Iron-57 NMR and Structural Study of [Fe(η⁵-Cp)(SnPh₃)(CO)(PR₃)] (PR₃ = Phosphine, Phosphite). Separation of Steric and Electronic σ and π Effects

Abstract

The complexes [Fe(Cp)(SnPh3)(CO)(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P(CH2Ph)3 (6), PPh3 (7), P(4-MeC6H4)3 (8), P(4-MeOC6H4)3 (9), P(4-FC6H4)3 (10), P(4-CF3C6H4)3 (11), P(NMe2)3 (12), P(OMe)3 (13), P(OPh)3 (14)), which have been characterized by X-ray crystallography (except for 1 and 4), infrared spectroscopy (carbonyl stretching frequency, νCO), and NMR spectroscopy (13C, 31P, 57Fe, 119Sn) offer some insight into the response of the iron nucleus to changes in the electronic and steric properties of the PR3 ligand. A fairly good correlation is found between the 57Fe chemical shift and the Tolman cone angle θ for PR3 and a rather poorer correlation between δ(57Fe) and νCO. However, for the subseries of complexes 7–11 having PR3 = P(4-XC6H4)3 (X = H, Me, MeO, F, CF3), the correlation between δ(57Fe) and νCO is very good. Since the steric properties of these ligands, from the point of view of the metal, are identical (θ = 145°), this provides a means of separating the steric and electronic contributions of PR3 to δ(57Fe). The electronic contribution of PR3 to δ(57Fe) can be further separated into σ and π components by making use of the finding that the π component of the Fe–P bond has a negligible influence on δ(57Fe), unlike its influence on νCO. The ligands PMe3, PnBu3, PCy3, PMe2Ph, PMePh2, and P(NMe2)3 are found to be “pure” σ donors, P(OMe)3 and P(OPh)3 are found to be π acceptors of differing strength, and P(4-XC6H4)3 is found to show weak but clearly distinguishable π acceptor properties.

A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible “Nonspectator” L/X‐Switching

“…This trend tracks qualitatively with the J Se‐P coupling constants for phosphorus selenides 1a ⋅Se ( J Se‐P = 907 Hz) and 1b ⋅Se ( J Se‐P = 784 Hz), suggesting to a first approximation that 1a is a weaker σ‐donor than 1b (see Table for collected metrical data). The 57 Fe NMR chemical shifts (obtained indirectly by 2D Fe−P correlation solution NMR experiments due to the low receptivity of the 57 Fe nucleus) for 1a ⋅Fp + ( δ =616 ppm) and 1b ⋅Fp + ( δ =688 ppm) are consistent with this interpretation, based on trends established for related cyclopentadienyliron complexes …”

“…The 57 Fe NMR chemical shifts (obtained indirectly by 2D Fe À Pc orrelation solution NMR experiments due to the low receptivity of the 57 Fe nucleus [28] )f or 1a•Fp + (d = 616 ppm) and 1b•Fp + (d = 688 ppm) are consistent with this interpretation, based on trends established for related cyclopentadienyliron complexes. [29] Further distinctions between 1a•Fp + and 1b•Fp + are manifest in structural analyses based on X-ray diffractometry data obtained with single-crystalline samples ( These metrics illustrate the enhanced nontrigonal local geometry about phosphorus for 1a•Fp + as compared to 1b•Fp + ,c onsistent with the structural distinctions between the free ligands. [26] Forr eference,t he N 2 -P-N 3 bond angle of 1a undertaken with the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) method [30] as implemented in the ADF modeling program [31] at the BP86/ def2-TZVP level of density functional theory (Table 1, see Supporting Information for full details).…”

“…The 57 Fe NMR chemical shifts (obtained indirectly by 2D Fe-P correlation solution NMR experiments due to the low receptivity of the 57 Fe nucleus 28 ) for 1a•Fp + (δ 616 ppm) and 1b•Fp + (δ 688 ppm) are consistent with this interpretation, based on trends established for related cyclopentadienyliron complexes. 29 Further distinctions between 1a•Fp + and 1b•Fp + are manifest in structural analyses based on X-ray diffractometry data obtained with single-crystalline samples (Figure 3). Most evidently, compound 1a•Fp + features a shorter Fe-P bond length (dFe-P = 2.1809(4) Å) as compared to compound 1b•Fp + (dFe-P = 2.2381(5) Å).…”

Bond dissociation energies of carbonyl gold complexes: a new descriptor of ligand effects in gold(i) complexes?

“…Next, we sought to evaluate the influence of electron density on the aromatic ring of triphenylphosphine by introduction of electron donating (OMe) and electron withdrawing (CF3) groups on para-position (complexes 9-pOMe and 10-pCF3, respectively) in order to compare our results with the CO probe which is often used to investigate the electronic properties of ligands by observation of CO of metal-carbonyl complexes. For example the CO frequencies reported for P(C6H4-p-OMe)3, PPh3, P(C6H4-p-CF3)3 in complexes FeCp(SnPPh3)(CO)(PR3) 40 are 1903, 1904 and 1914 cm -1 , respectively, indicating that P(C6H4-p-CF3)3 is the most electron poor ligand of this series. 41 By comparison with this probe, we would expect a higher BDE for the 9-pOMe complex, bearing an electron rich phosphine which could offer electron density to the metal and allow higher -retrodonation to CO, compared to the 10-pCF3 complex, an electron poor phosphine.…”

Exploring Phosphine Electronic Effects on Molybdenum Complexes: A Combined Photoelectron Spectroscopy and Energy Decomposition Analysis Study

“…Experimentally, several approaches in addition to Tolman's have indeed been proposed to probe these electronic effects. Spectroscopy (NMR, [20][21] photoelectron spectroscopy, [22][23] ..) is one of the method of choice as it enables to directly survey the electronic and/or vibrational structure of the studied systems. For instance,…”