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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Calcium Hydride Insertion Reactions with Unsaturated C−C Bonds Andrew S. S. Wilson, Michael S. Hill,* and Mary F. Mahon* Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.

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ABSTRACT: The dimeric β-diketiminato calcium hydride, [(BDI)CaH]2 (BDI = HC{(Me)CN-2,6-i-Pr2C6H3}2), reacts with terminal alkenes to provide the corresponding alkyl derivatives. With 1-octene, 3,3-dimethyl-1-butene, 3-phenyl-1propene, and 4-phenyl-1-butene, the ultimate products are dimeric alkyl species, which form sequentially via dinuclear calcium hydrido-alkyl intermediates. Reaction of diphenylacetylene with [(BDI)CaH]2 provides a tetra-calcium complex in which a stilbene dianion bridges symmetrically between two hydrido-bridged bis-β-diketiminato calcium units, while treatment with trans-stilbene yields a dicalcium benzyl-hydride derivative, [{(BDI)Ca}2(H)(PhCHCH2Ph)]. Reaction of [(BDI)CaH]2 and 1,5-hexadiene has previously been observed to provide facile 5-exo-trig cyclization to calcium cyclopentylmethyl derivatives. In contrast, analogous reaction with 1,7-octadiene provides exclusive intermolecular insertion to yield a dimeric open chain calcium oct-7-en-1-yl derivative. Reaction with the internal alkene, norbornene, enables the identification of a highly reactive dinuclear calcium norbornyl-hydride. The greater basicity of the calcium secondary alkyl species is emphasized by the subsequent isolation of a product of intramolecular deprotonation of a BDI iso-propyl methyl substituent.

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nal alkenes, ethane, 1-butene, and 1-hexene (Scheme 1).36 The formation of the dimeric compounds (8−10) was observed to ensue in a stepwise fashion through the intermediacy of mixed hydride-alkyl species (4−6). As part of a study of the ability of compound 3 to effect the hydrogenation of terminal alkenes, we have also very recently found that its reaction with 1,5hexadiene initially proceeds via similarly dinuclear hydrido-5alkenyl and 5-alkenyl species, 7 and 11. 37 Although spectroscopically identifiable, these compounds are consumed through favorable intramolecular 5-exo-trig carbocalciation reactions to provide the calcium cyclopentylmethyl derivatives, 12 and 13, the latter of which was crystallographically characterized. Although 8−10 were stable in aliphatic solvents, solutions in benzene at even mildly elevated temperatures (ca. 50 °C) were found to effect the unprecedented nucleophilic alkylation of a C−H bond of the arene solvent.36 In this contribution, we further elaborate the reactivity of compound 3 with unsaturated hydrocarbons.

INTRODUCTION Attempts to synthesize calcium Grignard analogues, RCaX (R = alkyl, X = halide), by the direct reaction of calcium metal and an alkyl or aryl halide date from the early 20th century.1 This direct method, however, was plagued by the low reactivity of elemental calcium, poor solubility and the high reactivity of any resulting organocalcium reagents such that the exact constitution of some of these reported derivatives is, at best, questionable.2−4 Westerhausen and co-workers have more recently demonstrated that the otherwise sluggish reactivity of calcium metal may be overcome by the removal of volatiles from a solution of calcium in liquid ammonia. The resultant metallic powder is highly reactive, enabling the synthesis of a plethora of calcium σ-aryl derivatives, ArCaI.5−18 Notably, these heavy aryl Grignard reagents are also observed to cleave THF above −30 °C. The successful isolation of well-defined calcium σ−alkyls has been historically dependent upon the use of sterically demanding and/or electronically stabilized (benzyl or α-triorganosilyl) organic anions. Lappert and co-workers, for example, reported the first dialkylcalcium, [(diox)2Ca{CH(SiMe3)2}2] (1, diox = 1,4-dioxane) in 1991, while similar utilization of the same or closely related sterically demanding silylalkyl ligands has since enabled the synthesis of a wide variety of both homo- and heteroleptic organocalcium derivatives.19−34 In contrast, the indisputable authentication of the methyl calcium derivative, [CaMe2]n (2), was only achieved by Anwander and co-workers in 2018.35 In a similar vein, we have recently reported that the isolation of longer chain calcium ethyl, n-butyl and n-hexyl homologues, compounds 8−10, may be achieved through reactions of the dimeric β-diketiminato calcium hydride, [(BDI)CaH]2 (BDI = HC{(Me)CN-2,6-i-Pr2C6H3}2), compound 3, with the termi© XXXX American Chemical Society

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RESULTS AND DISCUSSION Reactivity of 3 with Unsaturated Hydrocarbons. Harder and co-workers have previously demonstrated that 1,1-diphenylethene reacts with the THF-solvated analogue of compound 3 at 60 °C to afford the alkyl complex, [(BDI)Ca(PhC(CH3)Ph)(THF)].38 Similar treatment of equimolar quantities of 1,1-diphenylethene and 3 for one hour resulted in the complete consumption of the hydridic signal at δ 4.27 ppm along with the simultaneous appearance of two new BDI methine resonances at δ 4.86 and 4.81 ppm in Received: October 12, 2018

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DOI: 10.1021/acs.organomet.8b00743 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of Compounds 4−13

Figure 1. ORTEP representation of compound 14 with thermal ellipsoids at 25% probability. Hydrogen atoms and iso-propyl methyl groups have been removed for clarity. Selected bond lengths (Å) and bond angles (deg). Ca1−C32 2.9493(11), Ca1−C33 2.7935(10), Ca1−C34 2.7326(11), Ca1−C35 2.7230(13), Ca1−C36 372.7366(12), Ca1−C37 2.7804(11), C30−C31 1.5146(15), C31−C32 1.3832(17), C31−C38 1.4670(16), C32−C33 1.4568(15), C32−C37 1.4626(15), C33−C34 1.3766(16), C34−C35 1.4103(18), C35−C36 1.404(2), C36−C37 1.3810(19), C33− C32−C37 112.63(10), C30−C31−C32 119.31(10), C32−C31−C38 123.99(10), C30−C31−C38 116.69(10).

the resultant 1H NMR spectrum. While the methine signal at δ 4.81 ppm integrated with a 2:1 intensity relative to a further new hydridic signal at δ 4.17 ppm, suggestive of the formation of a mixed hydride-alkyl species similar to 4−7, the resonance at δ 4.86 ppm was assigned to the desired 1,1-diphenylethyl derivative, [(BDI)Ca(PhC(CH3)Ph)] (14). Recrystallization of the reaction product from hexane at −35 °C afforded orange blocks of 14, which were analyzed by single crystal X-ray diffraction analysis (Figure 1). As was its THF-solvated analogue, compound 14 is a mononuclear species in which the (Ph2CCH3)− anion coordinates to the calcium center through an η6 interaction to one of the phenyl substituents with close Ca−C contacts [2.7230(13) − 2.9493(11) Å] and a short [C32−C37] centroid to calcium distance of 2.4016(6) Å. The negative charge is delocalized through the coordinating phenyl ring, as

evidenced by the short C31−C32 distance [1.3832(17) Å], the long C32−C33, C32−C37 distances [1.4568(15), 1.4626(15) Å] and the narrow C33−C32−C37 angle [112.63(10)°]. The benzhydrylic carbon (C31) retains sp2 character as indicated by its essentially trigonal planar environment [C32−C31− C30, 119.31(10)°; C32−C31−C38, 123.99(10)° and C38− C31−C30, 123.99(10)°] and its effectively coplanar orientation with respect to the coordinated (C32 − C37) phenyl group. A handful of metal stilbene dianion [PhCHCHPh]2− complexes have been reported and derivatives of Li,39 Al,40 Y, La,41 Yb,42 Lu,43 Sm,44 and U45 have been structurally characterized. Of most relevance to the current study, however, are the amidinato calcium derivatives, [{(RC{NDipp}2)Ca}2(PhHCCHPh)] (R = t-Bu, Ad), which were reported in 2017 to result from the double insertion of diphenylacetylene B


Article

Organometallics Scheme 2. Synthesis of Compounds 15 and 16

a bridging hydride interaction [2.20(3) Å] together with close contacts to C64′, C65′, and C65 of the stilbene dianion [2.940(10), 2.707(5), and 2.891(4) Å, respectively]. As was observed previously,46 the similarity of the C64−C65 and C65−C65′ bond lengths [1.405(10), 1.414(11) Å], along with the elongation of the phenyl C−C bonds [1.390(18)−1.49(3) Å] are indicative of delocalization of the negative charge throughout the stilbene dianion. Delocalization of charge throughout phenyl moieties has been previously observed for heavier alkali metal (K and Rb)47 and lanthanide benzyl derivatives.48 A single complex resulting from the insertion of transstilbene into an M−H bond has been crystallographically characterized. Okuda and co-workers have reported that reaction of trans-stilbene with [Y(C5Me4CH2SiMe2NCMe3)(THF)(μ-H)] 2 affords the insertion product, [Y(C5Me4CH2SiMe2NCMe3){CH(CH2Ph)Ph}(THF)].49 The addition of 3 to an equimolar quantity of trans-stilbene provided an orange solution. Monitoring of the resultant 1H NMR spectrum did indeed reveal the formation of an initial insertion product, 16 (Scheme 2), which was identified by the appearance of a diagnostic triplet resonance assigned to the αhydrogen atoms of the benzylic anion at δ 2.44 ppm and which integrated in a 1:2 ratio with a new BDI methine signal at δ 4.77 ppm. Over the course of several days an insoluble purple component, identified as compound 15 by comparison of its unit cell, was observed to crystallize from the reaction mixture. Although, due to the insolubility of compound 15, it was not possible to interrogate this reaction further by solution NMR spectroscopy, based on these observations it is tentatively suggested that the stilbene dianion may form through the intermediacy of compound 16. Removal of the reaction solvent and cooling of a saturated hexane solution to −35 °C afforded crystals of compound 16 suitable for single crystal X-ray analysis (Figure 3). Each calcium center in the dimeric molecule of 16 is bound by a bridging hydride and a chelating β-diketiminate ligand. The ligation of Ca3 is completed by an η6 interaction with a phenyl substituent [Ca−Crange, 2.760(4)−3.185(3) Å], whereas the remaining coordination site of Ca4 is provided by the αcarbon of the benzylic anion [Ca4−C137, 2.608(4) Å]. The elongation and inequivalence of the C−C bonds across the C131−C137 unit [1.372(6)−1.446(4) Å] indicate that the negative charge is delocalized across the anion, which occurs with a simultaneous narrowing of the C131−C136−C135 [113.3(3)°] angle. The reduction of the CC bond to a C−C single bond is demonstrated by an elongation of C137−C138 [1.501(6) Å] relative to the CC bond of trans-stilbene [1.311(4) Å] and its close comparison to the analogous C−C bond of [Y(C5 Me 4CH2 SiMe 2 NCMe3 ){CH(CH 2 Ph)Ph}(THF)] [1.510(8) Å].49

into the Ca−H bonds of the corresponding calcium hydride complexes, [{(RC{NDipp}2)CaH]2.46 Addition of 3 to diphenylacetylene afforded the insoluble purple stilbene dianion complex, [{(BDI)Ca}4(H)2(PhHCCHPh)] (15) over the course of 3 days (Scheme 2). In a similar manner to the previously reported amidinate derivatives,46 the insolubility of compound 15 precluded meaningful solution analysis by NMR spectroscopy. X-ray quality crystals were deposited from the reaction solution, however, allowing unambiguous identification of its structure(Figure 2).

Figure 2. ORTEP representation of compound 15 with thermal ellipsoids at 25% probability. Hydrogen atoms, except H1 and H65, iso- propyl groups, occluded solvent and disordered atoms have been removed for clarity. Selected bond lengths (Å) and bond angles. Ca1−C64 2.958(10), Ca2−C64′ 2.946(7), Ca2−C65 2.891(4), Ca2−C65′ 2.708(4), C64−C65 1.405(10), C65−C65′ 1.415(9), C64−C65−C65′ 125.6(7). Primed labeled atoms are related to those in the asymmetric unit by the 1−x, −y, −z symmetry operator.

Despite significant disorder of the stilbene ligand, compound 15 was unambiguously determined to comprise two unique calcium environments. Ca1 is satisfied by two β-diketiminate nitrogen interactions [2.3458(17) and 2.3523(17) Å], a bridging hydride [2.27(3) Å] and an η6-arene interaction from the C64-containing phenyl substituent of the stilbene dianion with a short centroid [C59−C64] to calcium distance [2.497(9) Å]. Ca2 is similarly coordinated by a BDI ligand and C


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Organometallics

integrated in a ratio of 1:2 with upfield triplet or multiplet (range: δ − 0.61 to −0.82 ppm) resonances arising from the αmethylene protons of the relevant n-alkyl chains. For the respective reactions with 1-octene, 3-phenyl-1-propene, and 4phenyl-1-butene, an additional 24 h at room temperature afforded 1H NMR spectra that primarily contained either the desired calcium n- octyl (21) or residual excess alkene resonances due to the crystallization of the poorly soluble calcium alkyls (23 and 24). In contrast, the 1H NMR spectrum of the reaction of 3 with 3,3-dimethyl-1-butene, even after prolonged reaction times (1 week) at room temperature, continued to comprise a mixture of 3 (δ 4.83 and 4.27 ppm, 1:1), the dicalcium 3,3-dimethyl-1-butyl-hydride (18) (δ 4.96 and 4.77 ppm; 1:2 ratio), and the calcium 3,3-dimethyl-1-butyl (22) (δ 4.70 and −0.82 - −0.76 ppm, 1:2). The slower rate of reaction of 3 with 3,3-dimethyl-1-butene is rationalized to be a consequence of the steric demands of the tert-butyl substituent of 3,3-dimethyl-1-butene rather than any remote electronic or inductive effect. The limited solubility of the calcium alkyls (22−24) facilitated their direct isolation as single crystals from saturated C6D6 solutions at room temperature, while single crystals of the calcium n-octyl derivative (21) were isolated from a saturated toluene solution layered with hexane at −35 °C. The results of the subsequent X-ray diffraction analyses performed on 21−24 are shown in Figure 4 with relevant crystallographic measurements displayed in Table 1. The solid-state structures of the calcium alkyls 21−24 are strongly reminiscent of those of the previously reported derivatives, compounds 8−10,36 and, thus, require little discussion. All four compounds crystallize as centrosymmetric dimers in which each calcium is ligated by a bidentate βdiketiminate ligand [Ca−N, 2.32−2.35 Å] with dimerization enabled by two equivalent bridging alkyl ligands. Formal intra[2.47−2.51 Å] and intermolecular [2.56−2.59 Å] Ca−C bonding interactions may be discriminated between the calcium centers and the α-methylene carbon (C30) of each alkyl ligand. Reaction with 1,7-Octadiene. Whereas the previously described reaction of compound 3 with 1,5-hexadiene resulted in cyclization of the initially formed calcium alkenyl species through intramolecular insertion of the CC units into the Ca−C bonds (Scheme 1), a similar reaction performed between 3 and 1,7-octadiene provided contrasting observations. Inspection of the resultant 1H NMR spectrum after addition of three equivalents of 1,7-octadiene to a C6D6 solution of 3 revealed the presence of two new calciumcontaining species in solution. These were identified as the respective products, the hydrido-octenyl (25) and the calcium oct-7-en-1-yl derivative (26), resulting from the reaction of one and two equivalents of 1,7-octadiene with the Ca−H bonds of the dimeric hydride. Compound 25 was characterized by the appearance of new BDI methine, hydride singlet and upfield triplet resonances at δ 4.78, 4.75, and −1.08 ppm, which displayed respective intensities of 2:1:2. Compound 26, meanwhile, was identified by the emergence of BDI methine and upfield methine triplet signals at δ 4.71 and −0.72 ppm that integrated with a ratio of 1:2. In contrast to the analogous reaction with 1,5-hexadiene, storage of the reaction mixture at room temperature for 24 h provided a resultant 1H NMR spectrum evidencing complete conversion to the calcium 7octen-1-yl derivative (26). (Figure S29)

Figure 3. ORTEP representation of the Ca3/Ca4-containing component of compound 16 with thermal ellipsoids at 25% probability. Hydrogen atoms except H2 and H137, iso-propyl groups and the disordered Ca1/Ca2-containing molecule have been removed for clarity. Selected bond lengths (Å) and bond angles. Ca3−C131 3.021(3), Ca3−C132 2.846(3), Ca3−C133 2.760(4), Ca3−C134 2.840(3), Ca3−C135 3.005(3), Ca3−C136 3.185(3), Ca4−C136 3.182(3), Ca4−C137 2.608(4), C131−C132 1.372(6), C131−C136 1.432(5), C132−C133 1.400(6), C133−C134 1.408(5), C134−C135 1.375(5), C135−C136 1.446(4), C136−C137 1.404(5), C137−C138 1.501(6), C138−C139 1.507(6), C131−C136−C135 113.3(3), C136−C137−C138 122.4(3), C137−C138−C139 118.6(4).

Encouraged by the successful syntheses of compounds 8− 11, the further range of terminal n-alkenes, 1-octene, 3,3dimethyl-1-butene, 3-phenyl-1-propene, 4-phenyl-1-butene, was reacted with compound 3 (Scheme 3). Addition of the Scheme 3. Stepwise Formation of Calcium n-alkyls (21−24) from Reaction of Compound 3 with Terminal n-Alkenes via Dicalcium Alkyl-Hydride Intermediates (17−20)

liquid hydrocarbon alkenes to a C6D6 solution of 3 afforded 1H NMR spectra that comprised a mixture of 3, the corresponding dicalcium alkyl-hydride intermediates (17−20) and the desired calcium alkyl derivatives [(BDI)Ca(CH 2 ) 2 R] 2 (R = (CH2)5CH3, 21; R = C(CH3)3, 22; R = CH2Ph, 23; (CH2)2Ph, 24) after 24 h at room temperature. The intermediates (17−20) were identified through the appearance of new BDI methine (range: δ 4.79 to 4.72 ppm), hydride (range: δ 4.96 to 4.71 ppm) and diagnostic upfield triplet or multiplet (range: δ − 0.99 to −1.16 ppm) signals, which integrated in a 2:1:2 ratio in the resultant 1H NMR spectra. The alkyl derivatives, 21−24, were characterized by further BDI methine (range: δ 4.70 to 4.52 ppm) signals that D


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Organometallics

Figure 4. ORTEP representations of (a) compound 21, (b) compound 22, (c) compound 23, (d) compound 24 with thermal ellipsoids at 25% probability. Hydrogen atoms, except those attached to the α- and β-carbons of the organocalcium ligand, iso-propyl groups, disordered atoms, and cocrystallized solvent molecules, have been removed for clarity. Primed labeled atoms are related to those in the asymmetric unit by the −x, 2−y, −z (21), 1−x, 2−y, 1−z (22, 24) and 1/2−x, 3/2−y, 1−z (23) symmetry operators.

Table 1. Selected Bond Lengths (Å) and angles (deg) of Compounds 21−24 Ca1−N1 Ca1−N2 Ca1−C30 Ca1−C30’ N1−Ca1−N2 Ca1−C30−Ca1′ C31−C30−Ca1 C31−C30−Ca1′

21

22

23

24

2.3427(13) 2.3209(13) 2.4947(18) 2.5716(18) 82.88(5) 81.74(6) 161.79(12) 80.78(10)

2.3458(11) 2.3409(11) 2.5948(14) 2.5073(14) 83.11(4) 81.69(4) 78.58(8) 156.77(10)

2.3286(9) 2.3390(9) 2.4923(12) 2.5768(12) 82.86(3) 82.43(4) 164.12(9) 81.96(6)

2.3185(9) 2.3311(10) 2.4743(13) 2.5634(13) 83.79(3) 82.08(4) 162.05(9) 80.03(7)

Recrystallization of the reaction products from toluene afforded single crystals of compound 26, which were suitable for X-ray diffraction analysis (Figure 5). Although the retention of the double bond character within the octenyl units is evidenced by short C(65)−C(66) [1.176(10) Å] and C(73)− C(74) [1.203(12) Å] distances, the organic substituents interact with both calcium centers in an analogous fashion to that observed in the other saturated calcium organometallics, compounds 8−10 and 21−24. In contrast to the ready 5-exotrig cyclization which provided 13, the resistance of 26 to ring

closure and formation of either the cycloheptylmethyl or cyclooctyl derivatives may be ascribed to the requisite higher energy eight-membered (7-exo-trig) or nine-membered (8endo-trig) transition states, respectively. Reaction with Internal Alkenes. Encouraged by its reactivity with both trans-stilbene and terminal n-alkenes, reactions of 3 with a variety of unactivated internal alkenes were investigated. Although 2,3-dimethyl-2-butene, cyclopentene and cyclohexene were found to be completely unreactive, immediate assessment of the reaction between E


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Organometallics

Figure 5. ORTEP representation of compound 26 with thermal ellipsoids at 25% probability. Iso-propyl groups, and hydrogen atoms except those attached to C59−C74 have been removed for clarity. Selected bond lengths (Å) angles (deg): Ca1−N1 2.334(2), Ca1−N2 2.323(2), Ca2−N3 2.320(2), Ca2−N4 2.337(2), Ca1−C59 2.472(4), Ca1−C67 2.576(4), Ca2−C59 2.562(4), Ca2−C67 2.491(4), C65−C66 1.176(10), C73−C74 1.203(12), N1−Ca1−N2 83.23(8), Ca2−C67−Ca1 82.10(13).

Scheme 4. Reaction of 3 with Norbornene; Synthesis of Compound 27 and the Proposed Route to 28

three equivalents of norbornene and 3 in C6D6 by 1H NMR spectroscopy provided evidence for the formation of a proposed dicalcium norbornyl-hydride (27). The formation of compound 27 was signified by the emergence of new BDI methine, hydride, and upfield triplet signals at δ 4.76, 4.95, and −1.31 ppm with a 2:1:1 ratio by integration (Scheme 4). Further analysis by 1H NMR spectroscopy after 1 day at room temperature indicated that the predominant reaction product was compound 27 and provided no evidence for consumption of any further norbornene through reaction with the remaining Ca−H bond. Compound 27 was isolated by crystallization from benzene at room temperature as highly air- and moisture-sensitive single crystals. Compound 27 crystallized in space group P1, with one dinuclear molecule in the asymmetric unit plus a portion of solvent (Figure 6). Despite significant disorder pertaining to the main features in this structure, the connectivity was unambiguous. Each calcium center is ligated by two nitrogen interactions from a single β-diketiminate ligand, with bringing norbornyl and hydride interactions completing the distorted tetrahedral coordination sphere. The calcium to alkyl bonding appears little perturbed by the secondary nature of the norbornyl organyl substituent, and the Ca2−C65 [2.469(8) Å] and Ca1−C65 [2.619(8) Å] distances are commensurate with the bond lengths observed in the earlier described primary alkyl derivatives. Ca1 lies some 1.9 Å

out of the plane defined by the N1−C2−C3−C4−N2 atoms of the BDI ligand, suggestive of a sterically congested coordination environment. Consequently, it is reasonable to suggest that a second insertion of norbornene into the bridging Ca−H bond of 27 would necessitate the formation of a monomeric and highly reactive calcium norbornyl species (vide infra). Notably, storage of the reaction beyond 1 day was accompanied by a decrease of the signal intensity of the BDI methine signal of 27 (δ 4.76 ppm), which occurred with a concomitant increase of the BDI methine signal intensity of 3 at δ 4.83 ppm in the resultant 1H NMR spectrum. The accompanying hydridic (δ 4.27 ppm) signal of 3 was simultaneously observed to emerge at a low intensity with the triplet fine structure of the previously reported dicalcium hydride-deuteride (3-d1).36 Consequently, it is apparent that the dicalcium norbornyl-hydride (27) or a transient monomeric calcium secondary alkyl derivative reacts directly with C6D6, even at room temperature. In an effort to circumvent the reaction of 27 with deuterobenzene, three equivalents of norbornene were added to a methylcyclohexane solution of 3. After heating to 80 °C for 16 h, single crystals were deposited, and the subsequent Xray diffraction analysis identified compound 28 as a product of intramolecular ligand deprotonation (Figure 7). Although no further mechanistic investigation was attempted, the cyclometalated compound 28 is proposed to form via the F


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Organometallics

Figure 6. ORTEP representation of compound 27 with thermal ellipsoids at 25%. Hydrogen atoms (except H1 and those attached to C59, C64, and C65), iso-propyl groups, and disordered calcium and solvent atoms have been removed for clarity. Selected bond lengths (Å) and angles (deg): Ca1−N1 2.342(3), Ca1−N2 2.315(3), Ca1−C65 2.619(8), Ca2−C65 2.4692(8), Ca2−N3 2.370(3), Ca2−N4 2.331(3), N1−Ca1−C65 121.10(19), N2−Ca1−N1 85.01(10), N2−Ca1−C65 138.70(19), N4−Ca2−N3 81.34(10).

Figure 7. ORTEP representation of compound 28 with thermal ellipsoids at 25%. Hydrogen atoms, except those attached to C10, have been removed for clarity. Selected bond lengths (Å) and angles (deg): Ca1−N1 2.3766(13), Ca1−N2 2.3899(13), Ca1−C7′ 2.9670(15), Ca1−C10 2.5761(17), Ca1−C10′ 2.6142(16), Ca1−C11′ 2.9940(16), N1−Ca1−N2 77.64(5), Ca1−C10−Ca1′ 80.12(5).

intramolecular C−H activation of the methyl substituent of an iso-propyl group of the β-diketiminate ligand, most likely, by the in situ generated calcium norbornyl (Scheme 4). Each calcium center in the centrosymmetric structure of 28 is bound by a single β-diketiminate-derived dianion [Ca−N 2.3766(13) and 2.3899(13) Å] with dimer formation propagated by μ2 bridging interactions [2.5761(17), 2.6142(16) Å] from the

carbon atom (C10) of the deprotonated methyl group. Although elongated in comparison with the other calcium nalkyls described above, these methylene to calcium interactions lie within the range of previously reported calcium alkyls (ca. 2.5−2.7 Å).19−25,35,50 The coordination sphere of each calcium center is again supplemented by anagostic interactions between the bridging methylene groups and close contacts to two arene G


Article

Organometallics

(9 mg, 31%). The insolubility of compound 15 in deuterated aromatic solvents precluded the collection of meaningful NMR data. [{(BDI)Ca}2(H)(PhCHCH2Ph)] (16). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of compound 3 (38 mg, 0.08 mmol) and trans-stilbene (7 mg, 0.04 mmol). The reaction mixture was heated at 90 °C for 1 h, affording a red solution that contained primarily 16 as observed by 1H NMR spectroscopy (Figure S6). Although compound 16 could not be isolated in an analytically pure form, material suitable for single crystal diffraction analysis deposited from a hexane solution of crude 16 at −35 °C. 1H NMR (300 MHz, benzene-d6) δ 7.22−6.98 (m, ca. 17H, Ar-H), 6.04−5.99 (m, 1H, ArH), 5.60−5.54 (m, 2H, Ar-H), 5.47−5.38 (m, 2H, Ar-H), 4.77 (s, 2H, NC(CH3)CH)), 3.98 (s, 1H, CaH), 3.49−3.33 (br, 2H, CH(CH3)2) 3.27 (d, 3JHH = 7.3 Hz, 2H, PhCHCH2Ph), 3.24−2.94 (br, 6H, CH(CH3)2), 2.44 (t, 1H, 3JHH = 7.3 Hz, PhCHCH2Ph), 1.71−1.07 (m, ca. 60H, CH(CH3)2 and NC(CH3)CH) ppm. [(BDI)Ca{CH2(CH2)6CH3}]2 (21). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of compound 3 (60 mg, 0.13 mmol) and 1-octene (62 μL, 0.39 mmol). After 2 days at room temperature, the solvent was removed in vacuo, and the solid was redissolved in minimal toluene (1 mL) and layered with hexane (1 mL). Storage at −35 °C provided 21 as colorless crystals suitable for an X-ray diffraction analysis (19 mg, 26%). 1H NMR (500 MHz, benzene-d6) δ 7.19−7.12 (m, 6H, Ar-H), 4.70 (s, 1H, NC(CH3)CH), 3.20 (hept, 3 JHH = 6.8 Hz, 4H, CH(CH3)2), 1.59 (s, 6H, NC(CH3)CH), 1.51− 1.25 (m, 12H, CaCH2{CH2}6CH3), 1.20 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.04 (br d, 12H, CH(CH3)2), 0.92 (t, 3JHH = 7.1 Hz, 3H, CaCH 2 {CH 2 } 6 CH 3 ), − 0.70 (t, 3 J HH = 8.7 Hz, 2H, CaCH2{CH2}6CH3) ppm. 13C{1H} NMR (126 MHz, benzene-d6) δ 166.5 (NC(CH3)CH), 145.8 (Cipso), 142.1 (Cortho), 124.8 (Cpara), 124.0 (Cmeta), 93.6 (NC(CH3)CH), 38.8, 32.6, 30.9, 29.9, 29.4 (CaCH2{CH2}6CH3), 28.7 (CH(CH3)2), 28.2 (CaCH2{CH2}6CH3), 25.2, 24.5 (CH(CH3)2), 24.4 (NC(CH3)CH), 23.2 (CaCH2{CH2}6CH3), 14.4 (CaCH2{CH2}6CH3) ppm. [(BDI)Ca{(CH2)2C(CH3)3}]2 (22). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of 3 (30 mg, 0.065 mmol) and dimethylbut-1-ene (25 μL, 0.20 mmol). After 7 days at room temperature, colorless crystals of 22 suitable for X-ray diffraction analysis were deposited (19 mg, 54%). 1H NMR (500 MHz, benzened6) δ 7.15−7.13 (m, 6H, Ar-H), 4.70 (s, 1H, NC(CH3)CH), 3.29− 3.08 (br, 4H, CH(CH3)2), 1.57 (s, 6H, NC(CH3)CH), 1.40−1.36 (m, 2H, CaCH2CH2C(CH3)3), 1.22 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 1.17 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.16 (d, 3 JHH = 6.8 Hz, 3H, CH(CH3)2), 1.09 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 1.04 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 0.97 (s, 9H, CaCH2CH2C(CH3)3), − 0.77 - − 0.80 (m, 2H, CaCH2CH2C(CH3)3) ppm. 13C{1H} NMR (126 MHz, benzene-d6) 166.7 (NC(CH3)CH), 146.2 (Cipso), 141.9 (Cortho), 124.7 (Cpara), 124.0 (Cmeta), 93.4 (NC(CH3)CH), 45.8 (CaCH2CH2C(CH3)3), 33.3 (CaCH2CH2C(CH3)3), 29.1 (CaCH2CH2C(CH3)3), 28.6 (CH(CH3)2), 25.4, 25.4, (CH(CH3)2), 24.6 (NC(CH3)CH), 24.4, 24.4, 23.5 (CH(CH3)2), 18.3 (CaCH2CH2C(CH3)3) ppm. [(BDI)Ca{(CH2)3Ph}]2 (23). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of compound 3 (60 mg, 0.13 mmol) and allylbenzene (60 μL, 0.45 mmol). After 1 day at room temperature, colorless crystals of 23 suitable for X-ray diffraction analysis deposited (26 mg, 35%). 1H NMR (500 MHz, benzene-d6) δ 7.19−7.12 (m, 12H, Ar-H), 4.52 (s, 1H, NC(CH3)CH), 3.13 (hept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 2.74−2.71 (m, 2H, CaCH2CH2CH2Ph) 1.75−1.68 (m, 2H, CaCH2CH2CH2Ph), 1.49 (s, 6H, NC(CH3)CH), 1.16 (d, 3 JHH = 6.8 Hz, 3H, CH(CH3)2), 1.03−1.00 (br, 12H, CH(CH3)2), − 0.61 (t, 3JHH = 9.0 Hz, 2H, CaCH2CH2CH2Ph) ppm. 13C{1H} NMR (126 MHz, benzene-d6) δ 166.4 (NC(CH3)CH), 145.7 (DippCipso), 142.7 (PhCipso), 142.1 (DippCortho), 128.6 (PhCmeta), 128.3 (PhCortho), 126.0 (PhCpara), 124.8 (DippCpara), 124.0 (DippCmeta), 93.7 (NC(CH3) CH), 45.5 (CaCH2CH2CH2Ph), 34.0 (CaCH2CH2CH2Ph), 28.6 (CH(CH3)2), 28.1 (CaCH2CH2CH2Ph), 25.2, 24.5 (CH(CH3)2), 24.3 (NC(CH3)CH) ppm.

carbons (C7′ and C11′) of the neighboring di-iso-propylphenyl substituent (2.9670(15) and 2.9940(16) Å).

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CONCLUSIONS In summary, we have observed that a wide range of aliphatic organocalcium derivatives are available through insertion reactions of terminal, and in one case internal, alkenes with a dimeric β-diketiminato calcium hydride. We have previously observed that the highly polarized nature of the Ca−C bonding provides remarkably potent sources of alkyl nucleophiles. These observations underscore the view that heavier group 2 organometallic reactivity is not simply magnesium or even lanthanide “mimetic”, but displays features that are unique to each of the available elements.36,37 We are continuing to examine these possibilities.

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EXPERIMENTAL SECTION

General Considerations and Starting Materials. All manipulations were carried out using standard Schlenk line and glovebox techniques under an inert atmosphere of argon. NMR experiments were conducted in J Young tap NMR tubes prepared and sealed in a Glovebox. NMR spectra were collected on a Bruker AV300 spectrometer operating at 300.2 MHz (1H), 75.5 MHz (13C) or an Agilent ProPulse spectrometer operating at 500 MHz (1H), 126 MHz (13C). The spectra were referenced relative to residual protio solvent resonances. Solvents (Toluene, Hexane, Et2O) were dried by passage through a commercially available (Innovative Technologies) solvent purification system, under nitrogen and stored in ampules over 4 Å molecular sieves. C6D6 was purchased from Sigma-Aldrich Corp., dried over a potassium mirror before vacuum distilling under argon and storing over molecular sieves. Calcium iodide (99.95%) and phenylsilane (97%) were purchased from Sigma-Aldrich Corp. and used without further purification. Diphenylacetylene (98%) and transstilbene (96%) were purchased from Sigma-Aldrich Corp., recrystallized from ethanol and dried under high vacuum. Liquid alkenes; 1,1diphenylethene (97%), 3,3-dimethyl-1-butene (97%), 1-hexene (97%), 1,5-hexadiene (97%), 1-octene (98%), 1,7-octadiene (98%), allylbenzene (98%) and 4-phenyl-1-butene (99%) were purchased from Sigma-Aldrich Corp., dried over calcium hydride and distilled under argon before use. [(BDI)CaH]2 (3) was synthesized by a literature procedure.36 Despite multiple attempts, the extreme air- and moisture sensitivity of all the isolated compounds described in this work militated against the acquisition of meaningful and/or accurate CHN microanalysis. [(BDI)Ca{PhC(CH3)Ph}] (14). In a J Young NMR tube, 1,1diphenylethene (17.7 μL, 0.10 mmol) was added to a C6D6 (0.5 mL) solution of compound 3 (45 mg, 0.10 mmol). The reaction was heated for 1 h at 60 °C and then evaporated to dryness in vacuo, and the dark orange solid was redissolved in a minimum of hexane (0.5 mL). Storage at −35 °C provided compound 14 as dark orange crystals suitable for X-ray diffraction analysis (16 mg, 25%). 1H NMR (500 MHz, benzene-d6) δ 7.01 (s, 6H, N-2,6-iPrC6H3 Ar-H), 6.50− 6.46 (m, 4H, Ph2CCH3 Ar-Hortho), 6.46−6.41 (m, 4H, Ph2CCH3 ArHmeta), 5.72 (tt, 3JHH = 6.7 Hz, 5JHH = 1.5 Hz, 2H, Ph2CCH3 ArHpara), 4.85 (s, 1H, NC(CH3)CH), 2.93 (hept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 1.69 (s, 6H, NC(CH3)CH), 1.68 (s, 3H, Ph2CCH3), 1.13 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.11 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2) ppm. 13C{1H} NMR (126 MHz, benzene-d6) δ 165.3 (NC(CH3)CH), 144.3 (N-2,6-iPrC6H3 Ar-Cipso), 141.5 (N-2,6iPrC6H3 Ar-Cortho), 140.5 (Ph2CCH3 Ar-Cipso), 130.7 (Ph2CCH3 ArCmeta), 125.4 (N-2,6-iPr−C6H3 Ar-Cpara), 125.0 (N-2,6-iPrC6H3 ArCmeta), 118.2 (Ph2CCH3 Ar-Cortho), 107.5 (Ph2CCH3 Ar-Cpara), 95.0 (NC(CH3)CH), 93.0 (Ph2CCH3), 29.2 (CH(CH3)2), 24.4, 23.7 (CH(CH3)2), 23.5 (NC(CH3)CH), 19.6 (Ph2CCH3). [{(BDI)Ca}4(H)2(PhHCCHPh)] (15). In a vial, toluene (0.5 mL) was added to a mixture of compound 3 (30 mg, 0.065 mmol) and diphenylacetylene (3 mg, 0.017 mmol). After 1 day at room temperature, dark purple crystals of compound 15 were deposited H


Article

Organometallics Compounds 24, 26, 27. Despite numerous attempts, compounds 24, 26, and 27 could neither be formed nor isolated in analytically pure bulk form as indicated by 1H NMR analyses (Figures S28−S32). [(BDI)Ca({CH2}4Ph)]2 (24). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of 3 (30 mg, 0.065 mmol) and 4-phenyl1-butene (30 μL, 0.20 mmol). After 3 days at room temperature, colorless crystals of compound 24 suitable for X-ray diffraction analysis were deposited (19 mg). [(BDI)Ca({CH2}6CHCH2)]2 (26). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of 3 (90 mg, 0.20 mmol) and 1,7octadiene (87 μL, 0.59 mmol). After 1 day at room temperature, the solvent was removed in vacuo, and the solid was redissolved in a minimum of toluene (1 mL). Storage at −35 °C provided 26 as colorless crystals suitable for X-ray diffraction analysis (3 mg). [{(BDI)Ca}2(H)(CH(C6H10)] (27). In a J Young NMR tube, C6D6 (0.5 mL) was added to a mixture of 3 (90 mg, 0.20 mmol) and norbornene (57 mg, 0.61 mmol). After 1 day at room temperature colorless crystals of compound 27 suitable for X-ray diffraction analysis deposited (8 mg, 8%). 1H NMR (500 MHz, benzene-d6) δ 7.12−7.01 (m, 12H, Ar-H), 4.95 (s, 1H, CaH), 4.75 (s, 2H, NC(CH3)CH), 3.21 (hept, 3JHH = 6.8 Hz, 8H, CH(CH3)2), 2.14− 2.09 (m, 1H, C6H10), 1.64 (s, 12H, NC(CH3)CH), 1.50−1.47 (m, 1H, C6H10), 1.39−1.31 (m, 2H, C6H10), 1.19, 1.19 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.09 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.08 (m, 1H, C6H10), 1.06 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 0.76−0.66 (m, 3H, C6H10), 0.22 (d, 1H, C6H10), − 1.31 (t, 1H, CaCH) ppm. The reaction of compound 27 with C6D6 at room temperature, and poor solubility precluded definitive characterization by 13C NMR spectroscopy. [(DippNC(Me)CHC(Me){N-2,6-iPr,CH(Me)(CH2 )-C6 H3 })Ca] 2 (28). In a J Young NMR tube, methylcyclohexane (0.5 mL) was added to a mixture of compound 3 (30 mg, 0.065 mmol) and norbornene (6 mg, 0.20 mmol). Heating for 16 h at 80 °C and cooling to room temperature provided compound 28 as colorless crystals suitable for X-ray diffraction analysis (5 mg, 17%). 1H NMR (500 MHz, benzene-d6) δ 7.24−7.19 (m, 3H, 2,6-iPrC6H3), 6.89 (dd, 3 JHH = 7.7 Hz, 4JHH = 1.1 Hz, 1H, 2,6-iPr,CH(CH3)(CH2Ca)-C6H3), 6.69 (dd, 3JHH = 7.7 Hz, 4JHH = 1.1 Hz, 1H, 2,6-iPr,CH(CH3)(CH2Ca)-C6H3), 5.66 (t, 3JHH = 7.7 Hz, 1H, 2,6-iPr,CH(CH3)(CH2Ca)-C6H3), 3.43−3.34 (m, 1H, CH(CH3)(CH2Ca)), 3.34−3.25 (m, 2H, CH(CH3)2), 3.03 (hept, 3JHH = 6.8 Hz, 1H, CH(CH3)2), 1.71, 1.61 (s, 3H, NC(CH3)CH), 1.56 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 1.51 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)(CH2Ca)), 1.26, 1.16, 1.16, 1.13, 1.07 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 0.41 (dd, 2 JHH = 14.8 Hz, 3JHH = 7.3 Hz, 1H, CH(CH3)(CH2Ca)), − 1.30 (dd, 2 JHH = 14.8 Hz, 3JHH = 11.2 Hz, 1H, CH(CH3)(CH2Ca)) ppm. 13 C{1H} NMR (126 MHz, benzene-d6) δ 165.9, 164.6 (NC(CH3)CH), 148.4, 147.5, 146.5, 146.1, 142.6, 141.5, 125.3, 124.8, 124.7, 124.5, 124.2, 115.9 (C6H3) 94.4 (NC(CH3)CH), 46.3 (CH(CH3)(CH2Ca), 33.8 (CH(CH3)(CH2Ca)), 29.7, 28.3, 28.0 (CH(CH3)2), 26.7 (CH(CH3)(CH2Ca)), 26.2 (CH(CH3)2), 24.5 (NC(CH3)CH), 24.4, 24.4 (CH(CH3)2), 24.3 ((NC(CH3)CH)), 24.1, 23.9, 23.6, 23.5 (CH(CH3)2) ppm.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Michael S. Hill: 0000-0001-9784-9649 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC (U.K.) for the provision of a postgraduate studentship for ASSW from the University of Bath’s Doctoral training Programme.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00743. NMR spectra and X-ray crystallographic details (PDF) Accession Codes

CCDC 1857814−1857820 and 1857822−1857824 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data I


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