Synthesis of ansa‐[2]Boracyclopentadienylcycloheptatrienylchromium and Its Reaction to the ansa‐Platinabis(boryl) Complex by Oxidative Addition of the BoronBoron Bond

Abstract

Metalation reactions are of great importance in metallocene chemistry since they provide access to a wide variety of functionalized sandwich complexes. Among those, the dilithiation of ferrocene [1] and related compounds such as bis(benzene)chromium, [2] for which an alkyllithium base is employed in the presence of a chelating amine (typically N,N,N',N'-tetramethylethylenediamine = tmeda), are the most prominent examples to accomplish this goal. Recently, Mulvey and co-workers reported the selective tetrametalation of ferrocene [3] and monometalation of bis-(benzene)chromium [4] by the use of mixed alkali-metalmagnesium amide bases.Our interest focused on cyclopentadienylcycloheptatrienylchromium [(h 5 -C 5 H 5 )Cr(h 7 -C 7 H 7 )] (trochrocene 1), [5] which is isoelectronic to bis(benzene)chromium and was intensively studied by Fischer and co-workers. [6,7] Although the metalation of trochrocene was the subject of a series of studies, in which either n-amylsodium [7] or mixtures comprising nBuLi/tmeda [8] were employed, there is no precedence for the metalation of both the Cp and the Cht ring. We achieved the double lithiation of trochrocene by using a tBuLi/tmeda mixture, which resulted in the formation of the base-stabilized compound [(h 5 -C 5 H 4 Li)Cr(h 7 -C 7 H 6 Li)]·tmeda (2) in yields of up to 80 %. The 1 H NMR spectrum of 2 is in agreement with an overall C S -symmetric compound in solution and features two signals for the Cp ligand (d = 3.51 and 3.86 ppm) and two signals for the Cht ring (d = 5.23-5.30 and 5.70 ppm) with a relative intensity of 2:2:4:2. However, the 13 C NMR spectrum exhibits only five resonances for the aromatic carbon atoms; the ipso-carbon atoms are undetectable at room temperature, a fact which is probably due to the quadrupolar moment of the Li atom. [9] Dilithiotrocrocene (2) was subsequently treated with stoichiometric amounts of Br(NMe 2 )BÀB(NMe 2 )Br to afford the [2]boratrochrocenophane 3, which was isolated from pentane solutions at À30 8C as green crystals in 35-45 % yields (Scheme 1). The most noteworthy feature in the 1 H NMR spectrum of 3 is the splitting of the Cp resonances

Alternative synthesis and crystal structure of a [2]boraferrocenophane

“…Related complexes of the later transition metals are characterized by a pronounced reactivity of the bridging B-B-bond [6,17], and thus, served as facile diborane(4) precursors for the metal mediated 1,2-diboration of unsaturated organic substrates under homogeneous and even heterogeneous conditions [18]. [(g 5 -C 5 H 4 )-B(NMe 2 )-B(NMe 2 )-(g 5 -C 5 H 4 )Fe] (1) was obtained as the first [2]borametalloarenophane in 1997 by Herberhold and Wrackmayer from dilithiated ferrocene and B 2 (NMe 2 ) 2 Cl 2 , and thoroughly characterized in solution.…”

“…Strained boron-bridged ansa-complexes are obtained from salt elimination reactions between 1,1 0 -dilithiometallocenes and suitable dihaloboranes, while the non-strained group 4 borametallocenophanes have to rely on the deprotonation of ligand precursors of the type RB(g 1 -C x H y ) 2 (C x H y = C 5 H 5 , C 9 H 7 , C 13 H 9 ) [14,15] and their subsequent reaction with a suitable metal halide. An extension of these methods to related reactions of 1,1 0 -dilithiometallocenes with 1,2-dihalodiboranes(4) [6,7,[16][17][18] and diborane(4) based ligand precursors with metal halides (Eq. (1b)) [19], respectively, has led to corresponding [2]diborametalloarenophanes.…”

1,1′‐Diborylferrocenes from [2]Boraferrocenophanes by Boron–Boron Exchange

“…1 H NMR (C 6 D 6 ): δ = 7.02-6.94 (m, 4 H, C 6 H 4 ), 6.83-6.75 (m, 4 H, C 6 H 4 ), 4.67 (pseudot, 4 H, C 5 H 4 ), 4.15 (pseudo-t, 4 H, C 5 H 4 ) ppm. 11 B NMR (C 6 D 6 ): δ = 33.9 ppm. 13 …”

“…As a result, we proved the potential, particularly of the former, to readily insert low valent platinum and palladium complex fragments into the boronboron bond by oxidative addition. This led to various metal-mediated functionalisations of azobenzene [8] and alkynes (Scheme 3), [9][10][11] and most notably, to the first diboration of alkynes under conditions of heterogeneous catalysis. [12] It should be noted that [2]stannaferrocenophanes were also found to insert alkynes via homogeneous catalysis.…”

Singular Metal Activation of Diboron Compounds

“…[131][132][133][134] Conversely, oxidative addition of the BdB bond within [2]borametalloarenophanes 38 was reported to proceed smoothly and in high yields with platinum(0) phosphine complexes to give the corresponding bis(boryl)platinum(II) complexes 39 (Scheme 37). [135][136][137] Subsequent addition of these complexes to alkynes gave the ansa-bis(boryl)alkene products.…”