Monomeric Cyclopentadienylnickel Methoxo and Amido Complexes: Synthesis, Characterization, Reactivity, and Use for Exploring the Relationship between H−X and M−X Bond Energies

Molecular Dihydrogen Activation by (C5Me5)M/N (M=Rh, Ir) Transition Metal Frustrated Lewis Pairs: Reversible Proton Migration to, and Proton Abstraction from, the C5Me5 Ligand

The masked transition-metal frustrated Lewis pairs [Cp*M(κ3N,N',N''-L)][SbF6] (Cp*=η5-C5Me5; M=Ir, 1, Rh, 2; HL=pyridinyl-amidine ligand) reversibly activate H2 under mild conditions rendering the hydrido derivatives [Cp*MH(κ2N,N'-HL)][SbF6] observed as a mixture of the E and Z isomers at the amidine C=N bond (M=Ir, 3Z, 3E; M=Rh, 4Z, 4E). DFT calculations indicate that the formation of the E isomers follows a Grotthuss type mechanism in the presence of water. A mixture of Rh(I) isomers of formula [(Cp*H)Rh(κ2N,N'-HL)][SbF6] (5 a-d) is obtained by reductive elimination of Cp*H from 4. The formation of 5 a-d was elucidated by means of DFT calculations. Finally, when 2 reacts with D2, the Cp* and Cp*H ligands of the resulting rhodium complexes 4 and 5, respectively, are deuterated as a result of a reversible hydrogen abstraction from the Cp* ligand and D2 activation at rhodium.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

A Tutorial on Selectivity Determination in C(sp2)-H Oxidative Addition of Arenes by Transition Metal Complexes

A Tutorial on factors that determine the selectivity in C(sp²)–H activation and functionalization reactions involving two-electron oxidative addition processes with transition metals is presented. The interplay of the thermodynamics of C(sp²)–H oxidative addition and kinetic influences upon regioselectivity are presented alongside pedagogically valuable experimental and computational results from the literature. Mechanisms and energetics of chelate-assisted C(sp²)–H oxidative addition are examined, as are concepts related to chemoselectivity in the oxidative addition of C(sp²)–H or C(sp²)–X (X = F, Cl, Br, I) bonds with aryl halide substrates.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non-innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C-H activation at one methyl terminus of C5Me5 in [(η-C5Me5)Rh(PMe3)2] to form a new Rh-H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C-H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E-H bonds (E=O, N), including the N-H bonds in ammonia, can be understood in terms of bimetallic frustration.

Comparing Protonolysis and Transmetalation Reactions: Microcalorimetric Studies of C-AuI Bonds in [AuRL] Complexes

The protonolysis of C-Au bonds in [AuRL] organometallic complexes has been studied by calorimetry for 12 R groups. The experimental data have been combined with density functional theory calculations to obtain bond dissociation energy (BDE) values. The C-Au BDE values show a good correlation with the corresponding isolobal C-H BDE values. The heat released in the protonolysis of [AuRL] has also been measured for R = Ph and L = P(OPh)₃, PPh₃, PMe₃, PCy₃, and IPr, and these values strongly depend on the trans influence of L because of the mutual destabilization of the L-Au and Au-C bonds. The enthalpies of the transmetalation reaction [AuR(PPh₃)] + SnIBu₃ → [AuI(PPh₃)] + SnRBu₃ for seven R groups have been measured and compared with those of the corresponding [AuR(PPh₃)] protonolysis.

Nickel Pincer Complexes with Frequent Aliphatic Alkoxo Ligands [(iPrPCP)Ni-OR] (R = Et, nBu, iPr, 2-hydroxyethyl). An Assessment of the Hydrolytic Stability of Nickel and Palladium Alkoxides

A series of nickel pincer complexes with terminal alkoxo ligands [(^iPrPCP)Ni-OR] (R = Et, nBu, iPr, CH₂CH₂OH; ^iPrPCP is the 2,6-bis(diisopropylphosphinomethyl)phenyl pincer ligand) was synthesized and fully characterized. Together with the previously reported methoxo analogues of Ni and Pd, these complexes constitute a unique series of isostructural late transition-metal alkoxides. Spectroscopic and X-ray diffraction data provide direct indications of the strong polarization of their covalent Ni-OR bonds. One of the most salient features of this class of compounds is their facile hydrolysis with traces of moisture, leading to equilibrium mixtures with the corresponding hydroxides [(^iPrPCP)M-OH] (M = Ni or Pd) and alcohols, ROH. To compare the hydrolytic stability of nickel and palladium alkoxides, we performed NMR titrations of both hydroxides with several alcohols and determined the corresponding equilibrium constants. In general, these constants are ca. 1 order of magnitude smaller for M = Ni than Pd, indicating that Ni alkoxide complexes are more readily hydrolyzed than their Pd counterparts. For alkoxide complexes containing heteroatom-free R groups, the tendency to hydrolyze decreases as the parent alcohol ROH becomes more acidic, that is, R = Me > Et > iPr. This intuitive trend is broken for 2-methoxyethanol, the most acidic alcohol investigated. The hydroxo/2-methoxyethanol exchange equilibrium constants are comparable to those of ethanol (M = Ni) or methanol (M = Pd), showing that the corresponding 2-methoxyethoxide complexes are more prone to hydrolysis than anticipated. These experimental observations were rationalized in the light of density functional theory calculations.

The Essential Role of Bond Energetics in C-H Activation/Functionalization

The most fundamental concepts in chemistry are structure, energetics, reactivity and their inter-relationships, which are indispensable for promoting chemistry into a rational science. In this regard, bond energy, the intrinsic determinant directly related to structure and reactivity, should be most essential in serving as a quantitative basis for the design and understanding of organic transformations. Although C-H activation/functionalization have drawn tremendous research attention and flourished during the past decades, understanding the governing rules of bond energetics in these processes is still fragmentary and seems applicable only to limited cases, such as metal-oxo-mediated hydrogen atom abstraction. Despite the complexity of C-H activation/functionalization and the difficulties in measuring bond energies both for the substrates and intermediates, this is definitely a very important issue that should be more generally contemplated. To this end, this review is rooted in the energetic aspects of C-H activation/functionalization, which were previously rarely discussed in detail. Starting with a concise but necessary introduction of various classical methods for measuring heterolytic and homolytic energies for C-H bonds, the present review provides examples that applied the concept and values of C-H bond energy in rationalizing the observations associated with reactivity and/or selectivity in C-H activation/functionalization.

A terminal nickel(II) anilide complex featuring an unsymmetrically substituted amido pincer ligand: synthesis and reactivity

This work describes preparation and reaction chemistry of a terminal nickel(II) anilide complex supported by an unsymmetrically substituted diarylamido diphosphine ligand, [N(o-C(6)H(4)PPh(2))(o-C(6)H(4)P(i)Pr(2))](-) ([Ph-PNP-(i)Pr](-)). Treatment of NiCl(2)(DME) with H[Ph-PNP-(i)Pr] in THF at room temperature produced [Ph-PNP-(i)Pr]NiCl as green crystals in 82% yield. Salt metathesis of [Ph-PNP-(i)Pr]NiCl with LiNHPh(THF) in THF at -35 °C generated cleanly [Ph-PNP-(i)Pr]NiNHPh as a greenish blue solid. The anilide complex deprotonates protic (e.g., PhOH and PhSH) and aprotic (e.g., trimethylsilylacetylene, phenylacetylene, and acetonitrile) acids in benzene at room temperature to give quantitatively [Ph-PNP-(i)Pr]NiX (X = OPh, SPh, C≡CSiMe(3), C≡CPh, CH(2)CN). In addition, [Ph-PNP-(i)Pr]NiNHPh also behaves as a nucleophile to react with acetyl chloride to yield [Ph-PNP-(i)Pr]NiCl and N-phenylacetamide quantitatively. Carbonylation of [Ph-PNP-(i)Pr]NiNHPh with carbon monoxide affords cleanly the carbamoyl derivative [Ph-PNP-(i)Pr]Ni[C(O)NHPh]. The relative bond strengths of Ni-E in [Ph-PNP-(i)Pr]NiEPh (E = NH, O, S, C≡C) are assessed and discussed.

Rhodium and iridium amido complexes supported by silyl pincer ligation: ammonia N-H bond activation by a [PSiP]Ir complex

Ir silyl pincer complexes insert into the N-H bonds of anilines and ammonia under mild conditions to form isolable [Cy-PSiP]Ir(H)(NHR) complexes that are resistant to N-H bond reductive elimination at room temperature, even in the presence of arenes, alkenes, and phosphines.

Inside the black box — Perspectives on transformations in catalysis

Tandem catalysis and olefin metathesis are powerful tools in the development of sustainable synthetic practices. This Award Lecture describes our advances in designing new tandem metathesis-hydrogenation methodologies for the synthesis of “designer materials” and Ru-pseudohalide metathesis catalysts that amplify opportunities for tuning catalyst activity, selectivity, and lifetime. Also discussed is the operation of a previously unrecognized oligomerization-backbiting pathway in ring-closing metathesis of conformationally flexible α,ω-dienes, which has important implications for the sustainable synthesis of medium and large rings.Key words: tandem catalysis, green chemistry, olefin metathesis, ring-closing metathesis, mechanism.

Electronic structure and reactivity of three-coordinate iron complexes

[Reaction: see text]. The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (beta-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for pi interactions with ligands. Trends in sigma-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic sigma-bonding and the availability of pi-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong pi-backbonding to transfer charge from iron into coordinated alkynes and N 2, whereas iron(III) accepts charge from a pi-donating imido ligand. Though the imidoiron(III) complex is stabilized by pi-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the pi-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions and may be used by nature in transient intermediates of nitrogenase enzymes.