Models for Unsymmetrical Active Sites in Metalloproteins: Structural, Redox, and Magnetic Properties of Bimetallic Complexes with MII-(μ-OH)-FeIII Cores

Abstract

Bimetallic complexes are important sites in metalloproteins but are often difficult to prepare synthetically. We have previously introduced an approach to form discrete bimetallic complexes with M^II-(μ-OH)-Fe^III (M^II = Mn, Fe) cores using the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]^3-). This series is extended to include the rest of the late 3d transition metal ions (M^II = Co, Ni, Cu, Zn). All of the bimetallic complexes have similar spectroscopic and structural properties that reflect little change despite varying the M^II centers. Magnetic studies performed on the complexes in solution using electron paramagnetic resonance spectroscopy showed that the observed spin states varied incrementally from S = 0 through S = 5/2; these results are consistent with antiferromagnetic coupling between the high-spin M^II and Fe^III centers. However, the difference in the M^II ion occupancy yielded only slight changes in the magnetic exchange coupling strength, and all complexes had J values ranging from +26(4) to +35(3) cm^-1.

Figure 1

Examples of bimetallic systems using the ligand [MST]³⁻ with (A) redox-inactive metal ions and (B) transition metal ions.

Figure 2

ESI-MS spectra of (A)[Mn^II(OH)Fe^III]⁺, (B) [Fe^II(OH)Fe^III]⁺, (C) [Co^II(OH)Fe^III]⁺, (D) [Ni^II(OH)Fe^III]⁺, (E) [Cu^II(OH)Fe^III]⁺, (F) [Zn^II(OH)Fe^III]⁺, with the simulated spectra in grey.

Figure 3

(A) Absorbance and (B) FTIR spectra for [Mn^II(OH)Fe^III]⁺ (black solid line), [Fe^II(OH)Fe^III]⁺ (black dashed line), [Co^II(OH)Fe^III]⁺ (black dotted line), [Ni^II(OH)Fe^III]⁺ (grey solid line), [Cu^II(OH)Fe^III]⁺ (grey dashed line), (F) [Zn^II(OH)Fe^III]⁺ (grey dotted line). Absorbance measurements were performed on 0.1 mM CH₂Cl₂ solutions at room temperature, and FTIR measurements were performed in KBr discs.

Figure 4

Thermal ellipsoid diagrams depicting the molecular structures of (A) [Mn^II(OH)Fe^III]⁺, (B) [Fe^II(OH)Fe^III]⁺, (C) [Co^II(OH)Fe^III]⁺, (D) [Ni^II(OH)Fe^III]⁺, (E) [Cu^II(OH)Fe^III]⁺, and (F) [Zn^II(OH)Fe^III]⁺. Ellipsoids are drawn at the 50% probability level, and only the hydroxido H atoms are shown for clarity.

Figure 5

EPR spectra (red) and simulations (black) of Zn^IIFe^III at the temperatures listed. Microwave parameters: 9.647 GHz, 0.02 mW, B₁ ⊥ B. See Table 3 for simulation parameters.

Figure 6

EPR spectra (red) and simulations (black) of [Ni^II(OH)Fe^III]⁺ at the temperatures listed, showing changes in the relative intensities of the signals (g = 5.45, 5.05, 2.34, 1.65) from the S_C = 3/2 state and growth of the g = 8 signal from the S_C = 5/2 state. Microwave parameters: 9.663 GHz, 0.2 mW, B₁ ⊥ B. The simulations matched the temperature dependence of the signals and used a concentration that was in quantitative agreement with that measured for the complex.

Figure 7

Signal intensity × temperature versus temperature of the signals at g = 5.45(●), 5.05(■), 8.0(×) observed from [Ni^II(OH)Fe^III]⁺. The solid traces are % population of the indicated doublets calculated from eq 1 for J = 35 cm⁻¹ and appropriate parameters in Table 3.

Figure 8

EPR spectrum (red) and simulation (black) of [Cu^II(OH)Fe^III]⁺ attemperature of 7 K. Microwave parameters: 9.339 GHz, 0.02 mW, B₁ || B. See Table 3 for simulation parameters. The inset shows signal intensity × temperature and the corresponding percent population of the corresponding doublet for J = +33(3) cm⁻¹ with the error represented by the dashed lines.

Scheme 1

Preparation of [(TMTACN)M^II–(μ-OH)–Fe^IIIMST]⁺ complexes. M^II = Mn, Fe, Co, Ni, Cu, Zn, x = 2 for all syntheses except Ni^II where x = 5.