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Synthetic Endeavors toward Titanium Based Frustrated Lewis Pairs with Controlled Electronic and Steric Properties Adrien T. Normand,†,‡ Philippe Richard,†,§ Cédric Balan,† Constantin G. Daniliuc,‡,§ Gerald Kehr,‡ Gerhard Erker,*,‡ and Pierre Le Gendre*,† †

Institut de Chimie Moléculaire de l’Université de Bourgogne, ICMUB-UMR CNRS 6302, Université de Bourgogne, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France ‡ Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: A new family of cationic Ti complexes 4′ with a pendant phosphine of general formula [CpCpPTiOAr][BPh4] (Cp = η5-C5H5; CpP = η5-C5H4(CMe2)PR2) has been prepared in four steps from 6,6-dimethylfulvene. These complexes were designed to behave as Ti based frustrated Lewis pairs (FLPs). The key synthetic step is a reduction−oxidation sequence from [CpCpPTiClOAr] complexes 3 using lithium phosphide salts as the reductants and ferricinium tetraphenylborate as the oxidant. Four complexes have been structurally characterized by X-ray diffraction and show elongated Ti−P bonds, above 2.60 Å. One complex (4b′: OAr = 2,6-Me2C6H3; PR2 = PCy2) reacted with benzaldehyde to form a typical FLP activation product. Complex 4b′ also reacted with 2 equiv of trans-chalcone to form a 10-membered Ti phosphonium macrocycle (6b′) by extrusion of 6,6dimethylfulvene.

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INTRODUCTION Frustrated Lewis pairs (FLPs) are molecular systems containing non-interacting or weakly interacting Lewis acids and bases.1,2 This unquenched reactivitywhich usually stems from unfavorable steric interactionscan be exploited, for example in the activation and catalytic functionalization of small molecules such as H2 and CO2.3−5 The archetype of an FLP is perhaps the PtBu3/B(C6F5)3 mixture, a system based on two main-group elements with bulky substituents and considerable Lewis acidity/basicity, but devoid of observable specific interactions between them.6 However, this system is far from being a generic template for the numerous FLPs that have been reported to date. As a matter of fact, several FLPs do display some degree of adduct formation, as in the prototypical (Mes)2PCH2CH2B(C6F5)2;7 this does not seem to impede reactivity, as long as a dissociation pre-equilibrium takes place prior to reactions with other molecules. Another variant concerns the use of electron withdrawing substituents on the Lewis base (or electron donating ones on the acid). While this sometimes has the drawback of lowering the reactivity of the FLP,8−11 it also enables catalytic turnover by increasing the lability of the FLP−small molecule adduct.12−14 Finally, also © XXXX American Chemical Society

departing from the classical system described above are organometallic FLPs (om-FLPs), in which one of the components is a transition metal.15 Examples across the periodic table include Zr,16−24 Hf,19,24,25 and Ru.26−28 Although in these examples the metal acts as a Lewis acid, the use of low valent metals (e.g., Zr(II) or Pt(0)) as Lewis bases has been described in a few prominent cases.29−31 The notion that metal complexes can be integrated into the FLP paradigm was recognized early on by Stephan, who pointed out that the phosphinimide Ti cation [CpTi(NPtBu3)Me]+ was unable to coordinate bulky phosphine ligands such as P(o-tolyl)3. In retrospect, the observed activation of CH2Cl2 by this mixture could indeed be described in terms of the FLP reactivity of a Ti+/PR3 pair.32 Ironically, however, despite this pre-FLP era example and the numerous instances of Zr or Hf based FLPs, Ti has been somewhat neglected as a Lewis acid component for FLPs.33 Wass reported on the synthesis of titanocene−phosphinoaryloxide complexes and their reactivity toward H2, alongside catalytic studies of amine-borane Received: March 25, 2015

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DOI: 10.1021/acs.organomet.5b00250 Organometallics XXXX, XXX, XXX−XXX

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reaction of 2 with 2 equiv of MeLi); or (b) salt metathesis with ArONa (3) followed by reaction with MeLi (3′). Synthesis of cyclopentadienyl precursors 1. Crucial to the control of FLP reactivity, the choice of the phosphine arm was guided by the rigidity of the CpP backbone (CpP = phosphine-functionalized cyclopentadienyl ligand), the steric bulk around P, and its basicity. Scheme 2 shows the ligands initially selected for this study: 1a and 1b are new compounds, while 1c and 1d have been previously reported and were prepared according to the literature.36,37

dehydrogenation. Despite clear evidence for FLP-type behavior, no H2 adduct was observed; rather, a cationic Ti(III) hydride complex was obtained.34 Obviously the reactivity of Ti is governed not only by its Lewis acidity but also by its specific redox properties; this raises the question of whether Ti complexes can be useful at all in FLP chemistry. Prior to Wass’ report, we had designed a family of om-FLPs consisting of cationic aryloxy Ti (IV) complexes with a pendant phosphine arm (Figure 1),35 which seemed appropriate to investigate this matter.

Scheme 2. Selected Ligands and Synthesis of New Compounds

Figure 1. Assembly of Ti FLPs from simple building blocks.

The modular synthesis of these complexes starting from commercially available building blocks was expected to facilitate the systematic study of steric and electronic parameters. In this contribution, we describe the synthesis and characterization of five such complexes. In addition preliminary reactivity tests are reported.

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Ligands 1a and 1b were synthesized in good yields by nucleophilic addition onto 6,6-dimethylfulvene (the “fulvene route”, Scheme 2). 37,40 Formation of 1a was always accompanied by noticeable amounts (15−20%) of Cpisopropenyl lithium,41 consistent with deprotonation of the fulvene by PCy2Li. Initially, we could not eliminate this impurity, but it turned out not to be detrimental to the next step. We later optimized the synthesis of 1a by filtering the reaction mixture, thereby reducing the amount of impurity below 5% while still retaining a good yield of product (68%). The synthesis of 1b (81% yield) was more straightforward and no deprotonation took place, although this compound decomposes over time.42

RESULTS AND DISCUSSION The synthesis of the target compounds was envisaged according to the route depicted in Scheme 1. Thus, phosphine-functionalized cyclopentadienide ligands 136,37 would give functionalized titanocene dichloride 2 upon salt metathesis with CpR3TiCl3 (in this study, CpR3 = C5H5).38,39 Following this, aryloxy intermediates 3/3′ seemed particularly attractive because they could be transformed into the final compounds 4 either by X ligand abstraction (e.g., with B(C6F5)3) or by a reduction/oxidation sequence.34 Complexes 3/3′ could be accessed starting from 2 by either of two ways: (a) protonolysis of dimethyl precursors 2′ (obtained by

Scheme 1. Synthetic Routes toward Ti−Aryloxy Complexes Displaying FLP Behavior

B


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Organometallics Table 1. Selected NMR Spectroscopy Data for New Compoundsa,b

a

Compd (solvent)

δ (31P)

δ Cp (1H)

1a (d8-THF) 1b (d8-THF) 2a (CD2Cl2) 2a′ (C6D6) 2b (d8-THF) 3a (C6D6) 3b (C6D6) 3c (C6D6) 3d (C6D6) 3e (C6D6/CD2Cl2) 3f (C6D6) 3g (C6D6) 3h (C6D6) 4a (C6D6) 4a′ (CD2Cl2) 4b (CD2Cl2) 4b′ (CD2Cl2) 4c (C6D6) 4c′ (CD2Cl2) 4d′ (CD2Cl2) 4e′ (CD2Cl2) 5b′-A (CD2Cl2) 5b′-B (CD2Cl2) 6b′ (CD2Cl2)

33.1 −8.6 49.7 45.6 4.2 45.8 45.3 45.8 1.2 31.6 −15.3 −16.0 −15.3 −29.6 −29.6 −23.5 −23.5 −32.9 −30.4 −34.5 −28.8 13.9 1.9 34.4

N.A. N.A. 6.54 5.79 6.51 5.85 5.80 5.89 5.72 5.77 5.96 5.84 5.99 5.92 6.34 6.25 6.08 5.92 6.30 5.86 6.00 6.19 5.99 6.27

3

JPH Cp (1H)

δ CpP (1H)c

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 2.0 2.1 1.9 2.0 1.8 2.0 1.9 2.4 N.A. N.A. N.A.

5.62/5.59 5.69/5.58 6.64/6.51 5.90/5.73 6.54−6.51/6.47−6.42 6.62; 5.88/5.99; 5.66 6.68; 6.17/5.86; 5.30 6.64; 5.92/6.02; 5.69 6.68; 6.07/5.78; 5.18 6.30; 5.72/5.92; 5.33 6.31; 6.28; 6.05; 5.83 6.32; 6.17/6.03; 5.61 6.35; 6.07/6.33; 5.85 6.18; 5.92; 5.51; 5.33 6.19; 5.38/6.49−6.45; 6.15 6.89; 6.50; 6.36; 5.80 6.30; 5.51/6.55; 6.41 6.14; 5.65; 5.49; 5.38 6.22; 5.33/6.47; 6.12 6.62; 6.56; 6.40; 5.75 6.49; 5.79/6.69; 6.55 6.83; 6.30; 6.19−6.16; 5.96 6.67−6.62; 6.52; 6.34; 6.02 N.A.

δ CMe2 (1H) 1.44 1.51 1.57 1.23 1.79 1.69; 1.93; 1.72; 2.10; 1.81; N.A. N.A. N.A. 1.14; 1.71; 1.89; 1.86; 1.15; 1.72; 2.05; 1.76; 1.82; 1.74 N.A

3

JPH CMe2 (1H)

1.53 1.64 1.56 1.76 1.48

1.06 1.62 1.58 1.51 1.08 1.59 1.44 1.42 1.67

10.7 11.4 11.2 10.7 13.5 10.4; 11.6; 10.4; 13.5; 14.7; N.A. N.A. N.A. 11.6; 10.9; 11.3; 11.0; 11.6; 10.7; 11.5; 13.3; 13.7; 13.2 N.A.

12.4 10.5 12.5 13.5 14.7

10.9 11.4 10.8 11.6 10.5 11.6 13.6 12.9 13.0

δ in ppm, J in Hz. b4a−c characterized in situ. cH2; H2/H3; H3 (where possible).

Both compounds showed diagnostic peaks in the 1H NMR spectrum, with CpP signals appearing as two multiplets (1a, 5.62 and 5.59 ppm; 1b, 5.69 and 5.58 ppm). The 3JPC coupling constant between P and the C2 atoms of the CpP ring observed in the 13C NMR spectra allowed for the assignment of H2 and H3 signals in the 1H NMR spectra (through HSQC spectroscopy). This feature was also present in the Ti complexes, with a few exceptions. Finally, the CMe2 bridge signals appeared as a doublet due to coupling with P (1a, 1.44 ppm, 3JPH = 10.7 Hz; 1b, 1.51 ppm, 3JPH = 11.4 Hz). The 31 1 P{ H} NMR spectra showed peaks at 33.1 (1a) and −8.6 ppm (1b) (vide inf ra, Table 1). Synthesis of titanocene dichloride derivatives 2. Complexes 2a−d were prepared by salt metathesis with CpTiCl3 following literature procedures (2c and 2d are known compounds).38,39 Ligands 1a and 1b both reacted in a few minutes with CpTiCl3 at moderately low temperature (−10 and −15 °C, respectively) and precipitated out of the 1:1 THF/ Et2O reaction mixture to give 2a and 2b in 81 and 59% yield, respectively (Scheme 3).43 The 1H NMR spectra were qualitatively similar to those of the CpPLi precursors, except for a notable downfield shift of almost 1 ppm of the CpP signals and an additional peak for the Cp signal at 6.54 (2a) and 6.51 (2b) ppm. The chemical shifts

of the CMe2 signals were also slightly shifted downfield and the JPH coupling constant increased likewise (Table 1). Finally, the 31 P NMR spectra showed markedly downfield signals compared to the CpPLi precursors, with Δδ = +16.6 ppm (2a) and +12.8 ppm (2b). Synthesis of neutral aryloxy Ti complexes 3/3′. As shown in Scheme 1, this stage was important to install both the final elements of steric and electronic control, and the removable X ligand. We initially planned to synthesize Me derivatives 3′ via protonolysis of dimethyl precursors 2a′ because we felt that the methyl ligand would give us several options to synthesize the cationic FLPs. Indeed, methyl abstraction from Ti using B(C6F5)3 or [Ph3C][B(C6F5)4] has previously been reported.44,45 Therefore, we prepared complex 2a′ in 53% yield by reaction with 2 equiv of MeLi (Scheme 4).46 3

Scheme 4. Synthesis of TiMe2 Intermediate 2a′

The X-ray solid-state structure of 2a′ (Figure 2) revealed no interaction between Ti and P, as expected from NMR spectroscopy (no coupling between P and the Cp hydrogens in the 1H NMR spectrum). The chemical shift of 45.6 ppm in the 31P NMR spectrum is comparable to the values obtained for 2a and 3a−c (Table 1). Unfortunately, 2a′ did not react with phenol reagents (PhOH, p-methoxyphenol);47 only an excess of more acidic p-nitrophenol seemed to effect protonolysis of the TiMe bond,

Scheme 3. Synthesis of New Complexes 2a and 2b

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X-ray quality crystals were obtained for compounds 3a−c. Table 2 summarizes the relevant bond distances and angles. In Table 2. Selected Bond Distances (Å) and Angles (deg) for 3a−c Ti−O Ti−Cl Ti−Ct1 Ti−Ct2 O−Ar Ti−O−Ar O−Ti−Cl Ct1−Ti−Ct2 C11−Ct1−Ti−Ct2a

Figure 2. POV-Ray depiction of 2a′ (thermal ellipsoids drawn at the 50% probability level).

but the reaction was not clean and the products could not be separated. Therefore, this route to 3′ was abandoned. We next turned our attention to chloro−aryloxy complexes 3. Related Cp2TiClOAr compounds have previously been obtained by Jones and Stephan by reacting Cp2TiCl2 with a mixture of ArOH and NEt3.48,49 In our case, salt metathesis with ArONa was preferred in order to avoid protonation of the basic PCy2 moiety.50 Compounds 3a−h were obtained in moderate to good yields (Scheme 5).51

a

3a

3b

3c

1.8892(15) 2.4118(6) 2.0892(10) 2.0741(10) 1.351(2) 138.61(12) 95.59(5) 131.51(4) −87.96(8)

1.9045(10) 2.3823(5) 2.1060(8) 2.0766(8) 1.3558(18) 138.68(10) 99.55(3) 130.42(3) 155.45(5)

1.905(2) 2.4108(9) 2.0936(13) 2.0752(16) 1.342(3) 137.39(18) 95.54(7) 130.91(6) −91.31(10)

Defined as θ in Figure 4.

the three structures 3a−c, the phosphino alkyl substituents do not lie in the bisecting position with respect to the axial ligands. While in 3a and 3c the CpP is rotated about 90° from the bisecting position, in 3b the rotation is only about 25° with the phosphino alkyl group eclipsing the aryloxy ligand. This special conformation can be caused by van der Waals contacts between the methyl groups of the CpP ligand on the one hand, and those of the aryloxy ligand on the other hand (dH12c‑H33c = 2.18 Å; dH13b‑H32b = 2.51 Å, see Figure 3). Importantly, this feature was not observed in the loosely related [CpCp’TiCl(OAr)] complexes (Cp’ = η5-1-Me,3-iPrC5H3 or η5-1-Me,2-iPrC5H3; Ar = 2,6-Me2C6H3).50,53 Apart from this important difference, the structures of 3a−c are quite similar, with key distances and angles contained within 0.03 Å and 4° limits; no apparent trend could be related to the steric or electronic differences between the aryloxy ligands. Comparison of these parameters with those of previously reported structures of general formula [CpRCpR′TiClOAr] did not reveal major differences; in particular, the Ti−O and Ti−Cl bond distances (around 1.90 and 2.40 Å respectively) are quite typical.50,53−55 With complexes 3 in hand, we sought to exchange the Cl ligand for CH3. Therefore, we reacted 3a with 1 equiv of MeLi at −20 °C, but 1H NMR analysis of the crude reaction mixture revealed the presence of 2a′, the target [CpCpPTiMe(OPh] (3a′), and a third species which we tentatively identify as [CpCpPTi(OPh)2]. It became clear that ligand scrambling was an undesired outcome of this procedure. In light of the good results obtained via the reduction/oxidation sequence (vide inf ra), we therefore abandoned this route without further attempt to optimize reaction conditions. Synthesis of target compounds 4/4′. As it appeared that obtaining methyl-aryloxy complexes 3′ was going to be a challenging task, we wondered whether cationic aryloxy Ti complexes 4 could be obtained directly from 3. Indeed B(C6F5)3 and [Et3Si][B(C6F5]4 have been used to abstract Cl from Ti complexes derived from titanocene,19,56 and Wass recently showed that sequential reduction and oxidation of [Cp2TiClOAr] complexes afforded [Cp2TiOAr][B(C6F5)4] species.34 When complexes 3a and 3c were mixed with B(C6F5)3 in C6D6, the solutions immediately turned from redorange to red-brown (3a, 3c) or deep purple (3b). In the case of 3b, a biphasic mixture was formed and the signal-to-noise ratio of the measured NMR spectra (1H, 11B, 19 F, 31P) was low, which prompted us to perform the reaction in CD2Cl2.

Scheme 5. Synthesis of Chloro−Aryloxy Ti Precursors 3a−h

The most distinctive feature of the 1H NMR spectra of complexes 3 is the splitting of the CpP signals due to the chirality of the Ti center.52 It appears that the 2,6dimethylphenoxy ligand has a noticeable influence on the extent and the distribution of this splitting. Thus, while 3a and 3c have very similar H2 (3a: 6.62 and 5.88 ppm; 3c: 6.64 and 5.92 ppm) and H3 signals (3a: 5.99 and 5.66 ppm; 3c: 6.02 and 5.69 ppm), the values for 3b deviate significantly (up to 0.36 ppm for H3, see Table 1).The CMe2 signals in 3b are also shifted by 0.1−0.2 ppm compared to 3a and 3c. Additionally, restricted rotation of the aryloxy ligand is observed in 3b, 3d, 3e, and 3g at 300 K, whereas the smaller phenoxy and pmethoxyphenoxy rotate freely at this temperature. D


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Figure 3. POV-Ray depiction of 3a−c (thermal ellipsoids drawn at the 50% probability level).

−78.7 ppm) and by the small but noticeable 3JPH coupling constant between P and the Cp hydrogens (4a: 2.0 Hz; 4b: 1.9 Hz; 4c: 1.8 Hz) in the 1H NMR spectra (Table 1). Heteronuclear NMR spectroscopy data were consistent with the formation of the ClB(C6F5)3− anion;57−59 for instance, a broad singlet at −6.9 ppm was observed in the 11B NMR spectrum of 4b in CD2Cl2, along with a set of three signals (−135.2; −162.1; −166.9 ppm) in the 19F NMR spectrum. With these encouraging results in hand, we tried to isolate complexes 4a−c; however, we could not obtain these compounds in pure form: an impurity with a 31P NMR signal in the free phosphine region (4a: 43.7; 4b: 46.8; 4c: 43.2 ppm, compare to 3a−c, Table 1) was always observed (Figure 5). Moreover, the 1H NMR spectra showed an extra singlet in the Cp region (4a: 5.87; 4b: 6.28; 4c: 5.91 ppm), consistent with the hypothesis that a Ti complex with a noncoordinated pendant phosphine had formed. This seemed to suggest the formation of a tight ion-pair with Cl bridged between Ti and B. Indeed, when 4a and 4c were dissolved in d3-MeCN, crystals of 3a and 3c formed after several hours (Scheme 6). This constitutes strong evidence that the ClB(C6F5)3− anion is not stable in the presence of cationic Ti and that Cl− only binds reversibly to B. While the lability of the ClB(C6F5)3− is beyond doubt, the exact nature of the byproduct formed along with 4 is still unknown. In light of the potential complications associated with the ClB(C6F5)3− anion, we decided to use a different strategy in order to prepare the desired complexes. As mentioned above, a reduction/oxidation sequence had previously been employed by Wass in order to generate cationic Ti FLPs, using [(C5Me5)2Co] as the reductant and [Cp2Fe][B(C6F5)4] as the oxidant.34 While this strategy is clearly effective, it has the drawback of using expensive chemicals, and we felt it could be improved for increased practical convenience. Considering that simple alkali metal phosphides such as PPh2Li are known to reduce Ti(IV) into Ti(III), on the one hand;60−62 and that the BPh4− anion might be weakly coordinating enough in the context of FLP chemistry (in which polar groups are often present), on the other hand, we

Figure 4. Numbering of X-ray structures and definitions of the torsion angles θ and φ.

Figure 5. (4b).

31

P NMR spectra of 4a−c in C6D6 (4a, 4c) and CD2Cl2

Analysis of the spectra in C6D6 (3a, 3c) and CD2Cl2 (3b) revealed that the starting materials had been consumed and that species 4a−c had been formed along with 5−10% of byproduct. Coordination of P to Ti was indicated by the large upfield change of chemical shift of the signals in the 31P NMR spectra compared to 3a−c (Δδ: 4a: −75.4 ppm; 4b: −68.8 ppm; 4c:

Scheme 6. Putative Equilibria between Complexes 4 and 3 in the Presence of B(C6F5)3 and d3-MeCN

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spectra, indicative of P coordination to Ti. While the 1H NMR chemical shifts vary somewhat compared to 4a−c,65 the 31P NMR spectra of both types of cationic complexes show very similar chemical shift values; in fact, they are equal for 4a/4a′ and 4b/4b′ (Table 1). As mentioned above, coordination to Ti has a dramatic influence on these values; therefore, it is worth noting the lower field signal of 4b′ (−23.5 ppm) compared to 4a′ (−29.6 ppm) and 4c′ (−30.4 ppm). Interestingly, this discrepancy is paralleled in the solid state by the considerably longer Ti−P bond distance in 4b′: at 2.7137(8) Å, it is 2−4% longer than in 4a′ and 4c′ (Figure 6, Table 3). It is worth noting that,

devised a straightforward redox methodology using simple lithium phosphides as reductants and [Cp2Fe][BPh4] as the oxidant. Thus, when complexes 3a−e reacted with PPh2Li(OEt2)0.75 or PCy2Li in toluene, followed by removal of LiCl and treatment with [Cp2Fe][BPh4], complexes 4a′−e′ precipitated as pink-red to purple powders. Reprecipitation from CH2Cl2/ toluene afforded the pure compounds in moderate to good yields (Scheme 7).63 Scheme 7. Synthesis of Cationic Aryloxy Ti Complexes 4a′− e′a

Table 3. Selected Bond Distances (Å) and Angles (deg) for 4a′−d′ Ti−P Ti−O Ti−Ct1 Ti−Ct2 O−Ar Ti−O−Ar O−Ti−P Ct1−Ti− Ct2 Ct1−Ti−P P−C11− Ct1 Ti-Ct1-C11 C11−P−Ti C11−Ct1− Ti−Pb

4d′a

4á

4b′

4c′

2.6087(8) 1.872(2) 2.0529(15) 2.0521(18) 1.326(4) 160.88(18) 89.38(7) 132.89(7)

2.7137(8) 1.865(2) 2.0632(12) 2.0646(13) 1.361(3) 145.17(17) 102.31(6) 130.82(5)

2.6572(10) 1.8524(17) 2.0589(11) 2.0487(11) 1.348(3) 153.87(15) 101.11(6) 133.92(5)

2.693(2) 1.851(4) 2.058(4) 2.058(9) 1.385(8) 152.1(4) 107.9(2) 131.1(3)

91.79(4) 91.75(11)

88.45(4) 91.88(9)

90.85(4) 92.69(9)

88.57(11) 91.2(3)

82.52(8) 88.22(9) 14.90(7)

83.80(6) 87.40(8) 17.59(5)

84.63(6) 89.66(7) 9.12(5)

85.5(2) 88.8(2) −15.00(17)

a

a The structure of 4d′ contained a disordered Cp ligand. Values are given for the ligand with the highest occupancy (70%). bDefined as φ in Figure 4.

The reactions proceeded at room temperature, except for the reduction of 3d, which generated an intractable impurity and had to be conducted at −50 °C.64 On the other hand, complexes 4f′−h′ could not be obtained (instead giving complex oily mixtures), probably because of the rigidity of the CpPPh2 ligand, which prevents the stabilization of the Ti cation. Compounds 4a′−e′ were characterized by NMR spectroscopy, elemental analysis, high resolution ESI mass spectrometry, and single crystal X-ray diffraction (except 4e′). As in the case of 4a−c, a 3JPH coupling constant of ∼2 Hz causes the Cp signal to appear as a doublet in the 1H NMR

according to a recent survey of crystallographic data by Alvarez, the sum of covalent radii for Ti and P is 2.67(11) Å;66 clearly, all of our complexes can be described as “borderline” cases of stretched Ti−P bonds, with 4b′ being the most elongated. The P(o-tolyl)2 analogue 4d′ falls just short of stretching this distance further into FLP territory, with a value of 2.693(2) Å. In any case, these Ti−P bond lengths are high, although they remain quite below the 2.785(2) Å value obtained by Wass for a cationic Ti FLP.34 A notable feature of complexes 4a′−d′ is the important strain in the (Ti−Ct1−C11−P) ring resulting from the coordination of P to Ti. The rectangular shape of this ring is slightly offset by the torsion angle φ (Figure 4) which enables the R group on

Note: “PR2Li”: PPh2Li(OEt2)0.75 (4a’−c’, 4e’), PCy2Li (4d’). Reduction of 3d was performed at −50 °C.

Figure 6. POV-Ray depiction of 4a′−d′ (thermal ellipsoids drawn at the 30% probability level, BPh4− cation omitted for clarity). F


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Organometallics phosphorus (R = Cy or o-tolyl) closer to the aryloxy ligand to point away from it, either below (4a′) or above (4b′−d′). Comparing the structures of complexes 3a−c, on the one hand, and 4a′−c′, on the other hand, it appears that the cationic nature of the latter has a notable influence on several parameters (Tables 2 and 3). For instance, Ti−O bond distances are shortened by 0.019(3) (4a′), 0.040(2) (4b′), and 0.053(3) Å (4c′) in cationic complexes; apparently, the amount of shortening correlates with the electron-donating ability of the aryloxy ligand. The Ti−Ct1 and Ti−Ct2 distances also become smaller by respectively about 0.04 and 0.02 Å for all three complexes. These trends are consistent with a contraction of the coordination sphere of Ti. Even more considerable are the variations observed for the (Ti−O−Ar) angle, both between neutral and cationic complexes, and within cationic complexes themselves. Indeed, while this angle remains approximately constant in the 3a−c series (∼138°), the observed values in 4a′−c′ oscillate between 160.88(18) (4a′) and 145.17(17)° (4b′). It therefore seems that removing Cl induces complementary π-donation from the aryloxy ligand, which results in a more linear geometry around O. Finally, it is tempting to conclude that the 7° difference for Ti−O−Ar between 4b′ and 4d′ also originates from a combination of higher steric bulk and basicity of PCy2 compared to P(o-tolyl)2; however, in this case, crystal packing effects might be important, as evidenced by the presence of a 2.01 Å H−H contact distance between Me groups of the phosphine and aryloxy ligands in 4d′. Reactivity studies. Since the cationic complexes obtained in this work showed good evidence of a weakened Ti−P interaction despite the electrophilicity of the Ti+ center and the basicity of the phosphine arm, we felt that some of these complexes might display FLP reactivity. Complex 4b′ appeared as the best candidate, since it combined the longest Ti−P bond with the most basic phosphine. Therefore, we reacted 4b′ with a range of typical substrates that were previously found to react with FLPs. Disappointingly, 4b′ turned out to be completely unreactive toward gaseous substrates, such as CO, CO2, H2, or C2H2. Moreover, attempted catalytic hydrogenation (50 bar H2, room temperature) of styrene, tert-butylethylene, and phenylacetylene gave none of the hydrogenation products. Even a strongly activated alkyne such as dimethyl acetylenedicarboxylate was completely inert in the presence of 4b′ at 60 °C. However, 4b′ did react within minutes with benzaldehyde in THF or CD2Cl2 at room temperature, producing a 4:1 mixture of diastereoisomeric addition products (Scheme 8).67 Compound 5b′ shows two signals in the 31P NMR spectrum at 13.9 (major isomer, A) and 1.9 ppm (minor isomer, B). In the 1H NMR spectrum, the signal of the activated aldehyde group appears as a singlet at 6.21 ppm (A) and as a doublet at 5.42 ppm (2JPH = 2.6 Hz, B). Single crystals suitable for X-ray diffraction were obtained by diffusion of pentane into a CH2Cl2

solution of 5b′ and the crystal cell turned out to contain the racemic mixture of STi,SC/RTi,RC enantiomers (Figure 7).68

Figure 7. POV-Ray depiction of 5b′ (STi,SC isomer shown, thermal ellispoids drawn at the 30% probability level, BPh4− anion omitted for clarity.).

The crystals appeared homogeneous upon visual inspection, and NMR spectroscopic analysis of a batch obtained similarly (albeit at room temperature) indicated the presence of both diastereoisomers in the same 4:1 ratio. Therefore, we conclude that 5b′ exists as an equilibrium mixture of diastereoisomers in solution, and that crystallization drives the equilibrium toward the formation of only one diastereoisomer. This is consistent with the presence of traces of benzaldehyde in solutions of 5b′. Complex 4b′ was also reacted with 1 equiv of trans-chalcone in CD2Cl2 at room temperature, and the reaction was monitored by 31P NMR spectroscopy. Initially, a new peak appeared at 34.4 ppm along with several other minor peaks in the 30−50 ppm region. After several days the reaction appeared to halt and the mixture only consisted of 4b′ and the new compound. We therefore repeated the reaction with 2 equiv of chalcone and monitored the reaction by 1H and 31P NMR spectroscopy. Within a few hours, the presence of 6,6dimethylfulvene was detected, indicating that a rearrangement was occurring. After 9 days, the reaction was complete; upon workup, 10membered Ti phosphonium macrocycle 6b′ was obtained in 46% yield (Scheme 9). Scheme 9. Synthesis of Compound 6b′

Scheme 8. Reaction of 4b′ with benzaldehyde High resolution ESI mass spectrometry confirmed the presence of the cationic part of 6b′,69 and elemental analysis results were consistent with the proposed formula. However, several isomers are possible, as shown in Figure 8. Diastereoisomer A/A′ would result from a double 1,4-addition of P to chalcone (like 6b′ itself) while isomers B and C would result from 1,4/1,2 and double 1,2 addition, respectively. Isomers B and C can be ruled out on the basis of the 13C NMR G


Article

Organometallics

However, if I could be generated at room temperature from 4b′, the reactions shown in Scheme 9 would most certainly have had a different outcome. In particular, we never observed the formation of 6,6-dimethylfulvene in the presence of these reagents even upon prolonged standing and/or heating. Therefore, we propose that the formation of 6b′ proceeds via intermediate II. Extrusion of fulvene would then afford intermediate III, which could insert a second equivalent of chalcone to give 6b′. However, in the absence of more detailed studies (e.g., kinetic experiments, DFT calculations), this mechanism remains speculative.73

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CONCLUSION We have introduced a new family of cationic aryloxy Ti complexes with a pendant phosphine arm designed to behave as FLPs. Control of the steric and electronic properties of these complexes is easily achieved by variation of the aryloxy ligand and phosphine arm, as visible in the solid state structures of 4a′−d′. The complexes synthesized so far are unreactive toward typical FLP substrates such as H2 or CO2. However, one of the complexes (4b′) did react with benzaldehyde. This is a promising result considering the very limited examples of FLPs based on Ti, and it suggests that further steric and/or electronic tuning might enhance reactivity. Finally we have shown that phosphine-functionalized cyclopentadienyl ligands can decompose by fulvene extrusion, which is an intriguing and potentially important observation concerning the stability of metallocene complexes that incorporate similar ligands.

Figure 8. Possible isomers of 6b′ (BPh4− anion omitted for clarity).

chemical shifts of the CH carbons of the chalcone backbone,70 which appear as four doublets at 99.5 (JPC = 5.9 Hz), 97.4 (JPC = 6.2 Hz), 42.5 (JPC = 36.7 Hz), and 41.6 ppm (JPC = 32.2 Hz): indeed, the last two signals clearly indicate a double 1,4 addition. As for diastereoisomers A/A′, the symmetry plane (O−Ti− Ct) would give rise to magnetically equivalent chalcone moieties for each of them; thus, only two signals would appear for the CH groups, in both the 13C and 1H NMR spectra. Since this is clearly not the case (see Figure 9), we conclude that the

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EXPERIMENTAL SECTION

For general information and the spectroscopic and structural data of the new compounds see the Supporting Information. Preparation of compound 1a. Lithium dicyclohexylphosphide (7.00 g, 34.3 mmol) was suspended in 80 mL of Et2O and dimethylfulvene (3.64 g, 34.3 mmol) was diluted in 40 mL of Et2O. Both vessels were cooled at −80 °C for 10 min, then the solution of dimethylfulvene was cannulated onto the suspension of PCy2Li. The resulting mixture was stirred for 14 h, filtered over Celite to eliminate the insoluble Cp-propenyl salt, and evaporated. The material thus obtained was contaminated with ∼5% m/m of lithium Cp-propenyl, which was low enough for further synthesis; crystallization from Et2O afforded material suitable for elemental analysis. Elemental Analysis: calcd for C20H32LiP: C, 77.39; H, 10.39. Found: C, 77.23; H, 10.47. Preparation of compound 1b. Lithium di-o-tolylphosphide etherate (6.51 g, 23.6 mmol) was suspended in 15 mL of Et2O and 6, 6-dimethylfulvene (2.50 g, 23.6 mmol) was diluted in 15 mL of Et2O. Both vessels were cooled at −80 °C for 10 min, then the solution of dimethylfulvene was cannulated onto the suspension of phosphide. The resulting mixture was stirred for 8 h, during which time a white precipitate appeared. Pentane (50 mL) was added, the solid was filtered with a borosilicate filter fitted on a cannula stick, rinsed twice with 100 mL of pentane and dried in vacuo to give 1b as a white powder (6.20, 81%). The compound slowly decomposes in d8THF solutions and in the solid state, to give initially HP (o-tolyl)2 and 6, 6-dimethylfulvene (traces visible in the 1H and 31P NMR spectra). Elemental Analysis: calcd for C22H24LiP: C, 80.97; H, 7.41. Found: C, 79.60; H, 7.08. Preparation of complex 2a. Ligand 1a (6.00 g, 20.0 mmol) and CpTiCl3 (4.30 g, 19.6 mmol) were each dissolved in 45 mL of solvent (1:1 THF/Et2O mixture). Both vessels were cooled to −10 °C for 10 min, after which the solution of 1a was rapidly cannulated onto CpTiCl3. The reaction mixture was stirred a further 10 min, magnetic stirring was stopped to let a red solid settle, and the supernatant solution was filtered with a Teflon cannula fitted with a paper filter.

Figure 9. Detail of the 1H NMR spectrum of 6b′ in CD2Cl2 showing the CH signals of the chalcone backbone.

correct isomer is the one depicted in Scheme 9. Additional support for this conclusion came from a NOE experiment in which irradiation of the Cp signal induced a small but noticeable response from the proton pointing toward the Cp ligand, while the proton pointing away from it did not respond. The formation of 6b′ is intriguing, especially given the reactivity of 4b′ toward benzaldehyde. Indeed, no extrusion of fulvene was found to take place in this case, even in the presence of 2 equiv of reagent. The fact that only minor signals are observed by 31P NMR spectroscopy on the reaction path from 4b′ to 6b′ strongly suggests that reactive intermediates are formed. Two possible mechanisms are shown in Scheme 10 that could account for the observed product, while producing two intermediates. The first mechanism would involve extrusion of 6,6dimethylfulvene to generate the cationic planar phosphido Ti complex I.71 While such species are yet to be reported, we have recently found in a different context that they can be prepared and that they are indeed very reactive.72 H


Article

Organometallics Scheme 10. Possible Mechanisms for the Formation of 6b′ (BPh4− anion omitted for clarity)

Celite and evaporated. The solid residue was triturated in pentane and filtered to yield 3a as an orange powder (380 mg, 70%). Single crystals suitable for X-ray diffraction analysis were obtained by reacting 3a with B(C6F5)3, isolating the product, dissolving it in CD3CN and letting it stand in the NMR tube overnight. Elemental Analysis: calcd for C31H42ClOPTi: C, 68.32; H, 7.77. Found: C, 68.20; H, 7.59. Preparation of complex 3b. Complex 2a (487 mg, 1 mmol) and sodium 2,6-dimethylphenate etherate (205 mg, 1.2 mmol) were mixed in 20 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O (20 mL) at −40 °C to yield 3b as red crystals (392 mg, 66%). Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 3b in CH2Cl2 at −18 °C. Elemental Analysis: calcd for C33H46ClOPTi: C, 69.17; H, 8.09. Found: C, 69.00; H, 8.18. Preparation of complex 3c. Complex 2a (487 mg, 1 mmol) and sodium 4-methoxyphenate (176 mg, 1.2 mmol) were mixed in 20 mL of CH2Cl2 at room temperature for 17 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O (10 mL) to yield 3c as purple crystals (203 mg, 36%).A second crop was obtained after further crystallization at −18 °C overnight (43 mg, 43% combined yield). Single crystals suitable for Xray diffraction were grown by vapor diffusion of pentane into a solution of 3c in CH2Cl2 at −18 °C. Elemental Analysis: calcd for C32H44ClO2PTi: C, 66.84; H, 7.71. Found: C, 66.74; H, 7.69. Preparation of complex 3d. Complex 2b (3.10 g, 6.16 mmol) and sodium 2,6-dimethylphenate etherate (1.26 g, 7.40 mmol) were mixed in 100 mL of CH2Cl2 at room temperature for 13 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O (50 mL) at −40 °C and rinsed twice with 15 mL of pentane to yield 3h as an orange powder (2.82 g, 78%). The material obtained in this way was free of organic impurities, as evidenced by NMR spectroscopy, however elemental analysis was unexpectedly low in C, suggestive of the presence of residual NaCl. Preparation of complex 3e. Complex 2c (475 mg, 1 mmol) and sodium 2,6-dimethylphenate etherate (200 mg, 1.2 mmol) were mixed in 10 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O (15 mL) at −40 °C to yield 3e as red crystals (318 mg, 57%). The material obtained in this way was free of organic impurities, as evidenced by NMR spectroscopy, however elemental analysis was unexpectedly low in C, suggestive of the presence of residual NaCl.

The solid was rinsed once with 30 mL of solvent (1:1 THF/Et2O mixture), then three times with 20 mL of Et2O, and finally dried in vacuo. The product was isolated as a red powder (7.51 g, 81%) which decomposes in solution but is stable when stored under N2 at room temperature. Elemental Analysis: calcd for C25H37Cl2PTi: C, 61.62; H, 7.65. Found: C, 61.52; H, 7.55. Preparation of complex 2b. Ligand 1b (5.72 g, 17.5 mmol) and CpTiCl3 (3.84 mg, 17.5 mmol) were each dissolved in 100 mL of solvent (1:1 THF/Et2O mixture). Both vessels were cooled to −15 °C for 10 min, after which the solution of 1b was rapidly cannulated onto CpTiCl3. The reaction mixture was stirred a further 15 min, during which a purple-pink solid formed. The supernatant solution was filtered with a Teflon cannula fitted with a paper filter. The solid was rinsed once with 40 mL of solvent (1:1 THF/Et2O mixture), then twice with 40 mL of Et2O, and finally dried in vacuo. The product was isolated as a purple-pink powder (5.20 g, 59%) which decomposes in solution but is stable when stored under Ar at room temperature. The purity of the batch thus obtained was judged sufficient by NMR spectroscopy; however an analytically pure sample was prepared for elemental analysis in order to eliminate remaining traces of LiCl (recrystallization from saturated THF solution at −35 °C). Elemental Analysis: calcd for C27H29Cl2PTi: C, 64.44; H, 5.81. Found: C, 64.18; H, 5.79. Preparation of complex 2a′. Complex 2a (2.00 g, 4.10 mmol) was suspended in Et2O (60 mL). A commercial solution of MeLi (5 mL, 1.69 M in Et2O, 8.45 mmol) was diluted with Et2O (20 mL). Both vessels were cooled to −10 °C for 10 min, after which the solution of MeLi was added dropwise over 5 min to 2a by cannula. The cannula was rinsed with 5 mL of Et2O. The reaction mixture was stirred for 50 min, magnetic stirring was stopped during 10 min to let a yellow solid settle. The supernatant was filtered with a Teflon cannula fitted with a paper filter. The resulting solution blackened within minutes, therefore it was discarded. The yellow solid was extracted three times with 30 mL of Et2O then once with 10 mL of Et2O. Decomposition occurred at the surface of the cannula, therefore it was replaced after the second extraction. The resulting solution was filtered over Celite to eliminate insoluble that formed during the filtration, then evaporated to yield 2a′ as a yellow solid (0.96 g, 53%). Once isolated, 2a′ was stable both in the solid state and in solution if kept away from air. Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 2a′ in CH2Cl2 at −18 °C. HRMS (ESI-pos): calcd for C27H44PTi [M + H]+: 447.26572. Found: 447.26581 (+0.8 ppm). Preparation of complex 3a. Complex 2a (487 mg, 1 mmol) and sodium phenate (128 mg, 1.1 mmol) were mixed in 10 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over I


Article

Organometallics Preparation of complex 3f. Complex 2d (475 mg, 1.1 mmol) and sodium phenate (139 mg, 1.2 mmol) were mixed in 10 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O at −40 °C to yield 3f as orange crystals (375 mg, 70%). The product cocrystallized with 50 mol % Et2O which could not be removed even after prolonged drying in vacuo. Elemental Analysis: calcd for C28H24ClOPTi.0.5Et2O: C, 68.26; H, 5.54. Found: C, 68.35; H, 5.16. Preparation of complex 3g. Complex 2d (475 mg, 1.1 mmol) and sodium 2, 6-dimethylphenate etherate (200 mg, 1.2 mmol) were mixed in 10 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O to yield 3g as red crystals (441 mg, 77%). Elemental Analysis: calcd for C30H28ClOPTi: C, 69.45; H, 5.44. Found: C, 69.32; H, 5.37. Preparation of complex 3h. Complex 2d (475 mg, 1.1 mmol) and sodium 4-methoxyphenate (175 mg, 1.2 mmol) were mixed in 10 mL of CH2Cl2 at room temperature for 14 h. The reaction mixture was filtered over Celite and evaporated. The crude product was crystallized from Et2O to yield 3h as purple crystals (318 mg, 56%). Elemental Analysis: calcd for C29H26ClO2PTi: C, 66.88; H, 5.03. Found: C, 66.74; H, 4.97. Preparation of complex 4a′. Complex 3a (150 mg, 0.275 mmol) and PCy2Li (56 mg, 0.275 mmol) were mixed in toluene (4 mL) in the glovebox. The mixture was stirred for 90 min, then filtered over Celite into a vessel containing [Fc]BPh4 (132 mg, 0.262 mmol). The mixture was stirred for 75 min, during which a brown-purple precipitate appeared. The product was filtered, rinsed twice with 1.5 mL of toluene, and dissolved in CH2Cl2. The remaining insoluble solids were filtered off on a borosilicate glass fiber filter (1 μm pore size). The product was reprecipitated by addition of its CH2Cl2 solution to toluene (15 mL). The resulting precipitate was filtered, rinsed twice with 1.5 mL of toluene, twice with 1.5 mL of pentane, then dried in vacuo, yielding 4a′ (122 mg, 54%) as a mauve powder. Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 4a′ in CH2Cl2 at −18 °C. Elemental Analysis: calcd for C55H62BOPTi: C, 79.71; H, 7.54. Found: C, 79.61; H, 7.48. Preparation of complex 4b′. Complex 3b (573 mg, 1.0 mmol) and PCy2Li (204 mg, 1.0 mmol) were mixed in toluene (10 mL) in the glovebox. The mixture was stirred for 1 h, then filtered over Celite into a vessel containing [Fc]BPh4 (480 mg, 0.95 mmol). The mixture was stirred for 2 h, during which a brown-purple precipitate appeared. The product was filtered, rinsed twice with 3 mL of toluene, and dissolved in CH2Cl2. The remaining insoluble solids were filtered off on a borosilicate glass fiber filter (1 μm pore size). The product was recrystallized by addition of its CH2Cl2 solution to toluene (25 mL). The resulting solid was filtered, rinsed three times with 3 mL of toluene, then pentane, and finally dried in vacuo to yield 4b′ (634 mg, 74%) as a vermilion microcrystalline solid. Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 4b′ in CH2Cl2 at −18 °C Elemental Analysis: calcd for C57H66BOPTi: C, 79.90; H, 7.76. Found: C, 79.66; H, 7.66. Preparation of complex 4c′. Complex 3c (575 mg, 1.0 mmol) and PCy2Li (204 mg, 1.0 mmol) were mixed in toluene (10 mL) in the glovebox. The mixture was stirred for 1 h, then filtered over Celite into a vessel containing [Fc]BPh4 (480 mg, 0.95 mmol). The mixture was stirred for 2 h, during which a brown-purple precipitate appeared. The product was filtered, rinsed twice with 3 mL of toluene, and dissolved in CH2Cl2. The remaining insoluble solids were filtered off on a borosilicate glass fiber filter (1 μm pore size). The product was reprecipitated by addition of its CH2Cl2 solution to toluene (25 mL). The resulting solid was filtered, rinsed three times with 3 mL of toluene, then pentane, and finally dried in vacuo to yield 4c′ (409 mg, 48%) as a purple powder. Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 4c′ in CH2Cl2 at −18 °C.

Elemental Analysis: calcd for C56H64BO2PTi: C, 78.32; H, 7.51. Found: C, 78.18; H, 7.33. Preparation of complex 4d′. Complex 3d (589 mg, 1.00 mmol) and PPh2Li (OEt2)0.75 (245 mg, 0.99 mmol) were cooled to −50 °C in a Schlenk flask. Toluene (20 mL) was added. The mixture was stirred for 15 min, and a purple color was observed. The cold bath was removed and the mixture was allowed to warm up to room temperature over 1 h. A brown solution was obtained, which was filtered over Celite into a vessel containing [Fc]BPh4 (480 mg, 0.95 mmol). The mixture was stirred for 1H, during which a brown precipitate appeared. The product was filtered, rinsed three times with 5 mL of toluene and three times with 5 mL of pentane. The precipitate was dissolved in 7 mL of dichloromethane, reprecipitated from toluene (50 mL) at −40 °C, rinsed as previously with toluene and pentane, and finally dried in vacuo to yield 4d′ (320 mg, 39%) as a brownpurple powder. Single crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a solution of 4d′ in CH2Cl2 at −18 °C Elemental Analysis: calcd for C59H58BOPTi: C, 81.20; H, 6.70. Found: C, 80.16; H, 6.72. Preparation of complex 4e′. Complex 3e (180 mg, 0.37 mmol) and PCy2Li (66 mg, 0.37 mmol) were mixed in toluene (4 mL) in the glovebox. The mixture was stirred for 1 h, then filtered over Celite into a vessel containing [Fc]BPh4 (154 mg, 0.352 mmol). The mixture was stirred for 2H, during which a mauve precipitate appeared. The product was filtered, rinsed twice with 1.5 mL of toluene, then pentane, and finally dried in vacuo to yield 4e′ (217 mg, 69%) as a mauve powder. The material prepared in this way contained 80 mol % of toluene, which could be removed after drying in vacuo for several days. Elemental Analysis: calcd for C57H54BOPTi: C, 81.05; H, 6.44. Found: C, 80.85; H, 6.45. Preparation of complex 5b′. In a glovebox, complex 4b′ (171.0 mg, 0.20 mmol) and benzaldehyde (23.4 mg, 0.22 mmol) were mixed in THF (2 mL) until a homogeneous solution was obtained. The mixture was stored overnight in the fridge at −35 °C and orange crystals were obtained. These were dissolved in DCM and recrystallized by diffusion of pentane at room temperature, yielding 5b′ as a 4:1 mixture of diastereoisomers (major: A; minor: B) solvated by 1 molecule of DCM (61 mg, 29%). Single crystals suitable for X-ray diffraction were obtained similarly by diffusion at −35 °C. Elemental Analysis: calcd for C64H72BO2PTi.CH2Cl2: C, 74.51; H, 7.12. Found: C, 74.77; H, 7.25. Preparation of complex 6b′. In a glovebox, complex 4b′ (85.7 mg, 0.10 mmol) and trans-chalcone (41.6 mg, 0.20 mmol) were dissolved in CD2Cl2 (2 mL) and the solution was placed in a sealed NMR tube for periodic monitoring. After 9 days, the reaction mixture was returned to the glovebox and precipitated by addition to vigorously stirred pentane (15 mL). The resulting oil was dissolved in DCM (1 mL) and precipitated similarly. The operation was repeated one more time, yielding 6b′ as a yellow powder (54 mg, 46%). Elemental Analysis: calcd for C79H80BO3PTi: C, 81.30; H, 6.91. Found: C, 80.96; H, 7.19. Reaction of 3a−c with B (C6F5)3. Complexes 3a−c (0.275 mmol) were dissolved in 5 mL of toluene, and B(C6F5)3 (0.281 mmol, 1.02 equiv) was dissolved in 5 mL of toluene. The borane solution was added onto Ti at room temperature in the glovebox. The reaction mixture was placed into Schlenk vessels and removed from the glovebox, and the solvent was evaporated. The residue was taken up in 3 mL of toluene and the product was reprecipitated from pentane, filtered and dried in vacuo. The crude product was analyzed by 1H and 31 P NMR spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and NMR spectra of new compounds together with CIF files of X-ray characterized complexes (CCDC numbers: 2a′: 1052592; 3a: 1052593; 3b: 1052594; J


Article

Organometallics

(25) Sgro, M. J.; Stephan, D. W. Chem. Commun. 2013, 49, 2610− 2612. (26) Sgro, M. J.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51, 11343−11345. (27) Kalz, K. F.; Brinkmeier, A.; Dechert, S.; Mata, R. A.; Meyer, F. J. Am. Chem. Soc. 2014, 136, 16626−16634. (28) These are explicitly described as FLPs. As noted by Wass (ref 15) “...transition metal frustrated Lewis pairs can be viewed in the context of cooperative effects in catalysis, and inevitably, there are a great many examples of cooperativity or ligand-assisted reactions (often predating the development of main group FLPs) that could be viewed as FLPs···”. (29) Podiyanachari, S. K.; Fröhlich, R.; Daniliuc, C. G.; Petersen, J. L.; Mück-Lichtenfeld, C.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2012, 51, 8830−8833. (30) Forrest, S. J. K.; Clifton, J.; Fey, N.; Pringle, P. G.; Sparkes, H. A.; Wass, D. F. Angew. Chem., Int. Ed. 2015, 54, 2223−2227. (31) Devillard, M.; Bouhadir, G.; Bourissou, D. Angew. Chem., Int. Ed. 2015, 54, 730−732. (32) Cabrera, L.; Hollink, E.; Stewart, J. C.; Wei, P.; Stephan, D. W. Organometallics 2005, 24, 1091−1098. (33) Stephan also reported the reaction of a noninteracting [Cp2TiMe]+/PtBu3 pair with N2O, see ref 16. (34) Chapman, A. M.; Wass, D. F. Dalton Trans. 2012, 41, 9067− 9072. (35) This was described in a research proposal evaluated for funding by ANR and DFG during Summer 2011. (36) Casey, C. P.; Bullock, R. M.; Fultz, W. C.; Rheingold, A. L. Organometallics 1982, 1, 1591−1596. (37) Bosch, B.; Erker, G.; Fröhlich, R. Inorg. Chim. Acta 1998, 270, 446−458. (38) Le Gendre, P.; Richard, P.; Moïse, C. J. Organomet. Chem. 2000, 605, 151−156. (39) Le Gendre, P.; Picquet, M.; Richard, P.; Moïse, C. J. Organomet. Chem. 2002, 643−644, 231−236. (40) Knox, G. R.; Pauson, P. L. J. Chem. Soc. 1961, 4610−4615. (41) Schore, N. E.; LaBelle, B. E. J. Org. Chem. 1981, 46, 2306−2310. (42) We found that 1b was extremely sensitive to trace H2O, giving HP(o-tolyl)2 and 6,6-dimethylfulvene. Intriguingly, 1a was not so sensitive and normal handling/storage under inert atmosphere did not cause any decomposition. (43) Both compounds were found to be light sensitive and decomposed to Ti(III) species and (PR2)2 upon UV irradiation. Detailed investigations are being conducted and will be reported elsewhere in due course. (44) Dunlop-Brière, A. F.; Baird, M. C.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2013, 135, 17514−17527. (45) Gillis, D. J.; Tudoret, M. J.; Baird, M. C. J. Am. Chem. Soc. 1993, 115, 2543−2545. (46) This complex is stable when isolated, but it decomposes in the reaction mixture in the presence of PTFE objects (stir bar, cannula). This suggests a deleterious interaction between 2a′, MeLi, and fluorinated organic compounds. (47) These results corroborate those obtained by Wass in his study of phosphinoaryloxy titanium FLPs, see ref 34. (48) Firth, A. V.; Stewart, J. C.; Hoskin, A. J.; Stephan, D. W. J. Organomet. Chem. 1999, 591, 185−193. (49) Kalirai, B. S.; Foulon, J.-D.; Hamor, T. A.; Jones, C. J.; Beer, P. D.; Fricker, S. P. Polyhedron 1991, 10, 1847−1856. (50) Besancon, J.; Top, S.; Tirouflet, J.; Dusausoy, Y.; Lecomte, C.; Protas, J. J. Organomet. Chem. 1977, 127, 153−168. (51) Initially, we synthesized derivatives 3a−c to study the influence of added steric bulk and basicity of the aryloxy ligand. We later found (crystallographic and NMR spectroscopic study) that the 2,6dimethylphenoxy ligand provides the best template to weaken the Ti−P interaction; therefore, we limited our study of the CMe2PAr2 complexes to the synthesis of 3d and 3e. (52) While this feature should in principle allow for the complete assignment of the CpP signals with the combination of HSQC and

3c: 1052595; 4a′: 1052596; 4b′: 1052597; 4c′: 1052598; 4d′: 1053357; 5b′: 1053358). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00250.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

P.R. and C.G.D. were responsible for crystal structure analysis.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Agence Nationale de la Recherche and Deutsche Forschungsgemeinschaft (MENOLEP programme) and Conseil Régional de Bourgogne (PARI IME SMT08 program) is gratefully acknowledged.

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REFERENCES

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Article

Organometallics NOESY experiments, the latter only showed very weak correlation peaks between H2/H3 and the aryloxy ligand. This was deemed not to be solid enough evidence to assign all the CpP signals. (53) Besançon, J.; Szymoniak, J.; Moïse, C.; Toupet, L.; Trimaille, B. J. Organomet. Chem. 1995, 491, 31−39. (54) Köcher, S.; Walfort, B.; Rheinwald, G.; Rüffer, T.; Lang, H. J. Organomet. Chem. 2008, 693, 3213−3222. (55) Lecomte, C.; Dusausoy, Y.; Protas, J.; Tirouflet, J.; Dormond, A. J. Organomet. Chem. 1974, 73, 67−76. (56) Postigo, L.; Bellarosa, L.; Sánchez-Nieves, J.; Royo, P.; Lledós, A.; Mosquera, M. E. G. Organometallics 2010, 29, 642−655. (57) Courtenay, S.; Stephan, D. W. Organometallics 2001, 20, 1442− 1450. (58) Ghadwal, R. S.; Azhakar, R.; Roesky, H. W.; Propper, K.; Dittrich, B.; Goedecke, C.; Frenking, G. Chem. Commun. 2012, 48, 8186−8188. (59) Lawrence, E. J.; Oganesyan, V. S.; Wildgoose, G. G.; Ashley, A. E. Dalton Trans. 2013, 42, 782−789. (60) Issleib, K.; Wille, G.; Krech, F. Angew. Chem. 1972, 84, 582− 582. (61) Baker, R. T.; Krusic, P. J.; Tulip, T. H.; Calabrese, J. C.; Wreford, S. S. J. Am. Chem. Soc. 1983, 105, 6763−6765. (62) Dick, D. G.; Stephan, D. W. Organometallics 1991, 10, 2811− 2816. (63) Reprecipitation was required to completely remove the (PR2)2 and ferrocene byproducts. (64) On the basis of NMR spectroscopic data, we tentatively identify this impurity as [CpPCpTiCl][BPh4]. (65) This is possiblybut not onlydue to NMR solvent effects. (66) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832−2838. (67) Complex 4b gave identical results. (68) The following priority rules were applied for the substituent around Ti: 1: CpP; 2: Cp; 3: OAr; 4: OCHPh. See ref 55. (69) A small cluster of peaks corresponding to the extrusion of 1 molecule of chalcone was also observed. (70) Assigned on the basis of HSQC and HMBC experiments, see Supporting Information. (71) Rosenberg, L. Coord. Chem. Rev. 2012, 256, 606−626. (72) Normand, A. T.; Richard, P.; Balan, C.; Lecomte, V.; Le Gendre, P.; Erker, G. Manuscript in preparation. (73) We did run a trapping experiment with 2 atm CO2 and 1 eq of chalcone in order to influence the outcome of the reaction, but to no avail.

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Synthetic Endeavors toward Titanium Based ... - ACS Publications

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