Hydrogen Bonding Behavior of Amide-Functionalized α‑Diimine Palladium Complexes

A class of (<i>N,N</i>′-diaryl-α-diimine)­Pd complexes bearing amide substituents on the N-aryl rings is described. Hydrogen bonding interactions involving the amide groups influence the structures, isomer distributions, and ligand coordination behavior of these compounds. The amide-functionalized α-diimine ligands (2,6-ⁱPr₂-Ph)­NCMeCMeN­(2-C­(O)­NMe₂-6-ⁱPr-Ph) (<b>4a</b>), (2,6-ⁱPr₂-Ph)­NCMeCMeN­(2,6-(C­(O)­NMe₂)₂-Ph) (<b>4b</b>), and (2-C­(O)­NMe₂-6-ⁱPr-Ph)­NCMeCMeN­(2-C­(O)­NMe₂-6-ⁱPr-Ph) (<b>4c</b>) were prepared by condensation reactions of 2,3-butanedione and the appropriate anilines. The attempted preparation of (2,6-ⁱPr₂-Ph)­NCMeCMeN­(2-C­(O)­NHMe-6-ⁱPr-Ph) (<b>4d</b>) yielded the corresponding 1,2-dihydroquinazolinone derivative <b>4d</b>′ formed by nucleophilic attack of the amide nitrogen at the proximal imine carbon. <b>4a</b> and <b>4b</b> react with (cod)­PdMeCl to yield square planar (α-diimine)­PdMeCl complexes <b>5a</b>,<b>a</b>′ and <b>5b</b>,<b>b</b>′, respectively, which exist as two isomers that differ in the orientation (trans/cis) of the Pd–Me ligand and the amide-substituted arylimine unit. <b>4c</b> reacts with (MeCN)₂PdCl₂ and (cod)­PdMeCl to yield (<b>4c</b>)­PdCl₂ (<b>6c-</b><i><b>anti</b></i>,<i><b>syn</b></i>) and (<b>4c</b>)­PdMeCl (<b>5c-</b><i><b>anti</b></i>,<i><b>syn</b></i>), which exhibit anti/syn isomerism due to hindered rotation of the C_aryl–N bonds. In the solid state, the amide oxygen atoms in <b>6c-</b><i><b>anti</b></i> and <b>5c-</b><i><b>syn</b></i> engage in hydrogen bonding with cocrystallized CH₂Cl₂ solvent molecules. <b>4d</b>′ reacts with (MeCN)₂PdCl₂ via ring-opening metalation to afford the α-diimine complex (<b>4d</b>)­PdCl₂ (<b>6d</b>). Transmetalation of <b>6d</b> with SnMe₄ yields (<b>4d</b>)­PdMeCl (<b>5d</b>,<b>d</b>′) as a mixture of trans and cis isomers. The reaction of <b>5d</b>,<b>d</b>′ with AgOAc yields (<b>4d</b>)­PdMe­(OAc) (<b>7d</b>) as a single isomer in which the Pd–Me group is trans to the amide-functionalized arylimine unit. <b>5d</b>, <b>6d</b>, and <b>7d</b> exhibit intramolecular N–H···Cl and N–H···O hydrogen bonding interactions involving the amide NH units. The reactions of <b>5a</b>,<b>a</b>′, <b>5c-</b><i><b>anti</b></i>, and <b>5d</b>,<b>d</b>′ with AgSbF₆ in the presence of pyrazole yield the corresponding (α-diimine)­PdMe­(pz)⁺SbF₆^– salts (<b>8a</b>,<b>c</b>,<b>d</b>; pz = pyrazole), which exhibit an intramolecular hydrogen bond between the amide oxygen and the pyrazole NH unit. <b>8a</b>,<b>c</b>,<b>d</b> undergo partial dissociation of pyrazole in CD₃CN solution to generate the corresponding CD₃CN complexes <b>9a</b>,<b>c</b>,<b>d</b>. The non-hydrogen-bonded complex {(2,6-ⁱPr₂-Ph)­NCMeCMeN­(2,6-ⁱPr₂-Ph)}­PdMe­(pz)⁺SbF₆^– (<b>8e</b>) and its pyrazole dissociation product {(2,6-ⁱPr₂-Ph)­NCMeCMeN­(2,6-ⁱPr₂-Ph)}­PdMe­(CD₃CN)⁺SbF₆^– (<b>9e</b>) were generated in a similar fashion. The pyrazole dissociation constants, <i>K</i>_eq = [(α-diimine)­PdMe­(CD₃CN)⁺] × [pz] × [(α-diimine)­PdMe­(pz)⁺]⁻¹, vary in the order <b>8e</b> > <b>8d</b> > <b>8a</b> > <b>8c</b>, span more than 2 orders of magnitude, and reflect the enhancement of pyrazole binding in <b>8a</b>,<b>c</b>,<b>d</b> by amide–pyrazole hydrogen bonding. The intramolecular hydrogen bonding in <b>8c</b> strengthens pyrazole binding by a factor of ca. 120 (i.e., ΔΔ<i>G</i> = 2.8(1) kcal mol^–1) relative to the case of <b>8e