In this (tutorial overview) perspective we highlight the use of “redox non-innocent” ligands in catalysis. Two main types of reactivity in which the redox non-innocent ligand is involved can be specified: (A) The redox active ligand participates in the catalytic cycle only by accepting/donating electrons, and (B) the ligand actively participates in the formation/breaking of substrate covalent bonds. On the basis of these two types of behavior, four main application strategies of redox-active ligands in catalysis can be distinguished: The first strategy (I) involves oxidation/reduction of the ligand to tune the electronic properties (i.e., Lewis acidity/basicity) of the metal. In the second approach (II) the ligand is used as an electron reservoir. This allows multiple-electron transformations for metal complexes that are reluctant to such transformations otherwise (e.g., because the metal would need to accommodate an uncommon, high-energy oxidation state). This includes examples of (first row) transition metals that have a tendency to react via one-electron pathways, and even “oxidative addition” reactions for d0 transition metal complexes become possible with redox active ligands as electron reservoirs. The electron-reservoir function of the ligand tolerates the metal to maintain its most common or most stable oxidation state by delivering or accepting the electron density associated with the multielectron transformation (most typically two-electron transformations such as oxidative addition/reductive elimination). The third strategy (III) involves the generation of reactive ligand-radicals that actively participate in the making and breaking of chemical bonds during catalysis. Cooperative substrate activation by the redox non-innocent ligand and the metal allows reactions that are difficult to achieve otherwise. The last strategy (IV) involves (radical-type) activation of the substrates or modification of the substrate reactivity in cases where the substrate itself acts as a redox non-innocent ligand. These four approaches are illustrated by recent literature data.

The mechanism of cobalt(II) porphyrin-catalyzed benzylic C-H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides [N(3)C(O)OMe, N(3)SO(2)Ph, N(3)C(O)Ph, and N(3)P(O)(OMe)(2)] as nitrene sources was studied by means of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy. The DFT computational study revealed a stepwise radical process involving coordination of the azide to the metal center followed by elimination of dinitrogen to produce unusual "nitrene radical" intermediates (por)Co(III)-N(•)Y (4) [Y = -C(O)OMe, -SO(2)Ph, -C(O)Ph, -P(O)(OMe)(2)]. Formation of these nitrene radical ligand complexes is exothermic, predicting that the nitrene radical ligand complexes should be detectable species in the absence of other reacting substrates. In good agreement with the DFT calculations, isotropic solution EPR signals with g values characteristic of ligand-based radicals were detected experimentally from (por)Co complexes in the presence of excess organic azide in benzene. They are best described as nitrene radical anion ligand complexes (por)Co(III)-N(•)Y, which have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety. These key cobalt(III)-nitrene radical intermediates readily abstract a hydrogen atom from a benzylic position of the organic substrate to form the intermediate species 5, which are close-contact pairs of the thus-formed organic radicals R'(•) and the cobalt(III)-amido complexes (por)Co(III)-NHY ({R'(•)···(por)Co(III)-NHY}). These close-contact pairs readily collapse in a virtually barrierless fashion (via transition state TS3) to produce the cobalt(II)-amine complexes (por)Co(II)-NHYR', which dissociate to afford the desired amine products NHYR' (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R'(•)···(por)Co(III)-NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R'(•) by (por)Co(III)-NHY (via TS4) to form the corresponding olefin and (por)Co(III)-NH(2)Y, which dissociates to give Y-NH(2). This process for the formation of olefin and Y-NH(2) byproducts is also essentially barrierless and should compete with the collapse of 5 via TS3 to form the desired amine product. Alternative processes leading to the formation of side products and the influence of different porphyrin ligands with varying electronic properties on the catalytic activity of the cobalt(II) complexes have also been investigated.

The electronic structure, spectroscopic features, and (catalytic) reactivity of complexes with nitrogen-centered radical ligands are described. Complexes with aminyl ([M(˙NR2)]), nitrene/imidyl ([M(˙NR)]), and nitridyl radical ligands ([M(˙N)]) are detectable and sometimes even isolable species, and despite their radical nature frequently reveal selective reactivity patterns towards a variety of organic substrates. A classification system for complexes with nitrogen-centered radical ligands based on their electronic structure leads to their description as one-electron-reduced Fischer-type systems, one-electron-oxidized Schrock-type systems, or systems with a (nearly) covalent M-N π bond. Experimental data relevant for the assignment of the radical locus (i.e. metal or ligand) are discussed, and the application of complexes with nitrogen-centered radical ligands in the (catalytic) syntheses of nitrogen-containing organic molecules such as aziridines and amines is demonstrated with recent examples. This Review should contribute to a better understanding of the (catalytic) reactivity of nitrogen-centered radical ligands and the role they play in tuning the reactivity of coordination compounds.

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu(2)PCH(2)BPh(2) with H(2), CO(2), and isocyanates and supported computationally.

To fully characterize the CoIII–‘nitrene radical’ species that are proposed as intermediates in nitrene transfer reactions mediated by cobalt(II) porphyrins, different combinations of cobalt(II) complexes of porphyrins and nitrene transfer reagents were combined, and the generated species were studied using EPR, UV–vis, IR, VCD, UHR-ESIMS, and XANES/XAFS measurements. Reactions of cobalt-(II) porphyrins 1P1 (P1 = meso-tetraphenylporphyrin (TPP)) and 1P2 (P2 = 3,5-DitBu-ChenPhyrin) with organic azides 2Ns (NsN3), 2Ts (TsN3), and 2Troc (TrocN3) led to the formation of mono-nitrene species 3P1Ns, 3P2Ts, and 3P2Troc, respectively, which are best described as [CoIII(por)(NR″•−)] nitrene radicals (imidyl radicals) resulting from single electron transfer from the cobalt(II) porphyrin to the ‘nitrene’ moiety (Ns: R″ = –SO2-p-C6H5NO2; Ts: R″ = –SO2C6H6; Troc: R″ = –C(O)OCH2CCl3). Remarkably, the reaction of 1P1 with N-nosyl iminoiodane (PhI=NNs) 4Ns led to the formation of a bis-nitrene species 5P1Ns. This species is best described as a triple-radical complex [(por•−)CoIII(NR″•−)2] containing three ligand-centered unpaired electrons: two nitrene radicals (NR″•−) and one oxidized porphyrin radical (por•−). Thus, the formation of the second nitrene radical involves another intramolecular one-electron transfer to the “nitrene” moiety, but now from the porphyrin ring instead of the metal center. Interestingly, this bis-nitrene species is observed only on reacting 4Ns with 1P1. Reaction of the more bulky 1P2 with 4Ns results again in formation of mainly mono-nitrene species 3P2Ns according to EPR and ESI-MS spectroscopic studies. The mono- and bis-nitrene species were initially expected to be five- and six-coordinate species, respectively, but XANES data revealed that both mono- and bis-nitrene species are six-coordinate Oh species. The nature of the sixth ligand bound to cobalt(III) in the mono-nitrene case remains elusive, but some plausible candidates are NH3, NH2−, NsNH−, and OH−; NsNH− being the most plausible. Conversion of mono-nitrene species 3P1Ns into bis-nitrene species 5P1Ns upon reaction with 4Ns was demonstrated. Solutions containing 3P1Ns and 5P1Ns proved to be still active in catalytic aziridination of styrene, consistent with their proposed key involvement in nitrene transfer reactions mediated by cobalt(II) porphyrins.

In situ-generated metal carbonyl-complexed cyclopropenylphosphinidenes undergo a sequence of structural changes leading to phosphorus analogues of Pettit's seminal (η 4 -cyclobutadiene)iron tricarbonyl complex via multiple valence isomers along the reaction pathway and the elimination of one molecule of carbon monoxide.C yclobutadiene (CBD, C) has intrigued chemists for decades, 1 with landmarks such as the isolation of the parent C 4 H 4 inside a hemicarcerand cage, 2 the photoisomerization of a derivative to the highly strained tetrahedrane D, 3 and the formation of the seminal (η 4 -cyclobutadiene)iron tricarbonyl complex (E). 4 One effective route to CBD is ring expansion of the in situ-generated cyclopropenylcarbene B by N 2 elimination from cyclopropenyldiazomethane A. 5 This elegant rearrangement (B f C) has also been applied for sila 6 and aza analogues, 7,8 but detailed mechanistic insight is still lacking. In view of the diagonal CÀP relationship, it is surprising that the phosphacyclobutadiene ring structure (P 1 -CBD) 9 is known only as a rare transition-metal ligand (F) constructed from phosphaalkynes (R 1 CtP) and alkynes 10 and that no phosphatetrahedranes (P 1 -D) have ever been synthesized. 11 Here we report on the application of the phosphorus analogue of the cyclopropenylcarbeneÀ cyclobutadiene rearrangement for the selective formation of the P 1 analogues of Pettit's complex E. A detailed computational study has shown that the facile isomerization of the in situ-generated metal carbonyl-complexed cyclopropenylphosphinidenes P 1 -B 12,13 comes with surprises and has highlighted for the first time the connection of P 1 -B with the corresponding tetrahedranes P 1 -D.In this study, we focused on the rearrangement of the sterically shielded 1,2,3-tris(tert-butyl)cyclopropenyl-substituted phosphinidenes 14 4 bearing the metal carbonyl (ML n ) fragments Fe(CO) 4 (a), W(CO) 5 (b), and Mo(CO) 5 (c) (Scheme 1). These transient intermediates can be generated by thermal fragmentation of the corresponding 3H-3-benzo[d]phosphepines 3. 15 The starting material for 3, bis(trimethylsilyl)-1,2,3-tris(tert-butyl)cyclopropenylphosphane (1) ( 31 P NMR: δ = À134.6 ppm, 1 J PSi = 47.6 Hz) was prepared according to literature procedures 16 and converted quantitatively into its primary phosphane ( 31 P NMR: δ = À119.1 ppm, 1 J PH = 189.3 Hz) using 2.4 equiv of MeOH. Subsequent complexation with the appropriate metal carbonyl source afforded the air-and moisture-stable complexes 2aÀc after purification by column chromatography (54À64%; Scheme 1). Subsequently, the base-catalyzed double hydrophosphination of 1,2-diethynylbenzene with the primary phosphane complexes 2 afforded phosphepines 3, albeit in modest yields [12À41%; 31 P NMR: δ = 36.2 (3a), À16.9 (3b), 1.5 ppm (3c)], which we ascribed to the bulky nature of the P substituent (see the Supporting Information).Thermal fragmentation of 3aÀc at 100°C in toluene selectively generated the η 4 -phosphacyclobutadiene complexes 5 (Scheme 1), which were isol...

New, stable heterobidentate phosphane–olefin ligands based on the dibenzo[b,f]phosphepine backbone are reported together with their redox properties and coordination chemistry to rhodium(I). The X-ray crystal structures and DFT calculations show different conformations for the P-phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltammetry (vs Fc/Fc+) of 6a supported by UV–vis spectroelectrochemistry showed two cathodic waves, a reversible one at E 1/2 = −2.62 V (I f/I b = 1.0) and a quasi-reversible (I f/I b ≈ 1.2) one at E 1/2 = −3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a •]−, characterized by EPR spectroscopy, and dianion [6a]2–, for which an X-ray crystal structure was obtained. Both 6a and 6b bind to RhI centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two dibenzo[b,f]phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b showed two close-lying waves, a one-electron reversible wave at −1.45 V and a two-electron quasi-reversible wave at −1.80 V (I f/I b ≈ 1.3). 8a showed a reversible wave at −1.71 V and irreversible waves at lower potentials.

Der vorliegende Aufsatz gibt einen Überblick über die elektronische Struktur, die spektroskopischen Eigenschaften und die (katalytische) Reaktivität von Komplexen mit Stickstoffradikalliganden. Komplexe mit Aminyl‐ ([M(.NR2)]), Nitren/Imidyl‐ ([M(.NR)]) oder Nitridylradikalliganden ([M(.N)]) sind nachweisbare und manchmal auch isolierbare Spezies, die trotz ihres Radikalcharakters oft selektive Reaktionsmuster gegenüber einer Vielzahl organischer Substrate zeigen. Für Komplexe mit Stickstoffradikalliganden wird ein Klassifizierungssystem vorgestellt, das auf ihrer Elektronenstruktur basiert und sie als Einelektronen‐reduzierte Fischer‐Systeme, Einelektronen‐oxidierte Schrock‐Systeme oder Systeme mit (fast) kovalenter M‐N‐π‐Bindung beschreibt. Experimentelle, für die Bestimmung des Aufenthaltsortes des Radikals (d. h. Metall oder Ligand) relevante Ergebnisse werden diskutiert, und aktuelle Beispiele aus der Literatur belegen die Verwendung von Komplexen mit Stickstoffradikalliganden in den (katalytischen) Synthesen verschiedener organischer Stickstoffverbindungen wie Aziridine und Amine. Dieser Aufsatz soll dazu beitragen, die (katalytische) Reaktivität von Stickstoffradikalliganden und ihre Rolle bei der Feinabstimmung der Reaktivität von Koordinationsverbindungen besser zu verstehen.