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Polyhedron 24 (2005) 1861–1868

Mononuclear and binuclear ruthenium(II) complexes with4-(phenyl) thiosemicarbazone of benzaldehyde: A discussion on

the relative stabilities of the four-membered andfive-membered chelate rings formed by the ligand

Dipankar Mishra a, Subhendu Naskar a, Michael G.B. Drew b,*,Shyamal Kumar Chattopadhyay a,*

a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, Indiab School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK

Received 18 February 2005; accepted 11 May 2005Available online 21 July 2005

Abstract

Three new ruthenium complexes of the formulae cis-[Ru(PPh3)2(BzTscbz)2] (1a), [Ru2(PPh3)2(BzTscbz)4] (1b) and[Ru(PPh3)2(BzTscHbz)2](ClO4)2 (2) [BzTscHbz = 4-(phenyl) thiosemicarbazone of benzaldehyde] have been synthesized and char-acterized by various physicochemical methods including X-ray structure determinations for 1a and 1b. The relative stabilities of thefour-membered versus five-membered chelate rings formed by the deprotonated ligand BzTscbz are discussed on the basis of theexperimental results and some semi-empirical as well as DFT calculations.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Ruthenium(II) thiosemicarbazone complexes; Relative stability of four-membered and five-membered chelate rings; DFT calculations

1. Introduction

Thiosemicarbazides and thiosemicarbazones are well-known ligands (Fig. 1), coordinating through the sulfurand one of the hydrazinic nitrogen atoms (N2 or N1) [1].Coordination through N2 results in a four-memberedchelate ring, while if the N1 nitrogen participates incoordination a more stable five-membered chelate ringis formed. The R1 and R2 groups may provide addi-tional donor atoms.

In the vast majority of the structurally characterizedmetal complexes of these ligands (both protonated

0277-5387/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2005.05.018

* Corresponding authors. Tel.: +91 33 2668 4561; fax: +91 33 26682916 (S.K. Chattopadhyay).

E-mail address: shch20@hotmail.com (S.K. Chattopadhyay).

(LH) and deprotonated forms (L�)) the five-memberedchelate ring is found to be [1] the only species presentin the solid state. However, Bhattacharya et al. [2] re-cently reported a series of ruthenium(II) and osmium(II)complexes of the formula [M(PPh3)(L)2] and [M-(bpy)2-(L)]+, where L = benzaldehyde thiosemicarbazonate(1�)and salicylaldehyde thiosemicarbazonate(1�) (Fig. 1;R3 = R4 = H; R1 = H; R2 = phenyl or salicyl). Theinteresting observation was that in all the complexes re-ported by them the thiosemicarbazones coordinatethrough the sulfur and the deprotonated hydrazinicnitrogen N(2), resulting in a four-membered chelatering, which was unusual and quite rare in thiosemi-carbazide/thiosemicarbazone chemistry. Later, Casaset al. [3] reported a series of dimethyl thallium(III) com-plexes of acetylferrocene thiosemicarbazonates(1�)having various substituents on the N(4) position. They

HN

S

N

HN

H

N

S

N

HN

H

H

H

NS

N

H

N

H

NS

N

H

N

H

H

III

IIIIV

Fig. 2. Planar conformations of benzaldehyde 4-(phenyl)-thiosemicarbazone.

N N

S

C C

R1

R2

N NC C

R1

R2

S

H+

N

R3

R4N

R3

R4

H

1 2 3 4 1 2 3 4+

LH L-

5 5

Fig. 1. Protonated and deprotonated forms of the thiosemicarbazone ligands.

1862 D. Mishra et al. / Polyhedron 24 (2005) 1861–1868

found that the thiosemicarbazonate(1�) moiety is coor-dinated to dimethyl thallium(III) as a N,S chelatingligand, forming either a four-membered or a five-membered chelate ring depending on the substituenton the N(4) atom. The four-membered chelate ringwas also observed [4–6] for organometallic derivativesof group 13 elements (Al, Ga and In), for Pd as wellas another dimethyl thallium(III) complex. Bhattach-arya et al. [2] tried to rationalize the formation of thefour-membered chelate ring in terms of the known crys-tal structures [7] of benzaldehyde thiosemicarbazoneand the corresponding ruthenium(II) and osmium(II)complexes. They argued that in the solid state the benz-aldehyde thiosemicarbazone exists in form I (Fig. 2).Deprotonation of the N(2)–H proton can easily leadto the formation of a four-membered chelate ring. How-ever, formation of a five-membered chelate ring requiresrotation of 180� about the C(3)–N(2) bond to give formIII.

Bhattacharya et al. also showed that formation of thefive-membered chelate ring by form III results in a stericinteraction between the ortho hydrogen of the phenylring of the benzaldehyde moiety with the metal atom,as well as between the two thiosemicarbazone moieties.Accordingly they reported that it is this steric factorwhich is responsible for the stabilization of the relatively

uncommon four-membered chelate ring in these com-plexes compared to the more usual five-membered che-late rings. In this communication, we will examine thishypothesis more closely both from the viewpoint ofavailable experimental results as well as quantum chem-ical calculations. Quantum chemical calculations ofruthenium complexes are quite popular today as theyhelp to better understand the observed structural andexperimental data. The most common methodologiesadapted are DFT [8–11], INDO/S [11,12] and PM3[9,13].

2. Experimental

2.1. Materials

Commercial ruthenium trichloride (RuCl3 Æ xH2O),purchased from Arora Matthey (Kolkata, India), wasprocessed by repeated evaporation to dryness with conc.HCl. Ru(PPh3)3Cl2 was prepared by the published pro-cedure [14]. The ligand, 4-(phenyl)thiosemicarbazone ofbenzaldehyde (BzTscHbz),was prepared by a modifica-tion of the published procedure [15]. Acetonitrile usedfor spectroscopic and electrochemical studies was puri-fied according to a reported method [16]. Tetraethylam-

D. Mishra et al. / Polyhedron 24 (2005) 1861–1868 1863

monium perchlorate (TEAP), used for the electrochem-ical work, was prepared as reported in the literature [17].All other chemicals are of analytical grade and wereused without further purification.

2.2. Physical measurements

Elemental analyses were performed on a Perkin–Elmer 240 C,H,N analyzer. UV–Vis spectra were recordedusing a JASCO 7850 spectrophotometer. Magnetic sus-ceptibilities were measured with a PAR Model 155vibrating sample magnetometer with Hg[Co(SCN)4] asthe calibrant. 1H NMR spectra were recorded on a Bru-ker AVANCE DPX 300 MHz spectrometer usingSi(CH3)4 as internal standard. Cyclic voltammetry wascarried out using a PAR Versastat instrument drivenby E-chem software. A three-electrode configurationwith Pt working and auxiliary electrodes, Ag/AgCl ref-erence electrode and TEAP as supporting electrolytewas used. The potentials were calibrated against the fer-rocene/ferrocenium couple.

2.3. Synthesis of the complexes

Safety note! Perchlorate salts of organic and metallo-organic species are potentially explosive, and althoughwe have not encountered any problems in our work,nevertheless they should be handled with care.

2.3.1. Synthesis of cis-[Ru(PPh3)2(BzTscbz)2] (1a),[Ru2(PPh3)2(BzTscbz)4] (1b)

To a 30 ml methanolic solution of BzTscHbz (128mg, 0.5 mmol), Et3N (50 mg, 0.5 mol) was added, fol-lowed by the addition of solid [Ru(PPh3)3Cl2] (240 mg,0.25 mmol). After 3 h of reflux, a shining yellow precip-itate was collected by filtration and washed thoroughlywith cold methanol. Recrystallization of the productfrom 1:1 dichloromethane–hexane solution in nitrogenatmosphere leads to two types of crystals, golden yellow(1a) and red (1b). The products were separated mechan-ically under a microscope. Yield: 1a, 170 mg (60%), 1b,52 mg (12.5%). Anal. Calc. for RuC64H54N6S2P2 (1a): C,67.78; H, 4.76; N, 7.41% Found: C, 67.81; H, 4.80; N,7.42%; electronic spectra (dichloromethane solution,kmax/nm, e/L mol�1 cm�1) 431a (6178), 340 (20758),265a (43247), 227 (78586), (ashoulder); 1H NMR(CDCl3, 300 MHz), d 9.06 (s, 2H), 8.01 (s, 2H), 7.41–7.38 (m, 4H), 7.36–7.30 (m, 12H), 7.28–7.21 (m, 14H),7.17–7.12 (t, 8H, J = 7.1 Hz), 7.05–6.96 (m, 12H); Cyclicvoltammetric datab: (E1/2 in V (DEp in mV)) RuII/RuIII,0.51 (107); RuIII/RuIV, 1.25 (238); (bin CH2Cl2, 50 mVs�1 scan rate). Anal. Calc. for Ru2C92H78N12S4P2 (1b):C, 63.37; H, 4.47; N, 9.64% Found: C, 63.43; H, 4.51;N, 9.61%; electronic spectra (dichloromethane solution)

418a (8576), 340a (19113), 260a (41168), 228 (80866)(ashoulder); 1H NMR (CDCl3, 300 MHz), d 8.39 (s,2H), 8.27 (d, 2H, J = 6.9 Hz), 7.81 (d, 2H, J = 12.0Hz), 7.60 (d, 2H, 7.4 Hz), 7.26–7.20 (m, 14H), 7.09–6.94 (m, 24H), 6.92–6.79 (m, 24H), 6.69–6.67 (d, 8H,J = 7.7 Hz).

2.3.2. Synthesis of [Ru(PPh3)2(BzTscHbz)2](ClO4)2(2)

To a 30 ml hot methanolic solution of the ligandBzTscHbz (128 mg, 0.5 mmol), solid [Ru(PPh3)3Cl2](240 mg, 0.25 mmol) was added. The mixture was thenrefluxed for 3 h. The resulting orange yellow solutionwas concentrated to about 10 ml in a rotary evaporator;a 2 ml aqueous saturated solution of NaClO4 was thenadded. The resulting precipitate was collected by filtra-tion, washed with water and then dried over fusedCaCl2. Recrystallization of the crude product from 1:1dichloromethane–hexane solution gave the analyticallypure compound; yield 190 mg (57%). Anal. Calc. forRuC64H56N6S2P2Cl2O8: C, 57.57; H, 4.19; N, 6.29%Found: C, 57.73; H, 4.21; N, 6.31%; electronic spectra(dichloromethane solution, kmax/nm, e/L mol�1 cm�1)400a(3946), 337a(9866), 252(45387), (ashoulder); 1HNMR (CDCl3, 300 MHz), d 8.97 (s, 2H), 7.83 (s, 4H),7.68–7.61 (m, 8H), 7.52–7.43 (m, 12H), 7.32–7.16 (m,30H). Differential pulse voltammetric datab: (E1/2 in V)RuII/RuIII, 0.86; RuIII/RuIV, 1.28 (bin CH2Cl2, 20 mVs�1 scan rate).

2.4. Computation

The total energy (Etot) of the ligands and energy bar-riers (Eb) for transformation between the conformers de-picted in Fig. 2 were calculated by PM3 [18], as well asDFT [19] methods. The PM3 calculations were carriedout on the MM+ optimized structure of the moleculesusing CIS involving 10 occupied and 10 unoccupied orbi-tals. A window based HYPERCHEM program [20] was usedfor both MM+ and PM3 calculations and the defaultparameters of the program were adapted. The energybarriers for rotation around C(5)@N(1) and N(2)–C(3)bonds were calculated by changing the appropriate tor-sion angles between 0� and 180� and calculating the Etot

at each step and taking the difference of Etot between itsmaximum value and that for a torsion angle of 0�. TheDFT calculations on the ligand were carried out usingthe GAUSSIAN-98 program [21] and B3LYP/6-31+G*methodology, whereas for the calculations on the modeltransition metal complexes the ADF program [22] wasused. For all elements the ZORA approximation wasused together with the default TZP basis sets using asmall core. In addition Vosko, Wilk and Nusair�s localexchange correlation potential was used [23] togetherwith Becke�s non-local exchange [24] and Perdew�s corre-lation corrections [25]. With GAUSSIAN-98 and ADF

1864 D. Mishra et al. / Polyhedron 24 (2005) 1861–1868

programs, geometry optimization was carried out usingthe default criteria for convergence.

2.5. X-ray crystallography

Single crystals of the complex cis-[Ru(PPh3)2-(BzTscbz)2] (1a) were grown by slow evaporation froma dichloromethane–hexane solution of the complex innitrogen atmosphere. Data were measured with MoKa radiation using the MAR research Image PlateSystem The crystal was positioned at 70 mm from theImage Plate. 100 frames were measured at 2� intervalswith a counting time of 2 min. Data analysis was carriedout with the XDS program [26] to provide 10413 indepen-dent reflections. The structure was solved using directmethods with the SHELX-86 program [27]. Non-hydrogenatoms were refined with anisotropic thermal parameters.The hydrogen atoms bonded to carbon were included ingeometric positions and given thermal parametersequivalent to 1.2 times those of the atom to which theywere attached. An empirical absorption correction wasapplied using DIFABS [28]. The structure was refined onF2 using SHELXL [29] to R1 0.1105, wR2 0.1645 for 4179reflections with I > 2r(I). The high R-value was due to

Table 1Crystallographic data for 1a

Empirical formula C64H54N6S2P2RuFormula weight 1134.26T (K) 293(2)k (A) 0.71073Crystal system monoclinicSpace group P21/nUnit cell dimensions

a (A) 13.063(15)b (A) 28.870(30)c (A) 15.412(17)a (�) 90b (�) 98.23(10)c (�) 90

V (A3) 5752.5(109)Z 4Dcalc (g/cm

3) 1.310Absorption coefficient (mm�1) 0.446F (000) 2344Crystal size (mm3) 0.25 · 0.25 · 0.3h Range for data collection (�) 1.51–25.93Index ranges �15 6 h 6 15, �35 6 k 6 24,

�18 6 l 6 18Reflections collected 23900Independent reflections (Rint) 10413 (0.1173)Absorption correction semi-empirical from

equivalent reflectionsRefinement method full-matrix

least-squares on F2

Data/restraints/parameters 10403/3/677Goodness-of-fit on F2 1.265Final R indices [I > 2r(I)] R1 = 0.1105, wR2 = 0.1224R indices (all data) R1 = 0.2634, wR2 = 0.1650Largest difference in peakand hole (e A�3)

0.404 and �0.573

the poor quality of the crystals, which led to mis-shapenspots in the diffraction pattern. Crystal data for 1a ap-pear in Table 1.

3. Results and discussion

3.1. Synthesis and characterization

Reaction of Ru(PPh3)3Cl2 with BzTscHbz in reflux-ing methanol in the presence of Et3N results in the pre-cipitation of a yellow product. On recrystallization froma dichloromethane-hexane mixture in nitrogen atmo-sphere two types of crystals were obtained. The majorproduct was golden yellow needles, which was formu-lated as cis-[Ru(PPh3)2(BzTscbz)2] (1a) and the otherproduct was some red rhombic crystals formulated as[Ru2(PPh3)2(BzTscbz)4] (1b). [Ru(PPh3)2(BzTscHbz)2]-(ClO4)2 (2) was obtained using a similar procedure asabove, except Et3N was not added during the reactionand the compound was precipitated by addition of 2mlof saturated methanolic solution of NaClO4. Elementalanalyses and spectroscopic data for the complexes areconsistent with the proposed formulations. In theNMR spectra, the two C–H protons of the coordinatedthiosemicarbazones of 1a and 2 appear at d 9.06 and8.97, respectively, as a broad singlet, whereas the fourC–H protons of the four ligands of 1b appear at d8.39 and 8.27. The signals at d 8.01, 7.83 may be dueto the N–H protons of 1a and 2, respectively. The twodoublets at d 7.81 and 7.60 for 1b may be assigned asN–H protons. All other protons appear in the aromaticregion as multiplets and overlapping signals.

Room temperature magnetic moment measurementindicates all the complexes are diamagnetic; hence ruthe-nium is in the +2 oxidation state. Complex 2 behaves asa 1:2 electrolyte in MeCN solution (KM = 230 X�1 cm2

mol�1), indicating the thiosemicarbazone ligands act asneutral N–S donors in this complex. The products 1a

and 1b were separated mechanically under a microscopeand X-ray crystal structures of both were determined.

3.2. Description of the crystal structures for (1a) and

(1b)

The selected bond distances and bond angles for cis-[Ru(PPh3)2(BzTscbz)2] (1a) are summarized in Table 2.In the compound cis-[Ru(PPh3)2(BzTscbz)2] (1a) thecoordination geometry of Ru(II) is similar to that ob-served earlier [2], where two P atoms as well as twonitrogen atoms are cis to each other, while the sulfuratoms are trans to each other (Fig. 3).

The average bond distances are Ru–P, 2.314 A; Ru–S, 2.439 A; Ru–N, 2.140 A and the bite angle for thefour-membered chelate ring is 65.20�. This may be com-pared with the values reported [30] (average Ru–P,

Table 2Bond distances (A) and bond angles (�) of 1a

Ru1–N83 2.116(9)Ru1–N73 2.164(9)Ru1–P2 2.308(3)Ru1–P1 2.320(3)Ru1–S71 2.437(3)Ru1–S81 2.441(3)

N83–Ru1–N73 80.5(3)N83–Ru1–P2 91.3(2)N73–Ru1–P2 164.3(2)N83–Ru1–P1 168.4(2)N73–Ru1–P1 92.3(2)P2–Ru1–P1 97.76(12)N83–Ru1–S71 99.9(2)N73–Ru1–S71 65.5(2)P2–Ru1–S71 103.25(11)P1–Ru1–S71 85.14(11)N83–Ru1–S81 65.4(2)N73–Ru1–S81 98.4(2)P2–Ru1–S81 90.13(11)P1–Ru1–S81 107.13(11)S71-Ru1–S81 160.65(11)

Fig. 3. ORTEP diagram and atom numbering scheme for cis-[Ru(PPh3)2(BzTscbz)2] (1a). Hydrogen atoms are omitted for clarity.Thermal ellipsoids are at 25% probability.

Fig. 4. X-ray structure of centrosymmetric [Ru2(PPh3)2(BzTscbz)4](1b). Hydrogen atoms are omitted for clarity.

D. Mishra et al. / Polyhedron 24 (2005) 1861–1868 1865

2.382 A; Ru–S, 2.368 A; Ru–N, 2.145 A, bite angle forthe five-membered chelate is 81.65�) for cis-[Ru(PPh3)-(CyTscHPy)2](ClO4)2 (3) (where for CyTscPy R1 = H,R2 = Py, R3 = H, R4 = cyclohexyl in Fig. 1), which hasidentical donor atoms (cis-P2, cis-N2, trans-S2) to thosereported here but the two thiosemicarbazone moietiesare in the protonated form and coordinate in a moreconventional way forming five-membered chelate rings.Normally, the anionic chelates are expected to haveshorter bond lengths than the corresponding neutralvariety. However, in this case the Ru–P bonds are

shorter in 1a than in 3, while the Ru–S distances arelonger in 1a, clearly indicating that the Ru–S bond inthe four-membered chelate ring is much weaker com-pared to that in the five-membered ring.

The lack of good crystal quality of 1b resulted in poorrefinement of the structure to an R1 value of 0.25 so thatno detailed comment on the dimensions can be made.[Crystal data, 1b, Ru2C92H78N12S4P2, triclinic, Z = 4,a = 16.042(17) A, b = 16.471(19) A, c = 18.238(19) A,a = 76.63(1)�, b = 76.39(1)�, c = 66.36(1)�, V = 4448.1A3, Dcalc = 1.302 g cm�3]. However, the atoms in thestructure could be located well enough to show thatthere are two independent centrosymmetric dimers inthe unit cell in which the two Ru(II) centers are bridgedby two thiolato sulfur atoms. Each Ru(II) center is coor-dinated by one PPh3 and two thiosemicarbazonatemoieties, of which one thiosemicarbazonate forms afour-membered chelate ring while the other one formsa five-membered chelate ring, and it is the thiolato groupbelonging to the later variety which is responsible forbridging the Ru(II) centers (Fig. 4).

3.3. Probable structure of 2

We have also synthesized the complex [Ru(PPh3)2-(BzTscHbz)2](ClO4)2 (2) containing the non-deproto-nated thiosemicarbazone ligand. Although we have notbeen able to get a single crystal of the compound, thesimilarities of electronic spectra and the E0[Ru(III)/Ru(II)] values of this complex with those reported byGhosh et al. [30] suggest that it has a similar structureto one of their complexes, i.e., cis-P2, trans-S2, cis-N2,where the thiosemicarbazone is coordinated as a five-membered chelating N,S donor. This is further corrobo-rated by the fact that the addition of a dilute solution ofa base such as Et3N to a solution of 2 in dichlorometh-ane or acetonitrile does not generate the spectra of 1a.

1866 D. Mishra et al. / Polyhedron 24 (2005) 1861–1868

Similarly, addition of dilute acid to a dichloromethanesolution of 1a does not generate the spectra of 2. Ifthe thiosemicarbazone is coordinated to both 1a and 2

in a similar manner, then rapid interconversion betweenthem is expected due to acid or base addition, which isnot the case here. It may also be noted that so far inall the structurally characterized thiosemicarbazonecomplexes, when it is found to form a four-memberedchelate ring, it is always present in the deprotonatedform. Thus, on the basis of the above discussion, it islogical to conclude in 2 that thiosemicarbazone is pres-ent as a five-membered chelate ring.

3.4. Relative stability of four-membered versusfive-membered chelate rings

In the light of the above results, we can reassess thepossible binding modes of thiosemicarbazones in gen-eral and benzaldehyde thiosemicarbazone in particular.Benzaldehyde thiosemicarbazone can exist in the fourconformations depicted in Fig. 2. Some important re-sults of the quantum chemical calculations on these con-formers are summarized in Table 3. It may be notedfrom the table that although the Etot values for eachconformer substantially differ between the two methodsof calculations, the relative stabilities of the conformersas well as the Eb1 value (see below) given by the semi-empirical PM3 method and more accurate DFT methodare quite similar. In the solid state BzTscHbz exists inform I as revealed by the crystal structure determination[7]. When it coordinates in this form it has to undergodeprotonation of N(2)H followed by four-memberedchelate ring formation. Form II can also participate ina similar four-membered chelate ring formation. How-ever, conversion of form I to form II requires crossingof an energy barrier of �45 kcal mol�1 (Eb1), withoutany benefit of additional bond energy; rather the phenylring will undergo steric repulsion with the rings of thetri(phenyl) phosphine and thus this form has not beenfound to coordinate.

Table 3The Erel

tot values for the different conformers of BzTscHbz

Conformer Method of calculation Ereltot ðkcal mol�1Þ

I PM3 0.0DFT 0.0

II PM3 �0.40DFT 3.58

III PM3 10.85DFT 11.02

IV PM3 11.67DFT 15.59

Ereltot is the Etot values of different conformers relative to conformer I,

whose Etot value is �58295.40 (PM3)/�692628.45 (DFT).

From these energies, the barriers for rotation aboutthe C(5)@N(1) (Eb1) and N(2)–C(3) (Eb2) bonds canbe calculated as 48.0, 15.5 kcal mol�1, respectively, withPM3 and 45.6 and 31.3 kcal mol�1, respectively, withDFT methods.

Forming a five-membered chelate ring requires eitherform III or form IV. However, it was shown by Bhatt-achyarya et al. [2] that coordination in form III resultsin severe steric strain. So like form II, form III has neverbeen found to be coordinated in metal complexes. Thus,formation of a five-membered chelated complex requiresform IV and crossing of an energy barrier of 45–50 kcalmol�1, equivalent to the energy of a chemical bond,which is readily available in a chemical reaction likethe present one where a psuedo-five coordinated Ru(II)is converted to a six coordinated species and deprotona-tions take place (considering the crystal field stabilizationenergy (CFSE) of RuII � 350 kcal mol�1, solvation en-ergy of a proton in MeOH � 180 kcal mol�1 and protonaffinity of Et3N � 230 kcal mol�1). The benefit of stron-ger bond formation should lead to the coordination byform IV, in preference to form I, i.e., the metal complexcontaining the five-membered chelate ring with form IVshould be thermodynamically more stable compared toone containing the four-membered chelate ring withform I. This is particularly so as form I is only �16 kcalmol�1 more stable than form IV according to the DFTcalculations. The structure determination of two Ni(II)complexes [31] containing (4-fluoro)benzldehyde thiose-micarbazonate(�1) and (4-N,N-dimethylamino)benzal-dehyde S-benzyl dithiocarbazate(�1) as well as theRu(II) complex of the more sterically encumbered ligandpyridine-2-aldehyde(4-cyclohexyl) thiosemicarbazone,all containing the five-membered chelate ring, bears tes-timony to the above hypothesis.

3.5. DFT calculations

We have also carried out DFT calculations on Rucomplexes with these types of ligands using the ADF pro-gram. We first used two model complexes with lessbulky ligands, namely Ru(MeTscMe)2(PMe3)2, whereMeTscMe is the 4-(methyl)thiosemicarbazone of acetal-dehyde and Ru(BzTscbz)2(PMe3)2. We then performedcalculations on Ru(BzTscbz)2(PPh3)2 as well. All modelswere constrained in C2 symmetry and their geometryrefined to convergence. The crystal structure ofRu(BzTscbz)2(PPh3)2 has a structure only slightly dis-torted from C2 symmetry. The results of the ADF calcu-lations are given in Table 4.

From the results it is clear that five-membered chelatecontaining metal complexes are always thermodynami-cally more stable than the corresponding four-mem-bered chelate containing complexes, even when thesteric bulk of the axial phosphines are increased fromPMe3 to PPh3. Indeed it is clear that steric effects of

Table 4Results of DFT calculations on some thiosemicarbazone complexes

Complex E1tot ðkcal mol�1Þ E2

tot ðkcal mol�1Þ DEtot Ru–N (A) Ru–S (A)

Four-membered

Five-membered

Four-membered

Five-membered

trans-[Ru(MeTscMe)2(PMe3)2] �7425.02 �7450.74 25.72 2.140 2.117 2.396 2.337trans-[Ru(BzTscbz)2(PMe3)2] �12140.55 �12157.87 17.32 2.122 2.111 2.386 2.331trans-[Ru(BzTscbz)2(PPh3)2] �19139.66 �19171.09 31.43 2.129 2.088 2.372 2.341

DEtot ¼ E1tot � E2

tot (E1tot and E2

tot corresponds to four-membered and five-membered chelated complexes, respectively).

Table 5The frontier orbitals in the 5- and 4-chelation mode of trans-[Ru(BzTscbz)2(PPh3)2]

Energy (a.u.) Percentage of atomicorbital coefficients

4-Chelation modeLUMO �0.095649 no metal contributionHOMO �0.159906 31.6 Ru dxy, 14.4 S py, 5.1 S pzHOMO � 1 �0.166758 50.6 Ru dxz, 12.7 S px, 5.0 S pzHOMO � 2 �0.170253 20.1 Ru dz2 , 17.7 dxy, 9.8 S pyHOMO � 3 �0.175612 no metal contribution

5-Chelation modeLUMO �0.085458 no metal contributionHOMO �0.156133 20.3 Ru dxy, 18.85 S py,

16.0 Ru dx2�y2

HOMO � 1 �0.161880 35.5 Ru dxz, 17.07 S px, 8.9 S pz,8.0 Ru dz2

HOMO � 2 �0.176128 20.5 Ru dz2 , 12.0 dx2�y2 , 9.40 S pyHOMO � 3 �0.181913 no metal contribution

D. Mishra et al. / Polyhedron 24 (2005) 1861–1868 1867

the bulky phenyl rings compared to those of the methylgroups do not favour one type of chelation over an-other. It would appear that the electronic effects are ofparamount importance.

The bond lengths shown in Table 4 indicate that theRu–S and Ru–N bond lengths are significantly shorterin the five-membered chelation mode for all three com-plexes. Details of the frontier orbitals in the 5 and 4 che-lation mode of trans-[Ru(BzTscbz)2(PPh3)2] are given inTable 5.

As is apparent from the pattern of frontier orbitalsthere are very few differences between the 4 and 5 chela-tion modes. The HOMO–LUMO gaps are 0.064257 and0.070675 a.u., respectively, so the gap is slightly largerfor the 5 chelation mode. However, the contributionof the atoms to the HOMO orbitals are very similar withthe top three having major contributions only from themetal and sulfur atoms. However, HOMO � 3 and theLUMO both are orbitals, which do not include the me-tal atom in any way.

4. Conclusion

The quantum chemical calculations on the metalcomplexes discussed above as well as the crystal struc-ture data of the vast majority of structurally character-

ized thiosemicarbazone complexes clearly suggest thatfor the thiosemicarbazone complexes, five-memberedchelate rings are thermodynamically more stable thanthe four-membered chelate rings, as is intuitively ex-pected. In fact by suitable substitution on the N(4),and/or on the C atom attached to N(1) one can geteither the five-membered chelate or both the four- andfive-membered chelates as shown in this work as wellas in the works of Casas and Ghosh et al. [3,30]. Casasand co-workers [32] have also demonstrated that evenwhen R2 is a bulky substituent such as ferrocenyl, bothfive- and four-membered chelate complexes can be iso-lated. Similarly in [PbPh2(EtGTSC)2] (where HEt-GTSC = thiosemicarbazone of ethylglyoxalate), onearm of the thiosemicarbazone moiety forms a four-membered chelate ring whereas the other thiosemicarba-zone arm forms a five-membered chelate ring [33]. Theyconcluded that although the five-membered chelate ringmay be more stable, the four-membered ring retains theN(4)–H� � �N(1) hydrogen bond that is present in the freeligand. Thus, a small difference in weak interactions orin the steric requirement of the packing may decidethe actual coordination mode.

Acknowledgements

D.M. and S.N. acknowledge CSIR and UGC, respec-tively, for their fellowships. S.K.C. acknowledgesAICTE and UGC, New Delhi for financial support.We thank EPSRC (UK) and the University of Readingfor funds for the Image Plate System.

Appendix A. Supplementary data

The structure of 1a has been deposited with the Cam-bridge Crystallographic Data Centre and the depositionnumber is 248342. Copies of this information may beobtained free of charge from The Director, CCDC, 12Union Road, Cambridge CB2 1EZ, UK (fax: +441223 336 033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk). Supplementary data associatedwith this article can be found, in the online version, atdoi:10.1016/j.poly.2005.05.018.

1868 D. Mishra et al. / Polyhedron 24 (2005) 1861–1868

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