Biradicals are molecules which contain two unpaired electrons in two nearly degenerate non-bonding molecular orbitals. [1] Both electrons can either be antiparallel forming an open-shell singlet state or parallel describing a triplet state. [2] As a result of the two unpaired electrons such biradicals are usually transient species during the process of bond breaking and making. Introduction of steric strain by bulky substituents to prevent bond formation or dimerization, delocalization, and substitution of carbon atoms by suitable main-group elements can lead to a considerable stabilization of such biradicals, however, at the expense of the biradical character, which decreases. Thus the designation of such stabilized species as biradicaloids seems to be more appropriate. [3, 4] Singlet biradicals commonly show a relatively small energy gap between their lowest energy singlet and triplet state. The stability of biradicals is increased by increasing the HOMO-LUMO gap leading to a larger singlet-triplet splitting, and lower occupation of the LUMO. [1] However, when the LUMO occupation reaches zero, a closed-shell singlet is finally obtained, and such species cannot be referred to as biradical or biradicaloid, respectively, anymore.Following our interest in the heterocyclic chemistry of Group 15 elements, [5] we studied the reaction of four-membered rings of the type [XE(m-NR)] 2 E = Group 15 element, X = halogen) containing alternating pnictogen(III) and nitrogen centers, with reducing agents such as [Cp 2 Ti(btmsa)] (Cp = h-C 5 H 5 , btmsa = bis(trimethylsilyl)acetylene, Me 3 Si-C C-SiMe 3 ), [6] [{Cp 2 TiCl} 2 ] or Mg. [7] Upon chloride abstraction and reduction (Scheme 1), such cyclo-1,3-dipnicta(III)-2,4diazanes [ClE(m-NR)] 2[8] with bulky substituents R (R = terphenyl = Ter = 2,6-Mes 2 C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [9] should form remarkably tight ring structures of the type [E(m-NR)] 2 featuring two localized radical sites. The only known example, the [P(m-NR)] 2 biradicaloid, exhibits two radical centers in the

A series of ansa-titanocene triflate complexes are described as model compounds for the elementary steps of light-driven overall water splitting. Titanocene(III) triflate complexes are readily obtained by reaction of a titanocene source with Yb(OTf)3. Subsequent reactions with water and with/without TEMPO as hydrogen scavenger are studied. The as-obtained titanocene(IV) compounds can be photoreduced to give titanocene(III) triflate complexes, which can undergo further hydrolysis to form a closed catalytic cycle of water splitting. No further degradation of the photoreduced species was observed because of the presence of the OTf group. The stability of the system was evaluated in an experiment with high concentrations of water and TEMPO. X-ray crystallography on all titanocene complexes, EPR and NMR spectroscopy, and DFT were used to support our observations.

The synthesis of a series of ansa-titanocene dichlorides [Cp'2TiCl2] (Cp' = bridged η(5)-tetramethylcyclopentadienyl) and the corresponding titanocene bis(trimethylsilyl)acetylene complexes [Cp'2Ti(η(2)-Me3SiC2SiMe3)] is described. The ethanediyl-bridged complexes [C2H4(C5Me4)2TiCl2] (2-Cl2) and [C2H4(C5Me4)2Ti(η(2)-Me3SiC2SiMe3)] (2-btmsa; btmsa = η(2)-Me3SiC2SiMe3) can be obtained from the hitherto unknown calcocenophane complex [C2H4(C5Me4)2Ca(THF)2] (1). Furthermore, a heterodiatomic bridging unit containing both, a dimethylsilyl and a methylene group was introduced to yield the ansa-titanocene dichloride [Me2SiCH2(C5Me4)2TiCl2] (3-Cl2) and the bis(trimethylsilyl)acetylene complex [Me2SiCH2(C5Me4)2Ti(η(2)-Me3SiC2SiMe3)] (3-btmsa). Besides, tetramethyldisilyl- and dimethylsilyl-bridged metallocene complexes (structural motif 4 and 5, respectively) were prepared. All ansa-titanocene alkyne complexes were reacted with stoichiometric amounts of water; the hydrolysis products were isolated as model complexes for the investigation of the elemental steps of overall water splitting. Compounds 1, 2-btmsa, 2-(OH)2, 3-Cl2, 3-btmsa, 4-(OH)2, 3-alkenyl and 5-alkenyl were characterised by X-ray diffraction analysis.

Irradiation of a substituted ansa-titanocene(IV) dihydroxido complex with visible light induces Ti-O bond dissociation. In contrast to previous studies on structurally similar unbridged complexes, no side reactions are observed and formation of the Ti(III) species is highly selective. The formation of OH radicals was proved using a biradicaloid species.

Biradikale sind Moleküle mit zwei ungepaarten Elektronen, die sich in zwei nahezu entarteten, nicht-bindenden Molekülorbitalen befinden. [1] Beide Elektronen kçnnen entweder antiparallelen Spin besitzen, entsprechend einem offenschaligen Singulettzustand, oder aber parallelen Spin mit einem Triplettzustand. [2] Aufgrund der beiden ungepaarten Elektronen beobachtet man solche Biradikale gewçhnlich nur als transiente Spezies in Bindungsbruch-bzw. Bindungsbildungsprozessen. Die Einführung eines sterisch anspruchsvollen Substituenten, der eine Bindungsbildung oder Dimerisierung verhindern kann, Delokalisation sowie Substitution von C-Atomen durch geeignete Hauptgruppenelemente kann zu einer beachtlichen Stabilisierung solcher Biradikale führen, was jedoch zu Lasten eines abnehmenden Biradikalcharakters geht. Daher erscheint die Bezeichnung solcher stabilisierter Spezies als Biradikaloide geeigneter. [3, 4] Singulettbiradikale haben gewçhnlich eine relativ kleine Singulett-Triplett-Lücke. Eine Zunahme der Biradikalstabilität erhält man durch eine Vergrçßerung der HOMO-LUMO-Lücke, was wiederum zu einer grçßeren Singulett-Triplett-Lücke führt bzw. zu einer geringeren Besetzung des LUMO. [1] Wenn jedoch die Besetzungzahl des LUMO null erreicht, liegt letztlich eine geschlossenschalige Singulettspezies vor, die weder als Biradikal noch als Biradikaloid aufzufassen ist.Unserem Interesse an der Heterocyclenchemie der 15. Gruppe folgend [5] haben wir die Reaktion von viergliedrigen Ringen des Typs [XE(m-NR)] 2 (E = Element der 15. Gruppe, X = Halogen) mit verschiedenen Reduktionsmitteln wie [Cp 2 Ti(btmsa)] (Cp = Cyclopentadienyl, btmsa = Bis(trimethylsilyl)acetylen = Me 3 SiCCSiMe 3 ), [6] [{Cp 2 TiCl} 2 ] oder Mg näher untersucht. [7] Bei gleichzeitiger Chloridabstraktion sollte die Reduktion von cyclo-1,3-Dipnicta(III)-2,4-diazanen [ClE(m-NR)] 2 , [8] die über sperrige Substituenten R (R = Terphenyl = Ter = 2,6-Mes 2 -C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [9] ver-fügen (Schema 1), zu bemerkenswert gespannten Ringstrukturen des Typs [E(m-NR)] 2 mit zwei lokalisierten Radikalzentren führen. Das einzig bekannte Beispiel eines [P(m-NR)] 2 -Biradikaloids enthält zwei nahe beieinander liegende Radikalzentren, was einer stark geweiteten Einfachbindung ähnelt. [7] Biradikaloide des Typs [E(m-NR)] 2 kçnnen ausgehend vom kurzlebigen cyclo-Butandiyl [HC(m-CH 2 )] 2 durch formale, isolobale Substitution von CH durch E abgeleitet werden (Schema 2). Bahnbrechende Arbeiten auf diesem Gebiet wurde von Niecke et al. am Beispiel des [ClC-(m-PMes*)] 2 [10] und später für eine Reihe verschiedener Derivate durchgeführt. Darüber hinaus wurde über ein 1,3-Dibora-2,4-diphospha-cyclo-butan-1,3-diyl [tBuB(m-PiPr 2 )] 2 von Bertrand et al. berichtet. [11] Die Gruppen von Power und Lappert isolierten die ersten Biradikaloide mit schweren Elementen der 14. Gruppe: [RE(m-NR')] 2 (E = Sn, Ge; [ClSn(m-N-SiMe 3 )] 2 [12]und [RGe(m-N-SiMe 3 )] 2 (R = 2,6-Dipp 2 C 6 H 3 , Dipp = 2,6-iPr 2 C 6 H 3 )). [13] Erst kürzlich gelang den Schema 1. Synthese vierg...

Keywords: Metallocenes / Titanium / Water / IR spectroscopyThe synthesis of two pyridine-stabilised ansa-titanocene oxido complexes is described. Oxidation of the titanocene sources Cp b 2 Ti(η 2 -Me 3 SiC 2 SiMe 3 ) (Cp b 2 = bridged η 5 -cyclopentadienyl, 1a and 1b) with nitrous oxide yields corresponding oxido compounds 2a and 2b, respectively, in high yields. These can be hydrolysed to give dihydroxido complexes 3a

The mol­ecular structure of the title compound, C22H38Si2, features a trans arrangement of the cyclo­penta­dienyl rings to avoid steric strain [C—Si—Si—C torsion angle = −179.0 (5)°]. The Si—Si bond length is 2.3444 (4) Å. The most notable inter­molecular inter­actions in the mol­ecular packing are C—H⋯π contacts that lead to the formation of wave-like supra­molecular chains along the b axis.

In the cation of the title salt, C9H20NO+·CF3O3S−, the six-membered heterocyclic ring displays a chair conformation. In the crystal, centrosymmetric pairs of cations and anions are linked by N—H⋯O and O—H⋯O hydrogen bonds to form rings with a R 4 4(14) graph-set motif.