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

Abstract

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(Ph2pz)2Me2}] (3) and [Rh(COD){Al(Ph2pz)3Me}] (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  2 -coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar  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, t Bu on the pyrazolate ring afforded [Rh(R2pz)(COD)]2 and [R2pzAlMe2]2. Complexes 3 and 4 were investigated as homogenous 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(Ph2pz)2Me2}], and [Rh(COD){B(Ph2pz)3Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)2{B(Ph2pz)2Me2}], and [Rh(CO)2{B(Ph2pz)3Me}] were employed to probe the electronic situation of 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 analogs could be implicated in the better performance of the active catalytic species during polymerization of PA.

Catalytic activity of cyclobutadiene rhodium complexes in hydrosilylation and other transformations of alkynes

“…Diene rhodium complexes, such as [Rh(cod)Cl] 2 (cod = cyclooctadiene), are widely used in organometallic synthesis [1][2][3] and catalysis. [4][5][6] Related cyclobutadiene rhodium complexes have been known for a long time, 7,8 but have only been studied fragmentary before the general approach to their synthesis has been developed by our group in 2015. 9,10 Cyclobutadiene, despite its formal electron count as a diene, exhibits significantly different properties due to the conjugation of double bonds and its transformation into an aromatic system upon coordination to a metal.…”