Advances in the Coordination Chemistry of Amidinate and Guanidinate Ligands

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Boron Complexes with Propiolamidinato Ligands: Synthesis, Structure, and Photophysical Properties

Two series of boron derivatives with propiolamidinato ligands, [BPh2{C(C≡CAr)(NR)2}] (Ar = Ph, p-MeOPh, p-FPh, p-Me2NPh, or phen; R = iPr or p-tolyl), were synthesized and structurally characterized. The corresponding propiolamidine (or propargylamidine) proligands have been obtained through sustainable methods. One is the catalytic hydroalkynylation of diisopropylcarbodiimide with different terminal alkynes, using simple ZnEt2 as a precatalyst. Alternatively, to obtain propiolamidines with aromatic groups on the nitrogen atoms, the formation of lithiated derivatives of terminal alkynes by reaction with n-BuLi in air and at room temperature, and subsequent addition to the di-p-tolylcarbodiimide, under the same conditions and using 2-MeTHF as a sustainable solvent, has been used for the first time. After reaction with BPh3, the corresponding boron amidinates were obtained, which are emissive in the solution state. The influence of the different substituents introduced into the ligands on the photophysical properties of the boron compounds has been studied. One of the obtained compounds can be used as a ratiometric fluorescent pH sensor in the acidic range.

Towards understanding the first half-ALD cycle of Ag growth: adsorption and dissociation of silver(I) acetamidinates on the Ag(110) surface

The advancement of atomic layer deposition (ALD) techniques for the controlled growth of transition metal thin films is constantly growing due to the design and synthesis of novel organometallic (OM) precursors capable of facilitating precise deposition and clean film growth. In this context, acetamidinates have emerged as a highly promising family of OM precursors due to their exceptional attributes, including outstanding stability, favorable volatility, and reactivity at low evaporation and deposition temperatures. These unique properties make them a sought-after candidate for enabling ALD processes. Here we conducted an atomic-scale study to get an in-depth understanding of the first ALD partial reaction, which involves the adsorption and dissociation process of the silver acetamidinate on the Ag(110) surface. Our research sheds light on the multistep adsorption and breaking mechanism of the novel silver(I)-N,N'-dimethylacetamidinate precursor employed as the silver source. Since the difference in energy between the monomer and dimer phases of the precursor is only 1.92 eV, we have explored the adsorption states of both phases. The monomer adsorbs on the surface by occupying hollow (H) sites; after that, it dissociates and loses its ligand, adopting a perpendicular geometry via the formation of new Ag-N bonds with the pair of N atoms at the top sites of the surface. On the other hand, the dimer adsorbs on long-bridge sites (LB) with the pair of N atoms occupying top sites with the silver atoms from the surface. Next, the dimer loses a pair of N-Ag bonds on each ligand, reaching a more stable state of partial cleavage with a relative energy of -0.38 eV. After overcoming an energy barrier of 0.41 eV, the dimer loses the remaining pair of N-Ag bonds, and the silver atoms diffuse towards H sites. Finally, the ligands diffuse toward the adjacent channel in the [100] direction of the surface. A charge distribution analysis of the adsorption stages shows the evolution of the silver atoms from precursor to the metallic state.

Synthesis and Structural Characterization of p-Carboranylamidine Derivatives

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = ⁱPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C₂H₁₀B₁₀[C(NⁱPr)₂Li(DME)]₂ (2) and p-C₂H₁₀B₁₀[C(NCy)₂Li(THF)₂]₂ (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me₃SiCl) provided the silylated neutral bis(amidine) derivatives p-C₂H₁₀B₁₀[C{ⁱPrN(SiMe₃)}(=NⁱPr)]₂ (4) and p-C₂H₁₀B₁₀[C{CyN(SiMe₃)}(=NCy)]₂ (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot procedure to quickly produce multigram amounts of Hauser bases R2 N-MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R2 N-MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR2 )2 and MgBr2 depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X-ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Semistabilized Diazatrienyl Anions from Pyridine Imines and Acetylenes: An Access to (Z)-Stilbene/Imidazopyridine Ensembles, Benzyl Imidazopyridines, and Beyond

Semistabilized diazatrienyl anions are generated by the reaction of 2-pyridylarylimines with arylacetylenes in superbase systems MO^tBu (M = Li, Na, K)/DMSO at ambient temperature for 15 min. The initial intermediate N-centered propargyl-1,3-diaza-1,3,5-trienyl anions undergo intermolecular cyclization to benzyl imidazopyridine anions (formally [3 + 2] cycloaddition), further intercepting a second molecule of the starting pyridylimines or a proton of medium to afford (Z)-stilbene/imidazopyridine ensembles and benzyl imidazopyridines. The charge distribution in all intermediate anions and their synthetic evolution are consistent with quantum-chemical analysis (B2PLYPD/6-311+G**//B3LYP/6-31+G*).

Sc and Y bis(alkyl) complexes supported by bidentate and tridentate amidinate ligands. Synthesis, structure and catalytic activity in polymerization of isoprene and 1-heptene

A series of bis(alkyl) complexes {(tBu)C[N(2,6-Me₂C₆H₃)]₂}Ln(CH₂SiMe₃)₂(THF)_n (Ln = Y, n = 1 (1); Ln = Sc, n = 1 (2)), {2-[Ph₂P(O)]C₆H₄NC(tBu)N(2,6-Me₂C₆H₃)}Sc(CH₂SiMe₃)₂ (3), {2-[Ph₂P(NPh)]C₆H₄NC(tBu)N(2,6-Me₂C₆H₃)}Sc(CH₂SiMe₃)₂ (4) coordinated by bidentate (N,N) and tridentate (N,N,O; N,N,N) amidinate ligands are synthesized using an alkane elimination approach. Yttrium complex 1 demonstrated a half-life of ∼2.5 days at room temperature in benzene-D6 (C₆D₆) solution, whereas scandium complexes proved to be much more stable (25 d (2), 30 d (3) and 42 d (4)). Complexes 1-4 as a part of ternary catalytic systems 1-4/TB, HNB/AlR₃ (AlR₃ = AliBu₃, AliBu₂H; TB = [Ph₃C][B(C₆F₅)₄], HNB = [PhNHMe₂][B(C₆F₅)₄]) demonstrated high catalytic activity in isoprene polymerization and enable 80%-100% conversion of 1000 equivalents of monomer into polymer at 25 °C within 3-180 min. The isolated polyisoprenes feature predominantly cis-1,4-regularity (69.2%-87.3%) and polydispersities M_w/M_n = 2.26-8.92. Moreover, the binary (2/TB) and ternary (1-4/TB/10 AliBu₃) systems initiate 1-heptene polymerization providing 40%-100% conversion of 500 equivalents of monomer in 24 h at 25 °C giving polymer samples with M_n = 1.55-190.2 × 10³ and M_w/M_n = 1.55-3.87.

Reversible addition of tin(II) amides to nitriles

Lappert's stannylene (Sn[N(SiMe₃)₂]₂) has been reacted with various nitriles, dinitriles and trinitriles with the formation of heteroleptic amidotin(II) amidinates of the general formulae [RC(NSiMe₃)₂]SnN(SiMe₃)₂, R'{[C(NSiMe₃)₂]SnN(SiMe₃)₂}₂ and R''{[C(NSiMe₃)₂]SnN(SiMe₃)₂}₃, where R = Ph (1), 2-(CN)-C₆H₄ (2), 3-(CN)-C₆H₄ (3); R' = 1,3-C₆H₄ (4), 1,4-C₆H₄ (5) and R'' = 1,3,5-C₆H₃ (6). The reactions of amidotin(II) benzamidinate 1 with an excess of benzonitrile proceed to homoleptic tin(II) bis(benzamidinate) [PhC(NSiMe₃)₂]₂Sn, which reversibly eliminates benzonitrile and 1 when warmed. The premise of reversibility has been supported by a multinuclear time-dependent NMR study and a theoretical (DFT) description. On the other hand, magnesium(II) bis(benzamidinate) [PhC(NSiMe₃)₂]₂Mg (8) and lanthanum(II) tris(benzamidinate) [PhC(NSiMe₃)₂]₃La (7) have been synthesised from appropriate metal hexamethyldisilazides and benzonitrile.

Bis(4-benzhydryl-benzoxazol-2-yl)methane - from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (4-BzhH2 BoxCH2 ) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(4-BzhH2 BoxCH)] (M=Na (7), K (81 )) and dimeric [{M(4-BzhH2 BoxCH)}2 ] (M=K (82 ), Rb (9), Cs (10)) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)2 K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11), including the energetically favoured monoanionic (E,E)-(4-BzhH2 BoxCH) ligand. Further reaction of 4-BzhH2 BoxCH2 with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)2 K@(18-crown-6)}{K@(18-crown-6)K(4-Bzh BoxCH)}]n (n→∞) (12) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.

Synthesis and crystal structure analyses of tri-substituted guanidine-based copper(II) complexes

Three guanidine-derived tri-substituted ligands viz. N-pivaloyl-N′,N″-bis-(2-methoxyphenyl)guanidine (L1), N-pivaloyl-N′-(2-methoxyphenyl)-N″-phenylguanidine (L2) and N-pivaloyl-N′-(2-methoxyphenyl)-N″-(2-tolyl)guanidine (L3) were reacted with Cu(II) acetate to produce the corresponding complexes. The significance of the substituent on N″ for the resulting molecular structures and their packing in the solid state has been studied with respect to the structural specifics of the corresponding Cu(II) complexes. The key characteristic of the guanidine-based metal complexation with Cu(II) is the formation of an essentially square planar core with an N2O2 donor set. As an exception, in the complex of L1, the substituent’s methoxy moiety also interacts with the Cu(II) center to generate a square-pyramidal geometry. The hydroxyl groups of the imidic acid tautomeric forms of L1–L3, in addition to N″, are also bonded to Cu(II) in all three complexes rather than the nitrogen donor of the guanidine motif.

N-Heterocyclic carbene-carbodiimide (NHC-CDI) betaine adducts: synthesis, characterization, properties, and applications

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC-CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

Progress in propylene homo- and copolymers using advanced transition metal catalyst systems

Recent progress on advanced transition metal catalysts for propylene polymerization and copolymerization are reviewed.

Synthesis, structure, and catalytic activity of dinuclear aluminium bis(amidinate) and bis(guanidinate) complexes

The ring-opening polymerization grand prix: Bis(amidinate)s outpaced their bis(guanidinate) competitors and won the race, while the drivers’ engines using acidic co-initiators collapsed before take-off.

Half-sandwich complexes of osmium containing guanidine-derived ligands

Pyridinyl- and phosphano-guanidino complexes of formula [(η6-p-cymene)OsCl(H2L)][SbF6] (cymene = MeC6H4iPr; H2L = N,N'-bis(p-Tolyl)-N''-(2-pyridinylmethyl)guanidine, H2L1 (1) and N,N'-bis(p-Tolyl)-N''-(2-diphenylphosphanoethyl)guanidine, H2L2 (2)) have been prepared from the dimer [{(η6-p-cymene)OsCl}2(μ-Cl)2] and H2L in the presence of NaSbF6. Treatment of complex 2 with HCl renders the phosphano-guanidinium complex [(η6-p-cymene)OsCl2(H3L2)][SbF6] (3). Compounds 1 and 2 react with AgSbF6 rendering the cationic aqua complexes [(η6-p-cymene)Os(H2L)(OH2)][SbF6]2 (H2L = H2L1 (4), H2L2 (5)). Addition of monodentate ligands L to compound 4 affords complexes of formula [(η6-p-cymene)Os(H2L1)L][SbF6]2 (L = py (6), 4-(NHMe)py (7), CO (8), P(OMe)3 (9)). Treatment of complexes 4 and 5 with NaHCO3 renders the monocationic complexes [(η6-p-cymene)Os(κ3N,N',N''-HL1)][SbF6] (10) and [(η6-p-cymene)Os(κ3N,N',P-HL2)][SbF6] (11), respectively, in which the HL ligand adopts a fac-κ3 coordination mode. The new complexes have been characterised by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1-4, 6, 8, and 11, by X-ray diffractometric methods. The phosphano-guanidino complexes 2 and 5 exhibit a temperature dependent fluxional process in solution. The new 18 electron complexes 1, 2, 6, and 8-10 are active catalysts for the Friedel-Crafts reaction between trans-β-nitrostyrene and N-methyl-2-methylindole. Conversions greater than 90% were obtained. Proton NMR studies support a mechanism involving the Brønsted-acid activation of trans-β-nitrostyrene through the NH functionalities of the coordinated guanidine ligands.

Magnesium bis(amidinate) and bis(guanidinate) complexes: impact of the ligand backbone and bridging groups on the coordination behaviour

A library of ten dinucleating bis(amidine) and bis(guanidine) ligands, in which the bridging groups, terminal rests, and backbone substituents were systematically altered, has been synthesized and subsequently applied in metallation reactions using three different magnesium sources. Eight Mg complexes could be isolated and fully characterized, and in seven cases their solid-state structure could be determined by means of single crystal X-ray diffraction analysis. The results evidence a versatile coordination behaviour, which ranges from the first dinuclear heteroleptic magnesium iodide complex to dinuclear homoleptic complexes. These findings indicate the crucial impact of both the ligand and the magnesium source not only on the accessibility of well-defined dinuclear complexes but also on the aggregation in solution and in the solid state.

Lithium and Dilithium Guanidinates, a Starter Kit for Metal Complexes Containing Various Mono- and Dianionic Ligands

Comparative studies of the synthesis of lithium guanidinates via nucleophilic addition of lithium amides to carbodiimides were performed. Four combinations of small or sterically crowded carbodiimide and sterically crowded lithium amide or lithium amide containing an adjacent amino donor group give ten different types of complexes. In particular, 2,6-[(CH₃)₂CH]₂C₆H₃NHLi (DipNHLi, 1) reacts with (CH₃)₂CHN═C═NCH(CH₃)₂ upon the formation of the dissymmetric dimeric complex 2 with four-coordinate Li atoms. In contrast, 1 with DipN═C═NDip gives the mononuclear lithium guanidinate 3 with two-coordinate lithium by κ¹-guanidinate, solvent molecule, and additional interaction with a π-electron cloud of one of the Dip groups. Analogous reactions of 2-[(CH₃)₂NCH₂]C₆H₄NHLi (7) yield complexes 8 and 9, where the adjacent amino donors are always coordinated. Further deprotonation of 2, 3, 8, and 9 leads to dilithium guanidinates(2-)-4, 5, 10, and 11, among which only 5, containing three Dip groups, is monomeric with contacts to two π-electron systems of Dip groups. The rest of the complexes are tetranuclear with different structural patterns. In the central parts of molecules, toward which the nitrogen atoms of the guanidinates are oriented, lithium atoms are usually pseudotetrahedral, but trigonal in peripheral parts. Adjacent solvent molecules, chelating amino groups, and π-electron systems of Dip groups are coordinated in order to complete coordination polyhedra. Complexes 4 and 5 deoligomerize in solution upon the formation of fluxional monomeric dilithium species. Conversely, 11 is a dimer in solution due to the strong donation of an amino group. The silylated lithium amide {2-[(CH₃)₂NCH₂]C₆H₄}[(Si(CH₃)₃]NLi (12) reacts with both carbodiimides to give dinuclear 13 obtained from diisopropylcarbodiimide and monomeric 14 from the second carbodiimide. Complexes 13 and 14 structurally resemble 8 and 9, with the highest degree of the localization of π-electron density within the N₃C guanidinate system, η³-contact to the Dip ring, and a lack of the solvent molecule in 14.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization

A series of Rh(κ² -BHetA)(η² -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(μ-Cl)(IPr)(η² -coe)]₂ with the corresponding anionic BHetA species. The Rh^I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h^-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ¹ -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh^III H(C≡CPh)₂ (IPr)(py)₂ intermediate gives support for an alternative oxidative route.

Carbodiimide-based synthesis of N-heterocycles: moving from two classical reactive sites to chemical bond breaking/forming reaction

Carbodiimides are a unique class of heterocumulene compounds that display distinctive chemical properties. The rich chemistry of carbodiimides has drawn increasing attention from chemists in recent years and has made them exceedingly useful compounds in modern organic chemistry, especially in the synthesis of N-heterocycles. This review has outlined the extensive application of carbodiimides in the synthesis of N-heterocycles from the 1980s to today. A wide range of reactions for the synthesis of various types of N-heterocyclic systems (three-, four-, five-, six-, seven-, larger-membered and fused heterocycles) have been developed on the basis of carbodiimides and their derivatives.

Rare-earth complexes supported by an ansa-bis(amidinate) ligand with a rigid o-phenylene linker: synthesis, structure, and catalytic activity for polymerization of cyclic esters

A bis(amidinate) ligand with a rigid o-phenylene linker C₆H₄-1,2-[NC(^tBu)NH(C₆H₅)]₂ (1) was successfully employed for the syntheses of rare-earth amides 2-5 in good yields. The structures of the amides were established by X-ray diffraction analysis and revealed that the ligand framework coordinated to the lanthanide atoms in different fashions depending on the central ion size. Larger lanthanide metal ions La³⁺ and Nd³⁺ prefer to form dinuclear complexes with one linked bis(amidinate) ligand as a bridge, while the smaller sized Y³⁺ and Er³⁺ favor the formation of mononuclear complexes. These complexes exhibited good catalytic activity for ring-opening polymerization of ε-caprolactone and l-lactide and the mononuclear complexes display higher activity.

The chemistry of guanidinate complexes of the platinum group metals

In this Perspective article, recent advances in the chemistry of platinum group metal complexes containing mono- and dianionic guanidinate ligands, i.e. [(RN)2C-NR2]- and [(RN)2C[double bond, length as m-dash]NR]2-, respectively, are presented. Synthetic and structural aspects, reactivity studies, and applications of these compounds are discussed.