Reactions of Group 4 Amide Guanidinates with Dioxygen or Water. Studies of the Formation of Oxo Products

Reactions of the zirconium amide guanidinates (R₂N)₂M­[^<i>i</i>PrNC­(NR₂)­N^<i>i</i>Pr]₂ (R = Me, M = Zr, <b>1</b>; M = Hf, <b>2</b>; R = Et, M = Zr, <b>3</b>) with O₂ or H₂O give products that are consistent with the oxo dimers {M­(μ-O)­[^<i>i</i>PrNC­(NR₂)­N^<i>i</i>Pr]₂}₂ (R = Me, M = Zr, <b>4</b>; M = Hf, <b>5</b>; R = Et, M = Zr, <b>6</b>) and polymers {M­(μ-O)­[^<i>i</i>PrNC­(NR₂)­N^<i>i</i>Pr]₂}_<i>n</i> (R = Me, M = Zr, <b>7</b>; M = Hf, <b>8</b>; R = Et, M = Zr, <b>9</b>). Mass spectrometric (MS) analyses of the reactions of water in air with <b>1</b> and <b>2</b> show formation of the Zr monomer Zr­(O)­[^<i>i</i>PrNC­(NMe₂)­N^<i>i</i>Pr]₂ (<b>10</b>), oxo dimers <b>4</b> and <b>5</b>, and dihydroxyl complexes M­(OH)₂[^<i>i</i>PrNC­(NMe₂)­N^<i>i</i>Pr]₂ (M = Zr, <b>11</b>; Hf, <b>12</b>). Similar MS analyses of the reaction of diethylamide guanidinate <b>3</b> with water in air show the formation of Zr­(O)­[^<i>i</i>PrNC­(NEt₂)­N^<i>i</i>Pr]₂ (<b>13</b>), Zr­(OH)₂[^<i>i</i>PrNC­(NEt₂)­N^<i>i</i>Pr]₂ (<b>14</b>), <b>6</b>, and {(Et₂N)­Zr­[^<i>i</i>PrNC­(NEt₂)­N^<i>i</i>Pr]₂}⁺ (<b>15</b>). Kinetic studies of the reaction between <b>1</b> and a continuous flow of 1.0 atm of O₂ at 80–105 °C indicate that it follows pseudo-first-order kinetics with Δ<i>H</i>^⧧ = 8.7(1.1) kcal/mol, Δ<i>S</i>^⧧ = −54(3) eu, Δ<i>G</i>^⧧_358 K = 28(2) kcal/mol, and a half-life of 213(1) min at 85 °C