Rare Earth Complexes with Aliphatic Amide Type Ligands

Acknowledgments

N- Based Rare Earth Complexes

4.2 Rare Earth Complexes with Amide Type Ligands

4.2.1 Rare Earth Complexes with Aliphatic Amide Type Ligands

The aliphatic amine can be used to classify primary, secondary, tertiary, and quaternary amines.

Except for the quaternary amine, the other three types of amines are able to coordinate with the

Rare Earth Coordination Chemistry: Fundamentals and Applications Edited by Chunhui Huang

of the functional group substituted to the nitrogen atom, the space effect of the aliphatic amine, and the influence of the intermolecular hydrogen bonding interaction. Owing to the lack of integrated structural parameters, it is difficult to quantitatively depict the influence as a whole. In most instances, the factor affecting the coordination ability of the ligand was estimated according to experimental results. For example, when only the influence of the inductive effect is taken into consideration, the order of alkalescence of the aliphatic amines is tertiary>secondary>primary. However, this order does not always persist if the influence of the space effect is taken into account. In fact, the order of the alkalescence for aliphatic amines is secondary>primary>tertiary on the basis of experimental results.

Usually, the alkalescence of the aliphatic amines is strong. In order to avoid the hydrolysis of the rare earth ions, it is necessary to maintain a strictly anhydrous environment when complexes of the rare earths with aliphatic amine ligands are synthesized. Instead of rare earth chloride, nitrate, or perchlorate salts, the rare earth triflate salts RE(CF3SO3)3are often used as the start- ing material. The most characteristic feature of RE(CF3SO3)3is the stability and ease of elimi- nating water. The rare earth triflate salts are usually prepared from the corresponding rare earth oxide and trifluoromethanesulfonic acid CF3SO3OH [2]. Adding the rare earth oxide to an aque- ous solution of trifluoromethanesulfonic acid and removing the undissolved oxide by filtration, after evaporating the residue solution using a rotatory evaporator, the resulting solid is dried at 160–200^◦C under vacuum, and RE(CF3SO3)3is obtained and used without further purification.

Some representative aliphatic amine ligands are summarized in Figure 4.1. The synthesis and structure characteristics of example complexes will be introduced in the following sections.

[La(1)4·CF3SO3]CH3CN·(CF3SO3)2Complex: The preparation of this complex [3] is anal- ogous to the corresponding perchlorate first reported by Forsberg and Moeller. La(CF3SO3)3

reacted with ethylenediamine (1) in a ratio of 1 : 8 in acetonitrile under N2in a dry Schlenk tube. The resulting cloudy mixture was heated to reflux for about 5 min, then the mixture was- filtered. When the filtrate was evaporated and cooled to−20^◦C, a white crystalline solid was obtained. The lanthanum–ethylenediamine complex belongs to the triclinic system and crystal- lizes in the space groupP-1 witha=0.9526(2) (nm),b=1.2919(2) (nm),c=1.4077(2) (nm),

H₂N NH₂

N H

H N

NH₂ H₂N N

H₂N

H₂N NH₂

N HN

NH HN H₃C N

N H N NHN H NH₂H₂N

1 2 3

4 5 6

N NH

N N H H N N HN H NH

Figure 4.1 Six representative aliphatic amide type ligands.

α=102.62(1)^◦,β=91.38(1)^◦,γ=98.03(1)^◦,V=1.6713(9) nm³, andZ=2. The structure of the La(1)4(CF3SO3)²⁺cation is shown in Figure 4.2. As can be seen, the central lanthanum ion is nine-coordinate with eight nitrogen atoms from four ethyleneamines and one oxygen from a triflate anion. The coordination geometry can be described as a distorted tricapped trigonal prism. La–N bond lengths range from 0.2692(2) to 0.2741(2) nm with an average of 0.2705(2) nm, and the average ethylenediamine bite angle is 63.8^◦, ranging from 62.8 to 64.6^◦. The interligand N–N distance ranges from 0.315 to 0.341 nm. It seems that a weak hydrogen bonding interaction exists between the hydrogen of NH group and oxygen of the triflate, with the N–O distance ranging from 0.3000(4) to 0.3215(3) nm.

Pr(2)(3)(CF3SO3)3Complex: Raymond and coworkers [4] reported the synthesis and the crystal structure of the Pr(2)(3)(CF3SO3)3 complex. 1 equiv of both 2 and 3 were added simultaneously via two syringes into the suspended acetonitrile solution containing anhydrous Pr(CF3SO3)3. Most of the solid was dissolved after the addition of an appropriate amount of acetonitrile. The solution was then heated to reflux, briefly, and clarified by filtration. The resulting light green clear solution was evaporated to the required volume and cooled to about

−20^◦C for 6 h. After decantation of the solution, the target crystals were obtained. Further- more, additional crystals can be obtained if the mother liquor was cooled overnight again, with a total yield of about 72%. According to the data from the crystal structure, the formula of this complex can be expressed as Pr(2)(3)(CF3SO3)3. It belongs to the triclinic system and crystal- lizes in a space groupP-1 witha=0.9526(1) (nm),b=1.0660(1) (nm),c=1.7080(3) (nm), α=74.28(1)^◦,β=76.91(1)^◦,γ=85.50(1)^◦. The complex is nine-coordinate with eight amine donors and one oxygen donor from a coordinating triflate anion. The coordinated triflate anion is disordered. Figure 4.3 illustrates the labeling diagram for the cation [Pr(2)(3)(CF3SO3)]²⁺ on a schematic drawing of the complex. It is worth noting that the bond lengths and angles in this complex provide some informationwith respect to the appropriate encapsulation bridge lengths as well as typical values for these particular ligands. The tertiary amine Pr–N(1) distance in2is about of 0.2737(7) nm. The bond lengths of the three primary amines (N2, N3,

F2 F1 F3

O3 O2 O1 S1

N1 N4

N3 La1

N5 N7 N6

N8 N2

Figure 4.2 Structure of [La(1)₄CF₃SO₃]²⁺[3]. (Reproduced with permission from H. Paul, P.H. Smith and K.N. Raymond, “A lanthanide-amine template synthesis. Preparation and molecular structures of Ln(L)(CH3CN)(CF3SO3)3[L=1,9-bis(2-aminoethyl)-1,4,6,9,12,14-hexaazacyclohexadecane;

N5 N6 N8

N7 Pr1

N2 N4

N1 O1

O2 S1 O3

F2 F1

Figure 4.3 Structure of [Pr(2)(3)CF₃SO₃]²⁺ [4]. (Reproduced with permission from H. Paul, Z.E.R. Smith, C.W. Lee and K.N. Raymond, “Characterization of a series of lanthanide amine cage complexes,’’Inorganic Chemistry,27, 4154, 1988. © 1988 American Chemical Society.)

N4) Pr–N(2), Pr–N(3), and Pr–N(4) of 2are 0.2634 (7), 0.2642(7), and 0.2685(7) nm, respectively. The average of the three primary amine Pr–N bonds of 2is 0.2654(7) nm, which is significantly shorter than the tertiary amine Pr–N(1) distance. The primary amine (N5, N8), Pr–N(5), Pr–N(8) distances of3are 0.2690(7) and 0.2690(7) nm. The bond lengths of secondary amines (N6, N7), Pr–N(6), Pr–N(7) of 3are 0.2683(7) and 0.2687(7) nm. The average of the two primary amine Pr–N bonds of3is 0.2680 nm, in comparison with 0.2654 nm for the average of the two secondary amine Pr–N bonds of3. Obviously, the average length of the second amine Pr–N bonds of tren [tetradentate amine 2,2,2-tris(2-aminoethyl) amine] [4] (is almost the same as the primary amine Pr–N bonds. The average of all ethylene-bridged N–N distances is 0.268 nm, that for2is 0.287 nm, and that for3is 0.285 nm. The nitrogen hydrogens are involved in a weak hydrogen bonding network to the triflate oxygens.

RE(4)(CF3SO3)3CH3CN [RE=La, Yb; 4=1,9-bis(2-aminoethyl)-1,4,6,9,12,14 hex aza- cyclo hexadecane: With the use of a rare earth ion as a template, the complexes of RE(4)(CF3SO3)3CH3CN complexes (Figure 4.1) [4] can be easily synthesized. For example, La(4)(CF3SO3)3CH3CN was prpeared by the reaction of 2 equiv of 2 with 3 equiv of bis(dimethylamino)methane in the presence of 1 equiv of lanthanum triflate salt in acetonitrile at 70–80^◦C for 8 h. White crystals were obtained (yield 78%) after the reaction mixture was clarified by filtration, evaporated under vacuum, and then purified by repeated crys- tallization. For the complex of Yb(4)(CF3SO3)3CH3CN, although the reaction time extends to 24 h, only about 11% yield was obtained. Using this particular formaldehyde derivative, bis(dimethylamino)methane as a coupling reagent is criticalto the synthesis in this method.

The reaction of this compound with an amine produces dimethylamine, which is volatile and eventually leaves the reaction mixture as a gas. Thus it drives the reaction towards the desired product. No water, which often results in the hydrolysis of rare earths, was produced dur- ing the whole reaction process. Furthermore, the small bite angle of the N–CH2–N moiety favors a high coordination number around the rare earth metal. As can be seen, although

N1 N5

N6 N8

N2 N4

N7 N3

Yb1

(a) (b)

O1 N8 N5 N7 N6

N1 N2

N4 N3

La1

Figure 4.4 Structure of [La(4)(CF₃SO₃)₂]⁺and [Yb(4)(CF₃SO₃)₂]⁺[4]. (Reproduced with permission from H. Paul, Z.E.R. Smith, C.W. Lee and K.N. Raymond, “Characterization of a series of lanthanide amine cage complexes,’’Inorganic Chemistry,27, 4154, 1988. © 1988 American Chemical Society.)

the two complexes have the same composition they adopt significantly different structures.

For [La(4)(CF3SO3)2]CF3SO3CH3CN, the coordination number is ten, however, a nine- coordinate ytterbium ion is found in [Yb(4)(CF3SO3)](CF3SO3)2CH3CN, Figure 4.4a and b.

Owing to the smaller size of Yb³⁺, one of the CF3SO⁻₃ anions was packed outside. This phenomenon is often observed in rare earth coordination chemistry. In other words, the role of the metal in determining the macrocyclic ligand structure was evaluated by comparing [La(4)(CF3SO3)2]CF3SO3CH3CN and [Yb(4)(CF3SO3)](CF3SO3)2CH3CN.

For [La(1)(CF3SO3)2]CF3SO3CH3CN, the ten-coordinated lanthanum complex includes eight amine nitrogens from4 and two oxygens from two triflate anions, Figure 4.4a. The coordination geometry can be described as a bicapped square antiprism. If one ignores the ori- entation of the triflate anions, the complex has a noncrystallographicC2axis passing through the La ion and bisecting the O–La–O angle. The longest La–N distance among the eight La–N bonds comes from the tertiary amine La–N, which is 0.2819(3) and 0.2816(3) nm, respectively.

The bond lengths of primary amine La–N are 0.2669 (4) and 0.2677(7) nm. The secondary amine La–N lengths are 0.2756(3), 0.2701(3), 0.2685(3), and 0.2751(3), respectively. From these data, it can be seen that the longest La–N bond comes from the tertiary amine La–N.

However, the primary amine La–N bond is the shortest one. Clearly, thespace factor plays a cru- cial role. However, the ytterbium ion in [Yb(4)(CF3SO3)2]CF3SO3CH3CN is nine-coordinate with eight amine nitrogens from L and only one oxygen donor from a coordinated triflate anion. The coordination geometry can be described as a monocapped square antiprism, Fig- ure 4.4b. The Yb–N bond lengths range from 0.2442(3) to 0.2611(3) nm with an average of 0.2523 nm. Compared with the La–N bond, the Yb–N distance is slightly larger, which may be indicative of an increased intraligand N–N repulsion due to the shorter Yb–N distance.

In comparison with the complex of [Yb(4)(CF3SO3)2]CF3SO3CH3CN, a more encap- sulated ytterbium complex [Yb(5)(CF3SO3)3] CH3CN [5] was synthesized by a template reaction of 2 equiv of N(CH2CH2NH2)3 with 10 equiv of bis(dimethylamino)methane in the presence of 1 equiv of ytterbium triflate. The most exciting feature of this complex is its

N5 O4 O7 O1

N1 Nd1 N2 N4 N3

Figure 4.5 Structure of Nd(6)CH₃CN(CF₃SO₃)₂[7]. (Redrawn from P. Wei, T. Jin and G. Xu, “Synthesis and crystal structure of neodymium complex of 1-methyl-1,4,7,10-tetraazacyclododecane,’’Acta Chimica Sinica,50, 883, 1992.)

apparent stability towards hydrolysis. With the addition of water, the acetonitrile solution of [Yb(4)(CF3SO3)2]CF3SO3CH3CN forms precipiates of Yb(OH)3 immediately. In contrast, [Yb(5)(CF3SO3)3] CH3CN remains dissolved in water without producing a precipitate of Yb(OH)3.

1-Methyl-1,4,7,10-tetraazacyclododecane: This complex (6, Figure 4.2) was synthesized by Jin and coworkers using a straight synthesis method [6, 7]. By the reaction of 1 equiv of 6 with 1 equiv of a rare earth triflate salt (RE=La, Nd, Gd, and Eu) under N2and in an anhydrous environment in acetonitrile, the corresponding rare earth complexes were success- fully obtained. Except for La(6)CH3CN·(CF3SO3)3·H2O, almost all the rare earth complexes possess the same composition as RE(6)CH3CN·(CF3SO3)3 (RE=Nd, Gd, and Eu). In the structure of Nd(6)CH3CN(CF3SO3)3, the eight-coordinate neodymium ion coordinates with four amine nitrogen atoms from macrocycle ligand 6, one nitrogen atom is from one ace- tonitrole molecule, and three oxygen atoms are from three triflate anions, Figure 4.5. The coordination geometry can be described as a square antiprism. The bond lengths of its triflate anion Nd–O range from 0.2395(3) to 0.2412(4) nm, with an average of 0.2404 nm. The average Nd–N distance of the tetraaza heterocycle Nd–N is 0.2612 nm, ranging from 0.2575 to 0.2661 nm. Owing to theπ–d coordination interaction from the acetonitrile to the lanthanum ion, the acetonitrile Nd–N (0.2564 nm) length is shorter compared with the average Nd–N distance of the tetraaza heterocycle Nd–N. As a result of the larger ionic radius of lanthanum, the coordination number of complex La(6)CH3CN·(CF3SO3)3·H2O is nine with an addition of one molecule water.

Dalam dokumen rare earth coordination chemistry (Halaman 156-161)