A.S. Wells

Mössbauer studies on ferrocene complexes IX. Phosphaferrocenes and their protonated derivatives

Abstract A number of mono- and di-phosphaterrocenes have been synthesised and their NMR (1H, 31P) and Mossbauer spectra measured. The presence of a phosphorus atom in the cyclopentadienyl (Cp) moiety causes a reduction of about 0.3 mm s−1 in the quadrupole splitting (QS) compared with what of ferrocene. The effect appears to be additive, as does the effect of Cp ring substituents. Thus 2,5-diphenylphosphaferrocene (I), 3,3′,4,4′-tetramethyldiphosphaferrocene (II) and 2,2′,5,5′-tetraphenyldiphosphaferrocene (III) have QS values of 2.03, 1.82 and 1.52 mm s−1, respectively. III dissolves readily and reversibly in triflic acid (CF3SO3H) to give a phenyl ring protonated species. The Mossbauer spectrum of the frozen solution shows a significant increase in QS to 1.77 mm s−1 which is interpreted in terms of iron participation in the stabilisation of the σ complex. H/D exchange reactions and oxidation of phosphaferrocenes are briefly reported together with an improved synthesis of the important precursor, 1,2,5-triphenylphosphole.

A new synthetic route to monophosphaferrocenes

Abstract Phosphacyclopentadienyl anions ( A ) derived from 1 phenylphospholes cleave the η 6 -bonded arene from η 6 -arene-η 5 -cyclopentadienyliron( II ) cations ( B )via nucleophilic attack at iron to yield η 5 -cyclopentadienyl- η 5 -phosphacyclopentadienyliron( II ) sandwich compounds ( C ). The reaction provides a versatile synthetic route to phosphaferrocenes with one general reaction giving a pathway to phosphaferrocenes with greatly varying degrees of ring substitution.

Complexes of diphosphaferrocenes with Lewis acids and Cu(I), Ag(I) salts

Abstract Complexes of diphosphaferrocenes have been prepared using (a) Lewis acids BF 3 ·Et 2 O, A1C1 3 and (b) Cu(I) and Ag(I) salts. The 31 P spectra showed that BF 3 ·Et 2 O complexes at phosphorus (characterised by a large downfield shift of 103 ppm) whereas all the other adducts involved binding at iron (characterised by large upfield shifts of 36–94 ppm). Deep purple adducts were isolated and characterised for the reaction of 2, 2′, 5, 5′-tetraphenyldipho sphaferrocene with silver trifluoroacetate and triflate and also with copper(I) iodide. Similar complexes were isolated from 3, 3′, 4, 4′-tetramethyldiphosphaferrocene. 57 Fe Mossbauer spectroscopy of the adducts showed that unlike the ferrocene system, quadrupole splittings generally decreased on iron complexation. The results are discussed in terms of current MO theory of ferrocene and diphosphaferrocene.

Mössbauer and NMR spectroscopic studies of diphosphaferrocene in trifluoromethanesulphonic (triflic) acid

Abstract A fange of diphosphaferrocenes has been investigated in trifluoromethanesulphonic acid where iron protonated species result contrary to previous findings. 2,2′5,5′-Tetraphenyldiphosphaferrocene (TPDPF) has been shown to have an iron protonated structure rather than one arising from protonation of the phenyl side groups. Iron protonation has been demonstrated by 57 Fe Mossbauer spectroscopy and 31 p NMR. The iron bound proton is not always observed by 1 H NMR spectroscopy due to fast exchange which is evident by the loss of resolution of phosphorus-hydrogen coupling constants in the 31 p NMR. The structure of the protonated derivatives is discussed. An alternative synthetic route to alkyl diphosphaferrocenes has also been included.

An iron-57 mössbauer study of (η6-arene)(η5-cyclopentadienyl)iron(II) salts

57Fe Mossbauer spectra have been obtained for a series of (η6-arene)(η5-cyclopentadienyl)iron(II) complexes (ArFe+Cp). Simple Hammett plots of quadrupole splittings (Δ) for Ar ring substituents are sigmoidal, indicating limits to electron withdrawal and donation. For these simple plots the data are best fitted by use of σ−R constants, in keeping with the proposed ϵ2-π* back-bonding in the iron—arene interaction. The correlations are improved by use of a dual parameter approach with a 40% inductive component. A plot of Δ for (ArFe+Cp) complexes versus those of corresponding ferrocenes is approximately linear buth with a slope of 2.3, indicating enhanced substituent effects in the former series due to the positive charge on Fe. Substituent effects on the Cp ring are directly opposite to those found in ferrocenes, and Hammett correlations follow σ+R rather than the σ−R constants found to apply for the arene substituents. Electron-releasing groups in the arene ring have been shown to cause marked attenuation of the substituent effects on Δ for both arene and Cp substituents. Mossbauer parameters are also presented and discussed for polysubstituted complexes, polycyclic systems, and novel phosphacyclopentadienyl complexes and adducts. Finally, some tetracyanoquinodimethane complexes (TCNQ) have been synthesised and their Mossbauer parameters are discussed in terms of population of a conduction band.

The formation and spectroscopic identification of some α-phosphaferrocenyl carbenium ions

Abstract A number of α-phosphaferrocenyl alcohols have been synthesized and their behaviour in trifluoroacetic acid solution examined by investigating changes in 31 P NMR and 57 Fe Mossbauer spectra. The results indicate that dehydration occurs and carbenium ions are formed. These were found to have considerably lower stability than α-ferrocenyl carbenium ions.

The cleavage of cyclopentadienyl substituted η6-arene η5-cyclopentadienyl iron(II) cations by phospholyl anions. A facile route to 1′-substituted monophosphaferrocenes

Abstract η 6 -Arene η 5 -cyclopentadienyl iron PF 6 − salts bearing substituents on the cyclopentadienyl ring are cleaved by phosphacyclopentadienyl anions (PCp − M + ) to give 1′-substituted monophosphaferrocenes. Such derivatives are unavailable via chemical modification of monophosphaferrocenes such as Friedel-Crafts reaction or metallation, l′-Alkyl, acyl, N , N -dimethylaminomethyl and l′-carboxylic acids have been synthesised and characterised. Nucleophilic displacement of chloride by PCp − M + in the η 6 -chlorobenzene cation produces novel 1- phenylphospholes one of which has been isolated and characterised. The synthetic route was found to be unsuccessful in the production of azaferrocene.

Structural studies of protonated monophosphaferrocenes using mcolon; ssbauer and NMR spectroscopy

Some monophosphaferrocenes have been studied in strongly acidic media. In trifluoromethanesulphonic acid (triflic acid) the species are iron-protonated as shown by the presence of a hydrido signal in the 1H NMR spectra between tau; 0.8 and tau; 3.0 ppm. Such systems show novel 2JPH coupling transmitted via the iron centre. Evidence for iron protonation is also found in the 57Fe Mcolon; ssbauer spectra of triflic acid frozen solutions. Deuteriation of monophosphaferrocenes in triflic acid-d1 is discussed, together with the sulphonation of phenylated derivatives in concentrated sulphuric acid. The 13C NMR spectra of phenyl-substituted phosphaferrocenes are also reported, and compared with known data for diphosphaferrocenes.

Mössbauer and NMR studies of protonated acyl diphosphaferrocenes

Abstract The acyl derivatives of 3,3′,4,4′-tetramethyldi- phosphaferrocene (TMDPF) have been examined in strong acids by 57 Fe Mossbauer, 1 H and 31 P NMR spectroscopy. As with ferrocenyl ketones, protonation was found to occur at the keto function, the diphosphaferrocenyl ketones having comparable or, in some cases, reduced basicities compared to ferrocenyl ketones. [p ]Trends in the 57 Fe Mossbauer parameters are not as additive as in ferrocene systems due to steric crowding. The keto derivatives show some unusual deuteriation patterns and these have been compared with those of ferrocenyl ketones. The 13 C spectra of several derivatives have been reported to illustrate the rather complex stereochemistry found in these derivatives.

Synthesis and structure of η6-(biphenyl)-η5- (cyclopentadienyl)iron(II) hexaflourophosphates

Abstract A series of η 6 -(biphenyl)-η 5 -(cyclopentadienyl)iron(II) hexafluorophosphates have been prepared. Demethylation occured during the synthesis of the 2′-OMe derivatives to yield the correspoding 2′-OH product. The mechanism of this process is discussed. In all cases the complexation involved the unsubstituted phenyl ring. From 13 C NMR data, values of Hammett resonance parameters σ R , were calculated which show that the [CpfeC 6 H 5 ] + group behaves as an electron-withdrawing substituent comparable in strength to the cyano group. Approximate values of the biphenyl interplanar angle (θ) were obtained. θ appeared to be significantly lower when electron-releasing substituents were present. 57 Fe Mossbauer data support the strong electron acceptor properties of the [CpFe + C 6 H 5 ] moiety. In particular the quadrupole splitting ( QS ) shows a marked increase for the 4-OMe derivative relative to the unsubstituted comples. This is in direct contrast to the aryl ferrocenes. Here, the ferrocenyl and OMe substituents are electronically and so there is no ( QS ) enhancement.