Structure and bonding in reduced boron and aluminium complexes with formazanate ligands

Group 13 complexes of the type [(PhNNC(p-tol)NNPh)ZPh2]2- (Z = B, Al) containing a highly reduced, trianionic formazanate-derived ligand were studied and the differences in the structure, bonding and reactivity between the B and Al compounds were investigated. The increased ionic character in the bonding of the Al complex is evident from the enhanced charge delocalization onto the peripheral ligand substituents (N-Ph) via the π-framework, as shown by the rotation barrier around the N-C(Ph) bond. The electron-rich nature of these compounds allows facile benzylation at the ligand, and the structures of the products were analysed by X-ray crystallography. The products are inorganic analogues of 1-alkylated 1,2,3,4-tetrahydro-1,2,4,5-tetrazines ('leucoverdazyls'). The six-membered heterocyclic cores of the B and Al compounds are shown to be different, having envelope- and boat-type conformations, respectively. Homolysis of the N-C(benzyl) bond in these compounds was studied by NMR spectroscopy under conditions that trap the organic radical as TEMPO-Bn. Analysis of the reaction kinetics affords activation parameters that approximate the N-C(benzyl) bond strength. The ionic Al compound has one of the weakest N-C bonds reported so far in this type of inorganic leucoverdazyl analogues

Cation effects on dynamics of ligand-benzylated formazanate boron and aluminium complexes

The dynamic processes present in ligand-benzylated formazanate boron and aluminium complexes are investigated using variable temperature NMR experiments and lineshape analyses. The observed difference in activation parameters for complexes containing either organic countercations (NBu4+) or alkali cations is rationalized on the basis of a different degree of ion-pairing in the ground state, and the data are in all cases consistent with a mechanism that involves pyramidal inversion at the nitrogens in the heterocyclic ring rather than homolytic N-C(benzyl) bond cleavage. This journal i

Reversible On/Off Switching of Lactide Cyclopolymerization with a Redox-Active Formazanate Ligand

Redox-switching of a formazanate zinc catalyst in ring-opening polymerization (ROP) of lactide is described. Using a redox-active ligand bound to an inert metal ion (Zn2+) allows modulation of the catalytic activity by reversible reduction/oxidation chemistry at a purely organic fragment. A combination of kinetic and spectroscopic studies, together with mass spectrometry of the catalysis mixture, provides insight in the nature of the active species and the initiation of lactide ring-opening polymerization. The mechanistic data highlight the key role of the redox-active ligand and provide a rationale for the formation of cyclic polymer

Formazanate coordination compounds:Synthesis, reactivity, and applications

Formazans (Ar1-NH-NCR3-NN-Ar5), a class of nitrogen-rich and highly colored compounds, have been known since the late 1800s and studied more closely since the early 1940s. Their intense color has led to their widespread use as dyes, especially in cell biology where they are most often used to quantitatively assess cell-viability. Despite structural similarities to well-known ligand classes such as β-diketiminates, the deprotonated form of formazans, formazanates, have received relatively little attention in the transition metal and main group coordination chemistry arenas. Formazanate ligands benefit from tunable properties via structural variation, rich optoelectronic properties owing to their highly delocalized π-systems, low-lying frontier orbitals that stabilize otherwise highly reactive species such as radicals, and redox activity and coordinative flexibility that may have significant implications in their future use in catalysis. Here, we review progress in the coordination chemistry of formazanate ligands over the past two decades, with emphasis on the reactivity and applications of the subsequent complexes

Catalytic Conversion of Nitriles by Metal Pincer Complexes

The nitrile is an extremely useful functional group in organic synthesis: it can be transformed into amides, carboxylic acids, amines and imines; yet it is relatively stable and can be easily carried through several synthetic steps before being converted. The conversions of nitriles under mild conditions are thus very important transformations. Great progress has been made in the last decade in the use of metal pincer complexes as catalysts for quite a number of reactions of nitriles and nitrile-containing molecules. The selective hydrogenation of nitriles either to the amines or to the imines usually follows a Noyori-type outer-sphere mechanism. Coordination of aliphatic nitriles to the metal centre renders the α-proton rather acidic allowing deprotonation followed by carbon-carbon coupling reactions. The pyridine-based metal pincer complexes introduced by Milstein allow for novel mechanisms based on metal-ligand cooperativity in which the pyridine undergoes dearomatisation induced by deprotonation of one of the side arms. The nitrile can undergo a cycloaddition to the complex in its dearomatised form, creating a new bond between the nitrogen atom and the metal, whereas the nitrile carbon atom forms a C-C bond with the carbon atom of one of the pincer side-arms. The resulting metalimide undergoes nucleophilic addition more easily than the nitrile. It can also easily rearrange to the enamide, which can undergo C-C bond forming reactions. Also, oxo- and aza-Michael reactions are facilitated on the unsaturated nitriles, such as acrylonitriles or pentenitriles. Most reactions proceed under mild conditions in excellent yields.</p

Selective α-Deuteration of Cinnamonitriles using D₂O as Deuterium Source

The selective α-deuteration of α,β-unsaturated nitriles using the strong base tBuOK or a metal-ligand cooperative Ru pincer catalyst is described. With D2O as deuterium source and glyme as solvent at 70 °C, tBuOK is an efficient catalyst for deuteration at the α-C(sp 2) position of cinnamonitriles, providing access to a broad range of deuterated derivatives in good to excellent yields and with very high levels of deuterium incorporation. While the tBuOK-catalysed protocol does not tolerate base-sensitive functional groups, cinnamonitrile derivatives containing a benzylic bromide or ester moiety were deuterated in excellent yields using Milstein's ruthenium PNN pincer catalyst. Moreover, the activity for H/D exchange of the metal-ligand cooperative Ru catalyst is found to be significantly higher than that of tBuOK, allowing reactions to proceed well even at room temperature. A mechanistic proposal is put forward that involves deprotonation of the cinnamonitrile α-CH position when using tBuOK as catalyst, whereas H/D exchange catalysis with the Ru PNN pincer likely proceeds via (reversible) oxa-Michael addition of D2O

Hydration of nitriles using a metal-ligand cooperative ruthenium pincer catalyst

Nitrile hydration provides access to amides that are important structural elements in organic chemistry. Here we report catalytic nitrile hydration using ruthenium catalysts based on a pincer scaffold with a dearomatized pyridine backbone. These complexes catalyze the nucleophilic addition of H2O to a wide variety of aliphatic and (hetero)aromatic nitriles in (BuOH)-Bu-t as solvent. Reactions occur under mild conditions (room temperature) in the absence of additives. A mechanism for nitrile hydration is proposed that is initiated by metal-ligand cooperative binding of the nitrile

Switching Pathways for Reversible Ligand Photodissociation in Ru(II) Polypyridyl Complexes with Steric Effects

The effect of a minor difference in ligand structure is shown to have a large effect on the photochemical pathways followed by two ruthenium(II) polypyridyl based complexes [Ru(CH3CN) (LL)](2+), 1 and 2, where LL is MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis (pyridin-2-yl-methyl) ethan-1-amine) or N4Py (1,1-di (pyri din-2-yl)-N,N-bis (pyridin-2-yl-m ethyl) methanamine), respectively. In our earlier report we demonstrated near completely reversible two-way photochromism of 1, in which a pyridyl ring dissociated on irradiation with visible light to form the thermally stable 1P, [Ru(CH3CN)(2)(MeN4Py)](2+). Complex 1 was recovered upon irradiation in the near-UV. Here, we show that the methyl group in the ligand backbone is critical to the reversibility by impeding the dissociation of one of the two sets of pyridyl rings. Irradiation of 2, which does not bear the methyl group, with visible light results in formation of two thermally stable isomers 2a and 2b, which are characterized by UV-vis absorption, FTIR, H-1 NMR spectroscopy, ESI mass spectrometry, and X-ray crystallography. In contrast to 1P, in both 2a and 2b, a different pyridyl moiety is dissociated. Whereas UV irradiation returns 2a to its original state (2), the overall reversibility is limited by the relative stability of 2b. The changes to the structure of 2 made possible by the increased freedom for all four pyridyl moieties to dissociate allows access to coordination modes that are not accessible thermally opening opportunities toward new catalysts for oxidation chemistry, photochromism and photoswitching.</p