Isolobal Cationic\nIridium Dihydride and Dizinc Complexes:\nA Dual Role for the ZnR Ligand Enhances H<sub>2</sub> Activation

The reaction of [Ir(IPr)<sub>2</sub>H<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>] (<b>1</b>; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene;\nBAr<sup>F</sup><sub>4</sub> = B{C<sub>6</sub>H<sub>3</sub>(3,5-CF<sub>3</sub>)<sub>2</sub>}<sub>4</sub>) with ZnMe<sub>2</sub> proceeds\nwith CH<sub>4</sub> elimination to give [Ir(IPr)(IPr′)(ZnMe)<sub>2</sub>H][BAr<sup>F</sup><sub>4</sub>] (<b>3</b>, where (IPr′)\nis a cyclometalated IPr ligand). <b>3</b> reacts with H<sub>2</sub> to form tetrahydride [Ir(IPr)<sub>2</sub>(ZnMe)<sub>2</sub>H<sub>4</sub>][BAr<sup>F</sup><sub>4</sub>], <b>4</b>, that\nloses H<sub>2</sub> under forcing conditions to form [Ir(IPr)<sub>2</sub>(ZnMe)<sub>2</sub>H<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>5</b>. Crystallization of <b>3</b> also results\nin the formation of its noncyclometalated isomer, [Ir(IPr)<sub>2</sub>(ZnMe)<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>2</b>,\nin the solid state. Reactions of <b>1</b> and CdMe<sub>2</sub> form [Ir(IPr)<sub>2</sub>(CdMe)<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>6</b>, and [Ir(IPr)(IPr′)(CdMe)<sub>2</sub>H][BAr<sup>F</sup><sub>4</sub>], <b>7</b>, which reacts with\nH<sub>2</sub> to give [Ir(IPr)<sub>2</sub>(CdMe)<sub>2</sub>H<sub>4</sub>][BAr<sup>F</sup><sub>4</sub>], <b>8</b>, and [Ir(IPr)<sub>2</sub>(CdMe)<sub>2</sub>H<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>9</b>. Structures of <b>2</b>–<b>8</b> are determined crystallographically. Computational analyses show\nthe various hydrides in <b>3</b>–<b>5</b> sit on\na terminal to bridging continuum, with bridging hydrides exhibiting\ngreater Zn<sup>δ+</sup>···H<sup>δ−</sup> electrostatic interaction. The isolobal analogy between H and ZnMe\nligands holds when both are present as terminal ligands. However,\nthe electrostatic component to the Zn<sup>δ+</sup>···H<sup>δ−</sup> unit renders it significantly different to\na nominally isolobal H···H moiety. Thus, H<sub>2</sub> addition to <b>3</b> is irreversible, whereas H<sub>2</sub> addition to <b>1</b> reversibly forms highly fluxional [Ir(IPr)<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>)<sub>2</sub>H<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>11</b>. Computed mechanisms\nfor cyclometalation and H<sub>2</sub> addition showcase the role of\nthe bridging Zn<sup>δ+</sup>···H<sup>δ−</sup> moiety in promoting reactivity. In this, the Lewis acidic ZnMe ligand\nplays a dual role: as a terminal <i>Z</i>-type ligand that\ncan stabilize electron-rich Ir centers through direct Ir-ZnMe bonding,\nor by stabilizing strongly hydridic character via Zn<sup>δ+</sup>···H<sup>δ−</sup> interactions.