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)(2)Li{B(NDippCH)(2)} (1, Dipp = 2,6-iPr(2)C(6)H(3)) 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 (d(n), with n not equal 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 d(5) system [{(HCDippN)(2)B}Mn(THF)(mu-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 (13)C 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.