Synthesis and reactivity of a terminal uranium(<scp>iv</scp>) sulfide supported by siloxide ligands

Abstract

The reactions of the tetrasiloxide U(iii) complexes [U(OSi(O ^<i>t</i> Bu)₃)₄K] and [U(OSi(O ^<i>t</i> Bu)₃)₄][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(iv) sulfide complexes [SU(OSi(O ^<i>t</i> Bu)₃)₄K₂]₂, <b>1</b>, and [{SU(OSi(O ^<i>t</i> Bu)₃)₄K₂}₂(μ-18c6)], <b>2</b>, with concomitant formation of the U(iv) complex [U(OSi(O ^<i>t</i> Bu)₃)₄]. Addition of 1 equiv. of 2.2.2-cryptand to complex <b>1</b> resulted in the isolation of a terminal sulfide complex, [SU(OSi(O ^<i>t</i> Bu)₃)₄K][Kcryptand], <b>3</b>. The crucial role of the K⁺ Lewis acid in these reductive sulfur transfer reactions was confirmed, since the formation of complex <b>3</b> from the reaction of the U(iii) complex [U(OSi(O ^<i>t</i> Bu)₃)₄][Kcryptand] and 0.5 equiv. of PPh₃S was not possible. Reactivity studies of the U(iv) sulfide complexes showed that the sulfide is easily transferred to CO₂ and CS₂ to afford S-functionalized products. Moreover, we have found that the sulfide provides a convenient precursor for the synthesis of the corresponding U(iv) hydrosulfide, {[(SH)U(OSi(O ^<i>t</i> Bu)₃)₄][K18c6]}, <b>5</b>, after protonation with PyHCl. Finally, DFT calculations were performed to investigate the nature of the U-S bond in complexes <b>1</b>, <b>3</b> and <b>5</b>. Based on various analyses, triple-bond character was suggested for the U-S bond in complexes <b>1</b> and <b>3</b>, while double-bond character was determined for the U-SH bond in complex <b>5</b>.