A bromo(boryloxy) silylene and its heavier analogues.

A N-heterocylic boryloxy ligand equipped with bulky diazaboryl N-substituents is used to synthesize divalent and dicoordinate group 14 compounds which represent the first examples of acyclic halo(boryloxy) tetrylenes. The bromo(boryloxy) silylene reacts swiftly with benzophenone to a siloxindane.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents.

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Ferrocene-Based N-Heterocyclic Plumbylenes [Fe{(η5 -C5 H4 )NSiMe2 R}2 Pb:]: Influence of the Steric Demand of the N-Substituents on Their Dimerization via C-H Activation with PbII.

Ferrocene-based N-heterocyclic plumbylenes fc[(NSiMe2 R)2 Pb:] (1; fc=1,1'-ferrocenylene) are easily accessible by transamination from [(Me3 Si)2 N]2 Pb and the corresponding 1,1'-diaminoferrocene derivatives fc(NHSiMe2 R)2 . They may form unconventional dimers 2 by a process, which causes the cleavage of a cyclopentadienyl C-H bond and the formation of a Pb-C and an N-H bond. The monomer-dimer equilibrium (2 1⇆2) has been addressed experimentally and computationally. It critically depends on the steric demand of the N-substituents SiMe2 R, which has been varied systematically by using homologues with aliphatic (R=methyl, ethyl, isopropyl, tert-butyl) and aromatic units (R=phenyl, mesityl, ferrocenyl). Even in the sterically least congested case (R=methyl), dimerization is only slightly exergonic. It eventually becomes prohibitively endergonic with increasingly larger substituents and is thus not observed for R=tert-butyl, mesityl, and ferrocenyl. R=phenyl represents a borderline case, where the dimer is still detectable in the equilibrium mixture, albeit as a very minor component, in accord with the slightly endergonic Gibbs free energy change calculated for its formation. Addition of 4-dimethylaminopyridine (DMAP) to the monomer-dimer equilibrium mixtures cleanly affords the corresponding adducts [1(DMAP)], irrespective of the equilibrium composition.

Reactions of Mesityl Azide with Ferrocene-Based N-Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh2 -Functionalised Congeners.

The reactivity of ferrocene-based N-heterocyclic tetrylenes [{Fe(η5 -C5 H4 -NSitBuMe2 )2 }E] (E=Ge, Sn, Pb) towards mesityl azide (MesN3 ) is compared with that of PPh2 -functionalised congeners exhibiting two possible reaction sites, namely the EII and PIII atom. For E=Ge and Sn the reaction occurs at the EII atom, leading to the formation of N2 and an EIV =NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow-up products, either by [2+3] cycloaddition with MesN3 or, in the PPh2 -substituted case, by NMes transfer from the SnIV to the PIII atom. Whereas [{Fe(η5 -C5 H4 -NSitBuMe2 )2 }Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh2 -substituted congener forms an addition product with MesN3 , thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Planar-chiral 1,1'-diaminoferrocenes.

Planar-chiral homologues of 1,1'-diaminoferrocene, which bear a single additional substituent adjacent to one of the amino groups, are prepared as racemic mixtures in a few steps and in good yields from ferrocene. Various substituents relevant to steric shielding, coordination and further functionalisation are used, giving access to ferrocene-based planar-chiral diimines and diamines as well as stable N-heterocyclic carbenes and tetrylenes by transformations analogous to procedures established for 1,1'-diaminoferrocene.

Reactive Dimerization of an N-Heterocyclic Plumbylene: C-H Activation with PbII.

The N-heterocyclic plumbylene [Fe{(η5 -C5 H4 )NSiMe3 }2 Pb:] is in equilibrium with an unprecedented dimer in solution, whose formation involves the cleavage of a strong C-H bond and concomitant formation of a Pb-C and an N-H bond. According to a mechanistic DFT assessment, dimer formation does not involve direct PbII insertion into a cyclopentadienyl C-H bond, but is best described as an electrophilic substitution. The bulkier plumbylene [Fe{(η5 -C5 H4 )NSitBuMe2 }2 Pb:] shows no dimerization, but compensates its electrophilicity by the formation of an intramolecular Fe-Pb bond.

New Stable and Persistent Acyclic Diaminocarbenes.

The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr2N)CH(NRR')][PF6], for use as immediate carbene precursors. The corresponding ADACs (iPr2N)C(NRR') were sufficiently stable for isolation in the case of NRR' = 2-methylpiperidino (13), 3-methylpiperidino (14), 4-methylpiperidino (15), morpholino (17) and NiPrPh (20), but had to be trapped in situ in the case of NRR' = 2,2,6,6-tetramethylpiperidino (12) and NiPrMe (19). The tetraaryl-substituted ADACs (Ph2N)2C (22) and (Ph2N)C[N(C6F5)2] (24) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(µ-Cl)(cod)}2] did not work for 12 and 24. The (77)Se NMR chemical shifts, δ((77)Se), of the selenourea compounds derived from the new ADACs lie in the range 450-760 ppm, which indicates a much higher electrophilicity and π-accepting capability of ADACs in comparison with NHCs, which typically exhibit δ((77)Se)<200 ppm. The extreme low-field shift of 758 ppm observed for 12Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6-tetramethylpiperidino unit perpendicular to the N2C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.