Acknowledgments
N- Based Rare Earth Complexes
4.2 Rare Earth Complexes with Amide Type Ligands
4.2.2 Rare Earth Complexes with Silyl Amide Type Ligands
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 aver- age 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.
Si N H
Si
7 8
N
NH NH
ButSi Si SiN
But
But
Figure 4.6 Two representative silyl amide type ligands.
For example, according to the salt elimination reaction, some rare earth complexes with silyl amide type ligands can be used to prepare rare earth metallic compounds, which are further used in the homogenous catalytic reaction of C–H, C–C, and C–X bond forma- tion [8]. The most time-honored method to synthesize this type of rare earth complexes involves the reaction of anhydrous rare earth chlorides (usually the metal chloride–THF adduct, THF=tetrahydrofuran) with a theoretical amount of the alkali metal salts of the silyl amide ligands. The rare earth complexes containing the ligand–alkali metal are first obtained, which can be further used as precursors to form the organometal compounds according to the salt elimination reaction. In some cases, the anhydrous rare earth chloride dissolved in diethylether also proved to be of effective for the synthesis of these complexes. The first preparation of the simple rare earth silyl amide complexes Ln(4)3was reported by Bradleyet al. over 30 years ago [9]. Since then, many rare earth complexes with silyl amide type ligands have been prepared. Figure 4.6 shows two representative silyl amide type ligands.
The bis(trimethylsilyl)amido7, a very common ligand, is often used to coordinate with rare earth ions with low coordination number. To date, many homoleptic trivalent and divalent rare earth complexes in the form of {RE[N(SiMe3)2]3}n−[n=0 for RE(III), 1 for (REII)]
have been structurally characterized either in the solid state or in the gas phase for Sc(III), Ce(III), Pr(III), Nd(III), Eu(II), Eu(III), Dy(III), Er(III), Yb(II), and Yb(III) [10]. Usually, the pyramidal arrangement of the central MN3core is the common feature existing in this series of homoleptic rare earth complexes in addition to the analogous U(III) derivative. For example, in complex Eu(7)3the europium ion and three nitrogen atoms are coplanar [10e]. The center ion was found to be disordered between two equivalent positions above and below the plane of the three N atoms. The Si–N–Si plane for each ligand is tilted relative to the normal to the coplane, establishing aD3propeller arrangement for the three N(SiMe3)2ligands. The central europium ion was three-coordinate with three nitrogen atoms from three bis(trimethylsilyl)amido ligands.
The Eu–N bond length is 0.2259 nm and the pyramidal N–M–N angle is 116.6◦, similar to those in the transition metal7complexes.
Through the reaction of europium diiodide with sodium bis(trimethylsilyl)amide7in 1,2- dimethoxyethane (dme), the+2 charged six-coordinate yellow complex Eu(7)2(dme)2was obtained [11]. The whole coordination polyhedron has aC2 symmetry with the two bulky silylamide groups in a manner of surprising closeness to one another, Figure 4.7. This induces an increase in the N–Eu–N angle to 134.5◦, larger than that in complex Eu(7)3. The center atom is six-coordinate and bound to two nitrogen atoms from two bis(trimethylsily1)amido ligands and four oxygen atoms of the dme ligands. The mean Eu–N bond distance is 0.253 (4) nm and the Eu–O distances are 0.2634 (4) and 0.2756 (4) nm, respectively. The ethane carbon atoms in the 1,2-dimethoxyethane ligand are disordered.
Si1A Si2A N1A
O2A
Eu1 O1A
N1 Si2
Si1 O2
O1
Figure 4.7 The structure of complex Eu(7)2(dme)2[11]. (Reproduced with permission from T.D. Tilley, A. Zalkin, R.A. Andersen and D.H. Templeton, “Divalent lanthanide chemistry. Preparation of some four- and six-coordinate bis[(trimethylsilyl)amido] complexes of europium(II). Crystal structure of bis[bis(trimethylsilyl)amido]bis(1,2-dimethoxyethane) europiumII),’’ Inorganic Chemistry, 20, 551, 1981. © 1981 American Chemical Society.)
EuI2 + 2NaN(SiMe3)2
dme hexane
ether toluene
Eu[N(SiMe3)2]2(dme)2
NaEu[N(SiMe3)2]3
Figure 4.8 Synthesis of two types of europium complexes.
It must be pointed out thatwhen ether rather than dme was employed as the solvent, a discrete different anionic complex NaEu(7)3can be isolated after crystallization (Figure 4.8) [10d].
Comparison in the crystal structure of NaEu(7)3with Eu(7)2(dme)2and Eu(7)3is shown in Figure 4.9. As can be seen, the europium complex NaEu(7)3crystallizes in a monoclinic space group and contains two chemically equivalent but crystallographically independent molecules.
In each molecule, the sodium ion, three nitrogen atoms, and the europium ion are coplanar.
The central europium ion was three-coordinate with three nitrogen atoms from three ligands.
However, the nitrogen atom adopts a different mode. The coordination number of N(1) and N(4) is three with three donors from one europium atom and two silicon atoms. The N(2), N(3),
Si1 N1
Si2
Si3 Si5
Si6
Si4 N2
Na1 N3
Eu1
Figure 4.9 The structure of complex NaEu(7)3[10d]. (Reproduced with permission from T.D. Tilley, R.A. Andersen and A. Zalkin, “Divalent lanthanide chemistry. Preparation and crystal structures of sodium tris[bis(trimethylsilyl)amido]europate(II) and sodium tris[bis(trimethylsilyl)amido]ytterbate(II), NaM[N(SiMe3)2]3,’’Inorganic Chemistry,23, 2271, 1984. © 1984 American Chemical Society.)
N(5), and N(6) are four-coordinate due to their additional coordination with one sodium atom.
Compared with the six-coordinated complex Eu(7)2(dme)2, the average Eu–N bond length, 0.2446 nm, is slightly shorter. However, this distance is still 0.0019 nm longer than that in the +3 charged complex Eu(7)3. Similar to a previous report [12], the change in these bond length is in accordance with the change in metal radii as a function of oxidation state and coordination number.
The quadridentate triamidoamines [N(CH2CH2NR)3]3− (R=SiMe3, SiMe2But) have become established as an important class of ligands for the main group metals, transition metals, and actinide elements. Compared with the closely crowded ligand7, the triamidoamine ligands [N(CH2CH2NR)3] (R=SiMe3, SiMe2But) are expected to satisfy a lower steric demand. In 1998, Scott and coworkers reported the synthesis of rare earth complexes (RE=Y, La) with [N(CH2CH2NR)3]3−(R=SiMe2But) ligand8[13]. When the more sterically demanding tri- amidoamines R=SiPri3and SiMePha were used, no product was isolated. The complexes were obtained by the reaction between pure Li3[N(CH2CH2NR)3] (R=SiMe2But) and anhydrous [MCl3(thf)n] in THF. Adopting a similar method, a cerium complex with ligand8in the form of Ce[N(CH2CH2NR)3]3(R=SiMe2But) was also obtained [14]. When this compound was further reacted with a halogen, very different complexes of [{Ce(8)}2(µ-Cl)] (Figure 4.10a), [{Ce(8)}2(µ-Br)], and [Ce(8)I] (Figure 4.10b) were obtained. It has been found that the fas- cinating mixed valent Ce(III/IV) exists in complexes [{Ce(8)}2(µ-Cl)] and [{Ce(8)}2(µ-Br)].
However, the weakest oxidizing agent, iodine, unexpectedly gave the purple cerium(IV) iodide complex [Ce(8)I]. The complex [{Ce(8)}2(µ-Cl)] belongs to a trigonal space groupP-31c with the cell parametersa=1.25843(5) nm,c=2.5433(2) nm,γ=120◦,V=3.488 nm3, and Z=2. In the crystal, the threefold symmetric (triamidoamine)cerium fragments are crystallo- graphically equivalent. The presence of cerium with a mixed-valence in [{Ce(8)}2(µ-Cl)] was proved by the1H NMR result of this complex due to the observation of only one set of nuclear
(a) Si1C
N1A Si1A
Si1B
Ce1A
Si1E Si1D
Si1 N1
N2 N4
N3 Ce1
Si2
Si3 N1D I1
N2A N1 N1E
N2
Si1
Ce1 Cl1 NlB
N1C
(b)
Figure 4.10 The structure of complex (a) [{Ce(8)}2(µ-Cl)] and (b) [Ce(8)I] [14]. (Reproduced with permission from C. Morton et al., ‘Stabilization of cerium(IV) in the presence of an iodide ligand:
remarkable effects of Lewis acidity on valence state,’’Journal of the American Chemical Society,121, 11255, 1999. © 1999 American Chemical Society.)
and magnetic resonances for the triamidoamine ligands between 220 and 300 K. Different to [{Ce(8)}2(µ-Cl)], [Ce(8)I] crystallizes in the monoclinic space groupP21/n.