5 achieves a maximum conversion of only 20%. To account for this difference in
reactivity, it was proposed that the Cr centre electronically impacts the coordination
of the indenyl ligand to Rh facilitating an η
5 to η
3 ring slip which exposes the metal
towards reaction. This was supported by comparing the kinetics of rotation about the
Rh–indenyl bond for both mono- and bimetallic complexes where a larger η
3
component increased the rate of rotation for the bimetallic complex 4 compared to 5.
The bimetallic complex 6 containing half-sandwich RhCp* fragments (where
Cp* ¼ 1,2,3,4,5-pentamethylcyclopentadienyl) bridged by a borohydride scaffold
also showed moderate catalytic activity for the cyclotrimerisation of several
alkynes (Scheme 4) [18, 19]. 6 was recovered from the reaction mixture unchanged,
M
Cl
Cl
Cl
L
L
M
Cl
Cl
Cl
L
Nb
Cl
Cl
L
Nb
Cl
Cl
L
Cl
Cl
Me
Pr
Pr
Me
R
R
R
R
R
R
R
M= Nb (1), Ta (2)
L= tetrahydrothiophene
Nb
Cl
Cl
Cl
L
L
Nb
Cl
Cl
Cl
L
2-hexyne, -L
2-hexyne
Me
Me
Me
Pr
Pr
Pr
1
3
+
only observed with
monometallic catalysts:
[M(DME)Cl3]
Scheme 2 Catalysed cyclotrimerisation reaction of alkynes using bimetallic Nb (1) or Ta (2)
complexes
Scheme 3 Catalysed cyclotrimerisation reaction of electron-rich alkynes with a mono- and
bimetallic indenyl complex containing both Rh(I) and Cr(0)
108
M.J. Page et al.