Beyond phosphorus: synthesis, reactivity, coordination behaviour and catalytic properties of 1,1′-bis(diphenylstibino)ferrocene

Abstract

Compared to their widely studied phosphine counterparts, ferrocene stibines have received only marginal attention thus far. This paper describes the synthesis of 1,1′-bis(diphenylstibino)ferrocene (1*), which is an antimony analogue of the ubiquitous dppf, and our investigations into the reactivity and coordination behaviour of this compound. Thus, distibine 1 was oxidised to stiboranes fc(SbPh₂X₂)₂ (X = Cl, 2*; F, 6*; fc = ferrocene-1,1′-diyl) and to stibine-stiborane Ph₂SbfcSbPh₂F₂ (5*). Compounds 2 and 6 were easily hydrolysed to produce ferrocenophanes fc[SbPh₂XOSbPh₂X] (X = Cl, 3*; F, 7*). Removing the halogen from 3 with silver(I) salts afforded the corresponding ferrocenophanes with O-bound oxyanions, fc[SbPh₂ZOSbPh₂Z] (Z = NO₃, 4a*; ClO₄, 4b*), which were alternatively prepared from 2 and also converted back to 2 by adding a chloride source. Through investigations into the coordination behaviour of distibine 1, the following compounds were isolated and characterised: [(μ-ClO₄)₂{Ag(1-κ²Sb,Sb′)}₂] (8*), [Ag(1-κ²Sb,Sb′)₂]X (X = ClO₄, 9a; SbF₆, 9b*), [(μ(Sb,Sb′)-1){(arene)MCl₂}₂] (10*, 11*) and [(arene)MCl(1-κ²Sb,Sb′)][PF₆] (12*, 13*; 10, 12: M/arene = Ru/η⁶-p-cymene, 11, 13; Rh/η⁵-C₅Me₅), [(η⁵-C₅Me₅)RuCl(1-κ²Sb,Sb′)] (14), [MCl₂(1-κ²Sb,Sb′)] (M = Pd, 15*; Pt, 16*), [Pd(1-κ²Sb,Sb′)₂]X₂ (X = BF₄, 17a; SbF₆, 17b*), [Pd(η²-ma)(1-κ²Sb,Sb′)] (18*; ma = maleic anhydride), [(μ(Sb,Sb′)-1)(AuCl)₂] (19*), and [Au(1-κ²Sb,Sb′)₂]X (X = AuCl₂, 20a*; SbF₆, 20b*). Inspection of the structural parameters suggested that complexes featuring 1 exhibit less sterically strained structures than their dppf analogues due to longer M–Sb and Sb–C bonds, which reduce crowding around the ligated metal centre. Cyclic voltammetry and DFT calculations revealed that the primary electrochemical oxidation of 1 is reversible and occurs at the ferrocene unit. Based on preliminary catalytic tests in Suzuki–Miyaura biaryl coupling, Pd-1 complexes exhibited a lower efficiency than their respective Pd-dppf analogues. (An asterisk indicates that the crystal structure has been determined.)