Balancing Adduct Formation and Ligand Coupling with the Bulky Allyl

Jun 10, 2014 - Passage of CO at atmospheric pressure through solutions of A′2M (M = Fe, Co, Ni; A′ = [1,3-(SiMe3)2C3H3]−) in hexanes produces th...
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Balancing Adduct Formation and Ligand Coupling with the Bulky Allyl Complexes [1,3-(SiMe3)2C3H3]2M (M = Fe, Co, Ni) Nicholas R. Rightmire, Keith T. Quisenberry, and Timothy P. Hanusa* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *

ABSTRACT: Passage of CO at atmospheric pressure through solutions of A′2M (M = Fe, Co, Ni; A′ = [1,3-(SiMe3)2C3H3]−) in hexanes produces the corresponding allyl complexes A′2Fe(CO)2, A′Co(CO)3, and A′2Ni(CO), respectively. Although the iron and nickel species can be isolated as pure liquids, the cobalt complex is accompanied by the coupling product 1,3,4,6-tetrakis(trimethylsilyl)-1,5-hexadiene. A′Co(CO)3 was independently prepared from the reaction of Co2(CO)8, A′Br, and PhCH2N(C2H5)3+Cl− in aqueous base. The IR stretching frequencies of A′2Fe(CO)2 and A′Co(CO)3 are lower than those in the unsubstituted analogues, indicating that the trimethylsilated allyl ligand is a better electron donor than the parent version. Density functional theory calculations were performed on various conformations of the complexes, which reproduced the frequency-lowering effect of the trimethylsilyl groups. They also indicate that the thermodynamics of the formation of A′2Ni(CO) and the unknown (C3H5)2Ni(CO) are similar, suggesting that the thermal stability of the former is of kinetic origin. Oxidative coupling of the allyl ligands in A′2Fe and A′2Co is induced with I2; this is different from the case with A′2Ni, which has previously been shown to produce the mixed allyl halide complex [A′Ni(μ-I)2]2.

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(C3H5)3 Co + 3CO

INTRODUCTION

The field of π-allyl transition-metal chemistry was inaugurated with the synthesis of [(C3H5)PdCl]2 by Smidt and Hafner in 1959,1 and 2 years later Wilke had prepared a first-row allyl complex, (C3H5)2Ni.2 Since that time, π-allyl metal complexes have become broadly useful as reagents in organic chemistry and catalysis,3,4 and numerous examples have been synthesized for these purposes. Despite their uses, many poly(allyl) complexes of the firstrow transition metals are not easily handled; for example, the pyrophoric (C3H5)2Ni decomposes at 20 °C,5 and (C3H5)3Fe and (C3H5)3Co are even less stable (decomposition at −40 °C).5 The instability is often driven by the operation of low-energy decomposition pathways involving the coupling of the allyl ligands. In addition, the presence of additional donor ligands that could help saturate the metal coordination sphere and inhibit ligand loss may not lead to stable (C3H5)nMLm species. Although (C3H5)2NiPMe3 is stable enough to be crystallographically characterized, for example,6 the corresponding PPh3 adduct begins to dissociate in solution at −60 °C; at 0 °C, coupling of the allyl ligands occurs to give a hexadiene complex.6 Similarly, the 18-electron species (C3H5)2Fe(CO)2 evolves 1,5-hexadiene on standing at room temperature and requires storage at low temperature (−76 °C) under an inert atmosphere.7 Allyl coupling reactions are often induced during reactions of poly(allyl) complexes. Treatment of (C3H5)3Co with CO leads to the production of (π-allyl)cobalt tricarbonyl, for example, which is stable against further loss of the allyl ligand (eq 1).5,8 In contrast, Wilke found that the direct reaction of (C3H5)2Ni © XXXX American Chemical Society

→ (C3H5)Co(CO)3 + H 2CCH(CH 2)2 CHCH 2 (1)

with carbon monoxide goes completely to Ni(CO)4 and 1,5hexadiene, rather than yielding (C3H5)2Ni(CO) (1) (eq 2).2 (C3H5)2 Ni + 4CO → Ni(CO)4 + H 2CCH(CH 2)2 CHCH 2

(2)

There is some evidence that (2-MeC3H4)2Ni reacts with 1 equiv of CO at −80 °C to produce the unstable 1:1 adduct (2-MeC3H4)2Ni(CO).5 Nevertheless, warming the reaction mixture in the presence of additional CO leads to the production of Ni(CO)4 and the coupling product 2,5-dimethylhexal,5-diene or to the insertion product phorone (diisopropylidene acetone) if excess CO is present. Spectroscopic evidence for a bis(allyl)nickel carbonyl complex was obtained when Baker treated the bridged nickel allyl complex derived from 2,3,5-tris(methylene)hexamethylene with CO at −78 °C; the resulting red solution displayed a strong IR resonance at 2000 cm−1.9 Warming the reaction Special Issue: Catalytic and Organometallic Chemistry of EarthAbundant Metals Received: April 30, 2014

A

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mixture to −30 °C in the presence of additional CO caused the signal to disappear, however, and the cyclic insertion product 3,4,6-trimethylenecycloheptanone was identified (eq 3). The unisolated, thermally unstable species 1 was proposed as the source of the transient IR resonance.

in air. A solution of 2 in hexanes is also unstable, and a black precipitate is observed after standing overnight at room temperature. Only one set of resonances is found in the proton NMR spectrum for 2, suggesting that, unless there is rapid interconversion, only a single orientation of the allyl ligands is present (i.e., eclipsed or staggered). The proton NMR spectral pattern (s, s, d, d, and t from the two SiMe3 groups, the terminal hydrogens, and the central hydrogen,20 respectively) is indicative of a syn,anti arrangement of trimethylsilyl groups on the allyl ligands, as observed in the crystallographically characterized A′2Fe (see Figure 1).

Although less of a synthetic challenge, the instability associated with poly(π-allyl) transition-metal complexes is also encountered in their halide derivatives. (C3H5)2Ni reacts with bromine or iodine to form the heteroleptic (allyl)nickel halide species (C3H5)NiX, which in addition to possessing low thermal stability undergo ligand redistribution in donor solvents (eq 4).10 In a related reaction, iodine displaces an allyl from (C3H5)3Co to form [(C3H5)2CoI]n, which is thermally unstable with respect to allyl coupling.11 2(C3H5)NiX ⇄ (C3H5)2 Ni + NiX 2 X = Br,I

Figure 1. (a) syn,anti arrangement of SiMe3 groups on the A′ ligand, following the naming convention used for the hydrogens. (b) Corresponding syn,syn arrangement of SiMe3 groups.

The J values for the two doublets in the proton NMR spectrum of 2 are both 13.5 Hz; therefore, the syn and anti protons cannot be distinguished on that basis (in A′2Ni, in contrast, the J values are not the same (16 and 10 Hz); the larger coupling constant is associated with the anti proton of the allyl ligand).19 Whether the upfield or downfield resonance represents the syn or anti protons cannot be automatically assigned (see the discussion of the nickel complex below). The two carbonyls are equivalent in the 13C NMR spectrum of 2; a single resonance appears at 216.8 ppm. Two CO stretching peaks are present in the IR spectrum of 2 at 1931 and 1986 cm−1, which correspond to asymmetric and symmetric stretching modes, respectively. They are both 34 cm−1 lower than those reported for (C3H5)2Fe(CO)2 (1965, 2020 cm−1)7b and indicate that the SiMe3 groups on the allyl ligands are net electron donors. This point is discussed in more detail below. Reaction of A′2Co and CO. As noted above (eq 1), the orange-red (C3H5)Co(CO)3 (mp −33 to −32 °C)8,21 can be synthesized from the direct reaction of CO with (C3H5)3Co, but the usual routes start from the more conveniently obtained [Co(CO)4]− anion,8,21,22 which is derived from Co2(CO)8. A direct route is available for the synthesis of the trimethylsilylated version of the allyl cobalt complex. As carbon monoxide is added to an orange solution of A′2Co in hexanes, the solution turns dark red but eventually returns to orange. Removal of solvent results in an orange oil, which contains the spectrocopically identifiable cobalt complex A′Co(CO)3 (3) in addition to the allyl coupling product 1,3,4,6-tetrakis(trimethylsilyl)-1,5hexadiene. Colorless crystals of the latter grow out of the oil over several days at room temperature. The similar solubilities of 3 and the hexadiene has prevented the isolation of analytically pure samples. Nevertheless, the general chemical properties of 3 are consistent with those of the other allyl complexes described here. For example, like 2, 3 quickly decomposes in air, and it also decomposes in solution overnight, with the formation of a black precipitate. Owing to the complications involved in the synthesis of 3 by direct addition of CO, a more conventional route was used to confirm its properties. A known phase transfer catalyzed method for the synthesis of (C3H5)Co(CO)323 was adapted for the synthesis of 3. Addition of a dark brown benzene solution of equimolar Co2(CO)8 and 1,3-(SiMe3)2C3H3Br (A′Br)24 to a

(4)

As hinted at by the reactions of substituted bis(allyl)nickel compounds with CO noted above, it is possible to improve the kinetic stability of allyl complexes and their donor adducts by employing substituted allyl ligands. For example, (2-MeC3H4)2Fe(PMe3)212 is stable in solution to at least 30 °C (cf. 0 °C for the (C3H5)2Fe(PMe3)2 analogue),13 and unlike (C3H5)2Ni, the tetrasubstituted bis(1,1,3,3-tetraphenylallyl)nickel is stable indefinitely at 20 °C.14 The trimethylsilyl groups of the [1,3-(SiMe3)2C3H3]− (A′) ligand15 provide even more robust complexes, and A′ has been used to synthesize a variety of bis(allyl′)M species of the first-row transition metals, even in cases where the unsubstituted analogue is not known (e.g., A′2Fe,16 A′2Co3j,17).18 Furthermore, substituted allyl halide species of nickel (i.e., [A′Ni(μ-Br)]2, [A′Ni(μ-I)]2) are known that do not undergo Schlenk redistribution reactions.19 Nevertheless, use of the A′ ligand is not a panacea for instability, and we describe here reactions of first-row A′2M complexes with carbon monoxide and iodine, in which a balance exists between adduct formation and ligand coupling.

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RESULTS AND DISCUSSION Reactions with Carbon Monoxide. After carbon monoxide is passed through solutions of A′2M (M = Fe, Co, Ni) in hexanes, removal of the solvent leaves liquids that are the result of the addition of CO to the allyl complex. Reaction of A′2Fe and CO. The bis(allyl)iron complex (C3H5)2Fe is unknown, but the dicarbonyl complex (C3H5)2Fe(CO)2 has been prepared from the carbonyl halide (C3H5)Fe(CO)3I,7b which in turn is derived from Fe(CO)5. The trimethylsilylated version can be prepared directly from carbon monoxide. A CO purge at atmospheric pressure through an orange solution of A′2Fe in hexanes eventually causes the solution to become yellow. Removal of the solvent yields a viscous yellow liquid that was characterized with elemental and spectroscopic analysis as the dicarbonyl species A′2Fe(CO)2 (2). It does not crystallize after weeks at room temperature or for 1 week at low temperature (−40 °C). Although stable as the pure liquid under a nitrogen atmosphere, 2 quickly decomposes B

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colorless NaOH(aq) solution containing PhCH2NEt3+Cl− gradually produced an orange benzene layer above a blue aqueous layer. After extraction and drying of the upper layer, the dark orange 3 was isolated (eq 5).

approximately 7−10 days. Like 2 and 3, solutions of 4 are unstable; decomposition is evident within a few hours. As with 2, only one set of resonances is observed in the NMR data of 4. The 1H NMR spectrum contains a set of resonances that is consistent with trimethylsilyl substituents in syn,anti positions (see Figure 1) on the allyl ligands (i.e., s, s, d, d, dd). In the proton NMR spectrum, the two sets of doublets at δ 3.39 and 5.43 ppm have J values of 9.8 and 18.5 Hz, respectively. Since the proton resonance for the allyl framework’s anti proton of A′2Ni is associated with a larger J value than is that for its syn proton, the proton resonance at δ 5.43 is assigned to the anti proton in 4. Interestingly, this peak position is downfield from the resonance for the syn proton, making the relative positions of the peaks opposite those observed for A′2Ni.19 The 13C NMR peaks that are coupled to the corresponding proton peaks also follow the trend of upfield syn and downfield anti positions for 4. There is one peak in the 13C NMR spectrum that corresponds to bound CO (δ 204.1). The FT-IR data for 4 contain one CO stretching frequency at 2005 cm−1. It is close to the value of 2000 cm−1 reported for the CO stretch in 1, adding support to the latter’s identification as a nickel carbonyl complex. Reactions of A′2M (M = Fe, Co, Ni) and I2. Coupling of allyl ligands has been oxidatively induced in complexes of Mo(III) (with Cp2Fe+ 27) and Fe(II) (with Ag+ 28) and has been observed in the d0 complexes of the group 2 metals (induced with I218b,29). Halogens do not always initiate coupling, however. Direct reaction with halogens30 has been used to synthesize heteroleptic (π-allyl)nickel species (eq 6).

PhCH 2NEt3+Cl−

A′Br + Co2(CO)8 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ A′Co(CO)3 NaOH(aq), C6H6

(5)

The product matches the physical appearance and spectroscopic properties of the product from the treatment of A′2Co with CO; as the second synthesis reveals, 3 can tolerate brief exposure to water during synthesis. Nevertheless, even after drying, complete decomposition to a blue-green solid occurs within 12 h, perhaps accelerated by traces of remaining water. Regardless of the method of preparation of 3, there are two different species of it that can be identified from NMR data. One of them (3a) has a proton NMR spectral pattern consistent with trimethylsilyl groups in syn and anti positions on the allyl ligands (see Figure 1) (i.e., s, s, d, d, and dd). The two sets of doublets at 2.30 and 3.10 ppm have J values of 12.9 and 8.7 Hz, respectively. As in the proton NMR spectrum of A′2Ni, the upfield doublet resonance, with its larger J value, is considered to reflect the anti proton position, while the downfield resonance is associated with the syn protons. The corresponding carbons coupled to the syn and anti protons in 3a follow this trend in the 13C NMR spectrum. The other cobalt allyl species present (3b) has an NMR spectral pattern typical of allyl ligands with trimethylsilyl substituents in syn,syn positions (see Figure 1) (i.e., s, d, and t). The J value of 12.3 Hz for the doublet and triplet resonances is smaller than that either for K[A′] (15.6 Hz) or for the anti proton J value for A′2Ni (16 Hz) but is similar to the anti proton J value for 3a. The ratio of the syn,syn species to the syn,anti species in the proton NMR spectra varies somewhat from preparation to preparation, but the syn, syn complex is always the major product (by at least a factor of 1.7:1; note that in the formation of (1,3-Me2C3H3)Co(CO)3 from K[Co(CO)4] and 1,3-pentadiene, both syn,syn and syn,anti forms are generated, with the syn,syn form being the major species25). Although the syn (or syn,syn) forms of (allyl)Co(CO)3 complexes have been assumed to be more thermodynamically stable than the corresponding anti (or syn,anti) conformations,26 DFT calculations (discussed below) indicate that 3a,b have almost the same energy (within ∼1 kcal mol−1). Kinetic factors evidently play a role in the synthesis. The 13C NMR spectrum of 3 displays one peak at 204.5 ppm that corresponds to coordinated CO, suggesting that the CO environments of 3a,b are nearly equivalent. The FT-IR data for 3 exhibit carbonyl peaks at 1984 and 2053 cm−1. As the carbonyl resonance at 1984 cm−1 is roughly twice the width of the sharp peak at 2053 cm−1, the broad CO peak may reflect the presence of two unresolved, closely spaced peaks (the IR spectrum is found in the Supporting Information). Reaction of A′2Ni and CO. Owing to the anticipated low thermal stability of an allyl nickel complex, carbon monoxide was added to a solution of A′2Ni in hexane at −78 °C. The solution became red-brown, and removal of solvent produced a dark red-orange oil that was characterized as A′2Ni(CO) (4). Synthesis of 4 in THF was not successful; addition of CO to A′2Ni in THF produced a black reaction mixture from which no product could be isolated. In air, 4 rapidly decomposes, but even under a nitrogen atmosphere, decomposition is obvious after a few days, and complete decomposition is observed in

(C3H5)2 Ni + X 2 → C3H5X + (C3H5)NiX X = I,Br

(6)

The reaction products are believed to be dimeric in solution; in agreement with this, the reaction of A′2Ni with iodine or bromine leads to the formation of the structurally characterized [A′Ni(μ-X)]2 (X = I, Br) complexes.19 Similarly, the addition of iodine to (C3H5)3Co forms (C3H5)2CoI and C3H5I.31 On the basis of the precedent with A′2Ni, it was thought that similar reactions with A′2M (M = Fe, Co) might also lead to heteroleptic complexes. However, the addition of I2 to either A′2M complex (eq 7) results in the formation of A′2 M M = Fe,Co

+ I 2 → [1, 3‐(SiMe3)2 C3H3]2 + MI 2

(7)

the coupled allyl dimer 1,3,4,6-tetrakis(trimethylsilyl)-1,5hexadiene and insoluble black precipitates, presumably FeI2 and CoI2, respectively. No 1,3-(SiMe3)2C3H3I is observed in the NMR spectra, although its absence does not uniquely define the oxidative coupling mechanism. It is possible that the low electron counts of the iron and cobalt A′2M complexes are responsible for the formation of decomposition products with them. Calculations. Density functional theory calculations (PW91PW91/Def2-TZVP) were performed on model allyl carbonyl complexes with both the parent and the trimethylsilylated allyl ligands in order to clarify the effects of the substituents on ligand conformations and CO stretching frequencies.32 A variety of conformations were examined for the various complexes; only those that were found to be minima on the potential energy surface (as indicated by the absence of negative vibrational C

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Figure 2. Calculated structures of (a) eclipsed (C3H5)2Fe(CO)2 (C2), (b) eclipsed A′2Fe(CO)2 (C2), (e) distal (C3H5)Co(CO)3 (C1), (f) syn,anti A′Co(CO)3 (C1), (g) syn,syn A′Co(CO)3 (C1), (h) eclipsed-front (C3H5)2NiCO (C2v), (k) [η6-2,3,5-trimethylene-1,6-hexanediyl]nickel carbonyl, and (l) eclipsed-front A′2NiCO (C2). For clarity, hydrogen atoms have been omitted from trimethylsilyl groups.

narrow spread of 1943−1947 cm−1 (asymmetric) and 1985− 1993 cm−1 (symmetric). The C2-symmetric “eclipsed-front” conformation (Figure 2b) is the lowest in energy (ΔG°) by at least 6 kcal mol−1, and it shares the same basic arrangement as the stable form of (C3H5)2Fe(CO)2. Taking it as the most likely approximation to the solution structure of A′2Fe(CO)2, the ν(CO) values are calculated to be 21 and 24 cm−1 lower for the asymmetric and symmetric frequencies, respectively, than in (C3H5)2Fe(CO)2. This decrease parallels the 34 cm−1 drop for both frequencies observed experimentally between (C3H5)2Fe(CO)2 and A′2Fe(CO)2 and supports the conclusion that the SiMe3 groups on the allyl ligands are net electron donors. Cobalt Complexes. The conformation of (C3H5)Co(CO)3 with the CO group oriented distally to the center carbon of the allyl group was found to be a minimum on the potential energy

frequencies) are discussed in detail here. The lowest energy conformer in each case is depicted in Figure 2; illustrations of the others are available in the Supporting Information. Iron Complexes. Several conformations of (C3H5)2Fe(CO)2 were examined, but only the eclipsed arrangement depicted in Figure 2a was found to be a minimum on the potential energy surfance (Nimag = 0). A summary of its structural parameters is provided in Table 1. The calculated CO stretching frequencies of 1968 cm−1 (asymmetric) and 2009 cm−1 (symmetric) are close to the experimentally reported CO stetching frequencies (1965, 2020 cm−1).7b In contrast, calculations on 2 indicate that several local energy minima exist for it (Table 1). The angles between the C3 allyl planes differ substantially among the three conformations, but the calculated CO streteching frequencies are in the D

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Table 1. Selected Calculated Structural Parameters and ν(CO) Values in (R2C3H3)xM(CO)y Complexes angle between allyl planes (deg)

ν(CO) (cm−1)

complex

M−C range (Å)

M−CO (Å)

(a) (C3H5)2Fe(CO)2 (C2, eclipsed-front) (b) A′2Fe(CO)2 (C2, eclipsed-front) (c) A′2Fe(CO)2 (C2, eclipsed-back) (d) A′2Fe(CO)2 (C1, staggered) (e) (C3H5)Co(CO)3 (Cs, distal)

2.059−2.185 2.077−2.274 2.083−2.255 2.078−2.285 2.021−2.116

1.761 1.753 1.761 1.754, 1.759 1.811 (center); 1.771 (side)

(f) syn,anti A′Co(CO)3 (C1)

2.035−2.157

(g) syn,syn A′Co(CO)3 (C1)

2.039−2.195

(h) (C3H5)2Ni(CO) (C2v, eclipsed-front) (i) (C3H5)2Ni(CO) (C2, eclipsed-back) (j) (C3H5)2Ni(CO) (Cs, staggered) (k) [η6-2,3,5-trimethylene-1,6hexanediyl]nickel carbonyl (l) A′2Ni(CO) (C2, eclipsed-front) (m) A′2Ni(CO) (C2, eclipsed-back) (n) A′2Ni(CO) (C1, staggered)

2.021−2.103 2.032−2.194 1.995−2.087 2.014−2.138

1.802 (Co−C1); 1.777 (Co−C2); 1.761 (Co−C3) 1.760 (Co−C1); 1.800 (Co−C2); 1.775 (Co−C3) 1.795 1.794 1.818 1.782

8.6 85.0 54.0 28.0

1968 (asym); 2009 (sym) 1947 (asym); 1985 (sym) 1943 (asym); 1993 (sym) 1946 (asym); 1990 (sym) 2004 (asym); 2006 (asym); 2059 (sym) 1986 (asym); 1993 (asym); 2044 (sym) 1984 (asym); 1995 (asym); 2044 (sym) 2015 2005 2008 2014

2.038−2.193 2.017−2.187 2.020−2.334

1.789 1.859 1.795

12.9 71.6 40.7

1989 1979 1992

23.8 25.6 83.2 58.8

rel energy (kcal mol−1) 0.0 (ΔH°); 0.0 (ΔG°) 16.2 (ΔH°); 15.7 (ΔG°) 6.6 (ΔH°); 6.3 (ΔG°)

0.4 (ΔH°); 1.2 (ΔG°) 0.0 (ΔH°); 0.0 (ΔG°) 0.0 (ΔH°); 0.0 (ΔG°) 7.4 (ΔH°); 7.2 (ΔG°) 4.0 (ΔH°); 3.3 (ΔG°)

0.0 (ΔH°); 0.0 (ΔG°) 16.3 (ΔH°); 15.0 (ΔG°) 10.60 (ΔH°); 8.5 (ΔG°)

the 2000 cm−1 reported for 1 and supports the assignment of the latter as an allyl carbonyl complex. DFT calculations were also conducted on several forms of 4 (Table 1), but the “eclipsed-front” structure (Figure 2h) is the lowest in energy. The calculated Ni−C(allyl) bonding range (2.038−2.193 Å) and the allyl bending angle (12.9°) are somewhat larger than those calculated for the parent (C3H5)2Ni(CO); these changes reflect the increased steric strain imposed by the trimethylsilyl-substituted groups. The calculated bond lengths are also longer than those of the solidstate structure of A′2Ni (1.944(3)−2.037(3) Å), although the allyl bending angle is smaller (cf. 49.1° in A′2Ni (exptl)).19 However, the Ni−CO bond length is 1.789 Å, which is similar to that calculated for (C3H5)2Ni(CO) and shorter than that in Ni(CO)4 (1.836(2) Å, gas phase).35 The eclipsed-front structure has a CO stretching frequency of 1989 cm−1; this value is lower than that predicted for the unsubstituted allyl complexes, again reflecting the enhanced electron donation from the trimethylsilyl groups. The stretching frequency is close to the experimental CO stretching frequency measured for 4 (2005 cm−1). Formation of (C3R2H3)2Ni(CO). A series of calculations was performed to provide a measure of the thermodynamic driving force associated with the formation of an (C3R2H3)2Ni(CO) complex, historically the most elusive of the first-row allyl carbonyls. The reaction of (C3H5)2Ni with CO (eq 8) is

surface (Figure 2e, Table 1); with the (CO)3 group rotated by 180°, a transition structure is obtained with a single imaginary frequency (ν −76 cm−1). CO stretching frequencies of 2004 cm−1 (asymmetric), 2006 cm−1 (asymmetric), and 2059 cm−1 (symmetric) are predicted for the complex; these compare favorably with the experimental values of 1998 cm−1 (asymmetric; the individual stretches are not resolved) and 2065 cm−1 (symmetric).33 DFT calculations were performed on both the proposed syn,anti and syn,syn structures of 3. In the case of the syn,anti conformer (3a, Figure 2(f), Table 1), the structure displays three CO stretching frequencies at 1986 cm−1 (asymmetric), 1993 cm−1 (asymmetric), and 2044 cm−1 (symmetric); there is a 15−18 cm−1 reduction in the values in the trimethylsilated versions compared to the unsubstituted complex. These values are close to the experimental CO stretching frequencies in 3 (1984 and 2053 cm−1), taking into account that the two asymmetric stretching frequencies may be overlapping to produce the observed broad peak at 1984 cm−1. The calculated structure of syn,syn 3b (Figure 2g, Table 1) is lower in energy than the syn,anti structure by only 1.2 kcal mol−1 in ΔG° and 0.4 kcal mol−1 in ΔH°; i.e., 3a,b are essentially equienergetic. The syn,syn 3b with no symmetry (C1) has three calculated CO stretching frequencies at 1984 cm−1 (asymmetric), 1995 cm−1 (asymmetric), and 2044 cm−1 (symmetric).34 As in the case of 2 and 4 (below), this reflects greater electron donation from the trimethylsilated ligands relative to the parent species; the effect is less in 3, as only a single allyl ligand is contributing. Nickel Complexes. An “eclipsed-front” geometry is found in the structurally characterized (C3H5)2NiPMe3 complex,6 and although this conformation was determined by DFT calculations to be the lowest energy of the three (C3H5)2Ni(CO) complexes that were examined (Figure 2f, Table 1), all of them are minima on their potential energy surfaces (Nimag = 0). The calculated ν(CO) values are also similar (2005−2015 cm−1). The Ni−C(allyl) bond lengths and allyl bending angle in the eclipsed-front structure are close to those of (C3H5)2NiPMe3 (1.998(5)−2.092(5) Å and 3.6°, respectively). A calculation on the bridged 1 (Figure 2g) converged to a structure with features similar to those of (C3H5)2Ni(CO). Tellingly, its calculated ν(CO) value of 2014 cm−1 is close to

(C3H5)2 Ni + CO → (C3H5)2 Ni(CO) ΔG° = −10.1 kcal mol−1

(8)

predicted to have a ΔG° value of −10.1 kcal mol−1; this is reduced to −7.6 kcal mol−1 for the analogous reaction with A′2Ni (eq 9). These modest driving forces reflect competition between A′2 Ni + CO → A′2 Ni(CO)

ΔG° = − 7.6 kcal mol−1 (9)

the stable pseudosquare planar 16-electron (C3R2H3)2Ni species32 and their 18-electron. An alternative reaction of (C3R2H3)2Ni with CO to form Ni(CO)4 and the corresponding hexadienes (eqs 10 and 11) is strongly favored for both allyl ligands. E

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corresponding triemethylsilylated hexadiene [(SiMe3)2C3H3]2; in the process, a one-electron reduction of Co(II) to Co(I) occurs. The CO stretching frequencies observed in A′2Fe(CO)2 and A′Co(CO)3 are consistent with the A′ ligand being a better electron donor than unsubstituted allyl. All of the carbonyl drivatives are air-sensitive and degrade rapidly in solution. That they can be isolated at all, if even for a limited time, at room temperature is clearly a consequence of the presence of the trimethylsilyl substituents. The similar calculated thermodynamics for the reactions of CO with (C3R2H3)2Ni suggests that (C3R2H3)2Ni(CO) is only an intermediate on the way to forming nickel tetracarbonyl and the corresponding hexadienes for both R = H and SiMe3. Although oxidatively induced coupling of the allyl ligands with I2 is facile for A′2M when M = Fe, Co, that it does not occur when M = Ni indicates that the properties of both the ligand and metal must be considered when designing new reactions of allyl complexes for stoichiometric, and ultimately catalytic, purposes.

(C3H5)2 Ni + 4CO → Ni(CO)4 + H 2CCH(CH 2)2 CHCH 2 ΔG° = −61.1 kcal mol−1

(10)

A′2 Ni + 4CO → Ni(CO)4 + [(SiMe3)2 C3H3]2 ΔG° = −56.8 kcal mol−1

(11)

The fact that the reaction in eq 10 is experimentally observed,2 whereas eq 11 is not, or at least not immediately, is clearly a consequence of kinetic stabilization provided by the trimethylsilyl groups, rather than the result of major thermodynamic differences between the two systems. In this context, it is interesting to note that the reaction to form 1 (eq 12) is calculated to be

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise noted, all manipulations were performed with the rigorous exclusion of air and moisture using high-vacuum, Schlenk, or glovebox techniques. Proton and carbon (13C) NMR spectra were obtained on a Bruker DPX-300 spectrometer at 300 and 75.5 MHz, respectively, or on a Bruker AV-I-400 spectrometer at 400 and 101 MHz, respectively, and were referenced to the residual proton and 13C resonances of C6D6. Infrared data were obtained on an ATI Mattson-Genesis FT-IR spectrometer between KBr plates. Combustion analyses were performed by Columbia Analytical Services, Tuscon, AZ. ICP-OES measurements were made on a PerkinElmer Optima 2000 DV instrument. Materials. A′2M (M = Fe,16 Co,17 Ni19) and A′Br24 were prepared as previously described. Carbon monoxide (CP grade) was passed through a drying column (anhydrous CaSO4) before use. A Schlenk-line adapted needle, which was purged with CO for several minutes, was used to introduce CO to solutions. THF and hexanes were distilled under nitrogen from potassium benzophenone ketyl.44 Deuterated solvents were vacuum-distilled from Na/K (22/78) alloy prior to use. ICP-OES samples were digested in 4% HNO3 at 85 °C and diluted with deionized water until a transparent colorless solution was obtained. Other reagents were obtained from commercial sources and used as received. Reaction of A′2Fe and CO. A 125 mL Schlenk flask containing a magnetic stirring bar was charged with A′2Fe (0.123 g, 0.309 mmol) and 20 mL of hexanes. A needle was submerged in the solution, and CO was briskly added for 6 min. The solution was then degassed using the freeze−pump−thaw method. The solution was filtered, and hexanes was removed under reduced pressure, leaving a yellow-orange oil (0.130 g, 87% yield). Anal. Calcd for C20H42FeO2Si4: C, 49.76; H, 8.77; Fe, 11.57. Found: C, 50.20; H, 8.74; Fe, 11.32. 1H NMR (C6D6, 298 K): δ 0.18 (s, 18H, Si(CH3)3); 0.30 (s, 18H, Si(CH3)3); 1.15 (d, J = 13.5 Hz, 2H, anti/syn C−H); 3.02 (d, J = 13.5 Hz, 2H, anti/syn C−H); 5.29 (apparent t, J = 13.5 Hz, 2H, C(2)−H). 13C NMR (C6D6, 298 K): δ −0.22 (Si(CH3)3); −0.016 (Si(CH3)3); 47.20 (syn C−H); 69.30 (anti C−H); 106.85 (C(2)); 216.84 (CO). Principal IR bands (cm−1): 2953 (s), 2898 (s), 1986 (s), 1931 (s), 1697 (m), 1606 (w), 1603 (w), 1477 (m), 1415 (m), 1249 (s), 1204 (m), 1101 (m), 1019 (m), 852 (s), 732 (m), 688 (m), 601 (m), 563 (m). Reaction of A′2Co and CO. A 125 mL Schlenk flask containing a magnetic stirring bar was charged with A′2Co (0.140 g, 0.326 mmol) and 20 mL of hexanes. A needle was submerged in the solution, and CO was briskly added for 6 min. The solution was then degassed using the freeze−pump−thaw method. The solution was filtered, and hexanes was removed under reduced pressure, leaving an orange oil. Colorless crystals grew over a period of days and were confirmed with X-ray diffraction to be the dimerized propene [(SiMe3)2C3H3]2.19 Two other products were detected in NMR spectra, identified as the

more strongly favored (ΔG° = −30.2 kcal mol−1) than when the allyl ligand is C3H5 or A′; apparently the preorganization provided by the bridge optimizes the complex for subsequent rection with CO. Donor Properties of the A′ Ligand. The electronic effect of trimethylsilyl substitutents in organometallic systems is not always easily predictable. In consonance with the lower electronegativity of silicon in comparison to carbon, the group electronegativity of −SiMe3 relative to −CH3 is calculated to be lower (e.g., 1.92 vs 2.56, respectively,36 although other values have been suggested37). This might, perforce, indicate that that −SiMe3 would be a better donor than hydrogen, and indeed, a comparison of the free energies of ionization of substituted ruthenocene complexes suggest that the −SiMe3 group is comparable to −CH3 as an electron donor.38 Measurement of the inner-shell electron binding energies in [(SiMe3)nC5H5−n]2MCl2 (M = Zr, n = 0−3; M = Hf, n = 0, 3) indicates that −SiMe3 is ca. 1.25 times as strong a donor as is −CH3.39 In contrast to this evidence, however, the reduction potential (Ered1/2) of substituted Cp′2ZrCl2 compounds increases in the order (SiMe3)2 < SiMe3 < H < Me < Et, indicating that −SiMe3 is functioning as an electron acceptor relative to hydrogen.40 In other cases, especially when the steric bulk of the −SiMe3 comes into play, its electronic effect appears to be negligible.41 In the present case, the −SiMe3 groups on A′ are evidently electron donating relative to the protons on the unsubstituted allyl ligand, as manifested in the lowered CO stretching frequencies in both 2 and 3 relative to their parent complexes. In this regard, the −SiMe3 group has an effect similar to that observed when attached to the cyclobutadienyl ligand (e.g., CO stretches of 1995 and 2061 cm−1 are observed in (C4H4)Ru(CO)3,42 but stretches of 1980 and 2046 cm−1 are found in (C4(SiMe3)4)Ru(CO)343).

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CONCLUSIONS This work has shown that first-row A′2M complexes can be used to form kinetically stabilized carbonyl derivatives by direct reaction with CO. A′2Fe and A′2Ni accept carbon monoxide as a donor ligand to form the 18-electron carbonyl adducts A′2Fe(CO)2 and A′2Ni(CO), respectively. The addition of CO to A′2Co generates the 18-electron A′Co(CO)3, along with the F

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Article

syn,anti (3a), and syn,syn (3b) forms of A′Co(CO)3. 1H NMR (C6D6, 298 K): 3a, δ 0.00 (s, 9H, Si(CH3)3); 0.08 (s, 9H, Si(CH3)3); 2.30 (d, J = 12.9 Hz, 1H, anti C−H); 3.10 (d, J = 8.7 Hz, 1H, syn C−H); 4.89 (dd, J = 12.9 Hz, J = 8.7 Hz, 1H, C(2)−H). 13C NMR (C6D6, 298 K): δ −1.27 ppm (Si(CH3)3); 0.55 (Si(CH3)3); 63.48 (anti C−H); 67.92 (syn C−H); 96.06 (C(2)); 204.51 (CO); 3b, δ 0.10 (s, 18H, Si(CH3)3); 1.97 (d, J = 12.3 Hz, 2H, C(1,3)−H); 4.70 (t, J = 12.3 Hz, 1H, C(2)−H). 13C NMR (75 MHz, C6D6, 298 K): δ −1.13 ppm (Si(CH3)3); 67.30 (C(1,3)); 94.20 (C(2)); 204.51 (CO). Principal IR bands (cm−1): 2954 (s), 2898 (m), 2360 (w), 2344 (w), 2053 (s), 1984 (s, br), 1697 (w), 1599 (m), 1492 (w), 1444 (w), 1247 (s), 994 (m), 838 (s, br), 742 (m), 689 (m), 559 (m), 517 (m). Alternative Synthesis of A′Co(CO)3.23 A 250 mL Schlenk flask equipped with a magnetic stir bar was charged with PhCH2N(C2H5)3+Cl− (0.293 g, 1.29 mmol) dissolved in 40 mL of 5 M NaOH. The resulting clear colorless solution was degassed using the freeze− pump−thaw method. A 125 mL Schlenk flask with a magnetic stir bar was charged with Co2(CO)8 (0.461 g, 1.35 mmol), A′Br (0.367 g, 1.38 mmol), and 50 mL of benzene to yield a dark brown solution. The solution in benzene was cannulated into the NaOH solution at room temperature. The mixture was stirred vigorously for 2 h, resulting in an orange benzene solution above a blue aqueous layer. A liquid−liquid extraction was performed under argon to collect the benzene solution. The solution was dried with MgSO4 and gravity-filtered to remove any solid. The extracted orange solution was dried under vacuum and collected under an N2 atmosphere, yielding a dark orange oil (0.115 g, 26%). The product showed limited stability in solution and under an N2 atmosphere. Both in solution and as the dried compound, complete decomposition to a blue-green solid occurred within 12 h. Prior to decomposition, both products 3a,b were identified in the 1H and 13C NMR spectra. Their instability prevented the acquisition of elemental analysis data. Reaction of A′2Ni and CO. A 125 mL Schlenk flask containing a magnetic stirring bar was charged with A′2Ni (0.20 g, 0.56 mmol) and 50 mL of hexanes. The flask was cooled to −78 °C in a dry ice− acetone bath under nitrogen. A needle was purged with CO for 10 min and submerged into the solution, and CO was added for 15 min. Addition of CO resulted in a slight color change in the solution from dark brown to red-brown. The solution was degassed using the freeze−pump−thaw method and filtered, and hexanes was removed immediately under reduced pressure. A dark red-orange oil (0.18 g, 80% yield) was collected. Anal. Calcd for C19H42NiOSi4: Ni, 12.8. Found (ICP-OES): Ni, 11.2. As the neat oil, complete decomposition is observed within 1 week of synthesis; in solution, decomposition is much more rapid (2−3 h). 1H NMR (C6D6, 298 K): δ 0.08 (s, 18H, Si(CH3)3); 0.10 (s, 18H, Si(CH3)3); 3.39 (d, J = 9.8 Hz, 2H, syn C−H); 5.43 (d, J = 18.5 Hz, 2H, anti−C-H); 6.81 (dd, J = 18.0 Hz, J = 10.4 Hz, 2H, C(2)−H). 13C NMR (C6D6, 298 K): δ −2.50 ppm (Si(CH3)3); −1.00 (Si(CH3)3); 61.10 (syn C−H); 128.62 (anti C−H); 143.01 (C(2)); 204.14 (CO). Principal IR bands (cm−1): 2955 (s), 2898 (m), 2005 (m), 1655 (m), 1619 (s), 1597 (s), 1404 (m), 1341 (m), 1247 (s), 1210 (m), 1099 (s), 1020 (s), 945 (m), 908 (m), 840 (s, br), 753 (m), 690 (m), 637 (m). Reaction of A′2M (M = Fe, Co) with I2. A 125 mL Erlenmeyer flask containing a stirring bar was charged with A′2Fe (0.40 g, 0.94 mmol) and 30 mL of hexanes. Iodine (0.24 g, 0.94 mmol) was added, and the reaction mixture was stirred overnight. The solution eventually turned light yellow, and a finely divided black precipitate formed. The solution was decanted, and hexanes was removed under reduced pressure, leaving a yellow liquid that was identified as 1,3,4,6tetrakis(trimethylsilyl)-1,5-hexadiene, [(SiMe3)2C3H3]2 (0.28 g; 80% yield) from its characteristic 1H NMR spectrum.19 An analogously conducted reaction with A′2Co (1 equiv of I2) also produced the hexadiene (76% yield). Computational Details. Geometry optimization calculations were performed using the Gaussian 03 and 09 suites of programs.45 The PW91PW91 functional, which employs the 1991 gradient-corrected functional of Perdew and Wang for both correlation and exchange,46 was used. The triple-ζ polarized Def2-TZVP basis set47 and an ultrafine grid were used for all calculations. Stationary points were characterized

by the calculation of vibrational frequencies, and unless otherwise noted, all geometries were found to be minima (Nimag = 0). Use of the PW91PW91 functional in conjunction with the Def2-TZVP basis set predicts CO stretching frequencies with good accuracy even without scaling; the predicted values are generally lower than experimental values by 10−15 cm−1. This functional/basis set combination was used to calculate ν(CO) for CO itself and several benchmark carbonyl complexes; the values (cm−1) for CO (2135), Fe(CO)5 (2012, 2029), (CO)3Co(μ-CO)2Co(CO)3 (2033, 2040, 2041, 2065, 2102, terminal only), and Ni(CO)4 (2048) are in good agreement with the experimentally determined values (CO (2143), Fe(CO)5 (2013, 2034), Co2(CO)8 (2031, 2044, 2059, 2071, 2112), and Ni(CO)4 (2057)).48

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ASSOCIATED CONTENT

S Supporting Information *

Figures, xyz files, and text giving the FT-IR spectrum of [1,3(SiMe3)2C3H3]Co(CO)3, coordinates of the geometry optimized structures of 1a−n, illustrations of higher energy conformers of the allyl complexes, and the complete Gaussian 03 and 09 reference (ref 45). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.P.H.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Petroleum Research Fund, administered by the American Chemical Society (4-20-430-5872), and by the National Science Foundation (CHE-1112181) is gratefully acknowledged. The authors thank Toshia Wrenn for assistance with the ICP-OES analysis.

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REFERENCES

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dx.doi.org/10.1021/om5004573 | Organometallics XXXX, XXX, XXX−XXX