Synthesis, characterization, and single crystal X-raystructures of [CoIII(acacen)(thioacetamide)2]ClO4

and [CoIII((BA)2en)(thioacetamide)2]PF6 —Solvatochromic properties of[CoIII(acacen)(thioacetamide)2]ClO4

Mehdi Amirnasr, Vratislav Langer, Nahid Rasouli, Mehdi Salehi, andSoraia Meghdadi

Abstract: The trans-[CoIII(acacen)(ta)2]ClO4 (1) and trans-[CoIII((BA)2en)(ta)2]PF6 (2) complexes, where H2acacen =bis(acetylacetone)ethylenediimine, H2(BA)2en = bis(benzoylacetone)ethylenediimine, and ta = thioacetamide, have beensynthesized by a solid-state method, and characterized by elemental analyses, IR, UV–vis, and 1H NMR spectroscopy.The crystal and molecular structures of 1 and 2 were determined by X-ray crystallography. Both compounds crystallizein the monoclinic space group P2/n. The ClO4 and PF6 ions are both disordered, ClO4 on a twofold axis in 1 and PF6

on an inversion center in 2. Also bridging N-CH2-CH2-N is disordered in both compounds. The octahedral coordinationof Co(III) is slightly distorted in both cases. The thioacetamide ligands are S-bonded and occupy the axial position.The IR, UV–vis, and 1H NMR spectra of the two complexes and their solvatochromic properties are also discussed.The longest wavelength absorption that appears at 517 nm for 1 and at 528 nm for 2 in chloroform is solvent depend-ent, and is assigned as a metal-mediated ligand-to-ligand charge transfer (LLCT).

Key words: solid-state synthesis, thioactamide, Co(III) (Schiff base), crystal structure, solvatochromism, metal-mediatedLLCT.

Résumé : Utilisant une méthode en phase solide, on a synthétisé les complexes trans-[CoIII(acacen)(ta)2]ClO4 (1) ettrans-[CoIII((BA)2en)(ta)2]PF6 (2) dans lesquels H2acacen = bis(acétylacétone)éthylènediimine, H2(BA)2en = bis(benzoy-lacétone)éthylènediimine et ta = thioacétamide, et on les a caractérisés par le biais d’analyses élémentaires, de spectros-copies IR, UV–vis et RMN du 1H. On a déterminé les structures cristallines et moléculaires des composés 1 et 2 pardiffraction des rayons X. Les deux composés cristallisent dans le groupe d’espace monoclinique P2/n. Les ions ClO4 etPF6 sont tous les deux désordonnés, le ClO4 sur un axe binaire dans 1 et le PF6 sur un centre d’inversion dans 2. Lechaînon N-CH2-CH2-N qui agit comme pont est aussi désordonné dans les deux composés. La coordination tétraédriquedu Co(III) est légèrement déformée dans les deux cas. Les ligands thioacétamides sont liés par le soufre et occupentune position axiale. On discute aussi des spectres IR, UV–vis et RMN du 1H des deux complexes et de leurs propriétéssolvatochromiques. La bande d’absorption de plus grande longueur d’onde qui apparaît, dans le chloroforme, à 517 nmpour le composé 1 et à 528 nm pour le composé 2, dépend du solvant et elle est attribuée à un transfert de charge deligand à ligand aidée par le métal.

Mots clés : synthèse à l’état solide, thioacétamide, base de Shiff du Co(III), structure cristalline, solvatochromie, trans-fert de charge de ligand à ligand aidée par le métal.

[Traduit par la Rédaction] Amirnasr et al. 2081

Introduction

Cobalt(III) is normally considered a hard metal center thatfavors coordination to second-row atoms like oxygen and ni-trogen. This is clearly evident from numerous Co(III) com-

plexes bearing ambidentate ligands such as thiocyanate (1),selenocyanate (2), formamide, acetamide (3), methanesul-finamide, and methanesulfinate (4). However, when otherligands on the metal center tend to make the metal softer,bonding to soft donor atoms like S is favored. For instance,

Can. J. Chem. 83: 2073–2081 (2005) doi: 10.1139/V05-207 © 2005 NRC Canada

2073

Received 2 July 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 10 February 2006.

M. Amirnasr,1 N. Rasouli, M. Salehi, and S. Meghdadi. Department of Chemistry, Isfahan University of Technology, Isfahan84156-83111, Iran.V. Langer. Chalmers University of Technology, Department of Chemical and Biological Engineering, SE-412 96 Gothenburg,Sweden.

1Corresponding author (e-mail: amirnasr@cc.iut.ac.ir).

the S-bonded form of the complex [Co(CN)5SCN]3–, whichis the primary product of the reduction of Co(NH3)5NCS2+

by Co(CN)53–, is the stable linkage isomer (2).

Schiff base ligands also play an effective role in the modi-fication of the electronic structure of metal centers. Co(II)Schiff base complexes are among the most versatile com-pounds that are of potential interest in many different areassuch as catalysis (5, 6), modeling of antiviral agents (7), andoxygen-binding biomolecules (8). This remarkable diversityof functions is achieved by suitable modification of their ba-sic electronic structure. Several elegant theoretical studieshave been carried out to elucidate the electronic structure ofCo(II) Schiff base complexes and to rationalize their reactiv-ity (9, 10). The Schiff base ligand may modify the electrondensity on the metal center in such a way that the coordina-tion of a soft atom like S of thioacetamide to cobalt is fa-vored. In the course of our studies on the structural andelectronic effects that may influence the reactivity of cobaltSchiff base complexes (11) we report the synthesis, charac-terization, and X-ray crystal structure of trans-[CoIII(acacen)(ta)2]ClO4 (1) and trans-[CoIII((BA)2en)(ta)2]PF6(2). The structures of 1 and 2 are the first reported structuresof Co(acacen) and Co((BA)2en) complexes containing S-donor axial ligands.

Experimental

Caution! Transition-metal complex perchlorate salts areknown to be hazardous and must be treated with care, espe-cially in the presence of organic solvents.

Materials and methodsThe Schiff base ligands, bis(acetylacetone)ethyl-

enediimine (H2acacen) and bis(benzoylacetone)ethylene-diimine (H2(BA)2en), and their cobalt(II) complexes wereprepared as described in the literature (12). Silica gel 60(70–230 mesh, Merck) was used as the solid-state reactionmedia. All other chemicals were commercial reagent gradeand used as received from Aldrich and Merck. Elementalanalyses were performed by using a Heraeus CHN-O-RAPID elemental analyzer.

Electronic absorption spectra were recorded on a JASCOV-570 spectrophotometer. Fourier transform IR spectroscopyon KBr pellets was performed on a JASCO-680 Plus instru-ment. 1H NMR spectra were obtained on a Bruker AvanceDRX500 (500 MHz) spectrometer. Proton chemical shiftsare reported in part per million (ppm) relative to an internalstandard of Me4Si.

Synthesis

trans-[CoIII(acacen)(ta)2]ClO4 (1)A homogenized mixture of 951 mg (3 mmol) of

[CoII(acacen)], 900 mg (12 mmol) of thioacetamide, and1.2 g of silica gel in a porcelain dish was heated at 80 °C for30 min. The color changed from orange to dark purple. Thereaction mixture was then cooled to room temperature andtreated with 50 mL of chloroform. A solution of 368 mg(3 mmol) sodium perchlorate in 3 mL methanol was thenadded and the mixture was stirred for 10 min and finally fil-tered off. The filtrate was left in the open air and slow evap-oration of the solvent at room temperature gave a dark violet

microcrystalline powder. The powder was then dissolved inchloroform and the resulting solution was filtered off. Anequal volume of toluene was added to the filtrate and leftundisturbed at room temperature. Dark violet crystals suit-able for X-ray crystallography were obtained after 2 days.The crystals were isolated by filtration and washed with amixture of chloroform–toluene (1:9, v/v) and dried in vacuo.Yield: 340 mg (64%). IR (KBr, cm–1) νmax: 3323 (m, N-H),1515 (s, C=N), 1318 (m, C-N), 708 (m, C=S), 1116 (s,ClO4).

1H NMR (500 MHz, CDCl3) δ: 2.02 (s, 6Ha, CH3),2.20 (s, 6Hb, CH3), 2.37 (s, 6Hc, CH3), 3.5 (s, 4Hd, H2C-CH2), 5.05 (s, 2He, CH), 8.73 (s, 2Hg, NH), 10.22 (s, 2Hf,NH). Anal. calcd. for C16H28N4O6ClS2Co: C 36.20, H 5.30,N 10.50; found: C 36.20, H 5.40, N 9.80.

trans-[CoIII((BA)2en)(ta)2]PF6 (2)A homogenized mixture of 203 mg (0.5 mmol) of

[CoII((BA)2en)], 113 mg (1.5 mmol) of thioacetamide, and900 mg of silica gel in a porcelain dish was heated at 80 °Cfor 60 min. The bright orange color of the mixture turneddark orange. The reaction mixture was then cooled to roomtemperature, transferred to a 100 mL beaker, and treatedwith a mixture of 50 mL of dichloromethane and 20 mLethyl acetate. After adding 37.5 mg (0.5 mmol) of additionalthioacetamide, the reaction mixture was stirred in open airfor an extra 6 h. A solution of 98 mg (0.6 mmol) of NH4PF6in 10 mL ethyl acetate was then added and the reaction mix-ture was stirred for an additional 30 min and finally filteredoff. After adding 15 mL of carbon tetrachloride to the fil-trate, the resulting solution was left in open air, and slowevaporation of the solvent at room temperature gave darkcherry red crystals. Yield: 420 mg (60%). The product wasrecrystallized by slow evaporation of its solution in a mix-ture of ethyl acetate – benzene – dibutyl ether – carbon tetra-chloride (15:1:3:18, v/v) at room temperature and darkcherry red crystals suitable for X-ray crystallography wereobtained after 4 days. The crystals were isolated by filtrationand washed with a mixture of ethyl acetate – carbon tetra-chloride (1:9, v/v) and dried in vacuo. IR (KBr, cm–1) νmax:3386 (m, N-H), 1515 (s, C=N), 1311 (m, C-N), 699 (m,C=S), 852 (s, PF6).

1H NMR (500 MHz, CDCl3) δ: 2.23 (s,6Ha, CH3), 2.39 (s, 6Hc, CH3), 3.73 (s, 4Hd, H2C-CH2), 5.78(s, 2He, CH), 7.44–7.83 (m, 10H, Ar), 8.04 (s, 2Hg, NH),10.59 (s, 2Hf, NH). Anal. calcd. for C26H32N4O2S2PF6Co: C44.57, H 4.57, N 8.00; found: C 43.0, H 4.70, N 8.00.

X-ray crystallographyData were collected on a Siemens SMART CCD

diffractometer equipped with LT-2A low temperature device,using graphite-monochromated Mo Kα (λ = 0.710 73 Å) ra-diation. A full sphere of reciprocal lattice was scanned by0.3° steps in ω with a crystal-to-detector distance of 3.97 cm.Preliminary orientation matrices were obtained from the firstframes using SMART (13). The frames were integrated us-ing the preliminary orientation matrices, which were updatedevery 100 frames. Final cell parameters were obtained by therefinement on the positions of 6923 (for 1) and 8192 (for 2)reflections with I > 10σ(I), after integration of all the framesusing SAINT (13). The data were empirically corrected forabsorption and other effects using SADABS (14). The struc-tures were solved by direct methods and refined by the full-

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matrix least-squares method on F2 data using the SHELXTL(15). All non-H atoms were refined anisotropically. Oxygenatoms of both perchlorate ions, which are disordered on thetwofold axis with occupancy factors of 0.5, were refinedwith restraints to form regular tetrahedrons. Also PF6 ionwas found to be disordered on an inversion center and re-fined with geometrical restraints. The H atoms were con-strained to idealized geometries and refined isotropically.The crystallographic and refinement data are summarized inTable 1.

Results and discussion

SynthesisThe cobalt(III) complexes (1 and 2) were prepared by air

oxidation of a homogenized mixture of the correspondingcobalt(II) (Schiff base) complex and thioacetamide, in thepresence of silica gel at 80 °C. The solid reaction mixturewas heated in an oven for 30 min during which the colorchanged from orange to dark purple for 1 and from red or-ange to dark orange for 2. In addition to providing a suitablereaction medium to ensure that the reactive moleculesachieve contact with each other, silica gel is apparently ef-fective in adsorbing the water molecules that are loosely co-ordinated to Co(II) in [CoII(Schiff base)] complexes. It is

also likely that the nitrogen atom of thioacetamide bindsmore tightly to the Lewis acid sites of silica gel, thereforefacilitating the coordination of thioacetamide through its Satom.

It has been postulated that the tendency of a given[CoII(Schiff base)] complex towards oxidation is dependenton the available charge density on cobalt (10). For example,the [CoII(acacen)] complex is more readily oxidized in airthan [CoII(salen)]. The common explanation for this propertyis that the presence of the extended conjugation system inthe salen ligand reduces the charge density at oxygen and re-duces the reactivity of [CoII(salen)] towards oxidation. It isinteresting to note that the oxidation process in the prepara-tion of 1 from [CoII(acacen)] occurs within a short period,while it takes a bout 12 h in the synthesis of 2 from[CoII((BA)2en)]. This would be expected considering thepresence of two additional phenyl rings and extended conju-gation in the bis(benzoylacetone)ethylenediimine Schiff basecomplex.

Structures of trans-[Co(acacen)(ta)2]ClO4 (1) and trans-[Co((BA)2en)(ta)2]PF6 (2)

A view of the cations of 1 and 2 can be seen in Figs. 1and 2, respectively. The atom labeling scheme used is pre-sented in the figures with the unlabeled atoms being related

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Amirnasr et al. 2075

Empirical formula C16H28ClCoN4O6S2 (1) C16H32CoF6N4O2PS2 (2)

Formula weight 530.92 700.58Crystal system Monoclinic MonoclinicSpace group P2/n P2/na (Å) 13.409 9(4) 11.308 3(1)b (Å) 11.772 1(3) 11.168 3(1)c (Å) 16.109 4(4) 12.470 6(1)β (°) 109.306 (1) 94.19 3(1)V (Å3) 2 400.07(11) 1 570.7 5(2)Z 4 2Dc (Mg m–3) 1.469 1.481

Crystal size (mm) 0.20 × 0.20 × 0.08 0.50 × 0.26 × 0.22Radiation (λ, Å) Mo Kα, 0.710 73 Mo Kα, 0.710 73θ range (°) 2.19–30.53 1.82–32.87T (K) 183(2) 173(2)µ (mm–1) 1.038 0.796F(000) 1 104 720Index ranges –19 ≤ h ≤ 19,

–15 ≤ k ≤ 16,

–22 ≤ l ≤ 22

–17 ≤ h ≤ 16,

–16 ≤ k ≤ 16,

–18 ≤ l ≤ 18Reflections collected 30 332 26 822Independent reflections 7 335 (Rint = 0.040 8) 5 621 (Rint = 0.025 1)

Completeness of data 99.7%, θ = 30.53° 99.9%, θ = 30.00°Absorption correction Multi-scan Multi-scanMax and min transmission 0.921 6 and 0.819 3 0.844 3 and 0.691 5Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters 7 335/119/358 5 621/66/236Goodness-of-fit on F2 1.021 1.038Final R indices [I > 2σ (I)] R1 = 0.042 3, wR2 = 0.102 8 0.033 1, wR2 = 0. 090 5

R indices (all data) R1 = 0.067 3, wR2 = 0.116 8 0.040 8, wR2 = 0.096 3

Largest diff. peak and hole (e Å–3) 0.721 and –0.570 0.716 and –0.294

Table 1. Crystal data and structure refinement for trans-[CoIII(acacen)(ta)2]ClO4 (1) and trans-[CoIII((BA)2en)(ta)2]PF6 (2).

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to the labeled atoms by a twofold axis upon which the cobaltcation in 2 resides. Selected bond lengths and angles arelisted in Table 2. In both cases, the octahedral coordinationof Co(III) is slightly distorted, the Schiff base ligand(ONNO ligand) coordinates the cobalt ion in four equatorialpositions, and the two thioacetamide molecules occupy thetwo axial positions of the octahedral complexes.

The Co—O and Co—N bond distances of the acacen and(BA)2en ligands in the equatorial plane of 1 and 2 are sym-

metric within the limits of the experimental error. In trans-[CoIII(acacen)(ta)2]ClO4 (1), Co—O(1) = 1.8992(15) Å,Co—O(2) = 1.9023(15) Å, Co—N(1) = 1.9018(17) Å, Co—N(2) = 1.8967(18) Å, which compare well with the Co—Oand Co—N distances found in trans-[Co(acacen)(piperidine)2]NCS (Co—Oav = 1.897 Å, Co—Nav = 1.902 Å) (16) and trans-[Co(3-Cl-acacen)(NH3)2]BPh4(Co—O = 1.881(4) Å, Co—N = 1.8918(9) Å) (7). In trans-[CoIII((BA)2en)(ta)2]PF6 (2), Co—O(1) = Co—O(1)i =1.9004(9) Å, Co—N(1) = Co—N(1)i = 1.9010(10) Å, whichcompare well with the Co—O and Co—N distances found intrans-[Co((BA)2en)(morpholine)2]ClO4 (Co—Oav = 1.897 Å,Co—Nav = 1.902 Å) (17). The average cobalt–sulfur bondsin the axial positions are different in 1 and 2 (Co—Sav =2.3013(6) Å in 1 and 2.3169(3) Å in 2) and are in goodagreement with the Co—S distances in cobalt–thiourea (e.g.,[Co(NioxH)2(Thio)2]2SiF6, Co—S = 2.300–2.318 Å) com-plexes (18).

The C—S and C—N bond distances for the axialthioacetamide ligands, C(31)—S(3) = 1.675(3) Å, C(41)—S(4) = 1.682(2) Å, C(31)—N(3) = 1.305(4) Å, C(41)—N(4) = 1.288(3) Å in 1, and C(6)—S(1) = 1.6944(16) Å,C(6)—N(2) = 1.298(2) Å in 2, agree with those reported forthe related complexes (17) and are close to the C=S double

Fig. 1. Numbering scheme with thermal ellipsoids at 30% proba-bility level. The disordered section N1-C1B-C2B-N2 with higheroccupancy (0.686) is shown.

Fig. 2. Numbering scheme with thermal ellipsoids at 30% proba-bility level. The disordered section N1-C5-C5i-N1i with higheroccupancy (0.80) is shown. Symmetry code: –x + 3/2, y, –z +1/2 (i).

Bond lengths (Å) Bond angles (°)

trans-[CoIII(acacen)(ta)2]ClO4 (1)Co(1)—N(1) 1.9018(17) O(1)-Co(1)-N(2) 178.03(8)Co(1)—N(2) 1.8967(18) O(1)-Co(1)-N(1) 95.45(7)Co(1)—O(1) 1.8992(15) N(2)-Co(1)-N(1) 86.16(8)Co(1)—O(2) 1.9023(15) O(1)-Co(1)-O(2) 82.84(7)Co(1)—S(3) 2.2919(6) N(2)-Co(1)-O(2) 95.59(8)Co(1)—S(4) 2.3107(6) N(1)-Co(1)-O(2) 177.46(7)S(3)—C(31) 1.675(3) O(1)-Co(1)-S(3) 95.43(5)S(4)—C(41) 1.682(2) N(2)-Co(1)-S(3) 85.76(6)N(3)—C(31) 1.305(4) N(1)-Co(1)-S(3) 87.81(6)N(4)—C(41) 1.288(3) O(2)-Co(1)-S(3) 90.48(5)

O(1)-Co(1)-S(4) 91.78(5)N(2)-Co(1)-S(4) 87.17(6)N(1)-Co(1)-S(4) 86.79(6)O(2)-Co(1)-S(4) 95.13(5)S(3)-Co(1)-S(4) 171.37(3)

trans-[CoIII((BA)2en)(ta)2]PF6 (2)Co(1)—N(1) 1.9010(10) N(1)i-Co(1)-O(1) 178.83(4)Co(1)—N(1)i 1.9010(10) O(1)-Co(1)-N(1) 94.85(4)Co(1)—O(1) 1.9004(9) N(1)i-Co(1)-N(1) 86.22(6)Co(1)—O(1)i 1.9004(9) O(1)-Co(1)-O(1)i 84.09(5)Co(1)—S(1)i 2.3169(3) N(1)i-Co(1)-O(1)i 94.85(4)Co(1)—S(1) 2.3169(3) N(1)-Co(1)-O(1)i 178.83(4)S(1)—C(6) 1.6944(16) O(1)-Co(1)-S(1)i 94.36(3)N(2)—C(6) 1.298(2) N(1)i-Co(1)-S(1)i 85.13(3)

N(1)-Co(1)-S(1)i 91.45(3)O(1)i-Co(1)-S(1)i 89.13(3)O(1)-Co(1)-S(1)i 89.13(3)N(1)i-Co(1)-S(1) 91.45(3)N(1)-Co(1)-S(1) 85.12(3)O(1)i-Co(1)-S(1) 94.36(3)S(1)i-Co(1)-S(1) 175.309(19)

Table 2. Bond lengths (Å) and angles (°) for complexes 1 and 2.

bond (1.6854(10) Å) and C—N single bond (1.3126(12) Å)in free thioacetamide (19).

All angles around the Co center deviate significantly from90° indicating a rectangular distortion. The ligand-cobalt-ligand bond angles in the equatorial plane consist of two thatare larger than 90° (O(1)-Co(1)-N(1) = 95.45(7)° and N(2)-Co(1)-O(2) = 95.59(8)° in 1 and O(1)-Co(1)-N(1) = (N(1)i-Co(1)-O(1)i = 94.85(4)° in 2), and two smaller angles (O(1)-Co-O(2) = 82.84(7)° and N(1)-Co-N(2) = 86.16(8)° in 1 andO(1)-Co-O(1)I = 84.09(5)° and N(1)-Co-N(1)i = 86.22(6)°in 2) (Table 2), which are close to the corresponding values intrans-[Co(acacen)(piperidine)2]NCS (94.1(1)°, 95.2(1)°, 84.6(1)°,86.2(1)°) (16) and trans-[Co((BA)2en)(morpholine)2]ClO4(93.89(1)°, 94.26(11)°, 86.10(9)°, 85.87(11)°) (17).

The trans angles of 178.03(8)° and 177.46(7)° in 1 and178.83(4)° in 2 are indicative of a slight tetrahedral distor-tion of the basal coordination of Co(III), which in the caseof perfect planarity have values of 180°. This distortion isalso reflected by the torsion angles of ~2° in the six-membered rings of basal coordination in 1 and ~6 ° in 2 (Ta-ble 3), and the angle between the best least-squares planes(Co1-O1-C11-C12-C13-N1 and Co1-O2-C21-C22-C23-N2)in 1 being 5.61(9)°, and (Co1-O1-C1-C2-C3-N1 and Co1-O1i-C1i-C2i-C3i-N1i) in 2 being 0.08(4)°.

The (N-H) atoms of the axial thioacetamide ligands are in-volved in hydrogen bonding with the oxygen atoms of ClO4

–

(in 1) and PF6– (in 2) anions and the equatorial Schiff base

ligand (Fig. 3). As a result, the two axial thioacetamide lig-ands are tilted away from the Z axis by ~9° in 1 (S(3)-Co(1)-S(4) = 171.37(3)°) and ~5° in 2 (S(1)-Co(1)-S(1)i =175.309(19)°). Some features of this hydrogen bonding arealso observed in the IR and 1H NMR spectra of the com-pound (vide infra). The geometry of the hydrogen bonds isgiven in Table 4.

Spectral studiesThioamides are potential ambidentate ligands that can co-

ordinate to the transition-metal ions through either the sulfuror the nitrogen atoms. Thioacetamide exists as the two reso-nance structures presented in Scheme 1. When N-bonded, Iis the dominant structure and as a result the C—N stretchingfrequency is lowered while the C=S stretching frequency israised relative to the free thioacetamide. In the S-bondedcomplex, II is the dominant structure and consequently theC—N stretching frequency is raised while the C=S stretch-ing frequency is lowered relative to the free thioacetamide.

Yet another possibility for the S-bonded form is structureI in which the CS group acts as a thione. The shift in the vi-brational frequencies and the bond lengths of C—N and C=Sindicate which of the thione or thiolate structures is the ma-jor contributor to the ligand–metal coordination.

Except for the aromatic region, the IR spectral pattern isessentially the same for 1 and 2. The fairly strong band ap-pearing at 1305 cm–1 in the IR spectrum of free thio-acetamide, due to ν(CN) (20), shifts to 1318 cm–1 in 1 and1311 cm–1 in 2, respectively. The ν(CS), appearing at

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Amirnasr et al. 2077

1 2φ1 (C11-C12-C13-N1) 1.5(4) φ

1 (C1-C2-C3-N1) 7.4(2)φ2 (O1-C11-C12-C13) –0.1(4) φ

2 (O1-C1-C2-C3) –6.4(2)φ3 (C21-C22-C23-N2) 3.7(4) φ

3 (C1i-C2i-C3i-N1i) 7.4(2)φ4 (O2-C21-C22-C23) –1.8(4) φ

5 (N1-C5-C5i-N1i) –44.7(4)φ5 (N1-C1B-C2B-N2) –41.0(9) φ

6 (O1-C1-C11-C12) –18.98(18)

Table 3. Selected torsion angles (°) for compounds 1 and 2.

Fig. 3. (a) Hydrogen bonds in 1. Both perchlorate ions are disor-dered on the twofold axis. Hydrogens not taking part in thebonding scheme were omitted for clarity. (b) Hydrogen bonds in2 (for details and symmetry codes see Table 4). Both PF6 ionsare disordered on the inversion center. The main sections withoccupancy 0.702(5) and the other with occupancy 0.298(5) (atomlabels with a prime) are shown. Atomic displacement ellipsoidsare shown at 30% probability level. Hydrogens not taking part inthe bonding scheme were omitted for clarity.

708 cm–1 in free thioacetamide, appears at 708 cm–1 in 1 and699 cm–1 in 2, respectively. The small increase in ν(CN) andthe small decrease in ν(CS) of the coordinated thioacetamideis consistent with a metal sulfur coordination of the thioneform, and conform with the X-ray molecular structure. TheN—H stretching vibrational band of NH2 appears at 3323and 3386 cm–1 in 1 and 2, respectively. This band is rela-tively broad and appears at a lower frequency in the IR spec-trum of 1 because of the formation of strong hydrogen bondswith the oxygen atoms of the perchlorate anions and theequatorial Schiff base ligand.

The ν(C=N) of the equatorial Schiff base ligands that ap-pears at about 1540 cm–1 in the IR spectra of the free ligandsis shifted to a lower frequency by ~25 cm–1 and appears at1515 cm–1 in the IR spectra of the two complexes. Thestretching vibrations of ClO4

– (1116 cm–1) and PF6–

(852 cm–1) anions are observed in the corresponding regions(21).

The 1H NMR spectrum of 1 in CDCl3 is shown in Fig. 4(assignment as in Scheme 2). The signals at 2.02 and2.20 ppm are assigned to two sets of CH3 protons, Ha andHb, respectively. The four ethylenediamine protons (Hd) ap-pear as a singlet at 3.50 ppm. The singlet at 5.05 ppm corre-sponds to the He protons of acacen. The six methyl protons(Hc) of the axial thioacetamide ligands appear as a singlet at2.37 ppm.

The amide protons are usually observed at δ 5–8.5 ppm(22). The free thioacetamide in CDCl3 shows two NH protonresonances at 7.27 and 7.75 ppm. Upon coordination, thesetwo signals move to lower fields. Since the two N-H protonsof the coordinated thioacetamide (Hf and Hg) are involved indifferent hydrogen-bonding patterns (Fig. 3, Table 4), thetwo protons will presumably appear at considerably differentchemical shifts if the hydrogen bonding retains its characterin solution. This is most probably the origin of observing ahigher chemical shift for the Hf proton (10.22 ppm), whichis more strongly involved in an intramolecular hydrogenbonding relative to the Hg proton (8.73 ppm).

The 1H NMR spectrum of 2 in CDCl3 shows a patternsimilar to that of 1 except for the signals due to the aromaticprotons of the two phenyl rings that appear in the 7.44–7.83 ppm region. Other protons are also slightly shifteddownfield as expected from the electron-withdrawing char-acter of the rings. The signal at 2.23 ppm is assigned to thesix methyl protons (Ha) of the equatorial ligand. The sixmethyl protons (Hc) of the axial thioacetamide ligands ap-pear as a singlet at 2.39 ppm. The signal at 3.73 ppm is as-signed to the four ethylenediamine protons (Hd) and thesinglet at 5.78 ppm corresponds to the He protons of theequatorial ligand. The NH2 protons (Hg and Hf) of the coor-dinated thioacetamide appear as singlets at 8.04 and10.59 ppm, respectively.

UV–vis and solvatochromism studiesThe absorption spectrum of trans-[CoIII(acacen)(ta)2]ClO4

in chloroform features a band with a true maximum at518 nm. Based on the relatively high � (3654 (mol/L)–1 cm–1)of this band it may be assigned to a combination of d–d andCT transitions. The position of this band is solvent depend-ent and shows negative solvatochromic shifts in a dozen or-ganic solvents of various polarities (Table 5). This change

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2078 Can. J. Chem. Vol. 83, 2005

(a) trans-[CoIII(acacen)(ta)2]ClO4 (1).

D-H···A d(D-H) (Å) d(H···A) (Å) d(D···A) (Å) <(DHA) (°)

N(3)-H(3A)···O(13) 0.88 2.18 3.014(5) 158.3N(3)-H(3A)···O(13)#1 0.88 2.13 2.963(4) 157.1N(3)-H(3B)···O(1) 0.88 1.98 2.747(3) 144.9N(3)-H(3B)···O(2) 0.88 2.54 3.103(3) 122.2N(4)-H(4A)···O(22) 0.88 2.17 3.020(6) 161.8N(4)-H(4A)···O(24) 0.88 2.47 3.150(8) 134.0N(4)-H(4A)···O(21)#2 0.88 2.23 2.997(5) 145.3N(4)-H(4B)···O(2) 0.88 1.97 2.731(3) 143.4C(2B)-H(2B1)···S(4) 0.99 2.86 3.252(6) 104.5C(42)-H(42A)···O(22) 0.98 2.60 3.353(6) 134.1

(b) trans-[CoIII((BA)2en)(ta)2]PF6 (2).

D-H···A d(D-H) (Å) d(H···A) (Å) d(D···A) (Å) <(DHA) (°)

N(2)-H(2A)···F(1)iii 0.88 2.27 2.994(2) 38.9N(2)-H(2A)···F(3′)iv 0.88 2.28 3.124(6) 161.1N(2)-H(2A)···F(2)iii 0.88 2.40 3.162(3) 144.6N(2)-H(2A)···F(1′)iii 0.88 2.39 3.056(5) 132.5N(2)-H(2B)···O(1)i 0.88 1.91 2.7144(16) 150.9N(2)-H(2B)···O(1) 0.88 2.64 3.1031(18) 113.6

Note: Symmetry transformations used to generate equivalent atoms: –x + 1/2, y, –z + 1/2 (#1), –x – 1/2, y, –z + 1/2(#2), –x + 3/2, y, –z + 1/2 (i), –x + 1, –y, –z (ii), x, y + 1, z (iii), –x + 1, –y + 1, –z (iv).

Table 4. The hydrogen bonds for compounds (a) 1 and (b) 2.

S CNH2

R

(I)

S C

NH2

R

(II)

-

+

Scheme 1.

does not agree with the change in the polarity of the solvent.Therefore, it can be considered to correspond to the net ofseveral solvent effects such as polarity, basicity, and H-bondaccepting ability. The solvents can be divided into twogroups based on the degree of solvatochromic shift. Solventswith dielectric constants below 20.7 (Nos. 1–6), exhibitsmall solvatochromic shifts of the band (14 nm) and rela-tively large �. It is reasonable to assume that the first coordi-nation sphere of the bis(thioacetamide) complex remainsintact in these solvents. The structural integrity of the com-plex in solution is confirmed by its 1H NMR spectrum inCDCl3 (Fig. 4). The larger shift is observed in the solventpotentially capable of forming stronger hydrogen bonds withthe NH2 protons of the coordinated thioacetamide. This ob-servation may be rationalized based on the assumption thatthe CT band is mainly of LLCT character and the electronictransition occurs from the pπ(C=S) of the coordinatedthioacetamid to a π* of acacen chelate, mediated by the cen-tral Co atom. A stronger hydrogen bonding stabilizes theground state of the complex (Scheme 3) and has an unfavor-

able effect on the S → Co → acacen charge transfertransition and shifts the band to a higher energy.

In solvents of high dielectric constant and those that cancoordinate to the metal center such as methanol, acetonitrile,DMSO, and water, there is a drastic change in both the posi-tion (up to 72 nm) and � of the band. This hints at the possi-bility of a substitution process in which the two axialthioacetamide ligands are replaced by solvent molecules.Additional evidence for the substitution of the axial ligandscomes from the 1H NMR of the complex in DMSO-d6 andCD3OD. The appearance of a signal centered at about7.2 ppm, which can be attributed to the NH2 protons of thefree thioacetamide, is supportive of a dissociation processleading to the free ligand. A further clue to this point is ob-tained from the electronic absorption spectral changes of themethanolic solution of the complex in the presence of excessthioacetamide. This is achieved by adding increasingamounts of thioacetamide to a solution of trans-[CoIII(acacen)(ta)2]ClO4 in methanol and monitoring the ab-sorption spectral changes (Fig. 5). The absorption maximummoves from 450 nm in pure methanol to 510 nm (a red shiftof about 60 nm) after adding excess (56 times molar ratio)thioacetamide. While no isosbestic points are observed dur-ing the first nine additions, an isosbestic point at 382 nm ap-pears from then on and persists to the final stages of thereaction (Fig. 5. inset). These results indicate the existenceof a two-step reversible equilibrium in methanolic solution

© 2005 NRC Canada

Amirnasr et al. 2079

Scheme 2.

Fig. 4. 1H NMR spectrum of trans-[CoIII(acacen)(ta)2]ClO4 in CDCl3 at 298 K. The spectrum is labeled as to peak assignment dis-cussed in the text.

δ+

S C

NH2

RCo

δ-

Scheme 3. Co–thioacetamide fragment with a high contributionto the ground state.

involving association–dissociation of the thioacetamideligand as presented in eq. [1].

[1a] trans-[CoIII(acacen)(ta)2]+

+ Sol � trans-[CoIII(acacen)(ta)(Sol)]+ + ta

[1b] trans-[CoIII(acacen)(ta)(Sol)]+

+ Sol � trans-[CoIII(acacen)(Sol)2]+ + ta

The molar absorption coefficient � (3160 (mol/L)–1 cm–1)and the λmax (510 nm) in the final solution are close to thosemeasured in chloroform. Considering the microscopic re-versibility principle, the dissociation of the axial thioaceta-

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2080 Can. J. Chem. Vol. 83, 2005

1 2

Sample Solvent Dielectric constant λmax (nm) � ((mol/L)–1 cm–1) λmax (nm) � ((mol/L)–1 cm–1)

1 Chloroform 4.8 518 3654 528 30592 Dichloromethane 8.9 518 3189 528 29553 1,4-Dioxane 2.2 512 3621 518 34504 Ethyl acetate 6.0 510 3636 524 34415 Acetone 20.7 508 2593 518 24406 2-Butanone 18.5 504 2670 516 21707 Acetonitrile 37.5 489 1621 500(sh) 14738 Tetrahydrofurane 7.6 487 1199 496(sh) 11429 Ethanol 24.6 462 1463 474(sh) 1424

10 Dimethyl sulfoxide 46.6 458 910 468(sh) 147211 Methanol 32.7 454 1149 465(sh) 96712 Water 80.3 446 677 455(sh) 672

Table 5. Electronic absorption data for trans-[CoIII(acacen)(ta)2]ClO4 (1) and trans-[CoIII((BA)2en)(ta)2]PF6 (2) in solvents of variouspolarities.

Wavelength (nm)

Abs

orpt

ion

400 450 500 550 600 650 700

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Wavelength(nm)

400 450 500 550 600 650 700

Ab

so

rptio

n

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1

14

10

14

Fig. 5. The electronic absorption spectral traces of trans-[CoIII(acacen)(ta)2]ClO4 in methanol upon addition of thioacetamide at 298 K.The inset shows the spectral traces of samples 10–14 with the isosbestic point at 382.

mide ligands in methanol and other coordinating solvents isevident.

As a typical pseudooctahedral low-spin d6 complex, trans-[CoIII(acacen)(ta)2]

+ contains the thioacetamide ligand thatcan function as a CT donor ligand for ligand-to-metal chargetransfer (LMCT). The acacen is a π-donor and π-acceptorligand that can contribute to a metal-to-ligand charge trans-fer transition (MLCT). The relatively high � of the band at517 nm in a chloroform solution of 1 and the solvatochromicproperties of the complex are in agreement with the LLCTassignment of the longest wavelength absorption, which ismediated by the metal center. Some Co-centered character isalso possible. This is in analogy to certain low-spin d6 com-plexes such as Mo(diphos)(CO)(NO)(dtc) (23) and severald8, Ni(diimine)(dithiolate) (24), and d10, Zn(phen)(C6H5S)2(25) and Zn(SC6H4-CH3-p)2(dic), complexes (26).

The spectral properties of 2 fully support these conclu-sions (Table 5). The first transition in the electronic spec-trum of trans-[Co((BA)2en)(ta)2]PF6 in chloroform solutionappears at a longer wavelength (528 nm) relative to that ob-served for trans-[Co(acacen)(ta)2]ClO4 (518 nm). This is adirect consequence of increasing the π-acceptor character ofthe equatorial Schiff base ligand by the two additionalphenyl rings in (BA)2en, which facilitates the metal-mediated LLCT. The π–π* intraligand transition of theSchiff base is also red-shifted. While the first electronic ab-sorption of 1 in methanol is well-resolved (λ = 454 nm), it isobscured by the π–π* intraligand transition in 2 and appearsas a shoulder to the π–π* transition, which tails down to450 nm. The overall solvatochromic behavior of complex 2in different solvents is similar to that of 1 and can be used asfurther evidence for the LLCT nature of the first absorptionband in the electronic spectra of these complexes.

Conclusions

The preparation and detailed crystallographic and spectro-scopic characterization of trans-[CoIII(acacen)(ta)2]ClO4 (1)and trans-[Co((BA)2en)(ta)2]PF6 (2) have been carried out.2

The thioacetamide ligands are S-bonded and occupy the ax-ial positions. The donor ability of thioacetamide falls belowthat of common solvents: MeOH, CH3CN, Me2SO, andH2O. The compounds exhibit a long wavelength LLCT, me-diated by the metal center. The observed solvatochromism isexplained by the effect that solvents of low dielectric con-stant have on the LLCT and a facile replacement of thethioacetamide ligands by the coordinating solvents.

Acknowledgements

We gratefully acknowledge Professor Peter C. Ford of theUniversity of California, Santa Barbara for insightful discus-sions. M.A. wishes to thank the IUT Research Council forpartial support of this research under Grant 1 CHC 812.

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2 Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data, DocumentDelivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 4067. For more information on obtaining mate-rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 261132 and 261133 contain the crystallographic data for this manu-script. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge CrystallographicData Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).