Walter Siebert

Compounds containing a planar-tetracoordinate carbon atom as analogues of planar methane

Over the past years the number of examples of compounds containing a planar-tetracoordinate carbon atom has increased. However, the presence of a carbon atom with a 360° sum of angles does not imply that the species is a derivative of planar methane; there must be an appropriate electronic stabilization. In the case of complexes 21a and 21b the central carbon atom is indeed stabilized by σ-donors and π-acceptors, as required for planar methane.

Metallocenylborane III. Darstellung und eigenschaften von ferrocenyl- und cymantrenylboranen

Abstract Ferrocene, cymantrene and methylcymantrene react with BI 3 , BBr 3 , C 6 H 5 BI 2 and CH 3 BI 2 in boiling CS 2 or C 6 H 12 forming air-sensitive metallocenylhaloboranes. The direct dichloroborylation is only possible with ferrocene. Starting from metallocenyliodoboranes the corresponding substituted metallocenylboranes are obtained by halogen exchange with AsF 3 or AsCl 3 , by methylation with Sn(CH 3 ) 4 , by ether cleavage of (C 2 H 5 ) 2 O, by redox reaction with (CH 3 S) 2 and by reaction with R 2 NH. 1 H and 13 C NMR spectra indicate that in contrast to cymantrenylhaloboranes, in ferrocenylhaloboranes the 3,4-protons are more deshielded than the 2,5-protons. The metallocenylboranes, isoelectronic with α-metallocenylcarbenium ions, are weaker Lewis acids than phenylboranes; they form donor-acceptor compounds with pyridine and dimethylsulfane, respectively.

Borane‐Substituted Imidazol‐2‐ylidenes: Syntheses, Structures, and Reactivity☆

Reactions of the Lewis acids BH3 and BEt3 with trimethylimidazole (1) lead to the borane adducts 2a and 2b. Deprotonation of 2a with n-butyllithium results in the formation of the novel N-borane-substituted imidazol-2-ylidene anion 3a– whereas deprotonated 2b rearranges unexpectedly to the anionic compound 3b–. This can be transformed into the carbene–borane adduct 4 by methylation. The reaction of 3a– with [Mn(CO)5Br] yields the carbene complex 5. Surprisingly, 3a– attacks Fe(CO)5 at a carbon atom which leads to the iron acyl complex anion 6–. The compositions of the products follow from spectroscopic and analytical data and from X-ray structure analyses for Li(thp)+3a–, Li(thf)2+3b– and Li(thp)3+6–.

Diboryläthylenverbindungen als liganden in metallkomplexen ☆: V. Thiadiborolen-tricarbonyleisen-komplexe: darstellung, struktur- und bindungsverhältnisse

Abstract 3,4-Diethyl-1,2,5-thiadiborolenes (Ia—g, k) and Fe 2 (CO) 9 react to give stable thiadiborolenetricarbonyliron complexes (IIa—g, k). In IIa the BI function can be substituted by Me 2 NH, Et 2 O, AsF 3 and LiBH 4 to yield IIe, f, h and IIi, which leads to the synthesis of the unstable ligands 2,5-dihydro- and 2-fluoro-5-iodo 1,2,5-thiadiborolene in IIh and IIi. Rapid oxidation of IIe occurs with iodine liberating 2,5-bis(methylthio)-1,2,5-thiadiborolene, whereas IIa is only slowly attacked. From nuclear magnetic resonance ( 1 H, 11 B) and infrared spectroscopic data it is shown that the bonding situation between the ligand and the Fe(CO) 3 fragment is characteristically influenced by the Lewis acidity of the boryl group. The ligand acts as a two-electron acceptor, the complexation is accompanied by the formation of the thiadiborolene dianion. The X-ray structural-analyses of Ie and IIe exhibit a significant shortening of the BC-bond distances (0.04 A) Upon complexation of the planar 2,5-bis(dimethylamino)-1,2,5-thiadiborolene (Ie). Ie crystallizes in the space group Pccn with a = 5.6274(4) b = 14.9824(9), c = 15.921(1) A and four molecules per unit cell. The tricarbonyliron complex IIe crystallizes in the space group. P 1 with a = 9.0310(8), b = 9.7214(8), c = 11.0198(8) A, a = 74.598(8), β = 89.658(9), γ = 74.744°, and two molecules per unit cell.

Synthesis and Reactivity of Monoborylacetylene Derivatives

The catechol-substituted monoborylacetylenes 1a−d are obtained from the reaction of bis(diisopropylamino)borylacetylene with catechol derivatives and 2,2′-biphenol. The catalytic trimerization of 1a−d with [(η5-C5H5)Co(CO)2] yields isomeric mixtures of the triborylbenzene derivatives 2a,2a′, 2b,2b′, and 2c,2c′. The reaction of 2a,2a′ with mesityllithium provides the hexamesityl-substituted 1,3,5-triborylbenzene 2e. Hydroboration of 1a with catecholborane affords a mixture of 1,1-bis(1,3,2-benzodioxaborol-2-yl)ethene (3a) and trans-1,2-bis(1,3,2-benzodioxaborol-2-yl)ethene (4a). Hydroboration of 1a and 3a with one or two mol of HBCl2 and subsequent substitution of the chlorine atoms of the product with catechol leads in each case to the 1,1,1-trisborylmethane derivative 5a in 83 and 78% yield, respectively, which forms the tris(THF) adduct 5a(thf)3. Treatment of 5a with tBuLi yields 1,1,1-tris[di(tert-butyl)boryl]ethane 6a. [Co2(CO)8] reacts with 1a to give 3-(1,3,2-benzodioxaborol-2-yl)-1,2-bis(tricarbonylcobalta)tetrahedrane (9a). The new compounds have been characterized by NMR spectroscopy and mass spectrometry as well as by X-ray structure analyses for 1a, 3a, 5a, 5a(thf)3, and 9a.

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH 3 ·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH 3 adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li + ( 1b – d ) − . In the solid state (thf)Li + 1b − is dimeric. The acyl–iron complexes (thf) 3 Li + ( 3b , d ) − are formed from (thf)Li + ( 1b , d ) − and Fe(CO) 5 . (L)Li + ( 1a – c ) − react with [CpFe(CO) 2 X], however, the only complex obtained is [CpFe(CO) 2 1a ] (5a ). The analogous reaction of (L)Li + 1a − with the pentadienyl complex [(C 7 H 11 )Fe(CO) 2 Br] yields the corresponding iron compound 6a . Their compositions follow from spectroscopic data. Treatment of Cp 2 TiCl with (L)Li + 1a − leads to [Cp 2 Ti 1a ] ( 7a ), which could not be oxidized with PbCl 2 to give the corresponding Ti(IV) complex. The compounds [Li(py) 4 ] + 9a − and [Li(L) 4 ] + ( 10b – d ) − are obtained when (L)Li + 1 − are reacted with VCl 3 and ScCl 3 . The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V( 1a ) 4 ] − with two smaller and four larger CVC angles. The scandium compound [Li(dme) 2 + 10c − ] has a different structure: the distorted tetrahedron of the anion [Sc( 1c ) 4 ] − contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c − is formally a 16e complex.

Boron Heterocycles as Ligands in Transition-Metal Chemistry

Publisher Summary This chapter discusses the ligand properties of Lewis acid boron heterocycles and the newest developments in the expanding field of triple- and tetra-decker sandwich compounds. Replacement of carbon atoms by boron atoms in π-carbocyclic and heterocyclic systems leads to a wide variety of planar Lewis acid boron heterocycles. Six-membered Lewis acid ligands can be derived from benzene by substituting boron, nitrogen, and sulfur for carbon. As an alternative to the aromaticity formalism commonly used, compounds containing electron-deficient elements in addition to metal atoms can be regarded as clusters. In clusters composed solely of main-group elements, the following electronic relationship among closo, nido, and arachno structures has been found. In the paramagnetic electron-rich bis(borinato)cobalt complexes, the labilized metal-ligand interaction allows ligand exchange and transfer to other metals. Cyclic voltammetry of (borinato)cyclopentadienyliron and of bis(borinato) complexes of iron, chromium, and vanadium revealed oxidation of the iron complexes with varying degrees of reversibility, depending on the solvent. Between benzene and borazine, several isomeric boron-carbon-nitrogen heterocycles would be expected to exist as potential η 6 -ligands. In comparison with nitrogen, a sulfur atom is a “soft” electron donor, and therefore sulfur-containing ligands should readily form transition-metal complexes. The complexes decompose slowly in C 6 H 6 and CH 2 CI 2 solution but are stable in dioxane. The most interesting feature of electron-poor boron heterocycles in complex chemistry is their use as bridging ligands in di- and trinuclear complexes.

Coordinated B2 Bridges in Porphyrins—Unexpected Formation of a Diborane(4)‐ from a Diborylporphyrin

Fitting B2 bridges in porphyrins can be achieved by reaction of dilithiated porphyrins with B2 Cl4 as well as by reductive elimination from diborylporphyrins. Coordination takes place under rectangular distortion of the porphyrin framework.

Blue Tetrakis(diisopropylamino)‐cyclo‐tetraborane and Yellow Tetrakis(tetramethylpiperidino)tetrabora‐tetrahedrane

The colored boranes closo-1 and cyclo-2 are obtained upon dehalogenation of sterically hindered diaminodichlorodiborane(4) 3. The TMP groups of 1 cause the formation of a tetrahedrane, whereas in 2 the diisopropylamino substituents stabilize the bent four-membered ring. TMP=2,2,6,6-tetramethylpiperidino.

Borane-stabilized Boranediyls (Borylenes): Neutral nido-1-Borane-2,3,4,5,6-pentamethyl-2,3,4,5,6-pentacarbahexaboranes(6)

The reaction of (C5Me5)(2)Si Or (C5Me5)SiMe3 with B2Cl4 leads to the adducts (C5Me5)B-->BCl2SiCl3 (5a) or (C5Me5)B-->BCl3 (5b), in which the Lewis acids stabilize the boranediyl fragments. Spectroscopic data and X-ray structure analyses confirm the adducts.