Article pubs.acs.org/Organometallics
Allenylphosphonium Complexes of Rhodium and Iridium Ian A. Cade, Annie L. Colebatch, Anthony F. Hill,* and Anthony C. Willis Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia S Supporting Information *
ABSTRACT: The propargylic salt [Ph3PCH2CCH]PF6 ([1]PF6) undergoes a facile rearrangement to the allenyl isomer [Ph3PCHC CH2]PF6 ([2]PF6) upon coordination to rhodium and iridium. The η2allenylphosphonium salts [IrX(η2-CH2CCHPPh3)(CO)(PPh3)2]PF6 (X = Cl, [4]PF6; X = Br, [5]PF6) and [RhCl(η2-CH2CCHPPh3)(PPh3)2]PF6 ([7]PF6) have been synthesized and characterized, including X-ray crystallographic analyses. In the case of rhodium, it was found that [7]PF6 was also formed by the reaction of the α-alkynylphosphonium salt [Ph3PCCCH3]PF6 ([3]PF6) with [RhCl(PPh3)3]. The complex [RhCl(CO)(PPh3)2] was found to catalyze the isomerization of [1]+ to [2]+.
■
(PPh3)2]PF6, and [Pt{η2-C(CH2)CHPPh3}(PPh3)2]PF64 (Scheme 1). In the case of [Pt(C2H4)(PPh3)2], initial
INTRODUCTION Prop-1-ynes bearing heteroatom substituents (X) in the 3position, HCCCH2X, are thermodynamically predisposed toward prototropic rearrangements that bring the heteroatom first into conjugation with an allenyl π system (H2CC CHX) and ultimately a CC triple bond (H3CCCX). Kinetically, such rearrangements are typically acid or base catalyzed, reflecting the acidity of hydrogen atoms located between the alkyne and heteroatom. In principle, given that such rearrangements are exergonic, one might expect that transition-metal catalysts exist that would also be capable of mediating such processes. Propargylic rearrangements of p r o p a r g y l t r i p hen y l p h o s p h o ni u m br o m i de ([1] Br; [Ph3PCH2CCH]Br) were first reported by Schweizer in 1977 (Chart 1).1 Treatment of [1]Br with base gave the
Scheme 1. Metal-Mediated Rearrangements of [1]+ 3,4
Chart 1. Isomeric Structures: (a) Propargyl, (b) Allenyl, and (c) α-Alkynyl
formation of [Pt(η2-CH2CCHPPh3)(PPh3)2]PF6 is observed. On heating to 50 °C an equilibrium is established between this species and [Pt{η2-C(CH2)CHPPh3}(PPh3)2]PF6, in which the allene is coordinated via the internal double bond. We have attributed the lack of isomerism for the ruthenium system as arising from greater steric congestion about the five-coordinate ruthenium in comparison to threecoordinate platinum. To further explore the factors governing such isomerisms, we have now considered group 9 metals, intermediate between ruthenium and platinum. Previously reported mononuclear group 9 η2-allenylphosphonium complexes are limited to [Rh(acac){η2-CH(PR3)CCPh2}(PR3)]BF4 (R = Cy, iPr), reported by Esteruelas and Oro.5a The allenylphosphonium ligand in these complexes arises from the coupling of phosphine and σ-allenyl ligands, the latter being the product
allenylphosphonium salt [Ph3PCHCCH2]Br ([2]Br), while heating [1]Br in the presence of phenol gave the αalkynylphosphonium salt [Ph3PCCCH3]Br ([3]Br).1 More recently, Indizhikyan reported solvent-dependent propargylic rearrangements of [Ph3PCH2CCR]Br (R = H, Ph) to form the allenyl or α-alkynyl salts.2 We have shown that such propargylic rearrangements can be effected by coordination to a metal, e.g., in the reactions of [Ph3PCH2CCH]+ ([1]+) with either [Ru(CO)2(PPh3)3] or [Pt(C2H4)(PPh3)2], whereby coordination to ruthenium(0) or platinum(0) is accompanied by an isomerization of the ligand resulting in the allenylphosphonium complexes [Ru(η2-CH2 CCHPPh3)(CO)2(PPh3)2]Br,3 [Pt(η2-CH2CCHPPh3)© 2014 American Chemical Society
Received: April 29, 2014 Published: June 10, 2014 3198
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
Organometallics
Article
chemically equivalent trans-Ir(PPh3)2 ligands. In the 1H NMR spectrum the CH moiety appears as a doublet (2JPH = 26.0 Hz) at 6.17 ppm, while the magnetically inequivalent methylene protons give rise to a resonance at 0.23 ppm appearing as a triplet of doublet of doublets. This methylene resonance is perhaps the most diagnostic of the spectroscopic data due to its shape (AA′BXX′Y spin system), which is akin to that observed for [Ru(η2-CH2CCHPPh3)(CO)2(PPh3)2]+.3 For a more detailed discussion of the 1H NMR spectrum of this and other allenylphosphonium complexes, see the Supporting Information. The carbonyl stretching frequency of [4]PF6 (CH2Cl2: 1993 cm−1) appears toward the lower end of the range for π adducts formed between trans-[IrCl(CO)(PPh3)2] and C−C multiple bonds. Simple olefins do not coordinate strongly to Vaska’s complex,7 and so infrared data available for olefin complexes are biased toward those bearing electron-withdrawing substituents: e.g., C2F4 (2052 cm−1),7,8 F2CCC CF2 (2015 cm−1),9 C2(CN)4 (2057 cm−1),7 C60 (2014 cm−1),10 and C70 (2002 cm−1).11 Thus, although the phosphonium group would appear to activate the allene ligand in [4]+ sufficiently to irreversibly bind it to the iridium, retrodonation from iridium to the allene does not seem to compete effectively with the CO ligand. The characterization of [4]PF6 included an X-ray crystallographic study, the results of which are summarized in Figure 1.
of a hydride/allenylidene migratory insertion process (Scheme 2). Scheme 2. Synthesis of Rhodium η2-Allenylphosphonium Salts5a
Herein we report the reactions of [1]+ and [3]+ with a number of iridium(I) and rhodium(I) complexes to afford inter alia the first examples of iridium η2-allenylphosphonium complexes.
■
RESULTS AND DISCUSSION Vaska’s complex trans-[IrCl(CO)(PPh3)2] is known to form adducts with alkynes that are activated by electron-withdrawing substituents (CF3, CO2Me, etc.);6 however, simple aryl and alkyl alkynes do not bind effectively. Propiolic esters, in contrast, undergo oxidative addition of the alkynyl C−H bond to afford iridium(III) hydrido alkynyls: e.g., [IrHCl(C CCO2Me)(CO)(PPh3)2]. We therefore presumed that either reaction course might be available to the phosphonium salt [1]PF6 but instead observed the formation of the allenyl salt [IrCl(η 2 -CH 2 CCHPPh 3 )(CO)(PPh 3 ) 2 ]PF 6 ([4]PF 6) (Scheme 3) as the major product in 90% crude yield. Formation of [4]PF6 therefore parallels the metal-mediated propargylic rearrangement of [1]+ previously seen for Ru(0)3 and Pt(0)4 complexes. The 31P{1H} NMR spectrum of [4]PF6 shows a triplet at 12.5 ppm (4JPP = 2.9 Hz) corresponding to the allenylphosphonium resonance and a doublet at 4.4 ppm due to the
Figure 1. Molecular structure of [4]+ in a crystal of [4]PF6 (50% displacement ellipsoids; phenyl groups and PF6− counterion omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir1−C1 2.156(5), Ir1−C2 2.046(5), P1−C3 1.776(5), C1−C2 1.434(8), C2− C3 1.318(7); C1−Ir1−C2 39.8(2), Ir1−C1−C2 65.9(3), Ir1−C2−C3 143.5(4), C1- C2−C3 142.1(5), C2−C3−P1 123.4(5).
Scheme 3. Syntheses of Iridium Allenylphosphonium Salts
The geometry at iridium may be described as distorted trigonal bipyramidal (Cl1−Ir−C101 = 104.4(2)°, P2−Ir−P3 = 177.92(5)°), with the allene occupying a single equatorial coordination site. The trans-β geometry about the allenyl double bond C2C3 minimizes the steric interactions between the bulky PPh3 groups on C3 and Ir1. The bond lengths and angles are similar to those seen in the other example of a trigonal-bipyramidal η2-allenylphosphonium species, [Ru(η2-CH2CCHPPh3)(CO)2(PPh3)2]2Br(PF6),3 such that the coordinated C1−C2 bond (1.434(8) Å) is elongated relative to both the uncoordinated C2−C3 bond (1.318(7) Å) and that calculated for free allene (1.295 Å).12 The degree of bending of the allene upon coordination (C1− C2−C3 = 142.1(5)°) is comparable to that observed in the iridium bis(allene) complexes [Ir(H2CC CH2)2(PMe2Ph)3]+ (144.8, 145.2°)13 and [IrCl(H2CC CF2)2(PPh3)2] (135.1, 142.3°)14 such that the bulk of the PPh3 3199
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
Organometallics
Article
substituent does not appear to exert a pronounced steric influence. The 1H and 31P{1H} NMR spectra of [4]PF6 indicated the presence of two other species thought to be isomers of [4]PF6, denoted [4a]PF6 and [4b]PF6, in the [4]+:[4a]+:[4b]+ ratio 24:8:3. Efforts to further purify [4]PF6 by chromatography or crystallization were unsuccessful; however, satisfactory microanalytical data were obtained, thereby supporting the formulation of [4a]+ and [4b]+ as isomers of [4]+. The stereochemistry of the major product [4]PF6 is presumed to be that observed in the crystal structure determination (Figure 1). The NMR spectra of [4a]PF6 show the same number of environments, multiplicities, and magnitudes of couplings as [4]PF6 but with differing chemical shifts. This suggests that [4a]PF6 is the product in which the Cl and CO ligands have been transposed such that the sp3 CH2 moiety is trans to the Cl ligand rather than the CO ligand. Variable-temperature NMR experiments (−20 to +100 °C) showed no interconversion between [4]+ and [4a]+ on the 1H NMR time scale. The product [4b]PF6 contains three phosphorus environments, with the resonance for each appearing as a doublet of doublets in the 31P{1H} NMR spectrum at 12.5 ppm (JPP = 51.5, JPP = 1.8 Hz), −5.6 ppm (JPP = 29.2, JPP = 1.8 Hz) and −11.1 ppm (JPP = 29.2, JPP = 51.5 Hz). The couplings observed are consistent with a cis-Ir(PPh3)2 unit, for which a number of stereoisomers are conceivable. This isomer was not unequivocally identifiable in the 1H NMR spectrum; however, we note that in some instances π-adducts of “IrCl(CO)(PPh3)2” may adopt geometries with equatorial phosphines and a trans-axial Cl−Ir−CO arrangement.10,11 However, there was no evidence for formation of the isomer [IrCl{η2-C(CH2)CHPPh3}(CO)(PPh3)2]PF6, in which the allene is coordinated by the internal CC bond. This is the more common coordination mode of η2-allenylphosphonium complexes in the literature, and in some cases isomerization between the two modes is observed.4,15 The absence of coordination to the internal CC bond in this iridium species is presumably due to steric constraints. Such coordination would bring the phosphonium group one bond closer to the metal center (and the bulky phosphine co-ligands) and require it to be displaced from the equatorial plane as a result of the orthogonal nature of the π bonds in allenes. In an attempt to obviate the formation of stereoisomers, the synthesis of the bromide analogue [IrBr(η2-CH2C CHPPh3)(CO)(PPh3)2]PF6 ([5]PF6) was undertaken, utilizing a different synthetic approach. The reaction of Vaska’s complex with AgPF6 in MeCN affords the Ir(I) salt trans-[Ir(CO)(NCMe)(PPh3)2]PF6.16 The reaction of this compound with [1]Br gave the desired product [5]PF6 in 84% crude yield (Scheme 3). The spectroscopic features of [5]PF6 are similar to those of [4]PF6. The NMR spectra indicated the presence of a minor product, which we were unable to remove, as in the case of [4]PF6. Peaks corresponding to the isomer with the sp3 CH2 group trans to Br, denoted [5a]PF6, are observed in a [5]+: [5a]+ ratio of approximately 6:1. Unlike the case for [4]PF6, resonances due to a cis-Ir(PPh3)2 isomer were not observed. The crystal structure of [5]PF6 was obtained (Figure 2) and shows little deviation from that of [4]PF6. After the successful reaction of trans-[Ir(CO)(NCMe)(PPh3)2]PF6 with [1]Br, the reaction with [1]PF6 was undertaken in an attempt to obtain [Ir(η2-CH2C CHPPh3)(CO)(PPh3)2](PF6)2 (Scheme 3). A solution of trans-[Ir(CO)(NCMe)(PPh3)2]PF6 and [1]PF6 in CH2Cl2
Figure 2. Molecular structure of [5]+ in a crystal of [5]PF6 (50% displacement ellipsoids; phenyl groups and PF6− counterion omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir1−C1 2.134(7), Ir1−C2 2.024(8), P1−C3 1.773(8), C1−C2 1.411(11), C2−C3 1.321(11); C1−Ir1−C2 39.6(3), Ir1−C1−C2 66.0(4), C1− C2−C3 141.8(8), C2−C3−P1 123.4(7).
was monitored by infrared spectroscopy, but even after an extended reaction time (17 days) only the starting material was observed. The failure of trans-[Ir(CO)(NCMe)(PPh3)2]PF6 and [1]PF6 to react provides insight into the mechanism for the formation of [5]PF6, which must proceed via initial bromide coordination, restoring neutrality to the iridium center, followed by addition of the phosphonium unit. Chen and colleagues have reported the synthesis of a κ2 metallacyclic isomer of [4]+, [IrCl{κ2-CHC(PPh3)CH2}(CO)(PPh3)2]OTf ([6]OTf), in which the phosphorus is bound to the central carbon of the C3 unit.17 Because [4]PF6 and [6]OTf were both formed under mild conditions, the question arises as to whether they represent kinetic isomers en route to each other. We investigated the possibility of interconversion between [4]+ and [6]+ via either base-mediated or thermal isomerization (Scheme 4). Treatment of [4]PF6 or Scheme 4. Attempted η2−κ2 Interconversion
[6]OTf with the non-nucleophilic base DBU did not result in any isomerization as detected by NMR spectroscopy. After [6]OTf was heated for 1 week at 50 °C in chloroform, NMR spectroscopy showed some 50% of [6]+ remained, accompanied by a variety of unidentified decomposition products; however, resonances attributable to [4]+ were conspicuously absent. Under the same conditions, evidence for the decomposition of [4]PF6 was seen after 1 day, and after 1 week the product had completely decomposed. No evidence for interconversion between the two isomers was seen by heating either [4]+ or [6]+, though the possibility that any slow conversion of [6]+ to [4]+ is masked by its subsequent and much more rapid decomposition cannot be excluded. The iridium η2-alkynylphosphonium complex [IrCl(MeC CPPh3)(CO)(PPh3)2]PF6 represents a third isomer in the family of [4]+ and [6]+ which would provide an interesting comparison. Reaction of [3]PF6 with trans-[IrCl(CO)(PPh3)2] 3200
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
Organometallics
Article
The 1H NMR spectrum of [7]PF6 shows, in addition to the aromatic proton resonances, allenylphosphonium resonances consistent with coordination of the terminal rather than internal CC bond of the allene: a doublet at 6.64 ppm due to the methyne group (cf. 4.08 and 4.35 ppm in the parent allene) and a multiplet at 1.02 ppm due to the methylene moiety (cf. 0.53 ppm). In the 31P{1H} NMR spectrum the resonance due to the PPh3 ligands appears as a doublet of doublets at 34.1 ppm and the phosphonium resonance as a doublet of triplets at 8.2 ppm. The crystal structure of [7]PF6 (Figure 3) shows a squareplanar rhodium center with the allenylphosphonium ligand
was attempted with the aim of obtaining the alkynylphosphonium salt (Scheme 5). Stirring the two materials in CH2Cl2 at Scheme 5. Attempted Synthesis of an Iridium Alkynylphosphonium Complex
room temperature gave no reaction after 4 days. The reaction was then attempted at an elevated temperature (THF at reflux), but still no reaction was observed. As noted above, only alkynes activated by strongly electron withdrawing substituents form stable adducts with Vaska’s complex.6 The phosphonium group of [3]+ would therefore appear insufficiently electronegative to bind it in this case, in contrast to the stable adduct [Pt(η2MeCCPPh3)(PPh3)2]PF6 formed in the reaction of [3]PF6 with [Pt(η2-C2H4)(PPh3)2],4 though again, the steric clash between the α-phosphonium and iridium-bound phosphines presumably contributes to disfavor coordination. The successful preparation of [4]PF 6 prompted an investigation into the analogous rhodium compound. In the iridium case, the product is observed by IR spectroscopy after 10 min. In contrast to this, the reaction between trans[RhCl(CO)(PPh3)2] and [1]PF6 was monitored by IR spectroscopy over the course of 24 h but only the carbonyl stretching frequency of the starting material was observed (νCO 1978 cm−1). This is consistent with rhodium being a 4d transition metal and thus less π-basic than its 5d counterpart iridium and less prone to oxidative addition. The more electron rich complex [RhCl(PPh3)3] has been shown to react with alkynes RCCR (R = Ph, CF3) to produce [RhCl(RCCR)(PPh3)2]18 and with allene to yield [RhCl(η2-CH2CCH2)(PPh3)2].19 In an analgous manner, the allenylphosphonium salt [RhCl(η2-CH2CCHPPh3)(PPh3)2]PF6 ([7]PF6) was obtained from the reaction of [RhCl(PPh3)3] with [1]PF6 (Scheme 6).
Figure 3. Molecular structure of [7]+ in a crystal of [7]PF6 (50% displacement ellipsoids; phenyl groups and PF6− counterion omitted for clarity). Selected bond lengths (Å) and angles (deg): Rh1−C1 2.118(3), Rh1−C2 1.964(3), C1−C2 1.401(4), C2−C3 1.320(4), C3−P1 1.774(3); C1−Rh1−C2 39.93(11), Rh1−C1−C2 64.09(15), Rh1−C2−C3 133.3(2), C1−C2−C3 150.6(3), C2−C3−P1 122.3(2). The inset depicts a view along the P2−Rh1 vector.
orthogonal to the coordination plane to minimize steric interactions. This is in contrast to the trigonal-planar platinum complexes [Pt(η2-CH2CCHPPh3)(PPh3)2]PF6 and [Pt{η2-C(CH2)CHPPh3}(PPh3)2]PF6, where the allenyl ligand lies in the P−Pt−P coordination plane.4 A comparison with the reported complexes [RhCl(η2-CH2CCHR)(PMe3)2] (R = Ph, p-C6H4F)20 and [RhCl{η2-CH2C CHC(Ph)CHSiMe3}(PPh3)2]21 reveals subtle deviations in bond lengths and angles. A greater disparity is seen between the Rh1−C1 and Rh1−C2 bond lengths in [7]PF6 in comparison to [RhCl{η 2 -CH 2 CCHC(Ph)CHSiMe 3 }(PPh 3 ) 2 ], which gives rise to smaller Rh1−C1−C2 and Rh1−C2−C3 angles. The most notable feature of the coordination geometry is that not only are the C1−C2 and Cl1−Rh1 vectors not orthogonal (Cl1−Rh1−Centroid(C1,C2) = 166.3(3)°) but also the allene is displaced from the coordination axis such that the methylene is almost colinear with the Rh1−Cl1 bond (Cl1−Rh1−C1 = 174.6(3)°; inset of Figure 3). Other than weak hydrogen bonding between H31 and the PF6 counteranion (H31···F1 = 2.57 Å), there do not appear to be any intramolecular interactions to account for this deformation, which is not observed in other variously substituted allene
Scheme 6. Synthesis of Rhodium Allenylphosphonium Salts
3201
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
Organometallics
Article
coordination axis with the attendant development of negative charge on what is to become the β-vinylidene carbon. Were a similar slippage to occur for the proposed alkynylphosphonium complex, this would increase the basicity of the phosphoniosubstituted carbon while proximally positioning the propargylic protons for concerted transfer to this carbon. Although this would initially provide the sterically less favored allene isomer, unimolecular nondissociative allene isomerism is generally a low-energy process.26 The reaction of [7]PF6 with CO was investigated in the hopes of attaining the salt [RhCl(η2-CH2CCHPPh3)(CO)(PPh3)2]PF6 analogous to [4]PF6. The synthesis of this compound was attempted initially by treating [RhCl(CO)(PPh3)2] with [1]PF6, but no reaction was observed. In the case of [Rh(acac){η2−CH(PR3)CCPh2}(PR3)]BF4 (R = Cy, iPr) and [RhCl(η2-CH2CCH2)(PPh3)2], addition of CO results in the displacement of the allenylphosphonium ligand to give [Rh(acac)(CO)(PR3)] and [R3PCHC CPh2]BF4 or trans-[RhCl(CO)(PPh3)2] and CH2CCH2, respectively.5a,19 Similarly, when [7]PF6 was placed under 1 atm of CO, the allenylphosphonium ligand was displaced to give [2]PF6 and trans-[RhCl(CO)(PPh3)2] (Scheme 6, spectroscopically quantitative), identified on the basis of the 31 1 P{ H} NMR spectrum consistent with the published data.27 On the basis of this result, we revisited the reaction of trans[RhCl(CO)(PPh3)2] with [1]PF6. We found that there is indeed a reaction occurring, but it is the isomerization of [1]+ to [2]+, presumably via the allenylphosphonium complex (Chart 2). This reaction is in fact catalytic; when a solution of
complexes of the form [RhX(allene)(PR3)2] (X = Cl, I; R = Me, iPr, Ph).20−22 The reaction of the alkynylphosphonium salt [3]PF6 with [RhCl(PPh3)3] was carried out. On the basis of previous results, it was anticipated that this would provide either the η2alkynylphosphonium complex (as seen in the reaction of [Pt(C2H4)(PPh3)2] and [3]PF6)4 or no reaction (as seen for [IrCl(CO)(PPh3)2] and [3]PF6). Surprisingly, the reaction of [3]PF 6 with [RhCl(PPh 3 ) 3 ] in fact provides the η 2 allenylphosphonium complex [7]PF6 (Scheme 6). Calculations have shown that the free alkynylphosphonium ligand [3]+ is lower in energy than the allenylphosphonium salt [2]+,4 such that the [3]+ → [2]+ isomerization (endergonic in the free state) most likely occurs while the ligand is coordinated to the metal, thereby implicating the η2-alkynylphosphonium complex as an intermediate. This metal-based alkynylphosphonium to allenylphosphonium isomerization is not unprecedented. Wenger inferred a similar rearrangement upon treatment of the η2-alkynylphosphine complex [Pt(MeC CPPh2)(dcpe)] with MeI to give the η2-allenylphosphonium salt [Pt{η2−CH(PMePh2)CCH2}(dcpe)]I, presumably via the intermediacy of the η2-alkynylphosphonium salt [Pt{η2-C(PMePh2)CCH3}(dcpe)]I.23 In Wenger’s example the allene remains bound through the original alkyne CC linkage, whereas in our example the allene migrates to involve coordination via the terminal CC linkage. Two plausible mechanisms (Scheme 7) may account for the isomerism. (a) The transfer of a methyl hydrogen to rhodium Scheme 7. Mechanistic Proposals for Alkyne/Allene Isomerism: (a) Stepwise β-Rh−H Elimination/Reductive Elimination; (b) Concerted 1,4-Hydrogen Shift
Chart 2. Rhodium-Mediated Alkyne/Allene Isomerism
[1]PF6 in CD3CN is treated with a catalytic amount of trans[RhCl(CO)(PPh3)2], isomerization to [2]PF6 occurs quantitatively within 1 h. Given the observed rhodium coupling in the NMR spectra of the precursor [7]PF6, it can be assumed that in the absence of CO the allenylphosphonium ligand does not dissociate on the NMR time scale. It may therefore be concluded that the reaction with CO proceeds in an associative manner and that [RhCl(η2-CH2CCHPPh3)(CO)(PPh3)2]PF6 is formed as an intermediate from which [2]+ immediately dissociates. This is consistent with the coordinative unsaturation of the rhodium center in [7]PF6. The destabilization of [2]+ coordination upon the addition of a strongly competitive π-acceptor ligand suggests that retrodonation from the metal center to the allenyl π system is an important component of the overall binding of [2]+. Retrodonation to alkyne ligands would be expected to stabilize the putative but not observed alkyne complexes [RhCl(η2-HCCCH2PPh3)(PPh3)2]+ and [RhCl(η2-CH3CCPPh3)(PPh3)2]+; however, given the high d occupancy of rhodium(I), we suggest that π donation from the π bond orthogonal to the RhC2 plane would destabilize
(β-Rh−H elimination) initially isolates the hydrogen atom from the carbon to which it must travel were the resulting allenyl ligand to remain η3 coordinated. Dissociation of the newly formed CC bond would however afford a monodentate phosphonioallenyl ligand capable of undergoing reductive (C− H) elimination to afford the observed allene complex. There are numerous examples of phosphonioallenyl complexes,24 including the rhodium example [RhCl3{C(PiPr3)CCPh2}(PiPr3)],24a which generally result from the coupling of phosphine and allenylidene ligands. (b) The generally accepted concerted mechanism by which alkyne ligands rearrange to vinylidenes25 envisages a slippage of the alkyne off the 3202
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
Organometallics
Article
131.3 [s, C4(IrPPh3)]; 129.8 [d, C2,3,5,6(CPPh3), 2,3JPC = 12.9]; 128.6 [tv, C2,3,5,6(IrPPh3), JPC = 5.7]; 120.9 [d, C1(CPPh3), 1JPC = 90.2]; 91.8 (d, PCH, JPC = 80.5); 7.1 (s, broad, IrCH2). The resonances at 181.3 and 177.2 ppm could not be unequivocally differentiated between IrCO and C. The C1(IrPPh3) phenyl resonance could not be unequivocally assigned. 31P{1H} NMR (CDCl3) δ/ppm: 12.5 (t, CPPh3, 4JPP = 2.9); 4.4 (d, IrPPh3, 4JPP = 2.9); −143.5 (sep, PF6, 1JPF = 713.0). MS-ESI(+): m/z 1081.6 [M]+, 1053.5 [M − CO]+, 819.3 [M − PPh3]+. Accurate mass: found 1081.2256 [M]+, calcd for C58H48OP335Cl193Ir 1081.2236. Anal. Found: C, 56.82; H, 3.84; N, 0.00. Calcd for C58H48ClF6IrOP4: C, 56.80; H, 3.94; N, 0.00. Crystal data for C58H48ClIrOP3·PF6·2.5CH2Cl2: Mw = 1239.03, triclinic, P1̅ (No. 2), a = 14.0258(7) Å, b = 14.9269(9) Å, c = 16.7575(9) Å, α = 79.893(3)°, β = 83.090(3)°, γ = 75.388(3)°, V = 3331.6(3) Å3, Z = 2, ρcalcd = 1.24 Mg m−3, μ(Mo Kα) = 2.25 mm−1, T = 200(2) K, colorless prism, 0.38 × 0.19 × 0.14 mm, 11514 independent reflections, F2 refinement, R = 0.044, Rw = 0.122 for 9873 reflections (I > 2σ(I), 2θmax = 50°), 637 parameters. Data for minor products are as follows. [4a]PF6: 1H NMR (CDCl3) δ/ppm: 7.82−6.92 (m, 45H, C6H5); 6.57 (dd, 1H, PCH, 2JPH = 30.0, J = 2.0); 0.41 (tdd, 2H, IrCH2, 3JPH = 7.7, 4 JPH = 4.0, 4JHH = 2.8). 31P{1H} NMR (CDCl3) δ/ppm: 12.9 (t, CPPh3, 4JPP = 3.9); −0.2 (d, IrPPh3, 4JPP = 3.9); −143.5 (sep, PF6, 1JPF = 713.0). [4b]PF6: 31P{1H} NMR (CDCl3) δ/ppm: 12.5 (dd, CPPh3, trans-4JPP = 51.5 Hz, cis-4JPP = 1.8 Hz); −5.6 ppm (dd, IrPPh3, 2JPP = 29.2 Hz, cis-4JPP = 1.8 Hz); −11.1 ppm (dd, IrPPh3, 2JPP = 29.2 Hz, trans-4JPP = 51.5 Hz); −143.5 (sep, PF6, 1JPF = 713.0). Synthesis of [IrBr(η 2 -CH 2 CCHPPh 3 )(CO)(PPh 3 ) 2 ]PF 6 ([5]PF6). A mixture of trans-[Ir(CO)(NCCH3)(PPh3)2]PF6 (0.100 g, 0.107 mmol) and [1]Br (45 mg, 0.12 mmol) was stirred in CH2Cl2 (10 mL) for 72 h. The pale yellow solution was concentrated under reduced pressure with the gradual addition of toluene to afford a white precipitate. This precipitate was collected by vacuum filtration and washed with ether to yield the product as a white powder. Crystals suitable for crystallographic analysis were obtained by slow diffusion of n-hexane into a solution of [5]PF6 in CH2Cl2. Crude yield :115 mg (90.4 μmol, 84%). IR (Nujol) ν/cm−1: 1983 (CO), 1437 (PC), 1111 (PPh) 838 (PF6). IR (CH2Cl2) ν/cm−1: 1989 (CO), 1437 (PC), 1111 (PPh), 846 (PF6). 1H NMR (CDCl3) δ/ppm: 7.83−6.92 (m, 45H, C6H5); 6.40 (d, 1H, PCH, 2JPH = 25.6); 0.13 (tdd, 2H, IrCH2, 3JPH = 9.6, 4JPH = 4.4, 4JHH = 2.8). 13C{1H} NMR (CDCl3) δ/ppm: 180.5 (t, JPC = 4.0); 176.9 (t, JPC = 11.5); 134.7 [tv, C2,3,5,6(IrPPh3), JPC = 4.9]; 134.5 [d, C4(CPPh3), 4JPC = 2.3]; 133.5 [d, C2,3,5,6(CPPh3), 2,3JPC = 9.8]; 131.4 [s, C4(IrPPh3)]; 129.8 [d, C2,3,5,6(CPPh3), 2,3JPC = 12.0]; 128.7 [tv, C2,3,5,6(IrPPh3), JPC = 4.4]; 120.0 [d, C1(CPPh3), 1JPC = 88.3]; 92.2 (d, PCH, JPC = 79.5); 8.5 (s, broad, IrCH2). The C1(IrPPh3) resonance could not be unequivocally assigned. 31P{1H} NMR (CDCl3) δ/ppm: 12.9 (t, CPPh3, 4JPP = 2.9); 2.2 (d, IrPPh3, 4JPP = 2.9); −143.6 (sep, PF6, 1JPF = 713.2). MS-ESI(+): m/z 1125.6 [M]+, 1097.6 [M − CO + H]+, 863.5 [M − PPh3]+. Accurate mass: found 1125.1786 [M]+, calcd for C58H48OP379Br193Ir 1125.1725. Anal. Found: C, 55.02; H, 3.85; N, 0.00. Calcd for C58H48BrF6IrOP4: C, 54.81; H, 3.81; N, 0.00. Crystal data for C58H48BrIrOP3·PF6· 1.5CH2Cl2: Mw = 1398.42, triclinic, P1̅ (No. 2), a = 17.5245(7) Å, b = 20.0599(6) Å, c = 22.9773(8) Å, α = 65.5999(19)°, β = 86.674(2)°, γ = 66.190(2)°, V = 6672.3(4) Å3, Z = 4, ρcalcd = 1.392 Mg m−3, μ(Mo Kα) = 2.87 mm−1, T = 200(2) K, colorless needle, 0.31 × 0.19 × 0.18 mm, 28789 independent reflections, F2 refinement, R = 0.057, Rw = 0.157 for 18513 reflections (I > 2σ(I), 2θmax = 55°), 1360 parameters. Data for the minor product are as follows. [5a]PF6: 1H NMR (CDCl3) δ/ppm: 7.83−6.92 (m, 45H, C6H5); 6.56 (d, 1H, PCH, 2JPH = 30.4); 0.64 (tdd, 2H, IrCH2, couplings apparent but not resolved). 31P{1H} NMR (CDCl3) δ/ppm: 13.2 (t, CPPh3, 4JPP = 4.7); −2.0 (d, IrPPh3, 4JPP = 4.7); −143.6 (sep, PF6, 1JPF = 713.2). Synthesis of [RhCl(η2-CH2CCHPPh3)(PPh3)2]PF6 ([7]PF6). A mixture of [RhCl(PPh3)3] (200 mg, 0.216 mmol) and [1]PF6 (96 mg, 0.22 mmol) in CHCl3 (20 mL) was stirred for 90 min. The resulting red-orange suspension was concentrated to ca. 7 mL under reduced pressure and filtered. The product was precipitated by addition of n-hexane, collected by filtration, and washed with hexane. Crystals suitable for crystallographic and microanalytical analysis were
alkyne coordination relative to the allene isomers, for which only σ donation/π retrodonation operate. This is in contrast with the case for early-transition-metal centers ( 2σ(I), 2θmax = 56°), 667 parameters. Catalytic Isomerization of [1]PF6. [1]PF6 (20 mg, 0.045 mmol) and [RhCl(CO)(PPh3)2] (3 mg, 0.004 mmol, 10 mol % (not optimized)) were dissolved in CD3CN (0.5 mL), and the reaction was monitored by 1H NMR spectroscopy. After 20 min, ca. 20% conversion was observed. After 1 h the 1H NMR spectrum showed complete conversion of [1]PF6 to [2]PF6 (spectroscopically quantitative).
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
S Supporting Information *
CIF files giving crystallographic data for [4]PF6 (CCDC 932242), [5]PF6 (CCDC 932243), and [7]PF6 (CCDC 932244) and text and figures giving a discussion of 1H NMR features of allenylphosphonium complexes. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail for A.F.H.:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS This work was supported by the Australian Research Council (DP110101611). REFERENCES
(1) (a) Schweizer, E. E.; DeVoe Goff, S.; Murray, W. P. J. Org. Chem. 1977, 42, 200. (b) For a more general discussion of propargyl− allenyl isomerism see: Murray, M. In Methoden zur Herstellung und Umwandlung von Allenen bzw. Kumulenen; Houben-Weyl, Eds.; Thieme: Stuttgart, Germany, 1977; Vol. 5/2a, p 991. (2) (a) Bagdasaryan, G. B.; Pogosyan, P. S.; Panosyan, G. A.; Indzhikyan, M. G. Russ. J. Gen. Chem. 2008, 78, 1177. (b) Khachatrian, R. A.; Zalinian, S. A.; Bagdasarian, G. B.; Sarkisova, E. A.; Indzhikian, M. G. Russ. Chem. Bull., Int. Ed. 2002, 51, 148 and references therein. (3) Ang, W. H.; Cordiner, R. L.; Hill, A. F.; Perry, T. L.; Wagler, J. Organometallics 2009, 28, 5568. (4) Colebatch, A. L.; Cade, I. A.; Hill, A. F.; Bhadbhade, M. M. Organometallics 2013, 32, 4766. (5) (a) Esteruelas, M. A.; Lahoz, F. J.; Martin, M.; Onate, E.; Oro, L. A. Organometallics 1997, 16, 4572. Binuclear examples have been reported from the reactions of [Co2(μ-HCCCH2)(CO)6]BF4 and 3204
dx.doi.org/10.1021/om500450e | Organometallics 2014, 33, 3198−3204
