Long-distance magnetic coupling in dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands

Inorganica Chimica Acta 363 (2010) 1666–1678

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Long-distance magnetic coupling in dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands Jesús Ferrando-Soria a, María Castellano a, Consuelo Yuste a, Francesc Lloret a, Miguel Julve a,*, Oscar Fabelo b,1, Catalina Ruiz-Pérez b, Salah-Eddine Stiriba c, Rafael Ruiz-García c,d, Joan Cano e,f,* a

Departament de Química Inorgànica, Instituto de Ciencia Molecular (ICMol), Universitat de València, 46980 Paterna, València, Spain Departamento de Física Fundamental II, Facultad de Física, Universidad de la Laguna, 38204 La Laguna, Tenerife, Spain Departament de Química Orgànica, Instituto de Ciencia Molecular (ICMol), Universitat de València, 46980 Paterna, València, Spain d Fundació General de la Universitat de València (FGUV), Plaça del Patriarca 4-1, 46002 València, Spain e Departament de Química Inorgànica/Institut de Química Teòrica i Computacional (IQTC) and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain f Institució Catalana de Recerca i d’Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain b c

a r t i c l e

i n f o

Article history: Received 30 September 2009 Accepted 5 December 2009 Available online 21 December 2009 Keywords: Amines Ligand design Copper Polynuclear complexes Magnetic properties Density functional theory calculations

a b s t r a c t Two novel dinuclear copper(II) complexes of formulae [Cu2(tren)2(bpda)](ClO4)4 (2) and [Cu2(tren)2(tpda)](ClO4)4 (3) containing the tripodal tris(2-aminoethyl)amine (tren) terminal ligand and the 4,40 -biphenylenediamine (bpda) and 4,400 -p-terphenylenediamine (tpda) bridging ligands have been synthesized and structurally, spectroscopically, and magnetically characterized. Their experimentally available electronic spectroscopic and magnetic properties have been reasonably reproduced by DFT and TDDFT calculations. Single crystal X-ray diffraction analysis of 2 shows the presence of dicopper(II) cations where the bpda bridging ligand adopts a bismonodentate coordination mode toward two [Cu(tren)]2+ units with an overall non-planar, orthogonal anti configuration of the N–Cu–N threefold axis of the trigonal bipyramidal CuII ions and the biphenylene group. The electronic absorption spectra of 2 and 3 in acetonitrile reveal the presence of four moderately weak d–d transitions characteristic of a slightly distorted trigonal bipyramid stereochemistry of the CuII ions. TDDFT calculations on 2 identify these transitions as those taking place between the four lower-lying, doubly occupied a2 (dyz)2, b2 (dxz)2, b1 (dxy)2, and a1 (dx2 y2 )2 orbitals and the upper, singly occupied a1 (dz2 )1 orbital of each trigonal bipyramidal CuII ion. Variable-temperature magnetic susceptibility measurements of 2 and 3 show the occurrence of moderate (J = 8.5 cm1) to weak intramolecular antiferromagnetic couplings (J = 2.0 cm–1) [H = JS1S2 with S1 = S2 = SCu = ½] inspite of the relatively large copper–copper separation across the para-substituted biphenylene(r = 12.3 Å) and terphenylenediamine (r = 16.4 Å) bridges, respectively. DFT calculations on 2 and 3 support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the dz2 orbital of each trigonal bipyramidal CuII ion through the predominantly p-type orbital pathway of the oligo-p-phenylenediamine bridges, as reported earlier for the parent compound [Cu2(tren)2(ppda)](ClO4)42H2O (1) with the 1,4-phenylenediamine (ppda) bridging ligand. Finally, a rather slow exponential decay of the antiferromagnetic coupling (–J) with the number of phenylene repeat units, –(C6H4)n– (n = 1–3), has been found both experimentally and theoretically along this series of oligo-p-phenylenediamine-bridged dicopper(II) complexes. These results further support the ability of linear p-conjugated oligo-p-phenylene spacers to transmit the exchange interaction between the unpaired electrons of the two CuII centers with intermetallic distances in the range of 7.5–16.4 Å. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction and background

* Corresponding authors. E-mail addresses: [email protected] (M. Julve), [email protected] (J. Cano). 1 Present addresses: Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain; Institut Laue Langevin, 38000 Grenoble, France. 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.12.007

Polymetallic complexes with strong intramolecular electronic interactions have been a topic of large interest in molecular magnetism and molecular electronics [1]. Besides their interest as models for the fundamental research on electron exchange and electron transfer phenomena between distant metal centers through extended bridges, homo- and heterovalent polynuclear

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5+

N

N

n-1

H3N H3N

RuII

H3N

H3N

NH3

NH3

NH3

H3N

NH3

NH3

RuIII

n = 1-3 5+

H3N H3N

RuII

H3N

H3N

NH3 N

N n-1

NH3

NH3

RuIII H3N

NH3

NH3

n = 1-3 Chart 1. General chemical structure of the dinuclear mixed valence pentaammineruthenium(II, III) complexes with oligo-p-phenylenedipyridine bridging ligands illustrating the two octahedral metal redox sites separated by a oligo-p-phenylene spacer made up of several benzene rings as repeat units connected by carbon–carbon single bonds.

-80

100

-1

| J | (cm )

-60

J (cm -1 )

complexes are also of great importance in the ‘‘bottom-up” approach to molecular spintronic devices [2,3]. From a fundamental point of view, electron exchange and electron transfer are intimately related processes that rely on the transmission of spinand charge-based electronic effects, respectively [4]. Following the pioneering work of Creutz and Taube, intervalence electron transfer through extended p-conjugated aromatic bridges has been achieved in dinuclear mixed valence pentaamineruthenium complexes with either meta- or para-substituted oligophenylenedipyridine bridging ligands that allow the variation of the intermetallic distances between 8 and 20 Å (Chart 1) [5]. However, similar examples of long-range magnetic coupling in dinuclear exchange coupled complexes are still scarce when compared with the abundant examples of long-distance intervalence electron transfer in dinuclear mixed valence complexes [6]. Therefore, the design and synthesis of novel bridging ligands which can transmit spin coupling effects over long distances are a major goal in the field, which requires both a skillful organic synthesis and a deep understanding of the electron exchange mechanism. With the advent of calculations based on the density functional theory (DFT) combined with the broken-symmetry (BS) approach, it has been possible not only to reproduce but also to predict the electronic and magnetic properties of dinuclear complexes [7]. Dinuclear copper(II) complexes with one unpaired electron by metal center constitute the simplest systems to investigate electron exchange interactions through extended organic bridges from both experimental and theoretical points of view [8–15]. Typically, the intramolecular magnetic coupling between the two spin doublets of each CuII ion (S1 = S2 = SCu = 1/2) leads to a singlet (S = 0) and a triplet (S = 1) spin states which are separated by J, where J is the magnetic coupling parameter in the phenomenological spin Hamiltonian H = JS1S2. In particular, the research on longdistance magnetic coupling between two CuII ions have been addressed by several groups through the use of simple aromatic diamines and related pyridine- and carboxylate-group substituted derivatives as bridging ligands [16–18], with more or less satisfactory results (Table S1, Supplementary material). Hence, with a few exceptions [18], moderate to strong ferro- or antiferromagnetic couplings (|J| > 5 cm–1) have been observed for this family of structurally characterized dicopper(II) complexes with amine-based aromatic bridges affording rather long intermetallic distances (r > 5 Å) (Fig. 1) [16,17]. Interestingly, the absolute value of J shows an overall exponential decay with the intermetallic distance for each of the two series with meta- and para-substitution patterns

10

1

-40

0.1

-20

5

6

7

8

9

10 11 12 13

r (Å)

0

20 5

10

15

r (Å) Fig. 1. Plot of the experimental values of the magnetic coupling (J) with the intermetallic distance (r) for dicopper(II) complexes with diamine-based aromatic bridging ligands (data from Table S1). The inset shows the exponential decay law of the magnetic coupling with intermetallic distance for the series of dicopper(II) complexes with singly aromatic diamine bridges of meta- (d) and para- (j) substitution patterns. The solid lines correspond to the best-fit curves (see text).

as |J| = 3.02  105 exp(–1.84r) and |J| = 2.83  102 exp(–0.29r), respectively (inset of Fig. 1). Once again, the relative efficiency of para- over meta-substituted aromatic diamine bridges on the long-range magnetic coupling is clearly reflected in the calculated values of the exponential factor (b) of 0.29 and 1.84 Å–1, respectively (solid lines in the inset of Fig. 1). At this respect, the decay law of the magnetic coupling in the series of para-substituted aromatic diamine-bridged dicopper(II) complexes is rather slower than that predicted by Coffman and Buettner [8b] and the more recent ones based on experimental magneto-structural data on dicopper(II) complexes with a variety of p-conjugated aromatic bridging ligands (b = 1.5–1.8 Å–1) [15b]. In their seminal work [16a,b], Felthouse and Hendrickson reported a series of dicopper(II) complexes with p-phenylene- (ppda) and p-biphenylenediamine (bpda) bridges that possess strong to moderate intramolecular antiferromagnetic couplings (J values up to 70.2 and 9.0 cm–1, respectively) in spite of the relatively large intermetallic distances (r values of 7.5 and 12.3 Å, respectively) (Table S1, entries 3–5 and 7–9, respectively). Although they pointed out that the exchange interaction between the CuII ions through the para-substituted phenylene and biphenylene spacers mainly involved r-type orbital pathways, the alternative p-type orbital pathways cannot be totally neglected in the light of our recent results [16c]. In fact, we have reported on the structure and magnetic properties of the related pair of meta-(mpda) and paraphenylenediamine (ppda)-bridged dicopper(II) complexes with the tetradentate tris(2-aminoethyl)amine (tren) tripodal ligand (Scheme 1). In these two model complexes, the threefold axis of the trigonal bipyramidal copper(II)-tren units is perpendicular to the plane of the aromatic phenylenediamino group, owing to their mutual steric repulsion when they are brought closer by metal coordination. The overall orthogonal conformation of the resulting dicopper(II) complexes has allowed for an evaluation of the efficiency of the p- versus r-pathway for the propagation of the exchange interaction between the two unpaired electrons occupying the dz2 -type orbital of each trigonal bipyramidal CuII ion across the extended aromatic bridge. In spite of the relatively large intermetallic separations (r values from ca. 6.4–7.5 Å), moderate ferro- (J = +8.3 cm–1) to strong antiferromagnetic coupling (J = 51.4 cm–1) have been observed for these two dicopper(II) complexes with m- and p-phenylenediamine bridges, respectively (Table S1, entries 1 and 2, respectively). Both effects result from

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a

4+

N

NH2

H2N

N

CuII

CuII

NH2

H2N NH2

NH2

H2N H2N

b

4+

N

NH2

H2N

CuII

N CuII

NH2

H2N NH2

NH2

H2N

4+

N

CuII

H2N NH2

H2N

NH2 NH2

n

2.2.1. [Cu2(tren)2(bpda)](ClO4)4. (2) To a solution of Cu(ClO4)26H2O (0.74 g, 2.0 mmol) in water (15.0 cm3) was added tren (0.3 cm3, 2.0 mmol) under continuous stirring. An acetonitrile solution (10 cm3) of bpda (0.18 g, 1.0 mmol) was then added dropwise to the resulting deep blue reaction mixture. The final deep green solution was filtered off and allowed to evaporate at room temperature in the open air. X-ray quality green prisms of 2 appeared after a few days. They were collected on filter paper and air-dried (0.45 g, yield 45%). Anal. Calc. for C24H48Cl4Cu2N10O16 (2): C, 28.78; H, 4.83; N, 13.98. Found: C, 28.70; H, 4.81; N, 13.93%.

H2N

Scheme 1. Chemical structure of the dinuclear tris(2-aminoethyl)aminecopper(II) complexes with (a) meta- and (b) para-substituted phenylenediamine bridging ligands showing the different spin alignment resulting from the alternance of the spin densities on the phenylene spacer.

NH2

2.2. Preparation

CuII

2.2.2. [Cu2(tren)2(tpda)](ClO4)4. (3) To a solution of Cu(ClO4)26H2O (0.74 g, 2.0 mmol) in dimethylformamide (5.0 cm3) was added tren (0.3 cm3, 2.0 mmol) under continuous stirring. A dimethylformamide solution (7.5 cm3) of tpda (0.26 g, 1.0 mmol) was then added dropwise to the previous deep blue mixture and its color became deep green. Slow vapor diffusion of diethyl ether into the filtered solution gave green needles, which were not-suitable for X-ray diffraction, after one week at room temperature. They were collected on filter paper and airdried (0.33 g, yield 31%). Anal. Calc. for C30H52Cl4Cu2N10O16 (3): C, 33.43; H, 4.86; N, 13.00. Found: C, 33.38; H, 4.79; N, 12.98%.

N

H2N H2N

1-3 (n = 1-3) Chart 2. General chemical structure of the dinuclear tris(2-aminoethyl)aminecopper(II) complexes with oligo-p-phenylenediamine bridging ligands illustrating the two trigonal bipyramidal metal spin-bearing sites separated by an oligo-p-phenylene spacer made up of several benzene rings as repeat units connected by carbon– carbon single bonds.

the spin polarization of the extended p-conjugated bond system of the phenylene spacer with meta- and para-substitution patterns, as confirmed by DFT calculations [16c]. In order to further extend these promising results, we report herein the syntheses, X-ray structural characterization, spectroscopic and magnetic properties of the dinuclear copper(II) complexes of formulae [Cu2(tren)2(bpda)](ClO4)4 (2) and [Cu2(tren)2(tpda)](ClO4)4 (3) [tren = tris(2-aminoethyl)amine, bpda = 4,40 -biphenylenediamine, and tpda = 4,400 -p-terphenylenediamine]. Complexes 2 and 3, together with the parent compound of formula [Cu2(tren)2(ppda)](ClO4)42H2O (1) earlier reported [16c], constitute a unique series of dicopper(II) complexes with oligophenylenediamine bridging ligands of para-substitution pattern and various lengths, H2N–(C6H4)n–NH2 (n = 1–3) (Chart 2). This series offers a unique opportunity to examine the effect of intermetallic distance on the magnetic coupling in dicopper(II) systems with Cu–Cu separations in the range 7.5–16.4 Å. DFT and time-dependent DFT (TDDFT) calculations performed on these model compounds 1–3 have reproduced their electronic spectroscopic and magnetic properties but also have provided a reasonable molecular orbital interpretation about the type of mechanism which is involved in the propagation of the exchange interaction between the two CuII ions across the oligo-p-phenylenediamine bridges.

2.3. Physical techniques Elemental analysis (C, H, N) were performed by the Microanalytical Service of the Universidad Autónoma de Madrid (Spain). FT-IR spectra were recorded on a Nicolet-5700 spectrophotometer as KBr pellets. UV–Vis spectra were recorded on an Agilent Technologies-8453 spectrophotometer, equipped with a UV–Vis Chem Station. Variable-temperature (2.0–300 K) magnetic susceptibility measurements were carried out on powdered samples of 2 and 3 with a SQUID magnetometer under applied magnetic fields of 1.0 T (T P 25.0 K) and 250 G (T < 25.0 K). The susceptibility data were corrected for the diamagnetic contributions of the constituent atoms, the temperature-independent paramagnetism (tip), and the sample holder. 2.4. Crystal structure determination The X-ray diffraction data of 2 were collected with graphitemonochromated Mo Ka radiation using a Bruker-Nonius FR590 CCD area detector diffractometer. Data collection and data reduction were done with the collect and evalccd programs [19]. Empirical absorption corrections were carried out using sadabs [20]. The structure was solved by direct methods and refined with full-matrix least-squares technique on F2 using the SHELXS-97 and SHELXL97 programs [21]. All calculations for data reduction, structure solution, and refinement were done by standard procedures (WINGX) [22]. All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were calculated and refined with isotropic thermal parameters. The final geometrical calculations and the graphical manipulations were carried out with PARST97 and CRYSTAL MAKER programs, respectively [23]. A summary of the crystallographic data for 2 is listed in Table 1. 2.5. Theoretical calculations

2. Experimental and computational details 2.1. Materials All chemicals were of reagent grade quality and they were purchased from commercial sources and used as received.

The molecular geometries of the dinuclear models for 2 and 3 in their anti configuration were not optimized, but their bond lengths and interbond angles were taken from the crystal structure of 2 with an imposed perpendicular orientation of the trigonal axis of each copper atom with respect to the phenylene ring (u = 90°)

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Table 1 Summary of crystallographic data for 2. Formula

C24H48Cl4Cu2N10O16 –1

M (g mol ) Crystal system Spatial group a (Å) b (Å) c (Å) b (°) V (Å3) Z qcalc (g cm–3) F (0 0 0) l (mm–1) T (K) Reflections collected Reflections independent (Rint) Reflections observaation [I > 2r(I)] Data/restraints/parameters R1a [I > 2r(I)] (all) wR2b [I > 2r(I)] (all) Goodness-of-fit (GOF) Sc a b c

H2N

NH2 CuII

N

1001.60 monoclinic P21/n 10.5985(3) 16.0975(5) 11.2990(4) 91.023(2) 1927.41(11) 2 1.726 1032 1.462 293(2) 17299 4606 (0.0519) 3688 4606/0/253 0.0454 (0.0632) 0.1325 (0.1587) 1.117 1.117

NH2

H2N

NH2 NH2

CuII

N

H2N H2N

[CuII2(tren)2(bpda)]4+ (2) Yield: 45%

(a) 2+

N

2

H2N

CuII H2O

NH2

n

[CuII(tren)(H2O)]2+

P P R1 = (|Fo| – |Fc|)/ |Fo|. P P wR2 = [ w(F 2o  F 2c )2/ w(F 2o )2]1/2. P S = [ w(|Fo| – |Fc|)2/(No – Np)]1/2.

NH2

H2N

N H2

L = bpda (n = 2) tpda (n = 3) (b) 4+

and a coplanar conformation of the phenylene rings in the oligo-pphenylene spacers (w = 0°) (see structural discussion). The Ci symmetry constraint was considered in all the calculations for 2 and 3. DFT calculations were carried out on the BS singlet (1Ag) and triplet (3Au) spin states of the Ci-symmetric dinuclear model complexes 2 and 3 with the hybrid B3LYP method [24] combined with the ‘‘broken-symmetry” approach [25], as implemented in the GAUSSIAN 03 program [26], using the triple-f quality basis sets proposed by Ahlrichs and co-workers [27], with one extra p function for the metal atom. The electronic density data were obtained from Natural Bond Orbital (NBO) analysis [28]. TDDFT calculations were performed on the BS singlet (1Ag) and triplet (3Au) spin states of the Ci-symmetric dinuclear model complex 2 as implemented in the GAUSSIAN 03 program [26]. Solvation effects were introduced using a polarizable continuum model (PCM), where the cavity is created via a series of overlapping spheres [29], in order to accurately calculate the energy data of the frontier molecular orbitals (FMOs) as well as the transition energy data and transition strength force constants, which were deduced from transition electric dipole moments. 3. Results and discussion 3.1. Syntheses The cationic dicopper(II) complexes of general formula [CuII2(tren)2L]4+ (L = bpda and tpda) were synthesized by reacting stoichiometric amounts of [CuII(tren)]2+ (prepared in situ from a 1:1 mixture of CuII perchlorate hexahydrate and the aliphatic tetraamine ligand tren) and the corresponding aromatic diamine ligands bpda and tpda either in a water/acetonitrile mixture or in dimethylformamide (Scheme 2). The former method was used for the shortest oligo-p-phenylenediamine ligand (L = bpda) (Scheme 2a), while the latter method was used for the longest one (L = tpda) because of the ligand insolubility (Scheme 2b). They were isolated as the perchlorate salts of formula [Cu2(tren)2(bpda)](ClO4)4 (2) and [Cu2(tren)2(tpda)](ClO4)4 (3) in moderate yields (31–45%). The chemical identity of complexes 2 and 3 was established by elemental analyses, FT-IR and UV–Vis spectroscopies, and magnetic susceptibility measurements. The structure of 2 was further confirmed by single-crystal X-ray diffraction.

H2N

NH2 N

CuII

NH2

H2N

NH2 NH2

CuII

N

H2N H2N

[CuII2(tren)2(tpda)]4+ (3) Yield: 31% Scheme 2. Synthetic route to the dinuclear tris(2-aminoethyl)aminecopper(II) complexes with para-substituted bi- and triphenylenediamine bridging ligands. Reaction conditions: (a) H2O/CH3CN; (b) DMF.

3.2. Description of the structure The crystal structure of 2 consists of centrosymmetric, 4,40 -biphenylenediamine-bridged dicopper(II) complex cations, [Cu2(tren)2(l-bpda)]4+ (Fig. 2), and non-coordinated perchlorate anions. Within the dinuclear entity of 2, the bpda bridging ligand adopts a bismonodentate coordination mode through the two para amino groups toward the two [Cu(tren)]2+ cationic units. This situation is similar to that previously found in the related dinuclear copper(II) complex of formula [Cu2(tren)2(bpda)](NO3)4, which possesses two crystallographically independent, non-centrosymmetric 4,40 biphenylenediamine-bridged dicopper(II) complex cations in the crystal lattice [16a]. The intramolecular Cu(1)  Cu(1)I separation (r) across the 4,40 -biphenylenediamine bridge in 2 is 12.254(5) Å [symmetry code: (I) = 1  x, –y, –z], whereas it averages 12.178(2) Å in [Cu2(tren)2(bpda)](NO3)4 [16a]. Selected bond distances and angles for 2 are listed in Table 2. The two centrosymmetrically-related Cu(1) and Cu(1)I atoms from the dinuclear entity of 2 exhibit a five-coordinated, slightly distorted trigonal bipyramidal geometry. The CuN5 coordination environment is formed by three nitrogen atoms from the primary amino groups of the tren tripodal ligand in the equatorial plane, the axial positions being occupied by a nitrogen atom from the tertiary amino group of the tren tripodal ligand and the primary amino group of the bpda bridging ligand [Fig. 2(a)]. The average value of the axial metal–ligand bond lengths [Cu–Nax = 2.042(3) Å] is slightly shorter than that of the equatorial ones [Cu–Neq = 2.088(3) Å], as previously found in [Cu2(tren)2(bpda)](NO3)4

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Fig. 2. (a) Perspective view of the cationic dinuclear unit of 2 with the atomnumbering scheme [symmetry code: (I) = –x, –y, –z]. (b) Front and (c) top views of the dinuclear skeleton.

Table 2 Selected bond distances (Å) and angles (°) for 2.a,b

a b

Metal environment Cu(1)–N(1) Cu(1)–N(3) Cu(1)–N(5)

2.040(3) 2.082(3) 2.123(3)

Cu(1)–N(2) Cu(1)–N(4)

2.043(3) 2.060(3)

N(1)–Cu(1)–N(2) N(1)–Cu(1)–N(4) N(2)–Cu(1)–N(3) N(2)–Cu(1)–N(5) N(3)–Cu(1)–N(5)

178.17(12) 93.48(12) 84.59(12) 83.69(12) 109.24(12)

N(1)–Cu(1)–N(3) N(1)–Cu(1)–N(5) N(2)–Cu(1)–N(4) N(3)–Cu(1)–N(4) N(4)–Cu(1)–N(5)

96.01(11) 97.72(11) 84.80(13) 128.69(14) 119.19(14)

4,40 -p-Biphenylenediamine ligand N(1)–C(1) 1.444(4) C(1)–C(2) 1.380(5) C(2)–C(3) 1.378(5) C(3)–C(4) 1.396(4)

C(4)I–C(4) C(4)–C(5) C(5)–C(6) C(6)–C(1)

1.483(6) 1.389(5) 1.389(5) 1.380(5)

N(1)–C(1)–C(2) N(1)–C(1)–C(6) C(1)–C(2)–C(3) C(2)–C(3)–C(4) C(6)–C(1)–C(2)

C(4)I–C(4)–C(3) C(4)I–C(4)–C(5) C(3)–C(4)–C(5) C(4)–C(5)–C(6) C(5)–C(6)–C(1)

122.2(3) 121.5(3) 116.5(3) 122.4(3) 119.4(3)

120.8(3) 119.5(3) 120.2(3) 121.8(3) 119.6(3)

Estimated standard deviations are given in parentheses. Symmetry code: (I) = 1 – x, –y, –z.

[Cu–Nax = 2.0429(3) Å and Cu–Neq = 2.1017(3) Å] [16a]. The value of the axial metal–ligand bond angle [Nax–Cu–Nax = 178.16(11)°] is close to that expected for an ideal trigonal bipyramid (a = 180°), while the values of the equatorial metal–ligand bond angles [Neq–Cu–Neq = 109.24(14)–128.68(15)°] deviate slightly from that expected for an ideal trigonal bipyramid (b = 120°). The value of 0.82 for the trigonality parameter (s) also reflects this small distortion of the trigonal bipyramidal metal environment toward square pyramidal, so-called Berry pseudorotation [s = (a  b)/60 = 1 and 0 for ideal trigonal bipyramid and square pyramid, respectively] [30]. At this regards, the s value in 2 is somewhat smaller than those in [Cu2(tren)2(bpda)](NO3)4 (s = 0.95 and 0.98) [16a]. The bpda bridging ligand in 2 is strictly planar for symmetry considerations, the twist angle (w) between the two centrosymmetrically-related phenylene rings of the 4,40 -biphenylene spacer being 0°. In contrast, the two phenylene rings are slightly twisted around the carbon–carbon single bond in [Cu2(tren)2(bpda)](NO3)4 [w = 14.4(1)–22.5(1)°] [16a] as a result of the repulsive interactions between the ortho hydrogen atoms in the 4,40 -biphenylene spacer,

disfavoring thus the p-electron delocalization within the non-planar bpda bridging ligand. Hence, the value of 1.483(6) Å for the central C–C bond distance in 2 is slightly shorter than those in [Cu2(tren)2(bpda)](NO3)4 [1.4896(2) and 1.5409(2) Å] [16a], showing thus a small but non-negligible contribution from the quinoid canonical form in the planar bpda bridging ligand. When acting in a bismonodentate coordination mode through the two amino donor groups, the coordinating nitrogen atoms of the bpda bridging ligand adopt a sp3 hybridization [Fig. 2(a)]. In fact, the value of 114.8(2)° for the Cu–N–C bond angle (c) in 2 is close to that expected for an ideal sp3 hybridization (c = 109°), being within the range of those in [Cu2(tren)2(bpda)](NO3)4 [c = 111.9(2)– 117.9(2)°] [16a]. This situation minimizes the delocalization of the lone electron pair of the coordinated amino groups within the p-electron system of each aromatic phenylene ring. Indeed, a p–p interaction (N–C conjugation) between the pz-type orbitals of the nitrogen and carbon atoms within the p-system of the aromatic oligo-p-phenylenediamino bridge would occur if the coordinating nitrogen atoms of the bpda bridging ligand would adopt a sp2 hybridization. Thus, the value of 1.444(4) Å for the N–C bond distance of the coordinated amino groups in 2 is comparable to those found in [Cu2(tren)2(bpda)](NO3)4 [1.4272(2)–1.4565(2) Å] [16a], as expected for a single nitrogen–carbon bond in a benzenoid canonical form of the bpda bridging ligand. Interestingly, the two trigonal axis of each copper atom are disposed toward opposed sides of the bpda bridging ligand and they are oriented almost perpendicularly with respect to the mean plane of the phenylene rings [Fig. 2(b) and (c)]. This leads to an overall non-planar anti configuration for the entire dinuclear entity of 2, as previously reported for [Cu2(tren)2(bpda)](NO3)4 [16a]. The value of 79.1(3)° for the Cu–N–C–C torsion angle (u) in 2 deviates larger from an ideal orthogonal disposition (u = 90°) than those in [Cu2(tren)2(bpda)](NO3)4 [u = 83.46(3)–89.97(3)°] [16a]. This situation contrasts with that found in 1, where the entire dinuclear entity with an approximate C2 molecular symmetry adopts a nonplanar syn configuration with the two trigonal axis of each copper atom being disposed toward the same side of the ppda bridging ligand and perpendicularly to the phenylene ring [u = 80.9(2)– 86.5(2)°] [16c]. In this case, the syn configuration is favored over the alternative anti configuration because of the presence of intramolecular hydrogen bonds between the coordinated amine-nitrogen atoms of each terminal tren ligand through the intermediacy of the oxygen atoms from the perchlorate anions. In the crystal lattice of 2, the cationic dicopper(II) entities are well separated from each other by the perchlorate counteranions (Fig. 3). In fact, the occurrence of intermolecular p–p stacking interactions between the aromatic phenylene rings of neighboring dinuclear entities is precluded because of their anti configuration. Yet, there is a variety of intermolecular hydrogen bonds involving the oxygen atoms from the perchlorate anions and the coordinated amine-nitrogen atoms from both the bpda and the tren ligands [O  N = 3.047(4)–3.079(6) Å] [Fig. 3(a)]. The shortest intermolecular Cu(1)  Cu(1)II separation across the hydrogen-bonded perchlorate anions in 2 is 7.4102(5) Å for two neighboring dinuclear entities along the c axis [symmetry code: (II) = 2 – x, –y, 1 – z] [Fig. 3(b)]. 3.3. Infrared spectra The most relevant absorption bands in the IR spectra of the dinuclear copper(II) complexes 1–3 are listed in Table 3, together with those of the ppda, bpda, and tpda free ligands for comparison. Complexes 1–3 show the characteristic IR bands due to the N–H and C–H stretching vibrations of the tren ligand in the range of 3256–3345 and 2887–2962 cm–1, respectively. These bands obscure almost completely those corresponding to the ppda, bpda,

J. Ferrando-Soria et al. / Inorganica Chimica Acta 363 (2010) 1666–1678

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Fig. 3. (a) Perspective view of the asymmetric unit of 2. (b) Projection view of the crystal packing along the b axis. Hydrogen bonds are represented by dotted lines.

and tpda bridging ligands, which appear in the range of 3184–3549 (N–H stretching) and 3008–3030 cm–1 (C–H stretching) in the IR spectra of the free ligands (Table 3). In addition, complexes 1–3 show unique IR bands in the range of 1597–1614 and 1492–1519 cm–1 corresponding to the C@C and C–N stretching vibrations of the ppda, bpda, and tpda bridging ligands, together with a weak IR band at 1475 cm–1 which is assigned to the C–N stretching vibrations of the tren ligand (Table 3). Interestingly, the single IR band corresponding to the m(C–N) vibrational modes of the oligo-p-phenylenediamine bridging ligands for 1–3 is significantly shifted to lower frequencies from 1519 (1) to 1500 cm–1 (2), and then to 1492 cm–1 (3), a phenomenon which is also observed for the free ligands where it appears at 1516 (ppda), 1500 (bpda), and 1490 cm–1 (tpda) (Table 3). This batochromic shift of the C–N stretching vibrations suggests that

the delocalization of the lone electron pair of the amino donor groups into the oligo-p-phenylene spacer becomes more difficult upon increasing the number of phenylene units (n) from 1 to 3 along this series. This fact points to the occurrence of a r–p interaction (N–C hyperconjugation) within the p-electron system of the oligo-p-phenylenediamine bridges which involves the sp3-type orbitals of the nitrogen atoms that are orientated nearly parallel to the pz-type orbitals of the carbon atoms because of the non-planar orthogonal disposition of the trigonal axis of each CuII ion with respect to the mean plane of the benzene rings (u  90°) as evidenced by the crystal structure of 2. On the other hand, the main IR band corresponding to the m(C@C) vibrational modes of the oligo-p-phenylendiamine bridging ligand located at 1597 (1), 1597 (2), and 1607 cm–1 (3) are varyingly shifted to lower frequencies relative to that of the free ligands

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Table 3 Selected infrared spectroscopic data for 1–3 and the corresponding free ligands.a.

m(C–H) (cm–1)

m(C@C) (cm–1)

m(C–N) (cm–1)

1

3338s 3300s 3256s 3169s

2962w 2925w 2887w

1597s

1519s 1475w

2

3342s 3289s 3136m

2926w 2893w

1614(sh) 1597s

1500s 1475w

3

3345s 3281s 3126m

2924w 2895w

1607s

1492s 1475w

3374s 3303m 3199m

3008w

3431m 3394m 3322s 3203m

3030w

3549m 3385m 3283m 3184s

3026w

ppda

bpda

tpda

a

90000

ε (M-1 cm-1)

m(N–H) (cm–1)

1600

(a)

1200

(c)

60000

800

(b)

30000

ε (M-1 cm-1)

Complex

120000

400

(b) (a) (c)

1629s

1516s

0 200

1625(sh) 1605s

1500s

1607s

1490s

300

400

500

600

700

800

0 900

λ (nm) Fig. 4. Electronic absorption spectra of 1–3 in acetonitrile [curves (a–c), respectively].

In solid KBr.

centered at 1629 (ppda), 1605 (bpda), and 1607 cm–1 (tpda) (Table 3), reflecting thus a larger degree of p-conjugation within the aromatic benzene rings of the oligo-p-phenylene spacer upon coordination. The gradual decrease in the batochromic shift of the C@C stretching vibrations for 1–3 relative to their free ligands is likely explained in terms of a weaker metal–ligand coordinative interaction, as expected for the decrease in the basicity of the amino donor groups upon increasing the number of phenylene units (n) from 1 to 3 along this series. 3.4. Electronic spectra The electronic absorption spectra of the dicopper(II) complexes 2 and 3 in acetonitrile are shown in Fig. 4, together with that of the parent complex 1 for comparison. The most relevant bands in the electronic absorption spectra of this series of dinuclear copper(II) complexes 1–3 are listed in Table 4. Complexes 1–3 show two distinct bands in the UV region: an intense and sharp band centered at 200 nm (1) and 205 nm (2 and 3), together with a less intense and broad band located at 250 (1), 287 (2), and 313 nm (3) (Table 4). These two UV bands whose intensity vary almost proportionally with the number of phenylene units (n) in the oligo-p-phenylenediamine bridging ligand are commonly assigned to intraligand (IL) p–p* transitions (Fig. 4). Thus, the observed batochromic shift in the position of the low-energy UV band for 1–3 is as expected for an increase in the effective p-conjugation length upon increasing the number of phenylene units (n) from 1 to 3 along this series [31]. On the other hand, the broadness of the less intense, low-energy UV band for 1, with a distinct shoulder at 290 nm (Table 4), indicates the overlapping with the ligand-to-metal charge transfer (LMCT) transitions corresponding to the r(N)–d and p(N)–d excitations from the amino groups of the tren and bpda ligands coordinated to the CuII ions, respectively [Fig. 4, curve (a)] [16c]. These r(N)–d and p(N)–d LMCT transitions would be masked by the more intense, red-shifted p–p* IL transitions for 2 and 3 [Fig. 4, curves (b) and (c), respectively]. In addition, 2 and 3 show several band features of moderately weak intensity in the visible and NIR regions of the electronic absorption spectra [Fig. 4, curves (b) and (c), respectively]. These are the characteristic spin-allowed d–d transitions of trigonal bipy-

ramidal CuII ion (d9 electronic configuration). They appear as two distinct visible shoulders in the low-energy tail of the UV band centered at 370 and 455 nm for 2 or around 390 and 460 nm for 3, alongside a NIR maximum located at either 815 (2) or 820 nm (3) with a distinct visible shoulder at 655 (2) or 660 nm (3) which is responsible for the green color of the acetonitrile solutions of 2 and 3 (Table 4). By comparison, they appear as two well-resolved NIR and visible bands located at 805 and 640 nm for 1, together with a relatively strong visible band centered at 453 nm (Table 4). Interestingly, the observed d–d bands are overall shifted to lower energies for 1–3, indicating thus a weakening of the ligand field strength afforded by the coordinated amino groups of the oligop-phenylenediamine bridging ligand upon increasing the number of phenylene units (n) from 1 to 3 along this series. Alternatively, the high-energy visible band of 1 may have an important contribution from p(N)–d LMCT transitions from the ppda ligand [16c], accounting thus for the larger intensity of this band when compared to that of 2 and 3 (Table 4). This may be explained in terms of the stronger overlap between the p-type orbitals of the p-phenylenediamine bridging ligand and the singly occupied dz2 -type orbital of each CuII ion. As a matter of fact, only two d–d bands are expected for a trigonal bipyramidal CuII ion in ideal D3h symmetry corresponding to the individual transitions from each of the two pairs of lower-lying, doubly occupied degenerate orbitals, e00 (dxz, dyz)2 and e0 (dxy, dx2 y2 )2, to the upper, singly occupied a10 (dz2 )1 orbital [i.e., 2E00 (dxz, dyz) 2A10 (dz2 ) and 2E0 (dxy, dx2 y2 ) 2A10 (dz2 ) transitions, respectively [32]. When the local symmetry of the CuII ion is lowered from D3h to C2v due to a slight distortion of the trigonal bipyramidal environment toward square pyramidal (Berry pseudorotation) as evidenced by the crystal structure of 2, the degeneracy of the e00 (dxz, dyz) and e0 (dxy, dx2 y2 ) orbitals is lifted and they split into the b2 (dxz), a2 (dyz), b1 (dxy), and a1 (dx2 y2 ) orbitals (Scheme 3) [32]. Among the four possible transitions between these four lower-lying, doubly occupied b2 (dxz)2, a2 (dyz)2, b1 (dxy)2, and a1 (dx2 y2 )2 orbitals and the upper, singly occupied a1 (dz2 )1 orbital [i.e., 2B2 (dxz) 2A1 (dz2 ), 2A2 (dyz) 2A1 (dz2 ), 2B1 (dxy) 2A1 (dz2 ), and 2 A1 (dx2 y2 ) 2A1 (dz2 ) transitions, respectively], the first one is electric dipole forbidden, but it may occur with low intensity via vibronic coupling [32], explaining thus the four d–d bands observed in the electronic absorption spectra of 2 and 3. DFT and TDDFT calculations on the BS singlet (1Ag) and triplet (3Au) spin states of the Ci-symmetric dinuclear model complex 2, as a representative example of this series, give further support to the previous assignment of the experimental electronic transitions for 1–3. Selected calculated molecular orbital energy data and

J. Ferrando-Soria et al. / Inorganica Chimica Acta 363 (2010) 1666–1678 Table 4 Selected electronic spectroscopic data for 1–3.a,b

a b

Complex

kmax(p–p) (nm)

kmax(r–d) (nm)

kmax(p–d) (nm)

kmax(d–d) (nm)

1

200 (50 700) 250 (19 900)

290(sh) (10 300)

453 (1490)

640 (55) 805 (100)

2

206 (71 200) 287 (49 100)

370(sh) (520) 455(sh) (230) 655(sh) (125) 815 (215)

3

206 (107 000) 313 (80 800)

390(sh) (245) 460(sh) (80) 660(sh) (190) 820 (335)

In acetonitrile solution. The molar extinction coeficients (e) are given in parentheses (M–1 cm–1).

z 105° y x D3h

a1' (dz2)

C2v

a1 (dz2) a1 (dx2-y2)

e' (dxy, dx2-y2)

e” (dyz, dxz)

b1 (dxy) a2 (dyz) b2 (dxz)

Δ' δ'

Δ"

δ''

Scheme 3. Simplified energy level diagram of the splitting of the d-type metal orbitals of a trigonal bipyramidal CuII complex with the degree of Berry pseudorotation distortion.

transition energy data for the BS singlet (1Ag) and triplet (3Au) spin states of 2 are listed in Tables S2–S5 (Supplementary material). The energy level diagram of the FMOs for the BS singlet (1Ag) and the triplet (3Au) spin states of 2 is typical of a weakly coupled trigonal bipyramidal dicopper(II) complex with a terminal aliphatic tetramine ligand of large r-field strength like tren and a bridging aromatic diamine ligand of distinctly p donating nature such as bpda (Figs. 5 and 6, respectively). Overall, the order of levels for the symmetric and antisymmetric pairwise combinations of the five d-type orbitals of the two trigonal bipyramidal CuII ions is as depicted in Scheme 3: [a1g (dz2 )S, a1u (dz2 )AS] >> [a1g (dx2 y2 )AS, a1u (dx2 y2 )S] > [b1g (dxy)AS, b1u (dxy)S] >> [a2g (dyz)S, a2u (dyz)AS] > [b2g (dxz)S, b2u (dxz)AS].2 For both the BS singlet (1Ag) and the triplet (3Au) spin states of 2, the pair of upper, singly occupied orbitals [a1g (dz2 )1S, a1u (dz2 )1AS] are well above the two pairs of lower-lying, doubly occupied orbitals [a1g (dx2 y2 )2S, a1u (dx2 y2 )2AS] and [b1g (dxy)2S, b1u (dxy)2AS], which are in turn well separated from the two pairs of lowest, doubly occupied orbitals [a2g (dyz)2S, a2u (dyz)2AS] and [b2g (dxz)2S, b2u (dxz)2AS] (Fig. 5 and 6). Thus, for instance, the va-

2 Strictly speaking, there exist only two label types, noted ag and au, for the symmetric and antisymmetric orbitals of the triplet (3Au) spin state of 2 in the Ci point group. Of course, there are neither symmetric nor antisymmetric orbitals for the BS singlet (1Ag) spin state of 2. However, for both the BS singlet (1Ag) and triplet (3Au) spin states of 2, we will refer to the orbital symmetry labels of the distorted trigonal pyramidal CuII ion in the C2v local symmetry within this paper for simplicity.

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lue of the energy gap (D0 ) between the [a1g (dz2 )S, a1u (dz2 )AS] and the [b1g (dxy)S, b1u (dxy)AS] orbitals pair for the spin-down (b) manifold of the triplet spin state is 37107 cm–1, while the value of the energy gap (D00 ) between the [a1g (dz2 )S, a1u (dz2 )AS] and the [b2g (dxz)S, b2u (dxz)AS] orbitals pair is 52035 cm–1 (Table S3). On the other hand, there is a relatively important energy gap (d0 ) between the two pair of orbitals [b1g (dxy)S, b1u (dxy)AS] and [a1g (dx2 y2 )S, a1u (dx2 y2 )AS] originating from the splitting of the [eg0 (dxy, dx2 y2 )S, eu0 (dxy, dx2 y2 )AS] level for both the BS singlet (1Ag) and the triplet (3Au) spin states of 2 (Fig. 5 and 6). This situation contrasts with the smaller energy gap (d00 ) between the two pair of orbitals [b2g (dxz)S, b2u (dxz)AS] and [a2g (dyz)S, a2u (dyz)AS] originating from the splitting of the [eg” (dxz, dyz)S, eu” (dxz, dyz)AS] level for both the BS singlet (1Ag) and the triplet (3Au) spin states of 2 (Fig. 5 and 6). Thus, for instance, the value of d0 for the spindown (b) manifold of the triplet state is 2915 cm–1, while that of d” is only of 957 cm–1 (Table S3). Interestingly, there exists a highest-lying, doubly ocuppied ligand p bonding orbital, pc, which is close in energy to the lowest-lying, unoccupied ligand p antibonding orbital, pd*, for both the BS singlet (1Ag) and the triplet (3Au) spin states of 2 (Fig. 5 and 6). In addition, there are two nearly degenerate lower-lying, doubly occupied ligand p bonding orbitals, pa and pb, which are well separated from the aforementioned pd* orbital for both the BS singlet (1Ag) and the triplet (3Au) spin states of 2 (Fig. 5 and 6). Thus, for instance, the value of the energy gap between the pc and pd* orbitals for the spin-down (b) manifold of the triplet state is 39916 cm–1, while that between the (pa, pb) and pd* orbitals is 50481 cm–1 (Table S3). The pc orbital has a small contribution from the dz2 (M) orbitals, whereas the (pa, pb) and pd* orbitals have no contribution at all from the d-type metal orbitals (Fig. 6, left). The calculated energies and the nature of the lowest vertical excitations retaining the spin multiplicity of the BS singlet (1Ag) and triplet (3Au) spin states for 2 (Tables S4 and S5) provide a reasonable MO interpretation of the experimental electronic absorption spectra of 1–3. The most remarkable feature of the calculated spectrum of 2 are the low-energy LMCT transitions from the higher-lying, doubly occupied pc ligand orbital to the upper, singly occupied symmetric and antisymmetric combinations of the dz2 metal orbitals, which are located at 481 and 475 nm for the BS singlet spin state and at 480 and 451 nm for the triplet spin state (Tables S4 and S5, transitions 9 and 10). In addition, there exist several high-energy LMCT transitions from the lower-lying, doubly occupied pa and pb ligand orbitals to the upper, singly occupied symmetric and antisymmetric combinations of the dz2 metal orbitals, which are located in the ranges of 305–337 and 305–340 nm for the BS singlet and triplet spin states respectively (Tables S4 and S5, transitions 12–15), together with a low-energy IL transition from the higher-lying, doubly occupied pc ligand orbital to the unoccupied ligand pd* ligand orbital located at 381 and 382 nm for the BS singlet and triplet spin states, respectively (Tables S4 and S5, transition 11). All these LMCT and IL transitions are well-resolved in the experimental spectrum of 1, appearing as two distinct UV bands at 250 and 453 nm with a shoulder at 290 nm (Fig. 4 and Table 4). However, they appear as a unique UV band envelope centered at 287 and 313 nm for 2 and 3 respectively, which extends into the visible region (Fig. 4 and Table 4). On the other hand, the higher energy d–d transitions from the lower-lying, doubly occupied symmetric and antisymmetric combinations of the dxz and dyz metal orbitals to the upper, singly occupied symmetric and antisymmetric combinations of the dz2 metal orbitals are located at 526 and 538 nm respectively, for both the singlet [1B2g (dxz) 1A1g (dz2 ) and 1A2g (dyz) 1A1g (dz2 ) transitions, respectively] and the triplet [3B2u (dxz) 3A1u (dz2 ) and 3A2u (dyz) 3 A1u (dz2 ) transitions, respectively] spin states (Tables S4 and S5,

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0 d*

d*

(z2)S

(z2)S

Energy (au)

-0.1

-0.2 c

-0.3

(z2)AS (x2-y2)AS a, b (xy)S, (xy)AS (x2-y2)S (yz)S (xz)S (yz)AS (xz)AS

-0.4

c

(z2)AS (x2-y2)AS a, b (xy)S, (xy)AS (x2-y2)S (yz)S (xz)S (yz)AS (xz)AS

Fig. 5. Partial energy level diagram of the selected frontier MOs for the BS singlet spin state of 2 (data from Table S2). The isoelectronic surface corresponds to a value of 0.020 e bohr–3.

0 d*

d*

-0.1

Energy (au)

(z2)S, (z2)AS

-0.2 c c

(z )S, (z )AS a, b (x2-y2)S, (x2-y2)AS (xy)S, (xy)AS 2

-0.3

-0.4

2

(xz)S (yz)S, (yz)AS (xz)AS

(x2-y2)S, (x2-y2)AS a, b (xy)S, (xy)AS (yz)S, (yz)AS (xz)S, (xz)AS

Fig. 6. Partial energy level diagram of the selected frontier MOs for the triplet spin state of 2 (data from Table S3). The isoelectronic surface corresponds to a value of 0.020 e bohr–3.

transitions 5–8). Although these d–d transitions are obscured by the presence of the aforementioned low-energy LMCT band in the experimental spectrum of 1, they appear as two distinct shoulders in the low-energy tail of this band centered at 370 and 455 nm for 2 and at 390 and 460 nm for 3 (Fig. 4 and Table 4). The two remaining lower energy d–d transitions from the lower-lying, doubly occupied symmetric and antisymmetric combinations of the dxy and dx2 y2 metal orbitals to the upper, singly occupied symmetric and antisymmetric combinations of the dz2 metal orbitals are located at 735 and 872 nm for the BS singlet spin state [1B1g (dxy) 1A1g (dz2 ) and 1A1g (dx2 y2 ) 1A1g (dz2 ) transitions, respectively] and at 736 and 873 nm for the triplet spin state [3A1u (dx2 y2 ) 3A1u (dz2 ) and 3B1u (dxy) 3A1u (dz2 ) transitions] (Tables S4 and S5, transitions 1–4). These d–d transitions appear as two distinct bands centered at 640 and 805 nm in the experimental spectrum of 1, respectively, while they appear as a maximum centered at 815 and 820 nm with a distinct visible shoulder at 655 and 660 nm for 2 and 3, respectively (Fig. 4 and Table 4). 3.5. Magnetic properties The magnetic behavior of 2 and 3 in the form of the vMT and vM versus T plots (vM being the molar magnetic susceptibility per

dinuclear unit and T the temperature) is typical of moderate (2) to weak (3) antiferromagnetically coupled CuII2 pairs (Fig. 7). At room temperature, the values of vMT are 0.78 (2) and 0.79 cm3 mol1 K (3), values which are close to that expected for two magnetically isolated CuII ions [vMT = 2  (Nb2gCu2/3k)SCu(SCu + 1) = 0.83 cm3 mol–1 K with SCu = 1/2 and gCu = 2.1]. Upon cooling, vMT remains constant until ca. 100 (2) and 50 K (3), and then it decreases abruptly to reach vMT values of 0.01 (2) and 0.43 cm3 K mol–1 (3) at 2.0 K. In addition, vM shows a maximum at 7.5 K for 2 (inset of Fig. 7) which unambiguously supports the occurrence of a ground singlet (S = 0) spin state resulting from the intramolecular antiferromagnetic interaction between the two local spin doublets (SCu = 1/2) of each CuII ion. The analysis of the magnetic susceptibility data of 2 and 3 was carried out through the spin Hamiltonian for a dimer model, H = –JS1S2 + gb(S1 + S2)H (with S1 = S2 = SCu = 1/2), where J is the intramolecular magnetic coupling parameter and g is the average Landé factor of the CuII ions (g = g1 = g2 = gCu). The least-squares fit of the experimental data through the simple Bleaney–Bowers expression [Eq. (1)] gave J = –8.5 cm–1 and g = 2.054 for 2 and for J = –2.0 cm–1 and g = 2.055 for 3, with R = 8.0  10–5 (Table 5). The theoretical curves match very well the experimental ones for both 2 and 3 (solid lines in Fig. 7). In particular, they reproduce

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J. Ferrando-Soria et al. / Inorganica Chimica Acta 363 (2010) 1666–1678 Table 5 Best-fit magnetic parameters for 2 and 3.

0.8

0.6 0.2

χM (cm3 mol-1 )

χMT (cm3 mol-1 K)

0.25

a

0.15

0.4

b c

0.1

0.05

0.2

0

0

5

10

15

20

25

T (K)

0 0

50

100

150

200

250

300

T (K) Fig. 7. Temperature dependence of vMT for 2 (s) and 3 (h). The inset shows the vM plots in the low temperature region. The solid lines correspond to the best-fit curves (see text).

the observed maximum of vM for 2 at 7.5 K and the expected maximum of vM for 3 at 1.8 K (solid lines in the inset of Fig. 7).

vM T ¼ ð2Nb2 g 2 =kÞ=½3 þ expðJ=kTÞ

ð1Þ –1

The moderate antiferromagnetic coupling (J = –8.5 cm ) for 2 is quite remarkable, however, the weak but non-negligible antiferromagnetic coupling (J = –2.0 cm–1) for 3 is really spectacular given the relatively larger Cu–Cu separation. Indeed, this result indicates that strongly delocalized p-type orbital pathways through the para-substituted bi- and triphenylenediamine bridges are involved in 2 and 3, as previously observed for the related phenylenediamine-bridged dicopper(II) complex 1 which shows a moderately strong antiferromagnetic coupling (J = –51.4 cm–1) [16c]. This situation is as expected for an orthogonal disposition of the trigonal axis of the copper atoms with the oligo-p-phenylene spacer in either syn (1) or anti (2) configurations, the unpaired electron on each CuII ion occupying the dz2 -type orbital pointing toward the Cu–Nax bonds. At this respect, the J value for 2 is slightly smaller than that found in the other structurally characterized p-biphenylenediamine-bridged dicopper(II) complex of formula [Cu2(tren)2(bpda)](NO3)4 (J = 9.0 cm–1) in spite of the partial loss of p-conjugation in the non-planar biphenylene spacer as a result of the twisting around the C–C single bond for this latter complex [w = 14.4(1)–22.5(1)°] [16a]. This is likely associated to the deviations from orthogonality between the copper trigonal axes and the benzene rings of the planar biphenylene spacer for 2 [u = 79.1(3)°] when compared to the situation found for [Cu2(tren)2(bpda)](NO3)4 [u = 83.46(3)–89.87(3)°] [16a]. These results agree with the experimental magneto-structural correlation between the magnetic coupling parameter (J) and the Cu–N–C–C torsion angle (u) found for the related series of p-phenylenediaminebridged dicopper(II) complexes [16c]. More importantly, a slow rate of decay of the magnetic coupling with the number of benzene rings on the oligo-p-phenylene spacer is experimentally observed in 1–3. By comparison, the strong antiferromagnetic coupling across the p-phenylenediamine bridge previously reported for 1 (J = –51.4 cm–1) [16c] is no more than 6- and 25-fold that those observed for 2 (J = –8.5 cm–1) and 3 (J = –2.0 cm–1) through the biand triphenylenediamine bridges respectively, despite the significantly larger Cu–Cu separations. DFT calculations on the Ci-symmetric dinuclear model complexes 2 and 3 in their non-planar orthogonal anti configuration (u = 90° and w = 0°, see Computational Details) indicate a BS singlet (1Ag) ground state lying well bellow the corresponding triplet (3Au) excited state. The singlet–triplet energy gap (DEST = ES –

Complex

Ja (cm–1)

gb

Rc (105)

2 3

–8.5 –2.0

2.054 2.055

8.0 9.0

Magnetic coupling. Average Landé factor. P P Agreement factor defined as R = [(vMT)exp – (vMT)calcd]2/ [(vMT)exp]2.

ET = J) is 8.8 (2) and 5.4 cm–1 (3) with an estimated intermetallic distance (r) of 12.257 (2) and 16.417 Å (3). By comparison, the singlet–triplet energy gap is 26.4 cm–1 for the C2-symmetric dinuclear model complex 1 in the non-planar orthogonal syn configuration with an estimated intermetallic distance of 7.530 Å [16c]. Selected theoretical magneto-structural data for 1–3 are listed in Table 6. The calculated J value for 2 is in excellent agreement with the experimental one (J = 8.5 cm–1), whereas those for 1 and 3 are somewhat lower (J = 51.4 cm–1) and higher (J = 2.0 cm–1), respectively. Hence, although both experimental and theoretical J values for 1–3 decrease in an essentially exponential manner with the intermetallic distance as J = 7.89  102 exp(–0.37r) and J = 0.94  102 exp(–0.18r), respectively (Fig. 8), the experimental decay rate of magnetic coupling along this series is somewhat higher than that predicted by the DFT calculations as clearly reflected in the calculated values of the exponential factor (b) of 0.37 and 0.18 Å–1, respectively (solid lines in Fig. 8). They are however smaller than those based on previous magneto-structural correlations in dinuclear copper(II) complexes (b = 1.5–1.8 Å–1) [8b,15b]. Once again, this result confirms the relative efficiency of r- versus p-type orbital pathways for the propagation of an intramolecular electron exchange interaction between two copper(II) centers through extended aromatic bridges. Nevertheless, following the experimental decay law of the magnetic coupling with the intermetallic distance along this series of dicopper(II) complexes with oligo-p-phenylenediamine bridging ligands of varying length, H2N–(C6H4)n–NH2 (n = 1–3), two nanometers appears to be the upper limit for the observation of a non-negligible antiferromagnetic interaction in the longer homologue with n = 4 (J = –0.4 cm–1 with r = 20.577 Å). But, of course, even higher limits can be achieved in related series of dicopper(II) complexes with better p-conjugated aromatic diamine bridging ligands. Within the framework of a molecular orbital (MO) analysis of the electron exchange mechanism, the magnitude of the antiferromagnetic interaction (J) is governed by the energy separation (d) between the two singly occupied molecular orbitals (SOMOs) [8a]. This is illustrated in Scheme 4 for 2, where the two SOMOs, noted a1g* and a1u*, in the spin-restricted representation correspond to the spin-down (b) LUMO and LUMO+1 for the triplet (3Au) state (Fig. 6). They are composed by the nearly degenerate symmetric and antisymmetric a1g(M) and a1u(M) combinations, respectively, of the dz2 -type orbitals of the trigonal bipyramidal CuII ions mixed with the corresponding a1g(L) and a1u(L) combinations of the two p-type orbitals of the aromatic bridging ligand of appropriate symmetry (Scheme 4, top). The moderately large d value of 637 cm–1 for 2 is attributed to the stronger metal–ligand orbital mixing between the a1g(M) and a1g(L) orbitals relative to that of the a1u(M) and a1u(L) orbitals because of the smaller energy separation (DM–L) (Scheme 4, bottom). This fact is ultimately due to the large energy gap between the two a1g(L) and a1u(L) ligand orbitals (DL) before metal–ligand interaction, as a result of the antibonding contribution between the pz carbon orbitals of the central C–C single bond in the biphenylene spacer for the former (Scheme 4, top). As expected, the calculated d values decrease continuously along this series from 1332 (1) to 637 (2) and then to 367 cm–1

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Table 6 Selected theoretical magneto-structural data for 1–3.

a b c d e

Complex

Symmetry

ra (Å)

/b (°)

wc (°)

DESTd (cm–1)

de (cm–1)

1 2 3

C2 Ci Ci

7.530 12.257 16.417

90 90 90

0 0

–26.4 –8.8 –5.4

1337 637 367

a1g (M)

a1g (L)

a1u (M)

a1u (L)

Intermetallic distance. Cu–N–C–C torsion angle. Twist angle between the phenylene rings. Singlet–triplet energy gap. Energy gap between the two singly occupied molecular orbitals (SOMOs).

1000

30

-J (cm-1)

25

-J (cm-1)

100

20

a1g*

15

δ

10 a1u*

5 0

0

0.5

1

1.5

2

ΔM-L

a1g (M) a1u (M)

δ 2 x 106 (cm-2)

10

a1g (L)

ΔL

a1u (L)

1 6

8

10

12

14

16

a1ub

18

r (Å) a1gb

(3) (Table 6), in such a way that the calculated –J values vary fairly well with the square of d (inset of Fig. 8) according to the simplest orbital models of the exchange interaction [8a]. This situation reflects the parallel decrease of the dz2 (M)–p(L) metal–ligand orbital mixing for 1–3 and, consequently, the smaller delocalization of the unpaired electrons of the CuII ions onto the p-conjugated electron system of the oligo-p-phenylenediamine bridge with the increasing number of phenylene units (n) from 1 to 3 along this series (Fig. 9). Indeed, this result can be explained because of the appreciable attenuation of the p-conjugation associated with the successive incorporation of C–C single bonds connecting the phenylene groups in the oligo-p-phenylene spacer. Spin densities obtained by NBO analysis on the Ci-symmetric dinuclear model complexes 2 and 3 with a non-planar orthogonal anti configuration conform to a spin polarization mechanism for the propagation of the exchange interaction between the unpaired electrons of the two trigonal bipyramidal copper(II) ions through the p-type orbital pathways of the para-substituted bi- and triphenylenediamine bridges respectively, as reported earlier for the C2-symmetric dinuclear model complex 1 in the non-planar orthogonal syn configuration [16c]. The occurrence of a common spin polarization mechanism for this unique series of dicopper(II) complexes with oligo-p-phenylenediamine bridging ligands, independently of their syn or anti configuration, is clearly evidenced by the spin density distribution calculated for the ground BS singlet spin state of 2 and 3 (Fig. 10). In both cases, the r-type dz2 (Cu) orbitals containing the unpaired electrons are largely delocalized into the sp3(N) orbitals of the axial and equatorial amine-nitrogen donor atoms (‘magnetic orbitals’). The values of the spin density at the axial amine-nitrogen atom from the tren terminal ligand [±0.17 (2) and ±0.016 (3)] are greater than those of the equatorial ones

M

M

M-L-M

L

Scheme 4. Simplified energy level diagram of the interaction between the dz2 -type metal orbitals and the p-type ligand orbitals in 2.

-0.40

Energy (au)

Fig. 8. Plot of the experimental (s) and calculated (d) values of the magnetic coupling parameter (–J) with the intermetallic distance (r) for 1–3. The inset show the linear dependence of the calculated –J values (j) with the square of the energy gap (d) between the two SOMOs (data from Table 6). The solid lines correspond to the best-fit curves (see text).

-0.45

-0.50

Fig. 9. Partial energy level diagram of the SOMOs for the triplet spin state of 1 (left), 2 (middle) and 3 (right). The isoelectronic surface corresponds to a value of 0.020 e bohr–3.

[from ±0.03 to ±0.07 (2) and from ±0.03 to ±0.06 (3)] (Table S6, Supplementary material). On the other hand, the values of the spin density at the aminenitrogen atoms of the bpda and tpda bridging ligands are important [±0.09 (2) and ±0.10 (3)], and they have the same sign as in the copper atoms to which they are coordinated [±0.57 (2) and ±0.57 (3)] (Table S6). This result indicates that the spin delocalization from the metal toward the aromatic amino donor groups dominates over the spin polarization effects because of the strong covalency of the axial metal–ligand bonds in 2 and 3. In contrast, the sign alternation of the spin density at the carbon atoms of the oligo-p-phenylene spacers agrees with a spin polarization by

J. Ferrando-Soria et al. / Inorganica Chimica Acta 363 (2010) 1666–1678

Fig. 10. Perspective views of the calculated spin density distribution for the BS singlet spin state of 2 (a) and 3 (b). Yellow and blue contours represent positive and negative spin densities, respectively. The isodensity surface corresponds to a value of 0.001 e bohr–3.

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intermetallic distances. Together with an almost linear increase in the estimated intermetallic distance (r), a rather slow exponential decay of the antiferromagnetic coupling (–J) with the number of phenylene repeat units (n = 1–3) is experimentally observed in the corresponding dinuclear copper(II) complexes. The antiferromagnetic nature of the electron exchange interaction results from a spin polarization mechanism through the extended p-conjugated bond system of the oligophenylene spacers with para-substitution pattern, as confirmed by theoretical calculations. This unique series of dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands, which can be viewed as the simplest analogues of the well-known Creutz-Taube complexes with oligophenylenedipyridine bridging ligands, constitutes thus a rare example of long-distance magnetic coupling in transition metal complexes. We are currently investigating other series of dinuclear copper(II) complexes with aromatic diamine bridging ligands containing potential redox- or photoactive, p-conjugated aromatic spacers as synthetic and theoretical models for the fundamental study on electron exchange and electron- or photo-triggered electron exchange processes, which are two central topics in the emerging area of molecular spintronics, respectively. Indeed, these simple molecules appear as very promising candidates to get multifunctional molecular-based magnetic devices facilitating the spin communication (‘magnetic molecular wires’) or exhibiting multistable behavior (‘magnetic molecular switches’) for future applications in information processing and storage. Acknowledgements

a

+0.02 -0.01 +0.09

-0.02

+0.02 -0.01

+0.02

-0.01

-0.09

+0.01 -0.02 +0.01 -0.02 +0.01

+0.01 -0.01

+0.02 -0.01

-0.01

+0.02 -0.01

This work was supported by the Generalitat Valenciana (GV, Spain) (Project PROMETEO2009/108), MAT2007-60660, Ministerio de Ciencia e Innovación (MCIIN, Spain) (Projects CTQ2007-61690, MAT2007-60660, and Factoria de Cristalización, Consolider-Ingenio2010, CSD2006-00015). C.Y. and O.F. thank the MEC for Grants.

+0.02

+0.02

-0.02

b +0.10

+0.01 -0.02

+0.01

-0.02

+0.01 -0.01

-0.10

+0.01 -0.02

Scheme 5. Spin density distribution on the para-substituted bi- and triphenylenediamine bridging ligands for the ground BS singlet spin state of 2 (a) and 3 (b), respectively, with calculated atomic spin density values. Empty and full circles represent positive and negative spin densities respectively, with scaled surface areas (data from Table S6).

the aromatic amine-nitrogen donor atoms (Scheme 5), leading thus to significant values of the spin density of opposite sign at the carbon atoms of the two benzene groups to which they are directly attached [±0.02 (2) and ±0.01 (3)] (Table S6). Indeed, the values of the spin density at the carbon atoms of the central benzene group from the tpda bridging ligand are small but nonnegligible [±0.01 (3)] (Table S6), as expected because of the partial loss of p-conjugation with the incorporation of an additional C–C single bond (Scheme 5). Hence, a net decrease of the antiferromagnetic exchange interaction results for 3 when compared to 2, as observed experimentally. 4. Conclusions and perspectives In this work, we have shown both experimentally and theoretically that linear p-conjugated oligophenylenediamines with either one, two, or three para-substituted phenylene spacers, –(C6H4)n– (n = 1–3), are really effective to transmit electron exchange interactions between two CuII ions separated by relatively large

Appendix A. Supplementary material Magneto-structural data for structurally characterized dinuclear copper(II) complexes with amine-based aromatic bridges (Table S1) and theoretical data for model complexes 2 and 3 (Tables S2–S6). Crystallographic data (excluding structure factors) for 2 have been deposited with the Cambridge Crystallographic Data Center, CSD No. 738677. Copies of the data may be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.12.007. References [1] (a) O. Kahn, Molecular Magnetism, VCH Publishers, New York, 1993; (b) J. Jorner, M. Ratner (Eds.), Molecular Electronics, Blackwell Science, Oxford, 1997. [2] (a) C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541; (b) J.M. Tour, Acc. Chem. Res. 33 (2000) 791; (c) R.R. Caroll, C.B. Gorman, Angew. Chem., Int. Ed. 41 (2002) 4379; (d) N. Robertson, C.A. McGowan, Chem. Soc. Rev. 32 (2003) 96; (e) A. Dei, D. Gatteschi, C. Sangregorio, L. Sorace, Acc. Chem. Res. 37 (2004) 827. [3] (a) M.P. Shores, J.R. Long, J. Am. Chem. Soc. 124 (2002) 3512; (b) W. Liang, M.P. Shores, M. Bockrath, J.R. Long, H. Park, Nature 417 (2002) 725; (c) M. Fabre, J. Bonvoisin, J. Am. Chem. Soc. 129 (2007) 1434; (d) P. Hamon, F. Justaud, O. Cador, P. Hapiot, S. Rigaut, L. Toupet, L. Ouahab, H. Stueger, J.-R. Hamon, C. Lapinte, J. Am. Chem. Soc. 130 (2008) 17372. [4] (a) C. Creutz, Prog. Inorg. Chem. 30 (1983) 1; (b) G. Blondin, J.J. Girerd, Chem. Rev. 90 (1990) 1359; (c) R.J. Crutchley, Adv. Inorg. Chem. 41 (1994) 273; (d) B.S. Brunschwig, N. Sutin, Coord. Chem. Rev. 187 (1999) 233.

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J. Ferrando-Soria et al. / Inorganica Chimica Acta 363 (2010) 1666–1678

[5] (a) C. Creutz, H. Taube, J. Am. Chem. Soc. 95 (1973) 1086; (b) G.M. Tom, C. Creutz, H. Taube, J. Am. Chem. Soc. 96 (1974) 7827; (c) J.E. Sutton, P.M. Sutton, H. Taube, Inorg. Chem. 18 (1979) 1017; (d) Y. Kim, C.M. Lieber, Inorg. Chem. 28 (1989) 3990. [6] (a) J.A. McCleverty, M.D. Ward, Acc. Chem. Res. 36 (1998) 3447; (b) J.P. Launay, Chem. Soc. Rev. 30 (2001) 386; (c) W. Kaim, G.K. Lahiri, Angew. Chem., Int. Ed. 46 (2007) 1778. [7] (a) A. Bencini, D. Gatteschi, F. Totti, D. Nieto-Sanz, J.A. McCleverty, M.D.J. Ward, Phys. Chem. A 102 (1998) 10545; (b) E. Ruiz, A. Rodríguez-Fortea, S. Alvarez, Inorg. Chem. 42 (2003) 4881; (c) F. Nunzi, E. Ruiz, J. Cano, S. Alvarez, J. Phys. Chem. C 111 (2007) 618; (d) W. Li, F. Yang, Z. Wang, J. Hu, J. Ma, J. Phys. Chem. A 113 (2009) 3375; (e) R.G. Hadt, V.N. Nemykin, Inorg. Chem. 48 (2009) 3982. [8] (a) J.P. Hay, J.C. Thibeault, R. Hoffmann, J. Am. Chem. Soc. 97 (1975) 4884; (b) R.E. Coffman, G.R. Buettner, J. Phys. Chem. 83 (1979) 2387; (c) O. Kahn, Angew. Chem., Int. Ed. Engl. 24 (1985) 834. [9] (a) E.F. Hasty, T.J. Colburn, D.N. Hendrickson, Inorg. Chem. 12 (1973) 2414; (b) C.G. Pierpont, L.C. Francesconi, D.N. Hendrickson, Inorg. Chem. 16 (1977) 2367; (c) C.G. Pierpont, L.C. Francesconi, D.N. Hendrickson, Inorg. Chem. 17 (1978) 3470; (d) M. Hadded, D.N. Hendrickson, J.P. Cannady, R.S. Drago, D.S. Bieksza, J. Am. Chem. Soc. 101 (1979) 898; (e) J.T. Reinprecht, J.G. Miller, G.C. Vogel, M.S. Haddad, D.N. Hendrickson, Inorg. Chem. 19 (1980) 927. [10] (a) M. Verdaguer, J. Gouteron, S. Jeannin, Y. Jeannin, O. Kahn, J.M. Savariault, Inorg. Chem. 23 (1984) 4291; (b) F. Tinti, M. Verdaguer, O. Kahn, J.M. Savariault, Inorg. Chem. 26 (1987) 2380. [11] (a) M. Julve, M. Verdaguer, J. Faus, F. Tinti, J. Moratal, A. Monge, E. GutiérrezPuebla, Inorg. Chem. 26 (1987) 3520; (b) X. Solans, M. Aguiló, A. Gleizes, J. Faus, M. Julve, M. Verdaguer, Inorg. Chem. 29 (1990) 775; (c) I. Castro, J. Sletten, J. Faus, M. Julve, Y. Journaux, F. Lloret, S. Alvarez, Inorg. Chem. 31 (1992) 1889; (d) J. Cano, G. De Munno, J.L. Sanz, R. Ruiz, J. Faus, F. Lloret, M. Julve, A. Caneschi, J. Chem. Soc., Dalton Trans. 1997 (1915); (e) R. Hernández-Molina, A. Mederos, P. Gili, S. Domínguez, F. Lloret, J. Cano, M. Julve, C. Ruiz-Pérez, X. Solans, J. Chem. Soc., Dalton Trans. (1997) 4327; (f) J. Cano, G. De Munno, J.L. Sanz, R. Ruiz, F. Lloret, J. Faus, M. Julve, Anales de Química Int. Ed. 93 (1997) 174; (g) I. Castro, M.L. Calatayud, J. Sletten, F. Lloret, M. Julve, Inorg. Chim. Acta 287 (1999) 173. [12] (a) E.G. Bakalbassis, C.A. Tsipis, J. Mrozinski, Inorg. Chem. 24 (1985) 4231; (b) E.G. Bakalbassis, J. Mrozinski, C.A. Tsipis, Inorg. Chem. 25 (1986) 3684; (c) S.K. Hakhatreh, E.G. Bakalbassis, I. Brüdgam, H. Hartl, J. Mrozinski, C.A. Tsipis, Inorg. Chem. 30 (1991) 2801; (d) S.K. Hakhatreh, E.G. Bakalbassis, I. Brüdgam, H. Hartl, J. Mrozinski, C.A. Tsipis, Inorg. Chim. Acta 186 (1991) 113. [13] (a) P. Chaudhuri, K. Oder, K. Wieghardt, S. Gehring, W. Haase, B. Nuber, J. Weiss, J. Am. Chem. Soc. 110 (1988) 3657; (b) K.S. Bürger, P. Chaudhuri, K. Wieghardt, B. Nuber, Chem. Eur. J. 1 (1995) 583. [14] (a) A.R. Paital, T. Mitra, D. Ray, W.T. Wong, J. Ribas-Ariño, J.J. Novoa, J. Ribas, G. Aromí, Chem. Commun. (2005) 5172; (b) A.R. Paital, A.Q. Wu, G.G. Guo, G. Aromí, J. Ribas-Ariño, D. Ray, Inorg. Chem. 46 (2007) 2947. [15] (a) I. Fernández, R. Ruiz, J. Faus, M. Julve, F. Lloret, J. Cano, X. Ottenwaelder, Y. Journaux, M.C. Muñoz, Angew. Chem., Int. Ed. 40 (2001) 3039; (b) E. Pardo, J. Faus, M. Julve, F. Lloret, M.C. Muñoz, J. Cano, X. Ottenwaelder, Y. Journaux, R. Carrasco, G. Blay, I. Fernández, R. Ruiz-García, J. Am. Chem. Soc. 125 (2003) 10770; (c) E. Pardo, R. Carrasco, R. Ruiz-García, M. Julve, F. Lloret, M.C. Muñoz, Y. Journaux, E. Ruiz, J. Cano, J. Am. Chem. Soc. 130 (2008) 576. [16] (a) T.R. Felthouse, E.N. Duesler, D.N. Hendrickson, J. Am. Chem. Soc. 100 (1978) 618;

[17]

[18]

[19]

[20] [21]

[22] [23]

[24]

[25]

[26]

[27] [28]

[29] [30] [31] [32]

(b) T.R. Felthouse, D.N. Hendrickson, Inorg. Chem. 17 (1978) 2636; (c) C. Yuste, J. Ferrando-Soria, D. Cangussu, O. Fabelo, C. Ruiz-Pérez, N. Marino, G. De Munno, S.-E. Stiriba, R. Ruiz-García, J. Cano, F. Lloret, M. Julve, Inorg. Chim. Acta, doi: 10.1016/j.ica.2009.03.001. (a) S. Schindler, D. Szalda, C. Creutz, Inorg. Chem. 31 (1992) 2255; (b) T. Buchen, A. Hazell, L. Jessen, C.J. McKenzie, L.P. Nielsen, J.Z. Pedersen, D. Schollmeyer, J. Chem. Soc., Dalton Trans. (1997) 2697; (c) S.P. Foxon, G.R. Torres, O. Walter, J.Z. Pedersen, H. Toftlund, M. Hüber, K. Falk, W. Haase, J. Cano, F. Lloret, M. Julve, S. Schindler, Eur. J. Inorg. Chem. (2004) 335; (d) S.P. Foxon, O. Walter, R. Koch, H. Rupp, P. Müller, S. Schindler, Eur. J. Inorg. Chem. (2004) 344; (e) S. Turba, O. Walter, S. Schindler, L.P. Nielsen, A. Hazell, C.J. McKenzie, F. Lloret, J. Cano, M. Julve, Inorg. Chem. 47 (2008) 9612. (a) A. Mederos, P. Gili, S. Domínguez, A. Benítez, M.S. Palacios, M. HernándezPadilla, P. Martín-Zarza, M.L. Rodríguez, C. Ruiz-Pérez, F.J. Lahoz, L.A. Oro, F. Brito, J.M. Arrieta, M. Vlassi, G. Germain, J. Chem. Soc., Dalton Trans. (1990) 1477; (b) S. Domínguez, A. Mederos, P. Gili, A. Rancel, A.E. Rivero, F. Brito, F. Lloret, X. Solans, C. Ruiz-Pérez, M.L. Rodríguez, F. Brito, Inorg. Chim. Acta 255 (1997) 367. R.W.W. Hooft, COLLECT, Nonius BV, Delft, The Netherlands, 1999; (b) A.J.M. Duisenberg, L.M.J. Kroon-Batenburg, A.M.M. Schreurs, J. Appl. Crystallogr. 36 (2003) 220 (EVALCCD). SADABS, Version 2.03, Bruker AXS, Inc., Madison, WI, 2000. G. M. Sheldrick, SHELX97, Programs for Crystal Structure Analysis (Release 972), Institüt für Anorganische Chemie der Universität, Göttingen, Germany, 1998. L.J. Farrugia, (WINGX), J. Appl. Crystallogr. 32 (1999) 837. (a) M. Nardelli, J. Appl. Crystallogr. 28 (1995) 659; (b) D. Palmer, CRYSTAL MAKER, Cambridge University Technical Services, Cambridge, UK, 1996. (a) A.D. Becke, Phys. Rev. A 38 (1988) 3098; (b) C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785; (c) A.D. Becke, J. Chem. Phys. 98 (1993) 5648. (a) E. Ruiz, J. Cano, S. Alvarez, P. Alemany, J. Comput. Chem. 20 (1999) 1391; (b) E. Ruiz, A. Rodriguez-Fortea, J. Cano, S. Alvarez, P. Alemany, J. Comput. Chem. 24 (2003) 982. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J. Montgomery, J.A.T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C.02, Gaussian 03, Revision C.02 ed., Gaussian, Inc., Wallingford, CT, 2004. (a) A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 97 (1992) 2571; (b) A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. (a) F. Weinhold, J.E. Carpenter, J. Mol. Struct. 42 (1988) 189; (b) A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899; (c) F. Weinhold, J.E. Carpenter, The Structure of Small Molecules and Ions, Plenum, 1988. (a) M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 24 (2003) 669; (b) J. Tomasi, B. Mennucci, E. Cances, J. Mol. Struct.-Theochem. 464 (1999) 211. D. Casanova, J. Cirera, M. Llunell, P. Alemany, D. Avnir, S. Alvarez, J. Am. Chem. Soc. 126 (2004) 1755. M. Banerjee, R. Shukla, R. Rathore, J. Am. Chem. Soc. 131 (2009) 1780. M. Meyer, L. Fremond, E. Espinosa, R. Guilard, Z. Ou, K.M. Kadish, J. Chem. Soc., Inorg. Chem. 43 (2004) 5572. and references therein.