Syntheses of Heteroleptic Amidinate Strontium Complexes Using a

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

Syntheses of Heteroleptic Amidinate Strontium Complexes Using a Superbulky Ligand Benjamin Freitag, Jürgen Pahl, Christian Far̈ ber, and Sjoerd Harder* Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraβe 1, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Strontium complexes are presented with two different bulky amidinate ligands (Am): tBuC(N-DIPP)2 (DIPP = 2,6-diisopropylphenyl), abbreviated here as tBuAmDIPP, and (p-tolyl)C(N-Ar‡)2 (Ar‡ = 2,6-Ph2CH-4-iPrphenyl) abbreviated here as pTolAmAr‡. The amidine tBuAmDIPP-H was deprotonated by Sr[N(SiMe3)2]2 in benzene at 60 °C. Although the product, tBuAmDIPPSrN(SiMe3)2, could be characterized by NMR, attempts to isolate it led to ligand scrambling via a Schlenk equilibrium. Reaction of in situ prepared tBuAmDIPPSrN(SiMe3)2 with PhSiH3 gave PhH 2 SiN(SiMe 3 ) 2 and presumably the intermediate tBuAmDIPPSrH, but the latter is not stable and the homoleptic complex (tBuAmDIPP)2Sr was isolated and structurally characterized. Deprotonation of the bulkier amidine pTolAmAr‡-H with Sr[N(SiMe3)2]2 needed forcing conditions, inevitably giving rise to deprotonation of the Ph2CH substituent as well. Reaction of pTolAmAr‡-H with the less bulky and less basic Sr[N(SiHMe2)2]2, however, gave the heteroleptic product pTolAmAr‡SrN(SiHMe2)2, which has been structurally characterized. The latter was also at 60 °C stable toward ligand scrambling. Reaction with PhSiH3 did give hydride exchange, but the product pTolAmAr‡SrH decomposed even at −30 °C. Instead, an amidinate complex with a deprotonated Ph2CH substituent was isolated and structurally characterized (7). The latter catalyzed the intramolecular alkene hydroamination.

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INTRODUCTION Although the last decades have seen great progress in organocalcium chemistry,1 knowledge of the heaviest alkalineearth metals (Sr, Ba) is still underdeveloped.2 As the metal size increases and electronegativity decreases down the group from Be to Ba, bonds become more ionic, longer, and weaker. This results in higher reactivity (including air sensitivity) and increased dynamics. Fast ligand exchange hampers the isolation of well-defined heteroleptic complexes (defined here as complexes with two different anionic ligands) of the heavier alkaline-earth metals. Similar to the case for organocalcium chemistry, heteroleptic complexes of the heavier group 2 metals can be stabilized by using bulky bidentate β-diketiminate ligands. Although the heteroleptic complexes 1-Ca3 and 1-Sr4 have been reported (Scheme 1), the strontium complex is much more susceptible to ligand exchange reactions. Both react in a controlled way with H2O to give the well-defined hydroxide complexes 2-Ca5 and 2-Sr.4 Reaction with PhSiH3, however, gave only in the case of Ca access to a hydride complex (3-Ca).6 Attempts to isolate 3-Sr led to the homoleptic complex 4-Sr and precipitation of SrH2. In contrast, 3-Ca dissolved in benzene was stable for days at 75 °C. Recently we reported the use of bulky amidinate ligands (Am) for the isolation of the first calcium hydride complexes with amidinate ligands.7 Amidinate ligands have a considerably narrower bite angle in comparison to β-diketiminate ligands, © XXXX American Chemical Society

which makes them generally less bulky and unsuitable for stabilization of complexes with very large metals. Amidinate ligands with an N-aryl substituent, however, feature a unique binding mode in which the ligand coordinates in an (N,aryl)chelating instead of (N,N)-chelating mode, thus effectively shielding the metal center (Scheme 2). This binding mode is especially preferred with large groups (tBu or adamantyl) in the ligand backbone.8 A controlled reaction of 5-Ca with PhSiH3 gave the first solvent-free, neutral9 calcium hydride complex. A second example, i.e. dimeric 3-Ca without THF ligands, was reported only very recently.10 Complex 6-Ca crystallized, similarly to 3-Ca, as a hydride-bridged dimer.7 The steric protection of the (N,aryl)-chelating amidinate ligand in 6-Ca is therefore comparable to the combined sterics of a βdiketiminate and a THF ligand in 3-Ca. After the successful application of bulky amidinate ligands in Ca chemistry, we explored the possibility of making use of amidinate ligands in the synthesis of discrete Sr hydride complexes. In order to achieve such a challenging goal, a synthetic route to stable heteroleptic complexes of the type AmSrR (in which R is a reactive group) is a first requirement. Although various homoleptic Sr amidinate complexes have been reported,11 heteroleptic Sr complexes with amidinate Received: December 6, 2017

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DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Syntheses of β-Diketiminate Alkaline-Earth-Metal Hydroxide and Hydride Complexes

Scheme 3. Conversion of Heteroleptic Amidinate Strontium Amide Complexes with Phenylsilane

Scheme 2. Syntheses of Ca Hydride Complex 6-Ca Stabilized by an (N,aryl)-Bidentate Amidinate Ligand

ligands are unknown. Within the subgroup of guanidinate Sr complexes,12 only one example of a heteropletic Sr guanidinate complex has been reported, [iPr2NC(NiPr)2]SrCp*, which crystallizes as a dimer.12b Herein we describe unique heteroleptic Sr amidinate complexes of the type AmSrR and their reactivity with PhSiH3.13

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RESULTS AND DISCUSSION This study makes use of two Am ligands, the bulky tBuC(NDIPP)2 (DIPP = 2,6-diisopropylphenyl), abbreviated here as tBuAmDIPP, and the superbulky (p-tolyl)C(N−Ar‡)2 (Ar‡ = 2,6Ph2CH-4-iPr-phenyl), abbreviated here as pTolAmAr‡ (Scheme 3). The amidine precursor tBuAmDIPP-H has been reported earlier,14 and pTolAmAr‡-H was prepared according to a literature procedure of a similar amidine with a tBu substituent in the backbone.15 Although the latter, extremely bulky ligand has been converted to its K complex, no further metal complexes could be isolated.15 The ligand pTolAmAr‡, with a smaller p-tolyl backbone substituent, should be less bulky. Reaction of tBuAmDIPP-H with THF-free Sr[N(SiMe3)2]2 (obtained from Sn[N(SiMe3)2]2 and Sr) in C6D6 at 60 °C gave within 4 h full conversion to the heteroleptic complex tBuAmDIPPSrN(SiMe3)2 which could be characterized in situ by 1H and 13C NMR. This reaction also proceeded using the THF-containing educt Sr[N(SiMe3)2]2·2THF; however, since the latter Sr amide complex was obtained by salt metathesis in THF (2 KN(SiMe3)2 + SrI2), slight contamination of this

precursor with K amide led in this case to precipitation of tBuAmDIPPK, complicating product isolation. Therefore, the synthesis of tBuAmDIPPSrN(SiMe3)2 using THF-free Sr amide (prepared by reaction of Sn[N(SiMe3)2]2 with Sr) is preferred. Attempted isolation of the heteroleptic complex tBuAmDIPPSrN(SiMe3)2 by crystallization, precipitation, or removal of all volatiles failed. The only product that we have been able to isolate is the homoleptic complex (tBuAmDIPP)2Sr, which is likely formed by ligand exchange (Schlenk equilibrium). The synthesis of the Sr hydride complex tBuAmDIPPSrH was attempted by reacting in situ generated tBuAmDIPPSrN(SiMe3)2 B

DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

The complex (tBuAmDIPP)2Sr dissolves very well in alkanes or aromatic solvents. In C6D6 it shows a simple 1H NMR spectrum in which the iPr substituent at DIPP gives two signals for the Me substituents (one for the Me groups pointing to the inside and one for the outer Me groups) and one heptet, indicating free rotation of the DIPP substituents around the N−Cipso axis. Reaction of the considerably bulkier amidine pTolAmAr‡-H with Sr[N(SiMe3)2]2 (or its THF adduct) in C6D6 did not proceed at room temperature and needed more forcing conditions. The sluggishness of this deprotonation is likely due to the unusually high steric shielding of the amidine N−H unit. Heating to 60 °C led to immediate conversion; however, the drastic color change from light yellow to dark red suggested that under these conditions the relatively acidic Ph2CH substituents17 were deprotonated. In order to avoid this side reaction and steric stress during ligand deprotonation, we chose to use the weaker, less sterically hindered Sr base Sr[N(SiHMe2)2]2, which can be obtained by reaction of the stronger amide base Sr[N(SiMe3)2]2 with 2 equiv of HN(SiHMe2)2.18 This clearly indicates the lower pKa value of HN(SiHMe2)2 (22.8)19 versus that of HN(SiMe3)2 (25.8).20 Reaction of the amidine pTolAmAr‡-H with Sr[N(SiHMe2)2]2 in C6D6 at 60 °C gave full conversion to the heteroleptic complex pTolAmAr‡SrN(SiHMe2)2, which could be obtained pure by crystallization. The crystal structure of this heteroleptic Sr amidinate complex is shown in Figure 2. The (N,N)-chelating amidinate

in C6D6 with 2 equiv of PhSiH3. At room temperature a fast conversion was observed: the color changed from yellow to orange, and H2 gas was formed. The latter is due to the fact that in situ generation of tBuAmDIPPSrN(SiMe3)2 from tBuAmDIPPH and Sr[N(SiMe3)2]2 also produced the amine HN(SiMe3)2, which reacted with tBuAmDIPPSrH to give tBuAmDIPPSrN(SiMe3)2 and H2. The second equivalent of PhSiH3 is needed to reconvert tBuAmDIPPSrN(SiMe3)2 to tBuAmDIPPSrH. This Sr hydride complex was not stable toward Schlenk equilibria ,and homoleptic (tBuAmDIPP)2Sr was isolated in a yield of 67%. Running this reaction at lower temperatures resulted in longer reaction times but did not avoid ligand exchange to homoleptic species. The crystal structure of homoleptic (tBuAmDIPP)2Sr is shown in Figure 1. This is the first example of a monomeric, Lewis

Figure 1. Crystal structure of (tBuAmDIPP)2Sr. All H atoms have been omitted for clarity.

base free, bis-amidinate Sr complex. Another solvent-free bisamidinate Sr complex (with PhCC in the backbone and iPr substituents at N)11d crystallized as a dimer. There is, however, an example of a solvent-free bis-guanidinate Sr complex that crystallized as a monomer: [iPr2NC(N-DIPP)2]2Sr.12d The current amidinate complex crystallizes, like the latter, as a pseudo-C2-symmetric monomer in which both ligands (N,N)chelate the Sr metal. The Sr−N bond distances vary from 2.477(2) to 2.519(2) Å (average 2.496 Å) and are at the shorter end of the range previously reported for amidinate and guanidinate Sr complexes (2.506−2.646 Å).11,12 This is due to the fact that the Sr metal in (tBuAmDIPP)2Sr is only fourcoordinate. All other complexes contain between one and three ether donor ligands, except for [iPr2NC(N-DIPP)2]2Sr, which also shows short Sr−N interactions (2.506 Å).12d The coordination sphere of Sr is unusual. On account of the ligand geometry and the large metal size the N−Sr−N′ ligand bite angles are small (52.76(6) and 53.31(6)°). This results in large values for the remaining N−Sr−N′ angles: 127.33(6)− 178.13(6)°. More peculiar, half of the Sr coordination sphere is empty. The shortest distance between the iPr Me groups and Sr measures 3.621(3) Å (Sr···C42), which, given the fact that Ca···Me agostic interactions are in the 2.999−3.240 Å range,16 is too long for an extension of the Sr coordination sphere with Sr···Me agostic interactions. It is therefore likely that this peculiar Sr coordination geometry is caused by by ligand− ligand repulsion among the C atoms C16, C26, C42, and C57.

Figure 2. Crystal structure of pTolAmAr‡SrN(SiHMe2)2. For clarity the iPr substituents have been removed, some of the aryl rings are only partially shown by their Cipso atoms, and most of the H atoms have been omitted.

ligand is bound to Sr with Sr−N bonds of 2.576(2) and 2.595(2) Å. The Sr amide contact is much shorter: Sr−N3 = 2.469(2) Å. The amide ligand is additionally bound to Sr by a secondary Si1−H1···Sr interaction of 2.74(3) Å; the Si-bound H atoms were located and isotropically refined. The other Si− H functionality is not bound, and this results in significant tilting of the amide ligand, as demonstrated by strongly differentiated Sr−N3−Si1 (102.15(8)°) and Sr−N3−Si2 (131.9(1)°) angles. The Sr coordination sphere is further saturated by Sr−aryl π interactions with the Ph ring C32−C37 (Sr−C range 3.204(2)−3.418(2) Å, average 3.310 Å) and Ph ring C67−C72 (Sr−C range 3.133(2)−3.525(3) Å, average 3.332 Å). Since the complex pTolAm Ar‡ SrN(SiHMe 2 ) 2 C

DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

The 1H NMR spectrum of complex 7 in THF-d8 is rather complex. Deprotonation of one out of four Ph2CH substituents breaks molecular symmetry and gives rise to a multitude of partially overlapping resonances for the aromatic H atoms (the 13 C NMR spectrum is of similar complexity), but the Me group in the p-tolyl backbone substituent and the iPr substituents each show a single set of resonances. DOSY-NMR (Figure S6) shows that all signals belong to a single species. Complex 7, which could be considered as an amidinate complex with an intramolecular highly reactive trityl-type carbanion, is extremely air sensitive and decomposes by reaction with only traces of air. The latter could be conveniently monitored by immediate loss of its intensely red color. Complex 7 may therefore be active as a self-activating catalyst in the intramolecular alkene hydroamination. The advantage of using 7 in hydroamination catalysis is the fact that the formation of disturbing side products is avoided. For example, using 1-Sr (Scheme 1) as a catalyst for the intramolecular alkene hydroamination also generates 1 equiv of HN(SiMe3)2 during initiation. The latter, relatively acidic amine may be deprotonated by catalytic intermediates, thus retarding catalysis.21 Calculations on a catalyst with a Ca− N(SiMe3)2 active group show that reaction with a primary alkylamine indeed is endergonic by nearly 18 kcal/mol.22 Initiation with complex 7 would prevent formation of acidic HN(SiMe3)2. Preliminary catalytic runs for the conversion of several amino-alkene substrates, H2NCH2CR2CH2CHCH2 (CR2 = CPh2, CMe2, CCy), gave already at room temperature fast and full conversion to the cyclization products within 10, 25, and 60 min, respectively (10 mol % of the catalyst in a 5/1 mixture of C6D6 and THF). This demonstrates that, despite the significant bulk of this ligand, complex 7 can be a competitive catalyst for alkene hydroamination. The scope of catalyst 7 and other catalysts in which the ligand acts cooperatively is currently under investigation.

crystallizes also in the presence of Et2O and THF without any coordinating Lewis bases, these secondary Si−H···Sr and Ph··· Sr interactions are of considerable importance. The complex pTolAmAr‡SrN(SiHMe2)2 dissolves well in C6D6. 1H and 13C NMR spectra show only one set of signals for the p-tolyl backbone group, the iPr substituents, and the SiH and SiMe2 groups. The aromatic region is complex, but the Ph2CH substituents display only one singlet for the benzylic proton. This indicates that the Sr···aryl interactions are weak and dynamic. The heteroleptic complex pTolAmAr‡SrN(SiHMe2)2 is astoundingly stable toward Schlenk equilibria or decomposition by deprotonation of the Ph2CH substituents. Heating a C6D6 solution of pTolAmAr‡SrN(SiHMe2)2 for several days to 60 °C did not lead to any changes in the 1H NMR spectrum. Reaction of pTolAmAr‡SrN(SiHMe2)2 with PhSiH3 in C6D6 at room temperature led to an immediate color change from yellow to dark red. This color change, which is indicative of deprotonation of the Ph2CH substituents, could not be avoided by running the reaction at lower temperatures (−30 °C). Under a variety of reaction conditions and after recrystallization from a hexane/THF mixture, repeatedly the highly air sensitive dark red complex 7 is formed in a yield up to 42%. Since PhH2SiN(SiHMe2)2 was observed as a byproduct, it is presumed that complex 7 is formed through an intermediate hydride complex: pTolAmAr‡SrH.13 The crystal structure of 7 is shown in Figure 3. The superbulky amidinate ligand is (N,N)-chelating: Sr−N1 =

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CONCLUSIONS Amidinate ligands can be used to stabilize heteroleptic strontium complexes, provided there are bulky substituents. An amidinate ligand with a tBu substituent on the backbone C and two larger DIPP substituents at the N atoms, tBuAmDIPP, gave access to the heteroleptic amide complex tBuAmDIPPSrN(SiMe3)2, which on account of facile ligand scrambling could not be isolated. Further conversions were carried out with the in situ generated product. Any attempts to convert it to the hydride complex tBuAmDIPPSrH led to immediate Schlenk equilibria, and after precipitation of SrH2 the homoleptic complex (tBuAmDIPP)2Sr could be isolated. This is the first example of a monomeric, Lewis base free bis-amidinate Sr complex. The much bulkier amidine pTolAmAr‡-H, with a ptolyl substituent at the backbone C and two 2,6-Ph2CH-phenyl (Ar‡) substituents at the two N atoms, is considerably more sterically hindered and therefore rather difficult to deprotonate. Under the harsh reaction conditions the Ph2CH substituents were also deprotonated by the Sr[N(SiMe3)2]2 base (as was evident from a dark red color). Deprotonation with the smaller, less basic base Sr[N(SiHMe2)2]2 was successful and led to isolation of the heteroleptic Sr complex pTolAmAr‡SrN(SiHMe2)2. The latter crystallized even in the presence of THF or Et2O without any coordinated solvents, clearly demonstrating the bulk of this amidinate ligand. Part of the coordination sphere is filled with aryl···Sr and Si−H···Sr interactions. Attempts to convert pTolAmAr‡SrN(SiHMe2)2 to

Figure 3. Crystal structure of 7. For clarity the iPr substituents have been removed, some of the aryl rings are only partially shown by their Cipso atoms, and all of the H atoms have been omitted.

2.537(2) Å and Sr−N2 = 2.547(2) Å. The coordination sphere is further filled by two THF ligands: Sr−O1 = 2.541(2) Å and Sr−O2 = 2.520(2) Å. There is a weak π interaction with C56 in a neutral Ph ring (3.360(2) Å) and a much stronger π interaction with the Ph ring C16−C21: range 2.828(3)− 3.033(3) Å, average 2.950 Å. The Sr···C distances for this (η6)Ar···Sr interaction are much shorter than those observed in tBuAmAr‡SrN(SiHMe2)2 (Figure 2). This is due to delocalization of the negative charge on C15 into the C16−C21 ring. Charge delocalization is supported by the observation that the C15−C16 bond distance of 1.408(3) Å is much shorter than the remaining Cα−Cipso distances: 1.524(3)−1.535(3) Å, average 1.529(3) Å. The Sr−C15 distance of 3.499(3) Å is too long to be considered a strong bonding interaction. D

DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

pressure, filtered, and crystallized at −30 °C, thus yielding the ligand pTolAmAr‡-H as colorless plates with 0.5 equiv of cocrystallized diethyl ether. Yield: 9.69 g (9.03 mmol; 62%). 1 H NMR (400 MHz, C6D6, 25 °C): δ 0.97 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.00 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.12 (t, J = 7.0 Hz, 3H, (CH3CH2)2O), 1.87 (s, 3H, pTol−CH3), 2.45−2.57 (m, 2H, CH(CH3)2), 3.27 (q, J = 7.0 Hz, 2H, (CH3CH2)2O), 5.30 (bs, 1H, NH), 6.23 (d, J = 8.3 Hz, 2H, pTol-CH), 6.37 (s, 2H, CHPh2), 6.41 (s, 2H, Ar‡-o-CHarom), 6.67 (m, 2H, Ar-H), 6.86−7.20 (m, 44 H, Ar-H). 13 C{1H} NMR (151 MHz, C6D6, 25 °C): δ 15.6 (CH3CH2)2O), 21.1 (p-tol-CH3), 24.1 (CH(CH3)2), 33.6, (CH(CH3)2) 34.0, (CH(CH3)2), 52.0 (CHPh2), 53.8 (CHPh2), 65.9 (CH3CH2)2O), 125.5 (CHarom), 125.6 (CHarom), 126.2 (CHarom), 126.3 (CHarom), 126.8 (CHarom), 127.4 (CHarom), 127.9 (CHarom), 128.0 (CHarom), 128.1 (CHarom), 128.2 (CHarom), 128.3 (CHarom), 128.7 (CHarom), 129.0 (CHarom), 129.6 (CHarom), 129.8 (CHarom), 130.1 (CHarom), 132.4 (Carom), 134.5 (Carom), 136.5 (Carom), 138.5 (Carom), 141.7 (Carom), 143.4 (Carom), 143.7 (Carom), 143.9 (Carom), 144.5 (Carom), 145.8 (Carom), 146.3 (Carom), 147.5 (Carom), 155.5 (NCN). Anal. Calcd for C78H70N2· 0.5Et2O (1072.50 g/mol): C, 89.59; H, 7.05; N, 2.61. Found: C, 89.47; H, 7.08; N, 2.55. In situ Preparation of tBuAmDIPPSrN(SiMe3)2 by Reaction of Sr[N(SiMe3)2]2 with tBuAmDIPP-H. A Schlenk flask was charged with tBuAmDIPP-H (200 mg, 0.475 mmol) and Sr[N(SiMe3)2]2 (203.9 mg, 0.499 mmol), and the solids were dissolved in 6.0 mL of benzene. The reaction mixture was heated to 60 °C overnight, resulting in a slight shift in color from bright yellow to pale orange. Running the same reaction in deuterated solvents allowed for characterization of the reaction mixture by NMR methods. 1 H NMR (400 MHz, C6D6, 25 °C): δ 0.00 (s, 18H, Si(CH3)2), 1.14 (d, J = 7.0 Hz, 12H, CH(CH3)2), 1.27 (d, J = 6.7 Hz, 12H, CH(CH3)2), 1.47 (s, 9H, C(CH3)3), 3.25 (sept, J = 6.9 Hz, 4H, CH(CH3)2, 6.85−6.91 (m, 2H, p-Ar-H), 6.95−6.99 (m, 5H, o-Ar-H). 13 C{1H} NMR (151 MHz, C6D6, 25 °C): δ 6.0 (Si(CH3)2), 23.9 (CH(CH3)2), 28.6 (CH(CH3)2), 31.7 (C(CH3)3), 42.4 ((C(CH3)3), 121.7 (Carom), 125.1 (Carom), 142.6 (Carom), 153.7 (Carom), 169.4 (NCN). All attempts to isolate the complex either by crystallization or evaporation of all volatiles led to Schlenk equilibria and a mixture of homoleptic and heteroleptic products. Therefore, the in situ prepared tBuAmDIPPSrN(SiMe3)2 has been used for further conversion. Synthesis of (tBuAmDIPP)2Sr. In a Schlenk flask, charged with in situ prepared tBuAmDIPPSrN(SiMe3)2 (328 mg, 0.491 mmol) in 6.0 mL of benzene, was placed PhSiH3 (133 μL, 1.081 mmol, 2.2 equiv). Upon addition hydrogen evolution could be observed. The latter is due to the fact that in situ generation of tBuAmDIPPSrN(SiMe3)2 from tBuAmDIPP-H and Sr[N(SiMe3)2]2 also produces the amine HN(SiMe3)2, which can react with tBuAmDIPPSrH to give tBuAmDIPPSrN(SiMe3)2 and H2. The second equivalent of PhSiH3 reconverts tBuAmDIPPSrN(SiMe3)2 to tBuAmDIPPSrH. Conversion was accompanied by a color change from bright yellow to light orange and precipitation of insoluble SrH2. The reaction mixture was left at 60 °C overnight. The solvent was removed under reduced pressure, and the residue was dissolved in 5 mL of pentane and then stored at −30 °C. The complex (tBuAmDIPP)2Sr could be isolated in the form of colorless block-shaped crystals. Yield: 305.6 mg (0.329 mmol, 67%). 1 H NMR (400 MHz, C6D6, 25 °C): δ 0.86−0.95 (m, 24H, CH(CH3)2), 0.94 (s, 18H, C(CH3)3), 1.28 (d, J = 6.9 Hz, 24H, CH(CH3)2), 3.41 (sept, J = 7.0 Hz, 8H, CH(CH3)2), 6.95−7.02 (m, 12H, Ar-H). 13C{1H} NMR (151 MHz, C6D6, 25 °C): δ 22.1 (CH(CH3)2), 26.3 (CH(CH3)2), 28.7 (CH(CH3)2), 30.7 (C(CH3)3) 45.6 ((C(CH3)3), 122.3 (Carom), 123.2 (Carom), 140.2 (Carom), 146.7 (Carom), 173.1 (NCN). Anal. Calcd for C58H86N4Sr (926.98 g/mol): C, 75.15; H, 9.35; N, 6.04. Found: C, 74.99; H, 9.45; N, 6.05. Synthesis of pTolAmAr‡SrN(SiHMe2)2·2(toluene). A Schlenk flask was charged with Sr[N(Si(H)Me2)2]2 (215 mg, 0.610 mmol) and pTolAmAr‡-H (623 mg, 0.581 mmol). The reactants were suspended in 8.0 mL of toluene and stirred at 60 °C for 3 days. The yellowish solution was cooled to room temperature, concentrated to ca. 4 mL, and slowly cooled to −30 °C. The product was obtained as an off-

a Sr hydride complex by reactions with PhSiH3, led in all cases to immediate deprotonation of the Ph2CH substituent, as was evident from a color change to dark red. The product 7, an amidinate complex with an intramolecular Ph2(aryl)C− anion, was isolated and could be structurally characterized. It is extremely air sensitive and, despite the bulk of the amidinate ligand, can be used as a catalyst for the intramolecular alkene hydroamination.

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EXPERIMENTAL SECTION

General Considerations. All experiments were carried out in dry glassware under N2 using standard Schlenk techniques and freshly dried and degassed solvents (all solvents were dried over a column, except for THF, which was dried over Na and redistilled). The following reagents were obtained commercially: potassium bis(trimethylsilyl)amide (Sigma-Aldrich), SnCl 2 (Sigma-Aldrich), PhSiH3 (Sigma-Aldrich), iodine, 99% (ABCR), strontium (ABCR), 1,1,3,3-tetramethyldisilazane (ABCR), diphenylmethanol (SigmaAldrich), 2,6-diisopropylaniline 97% (ABCR), and p-toluoyl chloride (Sigma-Aldrich). The starting materials tBuAmDIPP-H,14 Sr[N(SiMe3)2]2,23 Sr[N(Si(H)Me2)2]2,18 and 2,6-Ph2CH-aniline24 have been prepared according to literature procedures. NMR spectra were measured on Bruker Avance III HD 400 MHz and Bruker Avance III HD 600 MHz spectrometers. Elemental analysis was performed with an Hekatech Eurovector EA3000 analyzer. All crystal structures have been measured on a SuperNova (Agilent) diffractometer with dual Cu and Mo microfocus sources and an Atlas S2 detector. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 1586616 ((tBuAm DIPP ) 2 Sr), 1586617 (pTolAm Ar‡ SrN(SiHMe 2 ) 2 ), and 1586618 (complex 7). Synthesis of pTolC(O)N(H)Ar‡. A mixture of 4-methylbenzoyl chloride (2.23 mL, 16.9 mmol), Ar‡NH2 (7.89 g, 16.9 mmol), and triethylamine (2.36 mL, 16.9 mmol) was dissolved in 500 mL of dichloromethane and heated to reflux for 3 days. The reaction mixture was cooled to room temperature and was washed three times with 100 mL of an aqueous solution of NaHCO3. The organic phase was dried over MgSO4, filtered ,and evaporated under reduced pressure, yielding a white powder. Yield: 8.54 g (14.6 mmol; 86.4%). 1 H NMR (400 MHz, CDCl3, 25 °C): δ 1.02 (d, J = 6.9 Hz, 6H, CH(CH3)2), 2.38 (s, 3H, pTol−CH3), 2.69 (sept, J = 6.9 Hz, 1H, CH(CH3)2), 5.66 (s, 2H, CHPh2), 6.42 (s, 1H, NH), 6.65 (s, 2H, Ar‡o-CHarom), 6.8−7.3 (m, 24H, CHarom) ppm. 13C{1H} NMR (151 MHz, CDCl3, 25 °C): δ 21.6 (pTol-CH3), 23.9 (CH(CH3)2), 33.9 (CH(CH3)2), 52.9 (CHPh2), 126.5 (CHarom), 126.9 (Ar‡-o-CHarom), 127.1 (CHarom), 128.5 (CHarom), 129.1 (CHarom), 131.1 (Carom), 131.6 (Carom), 142.1 (Carom), 142.3 (Carom), 143.2 (Carom), 147.8 (Carom), 165.6 (NCO) ppm. Anal. Calcd for C43H39NO (585.79 g/mol): C, 88.17; H, 6.71; N, 2.39. Found: C, 88.50; H, 6.65; N, 2.28. Synthesis of pTolAmAr‡-H·0.5Et2O. A round-bottom flask was dried carefully and charged with pTolC(O)N(H)Ar‡ (8.54 g, 14.6 mmol) and PCl5 (3.64 g, 17.5 mmol). The solids were suspended in 120 mL of toluene and heated to 60 °C, yielding a clear solution within 1 h. The mixture was heated for 3 days, and the reaction progress was monitored by 1H NMR spectroscopy. Upon completion of the reaction, all volatiles were removed under reduced pressure, yielding the chlorinated intermediate pTolC(Cl)NAr‡ as a waxy solid. Subsequently, Ar‡NH2 (6.82 g, 14.6 mmol) was added, and both reagents were suspended in 300 mL of toluene and heated to reflux for 3 days. Upon completion of the reaction, all volatiles were removed and the residue was dissolved in 200 mL of dichloromethane and washed three times with 100 mL of an aqueous solution of NaHCO3. Dichloromethane was removed under reduced pressure, leaving a mixture of the desired ligand pTolAmAr‡-H and the side product pTolC(O)N(H)Ar‡. The mixture was suspended in 750 mL of boiling Et2O, filtered, cooled to room temperature, and stored at +8 °C overnight. The side product pTolC(O)N(H)Ar‡ was removed via fractional crystallization at decreasing temperatures, yielding a solution of pTolAmAr‡-H in Et2O, which was concentrated under reduced E

DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Accession Codes

white material with cocrystallized toluene. Yield: 610.1 mg (0.424 mmol, 73%). 1 H NMR (400 MHz, C6D6, 25 °C): δ 0.37 (d, J = 2.8 Hz, 12H, Si(H)(CH3)2), 0.87 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.83 (s, 3H, ptol-CH3), 2.12 (s, 6H, toluene-CH3), 2.40 (sept, J = 6.9 Hz, 2H, CH(CH3)2), 4.76 (sept, J = 2.7 Hz, 2H, Si(H)(CH3)2), 6.17 (s, 4H, CHPh2), 6.37 (d, J = 7.8 Hz, 2H, p-tol-CH), 6.81 (s, 4H, Ar‡-m-CH), 6.94−7.34 (m, 52H, C-Harom). 13C{1H} NMR (151 MHz, C6D6, 25 °C): δ 5.2 (Si(H)(CH3)2), 21.1 (p-tol-CH3), 21.4 (toluene-CH3), 24.0 (CH(CH3)2), 33.5 (CH(CH3)2), 52.4 (CHPh2), 125.7 (CHtoluene), 126.4 (CHarom), 127.4 (CHarom), 127.5 (CHarom), 127.7 (CHarom), 128.4 (CHarom), 128.6 (CHarom), 128.6 (CHtoluene), 129.3 (CHtoluene), 129.4 (CHarom), 130.1 (CHarom), 130.5 (CHarom), 130.9 (CHarom), 132.6 (Carom), 136.2 (Carom), 137.9 (Ctoluene), 138.0 (Carom), 141.2 (Carom), 144.0 (Carom), 147.3 (Carom), 147.7 (Carom), 170.1 (NCN). Anal. Calcd for C96H99N3Si2Sr (1438.66 g/mol): C, 80.15; H, 6.94; N, 2.92. Found: C, 79.89; H, 6.97; N, 2.72. Synthesis of 7. A Schlenk flask was charged with pTolAmAr‡SrN(SiHMe2)2·2(toluene) (250 mg, 0.174 mmol), dissolved in 5.0 mL of benzene and 0.5 mL of THF, yielding a clear slightly yellow solution. Addition of PhSiH3 (21.4 μL, 18.8 mg, 0.174 mmol, 1.0 equiv) led to an immediate color change from slight yellow to an intensely dark red, and the solution was stirred for 1 h at 60 °C. Subsequently, the solvents were removed under reduced pressure and the crude product was washed with 2 × 3 mL of hexane. The red powder was recrystallized from THF at −30 °C, and the product was obtained as dark red crystalline blocks. It crystallized with two coordinated THF ligands and one free THF in the crystal lattice. Yield: 97.7 mg (0.073 mmol; 42%). 1 H NMR (400 MHz, THF-d8, 25 °C): δ 0.96 (d, J = 6.9 Hz, 3H, CH(CH3)2), 0.95 (d, J = 6.9 Hz, 3H, CH(CH3)2), 1.04 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.06 (d, J = 6.9 Hz, 3H, CH(CH3)2), 2.21 (s, 3H, ptol-CH3), 2.46 (sept, J = 7.2 Hz, 1H, CH(CH3)2), 2.68 (sept, J = 7.0 Hz, 1H, CH(CH3)2), 5.14 (t, J = 6.5 Hz, 1H, p-Ar-H), 5.36 (s, 1H, CHPh2), 5.53−5.57 (m, 1H, Ar-H), 5.65 (s, 1H, CHPh2), 5.72 (d, J = 7.9 Hz, 2H, Ar-H), 5.75 (d, J = 7.5 Hz, 1H, Ar-H), 5.87−5.92 (m, 1H, Ar-H), 6.02−6.10 (m, 1H, Ar-H), 6.54 (s, 1H, CHPh2), 6.40−7.30 (m, 37H, Ar-H), 7.45−7.50 (m, 1H, Ar-H), 7.51−7.60 (m, 4H, Ar-H). 13 C{1H} NMR (151 MHz, THF-d8, 25 °C): δ 20.2 (p-tol-CH3), 23.4 (CH(CH3)2), 23.5 (CH(CH3)2), 23.6 (CH(CH3)2), 24.0 (CH(CH3)2), 33.3 (CH(CH3)2), 33.5 (CH(CH3)2), 50.3 (CHPh2), 51.3 (CHPh2), 51.6 (CHPh2), 96.1 (CPh2), 98.1 (CHarom), 109.9 (CHarom), 115.4 (CHarom), 118.6 (CHarom), 123.8 (m-Ar‡-CHarom), 124.6 (CHarom), 124.6 (CHarom), 125.2 (CHarom), 125.4 (CHarom), 125.9 (CHarom), 126.1 (CHarom), 126.2 (CHarom), 126.2 (CHarom), 126.3 (CHarom), 126.5 (CHarom), 126.5 (m-Ar‡-CHarom), 127.2 (p-tolCHarom), 127.3 (CHarom), 127.4 (CHarom), 127.5 (CHarom), 127.5 (CHarom), 128.3 (CHarom), 128.8 (CHarom), 129.2 (CHarom), 129.5 (CHarom), 129.6 (m-Ar‡-CHarom), 129.7 (CHarom), 130.0 (CHarom), 130.0 (CHarom), 130.2 (CHarom), 131.5, (CHarom), 131.6, (Carom), 131.8 (CHarom), 134.3(Carom), 136.3 (Carom), 136.7 (CH3Carom), 139.4 (pAr‡-Carom), 139.8 (Carom), 141.3 (p-Ar‡-Carom), 142.5 (Carom), 144.5(Carom), 145.0 (Carom), 145.3 (Carom), 146.2 (Carom), 146.3 (Carom), 146.6 (Carom), 147.5 (Carom), 148.4 (Carom), 149.4 (Carom), 167.0 (NCN). Anal. Calcd for C78H69N2Sr·3THF (1122.04 g/mol): C, 80.83; H, 6.93; N, 2.09. Found: C, 80.82; H, 6.83; N, 2.13.

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CCDC 1586616−1586618 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.H.: [email protected]. ORCID

Sjoerd Harder: 0000-0002-3997-1440 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Christina Wronna for multiple elemental analyses on challenging highly sensitive complexes.

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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.7b00871. Crystallographic details, selected 1H and 13C NMR spectra, and DEPT-135, HSQC, HMBC and DOSY spectra (PDF) F

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DOI: 10.1021/acs.organomet.7b00871 Organometallics XXXX, XXX, XXX−XXX