Chemistry of Coordinated Nitroxyl. Reagent-Specific Protonations of trans-Re(CO)₂(NO)(PR₃)₂ (R = Ph, Cy) That Give the Neutral Nitroxyl Complexes cis,trans-ReCl(CO)₂(NHO)(PR₃)₂ or the Cationic Hydride Complex [trans,trans-ReH(CO)₂(NO)(PPh₃)₂⁺][SO₃CF₃^-]

The reactions of hydrochloric and triflic acids with the five-coordinate nitrosyl complexes trans-Re(CO)₂(NO)(PR₃)₂ (2a, R = Ph; 2b, R = Cy) have been investigated. Reaction of anhydrous HCl with 2 results in a formal protonation of the nitrosyl ligand and addition of chloride to the metal, giving the neutral nitroxyl complex cis,trans-ReCl(CO)₂(NHO)(PR₃)₂ (3a, R = Ph; 3b, R = Cy). Reaction of Brønsted bases with 3a or 3b results in clean conversion of 3 to 2 when the base is appropriately strong (pK_b ≈ 7). Addition of HOSO₂CF₃ to solutions of 2a results in protonation at the metal and formation of the cationic rhenium hydride [trans,trans-ReH(CO)₂(NO)(PPh₃)₂⁺][SO₃CF₃^-] (4) in 74% yield; the deuteride [trans,trans-Re(²H)(CO)₂(NO)(PPh₃)₂⁺][SO₃CF₃^-] (4-d) was analogously prepared from ²HOSO₂CF₃. 4 crystallized from CH₂Cl₂/Et₂O solution in the orthorhombic space group Pnma, with a = 17.2201(2) Å, b = 23.6119(3) Å, c = 9.2380(2) Å, and Z = 4. The least-squares refinement converged to R(F) = 0.039 and R(wF²) = 0.063 for the 4330 unique data with I > 2σ(I). The structure of 4 shows that the hydride (Re−H = 1.74 Å) occupies the position trans to the linear nitrosyl ligand (Re−N−O = 178.1(4)°) in the pseudooctahedral complex cation. Complex 4 does not react with chloride to give 3a. DFT calculations carried out on free nitroxyl and its model complexes [Re(CO)₅(NHO)⁺] (5), [mer,trans-Re(CO)₃(NHO)(PH₃)₂⁺] (6), and cis,trans-ReCl(CO)₂(NHO)(PH₃)₂ (7) indicate that coordinated nitroxyl acts as both a σ-donor and π-acceptor ligand, consistent with the observed trend for ν(NO) in free HNO (1563 cm^-1), [mer,trans-Re(CO)₃(NHO)(PPh₃)₂⁺] (1, 1391 cm^-1), 3a (1376 cm^-1), and 3b (1335 cm^-1).