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.

We report the reduction of bulky ferrocenyl-based NHC-stabilised aluminium(iii) diiodide [Fc*(NHC)AlI2] (Fc* = 2,5-bis(3,5-di-tert-butylphenyl)-1-ferrocenyl) in different hydrocarbon solvents (hexane, benzene, toluene, and p-xylene), which results in different outcomes. Reduction in hexane with an equivalent amount of KC8 generates the diiododialane [(Fc*(NHC)AlI)2], whereas complete reduction in hexane leads to an unusual C-H activation at an N-Me group of one NHC unit. In contrast, reaction in aromatic solvents result in hitherto unknown Birch-type reductions of the corresponding solvent molecules by transient aluminium radicals of the type [LAlR2]˙, which is ultimately bound to two aluminium centers.

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.

Aluminum(I)/Boron(III) Redox Reactions.

Reactions between BIII species and the novel nucleophilic cyclopentadienyl-stabilized AlI reagent (1) result in a diversity of complexes bearing different Al/B oxidation states and coordination geometries. With the triarylborane B(C6 F5 )3 , a simple AlI →BIII adduct is formed. In contrast, a bulky aryldihaloborane undergoes oxidative addition with the formation of a covalent bora-alane species. With an N-heterocyclic carbene-stabilized amino(bromo)borenium ion, a redox reaction was observed, where the product is a borylene-alane BI →AlIII complex. Additionally, reaction of 1 with BI3 results in complete scrambling of all of the Al/B-bound substituents, and the formation of a cyclopentadienylboron(I)→AlI3 complex. These latter reactions are the first examples of the reduction of a boron(III) compound to a borylene by a p-block reagent, and illustrate how subtle changes in the nature of the borane can result in highly divergent reaction outcomes.

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.

Bottleable Neutral Analogues of [B2 H5 ]- as Versatile and Strongly Binding η2 Donor Ligands.

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X=Cl, OTf, C6 F5 ) lies opposite the bridging hydrogen atom of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B-B bond account for their formation. Astoundingly, these copper σB-B complexes are inert to ligand exchange with pyridine under either heating or photoirradiation.

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.

Reactivity of an intramolecular fluorophosphonium fluoroborate.

The reactions of the intramolecular frustrated Lewis pair-adduct Ph(2) PC(p-Tol)=C(C(6) F(5))B(C(6)F(5))2 (CNtBu) with XeF(2) gave Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(F)(C(6)F(5))(2)(3). This species reacts with two equivalents of Al(C(6)F(5))(3)⋅C(7)H(8) producing the salt, [Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(C(6)F(5))(2)][F(Al(C(6)F(5))(3))(2)] (4), whereas reaction with HSiEt(3)/B(C(6)F(5))(3) gave Ph(2) P(F)C(p-Tol)=C(H)B(C(6)F(5))(3) (5). The photolysis of 3 resulted in aromatization affording the phenanthralene derivative Ph(2) P(F)C(p-Tol(o-C(6)F(4)))=CB(F)(C(6)F(5))(2) (6).

Ruthenium complexes of an abnormally bound, anionic N-heterocyclic carbene.

The abnormally bound, anionic NHC-borane complex [Ru(IDipp-BF3)(p-cymene)Cl]2 (4; IDipp-BF3=1,3-(2,6-iPr2C6H3)2-2-BF3(C3HN2)-4-yl) was synthesized by transmetalation from Li[(IDipp-BF3)2Ag]. Addition of donors gave species of the form [Ru(IDipp-BF3)(p-cymene)(L)Cl], whereas halide abstraction with Ag(Et2O)[B(C6F5)4] gave C-H activation of the methine position of the IDipp-BF3 ligand.