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Inorganica Chimica Acta 308 (2000) 97–102

Zinc complexes of the new N,N,S ligandN-(2-mercaptoisobutyl)(picolyl)amine

U. Brand, H. Vahrenkamp *Institut fur Anorganische und Analytische Chemie der Uni6ersitat Freiburg, Albertstr. 21, D-79104 Freiburg, Germany

Received 20 May 2000; accepted 29 May 2000

Abstract

The title ligand MBPAH was obtained from 2-picolylamine and 2,2-dimethylthiirane. It forms the 2:1 zinc complex(MBPA)2Zn, assumed to have a ZnN2S2 coordination with two uncoordinated nitrogen donor functions. Treatment of(MBPA)2Zn with alkylating agents or reaction of MBPAH with Zn(ClO4)2 yielded the complexes [(MBPA)4Zn3]X2 which werefound by structure determinations to contain one ZnS4 and two ZnN4S2 units in the trinuclear cations. Zinc acetate and MBPAHformed the thiolate-bridged dinuclear complex [(MBPA)Zn(OAc)]2 which according to its structure determination contains twoZnN2S2O units. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Crystal structures; Zinc complexes; Thiolate complexes; N,N,S-ligand complexes

1. Introduction

An efficient modeling of metal sites in enzymes re-quires the designing of polydentate ligands containingnitrogen and sulfur donors, as histidine and cysteine/methionine are the most common donor units for metalions in a protein environment. This is specifically so forzinc enzymes [1,2], and there is quite a number of suchenzymes, as exemplified by liver alcohol dehydrogenase[3], spinach carbonic anhydrase [4] or bovine amino-laevulinate dehydratase [5], in which the catalytic zincion is attached to the protein solely by a NxSy donorset. The quest to model such a donor environmentrequires the design of N,S-ligands which have the rightnumber of N and S donors, which use all these donorsas monodentate functions (i.e. do not allow thiolatebridging with concomitant formation of oligonuclearcomplexes), and which encapsulate the zinc ion suchthat there is room for just one coligand representing theenzymatic substrate.

The chemistry of zinc complexes with tri- and tetra-dentate NxSy ligands is well-developed. Basic results by

Kellogg [6], Sellmann [7] and Darensbourg [8] wererecently extended to the use of the favorable tripodalligands derived from the prototypes tris(pyraz-olyl)borate and tris(aminomethyl)methane. Thus thegroups of Parkin [9,10], Riordan [11] and Carrano [12]have designed N2S and NS2 ligands, the zinc complexesof which can serve as structural models for the relatedzinc enzymes. But until now very little functional mod-eling of enzymes with zinc in a NxSy donor environ-ment has been achieved.

Our own contributions to this field have ranged frombis(amine)thioether ligands [13] via thiazoline-derivedsystems [14] and chain-like NS2 [15] and N2S ligands[16,17] to tridentate [18] and tetradentate tripods [19].Of these the chain-like N2S ligands (2-mercap-toethyl)(picolyl)amine (MEPAH) [16] and (2-mercap-tophenyl)(picolyl)amine (MPPAH) [17] were found tohave a rich coordination chemistry with zinc, includingtetrahedral, square-pyramidal, trigonal-bipyramidaland octahedral coordination patterns. Yet in both lig-ands the tendency of the thiolate functions to bridgetwo zinc ions could not be suppressed. We thereforechose to apply steric hindrance in the vicinity of thesulfur atom, hoping to make the thiolate unit strictlymonodentate. The ligand thus designed in analogy toour previously described MEPAH [16] was N-(2-mer-

* Corresponding author. Tel.: +49-761-203 6120; fax: +49-761-203 6001.

E-mail address: vahrenka@uni-freiburg.de (H. Vahrenkamp).

0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 0 20 -1693 (00 )00220 -6

U. Brand, H. Vahrenkamp / Inorganica Chimica Acta 308 (2000) 97–10298

captiosobutyl)(picolyl)amine (MBPAH). This paper de-scribes its synthesis and zinc complex chemistry.

2. Results and discussion

2.1. Ligand MBPAH

The strategy behind the design of ligand MBPAHwas the favorable positioning of its three donor atomswhich allows the formation of two five-memberedchelate rings together with the steric hindrance near thesulfur atom which was meant to reduce the bridgingtendency of the thiolate function. Ideally the ligandwould support the formation of tetrahedral complexes(MBPA)Zn�X thus mimicking the protein environmentby the N,N,S coordination and the enzyme’s function-ality by the Zn�X unit. Five-membered chelate ringsare, however, also typical in octahedral and trigonal-bipyramidal complexes, and hence such coordinationgeometries could also be expected for zinc complexes ofMBPAH.

The ligand could be synthesized in a one-step proce-dure by ring opening of 2,2-dimethylthiirane with 2-pi-colylamine in boiling toluene and could easily bepurified by distillation. It is a liquid of unpleasant odor.Its spectroscopic property of highest diagnostic value isthe 1H NMR resonance of the methyl groups whichshows up as a singlet for the free ligand but often splitsup for the zinc complexes thereby yielding symmetryinformation.

2.2. Bis-ligand complex Zn(MBPA)2

The simplest combination of zinc and MBPAH, thebis-ligand complex, could not be obtained from zincsalts (see below). But using the acidity of the SHfunction to effect hydrolytic cleavage of Zn[N(SiMe3)2]2was an efficient procedure to obtain the ZnL2 complex1.

As crystals of 1 suitable for a X-ray analysis couldnot be obtained, its structure had to be deduced fromspectroscopic data. The simplest constitution would be1a, using all donor atoms in an octahedral arrange-ment. Although zinc in an environment of N and Sdonors prefers tetrahedral coordination, 1a is not un-likely as can be seen below in the structures of 3 and 4.Alternative 1b corresponds to the favorable ZnN2S2

coordination in a symmetrical fashion and is analogousto the known structure of the Zn(MEPA)2 complex[16]. The constitutions 1a and 1b would be in accordwith the 1H NMR spectrum of 1 which shows only oneset of signals for ligand MBPA. They are, however, indisagreement with the IR spectrum which shows oneband for the pyridine ring vibration at 1592 cm−1

which represents an uncoordinated pyridine (cf. theband at 1590 cm−1 in the free ligand) and one band at1602 cm−1 for a zinc-bound pyridine. We thereforefavor constitution 1c which is analogous to that ob-served for Zn(MPPA)2 [17]. In analogy with the obser-vations made for Zn(MPPA)2 the NMR data of 1indicate a fast exchange of the donor positions involv-ing both MBPA ligands.

2.3. Zn�MBPA complexes with non-coordinatinganions

Our initial attempts to obtain a ZnL2 complex ofMBPAH involved deprotonation with NaOH andtreatment with zinc salts of non-coordinating anions.But irrespective of the ratio of the reactants complexesof the type [Zn3L4]2+ were obtained, the product of thereaction with Zn(ClO4)2 being 2. This corresponds toour observations made with ligand MEPAH [16]. Thebinding of ligand MBPA to zinc in 2 represents theoctahedral ZnL2 coordination for the outer zinc ions,with the thiolate sulfur atoms making use of theirbridging tendency to provide the central zinc ion with atetrahedral ZnS4 coordination.

U. Brand, H. Vahrenkamp / Inorganica Chimica Acta 308 (2000) 97–102 99

Fig. 1. Molecular structure of the [Zn3(MBPA)4]2+ cations in compounds 3 and 4.

Complexes of this constitution were also obtainedwhen treating 1 with alkylating agents. The originalpurpose of these reaction was to liberate the thiolatedonors from zinc by converting them to thioether func-tions which are bad donors, as we have shown in ourstudies of modeling the enzyme class of cobalamine-in-dependent methionine synthases [20]. Treatment of 1with methyl iodide yielded compound 3, treatment withdimethyl sulfate led to 4. In both cases the methylatingagents were applied in a large excess, yet they effectedonly partial methylation, which can be expressed by Eq.(1). The reason for this seems to be the quick removalof the products 3 and 4 by precipitation. This demon-strates both the lability of 1 as already indicated by itsNMR spectrum and the high tendency of formation ofthe [Zn3L4]2+ complexes.

3Zn(MBPA)2+2MeX

� [Zn3(MBPA)4]X2+2MBPA-Me (1)

Our expectations [16] concerning the structure of the[Zn3L4]2+ complex cation were supported by the IRdata showing only absorptions for coordinated pyridineand by the NMR spectra showing two separate signalsfor the methyl group (see Section 4). They were con-firmed by structure determinations of 3 and 4. Fig. 1gives a view of the Zn3L4 cations in both compounds,Table 1 lists the relevant structural data.

As Table 1 shows, the Zn3L4 cations have verysimilar shapes for the three independent cases. Theirtwo molecular halves are nearly related by a S4 axisdefined by the Zn–Zn–Zn vector. The central zinc ionhas a severely distorted tetrahedral ZnS4 environment,and likewise the octahedral ZnS2N4 environment of theexternal zinc ions is quite distorted. The reason forboth are the angular requirements of the five-memberedchelate rings and the sulfur atoms. The overall shape ofthe Zn3L4 units is also nearly superimposable to that in

the [Zn3(MEPA)4]2+ complex [16], and just like therethe mer(cis,trans,cis) arrangement of the two MBPAligands on each of the external zinc ions represents thesame one of eleven possible isomers. The individualbond lengths and angles in 3 and 4 are in their normalranges, being typically different for the octahedrallyand tetrahedrally coordinated zinc ions. Yet none ofthese structural features can explain the preference forthe Zn3L4 composition and structure. Possibly the easeof crystallization extracts the Zn3L4 complexes fromequilibrium mixtures of various labile complexes in thereaction mixture.

2.4. MBPA zinc acetate complex

Attempts to combine ligand MBPA and coordinatinganions in a zinc complex, hopefully as tetrahedral(MBPA)Zn�X, were successful for X=acetate.Combining zinc acetate and MBPAH in the presence ofa base resulted in [(MBPA)Zn-OAc]2 (5). 5 has the

Table 1Selected bond lengths (A, ) and angles (°) for 3 and 4 (average valuesrepresenting a spread of less than 0.05 A, or 1.0°)

3 4 (molecule 1) 4 (molecule 2)

3.346(2) 3.325(1) 3.310(1)Zn···Zn%Zn�S 2.368(1)2.365(3) 2.370(1)Zn%�S 2.552(1)2.555(1)2.545(3)

2.148(3)Zn%�N(NH) 2.180(3) 2.155(3)2.230(9) 2.225(3)Zn%�N(py) 2.224(3)

98.6(1)S�Zn�S 99.6(1) 100.2(1)114.0(1)115.0(1) 114.6(1)

90.8(1)S�Zn%�S 89.5(1) 90.3(1)157.7(1)157.4(1)S�Zn%�N(py) 158.0(3)

176.0(3)N(NH) 177.3(1) 176.5(1)�Zn%�N(NH)

84.4(1)84.9(1)85.9(1)Zn�S�Zn%

U. Brand, H. Vahrenkamp / Inorganica Chimica Acta 308 (2000) 97–102100

Fig. 2. Molecular structure of dimeric 5. Important bond lengths (A, ) and angles (°): Zn···Zn 3.340(1), Zn�S 2.487(1), Zn�S% 2.407(1), Zn�N12.246(2), Zn�N2 2.120(2), Zn�O 2.009(2), S�Zn�S% 93.96(4), Zn�S�Zn% 86.04(4), S�Zn�N1 160.03(7).

expected composition, but it is dimeric like theanalogous Zn�MEPA complex [16].

The dimeric nature of 5 and the presence of bridgingthiolate ligands were ascertained by a structure determi-nation, see Fig. 2. The two molecular halves of thecomplex are related by a center of symmetry. Thecoordination geometry of zinc can be described asdistorted trigonal-bipyramidal with S and N1 on theapical positions, cf. the S�Zn�N1 angle and the unusu-ally long bond distances Zn�S and Zn�N1. This featuredistinguishes 5 from [(MEPA)Zn�OAc]2 [16] which hassquare-pyramidal zinc. The reason for the difference isthe orientation of the acetate ligand which in 5 is fixedby a hydrogen bond to the amine nitrogen N2 while in[(MEPA)Zn�OAc]2 it is fixed to the nitrogen of theopposite half-molecule.

3. Conclusions

Ligand MBPAH has extended our series of N,N,S-ligands which are suitable to form uncharged com-plexes LZn�X. Yet like its predecessors it did notrealize this binding capacity in the form of mononu-clear tetrahedral complexes. The steric hindrance intro-duced by the two methyl groups in a-position to thesulfur atom is not sufficient to prevent association viaZn�S�Zn bridging. More sophisticated N,N,S- orN,S,S-ligands will have to be designed in which the3-dimensional environment of the thiolate functionsprevents them from being available for bridging.

In compensation, ligand MBPA has displayed a vari-able coordination chemistry with zinc. The nuclearities1, 2 and 3 and the coordination numbers 4, 5 and 6were observed. In addition the coordinative behaviordiffers noticeably from that of the analogous ligandsMEPA and MPPA, the most striking feature being theway in which one, two or three donor atoms of the

ligands are used in the ZnL2 complexes. While weconfine our studies of these ligands to zinc complexes, itseems attractive to find out whether they have anequally variable coordination chemistry with othertransition metals.

4. Experimental

The general experimental methods, the synthesis ofthe starting materials and the measuring techniqueswere as in ref. [16]. Organic reagents were obtainedcommercially.

4.1. Ligand MBPAH

A solution of 9.53 g (10.0 ml, 8.1 mmol) of 2-picolyl-amine and 16.6 g (15.0 ml, 189 mmol) of 2,2-dimethylthiirane in 40 ml of toluene was heated toreflux for 30 h. The solvent was removed in vacuo andthe remaining oil fractionated in a high vacuum, yield-ing 11.7 g (67%) of MBPAH as an oily liquid, b.p.88°C/10−3 mbar. IR (film): 3318s (NH), 2542w (SH),1590s (py). 1H NMR (CDCl3): 1.33 [s, 6H, Me], 1.94 [s,br., 2H, NH and SH], 2.58 [s, 2H, CH2CS], 3.91 [s, 2H,NCH2], 7.08 [t, J=7.0, 1H, py], 7.28 [d, J=7.0, 1H,py], 7.57 [t, J=7.0, 1H, py], 8.48 [d, J=7.0, 1H, py].Anal. Calc. for C10H16N2S (196.3): C, 61.18; H, 8.21; N,14.27. Found: C, 60.93; H, 8.28; N, 14.29%.

4.2. Zinc complexes

1: MBPAH (1.20 g, 6.11 mmol) in 40 ml of toluenewas treated dropwise with stirring with a solution of1.17 g (3.03 mmol) of Zn[N(SiMe3)2]2 in 20 ml oftoluene and then stirred for 2 h. The resulting precipi-tate was filtered off, washed with toluene and diethylether and dried in vacuo. 870 mg (63%) of 1, m.p.

U. Brand, H. Vahrenkamp / Inorganica Chimica Acta 308 (2000) 97–102 101

Table 2Crystallographic details

3 4 5

C40H60I2N8S4Zn3·3C2H6OSFormula C42H66N8O8S6Zn3·1.5H2O C24H36N4O4S2Zn2

Molecular mass 639.41218.51465.50.6×0.4×0.3Crystal size (mm) 0.5×0.5×0.3 0.4×0.1×0.1

Space group P21/c P1( P1(Z 4 4 1Unit cell dimensions

9.313(2) 8.433(2)a (A, ) 16.267(3)14.523(3)b (A, ) 19.309(4) 9.044(2)

c (A, ) 26.856(5) 32.479(7) 10.383(2)90.34(3)72.72(3)a (°) 90

103.64(3)b (°) 85.82(3) 107.61(3)g (°) 90 84.77(3) 111.28(3)

6166(2) 5547(2)V (A, 3) 697.3(2)Dcalc (g cm−3) 1.521.471.58

293(2) 293(2)153(2)Temperature (K)2.44 1.57m (Mo Ka) (mm−1) 1.91

h : −12 to 12 h : −10 to 10hkl Range h : −16 to 17k : 0 to 15 k : −24 to 24 k : −11 to 11

l : −43 to 42l : −28 to 0 l : −12 to 127755 47 568Reflections measured 5762

Independent reflections 7556 24 958 2723Observed reflections (I\2s(I)) 5553 14 134 2281

627Parameters 1234 163Reflections refined 27237556 24 958R1 (observed reflections) 0.0320.0360.069

0.211wR2 (all reflections) 0.098 0.082Residual electron density +2.5 +0.9 +0.6

−1.7 −0.6 −0.3Largest difference peak and hole (e A, −3)

205°C (dec.), remained. IR (KBr): 3201s (NH), 1602s,1592s (py). 1H NMR (DMSO-d6): 1.22 [s, 12H, Me],2.32 [d, J=7.0, 4H, CH2CS], 4.07 [d, J=4.7, 4H,NCH2], 4.41–4.55 [m, 2H, NH], 7.41 [t, J=6.0, 2H,py], 7.49 [d, J=8.0, 2H, py], 7.88 [t, J=8.0, 2H, py],8.66 [d, J=6.0, 2H, py]. Anal. Calc. for C20H30N4S2Zn(456.0): C, 52.68; H, 6.63; N, 12.29. Found: C, 52.10;H, 6.51; N, 12.01%.

2: A solution of 609 mg (3.10 mmol) of MBPAH and125 mg (3.12 mmol) of NaOH in 30 ml of methanolwas stirred for 30 min A solution of 579 mg (1.55mmol) of Zn(ClO4)2·6H2O in 30 ml of methanol wasadded and the mixture stirred for 4 h. After reducingthe volume to 15 ml in vacuo the solution was layeredcarefully with 50 ml of diethyl ether and set aside for 8days. The precipitate was filtered off and washed withether. Recrystallization from hot methanol yielded 527mg (58%) of 2, m.p. 235°C (dec.). IR (KBr): 3272m(NH), 1607s (py), 1097vs (ClO4). 1H NMR (DMSO-d6):0.92 [s, 12H, Me], 1.60 [s, 12H, Me], 2.85–3.12 [m, 8H,CH2CS], 3.91–4.41 [m, 8H, NCH2], 5.97 [s, br., 4H,NH], 7.48 [t, J=6.0, 4H, py], 7.70 [d, J=7.0, 4H, py],8.06 [t, J=7.0, 4H, py], 8.54 [d, J=6.0, 4H, py]. Anal.Calc. for C42H66N8O8S6Zn3 (1176.3): C, 40.84; H, 5.14;N, 9.53. Found: C, 40.39; H, 5.34; N, 8.82%.

3: Compound 1 (86 mg, 0.19 mmol) in 10 ml ofdichloromethane was treated with 450 mg (0.2 ml, 3.2mmol) of methyl iodide. After a few min a precipitatewas formed. After 30 min of stirring the precipitate wasfiltered off, washed with dichloromethane and dried invacuo. 71 mg (91%) of 3, m.p. 249°C, remained. IR(KBr): 3249m (NH), 1605s (py). 1H NMR (DMSO-d6):0.92 [s, 12H, Me], 1.59 [s, 12H, Me], 2.86–3.14 [m, br.,8H, CH2CS], 3.89–4.44 [m, br., 8H, NCH2], 5.01 [s, br.,4H, NH], 7.48 [t, J=6.0, 4H, py], 7.71 [d, J=7.0, 4H,py], 8.07 [t, J=7.0, 4H, py], 8.54 [d, J=6.0, 4H, py].Anal. Calc. for C40H60N8I2S4Zn3 (1231.2): C, 39.02; H,4.91; N, 9.10. Found: C, 39.36; H, 4.86; N, 8.82%.

4: Like 3 from 96 mg (0.21 mmol) of 1 and 190 mg(1.5 mmol) of dimethyl sulfate. Yield 77 mg (92%) of 4,m.p. 231°C. IR (KBr): 3236m (NH), 1607s (py), 1251vs,1225vs (MeSO4). 1H NMR (DMSO-d6): 0.92 [s, 12H,Me], 1.59 [s. 12H, Me], 2.84–3.13 [m, br., 8H, CH2CS],3.35 [s, 9H, MeSO4 and H2O], 3.88–4.39 [m, br., 8H,NCH2], 5.00 [s, br., 4H, NH], 7.48 [t, J=6.0, 4H, py],7.70 [d, J=6.0, 4H, py], 8.06 [t, J=7.0, 4H, py], 8.54[d, J=6.0, 4H, py]. Anal. Calc. for C42H66N8-O8S6Zn3

. 1.5 H2O (1199.6+27.0): C, 41.12; H, 5.67; N,9.14. Found: C, 41.12; H, 5.57; N, 9.05%.

U. Brand, H. Vahrenkamp / Inorganica Chimica Acta 308 (2000) 97–102102

5: A solution of 1.07 g (5.43 mmol) of MBPAH in 20ml of methanol was treated with a solution of 225 mg(5.61 mmol) of NaOH in 10 ml of water–methanol(1:10). A suspension of 996 mg (5.43 mmol) ofZn(OAc)2 in 20 ml of methanol was added with stirring.After 30 h of stirring a clear solution had formed. 5 mlof water were added and then the volume reduced to 5ml in vacuo. The resulting precipitate was filtered off,washed with a small amount of cold water, dried invacuo and recrystallized from boiling ethanol, yielding1.13 g (65%) of 5, m.p. 167°C. IR (KBr): 3221s (NH),1599s, 1584s (py). 1H NMR (DMSO-d6): 1.27 [s, 12H,Me], 1.89 [s, 6H, OAc], 3.22 [d, J=5.0, 2H, CHCS],4.10 [d, J=5.0, 2H, CHCS], 4.19 [s, br., 4H, NCH2],4.93 [s, br., 2H, NH], 7.61–7.65 [m, 4H, py], 8.12 [t,J=8.0, 2H, py], 8.70 [d, J=6.0, 2H, py]. Anal. Calc.for C24H36N4O4S2Zn2 (639.5): C, 45.08; H, 5.67; N,8.76. Found: C, 44.90; H, 5.63; N, 8.66%.

4.3. Structure determinations

Crystals of 3 were obtained by layering a DMSOsolution with diethyl ether, crystals of 4 by recrystal-lization from hot methanol. Crystals of 5 were takenfrom the isolated product.

Diffraction data were taken by the v–2u techniqueon a Nonius CAD4 diffractometer using graphite-filtered Mo Ka radiation. They were treated without anabsorption correction. The structures were solved withdirect methods and refined anisotropically with theSHELX program suite [21]. Hydrogen atoms were in-cluded with fixed distances and isotropic temperaturefactors 1.2 times those of their attached atoms. Parame-ters were refined against F2. The R values are defined asR1=S�Fo−Fc�/SFo and wR2={S[w(Fo

2 −F c2)2]/

S[w(Fo2)2]}1/2. Drawings were produced with SCHAKAL

[22]. Table 2 lists the crystallographic data.

5. Supplementary material

Crystallographic data for the structural analysis hasbeen deposited with the Cambridge CrystallographicData Centre, CCDC Nos. 144268 (for 3), 144269 (for 4)and 144270 (for 5). Copies of this information may beobtained free of charge from: The Director, CCDC, 12

Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:http://www.ccdc.cam.ac.uk).

Acknowledgements

This work was supported by the Fonds der Chemis-chen Industrie. We thank Dr A. Trosch for the X-raymeasurements.

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