Cationic ligands - from monodentate to pincer systems

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

Structure and reactivity of germylene-bridged digold complexes

The bonding between gold and main-group metallic elements (M) featuring Au^δ−−M^δ+ polarity, has been studied recently. The gold in the bonds is expected to have the oxidation number of −1, and hence, nucleophilic. However, the knowledge of the reactivity of the gold-metal bonds remains limited. Here, we report digold-substituted germanes of the form of R’₂Ge(AuPR₃)(AuGeR’₂) (3a; R = Me, 3b; R = Et), featuring two Au-Ge(IV) and one Au-Ge(II) bonds. DFT calculations of 3a revealed the existence of high-lying σ(Ge-Au) type HOMO and low-lying LUMO with germylene p_π nature. A pendular motion of AuPR₃ group between Ge(IV) and Ge(II) of 3 occurs in the NMR time scale, suggesting that the Ge(II) center has an enhanced electrophilicity to be attacked by the nucleophilic gold (−I) atom. 3a reacts with nucleophilic Cl⁻ and electrophilic MeOTf reagents at Ge(II) and Ge(IV) centers, respectively.

The number of metal complexes featuring gold-germanium bonds is limited. Here the authors report the preparation of germylene-bridged digold complexes complexes and study their structure, bonding, and reactivity.

Chemistry of the Bis(imine)-Based Tetradentate Ligand Stabilized Group 14 E(II) Cations (E=Ge and Sn)

The ambiphilic Ge(II) and Sn(II) cationic species have been reported to be isolated through kinetic or thermodynamic stabilizations. Nonetheless, steric congestion or excessive coordination of donor atoms to the cationic center concurrently disfavors its prompt reactivity. Our research in this field revolves around the utilization of structurally non-rigid bis(imine) based tetradentate supporting ligands for the stabilization of Ge(II) and Sn(II) cationic species. Such E(II) cationic systems have been advantaged due to inherent flexibility present at the ligand backbone allowing disposal of E(II) orbitals through geometric rearrangements for further reactivity. The bifunctionality present in the ligand enables the first examples of Ge(II) bis-monocations. Furthermore, the redox-active nature of the ligand encourages participation in chemical transformations. In this personal account we have provided a detailed discussion of our published work in this direction in the last five years.

Coinage Metal Complexes of Germylene and Stannylene

Group 11 metal (M = Cu, Ag, Au) complexes of heavier tetrylenes (namely, germylene and stannylene) have been studied for several years now. However, the field is mainly unexplored for their potential application either in homogeneous catalysis or in photophysical properties regardless of how the current reports allude to the rich and fascinating chemistry of group 14 elements. In this mini-review, we attempted to summarize the recent advances of heavier tetrylene-stabilized coinage metal complexes, which are majorly dominated by structure elucidation studies. This comprehensive summary intends to help researchers design and fine-tune the tetrylene ligand framework that can lead to coinage metal complexes for future applications as photoemitters in organic light-emitting diode fabrication and efficient catalysts in homogeneous catalysis.

Reductively disilylated N-heterocycles as versatile organosilicon reagents

The reductively disilylated N-heterocyclic systems 1,4-bis(trimethylsilyl)-1-aza-2,5-cyclohexadiene (1Si), 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2Si) and its methyl derivatives (3Si and 4Si), and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (5Si) are proficient organosilicon reagents owing to their low first vertical ionization potentials and the heterophilicity of the polarized N-Si bonds. These have prompted their reactivity as two-electron reductants or reductive silylations. These reactions benefit from the concomitant rearomatization of the N-heterocycles and liberation of trimethylsilyl halides or (Me₃Si)₂O, which are mostly volatile or easily removable byproducts. In this perspective, we have discussed the utilization of these reductively disilylated N-heterocyclic systems as versatile reagents in the salt-free reduction of transition metals (A) and main-group halides (B), in organic transformations (C) and in materials syntheses (D).

Stabilization of the Elusive Antimony(I) Cation and Its Coordination Complexes with Transition Metals

Upon stabilization by 5,6-bis(diisopropylphosphino)acenaphthene to form compound 1, the fugitive antimony (I) cation exhibited nucleophilic behavior towards coinage metals. Compound 1 was strategically synthesized at room temperature from SbCl₃ , the bis(phosphine), and trimethylsilyl trifluoromethanesulfonate taken in a 1:2:3 ratio, whereby the bis(phosphine) plays the dual role of a reductant and a supporting ligand. The generation of 1 involves two-electron oxidation of the ligand to form a P-P bonded diphosphonium dication. Compound 1 was separated from this dication to give both products in pure form in moderate yields. Despite the overall positive charge, the Sb^I site in 1 was found to bind to metal centers, forming complexes with Au^I , Ag^I and Cu^I . Compound 1 reduced Cu^II to Cu^I and formed a coordination complex with the resulting Cu^I species. The effects of the electron-rich bis(phosphine) and the constrained peri geometry in stabilizing and enhancing the nucleophilicity of 1 have been rationalized through computational studies.

Pyramidal Dicationic Ge(II) Complexes with Homoleptic Neutral Pnictine Coordination: A Combined Experimental and Density Functional Theory Study

An unusual series of Ge(II) dicationic species with homoleptic phosphine and arsine coordination, [Ge(L)][OTf]₂, L = 3 × PMe₃, triphos (MeC(CH₂PPh₂)₃), triars (MeC(CH₂AsMe₂)₃), or κ³-tetraphos (P(CH₂CH₂PPh₂)₃) (OTf^- = O₃SCF₃^-) have been prepared by reaction of [GeCl₂(dioxane)] with L and 2 mol equiv of Me₃SiOTf in anhydrous CH₂Cl₂ (or MeCN for L = triars, triphos). X-ray crystal structures are reported for [Ge(PMe₃)₃][OTf]₂, [Ge(triars)][OTf]₂, and [Ge(κ³-tetraphos)][OTf]₂, confirming homoleptic P₃- or As₃-coordination at Ge(II) in each case and with the discrete OTf^- anions providing a charge balance. The Ge-P/As bond lengths are significantly shorter than those in neutral germanium(II) dihalide complexes with diphosphine or diarsine coordination. Solution NMR spectroscopic data indicate that the complexes are labile in solution. Using excess AsMe₃ and [GeCl₂(dioxane)] gives only the neutral product, [Ge(AsMe₂)₂(OTf)₂], the crystal structure of which shows four coordination at Ge(II), via two As donor atoms and an O atom from two κ¹-OTf^- ligands; further weak, long-range intermolecular interactions give a chain polymer. The electronic structure of the [Ge(PMe₃)₃]²⁺ dication has been investigated using density functional theory (DFT) calculations. The computed geometrical parameters for this dication are in good agreement with the experimental X-ray crystallographic values in [Ge(PMe₃)₃][OTf]₂. The results also indicate that the pyramidal arrangement of the [Ge(PMe₃)₃]²⁺ (computed P-Ge-P angle 96.8° at the B3LYP-D3 level) arises from a balance between electronic energy (E_elec) contributions, which favor a lower P-Ge-P angle, and nuclear-nuclear contributions (E_nn), which favor a higher P-Ge-P angle, to the total energy (E_TOT). An Atoms in Molecules (AIM) analysis reveals that one reason why E_elec decreases as the P-Ge-P angle decreases is because of C···H and H···H interactions between atoms on different CH₃ groups. The stability of the [Ge(PMe₃)₃]²⁺ dication is enhanced by the distribution of a significant part of the positive charge on Ge²⁺ to the atomic centers of the PMe₃ ligands. Similar results were obtained for [Ge(AsMe₃)₃][OTf]₂, showing the tris-AsMe₃ complex to be less stable compared to the PMe₃ analogue. Related calculations were also performed for the neutral [Ge(PMe₃)₂(OTf)₂] and [Ge(AsMe₃)₂(OTf)₂] complexes.

Intramolecularly Double-Donor-Stabilized Stannylene and Its Coordination towards Ag(I) and Au(I) Centers

The intramolecularly double-donor-stabilized stannylene 1 has been synthesized from the salt-metathesis reaction between two equivalents of lithium pyridine ene-amide L1 and SnCl₂ . Compound 1 exhibits dipolar behavior when reacted with B(C₆ F₅ )₃ leading to the zwitterionic compound 2. The reaction of 1 with one equivalent and 0.5 equivalent of AgOTf (OTf=trifluoromethane sulfonate) result in the formation of a stannylene-AgOTf complex 3 and a homoleptic distannylene-silver ionic complex 4, respectively. Analogous to complex 4, the gold(I) complex 5 has been synthesized from the reaction between two equivalents of 1 and 0.5 equivalent of AuCl.SMe₂ /Me₃ SiOTf. Complex 5 is the first example of homoleptic stannylene-Au(I) ionic complex among the very scarce reports on stannylene-gold(I) coordination complexes. All compounds have been structurally characterized using single crystal X-ray crystallography. Solution-state characterization have been performed using multinuclear NMR techniques. Detailed DFT calculations on the optimized geometries 1 o, 3 o-5 o reveal the change in sp- hybridization on the pyramidal Sn(II) center upon metal coordination and their bonding overlaps.

Stabilization of bis(chlorogermyliumylidene)s within bifunctional PNNP ligand frameworks and their reactivity studies

The diiminodiphosphine (L_im) and diaminodiphosphines (l-NH and l-NMe) with a bifunctional PNNP ligand framework have been employed to host two [GeCl]⁺ units leading to the formation of bis(chlorogermyliumylidene) 1-3, respectively. The synthetic route involves a 1 : 2 stoichiometric reaction between the PNNP ligand and GeCl₂·dioxane and the subsequent addition of two equivalents of chloride abstracting agent. Compound 1 is unstable towards coordinating solvents and Lewis bases, resulting in the displacement of the GeCl unit and the formation of rearranged products 4 and 5. However, the diaminodiphosphine coordinated Ge(ii) bis(monocation)s 2 and 3 proved to be stable and revealed their electrophilic behaviour towards the Lewis bases studied.

Versatile coordinating abilities of acyclic N4 and N2P2 ligand frameworks in conjunction with Sn[N(SiMe3)2]2

The conjoint effort of the –CH₂–CH₂– linker backbone and coordinating sites in the tetradentate ligand stabilizes a variety of stannylenes.

Bis(chlorogermyliumylidene) and its significant role in elusive reductive cyclization

Bis(chlorogermyliumylidene) 2 has been strategically obtained within redox-active bis(α-iminopyridine). Metal-free reduction of 2 followed by protonation led to elusive 2,3-di(pyridin-2-yl)piperazine with meso-stereoselectivity. Formation of persistent triplet diradicals upon reduction and isolation of piperazine stabilized Ge(ii) dication intermediates provide convincing evidence for the crucial role of [GeCl]+ units in reductive cyclization.

A Neutral η5 -Aminoborole Complex of Germanium(II)

The synthesis of two η⁵ -aminoborole complexes of germanium(II) from the reaction of a germole dianion with aminoboron dichlorides is reported. This reaction constitutes a remarkable example of a germole-to-borole transformation. The two aminoborole complexes of germanium(II) were fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, HRMS, and, in one case, by X-ray crystallography. The results of quantum-mechanical calculations favor the electronic structure of a half-sandwich complex of Ge^II over an ionic representation with a germanium dication stabilized by an aromatic aminoborole dianion.

Oxidation of a germanium(ii) dication to access cationic germanium(iv) fluorides

Herein we describe the synthesis and characterization of the tris(1-ethyl-benzoimidazol-2-ylmethyl)amine (BIMEt₃ = L) complex of Ge(ii)²⁺ and its oxidation with XeF₂ to access the cationic germanium fluorides [LGeF₂][OTf]₂ and [LGeF(OTf)][OTf]₂.

Two synthetic approaches for the preparation of tin(ii) dications

Facile access to weakly coordinated Sn(ii) salts is provided by reaction of NO[Al(OR^F)₄] with tin metal. Further use is indicated.