Iron(III) and cobalt(III) complexes with both tautomeric (keto and enol) forms of aroylhydrazone ligands: catalysts for the microwave assisted oxidation of alcohols

Abstract

Two Schiff bases derived from the condensation of 2-hydroxybenzohydrazide with 3,5-di-tert-butyl-2-hydroxybenzaldehyde (H₂L¹) or with 2,3-dihydroxy benzaldehyde (H₂L²) were used to synthesize the Fe(III) and Co(III) complexes [Fe(L¹)(HL¹)] (1) and [Co(L²)(HL²)] (2), respectively. The compounds were characterized by elemental analysis, IR, ESI-MS and single crystal X-ray analysis. Structural studies indicated the presence of both keto and enol tautomeric forms of the ligand in 1 and 2. The complexes (mainly 1) act as catalysts in the microwave-assisted solvent-free peroxidative oxidation (by tert-butylhydroperoxide, TBHP) of primary and secondary alcohols. A facile, efficient and selective synthesis of ketones was achieved with a yield up to 96% and a TON up to 500, after 30 min under low power (15 W) microwave irradiation (complex 1 as catalyst). 2-Pyrazinecarboxylic acid (Hpca) shows a promoting effect.

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1. Introduction

The coordination chemistry of aroylhydrazones has gained widespread interest. They are versatile in donor properties and coordination behaviour to which their tautomeric effect contributes.¹ Hydrazone Schiff bases exhibit keto–enol tautomerism in solution and form stable complexes with metal ions either in keto or in enol form.¹ In rare cases both forms simultaneously coordinate to the metal ion.² The existence of these tautomeric forms in the complexes is dependent on the pH of the medium and the nature of metal ions.^1,3 A number of such stable complexes shows interesting catalytic, magnetic and biological properties.⁴

Our group has been involved in exploring the catalytic activity of such complexes in various oxidation processes,⁵ including the selective oxidation of alcohols to the corresponding carbonyl compounds, a key reaction which is widely used in organic synthesis. Though there are many known stoichiometric oxidation methods, e.g. by pyridinium chlorochromate (PCC), Swern type, Dess–Martin periodinane and Oppenauer types, for economical and environmental reasons the development of efficient and selective catalysts for such an oxidation is a demanding field of study in the chemical industry.⁶ Hydrogen peroxide, tert-butylhydroperoxide (TBHP) or dioxygen are used as stoichiometric oxidants for such a reaction in transition metal catalytic systems.⁷ Recently, the use of microwave irradiation for the aerobic or peroxidative oxidation of alcohols received special attention due to the rapidity, simplicity and energy saving of this technique.⁸ In particular, iron⁹ and cobalt¹⁰ complexes can exhibit good catalytic properties in the oxidation of alcohols.

The main objective of this study was to synthesize Fe(III) and Co(III) complexes of aroylhydrazones where both the tautomeric forms (keto and enol) of the ligands exist simultaneously, thus expanding this rare type of coordinative combination. Exploration of the catalytic activity of these complexes towards peroxidative oxidation of alcohols under microwave-assisted solvent-free conditions is another objective of this study, aiming to contribute towards the establishment of a green catalytic system for such reactions. The influence of various parameters, such as reaction time, type and amount of catalyst, temperature and presence of additives, is also evaluated.

2. Results and discussion

2.1. Synthesis

Two hydrazone Schiff bases derived from the condensation of 2-hydroxybenzohydrazide with 3,5-di-tert-butyl-2-hydroxybenzaldehyde (affording H₂L¹) or 2,3-dihydroxy benzaldehyde (giving H₂L²), have been used to synthesize Fe(III) and Co(III) complexes (Scheme 1). Reaction of H₂L¹ with the acetate complex Fe(MeCOO)₂ or the pivalate complex [Fe₃(μ₃-O)(^tBuCOO)₆(H₂O)₃](^tBuCOO)¹¹ in ethanol yielded the neutral mononuclear complex [Fe(L¹)(HL¹)] (1), and reaction of H₂L² with the acetate complex Co(MeCOO)₂·4H₂O resulted in the formation of [Co(L²)(HL²)] (2), where one hydrazone ligand exhibits the monodeprotonated HL⁻ keto form and the other one displays the bideprotonated L²⁻ enol form, satisfying the +3 oxidation state of the metal cations. Generally, the pH of the reaction medium plays a crucial role to stabilize either the keto or the enol form of the ligand in the complexes.

Scheme 1 Synthesis of complexes 1 and 2.

In a previous case, reaction of FeCl₃·6H₂O with the aroylhydrazone 3,5-di-tert-butylsalicylidene benzoylhydrazine in the presence of Et₃N resulted in a neutral Fe(III) complex having both keto and enol forms of the ligand, while in the absence of that base a cationic Fe(III) complex with only the monodeprotonated keto form was obtained.² In the current study, the acetate (MeCOO⁻) or pivalate (^tBuCOO⁻) ion furnished the desired basic medium with a favorable pH to the formation of both the tautomeric forms. Complexes 1 and 2 were characterized by elemental analysis, IR, ESI-mass spectrometry and single crystal X-ray crystallography. The IR spectra of complexes 1 and 2 (Experimental section) contain all the characteristic bands of the corresponding coordinated tridentate anionic ligand (L¹)²⁻ or (L²)²⁻ in the enol form, viz., 3476, 3024, 1608, 1254 and 1159 cm⁻¹ for 1 and 3388, 3226, 2978, 1611, 1252 and 1068 cm⁻¹ for 2. Their ESI-MS spectra in ethanol solution (Experimental section) display the parent peaks at m/z = 790 [1 + H]⁺ (100%) and at m/z = 601 [2 + H]⁺ (100%), respectively, which also support the formulations.

2.2. General description of the crystal structures

X-ray quality crystals of complexes 1 and 2 were obtained upon slow evaporation of their ethanolic solutions, at room temperature. Crystallographic data are summarized in Table 1, representative plots are displayed in Fig. 1, and selected dimensions are presented in Table 2. Relevant hydrogen bond interactions are shown in Table 3.

Table 1 Crystal data and structure refinement details for H₂L², 1 and 2

	H2L^2	1	2
a R = ∑||Fo| − |Fc||/∑|Fo|.b wR(F^2) = [∑w(|Fo|^2 − |Fc|^2)^2/∑w|Fo|^4]^½.
Empirical formula	C14H12N2O4	C44H53FeN4O6	C30H27CoN4O9
Formula weight	272.26	789.75	646.48
Crystal system	Monoclinic	Monoclinic	Orthorhombic
Temperature/K	150(2)	296(2)	150(2)
Space group	P21/c	P21/c	Pna21
a/Å	8.9865(9)	17.1427(14)	18.5211(10)
b/Å	9.5514(7)	14.5854(13)	11.1131(6)
c/Å	14.1428(12)	17.4812(14)	12.9945(6)
α/°	90	90	90
β/°	90.203 (1)	100.604 (2)	90
γ/°	90	90	90
V (Å^3)	1213.92 (18)	4296.2 (6)	2674.6(2)
Z	4	4	4
Dcalc (g cm^−3)	1.490	1.221	1.605
μ(Mo Kα) (mm^−1)	0.111	0.401	0.709
Rfls. collected/unique/observed	7359/2121/1598	33 844/7677/3373	21 785/5032/3914
Rint	0.0440	0.1452	0.0657
Final R1^a, wR2^b (I ≥ 2σ)	0.0381, 0.1076	0.0705, 0.1486	0.0431, 0.852
Goodness-of-fit on F^2	1.005	0.868	1.025

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Fig. 1 Molecular structures of compounds H₂L², 1 and 2, with partial atoms labelling schemes. In 1 only one of the components of the disordered tert-butyl groups is shown. The H-atoms in the butyl groups (in 1) and the ethanol crystallization molecule (in 2) are omitted for clarity.

Table 2 Selected bond distances (Å) and angles (°) in the complexes 1 and 2

1
N1–Fe1	2.087 (3)	O2–Fe1	1.903 (3)
N3–Fe1	2.127 (3)	O4–Fe1	2.135 (3)
O1–Fe1	2.065 (3)	O5–Fe1	1.912 (3)
O2–Fe1–O5	94.58 (14)	O1–Fe1–N3	109.45 (12)
O2–Fe1–O1	157.27 (11)	N1–Fe1–N3	161.24 (15)
O5–Fe1–O1	88.53 (13)	O2–Fe1–O4	100.01 (13)
O2–Fe1–N1	83.89 (13)	O5–Fe1–O4	151.87 (12)
O5–Fe1–N1	116.30 (13)	O1–Fe1–O4	87.19 (12)
O1–Fe1–N1	74.61 (13)	N1–Fe1–O4	89.25 (13)
O2–Fe1–N3	93.28 (13)	N3–Fe1–O4	72.94 (13)
O5–Fe1–N3	82.38 (14)
2
N1–Co1	1.895 (7)	O3–Co1	1.920 (3)
N3–Co1	1.855 (7)	O2–Co1	1.941 (3)
O1–Co1	1.892 (3)	O5–Co1	1.888 (3)
N3–Co1–O5	94.9 (2)	O1–Co1–O8	89.97 (14)
N3–Co1–O1	90.6 (2)	N1–Co1–O8	93.3 (2)
O5–Co1–O1	92.34 (11)	N3–Co1–O2	92.0 (2)
N3–Co1–N1	174.03 (17)	O5–Co1–O2	89.38 (14)
O5–Co1–N1	88.7 (2)	O1–Co1–O2	176.8 (2)
O1–Co1–N1	94.0 (2)	N1–Co1–O2	83.3 (2)
N3–Co1–O8	82.8 (2)	O8–Co1–O2	88.42 (12)
O5–Co1–O8	176.8 (2)

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Table 3 Hydrogen-bond geometry (Å, °) in complexes 1 and 2

D–H⋯A	D–H	H⋯A	D⋯A	D–H⋯A	Symmetry code
1
N4–H4N⋯O6	0.96	1.83	2.615	136	—
O3–H3O⋯N2	0.84	1.78	2.565	154	—
O6–H6O⋯O1^i	0.86	1.90	2.674	150	−x + 1, −y, −z
2
O7–H7O⋯N4	0.87	1.77	2.529 (6)	144	—
O4–H4O⋯O1^i	0.88	2.04	2.896 (8)	164	−x + 3/2, y + 1/2, z + 1/2
O4–H4O⋯O3^i	0.88	2.33	2.834 (6)	116	−x + 3/2, y + 1/2, z + 1/2
O3–H3O⋯O6^ii	0.89	2.00	2.875 (5)	169	x + 1/2, −y + 1/2, z
N2–H2N⋯O4	0.88	1.95	2.619 (7)	131	—
O6–H6O⋯O7^iii	0.88	1.93	2.755 (7)	156	−x + 3/2, y − 1/2, z + 1/2

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Both metal cations in 1 and 2 exhibit octahedral geometries comprising two inequivalent tridentate ligands and giving rise to N₂O₄ coordination environments. Besides the M − O_ketone bond distances being longer than the M − O_enolate in both 1 and 2, in each complex the M − O lengths concerning one of the ligands are shorter than those involving the other, thus revealing the simultaneous different tautomeric forms involved. The enolate forms are more strongly coordinated to the metal centres with shorter M − O bond distances than the corresponding ones in the keto forms, as observed in other cases.^2,12 Although H₂L² is roughly planar with the least square planes of the aromatic rings making an angle of only 7.94°, upon coordination to the cobalt(III) cation in 2, these angles are of 20.02 and 29.51°; in complex 1, despite the sterically demanding ligands, those angles assume values of 18.56° and 25.75°. The greatest deviations from co-planarity concern the enol forms of the ligands.

Compounds H₂L², 1 and 2 are involved in extensive hydrogen bond interactions (Table 3). In all cases intramolecular H-contacts of types O–H⋯N and O–H⋯O_ketone, with graph sets S1,1(6), could be found. Possibly derived from such interactions, the hydroxybenzamide groups in 2 are twisted relative to the position it adopted in H₂L² (Fig. 1). The same effect can be found in 1.

2.3. Oxidation of primary and secondary alcohols

The catalytic study focused on the oxidation of various primary and secondary alcohols, mainly benzylalcohol and 1-phenylethanol, used as model substrates for the investigation of the catalytic properties of the new complexes 1 and 2. The study was undertaken following our previously developed procedure,⁸ under mild conditions using tert-butyl hydroperoxide (Bu^tOOH, 2 eq.) as oxidizing agent, under typical conditions of 80–120 °C, low power (5–15 W) microwave (MW) irradiation, 30–180 min reaction time and in the absence of any added solvent (Scheme 2 for the case of 1-phenylethanol). The ketones are the only oxidation products (benzaldehyde in the case of benzylalcohol) obtained from these MW-assisted transformations and the high selectivities observed (typically >98%) were confirmed by mass balances. Selected results are summarized in Tables 4 and 5.

Scheme 2 Solvent-free oxidation of 1-phenylethanol to acetophenone.

Table 4 Oxidation of various aromatic and aliphatic alcohols using 1 or 2 as catalyst precursors^a

Entry	Substrate	Catalyst	Time (min)	Product	Yield^b	TON (TOF)^c
a Reaction conditions (unless stated otherwise): 2.5 mmol of substrate, catalysts 1 or 2 (5 μmol, 0.2 mol% vs. substrate), 5 mmol of Bu^tOOH (aq. 70%), 80 °C, microwave irradiation (5 W).b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate determined by GC.c Turnover number = number of moles of product per mol of metal catalyst; turnover frequency = TON per hour.d In the presence of TEMPO.e In the presence of pyridine.f In the presence of 3,5-dimethylpyrazole.g In the presence of triethylamine.h Reaction performed at 120 °C (15 W).i Reaction performed at 120 °C (15 W) and in the presence of Hpca, n(acid)/n(catalyst) = 5.j For comparative purpose; [Fe3(μ3-O)] = [Fe3(μ3-O)(^tBuCOO)6(H2O)3](^tBuCOO); H2L^1 = (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide; H2L^2 = (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide.k In the presence of Hpca, n(acid)/n(catalyst) = 5, at 80 °C.
1		1	5		7	35 (420)
2		1	15		9	45 (180)
3		1	30		16	80 (160)
4		1	60		21	105 (105)
5		1	180		28	140 (47)
6^d		1	60		7	35 (35)
7^d		1	180		12	60 (29)
8^e		1	30		17	85 (170)
9^f		1	30		6	30 (60)
10^g		1	30		4	20 (40)
11^h		1	5		45	225 (2700)
12^h		1	15		54	270 (1100)
13^h		1	30		92	460 (920)
14^h		1	60		96	480 (480)
15^i		1	5		59	295 (3500)
16^i		1	15		61	305 (1220)
17^i		1	30		96	480 (960)
18		2	30		7	35 (70)
19^d		2	60		9	45 (45)
20		2	180		12	60 (20)
21^h		2	30		18	90 (180)
22^h		2	60		26	130 (130)
23^h^,^j		Fe(MeCOO)2	60		31	155 (155)
24^h^,^j		Co(MeCOO)2	60		27	135 (135)
25^h^,^j		[Fe3(μ3-O)]	60		17	85 (85)
26^h^,^j		H2L^1	60		8	41 (41)
27^h^,^j		H2L^2	60		10	51 (51)
28		1	30		23	115 (230)
29		2	30		4	20 (40)
30		1	30		10	50 (100)
31^d		1	30		4	20 (40)
32^e		1	30		Traces
33^h		1	30		19	95 (190)
34^k		1	30		19	95 (190)
35		2	30		6	30 (60)
36	CH3(CH2)5OH	1 or 2	60		Traces
37		1 or 2	60		Traces

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Table 5 Effect of the acid promoter-to-catalyst molar ratio on the oxidation of 1-phenylethanol catalyzed by 1 and 2^a

Entry	Catalyst	Acid	n(acid)/n(catalyst)	Yield^b (%)	TON^c
a Reaction conditions: 2.5 mmol of substrate, catalyst 1 (5 μmol, 0.2 mol% vs. substrate), 5 mmol of Bu^tOOH (aq. 70%), 80 °C, 30 min, microwave irradiation (5 W).b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate determined by GC.c Turnover number = number of moles of product per mol of metal catalyst.
1	1	Hpca	0	16	80
2			2	63	315
3			5	81	405
4			10	78	390
5			20	51	255
6		Hpic	2	13	65
7			5	19	95
8			10	17	85
9			20	5	25
10	2	Hpca	0	7	35
11			5	8	40
12			20	6	30
13		Hpic	5	6	30
14			20	7	35

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Complex 1 catalyzes moderately the peroxidative oxidation of 1-phenylethanol under a low power (5 W) MW irradiation, leading to 16% of acetophenone after 30 min of reaction at 80 °C (Table 4, entry 3), in the absence of any additive. Prolongation of the reaction time to 180 min increases the acetophenone product yield to 28% (Table 4, entry 5). Reactions performed under the same conditions (80 °C/5 W) but in the presence of complex 2 results only in 7 and 12% of acetophenone after 30 and 180 min of reaction (Table 4, entries 17 and 20, respectively), in accord with previously reported poor yields observed for the aerobic oxidation of benzylic alcohols by the water-soluble Co(II) complex bearing (3-/5-chloro-2-hydroxy-3-sulphenylhydrazo)pentane-2,4-dione ligand.¹³

Blank tests (in the absence of any catalyst) were performed under common reaction conditions and no considerable conversion was observed.

The Fe(III) catalytic system has also been tested towards other alcohols, including aliphatic and benzyl alcohols. The oxidation of cyclohexanol and benzyl alcohol in the presence of a catalytic amount of 1 (0.2 mol% vs. substrate) and in the absence of any additive, yielded 23% of cyclohexanone and 10% of benzaldehyde after 30 min of reaction at 80 °C (Table 4, entries 28 and 30, respectively). With 2 instead of 1 only 4 and 6% yield of cyclohexanone and benzaldehyde was achieved (Table 4, entries 29 and 35, respectively).

Both catalytic systems (1 or 2) failed in the oxidation of simple aliphatic alcohols such as 1-hexanol and 2-hexanol after 60 min reaction time (Table 4, entries 36 and 37).

The MW-assisted alcohol oxidation depends strongly on the temperature. The high yield of 92% of acetophenone is achieved after 30 min at 120 °C or 45% yield is already obtained after 5 min reaction time at the same temperature (Table 4, entries 11 and 13, respectively). These values are much higher than the 16 or 7% yield obtained for the corresponding reaction times but at 80 °C (Fig. 2).

Fig. 2 Product yield versus time for the microwave-assisted oxidation of 1-phenylethanol to acetophenone by the catalytic system 1 at 80 °C/5 W (▲) or at 120 °C/15 W (◆). Reaction conditions: 1-phenylethanol (2.5 mmol), catalyst (5 μmol), Bu^tOOH (5 mmol).

However, we should take into account that the temperature depends on the MW power. In fact, for a temperature of 80 °C, the MW irradiation reaches 10 W of power in the first 10 seconds, and then drops to ca. 5 W. When a temperature of 120 °C is required, ca. 40 W of power was used in the first 10 seconds, whereafter a 10–15 W power was observed. Hence, the reaction also strongly depends on the MW power.

In order to try to increase the catalytic performance of 1 in the solvent-free MW-assisted peroxidative oxidation of 1-phenylethanol, we have investigated the influence of different additives (co-catalysts)¹⁴ on the acetophenone product yield. For this purpose, heteroaromatic N-based acids, such as 2-pyrazinecarboxylic acid (Hpca) and 2-pyridinecarboxylic acid (Hpic), or bases such as pyridine, 3,5-dimethyl-1H-pyrazole or triethylamine were tested.

The addition of the heteroaromatic acids Hpca and Hpic has different effects. Only the presence of Hpca in the system catalysed by 1 has a promoting effect in the product yield. For the other cases the presence of such acids do not affect considerably the product yield, or even show an inhibiting effect for the higher amounts of acid. Since the effect is expected to depend on the quantity of the acid promotor,¹⁵ a search for the optimal acid promoter-to-catalyst ratio was carried out in the oxidations catalyzed by 1 (Table 5). The oxidation of 1-phenylethanol by the 1/Hpca system provides a yield of 81% and a TON of 405 (Table 5, entry 3, Fig. 3) for a considerable low molar ratio n(acid)/n(catalyst 1) = 5 (25 μmol of Hpca), remaining practically constant for a n(acid)/n(catalyst 1) molar ration of 10 (50 μmol of Hpca) (ca. 78% yield) (Table 5, entry 4, Fig. 3). When a high excess of Hpca is used, for e.g. 100 μmol, with n(acid)/n(catalyst 1) = 20, an important yield drop (51%, Table 5, entry 5, Fig. 3) is observed, but still much higher compared to the reaction carried out under the same conditions (5 μmol of catalyst, 80 °C, MW, 30 min) but in the absence of any additive (16%, entry 3 of Table 4 and entry 1 of Table 5). Moreover, when the oxidation reaction is performed in the presence of a n(acid)/n(catalyst 1) = 5 (25 μmol Hpca) and at 120 °C, the highest yield of 96% is achieved, after 30 min reaction (Table 4, entry 17).

Fig. 3 Acetophenone yield in the 1-catalysed oxidation of 1-phenylethanol as a function of Hpca/catalyst molar ratio. Reaction conditions: 1-phenylethanol (2.5 mmol), 70% aqueous solution of Bu^tOOH (5 mmol), catalyst (5 μmol), Hpca (0–100 μmol), 80 °C, 30 min.

The oxidation catalyzed by 2 proceeds less efficiently, even in the presence of different amounts of Hpca, yielding 8 and 6% of acetophenone for an n(acid)/n(catalyst 2) molar ratio of 5 and 20 (Table 5, entries 11 and 12, respectively) comparable with the product yield obtained in the absence of any acid (7%, entries 18 and 10 in Tables 4 and 5, respectively).

The crucial role of certain additives, in particular acids, on the activity of various transition metal complexes, e.g. Cu, Fe or V, in oxidation catalysis has been reported.^14,16–19 The acid promotor is believed to accelerate the oxidation reactions by improving the oxidation properties of the complexes and by creating unsaturated metal centres.^16a–20

The presence of 3,5-dimethylpyrazole and triethylamine has an inhibitory effect on the acetophenone product yield and e.g., the use of a n(base)/n(catalyst 1) = 20 molar ratio (100 μmol of base) results in a considerable yield drop (Table 4, entries 9 and 10, respectively). Pyridine has not an appreciable effect (entry 8, Table 4). The inhibiting effect of the base can be due to its competition with the substrate for metal coordination.

The metal sources Fe(MeCOO)₂, Co(MeCOO)₂ and [Fe₃(μ₃-O)(^tBuCOO)₆(H₂O)₃](^tBuCOO) were also tested under the same conditions as those of 1 and 2, for comparative purposes. The catalytic activity of 1 is much higher than those of the iron species Fe(MeCOO)₂ and [Fe₃(μ₃-O)(^tBuCOO)₆(H₂O)₃](^tBuCOO) (96% for 1 vs. 31 and 17%, respectively, entries 23 and 25, Table 4). However, similar yield values were obtained for 2 and Co(MeCOO)₂ (26 and 27%, respectively, entries 22 and 24, Table 4) showing that the ligands in 2 do not have a marked influence on the catalytic activity. The catalytic performances of the free ligands H₂L¹ and H₂L² were examined in the oxidation of 1-phenylethanol under similar reaction conditions, leading to much lower yields of acetophenone (8 and 10%, entries 26 and 27, Table 4, for H₂L¹ and H₂L², respectively) than those of 1 or 2 (96 or 26%, entries 17 and 22, Table 4).

2,2,6,6-Tetramethylpiperidyl-1-oxyl (TEMPO), a nitroxyl radical that is a known^16,21–23 promoter in aerobic oxidation of alcohols, was also evaluated but an inhibiting effect was usually observed either in the catalytic system 1 or 2 (entries 6, 7, 19 and 25, Table 4). TEMPO in our case conceivably behaves as a radical trap, what suggests the involvement of a radical mechanism.^23–25 The mechanism may involve the metal-assisted generation of t-BuOO˙ and t-BuO˙ radicals (upon oxidation and reduction of t-BuOOH by a M^III or M^II centre, respectively, M = Fe or Co)^26,27 the latter behaving as a H-atom abstractor from the alcohol.^26–28

The yields observed for our complex 1 are comparable to those observed for some efficient systems involving dichloro-iron(III) complexes derived from 3-amino-2-pyrazinecarboxylic acid,^9b iron(II) complexes bearing a N₂S₂-type ligand^16a or bis- and tris-pyridyl amino and imino thioether Fe complexes^16b for the MW-assisted oxidation of secondary alcohols to the corresponding ketones.

3. Conclusions

The aroylhydrazone complexes [Fe(L¹)(HL¹)] (1) and [Co(L²)(HL²)] (2), displaying the unusual concomitant keto (monodeprotonated/monoanionic) and enol (bisdeprotonated/dianionic) coordination forms of the ligands, were prepared by using carboxylate metal sources. The carboxylate species conceivably led to a pH favourable to stabilize both tautomeric forms, resulting in neutral metal(III) complexes.

Complexes 1 and, to a much lower extent, 2 act as catalysts in the MW-assisted peroxidative oxidation of 1-phenylethanol, benzylalcohol, cyclohexanol, 1-hexanol and 2-hexanol. The mononuclear iron(III) [Fe(L¹)(HL¹)] (1) complex leads to a maximum yield of 96% (TON = 480) of acetophenone, in 30 min, and in the presence of 2-pyrazinecarboxylic acid (Hpca) as a promoter. Another heteroaromatic N-based acid, 2-pyridinecarboxylic acid (Hpic), or bases such as pyridine, 3,5-dimethylpyrazole or triethylamine, did not promote the reaction or even acted as inhibitors.

4. Experimental

4.1. General materials and procedures

All synthetic work was performed in air. The reagents and solvents were obtained from commercial sources and used as received, i.e., without further purification or drying. The metal sources, [Fe₃(μ₃-O)(^tBuCOO)₆(H₂O)₃](^tBuCOO), Fe(MeCOO)₂ and Co(MeCOO)₂·4H₂O were used for the synthesis of complexes 1 and 2. The former metal starting material was prepared as described in the literature.¹¹

C, H, and N elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico. Infrared spectra (4000–400 cm⁻¹) were recorded on a Bruker Vertex 70 instrument in KBr pellets; wavenumbers are in cm⁻¹. The ¹H NMR spectra were recorded at room temperature on a Bruker Avance II + 400.13 MHz (UltraShieldTM Magnet) spectrometer. The chemical shifts are reported in ppm using tetramethylsilane as the internal reference. Mass spectra were run in a Varian 500 MS LC Ion Trap Mass Spectrometer equipped with an electrospray (ESI) ion source. For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 100 to 1200 in ethanol solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V). The catalytic tests under microwave (MW) irradiation were performed in a focused Anton Paar Monowave 300 microwave reaction fitted with a rotational system and an IR temperature detector, using a 10 mL capacity reaction tube with a 13 mm internal diameter. Gas chromatographic (GC) measurements were carried out using a FISONS Instruments GC 8000 series gas chromatograph with a FID detector and a capillary column (DB-WAX, column length: 30 m; internal diameter: 0.32 mm) and the Jasco-Borwin v.1.50 software. The temperature of injection was 240 °C. The initial temperature was maintained at 120 °C for 1 min, then raised 10 °C min⁻¹ to 200 °C and held at this temperature for 1 min. Helium was used as the carrier gas.

4.2. Syntheses of the pro-ligands H₂L¹ and H₂L²

The Schiff base pro-ligands (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H₂L¹) and (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide (H₂L²) (Scheme 1) were prepared by a reported method^1c,3c upon condensation of the salicylhydrazide with 3,5-di-tert-butyl-2-hydroxybenzaldehyde and 2,3-dihydroxybenzaldehyde, respectively. An ethanolic solution of H₂L² was refluxed for 30 min and then the clear solution was kept at room temperature for slow evaporation. After ca. 2 days good single crystals of H₂L² were obtained. Their IR and ¹H NMR data are consistent with those reported.^1c,3c

4.3. Synthesis of [Fe(L¹)(HL¹)] (1)

To an ethanolic suspension (30 mL) of H₂L¹ (0.552 g, 1.5 mmol), a 15 mL ethanol solution of Fe(MeCOO)₂ (0.130 g, 0.75 mmol) or [Fe₃(μ₃-O)(^tBuCOO)₆(H₂O)₃](^tBuCOO) (0.237 g, 0.25 mmol) was added and the resultant mixture was stirred at room temperature for 15 min. The reddish-brown solution slowly turned dark-brown. The mixture was then filtered and the solvent was allowed to evaporate slowly. After 4–5 days, single crystals suitable for X-ray diffraction were isolated, washed 3 times with cold ethanol and dried in open air.

Yield: 0.432 g (73%, with respect to Fe(MeCOO)₂). Anal. calcd for C₄₄H₅₃FeN₄O₆ (1): C, 66.92; H, 6.76; N, 7.09. Found: C, 66.86; H, 6.71; N, 7.07. IR (KBr; cm⁻¹): 3476 ν(OH), 3024 ν(NH), 1608 ν(C=N), 1254 ν(C–O) enolic and 1159 ν(N–N). ESI-MS(+): m/z 790 [M + H]⁺ (100%).

4.4. Synthesis of [Co(L²)(HL²)] (2)

To a 30 mL ethanol solution of H₂L² (0.544 g, 2.00 mmol), a 10 mL ethanol solution of Co(MeCOO)₂·4H₂O (0.250 g, 1.00 mmol) was added and the reaction mixture was stirred for 30 min at room temperature. The resultant dark red solution was filtered and the filtrate was kept in air. After 3 days, dark red single crystals suitable for X-ray diffraction analysis were isolated, washed 3 times with cold ethanol and dried in open air.

Yield: 0.456 g (76%, with respect to Co(MeCOO)₂·4H₂O). Anal. calcd for C₃₀H₂₇CoN₄O₉ (2): C, 55.73; H, 4.21; N, 8.67. Found: C, 55.68; H, 4.16; N, 8.61. IR (KBr; cm⁻¹): 3388, 3226 ν(OH), 2978 ν(NH), 1611 ν(C=N), 1252 ν(C–O) enolic and 1068 ν(N–N). ESI-MS(+): m/z 601 [M + H]⁺ (100%).

4.5. X-ray measurements

X-ray quality single crystals of H₂L², 1 and 2 were immersed in cryo-oil, mounted in Nylon loops and measured at a temperature of 150 (H₂L² and 2) or 296 K (1). Intensity data were collected using a Bruker AXS-KAPPA APEX II CCD or Bruker AXS PHOTON 100 diffractometers with graphite monochromated Mo-Kα (λ 0.71073) radiation. Data were collected using omega scans of 0.5° per frame and full sphere of data were obtained. Cell parameters were retrieved using Bruker SMART^29a software and refined using Bruker SAINT^29a on all the observed reflections. Absorption corrections were applied using SADABS.^29a Structures were solved by direct methods by using SIR97 (ref. 29b) and refined with SHELXL2014.^29c Calculations were performed using WinGX v2014.1.^29d All non-hydrogen atoms were refined anisotropically. Those H-atoms bonded to carbon were included in the model at geometrically calculated positions and refined using a riding model. U_iso(H) were defined as 1.2U_eq of the parent carbon atoms for phenyl and methyne residues and 1.5U_eq of the parent carbon atoms for the methyl groups. The other hydrogen atoms (O–H and N–H) were located in the difference Fourier synthesis and refined, in some cases with the help of distance restraints, their isotropic thermal parameter set at 1.5 times the average thermal parameter of the parent oxygen or nitrogen atom. The atomic positions of one of the tert-butyl carbon atoms in 2 were disordered over two orientations and were refined with the use of PART instruction. The occupancy of “a” and “b” components of each atom were refined to a ratio of 54% and 46%. The positions of the hydrogen atoms bonded to these carbon atoms were included in calculated positions by using the PART instructions in SHELXL and treated as riding atoms. Least square refinements with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic for the remaining atoms were employed.

4.6. Catalytic studies

Oxidation reactions of the alcohols were carried out in sealed cylindric Pyrex tubes under focused microwave irradiation, typically as follows: the alcohol (2.5 mmol), catalyst 1 and 2 (5 μmol) and a 70% aqueous solution of Bu^tOOH (5 mmol) were introduced in the tube. This was then placed in the microwave reactor and the system was left under stirring and under irradiation (5–10 W) for 5–180 min at 80 or 120 °C (15–40 W).

After cooling to room temperature, 150 μL of benzaldehyde (internal standard) and 2.5 mL of NCMe (to extract the substrate and the organic products from the reaction mixture) were added. The obtained mixture was stirred during 10 min and then a sample (1 μL) was taken from the organic phase and analysed by GC using the internal standard method.

Acknowledgements

Authors are grateful to the Fundação para a Ciência e a Tecnologia (FCT) (project UID/QUI/00100/2013), Portugal, for financial support. M.S. acknowledges the FCT, Portugal for a postdoctoral fellowship (SFRH/BPD/86067/2012). Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility.

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Footnote

† CCDC 1440204–1440206 for 1, 2 and H₂L² respectively. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra25774c

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