Variability in the Coordination Modes of 2-PyridineformamideThiosemicarbazone (HAm4DH) in some Zinc(II), Cadmium(II), andMercury(II) Complexes

Elena Bermejoa, Alfonso Castineirasa,*, Isabel Garcıa-Santosa, and Douglas X. Westb

a Santiago de Compostela/Spain, Departamento de Quımica Inorganica, Universidad de Santiago de Compostelab Seattle/USA, Department of Chemistry, University of Washington

Received November 19th, 2004; accepted February 11th, 2005.

Abstract. The reduction of 2-cyanopyridine by sodium in dry me-thanol in the presence of thiosemicarbazide produces 2-pyridine-formamide thiosemicarbazone, HAm4DH. The reactions of thepotentially tridentate ligand HAm4DH with salts of Zn, Cd,and Hg gave a variety of metal-ligand complexes. The complexeswere characterized by mass spectrometry as well as IR and multi-nuclear NMR (1H, 13C, 13C CP/MAS, 113Cd, 199Hg) spectro-scopy. The crystal structures of [Zn(Am4DH)(OAc)]2·H2O,[Hg(HAm4DH)2Br2]·C2H5OH and 1

�[Hg(μ-S-Am4DH)Br] wereobtained. Coordination of anionic Am4DH� occurs through thepyridyl nitrogen, imine nitrogen and thiolato sulfur atoms, while

1 Introduction

Thiosemicarbazones and their metal complexes have beenstudied extensively and have been shown to exhibit a rangeof biological activities, which is considered to be related totheir ability to chelate metals [1]. The versatility of thesesystems in terms of coordination and their ability to behaveas neutral or deprotonated ligands raised our interest in thiskind of compound. The solubility of these complexes is im-portant for human applications and strongly depends onthe peripheral groups of the ligands. It was envisaged thatattaching the thiosemicarbazone moiety to an amide car-bon, rather than an aldehyde or ketone carbon, may en-hance the water solubility. However, we recently reportedstudies concerning complexes of 2-pyridineformamidethiosemicarbazone, HAm4DH, prepared with zinc halides[2] and cadmium halides [3], as well as transition metal andGroup XII complexes with other N(4) substituted thiosemi-carbazones [4�14]. It was found that the proposed bettersolubility of the new complexes in water had not beenachieved.

In view of the pharmalogical properties of thiosemicar-bazones, we report here the spectral properties of binuclearzinc(II) and cadmium(II) complexes as well as mercury(II)

* Prof. Alfonso CastineirasUniversidad de Santiago de CompostelaDepartamento de Quımica InorganicaFacultad de FarmaciaE-15782 Santiago de CompostelaFax: �34 981 547 163E-mail: qiac01@usc.es

Z. Anorg. Allg. Chem. 2005, 631, 2011�2019 DOI: 10.1002/zaac.200570008 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2011

the neutral ligands in [Hg(HAm4DH)2Br2] coordinate as mono-dentate ligands through their thione sulfur atoms. One of the ace-tate ligands in [Zn(Am4DH)(OAc)]2·H2O is bridging monodentateand the other bridging bidentate. 1

�[Hg(μ-S-Am4DH)Br] featuresfive-coordinate mercury centers with bridging thiolato sulfuratoms. The intermolecular arrangement is dictated by hydrogenbonding from the amino groups and by π-π stacking of the pyri-dine rings.

Keywords: Thiosemicarbazones; Zinc; Cadmium; Mercury;π-π Stacking; Crystal structure

complexes of 2-pyridineformamide thiosemicarbazone(Fig. 1). The compounds were characterized by elementalanalysis, IR and multinuclear NMR (1H, 13C, 13C CP/MAS,113Cd, 199Hg) spectroscopy, mass spectrometry and, in somecases, X-ray diffraction. The crystal and molecular struc-tures of a binuclear zinc(II) complex and two mercury(II)complexes are included.

Fig. 1 Depiction of HAm4DH with the numbering scheme usedin the discussion of the spectral properties of the metal complexes.

2 Results and Discussion

The reduction of 2-cyanopyridine by sodium in dry meth-anol in the presence of thiosemicarbazide affords 2-pyri-dineformamide thiosemicarbazone (HAm4DH) [2]. The re-action of HAm4DH with Zn(OAc)2, Cd(OAc)2 or HgX2

(X � Cl, Br, or I) leads to coordination compounds of for-mulae [Zn(Am4DH)(OAc)]2·H2O (1), [Cd(Am4DH)(OAc)]2(2), and [Hg(HAm4DH)X2] (X � Cl, Br, or I) (3, 4 and 7,respectively). In addition, crystals of [Hg(HAm4DH)2Br2]·EtOH (5) and 1

�[Hg(μ-S-Am4DH)Br] (6) were formed. Thecomplexes are pale yellow solids that are moderately solublein common organic solvents but insoluble in water � exceptfor complex 1. The molar conductance values are in therange 5.72�17.23 ohm�1 cm2 mol�1 in DMF solution

E. Bermejo, A. Castineiras, I. Garcıa-Santos, D. X. West

(10�3 M), indicating non-electrolytic behavior. The massspectrum of [Zn(Am4DH)(OAc)]2 shows an ion fragmentat m/z � 258 {Zn(Am4DH)�}, but the spectrum of[Cd(Am4DH)(OAc)]2 shows an ion fragment at m/z � 503{Cd(Am4DH)2

�}. The mass spectra of the threemercury(II) complexes, unlike those of the other metalcomplexes in this study, do not show ion fragments but onlym/z fragments resulting from more extensive decompo-sition.

Table 1 Selected bond distances /A for 1, 5, and 6.

Bond 1a 5a 6b

M�N11 2.163(2) 2.159(2) 2.534(11)M�N12 2.054(2) 2.055(2) 2.293(11)M�S1 2.397(1) 2.382(1) 2.426(2) 2.633(4)M�S2 2.434(2) 2.499(4) (S1#1)M�X1 2.006(2) 2.049(2) 2.731(1) 2.632(2)M�X2 2.029(2) 2.051(2) 2.762(1)C17�S1 1.754(2) 1.745(2) 1.723(8) 1.712(8) 1.780(14)C16�N12 1.296(3) 1.292(3) 1.292(10) 1.308(12) 1.320(16)N12�N13 1.397(2) 1.390(3) 1.404(9) 1.392(10) 1.395(15)N13�C17 1.311(3) 1.313(3) 1.322(10) 1.307(11) 1.289(16)C17�N14 1.354(3) 1.360(3) 1.319(11) 1.326(11) 1.349(18)C16�N15 1.335(3) 1.358(3) 1.332(10) 1.329(13) 1.307(17)

a) The second number in a row refers to the ligand containing S2. b) Symme-try transformations used to generate equivalent atoms: #1 �x�1/2, y�1/2,�z�1/2.

Table 2 Selected bond angles /° for 1, 5, 6.

Angle 1a Angle 5a 6b

N15�C16�N12 124.2(2) 124.4(2) 126.9(7) 126.9(8) 123.6(1)C15�C16�N12 114.8(2) 115.8(2) 116.4(7) 117.1(9) 115.9(1)C16�N12�N13 116.6(2) 117.7(2) 113.4(7) 113.5(8) 111.8(1)N12�N13�C17 112.8(2) 112.4(2) 119.3(7) 120.1(7) 117.9(1)N13�C17�N14 116.9(2) 117.1(2) 118.3(7) 117.5(8) 118.2(1)N13�C17�S1 126.7(2) 127.7(2) 122.9(6) 123.6(6) 126.6(1)N14�C17�S1 116.4(2) 115.2(2) 118.8(6) 118.8(7) 115.0(1)Zn1�S1�C17 94.4(1) 94.6(1) Hg1�S1�C17 105.4(3) 106.1(3) 96.7(5)Zn1�N12�C16 120.1(2) 118.8(2) Hg#2�S1�C17 108.9(5)Zn1�N12�N13 123.0(1) 123.3(1) Hg1�N12�C16 125.1(9)O11�Zn1�O21 103.7(1) 99.8(1) Hg1�N12�N13 123.0(8)O11�Zn1�N12 116.4(1) 157.1(1) S1�Hg1�N12 75.2(3)O21�Zn1�N12 138.6(1) 102.5(1) S1�Hg1�N11 141.2(3)O11�Zn1�N11 101.0(1) 99.1(1) S1�Hg1�Br1 106.1(1) 104.8(1) 100.1(1)O21�Zn1�N11 87.3(1) S1�Hg1�Br2 105.1(1) 104.0(1)N12�Zn1�N11 75.9(1) 76.6(1) S1�Hg1�S1#1 122.3(1)O11�Zn1�S1 105.7(1) 98.5(1) N12�Hg1�N11 67.1(4)O21�Zn1�S1 98.4(1) 103.8(1) N12�Hg1�Br1 121.5(3)N12�Zn1�S1 81.1(1) 81.6(1) N12�Hg1�S1#1 119.7(3)N11�Zn1�S1 150.4(1) 157.3(1) N11�Hg1�Br1 92.0(3)Zn1�O11�C1 135.7(2) 107.1(1) N11�Hg1�S1#1 85.6(3)Zn1�O21�C3 130.4(2) 132.2(2) Br1�Hg1�S1#1 111.6(1)

Br1�Hg1�Br2 97.28(3)

a) The second number in a row refers to the S2 ligand. b) Symmetry transfor-mations used to generate equivalent atoms: #1 �x�1/2, y�1/2, �z�1/2; #2�x�1/2, y�1/2, �z�1/2

2.1 Crystal Structure of[Zn(Am4DH)(OAc)]2·H2O (1)

Selected bond distances are shown in Table 1 and selectedbond angles are presented in Table 2. As indicated pre-viously, five-coordinate [Zn(HAm4DH)X2] complexes

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 2011�20192012

(X � Cl, Br, I) were the subject of one of our previouscommunications [2]. Unlike those complexes, which in-volved the neutral ligand, complex 1 has anionic Am4DHcoordinating as NNS tridentate ligands and the five-coordi-nate zinc centers are connected by two bridging acetate li-gands (Fig. 2). In agreement with previously reported[Zn(Ac4E)(OAc)]2, where Ac4E is the anion of 2-acetylpyri-dine N(4)-ethylthiosemicarbazone, the two bridging acetateligands are different [15]. This manner of coordinationwas found in the complexes [Zn(Am4M)(OAc)]2 [8] and[Zn(Am4E)(OAc)]2 [16], where both acetate ligands are bis-monodentate bridges and Am4M and Am4E are the anionsof 2-pyridineformamide N(4)-methyl- and N(4)-ethylthiose-micarbazone. In the complexes [Zn(Ampip)(OAc)]2 [9] and[Zn(Amhexim)(OAc)]2·DMSO [14], where Ampip andAmhexim are the anions of 2-pyridineformamide 3-piperid-yl- and 3-hexamethyleneiminethiosemicarbazone, one of thebridging acetate ligands has a monodentate coordinationand the other bridges in a bidentate manner. This is a simi-lar situation to that in 1, where one acetate ligand bridgesin a syn-syn fashion through the O21�C3�O22 unit, withZn1�O21 and Zn1�O22 distances of 2.029(2) and2.051(2) A, respectively, and the other bridges with a singleoxygen, O11, with Zn1�O11� 2.006(2) A and Zn2�O11 �2.050(2) A. This arrangement results in two distinctZnN2O2S chromophores; the Zn1 polyhedron is closer to atrigonal bipyramid (τ � 0.20) [17] in which N11 and S1 areaxial and O11, O21 and N12 are equatorial. Zn2 is essen-tially a square pyramid (τ ca. 0) with O22 at its apex andN21, N22, S2 and O11 defining its planar base (Rms devi-ation 0.099 A of fitted atoms), which lies 0.309 A from thezinc atom (Table 3). Alternatively, if the Zn2�O12 distanceof 2.671(2) A and the associated O�Zn�X angles are con-sidered indicative of weak bonding (the sum of the van derWaals radii is 2.90 A [18]), then the second acetate ligandboth chelates and bridges and the geometry around Zn2 ispseudo-octahedral. Although there are small differences ineach of the parameters indicated above, this complex has

Fig. 2 View of a molecule of [Zn(Am4DH)(OAc)]2 showing theatom-labelling scheme. Displacement ellipsoids are draw at the50 % probability level.

Variability in the Coordination Modes

Table 3 Rms planes for 1, 5, and 6.

Compound Plane Rms deviation / A largest deviation / A Angle /° with previous plane

1 N11�C11�C12�C13�C14�C15 0.007 C15, 0.011(2) �C16�N12�N13�C17�S1�N14 0.085 N12, 0.148(2) 9.4(1)C26�N22�N23�C27�S2�N24 0.009 N23, 0.013(2) 29.6(1)N21�C21�C22�C23�C24�C25 0.003 C24, 0.005(2) 2.6(1)O11�S2�N22�N21 0.099 N22, 0.119(2) 9.0(1)O11�C1�O12�C2 0.001 C1, 0.002(2) 86.8(1)O21�C3�O22�C4 0.003 C3, 0.004(2) 10.2(2)

5 N11�C11�C12�C13�C14�C15 0.008 C11, 0.012(8) �C16�N12�N13�C17�S1�N14 0.112 N12, 0.219(7) 28.4(5)C26�N22�N23�C27�S2�N24 0.135 N22, 0.254(8) 5.5(5)N21�C21�C22�C23�C24�C25 0.002 C22, 0.004(9) 35.9(5)

6 N11�C11�C12�C13�C14�C15 0.008 C11, 0.01(2) �C16�N12�N13�C17�S1�N14 0.049 N13, 0.09(1) 8.2(6)

essentially the same structure as [Zn(Ac4E)(OAc)]2 [15].However, one major difference in the two binuclear com-plexes is the distance between Zn1 and Zn2, with values of3.462(1) A for 1 and 3.637(2) A for [Zn(Ac4E)(OAc)]2 [15].

Although the stereochemistry of the two zinc atoms in 1is different, the bond distances between zinc and the threedonor atoms of Am4DH are close to being within theircombined esd values. The Zn�S bonds show the greatestdifference, with Zn1�S1 � 2.3945(7) A and Zn2�S2 �2.3822(7) A. When compared to the bond distances of thethree donor atoms in [Zn(HAm4DH)Cl2] [2], the averagebonds in 1 are shorter � as one would expect for an anionicligand compared to a neutral ligand. The mean planes ofthe two thiosemicarbazone moieties in 1 have an angle be-tween them of 29.6(1)° (Table 3).

There is extensive intermolecular hydrogen bonding incomplex 1 (Table 4). The water molecule forms two hydro-gen bonds as a hydrogen donor (with the acetate oxygenatom uncoordinated and the S2 atoms as acceptors) andone as an acceptor (N15 as the donor). In addition, theuncoordinated nitrogen atoms participate in another hydro-gen bond, principally with the amine nitrogen atoms as do-

Fig. 3 View of a molecule of [Hg(HAm4DH)2Br2] showing theatom-labelling scheme and the intramolecular hydrogen bonding.Displacement ellipsoids are draw at the 50% probability level.

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nors and the hydrazine nitrogen atoms as acceptors. Thisbehavior gives rise to a supramolecular structure.

2.2 Crystal Structure of[Hg(HAm4DH)2Br2]·C2H5OH (5)

Mercury(II), in a similar manner to zinc(II) and cad-mium(II) [2, 3], formed [M(HAm4DH)X2] complexes witheach of the halo salts. However, neither of the crystals forwhich we have structural data have a similar composition.The mercury(II) compound discussed in this section hastwo neutral, monodentate thiosemicarbazone ligands coor-dinated through the thione sulfur atoms. The bromo ligandsalong with the thione sulfurs provide an approximate tetra-hedral polyhedron for Hg with bond angles ranging from97.28(3)° for Br1�Hg1�Br2 to 133.87(7)° for S1�Hg1�S2(Fig. 3). There is a substantial difference between the twoHg�Br bond distances and a smaller difference betweenthe two Hg�S bond distances (Tables 1 and 2). The struc-ture of another approximately tetrahedral four-coordinatecadmium complex, [Cd(4MeOBz4DH)2I2], with two 4-methoxybenzaldehyde thiosemicarbazone ligands coordi-nated through the thione sulfurs has recently been reported[19]. [Cd(4MeOBz4DH)2I2] is closer to tetrahedral than 5since its coordinate bond angles range from 100.02(3) to115.73(1)°. Small differences in bond distances and anglesbetween the HAm4DH ligands of 5 are within their com-bined esd values. The mean planes of the two thiosemicar-bazone moieties are at the largest angles from there pyridinerings, 28.45(48)° and 35.94(51)°, of any HAm4DH com-plexes studied to date [2, 3]. This situation is not unexpectedgiven the lack of chelation in 5. However, the thiosemicar-bazone moieties of the two ligands are almost coplanar, dif-fering by only 5.54(46)° (Table 3).

As in HAm4DH [2] and several of its complexes [2, 3],the coordinated HAm4DH ligands are extensively involvedin intra- and intermolecular hydrogen bonding (Table 4).All five N�H bonds participate as donors in a total of tenhydrogen bonds with a bromo ligand, pyridine nitrogen andazomethine nitrogen as acceptors in a curious fashion(Fig. 3). The amino N14 and N24 atoms in the molecule at

E. Bermejo, A. Castineiras, I. Garcıa-Santos, D. X. West

Table 4 Hydrogen bonding interactions for 1, 5, 6.

Com- D�H···A d(D�H) / A d(H···A) / A d(D···A) / A �(DHA) / °pound

1a) N14�H14A···N13#1 0.83(3) 2.28(3) 3.109(3) 170(3)N14�H14B···S1#2 0.87(3) 2.71(4) 3.562(3) 166(3)N15�H15B···O1 0.76(3) 2.26(3) 3.018(3) 173(3)N24�H24B···N23#3 0.88(4) 2.25(4) 3.110(3) 167(3)N25�H25A···S1 0.83(3) 2.89(3) 3.628(2) 149(3)N25�H25B···N24#3 0.87(3) 2.70(3) 3.463(3) 148(3)O1�H1A···S2 0.79(5) 2.62(5) 3.387(3) 167(5)O1�H1B···O22#4 0.84(5) 2.28(5) 3.082(3) 159(4)

5b) N13�H13···Br1 0.86 2.92 3.511(8) 127.9N13�H13···Br2 0.86 2.94 3.636(7) 139.6N14�H14A···Br1#1 0.86 2.86 3.576(8) 142.4N14�H14B···O1#1 0.86 2.13 2.980(7) 171.1N15�H15A···Br2 0.86 2.73 3.572(7) 165.3N15�H15B···N11#2 0.86 2.64 3.392(10) 146.1N23�H23···Br2 0.86 2.88 3.486(8) 129.4N23�H23···Br1 0.86 2.97 3.652(8) 137.7N24�H24A···Br2#3 0.86 2.77 3.491(8) 142.0N24�H24B···S1#4 0.86 3.02 3.746(9) 143.1N25�H25A···Br1 0.86 2.71 3.552(8) 167.0O1�H1···N14#3 0.82 2.18 2.980(7) 164.8

6c) N14�H14A···Br1#2 0.86 2.87 3.656(13) 153.6N14�H14B···Br1#1 0.86 3.04 3.662(13) 130.6N15�H15A···Br1#2 0.86 2.88 3.732(13) 171.0N15�H15B···Br1#3 0.86 2.78 3.508(12) 143.7

Symmetry transformations used to generate equivalent atoms:a) #1: �x�1/2, y�1/2, �z�1/2; #2: �x�1/2, y�1/2, �z�1/2; #3: �x, �y�1,�z; #4: x�1, y�1, zb) #1: x�1, y, z; #2: �x�1, �y�1, �z�2; #3: x�1, y, z; #4: �x�2, �y�2,�z�1.c) #1: �x�1/2, y�1/2, �z�1/2; #2: x�1, y, z; #3: �x�1, �y, �z�1.

(x, y, z) act as hydrogen bond donors, through H14A andH24A, respectively, to Br1 (x�1, y, z) and Br2 (x�1, y, z).This gives rise to a double chain running parallel to the[100] direction generated by translation (Fig. 4). The watermolecule is only hydrogen bonded to the amine N14 as botha donor and acceptor.

Fig. 4 One-dimensional hydrogen bonding network in 5 in theac plane.

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2.3 Crystal Structure of �1[Hg(μ-S-Am4DH)Br] (6)

The structure of the second mercury(II) complex derivedfrom a solution of [Hg(HAm4DH)Br2] is polymeric, withrepeating five-coordinate mercury(II) centers along the[010] direction (Fig. 5). Coordination around each mercuryatom involves the NNS tridentate Am4DH as in 1, but inthis case the thiolato sulfur bridges to the neighboring HgII

atom in the chain. A terminal Br ligand brings the coordi-nation number to 5 for the repeating Hg atoms, with thedistance between metal atoms being 4.384(1) A (Table 1).The bridging sulfur is significantly closer to the second Hg,2.501(3) A, than the Hg center for which it is a part of theNNS tridentate system, 2.633(4) A, and both Hg�S dis-tances are significantly longer than the Hg�S distances forthe four-coordinate mercury atom in 5. However, theHg�Br distance is significantly shorter in 6 compared toeither of the Hg�Br distances in 5. N11�Hg1�S1 is as-signed as β and N12�Hg1�Br1 as α, τ � 0.33 [24], indicat-ing considerable distortion toward a trigonal bipyramidalstructure about each of the mercury(II) atoms (Table 2).

Fig. 5 Section of the one-dimensional coordination polymer 6.The atoms marked with a and b are of symmetry position (�x�1/2, y�1/2, �z�1/2) and (�x�1/2, y�1/2, �z�1/2), respectively.

On considering the bonds of the thiosemicarbazonemoiety, it is noteworthy that C17�S1 is significantly longerthan in the other complexes because of the bridging be-tween mercury atoms. The bond angles of the thiosemicar-bazone moieties of the two mercury(II) complexes are simi-lar even though one complex has a neutral HAm4DH li-gand and the other has an anionic Am4DH ligand. Theangle between the mean planes of the pyridine ring and thethiosemicarbazone moiety are similar to other complexes inthis study that contain the anionic Am4DH ligand(Table 3).

Variability in the Coordination Modes

The hydrogen atoms attached to the nitrogen atoms ofone mercury(II) atom interact weakly with bromo ligandsof three neighboring molecules (Table 4), indicating that in-teraction occurs between the polymer chains (Fig. 6).

Fig. 6 Part of the crystal structure of 6 showing the linking of the[010] chains. The atoms marked with the sign (�) are of symmetryposition (�x�1/2, y�1/2, �z�1/2).

The packing diagram also shows the stacking of the pyri-dine rings [20]. The one-dimensional coordination polymer1�[Hg(μ-S-Am4DH)Br] generated in the crystals by sym-

Fig. 7 Partial packing diagram of 6, showing the formation of aone-dimensional strand and the slipped π-π interactions.

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metry along the [010] direction (lattice vector b) forms infi-nite columns (Fig. 7). The neighboring polymeric chains ofthe 1D complex are intercalated and the result is that allaromatic rings in one column are parallel. The distance be-tween ring centroids is 3.653 A (β � 24.75°) and the per-pendicular distance between ring systems of 3.317 A is lessthan the separations observed for stacking interactions(3.3�3.8 A) [20].

2.4 IR Spectroscopy

Coordination of the imine nitrogen, N2, often causes a shiftin νC�N to lower energy by 20�30 cm�1 in divalent andtrivalent transition metal ion complexes of thiosemicarba-zones [21]. A second peak in this region of the spectrum isassigned to νN3�C7, which formally becomes a doublebond in the coordinated anion. Coordination of the iminenitrogen results in a band at 440�480 cm�1, which is as-signed to νM�N2 in transition metal complexes [21], andat somewhat lower energy for Group XII metal complexes,particularly mercury(II) complexes. Coordination of theanion causes a shift to higher energy of 10�30 cm�1 forνN�N in these metal complexes, but such a shift is notobserved in the mercury(II) complexes with the neutralform of the thiosemicarbazone ligand. Coordination of S1results in a small decrease in the energy of the thioamideIV band, which has a large contribution from νC�S, incomplexes with neutral thiosemicarbazone ligands. A muchlarger shift to lower wavenumbers is found in complexeswith an anionic thiosemicarbazone ligand. Coordination ofeither form of the sulfur atom results in a band in the318�349 cm�1 region for the Group XII metal ion com-plexes.

The ν(CO2) bands are found at 1586 and 1406 cm�1 for2, which suggests bridging bidentate coordination for bothacetate ligands. However, two sets of bands assignable toν(CO2) at 1583/1422 cm�1 and 1635/1404 cm�1 are foundin the spectrum of 1. The former suggests bridging biden-tate coordination and the latter bridging monodentate co-ordination in this binuclear complex. Assignment of theνHg�X bands gives the following ratios: νHg�Br/νHg�Cl � 0.70 and νHg�I/νHg�Br � 0.54, which are inthe expected range [22].

Scheme 1

E. Bermejo, A. Castineiras, I. Garcıa-Santos, D. X. West

2.5 NMR Spectroscopy

In the 1H NMR spectrum, the N3H resonance is no longerobserved when the anionic Am4DH is coordinated to thevarious metal centers, but is found downfield in the mer-cury(II) complexes with neutral HAm4DH ligands whencompared to free HAm4DH. The downfield shift of N3His consistent with coordination of the thione sulfur in themercury(II) complexes, which results in a transfer of elec-tron density from N3 and a more polar N3�H bond. Simi-lar downfield shifts occur for the peaks due to N4H2 andN5H2 in the mercury complexes. However, upfield shifts oc-cur for N4H2 and N5H2 in complexes with the coordinatedanion due to the greater electron density along the thio-semicarbazone moiety. This electron density is likely to beaugmented by π-back donation back from the d8 and d10

metal ions.The HAm4DH ligand and its complexes present two dif-

ferent NH2 groups, one amide group (N(5)H2) and onethioamide group (N(4)H2). In the former case only observeonly one signal for the two protons is observed. However,the appearance of more than one N(4)H2 proton signal forsome complexes suggests the presence of more than onespecies in the solution phase (Scheme I). The N(4)H2

protons of the thioamide in the free thiosemicarbazone andthe mercury complexes, [Hg(HAm4DH)Cl2] and[Hg(HAm4DH)Br2], give rise to two peaks, which are attri-buted to the restricted rotation of this group about theC(7)�N(4) bond axis due to delocalization of the lone pairon the N(4)H2 nitrogen atom (I, II). These protons gener-ally show a single broad signal in the complexes,e.g., in [Zn(Am4DH)(OAc)]2, [Cd(Am4DH)(OAc)]2 and[Hg(HAm4DH)Br2], because deprotonation of N(3)H leadsto structure III and a reduction in the double-bond charac-ter of the C(7)�N(4) bond enables free rotation of the NH2

group about the C(7)�N(4) bond [23].In the 13C NMR spectrum, small changes occur in the

signal due to C1 on complexation and other protons at-tached to the pyridine ring experience similar or smallerchanges. Coordination of the thione sulfur atom in the mer-cury complexes causes a more marked change in chemicalshift for 13C7 than coordination of the thiolato sulfur atomin the complexes with the anion � once again suggestingsignificant back donation by metal ions. Back donation ofπ-electron density is presumably unimportant in the case ofmercury(II) due to poor overlap of the metal’s d orbitalswith ligand orbitals. The 13C6 signal also shifts on coordi-nation of the imine nitrogen in the various complexes. Theassignment of 1H and 13C spectra was aided by the use ofthe HMQC and HMBC techniques.

The proton signals of the methyl groups in the 1H NMRspectra of complexes 1 and 2 appear at 1.82 and 1.83 ppm,respectively. In the 13C spectrum the Me(OAc) and C(OAc)signals in the zinc complex appear at 22.82 and 175.03 ppm,respectively, but it was not possible to obtain this spectrumfor the cadmium complex due to its lack of solubility. The13C CP/MAS NMR spectra for 1 and 2 are shown in Fig. 8.The solid-state 13C NMR spectra of 1 and 2 were studied

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 2011�20192016

in order to elucidate the structural differences between Znand Cd complexes. The 13C CP/MAS spectrum of[Zn(Am4DH)(OAc)]2 presents two signals for the methyl ofthe acetate group, which is consistent with the different co-ordination of the two acetate ligands observed by X-ray dif-fraction. The single peak observed for this methyl groupin 2 indicates that the two acetate ligands have the samecoordination mode, a situation in agreement with the IRspectrum. Two peaks were observed for the other carbon ofthe acetate group in the zinc complex and one signal for thecorresponding carbon in the cadmium complex.

Fig. 8 The 13C CP/MAS NMR spectra of [Zn(Am4DH)(OAc)]2(top) and [Cd(Am4DH)(OAc)]2 (bottom).

The 113Cd NMR spectrum of 2 in DMSO shows two sig-nals. This is consistent with two cadmium atoms with differ-ent environments [24] and provides strong evidence for ex-change processes of the type:

[Cd(Am4DH)(OAc)]2 � DMSO p [Cd(Am4DH)2] �

Cd(OAc)2·nDMSO

The downfield signal, at 447 ppm, and the upfield signal,at 249 ppm, are due to [Cd(Am4DH)2] and the dimer ormonomer [Cd(Am4DH)(OAc)], probably with DMSO,respectively. The Cd(OAc)2 signal in DMSO, at �18 ppmwith respect to Cd(ClO4)2, is very broad is cannot ob-served.

Variability in the Coordination Modes

The experimental evidence can be summarized asfollows: (a) Recrystallization of [Cd(Am4E)(OAc)]2,[Cd(Ampip)(OAc)]2 and [Cd(Amhexim)(OAc)]2 fromDMSO gave [Cd(Am4E)2] [16], [Cd(Ampip)2] [9], and[Cd(Amhexim)2] [14], respectively; (b) all 113Cd NMR spec-tra of cadmium complexes of formula [Cd(Am4R)(OAc)]2contain two signals at around 250 and 450 ppm (Table 5);(c) the upfield shifts induced by substituents in the thio-semicarbazone on the downfield signal confirm that thissignal is due to Cd�thiosemicarbazone; (d) replacement ofsulfur and nitrogen atoms by oxygen atoms tends to givegreater shielding [25] and a decrease in the coordinationnumber tends to give greater deshielding. [26] Thus, thevalue of 249 ppm is a compromise between the two factorsoutlined above.

Table 5 113Cd Chemical shifts (ppm) versus Cd(ClO4)2.

Compound Ref.

[Cd(Am4DH)(OAc)]2 249 447 this work[Cd(Am4M)(OAc)]2 247 440 [28][Cd(Am4E)(OAc)]2 255 415 [16][Cd(Am4DM)(OAc)]2 251 451 [28][Cd(Ampip)(OAc)]2 251 450 [9][Cd(Amhxim)(OAc)]2 251 449 [14]

Consequently, the presence of two 113Cd resonances isindicative of rapid exchange between the five-coordinateCdN2O2S and six-coordinate CdN4S2 complexes on thebasis of comparison with the chemical shift differences be-tween these two species [27].

The 199Hg chemical shift in [Hg(HAm4DH)Cl2](�1058 ppm in DMSO-d6, relative to dimethylmercury) ischaracteristic of mercury chloride thione complexes. Thevalue is in good agreement with some analogous five-coor-dinate HgCl2N2S complexes [28] and indicates that only onemercuric species is present in solution. The accompanyingline-broadening is attributed to chemical shift anisotropy, acommon effect in 199Hg NMR spectroscopy. The HgCl2 hasa resonance at �1501 ppm [29] and this shifts to�1058 ppm for [Hg(HAm4DH)Cl2]; the difference inchemical shift is 443 ppm whereas for [Hg(HAm4M)Cl2]the difference is only 359 ppm [28]. This indicatesthat HAm4DH forms a stronger complex than[Hg(HAm4M)Cl2]. Such an observation is expected becausethe influence of the alkyl-substituent on the thioamide ni-trogen atom after the formation of the NH···S hydrogenbond would in turn affect the Hg�S bond character [30].

3 Conclusions

As pointed out previously [2, 5], hydrogen bonding is amuch more important factor in the structures of the com-plexes coordinated by thiosemicarbazones attached to anamide carbon as opposed to an aldehyde or ketone carbon.This situation could explain the failure of attempts to in-crease the water solubility of such metal complexes. Themultitude of hydrogen bonds that are formed in complexes

Z. Anorg. Allg. Chem. 2005, 631, 2011�2019 zaac.wiley-vch.de © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2017

with coordinated 2-pyridineformamide thiosemicarbazonesis an important factor in our ability to obtain numerouscrystals of diffraction quality. The large number of solvedcrystal structures has allowed us to study various modes ofcoordination by heterocyclic thiosemicarbazones to a vari-ety of different metal ions, as well as mononuclear, bi-nuclear and polynuclear complexes. Our studies of theseinteresting complexes continue.

Supplementary Material

Crystallographic data for the structures reported in this paper (ex-cluding structure factors) have been deposited with the CambridgeCrystallographic Data Centre as Supplementary Publication No.CCDC-257614, [Zn(Am4DH)(OAc)]2·H2O; CCDC-257615,[Hg(HAm4DH)2Br2]·C2H5OH; and CCDC-257616, 1

�[Hg(μ-S-Am4DH)Br]. Copies of the data can be obtained free of charge onapplication to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK(Fax: � 44-1223/336-033. E-mail: deposit@ccdc.cam.ac.uk).

4 Experimental

4.1 Synthetic Methods

HAm4DH was prepared by reducing 2-cyanopyridine in the pres-ence of thiosemicarbazide as described previously [2].

[Zn(Am4DH)(OAc)]2·H2O (1): A solution of HAm4DH (0.25 g,1.28 mmol) in ethanol (20 mL) was added to a solution ofZn(CH3COO)2·2H2O (0.28 g, 1.28 mmol) in ethanol (20 mL). Themixture was heated under reflux for 2 h. The resulting yellow solidwas filtered off and recrystallized from DMSO to give yellow crys-tals that were suitable for X-ray crystallography.Yield: 0.36 g (85.4 %). Yellow, m.p. > 300 °C. Analytical Data:C18H24N10O5S2Zn2 (aqua solvate) (655.33):C 32.4 (calc. 33.0); H3.35 (3.7); N 21.01 (21.4); S 8.5 (9.8) %.

IR (νmax/cm�1): 3439�3167 (OH, NH), 1602�1547 (C�N, C�C), 840 (C�S), 423 (Zn�N2), 413 (Zn�O), 339 (Zn�S).1H NMR (DMSO-d6, ppm): 8.44 (1H, s, H1); 8.06 (2H, s, H3�H4), 7.58(1H, s, H2), 6.86 (2H, s, N5H2), 5.94 (2H, s, N4H2), 1.82 (3H, s, Me(OAc)).13C NMR (DMSO-d6, ppm): 173.41 (C7), 147.8 (C1), 146.23 (C5), 139.68(C6), 138.67 (C3), 125.44 (C2), 119.98 (C4).

[Cd(Am4DH)(OAc)]2 (2): A solution of HAm4DH (0.25 g,1.28 mmol) in ethanol was added to a stirred solution ofCd(CH3COO)2·2H2O (0.34 g, 1.28 mmol) in ethanol (20 mL). Ayellow solid precipitated immediately. The mixture was heated un-der reflux for 2 h and the solid was filtered off.Yield: 0.44 g (93.8 %). Yellow, m.p. > 300 °C. Analytical data:

C18H22Cd2N10O4S2 (731.38): C 29.4 (calc. 29.6); H 3.2 (3.0); N 19.0(19.2); S 8.8 (8.8) %.

IR (νmax/cm�1): 3428�3154 (NH), 1586, 1563 (C�N, C�C), 846 (C�S), 426(Cd�N2), 405 (Cd�O), 349 (Cd�S).1H NMR (DMSO-d6, ppm): 8.41 (1H, s, H1); 8.04 (2H, m, H3�H4); 7.59(1H, t, H2), 6.71 (2H, s, N5H2); 5.95 (2H, s, N4H2); 1.83 (3H, s, Me(OAc)).

[Hg(HAm4DH)Cl2] (3): A solution of HgCl2 (0.35 g, 1.28 mmol)and HAm4DH (0.25 g, 1.28 mmol) in ethanol (20 mL) was heatedunder reflux for 2 h. A white solid precipitated and the productwas filtered off and dried under vacuum.Yield: 0.54 g (97 %). Pale yellow, m.p. 180 °C. Analytical data:

C7H9Cl2HgN5S (466.74): C 18.5 (calc. 18.0); H 2.0 (1.9); N 14.9(15.0); S, 6.5 (6.9) %.

E. Bermejo, A. Castineiras, I. Garcıa-Santos, D. X. West

IR (νmax/cm�1): 3431�3183 (NH), 1605�1550 (C�N, C�C), 845 (C�S),401 (Hg�N2), 327 (Hg�S), 292 (Hg�Cl).1H NMR (DMSO-d6, ppm): 11.04 (1H, s, N3H); 8.95, 8.73 (2H, s, s, N4H2);8.60 (1H, d, H1), 8.48 (1H, d, H4); 7.87 (1H, td, H3), 7.50 (1H, dd, H2),1.82 (3H, s, Me(OAc)). 13C NMR (DMSO-d6, ppm): 168.07 (C7), 149.42(C5), 148.51 (C1), 147.46 (C6), 137.34 (C3), 125.82 (C2), 122.11 (C4).

[Hg(HAm4DH)Br2] (4): A solution of HgBr2 (0.46 g, 1.28 mmol)and HAm4DH (0.25 g, 1.28 mmol) in ethanol (20 mL) was heatedunder reflux for 2 h. The resulting white solid was filtered off anddried under vacuum. Slow evaporation of the solvent yielded color-less crystals of [Hg(HAm4DH)2Br2]·EtOH (5). Recrystallizationfrom ethanol gave yellow crystals of 1

�[Hg(μ-S-Am4DH)Br] (6).Yield: 0.54 g (57.2 %). Pale yellow, m.p. 160 °C. Analytical data:

C7H9Br2HgN5S (555.64): C 15.2 (calc. 15.1); H 1.6 (1.6); N 12.6(12.6); S 6.0 (5.8) %.

IR (νmax/cm�1): 3430�3181 (NH), 1606�1544 (C�N, C�C), 846 (C�S),401 (Hg�N2), 324 (Hg�S), 203 (Hg�Br).1H NMR (DMSO-d6, ppm): 10.93 (1H, s, N3H); 8.67 (2H, s, N4H2); 8.59(1H, d, H1); 8.48 (1H, d, H4); 7.87 (1H, td, H3); 7.50 (1H, dd, H2); 7.27(2H, s, N5H2). 13C NMR (DMSO-d6, ppm): 168.49 (C7), 149.59(C5), 148.49(C1), 147.12 (C6), 137.30 (C3), 125.73 (C2), 122.11 (C4).

[Hg(HAm4DH)I2] (7): A solution of HAm4DH (0.25 g, 1.28 mmol)in ethanol (20 mL) was added to a solution of HgI2 (0.58 g,1.28 mmol) in ethanol (20 mL). The mixture was heated under re-flux for 2 h and the yellow solid was filtered off and dried undervacuum.Yield: 0.53 g (64.2 %). Yellow, m.p. 220 °C. Analytical data:

C7H9HgI2N5S (649.64): C 13.5 (calc. 12.9); H 1.2 (1.4); N 10.9(10.8); S, 4.4 (4.9) %.

IR (νmax/cm�1): 3408�3243 (NH), 1600�1535 (C�N, C�C), 849 (C�S),391 (Hg�N2), 318 (Hg�S), 159 (Hg�I).

Table 6 Crystal data and structure refinement for [Zn(Am4DH)(OAc)]2·H2O (1), [Hg(HAm4DH)2Br2]·C2H5OH (5), and1�[Hg(μ-S-Am4DH)Br] (6).

1 5 6

Empirical Formula C18H24N10O5S2Zn2 C16H24Br2HgN10OS2 C7H8BrHgN5SColor, Habit yellow, prism colorless, prism yellow, prismFormula weight 655.44 796.98 474.74Temperature, K 293(2) 293(2) 293(2)Crystal size (mm) 0.35 � 0.35 � 0.30 0.35 � 0.30 � 0.20 0.16 � 0.16 � 0.08Crystal System monoclinic triclinic monoclinicSpace Group P21/n (No. 14) P1 (No. 2) P21/n (No. 14)Unit Cell Dimensionsa / A 10.238(1) 8.206(1) 8.541(10)b / A 7.744(1) 10.631(1) 7.448(2)c/ A 32.152(4) 16.337(2) 17.293(10)� / ° � 77.311(11) �β / ° 96.03(1) 75.520(9) 99.81(6)γ / ° � 79.104(11) �Volume / A3 2534.8(5) 1332.5(3) 1084.0(14)Z 4 2 4Density (Mg/m3) 1.717 1.986 2.909Absorption coeff. / mm�1 4.324 15.584 18.050θ range for data collection / ° 2.76 to 75.89 2.84 to 76.09 2.39 to 26.21Index Ranges �12 � h � 12 0 � h � 10 �10 � h � 10

�9 � k � 9 �13 � k � 13 0 � k � 90 � l � 40 �19 � l � 20 �21 � l � 0

Absorption Correction Ψ-scan Ψ-scan Ψ-scanReflections collected 10337 5976 2249Ind. reflects., Rint 5284, 0.0202 5573, 0.0405 2178, 0.0904Data / parameters 5284 / 409 5573 / 266 2178 / 136Final R Indices [I>2σ(I)] R1 � 0.0330 R1 � 0.0578 R1 � 0.0463

wR2 � 0.0903 wR2 � 0.1618 wR2 � 0.0860R Indices (all data) R1 � 0.0357 R1 � 0.0620 R1 � 0.1806

wR2 � 0.0921 wR2 � 0.1653 wR2 � 0.1076Goodness-of-Fit 1.074 1.024 0.975Largest diff. peak/hole/ eA�3 0.446 / �0.617 1.992 / �2.063 1.518 / �2.528

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 2011�20192018

1H NMR (DMSO-d6, ppm): 10.93 (1H, s, br, N3H); 8.90, 8.66 (2H, d, s, br,N4H2); 8.59 (1H, d, H1); 8.48 (1H, d, H4); 7.88 (1H, td, H3); 7.51 (1H, m,H2); 7.21 (2H, s, N5H2). 13C NMR (DMSO-d6, ppm): 170.13 (C7),149.37(C1), 148.03 (C5), 145.76 (C6), 137.95 (C3), 124.68 (C2), 120.96 (C4).

4.2 Instrumetal methods

Elemental analyses were performed with a Carlos Erba 1108 micro-analyser. NMR spectra were obtained as d6-DMSO solutions witha Bruker AMX 300 spectrometer and chemical shifts are reportedin parts per million downfield from Me4Si. IR spectra were re-corded as KBr disks (4000�400 cm�1) or polyethylene-sandwichedNujol mulls (500�100 cm�1) with a Bruker IFS-66v spectrophoto-meter. The mass spectrum of HAm4DH was acquired using theelectron impact method with a Hewlett-Packard HP5988A massspectrometer and the spectra of the metal complexes were obtainedusing the FAB technique.

4.3 Crystal structure determination, refinement andsolution

Crystals of [Zn(Am4DH)(OAc)]2·H2O (1) were grown inDMSO, whereas [Hg(HAm4DH)2Br2]·C2H5OH (5) and 1

�[Hg(μ-S-Am4DH)Br] (6) were grown from ethanolic solutions. The crystalswere mounted on glass fibers and used for data collection. Reflec-tions were obtained with Enraf Nonius diffractometers: CAD4(λ � 1.54184 A) for 1 and 5 and MACH3 (λ � 0.71073 A) for 6.Cell constants and an orientation matrix for data collection wereobtained by least-squares refinement of the diffraction datafrom 25 reflections in the ranges 22.964° < θ < 45.331° for 1,

Variability in the Coordination Modes

19.389° < θ < 45.371° for 5 and 8.76 < θ < 20.87° for 6. Thestructures were solved by direct methods [31] and subsequent dif-ference Fourier maps, which revealed the positions of all non-hydrogen atoms, and refined on F2 by a full-matrix least-squaresprocedure using anisotropic displacement parameters [32]. Theatoms of ethanol in 5 were refined isotropically. Hydrogen atomswere located in difference Fourier maps or placed geometricallyand included as fixed contributions riding on attached atoms withisotropic thermal parameters 1.2 times those of their carrier atoms.In 5 the contribution of the density of other disordered solventmolecules was subtracted from measured structure factors usingthe SQUEEZE option [33]. Atom scattering factors were takenfrom the International Tables for Crystallography [34] and molecu-lar graphics were created using PLATON [33]. Table 6 containssummaries of crystal data and intensity collection for[Zn(Am4DH)(OAc)]2·H2O, [Hg(HAm4DH)2Br2]·C2H5OH and1�[Hg(μ-S-Am4DH)Br].

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