T-shaped 14 Electron Rhodium Complexes: Potential Active Species in C-H Activation.

Two T-shaped 14-electron rhodium complexes 2 a and 2 b, "framed" and thus stabilized by PNP pincer ligands have been synthesized. The bis(t-butyl)phosphine derived PNP-rhodium complex 2 a was isolated from pentane as the more stable cyclometalated Rh(III) hydrido complex and found to be in equilibrium with the T-shaped 14e Rh(I) complex 2 aT which itself could be directly crystallized upon change of the solvent. The cyclometallation is suppressed using an adamantyl substituted PNP ligand to give the analogous T-shaped Rh(I) species 2 b, stabilized through an agostic interaction with one of the adamantyl C-Hs.

Carbon-Carbon Bond Activation at Chromium(I): An 11-Electron Complex Cleaving Dialkynes.

An 11-electron T-shaped chromium(I) complex was obtained by reduction of a PNP-supported chromium(II) iodide complex. Its d high-spin electronic structure was characterized employing paramagnetic NMR, EPR, UV/Vis and magnetic measurements (SQUID). The complex readily reacts with conjugated dialkynes to cleave the internal C-C bond, forming the respective acetylide complexes.

A Direct Route to closo-SBCl Thiaboranes from Simple Electron-Precise Synthons: The Different Role of Chalcogen Bonding in SBCl and SBCl Crystals.

Six-vertex closo-SBCl (1) and ten-vertex closo-1-SBCl (2) thiaboranes have been prepared, besides the already known 12-vertex closo-SBCl (3), from the co-pyrolysis reaction of BCl with SCl at 280 °C in vacuo. The compounds are sublimable, off-white solids. Their elemental composition has been determined by high-resolution mass spectrometry.

Radicalizing CO by Mononuclear Palladium(I).

A mononuclear, T-shaped palladium(I) d metalloradical (3), stabilized by a bulky carbazole-based PNP-ligand, was obtained by reduction of palladium chloride or thermal Pd-C bond homolysis of the corresponding neopentyl complex. Pressurizing with CO gave the Pd(I) carbonyl complex, which was structurally characterized by X-ray diffraction. Delocalization of the unpaired electron to the carbonyl carbon was detected by EPR spectroscopy and theoretically modeled by DFT and ab initio methods.

Synthesis of Collidine from Dinitrogen via a Tungsten Nitride.

The synthesis of pyridines from dinitrogen in homogeneous solution is known to be challenging considering that an N cleavage step needs to be combined with two N-C coupling steps. Herein, a tungsten complex bearing a tailor-made 2,2'-(BuAs)-substituted tolane ligand scaffold was shown to split N to afford the corresponding tungsten nitride, which is not the case for the corresponding (PrAs)-substituted derivative. The former nitride was then reacted with 2,4,6-trimethylpyrylium triflate, which led to the formation of a tungsten oxo complex, along with collidine.

To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen.

Molybdenum, tungsten, and rhenium halides bearing a 2,2'-(PrAs)-substituted diphenylacetylene ([AsCCAs], ) were prepared and reduced under an atmosphere of dinitrogen in order to activate the latter substrate. In the case of molybdenum, a diiodo () and a triiodo molybdenum precursor () were equally suited for reductive N splitting, which led to the isolation of [AsCCAs]Mo≡N(I) () in each case. For tungsten, [AsCCAs]WCl () was reduced under N to afford {[AsCCAs]WCl}(N) (), which is best described as a dinuclear πδ-configured μ-(η: η)-N-bridged dimer.

Chiral -Alkynylated Tetraazaperylenes: Photophysics and Chiroptical Properties.

Fully -alkynylated octaazaperopyrene dioxide (OAPPDO) derivatives were accessible through Stille cross coupling reaction of the corresponding -chlorinated derivatives. This steric congestion of the area led to helically chiral fluorophores, and chiral resolution of two derivatives allowed the investigation of their chiroptical properties as well as their kinetics of enantiomerization and the related thermodynamic parameters depending on the size of the terminal alkynyl substituent. An increase of the latter resulted in stable OAPPDO atropisomers at room temperature.

Coordination-Induced Radical Generation: Selective Hydrogen Atom Abstraction via Controlled Ti-C σ-Bond Homolysis.

A method for the generation of transient alkyl radicals via homolytic Ti-C bond cleavage was developed by employing a tailor-made organotitanium half-cage complex. In contrast to established metal-mediated radical initiation protocols via thermal or photochemical M-C σ-bond homolysis, radical formation is triggered solely by coordination of a solvent molecule (thf) to a titanium(IV) center. During the reaction, the nonstabilized alkyl radical is formed along with a persistent titanium(III) metalloradical, thus taming the former transient radical (persistent radical effect).

Dioxygen Activation and Reduction by a Soluble Iron Phthalocyanine.

Iron ions in a square-planar coordination can bind molecules at the vacant axial positions and are able to transform them in stoichiometric and catalytic reactions. Nature takes advantage of these properties by incorporating iron into porphyrin systems, which play a key role not only in the binding and transport of oxygen, but also in catalytic oxidation and reduction reactions involving cytochrome P450. Although these systems have been studied extensively, there are still unresolved questions regarding the interplay between the iron ions and the surrounding ligands.

Zipping up tetraazaperylene: synthesis of tetraazacoronenes double coupling in the bay positions.

Substituted tetraazacoronene fluorophores have been obtained selectively by double Suzuki-Miyaura cross coupling of symmetrically substituted 1,2-bis(pinacolatoboryl)alkenes with a bay-substituted octaazaperopyrenedioxide (OAPPDO). Subsequent Scholl reaction of the dimethoxyphenylated derivative allowed further π-extension of the azaperylene core, yielding a highly redox-active bis(phenanthro)tetraazacoronene.

Triplet quenching pathway control with molecular dyads enables the identification of a highly oxidizing annihilator class.

Metal complex - arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, the corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X triplets.

Tetraazacoronenes and Their Dimers, Trimers and Tetramers.

Tetraazacoronenes were synthesized from bay-functionalized tetraazaperylenes by Zr-mediated cyclization and four-fold Suzuki-Miyaura cross coupling. In the Zr-mediated approach, an η -cyclobutadiene-zirconium(IV) complex was isolated as an intermediate to cyclobutene-annulated derivatives. Using bis(pinacolatoboryl)vinyltrimethylsilane as a C building block gave the tetraazacoronene target compound along with the condensed azacoronene dimer as well as higher oligomers.

[AsCCAs]-Coordinated Rhenium Hydrides and Their Reactivities Toward Unsaturated Hydrocarbons, Heterocumulenes, and CO.

An [AsCCAs] ligand featuring a central alkyne and two flanking arsenic donors was employed for the synthesis of a trihydrido rhenium complex, while the corresponding phosphorus ligand was shown to be less suited. The reactivity of the former trihydride [AsCCAs]ReH () was examined in detail, which revealed that two alternative reaction channels may be entered in dependence of the substrate. Upon reaction of with PhC≡CPh, ethylene, and CS, monohydrides of the general formula [AsCCAs]Re(L)H with L = η-PhC≡CPh (), η-HC═CH (), and η-CS () were formed along with H.

The Telluraboranes closo-TeB Cl and closo-TeB Cl with Exceptionally Long Body Diagonals: Synthetic and Bonding Motifs for Innovative Octahedral and Icosahedral Geometries.

Six-vertex closo-TeB Cl (1) and twelve-vertex closo-TeB Cl (2) telluraboranes have been prepared via co-pyrolysis of B Cl with TeCl in vacuo at temperatures between 360 °C and 400 °C. Both compounds are sublimable, off-white solids, and they have been characterized by one- and two-dimensional B NMR and high-resolution mass spectroscopy. Both ab initio/GIAO/NMR and DFT/ZORA/NMR computations support octahedral and icosahedral geometries for 1 and 2, respectively, as expected due to their closo-electron counts.

Martin's Phosphino-Triol Revisited: Unexpected P-C Bond Cleavage Reactions and Their Suppression via Complexation of Al and Sc.

A revised synthesis of Martin's phosphino-triols (two derivatives) is reported. Once isolated, these thermally sensitive triols were shown to disassemble selectively via an unexpected P-C bond cleavage reaction. This degradation pathway was effectively suppressed via complexation of Al and Sc.

Selective Reduction of CO to a Tantalum Formate Complex and Release of Methyl Formate from the Tantalum(V) Center.

A cyclometalated NHC-coordinated hydrido tantalum alkoxide was found to selectively react with CO to afford the genuine tantalum formate (NHC)(HCOO)Ta(OR) with OR = OC(CF)CH. In the solid state, the presence of a κ-,-formate moiety was established by single-crystal X-ray diffraction and ATR-IR spectroscopy, while NMR experiments and DFT modeling studies suggest that the κ--coordination mode is preferred in solution. Despite the accessibility of the latter κ--formate in solution, no over-reduction to a dinuclear methylene diolate was observed.

Molybdenum-Mediated N -Splitting and Functionalization in the Presence of a Coordinated Alkyne.

A new [PCCP]-coordinated molybdenum platform comprising a coordinated alkyne was employed for the cleavage of molecular dinitrogen. The coordinated η -alkyne was left unaffected during this reduction. DFT calculations suggest that the reaction proceeds via an initially generated terminal N -complex, which is converted to a dinuclear μ-(η :η )-N -bridged intermediate prior to N-N bond cleavage.

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes reversible C-P bond formation.

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (s) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two -positioned open coordination sites, is based on the controlled cyclisation of a ,'-diisopropyl-substituted 2,2'-diphosphinotolane () and leads to chelate complexes coordinated by a phosphine donor and the carbon atom. The C-P bond formation involved in the former partial cyclisation of proceeds under mild conditions and was shown to be applicable all over the d-block.

Reductive Hydrogenation under Single-Site Control: Generation and Reactivity of a Transient NHC-Stabilized Tantalum(III) Alkoxide.

One of the most attractive routes for the preparation of reactive tantalum(III) species relies on the efficient salt-free hydrogenolysis of tantalum(V) alkyls or tantalum(V) alkylidenes, a process known as reductive hydrogenation. For silica-crafted tantalum alkyls and alkylidenes, this process necessarily proceeds at well-separated tantalum centers, while related reductive hydrogenations in homogeneous solution commonly involve dimeric complexes. Herein, an NHC scaffold was coordinated to a novel tri(alkoxido)tantalum(V) alkylidene to circumvent the formation of dimers during reductive hydrogenation.

P-Protected Diphosphadibenzo[ a, e]pentalenes and Their Mono- and Dicationic P-Bridged Ladder Stilbenes.

The previously elusive diphosphadibenzo[ a, e]pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from RP-substituted 2,2'-diphosphinotolanes (R = Ph, Pr). The resulting P-protected diylidic compounds 4 (R = Ph, Pr) were converted to the corresponding P-bridged ladder stilbenes via two consecutive oxidation steps: upon selective one-electron oxidation, the persistent radical monocations 5 (R = Ph, Pr) were obtained and further oxidized to afford the respective fluorescent and air-stable dications 6 (R = Ph, Pr).

A Tautomeric λ/λ-Phosphane Pair and Its Ambiphilic Reactivity.

The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule's O-H bonds, which forms the basis for a tautomeric λ/λ-phosphane equilibrium. For the first time, this equilibrium was detected for a λ-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ-phosphane.

Synthesis of NPN-Coordinated Tantalum Alkyl Complexes and Their Hydrogenolysis: Isolation of a Terminal Tantalum Hydride Incorporating a Doubly Cyclometalated NPN Scaffold.

The closely related benzylene-linked diaminophosphines PhP(CHCH-o-NHPh) (AH) and PhP(CH-o-CHNHXyl) (BH with Xyl = 3,5-MeCH) were employed for the synthesis of tantalum(V) alkyls, which were then studied with respect to hydrogenolysis. In the case of AH, the tantalum trimethyl complex [Ta(A)Me] (1) and the tantalum hydrocarbyl complex [Ta(A)(CHSiMe)(η-EtC≡CEt)] (2) were prepared from the ligand's dilithium salt (A)Li(diox). Upon hydrogenolysis of 1 and 2, the formation of methane and SiMe, respectively, was observed, but well-defined tantalum hydrides could not be detected.

Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes.

The benzylene-linked [PNP] scaffolds HN(CH-o-CHPPh) ([A]H) and HN(CH-o-CHPPh) ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe) ([A]1-M) and [B]M(NMe) ([B]1-M) were prepared and converted to the iodides [A]MI ([A]2-M) and [B]MI ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn ([A]3-M) and [B]MBn ([B]3-M) (M = Zr, Hf) were obtained via reaction with BnMg(OEt).

Synthesis and reactivity of cyclometalated triamidophosphine complexes of niobium and tantalum.

The triamidophosphine protioligand 1 reacts with the homoleptic pentakis(dimethylamido) precursors of niobium and tantalum [M(NMe2)5, where M = Nb, Ta] to form cyclometalated complexes of the type [N2PCN-κ(5)-N,N,P,C,N]M(NMe2) (2-M). Apart from the three amido donors, one benzylic position of the ligand backbone is deprotonated over the course of this reaction, resulting in the formation of a new M-C bond. As a consequence, a metallaziridine substructure is formed, and the triamidophosphine moiety thus serves as a tetraanionic pentadentate ligand.

A tripodal benzylene-linked trisamidophosphine ligand scaffold: synthesis and coordination chemistry with group(IV) metals.

A new tripodal trisamidophosphine ligand (1) based on the trisbenzylphosphine backbone has been synthesized in three steps starting from NaPH2 and phthaloyl-protected 2-aminobenzyl bromide. At elevated temperatures, 1 reacts directly with M(NMe2)4 (M = Zr, Hf) to afford the dimethylamido complexes [PN3]M(NMe2) (M = Zr, Hf) (2), which are easily converted into the corresponding triflates [PN3]MOTf (M = Zr, Hf) (3) via reaction with triethylsilyl trifluoromethanesulfonate. The related titanium chloro complex [PN3]TiCl (4-Ti) is obtained from 1 and Bn3TiCl via protonolysis.