Unusual hetero-atomic RhSCNSb(Rh) co-ordination ring: Synthesis and X-ray structure of [Rh(N1,S2-2-thiopyrimidinato)2(N1(Sb),S2(Rh)-2-thiopyrimidinato){Sb(C6H5)3}] and long time sought structure of mer-[RhCl3{Sb(C6H5)3}3]

The reaction of RhCl3 · 3H2O with SbPh3 (Ph, C6H5) in ethanol produced mer-[RhCl3(SbPh3)3] (1), as the main product, together with trans-[RhCl2(Ph)(SbPh3)3] (2) (R. Cini, G. Giorgi, L. Pasquini, Inorg. Chim. Acta 196 (1992) 7; A. Cavaglioni, R. Cini, J. Chem. Soc., Dalton Trans. (1997) 1149). Ways to improve the yield for 1 and 2 were a subject of further works by this laboratory during the past decade and the reactivity of the organometallic compound 2 was investigated towards a variety of bases. Long sought single crystals of 1 suitable for X-ray diffraction have been obtained from hot acetone solution. The metal centre has a pseudo-octahedral coordination environment with the three chloride anions and the three SbPh3 molecules in mer positions. The Rh–Cl bond trans to Sb is significantly weakened by the high trans influence by antimony itself. This gives the rationale for the formation of 2 from RhCl3 and SbPh3 via the preliminary formation of 1, followed by dissociation of the activated chloride anion, and the subsequent formation of the Rh–C(Ph) linkage. The reaction of 1 with 2-thio-1,3-pyrimidine (Htpym) in ethanol produces crystalline [Rh(N1,S2-tpym)2(N1(Sb),S2(Rh)-tpym)(SbPh3)] (3). Single crystals suitable for X-ray diffraction studies as obtained from ethanol/diethyl-ether contain a complex molecule and a quarter of an ethanol molecule in the asymmetric unit. Two out of three tpym− anions chelate the metal centre via the N1 and S2 atoms, whereas the third tpym− anion behaves as mono-dentate through the S2 atom only. The sixth co-ordination position is occupied by the Sb atom. Interestingly, one of the N atoms of the monodentate tpym− anion behaves as electron donor to Sb and a Sb–N linkage forms (d(Sb–N), 2.846 Å). As a consequence, an unusual totally hetero-atomic five-membered co-ordination ring of the type RhSCNSb(Rh) forms. A molecular modelling analysis at the hybrid density functional level DFT-B3LYP was performed on selected co-ordination molecules.

Graphical abstractThe reaction of [RhCl3(SbPh3)3] with 2-thio-1,3-pyrimidine (Htpym) in ethanol produces crystalline [Rh(tpym)3(SbPh3)] that contains two tpym− N,S-chelating anions and a tpym− S-monodentate anion; this latter donates via a N atom to the Sb atom. The X-ray structure of [RhCl3(SbPh3)3] shows that the chloride anion trans to Sb is significantly activated towards dissociation, an event that gives a rationale to the formation of [RhCl2(Ph)(SbPh3)3] previously reported from this laboratory.Figure optionsDownload full-size imageDownload as PowerPoint slide