Novel metal-to-metal silyl-migration reactions in heterometallic complexes

An unprecedented, intramolecular metal-to-metal silyl ligand migration reac tion has been discovered in a series of phosphido-bridged iron-platinum com plexes and which may be triggered by an external nucleophile. Thus, reactio n of solutions of [(OC)(3)((R3Si)-Si-1)Fe(mu -(PRR3)-R-2)Pt(1,5-COD)] (1a R -1 = OMe, R-2 = R-3 = Ph; 1b R-1 = OMe, R-2 = R-3 = Cy; 1c R-1 = Ph, R-2 = R-3 = Ph; 1d R-1 = Ph, R-2 = R-3 = Cy; 1e R-1 = Ph, R-2 = H, R-3 = Ph) in C H2Cl2 with CO rapidly afforded the corresponding complexes [(OC)(4)Fe(mu -( PRR3)-R-2)Pt(SiR31)(CO)] (2a-e) in which the silyl ligand has migrated from Fe to Pt, while two CO ligands have been ligated, one on each metal. When la or Ic was slowly treated with two equivalents of tBuNC at low temperatur e, quantitative displacement of the COD ligand was accompagnied by silyl mi gration from Fe to Pt and coordination of an isonitrile ligand to Fe and to Pt to give [(OC)(3)(tBuNC)Fe(mu -PPh2)Pt{Si(OMe)(3)}(CNtBu)] (3a) and [(OC )(3)(tBuNC)Fe(mu -PPh2)Pt{SiPh3}(CNtBu)] (3c). Reaction of 2a with one equi valent of tBuNC selectively led to substitution of the Pt-bound CO to give [(OC)(4)Fe(mu -PCy2)Pt{Si(OMe)(3)}(CNtBu)] (4b), which reacted with a secon d equivalent of tBuNC to give [(OC)(4)Fe(mu -PCy2)Pt{Si(OMe)(3)}(CNtBu)(2)] (5b) in which the metal-metal bond has been cleaved. Opening of the Fe-Pt bond was also observed upon reaction of 3a with tBuNC to give [(OC)(3)(tBuN C)Fe(mu -PPh2)Pt{Si(OMe)(3)}(CNtBu)(2)] (6). The silyl ligand migrates from Fe, in which it is trans to mu -(PRR3)-R-2 in all the metal-metal-bonded c omplexes, to a position cis to the phosphido bridge on Pt. However, in 5a,b and 6 with no metal-metal bond, the Pt-bound silyl ligand is trans to the phosphido bridge. The intramolecular nature of the silyl migration, which m ay be formally viewed as a redox reaction, was established by a cross-over experiment consisting of the reaction of la and Id with CO; this yielded ex clusively 2a and 2d. The course of the silyl-migration reaction was found t o depend a) on the steric properties of the -SiR31 ligand, and for a given mu -(PRR3)-R-2 bridge (R-2 = R-3 = Ph), the migration rate decreases in the sequence Si(OMe)(3) > SiMe2Ph > SiMePh2 much greater than SiPh3; b) on the phosphido bridge and for a given silyl ligand (R-1 = OMe), the migration r ate decreases in the order mu -PPh2 much greater than mu -PHCy; c) on the e xternal nucleophile since reaction of Ic with two equivalents of P(OMe)(3), P(OPh)(3) or Ph2PCH2C(O)Ph led solely to displacement of the COD ligand wi th formation of 11a-c, respectively, whereas reaction with two equivalents of tBuNC gave the product of silyl migration 3c. Reaction of [(OC)(3){(MeO)(3)Si}Fe(mu -PPh2)Pt(PPh3)(2)] (7a) with tBuNC (e ven in slight excess) occurred stereoselectively with replacement of the PP h3 ligand trans to mu -PPh2, whereas reaction with CO led first to [(OC)(3) {(MeO)(3)Si}Fe(mu -PPh2)Pt(CO)(PPh3)] (8a), which then isomerized to the mi gration product [(OC)(4)Fe(mu -PPh2)Pt{Si(OMe)(3)}(PPH3)] (9a). Most I comp lexes were characterized by elemental analysis, IR and H-1, P-31, C-13, and Si-29 NMR spectroscopy, and in five cases by X-ray diffraction.