Amidinatogermylene Metal Complexes as Homogeneous Catalysts in

Article pubs.acs.org/Organometallics

Amidinatogermylene Metal Complexes as Homogeneous Catalysts in Alcoholic Media Lucía Á lvarez-Rodríguez, Javier A. Cabeza,* José M. Fernández-Colinas, Pablo García-Á lvarez,* and Diego Polo Departamento de Química Orgánica e Inorgánica-IUQOEM, Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de Oviedo-CSIC, E-33071 Oviedo, Spain S Supporting Information *

ABSTRACT: A series of transition-metal complexes containing the bulky amidinatogermylene Ge(tBu2bzam)tBu (1; tBu2bzam = N,N′-bis(tert-butyl)benzamidinate) as a ligand have been prepared and characterized. While the hydrolytic degradation of the germylene ligand of the square-planar complexes [MCl(η4cod){Ge(tBu2bzam)tBu}] (M = Rh (2), Ir (3); cod = 1,5cyclooctadiene) and [PdCl(η3-metallyl){Ge(tBu2bzam)tBu}] (4; metallyl = 2-methylallyl) is slow but clearly evident in carefully dried aprotic solvents, the octahedral complexes [RuCl2(η6cym){Ge(tBu2bzam)tBu}] (5; cym = p-cymene) and [IrCl2(η5Cp*){Ge(tBu2bzam)tBu}] (6; Cp* = pentamethylcyclopentadienyl) have proven to be stable even in alcoholic solvents. These latter complexes have been tested as catalyst precursors of reactions involving alcohols as substrates and/or solvents, and remarkably, they have been found to be active in the transfer hydrogenation of cyclohexanone with isopropyl alcohol (5 and 6), the N-alkylation of aniline with benzyl alcohol (5 and 6), and the deuteriation of acetophenone with CD3OD (6). The use of heavier carbene metal complexes as catalyst precursors of reactions involving alcohols as solvents is unprecedented.

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INTRODUCTION Many transition-metal (M) complexes featuring heavier carbene ligands, also known as heavier tetrylenes (HTs) or group 14 metalylenes, have been prepared since the 1970s.1,2 However, their usefulness as homogeneous catalysts has still been little investigated.3,4 This situation strongly contrasts with that of complexes containing carbene ligands (particularly Nheterocyclic carbenes, NHCs), since many of them have been shown to be very active catalysts for many useful organic transformations.5 Such a dissimilar situation can mainly be attributed to the generally low stability of HT-M complexes toward oxygen,6 moisture,7 and/or substitution processes.8,9 Amidinato-HT ligands, E(R1NCR2NR3)X (E = Si, Ge, Sn; n R , X = anionic group), have recently boosted the transitionmetal derivative chemistry of HTs. In fact, although the first amidinato-HT-M complex was reported only a few years ago, in 2008,10 more than 100 complexes of this type are currently known2b and it is worth noting that they are the majority3 among the relatively few HT-M complexes with recognized catalytic applications.3,4 Two key features of amidinato-HTs are responsible for the rapid expansion of their coordination chemistry: (a) they have proven to be very strongly electron donating ligands, comparable to, or even stronger than, NHCs3b,e−g,7a,11 (this fact alleviates some of the stability issues associated with the general lability of HT ligands and is also a key requirement when electron-rich metal complexes are needed to promote substrate activation), and (b) their © XXXX American Chemical Society

electronic and steric characteristics can be easily and extensively tuned because many different combinations of E, R1, R2, R3, and X are possible. In this context, we have recently discovered that the steric and electronic properties of the amidinatogermylene Ge( t Bu 2 bzam) t Bu (1; t Bu 2 bzam = 1,3-bis(tert-butyl)benzamidinate), which is equipped with tBu groups on the N and Ge atoms, are conveniently suited if one intends the synthesis of HT-M complexes of enhanced stability. For example, while the reactions of CuCl, AgCl, and [AuCl(tht)] (tht = tetrahydrothiophene) with 1 allowed a high-yield synthesis of the first amidinatogermylene group 11 metal complexes,12 the benzamidinium salt [tBu2bzamH2]Cl, a hydrolytic degradation compound, was the major product recovered from the reactions of the same coinage-metal precursors with the chlorogermylene Ge(tBu2bzam)Cl.12 In addition, while the extremely bulky amidinatogermylene Ge(tBu2bzam)(HMDS) (HMDS = N(SiMe3)2) is only capable of reacting with [Ru3(CO)12] at 90 °C, leading to intractable decomposition products,11d germylene 1 quickly reacts with this cluster at 20 °C to form the mononuclear derivatives [Ru{Ge(tBu2bzam)tBu}(CO)4] and [Ru{Ge(tBu2bzam)tBu}2(CO)3],11a mimicking analogous reactions of NHCs with [Ru3(CO)12].13 Having this in mind with the Received: May 27, 2016

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DOI: 10.1021/acs.organomet.6b00426 Organometallics XXXX, XXX, XXX−XXX

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Organometallics ultimate purpose of exploring the applicability of germylenecontaining transition-metal complexes in homogeneous catalysis, we decided to investigate the synthesis and to evaluate the stability of complexes resulting from the combination of germylene 1 with transition metals that are generally active in many homogeneously catalyzed transformations. We describe herein the synthesis and characterization of a series of complexes prepared by simple bridge cleavage of commonly used chloride-bridged dimeric precursors of rhodium(I), iridium(I), palladium(II), ruthenium(II), and iridium(III) with germylene 1. Remarkably, the ruthenium(II) complex [RuCl2(η6-cym){Ge(tBu2bzam)tBu}] (5; cym = pcymene) and the iridium(III) derivative [IrCl2(η5-Cp*){Ge(tBu2bzam)tBu}] (6; Cp* = pentamethylcyclopentadiene) proved to be stable toward the degradation of their germylene ligand in alcoholic solvents. This allowed us to explore their catalytic activity in processes involving alcohols as reagents and solvents, such as the transfer hydrogenation of cyclohexanone with isopropyl alcohol, the N-alkylation of aniline with benzyl alcohol, and the deuteriation of acetophenone with methanold4, which have never been previously investigated using complexes of this kind as catalyst precursors, as a consequence of the generally low stability of HT-M complexes in wet or protic solvents.

Scheme 1. Synthesis of Complexes 2−6

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RESULTS AND DISCUSSION Synthesis of Germylene−Transition-Metal Complexes. The reactions of Ge(tBu2bzam)tBu (1) with the dimeric metal precursors [M2(μ-Cl)2(η4-cod)2] (M = Rh, Ir; cod = 1,5-cyclooctadiene), [Pd2(μ-Cl)2(η3-metallyl)2] (metallyl = 2-methylallyl), [Ru2Cl2(μ-Cl)2(η6-cym)2], and [Ir2Cl2(μCl)2(η5-Cp*)2], in toluene at room temperature, gave the monomeric complexes [MCl(η4-cod){Ge(tBu2bzam)tBu}] (M = Rh (2), Ir (3)), [PdCl(η3-metallyl){Ge(tBu2bzam)tBu}] (4), [RuCl2(η6-cym){Ge(tBu2bzam)tBu}] (5), and [IrCl2(η5-Cp*){Ge(tBu2bzam)tBu}] (6), respectively, in high yields (Scheme 1). The X-ray diffraction (XRD) molecular structure of compound 2 (Figure 1) is that of a typical square-planar rhodium(I) complex. The Ge-bound tert-butyl group and the chlorine atom are arranged in a syn conformation with a C− Ge−Rh−Cl torsion angle of 34.32(6)°. The Rh−Ge bond distance, 2.4153(2) Å, lies within the range of Rh−Ge distances, 2.3366(9)−2.4499(8) Å, previously reported for the other four rhodium complexes equipped with terminal germylenes that have been characterized by XRD.7c,14 The Rh− C bond distances involving the olefinic group trans to the germylene are ca. 0.1 Å longer than those involving the alkene group trans to the chlorine atom, reflecting the higher trans influence of the germylene ligand. The 1H NMR spectrum of 2 shows only one signal for the two NtBu groups (δ(1H) 1.19 (s, 18 H)) and two broad resonances for the four cod olefinic protons (δ(1H) 5.70 (2 H), 4.34 (2 H)), indicating that the germylene ligand rotates about the Rh−Ge bond. The structure of the iridium derivative 3 could not be determined by XRD; however, it should be akin to that of 2 because the NMR spectra of both complexes are comparable. Only five [MCl(η4cod)(HT)] (M = Rh, Ir) complexes have been previously reported, three of which are germylene complexes (all of them with M = Rh)7c,14a and two are silylene derivatives (M = Rh, Ir).4e Unfortunately, no crystals of the palladium complex 4 suitable for an XRD analysis could be obtained; however, its

Figure 1. XRD molecular structure of compound 2 (thermal ellipsoids set at 40% probability; H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Rh1−Ge1 2.4153(2), Rh1−Cl1 2.3577(5), Rh1−C20 2.110(2), Rh1−C21 2.121(2), Rh1−C24 2.215(2), Rh1−C25 2.213(2), C20−C21 1.398(3), C21−C22 1.519(3), C22−C23 1.530(4), C23−C24 1.509(3), C24−C25 1.365(3), C25−C26 1.508(3), C26−C27 1.532(3), C20−C27 1.513(3), Ge1−C16 2.017(2), Ge1−N1 1.991(2), Ge1−N2 1.980(2), N1−C4 1.487(2), N1−C5 1.330(2), N2−C5 1.336(2), N2−C12 1.486(2), C5−C6 1.493(3); Rh1−Ge1−N1 121.59(4), Rh1−Ge1−N2 117.69(5), C16−Ge1−Rh1 128.57(6), C16−Ge1− N1 102.84(7), C16−Ge1−N2 102.38(7), N1−Ge1−N2 66.23(6), Ge1−N1−C5 92.3(1), Ge1−N2−C5 92.6(1), N1−C5−N2 108.9(2).

composition and structure (Scheme 1) was unambiguously inferred from a combination of analytical and spectrometric data. The 2-methylallyl group gives five 1H NMR resonances (δ(1H) 4.48 (s, br, 1 H), 3.53 (s, br, 1 H), 3.33 (s, br, 1 H), 2.13 (s, br, 1 H), 1.60 (s, 3 H)), indicating that all its syn and anti hydrogen atoms are inequivalent, as expected for an η3-allyl ligand in an asymmetric palladium(II) complex. The broad nature of these resonances hints at the existence of a dynamic process, possibly a rotation of the allyl ligand.15 In contrast to B


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Organometallics what was observed for complexes 2 and 3, the NtBu groups of the germylene ligand are not equivalent (δ(1H) 1.15 (s, 9 H), 1.09 (s, 9 H) ppm), indicating that the ligand rotation around the Ge−Pd bond is hindered. Four [PdCl(η3-allyl)(HT)] complexes, containing silylene,4g stannylene,16,17 or plumbylene17 ligands, have been previously reported. Compound 4, as for the other known [PdCl(η3-allyl)(HT)] complexes, proved to be unstable in solution at room temperature, decomposing gradually to palladium black. The XRD molecular structure of the ruthenium(II) complex 5 is displayed in Figure 2. It has the characteristic “three-legged

Figure 3. XRD molecular structure of compound 6 (thermal ellipsoids set at 40% probability, H atoms omitted for clarity; only one of the two independent but analogous molecules contained in the crystal asymmetric unit is shown). Selected bond lengths (Å) and angles (deg): Ir1−Ge1 2.4203(3), Ir1−Cl1 2.4009(7), Ir1−Cl2 2.4184(7), Ir1−C20 2.248(3), Ir1−C21 2.248(3), Ir1−C22 2.165(3), Ir1−C23 2.156(3), Ir1−C24 2.167(3), C−C(Cp*)av 1.44(2), C20−C25 1.493(5), Ge1−C16 2.025(3), Ge1−N1 2.002(2), Ge1−N2 1.992(2), N1−C4 1.485(4), N1−C5 1.333(4), N2−C5 1.326(4), N2−C12 1.488(4), C5−C6 1.498(4); Ir1−Ge1−N1 121.38(7), Ir1− Ge1−N2 120.58(7), C16−Ge1−Ir1 126.63(8), C16−Ge1−N1 102.1(1), C16−Ge1−N2 104.0(1), N1−Ge1−N2 65.7(1), Ge1− N1−C5 92.3(2), Ge1−N2−C5 92.9(2), N1−C5−N2 109.1(2).

quaternary carbon and the Ge and Ir atoms roughly bisects the ClIrCl angle and this results in an approximate Cs symmetry for the molecule. The Ir−Ge bond distance, 2.4203(3) Å, is longer than those of the three known iridium complexes featuring terminal germylene ligands that have been characterized by XRD.20 As expected, the NMR spectra of 6 confirms a symmetric situation in solution, since the two NtBu groups are equivalent (δ(1H) 1.18 (s, 18 H)). It is noteworthy that 6 is the first [IrCl2(η5-Cp)(HT)] complex ever reported. Complex Stability Tests. Having in mind (a) that HT-M complexes have a known propensity to react with oxygen6 and moisture,7 (b) that the concentration of a catalyst in a catalytic reaction solution is very small, and (c) that, therefore, trace amounts of water and oxygen present in carefully dried and degassed substrates and solvents may be sufficient to degrade a significant amount of the catalyst, we decided to check the air stability of complexes 2−6 since, considering a subsequent use of these complexes as possible catalyst precursors, these tests could help us select the appropriate complexes for catalytic reactions, as far as their stability in the chosen solvent is concerned. Toluene solutions and solid samples of 2−6 were left in air for 1 day (solutions) or 1 week (solids). All of the solution tests led to the precipitation of solids (palladium black was clearly formed in the solution of complex 4). After solvent removal in the vacuum line, the solid residues were analyzed by 1H NMR, which revealed the complete disappearance of complexes 2 and 3 from their corresponding solutions, while 5 and 6 were the major species in their corresponding samples. The complexes left in air as solids showed minor signs of decomposition (1H NMR analysis), except for the iridium(I) complex 3, which decomposed extensively. While the nature of the decomposition species of the solution and solid tests was not thoroughly investigated, the hydrolytic degradation of the

Figure 2. XRD molecular structure of compound 5 (thermal ellipsoids set at 30% probability; H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ru1−Ge1 2.4338(5), Ru1−Cl1 2.408(1), Ru1−Cl2 2.407(1), Ru1−C23 2.204(4), Ru1−C24 2.252(4), Ru1−C25 2.264(4), Ru1−C26 2.229(4), Ru1−C27 2.177(4), Ru1−C28 2.165(4), C−C(arene)av 1.41(3), C22−C23 1.507(8), C26−C29 1.527(7), Ge1−C16 2.029(4), Ge1−N1 1.989(3), Ge1−N2 2.006(3), N1−C4 1.488(5), N1−C5 1.326(5), N2−C5 1.330(5), N2−C12 1.487(5), C5−C6 1.496(5); Ru1−Ge1− N1 118.54(9), Ru1−Ge1−N2 119.05(9), C16−Ge1−Ru1 129.6(1), C16−Ge1−N1 103.5(2), C16−Ge1−N2 102.3(2), N1−Ge1−N2 65.8(1), Ge1−N1−C5 92.7(2), Ge1−N2−C5 91.8(2), N1−C5−N2 109.7(3).

piano-stool” geometry of [RuCl2(η6-arene)L] (L = twoelectron-donor ligand) complexes. In order to minimize steric hindrance, the plane defined by the GetBu quaternary carbon and the Ge and Ru atoms roughly bisects the ClRuCl angle. The Ru−Ge bond distance, 2.4338(5) Å, lies in the top limit of the range of Ru−Ge distances, 2.2821(6)−2.4363(7) Å, found for the other few ruthenium complexes equipped with terminal germylene ligands that have been characterized by XRD.4c,14a,11a,18 The conformation found for 5 in the solid state is not maintained in solution, since its 1H NMR spectrum shows only one signal for the two NtBu groups (δ(1H) 1.18 (s, 18 H)) and two signals for the four p-cymene ring H atoms (δ(1H) 5.52 (d, J = 6.0 Hz, 2 H), 5.38 (d, J = 6.0 Hz, 2 H)), indicating free rotation of the p-cymene ligand. Only two [RuCl2(η6-arene)(HT)] complexes have been previously reported, both of them featuring stannylene ligands.16,19 The XRD molecular structure of 6 is shown in Figure 3 (the crystal contains two analogous but independent molecules in the asymmetric unit). It can be described as resulting from the formal replacement of a “Ir(η5-Cp*)” unit for the “Ru(η6arene)” fragment of 5. In 6, the plane defined by the GetBu C


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Organometallics germylene ligand was confirmed in some samples, since they contained the benzamidine tBu2bzamH.21 The remarkable reluctance of 5 and 6 to hydrolytic degradation of their germylene ligand was further evidenced after stirring solutions of these complexes in isopropyl alcohol under air for 1 day. After solvent removal in the vacuum line, a 1 H NMR analysis of the solid residue revealed only minor signs of degradation.21 Catalytic Studies. The low stability of complexes 2 and 3 toward moisture and the tendency shown by the palladium(II) complex 4 to gradually decompose to palladium black discouraged us from using them as catalyst precursors. However, the remarkably high stability shown by complexes 5 and 6 in isopropyl alcohol led us to investigate their ability to promote catalytic reactions that involve alcohols as solvents and/or reagents because, to date, the use of M-HT complexes as catalyst precursors for such catalytic reactions is unprecedented. As complexes of the types [RuCl2(η6-arene)(NHC)] and [IrCl2(η5-Cp*)(NHC)] have been previously used as catalyst precursors for the transfer hydrogenation of ketones with alcohols,22 for the alkylation of amines with alcohols,23 and for H/D exchange reactions between organic substrates and deuterium sources,24 we decided to study (a) the transfer hydrogenation of cyclohexanone with isopropyl alcohol (reaction A), (b) the N-alkylation of aniline with benzyl alcohol (reaction B), and the deuteriation of acetophenone with CD3OD (reaction C), as model catalytic reactions (Scheme 2). Their results would allow for the first time a

Table 1. Catalytic Results reaction b

A

Bc

Cd

Scheme 2. Catalytic Reactions Studied in This Work

entry

catalyst

time

yield (%)a 0 19 55 91 >99 0 0 39 >99 0 0 18 22 30 31 52 76 95 4 15 3 5 1 15 30e (Me) 32e (ortho) 85e (meta) 82e (para) 32e (Me) 37e (ortho) 35e (meta) 32e (para) 78e (Me) 38e (ortho) 84e (meta) 84e (para)

1

5

2

6

3

[Ru2Cl2(μ-Cl)2(η6-cym)2]

4

[Ir2Cl2(μ-Cl)2(η5-Cp*)2]

5 6 7

5 + 3 AgOTf 6 + 3 AgOTf 5

8

6

9

[Ru2Cl2(μ-Cl)2(η6-cym)2]

10


11 12 13

5 + 3 AgOTf 6 + 3 AgOTf 6

15 min 90 min 24 h 15 min 90 min 90 min 24 h 90 min 24 h 90 min 90 min 24 h 48 h 4 days 24 h 48 h 4 days 7 days 24 h 48 h 24 h 48 h 24 h 24 h 24 h

14


24 h

15

6 + 3 AgOTf

24 h

a Conversions were determined by 1H NMR using an internal standard. b2 mol % of catalyst in isopropyl alcohol at 50 °C. c1 mol % of catalyst in toluene at 110 °C. d2 mol % of catalyst in methanol-d4 at 110 °C. eDeuterium incorporation (%).

additives, 5 converted 55% of cyclohexanone into cyclohexanol after 1 day (entry 1), while [Ru2Cl2(μ-Cl)2(η6-cym)2] showed no activity (entry 3). The iridium catalyst 6 proved to be much more active, achieving a quantitative conversion in 90 min (91% in 15 min; entry 2) at a considerably faster rate than for [Ir2Cl2(μ-Cl)2(η5-Cp*)2] (entry 4). Aiming at increasing the productivity of 5, we added AgOTf in order to activate the catalyst by chloride removal; however, this resulted in total deactivation of the catalyst (entry 5). The same deactivation was observed for 6 in the presence of AgOTf (entry 6). The catalytic activity shown by 6 is very remarkable, particularly considering that no base or halide abstractor is needed, as is commonly required for related complexes equipped with NHCs.22 Generally, base-free conditions apply only when a metal−ligand bifunctional mechanism is operative (e.g., when chelating amines are used as ligands25), since one of the ligand donor groups is acting as a basic center. Peris and co-workers have reported that [IrCl2(Cp*)(NHC)] derivatives are also capable of hydrogenating ketones and imines without base,

comparative analysis of the catalytic performance, in the same catalytic reactions, of complexes containing a germylene ligand versus similar complexes containing NHC ligands. Table 1 summarizes the catalytic results obtained from reactions A−C using complexes 5 and 6 as catalyst precursors. For comparison purposes, the table also includes the results afforded, under similar reaction conditions, by [Ru2Cl2(μCl)2(η6-cym)2] and [Ir2Cl2(μ-Cl)2(η5-Cp*)2], which are the metal precursors of 5 and 6, respectively. For reaction A, all tests were carried out at 50 °C with catalyst loadings of 2 mol %. In the absence of additional D


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Organometallics albeit requiring a 3-fold excess of AgOTf.22j The design of catalysts for base-free hydrogen transfer is of great importance, since the addition of base makes the reaction not suitable for substrates that are base-sensitive. For reaction B, catalyst loadings of 1 mol % and 110 °C were used as reaction conditions. Both complexes were more active than their metal precursors (Table 1, entries 7 and 8 vs entries 9 and 10, respectively) toward the formation of N-phenylbenzylamine in the absence of additives. Both catalysts proved to be very slow in comparison to related complexes equipped with NHCs,23 reaching conversions of 30% after 4 days (for 5) and 95% after 7 days (for 6). This can be attributed to the greater steric bulk exerted by the germylene ligand around the metal center, which has also been claimed as being responsible for the low activity observed for arene C−H borylations catalyzed by iridium LCL-pincer complexes, where L = amidinatosilylene or -germylene.3g Note that, while the ruthenium derivative seems to become deactivated (similar conversion from 48 h to 4 days; entry 7), the iridium complex slowly but constantly led to full conversion (entry 8). This fact agrees with the results of thermal stability tests, in which solutions of 5 and 6 were heated in toluene at 110 °C for 24 h, which indicated that, while the iridium derivative 6 showed no signs of decomposition or evolution to other products, the ruthenium complex 5 evolved quantitatively to the η6-toluene derivative [RuCl2(η6-toluene){Ge(tBu2bzam)tBu}].21 While the activity of [RuCl2(η6-toluene){Ge(tBu2bzam)tBu}] is expected to be comparable23a to that of 5, its stability might be compromised, since it has been reported that the arene lability of [RuCl2(η6-arene)(NHC)] complexes increases with less substituted arenes.22f Aiming at activating both catalysts, we also carried out the reactions in the presence of AgOTf; however, as observed for the transfer-hydrogenation reactions, the catalyst performances decreased dramatically (entries 11 and 12). We have no evidence to explain why the addition of AgOTf poisons the catalysts 5 and 6 in reactions A and B; however, a similar deactivation has been reported by O’Connor and co-workers in the base-free reduction of acetophenone catalyzed by [IrCl(Cp*)(pyridinesulfonamide)] complexes, where the addition of AgOTf lowered the conversion from 62% to 3%.25a Finally, having in mind the clearly superior activity exhibited by the iridium catalyst and that most of the transition metalcatalyzed H/D-exchange reactions are iridium-mediated,26 we tested complex 6 as a catalyst for reaction C. After 24 h (2 mol % of catalyst at 110 °C), 6 was capable of rendering a higher deuterium incorporation than [Ir2Cl2(μ-Cl)2(η5-Cp*)2] to the meta (85% vs 35%) and para (82% vs 32%) positions of the acetophenone ring, while both complexes showed similar activity (35−37%) with respect to deuteriation at the ortho position and methyl group (Table 1, entries 13 and 14). A blank experiment, carried out in the absence of any catalyst, afforded a 59% deuterium incorporation at the methyl group and no deuterium incorporation at the aromatic ring. This result clearly indicates that the deuteriation of the acetophenone ring is a metal-mediated process, while a keto−enolic equilibrium is responsible for the H/D exchange at the methyl position, which seems to be inhibited in the presence of [Ir2Cl2(μ-Cl)2(η5-Cp*)2] or 6. For this catalytic reaction (C), the addition of AgOTf to the reaction mixture had a positive effect on the process efficiency (entry 15), increasing the deuterium incorporation to the methyl group up to a level of 78% (higher than that observed in the blank experiment), while

it maintained that observed for 6 alone to the ortho, meta, and para positions (entry 13). This catalytic activity is comparable to that shown by [IrCl2(Cp*)(NHC)] complexes for the deuteriation of acetophenone under analogous reaction conditions.24c

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CONCLUDING REMARKS Five novel transition-metal compounds (2−6) featuring the amidinatogermylene Ge(tBu2bzam)tBu (1) have been prepared by simple bridge cleavage of commonly used chloride-bridged RhI, IrI, PdII, RuII, and IrIII dimeric complexes with the germylene. It is noteworthy that, despite the important role that the family of [IrCl2(η5-Cp*)(NHC)] complexes are playing in modern catalysis,22−24,26 no HT analogues (such as 6) have been reported previous to this work. Remarkably, the RuII and IrIII derivatives 5 and 6 proved to be very stable toward the hydrolytic degradation of their germylene ligand, probably as a consequence of the steric protection applied by the tert-butyl groups to the Ge atom of germylene 1,11a in conjunction with the saturation (18 electrons) of their M atom. This has allowed the evaluation of their activity in catalytic processes that, as a consequence of the general low stability of HT-M complexes in protic solvents, has never been tested before using complexes of this type as catalyst precursors. The catalytic activities presented by 5 and 6 in the tested reactions are clearly superior to those obtained with [Ru2Cl2(μCl)2(η6-cym)2] and [Ir2Cl2(μ-Cl)2(η5-Cp*)2], highlighting the enhanced catalytic activity induced by the germylene ligand. With the exception of the transfer hydrogenation of cyclohexanone catalyzed by 6, which is truly remarkable, the obtained catalytic activities of complexes 5 and 6 are not outstanding if they are compared to those reported for analogous NHC-derived complexes. However, this work has proven that HT-M complexes are capable of promoting catalytic reactions in alcoholic media. Having in mind that both the electronic and steric characteristics of amidinato-HTs can be easily and extensively tuned and that they do not require the use of base in the catalytic reactions (allowing the use of base-sensitive substrates), we feel that these results anticipate an interesting future for these types of complexes in the field of homogeneous catalysis.

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EXPERIMENTAL SECTION

General Procedures. Solvents were dried over appropriate desiccating reagents and were distilled under argon before use. All reactions, unless otherwise stated, were carried out under argon, using drybox and/or Schlenk−vacuum line techniques. All reaction products were vacuum-dried for several hours prior to being weighed and analyzed. The germylene Ge(tBu2bzam)tBu 11a and the metal complexes [Rh2(μ-Cl)2(η4-cod)2],27 [Ir2(μ-Cl)2(η4-cod)2],28 [Pd2(μCl)2(η3-metallyl)2],29 [Ru2Cl2(μ-Cl)2(η6-cym)2],30 and [Ir2Cl2(μCl)2(η5-Cp*)2]31 were prepared following published procedures. All remaining reagents were purchased from commercial sources. NMR spectra were run in C6D6 or CD2Cl2 on a Bruker DPX-300 instrument, using the solvent residual protic resonance for 1H (δ(C6HD5) 7.16 ppm; δ(CHDCl2) 5.32 ppm) and the solvent resonance for 13C (δ(C6D6) 128.1 ppm; δ(CD2Cl2) 53.8 ppm). Elemental analyses were obtained from a PerkinElmer 2400 microanalyzer. Low-resolution mass spectra (LRMS) were obtained using a Bruker Impact II mass spectrometer operating in the ESI-Q-ToF positive mode; data given correspond to the most abundant isotopomer of the molecular ion and/or of a fragment thereof. [MCl(η4-cod){Ge(tBu2bzam)tBu}] (2). Ge(tBu2bzam)tBu (0.50 mL of a 0.36 M solution in toluene, 0.180 mmol) was added to a solution E


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Organometallics of [Rh2(μ-Cl)2(η4-cod)2] (44 mg, 0.090 mmol) in toluene (4 mL), and the mixture was stirred at room temperature for 30 min. The color changed from yellow to orange. The reaction mixture was vacuumdried to give 2 as an orange solid (103 mg, 94%). Anal. Calcd for C27H44ClGeN2Rh (mol wt 607.62 amu): C, 53.37; H, 7.30; N, 4.61. Found: C, 53.40; H, 7.39; N, 4.51. (+)-ESI LRMS: unable to find a complete set of molecular formula, m/z 573 [M − Cl]+. 1H NMR (C6D6, 300.1 MHz, 293 K): δ 7.77 (d, J = 6.8 Hz, 1 H, 1 CH of Ph), 7.17−6.89 (m, 4 H, 4 CH of Ph), 5.70 (m, 2 H, 2 CH of cod), 4.34 (m, 2 H, 2 CH of cod), 2.38−2.25 (m, 2 H of CH2 of cod), 2.24−2.07 (m, 2 H of CH2 of cod), 1.92−1.83 (m, 2 H of CH2 of cod), 1.83− 1.63 (m, 2 H of CH2 of cod), 1.38 (s, 9 H, 3 CH3 of GetBu), 1.19 (s, 18 H, 6 CH3 of 2 NtBu) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): δ 170.1 (NCN), 134.5 (Cipso of Ph), 131.8−125.7 (5 CHs of Ph), 101.6 (d, J(Rh−C) = 6.8 Hz, 2 CH of cod), 62.7 (d, J(Rh−C) = 13.6 Hz, 2 CH of cod), 53.4 (2 C of 2 NtBu), 34.7 (2 CH2 of cod), 32.1 (6 CH3 of 2 NtBu), 29.8 (3 CH3 of GetBu), 28.7 (2 CH2 of cod) ppm. The signal corresponding to the quaternary carbon of the GetBu group is not observed, possibly due to overlapping with the resonance at 34.7 ppm (that of the analogous derivative 3 is at 35.1 ppm). [IrCl(η4-cod){Ge(tBu2bzam)tBu}] (3). Ge(tBu2bzam)tBu (0.40 mL of a 0.36 M solution in toluene, 0.144 mmol) was added to a solution of [Ir2(μ-Cl)2(η4-cod)2] (48 mg, 0.071 mmol) in toluene (4 mL), and the mixture was stirred at room temperature for 30 min. No color change was observed. The reaction mixture was vacuum-dried to give 3 as an orange solid (96 mg, 96%). Anal. Calcd for C27H44ClGeN2Ir (mol wt 696.93 amu): C, 46.53; H, 6.36; N, 4.02. Found: C, 46.66; H, 6.47; N, 3.95. (+)-ESI LRMS: unable to find a complete set of molecular formula, m/z 663 [M − Cl]+. 1H NMR (C6D6, 300.1 MHz, 293 K): δ 7.70 (d, J = 9.0 Hz, 1 H, 1 CH of Ph), 7.04−6.87 (m, 4 H, 4 CH of Ph), 5.23 (m, 2 H, 2 CH of cod), 4.08 (m, 2 H, 2 CH of cod), 2.32−2.13 (m, 2 H of CH2 of cod), 2.13−1.88 (m, 2 H of CH2 of cod), 1.63−1.25 (m, 4 H of CH2 of cod), 1.43 (s, 9 H, 3 CH3 of GetBu), 1.13 (s, 18 H, 6 CH3 of 2 NtBu) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): δ 170.8 (NCN), 134.4 (Cipso of Ph), 131.9 (1 CH of Ph), 129.8 (1 CH of Ph), 129.3 (1 CH of Ph), 128.0 (1 CH of Ph), 127.6 (1 CH of Ph), 87.6 (2 CH of cod), 53.4 (2 C of 2 NtBu), 46.3 (2 CH of cod), 35.5 (2 CH2 of cod), 35.1 (C of GetBu), 32.0 (6 CH3 of 2 NtBu), 29.7 (3 CH3 of GetBu), 29.4 (2 CH2 of cod). [PdCl(η3-metallyl){Ge(tBu2bzam)tBu}] (4). Ge(tBu2bzam)tBu (0.47 mL of a 0.30 M solution in toluene, 0.141 mmol) was added to a solution of [Pd2(μ-Cl)2(η3-metallyl)2] (28 mg, 0.071 mmol) in toluene (4 mL), and the mixture was stirred at room temperature for 2 h. The initially light yellow suspension turned darker due to the precipitation of some black solid. The reaction mixture was filtered, and the resultant solution was vacuum-dried to give 4 as a light brown solid (58 mg, 74%). Anal. Calcd for C23H39ClGeN2Pd (mol wt 558.05 amu): C, 49.50; H, 7.04; N, 5.02. Found: found: C, 49.37; H, 7.33; N, 4.86. (+)-ESI LRMS: unable to find a complete set of molecular formula, m/z 523 [M − Cl]+. 1H NMR (C6D6, 300.1 MHz, 293 K): δ 7.85 (d, J = 7.5 Hz, 1 H, 1 CH of Ph) 7.14−6.96 (m, 4 H, 4 CH of Ph), 4.48 (s, br, 1 H of metallyl), 3.53 (s, br, 1 H of metallyl), 3.33 (s, br, 1 H of metallyl), 2.13 (s, br, 1 H of metallyl), 1.60 (s, 3 H, CH3 of metallyl), 1.30 (s, 9 H, 3 CH3 of GetBu), 1.15 (s, 9 H, 3 CH3 of NtBu), 1.09 (s, 9 H, 3 CH3 of NtBu) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): δ 170.7 (NCN), 134.4 (Cipso of Ph), 131.7 (1 CH of Ph), 129.9 (1 CH of Ph), 129.2 (1 CH of Ph), 128.0 (1 CH of Ph), 126.4 (1 CH of Ph), 79.4 (CH2 of metallyl), 53.6 (2 C of 2 NtBu), 43.6 (CH2 of metallyl), 33.0 (C of GetBu), 32.1 (6 CH3 of 2 NtBu), 28.1 (3 CH3 of GetBu), 23.8 (1 CH3 of metallyl). The signal corresponding to the central carbon of the metallyl ligand, which has been reported to appear at around 131 ppm for [PdCl(η3-metallyl)(PR3)] complexes,32 is not observed, possibly due to overlapping with the resonances of the phenyl group. [RuCl2(η6-cym){Ge(tBu2bzam)tBu}] (5). Ge(tBu2bzam)tBu (0.40 mL of a 0.30 M solution in toluene, 0.120 mmol) was added to a solution of [Ru2Cl2(μ-Cl)2(η6-cym)2] (37 mg, 0.060 mmol) in toluene (4 mL), and the mixture was stirred at room temperature for 30 min. The color changed from orange to reddish orange. The reaction mixture was vacuum-dried to give 5 as a reddish orange solid (78 mg,

98%). Anal. Calcd for C29H46Cl2GeN2Ru (mol wt 667.27 amu): C, 52.20; H, 6.95; N, 4.20. Found: C, 52.31; H, 7.09; N, 4.31. (+)-ESI LRMS: unable to find a complete set of molecular formula, m/z 633 [M − Cl]+. 1H NMR (C6D6, 300.1 MHz, 293 K): δ 7.85 (d, J = 8.0 Hz, 1 H, 1 CH of Ph) 7.04−6.89 (m, 4 H, 4 CH of Ph), 5.52 (d, J = 6.0 Hz, 2 H, 2 CH of cym), 5.38 (d, J = 6.0 Hz, 2 H, 2 CH of cym), 2.90 (m, 1 H, 1 CH of cym iPr), 1.95 (s, 3 H, CH3 of cym), 1.50 (s, 9 H, 3 CH3 of GetBu), 1.25 (d, J = 7.0 Hz, 6 H, 2 CH3 of cym iPr), 1.18 (s, 18 H, 6 CH3 of 2 NtBu) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): δ 171.0 (NCN), 134.5 (Cipso of Ph), 131.9−127.0 (5 CHs of Ph), 104.3 (Cipso of cym), 92.6 (Cipso of cym), 87.4 (2 CH of cym), 83.3 (2 CH of cym), 54.0 (2 C of 2 NtBu), 36.8 (C of GetBu), 32.6 (6 CH3 of 2 NtBu), 31.2 (1 CH of cym iPr), 29.6 (3 CH3 of GetBu), 23.0 (2 CH3 of cym iPr), 18.3 (Me of cym) ppm. [IrCl2(η5-Cp*){Ge(tBu2bzam)tBu}] (6). Ge(tBu2bzam)tBu (18 mg, 0.050 mmol) was added to a solution of [Ir2Cl2(μ-Cl)2(η5-Cp*)2] (18 mg, 0.025 mmol) in toluene (4 mL), and the mixture was stirred at room temperature for 30 min. No color change was observed. The reaction mixture was vacuum-dried to give 6 as an orange solid (37 mg, 97%). Anal. Calcd for C29H47Cl2GeIrN2 (mol wt 759.43 amu): C, 45.86; H, 6.24; N, 3.69. Found: C, 46.01; H, 6.43; N, 3.78. (+)-ESI LRMS: unable to find a complete set of molecular formula, m/z 723 [M − Cl]+. 1H NMR (C6D6, 300.1 MHz, 293 K): δ 7.04−6.88 (m, 5 H, 5 CH of Ph), 1.60 (s, 15 H, 5 CH3 of Cp*), 1.55 (s, 9 H, 3 CH3 of GetBu), 1.18 (s, 18 H, 6 CH3 of 2 NtBu) ppm. 13C{1H} NMR (CD2Cl2, 75.5 MHz, 293 K): δ 171.3 (NCN), 133.3 (s, Cipso of Ph), 130.3−128.0 (5 CH of Ph), 90.2 (5 C of Cp*), 32.2 (6 CH3 of 2 NtBu), 31.1 (3 CH3 of GetBu), 9.5 (5 CH3 of Cp*). The signals corresponding to the quaternary carbons of the NtBu and GetBu groups are not observed, possibly due to overlapping with those of the solvent (CD2Cl2) and of the CH3 groups of the germylene ligand, respectively. Catalytic Hydrogenation of Cyclohexanone with Isopropyl Alcohol. All reactions were carried out in J. Young sealed 10 mL Schlenk tubes loaded with cyclohexanone (2 mL of a 0.45 M solution in iPrOH, 0.9 mmol), the catalyst precursor (2 mol %, 0.018 mmol), and AgOTf when appropriate (only for entries 5 and 6 of Table 1, 1/3 catalyst/AgOTf mole ratio). The resulting mixture was stirred at 50 °C. Aliquots (0.2 mL) were extracted from the reaction tubes and placed in an NMR tube containing CDCl3 (0.3 mL). Conversions were determined by 1H NMR using benzene as internal standard. Catalytic N-alkylation of Aniline with Benzyl Alcohol. All reactions were carried out in J. Young sealed NMR tubes loaded with benzyl alcohol (104 μL, 1.0 mmol), aniline (91 μL, 1.0 mmol), the catalyst precursor (1 mol %, 0.01 mmol), AgOTf when appropriate (only for entries 11 and 12 of Table 1, 1/3 catalyst/AgOTf mole ratio), and C6D5CD3 (0.5 mL). The resulting mixture was stirred at 110 °C. Conversions were determined by 1H NMR spectroscopy using p-xylene as internal standard. Catalytic Deuteriation of Acetophenone with CD3OD. All reactions were carried out in J. Young sealed NMR tubes loaded with CD3OD (0.5 mL), acetophenone (57 mg, 0.475 mmol), the catalyst precursor (2 mol %, 0.0095 mmol), and AgOTf when appropriate (only for entry 15 of Table 1, 1/3 catalyst/AgOTf mole ratio). The resulting mixture was stirred at 110 °C. Conversions were determined by 1H NMR using p-xylene as internal standard. X-ray Diffraction Analyses. Crystals of 2, 5, and 6 were analyzed by X-ray diffraction. A selection of crystal, measurement, and refinement data is given in Table S1 in the Supporting Information. Diffraction data were collected on Oxford Diffraction single-crystal diffractometers (Xcalibur Ruby Gemini with Mo Kα radiation for 2 and Xcalibur Onyx Nova with Cu Kα radiation for 5 and 6). Empirical absorption corrections were applied using the SCALE3 ABSPACK algorithm as implemented in CrysAlisPro RED.33 The structures were solved with SIR-97.34 Isotropic and full-matrix anisotropic leastsquares refinements were carried out using SHELXL.35 All non-H atoms were refined anisotropically. Hydrogen atoms were set in calculated positions and refined riding on their parent atoms. The WINGX program system36 was used throughout the structure determinations. The molecular plots were made with X-SEED.37 F


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CCDC deposition numbers: 1480090 (2), 1480091 (5), and 1480092 (6).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00426. Crystal, acquisition, and refinement XRD data, 1H and 13 C{1H} NMR spectra of compounds 2−6, and 1H NMR spectra of samples obtained from the air-stability experiments (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.A.C.: [email protected]. *E-mail for P.G.-A.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by MINECO-FEDER (CTQ2014-51912-REDC, CTQ2013-40619P, and RYC201210491) and Gobierno del Principado de Asturias (GRUPIN14009) research grants. We also thank the University of Oviedo and MINECO for predoctoral fellowships to L.A.-R. and D.P., respectively.

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REFERENCES

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Organometallics

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