Polydentate Amidinato-Silylenes, -Germylenes and -Stannylenes

This review article focuses on amidinatotetrylenes that potentially can (or have already shown to) behave as bi- or tridentate ligands because they contain at least one amidinatotetrylene moiety (silylene, germylene or stannylene) and one (or more) additional coordinable fragment(s). Currently, they are being widely used as ligands in coordination chemistry, small molecule activation and catalysis. This review classifies those that have been isolated as transition metal-free compounds into five families that differ in the position(s) of the donor group(s) (D) on the amidinatotetrylene moiety, namely: ED{R1NC(R2)NR1}, EX{DNC(R2)NR1}, EX{R1NC(D)NR1}, EX{DNC(R2)ND} and E{R1NC(R2)ND}2 (E=Si, Ge or Sn). Those that do not exist as transition metal-free compounds but have been observed as ligands in transition metal complexes are cyclometallated and ring-opened amidinatotetrylene ligands. This article presents schematic descriptions of their structures, the approaches used for their syntheses and a quick overview of their involvement (as ligands) in transition metal-catalysed reactions. The literature is covered up to the end of 2023.

Amidinatotetrylenes Donor Functionalized on Both N Atoms: Structures and Coordination Chemistry

E(hmds)(bqfam) (E = Ge (1a), Sn (1b); hmds = N(SiMe3)2, bqfam = N,N'-bis(quinol-8-yl)formamidinate), which are amidinatotetrylenes equipped with quinol-8-yl fragments on the amidinate N atoms, have been synthesized from the formamidine Hbqfam and Ge(hmds)2 or SnCl(hmds). Both 1a and 1b are fluxional in solution at room temperature, as the E atom oscillates from being attached to the two amidinate N atoms to being chelated by an amidinate N atom and its closest quinolyl N atom (both situations are similarly stable according to density functional theory calculations). The hmds group of 1a and 1b is still reactive and the deprotonation of another equivalent of Hbqfam can be achieved, allowing the formation of the homoleptic derivatives E(bqfam)2 (E = Ge, Sn). The reactions of 1a and 1b with [AuCl(tht)] (tht = tetrahydrothiophene), [PdCl2(MeCN)2], [PtCl2(cod)] (cod = cycloocta-1,5-diene), [Ru3(CO)12] and [Co2(CO)8] have been investigated. The gold(I) complexes [AuCl{κE-E(hmds)(bqfam)}] (E = Ge, Sn) have a monodentate κE-tetrylene ligand and display fluxional behavior in solution the same as that of 1a and 1b. However, the palladium(II) and platinum(II) complexes [MCl{κ3E,N,N'-ECl(hmds)(bqfam)}] (M = Pd, Pt; E = Ge, Sn) contain a κ3E,N,N'-chloridotetryl ligand that arises from the insertion of the tetrylene E atom into an M-Cl bond and the coordination of an amidinate N atom and its closest quinolyl N atom to the metal center. Finally, the binuclear ruthenium(0) and cobalt(0) complexes [Ru2{μE-κ3E,N,N'-E(hmds)(bqfam)}(CO)6] and [Co2{μE-κ3E,N,N'-E(hmds)(bqfam)}(μ-CO)(CO)4] (E = Ge, Sn) have a related κ3E,N,N'-tetrylene ligand that bridges two metal atoms through the E atom. For the κ3E,N,N'-metal complexes, the quinolyl fragment not attached to the metal is pendant in all the germanium compounds but, for the tin derivatives, is attached to (in the Pd and Pt complexes) or may interact with (in the Ru2 and Co2 complexes) the tin atom.

Redox Activity of IrIII Complexes with Multidentate Ligands Based on Dipyrido-Annulated N-Heterocyclic Carbenes: Access to High Valent and High Spin State with Carbon Donors

Synthetic strategies to access high-valent iridium complexes usually require use of π donating ligands bearing electronegative atoms (e. g. amide or oxide) or σ donating electropositive atoms (e. g. boryl or hydride). Besides the η5 -(methyl)cyclopentadienyl derivatives, high-valent η1 carbon-ligated iridium complexes are challenging to synthesize. To meet this challenge, this work reports the oxidation behavior of an all-carbon-ligated anionic bis(CCC-pincer) IrIII complex. Being both σ and π donating, the diaryl dipyrido-annulated N-heterocyclic carbene (dpa-NHC) IrIII complex allowed a stepwise 4e- oxidation sequence. The first 2e- oxidation led to an oxidative coupling of two adjacent aryl groups, resulting in formation of a cationic chiral IrIII complex bearing a CCCC-tetradentate ligand. A further 2e- oxidation allowed isolation of a high-valent tricationic complex with a triplet ground state. These results close a synthetic gap for carbon-ligated iridium complexes and demonstrate the electronic tuning potential of organic π ligands for unusual electronic properties.

Synthesis and Some Coordination Chemistry of Phosphane-Difunctionalized Bis(amidinato)-Heavier Tetrylenes: A Previously Unknown Class of PEP Tetrylenes (E = Ge and Sn)

The bis(amidinato)-heavier tetrylenes E(bzamP)2 (E = Ge (2a) and Sn (2b); bzamP = N-isopropyl-N'-(diphenylphosphanylethyl)benzamidinate), which are equipped with one heavier tetrylene (germylene or stannylene) and two phosphane fragments (one on each amidinate moiety) as coordinable groups, have been synthesized from the benzamidinum salt [H2bzamP]Cl and GeCl2(dioxane) or SnCl2 in 2:1 mol ratio. A preliminary inspection of their coordination chemistry has shown that their amidinate group can also be involved in the bonding with the metal atoms as tridentate ENP and tetradentate PENP' coordination modes have been observed for the ECl(bzamP)2 ligand of [Ir{κ3E,N,P-ECl(bzamP)2}(cod)] (E = Ge (3a) and Sn (3b); cod = η4-1,5-cyclooctadiene) and the E(bzamP)2 ligand of [Ni{κ4E,N,P,P'-E(bzamP)2}] (E = Ge (4a) and Sn (4b)), which are products of reactions of 2a and 2b with [IrCl(cod)]2 (1:0.5 mol ratio) and [Ni(cod)2] (1:1 mol ratio), respectively. These products contain a 5-membered NCNEM ring that results from the insertion of the metal M atom into an E-N bond of 2a and 2b. Additionally, while iridium(I) complexes 3a and 3b are chloridotetryl derivatives (insertion of the tetrylene E atom into the Ir-Cl bond has also occurred) that have an uncoordinated phosphane group, nickel(0) complexes 4a and 4b contain a tetrylene fragment that, maintaining the lone pair, behaves as a σ-acceptor (Z-type) ligand.

Effects of silylene ligands on the performance of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes

In order to study the effects of silylene ligands on the catalytic activity of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes, readily available model catalysts are required. In this contribution, a comparative study of the hydrosilylation of aldehydes and ketones catalyzed by tris(trimethylphosphine) cobalt chloride, CoCl(PMe₃)₃ (1), and bis(silylene) cobalt chloride, Co(LSi:)₂(PMe₃)₂Cl (2, LSi: = {PhC(N^tBu)₂}SiCl), is presented. It was found that both complexes 1 and 2 are good catalysts for the hydrosilylation of aldehydes and ketones under mild conditions. This catalytic system has a broad substrate scope and selectivity for multi-functional substrates. Silylene complex 2 shows higher activity than complex 1, bearing phosphine ligands, for aldehydes, but conversely, for ketones, the activity of complex 1 is higher than that of complex 2. It is worth noting that in the process of mechanistic studies the intermediates (PMe₃)₃Co(H)(Cl)(PhH₂Si) (3) and (LSi:)₂(PMe₃)Co(H)(Cl)(PhH₂Si) (4) were isolated from the stoichiometric reactions of 1 and 2 with phenylsilane, respectively. Further experiments confirmed that complex 3 is a real intermediate. A possible catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by 1 was proposed based on the experimental investigation and literature reports, and this mechanism was further supported by DFT studies. The bis(silylene) complex 4 showed complicated behavior in solution. A series of experiments were designed to study the catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by complex 2. According to the experimental results, the hydrosilylation of aldehydes catalyzed by 1 proceeds via a different mechanism than that of the analogous reaction with complex 2 as the catalyst. In the case of ketones, complex 4 is a real intermediate, indicating that both 1 and 2 catalyze the reaction by the same mechanism. The molecular structures of 3 and 4 were determined by single crystal X-ray diffraction analysis.

Dipyrromethane‐Based PGeP Pincer Germyl Rhodium Complexes

A family of germyl rhodium complexes derived from the PGeP germylene 2,2’‐bis(di‐isopropylphosphanylmethyl)‐5,5’‐dimethyldipyrromethane‐1,1’‐diylgermanium(II), Ge(pyrmPⁱPr₂)₂CMe₂ (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh₃)₃] and [RhCl(cod)(PPh₃)] (cod=1,5‐cyclooctadiene) to give, in both cases, the PGeP‐pincer chloridogermyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(CO)] (6), with HCl, HSnPh₃ and Ph₂S₂ rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (X=Cl, 7; SnPh₃, 8) and [Rh(SPh)₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (9), respectively, and with H₂ to give the hexacoordinate derivative [RhH₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4‐vinyltoluene and 4‐vinylfluorobenzene with catecholborane under mild conditions.

Stannylenes based on pyrrole-phosphane and dipyrromethane-diphosphane scaffolds: syntheses and behavior as precursors to PSnP pincer palladium(II), palladium(0) and gold(I) complexes

2-Ditertbutylphosphanylmethylpyrrole (H₂pyrmP^tBu₂) and 2,2'-bis(diisopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane ((HpyrmPⁱPr₂)₂CMe₂) have been used to synthesize new P-donor-stabilized stannylenes in which the Sn atom is attached to one, SnCl(HpyrmP^tBu₂) (1) and Sn{N(SiMe₃)₂}(HpyrmP^tBu₂) (2), or two pyrrolyl-phosphane scaffolds, Sn(HpyrmP^tBu₂)₂ (3), or to a dipyrromethane-diphosphane scaffold, Sn(pyrmPⁱPr₂)₂CMe₂ (4). It has been found that stannylenes 3 and 4 are excellent precursors to transition metal complexes containing PSnP pincer-type ligands. Their reactions with chlorido transition metal complexes have afforded [PdCl{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (6), [PdCl{κ³P,Sn,P-SnCl(pyrmPⁱPr₂)₂CMe₂}] (7) and [Au{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (8), which contain a PSnP pincer-type chloridostannyl ligand. While complexes 6 and 7 are square-planar palladium(II) complexes, compound 8 is an uncommon gold(I) complex having a T-shaped coordination geometry with a very long Sn-Au bond (3.120 Å). The T-shaped palladium(0) complex [Pd{κ³P,Sn,P-Sn(pyrmPⁱPr₂)₂CMe₂}] (9), which contains an unprecedented PSnP pincer-type stannylene that behaves as a Z-type (σ-acceptor) ligand, has been prepared from 4 and [Pd(η³-C₃H₅)(η⁵-C₅H₅)].

Origin of the Ir-Si bond shortening in Ir-NSiN complexes

The Ir-Si bond distances reported for Ir-(fac-κ3-NSiNOPy) and Ir-(fac-κ3-NSiN4MeOPy) species (NSiNOPy = bis(pyridine-2-yloxy)methylsilyl and NSiN4MeOPy = bis(4-methyl-pyridine-2-yloxy)methylsily) are in the range of 2.220-2.235 Å. These values are in the lowest limit of the Ir-Si bond distances found in the Cambridge Structural Database (CSD). To understand the origin of such remarkable shortening, a computational study of the bonding situation of representative examples of Ir-(fac-κ3-NSiN) species has been carried out. It is found that the Ir-Si bond can be described as an electron-sharing (i.e. covalent) bond. Despite that, this bond is highly polarized and as a result, the contribution of the electrostatic attractions to the bonding is rather significant. Indeed, there exists a linear relationship (R2 = 0.97) between the Ir-Si bond distance and the extent of the computed electrostatic interactions, which indicates that the ionic contribution to the bonding is mainly responsible for the observed Ir-Si bond shortening.

2-Pyridone-stabilized iridium silylene/silyl complexes: structure and QTAIM analysis

Iridium(iii) complexes of the general formula [Ir(X)(κ2-NSiiPr2)2] (NSiiPr2 = (4-methyl-pyridine-2-yloxy)diisopropylsilyl; X = Cl, 3; CF3SO3, 5; CF3CO2, 6) have been prepared and fully characterized, including X-ray diffraction studies and theoretical calculations. The presence of isopropyl substituents at the silicon atom favours the monomeric structure found in complexes 3 and 5. The short Ir-Si bond distances (2.25-2.28 Å) indicate some degree of base-stabilized silylene character of the Ir-Si bond in 3, 5 and 6 assisted by the 2-pyridone moiety. However, the shortening of these Ir-Si bonds might be a consequence of the constrained 2-pyridone geometry, and consequently the silyl character of these bonds can not be excluded. A DFT theoretical study on the nature of the Ir-Si bonds has been performed for complex 3 as well as for four other iridium complexes finding representative examples of different bonding situations between Ir and Si atoms: silylene, base-assisted silylene (both with an anionic base and with a neutral base), and silyl bonds, using the topological properties of the electron charge density. The results of these studies show that the Ir-Si bonds in Ir-NSiiPr2 complexes can be considered as an intermediate between the base-stabilized silylene and silyl cases, and therefore they have been proposed as 2-pyridone-stabilized iridium silylene/silyl bonds.

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.

Phosphane-functionalized heavier tetrylenes: synthesis of silylene- and germylene-decorated phosphanes and their reactions with Group 10 metal complexes

The stable phosphane-functionalized heavier tetrylenes E(tBu2bzam)pyrmPtBu2 (E = Si (1Si), Ge (1Ge); tBu2bzam = N,N'-ditertbutylbenzamidinate; HpyrmPtBu2 = ditertbutyl(2-pyrrolylmethyl)phosphane) have been prepared by reacting the amidinatotetrylenes E(tBu2bzam)Cl (E = Si, Ge) with LipyrmPtBu2. The reactions of 1Si and 1Ge with selected M0 and MII (M = Ni, Pd, Pt) metal precursors have allowed the synthesis of square-planar [MCl2{κ2E,P-E(tBu2bzam)pyrmPtBu2}] (M = Ni, Pd, Pt; E = Si, Ge), tetrahedral [Ni{κ2E,P-E(tBu2bzam)pyrmPtBu2}(cod)] (E = Si, Ge; cod = 1,5-cyclooctadiene) and triangular [M{κ2E,P-E(tBu2bzam)pyrmPtBu2}(PPh3)] (M = Pd, Pt; E = Si, Ge) complexes, showing that 1Si and 1Ge are excellent Si,P- and Ge,P-chelating ligands that, due to their large steric bulk, are able to stabilize three-coordinate Pd0 and Pt0 complexes.

Synthesis and Reactivity of a Hypersilylsilylene

Stabilization of an amidinatosilylene with a bulky tris(trimethylsilyl)silyl substituent was realized with the preparation of PhC(NtBu)₂Si{Si(SiMe₃)₃} (1) from PhC(NtBu)₂SiHCl₂ with K{Si(SiMe₃)₃} in more than 90% yield. The highly deshielded ²⁹Si NMR resonance (δ = 76.91 ppm) can be attributed to the absence of a π-donating substituent. The molecular structure of 1 shows a trigonal-planar geometry around the Si^II center with a Si^II-Si^IV bond length of 2.4339(13) Å. A series of reactions of 1 with Me₃NO, S, Se, and Te were performed. While siloxane derivatives (2 and 3) are obtained from reactions with Me₃NO, silachalcogenones (4-6) are formed with other chalcogens. The presence of Si═E (E = S, Se, and Te) bonds in 4-6 have been confirmed by single-crystal X-ray studies. Silaoxirane (7) formation was observed when 1 was treated with acetone, demonstrating the importance of the tris(trimethylsilyl)silyl group to kinetically and thermodynamically protect the silaoxirane derivative with less bulky substituents on the C atom.

(PhC(NtBu)2Al)2(SiH2)4 six-membered heterocycle: comparable in structure to cyclohexane

A (PhC(NtBu)₂Al)₂(SiH₂)₄ centrosymmetric six-membered heterocycle (LAl)₂(SiH₂)₄ (L = PhC(NtBu)₂) with chair conformation is prepared by reacting disilylene LSi–SiL with alane AlH₃·NEtMe₂. It is comparable in structure to cyclohexane containing four Si and two Al atoms as a core.

A dipyrromethane-based diphosphane–germylene as precursor to tetrahedral copper(i) and T-shaped silver(i) and gold(i) PGeP pincer complexes

A dipyrromethane-based PGeP germylene has allowed the synthesis of unusual tetrahedral copper(i) and T-shaped silver(i) and gold(i) germyl complexes.

Mesityl(amidinato)tetrylenes as ligands in iridium(i) and iridium(iii) complexes: silicon versus germanium and simple κ1-coordination versus cyclometallation

The reactivity of the germylene Ge(^tBu₂bzam)Mes with [Ir₂(μ-Cl)₂(η⁴-cod)₂] and [Ir₂Cl₂(μ-Cl)₂(η⁵-Cp*)₂] differs considerably from that previously communicated for the isostructural silylene Si(^tBu₂bzam)Mes.

Cyclometallation of a germylene ligand by concerted metalation-deprotonation of a methyl group

The reaction of [CH{(CMe)(2,6-iPr2C6H3N)}2]GeCl with LiN(SiMe3)2 was previously reported, which led to the formation of a hetero-fulvene type germylene, [CH{(CMe)(C[double bond, length as m-dash]CH2)(2,6-iPr2C6H3N)}2]Ge through the deprotonation of the C-H bond from the methyl substituents. In this paper, we attempted the analogous reaction with (Dipp)NCMeCHCOMeGeCl using LiN(SiMe3)2 which gave rise to a metathesis product, (Dipp)NCMeCHCOMeGeN(SiMe3)2 (2). However, the reactions of 2 with [M2Cl2(μ-Cl)2(η5-Cp*)2] (M = Rh and Ir) resulted in cyclometallated Rh and Ir complexes through the activation of the C-H bond from the germylene ligand. The complexes were characterized by single crystal X-ray analysis, which authenticated the presence of Ge-Rh and Ge-Ir bonds. DFT studies have been performed to understand the mechanism.