Titanocene and Zirconocene Complexes with Diaminoacetylenes

Article pubs.acs.org/Organometallics

Titanocene and Zirconocene Complexes with Diaminoacetylenes: Formation of Unusual Metallacycles and Fulvene Complexes Ò scar À rias,† Alex R. Petrov,† Thomas Bannenberg,† Kai Altenburger,‡ Perdita Arndt,‡ Peter G. Jones,† Uwe Rosenthal,*,‡ and Matthias Tamm*,† †

Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany

‡

S Supporting Information *

ABSTRACT: The reaction of dipiperidinoacetylene (pipC Cpip, pip = NC5H10, 1a) with [Cp2Ti(η2-btmsa)] (2) or with [Cp2Zr(η2-btmsa)(py)] (4) (btmsa = bis(trimethylsilyl)acetylene, py = pyridine) afforded the metallacyclopentadienes [Cp2M(C4pip4)] (3, M = Ti; 5, M = Zr), which in the solid state exhibit twisted five-membered metallacycles with an unusual half-chair conformation. In contrast, the sterically more demanding decamethyltitanocene (Cp*2Ti) and -zirconocene (Cp*2Zr) complex fragments can only accommodate one alkyne ligand. Thus, the titanacyclopropene [Cp*2Ti(C2pip2)] (7) was isolated from the reaction of 1a with [Cp*2Ti(η2-btmsa)] (6) or with [Cp*2TiCl] in the presence of magnesium, whereas the zirconacyclopropenes [Cp*2Zr(C2X2)] (8a, X = pip; 8b, X = NC5H9-4-Me; 8c, X = NEt2) were prepared by the reduction of [Cp*2ZrCl2] with magnesium in the presence of 1a, bis(4-methylpiperidino)acetylene (1b), and bis(diethylamino)acetylene (1c), respectively. NMR studies showed that complexes 8 are in equilibrium with their tucked-in tetramethylpentafulvene−diaminovinyl isomers [Cp*(η6-C5Me4CH2)Zr(CX=CHX)] (9) in solution, which are formed by intramolecular C−H-bond activation and hydrogen transfer from one Cp* methyl group to the alkyne ligand. Thermodynamic and kinetic parameters were derived by variabletemperature NMR spectroscopy and DFT experiments. The molecular structures of 3, 5, 7, 8a, [8a·MgCl2]2, 8b, and 8c were established by X-ray diffraction analyses.

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(MCPD)2 or with unsaturated organic substrates such as alkenes, aldehydes, ketones, imines, and nitriles.1 While the resulting metallacycles are interesting in their own right, they can also serve as valuable precursors for the preparation of highly functionalized organic molecules and heterocyclic main-groupelement compounds.1−3 The coordination chemistry of heteroatom-substituted alkynes (Scheme 1, X ≠ H, alkyl, aryl) toward group 4 metallocenes was also investigated, with bis(trimethylsilyl)acetylene (btmsa) being by far the most prominent alkyne employed, and complexes such as [Cp′2Ti(η2-btmsa)] and [Cp′2Zr(η2-btmsa)(L)] (L = pyridine, tetrahydrofuran)4 have been extensively studied.5 The btmsa ligand stabilizes the metallocene fragment, but it can be readily displaced by other substrates. These btmsa complexes are thus established as suitable sources of the highly reactive metallocene core, and substitution of the alkyne by other substrates can afford a wide range of products, including metallacyclopropenes5c and -cyclopentadienes,2,6 σ-alkenyl and σ-alkynyl complexes,7 and metallacyclocumulenes,8 along with other products of complexation,

INTRODUCTION

Alkyne complexes of group 4 metallocenes have been the subject of great interest in view of their application in organic synthesis.1 Various sources have become available for the generation of metallocene complex fragments Cp′2M (Cp′ = substituted or unsubstituted, bridged or unbridged cyclopentadienyl, M = Ti, Zr, Hf); in the presence of alkynes, these species can form 1:1 complexes, best described as metallacyclopropenes (MCP), as the result of oxidative alkyne addition (Scheme 1). Depending on the Cp′2M fragment and the alkyne substituents, the latter might further react with a second alkyne to afford metallacyclopentadienes Scheme 1. Oxidative Alkyne Addition to Metallocenes To Give Metallacyclopropenes (MCP) and Metallacyclopentadienes (MCPD)

Received: February 3, 2014 Published: March 17, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/om500121p | Organometallics 2014, 33, 1774−1786

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coupling, transfer, and bond cleavage or activation reactions.5,8 In contrast, group 4 metallocene complexes containing other heteroatom-substituted acetylenes of the type XCCX are rare and, except for X = SnMe3,9 all other examples with X = BR2,10 PR2,11 OR,12 SR13 were reported only very recently. It was shown that the nature of the heteroatom has a significant impact on the reactivity toward the metallocene complex fragment and can involve coordination of donor heteroatoms as well as element−carbon bond activation and cleavage. Diaminoacetylenes (ynediamines, X = NR2) represent another important class of heteroatom-functionalized alkynes;14 they have been only rarely employed for transition-metal complexation,15 which can be ascribed to the high reactivity of these electron-rich compounds and to the lack of reliable and adaptable synthetic protocols. This situation has been improved recently by a new synthetic protocol that provides facile access to several diaminoacetylenes in two steps from 1,1-ethenediamines of the type H2CC(NR2)2, (NR2 = piperidino, 4-methylpiperidino, homopiperidino, N-methylanilino).16 With these alkynes in hand, a series of mono- and bimetallic ruthenium, chromium, and tungsten complexes were prepared, as were heterobimetallic ruthenium−palladium complexes.17 In continuation of these studies, we were interested in the reactivity of diaminoacetylenes toward group 4 metallocenes and present herein our results obtained by the reaction of dipiperidinoacetylene (1a), bis(4-methylpiperidino)acetylene (1b), and bis(diethylamino)acetylene (1c) with several metallocene (Cp2M, M = Ti, Zr) and decamethylmetallocene (Cp*2M, M = Ti, Zr) precursors.

Addition of a second alkyne 1a to MCP, which can be expected to proceed via a bis(η2-alkyne)metallocene (BAM), followed by oxidative addition to furnish 3, is obviously fast.19 Accordingly, when the reagents were provided in a 2:1 ratio (1a:2), the starting materials were completely consumed to give 3 as the sole titanocene species, which was isolated in pure form in 52% yield after recrystallization from hexane solution. The composition of complex 3 was confirmed by NMR spectroscopy in C6D6, and the 1H NMR spectrum exhibits a singlet for the 10 Cp hydrogen atoms at 5.64 ppm. The hydrogen atoms of the piperidinyl groups give rise to overlapping multiplets in the range δ 1.41−1.57 ppm for the 24 methylene hydrogen atoms in the 3-, 4-, and 5-positions, whereas 2 relatively broad signals at 3.02 (ν1/2 = 33 Hz) and 3.40 ppm (ν1/2 = 15 Hz) can each be clearly assigned to 8 NCH2 hydrogen atoms in the 2,6-positions. The latter signals indicate the presence of two chemically and magnetically inequivalent piperidine rings and a relatively slow dynamic process on the NMR time scale at room temperature (vide infra). Similarly, the 13C NMR spectrum shows the Cp signals at 106.2 ppm and altogether six piperidinyl signals, with those of the NCH2 moieties in the 2,6-positions being observed as broadened singlets at 47.5 and 52.0 ppm. In addition, two peaks at 211.2 and 85.4 ppm are found, which can be assigned to the α- and β-carbon atoms of the five-membered metallacycle, respectively (Scheme 2). In comparison with other titanacyclopentadienes,20 these values lie outside the usual ranges of 192.1−204.3 ppm (α) and 112.4−137.5 ppm (β), which can be ascribed to the heteroatom substitution in 3. Single crystals of 3·1/2(n-C6H14) were grown from hexane solution at −30 °C; the molecular structure determined by X-ray diffraction analysis is shown in Figure 1 (top), and pertinent structural parameters are assembled in Table 1. As expected, the molecule exhibits a bent titanocene moiety with staggered Cp rings and a metallacyclopentadiene moiety that shows a pronounced short−long−short alternation of the intraring C−C bond lengths: viz., C1−C2 = 1.362(3) Å, C2−C3 = 1.501(2) Å, C3−C4 = 1.374(2) Å. In contrast to all other crystallographically characterized titanacyclopentadienes,18a,21 however, the TiC4 core in 3 deviates very distinctly from planarity, as indicated by large torsion angles (Table 1), with a maximum value of 83.5(2)° found for the C1−C2−C3−C4 angle. The metallacycle is best described as adopting a C2symmetric twist (or half-chair) conformation,22 in which the carbon atoms C2 and C3 lie above and below the plane defined by the Ti and the adjacent carbon atoms C1 and C4 (see Figure 1, bottom). The resulting puckering of the five-membered ring affords Ti−C distances shorter than those usually observed in planar titanacyclopentadienes,18a,21 and this involves not only the directly bonded carbon atoms C1 (2.135(2) Å) and C4 (2.134(2) Å) but also in particular the facing carbon atoms C2 (2.467(2) Å) and C3 (2.431(2) Å). Together with a noticeably elongated C2−C3 bond length of 1.501(2) Å, the bonding parameters in 3 could be interpreted in terms of a distortion (or mesomeric shift) toward a bis(η2-alkyne)metallocene structure (BAM), which might indicate the tendency to reverse the reactivity shown in Scheme 2 and to release a diaminoalkyne ligand 1a from 3. The pronounced deviation of the TiC4 ring in 3 from planarity can be ascribed to the alignment of the piperidino substituents, which at least partially sustain the conjugation of the nitrogen lone pairs with the metallacyclopentadiene system, despite the observation of substantially pyramidalized N atoms,

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RESULTS AND DISCUSSION Synthesis and Characterization of Metallacyclopentadienes. Addition of dipiperidinoacetylene (1a) to a toluene solution of the bis(trimethylsilyl)acetylene titanocene complex [Cp2Ti(η2-btmsa)] (2) resulted in release of the btmsa ligand and exclusive formation of the dark green titanacyclopentadiene 3 (Scheme 2), irrespective of the ligand to metal ratio emScheme 2. Synthesis of the Titanacyclopentadiene 3a

a

Abbreviations: btmsa, bis(trimethylsilyl)acetylene; pip, piperidin-1-yl.

ployed. Thus, the stoichiometric reaction between 1a and 2 afforded 3 together with unreacted 2, whereas the metallacyclopropene (MCP), which is expected to form as an intermediate,4b,18 could not be observed by NMR spectroscopy. 1775


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Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Metallacyclopentadienes 3 and 5

Figure 1. DIAMOND23 plots of 3 in 3·1/2(n-C6H14) with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms (top and bottom) and methylene carbon atoms (bottom) are omitted for clarity.

a CtCp = centroid of a Cp ring. bAngle between the least-squares planes of the Cp rings.

as evidenced by angle sums of 349.6° (N1), 346.3° (N2), 345.3° (N3), and 355.8° (N4). The angle sum correlates with the degree of π conjugation, which is most pronounced for nitrogen atom N4 with C4−N4 = 1.359(2) Å followed by N1 with C1−N1 = 1.380(2) Å, whereas the much longer C2−N2 and C3−N3 bond lengths of 1.439(2) and 1.443(2) Å indicate significantly less N−C π-bond character. These structural considerations are also consistent with the 13C NMR data, since π donation primarily from the α-piperidinyl groups is in agreement with the pronounced high-field shift observed for the β-carbon atoms (85.4 ppm, vide supra). In view of the C2-symmetric twist conformation in the solid state, a variable-temperature 1H NMR study of 3 in tetrahydrofuran-d8 was performed to investigate dynamic processes in solution. In principle, three processes must be considered: (a) inversion of the chair conformation of the sixmembered piperidine rings, for which a barrier of 11.9 kcal mol−1 can be estimated,24 (b) rotation around the exocyclic nitrogen−carbon bonds, and (c) inversion of the half-chair conformation of the five-membered titanacycle. If all processes were frozen out and if the complex retained at least C2 symmetry, eight separate signals would be expected for the 16 NCH2 hydrogen atoms. In fact, 8 broad signals could be resolved below ca. −90 °C (see the Supporting Information, pp S11−S12), which correlate with 4 13C NMR signals according

to HSQC experiments, indicating that we are dealing with 4 NCH2 groups, each with chemically inequivalent hydrogen atoms. This requires process b to be frozen out but does not permit a distinction between processes a (observation of separate signals for axial and equatorial H atoms) and c (observation of separate signals for diastereotopic CHH′ atoms). Nevertheless, the inversion barrier reported for N-methylpiperidine of 11.9 kcal mol−1 seems to be in good agreement with the coalescence range observed for 3, so that the 8 1H NMR signals can be tentatively assigned to the axial and equatorial NCH2 atoms of the piperidine rings in α- and β-positions. The preparation of the analogous zirconium complex 5 was accomplished by addition of 2 equiv of the diaminoacetylene 1a to a solution of [Cp2Zr(η2-btmsa)(py)] (4, py = pyridine) in hexane, which afforded an immediate color change from dark blue to violet and a dark violet precipitate, which was isolated in high yield (83%) by filtration. If toluene was used as the solvent, a purple solid formed in lower yield, which contained half a molecule of toluene per formula unit even after prolonged drying under vacuum. Alternatively, direct reduction of [Cp2ZrCl2] in THF with Mg turnings in the presence of 1a gave 5 in satisfactory yield (Scheme 3). The NMR spectroscopic characteristics of 5 are similar to those established for 3 (vide supra), revealing the presence of a 1776


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Scheme 3. Synthesis of the Zirconacyclopentadiene 5a

a

Abbreviations: btmsa, bis(trimethylsilyl)acetylene; py, pyridine.

zirconacyclopentadiene structure. Accordingly, the 1H NMR spectrum in C6D6 shows a singlet at 5.85 ppm for the Cp hydrogen atoms, and the piperidinyl groups give rise to 2 broad signals at 3.05 (ν1/2 = 34 Hz) and 3.35 ppm (ν1/2 = 19 Hz) for the 8 NCH2 hydrogen atoms together with overlapping signals in the range δ 1.36−1.67 ppm for the remaining 24 methylene hydrogen atoms. Likewise, the 13C NMR spectrum exhibits characteristic peaks at 203.2 and 93.0 ppm for the α- and β-carbon atoms and at 106.6 ppm for the Cp ligands. The molecular structure of 5 was additionally established by X-ray diffraction analysis of single crystals of 5·1/2C6H6 (Figure 2), corroborating the formation of a metallacyclopentadiene with structural parameters similar to those found for 3 (Table 1). Again, an unusual nonplanar metallacycle has formed,25 which unlike most other zirconacyclopentadienes6,26 shows a C2-symmetric twist conformation with an C1−C2− C3−C4 torsion angle of 89.5(6)°, even larger than in 3, thus resulting in a similar bond alternation pattern with short C1−C2 and C3−C4 bond lengths of 1.368(7) and 1.365(7) Å and a very long C2−C3 bond length of 1.525(7) Å. The pyramidalization of the nitrogen atoms again correlates with the lengths of the exocyclic C−N bonds, with the longest bond observed for N2 (angle sum 343.2°, C2−N2 = 1.463(6) Å), followed by N3 (344.3°/1.456(6) Å), N4 (345.5°/1.380(6) Å), and N1 (354.0°/1.361(6) Å). As mentioned above (Scheme 2), the distortion of the metallacyclopentadienes 3 and 5 toward a bis(η2-alkyne)metallocene structure (BAM) might suggest that a reversal of the oxidative addition reaction and release or substitution of an alkyne ligand in 3 and 5 might be feasible.27 However, the reaction with PMe3 at room temperature gave no indication of the formation of a monoalkyne−phosphine complex with displacement and release of 1a, and decomposition and formation of a complex mixture were observed at elevated temperature. In view of the widespread use of zirconacyclopentadienes for the synthesis of main-group-element heterocycles such as boroles, germanoles, stannoles, and phospholes,2,3,28 the use of 5 for the preparation of electron-rich tetraamino-substituted derivatives of these heterocyclic compounds will be an interesting future goal. Synthesis and Characterization of Metallacyclopropenes. Replacing the bis(trimethylsilyl)acetylene titanocene complex [Cp2 Ti(η 2 -btmsa)] (2) by the corresponding decamethyltitanocene [Cp*2Ti(η2-btmsa)] (6) allowed the introduction of only one ligand 1a, which was achieved by heating a benzene-d6 solution of stoichiometric amounts of 1a and 6 at 60 °C for 12 h. The gradual consumption of the starting materials (the signal for the η2-bound btmsa appears at

Figure 2. DIAMOND23 plots of 5 in 5·1/2C6H6 with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms (top and bottom) and methylene carbon atoms (bottom) are omitted for clarity.

δC 248.5 ppm)4b,29 with concomitant liberation of free btmsa (δC 114.0 ppm)4b,29 can be followed by 13C NMR spectroscopy. The resulting titanacyclopropene complex 7 was isolated as a deep red crystalline material in 67% yield after recrystallization from hexane. Alternatively, 7 could be prepared in higher yield (81%) by reduction of the titanium(III) species [Cp*2TiCl] with Mg turnings in THF solution in the presence of 1a at 60 °C (Scheme 4). A metallacyclopropene structure can be assigned unambiguously to compound 7, which is in agreement with the previous findings that the sterically more congested Cp*2Ti fragment can only accommodate one alkyne ligand. Thus, we find resonances at 1.39 ppm (3-, 4-, 5-CH2) and at 2.67 ppm (2,6NCH2, ν1/2 ≈ 8 Hz). In the 13C NMR spectrum, the metalbound carbon atoms give rise to two low-field signals at 121.0 ppm for the C5Me5 and at 202.3 ppm for the NCCN carbon atoms. The latter resonance is observed at considerably higher field in comparison with the starting material 6 (248.5 ppm) but falls in the range reported for other decamethyltitanocene complexes containing hydrocarbon alkynes such as 2-butyne (200.1 ppm),30 3-hexyne (202.0 ppm),31 and tolane (200.6 ppm).31 In agreement with the 1H NMR data, only three signals at 25.3, 27.0, and 54.3 ppm are found for the carbon atoms of the piperidine rings together with the Cp* methyl resonance at 12.7 ppm. 1777


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Scheme 4. Synthesis of the Titanacyclopropene 7a

a

is rather long in comparison to the range (1.300−1.313 Å) reported for related Cp2*Ti−alkyne systems.4b,29,31,33 Together with relatively short Ti−C1 and Ti−C2 distances of 2.0370(12) and 2.0369(12) Å, this situation indicates a strong metal− alkyne interaction and a pronounced metallacyclopropene character, as also supported by small C1−C2−N2 and C2−C1−N1 angles of 133.34(12) and 132.17(11)°, respectively. The exocyclic C−N bond lengths, viz. C1−N1 = 1.3989(16) Å and C2−N2 = 1.3913(16) Å, suggest a modest degree of π interaction involving markedly pyramidalized nitrogen atoms with angle sums of 348.6° (N1) and 352.4° (N2). Our efforts to prepare the corresponding zirconacyclopropene complex 8a in a fashion similar to that described for its titanium congener 7 failed, and the reaction of diaminoacetylene 1a with [Cp*2Zr(η2-btmsa)] was found to proceed at a much slower rate, leading to incomplete substitution of the btmsa ligand in various solvents (C6D6, THF, or toluene) even at elevated temperature. Above 60 °C, decomposition and formation of a complex reaction mixture was observed. Therefore, decamethylzirconocene dichloride [Cp*2ZrCl2] was employed, and its reduction with an excess of magnesium in THF in the presence of 1 equiv of 1a afforded an olive green solid (Scheme 5). NMR spectroscopic characterization suggested the presence of a mixture of complexes, and it was found impossible to completely remove the magnesium chloride formed as a side product. Even samples obtained after repeated extractions with hexane and filtration through Celite contained variable amounts of MgCl2, as indicated by elemental analyses. Nonetheless, the mass spectrum showed the molecular ion 8a•+ at m/z 552. From the many attempts to obtain single crystals of 8a, yellow plates and green prisms were grown from hexane solution at ca. −30 °C, and X-ray diffraction analyses established the molecular structures of 8a·MgCl2, which crystallized as a hexane solvate (yellow plate, Figure 4), and of the desired zirconacyclopropene 8a (green prism, Figure 5). The crystal lattice of the MgCl2 adduct of 8a is composed of centrosymmetric dimers of the type [8a·MgCl2]2, with the halves of the molecule being connected by an Mg2Cl2 square with symmetrically bridging chlorine atoms (Figure 4).

Abbreviation: btmsa, bis(trimethylsilyl)acetylene.

Figure 3. DIAMOND23 plot of 7 with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

The result of an X-ray diffraction analysis of red single crystals of 7 is shown in Figure 3, with pertinent structural data assembled in Table 2. The complex displays approximate C2 symmetry (rms deviation 0.07 Å) with the 2-fold axis running through the Ti atom and the center of the C1−C2 bond (CtC1C2). The η5-bound Cp* ligands adopt a staggered conformation, and the Ti atom resides in a pseudo-trigonal-planar coordination sphere with regard to CtC1C2 and the centroids of the Cp rings (CtCp). Thus, the sum of the three CtCp−Ti−CtCp and CtCp−Ti−CtC1C2 angles is 360.0°. The intra-ring C1−C2 bond length of 1.3325(17) Å implies a double bond,32 which

Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Metallacyclopropenes 7 and 8 C1−C2 M−C1 M−C2 C1−N1 C2−N2 M−CtCp1a M−CtCp2a CtCp1−M−CtCp2a CtCp1−M−CtC1C2a CtCp2−M−CtC1C2a C1−M−C2 M−C1−C2 M−C2−C1 N1−C1−C2 C1−C2−N2 N1−C1−C2−N2 ϕb

7 (M = Ti)

8a (M = Zr)

8b (M = Zr)c

8c (M = Zr)

1.3325(17) 2.0370(12) 2.0369(12) 1.3989(16) 1.3913(16) 2.1488(2) 2.1444(2) 135.741(10) 112.435(10) 111.817(9) 38.18(5) 70.90(7) 70.91(7) 132.17(11) 133.34(12) 2.7(2) 39.97(5)

1.344(2) 2.1566(14) 2.1545(13) 1.4042(19) 1.4025(19) 2.2642(1) 2.2667(1) 137.364(1) 111.142(3) 111.489(1) 36.32(5) 71.75(9) 71.92(9) 130.88(14) 130.95(14) 0.6(3) 40.29(6)

1.355(2) 2.1593(12)

1.345(2) 2.1709(14) 2.1663(13) 1.4060(19) 1.4090(17) 2.2642(2) 2.2602(2) 137.160(6) 111.511(6) 111.326(7) 36.14(5) 71.74(9) 72.12(8) 131.58(13) 131.77(13) 5.0(3) 41.30(6)

1.4060(16) 2.2615(1) 137.250(1) 111.375(2) 36.56(6) 71.72(3) 129.53(6) 2.7(2) 40.6(5)

a

Definitions: CtCp, centroid of a Cp ring; CtC1C2, center of the C1−C2 bond. bAngle between the least-squares planes of the Cp rings. cThe molecule displays crystallographic C2 symmetry. 1778


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Scheme 5. Synthesis of the Zirconacyclopropenes 8a

a

Abbreviation: pip, piperidin-1-yl.

Figure 4. DIAMOND23 plot of [8a·MgCl2]2 in [8a·MgCl2]2·n-C6H14 with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−C2 1.329(2), C1−N1 1.514(2), C2−N2 1.377(2), Zr−C1 2.3044(16), Zr−C2 2.2455(16), Zr−Ct Cp1 2.3256(1), Zr−Ct Cp2 2.3107(1), Zr−Cl1 2.6612(4), Mg−N1 2.0779(15), Mg−C1 2.2682(18), Mg−Cl1 2.3238(7), Mg−Cl2 2.3997(7), Mg−Cl2′ 2.4241(6); C1−Zr−C2 33.94(6), C1−C2−Zr 75.46(10), C2−C1−Zr 70.60(10), CtCp1−Zr−CtCp2 133.674(6), Zr− C1−N1 155.07(11), Zr−C2−N2 146.01(12), Mg−N1−C1 76.54(9), N1−Mg−C1 40.47(5), C1−Mg−Cl1 97.47(5), N1−Mg−Cl1 137.65(5), Cl2−Mg−Cl2′ 93.59(2), Mg−Cl2−Mg′ 86.41(2), ∑(N1) 340.9(4), ∑(N2) 355.1(4).

Figure 5. DIAMOND23 plots of 8a (top), 8b (center), and 8c (bottom) with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

The central four-membered ring is connected to the two complex fragments 8a through the Mg atoms, which are bound to the nitrogen and carbon atoms N1 and C1 of one diaminoalkyne ligand with Mg−N1 = 2.0779(15) Å and Mg−C1 = 2.2682(18) Å. In addition, one μ-Cl ligand on each side bridges the Zr and Mg atoms, affording a plane that also includes the N1−C1−C2−N2 fragment and that is almost perpendicular to the Mg2Cl2 plane (interplanar angle 88.0(8)°). MgCl2 coordination gives rise to a marked distortion of the regular metallacyclopropene structure expected for 8a, as indicated by significantly unequal Zr−C1 and Zr−C2 bond lengths of 2.3044(16) and 2.2455(16) Å and, in particular, by disparate C1−N1 and C2−N2 bond lengths of 1.514(2) and 1.377(2) Å. Moreover, the C1−C2 distance of 1.329(2) Å is longer than those observed for most reported zirconacyclopropenes1a,4a,c,d,10,34 but shorter than those in some decamethylzirconocene complexes35 such as complexes 8 (vide infra). Furthermore, the alkyne ligand is significantly twisted with respect to the zirconocene moiety in order to accommodate the additional chlorine ligand at the zirconium atom. It is also interesting to note that [8a·MgCl 2]2 represents a so-called “anti-van’t Hoff/LeBel compound” containing a

planar-tetracoordinate carbon atom (C1), which shows a deviation of 0.020(2) Å from the least-squares Zr−Mg−N1− C2 plane. Several examples of this class of compounds involve related zirconocene−alkyne species in which the square-planar carbon environment is typically stabilized by the interaction of Zr/Al and Zr/B pairs in a fashion similar to that achieved here by a Zr/Mg combination.36 The structure of the zirconacyclopropene 8a (Figure 5, top) is isotypic with that of its titanium analogue 7 (vide supra, Figure 3), and accordingly, all general structural considerations with regard to the approximate C2 symmetry of the complex and the pseudo-trigonal-planar coordination sphere around the Zr atom also apply to 8a (Table 2). Naturally,32,37 the Zr−C bond distances in 8a are longer than the Ti−C distances in 7: viz., Zr−C1 = 2.1566(14) Å, Zr−C2 = 2.1545(13) Å vs Ti−C1 = 2.0370(12) Å, Ti−C2 = 2.0369(12) Å. The C1−C2 bond in 8a (1.344(2) Å) is just slightly longer than in 7 (1.3325(17) Å), which is in agreement with the general trend found for titana- and zirconacyclopropenes.38 The C1−C2 1779


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remove MgCl2 from 8a−c, all three complexes gave rise to numerous signals in their NMR spectra in addition to those that can be assigned to symmetric zirconacyclopropene structures. For instance, the 1H NMR spectra showed several additional singlets in approximately 15:3:3:3:3 ratios, which fall in the range for Cp* methyl groups. Together with the observation of two doublets in each spectrum at markedly lower field (2.45−2.79 ppm) with 2J coupling constants of 5.3 Hz, we concluded that formation of the second complex involves C−H activation of one Cp* ligand in 8, affording “tucked-in” complexes of type 9 (Scheme 6).39 2D NMR

bond length in the latter complex again indicates a strong metal−alkyne interaction; it is longer than the corresponding bond in 8a·MgCl2 (1.329(2) Å) and also longer than that usually observed for other Cp2*Zr−alkyne species, which show a range of 1.312−1.340 Å.4d,34,35 The difficulties encountered with the separation of MgCl2 from 8a prompted us to employ bis(4-methylpiperidino)acetylene (1b), which, in contrast to 1a, is a crystalline compound at room temperature.16 Therefore, we expected that the corresponding zirconacyclopropene 8b might be easier to purify and separate from MgCl2 by fractional crystallization. The preparation of 8b was achieved in a fashion similar to that described for 8a from [Cp*2ZrCl2], magnesium, and 1b in THF solution (Scheme 5), but NMR spectroscopic characterization of the green material isolated after extraction with hexane indicated again that a mixture of complexes was present in solution. Elemental analyses indicated lesser amounts of MgCl2, but it was still difficult, albeit possible eventually (see the Experimental Section), to obtain satisfactory results by combustion analysis. However, green single crystals of 8b were conveniently obtained from pentane solution, and there was no indication for the cocrystallization of another material such as the MgCl2 adduct observed for 8a (vide supra). X-ray diffraction analyses confirmed the formation of a zirconacyclopropene, and the molecular structure of 8b displays crystallographic C2 symmetry with the rotational axis running through the Zr atom and the centroid of the coordinated C−C triple bond (Figure 5, center); the methyl groups are equatorially directed from the piperidine rings, as would be expected. The structural parameters are very similar to those established for 8a (Table 2). The C1−C1′ bond length in 8b is 1.355(2) Å, and comparison with the corresponding bond length of 1.2067(2) Å determined for the free alkyne 1b in the solid state reveals the expected elongation upon metal complexation.16 Finally, bis(diethylamino)acetylene (1c) was employed, which can be regarded as a sterically more demanding ynediamine because of the larger, acyclic NEt2 substituents in comparison with the cyclic, six-membered piperidino substituents in the alkynes 1a,b. Reduction of [Cp*2ZrCl2] with Mg in THF at 60 °C in the presence of 1 equiv of 1c afforded a deep red solution within a few minutes (Scheme 5). After extraction with hexane, large dichroic orange-green crystals precipitated from a saturated solution stored at −30 °C, and the molecular structure of 8c was determined by X-ray diffraction analysis (Figure 5, bottom). The structure displays approximate C2 symmetry (rms deviation 0.06 Å). Again, the overall structural parameters are very similar to those discussed for 8a,b (Table 2), although the zirconium−carbon distances Zr−C1 = 2.1709(14) Å and Zr−C2 = 2.1663(13) Å are slightly longer. The latter might indicate a somewhat greater steric demand of the alkyne ligand, in agreement with the trend found for the H−Zr−H cone angles involving the NCH2 hydrogen atoms, which increase in the order 113.6° (8b) < 116.0° (8a) < 120.9° (8c). Irrespective of the question whether steric reasons can be held responsible, 8c can be much more easily separated from, and crystallized without, MgCl2, allowing proof of its purity by elemental analysis. Yet again, the 1H and 13C NMR spectra showed the resonances for at least two different complexes, and therefore a more detailed investigation of the NMR spectroscopic properties of the zirconium complexes 8 was required, which is presented hereinafter. NMR Spectroscopic Characterization of the Interconversion between Decamethylzirconocene−Diaminoalkyne and Tetramethylfulvene−Diaminovinyl Zirconium Complexes. Despite our (partially successful) efforts to

Scheme 6. Equilibrium between Zirconacyclopropenes 8 and Fulvene Complexes 9

spectroscopy allowed the assignment of all resonances for the mixtures 8/9 (Table 3), in which the complexes are present in 1:1.3 (8a/9a), 1:1.3 (8b/9b), and 1:2.0 ratios (8c/9c), respectively. The number of NMR signals observed for complexes 8 reveals time-averaged C2v symmetry in solution, and the 13C NMR resonances of all metal-bound carbon atoms are in agreement with those established for other zirconacyclopropenes (δ 205.1−260.5 ppm),4a,c−e,34,35 even though the signals for the alkyne carbon atoms in 8 appear at slightly higher field. Since complexes 9 display C1 symmetry, they give rise to more complex NMR spectra, and in addition to the 1H NMR signals for the Cp* and tetramethylpentafulvene (C5Me4CH2) ligands (vide supra) and for the amine substituents, each complex shows a resonance at about 1.1 ppm, which can be assigned to the NCH hydrogen atom of a 1,2diaminovinyl moiety, generated by intramolecular C−H activation and hydrogen transfer to the respective diaminoalkyne ligand. The observation of these resonances at high field indicates a β-CH agostic interaction with the Zr atoms,40 which is further corroborated by the observation of the corresponding NCH 13C NMR resonances between 83.0 and 86.1 ppm with reduced coupling constants (1JC,H = 130 Hz in 9a; 1JC,H = 129 Hz in 9c).7b,40,41 As expected, the metal-bound carbon atoms of the vinyl moiety resonate at significantly lower field (ca. 180 ppm). Overall, the NMR resonances assigned to the pentamethylcyclopentadienyl−tetramethylpentafulvene unit in complexes 9 are in good agreement with those reported for other zirconium complexes of the type [Cp*Zr(η6-C5Me4CH2)X] (X = Ph,42,43 Me,44 C6F5,45 crotyl,46 C3H5).46,47 For instance, the α-CH2 group in the phenyl complex [Cp*Zr(η 6C5Me4CH2)(Ph)] gave rise to a 13C NMR signal at 63.5 ppm with 1JC,H = 145 Hz, whichas for complexes 9 (Table 3) falls in the region of σ-bonded Zr−C(sp3) data.42 It can therefore be concluded that the bonding in complexes 9 is also best represented by a ring-metalated (“tucked-in” methylenecyclopentadienyl) σ,η5-description rather than by a fulvene-like π-bonded situation.39,42−48 The latter complex was prepared by thermolysis of the diphenyl complex [Cp*2Zr(C6H5)2], affording [Cp*Zr(η6-C5Me4CH2)(C6H5)] and 1 equiv of benzene 1780


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Table 3. Assignment of Pertinent NMR Signals for Compounds 8 and 9a C5Me5 C5Me5 ZrCN C5Me4CH2 C5Me4CH2 C5Me4CH2 NCH a

1

H 13 C 13 C 13 C 1

H 13 C 1 H 13 C 13 C 1 H 13 C

8a

9a

8b

9b

8c

9c

1.96 (30H) 12.2 119.2 197.8

1.92 (15H) 12.5 115.2 179.6

1.97 (30H) 12.2 119.2 197.5

1.92 (15H) 12.5 115.2 179.4

1.95 (30H) 12.1 119.2 198.4

1.91 (15H) 12.4 115.3 179.7

9a

9b

9c

1.39, 1.75, 1.78, 2.05 10.9, 11.1, 12.8, 14.3 2.51, 2.79 (2J = 5.3 Hz) 55.8 (1JC,H = 145 Hz) 116.0, 116.1, 118.2, 121.4, 122.6 1.04 86.1 (1JC,H = 130 Hz)

1.38, 1.75, 1.79, 2.05 10.9, 11.2, 12.9, 14.3 2.50, 2.78 (2J = 5.3 Hz) 55.8 116.0, 116.1, 118.2, 121.4, 122.6 1.07 not obsd

1.37, 1.74, 1.79, 1.97 11.0, 11.3, 12.5, 13.9 2.45, 2.72 (2J = 5.3 Hz) 56.2 (1JC,H = 145 Hz) 116.0, 116.3, 118.0, 121.0, 122.6 1.18 83.0 (1JC,H = 129 Hz)

Values given in ppm. The solvent was C6D6.

(C6H6). Isotope-labeling studies revealed that this reaction proceeds via the benzyne intermediate [Cp*2Zr(η2-C6H4)], followed by intramolecular C−H bond activation of one Cp* methyl group and hydrogen atom transfer to the benzyne ligand.42 A transition state with possible tunneling contributions was suggested for this process, and we assume that a similar mechanism, by intramolecular hydrogen transfer to the diaminoalkyne ligand, can be considered for the conversion of complexes 8 into 9. As mentioned above, we were only able to crystallize complexes 8 from solutions containing both 8 and 9, and dissolution of these crystals again afforded the original mixtures. These observations suggest that the species are in equilibrium, which was tested by two-dimensional exchange NMR spectroscopy (2D EXSY) for 8a/9a and 8c/9c. At room temperature, however, 1H,1H EXSY spectra of these mixtures did not show exchange peaks between the signals that were assigned to the different isomers, indicating either that 8 and 9 are not in equilibrium or that the activation barrier for the intramolecular hydrogen transfer is too high to observe their interconversion at a sufficient rate at room temperature. The latter assumption was indeed confirmed by corresponding experiments at elevated temperatures, which unequivocally reveal the expected exchange peaks, and as an example, part of the spectrum recorded for 8c/9c at 103 °C can be found in the Supporting Information (pp S16−S17). For this pair, the conversion from 8c to 9c was also followed by 1H NMR spectroscopy. Hence, single crystals of 8c were dissolved in cold (−35 °C) toluene-d8 in an NMR tube, and NMR spectra were directly recorded in the temperature range from −30 to +20 °C (see the Supporting Information, p S17). Below 0 °C, the conversion is very slow and cannot be followed in a reasonable period of time. Therefore, the NMR sample was warmed to 20 °C, allowing us to monitor the conversion by integration of the Cp* signals for 8c and 9c. Following the established treatment for a first-order reaction proceeding to equilibrium,49,50 a plot of ln(A9c,∞ − A9c,t) against time confirmed the isomerization to be a first-order process with a rate constant of k293 = [2.27(7)] × 10−4 s−1, corresponding to a half-life of 50.9(15) min and a free activation energy of ΔG⧧ = 22.0(2) kcal mol−1 at T = 293 K (Figure 6 and Supporting Information, pp S18−S21). The observation of similar amounts of complexes 8 and 9 in the reaction mixtures with the equilibrium constants K = [9]/[8] ranging between 1.3 and 2.0 (vide supra) indicates

Figure 6. First-order kinetic plot for the conversion of 8c to 9c in toluene-d8 at 20 °C monitored by 1H NMR spectroscopy.

that these species are very similar in energy, with a slight thermodynamic preference for complexes 9 (ΔG298 = −0.16 kcal mol−1 for K = 1.3 and ΔG298 = −0.41 kcal mol−1 for K = 2.0). To assess their relative stabilities theoretically, DFT calculations on the pairs 8a/9a and 8c/9c were carried out employing the dispersion-corrected B97-D and M06-2X functionals.51 The standard Pople basis set 6-311g(d,p) was used for all main-group elements, and a Stuttgart−Dresden basis set and effective core potential were used for zirconium. Furthermore, we were able to locate the respective transition states for the interconversion of these isomers, and the resulting energy profile is shown in Figure 7. For both systems, the structurally characterized zirconacyclopropene isomer 8 is energetically favored by about 4−6 kcal mol−1, which is not entirely consistent with the experimental findings but could be ascribed to the neglect of solvation effects under the pseudogas-phase conditions applied. However, the activation barrier is of the order of 26−29 kcal mol−1, which falls in a range similar to that experimentally observed for 8c/9c (vide supra) and for the formation of [Cp*Zr(η6-C5Me4CH2)(C6H5)] and related species.42 The structural parameters derived for 8a,c with both functionals are in excellent agreement with those established by X-ray diffraction (Table 2), and a comparison of experimental and theoretical data can be found in the Supporting Information (Tables S4 and S5). Furthermore, the calculated structures of 9a,c resemble those of crystallographically characterized tetramethylfulvene complexes; for instance, the average Zr−C distance to the “tucked-in” methylene group is 1781


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Figure 7. Calculated Gibbs free energy profile for the intramolecular C−H activation and hydrogen transfer reaction involving the pairs 8a/9a and 8c/9c. The values indicate the relative B97-D and M06-2X/6-311g(d,p) energies; for full thermodynamic data and for calculated structural parameters, see the Supporting Information (pp S23−S26). DIAMOND23 drawings of the calculated B97-D structures are shown for the 8a/9a system.

thermodynamic data for this process were derived experimentally by NMR spectroscopy, and the mechanistic proposal was supported by DFT calculations. It should be noted that to the best of our knowledgethis type of intramolecular reaction is unprecedented for isolated group 4 metallocene alkyne complexes4a,d but had previously been proposed for a transient benzyne complex.42 These results give further evidence for the ability of heteroatom-functionalized acetylenes to give rise to different and unexpected reactivities in organotransition-metal chemistry, and this concept will be further exploited.

2.39 Å, which is very similar to the values established for Cp*Zr(η6-C5Me4CH2)X] (2.388(7) Å, X = Ph;42 2.371(5) Å, X = C6F5)45 and [Cp*Zr{−C(CCHtBu)CHtBuCH2−η6-C5Me3CH2−}] (2.389(5) Å).52 The 1,2diaminoalkenyl groups in 9a,c exhibit structural characteristics of β-agostic vinyl ligands:7b,c,41,53 viz., elongation of the metal-bound C−H bonds and virtually rectangular Zr−C1− C2 angles (see the Supporting Information, Tables S4 and S5 and Figures S2−S5). The shortest Zr−H distances (ca. 1.92 Å) are found in the transition states for both interconversions, with the hydridic H atom being located almost equidistant from the releasing and accepting carbon atoms C3 and C1, respectively. This situation is in agreement with a metal-centered C−H activation pathway for the conversion of complexes 8 into 9.42

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EXPERIMENTAL SECTION

General Considerations. All operations were performed under an atmosphere of dry argon using Schlenk and vacuum techniques. Solvents were purified and dried by an MBraun GmbH solvent purification system or by conventional methods54 (deuterated solvents). [Cp2TiCl2] (Acros), [Cp2ZrCl2] (Alfa Aesar), and bis(trimethylsilyl)acetylene (Acros) were obtained commercially and used without further purification. Mg turnings (99.9%) were purchased from Acros and activated by stirring them with iodine under an Ar atmosphere for 2 h. [Cp2Ti(η2-btmsa)] (2)4b and [Cp*2ZrCl2]4d were synthesized by slightly modified literature procedures (see the Supporting Information, p S2). Dipiperidinoacetylene (1a), 16 bis(4-methylpiperidino)acetylene (1b), 16 bis(diethylamino)acetylene (1c),55 [Cp 2Zr(η2 -btmsa)(py)] (4),4a [Cp*2Ti(η2-btmsa)] (6),4b and [Cp*2TiCl]56 were prepared according to the literature procedures. For sonication, a laboratory ultrasonic cleaning device, S15 Elmasonic (35 W), was applied. A Bruker Vertex 70 spectrometer was used for recording IR spectra. NMR spectra were recorded on Bruker DPX-200, Bruker AV II-300, Bruker AV II-400, Bruker DRX-400, and Bruker AV II-600 devices. The chemical shifts (δ) are expressed in ppm and are given relative to internal TMS (δ 0.00 ppm), to residual solvent 1H signals (toluene-d7, δH 2.08 ppm; tetrahydrofuran-d7, δH 3.58 ppm; C6HD5, δH 7.16 ppm), or to the 13C resonance of the solvents (toluene-d8, δC 20.43 ppm; tetrahydrofurand8, δC 67.21 ppm; C6D6, δC 128.06 ppm). The number of protons attached to each carbon was determined by 13C-DEPT135 experiments. If required, signal assignment was achieved by two-dimensional

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CONCLUSIONS The present study reveals significant differences in the coordination chemistry of diaminoacetylenes with group 4 metallocenes in comparison to conventional alkyne ligands. Even though the expected formation of metallacyclopentadienes is observed with the metallocene fragments Cp2M (M = Ti, Zr) by coupling of two alkyne ligands, the structures of the resulting five-membered metallacycles differ significantly from the planar geometry almost exclusively found for related species. Here, an unusual twisted (half-chair) conformation is observed for the complexes 3 and 5 in the solid state, which show a complex dynamic behavior in solution. In contrast, the sterically more demanding metallocene fragments Cp*2M (M = Ti, Zr) can only accommodate one diaminoacetylene ligand, affording the expected titana- and zirconacyclopropenes 7 and 8a−c. However, the latter species are found to be in equilibrium with the respective tucked-in tetramethylpentafulvene complexes 9a−c, which are formed by intramolecular C−H activation and hydrogen transfer to the alkyne ligand with generation of a 1,2-diaminovinyl moiety. Kinetic and 1782


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and hydrogen values, which we tentatively ascribe to partial loss of hexane. Synthesis of 1,1-Bis(η5-cyclopentadienyl)-2,3,4,5-tetrakis(1-piperidinyl)-1-zirconacyclopentadiene ([Cp2Zr(C4pip4)], 5) from [Cp2Zr(η2-btmsa)(py)] (4). To a solution of 4 (471 mg, 1.00 mmol) in hexane (3 mL) was added a solution of 1a (404 mg, 2.10 mmol, 2.1 equiv) in the same solvent (2 mL). The mixture was stirred for 5 min, while the color changed from deep blue to deep purple. Considerable amounts of a very dark violet precipitate were observed, and therefore the mixture was filtered through a frit and washed with hexane (2.5 + 1.5 + 1.0 mL), leaving deep red washings. The dark violet solid was dried under high vacuum and stored in a glovebox. Yield: 83% (503 mg). Alternatively, complex 5 was synthesized in toluene following the procedure described above. However, instead of a dark violet solid, a purple solid was obtained and the analyses showed the presence of a half molecule of toluene per molecule of 5. Yield: 50% (163 mg). Crystals suitable for X-ray diffraction analysis, which proved to be the benzene hemisolvate, were obtained by storing a concentrated C6H6 solution at ambient temperature. 1H NMR (C6D6, 300.1 MHz, 300 K): δ 1.36−1.67 (overlapped m, 24H, 3,4,5-CH2), 2.11 (Me, toluene), 3.05 (br s, ν1/2 ≈ 34 Hz, 8H, 2,6CH2), 3.35 (br s, ν1/2 ≈ 19 Hz, 8H, 2,6-CH2), 5.85 (s, 10H, C5H5), 6.99−7.14 (CH, toluene) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 300 K): δ 21.4 (Me, toluene) 25.5, 25.9 (2 × 4-CH2), 27.2, 27.9 (2 × 3,5-CH2), 47.7 (br, ν1/2 ≈ 5.5 Hz, 2,6-CH2), 93.0 (β-Cq), 106.6 (C5H5), 125.7, 128.6, 129.3 (5 × CD, toluene) 203.2 (α-Cq) ppm; one of the 2,6-CH2 signals was not observed, presumably because of extreme broadening. For NMR spectral data in THF-d8 see the Supporting Information (p S6). Anal. Calcd for C34H50N4Zr·1/2C7H8 (625.10): C, 69.07; H, 8.35; N, 8.59. Found: C, 69.00; H, 8.23; N, 8.60. Synthesis of 5 from [Cp2ZrCl2]. To a clear, yellowish solution of [Cp2ZrCl2] (59.7 mg, 0.20 mmol) and alkyne 1a (82 mg, 0.43 mmol, 2.1 equiv) in THF (0.6 mL) were added magnesium turnings (26.2 mg, 1.1 mmol, 5.4 equiv), and the reaction mixture was vigorously stirred overnight. The purple suspension was dried under high vacuum and then extracted with toluene (10 mL). The extract was filtered through a Celite pad and concentrated. After trituration with hexane, followed by filtration, the powder was dried under high vacuum for 1 h. Yield: 82% (98 mg). Synthesis of 1,1-Bis(η5-pentamethylcyclopentadienyl)-2,3-bis(1piperidinyl)titanacyclopropene ([Cp*2Ti(C2pip2)], 7) from [Cp*2Ti(η2-btmsa)] (6). A silylated and oven-dried NMR tube was charged with a benzene-d6 solution (0.50 mL) of 6 (68 mg, 0.139 mmol) and alkyne 1a (28 mg, 0.135 mmol). The reaction mixture was heated in an oil bath (60 °C), and the progress of the reaction was monitored by 1H NMR spectroscopy. After a conversion of ca. 95% was achieved in 12 h, the reaction mixture was cooled and all volatiles were removed under high vacuum. Crystallization from hexane (ca. 0.5 mL) at ambient temperature resulted in the formation of large, deep red crystals, which after removal of the mother liquor were dried under high vacuum. Yield: 67% (46 mg) of a highly air-sensitive solid. IR (Nujol): 1655 cm−1 (CC). 1H NMR (C6D6, 300.1 MHz, 300 K): δ 1.39 (br s, ν1/2 ≈ 17 Hz, 6H, 3,4,5-CH2), 1.91 (s, 15H, C5Me5), 2.67 (br s, ν1/2 ≈ 8 Hz, 4H, 2,6-CH2) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 300 K): δ 12.8 (C5Me5), 25.4 (4-CH2), 27.1 (3,5CH2), 54.4 (2,6-CH2), 121.0 (C5Me5), 202.3 (Ti−C(N)) ppm. Anal. Calcd for C32H50N2Ti (510.63): C, 75.27; H, 9.87. Found: C, 74.85; H, 9.85. Synthesis of 7 from [Cp*2TiCl]. A 10 mL Schlenk flask was charged with Mg turnings (19 mg, 0.8 mmol, 4.0 equiv), a THF solution (1.0 mL) of [Cp*2TiCl] (141.5 mg, 0.40 mmol), and a solution of 1a (81 mg, 0.42 mmol, 1.05 equiv) in the same solvent. The deep blue reaction mixture was heated in an oil bath (60 °C) for 1 h, and the color changed to brown. The solvent was removed under high vacuum; the residue was extracted with hexane and the extract then filtered through a Celite pad. After removal of the solvent under high vacuum a deep red solid was obtained in analytically pure form. Yield: 81% (165 mg).

H,H-COSY, H,H-NOESY, H,C-HSQC, and H,C-HMBC NMR experiments. The following abbreviations are used for spin multiplicity: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad. Elemental analyses (C, H, N, S) succeeded by combustion and gas chromatographic analysis on a VarioMICRO Tube instrument at the Technische Universität Braunschweig. MS (EI) measurements were performed on a Finnigan MAT 95-XP (Thermo Electron) mass spectrometer (70 eV) at the LIKAT Rostock. Theoretical Calculations. All computations were performed using the Gaussian 09 package.57 All structures were fully optimized on the DFT level employing the B97-D51a and M06-2X51b functionals. For all main-group elements (C, H, and N) the all-electron triple-ζ basis set (6-311G(d,p)) was used,58 whereas for the transition metal (Zr) the effective core potential double-ζ basis set (Stuttgart RSC 1997 ECP) was applied. X-ray Diffraction Studies. Data were recorded on an Oxford Diffraction Nova A diffractometer, using mirror-focused Cu Kα radiation (complexes 5, [8a·MgCl2] 2, and 8b), on an Oxford Diffraction Xcalibur E diffractometer, using graphite-monochromated Mo Kα radiation (complexes 3, 7, and 8c), or on a Bruker Kappa APEX II Duo diffractometer, using graphite-monochromated Mo Kα radiation (complex 8a). All structures were solved by direct methods (SHELXS-97) and refined anisotropically by full-matrix least-squares procedures on F2 using the SHELXL-97 program. 59 Hydrogen atoms were (i) located and refined isotropically (hydrogen atoms of the Cp rings in complex 3), (ii) included as idealized methyl groups allowed to rotate but not tip (all methyls), or (iii) placed geometrically and allowed to ride on their attached carbon atoms (all other H atoms). Numerical details are summarized in Table S3 (see the Supporting Information). Special features and exceptions: for 3 and [8a·MgCl 2] 2, the solvent could not be refined satisfactorily and was therefore removed mathematically using the routine SQUEEZE, which forms part of the PLATON suite (A. L. Spek, University of Utrecht, The Netherlands). The minute crystal of 5 displayed a very irregular and variable reflection profile, which is probably responsible for the high R(int) value; the final R values are acceptable, however. Complete data have been deposited at the Cambridge Crystallographic Data Centre under the numbers CCDC 983995 (3·1/ 2(n-C 6H14)), 983996 (5· 1/ 2C6H 6), 983997 (7), 983998 (8a), 983999 (8a·MgCl2·(n-C 6H14)), 984000 (8b), and 984001 (8c). These data can be obtained free of charge from www.ccdc.cam.ac. uk/data_request/cif. Experimental Procedures. Synthesis of 1,1-Bis(η5-cyclopentadienyl)-2,3,4,5-tetrakis(1-piperidinyl)-1-titanacyclopentadiene ([Cp2Ti(C4pip4)], 3). To a stirred, dark yellow solution of 2 (349 mg, 1.00 mmol) in hexane (10 mL) was added a solution of dipiperidinoacetylene (404 mg, 2.1 equiv) in the same solvent (5 mL) at ambient temperature. The reaction mixture gradually turned dark green. The homogeneous reaction mixture was left for 2 h without stirring, whereupon large, dark green crystals precipitated. The supernatant was discarded, and the solid was washed with the same solvent (2 × 5 mL) and, finally, dried under high vacuum. Yield: 52% (295 mg) of a fairly air-sensitive, dark green, crystalline solid. Crystals of composition 3·1/2(n-C6H14), suitable for X-ray diffraction analysis, were obtained by storing a concentrated n-hexane solution at −30 °C. 1 H NMR (toluene-d8, 600.1 MHz, 295 K): δ 1.42 (br s, ν1/2 ≈ 21 Hz, 4H, β-4-CH2), 1.51 (br s, ν1/2 ≈ 25 Hz, 4H, α-4-CH2), 1.53 (br m, ν1/2 ≈ 20 Hz, 8H, β-3,5-CH2), 1.56 (br m, ν1/2 ≈ 20 Hz, 8H, α-3,5-CH2), 2.98 (br s, ν1/2 ≈ 33 Hz, 4H, β-2,6-CH2), 3.37 (br s, ν1/2 ≈ 15 Hz, 4H, α-2,6-CH2), 5.56 (s, 5H, C5H5) ppm. 13C{1H} NMR (toluene-d8, 150.9 MHz, 295 K): δ 25.35 (α-4-CH2), 25.84 (β-4-CH2), 27.21 (α3,5-CH2), 27.91 (β-3,5-CH2), 47.40 (br, ν1/2 ≈ 3 Hz, β-2,6-CH2), 51.97 (br, ν1/2 ≈ 23 Hz, α-2,6-CH2), 85.23 (β-Cq), 106.14 (C5H5), 210.63 (α-Cq) ppm. For NMR spectral data at low temperature and in C6D6 see the Supporting Information (pp S11−S12). Anal. Calcd for C34H50N4Ti·1/2(n-C6H14) (605.74): C, 73.36; H, 9.48. Found: C, 71.75; H, 9.26. Combustion analysis repeatedly produced low carbon 1783


Organometallics

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Synthesis of 1,1-Bis(η5-pentamethylcyclopentadienyl)-2,3-bis(1-piperidinyl)-1-zirconacyclopropene ([Cp*2Zr(C2pip2)], 8a) and (η5-Pentamethylcyclopentadienyl)(cis-1,2-bis(1-piperidinyl)vinyl)(η6tetramethylpentafulvene)zirconium(IV) ([Cp*Zr(C 5 Me 4 CH 2 ){C(pip)CH(pip)}], 9a). [Cp*2ZrCl2] (216 mg, 0.50 mmol) and Mg (16 mg, 0.65 mmol, 1.3 equiv) were mixed with THF (1.5 mL), forming a yellow suspension. With stirring, a solution of 1a (101 mg, 0.53 mmol, 1.05 equiv) in THF (1 mL) was added and the mixture was stirred at 60 °C. After 30 min, the reaction mixture turned brown. The mixture was further stirred at 60 °C for 3 h. The solvent was removed, the residue was extracted with hexane (2 mL), and the extract was filtered through a Celite pad and washed with more hexane (2 × 1 mL). The solvent was removed under high vacuum, and a yellowish green solid was isolated. Yield (crude product): 70% (193 mg). Crystals suitable for X-ray diffraction analysis were obtained by storing a concentrated hexane solution at −42 °C. Complex [8a· MgCl2]2 crystallized from the filtrate at −30 °C as a hexane solvate. EIMS: m/z (rel abund) 552 (31.4) [8a]+, 468 (10.5) [8a − pip]+, 360 (68.3) [8a − 1a]+, 193 (100.0) [1a + H]+. IR (ATR): 1634 cm−1 (CC). 1H NMR (C6D6, 400.4 MHz, 297 K): ratio 8a:9a ca. 1:1.3; 8a, δ 1.43−1.53 (m, 4H, 4-CH2), 1.54−1.61 (br m, ν1/2 ≈ 15 Hz, 8H, 3,5-CH2), 1.96 (s, 30H, C5Me5), 2.88 (t, 3J2,6‑H,3,5‑H = 5.2 Hz, 8H, 2,6CH2); 9a, δ 1.04 (s, 1H, 19), 1.27−1.33 (br m, ν1/2 ≈ 21 Hz, 2H, 14), 1.39 (s, 3H, 5), 1.43−1.53 (m, 6H, 13 or 16 and 17), 1.54−1.61 (br m, ν1/2 ≈ 15 Hz, 4H, 13 or 16), 1.75 (s, 3H, 2), 1.78 (s, 3H, 3), 1.92 (s, 15H, 1), 2.05 (s, 3H, 4), 2.43−2.61 (br m, ν1/2 ≈ 28 Hz, 4H, 15), 2.51 (d, 2JH,H = 5.3 Hz, 1H, 6), 2.79 (dd, 2JH,H = 5.3 Hz, 3J6−19 = 0.5 Hz, 1H, 6), 3.25−3.43 (br m, ν1/2 ≈ 40 Hz, 2H, 14), 3.54−3.73 (br m, ν1/2 ≈ 42 Hz, 2H, 14). 13C{1H} NMR (C6D6, 100.7 MHz, 299 K): 8a, δ 12.2 (C5Me5), 25.7 (4-CH2), 27.4 (3,5-CH2), 54.4 (2,6-CH2), 119.2 (C5Me5), 197.8 (Zr−C(N)); 9a, δ 10.9 (3), 11.1 (2), 12.5 (C5Me5), 12.8 (4), 14.3 (5), 24.7 (12 or 17), 25.6 (12 or 17), 26.7 (16), 27.6 (13), 55.8 (6), 56.2 (15), 86.1 (19), 115.2 (C5Me5), 116.0 (9), 116.1 (8), 118.2 (7), 121.4 (11), 122.6 (10), 179.6 (18). The 13C signal for 14 was not observed. For additional NMR data see the Supporting Information (p S13). Elemental analysis did not succeed because of the presence of variable amounts of MgCl2.

35.9 (3,5-CH2), 53.5 (2,6-CH2), 119.2 (C5Me5), 197.5 (Zr−C(N)); 9b, δ 10.9 (3), 11.2 (2), 12.5 (C5Me5), 12.9 (4), 14.3 (5), 22.3 (21), 22.6 (20), 30.9 (17), 32.0 (12), 35.0, 35.3 (2 × 16), 36.1 (br, ν1/2 ≈ 8 Hz, 13), 36.2 (br, ν1/2 ≈ 10 Hz, 13), 54.5 (15), 55.8 (6), 56.6 (br, ν1/2 ≈ 22 Hz, 14), 115.2 (C5Me5), 116.0 (9), 116.1 (8), 118.2 (7), 121.4 (11), 122.6 (10), 179.4 (18). The 13C signal for 19 was not observed. Anal. Calcd for C34H54N2Zr (582.04): C, 70.16; H, 9.35; N, 4.81. Found: C, 69.79; H, 9.56; N, 5.06.

Synthesis of 1,1-Bis(η5-pentamethylcyclopentadienyl)-2,3-bis(diethylamine)-1-zirconacyclopropene ([Cp*2Zr{C2(NEt2)2}], 8c) and (η5-Pentamethylcyclopentadienyl)(cis-1,2-bis(diethylamine)vinyl)(η6tetramethylpentafulvene)zirconium(IV) ([Cp*(C5Me4CH2)Zr{C(NEt2) CH(NEt2)}], 9c). [Cp*2ZrCl2] (649 mg, 1.50 mmol) and Mg (110 mg, 4.5 mmol, 3.0 equiv) were mixed with THF (3 mL), forming a yellow suspension. With stirring, a solution of 1c (265 mg, 1.58 mmol, 1.05 equiv) in THF (2 mL) was added and the mixture was stirred at 60 °C. After 10 min, the yellow reaction mixture turned maroon. The mixture was further stirred at 60 °C for 1 h. The solvent was removed, the brown residue was extracted with hexane (10 mL), and the extract was filtered through a Celite pad and washed with more hexane (3 × 2 mL). The solvent was partially removed until crystallization started. At this point, the maroon solution was warmed to redissolve the precipitate and was stored at −30 °C. A brown precipitate formed that was collected by decantation and dried under high vacuum. Yield: 80% (638 mg). Orange crystals suitable for XRD analyses were obtained by recrystallization in hexane at −30 °C. The NMR spectra of the brown solid showed sufficient purity for further reactions. 1H NMR (C6D6, 400.4 MHz, 297 K): ratio 8c:9c ca. 1:2.0; 8c, δ 1.12 (t, 3JH,H = 7.2 Hz, 12H, CH3), 1.95 (s, 30H, C5Me5), 2.85 (q, 3JH,H = 7.2 Hz, 8H, CH2); 9c, δ 1.00 (t, 3JH,H = 7.1 Hz, 6H, 15), 1.08−1.18 (br m, ν1/2 ≈ 22 Hz, 6H, 13), 1.18 (s, 3J6,17 = 0.5 Hz, 1H, 17), 1.37 (s, 3H, 5), 1.74 (s, 3H, 2), 1.79 (s, 3H, 3), 1.91 (s, 15H, 1), 1.97 (s, 3H, 4), 2.45 (d, 2JH,H = 5.4 Hz, 1H, 6), 2.63, 2.65 (2 × q, 3J14,15 = 7.1 Hz, 2 × 2H, 14), 2.72 (dd, 2 JH,H = 5.3 Hz, 3J6,17 = 0.5 Hz, 1H, 6), 3.28−3.40 (br m, ν1/2 ≈ 29 Hz, 2H, 12), 3.50−4.10 (br m, ν1/2 ≈ 120 Hz, 2H, 12). 13C{1H} NMR (C6D6, 100.7 MHz, 298 K): 8c, δ 12.1 (C5Me5), 17.0 (CH3), 51.5 (CH2), 119.2 (C5Me5), 198.4 (Cq); 9c, δ 11.0 (3), 11.3 (2), 12.3 (15), 12.4 (C5Me5), 12.5 (4), 13.9 (5), 14.2−15.0 (br, 13), 40.0−41.5 (br, 12), 48.1 (14), 48.8−52.0 (br, 12), 56.2 (6), 83.0 (17), 115.3 (C5Me5), 116.0 (9), 116.3 (8), 118.0 (7), 121.0 (11), 122.6 (10), 179.7 (16). For additional NMR data see the Supporting Information (pp S14− S17). Anal. Calcd for C30H50N2Zr (529.97): C, 67.99; H, 9.51; N, 5.29. Found: C, 68.34; H, 9.53; N, 5.11.

Synthesis of 1,1-Bis(η5-pentamethylcyclopentadienyl)-2,3-bis(4methylpiperidin-1-yl)-1-zirconacyclopropene ([Cp*2Zr{C2(NC5H9CH3)2}], 8b) and (η5-Pentamethylcyclopentadienyl)(cis-1,2-bis(4-methylpiperidin-1-yl)vinyl)(η6-tetramethylpentafulvene)zirconium(IV) ([Cp*Zr(C5Me4CH2){C(NC5H9CH3)CH(NC5H9CH3)}], 9b). [Cp*2ZrCl2] (433 mg, 1.00 mmol) and Mg (97 mg, 4.0 mmol, 4 equiv) were mixed with THF (5 mL), forming a yellow suspension. With stirring, a solution of 1b (231 mg, 1.05 mmol, 1.05 equiv) in THF (1 mL) was added and the mixture was stirred at 60 °C. After 1 h the reaction mixture turned green and the solvent was removed. The residue was extracted with hexane (5 mL), and the extract was filtered through a Celite pad and washed with more hexane (2 × 2 mL). The solvent was removed under high vacuum, and a green solid was isolated. Yield: 80% (463 mg). Crystals suitable for X-ray diffraction analysis were obtained by storing a concentrated pentane solution at −30 °C. 1H NMR (C6D6, 399.9 MHz, 297 K): ratio 8b:9b ca. 1:1.3; 8b, δ 0.93 (m, 6H, 4Me), 1.24 (m, 4H, 3,5-CH2), 1.30 (m, 4H, 4-CH), 1.56 (m, 4H, 3,5CH2), 1.97 (s, 30H, C5Me5), 2.57 (br t, ν1/2 ≈ 25 Hz, 4H, 2,6-CH2), 3.20 (br d, ν1/2 ≈ 10 Hz, 4H, 2,6-CH2); 9b, δ 0.85 (m, 3H, 21), 0.95 (m, 3H, 20), 1.07 (s, 1H, 19), 1.19 (m, 1H, 17), 1.20 (m, 1H, 16), 1.28 (m, 1H, 12), 1.29 (m, 1H, 16), 1.31 (m, 2H, 13), 1.38 (s, 3H, 5), 1.41 (m, 1H, 16), 1.53 (m, 1H, 16), 1.58 (m, 2H, 13), 1.75 (s, 3H, 2), 1.79 (s, 3H, 3), 1.92 (s, 15H, 1), 2.00 (br s, 2H, 15), 2.05 (s, 3H, 4), 2.21 (m, 2H, 14), 2.50 (d, 2JH,H = 5.3 Hz, 1H, 6), 2.78 (d, 2JH,H = 5.3 Hz, 1H, 6), 2.81 (m, 2H, 15), 3.08 (m, 2H, 14). 13C{1H} NMR (C6D6, 100.6 MHz, 299 K): 8b, δ 12.2 (C5Me5), 22.6 (4-Me) 32.0 (4-CH),

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ASSOCIATED CONTENT

S Supporting Information *

CIF and mol2 files, text, figures, and tables giving crystallographic data for 3·1/2(n-C6H14), 5·1/2C6H6, 7, 8a, 8a·MgCl2· (n-C6H14), 8b, and 8c, NMR spectra of all compounds, details of the variable-temperature NMR studies for 3 and 8c/9c, including the calculation of the thermodynamic and kinetic parameters, details of the DFT calculations and all calculated 1784


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structures, and comparison of experimental and theoretical structural data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*U.R.: fax, (+49) 381-1281-51176; e-mail, Uwe.Rosenthal@ catalysis.de. *M.T.: fax, (+49) 531-391-5309; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. C. G. Daniliuc, Dr. M. Freytag, and Dr. A. Spannenberg for assistance in obtaining the X-ray crystallographic structures. Ò .À. gratefully appreciates helpful discussions with Dr. K. Ibrom, Dr. M. Walter, and Dr. C. Kleeberg.

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Organometallics

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