Diverse Reactivity of ECp* (E = Al, Ga) toward Low-Coordinate

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Diverse Reactivity of ECp* (E = Al, Ga) toward Low-Coordinate Transition Metal Amides [TM(N(SiMe3)2)2] (TM = Fe, Co, Zn): Insertion, Cp* Transfer, and Orthometalation Jana Weßing,† Christoph Göbel,‡ Birgit Weber,‡ Christian Gemel,† and Roland A. Fischer*,† †

Chair of Inorganic and Metal-Organic Chemistry, Technical University of Munich, D-85748 Garching, Germany Inorganic Chemistry II, University Bayreuth, D- 95440 Bayreuth, Germany

‡

S Supporting Information *

ABSTRACT: The reactivity of the carbenoid group 13 metal ligands ECp* (E = Al, Ga) toward low valent transition metal complexes [TM(btsa)2] (TM = Fe, Co, Zn; btsa = bis(trimethylsilyl)amide) was investigated, revealing entirely different reaction patterns for E = Al and Ga. Treatment of [Co(btsa)2] with AlCp* yields [Cp*Co(μ-H)(Al(κ2-(CH2SiMe2)NSiMe3)(btsa))] (1) featuring an unusual heterometallic bicyclic structure that results from the insertion of AlCp* into the TM−N bond with concomitant ligand rearrangement including C−H activation at one amide ligand. For [Fe(btsa)2], complete ligand exchange gives FeCp*2, irrespective of the employed stoichiometric ratio of the reactants. In contrast, treatment of [TM(btsa)2] (TM = Fe, Co) with GaCp* forms the 1:1 and 1:2 adducts [(GaCp*)Co(btsa)2] (2) and [(GaCp*)2Fe(btsa)2] (3), respectively. The tendency of AlCp* to undergo Cp* transfer to the TM center appears to be dependent on the nature of the TM center: For [Zn(btsa)2], no Cp* transfer is observed on reaction with AlCp*; instead, the insertion product [Zn(Al(η2-Cp*)(btsa))2] (4) is formed. In the reaction of [Co(btsa)2] with the trivalent [Cp*AlH2], transfer of the amide ligands without further ligand rearrangement is observed, leading to [Co(μ-H)4(Al(η2Cp*)(btsa))2] (5).

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INTRODUCTION Following their discovery in the early 1990s,1−4 the monovalent group 13 organyls ECp* (E = Al, Ga) and related low valent aluminum and gallium compounds soon emerged to be of interest as exotic ligands in the coordination chemistry of transition and even main group and rare earth metals, thus providing straightforward synthetic access toward intermetallic group 13 metal coordination compounds.5−8 In this regard, the abundance of synthetic strategies toward ECp*-ligated transition metal (TM) complexes is only the logical consequence of the unique steric and electronic features of the versatile ECp* ligand, i.e., the strong electron-donating, carbenoid character, the high flexibility of the Cp* binding modes, and the redox activity of the group 13 center. Established synthetic procedures include ligand substitutions at TM carbonyls and other, labile complexes [(TM)Ln], yielding hetero- and homoleptic complexes [TM(ECp*)xLy] and [TM(ECp*)x]. For example, reactions of dinuclear carbonyl complexes [M2(CO)x] (M = Fe, x = 9; M = Co, x = 8) with GaCp* yield the CO-analogous substitution products [Fe(GaCp*)(CO)4] and [Co2(CO)6(μ2-GaCp*)2],9 whereas the all-labile substituted [M(cod)2] (M = Ni, Pt) and [Pd(tmeda)Me2] afford homoleptic [M(GaCp*)4].10 For coordinatively unsaturated transition metal complexes such as [CpM(CO)2]2 (M = Mo, W), the formation of addition products like [(CO)2CpM(μ2-GaCp*)]2 occurs, contrary to classical carbonyl chemistry.11 Furthermore, the more polarized © XXXX American Chemical Society

TM−halide bonds undergo insertion reactions upon treatment with ECp* as first demonstrated with the synthesis of [Cp*Fe(GaCp*)2GaCl2·THF] and [Cp*(CO)2FeGaCl(η2Cp*)].12 Owing to the flexible redox behavior of the group 13 center and the flexible binding modes of the Cp* group, these reactions are often accompanied by complex arrays of competing side reactions, such as ligand rearrangement or C−H activations.12,13 Recent developments pointing toward the applicability of TM/E nanoclusters as molecular models for structural motifs of and reactivities at catalytically active TM/E nanophases present us with new challenges and requirements with regard to their underlying, fundamental coordination chemistry.14 In this regard, it is interesting to study the reactivity of ECp* toward transition metal amide complexes. Especially the more bulky amide ligands such as bis(trimethylsilyl)amide (btsa) are wellknown to stabilize metalloid clusters, as impressively demonstrated with the synthesis of a variety of metalloid group 13 and 14 metal clusters, with Schnöckel’s metalloid [AlxRy]n− as maybe the most prominent examples.15,16 Additionally, the btsa ligand can be cleaved, for instance via hydrogenation, a feature that has already been exploited in the synthesis of transition metal nanoparticles.17 Small mixed TM/E coordination compounds bearing btsa and Cp* ligands might thus provide Received: December 22, 2016

A

DOI: 10.1021/acs.inorgchem.6b03127 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Reactivity of [TM(btsa)2] (M = Fe, Co, Zn) toward ECp* (E = Al, Ga) and Cp*AlH2

AlCp*. Here, Cp* transfer occurs, along with the orthometalation of one PPh3 ligand, presumably at the strongly Lewis acidic Al atom of the AlBr2 fragment, to give [FeCp*(μ3-H)(κ(C6H4)PPh2)(AlCp*)(AlBr2)].13 Compound 1 is well soluble in nonpolar solvents like nhexane, toluene, or benzene, and it can be stored under an inert atmosphere at −30 °C for weeks without indication of decomposition. Suitable single crystals for X-ray diffraction studies are obtained from saturated n-hexane solutions within few days. 1 crystallizes in the monoclinic space group P21/n with four molecules per unit cell. The molecular structure consists of a disordered Cp*Co unit that is ligated by one aluminum, one carbon, and one hydrogen atom (Figure 1). The aluminum center adopts an almost perfectly trigonal planar geometry, additionally coordinated by two btsa ligands. The

us with new pathways to higher nuclearity clusters or mixed TM/E nanoparticles. Along these lines we wish to report on the interesting reactivity of ECp* toward the low-coordinate TM amides [TM(btsa)2] (TM = Fe, Co, Zn). The remarkably different reaction behavior of AlCp* and GaCp* will be compared and discussed, and potential means to avoid unwanted side reactions, i.e., competing Cp* transfer reactions, will be addressed.

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RESULTS AND DISCUSSION Treatment of the low-coordinate TM amides [TM(btsa)2] (TM = Fe, Co, Zn) with ECp* (E = Al, Ga) resulted in the formation of a small library of novel complexes following the general formula [TMEx(Cp*)x(btsa)2] (x = 1, 2), thus providing a straightforward access to new class of TM/E complexes being exclusively stabilized by easily cleavable organic ligands (Scheme 1). Depending on the choice of the transition metal and the group 13 metal, differing reactivities were observed, including insertion, Cp* transfer, and orthometalation reactions. The reaction of [Co(btsa)2] with stoichiometric amounts of AlCp* in toluene at 80 °C yields [Cp*Co(μ-H)(Al(κ2(CH2SiMe2)NSiMe3)(btsa))] (1) as dark red crystals in good, reproducible yields of 85%. 1 features the formation of a Co−Al bond with concomitant, formal ligand exchange between the two metal centers and C−H activation of the methyl group of one btsa ligand. The tendency of ECp* ligands to undergo Cp* transfer reactions in the presence of Lewis acidic first row transition metal centers is well-known and has already been demonstrated to be a relevant side reaction in the insertion of ECp* into TM−halide bonds.12,13 Mechanistically, insertion of AlCp* into the Co−N bond as well as btsa/Cp* rearrangement and C−H activation of the btsa ligand is obvious, and related to the reaction mechanism observed in the formation of [FeCp*(GaCp*)2(GaCl2·THF)] from FeCl2 and GaCp*.12 For the latter, the transition metal center is stabilized by the presence of further coordinating ligands. In the synthesis of 1, however, the absence of further ligands results in the activation of one methyl group to comply with the coordinatively unsaturated Co center and to counterbalance the formal oxidation of Al(I) to Al(III). A comparable behavior has been observed in the reaction of [Fe(PPh3)2Br2] with

Figure 1. Molecular structure of 1 (displacement ellipsoids shown on the 20% probability level, hydrogen atoms and the disorder of the Cp* ligand omitted for clarity). Selected interatomic distances (Å) and angles (deg): Co−Al 2.402(2), Co−Cp*centr 1.709 (1.689 for PART 2), Co−C(12) 2.095(2), Co−H(1) 1.566, Al−N(1) 1.818(4), Al− N(2) 1.827(4), Al−C(12) 2.149(2), Al−H(1) 1.705, Cp*centr−Co−Al 162.6, Co−Al−N(1) 120.4(2), Co−Al−N(2) 120.6(2), N(1)−Al− N(2) 119.1(2). B


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Inorganic Chemistry

Co(II) center with S = 1/2. This deviation is most probably explained by the presence of the Co−Al bond, which might give rise to additional contributions, e.g., from temperatureindependent Van Vleck paramagnetism.27−30 The χM−1 over T plot follows the Curie−Weiss law for paramagnetic compounds with the Curie constant being determined to be 0.58 cm3 K mol−1 and the Weiss constant to be −12.87 K. Indication for decomposition is observed above 250 K. On the basis of EPR data recorded for frozen solutions of 1 in toluene at 133.15 K, the average g-factor was determined to be 2.06. Note that the high complexity of the obtained spectrum did not allow for a simulation of the hyperfine coupling of 1. The analogous reaction of [Co(btsa)2] with one equivalent of GaCp* instead of AlCp* shows a completely different reaction behavior and does not feature the complex insertion or ligand rearrangement steps observed for 1. Instead, the simple 1:1 stoichiometric adduct [(GaCp*)Co(btsa)2] (2) is formed in almost quantitative yields at room temperature. Contrary to AlCp*, the lower nitrogen affinity of gallium in comparison to aluminum seems to preclude a transfer of the btsa ligands to the Ga center, which in turn retains its formal oxidation state +I. Along these lines, it appeared of interest to adjust the reaction conditions of the synthesis of 2 to those employed in the formation of 1 for a better comparison of the respective reaction pathways. Unfortunately, 2 proved to be too unstable in solution to be handled at elevated temperatures, whereas the reaction between [Co(btsa)2] and AlCp* does not proceed at room temperature. The latter is ascribed to the overall low solubility of AlCp* in common organic solvents, and the need to provide sufficient energy to compensate for the dissociation enthalpy of the [AlCp*]4 tetramer which is prevailing in solution at room temperature.3 Compound 2 dissolves readily in nonpolar solvents, and crystals suitable for X-ray diffraction can be obtained from saturated n-hexane solutions. It crystallizes in the monoclinic space group P21/c with two independent molecules in the asymmetric unit. The molecular structure of 2 features a planar, three-coordinate geometry at the cobalt center with a significant distortion from an ideal trigonal planar geometry toward a T-shape, similar to other reported three-coordinated bisamido complexes of d5−d7 transition metals (Figure 2).31−34 Presumably as a result of the lower steric demand of the GaCp* ligand, the N−Co−N angle of 146.1(2)° is significantly wider than in the closely related [PPh3Co(btsa)2] (130.7(7)°)33 but in a comparable range as the N−Co−N angle of the less sterically crowded [(py)Co(btsa)2] (140.7(2)°).31 The Co−Ga bond distance of 2.533(2) Å is notably elongated in comparison to those observed in the only literature-known Co−GaCp* complexes [(CO)3Co(μ-GaCp*)2Co(CO)3] (2.389 Å)9 and [Cp*Co(GaCp*)3][BArF]2 (⌀ 2.297 Å),35 which indicates a relatively weak dative bonding and may be a result of the strong electron-donating properties of the amide ligands. The Ga− Cp*centr distance of 1.939 Å is identical to that in [(CO)3Co(μGaCp*)2Co(CO)3] (1.939 Å). As expected, the 1H NMR spectrum of 2 in C6D6 shows two broad signals at 16.86 and −11.29 ppm for the protons of the Cp* and the btsa ligands, respectively, whereat the wide chemical shift range and a significant broadening of the signals are again the result of the paramagnetic nature of the Co(II) center. However, in contrast to 1, the sufficiently long relaxation delay times employed for the acquisition of the spectrum of 2 allow for its quantitative assessment, thus supporting the assignment of Cp* and btsa-correlated signals.

Co−Al bond distance of 2.402(2) Å is slightly longer than those found in [(μ-AlCp*)2Co2(CO)6] (⌀ 2.377 Å)18 or [(μAlEt)2(Co(ethylene)Cp*)2] (⌀ 2.335 Å),19 but shorter than in intermetallic CoAl phases, like Co2Al5 (2.43 Å) or Co2Al9 (2.47 Å).20 The Co−Cp*centr distance of 1.709 Å (1.689 Å for PART 2) is almost identical with the distance found in CoCp*2 (1.714 Å)21 and elongated in comparison to trivalent cobaltocenium salts [CoCp*2][X−] (e.g., 1.623 Å for X = PF6).22 One of the two btsa ligands is C−H activated (orthometalated), with both the CH2 group and the hydride ligand found in a Co−Al bridging position. The resulting geometry can be described as a [2.1.0] metalla-bicycle composed of a distorted Al−N(2)− Si(4)−C(12) square with bond lengths between 1.723(2) and 2.149(2) Å and angles of 80.02(2) to 98.51(1)°, and an almost symmetric Co−C(12)−Al triangle. The Co−C(12) and Al− C(12) bond lengths of 2.095(2) and 2.149(2) Å are in a typical range for comparable M−C bonds. The position of the hydride could be located on the differential Fourier map and was allowed to refine freely to be found in a Co−Al bridging position with Co−H and Al−H distances of 1.566 and 1.705 Å, respectively. Resulting from its paramagnetic nature, NMR spectroscopic analysis of 1 is not straightforward and demands broad spectral scopes and delay times to sufficiently resolve paramagnetically shifted signals. The latter renders a quantitative interpretation of the observed signals impossible as it considerably decreases the pulse repetition time and thus precludes a full relaxation of less paramagnetically influenced, excited nuclear spins. The 1H NMR spectrum of pure 1 in C6D6 shows clearly distinguishable signals at 1.15, 0.46, and −0.55 ppm, along with a very broad disturbance of the baseline around 81.01 ppm which we can neither reliably identify nor exclude as a signal. However, the significant paramagnetic shift and the broadness of the singlet at 81.01 ppm suggest that it corresponds to the Cp* ligand due to its close proximity to the paramagnetic cobalt center, whereas the remaining three signals at 1.15, 0.46, and −0.55 ppm appear to stem from the chemically nonequivalent H atoms of the asymmetric btsa ligands. The paramagnetic, non-protondecoupled 13C NMR spectrum gives rise to only one set of high-field signals that is composed of overlapping multiplets in a range between 9.58 and 4.30 ppm, assumedly corresponding to the btsa ligands in analogy to the paramagnetic 1H NMR spectrum. Attempts to detect the remaining Cp* signals in spectral ranges from −150 to 550 ppm proved to be futile. FTIR spectroscopic results of 1 reveal the characteristic set of absorption bands for Cp* at 2924, 2882, and 2835 cm−1 (νC−H), and 1423, 1386, and 1368 cm−1 (νC−C,C−Me), along with the very intense νSi−C and νSi−N−Si vibrational modes of the btsa ligands at 1232 and 821 cm−1, and 918 cm−1, respectively. An additional broad vibrational band at 1632 cm−1 provides evidence for the presence of a M−hydride species, which is in good agreement with a Co−H−Al bridging mode as concluded from the molecular structure. Literature values of terminal Al− H vibrations are usually found at higher wave numbers in the range of 1750 to 1890 cm−1,23,24 while terminal Co−hydride species give rise to a distinct signal in the range of 1900 to 2000 cm−1.25,26 For bridging M−H modes, however, a shift to lower energies accompanied by a significant broadening of the signal is observed.26 Magnetic properties of compound 1 were assessed using a SQUID magnetometer in a temperature range between 50 and 300 K. At ambient temperatures, the determined value for μeff of 2.1 is slightly above the calculated value of 1.7 for a low spin C


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Inorganic Chemistry

Figure 3. Molecular structure of 3 (displacement ellipsoids shown on the 20% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Fe−Ga 2.599(1), Ga−Cp*centr 1.979, Fe−N 1.956(3), N−Si(1) 1.716(3), N−Si(2) 1.720(3), Cp*centr−Ga−Fe 170.7, Ga−Fe−Ga′ 94.3(2), Ga−Fe−N 96.4(2), N−Fe−N′ 140.9(3).

Figure 2. Molecular structure of 2 (one molecule of the asymmetric unit, displacement ellipsoids shown on the 20% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Co(1)−Ga(1) 2.533(2), Ga(1)−Cp*centr 1.939, Co(1)−N(1) 1.887(5), Co(1)−N(2) 1.894(2), N−Si ⌀ 1.720, Cp* centr −Ga(1)−Co(1) 175.0, Ga(1)−Co(1)−N(1) 109.3(2), Ga(1)−Co(1)−N(2) 104.5(2), N(1)−Co(1)−N(2) 146.1(2).

3 crystallizes in the orthorhombic space group Pbcn with four molecules per unit cell. The molecular structure as determined by single crystal X-ray diffraction studies exhibits a central Fe(II) atom in a strongly distorted tetrahedral coordination environment with a large N−Fe−N and a small Ga−Fe−Ga angle of 140.9(3) and 94.3(2)°, respectively. This distortion is likely to arise from the steric demand of the organic groups. The strong π-donating properties of the amide ligands result in an almost perfectly planar coordination environment of the N atoms with a triangular sum of 359.8°. The strong electron donation of the amide group is probably also the reason for the significantly elongated Fe−Ga bond of 2.599(1) Å. For comparison, the Fe−Ga distances of literature-known terminal GaCp* motifs, e.g., in [(CO) 4 Fe(GaCp*)], 9 [Cp*Fe(GaCp*)3][BArF],35 or [Cp*(GaCp*)2Fe(GaCl2)·THF],12 vary between 2.273 and 2.314 Å. The Fe−N distance of 1.956(3) Å is comparable to those found in the starting compound (1.925 Å for the terminal btsa ligands) and [Fe(btsa)2(THF)] (1.916(5) Å).39 Magnetic measurements were carried out with a SQUID magnetometer between 50 and 300 K. The resulting μeff value of 4.5 is below the expected value of 4.9 for a high spin Fe(II) center with S = 2. This, in addition to the determined low Weiss constant of −27.36 K, provides evidence for a weak antiferromagnetic coupling. The χM−1 over T curve follows the Curie−Weiss law for paramagnetic compounds with the Curie constant being determined to be 2.75 cm3 K mol−1. Attempts to determine g on the basis of EPR measurements of frozen solutions of 3 in toluene at 133.15 K remained unsuccessful. While GaCp* does not react with [Zn(btsa)2] even at elevated temperatures up to thermal decomposition of the precursors, the reaction of [Zn(btsa)2] with two equivalents of AlCp* proceeds rapidly at 60 °C yielding [Zn(Al(η2-Cp*)(btsa))2] (4). Here, AlCp* not only acts as a reducing agent and inserts into the Zn−N bond but also stabilizes the thus generated low valent Zn center by formation of unsupported Zn−Al bonds without further Cp* transfer. In light of current developments in the field of low valent zinc coordination chemistry, a resemblance to the underlying mechanistic principles in the synthesis of recently reported cationic zinc

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C NMR spectroscopic measurements give rise to one slightly broadened signal at 32.62 ppm, corresponding to the Cp* methyl groups, and a sharp signal at 2.66 ppm which is attributed to the btsa ligands. A signal of the inner C atoms of the Cp* ligand could not be observed, most probably due to the paramagnetic influence of the Co(II) center. The IR spectrum shows comparable features to 1 with regard to characteristic νC−H and νC−C,C−Me vibrational modes of the Cp* ligand (2950, 2896, and 2857 cm−1; and 1423, 1386, and 1368 cm−1), and νSi−C and νSi−N−Si vibrations of the btsa ligands at 1246 and 818 cm−1, and 927 cm−1, respectively. Attempts to further characterize 2, e.g., by means of LIFDI-MS (liquid injection field desorption ionization mass spectrometry), elemental analysis, and magnetic measurements, were unsuccessful owing to the high sensitivity of the compound toward air and moisture and its low stability in solution. In a similar manner, [Fe(btsa)2] reacts with GaCp* in toluene to give the 1:2 adduct [(GaCp*)2Fe(btsa)2] (3, Figure 3). However, reaction of [Fe(btsa)2] with AlCp* does not lead to any products featuring Fe−Al bond formation, but instead forms FeCp*236,37 and [Al4Cp*3(btsa)].38 Compound 3 is stable for months when stored under an inert atmosphere at −30 °C and is well soluble in nonpolar solvents, like n-hexane, toluene, or benzene. 1H NMR spectroscopic measurements give rise to a broad signal at 45.74 ppm with low intensity which is assigned to the Cp* ligands, as well as a sharp singlet at 0.38 ppm attributable to the protons of the btsa ligands. The paramagnetic, non-1Hdecoupled 13C NMR spectrum of 3 shows one broad singlet at 245.28 ppm which corresponds to the Cp* ligand and a quartet around 5.52 ppm, probably attributable to the methyl groups of the btsa ligands. Efforts to properly resolve the second signal to be expected for the Cp* ligand within a range of −150 to 500 ppm remained futile, and only a very broad underlying signal was observed at 85.45 ppm. The FT-IR spectrum illustrates no unusual features and displays characteristic vibrational modes of the Cp* (νC−H at 2920, 2889, 2839 cm−1; νC−C,C−Me at 1469, 1409, 1375 cm−1) and btsa ligands (νSi−C at 1227, 818 cm−1; νSi−N−Si at 958 cm−1). D


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Inorganic Chemistry

to those found in [Al(btsa)3] (Al−N 1.78 Å)46 or the η2coordinated AlCp* moiety in [Fe(AlCp*)5] (Al−C 2.030(5) Å),47 thus hinting at steric crowding as the reason for the observed change in hapticity. However, the interatomic distance between Zn and C(9) and the distance to one H atom of the closest CH3 group are comparably short (Zn−C(9) 3.051(1), Zn−H 2.858(2) Å), i.e., weak intermolecular interactions cannot be entirely ruled out. Characterization of 4 by 1H NMR spectroscopy gives rise to two sharp singlets at 2.08 and 0.31 ppm attributable to the Cp* and btsa ligands, respectively. Here, the expected splitting of the Cp* signal into three signals owing to a change in hapticity from η5 to η2 could not be observed down to −80 °C, indicating a fast fluctuation of the Cp* ligand in solution. While it was not possible to obtain the 13C NMR spectrum of 4 without significant decomposition during the measurement, the 29 Si NMR spectrum shows one sharp signal at −3.96 ppm. The FT-IR spectrum of complex 4 shows overall similar features to those reported for 2 and 3, with characteristic νC−H and νC−C vibrational modes stemming from the Cp* ligands at 2946, 2901, 2859, and 1419 cm−1 and intense νSi−C and νSi−N−Si vibrations at 1246 and 826, and 939 cm−1, respectively. Motivated by the prospect of Cp* transfer inhibition and our experiences with the ECp*-ligated ruthenium polyhydride complexes [Ru(cod)(H)(GaCp*)3][BArF], [Cp*Ru(μ-H)(H)(μ-ECp*)]2, and [(Cp*Ru)3(μ-H)5(μ3-ECp*)],48 the reaction of the amide precursors [TM(btsa)2] with [Cp*AlH2] was investigated. For [Fe(btsa)2], again the formation of FeCp*2 was observed. The reaction of [Co(btsa)2] with two equivalents of [Cp*AlH2] in toluene at room temperature, however, gives [Co(μ-H)4(Al(η2-Cp*)(btsa))2] (5), which, in contrast to 1, features an insertion of the AlCp* unit into the Co−N bond without further ligand rearrangements. Complex 5 immediately crystallizes from the reaction mixture in form of microcrystalline, yellow plates in the triclinic space group P1,̅ featuring a centrosymmetric molecular structure with strong resemblance to that of 4 (Figure 5). The Co−Al bond length of 2.386(1) Å is comparable to those

clusters, i.e., [Zn3Cp*3][BArF] or [Zn10Cp*7Me][BArF], can certainly be found. Both result from the stabilization of coordinatively unsaturated, low valent zinc centers obtained via redox-active cleavage of stabilizing, organic ligands R from suitable primitive precursors Zn2Cp*2 or ZnR2 (R = Me, Cp*).40,41 Complex 4 is unstable in solution and undergoes fast decomposition under the reaction conditions, as indicated by a color change from light yellow to greenish-gray as well as metal precipitation at prolonged reaction times. Attempts to avoid decomposition by short reaction times and low-temperature workup, as well as subsequent purification steps at low temperatures, were only of limited success and failed to provide analytically pure 4. Note that an overall decrease of the reaction temperature is not possible due to the low solubility of AlCp* in nonpolar solvents and its preferential oligomerization in solution at room temperature. Suitable single crystals of 4 for X-ray diffraction measurements were grown from saturated n-hexane solutions. Complex 4 crystallizes in the monoclinic space group P1̅ . Its centrosymmetric molecular structure consists of a perfectly linear, central Al−Zn−Al arrangement resulting from the insertion of two AlCp* ligands into the Zn−N bonds of [Zn(btsa)2] (Figure 4). In contrast to 1, no further ligand

Figure 4. Molecular structure of 4 (displacement ellipsoids shown on the 20% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Zn−Al 2.448(2), Al−C(7) 2.210(2), Al−C(11) 2.181(2), Al−N 1.850(5), N−Si ⌀ 1.728, Zn−Al−C7/11 115.6, Zn−Al−N 127.6(2), C7/11−Al−N 116.7.

rearrangement occurs. Although a handful of zinc−gallyl complexes with unsupported Zn−Ga bonds have been reported,42−44 to the best of our knowledge, 4 represents the first isolated molecular Zn−Al complex, which is why no suitable experimental reference values for the given Zn−Al bond distance of 2.448(2) Å were found. Note that distances in intermetallic Zn−Al hydride model compounds were calculated to range between 2.60 and 2.79 Å.45 The coordination geometry of the Al atoms can be best described by a pseudo trigonal planar arrangement with Zn−Al−N and N−Al−C7/11 (centroids between two atoms will be denoted as Cx/y) angles close to the optimum value of 120 °C, and an angular sum of 359.8°. Interestingly, the Cp* ligands are bound in a η2coordination mode and somewhat tilted toward the Zn atom. The Al−N and Al−C bonds (Al−N 1.850(5) Å, Al−C 2.181(2), and 2.210(2) Å) are slightly elongated with regard

Figure 5. Molecular structure of 5 (displacement ellipsoids shown on the 20% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Co−Al 2.386(1), Al−C ⌀ 2.167, Al−N 1.833(1), Co−H ⌀ 1.510, Al−H ⌀ 1.758, Co− Al−C7/8 123.3, Co−Al−N 120.5, C7/8−Al−N 116.2. E


Article

Inorganic Chemistry observed for 1 or related Co−Al complexes (see above).18,19 Interatomic distances and angles around the pseudo trigonal planar Al center (neglecting the hydride ligands) are in the expected ranges, and selected data is listed in Figure 5. Based on the molecular structure, all four hydrides appear to remain at the complex and adopt Co−Al bridging positions, formally creating Co(II) and Al(III) centers. Although the bonding descriptions of multiply bridged heterobimetallic hydride complexes are still underdeveloped, for 5, the average Co−H and Al−H distances of 1.510 and 1.758 Å, along with the reasonably short Co−Al distance, suggest the formation of a true metal−metal adduct, rather than a TM−H−Al σ-complex or an oxidative addition product. The latter is particularly unlikely with regard to the Al−H distance which would be expected to be significantly increased to values >2.0 Å in the case of a complete hydride transfer to the Co(II) center.49 Note that hydrides were located on the differential Fourier map and allowed to refine freely. The thus determined bridging positions are in good agreement with the observed intense νM−H vibrational mode at 1672 cm−1 in the FT-IR spectrum. The suggested composition of 5 is in good accordance with results obtained from elemental analysis. Surprisingly, 1H NMR spectroscopic characterization of 5 proved to be nontrivial owing to the low solubility and/or stability of the compound in conventional deuterated NMR solvents. For C6D6 and acetonitrile-d3 low solubility was observed, whereas decomposition occurred in CD2Cl2 and CDCl3. Even though soluble in toluene-d8, no spectrum of 5 could be obtained, presumably owing to its paramagnetic nature. Upon dissolution of the compound in THF-d8, rapid liberation of H2 results in the formation of a yet unidentified, diamagnetic species which gives rise to clearly distinguishable, sharp signals at 1.85, 1.80, 0.40, 0.19, 0.11, and 0.04 ppm. The nature of this complex has not yet been elucidated. However, given the observed C−H activation of the btsa ligand for 1, an at least partial loss of the stabilizing hydride ligands might result in the C−H activation of the btsa ligand(s), thus generating chemically nonequivalent protons causing the observed signals from 0.40 to 0.04 ppm. The signals at 1.85 and 1.80 ppm are in good accordance with a η1- or η2-bound Cp* ligand, assuming a third signal underlying the residual solvent peak. Attempts at a full characterization of this compound are still ongoing in our laboratories. Magnetic measurements were carried out with a SQUID magnetometer in a temperature range between 50 and 300 K. At ambient temperatures, μeff was determined to be 2.7, which deviates considerably from the calculated values for Co(II) centers (low spin, 1.7; high spin, 3.9). Furthermore, the χM−1 over T curve shows nonlinear behavior, which precludes a reliable determination of the Curie constant and the Weiss temperature. Given the change in appearance of the sample during the measurement, decomposition of 5 under liberation of hydrogen as observed during 1H NMR spectroscopic measurements is likely. This is further supported by comparison of μeff to the calculated value for Co(I) (2.82), suggesting the reduction of the Co center.

undergoes adduct formation to form [(GaCp*)Co(btsa)2] (2) and [(GaCp*)2Fe(btsa)2] (3), the higher nitrogen affinity of aluminum prompts the insertion of AlCp* into the TM−N bond to form [Cp*Co(μ-H)(Al(κ2-(CH2SiMe2)NSiMe3)(btsa))] (1) and [Zn(Al(η2-Cp*)(btsa))2] (4). For 1, complete ligand exchange of the amide and Cp* ligands occurs between the metal centers, probably triggered by the comparably high Lewis acidity of the Co center which favors competing Cp* transfer reactions. This can be avoided by using the closely related, trivalent [Cp*AlH2], which reacts with [Co(btsa)2] to give the pure insertion product [Co(μ-H)4(Al(η2-Cp*)(btsa))2] (5). Here, the presence of the anionic hydride ligands appears to sufficiently lower the electron affinity of the Co center to prevent Cp* transfer. We are currently investigating to what extent these hydrides remain accessible for further reactions or may even facilitate intra- or intermolecular reductive elimination reactions to promote cluster formation. Moreover, it is reasonable to assume that the reported complexes may provide a novel access toward higher nuclearity TM/E complexes and clusters, or nanomaterials upon systematic removal of the all-labile Cp* and btsa ligands, i.e., via treatment under mild hydrogenation conditions. Respective experiments are currently ongoing in our laboratories.

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EXPERIMENTAL SECTION

Materials and Methods. Manipulation of air- and moisturesensitive compounds was performed under an atmosphere of purified argon using conventional Schlenk and glovebox techniques. Solvents were dried using an MBRAUN Solvent Purification System, whereat final H2O contents were checked by Karl Fischer titration and did not exceed 5 ppm. [Cp*AlH2],24 [AlCp*],24 GaCp*,9 [Fe(btsa)2],50 [Co(btsa)2]51 and [Zn(btsa)2]52 were prepared according to literature procedures. Instrumentation. Elemental analyses were performed by “Mikroanalytisches Laboratorium Kolbe”, Mülheim an der Ruhr, Germany, and the Microanalytical Laboratory of the Technical University of Munich, Germany. NMR spectra were measured in C6D6 or THF-d8 at 298 K, using a Bruker Advance DPX-250 or Bruker AV400 US spectrometer operating at the appropriate frequencies. For paramagnetic NMR spectroscopic measurements, specifically adapted pulse programs with broad spectral ranges, short relaxation times, and short prescan delays were employed, using a Bruker-DRX400 spectrometer. Chemical shifts are given relative to TMS, and spectra were referenced relative to the residual solvent signal. 29Si NMR spectra are referenced to TMS as an internal standard. FT-IR spectra were recorded on a Bruker Alpha FT-IR spectrometer with an ATR geometry, using a diamond ATR unit under argon atmosphere. Magnetic measurements were carried out on a Quantum Design MPMSXL-5 SQUID magnetometer under an applied field of 0.5 T over the temperature range from 50 to 300 K in the settle mode. To this end, the sample was placed in a gelatin capsule held within a plastic straw under inert conditions. The obtained data was corrected for the diamagnetic contributions of the ligands using tabulated Pascal constants and for the contributions of the gelatin capsule. EPR data of frozen toluene solutions was recorded on a Jeol JES-FA200 at 133.15 K. X-ray diffraction intensities for 1 to 4 were collected on an Agilent Technologies SuperNova diffractometer with an Atlas CCD, using Cu Kα radiation. For 5, X-ray diffraction intensities were collected on a Bruker diffractometer equipped with a CMOS detector (APEX III, κCMOS), a TXS rotating anode with Mo Kα radiation, and a Helios optic. Suitable crystals were coated in perfluoropolyether and mounted in the cooled nitrogen stream of the diffractometer on a loop. Diffraction data was processed with CrysAlisPro53 (1−4) or APEX III54 in conjunction with SAINT and SADABS.55 Molecular structures were solved and refined with the program package SHELXLE,56 using the programs SHELXS-9757 and SHELXL-2014.58 Crystallographic

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CONCLUSION In this contribution, we presented the reaction of the Fe(II), Co(II), and Zn(II) complexes [TM(btsa)2] with GaCp* and AlCp* as a first step of the in-depth investigation of the reactivity of the carbenoid group 13 metal ligands toward amide-stabilized coordination compounds. While GaCp* F


Article

Inorganic Chemistry

form of a yellow powder. Yield: 51 mg (0.07 mmol, 14%). 1H NMR (THF-d8, rt): δ = 1.85 (s, 6H), 1.80 (s, 6H), 1.73 (3H, overlapping with solvent residual signal), 0.40 (s, 2H), 0.19 (s, 3H), 0.11 (s, 9H), 0.04 (s, 3H). IR (ATR, neat, cm−1): 2927 (m), 2879 (w), 2839 (w), 1672 (m), 1430 (w), 1386 (w), 1366 (w), 1238 (m), 1126 (w), 1020 (w), 905 (m), 869 (s), 823 (s), 751 (m), 667 (m), 639 (m), 613 (w), 470 (m), 416 (m), 383 (m). Anal. Calcd for C32H70N2CoAl2Si4 (M = 708.15 g mol−1): C 54.27; H 9.96; N 3.96; Si 15.86; Co 8.32; Al 7.62. Found: C 54.22; H 9.84; N 3.95; Si 13.81; Co 8.26; Al 7.60.

details are provided in the Supporting Information, as is crystallographic data in cif format. CCDC-1523747 to CCDC-1523751 also contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Synthetic Procedures. [Cp*Co(μ-H)(Al(κ2-(CH2SiMe2)NSiMe3)(btsa))] (1). A mixture of [Co(btsa)2] (600 mg, 1.59 mmol) and AlCp* (255 mg, 1.59 mmol) was suspended in toluene (15 mL) and heated to 80 °C for 3 h. After cooling to room temperature, all volatiles were removed under reduced pressure and the residue was dissolved in n-hexane (10 mL) and filtered. The red filtrate was concentrated in vacuo and kept at −30 °C for crystallization. 1 was obtained as dark red needles within few days. Yield: 730 mg (1.35 mmol, 85%). 1H NMR (C6D6, rt): δ = 81.01 (vbr s, Cp*), 1.15 (s, btsa), 0.46 (s, btsa), −0.55 (s, btsa). 13C NMR (C6D6, rt): 9.58−4.30 (overlapping multiplets, btsa). IR (ATR, neat, cm−1): 2924 (m), 2882 (w), 2835 (w), 1632 (w), 1423 (w), 1386 (w), 1368 (w), 1232 (s), 1020 (s), 918 (m), 888 (s), 862 (s), 821 (vs), 770 (s), 741 (s), 667 (s), 667 (s), 632 (m), 613 (m), 561 (w), 527 (w), 478 (w), 443 (m), 396 (m), 374 (m). Anal. Calcd for C22H51N2CoAlSi4 (M = 541.91 g mol−1): C 48.76; H 9.49; N 5.17; Si 20.73; Co 10.88; Al 4.98. Found: C 48.61; H 10.34; N 4.99; Si 20.41; Co 10.54; Al 4.84. [(GaCp*)Co(btsa)2] (2). At room temperature, GaCp* (162 mg, 0.79 mmol) was added dropwise via syringe to a solution of [Co(btsa)2] (300 mg, 0.79 mmol) in toluene (9 mL). The resulting green solution was stirred for 3 h. Subsequent removal of all volatiles in vacuo gave crude 2 as a wax-like green solid with slight impurifications. Attempts on further purification via crystallization failed due to the lability of the compound. Yield: 266 mg (0.45 mmol, 86%). 1H NMR (C6D6, rt): δ = 16.86 (br s, 15H, Cp*), −11.29 (brs, 36H, −SiMe3). 13C NMR (C6D6, rt): 32.62 (C5Me5), 2.66 (−SiMe3). IR (ATR, neat, cm−1): 2950 (m), 2896 (m), 2857 (sh), 1475 (w), 1378 (m), 1246 (m), 1180 (w), 1079 (w), 999 (m), 927 (m), 818 (s), 750 (m), 717 (m), 667 (m), 618 (m), 488 (w). [(GaCp*)2Fe(btsa)2] (3). At room temperature, GaCp* (385 mg, 1.88 mmol) was added dropwise via syringe to a solution of [Fe(btsa)2] (354 mg, 0.94 mmol) in toluene (8 mL). The resulting yellow-green solution was stirred for 3 h. All volatiles were removed in vacuo, and the wax-like, yellow-brown residue was dissolved in nhexane and filtered. The filtrate was concentrated and kept at −30 °C for crystallization. After isolation via cannula filtration and drying in vacuo, 3 was obtained as violet-brown crystals. Repeated concentration and workup of the filtrate yields an additional amount of analytically pure 3. Yield: 458 mg (0.58 mmol, 62%). 1H NMR (C6D6, rt): δ = 45.74 (br s, 30H, Cp*), 0.38 (s, 36H, −SiMe3). 13C NMR (C6D6, rt): 245.28 (s, C5Me5), 88.45 (vbr s, C5Me5), 5.52 (q, −SiMe3). IR (ATR, neat, cm−1): 2920 (m), 2889 (w), 2839 (w), 1469 (w), 1409 (w), 1375 (w), 1227 (m), 958 (s), 818 (s), 770(m), 739 (m), 696 (m), 663 (m), 653 (m), 608 (m), 582 (m), 511(w). Anal. Calcd for C32H66N2FeGa2Si4 (M = 786.51 g mol−1): C 48.87; H 8.46; N 3.56; Si 14.28; Fe 7.10; Ga 17.73. Found: C 47.80; H 8.36; N 3.50; Si 13.44; Fe 8.13; Ga 15.60. [Zn(Al(η2-Cp*)(btsa))2] (4). At room temperature, [Zn(btsa)2] (300 mg, 0.78 mmol) was added to a suspension of AlCp* (252 mg, 1.55 mmol) in toluene (6 mL). The reaction mixture was heated to 60 °C for 30 min. After cooling to room temperature, the resulting green suspension was filtered and kept at −30 °C for crystallization. 4 was obtained in the form of yellow plates, which were isolated via cannula filtration and dried in vacuo. Yield: 73 mg (0.11 mmol, 14%). 1H NMR (C6D6, rt): δ = 2.08 (s, 30H, Cp*), 0.31 (s, 36H, −SiMe3). 13C NMR (C6D6, rt): decomposition. 29Si NMR (C6D6, rt): −3.96 ppm. IR (ATR, neat, cm−1): 2946 (m), 2901 (m), 2859 (w), 1419 (br, w), 1246 (s), 1028 (w), 939 (m), 867 (m), 826 (s), 756 (m), 719 (m), 672 (m), 641 (m), 616 (m), 595 (m), 455 (m), 433 (m). [Co(μ-H)4(Al(η2-Cp*)(btsa))2] (5). A mixture of [Co(btsa)2] (200 mg, 0.53 mmol) and [Cp*AlH2] (173 mg, 1.05 mmol) was dissolved in toluene (6 mL), resulting in immediate gas evolution. The dark red solution was stirred at room temperature for 3 h, and the yellow, microcrystalline precipitate was allowed to settle prior to filtration. Washing with toluene (2 × 1 mL) and drying in vacuo gave 5 in the

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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.inorgchem.6b03127. Crystallographic data (CIF) Selected crystallographic details, NMR, EPR and IR spectra, and magnetic characterization of the compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: roland.fi[email protected]. Fax: +49 (0)89 289 13194. ORCID

Birgit Weber: 0000-0002-9861-9447 Roland A. Fischer: 0000-0002-7532-5286 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W. is grateful for a Ph.D. scholarship provided by the German Chemical Industry Fund (https://www.vci.de/fonds). This work has been supported by the DFG (SPP 1708). We thank Dr. Alexander Pöthig and M.Sc. Philipp Altmann for support with single crystal XRD analysis, Maria Weindl for support with paramagnetic NMR measurements, and Dr. Carmen Haeßner for support with EPR measurements.

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REFERENCES

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