Heterobimetallic {PtMn} and {PtFe} lantern complexes with exceptionally long metallophilic contacts

Assessment of Iron-Based Spin-Orbit Coupling Effects in Pt-Fe Heterobimetallic Lantern Complexes via 57Fe Mössbauer Spectroscopy

A series of heterobimetallic lantern complexes, [PtFe(SOCR)4(pyX)] where R = Me, X = H (1), X = NH2 (2), X = SMe (3); R = Ph, X = H (4), X = NH2 (5), X = SMe (6), have been prepared and characterized spectroscopically. Compounds 1, 4, and 5 are reported herein for the first time. The high-spin iron(II) sites of 1-6 have been investigated using 57Fe Mössbauer spectroscopy. Although the isomer shift of these species is nearly identical, their quadrupole splitting exhibits a much larger variation. Moreover, the zero-field Mössbauer spectra of 3-5 show surprising changes over time which are likely indicative of small structural distortions. The field dependent Mössbauer study of 1 and 6 revealed a zero field splitting (ZFS) characterized by a relatively large and positive D value. The combined Density Functional Theory (DFT) and ab initio Complete Active Space Self-Consistent Field (CASSCF) investigation of 1-6 indicates that their ground state is best described using a linear combination of {|xz⟩, |yz⟩} states. Our theoretical analysis suggests that the ZFSs and magnitude of the quadrupole splitting of 1-6 are determined by the spin-orbit coupling of the three lowest orbital states which have a T2g parentage.

Metal-Metal Bond Umpolung in Heterometallic Extended Metal Atom Chains

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 Mo2M(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 Mo2-M interaction. The delocalized 3c/3e- interaction is discussed within the context of the analogous 3c/3e- π bonding in the vinoxy radical, CH2CHO. 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 Mo2···M covalency increases with increasing Δχ of the Mo and M atoms (ΔχMo-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 Mo2 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.

Antiferromagnetic Coupling Supported by Metallophilic Interactions: Theoretical View

The antiferromagnetic coupling supported by metallophilic interactions has been studied in the framework of the broken symmetry approach (BS) and multiconfigurational calculations (CASSCF). A series of heterobimetallic complexes of the form [PtCo(X)₄(Y)]₂ (X = tba thiobenzoate, SAc thioacetate, and Y = H₂O, NO₂py, py), previously reported, have been used as model systems. Magnetic coupling constants were found in good agreement with the experimental reports, and it could be concluded that axial ligands with a pure σ-donor character have a marked effect on the J value strengthening the antiferromagnetic coupling, as shown for [PtCo(SAc)₄(H₂O)]₂ and [PtNi(SAc)₄(H₂O)]₂. The latter complex, included for comparative purposes, also made it possible to evidence that the interaction between magnetic orbitals and low-level excitation in the Pt···Pt region is also relevant favoring the stronger antiferromagnetic coupling found in this case. A careful analysis of the energetic components involved in Pt···Pt interaction suggests that the stabilization arises from a combination of favorable orbital contributions, which allows a weak covalent Pt···Pt σ(dz²...dz²) bond. Theoretical tools evidence that the weak σ-bond found between monomeric units is responsible for a spin polarization mechanism resulting in the observed antiferromagnetic interaction. Multiconfigurational calculations finally allowed us to establish that the spin polarization mechanism involves not only the d_z² orbitals in the M-Pt···Pt-M bond direction but also the empty 6p_z orbitals of Pt atoms. The inclusion of these orbitals favors a correlation-induced delocalization of magnetic orbitals and therefore a better balance among direct and kinetic exchange. The results shown in this work are relevant in the molecular design of systems supported by metallophilic interactions not only between platinum atoms but also could be extended to other cases with similar interactions.

Hard-Soft Chemistry Design Principles for Predictive Assembly of Single Molecule-Metal Junctions

The achievement of atomic control over the organic-inorganic interface is key to engineering electronic and spintronic properties of molecular devices. We leverage insights from inorganic chemistry to create hard-soft acid-base (HSAB) theory-derived design principles for incorporation of single molecules onto metal electrodes. A single molecule circuit is assembled via a bond between an organic backbone and an under-coordinated metal atom of the electrode surface, typically Au. Here, we study molecular composition factors affecting the junction assembly of coordination complexes containing transition metals atoms on Au electrodes. We employ hetero- and homobimetallic lantern complexes and systematically change the coordination environment to vary the character of the intramolecular bonds relative to the electrode-molecule interaction. We observe that trends in the robustness and chemical selectivity of single molecule junctions formed with a range of linkers correlate with HSAB principles, which have traditionally been used to guide atomic arrangements in the synthesis of coordination complexes. We find that this similarity between the intermolecular electrode-molecule bonding in a molecular circuit and the intramolecular bonds within a coordination complex has implications for the design of metal-containing complexes compatible with electrical measurements on metal electrodes. Our results here show that HSAB principles determine which intramolecular interactions can be compromised by inter molecule-electrode coordination; in particular on Au electrodes, soft-soft metal-ligand bonding is vulnerable to competition from soft-soft Au-linker bonding in the junction. Neutral donor-acceptor intramolecular bonds can be tuned by the Lewis acidity of the transition metal ion, suggesting future synthetic routes toward incorporation of transition metal atoms into molecular junctions for increased functionality of single molecule devices.

Paramagnetic Metal-Metal Bonded Heterometallic Complexes

Significant progress has been made in the past 10-15 years on the design, synthesis, and properties of multimetallic coordination complexes with heterometallic metal-metal bonds that are paramagnetic. Several general classes have been explored including heterobimetallic compounds, heterotrimetallic compounds of either linear or triangular geometry, discrete molecular compounds containing a linear array of more than three metal atoms, and coordination polymers with a heterometallic metal-metal bonded backbone. We focus in this Review on the synthetic methods employed to access these compounds, their structural features, magnetic properties, and electronic structure. Regarding the metal-metal bond distances, we make use of the formal shortness ratio (FSR) for comparison of bond distances between a broad range of metal atoms of different sizes. The magnetic properties of these compounds can be described using an extension of the Goodenough-Kanamori rules to cases where two magnetic ions interact via a third metal atom. In describing the electronic structure, we focus on the ability (or not) of electrons to be delocalized across heterometallic bonds, allowing for rationalizations and predictions of single-molecule conductance measurements in paramagnetic heterometallic molecular wires.