Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions

Synthesis and characterization of homometallic cobalt complexes with metal-metal interactions

Complexes featuring metal-metal bonds play crucial roles in catalysis and small molecule activation due to the synergistic effects between the metals. Here, we report a series of homometallic cobalt complexes with metal-metal interactions that have been successfully stabilized by a multidentate ligand platform. Theoretical studies on metal-metal interactions in these cobalt complexes are discussed.

Flexible interactions of the rare-earth elements Y, La, and Lu with phosphorus in metallacyclohexane rings

A geometrically flexible bifunctional (bis)aminophosphine ligand was synthesized in a three-component, one-pot Kabachnik-Fields reaction using tert butylphosphine, paraformaldehyde, and 3,5-dimethyl aniline. The product, bis((3,5-dimethylphenyl)aminomethyl) tert butylphosphine (ArBiAMP t Bu), containing two secondary amines and a tertiary phosphine, was isolated in good yields. Deprotonation of both N-H groups with (trimethylsilyl)methylpotassium (K-CH2SiMe3), followed by salt metathesis with LaI3, YI3, and LuI3 generated the corresponding MI(ArBiAMP t Bu)(thf)3 complexes (M = Y (1), La (2), and Lu (3)) in good yields. A sterically encumbered indene, 1,3-diisopropyl-4,7-dimethyl-1H-indene, iPrMeInd, was deprotonated in situ and installed via salt-metathesis to generate the organometallic series of η5-indenide complexes, M(ArBiAMP t Bu)(η5-iPrMeInd)(thf) (M = Y (4), La (5), and Lu (6)). 1H, 31P, 13C, and 89Y NMR experiments, IR spectroscopy, and single crystal X-ray diffraction (SC-XRD), were used to characterize these complexes. The Y-P coupling constant was found to be variable depending on the modifiable coordination environment of the metal center, indicating potential as both a spectroscopic handle as well as providing insight into the influence of additional ligands on the metal center.

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

Rare-Earth Metalloligands for LowValent Cobalt Complexes: Fine Electronic Tuning via Co→RE Dative Interactions

Rare-earth metalloligand supported low-valent cobalt complexes were synthesized by utilizing a small-sized heptadentate phosphinomethylamine LsNH3 and a large-sized arene-anchored hexadentate phosphinomethylamine LlArH3 ligand precursors. The RE(III)-Co(-I)-N2 (RE = Sc, Lu, Y, Gd, La) complexes containing rare-earth metals including the smallest Sc and largest La were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The Co(-I)→RE(III) dative interactions were all polarized with major contributions from the 3dz2 orbital of the cobalt center, which was slightly affected by the identity of rare-earth metalloligands. The IR spectroscopic data and redox potentials obtained from cyclic voltammetry revealed that the electronic property of the Co(-I) center was finely tuned by the rare-earth metalloligand, which was revealed by variation of the ligand systems containing LsN, LmN, and LlAr. Unlike the direct alteration of the electronic property of metal center via an ancillary ligand, such a series of rare-earth metalloligand represents a smooth strategy to tune the electronic property of transition metals.

A scandium metalloligand supported Ni(0) complex with a heterobimetallocycle: versatile reactivity with unsaturated bonds

A N2-bridged tetranuclear Sc(III)-Ni(0) complex featuring a Ni → Sc interaction and a 4-membered [Sc-N-C-Ni] ring was synthesized and characterized. Bimetallic reactivity was demonstrated via reactions with a series of unsaturated compounds containing NC, CN, CC, CO and NN bonds.

Neutral and Anionic Square Planar Palladium(0) Complexes Stabilized by a Silicon Z-Type Ligand

Anionic [Pd(0)-X]- ate complex were proposed as key intermediates in Pd-catalyzed cross-coupling for decades, but their isolation remained elusive. Herein, a chelating Lewis acidic bis(amidophenolato)silane is introduced as a strong Z-type ligand which enables the characterization of the first anionic [Pd(0)-X]- ate complex. Intriguingly, these compounds and the neutral L-Pd(0) analogs exhibit a square planar coordination that is highly unusual for a d10 metal. Theoretical methods scrutinize the interaction between the Lewis acidic Si(IV) center and the late transition metal, while reactivity studies shed light on the potential role of anionic additives in oxidative addition reactions.

To Bond or Not to Bond: Metal-Metal Interaction in Heterobimetallic Rare-Earth Metal-Silver Complexes

The reaction of 2,4-tBu2-6-(PPh2)PhOH (HOArP) with silver(I) triflate in a 3:1 molar ratio gave the mononuclear coinage metal complex (HOArP-κP)3AgIOTf (1). Treatment of HOArP with LnIII[N(SiMe3)2]3 (Ln = La, Sm, Y, Yb) in a 3:1 molar ratio yielded the mononuclear rare-earth metal complexes LnIII(OArP-κ2O,P)3 (2-Ln). The heterobimetallic rare-earth metal-silver complexes LnIII(OTf)(μ-OArP-1κ1O,2κ1P)3AgI (3-Ln) were prepared from monometallic precursors by reactions of equimolar amounts of 1 with LnIII[N(SiMe3)2]3 or 2-Ln with silver(I) triflate, respectively. The compounds were characterized by NMR, ultraviolet-visible (UV-vis), and infrared (IR) spectroscopy, single-crystal X-ray diffraction, elemental analysis, and the effective magnetic moments of the paramagnetic complexes were determined via the Evans NMR method. Computational studies were conducted on 3-La and 3-Y.

Heterometallic Clusters with Cerium-Transition-Metal Bonding Supported by Nitrogen-Phosphorus Ligands

Ligands are known to play a crucial role in the construction of complexes with metal-metal bonds. Compared with metal-metal bonds involving d-block transition metals, knowledge of the metal-metal bonds involving f-block rare-earth metals still lags far behind. Herein, we report a series of complexes with cerium-transition-metal bonds, which are supported by two kinds of nitrogen-phosphorus ligands N[CH2CH2NHPiPr2]3 (VI) and PyNHCH2PPh2 (VII). The reactions of zerovalent group 10 metal precursors, Pd(PPh3)4 and Pt(PPh3)4, with the cerium complex supported by VI generate heterometallic clusters [N{CH2CH2NPiPr2}3Ce(μ-M)]2 (M = Pd, 2 and M = Pt, 3) featuring four Ce-M bonds; meanwhile, the bimetallic species [(PyNCH2PPh2)3Ce-M] (M = Ni, 5; M = Pd, 6; and M = Pt, 7) with a single Ce-M bond were isolated from the reactions of the cerium precursor 4 supported by VII with Ni(COD)2, Pd(PPh3)4, or Pt(PPh3)4, respectively. These complexes represent the first example of species with an RE-M bond between Ce and group 10 metals, and 2 and 3 contain the largest number of RE-M donor/acceptor interactions ever to have been observed in a molecule.

Structure and bonding patterns in heterometallic organometallics with linear Ln-Pd-Ln motifs

Complexes with short intermetallic distances between transition metal fragments and lanthanide (Ln) fragments are fascinating objects of study, owing to the ambiguity of the nature of the interaction. The addition of the divalent lanthanide fragments Cp*₂Ln(OEt₂) (Ln = Sm or Yb) to a Pd(ii) complex bearing the deprotonated form of the redox-active, non-symmetrical ligand, 2-pyrimidin-2-yl-1H-benzimidazole (Hbimpm), leads to two isostructural complexes, of the general formula (Cp*₂Ln)₂[μ-Pd(pyridyl)₂] (Ln = Sm (4) and Yb (5)). These adducts have interesting features, such as unique linear Ln-Pd-Ln arrangements and short Ln-Pd distances, which deviate from the expected lanthanide contraction. A mixed computational and spectroscopic study into the formation of these adducts gathers important clues as to their formation. At the same time, thorough characterization of these complexes establishes the +3 oxidation state of all the involved Ln centers. Detailed theoretical computations demonstrate that the apparent deviation from lanthanide contraction is not due to any difference in the intermetallic interaction between the Pd and the Ln, but that the fragments are joined together by electrostatic interactions and dispersive forces. This conclusion contrasts with the findings about a third complex, Cp*₂Yb(μ-Me)₂PdCp* (6), formed during the reaction, which also possesses a short Yb-Pd distance. Studies at the CASSCF level of theory on this complex show several orbitals containing significant interactions between the 4f and 4d manifolds of the metals. This demonstrates the need for methodical and careful analyses in gauging the intermetallic interaction and the inadequacy of empirical metrics in describing such phenomena.

Dinitrogen Complexes of Cobalt(-I) Supported by Rare-Earth Metal-Based Metalloligands

Sequential reactions of heptadentate phosphinoamine LH₃ with rare-earth metal tris-alkyl precursor (Me₃SiCH₂)₃Ln(THF)₂ (Ln = Sc, Lu, Yb, Y, Gd) and a low-valent cobalt complex (Ph₃P)₃CoI afforded rare-earth metal-supported cobalt iodide complexes. Reduction of these iodide complexes under N₂ allowed the isolation of the first series of dinitrogen complexes of Co(-I) featuring dative Co(-I) → Ln (Ln = Sc, Lu, Yb, Y, Gd) bonding interactions. These compounds were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The correlation of N-N vibrational frequencies with the pK_a of [Ln(H₂O)₆]³⁺ showed that strongest activation of N₂ was achieved with the least Lewis acidic Gd(III) ion. Interestingly, these Ln-Co-N₂ complexes catalyzed silylation of N₂ in the presence of KC₈ and Me₃SiCl with turnover numbers (TONs) up to 16, where the lutetium-supported Co(-I) complex showed the highest activity within the series. The role of the Lewis acidic Ln(III) was crucial to achieve catalytic turnovers and tunable reactivity toward N₂ functionalization.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

Recent advances in heterometallic clusters with f-block metal-metal bonds: synthesis, reactivity and applications

Due to the heterometallic synergistic effects from different metals, heterometallic clusters are of great importance in small-molecule activation and catalysis. For example, both biological nitrogen fixation and photosynthetic splitting of water into oxygen are thought to involve multimetallic catalytic sites with d-block transition metals. Benefitting from the larger coordination numbers of f-block metals (rare-earth metals and actinide elements), heterometallic clusters containing f-block metal-metal bonds have long attracted the interest of both experimental and theoretical chemists. Therefore, a series of effective strategies or platforms have been developed in recent years for the construction of heterometallic clusters with f-block metal-metal bonds. More importantly, synergistic effects between f-block metals and transition metals have been observed in small-molecule activation and catalysis. This tutorial review highlights the recent advances in the construction of heterometallic molecular clusters with f-block metal-metal bonds and also their reactivities and applications. It is hoped that this tutorial review will persuade chemists to develop more efficient strategies to construct clusters with f-block metal-metal bonds and also further expand their applications with heterometallic synergistic effects.

Phosphinoamido Ligand Supported Heterobimetallic Rare-Earth Metal-Palladium Complexes: Versatile Structures and Redox Reactivities

Heterobimetallic Ln(III)-Pd(0) complexes (Ln = Y, Sm, Gd, Yb) featuring tetranuclear structures with COD as bridges were obtained via the metallation of tris(phosphinoamido) rare-earth metal complexes [Ph2PNAd]3Ln (Ad = admantyl)...