Switching the Electron‐Donating Ability of Phosphines through Proton‐Responsive Imidazolin‐2‐ylidenamino Substituents

Abstract

Stimuli‐responsive ancillary ligands are valuable tools to control the activity and selectivity of transition‐metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton‐responsive imidazolin‐2‐ylidenamino substituents are reported. Determination of the ligand‐donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm−1, hence allowing for switching of the electron‐donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm−1).

Pyridinylidenaminophosphines: Facile Access to Highly Electron‐Rich Phosphines

“…Moreover, the lower TEP value of 4 d and 6 a compared with 4 a and 6 b can be attributed to the positive inductive effect of the additional methyl group. We recently showed that the π‐donor ability of substituents R 5 strongly responds to the interaction of the exocyclic nitrogen atom with protons . To exclude that the TEP values might be influenced by such interactions with CH protons in dichloromethane, we recorded the IR spectra of the nickel complexes of phosphines 2 a , 4 a , and 4 d in the solid state.…”

“…Recently, we discovered that the electron‐donating ability of phosphines can be significantly increased, beyond the range of N‐heterocyclic carbenes, by attaching up to three strong π‐donating imidazoline‐2‐ylidenamino substituents to the phosphorus atom . The resulting phosphines are promising ligands in catalysis and can activate and transform chemically rather inert species such as CO 2 or SF 6 …”

“…We recently showed that the p-donor ability of substituents R 5 strongly responds to the interaction of the exocyclic nitrogen atom with protons. [16] To exclude that the TEP values might be influenced by such interactions with CH protons in dichloromethane, we recorded the IR spectra of the nickel complexes of phosphines 2 a, 4 a, and 4 d in the solid state. In fact, the decrease in TEP values observed for the solid compounds (4 a: DTEP = 1.3 cm, 4 d: DTEP = 1.7, 2 a: DTEP = 2.9 cm À1 ) suggests that the TEP values in CH 2 Cl 2 are affected by the steric accessibility of the exocyclic nitrogen atom.…”

“…[14] The resulting phosphines are promising ligands in catalysis and can activate and transform chemically rather inert species such as CO 2 or SF 6 . [15][16][17][18][19][20] More recently, the family of highly electron-rich phosphines with p-donor substituents has been extended by the groups of Gessner and Sundermayer using phosphoniumylidyl (R 3 P= CRÀ) and phosphazenyl (R 3 P=NÀ) groups, respectively. [21,22] Although strongly donating phosphines have great potential as ligands in coordination chemistry and catalysis, [12,23] their broad application as ligands, but more importantly in stoichiometric reactions, is often hampered by their rather difficult synthesis.…”

Deutsche Bunsengesellschaft Awards

“…Fabian Dielmann (University of Münster), last featured in this section when he won a FCI Dozentenpreis in 2018, is the recipient of one of the Heinz Maier Leibnitz Prizes for 2019, presented by the German Research Foundation (Deutsche Forschungsgemeinschaft) to young scientists under the age of 40 years. Dielmann has recently published a Full Paper on switching the electron‐donating ability of phosphines in Chemistry—A European Journal …”