Synthesis and reactivity of a ferrocene-derived PCP-pincer ligand

Abstract

The 1,3-bis(diphosphinomethyl)ferrocene 3 readily reacts with [(C 2 H 4 ) 2 RhCl] 2 to form an equilibrating pair of diastereomers 8a and 8b by C-H insertion into the ferrocene.Pincer-type ligands have played a significant role in homogeneous catalysis. 1 With the recent development of effective catalysts for dehydrogenation that involve Rh, or more especially Ir, complexes related to compound 1, interest in their application has been heightened. 2 Among the factors that contribute to their reactivity in the critical C-H activation step will be the chelate bite angle and the electronic character of both the s-and n-bonding entities. Synthesis and complexation chemistry of a higher homologue 2 based on cycloheptatriene has already been reported. 3 For these reasons we are interested in the synthesis of the ferrocene analogue 3, and report its successful completion together with some preliminary reactions that demonstrate the access to pincer-type ligation.General methods for the preparation of 1,3-disubstituted ferrocenes are severely lacking. 4 In order to synthesise a suitable precursor, we prepared compound 4 according to published procedures, involving the photochemical displacement of p-xylene from CpFe[p-xylene] + BF 4 2 in the presence of 3-methoxycarbonyl-6-dimethylaminofulvene. 5,6 This was cleanly reduced by LiAlH 4 in refluxing THF to the primary diol 5a in 89% yield. 7 Attempts to form the diacetate 5b by conventional means (Ac 2 O, py, DMAP) were unsuccessful, but it was demonstrated that direct S N 1 reaction in AcOH at 80 °C proceeded cleanly, according to precedent. 8 Rather than isolate the diacetate according to conventional phosphination procedures, 9 diol 5a was converted directly into the desired diphosphine 3 by heating with HPBu t 2 (2 equiv.) in AcOH (80°C , 1 h), isolating the product † in 62% yield by precipitation from MeOH.The new ligand was surveyed for its reactivity towards simple unsaturated metal complexes under mild conditions. A model reaction was first established, using ligand 1b. By NMR (C 6 D 6 , 65 °C, 30 min), displacement of the pincer diamine ligand 10 in 6 by the diphosphine was observed. The formation of complex 7 was inferred, ‡ although it was not isolated. Repeating this reaction with ligand 3 gave no evidence of complexation, however. Reaction of the ferrocene-derived ligand 3 with (PPh 3 ) 3 RuCl 2 or (MeCN) 2 PdCl 2 likewise failed to give characterisable products.When ligand 3 was reacted with [(C 2 H 4 ) 2 RhCl] 2 in CD 2 Cl 2 at ambient temperature a clean transformation occurred within minutes, the product being characterised by NMR and MS. † Initially, two sets of resonances were observed in the 1 H NMR spectrum, in particular two distinct Rh-H signals at ca. 227 ppm. Equilibration occurred over 4 h so that the lower field signal, initially the minor component, became strongly predominant. The equilibration could also be followed by 31 P NMR, it being observed that the signal at 87.5 ppm, decreases monotonically over 3 h as the signal at 93.5 ppm increases. ...

Recent Advances on the Chemistry of POCOP–Nickel Pincer Compounds

“…However, two examples worthy of highlighting include the use of the aromatic ring on the POCOP backbone as an arene functional group. Although this idea was first reported for the case of POCOP complexes, the concept is not new and has been exploited before for NCN, SNS and PCP [69][70][71][72][73][74][75][76][77][78][79] …”

Ferrocene pincer ligands made easy

“…Their ligand synthesis uses 1-(ethoxycarbonyl)-3-formylferrocene as the key precursor as shown in Scheme 1. A similar approach to the synthesis is taken by Brown and van Koten [4] where the synthetic method of Lindsell et al [5] and Bickert et al [6] is again used to obtain the 1,3-disubstituted ferrocene. This synthesis uses a mixed arene-cyclopentadienyliron complex as a precursor.…”

Ruthenium complexes with ferrocene-based P,C,P pincer ligand

“…Our first attempt, as well as attempts of other researchers, 10 (4) have been synthesized earlier by the reactions of RuCl 2 (PPh 3 ) 3 with the corresponding benzene derivatives. In complexes 3 and 4, the Ru atom is in a square pyramidal coordina tion environment with the PPh 3 ligand in the apical posi tion.…”