Boryl Complexes Research Articles

Group VI metal aminoborylene complex-catalyzed demercuration reactions of bis(alkynyl)mercurials

The first catalytic application of the Group VI metal borylene complexes [(CO)(5)M[double bond]BN(SiMe(3))(2)] involves the demercuration reaction of bis(alkynyl)mercurials, [Hg(C[triple bond]CR)(2)], with formation of a series of buta-1,3-diynes.

Boryllithium: A novel boron nucleophile and its application in the synthesis of borylmetal complexes

Abstract The first boryl anion, boryllithium, was synthesized by a reduction of bromoborane precursor using lithium naphthalenide in tetrahydrofuran (THF) solvent at -45 °C. Structural and spectroscopic study revealed an ionic character of B-Li bond. Boryllithium could be utilized as a source of boryl ligand in the transition-metal chemistry. Structural and spectroscopic features of the resulting boryl complexes confirmed the large trans influence of boryl ligand.

Synthesis and Characterisation of Semi‐Bridging Molybdenum Borylene Complexes

AbstractInvestigation of the reactivity of the molybdenum borylene complex [(OC)5Mo=B=N(SiMe3)2] towards the late transition metal complexes [M(PCy3)2] (M = Pd, Pt) indicated the formation of [(OC)4Mo(μ‐CO){μ‐BN(SiMe3)2}Pd(PCy3)] and [(Cy3P)(OC)3Mo(μ‐CO){μ‐BN(SiMe3)2}Pt(PCy3)], respectively, as the first Mo‐based semi‐bridging borylene complexes. The new complexes were fully characterised spectroscopically and analysed by X‐ray diffraction. The conclusions drawn from the experimental data for the ligand‐metal interactions for this particular coordination mode of a borylene group were supported by DFT computations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Experimental Studies on the trans‐Influence of Boryl Ligands in Square‐Planar Platinum(II) Complexes

A series of platinum(II) boryl complexes of general formula trans-[(Cy(3)P)2Pt(Br)(BX2)], including the rare dibromoboryl species trans-[(Cy(3)P)2Pt(Br)(BBr2)], were synthesized by oxidative addition of the B-Br bond of a number of bromoboranes to [Pt(PCy3)2]. X-ray diffraction studies were performed on several such compounds. Comparison of the Pt--Br bond lengths allowed an empirical assessment of the trans-influence of different boryl ligands. A trans-influence scale was thus deduced and the results were compared with those previously computed for compounds of the type trans-[(Me(3)P)2Pt(Cl)(BX2)].

Reactivity of a Platinum Iminoboryl Complex toward Lewis and Brønsted Acids

Differently from typical iminoboranes, the iminoboryl complex trans-[(PCy3)2Pt(B≡NSiMe3)(Br)] (2) displays remarkable stability against oligomerization. Its clean reactivity toward Lewis acids led to the synthesis of the neutral platinum borylene complex trans-[(PCy3)2Pt{BN(SiMe3)(AlCl3)}(Br)], through coordination of AlCl3 to the nitrogen atom of the iminoboryl ligand. Surprising stability of the Pt−B linkage in 2 is further manifested in its reaction with MeOH, yielding the alkoxy(amino)boryl species trans-[(PCy3)2Pt{B(OMe)(N(H)SiMe3)}(Br)].

A Theoretical Study on the Mechanism of Boron Metathesis

The mechanism of the boron metathesis reaction of the transition-metal-aminoborylene complex Cp(CO)(2)FeBN(CH(3))(2+) (8) with EX, where EX = H(3)PO (9ap), H(3)AsO (9bp), H(3)PS (9aq), H(3)AsS (9bq), CH(3)CHCH(2) (9cr), (NH(2))(2)CCH(2) (9dr), H(2)CO (9ep), and (NH(2))(2)CO (9dp) is investigated at the B3LYP/LANL2DZ level. The analysis of bonding and charge distribution shows that the Fe-borylene complex (8) is a Fischer-type carbene analogue. The attack of the olefin takes place at the metal end of the M=C bond of the metal-carbene complex in olefin metathesis and proceeds via [2 + 2] cycloaddition, while in boron metathesis, the initial attack of the substrates takes place at the positively charged B atom of the Fe-borylene complex and forms the preferred acyclic intermediate. The energetics of boron metathesis is comparable to that of the olefin metathesis. Substrates that are polar and a have low-lying sigma* molecular orbital (weak sigma bond) prefer the boron metathesis reaction. The relative stability of the metathesis products is controlled by the strength of the Fe-E and B-X bonds of the products 13 and 14, respectively. We have also investigated the possibility of a beta-hydride-transfer reaction in the Fe-borylene complex.

Synthesis and Structure of a Cationic Platinum Borylene Complex

Making a B line: The boryl complex trans-[(Cy3P)2Pt(Br){B(Mes)Br}] was synthesized and converted into the first two-coordinate borylene complex of platinum, trans-[(Cy3P)2Pt(Br)(BMes)]+ (see picture), in which the boron atom is linearly coordinated. The bonding situation was analyzed by theoretical calculations and structural methods. Mes=2,4,6-Me3C6H2.

DFT Studies of Alkene Insertions into Cu−B Bonds in Copper(I) Boryl Complexes

DFT calculations have been carried out to study the insertion reactions of alkenes into the Cu−B bond in (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene). The nature of the insertion reactions and the relevant regiochemistry have been examined along with β-hydride eliminations, which are followed by reinsertion of the alkene into the Cu−H bond. Hyperconjugation (i.e., π bonding) between the Cu−C σ bond and the “empty” pz orbital on boron has been identified as the cause of the unexpectedly small Cu−C−B angle found experimentally by X-ray diffraction in α-borylalkyl Cu(I) complexes.

Cationic Terminal Aminoborylene Complexes: Controlled Stepwise Insertion into MB and BN Double Bonds

One thing leads to another: Reactions of the cationic BN vinylidene analogues 1-[BArF4] (R=Cy, iPr; ArF=3,5-(CF3)2C6H3) towards dicyclohexylcarbodiimide proceed by unprecedented insertion chemistry for terminal borylene complexes. Controlled, stepwise insertion into the FeB and BN bonds is demonstrated, sequentially forming four-membered rings linked at a spirocyclic boronium center.

Ruthenium-Catalyzed Hydroboration and Dehydrogenative Borylation of Linear and Cyclic Alkenes with Pinacolborane

The bis(dihydrogen)ruthenium complex RuH2(H2)2(PCy3)2 (1) catalyzes efficiently the borylation of linear and cyclic alkenes with pinacolborane. Similar results are obtained by using RuH[(μ-H)2Bpin](σ-HBpin)(PCy3)2 (2) as catalytic precursor. Selective hydroboration into the corresponding linear pinacol boronate is achieved in the case of 1-hexene, 1-octene, styrene, and allylbenzene. In the case of styrene, phenethyl pinacolboronate is isolated in 87% yield. Faster conversions of HBpin are obtained by increasing the alkene:borane ratio, with less than 10% of alkene hydrogenation. Isomerization into trans-β-methylstyrene is observed in the case of allylbenzene. Dehydrogenative borylation is competitive with ethylene and cyclic alkenes with large rings. Hydroboration of cyclohexene is highly favored, whereas for cyclodecene, vinylboronate is produced with only traces of allylboronate. Cyclooctene provides the two unsaturated boron-attached products, vinyl- and allylboronate in a 1:3 ratio, whereas only allylboronate is obtained from cycloheptene. The coupling of cyclodecenyl pinacolboronate with the aryl bromide (CF3)2C6H3Br, using standard catalytic Suzuki−Miyaura conditions, gives the corresponding product (CF3)2C6H3(C10H17) in 85% yield. Mechanistic investigations allow the characterization of the hydrido(boryl)(ethylene) complex RuH(Bpin)(C2H4)(PCy3)2 (3) as a key intermediate in the catalytic cycle.

Structure and coordinate bonding nature of the manganese–σ–borane complexes

The electronic and molecular structures of the complexes [(Cp′)Mn(CO) 2(σ-HBcat)] ( 1), [(Cp′)Mn(CO) 2(σ-HBpin)] ( 2) and [(Cp′)Mn(CO) 2(σ-HBMe 2)] ( 3) (Cp′ = η 5-MeC 5H 4) have been investigated at the DFT B3LYP and BP86 levels in order to understand the structures, bonding and energetics of the interactions between a transition metal and a σ-HBR 2 ligand. The calculated geometries are in excellent agreement with experimental values. These results are consistent with the description of [(Cp′)Mn(CO) 2(η 2-HBR 2)] as a Mn(I) complexes in which both hydrogen and boron of the [HBR 2] ligands have a bonding interaction with the manganese and B–H bond character is preserved. Upon coordination of [HBR 2], reduction in B–H bond order of about 1/3 was calculated. The LUMO of the distorted HBR 2 ligand is not a pure p π orbital, but is predominantly a hybrid (p π-s) from boron orbitals with some contribution from H s-orbitals. The coordination of the σ-borane ligand causes a rehybridization of the boron center. NBO analysis shows that the hybrid orbital of B–H bond obtains less 2s character and more 2p character upon coordination of borane to the manganese atom. The predominant effects of the coordination of the σ-borane ligand to manganese are the decrease in the B–H bond order and accompanying increase in the B–H bond distance. The nature of the metal–ligand interactions is quantitatively analyzed with an energy decomposition method. The [(Cp′)Mn(CO) 2]-[η 2-H-BR 2] bonding in borane complexes 1– 3 is more than half electrostatic. Indeed the three center-two electron bond in the Mn–H–B bridge may be regarded as a “protonated π-bond”. The metal-η 2-H-BR 2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M–B σ and π bonds with a boron atom lying in the plane defined by the metal and two R substituents.

Structural elucidation of a nickel boryl complex. A recyclable borylation Ni(ii) reagent of bromobenzene

The first nickel complex bearing a boryl moiety, (PNP)Ni[B(catechol)](PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), has been prepared, structurally characterized, and analyzed by DFT; this rare species is shown to be a recyclable reagent for the borylation of bromobenzene, via an unusual cycle, by applying the ingredients catecholborane and NaBH(4).

Structure and coordinate bonding nature of the rhenium–σ-borane complexes

The electronic and molecular structures of the complexes [(MeC 5H 4)Re(CO) 2(σ-HBcat)] 1, [(MeC 5H 4)Re(CO) 2(σ-HBpin)] 2 and [(MeC 5H 4)Re(CO) 2(σ-HBMe 2)] 3 have been investigated at the DFT BP86/TZ2P levels in order to understand the structures, bonding and energetic of the interactions between a transition metal and a σ-HBR 2 ligand. The nature of the Re–η 2-HBR 2 interactions was analyzed with charge and energy decomposition methods. The Re–B, B–H, Re–H bond distances are longer than that expected for single bond based on covalent radius predictions. The B–Re–H1 bond angles in the complexes [(η 5-MeC 5H 4)Re(CO) 2(η 2-HBR 2)]: 40.8° in 1, 41.5° in 2 and 38.5° in 3 are small and are consistent with B–H bonding. These results are consistent with the description of [(MeC 5H 4)Re(CO) 2(η 2-HBR 2)] as a Re(I) complexes in which both hydrogen and boron of the [HBR 2] ligands have a bonding interaction with the rhenium and B–H bond character is preserved. Upon coordination of [HBR 2], reduction in B–H bond order about 1/2 was calculated. The theoretically predicted bond dissociation energies of the borane ligands are −40.5 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBcat)], −39.7 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBpin)] and −43.3 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBMe 2)]. The [(η 5-MeC 5H 4)Re(CO) 2]–[η 2-H-BR 2] bonding in borane complexes 1– 3 is more than half electrostatic. All three complexes exhibit about 42–44% covalent bonding of the borane ligand to the metal fragment. Indeed the three center-two electron bond in the Re–H–B bridge may be regarded as a “protonated π-bond”. The metal–η 2-H-BR 2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M–B σ and π bonds with a boron atom lying in the plane defined by the metal and two R substituents.

Density Functional Theory Studies on the Mechanism of the Reduction of CO2 to CO Catalyzed by Copper(I) Boryl Complexes

The detailed reaction mechanism for the reduction of CO2 to CO catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition state structures. Our DFT calculations show that the reaction occurs through CO2 insertion into the Cu-B bond to give a Cu-OC(=O)-boryl species (i.e., containing Cu-O and C-B bonds), and subsequent boryl migration from C to O, followed by alpha-bond metathesis between pinB-Bpin (B2pin2, pin = pinacolate = OCMe2CMe2O) and (NHC)Cu(OBpin). The overall reaction is exergonic by 38.0 kcal/mol. It is the nucleophilicity of the Cu-B bond, a function of the very strong alpha-donor properties of the boryl ligand, rather than the oxophilicity of boron, which determines the direction of the CO2 insertion process. The boryl migration from C to O, which releases the product CO, is the rate-determining step and involves the "vacant" orbital orbital on boron. The (NHC)Cu(boryl) complexes show unique activity in the catalytic process. For the analogous (NHC)Cu(alkyl) complexes, the CO2 insertion into the Cu-C bond giving a copper acetate intermediate occurs with a readily achievable barrier. However, the elimination of CO from the acetate intermediate through a methyl migration from C to O is energetically inaccessible.

Transition‐Metal Complexes of Boron — New Insights and Novel Coordination Modes

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Cationic Terminal Borylene Complexes: Structure/Bonding Analysis and [4+1] Cycloaddition Reactivity of a BN Vinylidene Analogue

Cationic Terminal Borylene Complexes: Structure/Bonding Analysis and [4+1] Cycloaddition Reactivity of a BN Vinylidene Analogue

Cationic Terminal Borylene Complexes: Structure/Bonding Analysis and [4+1] Cycloaddition Reactivity of a BN Vinylidene Analogue

Charge changes everything: The cationic terminal aminoborylene complex [CpFe(CO)2(BNCy2)]+[BArF4]− (1, Cy= cyclohexyl, ArF=3,5-(CF3)2C6H3; Fe orange, B green, N blue, O red, C gray) was structurally characterized. DFT studies (see HOMO−2) indicate delocalized π bonding along the linear Fe-B-N framework and significant FeB and BN multiple-bond character. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2006/z602162_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis and Reactivity of Semibridging Borylene Complexes

Reaction of the terminal borylene complexes [(OC)5MBN(SiMe3)2] (M = Cr, W) with [M‘(PCy3)] (M‘ = Pd, Pt) at room temperature led to spontaneous formation of the heterodinuclear complexes [(OC)4M(μ-CO){μ-BN(SiMe3)2}M‘(PCy3)2] (M = Cr, M‘ = Pd; M = W, M‘ = Pd; M = Cr, M‘ = Pt) with concomitant liberation of PCy3. X-ray diffraction studies revealed that the borylene ligand adopts a semibridging position between the two different metal centers, thus establishing a new coordination mode for borylenes and providing further experimental evidence for the theoretically predicted close relationship between BR and CO. After prolonged reaction times the liberated phosphine substitutes the carbonyl group in trans-position to the borylene moiety, forming complexes of the type [(Cy3P)(OC)3M(μ-CO){μ-BN(SiMe3)2}M‘(PCy3)] (M = Cr, M‘ = Pt; M = W, M‘ = Pt; M = W, M‘ = Pd). Heating or photolytic activation of [(OC)4M(μ-CO){μ-BN(SiMe3)2}Pd(PCy3)] afforded the terminal borylene complexes trans-[(Cy3P)(OC)4MBN(SiMe3)2], which w...

Transition‐Metal Complexes of Boron—New Insights and Novel Coordination Modes

Since the publication of the last review in 1998, the transition-metal chemistry of boron has continued to raise unceasing interest. Boryl complexes, representing the most extensive subclass, have remained a focus of intense research, particularly for their implication in the metal-mediated functionalization of organic substrates. Absolute novelties such as borane complexes and terminal borylene complexes have been structurally authenticated. Upon further elaboration of these compounds, the known coordination modes of boron-based ligands have grown considerably. Combined structural and theoretical investigations have contributed to elucidate the fundamental electronic characteristics of the transition-metal-boron bond and are leading to applications of these compounds. The most useful synthetic strategies for the generation of transition-metal-boron bonds are highlighted here, and the most recent and intriguing compounds that have been reported are outlined and discussed.

Cationic Terminal Borylene Complexes: Interconversion of Amino and Alkoxy Borylenes by an Unprecedented Meerwein–Ponndorf Hydride Transfer

Cationic Terminal Borylene Complexes: Interconversion of Amino and Alkoxy Borylenes by an Unprecedented Meerwein–Ponndorf Hydride Transfer