Soft Scorpionate Anions as Platforms for Novel Heterocycles

Stereochemically (In)active lone pairs in Sb(III) complexes of a soft scorpionate ligand

Bis-homoleptic antimony complexes of hydrotris(3-methyl-2-thiooxoimidazolyl) borate (TMe) crystallize as either a penta- or hexa-coordinated isomer of [Sb(TMe)2]+; [Sb(κ3-TMe)(κ2-TMe)]+ or [Sb(κ3-TMe)2]+, depending on the counter anion. The known pentacoordinated [Sb(κ3-TMe)(κ2-TMe)]+ crystallizes in its TMe salt, while [Sb(κ3-TMe)2]+, which displays octahedral geometry, is isolated in a trifluoroacetate or PF6- salt. The non-coordinated arm of the bidentate TMe in [Sb(κ3-TMe)(κ2-TMe)]+ cannot be brought into the coordination sphere without a conformational rearrangement of the TMe amounting to inversion at the bridgehead BH group. Variable temperature 1H-NMR and 11B-NMR spectroscopy shows that [Sb(TMe)2]+ is labile in solution in the presence of the counter anionic TMe. The exchange process between the coordinated TMe and the TMe counter anion can be partially slowed by cooling the sample. DFT calculations support these observations with only very small energetic difference (2.22 kcal mol-1) between the two isomers, [Sb(κ3-TMe)2]+ and [Sb(κ3-TMe)(κ2-TMe)]+. The stereochemically active lone pair suggested by the geometry of [Sb(κ3-TMe)(κ2-TMe)]+ is supported by Electron Localisation Function plots for the two structures, showing that the Sb(III) lone pair can impose stereochemical constraints for the [Sb(κ3-TMe)(κ2-TMe)]+ ion, but that these are not present in the octahedral [Sb(κ3-TMe)2]+. A tetranuclear Sb(III) chloride complex of TMe and a dimetallic Sb(III) complex containing three TMe ligands and one methyl-imidazolethione originating from the hydrolysis of TMe have also been structurally characterized.

Exploring a tristhione scorpionate ligand as a suitable chelator for the theranostic pair antimony-119 and antimony-117

BACKGROUND

The highly potent Auger electron emitter antimony-119 (119Sb) and the SPECT-isotope antimony-117 (117Sb) comprise a true theranostic pair particularly suitable for cancer theranostics. Harnessing this potential requires development of a chelator that can rapidly form a stable complex with radioactive antimony ions at the low concentrations typical of radiopharmaceutical preparations. Stable Sb(III) complexes of hydrotris(methimazolyl)borate (TMe) are known, prompting our investigation of this chelator. Additionally, the production of radioantimony was optimized and the SPECT imaging properties of 117Sb was investigated, in an attempt to move towards biomedical implementation of the theranostic isotope pair of antimony.

RESULTS

A method for rapid and effective labelling of TMe using 117Sb was developed, yielding high radiochemical purities of 98.5 ± 2.7% and high radionuclidic purities exceeding 99%. Radiolabelling yielded an Sb(III) complex directly from the acidic Sb(V) solution. [1XXSb]Sb-TMe in aqueous acidic solution showed high stability in the presence of cysteine, however, the stability of the radiocomplex at increased pH was significantly decreased. The production method of 117Sb was optimized, enabling a production yield of up to 19.6 MBq/µAh and the production of up to 564 MBq at end of bombardment, following irradiation of a thin 117Sn-enriched solid target. Preclinical SPECT/CT scanning of a mouse phantom containing purified 117Sb demonstrated excellent SPECT imaging properties of 117Sb with high spatial resolution comparable to that of technetium-99m.

CONCLUSION

We have explored the TMe chelator for complexation of radioantimony and devised a rapid chelation protocol suitable for the short half-life of 117Sb (T1/2 = 2.8 h). [1XXSb]Sb-TMe (1XXSb = 117Sb, 118mSb, 120mSb and 122Sb) demonstrated a high stability in presence of cysteine, although low stability was observed at pH > 4. We have achieved a production yield of 117Sb high enough for clinical applications and demonstrated the excellent SPECT-imaging properties of 117Sb. The results contribute valuable information for the development of suitable chelators for radioantimony and is a step further towards implementation of the antimony theranostic pair in biomedical applications.

Photoinduced Reactivity of the Soft Hydrotris(6-tert-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts

The soft scorpionate ligand hydrotris(6-tert-butyl-3-thiopyridazinyl)borate (Tn) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-tert-butylpyridazine-3-thione (PnH) and 4,5-dihydro-6-tert-butylpyridazine-3-thione (H2PnH). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands.

Soft scorpionate coordination at alkali metals

Reported here are the single-crystal X-ray structure analyses of bis-μ-methanol-κ(4)O:O-bis{[hydrotris(3-phenyl-2-sulfanylidene-2,3-dihydro-1H-1,3-imidazol-1-yl)borato-κ(3)H,S,S'](methanol-κO)sodium(I)}, [Na2(C27H22BN6S3)2(CH4O)4] (NaTm(Ph)), bis-μ-methanol-κ(4)O:O-bis{[hydrotris(3-isopropyl-2-sulfanylidene-2,3-dihydro-1H-1,3-imidazol-1-yl)borato-κ(3)H,S,S'](methanol-κO)sodium(I)}-diethyl ether-methanol (1/0.3333/0.0833), [Na2(C18H28BN6S3)2(CH4O)4]·0.3333C4H10O·0.0833CH3OH (NaTm(iPr)), and a novel anhydrous form of sodium hydrotris(methylthioimidazolyl)borate, poly[[μ-hydrotris(3-methyl-2-sulfanylidene-2,3-dihydro-1H-1,3-imidazol-1-yl)borato]sodium(I)], [Na(C12H16BN6S3)] ([NaTm(Me)]n). NaTm(iPr) and NaTm(Ph) have similar dimeric molecular structures with κ(3)H,S,S'-bonding, but they differ in that NaTm(Ph) is crystallographically centrosymmetric (Z' = 0.5) while NaTm(iPr) contains one crystallographically centrosymmetric dimer and one dimer positioned on a general position (Z' = 1.5). [NaTm(Me)]n is a one-dimensional coordination polymer that extends along the a direction and which contains a hitherto unseen side-on η(2)-C=S-to-Na bond type. An overview of the structural preferences of alkali metal soft scorpionate complexes is presented. This analysis suggests that these thione-based ligands will continue to be a rich source of interesting alkali metal motifs worthy of isolation and characterization.