Chemistry and catalysis advances in organometallic chemistry and catalysis
Scheme 25.1 SYNTHESES AND MOLECULAR STRUCTURES 317
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Scheme 25.1 SYNTHESES AND MOLECULAR STRUCTURES 317 Figure 25.1 ORTEP view of 4 shown at 40% thermal ellipsoids probability. Selected bond lengths ( ˚ A) and angles ( ◦ ): Zr–O(1) 2.042(6); Zr–O(2) 2.035(6); Zr–N(1) 2.496(7); Zr–N(2) 2.095(7); Zr–N(3) 2.474(7); Zr–N(4) 2.077(7); O(1)–Zr–O(2) 90.7(3). Figure 25.2 ORTEP view of 9 shown at 50% thermal ellipsoids probability. Selected bond lengths ( ˚ A) and angles ( ◦ ): Zr–Cl 2.5292(13); Zr–N(5) 2.077(3); Zr–N(1) 2.507(3); Zr–N(2) 2.070(3); Zr–N(3) 2.507(3); Zr–N(4) 2.058(3); Cl–Zr–N(5) 87.69(8). The structural diversity of zirconium complexes suppored by (Bn 2 Cyclam) ancillary ligands also includes coordination numbers 7 and 8, as attested by the molecular structures of the chloro-hydrazido complex (Bn 2 Cyclam)ZrCl( κ 2 :N,N - N(Ph)NPh( n Bu)) (13) [10a] and of (Bn 2 Cyclam)Zr(( κ 2
2 ) 2 (14) [11] where both acetoximato ligands exhibit chelating bonding mode (Fig. 25.5). The hemilabile behavior of trans-disubstituted cyclams and the structural diversity exhibited by zirconium complexes incorporating these ligands are closely related to their properties in intramolecular hydroamination of aminoalkenes and ring-opening polymerization (ROP) of lactide, discussed in the following sections.
318 A NEW FAMILY OF ZIRCONIUM COMPLEXES ANCHORED BY DIANIONIC CYCLAM-BASED LIGANDS Figure 25.3 Solid-state molecular structure of 7 shown at 50% thermal ellipsoid probability. Selected bond lengths ( ˚ A) and angles (
◦ ): Zr–N(5) 2.0616(11); Zr–N(6) 2.0706(11); Zr–N(1) 2.833(1); Zr–N(2) 2.0860(10); Zr–N(3) 2.9768(10); Zr–N(4) 2.0998(10); N(2)–Zr–N(4) 114.35(4); N(5)–Zr–N(6) 99.74(5). X N Zr N N Ph Ph N Y α α Figure 25.4 Angle
α defined by the X–Zr–Y plane and the N amido
–Zr–N amido
plane. C28
C25 N5 N3 N1 N4 N2 N6 O2 O1 Zr1 Figure 25.5 ORTEP view of 14 shown at 40% thermal ellipsoids probability. Selected bond lengths ( ˚ A) and angles ( ◦ ): Zr–O(1) 2.128(2); Zr–O(2) 2.130(2); Zr–N(1) 2.445(2); Zr–N(2) 2.123(2); Zr–N(3) 2.453(2); Zr–N(4) 2.122(2); Zr–N(5) 2.317(2); Zr–N(6) 2,307(2); O(1)–Zr–O(2) 106.12(7). THERMALY INDUCED ORTHOMETALLATION AND INTRAMOLECULAR HYDROAMINATION OF AMINOALKENES 319 25.3 THERMALY INDUCED ORTHOMETALLATION AND INTRAMOLECULAR HYDROAMINATION OF AMINOALKENES Early transition metal imido complexes can be prepared by α-abstraction and extrusion of a good leaving group (e.g., alkane or amine) from a metal amido species. This process depends on how good the leaving group is, and on the acidity of the N–H proton, which is normally dictated by the substituent groups on the amido moiety [12]. In addition, the ancillary ligand plays a crucial role on the reactivity of the amido complexes. For example, the formation of diamido compounds by chloride metathesis, upon addition of two equivalents of a lithium amide to the zirconium dichloro starting material, as described in the previous section, finds a similar reactivity pattern in tetraazaannulene-based systems [7c]. However, in porphyrin or calixarene-based systems the addition of two equivalents of lithium amide generates a monomeric imido complex [13]. Finally, zirconocene imido complexes are readily accessible from amido species through amine or alkane elimination [12a]. The preparation of imido complexes through intramolecular amine elimination was not possible for trans-disubstituted- cyclam based zirconium compounds. The reaction of (Bn 2 Cyclam)Zr(NH t Bu)
2 (7) with one equivalent of a more acidic and sterically encumbered amine like 2,6-Me
PhNH 2 generates (Bn 2 Cyclam)Zr(NH 2,6-Me Ph)
2 (8). On the other hand, heating of a benzene solution of (Bn 2 Cyclam)Zr(NH t Bu)
2 in an NMR tube at 60 ◦ C led to a mixture of 7, free t BuNH
2 , and
the orthometallated-amido complex [(C 6 H 4 CH 2 )BnCyclam]Zr(NH t Bu) (15) (Scheme 25.2). α-Abstraction of the amine is reversible, and longer reaction times do not affect the relative amounts of these species. The dibenzyl complexes (Bn 2
2 Ph)
2 (12a) and ( 3,5-Me Bn
Cyclam)Zr(CH 2 Ph) 2 (12b) convert to the di-orthometallated complexes [(C 6
4 CH 2 ) 2 Cyclam]Zr (16a) and [(3,5-Me 2 C 6 H 2 CH 2 ) 2 Cyclam]Zr (16b) upon thermal activation, although the formation of the latter requires much longer reaction time (7 days vs 24 h at 115 ◦ C).
The ortho-metallation reaction was shown crucial for the catalytic performance of (Bn 2 Cyclam)ZrX 2 (X = NMe 2 , CH
2 Ph)
in the intramolecular hydroamination of aminoalkenes [14]. Addition of 2 equivalents of 2,2-diphenyl-pent-4-enylamine to the zirconium complex 16a in benzene-d 6 led to quantitative conversion to the diamido (Bn 2 Cyclam)Zr(N(H)R) 2 (R
2 CPh
2 CH 2 CH =CH
2 ) (17) that, after a few hours at room temperature, converted to the mono-orthometallated [(C 6
4 CH 2 )BnCyclam]ZrN(H)R (18) (R = CH
2 CPh
2 CH 2 CH =CH
2 ) and 2-methyl-4,4-diphenylpyrrolidine. Upon
heating the solution up to 90 ◦ C, the conversion is quantitative, and further heating produced the di-orthometallated [(C 6 H 4 CH 2 ) 2 Cyclam]Zr (16a) and the release of a second equivalent of 2-methyl-4,4-diphenylpyrrolidine (see Scheme 25.3). Taking into account that thermal conversion of diamido to imido species is not accessible to Bn 2 Cyclam zirconium complexes, the cyclization reaction is likely to occur by 1,2-insertion of the olefin moiety in the Zr–N bond (Scheme 25.4). This pathway was initially proposed by Tobin Marks for lanthanocene and constrained geometry zirconium complexes and was more recently also suggested for other group 4 metal catalysts [15, 16]. The activation of the olefin toward insertion may
320 A NEW FAMILY OF ZIRCONIUM COMPLEXES ANCHORED BY DIANIONIC CYCLAM-BASED LIGANDS Scheme 25.3 Scheme 25.4 ROP OF LACTIDE AND CYCLAM FUNCTIONALIZATION 321 be supported either by the ancillary ligand through the elongation of the Zr–N amine bonds or by increasing the coordination number of the metal center. The cleavage of the Zr–C bond may involve the NH proton of the amido ligand (path i) or the ortho-CH bond of the benzyl group (path ii) of species A in Scheme 25.4. The product of this step would be, in the first case, the imido complex or, in the second case, the mono-orthometalated species 18. The pathway disclosed in route (i) is in agreement with the formation of (Bn 2 Cyclam)Zr(NR) (R =2,6- i Pr) and CH 4 by reaction of (Bn 2 Cyclam)ZrCl 2 (2a) with MeMgCl [10c]. This route cannot be disclosed on the basis of the fact that it was not observed by NMR, as its conversion to 18 may be fast, but the results described below do not fit this hypothesis. Indeed, further insight into this problem was provided by ( 3,5-Me Bn
Cyclam)Zr(CH 2 Ph) 2 (12b). This complex (i) not only requires much harsh temperature conditions and longer reaction times than (Bn 2 Cyclam)Zr(CH 2 Ph)
2 (12a) to give the corresponding orthometallated compound (16b) but (ii) revealed inactive in the catalytic hydroamination of 2,2-diphenyl-pent-4-enylamine. If route (i) was a viable pathway to the cyclization reaction, the catalytic activity would not depend on the substitution of the cyclam benzyl rings and 12b would be expected to catalyze the hydroamination reaction. The lack of catalytic activity of 12b suggests that the ortho-metallation reaction is a requirement for the catalytic activity and seems to support that the formation of 2-methyl- 4,4-diphenylpyrrolidine takes place through path (ii). Further reactivity and DFT studies aiming the definite establishment of the reaction mechanism are in course and will be published soon. 25.4 ROP OF LACTIDE AND CYCLAM FUNCTIONALIZATION Cyclic ester polymers, in particular polylactic acid (PLA) and its copolymers, have noticeable applications that are related to their biocompatible and biodegradable properties [16]. These products may be obtained by ROP using alkoxide derivatives of electrophilic metals as catalysts [17], which provide living and/or stereoregular growing processes that are critical for many polymer applications. We have found that (Bn 2 Cyclam)Zr(O i Pr)
2 (3), (Bn 2 Cyclam)Zr(OPh) 2 (4), and (Bn 2 Cyclam)Zr(SPh) 2 (5) also catalyze the ROP of rac-lactide in a well-controlled way as attested by the PDI values and the direct proportionality between monomer conversions and the molecular weights of the polymers [10c]. The polymers obtained using 3 as catalyst have O i Pr-end groups in agreement with a polymerization reaction that is initiated by monomer insertion in the Zr–O bonds, as usually observed for metal alkoxido catalysts. Unexpectedly, the polymers obtained with (Bn 2 Cyclam)Zr(OPh) 2 or (Bn
2 Cyclam)Zr(SPh) 2 display Bn 2 Cyclam end-capped PLAs (Scheme 25.5). These results are compatible with an initial insertion of rac-lactide in the Zr–N amido
bonds of the cyclam ligand and subsequent chain propagation through insertion of incoming monomers into the newly formed Zr–O bonds. The first step of the polymerization reaction was investigated by DFT. It was assumed that the reaction started by the insertion of a carbonyl group in the Zr–N amido or the Zr–O bonds of (Bn 2 Cyclam)Zr(OR) 2 (R
i Pr, Ph). For each compound, the structures of the intermediates were optimized and the energy of the processes was calculated. In both cases, the coordination of the incoming ligand pushes one of the amine N-atoms of the cyclam ligand away from the metal. For (Bn 2
2 the overall energy balance favors the insertion in Zr–N amido bonds by
−3.9 kcal/mol, while for (Bn
2 Cyclam)Zr(O i Pr)
2 this pathway is essentially thermoneutral ( −0.2 kcal/mol). The formation of end-capped Bn 2 Cyclam polymers with complex 4 is thus thermodynamically favored and results from orbital controlled insertion. The global energy variation associated to the insertion in the Zr–O bond of 3 is 2.2 kcal/mol. Albeit this process is slightly disfavored in comparison with the insertion in the Zr–N amido
bond, the analysis of the charges located on the oxygen atoms of the O i Pr ligands suggests that, in the case of 3, the reaction is likely ruled by charge control [10c]. Although metal–amido bonds have been reported to initiate cyclic esters ROP, (Bn 2 Cyclam)Zr(OPh) 2 and (Bn
2 Cyclam)
Zr(SPh) 2 are the unique ROP catalysts reported to date that allow the simultaneous functionalization/polymerization of Scheme 25.5 322 A NEW FAMILY OF ZIRCONIUM COMPLEXES ANCHORED BY DIANIONIC CYCLAM-BASED LIGANDS lactide. The combination of a biocompatible and biodegradable polymer with a cyclam moiety, which proved useful in biochemical and sensoring applications [18], may set the ground for the design of new functional materials, obtained straightforwardly by insertion of adequate substrates in Zr–N bonds of dianionic cyclam-based ligands.
The results described reveal that dianionic diamido-diamine ligands derived from trans-dibenzylcyclams are able to support a variety of zirconium complexes of general formula (Bn’ 2 Cyclam)ZrXY. These compounds display varied structural motifs, with coordination numbers between 4 and 8, which attest for the hemilabile behavior of the ancillary ligand and revealed critical for the catalytic activity displayed by complexes of this family in intramolecular hydroamination and ROP of lactide. The ability to generate open coordination sites in the 6-coordinated precatalysts by cleavage of Zr–N amine
bonds of the macrocycle ligand and the possibility to reach higher coordination numbers have proved decisive in the catalytic activity observed. The intramolecular hydroamination of primary amines, typified in this study by 2,2-diphenyl-pent-4-enylamine, is likely to involve 1,2-insertion of the C =C bond in the Zr–NHR bond followed by σ-bond metathesis of the Zr–C bond and concomitant ortho-metallation of the benzyl groups appended to the cyclam nitrogen atoms. The ROP of rac-lactide with (Bn 2 Cyclam)ZrX 2 catalysts (X = O
Pr, OPh, SPh) may be modulated by X. For X = O
Pr the polymerization is initiated by substrate insertion in the Zr–O bonds, as usually observed for other metal alkoxidos. A completely different ROP occurs with (Bn 2 Cyclam)Zr(OPh) 2 and (Bn
2 Cyclam)Zr(SPh) 2 catalysts. The insertion of lactide takes place at the Zr–N amido
bonds of the cyclam, originating the growing of the polymer chain appended to the macrocycle nitrogen atoms. This process, which constitutes a straightforward strategy to the simultaneous ROP of lactide/functionalization of cyclam, will be further extended to other homo- and hetero-biopolymers.
Fundac¸˜ao para a Ciˆencia e Tecnologia is ackowledged for funding (PEst-OE/QUI/UI0100/2011). A. M. Martins thanks Prof. Luis F. Veiros (DFT calculations) and Dr. Samuel Dagorne (ROP catalysis) for their contributions.
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