Novel Stable Silyl, Germyl, and Stannyl Germanium(II) Compounds Containing an Amidinato Ligand

Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe3)3], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R'{Si(SiMe3)3}C(NR)2BCl [R' = Ph, R = Cy (3); R' = Ph, R = iPr (6); R' = tBu, R = Cy (8)], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy)2B(Ph)[Si(SiMe3)3]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11B NMR spectroscopic resonance and the distinct difference in the 29Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3, 6, and 8. Reaction of 3 with the Lewis acid AlCl3 led to the formation of complex 11 in which an unusual cleavage of one of the C-N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12, whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH3). All compounds (2-13) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.

Pyridylpyrrolido ligand in Ge(II) and Sn(II) chemistry: synthesis, reactivity and catalytic application

In our previous communication, we have reported the synthesis of a new chlorogermylene (B) featuring a pyridylpyrrolido ligand. This study details the preparation of a series of new germylenes and stannylenes starting from B. A transmetallation reaction between B and SnCl₂ led to the analogous chlorostannylene (1) with the simultaneous elimination of GeCl₂. This is a very unusual example of transmetallation between two elements of the same group. The preparation of 1via lithiation led to the formation of 2 as a side product, where the ortho C-H bond of the pyridine ring was activated and functionalized with a ⁿBu moiety. Subsequently, B and 1 were used as precursors to generate germylene (4) and stannylene (5) featuring tris(trimethylsilyl)silyl (hypersilyl) moieties. We also prepared tetrafluoropyridyl germylene (6) by reacting 4 with C₅F₅N with the simultaneous elimination of (Me₃Si)₃SiF by utilizing the fluoride affinity of the silicon atom. As there is scarcity of Sn(II) compounds as single-site catalysts, we investigated 5 as a catalyst towards the hydroboration of aldehydes, ketones, alkenes and alkynes. All the compounds have been characterized by single-crystal X-ray diffraction and by state of the art spectroscopic studies.

Low-Valent Germanylidene Anions: Efficient Single-Site Nucleophiles for Activation of Small Molecules

Rare mononuclear and helical chain low-valent germanylidene anions supported by cyclic (alkyl)(amino)carbene and hypermetallyl ligands were synthesised by stepwise reduction from corresponding germylene precursors via stable and isolable germanium radicals. The electronic structures of the anions can be described with ylidene and ylidone resonance forms with the Ge-C π-electrons capable of binding even weak electrophiles. The germanylidene anions reacted with CO2 to give μ-CO2 -κC:κO complexes, a rare coordination mode for low-valent germanium and inaccessible for the related neutral germylones. These results implicate low-valent germanylidene anions as efficient single-site nucleophiles for activation of small molecules.

Metal-metal bonded alkaline-earth distannyls

The first families of alkaline-earth stannylides [Ae(SnPh3)2·(thf) x ] (Ae = Ca, x = 3, 1; Sr, x = 3, 2; Ba, x = 4, 3) and [Ae{Sn(SiMe3)3}2·(thf) x ] (Ae = Ca, x = 4, 4; Sr, x = 4, 5; Ba, x = 4, 6), where Ae is a large alkaline earth with direct Ae-Sn bonds, are presented. All complexes have been characterised by high-resolution solution NMR spectroscopy, including 119Sn NMR, and by X-ray diffraction crystallography. The molecular structures of [Ca(SnPh3)2·(thf)4] (1'), [Sr(SnPh3)2·(thf)4] (2'), [Ba(SnPh3)2·(thf)5] (3'), 4, 5 and [Ba{Sn(SiMe3)3}2·(thf)5] (6'), most of which crystallised as higher thf solvates than their parents 1-6, were established by XRD analysis; the experimentally determined Sn-Ae-Sn' angles lie in the range 158.10(3)-179.33(4)°. In a given series, the 119Sn NMR chemical shifts are slightly deshielded upon descending group 2 from Ca to Ba, while the silyl-substituted stannyls are much more shielded than the phenyl ones (δ 119Sn/ppm: 1', -133.4; 2', -123.6; 3', -95.5; 4, -856.8; 5, -848.2; 6', -792.7). The bonding and electronic properties of these complexes were also analysed by DFT calculations. The combined spectroscopic, crystallographic and computational analysis of these complexes provide some insight into the main features of these unique families of homoleptic complexes. A comprehensive DFT study (Wiberg bond index, QTAIM and energy decomposition analysis) points at a primarily ionic Ae-Sn bonding, with a small covalent contribution, in these series of complexes; the Sn-Ae-Sn' angle is associated with a flat energy potential surface around its minimum, consistent with the broad range of values determined by experimental and computational methods.

Tetryl-Tetrylene Addition to Phenylacetylene

Phenylacetylene adds [Ar*GeH2 -SnAr'], [Ar*GeH2 -PbAr'] and [Ar'SnH2 -PbAr*] at rt in a regioselective and stereoselective reaction. The highest reactivity was found for the stannylene, which reacts immediately upon addition of one equivalent of alkyne. However, the plumbylenes exhibit addition to the alkyne only in reaction with an excess of phenylacetylene. The product of the germylplumbylene addition reacts with a second equivalent of alkyne and the product of a CH-activation, a dimeric lead acetylide, were isolated. In the case of the stannylplumbylene the trans-addition product was characterized as the kinetically controlled product which isomerizes at rt to yield the cis-addition product, which is stabilized by an intramolecular Sn-H-Pb interaction. NMR chemical shifts of the olefins were investigated using two- and four-component relativistic DFT calculations, as spin-orbit effects can be large. Hydride abstraction was carried out by treating [Ar'SnPhC=CHGeH2 Ar*] with the trityl salt [Ph3 C][Al(OC{CF3 })4 ] to yield a four membered ring cation.

Synthesis and reactivity of alkaline-earth stannanide complexes by hydride-mediated distannane metathesis and organostannane dehydrogenation

The synthesis of heteroleptic complexes with calcium- and magnesium-tin bonds is described. The dimeric β-diketiminato calcium hydride complex, [(BDI)Ca(μ-H)]2 (ICa) reacts with Ph3Sn-SnPh3 to provide the previously reported μ2-H bridged calcium stannanide dimer, [(BDI)2Ca2(SnPh3)(μ-H)] (3). Computational assessment of this reaction supports a mechanism involving a hypervalent stannate intermediate formed by nucleophilic attack of hydride on the distannane. Monomeric calcium stannanides, [(BDI)Ca(SnPh3)·OPPh3] (8·OPPh3) and [(BDI)Ca(SnPh3)·TMTHF] (8·TMTHF, TMTHF = 2,2,5,5-tetramethyltetrahydrofuran) were obtained from ICa and Ph3Sn-SnPh3, after addition OPPh3 or TMTHF. Both complexes were also synthesised by deprotonation of Ph3SnH by ICa in the presence of the Lewis base. The calcium and magnesium THF adducts, [(BDI)Ca(SnPh3)·THF2] (8·THF2) and [(BDI)Mg(SnPh3)·THF] (9·THF), were similarly prepared from [(BDI)Ca(μ-H)·(THF)]2 (ICa·THF2) or [(BDI)Mg(μ-H)]2 (IMg) and Ph3SnH. An excess of THF or TMTHF was essential in order to obtain 8·TMTHF, 8·THF2 and 9·THF in high yields whilst avoiding redistribution of the phenyl-tin ligand. The resulting Ae-Sn complexes were used as a source of [Ph3Sn]- in salt metathesis, to provide the known tristannane Ph3Sn-Sn(t-Bu)2-SnPh3 (11). Nucleophilic addition or insertion with N,N'-di-iso-propylcarbodiimide provided the stannyl-amidinate complexes, [(BDI)Mg{(iPrN)2CSnPh3}] (12) and [(BDI)Ca{(iPrN)2CSnPh3}·L] (13·TMTHF, 13·THF, L = TMTHF, THF). The reactions and products were monitored and characterised by multinuclear NMR spectroscopy, whilst for compounds 8, 9, 12, and 13·THF, the X-ray crystal structures are presented and discussed.

Deprotonation of Organogermanium and Organotin Trihydrides

Terphenyltin and terphenylgermanium trihydrides were deprotonated in reaction with strong bases, such as LiMe, LDA, or KBn. In the solid state, the Li salts of the germate anion 4 and 4a exhibit a Li-Ge contact. In the Li salt of the dihydridostannate anion 6a, the Li cation is not coordinated at the tin atom instead an interaction of the Li cation with the hydride substituents was found. Evidenced by ¹H-⁷Li-HOESY NMR spectroscopy the Li-salt of the deprotonated tin hydride 6a exhibits in toluene solution a contact between Li cation and hydride substituents, whereas in the ¹H-⁷Li-HOESY NMR spectrum of the homologous germate salt 4a, no crosspeak between hydride and Li signals was found. The organodihydridogermate and -stannate react as nucleophiles with low-valent Group 14 electrophiles. Thus, three compounds were synthesized: Ar-Ë'-EH₂-Ar (E', E = Sn, Ge; Pb, Ge; Pb, Sn; Ar = Ar', Ar*). Following an alternative synthesis Ar'SnH₂PbAr* was synthesized in reaction between [(Ar*PbH)₂] and [(Ar'SnH)₄] generated in situ. In reaction between low-valent organotin hydride [(Ar*SnH)₂] and organdihydridostannate [Ar*SnH₂]^- formation of distannate [Ar*₂Sn₂H₃]^- was found.

Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(i) compounds: multipurpose reagents for organometallic synthesis

Reactions of three magnesium(i) dimers, [{(ArNacnac)Mg-}2] (ArNacnac = [(ArNCMe)2CH]-; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-methylstyrene (MS), trans-stilbene (TS) or diphenylacetylene (DPA) led to the 1,2-addition of the Mg-Mg bond across the substrate, giving rise to the 1,2-dimagnesioethanes, [{(XylNacnac)Mg}2(μ-DPE)], [{(DepNacnac)Mg}2(μ-MS)], [{(ArNacnac)Mg}2(μ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{(MesNacnac)Mg}2(μ-DPA)]. The reactions involving the 1,1-substituted alkenes are shown to be readily redox reversible, in that the reaction products are in equilibrium with a significant proportion of the starting materials at room temperature. Variable temperature NMR spectroscopy and a van't Hoff analysis point to low kinetic barriers to these weakly exergonic reactions. [{(MesNacnac)Mg}2(μ-DPE)] and [{(MesNacnac)Mg}2(μ-DPA)] behave as 1,2-di-Grignard reagents in their reactions with very bulky amido-zinc bromides, yielding the first examples of a 1,2-dizincioethane, [(L*Zn)2(μ-DPE)] (L* = -N(Ar*)(SiPri 3); Ar* = C6H2Me{C(H)Ph2}2-4,2,6), and a 1,2-dizincioethene, [(TBoLZn)2(μ-DPA)] (TBoL = -N(SiMe3){B(DipNCH)2}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{(MesNacnac)Mg}2(μ-DPE)], which behaves as a two-electron reducing agent when treated with amido-cadmium and amido-magnesium halide precursors, yielding the cadmium(i) and magnesium(i) dimers, [PhBoLCdCdPhBoL] (PhBoL = -N(SiPh3){B(DipNCH)2}) and [L†MgMgL†] (L† = -N(Ar†)(SiMe3); Ar† = C6H2Pri{C(H)Ph2}2-4,2,6), respectively. A further class of reactivity for [{(MesNacnac)Mg}2(μ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, which gives a magnesio-germane, presumably via intramolecular C-H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(MesNacnac)}]. [{(MesNacnac)Mg}2(μ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.

Reaction of stannylene phosphorus Lewis pairs with dichlorides of germanium, tin and lead - the formation of base stabilized stannyl stannylenes/germylenes and redox reaction with PbCl2

The reaction of intramolecular stannylene phosphorus Lewis pairs with heavier dichlorides of group 14 (GeCl2, SnCl2, PbCl2) is reported. Phosphine base stabilized stannyl germylenes/stannylenes were formed by the oxidative addition of an E-Cl bond to the stannylene tin atom (E = Ge, Sn). In solution, a dynamic equilibrium between two diastereomeric configurations was observed. With PbCl2 a redox reaction towards elemental lead and the dichlorinated tin(iv) compound was found. All compounds were characterized by X-ray diffraction, NMR spectroscopy and elemental analysis.

First Insertions of Carbene Ligands into Ge-N and Si-N Bonds

The insertion of carbene ligands into Ge-N (three examples) and Si-N (one example) bonds has been achieved for the first time by treating Fischer carbene complexes (M=W, Cr) with bulky amidinatotetrylenes (E=Ge, Si). These reactions, which start with a nucleophilic attack of the amidinatotetrylene heavier group 14 atom to the carbene C atom, proceed through a stereoselective insertion of the carbene fragment into an E-N bond of the amidinatotetrylene ENCN four-membered ring, leading to [M(CO)₅ L] derivatives in which L belongs to a novel family of tetrylene ligands comprising an ECNCN five-membered ring.

Reactivity of an amidinato silylene and germylene toward germanium(ii), tin(ii) and lead(ii) halides

The coordination chemistry of an amidinato silylene and germylene toward group 14 element(ii) halides is described. The reaction of the amidinato silicon(ii) amide [LSiN(SiMe₃)₂] (1, L = PhC(NtBu)₂) with SnCl₂ and PbBr₂ afforded the amidinato silylene-dichlorostannylene and -dibromoplumbylene adducts [L{(Me₃Si)₂N}SiEX₂] (E = Sn, X = Cl (2); E = Pb, X = Br (3)), respectively, in which there is a lone pair of electrons on the Sn(ii) and Pb(ii) atoms. X-ray crystallography, NMR spectroscopy and theoretical studies show conclusively that the Si(ii)-E(ii) bonds are donor-acceptor interactions. Similar electronic structures were found in the amidinato germylene-dichlorogermylene and -dichlorostannylene adducts [L{(Me₃Si)₂N}GeECl₂] (E = Ge (5), Sn (6)), which were prepared by treatment of the amidinato germanium(ii) amide [LGeN(SiMe₃)₂] (4) with GeCl₂·dioxane and SnCl₂, respectively.

Reactivity of lithium n-butyl amidinates towards group 14 metal(II) chlorides providing series of hetero- and homoleptic tetrylenes

The new class of homo- and heteroleptic n-butyl-N,N'-disubstituted amidinato group 14 metal(II) complexes were prepared by salt elimination from starting lithium amidinates and metal(II) chlorides both in stoichiometric ratio 2:1 and 1:1, respectively. The target amidinates contain less bulky isopropyl or cyclohexyl as well as a sterically demanding aromatic substituent. Desired 1:1 Pb(II) complexes are not accessible by the described procedure. Ligand transfer from Pb to Sn is taking place if homoleptic Pb(II) compounds are reacted with SnCl(2). Prepared tetrylenes were characterized by (1)H, (13)C, (119)Sn and (207)Pb NMR spectroscopy in C(6)D(6) or THF-d(8). X-Ray diffraction studies of one heteroleptic Ge(II) monomeric where the coordination polyhedron of the three coordinated germanium atoms is a trigonal pyramid, two different dimeric structures of heteroleptic Sn(II) complexes, one amidine hydroiodide byproduct and the oxidation product of the heteroleptic chloro Sn(II) amidinate as a tetranuclear species with two Sn(IV) and two Sn(II) atoms in central Sn(2)O(2) planar ring were performed on appropriate single crystals. The dimer of one of the heteroleptic stannylenes reveals a new type of monomeric units connection, weak Sn-Cl contact and an interaction of the tin atom with delocalized N-C(C)-N system of the amidinato ligand of the second molecule.