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.