New Nitrosyl Derivatives of Diiron Dithiolates Related to the Active Site of the [FeFe]-Hydrogenases

Abstract

Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the versatile precursor Fe₂(S₂C_nH_2n)(dppv)(CO)₄ (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]BF₄ ([1(CO)₃]BF₄), the following compounds were prepared: [1(CO)₂(PMe₃)]BF₄, [1(CO)(dppv)]BF₄, NEt₄[1(CO)(CN)₂], and 1(CO)(CN)(PMe₃). Some if not all of these substitution reactions occur via the addition of two equiv of the nucleophile followed by dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)(PMe₃)₂]BF₄. This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(NO)(CO) derivative. Compounds [1(CO)₃]BF₄, [1(CO)₂(PMe₃)]BF₄, and [1(CO)(dppv)]BF₄ were also prepared by the addition of NO⁺ to the di-, tri- and tetrasubstituted precursors. In these cases, the NO⁺ appears to form an initial 36e-adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO.

Introduction

The substitution chemistry for compounds of the type [Fe₂(SR)₂(CO)_6-xL_x]^z is well developed for L = tertiary phosphines and cyanide.¹ These synthetic transformations were mainly developed en route to functional and structural mimics of the active site of the [FeFe]-hydrogenase enzymes. The enzymes catalyze the redox interconversion of protons and H₂, and do so at near-thermodynamic potentials.² The high efficiency of the Fe-based catalysts has motivated numerous studies on the synthesis and reactivity of diiron dithiolato carbonyls.

Recently, we reported a series of diiron dithiolate-PMe₃ complexes containing nitrosyl ligands. These compounds are of interest because they adopt ‘rotated’ structures as seen in related mixed-valence Fe^IFe^II complexes, the so-called H_ox models.³ Associated with their distinctive structure, these nitrosyl compounds are Lewis acidic, which is unknown for other [Fe^I]₂ species, but characteristic of the mixed valence H_ox state and its models. Our results indicated the likely stability of other nitrosyl complexes. We therefore have investigated the nitrosyl derivatives of the diiron(I) dithiolates supported by cis-1,2-bis(diphenylphosphinoethylene) (dppv). For studying the fundamental reactivity diiron(I) dithiolates, we have shown that Fe₂(S₂C_nH_2n)(CO)₄(dppv) (n = 2, 3) are useful reagents because of their high reactivity and the simplified structures of the products.⁴ We expected, incorrectly as we show, that NO⁺ would yield highly polar derivatives of Fe₂(S₂C_nH_2n)(CO)₄(dppv).

Nitrosyl derivatives of iron thiolates are of course well known. The first synthetic iron-sulfide cluster, Roussin's Red anion, [Fe₂S₂(NO)₄]^2-, led to the corresponding “esters,” Fe₂(SR)₂(NO)₄ (R = alkyl, aryl).⁵ The Roussin esters have come under renewed scrutiny because of their connection to the so-called dinitrosyl iron complexes (DNIC's).⁶ Nitric oxide inhibits [NiFe] hydrogenases,⁷ although the reactivity of NO with [FeFe] hydrogenases have not been reported.

Both NO and NO⁺ are electrophiles,⁸ hence it is worth mentioning the reactivity of other electrophiles with Fe₂(SR)₂(CO)_6-xL_x. Substituted diiron(I) dithiolates protonate readily.⁹ Sources of SMe⁺,¹⁰ Br⁺, I⁺,¹¹ and HgCl⁺,¹² have long been known to add across the Fe-Fe bond.

Results

[Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]⁺, a Versatile Synthetic Intermediate

The salts [Fe₂(S₂C_nH_2n)(NO)(CO)₃(dppv)]BF₄ (n = 2, [1(CO)₃]BF₄; n = 3, [2(CO)₃]BF₄) were found to form efficiently upon treatment of CH₂Cl₂ solutions of Fe₂(S₂C_nH_2n)(CO)₄(dppv) with NOBF₄ at 20 °C (Scheme 1). Indicative of their high electrophilicity, both [1(CO)₃]BF₄ and [2(CO)₃]BF₄ exhibit limited stability in MeCN solution at room temperature, although CH₂Cl₂ and THF solutions are stable for hours. These mononitrosyl complexes were unreactive toward additional equivs of NOBF₄ at room temperature.

Scheme 1.

Diiron Dithiolato Nitrosyl Complexes Described in this Work.

The crystallographically determined structure of [2(CO)₃]BF₄ revealed that the nitrosyl ligand is apical, the cation having idealized C_s symmetry (Figure 1). The complex exhibits a slight twisting distortion in both the Fe(dppv)(NO)⁺ and Fe(CO)₃ subunits, reminiscent of the structure seen for [Fe₂(S₂C_nH_2n)(CO)₃(PMe₃)₂(NO)]⁺.¹³

Figure 1.

Side-on (left) and end-on (right) views of [2(CO)₃]BF₄. Thermal ellipsoids are shown at 35% probability and are omitted on the phenyl rings for clarity, as are the anion (one phenyl group is perpendicular to the plane of the page). Selected bond distances (Å): Fe(1)–Fe(2), 2.5931(7); Fe(1)-P(1), 2.2791(10); Fe(1)-P(2), 2.3128(10); Fe(1)-N(1), 1.659(2); Fe(2)-C(4), 1.806(3); Fe(2)-C(5), 1.806(3); Fe(2)-C(6), 1.785(3).

In addition to the ν_NO band at 1796 cm^-1, the IR spectrum of [1(CO)₃]BF₄ features two ν_CO bands for the Fe(CO)₃ subunit. Relative to Fe₂(S₂C₂H₄)(CO)₄(dppv), these bands are shifted to higher energy by about 50 cm^-1. A similar trend is observed for the propanedithiolato complexes.

The ³¹P NMR spectrum of [1(CO)₃]BF₄ shows two singlets in a 1:2 ratio, consistent with two isomers that differ with respect to the position of the dppv chelate.⁴ The smaller signal is assigned to a highly fluxional apical-basal isomer; this signal was found to broaden upon cooling the sample to -80 °C. A singlet assigned to the major isomer proved temperature-invariant, consistent with the dibasal stereochemistry. A similar trend is seen for [2(CO)₃]BF₄, except that the equilibrium concentration of the apical-basal isomer is barely detectable (Scheme 2, Table 1). The related Fe₂(S₂C₂H₄)(CO)₄(dppv) exhibits similar dynamics, although the apical-basal isomer is far more stabilized (20:1). In the -80 °C ³¹P NMR spectrum of [2(CO)₃]⁺, four isomers are observed indicative of slowed folding of the propanedithiolate.

Scheme 2.

Isomerization Processes for the Fe(dppv)(NO)⁺ Subunit.

Table 1.

Ratios of Apical-Basal/Dibasal Isomers (dppv location) for [Fe₂(S₂C_nH_2n)(CO)_4-x(dppv)(NO)_x]^x+.

(S2C2H4)(CO)4	(S2C2H4)(CO)3(NO)	(S2C3H6)(CO)4	(S2C3H6)(CO)3(NO)
20:1	1:2	7:1	1:>25 (est.)

Substitution of [Fe₂(S₂C_nH_2n)(CO)₃(dppv)(NO)]BF₄ by PMe₃

The salt [1(CO)₃]BF₄ proved highly reactive toward nucleophiles. Thus, treatment of [1(CO)₃]BF₄ with 1 equiv of PMe₃ at room temperature gave [1(CO)₂(PMe₃)]BF₄. The ³¹P NMR spectrum of [1(CO)₂(PMe₃)]BF₄ displayed singlets in both the dppv and PMe₃ regionsm, which implicate axial PMe₃ and dibasal dppv. The dibasal disposition of the dppv ligand is further evidenced by the ³¹P NMR chemical shift (δ74.1), following a pattern seen for related complexes.⁴

In a test of its relative electrophilicity, equimolar amounts of Fe₂(S₂C₂H₄)(CO)₄(dppv) and [1(CO)₃]BF₄ were treated with one equiv of PMe₃ at 20 °C. IR and ³¹P NMR measurements revealed exclusive consumption of [1(CO)₃]BF₄. In an attempt to elucidate some details of the substitution mechanism, a solution of [1(CO)₃]BF₄ was treated with one equiv of PMe₃ at -20 °C; we observed complete disappearance of free PMe₃, partial consumption of [1(CO)₃]⁺, and the appearance of an intermediate species (ν_NO = 1727 cm^-1). This reaction appears to rapidly give the double adduct [1(CO)₃(PMe₃)₂]⁺, followed by a slower dissociation of PMe₃ that reacts with remaining starting materials (eqs 1-2).

$${[{\text{Fe}}_2({\text{S}}_2{\text{C}}_n{\text{H}}_{2n}){(\text{CO})}_3(\text{dppv})(\text{NO})]}^{+}+{2\text{PMe}}_3\to {[{\text{Fe}}_2({\text{S}}_2{\text{C}}_n{\text{H}}_{2n}){(\text{CO})}_3(\text{dppv}){({\text{PMe}}_3)}_2(\text{NO})]}^{+}$$ (1)

$${[{\text{Fe}}_2({\text{S}}_2{\text{C}}_n{\text{H}}_{2n}){(\text{CO})}_3(\text{dppv}){({\text{PMe}}_3)}_2(\text{NO})]}^{+}\to [{\text{Fe}}_2({\text{S}}_2{\text{C}}_n{\text{H}}_{2n}){(\text{CO})}_2(\text{dppv})({\text{PMe}}_3)(\text{NO})]{\text{BF}}_4+{\text{PMe}}_3+\text{CO}$$ (2)

Upon warming to room temperature, the mixture of [1(CO)₃]⁺ and the intermediate converted with good efficiency to [1(CO)₂(PMe₃)]⁺. When a solution of [1(CO)₃]⁺ was treated with excess PMe₃ at -20 °C, ESI-MS analysis of the reaction mixture confirmed the presence of a species corresponding to [1(CO)₃(PMe₃)₂]⁺. The related reaction of [2(CO)₃]⁺ with PMe₃ gave similar results. The ³¹P NMR spectrum of the intermediate in the PMe₃ region (∼δ25) revealed one major and two minor isomers, in each of which the PMe₃ groups are nonequivalent, as virtually required since only low symmetry products could be produced.

Solutions of [2(CO)₃(PMe₃)₂]BF₄ readily produced crystals suitable for analysis by X-ray diffraction. The structure of [2(CO)₃(PMe₃)₂]⁺ features both octahedral and trigonal bipyramidal iron centers, which are assigned as Fe(II) and Fe(0), respectively. The octahedral site features cis thiolates, cis phosphines, and cis carbonyl ligands. Viewing the pentacoordinate site as a trigonal bipyramid, the axial sites are occupied by the acceptor ligands, CO and NO⁺ (Figure 2). The crystallographic result shows that two molecules of PMe₃ added to the Fe(CO)₃ site, inducing migration of CO and scission of Fe-Fe and one Fe-S bonds.

Figure 2.

Structure of the cation in [2(CO)₃(PMe₃)₂]BF₄. Thermal ellipsoids are shown at 35% probability and are omitted on the phenyl rings for clarity, as are the anion. Selected bond distances (Å): Fe(1)-N(1), 1.618(9); Fe(1)-C(1), 1.860(11); Fe(1)-P(1), 2.231(3); Fe(1)-P(2), 2.270(3); Fe(1)-S(1), 2.323(3); Fe(2)-S(1), 2.376(2); Fe(2)-S(2), 2.306(3); Fe(2)-C(2), 1.749(11); Fe(2)-C(3), 1.741(11); Fe(2)-P(3), 2.312(3); Fe(2)-P(4), 2.270(3); Fe(1)-Fe(2), 3.969.

Cyano-nitrosyl Complexes

Treatment of [1(CO)₃]BF₄ with two equiv of Et₄NCN gave the dicyanide, Et₄N[1(CN)₂(CO)]. Shown in Figure 3, variable temperature ³¹P NMR measurements indicate that Et₄N[1(CN)₂(CO)] exists exclusively as a single diastereomer in solution wherein the dppv is apical-basal. We propose that this reaction proceeds via relocation of the nitrosyl to the Fe(CN)₂ site. The IR spectrum of Et₄N[1(CN)₂(CO)] resembles that for [1(CO)(dppv)]BF₄ in the ν_CO region, but ν_NO occurs at 41 cm^-1 lower energy. The switch of the dppv to apical-basal geometry is also consistent with a Fe(dppv)CO site; in Fe(dppv)NO sites, the dppv is invaribly dibasal in the absence of extreme steric effects.

Figure 3.

202 MHz ³¹P NMR spectra of Et₄N[1(CN)₂(CO)] (CD₂Cl₂ soln) at 20 (top) and -60 °C (bottom).

One equiv of [1(CO)₃]BF₄ was found to rapidly consume 2 equivs of Et₄NCN. This behavior is analogous to the reaction of PMe₃ and [1(CO)₃]BF₄ (see above). When examined by IR spectroscopy at -45 °C, the dicyanation of [1(CO)₃]BF₄ generated complex mixture characterized by several bands in the ν_CO and ν_CN region, although the region 1800 – 1600 cm^-1 was blank.¹³

Despite their high electrophilicity, neither [1(CO)₃]BF₄ nor [2(CO)₃]BF₄, were observed to form adducts with CO, even at -80 °C. This behavior contrasts with the monodentate bis(phosphine) nitrosyl complexes, [Fe₂(S₂C_nH_2n)(CO)₃(PMe₃)₂(NO)]BF₄ (n = 2, 3), which reversibly bind CO. Thus, it is not surprising that [1(CO)₃]BF₄ and [2(CO)₃]BF₄ react with excess PMe₃ only near -20 °C, whereas at comparable reactant concentrations, [Fe₂(S₂C_nH_2n)(CO)₃(PMe₃)₂(NO)]BF₄ react immediately with PMe₃ at -80 °C.

NO as a Trapping Agent for Mixed-Valence Species: [Fe₂(S₂C₂H₄)(CO)₂(dppv)₂(NO)]⁺ and Related Species

Treatment of [1(CO)₃]BF₄ with one equiv of dppv slowly and inefficiently afforded the bis(diphosphine), [1(CO)(dppv)]BF₄. Alternatively, [1(CO)(dppv)]BF₄ also arises in the reaction of the dicarbonyl¹⁴ Fe₂(S₂C₂H₄)(CO)₂(dppv)₂ with NOBF₄. This route suffered from the tendency of the mononitrosyl to further react with NOBF₄ to irreversibly yield, inter alia, Fe(dppv)(NO)₂. A potentially versatile route to diiron nitrosyl complexes entails a two-step process that involves one-electron oxidation of diiron(I) precursors followed by treatment with NO. Oxidation of Fe₂(S₂C₂H₄)(CO)₂(dppv)₂¹⁴ and trapping with NO gave the corresponding mononitrosyl [Fe₂(S₂C₂H₄)(NO)(CO)(dppv)₂]BF₄ ([1(dppv)(CO)]BF₄). The ³¹P NMR spectrum of [1(dppv)(CO)]BF₄ showed four broad singlets at 25 °C, which sharpened upon cooling the sample to -80 °C. We conclude that the two dppv ligands span apical-basal coordination sites, as observed crystallographically in the solid state (Figure 4), but that the dynamic racemization process is subject to a low activation barrier. The precursor complex also exhibits a related dynamic process, but at a higher barrier.¹⁴ The complex [1(dppv)(CO)]BF₄ is the only diiron ditholate nitrosyl observed in this series that features a basal nitrosyl ligand present in the major observed isomer. ³¹P NMR studies at low temperatures showed that [1(CO)(dppv)]BF₄ binds CO reversibly (see supporting info).

Figure 4.

Structure of [1(CO)(dppv)]. Thermal ellipsoids are shown at 35% probability and are omitted on the phenyl rings for clarity. Selected bond distances (Å): Fe(1)–Fe(2), 2.5528(13); Fe(2)-P(1), 2.253(2); Fe(2)-P(2), 2.237(2); Fe(2)-N(1), 1.695(6); Fe(1)-C(3), 1.719(7); Fe(1)-P(3), 2.2026(19); Fe(1)-P(4), 2.247(2).

Oxidation of Fe₂(S₂C₃H₆)(CO)₄(dppv) with one equiv FcBF₄ followed by the addition of about 1 equiv of NO gave [2(CO)₃]BF₄. We also prepared [1(CO)₂(PMe₃)]BF₄ via the mixed valence species [Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)]BF₄.⁴

Nitrosyl Migations: Fe₂(S₂C₂H₄)(CN)(CO)(dppv)(PMe₃)(NO)

Whereas Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃) is inert towards substitution by cyanide, its nitrosylated derivative [1(CO)₂(PMe₃)]BF₄ was found to react with one equiv of cyanide to produce 1(CN)(CO)(PMe₃). The ³¹P NMR spectrum of 1(CN)(CO)(PMe₃) consisted of two doublets in the dppv region, indicative of a single diastereomer. Apparently, rapid turnstile rotation^4,14 equilibrates the diastereomeric rotamers. The structure of this species was confirmed crystallographically; the compound is chiral and crystallizes as the racemate (Figure 5). Carbonyl, cyanide, and nitrosyl ligands were distinguished by their bond lengths; the crystallographic refinement also clearly favored one set of assignments. The dppv spans the apical-basal sites, as is typical for Fe(CO)(dppv) centers.⁴ Cyanide and PMe₃ are basal and NO is apical, as is typical in this series of compounds. Distinctively, the nitrosyl is no longer bound to the same Fe as the dppv, a finding that implicates the intramolecular migration of NO (eq 3).

Figure 5.

Structure of 1(CN)(CO)(PMe₃). Thermal ellipsoids are shown at 35% probability and are omitted on the phenyl rings for clarity. Distances (Å): Fe(1)–Fe(2), 2.275(4); Fe(1)-N(2), 1.631(12); Fe(1)-P(3), 2.275(4); Fe(1)-C(4), 1.926(15); Fe(2)-P(1), 2.183(4); Fe(2)-P(2), 2.208(4); Fe(2)-C(3), 1.713(14).

$${[{(\text{CO})}_2({\text{PMe}}_3)\text{Fe}({\text{S}}_2{\text{C}}_2{\text{H}}_4)\text{Fe}(\text{dppv})(\text{NO})]}^{+}+{\text{CN}}^{-}\to (\text{NO})(\text{CN})({\text{PMe}}_3)\text{Fe}({\text{S}}_2{\text{C}}_2{\text{H}}_4)\text{Fe}(\text{dppv})(\text{CO})+\text{CO}$$ (3)

A similar NO migration reaction had been implicated for the cyanation of [Fe₂(S₂C₃H₆)(CO)₄(PMe₃)(NO)]BF₄, which gives Fe₂(S₂C₃H₆)(CN)(CO)₃(PMe₃)(NO).¹³

Electrochemistry of Nitrosyl Derivatives

Cyclic voltammetric studies showed that replacement of CO by NO⁺ shifts the first reduction and first oxidation events by ca. 1 V anodically, an effect that is akin to that of protonation. Furthermore, the observed redox events are typically reversible: the species, [1(CO)₃]⁺ reduces quasi-reversibly at -630 and -1.090 mV versus Ag/AgCl. The bulky complex [1(CO)(dppv)]⁺ can be reversibly oxidized and reduced (see Table 3).

Table 3.

Redox Properties for Selected Nitrosyl Complexes. Potentials are Referenced Ag/AgCl. Voltammograms Recorded in CH₂Cl₂ Solution with 50 mM Et₄NPF₆, at a 100 mV/s.

Compound	Oxidation (V)	Reduction (V)
Fe2(S2C2H4)(CO)4(dppv)	-0.060	-2.070a
[1(CO)3]+	> 1	-1.150c, -1.610c
[2(CO)3]+	> 1	-1.210c, -1.730c
Fe2(S2C2H4)(CO)3(dppv)(PMe3)4	-0.300b
[1(CO)2(PMe3)]+	0.850	-1.51b, -1.99a
Fe2(S2C2H4)(CO)2(dppv)2	-0.695b
[1(CO)(dppv)]+	0.22b	-1.62b

^aIrreversible

^bReversible

^cReversible at scan rates < 100 mV/s.

Given their mild reduction potentials, we assayed the redox properties of the diiron nitrosyl complexes in the presence of acids. The simple dppv complexes [1(CO)₃]BF₄ and [2(CO)₃]BF₄ exhibited catalytic waves at their primary reductions, both of which were mild (-1.15 and -1.21 V, respectively, versus the Fc^0/+ couple see Fig. 7).

Figure 7.

Cyclic voltammograms of complex 1 mM [2(CO)₃]BF₄ in CH₂Cl₂ (∼50 mM Bu₄NPF₆) solution as a function of [CF₃CO₂H] (0, 1, 2, 3, 5, 7, and 10 expressed as molar equiv). Scan rate 100 mV s^-1 at a glassy carbon electrode 0.3 cm in diam.

At the concentration ratio of [CF₃CO₂H]/([2(CO)₃]⁺) ≤ 20, no catalysis was observed at the potential of the second reduction step. This behavior is explicable if the acid in the diffusion layer is consumed during the first reduction wave. A rough estimate of the catalytic efficiency of the FeNO derivatives can be obtained from the variation of the catalytic peak current with the acid concentration. As shown in Figure 8, the peak current varies linearly with the square root of the acid concentration, which indicates that the rate determining step in the catalytic reaction is first order in acid (see supplementary material).¹⁵ We estimated (see experimental) an overall catalytic rate constant k′ ∼ 130 and 100 M^-1 s^-1 for [1(CO)₃]⁺ and [2(CO)₃]⁺, respectively. These values compare well with those calculated by the same procedure for cobaloxime catalysts, which are known to be highly efficient.¹⁶ Note however that the overall rate constant k′ does not reflect precisely the rate determining step but is a composite of equilibrium and rate constants.

Figure 8.

Plots of the catalytic current as a function of the acid concentration for [1(CO)₃]⁺ (squares) and [2(CO)₃]⁺ (triangles): i_k and i_p are the peak current in the presence and in the absence of acid, respectively.

At high [H⁺], the catalytic current becomes almost independent of the acid concentration, indicative of a change in the rate determining step, which could be the release of H₂, as has been proposed previously.¹⁶ Catalysis by [1(CO)₃]⁺ and [2(CO)₃]⁺ is unaffected by the presence of CO, although the corresponding hexacarbonyl catalysts are poisoned by CO.¹⁷

Compounds [1(CO)₃]⁺ and [2(CO)₃]⁺ are not protonated even in the presence of excess triflic acid, consistent with proton reduction catalysis that is initiated by electron-transfer (E step) followed by protonation (C step). The sequence of the two subsequent steps, EC or CE, is unknown. Consistent with the EC mechanism, [2(CO)₃]BF₄ was chemically reduced with ∼1.1 equiv of Cp₂Co at -78 °C to generate the thermally unstable species 2(CO)₃. Following the addition of CF₃CO₂H, we observed regeneration of [2(CO)₃]BF₄.

Discussion

Nitrosylation provides an easy means to significantly modify the electronic environment of the diiron dithiolato carbonyls without altering its formal oxidation state. Replacement of CO by NO⁺ causes ν_CO to increase by 50 cm^-1 (Table 1). Such shifts in ν_CO are about half the effect induced of protonation of related diiron complexes.¹⁸ The nitrosyl derivatives exhibit enhanced tendency to undergo substitution reactions as well as to serve as proton reduction catalysts at potentials approaching the thermodynamic limit.¹⁹

As shown in this and the previous report, NO⁺ induces novel stereochemistry on the diiron site. In contrast to all other ligands evaluated on diiron(I) dithiolates, NO⁺ displays a high preference for the apical site. The complexes characterized in this work all exhibit apical NO⁺ ligands, except for the (dppv)₂ derivatives where steric factors destabilize the isomer containing with both dibasal and apical-basal diphosphines. Even this complex is less stereochemically rigid than the analogous carbonyl derivative.

The structure and reactivity of the complexes [Fe₂(S₂C₃H₆)(CO)₃(PR₃)₂(NO)]⁺ strongly depends on whether the two phosphines are dppv or PMe₃. The complex [Fe₂(S₂C₃H₆)(CO)₃(PMe₃)₂(NO)]⁺ adopts a strongly distorted “rotated” structure,¹³ whereas [1(CO)₃]⁺ adopts the normal distorted-C_2v motif. We suggest that the electronic asymmetry of the (PMe₃)₂ derivatives, which feature Fe(CO)₂(PMe₃) and Fe(NO)(CO)(PMe₃)⁺ sites, is greater than for the dppv derivatives described in this work.

The new nitrosyl diiron(I) complexes were prepared by three methods:

1. Direct electrophilic nitrosylation by attack of NO⁺. This method is effective but limited by the range of substituted diiron dithiolates, since the diiron center must contain some donor ligands; the hexacarbonyls do not form isolable derivatives upon reaction with NOBF₄. We propose that the electrophilic nitrosylation proceeds similarly to the protonation of diiron complexes, i.e. by attack of the electrophile at a terminal metal site.⁹ We have shown that related 36e diiron nitrosyl complexes are prone to decarbonylation.¹³

2. Nitrosylation by attack of NO on mixed valence diiron complexes. This method is complementary to the previous one. It has been shown that NO⁺ reacts with some metal carbonyls via inner sphere electron-transfer followed by dissociation of NO.²⁰ Many of the oxidized diiron carbonyl dithiolates studied by us previously react with NO.

3. Substitution of diiron nitrosyl complexes. The considerable electrophilicity of [1(CO)₃]BF₄ is indicated both by the rate and the stoichiometry of its substitution reactions. The presence of the nitrosyl allows the preparation of highly substituted diiron(I) complexes, which would be difficult to prepare without NO⁺. These include Et₄N[Fe₂(S₂C₂H₄)(CN)₂(CO)(dppv)(NO)], [Fe₂(S₂C₂H₄)(CO)(dppv)(PMe₃)₂(NO)]⁺, and the chiral-at-metal derivative Fe₂(S₂C₂H₄)(CN)(PMe₃)(CO)(dppv)(NO). The facility of the intermetallic migration of NO⁺ limits the range of isolable substituted products. Also impressive is the rate of substitutions: cyanation of Fe₂(S₂C₃H₆)(CO)₆ at 25 °C and [2(CO)₃]BF₄ at −78 °C were found to proceed at comparable rates. This temperature difference roughly corresponds to a 30% decrease in activation energy.

One striking finding is the tendency of [Fe₂(S₂C_nH_2n)(CO)₃(dppv)(NO)]⁺ to undergo substitution via the intermediacy of a 2:1 adduct, as illustrated by [(PMe₃)₂(CO)₂Fe(S₂C_nH_2n)Fe(CO)(dppv)(NO)]⁺ (Scheme 3). These adducts form via the scission of the Fe-Fe bond and one Fe-S bond. The 2:1 adducts illustrate the possibility that other [Fe(I)]₂ species substitute via mixed valence adducts. Such a mechanism may be relevant to the finding that attempted monocyanation of Fe₂(S₂C₃H₆)(CO)₆ mainly affords the dicyanide.²¹ The presence of the NO ligand, a very strong acceptor, favors such electron-transfer processes. Related Fe-S scission reactions occur upon the reduction of diiron dithiolates.^17,22

Scheme 3.

Pathway Proposed for Substitution of [2(CO)₃]⁺ by PMe₃.

Experimental Section

Methods have been recently described.²³ NOBF₄ was purified by rapid sublimation at 220 °C at ∼ 0.01 mm Hg and was stored in a dry glove box at -30 °C. Since NOBF₄ is poorly soluble, the solid was ground to a fine powder prior to use in sensitive reactions.

Electrochemistry

Cyclic voltammetry experiments were conducted in a ∼10-mL one-compartment glass cell, with a Pt wire (counter electrode) and a glassy carbon working electrode. All voltammagrams were referenced vs. an Ag/AgCl reference electrode (50 mM KCl).

[Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]BF₄, [1(CO)₃]BF₄

A slurry of 0.141 g (1.21 mmol) of pulverized NOBF₄ in 20 mL CH₂Cl₂ was treated with a solution of 0.865 g (1.21 mmol) of Fe₂(S₂C₂H₄)(CO)₄(dppv) in 100 mL of CH₂Cl₂. The reaction mixture was immediately cooled to 0 °C and after 10 h, the deep red reaction mixture was concentrated in vacuo. Addition of 50 mL of hexanes to the concentrated solution precipitated the dark red colored product. Yield: 0.912 g (94%). 500 MHz ¹H NMR (CD₂Cl₂): δ 8.6 – 7.2 (m, 20H, C₆H₅), 3.0 (dd, 2H, PCH), 2.05 (m, 1H, SCH), 1.6 (m, 1H, SCH), 1.3 (m, 1H, SCH), 0.8 (m, 1H, SCH). 202 MHz ³¹P NMR (CD₂Cl₂, 20 °C): δ 77.1 (s, dppv), 72.0 (s, dppv). ³¹P NMR (CD₂Cl₂, -80 °C): δ 78.5 (s, dppv), 73.3 (s, dppv). IR (CH₂Cl₂): ν_CO = 2070, 2006; ν_NO = 1796 cm^-1. ESI-MS: m/z = 714.1 ([Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]⁺). Anal. Calcd (Found) for C₃₁H₂₆BF₄Fe₂NO₄P₂S₂: C, 46.86 (46.86); H, 3.25 (3.27); N, 1.60 (1.75).

[Fe₂(S₂C₃H₆)(CO)₃(dppv)(NO)]BF₄, [2(CO)₃]BF₄

This compound was prepared following the method described for [1(CO)₃]BF₄. Yield: 2.0 g (88%). 500 MHz ¹H NMR (CD₂Cl₂, 20 °C): δ 8.5 – 8.2 (m, 20H, C₆H₅), 2.9 (bs, 2H, PCH), 2.6 (m, 4H, (SCH₂)₂CH₂), 2.0 (m, 2H, (SCH₂)₂CH₂). 202 MHz ³¹P NMR (CD₂Cl₂, 20 °C): δ 69.7 (s, dppv). ³¹P NMR (CD₂Cl₂, -80 °C): δ 76.7 (s, dppv), 75.1 (s, dppv), 72.4 (s, dppv), 69.5 (s, dppv). IR (CH₂Cl₂): ν_CO = 2069, 2005; ν_NO = 1788 cm^-1. ESI-MS: m/z = 728.1 ([Fe₂(S₂C₃H₆)(CO)₃(dppv)(NO)]⁺). Anal. Calcd (Found) for C₃₁H₂₆BF₄Fe₂NO₄P₂S₂: C, 47.15 (46.56); H, 3.46 (3.41); N, 1.72 (1.86).

[Fe₂(S₂C₂H₄)(CO)₂(NO)(PMe₃)(dppv)]BF₄, [1(CO)₂(PMe₃)]BF₄

To a mixture of 0.200 g (0.263 mmol) of Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)⁴ and 0.030 g (0.263 mmol) of finely pulverized NOBF₄ was added 15 mL of CH₂Cl₂. After stirring for 5 min., the solution was concentrated to 5 mL, and the dark red product was precipitated upon addition of 30 mL of hexanes. Crystals were grown via slow diffusion of hexanes into a CH₂Cl₂ solution of the complex. Yield: 0.19 g (86%). ³¹P NMR (CD₂Cl₂, 20 °C): δ 74.1 (s, dppv), 25.0 (s, PMe₃). ³¹P NMR (CD₂Cl₂, -70 °C): δ 78.1 (s, dppv), 75.7 (s, dppv), 26.8 (s, PMe₃), 23.0 (s, PMe₃). IR (CH₂Cl₂): ν_CO = 2002, 1958, ν_NO = 1775. In situ spectra (ReactIR™ 4000, Mettler-Toledo) indicated the presence of [Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)(NO)]BF₄ after 3 h of vigorous stirring at -78 °C: (CH₂Cl₂): ν_CO = 2037, 1992, 1957 ν_NO = 1775. ESI-MS: m/z = 762.2 ([Fe₂(S₂C₂H₄)(CO)(dppv)(PMe₃)(NO)]⁺). Anal. Calcd (Found) for C₃₃H₃₅BF₄Fe₂NO₃P₃S₂: C, 46.67 (46.66); H, 4.15 (4.37); N, 1.65 (1.62).

Synthesis via Oxidation and Trapping with NO

To a solution of 0.196 g (0.258 mmol) of Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)⁴ in 15 mL of CH₂Cl₂, cooled to -45 °C, was added 0.070 g (0.256 mmol) of FcBF₄. The IR spectrum of the purple solution displayed signals matching [Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)]BF₄.⁴ The reaction vessel was sealed and to the cooled solution was injected 6 mL (0.268 mmol) of NO gas. After 1 h, the IR spectrum of the resulting deep red solution matched that for [1(CO)₂(PMe₃)]BF₄. The product precipitated upon addition of 60 mL of hexanes. Yield: 0.116 g (53%). Analogous procedures were followed for the sequential oxidation and trapping of Fe₂(S₂C₃H₆)(CO)₄(dppv) to give [Fe₂(S₂C₃H₆)(CO)₃(dppv)(NO)]BF₄: To a solution of 0.045 g (0.062 mmol) of Fe₂S₂C₃H₆ in 5 mL of CH₂Cl₂, cooled to -45 °C, was added a solution of 0.017 g (0.062 mmol) of FcBF₄ in 5 mL of CH₂Cl₂. To the resultant reaction mixture was added 3.2 mL of NO (0.124 mmol), and the reaction vessel was sealed. After 20 min, the IR spectrum matched that of [2(CO)₃]BF₄, and the product was precipitated upon addition of 50 mL of hexanes. Yield: 77%.

[Fe₂(S₂C₂H₄)(CO)(dppv)₂(NO)]BF₄, [1(CO)(dppv)]BF₄

To a solution of 0.150 g (0.142 mmol) of Fe₂(S₂C₂H₄)(CO)₂(dppv)₂ in 20 mL of CH₂Cl₂, cooled to -45 °C, was added 0.039 g (0.142 mmol) of FcBF₄. An immediate IR spectrum of the dark brown solution displayed signals attributed to [Fe₂(S₂C₂H₄)(CO)(μ-CO)(dppv)₂]BF₄: ν_CO = 1959 (s), 1887 (w, br) cm^-1.23 The reaction vessel was sealed and to the cooled solution was injected 3.6 mL (0.161 mmol) of NO gas. After 1 h, the resultant dark brown solution displayed an IR spectrum corresponding to [1(CO)(dppv)]BF₄. The solution was warmed to room temperature and concentrated in vacuo to ∼ 5 mL. The product precipitated upon addition of 30 mL of Et₂O. Impurities observed in the ³¹P NMR spectrum can be removed after several recrystallizations from CH₂Cl₂-Et₂O. Yield: 0.080 g (49%). ¹H NMR (CD₂Cl₂): δ 8.2 – 6.8 (m, 40H, dppv), 1.7 – 0.8 (m, 4 H, SCH₂CH₂S). ³¹P NMR (CD₂Cl₂, 20 °C): δ 101.7 (br s, dppv), 87.8 (broad s, dppv), 81.2 (br s, dppv), 74.9 (br s, dppv). IR (CH₂Cl₂): ν_CO = 1928, ν_NO = 1760 cm^-1. ESI-MS: m/z = 1054.2 ([Fe₂(S₂C₂H₄)(CO)(dppv)₂(NO)]⁺). Acceptable CHN analyses were not be obtainable. Anal. Calcd (Found) for C₅₅H₄₈BF₄Fe₂NO₂P₄S₂: C, 57.87 (56.00); H, 4.24 (4.00); N, 1.23 (1.31). Excess of NOBF₄ gave Fe(dppv)(NO)₂:²⁴ ν_NO = 1718 and 1666 cm^-1; ESI-MS: m/z = 512.

Alternative routes were examined: addition of dppv to 1(CO)₃, and treatment of Fe₂(S₂C₂H₄)(CO)₂(dppv)₂ with NOBF₄. The raw product contained the targeted complex as well as unidentified impurities as indicated by the ³¹P NMR and IR spectra. Treatment of [Fe₂(S₂C₂H₄)(CO)₂(dppv)₂(NO)]BF₄ with NOBF₄ produced Fe(NO)₂(dppv).²⁴

[Fe₂(S₂C₂H₄)(CO)₃(dppv)(PMe₃)₂(NO)]BF₄, [1(CO)₃(PMe₃)₂]BF₄

Onto a frozen solution of 0.206 g (0.26 mmol) of [1(CO)₃]BF₄ in 10 mL of CH₂Cl₂ was distilled 1 mL of PMe₃. The mixture was allowed to thaw at -78 °C, warmed in an ice water bath for 1 min, followed by cooling again to -78 °C. The IR spectrum of the reaction mixture indicated [1(CO)₃(PMe₃)₂]BF₄. Addition of 50 mL of hexanes to the reaction mixture yielded a dark brown oil that was dried in vacuo. IR spectra of the redissolved material contained showed traces of [1(CO)₂(PMe₃)]BF₄, indicative of the thermal instability of [1(CO)₃(PMe₃)₂]BF₄. ³¹P NMR (CD₂Cl₂, 0 °C), A, B, and C correspond to three isomers: δ 98.4 (d, J_P-P = 48.3, dppv A), 98.0 (d, J_P-P = 44.3, dppv B), 97.3 (d, J_P-P = 39.7, dppv C), 81.5 (d, J_P-P = 48.9, dppv A), 81.3 (d, J_P-P = 45.8, dppv B), 78.8 (d, J_P-P = 38.4, dppv C), 25.3, 17.8 (d, J = 64.1, PMe₃ C), 17.0 (d, J = 62.5, PMe₃ A), 10.0 (AB_q, J = 292, J_AB = 245, PMe₃ B), 6.4 (d, J = 64.2, PMe₃ C), 5.4 (d, J = 65.6, PMe₃ A).

Crystallization of [Fe₂(S₂C₃H₆)(CO)₃(dppv)(PMe₃)₂(NO)]BF₄, [2(CO)₃(PMe₃)₂]BF₄

Approximately 0.2 mL of PMe₃ was distilled onto a frozen solution of 0.070 g of [1(CO)₃]BF₄ in 7 mL of CH₂Cl₂. Aliquots briefly warmed (<2 min.) to room temperature displayed an IR spectrum that indicated the presence of [2(CO)₃(PMe₃)₂(NO)]BF₄. IR (CH₂Cl₂): ν_CO = 2021, 1971, 1951. ν_NO = 1721 cm^-1. NMR (CD₂Cl₂, -20 °C): A, B, and C correspond to three isomers: δ 97.4 (d, J_P-P = 34.2, dppv A), 95.0 (d, J_P-P = 28.6, dppv B), 78.2 (d, J_P-P = 38.2, dppv A), 76.9 (d, J_P-P = 29.0, dppv B), 15.9 (d, J_P-P = 71.6, PMe₃ A), 15.2 (d, J_P-P = 72.4, PMe₃ C), 9.0 (AB_q, J = 964.5, J_AB = 193, PMe₃ B), 4.1 (d, J_P-P, PMe₃ A), 2.8 (d, J_P-P = 71.9, PMe₃ C). The dppv signals for isomer C are not reported because of overlap with isomer A's signals). The reaction mixture was thawed to -45 °C followed by transfer via cannula to a Schlenk tube cooled to -78 °C. This solution was layered with 50 mL of a 1/1 mixture of Et₂O and hexane and stored at -30 °C. After 1 week, red rhombs were visible.

Reaction of [1(CO)₃] with 1 equiv PMe₃

To a J. Young NMR tube was added a solution of 0.025 g (0.03 mmol) of [1(CO)₃]BF₄ in 1 mL of CD₂Cl₂. The solution was frozen in liquid nitrogen, and to it was added 0.3 mL of a 0.13 M solution of PMe₃ in CH₂Cl₂. The tube was immediately capped, immersed in liquid nitrogen, and evacuated. The contents were allowed to warm and were monitored by NMR spectroscopy at various temperatures. At -38 °C, no reaction was observed. Upon warming to 0 °C, partial consumption of [1(CO)₃(dppv)(NO)]BF₄ and nearly complete consumption of PMe₃ was accompanied by growth of several peaks. These peaks were assigned to one major (“B”) and two minor (“A” and “C”) intermediates (see above spectra assignments). Upon warming to room temperature overnight, conversion of the remaining intermediate(s) to [1(CO)₂(PMe₃)]BF₄ was observed.

NEt₄[Fe₂(S₂C₂H₄)(CN)₂(CO)(dppv)(NO)]

A solution of 0.503 g (0.63mmol) of [1(CO)3]BF4 in 30 mL of MeCN was cooled to -45 °C followed by treatment with a solution of 0.199 g (1.3 mmol) of NEt₄CN in 30 mL of MeCN, also cooled to -45 °C. The solution immediately darkened, and after 3 h, the dark brown solution was allowed to warm to room temperature followed by stirring overnight. After the solvent was removed in vacuo, the residue was extracted into 5 mL of CH₂Cl₂ and the product was precipitated by the addition of 30 mL of Et₂O. IR (CH₂Cl₂): ν_CN = 2100, ν_CO = 1923, ν_NO = 1719. ³¹P NMR (CD₂Cl₂, 20 °C): δ 96.1 (s, dppv). ³¹P NMR (CD₂Cl₂, -60 °C): δ 99.3 (d, J_P-P = 22.4, dppv), 94.1 (d, J_P-P = 22.4, dppv). MS ESI: m/z = 710.0 ([Fe₂(S₂C₂H₄)(CN)₂(CO)(dppv)(NO)]⁺), 682.0 ([Fe₂(S₂C₂H₄)(CN)₂(dppv)(NO)]⁺). Suitable CHN analyses were not be obstained.

[Fe₂(S₂C₂H₄)(CO)(CN)(dppv)(PMe₃)(NO)]BF₄, [1(CO)(CN)(PMe₃)]BF₄

A solution of 0.221 g (0.26 mmol) of [1(CO)₂(PMe₃)]BF₄ in 30 mL of CH₂Cl₂ was treated with a solution of 0.041 g (0.26 mmol) of NEt₄CN in 10 mL MeCN. After 60 min., the dark brown-colored reaction mixture was evaporated in vacuo, and the residue was extracted into 30 mL of toluene. The dark red-colored extract was concentrated followed by dilution with 30 mL of hexanes to precipitate the product. Yield: 0.065 g (33%). ¹H NMR (d⁸-toluene): δ 8.5 – 6.7 (m, C₆H₅ and P₂C₂H₂), 2.1 – 1.6 (m, S₂C₂H₄), 1.26 (d, J_P-H = 9.9, 9H, PCH₃). ³¹P NMR (CD₂Cl₂, 20 °C): δ 95.3 (d, J_P-P = 20.9, dppv), 74.2 (d, J_P-P = 24.5, dppv), 7.9 (s, PMe₃). IR (CH₂Cl₂): ν_CN = 2096, ν_CO = 1910, ν_NO = 1734. FD-MS: m/z = 760 ([Fe₂(S₂C₂H₄)(CN)(CO)(dppv)(PMe₃)(NO)]⁺). Anal. Calcd (Found) for C₃₃H₃₅Fe₂N₂O₂P₃S₂: C, 52.54 (52.13); H, 4.72 (4.64); N, 3.53 (3.68).

Relative Electrophilicity of [Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]⁺

To a solution of 0.015 g (0.02 mmol) of [Fe₂(S₂C₂H₄)(CO)₃(dppv)(NO)]BF₄ and 0.014 g (0.02 mmol) of Fe₂(S₂C₂H₄)(CO)₄(dppv) in 5 mL of CH₂Cl₂ was added 0.5 mL of 0.19 M solution of PMe₃ in CH₂Cl₂. The reaction was monitored by IR spectroscopy over the course of 7 h during which time the absorption bands for [1(CO)₃(NO)]BF₄ decayed and those for [1(CO)₃(PMe₃)(NO)]BF₄ increased. The bands for Fe₂(S₂C₂H₄)(CO)₄(dppv) remained unchanged.

Semi-quantitation of Catalytic Proton Reduction

An electron transfer followed by a fast catalytic reaction can be formulated by the following equations:

where k′ is the rate constant for the reaction of reagent Z with the reduced species R to give product P. When the concentration of Z is large compared to the concentration of the catalyst and the potential sufficiently negative with respect to E_1/2, the catalytic current i_k is given by the following relation:¹⁵

$$i_k=\text{nFA}{C}_o\sqrt{k\text{'}D{C}_z}$$

where n is the number of electrons involved in the catalytic process, F the Faraday constant, A the area of the electrode, and D the diffusion coefficient for the catalyst. For graphs of i_k against C_z^1/2, the slope is nFAC_O(k′D)^1/2. The value for AC_OD^1/2 can be obtained from the voltammetric peak current, i_p, of the catalyst in the absence of acid:

$$i_p=2.69\times {10}^5{n}^{3/2}A{D}^{1/2}{C}_o{\nu }^{1/2}$$

where ν is the potential scan rate.

X-ray Crystallography

Crystals were mounted on a thin glass fiber using Paratone-N oil (Exxon). Data, collected at 198 K on a Siemens CCD diffractometer, were filtered to remove statistical outliers. The integration software (SAINT) was used to test for crystal decay as a bilinear function of X-ray exposure time and sin(Θ). The data were solved using SHELXTL by Direct Methods; atomic positions were deduced from an E map or by an unweighted difference Fourier synthesis. H atom U's were assigned as 1.2U_eq for adjacent C atoms. Non-H atoms were refined anisotropically. Successful convergence of the full-matrix least-squares refinement of F² was indicated by the maximum shift/error for the final cycle. The assignment of NO vs CO was tested by refining the cations with these ligands interchanged. Optimal refinements were found only for cases with the Fe(dppv)(NO) centers.

Figure 6.

Cyclic voltammogram of 1 mM [1(CO)(dppv)]BF₄ in CH₂Cl₂ solution (50 mM Bu₄NPF₆, 20 °C, 100 mV/s).

Table 2.

IR Data (cm^-1, CH₂Cl₂ soln) for Selected Compounds.

Compound	νCO	νNO
[1(CO)3]+	2070, 2006	1796
[1(CO)2(PMe3)]+	2002, 1958	1775
[1(CO)(dppv)]+	1928	1760
1(CO)(PMe3)(CN)	1910	1734
[1(CN)2(CO)]-	1923	1719

Table 4.

Details of Data Collection and Structure Refinement.

	[2(CO)3]BF4	[1(CO)2(PMe3)]BF4
chemical formula	C32H28BF4Fe2NO4P2S2	C35H39BCl4F4Fe2NO3P3S2
Formula weight	815.12	1019.01
T (K)	193(2)	193(2)
Space group	P 21/c	P 21/n
a (Å)	10.094(4)	15.2650(11)
b (Å)	17.701(7)	19.3588(13)
c (Å)	19.493(8)	15.5567(11)
α (deg)	90	90
β (deg)	97.814(6)	95.573(3)
γ (deg)	90	90
Z	4	4
V (Å3)	3451(2)	4575.5(6)
λ (Å)	0.71073	0.71073
Dcalc (g cm-3)	0.001569	0.001479
μ (cm-1)	0.01114	0.01114
R(Fo2)	0.0278	0.0406
Rw(Fo2)	0.0568	0.1086
	[1(CO)(dppv)]BF4	[1(CN)(CO)(PMe3)]BF4	[1(CO)3(PMe3)2]BF4
chemical formula	C58H54BCl6F4Fe2NO2P4S2	C33H35Fe2N2O2P3S2	C38H46BF4Fe2NO4P4S2
Formula weight	1396.23	760.36	967.27
T	193(2)	193(2)	193(2)
Space group	P21	P na21	P-1
a (Å)	10.8808(16)	19.629(5)	13.4269(19)
b (Å)	17.757(3)	11.016(3)	13.637(2)
c (Å)	16.692(2)	32.186(9)	14.435(2)
α (deg)	90	90	74.158(9)
β (deg)	104.511(3)	90	73.229(9)
γ (deg)	90	90	79.917(9)
Z	2	8	2
V (Å3)	3122.3(8)	6959(3)	2421.2(6)
λ (Å)	0.71073	0.71073	0.71073
Dcalc (g cm-3)	0.001485	0.001451	0.001327
μ (cm-1)	0.00945	0.01124	0.00868
R(I >2σ)a	0.0610	0.0886	0.0773
Rw(I >2σ)b	0.1256	0.1186	0.1628

^aR = Σ‖F_o| - |F_c|/Σ|F_o|.

^bR_w = {[w(|F_o| - |F_c|)²]/Σ [wF_o²]}^1/2, where w = 1/σ²(F_o).