RNC Exchange in Chiral

Apr 6, 2012 - reaction, producing (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 with a high stereoselectivity of 2:98 in favor of (SFe,RC)-...
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16- and 17-Electron Intermediates in the MeCN/RNC Exchange in Chiral-at-Metal [CpFe(Prophos)NCMe]X (X = I, PF6) Henri Brunner,*,† Takaki Kurosawa,‡ Manfred Muschiol,† Takashi Tsuno,*,‡ and Hayato Ike‡ †

Institut für Anorganische Chemie, Universität Regensburg, 93040 Regensburg, Germany Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Chiba 275-8575, Japan

‡

S Supporting Information *

ABSTRACT: The compounds (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6), configurationally labile at the metal center, were used in the MeCN/ligand exchange reactions with cyclohexyl isocyanide (CyNC) and tert-butyl isocyanide (tBuNC). Kinetic measurements showed that the MeCN/CyNC exchange in diastereomerically pure (SFe,RC)-[CpFe(Prophos)NCMe]X proceeded via the slow SN1-type dissociation of the Fe−NCMe bond, already observed in the MeCN/phosphite exchange reactions. The product (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]X (X = I, PF6) was formed in diastereomer ratios between 40:60 and 60:40. However, specific for the MeCN/CyNC exchange in (SFe,RC)-[CpFe(Prophos)NCMe]PF6, in some of the samples a fast initial reaction interfered, initiated by traces of oxygen, which oxidized the cation in (SFe,RC)[CpFe(Prophos)NCMe]PF6 to (SFe,RC)-[CpFe(Prophos)NCMe]2+. This dipositive cation started an electrocatalytic chain reaction, producing (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 with a high stereoselectivity of 2:98 in favor of (SFe,RC)[CpFe(Prophos)CNCy]PF6. Deactivation processes terminated the chain reaction, depending on the varying amounts of (SFe,RC)-[CpFe(Prophos)NCMe]2+ present in the system. Larger amounts of oxygen or oxidants, such as I2 and AgPF6, caused immediate complete conversion to (RFe,RC)/(SFe,RC)-[CpFe(Prophos)CNR]PF6 in a diastereomer ratio of 2:98. In contrast to the hexafluorophosphate salt, addition of a crystal of iodine did not initiate the chain reaction in the iodide salt [CpFe(Prophos)NCMe]I, because I2 added to I− to form I3−, which did not oxidize the cation of [CpFe(Prophos)NCMe]I. Instead, there was slow conversion according to the dissociative pathway. The correlation between the configuration of (RFe,RC)and (SFe,RC)-[CpFe(Prophos)CNCy]X and the conformation of the Fe-Prophos chelate ring on the one hand and the correlation with the P−P coupling constants of the Prophos ligand on the other hand was corroborated.

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INTRODUCTION Ligand substitutions in 18-electron half-sandwich complexes, such as (R Ru ,R C )-/(S Ru ,R C )-[CpRu(Prophos)Cl] and (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6), which occur at ambient temperatures, take an SN1 course via dissociation of the Ru−Cl and Fe−NCMe bonds.1,2 The 16electron intermediates (RRu,RC)-/(SRu,RC)-[CpRu(Prophos)]+ and (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)]+, formed in firstorder reactions, retain their pyramidal geometry with the empty site in the remaining coordination position.3−6 The degree of retention of configuration at the metal atom in substitution reactions depends on the stability of the unsaturated intermediates toward pyramidal inversion. It is high for basilica-type energy profiles,6 in which the barrier for pyramidal inversion is much larger than the barriers for addition of other ligands, as established for epimerization and ligand exchange in (RRu,RC)-/(SRu,RC)-[CpRu(Prophos)Cl].1,5 It is low for Hall−Church energy profiles,6 in which the barrier for pyramidal inversion is as large as or even smaller than the barriers for addition of other ligands, as demonstrated for © 2012 American Chemical Society

epimerization and ligand exchange in (RFe,RC)-/(SFe,RC)[CpFe(Prophos)NCMe]X (X = I, PF6).2,5 In both types of energy profiles the intermediates are high-energy 16-electron species, which also control the ligand exchange reactions in the present paper. However, new 17-electron intermediates come into play, which may initiate fast substitution processes via electron transfer catalytic chain reactions.7,8 The substitution of MeCN in diastereomerically pure (SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6) by phosphites P(OR)3 in CDCl3 followed first-order kinetics. The rates depended on the cone angles of the phosphites and decreased in the series P(OCH2)3CMe (101°) > P(OMe)3 (107°) > P(OPh)3 (128°).9 PPh3, with a cone angle of 145°, was too large to enter the empty coordination site in the intermediates (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)]+.2 The rate of MeCN exchange in (SFe,RC)-[CpFe(Prophos)NCMe]PF6 in CDCl3 at room temperature (293 K) with the model ligand P(OMe)3 in Received: March 4, 2012 Published: April 6, 2012 3395

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Scheme 1. Synthesis of the Isocyanide Complexes (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNR]X (R = Cyclohexyl, tert-Butyl and X = I, PF6)

11-fold excess was k = 1.21 × 10−3 min−1, corresponding to a half-life of τ1/2 = 572 min. In the MeCN exchange with isocyanide ligands CNR we expected (and found) similar kinetics and mechanisms. However, in some of the MeCN/ RNC substitution reactions there was an unusual acceleration in the beginning, which was absent in the MeCN/P(OR)3 exchange reactions. This initial acceleration is due to the 17electron intermediates (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)]2+, which start chain reactions. Herein we describe the syntheses and characterization of the compounds (RFe,RC)-/(SFe,RC)[CpFe(Prophos)CNR]X (R = cyclohexyl (Cy), tert-butyl (tBu) and X = I, PF6), including kinetics and mechanisms.

(Prophos)CNCy]+ and (RFe,RC)-[CpFe(Prophos)CNCy]+ are compared. The SFe,RC cation of the hexafluorophosphate salt is very similar (Figure 5S, Supporting Information) to the SFe,RC cation of the iodide salt (Table 1). Its ORTEP plot is depicted in the Supporting Information together with the ORTEP plots of (SFe,RC)-[CpFe(Prophos)CNtBu]PF6 and (SFe,RC)-[CpFe(Prophos)CNtBu]I (Figures 6S and 7S). The isocyanide complexes (R Fe ,R C )-/(SFe ,R C)-[CpFe(Prophos)CNCy]X and (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu]X were configurationally stable at the Fe atom. Enriched samples of (RFe,RC)- and (SFe,RC)-[CpFe(Prophos)CNCy]PF6 did not epimerize on heating in CDCl3 for days, and on heating (RFe,RC)/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 in the presence of excess tert-butyl isocyanide in CDCl3 for days no ligand exchange was observed. MeCN/CyNC Exchange in (SFe,RC)-[CpFe(Prophos)NCMe]X via SN1-Type Dissociation of the Fe−NCMe Bond. The kinetics of the MeCN/CyNC exchange in diastereomerically pure (SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6), which did not contain oxidation products (see later), was measured with a 13-fold excess of CyNC in CDCl3 (Table 2). At 293 K the rate constant for the iodide salt was k = 1.19 × 10−3 min−1, corresponding to a half-life of τ1/2 = 582 min. At 313 K the half-life was down to 19 min. In the PF6 salt the MeCN/CyNC ligand exchange reaction was somewhat faster than in the iodide salt. This had also been observed in the MeCN/P(OR)3 exchange reactions.2 Starting with diastereomerically pure (SFe,RC)-[CpFe(Prophos)NCMe]X, X = I, PF6, the diastereomer ratio of the product (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]X at the end of the reaction typically was between 60/40 and 40/60. During the reaction the diastereomer ratio of the product changed slightly. The measurement at 293 K in Table 2 started with a 56/44 ratio of (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]I and ended with 44/56. This was different in the MeCN/phosphite exchange reactions, in which the diastereomer ratio had been constant during the reaction.2 The MeCN/CyNC exchange in (SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6) starts with the rate-determining cleavage of the Fe−NCMe bond (Scheme 1 of ref 2) to give the pyramidal intermediate (SFe,RC)-[CpFe(Prophos)]+ and the ligand MeCN in low concentrations. Quenching with a large excess of CyNC or pyramidal inversion followed by quenching with CyNC yields the substitution products (SFe,RC)- and (RFe,RC)-[CpFe(Prophos)CNCy]X (X = I, PF6). The unsaturated intermediates (SFe,RC)- and (RFe,RC)-[CpFe(Prophos)]+ include species such as (SFe,RC)- and (RFe,RC)-[CpFe(Prophos)(solvent)]+. The positive entropy of activation (Table 2) is in accord with the dissociative nature of the MeCN/CyNC exchange. MeCN/CyNC Exchange in (SFe,RC)-[CpFe(Prophos)NCMe]X via an Electrocatalytic Chain Reaction. In the MeCN/RNC exchange some of the samples (SFe,RC)-[CpFe-

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RESULTS AND DISCUSSION Synthesis of the Isocyanide Complexes (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)CNR]X (R = Cyclohexyl, tertButyl and X = I, PF6). The reaction of 5/95 (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)NCMe]X (X = I, PF6) with an excess of cyclohexyl isocyanide or tert-butyl isocyanide in dichloromethane or chloroform afforded after 10 half-lives (see later) the ligand exchange products (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNR]X (R = cyclohexyl, tert-butyl and X = I, PF6), according to Scheme 1, in quantitative yields with diastereomer ratios close to 50/50. The yellow isocyanide complexes are air-stable. They are sparingly soluble in hot toluene, but dissolve well in dichloromethane and THF. They can be chromatographed on silica with 100/1 CH2Cl2/THF. The two doublets for the two different phosphorus atoms in the Prophos ligand can easily be used to determine the diastereomer ratio. Configurational symbols are assigned on the basis of the ligand priority sequence Cp > PCHMe > PCH2 > CNR.10−12 Crystallization of 50/50 (R Fe ,R C )-/(S Fe ,R C )-[CpFe(Prophos)CNCy]I from chloroform/ether gave single crystals of (SFe,RC)-[CpFe(Prophos)CNCy]I, suitable for X-ray analysis. Single crystals of (RFe,RC)-[CpFe(Prophos)CNCy]I were obtained from a stoichiometric substitution reaction, run in dilute solution. In Figure 1 the cations (SFe,RC)-[CpFe-

Figure 1. Cation (SFe,RC)-[CpFe(Prophos)CNCy]+ (left side) of (S Fe,R C )-[CpFe(Prophos)CNCy]I and cation (R Fe ,R C )-[CpFe(Prophos)CNCy]+ (right side) of (RFe,RC)-[CpFe(Prophos)CNCy]I. Hydrogen atoms and iodide ion are omitted for clarity. 3396

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Table 1. Crystallographic Data for the Complexes (Mo Kα Radiation)

empirical formula formula wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (Mg/m3) abs coeff (mm−1) abs cor transmissn max/min F(000) cryst size (mm) θ range (deg) no. of measd/unique rflns Rint no. of data/params goodness of fit F2 R1/wR2 (I > 2σ(I)) R1/wR2 (all data) abs struct param largest diff peak/hole (e Å−3) CCDC no.

(SFe,RC)-[CpFe(Prophos)CNCy]I

(RFe,RC)-[CpFe(Prophos)CNCy]I

(SFe,RC)-[CpFe-(Prophos) CNCy]PF6

(SFe,RC)-[CpFe(Prophos)CNtBu]I

(SFe,RC)-[CpFe-(Prophos) CNtBu]PF6

C39H42FeNP2,I 769.43 orthorhombic P212121 9.664(2) 18.971(1) 21.112(8) 90 90 90 3442.1(18) 4 1.485 1.457 multiscan 0.9177/0.6075 1568 0.38 × 0.20 × 0.06 3.09−27.63 31914/7790

C39H42FeNP2,I 769.43 monoclinic P21 11.063(3) 14.721(4) 11.631(4) 90 117.106(15) 90 1686.2(9) 2 1.515 1.487 multiscan 0.7454/0.5543 784 0.45 × 0.36 × 0.21 3.04−27.53 15539/7619

C39H42FeNP2,F6P 787.50 monoclinic P21 10.010(4) 17.660(6) 10.919(4) 90 110.235(16) 90 1811.1(12) 2 1.461 0.61 multiscan 0.7667/0.7303 816 0.55 × 0.54 × 0.46 3.05−27.49 17530/7840

C37H40FeNP2,I 743.39 orthorhombic P212121 9.828(3) 16.854(3) 20.244(5) 90 90 90 3353.2(14) 4 1.473 1.492 multiscan 0.7855/0.7349 1512 0.37 × 0.19 × 0.18 3.02−27.49 32785/7657

C37H40FeNP2,F6P 761.46 monoclinic P21 10.202(3) 17.204(5) 11.280(3) 90 114.659(10) 90 1799.3(9) 2 1.406 0.611 multiscan 0.9701/0.8824 788 0.21 × 0.15 × 0.05 3.09−27.55 17805/8179

0.1029 7790/397 1.074 0.0704/0.164 0.1056/0.1846 −0.02(4) 2.084/−1.221

0.0373 7619/397 1.046 0.0399/0.0801 0.035/0.0774 0.007(13) 1.094/−0.475

0.0333 7840/451 1.041 0.038/0.091 0.0407/0.0922 0.004(13) 0.599/−0.454

0.063 7657/379 1.064 0.0397/0.0822 0.0519/0.0864 −0.02(2) 1.222/−0.966

0.1825 8179/433 0.97 0.103/0.1668 0.2153/0.2065 −0.01(4) 0.64/−0.398

813973

813970

813974

813971

813972

[CpFe(Prophos)CNCy]PF6. In other samples there was a fast initial reaction with 50, 20, and 10% conversion, followed by the slow MeCN/CyNC exchange according to the SN1 mechanism (Table 2) until equilibrium was reached. Soon it became clear that oxidation of [CpFe(Prophos)NCMe]PF6 was the reason for the fast initial reaction. Thus, on addition of CyNC to a CDCl3 solution of (SFe,RC)-[CpFe(Prophos)NCMe]PF6, stirred in air, immediate complete conversion took place with a high stereoselectivity of (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 (2/98). This was surprising, because (SFe,RC)-[CpFe(Prophos)NCMe]PF6 had been described to be air-stable2 and solutions of (SFe,RC)[CpFe(Prophos)NCMe]PF6 did not change color or deposit decomposition products on standing in air. Obviously, the degree of oxidation varied with the different contact to air, which the samples had prior to the substitution reaction. Interestingly, the phenomenon seems to be specific for the MeCN/RNC exchange in (SFe,RC)-[CpFe(Prophos)NCMe]PF6. Definitely, it is not observed in the related MeCN/P(OR)3 exchange reactions.2 Thus, we ascribe the fast initial reaction to traces of oxygen, which got into contact with the samples, in spite of using Schlenk techniques. We assume that the oxidation product is the dipositive cation (SFe,RC)-[CpFe(Prophos)NCMe]2+, a 17electron species. It is this species which starts the electrocatalytic chain reaction, shown in Scheme 2, by reacting with CyNC to give MeCN and (SFe,RC)-[CpFe(Prophos)CNCy]2+. Due to the π-acceptor ligand CyNC the iron center in the 17-electron intermediate (SFe,RC)-[CpFe(Prophos)CNCy]2+ is

Table 2. Kinetics of the MeCN/CyNC Exchange in (SFe,RC)[CpFe(Prophos)NCMe]X (X = I, PF6) in CDCl3a,b temp (K) 293 300 307 313

293

kc (min−1)

τ1/2 (min)

(RFe,RC)-/(SFe,RC)[CpFe(Prophos)CNCy]X

X=I 1.19 × 10−3 ± 582 ± 4 44/56 −4 0.1 × 10 163 ± 2 45/55 4.25 × 10−3 ± 0.6 × 10−4 1.37 × 10−2 ± 51 ± 2 45/55 0.5 × 10−4 19 ± 1 43/57 3.69 × 10−2 ± 1.9 × 10−3 activation enthalpy ΔH⧧ = 128 ± 3 kJ mol−1 activation entropy ΔS⧧ = 102 ± 7 J mol−1 s−1 Gibbs free energy ΔG⧧ = 97 ± 5 kJ mol−1 X = PF6 1.97 × 10−3 ± 382 ± 4 49/51 90.2 × 10−4

a

Kinetics determined using time-resolved 31P{1H} NMR spectroscopy. Complex ca. 10 mg (3.6 × 10−2 mol L−1) and ligand (0.47 mol L−1) in CDCl3 (0.4 mL). bActivation parameters for 293 K. cThe constants k are disappearance rates of (SFe,RC)-[CpFe(Prophos)NCMe]X.

(Prophos)NCMe]PF6 did not behave as expected on the basis of the SN1 dissociation, discussed in the previous section. On addition of CyNC to solutions of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 some samples showed an immediate color change from the red starting material (SFe,RC)-[CpFe(Prophos)NCMe]PF6 to the yellow substitution product (SFe,RC)3397

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Scheme 2. Mechanism of the Electrocatalytic Chain Reaction of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 with CyNC, Initiated by (SFe,RC)-[CpFe(Prophos)NCMe]2+

(Prophos)CNCy]PF6. As the radical chain reaction is extremely fast and the MeCN/CyNC exchange reaction according to the SN1 path sets in very slowly, the contribution of the electrocatalytic chain reaction to conversion and diastereomer ratio of the product can be determined by measuring an early 31 1 P{ H} NMR spectrum. Addition of a crystal of molecular iodine to the system (SFe,RC)-[CpFe(Prophos)NCMe]PF6/CyNC caused an immediate color change from red to yellow and full conversion by the chain mechanism to give the product (RFe,RC)-/(SFe,RC)[CpFe(Prophos)CNCy]PF6 in a high diastereomer ratio of 2/ 98. This was confirmed by a 31P{1H} NMR spectrum with 64 scans obtained after a few minutes. Usually, hexafluorophosphate and iodide salts, such as (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)NCMe]X (X = PF6, I), behave comparably; however, this is not true in the present case. Addition of a crystal of I2 to the iodide system (SFe,RC)-[CpFe(Prophos)NCMe]I/CyNC did not induce any initial reaction by the chain mechanism. Obviously, I2 added to the iodide ion to give the triiodide anion I3−, the oxidation potential of which was not high enough to start the electrocatalytic chain reaction. Instead, there was slow conversion according to the dissociative pathway (Table 2). Addition of an excess of iodine, however, immediately led to full conversion with high stereoselectivity. Addition of the oxidant AgPF6 to a solution containing (SFe,RC)-[CpFe(Prophos)NCMe]PF6 and CyNC resulted in an immediate color change from red to yellow and full conversion to (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 (2/98) according to the chain mechanism, initiated by (SFe,RC)[CpFe(Prophos)NCMe]2+. This is in accord with the fact that AgPF6 had been used to preparatively oxidize [CpFe(Diphos)X] (Diphos = Ph2PCH2CH2PPh2) to the paramagnetic 17-electron compound [CpFe(Diphos)X]PF6.13,14 Electrocatalytic chain reactions are well-known in ligand substitution reactions of half-sandwich CpFe compounds;7 however, this does not take the stereochemistry at the chiral Fe atom into account. Most of these reactions were initiated by reduction of the 18-electron substrate [CpFeL3] to the 19electron species [CpFeL3]− with a strong reductant, which lost a ligand to give the 17-electron species [CpFeL2]−.15−20 The oxidative pathway is rare, in which the 18-electron substrate [CpFeL3] is oxidized to the 17-electron species [CpFeL3]+, which starts the chain reaction.7,21 It is the electron-rich nature of the iron center in [CpFe(Prophos)NCMe]PF6 which favors

less electron-rich than the 17-electron dication (SFe,RC)[CpFe(Prophos)NCMe] 2+ . Therefore, (S Fe ,R C )-[CpFe(Prophos)CNCy]2+ will oxidize (SFe,RC)-[CpFe(Prophos)NCMe]+ to (SFe,RC)-[CpFe(Prophos)NCMe]2+, the starting point of a new cycle in the chain reaction, forming the final substitution product (SFe,RC)-[CpFe(Prophos)CNCy]+. As in many of the experiments described below, the conversion by the radical pathway was not quantitative and the chain length is not extremely high. Therefore, there must be a relatively effective deactivation mechanism which terminates the chain reactions. Obviously, the fast initial chain reaction is finished soon after addition of CyNC to the dissolved (SFe,RC)-[CpFe(Prophos)NCMe]PF6 and long before the first 31P{1H} NMR spectrum, obtained after 64 scans, can be recorded (a few minutes after mixing). Spectra measured after 128, 256, and 512 scans (another 5, 10, and 20 min later) show that after termination of the initial reaction the MeCN/CyNC exchange proceeds slowly, as expected for a dissociative mechanism. In contrast to the SN1 path the electrocatalytic chain reaction between (SFe,RC)-[CpFe(Prophos)NCMe]PF6 and CyNC, initiated by (SFe,RC)-[CpFe(Prophos)NCMe]2+, is highly stereoselective. Carried out with diastereomerically pure (SFe,RC)-[CpFe(Prophos)NCMe]PF6, it gave a 2/98 ratio of the substitution products (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 at full conversion. Using the equilibrium mixture (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)NCMe]PF6 (5/95), at full conversion a product diastereomer ratio of 8/92 was obtained. At lower conversions the ratio decreased in favor of (RFe,RC)[CpFe(Prophos)CNCy]PF6, and at very low conversions even (RFe,RC)-[CpFe(Prophos)CNCy]PF6 was formed in excess. The reason for this trend is the energy difference between the major and minor diastereomers of the starting material. The share of the minor diastereomer (RFe,RC)-[CpFe(Prophos)NCMe]PF 6 in the (R Fe ,R C)-/(S Fe ,R C)-[CpFe(Prophos)NCMe]PF6 equilibrium is only 5%. This means it is 2.5 kcal higher in energy than the major diastereomer (SFe,RC)[CpFe(Prophos)NCMe]PF6. Thus, (RFe,RC)-[CpFe(Prophos)NCMe]PF6 is more reactive and is more easily oxidized than (SFe,RC)-[CpFe(Prophos)NCMe]PF 6, manifested by the 31 1 P{ H} NMR spectra recorded after the chain reaction had stopped at low conversion. In these spectra the more reactive diastereomer (RFe,RC)-[CpFe(Prophos)NCMe]PF6 had completely disappeared and was converted to (RFe,RC)-[CpFe3398

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are configurationally labile at the metal center. Thus, these four structures together with the structures of previous reports2,22 with the same relative configuration support the correlation with the favored envelope conformation of the M-Prophos chelate ring, shown in Figure 2 (left side). One structure in this study with the opposite RFe,RC configuration at the metal atom deviates from this preferred conformation. The exception (RFe,RC)-[CpFe(Prophos)CNCy]I is unique, because the methyl group of Prophos adopts an axial position in the chelate ring (Figure 2, right side). This is surprising, as it implies a strong 1,3-interaction with the axial phenyl ring of the neighboring PPh2 group.23 It is generally accepted that the conformation of the M-Prophos chelate ring is controlled by the equatorial orientation of the methyl group to avoid such steric 1,3-repulsions and that the puckering of the chelate ring in enantioselective catalysis transmits the chiral information from the asymmetric carbon atom in the backbone via the phenyl “ears” of the PPh2 groups to the coordination sites at the metal center, where catalysis occurs.24,25 Thus, normally the R configuration of Prophos results in a λ conformation of the chelate ring (Figure 2, left side). The axial position of the methyl group in the chelate ring of (RFe,RC)-[CpFe(Prophos)CNCy]I enforces a δ conformation in the chelate ring (Figure 2, right side), exceptional for structures which contain chelate rings with Prophos.23 Another correlation links the configuration at the Fe atom and the P−P coupling constants in the Prophos ligand in compounds of the type (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)L].22 All the compounds in this paper with SFe,RC configuration have coupling constants (about 40 Hz) larger by far than those with RFe,RC configuration (27 Hz).

the rare electrocatalytic chain pathway via oxidation to the 17electron species [CpFe(Prophos)NCMe]2+ in our system. The electrocatalytic chain reaction in the MeCN/RNC exchange in (SFe,RC)-[CpFe(Prophos)NCMe]X can be completely suppressed by addition of reducing agents such as Cp2Co and Cr(C6H6)2. However, in such systems in addition to the signals of the desired substitution products (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)CNR]X, forming slowly according to the SN1 mechanism, the signals of (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)Cl]22 appeared, more quickly for Cr(C6H6)2 than for Cp2Co. The appearance of the signals of (RFe,RC)-/(SFe,RC)[CpFe(Prophos)Cl] is due to the reaction of Cr(C6H6)2 and Cp2Co with the solvent CDCl3. Samples of [CpFe(Prophos)NCMe]PF6, which contained oxidized material, could be purified by chromatography. Passing a solution of [CpFe(Prophos)NCMe]PF6 in THF under inert conditions through a silica column of 50 cm length gave a product, which on addition of RNC did not undergo a fast electrocatalytic chain reaction but only a slow dissociative substitution. With respect to the radical chain experiments the t BuNC complexes behaved similarly to the CyNC complexes. Conformations and Configurations. A correlation had been established between the conformation of the fivemembered metal-Prophos chelate ring and the thermodynamic stability of major/minor diastereomers in equilibrium mixtures for compounds of the type (R Fe ,R C )-/(S Fe ,R C)-[CpM(Prophos)X].2 This correlation had been extended to compounds of the type (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)L]X, which were configurationally stable at the metal atom.11 According to this correlation diastereomers (SFe,RC)-[CpFe(Prophos)L]X adopt the favored envelope conformation (Figure 2, left side), in which the Fe atom, the two P atoms,

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EXPERIMENTAL SECTION

General Considerations. IR: JASCO FT/IR4100ST. 1H/31P{1H} NMR: Bruker Avance 400 (400/162 MHz, T = 293, 300, 308, and 313 K) or Bruker Avance III (500/203 MHz, T = 300 K), TMS as internal standard, and H3PO4 as external standard. MS: Finnigan MAT 95 (EI, 70 eV) or ThermoQuest Finnigan TSQ 7000. All manipulations were carried out in purified nitrogen or argon. (RFe,RC)-/(SFe,RC)-(η5-Cyclopentadienyl)(isocyanocyclohexane)[propane-1,2-diylbis(diphenylphosphane-κP)]iron Iodide, (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]I. Method A. To a solution of (SFe,RC)[CpFe(Prophos)NCMe]I (88 mg, 0.13 mmol) in dichloromethane (10 mL) was added cyclohexyl isocyanide (0.20 mL, 1.65 mmol), and the solution was stirred for 35 h (10 half-lives) at room temperature. After evaporation of the solvent the residue was washed with ether/hexane to give (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)CNCy]I (44/56) as a pale yellow powder. Yield: 98% (101 mg). The diastereomer (SFe,RC)[CpFe(Prophos)CNCy]I, suitable for X-ray analysis, was obtained by crystallization from chloroform/ether. Method B. To a solution of (SFe,RC)-[CpFe(Prophos)NCMe]I (10 mg, 0.14 mmol) was added cyclohexyl isocyanide (172 μL, 0.14 mmol) in chloroform (0.8 mL). The solution was allowed to stand for

Figure 2. Fe-Prophos chelate rings in (SFe,RC)-[CpFe(Prophos)CNCy]I (left side, methyl group equatorial, λ conformation) and (RFe,RC)-[CpFe(Prophos)CNCy]I (right side, methyl group axial, δ conformation). At the P atoms only ipso-C atoms of phenyl rings are shown.

and the C atom of the CH2 group are in a plane.22 The C atom of the CHMe group deviates from this plane, orienting its methyl group equatorially away from the complex. As the diastereomers of the isocyanide complexes do not interconvert, their thermodynamic stability cannot be derived from their equilibrium mixtures. However, the four structures with SFe,RC configuration in Table 3 have the same relative configuration as the major diastereomers in complexes which

Table 3. Conformational Analysis of the Fe-Prophos Chelate Ring in the Compounds (RFe,RC)-/(SFe,RC)[CpFe(Prophos)CNR]X (R = Cyclohexyl, tert-Butyl and X = I, PF6) complex

chirality of conformation

∠(P−M−P−CHMe) (deg)

∠(P−M−P−CH2) (deg)

conformation type

(SFe,RC)-[CpFe(Prophos)CNCy]I (RFe,RC)-[CpFe(Prophos)CNCy]I (SFe,RC)-[CpFe(Prophos)CNCy]PF6 (SFe,RC)-[CpFe(Prophos)CNtBu]I (SFe,RC)-[CpFe(Prophos)CNtBu]PF6

λ δ λ λ λ

−14.0 20.8 −16.5 −10.6 −17.8

−5.8 4.6 −1.9 −7.6 −4.8

envelope axial Me group envelope envelope envelope

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(cation, 100). HRMS (LSI, glycerol): calcd for the cation C39H42FeNP2 616.1985, found: 616.2000. (R F e ,R C )-/(S Fe ,R C )-(η 5 -Cyclopentadienyl)(2-isocyano-2methylpropane)[propane-1,2-diylbis(diphenylphosphane-κP)] iron Hexafluorophosphate, (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu]PF6. Method A. To a solution of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 (30 mg, 0.042 mmol) in chloroform (1.2 mL) was added tert-butyl isocyanide (47 μL, 0.042 mmol) at room temperature. The mixture was allowed to stand for 4 days at room temperature. After evaporation of the solvent the residue was washed with ether to give a mixture of diastereomers (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu]PF6 (49/51) as a yellow powder in quantitative yield. Method B. To a solution of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 (10 mg, 0.014 mmol) (probably in the presence of some [CpFe(Prophos)NCMe]X2) in chloroform (0.4 mL) was added tert-butyl isocyanide (15.6 μL, 0.14 mmol) at room temperature. The solution immediately changed from red to yellow and resulted in a mixture of (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu]PF6 (2/98). The solution was concentrated, and a few drops of diethyl ether were added. The mixture was allowed to stand for several days to deposit crystals of (SFe,RC)-[CpFe(Prophos)CNtBu]PF6, suitable for X-ray analysis. IR (KBr): ν 2125 (CN), 838 cm−1 (P−F). 1H NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets, if distinguishable): δ 7.79−7.28 (m, 20H, ArH), 4.44 (s, 5H, CpH), [4.42 (s, 5H, CpH)], 3.18−2.93 (m, 1H, CH), 2.38−1.97 (m, 2H, CH2), 1.23 (dd, 3H, 3JP−H = 11.1 Hz, 3JH−H = 6.5 Hz, Me), [0.76 (dd, 3H, 3JP−H = 14.4 Hz, 3JH−H = 7.6 Hz, Me)], 0.85 (s, 9H, Me), [0.97 (s, 9H, Me)]. 31 P{ 1 H} NMR (CDCl 3 , 293 K, major diastereomer; minor diastereomer in brackets): δ 106.68 (d, 1P, 2JP−P = 39.6 Hz), [114.30 (d, 1P, 2JP−P = 25.9 Hz)], 84.11 (d, 2JP−P = 39.6 Hz), [97.26 (d, 1P, 2JP−P = 25.9 Hz)], −143.03 (septet, 1P, 1JP−F = 711.5 Hz). ESMS (MeCN): m/z 616 (cation, 100). HRMS (LSI, glycerol): calcd for the cation C37H40FeNP3+ 616.1985, found: m/z 616.1971.

4 days at room temperature. A mixture of diastereomers (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)CNCy]I (84/16) was obtained. To the solution were added a few drops of diethyl ether, and the mixture was allowed to stand for several days to deposit yellow prisms of (RFe,RC)-[CpFe(Prophos)CNCy]I, suitable for X-ray analysis. IR (KBr): ν 2134 cm−1(CN). 1H NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets, if distinguishable): δ 7.87−7.79 (m, 2H, ArH), 7.73−7.31 (m, 17H, ArH), 7.01−6.97 (m, 1H, ArH), 4.44 (s, 5H, CpH), [4.47 (s, 5H, CpH)], 3.64−1.44 (m, 3H), 2.07−0.89 (m, 11H), 1.25 (dd, 3H, 3JP−H = 11.6 Hz, 3JH−H = 6.0 Hz, Me), [0.80 (dd, 3H, 3JP−H = 14.3 Hz, 3JH−H = 7.3 Hz, Me)]. 31 P{ 1 H} NMR (CDCl 3 , 293 K, major diastereomer; minor diastereomer in brackets): δ 106.86 (d, 1P, 2JP−P = 40.1 Hz), [113.78 (d, 1P, 2JP−P = 27.2 Hz)], 84.76 (d, 2JP−P = 40.1 Hz), [96.96 (d, 1P, 2JP−P = 27.2 Hz)]. ES-MS (MeCN): m/z 642 (cation, 100). HRMS (LSI, glycerol): calcd for the cation C39H42FeNP2+ 642.2142, found m/z 642.2124 (M+). (RFe,RC)-/(SFe,RC)-(η5-Cyclopentadienyl)(isocyanocyclohexane)[propane-1,2-diylbis(diphenylphosphane-κP)]iron Hexafluorophosphate, (RFe,RC)-/ (SFe,RC)-[CpFe(Prophos)CNCy]PF6. Method A. A mixture of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 (30 mg, 0.042 mmol) and cyclohexyl isocyanide (61 μL, 0.056 mmol) in chloroform (1.2 mL) was allowed to stand for 4 days at room temperature. After evaporation of the solvent the residue was washed with ether to give a mixture of diastereomers (RFe,RC)-/(SFe,RC)[CpFe(Prophos)CNCy]PF6 (49/51) as a yellow powder in quantitative yield. Method B. To a solution of (SFe,RC)-[CpFe(Prophos)NCMe]PF6 (10 mg, 0.014 mmol) (in the presence of some [CpFe(Prophos)NCMe]X2) in CDCl3 (0.4 mL) was added cyclohexyl isocyanide (20.3 μL, 0.019 mmol) at room temperature. The solution changed immediately from red to yellow. The 31P{1H} NMR spectrum showed the presence of the diastereomers (R Fe,RC)-/(SFe,RC)-[CpFe(Prophos)CNCy]PF6 in a 2/98 ratio. Diastereomerically pure crystals of (SFe,RC)-[CpFe(Prophos)CNCy]PF6, suitable for X-ray analysis, were obtained by crystallization from chloroform/ether. IR (KBr): ν 2147 (CN), 838 cm−1 (P−F). 1H NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets, if distinguishable): δ 7.81−7.30 (m, 19H, ArH), 7.01−6.94 (m, 1H, ArH), 4.43 (s, 5H, CpH), [4.39 (s, 5H, CpH)], 3.46−0.85 (m, 14H), 1.21 (dd, 3H, 3 JP−H = 11.2 Hz, 3JH−H = 6.3 Hz, Me), [0.76 (dd, 3H, 3JP−H = 14.2 Hz, 3 JH−H = 7.3 Hz, Me)]. 31P{1H} NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets): δ 106.76 (d, 1P, 2JP−P = 39.6 Hz), [113.77 (d, 1P, 2JP−P = 27.2 Hz)], 84.37 (d, 2JP−P = 39.6 Hz), [96.97 (d, 1P, 2JP−P = 27.2 Hz)], −143.03 (septet, 1P, 1JP−F = 713.0 Hz). ES-MS (MeCN): m/z 642 (cation, 100). HRMS (LSI, glycerol): calcd for the cation C39H42FeNP2+ 642.2142, found m/z 642.2149 (cation). Anal. Calcd for C39H42F6FeNP3 (787.5): C, 59.48; H, 5.37; N, 1.79. Found: C, 59.29; H, 5.08; N, 1.94. (R F e ,R C )-/(S F e ,R C )-(η 5 -Cyclopentadienyl)(2-isocyano-2methylpropane)[propane-1,2-diylbis(diphenylphosphane-κP)] iron Iodide, (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu]I. To a solution of (RFe,RC)-[CpFe(Prophos)I] (100 mg, 0.15 mmol) in chloroform (5 mL) was added tert-butyl isocyanide (34 μL, 0.3 mmol) at room temperature. The mixture was stirred for 4 days. After evaporation of the solvent the residue was washed with ether to give a mixture of diastereomers (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)CNtBu] I (26/74) as a yellow powder in quantitative yield. Diastereomerically pure crystals of (SFe,RC)-[CpFe(Prophos)CNtBu]I, suitable for X-ray analysis, were obtained by crystallization from chloroform/ether. IR (KBr): ν 2137 cm−1 (CN). 1H NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets, if distinguishable): δ 7.90−6.93 (m, 20H, ArH), 4.48 (s, 5H, CpH), [4.43 (s, 5H, CpH)], 3.63−1.84 (m, 3H, CH2CH), 1.29 (dd, 3H, 3JP−H = 11.3 Hz, 3JH−H = 6.6 Hz, Me), [0.79 (dd, 3H, 3JP−H = 14.2 Hz, 3JH−H = 7.3 Hz, Me)], 0.89 (s, 9H, Me), [0.97 (s, 9H, Me)]. 31P{1H} NMR (CDCl3, 293 K, major diastereomer; minor diastereomer in brackets): δ 106.61 (d, 1P, 2 JP−P = 39.6 Hz), [114.35 (d, 1P, 2JP−P = 26.7 Hz)], 84.49 (d, 2JP−P = 39.6 Hz), [97.26 (d, 1P, 2JP−P = 26.7 Hz)]. MS (ES, MeCN): m/z 616

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H and 31P{1H} NMR spectra of all new compounds and ORTEP drawings of (S Fe ,R C )-[CpFe(Prophos)CNCy]PF 6 , (SFe ,R C)-[CpFe(Prophos)CN tBu]I, (SFe,RC)-[CpFe(Prophos)CNtBu]PF6, and CIF files giving crystallographic data for all the compounds listed in Table 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*H.B.: fax, +49-941-9434439; e-mail, henri.brunner@chemie. uni-regensburg.de. T.T.: fax, +81-47-474-2579; e-mail, tsuno. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the College of Industrial Technology, Nihon University. REFERENCES

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