Reactions of N-Monosubstituted Amidines with AlMe₃ and AlMeCl₂: Formation of Fused Triazaalane Heterocycles

Reactions of N-monosubstituted amidines of the formula HNC­(R)–NH­(R′) (R = Ph, 4-<i>tert</i>-butylphenyl, Me; R′ = 2,6-diisopropylphenyl, Ph) with AlMe₃ and AlMeCl₂ are reported. All the <i>N</i>-(Dipp)­amidines (Dipp = 2,6-diisopropylphenyl) react with AlMe₃ in a 1:1 ratio to yield tetrameric aluminum amidinates (<b>1</b>, <b>2</b>, and <b>3</b>) in good yields. In these compounds, the amidinate ligand chelates to Al while bridging to another Al. However, when <i>N</i>-phenylamidines are employed, tetracyclic (<b>4</b>–<b>9</b>) and pentacyclic (<b>10</b>) Al–N–C heterocycles are formed. In the case of <i>N</i>-phenylbenzamidine, formation of a hexacyclic Al–N–C heterocycle (<b>11</b>) is observed when a slight excess of AlMe₃ (1:1.2 equiv) is used. In these compounds, the amidinate ligand coordinates to Al atoms in a bridging fashion. Further, <i>N</i>-(Dipp)­acetamidine and <i>N</i>-(Dipp)­benzamidine are also treated with AlMeCl₂. Whereas <i>N</i>-(Dipp)­acetamidine affords an ionic 15-membered aluminum amidinate chain (<b>12</b>), <i>N</i>-(Dipp)­benzamidine gives a bicyclic heterocycle (<b>13</b>) and the AlCl₃ adduct of <i>N</i>-(Dipp)­benzamidine (<b>14</b>). However, from a reaction between <i>N</i>-phenylbenzamidine and AlMeCl₂, only the AlCl₃ adduct, <b>15</b>, was isolated. Compounds <b>3</b>, <b>4</b>, <b>6</b>, <b>8</b>, and <b>10</b>–<b>15</b> have been structurally characterized