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Structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and polynuclear [Mn(III)(salen)(NO3)]n
Inorganica Chimica Acta 290 (1999) 8 – 13
Structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and polynuclear [Mn(III)(salen)(NO3)]n Huey-Lih Shyu a,1, Ho-Hsiang Wei b,*, Yu Wang c a
Chung-Tai Institute of Health Science and Technology, Taichung, Taiwan, ROC b Department of Chemistry, Tamkang Uni6ersity, Tamsui, Taiwan, ROC c Instrumentation Center of College of Science, Taiwan Uni6ersity, Taipei, Taiwan, ROC Received 5 September 1998; accepted 17 February 1999
Abstract The synthesis, crystal structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 (1) and polynuclear [Mn(III)(salen)(NO3)]n (2) (salen=N,N%-bis(salicylideneaminato)ethylene) are reported. Compound 1 consists of a structurally dinuclear system in which two Mn ions are bridged by the oxygen atom of m-phenoxo of the salen ligands. Compound 2 is a linear polymeric chain in which the Mn ions are linked by NO3− groups. Cryomagnetic measurements show ferromagnetic interaction with J =6.30 cm − 1 and D= −1.70 cm − 1 for 1 and antiferromagnetic interaction with J = − 0.55 cm − 1 and D = −0.43 cm − 1 for 2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Magnetic properties; Manganese complexes; Schiff base complexes; Polynuclear complexes
1. Introduction Interest in the coordination chemistry of manganese has been driven by the important roles in metalloenzymes [1] and highly valuable catalysts in olefin expoxitation [2–6]. One of the most important processes in nature occurring in the oxygen evolving complexes of photosystem II (PS II) is believed to be catalyzed by a cluster of manganese ions [7]. The manganese coordination sphere is believed to be dominated by O and N donors from available amino acid side chains [8]. A variety of ligating systems has been employed to attain the high-valent manganese complexes and mimic the structural features of the active site; these Schiffbase ligands thus present suitable biometric ligands. Manganese(III) salen complexes (salen= N,N%-bis(salicylideneaminato)ethylene) are well studied. To date several X-ray crystal structures of mononuclear non* Corresponding author. Tel.: + 886-2-621 565; fax: + 886-2-622 8458. E-mail address: [email protected] (H.-H. Wei) 1 Also corresponding author.
substituted [Mn(III)(salen)X] complexes including those with X= Cl [9], S–Ph 10], O-o-C6H4(CH2CH2COCH3) [10], and substituted derivatives of [Mn(III)(7,7%-Ph2salen)(py)2] [2], [Mn(III)(5-Me-7-Memetsalen)Cl] [11], and a few correlated Mn(III) salpn complexes of known crystal structure (salpn= 1,3-bis(salicylideneiminato)propane) [12] have been reported already. To our knowledge, only three X-ray crystal structures of substituted binuclear Mn(III)-salenderivatives, diaqua-bridged [Mn(III)(3,5-Cl-salen) (H2O)2](ClO4)2 [13], m-phenoxy-bridged [Mn(III)(5-Brsalen)(H2O)]2(ClO4)2 [14] and [Mn(III)(3-MeOsalen)Cl]2 [15], and only one non-substituted polymeric [Mn(III)(salen)(OAc)]n [16], have been reported. However, non-substituted dinuclear and polynuclear Mn(III) salen complexes are relatively rare, and those that are structurally and magnetically characterized are still rarer. Herein we report the preparation, X-ray crystal structure and cryomagnetic behaviors of the m-phenoxybridged dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and nitrato-bridged polymeric [Mn(III)(salen)(NO3)]n complexes.
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H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
2. Experimental
2.1. Synthesis [Mn(salen)(H2O)]2(ClO4)2 (1) was synthesized by mixing an ethanol solution (15 cm3) of ethylenediamine (0.06 g, 1 mmol) and 0.244 g (2 mmol) of salicylaldehyde in 15 cm3 of ethanol, which was then stirred for 2 h, with subsequent addition of ethanol (20 cm3) of Mn(ClO4)2 · 6H2O (0.362 g, 1 mmol). Caution: although no problems have been encountered in the present work, perchlorates are potentially explosive and should be treated in small quantities with care. The resulting solution was stirred for 10 min, and then 0.10 g (1 mmol) of triethylamine added. A deep-brown solution was obtained, which was refluxed for 2 h, then allowed to stand at room temperature. Several weeks of standing, led to the growth of deep-brown crystals of 1 suitable for X-ray analysis. Anal. Calc for C32H32Mn2N4O14Cl2: C, 43.77; H, 3.65; N, 6.38. Found: C, 44.02; H, 3.59; N, 6.42%. IR (KBr, cm − 1): n(CN) 1619(vs), n(ClO4− ) 1097(vs), 628(s). [Mn(salen)(NO3)]n (2) was prepared in the same way as compound 1 but using the following quantities of starting reagents: Mn(NO3)2 · 6H2O (0.287 g, 1 mmol), salicylaldehyde (0.2444 g, 2 mmol), and triethylamine (0.10 g, 1 mmol). The resulting solution was allowed to stand in air at room temperature for about 3 weeks to yield large well-formed dark-brown crystals of 2. Anal. Calc. for C16H14Mn N3O5: C, 50.10, H, 3.65; N, 10.96. Found: C, 49.95; H, 4.01; N, 11.20%. IR (KBr, cm − 1): n(CN) 1625(vs), n(NO3− ) 1472(vs), 1305(vs), 805(s), 637(s). 2.2. Physical measurements Infrared spectra (4000 – 400 cm − 1) were recorded from KBr pellets on a Bio-Rad FTS40 FTIR spectrophotometer. The temperature dependence of the magnetic susceptibility of the polycrystalline samples was measured between 4 and 300 K at a field of 1.0 T using a Quantum Design model MPMS computercontrolled SQUID magnetometer. Diamagnetic corrections were made using Pascal’s constants [17].
2.3. X-ray structure determinations The X-ray single-crystal data for both compounds were collected on an Enraf – Nonius CAD 4 diffractometer equipped with graphite-monochromated Mo Ka radiation (l= 0.7107 A, ), 2u –u scan mode. N independent reflections and No with I \2.0s(I) were observed. The structures were solved by the location of heaving atoms using a Patterson map and refined (based on F) by the full-matrix least-squares method using the NRCVAX software package [18]; the func-
Table 1 Crystallographic data [Mn(salen)(NO3)]n (2)
for
Chemical formula fw Space group a (A, ) b (A, ) c (A, ) b (°) V (A, 3) Z T (K) l (Mo Ka) (A, ) dcalc (g cm−3) m (cm−1) Rf a Rwb a b
9
[Mn(salen)(H2O)]2(ClO4)2
(1)
and
1
2
C32H32Mn2N4O14Cl2 877.40 P21/c 7.080(3) 19.013(5) 13.382(3) 97.00(3) 1788.0(10) 2 298 0.7107 1.630 9.053 0.076 0.083
C16H14MnN3O5 383.24 R3( 22.538(3) 16.542(3) 7277.0(18) 18 298 0.7107 1.574 5.539 0.060 0.062
Rf =( Fo − Fc )/ Fo . Rw =w( Fo − Fc )2/ Fo 2.
tion minimized was w( Fo − Fc )2, where w=1/ s 2(Fo). All non-hydrogen atoms were located readily and refined with anisotropic thermal parameters. The data collection, crystallographic data, and data reduc-
Table 2 Selected bond distances (A, ) and angles (°) for [Mn(salen)(H2O)]2(ClO4)2 (1) and [Mn(salen)(NO3)]n (2) [Mn(salen)(H2O)]2(ClO4)2 (1) Mn–O(1) 1.864(5) Mn–O(2)% 2.412(6) Mn–N(1) 1.983(7) C(16)–O(2) 1.358(9) C(7)–N(1) 1.297(12)
Mn–O(2) Mn–O(3) Mn–N(2) C(1)–O(1) C(10)–N(2)
1.901(5) 2.219(6) 1.972(7) 1.356 (9) 1.297(11)
Mn–O(2)–Mn% O(1)–Mn–O(2) O(2)–Mn–O(2)% O(1)–Mn–O(3) O(1)–Mn–N(2) O(1)–Mn–N(1)
O(3)–Mn–O(2)% O(1)–Mn–O(2)% O(2)–Mn–O(3) O(2)–Mn–N(1) N(1)–Mn–N(2) O(2)–Mn–N(2)
172.65(20) 92.01(23) 95.31(24) 171.0(3) 84..5(3) 86.9(3)
Mn–O(2) Mn–O(4) Mn–N(2)
1.858(6) 2.293(6) 1.985(7)
O(1)–Mn–N(2) O(1)–Mn–O(3)% O(1)–Mn–O(2) O(1)–Mn–N(1) O(2)–Mn–O(4) O(3)%–Mn–N(1) Mn–O(4)–N(3) Mn–O(1)–C(1)
176.1(3) 87.59(24) 91.17(24) 91.8(3) 91.1(3) 84.88(25) 133.0(5) 128.5(5)
100.58(22) 92.74(25) 79.42(21) 92.74(25) 177.0(3) 92.6(3)
[Mn(salen)(NO3)]n (2) Mn–O(1) 1.864(5) Mn–O(3)% 2.324(6) Mn–N(1) 1.964(7) O(4)–N(3) 1.218(10) O(3)%–Mn–O(4) O(2)–Mn–N(1) O(1)–Mn–O(4) N(1)–Mn–N(2) O(2)–Mn–N(2) O(2)–Mn–O(3)% O(3)%–Mn–N(2) Mn–O(3)%–N(3)% Mn–O(2)–C(16)
168.38(24) 176.8(3) 83.44(23) 84.3(3) 92.7(3) 96.5(3) 91.7(3) 128.5(5) 130.1(5)
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
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Fig. 1. ORTEP drawing (30% thermal ellipsoid probability) of [Mn(salen)(H2O)]2(ClO4)2 (1) with the atom labeling scheme.
tion information are listed in Table 1. Selected bond distances and angles are listed in Table 2.
3. Results and discussion
3.1. Description of the structures 3.1.1. [Mn(III)(salen)(H2O)]2(H2O)2 (1) The structure of 1 consists of [Mn(salen)(H2O)]22 + cations (Fig. 1) and perchlorate anions. The structure has a centrosymmetric dimer in which the manganese(III) atoms are linked by m-phenoxo bridges from one of the phenolic oxygen atoms of each salen ligand to the opposite metal center, thus resulting in m-phenoxo Mn –O(2)% and Mn% – O(2) bond distances of 2.412(6) A, . The distance of Mn···Mn% separation and the angle of Mn – O(2)–Mn% are 3.334(3) A, and 100.58(22)°, respectively. The Mn···Mn separation is slightly shorter than the corresponding one of [Mn(3-MeO-salen)Cl]2 (3.572(5) A, ) [15]. Two nitrogen atoms and two oxygen atoms from salen ligands, and two oxygen atoms [O(3)] from capping water molecules occupy the coordination sites about each manganese. The equatorial Mn–O (phenoxo) [Mn–O(1) =1.864(5) A, , Mn – O(2)= 1.901(5) A, ] and Mn – N (imine) [Mn – N(1)= 1.983(7) A, , Mn–N(2)=1.972(7) A, ] bond distances are somewhat shorter than the axial Mn – O(water) [Mn – O(3)= 2.219(6) A, ] and Mn – O(m-phenoxo) [Mn – O(2)% = 2.412(6) A, ]. The equatorial atoms about the two manganese atoms are coplanar; consequently, the environment around each manganese atom can be described as a distorted N2O4 octahedron as a result of Jahn – Teller
effects. This results in four short and two long M–L bond lengths that correspond to a 5B1 ground state of Mn(III). In addition, hydrogen atoms of coordinated water in the dimer bind to the oxygen of the neighboring perchlorate anions with O(3)···O(ClO4)= 2.804(13) A, , and the shortest Mn···Mn separation of the interdimer through the ClO4− anion is larger than 7 A, .
3.1.2. [Mn(III)(salen)(NO3)]n (2) The structure (Fig. 2) of 2 consists of approximately planar Mn(III)(salen) moieties bridged by single nitrate
Fig. 2. ORTEP drawing (30% thermal ellipsoid probability) of [Mn(salen)(NO3)]n (2) with the atom labeling scheme.
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
Fig. 3. xm (--) and xmT (--) vs. T plots for compound 1. The solid line shows the best fit theoretical curve.
groups with anti–syn bridging arrangement. This anti– syn bridging arrangement generates the linear polymeric structure in which the closest Mn···Mn% separation is 5.7278(3) A, . This is the first example of nitrate group bridging in this fashion. Fig. 2 shows a view of one such moiety and defines the labeling scheme for the atoms. The Mn(III) ion is six-coordinated by virtue of the equatorial tetradentate salen ligand, that forms a tetragonal plane, and two axial nitrate oxygens [O(3), O(4)]. The equatorial Mn–O(phenoxo) [Mn–O(1) =1.864(5) A, , Mn – O(2)= 1.858(6) A, ] and Mn – N(imine) [Mn – N(1) = 1.964(7) A, , Mn –N(2) = 1.985(7) A, ] distances are shorter than those of the axial Mn – O(nitrate) [Mn – O(3)= 2.324(6) A, , Mn–O(4)% = 2.293(6) A, ] bonds. The resulting four short and two long Mn – N2O4 bond lengths are also due to the Jahn–Teller effect.
3.2. Magnetic properties The temperature dependences of magnetic susceptibility in an applied field of 1 T for compounds 1 and 2 were measured from 300 to 4 K and are shown in Figs. 3 – 4. The results for both compounds are identical within experimental error. The magnetic behavior of d4 high-spin Mn(III) compounds in a magnetic field is quite complex, being complicated by exchange interactions, low-symmetry ligand fields, and the fact that d4 ions are Jahn–Teller active [19,20].
3.2.1. Magnetic susceptibility of [Mn(III)(salen)(H2O)]2(ClO4)2 (1) Fig. 3 shows the xm and xmT versus T plots for 1.
11
The xmT per dinuclear manganese(III) unit increases from 5.935 cm3 mol − 1 K (6.89 mB) at 300 K to 8.41 cm3 mol − 1 K (8.20 mB) at 8 K, and decreases to 6.85 cm3 mol − 1 K (7.40 mB) at 3 K. The increase of xmT with decreasing temperature is characteristic of a ferromagnetic coupling between the two high-spin Mn(III) ions in 1, clearly this complex has an ST = 4 ground state. At low temperature the product xmT does not reach the predicted value of 10 cm3 mol − 1 K for an ST =4 ground state. The zero-field splitting (ZFS) and interdimer interactions between Mn(III) ions in the solid state may be responsible for this behavior. The latter point is believed to contribute little, because the closest contact distance between Mn(III) ions of the interdimer is large (\ 7 A, ). It was attempted initially to fit the data to the simple S= 2 dimer coupled model [21]. However, the calculated x values were always greater than the observed values in the temperature region below 15 K, suggesting the need to include a ZFS term. We have therefore attempted to fit the experimental data to the theoretical magnetic susceptibility calculated by the diagonalization of the effective spin Hamiltonian taking into account the single-ion and dipolar ZFS terms [22,23]: H= − 2JS1S2 + g mBSzHz + gÞmB[HxSx + HySy ] + D[S 2z − 1/3S(S+ 1)]+E(S 2x − S 2y)
(1)
As a first approximation, axial symmetry was assumed such that Sx = Sy, E=0, and the D values for the two Mn(III) ions were set equal. The best fit parameters obtained are J=6.30 cm − 1, and D= − 1.70 cm − 1 with g = gÞ = 2.0. The xm and xmT versus T plots calculated by using the best parameters are shown as solid lines in Fig. 3. The D values for Schiff-base complexes of Mn(III) have been found usually to fall in the range of −1.0 to − 3.0 cm − 1 [19]. Since the manganese environment is close to tetragonal geometry, mixing of the dz 2 and dx 2 − y 2 orbitals should be negligible so that the superexchange pathway from the dz 2 orbital of one manganese to the dx 2 − y 2 orbital of the other should be negligible. This implies that ferromagnetic exchange is propagated via a delocalized p-electron pathway.
3.2.2. Magnetic susceptibility of [Mn(III)(salen)(NO3)]n (2) The xmT versus T plot, as shown in Fig. 4 for 2, decreases from 2.87 cm3 mol − 1 K (4.79 mB) at 300 K to 1.80 cm3 mol − 1 K (3.80 mB) at 4 K, which is characteristic of a weak intrachain antiferromagnetic exchange interaction. We attempted initially to fit the data to the simple Heisenberg one-dimensional isotropic spin exchanging chain, using expression (2) derived by Fisher [24].
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
12
xo(chain)=Ng 2m 2BS(S +1)/3kT[(1 + u)/(1 −u)]
4. Supplementary material
with u=coth[2JS(S +1)/kT]−kT/2JS(S+1) (2) A least-squares fitting of the data to Eq. (2) gave −1 J= − 0.56 cm , g = 2.01. However, the data below ca. 30 K could not be fitted satisfactorily by this equation. Since the susceptibility does not turn over at the lowest temperatures studied, it is assumed that the interchain interaction is very small. To a first approximation, the analysis of the magnetic susceptibility of 2 was constructed by the combination of the anisotropic zero-field splitting and exchanged terms, i.e.
Tables containing complete crystallographic data, atom coordinates, anisotropic thermal parameters, and all bond distances and angles for 1 and 2 are available from the authors on request.
Acknowledgements We thank the National Science Council of Taiwan for support of this research under Grant NSC-872113-M-032-002.
xm(chain)= xo(chain)+ DxZFS
References
where
[1] (a) V.L. Pecoraro, (Ed.), Manganese Redox Enzymes, VCH, NY, 1992. (b) G. Christou, Acc. Chem. Res. 22 (1988) 328. (c) K. Wieghardt, Angew. Chem., Int. Ed. Engl. 33 (1994) 725. [2] K. Srinivasan, P. Michaud, J.K. Kochi, J. Am. Chem. Soc. 108 (1986) 2309. [3] (a) E.N. Jacobsen, W. Zhang, A.R. Muci, J.R. Ecker, L. Deng, J. Am. Chem. Soc. 113 (1991) 7063. (b) E.N. Jacobsen, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, VCH, New York, 1993, Chapter 4.2. [4] (a) N. Hosoya, A. Hatayama, R. Irie, H. Sasaki, T. Katsuki, Tetrahedron 50 (1994) 4311. (b) T. Katsuki, Coord. Chem. Rev. 140 (1995) 189. [5] K. Bernardo, S. Lippard, A. Robert, G. Commenges, F. Dahan, B. Meunier, Inorg. Chem. 35 (1996) 387. [6] (a) G.C. Dismukes, Photochem. Photobiol. 43 (1986) 99. (b) V.L. Pecoraro, Photochem. Photobiol. 48 (1988) 249. [7] (a) N. Tamara, M. Ikeuechi, Y. Inoue, Biochim. Biophys. Acta. 973 (1989) 281. (b) L.E. Andreasson, Biochim. Biophys. Acta. 973 (1989) 465. [8] V.L. Pecoraro, W.M. Butter, Acta Crystallogr., Sect C 42 (1986) 1151. [9] J.W. Gohdes, W.H. Armstrong, Inorg. Chem. 27 (1988) 1842. [10] X. Li, V.L. Pecoraro, Inorg. Chem. 28 (1989) 3403. [11] A.R. Oki, D.J. Hodgson, Inorg. Chim. Acta 170 (1990) 65. [12] (a) J.A. Bonadies, M.S. Lah, M.L. Kirk, D.P. Kessissoglou, W. Hatfield, V.L. Pecoraro, Inorg. Chem. 27 (1989) 2037. (b) N.A. Law, T.E. Machonkin, J.P. McGorman, E.J. Larson, J.W. Kampf, V.L. Pecoraro, J. Chem. Soc., Chem. Commun. (1995) 2015. [13] N. Aurangzeb, C.E. Hulme, M.R. Bermejo, A. Garcia-Deibe, M. Rey, J. Sarrmartin, A. Sousa, J. Chem. Soc., Chem. Commun. (1994) 1153. [14] M.R. Bermejo, A. Castin˜eiras, J.C. Garcia-Monteagudo, M. Rey, A. Sousa, M. Watikinson, C.A. McAuliffe, R.G. Pritchard, R.L. Beddoes, J. Chem. Soc., Dalton Trans. (1996) 2935 [15] B. Mabad, D. Luneau, S. Theil, F. Dahan, J.M. Savariault, J.P. Tuchagues, J. Inorg. Biochem. 43 (1991) 373. [16] J.E. Davies, B.M. Gatehouse, K.S. Murray, J. Chem. Soc. Dalton Trans. (1973) 2523. [17] O. Kahn, Molecular Magnetism, VCH, New York, 1993, p. 3. [18] E.J. Gabe, Y. Le Page, J.P. Charland, F.L. Lee, P.S. White, J. Appl. Crystallogr. 22 (1989) 384. [19] (a) B.J. Kennedy, K.S. Murry, Inorg. Chem. 24 (1985) 1552; (b) B.J. Kennedy, K.S. Murry, Inorg. Chem. 24 (1985) 1557. [20] M.L. Kirk, M.S. Lah, C. Raptopoutou, D.P. Kessissoglou, W.E. Hatifield, V.L. Pecoraro, Inorg. Chem. 30 (1991) 3900.
DxZFS =(x −xÞ) = − [2Ng 2m 2BS(S +1)/15kT] [{(1+ u)(1+ 6)/(1 − u)(1 −6)} +{2u/(1 − u)}] 6=1− [3ukT/2JS(S + 1)] (see Refs. [23–25]). Careful variation of D and J then gave a best fit with J = − 0.55 cm − 1, D = −0.43 cm − 1, and g =2.0 shown in Fig. 4. The weak antiferromagnetic interactions are the result of superexchange operating via bridging nitrate groups in the structure. The minus sign of D for compounds 1 and 2 reflects that the Mn(III) ions in 1 and 2 are in tetragonal elongated coordination sites which have a 5B1 electronic ground state with 5A1, 5B2 and 5E excited states [18,26]. .
Fig. 4. xm (--) and xmT (--) vs. T plots for compound 2. The solid line shows the best fit theoretical curve.
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13 [21] C.J. O’Connor, Prog. Inorg. Chem. 29 (1982) 203. [22] P. Garge, R. Chikate, S. Padhye, J.-M. Savariault, P. de Loth, J.-P. Tuchagues, Inorg. Chem. 29 (1990) 3315. [23] R.C. Reem, I.E. Solomon, J. Am. Chem. Soc. 109 (1987) 1216. [24] (a) M.E. Fisher, Am. J. Phys. 32 (1964) 343. (b) G.R. Wagner,
13
S.A. Friedberg, Phys. Lett. 9 (1964) 11. [25] V.R. Marathe, S. Mitra, J. Chem. Phys. 78 (1983) 915. [26] J.B. Vincent, H.L. Tsai, J.C. Blackman, S. Wang, P.D.W. Boyd, K. Folting, J.C. Huffman, E.B. Lobkorsky, D.N. Hendrickson, G. Christou, J. Am. Chem. Soc. 115 (1993) 12353.
. .
Structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and polynuclear [Mn(III)(salen)(NO3)]n Huey-Lih Shyu a,1, Ho-Hsiang Wei b,*, Yu Wang c a
Chung-Tai Institute of Health Science and Technology, Taichung, Taiwan, ROC b Department of Chemistry, Tamkang Uni6ersity, Tamsui, Taiwan, ROC c Instrumentation Center of College of Science, Taiwan Uni6ersity, Taipei, Taiwan, ROC Received 5 September 1998; accepted 17 February 1999
Abstract The synthesis, crystal structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 (1) and polynuclear [Mn(III)(salen)(NO3)]n (2) (salen=N,N%-bis(salicylideneaminato)ethylene) are reported. Compound 1 consists of a structurally dinuclear system in which two Mn ions are bridged by the oxygen atom of m-phenoxo of the salen ligands. Compound 2 is a linear polymeric chain in which the Mn ions are linked by NO3− groups. Cryomagnetic measurements show ferromagnetic interaction with J =6.30 cm − 1 and D= −1.70 cm − 1 for 1 and antiferromagnetic interaction with J = − 0.55 cm − 1 and D = −0.43 cm − 1 for 2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Magnetic properties; Manganese complexes; Schiff base complexes; Polynuclear complexes
1. Introduction Interest in the coordination chemistry of manganese has been driven by the important roles in metalloenzymes [1] and highly valuable catalysts in olefin expoxitation [2–6]. One of the most important processes in nature occurring in the oxygen evolving complexes of photosystem II (PS II) is believed to be catalyzed by a cluster of manganese ions [7]. The manganese coordination sphere is believed to be dominated by O and N donors from available amino acid side chains [8]. A variety of ligating systems has been employed to attain the high-valent manganese complexes and mimic the structural features of the active site; these Schiffbase ligands thus present suitable biometric ligands. Manganese(III) salen complexes (salen= N,N%-bis(salicylideneaminato)ethylene) are well studied. To date several X-ray crystal structures of mononuclear non* Corresponding author. Tel.: + 886-2-621 565; fax: + 886-2-622 8458. E-mail address: [email protected] (H.-H. Wei) 1 Also corresponding author.
substituted [Mn(III)(salen)X] complexes including those with X= Cl [9], S–Ph 10], O-o-C6H4(CH2CH2COCH3) [10], and substituted derivatives of [Mn(III)(7,7%-Ph2salen)(py)2] [2], [Mn(III)(5-Me-7-Memetsalen)Cl] [11], and a few correlated Mn(III) salpn complexes of known crystal structure (salpn= 1,3-bis(salicylideneiminato)propane) [12] have been reported already. To our knowledge, only three X-ray crystal structures of substituted binuclear Mn(III)-salenderivatives, diaqua-bridged [Mn(III)(3,5-Cl-salen) (H2O)2](ClO4)2 [13], m-phenoxy-bridged [Mn(III)(5-Brsalen)(H2O)]2(ClO4)2 [14] and [Mn(III)(3-MeOsalen)Cl]2 [15], and only one non-substituted polymeric [Mn(III)(salen)(OAc)]n [16], have been reported. However, non-substituted dinuclear and polynuclear Mn(III) salen complexes are relatively rare, and those that are structurally and magnetically characterized are still rarer. Herein we report the preparation, X-ray crystal structure and cryomagnetic behaviors of the m-phenoxybridged dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and nitrato-bridged polymeric [Mn(III)(salen)(NO3)]n complexes.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 0 8 9 - 4
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
2. Experimental
2.1. Synthesis [Mn(salen)(H2O)]2(ClO4)2 (1) was synthesized by mixing an ethanol solution (15 cm3) of ethylenediamine (0.06 g, 1 mmol) and 0.244 g (2 mmol) of salicylaldehyde in 15 cm3 of ethanol, which was then stirred for 2 h, with subsequent addition of ethanol (20 cm3) of Mn(ClO4)2 · 6H2O (0.362 g, 1 mmol). Caution: although no problems have been encountered in the present work, perchlorates are potentially explosive and should be treated in small quantities with care. The resulting solution was stirred for 10 min, and then 0.10 g (1 mmol) of triethylamine added. A deep-brown solution was obtained, which was refluxed for 2 h, then allowed to stand at room temperature. Several weeks of standing, led to the growth of deep-brown crystals of 1 suitable for X-ray analysis. Anal. Calc for C32H32Mn2N4O14Cl2: C, 43.77; H, 3.65; N, 6.38. Found: C, 44.02; H, 3.59; N, 6.42%. IR (KBr, cm − 1): n(CN) 1619(vs), n(ClO4− ) 1097(vs), 628(s). [Mn(salen)(NO3)]n (2) was prepared in the same way as compound 1 but using the following quantities of starting reagents: Mn(NO3)2 · 6H2O (0.287 g, 1 mmol), salicylaldehyde (0.2444 g, 2 mmol), and triethylamine (0.10 g, 1 mmol). The resulting solution was allowed to stand in air at room temperature for about 3 weeks to yield large well-formed dark-brown crystals of 2. Anal. Calc. for C16H14Mn N3O5: C, 50.10, H, 3.65; N, 10.96. Found: C, 49.95; H, 4.01; N, 11.20%. IR (KBr, cm − 1): n(CN) 1625(vs), n(NO3− ) 1472(vs), 1305(vs), 805(s), 637(s). 2.2. Physical measurements Infrared spectra (4000 – 400 cm − 1) were recorded from KBr pellets on a Bio-Rad FTS40 FTIR spectrophotometer. The temperature dependence of the magnetic susceptibility of the polycrystalline samples was measured between 4 and 300 K at a field of 1.0 T using a Quantum Design model MPMS computercontrolled SQUID magnetometer. Diamagnetic corrections were made using Pascal’s constants [17].
2.3. X-ray structure determinations The X-ray single-crystal data for both compounds were collected on an Enraf – Nonius CAD 4 diffractometer equipped with graphite-monochromated Mo Ka radiation (l= 0.7107 A, ), 2u –u scan mode. N independent reflections and No with I \2.0s(I) were observed. The structures were solved by the location of heaving atoms using a Patterson map and refined (based on F) by the full-matrix least-squares method using the NRCVAX software package [18]; the func-
Table 1 Crystallographic data [Mn(salen)(NO3)]n (2)
for
Chemical formula fw Space group a (A, ) b (A, ) c (A, ) b (°) V (A, 3) Z T (K) l (Mo Ka) (A, ) dcalc (g cm−3) m (cm−1) Rf a Rwb a b
9
[Mn(salen)(H2O)]2(ClO4)2
(1)
and
1
2
C32H32Mn2N4O14Cl2 877.40 P21/c 7.080(3) 19.013(5) 13.382(3) 97.00(3) 1788.0(10) 2 298 0.7107 1.630 9.053 0.076 0.083
C16H14MnN3O5 383.24 R3( 22.538(3) 16.542(3) 7277.0(18) 18 298 0.7107 1.574 5.539 0.060 0.062
Rf =( Fo − Fc )/ Fo . Rw =w( Fo − Fc )2/ Fo 2.
tion minimized was w( Fo − Fc )2, where w=1/ s 2(Fo). All non-hydrogen atoms were located readily and refined with anisotropic thermal parameters. The data collection, crystallographic data, and data reduc-
Table 2 Selected bond distances (A, ) and angles (°) for [Mn(salen)(H2O)]2(ClO4)2 (1) and [Mn(salen)(NO3)]n (2) [Mn(salen)(H2O)]2(ClO4)2 (1) Mn–O(1) 1.864(5) Mn–O(2)% 2.412(6) Mn–N(1) 1.983(7) C(16)–O(2) 1.358(9) C(7)–N(1) 1.297(12)
Mn–O(2) Mn–O(3) Mn–N(2) C(1)–O(1) C(10)–N(2)
1.901(5) 2.219(6) 1.972(7) 1.356 (9) 1.297(11)
Mn–O(2)–Mn% O(1)–Mn–O(2) O(2)–Mn–O(2)% O(1)–Mn–O(3) O(1)–Mn–N(2) O(1)–Mn–N(1)
O(3)–Mn–O(2)% O(1)–Mn–O(2)% O(2)–Mn–O(3) O(2)–Mn–N(1) N(1)–Mn–N(2) O(2)–Mn–N(2)
172.65(20) 92.01(23) 95.31(24) 171.0(3) 84..5(3) 86.9(3)
Mn–O(2) Mn–O(4) Mn–N(2)
1.858(6) 2.293(6) 1.985(7)
O(1)–Mn–N(2) O(1)–Mn–O(3)% O(1)–Mn–O(2) O(1)–Mn–N(1) O(2)–Mn–O(4) O(3)%–Mn–N(1) Mn–O(4)–N(3) Mn–O(1)–C(1)
176.1(3) 87.59(24) 91.17(24) 91.8(3) 91.1(3) 84.88(25) 133.0(5) 128.5(5)
100.58(22) 92.74(25) 79.42(21) 92.74(25) 177.0(3) 92.6(3)
[Mn(salen)(NO3)]n (2) Mn–O(1) 1.864(5) Mn–O(3)% 2.324(6) Mn–N(1) 1.964(7) O(4)–N(3) 1.218(10) O(3)%–Mn–O(4) O(2)–Mn–N(1) O(1)–Mn–O(4) N(1)–Mn–N(2) O(2)–Mn–N(2) O(2)–Mn–O(3)% O(3)%–Mn–N(2) Mn–O(3)%–N(3)% Mn–O(2)–C(16)
168.38(24) 176.8(3) 83.44(23) 84.3(3) 92.7(3) 96.5(3) 91.7(3) 128.5(5) 130.1(5)
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Fig. 1. ORTEP drawing (30% thermal ellipsoid probability) of [Mn(salen)(H2O)]2(ClO4)2 (1) with the atom labeling scheme.
tion information are listed in Table 1. Selected bond distances and angles are listed in Table 2.
3. Results and discussion
3.1. Description of the structures 3.1.1. [Mn(III)(salen)(H2O)]2(H2O)2 (1) The structure of 1 consists of [Mn(salen)(H2O)]22 + cations (Fig. 1) and perchlorate anions. The structure has a centrosymmetric dimer in which the manganese(III) atoms are linked by m-phenoxo bridges from one of the phenolic oxygen atoms of each salen ligand to the opposite metal center, thus resulting in m-phenoxo Mn –O(2)% and Mn% – O(2) bond distances of 2.412(6) A, . The distance of Mn···Mn% separation and the angle of Mn – O(2)–Mn% are 3.334(3) A, and 100.58(22)°, respectively. The Mn···Mn separation is slightly shorter than the corresponding one of [Mn(3-MeO-salen)Cl]2 (3.572(5) A, ) [15]. Two nitrogen atoms and two oxygen atoms from salen ligands, and two oxygen atoms [O(3)] from capping water molecules occupy the coordination sites about each manganese. The equatorial Mn–O (phenoxo) [Mn–O(1) =1.864(5) A, , Mn – O(2)= 1.901(5) A, ] and Mn – N (imine) [Mn – N(1)= 1.983(7) A, , Mn–N(2)=1.972(7) A, ] bond distances are somewhat shorter than the axial Mn – O(water) [Mn – O(3)= 2.219(6) A, ] and Mn – O(m-phenoxo) [Mn – O(2)% = 2.412(6) A, ]. The equatorial atoms about the two manganese atoms are coplanar; consequently, the environment around each manganese atom can be described as a distorted N2O4 octahedron as a result of Jahn – Teller
effects. This results in four short and two long M–L bond lengths that correspond to a 5B1 ground state of Mn(III). In addition, hydrogen atoms of coordinated water in the dimer bind to the oxygen of the neighboring perchlorate anions with O(3)···O(ClO4)= 2.804(13) A, , and the shortest Mn···Mn separation of the interdimer through the ClO4− anion is larger than 7 A, .
3.1.2. [Mn(III)(salen)(NO3)]n (2) The structure (Fig. 2) of 2 consists of approximately planar Mn(III)(salen) moieties bridged by single nitrate
Fig. 2. ORTEP drawing (30% thermal ellipsoid probability) of [Mn(salen)(NO3)]n (2) with the atom labeling scheme.
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
Fig. 3. xm (--) and xmT (--) vs. T plots for compound 1. The solid line shows the best fit theoretical curve.
groups with anti–syn bridging arrangement. This anti– syn bridging arrangement generates the linear polymeric structure in which the closest Mn···Mn% separation is 5.7278(3) A, . This is the first example of nitrate group bridging in this fashion. Fig. 2 shows a view of one such moiety and defines the labeling scheme for the atoms. The Mn(III) ion is six-coordinated by virtue of the equatorial tetradentate salen ligand, that forms a tetragonal plane, and two axial nitrate oxygens [O(3), O(4)]. The equatorial Mn–O(phenoxo) [Mn–O(1) =1.864(5) A, , Mn – O(2)= 1.858(6) A, ] and Mn – N(imine) [Mn – N(1) = 1.964(7) A, , Mn –N(2) = 1.985(7) A, ] distances are shorter than those of the axial Mn – O(nitrate) [Mn – O(3)= 2.324(6) A, , Mn–O(4)% = 2.293(6) A, ] bonds. The resulting four short and two long Mn – N2O4 bond lengths are also due to the Jahn–Teller effect.
3.2. Magnetic properties The temperature dependences of magnetic susceptibility in an applied field of 1 T for compounds 1 and 2 were measured from 300 to 4 K and are shown in Figs. 3 – 4. The results for both compounds are identical within experimental error. The magnetic behavior of d4 high-spin Mn(III) compounds in a magnetic field is quite complex, being complicated by exchange interactions, low-symmetry ligand fields, and the fact that d4 ions are Jahn–Teller active [19,20].
3.2.1. Magnetic susceptibility of [Mn(III)(salen)(H2O)]2(ClO4)2 (1) Fig. 3 shows the xm and xmT versus T plots for 1.
11
The xmT per dinuclear manganese(III) unit increases from 5.935 cm3 mol − 1 K (6.89 mB) at 300 K to 8.41 cm3 mol − 1 K (8.20 mB) at 8 K, and decreases to 6.85 cm3 mol − 1 K (7.40 mB) at 3 K. The increase of xmT with decreasing temperature is characteristic of a ferromagnetic coupling between the two high-spin Mn(III) ions in 1, clearly this complex has an ST = 4 ground state. At low temperature the product xmT does not reach the predicted value of 10 cm3 mol − 1 K for an ST =4 ground state. The zero-field splitting (ZFS) and interdimer interactions between Mn(III) ions in the solid state may be responsible for this behavior. The latter point is believed to contribute little, because the closest contact distance between Mn(III) ions of the interdimer is large (\ 7 A, ). It was attempted initially to fit the data to the simple S= 2 dimer coupled model [21]. However, the calculated x values were always greater than the observed values in the temperature region below 15 K, suggesting the need to include a ZFS term. We have therefore attempted to fit the experimental data to the theoretical magnetic susceptibility calculated by the diagonalization of the effective spin Hamiltonian taking into account the single-ion and dipolar ZFS terms [22,23]: H= − 2JS1S2 + g mBSzHz + gÞmB[HxSx + HySy ] + D[S 2z − 1/3S(S+ 1)]+E(S 2x − S 2y)
(1)
As a first approximation, axial symmetry was assumed such that Sx = Sy, E=0, and the D values for the two Mn(III) ions were set equal. The best fit parameters obtained are J=6.30 cm − 1, and D= − 1.70 cm − 1 with g = gÞ = 2.0. The xm and xmT versus T plots calculated by using the best parameters are shown as solid lines in Fig. 3. The D values for Schiff-base complexes of Mn(III) have been found usually to fall in the range of −1.0 to − 3.0 cm − 1 [19]. Since the manganese environment is close to tetragonal geometry, mixing of the dz 2 and dx 2 − y 2 orbitals should be negligible so that the superexchange pathway from the dz 2 orbital of one manganese to the dx 2 − y 2 orbital of the other should be negligible. This implies that ferromagnetic exchange is propagated via a delocalized p-electron pathway.
3.2.2. Magnetic susceptibility of [Mn(III)(salen)(NO3)]n (2) The xmT versus T plot, as shown in Fig. 4 for 2, decreases from 2.87 cm3 mol − 1 K (4.79 mB) at 300 K to 1.80 cm3 mol − 1 K (3.80 mB) at 4 K, which is characteristic of a weak intrachain antiferromagnetic exchange interaction. We attempted initially to fit the data to the simple Heisenberg one-dimensional isotropic spin exchanging chain, using expression (2) derived by Fisher [24].
H.-L. Shyu et al. / Inorganica Chimica Acta 290 (1999) 8–13
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xo(chain)=Ng 2m 2BS(S +1)/3kT[(1 + u)/(1 −u)]
4. Supplementary material
with u=coth[2JS(S +1)/kT]−kT/2JS(S+1) (2) A least-squares fitting of the data to Eq. (2) gave −1 J= − 0.56 cm , g = 2.01. However, the data below ca. 30 K could not be fitted satisfactorily by this equation. Since the susceptibility does not turn over at the lowest temperatures studied, it is assumed that the interchain interaction is very small. To a first approximation, the analysis of the magnetic susceptibility of 2 was constructed by the combination of the anisotropic zero-field splitting and exchanged terms, i.e.
Tables containing complete crystallographic data, atom coordinates, anisotropic thermal parameters, and all bond distances and angles for 1 and 2 are available from the authors on request.
Acknowledgements We thank the National Science Council of Taiwan for support of this research under Grant NSC-872113-M-032-002.
xm(chain)= xo(chain)+ DxZFS
References
where
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DxZFS =(x −xÞ) = − [2Ng 2m 2BS(S +1)/15kT] [{(1+ u)(1+ 6)/(1 − u)(1 −6)} +{2u/(1 − u)}] 6=1− [3ukT/2JS(S + 1)] (see Refs. [23–25]). Careful variation of D and J then gave a best fit with J = − 0.55 cm − 1, D = −0.43 cm − 1, and g =2.0 shown in Fig. 4. The weak antiferromagnetic interactions are the result of superexchange operating via bridging nitrate groups in the structure. The minus sign of D for compounds 1 and 2 reflects that the Mn(III) ions in 1 and 2 are in tetragonal elongated coordination sites which have a 5B1 electronic ground state with 5A1, 5B2 and 5E excited states [18,26]. .
Fig. 4. xm (--) and xmT (--) vs. T plots for compound 2. The solid line shows the best fit theoretical curve.
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. .