Photolabilisation of phosphine ligands bound to iron: photofragmentation voltammetry analysis

Abstract

Photofragmentation volammetry is used to demonstrate directly the primary loss of P(Ar), from [ (q5-C,H,)Fe(CO)(PAr,)=C(NHMe)Me]+BF,-(Ar = phenyll andp-tolyl2) on irradiation withvisible light. In contrast, no loss of carbon monoxide is observed.Contrary to accepted dogma that complexes containing both phosphine and carbon monoxide ligands preferentially lose the latter on photolysis, we have recently reported that, on irradiation of aminocarbenes of the type [(q 5-C5H5)Fe(CO)-(PAr,)=C(NHR)R] + BF,-, the primary photoreaction is loss of the phosphine ligand. The primary process of phosphine loss was demonstrated indirectly by stereochemical techniques and phosphine exchange experiments. We describe here the use of photofragmentation voltammetry (PFV) to monitor this process directly for the complexes [(q 5-C5H5)Fe(CO)(PAr,)= C(NHMe)Me]+BF,-(Ar = phenyl 1 and p-tolyl 2): in each case distinctive fingerprints for the relevant free phosphine and for the common residual iron fragment are observed. BF, ' p k 3 Me c BF, ' ! J Paper 3/01371E

NMR, Voltammetric, and Photoelectrochemical Studies on the Dark and Light-Catalyzed Reactions of α-[S2Mo18O62]4- with Aryl- and Alkylphosphines

“…This is expected as the measured oxidation potentials (ca. E p ox ) +0.6 to +0.7 V) for both these species are similar to that for Ph 3 P. [19][20][21][22][23] The final phosphine product was Ph 2 P(CH 2 )P(O)Ph 2 for the methane analogue (n ) 1) as the singlet in the 31 P NMR spectrum (δ ) -21.0 ppm) was converted to two doublets (δ P ) -27.4; δ PO ) 31.2; J( 31 P-31 P) ) 49 Hz), corresponding to inequivalent P atoms in the final product. 24 The polyoxomolybdate steady-state voltammetric response of a reaction mixture of [S 2 Mo 18 O 62 ] 4and Ph 2 P(CH 2 ) n PPh 2 (n ) 1 or 2) after 24 h was identical to that shown in Figure 1b after 48 h. Consequently, the final polyoxomolybdate species formed is identical to that produced on reaction with Ph 3 P.…”

“…Ph 3 P itself is irreversibly oxidized at +0.70 V under the conditions of Figure 1a, in agreement with other studies in different media. [19][20][21][22][23] In the presence of Ph 3 P, the nature of the voltammograms change slowly with time.…”

“…16 The platinum channel electrode was held at a potential of +0.30 V which will oxidize photoreduced forms of [S 2 Mo 18 O 62 ] 4-, so permitting rapid detection of such species. Neither Ph 3 P nor Ph 3 PO undergo redox processes at +0.30 V in acetonitrile, with the former species being oxidized only at potentials above +0.60 V. [19][20][21][22][23] Light (300-400 nm; intensity ∼10 mW cm -2 ) periodically irradiated the platinum working electrode surface, providing a phototransient response, similar to that observed on irradiation of solutions of [S 2 Mo 18 O 62 ] 4and toluene/THF. 16 The resultant photocurrent generated at the working electrode was measured as a function of the flow rate (V f ) 10 -3 -10 -1 cm 3 s -1 ) of the reaction solution over the electrode surface (Figure 5).…”

“…On the basis of the above results an overall reaction scheme can be proposed: Ph 3 P undergoes an irreversible one-electron oxidation at platinum electrodes in acetonitrile solution, and the initial product of this reaction is believed to be the phosphinium radical cation (Ph 3 P •+ ). [19][20][21][22][23]25 Hence it is postulated that, in the rate-determining step, Ph 3 P initially transfers one electron to [S 2 Mo 18 O 62 ] 4-:…”

“…16 The oxidizing power of polyoxometalate species has been reviewed recently. 7 Whereas toluene and THF have formal oxidation potentials greater than +1.1 V (vs Fc + / Fc) and require photoexcitation of the polyoxo-cluster to undergo oxidation, the peak potential (and hence the formal oxidation potential) of Ph 3 P is lower (+0.6 to +1.0 V [19][20][21][22][23] ) and it is oxidized slowly in the absence of light. Ferrocene has a much lower formal oxidation potential (0.0 V) and is oxidized rapidly in the absence of light.…”