Synthesis and characterization of cyano and isocyano complexes of bis(dithiolato) molybdenum using Me3SiCN: a route to a cyanide-bridged multimer to a monomer

Abstract

Cyanide- and isocyanide-bound molybdenum complexes containing maleonitriledithiolate (mnt = 1,2-dicyanoethylenedithiolate = S₂C₂(CN)₂) as coligands were prepared from the synthon [Et₄N]₂[Mo^IVO(mnt)₂] (1) as [Et₄N][(PPh₃)(mnt)₂³⁻˙Mo^III(μ-CN)Mo^III(PPh₃)(mnt)₂³⁻˙] (3), [Et₄N][Mo^III(PPh₃)(CN)(mnt)₂³⁻˙]·CH₂Cl₂ (4), and [Mo^III(CNSiMe₃)₂(mnt)₂³⁻˙](5) using trimethylsilylcyanide (Me₃SiCN). Triphenylphosphine (PPh₃) was used as a blocking group and also as a competing ligand to cyanide to control the geometry and reactivity for forming the cyanide-bridged molybdenum dimer 3 or with terminal cyanide for the monomer 4. The known tetramer [Et₄N]₄[Mo^III₄(μ-CN)₄(mnt)₈] (2) formed in the absence of PPh₃ showed a remarkable reaction with the excess Me₃SiCN, where the monomeric species [Mo^III(CNSiMe₃)₂(mnt)₂] (5) was formed with Me₃SiNC coordination. Overall, the Mo(IV) bis (dithiolate) complex showed diverse reactions with Me₃SiCN in the presence and absence of the coligand PPh₃ and under protic and aprotic media. The precursor molybdenum complex in the IV oxidation state was reduced to the III state in the complexes 3, 4, and 5, where one of the coordinating chelating ligand mnt²⁻ was oxidized to mnt¹⁻˙. This chemistry was supported by the EPR, electrochemical studies and X-ray crystallography results and corroborated by the DFT level of calculations. The complexes 3 and 4 were blue-emitting materials with a long lifetime in the millisecond range. Further DFT and TD-DFT calculations were carried out to understand the electronic states and the origin of the electronic absorptions.