Cyclooctadiene Rh(I) Bis- and Tris(pyrazolyl)aluminate Complexes and Their Catalytic Activity on the Polymerization of Phenylacetylene

In this work, we report the transfer of alkyl bis- and tris(pyrazolyl)aluminates metalloligands to an electron-rich organotransition metal center. The 16-electron heterobimetallic complexes of rhodium [Rh(COD){Al(Ph(2)pz)(2)Me-2}] (3) and [Rh(COD){Al(Ph(2)pz)(3)Me}] (4) were obtained by metathesis reaction of the sodium bis- (1) and tris(pyrazolyl)aluminate (2) with [RhCl(COD)](2). For 3, H-1 and C-13 NMR in solution along with DFT calculations are consistent with a kappa(2)-coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar kappa(2)-coordination mode of the tris(pyrazolyl)aluminate to Rh(I) with a pendant pyrazolyl moiety. The attempted synthesis of aluminate-rhodium complexes with R = CF3, tBu on the pyrazolate ring afforded [Rh(R(2)pz)(COD)](2) and [R2pzAlMe2]2. Complexes 3 and 4 were investigated as homogeneous catalysts in the polymerization of phenylacetylene (PA). Both complexes showed enhanced catalytic activity compared to analogous rhodium poly(pyrazolyl)borates. Optimized gas-phase DFT geometries of 3, 4, [Rh(COD){B(Ph(2)pz)(2)Me-2}], and [Rh(COD){B-(Ph(2)pz)(3)Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)2{B(Ph(2)pz)(2)M(e)2}], and [Rh(CO)(2){B(Ph(2)pz)(3)Me}] were employed to probe the electronic situation of the rhodium center through IR CO stretching modes. The wider bite angles and the less electron-rich rhodium center of the poly(pyrazolyl)aluminates compared with their borate analogues could be implicated in the better performance of the active catalytic species during polymerization of PA.

Automated summary

In this study, the transfer of alkyl bis- and tris(pyrazolyl)aluminates to an electron-rich organotransition metal center was reported, resulting in the formation of 16-electron heterobimetallic rhodium complexes. These complexes exhibited enhanced catalytic activity in the polymerization of phenylacetylene compared to analogous rhodium poly(pyrazolyl)borates.