B–H Bond Activation in a Rh(III) Hydrido Borohydride Complex [Rh(H)(K²‑BH₄)^<i>t</i>Bu4(PNCNP)] Gives a Rh(I) σ‑Dihydrogen Complex [Rh(η²‑H₂)^<i>t</i>Bu4(PNCNP)]: An Experimental and Theoretical Study

Rh­(III) pincer hydrido chloride and borohydride complexes [Rh­(H)­Cl^<i>t</i>Bu4(PNCNP)] (<b>1</b>) [^<i>t</i>Bu4(PNCNP) = 2,6-bis­((di-tert-butylphosphaneyl)­amino)­benzen-1-ide] and [Rh­(H)­(K²-BH₄)^<i>t</i>Bu4(PNCNP)] (<b>2</b>) have been synthesized and characterized. Heating the [Rh­(H)­(K²-BH₄)^<i>t</i>Bu4(PNCNP)] complex (<b>2</b>) at 336 K in tetrahydrofuran (THF) afforded the Rh­(I) σ-H₂ complex, [Rh­(η²-H₂)^<i>t</i>Bu4(PNCNP)] (<b>3</b>), and H₃B·THF via the B–H bond activation of the borohydride. The [Rh­(η²-HD)^<i>t</i>Bu4(PNCNP)] (<b>3-HD</b>) isotopomer was also prepared by heating the [Rh­(H)­(K²-BD₄)^<i>t</i>Bu4(PNCNP)] complex (<b>2-BD</b>_<b>4</b>) at 336 K in THF. The formation of [Rh­(η²-H₂)^<i>t</i>Bu4(PNCNP)] (<b>3</b>) was established using ¹<i>J</i>_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)^<i>t</i>Bu4(PNCNP)] complex (<b>3</b>) reverted back to the [Rh­(H)­(K²-BH₄)^<i>t</i>Bu4(PNCNP)] complex (<b>2</b>) upon reaction with H₃B·THF. The [Rh­(η²-H₂)^<i>t</i>Bu4(PNCNP)] complex (<b>3</b>) was also obtained independently by two alternative routes. The reaction of [Rh­(η²-H₂)^<i>t</i>Bu4(PNCNP)] (<b>3</b>) with N₂ at 273 K afforded the [Rh­(N₂)^<i>t</i>Bu4(PNCNP)] complex (<b>4</b>); this reaction was found to be reversible. The reaction of [Rh­(η²-H₂)^<i>t</i>Bu4(PNCNP)] (<b>3</b>) with CH₃CN, CO, and O₂ occurred instantaneously and yielded the corresponding products [Rh­(CH₃CN)^<i>t</i>Bu4(PNCNP)] (<b>5</b>), [Rh­(CO)^<i>t</i>Bu4(PNCNP)] (<b>6</b>), and [Rh­(η²-O₂)^<i>t</i>Bu4(PNCNP)] (<b>7</b>), 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.