Versatile Coordination Mode of a New Pyridine-Based Ditopic Ligand with Transition Metals: From Regular Pyridine to Alkyne and Alkenyl Bindings and Indolizinium Formation

The
new BPMPB ligand, namely, bis­[1-bis­(2-pyridylmethyl),1 (pyridyl)]­butyne,
can be very easily obtained as a side product in the known reaction
of picolyl chloride and sodium acetylide (which major product is the
known terminal alkyne-substituted tripod). This symmetrical ligand
contains two identical coordination sites with two methylenepyridines
and one pyridyl group on each side, linked by an alkyne function providing
a semirigid segment. Together with the molecular structure of the
ligand which is reported, we describe the preparation of complexes
with Fe­(II)­Cl2, Co­(II)­Cl2, Ni­(II)­Cl2, Cu­(I)­Cl, and Zn­(II)­Cl2 salts. All complexes have been
characterized by X-ray diffraction studies as well as by standard
spectroscopic techniques. The striking point in this work is the diversity
of the structures that are obtained. Co­(II) and Zn­(II) provide isostructural
dinuclear complexes in which both coordination sites are occupied
within a tetrahedral symmetry. The Cu­(I) complex is also a dinuclear
compound, but in that case, the copper atom is coordinated to the
alkyne moiety, two pyridines, and a bridging chloride. The 13C NMR spectrum of the copper complex confirms that the metal center
is coordinated to the alkyne in solution. The coordination of Ni­(II)
results in the formation of a mononuclear complex in which a pyridine
has fused with the alkyne moiety to generate an indolizinium group;
the structure of the corresponding alkenyl complex is reported. Finally,
the addition of FeCl2 to the ligand results in the formation
of a mononuclear complex with a free, noncoordinated indolizinium.
The sequence developed in the present work illustrates the possibility
for the metal centers to adopt various coordination modes which may
be relevant to the conversion of an alkyne and a pyridyl unit into
indolizinium.