Effect of the Positioning of Metal Centers on a Cavitand in the Ruthenium-Catalyzed N-Alkylation of Amines

Two bis-ruthenium(II) complexes, namely <i>N</i>,<i>N'</i>-{5,17-diamino-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arene}-bis-[dichloro-(<i>p</i>-cymene)-ruthenium(II)] (<b>1</b>) and <i>N</i>,<i>N'</i>-{5,11-diamino-4(24),6(10),12(16), 18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arene}-bis-[dichloro-(<i>p</i>-cymene)-ruthenium(II)] (<b>2</b>) were synthesized and tested as catalysts in the <i>N</i>-alkylation of primary amines with arylmethyl alcohol using the green "hydrogen borrowing" methodology. The catalytic results were compared with those obtained when the <i>N</i>-{5-amino-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-resorcin[4]arene}-[dichloro-(<i>p</i>-cymene)-ruthenium(II)] (<b>3</b>) complex was employed as catalyst. The rate of the <i>N</i>-alkylation of aniline with benzyl alcohol increased in the order <b>3</b> < <b>1</b> ≪ <b>2</b>, which highlights the importance of the relative positioning of the two metal centers on the upper rim of the resorcin[4]arene. Theoretical investigations suggest that the grafting of the two "RuCl₂(<i>p</i>-cymene)NH₂" moieties on two distal aromatic rings of the cavitand allows a cooperative effect between a ruthenium atom and the coordinated amine of the second metal center.