Rearrangement and co-ordination of 1-[(4-methylphenyl)- sulfonamido]-2-[1-(2-pyridylmethylidene)amino]benzene

Abstract

Two different compounds, L¹ {3-[ethoxy(2-pyridyl)methyl]-1-[(4-methylphenyl)sulfonyl]-2-(2-pyridyl)-2,3-dihydro-1H-benzo[d]imidazole} and L² {1-[(4-methylphenyl)sulfonyl]-2-(2-pyridyl)-2,3-dihydro-1H-benzo[d]imidazole} have been synthesised by the reaction of 2-pyridinecarboxaldehyde with N-tosyl-1,2-diaminobenzene. L¹ was crystallographically characterised. The interaction of L¹ with a nickel centre was checked. Electrochemical interaction of Ni with L¹ and N-(2-hydroxyphenyl)salicylidenimine (H₂L⁴) produces NiL¹L⁴·2H₂O 1. Its recrystallisation in dichloromethane leads to the isolation of single crystals of NiL³₂·0.75H₂O 2 (HL³=1-[(4-methylphenyl)sulfonamido]-2-[1-(2-pyridylmethylidene)amino]benzene), where L¹ underwent a rearrangement. In the reaction of L¹ with Ni and p-toluenesulfonic acid in an electrochemical cell, complex 2 is isolated as the bulk product. Nickel complexes have been characterised by elemental analysis, IR spectroscopy and magnetic measurements. The molecular structure of 2 has been determined by single X-ray diffraction studies.

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The co-ordination chemistry of Schiff base ligands has been widely investigated. The interest in these ligands arose not only from their versatility but also from their use for modelling the chemical environment in many metalloenzymes. The construction of models that can lead to a better understanding of their properties has attracted the attention of both biochemists and inorganic chemists. Many metallobiosites are formed by a metal atom surrounded by nitrogen atoms and some of them, such as cofactor F₄₃₀ , involve a nickel centre. Thus, Schiff bases containing nitrogen donors could act as versatile models of the active centre of these enzymes. In addition, nickel complexes with nitrogen donor atoms often show interesting structural, electric and magnetic properties.^1–6

Owing to our experience in the co-ordination chemistry of symmetrical and asymmetrical Schiff base ligands,^7,8 we extended our studies to the investigation of potentially tridentate asymmetrical Schiff bases containing N-donors. Herein we report our attempts to obtain the Schiff base 1 - [ ( 4- methylphenyl ) sulfonamido ] - 2 - [ 1 - ( 2 - pyridylmethylidene )amino]benzene (HL³) and its reorganisation in solution to yield two different ligands, L¹ and L². The interaction of L¹ with a nickel centre, under different reaction conditions, has also been investigated.

Results and discussion

N-Tosyl-1,2-diaminobenzene and 2-pyridinecarboxaldehyde were mixed in different ratios under various reaction conditions (room temperature, reflux, with or without traces of p-toluenesulfonic acid) in an attempt to obtain 1-[(4-methylphenyl ) sulfonamido ] - 2 - [ 1 - ( 2 - pyridylmethylidene ) amino ] benzene, HL³. All efforts were unfruitful and treatment of N-tosyl-1,2-diaminobenzene with 2-pyridinecarboxaldehyde led always to the isolation of two different products (L¹ and L²), depending on the reaction solvent (Scheme 1). The products can be formed through the reactions shown in Scheme 2.

Scheme 1 Reaction conditions (i absolute ethanol (99%), stir 5 h, room temperature, (ii) ethanol (96%), stir 5 h, room temperature or methanol, ethanol (96%) or chloroform, stir 3 h, reflux. Labels for the NMR spectra are also given.

Scheme 2 Proposed pathway for the formation of L¹ and L².

The isolation of L² seems to indicate that the reaction begins with an intramolecular nucleophilic attack on the imine carbon atom by the tosyl protected amino group, followed by an intermolecular nucleophilic attack on the carbonyl carbon of 2-pyridinecarboxaldehyde. The elimination of the hydroxyl group would lead to the formation of water and an intermediate containing, again, an imine group. The final product L¹ could be obtained from III, by nucleophilic attack by the reaction solvent. Although all these products seem to be in a dynamic equilibrium in solution, the displacement of the reaction towards the formation of L¹ could be explained by the low solubility of the latter product in ethanol at room temperature. This proposed reaction pathway is also consistent with the isolation of L² in an undried solvent: the higher quantity of water would disfavour the formation of III, reverting the equilibrium and producing L² in yields high enough to precipitate.

The stability and reactivity induced in these compounds by the pyridine ring, directly bonded to the imine carbon atom, is quite surprising. The related Schiff bases N-[(2-pyrrolyl)methylidene]-N′-tosylbenzene-1,2-diamine and 2-tosylamino(2-pyridyl)aniline have been previously reported.^7a,7c Both were easily obtained by condensation of the corresponding amine and aldehyde; the bases seem to be stable in solution and no decomposition or rearrangement reactions were reported for either of them.

Characterisation of L¹ and L²

The products L¹ and L² were characterised by elemental analyses, IR (Table 1) and ¹H NMR (Table 2) spectroscopies. A crystal structure determination on L¹ confirmed the nature of the compound.

Table 1 Elemental analyses and some physical properties of the ligands

Elemetal analysis^a				IR spectroscopy^b
Formula	Yield/%	Mp/°C	%C	%H	%N	%S	ν(NH)	ν(CNpy)	ν(SO2)
a Found (calcd.).b In KBr, cm^−1.
L^1	C27H26N4O3S	63.2	163–6	67.3 (66.7)	5.5 (5.3)	11.6 (11.5)	6.3 (6.6)	—	1593	1335, 1173
L^2	C19H17N3O2S	92.6	142–3	65.0 (65.0)	4.8 (4.8)	11.9 (12.0)	8.7 (9.1)	3205	1595	1349, 1165

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Table 2 Most significant ¹H NMR peaks (ppm) and mass spectral data for the ligands

H1+H1^′	Hα+Hα^′	Harom+NH	–CH2	–CH3^a	–CH3^b	m/z
a Overlapping peaks.
L^1	6.06 (s,1H),	8.51 (d,1H),	6.5–8 (m,15H)	2.38 (m,5H)^a	2.39 (m,5H)^a	0.58 (t,3H)	487
5.85 (s,1H)	8.24 (d,1H)
L^2	6.32 (s,1H),−	8.56 (d,1H)	6.4–7.8 (m,12H)	—	2.34 (s,3H)	—	352

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Crystal structure of L¹.. The nature of L¹ was determined by single X-ray diffraction studies (Fig. 1). The experimental details are given in Table 3 and main bond distances and angles in Table 4.

Fig. 1 An ORTEP view of the crystal structure of L¹.

Table 3 Crystal data and structure refinement for L¹ and 2.

L^1	2
Empirical formula	C27H26N4O3S	C38H33.5N6O4.75NiS
Formula weight	486.58	773.04
Crystal system	Monoclinic	Trigonal
Space group	P2/1	R3
Z	2	18
Temperature/K	296(2)	296(1)
a/Å	8.7002(1)	27.512(4)
b/Å	10.0675(3)	—
c/Å	14.1389(4)	25.092(3)
α/°	90	90
β/°	91.217(1)	90
γ/°	90	120
U/Å^3	1238.14(5)	16447(4)
µ/mm^−1	0.167	2.257
Reflections collected	5226	7985
Independent reflections	2494 (Rint=0.0537)	7557 (Rint=0.04)
Final Rw indices [I>2σ(I)]	0.0767	0.070
Rw indices (all data)	0.0884	0.049

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Table 4 Selected bond distance (Å) and angles (°) for L¹

S–O1	1.422(8)	N2–C27	1.461(10)
S–O2	1.424(8)	N21–C26	1.306(11)
S–N1	1.648(6)	N21–C22	1.348(14)
S–C6	1.754(8)	C34–N35	1.45(2)
N1–C16	1.421(11)	N35–C36	1.41(2)
N1–C27	1.492(10)	C37–O38	1.396(11)
N2–C15	1.382(12)	O38–C39	1.418(12)
N2–C37	1.439(11)	C26–N21–C22	116.1(8)
O1–S–N1	105.7(4)	C23–C22–N21	125.2(11)
O2–S–N1	105.7(4)	N21–C26–C25	123.3(8)
O1–S–C6	108.8(5)	N21–C26–C27	115.7(7)
O2–S–C6	108.4(5)	N2–C27–N1	103.7(6)
N1–S–C6	107.7(3)	N2–C27–C26	112.8(6)
C16–N1–C27	107.8(6)	N1–C27–C26	110.8(6)
C16–N1–S	121.8(6)	C33–C34–N35	117(2)
C27–N1–S	118.0(5)	C34–N35–C36	116.1(13)
O1–S–O2	119.9(4)	C31–C36–N35	122.7(11)
C15–N2–C37	126.7(7)	N35–C36–C37	118.8(10)
C15–N2–C27	109.2(7)	O38–C37–N2	112.3(7)
C37–N2–C27	121.4(7)	O38–C37–C36	109.1(8)
C16–C15–N2	110.6(7)	N2–C37–C36	112.2(8)
C14–C15–N2	127.9(8)	C39–O38–C37	113.6(8)
C11–C16–N1	131.0(9)	O38–C39–C40	109.7(11)
C15–C16–N1	108.5(7)

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All the angles and distances are in the range of those expected for organic compounds containing a tosyl group^8,9 and do not deserve further consideration. However, it is noteworthy that the crystal structure shows that L¹ has two tertiary carbon atoms, the compound being optically active.

IR spectroscopy.. The IR spectra of L¹ and L² show a strong band at ca. 1595 cm⁻¹, assigned to ν(CN_py). Besides, a broad and intense band at 3205 cm⁻¹, due to the NH stretching frequency,^7b is observed for L². No bands above 2500 cm⁻¹ were detected for L¹, indicating the absence of NH groups in the molecule, as the crystal structure shows. Very strong bands are observed at ca. 1340 and 1165 cm⁻¹ for L¹ and L², due to ν(SO₂)_as and ν(SO₂)_s, suggesting the presence of the tosyl group in both cases, in agreement with previous results.¹⁰

¹H NMR and mass spectroscopy.. L¹ shows two doublets at 8.51 and 8.24 ppm, corresponding to two protons ortho to the nitrogen atom of the pyridine ring, indicating the presence of two pyridine moieties in the ligand. Two singlets at ca. 6 ppm are assigned to two protons attached to two different tertiary carbon atoms.¹¹ The spectrum shows a very complicated multiplet (15 H), assigned to the aromatic protons, at 6.5–8 ppm. The ethyl chain of the ethoxy moiety gives two signals: one at 0.58 ppm (t, 3H) and a second one at 2.38 ppm, which appears as a multiplet as it is overlapped by the peak corresponding to the methyl radical of the tosyl group.

The spectrum of L² shows just one signal assigned to an α-proton of the pyridine ring (8.56 ppm, d) and a multiplet for the aromatic hydrogen atoms. The hydrogen on the tertiary carbon atom occurs at 6.32 ppm and the methyl radical of the tosyl group at 2.34 ppm. The signal of the NH proton was not assigned because of overlap by the aromatic protons. No singlets at ca. 8.5 ppm were detected so no imine groups are present in L¹ or L².¹²

It is remarkable to note that the spectrum of L¹ changes with time: many new signals, most of them assigned to L², increase in intensity with time. This seems to indicate that L¹ is unstable in solution and that many species are present. The equilibrium between L¹ and L² is in agreement with the proposed reaction scheme.

The mass spectra of L¹ and L² were recorded by the electrospray technique. Peaks at 487 and 352 amu are assigned to the molecular ions of L¹ and L², respectively.

Characterisation of the nickel complexes

The unfruitful attempts to obtain HL³, with the isolation of L¹ (or L²), focussed our efforts on the isolation of a nickel complex containing L¹.

The electrochemical reaction of a nickel centre with L¹ does not seem to occur and the ligand L¹ could be identified as one of the species after evaporation of the solution. Thus, an alternative method was tried.

Once the structure of L¹ was known, it was thought that its size would prevent the co-ordination of more than one ligand to the same metal centre. Besides, L¹ has no acidic protons. Thus, it should be necessary to react L¹ with a nickel centre in the presence of a ligand containing easily deprotonable groups, which could balance the charge of the central atom, in order to obtain a Ni(II) complex containing L¹. A simple potentially tridentate and dianionic Schiff base, such as H₂L⁴ [N-(2-hydroxyphenyl)salicylidenimine], was chosen for this purpose. The electrolysis of an acetonitrile solution containing L¹ and H₂L⁴, as described below, was done. Elemental analysis of the orange insoluble product isolated (Table 5) seems to indicate that the compound obtained is NiL¹L⁴·2 H₂O 1, although the evaporation of the mother waters of the electrochemical synthesis yields NiL³₂·0.75H₂O 2, crystallographically characterised. Attempts were made to determine the structure of NiL¹L⁴·2H₂O. However, its recrystallisation in dichloromethane led, again, to the isolation of complex 2.

Table 5 Elemental analyses and some physical properties for the nickel complexes

Elemental analysis^a				IR spectroscopy^b
Yield/%	Mp/°C	%C	%H	%N	%S	ν(CNimine)+ν(CNpy)	ν(OH)	μ/BM
a Found (calcd.).b In KBr, cm^−1.
1	37.6	>300	60.4 (60.3)	5.3 (5.4)	8.7 (8.8)	4.0 (3.4)	1609, 1596	3417	3.1
2	30.5	>300	58.8 (59.0)	4.1 (4.3)	10.8 (10.9)	8.1 (8.3)	1592	3477	3.1

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As a consequence of this unexpected result, we tried to substitute for ligand H₂L⁴ a weakly co-ordinating acid, which could be easily deprotonated and balance the charge of the nickel(II) cation, maybe by acting as a counter ion. The acid used in this case was p-toluenesulfonic acid (p-TosH). Electrolysis of an acetonitrile solution containing L¹ and p-TosH in a 1:2 ratio was performed in the presence of a nickel anode. Complex 2 was obtained after slow evaporation of the mother waters. Thus, this seems to indicate that L¹ is unstable in solution, as previously observed, and that the presence of a metal centre accelerates the reorganisation process. Therefore, the experimental findings indicate that the best results that can be achieved are the isolation of complexes containing [L³]⁻ as a ligand.

With these considerations in mind, it was thought that, it might be enough to catalyse the interaction of Ni and L¹ with a weak acid, in order to obtain complex 2. The electrochemical interaction of Ni and L¹, using traces of p-TosH, was checked. It was observed that quick evaporation of the solution produces a copious product of unknown nature. When the solution is left to stand and slowly evaporates, the isolated product is compound 2. These data show that L¹ rearranges to HL³ in solution. However, the reorganisation is not instantaneous and thus, a quick precipitation leads to a mixture of products, perhaps containing simultaneously derivatives of L¹ and HL³, as well as other fragments of the reorganisation process, which does not allow us to characterise the copious material isolated. On the contrary a slow evaporation of the solution allows L¹ to rearrange in higher quantity. The most insoluble product, in this case the nickel complex 2, precipitates as a crystalline product and can be isolated with high purity.

Crystal structure of NiL³₂·0.75H₂O 2.. The X-ray structure of 2 is shown in Fig. 2. Experimental details are given in Table 3 and significant bond lengths and angles are listed in Table 6. The unit cell contains 18 asymmetric units, accounting for the large volume. The asymmetric unit contains, in addition to the nickel complex, three disordered water fragments, with partial occupancies.

Fig. 2 An ORTEP view of the crystal structure of NiL³₂·0.75H₂O 2. Water solvent molecules have been omitted for clarity.

Table 6 Selected bond distances (Å) and angles (°) for 2

Ni–N1	2.090(5)	Ni–N2	2.030(5)
Ni–N3	2.105(5)	Ni–N4	2.146(5)
Ni–N5	2.025(5)	Ni–N6	2.125(6)
S1–O1	1.443(4)	S1–O2	1.444(4)
S1–N1	1.584(5)	S1–C13	1.780(7)
S2–O3	1.454(5)	S2–O4	1.456(5)
S2–N4	1.597(5)	S2–C32	1.783(7)
N1–C1	1.409(8)	N2–C6	1.401(8)
N2–C7	1.271(8)	N3–C8	1.347(8)
N3–C12	1.323(8)	N4–C20	1.398(8)
N5–C25	1.383(8)	N5–C26	1.283(8)
N6–C27	1.345(8)	N6–C31	1.317(8)
N1–Ni–N2	78.6(2)	N1–Ni–N3	156.7(2)
N1–Ni–N4	97.7(2)	N1–Ni–N5	108.5(2)
N1–Ni–N6	92.7(2)	N2–Ni–N3	78.5(2)
N2–Ni–N4	109.4(2)	N2–Ni–N5	169.5(2)
N2–Ni–N6	93.7(2)	N3–Ni–N4	86.5(2)
N3–Ni–N5	94.8(2)	N3–Ni–N6	92.3(2)
N4–Ni–N5	77.9(2)	N4–Ni–N6	156.1(2)
N5–Ni–N6	78.4(2)	O1–S1–O2	116.1(3)
O1–S1–N1	106.7(3)	O3–S2–O4	115.7(3)
O2–S1–N1	113.1(3)	N1–S1–C13	109.1(3)

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The structure of the Ni compound consists of a metal centre surrounded by six nitrogen atoms corresponding to two Schiff bases, which act as tridentate and monoanionic. The ligands adopt a mer configuration in the distorted octahedral complex. It is remarkable, in spite of the adopted configuration, that both ligands are not equivalent. This is clearly shown by a comparison of the Ni–N distances: while the Ni–N_imine bond lengths are identical for both ligands [Ni–N2=2.030(5) and Ni–N5=2.025(5) Å] remarkable differences are observed in the Ni–N_amide and Ni–N_pyridine bond lengths. These distances are significantly shorter for one of the ligands [Ni–N1=2.090(5) and Ni–N3=2.105(5) Å, respectively] than for the second one [Ni–N4=2.146(5) and Ni–N6=2.125(5) Å, respectively]. In addition, the distance Ni–N_amide is shorter than the distance Ni–N_pyridine for one of the ligands while is longer for the other one. Thus, the geometry of the complex can be considered as an octahedron with a tetragonal distortion due to the elongation of the apical bonds, with no interchangeable positions between both ligands. In this polyhedron, one of the Schiff base ligands uses its three donor atoms, N1, N2 and N3, to bind the Ni atom in the equatorial plane and the second one uses the imine nitrogen atom, N5, to link the Ni centre in the equatorial plane and the amide and pyridine nitrogen atoms, N4 and N6, to fill the apical positions.

The Ni–N distances in the equatorial plane are quite similar and shorter than those of the apical positions. All these distances are in the range of those expected for octahedral Ni complexes containing N-donor ligands^13,14 but are longer than those found for tetrahedral^15–17 and square planar^7b,18 Ni compounds. This is a reflection of the lower co-ordination number in the related complexes.

The equatorial angles around the Ni atom in the distorted octahedron vary from 77.9° to 108.5° and are quite different from the ideal value of 90°. The interaxial angle N4–Ni–N6 of 156.1(2)° further reinforces the distortion from the ideal geometry, which can be mainly attributed to the small bite angles of the tridentate ligand. This observation is in agreement with previous results, where chelate ligands induce a distortion from the ideal octahedral geometry.^14,15,19

IR spectroscopy and magnetic measurements.. The IR spectrum of complex 2 shows a strong and broad band at 1592 cm⁻¹, assigned to ν(CN_pyridine)+ν(CN_imine).^13,14 A broad band at 3477 cm⁻¹ is assigned to ν(OH) from the water of crystallisation. A comparison of the spectra of complexes 1 and 2 shows a clear difference. Complex 1 shows two strong bands in the range 1590–1620 cm⁻¹: a sharp and strong band at 1596 cm⁻¹ that can be assigned to ν(CN_pyridine) of L¹, present in the free ligand; a second strong band at 1609 cm⁻¹, assigned to ν(CN_imine) of [L⁴]²⁻.

The room temperature magnetic moments for the complexes are 3.1 BM and are very close to the ideal value of 2.9 for magnetically diluted Ni(II) ions. The magnetic moment value for 2 is in good agreement with the octahedral geometry of the compounds, deduced from the X-ray results. However, this value does not allow one to draw any unambiguous conclusions concerning the geometry of the chelate core in complex 1.³ This only allows one to reject the square planar geometry for the complex.

Conclusion

The reaction of N-Tosyl-1,2-diaminobenzene and 2-pyridinecarboxaldehyde yields two different optically active organic compounds, L¹ and L², depending on the reaction medium. However, the expected Schiff base, product of the condensation of the amine and carbonyl reactants, could not be isolated.

The electrochemical interaction of L¹ with nickel always leads to the isolation of NiL³₂ ·0.75H₂O, obtained by reorganisation of the ligand to the Schiff base (HL³) in solution.

Experimental

General

All solvents and 2-pyridinecarboxaldehyde are commercially available and were used without further purification. Nickel (Ega Chemie) was used as ca. 2×2 cm² plate. N-tosyl-1,2-diaminobenzene was obtained as previously reported by Malik and Sharma.²⁰

Elemental analyses were performed on a Carlo Erba EA 1108 analyser. NMR spectra were recorded on a Bruker AC-300 spectrometer using DMSO-d₆ as solvent. Infrared spectra were recorded as KBr pellets on a Bio-Rad FTS 135 spectrophotometer in the range 4000–600 cm⁻¹. Electrospray mass spectra were obtained on a Hewlett–Packard LC/MS spectrometer, in methanol as solvent. Room temperature magnetic measurements were performed using a Digital Measurement system MSB-MKI, calibrated using tetrakis(isothiocyanato)cobaltate(II).

Syntheses

L¹.. To an absolute ethanol (99%) solution (75 mL) of N-tosyl-1,2-diaminobenzene (0.524 g, 2 mmol) was added 2-pyridinecarboxaldehyde (0.2 mL, 2.1 mmol). The mixture was stirred for 5 h at room temperature until a white solid precipitated. The solid was collected by filtration, washed with diethyl ether (10 mL) and dried in air.

When the amine and aldehyde are mixed in a 1: 2 molar ratio in absolute ethanol, the same ligand is obtained. Crystals of L¹, suitable for X-ray diffraction studies, were obtained by recrystallisation of the white powder in hot ethanol.

L².. L² could be synthesised by two different methods.
Method A:. To an ethanol (96%) solution (75 mL) of N-tosyl-1,2-diaminobenzene (0.524 g, 2 mmol) was added 2-pyridinecarboxaldehyde (0.2 mL, 2.1 mmol). The mixture was stirred for 5 h at room temperature until a white solid precipitated. The solid was filtered, washed with diethyl ether (10 mL) and dried in air.
Method B:. To an ethanol (96%), chloroform or methanol solution (100 mL) of N-tosyl-1,2-diaminobenzene (0.524 g, 2 mmol) was added 2-pyridinecarboxaldehyde (0.2 mL, 2.1 mmol). The resultant yellow solution was refluxed for 3 h, removing the water with a Dean Stark. The solution was reduced in volume (25 mL) and the solid collected by filtration and dried in air.

NiL¹L⁴·2H₂O 1.. The compound was obtained using an electrochemical procedure. The cell can be summarised as: Pt₍₋₎ ∣L¹+H₂L⁴+MeCN∣Ni₍₊₎ . A platinum wire was used as the cathode and a nickel plate as the anode.

An acetonitrile solution (70 mL) of L¹ (0.2 g, 0.41 mmol) and H₂L⁴ (0.09 g, 0.41 mmol), containing about 10 mg of tetramethylammonium perchlorate, as supporting electrolyte, was electrolysed for 2 h and 12 min with a current of 10 mA. A small quantity of an insoluble orange solid of unknown nature was isolated (ca. 0.06 mg). The mother waters were reduced in volume (20 mL) and a second orange solid precipitated. The product was filtered, washed with diethyl ether and dried in air; the elemental analysis is in agreement with the formulation NiL¹L⁴·2H₂O 1 for the latter compound.

Recrystallisation of 1 in dichloromethane yielded small crystals of NiL³₂2·0.75H₂O 2.

NiL³₂·0.75H₂O 2.. Complex 2 could be synthesized by two other methods.
Method A:. An acetonitrile solution (70 mL) of L¹ (0.2 g, 0.41 mmol) and p-TosH (0.14 g, 0.82 mmol), containing about 10 mg of tetramethylammonium perchlorate as supporting electrolyte, was electrolysed for 2 h and 12 min with a current of 10 mA, as for 1. A small quantity (ca. 0.05 g) of an insoluble product of unknown nature was filtered. The mother waters were left to stand for one week. Slow evaporation of the solution yields small crystals, suitable for X-ray diffraction, of NiL³₂:0.75H₂O 2.
Method B:. An acetonitrile solution (70 mL) of L¹ (0.2 g, 0.41 mmol) containing about 10 mg of tetramethylammonium perchlorate, as supporting electrolyte, and 10 mg of p-toluenesulfonic acid as catalyst, was electrolysed for 2 h and 12 min with a current of 10 mA, as for 1. The resultant garnet-coloured solution was slowly evaporated and, after one week, small crystals of NiL³₂·0.75H₂O 2 were recovered and dried in air.

Crystallographic measurements

L₁.. White crystals, suitable for single-crystal X-ray studies, were obtained as previously described. Data were collected at 296 K with a CAD-4 diffractometer, employing graphite-monochromated Mo-Kα (λ=0.71073 Å) radiation. The structure was solved by direct methods²¹ and refined by full-matrix least squares on F². A semiempirical correction from psi scans was applied. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the structure factor calculation in idealised positions. The aromatic ring of the tosyl group (C1–C6) is disordered over two sites, with partial occupancies of 0.5.

NiL³₂·0.75H₂O 2.. Red needle crystals of 2 were obtained as previously described and mounted on a glass fiber. All measurements were made at 296 K on a Rigaku AFC5R diffractometer, employing Cu-Kα (λ=1.54178 Å) radiation and a rotating anode generator, using the ω−2θ scan technique (2θ_max=159.4°). The structure was solved by direct methods,²¹ expanded using Fourier techniques²² and refined by full-matrix least squares on F². An absorption correction was applied. Data were corrected for Lorentz and polarisation effects. Non-hydrogen atoms were refined anisotropically, except oxygen atoms of the disordered water fragment, which were refined isotropically. Hydrogen atoms were included in the structure factor calculation in idealised positions but not refined. All calculations were performed using the TEXSAN crystallographic software package.²³

CCDC reference 440/155. See http://www.rsc.org/suppdata/nj/a9/a907731f/ for crystallographic files of complex 2 in .cif format.

Acknowledgements

The authors would like to thank Xunta de Galicia (Spain) for financial support (XUGA/20903B 99).

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