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Square-planar cobalt(II)-systems have emerged as powerful carbene transfer catalysts for the synthesis of a variety of (hetero)cyclic compounds via redox non-innocent Co(III)-carbene radical intermediates. Spectroscopic detection and characterization of these reactive carbene radical intermediates has thus far been limited to a few scattered experiments, in part due to the fact that most studies have focused on mono-substituted carbene precursors. In this work, we demonstrate the unique formation of disubstituted cobalt(III)-carbene radicals in reactions between a cobalt(II)-porphyrin com-plex with acceptor-acceptor iodaneylidenes (iodonium ylides) as the carbene precursors. We report detailed spectroscopic characterization of the resulting reactive carbene radical species, and their application in styrene cyclopropanation. In particular, we demonstrate that iodonium ylides generate novel bis-carbenoid species leading to reversible substrate-promoted ligand modification of the commercially available [Co(TPP)]-catalyst. Two interconnected catalytic cycles are involved in the overall catalytic reaction with a mono-terminal carbene radical and an unprecedented N-enolate-carbene radical intermediate as the respective key species for the mono- and bis-carbene cycles. Notably, N-enolate formation is not a catalyst deactivation pathway, and both the N-enolate and the carbene radical moieties can be transferred as carbene units to styrene. The studies provide a detailed picture of the new [Co(TPP)]-catalyzed carbene transfer reactions from iodonium ylides. The findings are supported by detailed and unequivocal characterization of the reactive N-enolate & carbene radical intermediates and their deactivation products (EPR, UV-Vis, HR-MS, NMR, in-situ ATR-FT-IR, SC-XRD), Hammett analysis, mechanistic control experiments, DFT reaction pathway profiling and NEVPT2-CASSCF electronic structure calculations.

We herein report on the application and structural investigation of a new set of complexes that contain bidentate N-heterocyclic carbenes (NHCs) and primary amine moieties of the type [M(arene)Cl(L)] [M=Ru, Ir, or Rh; arene=p-cymene or pentamethylcyclopentadienyl; L=1-(2-aminophenyl)-3-(n-alkyl)imidazol-2-ylidine]. These complexes were tested and compared in the hydrogenation of acetophenone with hydrogen. Structural variations in the chelate ring size of the heteroditopic ligand revealed that smaller chelate ring sizes in combination with ring conjugation in the ligand are beneficial for the activity of this type of catalyst, favoring an inner-sphere coordination pathway. Additionally, increasing the steric bulk of the alkyl substituent on the NHC aided the reaction, showing almost no induction period and formation of a more active catalyst for the n-butyl complex relative to complexes with smaller Me and Et substituents. As is common in hydrogenation reactions, the activity of the complexes decreases in the order Ru>Ir>Rh. The application of [Ru(p-cym)Cl(L)]PF6, which outperforms its reported analogues, has been successfully extended to the hydrogenation of more challenging biomass-inspired substrates.

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 approximate to 1.2) one at E-1/2 = -3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a(center dot)](-), characterized by EPR spectroscopy, and dianion [6a](2-), for which an X-ray crystal structure was obtained. Both 6a and 6b bind to Rh-1 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 approximate to 1.3). 8a showed a reversible wave at -1.71 V and irreversible waves at lower potentials.

The synthesis of the novel C-3-symmetric tripodal, tetradentate ligand 1, bearing only phosphorus atoms as donor groups, is described, starting from commercially available o-tolyldiphenylphosphine, and its molecular structure has been determined by X-ray crystallographic analysis. Coordination to the cationic Ir-1 precursor [Ir(COE)(2)(acetone)(2)]PF6 led to a highly unsymmetrical species (90% yield) with four inequivalent phosphorus atoms, as evidenced by P-31 NMR spectroscopy. The corresponding H-1 NMR spectrum exhibited a pseudo doublet of quartets at delta -5.9 ppm with one large trans P-H coupling ((2)J(P-H) = 115.4 Hz) and a much smaller cis coupling ((2)J(P-H) = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir(H)(kappa P-5,P,P,P,C-1)]PF6, which is a rare example of a structurally characterized mononuclear Ir hydride species bearing an ortho-metalated phosphine ligand. This species does not react with hydride sources, but addition of 1 equiv of CF3COOH resulted in facile overall formal protonation of the Ir-C bond. DFT calculations support a pathway involving initial reductive elimination, forming the highly distorted four-coordinate species 2', followed by protonation at iridium to give the dicationic monohydride species 3, with an activation barrier Delta G(dagger) of 28.2 kcal mol(-1). Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of 2 with carbon monoxide, yielding the monocationic carbonyl complex [Ir-1(CO)(1)]PF6 as the sole product.

The previous mechanistic studies of the Rh-mediated polymerization of carbenes suggests the involvement of organometallic compounds that have been derived from Rh(diene) species that contain a five-membered chelate ring of the type Rh{κ2-C,O-[-CH(COOR)-CH(Pol)-C(OR)=O-]} (Pol = polymer chain), which are characterized by the coordination of the β-ester group of the growing polymer chain to the metal. Herein we present our efforts to characterize the possibly related RhI(cod){κ2-C,O-[-CH(COOR)-CHR′-C(OR)=O-]} (cod = 1,5-cyclooctadiene) species. These structures can be generated by means of olefin exchange with an allyl complex and with the concomitant release of the corresponding 1,3-diene, which is derived from the allyl ligand. The product is unstable and further reacts by means of a bimolecular C-H activation process to form a related dinuclear complex [(cod)Rh(μ-{-CH(COOR)-CH-C(OR)=O-})2Rh(cod)], in which the carbon dianion of the dimethyl or diethyl succinate bridges two Rh(cod) moieties. The complex was synthesized in an independent way and was structurally characterized. We investigated the importance of these complexes in carbene polymerization.

The high-yielding synthesis and detailed characterization of two well-defined, linkage isomeric tripodal, tetradentate all-phosphorus ligands 1-3 is described. Coordination to Cu(I) resulted in formation of complexes 4-6, for which the molecular structures indicate overall tridentate coordination to the copper atom in the solid state, with one dangling peripheral phosphine. The solution studies suggest fast exchange between the three phosphine side-arms. For these new Cu(I) complexes, preliminary catalytic activity in the cyclopropanation of styrene with ethyldiazoacetate (EDA) is disclosed. The anticipated well-defined tetradentate coordination in a C(3)-symmetric fashion was achieved with Rh(I) and Ir(I), leading to the overall five-coordinated complexes 7-12. Complex 11 has the norbornadiene (nbd) ligand coordinated in an unprecedented monodentate 2,3-eta(2) mode to Rh. Furthermore, unexpected but very interesting redox-chemistry and reactivity was displayed by the Rh(Cl)-complexes 7 and 8. Oxidation resulted in the formation of stable Rh(II) metalloradicals [7]PF(6) and [8]PF(6) that were characterized by X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, and electron paramagnetic resonance (EPR) spectroscopy. Subsequent redox-reactivity of these metalloradicals toward molecular hydrogen is described, resulting in the formation of Rh(III) hydride compounds.

We report the coordination behavior of ureaphosphane ligand 1[2-(diphenylphosphanyl)ethyl]-3-phenylurea (L1) towards different rhodium precursor complexes. Depending on the nature of the anion and the ligand/metal ratio, L1 acts either as a hybrid P, O-coordinating chelate, as an anionic P,N-coordinating chelate or as a supramolecular (hydrogen-bonded) bidentate ligand. The different types of complexes were investigated in the asymmetric hydrogenation of three olefinic substrates using ureaphosphanes L2 and L3, which are chiral analogues of L1. For methyl 2-acetamidoacrylate (Si) and dimethyl itaconate (S2) the neutral complex [Rh(L2-kappa P-2,N)(cod)], containing an anionic RN-coordinating ureaphosphane ligand, provided the best enantioselectivity (69.2 and 24.3 %, respectively). For N-(3,4-dihydro-2-naphth-alenyl)acetamide (S3) the best enantioselectivity was obtained with the cationic complex [Rh(L3-kappa P-2,O)(nbd)]BF4, containing a hybrid PO-coordinating ureaphosphane ligand (86.1 %).

This chapter gives an overview of the different spin-state crossing scenarios affecting the reactivity of organometallic compounds. It focuses on the effects of crossing spin states in a number of elementary reactions typically observed for organometallic compounds, such as ligand exchange, oxidative addition, reductive elimination, migration/insertion, β-hydride elimination, C-H bond activation and intramolecular electron transfer between a transition metal and a coordinated redox active ligand. These processes are highly relevant for catalysis. To address the influence of the spin-crossing on the reaction rate, Schroder et al. introduced the two-state reactivity (TSR) concept, which applies if the overall kinetics for product formation results from an interplay between the barriers for the spin-inversion and the barriers on the high- and low-spin surfaces. The chapter discusses examples of homogeneous first-row transition metal systems that catalyze (cyclo)oligomerization reactions of alkynes and alkenes, and which undergo a spin-crossing during the catalytic turnover.

The design of ancillary ligands used to modify the structural and reactivity properties of metal complexes has evolved into a rapidly expanding sub-discipline in inorganic and organometallic chemistry. Ancillary ligand design has figured directly in the discovery of new bonding motifs and stoichiometric reactivity, as well as in the development of new catalytic protocols that have had widespread positive impact on chemical synthesis on benchtop and industrial scales. Ligand Design in Metal Chemistry presents a collection of cutting-edge contributions from leaders in the field of ligand design, encompassing a broad spectrum of ancillary ligand classes and reactivity applications. Topics covered include: Key concepts in ligand design; Redox non-innocent ligands; Ligands for selective alkene metathesis; Ligands in cross-coupling. Ligand design in polymerizationThe design of ancillary ligands used to modify the structural and reactivity properties of metal complexes has evolved into a rapidly expanding sub-discipline in inorganic and organometallic chemistry. Ancillary ligand design has figured directly in the discovery of new bonding motifs and stoichiometric reactivity, as well as in the development of new catalytic protocols that have had widespread positive impact on chemical synthesis on benchtop and industrial scales. Ligand Design in Metal Chemistry presents a collection of cutting-edge contributions from leaders in the field of ligand design, encompassing a broad spectrum of ancillary ligand classes and reactivity applications. Topics covered include: Key concepts in ligand design; Redox non-innocent ligands; Ligands for selective alkene metathesis; Ligands in cross-coupling; Ligand design in polymerization.

Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined RhI complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (LH = PNCH = 2-di(tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh–CPh bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal–ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H2 evolution from putative Rh(CO)(H)(LH) complex A is very facile, proposedly encompassing formal C–H oxidative addition at Rh to give Cvia agostic intermediate B and subsequent reductive elimination of H2. Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H2, supporting a cooperative pathway. Reversible C–H activation is thus a viable mechanism for metal–ligand bifunctional catalysis.

A series of mononuclear Fe(II) triflate complexes based on the 3,3-bis(1-alkylimidazole-2-yl)propionate ester (BAIP) ligand scaffold are reported. I, these complexes, the tripodal N,N,O-BAIP ester ligand is varied by (i) changing the ester moiety (i.e., n-Pr, tert-Bu esters, n-Pr amide), (ii) changing the methylimidazole moieties to methylbenzimidazole moieties, and (iii) changing the methylimidazole moieties to 1-ethyl-4-isopropylimidazole moieties. The general structure of the resulting complexes comprises two facially capping BAIP ligands around a coordinatively saturated octahedral Fe(II) center, with either a transoid or cisoid orientation of the N,N,O-donor manifold that depends on the combined steric and electronic demand of the ligands, In the ligands. In the case of the sterically most encumbered ligand, a four-coordinate all N-coordinate complex is formed as well, which cocrystallizes with the six-coordinate complex. In combination with the catalytic properties of the new complexes in the epoxidation/cis-dihydroxylation of cyclooctene with H2O2, in terms of turnover number and cis-diol formation, these studies provide a number of insights for further ligand design and catalyst development aimed at Fe-mediated cis-dihydroxylation.

The first example of an iridium(I) species bearing a terminal phosphido (PR(2)(-)) ligand is reported. This stable compound shows well-behaved reactivity toward various electrophiles, owing to its exposed phosphorus lone pair, allowing reversible protonation, selective alkylation, isolation of a phosphidoborane of iridium, and generation of a phosphido-bridged iridium(I)-gold(I) dinuclear species.

The purpose of this review is to show that (cooperative) ligand radical reactivity can be effectively employed in synthetic organometallic chemistry and catalysis to achieve selectivity in radical-type transformations. The ‘redox non-innocence’ of ligands, and the controlled reactivity of ‘ligand radicals’ – giving rise to new, intriguing substrate transformations – allow unusual and selective radical-type substrate coupling reactions, ligand rearrangements and C-Y bond formations. In this review, several examples of fast and selective ligand-centered radical transformations in the open-shell organometallic chemistry of transition metals are described, focussing on radical-type reactions of olefin-, vinylidene-, vinyl-, alkyne-, allyl-, propargyl-, carbonyl-, and carbene ligands. Intriguing and selective substrate-substrate coupling reactions, (covalent) carbon-metal bond formations and hydrogen atom transfer reactions from and to ligand radicals are summarized. To conclude this chapter, an overview of recently disclosed new insights in the catalytic mechanism of CoII(por) catalysed olefin cyclopropanation is presented, showing that enzyme-like cooperative metal-ligand-radical reactivity is no longer reserved to real enzymes, but is a useful new concept to steer and control radical-type transformations in future bio-inspired organometallic catalysis.

The binuclear iridium complex [(cod)(Cl)Ir(bpi)Ir(cod)]PF6 (bpi = (pyridin-2-ylmethyl)(pyridin-2-ylmethylene)amine; cod = 1,5-cyclooctadiene) reveals a noteworthy asymmetric binuclear coordination geometry, wherein the bpi ligand acts as a heteroditopic ligand and has an unusual π-coordinated imine moiety. This species is an effective precatalyst for water oxidation. After a short incubation time the catalyst reveals a turnover frequency of 3400 mol mol-1 s-1 with an overall turnover number >1000. © 2011 American Chemical Society., Financial support from the European Research Council (ERC Grant Agreement 202886-CatCIR), NWO-CW (VIDI grant 700.55.426, VENI grant 700.59.410), the MEC/FEDER (Project CTQ2008-03860, Spain), and the University of Amsterdam is gratefully acknowledged.

Cationic rhodium and iridium complexes with dibenzo[a,e]cyclooctatetraene (dbcot) and bislutidylamine (bla) were synthesized from the respective [M(μ-Cl)(dbcot)]2 dimers and the bla ligand and were characterized by NMR, X-ray diffraction, and cyclic voltammetry. Both [MI(dbcot)(bla)]+ complexes are easily oxidized by one electron to form paramagnetic dicationic analogues [MII(dbcot)(bla)]2+ (M = Rh, Ir). In the solid state, the geometry changes from a distorted trigonal bipyramid to square pyramidal upon oxidation of the metal from MI to MII, as evidenced by X-ray diffraction studies of the iridium complexes. The paramagnetic complexes undergo facile one-electron reduction in the presence of base, most likely involving oxidation of the solvent or hydroxide anions. A control experiment using the corresponding [RhII(dbcot)(Bn-bla)]2+ complex, wherein Bn-bla is the N-benzyl-protected bla ligand, gives similar results, which strongly suggests that deprotonation of the N−H moiety of the [MII(dbcot)(bla)]2+ plays a minor role (if any at all) in the observed base-mediated reduction processes. Reaction of bla with [Ir(μ-Cl)(coe)]2 (coe = cis-cyclooctene) results in oxidative addition of the vinylic C−H bond of coe to the metal center, as evidenced by X-ray diffraction.

The synthesis of a new, all-phosphorus pincer PPNEt2P ligand L3NEt2, which is derived from 2-indolylphosphine and features a central N2P(NEt2) core, is described. This ‘PPP’ species shows coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PPHP ligands L1H and L2H, featuring a secondary phosphine core, show ‘ambivalent’ coordination, acting as persistent neutral triphosphine ligands with Rh, and as easily-formed monoanionic phosphido(bisphosphine) pincer ligands toward Pd. These subtle differences, which might be more general for group 9 and 10 metal complexes with this ligand set, are explained by comparative DFT calculations (BP86; def2-TZVP level of theory) for the Rh and Pd species involved, including those with the structurally related PNHP ligands. The optimized structure for complex PdCl(L2) indicates minimal overlap of available Pd d-orbitals with the lone pair of the central, deprotonated phosphorus atom (formally a phosphido fragment), suggesting that it behaves predominantly like a bulky phosphine instead of a phosphido fragment.

A series of complexes ML2(x+) (M = Mn-Zn, L = 2,6-bis(iminomethyl)pyridine) was investigated by theoretical methods. Electron transfer from the metal "t(2g)" orbitals to the ligand pi orbitals is reflected in the elongation of ligand C-N bonds and shortening of the C(py)-C(imine) bonds. Using zinc complexes as references, these deformations could be used to quantify the number of electrons transferred. Strong transfer is found in low-spin MnL2(+) (ca. 2 e) and in high-spin MnL2(+) and low-spin MnL2(2+), FeL2(2+), and CoL2(+) (ca. 1 e each). Smaller transfer is found in CoL(2)(2+), and the transfer is insignificant in high-spin MnL2(2+), NiL2(2+), and CuL2(2+). Analysis of the unpaired electron density on the metal (using the Staroverov-Davidson method) shows that the contribution of a biradical description, in which ligand radical anions are antiferromagnetically coupled to the metal center, is significant in most cases. In the case of CoL2(+) and high-spin MnL2(+), where the metal-ligand bond is weakened, it amounts to over 50% of the total transfer.