Diiron Azamonothiolates via Scission of Dithiadiazacyclooctanes by Iron Carbonyls

Abstract

The reaction of Fe₃(CO)₁₂ with the dithiadiazacyclooctanes [SCH₂N(R)CH₂]₂ affords Fe₂[SCH₂N(Me)CH₂](CO)₆ (R = Me, Bn). The methyl derivative 1^Me was characterized crystallographically (Fe-Fe = 2.5702(5) Å). Its low symmetry is verified by variable temperature ¹³C NMR spectroscopy which revealed that the turnstile rotation of the S(CH₂)Fe(CO)₃ and S(NMe)Fe(CO)₃ centers are subject to very different energy barriers. Although 1^Me resists protonation, it readily undergoes substitution by tertiary phosphines, first at the S(CH₂)Fe(CO)₃ center, as verified crystallographically for Fe₂[SCH₂N(Me)CH₂](CO)₅(PPh₃). Substitution by the chelating diphosphine dppe (Ph₂PCH₂CH₂PPh₂) gave Fe₂[SCH₂N(Me)CH₂](CO)₄(dppe), resulting from substitution at both the S(CH₂)Fe(CO)₃ and S(NMe)Fe(CO)₃ sites.

Introduction

The iron carbonyls form an immense range of complexes with organosulfur ligands. Curiously almost all such complexes result from degradation of organosulfur compounds. Only a fraction of organosulfur ligands bind intact to iron carbonyls.^[1] This trend highlights the tendency of iron(0) compounds to break C-S bonds, which in turn highlights the apparent high stability of iron carbonyl thiolates.

Iron carbonyl derivatives of S,N-compounds were uncommon, at least they were prior to the proposed,^[2] now confirmed,^[3] occurrence of the azadithiolate cofactor in the [FeFe]-hydrogenases (Figure 1).

Figure 1.

Active site of [FeFe]-hydrogenase highlighting the azadithiolate cofactor.

Although such diiron azadithiolates can be prepared by multiple routes,^[4] such methods involve multistep conversions. We speculated that the [(SCH₂)₂NR]^2- ligand (R-adt^2-) could arise from the degradation of 8- or 6-membered S,N-heterocycles by Fe₃(CO)₁₂. Sulfur-nitrogen heterocycles are attractive reagents because they can be made in high yields by condensation of primary amines with hydrogen sulfide and formaldehyde.^[5] They are structurally related to the P₂N₂ ligands popularized by DuBois et al. (Figure 2) ^[6].

Figure 2.

Heterocyclic compounds discussed in this work.

Our findings show that the S₂N^R₂ heterocycles are cleaved by Fe₃(CO)₁₂ to yield organoiron complexes with bridging -CH₂N(R)CH₂S- moieties.

Results and Discussion

Reactions of S₂N₂R₂ with Iron Carbonyls

The reaction of two equivalents of Fe₃(CO)₁₂ with S₂N^R₂ produced organoiron compounds with the formula Fe₂[SCH₂N(R)CH₂](CO)₆ (1^R, R = Me, Bn). The products were obtained in good yields as red colored powders after purification by column chromatography, which separates the product from Fe(CO)₅ and traces of Fe₂(SCH₂S)(CO)₆ (2). When the reaction was monitored by in-situ IR spectroscopy, we did not observe any intermediates. The synthesis can be represented by the idealized equation 1.

$$\begin{array}{c}\hfill {\mathrm{Fe}}_3{\left(\mathrm{CO}\right)}_{12}+\frac{1}{2}{\left[{\mathrm{SCH}}_2\mathrm{N}\left(\mathrm{R}\right){\mathrm{CH}}_2\right]}_2\to {\mathrm{Fe}}_2\left[{\mathrm{SCH}}_2\mathrm{N}\left(\mathrm{Me}\right){\mathrm{CH}}_2\right]{\left(\mathrm{CO}\right)}_6+\mathrm{Fe}{\left(\mathrm{CO}\right)}_5+\mathrm{CO}\hfill \\ \hfill 1^{\mathbf{Me}}(\mathrm{R}=\mathrm{Me})\hfill \\ \hfill 1^{\mathbf{Bn}}(\mathrm{R}=\mathrm{Bn})\hfill \end{array}$$ (1)

We propose that 2 arises via the reaction of Fe₃(CO)₁₂ with a small amount of S₂N^R generated by degradation of S₂(N^R)₂ during the synthesis of 1^Me. Consistent with this scenario, we demonstrated that 2 forms in moderate yields from the reaction of Fe₃(CO)₁₂ with S₂N^Me. The infrared spectra of 1^Me and 1^Bn in the ν_CO largely resemble that of Fe₂((SCH₂)₂NH)(CO)₆, the bands for 1^R being ~5 cm^-1 lower in energy than those for Fe₂((SCH₂)₂ NH)(CO)₆.

The structure of 1^Me is supported by spectroscopic and crystallographic analysis. Two dimensional ¹H-¹H and ¹H-¹³C correlation NMR spectra indicate that 1^Me is chiral, and that the Fe₂[SCH₂N(Me)CH₂] core is rigid. Signals for NCH₂S (δ4.62, 3.51, 2H; δ73.8, 1C) were assigned by comparison to the SCH₂S backbone of 2 (δ4.65, 2H); the FeCH₂N group was therefore assigned to be the more downshifted signals in both the ¹H and ¹³C NMR spectra (δ 2.73, 3.02, 2H; δ57.8, 1C). To assign the two C-H centers associated with the 4-bond coupling (“W-coupling”) in the spectrum of 1^Me, we examined the dihedral angles of the -CH₂-N-CH₂-S-moiety containing the hydrogen atoms of interest (Table 1). Large W-coupling is associated with small dihedral angles (Φ).^[7] Thus, since H_C and H_E are in planes with the smallest dihedral angle they will most likely exhibit the 4-bond coupling that is observed in the ¹H NMR spectrum (Table 1).

Table 1.

Dihedral Angles and H-H Coupling Constants for the -CH2-N-CH2-S- Ligand of 1Me.

Plane 1	Plane 2	Dihedral Angle (°)
HB-C5-N	N-C4-HD	52.9
HB-C5-N	N-C4-HE	126
HC-C5-N	N-C4-HD	130
HC-C5-N	N-C4-HE	10.7

Nucleus 1 (δ)	Nucleus 2 (δ)	″JHH (Hz)
HA (2.17)	-	-
HB (2.73)	HC (3.02)	7.34 (n = 2)
HC (3.02)	HE (4.62)	2.91 (n = 4)
HD (3.51)	HE (4.62)	7.54 (n =2)

Crystallographic analysis confirmed that two Fe(CO)₃ centers are bridged by thiolate and iminium-like^[8] ligands. The Fe centers differ in that one is bonded to nitrogen (Fe-N = 2.0332(17) Å), and the other to an alkyl ligand (Fe-CH₂ = 2.055(2) Å) (Figure 3). The Fe-Fe bond (2.5702(5) Å) is longer than related complexes with Fe₂S₂ cores (2.4924(7) Å).^[4b] This elongation is attributed to the extended Fe-N(Me)-CH₂-Fe linkage. Long has described a related Fe₂(CO)₆ complex with a μ-pyridyl group.^[9] The Fe-Fe distance in that complex is comparable to that in 1^Me.

Figure 3.

Two depictions of Fe₂[SCH₂N(Me)CH₂](CO)₆ (1^Me) (thermal ellipsoids at 50% probability level).

Cyclic voltammogram of 1^Me at various scan rates show a quasi-reversible reduction at E_pc = -1.85 V vs Fc^0/+ in CH₂Cl₂ (see SI). In comparison, Fe₂[(SCH₂)₂CH₂](CO)₆ has been reported in the range of E_pc = -1.61 to -1.74 V vs Fc^0/+ in MeCN, and Fe₂[(SCH₂)₂CH₂](CO)₆ is reduced at E_pc = -1.58 V vs Fc^0/+ in MeCN.^[10] The shift in potential for 1^Me in comparison to the iron dithiolate hexacarbonyl complexes may be partially attributed to the use of a less polar solvent. However, the more negative reduction potential for 1^Me may also be attributed to increased electron density on the metal centers in comparison to the iron dithiolate hexacarbonyl complexes. This electron density difference was observed by FTIR as aforementioned, in which 1^Me showed bands shifted to lower energy than Fe₂[(SCH₂)₂NH](CO)₆.

Dynamic NMR Studies of 1^Me

Since 1^Me is asymmetric, all six carbonyl ligands are inequivalent. At 22 °C, the ¹³C NMR spectrum (in the CO region) exhibits only three broad peaks at δ216, 212, and 209. These CO sites are located on what is referred to as the rigid iron center (Fe^R) because turnstile rotation of the CO sites is slow on the NMR timescale (Figure 4). At lower temperatures (< -36 °C), the remaining three signals for the CO ligands, those on the non-rigid iron center (Fe^NR), appear at δ216, 210, 205. At 41 °C four peaks are observed, three for the CO sites on the Fe^R center, and a broad singlet corresponding to the three CO sites on the Fe^NR center that have coalesced. Above 50 °C, only the coalesced signal for the CO sites on the Fe^NR center is observed. The differentiation of the Fe^NR(CO)₃ vs the Fe^R(CO)₃ centers is discussed below for the PMe₃-substituted derivative of 1^Me.

Figure 4.

Representation of dynamic processes of 1^Me; Fe^NR = non-rigid, Fe^R = rigid. ¹³C NMR spectra (125.7 MHz, d₈-toluene solution) of compound 1^Me at various temperatures (right). The average chemical shifts for the CO signals on the Fe^R and Fe^NR centers are indicated with * and **, respectively.

The temperature dependence of the rates of two CO interchange processes followed Arrhenius behavior. This analysis revealed that the dynamics at Fe^NRproceed with an activation energy barrier of 10.8 (±0.65) kcal/mol, whereas the dynamic process for the Fe^R center occurs with a barrier of 16 (±0.85) kcal/mol. A change in the slope of the Arrhenius plot for the dynamic behavior of the Fe^NR center is observed at the extremes of the temperatures used. This suggests that different dynamic process may be in effect at differing temperature ranges, and that the calculated value is an average energy barrier for these processes. It is worth noting that at low temperatures (approaching - 60 °C) the viscosity of the solution could influence the broadness of the peaks in the ¹³C NMR, and influence the calculations of the energy barriers.

Monosubstituted Derivatives of 1^Me

Treatment of 1^Me with phosphine ligands in the presence of Me₃NO·2H₂O afforded monosubstituted derivatives Fe₂(SCH₂N(Me)CH₂)(CO)₅(PR₃) (3^Me, PR₃ = PMe₃ ;4^Me, PR₃ = PPh₃). The ¹H NMR signals for the aminothiolate ligand are relatively unchanged in both cases from that of 1^Me. No ³¹P coupling to the ¹H NMR nor the ¹³C NMR signals is observed. The molecular structure of the PPh₃ derivative 3^Me is similar to that of 1^Me (Figure 5, Table 2). The phosphine is bound to the apical position of the Fe ^CH2₂ site (Figure 5). Owing to its low solubility, 3^Me was not examined by DNMR spectroscopy. The overall structure of the PMe₃ derivative 4^Me is again similar to that of 1^Me. The phosphine is however bound to a basal position of the Fe ^N₁ site (Figure 5).

Figure 5.

ORTEP of Fe₂[SCH₂N(Me)CH₂](CO)₅ (PPh₃) (3_Me , top left), Fe₂[SCH₂N(Me)CH₂](CO)₅(PMe₃) (4^Me, top right), and Fe₂[SCH₂N(Me)CH₂](CO)₄(dppe) (5^Me, bottom) with 50% probability thermal ellipsoids.

Table 2.

Bond distances of interest for crystallographically characterized complexes. All distances are given in angstroms.

	Fe1-Fe2	Fe1-N1	Fe2-C5	Fe1-S1	Fe2-S1	Fen-Pm
1Me	2.5702 (4)	2.033 (18)	2.055 (2)	2.258 (6)	2.248 (6)	-
3Me	2.5771 (4)	2.035 (16)	2.062 (2)	2.268 (6)	2.250 (6)	Fe2-P1: 2.219 (6)
4Me	2.623 (5)	2.039 (2)	2.066 (2)	2.260 (6)	2.249 (6)	Fe1-P1: 2.211 (8)
5Me	2.648 (4)	2.047 (18)	2.018 (18)	2.280 (6)	2.250 (5)	Fe1-P1: 2.217 (7)
						Fe2-P2: 2.222 (6)

The high solubility of 4^Me allowed examination of its low temperature NMR properties. Analogous to 1^Me, three signals, albeit broadened, are observed in its ¹³C NMR spectrum at 215.8, 210, and 208.8δ at 20 °C (Figure 6). This observation suggests that the Fe^R center in 1^Me and 4^Me is Fe^CH2(CO)₃. Upon cooling to - 60 °C all five CO sites of 4^Me can be resolved, with two new signals corresponding to the Fe^NR or Fe^NMe(CO)₂PMe₃ center present at 216.6 and 208.7δ (see SI). The molecular structure of 4^Me shows that the phosphine occupies the basal site of the Fe^NR center octahedral on the side of the sulfur atom. Based on the ¹³C NMR signals of 4^Me it can concluded that the CO site of 1^Me that is substituted to form 4^Me corresponded to the peak at 205δ in the ¹³C NMR of 1^Me. Coupling to phosphorus is observed for a non-rigid CO site, J_PC = 27 Hz, which cannot be specifically assigned based on this data alone. As in 1^Me, the CO sites on the Fe^R center begin to coalesce above 20 °C, and are too broad to be observed by NMR spectroscopy.

Figure 6.

¹³C NMR spectra (125.7 MHz, d₈-toluene solution) of 4^Me at various temperatures. Signals assigned to Fe^R(CO) (corresponding to Fe^CH2(CO)₃) and Fe^NR (CO) (corresponding to Fe^NMe(CO)₂PMe₃) are indicated with * and **, respectively.

Disubstituted Derivatives of 1^Me

Treatment of 1^Me with dppe (dppe = 1,2-bis(diphenylphosphino)ethane) in the presence of Me₃NO·2H₂O afforded the disubstituted derivative Fe₂[SCH₂N(Me)CH₂](CO)₄(dppe) (5^Me, Figure 5 and 7). The ³¹P{¹H} NMR spectrum shows two signals with J_PP of 11.8 Hz. Its ¹³ C NMR spectrum shows four signals in the CO region (δ225, 224, 221, 214), each coupled to one phosphorus with J_PC of ~ 17 Hz. Due to its poor solubility, 5^Me was not examined by low temperature NMR spectroscopy. The molecular structure of 5^Me confirms its similarity to 1^Me. As with the PMe₃ derivative 4^Me, the phosphine on the Fe ^N₁ side is trans to the amine, which causes an elongation of the Fe-N bond. The bridging of two metal centers by a dppe ligand in a similar fashion to that of 5^Me has been observed for related diiron complexes.^[11]

Figure 7.

Structures of the substituted derivatives of 1^Me.

Conclusions

S₂N^R₂ represent a fundamental class of S-N heterocycles, which have eluded study by the organometallic community. We show that they are readily cleaved by Fe(0) reagents to give diiron hexacarbonyls. The new azathiolate complexes are a rare examples of a chiral diiron hexacarbonyl in which the six diastereotopic carbonyl-carbon centers have been identified by NMR. Our ability to conduct these measurements is a result of the high solubility of 1^Me in both polar and non-polar solvents even at very low temperatures (-60 °C).

Compounds containing the S-CH₂-X group appear to be particularly reactive toward iron(0) carbonyls. In related work, the 1,3,5-trithiane is degraded by iron(0) carbonyls to give Fe₂(SCH₂SCH₂SCH₂)(CO)₆ as well as the methanedithiolate Fe₂(SCH₂S)(CO)₆ (Scheme 1).^[12]

Scheme 1.

Activation of saturated C-S heterocycles by iron carbonyls, which are reminiscent of the results in this report.^[12-13]

The selenium analogues of these complexes have recently been obtained from 1,3,5-triselenocyclohexane.^[14]

The pathway leading to 1^R (R = Me, Bn), as well as the SCH₂SCH₂ and SeCH₂SeCH₂ compounds is uncertain. With thioethers, iron carbonyls form complexes of the type Fe(CO)₄(SR₂)^[15] and Fe₃(CO)₈ (μ-SC₄H₈)₂. ^[16]

In related work, Hwang and coworkers described the compound Fe₂[HS(CH₂)₂NCHR](CO)₆ (R = 2-C₅H₃N-5-Me).^[17] The structure of this complex was assigned on the basis of X-ray crystallography. Thiol ligands bridging two metals have been observed spectroscopically, but are typically very labile.^[18] The results described in the present paper suggest that Huang's thiol complex might be reformulated as the Fe₂[S(CH₂)₂N(H)CHR](CO)₆ (Figure 8).

Figure 8.

Structures proposed by Huang et al (left)^[17] and by this work for Fe₂(RCHNC₂H₄S)(CO)₆ (R = 2-methylpyridyl) (right).

Experimental Section

General Considerations

Unless otherwise stated, reactions were conducted using standard Schlenk techniques, and all reagents were purchased from Sigma-Aldrich or Fisher. The heterocycles S₂N^Bn₂, S₂N^Me, and S₂N^Me₂ proceeded as reported in the literature.^{[5, 19]} Chromatography was conducted in air on 10 × 2.5 inch columns of silica gel (Silicycle, 40– 63μm). All other solvents were HPLC grade and purified using alumina filtration system (Glass Contour, Irvine, CA). NMR spectra were recorded on Varian UNITY 500 spectrometers. ¹H NMR spectra (500 MHz) and ¹³C NMR spectra (100, 128 MHz) are referenced to residual solvent referenced to TMS. ³¹P{¹H} NMR spectra (202 MHz) are referenced to external 85% H₃PO₄. FTIR spectra were recorded on a Perkin-Elmer 100 FT-IR spectrometer. Elemental analyses were done on Exeter Analytical CE 440, and Perkin Elmer 2440, series II analyzers. X-ray crystallography data was obtained on a Bruker SMART instrument with an Apex II detector, using a Copper radiation source.

N,N′-Dimethyl-1,5-dithia-3,7-diazacyclooctane (S₂N^Me₂)

The following is an optimized modification of the literature method.^[5] Aqueous methylamine (40 %, 20 mL, 0.23 mmol) diluted with water (50 mL) was treated with cold formalin (34.6 mL, 0.46 mmol). After being stirred for 5 min at 0 °C, this solution was purged with H₂S gas for 2 h causing precipitation of a white solid. After stirring the solution for an additional hour at 0 °C, the precipitate was collected and washed with cold ethanol (100 mL). The precipitate was purified by recrystallization by dissolving in 300 mL of hot acetone followed by cooling to 0 °C. Yield: 5.33 g (52%). ¹H NMR (500 MHz, CD₂Cl₂): δ 2.44 (s, 6H, NCH₃), 4.24 (d, 4H, NCH₂S), 4.52 (d, 4H, NCH₂S). ¹³ C NMR (500 MHz, CDCl₃): δ 38.2 (NCH₃), 65.4 (NCH₂S). ¹H NMR (500 MHZ, CD₂Cl_2, δ): 4.49 (d, 4H, CH₂); 4.25 (d, 4H, CH₂); 2.40 (s, 6H, NCH₃). Anal. Calcd for C₆H₁₄N₂S₂: C, 40.41; H, 7.91; N, 15.71. Found: C, 40.50; H, 7.90; N, 15.35. ESI-MS (m/z): 179 (MH⁺).

General Procedure for Reaction of S,N-Heterocycles with Fe₃(CO)₁₂

A slurry of the heterocycle and two mole equivalents of Fe₃(CO)₁₂ in THF was stirred for 20 h at room temperature under argon. The resulting dark red mixture was evaporated to dryness in vacuo, the residue was extracted in a minimum amount of hexane (15-30 mL). This extract was chromatographed on silica gel eluting with hexanes. Two closely spaced red bands were separated; the first band contained 2, and the second band contained 1 (see below). Each compound was recrystallized by dissolving in hexanes at room temperature followed by cooling to 0 °C.

Fe₂[SCH₂N(Me)CH₂] (CO)₆(1^Me)

The general procedure was applied to the reaction of S₂N^Me₂ (0.36 g, 2.0 mmol) and Fe₃(CO)₁₂ (2 g, 4.0 mmol) in 40 mL of THF. Yield, 2: 0.09 g (6%). Yield of 1Me: 0.62 g (84%). ¹³C NMR (127.7 MHz, d₈-toluene, 20 °C): δ 52.68 (s, 1C, NCH₃), 57.42(s, 1C, NCH₂Fe), 73.05 (s, 1C, NCH₂S). ¹ H NMR (500 MHz, CD₂Cl₂): δ 2.17 (s, 3H, NCH₃), 2.73 (d, 1H, NCH₂Fe), 3.03 (dd, 1H, NCH₂Fe), 3.52 (d, 1H, NCH₂S), 4.63 (dd, 1H, NCH₂S). IR (hexanes): ν_CO = 2068 (s), 2024 (vs), 1995 (vs), 1981 (vs), 1964 (s) cm^-1. Anal. Calcd for C₉H₇Fe₂NO₆S: C, 29.30; H, 1.91; N, 3.80. Found: C, 29.06; H, 1.83; N, 3.70.

Fe₂[SCH₂N(Bn)CH₂](CO)₆ (1^Bn)

The above general procedure was applied to the reaction of S₂N₂Bn₂ (0.22 g, 0.67 mmol) and Fe₃(CO)₁₂ (0.67 g, 1.3 mmol) in 20 mL of THF. Yield: 0.23 g (77%). ¹H NMR (500 MHz, CDCl₃): δ 2.49 (d, 1H, NCH₂Fe), 3.04 (dd, 1H, NCH₂S), 3.20 (d, 1H, NCH₂Fe), 3.31 (d, 1H, NCH₂), 3.42 (d, 1H, NCH₂), 4.69 (dd, 1H, NCH₂S), 7.16-7.41 (m, 5H, C₆H₅). IR (hexane): ν_CO = 2067 (s), 2024 (vs), 1994(vs), 1980(vs), 1963 (s) cm^–1. Anal. Calcd for C₁₅H₁₁Fe₂NO₆S: C, 40.48; H, 2.49; N, 3.15. Found: C, 40.55; H, 2.30; N, 3.14.

Fe₂(SCH₂S)(CO)₆(2)

The general procedure was applied to the reaction of S₂NMe (0.14 g, 1.0 mmol) and Fe₃(CO)₁₂ (1.0 g, 2.0 mmol) in 40 mL of THF. Yield: 0.12 g (33%). ¹H NMR (500 MHz, CD₂Cl₂): δ 4.65 (s, 2H, SCH₂S). IR (hexane): ν_CO 2079 (s), 2039 (vs), 2007 (vs), 1999 (vs), 1987 (w) cm^-1. ¹H NMR and IR spectra matched published data.^[12]

Fe₂[SCH₂N(Me)CH₂](CO)₅(PMe₃)(4^Me

A solution of 1^Me (0.30 g, 0.811 mmol) in MeCN (50 mL) was treated with Me₃NO·2H₂O (0.09 g, 0.811 mmol) in MeCN (30 mL). The solution was stirred for 10 min. at room temperature until all starting material was consumed as detected by FTIR. The resulting dark red solution was treated with PMe₃ (0.062 g, 0.811 mmol), and stirred for an additional 10 min. The solvent was evaporated in vacuo, and the solid was extracted into 5 mL of hexanes and chromatographed on silica (7 in by 0.5 in) in a glovebox. Hexanes were used remove first band followed by 95:5 hexanes: CH₂Cl₂ to obtain 4^Me. The solvent was removed to yield a red solid that was recrystallized from hexanes at -40 °C. Yield: 0.171 g (45 %). ¹³C NMR (127.7 MHz, d₈-toluene, 20 °C) δ 13.50 (d, 1C, P(CH₃)₃, J_PC = 28.9 Hz),47.29 (s, 1C, NCH₃), 52.01,(s, 1C, NCH₂Fe), 67.77 (s, 1C, NCH₂S), 207.9-215.13 (3s, 3 C, Fe(CO)₃). ³¹ P{¹H} NMR (202.3 MHz, d₂-CH₂Cl₂): δ 70.6 (s). IR (CH₂Cl₂): ν_CO = 2034 (w), 2021 (w), 1968 (s), 1939 (m), 1905 (w) cm^-1. Anal. Calcd for C₁₁H₁₆Fe₂NO₅PS: C, 31.68; H, 3.87; N, 3.36. Found: C, 31.67; H, 3.61; N, 3.39.

Reaction of 1^Me with PPh₃, and dppe

The diiron complex 1^Me and one mole equivalent of the phosphine were dissolved in MeCN or CH₂Cl_2, and treated dropwise with one mole equivalent of Me₃NO·2H₂O in MeCN. The resulting mixture was stirred for 3 h at room temperature, and the solvent was evaporated to dryness in vacuo. The red solid was extracted in a minimum amount of CH₂Cl₂ (~10 mL), and chromatographed on silica gel with CH₂Cl₂ as the eluent. The dried red or orange powders were recrystallized from CH₂Cl₂ with hexanes.

Fe₂[SCH₂N(Me)CH₂](CO)₅(PPh₃) (3^Me)

The above general procedure was applied to reaction of 1^Me (0.33 g, 0.75 mmol) and PPh₃ (0.20 g, 0.75 mmol) in MeCN (30 mL) followed by a solution of Me₃NO·2H₂O (0.083 g, 0.75 mmol) in MeCN (15 mL). Yield: 0.31 g (61%). ¹³C NMR (127.7 MHz, CD₂Cl₂, 20 °C): δ 64.68 (s, 1C, NCH₂Fe), 72.55 (s, 1C, NCH₂S). ¹ H NMR (500 MHz, CD₂Cl₂): δ 1.75 (d, 1H, NCH₂Fe), 1.79 (d, 1H, NCH₂Fe), 3.21 (d, 1H, NCH₂), 3.27 (dd, 1H, NCH₂S), 3.33 (d, 1H, NCH₂), 4.16 (dd, 1H, NCH₂S), 6.96-7.72 (m, 20H, C₆H₅). ³¹P{¹H} NMR ( 202.3 MHz, d₂-CH₂Cl₂): δ 70.6 (s). IR (hexane): ν_CO = 2042 (vs), 1979 (vs), 1971 (vs), 1957 (s), 1924 (s) cm^-1. Anal. Calcd. for C₂₆H₂₂Fe₂NO₅PS: C, 51.77; H, 3.68; N, 2.32. Found: C, 52.27; H, 3.70; N, 2.48.

Fe₂[SCH₂N(Me)CH₂(CO)₄(dppe) (5^Me)

The above general procedure was applied to reaction of 1^Me (1.00 g, 2.7 mmol) and dppe (1.08 g, 2.7 mmol) in CH₂Cl₂ (50 mL); treated with Me₃NO·2H₂O (0.30 g, 2.7 mmol) in MeCN (35 mL). Yield: 0.821 g (97%). ¹³C NMR (100.3 MHz, dCD₂Cl₂): δ20.8 (t, 1C, PCH₂CH₂P), 27.3 (t, 1C, PCH₂CH₂P), 559.74 (d, 1C, NCH₂Fe), 75.46 (d, 1C, NCH₂S), 213.54-224.93 (4d, 4C, Fe(CO)₂).¹ H NMR (500 MHz, d₂-CH₂Cl₂): δ2.06 (s, 3H, NCH₃), 2.56 (dd, 1H, NCH₂Fe), 3.02 (d, 1H, NCH₂Fe), 3.68 (dd, 1H, NCH₂S), 5.01 (d, 1H, NCH₂S), 0.9-2.10 (m, 10H, PCH₂CH₂P), 7.37 7.95 (m, 20H, P(C₅H₆)₂). ^[31]P ^20] NMR (202.3 MHz, d₂-CH₂Cl₂): δ 58.44 (d, PCH₂CH₂P, Jpp = 12 Hz), 65.65 (d, PCH₂CH₂P). IR (hexane): ν_CO = 2042 (vs), 1979 (vs), 1971 (vs), 1957 (s), 1924 (s) cm^-1. Anal. Calcd for C₃₃H₃₁Fe₂NO₄P₂S: C, 55.72; H, 4.39; N, 1.97. Found: C, 55.91; H, 4.34; N, 2.04.