Aminocarbyne coupling reactions at M(Ph2PCH2CH2PPh2)2(M = Mo or W) sites. Detailed mechanistic studies on the protonation of co-ordinated isocyanides and coupling of ligands in trans-[M(CNR)2(Ph2PCH2CH2PPh2)2](R = But or Me)
Abstract
The mechanism of protonation of trans-[M(CNR)2(dppe)2](M = Mo or W, R = Me or But, dppe = Ph2PCH2CH2PPh2) has been shown to involve the initial rapid protonation of an isocyanide ligand to form trans-[M(CNHR)(CNR)(dppe)2]+ which can then undergo rate-limiting intramolecular migration of the hydrogen to form [MH(CNR)2(dppe)2]+. For trans-[Mo(CNBut)2(dppe)2] an additional acidcatalysed pathway is observed involving the intermediate [MoH(CNHBut)(CNBut)(dppe)2]2+ which subsequently rapidly releases a proton from the aminocarbyne to produce the hydrido complex. The formation of the diaminoacetylene complex, trans-[MoCl(η2-MeHNC![[triple bond, length half m-dash]](https://www.rsc.org/images/entities/char_e007.gif)
CNHMe)(dppe)2]+ occurs in three distinct steps. First is the rapid protonation of the isocyanide ligand to form trans-[Mo(CNHMe)(CNMe)(dppe)2]+, which at low concentrations of acid undergoes intramolecular hydrogen migration to produce the hydrido complex but at high acid concentrations rapidly forms trans-[Mo(CNHMe)2(dppe)2]2+. It is this bis(aminocarbyne) species which in the second step is attacked by Cl– and subsequently undergoes intramolecular trans to cis isomerisation followed by coupling of the aminocarbyne ligands to form the product, trans-[MoCl(η2-MeHNCCNHMe)(dppe)2]+.
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