Electronic aspects of the phosphine-oxide / phosphinous acid tautomerism and the assisting role of transition metal centers Gabriele Manca, Maria Caporali, Andrea Ienco, Maurizio Peruzzini * , Carlo Mealli * Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy article info Article history: Received 23 September 2013 Received in revised form 22 October 2013 Accepted 23 October 2013 Keywords: Phosphine oxide Phosphinous acid Tautomerization Ruthenium Water chemistry DFT mechanistic studies abstract The H 3 P(O) / H 2 P(OH) tautomerism is addressed by experimental and DFT approaches. The process, disfavored for the free molecule, is easier over metal fragments of the type {CpRu II L 2 } n (L ¼ uncharged or anionic phosphine ligand), with an energy barrier reduced to one fourth. The free H 3 P(O) molecule is a very weak acid and hardly a proton migrates intra-molecularly towards the oxo atom, as expected for classic acid-base reactions. Rather, some electron density of the highly covalent PeH bond remains anchored to the H atom at least up to the TS with the barrier originated from the electronic repulsion with the approached O lone pair. Beyond TS, the H atom transforms into a proton after having released its electron portion at the P atom (lone pair). The calculations show the experimentally undetected inter- mediate [CpRu(PR 3 ) 2 (H)(H 2 PO)] n , at which the metal has induced a PeH oxidative addition. Consistent behaviors are found for all the molecules H n (OH) 3n P(O) (n ¼ 3, 2, 1), whereas some anomalies have been experimentally observed with the anionic TPPMS coligands [TPPMS ¼ PPh 2 (m-C 6 H 4 SO 3 )  ], used to favor the chemistry in water. In particular: i) the reaction with H 3 P(O) indicates that the product [CpRu(TPPMS) 2 {H 2 P(OH)}]  exists in two isomeric forms; ii) the tautomerization of H(OH) 2 P(O) is uniquely inhibited. Ad-hoc DFT calculations indicate that the features are attributable to the strong H- bonding networks between the sulphonate substituent and OH group(s) and water as well. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Amongst elements, phosphorus overcomes the frontiers of pure inorganic chemistry [1] being essential to all the known forms of life. For instance, phosphates, as components of the ATP molecule, provide energy to cells participating in metabolic processes. Phosphates are also central in industrial fields, such that of fertil- izers or cleansers [1a], hence they are associated to various pollu- tion problems arising from over-usage. Mastering in the recycle of phosphates and their derivatives has become one of the most important targets of the chemical industrial research, although a deeper knowledge of the element and its electronic behavior re- mains fundamental [1b]. The chemistry of phosphorus is well established for the highest oxidation state (þ5 in phosphates with four PeO bonds), whereas is more ambiguous for the lower ones, especially in molecules containing PeH bonds. In textbooks [2], the H and P atoms of PH 3 are formally assigned þ1 and 3 charges, respectively, although the electronegativity of the elements (2.20 and 2.19, respectively) [3] suggests bond apolarity. The given oxidation states are more appropriate for ammonia due to defi- nitely higher electronegativity of nitrogen (3.5), although in view of the known pK a values (27 vs. 35 [4]) it seems easier to extract a proton from PH 3 than NH 3 . This is explained in terms of the more diffuse negative charge of the associated base PH 2  , hence of its larger stability as separated anion with respect to NH 2  . In the H 3 P(O) / H 2 POH tautomerism, the inner proton transfer can be related to the PH 3 deprotonation, the formal P oxidation state being 1 rather than 3. This applies also to the hydroxo de- rivatives of general formula H n (OH) 3n P(O), where the P oxida- tion states are þ3 and þ2 for n ¼ 1 and 2. In any cases, the equilibrium is totally shifted towards the phosphine oxide spe- cies as in Scheme 1 . The easy availability of pure H 3 P(O), through our recently re- ported electrochemical synthesis from P 4 [5], and avoiding the extreme conditions of the PH 3 þ O 2 reaction [6], allowed a detailed study of the tautomerization mechanism also corroborated by a DFT study. The tautomerization process, excluded for the free molecule by NMR evidence, appears instead quantitative in pres- ence of Ru(II) complexes of the type CpL 2 RuX (L ¼ phosphine ligand, X ¼ weak 2e  donor such as H 2 O or CH 3 CN). In the products, * Corresponding authors. Fax: þ39 055 522 5203. E-mail addresses: maurizio.peruzzini@iccom.cnr.it (M. Peruzzini), mealli@iccom. cnr.it, carlomealli@gmail.com (C. Mealli). Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.10.043 Journal of Organometallic Chemistry xxx (2013) 1e9 Please cite this article in press as: G. Manca, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/ j.jorganchem.2013.10.043