Frustrated Lewis Pair Behavior of [Cp2ZrOCR2CH2PPh2]+ Cations

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Frustrated Lewis Pair Behavior of [Cp2ZrOCR2CH2PPh2]+ Cations Xin Xu, Gerald Kehr, Constantin G. Daniliuc, and Gerhard Erker* Organisch-Chemisches Institut, Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: Reactions of Cp2ZrMe2 with 1 equiv of 2-diphenylphosphino-substituted alcohols Ph2PCH2CR2OH (R = H, CH3) gave the neutral monomethyl zirconocene alkoxy complexes Cp2Zr(Me)OCR2CH2PPh2 (7) by means of methane elimination. Treatment of the complexes 7 with the alkyl abstraction reagent [Ph3C]+[B(C6F5)4]− resulted in the formation of the cationic complexes 8. X-ray crystal structure analysis shows that 8a (R = H) is a doubly oxygen bridged dimer and 8b (R = CH3) is a monomer with P−Zr coordination. Complexes 8 show some typical Zr+/P FLP reactivity. For example, complex 8a undergoes selective 1,4-addition to chalcone to produce the nine-membered metallacyclic product 13. Complex 8b reacts with benzaldehyde and an ynone reagent to form the respective Zr+/P addition products 14 and 15. Complex 8b also adds to the NO functionality of nitrosobenzene with formation of 20. Treatment of 8b with NO gave the six-membered metallaheterocycle 21 by oxidation of the phosphane. The reaction of compound 8b with 0.5 mol equiv of nitrobenzene afforded a mixture of complexes 20 and 21 in a 1:1 molar ratio.

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INTRODUCTION Frustrated Lewis pairs (FLPs) are mostly comprised of main group element derived Lewis acids and Lewis bases that are efficiently hindered from their usual neutralizing adduct formation by the presence of sufficiently bulky substituents at both these components.1 This generates situations where the active pairs of Lewis acids and bases in solution can undergo interesting (and sometimes new and unprecedented) cooperative reactions with added substrates. Dihydrogen activation by borane/phosphane or borane/amine FLPs (and other active Lewis acid/Lewis base combinations) is a prominent example,2−4 but FLPs were shown to react with a variety of other small molecules as well5−8 and they often undergo characteristic reactions with a variety of organic π systems,9 including α,β-unsaturated enones and ynones.10 Recently, there has been a tendency to use transition metal derived Lewis acids instead of the ubiquitous boron Lewis acid. In particular, the strongly electrophilic [Cp2ZrX]+ cation moiety11 has found some applications. Stephan et al. initially used it in electrophilic substitution reactions of preformed P/B FLP addition products.12,13 Wass et al. have prepared the system 1 (and related analogues) and very successfully applied it in typical FLP reactions (see Scheme 1).14 We had previously described the zirconocene-containing intramolecular Zr/P FLPs 215 and 316 and studied their reactions with small molecules and a variety of unsaturated substrates. In all of these compounds the cationic zirconocene moieties served as efficient charged Lewis acids that in conjunction with the phosphane Lewis base present underwent typical cooperative FLP reactions with various organic substrates. The compound 4 © XXXX American Chemical Society

Scheme 1. Examples of Previously Reported ZirconiumContaining FLPs

is an extraordinary example.17 In the Zr/B combination it is the zirconocene unit that serves as a metal Lewis base.18 It has served as a Zr/B frustrated Lewis pair that undergoes a variety of typical FLP reactions, among them the activation of dihydrogen under mild conditions. The Zr/B pair of compound 4 also adds cooperatively to some acetylenes, conjugated enynes, and diynes. We have now prepared a pair of Lewis acidic alkoxy zirconocene cations that bear a pendant PPh2 Lewis base at the single σ ligand. These were derived from 2-diphenylphosphinoethanol derivatives. We have characterized their structural features, which depended on the substituent pattern of the Special Issue: Mike Lappert Memorial Issue Received: December 20, 2014

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DOI: 10.1021/om501312a Organometallics XXXX, XXX, XXX−XXX

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Organometallics linker, and studied some of their chemical behavior with regard to FLP reactivity.

Table 1. Selected Structural Parameters of the Zirconium Complexes 7 and 8a

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7a

RESULTS AND DISCUSSION Synthesis and Characterization of the Zr+/P Systems. We first treated dimethylzirconocene (5)19 with 1 molar equiv of 2-diphenylphosphinoethanol (6a) in toluene solution. The reaction mixture was prepared at room temperature and then covered with pentane and stored at −35 °C. This eventually gave the product 7a as a crystalline solid in 83% yield (see Scheme 2). In solution it shows the 1H/13C NMR signals of the

Zr1−CH3 Zr1−O1 O1−C1 C1−C2 C2−P1 Zr1−P1 Zr1−O1A Zr1−Cp (centroid)

Scheme 2. Syntheses of the Zirconium Complexes 8

Zr1−O1−C1 ∑P1CCC a

8a b

2.278(6) 1.917(3) 1.432(7) 1.498(11) 1.861(7) 3.980 2.232, 2.232 152.7(3) 305.9

2.194(2) 1.436(3) 1.512(4) 1.810(3) 2.956(1) 2.248(2) 2.235, 2.235 126.6(1) 313.4

7b 2.287(2) 1.921(1) 1.404(3) 1.544(2) 1.857(1) 4.213

8b c

2.250, 2.247 178.9(1) 304.5

1.920(2) 1.423(3) 1.548(3) 1.833(2) 2.767(1) 2.209, 2.218 142.9(1) 318.1

Bond lengths in Å and angles in deg. bZr1−C3. cZr1−C5.

molar ratio. This resulted in a selective abstraction of the methyl group at zirconium. The resulting [Cp2ZrOCH2CH2PPh2]+ cation was not observed as a monomer, but it dimerized to give the doubly oxygen bridged dicationic product 8a(dimer), as is often formed in (alkoxy)zirconocenes (see Scheme 2).20,21 The X-ray crystal structure analysis of 8a(dimer) shows the diamond-shaped Zr2O2 core of the dication (see Figure 2 and Table 1). The PPh2 Lewis base is not

Cp2ZrCH3 moiety at δ 5.76/110.5 (Cp) and δ 0.34/19.3 (CH3), respectively. The 1H NMR signals of the OCH2CH2[P] tether occur at δ 4.04 and 2.22, respectively. Compound 7a shows a 31P NMR resonance at δ −22.7. Compound 7a was characterized by X-ray diffraction (see Figure 1 and Table 1). The X-ray crystal structure analysis

Figure 2. View of the bis(zirconocene) dication complex 8a(dimer) (thermal ellipsoids are shown with 30% probability).

essentially involved in bonding to the zirconium cation. The O1−C1−C2−PPh2 unit features a gauche-like conformation arrangement (θ(O1−C1−C2−P1) −64.2(2)°). The P1···Zr1 separation in the dizirconium dication amounts to 2.956(1) Å. The anions in 8a(dimer) are well separated from the cations. In solution the dimeric structure with noninteracting Zr+ Lewis acids and PPh2 Lewis bases seems to be retained. Complex 8a(dimer) shows a typical 31P NMR feature at δ −18.7, which indicates the presence of the tricoordinate phosphane moiety. It shows an 1H NMR Cp signal at δ 6.57 and 1H NMR resonances of the pendant O−CH2−CH2[P] unit at δ 4.44 and 2.52, respectively (13C δ 74.1, 33.5). The reaction of Cp2Zr(CH3)2 (5) with the reagent HOC(CH3)2CH2PPh2 (6b) was carried out in toluene. After workup we isolated the product 7b as a crystalline solid in 92% yield (see Scheme 2). Compound 7b was characterized by spectroscopy, by C,H-elemental analysis, and by X-ray

Figure 1. Molecular structure of the metallocene complex 7a (thermal ellipsoids are shown with 30% probability).

shows the pseudotetrahedral coordination geometry at zirconium (angle O1−Zr1−C3 97.5(2)°). The metal has a pair of σ ligands located in the typical σ ligand plane bisecting the Cp2Zr metallocene wedge. The OCH2CH2PPh2 σ ligand features a gauche-like conformation with the C2−P1 vector being oriented toward the back side of the metallocene wedge. We then reacted complex 7a with the alkyl abstraction reagent trityl tetrakis(pentafluorophenyl)borate (9) in a 1:1 B


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Organometallics diffraction. It has structural features similar to those of 7a (see Table 1 and the Supporting Information for details). In solution, it features a 31P NMR signal at δ −23.2. The subsequent reaction of complex 7b with the trityl salt 9 was carried out in bromobenzene. It gave the product 8b, which was isolated in 90% yield as a pale yellow crystalline solid (see Scheme 2). The X-ray crystal structural analysis of complex 8b revealed a markedly different structure than 8a. Compound 8b is a monomeric monocation. It features an internal Zr−P interaction (see Figure 3 and Table 1). The phosphorus

Scheme 3. FLP Reactions with Chalcone

the building block 8a and the PPh2 nucleophile from 8a to the Michael position of the α,β-unsaturated ketone reagent (see Figure 4). There is a new CC double bond (C4C5).

Figure 3. Molecular structure of the cation of complex 8b (thermal ellipsoids are shown with 30% probability).

atom is tetracoordinated. The zirconium atom bears the oxygen atom and the phosphane bonded in the σ ligand plane in front of the bent-metallocene wedge. The structure featuring a phosphanyl−zirconium coordination is retained in solution, as indicated from the characteristic 31P NMR chemical shift (δ +13.4). Compound 8b shows 1H NMR signals for the pair of Cp ligands at zirconium (δ 6.45), the geminal pair of methyl groups at carbon atom C1 of the tether (δ 1.46), and the adjacent CH2 group inside the heterocyclic ring system of 8b (δ 3.61, 2JPH = 9.8 Hz). Reactions of the Zr+/P Systems. It appeared as if the Zr+/ P dimer 8a was coordinatively saturated at the metal center by forming the doubly oxygen bridged dimer. Thus, this system was not expected to undergo any FLP-related reaction from the dimeric state. It remained to be clarified if the system 8a (dimer) might equilibrate22 with a monomer (8a). This would then contain an active 14-electron alkoxy zirconocene cation Lewis acid and an active PPh2 Lewis base. This combination might allow the observation of a typical FLP reaction with an added substrate. This was actually the case. We chose the reaction with chalcone 10. It had previously been shown that a variety of P/B FLPs, e.g. the intramolecular system 11, undergo 1,4-addition to this α,β-unsaturated ketone under mild conditions to give compound 12 (see Scheme 3).10 Stirring of a suspension of 8a(dimer) with 2 molar equiv of chalcone (10) at room temperature in dichloromethane (2 h) gave the 1,4-addition product of the monomeric Zr+/P FLP 8a, which was isolated in 66% yield after workup. The salt 13 was characterized by X-ray diffraction. It showed the medium-sized nine-membered-ring system that had been formed by addition of the chalcone carbonyl oxygen atom to the Cp2Zr cation of

Figure 4. View of the molecular structure of the cation of compound 13 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1 1.983(2), Zr1−O2 2.027(2), O2−C5 1.330(4), C5−C4 1.346(4), C2−P1 1.805(3), C3−P1 1.852(3), O1−Zr1−O2 94.4(1), C1−O1−Zr1 142.3(2), Zr1−O2− C5 141.9(2), C2−P1−C3 110.4(2), O1−C1−C2−P1 61.3(3).

Compound 13 contains a tetracoordinated phosphonium unit as part of the heterocyclic ring system and a pseudotetrahedral bis(alkoxide)zirconocene unit at the opposite side of the ring. The bond angles at both oxygen atoms are relatively large. In solution compound 13 features a 1H NMR pair of Cp signals at δ 6.60/6.23, the [P]CHPh signal at δ 6.01 (2JPH = 15.9 Hz) (13C δ 40.2 (1JPC = 39.5 Hz)), the 13C NMR signals of the [Zr]OCH2CH2[P] unit at δ 68.8 (2JPC = 5.8 Hz)/δ 29.1 (1JPC = 55.4 Hz), and the 13C NMR features of the Zr−enolate CC double bond at δ 165.9 (3JPC = 12.9 Hz) and δ 90.2 (2JPC = 5.0 Hz). The 31P NMR signal is at δ 26.1. We performed a series of related reactions with the monomeric cation 8b, which had a pair of methyl groups attached at the carbon atom adjacent to the ring oxygen atom. Benzaldehyde was added and the mixture then worked up after 1 h at room temperature to eventually give the product 14 as a yellowish crystalline solid in 87% yield (see Scheme 4). Compound 14 was characterized by spectroscopy, by C,Helemental analysis, and by X-ray diffraction. The X-ray crystal structure analysis of compound 14 shows the presence of a seven-membered heterocyclic ring system that was formed by Zr+/P FLP addition to the CO bond of benzaldehyde (see Figure 5). Carbon atom C1 bears the geminal pair of methyl C


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Organometallics

elemental analysis pointed to a 1:1 addition product. The 1,4addition reaction formally resulted in the formation of a metallacycle that contained an endocyclic allenic substructure. Consequently, we have observed a typical allene 13C NMR signal24,25 of the central C carbon atom of that unit at δ 229.0 (2JPC = 1.6 Hz) with signals of the adjacent allenic C(sp2) carbon atoms at δ 145.8 (3JPC = 14.1 Hz, [O]C) and δ 90.5 (1JPC = 75.1 Hz, C[P]). The allenic unit inside the ninemembered metallacycle represents an element of chirality. Therefore, we have observed the NMR signals of pairs of diastereotopic Cp ligands at zirconium (1H δ 6.43, 6.39; 13C δ 114.1, 113.6), as well as methyl groups and methylene hydrogen atoms derived from the former Zr+/P FLP starting material. We had previously shown that organic nitrosyl compounds undergo 1,2-addition reactions with P/B FLPs (e.g., to give compound 17; see Scheme 5).26 The Zr+/P FLP 8b reacts

Scheme 4. FLP Reactions of Complex 8b

Scheme 5. Typical FLP Products and the Reaction Product of 8b with ON-Containing Reagents

Figure 5. Molecular structure of the cation of complex 14 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1 1.928(2), Zr1−O2 2.042(2), O1−C1 1.397(4), O2−C5 1.378(4), C2−P1 1.813(4), C5−P1 1.881(4), O1− Zr1−O2 89.5(1), C2−P1−C5 110.1(2), C1−O1−Zr1 158.3(2), Zr1− O2−C5 134.1(2), Zr1−O2−C5−P1 −84.6(3), C5−P1−C2−C1 − 71.1(4), P1−C2−C1−O1 61.0(4).

analogously. The reaction of 8b with nitrosobenzene proceeds similarly. The reaction was carried out in a 1:1 molar ratio at room temperature (2 h); workup then gave the addition product 20 in 62% yield. The addition product was characterized by C,H-elemental analysis and by spectroscopy. The X-ray crystal structure analysis confirmed the overall atom bonding scheme of complex 20, but its quality was too poor to allow for a further discussion of the structural parameters. The 1 H NMR spectrum in CD2Cl2 solution shows a single sharp Cp singlet of relative intensity 10H at δ 6.28 (13C δ 114.4) and a methyl singlet of 6H relative intensity at δ 1.46 (4JPH = 1.9 Hz) (13C δ 32.5, 3JPC = 10.2 Hz). The methylene group adjacent to phosphorus gives rise to a 1H NMR singlet at δ 3.33 (2H relative intensity, 2JPH = 10.5 Hz) (13C 42.0, 1JPC = 67.5 Hz). Compound 20 features a 31P NMR resonance at δ 45.0 in the phosphonium range. Phosphanes react with NO by oxidation to give phosphinoxides and N2O.27 We had shown that many saturated vicinal P/B FLPs in contrast undergo N,N-addition to cleanly yield the persistent five-membered P/B FLP NO nitroxide radicals (e.g. 18 from 11 and NO).7,28 In this study we investigated which of the two competing reaction pathways might be favored for the Zr+/P pair 8b. The reaction of 8b with NO (1.5 bar) was carried out in dichloromethane at room temperature. The workup procedure gave compound 21, which was isolated in 89% yield. Compound 21 was identified as the phosphane oxidation product by an X-ray crystal structure analysis (see Figure 6). It showed the newly introduced oxygen atom that is

substituents originating from the starting material 8b. Carbon atom C5 has the phenyl group attached; it represents the newly formed chiral carbon center inside the framework of compound 14. We find a pseudotetrahedral dialkoxylzirconocene unit and a phosphonium moiety as part of the ring structure. The salt 14 contains well-separated metallacyclic cations and [B(C6F5)4]− anions in the crystal. In solution at 223 K complex 14 shows the 1H/13C NMR resonances of the pair of diastereotopic Cp ligands at zirconium (δ 6.42, 6.37/δ 114.0, 113.2) and a 31P NMR phosphonium signal at δ 16.5. The geminal methyl groups at the ring parameter are also diastereotopic, due to the chiral carbon inside the ring. They give rise to 1H NMR signals at δ 1.37 and 0.82, respectively (13C δ 33.2, 28.9). The 13C NMR signal of the [O]CHPh[P] methine carbon is at δ 87.0 (1JPC = 46.3 Hz; corresponding 1H NMR signal at δ 6.35). The methylene [P]CH2 group gives rise to 1H NMR features at δ 3.47 and 3.16 and a 13C NMR resonance at δ 43.9 (with 1JPC = 36.5 Hz). Compound 14 shows dynamic NMR spectra, which probably indicates reversible opening of the newly formed P1−C5 linkage (ΔG⧧(223 K) = 12.4 ± 0.3 kcal mol−1) (for further details see the Supporting Information).16a23 The reaction of complex 8b with the conjugated ynone 16 took place by Zr+/P 1,4-addition. We isolated the product 15 as a pale yellow oil in 67% yield (see Scheme 4). The C,HD


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bifunctional Zr+/phosphane systems. We shall explore their potential to serve as reactive FLP systems in small-molecule activation.

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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 Complex 7a. A solution of compound 6a (69 mg, 0.3 mmol) in toluene (1 mL) was added to a solution of Cp2ZrMe2 (75 mg, 0.3 mmol) in toluene (1 mL). After the reaction mixture was stored in the refrigerator (ca. −35 °C) overnight, it was covered with pentane (4 mL). After several days at −35 °C, compound 7a was obtained as a white crystalline solid (116 mg, 83%). Crystals suitable for an X-ray crystal structure analysis were obtained from a saturated solution of compound 7a in toluene at −35 °C. Anal. Calcd for C25H27OPZr: C, 64.48; H, 5.84. Found: C, 64.58; H, 5.64. Preparation of Complex 7b. A solution of compound 6b (258 mg, 1 mmol) in toluene (3 mL) was added dropwise to a solution of Cp2ZrMe2 (251 mg, 1 mmol) in toluene (3 mL). After 3 h at room temperature, the reaction mixture was filtered and the filtrate was dried in vacuo to give complex 7b as a white solid (452 mg, 92%). Crystals suitable for the X-ray crystal structure analysis were obtained from a saturated solution of compound 7b in toluene at −35 °C. Anal. Calcd for C27H31OPZr: C, 65.68; H, 6.33. Found: C, 66.28; H, 6.04. Preparation of Complex 8a. A solution of [Ph3C][B(C6F5)4] (307 mg, 0.33 mmol) in C6H5Br (2 mL) was added to a solution of complex 7a (155 mg, 0.33 mmol) in C6H5Br (2 mL). After standing at room temperature for 1 day, a large amount of crystalline material had formed which was collected and washed with pentane (3 × 2 mL) to finally give complex 8a as a pale yellow solid (257 mg, 69%). Crystals suitable for an X-ray crystal structure analysis were obtained from a C6H5Br solution of compound 8a. Anal. Calcd for C96H48B2F40O2P2Zr2: C, 51.03; H, 2.14. Found: C, 50.32; H, 2.20. Preparation of Complex 8b. A solution of [Ph3C][B(C6F5)4] (184 mg, 0.2 mmol) in C6H5Br (1 mL) was added to a solution of complex 7b (99 mg, 0.2 mmol) in C6H5Br (1 mL). After 1 h at room temperature, the reaction mixture was covered with cyclopentane (4 mL). A beige oil was formed after several days at −35 °C. The oil was separated. Then the oil was dissolved in CH2Cl2 (ca. 1 mL) and covered with cyclopentane (ca. 3 mL) to finally give complex 8b as a pale yellow crystalline solid (208 mg, 90%). Crystals suitable for an Xray crystal structure analysis were obtained from a two-layer procedure at −35 °C using a solution of compound 8b in CH2Cl2 covered with cyclopentane. Anal. Calcd for C50H28BF20OPZr: C, 51.87; H, 2.44. Found: C, 51.18; H, 2.79. Preparation of Complex 13. A solution of trans-chalcone (17 mg, 0.08 mmol) in CH2Cl2 (1 mL) was added to a suspension of complex 8a (90 mg, 0.04 mmol) in CH2Cl2 (1 mL). After the reaction mixture was stirred for 2 h at room temperature, it was covered with cyclopentane (4 mL) to eventually give complex 13 as a pale yellow solid (71 mg, 66% yield). Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure at room temperature using a solution of compound 13 in CH2Cl2 covered with cyclopentane. Anal. Calcd for C63H36BF20O2PZr: C, 56.56; H, 2.71. Found: C, 56.85; H, 3.24. Preparation of Complex 14. Complex 7b (49 mg, 0.1 mmol) and [Ph3C][B(C6F5)4] (92 mg, 0.1 mmol) were mixed in C6H5Br (2 mL). After the mixture stood at room temperature for 1 h, PhCHO (11 mg, 0.1 mmol) was added. After another 1 h, the reaction mixture was layered with cyclopentane (4 mL) to eventually give complex 14 as a pale yellow crystalline solid (110 mg, 87%). Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure at −35 °C using a solution of CH2Cl2 and compound 14 covered with cyclopentane. Anal. Calcd for C57H34BF20O2PZr: C, 54.17; H, 2.71. Found: C, 53.87; H, 3.06. Preparation of Complex 15. Following the procedure described for the preparation of compound 14, the reaction of complex 7b (49 mg, 0.1 mmol), [Ph3C][B(C6F5)4] (92 mg, 0.1 mmol), and the ynone 16 (14 mg, 0.1 mmol) gave compound 15 as a pale yellow oil (87 mg,

Figure 6. Molecular structure of the cation of the oxidation product 21 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1 1.932(4), Zr1−O2 2.123(3), P1−O2 1.529(3), P1−C2 1.810(6), O1−C1 1.423(6), C1−C2 1.550(8), C1−O1−Zr1 148.0(4), O1−Zr1−O2 85.5(2), Zr1−O2− P1 130.2(2), O2−P1−C2 112.2(2), O1−C1−C2−P1 49.5(6), C2− P1−O2−Zr1 10.2(4), P1−O2−Zr1−O1 10.9(3).

bonded to both the phosphorus and the zirconium atom. In solution the metallacycle 21 shows a 1H NMR Cp resonance at δ 6.48 (s, 10H) and a methyl singlet at δ 1.35 (4JPH = 1.6 Hz). Complex 21 features a 31P NMR resonance at δ 52.0. We have then also reacted the Zr+/P FLP system 8b with 0.5 mol equiv of nitrobenzene. The reaction was directly monitored by NMR to give a ca. 1:1 mixture of the known compounds 20 and 21. These were not isolated but identified from the reaction mixture by NMR spectroscopy. Apparently, the nitrobenzene reagent first serves to oxidize the system 8b to the five-membered Zr+/PO phosphane oxidation product 21. The resulting in situ formed nitrosobenzene is then apparently very efficiently scavenged by an additional 1 equiv of the Zr+/P system 8b to form the metallacyclic PhNO addition product 20.

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CONCLUSIONS The reaction of the zirconocene complexes 7 with trityl cation results in a clean methyl anion abstraction from the metal atom. In the case of the system 7a (R = H) this results in the formation of a doubly oxygen bridged dication, a structural motif that is often encountered in zirconocene chemistry. In this case the pendant phosphane is not involved in coordination. Nevertheless, this system shows some typical frustrated Lewis pair behavior. It may be assumed that dissociation of the dimer generates a Zr+/P FLP monomer, probably with equilibrating closed and open forms, that undergoes typical FLP addition reactions to unsaturated systems to yield the respective metallacyclic products. The more bulky system 7b (R = CH3) directly gives a monomeric cation upon methyl anion abstraction by trityl cation, which shows internal phosphane−Zr+ coordination. Despite the presence of this P−Zr+ bond the system 8b shows typical FLP addition reactions to organic carbonyl compounds and to nitrosobenzene; the latter was either added as such or apparently formed in situ by FLP deoxygenation of nitrobenzene and then very efficiently trapped. These reactions indicate some FLP reactivity potential of these readily available E


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Organometallics

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67%). Anal. Calcd for C60H36BF20O2PZr: C, 55.35; H, 2.79. Found: C, 55.48; H, 3.14. Preparation of Complex 20. A solution of PhNO (9 mg, 0.08 mmol) in CH2Cl2 (0.5 mL) was added to a solution of complex 8b (93 mg, 0.08 mmol) in CH2Cl2 (2 mL). After 2 h at room temperature the reaction mixture was covered with cyclopentane (4 mL) to finally give complex 20 as a light gray solid (63 mg, 62%). Anal. Calcd for C56H33BF20NO2PZr: C, 53.18; H, 2.63; N, 1.11. Found: C, 53.21; H, 2.54; N, 1.16. Preparation of Complex 21. A solution of complex 8b (93 mg, 0.08 mmol) in CH2Cl2 (2 mL) was degassed, and NO (1.5 bar) was introduced to the evacuated reaction flask for 10 min. The reaction mixture was then covered with cyclopentane (4 mL) to finally give compound 21 (83 mg, 89% yield) as a colorless solid. Crystals suitable for an X-ray crystal structure analysis were obtained by a two-layer procedure at −35 °C using a solution of compound 21 in CH2Cl2 covered with cyclopentane. Anal. Calcd for C50H28BF20O2PZr: C, 51.16; H, 2.40. Found: C, 51.05; H, 2.40. Reaction of Complex 8b with PhNO2 (NMR Scale). PhNO2 (2.5 mg, 0.02 mmol) was added to a solution of complex 8b (46 mg, 0.04 mmol) in CD2Cl2 (1 mL). After it stood at room temperature for 2 h, the reaction mixture was transferred to an NMR tube and the reaction mixture was characterized directly by NMR experiments. A mixture of complexes 20 and 21 in a 1:1 molar ratio was observed.

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ASSOCIATED CONTENT

* Supporting Information S

Text, tables, figures, and CIF files giving experimental and analytical details for all new compounds and crystallographic data for 7a,b, 8a,b, 13, and 14. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for G.E.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Alexander von Humboldt Stiftung and the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

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DEDICATION Dedicated to the memory of Professor Michael Lappert. REFERENCES

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Frustrated Lewis Pair Behavior of [Cp2ZrOCR2CH2PPh2]+ Cations

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