Inorganic & Coordination Chemistry, Short talk
IC-015

Chemistry at High Dilution: Dinuclear 99mTc complexes do exist!

R. Bolliger1, H. Braband1, G. Meola1, R. Alberto1*
1University of Zurich, Department of Chemistry, Winterthurerstrasse 190, 8057 Zürich

One of the most widely used radionuclides in nuclear medicinal diagnostics is 99mTc (half-life time t1/2 = 6h), the metastable form of the 99Tc-isotope (t1/2 = 221ky). Its nearly ideal decay properties, such as half life time and decay energy of 140 keV, makes 99mTc perfectly suited for Single Photon Emission Computed Tomography (SPECT).[1] All 99mTc applications start with [99mTcO4]– from the elution of a 99Mo/99mTc generator with saline. The 99mTc-concentrations are very small (10-7 – 10-9 M) and thus the formation of di- or polynuclear complexes is kinetically unfavoured and there has not been a single report about the formation of a dinuclear 99mTc-complex.[2] All reported 99mTc-compounds are mononuclear, despite numerous reports of polynuclear 99Tc-compounds.[3,4] Different reaction conditions on the tracer (99mTc) and on the macroscopic level (99Tc) might be responsible. Ligand concentrations are usually in 103 – 106 fold excess over 99mTc, while equimolar concentrations with 99Tc apply. As there was no report of a di- or polynuclear 99mTc complex, such complexes are believed to be synthetically inaccessible.

However, using appropriate bridging ligands (herein thiols HS-R) and a suitable 99mTc-precursor [99mTc(CO)3(H2O)3]+, we demonstrate that dinuclear 99mTc-complexes are formed at room temperature and at nanomolar concentrations. The dinuclear nature of [99mTc2(μ2-SR)3(CO)6]– was assessed by chromatographic comparison with its rhenium homologue and with its true 99Tc analogue. Kinetic studies showed that the rate limiting step is not the dimerization, but the formation of a mononuclear 99mTc-thiol complex, which rapidly dimerizes.[5]

[1] Shuang Liu, Chem. Soc. Rev., 2004, 33, 445-461.
[2] Roger Alberto, Henrik Braband, in: "Comprehensive Inorganic Chemistry II" J. Reedijk, K. Poppelmeier (ed.), 2013, Elsevier, Oxford, 785-817.
[3] Henrik Braband, Ulrich Abram, J. Organomet. Chem., 2004, 689, 2066-2072.
[4] Francesco Tisato, Cristina Bolzati, Adriano Duatti, Giuliano Bandoli, Fiorenzo Refosco, Inorg. Chem. 1993, 32, 2042-2048.
[5] Robin Bolliger, Angelo Frei, Henrik Braband, Giuseppe Meola, Bernhard Spingler, Roger Alberto, Chem. Eur. J., 10.1002/chem.201901161.