Spectroscopic studies of metal complexes with redox-active hydrogenated Schiff bases
Abstract
Synthesis and spectroscopic (IR, UV-visible, ESR) characterization of metal(II) complexes M(L’x )2, (where M=Co(II), NI(II), VO(II), Pd(II), L’x =L’1, L’2, L’3 are monoanion of unsubstituted, 5-Cl and 5-Br substituted-2-hy- droxybenzylamine) with redox-active N-(3,5-ditert-butyl-4-hydroxyphenyl)-2-hydroxybenzylamine ligands as well as radical species generated from these compounds by the oxidation with PbO2 are reported. ESR studies indicate that the VO(L’x )2 and Ni(L’x )2 complexes, in opposite to their salicylaldimine precursors, are more readily oxidized with lead dioxide and results in the formation of the indophenoxyl type stable radical. The formed radical species are very similar to each other and quite different from those of the salicylaldimine analogous according to their g-factors and hyperfine coupling constants. The nine line radical spectra observed in the oxidation of Co(L’x )2, on standing under vacuum, gradually converted to the signals characteristic of the low-spin Co(II) (gx,y=2.276, gz=1.998, ACo =122.7 G, ACo =150 G) and radical containing Co(III) intermediate with g =2.015, ACo =4.66 G, g =1.989, ACo =10 G were also observed. © 2001 Elsevier Science B.V.
Keywords: Redox-active benzylamine-complexes; Radical species; IR; UV-visible; ESR
1. Introduction
Intramolecular electron transfer is a fundamen- tal chemical phenomenon and relates specifically to biological and catalytic redox processes which occur in natural and synthetic electron transfer systems [1]. The ability of the metal ions to con- trol the oxidation potentials of organic molecules by complexing are significant in redox processes. A particularly interesting class of redox system is
provided by chelates of transition metal ions which are capable of oxidizing a coordinated or- ganic figand to the neutral radical. From this viewpoint, one interesting redox-active system consists of transition metal complexes containing sterically hindered phenol fragments. It is well known that sterically hindered phenol derivatives are effective antioxidants against photo-destruc- tion of materials such as polymers, petroleum products, and in the rancidification of fats and oils [2,3]. The fact that sterically hindered phenols can readily undergo one or two-electron transfer offers the possibility of preparing chelates with unusual oxidation states [4].
Although there have been intensive studies on coordination compounds of salicylaldimines with transition metal ions, relatively little is known about Schiff base chelates containing redox-ac- tive fragments. The coordination chemistry of some transition metals with potentially redox-ac- tive chelating ligands such as bi-, tri- and tetra- dentate salicylaldimines, naphthaldi mines, azoligands, β-ketoiminates and other species containing sterically hindered phenol fragments has been studied [4 – 6]. The present investigation was undertaken to explore the changes which can occur in redox reactivity of LxH and M(Lx)2 when the –CH =N–group was replaced by saturated –CH2–NH– linkage in the structure of salicylaldimine. In particular, it is of interest to know to what extent and how this structural change affects the redox properties of free and coordinated benzylamine ligands (L’xH).In this paper we describe the coordination and redox chemistry of new complexes of M(L’x)2 [M=Ni(II), Pd(II), VO(II)] on the basis of the redox-active N-(3,5-di-tert-butyl-4-hydroxyphenyl)- 2-hydroxybenzylamines (L’xH) ligands, prepared according to Scheme 1.
2. Experimental
Elemental analyses were performed by the Sci- ence and Technical Research Council of Turkey (TUBITAK) in Gebze. IR spectra were obtained using a Carl Zeiss Jena Specord M 80 spec- trophotometer in the 4000 – 400 cm−1 region as KBr discs. Electronic spectra were recorded on a Carl Zeiss Jena Specord M 40 spectrophotome- ter in the 200 – 900 nm region. Magnetic susceptibilities were measured by the Faraday technique at room temperature (25 ±2°C). The apparatus was calibrated by the use of Hg[- Co(SCN)4]. The molar susceptibilities were cor- rected for ligand diamagnetism using Pascal’s constants [7]. The ESR spectra of M(Lx)2 were recorded on a Radiopan model SE/X-2547 (X- Band) in solution and in polycrystalline state at 300 and 77 K using DPPH (g =2.0036) as the standard. The ESR parameters of the complexes were determined directly from the spectra. The magnetic field was calibrated using a standard Mn2+ sample. Reported values involve errors
within±0.005 for g-values and ±0.05 G A-val- ues of VO(Lx)2. The errors for radical g-factors and hyperfine splitting constant (hfsc) parame- ters are ±0.0005 and ±0.005 G, respectively.
3. Synthesis of ligands and complexes
Salicylaldimines LxH were synthesized by the method described previously 4a. N-(3,5-di-tert- butyl- 4 – hydroxyphenyl)-2- hydroxybenzylamines (L’xH) were prepared by using the following procedure: NaBH4 (0.05 mol) was added slowly with small portions of about 10 – 15 mg to 0.01 mol stirring solutions of LxH in 40 – 50 ml of isopropanol for 15 – 20 min. Stirring con- tinued for another 30 – 40 min and the mixture, cooled to room temperature, was then poured into 150 – 200 ml water. The resulting solution was vigorously stirred for 1 – 1.5 h and allowed to stand for 3 – 4 h. The precipitated white cry- stals were collected by filtration and then washed 6 – 8 times with water, dried under vacuum and recrystallized from hexane-acetone, yielded 85 – 93%.
The complexes of M(Lx)2 [M=Co(II), Ni(11), VO(II)] were prepared by the following general method. Metal(II) acetate (2 mmol) was dissolved in the minimum volume of methanol (10 – 15 ml) and added to an equivalent amount L’xH (4 mmol) in warm methanol (40 – 50 ml). Soon green or brown crystals were precipitated. After the mixture was warmed at 50 – 60°C with stirring for 50 – 60 min and left to cool. The precipitated crystals were by suction filtration, washed with methanol and diethyl ether, and then dried in the open air and recrystallized from methanol– ace- tone mixture. Pd(L’x)2 complexes were prepared according to previously reported procedure [6a] using Pd(II) acetate and L’xH in glacial acetic acid (20 – 25 ml). Analytical and spectroscopic data of the compounds synthesized are given in the Ta- bles 1 – 3.
3.1. Generation of radicals
L’xH and M(L’x)2 compounds were oxidized as follows. The compound, dissolved in 5 ml toluene or chloroform and PbO2 (70 – 90 mg), were trans- ferred into separate glass tubes on vacuum line.Solution was degassed under vacuum (10−3 – 10−4 mmHg) by four to five freeze– pump– thaw cycles. Then degassed solution was mixed with PbO2 (70 – 80 mg) and shaken for 30 s at room temperature. After the sedimentation of heteroge- neous phase, 1 ml of the solution was introduced into an ESR sample tube for monitoring the spectrum. The ESR spectra taken rapidly after the sedimentation of reaction mixture. Reduction of Pd(L’x)2 complexes with an excess of PPh3 was carried out by mixing their degassed CHCl3 solu- tions at room temperature under vacuum.
4. Results and discussion
The analytical data of the complexes (Table 1) indicate 2:1 (ligand/metal) stoichiometry and hence the figands are behaving as bidentate monobasic anions in the M(L’x)2 complexes. The examination show a higher complexation ability of hydrogenated L’xH ligands than that of corre- sponding salicylaldimines precursors. This behav- ior is undoubtedly caused by the higher basicity of the nitrogen atom in hydrogenated benzylamine ligands than in their salicylaldimine analogous. Additionally, the Ni(L’x)2 and VO(L’x)2 complexes, unlike their salicylaidiminate precursors, were readily oxidized by PbO2 giving very stable secondary radical. Note that the nonhindered bis(benzylaminato) complexes reported by West [8], does not undergo any oxidative conversion in the interaction with PbO2. The prepared ligands, except L’1H, are sensitive to air oxidation even in the solid state. However, the bis(benzylaminato) complexes (M(L’x)2, are quite stable in air.
5. Infrared spectra
The IR spectra of the complexes provide direct information about the coordination of the ligand via NH nitrogen atom and deprotonated oxygen of the OH group (Table 2). The LxH spectra exhibit broad medium bands at 3320 – 3345 cm−1 and a sharp strong band at 3610 – 3640 cm−1 due to the v(NH) and sterically hindered phenolic v(OH) stretching vibrations, respectively. A strong band in the salicylaldimines in the 1615 – 1635 cm−1 region due to the v(C=N) stretch disappeared in the spectra of hydrogenated L’xH and M(L’x)2 compounds. A strong band appeared in the IR spectra of L’xH at 1280 – 1310 cm−1 which was attributed to the hydrogen bond inplane bending mode, and very weak broad ab- sorption near 2700 cm−1 assignable to the intramolecular hydrogen bonding of O–H stretch- ing vibration were not observed in the complexes as a result of the displacement of the acidic proton by the metal ions. The v(V=O) frequencies of the VO(Lx)2 complexes occur in the range 910 – 960 cm−1 and this is in the usual range (960 ±50 cm−1) observed for the majority of VO(11) complexes [9].
6. LxH benzylamines
The electronic spectra of L’xH (Table 2) exhibit bands at about 230 and 250 – 289 nm attributable to the π → π* transition of benzene rings. The bands in the range 308 – 314 and 362 – 365 nm may assigned to n → π* charge transfer transition.These CT seems to originate from the OH and NH groups to a phenyl rings of the ligand. The low intensity broad band appeared at 446 – 454 nm which can be attributed to the bipolar ions formed as a result of the intramolecular proton transfer from OH to NH group, disappeared in the spectra of M(L’x)2 complexes. In all complexes the intraligand π → π* band at 230 nm is red shifted and appeared at 248 – 254 nm (Table 2).
In the oxidation of L’xH with PbO2 at 300 K in toluene, THF and CHCl3 solutions, the formation of less resolved asymmetric ESR spectra with similar magnetic resonance parameters have been observed. For oxidized L’1H and L’2H in CHCl3 or toluene, at room temperature, 7 line (g =
pling constant is positive and the orbital contribu- tion is almost completely quenched in oxovanadium(IV) complexes [10a]. The room temperature effective magnetic moments of the VO(L’x)2 (1.72 – 1.74 B.M.) are close to that ex- pected [10b], spin-only values of 1.73 B.M. and 2.0044, AH =1.85 G, AN=3.8 G) and 5 line indicate the magnetically diluted nature and absence of antiferromagnetic exchange interaction in these complexes at room temperature.
The electronic spectra of the VO(L’x)2 com- plexes in chloroform (Table 2) exhibit three ab- sorption bands in the range 485 – 490, 625 – 666 and 790 – 890 nm. These bands, according to Ball- hausen and Gray molecular orbital scheme [12],are assigned to the dxy→ dz 2, dxy→ dx 2 − y 2 and dxy→ dxz,yz transitions, respectively. This assign- ment also is agreement with our ESR data. Some charge-transfer bands are also observed at 365 – 385 mm.
7. Bis chelates of VO(11)
On mixing hot methanolic solutions equivmolar amount of L’xH and VO(acet)2 in a 2:1 ratio, the dark green complex precipitated simultaneously. Upon complexation of L’xH as evidenced by ESR, unlike parent salicylaldimine analogous, the for- mation of the radical species were not observed.
The oxovanadium(IV) complexes belong to S=1/2 system and it is expected that the mag- netic moments of magnetically dilute oxovanadiu- m(IV) complexes would be very close to the spin-only magnetic moment, as the spin-orbit direct from the spectra recorded at room tempera- ture (77 K) are presented in Table 3. The room temperature isotropic ESR spectra of VO(L’x)2 in chloroform consist of the usual eight-line hy- perfine pattern resulting from coupling of the unpaired electron to the 51V(I =7/2) nucleus (Fig. 1(a)). There was no observed additional splitting from 14N(I =1) nucleus. The isotropic vanadium electron spin– nuclear spin hyperfine coupling constants (Aiso), were obtained by averaging the separation of the −mI to mI transitions. The anisotropic spectra of these complexes at 77 K exhibit axial rather than rhombic symmetry (Fig.1(c)) with the values of Az>Ax=Ay and gz< gx=gy. The following expressions can be used as good approximations for the estimation of g values where α, β and g are modified molecular orbital coefficients of the dxy, dx 2 − y 2, and dxz, dyz orbitals, respectively, and h is the spin-orbit coupling con- stant of the vanadium ion in the complex [13]. The absence of hyperfine splitting from nitrogen nucleus in the ESR spectra, in agreement with Ballhausen and Gray, indicate that the unpaired electron occupies an essentially non-bonding vanadium dxy orbital which is largely metal in character and thus independent of the covalent character of the bonding orbitals. The one-electron oxidation of VO(L'x)2 with PbO2 in CHCl3 at room temperature, unlike their salicylaldiminate VO(II), readily leads to the for- mation of stable phenoxyl radicals and gradually disappearance of VO(II) signals. These spectral patterns are similar to each other and have quite close g-factors and hfsc values (Table 3), but quite different from those of the radicals generated from L'xH and parent salicylaldimine ligands. The spectra of all of the radicals generated from VO(L'x)2 exhibit well resolved equidistant nine- line pattern spacing of 1.129 G with an intensity distribution of 1:4:7:8:9:7:4:1 (Fig. 1(b)). These spectra can be analyzed in terms of an interaction of the unpaired electron spin density with one Fig. 1. ESR spectra of VO(L'1)2. (a) at 300 and (c) 77 K; (b) oxidized sample of VO(L'1)2 at 300 K, spectra detected for the oxidized samples of Ni(L'x )2 at 300 K: (d) Ni(L'2)2, (e) NI(L'3)2. All spectra recorded in CHCl3. 8. Bis chelates of Ni(II) All Ni(L'x)2 complexes are paramagnetic in the solid state with magnetic moments in the range 3.32 – 3.38 B.M. (Table 1), which are close to those found for tetrahedral Ni(II) complexes [14]. Electronic spectra of NI(L'x)2 in CHCl3 in the visible region show absorption bands in the range 458 – 590 and 685 – 790 nm and according to the values of coefficient extinction (c =320 – 450) can be assigned to 3T1 → 3A2 and 3T1 → 3T(P) in tetra- hedral symmetry [14]. The bands appeared at 364 – 385 nm, and can be attributed to the ligand- to-metal charge-transfer transitions. These complexes, unlike their salicylaldimine analogous, rather readily oxidized with PbO2 in CHCl3 at room temperature giving corresponding radical species. In the oxidation of Ni(L'x)2 low intensity unresolved signal centered at g =2.0038 was observed. However, under the same condi- tions, oxidation of Ni(L'2)2 and Ni(L'3)2 results in an ESR spectra which consist of equidistant nine- and ten-line patterns (Fig. 1(d) and (e)) with the parameters such as g =2.0041, AH=1.013 G and g =2.0038, AH=1.127 G, respectively. The nine- line spectrum, according to line shapes, number of lines, intensity ratios and the values of g-factors and hfsc (within experimental error) are very sim- ilar to those of the oxidized VO(Lx)2 and Pd(II) samples spectra. The highest field component of the spectrum in Fig. 1(e) most probably has an- other origin and equal spacing nine-line part of Cu(II) [15] complexes with the same ligands. These experimental observations allow us to con- clude that, the observed ESR spectra originated from secondary radical species. 9. Bis chelates of Pd(II) All the Pd(Lx)2 complexes are diamagnetic, sug- gesting a square planar geometry for these com- pounds. The electronic spectra of these compounds are different from those of the parent salicylaldimine Pd(II) chelates. In the electronic spectra of the Pd(L'x)2 complexes together intrali- gand absorptions in the UV region, the new bands at 378 – 385, 434 – 485 and 515 – 526 nm were also observed. The bands in the regions of 434 – 485 and 515 – 526 nm may be assigned to the 1Ag → 1A2g and 1Ag → 1B1g transitions, respectively [16]. The other intense band observed in the UV region at 378 – 385 nm, probably has metal to ligand charge transfer character. Upon treatment of Pd(L'1)2 and Pd(L'3)2 com- plexes with PbO2 in CHCl3 solution under the above mentioned conditions, well resolved ESR spectral patterns having quite close magnetic reso- nance parameters were observed. In all cases dur- ing oxidation the gradually precipitation of the metallic Pd occurred. Upon oxidation of Pd(L'1)2 and Pd(L'3)2 complexes, relatively stable radical species exhibiting nine line hyperfine splitting spectral features with an intensity distribution of 1:4:7:8:8:8:7:4:1 and ESR parameters such as g = 2.0062, AH=1.25 G and g =2.0045, AH=l.223 G, respectively, were observed (Fig. 2(e)). In the case of oxidized Pd(L'2)2 sample in CHCl3, at 300 K, the unusual spectrum having unresolved low field wing and a well resolved nine-line pattern on the high field side (g =2.0053, AH=1.166 G) was detected (Fig. 2(g)). This spectral pattern remains unchanged on varying in concentration of com- plex over range of 10−4 –10−3 M as well as with changes in gain and modulation amplitude from 0.05 to 2 G. The unusual broadness and the asymmetry of the overall pattern was probably due to the anisotropy of the hyperfine coupling tensors and that of the g-tensor [17]. At 77 K, for this sample, unresolved symmetric singlet centered at g =2.005 was detected. Fig. 2. ESR spectra of the oxidation products of Co(L'x )2 and Pd(L'x )2 in CHCl3 at 300 K. (a, b): Co(L'1)2, (c, d): Co(L'2)2, (e): Pd(L'1)2 and (g): Pd(L'3)2. The central multiplets (b) and (d) were recorded at scan range of 50 G and modulation amplitude of 0.2 G. Interaction of Pd(L'x)2 with an excess of PPh3 in CHCl3 solutions at 300 K under vacuum, by analogy with those which were observed for corre- sponding bis(salicylaldiminato) palladium(II) [6a], results in reduction of the complexes accompanied by the precipitation of the metallic Pd. Upon reduction of Pd(L'1)2 with PPh3 in CHCl3 the complicated ESR spectrum g =2.0045, AH=l.42 G was detected. But in the cases of Pd(L'2)2 and Pd(L'3)2, under similar conditions, along with immediate precipitation of Pd, a very weak inten- sity hardly detectable spectral patterns centered at g =2 were observed. 10. Bis chelates of Co(II) The values of the effective magnetic moments of these complexes ranging between 4.46 and 5.16 B.M. suggest a spin quartet state S=3/2, and electronic spectra in CHCl3 (c =102 – 614 l mol−1cm−1) in the visible region (Table 2) are indicative of tetrahedral coordinate Co(II) (d7) complexes [18]. The absorption band in the region of 556 – 610 nm may be assigned to the 4A2 → 4T1(P) transition in tetrahedral symmetry. Oxidation of Co(L'x)2 complexes in the toluene and CHC13 solutions under the above mentioned conditions resulted in ESR spectra which were quite different from those of the oxidized Ni(L'x)2, Pd(L'x)2 and Co(L'x)2 (Fig. 2(a), (c)). For instance, on treatment with PbO2, toluene solution of Co(L'1)2 gives a spectrum consisting of superimpo- sition of less resolved very intense radical signal (g =2.0035, AH=1.11 G) and low intensity octet pattern spacing of 6.2 G (g =2.01) due to interac- tion of the unpaired electron to the 59Co(I =7/2) nucleus (Fig. 2(a)) was observed. A frozen solu- tion (77 K) anisotropic spectrum of this sample shows resolved gxy and gz components with the parameters such as gx,y=2.276, gz=1.998, Ax,y(59Co) =122.7 G, and Az(59Co) =150 G clearly indicate a (dz 2) ground configuration char- acteristic of the low-spin Co(II) complexes [19]. In the presence of atmospheric oxygen the radical part of this spectrum was disappeared. ESR spec- trum of oxidized of Co(L'2)2 in toluene at room temperature, also consists of overlapped radical signal (g =2.0054, AH=1.18 G) and anisotropic eight-line pattern typical of the paramagnetic Co(III) with parameters as gx,y=2.015, Ax,y= 4.66 G, gz=1.989, Az=10 G (Fig. 2(c)). On the other hand, the spectrum generated from the com- plex of Co(L'3)2 under the similar conditions, ex- hibit six line pattern centered at g =2.0037 and has an hfsc of 4.09 G with an intensity distribu- tion of 1:2:3:3:2:1. At 77 K this spectrum trans- formed into two different signals centered at g =2.009 (ΔH =16.5 G) and g =2.0012 (ΔH = 3.3 G), respectively, However, exact analysis of this spectrum is not easy, the observed frozen solution signal undoubtedly indicate that the room temperature six-line pattern of the oxidized Co(L'1)2 sample, can be attributed to superim- posed two signals. The performed studies indicate that except Co(Lx)2 complexes practically all of the oxidized complexes independently of the ligand and metal nature display nearly identical ESR spectral fea- ture. The ESR parameters and intensity distribu- tion of 1:4:7:8:8:8:7:4:1 of the detected spectra are similar of those reported by Coppinger [20] in- dophenoxyl type radical 5 in Scheme 2. According to Scheme 2, the indirectly generated unstable radical 2 immediately distorted 3 and this radical product may result from dispropor- tion of the aminophenoxyl-aminophenoxyl ligand can be to give quinine-phenoxyl type stable radi- cals such as 5 or 6. It is necessary to note that the nine line spectral pattern with similar intensity ratios were also observed in the oxidation of some salicylaldimines and their complexes [4a,10]. Finally, this work clearly demonstrates the re- placement of the –C=N– group by the –CH2–NH– linkage in bis[N-(2,6-di-tert-butyl-l- hydroxyphenyl)salicylaldimine] metal(II) complexes significantly increases their oxidative reactivity towards lead dioxide. ESR studies indi- cate that bis(benzylaminato) complexes of Ni(II) and VO(II) containing peripheral sterically hin- dered phenol fragment, in opposite to their salicy- laldimine precursors are more readily oxidized with lead dioxide and results to the formation of secondary phenoxyl radicals. On the other hand in the cases of Pd(II) and Co(II) the radical intermediates generated from salicylaldiminate and benzylaminate are significantly different from each other. In the oxidation of benzylaminate, unlike their salicylaldiminate precursors, the de- tection of primery radical species In generally, ESR spectra of the generated radical species ac- cording to their g-factors and hyperfine coupling constants are very similar each other and essen- tially different from those of the salicylaldimine analogous and free benzylamine ligands. In addi- tion, the complexation ability of the hydrogenated benzylamine ligands was found rather higher than that of the parent salicylaldinine precursors.