Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a

Apr 19, 2017 - Patrick Steinhoff , Ralf Steinbock , Anna Friedrich , Benjamin G. Schieweck , Christopher Cremer , Khai-Nghi Truong , Michael E. Tauche...
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Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a Group 13 Metalloligand: Remarkable Effect of an AluminumMetalloligand in Hydrosilylation of CO2 Jun Takaya* and Nobuharu Iwasawa* Department of Chemistry, School of Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan S Supporting Information *

ABSTRACT: Efficient synthesis and catalysis of a series of palladium complexes having a group 13 metalloligand (Al, Ga, In) are reported utilizing 6,6″-bis(phosphino)terpyridine as a new scaffold for Pd−E bonds (E = Al, Ga, In). Systematic investigation revealed unique characteristics of the Al-metalloligand in both structure and reactivity, which exhibited the highest catalytic activity for hydrosilylation of CO2 ever reported (TOF = 19 300 h−1). This study demonstrated fine-tuning of catalyst activity by the precisely designed metalloligand is a promising approach for new catalyst development in synthetic organometallic chemistry.

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evelopment of new supporting ligands for transition metals has been an important challenge in synthetic organometallic chemistry. Reactivity of metals can be finely tuned by appropriate choice of elements and ligand structure, leading to high activity and new catalysis. Recently, group 13 elements have emerged as a new class of supporting ligands by utilizing rationally designed multidentate ligands as a scaffold for a M−E bond (E = group 13).1 Preintroduction of a group 13 element into the ligand followed by complexation with a metal (M) potentially enabled a facile synthesis of a variety of M−E bonds. The group 13 element (E) worked as a σ-acceptor ligand via dative bonding from the metal (M) to the ligand (E), realizing reversal of the role of the metal and the ligand.2 Strong trans influence and cooperative molecular activation were also reported.3 However, these studies mostly focused on boron, and utilization of heavier group 13 elements as a metalloligand has been rather limited (Figure 1).4−7 This is partly due to difficulty of incorporating and handling highly Lewis acidic, reactive metals as a supporting ligand for transition metals. In particular, the systematic synthesis and evaluation of a series of group 13 metalloligands have remained a formidable challenge despite the possibility of discovering and understanding unique catalysis by these metalloligands. In 2011, Lu and co-workers reported an efficient synthesis of various transition metal complexes having aluminum as a metalloligand utilizing a 3-fold N,P-multidentate ligand.6a They also demonstrated reactivity tuning of Ni complexes by varying group 13 metalloligands (Al, Ga, In) for catalytic hydrogenation of alkenes.8 However, the catalytic activity was not very high (TOF = ca. 12 h−1) compared to commonly employed transition metal catalysts. We envisioned that the suitable design of a scaffold for M−E bonds satisfying both synthetic © 2017 American Chemical Society

Figure 1. Various scaffolds for group 13 metalloligands.

ease and high reactivity is strongly required to realize efficient catalysis by group 13 metalloligands. Herein we report 6,6″bis(phosphino)terpyridine as an efficient scaffold enabling a facile synthesis of a series of palladium complexes having a group 13 metalloligand (Al, Ga, In). A remarkable effect of an Al-metalloligand is also demonstrated in hydrosilylation of CO2, achieving the highest catalytic efficiency ever reported. We focused on 6,6″-bis(diphenylphosphino)-2,2′:6′,2″-terpyridine 1 as a scaffold for M−E bonds (E = Al, Ga, In).9 We expected the terpyridine-based rigid and planar N3P2-structure would hold two metals tightly to stabilize the M−E bond effectively. Terpyridine is one of the most widely utilized polynitrogen ligand motifs, thus enabling incorporation of a wide range of metals as a metalloligand. In terms of reactivity, the planar, pincer-type coordination of the obtained metalloligand to a transition metal would allow easier access of substrates than the Lu’s triphosphine ligand, leading to high catalytic activity while keeping sufficient stability.10 The 6,6″-bis(diphenylphosphino)-2,2′:6′,2″-terpyridine 1 was easily synthesized by SNAr reaction of lithium diphenylphosphide to 6,6″-dibromoterpyridine (see the Supporting Information). Incorporation of aluminum into the terpyridine Received: March 14, 2017 Published: April 19, 2017 6074

DOI: 10.1021/jacs.7b02553 J. Am. Chem. Soc. 2017, 139, 6074−6077

Communication

Journal of the American Chemical Society Scheme 1. Synthesis of a Series of Pd Complexes Having a Group 13 Metalloligand (E = Al, Ga, In)

Table 1. Structural Data of E−Pd Complexes 3

E=

Al (3a)a

Ga (3b)

In (3c)

Pd−Cl (Å) Pd−E (Å) Pd−P1 (Å) Pd−P2 (Å) δ 31P

2.5475(9) 2.461(1) 2.273(1) 2.2898(8) 42.8b

2.457(1) 2.3956(7) 2.289(1) 2.293(1) 63.9c

2.426(1) 2.4802(5) 2.321(1) 2.347(1) 68.3c

a

The unit cell contained four independent molecules, and bond lengths of one of them are depicted. The lengths and angles of those four molecules are in a range of Pd−Cl = 2.510(1)−2.54759(9) Å, Pd−Al = 2.455(1)−2.472(1) Å, Pd−P = 2.271(1)−2.2798(8) Å. bIn D2O. cIn DMSO.

moiety was achieved by treatment of 1 with an excess amount of AlCl3 in THF at room temperature, affording a terpyridinealuminum complex 2a in 93% yield (Scheme 1). The structure of 2a was determined by X-ray analysis to be a trigonal bipyramidal, cationic N,N,N-AlCl2 complex having a [AlCl4]− as a counteranion (Figure 2a). The distance between two phosphorus atoms is ca. 4.88 Å, which would be suitable for complexation with various late transition metals. The reaction of 2a with Pd2(dba)3 proceeded smoothly at room temperature to give a palladium complex 3a bearing an Al-metalloligand in 67% yield as a pale yellow solid. Recrystallization of 3a from DMF-THF afforded a single crystal suitable for X-ray analysis, and the ORTEP diagram is depicted in Figure 2b,c. The geometry around palladium is square planar and aluminum is octahedral, forming a tetracoordinate palladium(II) chloride complex having a PAlP-pincer-type ligand. Furthermore, corresponding gallium and indium analogues 3b and 3c were also prepared successfully according to the similar procedure, demonstrating synthetic utility of this method for preparation of a variety of metalloligands and their metal complexes (Scheme 1). Structures of 2b, 2c and 3b, 3c were also fully characterized by NMR and X-ray analyses to be almost the same with those of 2a and 3a (see the Supporting Information). The Pd−Al distance of 3a (2.461(1) Å) is slightly longer than that of previously reported (ItBu)(PCy3)Pd(AlCl3) complex (2.3790(6) Å)11 having an unsupported dative bond from Pd to Al. This is probably due to the rather restricted

structure of the scaffold in 3a. The Pd−E distances are shorter than the sum of the corresponding covalent radii, supporting bonding between Pd and group 13 metals.12 Notable structural differences among Al-, Ga-, and In-metalloligands were found in the Pd−Cl bond length (Table 1). The Pd−Cl bond of 3a is largely elongated to 2.5475(9) Å compared to those of 3b and 3c (2.457(1) Å for 3b, 2.426(1) Å for 3c). This is the longest value among those of pincer-type palladium chloride complexes,13 suggesting the Pd−Cl bond is highly destabilized by the Al-metalloligand. Interestingly, the Pd−Cl bond lengths are seemingly in inverse proportion to the radii of group 13 metals. Moreover, the 31P NMR chemical shift of 3a is largely deviated from those of 3b and 3c (δ = 42.8 as a singlet for 3a vs 63.9 for 3b and 68.3 for 3c), invoking unique reactivity of aluminum among the heavier group 13 metalloligands.14 A more detailed investigation on the bonding and electronic property of palladium complexes 3 is described in the Supporting Information. After extensive screening of the reactivity of a series of palladium complexes, the Al−Pd complex 3a was found to be an active catalyst for hydrosilylation of carbon dioxide. CO2 is an abundant, but less reactive molecule, and its utilization as a C1 source has been a challenging task in current chemistry.14 In particular, hydrosilylation of CO2 is a widely investigated strategy for converting CO2 to useful compounds such as silyl

Figure 2. ORTEP drawings of 2a and 3a at 30% probability level. [AlCl4]− for 2a and hydrogen atoms are omitted for clarity. 6075

DOI: 10.1021/jacs.7b02553 J. Am. Chem. Soc. 2017, 139, 6074−6077

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Journal of the American Chemical Society Table 2. Evaluation of Catalytic Activity for Hydrosilylation of CO2a

entry

catalyst

time

yield (%)

1 2 3 4 5 6 7 8 9 10

3a − 3a w/o Cs-pivalate 2a Pd2(dba)2 + 1 + AlCl3c 3b 3c 5% Pd/C Pd(dba)2d PdCl2(PhCN)2d

3h 3h 3h 24 h 24 h 24 h 24 h 24 h 24 h 24 h

quant.b n.d. n.d. n.d. 2% 10% 7% 17% 4% 27%

Scheme 2. Large Scale Reaction Catalyzed by 3a

Although the detailed reaction mechanism is not clear yet, the unique electronic property of the Al-metalloligand would play a key role for the high catalytic activity as suggested in the structural evaluation. Further experimental and theoretical investigations on the reaction mechanism and origin of the high reactivity of the Al-metalloligand are in progress. In conclusion, 6,6″-bis(diphenylphosphino)-2,2′:6′,2″-terpyridine was established as an efficient scaffold for formation of M−E bonds, realizing a facile synthesis of a series of Pd complexes with group 13 metalloligands. The systematic evaluation of group 13 metals disclosed uniquely high catalytic activity of the Al-metalloligand, achieving the highest efficiency in hydrosilylation of CO2. This study demonstrated high utility of the precisely designed metalloligand for new catalyst development in synthetic organometallic chemistry. Further mechanistic studies and application to other catalytic reactions are ongoing in our group.

Dimethylphenylsilane (1.63 mmol), cesium pivalate (1.5 × 10−2 mmol), and catalyst (1.5 × 10−3 mmol) in DMF (1.0 mL) under CO2 (1 atm, balloon). The yield was calculated based on the amount of formic acid obtained after hydrolysis. b>95% silane was consumed in 1 h by GC analysis. cPd2(dba)3 (0.7 × 10−3 mmol), 1 (1.6 × 10−3 mmol), and AlCl3 (3.0 × 10−3 mmol) were used. dIn the presence of 1 (1.6 × 10−3 mmol). a

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formates, methoxysilanes, and methane because the reaction is thermodynamically favorable and proceeds under neutral conditions.15 Numerous catalysts including metals and organocatalysts have been developed; however, there still remain several problems in efficiency, reaction conditions, and product selectivity toward practical CO2-fixation.16 We found that in the presence of 0.1 mol % of 3a and 1.0 mol % of cesium pivalate as an additive, the reaction of dimethylphenylsilane with atmospheric pressure of CO2 in DMF proceeded smoothly at 25 °C, giving silyl formate in quantitative yield (Entry 1, Table 2).17 The reaction was highly selective for formate formation, and no over reduction was observed. Both 3a and a pivalate salt were crucial for this reaction although the role of the pivalate is not clear yet (Entries 2−4). In situ preparation of 3a in DMF was not successful (Entry 5). It should be noted that other Pd complexes having a Ga- or an In-metalloligand 3b and 3c afforded silyl formate in low yield even after extended reaction time (Entries 6 and 7), highlighting uniquely high reactivity of the Al-metalloligand among group 13 metals. Common palladium complexes such as Pd/C, Pd(dba) 2 , and PdCl2(PhCN)2 were not effective to promote the hydrosilylation (Entries 8−10). The Al−Pd complex 3a exhibited not only unique reactivity differences in Al triad but also the highest catalytic activity among known catalysts for hydrosilylation of CO2. Over 90% conversion was achieved with 4.8 × 10−3 mol % of 3a (s/c = ca. 21 000) at 25 °C under 1 atm CO2, affording silyl formate in 92% yield (Scheme 2). The turnover frequency (TOF) reached 19 300 h−1, which is the highest value ever reported. Previously, Baba reported a Cu-diphosphine complex as one of the most active catalysts for hydrosilylation of CO2 (TOF = 10 300 h−1) although the reaction required heating at 60 °C to afford 31% of product after 6 h.18 Therefore, the Al−Pd complex 3a clearly achieved the most efficient hydrosilylation of CO2 proceeding at ambient temperature under 1 atm CO2, demonstrating high practical utility of the Al−Pd catalyst for CO2-fixation.19

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02553. Preparative methods and spectral and analytical data (PDF) Data for 2a−c and 3a−c (CIF) Models for 3a−c (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Nobuharu Iwasawa: 0000-0001-6323-6588 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Numbers JP15H05800, 15KT0059, and 26620024. REFERENCES

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