Stereochemistry of Boron-Functionalized (Dioximato)iron(II) Complexes. Control of Nonbonded Interactions on Soft Conformational Surfaces

The structural and energetic consequences of nonbonded
interactions between axial ligands and minimally
superstructured borylated iron bis(dioxime) complexes are
described. Structural data for 20 low spin axially
ligated derivatives are compared. Complexes of
Fe((DMG)BPh2)2(Py)L, L =
NH3, TCNE, PMePh2, and Py and
Fe((DMG)BF2)2L2, L =
Py, and 4-t-BuPy, crystallize in the
C2v conformation.
The complexes, Fe((DMG)BPh2)2L2, L =
CH3CN,
CH3CH2CH2CN,
BuNH2, i-PrNH2, and piperidine
(PIP), and an Fe(III) derivative,
[Et4N][Fe((DMG)BF2)2(Cl)2],
all adopt the centrosymmetric
C2h conformation.
The cyclophane-like binding cavity in
the C2v complexes of
Fe((DMG)BPh2)2 opens or closes in
response to attractive or repulsive interactions between
the cavity and axial ligands. Face strain effects are largely
responsible for the binding order BuNH2 >
i-PrNH2
> PIP and 1−MeIm > Py in
Fe((DMG)BR2)2L2
complexes. They favor the exceptional
C2v conformer for
bis(pyridine) complexes, enforce an eclipsed conformation about the
N−Cα bond of i-PrNH2, and cause
a 0.12 Å
bond lengthening for PIP. The orientation of axial ligands in
FeN4 systems is controlled via the combined
effects
of axial interactions and face strain. Steric and Coulombic forces
have a significant effect on binding energetics
but London forces do not. Strategies for the effective use of
nonbonded interactions in conformationally complex
systems are described.