Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Stepwise reduction of a base-stabilised ferrocenyl aluminium(iii) dihalide for the synthesis of structurally-diverse dialane species

A bulky ferrocenyl-based NHC-stabilised aluminium(iii) dihalide was reduced in different solvents, leading to vastly different outcomes, including formation of a rare example of a dialane and a novel dialuminyl analogue of the Birch reduction.

Isolation and Reactivity of an Antiaromatic s-Block Metal Compound

The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation-state flexibility of the s-block metals has long stood in the way of their participation in sophisticated π-bonding arrangements, and truly antiaromatic systems containing s-block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis-base-coordination chemistry.

Phosphinoborylenes as stable sources of fleeting borylenes

Base-stabilised borylenes that mimic the ability of transition metals to bind and activate inert substrates have attracted significant attention in recent years. However, such species are typically highly reactive and fleeting, and often cannot be isolated at ambient temperature. Herein, we describe a readily accessible trimethylphosphine-stabilised borylborylene which was found to possess a labile P-B bond that reversibly cleaves upon gentle heating. Exchange of the labile phosphine with other nucleophiles (CO, isocyanide, 4-dimethylaminopyridine) was investigated, and the binding strength of a range of potential borylene "ligands" has been evaluated computationally. The room-temperature-stable PMe3-bound borylenes were subsequently applied to novel bond activations including [2 + 2] cycloaddition with carbodiimides and the reduction of dichalcogenides, revealing that PMe3-stabilised borylenes can effectively behave as stable sources of the analogous fleeting dicoordinate species under mild conditions.

Bond-Strengthening Backdonation in Aminoborylene-Stabilized Aminoborylenes: At the Intersection of Borylenes and Diborenes

Singly NHC-coordinated (aminoboryl)aminoborenium salts react with Na₂ [Fe(CO)₄ ] to yield stable coordination complexes of aminoborylene-stabilized aminoborylenes, which exhibit exceptional σ-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η³ -BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B π-orbital is observed. This bonding situation can be alternatively described as a Fe-diaminodiborene complex. In a related reduction of CAAC-stabilized (aminoboryl)aminoborenium with KC₈ , the reduced species can be captured with nucleophiles to form three-coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Complexation and Release of N-Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

The coordination chemistry and stability of aminoborylene ligands bearing different N-heterocyclic carbene (NHC) stabilizing groups has been investigated with Group VI and VIII metals. NHC-aminoborylene complexes have been accessed via reduction of NHC-dihaloaminoborane adducts with Na₂[M(CO) _x] species (M = Fe, Ru, Cr, W). Imidazol-2-ylidene-stabilized aminoborylene ligands were found to afford thermally robust metal-borylene complexes, which are inert to oxidation, hydrolysis, and insertion of unsaturated substrates. Such ligands have additionally been demonstrated to be significantly more electron releasing than NHCs and other carbon-based ligands by infrared spectroscopy, and can be regarded as unique examples of highly nucleophilic borylene ligands isolobal to classical NHCs. In contrast, cyclic alkylaminocarbene (CAAC)-bound dihaloaminoboranes were found to be reduced by one or two electrons upon reaction with Na₂[M(CO) _x] species to form either a stable borane-centered radical, or the free CAAC-aminoborylene complex, which further reacts to form a carbonyl-stabilized aminoborylene. Borylene-to-CO migration was also observed upon reaction of a ruthenium imidazol-2-ylidene aminoborylene complex with B(C₆F₅)₃, where the product borylene remains trapped by the Ru center.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group VI Terminal Borylene Complexes

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.

Monomeric 16-Electron π-Diborene Complexes of Zn(II) and Cd(II)

Despite the prevalence of stable π-complexes of most d¹⁰ metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d¹⁰ Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ion's limited capacity for π-backdonation. By employing more strongly donating π-ligands, namely neutral diborenes with a high-lying π(B═B) orbital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π-complexes were synthesized in good yields. Metal-B₂ π-interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB₂ planes significantly twist out of the MX₂ planes about the M-centroid(B-B) vector, with angles ranging from 47.0° to 85.5°, depending on the steric interactions between the diborene ligand and the MX₂ fragment.

Three and four coordinate Fe carbodiphosphorane complexes

Carbodiphosphoranes (CDPs) are a family of divalent carbon ligands that are known for their exceptional electron donor properties. Herein, the preparation and reactivity of a family of three and four co-ordinate Fe carbodiphosphorane complexes is described. Hexaphenylcarbodiphosphorane (HCDP) [1] is shown to react with FeCl₂(PPh₃)₂ to form the three coordinate adduct Fe(HCDP)Cl₂ [2], which is equilibrium with its four coordinate dimer. Reaction of [2] with two equivalents of benzyl Grignard yields the corresponding dialkyl complex (HCDP)FeBn₂ [3]. Combination of [2] with LiHMDS results in salt metathesis and the formation of the monosilylated derivative Fe(HCDP)Cl(N(SiMe₃)₂) [4]. Subsequent anion exchange leads to the three coordinate Fe(HCDP)(OTf)(N(SiMe₃)₂) [5] which was characterized crystallographically and in solution.

Synthesis of a Carbodicyclopropenylidene: A Carbodicarbene based Solely on Carbon

The first carbodicarbene stabilized by flanking cyclopropenylidenes is reported. Tetraphenylcarbodicyclopropenylidene (2) is accessed by deprotonation of the corresponding triafulvene cyclopropenium salt, and has been spectroscopically characterized in [D8 ]THF solution at -60 °C. Main-group and transition-metal complexes of 2 have been accessed, and have revealed the high sigma donating ability, and exclusive η(1) binding of this neutral all carbon ligand. Variable temperature NMR spectroscopy studies reveal varying degrees of free rotation in the flanking cyclopropenylidene groups of 2 in its coordination compounds.

Preparation and reactivity of a Ru(0) phosphino-carbene complex

The reaction of the phosphino-carbene MeNC3H2NPtBu2 with RuHCl(CO)(PPh3)3 is shown to give facile access to the Ru(0) species (MeNC3H2NPtBu2)Ru(CO)(PPh3)2 (). This species undergoes oxidative addition of H2 and silanes to give (MeNC3H2NPtBu2)RuH2(CO)(PPh3)2, (MeNC3H2NPtBu2)Ru H(SiRPh2)(CO)(PPh3) (R = Ph 5, H 6) and (MeNC3H2NPtBu2) RuH(PhSi(SCH2CH2)2O)(CO)(PPh3) .

Cyclic bent allene hydrido-carbonyl complexes of ruthenium: highly active catalysts for hydrogenation of olefins

A new family of ruthenium complexes bearing the carbodicarbene-type ligand "cyclic bent allene" (CBA) have been synthesized from the common precursor RuHCl(CO)(PPh3)3. Complexes were evaluated for catalytic activity in the room-temperature hydrogenation of unactivated olefins and were found to be significantly more active than known ruthenium hydrido-carbonyl phosphine or NHC complexes. In particular, RuH(OSO2CF3)(CO)(SIMes)(CBA) was found to be among the most active hydrogenation catalysts, achieving comparable activity to Crabtree's catalyst in the hydrogenation of unactivated trisubstituted olefins and superior activity in the hydrogenation of styrene derivatives in side-by-side catalytic runs. RuH(OSO2CF3)(CO)(SIMes)(CBA) was also found to be highly active in olefin selective hydrogenation in the presence of a variety of unsaturated functional groups, and can achieve exceptional diastereoselectivity in functional-group-directed hydrogenations at very low catalyst loadings.