Article pubs.acs.org/IC
Alkaline-Earth Derivatives of the Reactive [HB(C6F5)3]− Anion Mathew D. Anker, Merle Arrowsmith, Rory L. Arrowsmith, Michael S. Hill,* and Mary F. Mahon Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. S Supporting Information *
ABSTRACT: The β-diketiminato magnesium amidoboranes [HC{(Me)CNDipp}2Mg(NMe2BH2NMe2·BH3)] and [HC{(t-Bu)CNDipp}2Mg(NMe2·BH3)] are readily converted to the corresponding derivatives of the [HB(C6F5)3]− anion by treatment with B(C6F5)3. The bis(borohydride) derivatives of the heaviest alkaline-earth elements, strontium and barium, may be similarly synthesized by reaction of strontium or barium dimethylamidoboranes and B(C6F5)3 and by metathesis reactions of either SrI2 or BaI2 and 2 molar equiv of K(HB(C6F5)3). The strontium and barium compounds have been fully characterized in solution and in the solid state as the respective tris(diethyl ether) and tetrakis(tetrahydrofuran) adducts. The magnesium compound [HC{(Me)CNDipp}2Mg(HB(C6F5)3)] has been applied to the catalytic hydroboration of i-PrN CN-i-Pr with HBpin. In contrast to carbodiimide hydroboration catalyzed by the corresponding β-diketiminato magnesium hydride, which results in the exclusive production of the monoborylated amidine, use of the [HB(C6F5)3]− derivative provides the product of bis-borylation, the aminal H2C(N{Bpin}i-Pr)2, under mild conditions. A series of stoichiometric reactions highlight that, while this reactivity is likely to be primarily magnesium mediated, B(C6F5)3 plays a vital role both in the delivery of reactive hydride and through the Lewis acid activation of the heteroallene substrate and various reactive intermediates.
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INTRODUCTION The selective homogeneous catalytic reduction of polar heteroallenes, primarily carbon dioxide to produce formic acid and/or methanol equivalents but also including isoelectronic small-molecule isocyanate and carbodiimide derivatives, has elicited significant recent interest.1 For example, we have recently reported the competence of the β-diketiminato magnesium hydride [HC{(Me)CNDipp}2MgH]2 (1; Dipp = 2,6-diisopropylphenyl) as a catalyst for hydroboration of Nalkyl and N-aryl carbodiimides with pinacolborane (HBpin) to provide the corresponding N-borylated amidines (eq 1).2
also onward reduction via acetal derivatives and ultimate C−O cleavage to provide the methanol equivalent, CH3OBpin (Scheme 1).3,4 These observations were reminiscent of the ability of a decamethylscandocenium derivative of the same hydridoborate anion, [Cp*2Sc(HB(C6F5)3)], to effect the deoygenative hydrosilylation of CO2 to methane.5 In this case, experimental mechanistic and computational studies indicated a complex sequence of reaction steps and equilibria in which the [Cp*2Sc]+ Lewis acid activates CO2 toward acceptance of hydride from the reactive borate anion. While this process is similar to those proposed to be operative in “frustrated Lewis pairs”,6,7 a further more recent report provided by Okuda and co-workers has indicated that derivatives of the hydridotriphenylborate anion with Li, Na, and K and particularly Mg also catalyze the hydroboration of a wide range of unsaturated heteroallene substrates, including isocyanates and carbodiimides.8,9 In this latter respect the hydroboration of i-PrNCN-i-Pr with HBpin in the presence of 1 mol % of [Mg(THF)6][HBPh3]2 (eq 2) is particularly notable as, in contrast to our study of the use of compound 1,2 it resulted in further reduction to provide the bis(N-boryl)aminal product H2C(N{Bpin}i-Pr)2 (denoted herein as compound 11; vide infra). With these observations in mind, we have sought to expand our study of alkaline-earth derivatives of the [HB(C6F5)3]− anion. Sadow has reported that the synthesis of several calcium species may be achieved through treatment of [Ca{C-
This reaction was applicable to a wide variety of carbodiimide substituents, while the resultant amidine species were resistant to any further reduction of the remaining imine function, irrespective of the presence of additional reducing borane equivalents or the application of forcing conditions. In contrast to this latter observation, use of the magnesium and calcium tris(pentafluorophenyl)hydridoborate derivatives [H{(Me)CNDipp}2M(HB(C6F5)3)(THF)n] (2, M = Mg, n = 0; 3, M = Ca, n = 1) to the similar hydroboration of CO2 resulted in not only the generation of formate species (compound 4) but © 2017 American Chemical Society
Received: March 21, 2017 Published: April 27, 2017 5976
DOI: 10.1021/acs.inorgchem.7b00678 Inorg. Chem. 2017, 56, 5976−5983
Article
Inorganic Chemistry Scheme 1. Synthesis of Compound 4 and the Catalytic Deoxygenative Hydroboration of CO2
Scheme 2. Synthesis of Compounds 2 and 7
Figure 1. ORTEP representations of (a) compound 2 and (b) compound 7 with thermal ellipsoids set at 25% probability. The isopropyl methyl groups and all hydrogen atoms except for H1 are removed for clarity.
(SiHMe2)3}2(THF)2] with B(C6F5)3.10,11 In this contribution we describe synthetically convenient routes to [HB(C6F5)3]− derivatives of even the heaviest alkaline-earth elements and describe our initial attempts to deconvolute the mode of action of these species during the hydroboration of i-PrNCN-iPr.
readily removed under vacuum. A reaction performed between the recently reported magnesium amidoborane derivative [HC{(t-Bu)CNDipp}2Mg(NMe2·BH3)] (6)12 provided a similar result and the synthesis of the new compound [HC{(t-Bu)CNDipp}2Mg(HB(C6F5)3)] (7) through β-hydride abstraction (Scheme 2). This protocol is, thus, reminiscent of that previously applied to prepare both compound 2 and Sadow’s synthesis of a variety of potassium, calcium, and ytterbium derivatives of the [HB(C6F5)3]− anion, which utilized a hydride abstraction reaction with the appropriate tris(dimethylsilyl)methyls.10,11 The reactions are best monitored through their 11B NMR spectra, which comprised doublet resonances at δ −18.9 ppm (1JHB = 67 Hz) and δ −22.8 ppm (1JHB = 66 Hz) for compounds 2 and 7, respectively. These data are consistent with the formation of the four-coordinate [HB(C6F5)3]− anion and are closely comparable with the
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RESULTS AND DISCUSSION Synthesis of Alkaline-Earth [HB(C6F5)3]− Derivatives. We have previously reported that the synthesis of compound 2 may be conveniently achieved through treatment of the amidoborane derivative 5 with tris(pentafluorophenyl)borane (Scheme 2).3 Repetition of this procedure induced an apparent δ-hydride abstraction to form compound 2, which provided NMR spectroscopic data consistent with those reported previously, and the cyclic borazane [H2BNMe2]2, which was 5977
DOI: 10.1021/acs.inorgchem.7b00678 Inorg. Chem. 2017, 56, 5976−5983
Article
Inorganic Chemistry Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds 2, 7, 8, and 10 2a M(1)−N(1) M(1)−N(2) M(1)−F(1) M(1)−F(15) M(1)−H(1) B(1)−H(1) N(1)−M(1)−N(2) N(1)−M(1)−F(1) N(1)−M(1)−F(15) N(2)−M(1)−F(1) N(2)−M(1)−F(15) F(1)−M(1)−F(15)
2.0138(13) 2.0064(13) 2.0753(10) 2.0778(10) 1.8631 1.4190 96.38(5) 117.95(5) 145.58(5) 104.07(5) 89.55(5) 93.06(4)
7a 2.007(2) 2.013(2) 2.0634(16) 2.0800(17)b 2.01(2) 1.23(2) 99.44(10) 100.82(8) 91.74(8)c 102.65(8) 140.86(8)d 111.93(7)e
8f
10p g
2.511(2) 2.533(3)h 2.6337(16) 2.5899(17)i 2.57(3) 1.20(3) 79.23(5)j 122.92(6)k 95.34(7)l 143.79(4)m 75.67(4)n 74.10(5)o
2.708(2)q 2.768(2)r 2.8228(19) 2.8386(17)s 2.83(3) 1.15(3) 83.66(8)t 119.27(7)u 78.62(7)v 67.83(7)w 130.95(6)x 81.88(6)y
a
M = Mg. bMg(1)−F(6). cN(1)−Mg(1)−F(6). dN(2)−Mg(1)−F(6). eF(1)−Mg(1)−F(6). fM = Sr. gSr(1)−O(1). hSr(1)−O(2). iSr(1)−F(11). O(1)−Sr(1)−O(2). kO(1)−Sr(1)−F(1). lO(1)−Sr(1)−F(11). mO(2)−Sr(1)−F(1). nO(2)−Sr(1)−F(11). oF(1)−Sr(1)−F(11). pM = Ba. q Ba(1)−O(1). rBa(1)−O(2). sBa(1)−F(4). tO(1)−Ba(1)−O(2). uO(1)−Ba(1)−F(1). vO(1)−Ba(1)−F(4). wO(2)−Ba(1)−F(1). xO(2)− Ba(1)−F(4). yF(1)−Ba(1)−F(4). j
Scheme 3. Synthesis of Compounds 8 and 10
Figure 2. ORTEP representations of (a) compound 8 and (b) compound 10 with thermal ellipsoids set at 25% probability. All hydrogen atoms except for H1 are removed for clarity.
centers as tridentate κ3 donors through the B−H units and the o-fluorine atoms of two of the pentafluorophenyl rings in a fashion analogous to that observed in the various previously reported calcium species of this anion.3,10,11 In contrast to the pseudo-six-coordinate geometry induced by the incorporation of the additional THF ligand in the β-diketiminato calcium compound 3, the magnesium centers of compounds 2 and 7 adopt pseudo-five-coordinate geometries with one nitrogen donor and the two fluorine atoms from the [HB(C6F5)3]− ligand occupying the equatorial positions of a distortedtrigonal-bipyramidal coordination sphere. In comparison to the significant number of derivatives of the lighter alkaline earths, the chemistry of uniligated β-
data arising from a number of derivatives of this anion with electropositive metals.3,5,10,11 Single crystals suitable for X-ray diffraction analysis were obtained for both compounds 2 and 7 from toluene solutions. The results of both experiments are shown in Figure 1, while selected bond length and angle data are provided in Table 1. Although the differing steric demands of the β-diketiminate ligands result in some conformational adjustment to the various F−Mg−F and H−Mg−F bond angles, the solid-state structures of both compounds are otherwise analogous. Only minor variations are observed across the various Mg−N and Mg−F interactions and, in spite of the smaller radius of the magnesium dication, the hydridoborate anions interact with the metal 5978
DOI: 10.1021/acs.inorgchem.7b00678 Inorg. Chem. 2017, 56, 5976−5983
Article
Inorganic Chemistry
Figure 3. ORTEP representation of compound 11 with thermal ellipsoids set at 25% probability. All hydrogen atoms except those attached to C(4) are removed for clarity.
reminiscent of the THF adducts of the previously reported [M(HB(C6F5)3)2(THF)2] (M = Ca, Yb), although both of these compounds are limited to a coordination number of 8 through the incorporation of two additional THF donors.10,11 In contrast, the larger Sr2+ and Ba2+ centers (Yb2+ 1.02 Å, Ca2+ 1.00 Å, Sr2+ 1.21 Å, and Ba2+ 1.35 Å for the comparable 6coordinate ions)16 facilitate the adoption of the higher 9- and 10-coordinate geometries observed in the structures of compounds 8 and 10 through the inclusion of three diethyl ether and four THF molecules, respectively. Reactivity of Compound 2 and i-PrNCN-i-Pr. We have previously reported that compounds 2 and 3 are viable catalysts for the reductive hydroboration of CO2 with HBpin to provide the methanol-equivalent MeOBpin.3 Although the precise role of the [HB(C6F5)3]− anion during the course of the catalysis was not elucidated, some initial insight into its reactivity was provided by the identification of compound 4 (Scheme 1), which was isolated from a reaction between compound 2 and CO2 performed in the absence of HBpin. The isolation of compound 4 implies that the [HB(C6F5)3]− anion fulfills the role of a Lewis acidic activator of the CO2 substrate. In related research we have previously reported that selective hydroboration of isoelectronic N-alkyl and -aryl carbodiimides with HBpin may be catalyzed by the β-diketiminato magnesium hydride [HC{(Me)CNDipp}2MgH]2 (1) to provide exclusive access to the corresponding N-borylated formamidines with no further observable reaction with HBpin even under forcing conditions (eq 1).2 We were thus intrigued to note a report by Okuda and co-workers that a similar hydroboration of i-PrN CN-i-Pr may be catalyzed by [Mg(THF)6][HBPh3]2 to provide H2C(N(Bpin)i-Pr)2 (11) as the sole reaction product (eq 2).9 With these observations in mind, we carried out the reaction of i-PrNCN-i-Pr and 2 molar equiv of HBpin in the presence of 10 mol % of compound 2. This reaction provided >90% conversion of the carbodiimide substrate to the bis(Nboryl)aminal 11 at room temperature in 30 h or within 6 h on heating to 60 °C, as indicated by the appearance of a distinctive methylene singlet resonance at δ 4.61 ppm in the 1H NMR spectrum.
diketiminate derivatives of the heavier metals strontium and barium has seen only limited development, as a result of the greater propensity of these cations toward Schlenk-like solution equilibration to homoleptic species.13 With this restriction in mind, we turned our attention to the synthesis of the bis(hydridoborate) derivatives of strontium and barium. As described previously, addition of 2 molar equiv of Me2NH·BH3 to [Sr{N(SiMe2)2}2(THF)2] in toluene provides facile in situ access to a strontium bis(amidoborane) species.14 Subsequent addition of 2 equiv of B(C6F5)3 induced hydride abstraction and the formation of the bis(hydridoborate) derivative, compound 8, which was isolated as the tris(ether) adduct in high yield (86%) after crystallization from diethyl ether (Scheme 3). The 11B NMR spectrum of compound 8 comprised a doublet resonance (δ(11B) −24.5 ppm, 1JBH = 93 Hz), congruent with analogous data reported for compounds 2, 3, and 7.3 Although a similar protocol utilizing [Ba{N(SiMe2)2}2(THF)2] as the starting material provided the barium analogue of compound 8 in low (
