Single-Step Substitution of all the α, β‑Positions in Pyrrole: Choice of Binuclear versus Multinuclear Complex of the Novel Polydentate Ligand

The α and β-positions
of pyrrole were substituted with
3,5-dimethylpyrazolylmethyl groups in a single step that involved
the reaction between 2,5-dimethyl­amino­methylpyrrole and
3,5-dimethylpyrazole-1-carbinol, affording 2,3,4,5-tetrakis­(3,5-dimethylpyrazol-1-ylmethyl)­pyrrole LH in 40% yield. The coordination chemistry of this new polydentate
ligand LH was explored by synthesizing several Pd­(II),
Cu­(I), and Ag­(I) complexes. When LH was used as a neutral
ligand with [Pd­(COD)­Cl2], AgBF4, and CuX (X
= Cl and I), compartmental type binuclear Pd­(II) and Ag­(I) complexes
such as [Pd2Cl4(μ-C4HN-2,3,4,5-(CH2Me2pz)4-N,N,N,N)] and [Ag2(μ-C4HN-2,3,4,5-(CH2Me2pz)4-N,N,N,N)­(CH3CN)2]2+[BF4–]2 and cage-like copper­(I) complexes [Cu2(μ-X)­(μ-C4HN-2,3,4,5-(CH2Me2pz)4-N,N,N,N)]+[CuX2]− (X = Cl and I) containing a halide ion bridging
in a bent fashion were obtained, respectively. Conversely, when the
same metal precursors were treated with LH in the presence
of n-BuLi, the multinuclear complexes [Pd2Cl3(μ-C4N-2,3,4,5-(CH2Me2pz)4-N,N,N,N,N)], [(Cu2(μ-I)­{μ-C4N-2,3,4,5-(CH2Me2pz)4-N,N,N,N,N})2Cu]+I–, and [Ag4(μ-C4N-2,3,4,5-(CH2Me2pz)4-N,N,N,N)2]2+[BF4–]2 were
obtained. In addition, the treatment of LH with [Pd­(OAc)2] gave the mononuclear complex, [Pd­(OAc)­(C4N-2,3,4,5-(CH2Me2pz)4-N,N,N)]. The chloride analogue of this complex was
obtained by the reaction of LH with [Pd­(COD)­Cl2] in the presence of triethylamine. The structures of all complexes
were determined by single-crystal X-ray diffraction analyses, which
revealed interesting structural features, including pyrazole arms
adopting different conformations with respect to the pyrrole ring
plane and linear two- and three-coordinated copper­(I) and silver­(I)
atoms exhibiting weak interactions between the metal and the pyrrolic
carbon atoms and Ag­(I)···Ag­(I) interactions. The observed
shorter metal pyrrolide nitrogen (M–N) bond distances and the
elongation of the C–C double and single bond distances of the
pyrrole ring in these complexes probably indicates the presence of
π-donation/π-back bonding between the metal and the pyrrole
ring. These multinuclear complexes are novel, and their formations
are favored by the multidentating nature of the ligand LH.