Synthesis, Structure, and Dynamic Behavior of <i>ansa</i>-Ferrocenes with Pyrazabole Bridges^†

A variety of stereorigid ansa-ferrocenes 2 with o-phenylene-type bridges have been obtained by the reaction of 1,1‘-diborylferrocenes (1,1‘-Fc(BBrR)2; Fc = ferrocenyl; R = CH3, Br, OEt, NC4H8) with selected pyrazole derivatives. Subsequent intramolecular B−N adduct formation establishes the interannular bridges. The synthetic approach is based on the replacement of certain critical carbon−carbon links by the isoelectronic, self-assembling, dative boron−nitrogen bonds and thus follows the principles of noncovalent synthesis. The pyrazabole bridge does not seem to cause any significant distortion of the ferrocene core. Most substituents at boron or pyrazole lead to an ansa-ferrocene structure. However, the presence of both strong electron donors (i.e. pyrrolidine) at boron and strongly electron-withdrawing pyrazole substituents (i.e. ester groups) results in an open chain structure 2i with three-coordinate boron atoms. The tetravalent boron centers in ansa-1,1‘-Fc[B(Br)(μ-pz)]2, 2a, show an unusually high tendency to undergo nucleophilic substitution reactions, even though they are located at bridgehead positions. NMR spectroscopy, as well as chemical evidence argue against an unbridged open chain intermediate in the course of these reactions

Ferrocenophanes with Interannular Boron−Phosphorus Bridges: Synthesis, Structure, and Reactivity toward Nitrogen Bases

Starting from readily available diborylferrocenes 1,1‘-fc[B(Br)R]2 (fc = (C5H4)Fe(C5H4); R:  Br (1a), Me (1b), OEt (1d)), ferrocenophanes 1,1‘-fc[B(μ-PPh2)R]2 (R = Br (2a), Me (2b), PPh2 (2d)) and 1,1‘-fc[B(μ-PPh2)OEt][B(μ-PPh2)PPh2] (2c) are obtained upon reaction with LiPPh2. In contrast, 1,1‘-fc[B(Br)pyr]2 (pyr = pyrrolidinyl, 1e) and 2 equiv of LiPPh2 give an open-chain ferrocene 1,1‘-fc[B(PPh2)pyr]2 (2e). The dynamic behavior of the B2P2-bridged species has been investigated using variable-temperature NMR spectroscopy and the Forsén−Hoffman spin saturation transfer method. The reaction of 2a with Lipz (pz = pyrazolide) gives the ansa-ferrocene 1,1‘-fc[B(μ-PPh2)PPh2][B(μ-pz)Br] (3a), featuring an interannular (BNNBP) heterocyclic bridge. When Lipz* (pz* = 4-bromo-3,5-dimethylpyrazolide) was used rather than Lipz, two products could be isolated:  3b, which is analogous to 3a, and 1,1‘-fc[B(μ-PPh2)Br][B(μ-pz)Br] (3c), which bears Br substituents at both bridgehead boron centers. 2d and 3c have been characterized by X-ray crystallography. Treatment of 2a with excess γ-picoline (pic) results in an ansa-bridge opening and bromide substitution with the formation of the ionic species {1,1‘-fc[B(PPh2)pic2]2}Br2 (5a). The presence of three-coordinate phosphorus centers in 5a was confirmed by showing their ability to bind Cr(CO)5 fragments, which afforded the trimetallic complex 5b

1:1 Adduct of 1,1‘-Bis(dibromoboryl)ferrocene and 3,3‘,4,4‘-Tetramethyl-1,1‘-diphosphaferrocene

The Lewis acid−base adduct between 1,1‘-bis(dibromoboryl)ferrocene and 3,3‘,4,4‘-tetramethyl-1,1‘-diphosphaferrocene has been characterized by multinuclear variable-temperature NMR spectroscopy and single-crystal X-ray diffraction analysis. Irrespective of the crystallization conditions applied, the molecular structure of the adduct is that of an open-chain dinuclear complex featuring only one P−B bond

A Chloro-Bridged Dimanganese Complex and an Oxo-Bridged Dititanium Complex of a Ditopic Bis(pyrazol-1-yl)borate Ligand

The synthesis and crystal structure analysis of the ditopic p-phenylene-bridged bis(pyrazol-1-yl)borate [[p-C6H4(Bpz2tBu)2]Li2] (LLi2; pz = pyrazol-1-yl) is described. A salt metathesis reaction between LLi2 and MnCl2 in THF leads to the dinuclear complex [L[Mn(THF)]2(μ-Cl)2] featuring a central diamond MnII−(μ-Cl)2−MnII core (X-ray crystal structure analysis). Treatment of LLi2 with 2 equiv of [Ti(NMe2)3Cl] gives the dinuclear titanium compound [L[Ti(NMe2)3]2]. Upon reaction of LLi2 with [Ti(NMe2)2Cl2] and water, the μ-oxo-bridged dititanium species [L[Ti(NMe2)Cl]2(μ-O)] is obtained in excellent yield (X-ray crystal structure analysis)

Ru-Catalyzed Benzannulation Leads to Luminescent Boron-Containing Polycyclic Aromatic Hydrocarbons

A series of boron-containing polycyclic aromatic hydrocarbons (PAHs) have been synthesized through the Ru-catalyzed cyclization of aryl ene-ynes. The benchtop-stable products show deep blue photoluminescence. Reversible electrochemical reduction is possible at moderate electrode potentials (about −2.0 V vs FcH/FcH⁺); some of the compounds also underwent reversible oxidation. The systematic expansion of the PAH scaffolds permitted the analysis of even subtle structure–property relationships

Boron−Nitrogen Coordination Polymers Bearing Ferrocene in the Main Chain

The reaction of 1,1‘-fc(BMe2)2 (2) with pyrazine gives a novel poly(ferrocene) [2·pyz]n, which has been structurally characterized by X-ray crystallography [fc = (C5H4)2Fe; pyz = pyrazine]. The unusual dark green color of the solid material is indicative of charge-transfer interactions between the iron centers and the electron-poor pyrazine adduct bridges

Facile Synthesis of (3,5-(CF₃)₂C₆H₃)₂BX (X = H, OMe, F, Cl, Br): Reagents for the Introduction of a Strong Boryl Acceptor Unit

The reaction of (3,5-(CF3)2C6H3)Li ((Fxyl)­Li) with BH3·SMe2 in Et2O furnishes Li­[(Fxyl)­BH3] in an essentially quantitative yield. Hydride abstraction with Me3SiCl followed by the addition of a second equivalent of (Fxyl)Li gives Li­[(Fxyl)2BH2] in 71% yield. Treatment of Li­[(Fxyl)2BH2] with 1 equiv of Me3SiCl and a subsequent targeted methanolysis provide access to the methoxyborane (Fxyl)2BOMe. The latter compound serves as starting material for the synthesis of the haloboranes (Fxyl)2BX (X = F, Cl, Br) through the reaction with KHF2/Me3SiCl, BCl3, and BBr3, respectively. All three haloboranes, as well as key synthesis intermediates, have been structurally characterized by X-ray crystallo­graphy

B₂,N₄‑Doped Heptacenes: Ambipolar Charge-Transfer Compounds with Deep LUMO Levels

The B2,N4-doped heptacene H4 in which two N,N′-dihydrophenazine units are linked by two BMes bridges (Mes = mesityl) was synthesized via fourfold Buchwald−Hartwig coupling between 2,3,6,7-tetrachloro-9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene and o-phenylenediamine (tBuXPhos-Pd-G3, DBU/NaOTf, 2-MeTHF, 50 °C, 16 h). Upon exposure to ambient air, H4 is oxidized to its N,N′-dihydro form H2; further oxidation with MnO2 furnishes the di(phenazine) derivative H0. Stirring under a blanket of H2 in the presence of Pd/C hydrogenates H0 back to H2 and ultimately H4. Yellow-colored H0 is a remarkably good electron acceptor with a LUMO-energy level of −3.9 eV; upon irradiation with a 405 nm LED in the presence of THF or 1,4-cyclohexadiene, H0 accepts two H atoms to furnish H2. One-electron reduction of H0 yields the isolable radical-anion salt Li[H0] (lithium naphthalenide, THF, −30 °C to rt). The ambipolar compounds H2 and H4 possess a navy blue and deep purple color, respectively, due to charge-transfer interactions from the electron-rich N,N′-dihydrophenazine donor(s) to the electron-accepting B2C4 core

Mononuclear (<i>O</i>,<i>O</i>′ or <i>N</i>,<i>N</i>′) and Heterodinuclear (<i>O</i>,<i>O</i>′ and <i>N</i>,<i>N</i>′) Transition-Metal Complexes of <i>ortho</i>-Quinoid Bis(pyrazol-1-yl)methane Ligands

The ortho-hydroquinone-substituted bis­(3,5-dimethylpyrazol-1-yl)­methane ligand (HO)2C6H3–C­(H)­(pzMe,Me)2 (7) has been synthesized and fully characterized. Together with its bis­(3-tert-butylpyrazol-1-yl)­methane congener (HO)2C6H3–C­(H)­(pztBu)2 (6), 7 was employed in oxidation and complexation studies. 6 has been oxidized with 2,3-dichloro-5,6-dicyano-para-benzoquinone to the corresponding ortho-benzoquinone form 9 on a preparative scale. Pure samples of 9 are stable for several hours in solution and for approximately one day in the solid state. Attempts at a N,N′ coordination of [PdCl2] to 6 led to decomposition of the −C­(H)­(pztBu)2 moiety, whereas an introduction of [PdCl2] exclusively at the N,N′ site was possible in high yield for ligand 7. A selectively O,O′-chelated [(p-cym)­Ru]2+ complex was accessible for ligand 6, but not for 7. In contrast, [(ppy)2Ir]+ and [(Cp*)­Ir]2+ gave O,O′ complexes with both donors 6 and 7 (Hppy = 2-phenylpyridine; HCp* = pentamethylcyclopentadiene). The heterodinuclear complex [(Cp*)­Ir­(O)2C6H3–C­(H)­(pzMe,Me)2PdCl2] (16) was obtained from 14 and [(MeCN)2PdCl2]

Dilithio 9,10-Diborataanthracene: Molecular Structure and 1,4-Addition Reactions

9,10-Dihydro-9,10-diboraanthracene ([1]n) and its SMe2 adduct 1(SMe2)2 are readily reduced with lithium in THF to the dianionic 9,10-diborataanthracene Li2[1]. An X-ray crystal structure analysis of (Li(thf)2)2[1] revealed monomeric inverse sandwich complexes, each of them containing two Li(thf)2 moieties coordinated to both sides of the central B2C4 ring. Compared to 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene, the four B−CAr bonds of (Li(thf)2)2[1] are shorter by 0.046(4) Å, thereby indicating an increased degree of BCAr double-bond character. Consequently, (Li(thf)2)2[1] reacts with 4,4′-dimethylbenzophenone as a BCAr−CArB diene and undergoes a [4+2] cycloaddition reaction with formation of a bicyclic product. In contrast, tert-butylacetylene reacts with (Li(thf)2)2[1] under formal 1,4-addition of its methinic C−H group instead of its CC triple bond to the two boron atoms