Iron(II) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Abstract

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L¹), treatment of α,α′-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L²) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L³). Solvated crystals of [Fe(L¹)₂][BF₄]₂ (1[BF₄]₂) and [Fe(L²)₂][BF₄]₂ (2[BF₄]₂) are mostly high-spin, although one solvate of 1[BF₄]₂ undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF₄]₂ which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L³ complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)₂L³]Br·0.5H₂O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L⁴)₂][BF₄]₂ (L⁴ = 2,6-di{quinolin-2-yl}pyridine; 4[BF₄]₂) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF₄]₂ and 2[BF₄]₂ undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.

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Introduction

Spin-crossover (SCO) complexes continue to be heavily studied.^1–5 Controlling spin state properties is vital for the design of new catalysts^6–10 or photosensitisers^11,12 from base metal centres.¹³ Moreover, solid SCO materials are useful testbeds for the crystal engineering of phase transitions in molecular crystals,^14,15 while time-resolved spectroscopic and diffraction methods are shedding new light on the atomistic mechanisms of SCO-induced phase transitions.^16–18 SCO switching in thin films,^19,20 nanoparticles,^21,22 monolayers and single molecules^19,23–25 is well-established, leading to the use of SCO complexes in nanomolecular electronics.^26,27 Lastly, SCO molecules are useful switching components in functional materials which exploit their thermochromism,^28,29 or other materials properties associated with the spin transition.^30–35 Alternatively, SCO auxiliaries can control fluorescence,^36,37 conductivity,³⁸ magnetic^39–41 or ferroelectric⁴² properties in multifunctional composite crystals or soft materials.⁴³

Iron(II) complexes of tridentate tris-heterocyclic ligands are particularly fruitful for SCO chemistry.^44,45 Iron(II) complexes of ligands derived from trispyrazolylborate,⁴⁶ trispyrazolyl-methane,^46,47 2,6-di(1H-benzimidazol-2-yl)pyridine (bimpy)⁴⁸ and regioisomers of 2,6-di(pyrazolyl)pyridine (bpp)^49–52 can all exhibit SCO at accessible temperatures. These ligand classes can be functionalised using appropriate synthetic starting materials or through functional group manipulations, giving steric and electronic control over the spin states of their iron complexes.^{48,49,52–56} It also allows functional substituents, long alkyl chains or tether groups to be appended to SCO complexes, producing multifunctional materials or nanostructures based on SCO switches. Complexes of the bimpy^57,58 and bpp^59–64 ligand families have been particularly useful for the latter goal.

Notwithstanding its multifunctional derivatives, [Fe(bimpy)₂]²⁺ was an early example of a fluorescent SCO complex,⁶⁵ which have been increasingly developed during the last ten years.^36,37 With that in mind, we investigated [Fe(1-bip)₂]²⁺ (1-bip = 2,6-bis{indazol-1-yl}pyridine) and [Fe(2-bip)₂]²⁺ (2-bip = 2,6-bis{indazol-2-yl}pyridine), with the latter being an isosteric analogue of the bimpy complex.⁶⁶ These also exhibit a ligand-based fluorescence in solution at room temperature, but whose wavelength was unaffected by the spin state of the complexes.^66,67

Ruthenium complexes of 2,6-bis(imidazo-[1,2-a]pyridin-2-yl)pyridine (L¹; Scheme 1) and its derivatives have been studied by Song et al., as catalysts for a number of reactions.^68–72 Following our study of bip ligands, which are constitutional isomers of L¹, we now report the iron(II) complex chemistry of L¹ and two new analogues, 2,6-bis(imidazo-[1,2-a]pyrimidin-2-yl)pyridine (L²) and 2,6-bis(imidazo-[1,2-a]quinolin-2-yl)pyridine (L³; Scheme 1). We also include a re-investigation of previously reported [Fe(L⁴)₂]²⁺ (L⁴ = 2,6-di{quinol-2-yl}pyridine, dqp).⁷³

Scheme 1 The ligands investigated in this study.

Results and discussion

Reaction of 2,6-diacetylpyridine with N-bromosuccinimide in refluxing acetonitrile affords α,α′-dibromo-2,6-diacetylpyridine, in moderate yield. Treatment of that intermediate with 2 equiv. 2-aminopyridine in refluxing acetonitrile gives L¹ (Scheme 1), in ca. 60% yield after purification.⁶⁸ The latter reaction is sensitive to the purity of the α,α′-dibromo-2,6-diacetylpyridine starting material, which should be synthesised in situ and used immediately for the best results. We also screened other 2-aminoazine reagents in this protocol, which afforded L² (from 2-aminopyrimidine) and L³ (from 2-aminoquinoline) in NMR purity. An attempted synthesis of 2,6-bis(imidazo[1,2-a]pyrazinyl)pyridine from aminopyrazine by this method gave a product that was too insoluble to characterise, however (ESI†). Ligand L⁴ was synthesised for this study by a literature procedure.^73,74

Treatment of Fe[BF₄]₂·6H₂O with 2 equiv. L¹ or L² in acetonitrile affords [Fe(L¹)₂][BF₄]₂ (1[BF₄]₂) and [Fe(L²)₂][BF₄]₂ (2[BF₄]₂) as brown polycrystalline solids after the usual work up. Both complexes form solvated single crystals and, while 2[BF₄]₂ was obtained as a solvent-free material after drying in vacuo, samples of 1[BF₄]₂ always contained residual solvent or lattice water by microanalysis. Since the spin state properties of some solvated iron complexes depend significantly on the lattice solvent,^75–81 multiple solvates of 1[BF₄]₂ and 2[BF₄]₂ were isolated for characterisation.

Four crystalline solvates of 1[BF₄]₂ were characterised by X-ray diffraction, three of which can be considered together. The asymmetric unit of 1[BF₄]₂·MeCN and 1[BF₄]₂·mMeNO₂ (m ≈ 0.8; both orthorhombic, space group Pccn, Z = 4) contains half a formula unit. Their iron atom lies on a crystallographic C₂ axis, so the complex molecule has perfect C₂ symmetry (Fig. 1). The solvate 1[BF₄]₂·1.5MeOH (orthorhombic, Pna2₁, Z = 4) has similar unit cell dimensions and crystal packing as the first two solvates, but lacks their crystallographic inversion symmetry. All three solvates contain high-spin complex molecules at 120 K, with near-regular coordination geometries showing minor deviations from idealised D_2d symmetry (Table 1). The cations associate into zig-zag chains through strong face-to-face π⋯π interactions, involving one imidazopyridyl arm of each ligand. The chains alternate down the c axis, along the [11̄0] and [1̄10] crystal vectors in the Pccn solvates (the unit cell axes are exchanged in the Pna2₁ setting).

Fig. 1 Two [Fe(L¹)₂]²⁺ molecules in 1[BF₄]₂·MeCN, related by 1 − x, ½ + y, ½ − z. The intermolecular C–H⋯π contact that may inhibit thermal SCO in the three near-isomorphous solvates of 1[BF₄]₂ is shown in grey, which is positioned to inhibit displacement of those imidazopyridyl groups in the direction shown which would accompany SCO. A space-filling view of this interaction is in Fig. S10.† Colour code: C, white; H, pale grey; Fe, green; N, blue.

Table 1 Selected bond lengths and angular parameters for the solvate crystals of 1[BF₄]₂ at 120 K (Å, °, ų). A full list of bond lengths and angles is in Table S2 (ESI†)^a

	1[BF4]2·MeCN^b	1[BF4]2·mMeNO2^b	1[BF4]1.6[SiF6]0.2·1.7MeNO2·0.3Et2O	1[BF4]2·MeOH
a V Oh, Σ and Θ are indices showing the spin state of the complex,^82,83 while ϕ and θ measure the orientations of the two tridentate ligands in the molecule.^84,85 Typical values for these parameters in complexes related to 1[BF4]2 are given in ref. 51 and 52. b The complex cations in these crystals have crystallographic C2 symmetry.
Fe–N{pyridyl}	2.175(3)	2.168(3)	1.920(3), 1.924(3)	2.171(4), 2.171(4)
Fe–N{imidazopyridyl}	2.191(3), 2.199(4)	2.195(3), 2.199(3)	1.979(3)–2.002(3)	2.193(4)–2.201(4)
V Oh	12.901(15)	12.873(11)	9.896(8)	12.908(15)
Σ	144.4(5)	145.2(4)	81.5(4)	143.5(5)
Θ	473	476	268	470
ϕ	176.42(18)	175.81(14)	178.78(11)	175.96(14)
θ	89.66(3)	89.87(2)	88.89(2)	89.93(3)

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As discussed below, there is nothing in the molecular geometry of these three solvates that should prevent thermal SCO on cooling. Rather, we attribute their high-spin nature to an intermolecular C–H⋯π contact between molecules in neighbouring cation chains (Fig. 1). While the interaction is not notably short, it is positioned to inhibit the displacement of two of the imidazo-[1,2-a]pyridin-2-yl groups that would accompany contraction of those Fe–N bonds during SCO.

While samples of 1[BF₄]₂·MeCN and 1[BF₄]₂·1.5MeOH were homogeneous, red needles of 1[BF₄]₂·mMeNO₂ were often mixed with small black prisms which had the crystallographic composition 1[BF₄]_1.6[SiF₆]_0.2·1.7MeNO₂·0.3Et₂O (triclinic, P1̄, Z = 2). The fractional SiF₆²⁻ content of these crystals should arise from reaction of the silica crystallisation vials with adventitious F⁻, produced by hydrolysis of BF₄⁻ during the crystallisation process.⁸⁶ The complex in this crystal is low-spin at 120 K from its metric parameters, which confirms the low-spin state of 1[BF₄]₂ is thermodynamically accessible (Table 1).

1[BF ₄ ] _1.6 [SiF ₆ ] _0.2 ·1.7MeNO₂·0.3Et₂O differs from the other solvates of that complex, in adopting a ‘terpyridine embrace’ packing motif.⁸⁷ The complex cations interdigitate into four-fold layers in the (1̄10) crystal plane, through edge-to-face and face-to-face π⋯π interactions involving all their imidazopyridyl residues. Nearest neighbour molecules within the layers are related by crystallographic inversion centres, while the layers propagate in 3D by translation along a and b. While they are not perfectly isomorphous, the crystal packing in 1[BF₄]_1.6[SiF₆]_0.2·1.7MeNO₂·0.3Et₂O resembles its regioisomer [Fe(2-bip)₂][BF₄]₂·2MeNO₂, which adopts a closely related terpyridine embrace packing motif.⁶⁶

The solvates of 1[BF₄]₂ retain crystallinity by powder diffraction upon exposure to air (Fig. S15†). Magnetic susceptibility data show 1[BF₄]₂·MeCN and 1[BF₄]₂·1.5MeOH are both high-spin between 5–300 K, which is consistent with their crystal structures at 120 K. Another sample crystallised in small quantities from acetone/diethyl ether was also high-spin (Fig. S16†). The small crystal size of the MeNO₂ solvates made it impossible to separate them manually for magnetic characterisation. However, a mixed-phase sample was high-spin at room temperature with an abrupt, partial spin-transition at T_½ = 178 K occurring in ca. 45% of the material. That is consistent with a mixture of high-spin and SCO-active phases, as observed crystallographically. Iron(II) complexes related to 1[BF₄]_1.6[SiF₆]_0.2·1.7MeNO₂·0.3Et₂O with terpyridine embrace lattices often show similarly abrupt thermal spin-transitions.^66,88,89

Three solvates of 2[BF₄]₂ were also crystallised: isomorphous 2[BF₄]₂·1.5MeCN and 2[BF₄]₂·Me₂CO (both triclinic, P1̄, Z = 2) and 2[BF₄]₂·3.5MeNO₂·0.5Et₂O (also triclinic, P1̄, Z = 2 but not isomorphous with the other solvates). These are all crystallographically high-spin at 120 K. However, in contrast to 1[BF₄]₂, the complex coordination geometry in the 2[BF₄]₂ solvates is significantly distorted from its idealised D_2d symmetry (Fig. 2). This is particularly reflected in the trans-N{pyridyl}–Fe–N{pyridyl} angle (ϕ, Table 2) which lies between 156.26(12)–164.72(7)°, significantly lower than its ideal value of 180°. This type of distortion is well-known in high-spin [Fe(bpp)₂]²⁺ derivatives, and reflects an angular Jahn–Teller distortion of the complex along the O_h–D_3d coordinate.^51,52 The distortion has a shallow energy profile,^85,89,90 and [Fe(bpp)₂]²⁺ derivatives with 148 ≤ ϕ ≤ 180° have been reported.^91,92

Fig. 2 A centrosymmetric pair of [Fe(L²)₂]²⁺ molecules in 2[BF₄]₂·1.5MeCN, showing the intramolecular n⋯π, intermolecular π⋯π and intermolecular C–H⋯N contacts involving the L² ligands. Colour code: C, white; H, pale grey; Fe, green; N, blue.

Table 2 Selected bond lengths and angular parameters for the solvate crystals of 2[BF₄]₂ at 120 K (Å, °, ų). A full list of bond lengths and angles is in Table S4 (ESI†). See Table 1 and the ESI† for definitions of the structural indices^82–85

	2[BF4]2·1.5MeCN	2[BF4]2·Me2CO	2[BF4]2·3.5MeNO2·0.5Et2O
Fe–N{pyridyl}	2.148(2), 2.154(2)	2.1499(16), 2.1538(16)	2.160(3), 2.162(3)
Fe–N{imidazopyrimidyl}	2.183(2)–2.201(2)	2.1849(18)–2.2076(17)	2.196(3)–2.212(3)
V Oh	12.536(10)	12.586(6)	12.430(12)
Σ	140.3(3)	140.4(2)	144.8(4)
Θ	467	463	482
ϕ	163.52(9)	164.72(7)	156.26(12)
θ	87.59(3)	84.65(1)	88.37(2)

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Since the distortion is a property of the high-spin state, SCO in a distorted complex requires a significant structural rearrangement to the more regular coordination geometry preferred by the low-spin form. Hence, SCO becomes more disfavoured as ϕ (and θ, Table 2) deviate increasingly from their ideal values in crystalline complexes.^85,90 The distortions in 2[BF₄]₂ lie in the range where SCO is rarely observed (Fig. 3).^91,92 Consistent with that, all the solvates of 2[BF₄]₂ remain high-spin between 5–300 K from magnetic susceptibility data (Fig. S25†).

Fig. 3 Distortion parameters for [Fe(1-bpp)₂]²⁺ (1-bpp = 2,6-di{pyrazol-1-yl}pyridine) derivatives which are low-spin (grey triangles); high-spin and SCO-active (yellow squares); or remain high-spin on cooling (cyan circles).^84,85 Compounds in this work are plotted with the same symbols in darker colouration. The graph is adapted from ref. 92.

These distorted coordination geometries facilitate two different supramolecular interactions involving the L² ligands (Fig. 2). First, are intramolecular n⋯π contacts between the imidazopyrimidinyl N8 atom of one ligand, and a pyridyl C–N bond of the other ligand.⁹³ Second, is intermolecular π⋯π overlap between nearest neighbour molecules in the lattice. This is always accompanied by a weak intermolecular C–H⋯N contact to the other imidazopyrimidinyl N8 atom acceptor, from the same ligand that forms the intramolecular n⋯π interaction. One ligand in each molecule in isomorphous 2[BF₄]₂·1.5MeCN and 2[BF₄]₂·MeCO takes part in these interactions, associating the cations into centrosymmetric pairs. In contrast, both ligands in each molecule of 2[BF₄]₂·3.5MeNO₂·0.5Et₂O form the same set of pairwise interactions, affording zig-zag chains of strongly linked cations (Fig. S20 and S21†). The larger ϕ distortion in this solvate facilitates close approach of the neighbour molecule forming its second intramolecular n⋯π interaction.

Samples of 2[BF₄]₂·1.5MeCN and 2[BF₄]₂·Me₂CO retain crystallinity on exposure to air, but 2[BF₄]₂·3.5MeNO₂·0.5Et₂O decomposes through solvent loss to a poorly crystalline red powder. All these materials are high-spin above 5 K, as expected from their crystallographic coordination geometries (Fig. 3).

In contrast to L¹ and L², treatment of Fe[BF₄]₂·6H₂O with 2 equiv. L³ yields brown solids whose microanalyses are consistent with the formulae [FeL³][BF₄]₂·y(solvent); that is, with a 1 : 1 Fe : L³ stoichiometry. Consistent with that, freshly prepared solutions of a 1 : 2 molar mixture of Fe[BF₄]₂·6H₂O and L³ contain a significant amount of uncoordinated ligand by NMR, which is not observed in solutions of 1[BF₄]₂ and 2[BF₄]₂. Single crystals were ultimately obtained from a sample of formula [FeBr(py)₂L³]Br·0.5H₂O (py = pyridine; Fig. 4). The tridentate L³ ligand in that complex displays no obvious steric hindrance to explain its reluctance to form a homoleptic iron complex. Other physical characterisation was performed on a sample with the analytical formula [FeL³][BF₄]₂·H₂O, labelled 3′[BF₄]₂·H₂O.

Fig. 4 The complex cation in [FeBr(py)₂L³]Br·0.5H₂O. Displacement ellipsoids are at the 50% probability level, and H atoms are omitted for clarity. Colour code: C, white; Br, orange; Fe, green; N, blue.

Lastly, in 1969 Sinn et al. reported that [Fe(L⁴)₂][ClO₄]₂ (4[ClO₄]₂) is high-spin above 85 K in the solid state.⁷³ We have re-investigated this complex as its BF₄⁻ salt, 4[BF₄]₂. Single crystals of 4[BF₄]₂·1.39MeCN·0.125Et₂O·0.25H₂O (monoclinic, P2₁/c, Z = 16, i.e. Z′ = 4) contain four crystallographically unique complex molecules in their asymmetric unit. The four molecules are distinguishable by their molecular conformations, which are strongly flattened with a helical twist (Fig. 5). This is manifested by the dihedral angle between the least squares planes of the L⁴ ligands in each molecule (θ, Table 3 and Fig. 3).⁸⁴ Molecules A–D exhibit 57.75(5) ≤ θ ≤ 73.38(4) which is strongly reduced from the idealised value of 90°, while molecule A also has a significant ϕ distortion. The resultant helical ligand conformations give each metal centre a Λ or Δ chirality; the centrosymmetric crystal lattice contains equal numbers of Λ- and Δ-helical molecules. Space-filling models imply the helical distortion is induced by steric repulsion between the quinolyl C8–H groups on each ligand,⁹³ and the pyridyl ring of the other ligand in the molecule. One other homoleptic complex of L⁴ has also been crystallographically characterised, namely a solvate of [Co(L⁴)₂][PF₆]₂. That complex has a much smaller helical conformational distortion, with ϕ = 180 and θ = 79.5–80.9°.⁹⁴

Fig. 5 The four unique [Fe(L⁴)₂]²⁺ molecules in 4[BF₄]₂·1.39MeCN·0.125Et₂O·0.25H₂O, viewed along their N{pyridyl}–Fe–N{pyridyl} vectors. Both ligand disorder sites in molecule B are included and distinguished with pale and dark colouration. The crystallographic views of molecules C and D have been inverted, to give them the same handedness as molecules A and B. Displacement ellipsoids are at the 50% probability level, and H atoms are omitted for clarity. Colour code: C, white or dark grey; H, pale grey; Fe, green; N, pale or dark blue.

Table 3 Selected bond lengths and angular parameters for the four unique cation environments in crystals of 4[BF₄]₂·1.39MeCN·0.125Et₂O·0.25H₂O at 120 K (Å, °, ų). A full list of bond lengths and angles is in Table S7 (ESI†). See Table 1 and the ESI† for definitions of the structural indices^82–85

	Molecule A	Molecule B^a	Molecule C	Molecule D
a Ligand N(28B)–C(53B) is disordered over two sites, with refined occupancies of 0.61 : 0.39. Values involving both ligand disorder sites are given in the table.
Fe–N{pyridyl}	2.073(6), 2.102(6)	2.100(5), 2.148(9)/2.049(15)	2.088(5), 2.088(5)	2.114(5), 2.099(4)
Fe–N{quinolinyl}	2.221(5)–2.319(6)	2.235(10)–2.335(8)	2.252(5)–2.298(6)	2.249(5)–2.286(5)
V Oh	12.73(2)	12.85(3)/12.99(4)	12.94(2)	13.149(18)
Σ	174.6(7)	195.5(11)/172.9(13)	177.3(7)	153.6(7)
Θ	486	529/482	476	476
ϕ	164.9(2)	173.6(3)/173.1(5)	177.8(2)	177.6(2)
θ	57.75(5)	60.15(7)/61.04(9)	62.64(5)	73.38(4)

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The [Fe(L⁴)₂]²⁺ cations associate into homochiral helical columns in the crystal lattice, through weak intermolecular π⋯π interactions between their quinolyl groups. The molecules stack as ⋯A⋯B⋯D⋯C⋯ down the [101̄] crystal vector (Fig. S33–S35†), and left and right-handed columns are arranged into strips whose helicity alternates along c.

As discussed above, the twisted coordination geometries in 4[BF₄]₂ should strongly favour its high-spin state and inhibit SCO. Consistent with that, the solid complex is fully high-spin between 5–300 K (Fig. S36†).

Solutions of 1[BF₄]₂, 2[BF₄]₂ and 4[BF₄]₂ are paramagnetic, with just one C₂ or m-symmetric ligand environment by ¹H NMR (Fig. S37–S39†). The NMR spectrum of 4[BF₄]₂ in CD₃CN also contains ca. 0.1-equiv of uncoordinated L⁴, implying a minor degree of ligand dissociation in solution for that complex. No free ligand was detected in solutions of 1[BF₄]₂ and 2[BF₄]₂, however.

The spin states of the complexes were monitored by variable temperature Evans method measurements (Fig. 6). Each compound is fully, or predominantly, high-spin at 300 K which is consistent with their paramagnetic NMR spectra. Solutions of 1[BF₄]₂ and 2[BF₄]₂ clearly show the onset of SCO equilibria on cooling, which were fitted to the following parameters: for 1[BF₄]₂, T_½ = 237 ± 3 K, ΔH = 19.3 ± 1.3 kJ mol⁻¹, ΔS = 81 ± 6 J mol⁻¹ K⁻¹; for 2[BF₄]₂, T_½ = 251 ± 1 K, ΔH = 22.4 ± 0.7 kJ mol⁻¹, ΔS = 89 ± 3 J mol⁻¹ K⁻¹. The fitted thermodynamic data are typical for iron(II) complexes of N-donor ligands,^55,95,96 with no apparent contributions from other solution equilibria.^97,98 In contrast, 3′[BF₄]₂·H₂O and 4[BF₄]₂ remain high-spin over the liquid range of the solvent.

Fig. 6 Magnetic susceptibility data in CD₃CN solution for: 1[BF₄]₂ (black ●), 2[BF₄]₂ (red ■), 3′[BF₄]₂·H₂O (blue ▲), and 4[BF₄]₂ (green ◆). The lines show the best fit of the data for 1[BF₄]₂ and 2[BF₄]₂ to a thermodynamic SCO equilibrium (eqn (1) and (2), Experimental section).

Ligands L¹–L⁴ have a blue fluorescence in acetonitrile solution at room temperature (Fig. 7). Comparison with analogous model compounds implies the near-UV absorptions and emission maxima for L¹–L³ should be centred on their imidazo[1,2-a]azine residues.^99–101 The L² emission [λ^em_max = 457 nm] lies at longer wavelength than for L¹ [423 nm], L³ [434 nm] and L⁴ [435 nm], which might reflect additional dipolar interactions between L² and the solvent via its imidazopyrimidinyl N8 atoms.^93,102 Measurements in different solvents to probe that suggestion were hampered by the poor solubility of L², however. The absorption and emission spectra for L⁴ in Fig. 7 are consistent with literature data.^94,103,104

Fig. 7 Absorption spectra (left) and normalised emission spectra (right) from the compounds in this work, in acetonitrile solution at 298 K. The UV-vis extinction coefficients for L¹–L³ have larger errors, because of their poor solubility. Data from these spectra are listed in Table S10.†

The complexes have a weak blue emission under the same conditions, at 445 ≤ λ^em_max ≤ 453 nm for 1[BF₄]₂–3′[BF₄]₂, and λ^em_max = 499 nm for 4[BF₄]₂ (Fig. 7). In most cases, these emissions are red-shifted compared to the corresponding free ligand. The exception is 2[BF₄]₂, whose emission maximum [445 nm] is essentially identical to 1[BF₄]₂ [446 nm], but is slightly blue-shifted compared to L². That supports the involvement of solvation interactions in the longer wavelength emission of L² (see above). Those interactions would be quenched in 2[BF₄]₂ whose imidazopyrimidinyl N8 lone pairs⁹³ are oriented towards the interior of the complex (Fig. 2).

Gas phase DFT calculations were performed to confirm the spin state properties of the complexes, using the B86PW91 functional and def2-SVP basis set. Similar protocols performed well in benchmark studies comparing the spin states in different iron(II) complexes,^105–107 and we have used this method to elucidate structure:function relationships for SCO in several iron(II) complex families with heterocyclic ligands.^108–112 The hypothetical homoleptic complex [Fe(L³)₂]²⁺ (3²⁺) was included in the calculations, for comparison with [Fe(L¹)₂]²⁺ (1²⁺) and [Fe(L²)₂]²⁺ (2²⁺). The spin states of [Fe(bimpy)₂]²⁺ and [Fe(2-bip)₂]²⁺, which are isosteric constitutional isomers of 1²⁺, were also examined (Table 4). The corresponding metal-free ligands were minimised by the same protocol, in conformations consistent with tridentate coordination to a metal ion.

Table 4 Minimized gas-phase spin state energies for the complexes in this work, and their experimental solution-phase SCO mid-point temperatures (T_½; HS = high-spin). Previously published data for the reference molecule [Fe(bpp)₂]²⁺ computed by the same protocol are also given, for comparison. The lone pair orbital energies used to calculate E_av{LP} are given in Table S11†

	T ½, K	E(HS), Ha	E(LS), Ha	ΔE{HS–LS}, kcal mol^−1	ΔErel{HS–LS},^a kcal mol^−1	Free ligand Eav{LP}, eV
a A positive ΔErel{HS–LS} means the low-spin state is computed to be more stable than for 1^2+, and vice versa. b Not available. c In acetone solution.^48,116 d From ref. 112. e From ref. 66. This value has a larger error because of the limited temperature range of the measurement.
[Fe(L^1)2]^2+ (1^2+)	237 ± 3	−3273.967089	−3273.987838	+13.0	0	−5.66
[Fe(L^2)2]^2+ (2^2+)	251 ± 1	−3338.084900	−3338.111325	+16.6	+3.6	−5.73
[Fe(L^3)2]^2+ (3^2+)	—^b	−3888.377301	−3888.398581	+13.6	+0.3	−5.69
[Fe(L^4)2]^2+ (4^2+)	HS	−3362.198522	−3362.213259	+9.3	−3.7	−5.43
[Fe(bimpy)2]^2+	331 ± 5^c	−3274.011814	−3274.038130	+16.8	+3.5	−5.93^d
[Fe(2-bip)2]^2+	212 ± 10^e	−3273.839226	−3273.859677	+12.8	−0.2	−6.47

---

The computed Fe–N bond lengths lie within 2.4% of the available crystallographic data, in both spin states (Table S12†). That is a reasonable level of agreement for a calculation of this type. While the complexes mostly minimised to geometries with minor deviations from D_2d symmetry, [Fe(L⁴)₂]²⁺ (4²⁺) was computed with a helical conformation that closely resembles its crystal structure (Fig. S47†). Hence, the crystallographic geometry in 4[BF₄]₂ is not a crystal packing artefact, but is an intrinsic property of the molecule. It probably arises from intramolecular steric repulsion involving the quinolyl C8–H groups, as discussed above.⁹³

As a pure GGA functional, calculations using B86PW91 consistently overstabilise the low-spin states of metal complexes.^113,114 Hence, the spin state energies in Table 4 (ΔE_rel{HS–LS}) are expressed relative to 1²⁺.⁶⁶ A positive ΔE_rel{HS–LS} means the low-spin state is computed to be more stable than for 1²⁺ implying a higher T_½, and vice versa. The ΔE_rel{HS–LS} values follow the trend:

4²⁺ < [Fe(2-bip)₂]²⁺ < 1²⁺ ≈ 3²⁺ < 2²⁺ ≈ [Fe(bimpy)₂]²⁺

This agrees with the available experimental data, except that the computed low-spin state of 2²⁺ appears overstabilised compared to the other molecules (Fig. 8). With reference to our previous studies, the positive ΔE_rel{HS–LS} for 2²⁺ seems ca. 5× larger than expected when comparing two compounds whose T_½ values differ by just 14 K.^108–112

Fig. 8 Correlation between measured solution T_½ values, and the computed spin state energies in Table 4. The line shows the best fit linear regression of the data points for 1²⁺, [Fe(bimpy)₂]²⁺ and [Fe(2-bip)₂]²⁺.¹¹⁶ Compounds that experimentally show SCO are black circles, and high-spin 4²⁺ (T_½ ≤ 180 K) is a green triangle.

Gas phase minimisations of sterically crowded molecules using our protocol can overstabilise the high-spin state, with respect to analogous molecules lacking bulky substituents. We attributed this to intramolecular dispersion interactions between atoms from different residues which are in close contact in the molecule.^110,111 Intramolecular dispersion is not included in our protocol, and is difficult to model accurately in single-point DFT calculations.¹¹⁵1²⁺, 3²⁺, 4²⁺, [Fe(bimpy)₂]²⁺ and [Fe(2-bip)₂]²⁺ experience weak inter-ligand C–H⋯π contacts involving their distal benzo groups but 2²⁺ does not, since the relevant C–H groups are replaced by N atoms in that molecule. Hence, since 2²⁺ should experience less intramolecular dispersion, its ΔE_rel{HS–LS} may not be strictly comparable with the other molecules in the table.

The ligand Brønsted basicities were compared from the average energies of their three lone pair combination MOs (E_av{LP}, Table 4). These give the following trend, where the most basic ligand has the highest average lone pair energy:

2-bip < bimpy < L² ≈ L³ ≈ L¹ < L⁴

That is, 2-bip is the least σ-donating ligand in the series and L⁴ is the strongest. That ordering does not correlate perfectly with the spin states of their complexes, implying the ligand σ-bonding character is not the sole contributor to their different spin state properties. The high-spin nature of 4²⁺, containing the most σ-basic ligand L⁴, reflects the intramolecular steric hindrance in that molecule.¹¹¹ The spin states of the other complexes in the study should be less affected by steric factors, however.

Comparison of their frontier orbital energies shows L² and 2-bip are the most π-acidic of the remaining ligands, and that 2²⁺ and [Fe(2-bip)₂]²⁺ exhibit greater M–L π-back-bonding than the other complexes in the table. That explains the higher T_½ shown experimentally by 2[BF₄]₂, although this is over-estimated by the calculation. Similarly the stronger π-ligand field in [Fe(2-bip)₂]²⁺ partly offsets its weaker Fe–N σ-bonding, leading to just slightly lower ΔE_rel{HS–LS} and T_½ values than 1[BF₄]₂.

Unlike the other complexes in Fig. 8, the stabilised low-spin state in [Fe(bimpy)₂]²⁺ does not clearly relate to the electronic or steric properties of the bimpy ligand. Thus, no single factor contributing to its larger ΔE_rel{HS–LS} could be identified from its low-spin d-orbital energies. The π-orbitals of 1H-(benz)imidazolyl groups significantly rehybridise upon dative bond formation, which complicates the analysis of the bonding in that complex.¹¹⁷

The minimisations of 2²⁺ deviate only slightly from idealised D_2d symmetry, which contrasts with the distorted coordination geometry found in crystals of 2[BF₄]₂ (Fig. 2, Table 2). This was investigated by additional minimisations of high-spin 1²⁺ and 2²⁺, with ϕ fixed at values between 165 and 155°.⁸⁴ This range of distortions carries an energy penalty of <3 kcal mol⁻¹ for 1²⁺, and ≤2 kcal mol⁻¹ for 2²⁺ (ΔE{dist}, Table 5). Since these values lie above kT (0.6 kcal mol⁻¹ at 298 K), the population of distorted molecules in solution should be relatively small at room temperature. We conclude that the distorted geometries observed for 2[BF₄]₂ are probably induced by crystal packing effects.

Table 5 Computed energies for the minimised geometric distortions of high-spin [Fe(L¹)₂]²⁺ and [Fe(L²)₂]²⁺. ϕ is the trans-N{pyridyl}–Fe–N{pyridyl} bond angle^84,85

	ϕ, deg	E, Ha	ΔE{dist},^a kcal mol^−1
a ΔE(dist) is the energy relative to the corresponding undistorted molecule with ϕ = 180°. b Table 4. c Fixed during the calculation.
[Fe(L^1)2]^2+ (1^2+)	180	−3273.967089^b	0
	165^c	−3273.965652	+0.9
	160^c	−3273.964194	+1.8
	155^c	−3273.962500	+2.9
[Fe(L^2)2]^2+ (2^2+)	180	−3338.084900^b	0
	165^c	−3338.083875	+0.6
	160^c	−3338.083074	+1.2
	155^c	−3338.081760	+2.0

---

ΔE{dist} for 1²⁺ and 2²⁺ is 2–3× larger than for [Fe(1-bpp)₂]²⁺ (1-bpp = 2,6-di{pyrazol-1-yl}pyridine) at each value of ϕ, by the same computational protocol.¹⁰⁸ That indicates the steric influence of the annelated distal ligand donor groups on the distorted coordination geometries of 1²⁺ and 2²⁺.

Conclusions

We have surveyed the iron chemistry of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L¹), and two new derivatives L² and L³. While they undergo thermal SCO equilibria in solution, the homoleptic complexes 1[BF₄]₂ and 2[BF₄]₂ mostly form high-spin solvates in the crystalline phase. SCO may be inhibited by an intermolecular contact between complex molecules in the 1[BF₄]₂ solvates (Fig. 1), while 2[BF₄]₂ adopts a distorted coordination geometry in the crystal that favours a high-spin state (Fig. 2). In the latter case, the geometric distortion is coupled to a pattern of intramolecular n⋯π and intermolecular π⋯π and C–H⋯N interactions that is consistently found in all the solvates examined. However, crystalline 1[BF₄]_1.6[SiF₆]_0.2·1.7MeNO₂·0.3Et₂O is low-spin at 120 K, confirming that the low-spin state of that complex is thermally accessible.

Unexpectedly the homoleptic iron(II) complex of L³ was not isolated and is less stable in solution than 1[BF₄]₂ and 2[BF₄]₂ by ¹H NMR. A 1 : 1 Fe : L³ complex was structurally characterised, which shows no unexpected features (Fig. 4).

A crystal structure of a salt of [Fe(L⁴)₂]²⁺, 4[BF₄]₂, is also reported. That molecule adopts a strongly flattened six-coordinate geometry with a helical ligand conformation, which is imposed by an inter-ligand steric clash involving its quinolyl C8–H groups.⁹³ The molecular helicity in 4[BF₄]₂ is more pronounced than for the only other structurally characterised [M(L⁴)₂]ⁿ⁺ complex (Mⁿ⁺ = Co²⁺)⁹³ but was reproduced by a gas phase DFT minimisation, showing it is an intrinsic property of the molecule.

All the complexes in this work are high-spin in solution at room temperature and show a blue ligand-based emission under ambient conditions (Fig. 8). In most cases, this occurs at longer wavelength than the corresponding metal-free ligand emission. An exception is 2[BF₄]₂, whose emission maximum is similar to 1[BF₄]₂ but is slightly blue-shifted compared to L². That may reflect the influence of dipolar interactions between the solvent molecules and the L² imidazopyrimidinyl N8 atoms⁹³ on that ligand emission.¹⁰² Such interactions should be significant in the free ligand but less so in 2[BF₄]₂, where those N atoms become sterically inaccessible to the environment.

Gas phase DFT calculations mostly reproduce the spin state properties of 1²⁺–4²⁺, and two constitutional isomers of 1²⁺ from the literature (Table 4). However they overstabilise the low-spin state of 2²⁺ compared to the other molecules, which could reflect weaker intramolecular dispersion interactions in 2²⁺ arising from its reduced steric crowding. The high-spin state and helical molecular geometry of 4²⁺ are reproduced well by the calculation. The ΔE_rel{HS–LS} spin state energies of the other molecules do not correlate with the ligand σ-basicities, but can be understood as a combination of the metal–ligand σ-bonding and π-bonding character of each ligand.

As well as continuing our synthetic studies, we are seeking a more accurate computational protocol for the spin states of sterically hindered molecules, so the effect of steric crowding in such compounds can be accurately determined. That would have wider value for the design of base metal catalysts for oxidation reactions or other organic transformations, whose mechanisms and effectiveness depend on the spin state of the metal centre.^118,119

Experimental

Ligands L¹ (ref. 68) and L⁴ (Scheme 1)⁷⁴ were synthesised by the literature procedures. Other reagents were purchased commercially and used as supplied. Synthetic protocols for the new ligands L² and L³ and characterisation data for all the ligands used are given in the ESI.†

Synthesis of [Fe(L¹)₂][BF₄]₂ (1[BF₄]₂)

Reaction of Fe[BF₄]₂·6H₂O (0.054 g, 0.16 mmol) with L¹ (0.10 g, 0.32 mmol) in nitromethane (10 cm³) rapidly yielded a dark red solution. This was filtered to remove a small amount of brown precipitate, and the filtrate was concentrated to ca. 5 cm³ volume. Slow diffusion of diethyl ether vapour into the solution yielded a red powder, which was recrystallised from different solvents to afford the solvated crystals described below. Yield 0.065 g, 48%. Found C, 51.3; H, 3.39; N, 15.2%. Calcd for C₃₈H₂₆B₂F₈FeN₁₀·2H₂O C, 51.4; H, 3.40; N, 15.8%. ¹H NMR (CD₃CN) δ 2.7 (4H), 4.0 (4H), 5.8 (4H), 23.1 (2H), 23.4 (4H), 41.2 (4H), 63.6 (4H).

Synthesis of [Fe(L²)₂][BF₄]₂ (2[BF₄]₂)

Method as above, using L² (0.10 g, 0.32 mmol). The product was initially obtained as a cherry red powder, which was recrystallised from different solvents as below. Yield 0.060 g, 44%. Found C, 47.3; H, 2.51; N, 22.8%. Calcd for C₃₄H₂₂B₂F₈FeN₁₄ C, 47.7; H, 2.59; N, 22.9%. ¹H NMR (CD₃CN) δ 4.1 (4H), 9.9 (4H), 15.2 (2H), 25.0 (4H), 36.0 (4H), 53.3 (4H).

Synthesis of [FeL³][BF₄]₂·H₂O (3′[BF₄]₂·H₂O)

Method as above, using L³ (0.13 g, 0.32 mmol). Recrystallisation of the product from common organic solvent combinations consistently yielded a brown powder, which analysed as [FeL³][BF₄]₂·y(solvent). The sample reported here was recrystallised from undried nitromethane/diethyl ether. Yield 0.060 g, 44%. Found C, 49.9; H, 2.97; N, 10.3%. Calcd for C₂₇H₁₇B₂F₈FeN₅·H₂O C, 49.2; H, 2.91; N, 10.6%.

Synthesis of [FeBr₂(py)_xL³]·yH₂O (py = pyridine)

Addition of hydrated FeBr₂ (0.054 g, 0.16 mmol) to a solution of L³ (0.05 g, 0.12 mmol) in 2,2,2-trifluoroethanol (5 cm³) yielded a cloudy brown solution. The mixture was stored in a freezer for 1 h to coagulate the precipitate, then filtered. Layering the dark brown filtrate with pyridine at room temperature yielded dark brown prisms at the solvent interface. The crystals have the crystallographic formulation [FeBr(py)₂L³]Br·0.5H₂O. However, microanalysis was more consistent with the formula [FeBr(py)L³]Br·2H₂O, suggesting some of the pyridine content of the crystals may have been exchanged by atmospheric moisture on exposure to air. Yield 0.060 g, 44%. Found C, 51.5; H, 3.19; N, 11.0%. Calcd for C₃₂H₂₂Br₂FeN₆·2H₂O C, 51.8; H, 3.53; N, 11.3%.

Synthesis of [Fe(L⁴)₂][BF₄]₂ (4[BF₄]₂)

Method as for 1[BF₄]₂, using L⁴ (0.10 g, 0.32 mmol). The product crystallised as small red platelets, which rapidly collapsed to an amorphous pink powder upon isolation. Yield 0.060 g, 44%. Found C, 60.8; H, 3.47; N, 8.69%. Calcd for C₄₆H₃₀B₂F₈FeN₆·H₂O C, 60.4; H, 3.53; N, 9.19%. ¹H NMR (CD₃CN) δ −12.1 (2H), 2.5 (4H), 9.0 (4H), 9.5 (4H), 10.5 (4H), 30.3 (4H), 61.8 (4H), 73.6 (4H).

The perchlorate salt of this complex has been reported previously.⁷³

Single crystal structure analyses

Solvate crystals of 1[BF₄]₂, 2[BF₄]₂ and 4[BF₄]₂ were grown by slow diffusion of diethyl ether antisolvent vapour, into solutions of each complex in the appropriate solvent. The crystals of [FeBr(py)₂L³]Br·0.5H₂O were obtained from a solvent layering experiment, as described above.

Experimental details and refinement protocols for the structure determinations are given in the ESI.† All the structures were solved by direct methods (SHELX-TL¹²⁰), and developed by full least-squares refinement on F² (SHELXL2018 ¹²¹). Crystallographic figures were produced using XSEED,¹²² and other publication materials were prepared with OLEX2.¹²³ Unless otherwise stated, the following procedures were applied to the refinements.

Other measurements

Elemental microanalyses were performed by the microanalytical services at the University of Leeds and London Metropolitan University. Diamagnetic NMR spectra employed a Bruker AV3HD spectrometer operating at 400.1 MHz (¹H) or 100.6 MHz (¹³C), while paramagnetic ¹H NMR spectra were obtained with a Bruker AV3 spectrometer operating at 300.1 MHz. X-ray powder diffraction data were obtained with a Bruker D8 Advance A25 diffractometer using Cu-K_α radiation (λ = 1.5418 Å).

Solid state magnetic susceptibility measurements were performed on a Quantum Design MPMS-3 magnetometer, with an applied field of 5000 G and a scan rate of 5 K min⁻¹. A diamagnetic correction for the sample was estimated from Pascal's constants;¹²⁴ a diamagnetic correction for the sample holder was also applied. Solvated samples were protected against solvent loss by saturating the (tightly sealed) sample holder capsules with a drop of diethyl ether. Magnetic measurements in solution were obtained by Evans method using a JEOL ECA600ii or a Bruker AV500 spectrometer, operating at 600.13 and 500.05 MHz (¹H) respectively.^125,126 A diamagnetic correction for the sample,¹²⁴ and a correction for the variation of the density of the solvent with temperature,¹²⁷ were applied to these data. The parameters in Table 2 were derived by fitting these data to eqn (1) and (2):

ln[(1 − n_HS(T))/n_HS(T)] = ΔH/RT − ΔS/R (1)

ΔS = ΔH/T_½ (2)

UV-vis spectra were measured using a PerkinElmer Lambda 900 spectrophotometer. Fluorescence measurements under ambient conditions were obtained using a Horiba Fluoromax 3 fluorimeter with constant slit widths of 2 mm. A range of excitation wavelengths were sampled, and the data quoted are for the excitation wavelength that led to the most intense emission for each compound. The sample concentrations for the fluorescence spectra were between 1–4 × 10⁻⁵ mol dm⁻³.

DFT calculations were carried out using SPARTAN'20 for Windows,¹²⁸ with the B86PW91 functional and the def2-SVP basis set. Low-spin systems were treated as spin-restricted, and high-spin systems were treated as spin-unrestricted. The calculations were carried out in the gas phase, since a solvent gradient for iron is not implemented in SPARTAN'20. The molecules were constructed de novo in the program, then subjected to a preliminary molecular mechanics minimisation before the full DFT energy minimisation was undertaken.

Data availability

Data supporting this study are available in the ESI,† or at https://doi.org/10.5518/1413.

Author contributions

RK did all the experimental work in the publication. MAH conceived and supervised the study, assisted with the crystal structure refinements, performed the DFT calculations and wrote the manuscript draft. Both authors approved the final version of the publication.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the EPSRC (EP/K012576/1). The authors thank Simon Barrett (School of Chemistry, University of Leeds) for the Evans method study, and Dr Oscar Cespedes (School of Physics and Astronomy, University of Leeds) for help with the solid state magnetic measurements.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic protocols and characterisation data for the ligands in this work; experimental data, refinement details, figures and tables for the crystal structure determinations; additional solid and solution phase characterisation data; and details of the minimised structures from the DFT calculations. CCDC 2288873–2288881. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt02747c

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