Synthesis, Structure, Reactivity, and Intramolecular Donor–Acceptor Interactions in a Phosphinoferrocene Stibine and Its Corresponding Phosphine Chalcogenides and Stiboranes

Abstract

Ferrocene-based phosphines equipped with additional functional groups are versatile ligands for coordination chemistry and catalysis. This contribution describes a new compound of this type, combining phosphine and stibine groups at the ferrocene backbone, viz. 1-(diphenylphosphino)-1′-(diphenylstibino)ferrocene (1). Phosphinostibine 1 and the corresponding P-chalcogenide derivatives Ph2P(E)fcSbPh2 (1E, fc = ferrocene-1,1′-diyl, E = O, S, Se) were synthesized and further converted to the corresponding stiboranes Ph2P(E)fcSb(O2C6Cl4)Ph2 (6 and 6E) by oxidation with o-chloranil. All compounds were characterized by spectroscopic methods, X-ray diffraction analysis, cyclic voltammetry, and theoretical methods. Both NMR spectroscopy and DFT calculations confirmed the presence of P → Sb and PO → Sb donor–acceptor interactions in 6 and 6O, triggered by the oxidation of the stibine moiety into Lewis acidic stiborane. The corresponding interactions in 6S and 6Se were of the same type but significantly weaker. A coordination study with AuCl as the model metal fragment revealed that the phosphine group acts as the “primary” coordination site, in line with its higher basicity. The obtained Au(I) complexes were applied as catalysts in the Au-catalyzed cyclization of N-propargylbenzamide and in the oxidative [2 + 2 + 1] cyclization of ethynylbenzene with acetonitrile and pyridine N-oxides. The catalytic results showed that the stibine complexes had worse catalytic performance than their phosphine counterparts, most likely due to the formation of weaker coordination bonds and hence poorer stabilization of the active metal species. Nevertheless, the stibine moiety could be used to fine-tune the properties of the ligated metal center by changing the oxidation state or substituents at the “remote” Sb atom.

Pnictogen bonding at the service of gold catalysis: the case of a phosphinostiborane gold complex

“…Analysis of the resulting reaction mixture's 31 P{ 1 H} NMR spectrum indicated a partial oxidation of the phosphine center; however, no pure product was successfully isolated. This non-selective oxidation tendency of o -chloranil towards both a phosphine and a stibine moiety has been observed previously, 13 underscoring the excessively oxidizing nature of o -chloranil. Interestingly, when the reaction was repeated with a less potent o -quinone, namely 3,5-di- tert -butyl- o -benzoquinone (DtBQ), a complete conversion to a new species, designated as 3 , was observed.…”

“…Equipped with this knowledge, we became eager to test the ability of 3 -AuCl to act as a carbophilic catalyst that would self-activate through engagement of the gold-bound chloride by the stiborane. For comparative purposes, we also included the analogous ferrocene-based system 5 -AuCl which could be easily generated from the recently reported complex 4 -AuCl 13 with 3,5-di- tert -butyl- o -benzoquinone (Scheme 2). This complex, characterized by a 31 P{ 1 H} NMR resonance at 28.0 ppm, was isolated as a yellow, diamagnetic air stable solid and used as a carbophilic catalyst.…”

Synthesis, reactivity and coordination behaviour of a ferrocene phosphinostibine and intramolecular interactions in its P(v) and Sb(v) derivatives

“…The reaction between 3 and [AuCl(tht)] (1 equiv., tht = tetrahydrothiophene) afforded a phosphine complex [AuCl( 3 -κP)] ( 5 ) as the sole product, in line with our previous observations. 8…”

“…This suggested that both stibine and phosphine moieties were oxidised with o -chloranil but that the phosphorane was hydrolysed more easily to give phosphine oxide 8O . 8 Indeed, a reaction performed using dried reagents allowed isolation of the unstable intermediate Cy 2 P(O 2 C 6 Cl 4 )fcSbPh 2 (O 2 C 6 Cl 4 ) ( 9 ), which cleanly converted into 8O when treated with water. When water was present in the reaction mixture (introduced as a wet solvent), oxidation of 3 produced 8O as the sole product in good yield (85%).…”

“…6 In the following research, we focused on the analogous mixed-donor derivative 1-(diphenylphoshino)-1′-(diphenylstibino)ferrocene ( 2 ) and its P( v )- and Sb( v )-derivates. 8 These compounds were studied mainly because of possible intramolecular dative interactions between the P- and Sb-substituents, which were indeed identified in stiboranes resulting from oxidation of the stibine moiety. Unfortunately, studies on the coordination behaviour of 2 were hampered by the tendency of this compound to form disordered structures, in which the phosphine and stibine groups alternated in their positions.…”