Molybdenum Complex with Bulky Chelates as a Functional Model for Molybdenum Oxidases

The
novel bulky Schiff base chelate ligand [(4,5-diisopropyl-1H-pyrrole-2-yl)­methylene]-4-(tert-butyl)­aniline
(iPr2HL) bearing two isopropyl groups close to the pyrrole nitrogen atom
reacts with MoCl2(dme)­O2 (dme = 1,2-dimethoxyethane)
to give the sterically congested complex MoVI(iPr2L)2O2 (iPr21; OC-6–4–4 configuration). In spite
of the increased steric shielding of the [MoO2] unit iPr21 is active in oxygen-atom transfer to PMe3 and PPh3 to give OPMe3 and OPPh3, respectively.
Because of the increased steric bulk of the chelate ligand, formation
of dinuclear complexes [MoV(iPr2L)2O]2(μ-O) (iPr23) by comportionation is effectively prevented
in contrast to the highly favored formation of [MoV(H2L)2O]2(μ-O)
(H23) with the less bulky ligand H2HL. Instead, the smaller PMe3 ligand coordinates to the resulting pentacoordinate intermediate
MoIV(iPr2L)2O (iPr25), giving the hexacoordinate
complex MoIV(iPr2L)2O­(PMe3) (iPr22) with OC-6–3–3 configuration. The larger potential
ligands PPh3 and OPPh3 are only able to weakly
coordinate to iPr25, giving labile and sensitive MoIV(iPr2L)2O­(L) complexes (iPr26, L = PPh3; iPr27, L =
OPPh3). Traces of water and dioxygen in solutions of iPr26/iPr27 yield the di­(μ-oxido) complex [MoV(iPr2L)­O]2(μ-O)2 (iPr24) with reduced steric
congestion due to dissociation of the bulky chelate ligands. According
to electron paramagnetic resonance studies, the much more strongly
bound small PMe3 ligand in iPr22 can be slowly liberated by one-electron
oxidation to MoV, with Ag+ leaving a free coordination
site at MoV. Hence, essentially pentacoordinate MoIV and MoV complexes are accessible as a result
of the increased steric bulk.