Novel Metal‐to‐Metal Silyl‐Migration Reactions in Heterometallic Complexes

An unprecedented, intramolecular metal‐to‐metal silyl ligand migration reaction has been discovered in a series of phosphido‐bridged iron‐platinum complexes and which may be triggered by an external nucleophile. Thus, reaction of solutions of [(OC) 3 (R   1 3 Si) $\overline{{\rm Fe}(\mu -{\rm PR^{2}R^{3}}){\rm Pt}}$ (1,5‐COD)] ( 1 a R 1 =OMe, R 2 =R 3 =Ph; 1 b R 1 =OMe, R 2 =R 3 =Cy; 1 c R 1 =Ph, R 2 =R 3 =Ph; 1 d R 1 =Ph, R 2 =R 3 =Cy; 1 e R 1 =Ph, R 2 =H, R 3 =Ph) in CH 2 Cl 2 with CO rapidly afforded the corresponding complexes [(OC) 4 $\overline{{\rm Fe}(\mu -{\rm PR^{2}R^{3}}){\rm Pt}}$ (SiR   1 3 )(CO)] ( 2 a – e ) in which the silyl ligand has migrated from Fe to Pt, while two CO ligands have been ligated, one on each metal. When 1 a or 1 c was slowly treated with two equivalents of t BuNC at low temperature, quantitative displacement of the COD ligand was accompagnied by silyl migration from Fe to Pt and coordination of an isonitrile ligand to Fe and to Pt to give [(OC) 3 ( t BuNC) $\overline{{\rm Fe}(\mu -{\rm PPh_{2}}){\rm Pt}}$ {Si(OMe) 3 }(CN t Bu)] ( 3 a ) and [(OC) 3 ( t BuNC) $\overline{{\rm Fe}(\mu -{\rm PPh_{2}}){\rm Pt}}$ {SiPh 3 }(CN t Bu)] ( 3 c ). Reaction of 2 a with one equivalent of t BuNC selectively led to substitution of the Pt‐bound CO to give [(OC) 4 $\overline{{\rm Fe}(\mu -{\rm PCy_{2}}){\rm Pt}}$ {Si(OMe) 3 }(CN t Bu)] ( 4 b ), which reacted with a second equivalent of t BuNC to give [(OC) 4 Fe(μ‐PCy 2 )Pt{Si(OMe) 3 }(CN t Bu) 2 ] ( 5 b ) in which the metal–metal bond has been cleaved. Opening of the Fe−Pt bond was also observed upon reaction of 3 a with t BuNC to give [(OC) 3 ( t BuNC)Fe(μ‐PPh 2 )Pt{Si(OMe) 3 }(CN t Bu) 2 ] ( 6 ). The silyl ligand migrates from Fe, in which it is trans to μ‐PR 2 R 3 in all the metal–metal‐bonded complexes, to a position cis to the phosphido bridge on Pt. However, in 5 a , 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 may be formally viewed as a redox reaction, was established by a cross‐over experiment consisting of the reaction of 1 a and 1 d with CO; this yielded exclusively 2 a and 2 d . The course of the silyl‐migration reaction was found to depend a) on the steric properties of the −SiR   1 3ligand, and for a given μ‐PR 2 R 3 bridge (R 2 =R 3 =Ph), the migration rate decreases in the sequence Si(OMe) 3 >SiMe 2 Ph>SiMePh 2 ≫SiPh 3 ; b) on the phosphido bridge and for a given silyl ligand (R 1 =OMe), the migration rate decreases in the order μ‐PPh 2 ≫μ‐PHCy; c) on the external nucleophile since reaction of 1 c with two equivalents of P(OMe) 3 , P(OPh) 3 or Ph 2 PCH 2 C(O)Ph led solely to displacement of the COD ligand with formation of 11 a – c , respectively, whereas reaction with two equivalents of t BuNC gave the product of silyl migration 3 c . Reaction of [(OC) 3 {(MeO) 3 Si} $\overline{{\rm Fe}(\mu -{\rm PPh_{2}}){\rm Pt}}$ (PPh 3 ) 2 ] ( 7 a ) with t BuNC (even in slight excess) occurred stereoselectively with replacement of the PPh 3 ligand trans to μ‐PPh 2 , whereas reaction with CO led first to [(OC) 3 {(MeO) 3 Si} $\overline{{\rm Fe}(\mu -{\rm PPh^{2}}){\rm Pt}}$ (CO)(PPh 3 )] ( 8 a ), which then isomerized to the migration product [(OC) 4 $\overline{{\rm Fe}(\mu -{\rm PPh^{2}}){\rm Pt}}$ {Si(OMe) 3 }(PPh 3 )] ( 9 a ). Most complexes were characterized by elemental analysis, IR and 1 H, 31 P, 13 C, and 29 Si NMR spectroscopy, and in five cases by X‐ray diffraction.