Self-assembly of dinitrosyl iron units into imidazolate-edge-bridged molecular squares: characterization including Mössbauer spectroscopy

Abstract

Imidazolate-containing {Fe(NO)(2)}(9) molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)(2)(NO)(2) precursor. The structures of complex 1 [(imidazole)Fe(NO)(2)](4), (Ford, Li, et al.; Chem. Commun.2005, 477-479), 2 [(2-isopropylimidazole)Fe(NO)(2)](4), and 3 [(benzimidazole)Fe(NO)(2)](4), as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly coordinating (THF) and noncoordinating (CH(2)Cl(2)) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)(2)}(10) compounds A ((NHC-iPr)(2)Fe(NO)(2)), and C ((NHC-iPr)(CO)Fe(NO)(2)), and oxidized {Fe(NO)(2)}(9) compounds B ([(NHC-iPr)(2)Fe(NO)(2)][BF(4)]), and D ((NHC-iPr)(SPh)Fe(NO)(2)) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. Mössbauer data for the tetrameric complexes 1-3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)(2)}(9) units bound to one anionic ligand and one neutral ligand. At room temperature, the paramagnetic, S = (1)/(2) per iron, centers are not coupled.

Figure 1

Active site structures of a) [NiFe]-hydrogenase and b) bovine erythrocyte superoxide dismutaste demonstrating bridging cysteine or histidine (shown in blue) as found in metalloproteins.^,

Figure 2

Structure and ball and stick rendition of a RRE in which the {Fe(NO)₂}⁹ units are spaced 3.997 Å apart resulting in non-coupled S = ½, {Fe(NO)₂}⁹ centers.

Figure 3

a), b) Views of the thermal ellipsoid plot at 50% probability of [(Imid-benz)Fe(NO)₂]₄, complex 3. The labels correspond to the distance between opposite aryl C-C bonds at the widest point and at the closest point. Red: O, blue: N, black: C, orange: Fe, gray: H. c) Two molecules from the extended packing diagram to show the close contact of the benzyl groups of the closed portion to those of the open portion. d) Labeling scheme to demonstrate selected intermolecular C to C distances between benzyl groups of the closed portion to the open portion. Nitrosyl groups have been removed for clarity. C-C distances, Å: C8-C12 3.610; C8-C11 3.587; C11-C4 4.926; C11-C3 4.497; C11-C2 3.837; C10-C2 3.506; C10-C3 3.940.

Figure 4

Ball and stick representations of the structures of complexes 1 – 3 as derived from X-ray diffraction analysis and with canting of the imidazoles emphasized by shaded planes. In each case, the view on the right is from a rotation of 90° relative to the left.

Figure 5

Up and/or down orientations of imidazolate ligands of complexes 1 – 3 and an analogous Cu-containing molecular square. “Up” and “down” refers to the orientation of the ethenyl (HC=CH) group of the imidazolate with respect to the Fe₄ plane. Imidazole substituents (in the case of complex 2 and 3) have been removed for clarity. Red: O, blue: N, black: C, orange: Fe, dark red: Cu.

Figure 6

A view down the center of the squares demonstrating blocked cavities for complexes 1 and 2 and an open cavity for complex 3.

Figure 7

Experimental and computational structures and electrostatic potentials for 1 (left) and 3 (right), with the calculated coordinates shown taken from 3_quint. Both 3_calc. and 3_frozen are shown, with the imidazolate groups held frozen in the latter shown outlined in red. Electrostatic potentials were generated at an isosurface value of 0.01.

Figure 8

Orientational isomers of 1 (up/down, left) and 3 (up/up, right).

Figure 9

EPR spectra of complex 1 in THF at 170 K (line width = 30 G), reproduced with permission from ref. , 298 K (frequency at 9.45 GHz, line width = 14 G), and in CH₂Cl₂ at 298 K (frequency at 9.45, line width = 14 G); complex 2 in THF at 10 K (frequency at 9.49 GHz, line width = 43 G), 298 K (frequency at 9.45 GHz, line width = 13 G), and in CH₂Cl₂ at 298 K (frequency at 9.45, line width = 12 G); and complex 3 in THF at 10 K (frequency at 9.49 GHz, line width = 19 G), 298 K (frequency at 9.45 GHz, line width = 18 G), and in CH₂Cl₂ at 298 K (frequency at 9.44 GHz, line width = 14 G).

Figure 10

Scan reversals of the cyclic voltammograms to isolate successive waves of complexes a), 1; b), 2; c), 3 in 2 mM CH₂Cl₂ solution. All are referenced to Cp₂Fe/Cp₂Fe⁺.

Figure 11

5 K Mössbauer spectra for tetrameric complexes 1, 2, and 3 in frozen THF solution in an applied field of 700 G.

Figure 12

5 K Mössbauer spectra of test complexes A–D in frozen THF solution in an applied field of 700 G.