Structural Evidence for Lewis Acid Triggered Nitrosyl Bending in Rhenium(–I) Chloro Catalysts for Alkene Hydrogenation Reactions

The “catalytic nitrosyl effect” operating in reversible nitrosyl bending governs the performance of NO-functionalized chlororhenium derivatives as catalysts in 1-hexene hydrogenation reactions. Structural support for a Lewis acid (LA) induced bending at the NNO atom was provided by terminal attachment of a Lewis acid to one of the nitrosyl ligands in the [ReCl(PR3)2(NO)2] complexes (2: R = iPr a, R = Cy b) to give the products [ReCl(PR3)2(NO)(NO-LA)][Z] {3: LA = B(C6F5)3; 4: LA = Et+; Z = [B(C6F5)4]–; 5a: LA = SiEt3+, Z = [HB(C6F5)3]–; R = iPr a, Cy b}. X-ray diffraction studies revealed a moderately bent nitrosyl ligand in the nonderivatized complex 2a [159.3(5)°] and fully bent NO-LA groups in 3a [131.4(3)°] and 4b [132.7(3)°]. The catalytic activities of the chloro dinitrosyl complexes in the hydrogenation of prototypical 1-hexene were investigated. The NO-LA bent complexes 3b and 4b furnished excellent TONs of up to 1.0 × 104 and high TOFs of up to 5.4 × 104 h–1 when the Lewis acids were present as co-catalysts.