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

Heavier tetrylene- and tetrylyne-transition metal chemistry: it's no carbon copy

Since the late 19th century, heavier tetrylene- and tetrylyne-transition metal chemistry has formed an important cornerstone in both main-group and organometallic chemistry alike. Driven by the success of carbene systems, significant efforts have gone towards the thorough understanding of the heavier group 14 derivatives, with examples now known from across the d-block. This now leads towards applications in cooperative bond activation, and moves ultimately towards well-defined catalytic systems. This review aims to summarise this vast field, from initial discoveries of tetrylene and tetrylyne complexes, to the most recent developments in reactivity and catalysis, as a platform to the future of this exciting, blossoming field.

Disilicon-Mediated Carbon Monoxide Activation: From a 1,2,3-Trisila- to 1,3-Disilacyclopentadienes with Hypercoordinate λ4Si-λ3C Double Bonds

The facile reaction of the SiPh2-bridged bis-silylene (LSi:)2SiPh2 (L=PhC(NBut)2) with diphenylacetylene affords the unprecedented 1,2,3-trisilacyclopentadiene (LSi)2(PhC)2SiPh2 1 with a hypercoordinate λ4Si-λ3Si double bond. Compound 1 is very oxophilic and consumes three molar equivalents of inert N2O to form the bicyclic oxygenation product 2 through O-atom insertion in the Si=Si and Si-Si bonds. Strikingly, 1 can completely split the C≡O bonds of carbon monoxide under ambient conditions (1 atm, room temperature), yielding the 1,3-disilacyclopentadiene 3, representing the first hypercoordinate example of a cyclosilene with a λ4Si-λ3C double bond. Likewise, reaction of Xyl-NC (Xyl=2,6-dimethylphenyl), an isocyanide isoelectronic with CO, with 1 furnishes the related 1,3-disilacyclopentadiene 4 but with an amidinato silylene pendent attached to the Si=C carbon ring atom.

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.

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.

Germanium Complexes with ONO Tridentate Ligands: O-H Bond Activation Control According to DFT Calculations

Polydentate ligands are used for thermodynamic stabilization of tetrylenes-low-valent derivatives of Group 14 elements (E = Si, Ge, Sn, Pb). This work shows by DFT calculations how the structure (the presence or absence of substituents) and type (alcoholic, Alk, or phenolic, Ar) of tridentate ligands 2,6-pyridinobis(1,2-ethanols) [^AlkONO^R]H₂ and 2,6-pyridinobis(1,2-phenols) [^ArONO^R]H₂ (R = H, Me) may affect the reactivity or stabilization of tetrylene, indicating the unprecedented behavior of Main Group elements. This enables the unique control of the type of the occurring reaction. We found that unhindered [ONO^H]H₂ ligands predominantly led to hypercoordinated bis-liganded {[ONO^H]}₂Ge complexes, where an E(+2) intermediate was inserted into the ArO-H bond with subsequent H₂ evolution. In contrast, substituted [ONO^Me]H₂ ligands gave [ONO^Me]Ge: germylenes, which may be regarded as kinetic stabilized products; their transformation into E(+4) species is also thermodynamically favorable. The latter reaction is more probable for phenolic [^ArONO]H₂ ligands than for alcoholic [^AlkONO]H₂. The thermodynamics and possible intermediates of the reactions were also investigated.

First-Row Transition Metal Complexes of a Phosphine-Silylene-Based Hybrid Ligand

We have prepared two new silylene-phosphine-based hybrid ligands Si{N(R)C₆H₄(PPh₂)}{PhC(N^tBu)₂} [R = TMS {trimethylsilyl} (1) and TBDMS {tert-butyldimethylsilyl} (2)], which possess two donor sites. Furthermore, the treatment of the bidentate ligand 1 with base metal halides {FeBr₂, CoBr₂, NiCl₂·dme [nickel chloride(II) ethylene glycol dimethyl ether]} and 2 with NiBr₂·dme [nickel bromide(II) ethylene glycol dimethyl ether] afforded four-coordinate six-membered metal complexes 3-6, respectively, which feature coordination from both Si(II) and P(III) sites. Subsequently, complexes 3 [(FeBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 4 [(CoBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 5 [(NiCl₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], and 6 [(NiBr₂)Si{N(Si^tBuMe₂)C₆H₄(PPh₂)}{PhC(N^tBu)₂}] are studied for their redox and magnetic properties with the help of UV-vis spectroscopy, cyclic voltammetry, SQUID magnetometry, and theoretical calculations. Complexes 3-6 were found to display a paramagnetic behavior. All the compounds are well established by single-crystal X-ray diffraction studies.

Sn,P-coordinated Ru cation: a robust catalyst for aerobic oxidations of benzylamine and benzyl alcohol

A stable ionic κ²Sn,P-coordinated Ru complex shows excellent catalytic activity in aerobic oxidations of benzylamine and benzyl alcohol. This complex is stabilized by a stannylene-phosphine peri-substituted naphthalene ligand, which can act as either a reducing agent for a Ru(III) complex or as a κ²Sn,P-chelating ligand for Ru(II) compounds.

Coordination capabilities of bis-(2-pyridyl)amides in the field of divalent germanium, tin and lead compounds

The reactivity of two lithium amides derived from bis-(2-pyridyl)amine (dpa)H or its methyl-substituted congener bis-(6-methyl-2-pyridyl)amine (Me-dpa)H, i.e. (dpa)Li (1) and (Me-dpa)Li (2), toward ECl2 (where E = Ge (dioxane complex) and Sn) is reported. This study produced both heteroleptic complexes (dpa)GeCl (3), [(dpa)SnCl]2 (4), and (Me-dpa)GeCl (5) and homoleptic complexes (dpa)2E (E = Ge (6) or Sn (7)) and (Me-dpa)2E (E = Ge (8) or Sn (9)). The structures of all complexes were established by single-crystal X-ray diffraction analysis showing significant differences depending on the E atom and ligand used. By contrast, in solution, the majority of compounds showed a fluxional behaviour as demonstrated by the NMR study. Finally, it turned out that the Me-dpa ligand, unlike dpa, is able to form ate complexes [(Me-dpa)3E]Li (E = Ge (10), Sn (11) or Pb (12)), whose structures were determined by single-crystal X-ray diffraction analysis. This study revealed the formation of two isomers for Ge and Sn complexes depending on the coordination preference of the lithium atom being coordinated either by nitrogen donors (10-12) or solely by the Ge or Sn electron lone pair (10a and 11a). Furthermore, the NMR experiments proved that the germanium complex 10 exhibits only limited stability in solution and decomposes to germylene 8 and lithium amide 2.

Germylene-β-sulfoxide Hemilabile Ligand in Coordination Chemistry

We describe herein the synthesis of a germylene-β-sulfoxide ligand, 1, and its abilities in coordination chemistry. Treatment of 1 with metal complexes [W(cod)(CO)₄], [Mo(nbd)(CO)₄] and [Ni(cod)₂] afforded the corresponding (1)-chelated metal complexes (1)-W(CO)₄ (2a), (1)-Mo(CO)₄ (2b), and (1)-Ni(cod) (4a), clearly showing a bidentate ligation of the metal by the germanium(II) and sulfur centers. Coordination with [Ru(PPh₃)₃Cl₂] afforded an unprecedented bridged bis(ruthenium) complex 3b. In the case of 4a, the hemilability of the bidentate ligand 1 was demonstrated by sulfoxide substitution by a CO ligand.