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

The reaction of [Ir(IPr)2H2][BArF4] (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BArF4 = B{C6H3(3,5-CF3)2}4) with ZnMe2 proceeds with CH4 elimination to give [Ir(IPr)(IPr′)(ZnMe)2H][BArF4] (3, where (IPr′) is a cyclometalated IPr ligand). 3 reacts with H2 to form tetrahydride [Ir(IPr)2(ZnMe)2H4][BArF4], 4, that loses H2 under forcing conditions to form [Ir(IPr)2(ZnMe)2H2][BArF4], 5. Crystallization of 3 also results in the formation of its noncyclometalated isomer, [Ir(IPr)2(ZnMe)2][BArF4], 2, in the solid state. Reactions of 1 and CdMe2 form [Ir(IPr)2(CdMe)2][BArF4], 6, and [Ir(IPr)(IPr′)(CdMe)2H][BArF4], 7, which reacts with H2 to give [Ir(IPr)2(CdMe)2H4][BArF4], 8, and [Ir(IPr)2(CdMe)2H2][BArF4], 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 Znδ+···Hδ− electrostatic interaction. The isolobal analogy between H and ZnMe ligands holds when both are present as terminal ligands. However, the electrostatic component to the Znδ+···Hδ− unit renders it significantly different to a nominally isolobal H···H moiety. Thus, H2 addition to 3 is irreversible, whereas H2 addition to 1 reversibly forms highly fluxional [Ir(IPr)2(η2-H2)2H2][BArF4], 11. Computed mechanisms for cyclometalation and H2 addition showcase the role of the bridging Znδ+···Hδ− 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 Znδ+···Hδ− interactions.