2,2′-Diphosphino- and 2,2′-Diarsenotolanes and Their Fe‑, Co‑, and Ni-Complexes: Pnictogen-Dependent Cyclization Tendencies and Metal-Dependent Stability and Reactivity Patterns

Common iron(II), cobalt(II), and nickel(II) bromides
were reacted
with 2,2′-bis(diisopropylphosphino)tolane 1-P (R
= iPr), which led to the isolation of P-heterocyclic chelate complexes (2-M, M =
Fe, Co, and Ni) containing a new C–P and a new C–M bond.
Attempts to reduce complexes 2-M (M = Fe, Co, and Ni),
however, were unsuccessful, that is, no C–C coupling was observed,
while diphosphadibenzo[a,e]pentalene
formation was noticed in the majority of our experiments. To suppress
the latter reactivity pattern, the reduced stability of As-ylides (in comparison to P-ylides) was exploited
by preparing the arsa-derivative of the ligand (1-As, R = iPr), which was found
to resist diarsadibenzo[a,e]pentalene
formation. The lower tendency of 1-As to cyclize is also
reflected in its reactions with MBr2 (M = Fe, Co, and Ni).
For M = Co, a κ2-As,As-coordinated complex (3-Co) with an uncoordinated alkyne
backbone was obtained, while pincer formation was found to occur for
M = Fe (3-Fe). Only in the case of nickel, an As-heterocyclic species (3-Ni) akin to 2-Ni was isolated. Upon reduction of 3-Ni, a
trimetallic metallacyclopentadiene (4-Ni) was produced
via alkyne C–C coupling, inferring that 3-Ni is
prone to ring-opening under reducing conditions. Reduction of 3-Co was only successful in the presence of CO (2.5 atm),
which led to the formation of a new As–C bond and the isolation
of an As-heterocyclic cobalt tricarbonyl complex
(5-Co). For 3-Fe, neither intermolecular
C–C nor intramolecular As–C coupling was observed, but
an unusual mixed-valent dimer with an [Fe2(alkyne)2]+ core (4-Fe) was generated and isolated
upon reduction with Zn powder. Compounds 4-M (M = Ni
and Fe) are discussed in the context of alkyne cycloadditions, given
that related motifs have been proposed as key intermediates in these
transformations.