Classical and Nonclassical Nitrosyl Hydride Complexes of Rhenium in Various Oxidation States

The paramagnetic rhenium complex [NEt 4 ] 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)(η 2 -H 2 )(PR 3 ) 2 ] (R = i Pr, 2a; Cy, 2b ) and [Re(H)(BH 4 )(NO)(PR 3 ) 2 ] (R = i Pr, 3a; Cy, 3b ). The coordinated BH 3 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 = i Pr, 4a; Cy, 4b ) or [Re(H)(NO) 2 (PR 3 ) 2 ] (R = i Pr, 5a; Cy, 5b ). While [Re(H) 4 (NO)(PPh 3 ) 2 ] does not seem to be stable, [Re(H)(NO) 2 (PPh 3 ) 2 ] ( 5c ) has been obtained in a fashion similar to the preparation of 3a,b from the reaction of [Re(H)(BH 4 )(NO)(PPh 3 ) 2 ] and NOBF 4 . Detailed investigations of the reactions of 3a,b with NOBF 4 have revealed that the compounds initially formed are the isolable BF 3 adducts [Re(H)(NO)(NOBF 3 )(PR 3 ) 2 ] (R = i Pr, 6a; Cy, 6b ). The source of BF 3 is the nitrosonium salt. Dissociation of BF 3 from 6a,b takes place in donor solvents such as THF, affording the BF 3 -free compounds 5a,b, whereas in noncoordinating solvents such as toluene, benzene, or CH 2 Cl 2 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)(NOBF 3 )(PPh 3 ) 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)(η 2 -H 2 )(P i Pr 3 ) 2 ] ( 2a ), [Re(H)(BH 4 )(NO)(PR 3 ) 2 ] (R = i Pr, 3a; Ph, 3c ), [Re(H)(NO) 2 (P i Pr 3 ) 2 ] ( 5a ), and [Re(H)(NO)(NOBF 3 )(P i Pr 3 ) 2 ] ( 6a ). The hydrogen atoms of the η 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.