Iron-digermanium cluster: synthesis, Lewis-base-induced isomerization, dihydrogen activation, and alkene cycloaddition.

Heterometallic complexes containing multiple heavier tetrylidyne ligands (RE; E = Si, Ge, Sn, Pb) are promising for cooperative bond activation but are rare because RE units tend to oligomerize. We present the synthesis, electronic structure, and reactivity of an iron-digermanium complex, [Fe(GeAr')2] (1, Ar' = 2,6-Dipp2-C6H3, Dipp = 2,6- i Pr2-C6H3), featuring adjacent Ar'Ge units bonded to Fe through double and single Fe-Ge bonds, respectively. Complex 1 reacts readily with t BuNC, H2, and styrene via coordination, bond activation, and cycloaddition pathways, producing diverse structural outcomes. Coordination of t BuNC to Fe induces a formal reductive elimination of the two Ar'Ge units, forming an iron-digermyne complex (2) with a rare σ,σ-type double-bonding mode. Under mild conditions, H2 is activated, forming a germyl(iv)-iron-germylidyne complex (3). The reaction with styrene involves a [3 + 2]-cycloaddition, yielding a bis(germylene)-iron complex (4). Notably, the reaction with 2,3-dimethyl-1,3-butadiene (DMB) proceeds via an unusual kinetic [4 + 3]-cycloaddition pathway to produce a bis(germylene)-iron species (5), which then irreversibly rearranges into the thermodynamically favored [4 + 1] product (6). In contrast, the heavier iron-tin analogue exhibits distinct reactivity towards these substrates, highlighting fundamental differences between Fe-Ge and Fe-Sn bonding and reactivity.