Organometallics 2009, 28, 6707–6713 DOI: 10.1021/om9004143
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Half-Sandwich Lanthanide(III) Complexes Coordinated by Two r-Iminopyridine Radical Anions Alexander A. Trifonov,*,† Ivan D. Gudilenkov,† Joulia Larionova,‡ Carlos Luna,§,^ Georgy K. Fukin,† Anton V. Cherkasov,† Andrei I. Poddel’sky,† and Nikolai O. Druzhkov† †
G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, 603600 Nizhny Novgorod GSP-445, Russian Federation, ‡Institut Charles Gerhardt Montpellier, UMR5253 CNRS-UM2-ENSCM-UM1, Chimie Mol eculaire et Organisation du Solide, Universit e Montpellier 2, Pl. E. Bataillon, 34095 Montpellier Cedex 5, France, §Facultad de Ciencias Fı´sico-Matem aticas, Universidad Aut onoma de Nuevo Le on, Avenida Pedro de Alba s/n, San Nicol as de los Garza, 66450 Nuevo Le on, Mexico, and ^Centro de Innovaci on, Investigaci on y Desarrollo en Ingenierı´a y Tecnologı´a, Universidad Aut onoma de Nuevo Le on, Apodaca, 66600 Nuevo Le on, Mexico Received May 19, 2009
The reaction of ytterbocene (C5H4Me)2Yb(THF) with 2 equiv of N-2,6-diisopropylphenylimino2-pyridine (Ipy) occurs with oxidative cleavage of one Yb-Cp bond and oxidation of the ytterbium(II) ion and affords the half-sandwich Yb(III) complex [Yb(C5H4Me){(2,6-i-Pr2C6H3NCH(C5H4N)•-}2], containing two Ipy radical-anions. The indenyl-containing analogue [Yb(C9H7){(2,6-i-Pr2C6H3NCH(C5H4N)•-}2] was unexpectedly obtained in the metathesis reaction of (C9H7)2Yb(μ-Cl)2Li(Et2O)2 with an equimolar amount of [Ipy•-]Kþ instead of the sandwich Yb(III) complex [Yb(C9H7)2{(2,6i-Pr2C6H3NCH(C5H4N)•-}]. The gadolinium complex [Gd(C5Me5){(2,6-i-Pr2C6H3NCH(C5H5N)•-}2] was synthesized by the metathesis reaction of (C5Me5)GdCl2(THF)3 with a 2-fold molar excess of [Ipy•-]Kþ in THF. The structures of all complexes were proved by the X-ray single-crystal diffraction studies. The investigation of magnetic properties of complexes [LnCp{(2,6-i-Pr2C6H3NCH(C5H4N)•-}2] (Cp = C5H4Me, C5Me5, C9H7; Ln = Yb, Gd) in the temperature region 2-300 K revealed the presence of two types of antiferromagnetic interactions between the paramagnetic centers: between the Ln3þ ion and both radicals and between two radicals.
Introduction Redox non-innocent R,R0 -diimines have turned out to be a promising ligation system for transition metals that is widely involved in both academic and applied research. Combinations of these ligands possessing diverse coordination and redox properties with lanthanide metals (especially with ytterbium) have resulted in the development of rich and intriguing coordination chemistry.1 Thus the reductive reactivity of ytterbocenes toward R,R0 -diimines proved to be sterically and electronically tunable; depending on the extent of encumbering of the metal atom coordination sphere and the nature of metal-ligand bonding, these reactions can occur with metal atom
oxidation,2 C-C bond formation,3 and C-H bond activation.3 Moreover, manipulation of steric crowding of the ytterbium coordination sphere allows switching the reductive capacity of ytterbocenes in their reactions with R,R0 -diimines from one- to two-electron reduction.4 An expansion of ytterbocene reductive chemistry due to employment of R,R0 -diimines reveals new, for this field, phenomena: solvent-mediated redox tranformations2b-d and temperature-induced redox isomery.4 R-Iminopyridines that contain conjugated NdC-CdN fragment and also can be considered as diimine ligands until recently5 were not involved in lanthanide chemistry. However the first test of the reactivity of N-2,6-diisopropylphenylimino-2-pyridine (Ipy) with bis(indenyl) ytterbium complex (C9H7)2Yb(THF)2 resulted in the discovery of a new reaction: insertion of a CdN double bond in a formally η5-Ln-C bond.5 Interestingly for the complexes Cp*2Yb(THF)2 (Cp = C13H9, C5Me5) coordinated by bulkier η5-ligands having
*To whom correspondence should be addressed. E-mail: trif@iomc. ras.ru. (1) (a) Trifonov, A. A. Eur. J. Inorg. Chem. 2007, 3151–3167. (b) Walter, M. D.; Berg, D. J.; Andersen, R. A. Organometallics 2007, 26, 2296–2307. (c) Cui, P.; Chen, Y.; Wang, G.; Li, G.; Xia, W. Organometallics 2008, 27, 4013–4016. (d) Vasudevan, K.; Cowley, A. H. Chem. Commun. 2007, 3464–3466. (2) (a) Trifonov, A. A.; Kirillov, E. N.; Bochkarev, M. N.; Schumann, H.; Muehle, S. Russ. Chem. Bull. 1999, 48, 382–384. (b) Trifonov, A. A.; Kurskii, Yu. A.; Bochkarev, M. N.; Muehle, S.; Dechert, S.; Schumann, H. Russ. Chem. Bull. 2003, 52, 601–606. (c) Trifonov, A. A.; Fedorova, E. A.; Ikorskii, V. N.; Dechert, S.; Schumann, H.; Bochkarev, M. N. Eur. J. Inorg. Chem. 2005, 2812–2818. (d) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.; Ikorskii, V. N.; Kurskii, Yu. A.; Dechert, S.; Schumann, H.; Bochkarev, M. N. Russ. Chem. Bull. 2004, 53, 2736–2743.
(3) (a) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.; Druzhkov, N. O.; Bochkarev, M. N. Angew. Chem., Int. Ed. 2004, 5045–5048. (b) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.; Baranov, E. V.; Druzhkov, N. O.; Bochkarev, M. N. Chem.;Eur. J. 2006, 12, 2752–2757. (4) Trifonov, A. A.; Borovkov, I. A.; Fedorova, E. A.; Fukin, G. K.; Larionova, J.; Druzhkov, N. O.; Cherkasov, V. K. Chem.;Eur. J. 2007, 13, 4981–4987. (5) Trifonov, A. A.; Fedorova, E. A.; Borovkov, I. A.; Fukin, G. K.; Baranov, E. V.; Larionova, J.; Druzhkov, N. O. Organometallics 2007, 26, 2488–2491.
r 2009 American Chemical Society
Published on Web 11/10/2009
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Trifonov et al. Scheme 1. Reaction of 1 with Ipy
a different nature of metal-ligand bonding the same reaction results in an oxidative cleavage of the η5-Yb-Cp (Cp = C13H9, C5Me5) bond and formation of homo- [Yb{(2,6-iPr2C6H5NCH(C5H4N)•-}3] or heteroleptic [Yb(C5Me5){(2, 6-i-Pr2C6H3NCH(C5H4N)•-}2] complexes containing radical-anionic iminopyridine ligands.5 Thus it was shown that the pathway of reactions of ytterbocenes with N-2,6-diisopropylphenylimino-2-pyridine is mainly defined by both steric crowding in the coordination sphere of the metal atom and the bonding nature of the π-aromatic ligands bound to ytterbium. The tendency of the π-aromatic ligands coordinated to the ytterbium atom toward haptotropic rearrangements seems to play a crucial role in these transformations. In order to find out which factor predominates in driving these reaction toward insertion or cleavage processes, we investigated the reaction of less sterically crowded ytterbocene (C5H4Me)2Yb(THF) (1) with N-2,6-diisopropylphenylimino-2-pyridine and the metathesis reaction of (C9H7)2Yb(μ-Cl)2Li(Et2O)2 (2) with an equimolar amount of [Ipy•-]Kþ. Herein we report on the synthesis, structures, and magnetic properties of a new series of multispin halfsandwich complexes [LnCp{(2,6-i-Pr2C6H3NCH(C5H4N)•-}2] (Cp = C5H4Me, C5Me5, C9H7; Ln = Yb, Gd).
Results and Discussion It was found that despite the different size of cyclopentadienyl ligands coordinated to ytterbium atoms in starting ytterbocenes Cp2Yb(THF)n (Cp=C5H4Me, C5Me5, n=1, 2) the reaction of 1 with 2 equiv of Ipy occurs in THF similarly to that of the bis(pentamethylcyclopentadienyl) derivative.5 The reaction occurs with oxidative Yb-Cp bond cleavage and oxidation of the ytterbium atom and affords a half-sandwich complex of trivalent ytterbium, [Yb(C5H4Me){(2,6i-Pr2C6H3NCH(C5H5N)•-}2] (3) (Scheme 1). Complex 3 contains two radical-anionic Ipy ligands, which result from the reduction of neutral Ipy molecules: the YbII ion serves as a source for one electron, while the second one is obviously furnished by the C5H4Me anion, which is oxidized to a radical-anion in the course of the reaction. The ability of the related C5Me5 anion to reduce various substrates was reported in recent years by Evans et al.6 A reductive behavior of the C5Me5 anion versus diazabutadienes was also described.5 Recrystallization of the reaction product from a THFhexane mixture (1:5) allowed isolation of complex 3 as a deep green moisture- and air-sensitive crystalline solid in 75% yield. Complex 3 is soluble in THF and toluene but insoluble in hexane. (6) (a) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 2119–2136. (b) Evans, W. J. J. Organomet. Chem. 2002, 647, 2–11.
As mentioned above, the reaction of Ipy with bis(indenyl) ytterbium derivative (C9H7)2Yb(THF)2 resulted in an unprecedented insertion of double CdN bond into the formally η5-Ln-C9H7 bond. Unfortunately until now the mechanism of this insertion remains unclear. Is the tendency for multiple bond insertions a general property of the η5-Ln-C9H7 bond, or is the stage of an electron transfer from Yb(II) to the Ipy ligand necessary to make it feasible? Undoubtedly, the initial act of the reaction is coordination of Ipy to the ytterbium atom and formation of the mixed-ligand complex, while the sequence of electron transfer and insertion processes remains questionable. In order to answer these questions, we envisaged the preparation of sandwich complex (C9H7)2YbIII(Ipy•-) coordinated by radical-anionic iminopyridine ligand to find out if this complex undergoes the Ipy insertion reaction. The metathesis reaction of (C9H7)2Yb(μ-Cl)2Li(Et2O)2 (2) with an equimolar amount of [Ipy•-]Kþ in THF was used as a synthetic approach. Surprisingly it was found that regardless of the reactants’ molar ratio the reaction affords not sandwich, but half-sandwich Yb(III) complex [Yb(C9H7){(2,6-i-Pr2C6H3NCH(C5H5N)•-}2] (4) containing two radical-anionic iminopyridine ligands (Scheme 2). The reaction was carried out at room temperature. Separation of the precipitate of KCl, evaporation of THF, extraction of the solid residue by toluene, and subsequent recrystallization from THF allowed isolation of complex 4 as a deep green moisture- and air-sensitive crystalline solid in 40% yield. Complex 4 is soluble in THF and toluene and insoluble in hexane. For checking the possibility of CdN double-bond insertion into the η5-LnIII-C9H7 bond, the reactions of (C9H7)2Ln(μ-Cl)2Li(Et2O)2 (Ln = Yb, Lu) with equimolar amounts of Ipy were carried out (THF, toluene, 50 C, 24 h). However no evidence for reactions was found since Ipy was quantitatively recovered. Complexes of trivalent ytterbium-containing radical-anionic ligands are promising models for investigation of metal-ligand bonding, which provides grounds for a deeper understanding of the electronic structure of f-metal compounds and contribution of f-electrons in a bonding interaction. Moreover the complexes of ytterbium possessing two stable oxidation states with redox non-innocent ligands are attractive subjects for investigation of possible redox isomeric transformations.7 Unfortunately the interpretation of magnetic behavior of ytterbium complexes containing (7) (a) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102, 4951–4957. (b) Abakumov, G. A.; Nevodchikov, V. I.; Cherkasov, V. K. Dokl. Akad. Nauk SSSR 1984, 278, 641–645. For review articles, see: (c) Evangelio, E.; Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005, 2957–2971. (d) Pierpont, C. G. Coord. Chem. Rev. 2001, 219-221, 415–433. (e) Pierpont, C. G. Coord. Chem. Rev. 2001, 216-217, 99–125. (f) Sato, O; Tao, J.; Zhang, E.-J. Angew. Chem., Int. Ed. 2007, 46, 2152–2187.
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Scheme 2. Synthesis of Complex 4
Scheme 3. Synthesis of Complex 5
three paramagnetic centers is not trivial since the paramagnetic Yb3þ ion possesses a first-order angular momentum that prevents the use of a spin-only Hamiltonian for isotropic exchange and presents considerable difficulty.8 For this reason Yb3þ ions may be replaced by Gd3þ, which has an 8 S7/2 ground state without first-order angular momentum and thus may be anticipated. The gadolinium analogue [Gd(C5Me5){(2,6-i-Pr2C6H3NCH(C5H5N)•-}2] (5) was synthesized by the metathesis reaction of (C5Me5)GdCl2(THF)39 with a 2-fold molar excess of [Ipy•-]Kþ in THF at room temperature (Scheme 3). Separation of the precipitate of KCl and recrystallization of the product from THF allowed isolation of complex 5 as green air- and moisure-sensitive crystals in 57%. Complex 5 is soluble in THF and toluene and insoluble in hexane. Single crystals suitable for X-ray diffraction studies were obtained by slow condensation of hexane into a THF solution of 3, by slow concentration of a THF solution of 4 at room temperature, or by slow cooling of the concentrated THF solution of 5 from 60 C to room temperature. The molecular structures of complexes 3, 4, and 5 are depicted in Figures 1, 2, and 3 respectively, and the crystal and structural refinement data are summarized in Table 1. Complex 4 crystallizes as a solvate with two THF molecules and complex 5 with one THF molecule, while complex 3 does not contain molecules of solvents. The X-ray diffraction studies (Figures 1, 2, and 3) revealed that complexes 3, 4, and 5 have similar structures. The lanthanide atoms in all complexes are coordinated by one Cp (C5MeH4, C9H7, or Cp*) ligand in η5fashion and two iminopyridine ligands, thus resulting in coordination number seven of the central atom. The average Yb-C (Cp) bond lengths in complexes 3 (2.607(3) A˚) and 4 (2.670(2) A˚) are noticeably shorter than the corresponding values in the parent eight-coordinated Yb(II) complexes (8) Benelli, C.; Caneschi, A.; Gatteschi, D.; Guillou, O.; Pardi, L. Inorg. Chem. 1990, 29, 1750–1755. (9) Shen, Q.; Qi, M.; Lin, Y. J. Organomet. Chem. 1990, 399, 247–254.
Figure 1. Molecular structure of complex [Yb(C5H4Me){(2,6-iPr2C6H3NCH(C5H5N)•-}2] (3). Isopropyl groups of iminopyridine ligands are omitted. The terminal ellipsoids correspond to 30% probability. Selected bond lengths (A˚) and angles (deg): C(1)-Yb(1) 2.613(3), Yb(1)-N(4) 2.298(3), Yb(1)-N(2) 2.303(2), Yb(1)-N(1) 2.343(2), Yb(1)-N(3) 2.347(3), Yb(1)-C(5) 2.600(3), Yb(1)C(4) 2.602(3), Yb(1)-C(2) 2.610(3), Yb(1)-C(3) 2.613(3), Yb(1)C(12) 3.120(3), Yb(1)-C(30) 3.126(3), Yb(1)-C(11) 3.167(3), N(1)-C(11) 1.375(4), N(2)-C(12) 1.344(4), C(11)-C(12) 1.412(4), N(3)-C(29) 1.376(4), N(4)-C(30) 1.337(4), C(29)-C(30) 1.412(4), N(4)-Yb(1)-N(3) 71.27(9), N(2)-Yb(1)-N(1) 71.67(9), C(12)N(2)-C(13) 116.8(3), C(30)-N(4)-C(31) 116.8(3).
((C5MeH4)2Yb(DME) 2.723 A˚,10 (C9H7)2Yb(THF)2 2.73 A˚,11 (C9H7)YbI(DME)2 2.803 A˚12) and are comparable to those (10) Jiang, T; Shen, Q.; Lin, Y.; Jin, S. J. Organomet. Chem. 1993, 450, 121–124. (11) Jin, J. Z.; Jin, Z. S.; Chen, W. Q.; Zhang, Y. Chin. J. Struct. Chem. (Jiegou Huaxue) 1993, 12, 241–245. (12) Trifonov, A. A.; Kirillov, E. N.; Dechert, S.; Schumann, H.; Bochkarev, M. N. Eur. J. Inorg. Chem. 2001, 3055–3058.
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Figure 2. Molecular structure of complex [Yb(C9H7){(2,6-i-Pr2C6H3NCH(C5H5N)•-}2] (4). Isopropyl groups of iminopyridine ligands are omitted. The terminal ellipsoids correspond to 30% probability. Selected bond lengths (A˚) and angles (deg): Yb(1)-N(3) 2.280(2), Yb(1)-N(1) 2.281(2), Yb(1)-N(4) 2.345(2), Yb(1)N(2) 2.359(2), Yb(1)-C(40) 2.617(2), Yb(1)-C(39) 2.631(2), Yb(1)-C(38) 2.660(2), Yb(1)-C(41) 2.681(2), Yb(1)-C(37) 2.711(2), Yb(1)-C(31) 3.131(2), Yb(1)-C(13) 3.150(2), Yb(1)C(32) 3.170(2), N(1)-C(13) 1.368(3), N(2)-C(14) 1.372(3), N(3)C(31) 1.369(3), N(4)-C(32) 1.368(3), C(13)-C(14) 1.428(3), C(31)-C(32) 1.436(3), N(3)-Yb(1)-N(4) 72.18(6), N(1)-Yb(1)-N(2) 71.27(6), C(31)-N(3)-C(19) 114.16(17), C(13)-N(1)C(1) 113.81(17).
in related Yb(III) compounds,13 giving evidence of a trivalent oxidation state of the ytterbium atoms in 3 and 4. The noticeable difference of the bond distances between the Yb ion and the C atoms of the five-membered ring in complex 4 indicates a partial distortion of the indenyl ligand coordination toward η3 type. The average Gd-C (Cp*) bond distances in complex 5 (2.679(3) A˚) fall into the region characteristic for previously reported gadolinium cyclopentadienyl complexes.14 Both chelating iminopyridine ligands in 3 and 4 are bound to the ytterbium atom through two coordination Yb-N bonds (3: 2.343(2), 2.303(2), 2.298(3), 2.347(3); 4: 2.2809(17), 2.3588(17), 2.3453(16), 2.2799(18) A˚) whose lengths are close to those formerly reported for related complex [Yb(C5Me5){(2,6-iPr2C6H3NCH(C5H4N)•-}2].5 In the gadolinium complex 5 the Gd-N bond lengths are in the region 2.423(3)-2.445(3) A˚. Resulting five-membered metallacycles in complexes 3-5 are planar with maximum deviation from the YbNCCN plane of 0.06 A˚. The dihedral angles between planes of NCCN fragments in 3-5 are 50.3, 52.5, and 53.2, respectively. The bonding (13) (a) Deacon, G. B.; Forsyth, C. M.; Scott, N. M. Dalton Trans. 2003, 3216–3220. (b) Schumann, H.; Karasiak, D.; Muehle, S. Z. Anorg. Allg. Chem. 2000, 626, 1434–1443. (c) Gilbert, A. T.; Davis, B. L.; Emge, T. J.; Broene, R. D. Organometallics 1999, 18, 2125–2132. (14) (a) Mandel, A.; Magull, J. Z. Anorg. Allg. Chem. 1996, 622, 1913–1919. (b) Gao, F.; Wei, G.; Jin, Z.; Chen, W. J. Organomet. Chem. 1992, 438, 289–295. (15) (a) Gibson, V. C.; O’Reily, R. K.; Wass, D. F.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 2824–2830. (b) Laine, T. V.; Klinga, M.; Leskel€a, M. Eur. J. Inorg. Chem. 1999, 959–964. (c) Laine, T. V.; Piironen, U.; Lappalainen, K.; Klinga, M.; Aitolla, E.; Leskel€a, M. J. Organomet. Chem. 2000, 606, 112–124.
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Figure 3. Molecular structure of complex [Gd(Cp*){(2,6-i-Pr2C6H3NCH(C5H5N)•-}2] (5). Isopropyl groups of iminopyridine ligands are omitted. The terminal ellipsoids correspond to 30% probability . Selected bond lengths (A˚) and angles (deg): Gd(1)-N(4) 2.423(3), Gd(1)-N(2) 2.436(3), Gd(1)-N(3) 2.443(3), Gd(1)-N(1) 2.445(3), Gd(1)-C(38) 2.647(3), Gd(1)-C(39) 2.661(4), Gd(1)-C(37) 2.684(3), Gd(1)-C(41) 2.701(4), Gd(1)-C(40) q2.702(4), N(1)-C(5) 1.396(4), N(2)-C(6) 1.345(5), N(3)-C(23) 1.393(4), N(4)-C(24) 1.333(4), C(23)-C(24) 1.406(5), C(5)-C(6) 1.390(5), N(2)-Gd(1)-N(1) 69.9(1), N(4)-Gd(1)-N(3) 69.39(9), C(24)-N(4)-C(25) 116.0(3), C(6)-N(2)-C(7) 117.0(3). Table 1. Crystallographic Data and Structure Refinement Details for Complexes 3-5
empirical formula fw cryst size, mm T/K space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z calcd density, mg/mm3 absorp coeff, mm-1 Tmin/Tmax F(000) 2θ, deg unique reflns collected (Rint) R1 (I > 2σ(I)) wR2 (all data) parameters goodness-of-fit on F2 largest diff hole and peak, e/A˚3
3
4
5
C42H51N4Yb 784.91 0.12 0.09 0.05 100(2) P1 10.5384(6) 11.1775(7) 17.745(1) 80.645(1) 88.958(1) 63.402(1) 1840.7(2) 2 1.416
C53H67N4O2Yb 965.15 0.15 0.15 0.05 100(2) P1 11.1734(8) 12.2826(9) 17.681(1) 81.317(1) 79.604(1) 76.909(1) 2309.3(3) 2 1.388
C50H67N4OGd 897.33 0.45 0.31 0.08 100(2) P2(1)/c 15.876(1) 13.479(1) 20.954(2) 90 94.348(2) 90 4470.8(6) 4 1.333
2.574
2.069
1.523
0.7475/0.8821 802 60 9752 (0.0300)
0.7466/0.9036 998 52 9010 (0.0204)
0.5472/0.8878 1868 48 6989 (0.0601)
0.0326 0.0750 628 1.059
0.0285 0.0729 587 1.094
0.0420 0.1087 505 1.052
-0.646/1.736
-1.674/1.269
-0.952/1.258
situations within the NCCN fragment of the iminopyridine ligand in 3-5 are noticeably different from those in d-transition metal complexes with neutral Ipy.15 Thus the C-N bonds of former
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Figure 4. X-band EPR spectrum of 3: (top) experimental curve (toluene-CH2Cl2 (5:1) frozen solution at 5 K); (bottom) computer-simulated (g1 = 4.43, g2 = 2.23, g3 = 2.15; line width ΔH1 = 270 G, ΔH2 = 500 G, ΔH3 = 450 G, Lorentzian/ Gaussian = 0.4).
imino groups in complexes 3-5 (3: N(2)-C(12) 1.344(4), N(4)-C(30) 1.337(4); 4: N(1)-C(13) 1.368(3), N(3)-C(31) 1.369(3); 5: N(2)-C(6) 1.345(5), N(4)-C(24) 1.333(4) A˚) are elongated, while the C-C bonds are shortened (3: C(11)C(12) 1.412(4), C(29)-C(30) 1.412(4); 4: C(13)-C(14) 1.428(3), C(31)-C(32) 1.436(3); 5: C(5)-C(6) 1.390(5), C(23)-C(24) 1.406(5) A˚) compared to the appropriate bonds in complexes containing a neutral Ipy ligand ((Ipy)PdCl2: C-N 1.279(4), C-C 1.457(5) A˚; [(Ipy)NiCl2]2: C-N 1.278(3), C-C 1.463(4) A˚).15b The geometric parameters of the iminopyridine ligands in 3-5 give evidence of their reduced radical-anionic character.16 Ytterbium complexes 3 and 4 display rhombic EPR spectra in toluene-CH2Cl2 (1:5) frozen glass at 5 and 8 K, respectively, with large anisotropy of the g tensor (g1 = 4.43, g2 = 2.23, g3 = 2.15 for 3 and g1 = 4.47, g2 = 2.32, g3 = 2.10 for 4) without observable resolved hyperfine structure. The X-band EPR spectrum of 3 at 5 K is shown in Figure 4 (for EPR spectrum of 4 see Supporting Information). The calculated isotropic g values are 2.94 for 3 and 2.96 for 4. The EPR characteristics (high g value, large g anisotropy with concomitant line broadening) signify apparent mixing of metal and ligand π-orbitals in these complexes and, as a result, a considerable contribution of ytterbium metallic orbitals to the singly occupied MO. So, both ytterbium complexes possess a doublet ground state at low temperature (5-8 K), which is attained through antiferromagnetic ligand-ligand or metal-ligand coupling (see below for magnetic studies). Complexes 3 and 4 are EPR silent in the solid state and THF solutions in the temperature range 150-293 K. The fact that an EPR spectrum for these complexes may be observed only at very (15) (a) Gibson, V. C.; O’Reily, R. K.; Wass, D. F.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 2824–2830. (b) Laine, T. V.; Klinga, M.; Leskel€a, M. Eur. J. Inorg. Chem. 1999, 959–964. (c) Laine, T. V.; Piironen, U.; Lappalainen, K.; Klinga, M.; Aitolla, E.; Leskel€a, M. J. Organomet. Chem. 2000, 606, 112–124. (16) For complexes with radical-anionic iminopyridine ligands see: (a) Lu, C. C.; Bill, E.; Weyherm€ uller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008, 130, 3181–3197. (b) Lu, C. C.; De Beer George, S.; Weyherm€ uller, T.; Bill, E.; Bothe, E.; Wieghardt, K. Angew. Chem., Int. Ed. 2008, 47, 6384–6387.
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Figure 5. UV-vis spectra: (1) 3 (THF), (2) 4 (THF), (3) 5 (THF), (4) Ipy (hexane), (5) [Ipy•-]Naþ (THF).
low temperatures can be attributed to a substantial broadening of the signal of iminopyridine radical anions in the field of the paramagnetic YbIII ion and the thermal population of states with higher spin multiplicity as the temperature increases. The EPR spectrum of the gadolinium analogue 5 displays a highly anisotropic complex signal (see Supporting Information) in the temperature region 6-150 K. The UV-vis spectra of complexes 3-5 in THF show strong absorptions at 362, 361, and 371 nm, respectievly, which correspond well with the strong band of [Ipy•-]Naþ in THF at 358 nm (Figure 5), thus providing further proof of the reduced character of the iminopyridine ligands in solution. As previously mentioned, the difficulty in studying the magnetic properties of compounds containing paramagnetic YbIII ions arises from the fact that these ions (with the exception of GdIII) possess a first-order angular momentum, which prevents the use of a spin-only Hamiltonian for isotropic exchange.17 The YbIII ion has a 2F7/2 ground state with a first-order orbital momentum. In the low-symmetry site, this state is split into Stark components by the crystal field, each of them being a Kramers doublet. At high temperature, all the Stark levels are populated, but as the temperature decreases, the effective magnetic moment of the YbIII ion will change by thermal depopulation of the Stark sublevels. For this reason, complex 5 with GdIII and two anion-radicals has been synthesized and its magnetic properties have been investigated. The χT vs T plot for 5 is shown in Figure 6 in the temperature range 2-300 K with an applied magnetic field of 5 kOe. At 300 K, the χT value is equal to 8.59 emu K mol-1; then it slightly decreases as the temperature decreases and below 50 K decreases very rapidly to 2.56 emu K mol-1 at 2 K. The room-temperature value of χT corresponds well to the calculated one for the noninteracting GdIII ion (S = 7/2 with ground state 8S7/2) and two anionradicals (S = 1/2) (8.624 emu K mol-1). As the crystal structure showed that in the solid state there are isolated molecules containing one gadolinium and two radicals, the magnetic behavior was analyzed as due to the three types of interactions: GdIII-radical 1, GdIIIradical 2, and intramolecular radical-radical interactions. (17) Carlin R. L. Magnetochemistry; Springer: Berlin, 1997.
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Figure 6. Temperature dependence of the χT for complex 5 performed with an applied magnetic field of 5 kOe. The solid lines represent the best fit of the data with values in the text.
The exchange Hamiltonian can be written as
H ex ¼ -2
X i
