Mixed-Ligand Approach to Changing the Metal Ratio in Bismuth–Transition Metal Heterometallic Precursors

A new series of heteroleptic bismuth–transition
metal β-diketonates [BiM­(hfac)3(thd)2]
(M = Mn (1), Co (2), and Ni (3); hfac = hexafluoroacetylacetonate, thd = tetramethylheptanedionate)
with Bi:M = 1:1 ratio have been synthesized by stoichiometric reactions
between homometallic reagents [BiIII(hfac)3]
and [MII(thd)2]. On the basis of analysis of
the metal–ligand interactions in heterometallic structures,
the title compounds were formulated as ion-pair {[BiIII(thd)2]+­[MII(hfac)3]−} complexes. The direct reaction between homometallic
reagents proceeds with a full ligand exchange between main group and
transition metal centers, yielding dinuclear heterometallic molecules.
In heteroleptic molecules 1–3, the
Lewis acidic, coordinatively unsaturated BiIII centers
are chelated by two bulky, electron-donating thd ligands and maintain
bridging interactions with three oxygen atoms of small, electron-withdrawing
hfac groups that chelate the neighboring divalent transition metals.
Application of the mixed-ligand approach allows one to change the
connectivity pattern within the heterometallic assembly and to isolate
highly volatile precursors with the proper Bi:M = 1:1 ratio. The mixed-ligand
approach employed in this work opens broad opportunities for the synthesis
of heterometallic (main group–transition metal) molecular precursors
with specific M:M′ ratio in the case when homoleptic counterparts
either do not exist or afford products with an incorrect metal:metal
ratio for the target materials. Heteroleptic complexes obtained in
the course of this study represent prospective single-source precursors
for the low-temperature preparation of multiferroic perovskite-type
oxides.