J . Am. Chem. SOC.1992, 114, 8742-8743
8742
in C D 3 0 Dand DMSO-d6 solution, consistent with dissociation (es 1). Fe2(H20)(02CR)4(tmen)2 2Fe(02CR),(tmen) +H20 (1)
-
The solid-state structures of 1 and 2 were confirmed by single-crystal X-ray determinations (2 is shown in Figure 1 and 1 in Figure S-I in the supplementary material). Each complex consists of a g a q u a bis(p-02CR) diiron core with additional terminal carboxylate ligands coordinated to each Fe and arranged such that the uncoordinated oxygens O(7) and O(9) are involved in strong hydrogen bonding with the bridging water [0(1)-0(7), 2.57 8, in 1 and 2.54 A in 2; 0(1)--0(9), 2.58 A in 1 and 2.57 A in 21. This results in a twisting of the bridging water oxygen away from a tetrahedral geometry as reflected in FeO-H angles [in 2: Fe(l)-O(l)-H(lA), 126.3 (3)'; Fe(2)-O(l)-H(lB), 118.4 (2)'; Fe(1)-O(1)-H(lB), 97.0 (2)'; Fe(2)-O(l)-H(lA), 95.6 (2)'; but H(lA)-O(l)-H(lB), 108.7 (3)OI. The hydrogen bonds in 2 are symmetric [O(l)-H(lA), 1.08 A; O(1)-H(lB), 0.98 A], but in 1 they are asymmetric [0(1)-H( lA), 0.85 A; O(1)-H( lB), 1.37 A]. Fe-0 bond lengths for oxygens trans to the terminal carboxylates are considerably shorter thap those trans to the nitrogen donors in 1, but not in 2. The Fe-Fe separations and the Fe-(p-0) bonds are considerably larger than those found in diiron(I1) complexes with bridging OH-, [Fe2(0H)(02CCH3),(Me,TACN),]+ (3.32 A),6 OR-, [Fe2BPMP(02CCH2CH3)21+ (3.348 &,I5 [Fe2(N-Et-HPTB)(O2CPh),I2+ (3.473 A),'6 or 0,OCHO-, [Fe2(BIPhMe)2(HC02)4](3.585 A),'' which is consistent with a neutral bridging ligand. The Mossbauer spectra confirm the high-spin iron(I1) oxidation state.I8 Only one doublet is observed for 2, but the appearance of two resolvable doublets for 1 indicates that the structural asymmetry is reflected in slightly different electronic environments for the two iron atoms. These values are very similar to a fit of the spectrum of the diiron(I1) form of RRB2 from Escherichia coli.19 Preliminary magnetic susceptibility measurements on powdered samples and frozen solutions of 1 and 2 indicate substantially weaker magnetic coupling between iron atoms than is observed for OH--bridged 3" ( J = -13.1 cm-I) and deoxyhemerythrin ( J = -12 to -38 cm-' by MCD-ESR,4 and -15 cm-' by 'H NMRZ0). This decreased coupling in 1 and 2 is consistent with the weakly ferromagnetic behavior found in d e ~ x y H r N ~Complexes .~ 1 and 2 are EPR silent,,' in contrast to deoxyHrN,, which exhibits a low-field EPR ~ i g n a l . ~However, ,~~ either type of EPR behavior is consistent with an integer-spin ground state.'5b%23.24 In conclusion, the first examples of diiron(I1) complexes containing a bridging water have been prepared. The structure of 2 has been shown by IH N M R to be maintained in noncoordinating aprotic solvents, but not in protic or strongly coordinating solvents. Protonation of the bridging oxygen results in the longest Fe(II).-Fe(II) and Fe-Obridgedistances yet observed in similar (15) (a) Borovik, A. S.;Que, L., Jr. J . A m . Chem. SOC.1988, 110, 2345-2347. (b) Borovik, A. S.;Hendrich, M. P.; Holman, T. R.; Munck, E.; Papaefthymoiu, V.; Que, L., Jr. J . A m . Chem. SOC.1990, 112, 6031-6038. (16) Menage, S.;Brennan, B. A.; Juarez-Garcia, C.; Munck, E.; Que, L., Jr. J . Am. Chem. SOC.1990, 112, 6425-6426. (17) (a) Tolman, W. B.; Bino, A.; Lippard, S. J. J . Am. Chem. SOC.1989, 111, 8522-8523. (b) Tolman, W. B.; Liu, S.;Bentsen, J. G.; Lippard, S.J. J . Am. Chem. SOC.1991, 113, 152-164. (18) For measurements made at 4.2 K and referenced to iron metal at room temperature: for 1, &?,(I) = 3.11, 6(1) = 1.25, and &?Q(2) = 2.70, 6(2) = 1.26; for 2, AEQ = 2.75, 6 = 1.27 mm/s. (19) AEQ(I)= 3.28 mm/s, 6 ( l ) = 1.27 mm/s, and hEQ(2) = 2.93 mm/s, 6(2) = 1.26 mm/s. Lynch, J. B.; Juarez-Garcia, C.; Munck, E.; Que, L., Jr. J . Biol. Chem. 1989, ?54, 8091-8096. (20) Maroney, M. J.; Kurtz, D. M., Jr.; Nocek, J. M.; Pearce, L. L.; Que, L., Jr. J . Am. Chem. SOC.1986, 108, 6871-6879. (21) Spectra of frozen (
