A new amido-phosphane as ligand for copper and silver complexes. Synthesis, characterization and catalytic application for azide–alkyne cycloaddition in glycerol

Abstract

The new sterically hindered amido-phosphane 1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane-3,7-diylbis(phenylmethanone), DBPTA (1), has been obtained via an open-cage double N-acylation of 1,3,5-triaza-7-phosphadamantane (PTA) using benzoic anhydride. DBPTA is the only acyl derivative of PTA that contains an aromatic appendage. Due to the bulky nature of the benzoyl C(O)Ph groups, they exhibit mutual anti configuration as confirmed by solution NMR and single crystal X-ray diffraction. Compound 1 is readily soluble in common polar organic and green solvents, making it a very versatile ligand that could be used in a variety of reaction systems. To assess the coordination characteristics of the new phosphane, seven copper complexes of formulas [Cu(DBPTA)₄]BF₄ (2), [CuX(DBPTA)₃] {X = Br (3) and I (4)}, [Cu(μ-X)(DBPTA)₂]₂ {X = Br (5) and I (6)}, [Cu(bpy)(DBPTA)₂]Y {Y = BF₄ (7) and BPh₄ (8)} {bpy = 2,2′-bipyridine}, and three silver complexes with formulas [Ag(DBPTA)₄]NO₃ (9), [Ag(Tpm*)(DBPTA)]NO₃ (10) and [Ag(Tpms)(DBPTA)] (11) {Tpm* = tris(3,5-dimethyl-1-pyrazolyl)methane, Tpms = tris(pyrazol-1-yl)methanesulfonate} have been synthesised. Compounds 1–11 were characterized by elemental analyses and electrospray ionization mass spectrometry (ESI-MS), as well as by FT-IR and NMR (¹H, ¹³C, ³¹P, COSY and HSQC) spectroscopic techniques. The catalytic activity of the complexes has been investigated for 1,3-dipolar azide–alkyne cycloaddition reaction using glycerol as a reaction medium to afford 1,4-disubstituted-1,2,3-triazoles. Complex 7 was found to be the most efficient catalyst, affording triazoles in yields up to 97% after 18 h under standard bench experimental conditions (at 23 °C, aerobic conditions and in the absence of any additional bases) and up to 98% after 15 minutes under microwave irradiation (125 °C, 30 W). The catalysis proceeds with a broad substrate scope according to “Click” rules providing a significant contribution to “Green Chemistry”.