Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(I1) ChlorideStarting with a nearly quantitative coupling of cyclonondtetraenide 7 to 1, 1'-dihydrononafulvalene 8 by means of AgBF,, a simple general synthetic concept for fulvalenes is outlined (Scheme 2), consisting in an oxidative coupling of 'Huckel anions' like 2 and 7 to 1, 1'-dihydrofulvalenes 10 with Ag(1) or Cu(I1) salts, followed by deprotonation (-11) and oxidation (412); it has been realised in the case ofpentafulvalene (1; overall yield 61 %; Scheme 3 ) and 1,2:5,6-dibenzopentafulvalene (18; overall yield 66%, Scheme 4 ) . NMR-spectroscopic investigations show that 1 is a non-aromatic compound with strongly alternating bond-lengths, its n-system being even more locdlised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above -50". Resides of polymerisations, Diets-Alder dimerisation 1 +19 followed by a rearrangement 19-20 takes place (Scheme 5 ) . 1.Einleitung. -Pentafulvalen (1) ist eine sowohl synthetisch wie theoretisch attraktive Verbindung, deren spektroskopische Daten wichtige Riickschliisse auf die n -Delokalisation und Ladungsdichteverteilung in gekreuzt konjugierten Systemen erlauben wiirden. Da 1 das n-System von Pentafulvenen aufweist, welches mit einer Butadien-Einheit iiberbriickt worden ist, konnte ein spektroskopischer Vergleich von 1 mit Pentafulvenen [ 1 J von Interesse sein.

~ ~ _ _ _ _ _ _~Nonapentafulvalene Nonapentafulvalene (1) has been prepared by oxidative coupling of sodium cyclopentadienide (6) and sodium cyclononaietraenide (7) with CuCI, in THF, two-fold deprotonation of cyclopentadienyl-cyclononatetraene 8 to give dianion 16, and oxidative treatment of 16 with CuCI, (Schemm 2 and 3). Compound 1 is a highly reactive and thermally instable molecule, since valence isomerisation 1+ 17 proceeds easily even at low temperature (the half-life of 1 is ca. 30 min at -15" in CDCI,). NMR investigations show that nonapentafulvalene is an olefinic molecule with strongly alternating bond lengths, its nine-membered ring deviating strongly from planarity.Comparison of the NMR data of 1 with those of a series of sterically similar pentafulvenes 18 and nonafulvenes 19 (Tables 1 and 2) demonstrates that a ) with regard to the pentafulvene unit of 1, the cyclononatetraene ring acts as very weak electron-donating group, while b ) with regard to the nonafulvene unit of 1, the cyclopentadiene ring acts as weak electron-accepting group. So nonapentafulvalene may be regarded as a 'nonafulvene of inverse n-polarisation'. Einleitung

Following a general strategy (Scheme 2) for the synthesis of fulvalenes by oxidative coupling of ffiickel anions, the synthesis of nonafulvalene (1) has been realised (Scheme 3 ) . The most tricky step is a twofold deprotonation of bi(cyclononatetraeny1) 3 with K(t-BuO) to give 4; it is only possible after equilibration 3~3 a ; otherwise, deprotonation of one cyclononatetraene unit of 3 followed by oxidative coupling with AgBF4 gives quater(cyclononatetraeny1) I2 as an isomeric mixture. The reaction of 3 with K gives potassium ccct-cyclononatetraenide 11 (M = K; Scheme 5 ) and not the dianion 4 as reported in the literature. Nonafulvalene (1) is extremely reactive due to its very easy valence isomerisation to give an isomeric mixture of tetrahydro-dibenzopentafulvalenes 13 (Scheme 4 ) . The structure of crystalline isomer (E-anti)-13 has been proved by X-ray analysis. 1.Einleitung. -Nonafulvalen (1) ist sowohl synthetisch reizvoll wie theoretisch attraktiv, denn spektroskopische Untersuchungen von 1 konnten in Analogie zu den NMR-Ergebnissen von Nonafulvenen [3] wichtige Aufschlusse uber die Geometrie sowie die Bindungslangenalternanz in diesen cyclisch gekreuzt konjugierten 71 -Systemen geben. Bisher sind jedoch nur vereinzelte hochanellierte und arylierte Nonafulvalene bekannt geworden [4], die keine sicheren Aussagen uber die Grundzustandseigenschaften und die Reaktivitat des Grundkorpers 1 zulassen. Die praparativen Erfahrungen mit einfachen Nonafulvenen [5-71 lassen vermuten, dass 1 sowohl schwer zuganglich als auch ausserst reaktiv sei und sich der Isolierung durch rasche (6n +4n + 26)-Valenzisomerisierung der beiden Neunringe entziehen konnte.Hafner et al. beobachteten 1975 [S], dass Lithium-cyclononatetraenid 2 (M = Li) in Gegenwart von I, oder SbCl, n i t rund 30% Ausbeute zu Bi(cyclononatetraeny1) 3

Durch oxidative Kupplung der Natriumcyclopentadienide (I) mit KupferUH‐chlorid oder Silbertetrafluoroborat, Depmtonierung der gebildeten l,l ′‐Dihydrofulvalene (II) mit Butyllithium und erneute Oxidation der Dilithiumsalze (III) werden die Fulvalene (IV) hergestellt.

of the anion closely resembles that of [Moz05(o-benzened i~l a t e )~]~~~'~ and of [MoZO5(3,5-di-tert-1,2-benzenedioland possesses two cis-dioxomolybdenum(v1) centers bridged by one pz-oxido-and two pz-O-2-sulfidophenolate ligands. The last-mentioned bridges are asymmetr!c and involve Mo-0 distances of 2.117(2) and 2.482(2) A. The Mo atom has a very distorted octahedral environment of five 0 and one S donor atoms, with the largest and the smallest bond angles being 02Mo04'= 167.4(1)" and 03Mo04'= 67.q l ) O , respectively. The UV/VIS spectrum of [PPh4]Z[Moz05(SC6H40)z] contains two bands of a comparable intensity with A,,,,, = 400 and 350 nm and an ab-