Addition of H 2 Across a Cobalt–Phosphorus Bond

Synthesis and Reactivity of the Rare-Earth Metal Complexes Bearing the Indol-2-yl-Based NCN Pincer Ligand

The pincer rare-earth dialkyl complexes [κ3-LRE(CH2SiMe3)2 (RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e))] with the indol-2-yl-based NCN pincer ligand were synthesized by the reactions of the proligand HL (L = 1-Me2NCH2CH2-3-(2-iPrC6H5N═CH)C8H4N) with RE(CH2SiMe3)3(THF)2. These complexes exhibited a variety of reactivities toward organic compounds such as amines, triphenylphosphine ylide, N-phenylimidazole, pyridine derivatives, and o-carborane leading to σ-bond metathesis, migration insertion, and redox reaction products. The reactions of the dialkyl rare-earth metal complexes with o-carborane afforded the novel NCN pincer-ligated carboryne-based metallacyclopropanes which reacted with diphenyl ketone to give insertion products of the RE-C2-ind and one of the RE-Ccage bonds, while the reaction of the carboryne-based metallacyclopropanes with diphenyldiazomethane produced the di-aza-metallacyclopentanes via the insertions of the N═N bond of the diphenyldiazomethane into two RE-Ccage bonds and the RE-C2-ind bond. The reactions of the dialkyl complexes with 2 equiv of 2,2'-bipyridine afforded the pincer-ligated bis(2,2'-bipyridyl monoanionic radical) complexes via the homolytic redox reaction.

Facile synthesis of five-membered cyclic RE2P-H iron(0) complexes

The synthesis of five-membered P-heterocyclic iron(0) complexes possessing a P-H unit and two heteroatoms (E = O, N), directly bound to phosphorus, is described. Initial problems to achieve access via "classical" reduction of P-Cl bonds of P-heterocycle complexes, e.g., leading to P-P coupling, could be solved by a "combined two-step" reduction/complexation. The use of K[Fe(CO)4H] not only opened access to such heterocyclic phosphane Fe(CO)4 complexes but also allowed the synthesis of sterically non-shielded 'secondary' heterocyclic phosphane complexes. Spectroscopic and structural parameters are discussed.

Cationic ligands - from monodentate to pincer systems

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

Gradual Coordination and Reversible P-P Bond Activation of a P3 -Unit with Transition Metal Carbonyls

Coordination of a stereochemically defined P3 -chain to a series of transition metal carbonyls [M(CO)x ]z- (M = Mn (x = 5, z = 1), Fe (x = 4, z = 2) or Co (x = 4, z = 1)) is explored using a [3]ferrocenophane scaffold. A gradual transition from η1 -, η2 - to η3 -coordination is observed where in the η2 -mode the terminal positions of the phosphorus chain are bridged. With an excess of cobalt carbonyl successive P-P bond activation and gradual separation of the central phosphorus atoms from the phosphorus chain has been observed. This process is reversible and with suitable reagents such as methyl lithium, the P3 -unit is regenerated in stereospecific manner. The bonding situation and steps of the gradual P-P bond activation are investigated by DFT calculations as well as experimental methods (e.g., NMR spectroscopy, X-ray crystallography).

Reversible Binding of Hydrogen and Styrene Coordination on a Manganese Phosphenium Complex

The reactions of two complexes [(R NHP)Mn(CO)4 ] (R NHP=N-arylated N-heterocyclic phosphenium) with H2 at elevated pressure (≈4 bar) were studied by NMR spectroscopy. Irradiation with UV light initialized in one case (5 a, R=Dipp) the unselective formation of (R NHP-H)MnH(CO)4 ] (6 a) via cooperative addition of H2 across the Mn=P double bond. In the other case (5 b, R=Mes), addition of H2 was unobservable and the reaction proceeded via decarbonylation to a dimeric species [(R NHP)2 Mn2 (CO)7 ] (7 b) that was isolated and identified spectroscopically. Taking into account the outcome of further reaction studies under various conditions in the absence and presence of H2 , both transformations can be explained in the context of a common mechanism involving decarbonylation to 7 a,b as the first step, and the different outcome is attributable to the fact that 7 b is unreactive towards both H2 and CO while 7 a is not. DFT studies relate this divergence to deviations in the molecular constitution and stability arising from a different level of steric congestion. Preliminary studies suggest further that 5 a/H2 as well as 6 a enable the photo-induced hydrogenation of styrene to ethyl benzene, even if the mechanism and possibly catalytic nature of this process remain yet unknown.

Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(iv) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(iii)-aluminum(i) and -gallium(i) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)2(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H2 with the U(iii)-Al(i) complex affords a trihydroaluminate complex, [(C5Me5)2(MesO)U(μ2-(H)3)-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C5Me5)2(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C5Me5)Al]4 are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)2(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)2(MesO)U(CH2CH3)] or treatment of [(C5Me5)2U(i)(Me)] with KOMes forms [(C5Me5)2(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)2(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.

Synthesis, Characterization, and Reactivity of a (PPP) Pincer-Ligated Manganese Carbonyl Complex: Polarity Reversal Imparted by the Electrophilic Nature of a Planar Mn-P(NR2)2 Fragment

The bonding interactions of a synthesized pincer-ligated manganese dicarbonyl complex featuring an N-heterocyclic phosphenium (NHP+) central moiety are explored. The pincer ligand [PPP]Cl was coordinated to a manganese center using Mn(CO)5Br and 254 nm light to afford the chlorophosphine complex (PPClP)Mn(CO)2Br (2) as a mixture of halide exchange products and stereoisomers. The target dicarbonyl species (PPP)Mn(CO)2 (3) was prepared by treatment of 2 with 2 equiv of the reductant KC8. Computational investigations and analysis of structural parameters were used to elucidate multiple bonding interactions between the Mn center and the PNHP atom in 3. The generation of a product of formal H2 addition, (PPHP)Mn(CO)2H (4), was achieved through the dehydrogenation of NH3BH3, affording a 2:1 mixture of 4syn:4anti stereoisomers. The nucleophilic nature of the Mn center and the electrophilic nature of the PNHP moiety were demonstrated through hydride addition and protonation of 3 to produce K(THF)2[(PPHP)Mn(CO)2] (6) and (PPClP)Mn(CO)2H (5), respectively. The observed reactivity suggests that 3 is best described as a Mn-I/NHP+ complex, in contrast to pincer-ligated dicarbonyl manganese analogues typically assigned as MnI species.

Electronic Effect on Phenoxide Migration at a Nickel(II) Center Supported by a Tridentate Bis(phosphinophenyl)phosphido Ligand

A phosphide nickel(II) phenoxide pincer complex (2) reacts with CO(g) to give a pseudo-tetrahedral nickel(0) monocarbonyl complex (3) possessing a phosphinite moiety. This metal-ligand cooperative (MLC) transformation occurs with a (PPP)Ni scaffold (PPP^- = P[2-PⁱPr₂-C₆H₄]₂^-), which can accommodate both square planar and tetrahedral geometries. The 2-electron reduction of a nickel(II) species induced by CO coordination involves group transfer to generate a P-O bond. For better mechanistic understanding, a series of nickel(II) phenolate complexes (2a-2e, XC₆H₄O^- (X = OMe, Me, H, and CF₃) and pentafluorophenolate) were prepared. Kinetic experimental data reveal that a phenolate species with an electron-withdrawing group reacts faster than those with electron-donating groups. The reaction kinetic experiments were conducted in pseudo-first order conditions at room temperature monitored by UV-vis spectroscopy. A pentafluorophenolate nickel(II) complex (2e) reveals instantaneous reactions even at -40 °C to give a nickel(0) monocarbonyl species (3e) and the reverse reaction is also possible. According to kinetic experiments, the rate determining step (RDS) would be the formation of a 5-coordinate intermediate 4 with a negative entropy value (ΔS^‡ < 0), and a positive ρ value based on the Hammett plot indicates that the electron-deficient phenolate leads to a faster CO association. Furthermore, scramble experiments suggest that phenolate de-coordinates from the intermediate 4, which gives a (PPP)Ni-CO species 6. The cationic nickel monocarbonyl intermediate can possess a P^--Ni(II), P•-Ni(I), or even a P⁺-Ni(0) character. Such an inner-sphere electron transfer is suggested when a π-acidic ligand such as CO coordinates to a metal ion. Another possible reaction is homolysis of a Ni-O bond to give P^--Ni(I) or P•-Ni(0), when a phenoxyl radical is liberated. Considering the P-O bond formation, closed-shell nucleophilic and open-shell radical pathways are suggested. A phenolate pathway reveals a lower energy state for 2e relative to other complexes (2c and 2d), while its radical pathway undergoes via a higher energy state. Therefore, the formation of a P-O bond may occur with the binding of a closed-shell phenolate to the electron-deficient P center.

Two-Site O-H Addition to an Iridium Complex Featuring a Nonspectator Tricoordinate Phosphorus Ligand

The synthesis and reactivity of an ambiphilic iridium complex IrCl(PPh3)(L1) (1; L1 = P(N(o-N(2-pyridyl)C6H4)2)) featuring a chelating nontrigonal phosphorus triamide ligand is reported. The tandem Lewis basic Ir and Lewis acidic P of 1 achieve a two-site oxidative addition of phenol giving the iridaphenoxyphosphorane species IrHCl(PPh3)(L1OPh) (3'). In contrast, reactions of 1 with benzenethiol and benzeneselenol do not engage L1 and instead proceed via metal-centered oxidative addition of the chalcogen-hydrogen bond. These findings establish metal-ligand cooperation involving nonspectator reactivity of tricoordinate phosphorus ligands.

Carbocyclic pincer carbene complexes of ruthenium: syntheses and reversible hydrogenation

The ruthenium carbene pincer complex 2 was synthesized treating the benzo annulated cycloheptatriene bisphosphine 1 with RuCl₃. Addition of three equivalents of hydrogen to the carbocyclic carbene complex 2 was achieved in reaction of 2 with hydrogen at elevated temperatures. Hydrogenated complex 4, exhibiting a rigid chair conformation in solution, was dehydrogenated by heating a toluene solution of complex 4 to reflux for 5-7 d. In reaction with ethylene, complex 4 transfers one equivalent of hydrogen, forming ethane and alkyl complex 5.

Enthalpy-Controlled Insertion of a "Nonspectator" Tricoordinate Phosphorus Ligand into Group 10 Transition Metal-Carbon Bonds

Insertion of a tricoordinate phosphorus ligand into late metal-carbon bonds is reported. Metalation of a P^P-chelating ligand (L1), composed of a nontrigonal phosphorous (i.e., P(III)) triamide moiety, P(N(o-N(Ar)C6H4)2, tethered by a phenylene linker to a -PiPr2 anchor, with group 10 complexes L2M(Me)Cl (M = Ni, Pd) results in insertion of the nontrigonal phosphorus site into the metal-methyl bond. The stable methylmetallophosphorane compounds thus formed are characterized spectroscopically and crystallographically. Metalation of L1 with (cod)PtII(Me)(Cl) does not lead to a metallophosphorane but rather to the standard bisphosphine chelate (κ2-L1)Pt(Me)(Cl). These divergent reactivities within group 10 are rationalized by reference to periodic variation in M-C bond enthalpies.

Reversible cooperative dihydrogen binding and transfer with a bis-phosphenium complex of chromium

The reversible reaction of H₂ with a bis-phosphenium complex of chromium provides a rare example of 3d transition metal/phosphenium cooperativity. Photolysis induces the activation of H₂ and yields a spectroscopically detectable phosphenium-stabilized (σ–H₂)-complex, readily showing exchange with gaseous H₂ and D₂. Further reaction of this complex affords a phosphine-functionalized metal hydride, representing a unique example of reversible H₂ cleavage across a 3d M Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P bond. The same species is also accessible via stepwise H⁺/H⁻ transfer to the bis-phosphenium complex, and releases H₂ upon heating or irradiation. Dihydrogen transfer from the H₂-complex to styrene is exploited to demonstrate the first example of promoting hydrogenation with a phosphenium complex.

Photolysis of a phosphenium complex enables reversible activation of H₂ to yield a dihydrogen complex which stimulates H₂ cleavage or catalytic hydrogenation.

PSb+P Ligand: Platform for a Stibenium to Transition-Metal Interaction

The coordination chemistry of cationic divalent pnictogen ligands, such as nitrenium and phosphenium, has been well-explored in recent years. However, corresponding studies of a heavier congener, stibenium ion, are rare. To better facilitate a Sb⁺-metal interaction, a tridentate P-Sb⁺-P ligand with two phosphine buttresses was designed and synthesized, and its coordination chemistry toward late transition metals was investigated. The stibenium ligand was delivered as an activated P(SbCl)P-AgOTf complex (2) that releases AgCl and the P-Sb⁺-P ligand upon the treatment with transition metals. Reacting 2 with Rh(I) and Ir(I) metals yielded the anticipated stibenium-transition-metal complexes [(Rh(COD)Cl)₂(μ-PSb⁺P)] OTf ([3][OTf]) and [(Ir(COD)Cl)₂(μ-PSb⁺P)] OTf ([4][OTf]). The M-Sb⁺-M bridging structure was confirmed by single-crystal X-ray crystallography, and the bonding situation was examined computationally. Theoretical studies revealed the presence of three-center delocalized M-Sb⁺-M bonding interactions in [3][OTf] and [4][OTf].

A Series of Dimeric Cobalt Complexes Bridged by N-Heterocyclic Phosphido Ligands

A tridentate [PPP] ligand has been used to construct a series of dimeric cobalt complexes and explore cooperative multielectron redox processes that are both metal- and ligand-centered. Reduction of (PP^ClP)CoCl₂ (1) with excess magnesium affords the Co^ICo^I N-heterocyclic phosphido (NHP^-)-bridged symmetric dimer [(μ-PPP)Co]₂ (2). Two-electron oxidation of 2 with FcPF₆ generates an asymmetrically bridged dication [(μ-PPP)Co]₂[PF₆]₂ (3) in which the oxidation has occurred in a delocalized fashion throughout the Co₂P₂ core. In contrast, [(μ-PPP)Co]₂⁺ (5), which can be generated either by one-electron oxidation of 2 with FcPF₆ or comportionation of 2 and 3, features an asymmetric geometry and localized mixed valence. Treatment of 1 with the milder reductants CoCp₂ and KBEt₃H does not lead to formation of 2, 3, or 5 but instead generates dimeric species [(PPP)CoCl]₂ (6) and [(PPP)CoH]₂ (7). Unlike 2-5, where the phosphine side arms of the tridentate [PPP] ligand span the two Co centers, complex 6 and 7 are connected solely by NHP^- ligands that bridge the two (PPP)Co fragments.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible "Nonspectator" L/X-Switching

We report here a "nonspectator" behavior for an unsupported L-function σ³ -P ligand (i.e. P{N[o-NMe-C₆ H₄ ]₂ }, 1a) in complex with the cyclopentadienyliron dicarbonyl cation (Fp⁺ ). Treatment of 1a⋅Fp⁺ with [(Me₂ N)₃ S][Me₃ SiF₂ ] results in fluoride addition to the P-center, giving the isolable crystalline fluorometallophosphorane 1a^F ⋅Fp that allows a crystallographic assessment of the variance in the Fe-P bond as a function of P-coordination number. The nonspectator reactivity of 1a⋅Fp⁺ is rationalized on the basis of electronic structure arguments and by comparison to trigonal analogue (Me₂ N)₃ P⋅Fp⁺ (i.e. 1b⋅Fp⁺ ), which is inert to fluoride addition. These observations establish a nonspectator L/X-switching in (σ³ -P)-M complexes by reversible access to higher-coordinate phosphorus ligand fragments.

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

N-Heterocyclic phosphenium (NHP) and nitrosonium (NO⁺) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO) _n(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)₃(NO)] or PPN[Cr(CO)₄(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and π-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO^-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/NO complexes is closely akin to that of the Hieber anion, [Fe(CO)₃(NO)]^-. The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)₂(NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H⁺, SnPh₃⁺) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M═P double bond.

Cooperative activation of O–H and S–H bonds across the Co–P bond of an N-heterocyclic phosphido complex

A cobalt N-heterocyclic phosphido complex is shown to cleave element–hydrogen bonds via a metal–phosphorus ligand cooperative pathway.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Insertion of a Nontrigonal Phosphorus Ligand into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane

The synthesis and reactivity of an NPN-chelating ligand containing a nontrigonal phosphorous triamide center (L1 = P(N( o-N(2-pyridyl)C₆H₄)₂) is reported. Metalation of L1 with RuCl₂(PPh₃)₃ gives RuCl₂(PPh₃)(L1) (2). By contrast, metalation of L1 with RuHCl(CO)(PPh₃)₃ yields RuCl(CO)(PPh₃)(L1^H) (3), a chelated 10-P-5 ruthenahydridophosphorane, via net insertion into the Ru-H bond. Hydride abstraction from 3 with Ph₃CPF₆ gives [RuCl(CO)(PPh₃)(L1)]PF₆ (4); reaction of 4 with NaBH₄ returns 3.