Synthesis, Characterization, and Properties of Poly(aryl)phosphinoboranes Formed via Iron‐Catalyzed Dehydropolymerization

Abstract

The dehydropolymerization of the primary phosphine–boranes, RPH2•BH3 (1a–f) (R = 3,4‐(OCH2O)C6H3 (a), Ph (b), p‐(CF3O)C6H4 (c), 3,5‐(CF3)2C6H3 (d), 2,4,6‐(CH3)3C6H2 (e), 2,4,6‐tBu3C6H2 (f)) is explored using the precatalyst [CpFe(CO)2OTf] (I) (OTf = OS(O)2CF3), based on the earth abundant element Fe. Formation of polyphosphinoboranes [RPH‐BH2]n (2a–e) is confirmed by multinuclear NMR spectroscopy, but no conversion of 1f to 2f is detected. Analysis by electrospray ionization mass spectrometry confirms the presence of the anticipated polymer repeat units for 2a–e. Gel permeation chromatography (GPC) confirms the polymeric nature of 2a–e and indicates number‐average molecular weights (Mn) of 12 000–209 000 Da and polydispersity indices between 1.14 and 2.17. By contrast, thermal dehydropolymerization of 1a–e in the absence of added precatalyst leads to formation of oligomeric material. Interestingly, polyphosphinoboranes 2c and 2d display GPC behavior typical of polyelectrolytes, with a hydrodynamic radius dependent on concentration. The thermal transition behavior, thermal stability, and surface properties of thin films are also studied.

Synthesis and Post‐Polymerization Functionalization of Halogen‐Substituted Polyphosphinoboranes to Access Alkyne‐Functionalized Derivatives

“…The higher tendency of 1a for chain‐branching (or macrocondensation) is likely favoured by the presence of the electronegative para ‐bromo substituents in the polymer side chain, which activates the P–H group by increasing its protic character. In a similar fashion, a significant tendency for chain‐branching or macrocondensation has previously been reported for the dehydropolymerization of the phosphine–borane [3,5‐(CF 3 ) 2 ‐C 6 H 3 ]PH 2 ⋅BH 3 with [Fe] (2 mol%) for 24 h at 100 °C …”

“…The dehydropolymerization of 1a to give the bromo‐functionalized polyphosphinoborane [(4‐Br‐C 6 H 4 )PH–BH 2 ] n ( 2a ) was successfully achieved by heating a toluene solution of 1a with 2 mol% of the iron precatalyst [Fe] under N 2 to 100 °C for 3 h ( Scheme ). As previously reported, the dehydropolymerization reaction is accompanied by an initial colour change from red to yellow upon heating . Repeated precipitation of the polymer into cold n ‐pentane (−78 °C) afforded 2a as a spectroscopically pure pale yellow solid in 81% yield.…”

“…Since these early reports, other approaches to access polyphosphinoboranes via metal‐free or metal‐catalyzed routes3e, have also been described. Recently, we reported a significantly improved approach to access high molecular weight aryl‐substituted polyphosphinoboranes via dehydropolymerization of primary phosphine–boranes using the Fe II precatalyst [CpFe(CO) 2 (OTf)] (abbreviated hereafter as [Fe] ; Cp = η 5 ‐C 5 H 5 ) . In this case, the polymerization reaction can be carried out in solution phase at lower temperatures (100 °C), providing access to mainly linear high molecular weight polyphosphinoboranes of significantly lower dispersity.…”

“…Despite these advances, only a limited number of functionalized polyphosphinoboranes are known to date and are limited to chemically inert groups which are not suitable for further post‐polymerization modification 5b,c,. The establishment of a synthetic protocol to polyphosphinoboranes with reactive groups in the side chain would represent a significant advance in terms of the future development of these polymers, potentially enabling access to a wide range of functional P–B materials via post‐polymerization modification and facilitating the incorporation of polyphosphinoboranes into larger macromolecular architectures .…”

“…The halogen‐functionalized phosphine–borane monomers 1a and 1b were synthesized by a multistep procedure,5a,b, which involves a) monolithiation of the respective 1,4‐dihalobenzene with one equivalent of n BuLi, b) subsequent reaction with ClP(N i Pr 2 ) 2 to provide the diaminophosphines (4‐X‐C 6 H 4 )P(N i Pr 2 ) 2 , c) deprotection at phosphorus by addition of HCl/Et 2 O to give the dichlorophosphines (4‐X‐C 6 H 4 )PCl 2 , and d) reaction with LiBH 4 to generate the phosphine–boranes 1a and 1b ( Scheme ). Final crystallization of the products after the last step at −60 °C from n ‐hexane afforded 1a and 1b in overall yields of 52% and 69%, respectively.…”