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.