Metal Borane Research Articles

Hydroboration reactions using transition metal borane and borate complexes: an overview.

In recent years, the chemistry of transition metal borane and borate complexes has advanced fast with reasonable growth. Frequent utilisation of these complexes in hydrofunctionalisation reactions is one of the key driving forces for their development. As a result, the important role of borate complexes in the hydroboration/hydrosilylation/hydroamination of unsaturated organic species has been successfully demonstrated together with the isolation of many different boron-containing transition metal complexes such as borataallyl, vinylborane, silyl complexes featuring the known bonding modes of boron. Both the uncatalysed and catalysed hydroboration reactions using the transition metal borane/borate complexes are known, which show these complexes' huge potential. Careful investigation and fine-tuning of the electronic and steric properties of the borane/borate ligands has facilitated the synthesis of these transition metal complexes which are convenient for use in the hydroboration reactions. Furthermore, the systematic development of this field has established the connection between the structure and reactivity of these complexes and their utilisation in hydroboration reactions. This Frontier sheds light on the recent developments that have been made with hydroboration reactions using transition metal borane/borate complexes. Also, in this Frontier we have provided meaningful synthetic methods to make new boron-containing transition metal complexes together with mechanistic insights for some of these reactions.

Joshua S. Figueroa

Joshua S. Figueroa

ChemInform Abstract: 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

AbstractReview: [73 refs.

Carbonyl Chalcogenides Clusters Existing as Disguised Forms of Hydrocarbon Isomers

<p>The 4n Series Method has been utilized to categorize, analyze and predict structures for transition metal carbonyl, borane, hydrocarbon and Zintl ion clusters. The method is being extended to study carbonyl chalcogenide clusters. Adequate examples have been given to demonstrate the application of the 4n series method to categorize clusters and where possible predict their possible skeletal structures. In this paper, the method is being applied to the study of carbonyl chalcogenide cluster complexes. What has been found is the striking structural similarity of a wide range of carbonyl chalcogenide clusters to those of corresponding hydrocarbon clusters. It was observed that when a derived hydrocarbon from a cluster, F<sub>CH</sub> = C<sub>n</sub>H<sub>q</sub>, is such that n<q, the cluster portrays structural similarity with an equivalent hydrocarbon. On the other hand when n>q, the ‘hydrocarbon character’ becomes reduced and the typical cluster tendencies increase. When n = q, the situation becomes more or less a borderline case. When q=0, then F<sub>CH</sub> = C<sub>n</sub>. When the series becomes bi-capped or more, then the equivalent carbon cations are obtained.</p>

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.

Cooperative transition metal/Lewis acid bond-activation reactions by a bidentate (boryl)iminomethane complex: a significant metal-borane interaction promoted by a small bite-angle LZ chelate.

The synthesis of a three-coordinate Pt-borane complex featuring a bidentate "LZ" (boryl)iminomethane (BIM) ligand is reported. Unlike other LZ-type borane ligands featuring a single-donor buttress, the small bite angle enforced by the BIM ligand is shown to promote a significant metal-borane reverse-dative σ-interaction akin to multiply strapped metalloboratranes. The steric accessibility of the reactive Pt → B bond fostered by the BIM ligand allows for a rich reactivity profile toward small molecules that exploit metal-borane cooperative effects. The unligated (boryl)iminomethane BIM is also synthetically accessible and functions as a Frustrated Lewis Pair (FLP). The ability of the free BIM to effect bond activation reactions is contrasted with the behavior seen in the corresponding platinum-bound complexes.

Facile Si–H bond activation and hydrosilylation catalysis mediated by a nickel–borane complex

Metal–borane complexes are emerging as promising systems for study in the context of bifunctional catalysis. Herein we describe diphosphineborane nickel complexes that activate Si–H bonds and catalyze the hydrosilylation of aldehydes. Treatment of [MesDPBPh]Ni (1) ([MesDPBPh] = MesB(o-Ph2PC6H4)2) with organosilanes affords the complexes [MesDPBPh](μ-H)NiE (E = SiH2Ph (3), SiHPh2 (4)). Complex 4 is in solution equilibrium with 1 and the thermodynamic and kinetic parameters of their exchange have been characterized by NMR spectroscopy. Complex 1 is a catalyst for the hydrosilylation of a range of para-substituted benzaldehydes. Mechanistic studies on this reaction via multinuclear NMR spectroscopy are consistent with the intermediacy of a borohydrido-Ni-siloxyalkyl species.

Insight into the Hydrogen Migration Processes Involved in the Formation of Metal–Borane Complexes: Importance of the Third Arm of the Scorpionate Ligand

The reactions of [Ir(κ3N,N,H-Tai)(COD)] and [Ir(κ3N,N,H-PhBai)(COD)] (where Tai = HB(azaindolyl)3 and PhBai = Ph(H)B(azaindolyl)2) with carbon monoxide result in the formation of Z-type iridium–borane complexes supported by 7-azaindole units. Analysis of the reaction mixtures involving the former complex revealed the formation of a single species in solution, [Ir(η1-C8H13){κ3N,N,B-B(azaindolyl)3}(CO)2], as confirmed by NMR spectroscopy. In the case of the PhBai complex, a mixture of species was observed. A postulated mechanism for the formation of the new complexes has been provided, supported by computational studies. Computational studies have also focused on the reaction step involving the migration of hydrogen from boron (in the borohydride group) to the iridium center. These investigations have demonstrated a small energy barrier for the hydrogen migration step (ΔG298 = 10.3 kcal mol–1). Additionally, deuterium labeling of the borohydride units in Tai and PhBai confirmed the final position of the former borohydride hydrogen atom in the resulting complexes. The importance of the “third azaindolyl” unit within these transformations and the difference in reactivity between the two ligands are discussed. The selective coordination properties of this family of metallaboratrane complexes have also been investigated and are discussed herein.

Estimating the strength of the M–H–B interaction: a kinetic approach

Photolysis of BzCr(CO)3 and TpMn(CO)3 in the presence of H3B·NEt3 yields a metal-borane σ complex. Displacement of the weakly coordinated borane by P(OEt)3 proceeds by a dissociative mechanism and an Eyring analysis yields the first estimate of 20 kcal mol(-1) for the strength of this novel interaction.

ChemInform Abstract: Hydrogen Atom Storage upon Z‐Class Borane Ligand Functions: An Alternative Approach to Ligand Cooperation

AbstractReview: 69 refs.

Hydrogen atom storage upon Z-class borane ligand functions: an alternative approach to ligand cooperation

Ligand cooperation has become an important strategy in the development of new transition metal based transformations. By using this approach some remarkable new catalytic transformations have been achieved, all in the space of only a few years. The purpose of this tutorial review is to explore the potential utilisation of ligands containing borohydride and borane functionalities as reversible hydrogen atom stores. At the heart of this review will be a discussion on hydrogen transfer reactions and the transformation between borohydride and borane moieties. An outline of the various synthetic routes to metal-borane (metallaboratrane) complexes will be provided together with a discussion of their further reactivity including key transformations such as 1,2 additions across the metal-boron bond and 'recharging' the borane functional group back to borohydride. Finally, an evaluation of the potential future applications of such reactivity will be provided.

Double addition of H2to transition metal–borane complexes: a ‘hydride shuttle’ process between boron and transition metal centres

The addition of H(2) across a transition metal-borane bond is reported for the first time providing a mechanism for recharging borane functional groups to borohydride.

Crystal field arguments to explain the trans labilisation within transition metal–borane complexes

There has been great interest in metal–borane complexes following the discovery of metallaboratrane complexes in 1999. These complexes contain unusual Z-type (σ-acceptor) coordination of borane functional groups and have been shown to provide unexpected coordination geometries and properties for a transition metal in a particular oxidation state. Additionally, such ligands exhibit unusually high trans influences only in certain geometries. Herein, a simple qualitative description, based on crystal field theory arguments, is presented for the explanation of the geometries and atypical trans labilisations observed. Trans labilisation: This discussion highlights the geometries observed within metal–borane complexes and discusses the origin of the labilisation of ligands in the site trans to boron.

A Simple Description of the Bonding in Transition-Metal Borane Complexes

The use of oxidation numbers to determine the dn configuration of metal borane compounds (M→BX3) is problematic due to differences in opinion concerned with the charge assigned to the BX3 ligand; a molecular orbital analysis, however, indicates that the dn configuration must be reduced to dn-2 upon coordination of BX3.

The role of the metal formal oxidation state in metallaborane structures relating to closo/ isocloso and nido/ isonido pairs

Relationships have been observed between the metal formal oxidation state and the metal valence electron counts in closo/ isocloso and nido/ isonido pairs of polyhedral metallaboranes. In general, the metal formal oxidation state increases by two units in a closo → isocloso or nido → isonido structural transformation, generally involving diamond–square–diamond processes relating to internal metal oxidative addition reactions. More exotic “ isoiso” structures are found for metallaboranes containing metals exhibiting relatively high oxidation states such as molybdenum, tungsten, and rhenium. The free borane ligand obtained by removal of the metal vertex from a metallaborane structure can be classified as symmetric or asymmetric depending upon the nature of the metal–ligand bond. In symmetric borane ligands found in closo metallaboranes, the metallaborane structure is generated by symmetrical “centroid” metal capping of the ligand open face. However, in asymmetric borane ligands found in nido metallaboranes, the metallaborane structure is generated by skew-symmetrical or offset metal capping of the ligand open face. Capping regime descriptors are defined to describe these types of metal–borane bonding. In addition, electrostructural correlation tables are described for closo/ nido and nido/ arachno pairs relating metal formal oxidation states, the numbers of metal valence electrons, and the capping regime descriptors.

A Computational Study on Fluxional Behavior of Group 6 and 7 Transition-metal Complexes of Borane–Lewis Base Adducts

Abstract Fluxional behavior of group 6 and 7 metal borane complexes was investigated with use of density functional theory (DFT). Site exchange of BH hydrogen atoms of the borane ligand BH3·L or B2H4·2PMe3 proceeds via a transition state in which the borane ligand interacts with metal with a bidentate fashion. Calculated values of the activation energy were in good agreement with experimentally observed barriers.

Density functional theory study on structural isomers and bonding of model complexes M(CO) 5(BH 3 · PH 3) (M = Cr, Mo, W) and W(CO) 5(BH 3 · AH 3) (A = N, P, As, Sb)

The influence of group 15 various substituents and effect of metal centers on metal–borane interactions and structural isomers of transition metal–borane complexes W(CO) 5(BH 3 · AH 3) and M(CO) 5(BH 3 · PH 3) (A = N, P, As, and Sb; M = Cr, Mo, and W), were investigated by pure density functional theory at BP86 level. The following results were observed: (a) the ground state is monodentate, η 1, with C 1 point group; (b) in all complexes, the η 1 isomer with C S symmetry on potential energy surface is the transition state for oscillating borane; (c) the η 2 isomer is the transition state for the hydrogens interchange mechanism; (d) in W(CO) 5(BH 3 · AH 3), the degree of pyramidalization at boron, interaction energy as well as charge transfer between metal and boron moieties, energy barrier for interchanging hydrogens, and diffuseness of A increase along the series A = Sb < As < P < N; (e) in M(CO) 5(BH 3 · PH 3), interaction energy is ordered as M = W > Cr > Mo, while energy barrier for interchanging hydrogens decreases in the order of M = Cr > W > Mo.

Metal–dihydrogen and σ-bond coordination: the consummate extension of the Dewar–Chatt–Duncanson model for metal–olefin π bonding

There is a marvelous analogy between the metal–olefin π bonding model first brought to light by Dewar 50 years ago and that of σ-bond coordination discovered by us 17 years ago. In some ways it is surprising that 33 years elapsed between the two parallel bonding situations. However this difference pales in comparison to that between the actual synthesis of the first olefin complex, Zeise's salt in 1837, and the first recognized dihydrogen complex nearly 150 years later. This article delineates the principles of σ-bond coordination and activation inspired by the Dewar–Chatt–Duncanson model and illuminates the often-spectacular interplay between theory and experiment in this field. Aside from HH bond coordination and activation towards cleavage, the structure and bonding principles apply to SiH, CH, and virtually any two-electron XH or XY bond. Metal d to σ* XH backdonation is the key to stabilizing σ-bond coordination and is also crucial to homolytic cleavage (oxidation addition). There are some differences in bonding depending on X, and, in the case of BH bond coordination, in metal–borane complexes, backdonation to boron p orbitals occurs. For electrophilic complexes, particularly cationic systems with minimal backdonation, heterolytic cleavage of XH is common and is a key reaction in industrial and biological catalysis. Thus there are two separate pathways for σ-bond activation that directly depend on the electronics of the metal σ-ligand bonding.

σ-Borane Complexes of Manganese and Rhenium

The late transition metal borane complexes (MeCp)Mn(CO)2(HBR2) (R = alkyl, alkoxy) and Cp*Re(CO)2(HBpin) (HBpin = pinacolborane, Cp* = pentamethylcyclopentadienyl) have been synthesized and isolated. All complexes were characterized spectroscopically, and X-ray crystal structure analyses were performed for (MeCp)Mn(CO)2(HBpin), (MeCp)Mn(CO)2(HBcat) (HBcat = catecholborane) and (MeCp)Mn(CO)2(HBCy2) (Cy = cyclohexyl). These data show that the complexes contain a borane ligand with a weakened but intact B−H bond. The borane ligand in (MeCp)Mn(CO)2(HBcat) is replaced by PhCCPh, Ph3SnH, Ph2MeSiH, CO, and excess HBpin in ligand substitution reactions. The mechanism of substitution by PhCCPh was investigated. Kinetic studies showed that the reaction is first-order in complex and that borane and PhCCPh react competitively with the 16-electron intermediate, (MeCp)Mn(CO)2. The ΔH⧧ value for borane dissociation was 24 ± 3 kcal/mol for (MeCp)Mn(CO)2(HBcat) and 21 ± 1 kcal/mol for (MeCp)Mn(CO)2(HBCy2), which provided an upper limit on the metal−borane binding energies. Dynamic NMR spectroscopy of (MeCp)Mn(CO)2(HBcat) revealed two types of rotations of the borane ligand.

The crystal and molecular structure of [1,1,1,1-(CO)H(PPh3)2-arachno-1-IrB3H7] and some bonding considerations in arachno-type four-vertex metal ‘π-allyl’ and ‘borallyl’ clusters

The detailed molecular structure of [(CO)(PPh3)2HIrB3H7] has been determined by X-ray diffraction analysis. The colourless crystals are monoclinic, space group P21/n with a= 1 182.0(3), b= 2 189.4(4), c= 1 355.7(3) pm, β= 104.27(2)°, and Z= 4. The molecular structure (R= 0.032 for 4 119 reflections) features a four-vertex butterfly cluster (IrB3) with Ir on a ‘hinge’ position. The structure is interpreted in terms of a ‘capped octahedral’ seven-orbital 18-electron d4 iridium(V) complex in which the metal–borane bonding occurs predominantly via three two-electron two-centre iridium–boron bonds. The cluster bonding implications are discussed with reference to the ‘π-allyl’ and ‘borallyl’ bonding modes.