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Reactivity of Transition-Metal Borylene Complexes: Recent Advances in B−C and B−B Bond Formation via Borylene Ligand Coupling Holger Braunschweig* and Rong Shang Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
ABSTRACT: Terminal borylene complexes of group 6−10 metals have been prepared through a number of synthetic strategies, displaying a wide structural diversity with numerous coordination modes of the borylene ligand [:BR]. Earlier, amino-substituted borylenes were found to serve as borylene ligand transfer reagents to a range of organic and organometallic substrates. In contrast, aryl- and alkyl-substituted borylenes display metathesis behavior when combined with ketones. Recent studies on these complexes revealed new reactivity patterns involving borylene−borylene and carbonyl−borylene ligand coupling. Herein a brief account on the recent progress of these borylene ligand coupling reactions made in our laboratories is provided. These reactions offer unprecedented alternative methods for electron-precise B−B and B−C bond formation.
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INTRODUCTION
Despite being very reactive species, borylenes can be stabilized in the coordination sphere of transition-metal fragments, forming complexes that are thermally stable and isolable under ambient conditions. Since the first structural report in 1995,30 the library of borylene complexes has been growing steadily and now demonstrates an impressive structural diversity (Figure 1). These complexes are now accessible using several synthetic strategies. These and the structural diversity of these complexes have received numerous comprehensive reviews.31−34 Recent developments in our laboratory on the reactivity of alkyl- and aryl-substituted terminal borylene complexes revealed that the low-valent borylene ligand can couple with itself, or an isolobal ligand such as carbon monoxide (CO), to form B−B and B−C bonds. In contrast to the vast knowledge accumulated on C−C and C−B bond formation, there is very little known about the chemistry concerning electron-precise B−B bond formation. Despite the reasonably high homonuclear σ-bond enthalpy of boron (D0 = 293 kJ mol−1, compared to that of C−C, D0 = 345 kJ mol−1, and Si−Si, D0 = 222 kJ mol−1),35 compounds containing multiple boron atoms intrinsically tend to form nonclassical cluster structures36 instead of electron-precise chains as in the cases of carbon and silicon. Since the first report on the synthesis of B2Cl4 by
Transition-metal complexes with multiple metal−main group element bonds have proven to be of crucial importance in industrial-scale chemical transformations.1−4 Unarguably, the most well-known classes of these compounds are alkylidene and alkylidyne complexes, which contain metal−carbon multiple bonds. These complexes are noted for their roles in homogeneous catalysis, particularly olefin metathesis and the Fischer−Tropsch process, as well as many other useful applications in laboratory transformations such as natural product syntheses.5−9 The demonstrated diverse reactivity of complexes containing transition metal−main group element multiple bonds and their profound usefulness have accordingly attracted intense interest from chemists. Our laboratory is interested in the chemistry of transitionmetal borylene complexes. In contrast to singlet carbenes [:CR2], which are (in many cases) isolable species, free borylenes [:BR] are highly reactive species, the generation of which requires harsh reaction conditions.10−14 In addition, the trapping of the in situ generated borylene species requires extremely low temperatures and suffers from selectivity issues.15−21 Only recently, carbon-centered Lewis bases such as N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes have been found to facilitate the generation and stabilization of base-stabilized borylene species under milder reaction conditions with good selectivity.22−29 © 2015 American Chemical Society
Received: January 15, 2015 Published: March 11, 2015 3099
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are only a handful of examples, including cationic60−65 and neutral59,66−69 alkyl-, aryl-, and silyl-substituted terminal borylenes, available in the literature. This intrinsic electronic distinction between amino- and alkyl-/arylborylenes differentiates their chemical reactivity. Reactivity studies have shown that group 6 aminoborylenes serve as an excellent source for the borylene [:BR] fragment, which allows direct boron functionalization of both organic70−73 and organometallic substrates.70,74−98 One particularly useful application of this chemistry is in the synthesis of new terminal and bridging borylene complexes. The first terminal bis(borylene) [(η5-C5Me5)Ir{BN(SiMe3)2}2] (2) was synthesized (Scheme 1, A) by adding 2 equiv of the chromium
Figure 1. Coordination modes of borylene and related low-coordinate boron ligands in transition-metal complexes.
Scheme 1. Syntheses of Terminal Bis(borylene) Complexes via Borylene Transfer
Stock et al. almost a century ago,37 the reductive coupling of πstabilized boranes first reported by Brotherton et al.38 remained the only scalable method to prepare compounds containing electron-precise B−B bonds in the 20th century.39 It was not until 2007 that alternative methods via metal-free as well as metal-catalyzed dehydrocoupling were achieved by the pioneering work of Himmel et al.40−50 More recently, Macgregor, Weller, and co-workers have reported metal-assisted B−B homocoupling of amine−boranes.51 Since 2011, our group has discovered a series of dehydrocoupling reactions catalyzed (both homo- and heterogeneously) by group 4, 9, and 10 transition metals, which provided a more direct and atomeconomical route to the increasingly useful reagents diboranes(4).52−54 More recently, the emerging metal-mediated ligand coupling reactions of borylene complexes also represent an unprecedented alternative method for electron-precise B−B as well as B−C bond formation, which will be the emphasis of this account.
aminoborylene complex [(OC)5CrBN(SiMe3)2] (1) as a borylene source to 1 equiv of the iridium carbonyl complex [(η5-C5Me5)Ir(CO)2] under photolytic conditions.83 Complex 2 remained the only example of a terminal bis(borylene) complex until recently, when iron bis(borylene) complex [(OC)3Fe(BDur){BN(SiMe3)2}] (5, DUR = 2,3,5,6-tetramethylphenyl) was synthesized from a transfer reaction from [(OC)5MoBN(SiMe3)2] (3)99 to the arylborylene complex [(OC)2(Me3P)Fe(BDur)] (4) under mild thermal conditions.68 In contrast to 2, complex 5 possesses two different terminal borylene ligands, which significantly interact with each other. This is evident in several aspects: (i) the 11B NMR spectrum revealed an upfield shift for both boron nuclei, at 129 (BDur) and 78 ppm (BN(SiMe3)2), compared to its precursors (146 and 90 ppm respectively), (ii) the B−Fe−B angle of 65.9° is much more acute than that of 2 (78.4°), and (iii) the B−B separation of 1.982(3) Å is comparable to the theoretically predicted B−B distance of a metal-coordinated difluorodiborene and significantly shorter than that observed in 2 (2.36 Å). This was further confirmed by density functional theory calculations on 5 at the OLYP/TZVP level of theory, which showed a significant bonding interaction between the two boron atoms, with a large Wiberg bond index (WBI) of 0.78, corresponding to almost complete coupling of the boron atoms. On the other hand, the boron substituents are more aligned with the FeB bond than the B−B interactions, reflected by the very obtuse Fe−B−N [172.0 (2)°] and Fe−B− C [174.9(2)°] angles. This is in contrast to the η2-B,Bcoordinated platinum complex [Pt(η2-B2Dur2)(PMe3)2], in which the boron substituents are more aligned along the B−B bond [angles B−B−R: 166.0(8) and 164.2(8) Å].68
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BONDING Computational studies55−58 on borylenes have revealed that such species possess a high-energy highest occupied molecular orbital (HOMO) of σ geometry and two orthogonal π* orbitals of moderate energy as lowest unoccupied molecular orbitals (LUMOs), which account for the ligand’s much stronger σdonor ability and comparable π-acceptor ability compared to its isoelectronic analogues (such as CO and N2). The strong σdonor ability of the borylene ligand overwhelms its π-acceptor ability, resulting in an accumulation of positive charge on the borylene moiety. This and the low coordination number of the boron atom lead to enhanced susceptibility of terminal borylene ligands to nucleophilic attack. This intrinsic kinetic instability can be mitigated either (i) by electronic stabilization via strong π-donor substituents at boron or (ii) by steric protection through the introduction of bulky borylene substituents. As such, the π-acceptor characteristics of the borylene ligand are strongly influenced by the borylene substituent. The first strategy, employing electronically stabilizing borylene substituents, is responsible for the isolation of aminoborylenes [LnMBNR2], which constitute the largest class of terminal borylenes. Without exception, the nitrogen atom in these complexes adopts a planar geometry. Its lone electron pair competes with the filled d orbitals of the metal center for stabilization of the electron-deficient boron atom, increasing the B−N bond order while decreasing the M− B bond order. The second synthetic strategy, stabilization by steric production, has proven more challenging. To date, there 3100
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BORYLENE−BORYLENE COUPLING (HOMOCATENATION) In 2002, our laboratory reported a reaction involving the bridging chloroborylene complex [μ-BCl({η5-C5H4CH3)Mn(CO)2}2] (6) and CO, from which an orange crystalline solid of structure [(μ-BCl)2({η5-C5H4CH3)Mn(CO)2}2] (8) was isolated under photolytic conditions.100 Although the detailed mechanism is still unknown, the reaction is likely to proceed via the formation of terminal chloroborylene intermediate [(η5C5H4CH3)(OC)2MnBCl] (7) followed by intermolecular coupling with another molecule, which would represent the first example of a borylene−borylene coupling reaction. Calculations have shown that both formation of the proposed terminal chloroborylene intermediate from its bridged borylene precursor and its dimerization are highly exothermic processes and have small reaction barriers.101 Later, computational studies on the coupling of iron fluoroborylene systems also predicted similar structures.102 The central Mn2B2 core adopts a bicyclobutane geometry with a dihedral angle of 150.9° between the two MnB2 triangles. The B−B separation is 1.695(7) Å, and the two boron-bound chloride ligands are located in exo positions. These comply with the electron count and geometry of a dimetalla-nido-tetraborane. More recently, we have found that under photolytic conditions, the bis(borylene) complex 5 dimerizes to form the dinuclear tetra(borylene) complex 9, which in the presence of CO selectively extrudes one iron center under thermal conditions to form a mononuclear tetraboron complex (10) with a catenated B4 chain (Scheme 2). This series of reactions,
[Fe2−B2, 2.033(2) Å]. The Fe−Fe distance of 2.3554(5) Å is significantly shorter than that observed in the classical [Fe2(CO)9] [2.523(1) Å]103 and therefore can be considered a double bond, which would also comply with the 18-electron rule at each iron center. In contrast to its transient parent compound [Fe2(CO)8], 9 possesses considerable stability in the solid state. In solution though, it reacts with CO at 80 °C to afford the catenation product 10. The solid structure of 10 revealed an almost planar central Fe−B1−B2−B3−B4 ring. The acute angles of Fe−B1− B2 (64.2°) and Fe−B4−B3 (63.8°) remain, and concomitantly the B2−B3 bond is held close to the iron center, which thus suggests a significant interaction between the three atoms. The Fe−B1 and Fe−B4 separations (1.90 Å) are lengthened by more than 3% in comparison to those observed in 5, consistent with the increased coordination number at boron. The B1−B2 [1.792(6) Å], B2−B3 [1.686(6) Å], and B3−B4 [1.814(6) Å] distances fall in the range expected for B−B single bonds in structures such as cyclo-B6(NMe2)6.104 The WBIs calculated for B1−B2, B2−B3, and B3−B4 are 0.87, 1.00, and 0.86, respectively, suggesting three fully formed single B−B bonds. These predicted strengths of the covalent interactions are consistent with the observed bond distances.
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BORYLENE−CARBONYL COUPLING Base-Induced Intramolecular Coupling. The borylene ligands in 5 are not fully coupled owing to the strong electron donation from the filled B−B σ bond to an empty orbital on the iron center. By introducing a strong σ-donor ligand to populate this empty iron-based orbital, we can decrease the amount of B−B σ donation to iron and facilitate B−B coupling. This has been realized by the thermal reaction of 5 with a slight excess of phosphine, from which the intramolecular coupling product 11 was isolated (Scheme 3). In this reaction, one carbonyl ligand
Scheme 2. Boron Catenation via Borylene−Borylene Coupling
Scheme 3. Carbonyl−Borylene Coupling of Iron Bis(borylene) Complexes
also couples with one of the two borylene ligands. The 11B NMR spectrum revealed high-field-shifted resonances at 74 ppm (BN) and 51 ppm (BDur) for both boron centers, which are consistent with the increased coordination number of the boron atoms. The B−B bond distance of 1.646(4) Å is comparable to the B2−B3 distance of 10 [1.686(6) Å] and significantly shorter than that observed in 5 [1.982(3) Å]. Under photolytic conditions, complex 11 releases one CO to form phosphine-substituted bis(borylene) complex 12, which is
with structural characterization of 9 and 10, demonstrates a stepwise coupling of the borylene ligands of 5, which remains the only example of controlled boron catenation to date. Although the intermediate complex 9 possesses a slightly longer B1−B2 distance [2.102(3) Å] compared to 5 [1.982(3) Å], the interactions between B1 and B2 (also between B3 and B4) are evident in the solid-state structure in a similar way. Boron atom B1 bends more toward B2 [165.6(2) Å], while the bridging B2 is closer to Fe1 [Fe1−B2, 1.963(2) Å] than to Fe2 3101
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initiate the coupling. In contrast, in the case of the manganese borylene complex 14, coordination of the Lewis base at the low-valent boron center appears to activate the coupling. This has been demonstrated by reactions that involve nitrogen- and carbon-based ligands (Scheme 5).107 The base-stabilized
analogous to 5. Upon the further addition of a second 1 equiv of phosphine, a bis(phosphine) iron complex (13) is formed, in which both carbonyl ligands couple with the two central borylene ligands to form a CBBC chain bound to the metal center (Scheme 3). Complex 13 is electronically and structurally analogous to 10, replacing two borylene units with two isoelectronic carbonyl groups. It is worth noting that the (O)C−B(N) distance [2.030(5) Å] is significantly longer than the (O)C−B(Dur) distance [1.692(5) Å] in complex 13. This has been attributed to a significantly stronger backbonding interaction from the metal dz2 orbital to the antibonding orbital of (O)C−B(N) in comparison to that from the same metal dz2 orbital to the (O)C−B(Dur) antibonding orbital, which further reflects the dramatic difference between amino- and alkyl-substituted borylenes. Borylene−carbonyl ligand coupling has also been observed with the manganese alkylborylene complex 14. This complex reacts with 2 equiv of supermesityl isonitrile (Mes*NC, Mes* = 2,4,6-tritertbutylphenyl) to form complex 15, which features a bridging and a semibridging carbonyl between the boron atom and the metal center (Scheme 4).105 The calculated106 bridging
Scheme 5. Base-Induced Borylene−Carbonyl Coupling Reactions
borylene complexes 17 and 18 have been isolated from reactions of 14 with (dimethylamino)pyridine (DMAP) and the NHC IMe [(MeNCH)2C:], respectively. Upon the addition of CO, both 17 and 18 incorporate 1 equiv of CO to form the corresponding coupling products 19 and 20. Intermolecular Coupling. Aside from intramolecular coupling reactions, the bis(borylene) complex 5 has been shown to couple with alkynes to form 1,4-dibora-1,3-butadiene and 1,4-diboracyclohexadiene complexes, depending on the steric bulk of the acetylene substituents, under photolytic conditions (Scheme 6, top).87 The reaction of 5 with the more sterically demanding bis(trimethylsilyl)acetylene affords the iron complex 21. In this reaction, both borylene ligands of the iron complex couple to one acetylene substrate to form a 1,4dibora-1,3-butadiene moiety. The overall bonding situation can be described by both the mesomeric form A, an iron cis-diboryl
Scheme 4. Carbonyl−Borylene Coupling of the Manganese Borylene Complex 14
(O)C2−B distance of 1.586 Å falls within the range expected for a B−C single bond. The (O)C2−Mn distance of 1.938 Å is significantly longer than a typical terminal (O)C−Mn distance (1.75−1.85 Å), which is consistent with the reduced bond order expected for the bridging coordination mode. The second carbonyl (C1O) binds to the boron, although the interaction is very weak. The C1−Mn distance of 1.784 Å is still within the range expected for a terminally bound CO. The C1−B distance of 2.229 Å is very long for a B−C interaction, although the IR CO absorption band at 1903 cm−1 lies within the range expected for a semibridging carbonyl (cf. 14: 1968 and 1912 cm−1). Furthermore, the WBI of 0.21 calculated for B−C1 is small but not negligible. A similar degree of interaction is also found between the boron and manganese atoms, reflected by the WBI of 0.24. The isonitriles involved in this reaction of 14 serve as ligands. One coordinates to the manganese center, while the other coordinates to the boron atom. This result prompted further investigation with different Lewis bases. For example, 14 also reacts with PMe3 in the presence of CO to afford 16, in which the borylene ligand couples with two carbonyl ligands completely (Scheme 4).107 In both the iron bis(borylene) and manganese borylene cases, the addition of a Lewis base induces the intramolecular coupling of the borylene and carbonyl ligands. In the iron bis(borylene) system, the Lewis base attacks the metal center to
Scheme 6. (Top) Metal-Mediated Borylene−Alkyne Coupling Reactions of 5 and (Bottom) Mesomeric Forms of Complex 21
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substituent in the unit cell of 26 (1.575 Å) is elongated in comparison to that observed in 25 [1.537(6) Å]. The Cr−B separation of 2.067(4) Å is only slightly longer than a typical 3d metal−boron single bond.31,32,89,108 This reductive coupling shown by 25 is in stark contrast to the reduction of other well-known group 6 carbonyl complexes such as [Cr(CO)6]109,110 and [(OC)5Cr(NMe3)],111 where one ligand cleaves from the metal center upon reduction. Indeed, when the same reaction was carried out with the analogous aminoborylene complex 1, the borylene ligand was liberated and the dichromium complex K2[Cr2(CO)10]·2THF· 2(18-crown-6) was isolated in 70% yield. This notable difference between 25 and 1 in their respective behavior toward reduction was attributed to the difference in their electronic structures. The LUMO of 25 is a nonbonding orbital centered predominantly on the [BAr′] side of the complex. Population of this orbital upon reduction does not lead to facile liberation of the borylene unit. In contrast, the LUMO of 1 comprises antibonding interactions spread all around its metal center, the population of which leads to liberation of the boroncontaining moiety from the chromium scaffold.
complex with a side-on-coordinated alkenyl function, and the mesomeric form B, an iron 1,4-dibora-1,3-butadiene complex (Scheme 6, bottom). Monitored by 11B NMR experiments, the reaction of 5 with the smaller alkyne diphenylacetylene proceeds to form complex 22 initially, which shows chemical shifts (δB = 80 and 61 ppm) comparable to those observed for 21 (δB = 93 and 60 ppm). Then a second diphenylacetylene molecule couples with the iron-coordinated 1,4-dibora-1,3-butadiene ligand to form the 1,4-diboracyclohexadiene iron complex 23 (δB = 35 and 26 ppm; Scheme 6, top). On the basis of the structural parameters obtained from a single-crystal crystallographic study, the diboracyclohexadiene ring of 23 is best described as a fourelectron donor with olefinic η2−η2 coordination and a comparatively weak interaction between the vacant p orbitals of the boron atoms and a filled d orbital of iron. In contrast to the previous two cases, the reaction of 5 with but-2-yne under the same photolytic conditions only affords the 1,4-diboracyclohexadiene complex 24, isostructural to 23. The reaction proceeds to completion in 24 h at room temperature, during which the intermediate 1,4-dibora-1,3-butadiene complex is not detectable by NMR spectroscopy. This suggests that the addition of the first 1 equiv of alkyne is rate-limiting and the addition of the second alkyne is much faster than that in the previous cases. This has been attributed to the smaller steric hindrance of but-2-yne in comparison to the phenyl- and trimethylsilyl-substituted alkynes.
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CONCLUSIONS Recent reactivity studies on aryl- and alkyl-substituted terminal borylenes have revealed a number of ligand coupling reactivity patterns that have not been observed for terminal aminoborylene complexes, including intermolecular borylene−borylene homocoupling, base-induced intramolecular borylene− carbonyl coupling, intermolecular borylene−acetylene coupling, and reductive borylene−carbonyl coupling. Some of these reactions closely resemble those observed for Fischer carbyne complexes, which is reflective of the electronic similarity between metal borylene (MBR) and metal carbyne (M CR) multiple bonds. The fact that a borylene ligand can homocouple with another borylene, as well as carbonyl ligands, is reflective of the isolobality of carbonyl and borylene ligands. These reactions allow us to better understand metal-mediated B−B and B−C bond formation and therefore have important implications in organic and organometallic syntheses.
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REDUCTIVE COUPLING The latest addition to the library of alkyl-substituted terminal borylene complexes is the bulky aryl-substituted chromium borylene complex [(OC)5CrBAr′] (25; Ar′ = 2,6-(2,4,6iPr3C6H2)2C6H3), which has been synthesized via a double salt elimination reaction between Na2[Cr(CO)5] and Cl2BAr′. Besides the Lewis-base-induced coupling chemistry analogous to that of the manganese system, complex 25 has also been found to carry out reductive borylene−carbonyl coupling in the presence of a strong reducing agent.66 The reaction of 25 with 2 equiv of potassium graphite leads to the quantitative formation of 26 as a red solid. An X-ray crystallographic study of single crystals of 26 revealed its structure to be [K(THF)2]2[(OC)3Cr{η3-C(O)B(Ar′)C(O)}], where all metal-bound ligands of 22 are retained, among which two carbonyl ligands are coupled with the borylene ligand, while the other three remain terminal (Scheme 7). The average Cr−B distance of 2.067 Å is longer than that observed in 22 [1.904(5) Å]. The B−CO bond distances (1.504−1.704 Å) are significantly shorter than those observed in the previously discussed manganese-based complexes (e.g., 12, 13, 16, and 17).105,107 The average B−C bond between boron and the aryl
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest. Biographies
Scheme 7. Reductive Borylene−Carbonyl Coupling
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(2) Ivin, K. J. Olefin Metathesis; Academic Press: London, 1983. (3) Applications of Transition Metal Catalysis in Drug Discover and Development: An Industrial Perspective; John Wiley & Sons, Inc.: Hoboken, NJ, 2012. (4) Handbook of Metathesis. Three Volume Set; John Wiley and Sons Ltd.: New York, 2003; p 1234. (5) Herndon, J. W. Coord. Chem. Rev. 2014, 272, 48−144. (6) Herndon, J. W. Coord. Chem. Rev. 2013, 257, 2899−3003. (7) Herndon, J. W. Coord. Chem. Rev. 2012, 256, 1281−1376. (8) Herndon, J. W. Coord. Chem. Rev. 2011, 255, 3−100. (9) Herndon, J. W. Coord. Chem. Rev. 2010, 254, 103−194. (10) Bettinger, H. F. J. Am. Chem. Soc. 2006, 128, 2534−2535. (11) Lavrov, B. P.; Osiac, M.; Pipa, A. V.; Roepcke, J. Plasma Sources Sci. Technol. 2003, 12, 576−589. (12) Timms, P. L. J. Am. Chem. Soc. 1967, 89, 1629−1632. (13) Timms, P. L. Acc. Chem. Res. 1973, 6, 118−123. (14) Timms, P. L. J. Am. Chem. Soc. 1968, 90, 4585−4589. (15) Meller, A.; Bromm, D.; Maringgele, W.; Heine, A.; Stalke, D.; Sheldrick, G. M. Chem. Commun. 1990, 741−742. (16) Meller, A.; Seebold, U.; Maringgele, W.; Noltemeyer, M.; Sheldrick, G. M. J. Am. Chem. Soc. 1989, 111, 8299−8300. (17) Schlögl, R.; Wrackmeyer, B. Polyhedron 1985, 4, 885−892. (18) Pachaly, B.; West, R. Angew. Chem., Int. Ed. 1984, 23, 454−455. (19) van der Kerk, S. M.; Budzelaar, P. H. M.; van der Kerk-van Hoof, A.; van der Kerk, G. J. M.; Schleyer, P. v. R. Angew. Chem., Int. Ed. 1983, 22, 48−48. (20) van der Kerk, S. M.; Budzelaar, P. H. M.; van der Kerk-van Hoof, A.; van der Kerk, G. J. M.; Schleyer, P. v. R. Angew. Chem. 1983, 95, 61−61. (21) van der Kerk, S. M.; Boersma, J.; van der Kerk, G. J. M. Tetrahedron Lett. 1976, 17, 4765−4766. (22) Braunschweig, H.; Maier, J.; Radacki, K.; Wahler, J. Organometallics 2013, 32, 6353−6359. (23) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking, G.; Bertrand, G. Science 2011, 333, 610−613. (24) Bissinger, P.; Braunschweig, H.; Kraft, K.; Kupfer, T. Angew. Chem., Int. Ed. 2011, 50, 4704−4707. (25) Bissinger, P.; Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Kupfer, T.; Radacki, K.; Wagner, K. J. Am. Chem. Soc. 2011, 133, 19044−19047. (26) Grigsby, W. J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 7981− 7988. (27) Ruiz, D. A.; Melaimi, M.; Bertrand, G. Chem. Commun. 2014, 50, 7837−7839. (28) Dahcheh, F.; Martin, D.; Stephan, D. W.; Bertrand, G. Angew. Chem., Int. Ed. 2014, 53, 13159−13163. (29) Kong, L.; Li, Y.; Ganguly, R.; Vidovic, D.; Kinjo, R. Angew. Chem., Int. Ed. 2014, 53, 9280−9283. (30) Braunschweig, H.; Wagner, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 825−826. (31) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924−3957. (32) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin) 2008, 130, 1−27. (33) Braunschweig, H.; Rais, D. Heteroat. Chem. 2006, 17, 238. (34) Vidovic, D.; Pierce, G. A.; Aldridge, S. Chem. Commun. 2009, 1157−1171. (35) Huheey, J.; Keiter, E.; Keiter, R. Principles of Structure and Reactivity, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1997. (36) Osorio, E.; Olson, J. K.; Tiznado, W.; Boldyrev, A. I. Chem. Eur. J. 2012, 18, 9677−9681. (37) Stock, A.; Brandt, A.; Fischer, H. Ber. Dtsch. Chem. Ges. B 1925, 58B, 643−57. (38) Brotherton, R. J.; McCloskey, A. L.; Petterson, L. L.; Steinberg, H. J. Am. Chem. Soc. 1960, 82, 6242−5. (39) Braunschweig, H.; Dewhurst, R. D. Angew. Chem., Int. Ed. 2013, 52, 3574−3583. (40) Stasch, A.; Jones, C. Dalton Trans. 2011, 40, 5659−5672.
Prof. Dr. Holger Braunschweig was born in 1961 in Aachen, Germany. He obtained his Ph.D. (1991) and Habilitation (1998) from the RWTH Aachen with P. Paetzold, while between these completing a postdoctoral stay with M. F. Lappert, FRS at the University of Sussex, U.K. After 2 years at Imperial College as Senior Lecturer and Reader, he moved to a chair for inorganic chemistry at the Julius-MaximiliansUniversity Würzburg in 2002. In 2009, he was awarded the Gottfried Wilhelm Leibniz prize of the DFG and was elected to the Bavarian Academy of Sciences. In 2011, Braunschweig was awarded a prestigious Advanced Investigator grant of the European Research Council and was elected to the German National Academy of Sciences (Leopoldina). The 2012 discovery of the first molecule with a boron− boron triple bond in his laboratories attracted extensive media attention, including from The Times of London, New Scientist, Nature, Science Magazine, and Spektrum der Wissenschaf t (Germany). Braunschweig was recently awarded the 2014 Main Group Award from the Royal Society of Chemistry. His research interests lie in the areas of boron chemistry, organometallic synthesis, and catalysis and are currently focused on borametallocenophanes, boron heterocycles, boron−boron multiple bonds, and transition-metal complexes of boron.
Dr. Rong Shang was born in China. She obtained her B.Sc. (1st Class Hons) degree from the University of Canterbury in New Zealand in 2007. Supported by an international Ph.D. student scholarship, she then joined the laboratory of Prof. Anthony F. Hill at the Research School of Chemistry, Australian National University, working on maingroup element-substituted group 16 carbyne complexes. Since 2011, she has been a postdoctoral research fellow in Prof. Holger Braunschweig’s group at the University of Würzburg (Germany), working on the reactivity of transition-metal borylene complexes. She has recently been appointed as an Assistant Professor at Hiroshima University in Japan. Her research interests lie in the areas of maingroup and organometallic chemistry.
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ACKNOWLEDGMENTS The authors thank the talented researchers involved in the synthesis and study of these fascinating molecules, namely, Dr. Rian Dewhurst, Katharina Ferkinghoff, Kai Hammond, Dr. J. Oscar C. Jimenez-Halla, Thomas Kramer, Marco Nutz, Christian Saalfrank, Thomas Steffenhagen, Dr. Krzysztof Radacki, Thomas Scheller, Dr. Eva Siedler, Dr. Christopher W. Tate, Benedikt Wennemann, Christine Werner, Dr. Alfredo Vargas, and Dr. Qing Ye. This work was generously supported by the European Research Council.
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REFERENCES
(1) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis Polymerization; Academic Press: San Diego, 1997. 3104
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(41) Schulenberg, N.; Ciobanu, O.; Kaifer, E.; Wadepohl, H.; Himmel, H.-J. Eur. J. Inorg. Chem. 2010, 5201−5210. (42) Ciobanu, O.; Fuchs, A.; Reinmuth, M.; Lebkuecher, A.; Kaifer, E.; Wadepohl, H.; Himmel, H.-J. Z. Anorg. Allg. Chem. 2010, 636, 543−550. (43) Ciobanu, O.; Kaifer, E.; Enders, M.; Himmel, H.-J. Angew. Chem., Int. Ed. 2009, 48, 5538−5541. (44) Himmel, H.-J.; Kaifer, E.; Ciobanu, O.; Roquette, P.; Siebert, W. Hydrogen storage and hydrogen transfer reagents for hydrogenation reactions. 2008-EP3003, 2008141705, 20080415, 2008. (45) Ciobanu, O.; Leingang, S.; Wadepohl, H.; Himmel, H.-J. Eur. J. Inorg. Chem. 2008, 322−329. (46) Ciobanu, O.; Emeljanenko, D.; Kaifer, E.; Mautz, J.; Himmel, H.-J. Inorg. Chem. 2008, 47, 4774−4778. (47) Ciobanu, O.; Allouti, F.; Roquette, P.; Leingang, S.; Enders, M.; Wadepohl, H.; Himmel, H.-J. Eur. J. Inorg. Chem. 2008, 5482−5493. (48) Dinda, R.; Ciobanu, O.; Wadepohl, H.; Huebner, O.; Acharyya, R.; Himmel, H.-J. Angew. Chem., Int. Ed. 2007, 46, 9110−9113. (49) Ciobanu, O.; Roquette, P.; Leingang, S.; Wadepohl, H.; Mautz, J.; Himmel, H.-J. Eur. J. Inorg. Chem. 2007, 4530−4534. (50) Ciobanu, O.; Himmel, H.-J. Eur. J. Inorg. Chem. 2007, 3565− 3572. (51) Johnson, H. C.; McMullin, C. L.; Pike, S. D.; Macgregor, S. A.; Weller, A. S. Angew. Chem., Int. Ed. 2013, 52, 9776−9780. (52) Braunschweig, H.; Guethlein, F. Angew. Chem., Int. Ed. 2011, 50, 12613−12616. (53) Braunschweig, H.; Claes, C.; Guethlein, F. J. Organomet. Chem. 2012, 706−707, 144−145. (54) Braunschweig, H.; Bertermann, R.; Brenner, P.; Burzler, M.; Dewhurst, R. D.; Radacki, K.; Seeler, F. Chem.Eur. J. 2011, 17, 11828−11837. (55) Blank, B.; Colling-Hendelkens, M.; Kollann, C.; Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. R.; Braunschweig, H. Chem.Eur. J. 2007, 13, 4770−4781. (56) Uddin, J.; Boehme, C.; Frenking, G. Organometallics 2000, 19, 571−582. (57) Ehlers, A. W.; Baerends, E. J.; Bickelhaupt, F. M.; Radius, U. Chem.Eur. J. 1998, 4, 210−221. (58) Pandey, K. K.; Braunschweig, H.; Lledos, A. Inorg. Chem. 2011, 50, 1402−1410. (59) Braunschweig, H.; Burzler, M.; Kupfer, T.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 7785−7787. (60) Braunschweig, H.; Kollann, C.; Englert, U. Angew. Chem., Int. Ed. 1998, 37, 3179−3180. (61) Arnold, N.; Braunschweig, H.; Brenner, P.; Jimenez-Halla, J. O. C.; Kupfer, T.; Radacki, K. Organometallics 2012, 31, 1897−1907. (62) Braunschweig, H.; Radacki, K.; Uttinger, K. Angew. Chem., Int. Ed. 2007, 46, 3979−3982. (63) Braunschweig, H.; Radacki, K.; Rais, D.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2005, 44, 5651−5654. (64) Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones, C.; Willock, D. J. Organometallics 2004, 23, 2911−2926. (65) Coombs, D. L.; Aldridge, S.; Jones, C.; Willock, D. J. J. Am. Chem. Soc. 2003, 125, 6356−6357. (66) Braunschweig, H.; Dewhurst, R. D.; Hörl, C.; Radacki, K.; Tate, C. W.; Vargas, A.; Ye, Q. Angew. Chem., Int. Ed. 2013, 52, 10120− 10123. (67) Braunschweig, H.; Ye, Q.; Radacki, K. Chem. Commun. 2012, 48, 2701−2703. (68) Braunschweig, H.; Ye, Q.; Vargas, A.; Dewhurst, R. D.; Radacki, K.; Damme, A. Nat. Chem. 2012, 4, 563−567. (69) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878−12879. (70) Braunschweig, H.; Herbst, T.; Rais, D.; Ghosh, S.; Kupfer, T.; Radacki, K.; Crawford, A. G.; Ward, R. M.; Marder, T. B.; Fernandez, I.; Frenking, G. J. Am. Chem. Soc. 2009, 131, 8989−8999. (71) Braunschweig, H.; Dewhurst, R. D.; Herbst, T.; Radacki, K. Angew. Chem., Int. Ed. 2008, 47, 5978−5980.
(72) Apostolico, L.; Braunschweig, H.; Crawford, A. G.; Herbst, T.; Rais, D. Chem. Commun. 2008, 497−498. (73) Braunschweig, H.; Herbst, T.; Rais, D.; Seeler, F. Angew. Chem., Int. Ed. 2005, 44, 7461−7463. (74) Braunschweig, H.; Colling, M.; Kollann, C.; Stammler, H. G.; Neumann, B. Angew. Chem., Int. Ed. 2001, 40, 2298−2300. (75) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003, 42, 205−208. (76) Braunschweig, H.; Radacki, K.; Scheschkewitz, D.; Whittell, G. R. Angew. Chem., Int. Ed. 2005, 44, 1658−1660. (77) Braunschweig, H.; Forster, M.; Radacki, K. Angew. Chem., Int. Ed. 2006, 45, 2132−2134. (78) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Organometallics 2006, 25, 5159−5164. (79) Braunschweig, H.; Fernandez, I.; Frenking, G.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 5215−5218. (80) Braunschweig, H.; Forster, M.; Radacki, K.; Seeler, F.; Whittell, G. R. Angew. Chem., Int. Ed. 2007, 46, 5212−5214. (81) Braunschweig, H.; Forster, M.; Kupfer, T.; Seeler, F. Angew. Chem., Int. Ed. 2008, 47, 5981−5983. (82) Braunschweig, H.; Ye, Q.; Radacki, K. Chem. Commun. 2009, 6979−6981. (83) Bertsch, S.; Braunschweig, H.; Christ, B.; Forster, M.; Schwab, K.; Radacki, K. Angew. Chem., Int. Ed. 2010, 49, 9517−9520. (84) Braunschweig, H.; Ye, Q.; Radacki, K.; Brenner, P.; Frenking, G.; De, S. Inorg. Chem. 2011, 50, 62−71. (85) Braunschweig, H.; Dewhurst, R. D.; Kraft, K.; Radacki, K. Chem. Commun. 2011, 47, 9900−9902. (86) Braunschweig, H.; Ye, Q.; Damme, A.; Kupfer, T.; Radacki, K.; Wolf, J. Angew. Chem., Int. Ed. 2011, 50, 9462−9466. (87) Braunschweig, H.; Ye, Q.; Radacki, K.; Damme, A. Angew. Chem., Int. Ed. 2012, 51, 7839−7842. (88) Braunschweig, H.; Herbst, T.; Radacki, K.; Tate, C. W.; Vargas, A. Chem. Commun. 2013, 49, 1702−1704. (89) Braunschweig, H.; Dewhurst, R. D.; Gessner, V. H. Chem. Soc. Rev. 2013, 42, 3197−3208. (90) Bose, S. K.; Roy, D. K.; Shankhari, P.; Yuvaraj, K.; Mondal, B.; Sikder, A.; Ghosh, S. Chem.Eur. J. 2013, 19, 2337−2343. (91) Addy, D. A.; Bates, J. I.; Kelly, M. J.; Riddlestone, I. M.; Aldridge, S. Organometallics 2013, 32, 1583−1586. (92) Braunschweig, H.; Ye, Q.; Damme, A.; Radacki, K. Chem. Commun. 2013, 49, 7593−7595. (93) Bertsch, S.; Braunschweig, H.; Dewhurst, R. D.; Radacki, K.; Saalfrank, C.; Wennemann, B.; Ye, Q. Organometallics 2014, 33, 3649−1651. (94) Bertsch, S.; Bertermann, R.; Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Phukan, A. K.; Saalfrank, C.; Vargas, A.; Wennemann, B.; Ye, Q. Angew. Chem., Int. Ed. 2014, 53, 4240−4243. (95) Braunschweig, H.; Ye, Q.; Vargas, A.; Dewhurst, R. D.; Hupp, F. J. Am. Chem. Soc. 2014, 136, 9560−9563. (96) Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Kelch, H.; Macha, B. B.; Radacki, K.; Vargas, A.; Ye, Q. Chem.Eur. J. 2014, 21, 2377−2386. (97) Braunschweig, H.; Dewhurst, R. D.; Radacki, K.; Tate, C. W.; Vargas, A. Angew. Chem., Int. Ed. 2014, 53, 6263−6266. (98) Braunschweig, H.; Dewhurst, R. D.; Kramer, T.; Siedler, E. Organometallics 2014, 33, 3877−3881. (99) Braunschweig, H.; Radacki, K.; Uttinger, K. Eur. J. Inorg. Chem. 2007, 4350−4356. (100) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2002, 41, 1359−1361. (101) Pandey, K. K.; Braunschweig, H.; Dewhurst, R. D. Eur. J. Inorg. Chem. 2011, 2045−2056. (102) Xu, L.; Li, Q.-s.; King, R. B.; Schaefer, H. F. Organometallics 2011, 30, 5084−5087. (103) Fletcher, S. C.; Poliakoff, M.; Turner, J. J. Inorg. Chem. 1986, 25, 3597−3604. (104) Nöth, H.; Pommerening, H. Angew. Chem., Int. Ed. 1980, 19, 482−483. 3105
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(105) Braunschweig, H.; Radacki, K.; Shang, R.; Tate, C. W. Angew. Chem., Int. Ed. 2013, 52, 729−733. (106) Complex 12 is heavily disordered. Although the electron density has been modeled well in the X-ray crystallographic study, the bond distances are taken from the calculated values (at B3LYP/Def2SVP level of theory). For more information, see ref 105. (107) Braunschweig, H.; Kramer, T.; Radacki, K.; Shang, R.; Siedler, E.; Werner, C. Chem. Sci. 2014, 5, 2271−2276. (108) Boehme, C.; Uddin, J.; Frenking, G. Coord. Chem. Rev. 2000, 197, 249−276. (109) Imwinkelried, R.; Hegedus, L. S. Organometallics 1988, 7, 702− 706. (110) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. Organometallics 1990, 9, 2814−2819. (111) Maher, J. M.; Beatty, R. P.; Cooper, N. J. Organometallics 1985, 4, 1354−1361.
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