Synthesis, structure and catalytic application of lead(ii) complexes in cyanosilylation reactions

Abstract

Hydrothermal reactions of a lead(II) salt with 3-aminopyrazine-2-carboxylic acid (HL1) gave rise to a series of lead(II) coordination compounds (1–6) having zero, one, two and three dimensional structures. X-ray diffraction structural analyses reveal that complexes [Pb(L1)₂]₂ (1) and [Pb(L1)(OCHNH₂)(η-OCHO)]₂ (2) possess dinuclear structures, containing a centre of symmetry. Complexes [Pb₂(L1)₄(NHMeCHO)₂]_n (3) and [Pb₂(L1)₄]_n·(H₂O)_n·(2.5DMF)_n (4) have 1D chain like structures, and [Pb₅(L1)₇(η-NO₃)(μ-HCOO)(η-HCOO)]_n·(DMF)_n·(MeOH)_n (5) shows a 2D sheet like structure constructed by the [Pb₅O₅(HCOO)] cluster and 3-aminopyrazine-2-carboxylate anions. The hydrothermal reaction of lead(II) nitrate with HL1 in DMF led to in situ formation of 3,3′-(methylenebis(azanediyl))bis(pyrazine-2-carboxylic acid) [H₂L2] which produces the 3D framework [Pb₂(L2)₂]_n·(2DMF)_n·(H₂O)_n (6). The L1⁻ and L2²⁻ ligands bind the metal cations by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. The carboxylate group of L1⁻ presents a diversity of coordination modes, viz., monodentate (1 and 3), bridging μ₂ (3) and bridging μ₃ (4), monodentate bridging μ₂ (1, 2, 4, 5 and 6) and bridging chelate μ₂ (5). The carboxylate moiety of L2²⁻ in 6 binds the metal in a bridging μ₂ fashion. The Pb(II) ions display coordination numbers from 5 to 8 with hemi- or holodirected coordination environments. The Pb(II) complexes act as heterogeneous catalysts for the cyanosilylation reaction, at 15 °C, of different aldehydes with trimethylsilyl cyanide (TMSCN) and can be recycled at least three times without losing activity.