Planar tetracoordinate carbon in tungstenacyclobutadiene from alkyne metathesis and expanded structures.

The planar tetracoordinate (ptC) character of Cβ in the tungstenacyclobutadiene (WCBD) from alkyne metathesis is analyzed with the support of structural, electronic, molecular orbital and electron density data obtained from density functional theory calculations. The ptC character of Cβ is due to 1,3-WC bonding, which is established on the basis of the single bond-like WCβ distance in X-ray structures and calculated structures, a catastrophe ring critical point for the WCBD ring in quantum theory of atoms-in-molecule analysis (QTAIM) and a large difference in the (13)C-NMR data of Cα and Cβ atoms. The metalloaromatic character of WCBD is revealed by nuclear independent chemical shift (NICS) values and diatropic ring current observed in the anisotropy of the induced current density (AICD) plot. These WCBD structural motifs provide a new strategy to build 1-, 2- and 3-dimensional organometallic polymeric structures containing multiple ptC centers. Several such structures are reported, and their 3-dimensional extensions provide access to novel ptC-incorporated metal-organic frameworks.