Isolobal Cationic Iridium Dihydride and Dizinc Complexes: A Dual Role for the ZnR Ligand Enhances H2 Activation

The reaction of [Ir(IPr) 2 H 2 ][BAr F 4 ] (1; IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BAr F 4 = B-{C 6 H 3 (3,5-CF 3 ) 2 } 4 ) with ZnMe 2 proceeds with CH 4 elimination to give [Ir(IPr)(IPr)(ZnMe) 2 H][BAr F 4 ] (3, where (IPr) is a cyclometalated IPr ligand).3 reacts with H 2 to form tetrahydride [Ir(IPr) 2 (ZnMe) 2 H 4 ][BAr F 4 ], 4, that loses H 2 under forcing conditions to form [Ir(IPr) 2 (ZnMe) 2 H 2 ][BAr F 4 ], 5. Crystallization of 3 also results in the formation of its noncyclometalated isomer, [Ir(IPr) 2 (ZnMe) 2 ][BAr F 4 ], 2, in the solid state.Reactions of 1 and CdMe 2 form [Ir(IPr) 2 (CdMe) 2 ][BAr F 4 ], 6, and [Ir(IPr)(IPr)-(CdMe) 2 H][BAr F 4 ], 7, which reacts with H 2 to give [Ir-(IPr) 2 (CdMe) 2 H 4 ][BAr F 4 ], 8, and [Ir(IPr) 2 (CdMe) 2 H 2 ][BAr F 4 ], 9. Structures of 2-8 are determined crystallographically. Computational analyses show the various hydrides in 3-5 sit on a terminal to bridging continuum, with bridging hydrides exhibiting greater Zn + H -electrostatic interaction.The isolobal analogy between H and ZnMe ligands holds when both are present as terminal ligands.However, the electrostatic component to the Zn + H -unit renders it significantly different to a nominally isolobal HH moiety.Thus, H 2 addition to 3 is irreversible, whereas H 2 addition to 1 reversibly forms highly fluxional [Ir(IPr) 2 ( 2 -H 2 ) 2 H 2 ][BAr F 4 ], 11.Computed mechanisms for cyclometalation and H 2 addition showcase the role of the bridging Zn + H - moiety in promoting reactivity.In this, the Lewis acidic ZnMe ligand plays a dual role: as a terminal Z-type ligand that can stabilize electron-rich Ir centers through direct Ir-ZnMe bonding, or by stabilizing strongly hydridic character via Zn + H -interactions.