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Boratabenzene rare-earth metal complexes
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CCR-112117; No. of Pages 12
Coordination Chemistry Reviews xxx (2015) xxx–xxx
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Boratabenzene rare-earth metal complexes Peng Cui, Yaofeng Chen ∗ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China
Contents 1. 2. 3.
4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Mono(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Bis(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Bis(boratabenzene) divalent rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. Bis(boratabenzene) trivalent rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Tris(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 NMR Spectroscopic features of boratabenzene rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
a r t i c l e
i n f o
Article history: Received 29 May 2015 Received in revised form 22 July 2015 Accepted 22 July 2015 Available online xxx Keywords: Rare-earth metals Boron Heterocycles Complex
a b s t r a c t Boratabenzenes are heterocyclic 6-electron aromatic anions that can serve as surrogates of cyclopentadienyl-type ligands in the rare-earth metal chemistry. The synthesis of mono-, bis- and tris(boratabenzene) rare-earth metal complexes, including the divalent and trivalent rare-earth metal derivatives, their halides, amides, and alkyls are summarized in this review. Their structural features, stoichiometric reactivity, and catalytic activity are also presented. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The organometallic chemistry of rare-earth metals (Sc, Y and lanthanide metals) began with the synthesis of their tris-Cp complexes as early as 1954 [1,2], and were considered as curiosities at that time. Rapid development of rare-earth metal complexes containing more reactive M C bonds became possible when their bis-Cp halide precursors became synthetically available [3].
Abbreviations: Cp, cyclopentadienyl and cyclopentadienyl derivatives; Cp*, 1,2,3,4,5-pentamethyl cyclopentadienyl; i Pr, isopropyl; Me, methyl; Et, ethyl; Ph, phenyl; Cy, cyclohexyl; i Bu, isobutyl; THF, tetrahydrofuran; THP, tetrahydropyran; COD, 1,5-cyclooctadiene; NOESY, nuclear overhauser effect spectroscopy; TOF, turn of frequency; PS, polystyrene; PE, polyethylene; PDI, polydispersity index; GPC, gel permeation chromatography; MMAO, modified methylaluminoxanes; MMA, methyl methacrylate. ∗ Corresponding author. Tel.: +86 21 54925149. E-mail address: [email protected] (Y. Chen).
Benefit from the large ionic radii and strong Lewis acidity of rare-earth metal ions, as well as the highly reactive nature of their M C and M H bonds [4,5], these bis-Cp rare-earth metal complexes have exhibited high catalytic activity in a variety of useful transformations involving alkenes, such as polymerization [6,7], hydrogenation, hydrosilylation, hydroboration, hydroamination and hydrophophination [8,9]. Further reducing the number of ancillary cyclopentadienyl ligand ultimately led to the synthesis of mono-Cp rare-earth metal complexes that possess two reactive -bonded ligands [10]. The mono-Cp rare-earth metal dialkyl complexes are highly active and versatile olefin polymerization precatalysts [11,12]. Ancillary ligands other than Cp were developed, among which one of the closest system is the heterocyclic 6-electron ligands derived from boratabenzenes (Scheme 1). The first boratabenzene derivative [CpCoC5 H5 BPh]+ was reported by Herberich and co-workers in 1970 [13]. One year later, Ashe III and co-workers described the synthesis of lithium 1-phenylboratabenzene [14].
http://dx.doi.org/10.1016/j.ccr.2015.07.014 0010-8545/© 2015 Elsevier B.V. All rights reserved.
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Scheme 1. Cp and boratabenzenes.
Their pioneering research opened the fascinating boratabenzene chemistry. Studies on the transition metal and main group metal boratabenzene complexes showed that the boratabenzenes are powerful ligands [15–17], in particular with group 4, 6 and 8 metals, and some of them show excellent activities in the polymerization of olefins [18–23]. Though both are 6-electron donors, boratabenzenes are distinct from Cp-type ligands in that: (1) the electron deficient boron on the ring weakens the donating ability of boratabenzenes, which would generate a more electron deficient metal center in their metal complexes; (2) the Lewis acidic boron center is potentially non-innocent, which may be combined with the Lewis acidity of metal center to facilitate substrate binding and activation, and the B–R functionality may participate in certain reactions; (3) the electronic property of boratabenzenes can be readily tuned by the groups on boron, which might not be straightforward for Cp-type ligands. Combining these prominent features of boratabenzenes with properties of rare-earth metals would provide their complexes versatile coordination chemistry and reactivity. In the present review, the synthesis, structure, stoichiometric reactivity and catalytic activity of mono-, bis- and tris(boratabenzene) rare-earth metal complexes are covered. The contents are arranged in a rough chronological order so that readers can follow the entire development of this emerging field. 2. Mono(boratabenzene) rare-earth metal complexes The first rare-earth metal complex bearing boratabenzene was synthesized by an intramolecular nucleophilic substitution of coordinated borabenzene instead of the traditional salt metathesis reaction. Treatment of ScPh3 (THF)2 with 1 equiv. of borabenzene-phosphine adduct [C5 H5 B ← PMe3 ] afforded the mono(boratabenzene) scandium diphenyl complex (C5 H5 BPh)ScPh2 (THF) (1) in high yield (Scheme 2) [24]. Complex 1 reacted further with 1 equiv. of [C5 H5 B ← PMe3 ] to form bis(boratabenzene) scandium phenyl complex (C5 H5 BPh)2 ScPh(THF) (2). Removal of THF by repeated chlorobenzene condensation/evaporation cycles produced a dimer [(C5 H5 BPh)2 ScPh]2 (22 ). Reactions of [C5 H5 B ← PMe3 ] with Sc(CH2 SiMe3 )3 (THF)2 or Sc(CH2 C(Ph)Me2 )3 (THF) produced similar products as that observed for ScPh3 (THF)2 , however, their thermal instability prevented the isolation. The catalytic activity of 1 was not reported; 2 and 22 are inactive for ethylene polymerization. Pretreatment of 22 with H2 generated an active catalyst, which catalyzed ethylene oligomerization to produce 1-alkenes at moderate activity (7.4 kg 1-alkenes/(mol of Sc) h). Besides the intrinsic thermal instability of rare-earth metal alkyl complexes, the challenges in synthesis of mono(boratabenzene) rare-earth metal complexes are also associated with ligand redistribution, which is well known for the mono-Cp rareearth metal complexes [10]. Sterically demanding Cp ligands are generally required in the synthesis of mono-Cp rare-earth metal dialkyl complexes [12]. For the purpose of synthesis of mono(boratabenzene) rare-earth metal dialkyl complexes, lithium salts of multi-substituted boratabenzenes, Li[3,5-Me2 C5 H3 BR]
(R = NEt2 , Ph), were prepared from a modified procedure reported by Herberich and co-workers [25]. Salt metathesis reactions of Li[3,5-Me2 C5 H3 BR] with LnCl3 (THF)x (Ln = Sc, x = 3; Ln = Lu, x = 0) gave the dichloride complexes, which were subsequently treated with 2 equiv. of LiCH2 SiMe3 to afford four mono(boratabenzene) dialkyl complexes, [(3,5-Me2 C5 H3 BR)Ln(CH2 SiMe3 )2 (THF)] (3: R = NEt2 , Ln = Sc; 4: R = NEt2 , Ln = Lu; 5: R = Ph, Ln = Sc; 6: R = Ph, Ln = Lu), in high yields (Scheme 3) [26]. Solid-state structures of 3 and 4 were determined by X-ray crystallography. Complexes 3 and 4 both exhibit rather long Ln Cboratabenzene distances compared to their mono-Cp analogues, indicating the weaker donor property of boratabenzene. Variable-temperature 1 H NMR studies indicated that the energy barrier for the rotation of aminoboratabenzene in 3 (G‡ ≈ 71 kJ/mol) is higher than that of phenylboratabenzene in 5 (G‡ ≈ 59 kJ/mol), which is consistent with the fact that the aminoboratabenzene is a better electron donor than the phenylsubstituted one due to the strong -interaction between boron and nitrogen. The scandium complexes 3 and 5, upon activating with [Ph3 C][B(C6 F5 )4 ] and Al i Bu3 , act as good catalytic systems for syndiotactic styrene polymerization, the activities are up to 1944 and 2061 kg PS/((mol of Sc) h). Attempts to prepare the yttrium analogues failed as the formed mono(boratabenzene) yttrium dialkyls readily decomposed at room temperature. A series of amidino-boratabenzenes, [C5 H5 BN(i Pr)C(CH3 ) N(i Pr)]− and [Me2 C5 H3 BN(R1 )C(R2 )N(R1 )]− (R1 = i Pr, R2 = Me; R1 = Cy, R2 = Me; R1 = i Pr, R2 = Ph), were developed. This linked ligand system is of interest in light of its structural remembrance to the famed Cp-amido constrained geometry ligand system [C5 R4 SiMe2 NR ]2- , but it is uniquely mono-anionic, as compared to the dianionic Cp-amido ligand [27–29]. Li[C5 H5 BN(i Pr)C(CH3 )N(i Pr)] (LiL1) was prepared by nucleophilic substitution of [C5 H5 B ← PMe3 ] with lithium amidinate (Scheme 4). The potassium salts of bulkier 3,5dimethyl-substituted amidino-boratabenzenes, K[Me2 C5 H3 BN(R1 )C(R2 )N(R1 )] (R1 = i Pr, R2 = Me; R1 = Cy, R2 = Me; R1 = i Pr, R2 = Ph) (KL2–KL4), were synthesized by reacting the lithium amidinate Li[N(R1 )C(R2 )N(R1 )] with 1-chloro-3,5dimethyldihydroborinine, followed by deprotonation with KN(SiMe3 )2 (Scheme 4) [30]. Subsequent reactions of LnCl3 (THF)x (Ln = Sc, x = 3; Ln = Y, x = 0; Ln = Lu, x = 0) with lithium or potassium salts of amidino-boratabenzenes, followed by the treatment with LiCH2 SiMe3 afforded the rare-earth metal dialkyl complexes (7–9 and 12–18) in high yields (Scheme 5). The corresponding zirconium trichloride complexes (10 and 19) and chromium dichloride complexes (11 and 20–22) were also synthesized. X-ray diffraction analysis revealed that the amidino-boratabenzene ligand coordinates to the metal ion both through the boratabenzene ring and one amidinate nitrogen, showing a similar constrained geometry to that observed for [C5 R4 SiMe2 NR ]2− [27–29] and its boratabenzene analogue [4-(SiMe2 N(t Bu))C5 H5 BN(i Pr)2 ]2− [31]. Interestingly, the amidinate fragment is nearly perpendicular to the boratabenzene ring, thus offering a minimum overlap between the orbitals of the amidinate and those of the boratabenzene. Upon activation with [Ph3 C][B(C6 F5 )4 ] and Ali Bu3 , the scandium dialkyl complexes (7 and 12–14) showed high catalytic activity for ethylene polymerization (1.24 × 106 –31.94 × 106 g of PE/((mol of cat) h (mol/L of C2 H4 )). GPC traces displayed bimodal peaks for some polymer samples, indicating there are more than one catalytic species during the reaction. The yttrium and lutetium dialkyl complexes 8–9 and 15–18 were inactive under same conditions. The zirconium trichloride complexes (10 and 19) showed no catalytic activity under various MMAO/Zr ratios and temperatures, but the chromium dichlorides (11 and 20–22) exhibited very high activity (up to 170 × 106 g of PE/((mol of cat) h (mol/L of C2 H4 )) in the presence of MMAO to produce high molecular weight polymers with narrow PDIs (Mw = 7.7–165 × 103 , PDI ∼ 2).
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Scheme 2. Synthesis of complexes 1–2.
Scheme 3. Synthesis of complexes 3–6.
3. Bis(boratabenzene) rare-earth metal complexes 3.1. Bis(boratabenzene) divalent rare-earth metal complexes Metathesis reactions between SmI2 (THF)2 and 2 equiv. of K[C5 H5 BXPh2 ] (X = N, P) in THF afforded black bis(boratabenzene) divalent samarium complexes [C5 H5 BXPh2 ]2 Sm(THF)2 (X = N (23), P (24)) in moderate yields (Scheme 6) [32]. Both complexes were characterized by X-ray diffraction, which revealed the typical bend metallocene-type structures. The average Sm Cboratabenzene bond ˚ and 24 (2.93 A) ˚ are longer than the average lengths in 23 (2.93 A) ˚ a result that from the Sm Ccp distance in Cp*2 Sm(THF)2 (2.86 A), weaker donor property of boratabenzene. Solid-state structure also indicated a C2 -symmetry for 23, but a C1 -symmetry for 24. Strong -donation from the nitrogen atom of NPh2 group to the boron was observed in 23, while there is no such -donation in 24. Complexes 23 and 24 initiated the polymerization of MMA to produce
syndiotactic (mm: mr: rr = 0.4: 11.7: 87.9) and atactic polymers, respectively. The diamagnetic divalent ytterbium complex [C5 H5 BNPh2 ] Yb(THF)2 (25) was synthesized from the reaction of YbI2 (THF)2 with 2 equiv. of K[C5 H5 BNPh2 ] in THF [33]. The crystal structure of 25 is similar to its samarium analogue 23. The redox reactivity of 25 toward ␣-diimines was studied to investigate the intramolecular metal-ligand electron transfer process [34,35]. In toluene, the dark-red 25 was instantly oxidized by 1 equiv. of ␣-diimine PhNC(Me)C(Me)NPh to give a dark-green paramagnetic trivalent ytterbium complex [C5 H5 BNPh2 ]2 Yb[PhNC(Me)C(Me)NPh] (26) [33]. X-ray diffraction studies of 26 showed that the Yb3+ ion is coordinated by the radical anion [PhNC(Me)C(Me)NPh]−• in a -2 fashion, where the radical is not delocalized through the ␣-diimine NCCN moiety but located in one of the C N bond as a radical anion [C N]−• .
Scheme 4. Synthesis of LiL1, KL2–KL4.
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Scheme 5. Amidino-boratabenzene complexes 7–22.
Scheme 6. Synthesis of complexes 23–24.
The one-electron transfer between ytterbium ion and ␣-diimine is reversible which is governed by the nature of solvent (Scheme 7). Dissolving the dark-green complex 26 in donor solvent THF-d8 gradually regenerated the dark-red solution of 25 within 8 h. This solvent-mediated reversible transformation is attributed to the moderate redox potential of Yb2+ /Yb3+ (−1.15 V vs NHE) [36], therefore the interaction of donor solvent THF to ytterbium ion can compete with the electron transfer process between ytterbium ion and ␣-diimine. Accordingly, complex 25 did not react with the same ␣-diimine when THF was used as the solvent. However, the samarium analogue 23 only irreversibly reacted with PhNC(Me)C(Me)NPh due to a higher Sm2+ /Sm3+ transformation potential (−1.55 V vs NHE) [36]. Reaction of solvent-free divalent ytterbium amide Yb[N(SiMe3 )2 ]2 with 2 equiv. of [C5 H5 B ← PMe3 ] in toluene afforded an ansa-heteroborabenzene divalent ytterbium amide [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 (27) in moderate yield (Scheme 8) [37]. Complex 27 has several remarkable features: (1) the ytterbium ion is ligated by both neutral borabenzene and anionic boratabenzene rings; (2) two aromatic rings are linked through covalent and coordination bonds, respectively; (3) despite being a divalent lanthanide complex, the structure is reminiscent
Scheme 7. Solvent mediated reversible redox reaction between 25 and ␣-diimine.
of a trivalent ansa-Cp lanthanide complex. Monitoring the reaction of [C5 H5 B ← PMe3 ] with Yb[N(SiMe3 )2 ]2 in C6 D6 by 1 H and 31 P{1 H} NMR spectroscopy indicated an initial coordination of the borabenzene ring to the ytterbium ion, followed by a slow C H bond cleavage of PMe3 to release free HN(SiMe3 )2 and generate an intermediate A. The intermediate A underwent a nucleophilic aromatic substitution on a second [C5 H5 B ← PMe3 ] to form the final product 27 (Scheme 9). The initial coordination of the borabenzene ring to the metal center also seems to be crucial, since the reaction
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Scheme 8. Synthesis of complex 27.
was retarded in the presence of small amount of THF or by using the THF solvated Yb[N(SiMe3 )2 ]2 (THF)2 as the starting material. No reaction occurred between [C5 H5 B ← PMe3 ] and the bulkier trivalent lanthanide amides Ln[N(SiMe3 )2 ]3 (Ln = Yb, Lu). Due to its unique structural features, complex 27 displayed versatile reactivity (Scheme 10). Insertion of N,N diisopropylcarbodiimide into the Yb N bond of 27 afforded the corresponding divalent ytterbium guanidinate complex 28 [37]. The structure of 28 was confirmed by X-ray diffraction studies, indicating a similar insertion reactivity as that reported for the bis-Cp trivalent lanthanide amides [38,39]. The protonolysis of 27 with cyclopentadiene in benzene provided the first solvent-free bis(boratabenzene) divalent complex [C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]2 Yb (29) [40]. The [(C5 H5 )2 Yb]n [41] was also formed as the by-product resulting from a ligand redistribution of the heteroleptic intermediate [C5 H5 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb. In sharp contrast to the polymeric chain structure formed through the intermolecular Yb Cp interactions in [(C5 H5 )2 Yb]n [41], complex 29 exists as a monomer and features interesting intramolecular 1 Yb C interactions between the pendant neutral borabenzene rings and the ytterbium ion. Using substituted 1-trimethylsilylcyclopentadiene and 1,3-bis(trimethylsilyl)cyclopentadiene suppressed the ligand redistribution, and the solvent-free heteroleptic complexes [C5 H3 R1 R2 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (30: R1 = H, R2 = SiMe3 31: R1 = R2 = SiMe3 ) were synthesized [40]. Unlike the 1 Yb C interaction observed for 29, the crystal structure of 31 indicated an 6 interaction between the pendant neutral borabenzene and the ytterbium ion. Single crystals of 30 bearing the relatively less bulky 1-trimethylsilylcyclopentadienyl ligand were not obtained due to the redistribution to form 29 during recrystallzation. Detailed NMR spectroscopy studies showed that the interactions
between the pendant neutral borabenzene ring and the ytterbium ion for 29–31 are retained in non-donor solvent C6 D6 , but can be easily interrupted by the addition of -donor solvent THF-d8 . Complex 30 exists as two isomers in THF-d8 depending on the orientation of -SiMe3 group to the pendant borabenzene ring (30-syn: 30-anti = 2:1) (Scheme 11), which were identified by a NOESY experiment. Treatment of 27 with 1 equiv. of KC5 Me5 afforded a heterometallic Yb K complex 32 with a polymeric structure, in which the neutral borabenzene was displaced by KC5 Me5 at the ytterbium center (Scheme 10) [42]. While in the reaction of 27 with 1 equiv. of NaOi Pr, displacement of the anionic amido ligand by the isopropoxyl group occurred to produce a heterometallic Yb Na complex 33, which can be described as centrosymmetric dimers of ansa-heteroborabenzene divalent ytterbium alkoxide linked by centrosymmetric dimers of sodium amide. Complex 27 reacted with LiNEt2 to give ytterbium complex 34 and lithium complex 35. X-ray diffraction studies showed that 35 exists as a centrosymmetric square tetramer, in which the four nitrogen atoms of [N(SiMe3 )2 ]− ligands occupy the corners and four [Li boratabenzene Li] units make up the sides. The formation of 35 indicated a nucleophilic attack of neutral borabenzene of 27 by [NEt2 ]− ligand, which led to the cleavage of the P → B coordination bond. The complex 34 has far evaded crystallographic analysis due to its poor solubility in aliphatic solvents and the formation of oil in the presence of THF or pyridine. The 1 H and 31 P{1 H} NMR spectra of 34 in THF-d8 indicated a complicated aggregation with the PMe2 moiety free of coordination, the exact nature of this complex is still elusive. Similarly, the reactions of 27 with potassium benzyls, such as KCH2 C6 H5 , KCH2 C6 H3 -3,5-Me2 , as well as KCH2 C6 H4 -o-NMe2 also led to the cleavage of P → B coordination bond, but all products precluded crystallographic analysis.
Scheme 9. Reaction pathway for the formation of complex 27.
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Scheme 10. Reactivity of complex 27.
3.2. Bis(boratabenzene) trivalent rare-earth metal complexes A series of bis(boratabenzene) trivalent rare-earth metal chloride complexes were synthesized through salt metathesis between lithium boratabenzenes and rare-earth metal trichlorides. The reactions in toluene provided complexes 36–43 (Scheme 12); the structures of these complexes largely depend on the substituents on the boron and the radius size of metal ion [43–45]. The smallest rare-earth metal scandium afforded dimeric complexes [(C5 H5 BMe)2 ScCl]2 (36) and [(3,5-Me2 C5 H3 BNMe2 )2 ScCl]2 (38) in the case of small Me or NMe2 group on boron. Similar dimers of 37 and 39 were obtained with yttrium. With larger substituent on the boron, monomeric chloride complexes 40–43 were obtained for scandium, yttrium and lutetium. Reactions of lithium boratabenzenes with rare-earth metal trichlorides in THF or THP gave the THF or THP solvated “ate” complexes 44–46, in which the salt LiCl cannot be simply removed by recrystallization from toluene [45]. In 46, the lithium ion also coordinates to the amino group on boron, and the YCl2 Li ring is folded by 51.7(3)◦ . Trivalent
rare-earth metal chloride complexes bearing a B,B -bridged ansabis(boratabenzene), 47 and 48, were also synthesized for lutetium, where the rac-stereomer was obtained exclusively (Scheme 12) [45]. Reactions of bis(boratabenzene) yttrium chloride complexes (37, 49 and 50) with 2 equiv. of KN(SiMe3 )2 in toluene afforded bis(boratabenzene) yttrium amides 51–53 (Scheme 13) [46].
Scheme 11. Two isomers of complex 30 in THF.
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Scheme 12. Bis(boratabenzene) trivalent rare-earth metal chloride complexes 36–48.
Complex 53 was characterized by X-ray diffraction, which features a -agostic interaction between the electron deficient yttrium center and the Si C bond that led to a nonequivalent Si N Y angles (107.2(2)◦ and 126.8(3)◦ ). The nitrogen atom of the amido ligand displays a trigonal planar geometry ( NY = 360.0◦ ), indicating a donation of the lone electron pair on nitrogen to the highly electron-deficient metal center of the boratabenzene complex. Rare-earth metal amide complexes bearing Cp-type ligands are usually unreactive toward excess of KN(SiMe3 )2 , however, complexes 51–53 can react with KN(SiMe3 )2 that led to a displacement of boratabenzene by amido ligand to generate Y[N(SiMe3 )2 ]3 and
potassium boratabenzenes. Monitoring the reactions of the chloride complex 49 with 2, 4 and 6 equiv. of KN(SiMe3 )2 in C6 D6 indicated that the amide 52 was formed initially, which was slowly transformed to K[C5 H5 BNEt2 ] and Y[N(SiMe3 )2 ]3 . The displacement of boratabenzene is attributed to the weaker donor property of boratabenzene in comparison with the Cp-type ligands. The rate of displacement of boratabenzene is greatly influenced by the substitutent on boron, which decreases in the order of Me > NPh2 > NEt2 . Therefore, the -ligand displacement is slower in the more electron-donating aminoboratabenzene. All these amide complexes 51–53 are able to catalyze the
Scheme 13. Synthesis of bis(boratabenzene) yttrium amide complexes 51–53.
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Scheme 14. Synthesis of bis(boratabenzene) rare-earth metal alky complexes 59–66.
Table 1 Selected 1 H and 13 C{1 H} signal for Ln-C˛ H˛ of yttrium and lutetium alkyl complexes. Complexes
Solvent
ı Ln-C␣ H˛ (ppm)
ı Ln-C˛ H␣ (ppm)
Ref
(C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BCH3 )2 YCH(SiMe3 )2 (61) (C5 H4 Me)2 YCH(SiMe3 )2 (C5 Me5 )2 YCH(SiMe3 )2 [Et2 Si(C5 H4 )(C5 Me4 )]YCH(SiMe3 )2 (R,S)-Me2 Si(C5 Me4 )[(+)-neomenthyl-C5 H3 ]YCH(SiMe3 )2 (C5 Me5 )(OAr)YCH(SiMe3 )2 (2,4,7-Me3 C9 H4 )2 YCH(SiMe3 )2 (C9 Me7 )2 YCH(SiMe3 )2 Y[CH(SiMe3 )2 ]3 (C5 H5 BNEt2 )2 LuCH(SiMe3 )2 (64) (C5 H5 BNPh2 )2 LuCH(SiMe3 )2 (65) (C5 H5 BCH3 )2 LuCH(SiMe3 )2 (66) (C5 Me5 )2 LuCH(SiMe3 )2 [Me2 Si(C5 Me4 )2 ]LuCH(SiMe3 )2 [Me2 Si(C5 H4 )(C5 Me4 )]LuCH(SiMe3 )2 [Et2 Si(C5 H4 )(C5 Me4 )]LuCH(SiMe3 )2 (R,S)-Me2 Si(C5 Me4 )[(+)-neomenthyl-C5 H3 ]LuCH(SiMe3 )2 [Me2 Si(C5 Me4 )(tBuN)]LuCH(SiMe3 )2
C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 C6 D6 Tol-d8 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 C6 D6 C6 D6 C6 D6 C6 D6
0.79 0.68 0.95 0.30 −0.10 −0.59 −0.55, −0.56 −0.39 0.16 −0.37 −0.62 0.92 0.74 0.99 −0.02 −0.411 −0.50 −0.52 0.06, −0.48 −1.14
33.88 39.06 38.7 27.39 25.19 29.3 – 32.0 26.03 27.5 50.0 34.1 38.6 38.6 – – 29.1 – – 44.7
[47] [47] [47] [47] [48] [49] [50] [51] [52] [53] [54] [47] [47] [47] [55] [56] [49] [49] [50] [57]
intramolecular hydroamination of 2,2-dimethyl-1-aminopent-4ene to give 2,4,4-trimethylpyrrolidine. Bis(boratabenzene) rare-earth metal alkyl complexes 59–66 were synthesized by metathesis reactions between the chloride complexes and LiCH(SiMe3 )2 in toluene (Scheme 14) [47]. Crystal structures of 60, 63 and 65 showed unsymmetrical coordination of CH(SiMe3 )2 caused by the -agostic interaction between the electron-deficient metal center and the Si C bond, which is even more profound than that in the amide complex 53 due to the lack of -donation from the CH(SiMe3 )2 group. 1 H and 13 C{1 H} NMR spectroscopy of 59–61 and 64–66 revealed downfield shifts of C˛ H˛ signals, compared to the alkyl ( CH(SiMe3 )2 ) complexes with Cp-type ligands (Table 1). For example, the Y C˛ H˛ of complexes 59–61 were observed at ı = 0.79, 0.68, 0.95 ppm and ı = 33.9, 39.1, 38.7 ppm in the 1 H and 13 C{1 H}NMR spectra, respectively, both are downfield shifted compared to those of bis-Cp complexes (−0.59 to 0.30 ppm and 27.5 to 29.3 ppm for 1 H and 31 C{1 H} NMR signals, respectively). The catalyzed dehydrocoupling of amine boranes has attracted great interest recently, due to the potential usage of amine boranes as H2 storage materials in the future clean energy application. Bis(boratabenzene) rare-earth metal alkyl complexes 59, 61, 64 and 66 are active for the dehydrocoupling of Me2 NH·BH3 (Scheme 15) [58]. The yttrium complex 61 with 1-methyl boratabenzene showed very high activity: with 0.5 mol% of 61, 99% of conversion
Scheme 15. Dehydrocoupling of Me2 NH·BH3 .
was achieved within 12 min (98% to the cyclic dimer [Me2 N–BH2 ]2 , the reminder to Me2 N = BH2 ) with a TOF up to 1000 h−1 , which represented the most active early-transition metal catalyst for the dehydrocoupling of Me2 NH·BH3 . The activity of lutetium complex 66 (TOF of 208 h−1 ) is lower than that of 61, however, both of them are much more active than complexes 59 (TOF of 9.4 h−1 ) and 64 (TOF of 11 h−1 ) which contain 1-diethylamino boratabenzene. The less electron-donating 1-methyl boratabenzene provides a more electron-deficient metal center, which is probably the key factor that facilitates the reaction. On the other hand, the hydridic Hs on boron in Me2 NH·BH3 or the reaction intermediates may also interact with the electron-deficient boron atom of 1methyl boratabenzene to accelerate the reaction. Other rare-earth metal alkyl complexes, Ln(CH2 SiMe3 )3 (THF)2 (Ln = Sc, Y, Lu; TOF of 1.8–7.7 h−1 ), (MeC5 H4 )2 YCH(SiMe3 )2 (TOF of 74 h−1 ) and [1,3(SiMe3 )2 C9 H5 ]Y(CH2 SiMe3 )2 (THF) (TOF of 2.3 h−1 ), are also active for the dehydrocoupling of Me2 NH·BH3 , however, their activity are much lower than that of 61 and 66 [58].
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Scheme 16. Hydroboration reactivity of complex 67.
Of the various boratabenzene frameworks, 1-H-boratabenzene is special due to the presence of a reactive B H bond, as compared to the analogous C H bond on the ubiquitous Cp ligand. 1-H-boratabenzene yttrium chloride complex [(C5 H5 BH)2 YCl]2 (67) is able to undergo a series of hydroboration reactions with alkenes, alkynes, imines and carbodiimides that allow the rapid entry to functionalized boratabenzene yttrium chloride complexes (Scheme 16) [59]. The hydroboration of 1-hexene with 67 afforded the anti-Markovnikov product 68 in high yield and selectivity. The allyl propyl ether that contains an ether functional group also underwent anti-Markovnikov hydroboration with 67 to form complex 69 in which the coordination of ether functionality is labile as indicated by Variable-temperature 1 H NMR spectroscopy. When 0.9 equiv. of diallyl ether was used, double B H bond additions to the same molecule were achieved and gave an ansa-bis(boratabenzene) yttrium chloride 70. In the presence of a large excess of diallyl ether (10 equiv.), complex 71 containing mono-addition of two diallyl ethers was obtained in high yield. In both cases, the formation of a mono-addition intermediate B was observed by 1 H NMR spectroscopy. The anti-Markovnikov hydroboration selectivity for 68–71 was further confirmed by the deuterium-labeling experiment on the reaction of [(C5 H5 BD)2 YCl]2 (67-D2 ) with allyl propyl ether. The hydroboration of 3-hexyne with 67 readily occured at room temperature to afford the alkenyl-substituted
boratabenzene complex 72. Benzylidene-n-propylamine reacted with 67 at 75 ◦ C to give the amino-substituted boratabenzene complex 73. The reaction with N,N -diisopropylcarbodiimide also occurred at room temperature, but only afforded the monoaddition product 74 in high yield, further addition did not occur due to the steric bulk of the [C5 H5 BN(i Pr)CHN(i Pr)]− ligand. 1-H-borabenzene lutetium chloride complex [(C5 H5 BH)2 LuCl]2 (75) also underwent hydroboration with 1-hexene and N,N diisopropylcarbodiimide to form [C5 H5 B(CH2 )5 CH3 ]2 LuCl (76) and [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]LuCl (77) in high yields, respectively. In comparison, 1-H-boratabenzene zirconium complex [C5 H5 BH]2 ZrCl2 (78) did not react with 1-hexene or 3-hexyne, only reacted with benzylidene-n-propylamine to afford the hydroboration product [C5 H5 BN(n Pr)CH2 Ph]2 ZrCl2 (79) [59]. The reaction with N,N -diisopropylcarbodiimide gave an unidentified mixture. Rhodium complex [C5 H5 BH]Rh(PPh3 )2 (80) only slowly reacted with N,N -diisopropylcarbodiimide at 75 ◦ C for 2 weeks to form the hydroboration product [C5 H5 BN(i Pr)CHN(i Pr)]Rh(PPh3 )2 (81), and rhodium complex [C5 H5 BH]Rh(COD) (82) is inert toward all above mentioned substrates. Therefore, the B H addition reactivity of the 1-H-boratabenzene transition metal complexes decreases in the following order: 67 (Y) > 78 (Zr) > 80 (Rh) > 82 (Rh), showing that the hydroboration at the 1-H-boratabenzene ligand is sensitive to the electronic property of the metal
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lutetium (75) complexes [60]. While 1-hexene and 3-hexyne underwent hydroboration with all three 1-H-boratabenzenes of 85 to afford [C5 H5 B(CH2 )5 CH3 ]3 La·LiCl(THF) (88) and [C5 H5 BC(C2 H5 ) = CH(C2 H5 )]3 La·LiCl(THF) (89) in good yields, the reaction of 85 with N,N -diisopropylcarbodiimide only provided the mono-hydroboration product [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]2 La (90) due to the steric bulk of the [C5 H5 BN(i Pr)CHN(i Pr)]− ligand. The remaining two 1-H-boratabenzenes of 90 can be functionalized further with 3-hexyne or phenyl acetylene (Scheme 18). The 3-hexyne slowly reacted with 90 to form the hydroboration product 91, while for the phenyl acetylene, the dehydrogenative coupling product 92 was obtained in high yield instead of forming the hydroboration product, presumably due to the more acidity of the terminal acetylene. Effort to synthesize tris(boratabenzene) complex [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BC CPh][C5 H5 BH]La (93) that contains three different boratabenzenes by controlling the molar ratio of phenyl acetylene to 90 only led to the formation of mixture of 90, 92 and 93. 5. NMR Spectroscopic features of boratabenzene rare-earth metal complexes
Scheme 17. Ligand redistribution between boratabenzene yttrium complexes and rhodium chloride complexes.
complex. It was also found that the boratabenzene yttrium chloride complexes, 67, 68 and 72, are capable of redistributing ligand with Rh(PPh3 )3 Cl or [Rh(COD)Cl]2 to give rhodium complexes 80 and 82–84 (Scheme 17) [59]. Therefore, although the 1-alkyl- (or 1-alkenyl-) functionalized boratabenzene rhodium complexes could not be directly synthesized via the hydroboration of 1-H-boratabenzene rhodium complexes, the highly efficient hydroboration with 1-H-boratabenzene yttrium complex followed by the ligand redistribution provides a synthetic pathway to these complexes. 4. Tris(boratabenzene) rare-earth metal complexes Salt metathesis reactions of LaCl3 with 3 equiv. of Li[C5 H5 BH] or Li[C5 H5 BNEt2 ] afforded the corresponding tris(boratabenzene) lanthanum complexes [C5 H5 BH]3 La·LiCl (85) and [C5 H5 BNEt2 ]3 La·LiCl(THF) (86) in high yields [60]. Attempts to obtain the alkali-metal free tris(boratabenzene) complexes by sublimation of complexes 85 and 86 under vacuum did not succeed. Recrystallization of 85 in a THF/toluene mixture gave a THF solvated discrete cation–anion pair [(C5 H5 BH)3 La(Cl)La(C5 H5 BH)3 ][Li(THF)4 ] (87), where the two (C5 H5 BH)3 La units are bridged by a chloride to form the [(C5 H5 BH)3 La(Cl)La(C5 H5 BH)3 ]− anion and the Li+ ion is coordinated by four THF molecules. Complex 85 showed hydroboration reactivity as that observed for the bis(1-H-boratabenzene) yttrium (67) and
As shown in Table 2, for the diamagnetic rare-earth metal ions Sc3+ , Y3+ , La3+ , Yb2+ and Lu3+ , 11 B NMR resonances of their boratabenzene complexes are observed in the range of 29–46 ppm, which largely depend on the substituent on boron while the metal ion itself has little affect on the value. In all cases, the 1-methyl boratabenzenes show resonances at the lowest field (42–45 ppm, in complexes 37, 51, 61, 66). Complexes 70 and 72 containing alkyl and alkenyl substituted boratabenzenes also show relatively low field shifts at ∼40 ppm. In the divalent ytterbium complexes (27–32) containing ansa-heteroborabenzene ligand, 11 B NMR resonances for the anionic boratabenzene and neutral borabenzene were observed at 31–37 and 23–29 ppm, respectively. The 1 JPB coupling constants for the neutral borabenzenes are ∼100 Hz. Although the direct observation of rare-earth metal ions by solution NMR spectroscopy method was hampered by the paramagnetic nature of majority of rare-earth metal complexes (except those of Sc3+ , Y3+ , La3+ , Yb2+ , and Lu3+ ) as well as the large quadruple moments of these elements, the diamagnetic properties of Y3+ and Yb2+ allow 89 Y (I = 1/2, 100% abundance) and 171 Yb nuclei (I = 1/2, 14.27% abundance) to be amenable to an NMR study. Table 3 shows the 89 Y NMR spectroscopy data of bis(boratabenzene) yttrium alkyl complexes 59, 60 and 61 at 176.1, 170.0 and 162.2 ppm, respectively, which are significantly shifted to lower field compared to that observed for (C5 H4 Me)2 YCH(SiMe3 )2 (44.0 ppm) [47]. The movement of 89 Y NMR resonances to the lower field indicates a decrease of electronegativity and -donating ability of the ligands. The group contributions of various ligands to the 89 Y nuclear shielding have been estimated before as C5 Me5 (ca. −100 ppm) > alkoxides (ca. 15 ppm) > aryloxides (ca. 56 ppm) > amides (ca. 190 ppm) > alkyls (ca. 300 ppm) [51]. In complexes 59, 60 and 61, the group contributions from boratabenzenes [C5 H5 BNEt2 ]- , [C5 H5 BNPh2 ]− and [C5 H5 BMe]− to the 89 Y nuclear shielding are −61, −64 and −68, respectively. The lower values for the boratabenzenes compared to that of −128 for the [C5 H4 Me]− are consistent with the weaker -donor ability of boratabenzenes than Cp ligands. 6. Outlook The boratabenzene rare-earth metal chemistry is in the middle stage. A system that contains mono-, bis- and tris(boratabenzene) rare-earth metal complexes has been established, which is paralleled with the system of Cp ligands, but also leaves a wide space for
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Scheme 18. Hydroboration and dehydrogen-coupling reactions of complex 90.
the further development in the near future. The weaker -donor property of boratabenzenes have been exhibited by structures and reactivity of the complexes, the 1-H-boratabenzene complexes showed hydroboration reactivity with unsaturated molecules, but it will be more interesting to investigate the role of Lewis
acidity of boron, and how it might cooperate with the Lewis acidic rare-earth metal center in the activation of substrates. As was indicated in the dehydrocoupling of Me2 NH·BH3 , some intrinsic property of the boratabenzene rare-earth metal complexes needs to be discovered. Another important aspect of boratabenzenes is
Table 2 Selected 11 B NMR spectroscopy data of boratabenzene rare-earth metal complexesa . Complexes
Solvent
ı (ppm)
Ref.
(C5 H5 BPh)ScPh2 (THF) (1) [3,5-Me2 C5 H3 BNEt2 ]Sc(CH2 SiMe3 )2 (THF) (3) [3,5-Me2 C5 H3 BNEt2 ]Lu(CH2 SiMe3 )2 (THF) (4) [3,5-Me2 C5 H3 BPh]Sc(CH2 SiMe3 )2 (THF) (5) [3,5-Me2 C5 H3 BPh]Lu(CH2 SiMe3 )2 (THF) (6) [C5 H5 BN(i Pr)C(CH3 )N(i Pr)]Sc(CH2 SiMe3 )2 (7) [C5 H5 BN(i Pr)C(CH3 )N(i Pr)]Y(CH2 SiMe3 )2 (8) [Me2 C5 H3 BN(i Pr)C(CH3 )N(i Pr)]Sc(CH2 SiMe3 )2 (12) [C5 H5 BNPh2 ]2 Yb(THF)2 (25) [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 (27) [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(i Pr)CN(i Pr)N(SiMe3 )2 (28) [C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]2 Yb (29) [C5 H4 (SiMe3 )][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (30) [C5 H3 (SiMe3 )2 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (31) {[C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 K(C5 Me5 )}n (32) {[C5 H5 BCH2 (CH3 )2 P]YbNEt2 }n (34) [(C5 H5 BMe)2 YCl]2 (37) (Me2 C5 H3 BNi Pr2 )2 ScCl (40) [Me2 C5 H3 BN(SiMe3 )2 ]2 LuCl (43) [(C5 H5 BNPh2 )2 YCl]2 (50) (C5 H5 BNEt2 )2 YN(SiMe3 )2 (52) (C5 H5 BNPh2 )2 YN(SiMe3 )2 (53) (C5 H5 BMe)2 YN(SiMe3 )2 (51) (C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BMe)2 YCH(SiMe3 )2 (61) (C5 H5 BMe)2 LuCH(SiMe3 )2 (66) [(C5 H5 BH)2 YCl]2 (67) [C5 H5 B(CH2 )3 O(CH2 )3 BC5 H5 ]YCl (70) [C5 H5 BC(C2 H5 ) = CH(C2 H5 )]2 YCl (72) [C5 H5 BN(n Pr)CH2 Ph]2 YCl (73) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]YCl (74) [C5 H5 BH]3 La•LiCl (85) [C5 H5 BNEt2 ]3 La•LiCl (THF) (86) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]2 La (90) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BCCPh]2 La (92)
C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 THF-d8 C6 D6 C6 D6 THF-d8 THF-d8 C6 D6 THF-d8 THF-d8 C6 D6 Tol-d8 C6 D6 CDCl3 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 CDCl3 C6 D6 C6 D6 C6 D6 C6 D6 THF-d8 C6 D6 C6 D6 C6 D6
37.0 32.7 32.9 36.7 37.0 35.9 36.1 36.2 33.8 33.7 (CP); 25.8 (BP) 37.0 (CP); 29.1 (BP) 34.0 (CP); 23.9 (BP) 33.7 (CP); 23.9 (BP) 34.2 (CP); 25.5 (BP) 31.7 (CP); 24.0 (BP) 31.4 43.2 34.7 38.9 34.4 32.4 33.3 45.4 33.1 34.1 42.6 42.8 37.3 40.1 39.3 33.6 36.9, 34.0 32.3 32.1 36.9, 34.4 36.1, 29.0
[24] [26] [26] [26] [26] [30] [30] [30] [33] [37] [37] [40] [40] [40] [42] [42] [44] [45] [45] [46] [46] [46] [46] [47] [47] [47] [47] [59] [59] [59] [59] [59] [60] [60] [60] [60]
a 11
B NMR spectra were measured at 298 K relative to BF3 ·OEt2 .
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Table 3 Selected 89 Y NMR spectroscopy data of yttrium complexesa . Complexes
Solvent
ı (ppm)
Ref.
(C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BMe)2 YCH(SiMe3 )2 (61) (C5 H4 Me)2 YCH(SiMe3 )2 Y[CH(SiMe3 )2 ]3 (C5 H4 Me)3 Y(THF)
C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 THF-d8
176.1 170.0 162.2 44.0 895 −371
[47] [47] [47] [47] [51] [61]
a 89
Y NMR spectra were measured at 298 K relative to 3 M YCl3 in D2 O.
their readily tunable electronic property through the substituents on boron, systematic studies of this property would require a comprehensive DFT calculation, but eventually should be demonstrated in their reactivity such as the rate and selectivity. Moreover, the electronic property of boratabenzene ligand also affects the redox property of the divalent rare-earth metal complexes, which might be a suitable platform to investigate the electronic effect of ligand on the redox power of the metal complex. Finally, the whole system of boratabenzene rare-earth metal chemistry is not complete yet, the fascinating hydride complexes including mono-, dihydride have not been achieved, and there will be more variants if cationic ones are included. Clearly, more boratabenzene ligands also can be developed in the future studies. Acknowledgements The authors wish to thank the State Key Basic Research & Development Program, the Chinese National Ministry of Science and Technology (Grant No. 2011CB808705), the National Natural Science Foundation of China (Grant Nos. 21272256, 21132002, 21325210 and 21421091), and Chinese Academy of Sciences for financial support. The authors also wish to thank all of their coworkers whose names appear in the references for their dedicated effort to this chemistry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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Contents lists available at ScienceDirect
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Boratabenzene rare-earth metal complexes Peng Cui, Yaofeng Chen ∗ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China
Contents 1. 2. 3.
4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Mono(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Bis(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Bis(boratabenzene) divalent rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. Bis(boratabenzene) trivalent rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Tris(boratabenzene) rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 NMR Spectroscopic features of boratabenzene rare-earth metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
a r t i c l e
i n f o
Article history: Received 29 May 2015 Received in revised form 22 July 2015 Accepted 22 July 2015 Available online xxx Keywords: Rare-earth metals Boron Heterocycles Complex
a b s t r a c t Boratabenzenes are heterocyclic 6-electron aromatic anions that can serve as surrogates of cyclopentadienyl-type ligands in the rare-earth metal chemistry. The synthesis of mono-, bis- and tris(boratabenzene) rare-earth metal complexes, including the divalent and trivalent rare-earth metal derivatives, their halides, amides, and alkyls are summarized in this review. Their structural features, stoichiometric reactivity, and catalytic activity are also presented. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The organometallic chemistry of rare-earth metals (Sc, Y and lanthanide metals) began with the synthesis of their tris-Cp complexes as early as 1954 [1,2], and were considered as curiosities at that time. Rapid development of rare-earth metal complexes containing more reactive M C bonds became possible when their bis-Cp halide precursors became synthetically available [3].
Abbreviations: Cp, cyclopentadienyl and cyclopentadienyl derivatives; Cp*, 1,2,3,4,5-pentamethyl cyclopentadienyl; i Pr, isopropyl; Me, methyl; Et, ethyl; Ph, phenyl; Cy, cyclohexyl; i Bu, isobutyl; THF, tetrahydrofuran; THP, tetrahydropyran; COD, 1,5-cyclooctadiene; NOESY, nuclear overhauser effect spectroscopy; TOF, turn of frequency; PS, polystyrene; PE, polyethylene; PDI, polydispersity index; GPC, gel permeation chromatography; MMAO, modified methylaluminoxanes; MMA, methyl methacrylate. ∗ Corresponding author. Tel.: +86 21 54925149. E-mail address: [email protected] (Y. Chen).
Benefit from the large ionic radii and strong Lewis acidity of rare-earth metal ions, as well as the highly reactive nature of their M C and M H bonds [4,5], these bis-Cp rare-earth metal complexes have exhibited high catalytic activity in a variety of useful transformations involving alkenes, such as polymerization [6,7], hydrogenation, hydrosilylation, hydroboration, hydroamination and hydrophophination [8,9]. Further reducing the number of ancillary cyclopentadienyl ligand ultimately led to the synthesis of mono-Cp rare-earth metal complexes that possess two reactive -bonded ligands [10]. The mono-Cp rare-earth metal dialkyl complexes are highly active and versatile olefin polymerization precatalysts [11,12]. Ancillary ligands other than Cp were developed, among which one of the closest system is the heterocyclic 6-electron ligands derived from boratabenzenes (Scheme 1). The first boratabenzene derivative [CpCoC5 H5 BPh]+ was reported by Herberich and co-workers in 1970 [13]. One year later, Ashe III and co-workers described the synthesis of lithium 1-phenylboratabenzene [14].
http://dx.doi.org/10.1016/j.ccr.2015.07.014 0010-8545/© 2015 Elsevier B.V. All rights reserved.
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Scheme 1. Cp and boratabenzenes.
Their pioneering research opened the fascinating boratabenzene chemistry. Studies on the transition metal and main group metal boratabenzene complexes showed that the boratabenzenes are powerful ligands [15–17], in particular with group 4, 6 and 8 metals, and some of them show excellent activities in the polymerization of olefins [18–23]. Though both are 6-electron donors, boratabenzenes are distinct from Cp-type ligands in that: (1) the electron deficient boron on the ring weakens the donating ability of boratabenzenes, which would generate a more electron deficient metal center in their metal complexes; (2) the Lewis acidic boron center is potentially non-innocent, which may be combined with the Lewis acidity of metal center to facilitate substrate binding and activation, and the B–R functionality may participate in certain reactions; (3) the electronic property of boratabenzenes can be readily tuned by the groups on boron, which might not be straightforward for Cp-type ligands. Combining these prominent features of boratabenzenes with properties of rare-earth metals would provide their complexes versatile coordination chemistry and reactivity. In the present review, the synthesis, structure, stoichiometric reactivity and catalytic activity of mono-, bis- and tris(boratabenzene) rare-earth metal complexes are covered. The contents are arranged in a rough chronological order so that readers can follow the entire development of this emerging field. 2. Mono(boratabenzene) rare-earth metal complexes The first rare-earth metal complex bearing boratabenzene was synthesized by an intramolecular nucleophilic substitution of coordinated borabenzene instead of the traditional salt metathesis reaction. Treatment of ScPh3 (THF)2 with 1 equiv. of borabenzene-phosphine adduct [C5 H5 B ← PMe3 ] afforded the mono(boratabenzene) scandium diphenyl complex (C5 H5 BPh)ScPh2 (THF) (1) in high yield (Scheme 2) [24]. Complex 1 reacted further with 1 equiv. of [C5 H5 B ← PMe3 ] to form bis(boratabenzene) scandium phenyl complex (C5 H5 BPh)2 ScPh(THF) (2). Removal of THF by repeated chlorobenzene condensation/evaporation cycles produced a dimer [(C5 H5 BPh)2 ScPh]2 (22 ). Reactions of [C5 H5 B ← PMe3 ] with Sc(CH2 SiMe3 )3 (THF)2 or Sc(CH2 C(Ph)Me2 )3 (THF) produced similar products as that observed for ScPh3 (THF)2 , however, their thermal instability prevented the isolation. The catalytic activity of 1 was not reported; 2 and 22 are inactive for ethylene polymerization. Pretreatment of 22 with H2 generated an active catalyst, which catalyzed ethylene oligomerization to produce 1-alkenes at moderate activity (7.4 kg 1-alkenes/(mol of Sc) h). Besides the intrinsic thermal instability of rare-earth metal alkyl complexes, the challenges in synthesis of mono(boratabenzene) rare-earth metal complexes are also associated with ligand redistribution, which is well known for the mono-Cp rareearth metal complexes [10]. Sterically demanding Cp ligands are generally required in the synthesis of mono-Cp rare-earth metal dialkyl complexes [12]. For the purpose of synthesis of mono(boratabenzene) rare-earth metal dialkyl complexes, lithium salts of multi-substituted boratabenzenes, Li[3,5-Me2 C5 H3 BR]
(R = NEt2 , Ph), were prepared from a modified procedure reported by Herberich and co-workers [25]. Salt metathesis reactions of Li[3,5-Me2 C5 H3 BR] with LnCl3 (THF)x (Ln = Sc, x = 3; Ln = Lu, x = 0) gave the dichloride complexes, which were subsequently treated with 2 equiv. of LiCH2 SiMe3 to afford four mono(boratabenzene) dialkyl complexes, [(3,5-Me2 C5 H3 BR)Ln(CH2 SiMe3 )2 (THF)] (3: R = NEt2 , Ln = Sc; 4: R = NEt2 , Ln = Lu; 5: R = Ph, Ln = Sc; 6: R = Ph, Ln = Lu), in high yields (Scheme 3) [26]. Solid-state structures of 3 and 4 were determined by X-ray crystallography. Complexes 3 and 4 both exhibit rather long Ln Cboratabenzene distances compared to their mono-Cp analogues, indicating the weaker donor property of boratabenzene. Variable-temperature 1 H NMR studies indicated that the energy barrier for the rotation of aminoboratabenzene in 3 (G‡ ≈ 71 kJ/mol) is higher than that of phenylboratabenzene in 5 (G‡ ≈ 59 kJ/mol), which is consistent with the fact that the aminoboratabenzene is a better electron donor than the phenylsubstituted one due to the strong -interaction between boron and nitrogen. The scandium complexes 3 and 5, upon activating with [Ph3 C][B(C6 F5 )4 ] and Al i Bu3 , act as good catalytic systems for syndiotactic styrene polymerization, the activities are up to 1944 and 2061 kg PS/((mol of Sc) h). Attempts to prepare the yttrium analogues failed as the formed mono(boratabenzene) yttrium dialkyls readily decomposed at room temperature. A series of amidino-boratabenzenes, [C5 H5 BN(i Pr)C(CH3 ) N(i Pr)]− and [Me2 C5 H3 BN(R1 )C(R2 )N(R1 )]− (R1 = i Pr, R2 = Me; R1 = Cy, R2 = Me; R1 = i Pr, R2 = Ph), were developed. This linked ligand system is of interest in light of its structural remembrance to the famed Cp-amido constrained geometry ligand system [C5 R4 SiMe2 NR ]2- , but it is uniquely mono-anionic, as compared to the dianionic Cp-amido ligand [27–29]. Li[C5 H5 BN(i Pr)C(CH3 )N(i Pr)] (LiL1) was prepared by nucleophilic substitution of [C5 H5 B ← PMe3 ] with lithium amidinate (Scheme 4). The potassium salts of bulkier 3,5dimethyl-substituted amidino-boratabenzenes, K[Me2 C5 H3 BN(R1 )C(R2 )N(R1 )] (R1 = i Pr, R2 = Me; R1 = Cy, R2 = Me; R1 = i Pr, R2 = Ph) (KL2–KL4), were synthesized by reacting the lithium amidinate Li[N(R1 )C(R2 )N(R1 )] with 1-chloro-3,5dimethyldihydroborinine, followed by deprotonation with KN(SiMe3 )2 (Scheme 4) [30]. Subsequent reactions of LnCl3 (THF)x (Ln = Sc, x = 3; Ln = Y, x = 0; Ln = Lu, x = 0) with lithium or potassium salts of amidino-boratabenzenes, followed by the treatment with LiCH2 SiMe3 afforded the rare-earth metal dialkyl complexes (7–9 and 12–18) in high yields (Scheme 5). The corresponding zirconium trichloride complexes (10 and 19) and chromium dichloride complexes (11 and 20–22) were also synthesized. X-ray diffraction analysis revealed that the amidino-boratabenzene ligand coordinates to the metal ion both through the boratabenzene ring and one amidinate nitrogen, showing a similar constrained geometry to that observed for [C5 R4 SiMe2 NR ]2− [27–29] and its boratabenzene analogue [4-(SiMe2 N(t Bu))C5 H5 BN(i Pr)2 ]2− [31]. Interestingly, the amidinate fragment is nearly perpendicular to the boratabenzene ring, thus offering a minimum overlap between the orbitals of the amidinate and those of the boratabenzene. Upon activation with [Ph3 C][B(C6 F5 )4 ] and Ali Bu3 , the scandium dialkyl complexes (7 and 12–14) showed high catalytic activity for ethylene polymerization (1.24 × 106 –31.94 × 106 g of PE/((mol of cat) h (mol/L of C2 H4 )). GPC traces displayed bimodal peaks for some polymer samples, indicating there are more than one catalytic species during the reaction. The yttrium and lutetium dialkyl complexes 8–9 and 15–18 were inactive under same conditions. The zirconium trichloride complexes (10 and 19) showed no catalytic activity under various MMAO/Zr ratios and temperatures, but the chromium dichlorides (11 and 20–22) exhibited very high activity (up to 170 × 106 g of PE/((mol of cat) h (mol/L of C2 H4 )) in the presence of MMAO to produce high molecular weight polymers with narrow PDIs (Mw = 7.7–165 × 103 , PDI ∼ 2).
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3
Scheme 2. Synthesis of complexes 1–2.
Scheme 3. Synthesis of complexes 3–6.
3. Bis(boratabenzene) rare-earth metal complexes 3.1. Bis(boratabenzene) divalent rare-earth metal complexes Metathesis reactions between SmI2 (THF)2 and 2 equiv. of K[C5 H5 BXPh2 ] (X = N, P) in THF afforded black bis(boratabenzene) divalent samarium complexes [C5 H5 BXPh2 ]2 Sm(THF)2 (X = N (23), P (24)) in moderate yields (Scheme 6) [32]. Both complexes were characterized by X-ray diffraction, which revealed the typical bend metallocene-type structures. The average Sm Cboratabenzene bond ˚ and 24 (2.93 A) ˚ are longer than the average lengths in 23 (2.93 A) ˚ a result that from the Sm Ccp distance in Cp*2 Sm(THF)2 (2.86 A), weaker donor property of boratabenzene. Solid-state structure also indicated a C2 -symmetry for 23, but a C1 -symmetry for 24. Strong -donation from the nitrogen atom of NPh2 group to the boron was observed in 23, while there is no such -donation in 24. Complexes 23 and 24 initiated the polymerization of MMA to produce
syndiotactic (mm: mr: rr = 0.4: 11.7: 87.9) and atactic polymers, respectively. The diamagnetic divalent ytterbium complex [C5 H5 BNPh2 ] Yb(THF)2 (25) was synthesized from the reaction of YbI2 (THF)2 with 2 equiv. of K[C5 H5 BNPh2 ] in THF [33]. The crystal structure of 25 is similar to its samarium analogue 23. The redox reactivity of 25 toward ␣-diimines was studied to investigate the intramolecular metal-ligand electron transfer process [34,35]. In toluene, the dark-red 25 was instantly oxidized by 1 equiv. of ␣-diimine PhNC(Me)C(Me)NPh to give a dark-green paramagnetic trivalent ytterbium complex [C5 H5 BNPh2 ]2 Yb[PhNC(Me)C(Me)NPh] (26) [33]. X-ray diffraction studies of 26 showed that the Yb3+ ion is coordinated by the radical anion [PhNC(Me)C(Me)NPh]−• in a -2 fashion, where the radical is not delocalized through the ␣-diimine NCCN moiety but located in one of the C N bond as a radical anion [C N]−• .
Scheme 4. Synthesis of LiL1, KL2–KL4.
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Scheme 5. Amidino-boratabenzene complexes 7–22.
Scheme 6. Synthesis of complexes 23–24.
The one-electron transfer between ytterbium ion and ␣-diimine is reversible which is governed by the nature of solvent (Scheme 7). Dissolving the dark-green complex 26 in donor solvent THF-d8 gradually regenerated the dark-red solution of 25 within 8 h. This solvent-mediated reversible transformation is attributed to the moderate redox potential of Yb2+ /Yb3+ (−1.15 V vs NHE) [36], therefore the interaction of donor solvent THF to ytterbium ion can compete with the electron transfer process between ytterbium ion and ␣-diimine. Accordingly, complex 25 did not react with the same ␣-diimine when THF was used as the solvent. However, the samarium analogue 23 only irreversibly reacted with PhNC(Me)C(Me)NPh due to a higher Sm2+ /Sm3+ transformation potential (−1.55 V vs NHE) [36]. Reaction of solvent-free divalent ytterbium amide Yb[N(SiMe3 )2 ]2 with 2 equiv. of [C5 H5 B ← PMe3 ] in toluene afforded an ansa-heteroborabenzene divalent ytterbium amide [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 (27) in moderate yield (Scheme 8) [37]. Complex 27 has several remarkable features: (1) the ytterbium ion is ligated by both neutral borabenzene and anionic boratabenzene rings; (2) two aromatic rings are linked through covalent and coordination bonds, respectively; (3) despite being a divalent lanthanide complex, the structure is reminiscent
Scheme 7. Solvent mediated reversible redox reaction between 25 and ␣-diimine.
of a trivalent ansa-Cp lanthanide complex. Monitoring the reaction of [C5 H5 B ← PMe3 ] with Yb[N(SiMe3 )2 ]2 in C6 D6 by 1 H and 31 P{1 H} NMR spectroscopy indicated an initial coordination of the borabenzene ring to the ytterbium ion, followed by a slow C H bond cleavage of PMe3 to release free HN(SiMe3 )2 and generate an intermediate A. The intermediate A underwent a nucleophilic aromatic substitution on a second [C5 H5 B ← PMe3 ] to form the final product 27 (Scheme 9). The initial coordination of the borabenzene ring to the metal center also seems to be crucial, since the reaction
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Scheme 8. Synthesis of complex 27.
was retarded in the presence of small amount of THF or by using the THF solvated Yb[N(SiMe3 )2 ]2 (THF)2 as the starting material. No reaction occurred between [C5 H5 B ← PMe3 ] and the bulkier trivalent lanthanide amides Ln[N(SiMe3 )2 ]3 (Ln = Yb, Lu). Due to its unique structural features, complex 27 displayed versatile reactivity (Scheme 10). Insertion of N,N diisopropylcarbodiimide into the Yb N bond of 27 afforded the corresponding divalent ytterbium guanidinate complex 28 [37]. The structure of 28 was confirmed by X-ray diffraction studies, indicating a similar insertion reactivity as that reported for the bis-Cp trivalent lanthanide amides [38,39]. The protonolysis of 27 with cyclopentadiene in benzene provided the first solvent-free bis(boratabenzene) divalent complex [C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]2 Yb (29) [40]. The [(C5 H5 )2 Yb]n [41] was also formed as the by-product resulting from a ligand redistribution of the heteroleptic intermediate [C5 H5 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb. In sharp contrast to the polymeric chain structure formed through the intermolecular Yb Cp interactions in [(C5 H5 )2 Yb]n [41], complex 29 exists as a monomer and features interesting intramolecular 1 Yb C interactions between the pendant neutral borabenzene rings and the ytterbium ion. Using substituted 1-trimethylsilylcyclopentadiene and 1,3-bis(trimethylsilyl)cyclopentadiene suppressed the ligand redistribution, and the solvent-free heteroleptic complexes [C5 H3 R1 R2 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (30: R1 = H, R2 = SiMe3 31: R1 = R2 = SiMe3 ) were synthesized [40]. Unlike the 1 Yb C interaction observed for 29, the crystal structure of 31 indicated an 6 interaction between the pendant neutral borabenzene and the ytterbium ion. Single crystals of 30 bearing the relatively less bulky 1-trimethylsilylcyclopentadienyl ligand were not obtained due to the redistribution to form 29 during recrystallzation. Detailed NMR spectroscopy studies showed that the interactions
between the pendant neutral borabenzene ring and the ytterbium ion for 29–31 are retained in non-donor solvent C6 D6 , but can be easily interrupted by the addition of -donor solvent THF-d8 . Complex 30 exists as two isomers in THF-d8 depending on the orientation of -SiMe3 group to the pendant borabenzene ring (30-syn: 30-anti = 2:1) (Scheme 11), which were identified by a NOESY experiment. Treatment of 27 with 1 equiv. of KC5 Me5 afforded a heterometallic Yb K complex 32 with a polymeric structure, in which the neutral borabenzene was displaced by KC5 Me5 at the ytterbium center (Scheme 10) [42]. While in the reaction of 27 with 1 equiv. of NaOi Pr, displacement of the anionic amido ligand by the isopropoxyl group occurred to produce a heterometallic Yb Na complex 33, which can be described as centrosymmetric dimers of ansa-heteroborabenzene divalent ytterbium alkoxide linked by centrosymmetric dimers of sodium amide. Complex 27 reacted with LiNEt2 to give ytterbium complex 34 and lithium complex 35. X-ray diffraction studies showed that 35 exists as a centrosymmetric square tetramer, in which the four nitrogen atoms of [N(SiMe3 )2 ]− ligands occupy the corners and four [Li boratabenzene Li] units make up the sides. The formation of 35 indicated a nucleophilic attack of neutral borabenzene of 27 by [NEt2 ]− ligand, which led to the cleavage of the P → B coordination bond. The complex 34 has far evaded crystallographic analysis due to its poor solubility in aliphatic solvents and the formation of oil in the presence of THF or pyridine. The 1 H and 31 P{1 H} NMR spectra of 34 in THF-d8 indicated a complicated aggregation with the PMe2 moiety free of coordination, the exact nature of this complex is still elusive. Similarly, the reactions of 27 with potassium benzyls, such as KCH2 C6 H5 , KCH2 C6 H3 -3,5-Me2 , as well as KCH2 C6 H4 -o-NMe2 also led to the cleavage of P → B coordination bond, but all products precluded crystallographic analysis.
Scheme 9. Reaction pathway for the formation of complex 27.
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Scheme 10. Reactivity of complex 27.
3.2. Bis(boratabenzene) trivalent rare-earth metal complexes A series of bis(boratabenzene) trivalent rare-earth metal chloride complexes were synthesized through salt metathesis between lithium boratabenzenes and rare-earth metal trichlorides. The reactions in toluene provided complexes 36–43 (Scheme 12); the structures of these complexes largely depend on the substituents on the boron and the radius size of metal ion [43–45]. The smallest rare-earth metal scandium afforded dimeric complexes [(C5 H5 BMe)2 ScCl]2 (36) and [(3,5-Me2 C5 H3 BNMe2 )2 ScCl]2 (38) in the case of small Me or NMe2 group on boron. Similar dimers of 37 and 39 were obtained with yttrium. With larger substituent on the boron, monomeric chloride complexes 40–43 were obtained for scandium, yttrium and lutetium. Reactions of lithium boratabenzenes with rare-earth metal trichlorides in THF or THP gave the THF or THP solvated “ate” complexes 44–46, in which the salt LiCl cannot be simply removed by recrystallization from toluene [45]. In 46, the lithium ion also coordinates to the amino group on boron, and the YCl2 Li ring is folded by 51.7(3)◦ . Trivalent
rare-earth metal chloride complexes bearing a B,B -bridged ansabis(boratabenzene), 47 and 48, were also synthesized for lutetium, where the rac-stereomer was obtained exclusively (Scheme 12) [45]. Reactions of bis(boratabenzene) yttrium chloride complexes (37, 49 and 50) with 2 equiv. of KN(SiMe3 )2 in toluene afforded bis(boratabenzene) yttrium amides 51–53 (Scheme 13) [46].
Scheme 11. Two isomers of complex 30 in THF.
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Scheme 12. Bis(boratabenzene) trivalent rare-earth metal chloride complexes 36–48.
Complex 53 was characterized by X-ray diffraction, which features a -agostic interaction between the electron deficient yttrium center and the Si C bond that led to a nonequivalent Si N Y angles (107.2(2)◦ and 126.8(3)◦ ). The nitrogen atom of the amido ligand displays a trigonal planar geometry ( NY = 360.0◦ ), indicating a donation of the lone electron pair on nitrogen to the highly electron-deficient metal center of the boratabenzene complex. Rare-earth metal amide complexes bearing Cp-type ligands are usually unreactive toward excess of KN(SiMe3 )2 , however, complexes 51–53 can react with KN(SiMe3 )2 that led to a displacement of boratabenzene by amido ligand to generate Y[N(SiMe3 )2 ]3 and
potassium boratabenzenes. Monitoring the reactions of the chloride complex 49 with 2, 4 and 6 equiv. of KN(SiMe3 )2 in C6 D6 indicated that the amide 52 was formed initially, which was slowly transformed to K[C5 H5 BNEt2 ] and Y[N(SiMe3 )2 ]3 . The displacement of boratabenzene is attributed to the weaker donor property of boratabenzene in comparison with the Cp-type ligands. The rate of displacement of boratabenzene is greatly influenced by the substitutent on boron, which decreases in the order of Me > NPh2 > NEt2 . Therefore, the -ligand displacement is slower in the more electron-donating aminoboratabenzene. All these amide complexes 51–53 are able to catalyze the
Scheme 13. Synthesis of bis(boratabenzene) yttrium amide complexes 51–53.
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Scheme 14. Synthesis of bis(boratabenzene) rare-earth metal alky complexes 59–66.
Table 1 Selected 1 H and 13 C{1 H} signal for Ln-C˛ H˛ of yttrium and lutetium alkyl complexes. Complexes
Solvent
ı Ln-C␣ H˛ (ppm)
ı Ln-C˛ H␣ (ppm)
Ref
(C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BCH3 )2 YCH(SiMe3 )2 (61) (C5 H4 Me)2 YCH(SiMe3 )2 (C5 Me5 )2 YCH(SiMe3 )2 [Et2 Si(C5 H4 )(C5 Me4 )]YCH(SiMe3 )2 (R,S)-Me2 Si(C5 Me4 )[(+)-neomenthyl-C5 H3 ]YCH(SiMe3 )2 (C5 Me5 )(OAr)YCH(SiMe3 )2 (2,4,7-Me3 C9 H4 )2 YCH(SiMe3 )2 (C9 Me7 )2 YCH(SiMe3 )2 Y[CH(SiMe3 )2 ]3 (C5 H5 BNEt2 )2 LuCH(SiMe3 )2 (64) (C5 H5 BNPh2 )2 LuCH(SiMe3 )2 (65) (C5 H5 BCH3 )2 LuCH(SiMe3 )2 (66) (C5 Me5 )2 LuCH(SiMe3 )2 [Me2 Si(C5 Me4 )2 ]LuCH(SiMe3 )2 [Me2 Si(C5 H4 )(C5 Me4 )]LuCH(SiMe3 )2 [Et2 Si(C5 H4 )(C5 Me4 )]LuCH(SiMe3 )2 (R,S)-Me2 Si(C5 Me4 )[(+)-neomenthyl-C5 H3 ]LuCH(SiMe3 )2 [Me2 Si(C5 Me4 )(tBuN)]LuCH(SiMe3 )2
C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 C6 D6 Tol-d8 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 C6 D6 C6 D6 C6 D6 C6 D6
0.79 0.68 0.95 0.30 −0.10 −0.59 −0.55, −0.56 −0.39 0.16 −0.37 −0.62 0.92 0.74 0.99 −0.02 −0.411 −0.50 −0.52 0.06, −0.48 −1.14
33.88 39.06 38.7 27.39 25.19 29.3 – 32.0 26.03 27.5 50.0 34.1 38.6 38.6 – – 29.1 – – 44.7
[47] [47] [47] [47] [48] [49] [50] [51] [52] [53] [54] [47] [47] [47] [55] [56] [49] [49] [50] [57]
intramolecular hydroamination of 2,2-dimethyl-1-aminopent-4ene to give 2,4,4-trimethylpyrrolidine. Bis(boratabenzene) rare-earth metal alkyl complexes 59–66 were synthesized by metathesis reactions between the chloride complexes and LiCH(SiMe3 )2 in toluene (Scheme 14) [47]. Crystal structures of 60, 63 and 65 showed unsymmetrical coordination of CH(SiMe3 )2 caused by the -agostic interaction between the electron-deficient metal center and the Si C bond, which is even more profound than that in the amide complex 53 due to the lack of -donation from the CH(SiMe3 )2 group. 1 H and 13 C{1 H} NMR spectroscopy of 59–61 and 64–66 revealed downfield shifts of C˛ H˛ signals, compared to the alkyl ( CH(SiMe3 )2 ) complexes with Cp-type ligands (Table 1). For example, the Y C˛ H˛ of complexes 59–61 were observed at ı = 0.79, 0.68, 0.95 ppm and ı = 33.9, 39.1, 38.7 ppm in the 1 H and 13 C{1 H}NMR spectra, respectively, both are downfield shifted compared to those of bis-Cp complexes (−0.59 to 0.30 ppm and 27.5 to 29.3 ppm for 1 H and 31 C{1 H} NMR signals, respectively). The catalyzed dehydrocoupling of amine boranes has attracted great interest recently, due to the potential usage of amine boranes as H2 storage materials in the future clean energy application. Bis(boratabenzene) rare-earth metal alkyl complexes 59, 61, 64 and 66 are active for the dehydrocoupling of Me2 NH·BH3 (Scheme 15) [58]. The yttrium complex 61 with 1-methyl boratabenzene showed very high activity: with 0.5 mol% of 61, 99% of conversion
Scheme 15. Dehydrocoupling of Me2 NH·BH3 .
was achieved within 12 min (98% to the cyclic dimer [Me2 N–BH2 ]2 , the reminder to Me2 N = BH2 ) with a TOF up to 1000 h−1 , which represented the most active early-transition metal catalyst for the dehydrocoupling of Me2 NH·BH3 . The activity of lutetium complex 66 (TOF of 208 h−1 ) is lower than that of 61, however, both of them are much more active than complexes 59 (TOF of 9.4 h−1 ) and 64 (TOF of 11 h−1 ) which contain 1-diethylamino boratabenzene. The less electron-donating 1-methyl boratabenzene provides a more electron-deficient metal center, which is probably the key factor that facilitates the reaction. On the other hand, the hydridic Hs on boron in Me2 NH·BH3 or the reaction intermediates may also interact with the electron-deficient boron atom of 1methyl boratabenzene to accelerate the reaction. Other rare-earth metal alkyl complexes, Ln(CH2 SiMe3 )3 (THF)2 (Ln = Sc, Y, Lu; TOF of 1.8–7.7 h−1 ), (MeC5 H4 )2 YCH(SiMe3 )2 (TOF of 74 h−1 ) and [1,3(SiMe3 )2 C9 H5 ]Y(CH2 SiMe3 )2 (THF) (TOF of 2.3 h−1 ), are also active for the dehydrocoupling of Me2 NH·BH3 , however, their activity are much lower than that of 61 and 66 [58].
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Scheme 16. Hydroboration reactivity of complex 67.
Of the various boratabenzene frameworks, 1-H-boratabenzene is special due to the presence of a reactive B H bond, as compared to the analogous C H bond on the ubiquitous Cp ligand. 1-H-boratabenzene yttrium chloride complex [(C5 H5 BH)2 YCl]2 (67) is able to undergo a series of hydroboration reactions with alkenes, alkynes, imines and carbodiimides that allow the rapid entry to functionalized boratabenzene yttrium chloride complexes (Scheme 16) [59]. The hydroboration of 1-hexene with 67 afforded the anti-Markovnikov product 68 in high yield and selectivity. The allyl propyl ether that contains an ether functional group also underwent anti-Markovnikov hydroboration with 67 to form complex 69 in which the coordination of ether functionality is labile as indicated by Variable-temperature 1 H NMR spectroscopy. When 0.9 equiv. of diallyl ether was used, double B H bond additions to the same molecule were achieved and gave an ansa-bis(boratabenzene) yttrium chloride 70. In the presence of a large excess of diallyl ether (10 equiv.), complex 71 containing mono-addition of two diallyl ethers was obtained in high yield. In both cases, the formation of a mono-addition intermediate B was observed by 1 H NMR spectroscopy. The anti-Markovnikov hydroboration selectivity for 68–71 was further confirmed by the deuterium-labeling experiment on the reaction of [(C5 H5 BD)2 YCl]2 (67-D2 ) with allyl propyl ether. The hydroboration of 3-hexyne with 67 readily occured at room temperature to afford the alkenyl-substituted
boratabenzene complex 72. Benzylidene-n-propylamine reacted with 67 at 75 ◦ C to give the amino-substituted boratabenzene complex 73. The reaction with N,N -diisopropylcarbodiimide also occurred at room temperature, but only afforded the monoaddition product 74 in high yield, further addition did not occur due to the steric bulk of the [C5 H5 BN(i Pr)CHN(i Pr)]− ligand. 1-H-borabenzene lutetium chloride complex [(C5 H5 BH)2 LuCl]2 (75) also underwent hydroboration with 1-hexene and N,N diisopropylcarbodiimide to form [C5 H5 B(CH2 )5 CH3 ]2 LuCl (76) and [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]LuCl (77) in high yields, respectively. In comparison, 1-H-boratabenzene zirconium complex [C5 H5 BH]2 ZrCl2 (78) did not react with 1-hexene or 3-hexyne, only reacted with benzylidene-n-propylamine to afford the hydroboration product [C5 H5 BN(n Pr)CH2 Ph]2 ZrCl2 (79) [59]. The reaction with N,N -diisopropylcarbodiimide gave an unidentified mixture. Rhodium complex [C5 H5 BH]Rh(PPh3 )2 (80) only slowly reacted with N,N -diisopropylcarbodiimide at 75 ◦ C for 2 weeks to form the hydroboration product [C5 H5 BN(i Pr)CHN(i Pr)]Rh(PPh3 )2 (81), and rhodium complex [C5 H5 BH]Rh(COD) (82) is inert toward all above mentioned substrates. Therefore, the B H addition reactivity of the 1-H-boratabenzene transition metal complexes decreases in the following order: 67 (Y) > 78 (Zr) > 80 (Rh) > 82 (Rh), showing that the hydroboration at the 1-H-boratabenzene ligand is sensitive to the electronic property of the metal
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lutetium (75) complexes [60]. While 1-hexene and 3-hexyne underwent hydroboration with all three 1-H-boratabenzenes of 85 to afford [C5 H5 B(CH2 )5 CH3 ]3 La·LiCl(THF) (88) and [C5 H5 BC(C2 H5 ) = CH(C2 H5 )]3 La·LiCl(THF) (89) in good yields, the reaction of 85 with N,N -diisopropylcarbodiimide only provided the mono-hydroboration product [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]2 La (90) due to the steric bulk of the [C5 H5 BN(i Pr)CHN(i Pr)]− ligand. The remaining two 1-H-boratabenzenes of 90 can be functionalized further with 3-hexyne or phenyl acetylene (Scheme 18). The 3-hexyne slowly reacted with 90 to form the hydroboration product 91, while for the phenyl acetylene, the dehydrogenative coupling product 92 was obtained in high yield instead of forming the hydroboration product, presumably due to the more acidity of the terminal acetylene. Effort to synthesize tris(boratabenzene) complex [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BC CPh][C5 H5 BH]La (93) that contains three different boratabenzenes by controlling the molar ratio of phenyl acetylene to 90 only led to the formation of mixture of 90, 92 and 93. 5. NMR Spectroscopic features of boratabenzene rare-earth metal complexes
Scheme 17. Ligand redistribution between boratabenzene yttrium complexes and rhodium chloride complexes.
complex. It was also found that the boratabenzene yttrium chloride complexes, 67, 68 and 72, are capable of redistributing ligand with Rh(PPh3 )3 Cl or [Rh(COD)Cl]2 to give rhodium complexes 80 and 82–84 (Scheme 17) [59]. Therefore, although the 1-alkyl- (or 1-alkenyl-) functionalized boratabenzene rhodium complexes could not be directly synthesized via the hydroboration of 1-H-boratabenzene rhodium complexes, the highly efficient hydroboration with 1-H-boratabenzene yttrium complex followed by the ligand redistribution provides a synthetic pathway to these complexes. 4. Tris(boratabenzene) rare-earth metal complexes Salt metathesis reactions of LaCl3 with 3 equiv. of Li[C5 H5 BH] or Li[C5 H5 BNEt2 ] afforded the corresponding tris(boratabenzene) lanthanum complexes [C5 H5 BH]3 La·LiCl (85) and [C5 H5 BNEt2 ]3 La·LiCl(THF) (86) in high yields [60]. Attempts to obtain the alkali-metal free tris(boratabenzene) complexes by sublimation of complexes 85 and 86 under vacuum did not succeed. Recrystallization of 85 in a THF/toluene mixture gave a THF solvated discrete cation–anion pair [(C5 H5 BH)3 La(Cl)La(C5 H5 BH)3 ][Li(THF)4 ] (87), where the two (C5 H5 BH)3 La units are bridged by a chloride to form the [(C5 H5 BH)3 La(Cl)La(C5 H5 BH)3 ]− anion and the Li+ ion is coordinated by four THF molecules. Complex 85 showed hydroboration reactivity as that observed for the bis(1-H-boratabenzene) yttrium (67) and
As shown in Table 2, for the diamagnetic rare-earth metal ions Sc3+ , Y3+ , La3+ , Yb2+ and Lu3+ , 11 B NMR resonances of their boratabenzene complexes are observed in the range of 29–46 ppm, which largely depend on the substituent on boron while the metal ion itself has little affect on the value. In all cases, the 1-methyl boratabenzenes show resonances at the lowest field (42–45 ppm, in complexes 37, 51, 61, 66). Complexes 70 and 72 containing alkyl and alkenyl substituted boratabenzenes also show relatively low field shifts at ∼40 ppm. In the divalent ytterbium complexes (27–32) containing ansa-heteroborabenzene ligand, 11 B NMR resonances for the anionic boratabenzene and neutral borabenzene were observed at 31–37 and 23–29 ppm, respectively. The 1 JPB coupling constants for the neutral borabenzenes are ∼100 Hz. Although the direct observation of rare-earth metal ions by solution NMR spectroscopy method was hampered by the paramagnetic nature of majority of rare-earth metal complexes (except those of Sc3+ , Y3+ , La3+ , Yb2+ , and Lu3+ ) as well as the large quadruple moments of these elements, the diamagnetic properties of Y3+ and Yb2+ allow 89 Y (I = 1/2, 100% abundance) and 171 Yb nuclei (I = 1/2, 14.27% abundance) to be amenable to an NMR study. Table 3 shows the 89 Y NMR spectroscopy data of bis(boratabenzene) yttrium alkyl complexes 59, 60 and 61 at 176.1, 170.0 and 162.2 ppm, respectively, which are significantly shifted to lower field compared to that observed for (C5 H4 Me)2 YCH(SiMe3 )2 (44.0 ppm) [47]. The movement of 89 Y NMR resonances to the lower field indicates a decrease of electronegativity and -donating ability of the ligands. The group contributions of various ligands to the 89 Y nuclear shielding have been estimated before as C5 Me5 (ca. −100 ppm) > alkoxides (ca. 15 ppm) > aryloxides (ca. 56 ppm) > amides (ca. 190 ppm) > alkyls (ca. 300 ppm) [51]. In complexes 59, 60 and 61, the group contributions from boratabenzenes [C5 H5 BNEt2 ]- , [C5 H5 BNPh2 ]− and [C5 H5 BMe]− to the 89 Y nuclear shielding are −61, −64 and −68, respectively. The lower values for the boratabenzenes compared to that of −128 for the [C5 H4 Me]− are consistent with the weaker -donor ability of boratabenzenes than Cp ligands. 6. Outlook The boratabenzene rare-earth metal chemistry is in the middle stage. A system that contains mono-, bis- and tris(boratabenzene) rare-earth metal complexes has been established, which is paralleled with the system of Cp ligands, but also leaves a wide space for
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Scheme 18. Hydroboration and dehydrogen-coupling reactions of complex 90.
the further development in the near future. The weaker -donor property of boratabenzenes have been exhibited by structures and reactivity of the complexes, the 1-H-boratabenzene complexes showed hydroboration reactivity with unsaturated molecules, but it will be more interesting to investigate the role of Lewis
acidity of boron, and how it might cooperate with the Lewis acidic rare-earth metal center in the activation of substrates. As was indicated in the dehydrocoupling of Me2 NH·BH3 , some intrinsic property of the boratabenzene rare-earth metal complexes needs to be discovered. Another important aspect of boratabenzenes is
Table 2 Selected 11 B NMR spectroscopy data of boratabenzene rare-earth metal complexesa . Complexes
Solvent
ı (ppm)
Ref.
(C5 H5 BPh)ScPh2 (THF) (1) [3,5-Me2 C5 H3 BNEt2 ]Sc(CH2 SiMe3 )2 (THF) (3) [3,5-Me2 C5 H3 BNEt2 ]Lu(CH2 SiMe3 )2 (THF) (4) [3,5-Me2 C5 H3 BPh]Sc(CH2 SiMe3 )2 (THF) (5) [3,5-Me2 C5 H3 BPh]Lu(CH2 SiMe3 )2 (THF) (6) [C5 H5 BN(i Pr)C(CH3 )N(i Pr)]Sc(CH2 SiMe3 )2 (7) [C5 H5 BN(i Pr)C(CH3 )N(i Pr)]Y(CH2 SiMe3 )2 (8) [Me2 C5 H3 BN(i Pr)C(CH3 )N(i Pr)]Sc(CH2 SiMe3 )2 (12) [C5 H5 BNPh2 ]2 Yb(THF)2 (25) [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 (27) [C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(i Pr)CN(i Pr)N(SiMe3 )2 (28) [C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]2 Yb (29) [C5 H4 (SiMe3 )][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (30) [C5 H3 (SiMe3 )2 ][C5 H5 BCH2 (CH3 )2 P → BC5 H5 ]Yb (31) {[C5 H5 BCH2 (CH3 )2 P → B(C5 H5 )]YbN(SiMe3 )2 K(C5 Me5 )}n (32) {[C5 H5 BCH2 (CH3 )2 P]YbNEt2 }n (34) [(C5 H5 BMe)2 YCl]2 (37) (Me2 C5 H3 BNi Pr2 )2 ScCl (40) [Me2 C5 H3 BN(SiMe3 )2 ]2 LuCl (43) [(C5 H5 BNPh2 )2 YCl]2 (50) (C5 H5 BNEt2 )2 YN(SiMe3 )2 (52) (C5 H5 BNPh2 )2 YN(SiMe3 )2 (53) (C5 H5 BMe)2 YN(SiMe3 )2 (51) (C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BMe)2 YCH(SiMe3 )2 (61) (C5 H5 BMe)2 LuCH(SiMe3 )2 (66) [(C5 H5 BH)2 YCl]2 (67) [C5 H5 B(CH2 )3 O(CH2 )3 BC5 H5 ]YCl (70) [C5 H5 BC(C2 H5 ) = CH(C2 H5 )]2 YCl (72) [C5 H5 BN(n Pr)CH2 Ph]2 YCl (73) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]YCl (74) [C5 H5 BH]3 La•LiCl (85) [C5 H5 BNEt2 ]3 La•LiCl (THF) (86) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BH]2 La (90) [C5 H5 BN(i Pr)CHN(i Pr)][C5 H5 BCCPh]2 La (92)
C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 THF-d8 C6 D6 C6 D6 THF-d8 THF-d8 C6 D6 THF-d8 THF-d8 C6 D6 Tol-d8 C6 D6 CDCl3 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 C6 D6 CDCl3 C6 D6 C6 D6 C6 D6 C6 D6 THF-d8 C6 D6 C6 D6 C6 D6
37.0 32.7 32.9 36.7 37.0 35.9 36.1 36.2 33.8 33.7 (CP); 25.8 (BP) 37.0 (CP); 29.1 (BP) 34.0 (CP); 23.9 (BP) 33.7 (CP); 23.9 (BP) 34.2 (CP); 25.5 (BP) 31.7 (CP); 24.0 (BP) 31.4 43.2 34.7 38.9 34.4 32.4 33.3 45.4 33.1 34.1 42.6 42.8 37.3 40.1 39.3 33.6 36.9, 34.0 32.3 32.1 36.9, 34.4 36.1, 29.0
[24] [26] [26] [26] [26] [30] [30] [30] [33] [37] [37] [40] [40] [40] [42] [42] [44] [45] [45] [46] [46] [46] [46] [47] [47] [47] [47] [59] [59] [59] [59] [59] [60] [60] [60] [60]
a 11
B NMR spectra were measured at 298 K relative to BF3 ·OEt2 .
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ARTICLE IN PRESS P. Cui, Y. Chen / Coordination Chemistry Reviews xxx (2015) xxx–xxx
12
Table 3 Selected 89 Y NMR spectroscopy data of yttrium complexesa . Complexes
Solvent
ı (ppm)
Ref.
(C5 H5 BNEt2 )2 YCH(SiMe3 )2 (59) (C5 H5 BNPh2 )2 YCH(SiMe3 )2 (60) (C5 H5 BMe)2 YCH(SiMe3 )2 (61) (C5 H4 Me)2 YCH(SiMe3 )2 Y[CH(SiMe3 )2 ]3 (C5 H4 Me)3 Y(THF)
C6 D6 C6 D6 C6 D6 C6 D6 Tol-d8 THF-d8
176.1 170.0 162.2 44.0 895 −371
[47] [47] [47] [47] [51] [61]
a 89
Y NMR spectra were measured at 298 K relative to 3 M YCl3 in D2 O.
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