Isomerization of an Enantiomerically Pure Phosphorus-Bridged [1

Jun 23, 2014 - The enantiomerically pure phospha[1]ferrocenophane 4-C1 was prepared through a salt-metathesis reaction between tBuPCl2 and a chiral di...
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Isomerization of an Enantiomerically Pure Phosphorus-Bridged [1]Ferrocenophane Elaheh Khozeimeh Sarbisheh,† Jennifer C. Green,‡ and Jens Müller*,† †

Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom

‡

S Supporting Information *

ABSTRACT: The enantiomerically pure phospha[1]ferrocenophane 4-C1 was prepared through a salt-metathesis reaction between tBuPCl2 and a chiral dilithioferrocene derivative (Li2fc3‑Pen), which was equipped with two 3-pentyl groups in α positions with respect to lithium on the Cp rings (Sp,Sp isomer; C2 symmetry). The chiral 4-C1 isomerizes in reaction mixtures to give the Cssymmetrical phospha[1]ferrocenophane 4-Cs. This thermal isomerization involves haptotropic η5 to η1 shifts of Cp rings and is catalyzed by the chelating ligand 1,1′(tBuClP)2fc3‑Pen, which is a byproduct of the salt-metathesis reaction. Both phospha[1]ferrocenophanes, 4-C1 and 4-Cs, were isolated and characterized as pure compounds; the molecular structure of 4-Cs was determined by a single-crystal X-ray analysis. According to DFT calculations, the equilibrium constant K° for 4-C1 ⇌ 4-Cs is 4.43 (ΔESCF = −3.74 kcal/mol; ΔH° = −3.81 kcal/mol; ΔG° = −3.68 kcal/mol). As deduced from calculated molecular geometries, the thermodynamic difference between both isomers is mainly caused by a steric repulsion between the tBu group on phosphorus and one of the 3-pentyl groups on a Cp ring in the isomer 4-C1.

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INTRODUCTION Shortly after the description of the first [1]ferrocenophane ([1]FCP) by Osborne and Whiteley in 1975 (A; Chart 1),1 the

characterizations revealed a narrow range of 26.9−27.9° for the tilt angles between the two planes of the Cp rings (α angle in B; Chart 1).6b Recently, spurred by existing problems in aluminum- and gallium-bridged [1]FCPs,15 we employed ferrocene moieties equipped with two iPr groups in α positions to the bridging element. This approach led to the new [1]FCPs of the type C (Chart 1), equipped with boron, gallium, indium, silicon, and tin in bridging positions.13,16 These new [1]FCPs were prepared as enantiomerically pure compounds using a saltmetathesis route, starting from the planar-chiral ferrocene dibromide D (Sp,Sp isomer; Chart 1) as the precursor for the dilithioferrocene Li2fciPr.13 The iPr groups on the Cp rings sterically protect the bridging element and also increase the solubility of monomers and, more importantly, of resulting polymers. Here, we report on our first results to prepare enantiomerically pure phospha[1]ferrocenophanes where the chirality stems from a planar-chiral ferrocene moiety. One of the motivations for these investigations is to prepare phosphoruscontaining polymers with enantiomerically pure repeating units as ligands in asymmetric catalysis.

Chart 1. Known [1]Ferrocenophanes (A, C), Chiral Dibromide (D), and Structural Parameters in [1]Ferrocenophanes (B)

first phosphorus-bridged species could be characterized.2,3 After these early discoveries, it took more than a decade to show that silicon-bridged [1]FCPs can be polymerized to give highmolecular-weight metallopolymers.4,5 Over the last two decades, the preparation of new strained sandwich compounds and their ring-opening polymerization (ROP) have been explored.6 However, from the large family of [1]FCPs, only those bridged by silicon,5,7,8 germanium,9 or phosphorus10 could be polymerized with control over molecular weights and molecular weight distributions.5 Two aspects of phosphorusbridged [1]FCPs are particularly interesting. First, the small radius of phosphorus results in one of the most strained FCPs; only sulfur-bridged11 (α = 31.1°) and boron-bridged (α = 31.0−32.4°)12,13 [1]FCPs are more strained. Second, the donor property of phosphorus should allow for functionalization of metallopolymers through metal coordination. To date, many phospha[1]ferrocenophanes are known3,10,14 and structural © 2014 American Chemical Society

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RESULTS AND DISCUSSION The alkyl groups in the recently prepared dibromoferrocene D (Chart 1)13 were introduced onto the Cp rings through a known multistep, diastereoselective synthesis using “Ugi amine” chemistry.17,18 For the results discussed here, we used a similar Received: April 22, 2014 Published: June 23, 2014 3508

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synthetic methodology to introduce 3-pentyl instead of the isopropyl groups (Scheme 1). The move from iPr- to 3-pentylScheme 1. Synthesis of Phospha[1]ferrocenophanes

Figure 1. Molecular structure of 4-Cs with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Only one of two independent molecules is shown. Selected bond lengths and bond angles are shown in Table 1 (see Table S2 (Supporting Information) for additional bond lengths and angles; for crystal and structural refinement data see Table S1 (Supporting Information)).

substituted ferrocene derivatives originated from our group 13 chemistry, where sterics play a very important role in the formation of [1]FCPs in salt-metathesis reactions.13,15,16 The targeted phospha[1]ferrocenophane 4-C1 was prepared starting from the known “double Ugi amine” 1,18 which was lithiated and brominated to give the dibromide 2. Through a known two-step process the amino groups in 2 were replaced by ethyl groups, resulting in the known planar-chiral compound 3 ((Sp,Sp)-1,1′-dibromo-2,2′-bis(3-pentyl)ferrocene).19,20 Species 3 can be cleanly lithiated using a procedure that we applied for its isopropyl analogue D before (Chart 1).13 Addition of tBuPCl2 gave the targeted [1]FCP 4-C1, which could be clearly identified in the reaction mixture by its distinct pattern of six Cp signals in 1H NMR spectra (Scheme 1). The first attempt to purify this chiral [1]FCP through vacuum sublimation gave an oily solid with the expected dark red color of a phosphorus-bridged [1]FCP. However, even though the 1 H NMR spectrum of this sublimed species showed the expected peaks for the various groups, only half of the expected number of signals were present (e.g., three instead of the six Cp signals). The reduced number of signals revealed the presence of a 2fold symmetry element, which, together with the observation of the phospha[1]ferrocenophane characteristic color, led to the conclusion that 4-C1 had isomerized to the nonchiral [1]FCP 4-Cs (Scheme 1). It should be noted that the sublimation resulted in impure 4-Cs, where signals in the proton NMR spectrum revealed that still small amounts of 4-C1 were present (Figure S6 (Supporting Information)). Analytically pure 4-Cs was obtained by crystallization from hexanes at −80 °C. The targeted species 4-C1 can be isolated if, instead of the sublimation, flash column chromatography is used, which gave 4-C1 as a red oil in a yield of 32%. The interpretation of the NMR data was confirmed by a single-crystal X-ray analysis of 4-Cs (Figure 1, Table 1, and Tables S1 and S2 (Supporting Information)). 4-Cs crystallized in the monoclinic space group P21/c with two independent molecules per asymmetric unit (one is shown in Figure 1). The most interesting structural parameters are the common angles to describe distortions in [1]FCPs (B; Chart 1).6b The measured tilt angles α of 27.20(6) and 26.78(6)°, respectively, fit right in the narrow range of 26.9−27.9° known for a variety of phosphorus-bridged [1]FCPs.6b The isomerization from 4-C1 to 4-Cs must involve flipping of one Cp ligand. As illustrated in Scheme 2, Miyoshi et al.

described a haptotropic shift of a Cp moiety in phospha[1]ferrocenophanes.10d Employing mono- or bidentate phosphines, Manners et al. investigated photocontrolled η5 to η1 isomerizations in sila[1]ferrocenophanes and dicarba[2]ferrocenophanes.21 The more highly strained dicarba[2]ruthenophanes underwent similar isomerizations but did not require activation by irradiation.21 Whereas the isomerizations of the [1]- and [2]FCPs were reversible, those of [2]ruthenophanes were irreversible. On the basis of this knowledge, the isomerization from 4-C1 to 4-Cs likely follows a similar reaction path. This is illustrated in Scheme 3, where we speculate that the intermediate I is stabilized either by one bidentate phosphine or by two monodentate phosphines. An NMR sample of pure 4-C1 in C6D6 did not change over the course of one week, whereas a similar sample of the reaction mixture, which contains 4-C1 as the main product, changed at ambient temperature over the same time period to give isomer 4-Cs as the main product. These facts indicate that probably a byproduct in the reaction mixture acts as a catalyst for the isomerization. 31P NMR spectra of the reaction mixture show, in addition to the peaks for 4-C1 and 4-Cs at δ 29.4 and 16.2 ppm, respectively, two peaks at δ 112.8 and 113.0 ppm. These chemical shifts are nearly identical with those of the reported mixture of diastereomers of 1,1′-(tBuClP)2fc (fc = (C5H4)2Fe) found at δ 111.2 and 112.5 ppm,22 clearly indicating that the respective species 1,1′-(tBuClP)2fc3‑Pen (fc3‑Pen = (Sp,Sp)-[(Et2HC)C5H3]2Fe) was formed in the salt-metathesis reaction (Scheme 1f). In order to test if these species indeed act as the catalyst, we attempted their isolation but could not obtain any isomer as a pure compound. However, small amounts of one isomer of 1,1′-(tBuClP)2fc3‑Pen were left behind in a reaction flask after all volatile materials were removed by vacuum sublimation (100 °C oil bath temperature). The so-obtained sample exhibits mainly one 31P resonance at 112.8 ppm and three main peaks in the Cp range of the 1H NMR spectrum, revealing its C2 symmetry. An NMR sample of a mixture of pure 4-C1 and 1,1′(tBuClP)2fc3‑Pen in C6D6 (approximate molar ratio of 1:0.5) showed the isomerization of 4-C1 to 4-Cs with a rate similar to that observed for reaction mixtures. On the basis of these facts, it can be assumed that the byproduct 1,1′-(tBuClP)2fc3‑Pen is the catalyst of the isomerization, as it acts as a bidentate ligand to form the intermediate I (Scheme 3). For all of the NMR experiments the conversion of 4-C1 to 4-Cs did not proceed to 3509

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Table 1. Calculated and Measured Angles (deg) and Atom−Atom Distances (Å) in Phospha[1]ferrocenophanes 4-Cs α β/β′ θ δ P1−C1 P1−C6 P1−C21 α-C···α-C CCp−P−CtBu C−P−C ∑(C−P−C) a

4-C1

exptla

calcdb

calcd

27.20(6) {26.78(6)} 34.35(9)/35.48(9) {35.06(10)/33.87(10)} 91.63(6) {91.63(6)} 159.87(1) {160.11(1)} 1.8711(15) {1.8749(14)} 1.8489(14) {1.8527(14)} 1.8638(14) {1.8657(14)} 3.3123(19) {3.2845(18)} (C2−C7) 2.8673(20) {2.8969(19)} (C5−C10) 108.57(6) {108.96(6)} (C6−P1−C21) 109.38(7) {107.85(6)} (C1−P1−C21) 91.63(6) {91.63(6)} 309.6 {308.4}

27.41 33.92/33.92 91.22 160.88 1.880 1.880 1.899 3.336 (C2−C7) 2.910 (C5−C10) 109.40 109.40 91.22 310.02

25.72 33.60/35.86 90.98 161.95 1.876 1.892 1.906 3.149 (C2−C10) 3.062 (C5−C7) 115.56 108.83 90.98 315.37

Values for the second independent molecule are given in braces. bThe calculated symmetry is Cs.

Scheme 2. Known η5 to η1 Isomerization in Phospha[1]ferrocenophanes10d

Scheme 3. Proposed Isomerization of 4-C1 to 4-Csa

Only one of the possible positions of double bonds in the η1-Cp ring is illustrated. a

completion. Unfortunately, an equilibrium constant could not be determined, as the conversion was slow, and after approximately 3 days, NMR solutions started to turn cloudy and some new peaks appeared in 1H and 31P NMR spectra. We could only estimate that the relative amount of 4-C1 in an equilibrium mixture of the two isomers is less than 30%. Additional tests were done to see if photons play a role in the isomerization. Therefore, light was excluded from an NMR sample containing a freshly prepared reaction mixture; however, the isomerization also happened in the dark. Second, two NMR samples of 4-C1, one with only C6D6 and one with a mixture of C6D6 and thf as solvent, were exposed to light from a 150 W medium-pressure Hg lamp for 1 h. According to 1H and 31P NMR spectroscopy both samples did not change. DFT Calculations. In order to get structural and thermodynamic information about isomers 4-C1 and 4-Cs, DFT calculations were performed at the BP86/TZ2P level of theory.23 Results of the calculated ground-state geometries are illustrated in Figure 2; selected structural parameters are shown in Table 1 and compared to respective values of the measured molecular structure of 4-C s. First, the calculated and experimental structures of 4-Cs match very well. For example, the calculated tilt angle α of 27.41° differs from the experimental values of the two independent molecules by only 0.21 and 0.63°, respectively (α = 27.20(6) and 26.78(6)°) (Table 1). To our surprise, isomer 4-C1 shows at 25.72° a

Figure 2. Calculated molecular structures of phospha[1]ferrocenophanes 4-C1 and 4-Cs. Hydrogen atoms are omitted for clarity. Atom labeling is similar to that shown in Figure 1 (see the Supporting Information for Cartesian coordinates).

smaller α angle in comparison to that of its higher symmetric cousin 4-Cs. The difference is not large but still significant enough to imply that 4-C1 is less strained and, hence, thermodynamically more stable than 4-Cs. However, as discussed before, the opposite is the case, as 4-C1 converts to 4-Cs as the major isomer in solution. This shows that the Cs symmetric species is less strained even though it has a larger tilt angle α than the C1-symmetric compound. The equilibrium constant K° was calculated as 4.43, showing that a mixture of 18.4 (4-C1) to 81.6% (4-Cs) should form at 25 °C (ΔESCF = −3.74 kcal/mol; ΔH° = −3.81 kcal/mol; ΔG° = −3.68 kcal/ mol). A closer look at the molecular geometries of both isomers gives some indications where the additional strain in 4-C1 might originate. The geometry around phosphorus is significantly different in both species. Whereas the two CCp−P−CtBu bond angles in 4-Cs are the same (calculated value 109.40°),24 those in 4-C1 differ by 6.7° (calculated values 108.83 and 115.56°). This difference is mainly due to a widening of one of the two 3510

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Reagents. The compound (Sp,Sp)-1,1′-dibromo-2,2′-bis(3-pentyl)ferrocene (3) was synthesized according to the literature procedure.19 nBuLi (2.5 M in hexanes), tBuPCl2 (1.0 M in diethyl ether), and Al2O3 (Brockmann I, activated neutral standard grade, particle size ∼150 mesh) were purchased from Sigma-Aldrich. Synthesis of Phospha[1]ferrocenophane 4-C1. nBuLi (2.5 M in hexanes, 0.82 mL, 2.0 mmol) was added dropwise to a cold (0 °C) solution of 3 (0.467 g, 0.965 mmol) in a mixture of thf (1 mL) and hexanes (9 mL). The reaction mixture was stirred at 0 °C for 30 min, resulting in an orange solution. tBuPCl2 (1.0 M in Et2O, 1.03 mL, 1.0 mmol) was added dropwise within 10 min to the 0 °C cold solution by applying a syringe pump. The reaction mixture changed from orange to dark red along with formation of a white precipitate. After the reaction mixture was stirred at room temperature for 30 min, all volatiles were removed and the resulting red residue was dissolved in hexanes (15 mL). After the removal of LiCl by filtration, solvents were removed under vacuum. Flash column chromatography (neutral alumina, hexanes, 2% Et3N) under N2 pressure yielded 4-C1 as a red oil (0.126 g, 32%). 1H NMR (C6D6): δ 0.71 [t, 3H, CH(CH2CH3)2], 0.89 [t, 3H, CH(CH2CH3)2], 1.07 [t, 3H, CH(CH2CH3)2], 1.09 [t, 3H, CH(CH2CH3)2], 1.30 [m, 2H, CH(CH2CH3)2], 1.46 [d, J(1H/31P) = 15.0 Hz, 9H, PC(CH3)3], 1.42−1.62 [m, 3H, CH(CH2CH3)2], 1.75 [m, 1H, CH(CH2CH3)2], 2.04 [m, 2H, CH(CH 2 CH 3 ) 2 ], 2.64 [m, 1H, CH(CH 2 CH 3 ) 2 ], 3.16 [m, 1H, CH(CH2CH3)2], 4.07 (m, 1H, Cp), 4.17 (m, 1H, Cp), 4.19 (m, 1H, Cp), 4.20 (m, 1H, Cp), 4.25 (t, 1H, Cp), 4.43 (t, 1H, Cp). 13 C{1H} NMR (C6D6): δ 9.40 [s, CH(CH2CH3)2], 10.79 [s, CH(CH2CH3)2], 13.17 [s, CH(CH2CH3)2], 22.18 [d, C-PtBu, J(13C/31P) = 55 Hz], 25.80 [s, CH(CH2CH3)2], 26.72 [d, C-PtBu, J(13C/31P) = 72 Hz], 26.88 [s, CH(CH2CH3)2], 29.55 [s, CH(CH2CH3)2], 29.77 [s, CH(CH2CH3)2], 30.10 [d, PC(CH3)3, J(13C/31P) = 20 Hz], 32.02 [d, PC(CH3)3, J(13C/31P) = 23 Hz], 39.30 [d, CH(CH2CH3)2, J(13C/31P) = 12 Hz], 40.80 [s, CH(CH2CH3)2], 74.84 [d, Cp, J(13C/31P) = 3.9 Hz], 74.97 [d, Cp, J(13C/31P) = 14 Hz], 75.74 [d, Cp, J(13C/31P) = 1.8 Hz], 76.70 (s, Cp), 83.39 [d, Cp, J(13C/31P) = 7.4 Hz], 85.21 [d, Cp, J(13C/31P) = 49 Hz], 104.11 [d, C-CHEt2, J(13C/31P) = 11 Hz], 104.99 [d, C-CHEt2, J(13C/31P) = 27 Hz]. 31P{1H} NMR (C6D6): δ 29.4. MS (70 eV): m/z (%) 412 (100) [M+], 355 (19) [M+ − tBu], 326 (8) [M+ − tBuP]. HRMS (70 eV; m/z): calcd for C24H37FeP 412.1982, found 412.1985. As 4-C1 was obtained as a sticky oil, a CHN analysis was not obtained. Synthesis of Phospha[1]ferrocenophane 4-Cs. Employing nBuLi (2.5 M in hexanes, 0.84 mL, 2.1 mmol), 3 (0.484 g, 1.00 mmol), and tBuPCl2 (1.0 M in Et2O, 1.04 mL, 1.0 mmol), the procedure as described for 4-C1 was followed. However, instead of flash column chromatography a flask-to-flask condensation (100 °C oil bath temperature; p ≈ 10−2 mbar) was performed. The resulting mixture of a crystalline and an oily red substance was dissolved in hexanes (∼2 mL) and left at −80 °C for 16 h, resulting in 4-Cs as dark red crystals (0.178 g, 43%), suitable for single-crystal analysis. 1H NMR (C6D6): δ 0.76 [t, 6H, CH(CH2CH3)2], 1.00 [t, 6H, CH(CH2CH3)2], 1.30 [d, J(1H/31P) = 14.5 Hz, 9H, PC(CH3)3], 1.37 [m, 2H, CH(CH2CH3)2], 1.59 [m, 2H, CH(CH2CH3)2], 1.71− 1.82 [m, 4H, CH(CH2CH3)2], 2.96 [m, 2H, CH(CH2CH3)2], 4.19 (m, 2H, Cp), 4.32 (m, 2H, Cp), 4.40 (m, 2H, Cp). 13C{1H} NMR (C6D6): δ 8.81 [s, CH(CH2CH3)2], 12.31 [s, CH(CH2CH3)2], 21.25 [d, C-PtBu, J(13C/31P) = 58 Hz], 23.82 [s, CH(CH2CH3)2], 28.32 [d, CH(CH2CH3)2, J(13C/31P) = 1.3 Hz], 29.02 [d, PC(CH3)3, J(13C/31P) = 18 Hz], 31.58 [d, PC(CH3)3, J(13C/31P) = 16 Hz], 36.30 [d, CH(CH2CH3)2, J(13C/31P) = 8.2 Hz], 74.75 (s, Cp), 76.19 [d, Cp, J(13C/31P) = 4.6 Hz], 78.29 [d, Cp, J(13C/31P) = 7.4 Hz], 106.56 [d, C-CHEt2, J(13C/31P) = 26 Hz]. 31P{1H} NMR (C6D6): δ 16.2. MS (70 eV): m/z (%) 412 (100) [M+], 355 (21) [M+ − tBu], 327 (10) [M+ − tBuP]. HRMS (70 eV; m/z): calcd for C24H37FeP 412.1982, found 412.1981. Anal. Calcd for C24H37FeP (412.20): C, 69.90; H, 9.04. Found: C, 70.08; H, 9.04. Crystal Structure Determination. A red platelike crystal of 4-Cs was coated with Paratone oil, mounted using a Micromount (MiTeGen - Microtechnologies for Structural Genomics), and frozen in the cold stream of the Oxford cryojet attached to the diffractometer.

angles. The larger of the two angles is C6−P−C21 at 115.56° (Figure 2), and the increased value must be caused by a steric repulsion between the tBu group at phosphorus and the 3pentyl group attached to the Cp ring (Figure 2). Such a steric interaction is impossible for the Cs-symmetric isomer, as both 3-pentyl groups are at the maximum possible distance away from the tBu group. Hence, it can be assumed that the energy difference between the two isomers is mainly caused by the steric repulsion between one 3-pentyl and the tBu group in isomer 4-C1.

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SUMMARY AND CONCLUSIONS The planar chiral phospha[1]ferrocenophane 4-C1 was prepared as an enantiomerically pure compound through a multistep synthesis. To the best of our knowledge, five other chiral phosphorus-bridged [1]FCPs are known to date: three species with a planar chiral ferrocene moiety14c and two others with stereogenic carbon atoms present in the R group at phosphorus.14h Species 4-C1 can be isolated through flash column chromatography; however, if it is left in the solution of the reaction mixture, it isomerizes to the nonchiral phospha[1]ferrocenophane 4-Cs. This process is catalyzed by the byproduct 1,1′-(tBuClP)2fc3‑Pen of the salt-metathesis reaction and is reminiscent of other isomerizations of strained FCPs that involve haptotropic η5 to η1 shifts.10d,21 In contrast to the known FCPs where activation by photons is required for haptotropic rearrangements, the isomerization of 4-C1 to 4-Cs is a thermal process. One might be tempted to conclude that isomer 4-C1 with the less tilted Cp rings (calculated angle α = 25.72°) is less strained than the nonchiral isomer 4-Cs with a higher degree of tilting (calculated angle α = 27.41°), but the opposite is the case. Inspection of the calculated geometries revealed that the steric repulsion between one 3-pentyl and the tBu in 4-C1 must be causing additional strain so that thermodynamically its nonchiral cousin 4-Cs is more stable (ΔG° = −3.68 kcal/mol). As mentioned in the Introduction, we are interested in preparing phosphorus-containing polymers with enantiomerically pure repeating units, as they might find applications as ligands in asymmetric catalysis. The results described within this report show that a careful choice of the R group at phosphorus and the polymerization conditions are needed in order to avoid the loss of the planar chirality.

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EXPERIMENTAL SECTION

Syntheses. All syntheses were carried out using standard Schlenk and glovebox techniques. Solvents were dried using an MBraun Solvent Purification System and stored under nitrogen over 3 Å molecular sieves. All solvents for NMR spectroscopy were degassed prior to use and stored under nitrogen over 3 Å molecular sieves. 1H, 13 C, and 31P NMR spectra were recorded on a 500 MHz Bruker Avance NMR spectrometer at 25 °C in C6D6. 1H chemical shifts were internally referenced to the residual protons of the deuterated solvent peaks (δ 7.15 ppm for C6D6), and 13C chemical shifts were internally referenced to the C6D6 signal at δ 128.00 ppm. Assignments for 4-C1 and 4-Cs were supported by additional NMR experiments (DEPT, HMQC, COSY). Mass spectra were measured on a VG 70SE instrument and are reported in the form m/z (relative intensity) [M+]. The intensities are reported relative to the most intense peak, and M+ is the molecular ion peak or a fragment; only characteristic mass peaks are listed. For isotopic patterns, only the mass peak of the isotopologue or isotope with the highest natural abundance is listed. Elemental analyses were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer using V2O5 to promote complete combustion. 3511

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Crystal data were collected at −100 °C using the beamline 08B1-1 (CMCF-BM; Canadian Light Source) equipped with a ACCEL MD2 microdiffractometer, a mini-κ goniometer head, and a 300 mm 16 K Rayonix MX300 HE CCD detector (λ = 0.68878 Å; detector distance 150 mm). The initial screening and data collection were performed with the Macromolecular Crystallography Data Collector (MXDC) graphical user interface. A series of data frames at 1° increments of ω were collected. The integrated intensity data were merged and corrected for absorption using the XDS software package.25 The final unit cell parameters are based upon the refinement of the XYZ weighted centroids of 76599 reflections above 20σ(I) with 0.790° < 2θ < 27.334°. Data reduction was performed with the XDS software,25 which corrects for beam inhomogeneity, possible crystal decay, and Lorentz and polarization effects. A multiscan absorption correction was applied. Transmission coefficients were calculated using SHELXL2012.26 The structure was solved using direct methods (SIR-2004)27 and refined by full-matrix least-squares methods on F2 with SHELXL2012.26 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included at geometrically idealized positions (C−H bond distances 1.00/0.98/0.95 Å) and were not refined. The isotropic thermal parameters of these hydrogen atoms were fixed at 1.2 or 1.5 times that of the preceding carbon atom. Computational Details. Theoretical calculations were carried out using the Amsterdam Density Functional package (version ADF2012.01).23 The Slater-type orbital (STO) basis sets were of triple-ζ quality augmented with two polarization functions (ADF basis TZ2P). Core electrons were frozen (C and N, 1s; Fe and P, 2p) in our model of the electronic configuration for each atom. Relativistic effects were included by virtue of the zero-order regular approximation (ZORA).28 The local density approximation (LDA) by Vosko, Wilk, and Nusair (VWN)29 was used together with the exchange correlation corrections of Becke30 and Perdew31 (BP86).30,31 Tight optimization conditions were used for all compounds. Frequency calculations were used to confirm minima and provide thermodynamic information. The notation used for ΔH° and ΔG° indicate standard conditions (p = 105 Pa and T = 298.15 K). Graphical illustrations of calculated results were done with the help of ORTEP-3 for Windows (version 2.02);32 extraction of structural parameters (see Table 1) from the calculated coordinates of [1]FCPs was done with the help of Mercury (version 3.3).33

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

A CIF file giving crystallographic data for 4-Cs, bond lengths and angles and crystal and structural refinement data for 4-Cs (Tables S1 and S2); an xyz file giving Cartesian coordinates for 4-C1 and 4-Cs, and 1H, 13C, and 31P NMR spectra (Figures S1− S11). This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant, J.M.) for support. We thank the Canada Foundation for Innovation (CFI) and the government of Saskatchewan for funding of the X-ray and NMR facilities in the Saskatchewan Structural Sciences Centre (SSSC). We thank Dr. G. Schatte (SSSC) for crystallographic data of 4-Cs and Dr. K. Brown (NMR) and K. Thoms (CHN analysis and MS) for support and measurements. 3512

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Organometallics

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