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

Abstract

Rh(III) pincer hydrido chloride and borohydride complexes [Rh(H)Cl tBu4 (PNCNP)] (1) [ tBu4 (PNCNP) = 2,6bis((di-tert-butylphosphaneyl)amino)benzen-1-ide] and [Rh(H)-(K 2 -BH 4 ) tBu4 (PNCNP)] (2) have been synthesized and characterized. Heating the [Rh(H)(K 2 -BH 4 ) tBu4 (PNCNP)] complex (2) at 336 K in tetrahydrofuran (THF) afforded the Rh(I) σ-H 2 complex, [Rh(η 2 -H 2 ) tBu4 (PNCNP)] (3), and H 3 B•THF via the B−H bond activation of the borohydride. The [Rh(η 2 -HD) tBu4 (PNCNP)] (3-HD) isotopomer was also prepared by heating the [Rh(H)(K 2 -BD 4 ) tBu4 (PNCNP)] complex (2-BD 4 ) at 336 K in THF. The formation of [Rh(η 2 -H 2 ) tBu4 (PNCNP)] (3) was established using 1 J 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(η 2 -H 2 ) tBu4 (PNCNP)] complex (3) reverted back to the [Rh(H)(K 2 -BH 4 ) tBu4 (PNCNP)] complex (2) upon reaction with H 3 B•THF. The [Rh(η 2 -H 2 ) tBu4 (PNCNP)] complex (3) was also obtained independently by two alternative routes. The reaction of [Rh(η 2 -H 2 ) tBu4 (PNCNP)] (3) with N 2 at 273 K afforded the [Rh(N 2 ) tBu4 (PNCNP)] complex (4); this reaction was found to be reversible. The reaction of [Rh(η 2 -H 2 ) tBu4 (PNCNP)] (3) with CH 3 CN, CO, and O 2 occurred instantaneously and yielded the corresponding products [Rh(CH 3 CN) tBu4 (PNCNP)] (5), [Rh(CO) tBu4 (PNCNP)] ( 6), and [Rh(η 2 -O 2 ) tBu4 (PNCNP)] (7), respectively. These observations together with computational studies showed that the binding strengths of CH 3 CN, CO, and O 2 with the Rh center are significantly greater than those of H 2 and N 2 .