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

The reaction of [Ir(IPr)₂H₂][BAr^F₄] (<b>1</b>; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BAr^F₄ = B{C₆H₃(3,5-CF₃)₂}₄) with ZnMe₂ proceeds with CH₄ elimination to give [Ir(IPr)(IPr')(ZnMe)₂H][BAr^F₄] (<b>3</b>, where (IPr') is a cyclometalated IPr ligand). <b>3</b> reacts with H₂ to form tetrahydride [Ir(IPr)₂(ZnMe)₂H₄][BAr^F₄], <b>4</b>, that loses H₂ under forcing conditions to form [Ir(IPr)₂(ZnMe)₂H₂][BAr^F₄], <b>5</b>. Crystallization of <b>3</b> also results in the formation of its noncyclometalated isomer, [Ir(IPr)₂(ZnMe)₂][BAr^F₄], <b>2</b>, in the solid state. Reactions of <b>1</b> and CdMe₂ form [Ir(IPr)₂(CdMe)₂][BAr^F₄], <b>6</b>, and [Ir(IPr)(IPr')(CdMe)₂H][BAr^F₄], <b>7</b>, which reacts with H₂ to give [Ir(IPr)₂(CdMe)₂H₄][BAr^F₄], <b>8</b>, and [Ir(IPr)₂(CdMe)₂H₂][BAr^F₄], <b>9</b>. Structures of <b>2</b>-<b>8</b> are determined crystallographically. Computational analyses show the various hydrides in <b>3</b>-<b>5</b> sit on a terminal to bridging continuum, with bridging hydrides exhibiting greater Zn^δ+···H^δ- 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^δ+···H^δ- unit renders it significantly different to a nominally isolobal H···H moiety. Thus, H₂ addition to <b>3</b> is irreversible, whereas H₂ addition to <b>1</b> reversibly forms highly fluxional [Ir(IPr)₂(η²-H₂)₂H₂][BAr^F₄], <b>11</b>. Computed mechanisms for cyclometalation and H₂ addition showcase the role of the bridging Zn^δ+···H^δ- moiety in promoting reactivity. In this, the Lewis acidic ZnMe ligand plays a dual role: as a terminal <i>Z</i>-type ligand that can stabilize electron-rich Ir centers through direct Ir-ZnMe bonding, or by stabilizing strongly hydridic character via Zn^δ+···H^δ- interactions.