Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes
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Abstract
Microwave irradiation of 1,6-diynes, RC≡C(CH2)4C≡CR, with Fe(CO)5 in dimethylether leads to the facile and clean formation of cyclopentadienone complexes [{η4-C4R2C(O)C4H8}Fe(CO)3] in good yields resulting from a [2 + 2 + 1] cycloaddition. The molecular structures of three examples (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) have been obtained. The addition of HBF4 leads to the clean and reversible formation of cationic hydroxycyclopentadienyl complexes [{η5-C4R2C(OH)C4H8}Fe(CO)3][BF4]. Sequential addition of hydroxide and acid has also been carried out in an attempt to prepare hydroxycyclopentadienyl–hydride complexes. These were largely unsuccessful but in one case a Shvo-type complex with a bridging hydride was detected by 1H NMR spectroscopy. Reasons for the differing behaviour of [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)3] and the related aryl-functionalised derivatives are considered.
Graphical Abstract
Introduction
Proposed equilibrium between dimeric Shvo catalyst and mononuclear species
Sequential conversion of cyclopentadienone 1 into hydroxycyclopentadienyl–hydride complex B
The synthesis of cyclopentadienone complexes of this type is typically achieved via the [2 + 2 + 1] cycloaddition of 1,6-hexadiynes and iron carbonyls [14, 15, 16, 17, 46, 47, 48, 49, 50, 51, 52, 53, 54]. Such processes require high temperatures and long reaction times and generally provide the desired cyclopentadienone complexes in only low-to-moderate yields. Further, they are often carried out in sealed tubes, thus requiring specialist handling techniques in order to minimise risks. The use of microwave reactors to accelerate organic transformations is now well documented [55, 56], and there is an increasing literature concerning the use of microwaves in organometallic synthesis [57, 58, 59, 60]. In order to develop the chemistry of cyclopentadienone iron tricarbonyl complexes as catalysts for hydrogen transfer reactions, we sought a simple, efficient and high-yielding synthesis of range of these complexes and consequently sought to develop a microwave-assisted synthesis. Herein, we report the success of this route together with the molecular structures of three of these complexes with the aim of better understanding the correlation between structural parameters and catalytic efficiency. We also report attempts to prepare Shvo-type hydroxycyclopentadienyl–hydride complexes upon sequential addition of hydroxide and proton sources. These were largely unsuccessful, but in one case an iron hydride was identified by 1H NMR spectroscopy.
Results and discussion
Synthesis and characterisation of cyclopentadienone complexes
Microwave-accelerated syntheses of cyclopentadienone complexes 1–5
Characterisation was straightforward, IR spectra being particularly characteristic with each showing (in CH2Cl2) three strong absorptions between 2073 and 1987 cm−1 associated with the metal-bound carbonyls and a fourth at 1610–1638 cm−1 attributed to the ketonic carbonyl. NMR spectra were in accordance with the proposed structures, notable features being the observation of ketonic and metal-bound carbonyl groups at 169–170 and 208–210 ppm, respectively, in the 13C spectra of 2–5. It is perhaps noteworthy that while the metal-bound carbonyls in 1 also appear within the expected range, the ketonic carbonyl was shifted considerably downfield, appearing at 181.6 ppm. This suggests that while the metal centres in the aryl (2–5)-functionalised and trimethylsilyl (1)-functionalised complexes are electronically comparable, the nature of the ketonic carbonyl differs which may account for observed differences in reactivity.
Structural studies
Molecular structures of a 2, b 3 and c 4
Selected bond lengths (Å) and angles (o) for cyclopentadienone complexes
2 | 3 | 4 | 6 c | |
---|---|---|---|---|
Fe–C(4)O | 2.419 (2) | 2.386 (2) | 2.373 (4) 2.412 (4) | 2.362 (9) |
Fe–C(5)R | 2.137 (2) | 2.107 (2) | 2.141 (4) 2.151 (4) | 2.114 (9) |
Fe–C(8)R | 2.122 (2) | 2.140 (2) | 2.139 (4) 2.147 (4) | 2.131 (9) |
Fe–C(6)CH2 | 2.094 (2) | 2.080 (2) | 2.076 (4) 2.084 (4) | 2.057 (9) |
Fe–C(7)CH2 | 2.074 (2) | 2.099 (2) | 2.092 (4) 2.086 (4) | 2.053 (9) |
C(4)–O(4) | 1.223 (2) | 1.228 (2) | 1.240 (5) 1.237 (5) | 1.237 (10) |
C(4)–C(5) | 1.490 (2) | 1.480 (2) | 1.487 (5) 1.502 (5) | 1.484 (13) |
C(4)–C(8) | 1.492 (2) | 1.494 (2) | 1.496 (6) 1.487 (5) | 1.482 (12) |
C(5)–C(6) | 1.429 (2) | 1.437 (2) | 1.453 (6) 1.444 (5) | 1.440 (12) |
C(7)–C(8) | 1.446 (2) | 1.440 (2) | 1.439 (5) 1.432 (5) | 1.418 (13) |
C(6)–C(7) | 1.430 (2) | 1.429 (2) | 1.421 (6) 1.425 (6) | 1.408 (12) |
C(5)–C(4)–C(8) | 103.3 (1) | 103.6 (1) | 104.9 (3) 104.3 (3) | 105.2 (9) |
C(4)a | 0.308 | 0.280 | 0.246 0.276 | 0.257 |
O(4)a | 0.639 | 0.570 | 0.507 0.577 | 0.477 |
C(13)–C(18)b | 25.5 | 40.5 | 29.8 17.4 | – |
C(19)–C(24)b | 59.9 | 40.0 | 73.0 50.1 | – |
As shown in Fig. 1, in all three complexes the aryl rings are rotated out of the plane of the cyclopentadienone moiety. This can be quantified by considering the torsion angles of the aryl rings relative to the diene unit which vary between 17.4 and 73.0°, the smallest and largest values both being associated with the independent molecules of 4 (Table 1). In 2 and 4 (both molecules), there are significant differences in the positions of the two aryl rings, while in 3 both are rotated by ca. 40° with respect to the cyclopentadienone group. In all cases, the two rings are rotated in opposite directions. Closer inspection of the structures shows that in each there is a close contact between an ortho-proton on the aryl ring and the ketonic oxygen atom, O(4). Thus in 1, O(4) lies close to both H18A and H20A [O–H 2.573 and 2.681 Å], while in other molecules O–H contacts range from 2.284 to 2.649 Å. In 3 there is a close contact with one proton [O(4)–H(24A) 2.325 Å] and also one of the fluorine substituents on the other ring [O(4)–F(1) 2.968 Å].
Synthesis of bis(acetonitrile) adduct of 1
Space-filling diagrams for a 2 and b [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)(NCMe)2] (6)
Protonation experiments
Protonation of cyclopentadienone complexes 2–4
Attempts to prepare Shvo-type hydroxycyclopentadienyl–hydride complexes
A key feature of the ability of 1 to act as hydrogen transfer catalyst is its conversion to the hydroxycyclopentadienyl–hydride B formed upon sequential addition of hydroxide and proton sources (Scheme 2) [12, 13, 52]. The first step in this sequence is the nucleophilic attack of hydroxide at the metal-bound carbonyl followed by subsequent elimination of CO2 in a Hieber base reaction [64, 65, 66]. The resulting hydrido-anion, A, is then protonated at the ketonic carbonyl. As seen above, the latter is facile for all neutral cyclopentadienone complexes and thus should not vary upon changing substituents. Nucleophilic attack of hydroxide at the metal-bound carbonyl is the first step and should be controlled by the electrophilic nature of the metal-bound carbon, which in turn should be reflected by the position of these carbons in the 13C NMR spectrum. As the chemical shift of the carbonyls in aryl-substituted 2–5 (208.8–209.6 ppm) are in accord with those in 1 (209.1 ppm), it seems reasonable to expect a similar degree of electrophilicity across these complexes. We have attempted to prepare related hydroxycyclopentadienyl–hydride complexes starting from the aryl-substituted cyclopentadienone complexes.
Proposed transformations upon sequential addition of NaOH and acids to cyclopentadienone complexes 2–5
The addition of H3PO4 or HBF4 (used in NMR studies) to the reaction mixture at this stage resulted in formation of a clear red solution in each case. IR spectra showed the generation of two new carbonyl bands coming at 1996 and 1946 cm−1 for 2. While clearly not the same species seen in solution prior to protonation, the small shifts (− 8 and + 4 cm−1) and similar pattern suggest Fe(CO)2X unit(s). The 1H NMR spectrum of the crude material after addition of acid was complex, and little information could be obtained; however, there were no signals associated with terminal hydrides, but a sharp singlet was observed at δ − 22.3 attributed to a bridging hydride. Attempts were made to separate reaction products by chromatography on silica but without much success. Most notably, from the reaction of 5 a red band was isolated with carbonyl resonances at 2018vs, 1985 m, 1961 cm−1 in the IR spectrum, while the three hydride resonances prior to addition of acid (see above) were replaced by a single sharp resonance at δ − 22.87. Further, in the aromatic region of the 1H NMR spectrum, a pair of AB doublets shows that the two aromatic groups remain equivalent, while in the aliphatic region signals associated with the methylene and methyl resonances were observed. We suggest that these observations are consistent with conversion of all three components of the reaction mixture prior to acidification into Shvo-type complexes (Scheme 6). Unfortunately, all our attempts to isolate pure products have been unsuccessful.
Discussion
From these studies, we believe that the aryl-substituted cyclopentadienone iron tricarbonyl complexes 2–5 undergo a broadly similar Hieber base reaction upon addition of sodium hydroxide to that described by Knölker et al. for 1 [52]. A competing reaction in the case of the aryl complexes is the decomplexation of the cyclopentadienone ligand which leads to the formation of a strong red colouration of the reaction mixture and some precipitate. This is, however, a relatively minor side reaction and may also occur to some extent with 1, but since the free cyclopentadienone is yellow [69], its formation is not easily detected. A second competing reaction appears to be the “back reaction” of the generated hydride anion with unreacted cyclopentadienone complex to afford, after CO loss, a dimeric complex with a bridging hydride. Such a “dimerisation” may be precluded in the case of the trimethylsilyl derivative on the basis of steric considerations (Fig. 2). This also becomes important upon protonation, which in the case of 1 yields the desired hydroxycyclopentadienyl–hydride B, but for the aryl derivatives generates Shvo-type complexes instead. Guan et al. have described similar attempts to generate an iron(II) hydride from 2 [16]. They reported that “rapid decomposition reactions precluded purification and full characterisation of the desired” hydride, which at first sight appears to contradict our own observations. However, they also reported that upon addition of acetone to the crude reaction mixture in d8-toluene a signal was observed at δ − 22.36 in the 1H NMR spectrum which is the same species as we observed (at δ − 22.32 in CD2Cl2). Guan suggests that the observed differences between 1 and 2 are likely due to the propensity of the bulky trimethylsilyl groups to block the formation of dimeric products. We would broadly concur with this view, while also noting the clear difference in stability between the iron(II) cyclopentadienyl–hydride B and the related species [(η5-C5H5)Fe(CO)2H] studied by Baird and co-workers [70]. The latter is rapidly degraded in the presence of oxidants via a free-radical chain process which generates hydrogen and the iron(I) dimer [(η5-C5H5)Fe(CO)2]2. Hence, it might be that the stability of iron(II) hydride complexes of the general type [(η5-cyclopentadienyl)Fe(CO)2H] is very sensitive to the nature of the substituents on the cyclopentadienyl ligand. Further studies are required to fully ascertain this.
Summary and conclusions
In this contribution, we have shown that microwave irradiation of 1,6-diynes, RC≡C(CH2)4C≡CR, with Fe(CO)5 provides a simple and high-yielding route to cyclopentadienone complexes [{η4-C4R2C(O)C4H8}Fe(CO)3] (1–5) and crystal structures of three aryl derivatives (2–4) are reported. Protonation results in the facile and reversible conversion of the cyclopentadienone ligand into a hydroxyl-cyclopentadienyl moiety (7–9). Sequential addition of hydroxide and acid was carried out in an attempt to prepare hydroxycyclopentadienyl–hydride complexes but was largely unsuccessful, and these results are in line with those of Guan et al. [16]. We also find that mild heating of aryl-substituted cyclopentadienone complexes 2–5 over slowly leads to elimination of the free cyclopentadienone, a process that is accelerated upon microwave irradiation in the presence of the decarbonylation agent, Me3NO. Thus, while the trimethylsilyl derivative [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)3] (1) and related silyl-substituted complexes find widespread use in catalysis, seemingly similar aryl derivatives apparently generate less stable 16-electron dicarbonyl and 18-electron hydroxycyclopentadienyl–hydride species and thus are less useful in a catalytic context. Very recently, Wills and co-workers [71] have reported that 2 and related aryl-substituted iron cyclopentadienone complexes are competent catalysts for ketone reductions and alcohol oxidations. Ketone reduction takes place under a H2 atmosphere, and in a model reaction (4 bar H2 and UV irradiation) on a phosphine derivative, hydroxycyclopentadienyl–hydride complexes (two diastereoisomers) were clearly observed by 1H NMR (hydride signals at δ − 12.11 and − 12.18). Clearly, such species are accessible and may be catalytically active.
Experimental
All reactions were carried out under a nitrogen atmosphere in dried degassed solvents unless otherwise stated. Diynes were prepared by standard methods, and Fe(CO)5 was purchased from Aldrich and used as supplied. NMR spectra were run on Bruker AC300 or AMX400 spectrometers and referenced internally to the residual solvent peak. Infrared spectra were run on Nicolet 205 or Shimadzu 8700 FTIR spectrometers in a solution cell fitted with calcium fluoride plates, subtraction of the solvent absorptions being achieved by computation. Fast atom bombardment mass spectra were recorded on a VG ZAB-SE high-resolution mass spectrometer, and elemental analyses were performed in-house. Microwave irradiation was carried out in a CEM 150-W microwave reactor.
Synthesis and spectroscopic data
Diyne (0.8 mmol), Fe(CO)5 (200 μL, 1.6 mmol) and anhydrous dimethylether (2.5 ml) were added to a microwave reactor tube. The tube was flushed with nitrogen and irradiated for 15 min at 200 W and 140 °C in the microwave reactor. The cooled reaction mixture was vented in a fume hood, filtered and the solvent removed in vacuo. Flash chromatography of the residue on silica gel (10% ethyl acetate/hexane) gave two bands, the second yielding 1–5 as dry yellow solids in 70–80% yields. Complex 1 was characterised by comparison with literature spectroscopic data [12, 13]. Protonation studies were carried out by adding a slight excess of HBF4.Et2O to CH2Cl2 or CD2Cl2 solutions of 2–4 in air. The addition of NEt3 resulted in regeneration of the cyclopentadienone complexes. Attempts to generate Shvo-type species resulted from addition of aqueous NaOH (0.8 mol dm−3) to THF solutions of 2–5 which was followed by addition of a slight excess of H3PO4 or HBF4 (for NMR studies).
2 1H NMR (400 MHz, CD2Cl2): δ 1.95 (m, 4H, CH2), 2.77 (m, 2H, CH2), 7.36–7.48 (m, 6H, Ph), 7.72 (dd, J 8.0, 0.8 Hz, 4H, Ph). 13C NMR δ 22.2 (CH2), 23.5 (CH2), 81.9 (C–CH2), 100.99 (CPh), 127.87, 128.33, 129.87, 131.59 (Ph), 169.71 (C=O), 209.24 (CO). IR (CH2Cl2) 2066 s (CO), 2009 s (CO), 1994 s (CO), 1632 m (C=O) cm−1.
3 1H NMR (400 MHz, CD2Cl2): δ 1.84 (br, 4H, CH2), 2.59 (m, 2H, CH2), 6.97 (m, 4H, Ar), 7.60 (q, J 10.8, 2H, Ar). 19F NMR δ − 103.6 (s), − 110.1 (s). 13C NMR δ 22.5 (CH2), 23.0 (CH2), 77.3 (C–CH2), 102.6 (CAr), 104.7 (t, J 26.2, Ar), 112.0 (d, J 21.0, Ar), 115.2 (d, J 16.0, Ar), 134.8 (d, J 4.0, Ar), 168.0 (C=O), 208.8 (CO). IR (CH2Cl2) 2072 s (CO), 2016 s (CO), 2003 s (CO), 1638 m (C=O) cm−1.
4 1H NMR (400 MHz, CD2Cl2): δ 1.63 (br, 4H, CH2), 2.77 (m, 2H, CH2), 3.85 (s, 6H, OMe), 6.96 (d, J 8.0, 4H, Ar), 7.71 (d, J 8.0, 4H, Ar). 13C NMR δ 22.5 (CH2), 23.7 (CH2), 81.8 (C–CH2), 100.3 (CAr), 113.7, 123.54, 130.9, 159.2 (Ar), 169.56 (C=O), 209.6 (CO). IR (CH2Cl2) 2062 s (CO), 2005 s (CO), 1991 s (CO), 1627 m (C=O) cm−1; (THF) 2057 s, 2001 s, 1981 s, 1643 m cm−1.
5 1H NMR (400 MHz, CDCl3): δ 1.79 (m, 4H, CH2), 2.34 (s, 6H, Me), 2.48 (m, 2H, CH2), 7.11 (d, J 7.6, 4H, Ar), 7.31 (d, J 7.6, 4H, Ar). IR (CH2Cl2) 2064 s (CO), 2006 s (CO), 1992 s (CO), 1631 m (C=O) cm−1.
7 1H NMR (400 MHz, CD2Cl2): δ 1.83 (m, 2H, CH2), 1.97 (m, 2H, CH2), 2.51 (d, J 16.0, 2H, CH2), 2.73 (d, J 16.0, 2H, CH2), 7.58 (s, 6H, Ph), 7.62 (s, 4H, Ph). 13C NMR δ 21.4 (CH2), 21.9 (CH2), 87.5 (C–CH2), 103.3 (CPh), 124.4, 129.5, 130.6, 130.6 (Ph), 144.0 (C–OH), 209.5 (CO). IR (CH2Cl2) 2099 s (CO), 2046 s (CO) cm−1.
8 1H NMR (400 MHz, CD2Cl2): δ 1.80 (m 2H, CH2), 1.99 (m, 2H, CH2), 2.38 (m, 2H, CH2), 2.68 (m, 2H, CH2), 7.10 (br, 4H, Ar), 7.50 (br, 1H, Ar), 7.70 (m, 1H, Ar). 19F NMR δ − 101.4 (s), − 105.6 (s), − 150.8 (br). IR (CH2Cl2) 2104 s (CO), 2053 s (CO) cm−1.
9 1H NMR (400 MHz, CD2Cl2): δ 1.82 (m, 2H, CH2), 1.96 (m, 2H, CH2), 2.55 (m, 2H, CH2), 2.72 (m, 2H, CH2), 3.82 (s, 6H, OMe), 7.09 (d, J 7.6, 4H, Ar), 7.58 (d, J 7.6, 4H, Ar). IR (CH2Cl2) 2097 s (CO), 2046 s (CO) cm−1.
Crystallography
Crystallographic data and structure refinement information for 2–4
2 | 3 | 4 | |
---|---|---|---|
Empirical formula | C24H18O4Fe | C24H14O4F4Fe | C26H22O6Fe |
Formula weight (Å) | 426.23 | 498.20 | 1945.14 |
Crystal system | Triclinic | Monoclinic | Triclinic |
Space group | P \( \bar{1} \) | P21/n | P \( \bar{1} \) |
a (Å) | 9.137 (1) | 12.954 (2) | 11.332 (3) |
b (Å) | 9.715 (1) | 10.543 (1) | 13.616 (3) |
c (Å) | 11.565 (1) | 14.940 (2) | 16.431 (4) |
α (°) | 80.552 (2) | 90 | 105.536 (4) |
β (°) | 70.070 (2) | 98.511 (2) | 92.801 (4) |
γ (°) | 77.609 (2) | 90 | 110.047 (4) |
Volume (Å3) | 937.99 (18) | 2018.4 (4) | 2266.8 (9) |
Z | 2 | 4 | 4 |
Dcalc (Mg/m3) | 1.509 | 1.639 | 1.425 |
Absorption coefficient (mm−1) | 0.833 | 0.815 | 0.705 |
F(000) | 440 | 1008 | 1008 |
Crystal size (mm) | 0.32 × 0.20 × 0.12 | 0.42 × 0.40 × 0.20 | 0.18 × 0.18 × 0.03 |
θ Range for data collection (°) | 2.72–28.27 | 2.25–28.29 | 1.67–28.25 |
Index ranges | − 11 ≤ h ≤ 12, | − 16 ≤ h ≤ 17, | − 14 ≤ h ≤ 14, |
− 12 ≤ k ≤ 12, | − 13 ≤ k ≤ 13, | − 18 ≤ k ≤ 17, | |
− 14 ≤ 1 ≤ 14 | − 19 ≤ 1 ≤ 19 | − 21 ≤ 1 ≤ 20 | |
Reflections collected | 7957 | 16,599 | 18,738 |
Independent reflections | 4238 [Rint = 0.0211] | 4813 [Rint = 0.0223] | 10,022 [Rint = 0.0548] |
Data/restraints/parameters | 4238/0/334 | 4813/0/299 | 10,022/0/595 |
Goodness of fit on F2 | 1.050 | 1.052 | 0.963 |
Final R indices [I > 2σ(I)] | R1 = 0.0350, wR2 = 0.0979 | R1 = 0.0322, wR2 = 0.0917 | R1 = 0.0716, wR2 = 0.1794 |
R indices (all data) | R1 = 0.0374, wR2 = 0.1000 | R1 = 0.0345, wR2 = 0.0930 | R1 = 0.1075, wR2 = 0.2050 |
Largest diff. peak and hole(e. Å−3) | 0.528 and − 0.400 | 0.429 and − 0.412 | 0.942 and − 1.122 |
Supplementary material
Crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 1520535 (2), 1520536 (3) and 1520539 (4). Copies of this information can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1FZ, UK (E-mail: deposit@ccdc.cam.ac.uk, or on the Web at http://www.ccdc.ac.uk).
Supplementary material
References
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