Synthesis, Structure, Spectroscopy, and Reactivity of Azapentadienyl

Article pubs.acs.org/Organometallics

Synthesis, Structure, Spectroscopy, and Reactivity of Azapentadienyl−Cobalt−Phosphine Complexes1 John R. Bleeke* and Wipark Anutrasakda Department of Chemistry, Washington University, St. Louis, Missouri 63130, United States

Nigam P. Rath Department of Chemistry and Biochemistry, University of MissouriSt. Louis, One University Boulevard, St. Louis, Missouri 63121, United States S Supporting Information *

ABSTRACT: We report the synthesis, structure, spectroscopy, and reactivity of the first examples of azapentadienyl−cobalt complexes. Treatment of (Cl)Co(PMe3 ) 3 with potassium tert-butylazapentadienide produces ((1,2,3-η 3 )-5-tertbutylazapentadienyl)Co(PMe3)3 (1), a dark red solid. Compound 1 undergoes ligand substitution reactions with trimethyl phosphite and carbon monoxide, producing ((1,2,3-η3)-5-tert-butylazapentadienyl)Co(PMe3)2[P(OMe)3] (2) and ((1,2,3-η3)-5tert-butylazapentadienyl)Co(PMe3)2(CO) (3), respectively. Compounds 1−3 react with 1 equiv of triflic acid (HO3SCF3) at the azapentadienyl nitrogen, yielding the corresponding η4-(tert-butylamino)butadiene−cobalt complexes (4−6). These cations have unusually long Co−C4 bond distances, indicating that η3 resonance structures are contributing to the overall bonding picture. Treatment of compound 4 with P(OMe)3 or CO leads to displacement of (tert-butylamino)butadiene (rather than PMe3 ligand exchange) and formation of Co(PMe3)2[P(OMe)3]3+O3SCF3− (7) or Co(PMe3)3(CO)2+O3SCF3− (8), respectively. Compound 5 reacts readily with a another 1 equiv of triflic acid, again at the nitrogen center. The dicationic product, 9, possesses an η4-(tertbutylammonium)butadiene ligand with a strong Co−C4 interaction. The X-ray crystal structures of compounds 1−4, 6, 7, and 9 have been obtained.

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INTRODUCTION Heteropentadienyl ligands (i.e., pentadienyl analogues in which one terminal CH2 group has been replaced by a heteroatom)2 have attracted increasing attention due to their ability to adopt a variety of bonding modes, a feature which can enhance both stoichiometric and catalytic reactivity.3 Over the past 20 years, our group has undertaken a systematic investigation of heteropentadienyl−transition-metal−phosphine complexes, with a focus on electron-rich late-transition-metal systems.4−8 While much of this work has involved rhodium and iridium, our recent investigations have shifted to cobalt, which is attractive because of its unique properties and low cost. In recent papers, we have described the synthesis, structure, and reactivity of thiapentadienyl−9 and oxapentadienyl−cobalt−phosphine1 complexes. We now report the results of a parallel study involving tert-butylazapentadienyl−cobalt−phosphine complexes. While this system shows many similarities to the closely © 2012 American Chemical Society

related oxapentadienyl system, some important differences have also emerged.

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RESULTS AND DISCUSSION Synthesis of ((1,2,3-η3)-5-tert-butylazapentadienyl)Co(PMe3)3 (1). As shown in Scheme 1, treatment of (Cl)Co(PMe3)310 with potassium tert-butylazapentadienide6,11 in tetrahydrofuran solvent leads to the production of dark red ((1,2,3,η3)-5-tert-butylazapentadienyl)Co(PMe3)3 (1) in 85% yield. The 1H NMR spectrum of 1 shows a downfield peak at δ 6.21 due to H4 on the uncoordinated carbon, C4. The signal is a doublet (J = 9.0 Hz) as a result of coupling to H3. The protons on the coordinated allyl portion of the ring all appear more upfield at δ 4.46 (H2), 3.20 (H3), 0.91 (H1), and 0.47 (H1). The tert-butyl group gives rise to a sharp singlet at δ 1.02. Received: November 15, 2011 Published: March 1, 2012 2219

dx.doi.org/10.1021/om2011326 | Organometallics 2012, 31, 2219−2230

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Scheme 1

Similarly, in the 13C{1H} NMR, C4 resonates at δ 156.9, while the allylic carbons all appear much further upfield at δ 66.9 (C2), 60.0 (C3), and ∼30 (C1). The tert-butyl group gives rise to peaks at δ 56.0 (central C) and ∼30 (Me’s). The 31P{1H} NMR spectrum at room temperature consists of a single peak at δ 1.5, indicating a fluxional process that exchanges the chemical environments of the three phosphine ligands. The simplest picture of this dynamic process is a rapid rotation of the azapentadienyl group on the allyl−cobalt axis.12 Upon cooling to −70 °C in acetone, the signal broadens slightly but does not split into separate peaks. The infrared spectrum shows an intense sharp peak at 1594 cm−1, assignable to the azapentadienyl CN stretch. The X-ray crystal structure of 1 is presented in Figure 1; selected bond distances are reported in the caption. The

As shown in Scheme 1, a likely (but unobserved) intermediate on the path to 1 is the syn isomer A. Given that the potassium tert-butylazapentadienide reagent is Wshaped,13 the initial cobalt product most likely retains that geometry before quickly isomerizing to the observed anti isomer. The reason that the anti isomer is preferred may be the contribution of a second resonance structure (II, Chart 1), Chart 1

featuring an η4-azapentadienyl ligand. Consistent with this bonding picture is the relatively short bond length of C3−C4 (1.449(7) Å). Note that in this resonance structure a formal positive charge is localized at cobalt, where it can be stabilized by the electron-donating phosphine ligands, while the formal negative charge resides on the electronegative nitrogen atom. A likely mechanism for the conversion of syn isomer A to the observed anti product 1 is shown in Scheme 2.14 The complex first isomerizes to 16e 3-η1-azapentadienyl−cobalt intermediate B. Rotation around the single bond C2−C3, followed by recoordination to the η3-allyl moiety, produces 1. Synthesis of ((1,2,3-η3)-5-tert-butylazapentadienyl)Co(PMe3)2[P(OMe)3] (2). When trimethyl phosphite is added to a solution of 1 in THF, ((1,2,3-η 3)-5-tertbutylazapentadienyl)Co(PMe3)2[P(OMe)3] (2) is produced in 74% yield. As shown in Scheme 3, this reaction most likely proceeds via an associative mechanism involving an η1azapentadienyl−Co intermediate (D).15 The 1H and 13C{1H} NMR spectra of 2 are very similar to those of 1, the main difference being the appearance of peaks due to P(OMe)3 at δ 3.47 in the 1H NMR and δ 50.1 in the 13C{1H} NMR. In the 31 1 P{ H} NMR, the P(OMe)3 signal appears at δ 169.6, while the PMe3 ligands resonate at δ 1.8. In the infrared spectrum of 2, the CN stretch comes at 1602 cm−1, as compared to 1594 cm−1 for 1. This shift to higher energy may reflect a reduced contribution from the η4-azapentadienyl resonance structure (II, Chart 1) due to the less electron-rich environment at cobalt. The X-ray crystal structure of 2 is presented in Figure 2. As in 1, the azapentadienyl ligand is anti, exhibiting cis geometry

Figure 1. Molecular structure of 1, using thermal ellipsoids at the 30% probability level. The methyl H’s on the PMe3 ligands and on the tertbutyl group are not shown. Selected bond distances (Å): Co1−P1, 2.1659(12); Co1−P2, 2.1717(13); Co1−P3, 2.1609(13); Co1−C1, 2.039(5); Co1−C2, 1.946(4); Co1−C3, 2.117(5); C1−C2, 1.392(8); C2−C3, 1.426(7); C3−C4, 1.449(7); C4−N1, 1.288(6); N1−C5, 1.466(7).

azapentadienyl ligand in 1 is anti, meaning that there is cis geometry around the C2−C3 bond. However, the bulky tertbutylimine group is rotated substantially out of the allyl plane, resulting in a C1−C2−C3−C4 torsion angle of 37.0(7)°. This rotation causes C4 to lie 0.721 Å out of the C1−C2−C3 plane and 2.982 Å from cobalt. Trans geometries are observed around bonds C3−C4 and C4−N1, as reflected in torsion angles of 164.4(5) and 176.6(5)° for C2−C3−C4−N1 and C3−C4−N1−C5, respectively. The bonding within the allyl portion of the azapentadienyl ligand is delocalized, as expected, with C1−C2 = 1.392(8) Å and C2−C3 = 1.426(7) Å. 2220


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Scheme 2

Scheme 3

through a solution of 1 in THF, the color changes from dark red to orange and ((1,2,3-η3)-5-tert-butylazapentadienyl)Co(PMe3)2(CO) (3) is produced in 55% yield (see Scheme 4, top line). The 1H NMR of 3 shows the presence of an equilibrium mixture of two isomers, 3a,b, in an approximate 10:90 ratio. The spectrum of the principal isomer, 3b, is qualitatively different from those for 1 and 2, suggesting a different ligand geometry. In particular, the H4 signal is shifted substantially downfield to δ 7.43 (from δ 6.21 in 1), while H3 is shifted upfield to δ 1.88 (from δ 3.20 in 1). The H4 signal is split into a doublet by H3 (J = 8.7 Hz). The minor isomer 3a shows a small doublet at δ 6.34 (J = 9.0 Hz) for H4 and a small multiplet at δ 3.70 for H3. These peaks are very similar to those observed for 1 and 2, suggesting that 3a has the same ligand geometry as 1 and 2. Signals for both isomers are also observed in the 31P{1H} NMR spectrum. The X-ray crystal structure of 3b is presented in Figure 3. As anticipated, the azapentadienyl ligand is syn, meaning that there is trans geometry around bond C2−C3. Trans geometries are also observed around C3−C4 and C4−N1, leading to a zigzagshaped ligand. The torsion angle C1−C2−C3−C4 is 170.87(14)°, and C4 is displaced 0.198 Å from the C1−C2− C3 plane. Torsion angles C2−C3−C4−N1 and C3−C4−N1− C5 are 163.12(14) and 179.12(13)°, respectively. The small CO ligand lies under the bulkier tert-butylimine side of the azapentadienyl ligand, while the PMe3’s are situated under the open “mouth” and the less congested side of the azapentadienyl.17 The shift to the syn isomer as the dominant isomeric form for 3 probably results from the destabilization of the η4-azapentadienyl resonance structure (see II, Chart 1) by the presence of the electron-withdrawing CO ligand. With the destabilization of this resonance structure, the azapentadienyl ligand rearranges to the less sterically encumbered syn isomer. Consistent with the absence of the η4 resonance structure is the higher CN stretch in 3, 1628 cm−1, as compared to 1594 and 1602 cm−1 for 1 and 2, respectively. However, it should be

Figure 2. Molecular structure of 2, using thermal ellipsoids at the 50% probability level. The methyl H’s on the PMe3 and P(OMe)3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co1−P1, 2.0961(3); Co1−P2, 2.1904(3); Co1−P3, 2.1652(3); Co1−C1, 2.0487(10); Co1−C2, 1.9573(10); Co1−C3, 2.1065(10); C1−C2, 1.4177(17); C2−C3, 1.4253(14); C3−C4, 1.4546(13); C4− N1, 1.2765(13); N1−C5, 1.4730(13).

around C2−C3. The C1−C2−C3−C4 torsion angle is 38.91(14)°, causing C4 to be displaced by 0.763 Å from the C1−C2−C3 plane. Trans geometries are observed around C3− C4 and C4−N1, as reflected in torsion angles of 160.95(10) and 178.18(9)° for C2−C3−C4−N1 and C3−C4−N1−C5, respectively. The smaller P(OMe)3 ligand (cone angle 107°)16 is situated under the bulkier tert-butylimine side of the azapentadienyl ligand, while the PMe3 ligands (cone angle 118°)16 lie under the open “mouth” and less congested side of the azapentadienyl.17 Synthesis of ((1,2,3-η3)-5-tert-butylazapentadienyl)Co(PMe3)2(CO) (3). When carbon monoxide is bubbled 2221


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Scheme 4

reaction is an η1-azapentadienyl−cobalt species (F, Scheme 4), which loses a CO to produce 3. Reaction of Compound 1 with Triflic Acid. Compound 1 possesses two potential sites for electrophilic addition: the azapentadienyl nitrogen atom and the electron-rich Co(I) center. In order to probe the relative reactivities of these sites, we treated 1 with 1 equiv of triflic acid in diethyl ether. As shown in Scheme 5, the acid attacks exclusively at the Scheme 5

Figure 3. Molecular structure of 3b, using thermal ellipsoids at the 50% probability level. The methyl H’s on the PMe3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co1−P1, 2.2009(4); Co1−P2, 2.1531(4); Co1−C9, 1.7442(15); Co1−C1, 2.0696(17); Co1−C2, 1.9528(15); Co1−C3, 2.0733(14); C1−C2, 1.405(2); C2−C3, 1.443(2); C3−C4, 1.449(2); C4−N1, 1.271(2); N1−C5, 1.473(2); C9−O1, 1.1557(19).

noted that in the analogous oxapentadienyl complex ((1,2,3η3)-oxapentadienyl)Co(PMe3)2(CO), which we reported earlier,1 the oxapentadienyl ligand retains its anti bonding mode. Thus, in that case the presence of an electron-withdrawing carbonyl is not enough to promote the ligand isomerization. Hence, it seems likely that the appearance of the syn isomer results from a combination of both electronic and steric effects. A strong CO stretch is observed at 1915 cm−1. Compound 3 can also be synthesized from the alternative starting material (Cl)Co(PMe3)2(CO)2.10 As shown in Scheme 4 (bottom line), treatment of this species with potassium tertbutylazapentadienide in THF produces the equilibrium mixture of 3a and 3b in 47% yield. The likely intermediate in this

azapentadienyl nitrogen, causing immediate precipitation of purple (η4-(tert-butylamino)butadiene)Co(PMe3)3+O3SCF3− (4) in 88% yield. The 1H NMR spectrum of 4 shows a doublet at δ 5.25, due to the NH group. The observed coupling (J = 11.4 Hz) is to H4 on the neighboring carbon. Meanwhile, the peak due to H4 is shifted substantially upfield to δ 3.94 from δ 6.21 in 1, indicating that C4 is now interacting with cobalt. The remaining η4-diene protons resonate at δ 4.94 (H2), 4.90 (H3), 0.99 (H1), and −0.63 (H1). In the 13C{1H} NMR, C4 shifts upfield to δ 109.6 (from δ 156.9 in 1) while the remaining diene carbons appear at δ 73.3 (C3), 68.2 (C2), and 29.0 (C1). The 31P{1H} NMR spectrum of 4 at room temperature is a broad singlet, most likely due to facile rotation of the diene ligand about the η4-diene−cobalt axis, which 2222


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for resonance structure III (Chart 2). However, the C4−N1 bond length of 1.348(2) Å is clearly intermediate between a normal C−N double bond (1.28 Å) and single bond (1.47 Å),19 supporting the contributions of both resonance structures III and IV. In the infrared spectrum, a peak at 1578 cm−1 is observed for the CN stretch. The torsion angle C1−C2−C3−C4 is 12.71(19)°, leading to a 0.258 Å displacement of C4 out of the C1−C2−C3 plane. (Recall that C4 was displaced by 0.721 Å in the structure of 1.) Torsion angles C2−C3−C4−N1 and C3−C4−N1−C5 are 178.28(12) and 156.71(13)°, respectively. The hydrogen atom on nitrogen (HN) was located and refined. The HN−N1−C4 and HN−N1−C5 angles are 114.2(12) and 113.8(12)°, intermediate between tetrahedral (109.5°) and trigonal planar (120°). Again, this observation supports the dual contributing resonance structures drawn in Chart 2. Reaction of Compound 2 with Triflic Acid. Treatment of compound 2 with 1 equiv of triflic acid in diethyl ether produces (η 4 -(tert-butylamino)butadiene)Co(PMe 3 ) 2 [P(OMe)3]+O3SCF3− (5) in 65% yield (see Scheme 5). The 1H NMR spectrum of 5 is very similar to that of 4, strongly suggesting an analogous structure. Each 1H NMR signal in 5 is observed downfield from its position in 4, probably as a result of the reduced electron richness at the cobalt center. For example, NH in 5 resonates at δ 5.99, as compared to δ 5.25 in 4. Similarly, H4 shifts to δ 4.24 vs δ 3.94 in 4, while H3, H2, and the two H1’s each exhibit downfield shifts ranging from 0.02 to 0.45 ppm. In the 31P{1H} NMR, separate peaks for the three phosphorus-containing ligands are observed, with the P(OMe)3 signal far downfield (δ 156.7) from the PMe3 signals (δ 5.3 and 3.4). In the IR spectrum, a CN stretch is observed at 1595 cm−1. Reaction of Compound 3 with Triflic Acid. Treatment of compound 3 with 1 equiv of triflic acid in diethyl ether produces (η4-(tert-butylamino)butadiene)Co(PMe3)2(CO) (6) in 45% yield (see Scheme 5). Again, the similarity of compound 6's 1H NMR spectrum to those of 4 and 5 strongly suggests analogous structures. Compound 6 is significantly less electronrich than 4 or 5, and again this is reflected in the downfield chemical shifts of the ligand hydrogens. In particular, the NH in 6 resonates at δ 7.17 (vs δ 5.25 in 4 and δ 5.99 in 5) while H4 shifts to δ 5.10 (vs δ 3.94 in 4 and δ 4.24 in 5). Protons H3, H2, and the two H1′s also shift downfield from their positions in 4 and 5. In the 31P{1H} NMR, the PMe3’s resonate at δ 6.8 and 5.0. In the IR, a strong CO stretch appears at 1957 cm−1, while a weaker CN stretch is observed at 1598 cm−1. Compound 6 crystallizes with two independent molecules in the unit cell; these structures (molecules A and B) are presented in Figures 5 and 6, respectively. In both molecules, the η4-(tert-butylamino)butadiene ligand is anti (i.e., cis with respect to bond C2−C3). This is in contrast to the structure of precursor 3, where the principal isomer is syn. Undoubtedly, this rearrangement occurs so that C4 can interact with the cobalt center. Molecules A and B are rotamers; i.e., they exhibit different orientations of the PMe3 and CO ligands with respect to the (tert-butylamino)butadiene ligand. Projection drawings of the two rotamers are presented in Chart 3. In molecule A, the smallest ligand, CO, is situated beneath the bulky tertbutylamino side of the ligand, a structure that minimizes steric interactions. Recall that the structures of compounds 2 and 3 likewise feature the smallest ligand (P(OMe)3 or CO) situated beneath the bulky tert-butylimino moiety. This favorable steric

exchanges the chemical environments of the three phosphine ligands.18 When the temperature is lowered to −40 °C, the rotational process is slowed, and the singlet splits into three separate peaks. Further sharpening of the signals is observed as the sample is cooled to −70 °C. The X-ray crystal structure of 4 has been obtained and is presented in Figure 4. While the NMR spectra of 4 clearly

Figure 4. Molecular structure of the cation of 4, using thermal ellipsoids at the 50% probability level. The methyl H’s on the PMe3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co1−P1, 2.1896(4); Co1−P2, 2.2012(4); Co1−P3, 2.1952(4); Co1−C1, 2.0484(13); Co1−C2, 1.9797(13); Co1−C3, 2.1389(13); C1−C2, 1.427(2); C2−C3, 1.4104(19); C3−C4, 1.3972(19); C4−N1, 1.3483(19); N1−C5, 1.4815(18). The dashed line indicates a weak interaction; the Co1−C4 distance is 2.577 Å.

indicate an interaction between C4 and Co (vide supra), the Xray structure reveals a more nuanced picture, where both η4 and η3 resonance structures (III and IV, Chart 2) are significant Chart 2

contributors. The most striking feature of the structure is the very long distance between Co and C4: 2.577 Å. For comparison, the distances from cobalt to the other three diene carbons (C1−C3) fall in the range 1.9797(13)− 2.1389(13) Å. Another interesting comparison is with (η4butadienol)Co(PMe3)3+O3SCF3−, which we previously synthesized by protonating (η3-oxapentadienyl)Co(PMe3)3.1 In this structure, Co−C4 = 2.173(2) Å. Within the protonated azapentadienyl ligand in 4, the three diene C−C bonds are delocalized (ranging from 1.397(2) to 1.427(2) Å), as expected 2223


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Chart 3

11.0(11)°, causing C4 to lie 0.2411 Å out of the C1−C2−C3 plane. The second independent molecule of 6 (see Figure 6 and B, Chart 3) has the carbonyl ligand situated under the open “mouth” of the (tert-butylamino)butadiene ligand and the phosphines under the edges. Sterically, this is not as favorable as rotamer A, and this is reflected in the significantly longer Co−C4 distance (2.593 Å for Co2−C54 in B vs 2.450(6) Å for Co1−C4 in A). The Co−C4 distance in molecule B is similar to that in compound 4, where a bulky PMe3 ligand similarly resides under the tert-butylamino side of the molecule. The bond distances within the (tert-butylamino)butadiene ligand in B are very similar to those observed in A (see Figure 6 caption). The torsion angle C51−C52−C53−C54 is 13.6(10)°, leading to a 0.2877 Å displacement of C54 from the C51−C52−C53 plane. Although rotamer B is more sterically encumbered than A, it is perhaps electronically preferred because it places the electron-donating PMe3 ligands approximately trans to butadiene carbons C1 and C4, which bear a significant portion of the molecule’s positive charge. The NMR spectra of crystalline 6 do not reflect the presence of two separate rotamers, even when the sample is cooled to −70 °C. Therefore, it appears that one rotamer is strongly preferred in solution.20 Reactions of Compound 4 with P(OMe)3 and CO. In order to investigate the possibility of forming compounds 5 and 6 from simple PMe3 ligand substitution reactions, we treated compound 4 with excess P(OMe)3 and excess CO. However, as shown in Scheme 6, these reactions lead instead to the displacement of tert-butylaminobutadiene21 and the formation of Co(PMe 3 ) 2 [P(OMe) 3 )] 3 + O 3 SCF − (7) and Co(PMe3)3(CO)2+O3SCF3− (8), respectively. In the 1H NMR, compound 7 exhibits signals at δ 3.80 (P(OMe)3’s) and δ 1.40 (PMe3’s) in a 3:2 ratio. The corresponding signals in the 13C{1H} NMR come at δ 51.9 and δ 20.6. In the 31P{1H} NMR, the P(OMe)3 signal appears as a broad singlet at δ 141.6, while the PMe3 signal is a binomial quartet (JP−P = 107.8 Hz) centered at δ 22.7. The X-ray crystal structure of 7 has been obtained and is presented in Figure 7. The compound adopts a trigonal-bipyramidal coordination geometry with the bulkier PMe3 ligands occupying the axial positions and the three smaller P(OMe)3 ligands lying in the equatorial plane. As expected, the three equatorial P−Co−P angles are all close to 120°, ranging from 119.28(3)° to 120.24(3)°, while the axial P−Co−P angle is 179.65(4)°. Compound 8 has been previously reported,1,22 and the NMR spectra of our product matched the reported spectra. The unit cell of our product also matched that of the published crystal structure.

Figure 5. Molecular structure of the cation of 6, molecule A, using thermal ellipsoids at the 30% probability level. The methyl H’s on the PMe3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co1−P1, 2.185(2); Co1−P2, 2.1986(19); Co1−C9, 1.757(7); Co1−C1, 2.079(8); Co1−C2, 1.978(8); Co1−C3, 2.077(8); Co1−C4, 2.450(6); C1−C2, 1.389(14); C2−C3, 1.363(13); C3−C4, 1.463(12); C4−N1, 1.327(9); N1−C5, 1.516(12).

Figure 6. Molecular structure of the cation of 6, molecule B, using thermal ellipsoids at the 30% probability level. The methyl H’s on the PMe3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co2−P3, 2.200(2); Co2−P4, 2.1874(19); Co2−C59, 1.760(7); Co2−C51, 2.057(7); Co2−C52, 1.998(7); Co2−C53, 2.150(7); C51−C52, 1.390(15); C52−C53, 1.410(14); C53−C54, 1.423(9); C54−N2, 1.303(11); N2−C55, 1.446(9). The dashed line indicates a weak interaction; the Co2−C54 distance is 2.593 Å.

situation is reflected in the relatively short Co−C4 contact, 2.450(6) Å (as compared to a Co−C4 distance of 2.577 Å in 4). The distances from cobalt to the other three carbons in the η4-butadiene moiety are much shorter, ranging from 1.978(8) to 2.079(8) Å. As in compound 4, the N1−C4 bond distance of 1.327(9) Å is intermediate between double- and single-bond lengths and reflects contributions from both η3 and η4 resonance structures. The torsion angle C1−C2−C3−C4 is 2224


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Scheme 6

downfield region (at δ 7.36 and 6.95), attributable to the two H′s on N. In addition, H4 is shifted dramatically upfield to δ ∼1.5 from its position of δ 4.24 in 5. This shift suggests that H4 resides closer to the metal center in 9 than in 5, which in turn implies that C4 is more tightly bound to cobalt. This has been confirmed by X-ray diffraction (vide infra). All of the other H′s on the butadiene moiety are shifted downfield from their positions in 5, probably due to the greater overall charge on the complex. In the 13C{1H} NMR, C4 shifts upfield to δ 59.1 (from δ 116.4 in 5). The remaining butadiene carbons (C1− C3) shift somewhat downfield from their positions in 5. The 31 1 P{ H} NMR spectrum of 9 shows three separate peaks at δ 141.6 (P(OMe)3), 11.1 (PMe3), and 4.0 (PMe3). In the infrared spectrum, there are no peaks between 1500 and 1650 cm−1, indicating the absence of a CN stretch. The X-ray crystal structure of 9 has been obtained and is presented in Figure 8. As expected from steric considerations, the smaller P(OMe)3 ligand is situated beneath the bulky tertbutylammonium moiety, while the PMe3 ligands sit under the

Figure 7. Molecular structure of the cation in 7 using thermal ellipsoids at the 50% probability level. The methyl H’s on the PMe3 and P(OMe)3 ligands are not shown. Selected bond distances (Å): Co1−P1, 2.1350(8); Co1−P2, 2.1309(9); Co1−P3, 2.1340(8); Co1− P4, 2.2429(8); Co1−P5, 2.2471(9).

Double Protonation of Compound 2. As shown in Scheme 7, treatment of compound 2 with 2 equiv of triflic acid Scheme 7

leads to double protonation at nitrogen and formation of (η4(tert-butylammonium)butadiene)Co(PMe3)2[P(OMe)3]2+(O3SCF3−)2 (9) in 63% yield. This compound is easily distinguished from its monoprotonated relative, 5, by its yellow color and its insolubility in THF. Fortunately, 9 is soluble in acetone, which has allowed full characterization by NMR spectroscopy and X-ray crystallography. Compound 9 can also be synthesized by treating compound 5 with an additional 1 equiv of triflic acid. The 1H NMR spectra for 9 appear quite different from those for monoprotonated 5. First, there are two signals in the

Figure 8. Molecular structure of the dication of 9, using thermal ellipsoids at the 50% probability level. The methyl H’s on the PMe3 and P(OMe)3 ligands and on the tert-butyl group are not shown. Selected bond distances (Å): Co1−P1, 2.1743(5); Co1−P2, 2.2204(5); Co1−P3, 2.2608(5); Co1−C1, 2.0845(18); Co1−C2, 2.0387(19); Co1−C3, 2.0543(18); Co1−C4, 2.0994(17); C1−C2, 1.419(3); C2−C3, 1.410(3); C3−C4, 1.419(3); C4−N1, 1.478(2); N1−C5, 1.542(2). 2225


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open “mouth” and under the less congested edge of the butadiene ligand.17 Unlike the structures of 4 and 6, torsion angle C1−C2−C3C4 is 0°, and C4 is coplanar with C1−C2−C3. As a result, the Co−C4 distance is much shorter, 2.0994(17) Å, as compared to 2.577 Å for 4, 2.450(6) Å for 6, molecule A, and 2.593 Å for 6, molecule B. The C4−N1 distance in 9 is 1.478(2) Å, which is substantially longer than the C4−N1 bonds in 4 and 6 and is a typical carbon−nitrogen single bond length.19 The carbon− carbon bonds within the butadiene moiety are fully delocalized, ranging from 1.410(3) to 1.419(3) Å. The two hydrogens on nitrogen were located and refined, and the angles around nitrogen are normal for a tetrahedral sp3 center. All of this structural evidence points toward a single contributing resonance structure for 9, which is pictured in Scheme 7. Treatment of Compounds 1 and 3 with 2 equiv of Triflic Acid. Treatment of compound 1 with 2 equiv of triflic acid leads to the immediate precipitation of a yellow solid in THF.23 This yellow product appears to be the tris-PMe3 analogue of diprotonated compound 9 but unfortunately is either insoluble in or reactive toward all solvents that we have tried, preventing full characterization. In the presence of polar solvents such as acetone, methanol, DMSO, and acetonitrile, the yellow solid is deprotonated, producing a deep purple solution of monoprotonated 4. Treatment with 1 equiv of triethylamine also produces 4 cleanly. Chlorinated solvents such as methylene chloride lead to an unidentified green decomposition product. In contrast, compound 3 does not undergo double protonation. When 3 is treated with 2 equiv of triflic acid, dark red 6 is produced but no yellow solid precipitates from the THF solution. Apparently, the presence of the carbonyl ligand in 3 reduces its electron richness enough that a second protonation at nitrogen is not possible.

the oxapentadienyl ligand is limited to a single protonation. This reflects the greater basicity of nitrogen vs oxygen.

■

FINAL WORD Work on azapentadienyl−cobalt complexes is continuing in our group, with a focus on molecules containing larger ancillary ligands that can stabilize the η5-azapentadienyl bonding mode. These complexes are expected to display enhanced reactivity, possibly even catalytic activity, by shuttling among accessible η5, η3, and η1 bonding modes.

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EXPERIMENTAL SECTION

General Comments on Experimental Techniques. All manipulations were carried out under a nitrogen atmosphere, using either glovebox or double-manifold Schlenk techniques. Solvents were stored under nitrogen after being distilled from the appropriate drying agents. Deuterated NMR solvents were obtained in sealed vials and used as received. (Cl)Co(PMe3)3 and (Cl)Co(PMe3)2(CO)2 were prepared by the procedure of Klein and Karsh.10 Potassium tert-butylazapentadienide was prepared by literature procedures.6 High-purity carbon monoxide gas was obtained from Praxair and used as received. Trimethyl phosphite and triflic acid (trifluoromethanesulfonic acid) were obtained from Aldrich and used as received. NMR experiments were performed on a Varian Unity Plus-300 spectrometer (1H, 300 MHz; 13C, 75 MHz; 31P, 121 MHz), a Varian Mercury-300 spectrometer (1H, 300 MHz; 13C, 75 MHz; 31P, 121 MHz), a Varian Unity Plus-500 spectrometer (1H, 500 MHz; 13C, 125 MHz; 31P, 202 MHz), or a Varian Unity-600 spectrometer (1H, 600 MHz; 13C, 150 MHz; 31P, 242 MHz). 1H and 13C spectra were referenced to tetramethylsilane, while 31P spectra were referenced to external H3PO4. HMQC (1H-detected multiple quantum coherence), HMBC (heteronuclear multiple-bond correlation), and COSY (correlation spectroscopy) experiments aided in assigning some of the 1H and 13C peaks. In all of the NMR spectra, carbon atoms and associated hydrogens are numbered by starting at the end of the chain opposite nitrogen. Carbonyl ligands always gave rise to very weak 13C NMR signals and could not be reliably assigned. Synthesis of ((1,2,3-η 3)-5-tert-butylazapentadienyl)Co(PMe3)3 (1). Potassium tert-butylazapentadienide (0.46 g, 2.8 mmol) and (Cl)Co(PMe)3 (0.45 g, 1.4 mmol) were dissolved in 30 mL of tetrahydrofuran (THF), and the mixture was stirred at room temperature for 2 h. The resulting dark red solution was evacuated to dryness, and the residue was extracted with pentane before filtering through Celite. The extract was evacuated to dryness and then dissolved in a minimal quantity of pentane. The solution was cooled to −30 °C, producing dark red crystals of 1 overnight. Yield: 0.49 g (85%). Anal. Calcd for C17H41CoNP3: C, 49.62; H, 10.06. Found: C, 49.55; H, 9.84. 1H NMR (acetone-d6, 22 °C): δ 6.21 (d, JH−H = 9.0 Hz, 1, H4), 4.46 (m, 1, H2), 3.20 (m, 1, H3), 1.25 (br s, 27, PMe3’s), 1.02 (s, 9, tert-butyl), 0.91 (m, 1, H1), 0.47 (apparent quintet, J = 8.1 Hz, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 156.9 (s, C4), 66.9 (s, C2), 60.0 (s, C3), 56.0 (s, tert-butyl C), ∼30 (obscured, C1), ∼30 (obscured, tert-butyl Me’s), 22.5 (br m, PMe3’s). 31P{1H} NMR (acetone-d6, 22 °C): δ 1.5 (br s, PMe3’s). NOTE: This signal did not split upon cooling to −70 °C. IR (Nujol mull): 1594 cm−1 (CN). Synthesis of ((1,2,3-η 3)-5-tert-butylazapentadienyl)Co(PMe3)2[P(OMe)3] (2). Compound 1 (0.30 g, 0.73 mmol) was dissolved in 30 mL of THF and cooled to −30 °C. Cold trimethyl phosphite (0.27 g, 2.2 mmol) was then added to the solution. The mixture was slowly warmed to room temperature with stirring for 90 min. After removal of the solvent under vacuum, the residue was extracted with pentane and filtered through Celite. The extract was then evacuated to dryness and dissolved in a minimal quantity of pentane. The solution was cooled to −30 °C, producing dark red crystals of 2 overnight. Yield: 0.25 g (74%).

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COMPARISON TO THE ANALOGOUS OXAPENTADIENYL SYSTEM While the tert-butylazapentadienyl−cobalt chemistry reported here is similar to that of the related oxapentadienyl−cobalt system reported earlier by our group,1 several important differences have emerged. First, we have observed a stable synη3-tert-butylazapentadienyl bonding mode in compound 3b, while the analogous oxapentadienyl compound exhibits anti geometry. The stability of the syn bonding mode appears to result from the steric bulk of the tert-butylimine moiety, coupled with the presence of the electron-withdrawing carbonyl ligand. Second, the structures of the monoprotonated tertbutylazapentadienyl−cobalt and oxapentadienyl−cobalt complexes are quite different. In both cases, the protonation occurs at the heteroatom (nitrogen or oxygen), resulting in a η4butadiene−cobalt product. However, the products that result from protonation of the oxapentadienyl−cobalt complexes exhibit “normal” Co−C bond distances to all four butadiene carbons, while protonation of the tert-butylazapentadienyl compounds yields products with very long Co−C4 distances. A likely explanation is that the protonated imine moiety is far more stable than the protonated formyl, leading to a greater contribution of the η3 protonated imine resonance structure to the overall bonding picture. A third and final difference is that the tert-butylazapentadienyl ligand can undergo double protonation at nitrogen, while 2226


Organometallics

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at −40 °C. Further cooling led to better resolution of the three peaks. P{1H} NMR (acetone-d6, −70 °C): δ 16.3 (br s, 1, PMe3), 8.5 (br s, 1, PMe3), 2.6 (br s, 1, PMe3). IR (Nujol mull): 1578 cm−1 (CN). Synthesis of (η4-(tert-butylamino)butadiene)Co(PMe3)2[P(OMe)3]+O3SCF3− (5). Compound 2 (0.21 g, 0.46 mmol) was dissolved in 15 mL of diethyl ether. A 0.11 M solution of triflic acid in diethyl ether (4.2 mL, 0.46 mmol) was then added dropwise. The reaction mixture was swirled after each drop of triflic acid solution added, causing a dark red residue to form on the bottom of the flask. After the solution was poured off, the remaining dark red residue was washed with pentane and then extracted with THF and filtered through Celite. The extract was then evacuated to dryness, producing crude 5 as a dark red solid. When 5 was dissolved in a minimal quantity of THF and cooled to −30 °C, compound 9 (not 5) crystallized, presumably through a disproportionation reaction that simultaneously produced compound 2.24 Crude yield of 5: 0.18 g (65%). 1 H NMR (acetone-d6, 22 °C): δ 5.99 (br d, JH−H = 13.2 Hz, 1, NH), 5.16 (m, 1, H2), 4.92 (m, 1, H3), 4.24 (dd, JH−H = 13.2 Hz, 10.5 Hz, 1, H4), 3.87 (d, JH−P = 10.5 Hz, 9, P(OMe)3), 1.50 (d, JH−P = 8.1 Hz, 9, PMe3), 1.45 (d, JH−P = 8.1 Hz, 9, PMe3), 1.39 (obscured, 1, H1), 1.29 (s, 9, tert-butyl), −0.18 (m, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 116.4 (s, C4), 68.6 (s, C3), 67.3 (s, C2), 52.8 (s, tert-butyl C), 51.9 (d, JC−P = 8.3 Hz, P(OMe)3), 30.0 (obscured, C1), 27.9 (s, tertbutyl Me’s), 18.5 (ddd, JC−P = 24.9 Hz, 3.8 Hz, 3.3 Hz, PMe3), 18.3 (ddd, JC−P = 24.3 Hz, 3.8 Hz, 3.3 Hz, PMe3). 31P{1H} NMR (acetoned6, 22 °C): δ 156.7 (br s, 1, P(OMe)3), 5.3 (s, 1, PMe3), 3.4 (s, 1, PMe3). IR (Nujol mull): 1595 cm−1 (CN). S yn t h e sis of (η 4 -(tert-butylamino)butadiene)Co(PMe3)2(CO)+O3SCF3− (6). Compound 3 (0.17 g, 0.47 mmol) was dissolved in 20 mL of diethyl ether and cooled to −30 °C. A cold 1.33 M solution of triflic acid in diethyl ether (0.35 mL, 0.47 mmol) was then added dropwise, causing a dark red precipitate to form immediately. The reaction mixture was stirred for 10 min. The resulting precipitate was then filtered and washed with diethyl ether and pentane, giving a dark red powder. The powder was dissolved in an acetone/diethyl ether mixture and stored at −30 °C. Dark red crystals of 6 formed overnight: Yield: 0.11 g (45%). Anal. Calcd for C16H33CoF3NO4P2S: C, 37.43; H, 6.49. Found: C, 36.93; H, 6.56. 1H NMR (acetone-d6, 22 °C): δ 7.17 (br d, 1, NH), 5.33 (br m, 1, H2), 5.10 (complex m, 2, H3 and H4), 1.75 (m, 1, H1), 1.60 (dd, JH−P = 8.7 Hz, 2.7 Hz, 9, PMe3), 1.46 (dd, JH−P = 9.0 Hz, 3.0 Hz, 9, PMe3), 1.29 (s, 9, tert-butyl), 0.70 (m, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 121.8 (s, C4), 76.8 (s, C2), 68.9 (s, C3), 53.5 (s, tert-butyl C), 37.8 (d, JC−P = 5.6 Hz, C1), 28.2 (s, tert-butyl Me’s), 19.5 (dd, JC−P = 28.1 Hz, 3.3 Hz, PMe3), 18.2 (dd, JC−P = 25.4 Hz, 2.3 Hz, PMe3). 31P{1H} NMR (acetone-d6, 22 °C): δ 6.8 (br s, 1, PMe3), 5.0 (br s, 1, PMe3). IR (Nujol mull): 1957 cm−1 (CO), 1598 cm−1 (CN). Reaction of Compound 4 with P(OMe)3. Compound 4 (0.25 g, 0.45 mmol) was dissolved in 30 mL of THF and cooled to −30 °C. Cold (−30 °C) trimethyl phosphite (0.17 g, 1.37 mmol) was then added dropwise to the solution. The solution was slowly warmed to room temperature with stirring for 1 h. The resulting orange solution was evacuated to dryness, and the residue was washed twice with pentane. It was then dissolved in a minimal amount of THF and cooled to −30 °C. Orange crystals of Co(PMe 3 ) 2 [P(OMe)3]3+O3SCF3− (7) formed overnight. Yield: 0.21 g (64%). Anal. Calcd for C16H45CoF3O12P5S: C, 26.24; H, 6.21. Found: C, 27.09; H, 6.23. 1H NMR (acetone-d6, 22 °C): δ 3.80 (m, 27, P(OMe)3’s), 1.40 (m, 18, PMe3’s). 13C{1H} NMR (acetone-d6, 22 °C): δ 51.9 (d, JC−P = 3.9 Hz, P(OMe)3’s), 20.6 (complex m, PMe3’s). 31 1 P{ H} NMR (acetone-d6, 22 °C): δ 141.6 (br s, 3, P(OMe)3’s), 22.7 (q, JP−P = 107.8 Hz, 2, PMe3’s). Reaction of Compound 4 with CO. Compound 4 (0.20 g, 0.36 mmol) was dissolved in 30 mL of THF. Carbon monoxide was then bubbled through the solution for 1 h. After removal of the solvent under vacuum, the residue was washed twice with pentane. It was then dissolved in a THF/diethyl ether mixture and stored at −30 °C.

Anal. Calcd for C17H41CoNO3P3: C, 44.44; H, 9.01. Found: C, 43.85; H, 8.61. 1H NMR (acetone-d6, 22 °C): δ 6.30 (d, JH−H = 9.0, 1, H4), 4.70 (br s, 1, H2), 3.47 (d, JH−P = 10.5 Hz, 9, P(OMe)3), 3.32 (br s, 1, H3), 1.43 (br s, 1, H1), 1.29−1.23 (2 doublets, 18, PMe3’s), 1.02 (s, 9, tert-butyl), 0.90 (br t, J = 6.6 Hz, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 157.5 (s, C4), 67.3 (s, C2), 58.3 (s, C3), 55.6 (s, tert-butyl C), 50.1 (d, JC−P = 3.8 Hz, P(OMe)3), 31.0 (br s, C1), 29.5 (s, tert-butyl Me’s), 21.3 (dt, JC−P = 18.8 Hz, 5.0 Hz, PMe3), 21.1 (dt, JC−P = 19.3 Hz, 5.0 Hz, PMe3). 31P{1H} NMR (acetone-d6, 22 °C): δ 169.6 (br s, 1, P(OMe)3), 1.8 (br s, 2, PMe3’s). IR (Nujol mull): 1602 cm−1 (CN). Synthesis of ((1,2,3-η 3 )-5-tert-butylazapentadienyl)Co(PMe3)2(CO) (3). Compound 1 (0.35 g, 0.85 mmol) was dissolved in 30 mL of THF. Carbon monoxide was then bubbled through the solution for 1 h, and the solution changed from dark red to orange. The solvent was removed under vacuum, and the residue was extracted with pentane. After filtration through Celite, the solution was evacuated to dryness. The orange residue was redissolved in a pentane/diethyl ether mixture and stored at −30 °C, causing orange crystals of 3b (the syn isomer) to form after a few days. Yield: 0.17 g (55%). Alternative Synthesis of 3. Potassium tert-butylazapentadienide (0.36 g, 2.2 mmol) and (Cl)Co(CO)2(PMe3)2 (0.33 g, 1.1 mmol) were dissolved in 30 mL of THF, and the mixture was stirred at room temperature for 3 h. After removal of the solvent under vacuum, the residue was extracted with pentane and filtered through Celite. The extract was evacuated to dryness and dissolved in a minimal quantity of pentane. The solution was cooled to −30 °C, producing orange crystals of 3b (the syn isomer) after a few days. Yield: 0.19 g (47%). Anal. Calcd for C15H32CoNOP2: C, 49.58; H, 8.89. Found: C, 48.89; H, 8.46. Note: in acetone-d6, 3 established an equilibrium mixture of 3b (syn) and 3a (anti) in a ∼90:10 ratio. NMR spectra for 3b are as follows. 1H NMR (acetone-d6, 22 °C): δ 7.43 (d, JH−H = 8.7 Hz, 1, H4), 4.80 (m, 1, H2), 1.88 (m, 1, H3), 1.72 (ddd, J = 15.9 Hz, 5.7 Hz, 2.4 Hz, 1, H1), 1.31 (d, JH−P = 8.4 Hz, 9, PMe3), 1.28 (d, JH−P = 6.0 Hz, 9, PMe3), 1.09 (s, 9, tert-butyl), 1.00 (m, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 162.6 (s, C4), 73.2 (s, C2), 58.0 (s, C3), 55.6 (s, tert-butyl C), 41.2 (s, C1), 29.5 (s, tertbutyl Me’s), 20.2 (dd, JC−P = 25.6 Hz, 6.1 Hz, PMe3), 18.9 (dd, JC−P = 18.2 Hz, 2.3 Hz, PMe3). 31P{1H} NMR (acetone-d6, −70 °C): δ 22.7 (br s, 1, PMe3), −4.9 (br s, 1, PMe3). NMR spectra for 3a are as follows. 1H NMR (acetone-d6, 22 °C): δ 6.34 (d, JH−H = 9.0 Hz, 1, H4), 4.79 (m, 1, H2), 3.70 (m, 1, H3), 1.85 (m, 1, H1), 1.44 (m, 1, H1), 1.32 (d, JH−P = 8.7 Hz, 9, PMe3), 1.28 (d, JH−P = 6.0 Hz, 9, PMe3), 1.09 (s, 9, tert-butyl). 13C{1H} NMR (acetone-d6, 22 °C): δ 155.1 (s, C4), 72.8 (s, C2), 65.8 (s, C3), 55.5 (s, tert-butyl C), 34.8 (s, C1), 29.4 (s, tert-butyl Me’s), 20.4 (dd, JC−P = 21.5 Hz, 3.9 Hz, PMe3), 19.9 (dd, JC−P = 22.6 Hz, 4.0 Hz, PMe3). 31 1 P{ H} NMR (acetone-d6, −70 °C): δ 8.1 (br s, 1, PMe3), 3.9 (br s, 1, PMe3). IR (Nujol mull): 1915 cm−1 (CO), 1628 cm−1 (CN). Synthe sis of (η 4 -(tert- but yl am ino)butad iene )Co(PMe3)3+O3SCF3− (4). Compound 1 (0.30 g, 0.73 mmol) was dissolved in 20 mL of diethyl ether and cooled to −30 °C. A cold 1.33 M solution of triflic acid in diethyl ether (0.55 mL, 0.73 mmol) was then added dropwise, causing a purple precipitate to form immediately. The reaction mixture was stirred for 10 min before the resulting precipitate was filtered and washed with diethyl ether and pentane, giving a purple powder. The powder was dissolved in a small quantity of acetone and cooled to −30 °C. Purple crystals of 4 formed overnight. Yield: 0.36 g (88%). Anal. Calcd for C18H42CoF3NO3P3S: C, 38.50; H, 7.55. Found: C, 38.46; H, 7.68. 1H NMR (acetone-d6, 22 °C): δ 5.25 (br d, JH−H = 11.4 Hz, 1, NH), 4.94−4.90 (complex m, 2, H2 and H3), 3.94 (dd, JH−H = 11.4 Hz, 10.5 Hz, 1, H4), 1.47 (d, JH−P = 7.2 Hz, 27, PMe3’s), 1.22 (s, 9, tert-butyl), 0.99 (br s, 1, H1), −0.63 (br m, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 109.6 (s, C4), 73.3 (s, C3), 68.2 (s, C2), 53.3 (s, tert-butyl C), 29.0 (s, C1), 28.7 (s, tert-butyl Me’s), 19.4 (d, JC−P = 23.2 Hz, PMe3’s). 31P{1H} NMR (acetone-d6, 22 °C): δ 3.6 (v br s, PMe3’s). Upon cooling, this peak broadened and split into three peaks

31

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Table 1. X-ray Diffraction Structure Summary formula fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z cryst dimens, mm calcd density, g/cm3 radiation: λ, Å temp, K θ range, deg index ranges h k l total decay no. of data collected no. of unique data Mo Kα linear abs coeff, mm−1 abs cor applied data to param ratio final R indices (obsd data)a R1 wR2 R indices (all data) R1 wR2 goodness of fit largest diff peak/hole, e Å−3 formula fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z cryst dimens, mm calcd density, g/cm3 radiation: λ, Å temp, K θ range, deg index ranges h k l total decay no. of data collected no. of unique data

1

2

3b

C17H41CoNP3 411.35 monoclinic P21/c 12.9502(5) 11.6282(4) 15.9231(6) 90 105.476(2) 90 2310.88(15) 4 0.23 × 0.15 × 0.12 1.182 0.710 73 253(2) 2.20−26.02

C17H41CoNO3P3 459.35 monoclinic P21/c 8.9084(6) 13.4870(10) 20.1294(14) 90 92.407(3) 90 2416.4(3) 4 0.33 × 0.31 × 0.14 1.263 0.710 73 100(2) 2.03−34.17

C15H32CoNOP2 363.29 monoclinic P21/n 8.1849(7) 9.6528(8) 25.202(2) 90 95.097(4) 90 1983.3(3) 4 0.27 × 0.23 × 0.15 1.217 0.710 73 100(2) 2.26−30.69

−15 to +15 −14 to +14 −19 to +19 none obsd 65 107 4518 0.949 semiempirical 21.41



0.0617 0.1683

0.0267 0.0661

0.0291 0.0701

0.0816 0.1845 1.055 1.265/−0.407

0.0374 0.0717 1.045 0.816/−0.411

0.0381 0.0751 1.022 0.787/−0.598 9·(acetone)

4

6

7·THF

C18H42CoF3NO3P3S 561.43 monoclinic P21/c 16.8708(16) 10.7610(10) 16.1695(15) 90 109.678(5) 90 2764.1(4) 4 0.16 × 0.13 × 0.11 1.349 0.710 73 100(2) 2.29−27.65

C16H33CoF3NO4P2S 513.36 orthorhombic Pca21 18.8272(15) 16.3686(13) 15.3104(13) 90 90 90 4718.3(7) 8 0.26 × 0.26 × 0.22 1.445 0.710 73 100(2) 1.65−25.00

C20H53CoF3O13P5S 804.46 monoclinic P21/n 10.6746(7) 19.3537(13) 17.1144(11) 90 90.133(4) 90 3535.7(4) 4 0.37 × 0.23 × 0.18 1.511 0.710 73 100(2) 1.59−26.48

C22H49CoF6NO10P3S2 817.58 monoclinic P21/n 15.2319(16) 11.8561(12) 20.623(2) 90 97.346(5) 90 3693.7(7) 4 0.30 × 0.26 × 0.10 1.470 0.710 73 100(2) 1.78−27.49

−22 to +21 −13 to +13 −21 to +21 none obsd 122 412 6396

−22 to +22 −19 to +19 −18 to +18 none obsd 122 090 8306

−13 to +13 −24 to +24 −21 to +18 none obsd 74 445 7282

−19 to +19 −15 to +15 −26 to +26 none obsd 80 206 8449

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Table 1. continued 4 Mo Kα linear abs coeff, mm−1 abs cor applied data to param ratio final R indices (obsd data)a R1 wR2 R indices (all data) R1 wR2 goodness of fit largest diff peak/hole, e Å−3 a

6

7·THF

9·(acetone)

0.908 semiempirical 22.29




0.0232 0.0515

0.0616 0.1640

0.0387 0.0853

0.0300 0.0631

0.0324 0.0558 1.054 0.393, −0.321

0.0666 0.1705 1.029 1.149, −0.582

0.0577 0.0938 1.107 0.492, −0.501

0.0425 0.0697 1.045 0.412, −0.381

I > 2σ(I).

Orange crystals of Co(PMe3)3(CO)2+O3SCF3− (8) formed overnight. Yield: 0.13 g (72%). The NMR spectra and unit cell parameters for this previously reported compound1 matched those of an authentic sample. Synthesis of (η 4 -(tert-butylammonium)butadiene)Co(PMe3)2[P(OMe)3]2+(O3SCF3−)2 (9). Compound 2 (0.20 g, 0.44 mmol) was dissolved in 20 mL of THF and cooled to −30 °C. A cold 1.33 M solution of triflic acid in diethyl ether (0.66 mL, 0.88 mmol) was then added dropwise. A yellow precipitate formed with stirring for 45 min. The resulting precipitate was collected by filtration and washed with diethyl ether and pentane, giving a yellow powder. The powder was dissolved in a minimal quantity of acetone and stored at −30 °C. Yellow crystals of 9 formed overnight. Yield: 0.21 g (63%). Alternative Synthesis of 9. Compound 5 (0.15 g, 0.25 mmol) was dissolved in 15 mL of THF and cooled to −30 °C. A cold 1.33 M solution of triflic acid in diethyl ether (0.18 mL, 0.24 mmol) was then added dropwise. A yellow precipitate formed with stirring for 45 min. The resulting precipitate was collected by filtration and washed with diethyl ether and pentane, giving a yellow powder. The powder was dissolved in a minimal quantity of actone and stored at −30 °C. Yellow crystals of 9 formed overnight. Yield: 0.096 g (54%). Anal. Calcd for C19H43CoF6NO9P3S2: C, 30.04; H, 5.72. Found: C, 29.61; H, 5.63. 1H NMR (acetone-d6, 22 °C): δ 7.36 (br s, 1, NH), 6.95 (br s, 1, NH) 6.35 (br q, J = 5.1 Hz, 1, H3), 5.77 (br s, 1, H2), 3.99 (d, JH−P = 10.8 Hz, 9, P(OMe)3), 2.30 (m, 1, H1), 1.86 (d, JH−P = 9.3 Hz, 9, PMe3), 1.53 (s, 9, tert-butyl), 1.51 (d, JH−P = 9.9 Hz, 9, PMe3), ∼1.5 (obscured, H4), 0.49 (m, 1, H1). 13C{1H} NMR (acetone-d6, 22 °C): δ 84.0 (s, C2), 82.4 (s, C3), 59.1 (br s, C4), 53.8 (d, JC−P = 8.8 Hz, P(OMe)3), ∼53.8 (obscured, tert-butyl C), 46.1 (br s, C1), 24.0 (s, tert-butyl Me’s), 19.4 (d, JC−P = 29.2 Hz, PMe3), 17.7 (d, JC−P = 29.8 Hz, PMe3). 31P{1H} NMR (acetone-d6, −70 °C): δ 141.6 (d, JP−P = 109.1 Hz, 1, P(OMe)3), 11.1 (d, JP−P = 109.1 Hz, 1, PMe3), 4.0 (s, 1, PMe3). IR (Nujol mull): no peaks observed between 1500 and 2000 cm−1. Reaction of Compound 1 with 2 Equiv of Triflic Acid. Compound 1 (0.30 g, 0.73 mmol) was dissolved in 20 mL of THF and cooled to −30 °C. A cold 1.33 M solution of triflic acid in diethyl ether (1.1 mL, 1.5 mmol) was then added dropwise, causing a yellow precipitate to form immediately. The reaction mixture was stirred for 10 min before the resulting precipitate was collected by filtration and washed with diethyl ether and pentane, giving a yellow powder. The yellow powder was similar in appearance to 9 and was most likely (η4(tert-butylammonium)butadiene)Co(PMe3)32+(O3SCF3−)2, but its insolubility and reactivity toward polar solvents prevented full characterization. Yield: 0.48 g (92%). Note: this compound could also be synthesized by treating compound 4 with 1 equiv of triflic acid. Yield: 88%. X-ray Diffraction Studies. Crystals of X-ray diffraction quality were obtained for compounds 1, 2, 3b, 4, 6, 7, and 9. In all cases crystals of appropriate dimensions were mounted on MiTeGen microloops25 in random orientations. Preliminary examination and data collection were performed using a Bruker Kappa Apex II charge coupled device (CCD) detector system single-crystal X-ray diffrac-

tometer equipped with an Oxford Cryostream LT device. All data were collected using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) from a fine-focus sealed-tube X-ray source. Preliminary unit cell constants were determined with a set of 36 narrow frame scans. Typical data sets consisted of combinations of ω and ϕ scan frames with a typical scan width of 0.5° and counting time of 15−30 s/frame at a crystal to detector distance of 4.0 cm. The collected frames were integrated using an orientation matrix determined from the narrow frame scans. Apex II and SAINT software packages26 were used for data collection and data integration. Analysis of the integrated data did not show any decay. Final cell constants were determined by global refinement of reflections from the complete data set. Collected data were corrected for systematic errors using SADABS or TWINABS.26 Crystal data and intensity data collection parameters are given in Table 1. Structure solution and refinement were carried out using the SHELXTL-PLUS software package.27 The structures were solved by direct methods and refined with full-matrix least-squares refinement by minimizing ∑w(Fo2 − Fc2)2. All non-hydrogen atoms were refined anisotropically to convergence. Specific experimental details for individual structures are given below. For compounds 1, 2, 3b, 6, and 7, all H atoms were added in calculated positions and were refined using appropriate riding models (AFIX m3). For 4, the H atom on N (H1) was located and refined freely. All other H atoms were added in calculated positions and were refined using appropriate riding models. For 9, the H’s on the butadiene moiety (H1A, H1B, H2, H3, and H4) and the H’s on N (H1C and H1D) were located and refined freely. All other H atoms were added in calculated positions and were refined using appropriate riding models. In 3b, the methyl groups of the tert-butyl group exhibited a 2-fold rotational disorder, which was successfully modeled. The rotamers were present in a 59:41 ratio. In 6, both trifluoromethanesulfonate anions exhibited rotational disorder, which was successfully modeled. In 7, the THF solvent molecule exhibited a 2-fold disorder, which was successfully modeled. The two orientations were present in a 69.5:30.5 ratio.

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ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF files giving structure determination summaries, final atomic coordinates, thermal parameters, bond lengths, bond angles, and torsion angles for compounds 1, 2, 3b, 4, 6, 7, and 9. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 2229


Organometallics

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Article

4.38 (dd, JH−H = 10.5 Hz, 2.1 Hz, 1, H1syn), 1.21 (s, 9, tert-butyl). The NH signal was obscured. (22) For the synthesis of the analogous BPh4− salt, see: Attali, S.; Poilblanc, R. Inorg. Chem. Acta 1972, 6, 475−479. (23) The same product is obtained upon treatment of 4 with an additional 1 equiv of triflic acid. (24) Consistent with this idea was the observation that compounds 2 and 9 reacted rapidly with each other in acetone-d6 to produce a dark red solution of 5. (25) MiTeGen, LLC, PO Box 3867, Ithaca, NY, 14852. (26) Apex II and SAINT; Bruker Analytical X-Ray, Madison, WI, 2008. (27) Bruker-SHELXTL: Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.

ACKNOWLEDGMENTS The regional X-ray Facility at the University of MissouriSt. Louis was funded in part by the National Science Foundation’s MRI Program (No. CHE-0420497).

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REFERENCES

(1) Pentadienyl−Metal−Phosphine Chemistry. 38. For Part 37, see: Bleeke, J. R.; Lutes, B. L.; Lipschutz, M.; Sakellariou-Thompson, D.; Lee, J. S.; Rath, N. P. Organometallics 2010, 29, 5057−5067. (2) In principle, the heteroatom could be located anywhere along the five-atom chain. However, for the vast majority of heteropentadienyl− metal complexes, the heteroatom resides in the terminal position, and our work has focused exclusively on “terminal” heteropentadienyl ligands. (3) For recent reviews of heteropentadienyl−metal chemistry, see: (a) Bleeke, J. R. Organometallics 2005, 24, 5190−5207. (b) PazSandoval, M. A.; Rangel-Salas, I. I. Coord. Chem. Rev. 2006, 250, 1071−1106. (4) For oxapentadienyl−metal−phosphine complexes, see: (a) Bleeke, J. R.; Haile, T.; New, P. R.; Chiang, M. Y. Organometallics 1993, 12, 517−528. (b) Bleeke, J. R.; Donnay, E.; Rath, N. P. Organometallics 2002, 21, 4099−4112. (5) For thiapentadienyl−metal−phosphine complexes, see: (a) Bleeke, J. R.; Ortwerth, M. F.; Rohde, A. M. Organometallics 1995, 14, 2813−2826. (b) Bleeke, J. R.; Shokeen, M.; Wise, E. S.; Rath, N. P. Organometallics 2006, 25, 2486−2500. (6) For azapentadienyl−metal−phosphine complexes, see: Bleeke, J. R.; Luaders, S. T.; Robinson, K. D. Organometallics 1994, 13, 1592− 1600. (7) For phosphapentadienyl−metal−phosphine complexes, see: Bleeke, J. R.; Rohde, A. M.; Robinson, K. D. Organometallics 1995, 14, 1674−1680. (8) For silapentadienyl−metal−phosphine complexes, see: Bleeke, J. R.; Thananatthanachon, T.; Rath, N. P. Organometallics 2008, 27, 2436−2446. (9) Bleeke, J. R.; Lutes, B. L.; Rath, N. P. Organometallics 2009, 28, 4577−4583. (10) Klein, H.-F.; Karsch, H. H. Inorg. Chem. 1975, 14, 473−477. (11) For synthesis of the analogous Li salt, see: Wolf, G.; Wurthwein, E.-U. Chem. Ber. 1991, 124, 889−896. (12) Alternatively, a Berry pseudorotation may be responsible, but the small “bite angle” of the allyl moiety makes this pathway more strained and, in our view, less likely. See: Albright, T. A.; Hofmann, P.; Hoffmann, R. J. Am. Chem. Soc. 1977, 99, 7546−7557. (13) The E,E (W-shaped) geometry of the lithium salt was established by Wurthwein.11 The NMR spectra of the potassium salt are identical with those of the lithium salt. (14) Similar η3 to η1 isomerizations are common in allyl−metal chemistry: Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; pp 175−181. (15) By “associative,” we simply mean that P(OMe)3 adds before PMe3 dissociates. (16) Tolman, C. A. Chem. Rev. 1977, 77, 313−348. (17) This ligand arrangement may result from crystal packing forces rather than from a minimization of intramolecular steric interactions. (18) As with compound 1 (vide supra), the dynamic behavior could be due to a Berry pseudorotation, but this is less likely because of the small “bite angle” of the diene moiety. (19) Huheey, J. E. Inorganic Chemistry: Principles of Structure and Reactivity; Harper and Row: New York, 1972; pp 691−702. (20) It is possible that the rotamers are rapidly interconverting, even at −70 °C, but this seems unlikely given that rotational exchange in 4 is arrested at low temperature. (21) The tert-butylaminobutadiene was identified by 1H NMR. 1H NMR (acetone-d6, 22 °C): δ 6.41 (apparent t, JH−H = 13.2 Hz, 1, H4), 6.24 (ddd, JH−H = 17.1 Hz, 11.4 Hz, 10.5 Hz, 1, H2), 5.27 (dd, JH−H = 13.2 Hz, 11.4 Hz, 1, H3), 4.58 (dd, JH−H = 17.1 Hz, 2.1 Hz,1, H1anti), 2230


Synthesis, Structure, Spectroscopy, and Reactivity of Azapentadienyl

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