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.

Back to the future of organolanthanide chemistry.

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp3 complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization.

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

Application of the Redox-Transmetalation Procedure to Access Divalent Lanthanide and Alkaline-Earth NHC Complexes*.

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands have been accessed by redox-transmetalation between air-stable NHC-AgI complexes and the corresponding metals. By using the small ligand 1,3-dimethylimidazol-2-ylidene (IMe), two series of isostructural complexes were obtained: the tetra-NHC complexes [LnI2 (IMe)4 ] (Ln=Eu and Sm) and the bis-NHC complexes [MI2 (IMe)2 (THF)2 ] (M=Yb, Ca and Sr). In the former, distortions in the NHC coordination were found to originate from intermolecular repulsions in the solid state. Application of the redox-transmetalation strategy with the bulkier 1,3-dimesitylimidazol-2-ylidene (IMes) ligand yielded [SrI2 (IMes)(THF)3 ], while using a similar procedure with Ca metal led to [CaI2 (THF)4 ] and uncoordinated IMes. DFT calculations were performed to rationalise the selective formation of the bis-NHC adduct in [SrI2 (IMe)2 (THF)2 ] and the tetra-NHC adduct in [SmI2 (IMe)4 ]. Since the results in the gas phase point towards preferential formation of the tetra-NHC complexes for both metal centres, the differences between both arrangements are a result of solid-state effects such as slightly different packing forces.

Influence of a Lanthanide Ion on the Ni Site of a Heterobimetallic 3d-4f Mabiq Complex.

This work presents the synthesis and characterization of a 3d-4f bimetallic complex based on the redox-active macrocyclic biquinazoline ligand, Mabiq. The mixed Yb-Ni complex, [(Cp*)2Yb(Mabiq)Ni]BArF (3), was synthesized upon reaction of [NiII(Mabiq)]BArF (2) with (Cp*)2YbII(OEt2). The molecular structures of 3 and its sister complex, [(Cp*)2Yb(Mabiq)Ni][(Cp*)2Yb(OTf)2] (1), confirmed the presence of a Yb(III) center and a reduced Ni-Mabiq unit. Spectroscopy (absorption and NMR), cyclic voltammetry, and magnetic susceptibility studies were employed to analyze the electronic structure of 3, which is best described by the [(Cp*)2YbIII(Mabiq•)NiII]+ formulation. Notably, the ligand-centered radical is delocalized over both the diketiminate and bipyrimidine units of the Mabiq ligand. The magnetic susceptibility and variable temperature NMR studies for 3 denote coupling between the Ni-Mabiq site and the peripheral Yb center-previously unobserved in 3d-3d Mabiq complexes. The complex nature of the exchange interactions is highlighted by the multiconfigurational ground state for 3, comprising nearly degenerate singlet and triplet states.

Intermediate Valence States in Lanthanide Compounds.

Over more than 50 years, intermediate valence states in lanthanide compounds have often resulted in unexpected or puzzling spectroscopic and magnetic properties. Such experimental singularities could not be rationalised until new theoretical models involving multiconfigurational electronic ground states were established. In this minireview, the different singularities that have been observed among lanthanide complexes are highlighted, the models used to rationalise them are detailed and how such electronic effects may be adjusted depending on energy and symmetry considerations is considered. Understanding and tuning the ground-state multiconfigurational behaviour in lanthanide complexes may open new doors to modular and unusual reactivities.

Bis-cyclooctatetraenyl Thulium(II): Highly Reducing Lanthanide Sandwich Single Molecule Magnets.

Divalent lanthanide organometallics are well-known highly reducing compounds usually used for single electron transfer reactivity and small molecule activation. Thus, their very reactive nature prevented for many years the study of their physical properties, such as magnetic studies on a reliable basis. In this article, the access to rare organometallic sandwich compounds of TmII with the cyclooctatetraenyl (Cot) ligand impacts on the use of divalent organolanthanide compounds as an additional strategy for the design of performing Single Molecule Magnets (SMM). Herein, the first divalent thulium sandwich complex with f13 configuration behaving as a Single Molecule Magnet in absence of DC field is highlighted.

Synthesis and Reactivity of Bis(silylene)-Coordinated Calcium and Divalent Lanthanide Complexes.

Divalent lanthanide complexes of Eu (1) and Yb (2) coordinated by a chelating pyridine-based bis(silylene) ligand were isolated and fully characterized. Compared to the EuII complex 1, the YbII complex 2 presents a lower thermal stability, resulting in the activation of one SiII -N bond and formation of an YbIII complex (3), which features a unique silylene-pyridyl-amido ligand. The different thermal stability of 1 and 2 points towards reduction-induced cleavage of one SiII -N bond of the bis(silylene) ligand. Successful isolation of the corresponding redox-inert bis(silylene) CaII complex (5) was achieved at low temperature and thermal decomposition into a CaII complex (4) bearing the same silylene-pyridyl-amido ligand was identified. In this case, the thermolysis reaction proceeds through another, non-redox induced, mechanism. An alternative higher yielding route to 4 was developed through an in situ generated silylene-pyridyl-amine proligand.

A bis(germylene) functionalized metal-coordinated polyphosphide and its isomerization.

Pentaphosphaferrocene, [Cp*Fe(η5-P5)] (Cp* = η5-C5Me5), was used as a polyphosphorus source to obtain germylene-polyphosphide complexes. A stepwise reactivity was observed between the di-germylene, [LGe-GeL] (L = {PhC(NtBu)2}), and [Cp*Fe(η5-P5)]. Firstly, reductive homolytic cleavage of the Ge-Ge single bond in [LGe-GeL] led to [(LGe)2{(µ,η4-P5)FeCp*}]. This complex showed an unprecedented isomerization.

3d-4f heterometallic complexes by the reduction of transition metal carbonyls with bulky LnII amidinates.

The redox chemistry between divalent lanthanide complexes bearing bulky amidinate ligands has been studied with 3d transition metal carbonyl complexes (iron and cobalt). The reaction of [(DippForm)2SmII(thf)2] (DippForm = N,N'-bis(2,6-diisopropylphenyl)formamidinate) with [Co2(CO)8] resulted in the formation of a tetranuclear Sm-Co complex, [{(DippForm)2SmIII(thf)}2{(µ-CO)2Co(CO)2}2]. The product of the reaction of [(DippForm)2YbII(thf)2] and [Co2(CO)8] gives the dinuclear Yb-Co complex [{(DippForm)2YbIII(thf)}{(µ-CO)Co(CO)3}] in toluene. The reaction of [(DippForm)2SmII(thf)2] was also carried with the neighbouring group 8 carbonyl complexes [Fe2(CO)9] and [Fe3(CO)12], resulting in a pentanuclear SmIII-Fe complex, [{(DippForm)2SmIII}2{(µ3-CO)2Fe3(CO)9}], featuring a triangular iron carbonyl cluster core.

Regioselective Insertion of Aluminum(I) in the cyclo-P5 Ring of Pentaphosphaferrocene.

A route to directly access mixed Al-Fe polyphosphide complexes was developed. The reactivity of pentaphosphaferrocene, [Cp*Fe(η5 -P5 )] (Cp*=C5 Me5 ), with two different low-valent aluminum compounds was investigated. The steric and electronic environment around the [AlI ] centre are found to be crucial for the formation of the resulting Al-Fe polyphosphides. Reaction with the sterically demanding [Dipp-BDIAlI ] (Dipp-BDI={[2,6-i Pr2 C6 H3 NCMe]2 CH}- ) resulted in the first Al-based neutral triple-decker type polyphosphide complex. For [(Cp*AlI )4 ], an unprecedented regioselective insertion of three [Cp*AlIII ]2+ moieties into two adjacent P-P bonds of the cyclo-P5 ring of [Cp*Fe(η5 -P5 )] was observed. The regioselectivity of the insertion reaction could be rationalized by isolating an analogue of the reaction intermediate stabilized by a strong σ-donor carbene.

Highly Selective Substitution and Insertion Reactions of Silylenes in a Metal-Coordinated Polyphosphide.

Selective substitution and insertion reactions of silylenes into the cyclo-P5 ring of [Cp*Fe(η5-P5)] are reported. The selective substitution of one P atom by an isoelectronic [LSi] fragment (L = PhC(NtBu)2) leads to [(η4-P4SiL)FeCp*] and [LSi(Cl)═P-SiL(Cl)2]. To elucidate the reaction mechanism, [{LSi(N(SiMe3)2)}{(η4-P5)FeCp*}], in which the silicon atom binds to the cyclo-P5 ring, was synthesized as a model compound for the reaction intermediate. The insertion of [LSi-SiL] into the cyclo-P5 ring of [Cp*Fe(η5-P5)] resulted in [{η4-P5(SiL)2}FeCp*] featuring a cyclo-P5(SiL)2 ring, which corresponds to the largest silicon-polyphosphorus ring known in a complex.

A Stable Aromatic Tetrasilacyclobutadiene Dication.

The isolation and characterization of the first silicon analogue of the aromatic cyclobutadiene dication is reported. The aromatic 2π-electron tetrasilacyclobutadiene dication in [(LPhSi)4](BPh4)2 (LPh = PhC(NtBu)2) has been obtained in a simple and straightforward synthesis, from [LPhSiCl], [LPhSiSiLPh] and NaBPh4, and fully characterized. The molecular structure reveals a four-membered Si4-dication in an almost perfect square-planar geometry. Theoretical calculations support the interpretation of the title compound as a classical 2π-aromatic species with a singlet ground state.

Reaction of an arsinoamide with chloro tetrylenes: substitution and As-N bond insertion.

Reaction of the arsinoamide [(Mes2AsNPh){Li(OEt2)2}] with the low-valent group 14 compounds, [{PhC(tBuN)2}ECl] (E = Si, Ge) and GeCl2·dioxane, resulted in two different reaction pathways: simple substitution or substitution accompanied by an insertion step. As a result, either insertion products with an As-Si[double bond, length as m-dash]N unit and an As-Ge bond, or substitution products, in which the intact arsinoamide binds to the group 14 elements via the N atom, were obtained.

Rhenium is different: CO tetramerization induced by a divalent lanthanide complex in rhenium carbonyls.

The reduction of M2(CO)10 (M = Mn, Re) with different divalent lanthanide (Ln = Sm, Yb) compounds was investigated. Depending on the steric demand of the ligand, either unusual CO tetramerization or formation of a new Re carbonyl anion occurred in the case of Re. Theoretical calculations were performed for a better understanding of the nature of bonding in the newly formed species.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt.

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Access to divalent lanthanide NHC complexes by redox-transmetallation from silver and CO2 insertion reactions.

Through a redox-transmetallation procedure, divalent NHC-LnII (NHC = N-heterocyclic carbene; Ln = Eu, Yb) complexes were obtained from the corresponding NHC-AgI. The lability of the NHC-LnII bond was investigated and treatment with CO2 led to insertion reactions without oxidation of the metal centre. The EuII complex [EuI2(IMes)(THF)3] (IMes = 1,3-dimesitylimidazol-2-ylidene) exhibits photoluminescence with a quantum yield reaching 53%.

Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si-H bond activation via metal-ligand cooperation.

The double aminolysis reaction of [Co{N(SiMe3)2}2] by the salt 1-(6-((dicyclohexylphosphaneyl)methyl)pyridin-2-yl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide, which contains one phosphane, one pyridine and one imidazolium groups, of formula [o-Cy2PCH2(C5H3N)(o-C3H3N2DiPP)]Br and abbreviated as (CyPNpyrCim)Br, was previously shown to afford the Co(ii) complex [Co(CyP*NaCNHC)Br] (1) containing a dearomatised picolyl moiety in the tridentate, anionic donor ligand CyP*NaCNHC (Na = anionic amido, P* = vinylic P donor). We now report that formation of 1 is preceded by an intermediate tentatively assignable to the 5-coordinate Co(ii) complex [Co(CyPNpyrCNHC){N(SiMe3)2}Br] (2). The reaction of 1 with LiCH2SiMe3 afforded the dark purple, paramagnetic [Co(CyP*NaCNHC)CH2SiMe3] (3) with a low spin d7 CoII; the electronic configurations of 1 and 3 were corroborated by EPR spectroscopy. Addition of excess (≥4 equiv.) H2SiPh2 to a solution of 1 gave the diamagnetic [Co(CyP(SiHPh2)NCNHC)Br] (4) following Si-H activation and silylation of the ligand backbone at the α-CHP. Reduction of CoII to CoI by silanes is uncommon.

Non-symmetrical, potentially redox non-innocent imino NHC pyridine 'pincers' via a zinc ion template-assisted synthesis.

New non-symmetrical, redox-active imino NHC pyridine pincer ligands, 2-(R1-imidazol-2-ylidene)-6-(R2N[double bond, length as m-dash]CH)-pyridine (R1 = 2,6-diisopropylphenyl (DiPP), R2 = 2,4,6-trimethylphenyl (Mes), 4B; R1 = R2 = DiPP, 4C), have been accessed by a ZnII-promoted modular synthesis involving the quaternization of R1-imidazole by [Zn(κNimineκNpyridine)(2-(R2N[double bond, length as m-dash]CH)-6-bromo-pyridine)Cl2], followed by ZnII removal and deprotonation of imidazolium pro-ligands. Redox active forms of 4B were implicated in the two complexes obtained by its reaction with FeBr2/KC8; metrical data analysis pointed to the occurrence of radical anionic and dianionic redox states of 4B.

Coinage metal complexes with bridging hybrid phosphine-NHC ligands: synthesis of di- and tetra-nuclear complexes.

A series of P-NHC-type hybrid ligands containing both PR2 and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L(Cy), L(tBu) and L(Ph) (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L(Ph)·n(HBr) (n = 1, 2) with Ag2O gave the pseudo-cubane [Ag4Br4(L(Ph))2], isostructural to [Ag4Br4(L(R))2] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of ·HBF4 (R = Ph, tBu) led to the dinuclear complexes [Ag2(L(R))2](BF4)2 which, in the solid state, feature heteroleptic Ag centres and a 'head-to-tail' (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic 'head-to-head' (HH) isomers is facilitated by ligand fluxionality. 'Head-to-tail' [Cu2Br2(L(R))2] (R = Cy, tBu) dinuclear complexes were obtained from L(R)·HBr and [Cu5(Mes)5], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L(R)l ed to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag4Br4(L(tBu))2] to AuI by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L(tBu) transfer and to the dinuclear complex [Au2Cl2L(tBu)] with one L(tBu) ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.