Stabilization of the Tautomers HP(OH)2 and P(OH)3 of Hypophosphorous and Phosphorous Acids as Ligands

Treatment of [CpRu(PPh3)2Cl] 1 with the stoichiometric amount of H3PO2 or H3PO3 in the presence of chloride scavengers (AgCF3SO3 or TlPF6) yields compounds of formula [CpRu(PPh3)2(HP(OH)2)]Y (Y = CF3SO3 2a or PF6 2b) and [CpRu(PPh3)2(P(OH)3)]Y (Y = CF3SO3 3a or PF6 3b) which contain, respectively, the HP(OH)2 and P(OH)3 tautomers of hypophosphorous and phosphorous acids bound to ruthenium through the phosphorus atom. The triflate derivatives 2a and 3a react further with hypophosphorous or phosphorous acids to yield, respectively, the complexes [CpRu(PPh3)(HP(OH)2)2]- CF3SO3 4 and [CpRu(PPh3)(P(OH)3)2]CF3SO3 5 which are formed by substitution of one molecule of the acid for a coordinated triphenylphosphine molecule. The compounds 2 and 3 are quite stable in the solid state and in solutions of common organic solvents, but the hexafluorophosphate derivatives undergo easy transformations in CH2Cl2: the hypophosphorous acid complex 2b yields the compound [CpRu(PPh3)2(HP(OH)2)]PF2O2 6, whose difluorophosphate anion originates from hydrolysis of PF6 -; the phosphorous acid complex 3b yields the compound [CpRu(PPh3)2(PF(OH)2)]PF2O2 7, which is produced by hydrolysis of hexafluorophosphate and substitution of a fluorine for an OH group of the coordinated acid molecule. All the compounds have been characterized by elemental analyses and NMR measurements. The crystal structures of 2a, 3a and 7 have been determined by X-ray diffraction methods. Introduction The hypophosphorous and phosphorous acids (systematic names phosphinic and phosphonic acid, respectively) have tetrahedral structures, [H2PO(OH)]1 and [HPO(OH)2]2 with two and, respectively, one hydrogen atom directly bound to the phosphorus. The hypophosphorous acid therefore acts as a relatively strong monobasic acid (pKa = 1.24),3 yielding hypophosphites [H2PO2]-; the phosphorous acid is dibasic and its completely deprotonated anion is [HPO3]2-.The coordination properties of the above species have been seldom considered and the sparse results suggest that the acids and their anions act as ligands via their oxygen atoms, while the phosphorus atom is devoid of donor properties, lacking a lone pair.4 Both hypophosphorous and phosphorous acids exhibit tautomeric equilibria between tetrahedral (a) and pyramidal (b) structures (Scheme 1) and it is well established that the phosphoryl tautomer (a) predominates.5 Accordingly, it has been difficult to trace the less stable tautomer (b) of each acid. They have been postulated as intermediates in some palladium catalytic reactions6 and in the oxidation of phosphorous acid by metals.7 Recently the (b) tautomeric form of both hypophosphorous8 and the phosphorous9 acid has been stabilized by coordination to nickel and palladium inM3MQ4(H2O)9 (M =Mo, W; M = Ni, Pd; Q = aDipartimento di Chimica, Universit`a di Firenze, via della Lastruccia n. 3, 50019, Sesto Fiorentino, Firenze, Italy. E-mail: piero.stoppioni@unifi.it bICCOM CNR, Via Madonna del Piano, snc-50019, Sesto Fiorentino, Firenze, Italy. Email: mperuzzini@iccom.cnr.it Scheme 1