Jelena Wiecko

Mixed‐Metal Lanthanide–Iron Triple‐Decker Complexes with a cyclo‐P5 Building Block

Tripleand multidecker sandwich complexes have been discussed in the last decades for their unique electrical and magnetic properties. The organic spacer between the metals may facilitate intermetallic electronic communication, which has a high potential for molecular electronics. A number of one-dimensional organometallic sandwich molecular wires (SMWs) have been extensively studied. Thus, the multilayer vanadium–arene (Ar) organometallic complexes [Vn(Ar)m], which can be produced in a molecular beam by laser vaporization, are a class of one-dimensional molecular magnets. Ferrocene-based molecular wires have been synthesized in the gas phase and characterized by mass spectroscopy. It was calculated that these compounds have half-metallic properties with 100 % negative spin polarization near the Fermi level in the ground state. In contrast to this investigation in the gas phase, studies on related organometallic tripleand multidecker sandwich complexes containing f-block elements (lanthanides or actinides) in condensed phase remain rare; studies were mostly on the cyclooctatetraene ligand and its derivatives. The only rare-earth-element triple-decker complex with heterocycles is the low-valent scandium 1,3,5-triphosphabenzene complex [{(hP3C2tBu2)Sc}2(m-h 6 :h-P3C3tBu3)], which was obtained by cocondensation of scandium vaporized in an electron beam with an excess of the phosphaalkyne tBuC P. Apart from organometallic compounds, tripleand multidecker sandwich complexes of the 4f elements consisting of “salen” type Schiff base ligands, phthalocyanines, and porphyrins have been extensively studied because these compounds exhibit tunable spectroscopic, electronic, and redox properties, and different extents of intramolecular p–p interactions. Despite these promising physical properties further investigations on 4f elements based tripleand multidecker sandwich complexes are obviously hampered by the limited variety of ligands that have been attached to the metal centers to date. Based on these considerations, we present herein mixed d/f-block-metal triple-decker complexes with a purely inorganic all-phosphorus middle deck. In contrast to d-block chemistry, where purely inorganic ring systems of Group 15 elements such as P5 and P6, [9] As5, [9c] and Sb5 [10] are well-established, there is no analogy with the fblock elements to date. On the other hand, it was shown only recently that rare-earth elements can stabilize highly reactive main-group species such as N2 3 . Although some heavier Group 15–lanthanide compounds, such as phosphides (Ln PR2), [12] arsenides (Ln AsR2), 13] stibides (Ln Sb3), and bismutides (Ln Bi Bi Ln) are known, the first molecular polyphosphide of the rare-earth elements, [(Cp*2Sm)4P8] (Cp* = h-C5Me5), was recently synthesized. [16] The structure of the complex is very rare and can be described as a realgartype P8 4 ligand trapped in a cage of four samarocenes. As no triple-decker sandwich complex of the rare-earth elements with a polyphosphide middle-deck bridging the metal centers is known, we focused our interest on the cyclo-P5 ligand. The structure and properties of this ligand are very similar to the well-known cyclopentadienyl anion (Scheme 1) and could therefore have many possible coordination modes.

Bis-N-heterocyclic carbene aminopincer ligands enable high activity in Ru-catalyzed ester hydrogenation

Bis-N-heterocyclic carbene (NHC) aminopincer ligands were successfully applied for the first time in the catalytic hydrogenation of esters. We have isolated and characterized a well-defined catalyst precursor as a dimeric [Ru2(L)2Cl3]PF6 complex and studied its reactivity and catalytic performance. Remarkable initial activities up to 283,000 h(-1) were achieved in the hydrogenation of ethyl hexanoate at only 12.5 ppm Ru loading. A wide range of aliphatic and aromatic esters can be converted with this catalyst to corresponding alcohols in near quantitative yields. The described synthetic protocol makes use of air-stable reagents available in multigram quantities, rendering the bis-NHC ligands an attractive alternative to the conventional phosphine-based systems.

Synthesis and Photoluminescence Properties of an Unprecedented Phosphinine–Cu4Br4 Cluster

A hitherto unprecedented polynuclear phosphinine-Cu(I) complex has been prepared and crystallographically characterized. The molecular structure in the crystal verifies the presence of the heterocubane-type tetrameric cluster [LCuBr]4 (L = 2,4-diphenyl-5-methyl-6-(2,3-dimethylphenyl)phosphinine), which unexpectedly shows in the solid state temperature-independent orange phosphorescence solely from an (3)XMLCT state, in clear contrast to the well-established dual-emissive-state model for luminescent Cu4X4L4 clusters.

Pyridyl-Functionalised 3H-1,2,3,4-Triazaphospholes: Synthesis, Coordination Chemistry and Photophysical Properties of Low-Coordinate Phosphorus Compounds

Novel conjugated, pyridyl-functionalised triazaphospholes with either tBu or SiMe3 substituents at the 5-position of the N3 PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole-based systems. Photoexcitation of the 2-pyridyl-substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all-nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2-pyridyl substituted systems have a more rigid and planar structure than their 3- and 4-pyridyl isomers. Time-dependent (TD) DFT calculations show that only the 2-pyridyl-substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N-hybrid ligand forms a Re(I) complex of the type [(N^N)Re(CO)3 Br] through the coordination of nitrogen atom N(2) to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π-accepting character of the triazaphosphole, which is again in contrast to that of the all-nitrogen-containing triazoles. The synthesis and photophysical properties of a new class of phosphorus-containing extended π systems are described.

2,4,6-Triarylphosphinines versus 2,4,6-Triarylpyridines: An Investigation of the Differences in Reactivity between Structurally Related Aromatic Phosphorus and Nitrogen Heterocycles

The novel atropisomeric pyridine derivative rac-10 has been synthesized and structurally characterized. In contrast to its phosphorus analogue 3, axially chiral 10 has a considerably lower rotational barrier as estimated by DFT calculations. However, the presence of the two enantiomers could be confirmed by means of chiral analytical HPLC analysis and by protonation experiments with a chiral acid. Compound rac-10 could be further dehydrogenated by treatment with DDQ to the benzo(h)quinoline derivative rac-12. This conversion failed for the phosphorus analogue rac-3. Interestingly, although 2,4,6-triarylphosphinines undergo facile CH activation with [Cp*IrCl2 ]2 in the presence of NaOAc, this reaction does not proceed with the corresponding pyridine derivatives. On the other hand, the latter ones can be selectively ortho-metalated with Pd(OAc)2 , leading to acetate-bridged dimeric species, which could be unambiguously confirmed by means of X-ray crystal structure analysis. The treatment of phosphinines with Pd(OAc)2 led instead to the formation of the unusual cofacial oxidative coupling products 16 and 17, which consist of a phosphorus-containing cage structure.

Functionalized 3H-1,2,3,4-triazaphosphole derivatives: Synthesis and structural characterization of novel low-coordinate phosphorus heterocycles

GRAPHICAL ABSTRACT ABSTRACT The synthesis of novel functionalized 3H-1,2,3,4-triazaphosphole derivatives is presented, making use of the modular [3+2] dipolar cycloaddition reaction between an azide and a phosphaalkyne. These low-coordinate phosphorus heterocycles were prepared in high yield and could be characterized crystallographically. This strategy provides the possibility to access hitherto unknown chelating and potentially chiral P^O ligands based on triazaphospholes.

New trifluoromethyl-substituted heterocycles by multicomponent reactions of siloxycyclopropanes

Multicomponent reactions of ethyl 1-trifluoromethyl-2-(trimethylsilyloxy)cyclopropanecarboxylate, isonitriles, and either 2-aminopyridine or amino acids provided imidazo[1,2-a]pyridines or γ-lactams, respectively. This access to trifluoromethyl-substituted heterocycles demonstrates again that siloxycyclopropanes can serve as hidden functionalized aldehydes and that this type of donor–acceptor-substituted cyclopropanes is particularly versatile.

Tuning the electronic effects of aromatic phosphorus heterocycles

A hitherto unprecedented electronic situation has been observed for a substituted, pyridyl-functionalized phosphinine. In contrast to previous studies, this compound shows considerable π-donor properties as the result of the rather strong +M effect of the CH3S-substituent, changing the electronic properties of this low-coordinate and aromatic phosphorus heterocycle substantially.