On the reaction of Ph₂PNHPPh₂ with RNCS (R = Et, Ph, p-NO₂C₆H₄): Preparation of the zwitterionic ligand EtNHC(S)Ph₂P=NPPh₂C(S)NEt (HSNS) and the zwitterionic metalate [(SNS)Rh(CO)]

The reaction of Ph₂PNHPPh₂ (PNP) with RNCS (Et, Ph, p-NO₂-(C₆H₄)) gives addition products resulting from the attack of the P atoms of PNP on the electrophilic carbon atom of the isothiocyanate. When PNP is reacted with EtNCS in a 1:2 molar ratio, the zwitterionic molecule EtNHC(S)PPh₂=NP⁺Ph ₂C(S)N-Et (HSNS) is obtained in high yield. HSNS can be protonated (H₂SNS⁺) or deprotonated (SNS^-), behaving in the latter form as an S,N,S-donor pincer ligand. The reaction of HSNS with [(acac)Rh(CO)2] (acac = acetylacetonate) affords the zwitterionic metalate [(SNS)Rh(CO)]. Other products can be obtained depending on the R group, the PNP/RNCS ratio (1:1 or 1:2), and the reaction temperature. The proposed product of the primary attack of PNP on RNCS, Ph₂PN=PPh₂C(S)NHR (A), cannot be isolated. Reaction of A with another RNCS molecule leads to 1:2 addition compounds of the general formula RNHC(S)PPh₂=NP⁺ Ph₂C(S)N^-R (1), which can rearrange into the non-zwitterionic product RNHC(S)PPh₂=NP(S)Ph2 (2) by eliminating a molecule of RNC. Two molecules of A can react together, yielding 1:1 PNP/RNCS zwitterionic products of the formula RNHCH[PPh₂= NP(S)Ph ₂]PPh₂=NP⁺Ph₂C(S)N^-R (3). Compound 3 can then rearrange into RNHCH[PPh₂=NP(S)Ph ₂]₂ (4) by losing a RNC molecule. When R = Et (a), compounds 1a, 2a (HSNS), and 4a have been isolated and characterized. When R = Ph (b), compounds 2b and 4b can be prepared in high yield. When R = p-NO ₂C₆H₄ (c), only compound 3c is observed and isolated in high yield. The crystal structures of HSNS, [(SNS)Rh(CO)], and of the most representative products have been determined by X-ray diffraction methods.