Conjugated Polymers and Aromaticity

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Metallaaromatic Chemistry: History and Development.

Das Iminoboran tBuB = NtBu (1) bildet sich in der Gasphase bei 530°C aus dem Aminoboran Cl(tBu)B – N(tBu)SiMe3 durch Abspaltung von Me3SiCl. Das Boran 1 dimerisiert langsam zum Diazadiboretidin [tBuBNtBu]2 (2). Fur Iminoborane typische Reaktionen, namlich die Ethyloborierung mit BEt3, di [2 + 3]-Cycloaddition mit PhN3 und die Azidosilierung mit Me3SiN3, fuhren auch mit 1 zu den erwarteten Produkten. Die Molekul- und Kristallstrukturen von 1 und 2 werden angegeben; die zentrale CBNC-Kette von 1 ist linear gebaut, der BN-Abstand betragt 125.8 pm; das Vierringgerust von 2 wird durch die sperrigen Ring-Liganden aus der Planaritat gedrangt. In seinen strukturellen Eigenschaften ist das Iminoboran 1 mit dem isoelektronischen Alkin tBuCCtBu (3), das Cyclodimere 2 mit dem isoelektronischen Cyclobutadien [tBuC = CtBu]2 (4) vergleichbar. Die Starke der Dreifachbindung fallt von 3 nach 1 in ahnlicher Weise ab wie beim Ubergang von N2 zu CO.



Preparation, Reactions, and Structure of tert-Butyl(tert-butylimino)borane



The iminoborane tBuB = NtBu (1) is formed from the aminoborane Cl(tBu)B – N(tBu)SiMe3 by the elimination of Me3SiCl in the gas phase at 530°C. Compound 1 slowly dimerizes to the diazadiboretidine [tBuBNtBu]2 (2). In a way typical for iminoboranes, the borane 1 undergoes an ethyloboration with BEt3, a [2 + 3]-cycloaddition with PhN3, and an azidosilation with Me3SiN3. The crystal and molecular structure of 1 and 2 are described; there is a linear central CBNC-chain in 1 with a BN-bond distance of 125.8 pm; the ring-skeleton of 2 deviates from planarity by the influence of the bulky ring ligands. The structural properties of the iminoborane 1 are comparable to those of the isoelectronic alkyne tBuCCtBu (3), and the properties of the cyclodimer 2 are comparable to those of the isoelectronic cyclobutadiene [tBuC = CtBu]2 (4). The strength of the triple bond decreases from 3 to 1 in a similar manner as by going from N2 to CO.

Darstellung, Reaktionen und Struktur von tert-Butyl(tert-butylimino)boran

Tetra-tert-butyltetrahedran – ein nutzloses Molekul? Eine Antwort auf diese provozierende Frage wird im ersten Teil des vorliegenden Forschrittsberichts versucht. Der Leser soll nachvollziehen konnen, warum es sich lohnt, einem derart esoterischen Molekul nachzujagen. Zweifelsfrei identifiziert wurde bisher nur Tetra-tert-butyltetrahedran, der gespannteste gesattigte Kohlenwasserstoff uberhaupt. Der Beschreibung der ungewohnlichen Eigenschaften dieses Derivats folgt eine Zusammenfassung uber Versuche zur Synthese der Stammverbindung. Im Vordergrund dieser beiden Abschnitte stehen die Ergebnisse eigener Arbeiten. Da die Diskussion von Tetrahedranen die entsprechenden Cyclobutadiene zwangslaufig einschliesen mus, hat es sich angeboten, den Beitrag zugleich als eine Art Abschlusbericht zum Cyclobutadien-Problem zu konzipieren. Masamune hat 1980 seinen vorzuglichen Report in der Zeitschrift Tetrahedron als ein Komma in der Geschichte des Cyclobutadiens bezeichnet. Inzwischen sind die wesentlichen Fragen beantwortet. Die Zeit ist nunmehr reif, anstelle eines Kommas einen Punkt zu setzen.

Tetrahedran und Cyclobutadien

Tetra-tert-butyltetrahedrane—a useless molecule? In the first part of this progress report an attempt is made to answer this provoking question in order that the reader can evaluate why it is worthwhile to track such an esoteric molecule. An account of the unusual properties of tetra-tert-butyltetrahedrane, the only tetrahedrane unequivocally identified so far, and the most highly strained saturated hydrocarbon of all, is followed by a summary of attempts to synthesize the parent compound. The emphasis in these two sections is on our own research achievements. Since any discussion of the tetrahedranes must necessarily include the corresponding cyclobutadienes, it was found appropriate to organize the article at the same time as a final account of the cyclobutadiene problem. Masamune[8c] characterized his excellent report in 1980 in the journal Tetrahedron as a comma in the history of cyclobutadiene. Meanwhile, the significant questions have been answered. The time is now ripe to replace the comma by a full stop.

Tetrahedrane and Cyclobutadiene

Structure of Tetra-tert-butylcyclobutadiene†‡

Struktur des Tetra‐tert‐butylcyclobutadiens

Die Molekulstruktur der Cyclobutadien-Derivate 1–3 wird mit Rontgenbeugung bestimmt. Das Derivat 2 kristallisiert in zwei verschiedenen Modifikationen mit unterschiedlichen molekularen Konformationen. Die Form 2a enthalt sesselformige Siebenringe wie 1, die Form 2b twistformige Siebenringe. Die Vierringe von 1 und 2a sind planar, von 2b und 3 gefaltet (176.9° und 170.2°). Sie sind deutlich nicht-quadratisch. Ihre Bindungslangen sind fur 1 und 2 stark alternierend: 1.34 und 1.59–1.60 A. Eine Tieftemperaturmessung (123 K) fur 3 ergab 1.44 und 1.53 A. Bei Raumtemperatur fuhren Fehlordnungseffekte zur Aquilibrierung der Vierringbindungen von 3.



Structure Determination of Cyclobutadiene Derivatives



The molecular structures of the cyclobutadiene derivatives 1–3 have been determined by X-ray structure analyses. Compound 2 crystallizes in two different modifications with different molecular conformations. Form 2a has chair like seven-membered rings as has 1, in form 2b the seven-membered rings are twisted. The fourmembered rings of 1 and 2a are plane, of 2b and 3 they are folded (176.9° and 170.2°). All of them are significantly non-square. The bond lengths alternate strongly for 1 and 2: 1.34 and 1.59–1.60 A. From a low temperature measurement (123 K) we obtained for 3 1.44 and 1.53 A. At room temperature disorder effects equilibrated the bond lengths of the four-membered ring of 3.

Strukturbestimmungen von Cyclobutadien-Derivaten

Structure of Cyclobutadiene Derivatives

Structure of tetra-tert-butylcyclobutadiene

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