173467-68-0Relevant articles and documents
Stereoelectronic factors that influence kinetic and thermodynamic sites of protonation of (η6-arene)molybdenum(phosphine)3 complexes
Ashby, Michael T.,Asirvatham, Victor S.,Kowalski, Angela S.,Khan, Masood A.
, p. 5004 - 5016 (2008/10/08)
We have previously reported (J. Am. Chem. Soc. 1995, 117, 12639) the arene complex (η6-C6H6)Mo(TRIPOD) (1), where TRIPOD = 1,1,1-tris(diphenylphosphinomethyl)ethane, is protonated upon addition of 1 equiv of D+ to yield the metal-hydride [η6-C6H5D)Mo-(TRIPOD)H]+ (1H+-d1) via exo addition of D+ to the arene ligand followed by migration to the metal of the endo proton of the putative η5-cyclohexadienyl complex [(η5-C6H6D)Mo-(TRIPOD)]+ (1?). The opposite isotopomer is obtained when (η6-C6D6)Mo(TRIPOD) (1-d6) is employed to give [(η6-C6D5H)Mo(TRIPOD)D]+ (1D+-d6). Our recent investigation of the electronic structure of 1 and its one-electron oxidized derivative [(η6-C6H6)Mo(TRIPOD)]+ (1+) suggested the mechanism of protonation is not driven entirely by electronic factors; steric factors are likely important for indirect protonation of 1. Consistent with that conclusion, we report herein that some other phosphine derivatives are protonated directly at the metal center. Thus, under the same reaction conditions the monodentate phosphine derivatives (η6-C6D6)Mo(PR3)3, where PR3 = PPh2Me (3-d6), PPhMe2 (4-d6), and PMe3 (5-d6), are protonated directly at the metal 0%, 20%, and 40% of the time, respectively. The remainder of the protonations take place via the aforementioned indirect protonation of the arene ligand. Surprisingly, (η6-C6D6)Mo(TRIPHOS) (2-d6), where TRIPHOS = bis(2-diphenylphosphinoethyl)phenylphosphine, reacts with H+ by direct protonation of the metal center 80% of the time to give a C1-symmetric kinetic hydride [(η6-C6D6)Mo(TRIPHOS)H]+ (2KH+-d6). The other 20% of the time 2-d6 reacts via the arene mechanism to give [(η6-C6D5H)Mo(TRIPHOS)D]+ (2KD+-d6). The two enantiomeric forms of 2KH+ are observed to be in rapid equilibrium (k = 6.4 × 103 s-1 at -87°C). Compound 2KH+ eventually isomerizes (k = 6.1 × 10-4 s-1 at -60°C) to the Cs-symmetric thermodynamic hydride [(η6-C6H6)Mo-(TRIPHOS)H]+ (2TH+). We conclude from the crystal structures of 1-4, tracer studies, and electronic structure calculations of the five derivatives 1-5 presented herein that indirect protonation does not represent a general mechanism for this class of compounds. In addition to electronic factors that favor direct protonation of the metal center from an equatorial trajectory, subtle steric demands of the phosphine ligands play an important role in dictating whether a proton initially attacks the arene ligand or the metal of 1-5. The semiempirical PM3(tm) method may be used to map metal-based frontier orbitals of the complexes 1-5 onto plots of their surface electron densities, thereby revealing sterically accessible metal electron density. Such plots provide straightforward models that predict the mechanisms of protonation.
Synthesis and characterization of arene, mono-and dihydrido-arene, monohydrido-cyclopentadienyl, and phosphite complexes of molybdenum containing the tridentate ligand PhP (CH2CH2PPh2)2
Hammud, Hassan H.,George, T. Adrian,Kurk, David N.,Shoemaker, Richard K.
, p. 153 - 159 (2008/10/08)
The reduction of [MoCl3(TRI)], where TRI = PhP(CH2CH2PPh2)2, with sodium amalgam in benzene, toluene or anisole resulted in the formation of the corresponding [Mo(η6-arene) (TRI)] complex. Each complex displayed a reversible one-electron oxidation in the cyclic voltammogram at ～ - 1.0 V versus the ferrocenium/ferrocene couple at 0.0 V, corresponding to the oxidation of Mo(0) to Mo(1+). A second pseudo-reversible oxidation occurred at ～0.7 V more positive. Monoprotonation of the arene complexes with CF3COOH in THF resulted in the isolation of [Mo(H)(η6-arene)(TRI)][CF3COO] (δ(MoH) ～ - 6 ppm). In neat CF3COOH or HBF4, evidence of diprotonation of the arene complexes was observed in the 1H NMR spectra. Upon work-up, only the monoprotonated product was isolated. [Mo(TRI)(P(OMe)3)] was formed by the reduction of [MoCl3(TRI)] in the presence of a small excess of P(OMe)3. Attempts to prepare [Mo(TRI)(PMe3)3] by a similar method resulted in a product that readily absorbed N2 to form fac-[Mo(N2)(TRI)(PMe3)2]. Yellow [Mo(H)(η5-C5H5)(TRI)] was formed by (i) the reduction of [MoCl3(TRI)] with sodium amalgam in the presence of cyclopentadiene, or (ii) heating fac-[Mo(N2)(TRI)(PMe3)2] with cyclopentadiene in heptane, or (iii) heating trans-[Mo(N2)2(TRI)(PPh3)] with cyclopentadiene in THF.
Synthesis and characterization of low valent complexes of molybdenum containing the tridentate ligand PhP(CH2CH2PPh2)2(TRI)
George, T. Adrian,Hammund, Hassan H.
, p. C1 - C3 (2007/10/03)
Reduction of with sodium amalgam in the presence of potential ligands results in the formation of a series of new complexes containing η3-TRI.These include 6-C6H5R)(TRI) (where R = H, Me, MeO), 5-C5H5)(H)(TRI)>, (where R = Me, Et), and fac-.Protonation of the arene complexes results in the formation of both mono- and diprotonated complexes.Keywords: Molybdenum; Phosphine; Phosphite; Dinitrogen; η6-Benzene complex; η6-Arene complex.
