Terminal vs bridging hydrides of diiron dithiolates: protonation of Fe2(dithiolate)(CO)2(PMe3)4

Abstract

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(4), where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C(2)-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1](+), establishing the non-intermediacy of the terminal hydride ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[μ-H1](+) indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(4)](+) ([S-H2](+)). This intermediate converts to [t-H2](+) and [μ-H2](+) by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe(3) ligands are basal.

Figure 1

Structure of the active site of the [FeFe]-hydrogenase in the H_ox state (left). The structure of the H_red state remains uncertain - both structures are consistent with available observations. The amine cofactor is shown in the protonated form, although its protonation state is not known.

Figure 2

Structure of 1 showing 50% thermal ellipsoids. Selected distances (Å) and angles (deg): Fe1-Fe2, 2.6164(6); Fe-P_avg, 2.204; S(1)-Fe(1)-Fe(2)-S(2), 103.1; P(1)-Fe(1)-Fe(2)-P(3), 94.24.

Figure 3

Temperature dependence of the product ratio [t-H1]⁺/[μ-H1]⁺ resulting from the protonation of CD₂Cl₂ solutions of 1 with H(OEt₂)₂BAr^F₄ at various temperatures.

Figure 4

Structure of 2 showing 50% thermal ellipsoids. Selected distances (Å) and angle (deg): Fe1-Fe2, 2.6252(7); Fe-P_avg, 2.212; S(1)-Fe(1)-Fe(2)-S(2), 109.69; P(2)-Fe(1)-Fe(2)-P(4), 90.05.

Figure 5

³¹P{¹H} NMR spectra for protonation of Fe₂(pdt)(PMe₃)₄(CO)₂ with one equiv of H(OEt₂)₂BAr^F₄, starting at − 90 °C (bottom). The same solution at −60 °C is shown above. In the − 90 °C spectrum, the four singlets assigned to a S-protonated species ([S-H2]⁺) are indicated with *. Signals assigned to [t-H2]⁺ are indicated with o. The two doublets are assigned to [μ-H2]⁺.

Figure 6

Product distribution for the protonation of 2 with various deficiencies of the acid H(OEt₂)₂BAr^F₄. The fit on the right is for 14.4[Fe₂], = %[μ-H2⁺]/%[t-H2⁺], where

Figure 7

¹H NMR spectra in the adt region of a CD₂Cl₂ solution of [t-H3]⁺ . Top is the result of 1-D nOe experiment upon irradiation at δ −2.11. Middle and bottom: spectra before and after addition of D₂O, respectively (the Fe-H signal also vanishes). The shoulder on the δ 3.6 multiplet arises from [t-H3′]⁺.

Figure 8

Structures of the cations in [t-H3]BAr^F₄/[t-H3′]BAr^F₄ showing 50% thermal ellipsoids. Selected distances (Å) and angles (°) for [H3]⁺ and [H3′]⁺: Fe1B-Fe2B, 2.660(6); Fe1-Fe2, 2.659(6); O2-C2-Fe2, 152(1); O2B-C2B-Fe2B, 156(4). Fe1-C2, 2.50(1); Fe1B-C2B, 2.46(4).

Figure 9

DFT-optimized structure of the transition state for proton transfer from S to Fe in [Fe₂(Hedt)(CO)₂(PMe₃)₄]⁺ and [Fe₂(Hpdt)(CO)₂(PMe₃)₄]⁺. Note that these images are for the enantiomer of the structures shown elsewhere.

Scheme 1

Scheme 2