Planar tetracoordinate carbon in tungstenacyclobutadiene from alkyne metathesis and expanded structures

The planar tetracoordinate (C-pt) character of C-beta 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 C-pt character of C-beta is due to 1,3-WC bonding, which is established on the basis of the single bond-like WC beta 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 C-13-NMR data of C-alpha and C-beta 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 C-pt centers. Several such structures are reported, and their 3-dimensional extensions provide access to novel C-pt-incorporated metal-organic frameworks.