Ilnett García-Ventura,
Marcos Flores-Alamo and
Juventino J. García*
Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico City, 04510, Mexico. E-mail: juvent@unam.mx
First published on 20th October 2016
The reaction between [(dippe)NiH]2 (1) (dippe = 1,2-bis(diisopropylphosphino)-ethane) and 2- and 3-furonitrile (2-FN and 3-FN) was carried out at room temperature. Both furonitriles initially react with H2 loss and coordinating a [Ni(dippe)] fragment to the nitrile substituent in a η2-CN way to yield complexes [(dippe)Ni(η2-C,N-2-FN)] (2) and [(dippe)Ni(η2-C,N-3-FN)] (10), respectively. After monitoring this reaction for 6 days at room temperature in the presence of a slight excess of each nitrile, the C–CN bond activation products [(dippe)Ni(CN)(η1-C-2-FN)] (7) and [(dippe)Ni(CN)(η1-C-3-FN)] (11) were observed as the main products. These complexes were fully characterized by 1H, 31P{1H} and 13C{1H} NMR and by single crystal X-ray determinations. After isolation and characterization of (7) and (11), a thermal stability study was carried out, and it was found that the –C–CN bond activation is reversible at 100 °C for both compounds. Particularly, the thermolysis of complex (7) allowed the further production of a C–O ring opening at the furan to yield complex [(dippe)Ni(κ2-O,C–OCH
CH–CH
C(CN))] (9), which was also isolated and characterized by multinuclear NMR.
Our group has been interested in these kinds of transformations using nickel-based compounds in the presence of aromatic,4–6 hetero-aromatic6 and alkyl nitriles.7 These studies have contributed to a better understanding of the C–CN oxidative addition process in the presence of a Ni(0) center, allowing an understanding of the observed reactivity by the full characterization of a variety of different intermediates, including the observation of reversibility in some cases of C–CN oxidative addition.4,5 The use of the [Ni(dippe)] fragment has shown that oxidative addition of the C–CN bond is preceded by the η2-coordination of the corresponding nitrile. Additionally, the presence of a bidentate phosphine as an ancillary ligand has been helpful to establish the oxidation state of the metal center by the magnitude of the 2JP–P coupling constant. Thus, Ni(0) complexes with a η2-CN bond have typical 2JP–P values around 60 Hz, and Ni(II) complexes derived from the C–CN oxidative addition have 2JP–P values lower than 30 Hz.5–7
In this context, the use of the complex [(dippe)NiH]2 as a source of the [Ni(dippe)] fragment by Jones and Vicic8 has been a very useful tool for mechanistic studies related to complex catalytic systems. For example, it was found that the [Ni(dippe)] fragment was able to cleave C–S bond in stable molecules such as thiophene, benzothiophenes and dibenzothiophenes, unveiling mechanistic insights in hydrodesulfurization (HDS) reactions.
Recently, the same nickel complex has been used in a study of reactivity with 2-cyanothiophene;9 for that particular substrate, two competitive reaction pathways can be envisaged: C–CN or C–S bond activation. Both were observed, and several nickel intermediates were characterized at low temperature by considering their 2JP–P values. Additionally, in this study, a full characterization of the Ni(II) complexes derived from C–CN and C–S bond activation was reported, and it was possible to establish that C–S bond activation was reversible at 85 °C to ultimately yield the thermodynamically favored C–CN bond activation complex.
With the continuous interest of our group in the activation of nitriles to be used as building blocks of heterocycles, we decided to explore the reactivity of 2- and 3-furonitriles with the nickel complex [(dippe)NiH]2 to assess the reactivity of these substrates towards a C–CN or C–O bond activation. The reactivity of 3-furonitrile in the presence of a nickel complex containing a N-heterocyclic carbene ligand has been reported by Radius et al.,10 but it was only characterized by 1H-NMR without any further evidence. We report here that both furonitriles reacted with preference for a side on nitrile coordination, followed by C–CN oxidative addition as the kinetic favored reaction at room temperature, which is shown to be reversible at 100 °C for 2-furonitrile to yield the C–O bond ring-opening cleavage as the thermodynamically favored product.
At room temperature, complex (2) was formed as the major product, and this compound displayed two broad doublets on 31P{1H}-NMR spectrum, at δ 79.7 and 67.9, with 2JP–P = 64 Hz, which is indicative of two asymmetric phosphorous in a Ni(0) complex in agreement with previous reports of closely related compounds.4–7 The 1H-NMR spectrum shows three resonances at δ 7.56, 6.77 and 6.44 (signals for free 2-FN at δ 7.82, 7.31 and 6.63) that shift to high field according with the expected loss of π electron density in the hetero-aromatic ring due to side on coordination of the CN bond to nickel.
Additionally, the 31P{1H}-NMR spectrum of initial reaction mixture also displays two small broad doublets at δ 66.90 and 58.74 with 2JP–P = 80 Hz, which were assigned to complex (3). To note, this coupling constant is larger compared to the typical values reported for Ni(0) complexes side-on coordinated to nitriles (which are between 60 and 70 Hz). However, larger coupling constants have been observed for aromatic nitriles in which the aromatic or heteroaromatic ring has been coordinated through a double bond in a η2-CC fashion.9,11 Besides, the 1H-NMR spectrum of initial reaction shows another three minor signals at δ 7.22, 6.09 and 5.66. These signals presented a larger change in their chemical shifts compared with the above quoted signals for complex (2), which is consistent with a larger loss of aromaticity in the furan ring due to the direct η2-coordination of nickel to the C
C moiety.
A closely related complex to (3) has been reported for 2-cyanothiophene. DFT calculations showed that this kind of complexes are key intermediates in the case of 2-cyanothiophene leading to C–S and later to a C–CN bond activation.9 Considering the above, 2-FN can react either by two ways, i.e., by C–O or by C–CN bond activation, but we expect the C–CN oxidative addition as the favored one in 2-FN, considering the lower Ni(0)–oxygen affinity compared to Ni(0)–sulfur affinity.12
Thus, after 6 days of reaction at room temperature monitored by 31P{1H} NMR, the reaction mixture displayed the above quoted signals for (2) and (3) in a lower amount and the formation of a new signal located at δ 62.5 (ESI, Fig. S1†) assigned to complex [(dippe)2Ni2], (4).7 However, the 1H NMR spectrum does not show signals corresponding to free 2-FN (ESI, Fig. S2†). This spectrum displays characteristic signals corresponding to compound 2′,2-bifuryl (5) instead, which are assigned by comparison of the reported signals.13 Additionally, complex [(dippe)Ni(CN)2] (6) crystallized out of the solution as yellow crystals, the structure was confirmed by an X-ray structure determination and crystallographic parameters compared with the ones previously reported.7
According to this evidence, it is likely that compounds (2) and (3) yield an oxidative addition product such as [(dippe)Ni(CN)(η1-C-2-FN)] (7) that is not observed under the reaction conditions and that quickly disproportionates to yield compounds (6) and (8) (see Scheme 2). Thus, a further reductive elimination reaction from complex (8) yields 2′,2-bifuryl (5) along with the [(dippe)Ni] fragment; the last of which ultimately forms complex (4) (Scheme 2).
The reaction using an excess of 2-FN with complex (1) monitored by 31P{1H}-NMR initially gave a major set of broad doublets, i.e., the previously quoted signals at δ 79.7 and 67.9 with 2JP–P = 64 Hz, along with the corresponding signals in 1H-NMR spectrum at δ 7.56, 6.75 and 6.44 and the signals for the free 2-FN (ESI, Spectra S1 and S2†). These are the same signals assigned before to complex (2). Complex (3) was not observed under the current reaction conditions.
Again, the reaction mixture was monitored for 6 days at room temperature by 1H and 31P{1H}-NMR (ESI, Fig. S3 and S4†); after this time, the reaction selectively yielded the formation of complex (7). The 31P{1H}-NMR spectrum for (7) showed two close doublets at δ 83.9 and 83.5 with a 2JP–P = 32 Hz. This P–P coupling constant value is characteristic for a Ni2+ which is formed via oxidative addition of the C–CN bond.4–7 Additionally, the 1H-NMR spectrum shows three signals shifted to high field at δ 7.50, 6.44 and 6.17, as expected for the furan ring. The reaction mixture was placed in a dry box fridge at −30 °C and layered with hexanes to obtain suitable crystals for X-ray diffraction for complex (7), allowing to confirm the characterization of the C–CN oxidative addition product. The corresponding ORTEP representation is shown in Fig. 1.
As observed in the ORTEP diagram for complex (7), the nickel center presents a square planar geometry, which is expected for a Ni(II) complex, with a slight elongation of the CN bond [1.14 Å] due to the back-donation from nickel to –C
N anti-bonding orbitals. The heteroaromatic ring was aligned with the P–Ni–P plane. This is an uncommon orientation of the aromatic fragment compared to Ni2+ complexes that have been reported, which derived from C–CN bond activation in aromatic nitriles.5–7,9 In this case, probably this orientation of the hetero-aromatic ring favored π-back donation from nickel to π* anti-bonding orbitals of C
C moiety, which is consistent with a Ni–Cipso bond length in complex (7) that is shorter [1.906(3) Å] compared with related distances observed for benzonitrile [1.935(2) Å] and 2-cyanotiophene [1.918(3) Å] analogues.5,9
The very same crystals were re-dissolved in THF-d8, and in this way, the previous assignations on 1H-NMR and 31P{1H}-NMR spectra were confirmed for complex (7) (vide supra). Additionally, the 13C{1H} NMR spectrum was determined with key resonances assigned to the σ-coordinated CN ligand at δ 135.2 (dd, 2JP–C = 85, 46 Hz) and also the signal corresponding to ipso heteroaromatic carbon at δ 170.4 (dd, 2JP–C = 98, 39 Hz) (ESI, Spectrum S3†).
After leaving a reaction mixture containing complex (7) as a major component in solution (THF-d8) for several days at room temperature, the 31P{1H}-NMR spectrum shows the formation of a new singlet signal at δ 73.5 along with the formation of a yellow crystalline solid precipitating out of the solution (Fig. 2).
The yellow solid corresponds again to complex (6), and consequently the new singlet at 73.5 ppm was assigned to complex (8), which is consistent with the increasing intensity of this signal as long as (7) was consumed. As discussed before, complex (8) results in the intermediate formation of 2,2′-bifuryl, which was confirmed by GC-MS determination (ESI, Chromatogram S1 and Spectrum S4†).
The value of this coupling constant is relatively small to be assigned to a Ni(0) complex and is also quite large for a Ni(II) complex; however, it has a similar value to the reported Ni(II) complex derived from the C–S bond activation of 2-cyanothiophene (2JP–P = 34 Hz).9 Additionally, the 1H-NMR spectrum shows three new up-field signals at δ 7.16 [1H, ddd, J = 11.7, 4.5, 1.8 Hz], δ 6.90 [1H, ddd, J = 16.4, 7.2, 3.6 Hz] and δ 5.10 [1H, dd, J = 7.2, 4.5 Hz] that grow along with the new phosphorous signals as the reaction takes place (ESI, Fig. S5 and S6†). The multiplicity of these signals and the coupling constant values are consistent with a 4JP–H coupling.14 Therefore, complex (9) is proposed as the Ni(II) complex derived from the ring opening oxidative addition of C–O bond in 2-FN (Fig. 3).
Complex (9) was independently synthesized in pure form using a higher quantity of starting complex (7) and toluene as solvent (Experimental section 4.5); thus, 31P{1H} and 1H-NMR assignations were confirmed (ESI, Spectrum S5 and S6†).
Additionally, a GC-MS analysis at the end of the thermolysis for complex (7) revealed the presence of compound (5) and free 2-FN (ESI, Chromatogram S2, Spectrum S7 and S8†). Since compound (7) was used in pure form, the only reason to obtain free 2-FN is the reversibility of the C–CN oxidative addition at 100 °C, ultimately leading to the formation of complex (9) as the main product. Closely related compounds to this nickelaoxacyclohaxadiene (9) have been reported using iridium and alkylphosphines by Bleeke and coworkers,15 but by using a rather different synthetic procedure.
Considering all these results, a mechanistic proposal is depicted in Scheme 4.
The reaction initially yields complex (10) as major product. This compound displays two major broad doublets in 31P{1H}-NMR spectrum at δ 79.2 and 66.7 with 2JP–P = 68 Hz, which is characteristic of a Ni(0) complex. The 1H-NMR spectrum shows three up-field signals at δ 7.73, 7.46 and 6.73. The reaction was further monitored by 1H and 31P{1H}-NMR at room temperature for 6 days (ESI, Fig. S7 and S8†); after this time, the C–CN oxidative addition complex (11) was obtained as the main product. Complex (11) displays two closely broad doublets at δ 84.2 and 78.4 with 2JP–P = 29 Hz on the 31P{1H}-NMR spectrum, and the coupling constant is characteristic of a Ni(II) complex. The 1H-NMR spectrum exhibits three up-field signals at δ 7.34, 6.63 and 6.26 (ESI, Fig. S9†).
The reaction mixture was placed in a vial, layered with hexanes and stored at −30 °C in a dry box fridge to obtain suitable crystals for X-ray diffraction. The corresponding ORTEP diagram for complex (11) is shown in Fig. 4.
Complex (11) shows a square planar geometry that is slightly deformed, where the C(15)–Ni(1)–C(19) angle is barely less than 90°; this is in agreement with the values reported for similar compounds, such as the Ni(II) complexes derived from C–CN bond activation in benzonitrile,5 2-cyanoisoquinoline6 and 2-cyanothiopene,9 which displays C–Ni–CN angles values of 88.56(3), 89.15(3) and 88.49(3) degrees, respectively. In complex (11), the furan ring is facing out from the P(1)–Ni(1)–P(2) plane (with an inter-planar angle of 54°). The Ni(1)–C(15) [1.951(4) Å] bond distance is longer in complex (11) compared to the same distance in complex (7) [1.906(3) Å] and C(15)–C(18) distance in complex (11) [1.335(4) Å] is shorter compared to the same distance in complex (7) [1.355(6) Å], probably due to a minor back bonding contribution to π* anti-bonding orbitals on the furan ring in this complex.
As the formation of complex (11) takes place, the formation of complex (6) as a yellow crystalline solid along with complex (12) is observed in 31P{1H}-NMR as small singlets at δ 91.7 and δ 72.6, respectively. Pure crystals for complex (11) were re-dissolved in THF-d8 to confirm the previous assignation on 1H- and 31P{1H}-NMR along with the 13C{1H}-NMR spectrum (ESI, Spectrum S9†). These are key signals corresponding to the σ-coordinated CN fragment assigned at δ 135.7 (dd, 2JP–C = 80 (trans), 30 (cis) Hz), with similar values observed for complex (7) and closely related compounds.5–7
Additionally, the formation of two new, low intensity, broad doublets was observed at δ 80.8 and 68.6 with 2JP–P = 24 Hz. Considering the chemical shifts and the 2JP–P value, these signals can be assigned to the complex derived from the C–O bond activation of 3-FN, (14). However, this complex could not be isolated or crystallized due to its low abundance in the reaction mixture. Also, free 3-FN was detected by GC-MS analysis (ESI, Chromatogram S3 and Spectrum S11†) and complex [(dippe)2Ni2] (4) was formed as the main product, showing a singlet at δ 62.5 in 31P{1H}-NMR spectrum.
Crystallographic data were collected with an Oxford Diffraction Gemini “A” diffractometer with a CCD area detector with λMoKα = 0.71073 Å and monochromator of graphite at 130 K. Crystals of complex (7) and (11) were covered with Paratone and immediately placed under a cold stream of nitrogen in the diffractometer. CrysAlisPro and CrysAlis RED software packages were used for data collection and integration.17 The double-pass method of scanning was used to exclude any noise. The collected frames were integrated by using an orientation matrix determined from the narrow frame scans. Final cell constants were determined by a global refinement; collected data were corrected for absorbance by using analytical numeric absorption correction using a multifaceted crystal model based on expressions upon the Laue symmetry using equivalent reflections.18
Structure solution and refinement were carried out with the SHELXS-2014 and SHELXL-2014 packages;19 WinGX v2014.1 software was used to prepare material for publication.20 Full-matrix least-squares refinement was carried out by minimising the following: (Fo2 − Fc2)2. All non-hydrogen atoms were anisotropically refined. H atoms attached to C atoms were placed in geometrically idealized positions and refined as riding on their parent atoms with C–H = 0.95–1.00 Å and with Uiso(H) = 1.2Ueq(C) for aromatic, methylene and methyne groups and Uiso(H) = 1.5Ueq(C) for methyl groups.
Footnote |
† Electronic supplementary information (ESI) available: Includes selected NMR spectra, GC-MS determinations of all products and crystallographic tables. The crystallographic material for compounds 7 and 11 has been deposited at the CCDC 1497182 and 1497183 respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra20244f |
This journal is © The Royal Society of Chemistry 2016 |