Early-late heterobimetallics

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.

Syntheses, Characterization, and Redox Activity of Ferrocene-Containing Titanium Complexes

The chemistry of bis(π-η5:σ-η1-pentafulvene)titanium complexes is characterized by a broad range of E-H activation and Ti-C functionalization reactions, whereas ferrocene derivatives are easily accessible and redox-active compounds. The reaction of ferrocenealdehyde and -ketones with bis(π-η5:σ-η1-pentafulvene)titanium complexes result in the formation of bimetallic complexes via insertion of the C=O double bond of the aldehyde/ketone into the Ti-Cexo bond of the pentafulvene moiety. The reaction of bis(π-η5:σ-η1-pentafulvene)titanium complexes with ferrocenyl alcohols leads to alcoholate complexes via deprotonation of the OH group by the pentafulvene ligand. Because of the one remaining pentafulvene unit, further functionalization of the complexes is possible. In this work, we proceeded with 1,1'-bifunctionalized ferrocene derivatives for intramolecular follow-up reactions. 1,1'-Ferrocenedimethanol reacts with bis(π-η5:σ-η1-pentafulvene)titanium complexes in a double O-H deprotonation reaction to yield the dialcoholate complex. 1,1'-bis(phenylphosphine)ferrocene reacts differently as the double P-H deprotonation reaction results in the formation of a P-P linked phosphine. Therefore, we studied the reactivity of 1,1'-bis(phenylphosphine)ferrocene toward Rosenthal's reagent. As Rosenthal's reagent is regarded as a masked titanocene(II) species, it undergoes redox reactions toward H-acidic substrates, forming a paramagnetic Ti(III) complex.

Mechanism and Stereoselectivity Control of Terminal Alkyne Dimerization Activated by a Zr/Co Heterobimetallic Complex: A DFT Study

Heterobimetallic complexes have recently garnered considerable attention in organic synthesis owing to their high activity and selectivity, which surpass those of monometallic complexes. In this study, the detailed mechanisms of terminal alkyne dimerization activated by the heterobimetallic Zr/Co complex, as well as the different stereoselectivities of Me3SiC≡CH and PhC≡CH dimerization, were investigated and elucidated by using density functional theory calculations. After excluding the three-molecule reaction and outer-sphere mechanisms, the inner-sphere mechanism was determined as the most optimal process. The inner-sphere mechanism involves four processes: THF dissociation and coordination of the first alkyne; ligand migration and C-H activation; N2 dissociation and insertion of the second alkyne; and reductive elimination. The stereoselectivity between the E-/Z- and gem-isomers is determined by the C-C coupling mode of the two alkynes and that of the E- and Z-isomers is determined by the sequence of the C-C coupling and hydrogen migration in the reductive elimination process. Me3SiC≡CH dimerization yields only an E-isomer owing to the large differences in the distortion and interaction energies, whereas PhC≡CH dimerization produces an E-, Z-, and gem-isomers owing to the reduced interaction energy differences.

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.

Luminescent early-late-hetero-tetranuclear group IV - Au(I) bisamidinate complexes

The versatile metalloligand [{HCCC(NDipp)₂}₂Au₂] (dipp = 2,6-diisopropylphenyl) was converted into early-late heterotetrametallic complexes [{ClCp₂MCCC(NDipp)₂}₂Au₂] (M = Ti, Zr). These compounds show photoluminescence with either remarkably different (Ti) or similar (Zr) features as compared to related solely coinage metal containing acetylide amidinate complexes.

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)...

Heterobimetallic complexes stabilized by the P2N2 macrocyclic ligand system: synthesis and reactivity of a rhodium-copper system that activates molecular hydrogen

The synthesis and reactivity of the bimetallic rhodium-copper complex, Rh(COE)[P₂N₂]Cu, which is stabilized by the P₂N₂ macrocycle, is reported. In the solid state, the rhodium and copper centers are on opposite sides of the macrocyclic ring with the Cu(I) in a linear environment and the Rh(I) in a square planar array. However, in solution a very symmetrical structure is suggested on the basis of the ¹H NMR data, which is consistent with at least two separate fluxional processes, rotation of the cyclooctene unit and movement of the Rh(I) unit between the two amido donors. Addition of H₂ to Rh(COE)[P₂N₂]Cu results in the formation of ([P₂N₂H]RhH(μ-H)₂Cu)₂via hydrogenation of the coordinated cyclooctene unit, oxidative addition of H₂ to the rhodium center and hydrogenolysis of the copper amido unit. Monitoring the reaction of H₂ by NMR spectroscopy indicated the formation of a number of intermediates which suggests hydrogenolysis of the copper amido linkage occurs to generate CuH in some form, along with Rh(COE)[P₂N₂H], which is converted to Rh(H)₂[P₂N₂H] by hydrogenation of the cyclooctene, which then recombines with the CuH present to generate the final product. Deuteration studies indicate that there is considerable H/D scrambling in the cyclooctane produced that we attribute to reversible beta-elimination, migratory insertion steps.

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.

A Phosphine Functionalized β-Diketimine Ligand for the Synthesis of Manifold Metal Complexes

A bis(diphenyl)-phosphine functionalized β-diketimine (PNac-H) was synthesized as a flexible ligand for transition metal complexes. The newly designed ligand features symmetrically placed phosphine moieties around a β-diketimine unit, forming a PNNP-type pocket. Due to the hard and soft donor atoms (N vs. P) the ligand can stabilize various coordination polyhedra. A complete series ranging from coordination numbers 2 to 6 was realized. Linear, trigonal planar, square planar, tetrahedral, square pyramidal, and octahedral coordination arrangements containing the PNac-ligand around the metal center were observed by using suitable metal sources. Hereby, PNac-H or its anion PNac- acts as mono-, bi- and tetradendate ligand. Such a broad flexibility is unusual for a rigid tetradentate system. The structural motifs were realized by treatment of PNac-H with a series of late transition metal precursors, for example, silver, gold, nickel, copper, platinum, and rhodium. The new complexes have been fully characterized by single crystal X-ray diffraction, NMR, IR, UV/Vis spectroscopy, mass spectrometry as well as elemental analysis. Additionally, selected complexes were investigated regarding their photophysical properties. Thus, PNac-H proved to be an ideal ligand platform for the selective coordination and stabilization of various metal ions in diverse polyhedra and oxidation states.

Heterobimetallic complexes of IrM (M = FeII, CoII, and NiII) core and bridging 2-(diphenylphosphino)pyridine: electronic structure and electrochemical behavior

Three complexes based on an Ir-M (M = Fe^II, Co^II, and Ni^II) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph₂PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph₂PPy)₂] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph₂PPy)₂FeCl₂]·2CH₂Cl₂ (2), [IrCl(CO)(μ-Ph₂PPy)₂CoCl₂]·2CH₂Cl₂ (3), and [Ir(CO)(μ-Cl)(μ-Ph₂PPy)₂NiCl₂]·2CH₂Cl₂ (4). Time-dependent DFT computations suggest a donor-acceptor interaction between a filled 5d_z² orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high-spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through an ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals.

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.

Heterobimetallic scandium–group 10 metal complexes with LM → Sc (LM = Ni, Pd, Pt) dative bonds

Heterobimetallic scandium–group 10 metal complexes featuring notable LM → Sc (LM = Ni, Pd, Pt) dative bonding interactions.

A dimolybdenum paddlewheel as a building block for heteromultimetallic structures

A series of heteromultimetallic complexes, containing a Mo₂⁴⁺core unit, were synthesized based on a bifunctional phosphine–carboxylic acid ligand system, leadingi.e.to the formation of supramolecular structuresviaaurophilic interactions.

Crystal structure of dicarbon-yl[μ2-methyl-enebis(di-phenyl-phosphane)-κ2 P:P'][μ2-2-(2,4,5-tri-methyl-phen-yl)-3-oxoprop-1-ene-1,3-di-yl](tri-phenyl-phosphane-κP)ironplatinum(Fe-Pt)-di-chloro-methane-toluene (1/1/2), [(OC)2Fe(μ-dppm)(μ-C(=O)C(2,4,5-C6H2Me3)=CH)Pt(PPh3)]

The title compound, [FePt(C12H12O)(C18H15P)(C25H22P2)(CO)2]·2C7H8·CH2Cl2 or [(OC)2Fe(μ-dppm)(μ-C(=O)C(2,4,5-C6H2Me3)=CH)Pt(PPh3)], represents an example of a diphosphane-bridged heterobimetallic dimetalla-cyclo-pentenone complex resulting from a bimetallic activation of 1-ethynyl-2,4,5-tri-methyl-benzene and a metal-coordinated carbonyl ligand. The bridging μ2-C(=O)C(2,4,5-C6H2Me3)=CH unit (stemming from a carbon-carbon coupling reaction between CO and the terminal alkyne) forms a five-membered dimetalla-cyclo-pentenone ring, in which the C=C bond is π-coordinated to the Fe centre. The latter is connected to the Pt centre through a short metal-metal bond of 2.5770 (5) Å. In the crystal, the complex is solvated by one di-chloro-methane and two toluene mol-ecules.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Titanium thiosalicylate complexes: functional metalloligands for the construction of redox-active heterometallic architectures

The titanium complex [TiCp*(thiosal)(thiosalH)] (1) has been synthesised by reaction of [TiCp*Me₃], Cp* = η⁵-C₅Me₅, with thiosalicylic acid (H₂thiosal). Complex 1 reacts with [M(μ-OH)(COD)]₂ (M = Rh, Ir) to yield the corresponding early-late heterobimetallic complexes [TiCp*(thiosal)₂M(COD)] (M = Rh (2); Ir (3)). Carbon monoxide replaces the COD ligand in 2 and 3 leading to the respective dicarbonyl complexes [TiCp*(thiosal)₂M(CO)₂] (M = Rh (4); Ir (5)). Compound 4 reacts with PPh₃ to yield the monocarbonyl derivative [TiCp*(thiosal)₂Rh(CO)(PPh₃)] (6). The reaction of compound 1 with LiⁿBu yields the tetrametallic complex [{TiCp*(thiosal)₂Li}₂(THF)₃(H₂O)] (7). Compound 7 reacts with [RuCp*Cl(COD)] yielding the heterometallic complex [TiCp*(thiosal)₂RuCp*] (8). The molecular structures of compounds 4, 5 and 7 have been studied by X-ray diffraction. From cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments, we observed that attachment of the titanium moiety of precursor 1 to a late transition metal moiety through the sulfur atoms has a significant influence on the reduction behaviour of the Ti(iv) metal centre. Thus, monometallic 1 exhibits an irreversible reduction process at -1.15 V vs. SCE, whereas the CVs of heterobimetallic compounds 2-6 are characterized by the reversible or quasi-reversible one-electron reduction of the Ti(iv)/Ti(iii) system, suggesting a significant stabilization of the Ti(iii) reduced species. Likewise, substitution of the M(COD) diolefin fragment in 2 and 3 by the M(CO)₂ carbonyl-containing moiety (in compounds 4 and 5) leads to a significant anodic shift in the titanium E_1/2 reduction redox potentials.

Playing with Pearson's concept: orthogonally functionalized 1,4-diaza-1,3-butadienes leading to heterobinuclear complexes

By reacting 1,2-diketones and ortho- diphenylphosphinoyl aniline in the presence of zinc(ii) as a templating agent, cationic zinc(ii) complexes of novel phosphine oxide functionalized 1,4-diaza-1,3-butadiene ligands are acessible. Herein the zinc(ii) site is bound to all four donor atoms of the ligand. Depending on the flexibility of the 1,4-diaza-1,3-butadiene backbone, the bonds to zinc(ii) from the 1,4-diaza-1,3-butadiene donors can be broken. Reaction with oxalate cleaves the zinc(ii) coordination completely and makes accessible the free ligands possessing orthogonal (N,N: soft; O,O: hard) sets of donor sites. This allows for the specific coordination of soft and hard Lewis acids and thus for the generation of heterobimetallic complexes, here exemplarily shown for the combination of palladium(ii) (soft) and zinc(ii) (hard) centres.

Crystal structure of homodinuclear platinum complex containing a metal-metal bond bridged by hydride and phosphide ligands

In the title compound, μ-di-phenyl-phosphido-μ-hydrido-bis-[bromido-(tri-phenyl-phosphane-κP)platinum(II)] diethyl ether monosolvate, [Pt2Br2(C12H10P)H(C18H15P)2]·C4H10O or [Pt2(μ-H)(μ-PPh2)Br2(PPh3)2]·(C2H5)2O, the PtII atoms are coordinated in a distorted square-planar arrangement, with one hydrido and one phosphido ligand bridging in a trans position. In the lattice, C-H⋯·O and C-H⋯π interactions are present. This complex has a total number of 32 electrons, 16 electrons for each PtII atom. One of the Br atoms is disordered over two positions in a 0.92:0.08 ratio.

Magneto-structural correlations in oxalate-bridged Sr(ii)Cr(iii) coordination polymers: structure, magnetization, X-band, and high-field ESR studies

Magneto-structural correlations in 1D oxalate-bridged Sr(ii)Cr(iii) coordination polymers with two crystallographically and magnetically non-equivalent Cr(iii) ions are studied by HF-ESR spectroscopy.

Tantallacyclopentadiene as a unique metal-containing diene ligand coordinated to nickel for preparing tantalum–nickel heterobimetallic complexes

The mononuclear tantallacyclopentadiene, TaCl₃(C₄H₂^tBu₂), coordinates to Ni to form heterobimetallic complexes of Cl₃Ta(μ-C₄H₂^tBu₂)Ni(L) (L = COD, phosphines, IPr).

Bimetallic rare-earth/platinum complexes ligated by phosphinoamides

The heterometallic early-late 5d/4f binuclear phosphinoamido Ln/Pt(0) complexes [(Ph₂PNHPh)Pt{μ-(Ph₂PNPh)}₃Ln(μ-Cl)Li(THF)₃] (Ln = Y, Lu) and [(Ph₂PNHPh)Pt{μ-(Ph₂PNPh)}₃Ln{η²-(Ph₂PNPh)}][Li(THF)₄] (Ln = Y, Lu) are reported.