Classical and nonclassical nitrosyl hydride complexes of rhenium in various oxidation states

The paramagnetic rhenium complex [NEt4](2)[Re(Br)(5)(NO)] (1) has been used to prepare a series of novel mononitrosyl hydride and dihydrogen rhenium complexes: [Re(Br)(2)(NO)(eta(2)-H-2)(PR3)(2)] (R = Pr-i, 2a; Cy, 2b) and [Re(H)(BH4)(NO)(PR3)(2)] (R = Pr-i, 3a; Cy, 3b). The coordinated BH3 of the derivatives 3 can be replaced by the H-2 or the NO ligand, thus leading to the tetrahydride and dinitrosyl species [Re(H)(4)(NO)L-2] (R = Pr-i, 4a; Cy, 4b) or [Re(H)(NO)(2)(PR3)(2)] (R = Pr-i, 5a; Cy, 5b). While [Re(H)(4)(NO)(PPh3)(2)] does not seem to be stable, [Re(H)(NO)(2)(PPh3)(2)] (5c) has been obtained in a fashion similar to the preparation of 3a,b from the reaction of [Re(H)(BH4)(NO)(PPh3)(2)] and NOBF4. Detailed investigations of the reactions of 3a,b with NOBF4 have revealed that the compounds initially formed are the isolable BF3 adducts [Re(H)(NO)(NOBF3)(PR3)(2)] (R = Pr-i, 6a; Cy, 6b). The source of BF3 is the nitrosonium salt. Dissociation of BF3 from 6a,b takes place in donor solvents such as THF, affording the BF3-free compounds 5a,b, whereas in noncoordinating solvents such as toluene, benzene, or CH2Cl2 only the species 6a,b are observable. Apparently due to an unfavorable position of the dissociation equilibrium, the existence of the complex [Re(H)(NO)(NOBF3)(PPh3)(2)] could only be made plausible from dynamic NMR spectroscopic observations. Attempts to isolate it failed even from nonpolar solvents. X-ray diffraction studies have been carried out on the complexes [Re(Br)(2)(NO)(eta(2)-H-2)((PPr3)-Pr-i)(2)] (2a), [Re(H)(BH4)(NO)(PR3)(2)] (R = Pr-i, 3a; Ph, 3c), [Re(H)(NO)(2)((PPr3)-Pr-i)(2)] (5a), and [Re(H)(NO)(NOBF3)((PPr3)-Pr-i)(2)] (6a). The hydrogen atoms of the eta(2)-H-2 moiety of 2a could not be located in the X-ray diffraction study, but their most probable position in the molecule has been traced by an extensive search based on DFT calculations.