Coenzyme M studies with nickel(II) compounds

Polyhedron 18 (1999) 3383–3390 www.elsevier.nl / locate / poly

Coenzyme M studies with nickel(II) compounds Synthesis, structure, spectra and redox behaviors ´ ` C. Duran ´ de Bazua ´ T. Pandiyan*, M.A. Rios Enrıquez, S. Bernes, ´ ´ ´ ´ , 04510 Mexico ´ ´ , Universidad Nacional Autonoma de Mexico ( UNAM), Ciudad Universitaria, Coyoacan D.F., Mexico Facultad de Quımica Received 15 February 1999; accepted 18 August 1999

Abstract Nickel(II) complexes of 1,6-bis(pyridyl)-2,5-dithiahexane (L 1 ), 1,7-bis(29-pyridyl)-2,6-dithiaheptane (L 2 ) and 1,9-bis(29-pyridyl)2,5,8-trithianonane (L 3 ) have been prepared and their spectroscopic and redox behaviors were studied. [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 , and [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 were crystallized in single crystal form; their structures were solved by X-ray crystallography. The structures of the complexes are of distorted octahedral geometry. A red shift in the electronic spectra and a positive potential shift in electrochemical studies were detected during the addition of the sodium salt of 2-mercaptoethanesulfonic acid (CoM) to Ni(II) complexes containing L 1 and L 2 . The high redox potential shifting difference (PSD) was observed with the addition of CoM to [NiL 1 ] 21 , which accounts for the axial coordination of CoM with the nickel ion. However, [Ni(II)L 3 ] 21 does not respond well with CoM addition due to the structural limitation around the Ni(II) ion. A destabilization of [Ni(II)L 1 ] 21 and [Ni(II)L 2 ] 21 complexes and stabilization for [Ni(II)L 3 ] 21 were noticed in their redox studies and these trends were inversely changed during anaerobic CoM addition to Ni(II) complexes. A nephelauxetic effect (b values) has been shown to establish a good relation with PSD.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Nickel(II) complexes; Nephelauxetic effect; Coenzyme M; Redox potentials

1. Introduction Organometallic chemistry of nickel(II) attracts much attention due to an involvement of organonickel species in the mechanisms of metalloenzymes reactions such as methyl-S-coenzyme M reductase, carbon monoxide dehydrogenase (CODH), and acetyl coenzyme A synthesis, found in certain methanogenic, acetogenic, and sulfatereducing bacteria. The intermediate complex containing an apical methyl ligand coordinated to the Ni(II) ion has been proven to be a crucial intermediate in the enzymate cycle CODH and its existence has been proposed in the case of methyl-coenzyme reductase and acetyl coenzyme A synthase [1–4]. The cofactor coenzyme M reductase [5,6] contains F 430 [7–9] because of its absorbance at 430 nm involved in the terminal steps of methane formation from methanogenic bacteria and has attracted much attention [10,11]. The function of F 430 in methane formation was demonstrated by Ankel-Fuchs et al. [12]. A mechanism has been suggested [13] for the function of methyl-coenzyme M reductase where Ni(II)F 430 is first reduced to Ni(I)F 430 , *Corresponding author. Tel.: 152-56-22-3712; fax: 152-56-22-3712. E-mail address: [email protected] (T. Pandiyan)

which homolytically cleaves methyl-CoM to produce methyl-Ni(I)F 430 followed by the protonation of methylNi(I)F 430 to yield CH 4 and Ni(II)F 430 [14–16]. Nickel(I)– tetraazamacrocyclic complexes are only somewhat more effective in producing methane from methyl-CoM in aqueous solution [17]. It is clearly illustrated that the ligand activates Ni(II) toward methyl-CoM and the ligand in F 430 might also play an important role in activating nickel toward methyl-CoM. The capacity of nickel to bind both hard and soft donor ligands allow its coordination chemistry to encompass a variety of geometries and oxidation states with reactivity ranging from that in biological systems to organometallic chemistry. Studies of active sites of various metalloproteins / enzymes has been used in elucidating the relationship between their structure and activity [18–24]. The correlation between a metal coordination sphere and its redox chemistry has been emphasized in studies aimed at elucidating the catalytic function of metalloenzymes. Although the N 4 macrocyclic system found in the factor F 430 of coenzyme M reductase, is one of the responsible characters for producing methane since the energy of the Ni d x 2 2y 2 orbital appears to be very sensitive to the Ni–N distances in F 430 ; we aimed to demonstrate the effect of an

0277-5387 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0277-5387( 99 )00243-0

T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390

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software which was employed to carry out the experiments. The reference electrode was Ag(s) /(AgNO 3 ) in methanol. The solutions were deoxygenated by bubbling with research grade nitrogen. Methanol for electrochemistry was distilled over magnesium turnings.

2.3. Synthesis of ligands 1,6-Bis(2-pyridyl)-2,5-dithiahexane (L 1 ) and 1,9-bis(2pyridyl)-2,5,8-trithianonane (L 3 ) were synthesized as reported elsewhere [26].

Fig. 1. Structures of the ligands.

acyclic system with soft thioether sulfur donor atoms on biogas evolution and, also to understand the relationship between the structure around the nickel ion and the efficiency of methane production in anaerobic conditions. The ligands containing thioethers with different chelate ring systems were prepared (Fig. 1) and complexed with nickel(II) salts. The nickel(II) compound of L 1 and L 3 were crystallized and their structures were solved by X-ray crystallography. The electronic and redox potentials were recorded for the nickel(II) complexes.

2.3.1. 1,7 -Bis(29 -pyridyl)-2,6 -dithiaheptane ( L 2) The hydrochloride of 2-chloromethylpyridine (2.12 g, 13 mmol) dissolved in 25 ml of ethanol were slowly added, under nitrogen and vigorous stirring, to a solution of KOH (46 g, 26 mmol) and of 1,3-propanedithiol (1.0 g, 6.5 mmol) in 25 ml of nitrogen-flushed ethanol. A white precipitate of KCl was formed immediately. The reaction mixture was stirred for 30 min and refluxed for 1 h, then it was cooled to room temperature and the solvent removed by rotary evaporation. Water was then added, followed by KOH to bring the pH to 13, after which the product was extracted with dichloromethane. After the dichloromethane solution was dried with magnesium sulfate and stripped off the solvent by rotary evaporation, the product was obtained as a yellowish oil. Characterization by 1 H NMR: dH (300 MHz; solvent CDCl 3 ; TMS used as internal standard): 8.52 (d, J54 Hz, 2H; 6-pry), 7.67 (dd, J58 Hz, 2H, 4-pry), 7.38 (d, J58 Hz, 2H, 3-pyr), 7.18 (dd, J58 Hz, J54 Hz, 2H, 5-pyr), 3.87 (s, 4H, pyr–CH 2 –S) and 2.68 (tt, 6H, S–C 3 H 6 –S).

2. Experimental

2.4. Synthesis of complexes

2.1. Materials

[Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 : To the ligand L 1 (0.28 g, 1.0 mmol) dissolved in methanol (10.0 ml) was added a solution of Ni(ClO 4 ) 2 ?6H 2 O (0.36 g, 1.0 mmol) in methanol (5.0 ml). The blue compound obtained was collected, washed with methanol and dried over P4 O 10 in vacuum (yield, 90%). Recrystallization from methanol afforded suitable crystals for X-ray crystallography. [Ni(II)L 2 ](ClO 4 ) 2 (H 2 O): A solution of Ni(ClO 4 ) 2 ? 6H 2 O (0.36 g, 1.0 mmol) in methanol (5.0 ml) was added under stirring to L 2 (0.29 g, 1.0 mmol) dissolved in methanol (10.0 ml). The blue crystals obtained were collected, washed with small amounts of methanol and dried over P4 O 10 in vacuum. (yield, 85%). [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 : The compound L 3 (0.34 g, 1.0 mmol) was dissolved in methanol (10.0 ml) and a solution of Ni(ClO 4 ) 2 ?6H 2 O (0.36 g, 1.0 mmol) in methanol (5.0 ml) was added. The purple crystals obtained were collected, washed with a small amount of methanol and dried over P4 O 10 in vacuum (yield, 95%). Suitable single crystals were prepared by slow evaporation of a methanol solution.

The following reagents were used as received: Ni(II)Br 2 ?xH 2 O, Ni(II)Cl 2 ?6H 2 O, Ni(II)(ClO 4 ) 2 ?6H 2 O, 2chloromethylpyridine, 1,2-ethanedithiol, 1,3-propanedithiol (Aldrich); sodium salt of 2-mercaptoethanesulfonic acid (coenzyme M) (Fluka). Tetra-n-hexylammonium perchlorate (G.F. Smith) was recrystallized twice from aqueous ethanol.

2.2. Physical measurements Elemental analyses were carried out in the Faculty of ´ chemistry, UNAM, Mexico. Nickel was determined using voltammetry technique [25]. The diffuse reflectance and absorption spectra in methanol were measured on an Hitachi U-3400 double beam UV/ VIS / NIR spectrophotometer. All voltammetric experiments were performed in a single-compartment cell with three-electrode configuration on a EG&G PAR 273 potentiostat / galvanostat interfaced with a computer along with EG&G M270

T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390 Table 1 Elemental analysis of the Ni(II) complexes with calculated values in parenthesis Compounds

C (%)

H (%)

N (%)

Ni (%)

[Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2

29.47 (29.49) 41.35 (41.40) 31.80 (31.82) 39.30 (39.49) 31.30 (31.39) 40.90 (40.98) 33.75 (33.74)

3.51 (3.54) 3.90 (3.97) 3.45 (3.56) 4.80 (4.86) 3.50 (3.62) 4.80 (4.86) 4.30 (4.33)

4.71 (4.91) 6.85 (6.90) 4.75 (4.95) 6.10 (6.14) 4.47 (4.58) 5.60 (5.62) 4.70 (4.63)

10.35 (10.29) 14.40 (14.45) 10.20 (10.37) 12.90 (12.87) 9.45 (9.59) 11.60 (11.78) 9.70 (9.70)

Ni(II)(L 1 )Cl 2 2

Ni(II)(L )(ClO 4 ) 2 ?H 2 O Ni(II)(L 2 )Cl 2 ?2H 2 O [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 Ni(II)(L 3 ) Cl 2 ?MeOH Ni(II)(L 3 ) Br 2 MeOH?H 2 O

Ni(II)L 1 Cl 2 , Ni(II)(L 2 )Cl 2 ?2H 2 O, Ni(II)L 3 Cl 2 ?MeOH and Ni(II)L 3 Br 2 ?MeOH?H 2 O were prepared as indicated in the above procedures. Elemental analyses of Ni(II) compounds are presented in Table 1. Caution: Although no accident occurred with nickel perchlorate complexes during the experimental work, it should be remembered that perchlorates are potentially explosive.

2.5. Reaction of Ni( II) compounds with CoM The experimental studies of the sodium salt of 2-mercaptosulfonic acid as CoM (1.0 mmol) with the nickel(II) complexes (1.0 mmol) were carried out. The color changes were observed from blue to yellow during the addition of CoM (0.1 mmol) to Ni(II) solution (0.1 mmol) under anaerobic conditions. Electronic spectra and the redox potential were measured for the nickel (II) complex and

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also for the adduct of nickel(II) complexes with CoM solutions. When keeping the complexes for a long time in CoM solution, a precipitation was noticed.

2.6. Crystallography Details of the data collection and structure refinements for [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 and [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 are presented in Table 2. The diffraction data were collected at 298 K on a Siemens P4 diffractometer using a standard procedure [27]. A semi-empirical absorption was applied for both complexes, using c-scans. The structures were solved and refined without constraints nor restraints using the SHELX package [28]. The hydrogen atoms were introduced in an idealized position (riding refinement) except for the water molecules, for which they were omitted. For [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 , a minor disorder was observed for a perchlorate ion: O(22), O(23) and O(24) are disordered with O(229), O(239) and O(249), respectively, and were refined with respective site occupation factors of 2 / 3 and 1 / 3.

3. Results and discussion [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 : The asymmetric unit of the unit cell contains a nickel atom, a half-part of L 1 , one water molecule and one perchlorate ion. The metal lies on a 2-fold axis of the C2 /c space group, which yield the cation depicted in Fig. 2 with 2 /m point group, not observed in the related complex [Ni(II)L 1 (CH 3 CN) 2 ](ClO 4 ) 2 which crystallizes in a 1¯ symmetry reported by Adhikary et al. [26]. The metal center is coordinated (Table 3) by two thioether sulfur atoms, two pyridyl nitrogen atoms and two water molecules. The greatest deviation from an idealized octahedral geometry is

Table 2 Crystallographic data Empirical Formula

C 14 H 20 Cl 2 N 2 Ni 1 O 10 S 2

C 16 H 22 Cl 2 N 2 NiO 9 S 3

Formula weight Crystal system Space group ˚ 8) Unit cell dimensions (A,

570.05 Monoclinic C2 /c a520.149 (1) b510.299 (1) c511.609 (1) b 5114.59 (1) 2190.6 (3) 4 1.729 1.37 3181 2642 (R int 52.04%) 1899 [Fo .4s (Fo )] R 1 55.04%, wR 2 512.11% R 1 57.65%, wR 2 513.70%

612.15 Monoclinic P2 1 /n a58.986 (2) b523.806 (3) c511.318 (2) b 5104.32 (1) 2346.0 (7) 4 1.733 1.373 5239 4316 (R int 54.46%) 3054 [Fo .4s (Fo )] R 1 55.34%, wR 2 514.22% R 1 57.94%, wR 2 516.18%

3

˚ ) Volume (A Z Density (calc.) (g cm 23 ) Absorption coefficient (mm 21 ) Reflections collected Independent reflections a Observed reflections Final R indices (I . 2 s (I))a Final R indices (all data)a a

]]]] uF 2o 2kF 2o lu iFo u2uFc i w(F 2o 2F 2c )2 ]]] ]]]] R int 5 ]]] , , wR 5 . 2 2 2 2 uF o u uFo u w(F o )

O

O

O

O

O œO

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T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390

Fig. 2. Crystal structure drawing of the [Ni(II)L 1 (H 2 O) 2 ] 21 cation showing the atom numbering and the thermal motion ellipsoids (30% probability level). Hydrogen atoms are omitted for clarity.

observed for the pyridyl nitrogen atoms, N(1)–Ni(1)– N(1)[15169.3 (2)8 (Table 3), probably as a consequence of the steric hindrance of the three 5-membered rings involving L 1 . Moreover, calculated deviations from pla˚ for the ring narity for these rings are large, 0.273 A ˚ for the ring including C(2). containing C(1) and 0.114 A Similar reported structures (555 system) were analyzed with imidazole ligands [29–33], but none with pyridyl systems. An unexpected bite angle is observed for the cis water molecules, O(1)–Ni(1)–O(1)[1583.3(2)8, substantially shorter that the analogue angle in [Ni(II)L 1 (CH 3 CN) 2 ](ClO 4 ) 2 , 88.9(3)8 [26] and [Ni(II)L 1 Cl 2 ](ClO 4 ) 2 , 96.00(3)8 [34]. Remaining coordination distances and angles are identical within experimental errors to those reported for similar complexes [22]. Finally, the compound is stabilized in the solid state by intermolecular hydrogen bonds between the water mole-

Fig. 3. Crystal structure drawing of the [Ni(II)L 3 (H 2 O)] 21 cation showing the atom numbering and the thermal motion ellipsoids (30% probability level). Hydrogen atoms are omitted for clarity.

cule and the perchlorate ions, as indicated by the short contacts O(1) . . . O(4)52.835 and O(1) . . . O(5)52.851 ˚ A. [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 : The crystal structure of this compound consists of a nickel ion surrounded by L 3 coordinated by three thioether sulfur atoms and two nitrogen atoms of the pyridyl groups, one coordinated water molecule and two perchlorate ions, with all atoms in general positions (Fig. 3). The composition is identical to that of an earlier report [26], the main difference being that in the present work, no solvent molecule was included in the cell, which increases the Laue symmetry from 1¯ to 2 /m. The ligand L 3 forms a system of four 5-membered

Table 3 ˚ and bond angles a (8) of [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 Selected bond lengths a (A) Ni(1)–O(1) Ni(1)–N(1) Ni(1)–S(1) S(1)–C(2) N(1)–C(3) C(1)–C(1)[1 C(3)–C(4) Cl(1)–O(3) Cl(1)–O(5) O(1)–Ni(1)–O(1)[1 O(1)[1–Ni(1)–N(1) O(1)[1–Ni(1)–N(1)[1 O(1)–Ni(1)–S(1) N(1)–Ni(1)–S(1) O(1)–Ni(1)–S(1)[1 N(1)–Ni(1)–S(1)[1 S(1)–Ni(1)–S(1)[1 a

2.075(3) 2.095(3) 2.3974(11) 1.793(5) 1.333(5) 1.519(8) 1.368(6) 1.394(5) 1.398(4) 83.3(2) 96.69(13) 91.32(13) 93.10(10) 84.01(9) 176.35(10) 88.46(10) 90.49(5)

Ni(1)–O(1)[1 Ni(1)–N(1)[1 Ni(1)–S(1)[1 S(1)–C(1) N(1)–C(7) C(2)–C(3) C(4)–C(5) Cl(1)–O(2) Cl(1)–O(4) O(1)–Ni(1)–N(1) O(1)–Ni(1)–N(1)[1 N(1)–Ni(1)–N(1)[1 O(1)[1–Ni(1)–S(1) N(1)[1–Ni(1)–S(1) O(1)[1–Ni(1)–S(1)[1 N(1)[1–Ni(1)–S(1)[1

Symmetry transformation used to generate equivalent atoms: [1: 2x11, y, 2z13 / 2.

2.075(3) 2.095(3) 2.3974(11) 1.804(4) 1.354(5) 1.519(6) 1.372(8) 1.396(5) 1.426(5) 91.32(13) 96.69(13) 169.3(2) 176.35(10) 88.46(10) 93.10(10) 84.01(9)

T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390 Table 4 Selected bond lengths [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 Ni(1)–N(2) Ni(1)–O(1) Ni(1)–S(1) N(1)–C(1) S(2)–C(9) C(12)–N(2) N(2)–Ni(1)–N(1) N(1)–Ni(1)–O(1) N(1)–Ni(1)–S(3) N(2)–Ni(1)–S(1) O(1)–Ni(1)–S(1) N(2)–Ni(1)–S(2) O(1)–Ni(1)–S(2) S(1)–Ni(1)–S(2)

˚ (A)

2.072(4) 2.100(3) 2.4138(13) 1.341(6) 1.826(6) 1.341(6) 99.69(16) 89.33(15) 177.55(12) 84.46(11) 170.63(11) 164.73(11) 99.09(10) 87.22(5)

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3.1. Electronic spectra and

bond

angles

Ni(1)–N(1) Ni(1)–S(3) Ni(1)–S(2) C(6)–S(1) S(3)–C(11) C(12)–C(13) N(2)–Ni(1)–O(1) N(2)–Ni(1)–S(3) O(1)–Ni(1)–S(3) N(1)–Ni(1)–S(1) S(3)–Ni(1)–S(1) N(1)–Ni(1)–S(2) S(3)–Ni(1)–S(2)

(8)

of

2.081(4) 2.3797(14) 2.4155(14) 1.798(6) 1.820(5) 1.388(7) 90.84(14) 82.72(12) 90.23(10) 83.50(11) 97.19(5) 92.05(12) 85.65(5)

rings, with the pyridyl nitrogen in a trans geometry (Table 4), N(1)–Ni(1)–N(2)599.69(16)8, while the ligand L 1 produced a cis configuration due to the reduction of one 5-membered ring. A number of weak hydrogen bonds ˚ between the perchlorate anions and (2.394 to 2.566 A) hydrogen atoms from L 3 establishes the packing in the solid state.

The reflectance spectra of nickel(II) compounds display a broad band in the visible region. On dissolution in methanol, relatively sharp well-separated bands were obtained, indicating the probable involvement of the solvent molecule in the coordination sphere. The spectral data and band assignment are presented in Table 5. The presence of more than one transition in the visible region are accountable for the asymmetric nature of the bands [24,35]. Assignments of the bands position are 3 A 2g → 3 T 2g (1110.0–835.0 nm), 3 A 2g → 3 T 1g (F) (715.0–555.0 nm) and a higher energy band starting around 400 nm [ 3 A 2g → 3 T 1g (P)] which is obscured by a charge-transfer [36]. A lower ligand field energy band was noticed between 900 and 945 nm for all nickel(II) complexes; however, [Ni(II)L 2 ] 21 exhibits somewhat higher ligand field energy then Ni(II)L 1 ] 21 and [Ni(II)L 3 ] 21 compounds. The charge-transfer bands with lower e values were observed in the 400–380 nm range revealing less interaction of the s-orbital and a higher overlap of the p-orbital of the ligands with Ni(II) ion. When CoM is added to [Ni(II)L 1 ] 21 and [Ni(II)L 2 ] 21 , a band shift towards the red region in the electronic spectra is observed

Table 5 Electronic absorption spectral data in nm, with e / dm 3 mol 21 cm 21 in parentheses Compounds

Medium

Ligand field a

[Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2

Solid Methanol

538 935(12) 552(8) 552 952(14) 559(9) 543 870(11) 562(8) 559 892(32) 578(19) 901 562 917(33) 581(22) 917 581 926(34) 580(21) 901 575 926(20) 592(29) 1100 (4) 751(5) 1048 (14) 632(10) 1002(15) 590(11)

Ni(II)(L 1 )Cl 2 Ni(II)L 2 (ClO 4 ) 2 ?H 2 O Ni(II)L 2 Cl 2 ?2H 2 O [Ni(II)L 3 (H 2 O)](ClO 4 ) 2

Solid Methanol Solid Methanol Solid Methanol Solid Methanol

Ni(II)L 3 Cl 2 ?MeOH

Solid Methanol

Ni(II)(L 3 )Br 2 ?MeOH?H 2 O

Solid Methanol

[Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 1CoM

Methanol

Ni(II)L 2 (ClO 4 ) 2 ?H 2 O1CoM

Methanol

[Ni(II)L 3 (H 2 O)](ClO 4 ) 2 1CoM

Methanol

a b

Concentration 5310 22 mol dm 23 . Calculated from B 5 2n 21 1 n 22 2 3n1 n2 /(15n2 2 27n1 ); in the free ion B 5 1038, b 5 Bcomplex /Bfree

b values b

Charge-transfer a

1.37

497 398(23)

1.45

398 402(26)

0.73

397 400(23)

0.70

394 398(34) 382

0.77

386(65) 380

0.82

388(62) 386

ion

0.72

391(55)

0.43

406(14)

0.97

390(18)

1.32

401(20)

and 3 A 2g – 3 T 2g (n1 ) and 3 A 2g – 3 T 1g (n2 ).

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T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390

Fig. 4. Electronic absorption spectra of [Ni(II)L 1 ] 21 (–) and with Coenzyme M ( . . . . . . ) in methanol.

irrespective of the chelate rings effects (Fig. 4). The complexes [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 and [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 are not perfectly octahedral, indicated in the crystallographic structural analysis, and it is treated as a distorted octahedral or tetragonal geometry. A tetragonal distortion from octahedral symmetry often occurs even when all six ligand atoms of a complex are the same. The X-ray crystallographic analysis is a known technique for determination of the structure of the compounds in solid state, however, it is limited to solutions. The electronic spectral studies are often useful for investigating the geometries of the metal complexes in the solutions. We calculated the nephelauxetic ratios ( b ) [37] from the electronic repulsion parameter known as Racah parameters (B) ( b 5Bcomplex /Bfree ion ) for the complexes and its adducts of CoM. When the apparent value of Bcomplex is smaller than that of the free ion, it is attributed to the delocalization of the metal electrons over molecular orbitals that encompass both the metal and the ligands. As a consequence of this delocalization or cloud expanding, the average interelectronic repulsion is reduced and it leads to a value of b always less than one and it decreases with increasing delocalization. The experimental data obtained show that a greater nephelauxetic effect was observed for [Ni(II)L 1 ] 21 when compared to [Ni(II)L 2 ] 21 and [Ni(II)L 3 ] 21 compounds. An earlier report revealed that the thioethers and amines exert roughly a comparable ligand field strength [38], but the thioethers manifest a much greater nephelauxetic effect. Moreover, thioethers generally bind weakly because they are both weak s-donors and weak p-acceptors [39]. Thus, in the absence of other effects, complexes containing more thioether donors should exhibit a greater nephelauxetic effect. The [Ni(II)L 1 ] 21 complex exhibits a higher b value and the value is forced to reduce 0.43 in the presence of CoM. The low nephelauxetic ratio ( b ) for the complexes implies considerable mixing between the ligand and metal orbitals. For the [Ni(L 1 )] 21 complex, the higher b value is observed, since the cavity of the chelate ring (555)

is small and it pushes the metal above the plane and consequently interaction of the metal orbits with the ligand is less and it leads to a higher b value. With the addition of CoM to [Ni(II)L 1 ] 21 , the nickel ion, which is situated above the plane, interacts effectively with the axially incoming sulfur atom from CoM, and leads to the significant lowering of the b value. This manifests the favorable coordination of CoM to the Ni(II) ion. However, the bonding of CoM is not favorable for [Ni(II)L 3 ] 21 due to changes of the chain length (5555) between donors sites which may alter the fit of the metal ion to the coordination sphere provided by the ligand and decreases the effectiveness of orbital overlap and thus, a much higher b value was derived. There is no considerable change in b values for [Ni(II)L 2 ] 21 during CoM addition, it may be due to ineffective mixing of the metal orbital with CoM.

3.2. Electrochemical properties The redox behavior of the present Ni(II) complexes was investigated using cyclic voltammetry (CV) on a stationary platinum electrode. Electrochemical experiments were carried out in methanol, with tetra-n-hexylammonium perchlorate used as a supporting electrolyte. Nickel(II) complexes of L 3 revealed quasi-reversible redox behaviors, however, an irreversible Ni(I) / Ni(II) redox process was recorded for other nickel(II) complexes. A Differential Pulse Voltammetry (DPV) technique was used for measuring the E1 / 2 of the complexes. The electrochemical data of the complexes are presented in Table 6. The non-Nernstian behavior may be ascribed to heterogeneous electron-transfer kinetics. This is evident from the cathodic shift of the reduction peak and the anodic shift of the oxidation wave that increases in the scan rate [40]. The electrochemical data of Ni(II) compounds show that the nickel complex of L 3 exhibits a more positive potential than nickel complexes of L 2 and L 1 . It can be ascribed to a chelate ring size effect (5555) which plays greater flexibility around the nickel cation and it facilitates the reduction

T. Pandiyan et al. / Polyhedron 18 (1999) 3383 – 3390 Table 6 Electrochemical data a of Ni(II) complexes at 258C Compounds 1

[Ni(II)L (H 2 O) 2 ](ClO 4 ) 2 Ni(II)(L 1 )Cl 2 Ni(II)(L 2 )(ClO 4 ) 2 ?H 2 O Ni(II)(L 2 )Cl 2 ?2H 2 O [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 Ni(II)(L 3 )Cl 2 ?MeOH [Ni(II)L 1 (H 2 O) 2 ](ClO 4 ) 2 1CoM Ni(II)L 1 Cl 2 1CoM Ni(II)L 2 (ClO 4 ) 2 ?H 2 O1CoM Ni(II)L 2 Cl 2 ?2H 2 O1CoM [Ni(II)L 3 (H 2 O)](ClO 4 ) 2 1CoM Ni(II)L 3 Cl 2 ?MeOH1CoM

E1 / 2 b (V)

PSD c (V)

DEp (V)

20.764 20.761 20.834 20.814 20.603 20.590 20.690 20.698 20.775 20.778 20.605 20.607

– – – – – – 0.074 0.063 0.059 0.036 0.002 0.017

– – – – 0.130 0.089 – – – – – 0.146

a

Measured vs. non-aqueous silver reference electrode; add 544 mV to convert into normal hydrogen electrode (NHE); scan rate 50 mV s 21 , supporting electrolyte tetra-n-hexylammonium perchlorate (0.1 mol dm 23 ); complex concentration 1.0 mM dm 23 . CoM5Coenzyme M (sodium salt of 2-mercaptosulfonic acid). b Difference Pulse Voltammetry, scan rate 1.0 mV s 21 , pulse height 50 mV. c Potential shifting difference (PSD)5E1 / 2 (complex1CoM ) 2E1 / 2 complex .

of the Ni(II) ion. The redox potentials [Ni(II) / Ni(I)] of perchlorates (Table 6) follow the order [Ni(II)L 2 ] 21 , [Ni(II)L 1 ] 21 ,[Ni(II)L 3 ] 21 and this reflects the destabilization and stabilization of the Ni(II) ion by 5555 and 565 chelate rings, respectively. This also indicates that the 565 chelate ring is ideally suited to establish strong in-plane s interactions with the Ni(II) ion rather than 555 or 5555 rings. The greater number of thioether coordination in the [Ni(II)L 3 ] 21 is accountable for the higher order in the redox potentials series than other nickel(II) complexes. The potential difference between the complexes and its CoM adducts are in the order [Ni(II)L 1 ] 21 .[Ni(II)L 2 ] 21 . [Ni(II)L 3 ] 21 (0.074, 0.059 and 0.002 V, respectively). A potential shift towards the positive side is detected while addition of CoM to [Ni(II)L 1 ] 21 and [Ni(II)L 2 ] 21 and it manifests the CoM coordination with Ni(II) complexes. The redox potential shifting difference (PSD) between the nickel(II) complex of L 1 and its CoM adducts is 0.074 V, which indicates that the lower b and higher PSD values favor the axial CoM ligation with the nickel ion. However, very small potential changes were recorded during the anaerobic addition of CoM to the nickel complex of L 3 ; it is an indication of Ni(II) not binding with CoM, indicated also in the electronic spectra. Even though this redox shift difference value (0.074 V) considerably changes the biological functions in natural biological systems, but it is not known whether it is enough to produce methane from the reaction center. However, the nickel(II) complex of L 1 might have more tendency of coordination towards CoM than [Ni(II)L 3 ] 21 . In addition to investigating the influence of the redox potentials and the ligand field effects on methane production from the methyl-coenzyme reactions, we synthesized another series of nickel(II) compounds with different types of N 4 chromophores and these are

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studied with methyl-CoM addition; we detected methane evolution in the reaction center by using gas chromatography [41].

4. Conclusions The present investigation illustrates that a distorted octahedral geometry is found in [Ni(II)L 1 (H 2 O) 2 ] 21 and [Ni(II)L 3 (H 2 O)] 21 . The sterically hindered 555 chelate ring of L 1 around the nickel ion leads to an elevation of the nickel cation above the plane which causes its high ligand field energy with lower intensities in the visible region. The spectral results and b values, indicate the binding of CoM to nickel(II) compounds leads to a lower symmetry in the nickel(II) environment and provokes their red shift in the electronic spectra; however, the nickel ion bonding with CoM is not favorable for Ni(L 3 )21 due to the overcrowded and flexible arrangement around the metal ion. Electrochemical data show that the nickel(II) compounds of L 1 and L 2 have more negative potentials than that of L 3 . The destabilization is favored for the Ni(II) ion by the 5555 chelate ring, and stabilization of the Ni(II) cation is observed with 565 and 555 rings in the redox potential. On the other hand, when addition of CoM to Ni(II) compounds of L 1 and L 2 favors destabilization, which shifts the potentials to a more positive side. The CoM adduct on Ni(L 3 )21 causes very little effect in the potentials; it may be ascribed to limited structural changes during the addition.

Acknowledgements ´ The authors acknowledge the Direccıon General de ´ Asuntos del Personal Academico (Project No. IN205398) for their economic support. The authors also thank the ´ Facultad Unidad de Servicios de Apoyo a la Investigacion, ´ de Quımica for providing diffractometer facilities. Q. ´ ´ Marisela Gutierrez Franco and Q.F.B. Graciela Chavez ´ are thanked for recording electronic spectra. Beltran

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