyridine and vinyl-pyridine using acetic acid as a catalyst and waspurified by distillation and/or chromatography. Mixingpmap, zinc perchlorate, and triethylamine in methanolmost likely leads first to a zinc pmap hydroxide com-plex which could be mononuclear or dinuclear withbridging hydroxides, as was observed for[Zn2(tmpa)2(OH)2](ClO4)2 [10]. This complex can thenreact further with carbon dioxide to [m-CO3{Zn(pmap)}2](ClO4)2. This product was formed inan acceptable yield and recrystallization from methanoltogether with a small amount of triethylamine led tothe formation of white crystals suitable for X-ray anal-ysis. The structure of the cation of the complex isshown in Fig. 3.

Bond lengths are 2.079(3) and 2.067(4) A for thenon-bridging and 2.357(4) and 2.366(4) A for the bridg-ing Zn–O bonds. These bond lengths are in a normal

range compared with the other structurally character-ized zinc carbonate complexes. They compare very wellwith the bond lengths of 2.305(6) A for the bridgingand 1.946(5) A for the non-bridging Cu–O bonds ofthe dinuclear copper carbonate complex [(Cu(pet-dien))2(CO3)](ClO4)2 [18], (Table 2). All other knowndinuclear copper carbonate complexes show muchshorter Cu–O bond lengths for the bridging O atom. Ingeneral, it can be said that the bond lengths, M–O, ofthe carbonate complexes of zinc or copper can varyover quite a large range depending on the ligand systemused. The Zn–N bonds in [m-CO3{Zn(pmap)}2](ClO4)2

are quite similar and only vary from 2.106(4) to2.181(5). The bond lengths are slightly longer in otherzinc carbonate complexes but compare well with thosein [(Zn(tmpa))3(CO3)](ClO4)4 [10]. The coordinationaround the central ions of the complex can be describedas distorted octahedral. This differs from the analogouscopper carbonate complexes which are either four- orfive-coordinate. The bond angles for O–Zn–O are60.1(1) and 59.4(1)° and again they resemble the O–Cu–O unit in [(Cu(petdien))2(CO3)](ClO4)2 where abond angle of 61.6(2)° was found, while in the othercopper carbonate complexes this bond angle is in arange from 63.7(2) to 66.1(4)° [18,19].

Even though dinuclear carbonate bridged zinc com-plexes do not seem to be involved in the reactions ofcarbonic anhydrases, it is interesting to see how zinccomplexes can bind carbonate as an additional ligand.While complexes of the binding modes shown in Fig.1(a–c) are well known, so far binding mode 1d has notbeen observed for zinc complexes. In order to synthe-size a dinuclear zinc complex with this kind of bindingmode for carbonate, it was necessary to find a suitableligand. From our findings we can conclude that the twochelate rings, increased in size in pmap compared totmpa, provide an optimized environment for forming a

Fig. 3. Molecular structure of the cation of [m-CO3(Zn(pmap))2]-(ClO4)2·7H2O.

J. Dietrich et al. / Inorganica Chimica Acta 288 (1999) 206–209 209

symmetrically dinuclear bridged zinc carbonatecomplex.

4. Supplementary material

Crystallographic data (excluding structure factors)for the structure reported in this paper have beendeposited with the Cambridge Crystallographic DataCentre as supplementary publication no. CCDC-108006. Copies of the data can be obtained free ofcharge from The Director, CCDC, 12 Union Road,Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033;e-mail: deposit@ccdc.cam.ac.uk or www:http//www.ccdc.cam.ac.uk).

Acknowledgements

The authors gratefully acknowledge financial supportfrom the Bundesministerium fur Bildung, Wissenschaft,Forschung und Technologie (BMBF) and the DeutscheForschungsgemeinschaft (DFG). Furthermore, wewould like to thank Professor Rudi van Eldik (Univer-sity of Erlangen-Nurnberg) for his support of thiswork.

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