B–H Bond Activation in a Rh(III) Hydrido Borohydride Complex [Rh(H)(K²‑BH₄)^tBu4(PNCNP)] Gives a Rh(I) σ‑Dihydrogen Complex [Rh(η²‑H₂)^tBu4(PNCNP)]: An Experimental and Theoretical Study

Rh­(III) pincer hydrido chloride and borohydride complexes [Rh­(H)­Cl^tBu4(PNCNP)] (1) [^tBu4(PNCNP) = 2,6-bis­((di-tert-butylphosphaneyl)­amino)­benzen-1-ide] and [Rh­(H)­(K²-BH₄)^tBu4(PNCNP)] (2) have been synthesized and characterized. Heating the [Rh­(H)­(K²-BH₄)^tBu4(PNCNP)] complex (2) at 336 K in tetrahydrofuran (THF) afforded the Rh­(I) σ-H₂ complex, [Rh­(η²-H₂)^tBu4(PNCNP)] (3), and H₃B·THF via the B–H bond activation of the borohydride. The [Rh­(η²-HD)^tBu4(PNCNP)] (3-HD) isotopomer was also prepared by heating the [Rh­(H)­(K²-BD₄)^tBu4(PNCNP)] complex (2-BD₄) at 336 K in THF. The formation of [Rh­(η²-H₂)^tBu4(PNCNP)] (3) was established using ¹J_H,D coupling constant and variable temperature spin–lattice relaxation time measurements and supported by density functional theory (DFT) calculations. At 273 K, the [Rh­(η²-H₂)^tBu4(PNCNP)] complex (3) reverted back to the [Rh­(H)­(K²-BH₄)^tBu4(PNCNP)] complex (2) upon reaction with H₃B·THF. The [Rh­(η²-H₂)^tBu4(PNCNP)] complex (3) was also obtained independently by two alternative routes. The reaction of [Rh­(η²-H₂)^tBu4(PNCNP)] (3) with N₂ at 273 K afforded the [Rh­(N₂)^tBu4(PNCNP)] complex (4); this reaction was found to be reversible. The reaction of [Rh­(η²-H₂)^tBu4(PNCNP)] (3) with CH₃CN, CO, and O₂ occurred instantaneously and yielded the corresponding products [Rh­(CH₃CN)^tBu4(PNCNP)] (5), [Rh­(CO)^tBu4(PNCNP)] (6), and [Rh­(η²-O₂)^tBu4(PNCNP)] (7), respectively. These observations together with computational studies showed that the binding strengths of CH₃CN, CO, and O₂ with the Rh center are significantly greater than those of H₂ and N₂.