Article pubs.acs.org/Organometallics
Deprotonated P‑ylides As Templates for Novel Cyclopentadienyl Phosphonioalkyl, -alkylidene, and -alkylidyne (CpPC) ConstrainedGeometry Complexes Fabian G. Schröder, Crispin Lichtenberg,† Michael Elfferding, and Jörg Sundermeyer* Fachbereich Chemie der Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany S Supporting Information *
ABSTRACT: Cyclopentadienylidene phosphoranes of the general formula Cpx-PR2-CH3 (1a−d) and anionic derivatives thereof have been investigated as ligands in the coordination sphere of zirconium (R = NMe2, tBu, Ph; Cpx = C5H4, C5Me4, C5H2(CMe2)2CH2). The ligand set includes a full series of neutral and mono-, di-, and trianionic phosphonium ylides, which are formed by successive deprotonation of the PCH3 group. The degree of deprotonation can be controlled by choice of the ylides 1a−d and the zirconium precursor. The resulting zirconium complexes have been analyzed by NMR spectroscopy, elemental analyses, and single-crystal X-ray diffraction analyses. Our findings include the first fully characterized zirconium complex bearing a neutral cyclopentadienylidene phosphorane ligand, [Zr(H3C-P(NMe2)2-C5Me4)(CH2SiMe3)Cl3] (3a). New constrained-geometry complexes with chelating Cpx-phosphonio-alkyl ([Zr(CH2-PtBu2-C5H4)R′3]: 4b, R′ = Bn; 4b′, R′ = CH2SiMe3)) and Cpx-phosphonio-alkylidene ligands ([Zr(CH-PR2-Cpx)(CH2SiMe3)2]: 5a, R = NMe2, Cpx = C5Me4; 5c, R = Ph, Cpx = C5H2(CMe2)2CH2; 5d, R = Ph, Cpx = C5Me4) have been isolated and characterized. [Zr(C-PR2-C5Me4)Bn]2 (6a, R = NMe2) is a rare example of a compound featuring a bridging trianionic phosphonium ylide (phosphonio-alkylidyne) ligand.
■
INTRODUCTION On the basis of the seminal contributions of Wittig,1 phosphonium ylides have been established as important reagents in organic synthesis.2 Subsequently, they have been investigated in detail as neutral or monoanionic ligands in the coordination sphere of a large variety of main-group and transition metals.3−5 In a series of publications Erker et al. investigated the synthesis and reactivity of nonchelating phosphonio-alkylidene complexes [Cp2Zr(CH-PPh3)X].6 Doubly deprotonated methylene phosphoranes R3P-CH2 can form terminal phosphonio-alkylidene ligands with metal centers that facilitate M−C triple bonds, such as Nb, Ta, W, and Re.7 In the coordination sphere of other (especially s-block and group 4) metals, doubly deprotonated phosphonium ylides prefer bridging or chelating bonding modes.8 Overall, neutral, anionic, and dianionic phosphonium ylides show a rich coordination chemistry, which includes terminal, bridging, and chelating bonding modes with denticities ranging from 1 to 3 (Scheme 1).9 As a result of these versatile ligand properties, a broad spectrum of applications has been reported for metal complexes of phosphonium ylides. They have been utilized as stoichiometric reagents in organic synthesis,10 as pharmacologically active compounds,11 and as catalysts in enantioselective allylic substitution reactions,12 the carbonylation of methanol,13a olefin hydrogenation,13 hydrosilylation of ketones,13b and olefin polymerization.14 Within the phosphonium ylide ligand family, cyclopentadienyl-substituted species such as neutral phosphonium cyclo© 2013 American Chemical Society
Scheme 1. Coordination Modes of Neutral and Deprotonated Phosphonium Ylidesa
a
R = alkyl, aryl.
pentadienylides3 (Scheme 2, A) and monoanionic phosphonium-bridged ansa-metallocene ligands14b,c,15 (Scheme 2, B) Scheme 2. Cyclopentadienyl-Substituted Phosphonium (Di)ylides without (A, B) or with (C) Additional CH Acidic Functionalities
Received: June 14, 2013 Published: September 9, 2013 5082
dx.doi.org/10.1021/om4005548 | Organometallics 2013, 32, 5082−5091
Organometallics
Article
situ generated bulky phosphane tBu2PC5H5 to undergo quaternization with methyl iodide (for details, including a single-crystal X-ray analysis of 1b, see the Supporting Information). The phosphonium ylides 1a−d cover a broad spectrum of electronic and steric properties (31P NMR (C6D6) δ 2.1 (1d), 52.6 (1a) ppm; high (1c) or low (1b) steric demand of the Cp moiety); thus, significant differences with respect to their coordination behavior may be expected. 1a−d can be utilized as neutral ligands or as CH-acidic precursors for anionic ligands. In addition, the corresponding lithium phosphonium diylides 2a−d were prepared as CpPCH2− transfer reagents for salt elimination reactions (Scheme 4, 2a−d; Z = Li). These compounds were synthesized according to protocols that have been reported by us for the known species 2d.18 The increased electron density in the anionic ligands in comparison to their neutral counterparts is reflected by spectroscopic parameters such as high-field shifts of the 1H NMR resonances for the CH2(H/Li) groups (e.g. 1H NMR (C6D6/THF-d8 5/1) δ 1.32 ppm (1a) vs −0.43 ppm (2a)) and downfield shifts of the 31P NMR resonances (e.g. 31P NMR (C6D6/THF-d8 5/1) δ 30.2 ppm (1b) vs 51.0 ppm (2b)). Compounds 2a,b were also characterized by single-crystal X-ray analysis and show a dinuclear head-to-tail arrangement in the solid state, as previously reported for 2d (for details see the Supporting Information).18 Zr Compounds with Neutral CpPCH3 Ligands. Whereas anionic cyclopentadienide ligands are common motifs in organozirconium chemistry, neutral phosphonium cyclopentadienylides (R3PC5H4) as ligands bound to zirconium have rarely been reported.3 The only known compound of this type, [Zr(C5H4PPh3)Cl4], was isolated from the reaction of ZrCl4 with Ph3PC5H4 but gave unsatisfying results in elemental analysis and could not be recrystallized.24 Whereas reactions of ZrCl4 with phosphonium ylides from our ligand set 1a−d led to product mixtures,25 reaction of 1a with in situ generated Zr(CH2SiMe3)Cl3 allowed the isolation of the adduct [Zr(H3CP(NMe2)2-C5Me4)(CH2SiMe3)Cl3] (3a) in 58% yield (Scheme 5). At ambient temperature, 3a is only modestly soluble in
have been investigated in some detail due to their structural analogies with related silyl-bridged ansa-metallocenes.15b However, multiple deprotonation of cyclopentadienyl (Cp) groups cannot be achieved, reducing the possibilities of charge variation in these ligands. Ylides of type C (Scheme 2) featuring an additional, multiple CH acidic potential donor functionality combine the stabilizing effect of the Cp group and the possibility of charge variation by successive deprotonation in a chelating constrained-geometry ligand regime. So far only compounds of group 6 with neutral type C ligands have been investigated with respect to M−ligand bonding interactions.16,17 Recently, we reported on monoanionic forms derived from neutral type C ylides as bridging ligands in dinuclear lithium or copper(I) complexes.18,19 The ease of their deprotonation motivated us to search for a new class of constrained-geometry chelate ligands CpPC2− which are isolobal (and even isoelectronic) analogues of well-investigated cyclopentadienylsilylamido20 (CpSiN2−) and cyclopentadienylphosphazene21 (CpPN−) ligand classes (Scheme 3). Scheme 3. Isolobal Analogy of Cyclopentadienylsilylamido (CpSiN2−), Cyclopentadienylphosphazene (CpPN−), and Cyclopentadienylphosphonioalkylidene (CpPC2−) Ligands
Here we present a full series of monoanionic (CpxPCH2−), dianionic (CpxPCH2−), and trianionic (CpxPC3−) phosphorus ylide ligands derived from neutral cyclopentadienylidene phosphoranes of type C (CpxPCH3). Compounds of each type have been isolated and characterized in the coordination sphere of zirconium. They reveal unprecedented Zr−ylide interactions.
■
RESULTS AND DISCUSSION Phosphonium Ylide and Phosphonium Diylide Ligands. The phosphonium ylides shown in Scheme 4 represent
Scheme 5. Reaction of C5Me4-P(NMe2)2-CH3 (1a) with in Situ Generated Zr(CH2SiMe3)Cl3 To Give Adduct [Zr(H3CP(NMe2)2-C5Me4)(CH2SiMe3)Cl3] (3a)
Scheme 4. Phosphonium Ylides 1a−d (Z = H) and Corresponding Lithium Phosphonium Diylides 2a−d (Z = Li)
aromatic solvents but readily soluble at elevated temperatures or in the presence of small amounts of additional coordinating solvents. The neutral phosphonium ylide ligand coordinates to the metal center in solution, as shown by significant shifts of 1H and 13C NMR resonances of 3a in comparison to those of metal-free 1a (e.g.: 1H NMR (C6D6/THF-d8 5/1) δ 1.32 ppm (PCH3 in 1a) vs 2.31 ppm (PCH3 in 3a)). In comparison, only a slight downfield shift of the 31P NMR resonance is observed (e.g. 31P NMR (C6D6/THF-d8 5/1) δ 52.0 ppm (1a) vs 55.6 ppm (3a)). Single crystals of 3a were grown from a solution in hot toluene by gradual cooling to ambient temperature. 3a crystallizes in the monoclinic space group P21/n with Z = 4 (Figure 1). The zirconium center is found in a distortedsquare-pyramidal coordination geometry with the sterically
the ligand set used in this work (1a−d, Z = H). Compound 1a was synthesized by reaction of in situ generated (Me2N)2PC5Me4H with methyl iodide followed by dehydrodehalogenation with KH, as previously reported for 1c,d.18,22,23 A modified literature method22 had to be applied for the synthesis of 1b, which was ascribed to the reluctance of the in 5083
dx.doi.org/10.1021/om4005548 | Organometallics 2013, 32, 5082−5091
Organometallics
Article
Scheme 6. Synthesis of Zirconium Phosphonium Diylides 4b,b′ via an Alkane or Salt Elimination Reaction, Respectivelya
a
4b, R′ = Bn; 4b′, R′ = CH2SiMe3.
[(C5Me4)2PMe2]−.8b,15b,31 The 31P NMR resonances are shifted downfield in comparison to the free ylide, which is in good agreement with a monoanionic ligand fragment (e.g. 31P NMR (C6D6) δ 45.9 ppm (4b) vs 30.7 ppm (1b)). The 1H NMR resonances for the CH2 groups show a high-field shift (e.g. 1H NMR (C6D6) δ −0.05 ppm (4b) vs 1.01 ppm (1b)). The above-mentioned structural proposal was corroborated by a single-crystal X-ray analysis of 4b, which crystallizes in the triclinic space group P1̅ with Z = 2 (Figure 2). The
Figure 1. Molecular structure of 3a. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms (except for those bound to C10) and one disordered toluene molecule per asymmetric unit in the crystal lattice are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr1−Cpcentroid, 2.2437(8); Zr1−C15, 2.261(2); Zr1− Cl1, 2.4764(5); Zr1−Cl2, 2.5031(5); Zr1−Cl3, 2.4762(5); P1−C1, 1.780(2); P1−C10, 1.7786(19); C1−C2, 1.442(3); C2−C3, 1.402(3); C3−C4, 1.427(3); C4−C5, 1.411(3); C1−C5, 1.443(3); Cpcentroid− Zr1−C15, 106.47(5); C15−Zr1−Cl1, 83.12(5).
demanding phosphonium ylide ligand in the apical position. The Zr1−Cl1(Cl2, Cl3) (2.4762(5)−2.5031(5) Å) and Zr1− C15 (2.261(2) Å) bond lengths are in the expected ranges, with Cl2 experiencing a trans influence by the alkyl group.26 The Zr1−Cpcentroid distance amounts to 2.2437(8) Å which is, as expected, at the upper limit of corresponding values of 2.19− 2.24 Å that have been reported for chloro zirconium species with monoanionic cyclopentadienide ligands [Zr(C5R5)Cl3−nR′nLm] (R = H, Me; R′ = alkyl; L = neutral ligand; n = 0, 1; m = 0−2).27 The relatively long P1−C1 bond (1.780(2) Å)28 and small alternations in the C−C bond lengths within the five-membered ring (ΔC−C ≤ 0.04 Å) corroborate an increased charge separation Pδ+−(C5Me4)δ− induced by interaction of the cyclopentadienylide group with the Lewis acidic metal center. 3a is the first fully characterized zirconium compound featuring a phosphonium cyclopentadienylide ligand.29 These findings suggest that adduct formation between phosphonium cyclopentadienylides and Lewis acidic zirconium compounds is the first step in related reactions that lead to zirconium phosphonium diylides via alkane elimination (vide infra).30 Zr Compounds with Monoanionic CpPCH2− Ligands. In order to study a monoanionic CpPCH2− ligand in the coordination sphere of zirconium, the phosphonium ylide 1b was reacted with Brønsted basic zirconium precursors. Whereas the reaction of 1b with Zr(CH2SiMe3)4 showed only low conversions after extended reaction times, reaction with ZrBn4 (Bn = benzyl) gave [Zr(CH2-PtBu2-C5H4)Bn3] (4b) as a product of toluene elimination (Scheme 6, top). The corresponding alkyl compound [Zr(CH 2 -PtBu 2 -C 5 H 4 )(CH2SiMe3)3] (4b′) was accessible in a salt metathesis reaction between lithium phosphonium diylide [Li(CH2-PtBu2-C5H4)] (2b) and in situ generated Zr(CH2SiMe3)3Cl (Scheme 6, bottom). Whereas the synthesis of 4b involves elevated temperatures of up to 50 °C, 4b′ undergoes thermal degradation when warmed to temperatures above 0 °C in solution or in bulk. The phosphonium diylide in benzene soluble complexes 4b and 4b′ is expected to act as a bidentate ligand, as reported for the related species [(CH2)2PMe2]− and
Figure 2. Molecular structure of 4b. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms (except those bound to C6) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr1−Cpcentroid, 2.2861(12); Zr1−C6, 2.400(2); Zr1− C15, 2.301(2); Zr1−C22, 2.364(2); Zr1−C29, 2.324(3); P1−C1, 1.809(2); P1−C6, 1.754(2); Cpcentroid−Zr1−C6, 92.92(7); Cpcentroid− Zr1−C22, 171.55(7); C6−Zr1−C15, 123.39(9); C1−P1−C6, 96.75(11); C1−P1−C7, 109.85(12).
coordination geometry around the zirconium center is distorted trigonal bipyramidal. The phosphonium diylide ligand adopts a chelating η1:η5 coordination mode, which leads to a small Cpcentroid−Zr1−C6 angle of 92.92(7)°, classifying 4b as a constrained-geometry complex (CGC).32 The chelation also induces a strong decrease of the C1−P1−C6 angle (96.75(11)°) in comparison to free ligand 1b (108.45(13)°) or to the dimeric, CpPCH2−-bridged lithium analogue 2b (108.82(7)°).33 The ylidic Cmethylene atom shows a stronger interaction with P1 than the other ylidic CCp atom, which was ascribed to resonance stabilization of electron density in the cyclopentadienyl fragment (P1−C6, 1.754(2) Å; P1−C1, 1.809(2) Å). The methylene group also acts as the stronger σ donor toward Zr1 and therefore is found in an equatorial position of the trigonal bipyramid. The Zr1−C6 distance of 2.400(2) Å is short in comparison to the corresponding value in [Zr(CH2)-PMe2-CH2)(C5Me5)2H] (2.467(6) Å).31 The cyclopentadienyl group of 4b is found in an axial position. The Zr1− Cpcentroid distance of 2.2861(12) Å is significantly longer than in 3a, which was attributed to the higher substitution of the Lewis acid center with strong and sterically demanding benzyl donor substituents. Two of the benzyl ligands in 4b are found in 5084
dx.doi.org/10.1021/om4005548 | Organometallics 2013, 32, 5082−5091
Organometallics
Article
characteristics of the CpPCH2− ligands in 5a,c,d are unique in that they represent dianionic, bidentate phosphonium diylides with nonidentical donor functionalities. Compounds 5a,c,d were characterized by single-crystal X-ray analysis (Figure 3). The zirconium centers adopt distorted-tetrahedral coordination geometries. The deprotonated phosphonium diylides CpPCH2− act as chelating ligands, resulting in essentially planar ZrCmethinePCpcentroid units (torsion angles
