Molecule-Based Magnetic Materials - American Chemical Society

Chapter 11

Magnetostructural Correlations in Dinuclear Cu(II) and Ni(II) Complexes Bridged by μ -1,1-Azide and μ -Phenoxide Downloaded by UNIV MASSACHUSETTS AMHERST on September 10, 2013 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch011

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L. K. Thompson , S. S. Tandon , M . E. Manuel , M. K. Park , and M . Handa 1

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Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, A1B 3X7, Canada Department of Materials Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, Nishikawatsu, Matsue 690, Japan 2

Magnetostructural correlations for dinuclear copper(II) complexes bridged by a combination of μ -1,1-azide and a diazine, and macrocyclic complexes bridged by phenoxide are discussed. For the azide bridged complexes the magnetic properties change from ferromagnetic to antiferromagnetic at =108° (∆2J=46 cm /deg.), with evidence to suggest a temperature variable exchange integral. The phenoxide bridge angle/exchange integral correlation does not extrapolate to a normal crossover from antiferromagnetic to ferromagnetic behavior, and π conjugation routes within the macrocyclic ligand are implicated in the total exchange process. The nickel/phenoxide case conforms to the expected oxygen bridged correlation with -2J=0 cm at =97°. 2

-1

-1

Intramolecular magnetic exchange interactions in dinuclear transition metal complexes bridged by simple ligands have fascinated coordination chemists and physicists, and theorists, since the pioneering work of Bleaney and Bowers in the early 1950's on the dimeric copper acetate molecule (7). The variable temperature magnetic properties of simple dinuclear copper(H) complexes can be described by the Bleaney-Bowers equation (equation 1; terms have their usual meaning with x expressed per mole of copper, and based on the exchange Hamiltonian H = -2JS1.S2) (1). Single atom bridging ligands e.g. hydroxide, have been studied widely,

X

=

" 3?r-6) [

1 + 1

/ 3 e x p ( - 2 c 7 / i c r ) ] - M l - p ) ^ "^g

1

P +to

(1)

particularly for copper(H) complexes, because of the ease with which such 0097-6156/96/0644-0170$15.00/0 © 19% American Chemical Society

In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

11. THOMPSON ET AL.

Dinuclear Cu(II) & Ni(II) Complexes

111

complexes form in aqueous solvent media, even in the absence of base. The now classical study by Hatfield et al (2) on simple dirrydroxo-bridged dicopper(II) complexes, with d 2-y2 copper ground states, revealed the importance of bridge angle as a fundamental factor controlling spin exchange, with a demonstrated linear relationship between Cu-OH-Cu bridge angle and exchange integral. The bridge angles ranged from 95.6° to 104.1°, which fortuitously spanned the critical angle at which the magnetic properties changed* from antiferromagnetic to ferromagnetic (97.5°). These experimental observations were consistent with extended Huckel MO calculations, which predicted that accidental orthogonality (the situation in which the symmetric and antisymmetric MO combinations involving the metal magnetic orbitals and appropriate symmetry bridge orbitals, in the triplet state, are degenerate) would occur at 92° (3,4). Subsequent studies on alkoxide bridged dicopper(II) complexes revealed a similar situation (5,6). While numerous examples of open chain complexes with hydroxide or alkoxide bridges were known, with a reasonable range of bridge angles, this was not true for phenoxide bridged systems, and in particular those involving macrocyclic ligands. Macrocyclic phenoxide bridged dimckel(II) and dicopper(H) complexes will be discussed in this report, along with consideration of inductive contributions to exchange. The step from simple oxygen bridged dicopper(H) complexes to ^-1,1-azide bridged analogues followed logically, and a few examples of systems with small CuN -Cu angles ( < 106°) were reported in the early 1980's (3,7,8). These were found to be exclusively ferromagnetic, and the ability of the ^-l.l-azide to propagate ferromagnetic coupling was attributed to a spin polarization effect, involving an interaction between the two copper d^ orbitals and the x MO on the azide (8). The theoretical argument to support this explanation arose from the fact that in this angle range the energy splitting between the two molecular orbitals constructed from the d magnetic orbitals in the triplet state, and appropriate symmetry ligand orbitals (A), according to extended Huckel calculations (5), was considered to be very small, and so any antiferromagnetic term ( J ^ ) would be small and not compensate for or exceed any inherent ferromagnetic term (J )(equation 2; S is the overlap integral and C the two-electron exchange integral for a system with two thermally populated spin levels).

Downloaded by UNIV MASSACHUSETTS AMHERST on September 10, 2013 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch011

x

3

g

xy

F

J = JAF + JF

JAF =

-2AS; J = 2C F

(2)

This lead to the suggestion that perhaps all 1,1-azide bridged complexes would be ferromagnetic, regardless of angle. This chapter will explore the previously uncharted antiferromagnetic realm of the ^-1,1-azide bridge, and also the fundamental question of the temperature dependence of the exchange integral itself, or more appropriately the result of A being temperature dependent. Phenoxide Bridged Macrocyclic Dinickel(H) Complexes Phenoxide bridged macrocyclic complexes derived by template condensation of diformylphenols and diamines have been produced in large numbers since the original report by Robson in 1970 (9) describing the dinuclear species [M(L)] 2+



172

MOLECULE-BASED MAGNETIC MATERIALS

R = ( C H ) (n=2-4), Q H j , C F4,C (CN)2 R'=H, C H R"=CH ,t-Bu, C F 2

6

n= 2,3

n

2

3

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3

7

Figure 1. Macrocyclic dicopper and dinickel complexes.

20 r

-80 ' 95

•—•

1

98

•

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101

1

104

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107

Ni-0-Ni(deg.) Figure 2. Plot of excnange integral against N i - O p ^ ^ ^ - N i bridge angle for macrocyclic dinickel(II) complexes.



11. THOMPSON ET AL.

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(M=Cu,Ni,Co,Fe,Mn) (Figure 1). However, despite many studies describing both structures and magnetic properties of such systems, no meaningful magnetostructural correlations relating exchange interaction with M-OPh-M angle were reported until recently. In order to develop such a correlation a series of structurally related complexes is required, within which factors contributing to the exchange process other than the phenoxide bridge angle are kept essentially constant. A series of dinickel(II) complexes of the related saturated macrocyclic ligand L ' , with phenoxide bridge angles in the range 99-106° (Figure 1, n=3), which were reported recently (10), showed a linear relationship between Ni-OPh-Ni angle and exchange integral and a crossover from antiferromagnetic to ferromagnetic behavior at « 9 7 ° , consistent with the dihydroxy-bridged copper systems (Figure 2). Further support for this correlation comes from the complex [Ni (L ' )(CH COO)J.10H O (Ni-OPh-Ni 95.6°, J=10.1 c m ; L = L ' ( F i g u r e 1; n=2)) (77), which fits exactly on the line, and extends the correlation over a range of angles of 10°. The similarity between dicopper(H) and dinickel(II) complexes, where exchange depends only on the bridging oxygen, is considered to be reasonable despite the fact that for mckel(H) both the d 2-y2 and dp orbitals are magnetic orbitals. However the involvement of symmetric dp orbitals in exchange coupling will be minimal in complexes of this sort where the exchange process is dominated by equatorial interactions. 2 2

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x

Phenoxide Bridged Macrocyclic Dicopper(II) Complexes Numerous dicopper(II) complexes of unsaturated ligands L (e.g. n=2-4, X = H 0 , CI, Br, I, C10 ) (Figure 1) have been synthesized and studied, mostly with squarepyramidal copper centers. Control over the dinuclear center dimensions, e.g. Cu-Cu separation and Cu-O-Cu angle, is somewhat limited, but varying the linker group size (R) in general leads to increased angles and metal-metal separations as the chain length increases (12-18). The presence of electron withdrawing ligands (e.g. X=C1, Br, I) were demonstrated to exert an inductive effect, which modulated the exchange process, leading to reduced antiferromagnetic coupling in comparison with structurally related complexes, and electron withdrawing groups on the macrocyclic ligand itself also had a similar effect (73,77,18). By choosing related complexes with no inductive perturbations, and consistent copper ion magnetic ground state (d 2-y2), and with miriimal distortions of e.g. the copper ion coordination sphere and the C u 0 dinuclear center itself, a series of suitable complexes for a magnetostructural correlation can be selected. A typical example, [ C i ^ C L J C H ^ J C B F ^ , is shown in Figure 3 ( L C R - C C H ^ , R = C H , R " = C H ) , Cu-O-Cu 98.8(4)°, Cu-Cu 2.997(3) A ; -2J = 689 cm ). Figure 4 illustrates a plot of -2J against averaged Cu-OPh-Cu angle for this series (72,74,77,79,20,27) (Thompson, L . K . ; Mandal S.K.; Tandon, S.S.; Bridson, J . N . ; Park, M . K . , Inorg. Chem., in press.), with the best fit line. Although the correlation is not as precise as in the nickel case the trend is clear, and assuming that a linear relationship is appropriate the data extrapolate to an angle of « 7 7 ° for -2J=0 cm' . This is clearly inconsistent with the dinickel case, and also the copper/hydroxide case, and begs the question as to why -2J values are so high at angles close to the normal experimental angle of accidental orthogonality for a dioxygen bridged system. 2

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MOLECULE-BASED MAGNETIC MATERIALS


174

Figure 4. Plot of exchange integral (-2J) against C u - O - C u bridge angle for macrocyclic dicopper(II) complexes (data from refs. 12,14,17,19,20,21 ( A ) ; Thompson, L . K . et al, Inorg. Chem., in press (*)). phcnoxidc


11. THOMPSON ET AL.


A test of the validity of the relationship is obtained by considering comparable complexes with electron withdrawing ligand groups. The squarepyramidal complexes [Cu (L)XJ ( I / R ^ C H ^ , R'=H, R"=CH ); X=C1, Br) in which the halogens are axially bonded to the copper centers have much lower -2J values than predicted based on their Cu-O-Cu angles (77). The pseudo-octahedral complexes [(^(LXCIO^J (L (R=C F , R*=H, R"=tBu) and [ C u ^ X C ^ J (L (R=C F , R'=H, R"=C F ), which have perfluoro-phenyl linker groups (R), both have small Cu-O-Cu angles (99.2(1)°, 100.8(1)° respectively)(76), and are more weakly antiferromagnetically coupled (-2J=581 cm , 526 cm' respectively) than would be expected based on their bridge angles, consistent with the presence of electron withdrawing fluorine atoms on the macrocyclic ligand. The complex [Cu2(L)](C10 ) .3H O.CH OH (L(R=C (CN) , R* =H, R" =CH ) has peripheral CN groups bound much more closely to the copper centers (Figure 5), and although it is still strongly antiferromagnetically coupled, the coupling is weak in comparison with all the other complexes in this class. The five-membered chelate rings subtended at the dicyano-alkene bridges would dictate small phenoxide bridge angles ( » 99°), consistent with the perfluoro-phenyl complexes. That this complex is much more weakly coupled is, no doubt, the result of a cyanide inductive effect at the copper centers, attenuating the exchange. While no x-ray structure is available for this, or related compounds, a neutral derivative produced by recrystallization from acetone (Figure 5) in which a most unusual conjugated addition of acetone has occurred across two imine groups, is still antiferromagnetically coupled despite having Cu-OPh-Cu angles of 92.0(2)° and 92.8(2)°. The complex is severely bent along the phenoxide 0-0 axis with an angle between the CuN 0 mean planes of 151.1°, and pronounced pyramidal distortion at the oxygen bridges themselves (MacLachlan, M.J.; Park, M.K.; Thompson, L.K., unpublished results). These factors, of necessity, would diminish antiferromagnetic coupling, strengthening the argument that an unusual exchange situation prevails with these complexes. A common feature in all cases (Figures 1,5) involves x-conjugated macrocyclic fragments within each aromatic half of the molecule. These provide additional sixbond pathways through which exchange coupling could occur between the copper centers. This would involve a long exchange distance (

Molecule-Based Magnetic Materials - American Chemical Society

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