Reactions of the amidinate-stabilized silicon(II) bis(trimethylsilyl)amide [LSiN(SiMe₃)₂], {L = PhC(NtBu)₂, 1} with a stiochiometric amount of elemental sulfur, selenium and tellurium afforded the first stable silanechalcogenones comprising an acyclic amido substituent [L{N(SiMe₃)₂}Si=E] {E = S (2), Se (3) or Te (4)}. All compounds were characterized by X-ray crystallography and multinuclear NMR spectroscopy. The results illustrate that these compounds possess some silicon–chalcogen double bond character.

Invited for the cover of this issue is the group of Cheuk-Wai So at the Nanyang Technological University, Singapore. The cover image shows the isolation of the first stable four-coordinate silanechalcogenones comprising an acyclic amido substituent by a straightforward oxidation of the amidinato silicon(II) bis(trimethylsilyl)amide with chalcogens.

The syntheses and structures of heteroleptic germanium(II) and tin(II) chlorides supported by anionic ligands derived from 2,3-dimethyl-1,4-diaza-1,3-butadiene are described. The reaction of ArN=C(Me)C(Me)=NAr (L, Ar = 2,6-iPr₂C₆H₃) with MeLi in Et₂O, followed by treatment with GeCl₂·dioxane or SnCl₂, afforded [L¹ECl] [L¹ = ArN=C(Me)C(Me)₂NAr; E = Ge (1), Sn (2)]. Moreover, L²Li [L² = ArN=C(Me)C(=CH₂)NAr] reacted with GeCl₂·dioxane in Et₂O to form [{C(=CH)NAr}{C(Me)=NAr}]₂(GeCl)₂ (3). The byproducts are [{C(CH₂)NAr}{C(Me)=NAr}Ge]₂ (4) and L²H (5). Furthermore, the reaction of L with LiNiPr₂ in Et₂O, followed by treatment with SnCl₂, afforded [L²SnCl] (6). Compounds 1–4 and 6 have been characterized by NMR spectroscopy and X-ray crystallography.

The synthesis of an N-heterocyclic silylene-stabilized digermanium(0) complex is described. The reaction of the amidinate-stabilized silicon(II) amide [LSiN(SiMe₃)₂] (1; L=PhC(NtBu)₂) with GeCl₂⋅dioxane in toluene afforded the Si^II–Ge^II adduct [L{(Me₃Si)₂N}SiGeCl₂] (2). Reaction of the adduct with two equivalents of KC₈ in toluene at room temperature afforded the N-heterocyclic carbene silylene-stabilized digermanium(0) complex [L{(Me₃Si)₂N}Si GeGeSi{N(SiMe₃)₂}L] (3). X-ray crystallography and theoretical studies show conclusively that the N-heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor–acceptor interaction.

The two-electron reduction of a Group 14-element(I) complex [RË⋅] (E=Ge, R=supporting ligand) to form a novel low-valent dianion radical with the composition [RË:]^. 2− is reported. The reaction of [LGeCl] (1, L=2,6-(CHNAr)₂C₆H₃, Ar=2,6-iPr₂C₆H₃) with excess calcium in THF at room temperature afforded the germylidenediide dianion radical complex [LGe]^. 2−⋅Ca(THF)₃²⁺ (2). The reaction proceeds through the formation of the germanium(I) radical [LGe⋅], which then undergoes a two-electron reduction with calcium to form 2. EPR spectroscopy, X-ray crystallography, and theoretical studies show that the germanium center in 2 has two lone pairs of electrons and the radical is delocalized over the germanium-containing heterocycle. In contrast, the magnesium derivative of the germylidendiide dianion radical is unstable and undergoes dimerization with concurrent dearomatization to form the germylidenide anion complex [C₆H₃-2-{C(H)NAr}Ge-Mg-6-{C(H)-NAr}]₂ (3).

The two-electron reduction of a Group 14-element(I) complex [RË⋅] (E=Ge, R=supporting ligand) to form a novel low-valent dianion radical with the composition [RË:]^. 2− is reported. The reaction of [LGeCl] (1, L=2,6-(CHNAr)₂C₆H₃, Ar=2,6-iPr₂C₆H₃) with excess calcium in THF at room temperature afforded the germylidenediide dianion radical complex [LGe]^. 2−⋅Ca(THF)₃²⁺ (2). The reaction proceeds through the formation of the germanium(I) radical [LGe⋅], which then undergoes a two-electron reduction with calcium to form 2. EPR spectroscopy, X-ray crystallography, and theoretical studies show that the germanium center in 2 has two lone pairs of electrons and the radical is delocalized over the germanium-containing heterocycle. In contrast, the magnesium derivative of the germylidendiide dianion radical is unstable and undergoes dimerization with concurrent dearomatization to form the germylidenide anion complex [C₆H₃-2-{C(H)NAr}Ge-Mg-6-{C(H)-NAr}]₂ (3).

The syntheses of a zwitterionic base-stabilized digermadistannacyclobutadiene and tetragermacyclobutadiene supported by amidinates and low-valent germanium amidinate substituents are described. The reaction of the amidinate Ge^I dimer, [LGe:]₂ (1, L=PhC(NtBu)₂), with two equivalents of the amidinate tin(II) chloride, [LSnCl] (2), and KC₈ in tetrahydrofuran (THF) at room temperature afforded a mixture of the zwitterionic base-stabilized digermadistannacyclobutadiene, [L₂Ge₂Sn₂L′₂] (3; L′=LGe:), and the bis(amidinate) tin(II) compound, [L₂Sn:] (4). Compound 3 can also be prepared by the reaction of 1 with [L^ArSnCl] (5, L^Ar=tBuC(NAr)₂, Ar=2,6-iPr₂C₆H₃) in THF at room temperature. Moreover, the reaction of 1 with the “onio-substituent transfer” reagent [4-NMe₂-C₅H₄NSiMe₃]OTf (8) in THF and 4-(N,N-dimethylamino)pyridine (DMAP) at room temperature afforded a mixture of the zwitterionic base-stabilized tetragermacyclobutadiene, [L₄Ge₆] (9), the amidinium triflate, [PhC(NHtBu)₂]OTf (10), and Me₃SiSiMe₃ (11). X-ray structural data and theoretical studies show conclusively that compounds 3 and 9 have a planar and rhombic charge-separated structure. They are also nonaromatic.

The synthesis and reactivity of a silyliumylidene cation stabilized by an amidinate ligand and 4-dimethylaminopyridine (DMAP) are described. The reaction of the amidinate silicon(I) dimer [LSi:]₂ (1; L=PhC(NtBu)₂) with one equivalent of N-trimethylsilyl-4-dimethylaminopyridinium triflate [4-NMe₂C₅H₄NSiMe₃]OTf and two equivalents of DMAP in THF afforded [LSi(DMAP)]OTf (2). The ambiphilic character of 2 is demonstrated from its reactivity. Treatment of 2 with 1 in THF afforded the disilylenylsilylium triflate [L′₂(L)Si]OTf (3; L′=LSi:) with the displacement of DMAP. The reaction of 2 with [K{HB(iBu)₃}] and elemental sulfur in THF afforded the silylsilylene [LSiSi(H){(NtBu)₂C(H)Ph}] (4) and the base-stabilized silanethionium triflate [LSi(S)DMAP]OTf (5), respectively. Compounds 2, 3, and 5 have been characterized by X-ray crystallography.

The synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl, [LSi(μ-CNAr)₂SiL] (2, L: PhC(NtBu)₂, Ar: 2,6-iPr₂C₆H₃), and a silylenylsilaimine, [LSi(NAr)SiL] (3), are described. The reaction of three equivalents of the disilylene [LSiSiL] (1) with two equivalents of ArNCNAr in toluene at room temperature for 12 h afforded [LSi(μ-CNAr)₂SiL] (2) and [LSi(NAr)SiL] (3) in a ratio of 1:2. Compounds 2 and 3 have been characterized by NMR spectroscopy and X-ray crystallography. Compound 2 was also investigated by theoretical studies. The results show that compound 2 possesses singlet biradicaloid character with an extensive electronic delocalization throughout the Si₂C₂ four-membered ring and exocyclic CN bonds. Compound 3 is the first example of a silylenylsilaimine, which contains a low-valent silicon center and a silaimine substituent. A mechanism for the formation of 2 and 3 is also proposed.

The synthesis and characterization of new amidinate-stabilized germatrisilacyclobutadiene ylides [L₃Si₃GeL′] (L=PhC(NtBu)₂; L′=ËL; Ë=Ge (3), Si (7)) are described. Compound 3 was prepared by the reaction of [LSiSiL] (1) with one equivalent of [LGeGeL] (2) in THF. Compound 7 was synthesized by the reaction of 2 with excess 1 in THF. The bisamidinate germylene [L₂Ge:] (4) is a by-product in both reactions. Moreover, compound 7 was prepared by the reaction of 3 with one equivalent of 1 in THF. Compounds 3 and 7 have been characterized by NMR spectroscopy, X-ray crystallography, and theoretical studies. The results show that compounds 3 and 7 are not antiaromatic. The puckered Si₃Ge four-membered rings in 3 and 7 have a ylide structure, which is stabilized by amidinate ligands and the electron delocalization within the Si₃Ge four-membered ring.

The reactivity of the silylsilylene [{PhC(NtBu)₂}SiSi(Cl){(NtBu)₂C(H)Ph}] (2) towards diphenylacetylene, azobenzene, 2,6-diisopropylphenyl azide, sulfur, and selenium is described. The reaction of 2 with one equivalent of azobenzene in toluene afforded compound 3, which is the first example of a 1,2-diaza-3,4-disilacyclobutane containing a pentacoordinate silicon center. The formation of 3 can be explained by a [1+2] cycloaddition of the divalent Si center in 2 with PhNNPh to form a diazasilacyclopropane intermediate, which then undergoes a 1,2-chlorine shift to release the ring strain to form 3. Similarly, the reaction of 2 with one equivalent of diphenylacetylene in toluene afforded the 1,2-disilacyclobutene 4, which contains a pentacoordinate silicon center. The reaction of 2 with 1.6 equivalents of 2,6-diisopropylphenylazide in toluene afforded the silaimine [LSi(NAr)N(Ar)L′] (5, L=PhC(NtBu)₂, L′=Si(Cl){(NtBu)₂C(H)Ph}, Ar=2,6-iPr₂C₆H₃). The formation of 5 can be explained by an oxidative addition of the divalent Si center in 2 with ArN₃ to afford a silaimine intermediate, which then reacts with another molecule of ArN₃ to give compound 5. The reaction of 2 with elemental sulfur in toluene afforded the chlorosilanethione [LSi(S)Cl] (6) and dithiodisiletane [{Ph(H)C(NtBu)₂}Si(μ-S)]₂ (7). Treatment of 2 with elemental selenium in THF afforded the di(silaneselone) [LSi(Se)Si(Se)L] (8). Evidently, the divalent Si center in 2 undergoes oxidative addition with chalcogens to afford a silylsilanechalcogenone intermediate, which then displaces “:Si{(NtBu)₂C(H)Ph}” and “ClSi{(NtBu)₂C(H)Ph}” to form 6 and 8, respectively. Moreover, compound 8 was synthesized by the reaction of [{PhC(NtBu)₂}Si:]₂ (10) with elemental selenium in THF. The results show that the reactions of 2 are initiated by oxidative addition of the divalent silicon center, and then the intermediate formed undergoes a rearrangement involving the diaminochlorosilyl substituent to form compounds 3–8. These products have been characterized by NMR spectroscopy and X-ray crystallography.

The synthesis and characterization of novel cis-1,2-disilylenylethene [cis-LSi{C(Ph)C(H)}SiL] (2; L=PhC(NtBu)₂) and a singlet delocalized biradicaloid [LSi(μ₂-C₂Ph₂)₂SiL] (3) are described. Compound 2 was prepared by the reaction of [{PhC(NtBu)₂}Si:]₂ (1) with one equivalent of PhCCH in toluene. Compound 3 was synthesized by the reaction of 1 with two equivalents of PhCCPh in toluene. The results suggest that the reaction proceeds through an [LSi{C(Ph)C(Ph)}SiL] intermediate, which then reacts with another molecule of PhCCPh to form 3. Compounds 2 and 3 have been characterized by X-ray crystallography and NMR spectroscopy. X-ray crystallography and DFT calculations of 3 show that the singlet biradicals are stabilized by the amidinate ligand and the delocalization within the “Si(μ₂-C₂Ph₂)₂Si” six-membered ring.

The synthesis and characterization of novel monomeric silylsilylenes [{PhC(NtBu)₂}SiSi{(NtBu)₂C(H)Ph}R] (R=Cl (2), H (4)) are described. Compound 2 was prepared by the treatment of [{PhC(NtBu)₂}SiHCl₂] (1) with two equivalents of potassium graphite, whereas compound 4 was synthesized by the treatment of 1 with four equivalents of potassium graphite. The results suggest that silicon(II) hydride intermediate [{PhC(NtBu)₂}SiH] was formed in the reduction, which underwent a hydrosilylation with the amidinate ligand of [{PhC(NtBu)₂}SiR] (R=Cl or H) to form 2 and 4, respectively. The existence of [{PhC(NtBu)₂}SiH] in solution was demonstrated by the treatment of [{PhC(NtBu)₂}SiCl] (3) with [K{HB(iBu)₃}]. Compounds 2 and 4 have been characterized by X-ray crystallography and NMR spectroscopy. The results show that compounds 2 and 4 are stable in solution or the solid state, and do not dimerize to form the corresponding disilene. DFT calculations show that the SiSi bonds in 2 and 4 do not have multiple-bond character.