Analogies between Metallaboratranes, Triboronates, and Boron Pincer Ligand Complexes

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation

Pre-coordination to a transition metal by the terminal donor groups of a tri-dentate ligand is a common strategy to stabilise elusive groups, to achieve unprecedented bond activation and to develop novel modes of metal-ligand-cooperation for catalysis. In the current manuscript, we demonstrate that the oxidative addition of a central E-H-bond after pre-coordination to the metal centre is disfavoured for metals with d¹⁰ electron configuration. For exemplary pincer ligands and metals with d¹⁰ electron configuration, quantum chemical calculations suggest a second barrier, which is associated with the rearrangement of the saw-horse structure, obtained after oxidative addition, to the expected square planar geometry for the resulting d⁸ electron configuration. In the case of PBP-type ligands with a central L₂BH₂-group (L = R₃P) the reaction with Pt⁰ precursors proceeds via an alternative pathway of activation, which involves the backside attack of a nucleophile to the boron atom, which facilitates the nucleophilic attack of the Pt⁰ centre and formation of a boryl complex (LBH₂). As the corresponding reaction with a Pt^II precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a L₂BH donor, our results show that ligand-stabilized borylenes (L₂BH) can in principle be converted to boryls (LBH₂) via boronium salts (L₂BH₂⁺).

Ethylene Dimerization and Oligomerization Using Bis(phosphino)boryl Supported Ni Complexes

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (^RPBP)NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol_ethylene·mol_Ni^-1·s^-1 for the formation of butenes with 41(1)% selectivity for 1-butene using (^PhPBP)NiBr, and 68 mol_ethylene·mol_Ni^-1·s^-1 for butenes production with 87.2(3)% selectivity for 1-butene using (^tBuPBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and (^tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C₄ through C₂₀ products was achieved, while the use of (^PhPBP)NiBr as the pre-catalyst retained selectivity for C₄ products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni-B unit of the (^RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The kinetics of carbon dioxide insertion into a pincer-supported palladium methyl complex are studied. The complex inserts carbon dioxide at room temperature, and we explore both solvent and Lewis acid effects on carbon dioxide insertion.

The Stability of Diphosphino-Boryl PBP Pincer Backbone: PBP to POP Ligand Hydrolysis

Since moisture may frequently be present in many solvents, it is important to know the reactivity of a catalyst against water for catalytic reactions. In order to explore the stability and understand the transformation process of diphosphino-boryl-based PBP pincer platform, [PdCl{B(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (1) was treated with PdCl₂ , HB(NCH₂ PPh₂ )₂ -o-C₆ H₄ was reacted with [PdCl₂ (cod)] (cod=cyclo-octa-1,5-diene) and [Pd₂ (dba)₃ ] (dba=dibenzylideneacetone), respectively, in the presence of water. Some novel palladium POP complexes, [Pd₂ Cl₂ (μ-Cl){μ-κ³ -P,O,P-OB(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (2 a), [Pd₄ (μ-Cl)₂ (μ-O)₂ {μ-κ³ -P,O,P-OB(NCH₂ PPh₂ )₂ -o-C₆ H₄ }₂ ] (2 b), [Pd₂ {μ-κ⁴ -P,P,P,P-O(B(NCH₂ PPh₂ )₂ -o-C₆ H₄ )₂ }{μ-κ² -P,P-(NHCH₂ PPh₂ )₂ -o-C₆ H₄ }] (3), were obtained. It was found that the PBP pincer backbone can easily be converted into a POP backbone in the presence of water. From the crystal structures of the resultant palladium complexes, possible pincer backbone transformation pathways were discussed.

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2

The concept of metal-ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel-methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

Palladium(ii) complexes supported by PBP and POCOP pincer ligands: a comparison of their structure, properties and catalytic activity

A Pd(ii) chloride complex supported by a Yamashita-Nozaki PBP pincer ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdCl (1a), was synthesized. The structure, properties and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C6H3-2,6-(OPtBu2)2]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that in complex 2a; complex 1a is a much better catalyst for Suzuki-Miyaura cross-coupling reactions than complex 2a. Starting from complexes 1a and 2a, two series of Pd(ii) pincer complexes bearing a SH, BH4, N[combining low line]CS, N[combining low line]CSe or N3 covalent ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdY (Y = SH, 1b; BH4; 1c; N[combining low line]CS, 1d; N[combining low line]CSe, 1e; and N3, 1f) and [C6H3-2,6-(OPtBu2)2]PdY (Y = SH, 2b; BH4, 2c; N[combining low line]CS, 2d; N[combining low line]CSe, 2e; and N3, 2f), were synthesized and fully characterized. Single crystal X-ray diffraction analysis indicated that the Pd centre is less tightly chelated in PBP pincer complexes. The strong σ-donor ability of the PBP pincer ligand has little influence on the structure of the covalent ligand possessing both σ-donor and π-acceptor properties. However, the stretching vibrational frequencies of N[combining low line]CS, N[combining low line]CSe and N3 ligands and the coordination mode of the BH4 ligand are significantly different in these two types of palladium pincer complexes.

Functional group migrations between boron and metal centres within transition metal-borane and -boryl complexes and cleavage of H-H, E-H and E-E' bonds

This feature article examines some of the recent advances in the chemistry of Z-type transition metal-borane and X-type transition metal-boryl complexes. It focuses on the employment of these boron-based functionalities acting as stores and transfer agents for functional groups such as hydrides, alkyl groups and aryl groups which can either be abstracted or delivered to the metal centre. The review also explores the rather novel reactivity involving the cleavage of H-H, E-H and E-E' bonds (where E and E' are a range of groups) across the transition metal-boron bond in such complexes. It explores the early examples of the addition of H-H across transition metal-borane bonds and describes the new transformation in the context of other known modes of hydrogen activation including classic oxidative addition and heterolytic cleavage at transition metal centres as well as Frustrated Lewis Pair chemistry. Similar reactivity involving transition metal-boryl complexes are also described particularly those which undergo both boryl-to-borane and borane-to-borohydride transformations. The delivery of hydride to the metal centre in combination with the potential to regenerate the borohydride functional group via a recharging process is explored in the context of providing a new strategy for catalysis. Finally, a light-hearted look at the analogy of the 'stinging processes' involving Trofimenko type ligands is taken one step further to determine whether it is indeed in the nature of scorpionate ligands to repeatedly 'sting' just as the real life scorpions do.

Rapid reversible borane to boryl hydride exchange by metal shuttling on the carborane cluster surface

In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh3) pincer complex (POBOP = 1,7-OP(i-Pr)2-m-2-carboranyl) features extreme distortion of the two-center-two-electron Ru-B bond due to the presence of a strong three-center-two-electron B-H···Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh3) pincer complex, which possesses B-Ru, B-H···Ru, and Ru-H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal-boron and metal-hydrogen bonds, is unexpectedly facile at temperatures above -50 °C corresponding to an activation barrier of 12.2 kcal mol-1. Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru(iv) intermediates, respectively. The presence of this hemilabile vicinal B-H···Ru interaction in (POBOP)Ru(H)(PPh3) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C.

Triaminoborane-bridged diphosphine complexes with Ni and Pd: coordination chemistry, structures, and ligand-centered reactivity

The synthesis, coordination chemistry, and reactivity of two diphosphines containing the cyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD) are described.

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B-H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)Ru(CO)2 (POBBOP = 1,7-OP(i-Pr)2-2,6-dehydro-m-carborane) was synthesized by double B-H activation within the strained m-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B-Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne-carboranyl transformations reported herein represent a new mode of cooperative metal-ligand reactivity of boron-based complexes.

Isolation and properties of a palladium PBP pincer complex featuring an ambiphilic boryl site

The ambiphilic boryl site in the PBP pincer [{(o-PPh₂C₆H₄)₂B}Pd^III] reacts with Lewis bases.

A long-tethered (P–B–P)-pincer ligand: synthesis, complexation, and application to catalytic dehydrogenation of alkanes

A new long-tethered boron-containing (P–B–P)-pincer ligand has been synthesized. This ligand was introduced to Ir to form (P–B–P)Ir(H)Cl complex. Subsequent reaction with nBuLi led to the formation of dihydride complex (P–B–P)Ir(H)₂. Both complexes were found to be moderately active for the catalytic dehydrogenation of alkanes.

Boryl-assisted hydrogenolysis of a nickel-methyl bond

A stable nickel(II) methyl complex containing a diphosphino-boryl (PBP) pincer ligand is described. Mechanistic studies on the hydrogenolysis of the Ni-Me bond suggest a metal ligand cooperation mechanism that involves the intermediacy of a σ-B-H Ni(0) species that further undergoes B-H oxidative addition to form a Ni(II) hydride complex.

Toward a comprehensive definition of oxidation state (IUPAC Technical Report)

A generic definition of oxidation state (OS) is formulated: “The OS of a bonded atom equals its charge after ionic approximation”. In the ionic approximation, the atom that contributes more to the bonding molecular orbital (MO) becomes negative. This sign can also be estimated by comparing Allen electronegativities of the two bonded atoms, but this simplification carries an exception when the more electronegative atom is bonded as a Lewis acid. Two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology. Both provide the same generic OS because both the ionic approximation and structural formula obey rules of stable electron configurations. A sufficiently simple empirical formula yields OS via the algorithm of direct ionic approximation (DIA) by these rules. The topological algorithm works on a Lewis formula (for a molecule) or a bond graph (for an extended solid) and has two variants. One assigns bonding electrons to more electronegative bond partners, the other sums an atom’s formal charge with bond orders (or bond valences) of sign defined by the ionic approximation of each particular bond at the atom. A glossary of terms and auxiliary rules needed for determination of OS are provided, illustrated with examples, and the origins of ambiguous OS values are pointed out. An electrochemical OS is suggested with a nominal value equal to the average OS for atoms of the same element in a moiety that is charged or otherwise electrochemically relevant.

A bidentate boryl ligand: syntheses of platinum and iridium complexes

A bidentate diphenylphosphino substituted diazaborole ligand was obtained and coordinated at platinum and iridium.