Structural variety of zinc and copper complexes based on a 2,3-disubstituted 1,2,3,4-tetrahydroquinazoline ligand

Abstract

The ring-chain tautomerism of 2-(3-tosyl-1,2,3,4-tetrahydroquinazolin-2-yl)quinolin-8-ol (H(2)L(ring)) has been exploited to produce mononuclear complexes or, alternatively, dinuclear complexes, as desired, by varying the stoichiometry of the ligand. Cu(2+) and Zn(2+) stabilise the ring tautomeric form of the ligand in their mononuclear complexes M(HL(ring))(2). The structural characterisation of Zn(HL(ring))(2)·2MeOH·0.5H(2)O shows O,N-donor behaviour of the ring tautomer. The 1,2,3,4-tetrahydroquinazoline undergoes a ring-opening reaction upon formation of phenoxo-bridged dinuclear complexes M(2)(L(chain))(2) in which the chain tautomer is acting as O,N,N,N-donor. The crystal structure of Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH evidenced the sensitivity of H(2)L(ring) to the copper-mediated aerobic oxidation, which results in two derivatives of the ligand, a quinazoline and an amide. The quinazoline ligand is acting as monoanionic and mononucleating through its O,N,N binding site, while the amide ligand behaves as a trianionic and binucleating through its O,N,N,N and O,O binding sites in Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH.

Controlling ring-chain tautomerism through steric hindrance

“…The half-chair conformation, which is adopted by the tetrahydroquinazoline ring in chiral H 2 L 2 TQ (ESI †), contrasts with the envelope conformation found for the achiral 3-tosyl-1,2,3,4tetrahydroquinazoline, 24 explaining the diasterotopic nature of the methylene protons in H 2 L 2 TQ (H-7 eq and H-7 ax ). Regarding to other aspects related to conformation, H 2 L 2 TQ displays the preferred conformation of sulfonamides, i.e., with the lone pair of the N atom bisecting the O]S]O angle and practically perpendicular to the tosyl ring.…”

New Cyclodextrin‐Bearing 8‐Hydroxyquinoline Ligands as Multifunctional Molecules

“…[44] In fact, amide derivatives of 4 have been characterized and the coordination of the deprotonated amide nitrogen has been reported. [45,46] The amide N À H deprotonation occurs at relatively low pH promoted by copper(II) ion complexation. [44] In the case of tertiary amides of 4, the coordination of the carboxylic oxygen has been reported.…”

Mechanistic insights into a Zn-assisted ring-chain tautomerism process involving a 1,2,3,4-tetrahydroquinazoline and a Schiff base

“…The reaction of H 2 L ring /H 2 L chain and Zn(OAc) 2 in a 1:1 and 2:1 molar ratios were performed as previously reported. 2 The 1 H NMR spectroscopic monitoring of the reactions was performed taking aliquots of the reaction mixtures at different times and temperatures and recording their 1 H NMR spectra.…”

“…1) that in the absence of metal ions the chain tautomer N-{2-[(8-hydroxyquinolin-2-yl)methyleneamino]benzyl}-4-methylbenzenesulfonamide (H 2 L chain ) easily undergoes an intramolecular cyclization reaction to give the final more thermodynamically stable product, the ring tautomer 2-(3-tosyl-1,2,3,4tetrahydroquinazolin-2-yl)quinolin-8-ol (H 2 L ring ). In the presence of Pd 2+ the tetrahydroquinazoline-imine tautomeric equilibrium is shifted towards the chain form, yielding Pd(HL chain ) 2 , while Ni 2+ shifts the equilibrium towards the ring form to afford Ni(HL ring ) 2 (Sch. 2, I).…”