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.