Synthesis and protonation reactions of nitrosyl complexes [M2Cp(μ-κ:η5-C5H4)(μ-PR2)(CO)(NO)2] to give κ:η-NO-bridged derivatives (M = Mo, W; R = Cy, tBu)

The title compounds were prepared upon reaction of BAr’₄⁻ salts of the known cations [M₂Cp₂(μ-PR₂)(CO)₂(NO)₂]⁺ (M = Mo, W; R = Cy, ^tBu; Ar’ = 3,5-C₆H₃(CF₃)₂), with commercial Me₃NO·2H₂O in fluorobenzene solution at 273 K, this promoting decarbonylation and cyclopentadienyl deprotonation to yield complexes with cyclopentadienylidene ligands bridging the metal centres (W−W = 2.9917(5) Å when R = Cy). The use of dichloromethane as solvent, however, triggered a competitive chloride abstraction reaction by the Mo compounds that yielded the chloride complexes [Mo₂Cp₂Cl(μ-PR₂)(CO)(NO)₂], specifically formed with an anti disposition of the Cp ligands relative to the Mo₂P plane, and cis positioning of carbonyl and chloride ligands relative to the P atom (Mo−Cl = 2.336(4) Å when R = ^tBu). Protonation of the cyclopentadienylidene-bridged complexes with [H(OEt₂)₂](BAr’₄) yielded the BAr’₄⁻ salts of the dicyclopentadienyl cations [M₂Cp₂(μ-κ:η-NO)(μ-PR₂)(CO)(NO)]⁺ (Mo−Mo = 3.1027(2) Å when R = ^tBu; W−W = 3.0446(7) Å when R = Cy), which display bridging nitrosyl ligands in the rare linear end-on:side-on coordination mode, and are kinetic rather than thermodynamic products, according to density functional theory calculations. When using HBF₄·OEt₂ as protonation reagent on the P^tBu₂-bridged complexes, however, coordination of the external anion (in the κ¹-mode) also takes place, to yield the tetrafluoroborate complexes [M₂Cp₂(κ¹-BF₄)(μ-P^tBu₂)(CO)(NO)₂] as unique (Mo) or major (W) products. Methylation of the same cyclopentadienylidene complexes with [Me₃O](BF₄) was only observed for the tungsten cation, and took place specifically at the O atom of one nitrosyl ligand to give [W₂Cp(μ-κ:η⁵-C₅H₄)(NOMe)(μ-P^tBu₂)(CO)(NO)](BF₄), a complex displaying a linear methoxyimido ligand (W−N = 1.746(4) Å; W−N−O = 163.5(4)^o).