N–O Bond Activation and Cleavage Reactions of the Nitrosyl-Bridged Complexes [M₂Cp₂(μ-PCy₂)(μ-NO)(NO)₂] (M = Mo, W)

The title complexes (1a,b) were prepared in two steps by first reacting the hydrides [M2Cp2(μ-H)­(μ-PCy2)­(CO)4] with [NO]­(BF4) in the presence of Na2CO3 to give dinitrosyls [M2Cp2(μ-PCy2)­(CO)2(NO)2]­(BF4), which were then fully decarbonylated upon reaction with NaNO2 at 323 K. An isomer of the Mo2 complex having a cisoid arrangement of the terminal ligands (cis-1a) was prepared upon irradiation of toluene solutions of 1a with visible–UV light at 288 K. The structure of these trinitrosyl complexes was investigated using X-ray diffraction and density functional theory (DFT) calculations, these revealing a genuine pyramidalization of the bridging NO that might be associated in part to an increase of charge at the N atom and anticipated a weakening of the N–O bond upon reaction with bases or reducing reagents. Complexes 1a,b reacted with [FeCp2]­(BF4) to give first the radicals [M2Cp2(μ-PCy2)­(μ-NO)­(NO)2]­(BF4) according to CV experiments, which then underwent H-abstraction to yield the nitroxyl-bridged complexes [M2Cp2(μ-PCy2)­(μ-κ1:η2-HNO)­(NO)2]­(BF4), alternatively prepared upon protonation with HBF4·OEt2. The novel coordination mode of the nitroxyl ligand in these products was thermodynamically favored over its tautomeric hydroximido form, according to DFT calculations, and similar nitrosomethane-bridged cations [M2Cp2(μ-PCy2)­(μ-κ1:η2-MeNO)­(NO)2]+ were prepared by reacting 1a,b with CF3SO3Me or [Me3O]­BF4. Complexes 1 reacted with M­(Hg) (M = Zn, Na) in tetrahydrofuran to give the amido-bridged derivatives [M2Cp2(μ-PCy2)­(μ-NH2)­(NO)2] with retention of stereochemistry, a transformation also induced by using mild O atom scavengers such as CO and phosphites in the presence of water. In the absence of water, phosphites accomplished a deoxygenation of the bridging NO of the Mo2 complexes to yield the phosphoraniminato-bridged derivatives [Mo2Cp2(μ-PCy2)­{μ-NP­(OR)3}­(NO)2] (R = Et, Ph), also with retention of stereochemistry