Synthesis and characterisation of ferrocenyl-phosphonic and -arsonic acids

The ferrocene-derived acids FcCH2CH2E(O)(OH)2 [4, E=P; 10, E=As; Fc=Fe(η5-C5H5)(η5-C5H4)] have been synthesized by the reaction of FcCH2CH2Br with either P(OEt)3 followed by hydrolysis, or with sodium arsenite followed by acidification. Reaction of FcCH2OH with (EtO)2P(O)Na gave FcP(O)(OEt)(OH), which was converted to FcCH2P(O)(OH)2 (3) by silyl ester hydrolysis using Me3SiBr–Et3N followed by aqueous work-up. Similarly, the known phosphonic acid FcP(O)(OH)2 and the new derivatives 1,1′-Fc′[P(O)(OH)2]2 [Fc′=Fe(η5-C5H4)2] and 1,1′-Fc′[CH2P(O)(OH)2]2 (7) have been synthesized via their corresponding esters. X-ray crystal structure determinations have been carried out on 3 and 7, and the hydrogen-bonding networks discussed. Electrospray mass spectrometry has been employed in the characterization of the various acids. Phosphonic acids give the expected [M–H]− ions and their fragmentation at elevated cone voltages has been found to be dependent on the acid. FcP(O)(OH)2 fragments to [C5H4PO2H]−, but in contrast Fc(CH2)nP(O)(OH)2 (n=1, 2) give Fe{η5-C5H4(CH2)nP(O)O2]− ions, which are proposed to have an intramolecular interaction between the Fe atom and the phosphonate group. In contrast, arsonic acid (10), together with PhAs(O)(OH)2 for comparison, undergo facile alkylation (in methanol or ethanol solvent), and at elevated cone voltages (e.g. >60 V) undergo carbon–arsenic bond cleavage giving [CpFeAs(O)(OR)O]− (R=H, Me, Et) and ultimately [AsO2]− ions.