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136 CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL R R
H R H (a) (b)
(c) nongeminal mixture of 3 isomers geminal
Figure 10.6 (a–c) Regio-isomers of substituted tetraalkylcyclopentadienyl ligands. (i) CpLi N N O S O O N N Me + N N Me N N Me (i) IndLi (ii) MeI I (ii) MeI I I N N OH Pyridine TsCl H 2 N OH NH 4 OAc, glyoxal formaldehyde
Synthesis of chiral cyclopentadienyl- and indenyl-imidazolium pro-ligands. the procedure introduced by Saigo by the condensation reaction of the amino alcohol with ammonium acetate, glyoxal, and formaldehyde [45]. The alcohol is then converted to the corresponding tosylate, which is a better leaving group [46]. Subsequent reaction of the imidazole tosylate with cyclopentadienyl lithium salt, followed by treatment with iodomethane, affords the corresponding chiral cyclopentadienyl-functionalized imidazolium pro-ligand [32] (Fig. 10.7). This method is limited to preparing unsubstituted cyclopentadienyls because of the lack of regioselectivity in the reaction with alkylsubstituted cyclopentadienides previously discussed, but it can be extended to the synthesis of indenyl- functionalized NHCs.
Iron and other first-row transition metal–NHC complexes are generally prepared either via free carbenes, generated by deprotonation of imidazolium salts with a strong base (NaH, n-BuLi, t-BuOK) [47–51], or by the reaction of a metal amide with an imidazolium salt [52–55]. Transmetallation from silver NHC adducts has rarely been used in iron chemistry [56, 57]. Piano-stool iron complexes bearing cyclopentadienyl-functionalized NHCs are prepared by treating the bifunctional Cp
-NHC pro-ligands with two equivalents of n-BuLi to provide the corresponding lithium salts (Cp
-NHC)Li(THF) n , which
are used in situ to form the half-sandwich Fe–NHC complexes, after addition of the appropriate iron precursor (Fig. 10.8). The lithium salts (Cp x -NHC)Li(THF) n are stable enough to be isolated and characterized by nuclear magnetic resonance (NMR) spectroscopy. Following this synthetic pathway, the iron(II) carbonyl complexes of general type (Cp
-NHC)Fe(CO)I (Fig. 10.9a– c), and the coordinatively and electronically unsaturated iron(II) species (Cp*-NHC)FeCl (Fig. 10.9d), are prepared [31]. Remarkably, the iron complexes of the general type (Cp
-NHC)Fe(CO)I can also be obtained by direct reaction of the corresponding imidazolium pro-ligands with commercially available triiron dodecacarbonyl, Fe 3 (CO) 12 , in refluxing toluene (Fig. 10.10). This reaction gives the iron carbonyl complexes with high yield and purity, providing an easy access to these organometallic half-sandwich Fe–NHC complexes [34]. Moreover, the reaction can be performed in large scale (up to 2 g). The main advantage of this one-step synthesis is that it avoids the generation of free carbenes, which requires harsh conditions such as strong base (n-BuLi), and in addition, it does not require the preparation of the iron precursor FeI 2
4 . The formation of the chelating iron complexes (Cp x -NHC)Fe(CO)I probably implies the oxidative addition of the CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL 137 Fe N N Me Ph CO I N N Me I H n-BuLi Fe N N Me Ph Cl FeI
2 (CO)
4 FeCl
2 Figure 10.8 Synthesis of iron(II) complexes bearing tetramethylcyclopentadienyl-functionalized NHC ligands. Fe N
Me Ph CO I Fe N N Me Ph CO I Fe N N Me Ph CO I Bz Bz Bz Bz Ph Ph Ph (a)
(b) (c)
Fe N N Me Ph Cl (d) Figure 10.9 (a–d) Iron(II) complexes with Cp-tethered NHC ligands. Fe N
Me Ph CO I N N Me I H 1/3 Fe 3 (CO) 12 reflux in toluene Figure 10.10 Direct reaction of Cp*-functionalized imidazolium pro-ligands with Fe 3 (CO)
12 . imidazolium salt to Fe(0), followed by the elimination of the Fe–H intermediate and the cyclopentadienyl proton. A similar reaction pathway was reported for the synthesis of related ruthenium complexes [23, 30]. However, C–H activation of an imidazolium salt mediated by an iron(0) species has no precedent in Fe–NHC chemistry. Remarkably, the presence of base is not required in this reaction. Following this synthetic method, substituted and unsubstituted cyclopentadienyl-functionalized NHCs of Fe(II) can be prepared. These iron complexes, isolated as green crystalline solids, are very stable in the solid state and can be handled in air.
Infrared spectroscopy provides valuable information on the donor strength of the Cp-NHC ligands. The lower frequency of the CO-stretching band in the tetramethylcyclopentadienyl complex (Cp * -NHC)Fe(CO)I [ ν(CO) 1906 cm −1 ] indicates the stronger electron donation of Cp* compared to Cp and Cp Bz in (Cp-NHC)Fe(CO)I [ ν(CO) 1949 cm −1 ] and (Cp Bz - NHC)Fe(CO)I [ ν(CO) 1920 cm −1 ] complexes, respectively. The presence of the NHC fragment also has a clear influence on the frequency of the band, as we can see if we compare the bands shown for the (Cp-NHC)Fe(CO)I complex with the bands displayed by CpFe(CO) 2 I (
ν(CO) 2035, 2000 cm −1 ) [58]. Interestingly, the linker between the Cp and the NHC fragment seems to have an effect on the electronic situation of the metal center; the ν(CO) of the tethered complex (Cp- NHC)Fe(CO)I appears at higher wave numbers (1949 cm −1 ) than those for the related nonlinked Cp, NHC systems, which display a CO-stretching band at circa 1936 cm −1 [49, 59]. This shift suggests lower electron density on the metal center in the tethered (Cp-NHC)Fe(CO)I complex compared to the nonlinked systems (Fig. 10.11). The presence of the sterogenic centers at the aliphatic linker between the NHC and the cyclopentadienyl ring, and at the metal center, implies that a mixture of diastereomeric complexes is expected. NMR experiments (variable temperature
138 CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL Fe N
Me Me OC I Fe N N Me Ph OC I Ph Ph Fe N N Mes
Mes OC I Figure 10.11 Tethered and untethered Fe(II) complexes bearing Cp and NHC ligands. NMR studies and NMR carried out in different solvents) suggest that only one distereomer is formed in the reaction [60]. The
13 C NMR spectra display the characteristic Fe–C carbene signals at 188 (for a, 10.9a), 195 (for b, 10.9b), and 193 (for c, 10.9c) as expected because of the stronger electron donor capability of the Cp* ligand in complex B compared to Cp and Cp Bz (Fig. 10.9a–c). The known crystal structures of the cyclopentadienyl-functionalized NHC iron carbonyl complexes show distorted four- legged piano-stool geometry [31]. The cyclopentadienyl-NHC ligand adopts a bidentate coordination mode, chelating through the five-membered ring of the cyclopentadienyl and the NHC groups (Fig. 10.12). The small difference (ca. 3 ◦ ) in the
exocyclic and endocyclic N–C NHC
–Fe angles indicates low strain in the chelate. The Fe–carbene bond distance of 1.942(4) ˚ A is slightly shorter than in the nonlinked monocarbene complex CpFe(IMes)(CO)I 1.980(5) ˚ A [59], probably as a consequence of both steric constrains of the chelate and the steric effect of the bulkier mesityl substituent of the imidazole ring compared to the methyl group in the tethered (Cp-NHC)Fe(CO)I complex. The chelating configuration can also explain the slightly longer distance of the Fe–I bond in (Cp-NHC)Fe(CO)I (2.6452 ˚ A) compared to that distance in the related nonlinked system CpFe(IMes)(CO)I (2.6311 ˚ A). The nickel complex bearing the tetramethylcyclopentadienyl-functionalized NHC (Cp*-NHC)NiCl is prepared by the reaction of NiCl 2 (DME) (DME, dimethoxyethane) with the corresponding lithium salt (Cp*-NHC)Li(THF) n [35] (Fig. 10.13). This complex is thermally stable; its toluene solutions are heated up to 120 ◦ C without notice of any decomposition. However, it is rather sensitive to air and moisture and it has to be manipulated under nitrogen atmosphere. The
13 C NMR spectrum of (Cp*-NHC)NiCl displays the characteristic signal for the Ni–C carbene at 176 ppm, which appears in the region of previously reported Ni–NHC complexes (170–180 ppm) [61–67]. The related nonlinked Cp*Ni(IMe)I (IMe, 1,3-dimethylimidazol-2-ylidene) complex exhibits similar resonance for its Ni–C carbene at 175 ppm [61]. The only other known example of a Ni complex bearing a cyclopentadienyl-functionalized NHC ligand was reported in 2009 by Shen et al. [22]. Reaction of NiCl 2 (DME) with the in situ generated lithium salt of the indenyl-functionalized NHC Figure 10.12 Molecular diagram of (Cp-NHC)Fe(CO)I. Selected bond distances ( ˚ A) and angles (deg): Fe(1)-C(3) 1.942(4), Fe(1)- I(1) 2.6452(6), Fe(1)-C(5) 1.759(5), C(5)-O(1) 1.134(5), Fe(1)-Cpcentroid 2.0972, C(3)-Fe(1)-I(1) 90.69(11), C(3)-Fe(1)-C(5) 97.46(18), C(5)-Fe(1)-I(1) 94.06(14), Cpcentroid-Fe(1)-C(3) 120.538, Cpcentroid-Fe(1)-I(1) 118.462, Cpcentroid-Fe(1)-C(5) 117.06.
HALF-SANDWICH IRON AND NICKEL NHC COMPLEXES AS CATALYSTS FOR REDUCTIONS 139 Ni N N Me Ph Cl N N Me I H (i) n-BuLi (ii) NiCl 2 (DME)
Figure 10.13 Synthesis of a nickel tetramethylcyclopentadienyl-functionalized NHC complex. Br (i) n-BuLi (ii) NiCl 2 (DME) N N N Ni Br N Figure 10.14 Synthesis of a nickel Ind-functionalized NHC complex. ligand afforded the half-sandwich (Ind-NHC)NiBr complex (Fig. 10.14). The Ni–C carbene
signal in its 13 C NMR spectrum (170 ppm) reflects the weaker donation of the indenyl ligand compared to the tetramethylcyclopentadienyl ligand. 10.4 HALF-SANDWICH IRON AND NICKEL NHC COMPLEXES AS CATALYSTS FOR REDUCTIONS In recent years, there has been significant interest in developing catalytic processes based on first-row transition metals. Global efforts in sustainability, coupled with increasing prices and concerns over long term supplies of precious metals, have pressured researchers to look for cheaper alternatives. The use of iron- and nickel-based catalysts is highly attractive, not only because of their low price and high abundance, but also because these metals are typically used in enzymes for numerous transformations of natural products. Among catalytic applications being developed, the reduction of carbonyl groups mediated by first-row transition metals is emerging as an exciting area of research [68–70]. The reduction of carbonyl groups is a fundamental process in organic synthesis. The catalytic reduction of aldehydes and ketones to the corresponding alcohols can be carried out either by hydrosilylation followed by hydrolysis or by transfer hydrogenation usually in the presence of an alcohol as a hydrogen donor and a base, or else by hydrogenation, which uses hydrogen as a reducing agent [71]. So far, the majority of the homogeneously catalyzed processes for the reduction of carbonyl groups are based on precious metals such as palladium, platinum, ruthenium, and rhodium. Replacement of these rare and expensive metals by first-row transition metals is highly desirable. Recent developments on iron catalysis have shown the enormous potential of this metal in the reduction of carbonyl groups with hydrogen or using transfer hydrogen and silanes as reducing agents [69]. However, the use of well-defined iron–NHC complexes as catalysts for these reduction processes is rather undeveloped [72]. Cyclopentadienyl-functionalized NHCs of iron have shown to be active catalysts in the reduction of organic functionalities. Catalytic studies showed the potential of (Cp x -NHC)Fe(CO)I and (Cp-NHC)FeCl complexes (Fig. 10.9a–d) catalyzing the reduction of aldehydes and ketones through hydrogen transfer and hydrosilylation reactions [31]. A series of (Cp
- NHC)Fe(CO)I complexes catalyzes the hydrogenation of C =O via hydrogen transfer from 2-propanol in the presence of a strong base (KOH) at 80 ◦ C, achieving quantitative conversion to the corresponding alcohol in 6 h of reaction. Catalytic studies showed that different substitution on the cyclopentadienyl ring does not affect the catalytic performance in the hydrogen transfer of acetophenone. Comparable reactivity is displayed by the coordinatively unsaturated iron (Cp*-NHC)FeCl complex, which exhibited also good catalytic efficiency in the hydrosilylation of aldehydes [31]. In 2011, Sortais and Darcel described the activity of the related nonlinked half-sandwich iron complex [CpFe(IMes)(CO) 2 ]I [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] (Fig. 10.15c) in the hydrosilylation of alde- hydes and ketones [73], and more recently, they extended these studies to the reduction of amides [74], nitriles [74], imines 140 CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL Fe N
Mes Mes
CO CO Fe N N Me Ph CO I Ph Ph I Fe N N Me Ph Cl Royo [30] Sortais and Darcel [73] (a) (b)
(c) Figure 10.15 (a, b)Tethered and (c) untethered Fe(II) catalysts for the reduction of organic functionalities. [75], and esters [76]. The cationic [CpFe(IMes)(CO) 2 ]I species catalyzes the reaction under solvent-free conditions with light irradiation. The role of light and the nature of the active catalytic species are undisclosed. The formation of the neutral species CpFe(IMes)(CO)I by light irradiation of [CpFe(IMes)(CO) 2 ]I complexes is known [44], but the effect of light irradiation in the catalytic reaction remains unclear. Despite the mechanism of the hydrosilylation reactions catalyzed by the nonlinked [CpFe(IMes)(CO) 2 ]I and the tethered (Cp-NHC)Fe(CO)I and (Cp-NHC)FeCl complexes is currently unknown, the ability of the NHC ligand to stabilize coordinatively unsaturated iron complexes seems to play a crucial role. The implications that the presence of the linker between the Cp and the NHC units may have in the stability and reactivity of the catalytically active species is an interesting issue that undoubted will be explored in the future. More recently, a novel catalytic system based on the iron complex (Cp-NHC)Fe(CO)I (Fig. 10.15a) combined with AgBF 4
3 resulted to be an efficient and selective method for the reduction of sulfoxides to sulfides [34]. The catalytic reaction is suitable for a variety of sulfoxides including aromatic and aliphatic. Radical scavenger experiments indicate the presence of free radicals in the catalytic reaction, suggesting a radical-base mechanism. The addition of both carbon- and oxygen-centered spin traps such as TEMPO (2,2,6,6-tetramethyl-piperidinyloxy) and BHT (2,6-di-tert-butyl-4-methylphenol) has a clear effect on the efficiency of the catalytic reaction. So far, the nature of the radical species is unknown. Regarding the use of well-defined nickel complexes as catalysts for reduction of carbonyl groups, only three examples are described in the literature. In 2009, Guan and coworkers [77] described the efficiency of a nickel PCP-pincer complex performing the hydrosilylation of aldehydes. In the same year, the catalytic hydrosilylation of ketones via a transient Ni–H complex supported by a monoanionic bidentate amidophosphine ligand was reported by Mindiola [78]. Later, Jones investigated well-defined PNP nickel pincer complexes, which catalyzed the hydrosilylation of aldehydes [79] (Fig. 10.16). Half-sandwich tetramethylcyclopentadienyl-functionalized NHC nickel complexes have proved to be very effective catalysts in the reduction of aldehydes and ketones. The nickel alkoxide (Cp*-NHC)Ni(OBu
), prepared by treatment of (Cp*-NHC)NiCl with KOBu
(Fig. 10.17), was isolated as a rather stable crystalline solid, which could be stored for weeks without noticeable decomposition. The tethered (Cp*-NHC)Ni(OBu
) compound displays remarkable catalytic activity in the hydrosilylation of aldehydes using phenylsilane as a reducing agent. The catalytic reactions afford quantitative conversion to the corresponding alcohol in 5 min of reaction, at 25 ◦ C. Ketones are also reduced, although longer reaction times and higher temperatures are required to achieve good conversions of the corresponding alcohols. Mechanistic studies, based on stoichiometric reactions, revealed that the transient nickel hydride (Cp*-NHC)NiH complex, generated by reaction of (Cp*-NHC)Ni(OBu
) with phenylsilane, is the active species in the hydrosilylation of aldehydes. The identity of (Cp*-NHC)NiH was demonstrated by NMR spectroscopy (the characteristic hydride resonance appeared at −18 ppm). Surprisingly, the Ni–H is fairly stable in solution; no decomposition is observed after being kept in solution for O (a)
(b) (c)
Ni P(Pr
i ) 2 O P(Pr i ) 2 H (Pr i ) 2 P Ni N P(Pr i ) 2 (Pr
i ) 2 P P(Pr
i ) 2 Br 2 N O Ni P(Bu t ) 2 O P(Bu t ) 2 H Guan [77] Jones [79] Mindiola [78] Figure 10.16 (a–c) Well-defined Ni(II) complexes catalyzing the hydrosilylation of carbonyl groups. REFERENCES 141 Ni N N Me Ph Cl KOBu
t Ni N N Me Ph OBu t Figure 10.17 Synthesis of a nickel alkoxo complex bearing a Cp*-functionalized NHC ligand. several days. This Ni hydride complex readily reacts with benzaldehyde to give the corresponding nickel benzyloxide. The fast C
=O insertion of PhCHO into the nickel hydride suggests that the catalytic reaction may occur through the classical hydride mechanism, already proposed for other Cu- and Ni-alkoxo catalysts [78, 80–82]. This mechanistic proposal is based on the formation of a metal hydride intermediate on the addition of silane to the corresponding nickel alkoxo compound, followed by carbonyl insertion into the M–H bond to give an alkoxide (the hydride mechanism) [68]. However, isotopic labeling experiments carried out with deuterated silanes show that the hydride ligand does not directly participate in the reduction product, ruling out the classical hydride mechanism [35, 83]. Similar mechanistic details have been found in the hydrosilylation reaction catalyzed by a Fe–H complex recently reported by Guan [84]. Further mechanistic studies are needed to clarify the true role of the hydride ligand in the reduction of carbonyl groups mediated by the tethered half- sandwich nickel (Cp*-NHC)Ni(OBu t ) complexes. Related nonlinked half-sandwich nickel NHC hydride complex has been described by Darcel and Sortais [85]. The use of these relatively new tethered Cp-NHC ligands in iron and nickel chemistry reveals an interesting potential in catalysis that certainly will be further exploited in the near future.
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