Ferrous Carbonyl Dithiolates as Precursors to FeFe, FeCo, and FeMn Carbonyl Dithiolates.

Reported are complexes of the formula Fe(dithiolate)(CO)2(diphos) and their use to prepare homo- and heterobimetallic dithiolato derivatives. The starting iron dithiolates were prepared by a one-pot reaction of FeCl2 and CO with chelating diphosphines and dithiolates, where dithiolate = S2(CH2)22- (edt2-), S2(CH2)32- (pdt2-), S2(CH2)2(C(CH3)2)2- (Me2pdt2-) and diphos = cis-C2H2(PPh2)2 (dppv), C2H4(PPh2)2 (dppe), C6H4(PPh2)2 (dppbz), C2H4[P(C6H11)2]2 (dcpe). The incorporation of 57Fe into such building block complexes commenced with the conversion of 57Fe into 57Fe2I4( i PrOH)4, which then was treated with K2pdt, CO, and dppe to give 57Fe(pdt)(CO)2(dppe). NMR and IR analyses show that these complexes exist as mixtures of all-cis and trans-CO isomers, edt2- favoring the former and pdt2- the latter. Treatment of Fe(dithiolate)(CO)2(diphos) with the Fe(0) reagent (benzylideneacetone)Fe(CO)3 gave Fe2(dithiolate)(CO)4(diphos), thereby defining a route from simple ferrous salts to models for hydrogenase active sites. Extending the building block route to heterobimetallic complexes, treatment of Fe(pdt)(CO)2(dppe) with [(acenaphthene)Mn(CO)3]+ gave [(CO)3Mn(pdt)Fe(CO)2(dppe)]+ ([3d(CO)]+). Reduction of [3d(CO)]+ with BH4- gave the Cs -symmetric μ-hydride (CO)3Mn(pdt)(H)Fe(CO)(dppe) (H3d). Complex H3d is reversibly protonated by strong acids, the proposed site of protonation being sulfur. Treatment of Fe(dithiolate)(CO)2(diphos) with CpCoI2(CO) followed by reduction by Cp2Co affords CpCo(dithiolate)Fe(CO)(diphos) (4), which can also be prepared from Fe(dithiolate)(CO)2(diphos) and CpCo(CO)2. Like the electronically related (CO)3Fe(pdt)Fe(CO)(diphos), these complexes undergo protonation to afford the μ-hydrido complexes [CpCo(dithiolate)HFe(CO)(diphos)]+. Low-temperature NMR studies indicate that Co is the kinetic site of protonation.

Introduction

Compounds of the type Fe2(SR)2(CO)6–L are topical because of their relationship to the active sites of the [FeFe]-hydrogenases (H2ases).[1] The quest for a deeper understanding of the enzyme mechanism, as well as interest in base metal H2-processing catalysts,[2] has led chemists to prepare many examples of these dithiolates.[3] Since the [NiFe]-H2ases feature related Ni(SR)2Fe(CO)L2 centers, a new methodology addresses both families of enzymes. This paper describes a new route to models of these two families of enzyme active sites.

Traditionally, compounds of the type Fe2(SR)2(CO)6–L are prepared by ligand (L) substitution of hexacarbonyls Fe2(SR)2(CO)6,[4] which in turn are obtained by treating Fe3(CO)12 with thiols. Complementarily, Fe2(S2)(CO)6[5] is an ideal precursor to diiron complexes[6] of more elaborate organosulfur ligands.[7] Diiron dithiolato carbonyls can also be prepared from more oxidized iron sources, as exemplified by the reductive carbonylation of ferrous halides in the presence of dithiolates (Scheme 1).[8] The method, which proceeds in at least modest yields, is suited for generating 57Fe-labeled derivatives, which are of interest for nuclear resonance vibrational spectroscopy (NRVS)[9] and Mössbauer measurements.[10]

Scheme 1

Illustrative Routes to Fe2(SR)2(CO)6[4,8,11]

Promising precursors to dimetallic dithiolato complexes are monoiron complexes of the type Fe(dithiolate)(CO)2(PR3)2. These compounds should be obtainable from complexes of the form FeX2(CO)2(PR3)2,[12−14] which in turn are accessed by mild carbonylation of ferrous halides in the presence of phosphine ligands. Ferrous bis(thiolato) complexes appear especially amenable to carbonylation[15,16] (eq 1). Routes to ferrous dithiolato carbonyls have been established in the case of Fe(edt)(CO)2(PMe3)2 (edt2– = ethanedithiolate)[17] and related complexes containing bidentate phosphine ligands (Scheme 2).[18] The benzenedithiolates Fe(S2C6H2X2)(CO)2(PMe3)2 (X = H, Cl) are formed from diiron dithiolato precursors,[19] which undergo cleavage upon treatment with PMe3. With regard to structure, complexes of the type FeX2(CO)2(chel) exist in various isomeric forms.[13,20] Fe(SPh)2(CO)2(dppe) crystallizes as the all-cis isomer.[15] Complexes of formula FeX2(CO)2(PR3)2, in which all the ligands are monodentate, exist as both the cis,cis,trans and all-trans isomers, depending on the phosphine ligand.[21]

Scheme 2

Synthetic Routes to Fe(dithiolate)(CO)2L2

Ferrous carbonyl thiolates have been examined as metalloligands, analogous to the use of metal dithiolates to prepare diverse dithiolato-bridged dimetallic complexes.[22] Thiolates retain significant basicity even when bound to a metal, as evidenced by the formation of adducts of Fe(SPh)2(CO)2(dppe) with HgCl2.[15] Similarly, the bis(chelate) complex Fe(pdt)(CO)2(dppe) is a useful precursor to bimetallic species, including Ni–Fe dithiolates (eq 2).[23]

Presented here is a general route to monoiron precursors. Reactions of these monoiron precursors with iron, manganese, and cobalt carbonyl complexes afford the respective FeFe, MnFe, and CoFe complexes. In the course of this work, one of the authors (J.C.) published some of these results without the knowledge of the other authors.[24] The new compounds and new synthetic routes demonstrate the broad scope for first-row analogues of the active sites of the [FeFe]- and [NiFe]-H2ases.

Results

Ferrous Dithiolato Carbonyls

New diphosphine-substituted ferrous carbonyl dithiolates were prepared by carbonylation of a slurry of anhydrous FeCl2 and the diphosphine ligand, followed by the addition of the sodium dithiolate (Scheme 3). The reaction affords a mixture of the two isomers possible for an octahedral complex of type M(chel)(chel′)L2. The complex FeCl2(CO)2(dppe)[13] was observed spectroscopically as an intermediate in the preparation of 1d. Although yields ranged from 10 to 50%, the necessary reagents are readily available. A complementary route to 1d involving the reaction of Fe(CO)3(dppe) and pdtH2 did not proceed.

Scheme 3

Main Route to Fe(dithiolate)(CO)2(diphosphine)

Spectroscopic measurements on the new compounds indicate the presence of two isomers, with the 31P{1H} NMR spectra exhibiting three signals: a singlet corresponding to the symmetrical isomer and a pair of doublets corresponding to the unsymmetrical isomer (Table 1). For 1,2-ethanedithiolate (edt2–) complexes 1a,b, the unsymmetrical isomer is the major species, whereas the symmetrical isomers predominate for complexes of 1,3-propanedithiolate (pdt2–) and 2-dimethyl-1,3-propanedithiolate (Me2pdt2–), 1c–f. (Table 1, Scheme 3). The IR spectra of the edt2– complexes 1a,b exhibit two or three bands in the νCO region. The pair of bands at ∼2000 and 1960 cm–1 is assigned to the unsymmetrical isomer, and one band at 1970 cm–1 is assigned to the symmetrical trans-dicarbonyl isomer. The complex Fe(pdt)(CO)2(dcpe) (1f) was also prepared. As with the other pdt2– complexes, it exists as a mixture of symmetrical and unsymmetrical isomers. Relative to the dppe analogue 1b, the νCO bands for 1f are shifted by 20 cm–1 to lower energy. Unlike complexes 1a–g, Fe(edt)(CO)2(PMe3)2 is present in solution as a single isomer with mutually trans PMe3 ligands.[17] The IR spectrum of solid 1d indicates that the trans isomer dominates, which is also the predominant isomer in THF solution.

Table 1

Spectroscopic Data for Ferrous Carbonyl Complexesa

complex	isomer ratio unsym:sym (20 °C)	νCO (cm–1) (THF)	31P{1H} NMR (δ) unsym isomer	JP–P (Hz)	31P{1H} NMR (δ) sym isomer
Fe(edt)(CO)2(dppv) (1a)	2:1	2013 (s), 1978 (s), 1960 (s)	89.4 (d)	21.1	87.7 (s)
			59.9 (d)	21.1
Fe(edt)(CO)2(dppe) (1b)	3:1	2009 (s), 1973 (s), 1959 (s)	78.3 (d)	29.1	77.5 (s)
			48.1 (d)	29.1
Fe(pdt)(CO)2(dppv) (1c)	1:4	2010 (m), 1975 (s)	87.3 (d)	22.0	81.2 (s)
			60.4 (d)	22.1
Fe(pdt)(CO)2(dppe) (1d)	2:3	2004 (m), 1969 (s), 1958 (sh)	78.0 (d)	30.9	73.7 (s)
			51.2 (d)	30.6
Fe(pdt)(CO)2(dppbz) (1e)	1:7	2012 (m), 1970 (s)	78.5 (d)	38.6	80.8 (s)
			68.2 (d)	39.3
Fe(pdt)(CO)2(dcpe) (1f)	1:2	1990 (m), 1940 (s)	79.19 (d)	32.9	81.34 (s)
			55.8 (d)	32.9
Fe(Me2pdt)(CO)2(dppe) (1g)	1:3	2006 (m), 1969 (s)	78.3 (d)	30.5	73.8 (s)
			52.6 (d)	30.8

IR spectra are reported for THF, wherein signals are better resolved. 31P{1H} NMR spectra were recorded in CD2Cl2 solution, but isomer ratios were similar in THF solution.

Diffraction-quality crystals of 1a and 1c were obtained, and the molecular structures were determined by X-ray crystallography (Figures 1 and 2). In both cases, the major solution isomer crystallized.

Figure 1

Structure of the unsymmetrical isomer of Fe(edt)(CO)2(dppv) (1a). Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms are omitted for clarity. Key distances (Å) and angles (deg): Fe1–C1, 1.777(3); Fe1–C2, 1.790(3); Fe1–P2, 2.2304(8); Fe1–P1, 2.2648(7); Fe1–S1, 2.3313(8); Fe1–S2, 2.3471(7); C1–Fe1–C2, 93.60(12); C1–Fe1–P2, 94.43(8); C2–Fe1–P2, 95.80(9); C1–Fe1–P1, 95.90(8); C2–Fe1–P1, 170.41(9); P2–Fe1–P1, 84.80(3); C1–Fe1–S1, 85.78(8); C2–Fe1–S1, 89.16(9); P2–Fe1–S1, 175.01(3); P1–Fe1–S1, 90.23(3); C1–Fe1–S2, 174.68(8); C2–Fe1–S2, 85.05(9); P2–Fe1–S2, 90.80(3); P1–Fe1–S2, 85.32(3); S1–Fe1–S2, 89.07(3).

Figure 2

Structure of the symmetrical isomer of Fe(pdt)(CO)2(dppv) (1c). Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms are omitted for clarity. Key distances (Å) and angles (deg): Fe1–C1, 1.808(2); Fe1–C2, 1.799(2); Fe1–P2, 2.2204(1); Fe1–P1, 2.214(1); Fe1–S1, 2.3441(6); Fe1–S2, 2.3462(6); C1–Fe1–C2, 169.3(1); C1–Fe1–P2, 91.10(7); C2–Fe1–P2, 95.25(7); C1–Fe1–P1, 93.85(7); C2–Fe1–P1, 91.80(7); P2–Fe1–P1, 86.76(2); C1–Fe1–S1, 87.82(7); C2–Fe1–S1, 82.97(7); P2–Fe1–S1, 177.54(2); P1–Fe1–S1, 91.58(2); C1–Fe1–S2, 83.20(7); C2–Fe1–S2, 91.74(6); P2–Fe1–S2, 89.50(2); P1–Fe1–S2, 175.07(2); S1–Fe1–S2, 92.25(2).

Starting from 57Fe2I4(iPrOH)4, which can be derived from metallic 57Fe, the isotopically labeled complex 57Fe(pdt)(CO)2(dppe) (1d) was prepared (eqs 3 and 4). Characterization of cis/trans-571d included the observation of 57Fe–31P coupling in the 31P{1H} NMR spectrum (1JPFe = 35 and 38 Hz for cis and trans isomers, respectively, Figure 3).[25]

Figure 3

31P{1H} NMR spectrum of a mixture of the two isomers of 57Fe(pdt)(CO)2(dppe) (CD2Cl2 solution). Expanded regions show the additional coupling to 57Fe.

Diiron Dithiolato Carbonyl Complexes via Comproportionation

A comproportionation reaction was applied to the synthesis of unsymmetrically disubstituted subferrous diiron dithiolates (eq 5; xdt = edt, pdt; diphos = diphosphine; bda = benzylideneacetone). Thus, the reaction 1a + (bda)Fe(CO)3 gave the known complex Fe2(edt)(CO)4(dppv) (2a).[26] The reaction occurred over the course of several hours at room temperature, giving 83% isolated yield. The complex Fe2(pdt)(CO)4(dppe) (2d) was prepared analogously in about 60% yield. This species can be obtained in modest yields under very specific conditions by substitution of Fe2(pdt)(CO)6.[27,28] The low yields result from complications arising from the flexibility of the dppe ligand, which allows other intra- and intermolecular processes (Scheme 4).[27,28]

Scheme 4

Routes to Fe2(pdt)(CO)4(dppe) and Related Complexes

The reaction of the dcpe complex 1f with (bda)Fe(CO)3 afforded Fe2(pdt)(CO)4(dcpe) (2f) in 81% yield. As with the analogous dppe complex 2d, 2f exists in solution as a 3:1 mixture of apical–basal and dibasal isomers. The reaction of Fe2(pdt)(CO)6 and dcpe gives only a low yield of 2f, highlighting the advantage of the building block method. When, however, Fe(pdt)(CO)2(PMe3)2 was treated with (bda)Fe(CO)3, the product is the known complex Fe(pdt)(CO)4(PMe3)2,[29] where the phosphine ligands are bound to different iron centers.

Synthesis of MnIFeII Complexes

The salt [(CO)3Mn(pdt)Fe(CO)2(dppe)]BF4 ([3d(CO)]BF4) forms upon treatment of 1d with [(acenaphthene)Mn(CO)3]BF4, a well-known source of Mn(CO)3+ (Scheme 5).[30] When monitored by 31P{1H} NMR spectroscopy, the reaction was found to produce two isomeric MnFe compounds: an unsymmetrical species (doublets at δ 48 and 76) and a symmetrical species (singlet at δ 58). In these species the diphosphine ligands occupy apical–basal and dibasal sites on the Fe centers (Scheme 5). The initial isomer ratio matches that of the starting iron complex. The unsymmetrical isomer was found to convert to the symmetrical isomer over the course of 24 h (Figure 4). Comparably slow isomerism is observed in related diiron(II) dithiolates.[31]

Scheme 5

Synthesis of [(CO)3Mn(pdt)Fe(CO)2(dppe)]BF4 from Isomers of 1d

Figure 4

31P{1H} NMR spectra of a CD2Cl2 solution of (a) Fe(pdt)(CO)2(dppe) (triangle, unsym isomer; circle, sym isomer), (b) the same solution 30 min after addition of [(acenaphthene)Mn(CO)3]BF4 (diamond, unsym isomer; star, sym isomer), and (c) the same solution after 12 h.

Analogous complexes featuring different dithiolate and diphosphine ligands were also examined. Reaction of [(acenaphthene)Mn(CO)3]BF4 with the ethanedithiolate 1b afforded a mixture of isomers even after extended time for equilibration. The result is not surprising, since edt2– vs pdt2– affects the isomer ratio in 1b and 1d. The complexes [(CO)3Mn(pdt)Fe(CO)2(dppbz)]BF4, [(CO)3Mn(pdt)Fe(CO)2(dcpe)]BF4, and [(CO)3Mn(Me2pdt)Fe(CO)2(dppe)]BF4 form in a manner analogous to that for [3(CO)]BF4, and their spectroscopic data were similar.

Synthesis of MnIFeII Hydride

Reaction of [3d(CO)]BF4 with [Bu4N]BH4 results in loss of one CO ligand and formation of the neutral hydride complex (CO)3Mn(pdt)(μ-H)Fe(CO)(dppe) (H3d) (eq 6). The 1H NMR spectrum of H3d consists of a triplet at δ −12.3, indicating coupling to two equivalent phosphorus centers. The 31P{1H} NMR spectrum exhibits a singlet at δ 80.8, which confirms that the two phosphorus centers are equivalent, both occupying basal positions.

The structure of H3d was confirmed by X-ray crystallography, and the details are consistent with the NMR data (Figure 5). The phosphorus centers on the dppe ligand both occupy basal positions. The bridging hydrido ligand, whose location was identified on the difference map, refined to a position slightly closer to Fe (1.62(2) Å) than to MnH (1.75(2) Å). The d6d6 36e-complex (H3d is 34e-) FeMn(μ-SPh)3(CO)6 has been described.[32] The cyclic voltammogram of H3d consists of a reversible oxidation event at 0.125 V vs Fc+/0, characterized by the linear dependence of ip on (scan rate)1/2 (Supporting Information).

Figure 5

Structure of (CO)3Mn(pdt)(μ-H)Fe(CO)(dppe) (H3d) with thermal ellipsoids drawn at the 50% probability level. Selected distances (Å): Fe1–Mn1, 2.6433(4); Fe1–C27, 1.753(2); Fe1–P1, 2.2139(5); Fe1–P2, 2.2086(5); Fe1–S1, 2.2759(5); Fe1–S2, 2.2648(5); Fe1–H1, 1.62(2); Mn1–C31, 1.789(2); Mn1–C32, 1.813(2); Mn1–C33, 1.799(2); Mn1–S1, 2.3361(6); Mn1–S2, 2.3042(5); Mn1–H1, 1.75(2).

The acid–base behavior of H3d was investigated. Upon treatment of H3d with [H(Et2O)2]BArF4, the νCO bands shifted by 20–50 cm–1 to higher energy. Protonation of H3d was anticipated to give H2 or a dihydrogen complex (see [(H2)Mn(CO)3(dppe)]+).[33] Treatment of the acidified reaction mixture with Et3N gave back H3d (Figure 6). The reversible protonation of H3 is proposed to occur at sulfur. Similar S-protonations have been proposed for related charge-neutral diiron dithiolate complexes.[34] In contrast to the behavior of H3d, the closely related complex [(μ-H)Fe2(pdt)(CO)4(dppv)]+ is unreactive toward [H(Et2O)2]BArF4, illustrating the effect of charge on the basicity of the thiolate S centers.

Figure 6

IR spectra in CH2Cl2 of (a) (CO)3Mn(pdt)(μ-H)Fe(CO)(dppe) and this solution (b) after addition of 1 equiv of H(Et2O)2BArF4, (c) after addition of 0.5 equiv of Et3N, and (d) after addition of 1 equiv total of Et3N.

31P{1H} and 1H NMR (hydride region) resonances for H3d were broadened upon protonation of this complex (Supporting Information). In the presence of 2 equiv of acid, a pair of doublets appears at 79.6 and 78.5 in the 31P{1H} NMR spectrum, assigned to the nonequivalent phosphorus centers in [H3dH]+.

Reduction of MnIFeII Complexes

The electrochemical behavior of [3d(CO)]BF4 was investigated by cyclic voltammetry. At −1.0 V, an irreversible reduction is observed, followed by a reversible event centered at −1.3 V. Consistent with its being reductive decarbonylation, the irreversible event at −1.0 V diminishes upon further electrochemical cycling (i.e., as decarbonylation proceeds to completion) but is less irreversible at low temperatures. To probe this irreversible chemical process, a solution of [3d(CO)]BF4 was treated with 1 equiv of cobaltocene, producing a new species characterized by νCO 1997 and 1902 cm–1. In view of the electrochemical and chemical reduction results, the initial reduction is proposed to induce decarbonylation, generating (CO)3Mn(pdt)Fe(CO)(dppe) ([3d(CO)]0). Analytically pure [3d(CO)]0 was obtained once it was determined that its stability was greater in THF than in CH2Cl2 solution. Cyclic voltammetry of this salt exhibits the quasi-reversible couple at −1.3 V.

Cobalt–Iron Dithiolates

Treatment of the ferrous dithiolato carbonyls with CpCo(CO)2 in refluxing toluene or THF gave complexes of the type CpCo(pdt)Fe(CO)(dppx) (4a,d). The yields for these preparations were inconsistent, however, varying from 50% to almost nothing. A related but more reliable route to the same compounds involved the reactions of the ferrous dithiolates with CpCoI2(CO) followed by reduction. For example, 1d and CpCoI2(CO) react rapidly to give an isomeric mixture of the intermediate [CpCoI(pdt)Fe(CO)(dppe)]+ (νCO 1952 cm–1). When this reaction is monitored by 31P{1H} NMR spectroscopy, two isomeric Co–Fe complexes are observed. Akin to the MnFe systems, the 31P{1H} NMR spectrum showed that one isomer is unsymmetrical (doublets at δ 47 and 90) and the second isomer is characterized by a singlet at δ 59. In these isomers the phosphine ligands occupy apical–basal and dibasal sites, respectively. Reduction of [CpCoI(pdt)Fe(CO)(dppe)]+ with 2 equiv of Cp2Co gave 4d in good yield (eqs 7 and 8).

These CoFe complexes are analogues of Fe2(S2CH2)(CO)4(dppx), in which CpCo replaces the Fe(CO)3 center. Like the corresponding diiron complexes, 4d oxidizes reversibly, at a potential (−0.6 V vs Fc+/0) between those for Fe2(pdt)(CO)2(dppv)2 (−0.19 V) and Fe2(pdt)(CO)4(dppv) (−0.94 V).[26,35] Crystallographic and spectroscopic characterization of the ethanedithiolate 4a proved mutually consistent (Figure 7). The complex can be described as a pair of five-coordinate metal centers linked by a Co–Fe bond (2.5038(5) Å). A distinctive feature of the structure is the acute angle for the FeP2 plane relative to the FeS2 plane.

Figure 7

Structure of CpCo(edt)Fe(CO)(dppv) (4a) with thermal ellipsoids drawn at the 50% probability level. Selected distances (Å): Fe1–Co1, 2.5038(5); Fe1–C1, 1.754(3); Fe1–P1, 2.2038(5); Fe1–S4, 2.2318(5); Co1–S1, 2.1748(6); Co–Cp centroid, 1.6727(3).

The 31P{1H} NMR spectrum of 4d established the presence of only one isomer, even at low temperatures. Below −60 °C, the spectrum consists of a pair of singlets with ∼4:1 intensities, attributed to conformational isomers arising from the pdt2– backbone. Using an internal integration standard, >90% of the sample was verified to be in solution at −90 °C.

CoIIIFeII Hydrides

At room temperature, the Co–Fe complexes protonate to give the corresponding μ-hydrido derivatives. Protonation shifts the νCO band from 1890 to 1975 cm–1. The 31P{1H} and 1H NMR spectra indicate that the hydride complex is C-symmetric. On the basis of its chemical shift and J(31P,[1]H) value of 25 Hz, the hydride ligand is bridging the Fe and Co centers.

NMR studies on the protonation at low temperatures revealed at least two intermediates. The initial product of protonation, formed quantitatively at −85 °C, is characterized by singlets at δ −9.0 and δ 70.7 in the 1H and 31P{1H} NMR spectra, respectively (Figures 8 and 9). These signals are assigned to a terminal hydride complex with the hydride ligand on the CpCo center (Scheme 6). When the temperature is raised to −50 °C, this terminal hydride converts to approximately equal amounts of two species characterized by a doublet at δ −14.7 (J = 30 Hz) and triplet at δ −15.5 (J = 25 Hz). These species correspond to the bridging hydrides with apical–basal and dibasal phosphines. Bridging hydrides typically exhibit a ∼25 Hz coupling to the cis phosphine, whereas coupling to the trans phosphine is often weak or is not observed.[36] At room temperature, the unsymmetrical isomer, labeled a,b for apical–basal, converts to the dibasal isomer. Comparable isomerizations have been observed for diiron complexes.

Figure 8

31P{1H} NMR spectra of a CD2Cl2 solution of 4d and 1 equiv of [H(OEt2)2]BArF4. The solution was prepared at −85 °C and then warmed to the indicated temperatures.

Figure 9

1H NMR spectra of a CD2Cl2 solution of 4d and 1 equiv of [H(OEt2)2]BArF4. The solution was prepared at −85 °C and then warmed to the indicated temperatures.

Scheme 6

Protonation of 4d and Isomerization of Resulting Hydride Complexes

Discussion

Ferrous dicarbonyl dithiolato diphosphine complexes are versatile precursors to dimetallic complexes, as illustrated by their conversion to FeFe, MnFe, and CoFe dithiolate complexes described above. The new synthesis of Fe2(pdt)(CO)4(dppe) avoids side reactions that hamper the installation of flexible chelating ligands.[27,28] Related complexes can be prepared via FeX2(CO)4.[14,37] The new routes are potentially appealing because the iron complexes are derived from FeCl2.[8] Conveniently available sources of 57Fe are easily converted to the dichloride and 57Fe2I4(iPrOH)4. Although the preparations reported herein proceed in modest yields, the precursors are readily available and the product workup is relatively simple. The methods lend themselves to the incorporation of 57Fe (I = −1/2) into ferrous carbonyl building blocks, as illustrated by the synthesis of 57Fe(pdt)(CO)2(dppe). Interest in 57Fe labeling stems from the wealth of information available from, among other techniques, Mössbauer and NRVS spectroscopy.

This building block approach allowed the synthesis of the first dithiolato-bridged MnFe complexes. Related dimanganese dithiolate complexes had been reported by Treichel.[38] The most curious property of [(CO)3Mn(pdt)Fe(CO)2(dppe)]+ is its tendency to decarbonylate upon 1e-reduction. The resulting complex, (CO)3Mn(pdt)Fe(CO)(dppe), is electronically related to the mixed-valence “Hox-models” [Fe2(pdt)(CO)6–L]+.[39] Similarly, the hydride (CO)3Mn(pdt)HFe(CO)(dppe), which was characterized crystallographically, is isoelectronic with [HFe2(pdt)(CO)4(dppe)]+.[28] The MnFe complex is sufficiently electron rich that it undergoes a mild one-electron oxidation (∼0.1 V vs Fc+/0), whereas oxidations of analogous diferrous hydrides occur only at very positive potentials.[40]

This building block approach also allowed the synthesis of dithiolato-bridged iron–cobalt complexes. Two complementary routes to these CoFe complexes were devised: a direct Co(I) + Fe(II) pathway and an less direct but more reliable route via Co(III) + Fe(II), followed by 2e-reduction. The latter method is modeled after the route to (CO)3Fe(pdt)Ni(dppe) from FeI2(CO)4 + Ni(pdt)(dppe) followed by reduction.[41] The complexes CpCo(dithiolate)Fe(CO)(diphos) are electronically related to Fe2(dithiolate)(CO)4(diphos), both being of d7d7 configuration. Like FeIFeI dithiolates, the CoIIFeI species undergo protonation and redox reactions.

Experimental Section

Methods used in this work have been recently described.[35] Chromatography was performed using silica gel (40–63 μm, 230–400 mesh) as the stationary phase. K2pdt,[42] [(acenaphthene)Mn(CO)3]BF4,[30]57Fe2I4(iPrOH)4,[43] and CpCo(CO)I2[44] were prepared according to the literature methods. ESI-MS data were recorded on dilute CH2Cl2 solutions on a Waters Micromass Quattro II spectrometer. ATR data were collected on a PerkinElmer Spectrum 100 FT-IR instrument. 1H NMR spectra were recorded at 500 MHz and 31P{1H} NMR spectra at 202 MHz. Coupling constants are reported in Hz. IR measurements, reported in cm–1, were recorded only in the νCO region.

Illustrative Preparation: Fe(pdt)(CO)2(dppe) (1d)

Under a CO atmosphere, a solution of 1.9 g of FeCl2 (15.0 mmol) in 250 mL of acetone was treated with a solution of 5.98 g (15.0 mmol) of dppe in 60 mL of THF. The solution changed from pale orange to green and then to dark orange, signaling formation of FeCl2(CO)2(dppe). Separately, 1.71 mL (17.0 mmol) of C3H6(SH)2 and 0.72 g (30.0 mmol) of NaH were combined in 50 mL of THF. After 1 h, the resulting solution of Na2S2C3H6 was added to the solution of FeCl2(CO)2(dppe). After being allowed to react for 16 h, the mixture was filtered through Celite, and the solvent was evaporated from the filtrate. The residue was extracted into 15 mL of CH2Cl2 and purified by flash column chromatography on a 4 × 50 cm column of silica gel. After a yellow band eluted with CH2Cl2, the red band containing the product eluted with 5/1 CH2Cl2/Et2O. Evaporation of solvent from this band afforded 1d as a red solid. Yield: 3.34 g (36%). 1H NMR (CD2Cl2): δ 7.89–7.32 (m, 20H, C6H5), 2.63 (m, 4H, PCH2), 2.50 (d, 4H, SCH2), 1.97 (m, 2H, SCH2CH2). IR (CH2Cl2): 2010, 1968. Anal. Calcd for C31H30FeO2P2S2·0.3CH2Cl2 (found): C, 58.56 (58.24); H, 4.80 (4.7).

Fe(edt)(CO)2(dppv) (1a)

Conducted as for 1d. Yield: 12–50%. 1H NMR (CD2Cl2): δ 8.14–7.32 (m, 20H, C6H5), 2.53 (s, 2H, CH2), 2.46 and 2.02 (d each, 1:1 H, CH2), 2.18 and 0.21 (t each, 1:1 H, CH). IR (CH2Cl2): 2013 (s), 1978 (s), 1960 (s). Anal. Calcd for C30H26FeO2P2S2·0.5CH2Cl2 (found): C, 56.98 (57.35); H, 4.23 (4.17).

Fe(edt)(CO)2(dppe) (1b)

Conducted as for 1d. Yield: 50%. 1H NMR (CD2Cl2): δ 7.87–7.27 (m, C6H5), 2.62 (s, 4H, S CH2, sym), 2.50 (s, 4H, PCH2, sym), 2.78 (d 2H PCH2, unsym), 2.16 (d 2H PCH2, unsym), 1.22 (s, 3H, SCH2, unsym), 0.34 (s, 1H, SCH2, unsym). IR (THF): 2009 (s), 1973 (s), 1959 (s). Anal. Calcd for C30H28FeO2P2S2 (found): C, 59.81 (59.78); H, 4.68 (5.0).

Fe(pdt)(CO)2(dppv) (1c)

Conducted as for 1d, but the column was eluted with 10/1 CH2Cl2/Et2O. Yield: 78%. 1H NMR (CD2Cl2): δ 7.94–7.32 (m, 20H, C6H5), 2.44 (d, 4H, CH2), 1.96 (d, 2H, SCH2). IR (CH2Cl2): 2014, 1975. Anal. Calcd for C31H28FeO2P2S2·0.7CH2Cl2 (found): C, 56.49 (56.50); H, 4.40 (4.40).

Fe(pdt)(CO)2(dppbz) (1e)

Under a CO atmosphere, a solution of 0.50 g of FeCl2 (3.94 mmol) in 100 mL of acetone was treated with a solution of 1.76 g (15.0 mmol) of dppbz in 40 mL of THF. The solution changed from pale orange to dark orange, signaling formation of FeCl2(CO)2(dppbz). Separately, 181 μL (4.33 mmol) of C3H6(SH)2 and 188 mg (7.88 mmol) of NaH were combined in 15 mL of THF. After 1 h, the resulting solution of Na2S2C3H6 was added to the solution of FeCl2(CO)2(dppbz). A large amount of red precipitate had formed after 15 h. The solution was filtered, the filtrate was discarded, and the red solid was extracted into 50 mL of CH2Cl2. The product was recrystallized by addition of hexanes to a concentrated CH2Cl2 solution. Yield: 400 mg (20%). 1H NMR (CD2Cl2): δ 7.4–7.6 (m, 24H, C6H5 and C6H4), 2.39 (s, 4H, SCH2CH2), 1.94 (s, 2H, SCH2CH2). IR (CH2Cl2): 2012, 1970. Anal. Calcd for C35H30FeO2P2S2·0.2CH2Cl2 (found): C, 62.03 (62.22); H, 4.5 (4.83).

Fe(pdt)(CO)2(dcpe) (1f)

Conducted as for 1d, but instead of chromatography, the product was extracted into ∼100 mL of hexanes. The solution volume was reduced to ∼20 mL and the solution cooled to 0 °C, resulting in the formation of dark red crystals. Yield: 430 mg (35%). IR (CH2Cl2): 1997 (w), 1945 (s). Anal. Calcd for C31H54FeO2P2S2·0.2CH2Cl2 (found): C, 56.98 (56.94); H, 8.34 (8.71).

57Fe2I4(iPrOH)4

This complex was obtained as a green powder in 95% yield analogously to the published method for Fe2I4(iPrOH)4,[43] using 57Fe as the precursor. ESI-MS: m/z 674.8 [M – iPrOH – I–]+, 364.1 [M – iPrOH – 57Fe2+ – 3I–]+. Anal. Calcd for C12H32O4I457Fe2 (found): C, 16.72 (16.16); H, 3.74 (3.44); N, 0.00 (0.00).

57Fe(pdt)(CO)2(dppe) (571d)

A mixture of 86.2 mg (100 μmol) of 57Fe2I4(iPrOH)4 and 79.7 mg (200 μmol) of dppe in 6 mL of 1/1 iPrOH/THF was stirred under 1 atm of CO. The suspension, which had developed a red color, was treated with 36.8 mg (200 μmol) of K2pdt in 3 mL of iPrOH. After it was stirred for 24 h in the absence of light, the mixture was evaporated to dryness. The dark residue was extracted with 2 × 5 mL of CH2Cl2, and these extracts were concentrated to ∼0.5 mL and chromatographed on a ∼5 cm column of silica gel, with CH2Cl2 as eluent. The second band, deep red, was collected, concentrated to ∼2 mL, and treated with 10 mL of Et2O. Any dark solids that formed were removed by filtration, and the filtrate was treated with 15 mL of pentane. The mixture was allowed to stand at −28 °C for 1 h, after which the solid that formed was isolated by filtration, washed with additional pentane, and dried briefly to afford the title compound as a pink powder (23.6 mg, 38.2 μmol, 19%). 31P{1H} NMR (CH2Cl2): δ 76.2 (dd, 1JPFe = 35, 2JPP = 32, cis isomer), 71.8 (d, 1JPFe = 38, trans isomer), 49.2 (dd, 1JPFe = 35, 2JPP = 32, cis isomer). FTIR (CH2Cl2): 2010 (cis), 1969 (overlapping cis/trans). ESI-MS: m/z 618.2 [M + H+]+ (similar analysis for the unlabeled complex gave m/z 617.1).

Fe(Me2pdt)(CO)2(dppe) (1g)

Conducted as for 1d. Yield: 1.03 g (40%). 1H NMR (CD2Cl2): δ7.4–7.8 (m, 20H, C6H5), 2.65 (dd, 4H, P2CH2CH2), 2.23 (s, 4H, SCH2), 1.01 (s, 6H, CCH3). IR (CH2Cl2): 2006 (w), 1969 (s). Anal. Calcd for C33H34FeO2P2S2·0.2CH2Cl2 (found): C, 60.28 (60.0); H, 5.24 (5.41).

Fe2(edt)(CO)4(dppv) (2a) from 1a

A mixture of 1a (125 mg, 0.208 mmol) and 50 mg (0.208 mmol) of (bda)Fe(CO)3 in 20 mL of toluene gradually darkened over the course of several hours to a deep red solution. The reaction was monitored by IR for the disappearance of νCO bands for (bda)Fe(CO)3 and 1a and the appearance of 2a. After 24 h, the reaction solution was concentrated, and the crude product was chromatographed on silica gel in air, with toluene as eluent. The first brown-red band was collected and dried in vacuo. Yield: 123 mg (83%). The IR, 1H NMR, and 31P{1H} NMR spectra of the product match reported data.[26]

Fe2(pdt)(CO)4(dppe) (2d) from 1d

An orange mixture of 1d (250 mg, 0.40 mmol) and (bda)Fe(CO)3 (115 mg, 0.40 mmol) in 70 mL of toluene gradually darkened over the course of several hours to a deep red solution. The reaction was monitored by IR spectroscopy. After 24 h, the reaction solution was concentrated, and the crude product was chromatographed on silica gel in air, with toluene as eluent. The first brown-red band was collected. Yield: 150 mg (60%). The IR, 1H NMR, and 31P{1H} NMR spectra of the product match reported data.[28]

Fe2(pdt)(CO)4(dcpe) (2f) from 1f

A 5 mL solution of (bda)Fe(CO)3 (48 mg, 0.17 mmol) was added to a red solution of 1f (107 mg, 0.17 mmol) in 20 mL of toluene. The solution gradually darkened over the course of several hours to a deep red-brown solution. The reaction was monitored by IR spectroscopy. After 24 h, the reaction solution was concentrated, and the crude product was chromatographed on silica gel in a glovebox, with toluene as eluent. The first brown-red band was collected. Yield: 101 mg (81%). 31P{1H} NMR (CD2Cl2): δ 89.27 (s), apical–basal; 74.90 (s), dibasal. IR (CH2Cl2): 2013 (s), 1939 (br), 1882 (br).

[(CO)3Mn(pdt)Fe(CO)2(dppe)]BF4 ([3(CO)]BF4)

A solution of 500 mg (1.32 mmol) of [(acenaphthene)Mn(CO)3]BF4 in 125 mL of CH2Cl2 was treated with a solution of 810 mg (1.32 mmol) of 1b in 50 mL of CH2Cl2. The solution was stirred for 20 h and gradually became dark brown. The IR spectrum of the solution showed bands for the product. The solution was evaporated to dryness. The resulting brown residue was extracted into ∼30 mL of CH2Cl2, and the extract was filtered through a pad of Celite. The brown solution was concentrated to ∼10 mL and then diluted with 100 mL of hexanes. Upon storage of the solution at 0 °C, brown microcrystals formed. Yield: 1.00 g (90%). Crystals were obtained by layering a CH2Cl2 solution with pentane. 1H NMR (CD2Cl2): δ 7.41–7.80 (m, 20H, C6H5), 3.50 (m, 2H, PCH2), 3.28 (m, 2H, PCH2), 3.08 (m, 2H, SCH2), 2.84 (m, 3H, SCH2CH2), 2.03 (m, 1H, SCH2CH2). 31P{1H} NMR (CD2Cl2): δ 58 (s). IR (CH2Cl2): 2053 (w), 2027 (s), 1992 (s), 1974 (s), 1905 (br). Anal. Calcd for C34H30BF4FeMnO5P2S2 (found): C, 48.48 (48.66); H, 3.59 (3.81).

(CO)3Mn(pdt)(μ-H)Fe(CO)(dppe) (H3)

A solution of 590 mg (0.70 mmol) of [3(CO)]BF4 in 100 mL of CH2Cl2 was cooled to −78 °C and treated with a precooled solution of 257 mg (0.70 mmol) of [Bu4N]BH4 in 60 mL of CH2Cl2 over the course of 90 min, during which time the solution changed from brown to dark red. The reaction progress was monitored by IR spectroscopy. The solution was warmed to room temperature and stirred at room temperature for 15 h before being evaporated to dryness. An extract of the red residue in ∼20 mL of toluene was chromatographed on silica gel with a 3/1 toluene/hexanes mixture as eluent. The product (an orange band) eluted first, followed by a brown band. The orange band was evaporated under vacuum, and the resulting orange residue was extracted into ∼25 mL of toluene. This extract was filtered through Celite, concentrated to half volume, and then diluted with 60 mL of hexanes. After storage of the solution at 0 °C, orange crystals formed. Yield: 98 mg (20%). Diffraction-quality crystals were grown at 0 °C by layering a toluene solution with hexanes. 1H NMR (CD2Cl2): δ 7.38–7.84 (m, 20H, C6H5), 2.76 (m, 4H, PCH2), 2.58 (m, 2H, SCH2), 2.41 (m, 3H, SCH2CH2), 1.91 (m, 1H, SCH2CH2), −12.26 (t, 1H, Mn–H–Fe). 31P{1H} NMR (CD2Cl2): δ 80.80 (s). IR (CH2Cl2): 2002 (s), 1935 (br), 1905 (br). Anal. Calcd for C33H31FeMnO4P2S2 (found): C, 54.41 (54.35); H, 4.29 (4.49).

(CO)3Mn(pdt)Fe(CO)(dppe)

A solution of [4d(CO)]BF4 (71 mg, 84 μmol) in THF (10 mL) was treated with a THF solution of Cp2Co (16 mg, 84 μmol). The solution was filtered through Celite, and then the solvent was removed under vacuum to yield a red solid. Yield: 46 mg (75%). IR (CH2Cl2): 1997 (s), 1902 (s). Anal. Calcd for C33H30FeMnO4P2S2 (found): C, 54.49 (54.54); H, 4.16 (4.55).

CpCo(edt)Fe(CO)(dppv) (4a) from CpCoI2(CO)

A mixture of 1a (100 mg, 0.17 mmol) and CpCo(CO)I2 (68 mg, 0.17 mmol) was stirred in CH2Cl2 (20 mL) at room temperature for 2 h to give a dark brown solution with a predominant IR band at 1978 cm–1. To this solution was added a solution of Cp2Co (68 mg, 0.36 mmol) in 20 mL of CH2Cl2. The IR spectrum of the resulting solution revealed a prominent band at 1890 cm–1. After the mixture was stirred for 30 min, solvent was removed, and the residue was purified by column chromatography, initially with a 1/1 mixture of CH2Cl2 and pentane as eluent and gradually increasing the CH2Cl2 content. After an initial green band, the product eluted as a brown band using CH2Cl2. Evaporation of the solution gave the brown product. Yield: 45 mg (39%). 1H NMR (CD2Cl2, 23 °C): δ 8.30–7.12 (m, 20 H, C6H5), 4.75 (s, 2 H, PCH), 3.36 (s, 5 H, C5H5), 2.26 and 1.84 (d each, 2 H each, SCH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 93.56 (s). IR (CH2Cl2): 1890.

CpCo(edt)Fe(CO)(dppv) from CpCo(CO)2

A mixture of 1a (200 mg, 0.34 mmol) and CpCo(CO)2 (92 μL, 0.68 mmol) was stirred in toluene (100 mL) at reflux for 2.0 h to give a dark brown solution containing a small amount of red-brown solid. The IR spectrum at this stage revealed a new band at 1896 cm–1. The solution was filtered, and the brown filtrate was evaporated. The brown residue was washed with ∼10 mL each of hexanes and Et2O. The brown residue was extracted into ∼4 mL of CH2Cl2, and the extract was layered with 10 mL of hexane. Dark brown crystals of 4a formed overnight. Yields were variable. Anal. Calcd for C34H31CoFeOP2S2 (found): C, 58.63 (58.12); H, 4.49 (4.37). IR (CH2Cl2): 1890.

CpCo(pdt)Fe(CO)(dppv) (4c)

A mixture of 1c (200 mg, 0.32 mmol) and CpCo(CO)2 (67 μL, 0.65 mmol) was stirred in toluene (100 mL) at reflux temperature for 2.0 h to give a dark brown solution containing a small amount of red-brown solid. The IR spectrum at this stage revealed a new band at 1892 cm–1. The solution was filtered, and the brown filtrate was evaporated. The brown residue was washed with ∼10 mL of hexanes and then extracted into ∼5 mL of CH2Cl2. This extract was layered with 10 mL of hexane to yield dark brown crystals of 4b upon standing overnight. 1H NMR (CD2Cl2, 23 °C): δ 8.17–7.30 (m, 20 H, C6H5), 3.51 (s, 5 H, C5H5), 2.30 and 2.20 (d each, 2 H each, SCH2), 1.87 (m, 2 H, SCH2CH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 90.17 (s). IR (CH2Cl2): 1885.

CpCo(pdt)Fe(CO)(dppe) (4d) from CpCoI2(CO)

A solution of Fe(pdt)(CO)2(dppe) (50 mg, 0.08 mmol) and CpCo(CO)I2 (31 mg, 0.08 mmol) in CH2Cl2 (20 mL) was stirred overnight. Formation of a new product was detected by an IR band at 1952 cm–1. The reaction solution was treated with a solution of Cp2Co (38 mg, 0.20 mmol) in CH2Cl2 (5 mL). After the solution was stirred for 5 min, the dominant IR band shifted to 1879 cm–1. The solution was filtered through Celite, and the product was purified by column chromatography on silica gel, initially with 1/1 CH2Cl2/pentane as eluent, which produced a green band. Eluting with CH2Cl2 gave the product as a brown band, which was evaporated to leave a brown solid. Yield: 23 mg (40%). 1H NMR (CD2Cl2, 23 °C): δ 7.82–7.29 (m, 20 H, C6H5), 3.84 (s, 5 H, C5H5), 2.98 (m, 2 H, PCH2), 2.58 and 2.26 (m each, 2 H each, SCH2), 2.17 and 1.89 (m each, 1 H each, PCH2), 1.47 (m, 2 H, S(CH2)2CH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 83.48 (s). IR (CH2Cl2): 1880.

CpCo(pdt)Fe(CO)(dppe) (4d) from CpCo(CO)2

A mixture of 1d (200 mg, 0.32 mmol) and CpCo(CO)2 (67 μL, 0.65 mmol) was stirred in toluene (100 mL) at reflux temperature for 7 h to give a dark brown solution. The IR spectrum at this stage revealed a new band at 1883 cm–1. The solution was filtered, and the brown filtrate was evaporated. The brown residue was washed with ∼15 mL of toluene and then extracted into ∼4 mL of CH2Cl2. This extract was layered with 10 mL of hexane to yield dark brown crystals of 4d upon standing overnight. Yields were variable. 1H NMR (CD2Cl2, 23 °C): δ 7.82–7.29 (m, 20 H, C6H5), 3.84 (s, 5 H, C5H5), 2.98 (m, 2 H, PCH2), 2.56 and 2.25 (m each, 2 H each, SCH2), 2.18 and 1.87 (m each, 1 H each, PCH2), 1.51 (m, 2 H, CH2SCH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 85.54 (s). Anal. Calcd for C35H35CoFeOP2S2 (found): C, 59.00 (58.89); H, 4.95 (4.83). IR (CH2Cl2, cm–1): νCO 1879. Data for the low-temperature intermediate, terminal hydride are as follows. 1H NMR (CD2Cl2, 23 °C): δ 7.73–7.35 (m, 20 H, C6H5), 3.61 (s, 5 H, C5H5), 2.98 (m, 2 H, PCH2), 2.56 and 2.25 (m each, 2 H each, SCH2), 2.18 and 1.87 (m each, 1 H each, PCH2), 1.51 (m, 2 H, S(CH2)2CH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 85.54 (s). Data for the low-temperature intermediate, apical–basal μ-hydride are as follows. 1H NMR (CD2Cl2, 23 °C): δ 7.82–7.29 (m, 20 H, C6H5), 3.84 (s, 5 H, C5H5), 2.98 (m, 2 H, PCH2), 2.56 and 2.25 (m each, 2 H each, SCH2), 2.18 and 1.87 (m each, 1 H each, PCH2), 1.51 (m, 2 H, CH2SCH2). 31P{1H} NMR (CD2Cl2, 23 °C): δ 85.54 (s).

Low-Temperature Protonation of CpCo(pdt)Fe(CO)(dppe) (4d)

In a J. Young NMR tube, ∼1 mL of CD2Cl2 was distilled and frozen onto 4d (5 mg, 0.007 mmol) and [H(OEt2)2]BArF24 (8.5 mg, 0.008 mmol) in a liquid-N2 bath. The sample was then thawed, placed in an NMR spectrometer (probe precooled to −85 °C), and analyzed by NMR spectroscopy. 1H NMR (CD2Cl2, −85 °C): δ 3.61 (s, C5H5, [term-H4d]+); −9.0 (s, Co-H, [term-H4d]+). 31P{1H} NMR (CD2Cl2, −85 °C): δ 70.65 (s, [term-H4d]+). When the sample was warmed to −50 °C, the NMR spectra indicated the presence of two new species. 1H NMR (CD2Cl2, −50 °C): δ 3.61 (s, C5H5, [term-H4d]+); −9.0 (s, Co-H, [term-H4d]+); 5.29 (s, C5H5, a,b-[μ-H4d]+); −14.68 (d, JP–H = 30, Co-H-Fe, a,b-[μ-H4d]+); 4.69 (s, C5H5, b,b-[μ-H4d]+); −15.48 (t, JP–H = 25, Co-H-Fe, b,b-[μ-H4d]+). 31P{1H} NMR (CD2Cl2, −50 °C): δ 70.65 (s, [term-H4d]+); 88.17, 85.58 (d, a,b-[μ-H4d]+); 85.3 (s, b,b-[μ-H4d]+). When the sample was warmed above −20 °C, the signals for [term-H4d]+ in the 1H and 31P{1H} NMR spectra disappeared. At room temperature, a,b-[μ-H4d]+ converts entirely to b,b-[μ-H4d]+.

[CpCo(edt)(H)Fe(CO)(dppv)]BF4 ([H4a]BF4)

A solution of 4a (20 mg, 0.03 mmol) in CH2Cl2 (10 mL) was treated with 2 equiv of HBF4·Et2O. The mixture was stirred for 15 min. The volume was then reduced under vacuum, and Et2O was added to precipitate a brown powder. 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 8.50–7.26 (m, 20 H, C6H5), 4.31 (s, 5 H, C5H5), 3.06 and 2.70 (d each, 2 H each, SCH2), −18.14 (t, JPH = 26.8, 1H, Co-H-Fe). 31P{1H} NMR (CD2Cl2, 23 °C): δ 89.72 (d). IR (CH2Cl2): 1975.

[CpCo(pdt)(H)Fe(CO)(dppe)]BF4 ([H4d]BF4)

A solution of 4d (20 mg, 0.03 mmol) in CH2Cl2 (10 mL) was treated with 2 equiv of HBF4·Et2O. After it was stirred for 15 min, the solution was concentrated and then diluted with Et2O to precipitate a brown powder. 1H NMR (CD2Cl2, 25 °C): δ 7.66–7.30 (m, 20H, C6H5), 4.79 (s, 5H, C5H5), 3.30 (m, 2H, PCH2), 3.01 and 2.71 (m each, 2H each, SCH2), 2.61 (m, 1H, PCH2), 2.15 (m, 3H, PCH2 and CH2SCH2), −15.19 (t, JPH = 27, 1H, Co-H-Fe). 31P{1H} NMR (CD2Cl2, 25 °C): δ 82.91 (d). Anal. Calcd for C35H36BCoF4FeOP2S2·0.2CH2Cl2 (found): C, 51.73 (51.90); H, 4.49 (4.61). IR (CH2Cl2): 1962.