Boryl Complexes Research Articles

Heavy metal boryl chemistry: complexes of cadmium, mercury and lead

Synthetic routes to the first boryl complexes of cadmium and mercury are reported via transmetallation from boryllithium; the syntheses of related group 14 systems highlight the additional factors associated with extension to more redox-active post-transition elements.

Versatile reactivity of a rhodium(i) boryl complex towards ketones and imines

The rhodium(I) boryl complex [Rh(Bpin)(PEt3)3] (1) reacts with the ketones α,α,α-trifluoroacetophenone and 9-fluorenone by insertion of the C=O bond to give [Rh{η(3)-C(CF3)(OBpin)C6H5}(PEt3)2] (4) and [Rh{η(5)-C13H8(OBpin)}(PEt3)2] (6), whereas the reaction with acetophenone leads to the formation of [Rh(H)(PEt3)3] (2), [Rh(OBpin)(PEt3)3] (3) and (E)-(Ph)CH=CHBpin. Treatment 1 of with ketimines generates [Rh{η(3)-C6H5=C(Ph)N(Ph)(Bpin)}(PEt3)2] (7), [Rh{(η(3)-C12H8)N(Ph)(Bpin)}(PEt3)2] (8) or [Rh{CPh2N(H)(Bpin)}(PEt3)2] (9). The insertion of aldimines into the Rh-B bond gives access to [Rh[η(3)-CH{N(C6H13)Bpin}C6H5](PEt3)2] (11) or [Rh[η(3)-CH{N(Ph)Bpin}C6H5](PEt3)2] (12). The latter is converted into the C-H activation product [Rh{(C6H4)-o-N(Bpin)(CH2Ph)}(PEt3)3] (13). Complex 13 reacts with B2pin2 to yield the boryl complex 1 and the amine PhCH2N(Bpin)(C6H4-o-Bpin).

A bidentate boryl ligand: syntheses of platinum and iridium complexes

A methylenediphenylphosphine substituted diazaborole was obtained in a two-step procedure in amounts around 4 g. This B-P ligand was coordinated at platinum and iridium and the structures of the boryl complexes are discussed. In the case of the iridium derivative an interesting boron-boron contact was found.

Synthesis of Rhodium(I) Boryl Complexes: Catalytic N–H Activation of Anilines and Ammonia

AbstractThe treatment of the rhodium methyl complex [Rh(Me)(PEt3)3] (1) with diboranes B2[(OR)2]2 gave the 16‐valence‐electron rhodium(I) boryl complexes [Rh{B(OR)2}(PEt3)3] [2: (OR)2 = pin = pinacolato, O2C2Me4; 3: (OR)2 = cat = catecholato, O2C6H4; 4: (OR)2 = O2C5H10]. The reactions of 2 and 4 with anilines yielded the amidoboranes HN[B(OR)2](R′) [6: B(OR)2 = Bpin, R′ = C6F5; 7: B(OR)2 = Bpin, R′ = Ph; 8: B(OR)2 = BO2C5H10, R′ = Ph] and the rhodium hydrido complex [Rh(H)(PEt3)3] (5) by N–H activation. The addition of ammonia to 2 resulted in the generation of the amine HN(Bpin)2 (9) as well as 5. The amidoboranes 6, 7, and 9 can also be synthesized catalytically with turnover numbers of 10 for 9 and 8 for 6 and 7.

New Heteronuclear Bridged Borylene Complexes That Were Derived from [{Cp*CoCl}2] and Mono‐MetalCarbonyl Fragments

The synthesis, structural characterization, and reactivity of new bridged borylene complexes are reported. The reaction of [{Cp*CoCl}2] with LiBH4·THF at -70 °C, followed by treatment with [M(CO)3(MeCN)3] (M=W, Mo, and Cr) under mild conditions, yielded heteronuclear triply bridged borylene complexes, [(μ3-BH)(Cp*Co)2(μ-CO)M(CO)5] (1-3; 1: M=W, 2: M=Mo, 3: M=Cr). During the syntheses of complexes 1-3, capped-octahedral cluster [(Cp*Co)2(μ-H)(BH)4{Co(CO)2}] (4) was also isolated in good yield. Complexes 1-3 are isoelectronic and isostructural to [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (5) and [(μ3-BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (6), with a trigonal-pyramidal geometry in which the μ3-BH ligand occupies the apical vertex. To test the reactivity of these borylene complexes towards bis-phosphine ligands, the room-temperature photolysis of complexes 1-3, 5, 6, and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] (7) was carried out. Most of these complexes led to decomposition, although photolysis of complex 7 with [Ph2P(CH2)(n)PPh2] (n=1-3) yielded complexes 9-11, [3,4-(Ph2P(CH2)(n)PPh2)-closo-1,2,3,4-Ru2Fe2(BH)2] (9: n=1, 10: n=2, 11: n=3). Quantum-chemical calculations by using DFT methods were carried out on compounds 1-3 and 9-11 and showed reasonable agreement with the experimentally obtained structural parameters, that is, large HOMO-LUMO gaps, in accordance with the high stabilities of these complexes, and NMR chemical shifts that accurately reflected the experimentally observed resonances. All of the new compounds were characterized in solution by using mass spectrometry, IR spectroscopy, and (1)H, (13)C, and (11)B NMR spectroscopy and their structural types were unequivocally established by crystallographic analysis of complexes 1, 2, 4, 9, and 10.

Base‐Stabilized Boryl and Cationic Haloborylene Complexes of Iron

The synthesis of base-stabilized boryl and borylene complexes is reported. An N-heterocyclic carbene (NHC)-stabilized iron-dihydroboryl complex was prepared by two different routes including methane liberation and salt elimination. A range of base-stabilized iron-dichloroboryl complexes was prepared by addition of Lewis bases to boryl complexes. Base-stabilized, cationic monochloroborylene complexes were synthesized from these boryl complexes by halide abstraction by using weakly coordinating anions.

B═B and B≡E (E = N and O) Multiple Bonds in the Coordination Sphere of Late Transition Metals

Because of their unusual structural and bonding motifs, multiply bonded boron compounds are fundamentally important to chemists, leading to enormous research interest. To access these compounds, researchers have introduced sterically demanding ligands that provide kinetic as well as electronic stability. A conceptually different approach to the synthesis of such compounds involves the use of an electron-rich, coordinatively unsaturated transition metal fragment. To isolate the plethora of borane, boryl, and borylene complexes, chemists have also used the coordination sphere of transition metals to stabilize reactive motifs in these molecules. In this Account, we summarize our results showing that increasingly synthetically challenging targets such as iminoboryl (B≡N), oxoboryl (B≡O), and diborene (B═B) fragments can be stabilized in the coordination sphere of late transition metals. This journey began with the isolation of two new iminoboryl ligands trans-[(Cy3P)2(Br)M(B≡N(SiMe3))] (M = Pd, Pt) attached to palladium and platinum fragments. The synthesis involved oxidative addition of the B-Br bond in (Me3Si)2N═BBr2 to [M(PCy3)2] (M = Pt, Pd) and the subsequent elimination of Me3SiBr at room temperature. Variation of the metal, the metal-bound coligands, and the substituent at the nitrogen atom afforded a series of analogous iminoboryl complexes. Following the same synthetic strategy, we also synthesized the first oxoboryl complex trans-[(Cy3P)2BrPt(BO)]. The labile bromide ligand adjacent to platinum makes the complex a viable candidate for further substitution reactions, which led to a number of new oxoboryl complexes. In addition to allowing us to isolate these fundamental compounds, the synthetic strategy is very convenient and minimizes byproducts. We also discuss the reaction chemistry of these types of compounds. In addition to facilitating the isolation of compounds with B≡E (E = N, O) triple bonds, the platinum fragment can also stabilize a diborene (RB═BR) moiety, a bonding motif that thus far had only been accessible when stabilized by N-heterocyclic carbenes (NHCs). In the new π-diborene [(Et3P)2Pt(B2Dur2)] (Dur = 2,3,5,6-Me4-C6H) complex, the diborene ligand receives electron density from Pt, leading to a strong Pt-B bond and a B═B bond. We attribute this result to the very short B═B bond distance (1.51(2) Å) while coordinated to platinum. Overall, an increasing number of chemists are examining the chemistry of multiply bound boron compounds. The isolation of an oxoboryl complex is of special interest not only from a structural standpoint but also because of its orbital similarities to the ubiquitous CO ligand. Detailed computational studies of the π-diborene complex [(Et3P)2Pt(B2Dur2)] show that the bonding properties of this molecule violate the widely accepted Dewar-Chatt-Duncanson (DCD) bonding model.

ChemInform Abstract: Transition Metal Borylene Complexes

AbstractReview: 47 refs.

A Base-stabilized Iodoborylene Complex of Platinum(II)

Abstract The first base-stabilized iodoborylene platinum complex was prepared through the addition of 4-picoline (4-Pic) to a suspension of an iodo-bridged binuclear iodoboryl complex. An X-ray structure determination of the title compound has characterized the molecular structure as cis-[Pt{BI(4- Pic)}I2(PCy3)]. The bond lengths lie in the expected range for neutral, base-stabilized borylene complexes. The strong trans influence of the borylene moiety is reflected in the longer Pt-1 distance for the iodo ligand opposite the borylene, compared to that opposite the phosphine ligand

Metathesis Reactions of a Manganese Borylene Complex with Polar Heteroatom–Carbon Double Bonds: A Pathway to Previously Inaccessible Carbene Complexes

A comprehensive study has been carried out to investigate the metathesis reactivity of the terminal alkylborylene complex [(η(5)-C5H5)(OC)2Mn═B(tBu)] (1). Its reactions with 3,3',5,5'-tetrakis(trifluoromethyl)benzophenone, 4,4'-dimethylbenzophenone, 2-adamantanone, 4,4'-bis(diethylamino)benzophenone, and 1,2-diphenylcyclopropen-3-one afforded the metathesis products [(η(5)-C5H5)(OC)2Mn═CR2] (R = C6H3-3,5-(CF3)23a, C6H4-4-Me 3b, C6H4-4-NEt23d; CR2 = adamantylidene 3c, cyclo-C3Ph23e). The cycloaddition intermediates were detected by NMR spectroscopy from reactions involving ketones with more electron-withdrawing substituents. The reaction of 1 with dicyclohexylcarbodiimide (DCC) only proceeds to form the cycloaddition product [(η(5)-C5H5)(OC)2Mn{κ(2)-C,B-C(═NCy)N(Cy)B(tBu)}] (4), which upon warming, rearranges to afford complex [(η(5)-C5H5)(OC)2Mn{CN(Cy)B(tBu)CN(Cy)}] (5). The reaction of 1 with triphenylphosphine sulfide SPPh3 also yields the metathesis product [(η(5)-C5H5)(OC)2Mn(PPh3)] via an intermediate which is likely to be a η(2)-thioboryl complex [(η(5)-C5H5)(OC)2Mn{(η(2)-SB(tBu)}] (6). Similar reactions have been studied using an iron borylene complex [(Me3P)(OC)3Fe═B(Dur)] (Dur = 2,3,5,6-tetramethylphenyl, 9). Extensive computational studies have been also carried out to gain mechanistic insights in these reactions, which provided reaction pathways that fit well with the experimental data.

Cationic Dihydride Boryl and Dihydride Silyl Osmium(IV) NHC Complexes: A Marked Diagonal Relationship

The complex [OsCl(η6-p-cymene)(IPr)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = trifluoromethanesulfonate) reacts with pinacolborane (HBpin) and catecholborane (HBcat) to give the dihydride boryl osmium(IV) derivatives [OsH2(Bpin)(η6-p-cymene)(IPr)]OTf (2) and [OsH2(Bcat)(η6-p-cymene)(IPr)]OTf (3), which undergo hydrolysis to afford the trihydride [OsH3(η6-p-cymene)(IPr)]OTf (4). In agreement with the boron–silicon diagonal relationship, complex 1 also reacts with silanes, HSiR3. The reactions lead to the dihydride silyl species [OsH2(SiR3)(η6-p-cymene)(IPr)]OTf (SiR3 = SiPh3 (5), SiEt3 (6), SiHPh2 (7)), which also undergo hydrolysis to yield 4. Complexes 2 and 5 have been characterized by X-ray diffraction analysis. Their four-legged piano-stool geometries with transoid hydride ligands are similar. In solution, the arene rotates over the remaining four ligands. The activation barrier of the process depends upon the size of the boryl or silyl groups. The nature of the Os–B and Os–Si...

Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes

Thermolysis of [(Cp*RuCO)2B2H6] (1; Cp* = η5-C5Me5) with [Ru3(CO)12] yielded the trimetallaborane [(Cp*RuCO)3(μ3-H)BH] (2) and a number of homometallic boride clusters: [Cp*RuCO{Ru(CO)3}4B] (3), [(Cp*Ru)2{Ru2(CO)8}BH] (4), and [(Cp*Ru)2{Ru4(CO)12}BH] (5). Compound 2 is isoelectronic and isostructural with the triply bridged borylene compounds [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (6) and [(μ3-BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (7), where the [μ3-BH] moiety occupies the apical position. To test if compound 2 undergoes hydroboration reactions with alkynes, as observed with 6, we performed the reaction of 2 with the same set of alkynes under photolytic conditions. However, neither 2 nor 7 undergoes hydroboration to yield a vinyl–borylene complex. On the other hand, thermolysis of 6 with trimethylsilylethylene yielded the novel diruthenacarborane [1,1,7,7,7-(CO)5-2,3-(Cp*)2-μ-2,3-(CO)-μ3-1,2,3-(CO)-5-(SiMe3)-pileo-1,7,2,3,4,5-Fe2Ru2C2BH] (8). The solid-state X-ray diffraction results suggest that 8 exhibits a pentagonal -bipyramidal geometry with one additional CO capping one of its faces. Cluster 3 is a boride cluster where boron is in the interstitial position of a square-pyramidal geometry, whereas compound 4 can be described as a tetraruthenium boride in which the Ru4 butterfly skeleton has an interstitial boron atom. Electronic structure calculations of compound 2 employing density functional theory (DFT) generate geometries in agreement with the structure determinations. The existence of a large HOMO–LUMO gap in 2 is in agreement with its high stability. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. Furthermore, the B3LYP-computed 11B and 1H chemical shifts for compound 2 precisely follow the experimentally measured values. All the compounds have been characterized by IR and 1H, 11B, and 13C NMR spectroscopy, and the geometries of the structures were unambiguously established by crystallographic analyses of 2–4 and 8.

ChemInform Abstract: Modern 1,3,2‐Diazaborole Chemistry — A Metamorphosis from Electrophilic to Nucleophilic Boron

AbstractReview: ca. 100 refs.

Aminoborane σ Complexes: Significance of Hydride Co-ligands in Dynamic Processes and Dehydrogenative Borylene Formation

Systems of the type [(p-cym)Ru(PR3)(H)(H2BNiPr2)]+ (R = Cy, Ph) can be synthesized from (p-cym)Ru(PR3)Cl2 and H2BNiPr2/Na[BArf4] and are best formulated as (hydrido)ruthenium κ1-aminoborane complexes. VT-NMR measurements have been used to probe the σ-bond metathesis process leading to Ru–H/H–B exchange, yielding an activation barrier of ΔG⧧ = 7.5 kcal mol–1 at 161 K. Moreover, in contrast to the case for related non-hydride-containing systems, reactivity toward alkenes constitutes a viable route to a metal borylene complex via sacrificial hydrogenation.

Maximizing Coordinative and Electronic Unsaturation: Three-Coordinate Dicationic Platinum Complexes

Halide abstraction from cationic base-stabilized borylene complexes led to extremely electronically and coordinatively unsaturated platinum complexes. The formally 14-electron borylene complexes show no agostic interactions with the phosphine ligands, and only small Lewis donor ligands could be added to the Pt center.

Transition metal borylene complexes

Borylene ligands (:BR) are isolobal to CO and other iconic organometallic ligands. As such, borylene ligands enjoy some parallels to these ligands, but in many ways, their chemistry is distinct. This tutorial review gives an introduction to the synthesis, properties and reactivity of the major classes of transition metal borylene complexes, including terminal and bridging examples, pseudoborylenes, and metalloborylenes (borido complexes).

Cyclic (amino)(imino)carbene complexes by borylene transfer to isocyanides

Here we report the facile formation of boron-containing cyclic (amino)(imino)carbene complexes by reaction of a chromium borylene complex [(OC)(5)Cr=B=N(SiMe(3))(2)] with isocyanides.

Borole-Derived Spirocyclic Tetraorganoborate

Preparation of a novel conjugated tetraorganoborate is presented in a facile two-step procedure. Successful utilization as a halide abstraction reagent is demonstrated in a metathesis reaction with the platinum(II) boryl complex [(Cy3P)2Pt(Br)(BC4Ph4)], which was obtained by oxidative addition of 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0) species. The novel compounds were investigated by means of spectroscopic and X-ray diffraction techniques.

Modern 1,3,2‐Diazaborole Chemistry – A Metamorphosis from Electrophilic to Nucleophilic Boron

AbstractDiazaborolyl anions are species with electronically and coordinatively unsaturated boron centers, and as six‐electron systems they are analogues of the more familiar N‐heterocyclic carbenes and N‐heterocyclic silylenes. In contrast to the latter compounds lithium diazaborolyls are thermally unstable, they are extremely strong bases and versatile nucleophiles. With these properties they overcome the paradigm that low‐coordinated borane derivatives are mainly electrophiles. In keeping with this, the metal diazaborolyls discussed here serve as excellent transfer reagents of the borolyl anion to main group elements and transition and rare earth metals. Thereby they establish a novel synthetic approach to the class of metal boryl complexes. A series of iridium complexes produced by this protocol have meanwhile found application as emitter materials in organic light‐emitting diodes (OLEDs).

A Theoretical Study of an Unusual Y-Shaped Three-Coordinate Pt Complex: Pt(0) σ-Disilane Complex or Pt(II) Disilyl Complex?

The unusual Y-shaped structure of the recently reported three-coordinate Pt complex Pt[NHC(Dip)(2)](SiMe(2)Ph)(2) (NHC = N-heterocyclic carbene; Dip = 2,6-diisopropylphenyl) was considered a snapshot of the reductive elimination of disilane. A density functional theory study indicates that this structure arises from the strong trans influence of the extremely σ-donating carbene and silyl ligands. Though this complex can be understood to be a Pt(II) disilyl complex bearing a distorted geometry due to the Jahn-Teller effect, its (195)Pt NMR chemical shift is considerably different from those of Pt(II) complexes but close to those of typical Pt(0) complexes. Its Si···Si bonding interaction is ~50% of the usual energy of a Si-Si single bond. The interaction between the Pt center and the (SiMe(2)Ph)(2) moiety can be understood in terms of donation and back-donation interactions of the Si-Si σ-bonding and σ*-antibonding molecular orbitals with the Pt center. Thus, we conclude that this is likely a Pt(0) σ-disilane complex and thus a snapshot after a considerable amount of the charge transfer from disilane to the Pt center has occurred. Phenyl anion (Ph(-)) and [R-Ar](-) [R-Ar = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3)] as well as the divalent carbon(0) ligand C(NHC)(2) also provide similar unusual Y-shaped structures. Three-coordinate digermyl, diboryl, and silyl-boryl complexes of Pt and a disilyl complex of Pd are theoretically predicted to have similar unusual Y-shaped structures when a strongly donating ligand coordinates to the metal center. In a trigonal-bipyramidal Ir disilyl complex [Ir{NHC(Dip)(2)}(PH(3))(2)(SiMe(3))(2)](+), the equatorial plane has a similar unusual Y-shaped structure. These results suggest that various snapshots can be shown for the reductive eliminations of the Ge-Ge, B-B, and B-Si σ-bonds.