Iron(II), manganese(II) and cobalt(II) complexes containing tetradentate biphenyl-bridged ligands and their application in alkane oxidation catalysis

Abstract

A series of manganese(II), iron(II) and cobalt(II) bis(triflate) complexes containing linear tetradentate bis(imine) and bis(amine) ligands with a biphenyl bridge have been synthesized. The twist in the ligand backbone due to the biphenyl unit leads in the case of the bis(imine) ligands (1 and 2) containing sp² hybridised N donors, to a distorted cis-α coordination geometry, whereas in the case of the biphenyl- and biphenylether-bridged bis(amine) ligands (7–9 and 12), a trans coordination geometry is observed. The catalytic properties of the complexes for the oxidation of cyclohexane, using H₂O₂ as the oxidant, have been evaluated. Only the iron complexes show any catalytic activity under the conditions used, but the low conversions and selectivies observed indicate that these catalysts lead predominantly to free radical auto-oxidation.

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Introduction

In recent years, several non-heme iron-based catalysts for the oxidation of alkanes with H₂O₂ have been reported.^1,2 Important examples are the tetradentate tpa (A) and bpmen (B) complexes shown in Fig. 1, where X represents a weakly bound co-ligand, typically CH₃CN or triflate (OTf⁻). The combination of catalytic amounts of these complexes and H₂O₂ as the oxidant have shown unusual product selectivities such as remarkably high selectivities and activities for the formation of cyclohexanol in the oxidation of cyclohexane.^3–5 Oxygen labeling studies have established that, unlike in Fenton chemistry, O₂ is not involved in these oxidations.⁴ In addition, stereospecific hydroxylation has been observed with a prochiral substrate.^6,7 These observations, combined with relatively large kinetic isotope effects, have led to the suggestion that alkane hydroxylations catalysed by A or B operate via a metal-based mechanism rather than the unselective radical chain auto-oxidation mechanism. The existence of high-valent iron-oxo species, first proposed in 1932,⁸ has gained strong support recently through the first crystallographic characterization of two Fe(IV) oxo complexes.^9,10 Catalytic efficiencies are still rather low, partly due to decomposition of H₂O₂ (‘catalase’ activity)¹¹ and probably also due to oxidative degradation of the catalyst, a common problem in oxidation catalysis.¹² These problems may be overcome by improvements in catalyst design, which is the aim of our research.

Fig. 1 Non-heme iron complexes containing tetradentate ligands.

Based on the success obtained with non-heme iron catalysts A and B it appears that multidentate ligands containing nitrogen (preferably pyridine) donors and a cis orientation of two labile co-ligands are desirable criteria in catalyst design. We have previously reported the use of metal complexes containing tridentate pyridine bis(imine) and pyridine bis(amine) ligands and their application as catalysts for the oxidation of cyclohexane. It was found that instead of just two, sometimes three labile sites were present at the metal centre and these catalyst systems resulted in Fenton-type reactivity only.¹³ We therefore turned our attention to tetradentate ligands. Initially, tripodal ligands of type A and the effect of pyridine versus amine donors on the catalytic properties of iron(II) bis(triflate) complexes in cyclohexane oxidation was investigated.¹⁴ Here we report our results on the synthesis and characterisation of a series of metal(II) complexes containing linear tetradentate bis(imine) and bis(amine) ligands containing a biphenyl-bridged ligand backbone (types C and D). The biphenyl bridge, in particular the dimethyl-substituted biphenyl unit, has been used previously to enforce cis-α geometries in transition metal complexes.¹⁵ This geometry, which leaves two labile cis co-ligands at the metal centre, is also seen in bpmen B complexes and is believed to be important for high catalytic activity in the oxidation of cyclohexane with H₂O₂. Compared to iron, the next-door neighbours manganese and cobalt have received much less attention as potential catalysts for the oxidation of alkanes,^16–18 despite their large-scale industrial application in the oxidation of p-xylene and cyclohexane.^19,20 In addition to iron(II), we have therefore also prepared and evaluated manganese(II) and cobalt(II) complexes of the biphenyl-bridged ligands.

Results and discussion

Synthesis of ligands and complexes

Biphenyl bridged bis(pyridylimines) have been known for some time,²¹ but their coordination chemistry has only recently been investigated, in particular binaphthyl-bridged^22–24 and 6,6-dimethyl biphenyl-bridged variations such as 1,²⁵ which form atropisomers and may find application as ligands in asymmetric catalysis. Whilst this work was in progress, the synthesis of iron(II) and cobalt(II) dichloro complexes of ligand 1 was reported and their catalytic properties for olefin polymerization were investigated.²⁶ The ketimine derivative 2 was also described but could not be obtained in pure form. We found that by using Si(OEt)₄ as the dehydrating agent, compound 2 could be obtained cleanly. Both ligands 1 and 2 react readily with various metal salts and the manganese(II), iron(II) and cobalt(II) bis(triflate) complexes have been successfully prepared (see Scheme 1).

Scheme 1

The non-methylated pyridylamine precursors 3–6 have been generally prepared by two-step procedures via reduction of the bis(pyridylimine).^21,25,27,28 We have prepared compounds 3–6 by a more convenient high yielding one-pot reductive amination protocol using NaBH(OAc)₃. These secondary amines were subsequently methylated to give the tertiary amines 7–10 (Scheme 2). The potentially pentadentate 12, containing a biphenylether backbone was prepared by a similar procedure. The unsubstituted ligand 7 and the ligands bearing two methyl groups 8 and 9 reacted readily with Fe(OTf)₂(CH₃CN)₂ in dichloromethane to give the corresponding iron(II) bis(triflate) complexes. In the case of ligand 8, containing methyl groups in the pyridyl 6-position, the resulting complex [Fe(8)OTf₂] was found to be significantly more air-sensitive than the other complexes. The reaction of ligand 10, containing four methyl substituents, with Fe(OTf)₂(CH₃CN)₂ did lead to a product but this could not be isolated in pure form due to its instability.

Scheme 2

All metal complexes have been characterised by MS, CHN analysis and magnetic moment. The complexes are paramagnetic, with magnetic moments corresponding to a high-spin d⁵, d⁶ and d⁷ electronic configuration for Mn(II), Fe(II) and Co(II), respectively. ¹H and ¹⁹F NMR spectra have been recorded and assigned for Fe(II) and Co(II) complexes (see Experimental and ESI†). In some cases ¹H COSY NMR spectra have been recorded to aid peak assignments (see ESI†). The metal complexes of the diimine ligands are generally brightly coloured, whereas the diamine iron(II) complexes are typically off-white solids, displaying rather featureless UV-VIS spectra (see ESI†). Complexes [Fe(1)OTf₂] and [Fe(9)OTf₂] have been characterised by X-ray crystallography.

Solid state structures

A solid state structure of [Fe(1)OTf₂] has been determined previously, but was not discussed in any detail.²⁶ The two structures for [Fe(1)OTf₂] are identical and similar to the dichloro analogue [Fe(1)Cl₂],²⁶ again having crystallographic C₂ symmetry about an axis that passes through the metal centre and bisects the C(9)–C(9′) aryl–aryl linkage of the ligand (Fig. 2). The geometry at the iron centre is significantly distorted octahedral (Table 1) and the ligand has adopted a distorted cis-α geometry. The restricted bite [74.67(9)°] of the two 5-membered chelate rings [involving N(1) and N(7)] causes the two imine nitrogens N(7) and N(7′) to be displaced out of the equatorial {FeO₂} coordination plane such that the {Fe,O(1),O(1′)} and {Fe,N(7),N(7′)} planes are inclined by ca. 35°. These 5-membered chelate rings have envelope conformations, N(1) lying ca. 0.23 Å out of the {Fe,C(6),C(7),N(7)} plane which is coplanar to within ca. 0.01 Å. Though larger, the twisted 7-membered N(7),N(7′) chelate ring also has a restricted bite angle [N(7)–Fe–N(7′) 80.03(13) Å]; the aryl–aryl linkage is a clear single bond [C(9)–C(9′) 1.501(6) Å] with a torsion angle of ca. 67°. Compared to the dichloro analogue, the pyridine nitrogens in [Fe(1)OTf₂] are bent slightly towards the triflate ligands, the N(1)–Fe–O(1) and N(1)–Fe–O(1′) angles being 88.07(10) and 84.62(10)° (cf. 91.4(1) and 89.4(1)° in [Fe(1)Cl₂]) and this is reflected in the axial N(pyridine)–Fe–N(pyridine) angles of 168.17(14)° in [Fe(1)OTf₂], and 178.6(2)° in [Fe(1)Cl₂].

Table 1 Selected bond lengths (Å) and angles (°) for complex [Fe(1)OTf₂]

Fe–N(1)	2.199(2)	Fe–N(7)	2.190(2)
Fe–O(1)	2.097(2)	C(7)–N(7)	1.262(4)
C(9)–C(9′)	1.501(6)
N(1)–Fe–N(7)	74.67(9)	N(1)–Fe–O(1)	88.07(10)
N(1)–Fe–N(1′)	168.17(14)	N(1)–Fe–N(7′)	114.98(10)
N(1)–Fe–O(1′)	84.62(10)	N(7)–Fe–O(1)	93.33(11)
N(7)–Fe–N(7′)	80.03(13)	N(7)–Fe–O(1′)	152.77(11)
O(1)–Fe–O(1′)	103.6(2)

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Fig. 2 The molecular structure of the C₂ symmetric complex Fe(1)OTf₂.

Differences between [Fe(1)Cl₂] and [Fe(1)OTf₂] can also be seen in the iron coordination distances. Though the Fe–N(pyridine) bond lengths are just about statistically the same, being 2.199(2) and 2.214(3) Å in [Fe(1)OTf₂] and [Fe(1)Cl₂], respectively, those to the imine nitrogens differ by ca. 0.1 Å {2.190(2) and 2.296(4) Å in [Fe(1)OTf₂] and [Fe(1)Cl₂], respectively}, presumably a consequence of the varying trans influences exerted by triflate and chloride; the respective Fe–X distances are 2.097(2) Å to O(1) in Fe(1)OTf₂, and 2.397(2) Å to Cl in [Fe(1)Cl₂]. Additionally, the X–Fe–X′ angle between the two cis anionic co-ligands is 103.6(2)° in [Fe(1)OTf₂] (cf. 111.6(1)° in [Fe(1)Cl₂]).

In contrast to [Fe(1)OTf₂], the X-ray crystal structure of [Fe(9)OTf₂] (which has molecular rather than crystallographic C₂ symmetry about an axis that passes through the metal centre and bisects the C(14)–C(15) aryl–aryl linkage) shows the tetradentate N,N′,N″,N‴ diamine ligand 9 to have adopted a so-called trans geometry around the metal centre (Fig. 3), the two triflate co-ligands being mutually trans [O(1A)–Fe–O(2A) 173.75(15)°]. The geometry at the metal is again distorted octahedral (Table 2), though the distortions are less than seen in [Fe(1)OTf₂]. The bite angles [N(1)–Fe–N(8) 78.63(16)°, N(21)–Fe–N(28) 77.26(16)°] compare to the bite angle of 74.67(9)° for the equivalent N(pyridyl),N(imine) chelate ring in [Fe(1)OTf₂]. Similar to the conformations seen in [Fe(1)OTf₂], here in [Fe(9)OTf₂] both of the 5-membered chelate rings have envelope conformations, though distinctly more folded than in the earlier structure. For the N(1),N(8) ring the methylene carbon C(7) lies ca. 0.59 Å out of the {Fe,N(1),C(6),N(8)} plane (which is coplanar to within ca. 0.07 Å), whilst for the N(21),N(28) ring it is the amino nitrogen N(21) which lies ca. 0.74 Å out of the {Fe,C(22),C(23),N(28)} plane (which is coplanar to within ca. 0.02 Å). One of the more noticeable differences between the coordination behaviour of ligands 1 and 9 is the bite angle of the 7-membered N,N′ chelate ring. Here in [Fe(9)OTf₂] the bite angle for this ring is 103.66(15)°, (cf. 80.03(13)° in [Fe(1)OTf₂]), very clearly reflecting the release of strain on going from imino to amino nitrogens. Concurrently, the torsion angle about the aryl–aryl linkage C(14)–C(15) [1.504(7) Å] increases to ca. 81° (cf. 67° in [Fe(1)OTf₂]). The equatorial plane formed by the iron centre and the four donor nitrogen atoms of the tetradentate ligand 9 is noticeably twisted, the {Fe,N(1),N(8)} and {Fe,N(21),N(28)} planes being inclined by ca. 15°.

Table 2 Selected bond lengths (Å) and angles (°) for complex [Fe(9)OTf₂]

Fe–N(1)	2.151(4)	Fe–N(8)	2.295(4)
Fe–N(21)	2.282(4)	Fe–N(28)	2.153(4)
Fe–O(1A)	2.144(4)	Fe–O(2A)	2.116(4)
C(14)–C(15)	1.504(7)
N(1)–Fe–N(8)	78.63(16)	N(1)–Fe–N(21)	169.85(16)
N(1)–Fe–N(28)	101.96(17)	N(1)–Fe–O(1A)	95.30(16)
N(1)–Fe–O(2A)	88.50(16)	N(8)–Fe–N(21)	103.66(15)
N(8)–Fe–N(28)	171.56(16)	N(8)–Fe–O(1A)	82.39(14)
N(8)–Fe–O(2A)	93.54(15)	N(21)–Fe–N(28)	77.26(16)
N(21)–Fe–O(1A)	94.80(15)	N(21)–Fe–O(2A)	81.51(15)
N(28)–Fe–O(1A)	89.17(16)	N(28)–Fe–O(2A)	94.89(16)
O(1A)–Fe–O(2A)	173.75(15)

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Fig. 3 The molecular structure of Fe(9)OTf₂.

The most obvious difference in the Fe–N bond lengths seen in [Fe(1)OTf₂] and those in [Fe(9)OTf₂] is unsurprisingly associated with the change from imine in [Fe(1)OTf₂] [Fe–N(imine) 2.190(2) Å] to amine in [Fe(9)OTf₂] [Fe–N(amine) 2.282(4), 2.295(4) Å]. The Fe–N(pyridine) distances also vary between the two structures {2.199(2) Å in [Fe(1)OTf₂], cf. 2.151(4) and 2.153(4) Å in [Fe(9)OTf₂]}. The distances to the mutually trans triflate co-ligands [2.116(4) and 2.144(4) Å] are comparable with the unique Fe–O bond length of 2.097(2) Å seen for the cis oriented triflate ligands in [Fe(1)OTf₂]; the reason for the slight asymmetry between the two Fe–O bonds in [Fe(9)OTf₂] is not immediately apparent, especially given the approximate C₂ symmetry of the complex. There are no intermolecular packing interactions of note.

Conformational analysis of linear tetradentate ligands in 6-coordinate complexes

Linear tetradentate ligands are capable of arranging their donors around a metal centre in three different ways: cis-α, cis-β and trans (see Table 3).^15,29,30 The trans geometry observed in [Fe(9)OTf₂] was initially surprising as the twisted biphenyl backbone was expected to enforce a cis-α coordination mode. However, when comparing coordination geometries observed in metal complexes of a series of linear tetradentate bis(pyridylmethyl) diamine ligands that have been structurally characterised, as shown in Table 3, a trend becomes apparent. Metal complexes of the ethylene-bridged bpmen ligand (I) are invariably cis-α, whereas for the slightly more constrained cyclohexyl backbone, both cis-α and cis-β conformations have been observed. The more flexible propylene-bridged derivative III yields, with a few exceptions, cis-β geometries. The very rigid double ethylene-bridged derivative IV can only give a trans geometry, which is distorted to a trigonal prismatic geometry when the bridges are replaced by propylene units as in V. The ethylene-bridged bis(imine) ligand VI, containing sp² hybridized nitrogen donors, also prefers the trans geometry, unless the backbone is twisted by a biphenyl bridge, as shown for ligand 1. There is, however, considerable strain in this conformation and the addition of stronger field co-ligands such as iso-nitriles has been shown to result in a cis-β coordination geometry.²⁶ Finally, the twist-effect of the biphenyl unit is significantly reduced when the nitrogen donors are sp³ hybridized, as in 9, and the backbone behaves as a relatively flexible C₄-linkage, resulting in a distorted trans geometry. Thus, it appears that for singly bridged bis(pyridylmethylamine) ligands with sufficient flexibility in the ligand backbone, the preferred coordination geometry is trans. Shortening the backbone, for example by using a propylene bridge, results in a move of one of the pyridylmethyl arms out of the coordination plane to give a cis-β geometry. Further restriction of the flexibility, as in bpmen, leads to the least favourable cis-α geometry. Considering the requirement for cis labile sites in non-heme alkane oxidation catalysis, these observations may be related to the catalytic activity of the complexes.

Table 3 Variations in coordination geometries observed in complexes containing linear tetradentate bis(pyridylmethyl) diamine ligands

	cis-α	cis-β	trans			cis-α	cis-β	trans
	Mn(II)^43							Fe(II): trigonal prismatic^65
	Fe(II)^4,^44–48
	Fe(III)^49
	Cr(III)^50	Fe(II)^52,55						Mn(II)^66,^67
	Mn(II)^51	Co(III)^56
	Fe(II)^52
	Co(II)^53
	Co(III)^54
	Mn(IV)^57	Cr(III)^58–60				Fe(II)^26, this work	Fe(II)^26
	Fe(II)^45	Mn(II)^43
		Fe(III)^61,^62
		Co(III)^63
			Mn(II)^64					Fe(II), this work.
			Co(II)^64

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Solution behaviour

The solution behaviour of the iron(II) and cobalt(II) complexes was investigated by ¹H and ¹⁹F NMR. The bis(imine) complexes [Fe(1)OTf₂], [Fe(2)OTf₂] and [Co(1)OTf₂] show nine paramagnetically shifted resonances in the ¹H NMR (cf. Fig. S5, ESI†), which is consistent with the C₂-symmetric cis-α geometry seen in the solid state for [Fe(1)OTf₂]. The peaks have been assigned on the basis of their integration and the proximity of the protons to the metal centre.²⁶ In addition, correlation spectroscopy (COSY) was used to identify coupling interactions (Fig. S6, see ESI†).

The ¹H NMR spectra of the bis(amine) complexes [Fe(7)OTf₂] and [Fe(9)OTf₂] show 11 signals in CDCl₃, consistent with the C₂-symmetric trans geometry seen in the solid state for [Fe(9)OTf₂] (see Fig. S7, ESI†). Complex [Fe(8)OTf₂] also shows a single C₂-symmetric species in CDCl₃ solution (11 signals), but the chemical shifts are significantly different, indicating the possibility of the less favourable cis-α geometry in this case. This may be caused by a steric clash between the two methyl groups in the pyridyl 6-position. These opposing effects, the biphenyl bridge dictating a trans geometry and the clash between the methyl groups distorting the geometry to cis-α, should lead to a tension within the complex, which may be the reason for the decreased stability of this complex in solution and the notably increased sensitivity to air oxidation. In addition, complex [Fe(10)OTf₂] containing the 6,6′-dimethylbiphenyl bridge, was found to be very unstable and could not be isolated in pure form.

Triflate anions are generally weakly coordinating ligands and are often displaced by acetonitrile ligands in solution. In CD₃CN, [Fe(7)OTf₂] and [Fe(9)OTf₂] gave ¹H NMR spectra containing large numbers of resonances, indicative of the presence of multiple species. This could be due to geometrical isomers, but more likely is the presence of an equilibrium mixture between bis(triflate) [Fe(L)OTf₂], monotriflate [Fe(L)(OTf)(CD₃CN)]⁺ and bis(acetonitrile) [Fe(L)(CD₃CN)₂]²⁺ complexes. One of the contributing species in the CD₃CN spectra was found to exhibit a set of peaks with very similar chemical shifts compared to the CDCl₃ spectra, indicating the presence of the bis(triflate) complex. In contrast, [Fe(8)OTf₂] gave cleanly a spectrum containing 11 lines in CD₃CN, consistent with a single C₂-symmetric complex, most likely [Fe(8)(CD₃CN)₂]²⁺ with a cis-α geometry (Fig. S8, see ESI†). Complex [Fe(12)OTf₂] displayed 10 signals both in CDCl₃ and in CD₃CN solution (see Fig. S9, see ESI†), indicative of the trans complexes [Fe(12)OTf₂] and [Fe(12)(CD₃CN)₂]²⁺, respectively.

In order to understand the complex solution behaviour of the iron complexes containing ligands 7–9 and 12 in more detail, we have carried out a series of ¹⁹F NMR experiments. ¹⁹F NMR spectroscopy is particularly useful for determining whether triflate anions are coordinated to a metal centre. In diamagnetic compounds the ¹⁹F chemical shift for a triflate group in CD₂Cl₂ at room temperature can vary between −78.7 ppm for a covalently bound triflate such as Me₃SiOTf³¹ and −80.5 ppm for ionic triflate in [PPN]OTf (PPN = Ph₃P=N=PPh₃⁺).³² Diamagnetic transition metal triflate complexes also generally show ¹⁹F resonances between −77 and −79 ppm.³³ In paramagnetic iron(II) complexes much larger differences in chemicals shifts are observed, ranging from ca. +60 ppm (bridging triflate), to ca. −10 ppm (terminal) and ca. −80 ppm (free triflate).^14,34,35 In CDCl₃ or CD₂Cl₂, the ¹⁹F NMR spectra of complexes [Fe(7)OTf₂] and [Fe(9)OTf₂] show a single peak at −11.1 and −12.1, respectively, indicating that for these complexes the triflate anions are coordinated. The same applies to complex [Fe(12)OTf₂] with a chemical shift of −0.3 ppm, which also indicates that the ether oxygen donor is probably not coordinated to the iron centre. Complex [Fe(8)OTf₂] shows a peak at −20.5 ppm, slightly upfield, which indicates increased lability of the triflate ligands in CD₂Cl₂. In CD₃CN, the complexes [Fe(8)OTf₂] and [Fe(12)OTf₂] show relatively sharp peaks (ν_1/2 < 100 Hz) at −77 ppm, indicative of uncoordinated triflate anions. Complexes [Fe(7)OTf₂] and [Fe(9)OTf₂] on the other hand, give significantly broadened peaks (ν_1/2 > 1000 Hz) between −70 and −72 ppm. This suggests partial coordination of the triflate anions, which is consistent with the multiple species observed in the ¹H NMR spectra of these complexes.

Catalytic oxidation of alkanes

The catalytic properties of the iron(II), manganese(II) and cobalt(II) bis(triflate) complexes containing ligands 1, 7–9 and 12 for the oxidation of cyclohexane with H₂O₂ have been evaluated (eqn (3)).

The oxidation reactions were carried out in acetonitrile as the solvent at room temperature under air. Hydrogen peroxide solution (70 mM, 10 equiv.) was added to an acetonitrile solution containing the catalyst (2.1 µmol, 1 equiv.) and cyclohexane (2.1 mmol, 1000 equiv.). A large excess of substrate was used to minimize over-oxidation of cyclohexanol (A) to cyclohexanone (K). The addition of dilute H₂O₂ was carried out slowly using a syringe pump, in order to minimise H₂O₂ decomposition. The yields are based on the amount of oxidant H₂O₂ converted into oxygenated products. Two series of catalytic experiments were carried out using 10 (70 mM) and 100 (700 mM) equiv. of H₂O₂. In some cases, cyclohexyl hydroperoxide (P) could also be identified by GC-MS as one of the reaction products. Cyclohexyl hydroperoxide is a relatively stable peroxide which can be isolated in pure form,^36,37 but does decompose to some extent during GC analysis.³⁸ We have therefore not quantified the amount of P, but have merely indicated a qualitative (Y/N) observation in Table 4.

Table 4 Catalytic oxidation of cyclohexane with hydrogen peroxide^a

Run	Catalyst	Equiv. of H2O2	Yield of A + K (%)^b	A/K^c	P ^d
a Conditions: b Total percentage yield of cyclohexanol (A) + cyclohexanone (K), expressed as moles of product per mole of H2O2. c Ratio of moles of cyclohexanol (A) to moles of cyclohexanone (K). d Indication of detection of cyclohexyl hydroperoxide (P): Y = Yes, N = No.
1	Fe(OTf)2(CH3CN)2	10	3.8	1.6	Y
2	Fe(OTf)2(CH3CN)2	100	3.4	2.4	Y
3	[Fe(tpa)(OTf)2]	10	32	12.0	N
4	[Fe(tpa)(OTf)2]	100	29	5.9	N
5	[Fe(bpmen)(OTf)2]	10	65	9.5	N
6	[Fe(bpmen)(OTf)2]	100	48	2.5	N
7	[Fe(1)(OTf)2]	10	10	1.3	N
8	[Fe(1)(OTf)2]	100	2.6	2.0	Y
9	[Fe(7)(OTf)2]	10	14	1.9	N
10	[Fe(8)(OTf)2]	10	0	0	N
11	[Fe(9)(OTf)2]	10	9.5	2.1	N
12	[Fe(12)(OTf)2]	10	3.7	2.8	Y

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The two iron bis(triflate) complexes containing the ligands bpmen and tpa are used as benchmarks against which we compare our other catalysts. Using 10 equiv. of H₂O₂ the most active catalyst [Fe(bpmen)OTf₂] converts 65% of the H₂O₂ added into oxygenated products, with a large A/K ratio. The complex [Fe(tpa)OTf₂] gives a conversion of 32%, but with a better A/K ratio. These results are consistent with those reported previously by Que and co-workers for complexes [Fe(bpmen)(CH₃CN)₂](ClO₄)₂ and [Fe(tpa)(CH₃CN)₂](ClO₄)₂, respectively.⁷ Similar to previous observations with manganese and cobalt complexes containing bis(imino)pyridine ligands,¹³ the complexes [Mn(1)OTf₂] and [Co(1)OTf₂] showed no catalytic activity under these conditions.

When using 70 mM solutions of hydrogen peroxide, the complexes [Fe(1)OTf₂], [Fe(7)OTf₂] and [Fe(9)OTf₂] exhibit higher yields of product (A + K) (10, 9.5 and 14%, respectively) than Fe(OTf)₂(CH₃CN)₂ (3.8%). Although no cyclohexyl hydroperoxide P is observed in the product mixtures, it should be noted that the A/K ratios are small (1.3, 2.1 and 1.9, respectively). This implies that Fenton type chemistry is dominant, although partial involvement of iron-based oxidants is possible. In line with previous results,^13,14 at increased hydrogen peroxide concentration (700 mM) using complex [Fe(1)OTf₂] as the catalyst, the A/K ratio increased and the production of P was observed. The yield of (A + K), however, was seen to decrease dramatically from 10% to 2.6%. This apparent deactivation of the catalyst is assumed to be the result of catalyst decomposition, most likely via oxidation or hydrolysis of the imine functionality.

Considering the moderate activity of the complexes [Fe(7)OTf₂] and [Fe(9)OTf₂], it was rather surprising that [Fe(8)OTf₂] displayed no catalytic activity under the conditions used here. However, as mentioned previously, exposure of an acetonitrile solution of [Fe(8)OTf₂] to air caused the colourless solution to turn yellow–brown within minutes. This colour change probably reflects oxidation by dioxygen and it is the product of oxidation that is found to be unable to catalyse the oxidation. The robustness of the product of oxidation of [Fe(8)OTf₂] indicates either formation of multimetallic oxo-bridged species, or metal-mediated ligand oxidation. The high sensitivity of [Fe(8)OTf₂] to oxidation by dioxygen is likely to be related to the destabilising effect of the methyl groups substituted at the 6-position of the pyridyl rings.

The complex [Fe(12)OTf₂] was found to be a very poor catalyst for the oxidation of cyclohexane with an (A + K) yield and A/K ratio of 3.7% and 2.8, respectively. Additionally, cyclohexyl hydroperoxide (P) was observed in the GC trace of the product mixture. These observations are all indicative of catalysis proceeding predominantly via Fenton-type chemistry.

In conclusion, we have prepared and characterized transition metal complexes of biphenyl-bridged bis(pyridylmethylimine) and bis(pyridylmethylamine) ligands. The twist of the biphenyl bridge was shown in the case of the iron(II) bis(triflate) complexes containing the bis(imine) ligand 1 to give a distorted cis-α coordination geometry, whereas the amine ligand 9, rather unexpectedly, gave a trans geometry. A similar geometry is assumed for the unsubstituted derivative 7. The introduction of methyl groups in the 6 position of the pyridyl groups, as in [Fe(8)OTf₂], is believed to distort this trans geometry to give a cis-α geometry. This complex is inherently less stable than the previous complexes, leading to an increased sensitivity to air oxidation. Complex [Fe(10)OTf₂], containing both methyl groups on the pyridine rings and on the biphenyl backbone, was too unstable to be isolated in pure form. The biphenylether ligand 12 forms an iron(II) bis(triflate) complex where, based on ¹⁹F NMR studies, both triflate ligands are coordinated in CH₂Cl₂ solution. This indicates that the ether oxygen is not coordinated and the coordination geometry is most likely trans. The catalytic properties of the iron(II) complexes for the oxidation of cyclohexane with H₂O₂ in acetonitrile have been evaluated and it can be concluded that all iron(II) bis(triflate) complexes oxidize cyclohexane predominantly via a radical chain auto-oxidation mechanism, which results typically in low conversions (<10% of the oxidant), low selectivity (A/K ≈ 1) and extensive decomposition of H₂O₂. In the case of the bis(imine) ligands this is believed to be due to the sensitivity of the imine moieties to oxidation or hydrolysis, whereas the biphenyl bridged bis(amine) ligands give rise to complexes with the unfavourable trans geometry. These results highlight the importance of a cis disposition of the labile co-ligands in non-heme alkane oxidation catalysis, i.e. the use of tetradentate ligands that enforce a rigid cis-α coordination geometry around the iron centre.

Experimental

General

All moisture-sensitive compounds were manipulated using standard vacuum line, Schlenk or cannula techniques, or in a conventional nitrogen-filled glove box. NMR spectra were recorded either on a Bruker AC-250 or a DRX-400 spectrometer; chemical shifts for ¹H and ¹³C NMR are referenced to the residual protio impurity and to the ¹³C-NMR signal of the deuterated solvent. Mass spectra were recorded on either a VG Autospec or a VG Platform II spectrometer. Elemental analyses were performed by the Science Technical Support Unit at the London Metropolitan University. GC analysis was carried out on an Agilent 6890A gas chromatograph with a HP-5 column (30 m × 0.32 mm, film thickness 0.25 µm). Toluene was used as the standard for quantitative analysis. Magnetic moments were determined by the Evans’ NMR method in CD₂Cl₂, unless otherwise stated.³⁹

Solvents and reagents

Toluene and pentane were dried by passing through a column, filled with commercially available Q-5 reagent (13 wt% CuO on alumina) and activated alumina (pellets, 3 mm). Diethyl ether and tetrahydrofuran were dried over potassium metal with a benzophenone ketyl indicator, whereas dichloromethane and acetonitrile were dried over CaH₂. The metal complex precursors Mn(OTf)₂ and Fe(OTf)₂(CH₃CN)₂,^40,41 and N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)diamine⁴² and N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylmethyl)diimine (1)²⁵ as well as the cobalt complex [Co(1)Cl₂]²⁶ have been reported previously. All other chemicals and NMR solvents were obtained commercially and used as received.

Synthesis of ligands

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylethyl)diimine (2). 0.56 g of 2,2′-diamino-6,6′-dimethylbiphenyl (2.64 mmol), 2.2 equiv. of 2-acetylpyridine (0.65 ml, 5.80 mmol), 2.2 equiv. of tetraethoxy silane (1.29 ml, 5.80 mmol) and 1 drop of sulfuric acid were heated, in a flask equipped for distillation, at 160 °C for 16 h. Upon cooling, the residue was dissolved in diethyl ether and washed with saturated sodium hydrogen carbonate solution. The organic layer was separated, washed with water, dried (magnesium sulfate) and reduced to dryness. The residue was recrystallised from ethanol to give 2 as a brick red crystalline solid (0.49 g, 44.3%). ¹H-NMR (CDCl₃): δ 8.54 (d, 2H, 6-PyH), 7.60 (m, 4H, 3-PyH and 4-PyH), 7.24 (m, 2H, 5-PyH), 7.17 (t, 2H, PhH_p), 7.02 (d, 2H, PhH_m), 6.55 (d, 2H, PhH_m), 2.27 (s, 6H, MeC=N), 2.13 (s, 6H, PhMe); MS (EI): m/z (%) 418 (100), 403 (34), 340 (45) [M–Py], 285 (48), 202 (47); Anal. Calcd. (found) for C₂₈H₂₆N₄: C, 80.35 (80.47); H, 6.26 (6.23); N, 13.39 (13.28).

N,N′-(biphenyl-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)diamine (3). Two equiv. of 2-pyridine carboxaldehyde (0.52 ml, 5.42 mmol) was added to a mixture of 1 equiv. of 2,2′-diamino-biphenyl (0.50 g, 2.71 mmol) and 2.8 equiv. of sodium trisacetoxyborohydride (1.61 g, 7.60 mmol) in DCM and stirred overnight, under a nitrogen atmosphere. Saturated aqueous sodium hydrogen carbonate solution was added to the resultant mixture, stirred for 15 min and extracted with ethyl acetate. The organic layer was dried (magnesium sulfate) and reduced to dryness. The oil obtained was dissolved in diethyl ether, filtered to remove any insoluble material and reduced to dryness to give 3 as a pale yellow oil (0.70 g, 70%). Compounds 4, 5 and 6 were also prepared using this procedure in 97%, 96% and 92% yield, respectively. Analytical data was identical to that previously reported.^25,28

N,N′-(biphenyl-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyldiamine (7). 2.29 ml of aqueous formaldehyde (37%) was added to a solution of 0.68 g (1.86 mmol) of 3 and 7 ml of acetic acid in acetonitrile (50 ml), and allowed to stir for 30 min. Subsequently, 0.47 g (12.4 mmol) of sodium borohydride was added and the resultant mixture was stirred overnight. All solvents were removed and the residue was made strongly basic with 3 M aqueous sodium hydroxide. This aqueous mixture was extracted several times with DCM. The organic layers were combined, dried (magnesium sulfate) and reduced to dryness to give 7 as a pale yellow oil (0.58 g, 79.2%). ¹H-NMR (CDCl₃): δ 8.40 (d, 2H, 6-PyH), 7.41 (t, 2H, 4-PyH), 7.33 (m, 4H, 5-PyH and 6-PhH), 7.17 (d, 2H, 3-PyH), 7.07 (t, 4H, 4-PhH and 5-PhH), 6.76 (d, 2H, 3-PhH), 4.11 (dd, 4H, PhNCH₂), 2.54 (s, 6H, NMe). ¹³C-NMR (CDCl₃): δ 159.6, 151.1, 148.5, 136.2, 135.5, 132.0, 128.0, 122.4, 122.0, 121.6, 120.1, 62.8 (NCH₂), 40.3 (NMe). MS (EI): m/z (%) 394 (19) [M⁺], 317 (36) [(M − C₅H₃N)⁺], 302 (28) [(M–PyCH₂)⁺], 225 (62) [(M − (Py)(PyCH))⁺], 209 (46) [(M − (PyCH₂)(PyCH₃))⁺], 194 (65) [(M − (PyCH₃)(PyCH₂)(CH₃))⁺], 180 (39) [(M − (PyCH₂NH)(PyCH₃))⁺], 93 (100) [(PyCH₃)⁺].

N,N′-(biphenyl-2,2′-diyl)-N,N′-bis[(6-methyl-2-pyridylmethyl)]-N,N′-dimethyldiamine (8). 8 was prepared using the same procedure as for 7 from 1.20 g (3.04 mmol) of 4, 16.5 ml of acetic acid, 110 ml of acetonitrile, 5.36 ml of aqueous formaldehyde (37%) and 1.09 g (28.8 mmol) of sodium borohydride to give 8 as a pale yellow oil (0.84 g, 65.4%). ¹H-NMR (CDCl₃): δ 7.32 (m, 4H, 4-PyH and 4-PhH), 7.24 (d, 2H, 6-PhH), 7.14 (d, 2H, 3-PyH), 7.02 (t, 2H, 5-PhH), 6.89 (d, 2H, 5-PyH), 6.63 (d, 2H, 3-PhH), 4.10 (dd, 4H, NCH₂), 2.53 (s, 6H, NMe), 2.45 (s, 6H, PyMe). ¹³C-NMR (CDCl₃): δ 159.1, 157.1, 151.1, 136.4, 135.2, 132.0, 127.9, 122.1, 121.0, 120.0, 118.7, 62.6 (NCH₂), 40.4 (NMe), 24.3 (PyMe). MS (EI): m/z (%) 422 (16) [M⁺], 316 (20) [(M − PicCH₂)⁺], 223 (24) [(M − (PicH)(PicCH₂))⁺], 209 (18) [(M − (PicCH₂)(PicCH₃))⁺], 194 (34) [(M − (PicCH₃)(PicCH₂)(CH₃))⁺], 107 (100) [(PicCH₃)⁺].

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyldiamine (9). 9 was prepared using the same procedure as for 7 from 0.47 g (1.19 mmol) of 5, 3.0 ml of acetic acid, 20 ml of acetonitrile, 0.98 ml of aqueous formaldehyde (37%) and 0.20 g (5.29 mmol) of sodium borohydride to give 9 as a pale yellow oil (0.43 g, 85.5%). ¹H-NMR (CDCl₃): δ 8.37 (d, 2H, 6-PyH), 7.36 (t, 2H, 4-PyH), 7.23 (d, 2H, 3-PyH), 7.17 (d, 2H, 3-PhH), 7.00 (m, 4H, 5-PyH and 4-PhH), 6.39 (d, 2H, 5-PhH), 3.96 (dd, 4H, PhNCH₂), 2.41 (s, 6H, NMe), 2.03 (s, 6H, PhMe). ¹³C-NMR (CDCl₃): δ 160.0 (ipso), 152.2, 148.2, 147.5, 136.3, 135.5, 127.5, 125.4, 121.8, 121.5, 118.7, 63.4 (NCH₂), 40.5 (NMe), 20.2 (PhMe). MS (EI): m/z (%) 422 (45) [M⁺], 407 (11) [(M − Me)⁺], 330 (100) [(M − PyCH₂)⁺], 237 (44) [(M− (PyCH₃)(PyCH₂))⁺], 223 (32) [(M − (PyCH₂NMe)(Py))⁺], 208 (23) [(M − (PyCH₂NMe)(PyMe))⁺], 93 (76) [(PyCH₂NH)⁺].

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)-N,N′-bis[(6-methyl-2-pyridylmethyl)]-N,N′-dimethyldiamine (10). 10 was prepared using the same procedure as for 7 from 0.88 g (2.08 mmol) of 6, 6.65 ml of acetic acid, 45 ml of acetonitrile, 2.17 ml of aqueous formaldehyde (37%) and 0.44 g (11.6 mmol) of sodium borohydride to give 10 as a pale yellow oil (0.61 g, 65.2%). ¹H-NMR (CDCl₃): δ 7.24 (m, 4H, 4-PyH and 4-PhH), 7.12 (d, 2H, 3-PyH), 6.97 (d, 2H, 5-PyH), 6.87 (d, 2H, 5-PhH), 6.27 (d, 2H, 3-PhH), 3.95 (dd, 4H, PhNCH₂), 2.45 (s, 6H, PyMe), 2.41 (s, 6H, NMe), 2.01 (s, 6H, PhMe). ¹³C-NMR (CDCl₃): δ 159.5, 156.7, 152.3, 137.5, 136.4, 135.6, 127.6, 125.3, 123.8, 120.9, 118.7, 63.4 (NCH₂), 40.7 (NMe), 24.3 (PyMe), 20.3 (PhMe). MS (EI): m/z (%) 450 (4) [M⁺], 359 (32) [(M − Pic)⁺], 344 (25) [(M − PicCH₂)⁺], 222 (18) [(M − (PicCH₃)(PicCH₂)(CH₃))⁺], 107 (100) [(PicCH₃)⁺].

N,N′-(biphenylether-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)diamine (11). 11 was prepared using the same procedure as for 3 from 0.84 g (4.19 mmol) bis(2-aminophenyl)ether, 80 ml (8.39 mmol) 2-formylpyridine and 2.49 g (11.7 mmol) sodium triacetoxyborohydride to give a pale yellow oil (1.60 g, 88%). ¹H-NMR (CDCl₃): δ 8,54 (d, 2H, 6-PyH), 7.60 (t, 2H, 4-PyH), 7.30 (d, 2H, 3-PyH), 7.15 (t, 2H, 5-PyH), 6.96 (t, 2H, 5-PhH), 6.83 (d, 2H, 3-PhH), 6.64 (t, 4H, 4-PhH and 6-PhH), 5.29 (broad s, 2H, NH), 4.54 (s, 2H, NCH₂). ¹³C-NMR (CDCl₃): δ 158.9, 149.3, 143.7, 139.4, 136.7, 124.3, 122.1, 121.3, 117.6, 117.1, 111.6, 49.2 (NCH₂). MS (EI): m/z (%) 382 (22) [M⁺], 291 (50) [(M − PyCH)⁺], 211 (36) [(M − (PyH)(PyCH₂))⁺], 183 (100) [(PhNHCH₂Py)⁺], 93 (50) [(PyCH₃)⁺].

N,N′-(biphenylether-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyldiamine (12). 12 was prepared using the same procedure as for 7 from 1.50 g (3.92 mmol) of 11, 10.0 ml of acetic acid, 70.0 ml of acetonitrile, 3.46 ml of aqueous formaldehyde (37%) and 0.70 g (18.5 mmol) of sodium borohydride to give a pale yellow oil (1.15 g, 72%). ¹H-NMR (CDCl₃): δ 8.46 (d, 2H, 6-PyH), 7.44 (t, 2H, 4-PyH), 7.18 (d, 2H, 3-PyH), 7.04 (m, 6H, 4-, 5-, and 6-PhH), 6.89 (m, 2H, 5-PyH), 6.78 (d, 2H, 3-PhH), 4.39 (s, 4H, NCH₂), 2.70 (s, 6H, NMe). ¹³C-NMR (CDCl₃): δ 159.7 (ipso), 149.3, 148.7, 143.6, 136.4, 129.4, 123.7, 122.0, 121.7, 119.4, 119.3, 61.5 (NCH₂), 39.9 (NMe). MS (EI): m/z (%) 410 (10) [M⁺], 333 (27) [(M–C₅H₃N)⁺], 318 (10) [(M − PyCH₂)⁺], 254 (100) [(M − (Py)₂)⁺], 210 (29) [(M − (PyCH₂)(PyCH₃)(CH₃))⁺], 197 (38) [(PhNMeCH₂Py)⁺], 134 (50) [(OPhN(CH₂)₂⁺], 120 (65) [(PhNMe₂)⁺], 93 (72) [(PyCH₃)⁺].

Synthesis of complexes

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylmethyl)diimine manganese(II) bis(triflate).
[Mn(1)(OTf)₂]. A solution of 0.30 g (0.77 mmol) of the ligand 1 and 0.27 g (0.77 mmol) of Mn(OTf)₂ in 25 ml of DCM was stirred overnight, under a nitrogen atmosphere. The solution was then filtered, reduced to approximately 5 ml, under vacuum, and pentane added to precipitate the product. This solid was subsequently washed with diethyl ether and dried under vacuum to give [Mn(1)(OTf)₂] as a yellow powder (0.31 g, 55%). ¹⁹F-NMR (CD₃CN): δ −53 (ν_1/2 ≈ 4000 Hz). MS (+FAB): m/z (%) 743 [M⁺], 594 [(M − OTf)⁺], 445 [(M − (OTf)₂)⁺]; Anal. Calcd. (found) for C₂₈H₂₂F₆MnN₄O₆S₂: C, 45.23 (45.19); H, 2.98 (2.91); N, 7.54 (7.55). µ_eff (CD₂Cl₂) = 5.99 µ_B.

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylmethyl)diimine iron(II) bis(triflate).
[Fe(1)OTf₂]. This complex was prepared by a similar procedure as used for complex [Mn(1)OTf₂] using a solution of 0.40 g (1.02 mmol) of 1 and 0.45 g (1.02 mmol) of Fe(OTf)₂(CH₃CN)₂ in 25 ml of DCM to give [Fe(1)OTf₂] as a burgundy powder (0.66 g, 86%). ¹H-NMR (CD₂Cl₂): δ 180.2 (2H, N=CH), 142.0 (2H, PyHα), 52.5 (2H, PyHβ), 44.3 (2H, PyHβ′), 13.6 (2H, PyHγ), 9.9 (2H, 4-PhH), −2.4 (6H, PhMe), −7.1 (4H, 3-PhH and 5-PhH); MS (+FAB): m/z 744 [M⁺], 595 [(M − OTf)⁺], 446 [(M − (OTf)₂)⁺]; Anal. Calcd. (found) for C₂₈H₂₂F₆FeN₄O₆S₂: C, 45.17 (45.22); H, 2.98 (2.89); N, 7.53 (7.60). µ_eff (CD₂Cl₂) = 4.95 µ_B.
Crystal data for [Fe(1)OTf₂]. C₂₈H₂₂F₆FeN₄O₆S₂, M = 744.47, monoclinic, C2/c (no. 15), a = 16.7167(13), b = 11.4815(8), c = 16.6129(10) Å, β = 95.470(5)°, V = 3174.0(4) ų, Z = 4 (C₂ symmetry), D_c = 1.558 g cm⁻³, µ(Mo-Kα) = 0.690 mm⁻¹, T = 293 K, orange/red prisms; 2787 independent measured reflections, F² refinement, R₁ = 0.043, wR₂ = 0.099, 2119 independent observed absorption-corrected reflections [|F_o| > 4σ(|F_o|), 2θ_max = 50°], 242 parameters. CCDC 270626. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b513886h.

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylethyl)diimine iron(II) bis(triflate).
[Fe(2)OTf₂]. This complex was prepared by a similar procedure as used for complex [Mn(1)OTf₂] using 2 (0.20 g, 0.48 mmol) and Fe(OTf)₂(CH₃CN)₂ (0.21 g, 0.48 mmol) in DCM to give [Fe(2)OTf₂] as a dark red solid (0.24 g, 64%). ¹H-NMR (CD₂Cl₂): δ 130.1 (2H, PyHα), 65.1 (2H, PyHβ), 49.3 (2H, PyHβ′), 15.0 (6H, N=C–Me), 11.2 (2H, 4-PhH), 11.1 (2H, PyHγ), −3.9 (6H, PhMe), −11.7 (2H, 5-PhH), −18.5 (4H, 3-PhH); ¹⁹F-NMR (CD₂Cl₂): δ −25 (ν_1/2 ≈ 530 Hz). ¹⁹F-NMR (CD₃CN): δ −78.3 (ν_1/2 = 80 Hz). MS (+FAB): m/z 772 [M⁺], 623 [(M − OTf)⁺], 474 [(M − (OTf)₂)⁺]; Anal. Calcd. (found) for C₂₆H₂₂Cl₂FeN₄: C, 46.64 (46.80); H, 3.39 (3.25); N, 7.25 (7.13). µ_eff (CD₂Cl₂) = 5.08 µ_B.

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylmethyl)diimine cobalt(II) bis(triflate).
[Co(1)(OTf)₂]. A mixture of 0.19 g (0.37 mmol) of [Co(1)Cl₂] and 0.19 g of silver(I) triflate (0.73 mmol) in 80 ml of DCM was stirred overnight, under a nitrogen atmosphere. The solution was then filtered through Celite and the Celite washed with further DCM. The filtrate was reduced to dryness and a Soxhlet extraction, using DCM, performed on the solid obtained. The DCM solution obtained was reduced in volume to approximately 5 ml, under vacuum, and pentane added to precipitate the product. This mixture was subsequently filtered and the solid washed with diethyl ether and dried under vacuum to give [Co(1)(OTf)₂] as an orange–yellow powder (0.14 g, 58.7%). ¹H-NMR (CD₂Cl₂): δ 144.0 (2H, N=CH), 111.2 (2H, PyHα), 80.1 (2H, PyHβ), 60.5 (2H, PyHβ′), 23.1 (2H, PyHγ), 2.6 (2H, 4-PhH), −3.7 (6H, PhMe), −14.6 (2H, 5-PhH), −52.3 (2H, 3-PhH); ¹⁹F-NMR (CD₂Cl₂): δ −39 (ν_1/2 ≈ 2000 Hz). ¹⁹F-NMR (CD₃CN): δ −73.1 (ν_1/2 = 46 Hz). MS (+FAB): m/z 747 [M⁺], 598 [(M − OTf)⁺], 449 [(M − (OTf)₂)⁺]; Anal. Calcd. (found) for C₂₈H₂₂CoF₆N₄O₆S₂: C, 44.99 (45.17); H, 2.97 (2.80); N, 7.49 (7.35). µ_eff (CD₂Cl₂) = 3.86 µ_B.

N,N′-(biphenyl-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyldiamine iron(II) bis(triflate).
[Fe(7)(OTf)₂]. A mixture of 0.55 g (1.39 mmol) of 7 and 0.55 g (1.27 mmol) of Fe(OTf)₂(CH₃CN)₂ was stirred in DCM (50 ml) overnight. The mixture obtained was reduced in volume and pentane added to precipitate a solid. The solid was washed with diethyl ether and dried under vacuum. It was, subsequently, recrystallised by slow diffusion from a DCM solution layered with pentane to give [Fe(7)(OTf)₂] as an off-white crystalline solid (0.53 g, 55%). ¹H-NMR (CDCl₃): δ 90.3 (8H, PyHα and NMe), 69.4 (4H, PyCH₂N and PyHβ), 55.6 (2H, PyCH₂N), 47.4 (2H, PyHβ′), 8.9 (2H, PyHγ), 4.5 (2H, 4-PhH), 3.5 (2H, 3-PhH), −2.5 (2H, 6-PhH), −10.7 (2H, 5-PhH). ¹H-NMR (CD₃CN): very complex spectrum arising from multiple species in solution. ¹⁹F-NMR (CDCl₃): δ −11.1. ¹⁹F-NMR (CD₃CN): δ −71.8. MS (+FAB): m/z (%) 748 [M⁺], 599 [(M − OTf)⁺], 395 [(M − Fe(OTf)₂)⁺]. Anal. Calcd. (found) for C₂₈H₂₆F₆FeN₄O₆S₂: C, 44.93 (44.54); H, 3.50 (3.36); N, 7.49 (7.33). µ_eff (CD₂Cl₂) = 5.23 µ_B.

N,N′-(biphenyl-2,2′-diyl)-N,N′-bis[(6-methyl-2-pyridylmethyl)]-N,N′-dimethyl-diamine iron(II) bis(triflate).
[Fe(8)(OTf)₂]. Similar procedure as for [Fe(7)(OTf)₂] using 0.74 g (1.75 mmol) of 8 and 0.69 g (1.59 mmol) of Fe(OTf)₂(CH₃CN)₂, to give [Fe(8)(OTf)₂] as an off-white crystalline solid (0.67 g, 55%). ¹H-NMR (CDCl₃): δ 112.6 (6H, NMe), 72.2 (2H, PyCH₂), 55.1 (2H, PyHβ), 53.7 (2H, PyHβ′), 8.3 (2H, PyHγ), 5.8 (2H, 4-PhH), −1.0 (2H, 6-PhH), −9.6 (2H, 5-PhH), −16.4 (4H, 3-PhH and PyCH₂), −29.7 (6H, PyMe). ¹H-NMR (CD₃CN): δ 139.3 (2H, PyCH₂), 133.8 (6H, NMe), 71.3 (2H, PyHβ), 64.9 (2H, PyHβ′), 9.9 (2H, PyHγ), 1.5 (2H, 4-PhH), −16.6 (2H, 6-PhH), −18.5 (2H, 5-PhH), −24.2 (2H, PyCH₂), −32.7 (2H, 3-PhH), −62.4 (6H, PyMe). ¹⁹F-NMR (CDCl₃): δ −20.5. ¹⁹F-NMR (CD₃CN): δ −76.7. MS (+FAB): m/z (%) 573 [(M − (C₂F₆O₂S)⁺], 423 [(M − Fe(OTf)₂)⁺]. Anal. Calcd. (found) for C₃₀H₃₀F₆FeN₄O₆S₂: C, 46.40 (46.58); H, 3.89 (3.82); N, 7.22 (7.16). µ_eff (CD₂Cl₂) = 5.10 µ_B.

N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyl-diamine iron(II) bis(triflate).
[Fe(9)(OTf)₂]. Similar procedure as for [Fe(7)(OTf)₂], using 0.43 g (1.02 mmol) of 9 and 0.42 g (0.97 mmol) of Fe(OTf)₂(CH₃CN)₂, to give [Fe(9)(OTf)₂] as an off-white crystalline solid (0.49 g, 65.4%). Crystals suitable for X-ray diffraction were grown by slow diffusion from a DCM solution layered with pentane. ¹H-NMR (CDCl₃): δ 96.2 (2H, PyHα), 92.9 (6H, NMe), 69.8 (4H, PyHβ and PyCH₂N), 53.6 (2H, PyCH₂N), 47.6 (2H, PyHβ′), 8.5 (2H, PyHγ), 5.0 (4H, 4-PhH and 3-PhH), −9.8 (2H, 5-PhH), −10.3 (6H, PhMe). ¹H-NMR (CD₃CN): very complex spectrum arising from multiple species in solution. ¹⁹F-NMR (CDCl₃): δ −12.1. ¹⁹F-NMR (CD₃CN): δ −70.1. MS (+FAB): m/z (%) 776 [M⁺], 627 [(M − OTf)⁺], 423 [(M − Fe(OTf)₂)⁺]. Anal. Calcd. (found) for C₃₀H₃₀F₆FeN₄O₆S₂: C, 46.40 (46.72); H, 3.89 (4.05); N, 7.22 (7.06). µ_eff (CD₂Cl₂) = 5.16 µ_B.
Crystal data for [Fe(9)OTf₂]. C₃₀H₃₀F₆FeN₄O₆S₂, M = 776.55, triclinic, P1̄ (no. 2), a = 10.2990(17), b = 11.717(4), c = 15.587(2) Å, α = 94.10(3), β = 102.471(18), γ = 115.10(2)°, V = 1634.6(7) ų, Z = 2, D_c = 1.578 g cm⁻³, µ(Mo-Kα) = 0.673 mm⁻¹, T = 203 K, colourless blocks; 5645 independent measured reflections, F² refinement, R₁ = 0.062, wR₂ = 0.172, 4137 independent observed absorption-corrected reflections [|F_o| > 4σ(|F_o|), 2θ_max = 50°], 503 parameters. CCDC 270627. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b513886h.

N,N′-(biphenylether-2,2′-diyl)-N,N′-bis(2-pyridylmethyl)-N,N′-dimethyldiamine iron(II) bis(triflate).
[Fe(12)(OTf)₂]. Similar procedure as for [Fe(7)(OTf)₂], using 0.22 g (0.52 mmol) of 12 and 0.20 g (0.46 mmol) of Fe(OTf)₂(CH₃CN)₂, to give [Fe(12)(OTf)₂] as a tan-coloured crystalline solid (0.23 g, 65%). ¹H-NMR (CDCl₃): δ 75.4 (2H, PyHα), 67.5 (2H, PyCH₂), 49.1 (6H, NMe), 48.0 (2H, PyHβ), 42.1 (2H, PyHβ′), 21.3 (2H, PyHγ), 10.7 (2H, 5-PhH), 5.3 (2H, 3-PhH), 2.2 (2H, 4-PhH), −2.4 (2H, 6-PhH). ¹H-NMR (CD₃CN): δ 70.1 (2H, PyHα), 62.0 (2H, PyCH₂), 43.9 (6H, NMe), 42.7 (2H, PyHβ), 36.7 (2H, PyHβ′), 16.0 (2H, PyHγ), 5.1 (2H, 5-PhH), 0.2 (2H, 3-PhH), −2.8 (2H, 4-PhH), −7.4 (2H, 6-PhH). ¹⁹F-NMR (CDCl₃): δ −0.3. ¹⁹F-NMR (CD₃CN): δ −76.8. MS (+FAB): m/z (%) 615 [(M − OTf)⁺], 411 [(M − Fe(OTf)₂)⁺]. Anal. Calcd. (found) for C₂₈H₂₆F₆FeN₄O₇S₂: C, 43.99 (44.12); H, 3.43 (3.63); N, 7.33 (7.11). µ_eff (CD₂Cl₂) = 5.04 µ_B.

Acknowledgements

We are grateful to BP Chemicals Ltd. for a CASE award to J. E. Mr Richard Sheppard and Mr Peter Haycock are thanked for their assistance in NMR measurements.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S10. See DOI: 10.1039/b513886h

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