Fluxionality of [(Ph₃P)₃Rh(X)]: The Extreme Case of X = CF₃

Abstract

[(Ph(3)P)(3)Rh(F)] reacts with CF(3)SiMe(3) to produce trans-[(Ph(3)P)(2)Rh(CF(2))(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph(3)P shifts all of the equilibria to [(Ph(3)P)(3)Rh(CF(3))] (2; X-ray) as the only NMR-observable and isolable (84%) species. Complex 2 is uniquely highly fluxional in solution, maintaining ligand exchange even at -100 degrees C (12.1 s(-1)). Activation parameters have been determined (variable-temperature (31)P NMR) for the similar but slower exchange in the Me analogue of 2, [(Ph(3)P)(3)Rh(CH(3))]: E(a) = 16.5 +/- 0.6 kcal mol(-1), DeltaG(double dagger) = 12.9 kcal mol(-1) (calculated at -30 degrees C), DeltaH(double dagger) = 16.0 +/- 0.6 kcal mol(-1), and DeltaS(double dagger) = 12.8 +/- 2.3 e.u. Intramolecular exchange in [(R(3)P)(3)Rh(X)] occurs (DFT, MP2//BP86) via a distorted trigonal transition state (TS) with X in an axial position trans to a vacant site. The rearrangement is governed by a combination of steric and electronic factors and is facilitated by bulkier ligands on Rh as well as by strongly donating X that stabilize the TS. The Rh atom in [(H(3)P)(3)Rh(X)] has been shown to be more negatively charged (NPA) for X = CF(3) than for X = CH(3), despite the strongly oppositely charged carbon atoms of the CF(3) (+0.79e) and CH(3) (-0.96e) ligands. Clarification of stereochemical rigidity (X = halide, CN, OR, NR(2)) versus fluxionality (X = H, Alk, Ar, CF(3)) is provided, along with a resolution of the long-standing contradiction between the electron-withdrawing effect of CF(3) in organic compounds and its strong trans influence (electron donation) in metal complexes.

Fluorocarbene, fluoroolefin, and fluorocarbyne complexes of Rh

“…, MCF 2 ). 5,10a,23,24 The observed 2 J Rh–F = 49 Hz was also similar to that of Grushin's trans -(Ph 3 P) 2 (F)RhCF 2, which possessed a 2 J Rh–F of 33 Hz. 23 …”

“…23 Indeed, treatment of (PNP)Rh(TBE) (TBE = tert -butylethylene) with CsF/Me 3 SiCF 3 (Ruppert's reagent) resulted in complete consumption of (PNP)Rh(TBE) and the formation of (PNP)RhCF 2 and (PNP)Rh(C 2 F 4 ) in about 85 : 15 ratio (NMR evidence). 25 We were able to isolate (PNP)RhCF 2 in 52% yield and of >98% purity by recrystallization.…”

Simple, Stable, and Easily Accessible Well‐Defined CuCF3 Aromatic Trifluoromethylating Agents

“…This situation might be due to nearly equal electron donation from the CH 3 and the CF 3 ligands, [2] as is the case for the [(Ph 3 P) 3 Rh(CF 3 )]/[(Ph 3 P) 3 Rh(CH 3 )] pair. [16] Room-temperature 31 P and 19 F NMR spectra of 1 (CD 2 Cl 2 ) displayed singlets at d = À0.8 and À26.2 ppm, respectively, in accord with the literature data.…”

Simple, Stable, and Easily Accessible Well‐Defined CuCF3 Aromatic Trifluoromethylating Agents

“…This situation might be due to nearly equal electron donation from the CH 3 and the CF 3 ligands, [2] as is the case for the [(Ph 3 P) 3 Rh(CF 3 )]/[(Ph 3 P) 3 Rh(CH 3 )] pair. [16] Room-temperature 31 P and 19 F NMR spectra of 1 (CD 2 Cl 2 ) displayed singlets at d = À0.8 and À26.2 ppm, respectively, in accord with the literature data. [11] These patterns did not change considerably in the temperature range of + 25 to À90 8C.…”