Bimetallic complexes for enhancing catalyst efficiency: probing the relationship between activity and intermetallic distance

A series of new homoditopic ligands (14–17) containing two bis(pyrazol-1-yl)methane moieties connected to either flexible (1,6-bis(bis(pyrazol-1-yl)methyl)hexane, L6C (14); 1,7-bis(bis(pyrazol-1-yl)methyl)heptane, L7C (15)) or rigid scaffolds (4,5-bis(bis(pyrazol-1-yl)methyl)-9,9-dimethylxanthene, LXan (16); 4,6-bis(bis(pyrazol-1-yl)methyl)dibenzofuran, LDib (17)) were synthesized. A series of bimetallic rhodium(I) complexes [Rh2(CO)4(LX)][BArF4]2 (X = Xan (8), Dib (9), Fc ((1,1′-bis(bis(pyrazol-1-yl)methyl)ferrocene) (10)), 6C (11), 7C (12)) and [Rh2(COD)2(LX)][BArF4]2 (COD = 1,5-cyclooctadiene, X = 6C (21), 7C (22)) as well as the monometallic complexes [Rh(CO)2(LPh)][BArF4] (7, LPh = α,α-bis(pyrazol-1-yl)toluene) and [Rh(COD)(LPh)][BArF4] (20) were synthesized. The solid-state structures of 8, 10, 16, 17, and 21 were determined using single-crystal X-ray diffraction analysis. The catalytic activity of complexes 7–12 was established for the dihydroalkoxylation of the alkynediols 2-(5-hydroxypent-1-ynyl)benzyl alcohol (I) and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (II). The rigid bimetallic scaffolds LXan and LDib were found to yield the most active catalysts, 8 and 9, respectively, with 9 achieving a reaction rate 5–6 times faster than the monometallic complex 7 for the dihydroalkoxylation of I. Density functional theory calculations were used to examine the intermetallic Rh···Rh distances in 8 and 9, and these were compared with those of three other related bimetallic catalysts reported previously. The calculations showed all these species to be very flexible at minimal energetic cost, both in terms of the Rh···Rh distance and in being able to access a range of different conformations. No clear correlation between Rh···Rh distance and catalytic activity was established here, which suggests that the observed experimental correlation between catalyst structure and activity may derive from the structures of key reaction intermediates