Ruthenium Complexes of Triazole-Based ... - ACS Publications

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Ruthenium Complexes of Triazole-Based Scorpionate Ligands Transfer Hydrogen to Substrates under Base-Free Conditions Mukesh Kumar,† Joseph DePasquale,† Nicholas J. White,† Matthias Zeller,‡ and Elizabeth T. Papish*,† †

Department of Chemistry, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States Department of Chemistry, Youngstown State University, 1 University Plaza, Youngstown, Ohio 44555, United States

‡

S Supporting Information *

ABSTRACT: The first ruthenium complexes of bulky tris(triazolyl)borate (Ttz) ligands were synthesized, fully characterized, and studied as transfer hydrogenation catalysts. The structures of the complexes were (η6-arene)RuCl(N,N), where in each case N,N is a κ2-Ttz or bis(triazolyl)borate (Btz) ligand (arene = p-cymene (1, 3, 5, 6), benzene (2), C6Me6 (4); N,N = TtzPh,Me* (1, 2), TtzMe,Me (3, 4), Ttz (5), Btz (6)). All but 5 were crystallographically characterized, and notably for 1 and 2 a rearranged ligand structure is observed (as indicated by an asterisk). These complexes were all effective catalysts for transfer hydrogenation of aryl ketones in isopropyl alcohol with base co-catalyst, with rates that were accelerated by moisture-free conditions. Complexes 1 and 2 are also effective catalysts for base-free transfer hydrogenation, and with 1 hydrogenation of several base-sensitive substrates was demonstrated. The ability of 1 to serve as a hydrogenation catalyst without base is attributed primarily to steric bulk, and a preliminary mechanism for formation of that active catalyst is proposed.

1. INTRODUCTION We report the first series of ruthenium scorpionate complexes based upon 1,2,4-triazole ligands of varied steric bulk. Scorpionate1,2 complexes of ruthenium have most commonly been based upon pyrazole3 (in Tp4−10 and related complexes11 and N,N,O complexes12−16) but have also been reported with 1,2,3-triazole derivatives,17 other N donor heterocycles,18 and S donors.19−22 Many of these complexes are promising catalysts23 or form interesting structures. Specifically, C−H bond activation is catalyzed by various (Tp)Ru(arene) complexes,24 and complexes with agostic B−H to metal interactions have been reported.4,25,26 Our work herein shows that 1,2,4-triazole-based scorpionate complexes are useful as base-free transfer hydrogenation catalysts, and they show new structural variations. Transfer hydrogenation with ruthenium(II) arene catalysts is a mature area of research;27 in contrast, base-free transfer hydrogenation is a more recent development28−33 with significant room for improvement, and very few catalysts use ruthenium.34−37 Recently, Manzano et al. reported a series of Ru complexes of poly(pyrazolyl)methane ligands that catalyze transfer hydrogenation of ketones in the absence of base (with isopropyl alcohol as the H2 source).38 The activation of most transfer hydrogenation catalysts requires a base (e.g. NaOH, KOH, or KOtBu) to produce a ruthenium hydride species (often via β-H elimination from a ruthenium alkoxide intermediate)27 as the active catalyst; base-free hydrogenation is also possible but often occurs much more slowly.28,37 In many cases, an internal strong base is present in the ligand framework or the Ru−H species is preformed, and cases without these features (as reported herein and by Manzano et al.38) are few. Base-free © 2013 American Chemical Society

hydrogenation reactions are desirable in industry to avoid corrosion and in asymmetric synthesis to avoid racemization of chiral centers.29,39,40 Tris(triazolyl)borate (Ttz) ligands are an interesting class of scorpionate ligands, which when formed from nonbulky 1,2,4triazole ligands41 often give rise to coordinatively saturated homoleptic bis ligand complexes and coordination polymers (via coordination of each ligand to multiple metals) (see ref 42 and references therein). Our group synthesized bulky Ttz ligands for formation of low-coordinate complexes,43−50 and despite the increased bulk, metal coordination to or protonation of the fourth-position N still occurs under certain circumstances.47 The basicity of the uncoordinated distal nitrogens47,51,52 combined with the fact that triazole is a weaker net donor than pyrazole (from our studies of [(TtztBu,Me)CuCO])45 suggests to us that triazole-based chelate complexes behave quite differently from pyrazole-based chelate complexes. Thus, here we report the synthesis and characterization of new LRu(arene) complexes (where L = Ttz, bis(triazolyl)borate (Btz)) and their use as catalysts for transfer hydrogenation of various polar double bonds with and without exogenous base present.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of 1−6. As shown in Scheme 1 and Table 1, complexes 1−6 were synthesized via a salt elimination approach by combining [(η6-arene)RuCl2]2 Received: December 28, 2012 Published: March 21, 2013 2135

dx.doi.org/10.1021/om301260j | Organometallics 2013, 32, 2135−2144

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pressure, and consistent with this explanation is the fact that the bulky TtztBu,Me ligand (with either K or Tl counterion) did not coordinate to ruthenium using [(η6-p-cymene)RuCl2]2. The rearrangement to form TtzPh,Me* has been observed previously in Cu and Ni(II) bis ligand complexes,48 and a similar rearrangement is observed in the crowded [(TtztBu,Me*)ZnSPh] complex.47 In the Tp literature, these sorts of rearrangements are described as 1,2-borotropic shifts;53 they occur most commonly with Tp ligands of intermediate steric bulk (e.g., TpiPr,Me, TpPh,Me), and we have observed a similar trend with Ttz ligands thus far. 2.2. X-ray Crystal Structures of Complexes 1−4 and 6. The structural data for complexes 1−4 and 6 all show a pianostool geometry with a κ2-Ttz or Btz ligand (N,N donor mode) occupying two legs of the stool (see molecular diagrams in Figures 1−5, crystal data in Table SI-1 (Supporting Information), and bond lengths and angles in Table 2). The average Ru−N bond distances in 1−4 and 6 range from 2.0872(2) to 2.116(2) Å, and thus there are no significant differences between the complexes. Similarly, variations in the Ru−Cl bond distances (2.3957(6)−2.4127(6) Å) are not significant. The Ttz or Btz chelate ring in each case forms a boat conformation (Figure 6). On this chelate ring, the uncoordinated triazole ring in Ttz complexes is anti to the chloride in two cases (1 and 2), and this creates a very close steric interaction between the arene and the triazole rings (the closest interaction is the H34 (cymene) to N6 distance of 2.332(2) Å for 1; similarly there is an H32 (benzene) to N7 distance of 2.518(2) Å in 2). In the space-filling models of 1 and 2, the arene ring appears to be touching this triazole ring. In contrast, for 3 and 4, the uncoordinated triazole ring is syn to the chloride; thus, a steric clash with the Ru-bound arene is avoided, and the triazole ring sits between the 5-position methyl groups of the coordinated triazole rings. Thereby, H (of 5-Me) to N of triazole distances are at their shortest with values of ∼2.4−2.5 Å (in 3 and 4). It appears (from the space-filling models) that there is less of a steric clash in 3 and 4 (cf. 1 and 2), since fewer atoms are in close proximity of each other. The 5-position phenyl on the rearranged triazole ring in 1 and 2 appears to block placement of the uncoordinated triazole ring away from the arene ligand. The chelate angle N−Ru−N decreases in the order 2 (87.9(1)°) < 1 (87.1(1)°) < 3 (86.4(1)°) < 4 (83.8(1)°); this angle is smallest and most different from the others in 4 and appears to reflect slight differences in the boat conformation for 4. Although the steric bulk on Ttz appears to alter the placement of the uncoordinated triazole ring, it does not otherwise perturb the bonds lengths and angles. This is clear from a comparison of complexes 1 and 6 (Figures 1 and 5; Table 2): 6 with the unhindered Btz ligand has a geometry and metric parameters similar to those of 1. Complex 4 crystallized with two molecules of [(η6-C6H6)RuCl(κ2-TtzMe,Me)] linked by two water molecules in the asymmetric unit (Figure 4b). The adventitious water (from recrystallization in air) participates in OH···N hydrogen bonds (N···O = 2.884(2), 2.902(2), 2.894(2) Å; consistent with hydrogen bond definitions54) to the 4-position nitrogen of the coordinated triazole rings. In this case, water-to-water hydrogen bonds (O···O = 2.995(3) Å) are longer and weaker than the triazole−water interactions. We have frequently observed that one water molecule will bridge two Ttz complexes (of Zn or Cu),43,45,46 but structures bridged by two water molecules have not been observed before with Ttz (we do have a case of two

Scheme 1. Procedure for the Synthesis of Complexes 1−6 and Structures of the Products

Table 1. Starting Materials and Products Used in Scheme 1 for the Synthesis of Complexes 1−6a in starting materials arene

L

product complex

p-cymene benzene p-cymene C6Me6 p-cymene p-cymene

TtzPh,Me TtzPh,Me TtzMe,Me TtzMe,Me Ttz Btz

(η6-p-cymene)RuCl(κ2-TtzPh,Me*) (1) (η6-benzene)RuCl(κ2-TtzPh,Me*) (2) (η6-p-cymene)RuCl(κ2-TtzMe,Me) (3) (η6-C6Me6)RuCl(κ2-TtzMe,Me) (4) (η6-p-cymene)RuCl(κ2-Ttz) (5) (η6-p-cymene)RuCl(κ2-Btz) (6)

a In TtzPh,Me* (1, 2), the asterisk indicates a rearranged ligand with one triazole ring having swapped 3- and 5-position substituents.

and M(TtzR,R′) or M(Btz) (M = Tl, K) in a 1:2 molar ratio in dichloromethane. After they were stirred for an appropriate time period, the solutions were filtered to remove MCl and the resulting complexes were purified (see the Experimental Section). Complexes 1−5 were isolated as air- and moisturestable orange-yellow solids in high yields (61−83%) and were purified by recrystallization in a suitable solvent(s) in air. Complex 6 was synthesized from impure K(Btz) (90% with 10% Ttz, as this ligand is difficult to obtain in pure form), and 6 was obtained in 46% yield after recrystallization from dichloromethane and toluene. Once isolated, all products were stored under N2 as a precaution, but they are believed to be air stable. Complexes 1−5 can also be prepared from the potassium salts of the Ttz ligands, but the yields are much lower (e.g. 1 is prepared in 81% yield from Tl(TtzPh,Me), and the yield drops to 30% with K(TtzPh,Me)). Thallium salts are very toxic and were used here for reasons of product yield and purity (see the Experimental Section), but in some cases the lower-yielding procedure using potassium salts may be preferable for largescale procedures or safety reasons. These complexes (1−6) were characterized by 1H and 13C NMR, FT-IR, HR-MS, elemental analysis, and single-crystal X-ray diffraction (except for 5). It is interesting to note that the TtzPh,Me ligand isomerized to TtzPh,Me* (for 1 and 2) during the reaction at room temperature. We surmise that this rearrangement relieves steric 2136


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Figure 1. Molecular diagram of (TtzPh,Me*)Ru(p-cymene)Cl (1). Ellipsoids are shown at the 50% probability level. Most hydrogen atoms are omitted for clarity.

Figure 2. Molecular diagram of (TtzPh,Me*)Ru(benzene)Cl (2). Ellipsoids are shown at the 50% probability level. Most hydrogen atoms and noncoordinated toluene solvent are omitted for clarity.

Figure 3. Molecular diagram of (TtzMe,Me)Ru(p-cymene)Cl (3). Ellipsoids are shown at the 50% probability level. Most hydrogen atoms and noncoordinated dichloromethane solvent are omitted for clarity.

methanol molecules bridging two molecules of [(TtzPh,Me)Na(CH3OH)3].44 2.3. Transfer Hydrogenation with Catalysts 1−6. Catalysts 1, 2, 5 and 6 were prepared first and used for initial studies of transfer hydrogenation of aryl ketones in isopropyl alcohol. Initial results with 100% base (KOH, relative to substrate, Table SI-2 (Supporting Information)) show that the hydrogenation is sluggish with limited impact of 1 mol % catalyst. Yields of 1-phenylethanol were 97−99.5% after 24 h (and similarly 70−99% hydrogenation product is reported with other aryl ketones in Table SI-2), but most of this activity can be attributed to the base acting as a catalyst for transfer hydrogenation.55−57 Without the Ru catalyst, KOH alone is

reported to produce 60−70% yield for acetophenone hydrogenation under similar conditions.58,59 Moreover, in these trials the solvent was not rigorously dried, and our studies below suggest that moisture is a poison for these Ru catalysts. By rigorous degassing and drying of all reaction components (base, substrate, catalyst, and isopropyl alcohol) and by assembling the reaction components in the glovebox (rather than on the benchtop), we were able to greatly improve catalytic rates (Table 3). From control experiments in Table 4, it appears that moisture is the poison that deactivates the 2137


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Figure 4. (a) Molecular diagram of (TtzMe,Me)Ru(C6Me6)Cl·H2O (4·H2O). Ellipsoids are shown at the 50% probability level. Most hydrogen atoms and noncoordinated dichloromethane solvent are omitted for clarity. (b) H-bonding interaction with two water molecules connecting two molecules of 4. Color code: O = red; N = blue; Ru = purple; B = pink; C = gray; H = white; Cl = green.

catalyst; a comparison of entries 2 and 3 suggests that air does not significantly impact activity. Table 3 shows that, under rigorously dry conditions, effective hydrogenation results (91− 99% conversion, see Roman entries in Table 3) are obtained with less catalyst (0.2 mol %), less base (25 mol %), and less time (7 h) relative to the results in Table SI-2. Furthermore, at this concentration of base (0.083 M KOH), the background catalysis by KOH is more negligible (entry 1 in Table 3 shows 25% conversion without Ru catalyst). Our best TON and TOF values (495 and 193 h−1 for catalyst 5 [(η6-p-cymene)RuCl(κ2-Ttz)] with base present) are now on a par with those of industrial catalysts.60 Nonetheless, the differences in reactivity between catalysts are slight with base present; while going from the slowest (2) to the fastest catalyst (5) shows an increase in activity by a factor of 2.75 at short times (2 h), these differences become negligible after 7 h. In contrast, when base is absent (italic entries in Table 3), an increase in steric bulk of the Ttz

ligand is accompanied by a dramatic increase in transfer hydrogenation activity. Catalysts 5 and 6 (with Ttz and Btz, respectively) are inactive, catalysts 3 and 4 (with TtzMe,Me) are very slow catalysts, and catalysts 1 and 2 (with TtzPh,Me*) are reasonably active for base-free conditions. A comparison of catalysts 3 and 4 indicates that increased arene steric bulk improves the catalysis slightly, as 3 and 4 differ only in terms of p-cymene vs hexamethylbenzene as the arene group. However, phenyl groups on the triazole rings (in 1 and 2) appear necessary for significant rate enhancements, and here a bulkier arene (p-cymene) produces a significant rate enhancement (initial TOF = 115 and 65 h−1 for 1 and 2, respectively). The high activity of catalyst 1 under base-free conditions (TOF = 115 h−1) is comparable to but slightly less than that with Manzano’s best catalysts under similar conditions ([RuCl(p-cymene)(N,N)]+, where N,N = (bis(3,5-dimethylpyrazolyl)methane)ArX; X = OH, NO2; TOF = 285, 312 h−1, 2138


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substrates (Table 5). For example, we found that 4′-nitroacetophenone is base sensitive when we attempted transfer hydrogenation of this substrate with certain ruthenium N-heterocyclic carbene (NHC) catalysts.37 Entry 1 shows that this substrate is hydrogenated effectively with catalyst 1 (92%, TON = 460), with exclusive ketone (rather than nitro) reduction. Similarly, aldehydes are often subject to the aldol condensation reaction under basic conditions, yet entries 2 and 3 show that the hydrogenation of PhCH2CHO occurred, albeit with low yield. Finally, benzaldehyde is susceptible to the Cannizaro and Tishchenko reactions with base,61 but entries 4 and 5 show that benzaldehyde can be catalytically hydrogenated in moderate yields. Thus, 1 shows promise for hydrogenation of base-sensitive complex molecules. The nature of this rate enhancement may be that increased steric bulk leads TtzPh,Me* to readily form a κ1 intermediate. The β-hydride elimination reaction is well-known to usually require a free site,62 and here two free sites are needed: one for isopropyl alcohol coordination and one for β-hydride elimination. Decoordination of a triazole ring may lead to one free site and may occur more readily with bulky groups (similarly, faster rates for dissociative mechanisms are wellknown with bulky phosphines).63 Chloride loss or substitution can generate the other free site. It is possible that this reaction could occur with bulky κ2-Tp complexes of Ru as well, but furthermore the weak donor properties of Ttz vs Tp38 may also accelerate decoordination of an azole ring with Ttz. It is difficult

Figure 5. Molecular diagram of (Btz)Ru(p-cymene)Cl (6). Ellipsoids are shown at the 50% probability level. Hydrogen atoms on the arene ring are omitted for clarity.

respectively).38 In Manzano’s study bulky groups appeared to increase transfer hydrogenation rates in certain cases (most notably, catalysts with arene = p-cymene were much more active than with arene = benzene),38 and this is similar to results seen in our study. The ability of complex 1 to catalyze base-free transfer hydrogenation can be useful for the hydrogenation of base-sensitive Table 2. Selected Bond Lengths (Å) and Angles (deg) (TtzPh,Me*)Ru(Cl)(p-cymene) (1)

(TtzMe2)Ru(Cl)(C6Me6) (4)

Cl2−Ru1 N9−Ru1 N9−Ru1−N2 N2−Ru1−C34 N2−Ru1−C31 N2−Ru1−C30 N2−Ru1−C33 N2−Ru1−C29 N2−Ru1−C32 N2−Ru1−Cl2

2.3972(5) N2−Ru1 2.1128(2) 87.14(6) N9−Ru1−C34 95.90(7) N9−Ru1−C31 150.34(6) N9−Ru1−C30 162.06(7) N9−Ru1−C33 91.74(7) N9−Ru1−C29 124.38(7) N9 -Ru1- C32 114.11(6) N9−Ru1−Cl2 83.10(4) (TtzPh,Me*)Ru(Cl)(benzene) (2)

2.1192(2)


2.3980(8) N1−Ru1 2.116(2) 87.86(8) N1−Ru1−C32 90.08(1) N1−Ru1−C31 113.02(1) N1−Ru1−C28 123.71(1) N1−Ru1−C30 150.06(1) N1−Ru1−C29 161.41(1) N1−Ru1−C33 95.29(1) N1−Ru1−Cl2 82.56(6) (TtzMe2)Ru(Cl)(p-cymene) (3)

2.110(2)


2.3957(6) 2.0843(2) 86.35(7) 91.96(8) 161.62(7) 151.19(7) 96.45(7) 148.12(7) 96.05(7) 84.67(5)

N2−Ru1 N5−Ru1−C18 N5−Ru1−C15 N5−Ru1−C16 N5−Ru1−C19 C16−Ru1−C19 N2−Ru1−C14 N2−Ru−C17 N2−Ru1−Cl1

113.28(7) 121.67(6) 93.34(6) 150.80(7) 89.36(6) 158.74(6) 86.11(4)

126.02(1) 95.93(1) 147.40(1) 89.87(1) 110.70(1) 163.22() 85.99(7)

2.0901(2) 125.86(7) 111.83(7) 90.41(7) 163.42(7) 79.25(8) 124.09(7) 114.21(7) 83.44(5) 2139

Cl1−Ru1 N1−Ru1 N10−Ru2 O1−H1A O2−H2A N1−Ru1−N4 N4−Ru1−C18 N4−Ru1−C14 N4−Ru1−C17 N4−Ru1−C16 N4−Ru1−C13 N4−Ru1−C15 N4−Ru1−Cl1 N10−Ru2−C40 N10−Ru2−C41 N10−Ru2−C42 N10−Ru2−C37 N10−Ru2−C38 N10−Ru2−C39 C38−Ru2−C39 N13−Ru2−Cl2

2.4028(4) Cl2−Ru2 2.1034(2) N4−Ru1 2.0999(2) N13−Ru2 0.907(2) O1−H1B 0.899(2) O2−H2B 83.80(5) N1−Ru1−C18 153.56(5) N1−Ru1−C14 91.70(5) N1−Ru1−C17 159.27(5) N1−Ru1−C16 121.26(5) N1−Ru1−C13 116.21(5) N1−Ru1−C15 94.37(5) N1−Ru1−Cl1 86.62(3) N10−Ru2−N13 91.75(5) N13−Ru2−C40 116.95(5) N13−Ru2−C41 154.58(5) N13−Ru2−C42 157.84(5) N13−Ru2−C37 119.96(5) N13−Ru2−C38 93.67(5) N13−Ru2−C39 36.92(5) N10−Ru2−Cl2 86.26(4) (Btz)Ru(Cl)(p-cymene) (6)

2.4073(4) 2.1106(2) 2.1022(2) 0.9212) 0.910(2) 92.93(5) 121.98(6) 116.68(5) 154.18(5) 95.41(5) 159.78(5) 85.65(4) 83.75(5) 120.35(6) 94.31(5) 93.35(5) 118.12(5) 155.71(5) 158.10(5) 87.33(3)


2.4127(4) 2.0966(1) 85.16(5) 156.21(6) 118.41(6) 121.05(6) 158.72(6) 95.12(6) 93.39(6) 85.23(4)

2.0846(1)

N1−Ru1 N1−Ru1−C8 N1−Ru1−C9 N1−Ru1−C6 N1−Ru1−C7 N1−Ru1−C5 N1−Ru1−C10 N1−Ru1−Cl2

91.23(6) 93.84(6) 152.94(6) 115.25(6) 158.75(6) 120.97(6) 83.87(4)


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Figure 6. Six-membered chelate rings in a boat conformation for 1 (a), 2 (c), 3 (b), and 4 (d). Color code: N = blue; Ru = magenta; B = pink; H = white; Cl = green. The BH group is syn with respect to Cl in 1 and 2 and anti in 3 and 4.

Table 3. Transfer Hydrogenation of Acetophenone Catalyzed by 1−6 and without Ru Catalyst (entry 1) in the Presence and Absence of Basea

Table 4. Transfer Hydrogenation of Acetophenone to 1-Phenylethanol with 1a entry

airfree?

moisture free?

conversn, % (time, h)b

TON

final TOF, h−1

init (2 h) TOF, h−1)

1 2 3c

yes no yes

yes no no

98 (7) 15 (7) 10 (7)

490 75 50

70 11 7

138 20 15

a This table compares the percent conversions obtained under different reaction conditions. For all reactions 0.2 mol % of 1 and 25 mol % of KOH were used. bPercent conversion to 1-phenylethanol was determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard and were reported as an average of two runs. cThe reaction mixture was prepared as described in the Experimental Section, except that 0.7 mL of degassed water was added prior to the introduction of catalyst, resulting in a 90/10 isopropyl alcohol/water solvent system.

minor factor here, as triazole rings in TtzPh,Me* should be the least basic of the three Ttz ligands studied. Thus, we propose a mechanism in which free sites are needed for β-hydride elimination in the absence of base (Scheme 2). A mechanism involving a decrease in ligand denticity was also proposed by Manzano et al.,38 although it differed somewhat in the details. Here, we propose that decoordination of one triazole ring is the rate-determining step; this is consistent with the observed decrease in base-free catalytic activity: 1 > 2 ≫ 4 > 3 ≫ 5 and 6 (inactive). It is also equally plausible that halide loss occurs before triazole ring decoordination, but the effect of steric bulk suggests that triazole ring decoordination is the bottleneck. In step 2 (Scheme 2), coordination of isopropyl alcohol may proceed with protonation of one of the dangling triazole rings. Thus, decoordinated triazole may be acting as a pendent base; the 2- and 4-positions of the triazole rings have been demonstrated to be Lewis basic sites by binding metals and protons.42−45,47,66 Then, after halide loss the complex has a free site and is poised for β-hydride elimination to give a hydride complex with concomitant acetone loss. We have evidence that hydrides are viable species here; the reaction of 1 with Et3SiH (2 equiv) in toluene led to formation of a hydride species that was characterized by 1H NMR and MS (see the Experimental Section for details). Once a hydride complex is formed, one triazole ring can potentially recoordinate to the metal center, though it may also act as a hemilabile group to allow binding of substrate. The end product proposed in Scheme 2 is poised to

a

Entries in white are with 25% base (KOH) relative to substrate, and similar but base-free conditions are shown in the grey cells. The catalyst concentration for experiments using 1−6 was 0.2 mol % relative to substrate. bPercent conversion to 1-phenylethanol was determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard and were reported as an average of two runs.

to separate steric and electronic effects here, as TtzPh,Me* is the bulkiest ligand studied and is also the weakest donor due to the electron withdrawing Ph group.45 However, the increase in base-free activity upon going from Ttz (5) to TtzMe,Me (3) can only be attributed to sterics, as TtzMe,Me is also a stronger donor,64 and if electronics were to dominate, then a different trend would be observed. Inherent basicity appears to be a 2140


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Table 5. Base-Free Transfer Hydrogenation of Base-Sensitive Substrates in Isopropyl Alcohol with [(η6-p-cymene)RuCl(κ2-TtzPh,Me*)] (1) as the Catalystb

a

Percent conversion was determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard and were reported as an average of two runs, unless otherwise stated. bAll reactions were carried out in isopropyl alcohol (6 mL) at 85 °C under anhydrous and air-free conditions.

Scheme 2. Proposed Mechanism for Formation of the Active Ruthenium Hydride Catalyst from Precatalyst 1 in the Absence of Basea

a The protonated triazole ring represented by NH may have the proton at either N2 or N4. Inner-sphere transfer of H2 to substrate is shown. Similarly, inner-sphere transfer of H2 from isopropyl alcohol to the ruthenium complex can regenerate the active hydride species.

transfer H2 to substrate as hydride from Ru and H+ from a protonated dangling triazole ring, by a metal−ligand bifunctional approach. The net transfer of H2 most likely proceeds via an inner sphere mechanism (with substrate coordinated once a triazole ring leaves) (Scheme 2). Then, the active catalyst can be regenerated by dehydrogenation of isopropyl alcohol via H+ transfer to a triazole ring and H− transfer to the ruthenium center to produce a ruthenium hydride. Thus, in 1 the base-free transfer hydrogenation activity appears to be due primarily to sterics, as the bulkiest catalyst studied is the most active. Also, the basicity of uncoordinated triazole rings may play some role in accelerating proton transfer events that occur after the rate-determining step.

analogues, including crystal structures of complexes 1−4 and 6. Most notable from these studies is the fact that the TtzPh,Me complexes are rearranged (such that one Ph is at the 5-position) and these complexes (1 and 2) are the most crowded, with the methyl of the arene in 1 making contact with one triazole ring. These steric interactions likely lead to facile triazole ring decoordination, for reduced Ttz denticity in solution. It appears that steric bulk accelerates base-free transfer hydrogenation, and thus we propose that a change in denticity is the rate-determining step. Catalyst 1 shows the best activity for base-free transfer hydrogenation of sensitive substrates, and notably, the acceleration with base is quite modest (TOF of 115 h−1 goes to 138 h−1 with base; Table 3). Consistent with these observations is the hypothesis that decoordinated triazole is a good pendent base that may further assist in the transfer hydrogenation mechanism. We ascribe the high activity of 1 for base-free transfer hydrogenation to sterics principally, but

3. CONCLUSIONS The first ruthenium complexes of bulky Ttz ligands have been synthesized and fully characterized, along with their less bulky 2141


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(experimental), 506.167882 [M + H]+ (calculated); all peaks showed the expected isotopic pattern. Caution! Thallium salts are toxic, and extra care must be taken to handle the products and the waste. 4.3.3. [(TtzPh,Me*)Ru(p-cymene)Cl] (1; * = Rearranged Ligand). A solution of Tl(TtzPh,Me) (0.054g, 0.078 mmol) in 10 mL of CH2Cl2 was combined with [RuCl2(p-cymene)]2 (0.024g, 0.039 mmol) in a flask under an N2 atmosphere. The contents were then stirred at room temperature for 16 h. A white precipitate formed during the reaction and was filtered off, and an orange solution was obtained. After removal of solvent from the filtrate an orange solid was isolated, which was washed with hexane (2 × 5 mL) and dried under vacuum. The crude product was purified by recrystallization from dichloromethane solution layered with hexane in air at room temperature to produce exclusively [(TtzPh,Me*)Ru(p-cymene)Cl] (1) in 81% yield (0.048 g, 0.063 mmol) (where the asterisk indicates a rearranged ligand). 1 H NMR (CDCl3): δ 0.65 (d, 6H, CH3 (iPr), 3JH−H = 7.0 Hz), 1.19 (m, 1H, CH(iPr)), 1.35(s, 3H, CH3 (cymene)), (note that the expected number of cymene resonances are not observed, perhaps due to a fluxional process), 2.07 (s, 3H, CH3(tz)), 2.75 (s, 6H, CH3(tz)), 4.36 (d, 2H (cymene), 3JH−H = 6.0 Hz), 4.58 (d, 2H (cymene), 3 JH−H = 6.0 Hz), 7.35−7.54 (m, 9H, meta, para Ph), 8.02−8.04 (m, 6H, ortho Ph). 13C NMR (CDCl3): δ 12.64, 15.20, 17.62, 22.98 (CH3), 30.00(CH(iPr)), 81.55, 83.38, 101.65, 107.98 (cymene), 126.26, 128.46, 128.80, 130.08, 130.39, 131.20, 132.22, 133.39 (phenyl), 155.85, 162.09 (5-tz), 161.71, 168.67 (3-tz). IR (cm−1): 2489.97 (νB−H). Anal. Found: C, 58.46; H, 5.27; N, 16.37. Calcd for 1: C, 58.70; H, 5.19; N, 16.65. In a manner similar to the preparation of 1, complexes 2−4 were prepared using the amounts of reagent, temperature and time noted. 4.3.4. [(TtzPh,Me*)Ru(benzene)Cl] (2). Tl(TtzPh,Me) (0.143 g, 0.207 mmol), [RuCl2(benzene)]2 (0.052 g, 0.10 mmol), room temperature, 16 h. The product was purified by recrystallization from a mixture of toluene and hexane (2/1) at −35 °C to produce [(TtzPh,Me*)Ru(benzene)Cl] (2) in 72% yield (0.105 g, 0.150 mmol). 1H NMR (CDCl3): δ 2.05 (s, 3H, CH3), 2.76 (s, 6H, CH3), 4.51 (s, 6H (benzene)), 7.33−7.52 (m, 9H, meta, para Ph), 7.99−8.09 (m, 6H, ortho Ph). 13C NMR (CDCl3): δ 11.77, 13.47, 13.97, 15.69, 17.42 (CH3), 84.17, 86.49 (benzene), 124.97−132.93 (17 overlapping resonances (phenyl)), 155.92, 161.81 (5-tz), 161.20, 168.54 (3-tz) (some dynamic exchange processes may be occurring, perhaps to interchange which 3-phenyl-5-methyltriazole ring is coordinated). IR (cm−1): 2469.11 (νB−H). HR FAB-MS: m/z 702.160316 [M + H]+ (experimental), 702.160573 [M + H]+ (calculated); all peaks showed the expected isotopic pattern. Anal. Found: C, 56.25; H, 4.59; N, 17.80. Calcd for 2: C, 56.54; H, 4.46; N, 17.98. 4.3.5. [(TtzMe,Me)Ru(p-cymene)Cl] (3). Tl(TtzMe,Me) (0.073 g, 0.14 mmol), [RuCl2(p-cymene)]2 (0.044 g, 0.072 mmol), room temperature, 16 h. The product was purified by recrystallization from a mixture of dichloromethane and hexane (2/1) at room temperature to produce [(TtzMe,Me)Ru(cymene)Cl] (3) in 67% yield (0.055g, 0.096 mmol). 1H NMR (CDCl3): δ 1.11 (d, 6H, CH3 (iPr), 3JH−H = 6.9 Hz), 1.77(s, 6H, CH3), 1.98 (s, 3H, CH3), 2.25 (s, 3H, CH3 of cymene), 2.36 (s, 3H, CH3), 2.72 (s, 6H, CH3), 5.29−5.37 (m, 4H, cymene) (CH resonance of iPr group not observed). 13C NMR (CDCl3): δ 12.80, 14.52, 15.34, 16.24, 17.07, 21.61 (CH3), 24.99 (CH), 83.83, 85.92, 101.71, 106.95 (cymene), 160.49 (5-tz), 165.74 (3-tz). IR (cm−1): 2436.79 (νB−H). HR CIMS: m/z 572.176558 [M + H]+ (experimental), 572.176223 [M + H]+ (calculated); all peaks showed the expected isotopic pattern. Anal. Found: C, 46.04; H, 5.54; N, 21.90. Calcd for 3: C, 46.28; H, 5.83; N, 22.08. 4.3.6. [(TtzMe,Me)Ru(C6Me6)Cl] (4). Tl(TtzMe,Me) (0.191 g, 0.379 mmol), [RuCl2(C6Me6)]2 (0.124 g, 0.185 mmol), room temperature, 20 h. The product was purified by recrystallization from a mixture of dichloromethane and hexane (2/1) at room temperature to produce [(TtzMe,Me)Ru(C6Me6)Cl] (4) in 84% yield (0.187 g, 0.312 mmol). 1 H NMR (CDCl3): δ 1.95 (s, 12H, CH3 of C6Me6), 2.01 (s, 6H, Me), 2.35 (s, 3H, Me), 2.36 (s, 3H, Me), 2.59 (s, 6H, Me). 13C NMR (CDCl3): δ 13.73, 13.99, 16.11, 16.57 (CH3), 94.17 (CH3 of C6Me6), 125.58, 128.51, 129.32 (C of C6Me6), 161.23 (5-tz), 165.53 (3-tz). IR

triazole basicity may play a secondary role once the active catalyst is formed.

4. EXPERIMENTAL SECTION 4.1. Reagents and Physical Measurements. All experiments were performed under an atmosphere of dry N2 using Schlenk techniques and an M. Braun UNILAB glovebox. K(Ttz), 41 K(TtzMe,Me),64 Tl(TtzPh,Me),46 and [RuCl2(C6Me6)]265 were prepared by following the literature procedures. [RuCl2(p-cymene)]2 [RuCl2(C6H6)]2, all ketones and aldehydes, and styrene were obtained commercially. Solvents were dried using an M. Braun solvent purification system with alumina columns or were freshly distilled from drying agents using standard methods. NMR spectra were recorded using either a 300 MHz or a 500 MHz Varian Unity Inova NMR spectrophotometer. IR spectra were recorded on a Perkin-Elmer Spectrum One Fourier-transform IR absorption spectrophotometer. High-resolution (HR) mass spectrometry was performed on a VG70SE double-focusing, triple-quadrupole mass spectrometer equipped with FAB or CI ionization capability. Samples for LIFDI MS were analyzed at the University of California at Riverside. Elemental analyses were performed by Robertson Microlit, Ledgewood, NJ. 4.2. Single Crystal X-ray Diffraction Studies. Diffraction data for compounds 1−4 and 6 were collected using a Bruker AXS SMART APEX CCD diffractometer using monochromated Mo Kα radiation with the ω-scan technique. Single crystals of compounds were mounted on Mitegen micromesh mounts, and data were collected at 100 K. Data were collected, unit cells were determined, and the data were integrated and corrected for absorption and other systematic errors using the Apex2 suite of programs. The structures were solved by direct methods and refined by full-matrix least squares against F2 with all reflections using SHELXTL 6.14. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and were refined with isotropic displacement parameters 1.2 (C−H) or 1.5 (CH3) times that of the adjacent carrier atom. CIF files for structures of compounds were deposited with the Cambridge Structural Database as CCDC Nos. 917041−917045. Further details are given below. In the structure of 2 one of the two crystallographically independent toluene molecules is located on and disordered around a crystallographic inversion center. The disorder also affects the methyl end of one of the triazole ligands, and the NCCH3 unit is also disordered in a 1:1 ratio. In the disordered regions the aromatic atoms of the toluene were constrained to resemble an ideal hexagon with C−C bond lengths of 1.39 Å. In the disordered triazole equivalent bond distances were restrained to be similar and equivalent atoms constrained to have identical ADPs. The CNC(CH3)N units were restrained to be flat. 4.3. Synthesis. 4.3.1. Tl(Ttz). A 5 mL methanol solution of K(Ttz) (0.554 g, 2.17 mmol) was combined with 5 mL of a water solution of TlNO3 (0.600 g, 2.25 mmol). The contents were then stirred at room temperature for 24 h. The solvents were removed, the product was extracted into 50 mL of dichloromethane, and the extract was filtered. The filtrate was dried to afford a white solid that was washed with 5 mL of hexane and dried under vacuum. The yield of Tl(Ttz) was 49% (0.448 g, 1.07 mmol). This complex was recrystallized from dichloromethane at 0 °C. 1H NMR (CDCl3): δ 8.05 (s, 3H, tz), 8.34 (s, 3H, tz). IR (cm−1): 2449.78 (νB−H). HR CIMS: m/z 422.074438 [M + H]+ (experimental), 422.073982 [M + H]+ (calculated); all peaks showed the expected isotopic pattern. 4.3.2. Tl(TtzMe,Me). A 10 mL methanol solution of K(TtzMe,Me) (0.370 g, 1.09 mmol) was combined with 5 mL of a water solution of TlNO3 (0.298 g, 1.12 mmol). The contents were then stirred at room temperature for 24 h. A white precipitate formed and was collected by filtration. The precipitate obtained was dissolved in 30 mL of dichloromethane and the solution filtered. The filtrate was then dried to afford a white solid that was washed with 5 mL of hexane and dried under vacuum. The yield of Tl(TtzMe,Me) is 48% (0.262 g, 0.519 mmol). This complex was recrystallized from dichloromethane at 0 °C. 1H NMR (CDCl3): δ 2.39 (s, 9H, CH3), 2.53 (s, 9H, CH3). IR (cm−1): 2530.06 (νB−H). HR CIMS: m/z 506.168951 [M + H]+ 2142


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(cm−1): 2481.06 (νB−H). HR CI-MS: m/z 600.207831 [M + H]+ (experimental), 600.207523 [M + H]+ (calculated); all peaks showed the expected isotopic pattern. Anal. Found: C, 48.64, H, 6.42, N, 20.52, Calcd for 4: C, 48.13; H, 6.23; N, 21.05. 4.3.7. [(Ttz)Ru(p-cymene)Cl] (5). Tl(Ttz) (0.168 g, 0.400 mmol), [RuCl2(p-cymene)]2 (0.122 g, 0.199 mmol), room temperature, 20 h. The product was purified by precipitation of dichloromethane solution (2 mL) of the crude product with diethyl ether (20 mL). The yellow precipitate that was obtained was washed with hexane (5 mL) and dried under vacuum. The yield of [(Ttz)Ru(p-cymene)Cl] (5) is 61% (0.118 g, 0.242 mmol). 1H NMR (CDCl3): δ 1.33 (d, 6H, CH3 (iPr), 3 JH−H = 6.5 Hz), 2.17 (s, 3H, CH3), 2.93 (m, 1H, CH(iPr)), 5.55 (d, 2H (cymene), 3JH−H = 6.5 Hz), 5.73 (d, 2H (cymene), 3JH−H = 6.5 Hz), 7.72 (s, 1H, tz), 8.15−8.23 (unresolved resonances, 4H, tz), 8.58 (s, 1H, tz). 13C NMR (CDCl3): δ 18.63, 22.49 (CH3), 30.97(CH), 83.98, 85.43, 99.98, 104.04 (cymene), 148.61, 148.93, 151.55, 152.48, 155.11, 156.73 (tz). IR (cm−1): 2464.28 (νB−H). HR CIMS: m/z 452.1 [M − Cl]+ (experimental), 452.3 [M − Cl]+ (calculated); all peaks showed the expected isotopic pattern. Anal. Found: C, 39.67; H, 4.59; N, 25.27. Calcd for 5: C, 39.48; H, 4.35; N, 25.90. 4.3.8. [(Btz)Ru(p-cymene)Cl] (6). A solution of K(Btz) (0.255 g, 1.36 mmol) contaminated with a minor amount of K(Ttz) in 50 mL of dichloromethane was mixed with [RuCl2(p-cymene)]2 (0.270 g, 0.441 mmol) at room temperature, and the contents were then stirred for 24 h. After that, the solution was filtered and the solvent from the filtrate was removed under vacuum. 1H NMR and MS spectra of the crude sample showed the formation of [(Btz)Ru(p-cymene)Cl] (6) as the major product and [(Ttz)Ru(p-cymene)Cl] (4) as the minor product (∼10%) (compound 4 was prepared in a good yield, as described above in a separate synthesis). The crude product was then purified by recrystallization from a dichloromethane solution layered with toluene in air to produce analytically pure [(Btz)Ru(p-cymene)Cl] (6) in 46% yield (0.169 g, 0.403 mmol). 1H NMR (CDCl3): δ 1.30 (d, 6H, CH3 (iPr), 3JH−H = 6.0 Hz), 2.00 (s, 3H, CH3), 2.92 (m, 1H, CH(iPr)), 5.46 (d, 2H (cymene), 3JH−H = 5.5 Hz), 5.65 (d, 2H (cymene), 3JH−H = 5.5 Hz), 8.13 (s, 2H, tz), 8.14 (s, 2H, tz). 13C NMR (CDCl3): δ 18.50, 22.49 (CH3), 30.89 (CH), 83.81, 85.30, 99.72, 106.19 (cymene), 149.56 (5-tz), 156.02 (3-tz). IR (cm−1): 2426.58 (νB−H). HR CIMS: m/z 420.055844 [M]+ (experimental), 420.057450 [M]+ (calculated); all peaks showed the expected isotopic pattern. Anal. Found: C, 40.01; H, 4.69; N, 19.95. Calcd for 6: C, 40.07; H, 4.80; N, 20.02. 4.3.9. Synthesis of a Hydride Species from 1. A solution of 1 (0.055 g, 0.073 mmol) in toluene was combined with Et3SiH (0.25 g, 0.22 mmol, 3 equiv) and refluxed under nitrogen for 2 h. The solution was filtered, and the solvent was removed. 1H NMR in C6D6 showed that hydride species was formed at δ −13.51 ppm and there were several new resonances and starting material present. MS analysis showed a peak at m/z 722.3, corresponding to [M − H]+; [M]+ calcd m/z 723.3. 4.4. General Hydrogenation Procedure. In a typical run (as shown in Table 3) acetophenone (240 mg, 2.0 mmol), KOH (0.028 g, 0.5 mmol), and 1,3,5-trimethoxybenzene (111 mg, 0.667 mmol) were mixed in 5 mL of isopropyl alcohol in a three-neck flask. The desired catalyst (1.0 mL, 4 × 10−6 mol, from 3.96 mM solution in isopropyl alcohol) was injected into the flask, which was attached to a reflux condenser and placed in an oil bath at 85 °C. The reaction was stopped at the desired time, and the product(s) was extracted in hexane to remove any insoluble impurity such as KOH and the catalyst. The products were analyzed by 1H NMR spectra, and the percent conversion was calculated using the signal of the internal standard and the product.

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AUTHOR INFORMATION

Corresponding Author

*E.T.P.: tel, 215-895-2666; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from NSF CAREER (Grant CHE-0846383), ACS-PRF (Grant 48295-AC3), and Drexel University. The diffractometer was funded by NSF grant 0087210, by Ohio Board of Regents Grant CAP-491, and by YSU. We also thank Tim Wade (Drexel University) for MS analysis. A Drexel University Career Development award funded travel to seminars and conferences to discuss this project with colleagues, and in particular our discussions with Alan S. Goldman were helpful. We also thank Jack R. Norton for helpful discussions. Finally, we thank the members of the Papish research group for assistance and suggestions.

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REFERENCES

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S Supporting Information *

Tables and CIF files giving data for transfer hydrogenation of aryl ketones catalyzed by 1, 2, 5, and 6 and crystallographic data for 1−4 and 6. This material is available free of charge via the Internet at http://pubs.acs.org. 2143


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