Circumventing Redox Chemistry: Synthesis of Transition Metal Boryl Complexes from a Boryl Nucleophile by Decarbonylation

The very strong reducing capabilities
of the boryllithium nucleophile
(THF)2Li­{B­(NDippCH)2} (1, Dipp
= 2,6-iPr2C6H3)
render impractical its use for the direct introduction of the {B­(NDippCH)2} ligand via metathesis chemistry into the immediate coordination
sphere of transition metals (dn, with n ≠ 0 or 10). Instead, 1 typically reacts with metal halide, amide and hydrocarbyl
electrophiles either via electron transfer or halide abstraction.
Evidence for the formation of M–B bonds is obtained only in
the case of the d5 system [{(HCDippN)2B}­Mn­(THF)­(μ-Br)]2. Lower oxidation state
metal carbonyl complexes such as Fe­(CO)5 and Cr­(CO)6 react with 1 via nucleophilic attack at the
carbonyl carbon atom to give boryl-functionalized Fischer carbene
complexes Fe­(CO)4{C­(OLi­(THF)3)­B­(NDippCH)2} and Cr­(CO)5{C­(OLi­(THF)2)­B­(NDippCH)2}. Although C-to-M boryl transfer does not occur for these
formally anionic systems, more labile charge neutral bora-acyl derivatives
of the type LnM­{C­(O)­B­(NDippCH)2} [LnM = Mn­(CO)5, Re­(CO)5, CpFe­(CO)2] can be synthesized, which cleanly
lose CO to generate M–B bonds. From a mechanistic standpoint,
an archetypal organometallic mode of reactivity, carbonyl extrusion,
has thus been shown to be applicable to the boryl ligand class, with 13C isotopic labeling studies confirming a dissociation/migration
pathway. These proof-of-methodology synthetic studies can be extended
beyond boryl complexes of the group 7 and 8 metals (for which a number
of versatile synthetic routes already exist) to provide access to
complexes of cobalt, which have hitherto proven only sporadically
accessible.