Scientific paper
New Molybdenum(VI) Complexes with Thiosemicarbazone Ligands Containing 4-hydroxy-2-pyrone Ring
Vi{nja Vrdoljak,a'* Biserka Prugove~ki,a Marina Cindri},a Dubravka Matkovi}-Calogovi}a and Ana Brbot-Saranovi}
"Laboratory of General and Inorganic Chemistry, Chemistry Department, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia.
* Corresponding author: E-mail: visnja@chem.pmf.hr Fax:+385-1-4606341
Received: 28-03-2008
Dedicated to the memory of Professor Ljubo Golic
Abstract
New molybdenum(VI) complexes have been prepared by the reaction of [MoO2(acac)2] (acac = acetylacetonate ion) with the thiosemicarbazone ligands derived from dehydroacetic acid and thiosemicarbazide (H2L1) or 4-phenylthiose-micarbazide (H2L2). All complexes consist of the MoO22+ core with molybdenum coordinated by ligand L via three donor atoms: pyran-4-ol-oxygen, imine-nitrogen and thiol-sulphur. Unlike the monomeric complexes [MoO2L2D], where the octahedral coordination around the molybdenum atom is achieved by additional coordination of one neutral solvent molecule D (methanol in 2a or ethanol in 2b), in the polymeric complexes [MoO2L]n (1 and 2) the octahedral coordination is accomplished by an intermolecular metaloxygen interaction (metal...oxygen//Mo=O---Mo=O). All compounds were characterized by means of elemental analyses, IR spectra, thermal analysis and some of them by X-ray crystallography (ligand HL1 and complex 2b).
Keywords: Molybdenum(VI) complexes; dehydroacetic acid, thiosemicarbazone; pyrone; crystal structure
1. Introduction
Various biological activities (including antibacterial, antiviral and antitumor activities) of thiosemicarba-zones and their metal complexes have inspired numerous investigations concerning their sytheses, characterization and bioinorganic relavance.1-5 2-Pyrone derivatives and their metal complexes have received similar attention in
chemistry and biology.6-10 However, little is known about complexes with thiosemicarbazone ligands containing the 2-pyrone ring.11-14 As far as we know only the cop-per(II) complex [Cu(DAhexim-H)Cl] • MeCN (DAhe-xim-H = 3-hexamethyleneiminyl-N-substituted thiosemi-carbazone of dehydroacetic acid) has been structurally characterized.11
Scheme 1
H2L1
h2l2
Due to the biological properties of such compounds we are interested in nature of coordination as well as in the structural and spectral properties of molybdenum complexes with thiosemicarbazone ligands. Recently we have published syntheses and structures of several molyb-denum(VI) complexes with aromatic thiosemicarbazones (salicylaldehyde, 2-hydroxy-1-naphthaldehyde, 2-hy-droxy-3-methoxybenzaldehyde or 4-(diethylamino)sa-licylaldehyde).15-18 Continuing with our research on thio-semicarbazones, we report here the synthesis of novel molybdenum(VI) complexes [MoO2L]n (1 and 2), [Mo-02LD], (2a, D = methanol and 2b, D = ethanol) with thio-semicarbazone ligands containing 4-hydroxy-2-pyrone ring derived from dehydroacetic acid (Scheme 1). The molecular structures of molybdenum(VI) complex (2b), as well as of one ligand (H2L1) have been determined by a single crystal X-ray diffraction method.
The ligands and complexes have also been characterized by means of chemical analyses, thermal analysis and IR spectroscopy.
2. Experimental
All chemicals were of reagent grade and used as purchased. The starting complex [MoO2(acac)2] was prepared as described in the literature.19 Methanol and etha-nol were dried using magnesium turnings and iodine, and then distilled. Acetonitrile was dried over P2O5. C, H, N and S analyses were provided by the Analytical Services Laboratory of Rudjer Boskovic Institute, Zagreb. Infrared spectra were recorded in KBr with an FTIR 1600, Fourier transform spectrophotometer in the 4500-450 cm-1 region. Thermogravimetric (TG) analyses were measured on a Mettler TG 50 thermobalance using aluminum crucibles under oxygen atmosphere with the temperature range from 25 to 600 °C. Melting points of the H2L1 and H2L2 were determined using differential scanning calori-metry on a Mettler DSC 30 instrument under nitrogen atmosphere with the temperature range from 25 to 250 °C. All experiments were recorded with a heating rate of 5 °C min-1 in a dynamic atmosphere with a flow rate of 200 cm3 min-1. The results were developed by applying the Mettler STARe 6.1. programme.
2. 1. Synthesis of Thiosemicarbazones
(H2L1, H2L2)
Thiosemicarbazones H2L1 and H2L2 (Scheme 1) were prepared by the reaction of dehydroacetic acid (dha = 3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one) and appropriate thiosemicarbazides H2NNHC(S)NHR (where R = H, or C6H5) according to already described procedure.20 From the DSC measurements it follows that the melting points for ligands were 196 °C and 184 °C for H2L1 and H2L2, respectively. Single crystals of H2L1 suitable for the
X-ray diffraction study were obtained by recrystallization of H2L1 from ethanol.
2. 2. Synthesis of Thiosemicarbazone Complexes
2. 2. 1. Synthesis of [MoO2L1]n (1)
[MoO2(acac)2] (0.1 g, 0.3 mmol) was dissolved in dry acetonitrile (20 mL) and H2L1 (0.3 mmol) was added. The mixture was refluxed for 12 h and a greenish brown product deposited during warming of the reaction mixture. It was filtered off, washed with acetonitrile, methanol and dried.
Yield: 0.03 g, 27%. Found: C, 29.04; H, 2.23; N, 11.13; S, 8.25%. Anal. Calcd. for C H MoN O S: C, 29.44; H, 2.47; N, 11.44; S, 8.73%. tg: 938.85% 5MoO3 (Calcd. 39.20%)	3
Selected IR data (cm-1): 1717 (C=0), 1640 (C=0), 1559 (C=N), 1507 (C-0), 905 (MoO2), 829 (Mo=O-Mo), 752 (C-S).
2. 2. 2. Synthesis of [MoO2L2]n (2)
[MoO2(acac)2] (0.1 g, 0.3 mmol) was dissolved in dry acetonitrile (20 mL) and H2L2 (0.3 mmol) was added. The mixture was refluxed for 6 h and a greenish brown product deposited during warming of the reaction mixture. It was filtered off, washed with acetonitrile and dried.
(2) Yield: 0.11 g, 81%. Found: C, 40.11; H, 2.50; N, 9.21; S, 7.05%. Anal. Calcd. for C H MoN O S: C, 40.64; H, 2.96; N, 9.48; S, 7.23%. tÌ^g:1 331.93% 5MoO3 (Calcd. 32.47%)	3
Selected IR data (cm-1): 1700 (C=0), 1645 (C=0), 1568 (C=N), 1528 (C-0), 918 (MoO2), 827 (Mo=O-Mo), 759 (C-S).
2. 2. 3. Synthesis of [MoO2L2(ROH)] (2a, 2b)
[MoO2(acac)2] (0.15 g, 0.46 mmol) was dissolved in dry alcohol (30 mL) and H2L2 (0.46 mmol) was added. The mixture was warmed up for 5 h, the solution was evaporated to one third of its volume under reduced pressure and left at room temperature. Upon standing for a few days the red crystalline products were filtered off and dried.
[MoO^L'iCH^OH)] (2a) Red needle shaped crystals of 2a obtained from the methanol solution were readily loosing solvated methanol molecules at room temperature. They were left in a dessicator up to constant weight and analyzed as unsolvated species.
Yield: 0.04 g, 81%. Found: C, 40.11; H, 2.50; N, 9.21; S, 7.05%. Anal. Calcd. for C1gH17MoN30gS: C, 40.43; H, 3.60; N, 8.84; S, 6.75%. TG: 29.93% MoO3 (Calcd. 30.28%), 7.04% CH3OH (Calcd. 6.74%).
Selected IR data (cm-1): 1699 (C=0), 1636 (C=0), 1567 (C=N), 1520 (C-0), 934 (MoO2 asym), 889 (Mo-
02_sym), 754 (C-S).	_
[MoO2L2(CH3CH2OH)] (2b) Red plate like crystals. Yield: 0.06 g, 81%. Found: C, 40.64; H, 2.96; N, 9.48; S, 7.23%. Anal. Calcd. for C H MoN O6S: C, 41.72; H, 3.91; N, 8.59; S, 6.55%. C1,7 40964; H3, ^.96; N, 9.48; S,
best quality because of slight decomposition of the crystal.
Solution, refinement and analysis of the structures were done using the programs integrated in the WINGX
7.23%. TG: 28.87% MoO3 (Calcd. 29.42%); 9.22% system.22 The structures were solved by direct methods us-
C2H5OH (Calcd. 9.41%)
Selected IR data (cm-1): 1694 (C=O), 1637 (C=O), 1569 (C=N), 1521 (C-O), 942 (MoO2 asym), 902 (Mo-O2sym), 758 (C-S).
2. 3. X-ray Crystallography
Crystals of H2L1 were stable and diffraction data could be collected at room temperature. On contrary, crystals of 2b were unstable upon prolonged exposure to air. Room temperature data of 2b crystal in a capillary was extremely poor. A crystal protected with vacuum grease at low temperature still decomposed so the method of protein crystal mounting was adapted for the second data collection. The crystal was mounted in a loop in a mixture of ethanol and glycerol, and cooled in nitrogen vapor stream at 110 K. Diffraction intensity data for both compounds, H2L1 and 2b were collected by ra-scans on an Oxford Diffraction Xcalibur CCD diffracto-meter with graphite monochromated Mo-^a radiation (X = 0.71073 À) and reduced using the CRYSALIS software package.21 Analytical absorption correction gave no improvement of the 2b data set. The data for 2b is not of
ing the SHELXS97 program.23 The refinement procedure by the full-matrix least-squares method based on F2 against all reflections included anisotropic displacement parameters for all non-hydrogen atoms. All hydrogen atoms were placed in calculated positions. Refinement was performed using the SHELXL97 program.24 Geometrical calculations and the figures were performed using the PLATON program.25 Crystal data, experimental conditions and refinement parameters are summarized in Table 1.
Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary material with the deposition numbers CCDC 682 602 and CCDC 682 603. Copies of the data can be obtained, free of charge via http:/www.ccdc.cam. ac.uk/const/retrieving.html.
3. Results and Discussion
The complexes are prepared by the reaction of [Mo-O2(acac)2] (acac = acetylacetonate ion) with the thiosemi-carbazone ligands derived from dehydroacetic acid (dha)
Table 1. Crystallographic data for compounds H2L1 and 2b.
Compound	h2l1	2b
Empirical formula	C9H„N303S	C17H19MoN3O6S
Formula weight	241.27	4^9.3^
Crystal system	monoclinic	tri-clinic
Space group	P21/c	P-
a (À)	7.885(2)	8.531(5)
b (À)	12.201(3)	10.157(7)
c (À)	11.679(3)	11.564(5)
a(°)	90	97.540(5)
ß(°)	102.84(2)	104.300(5)
y(°)	90	91.120(5)
V (À3)	1095.5(5)	961.2(10)
Z	4	2
T (K)	295(2)	110(2)
Đcalc(g cm-3)	1.463	1.687
^ (MoKa) (mm-1)	0.292	0.831
F(000)	504	496
Ranges of h, k, l	-9-9, -14-14, -13-13	-10-9, -12-12, -13-11
Reflections collected/ unique /	5966/1916/1685	6543/3347/2538
observed I > 2.0 a(I)		
Data/restrains/parameters	1916/0/189	3347/0/253
Goodness of fit on F2	1.11	1.07
Ra ^ wR^	0.0403, 0.1108	0.0854, 0.1924
(Shift/s.u.)max	<0.001	<0.001
Largest diff. peak & hole/ (e À-3)	0.15, -0.22	1.19, -1.09
a R =
^IFoi-FcF ^ifoi
= rTw(F 2-f 2)2/
wR = rSw(Fo2 -Fc2)2/Sw(Fo2)2]
b
and thiosemicarbazide (H2L1) or 4-phenylthiosemicarba-zide (H2L2). If the reaction is carried out using acetonitri-le as solvent the polymeric greenish brown complex [Mo-O2L]n is obtained. The monomeric red coloured complexes [MoO2L2D] containing methanol (2a) or ethanol (2b) as the additional ligand are obtained only from the reactions of H2L2 with [MoO2(acac)2] using the appropriate alcohol as solvent. From the analogous reaction with H2L1 we were not able to isolate the monomeric complex suggesting that the polymeric chains Mo=O-Mo=O - are not easily cleaved by donor solvents like methanol or et-hanol. The brown and red colours of the compounds are typical for the thiosemicarbazonato polymeric and monomeric molybdenum(VI) complexes, respectively. The electronic spectra of similar thiosemicarbazonato complexes are well-known in the literature.26-28
All complexes consist of the MoO22+ core coordinated by ligand L via three donor atoms, pyran-4-ol-oxygen, imine-nitrogen and thiol-sulphur. The octahedral coordination around the molybdenum atoms is completed by the neutral solvent molecule D in [MoO2L2D] (2a, D = methanol and 2b, D = ethanol) or by an intermolecular metaloxy-gen interaction (metal...oxygen Mo=O-Mo=O) in [Mo-O2L]n (1 and 2).
The IR spectra support such structures. The single strong absorption attributed to Mo=O at 905 cm-1 (1) or 918 cm-1 (2), accompanied with a broad band at 827 cm-1 (1) or 829 cm-1 (2) is indicative for the Mo=O-Mo=O - interaction, which is in agreement with literature values. The structure of the [MoO2L]n complexes via such interactions in some cases is confirmed also by X-ray method.29,30 The main characteristic of the IR spectra of the monomeric complexes is appearance of vasym(MoO2) at 934 cm-1 (for 2a) and 942 cm-1 (for 2b), vsyma(MoO2) at 889 cm-1 (for 2a) and 902 cm-1 (for 2b), together with absence of the broad band indicative for the Mo=O-Mo bridging. The stretching frequencies attributed to coordinated groups are found at
Fig. 2: Hydrogen bonding in the crystal structure of the ligand
about 1560 cm-1 (for C-O,
), 1520 cm-1 (for
pyran-4-ol-oxygen
C=N) and 750 cm-1 (for C-S). Two bands at 1717-1694 cm-1 and 1645-1636 cm-1 are assigned to the stretching vi-
H2L1
brations of the carbonyl group from the ester COOC2H5 and 2-pyrone, respectively. The remaining frequencies in the IR spectra are due to the vibrations within the ligand.
The monomeric complexes [MoO2L2(CH3OH)] (2a) and [MoO2L2(CH3CH2OH)] (2b) show similar thermal behaviour. While heated the first weight loss is attributed to the loss of the coordinated alcohol molecule and conversion into a stable species [MoO2L2]. On further heating, significant weight losses are indicative of complex decomposition resulting in the solid residue identified as MoO3. The weight loses of the polymeric complexes [Mo-O2L1]n (1) and [MoO2L2]n (2) correspond to the decomposition resulting in the formation of the MoO3. The agreement between theoretical and experimental mass losses is within the experimental error.
3. 1. X-ray Crystallography
Comparison of the selected bond distances for H2L1 and 2b is given in Table 2. Hydrogen bonding parameters are listed in Table 3.
Fig. 1: ORTEP plot of the ligand molecule HjL1 with the atom labelling scheme. Ellipsoids are drawn at the 50% probability level.
Fig. 3: ORTEP plot of the complex molecule 2b with the atom labelling scheme. Ellipsoids are drawn at the 50% probability level.
Table 2. Selected bond distances (À) and angles (°) for HjL1 and 2b.
	HjL1	2b
S1-C1	1.673(2)	1.780(8)
N1-C1	1.324(3)	1.354(10)
C1-N2	1.350(2)	1.298(11)
N2-N3	1.364(2)	1.391(9)
N3-C2	1.286(2)	1.336(12)
C2-C3	1.465(2)	1.481(11)
C2-C8	1.493(3)	1.507(12)
C3-C4	1.397(2)	1.388(11)
C3-C7	1.425(2)	1.439(12)
C7-O3	1.217(2)	1.226(9)
C4-O1	1.309(2)	1.325(11)
Mo1-S1		2.432(3)
Mo1-O1		1.974(6)
Mo1-O4		2.349(6)
Mo1-O5		1.706(7)
Mo1-O6		1.702(6)
Mo1-N3		2.295(6)
N1-C1-N2	113.65(18)	121.0(7)
S1-C1-N1	123.45(16)	112.6(6)
N3-C2-C3	121.89(16)	121.3(7)
N3-C2-C8	121.89(16)	119.4(7)
O5-Mo1-O6		105.4(3)
S1-Mo1-O1	153.91(15)	
S1-Mo1-O4		81.25(14)
S1-Mo1-O5		97.0(2)
S1-Mo1-O6		92.9(2)
S1-Mo1-N3		76.83(17)
O1-Mo1-O4		79.5(2)
O1-Mo1-O5		98.9(3)
O1-Mo1-O6		102.6(3)
O1-Mo1-N3		81.7(2)
O4-Mo1-O5		169.9(2)
O4-Mo1-O6		84.8(3)
O4-Mo1-N3		76.7(2)
O5-Mo1-N3		93.3(3)
O6-Mo1-N3		159.7(3)
ORTEP plot of H2L1 is shown in Fig. 1. The ligand molecule H2L1 is found to be in the keto tautomeric form with the distances C1-S1 of 1.673(2) À and N2-C1 of 1.350(2)À. It consists of two nearly planar fragments connected by a single C2-C3 bond: first fragment (S1, N1, N2, N3, C1, C2, C3, C8) and second fragment (O1, C4, C3, C7, O3, O2, C6, C5, C9). Dihedral angle between the-
se two planes is 11.38(7)°. The C4 atom is in the syn conformation in respect to the N3 atom (torsion angle C4-C3-C2-N3=10.0(2)°) stabilized by an intramolecular O1-H^N3 hydrogen bond. In the crystal structure of H2L1 molecules are interconnected by two strong N1-H^O3 [2-x,-1/2+y, 3/2-z] hydrogen bonds and one N1-H^S1r1-x,-y, 1-z] hydrogen bond (Fig. 2).
Complex molecule found in the crystal structure of 2b is shown in Fig. 3. The molybdenum atom in this complex has a distorted octahedral coordination. Molecule 2b consists of the cis-[MoO2]2+ moiety, with the tridentate thiosemicarbazone ligand bonded to molybdenum through the phenolic-oxygen, imine-nitrogen and thiola-to-sulphur atoms. The sixth coordination site is occupied by an oxygen atom from the ethanol molecule. The ligand in the complex is bonded in its enthiol form indicated by the bond distances N2-C1r1.295(11) À] and C1S1r1.781 (8) À] The Mo=O bond lengths [1.702(6) and 1.706(7) À] and the O=Mo=O [105.4(3)°] bond angle are similar to those found in cis-dioxomolybdenum(VI) complexes. The bond distances Mo1-S1 [2.432(3) À], Mo1-O1r1.974(6) À], Mo1-N3r2.295(6) À] are similar to those found in other thiosemicarbazonato molybdenum(VI) comple-xes.15,16,18 The bond distance Mo1-O4 [2.349(6) À] is significantly larger in comparison to Mo-O1. In the crystal structure of 2b molecules are bonded by two hydrogen bonds O4-H4■■■O3r2-x,1-y,- z] and N1-H1■■■Slrl-x,1- y,- z].
4. Conclusions
New thiosemicarbazonato molybdenum(VI) complexes consist of the MoO22+ core coordinated by tridentate ONS donor ligand through pyran-4-ol-oxygen, imine-nitrogen and thiol-sulphur. The octahedral coordination of each molybdenum atom is completed either by one neutral solvent molecule D in rMoO2L2D] (methanol or etha-nol) or by the oxygen atom of M=O unit from the neighbouring molecule.
5. Acknowledgements
This research was supported by the Ministry of Science and Technology of the Republic of Croatia (Grant Nos. 119-1191342-1082 and 119-1193079-1084).
Table 3. Hydrogen bonding geometry in structures H2L1 and 2b.
	D-H-A	d(D-H) /À	d(H^^^A) /À	d(D^^^A) /À	<(DHA) /°
HjL1	N1-H1NA•••O3r2-x, -1/2+y, 3/2- z]	0.94(2)	2.10(3)	2.896(3)	142(2)
	N1-H1NB•••Slrl-x, -y, 1-z]	0.86(2)	2.53(2)	3.368(2)	164(2)
	N2-H2N•••O3r2-x, -1/2+y, 3/2- z]	0.87(2)	2.11(2)	2.876(2)	146(2)
	O1-H1O•••S1	0.91(3)	2.84(4)	3.5284(19)	134(2)
	O1-H1O-N3	0.91(3)	1.67(3)	2.489(2)	149(3)
2b	N1-H1-S1r1-x,-y,- z]	0.8600	2.6700	3.475(8)	157.00
	O4-H4-O3r2-x,1-y,- z]	0.8600	1.9000	2.711(7)	157.00
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Povzetek
Z reakcijo med [MoO2(acac)2] (acac = acetilacetonatni ion) z tiosemikarbazonom iz dehidroocetne kisline in tiosemi-karbazida (H2L1) ali iz 4-feniltiosemikarbazida (H2L2) so bili pripravljeni novi molibdenovi(VI) kompleksi. Osnova v vseh kompleksih je jedro MoOj^^. Molibdenov atom je koordiniran z ligandom L preko treh donorskih atomov: kisika v piran-4-olu, iminskega dušika in tiolnega žvepla. Vse spojine so bile okarakterizirane z elementno analizo, z IR spektri, s termično analizo in nekatere tudi z rentgenskimi kristalografskimi metodami.