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.