FULL PAPER DOI: 10.1002/ejic.200800075 A Study on the Coordinative Versatility of the Zwitterionic S,N,S Ligand EtNHC(S)Ph 2 P=NPPh 2 C(S)NEt in Its Anionic, Neutral and Cationic Forms – Determination of Absolute pK a Values in CH 2 Cl 2 of Rh I Complexes Massimiliano Delferro, [a] Daniele Cauzzi,* [a] Roberto Pattacini, [a] Matteo Tegoni, [a] Claudia Graiff, [a] and Antonio Tiripicchio [a] Keywords: Rhodium / Zwitterions / Coordination Modes / Acid–base properties The coordination properties of EtNHC(S)Ph 2 P=NPPh 2 C(S)- NEt (HEtSNS) towards Rh I species derived from [Rh(CO) 2 - Cl] 2 and [Rh(cod)Cl] 2 (cod = 1,5-cyclooctadiene) were studied. This ligand is an amphoteric zwitterion, which forms the H 2 EtSNS + cation upon protonation and the EtSNS – di- anion–cation upon deprotonation. All three forms coordinate to metal centers. Their geometrical versatility allows many coordination fashions: S-monodentate; S,S-bidentate (with a bite angle spanning from 90 to 180°); S,N,S-tridentate; N,N,N-tridentate; and S,S-bridging, as determined by X-ray Introduction The zwitterionic ligand EtNHC(S)Ph 2 P=NPPh 2 NEt (HEtSNS, Scheme 1) can be prepared in high yield by double addition of ethylisothiocyanate (EtNCS) to bis(di- phenylphosphanyl)amine (Ph 2 PNHPPh 2 , dppa), [1,2] which forms one thioamidic and one thioamidyl functionality bound to the phosphorus atoms. The opposite charges are formally located on the zwitterionic thioamidyl–phospho- nium, N – C(S)P + , functional group. In solution, a rapid pro- ton exchange between the thioamidic–thioamidyl function- alities is present. This exchange is facilitated by the forma- tion of a pseudoring as a result of an intramolecular N···H···N hydrogen bond, as found in the solid-state struc- ture. [1] Thus, the positive charge can be shared by the two P atoms of the P–N–P group and the negative charge can be delocalized on the two EtNC(S) groups (Scheme 1). As previously reported, EtSNS – acts as an S,N,S-triden- tate or S,S-bidentate ligand towards M I species (M = Rh [1] and Cu, Ag, Au [2] ). The [–Ph 2 PNPPh 2 –] + nitrogen atom can be coordinated as in [Rh(CO)(EtSNS)] and [Cu(EtSNS)] and it weakly interacts with the metal center in [Ag- (EtSNS)] 2 , or it can be not coordinated as in the case of [Au(EtSNS)]. The [–Ph 2 PNPPh 2 –] + group is reminiscent of [a] Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Università degli Studi di Parma, Viale G.P. Usberti 17/A, 43100, Parma, Italy Fax: +39-0521905557 E-mail: cauzzi@unipr.it Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2008, 2302–2312 2302 diffraction methods. The [Rh(CO)EtSNS] complex is a bipro- tic base; its conjugated acids are [Rh(CO)HEtSNS] + and [Rh(CO)H 2 EtSNS] 2+ , and their pK a values were determined in dichloromethane solutions. In this triad of compounds, which are geometrically similar, the Rh I metal center features variable charge density as confirmed by the νCO infrared absorption frequency. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Scheme 1. Lewis structures and formal charge distribution of HEtSNS and its deprotonated (EtSNS – ) and protonated (H 2 EtSNS + ) forms. the classical bis(triphenylphosphane)iminium cation [Ph 3 PNPPh 3 ] + (PPN + ), whose N atom, as evidenced by its bent geometry, may coordinate a metal center, although this behavior has never been observed. [3] We also became inter- ested in studying the coordination properties of HEtSNS and H 2 EtSNS + . Their reactivity towards Rh I species de- rived from [Rh(CO) 2 Cl] 2 and [Rh(cod)Cl] 2 (cod = 1,5 cyclo- octadiene) is reported. Acid–base properties of the [Rh- (CO)(EtSNS)] complex are discussed together with a gene- ral method for the determination of absolute pK a values in dichloromethane.