Synthesis, Characterization and Reactivity of New Dinuclear Guanidinate Diimidoniobium Complexes

Guanidinates as Alternative Ligands for Organometallic Complexes

For decades, ligands such as phosphanes or cyclopentadienyl ring derivatives have dominated Coordination and Organometallic Chemistry. At the same time, alternative compounds have emerged that could compete either for a more practical and accessible synthesis or for greater control of steric and electronic properties. Guanidines, nitrogen-rich compounds, appear as one such potential alternatives as ligands or proligands. In addition to occurring in a plethora of natural compounds, and thus in compounds of pharmacological use, guanidines allow a wide variety of coordination modes to different metal centers along the periodic table, with their monoanionic chelate derivatives being the most common. In this review, we focused on the organometallic chemistry of guanidinato compounds, discussing selected examples of coordination modes, reactivity and uses in catalysis or materials science. We believe that these amazing ligands offer a new promise in Organometallic Chemistry.

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.

Aromatic guanidines as highly active binary catalytic systems for the fixation of CO2 into cyclic carbonates under mild conditions

The formation of hydrogen bonding causes a considerable decrease in the reaction temperature and CO₂ pressure used in this process.

Synthesis of (Arylmido)niobium(V) Complexes Containing Ketimide, Phenoxide Ligands, and Some Reactions with Phenols and Alcohols

Reactions of Nb(NAr)(N=C t Bu2)3 (3a, Ar = 2,6-Me2C6H3) with 1.0, 2.0, or 3.0 equiv of Ar'OH (Ar' = 2,6- i Pr2C6H3) afforded Nb(NAr)(N=C t Bu2)2(OAr'), Nb(NAr)(N=C t Bu2)(OAr')2, or Nb(NAr)(OAr')3, respectively (at 25 °C), whereas the reaction with 2.0 equiv of 2,6- t Bu2C6H3OH afforded Nb(NAr)(N=C t Bu2)2(O-2,6- t Bu2C6H3) upon heating (70 °C) without the formation of bis(phenoxide) and the reaction of 3a with 2.0 equiv of 2,4,6-Me3C6H2OH afforded Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2). Similar reactions of 3a with 1.0 equiv of (CF3)3COH or 2.0 equiv of (CF3)2CHOH afforded Nb(NAr)(N=C t Bu2)2[OC(CF3)3](HN=C t Bu2) or Nb(NAr)(N=C t Bu2)[OCH(CF3)2]2(HN=C t Bu2), respectively. On the basis of their structural analyses and the reaction chemistry, it was suggested that these reactions proceeded via coordination of phenol (alcohol) to Nb and the subsequent proton (hydrogen) transfer to the ketimide (N=C t Bu2) ligand. The reaction of Nb(NAr)(N=C t Bu2)2(OAr') with 1.0 equiv of 2,4,6-Me3C6H2OH gave the disproportionation products Nb(NAr)(N=C t Bu2)(OAr')2 and Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2) with 1:1 ratio, clearly indicating the presence of the above mechanism and the fast equilibrium (between the ketimide and the phenoxide). The reaction of 3a with 1.0 or 2.0 equiv of C6F5OH afforded Nb(N=C t Bu2)2(OC6F5)3(HN=C t Bu2) as the sole isolated product, which was formed from once generated Nb(NAr)(N=C t Bu2)2(OC6F5)(HN=C t Bu2) by treating with C6F5OH.

Insertion reactions of small unsaturated molecules in the N-B bonds of boron guanidinates

We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N-B bonds of two boron guanidinates, (Me₂N)C(NⁱPr)₂BCy₂ (1) and {ⁱPr(H)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (2), and two bisboron guanidinates(2-), {ⁱPr(BCy₂)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (3) and {ⁱPr(C₈H₁₄B)N}C(NⁱPr){N(p-Me-C₆H₄)}BC₈H₁₄ (4), the latter being prepared for the first time by double deprotonation of the corresponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC₈H₁₄)₂. Compounds 1-4 easily insert aromatic isonitriles, XylNC (Xyl = 2,6-Me₂-C₆H₃) and (p-MeO-C₆H₄)NC, to give the expected diazaboroles 5-12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC₈H₁₄ derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products, whose thermodynamic parameters are reported here. A correlation between these equilibria and the puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded, and only one product, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO)BCy₂ (13), could be isolated in moderate yields. The 1,2-insertions of benzaldehyde in compounds 1, 2 and 4 are reversible reactions in all cases, and only one of the insertion products, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(PhHCO)BCy₂ (16a), was isolated and diffractrometrically characterised. Likewise, CO₂ reversibly inserts into a N-B bond of 2 to give {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO₂)BCy₂ (19) with a conversion of ca. 9%. In all these equilibria, de-insertion is always favoured upon increasing the temperature.

Structural and Mechanistic Insights into s-Block Bimetallic Catalysis: Sodium Magnesiate-Catalyzed Guanylation of Amines

To advance the catalytic applications of s-block mixed-metal complexes, sodium magnesiate [NaMg(CH₂ SiMe₃ )₃ ] (1) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed-metal system is much greater than that of its homometallic components [NaCH₂ SiMe₃ ] and [Mg(CH₂ SiMe₃ )₂ ]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N'-diisopropylcarbodiimide with 4-tert-butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine-assisted rate-determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.

Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines

Tris(pentafluorophenyl)borane, [B(C6F5)3], has been used as an efficient catalyst in the guanylation reaction of amines with carbodiimide under mild conditions. A combined approach involving NMR spectroscopy and DFT calculations was employed to gain a better insight into the mechanistic features of this process. The results allowed us to propose a new Lewis acid-assisted Brønsted acidic pathway for the guanylation reaction. The process starts with the interaction of tris(pentafluorphenyl)borane and the amine to form the corresponding adduct, [(C6F5)3B-NRH2] , followed by a straightforward proton transfer to one of the nitrogen atoms of the carbodiimide, (i)PrN[double bond, length as m-dash]C[double bond, length as m-dash]N(i)Pr, to produce, in two consequent steps, a guanidine-borane adduct, [(C6F5)3B-NRC(N(i)PrH)2] . The rupture of this adduct liberates the guanidine product RNC(N(i)PrH)2 and interaction with additional amine restarts the catalytic cycle. DFT studies have been carried out in order to study the thermodynamic characteristics of the proposed pathway. Significant borane adducts with amines and guanidines have been isolated and characterized by multinuclear NMR in order to study the N-B interaction and to propose the existence of possible Frustrated Lewis Pairs. Additionally, the molecular structures of significant components of the catalytic cycle, namely 4-tert-butylaniline-[B(C6F5)3] adduct and both free and [B(C6F5)3]-bonded 1-(phenyl)-2,3-diisopropylguanidine, and respectively, have been established by X-ray diffraction.

Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions

Solutions of some of the title compounds reached an equilibrium with the aminoboranes and the corresponding carbodiimides at room temperature.

Recent development of synthetic preparation methods for guanidines via transition metal catalysis

This article provides an overview of guanidine synthesis via transition-metal-catalyzed reactions including cycloaddition, guanylation and tandem guanylation/cyclization.

C–F sp2 bond functionalization mediated by niobium complexes

The functionalization of fluorobenzene is described via insertion of isocyanide after the oxidative addition of the C–F bond.

Early main group metal catalysis: how important is the metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal-alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca(2+) . The role of the metal was evaluated by a study using the metal-free catalysts: [Ph2 N(-) ][Me4 N(+) ] and [Ph3 C(-) ][Me4 N(+) ]. These "naked" amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of ArNCO and RNCNR (R=alkyl) by Ph2 NH. For the intramolecular hydroamination of unactivated CC bonds in H2 CCHCH2 CPh2 CH2 NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

Theoretical characterization and design of highly efficient iridium (III) complexes bearing guanidinate ancillary ligand

A density functional theory/time-depended density functional theory was used to investigate the synthesized guanidinate-based iridium(III) complex [(ppy)2Ir{(N(i)Pr)2C(NPh2)}] (1) and two designed derivatives (2 and 3) to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including spin-orbital coupling (SOC) matrix elements, zero-field-splitting parameters, radiative rate constants (kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings cannot only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the photoluminescence quantum efficiency (ΦPL) of 3 is significantly higher than that of 1. This can be explained by its large SOC value<T1|HSO|Sn>(n=3-4) and large transition electric dipole moment (μS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy (ET1) and smaller <S0|HSO|T1>(2) value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.

Guanidines: from classical approaches to efficient catalytic syntheses

This review focuses on the metal-mediated catalytic addition of amines to carbodiimides as an atom-economical alternative to the classical synthesis of guanidines.