Metal–Metal BondUmpolungin Heterometallic Extended Metal Atom Chains

Abstract

Understanding the fundamental properties governing metal− metal interactions is crucial to understanding the electronic structure and thereby applications of multimetallic systems in catalysis, material science, and magnetism. One such property that is relatively underexplored within multimetallic systems is metal−metal bond polarity, parameterized by the electronegativities (χ) of the metal atoms involved in the bond. In heterobimetallic systems, metal−metal bond polarity is a function of the donor−acceptor (Δχ) interactions of the two bonded metal atoms, with electropositive early transition metals acting as electron acceptors and electronegative late transition metals acting as electron donors. We show in this work, through the preparation and systematic study of a series of Mo 2 M(dpa) 4 (OTf) 2 (M = Cr, Mn, Fe, Co, and Ni; dpa = 2,2′dipyridylamide; OTf = trifluoromethanesulfonate) heterometallic extended metal atom chain (HEMAC) complexes that this expected trend in χ can be reversed. Physical characterization via single-crystal X-ray diffraction, magnetometry, and spectroscopic methods as well as electronic structure calculations supports the presence of a σ symmetry 3c/3e − bond that is delocalized across the entire metal-atom chain and forms the basis of the heterometallic Mo 2 −M interaction. The delocalized 3c/3e − interaction is discussed within the context of the analogous 3c/3e − π bonding in the vinoxy radical, CH 2 CHO. The vinoxy comparison establishes three predictions for the σ symmetry 3c/3e − bond in HEMACS: (1) an umpolung effect that causes the Mo−M interactions to become more covalent as Δχ increases, (2) distortion of the σ bonding and non-bonding orbitals to emphasize Mo−M bonding and de-emphasize Mo−Mo bonding, and (3) an increase in Mo spin population with increasing Mo−M covalency. In agreement with these predictions, we find that the Mo 2 •••M covalency increases with increasing Δχ of the Mo and M atoms (Δχ Mo−M increases as M = Cr < Mn < Fe < Co < Ni), an umpolung of the trend predicted in the absence of σ delocalization. We attribute the observed trend in covalency to the decreased energic differential (ΔE) between the heterometal d z 2 orbital and the σ bonding molecular orbital of the Mo 2 quadruple bond, which serves as an energetically stable, "ligand"-like electron-pair donor to the heterometal ion acceptor. As M is changed from Cr to Ni, the σ bonding and nonbonding orbitals do indeed distort as anticipated, and the spin population of the outer Mo group is increased by at least a factor of 2. These findings provide a predictive framework for multimetallic compounds and advance the current understanding of the electronic structures of molecular heteromultimetallic systems, which can be extrapolated to applications in the context of mixed-metal surface catalysis and multimetallic proteins.

Rh(III)-Catalyzed N-Amino-Directed C–H Coupling with 3-Methyleneoxetan-2-ones for 1,2-Dihydroquinoline-3-carboxylic Acid Synthesis

“…The opposite and complementary polarity between the two sites in the reaction partners, one electronically rich (nucleophilic) and one electronically poor (electrophilic), creates a working mechanism for their engagement in a bonding state. As such, polarity analysis, the formal examination and assignment of electronic character of organic molecule sites, and corresponding electronic character complementarity, has developed into a core synthetic planning practice . Thus far, polarity analysis is executed primarily in the regime of conventional organic chemistry and in the form of static polarity analysis, assignment of electronic character exclusively based on the electronegativity (with resonance effect included) of each site (static polarity: regular polarity and umpolung polarity inversion) and associated complementarity .…”