Nucleophilic attack at co-ordinated isocyanides promoted by the 2-pyridyl ligand

Abstract

The co-ordinated isocyanide in the 2-pyridyl intermediate [Pt(CNR)(C₅H₄N-C²)(dppe)]⁺[R = C₆H₄-OMe-4, dppe = 1,2-bis(diphenylphosphino)ethane] is readily attacked by protic nucleophiles, such as water, alcohols and primary amines. It is hydrolysed to CO and RNH₂ by trace amounts of water in commercial benzene or acetone, and also reacts with the small amount of ethanol, present as a stabilizer in commercial chloroform, to yield the (ethoxy)aminocarbene complex [Pt{C(OEt)NHR}(C₅H₄N-C²)(dppe)]⁺. The reaction with p-anisidine (RNH₂) is remarkably fast and gives the diaminocarbene complex [Pt{C(NHR)₂}(C₅H₄N-C²)(dppe)]⁺. Similar but slower reactions with H₂O and RNH₂ occur for the 2-pyrazyl derivative [Pt(CNR)(C₄H₃N₂-C²)(dppe)]⁺. The cationic complex [Pt(CNR)(C₅H₄NMe-C²)(dppe)]²⁺(C₅H₄NMe-C²= 1 -methyl-2-pyridylium) also reacts with RNH₂ to form [Pt{C(NHR)₂}(C₅H₄NMe-C²)(dppe)]²⁺, but at significantly lower rates. The enhanced reactivity of the isocyanide ligand in [Pt(CNR)(C₅H₄N-C²)(dppe)]⁺ is ascribed to the highly basic 2-pyridyl group in cis position, which assists the nucleophilic attack at the isocyanide carbon by hydrogen bonding with the incoming protic nucleophile. The complex cations [Pt{C(NHR)₂}(C₅H₄N-C²)(dppe)]⁺ and [Pt{C(OEt)(NHR)}(C₅H₄N-C²)(dppe)]⁺ are better characterized through their ZnCI₂ adducts, upon deprotonation of the carbene ligand with NEt₃. The crystal and molecular structure of the binuclear complex [(dppe)Pt{C(NHR)NR}(C₅H₄N-C²)ZnCl₂] has been determined by an X-ray analysis. This compound crystallizes in the orthorhombic system: space group Pna2₁, with a= 24.206(3), b= 14.269(3), c= 1 2.875(3)Å and Z= 4. A total of 3265 reflections have been used in the refinement, resulting in a final R value of 0.029 and R′ of 0.027. The structure is characterized by a six-membered ring of boat conformation, formed by the platinum and zinc metal centres and the bridging carbon and nitrogen atoms of the 2-pyridyl and amidino ligands. A distorted square geometry is present around the platinum centre, with bond distances in the normal ranges: Pt–P 2.305(3) and 2.297(3)Å, Pt–C(2-pyridyl) 2.044(10) and Pt–C(amidino) 2.035(8)Å. Some distortion from tetrahedral geometry is present also around the zinc centre. The Zn ⋯ Pt distance of 3.238(1)Å appears too long for any metal–metal bonding interaction.