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 , 1 H and 13 C NMR in solution along with DFT calculations are consistent with a κ 2 -coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar κ 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 = CF 3 , Bu on the pyrazolate ring afforded [Rh(R 2 pz)(COD)] 2 and [R 2 pzAlMe 2 ] 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 Me 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.