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(C 6H 5) 3}] and long time sought structure of mer-[RhCl 3{Sb(C 6H 5) 3} 3

Abstract

The reaction of RhCl 3 · 3H 2O with SbPh 3 (Ph, C 6H 5) in ethanol produced mer-[RhCl 3(SbPh 3) 3] ( 1), as the main product, together with trans-[RhCl 2(Ph)(SbPh 3) 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 SbPh 3 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 RhCl 3 and SbPh 3 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( N 1, S 2-tpym) 2( N 1(Sb), S 2(Rh)-tpym)(SbPh 3)] ( 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 N 1 and S 2 atoms, whereas the third tpym − anion behaves as mono-dentate through the S 2 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.