Complexation and Release of N-Heterocyclic Carbene

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Complexation and Release of N-Heterocyclic CarbeneAminoborylene Ligands from Group VI and VIII Metals Conor Pranckevicius, J. Oscar C. Jimenéz-Halla, Marius Kirsch, Ivo Krummenacher, and Holger Braunschweig J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05398 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Journal of the American Chemical Society

Complexation and Release of N-Heterocyclic CarbeneAminoborylene Ligands from Group VI and VIII Metals Conor Pranckevicius,a,b J. Oscar C. Jimenéz-Halla,c Marius Kirsch,a,b Ivo Krummenacher,a,b Holger Braunschweig.*a,b a

Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany). b Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany). c Department of Chemistry, Division of Natural and Exact Sciences, Universidad de Guanajuato, Noria Alta s/n 36050 Guanajuato (Mexico). ABSTRACT: The coordination chemistry and stability of aminoborylene ligands bearing differing 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.

INTRODUCTION Borylenes are typically actor ligands when coordinated to a transition metal center. Two- and three-coordinate terminal borylene ligands are electronically unsaturated,1-10 and undergo a wide array of insertion, oxidation, reduction, and addition reactions involving the M-B unit.11-25 Furthermore, the addition of ancillary ligands, including CO and isonitrile, to metalloborylenes has emerged as an important method for the metal-templated assembly and release of novel doubly basestabilized borylene species.26-30 Outside of the coordination sphere of a transition metal complex, free borylenes are most commonly stabilized via the coordination of two donor ligands to a monovalent boron centre (Figure 1; A, B),31-37 which are electronically analogous to ‘carbone’-type ligands.38-40 The only examples of isolable two-coordinate borylenes are ‘pushpull’ aminoborylenes bearing π-acidic N-heterocyclic carbene (NHC) ligands (Figure 1; C).41,42 Such species are isoelectronic to Fischer carbenes and were first reported by Bertrand, Stephan and coworkers in 2014. While having a linear groundstate reminiscent of N=B=C heteroallenes,43-46 these species have been demonstrated to bind CO and cleave H2, reactions evocative of cyclic alkylamino-carbenes (CAACs) and other electrophilic carbene ligands,41,47,48 and which further support the emerging metallomimetic reactivity of low-valent boron.27,49 Nevertheless, unlike tricoordinate borylenes,31,37 these dicoordinate borylene species are not known to act as nucleophiles to transition metal or main group Lewis acids.

Figure 1. Examples of free and metal bound borylenes.

As more electropositive analogues of singlet carbenes, we became interested in accessing nucleophilic ‘NHClike’ base-stabilized aminoborylenes as ligands for transition metal complexes. In order to disfavor a linear ‘heteroallenelike’ ground-state and increase the nucleophilicity at boron, we chose to explore the chemistry of aminoborylenes bearing weakly π-acidic NHC supporting ligands. While basestabilized aminoborylene ligands have been reported in the literature,18,25 surprisingly none have been explored bearing classic Arduengo-type imidazol-2-ylidene ligands.50 To afford additional kinetic stabilization of the borylene, we selected sterically encumbered imidazol-2-ylidene ligands to shield the boron center.

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RESULTS AND DISCUSSION An appropriate imidazol-2-ylidene dihaloaminoborane adduct 1a can be readily synthesized by the combination of 1,3dimesitylimidazol-2-ylidene (IMes) with Et2NBBr2 in toluene, and was isolated in a 70% yield upon recrystallization from THF (Scheme 1). An X-ray crystal structure determination of 1a revealed a three-coordinate boron center where one bromide anion is outer sphere (see SI). Anticipating the high degree of nucleophilicity in the reduced NHC-aminoborylene complexes, we attempted reductions with low-valent transition metal complexes, in order that the product borylene may be immediately captured by an unsaturated metal center. In an initial experiment, 1a was combined with Collman’s reagent Na2[Fe(CO)4] at ambient temperature in THF, resulting in the immediate formation of an orange suspension. Upon purification, yellow single crystals of the NHC-aminoborylene Fe complex 2 were isolated in a 43% yield (Scheme 1). 1H NMR spectroscopy revealed two inequivalent N-ethyl environments, indicating restricted rotation around the B-N bond. A broad 11 B NMR signal at 65 ppm is also observed, somewhat to lower field than those of known base-stabilized aminoborylenes, which typically fall in the range of 49 – 60 ppm.25 An X-ray crystal structure determination revealed a Fe-B distance of 2.086(2) Å (Figure 2), within the range reported for Fe basestabilized aminoborylenes (2.05 – 2.10 Å),25 and is indicative of a single dative bond to the metal center. Compound 2 is the first example of a base-stabilized borylene ligand formed by the reduction of a singly ligated dihaloborane, as basestabilized borylenes are typically formed by reaction of terminal metalloborylenes with the corresponding bases.

Na2[M(CO)4] N N Mes

THF, 23 οC

Mes Et N B

Mes Et N N N Et B Mes OC M CO OC CO 2: M = Fe 3: M = Ru

Et Br

Br

Na2[M(CO)5]

1a

THF, −78 οC

N N Mes

Mes Et

OC OC

observed with oxidants including S8 and PhSe-SePh. Computational investigation of the frontier molecular orbitals (Figure 3) has revealed that the HOMO is metal-centered with some mixing of the C-O π* orbitals. The LUMO is delocalized over the supporting NHC ligand, but most closely resembles the NC π* orbital. This is in contrast to known metalloborylenes, which often have a B-centered LUMO,24 supporting the relative inertness of 2 towards nucleophilic attack.

Figure 2. Molecular structures of 2 (left) and 5 (right) with thermal ellipsoids drawn at 50% probability. Hydrogen atoms removed for clarity. Selected bond lengths (Å): 2: Fe-B; 2.087(2), B-N1; 1.406(3), B-C5; 1.622(3), Fe-C1; 1.785(2), Fe-C2; 1.768(2), Fe-C3; 1.782(2), Fe-C4; 1.772(2), C1-O1; 1.151(2), C2O2; 1.159(2), C3-O3; 1.158(2), C4-O4; 1.164(2). 3 (isostructural numbering with 2): Ru-B; 2.209(2), B-N1; 1.406(2), B-C5; 1.625(2), Ru-C1; 1.948(2), Ru-C2; 1.910(2), Ru-C3; 1.923(2), Ru-C4; 1.922(2), C1-O1; 1.139(2), C2-O2; 1.153(2), C3-O3; 1.149(2), C4-O4; 1.150(2). 4 (isostructural numbering with 5): Cr-B; 2.281(4), B-N1; 1.401(4), B-C6; 1.623(4); Cr-C1; 1.856(3), Cr-C2; 1.887(3), Cr-C3; 1.881(3), Cr-C4; 1.863(3), Cr-C5; 1.879(4), C1-O1; 1.159(4), C2-O2; 1.155(3), C3-O3; 1.160(4), C4-O4; 1.163(3), C5-O5; 1.155(4). 5: W-B; 2.405(3), B-N1; 1.410(3), B-C6; 1.612(3); W-C1; 1.993(3), W-C2; 2.036(2), WC3; 2.039(3), W-C4; 2.007(2), W-C5; 2.026(3), C1-O1; 1.157(3), C2-O2; 1.148(3), C3-O3; 1.148(3), C4-O4; 1.151(3), C5-O5; 1.145(3).

N B

Et

CO M CO CO 4: M = Cr 5: M = W

Scheme 1. Synthesis of Group VI and VII imidazol-2-ylideneaminoborylene complexes.

An IR spectrum of 2 in CH2Cl2 solution revealed an A1 symmetric carbonyl stretch at 1995 cm–1. For reference, known [(NHC)Fe(CO)4] complexes have A1 symmetric stretching frequencies in the region of 2034-2036 cm–1.51,52 Furthermore, a Fe-carbonyl complex bearing a strongly donating carbodicarbene type ligand has a reported A1 symmetric stretch at 2026 cm–1,53 suggesting that the NHCaminoborylene ligand confers exceptional electron density to the metal centre compared to carbon-based donor ligands. Despite this, complex 2 is stable to air and moisture, both as a solid and in solution. Initial reactivity studies indicated that 2 is inert to many of the transformations typical of terminal borylenes. Complex 2 does not react with nucleophiles or unsaturated substrates including DMAP, CO, acetone, and 2,6dimethylphenylisocyanide. Additionally, no reaction with 2 is

Figure 3. LUMO (left), HOMO (centre), and HOMO−3 (right) of compound 2.

An analogous Ru complex 3 may also be accessed by combination of 1 with Na2[Ru(CO)4] in THF solution at room temperature (Scheme 1). An 11B NMR signal at 62 ppm is observed for 3, while an X-ray crystal structure determination revealed an elongated Ru-B distance of 2.209(2) Å (Figure 2). The A1 symmetric CO stretching frequency of 3 is observed at 2008 cm–1, likewise much lower than that observed with (IMes)Ru(CO)4 of 2044 cm-1.54 Additionally, we have also explored the reactivity of 1a with Group VI dianions. Combination of 1a with in-situ-generated solutions of Na2[M(CO)5] (M = Cr, W) in THF affords the corresponding NHCstabilized aminoborylene complexes 4 and 5 (Scheme 1). The


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Journal of the American Chemical Society M-B distances in 4 and 5 are further elongated to 2.281(4) and 2.405(3) Å, respectively, significantly longer than that observed in terminal aminoborylene complexes (1.996(6) (Cr) and 2.151(7) Å (W) for [(OC)5M{BN(SiMe3)2}]).55 11B NMR spectroscopic shifts of 70 and 66 ppm for 4 and 5, respectively, are also observed. Complexes 4 and 5 display similar robustness to complex 2 and are inert to treatment with CO, S8 and PhSe-SePh. The unique robustness and purely σ-donor nature of these M-B linkages (Figure 3, right) is highly reminiscent of NHC-metal complexes. Indeed, reaction of 2 with the cationic coinage metal species IDippCuOTf results in the formation of the “metal-only Lewis pair” (MOLP)56-58 species 6 (Scheme 2), where a Fe-Cu bond has been formed. This observation stands in contrast to known metalloborylene reactivity with coinage metals, where coordination to the borylene M-B πbond has been described.59 However, in the case of 2, the lack of borylene-centered frontier molecular orbitals results in robustness of the M-B linkage while conferring exceptional electron density to the metal center. Mes Et N N N Et B Mes OC Fe CO OC CO 2

Mes OTf

O

IDippCuOTf C6H6

Et 2N

Dipp

Fe

Cu

N

C N

B

C

O Dipp

C

N N Mes

with a broad signal at 73 ppm in its 11B NMR spectrum, and a 1 H NMR environment indicative of a linear ‘heteroallene’-type species (Figure 1; C), suggesting the formation of free borylene 7 (Scheme 3). However, upon warming above –40 °C, a new product with a sharp 11B NMR spectroscopic resonance at –2.7 ppm is formed with corresponding loss of the resonances for 7. This new species has been identified as the aminoborylene-CO adduct 8 (Scheme 3). IR spectroscopy of isolated 8 revealed a strong band at 1928 cm–1, similar to known borylene-CO complexes (Scheme 3). The formation of 8 suggests transient formation of a CAAC-supported Crborylene species, which then eliminates the borylene-carbonyl adduct 8. This transformation was examined computationally (Scheme 4); DFT calculations suggest that the initial reduction of 1b to borylene 7 is very exergonic (∆G0R1 = −13.2 kcal⋅mol−1), as is complexation of 7 with a ‘Cr(CO)5’ fragment to form the complex I− −1 (∆G0R2 = −1.9 kcal⋅mol−1) (see SI for further discussion of this step). Subsequent nucleophilic migration of the borylene to CO to form an intermediate Crstabilized borylene-carbonyl species I− −2 is followed by release of compound 8 from the coordination sphere of the metal (∆G0R3 = −7.0 kcal⋅mol−1). Compound 8 is similarly formed in the reaction between 1b and Na2[Fe(CO)4] in C6D6 at ambient temperature.

CO 6

O

R N

N Dipp

B

Et 1b R X

X

Scheme 2. Synthesis of the metal-only Lewis pair complex 6.

SiMe3 N Dipp

N B Cl 9

SiMe3

N

Na2[Cr(CO) 5], THF, −50 οC

1b: R = Et, X = Br 1c: R = SiMe3, X = Cl Na2[Fe(CO)4] 1c THF, 23 οC

B N Dipp

Et 7

+ '[Cr(CO) 5]'

Na

1b

23 οC

2 [F e( 6D C 6, 23 O)4 ο ],

C

C

N Dipp

B

NEt2

CO 8

Scheme 3. Reactions of CAAC-dihaloaminoboranes with Na2[M(CO)x] complexes (M = Cr, Fe).

Figure 4. Molecular structure of 6 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms and ellipsoids of peripheral groups removed for clarity. Selected bond lengths (Å) and angles (°): Fe-B; 2.135(6), B-C4; 1.619(7), B-N1; 1.394(7), Fe-Cu; 2.4057(9), Fe-C1; 1.813(6); C1-O1; 1.144(6); Fe-C2; 1.775(6), C2-O2; 1.169(7), Fe-C3; 1.788(6), C3-O3; 1.143(6), Cu-C3; 2.369(5). B-Fe-Cu; 109.8(2), C1-Fe-Cu; 74.4(2).

In order to assess the electronic effect of imidazol-2ylidenes as stabilizing ligands in these metalloborylene complexes, analogous metalation reactions were examined with the 1-(2,6-di-isopropylphenyl)-2,2,4,4-tetramethylpyrrolid-5ylidene (MeCAAC) adduct 1b (Scheme 3). Combination of 1b with Na2[Cr(CO)5] at –78 °C resulted in the formation of an orange solution upon warming to –40 °C. Spectroscopic analysis of this mixture by NMR at –40 °C revealed a new product

Scheme 4. Reaction coordinate for the free energy of formation of 8. Numeric values in kcal/mol.



Additionally, we have also assessed the reaction of CAAC-BCl2N(SiMe3)2 (1c) with Na2[Fe(CO)4] (Scheme 3). Surprisingly, rather than complexation or borylene-carbonyl release, we instead observed selective reduction to the boronradical species 9, which was isolated from the reaction mixture in an 87% yield as a pale yellow crystalline solid. An X-ray crystal structure determination of 9 revealed a contracted BCCAAC bond with a length of 1.524(2) Å, suggesting a bond order greater than unity (See SI). An EPR spectrum obtained in toluene solution displays a characteristic signal for CAACstabilized, boron-centered radicals with hyperfine coupling constants of a(14N) = 18.9 MHz, a(11B) = 5.0 MHz, and a(35Cl) = 1.9 MHz.41 The intermediacy of the free borylene 7 in the reaction of 1b with Na2[Cr(CO)5] was further supported by the reaction of 1b with two equivalents of Na[C10H8] at –78 °C, from which a sole species bearing identical NMR signals to 7 was spectroscopically identified below 0 °C (Scheme 5). Complex 7 may also be trapped at this stage by addition of CO or [Cr(CO)5(NMe3)] to form the borylene-CO species 8.41 However, when the free borylene 7 was warmed to 23 °C, approximately 50% conversion to a new product with an 11B NMR resonance at 43 ppm was observed, and conversion was complete upon heating the reaction mixture at 60 °C for 1 h. A crystallographic study indicated this new species 10 is derived from insertion of the borylene moiety into the Ar-iPr C-C bond, followed by migration of the iPr group to the electrophilic carbene center (Figure 5), the mechanism of which has been supported computationally (see SI). Complex 10 may likewise be formed by heating the CO-borylene complex 8 at 60 °C for 18 h. The formation of 10 is interesting considering the stability of Bertrand and Stephan’s borylene featuring N(SiMe3)2 groups, which indicates significant stabilization of the electrophilic borylene center by the β-Si effect.60 Conversely, the reaction of 1a with two equivalents of Na[C10H8] at –78 °C results in formation of the C-H-activated product 11 upon warming to ambient temperature,61 which displays a sharp doublet in the 11B NMR spectrum at –12.2 ppm (Scheme 4, Figure 4). No intermediates were observed in the reaction mixture at lower temperatures.

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Me

Et L

N B

N N

1a

Et Br

Mes B

2 Na[C 10H 8] THF, −78 οC

Br

H 11

1a; L = IMes 1b; L = MeCAAC

Et 1b

B N N

2 Na[C 10H 8] THF, −78 οC

Dipp

B

Et

Dipp

stable to 0 οC 7

CO N

NEt2

23 οC

NEt2

CO 8

60 οC, 18 h N

- CO iPr

iPr Et B N Et 10

Scheme 5. Reduction of NHC-dihaloaminoborane complexes with Na[C10H8].

Figure 5. Molecular structure of 10 (left) and 11 (right) with thermal ellipsoids drawn at 50% probability. Hydrogen atoms, except those on boron, are removed for clarity. Selected bond lengths (Å): Compound 10: B1-N1; 1.397(2), B1-C1; 1.626(2); B1-C2; 1.567(2), C1-N2; 1.516(1). Compound 11: B1-N1; 1.526(2), B1-C1; 1.635(2), B1-C2; 1.634(2).

While the NHC-aminoborylene complexes 2-4 are thermally robust to at least 80 °C, we wanted to determine whether a Lewis acid could be utilized to prompt borylene migration to a carbonyl group analogous to that calculated for the CAAC-aminoborylene system (Scheme 4). Indeed, combination of complex 3 with B(C6F5)3 at ambient temperature resulted in the immediate formation of new species 12 (Scheme 6). 19F NMR indicated the quaternization of the B(C6F5)3 moiety, and a new broad signal at –2 ppm was observed in the 11B NMR spectrum, suggesting the formation of an adduct of B(C6F5)3. A molecular structure revealed that B(C6F5)3 had coordinated to a CO moiety, which in turn had been attacked by the nucleophilic borylene to form the Rusupported CO-borylene coupled product 12 (Figure 6). This interaction is supported by the coordination of the amino moiety to the Ru center. This complex is structurally similar to the computed intermediate I−2 (Scheme 4) in the release of CAAC-carbonyl borylene species 8. Mes Et N N N Et B Mes OC M CO OC CO 2: M = Fe 3: M = Ru

Mes B(C6F5)3 toluene

N

N

Mes

B C

Et2N

O

M

B(C6F5)3 CO CO 12: M = Ru (isolated) 13: M = Fe (observable ) OC

Scheme 6. Synthesis of metal-supported borylene-carbonyl complexes. In order to shed light on the bonding situation in 12, its frontier molecular orbitals were investigated computationally (Figure 7). The HOMO of 12 resembles a B-C π bond with some contribution from the d orbitals on Ru. Wiberg bond order calculations indicate partial B-C and C-O double bond character, with bond orders of 1.09 and 1.34 respectively, which is further supported by crystallographically-determined bond lengths of 1.525(3) and 1.302(2) Å. The B-N bond length is 1.495(3) Å, and the respective bond order was calculated to be 0.90, supporting the absence of significant B-N multiple bonding in this species. Collectively, this data supports our formulation of 12 as a metal-supported CO-aminoborylene species. The analogous reaction of 3 and B(C6F5)3 appears to


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Journal of the American Chemical Society result in the formation of a similar species 13 by NMR spectroscopy (see SI), however, this quickly decomposes in solution to a mixture of products. Complexes 4 and 5 do not observably react with B(C6F5)3.

ASSOCIATED CONTENT Experimental, computational and crystallographic details are available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT C.P. acknowledges financial support from the Alexander von Humboldt foundation and the Natural Sciences and Engineering Research Council of Canada. J.O.C.J.-H. thanks CONACyT (Mexico) for funding (project CB2014-241803) for a research stay at Julius-Maximilians-Universität Würzburg and the use of their supercomputing facilities.

REFERENCES

Figure 6. Molecular structure of 12 with thermal ellipsoids drawn at 50% probability. Selected bond lengths (Å) and angles (°): RuN1; 2.204(2), Ru-B1; 2.265(2), Ru-C1; 1.980(2), N1-B1; 1.495(3), B1-C1; 1.525(3), B1-C2; 1.603(3), C1-O1; 1.302(2), O1-B2; 1.527(3). N1-B1-C1; 108.0(2), B1-C1-O1; 133.3(2), B2O1-C1; 127.1(2).

Figure 7. HOMO of complex 12.

SUMMARY & CONCLUSIONS In summary, we have synthesized the first examples of imidazol-2-ylidene-stabilized aminoborylenes acting as ligands to transition metals, demonstrated their exceptional electron releasing ability and robustness, and have explored their reactivity with Lewis acids IDippCuOTf and B(C6F5)3. Notably, NHC-aminoborylene ligands are only stable in the presence of weakly π-acidic NHC supporting ligands, as the more π-acidic CAAC-substituted analogues result in favorable borylene migration to CO and release of a CO-borylene adduct from the metal center. Such reactivity highlights the often diverging trends in the stabilization of free borylenes and metalloborylenes. The catalytic properties of NHC-aminoborylene complexes are currently under investigation.

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Complexation and Release of N-Heterocyclic Carbene

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