Inorg. Chem. 1995, 34, 134-139
134
Structural and Spectroscopic Properties of Antiferromagnetically Coupled FeUIMnUand FeI1Mn1IComplexes Theodore R. Holman, Zhigang Wang, Michael P. Hendrich, and Lawrence Que, Jr.* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received August 19, 1994@
The heterobimetallic complexes [FemMnnBPMP(02CCH2CH3)z1(BPh& (11, [FemMn11BPMP(0~CCH3)~l(BPh& (1'), and [FenMnnBPMP(02CCH2CH3)~](BPb) (2), in which BPMP is the anion of 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol,were synthesized and characterized by proton NMR, EPR, and magnetization measurements. The high-spin Fe(II1) (S = V2) and high-spin Mn(I1) centers (S = V 2 ) in 1 and 1' are antiferromagnetically coupled, resulting in an S = 0 ground state in these complexes. Variable temperature magnetic susceptibility measurements show that J = 23 cm-' for 1' (H= JSz*S2). The solid state structure of the FeIIMn" complex (2) was determined by X-ray crystallography. Complex 2 crystallizes in the triclinic space group Pi with the following unit cell parameters: a = 12.613(6) A, b = 15.037(13) A, c = 16.62(2) A, a = 82.070(5)", p = 81.180(5)", y = 67.940(5)", and 2 = 2. Its structure contains a @-phenoxo)bis@-carboxy1ato)dimetal cluster with an iron-manganese distance of 3.360(4) A. Complex 2 is remarkably rich in spectroscopic characteristics. The high-spin Fe(I1) (S = 2) and high-spin Mn(1I) (S = 5/2) ions are antiferromagnetically coupled affording an S = I12 ground state and an unusual EPR signal at 2.5 K. This resonance is broad with g, < 2 and exhibits hyperfine features due to the nuclear spin of the Mn(I1) ion ( I = V2). At temperatures above 10 K, a second resonance signal appears at g = 5.8 which is ascribed to a doublet within the first excited S = 3/2 manifold. Variable temperature studies of both the ground and excited state EPR signals have yielded values of J = 8 cm-l and = 5 cm-', which approximately agree with the multifield, variable temperature magnetic susceptibility data on the polycrystalline material.
The study of metal-metal interactions has elicited the interest of a broad range of inorganic chemists. Our efforts in this area have been stimulated by the emergence of metalloproteins with carboxylate-bridged bimetallic active sites. In general, such proteins have homodinuclear active sites involving first row transition metals like Fe (hemerythrin,2 ribonucleotide redu~ a t a s emethane ,~ monooxygenase," purple acid phosphatase5), Co (methionine aminopeptidase): and Zn (alkaline phosp h a t a ~ e Klenow ,~~ fragment of DNA p o l y m e r a ~ e ~ ~A) .heterobimetallic FeInZn" active site has been found for the purple acid phosphatase from kidney bean,* and the related diiron phosphatases from bovine spleen and porcine uterus can be converted into Fe1*IMI1forms by metal sub~titution.~ The Fe1I1CoI1form of the porcine enzyme, in particular, has sufficiently altered electronic properties for the bimetallic site as Abstract published in Advance ACS Abstracts, December 1, 1994. (1) (a) Karlin, K. D. Science 1993,261,701-708.(b) Que, L., Jr.; True, A. E. Prog. Inorg. Chem. 1990,38, 97-200. (2)(a) Holmes, M. A.; Stenkamp, R. E. J. Mol. Biol. 1991,220,723737. (b) Holmes, M. A.; Trong, I. L.; Turley, S . ; Sieker, L. C.; Stenkamp, R. E. J. Mol. Biol. 1991,218,583-593. (3) (a) Nordlund, P.; Eklund, H. J. Mol. Biol. 1993,232, 123-164. (b) Atta, M.; Nordlund, P.; Aberg, A.; Eklund, H.; Fontecave, M. J. Biol. Chem. 1992,267,20682-20688. (4)Rosenzweig, A. C.; Frederick, C. A,; Lippard, S . J.; Nordlund, P. Nature 1993,366,537-543. (5)True, A. E.;Scarrow, R. C.; Randall, C. R.; Holz, R. C.; Que, L., Jr. J. Am. Chem. Soc. 1993,115,4246-4255. (6)Roderick, S.L.; Matthews, B. W. Biochemistry 1993,32,3907-3912. (7)(a) Kim, E. E.; Wyckoff, H. W. J. Mol. Biol. 1991,218, 449. (b) Beese, L. S . ; Steitz, T. A. EMBO J. 1991,10,25-33. (8) Beck, J. L.; McConachie, L. A.; Summors, A. C.; h o l d , W. N.; de Jersey, J.; Zemer, B. Biochim. Biophys. Acta 1986,869,61-68. (9)(a) Beck, J. L.; Keough, D. T.; de Jersey, J.; Zemer, B. Biochim. Biophys. Acta 1984,791,357-363. (b) Davis, J. C.; Averill, B. A. Proc. Natl. Acad. Sci. U.S.A. 1982,79,4623-4621. (c) David, S.S . ; Que, L., Jr. J. Am. Chem. Soc. 1990, 112,6455-6463. (c) Holz, R. C.; Que, L., Jr.; Ming, L.-J. J. Am. Chem. Soc. 1992,114,4434@
4436.
0020- 166919511334-0134$09.0010
to allow the observation of NOESY cross peaks relating a number of the paramagnetically shifted NMR r e ~ o n a n c e s . ~ ~ The ability of the dinucleating ligand H-BPMP'O to form heterobimetallic complexes with iron being one of the metal ions has allowed us to investigate the novel properties of a number of heterobimetallic c ~ m p l e x e s . ~ ~ The - ' ~ Fen1NiI1 complex was the first example of a coupled dinuclear complex wherein EPR signals from both ground and excited spin states were observed.12 Indeed the temperature dependence of the excited state EPR signal provided an estimate of the antiferromagnetic coupling constant which was corroborated by a variable temperature magnetic susceptibility study. Several FernCu" derivatives were synthesized, all of which were shown to exhibit low field integer spin EPR ~igna1s.l~One of these provided a test for the spin quantitation protocol developed for complexes with integer spin.13b In this paper, we explore the effects of replacing Fe(II1) with Mn(I1) in a dinuclear BPMP complex to afford an FenMnn complex which is isoelectronic to the Fe'"Fe" center. We report the synthesis and properties of the heterobimetallic complexes [Fe"'MnUBPMP(02CCH2CH3)2]+ (l),[Feu1Mn11BPMP(O+2CH3)2]+(l'),and [FemMnI1BPMP(02CCH2CH3)2]+ (2). (10) Abbreviations used: H-BPMP, 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol; H-BIMP, 2,6-bis[(bis((l-methylimidazol2-yl)methyl)amino)methyl]-4-methylphenol; TACN, 1,4,7-triazacy-
clononane. (11)Borovik, A. S.;Papaefthymiou, V.; Taylor, L. F.; Anderson, 0. P.; Que. L.. Jr. J. Am. Chem. Soc. 1989. 111. 6183-6195. (12)Holman, T. R.; Juarez-Garcia, C.; Hendrich, M. P.; Que, L., Jr.; Munck, E. J. Am. Chem. Soc. 1990,112,7611-7618. (13) (a) Holman, T. R.; Andersen, K. A.; Anderson, 0. P.; Hendrich, M. P.; Juarez-Garcia, C.; Miinck, E.; Que, L., Jr. Angew. Chem., Intl. Ed. Engl. 1990,29, 921-923. (b) Juarez-Garcia, C.;Hendrich, M. P.; Holman, T. R.; Que, L.; Miinck, E. J. Am. Chem. Soc. 1991.113, 518-525.
0 1995 American Chemical Society
Inorganic Chemistry, Vol. 34, No. I, 1995 135
FemMnn and FenMnn Complexes
Experimental Section All reagents and solvents were purchased from commercial sources and used as received unless noted otherwise. Microanalysis were performed by Desert Analytics, Inc., Tuscon, AZ, while the metal analyses were performed by the Soil Research Laboratory at the University of Minnesota, St. Paul, MN. The solvents CHzClz and CH3CN were distilled from CaHz under argon before use. The ligand 2,6bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (HBPMP) was synthesized according to published methods.11J2 (Bis-p-O,O'-propionato)(2,6-bis[(bis(2-pyridylmethyl)~no)m-
ethyl]-4methyIphenolato)iron(III)mangan~Bis(tetrapheny1borate), FPMnnBPMP(O~CCHzCH~)z](B~)zCH~COC& (1). [FernMnnBPMP(02CCH2CH3)2](BPh&CH3COCH3 (1) was synthesized
Table 1. Crystallographic Experiments and Computations for 2 fO~Ula a, A b, %, c, A a, deg /?,deg y , deg V, A3 Z
C&6sBClzFeMnN605 12.613(6) 15.037(13) 16.62(2) 82.070(5) 81.180(5) 67.940(5) 2876(5) 2
fW space group
?Ip9
D(calc), g cm-3 p , cm-' Ra
Ra
1190.76 P1 175 0.7107 1.375 6.10 0.068 0.072
" R = (XI(Fo- F c ) 1 ) / ( 5 J ;R, = ((2 wlF0 - FCl2)/(XW ( F ~ ) ~ ) } " ~ .
in these intensities was observed during the course of data acquisition. Lorentz and polarization corrections were applied to the data, and absorption corrections based on Zy scans were carried out (correction factors 0.87-1.14) using the program, DIFABS.16 The structure was solved by using Patterson and Fourier methods using 10087 ( I > a(4)out of 10288 reflections. Neutral atom scattering factors (including anomalous scattering) were used.17 All non-H atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in calculated positions (C-H = 0.95 A, EH = 1.2 BA, where E A = isotropic equivalent thermal parameter for the atom to which the proton is attached). Weighted (w = [@(I?) gF1-l) leastsquares refinement on F was carried out by altematively refining the cation or the anions plus solvent molecules until the largest shifdesd ratio was equal to 0.02. In the final AF map, the highest peaks were located near the partially occupied CHzC12 solvate positions. Atomic coordinates for the non-hydrogen atoms and selected bond lengths and angles of compound 2 are listed in Tables 2 and 3, respectively. Complete tables of fractional atomic coordinates, thermal parameters, (sis-~-O,O'-acetato)(2,6-bis[(bh(2-pyridylmethyl)amino)methyl]bond lengths, and bond angles for 2 can be found in the supplementary 4-methylphenolato)iron(III)manganese(II) Bis(tetraphenylborate), material. [FemMnnBPMP(OzCCH&](BPh&CH3COCH3 (1'). This complex Physical Methods. 1D and 2D 'H NMR experiments were was prepared by using the same experimental procedures outlined for performed on a Varian VXR 300 NMR spectrometer. The sample was 1 except that Mn(NO& and NaOzCCzH5 were replaced by 0.046 g of dissolved in CHzClz solvent. The 1D spectra were obtained using a Mn(OzCCH3)24H20 (75% yield). Anal. Calcd for CggHssB2Fe90" pulse with 16K data points. An inversion-recovery pulse sequence h " 6 0 6 : C, 72.64; H, 5.89; N, 5.78; Fe, 3.84; Mn, 3.78. Found, C, was used to obtain non-selective proton longitudinal relaxation times 72.83; H, 6.01; N, 5.97; Fe, 3.92; Mn, 3.71. W - v i s (acetone): A, (TI) with carrier frequency set at several different positions to ensure 385 nm (sh), 596 nm ( 6 = 980 M-' cm-l). the validity of the measurements. A typical magnitude COSY spectrum (Bis-p-O,O'-propionato)(2,6-bis[(bis(2-pyridylmethyl)amino)was obtained by collecting 1024 data points in tz and 256 data points methyl]-4-methylphenolato)iron(II)manganese(II) Tetraphenylborate, [F~MnnBPMP(OzCCHzCH3)~]BPh4.0.8CH~C1~(2). [FenMnnin t1 with a repetition time of
