Synthesis, structure and reactivity of N,O-metallacyclic (dicarbonyldiazene) platinum complexes
Reaction of cis-Pt(PR13)2Cl2 (R1=Me, Ph) with diacyl hydrazine R2OCNHNHCOR2 (R2=Me, Ph) in refluxing ethanol and NaHCO3 afforded platinum diazene diacyl complexes Pt(PR13)2(R2OCNNCOR2) (1, R1=R2=Ph; 3, R1=Me, R2=Ph; 4, R1=R2=Me). Reaction of Pt(PPh3)2(C2H4) with diazene dicarboxylates R2OCNNCOR2 (R2=OEt, OPri) resulted in the formation of Pt(PPh3)2(R2OCNNCOR2) (2, R2=OEt; 5, R2=OPri). Multinuclear NMR spectroscopy on 1–5 revealed that the dicarbonyl-substituted diazene ligand is coordinated asymmetrically, consistent with a five membered Pt–N–N–C–O ring. Compound 5 crystallises from THF–benzene as the solvate Pt(PPh3)2(PriO2CNNCO2Pri)·(C6H6)2; its X-ray crystal structure shows that the coordination sphere of platinum is essentially square planar and coplanar with the five membered Pt1–O1–C5–N2–N1 ring. The Pt1–O1 distance is 2.027(4) Åand the Pt1–N1 distance is 2.050(5) Å. The N1–N2 distance is 1.421(6) Åwhile the N2–C5 and C5–O1 distances are 1.273(7) and 1.314(7) Å, respectively. Complexes 1–5 show sensitivity towards chlorinated solvents (CH2Cl2, CHCl3) under photolysis conditions, forming the corresponding cis-platinum bis(phosphine) dichloride complexes; the same products are formed in a slower thermal reaction with 2 and 5. Complexes 2 and 5 react photochemically with ethylene in [2H8]THF yielding Pt(PPh3)2(C2H4) but 1, 3 and 4 are inert. Addition of an excess of dppe ligand to 2 and 5 resulted in the displacement of PPh3 and the formation of Pt(dppe)(EtO2CNNCO2Et) and Pt(dppe)(PriO2CNNCO2Pri) [dppe=1,2-bis(diphenylphosphino)ethane].