Hydrothermal synthesis of Group 13 metal tri fl uoride complexes with neutral N-donor ligands † ‡

of the complexes [MF3(terpy)]·3H2O, [MF3(bipy)(OH2)]·2H2O and [MF3(phen)(OH2)]. X-Ray crystal structures of [M’F3(terpy)]·3H2O (M’ = Al or Ga), [M’F3(bipy)(OH2)]·2H2O and [GaF3(phen)(OH2)] show that all of them contain distorted octahedral geometries at the metal with mer-trifluoride coordination. Extensive H-bonding (F⋯H–OH) links the molecules. The complexes have been further characterised by microanalysis, IR, H, F{H} and Al NMR spectroscopy. In contrast, reactions of the trifluorides with the acyclic

An alternative method for the production of the metalfluoro complexes is to first synthesise the metal chloride analogue and then perform a halide exchange reaction using reagents such as Me 3 SiF or Me 3 SnF.For example, [AlF 2 ( py) 4 ]Cl was successfully formed from the reaction of [AlCl 3 ( py) n ] (n = 1 to 3) with Me 3 SiF in pyridine. 16he use of aluminium 15 and gallium 17 fluoride complexes incorporating 18 F as a radiolabel, has attracted much recent interest as diagnostic imaging agents for PET ( positron emission tomography).Key to their potential clinical suitability is the ability to incorporate the short-lived 18 F isotope (t 1/2 = 110 min.)rapidly and cleanly in water; the resulting aluminium fluoride complexes are stable under physiological conditions, 15 while the gallium fluoride radio-product is stable in phosphate buffered saline (PBS) solution. 17A fuller understanding of the coordination chemistry of these Group 13 fluorides is expected to contribute to advancing the design of the next generation of 18 F imaging agents.
Here we report on the systematic study of the preparation, spectroscopic and structural features of several series of complexes of the three Group 13 trifluorides with bi-and polydentate N-donor ligands, to explore the suitability of hydrothermal synthesis for other neutral ligands.The work also investigates the effect of replacing amine with neutral N-heterocyclic ligands, and introducing the mer-trifluoride geometry, rather than the fac geometry present in the [MF 3 (R 3 -tacn)] systems.
[AlF 3 (terpy)]•3H 2 O AlF 3 •3H 2 O (0.100 g, 0.72 mmol) was suspended in freshly distilled water (7 mL) and terpy (0.169 g, 0.72 mmol) was then added.The suspension was transferred into a Teflon container and loaded into a stainless steel high pressure vessel (Parr) and heated to 180 °C for 15 h.The vessel was then allowed to cool.A pale yellow solution had formed, a small aliquot of which was retained to grow crystals.For the remaining reaction mixture the solvent was removed in vacuo, yielding a pale orange solid.Yield: 0.  27 Al NMR (298 K): δ = 16.7 (br).Microanalyses on several batches, which were pure by spectroscopic analysis (including the single crystals), consistently gave H and N content as expected, but very variable (low) C content for this complex.Slow evaporation of the reaction solvent gave crystals suitable for X-ray diffraction.Method 2: A suspension of [GaCl 3 (terpy)] (0.06 g, 0.15 mmol) in anhydrous MeCN (5 mL) was treated with 0.45 mL (0.45 mmol) of a 1.0 M solution of [NBu 4 ]F in thf.Addition of the fluoride source resulted in the dissolution of the chloride precursor and the formation of a pale yellow solution.The mixture was stirred at room temperature for 1 h and then the volatiles were removed in vacuo to yield a yellow gum.This was dissolved in a minimum volume of CH 2 Cl 2 (ca. 2 mL) and layered with hexane.A pale yellow precipitate formed overnight.Yield 0.042 g, 67%.Spectroscopic data matched that observed for Method 1 and recrystallisation from CH 2 Cl 2 / hexane yielded small crystals whose unit cell dimensions matched those of the crystals obtained via Method 1.
Method 3: [GaCl 3 (terpy)] (0.020 g, 0.050 mmol) was suspended in anhydrous MeCN (5 mL).A solution of [K(2.2.2-crypt)]F (0.067 g, 0.150 mmol) in 3 mL anhydrous MeCN was added dropwise to the chloride precursor.Addition of the fluoride source resulted in the dissolution of the chloride precursor and the formation of a colourless solution.The mixture was stirred at room temperature for 1 h, then the volatiles were removed in vacuo to give a white solid containing both the expected fluoride complex and the [K(2.2.2-crypt)]Cl by-product.Spectroscopic data for the former matched that observed from Method 1.No further purification was undertaken.
[{Ga(terpy    ).Slow evaporation of the reaction solvent gave crystals suitable for X-ray diffraction.

[InF 3 ( phen)(OH 2 )]
Method as for [AlF 3 (terpy)]•3H 2 O, but using InF  A solution of t Bu 3 -terpy (0.111 g, 0.278 mmol) in anhydrous CH 2 Cl 2 (5 mL) was added dropwise to a solution of GaCl 3 (0.025 g, 0.139 mmol) in anhydrous CH 2 Cl 2 (5 mL).Addition of the ligand resulted in the formation of a white precipitate.The mixture was stirred at room temperature for 1 h.The corresponding gallium and indium fluoride reactions were conducted similarly.A small number of crystals obtained from the Ga and In reaction mixtures were found to be [⊂Me 2 N(CH 2 ) 2 NMe(CH 2 ) 2 ]Cl (see text and ESI ‡).

X-Ray experimental
Details of the crystallographic data collection and refinement parameters are given in Table 1.Crystals suitable for single crystal X-ray analysis were obtained as described above.Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 µm focus) with the crystal held at 100 K (N 2 cryostream).Structure solution and refinements were performed with either SHELX (S/L)97 or SHELX(S/L)2013 18

Results and discussion
The unreactive and poorly soluble nature of the MF 3 •3H 2 O makes reaction with neutral ligands in organic solvents difficult or impossible.We therefore used the hydrothermal approach (180 °C/15 h) and found this gave high yields of [MF 3 (terpy)]•3H 2 O (below).The same approach was then extended to reactions with N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDTA), and to the diimines, 2,2′-bipyridyl and 1,10-phenanthroline.
The reaction of the MF 3 •3H 2 O with terpy in a 1 : 1 molar ratio in water at 180 °C/15 h, followed by removal of the solvent in vacuo at room temperature, gave high yields of the [MF 3 -(terpy)]•3H 2 O as pale orange solids.Slow evaporation of a small portion of the mother liquor at ambient temperatures gave crystals of [MF 3 (terpy)]•3H 2 O (M = Al or Ga).For M = Al, the structure shows (Fig. 1) a distorted octahedral geometry about the aluminium, the distortions being largely due to the rigid terpy ligand which results in N-Al-N angles significantly less than 90°/180°, whereas the F-Al-F and (cis) F-Al-N angles are close to those expected for an octahedron.
There is extensive H-bonding between the fluoride ligands and the lattice water molecules (Fig. 2), as well as π-stacking of the aromatic rings (Fig. 3).
The Al-F bond lengths are very similar to those observed in fac-[AlF 3 (Me 3 -tacn)]•4H 2 O, 15 but the Al-N bonds are shorter by ∼0.05 Å, suggesting the macrocyclic ring may limit the close approach of the N atoms to the metal centre.The molecular structure of [GaF 3 (terpy)]•3H 2 O is very similar to that of the Al(III) complex (Fig. 4) with the Ga-F bonds ca.0.1 Å longer than the corresponding Al-F, whilst the Ga-N and Al-N are little different.
As found in the aluminium complex, the structure of [GaF 3 (terpy)]•3H 2 O shows extensive H-bonding as shown in  15 The Ga-N distances are also not significantly different to those found in the [GaX 3 (terpy)] (X = Cl or Br), 19 although the extensive H-bonding in the fluoride complex is absent in the structures of the heavier halides.The single bond covalent radii of Al(III) and Ga(III) are quoted in standard texts as nearly identical (∼1.25 Å), although the ionic radius of Ga 3+ is ∼0.07 Å larger than that of Al 3+ . 20The limited number of structurally characterised complexes of the trifluorides limits detailed comparisons, but it seems that the metaldonor bond length may be very sensitive to the electronegativity of the donor atom, with little difference between Al-L and Ga-L with heavier donor ligands and halides, 21 but significant differences in the M-F bond lengths.The effects of solvent molecules and hydrogen bonding also complicates the interpretation of small differences in metal-donor bond lengths, 19,22 and these may be a significant factor in the extensively hydrogen bonded metal fluoride complexes.Interpretation of the spectroscopic properties (Experimental section) of the three terpy complexes is straightforward.The IR spectra show strong, very broad features due to the ν(OH) and δ(HOH) modes of the water and three ν(MF) stretches, as expected for a mer-trifluoride (theory 2A 1 + B 1 ).The 1 H NMR spectra in CD 3 OD exhibit sharp multiplet resonances for the terpy protons and a broad signal for the water, whilst the 19 F{ 1 H} NMR spectra show two resonances in a 2 : 1 ratio due to F transF and F transN respectively.The 19 F{ 1 H} resonances for the [AlF 3 (terpy)] show doublet and triplet 2 J FF couplings of 23 Hz, but for the gallium and indium complexes only broad singlets are observed over the temperature range 298-183 K. 23 The aluminium complex also exhibited a 27 Al NMR resonance at δ = 16.7 as a broad singlet with no resolved 1 J AlF coupling, which is in the range expected for six-coordinate aluminium. 24either the gallium nor indium complex exhibited a metal nucleus resonance, probably due to fast quadrupolar relaxation.The multinuclear NMR data show that the molecular structures of these complexes are maintained in MeOH solution.2.2.2-crypt)]F was also successful at 80 °C in MeCN, confirming the gallium species is thermally stable under these conditions.The [GaCl 3 (terpy)] was relatively poorly soluble in organic solvents and in an attempt to increase the solubility, the corresponding complex of 4,4′,4″-tris-t-butyl,-2,2′:6′,2″-terpyridyl, [GaCl 3 ( t Bu 3 -terpy)], was prepared.Its properties were very similar to the terpy complex (Experimental section), but it was in fact rather less soluble in common organic solvents.However, crystals grown from the filtrate from one synthesis were found to be [GaCl 2 ( t Bu 3 -terpy)][GaCl 4 ]•CH 2 Cl 2 , containing a five-coordinate cation.This minor by-product probably results from the presence of a small excess of GaCl 3 in the synthesis, which extracts a chloride from the neutral species to form the stable [GaCl 4 ] − anion.The structure of the cation is shown in Fig. 6.The geometry is a distorted trigonal bipyramid with apical N, distorted by the steric constraints of the t Bu 3terpy (N3-Ga1-N1 = 155.1(2)°),as also found in the neutral [GaCl 3 (terpy)] complex above.Comparisons of the bond lengths between [GaCl 2 ( t Bu 3 -terpy)] + and [GaCl 3 (terpy)], 19 as expected, show shorter bonds in the five-coordinate cation (Ga-Cl = 2.169(2), 2.195(2), Ga-N = 1.995(5)-2.092(5)Å) compared to the six-coordinate neutral complex (Ga-Cl = 2.2511(5) −2.4118(6), Ga-N = 2.0412(5)-2.1024(15)Å). [{Ga(terpy The fac-[GaF 3 (BzMe 2 -tacn)] has been shown to function as a neutral 'metalloligand' through the coordinated fluorides towards alkali metal and ammonium cations in aqueous solu-tion, leading to supramolecular arrays with Ga-F-M linkages, 25 whilst combination of Gd 3+ and fac-[GaF 3 (Me 3 -tacn)] leads to '[Gd 3 Ga 2 ]' cores that are of interest as molecular magnets. 26A key feature of these systems is that the three facial Ga-F bonds remain intact throughout.To explore if the meridional trifluoride arrangement present in [GaF 3 (terpy)] could function in a similar manner, a H 2 O-MeCN solution containing [GaF 3 -(terpy)] and [NH 4 ][PF 6 ] was allowed to evaporate slowly.Orange crystals formed and an X-ray structure analysis on these showed that rather than forming an ammonium-metalloligand complex, the dimer, [{Ga(terpy The centrosymmetric cation (Fig. 7) contains six-coordinate gallium, severely distorted from regular octahedral by the steric constraints of the terpy ligand (N1-Ga1-N3 = 153.85(10)°),and the fluoride bridges are asymmetric (Ga1-F1 = 1.889(2),Ga1-F1a = 2.003(2) Å).There is extensive hydrogen bonding linking the lattice water molecules, the cations and the [PF 6 ] − anions (Fig. 8).The complex, formed by dissociation of one fluoride from each gallium centre, followed by dimerisation, dissolves in CD 3 OD with decomposition and formation of a white precipitate.The 19 F{ 1 H} NMR spectrum of the supernatant shows only [GaF 3 (terpy)] and [PF 6 ] − as significant species.
Overall, the structural data reinforce earlier conclusions that trends in the bond lengths in comparable Al and Ga complexes often differ from those predicted on the basis of simple Lewis acidity in the gas phase; 22,[27][28][29] other factors, including the presence or absence of lattice solvent and hydrogen bonding also need to be considered. 13,19,21,22The effects are less noticeable at indium, where corresponding bonds are typically ∼0.2 Å longer than for Ga, reflecting the increased radius of the metal centre.
The IR spectra of the [MF 3 ( phen)(OH 2 )] complexes show quite weak features due to ν(OH) and δ(HOH), whereas in the hydrated [MF 3 (bipy)(OH 2 )]•2H 2 O the corresponding features       16) has been obtained previously as the [NMe 4 ] + salt, 32 and breaks up in solution to form [AlF 4 ] − , which was identified by a combination of 27 Al (δ = 48.7 (s)) and 19 F{ 1 H} NMR data (δ = -194.6,6 lines, 1 J AlF = 38 Hz). 32n the cases of the gallium and indium reactions, a few crystals of the same cation were obtained as the chloride salt, from traces of chloride in the reaction.The structure of [⊂Me 2 N-(CH 2 ) 2 NMe(CH 2 ) 2 ]Cl has been reported previously 31 and the crystals obtained in this study were identical, and hence are not discussed further (see ESI ‡).The data on the bulk product from the gallium reaction fitted the constitution [⊂Me 2 N-(CH 2 ) 2 NMe(CH 2 ) 2 ] 2 [Ga 2 F 8 (OH 2 ) 2 ]•2H 2 O, analogous to the aluminium complex, although in the absence of X-ray structural data, the anion present cannot be confirmed.The relative instability of the fluoro-metallate anions in solution and the sensitivity of the 19 F chemical shifts to solvent, 30,32 make identification of the anions uncertain without structural data.The reactions of the MF 3 •3H 2 O with PMDTA were also attempted in refluxing methanol solution, since it was reasoned that the milder conditions (compared to the hydrothermal preparations) might have prevented cleavage of the PMDTA.No reaction occurred in the case of indium fluoride, whilst with AlF 3 •3H 2 O, 1 H and 19 F{ 1 H} NMR spectra of the crude product showed protonated PMDTA and [AlF 4 ] − as the only significant species.The contrast between the instability of PMDTA and the robust Me 3 -tacn which has similar groups in these Group 13 fluoride reactions may be due to the ring structure of the latter preventing close approach of an amine function polarised by coordination to the metal, to the next CH 2 NMe unit, which is presumably the first stage in C-N bond fission and formation of the small ring.

Conclusions
Hydrothermal synthesis using MF 3 •3H 2 O as the metal source has been shown to be a convenient method to form a range of complexes of the otherwise rather intractable Group 13 trifluorides with nitrogen heterocycles in high yield.X-Ray crystallographic studies show all of the new complexes contain a mer arrangement of fluorides, contrasting with the fac geometry present in the triaza-macrocyclic complexes 15 reported previously.Extensive H-bonding and π-stacking networks are present in the complexes of all three imines with the three metal ions, although the details differ.These studies significantly extend the known coordination chemistry of the Group 13 trifluorides.The relatively high stability of the trifluoride complexes contrasts with the moisture sensitivity of complexes of the Group 13 elements with heavier halides.However, this work has also shown that unlike [GaF Future work will aim to establish whether the hydrothermal approach is also suitable for oxygen donor ligands and whether soft donor ligands such as thioethers or phosphines can form complexes with the Group 13 fluorides.
and were straightforward, except where detailed below.H atoms bonded to C were placed in calculated positions using the default C-H distance and refined using a riding model.In the case of the [AlF 3 (bipy)(OH 2 )]•2H 2 O structure, the H-atoms on the co-crystallised water molecules were not located in the difference map.While not included in the refinement, the H-atoms are inferred from the H-bonding distances of F⋯O and O⋯O and are thus included in the formulae.The H-atoms on both the coordinated and co-crystallised water molecules could not be located in the structure of [⊂Me 2 N(CH 2 ) 2 NMe(CH 2 ) 2 ] 2 [Al 2 F 8 (OH 2 ) 2 ]•2H 2 O.While not included in the refinement, the H-atoms are similarly inferred and included in the formulae.CCDC numbers 1053047-1053048 and 1053152-1053158.

Fig. 5
Fig. 5 as well as π-stacking interactions of 3.57 and 3.62 Å (see ESI Fig. S1 ‡), although the arrangements differ in detail.The Ga-F distances are very similar to those found in fac-[GaF 3 (Me 3 -tacn)]•4H 2 O.15 The Ga-N distances are also not significantly different to those found in the [GaX 3 (terpy)] (X = Cl or Br),19 although the extensive H-bonding in the fluoride complex is absent in the structures of the heavier halides.The single bond covalent radii of Al(III) and Ga(III) are quoted in standard texts as nearly identical (∼1.25 Å), although the ionic radius of Ga 3+ is ∼0.07 Å larger than that of Al 3+ .20The limited number of structurally characterised complexes of the trifluorides limits detailed comparisons, but it seems that the metaldonor bond length may be very sensitive to the electronegativity of the donor atom, with little difference between Al-L

Table 1
Crystallographic parameters a GaN 2 O C 14 H 18.5 Cl 4 GaN 1.5 C 14 H 42 Al 2 F 8 N 4 O 4 C 30 H 30 F 16 Ga 2 N 6 O 4 P 2 3 13H 2 O with PMDTA PMDTA, Me 2 N(CH 2 ) 2 NMe(CH 2 ) 2 NMe 2 , is a flexible, aliphatic acyclic triamine analogue of Me 3 -tacn and terpy, and its reactions with the hydrated Group 13 fluorides were explored to provide a third series of complexes for comparison.In contrast to the reactions with the other two N 3 -donor ligands, the hydrothermal route using PMDTA resulted in cleavage of the triamine and the formation of the 1,1,4-trimethylpiperazinium cation, [⊂Me 2 N(CH 2 ) 2 NMe(CH 2 ) 2 ] + .After removing all volatiles from the reaction mixtures and washing the residue with MeCN, the 1 H and13C{ 1 H} NMR spectra show the cyclic cation to be the only organic species in the bulk products.For the aluminium reaction, crystals were obtained, showing the solid contained [⊂Me 2 N(CH 2 ) 2 NMe(CH 2 ) 2 ] 2 [Al 2 F 8 (OH 2 ) 2 ]•2H 2 O.The anion (Fig.