Influence of the Cis Ligand on the H−H Separation and the Rotation Barrier of the Dihydrogen in Osmium-Elongated Dihydrogen Complexes Containing an Ortho-Metalated Ketone

Treatment of the complex ( 1 ) with HBF 4 ·OEt 2 in diethyl ether−acetone (2:1) affords the elongated dihydrogen derivative ( 2 ), which reacts with NaCl and CsF to give (X = Cl ( 3 ), F ( 4 )). The X-ray diffraction studies on 2 and 4 and DFT calculations on the model complexes ( 1t ), ( 2t ), and (X = Cl ( 3t ), F ( 4t )) suggest that one of the hydrogen atoms bonded to the osmium atom undergoes a cis electrostatic attraction with the L ligand (L = H ( 1, 1t ), (CH 3 ) 2 CO or H 2 O ( 2, 2t ), Cl ( 3, 3t ), F ( 4, 4t )), which increases in the sequence (CH 3 ) 2 CO or H 2 O < Cl ≤ F < H. This interaction provokes a lengthening of the hydrogen−hydrogen bond and an increase in the rotation barrier of the elongated dihydrogen ligand. Thus, it is observed that the separation between the hydrogen atoms of the dihydrogen decreases in the sequence 1t (1.695 Å) > 4t (1.544 Å) > 3t (1.489 Å) > 2t (1.455 Å), whereas the rotation barrier of the dihydrogen increases in the sequence 2 (<9 kcal·mol - 1 ) < 3 (≅9 kcal·mol - 1 ) < 4 (10.1 ± 0.8 kcal·mol - 1 ) < 1 (13.9 ± 0.3 kcal·mol - 1 ).