Assessing the Metal–Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties

Polarized metal-metal multiple bonding and reactivity of phosphinoamide-bridged heterobimetallic group IV/cobalt compounds

Heterobimetallic complexes are studied for their ability to mimic biological systems as well as active sites in heterogeneous catalysts. While specific interest in early/late heterobimetallic systems has fluctuated, they serve as important models to fundamentally understand metal-metal bonding. Specifically, the polarized metal-metal multiple bonds formed in highly reduced early/late heterobimetallic complexes exemplify how each metal modulates the electronic environment and reactivity of the complex as a whole. In this Perspective, we chronicle the development of phosphinoamide-supported group IV/cobalt heterobimetallic complexes. This combination of metals allows access to a low valent Co-I center, which performs a rich variety of bond activation reactions when coupled with the pendent Lewis acidic metal center. Conversely, the low valent late transition metal is also observed to act as an electron reservoir, allowing for redox processes to occur at the d0 group IV metal site. Most of the bond activation reactions carried out by phosphinoamide-bridged M/Co-I (M = Ti, Zr, Hf) complexes are facilitated by cleavage of metal-metal multiple bonds, which serve as readily accessible electron reservoirs. Comparative studies in which both the number of buttressing ligands as well as the identity of the early metal were varied to give a library of heterobimetallic complexes are summarized, providing a thorough understanding of the reactivity of M/Co-I heterobimetallic systems.

Modulation of Fe-Fe distance and spin in diiron complexes using tetradentate ligands with different flanking donors

Here we report the synthesis and characterization of diiron complexes containing triaryl N4 and N2S2 ligands derived from o-phenylenediamine. The complexes display significant differences in Fe-Fe distances and magnetic properties that depend on the identity of the flanking NMe2 and SMe donor groups.

Reactivity of Ir(I)-aminophosphane platforms towards oxidants

The iridium(I)-aminophosphane complex [Ir{κ3C,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}2; SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}2). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(η3-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(η3-C3H5)(CNtBu)(SiNP)]Cl via the observed intermediate [IrCl2(η1-C3H5)(CNtBu)(SiNP)]. On the other hand, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{κ3C,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2]+ (1 : 2) in acetonitrile affording [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)3]2+ isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with I2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I, containing the η2,η3-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir2{κ3C,P,P'-(SiNP-H)}2(μ-I)3]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{κ3C,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the η2,η3-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

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.

Cyclometalated iridium complexes based on monodentate aminophosphanes

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with [Ir(μ-Cl)(cod)]₂ affording tetra- or pentacoordinate complexes of formula [IrCl(L)_n(cod)] (L = HNP, n = 1, 2; L = SiMe₃NP, n = 1). The reaction of [IrCl(SiMe₃NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)₃(SiMe₃NP)₂][IrCl₂(CO)₂]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, SiMe₃) as a result of the intramolecular oxidative addition of the tolyl C²-H bond to iridium. The straighforward formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺, including the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ and [Ir(cod)(HNP)₂]⁺. The "mixed" complex [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ was obtained upon reaction of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] with SiMe₃NP at 233 K. Finally, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP resulted in the formation of [Ir(CH₃CN)₂(SiMe₃NP)₂][PF₆] and [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)₂(cod)] being the most active, with TOF_{1 min} of about 2300 h^-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand

The iridium(III) hydride compound [IrH{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (1PF₆) was obtained by reaction of [Ir(SiNP)(cod)][PF₆] with CN^tBu as the result of the intramolecular oxidative addition of the SiCH₂-H bond to iridium(I) [SiNP = Si(CH₃)₂{N(4-tolyl)PPh₂}₂, SiNP-H = CH₂Si(CH₃){N(4-tolyl)PPh₂}₂]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆) forms in the first place and that further reacts with CN^tBu, affording the square planar intermediate [Ir(SiNP)(CN^tBu)₂][PF₆] (3PF₆) that finally undergoes the intramolecular oxidative addition of the SiCH₂-H bond. The reactivity of 1PF₆ was investigated. On one hand, the reaction of 1PF₆ with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (X = Cl, 4PF₆; Br, 5PF₆), and the reaction of 5PF₆ with AgPF₆ in the presence of acetonitrile affords the solvato species [Ir{κ³C,P,P'-(SiNP-H)}(CH₃CN)(CN^tBu)₂]²⁺ (6²⁺) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF₆ with HBF₄ gives the iridium(III) compound [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄) as the result of the formal addition of hydrogen fluoride to the Si-N bonds of 1⁺ [HNP = HN(4-tolyl)PPh₂]. A similar outcome was observed in the reaction of 1PF₆ with CF₃COOH rendering 7PO₂F₂. In this case the intermediate [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si-N-P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF₆, 4PF₆ and 7BF₄ have been determined by X-ray diffraction measurements.

Deciphering Cryptic Behavior in Bimetallic Transition-Metal Complexes with Machine Learning

We demonstrate an alternative, data-driven approach to uncovering structure-property relationships for the rational design of heterobimetallic transition-metal complexes that exhibit metal-metal bonding. We tailor graph-based representations of the metal-local environment for these complexes for use in multiple linear regression and kernel ridge regression (KRR) models. We curate a set of 28 experimentally characterized complexes to develop a multiple linear regression model for oxidation potentials. We achieve good accuracy (mean absolute error of 0.25 V) and preserve transferability to unseen experimental data with a new ligand structure. We also train a KRR model on a subset of 330 structurally characterized heterobimetallics to predict the degree of metal-metal bonding. This KRR model predicts relative metal-metal bond lengths in the test set to within 5%, and analysis of key features reveals the fundamental atomic contributions (e.g., the valence electron configuration) that most strongly influence the behavior of these complexes. Our work provides guidance for rational bimetallic design, suggesting that properties, including the formal shortness ratio, should be transferable from one period to another.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Molecular Complexes Featuring Unsupported Dispersion-Enhanced Aluminum-Copper and Gallium-Copper Bonds

The reaction of the copper(I) β-diketiminate copper complex {(Cu(BDI^Mes))₂(μ-C₆H₆)} (BDI^Mes = N,N'-bis(2,4,6-trimethylphenyl)pentane-2,4-diiminate) with the low-valent group 13 metal β-diketiminates M(BDI^Dip) (M = Al or Ga; BDI^Dip = N,N'-bis(2,6-diisopropylphenyl)pentane-2,4-diiminate) in toluene afforded the complexes {(BDI^Mes)CuAl(BDI^Dip)} and {(BDI^Mes)CuGa(BDI^Dip)}. These feature unsupported copper-aluminum or copper-gallium bonds with short metal-metal distances, Cu-Al = 2.3010(6) Å and Cu-Ga = 2.2916(5) Å. Density functional theory (DFT) calculations showed that approximately half of the calculated association enthalpies can be attributed to London dispersion forces.

Synthesis and Characterization of Tantalum-Based Early-Late Heterobimetallic Complexes Supported by 2-(diphenylphosphino)pyrrolide Ligands

The synthesis of the metalloligand Ta(κ²-NP)₃Cl₂ (NP = 2-diphenylphosphinopyrrolide) and its coordination chemistry with group 9 and 10 metals is reported. Treatment of Ta(κ²-NP)₃Cl₂ with group 9 and 10 metals resulted in clean formation of the heterobimetallic complexes Cl₂Ta(μ₂-NP)₃M (M = Ni (2), Pd (3)) or Cl₂Ta(μ₂-NP)₃MCl (M = Rh (4), Ir (5)). Each pair of complexes is isostructural and contains three phosphinopyrrolide ligands that bridge the metal centers. The d¹⁰ or d⁸ complexes are all diamagnetic and X-ray crystallographic analysis reveals similarly short metal-metal distances, ranging from 2.2979(5) Å to 2.4366(2) Å. Despite the similar bonding metrics in 2-5, treatment with an L type donor (2,6-dimethylphenylisocyanide (CNXylyl)) reveals 3 different coordination geometries in TaNi(CNXylyl) (6), TaPd(CNXylyl) (7), and TaIr(CNXylyl) (8). While complexes 6, 7, and 8 all bind the isocyanide at the late metal, ligand rearrangements are observed in the first row complex 6. Complex 7 binds the isocyanide in the axial position while equatorial binding is observed in 8. All isocyanide adducts maintain close metal-metal contacts in the solid state.

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.