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 Ph(2)SbfcSbPh(2)F(2) (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[SbPh(2)ZOSbPh(2)Z] (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-κ(2)Sb,Sb')}(2)] (8*), [Ag(1-κ(2)Sb,Sb')(2)]X (X = ClO4, 9a; SbF6, 9b*), [(μ(Sb,Sb')-1){(arene)MCl2}(2)] (10*, 11*) and [(arene)MCl(1-κ(2)Sb,Sb')][PF6] (12*, 13*; 10, 12: M/arene = Ru/η(6)-p-cymene, 11, 13; Rh/η(5)-C5Me5), [(η(5)-C5Me5)RuCl(1-κ(2)Sb,Sb')] (14), [MCl2(1-κ(2)Sb,Sb')] (M = Pd, 15*; Pt, 16*), [Pd(1-κ(2)Sb,Sb')(2)]X-2 (X = BF4, 17a; SbF6, 17b*), [Pd(η(2)-ma)(1-κ(2)Sb,Sb')] (18*; ma = maleic anhydride), [(μ(Sb,Sb')-1)(AuCl)(2)] (19*), and [Au(1-κ(2)Sb,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.)