This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article pubs.acs.org/Organometallics
Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands Andrea Causero, Gerd Ballmann, Jürgen Pahl, Harmen Zijlstra, Christian Far̈ ber, and Sjoerd Harder* Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: A range of symmetric amidinate ligands RAmAr (R is backbone substituent, Ar is N substituent) have been investigated for their ability to stabilize calcium hydride complexes of the type RAmArCaH. It was found that the precursors of the type RAmArCaN(SiMe3)2 are only stable toward ligand exchange for Ar = DIPP (2,6-diisopropylphenyl). The size of the backbone substituent R determines aggregation and solvation. The following complexes could be obtained: [RAmDIPPCaN(SiMe3)2]2 (R = Me, p-Tol), RAmDIPPCaN(SiMe3)2·Et2O (R = Np, tBu), AdAmDIPPCaN(SiMe3)2·THF, and AdAmDIPPCaN(SiMe3)2. Reaction of these heteroleptic calcium amide complexes with PhSiH3 gave only for larger backbone substituents (R = tBu, Ad) access to the dimeric calcium hydride complexes (RAmArCaH)2. (N,aryl)-coordination of the amidinate ligand seems crucial for the stability of these complexes, and the aryl···Ca interaction is found to be strong (17 kcal/mol). Addition of polar solvents led to a new type of trimeric calcium hydride complex exemplified by the crystal structures of (tBuAmDIPPCaH)3·2Et2O and (AdAmDIPPCaH)3·2THF. The overall conclusion of this work is that minor changes in sterics (tBu vs Ad) or coordinated solvent (THF vs Et2O) can have large consequences for product formation and stability.
■
INTRODUCTION Metal hydride complexes are well-known for their wide range of applications in catalysis and reduction chemistry. Although this chemistry has long been a prerogative of transition-metal and late-main-group-metal chemists, early-main-group-metal hydride complexes have received a growing popularity.1 This current interest is especially for the alkaline-earth-metal hydrides fueled by applications. Calcium hydride complexes are key intermediates in alkene hydrosilylation2 or hydrogenation3 catalysis, whereas magnesium hydride complexes can be intermediates in pyridine hydroboration,4 although this is controversial.5 Larger magnesium hydride clusters are of particular interest as molecular model systems for the hydrogen storage material MgH2.6 Despite the current interest in group 2 metal hydride complexes, their syntheses and isolation are challenging. They display considerable ionic bond character7 which translates to weakly bound ligands and ligand exchange. This is the origin of the Schlenk equilibrium (Scheme 1a) established for Grignard reagents already at an early stage by Schlenk.8 Such ligand scrambling is fatal for the isolation of well-defined LAeH complexes (Ae = alkaline-earth metal). The enormous lattice energies for saline (AeH2)∞ range from 2171 to 3205 kJ/mol9 and lead to immediate precipitation of AeH2, redirecting the equilibrium to the homoleptic L2Ae complexes. Although lattice energies for (AeH2)∞ increase with decreasing cation size, difficulties in isolating LAeH complexes counterintuitively © XXXX American Chemical Society
Scheme 1. (a) Schlenk Equilibrium Leading to Insoluble (AeH2)∞ Salts and (b) Steering the Schlenk Equilibrium to the Heteroleptic Side by Using Bulky Spectator Ligands
increase with increasing metal size in the order Be < Mg < Ca < Sr < Ba. Various examples of beryllium hydride complexes were already reported in the middle of the last century.10 Defined magnesium hydride complexes have been known since 2008,11 and hitherto at least 20 further crystal structures have been reported.12 The first structurally established calcium Received: July 15, 2016
A
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 2. Synthetic Routes to Calcium Hydride Complexes 1−3
amide-triamine ligand Me3TACD− to generate the cationic calcium hydride complex (Me3TACD)3Ca3H2+ (2; Scheme 2) using the “silane method”.14 Very recently, a hydride-rich cationic calcium complex of a similar but neutral ligand, Me4TACD, has been reported: (Me4TACD)2Ca2H3+ (3).15 The latter was generated in situ by hydrogenation of a calcium silanide complex with H2. It is of interest to note that use of multidentate Me3TACD or Me4TACD hitherto always gave cationic calcium hydride complexes. This is likely due to strong, multiple coordination of the aza-crown ligands, causing dissociation of either a hydride ligand (captured by phenylsilane) or a resonance-stabilized Ph3Si− anion. This clearly demonstrates the weakness of Ca−ligand bonds and demonstrates that for isolation of neutral complexes the ligand denticity should not be too high. Given the importance of calcium hydride complexes as intermediates in Ca-catalyzed reactions, there is 10 years after the isolation of the first calcium hydride complex only very little known about this compound class. This holds especially for neutral well-defined calcium hydride complexes, of which only one example has been reported to date.13 In order to broaden our knowledge of calcium hydride chemistry, we actively follow the goal to extend this series of complexes with new examples based on new ligands. Herein, we report our recent investigations on using amidinate ligands for the stabilization of neutral calcium hydride complexes.
hydride complex dates from 2006,13 and only since 2012 have two further structural motives been reported.14,15 To the best of our knowledge, there is currently no access to strontium and barium hydride complexes and crystal structures of well-defined complexes are hitherto unknown. Problems in preparing “heavy” alkaline-earth-metal hydride complexes arise from their inherently weak metal−ligand bonds. As ionic character and bond lengths increase along the row in the order Be < Mg < Ca < Sr < Ba, bond energies decrease along the same row in the order Be > Mg > Ca > Sr > Ba. Ligand scrambling according to the Schlenk equilibrium is therefore faster and easier for the heavier metals, which explains the difficulties in isolating their hydride complexes. The key to the synthesis of the heavier alkaline-earth metal hydride complexes is the in situ generation of a heteroleptic LAeH complex and control over the Schlenk equilibrium by the spectator ligand L (Scheme 1b). The following requirements are imposed on the ligand: (i) it should encapsulate the metal and be bulky in order to prevent formation of L2Ae, and thus insoluble AeH2, by steric stress and (ii) it should be strongly bound and therefore preferably bi- or multidentate, in order to avoid ligand scrambling. Achieving these goals is increasingly challenging for the larger metals (Ca, Sr, Ba). Using the bulky β-diketiminate ligand DIPP-nacnac, Harder and co-workers isolated the first example of a well-defined calcium hydride complex (1; Scheme 2).13 Complex 1 was generated in situ by the reaction of (DIPPnacnac)CaN(SiMe3)2·THF with PhSiH3. This “silane method”, known from lanthanide chemistry, has now been proven very successful in group 2 chemistry. In 2012 Okuda and coworkers introduced the monoanionic tetradentate macrocyclic
■
RESULTS AND DISCUSSION The very versatile N,N-bidentate amidinate (or related guanidinate) ligands are widely used in organometallic chemistry.16 They are easily accessible, can be tuned by B
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
of (N, aryl)-coordination allows for efficient encapsulation of even large metal cations such as K+, Ba2+, Yb2+, and Sm2+.17 Recent work by the Westerhausen group has shown a similar coordination mode in calcium amidinate chemistry.18 The strategy followed in synthesizing heteroleptic calcium hydride complexes supported by the amidinate ligand is shown in Scheme 3b (the amidinate ligand is abbreviated by RAmAr in which R represents the backbone substituent and Ar the N substituents). The first target is syntheses of a range of heteroleptic RAmArCaN(SiMe3)2 complexes. In a second step, these will be converted to hydride complexes by the silane route. The influence of substituents R and Ar on the Schlenk equilibrium will be evaluated. Synthesis and Structures of Heteroleptic Intermediates: RAmArCaN(SiMe3)2. Using standard methods, two new amidine ligands have been prepared (R = Np, Ar = DIPP and R = Ad, Ar = DIPP) and four ligands were synthesized according to the literature. To map the steric effects of the backbone substituent (R) and the N substituents (Ar) on the stability of the product, we chose substituents of varying sizes as shown in Scheme 4. Conversion of the amidine ligand RAmArH into RAmArCaN(SiMe3)2 can be achieved by reacting it with 2 equiv KN(SiMe3)2 and CaI2 (route A, Scheme 3b). This is the most direct one-pot synthesis developed by Chisholm et al. for the synthesis of the analogue DIPPnacnacCaN(SiMe3)2·THF complex19 and has been employed successfully for the syntheses of amidinate calcium amide complexes by the groups of Westerhausen18 and Cui.20 We introduce here the direct deprotonation route in which RAmArH is deprotonated by Ca[N(SiMe3)2]2·2Et2O (route B). Although most Ca amidinate complexes are prepared in THF, we use the more volatile and less coordinating solvent Et2O. This may allow for removal of coordinated solvent in the final product, as has been shown before in Ca chemistry,21 and gives the additional possibility of controlling product composition by removal/addition of a Lewis base.
variation of backbone and/or N substituents, and, being bidentate, are often used when strongly bound sterically encumbered ligands are needed. In contrast to the βdiketiminates, their bite angle is somewhat smaller due to a decrease of the six- to four-membered metallacycle. Their enormous flexibility in coordination modes, however, easily compensates for this shortcoming (Scheme 3a). The possibility Scheme 3. (a) Representative Coordination Modes of the Amidinate Ligand and (b) Potential Synthetic Routes toward Heteroleptic RAmArCaH Complexes
Scheme 4. Effect of the Backbone Substituent (R) and the N Substituents (Ar) on the Nature and Stability of RAmArCaN(SiMe3)2 Intermediates and the Final Product RAmArCaHa
a
Abbreviations: p-Tol = p-tolyl, Np = neopentyl, Ad = 1-adamantyl, Mes = mesityl, DIPP = 2,6-diisopropylphenyl. C
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics We were able to isolate five new heteroleptic calcium amide complexes supported by an amidinate ligand and characterized these by 1H and 13C NMR as well as by X-ray diffraction (vide infra). The use of a DIPP substitituent at N is absolutely essential for the stabilization of complexes of the type RAmArCaN(SiMe3)2. Attempted synthesis of a heteroleptic species with a Mes at N gave the homoleptic complex (tBuAmMes)2Ca·Et2O, for which a crystal structure was determined (Supporting Information). The size of the backbone substituent R is less critical for stabilization of the heteroleptic calcium amide complexes and can vary between Me and Ad. It has, however, a strong influence on the composition and structure of the calcium amide intermediates. Small substituents (R = Me, p-Tol) gave Et2O-free dimeric products in which the amido groups bridge two Ca2+ ions. Larger substituents (R = Np, tBu) gave monomeric products of the type RAmDIPPCaN(SiMe3)2·Et2O. The largest backbone substituent (R = Ad) gave an Et2O-free heteroleptic complex in which the ligand is bound by (N,aryl)-coordination. With few exceptions, all complexes can be obtained by routes A and B. The complex [MeAmDIPPCaN(SiMe3)2]2 could only be isolated by the direct deprotonation route B. The one-pot salt metathesis route A gave the ate complex (MeAmDIPP)[N(SiMe3)2]2CaK (see the Supporting Information for the crystal structure). Formation of K-containing ate complexes is often a problem of the salt-metathesis route. 22 The complex AdAmDIPPCaN(SiMe3)2 could only be obtained in good yields by using method B (method A gave poor yields of 5−14%). The direct deprotonation route B is therefore the method of choice. Representative crystal structures of the three types of heteroleptic calcium amide complexes are shown in Figure 1; selected geometrical data can be found in Table 1. Despite crystallization from Et2O, the complex AdAmDIPPCaN(SiMe3)2 represents an unusual solvent-free product in the series (Figure 1a). The (N,aryl)-chelating ligand could be regarded as an amide/imine ligand with a localized negative charge on N2. This is corroborated by a long N1−C (1.348(3) Å) and short N2−C (1.319(2) Å) bond. Saturation of calcium’s coordination sphere is achieved partially by aryl coordination (Ca···C distances range from 2.745(2) to 2.925(2) Å) but also by a SiMe···Ca agostic interaction (Ca···C 2.823(2) Å). This causes the amide ligand to coordinate asymmetrically as shown by the significantly different distances Ca···Si1 3.1396(8) Å/Ca···Si2 3.6075(8) Å and angles Ca−N3−Si1 104.31(9)°/Ca−N3−Si2 130.5(1)°. The structure of solvent-free DIPPnacnacCaN(SiMe3)2 shows a similar asymmetric coordination of the amide ligand. 21 It is of interest to note that solvent-free AdAmDIPPCaN(SiMe3)2 can only be obtained by synthesis in Et2O. The reaction in THF gave a product of composition AdAmDIPPCaN(SiMe3)2·THF, in which the amidinate ligand is (N,N)- instead of (N,aryl)-chelating (see Supporting Information). This shows that small changes in solvent polarity can have large effects on the structure of the product. A small change of the backbone substituent R from Ad to tBu gives a dramatic change in product composition (Figure 1b). It crystallizes as a monomer with an (N,N)-chelating amidinate ligand and is solvated by an additional Et2O ligand. The geometry of tBuAmDIPPCaN(SiMe3)2·Et2O is closely related to that of tBuAmDIPPCaN(SiMe3)2·THF recently reported by Westerhausen and co-workers.18 Further decrease of the backbone substituent results again in solvent-free products that crystallize as dimers with bridging
Figure 1. Crystal structures of (a) AdAmDIPPCaN(SiMe3)2, (b) tBuAmDIPPCaN(SiMe3)2·Et2O, and (c) [MeAmDIPPCaN(SiMe3)2]2 (iPr groups omitted for clarity).
amide ligands, as shown for example by the structure of [MeAmDIPPCaN(SiMe3)2]2 (Figure 1c). Although this dimer lacks crystallographic symmetry, it is pseudo C2 symmetric (axis through N3 and N4). The unusual Ca coordination geometry, which is strongly distorted from tetrahedral, is noteworthy. This is due to a twist of the amidinate ligand: the orientation between the planes defined by N1−Ca1−N2 and N3−Ca1− N4 deviates strongly from an expected perpendicular arrangement. Instead, an interplanar angle of 53.69(8)° is found. This is likely caused by repulsion between bulky amidinate and amide ligands. This series of structures nicely demonstrates that DIPP substituents are needed to stabilize heteroleptic complexes and that secondary effects from the backbone substituent R are decisive for product composition. A large backbone substituent pushes the DIPP groups forward, thus creating a bulkier ligand. In the extreme case, ligand coordination switches from (N,N)chelation to (N,aryl)-chelation. Angles and bond distances for each structure type are given in Table 1 and support this conclusion. It can be generally observed that N−C−N′ angles decrease with a larger backbone substituent R. The C−N−CAr angles increase with increasing bulk of R. This affects amidinate−Ca coordination: the Ca−N−CAr angles decrease with increasing bulk of R, thus showing the important D
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Table 1. Selected Bond Distances (Å) and Angles (deg) for Crystal Structures of Heteroleptic RAmArCaN(SiMe3)2 Complexes R
Ca−NAm
Ca−NTMS
Ad
2.397(2)
2.264(2)
tBu
2.375(1) 2.371(1) 2.366(2) 2.375(2)
2.282(1)
2.429(2) 2.430(2) 2.424(1) 2.418(1) 2.418(1) 2.409(1)
2.465(2) 2.480(2) 2.497(1) 2.514(1) 2.500(1) 2.490(1)
Np
p-Tol Me
2.275(2)
NAm−C−NAm′
C−N−CAr
Unsolvated Monomer 119.8(2) 131.0(2) Et2O-Solvated Monomer 112.4(1) 129.6(1) 127.4(1) 115.0(2) 122.7(2) 120.7(2) Unsolvated Dimer 115.5(2) 122.0(2) 122.9(2) 117.3(1) 118.5(1) 116.6(1) 119.2(1)
secondary effect of the backbone substituent. It is not ruled out that also different electronic effects (alkyl vs aryl) may play a role in the stability of these heteroleptic complexes, but the trends seen in N−C−N and C−N−CAr angles led us to believe that steric factors are more important. Synthesis and Structures of Calcium Hydride Complexes: RAmDIPPCaH. The five heteroleptic calcium amide complexes in Scheme 4 were converted with PhSiH3 in C6D6 at room temperature to calcium hydride complexes. In all cases the conversion could be monitored by production of PhSiH2N(SiMe3)2. We were, however, not able to isolate defined calcium hydride complexes. Samples became turbid and 1 H NMR signals for homoleptic calcium amidinate complexes appeared. When the solvent was changed to hexane and the temperature lowered to −30 °C, isolation of calcium hydride complexes were successful but only for the bulkier ligands with R = tBu, Ad. Complexes tBuAmDIPPCaH and AdAmDIPPCaH could be isolated in crystalline yields of 80% and 69%, respectively. The crystal structure of tBuAmDIPPCaH shows a dimeric aggregate in which the hydride ligands bridge the Ca2+ ions and the terminal amidinate ligand is bound by (N,aryl)-chelation. Unfortunately, the structure is heavily modulated (see Supporting Information for details). Repeated measurement of different samples (partially crystallized from different solvents) at different temperatures did not solve the modulation problems; however, the composition and even positions of the hydride ligands could be estimated by refinement in a supercell. The structure of (tBuAmDIPPCaH)2 is closely similar to that of the Yb analogue (tBuAmDIPPYbH)2 recently reported by Trifonov and co-workers.23a Given the close relationship between Ca2+ and Yb2+ chemistry,24 this is not surprising. Both compounds crystallize in an isomorphous form, but modulation problems have not been reported for (tBuAmDIPPYbH)2. Recently, Jones et al. reported the structure of (tBuAmDIPPMnH)2, which has a similar buildup and unit cell (no modulation was observed).23b As the structure of tBuAmDIPPCaH is essentially similar to that of AdAmDIPPCaH, which is not modulated, we decided to give only detailed information for the latter. The crystal structure of the centrosymmetric dimer (AdAmDIPPCaH)2 is shown in Figure 2. The hydride ligands were located in the difference Fourier map and refined isotropically. The Ca−H distances (2.17(2) and 2.19(2) Å) compare well to those in (DIPPnacnacCaH·THF)2: range
NAm−Ca−NAm′
Ca−N−CAr
Ca−NTMS−Ca′
100.8(1) 56.08(4) 56.99(7)
55.61(7) 56.16(5) 55.99(5)
134.0(1) 134.5(1) 142.9(2) 144.6(2) 142.2(2) 141.1(2) 147.6(1) 147.9(1) 147.4(1) 147.6(1)
95.8(1) 95.8(1) 95.76(4) 95.56(4)
Figure 2. Crystal structure of (AdAmDIPPCaH)2. The hydride ligands (observed and refined) are shown, but the remaining H atoms have been omitted for clarity.
2.09(4)−2.21(3) Å and average 2.15(4) Å.13 They are somewhat shorter than those observed in complexes 2 (average 2.33(9) Å), but the standard deviation in the latter is quite large; Ca−H bond distances for 3 are not reported. The nonbonding Ca···Ca′ distance in 3 (3.3696(6) Å) is significantly shorter than that in (DIPPnacnacCaH·THF)2 (3.524(4) Å). Whereas the Ca−N2 distance of 2.336(2) Å is slightly shorter than that in the monomer AdAmDIPPCaN(SiMe3)2, the CAr···Ca distances (2.739(2)−2.894(2) Å) compare well. The amide/imine character of the AdAmDIPP ligand is supported by a long C−N2 (1.352(2) Å) and short C−N1 (1.318(2) Å) distance. The flexibility of the amidinate ligand for metal complexation seems the key to the stability of these new amidinate calcium hydride complexes. Bidentate (N,aryl)···Ca coordination allows for isolation of a solvent-free calcium hydride complex (despite the presence of 1 equiv of Et2O in the synthesis of tBuAmDIPPCaH). Their ability for (N,aryl)···Ca coordination makes these ligands more sterically hindered than the DIPPnacnac ligand: (DIPPnacnacCaH·THF)2 crystallized as a THF adduct. The η6-aryl···Yb interaction in (tBuAmDIPPYbH)2 has been described as being quite robust. Addition of TMEDA did not lead to cleavage of the aryl···metal bond, and recrystallization from THF gave crystals of unsolvated (tBuAmDIPPYbH)2. In addition, the calcium hydride complexes (tBuAmDIPPCaH)2 and (AdAmDIPPCaH)2 show aryl···metal bonds of significant strength. NMR investigations in C6D6 display two heptets and four doublets for the iPr groups, indicating asymmetric E
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
replaced by the coordinative aryl group in comparison to THF, thus leading to the dimer (tBuAmDIPPCaH)2. The latter product does not dissolve well in Et2O, but keeping a suspension of thin colorless needlelike crystals of (tBuAmDIPPCaH)2 in Et2O at 40 °C led to slow dissolution and concomitant formation of large colorless blocklike crystals. Xray diffraction analysis showed conversion to a trimeric calcium hydride complex with two coordinated Et2O ligands. The crystal structure of (tBuAmDIPPCaH)3·2Et2O represents a new structural type and is shown in Figure 3a. It is similar to the
bonding of the amidinate ligands. Aryl···Ca bonding is also supported by the unusually high field shift for the p-H of the coordinated DIPP substituent: triplets at 6.52 and 6.56 ppm have been observed for tBuAmDIPP and AdAmDIPP, respectively. Heating a solution of (tBuAmDIPPCaH)2 in C6D6 gave at 350 K coalescence of the two heptets (CHMe2) into one signal and four doublets (CHMe2) into two signals (see the Supporting Information). In addition, the triplet at 6.52 ppm (p-H coordinated DIPP) changed to a very broad signal due to coalescence. The activation energy for fast exchange between the two different sides of the amidinate ligand has been estimated as ΔG⧧ = 16.8 kcal/mol and supports the relatively strong aryl···Ca bond. We presume exchange takes place through a transition state with a symmetrically (N,N)-chelating amidinate ligand. The slightly higher activation energy for exchange in (AdAmDIPPCaH)2 (ΔG⧧ = 17.7 kcal/mol) is in line with the slightly bulkier adamantyl backbone substituent. The effect of additional Lewis bases on aryl···Ca bonding has been investigated and is discussed in detail below. The 1H NMR chemical shifts of the hydride ligands in (tBuAmDIPPCaH)2 and (AdAmDIPPCaH)2 at 3.09 and 3.07 ppm, respectively, are significantly upfield in comparison to hydride chemical shifts in the calcium hydride complexes 1 (4.45 ppm),13 2 (3.99−4.00 ppm),14 and 3 (4.72 ppm).15 Solutions of the dimer (AdAmDIPPCaH)2 in toluene-d8 are remarkably thermally stable: keeping the solution for 1 week at 110 °C gave less than 10% decomposition. In contrast, prolonged heating of a benzene solution of 1 at 75 °C gave decomposition into (DIPP-nacnac)2Ca and CaH2 with a halflifetime of ca. 24 h.25 Solvent Effects. During the course of our investigations it became clear that small changes in the solvent medium or the presence and type of coordinated solvents can have a large impact on the stabilities and structures of amidinate calcium hydride complexes. Apart from the solvent, also the backbone substituent R (tBu or Ad) has a pronounced influence on product formation. Details of our studies are summarized in Scheme 5. Scheme 5. Effects of Solvent (Medium or Coordinated Ligands) and Backbone Substituent on Product Formation
Figure 3. Crystal structures of the trimers (a) (tBuAmDIPPCaH)3· 2Et2O and (b) (AdAmDIPPCaH)3·(THF)2. For clarity, the amidinate ligands are partially shown: for tBu, Ad, and DIPP substituents only the pivot C atoms are shown.
A striking example of the effect of coordinated solvent is shown by the observation that it is impossible to convert literature-known tBuAmDIPPCaN(SiMe3)2·THF18 into a calcium hydride complex by reaction with PhSiH3. Instead, only indications for formation of the homoleptic products (tBuAmDIPP)2Ca and CaH2 have been found. In contrast, the same reaction with the etherate tBuAmDIPPCaN(SiMe3)2·Et2O gave an 80% yield of (tBuAmDIPPCaH)2. We assume Et2O is slightly more weakly bound to Ca and can be more easily
manganese hydride complex (tBuAmDIPPMnH)3 recently reported by Jones et al.,23b but due to the larger ionic radius of Ca2+, additional ether ligands are bound. Coordination of Et2O ligands to only two of the Ca2+ ions, however, breaks the symmetry of this trimer and results in deformation of a regular hexagon into a ladder-type structure. The Ca2+ ion that is free from Et2O coordination seems to form an additional Ca···H bond along the diagonal of the six-membered Ca3H3 ring. The Ca3−H1 distance of 3.10(2) Å is much longer than the regular Ca−H bonds, and although it may seem questionable whether this should be accepted as a bonding interaction, there should be significant electrostatic attraction.26 Selected bond distances and angles are shown in Table 2. The regular Ca−H bond distances within the six-membered ring vary strongly from 2.16(3) to 2.32(2) Å. This large variation is due to the unusual bonding situation for H1 that shows a nearly linear Ca1−H1− Ca2 angle of 174(1)°, whereas angles around H2 and H3 are 124(1)°. The Ca−H1 bond distances are longer due to the facts that (i) Ca1 and Ca2 are additionally solvated by Et2O, which results in elongation of bonds in comparison to those to four-coordinate Ca3, and (ii) H1 interacts weakly with Ca3, F
DOI: 10.1021/acs.organomet.6b00566 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
homoleptic species was observed instead. Similarly, attempts to abstract the THF ligand by dissolution/suspension in toluene and evaporation of the solvent failed due to formation of homoleptic products. The instability of this trimer in polar as well as apolar solvents prevents further NMR characterization. Attempts to synthesize the Et2O complex (AdAmDIPPCaH)3· 2Et2O by addition of Et2O to the solvent-free dimer failed and only (AdAmDIPPCaH)2 was isolated. This shows the importance and strength of the aryl···Ca coordination, which in this case can compete with ether coordination. Requirements and limitations for such calcium π interactions with aryl substituents have been discussed previously by Westerhausen and coworkers.27 The experiment also shows that simple exchange of the backbone substituent tBu for the slightly larger Ad strengthens the aryl···Ca coordination and prevents Et2O coordination. It demonstrates how minor steric changes can have large impacts on product formation.
Table 2. Selected Bond Distances (Å) and Angles (deg) in Trimeric Calcium Hydride Complexes Ca1−H1 Ca1−H3 Ca2−H1 Ca2−H2 Ca3−H2 Ca3−H3 Ca3···H1 Ca1−N1 Ca1−N2 Ca1−O1 Ca2−N3 Ca2−N4 Ca2−O2 Ca3−N5 Ca3−N6 H1−Ca1−H3 H1−Ca2−H2 H2−Ca3−H3 Ca1−H1−Ca2 Ca2−H2−Ca3 Ca3−H3−Ca1 a
(tBuAmDIPPCaH)3·2Et2Oa
(AdAmDIPPCaH)3·2THF
2.32(2) 2.16(3) 2.29(2) 2.16(2) 2.16(2) 2.16(3) 3.10(2) 2.417(2) 2.364(2) 2.378(2) 2.372(2) 2.402(2) 2.384(2) 2.368(2) 2.370(2) 82(1) 83(1) 131(1) 174(1) 124(1) 124(1)
2.23(3) 2.17(3) 2.23(3) 2.12(3) 2.16(3) 2.13(3) 4.08(3) 2.413(2) 2.391(2) 2.356(2) 2.392(1) 2.402(2) 2.373(2) 2.351(2) 2.342(2) 96(1) 96(1) 105(1) 144(1) 139(1) 137(1)
■
CONCLUSIONS A series of heteroleptic RAmDIPPCaN(SiMe3)2·nEt2O complexes has been prepared and characterized. Bulky DIPP substituents at the amidinate N atoms are necessary to prevent ligand exchange to homoleptic species. The size of the backbone substituent can vary over a wide range but determines solvation and aggregation. With small substituents (R = Me, p-Tol) unsolvated dimers are found. Slightly larger substituents (R = Np, tBu) gave Et2O-solvated monomers. The largest backbone substituent (R = Ad) led to a solvent-free monomer in which the amidinate ligand shows (N,aryl)coordination. Only the complexes with the largest backbone substituent (R = tBu, Ad) could be converted to calcium hydride complexes by reaction with PhSiH 3 and crystallized as dimers (RAmDIPPCaH)2. Success in preventing ligand exchange is, however, strongly dependent on the reaction conditions. Reactions should be run in a very apolar solvent such as hexane (aromatic solvents led to ligand exchange) and the temperature should be kept low (−30 to 0 °C). Once formed and isolated, the dimeric calcium hydride complexes are extremely thermally stable: reflux in toluene for 1 week gave
