Rhenium Dihydrogen Complexes with Isonitrile Coligands:  Novel Displacement of Chloride by Hydrogen

Abstract

The syntheses, properties, and characterization of several new complexes, Re(CN t Bu) 3 (PCy 3 ) 2 H ( 1a ), Re(CN t Bu) 3 (PR 3 ) 2 Cl [R = Cy, Ph ( 2a, 2b )], the 17-electron [Re(CN t Bu) 3 (PCy 3 ) 2 Cl] + ( 3a ), the dihydrogen complex, [Re(CN t Bu) 3 (PR 3 ) 2 (H 2 )] + [R = Cy, Ph ( 4a, 4b )], the coordinatively unsaturated [Re(CN t Bu) 3 (PCy 3 ) 2 ] + ( 5a ), and [Re(CN t Bu) 4 (PCy 3 ) 2 ] + ( 6a ) are reported. In addition, spectroscopic evidence for the dinitrogen complex [Re(CN t Bu) 3 (PCy 3 ) 2 (N 2 )] + and the dihydrogen complex [Re(CN t Bu) 5 (H 2 )] + is presented. Thermodynamic parameters have been obtained for the equilibrium system, Re(CN t Bu) 3 (PCy 3 ) 2 Cl ( 2a ) + H 2 ⇌ [Re(CN t Bu) 3 (PCy 3 ) 2 (H 2 )]Cl ( 4a ). 1 H and 31 P{ 1 H} NMR studies (CD 2 Cl 2 ) over the temperature range 286−316 K afford values of Δ H ° = −18.0 ± 0.7 kcal/mol, Δ S ° = −44 ± 2 eu, and Δ G ° 298 = −4.8 ± 1.3 kcal/mol for this equilibrium. The complexes 4a and 4b are characterized as dihydrogen complexes. Their J HD (R = Cy, 30.3 Hz; R = Ph, 30.9 Hz) and T 1(min) values of 8 ms (300 MHz) are consistent with H−H distances of ca . 0.80 Å. [Re(CN t Bu) 5 (HD)] + is also characterized as a dihydrogen complex based on a J HD of 33.4 Hz. This complex could not be isolated at room temperature, even with non-coordinating counteranions, due to the lability of the H 2 ligand. [Re(PMe 3 ) 5 HD] + shows no H−D coupling and is characterized as a dihydride complex. The unsaturated complex [Re(CN t Bu) 3 (PCy 3 ) 2 ] + ( 5a ) is found to undergo a dynamic isonitrile rearrangement on the NMR time scale. 1 H NMR spin saturation transfer studies over a temperature range of 254−297 K afford the activation parameters Δ H ⧧ = 11.4 ± 0.8 kcal/mol, Δ S ⧧ = −17.6 ± 1.6 eu, and Δ G ⧧ 298 = 16.6 ± 1.2 kcal/mol. The molecular structures of 2a and 3a have been determined by X-ray crystallography. 2a shows an unusually long Re−Cl bond distance of 2.596(2) Å.