Peripheral Methyl Activation in η4-1,2,3,4

Aug 29, 2014 - These reactions give rise to the piano-stool complexes [(η4-C4Me4)Co(dppe)-. (NCCH3)]+ (2), [(η4-C4Me4)Co(dppe)(PH2Ph)]+ (3), ...
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Peripheral Methyl Activation in η4‑1,2,3,4Tetramethylcyclobutadienylcobalt Complexes: Template Synthesis and Subsequent Reactivity of Triphosphamacrocycles Peter G. Edwards,* Benson M. Kariuki, Paul D. Newman,* and Wenjian Zhang School of Chemistry, Cardiff University, Park Place, Cardiff, CF10 3AT, U.K. S Supporting Information *

ABSTRACT: The cationic complex (η 4-1,2,3,4-tetramethylcyclobutadienyl)cobalt(trisacetonitrile), [(η 4-C4Me4)Co(NCCH3)3]+ (1), allows the stepwise introduction of suitable phosphine precursors to the [(η4-C4Me4)Co]+ fragment by replacement of the labile acetonitrile ligands. These reactions give rise to the piano-stool complexes [(η4-C4Me4)Co(dppe)(NCCH3)]+ (2), [(η4-C4Me4)Co(dppe)(PH2Ph)]+ (3), [(η4-C4Me4)Co(dfppb)(NCCH3)]+ (4), and [(η4-C4Me4)Co(dfppb)(PH2Ph)]+ (5), where dfppb = 1,2-bis{di(2-fluorophenyl)phosphino}benzene and dppe = 1,2-bis(diphenylphosphino)ethane. Complex 5 is a template for the synthesis of the P3 macrocycle complex [(η4-C4Me4)Co{1,4-bis(2-fluorophenyl)-7phenyl[b,e,h]tribenzo-1,4,7-triphosphacyclononane}]+ (6), through base-promoted intramolecular macrocyclization. The hydrogens of two of the ring methyls of the tetramethylcyclobutadienyl ligand in the macrocycle complex 6 are sufficiently acidic to undergo deprotonation by KOtBu, promoting nucleophilic attack at the fluorine-bearing ortho-carbons of the 2fluoroaryl groups on two of the phosphorus donors in 6. The resultant hemi-incarcerand complex [{η4,κP,κP,κP-Me2C4-[1,4bis(2-CH2C6H4)-7-C6H5-[b,e,h]tribenzo-1,4,7-triphosphacyclononane]-1,2}Co]+ (cis-7) contains a hybrid phosphorus/carbon donor ligand where the P3 macrocycle is connected to the cyclobutadienyl function through two cis-2-methylphenyl links. The new complexes have been characterized fully by spectroscopic and analytical techniques including single-crystal X-ray structure determinations of 2, 3, 4, 5, 6, and cis-7.

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INTRODUCTION In contrast to triaza and trithia macrocycles with variable ring sizes, related P-donor triphospha macrocycle ligands, which may have the ability to stabilize low and unusual metal oxidation states, remain rare. The common method of combining suitably functionalized precursors in dilute solution to yield the desired macrocycle cannot be applied easily to the synthesis of phosphorus macrocycles because of experimental difficulties associated with handling the often volatile, noxious, toxic, and air-sensitive phosphine precursors and intermediates. Aside from the necessary need for large volumes of solvent, a further drawback of the direct method is a lack of stereochemical control, which results in an isomeric mixture of phosphamacrocycles due to the relatively high inversion energy barrier at P; this has recently been confirmed in the highdilution solution synthesis of 1,4,7-triphenyl-1,4,7-triphosphacyclononane (9-aneP3Ph3).1 Template methods provide a © XXXX American Chemical Society

means of controlling the stereochemistry but are often hampered by difficulties associated with the release of the macrocycle. Details of successful direct and template approaches to macrocyclic phosphines and aspects of their coordination chemistry appear in a recent review by Swor and Tyler.2 Much of our interest in triphosphorus macrocycles stems from their expected ability to form thermodynamically and kinetically robust metal−ligand fragments that may have application in catalysis. The preparation of 1,5,9-triphosphacyclododecane, 12-aneP3H3, is an early example of a templateassisted synthesis where Norman and co-workers employed zerovalent molybdenum or tungsten systems for the controlled formation of the triphosphorus macrocycle from three mutually Received: July 21, 2014

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Scheme 1. Synthesis of the Model Complexes 2 and 3

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cis allylphosphine ligands.3 We have subsequently exploited the chemistry of the 12-aneP3R3 (where R = alkyl) derivatives after developing a method for demetalation of the Mo/Cr complexes to give the free ligands.4 These studies reveal that these facially capping phosphorus macrocycles do support interesting chemistry of transition metals.4e,h However, the 12-aneP3R3 ligands are relatively flexible donors, which limits their ability to form stable complexes with certain metal ions. Smaller ring triphosphamacrocycles should provide more rigid, robust complexes, enabling access to these hitherto elusive species. Norman’s approach has not been successful for the formation of smaller ring sizes,4b and so we have sought to develop other template systems that might prove more versatile. The smallest ring triphosphamacrocycle, namely, 1,4,7-triphosphacyclononane (9-aneP3H3), was first synthesized by us using a CpFe+ template.5 A key advantage of this system (bearing parent or functionalized Cp ligands) is that it supports a range of ringclosing P−C bond forming hydrophosphinations, either by radical- or base-promoted addition to alkenyl phosphines.4f,5 We have extended the CpFe+ template method for the synthesis of nine-membered macrocyclic triphosphines with benzannulated (i.e., o-phenylene or 1,2-benzenediyl) backbone functions via intermolecular nucleophilic displacement of fluoride from 2-fluorophenyl phosphines.4g The resulting complexes proved very robust such that liberation of the macrocycle was difficult without destruction of the ligand and has only been achieved through exhaustive oxidation with H2O2 or Br2/H2O. The resulting 9-aneP3R3 trioxides could not be reduced readily back to the desired P(III) species without decomposition (arising from P−C bond cleavage). Alternative routes to these macrocyclic ligands are still required in order to develop their chemistry. The isoelectronic [(η4-C4Me4)Co(NCCH3)3]+ appears suitable as a potential candidate, as it restricts the labile ligands to a rigorously facial coordination configuration and allows selective, sequential replacement of the acetonitrile ligands.6 In addition, the Co(III) center has a smaller radius and is a harder Lewis acid than Fe(II), which might favor smaller ring formation.7 However, for [(η4-C4Me4)Co(NCCH3)3]+ to be a potential template for the assembly of P3 macrocycles, all three acetonitrile ligands need to be sufficiently labile to allow their substitution by the appropriate P-containing ligands. The chemistry of these systems is not well established, and there is a dearth of information in the literature concerning the synthesis of complexes of the type [(η4-C4Me4)Co(PR3)2(PR3′)]+, where (PR3) and (PR3′) may be the same or preferably different. This paper details our studies on [(η4-C4Me4)Co(CH3CN)3]+ systems and their application as alternative templates for the preparation of a number of 9-aneP3R3 macrocycles bearing benzannulated “backbone” functions.

RESULTS AND DISCUSSION Ligand Substitution Reactions of [(η4-C4Me4)Co(NCCH3)3]+. The pattern of substitution kinetics in [(η4C4Me4)Co(NCCH3)3]+, 1, previously reported6 ideally lends itself to the selective sequential incorporation of suitable macrocycle precursor phosphines. However, as reports of [(η4C4Me4)Co(PR3)3]+ complexes are rare and the chemistry of these complexes remains poorly understood, it was thought prudent to test this through the model reaction shown in Scheme 1. The reaction involves the initial substitution of two MeCN ligands in 1 by dppe prior to replacement of the final acetonitrile ligand with phenylphosphine. Treatment of 1 in acetonitrile with a molar equivalent of dppe causes the rapid displacement of two CH3CN ligands, affording the diphosphine-monoacetonitrile complex [(η 4 -C 4 Me 4 )Co(dppe)(NCCH3)]+, 2, which was identified by a singlet at δ 70.2 ppm in the 31P{1H} NMR spectrum. The chemical shift is consistent with the formation of a five-membered chelate ring.8 The isolated orange solid had characteristic peaks in the 1H NMR spectrum at 0.92 and 0.80 ppm for the C4Me4 methyls and the coordinated acetonitrile, respectively, in addition to the resonances for the dppe. The 13C{1H} NMR spectrum showed the C4Me4 ring carbons at 85.6 ppm and the peripheral methyls at 8.9 ppm with the CH3CN signal at 0.8 and the dppe CH2 resonance at 28.5 ppm. The IR spectrum contained a peak at 2255 cm−1 for the CN stretch of the coordinated acetonitrile, and the elemental analysis confirmed the composition of the complex. In contrast to the reported [(η 4-C4Me4)Co(PMe3) 3]+ complex,6 the substitution of the remaining CH3CN ligand in 2 occurred readily upon addition of a molar equivalent of phenylphosphine to give the diphosphine-monophosphine complex [(η4-C4Me4)Co(dppe)(PH2Ph)]+, 3. The 31P{1H} NMR spectrum of 3 showed two resonances at δ 70.2 and −18.6 ppm for coordinated dppe and PhPH2, respectively. Although no P−P coupling could be observed due to peak broadening, the latter resonance is confirmed as the coordinated primary phosphine in the 31P NMR spectrum, where it appears as a triplet (1JP−H = 336 Hz). The 1H and 13 C{1H} NMR spectra were as expected (see Experimental Section), and the IR spectrum confirmed the absence of coordinated CH3CN and the presence of the phenylphosphine with bands due to νPH stretches at 2311 and 2333 cm−1. Complexes 2 and 3 were further characterized by determination of their solid-state structures by single-crystal X-ray diffraction techniques. The molecular structures of the two complexes are shown in Figure 1 with pertinent bond lengths and angles in Table 1. Surprisingly, a search of the Cambridge Crystallographic Database did not reveal any structures for complexes of the type [(η 4 -C 4Me 4 )Co(PR3)2(L)]+, emphasizing the rare nature of these compounds. B

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last coordination site being occupied by CH3CN in 2 or phenylphosphine in 3. The distance from the C4 centroid to cobalt is longer in 3 compared to 2 and 1 [1.670(10) Å],6 reflecting the different stereoelectronic properties of the two trans C4 ligand sets. In 3, the plane defined by the P atoms (plane P3) is essentially parallel to the C4 plane with a deviation of only 2°, and there is little distinction in the three Co−P bond lengths (Table 1). Although the dppe chelate bite angle is close to 90°, the P−Co−P angles involving the phenylphosphine donor are appreciably more obtuse. The nonbonded P···P distances can provide a crude measure of the ability of a template to enable the formation of small-ring triphosphamacrocycles. The values of 3.29 and 3.36 Å observed in 3 are on the order seen in Cr(0) templates used for the construction of 12aneP3 frameworks. The precursor Fe(II) complexes that are effective for the preparation of 9aneP3 macrocycles have nonbonded P···P contacts typically around 3.1 Å, so that this measure alone would not seem to support the use of the current Co(III) systems as templates for small-ring P3 derivatives. Synthesis of Tribenzannulated Nine-Membered Triphosphamacrocycles Based on Tetramethylcyclobutadienylcobalt Templates. The model system above has demonstrated that both diphosphine and monophosphine ligands may be sequentially incorporated into the [(η4C4Me4)Co]+ fragment by substitution of the labile acetonitriles in 1. Thus, the [(η4-C4Me4)Co]+ fragment appears suitable as a template for preorganizing phosphine precursors to facilitate the formation of P3 macrocycles. Although some of the metrics observed in the structure of 3 were not encouraging (see above), the possibility of template-enabled formation of 9aneP3 macrocycles needed to be assessed. In order to do this, the dppe ligand in 3 must be replaced by an alternative diphosphine bearing appropriate functionality to encourage intramolecular ring closure. Our strategy is based upon the methodology established using the CpRFe+ template and involves the use of 1,2-bis{di(2-fluorophenyl)phosphine}benzene for the synthesis of tribenzannulated triphosphacyclononanes as outlined in Scheme 2.9a This protocol has also been used for the preparation of the first diphosphine-carbene macrocycle,9b which has been further developed by Hahn and co-workers.9c In a similar manner to 2, treatment of 1 in CH3CN with an equimolar amount of dfppb caused an immediate color change from red to orange upon rapid displacement of two CH3CN ligands to afford the diphosphinemonoacetonitrile complex [(η4-C4Me4)Co(dfppb)(NCCH3)]+ (4) in quantitative yield. The progress of the reaction was monitored by 31P{1H} NMR spectroscopy, which showed the growth of a new peak at δP 64.3 ppm due to coordinated dfppb and the gradual loss of the peak at δP −34.4 ppm for uncoordinated dfppb. The 19F NMR spectrum of the isolated orange solid has two resonances at δ −96.5 and −97.1 ppm, respectively, indicating the presence of two sets of magnetically inequivalent fluorine atoms (no resolved P−F coupling) arising from the two possible rotameric orientations of the ArF groups (toward or away from the η4-C4Me4 ring). Due to the predominance of aromatic hydrogens, the 1H NMR spectrum of 4 is relatively uninformative, although the coordinated acetonitrile is seen at 0.79 ppm and the IR spectrum shows the CN stretch for the CH3CN at 2269 cm−1. The CH3CN ligand in 4 is readily exchanged by phenylphosphine to give the diphosphine-monophosphine complex [(η4-C4Me4)Co(dfppb)(PH2Ph)]+ (5). No obvious color

Figure 1. ORTEP view of the cations 2 (top) and 3 (bottom).

Table 1. Pertinent Bond Length and Angle Data for the Complexes complex 2

Co−N (Å) 1.929(3)

3

4 5

6

cis-7

1.934(2)

Co−P (Å) 2.2403(8) 2.2086(8) 2.2043(7) 2.2116(7) 2.2196(8) 2.2241(8) 2.2260(8) 2.2190(8) 2.2139(8) 2.2125(7) 2.1533(11) 2.1614(11) 2.1643(11) 2.1216(15) 2.1241(16) 2.1262(16)

Co−C4 centroid (Å) 1.752(3)

N−Co−P (deg)

P−Co−P (deg)

91.99(7) 90.92(7)

87.15(3)

1.782(2)

1.773(3) 1.789(3)

1.762(5)

1.716(8)

95.79(7) 94.46(7)

89.88(3) 96.18(3) 98.58(3) 85.85(3) 88.86(3) 86.90(3) 97.79(3) 88.10(4) 87.75(4) 87.37(4) 90.11(6) 89.35(6) 89.83(6)

Both 2 and 3 are three-legged piano-stool complexes with pseudo-octahedral cobalt centers. The η4-tetramethylcyclobutadiene ligand coordinates in a facial fashion to the metal with dppe in two mutually cis coordination sites as necessary and the C

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Scheme 2. Synthetic Scheme for the Preparation of Complexes 4, 5, and 6

expected, the structures resemble closely those of 2 and 3 with an obvious three-legged piano-stool geometry, and the η4tetramethylcyclobutadiene ligand coordinated in a facial fashion to the pseudo-octahedral Co(III) center. The distance from the Co to the cyclobutadienyl ring centroid is slightly longer in both 4 and 5 compared to 2 and 3 (Table 1). The Co−N and Co−P bond lengths in 4 are directly comparable to those in 2, and the Co−PH2Ph bond length in 5 is unchanged from that in 3. The dihedral angle between the C4 and P3 planes is 8.8(1)° in 5, and the o-phenylene carbons and the two chelating phosphorus atoms are roughly coplanar in 4. The dihedral angle between this plane and the trigonal plane defined by the two chelating P donors and the Co atom is 10.6(1)°. The equivalent dihedral angle in 5 is expanded to 27.2(1)°. This could be due to a larger repulsion between the phenylphosphine and the dfppb ligand compared to that between acetonitrile and dfppb in 4. The dfppb bite angle is more acute than that seen for dppe, with values of 85.85(3)° in 4 and 86.30(3)° in 5. The remaining P−Co−P angles in 5 are distinctly disparate (Table 1), and the P···P nonbonded distances between the two phosphines of the chelate and the phenylphosphine are 3.103(1) and 3.341(1) Å. The cyclization reaction to form the macrocycle complex [(η 4 -C 4 Me 4 )Co{1,4-bis(2-fluorophenyl)-7-phenyl[b,e,h]tribenzo-1,4,7-triphosphacyclononane}]+ (6) was promoted by addition of 2 molar equiv of KOtBu to a solution of complex 5 in THF. The mechanism is presumed to involve initial deprotonation of the coordinated phenylphosphine to produce the bound phosphide anion with an available lone pair for nucleophilic attack at the fluorine-bearing carbon on an adjacent −P(o-ArF)2 group. This is expected to be aided by coordination-enhanced electrophilic activation of the orthocarbon of the 2-fluoroarylphosphine unit. Addition of 2 molar equiv of KOtBu to a solution of 5 in THF caused an immediate color change from orange to red, which reverted back to orange within a matter of minutes. The short-lived red color is consistent with the transient formation of the intermediate phosphide complex, which reacts quickly with a neighboring 2fluoroaryl group. 31P NMR analysis of the reaction mixture revealed the complete absence of 5 and the presence of a new species characterized by two new signals at δP 100.9 and 94.8 ppm with relative intensities of 1:2, respectively (AB2 pattern). The chemical shifts of these resonances are significantly downfield of those seen for 5 but are entirely consistent with the shifts expected upon formation of new chelate rings.8 No

change (following addition of phenylphosphine) was observed; however, a new resonance at δP −19.6 ppm in the 31P spectrum confirms coordination of phenylphosphine (1JP−H = 339 Hz) along with a shift for the coordinated dfppb to δP 61.7 ppm. Once again, P−P coupling could not be resolved. The 19F NMR spectrum shows two resonances at δF −96.3 and −97.2 ppm (no resolved P−F coupling) for the two rotamers of the complex, and the IR spectrum of 5 shows PH bond stretches at 2304 and 2335 cm−1. The molecular structures of 4 and 5 determined by singlecrystal X-ray crystallography are shown in Figure 2. As

Figure 2. ORTEP view of the cations 4 (top) and 5 (bottom). D

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C4Me4 ligand and the terminal phenyl or fluorophenyl group in 6 also causes these groups to be bent away from the cobalt atom with the Co−P−Cexo (exocyclic phenyl or fluorophenyl group) angles expanded to 123.1(1)°, 126.7(1)°, and 126.9(1)°, respectively, whereas the Co−P−Cring angles are close to tetrahedral [av 109.5(1)°]. It is likely that the two larger Co−P−Cexo angles belong to the more sterically demanding fluorophenyl groups, which is consistent with the bond length data above. Intramolecular C−C Bond Coupling between the Phosphine Macrocycle and Tetramethylcyclobutadiene. As noted above, the addition of 2 molar equiv of a strong base to a solution of 5 in THF results in an intramolecular SN2Ar attack of a transient phosphide at a neighboring fluoroaryl group to generate ultimately complex 6. However, if an excess of base is used for the ring-closure reaction of 5, a second product is formed in addition to 6. This observation was a little surprising given that the reaction with exactly 2 molar equiv of base was highly selective, giving 6 in essentially quantitative yield, and suggested that the macrocyclic complex was undergoing further reaction (most likely involving the two remaining 2-F-aryl groups) upon addition of excess base. In order to establish whether this was indeed the case, preformed 6 was treated with 2 molar equiv of KOtBu. The addition of the base to an orange solution of 6 in THF gave an immediate red color, which again quickly reverted back to orange. Observation of the 31P{1H} NMR spectrum at this stage showed two peaks at δP 106 and 102 ppm with no evidence of the resonances for 6. The addition of 1 equiv of KOtBu to a solution of 6 in THF did not lead to the observation of intermediate 7a in the 31 1 P{ H} NMR spectrum, and only resonances assignable to 6 and 7 were seen; this indicates rapid activation of the second CH3 group after formation of the first linkage. The 31P{1H} NMR spectrum of the isolated orange complex in CD2Cl2 shows two resonances shifted downfield relative to 6 at δP 106.3 and 102.2 ppm of 2:1 relative intensity and no observable JP−P coupling (i.e., JP−P was unresolvable within the line-width of the resonance). The 19F NMR spectrum of the solid showed only a doublet at δF −72.9 ppm (1JF−P = 710 Hz) due to the PF6− counterion, confirming the absence of the ortho-fluorophenyl fluorines and loss of HF. The 13C{1H} DEPT spectrum revealed a new CH2 resonance at δC 26.5 ppm and two sets of peaks for the η4-C4Me4 ring carbons at δC 86.6 and 78.3 ppm. The data are consistent with a reaction occurring at two of the methyl carbons of the η4-C4Me4 ligand presumably through a nucleophilic attack (after deprotonation) of a methylenide anion at the fluorine-bearing carbons of the fluoroaryl groups of 6, as indicated in Scheme 3. The two methylene protons of the CH2 bridges are diastereotopic and appear as a doublet at δH 3.08 ppm (2JH−H = 16.8 Hz) and a doublet of triplets at δH 3.57 ppm through further coupling to one aryl hydrogen and one phosphorus in the 1H NMR spectrum of 7. As noted in the scheme, there are two structural possibilities for 7, namely, cis-7, where two neighboring methyl groups have reacted to form the methylene bridges to form a hemi-incarcerand species, or the trans-7 isomer, where the two methyls in the 1,3 positions on the C4 ring have reacted to give an incarcerand complex. In order to establish which of these is formed (it is clear from the spectroscopic data that a single product is obtained), the complex was crystallized from MeCN/Et2O as its SbF6− salt and the molecular structure determined by single-crystal X-ray diffraction methods (Figure 4). The cobalt in 7 has a pseudooctahedral geometry similar to that in complexes 1−6.

1

JP−H coupling was observed in the 31P NMR spectrum, confirming that both resonances were due to tertiary phosphines. The air- and moisture-stable orange compound 6 was subsequently isolated as its PF6− salt in >95% yield. The 31 1 P{ H} NMR spectrum was broadened to the extent that no individual P−P or P−F couplings were observed (a complex pattern would be anticipated for the ABB′XX′ spin system). The 19F NMR spectrum of 6 does show a temperature dependence suggesting some fluxionality in solution presumably arising from hindered rotation of the o-fluorophenyl groups about the P−Cipso bond. The hindered rotation renders the F atoms magnetically inequivalent, and four broad singlets are observed in the 19F NMR spectrum at ambient temperature. Upon heating to 70 °C in (CD3)2SO, the spectrum changes so that only two signals are observed; however, further heating did not lead to the observation of a single peak. The mass spectrum of compound 6 showed a molecular ion at m/z = 755 amu consistent with the formation of the macrocyclic complex. The crystal structure of compound 6 (Figure 3) confirms the formation of the triphosphamacrocycle. The Co atom is

Figure 3. ORTEP view of the cation 6.

contained in a sandwich structure where the macrocycle occupies an octahedral face opposite the η4-cyclobutadienyl ligand and ligates the metal center via three fused fivemembered rings as a tridentate crown. The roughly parallel P3 and C4 planes have a dihedral angle of 4.1(1)°, and the Co−P bond lengths in 6 are significantly shorter than those in 5 (Table 1). This exemplifies the macrocycle coordination effect, leading to strong bonding between the metal center and the macrocyclic ligand. The fluorine positions in the structure are disordered, and it was not possible (from structural data) to identify definitively the two phosphorus atoms bearing the ofluorophenyl substituents. However, two of the Co−P bond lengths are slightly longer than the third, and it is likely that these two longer contacts are to the o-fluorophenyl-bearing phosphines (for steric reasons). The average dihedral angle between the plane defined by the o-phenylene carbons and their phosphorus atoms and the trigonal plane formed by the two phosphorus donors and the Co(III) atom in 6 is 9.5(1)°, and the P−Co−P angles of the three fused five-membered chelate rings are 88.10(5)°, 87.74(4)°, and 87.35(4)° with nonbonded P···P distances averaging 3.002(1) Å. In all five of the structures 2−6, the methyl groups of the cyclobutadienyl unit are displaced out of the C4 ring plane away from the ligands on the opposing face, and in 6 this is on the order of 8°. This intramolecular repulsion between the η4E

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Scheme 3. Formation of cis-7

leads to a shortening of all the metal−ligand bonds in the complex compared to the other complexes, with the Co−C4 centroid length being 1.716(8) Å and the average Co−P distance being 2.123(1) Å. The Co−P−C (C1 and C20 on the coupled methylenephenyl) angles average 119.5(2)° and are smaller than the remaining Co−P−Cipso angle to the untethered phenyl group, which is 124.2(2)°. The Co−P−C1 and Co−P−C20 angles are both significantly smaller than those in the uncoupled precusor 6 due to the constraints imposed as a result of the coupling. The related C−H bond activation of ring methyls in η51,2,3,4,5-pentamethylcyclopentadienyl complexes has previously been reported. Saunders and others have observed intramolecular coupling to the o-aryl position of a fluorophenyl phosphine ligand in a number of transition metal complexes.10 The precise nature of the ring methyl activation of η5-C5Me5 or η6-arene ligands, followed by intramolecular or intermolecular coupling reactions, remains speculative, and some are sensitive to the nature of the organometallic base used. In certain cases only KOtBu is successful,10c,11 and, occasionally, no additional base is necessary.10a,b Metal-dependent, multiple alkylations have been reported with η5-C5Me5.11 Although C−H activation in η5-C5Me5 complexes is well established, to the best of our knowledge, the current example is the first report of the ring methyl C−H bond of an η4-tetramethylcyclobutadienyl ligand undergoing such an intramolecular coupling. The reaction of 6 to give cis-7 can be viewed simply as an overall addition− elimination reaction or a nucleophilic aromatic substitution, and a mechanism similar to that proposed by Saunders10b,d and Hughes10c is likely to operate. The cobalt center is clearly influential and brings the ring methyl of the η4-C4Me4 ligand and the o-fluorophenyl group of the P3 macrocycle ligand into

Figure 4. ORTEP view of the cation cis-7.

There are, however, obvious structural differences between 7 and the other complexes reported herein. The cobalt center is encompassed by the two parts of a hybrid triphosphamacrocycle−cyclobutadienyl ligand, and although the two methylene carbon atoms and a ring carbon atom of the cyclobutadiene in 7 are disordered, the structure clearly shows the triphosphamacrocycle linked to the cyclobutadiene group at two adjacent (cis) positions of the cyclobutadiene ring. Thus, it is the cis-7 isomer that is formed selectively, as indicated in Scheme 3. Although the metal is encapsulated within the ligand framework, there is a partially open face on the side of the two unreacted peripheral methyl groups of the Cb* unit. This open face would not be present in the trans form, where the cobalt would be truly incarcerated. Hence complex 7 can be described as a hemi-incarcerand. The formation of the methylene straps F

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Organometallics

Article

spectra were recorded on a PerkinElmer 1600 spectrophotometer using KBr pellets for solid samples. Mass spectra were carried out on a VG Platform II Fisons mass spectrometer. The NMR spectra were recorded on a Bruker DPX-400 instrument at 400 MHz (1H) and 100 MHz (13C) or a Jeol Lamda Eclipse 300 at 121.65 MHz (31P), 300.52 MHz (1H), 75.57 MHz (13C), and 282.78 MHz (19F). Elemental analyses were performed by the Warwick Analytical Service. [(C4Me4)Co(NCCH3)3]PF6,13 1(PF6), phenylphosphine,14 1,2-bis(dichlorophosphino)benzene, 15 and 1,2-bis{di(2-fluorophenyl)phosphine}benzene9b were prepared by literature procedures. Crystallography. Complexes 2−6 were crystallized as their PF6− salts and complex cis-7 was crystallized as the SbF6− salt by vapor diffusion of diethyl ether into solutions of the complexes in acetonitrile. X-ray diffraction data collection was carried out on a Bruker Kappa CCD diffractometer at 150(2) K with Mo Kα irradiation (graphite monochromator). Empirical absorption corrections were performed using equivalent reflections. For the solution and refinement of the structures, the program package SHELX was employed.16 H atoms were placed into calculated positions, and a riding model was used during refinement. One phenyl group in complex 6 is disordered with occupancies of 0.74:0.26. The two F atoms in the complex are refined and distributed between four positions (two ordered and one disordered phenyls) with a disordered model of 0.72:0.83:0.44:0.01. In complex cis-7, the η4-C4Me4 ring and the associated CH2 and phenyl groups are disordered with occupancies of 0.62:0.38. Pertinent data are given in Table 1. Figures were drawn using ORTEP-3 for Windows;17 in all cases, thermal ellipsoids are drawn at the 30% probability level and hydrogen atoms are omitted in all figures for clarity of presentation. A table of crystallographic data is available in the Supporting Information. [(η4-C4Me4)Co(dppe)(NCCH3)]PF6, 2(PF6). To a solution of [(η4C4Me4)Co(NCCH3)3]PF6, 1 (0.10 g, 0.23 mmol), in CH3CN (20 mL) was added dppe (0.09 g, 0.23 mmol) with stirring. The deep-red color of the solution changed to yellow-brown immediately, and a resonance at δP 70.2 ppm was observed in the 31P{1H} NMR spectrum of the solution. The reaction mixture was filtered and concentrated to small volume. Orange crystals of 2 were obtained by vapor diffusion of diethyl ether into the solution. Yield = 156 mg (90%). 31P{1H} (CDCl3): 74.3 (dppe), −143.9 (septet, PF6, 1JP−F = 711 Hz) ppm. 13 C{1H} NMR (CDCl3): 135−115 (aromatics), 85.5 (C4Me4), 28.5 (m, CH2), 8.9 (C4Me4), 1.4 (MeCN) ppm. 1H NMR (CD2Cl2): 7.9 to 6.9 (m, 20H, Haryl), 3.08 (m, 2H), 2.65 (m, CH2), 0.92 (s, 12H, C4Me4), 0.80 (s, 3H, CH3CN) ppm. IR: 2255 cm−1 (CN). Anal. Calcd for C36H39NF6P3Co (M = 751.60 g mol−1): C, 57.53; H, 5.24; N, 1.86. Found: C, 57.2; H, 5.1; N, 1.8. [(η4-C4Me4)Co(dppe)(PH2Ph)]PF6, 3(PF6). To a solution of 2 (0.10 g, 0.12 mmol) in CH2Cl2 (20 mL) was added phenylphosphine (17 mg, 0.15 mmol), and the mixture stirred for 20 min, whereupon two resonances at δP 70.2 and −18.6 ppm were observed in the 31P NMR spectrum. The volatile materials were then removed in vacuo, and the solid residue was triturated with 40/60 petroleum ether, then dissolved in CH2Cl2 before filtering and concentrating to small volume. Orange crystals of 3 were obtained upon vapor diffusion of diethyl ether into the solution. Yield = 87 mg (88%). 31P{1H} NMR (CDCl3): 70.2, −18.6 ppm. 31P NMR (CDCl3): 70.2, −18.6 (t, 1JP−H = 336 Hz), −143.8 (septet, PF6, 1JP−F = 711 Hz) ppm. 13C{1H} NMR (CDCl3): 135−115 (aromatics), 83.7 (C4Me4), 29.0 (m, CH2), 6.72 (C4Me4) ppm. 1H NMR (CD2Cl2): 7.9 to 6.8 (m, 25H, Haryl), 4.40 (dm, 2H, PH, 1JP−H = 340 Hz), 3.11 (m, 2H), 2.48 (m, 2H), 0.88 (s, 12H, C4Me4) ppm. IR: 2311 cm−1 (PH), 2333 cm−1 (PH). Anal. Calcd for C40H43F6P4Co (M = 820.64 g mol−1): C, 58.54; H, 5.29. Found: C, 58.4; H, 5.1. [(η 4 -C 4 Me 4 )Co{(o-C 6 H 4 F) 2 PC 6 H 4 P(o-C 6 H 4 F) 2 }(NCCH 3 )]PF 6 , 4(PF6). To a stirred solution of 1 (0.10 g, 0.23 mmol) in CH3CN (20 mL) was added dfppb (0.12 g, 0.23 mmol), whereupon the color of the solution changed instantly from red to orange. 31P{1H} NMR analysis of the solution showed a new resonance at δP 64.3 ppm. The reaction mixture was filtered and concentrated to small volume. Orange crystals of 4 were obtained by vapor diffusion of diethyl ether into the solution. Yield = 156 mg (78%). 31P{1H} NMR (CD2Cl2):

close proximity, facilitating the nucleophilic attack of the transient methylenide carbanion at the 2-F-aryl group. Since the yield of the macrocycle compound 6 obtained by stoichiometric reaction with base and precursor 5 is essentially quantitative with no evidence of the formation of cis-7, the C−H activation is clearly not competitive with the macrocyclization. The cisselectivity is presumably the result of constraints imposed by the presence of the first methylene bridge, which encourages reaction at a neighboring ring CH3 rather than the opposing CH3. The dehydrofluorinative C−C coupling reaction is fast on the NMR time scale, and the intermediate 7a could not be detected by NMR spectroscopy even when a deficiency of base was added to solutions of 6; in this case only a mixture of 6 and 7 was seen. Several routes for macrocycle liberation of the 1,4,7tribenzannulated triphosphacyclononane ligand from complex 6 were attempted without success. The basic premise for liberation is to transform the inert 18-electron cobalt complex to a more kinetically active 17- or 19-electron complex by oxidation or reduction. The addition of 2 molar equiv of LiHB(C2H5)3 to a solution of 6(PF6) in THF caused the solution color to change immediately from yellow to red and subsequently quickly back to yellow. The 31P{1H} NMR spectrum showed only cis-7, and it was clear that the hydride was acting as a base rather than a reductant. The reaction of 6(PF6) with metallic lithium in THF was also attempted. After stirring overnight, no 31P{1H} NMR signal was observed for the solution, suggesting reduction to a paramagnetic species. However, efforts to work up this mixture gave no tractable products. Oxidation of 6(PF6) was performed in acetonitrile solution with ceric ammonium nitrate. The reaction proceeds quite slowly compared to the Fe analogue.12 After stirring overnight, the 31P{1H} NMR spectrum showed no resonances, presumably due to the formation of a paramagnetic complex. However, as above, efforts to isolate identifiable products using workup procedures similar to those employed using the Fe(II) templates were unsuccessful. Similar oxidations using Br2 or H2O2 also produced intractable mixtures.

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CONCLUSION The lability of the acetonitrile ligands in the cationic [(η4C4Me4)Co(NCCH3)3]+ complex enables the selective stepwise coordination of di- and monophosphines to the electrophilic [(η4-C4Me4)Co]+ fragment to give the macrocycle precursor complex [(η4-C4Me4)Co(dfppb)(PH2Ph)]+ (5). The template precursor complex 5 undergoes base-promoted ring closure to produce a nine-membered 1,4,7-triphosphorus macrocycle complex (6) with a tribenzannulated backbone via intramolecular dehydrofluorination. In the presence of excess base (KOtBu), two mutually cis methyls of the η4-C4Me4 ligand in complex 6 undergo C−H bond activation and concomitant coupling to the ortho position of two coordinated PArF groups to produce the hemi-incarcerand complex cis-7 bearing a 12electron-donor cyclobutadienyl/triphosphamacrocycle hybrid ligand.

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EXPERIMENTAL SECTION

General Information. Unless stated otherwise, all reactions were performed under a nitrogen atmosphere using standard Schlenk techniques and, where appropriate, an inert-atmosphere glovebox. Solvents were dried and degassed by refluxing over standard drying agents under dinitrogen and distilled immediately prior to use. Infrared G

dx.doi.org/10.1021/om500734b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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64.3 (dfppb), −143.9 (septet, PF6, 1JP−F = 711 Hz) ppm. 19F NMR (CD2Cl2): −96.5 (dfppb), −97.1 (dfppb), −73.4 (d, PF6) ppm. 13 C{1H} NMR (CD2Cl2): 135−115 (aromatics), 85.1 (C4Me4), 8.2 (C4Me4), 1.4 (MeCN) ppm. 1H NMR (CD2Cl2): 7.6 to 6.4 (20H, m, Haryl), 0.91 (12H, s, C4Me4), 0.79 (3H, s, CH3CN) ppm. IR: 2269 cm−1 (CN). Anal. Calcd for C40H35NF10P3Co (M = 871.56 g mol−1): C, 55.12; H, 4.05; N, 1.61. Found: C, 54.8; H, 4.1; N, 1.5. [(η 4 -C 4 Me 4 )Co{(o-C 6 H 4 F) 2 PC 6 H 4 P(o-C 6 H 4 F) 2 }(PH 2 Ph)]PF 6 , 5(PF6). To a solution of 4 (0.20 g, 0.23 mmol) in CH2Cl2 (20 mL) was added phenylphosphine (0.03 g, 0.27 mmol), and the solution was stirred for 15 min. Inspection of the 31P{1H} NMR spectrum of the mixture showed two resonances at δP 61.7 and −19.6 ppm. The reaction mixture was filtered, and the solvent removed in vacuo. The residue was triturated with diethyl ether to remove excess phenylphosphine, and the remaining yellow powder was dissolved in CH2Cl2 and concentrated to small volume. Orange crystals of 5 were obtained after diffusion of diethyl ether vapor into the concentrated solution over several days. Yield = 147 mg (68%). 31P NMR (CD2Cl2): 61.7, −19.6 (t, 1JP−H = 339 Hz), −143.8 (septet, PF6, 1JP−F = 711 Hz) ppm. 19 F NMR (CD2Cl2): −96.3 (dfppb), −97.2 (dfppb), −73.3 (d, PF6) ppm. 13C{1H} (CD2Cl2): 135−115 (aromatics), 83.7 (C4Me4), 6.72 (C4Me4) ppm. 1H NMR (CD2Cl2): 7.4−6.7 (m, 25H, Haryl), 4.0 (dm, 2H, PH, 1JP−H = 336 Hz), 0.86 (s, 12H, C4Me4) ppm. IR: 2304 cm−1 (PH), 2335 cm−1 (PH). Anal. Calcd for C44H39F10P4Co (M = 940.61 g mol−1): C, 56.19; H, 4.18. Found: C, 55.8; H, 4.0. [(η 4 -C 4 Me 4 )Co{1,4-bis(2-fluorophenyl)-7-phenyl[b,e,h]tribenzo-1,4,7-triphosphacyclononane}]PF6, 6(PF6). To a solution of 5 (0.27 g, 0.29 mmol) in THF (20 mL) was added KOtBu (65 mg, 0.58 mmol), whereupon the solids dissolved quickly and the color of the solution changed immediately from orange to red, then back to orange again. After stirring for 15 min, the 31P NMR spectrum showed two resonances at δP 102.0 and 93.9 ppm with relative intensities of 1:2. The solvent was removed in vacuo, the residue dissolved in dichloromethane, and the mixture filtered. The solvent was removed in vacuo, and the solids were dissolved in the minimum amount of acetonitrile. Orange crystals of 6 as an acetonitrile solvate were obtained by diffusion of diethyl ether vapor into the solution. Yield = 218 mg (80%). 31P{1H} NMR (CD2Cl2): 100.9 (m), 94.8 (m), −143.8 (septet, PF6, 1JP−F = 711 Hz) ppm. 19F NMR (d6-DMSO, 70 °C): −96.4, −96.9, −73.3 (d, PF6) ppm. 13C{1H} NMR (CD2Cl2): 135−115 (aromatics), 82.4 (C4Me4), 6.2 (C4Me4) ppm. 1H NMR (CD2Cl2): 7.6 to 7.1 (m, 25H, Haryl), 1.8 (s, 3H, CH3CN), 0.7 (s, 12H, C4Me4) ppm. MS (APCI): 755 (M+). Anal. Calcd for C46H40NF8P4Co (M = 941.65 g mol−1): C, 58.67; H, 4.28; N, 1.49. Found: C, 58.6; H, 4.2; N, 1.3. [{η 4 ,κP,κP,κP-Me 2 C 4 -[1,4-bis(2-CH 2 C 6 H 4 )-7-C 6 H 5 -[b,e,h]tribenzo-1,4,7-triphosphacyclononane]-1,2}Co]PF6, cis-7(PF6). To a solution of 6 (0.10 g, 0.1 mmol) in THF (20 mL) was added KOtBu (24 mg, 0.2 mmol). The solids dissolved quickly, and the color of the solution changed immediately from orange to red, then back to orange. The 31P{1H} NMR spectrum showed two new resonances at δP 106 and 102 ppm with relative intensities of 2:1. The solvent was removed in vacuo, and the residue was dissolved in dichloromethane and filtered. The solvent was removed in vacuo, and the residue dissolved in acetonitrile. Orange crystals of cis-7 were obtained upon vapor diffusion of diethyl ether into the solution. Yield = 56 mg (65%). 31 1 P{ H} NMR (CD2Cl2): 106.3, 102.2, −143.8 (septet, PF6, 1JP−F = 711 Hz) ppm. 19F NMR (CD2Cl2): −72.9 (d, PF6, 1JP−F = 712 Hz) ppm. 13C{1H} NMR (CD2Cl2): 135−115 (aromatics), 84.6 (C), 78.3 (C), 26.5 (CH2), 7.0 (CH3) ppm. 1H NMR (CD2Cl2): 7.3 to 8.4 (m, 25H, Haryl), 3.57 (dt, 2H, CH2, 2JH−H = 16.8, 4JH−H/H‑P = 7.2 Hz), 3.08 (d, 2H, CH2, 2JH−H = 16.8 Hz), 1.08 (s, 6H, Me2C4) ppm. MS (APCI): 714 (M − 2H), 713 (M − 3H). Anal. Calcd for C44H35F6P4Co (M = 860.60 g mol−1): C, 61.40; H, 4.11. Found: C, 61.0; H, 4.0.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: edwardspg@cardiff.ac.uk. *E-mail: newmanp1@cardiff.ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC for funding and for support for NMR spectroscopy and mass spectrometry facilities. We are very grateful to Dr. R. L. Jenkins and Mr. R. Hicks for help in collecting spectroscopic data.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic data for 2, 3, 4, 5, 6, and cis-7 (CIF) are available free of charge via the Internet at http://pubs.acs.org. H

dx.doi.org/10.1021/om500734b | Organometallics XXXX, XXX, XXX−XXX