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Phase transfer catalysis has been adapted to the synthesis of triisopropylcyclopentadiene isomers 1 in high yield and purity. Alkylation of sodium triisopropylcyclopentadienides with isopropyl bromide in tetrahydrofuran has been shown to be the currently most efficient way to generate tetraisopropylcyclopentadiene isomers 2, which give sodium tetraisopropylcyclopentadienide (3) upon metalation. The crucial step for the introduction of the fifth isopropyl group, the selective attack of an iminium salt at the tetraisopropylcyclopentadienide anion in the 5-position with generation of the corresponding 1,2,3,4-tetraisopropyl(6-dimethylamino)fulvene ( 4), has been carried out according to a published procedure. Addition of 1-naphthyllithium to the dimethylaminofulvene 4 yielded extremely bulky lithium dimethylamino-(1-naphthyl)methyltetraisopropylcyclopentadienide (7). Pure 1,2,4,1′,2′,4′-hexaisopropylnickelocene (8) was obtained from sodium triisopropylcyclopentadienide and nickel(II) bromide. The tetraisopropylcyclopentadienylnickel(II) bromide [(C 5

A reaction of the bulky alkylcyclopentadienyliron(II) high-spin complex [Cp 000 Fe(μ-Br)] 2 (1a) (Cp 000 = C 5 H 2 (CMe 3 ) 3 -1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex [{Cp 000 Fe} 2 (μ,η 5 :η 5 -H 5 C 6 dC 6 H 5 )] (2) with a bridging bis(cyclohexadienylidene) ligand. Its structural analysis shows a centrosymmetric dimer. Each tri(tert-butyl)cyclopentadienyliron fragment is η 5coordinated to a cyclohexadienylidene moiety in which one carbon atom is bent out of the plane by 0.39 A ˚, exhibiting a bond length of 1.370 A ˚to its symmetry equivalent. Electrospray ionization mass spectra (ESI-MS) from acetonitrile solution confirm nicely the elemental composition of 2 by way of their isotope patterns. Reaction of 1a or its tetraisopropylcyclopentadienyl analogue [ 4 CpFe(μ-Br)] 2 (1b) ( 4 CpdC 5 H(CHMe 2 ) 4 ) with 2,6-diisopropylphenylmagnesium bromide affords the extremely air-sensitive, paramagnetic σ-aryl complexes [Cp 000 Fe(C 6 H 3 i Pr 2 )] (3a) or [ 4 CpFe(C 6 H 3 i Pr 2 )] (3b), whose 4 Cp-Fe distance of 1.92 A ˚is typical for cyclopentadienyliron high-spin complexes. In reactions with copper(I) halides 3a is rearranged to a diamagnetic π complex and coordinated via the ipso carbon atom of the six-membered ring to copper(I) halide fragments to form heterodinuclear complexes [Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )CuCl] (4-Cl) and [Cp 000 Fe( μ,η 5 :η 1 -C 6 H 3 i Pr 2 )CuBr] (4-Br). ESI mass spectra of complexes 4 do not show the molecular cations, but fragmentation to cyclopentadienyliron arene cations and formation of the hexa(tert-butyl)ferrocenium cation on one hand and fusion of complex fragments to oligonuclear complexes with or without inclusion of oxygen or fragments of solvent molecules on the other hand. Three of these oligonuclear complexes formed under the conditions of the ESI-MS experiment, whose elemental composition could be derived from isotope patterns, have been interpreted as [Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )Cu(μ,η 1 :η 5 -OC 6 H 3 i Pr 2 )FeCp 000 ] þ and [{Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )Cu} 2 X] þ (X=Cl, Br). DFT calculations support the structural analysis of 2 and predict the structure of the dication 2 2þ . The crystal structures obtained by X-ray diffraction for 2 and 3b are reported.

The σ‐aryliron complexes [5CpFe(C6H3iPr2‐2,6)] (2, 5Cp = η5‐C5iPr5) and [4CpFe(C6H3iPr2‐2,6)] (6, 4Cp = η5‐C5HiPr4) are available from the halides [5CpFe(μ‐Br)]2 (1) and [4CpFe(μ‐Br)]2 (4) and have been used for σ/π rearrangement reactions with trialkylaluminum compounds. Whereas trimethylaluminum can be added to 2 and 6 to form the aryltrimethylaluminate complexes [5CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlMe3] (3) and [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlMe3] (7) in clean σ/π rearrangement reactions, the addition of tripropylaluminum to 6 afforded a mixture containing the aryl(bromo)dipropylaluminate complex [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlBr(C3H7)2] (8). With triethylaluminum, a similar product mixture was obtained and could be separated to yield the aryl(bromo)diethylaluminate [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6) AlBr(C2H5)2] (9), the aryltriethylaluminate [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)Al(C2H5)3] (10), the arene complex [4CpFe(η6‐C6H4iPr2‐1,3)]Br (11), and the 2‐butyne complex [(4CpFe)2(μ,η2:η2‐MeCCMe)] (12). The formation of 12 involves the coupling of two ethyl groups with the loss of four hydrogen atoms and could also be observed in reactions of the bromide dimer 4 or the σ aryl complex 6 with ethylmagnesium bromide. The aluminates 3, 7, 8, and 10, the arene sandwich 11, the 2‐butyne complex 12, as well as the alkylcyclopentadienyliron(II) halides [5CpFe(μ‐Br)]2 (1), [4CpFe(μ‐Br)]2 (4), and [4CpFe(μ‐I)]2 (5) have been crystallographically characterized.

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.

Four new acyclic diaminocarbenes (ADACs), viz. [(cyclo‐CnH2n−1)2N]2C (n=5–7) and iPr2N‐C‐N(cyclo‐C6H11)2, were synthesised by reacting the corresponding formamidinium hexafluorophosphates with NaN(SiMe3)2. Their nucleophilicities and electrophilicities were respectively judged from the 1JCH values determined for the N2CH unit of the corresponding formamidinium cations and from the 77Se NMR chemical shifts of the selenourea derivatives obtained from the reaction of elemental selenium with the corresponding ADACs. An ambiphilic profile essentially identical to that of the “Alder carbene” (iPr2N)2C was found in each case. Similar to the latter carbene, the new ADACs undergo a well‐defined thermal decomposition by β‐fragmentation, affording an alkene and a formamidine. The stabilities of [(cyclo‐CnH2n−1)2N]2C depend strongly on the value of n, following the order 6>5>7, with the latter congener being too unstable for isolation. [(cyclo‐C6H11)2N]2C shows no thermal decomposition at room temperature in solution and is thus significantly more stable than (iPr2N)2C. The stability of iPr2N‐C‐N(cyclo‐C6H11)2 is intermediate between that of (iPr2N)2C and [(cyclo‐C6H11)2N]2C, its β‐fragmentation selectively affording propene and iPrN=CH‐N(cyclo‐C6H11)2. [(cyclo‐CnH2n−1)2N]2C (n=5–7) react readily with CO under mild conditions, selectively affording trisubstituted spirocyclic β‐lactam derivatives with an antimicrobial activity spectrum similar to that of penicillin G.

Close relatives! The Pd‐catalyzed hydrogenation of the “isolated” CC double bonds of dibenzo[c,g]fluorenide coordinated to an FeII center confirms that the electronic structure of this ligand in organometallic compounds is related to that of cyclopentadienide. The resulting 3,4,3′,4′‐tetrahydrodibenzo[c,g]fluorenide is a new chiral cyclopentadienide ligand (see picture for the molecular structure of the Fe complex).

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