[Fe(µ2-OH)6]3− Linked Fe3O Triads: Mössbauer Evidence for Trigonal µ3-O2− or µ3-OH− Groups in Bridged versus Unbridged Complexes

The syntheses, coordination chemistry, and Mössbauer spectroscopy of hepta-iron(III) complexes using derivatised salicylaldoxime ligands from two categories; namely, ‘single-headed’ (H2L) and ‘double-headed’ (H4L) salicylaldoximes are described. All compounds presented here share a [Fe3-µ3-O] core in which the iron(III) ions are µ3-hydroxo-bridged in the complex C1 and µ3-oxo-bridged in C2 and C3. Each compound consists of 2 × [Fe3-µ3-O] triads that are linked via a central [Fe(µ2-OH)6]3− ion. In addition to the charge balance and microanalytical evidence, Mössbauer measurements support the fact that the triads in C1 are µ3-OH bridged and are µ3-O bridged in C2 and C3.

Salicylaldoxime-based ligands are of particular interest due to the ease in derivatizing the aromatic ring and the inherent ability of the oximato moiety to coordinate multiple metal centres in close proximity.We herein report the syntheses and structures of three analogues (C1-C3) of the hepta-iron(III) complex 1 and use Mössbauer spectroscopy to evidence unexpected speciation of the central µ3-oxygen atom.

Discussion of the Crystal Structure of the Fe7 Complex of a 'Single-Headed' Derivatised Salicylaldoxime (C1)
The first hepta-iron(III) compound, C1, was synthesised using a simple derivatised salicylaldoxime ligand, H2L1 (2-hydroxy-5-tert-butyl-3-(N-piperidinylmethyl)benzaldehyde oxime) (Figure 2   One sixth of the complex C1 represents the asymmetric unit, and the full complex is generated by an S 6 − 3 improper rotation.There are six molecules of the ligand H 2 L1 in the di-anionic form, H 2 L1-2H, in the complex, which are directly connected to six iron atoms Molecules 2024, 29, 3218 3 of 12 (6 × µ 3 -Fe2) that form two metal triads of [Fe III 3 (µ 3 -OH)] 8+ , which are exactly parallel to each other (Figure 3).The central oxygen of the triad is formulated as a hydroxo species based on Mössbauer spectroscopy (see below).These triads are linked via six hydroxo groups that provide the coordination sphere to a seventh iron atom (Fe1), which sits in the middle of the complex as an anion [Fe(µ 2 -OH) 6 ] 3− and is located 3.118 Å from the metal triads (the distance between the metal planes is 6.237 Å).Each triangle consists of three doubly deprotonated ligands (H 2 L1-2H), three iron(III) bound to a µ 3 -OH and three capping pyridine molecules (pyr).Thus, the positive charge (+21) provided by the seven Fe III is overbalanced by −12 from the six ligands, −8 from hydroxo groups [2 × (µ 3 -OH) + 6 × (µ 2 -OH)], and −2 from 2 × BF 4 − ions present within the lattice.Charge neutrality is achieved by a single proton distributed randomly over the 6 piperidinyl groups of the salicylaldoximato ligand.
Each iron atom of the complex is hexa-coordinated and sits in an approximately octahedral geometry.Equatorial sites around each iron atom of the triads (Fe2) are occupied by a phenolato oxygen (O1) atom and an oximato nitrogen (N212) atom from one ligand and an oximato oxygen (O213) atom from a neighbouring ligand and a central oxygen atom (µ3-O).A pyridine group (N100) and a hydroxo group (µ2-OH) are axially coordinated to each iron atom (Fe2) of the triangles.The iron centres of each metal triangle are held together by three N-O groups from the ligands resulting in a bridge between two neighbouring iron atoms.The bridging sequence is as Fe-O-N-Fe on both metal triangles.The central oxygen atom, µ3-O, of the metal triangle is displaced out of the metal planes by 0.314(6) Å away from the centre of the complex.The consequence is that the axial pyridyl groups tilt slightly away from each other, relieving steric strain.The Fe atom from [Fe(µ2-OH)6] 3− sits in an almost perfect octahedral coordination environment, as a consequence of sitting on the S6-3 axis.The hourglass-like metallic core of C1 is illustrated in Figure 3, and selected bond lengths and angles around Fe1 and Fe2 are shown in Table 1.Additionally, water and pyridine molecules exist within the lattice.The hydroxo groups (µ2-OH) form strong hydrogen bonds (1.880 (10) Å) with water molecules and also moderately strong hydrogen bonds with neighbouring phenolate oxygen atoms, O1 (2.559Each iron atom of the complex is hexa-coordinated and sits in an approximately octahedral geometry.Equatorial sites around each iron atom of the triads (Fe2) are occupied by a phenolato oxygen (O1) atom and an oximato nitrogen (N212) atom from one ligand and an oximato oxygen (O213) atom from a neighbouring ligand and a central oxygen atom (µ 3 -O).A pyridine group (N100) and a hydroxo group (µ 2 -OH) are axially coordinated to each iron atom (Fe2) of the triangles.The iron centres of each metal triangle are held together by three N-O groups from the ligands resulting in a bridge between two neighbouring iron atoms.The bridging sequence is as Fe-O-N-Fe on both metal triangles.The central oxygen atom, µ 3 -O, of the metal triangle is displaced out of the metal planes by 0.314(6) Å away from the centre of the complex.The consequence is that the axial pyridyl groups tilt slightly away from each other, relieving steric strain.The Fe atom from [Fe(µ 2 -OH) 6 ] 3− sits in an almost perfect octahedral coordination environment, as a consequence of sitting on the S 6 -3 axis.The hourglass-like metallic core of C1 is illustrated in Figure 3, and selected bond lengths and angles around Fe1 and Fe2 are shown in Table 1.
Additionally, water and pyridine molecules exist within the lattice.The hydroxo groups (µ 2 -OH) form strong hydrogen bonds (1.880 (10) Å) with water molecules and also moderately strong hydrogen bonds with neighbouring phenolate oxygen atoms, O1 (2.559 (6) Å) [34].The composition of the crystal structure of this complex is confirmed by microanalytical data, charge balance, and Mössbauer.

Discussion of the Crystal Structures of Fe 7 Complexes of Linked/'Double-Headed' Derivatised Salicylaldoximes
The complexes, C2 and C3 are double-headed, µ 3 -oxo-bridged hepta-iron(III) compounds produced in the form of dark red rhombic crystals.Both were obtained by slow evaporation of filtered reaction mixtures of the iron salt Fe(BF 4 ) 2 •6H 2 O and the corresponding ligand (H 4 L2 and H 4 L3, respectively) in the presence of NaPF 6 at a 1:2:2 ratio in a methanol-pyridine solution.These complexes are analogues of C1.Despite the different amine linkers present in the ligands (Figure 4) and the additional non-coordinated species present within the lattices, C2 and C3 are structurally very similar.Each of these clusters contains two approximately parallel oximato-and oxo-bridged metal triangles connected to a central Fe(III) atom via six hydroxo groups.X-ray crystal structures of the heptairon(III)clusters, C2 and C3, are described in this section.Selected structural parameters for these complexes can be found in Table 2.    Of particular note are the Fe-µ 3 -oxo bond lengths and displacements of the triplybridging oxygen atom from the planes of the III -µ 3 -O] 7+ on the basis of Mössbauer spectroscopy.In C2, the oximato bridging sequence on the upper triangle is -N-O-, whereas it is -O-N-on the lower triangle.On the other hand, the same oximato bridging sequence occurs on both triangles of C3.As the ligands utilised for C2 and C3 are flexible linked salicylaldoximes containing salicylaldoxime units on either side, only three ligand molecules are required to form a hepta-iron(III)complex, unlike those used for C1.Three of these 'salicylaldoxime heads' from three ligand molecules form a lower triangle and the other three 'heads' form an upper triangle (Figure 1).Due to the flexibility of the di-amine linker between the salicylaldoxime 'heads', these complexes take a twisted helical shape (Figure 5).

Mössbauer Results and Discussion
57 Fe Mössbauer measurements were performed on complexes C1-C3 at low and room temperature.Integral fits of the transmission were carried out for the data obtained at room temperature.The parameters for each of the samples are listed in Table 3.

Mössbauer Results and Discussion
57 Fe Mössbauer measurements were performed on complexes C1-C3 at low and room temperature.Integral fits of the transmission were carried out for the data obtained at room temperature.The parameters for each of the samples are listed in Table 3.The spectra that were recorded at 293 K illustrate two distinctive fitting lines (red and blue) (Figures 6-8).These two lines can be unambiguously attributed to the two different iron environments present in each complex.The intensity of the blue peaks on the Mössbauer spectra of these complexes is much higher than that of the red peaks.The intensity ratio between the two iron species of each hepta-iron(III)compound was observed to be approximately 7:3, near enough to the expected value of 6:1 given by the crystallographic results, given that the central Fe atom is very tightly constrained relative to the iron triads.The isomer shift values of these complexes indicate the +3 oxidation state and high-spin state of the iron sites [35], and these numbers do not differ significantly among complexes C1-C3 at 293 K.The quadrupole splitting value for C1 (0.50 mm −1 and 0.87 mm −1 ), on the other hand, is significantly different from the values obtained for C2 and C3 (0.45-0.55 mm −1 and 1.50-1.55mm −1 ) (see Table 3).The large quadrupole splitting (and relative intensity compared to the other doublet) is consistent with µ 3 -oxo groups for C2 and C3.The smaller quadrupole splitting of the doublets of weaker intensity for C2 and C3 and the pair of quadrupole doublets for C1 are consistent with µ 2 -hydroxo groups and for C1 the µ 3 -hydroxo groups.  5Fe Mössbauer spectra of the complex C1 at high and low temperature and are overlai with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 57 Fe Mössbauer spectra of the complex C1 at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 6. 57Fe Mössbauer spectra of the complex C1 at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 7. 57 Fe Mössbauer spectra of the complex C2 at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 8. 57 Fe Mössbauer spectra of the complex at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.

Materials and Methods
All reactions were performed under aerobic conditions using chemicals and solvent as received, unless otherwise stated. 1 H and 13 C NMR spectra were recorded on a Bruke Avance 500 MHz spectrometer (Bruker, Billerica, MA, USA); δ values are relative to TMS or the corresponding solvent.Mass spectra were obtained using a Micromass ZMD 40 electrospray spectrometer (Waters Corporation, Millford, MS, USA).IR spectra were rec orded on a Nicolet 5700 FT-IR spectrometer from Thermo Electron Corporation (Thermo Electron Scientific Instruments Corp., Madison, WI, USA) using an ATR sampling Figure 7. 57 Fe Mössbauer spectra of the complex C2 at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 6. 57Fe Mössbauer spectra of the complex C1 at high and low temperature and are overlai with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 7. 57 Fe Mössbauer spectra of the complex C2 at high and low temperature and are overlai with corresponding fits using the parameters given in Table 3 at high temperature.
Figure 8. 57 Fe Mössbauer spectra of the complex C3 at high and low temperature and are overlai with corresponding fits using the parameters given in Table 3 at high temperature.

Materials and Methods
All reactions were performed under aerobic conditions using chemicals and solvent as received, unless otherwise stated. 1 H and 13 C NMR spectra were recorded on a Bruke Avance 500 MHz spectrometer (Bruker, Billerica, MA, USA); δ values are relative to TM or the corresponding solvent.Mass spectra were obtained using a Micromass ZMD 40 electrospray spectrometer (Waters Corporation, Millford, MS, USA).IR spectra were rec orded on a Nicolet 5700 FT-IR spectrometer from Thermo Electron Corporation (Therm Electron Scientific Instruments Corp., Madison, WI, USA) using an ATR samplin Figure 8. 57 Fe Mössbauer spectra of the complex C3 at high and low temperature and are overlaid with corresponding fits using the parameters given in Table 3 at high temperature.

Materials and Methods
All reactions were performed under aerobic conditions using chemicals and solvents as received, unless otherwise stated. 1 H and 13 C NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer (Bruker, Billerica, MA, USA); δ values are relative to TMS or the corresponding solvent.Mass spectra were obtained using a Micromass ZMD 400 electrospray spectrometer (Waters Corporation, Millford, MS, USA).IR spectra were recorded on a Nicolet 5700 FT-IR spectrometer from Thermo Electron Corporation (Thermo Electron Scientific Instruments Corp., Madison, WI, USA) using an ATR sampling accessory.Elemental analyses were determined by the Campbell Microanalytical Laboratory at the University of Otago.

Synthesis of Ligands H 4 L2 and H 4 L3
The starting material of the multi-step ligand synthesis, 5-methylsalicylaldehyde, was synthesised as described in the literature [31].The preparation of 3-(bromomethyl)-2hydroxy-5-methylbenzaldehyde (1) and precursors L2a and L3a were carried out by the procedure of Tasker and Schröder [36].The preparation of N,N ′ -dimethyl-p-xylenediamine (2), and the oximations were carried out according to the procedure by Plieger et al. [32] The ligand H 2 L1 was synthesised using the protocols in Tasker et al. [33] and Plieger et al. [37].(20 mL).The resulting yellow solution was stirred for 24 h at room temperature (RT).The solution was washed with water (3 × 70 mL) and the organic phase dried over anhydrous Na 2 SO 4 .Removal of the solvent afforded a brown solid, which was purified by adding ethanol to a concentrated solution of the compound in CHCl 3 affording a pale brown powder, which was in vacuo.Yield (0.90 g, 85%).MP 221-222 • C. υ max /cm −1 1679 (s).Found: C, 68.14; H, 6.74; N, 7.26.Calc for C 22 H 26 N 2 O 4 •0.3C 2 H 5 OH: C, 68.44; H, 7.09; N, 7.04. 1 H NMR (500 MHz; CDCl 3 ) δ: 2.29 (s, 6H), 2.66 (br, 8H), 3.70 (s, 4H), 7.17 A solution of hydroxylamine hydrochloride (0.400 g, 5.76 mmol) in dry ethanol (60 mL) was neutralised with potassium hydroxide (0.324 g, 5.76 mmol) in dry ethanol (60 mL).The resulting white precipitate was removed, and the filtrate was added to a solution of L2a (0.727 g, 1.90 mmol) in 5 mL chloroform and 95 mL dry ethanol over 30 min.The pale yellow solution was stirred for a further 24 h at RT, during which time a pale yellow precipitate was formed.The precipitate was filtered, and the remaining solvent was removed under reduced pressure.The combined pale yellow residues were then washed with cold chloroform (3 x 30 mL) and dried in vacuo.Yield (0.321 g, 41%  (20 mL).The yellow solution was stirred for 24 h at RT.The solution was washed with water (3 × 70 mL), and the organic phase dried over anhydrous Na 2 SO 4 .Removal of the solvent afforded a pale yellow solid, which was recrystallised by adding ethanol to a concentrated solution of the compound in CHCl 3 affording yellow crystals, which were dried in vacuo.Yield (1.16A solution of hydroxylamine hydrochloride (0.377 g, 5.43 mmol) in dry ethanol (60 mL) was neutralised with potassium hydroxide (0.323 g, 5.76 mmol) in dry ethanol (60 mL).The resulting white precipitate was removed, and the filtrate was added to a solution of L3a (1.00 g, 2.17 mmol) in 5 mL chloroform and 95 mL dry ethanol over 30 min.The pale yellow solution was stirred for a further 48 h at RT, after which time a pale yellow precipitate was obtained.The combined residues were filtered, washed with cold chloroform (3 × 30 mL) followed by cold ethanol (3 × 30 mL), and dried in vacuo.Yield (0.978 g, 92% To the ligand H 2 L1 (0.145 g, 0.50 mmol), dissolved in MeOH (12.5 mL), was added Fe(BF 4 ) 2 •6H 2 O (0.169 g, 0.50 mmol) in MeOH (12.5 mL).After full dissolution, NaPF 6 (0.167 g, 1.00 mmol) and pyridine (2 mL) were added to the maroon-coloured solution.The mixture was stirred for 3 h and filtered, and the filtrate was left to evaporate slowly.X-ray quality crystals were produced after 2 weeks (CCDC 2331487).Yield (0.180 g, 67% To the ligand H 4 L2 (0.206 g, 0.50 mmol), suspended in MeOH (12.5 mL), was added Fe(BF 4 ) 2 •6H 2 O (0.348 g, 1.00 mmol) dissolved in MeOH (12.5 mL).After full dissolution, NaPF 6 (0.167 g, 1.00 mmol) and pyridine (2 mL) were added to the maroon-coloured solution.The solution was stirred for 3 h and filtered, and the filtrate was left to evaporate slowly.X-ray quality crystals were produced after 2 weeks (CCDC 2331488 To the ligand H 4 L3 (0.245 g, 0.50 mmol), suspended in MeOH (12.5 mL), was added Fe(BF 4 ) 2 •6H 2 O (0.337 g, 1.00 mmol) dissolved in MeOH (12.5 mL).After full dissolution, NaPF 6 (0.167 g, 1.00 mmol) and pyridine (2 mL) were added to the maroon-coloured solution.The mixture was stirred for 3 h and filtered, and the filtrate was left to evaporate slowly.X-ray quality crystals were produced after 2 weeks (CCDC 2331489

X-ray Structure Determination
X-ray data of complexes C1 and C2 were recorded at low temperature with a Rigaku-Spider X-ray diffractometer, comprising a Rigaku MM007 microfocus copper rotating-anode generator, high-flux Osmic monochromating and focusing multilayer mirror optics (Cu K α radiation, λ = 1.54178Å), and a curved image plate detector.CrystalClear [38] was utilized for data collection and FSProcess in PROCESS-AUTO [39] for cell refinement and data reduction.
Single-crystal diffraction data for C3 were collected at 100 K on the MX2 beamline (λ = 0.7093 Å) at the Australian Synchrotron, Victoria, Australia.The dataset was processed and evaluated using XDS [40].The resulting reflections were scaled using AIMLESS140 from the CCP4 program suite [41].All structures were solved employing direct methods and expanded by Fourier techniques [42].All nonhydrogen atoms were refined using anisotropic thermal parameters.The hydrogen atoms were included in the ideal positions with fixed isotropic U value and were riding on their respective non-hydrogen atoms.Crystal data and refinement parameters for C1-C3 are given in Table A1 (pyr) 6 ]•5BF 4 •6H 2 O•14MeOH, 1•2BF 4 •6H 2 O•14MeOH,consisting of two triangles of [Fe 3 O] 7+ , which are linked via a central [Fe(OH) 6 ] 3− ion and three helical (H 4 L-2H) ligands.

Figure 2 .
Figure 2. The chemical structure if the non-linked/single headed saicylaldoxime ligand, H2L1, utilised for the synthesis of the Fe7 complex C1.
Crystal Structure of the Fe 7 Complex of a 'Single-Headed' Derivatised Salicylaldoxime (C1)

Figure 2 .
Figure 2. The chemical structure if the non-linked/single headed saicylaldoxime ligand, H2L1, utilised for the synthesis of the Fe7 complex C1.

Figure 2 .
Figure 2. The chemical structure if the non-linked/single headed saicylaldoxime ligand, H 2 L1, utilised for the synthesis of the Fe 7 complex C1.
Fe 3 moiety, which are not significantly different for the [Fe 3 III -µ 3 -OH] 8+ moiety of C1, and the [Fe 3 III -µ 3 -O] 7+ of 1 and C2 and C3.Therefore, there is no crystallographic evidence to distinguish µ 3 -O atoms being hydroxo in C1 from their being oxo in C2 and C3.In contrast to the [Fe 3 III -µ 3 -OH] 8+ of C1, the metal triads are formulated as [Fe 3

Figure 5 .
Figure 5. X-ray crystal structure of complex C2.Anions and non-interacting H atoms omitted for clarity.Fe = orange, N = blue, O = red; thermal ellipsoids shown at 30% probability level.

Molecules 2024 ,
29,  x FOR PEER REVIEW 7 of 1 the pair of quadrupole doublets for C1 are consistent with µ2-hydroxo groups and for C the µ3-hydroxo groups.

Figure 6 .
Figure 6.57Fe Mössbauer spectra of the complex C1 at high and low temperature and are overlai with corresponding fits using the parameters given in Table3at high temperature.

Table 2 .
Selected structural parameters for C2 and C3.