N-Substituted Derivatives of the Azadithiolate Cofactor from the [FeFe] Hydrogenases: Stability and Complexation

Abstract

Experiments are described that probe the stability of N-substituted derivatives of the azadithiolate cofactor recently confirmed in the [FeFe] hydrogenases (Berggren, G., et al. Nature 2013, 499, 66). Acid-catalyzed hydrolysis of bis(thioester) BnN(CH₂SAc)₂ gives [BnNCH₂SCH₂]₂ rather than azadithiol BnN(CH₂SH)₂. Treatment of BnN(CH₂SAc)₂ with NaO^tBu generates BnN(CH₂SNa)₂, which was trapped with NiCl₂(diphos) (diphos = 1,2-C₂H₄(PR₂)₂; R = Ph (dppe) and Cy (dcpe)) to give fully characterized complexes Ni[(SCH₂)₂NBn](diphos). The related N-aryl derivative Ni[(SCH₂)₂NC₆H₄Cl](diphos) was prepared analogously from 4-ClC₆H₄N(CH₂SAc)₂, NaO^tBu, and NiCl₂(dppe). Crystallographic analysis confirmed that these rare nonbridging [adt^R]²⁻ complexes feature distorted square planar Ni centers. The analogue Pd[(SCH₂)₂NBn](dppe) was also prepared. ³¹P NMR analysis indicates that Ni[(SCH₂)₂NBn](dppe) has basicity comparable to typical amines. As shown by cyclic voltammetry, the couple [M[(SCH₂)₂NBn](dppe)]^+/0 is reversible near −2.0 V versus Fc^+/0. The wave shifts to −1.78 V upon N-protonation. In the presence of CF₃CO₂H, Ni[(SCH₂)₂NBn](dppe) catalyzes hydrogen evolution at rate of 22 s⁻¹ in the acid-independent regime, at room temperature in CH₂Cl₂ solution. In contrast to the instability of RN(CH₂SH)₂ (R = alkyl, aryl), the dithiol of tosylamide TsN(CH₂SH)₂ proved sufficiently stable to allow full characterization. This dithiol reacts with Fe₃(CO)₁₂ and, in the presence of base, NiCl₂(dppe) to give Fe₂[(SCH₂)₂NTs](CO)₆ and Ni[(SCH₂)₂NTs](dppe), respectively.

INTRODUCTION

The identity of the dithiolate cofactor that supports the activity of the [FeFe] hydrogenases has been actively discussed since the original crystallographic descriptions of the enzymes¹ from D. desulfuricans and C. pasteurianum. Model studies show that the amine in azadithiolates facilitates protonation of its Fe(I)Fe(I) derivatives (Figure 1).^2,3 Furthermore, amine-containing dithiolates are required for oxidation of H₂ by mildly electrophilic Fe(II)Fe(I) species.⁴ The idea⁵ of an amine poised close but not coordinated to a metal center has spawned the development of homogeneous catalysts featuring proton relays⁶ and is a foundation of our understanding of the second coordination sphere.⁷

Figure 1.

Active site of the H-cluster in [FeFe] hydrogenase enzyme (left) and typical model complex (right).

The first spectroscopic evidence for the azadithiolate cofactor versus, say, 1,3-propanedithiolate, came from electron–nuclear double resonance studies on [FeFe] hydrogenases that revealed coupling of Fe(I) to two ¹⁴N centers, proposed to be the CN⁻ cofactor and the azadithiolate bridge.⁸ This assignment is supported by recent work on the C¹⁵N-enriched enzyme.^9,10 The nature of the dithiolate has finally been settled, with experiments showing that synthetic [Fe₂[(SCH₂)₂NH]-(CN)₂(CO)₄]²⁻ reconstitutes apo-HydA1 from C. reinhardtii to give a highly active catalyst. Isostructural diiron complexes [Fe₂(S₂C₃H₆)(CN)₂(CO)₄]^{2 −} and [Fe₂[(SCH₂)₂O]-(CN)₂(CO)₄]²⁻ are also accepted by the apoenzyme,^11,12 but the resulting proteins exhibit very low catalytic activity.¹²

Now that the azadithiolate cofactor has been confirmed, it is of interest to examine the cofactor itself, free of the protein and, if possible, metals. First reported in 2001,¹³ complexes of [adt^R]²⁻ can be prepared by many routes, almost all of which involve installation of the cofactor on diiron centers (Scheme 1).^13-16 Typical routes include condensation of CH₂O and amines with Fe₂(SH)₂(CO)₆^14,16 and alkylation of Fe₂(μ-SLi)₂(CO)₆, the latter route only suited for tertiary amines of the type RN(CH₂Cl)₂.¹³ Related complexes have been developed with an azadiphosphide group (RN(CH₂PR)₂²⁻)¹⁷ and a azadiselenide (RN(CH₂Se)₂²⁻)¹⁸ bridging two iron centers.

Scheme 1.

Synthetic Routes to Fe₂[(SCH₂)₂NR](CO)₆

In terms of obtaining the free cofactor, the diiron dithiolates represent obvious precursors to RN(CH₂SH)₂. Azadithiolato diiron complexes are, however, robust and exhibit no tendency to release the cofactor even in the presence of strong acids.

Free HN(CH₂SH)₂ and related derivatives might be anticipated to be unstable with respect to loss of hydrogen sulfide. This anticipated reactivity necessitates that the azadithiolates or their protonated derivatives be generated under mild reaction conditions. The bis(thioester) compounds of type RN(CH₂SAc)₂ emerged as attractive precursors to the azadithiolates. Indeed, hydrolysis of one such thioester has been claimed to afford [HOC₂H₄N(H)(CH₂SH)₂]Cl, the conjugate acid of an azadithiol.¹⁹

RESULTS AND DISCUSSION

Preparation and Hydrolysis of BnN(CH₂SAc)₂

Hydroxymethylation of primary amines in the presence of thioacetic acid is known to efficiently give bis(thioesters) RN-(CH₂SAc)₂.²⁰ The relevant transformations are given in eqs 1 and 2.

$${\mathrm{RNH}}_2+2{\mathrm{CH}}_2\mathrm{O}\to \mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{OH}\right)}_2$$ (1)

$$\mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{OH}\right)}_2+2\mathrm{AcSH}\to \mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{SAc}\right)}_2+2{\mathrm{H}}_2\mathrm{O}$$ (2)

This method appears general, and the initial phases of this study focused on benzyl derivatives. The acid-catalyzed hydrolysis of BnN(CH₂SAc)₂ was investigated as a route to the dithiol BnN(CH₂SH)₂. Upon treatment with aqueous HCl in EtOH, BnN(CH₂SAc)₂ was rapidly consumed with formation of a precipitate. ¹H NMR analysis of the crude product with an internal standard showed that [BnNCH₂SCH₂]₂ was present in ~60% yield. The reaction does not yield NMR-detectable amounts of trithiane (SCH₂)₃, a result consistent with the stoichiometry in eq 3.

$$\begin{array}{c}\hfill 2\mathrm{BnN}{\left({\mathrm{CH}}_2\mathrm{SAc}\right)}_2+2{\mathrm{H}}_2\mathrm{O}\hfill \\ \hfill \to {\left[{\mathrm{BnNCH}}_2{\mathrm{SCH}}_2\right]}_2+2{\mathrm{H}}_2\mathrm{S}+4\mathrm{HOAc}\hfill \end{array}$$ (3)

Experiments were conducted testing the utility of [RNCH₂SCH₂]₂ and the bis(thioesters) as precursors to diiron azadithiolate complexes. Reaction of [RNCH₂SCH₂]₂ with Fe₃(CO)₁₂ yields Fe₂(SCH₂N(R)CH₂)(CO)₆ instead of the typical Fe₂[(SCH₂)₂NBn](CO)₆ derivatives featuring Fe₂S₂ tetrahedranes.²¹ Additionally, reaction of BnN(CH₂SAc)₂ with Fe₃(CO)₁₂ gave complex mixtures containing only traces of Fe₂[(SCH₂)₂NBn](CO)₆.

Trapping RN(CH₂SNa)₂ (R = Bn or 4-ClC₆H₄)

Given the instability of azadithiol RN(CH₂SH)₂ implicated by the previous experiments, the preparation of the corresponding dithiolate dianion was investigated. Treatment of BnN-(CH₂SAc)₂ with NaOMe in MeOH afforded mainly the known diether BnN(CH₂OMe)₂. A bulkier nucleophile might favor attack at the carbonyl centers instead of the methylene groups, and avoidance of protic solvents would preclude formation of any thiols, these being prone to the degradation pathway in eq 3. Indeed, experiments suggest that the dithiolate can be generated by treatment of RN(CH₂SAc)₂ (R = Bn, 4-ClC₆H₄) with NaO^tBu in tetrahydrofuran (THF) at −78 °C. The putative RN(CH₂SNa)₂ was derivatized by treatment with NiCl₂(diphos) (diphos = 1,2-C₂H₄(PR′₂)₂; R′ = Ph (dppe) and Cy (dcpe)). These trapping experiments afforded modest yields of orange microcrystalline solids identified as Ni[(SCH₂)₂NR]-(diphos) (eqs 4 and 5).

$$\begin{array}{c}\hfill \mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{SAc}\right)}_2+2{\mathrm{NaO}}^{\mathrm{t}}\mathrm{Bu}\hfill \\ \hfill \to 2^{\mathrm{t}}\mathrm{BuOAc}+\mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{SNa}\right)}_2\hfill \end{array}$$ (4)

$$\begin{array}{c}\hfill \mathrm{RN}{\left({\mathrm{CH}}_2\mathrm{SNa}\right)}_2+2{\mathrm{NiCl}}_2\left(\mathrm{diphos}\right)\hfill \\ \hfill \to \mathrm{Ni}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{NR}\right]\left(\mathrm{diphos}\right)+2\mathrm{NaCl}\hfill \end{array}$$ (5)

In the case of the electrophile NiCl₂(dppe), the derivative Ni[(SCH₂)₂NR](dppe) was isolated and characterized according to ¹H and ³¹P{¹H} NMR and electrospray ionization mass spectrometry (ESI-MS) data. The complexes were stable as solids and in THF, CH₂Cl₂, and MeCN solution under an inert atmosphere. Single crystals of Ni[(SCH₂)₂NR](dppe) were grown from CH₂Cl₂/pentane. The solid-state structures of the two compounds are presented in Figures 2 and 3.

Figure 2.

ORTEP of Ni[(SCH₂)₂NBn](dppe) with ellipsoids drawn at the 50% probability level. H atoms and disorder are omitted for clarity. Selected distances (Å): Ni1−S1, 2.1925(10); Ni1−S2, 2.1766(9); Ni1−P1, 2.1840(10); Ni1−P2, 2.1675(9). Selected angles (deg): S1−Ni1−S2, 100.46(3); P1−Ni1−P2, 85.84(3); P1−Ni1−S1, 165.05(3); P2−Ni1−S1, 88.93(3); P1−Ni1−S2, 88.20(3); P2−Ni1−S2, 163.16(3); C27−N1−C28, 112.8(2), C27−N1−C29, 114.6(3); C28−N1−C29, 113.7(2).

Figure 3.

ORTEP of Ni[(SCH₂)₂NC₆H₄Cl](dppe) with ellipsoids drawn at the 75% probability level. H atoms and a solvent molecule are omitted for clarity. Selected distances (Å): Ni1−S1, 2.1737(9); Ni1−S2, 2.1792(10); Ni1−P1, 2.1651(10); Ni1−P2, 2.1663(10). Selected angles (deg): S1−Ni1−S2, 104.66(4); P1−Ni1−P2, 85.88(4); P1−Ni1−S1, 156.77(4); P2−Ni1−S1, 87.44(4); P1−Ni1−S2, 89.04(4); P2−Ni1−S2, 158.65(4); C27−N1−C28, 112.7(3), C27−N1−C29, 118.5 (3); C28−N1−C29, 112.00(2).

Crystallographic analysis revealed that Ni[(SCH₂)₂NBn]-(dppe) features a twisted NiS₂P₂ coordination. The average Ni−S (2.185 Å) and Ni−P bond lengths (2.176 Å) are similar to those in the propanedithiolate Ni(S₂C₃H₆)(dppe) (2.203 and 2.160 Å, respectively).²² Substitution, however, of CH₂ with NBn causes distortion of the ligand environment such that the NiS₂ and NiP₂ planes in Ni[(SCH₂)₂NBn](dppe) are twisted by 19.6° (this value being 7.6° for the propane-dithiolate). In the crystal, the Bn group participates in weak edge-to-face π−π stacking (4.3 Å) with the dppe ligand of a neighboring complex, orienting Bn toward Ni. In addition to stereoelectronic effects, this interaction causes the N lone pair to be directed away from the Ni site.

Crystallographic analysis of the complex Ni[(SCH₂)₂-NC₆H₄Cl](dppe) again reveals a twisted NiS₂P₂ core (Figure 3). The average Ni−S (2.179 Å) and Ni−P bond lengths (2.166 Å) are shortened relative to the benzyl derivative, highlighting a decrease in electron density at the metal center. The distortion of the ligand environments with R = 4-ClC₆H₄ is increased relative to the complex with R = Bn, with a twist of the NiS₂ and NiP₂ planes of 27.6° for the former derivative. The changes in bond lengths and coordination geometry likely reflect the weaker donicity of the [adt^C6H4Cl]²⁻ ligand. Containing a more basic diphosphine, NiCl₂(dcpe) was converted to Ni[(SCH₂)₂NBn](dcpe), albeit in lower yield than in the dppe case. The ¹H and ³¹P NMR (δ 73.6) and ESI-MS (m/z 676.3) data confirm formation of the target although the sample was not obtained in high purity. Nevertheless, the solid-state structure could be determined by diffraction (Figure 4).

Figure 4.

ORTEP of Ni[(SCH₂)₂NBn](dcpe)·CH₂Cl₂, showing one of the crystallographically independent complexes with ellipsoids drawn at the 50% probability level. H atoms, solvate, and disorder are omitted for clarity. Selected distances (Å): Ni1−S1, 2.1952(19); Ni1−S2, 2.1841(18); Ni1−P1, 2.1884(18); Ni1−P2, 2.1866(18); Ni2−S3, 2.1860(19); Ni2−S4, 2.176(2); Ni2−P3, 2.1774(19); Ni2−P4, 2.1797(18). Selected angles (deg): S1−Ni1−S2, 100.01(7); P1−Ni1−P2, 88.07(7); P1−Ni1−S1, 85.69(7); P2−Ni1−S1, 173.39(7); P1−Ni1−S2, 173.94(8); P2−Ni1−S2, 86.31(7); C27−N1−C28, 113.5(6), C27−N1−C29, 111.3(6); C28−N1−C29, 115.0(6). S4−Ni2−S3, 100.01(8); P3−Ni2−P4, 88.77(7); P3−Ni2−S3, 85.97(7); P4−Ni2−S3, 170.30(8); S4−Ni2−P3, 170.71(9); S4−Ni2−P4, 86.28(7); C62−N2−C63, 112.5(6); C62−N2−C64, 114.7(6); C63−N2−C64, 114.5(6).

The solid-state structure of Ni[(SCH₂)₂NBn](dcpe) mirrors that of the analogous dppe compound. While the Ni−P and Ni−S bond lengths in the two complexes are virtually identical, the ligand environment in the dcpe complex is less distorted, with the NiS₂ and NiP₂ planes being 2.9° and 10.5° apart in the two crystallographically independent complexes. This planarity might result from the greater size and σ-donicity of dcpe versus dppe. The anion in BnN(CH₂SNa)₂ was also trapped using PdCl₂(dppe) as the electrophile, affording crystalline Pd-[(SCH₂)₂NBn](dppe), whose ¹H NMR spectrum is similar to that for Ni[(SCH₂)₂NBn](dppe).

Protonation of Ni[(SCH₂)₂NBn](dppe)

Treatment of Ni[(SCH₂)₂NBn](dppe) with HOTf (1 equiv) in CD₂Cl₂ resulted in protonation, signaled by a change in color from orange to yellow-orange. The ³¹P NMR singlet only shifted by 3 ppm downfield upon protonation of the complex, but the ¹H NMR spectrum drastically changed (see Supporting Information)—a new singlet was observed at δ 8.84, assigned to N_H, and the symmetry of the Ni[(SCH₂)₂NBn](dppe) is lifted. While the SCH₂N signals appeared as a broad multiplet for the neutral complex, they appear as doublets-of-triplets for the ammonium species. Consistent with a C_s-symmetric cation, the CH₂Ph signal remains a doublet after protonation. The pK_a^MeCN of the amine was not determined since protonation labilizes the complex in this solvent. Nonetheless, addition of increasing amounts of BnNH₃⁺ (pK_a^MeCN = 16.76) or Bu₃NH⁺ (pK_a^MeCN = 18.03) to Ni[(SCH₂)₂NBn](dppe) shift the ³¹P NMR signals toward those of the fully protonated species (see Supporting Information). The complex Ni[(SCH₂)₂-NC₆H₄Cl)](dppe) did not survive protonation.

Electrochemical Behavior

Cyclic voltammograms (CVs) of Ni[(SCH₂)₂NBn](dppe) in CH₂Cl₂ feature irreversible oxidations at 0.11 V and a reduction at −2.02 V versus Fc^0/+. The cathodic wave is shifted to −1.55 V upon addition of 1 equiv of CF₃CO₂H in CH₂Cl₂. The anodic shift is attributed to protonation of the amine and more facile reduction of the cation [Ni[(SCH₂)₂N(H)Bn](dppe)]⁺ relative to its conjugate base. Anodic shifts of this magnitude are observed for diiron azadithiolate catalysts.^3,23 In the presence of CF₃CO₂H (1 equiv), no anodic peaks are observed when scanning in the positive direction. However, once [Ni[(SCH₂)₂NHBn]-(dppe)]⁺ has been reduced, the anodic peak corresponding to [Ni[(SCH₂)₂NBn](dppe)]^0/+ reappears. These observations are consistent with the series of electron transfer and chemical steps in eqs 6–10.

$$\begin{array}{c}\hfill {\mathrm{M}}^{\mathrm{II}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{NBn}\right]\left(\mathrm{dppe}\right)+{\mathrm{H}}^{+}\hfill \\ \hfill \to {\left[{\mathrm{M}}^{\mathrm{II}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)\right]}^{+}\hfill \end{array}$$ (6)

$$\begin{array}{c}\hfill {[{\mathrm{M}}^{\mathrm{II}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)]}^{+}+{\mathrm{e}}^{-}\hfill \\ \hfill \to {\left[{\mathrm{M}}^{\mathrm{I}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)\right]}^0\hfill \end{array}$$ (7)

$$\begin{array}{c}\hfill {[{\mathrm{M}}^{\mathrm{I}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)]}^0+{\mathrm{H}}^{+}\hfill \\ \hfill \to {\left[{\mathrm{HM}}^{\mathrm{III}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)\right]}^{+}\hfill \end{array}$$ (8)

$$\begin{array}{c}\hfill {[{\mathrm{HM}}^{\mathrm{III}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{N}\left(\mathrm{H}\right)\mathrm{Bn}\right]\left(\mathrm{dppe}\right)]}^{+}\hfill \\ \hfill \to {\left[{\mathrm{M}}^{\mathrm{III}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{NBn}\right]\left(\mathrm{dppe}\right)\right]}^{+}+{\mathrm{H}}_2\hfill \end{array}$$ (9)

$$\begin{array}{c}\hfill {[{\mathrm{M}}^{\mathrm{III}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{NBn}\right]\left(\mathrm{dppe}\right)]}^{+}{\mathrm{e}}^{-}\hfill \\ \hfill \to \left[{\mathrm{M}}^{\mathrm{II}}\left[{\left({\mathrm{SCH}}_2\right)}_2\mathrm{NBn}\right]\left(\mathrm{dppe}\right)\right]\hfill \end{array}$$ (10)

Upon addition of increasing amounts of CF₃CO₂H to [Ni[(SCH₂)₂NHBn](dppe)]⁺, current increases are observed, which are attributed to catalytic reduction of the acid to give H₂. The dependence of current on [CF₃CO₂H] plateaus at 500 mM of acid, which corresponds to a turnover frequency of 22 s⁻¹ according to the usual analysis (see Supporting Information).²⁴

The CV of Ni[(SCH₂)₂NC₆H₄Cl](dppe) in CH₂Cl₂ features an irreversible oxidation at 0.22 V and a quasi-reversible reduction at E_1/2 = −1.88 V, both versus Fc^0/+. The shifts to more positive potentials for the cathodic and anodic events for the ClC₆H₄ versus Bn derivatives further reflects the influence of the amine substituent on the metal center, despite its remoteness.

TsN(CH₂SH)₂ and Its Fe and Ni Derivatives

The lability of free azadithiols is proposed to be correlated to the basicity of the amine group. This hypothesis suggests that free azadithiols with electron-withdrawing groups on the nitrogen center could be stable. To address this question, the tosylamide (MeC₆H₄SO₂NR₂, abbreviated TsNR₂) platform was selected since such sulfonamides are known to be nonbasic.²⁵ The targeted TsN(CH₂SH)₂ was prepared from bis(thioester) TsN(CH SAc), which in turn was prepared from dichloride TsN(CH₂Cl)₂ (Scheme 2). Acid-catalyzed hydrolysis of the thioester afforded dithiol TsN(CH₂SH)₂ as analytically pure white crystals, the ¹H NMR spectrum of which is readily assigned.

Scheme 2.

Synthesis of TsN(CH₂SH)₂ and Its Complexes

The robustness of TsN(CH₂SH)₂ is further indicated by its efficient conversion to dithiolato complexes. Thus, treatment of the dithiol with Fe₃ (CO)₁₂ in hot toluene gave Fe₂[(SCH₂)₂NTs](CO)₆, isolated as red crystals. This synthetic method is analogous to the use of alkyl- and arylthiols.²⁶ In addition to its characterization by IR and NMR spectroscopy, Fe₂[(SCH₂)₂NTs](CO)₆ was examined by single-crystal X-ray diffraction (Figure 5). Hexacarbonyl Fe₂[(SCH₂)₂NTs](CO)₆ is structurally similar to many diiron(I) dithiolates. The Fe centers are within the sum of their covalent radii (2.64 Å for low-spin Fe) with Fe−Fe distance (2.5145(4) Å) being typical for such compounds, for example, 2.5047(6) Å for Fe₂[(SCH₂)₂NPh](CO)₆.¹⁴ Like this phenyl analogue, the N center in the Ts derivative is roughly trigonal planar, the sum of its bond angles being 357°. Reflecting the nonbasic nature of this tosylamido center, a solution of the diiron complex was found to be unaffected by HBF₄. The nonbasicity of Fe₂[(SCH₂)₂NTs](CO)₆ is in contrast to the easy protonation of other azadithiolato complexes with R = alkyl or aryl.³

Figure 5.

ORTEP of Fe₂[(SCH₂)₂NTs](CO)₆ with ellipsoids drawn at the 50% probability level and H atoms omitted for clarity. Selected distances (Å): Fe1−Fe2, 2.5145(4); Fe1−S1, 2.2537(5); Fe2−S2, 2.2547(5); Fe2−S1, 2.2619(5); Fe2−S2, 2.2547(5). Selected angles (deg): Fe1−S1−Fe2, 67.675(15); Fe1−S2−Fe2, 67.718(15); S1−Fe1−S2, 85.735(17); S1−Fe1−Fe2, 56.318(14); C7−N1−S3, 116.45(14); C7−N1−S3, 119.70(11); C8−N1−S3, 120.63(11).

In the presence of base, TsN(CH₂SH)₂ was also found to react with NiCl₂(dppe) to give Ni[(SCH₂)₂NTs](dppe), isolated as air-stable red crystals. The structure of this Ni complex was verified by X-ray diffraction (Figure 6).

Figure 6.

ORTEP of Ni[(SCH₂)₂NTs](dppe) with ellipsoids drawn at the 50% probability level. H atoms and disorder are omitted for clarity. Selected distances (Å): Ni1−S1, 2.1943(5); N1−S2, 2.1963(5); Ni1−P1, 2.1804(5); Ni1−P2, 2.1873(5). Selected angles (deg): S1−Ni1−S2, 94.568(19); P1−Ni1−P2, 87.143(18); P1−Ni1−S1, 90.505(18); P2−Ni1−S1, 168.68(2); P1−Ni1−S2, 173.82(2); P2−Ni1−S2, 88.549(18); C27−N1−C28, 116.95(15), C27−N1−S3, 119.69(13); C28−N1−S3, 117.26(13).

The complex Ni[(SCH₂)₂NTs](dppe) was found to be virtually isostructural to its NBn congener, however, with less distortion of the Ni center (the NiS₂ and NiP₂ interplanar angle being 11.7°). As with the diiron complex, the N atom is approximately planar (angle sum = 354°), but the Ni complex differs in that the sp²-hybridization causes puckering of the chelate ring, in contrast to the idealized chair/boat conformation in Fe₂[(SCH₂)₂NTs](CO)₆.

CONCLUSIONS

The present work was motivated by the recent confirmation of the azadithiolate ([adt^H]²⁻) cofactor in the [FeFe] hydrogenases.¹¹ Our studies were intended to probe the stability of the free azadithiol and its salts. A simplifying aspect of our endeavor was the use of the N-substituted analogues of [adt^H]²⁻ rather than the secondary amine itself. This compromise was necessary because the precursor HN-(CH₂SAc)₂ is unknown, and a synthesis of this compound could not be devised. The work described here led us to two instructive conclusions.

1. Azadithiolate Salts and Azadithiols Are Labile

The instability is associated with the proximity of the basic nitrogen centers to the thiol groups, this labile motif also being present in the yet-to-be-isolated thioaminals (R₂NCH₂SH). In contrast, related compounds are stable when SH is replaced by S-alkyl^27,28 or when the spacer between the thiol and the amine is extended, as is the case for cysteamine derivatives (R₂NCH₂CH₂SH).²⁷ When the N-substituent is electron-withdrawing, the corresponding dithiolates appear more stable. This effect is indicated by the improved yields of Ni[(SCH₂)₂-NC₆H₄Cl](dppe) relative to the N-benzyl derivative. More dramatically, N-substitution with p-toluenesulfonyl (Ts) allowed full characterization of the dithiol TsN(CH₂SH)₂. This dithiol was shown to react efficiently with iron carbonyls to give the corresponding azadithiolatodiiron(I) complex. In this complex, however, the nitrogen center is insufficiently basic to sustain protonation. In terms of their ease of synthesis and low basicity, the [adt^Ts]²⁻ derivatives resemble analogous N-acetylated ligands [adt^Ac]²⁻.²⁹

2. Azadithiolate is Stabilized by Coordination, Even to One Metal

Although hundreds of [adt^R]²⁻ complexes have been reported, few are not components of a diiron complex. In this paper, the N-substituted [adt^R]²⁻ ligands were stabilized through formation of complexes of the familiar Ni(dithiolate)-(dppe) motif.³⁰ Aside from the nickel complexes reported in this paper, the titanocene derivatives (C₅H₄R)₂Ti(adt^R) (prepared from (C₅H₄R′)₂Ti(SH)₂ and (CH₂NR)₃; R′ = H, Me; R = Ph, Me) feature nonbridging [adt^R]²⁻ complexes.¹⁵

Finally, these results have some bearing on the biosynthesis of [FeFe] hydrogenase. Containing three unusual cofactors, CO, CN⁻, and [adt^H]²⁻, as well as the attached 4Fe−4S cluster, the active site is assembled following a multistep maturation pathway.^10,31 The source of CO and CN⁻ has been elucidated, but the origin of [adt^H]²⁻ remains unsolved. As demonstrated here, the cofactor in its various protonated forms [H_nadt^H]⁽²⁻ⁿ⁾⁻ is expected to be too unstable to persist in the absence of metal ions. Indeed, the biochemical evidence³² and some literature precedents³³ suggest that the azadithiolate is biosynthesized on an Fe-containing scaffold.

EXPERIMENTAL SECTION

General Considerations

Unless otherwise stated, reactions were conducted using standard Schlenk techniques, and all reagents were purchased from Sigma-Aldrich or Fisher. Unless otherwise noted, all solvents were HPLC grade and purified using an alumina filtration system (Glass Contour, Irvine, CA). ESI-MS data were acquired using a Waters Micromass Quattro II or ZMD spectrometer with analytes in dilute CH₂Cl₂ solution. Analytical data were acquired using an Exeter Analytical CE-440 elemental analyzer. NMR data were acquired using Varian U400, U500, and VXR500 spectrometers. Chemical shifts (in parts per million) were referenced to residual solvent peaks (for ¹H, ¹³C) or external 85% H₃PO₄ (for ³¹P). Solution IR spectra were recorded on a PerkinElmer Spectrum 100 FTIR spectrometer. Crystallographic data were collected using a Siemens SMART diffractometer equipped with a Mo Kα source (λ = 0.710 73 Å) and an Apex II detector.

BnN(CH₂SAc)₂

In a modification of the method of Izawa,²⁰ a solution of benzylamine (10.72 g, 100 mmol) in 100 mL of 95% EtOH was treated with formaldehyde (37% in MeOH/H₂O, 24 mL). The mixture was heated at 60 °C for 30 min and then treated with AcSH (14.1 mL, 200 mmol). After 2 h, the reaction mixture was cooled to −17 °C, when colorless crystals formed. The crystals were isolated by filtration, washed with cold 95% EtOH, and dried briefly under vacuum. Yield: 22.52 g (79%). ¹H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d₆), 298 K): δ 7.34−7.23 (m, 5H, C₆H₅), 4.51 (s, 4H, C(O)CH₂), 3.61 (s, 2H, PhCH₂), 2.35 (s, 6H, CH₃). ¹³C NMR (75 MHz, DMSO-d₆, 298 K): δ 196 (C=O), 137 (Ph−C1), 129 (PhC2, PhC2′), 128 (PhC2, PhC2′), 128 (Ph−C1), 56.4 (C(O)−CH₂), 53.6 (PhCH₂), 31.5 (CH₃). Anal. Calcd for C₁₃H₁₇NO₂S₂: C, 55.09; H, 6.05; N, 4.94. Found: C, 55.24; H, 6.02; N, 4.98%.

Hydrolysis of BnN(CH₂SAc)₂

A solution of BnN(CH₂SAc)₂ (0.58 g, 2.00 mmol) in 10 mL of EtOH was treated with a 2 M solution of HCl in Et₂O (4.00 mL, 8.00 mmol) under Ar. After the mixture was stirred for 15 h, the white precipitate was isolated by filtration in air and washed with 2 × 5 mL of EtOH, yielding 0.370 g of crude product. ¹H NMR analysis (1,3,5-C₆H₃(OMe)₃ integration standard) of the precipitate showed 63% [BnNCH₂SCH₂]₂, indicating a yield of 0.23 g of [BnNCH₂SCH₂]₂ (theoretical yield: 330 mg). The reaction of BnN(CH₂SAc)₂ with NaOMe afforded the known compound BnN(CH₂OMe)₂.³⁴

[BnNCH₂SCH₂]₂

The following is an adaptation of a method for the α-methylbenzylamine derivative.³⁵ A solution of benzylamine (4.9 g, 0.045 mol) in 90 mL of MeOH was added to cold 37% aqueous formaldehyde (27 mL, 0.352 mol) at 0 °C. After it was stirred for 5 min at 0 °C, the solution was treated with a solution of NaSH·xH₂O (13.53 g, 0.12 mol) in 115 mL of MeOH. After it was stirred for 5 min at 0 °C for 24 h, the cloudy solution was concentrated to half its volume under vacuum causing an oil to precipitate. Approximately 30 mL of the oil was separated from the reaction mixture. The oil was mixed with 30 mL of acetone and diluted with 60 mL of EtOH. The solution was concentrated under vacuum until white crystals formed. The crystals were isolated by filtration and washed with 2 × 20 mL of EtOH. Yield: (0.560 g, 18%). ¹H NMR (500 MHz, DMSO-d₆) δ 7.24−7.34 (m, J = 15.8, 7.4 Hz, 10H), 4.54/4.51/4.38/4.35 (ABq, 8H), 3.87 (s, 4H). These data match those from samples prepared according to literature methods.^23,24 Anal. Calcd for C₁₈H₂₂N₂S₂: C, 65.41; H, 6.71; N, 8.48. Found: C, 65.18; H, 6.66; N, 8.43%. ESI-MS (m/z): 331 (MH⁺).

Ni[(SCH₂)₂NBn](dppe)

A slurry was prepared of BnN(CH₂SAc)₂ (283.4 mg, 1.00 mmol), NaO^tBu (192.2 mg, 2.00 mmol), and NiCl₂(dppe) (528.0 mg, 1.00 mmol) in THF (45 mL) at −78 °C with stirring. After 4 h, the purple-red mixture was allowed to warm to room temperature and filtered, with the filtrate being carefully layered with pentane. Red-orange crystals of the product were carefully isolated using a pipet, the remainder of the material (an orange powder) being recrystallized from CH₂Cl₂/pentane. The combined crops were washed with pentane (2 × 10 mL) and dried to afford the title compound as red-orange crystals. Yield: 210 mg (32%). ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ 7.89 (m, 8H, H2), 7.53 (m, 4H, H4), 7.49 (m, 8H, H3), 7.33 (d, ³J_HH = 7.5 Hz, 2H, H2-CH₂Ph), 7.24 (dd, ³J_HH = 7.5 Hz, ³J_HH = 7.5 Hz, 2H, H3-CH₂Ph), 7.19 (d, ³J_HH = 7.5 Hz, 1H, H4-CH₂Ph), 3.99 (s, 2H, CH₂Ph), 3.90 (d, ⁴J_PH = 4.3 Hz, 4H, SCH₂), 2.18 (d, ³J_PH = 16.9 Hz, 4H, PCH₂). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ 57.3. ESI-MS: m/z 654.1 (MH⁺). Red prisms of Ni[(SCH₂)₂NBn](dppe) formed upon slow diffusion of pentane layered onto a concentrated CH₂Cl₂ solution of the title compound at −28 °C.

Protonation of Ni[(SCH₂)₂NBn](dppe)

A solution of Ni-[(SCH₂)₂NBn](dppe) (5.4 mg, 8.25 μmol) in 0.5 mL of CD₂Cl₂ in a J-young tube was treated with HOTf (8.4 μL, 1 M) resulting in a color from orange to yellow-orange. ¹H NMR (500 MHz, CD₂Cl₂): δ 8.84 (br. s, 1H, N(H)Bn), 7.80−7.36 (m, 20 H, PPh₂), 4.19 (dt, ³J_HH = 12.4, 4.0 Hz, 2H, SCH₂N), 3.79 (dt, ³J_HH = 12.4, 4.2 Hz, 2H, SCH₂N). (ddt, ³J_PH = 14.3 Hz, ³J_HH = 7.9, 3.7 Hz, 4H, PCH₂). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ 61.03.

Ni[(SCH₂)₂NBn](dcpe)

This compound was prepared as yellow-orange crystals analogously to Ni[(SCH₂)₂NBn](dppe), using NiCl₂(dcpe) as the precursor. Yield: 28%. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ 7.42−7.18 (m, 5H, Ph), 3.95 (s, 2H, CH₂Ph), 3.89 (d, ⁴J_PH = 3.0 Hz, 4H, SCH₂), 2.58 (d, ³J_PH = 12.7 Hz, 2H, PCH), 2.37 (d, ³J_PH = 12.7 Hz, 4H, PCH₂), 2.28 (d, ³J_PH = 13.8 Hz, 2H, PCH), 2.20−1.46 (m, 40H, (CH₂)₅). ³¹P NMR (202 MHz, CD₂Cl₂, 298 K): δ 73.6. ESI-MS: m/z 676.3 (MH⁺). Orange blocks of Ni[(SCH₂)₂-NBn](dcpe)·CH₂Cl₂ formed upon slow diffusion of pentane layered onto a concentrated CH₂Cl₂ solution of the title compound at −28 °C.

Pd[(SCH₂)₂NBn](dppe)

This compound was prepared as orange crystals analogously to Ni[(SCH₂)₂NBn](dppe), using PdCl₂(dppe) as the precursor. Yield: 24%. ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ 7.82 (m, 8H, H2), 7.52 (m, 4H, H4), 7.48 (m, 8H, H3), 7.35 (d, ³J_HH = 7.6 Hz, 2H, H2-CH₂Ph), 7.24 (dd, ³J_HH = 7.9 Hz, ³J_HH = 7.9 Hz, 2H, H3-CH₂Ph), 7.18 (m, 1H, H4-CH₂Ph), 4.19 (s, 2H, CH₂S), 4.19 (s, 2H, CH₂S), 4.17 (s, 2H, CH₂Ph), 2.41 (s, 2H, PCH₂), 2.37 (s, 2H, PCH₂). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ 50.3. ESI-MS: m/z 702.1 (MH⁺). Anal. Calcd for C₃₅H₃₅NP₂Pd₂S₂: C, 59.87; H, 5.02; N, 1.99. Found: C, 59.55; H, 5.12; N, 1.64%.

ClC₆H₄N(CH₂SAc)₂

A solution of 4-chloroaniline (12.76 g, 100 mmol) in 100 mL of 95% EtOH was treated with formaldehyde (37% in methanol/H₂O, 24 mL). The mixture was heated at 60 °C for 30 min and then treated with AcSH (14.1 mL, 200 mmol). After 2 h, the reaction mixture was poured into ~100 mL of ice−water, when yellow crystals formed. The crystals were collected by filtration and washed with 100 mL of cold EtOH. Yield: 24.6 g (81%). ¹H NMR (500 MHz, DMSO-d₆, 298 K): δ 7.29 (d, ³J_HH = 7.69 Hz, 2H, C₆H₄), 6.80 (d, ³J_HH = 7.69 Hz, 2H, C₆H₄), 5.10 (s, 4H, C(O)CH₂), 2.35 (s, 6H, CH₃). ¹³C NMR (75.47 MHz, DMSO-d₆, 298 K): δ 196 (C=O), 143 (C₆H₄), 129 (C₆H₄), 124 (C₆H₄), 116 (C₆H₄), 52.1 (C(O)CH₂), 31.2 (CH₃). Anal. Calcd for C₁₂H₁₄ClNO₂S₂: C, 47.44; H, 4.64; N, 4.61. Found: C, 47.13; H, 4.5; N, 4.61%. Also prepared analogously was 4-MeC₆H₄N(CH₂SAc)₂. Anal. Calcd for C₁₃H₁₇NO₂S₂: C, 55.09; H, 6.05; N, 4.94. Found: C, 55.24; H, 6.02; N, 4.98%.

Ni[(SCH₂)₂NC₆H₄Cl](dppe)

A slurry was prepared of ClC₆H₄N-(CH₂SAc)₂ (304.8 mg, 1.00 mmol), NaO^tBu (192.2 mg, 2.00 mmol), and NiCl₂(dppe) (528.0 mg, 1.00 mmol) in THF (45 mL) at −78 °C with stirring. After 4 h, the purple-red mixture was allowed to warm to room temperature, resulting in a red solution and some green solid precipitate. The solids were removed by filtration and carefully layered with pentane, affording red-orange crystals. Yield: 505 mg (75%). ¹H NMR (500 MHz, CD₂Cl₂, 298 K): δ 7.89 (m, 8H, H2), 7.53 (m, 4H, H4), 7.49 (m, 8H, H3), 7.33 (d, ³J_HH = 7.5 Hz, 2H, H2-CH Ph), 7.24 (dd, ³J_HH = 7.5 Hz, ³J_HH = 7.5 Hz, 2H, H3-CH₂Ph), 7.19 (d, ³J_HH = 7.5 Hz, 1H, H4-CH₂Ph), 3.99 (s, 2H, CH₂Ph), 3.90 (d, ⁴J_PH = 4.3 Hz, 4H, SC_H2), 2.18 (d, ³J_PH = 16.9 Hz, 4H, PCH₂) (NMR analysis of the green solid was the same, except for the presence of THF). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 298 K): δ 57.3. ESI-MS: m/z 654.1 (MH⁺). Red prisms of Ni[(SCH₂)₂NBn](dppe) formed upon slow diffusion of pentane layered onto a concentrated CH₂Cl₂ solution of the title compound at °C. Anal. Calcd for C₃₄H₃₂ClNNiP₂S₂·CH₂Cl₂: C, 55.33; H, 4.51; N, 1.84. Found: C, 55.72; H, 4.35; N, 2.08%.

TsN(CH₂SAc)₂

A suspension of KSAc (2.73 g, 23.9 mmol) and TsN(CH₂Cl)₂³⁶ (3.06 g, 11.4 mmol) in THF (60 mL) was stirred for 27 h, after which the reaction mixture was filtered to remove NaCl and the colorless filtrate was evaporated to afford a yellow oily residue. A CH₂Cl₂ extract of the crude product was chromatographed on silica gel eluting with 4:1 hexane/EtOAc. The second band was collected, and the solvent was removed to yield the product TsN(CH₂SAc)₂ as colorless crystals. Yield: 3.04 g (77%). ¹H NMR (400 MHz, CDCl₃, 293 K): δ 7.69 (d, 2H, H2), 7.32 (d, 2H, H3), 4.89 (s, 4H, NCH₂S), 2.45 (s, 3H, CH₃), 2.31 (s, 6H, C(O)CH₃). Anal. Calcd for C₁₃H₁₇NO₄S₃: C, 44.94; H, 4.93; N, 4.03. Found: C, 45.17; H, 4.76, N, 4.32%.

TsN(CH₂SH)₂

A solution of HCl (12 M aqueous, 4.88 mL, 5.76 mmol) in MeOH (100 mL) was sparged with N₂ and then added to a solution of TsN(CH₂SAc)₂ (0.993 g, 2.86 mmol) in MeOH (20 mL). The resulting homogeneous solution was stirred for 21 h at 50 °C, afforded a white residue. A CH₂Cl₂ after which solvent evaporation a extract of this solid was chromatographed (SiO₂, CH₂Cl₂ eluent). The first band was collected and evaporated to dryness to give the dithiol as a white solid. Yield: 310 mg (41%). ¹H NMR (400 MHz, CDCl₃, 293 K): δ 7.72 (d, 2H, H2), 7.34 (d, 2H, H3), 4.66 (d, 4H, NCH₂S), 2.44 (s, 3H, CH₃), 1.79 (t, 2H, SH). Anal. Calcd for C₉H₁₃NO₂S₃: C, 41.04; H, 4.97; N, 5.32. Found: C, 41.31; H, 4.99; N, 5.19%. mp 51−53 °C.

Fe₂[(SCH₂)₂NTs](CO)₆

A solution of TsN(CH₂SH)₂ (263.4 mg, 1.00 mmol) and Fe₃(CO)₁₂ (514.1 mg, 1.02 mmol) in toluene (30 mL) was stirred at 90 °C for 3 h, during which time the solution assumed a red color. The reaction mixture was evaporated to yield 472 mg of brick-red solid, which crystallized from CH₂Cl₂ (5 mL) upon the addition of hexanes followed by cooling. Yield: 189 mg (35%). ¹H NMR (400 MHz, CDCl₃, 293 K): δ 7.50 (d, 2H), 7.30 (d, 2H), 3.65 (s, 4H), 2.43 (s, 3H). FTIR (CH₂Cl₂): ν_CO 2080, 2043, 2005, 1985 cm⁻¹. Anal. Calcd for C₁₅H₁₁Fe₂NO₈S₃: C, 33.29; H, 2.05; N, 2.59. Found: C, 33.10; H, 1.85; N, 2.63%. Red prisms of Fe₂[(SCH₂)₂NTs]-(CO)₆ formed upon slow diffusion of hexane layered onto a concentrated CH₂Cl₂ solution of the title compound.

Ni[(SCH₂)₂NTs](dppe)

A solution of TsN(CH₂SH)₂ (1.57 g, 6.98 mmol) in 100 mL of CH₂Cl₂ was added to a slurry of NiCl₂(dppe) (2.63 g, 5.00 mmol) and 150 mL of CH₂Cl₂. A solution of NaOEt (0.68 g, 10.0 mmol) in 45 mL of EtOH was added into the mixture. The mixture darkened in color, the NiCl₂(dppe) dissolved, and NaCl precipitated. After the reaction mixture was stirred for 3 h, solvent was removed under vacuum. The product was extracted into CH₂Cl₂ (90 mL). The extract was concentrated to a volume of 10 mL. Brick-red powder precipitated from the concentrate upon the addition of 90 mL of hexane. Yield: 3.15 g (88%). ¹H NMR (500 MHz, CD₂Cl₂, 293 K): δ 7.74 (dd, 8H, Ph), 7.63 (d, 2H, Ts), 7.52 (t, 4H, Ph), 7.46 (t, 8H, Ph), 7.21 (d, 2H, Ts), 4.21 (d, 4H, NCH₂S), 2.38 (s, 3H, CH₃), 2.19 (d, 4H, PCH₂). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂, 293 K): δ 143.2, 134, 132, 130, 129, 128, 125, 121, 43.7, 27.5, 21.6. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 293 K): δ 57.6. Anal. Calcd for C₃₅H₃₅NNiO₂P₂S₃: C, 58.51; H, 4.91; N, 1.95. Found: C, 57.88; H, 4.74; N, 2.06%. Red prisms of Ni[(SCH₂)₂NTs](dppe) formed upon slow diffusion of hexane layered onto a concentrated CH₂Cl₂ solution of the title compound.