⁵⁷Fe NMR Study of Ligand Effects in Cyclopentadienyliron Complexes

Abstract

The first systematic 57Fe NMR study of ligand effects in cyclopentadienyliron complexes is presented. Four series encompassing a total of 35 compounds have been studied. Among the compounds are five new ring-substituted complexes with the formula (C5H4Y)Fe(CO)(PPh3)(Me), Y = Me, SiMe3, NEt2, I, and Ph. The crystal structure of (C5H4I)Fe(CO)(PPh3)(Me) (32) was determined. For the series CpFe(CO)2R (type I), 57Fe shielding was found to decrease with the bulkiness of the alkyl ligand R and to correlate with the CO-insertion rate of the complex. In type III complexes CpFe(CO)(L)(COMe), L = PR3, and CO, it is again the steric requirement of the ligand L that dominates the 57Fe chemical shift which increases with larger cone angles (θ). In contrast, a strong electronic effect was found for type II complexes CpFe(CO)(PPh3)X, X = H, Me, and I. Ligands with higher electronegativity induce a shift of the 57Fe resonance to higher frequencies. In the complexes (C5H4Y)Fe(CO)(PPh3)(Me), Y = Me, H, SiMe3, NEt2, I, Ph, COOMe, and COiPr (type IV), again electronic effects are dominant, whereby electron acceptor substituents (e.g. Y = COiPr, COOMe) cause a deshielding of the iron nucleus, relative to complexes with electron donor substituents (e.g. Y = Me, H, SiMe3). Dominant electronic ligand effects in type IV complexes are also apparent from the correlation of δ(57Fe) with the electronic substituent parameter σI. The 57Fe chemical shift is shown to be very sensitive to changes in the ligand sphere, and the effects are discussed in terms of the paramagnetic shielding constant (σpara). The 1 J(57Fe,13C) coupling constants and longitudinal relaxation times T 1 of 57Fe in selected complexes have been determined and ligand effects are discussed.

Correlating Reactivity and Selectivity to Cyclopentadienyl Ligand Properties in Rh(III)-Catalyzed C–H Activation Reactions: An Experimental and Computational Study

“…27 As an alternative approach for electronic parameterization of Cp X ligands, we converted the dimeric [Cp X RhCl 2 ] 2 complexes ( Rh ) into their corresponding monomeric triethylphosphite adducts ([Cp X RhP(OEt) 3 Cl 2 ], Rh” ). We then used 31 P NMR to measure the chemical shift of the phosphorus nucleus (δ P ) and the coupling constant between phosphorus and rhodium ( J Rh-P ).…”

Rhodium‐103 NMR of [Rh(X)(PPh3)3] [X = Cl, N3, NCO, NCS, N(CN)2, NCBPh3, CNBPh3, CN] and derivatives containing CO, isocyanide, pyridine, H2 and O2. Ligand, solvent and temperature effects

“…The complexes [Rh(X) PPh 3 3 2 , NCS, NCO, N 3 , Cl 25 and the trans isomer with X D CN, CNBPh 3 ] was prepared by dissolving [Rh(X) PPh 3 3 ] (7-11 mg giving 0.010-0.020 M in 0.6 ml) in the NMR solvent (toluene-d 8 /toluene or CDCl 3 ) containing ¾10% (v/v) of pyridine; the dihydro complexes 26 were prepared by bubbling H 2 through solutions containing [Rh(X) PPh 3 3 ] or [Rh(X) PPh 3 2 (py)] for 1-2 min; the O 2 complexes 27 were prepared by filling the NMR tube above the solution (of [Rh(X) PPh 3 3 ] (with ¾0.06 M PPh 3 to enhance stability) or [Rh(X) Ph 3 2 (py)]) with O 2 and shaking for 10-20 s. The coordination geometry of the dihydro complexes [Rh(X) H 2 PPh 3 2 (py)] was established to be trans (i.e. phosphines in axial positions, hydrides trans to X and pyridine) and of the dioxygen complexes [Rh(X) O 2 PPh 3 2 (py)] was established to be cis (one phosphine axial, one equatorial, positioned trans to oxygen) by 31 P NMR, which showed a single doublet for the dihydro complexes and two doublets of doublets for the dioxygen complexes (see Table 1 …”

Theoretical investigations of NMR chemical shifts and reactivities of oxovanadium(v) compounds

“…puted Fe-C alkyl bond dissociation energy decreases as R becomes bulkier, 19 consistent with the trend observed for the rate constants for PPh -in-3 duced CO insertion into these bonds. 5 In the w Ž . Ž .x Ž Rh CO C H X q PH model system X s .…”