PN3(P)-Pincer Complexes: Cooperative Catalysis and Beyond | ACS

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PN3(P)-Pincer Complexes: Cooperative Catalysis and Beyond Huaifeng Li, Théo P. Gonçalves, Daniel Lupp, and Kuo-Wei Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04495 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Catalysis

PN3(P)-Pincer Complexes: Cooperative Catalysis and Beyond Huaifeng Li,# Théo P. Gonçalves,# Daniel Lupp, and Kuo-Wei Huang* KAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia KEYWORDS: PN3(P)-pincer, Catalysis, Metal–ligand cooperation, pseudo-aromaticity 1. INTRODUCTION Organometallic catalysis takes advantage of the ability to introduce highly distinctive reactivity of the corresponding metal complexes by modifying the ligand steric and electronic properties. The tridentate pincer ligands, that enforce the meridional coordination around the metal center upon complexation, have become a powerful platform in catalyst design because of the enhanced stability and flexibility for synthetic modifications.1 Considerable progress has been made in the pincer chemistry after the pioneering reports in the 1970s by Nelson and the Shaw groups (Scheme 1).1,2 Among them, the pyridine-based pincer complexes are particularly attractive. The studies on the synthesis and application of numerous categories of so-called PNP (or PNN) pincer complexes started from the first examples in 1971.2a While the deprotonation of one of the CH2 arms was observed in the synthesis of PNP palladium and platinum complexes by Sacco et. al,3 it was not until 2005 when a remarkable discovery of a new mode of metal-ligand cooperation (MLC) involving dearomatization/rearomatization processes of pyridine-based pincer complexes was made by Milstein and co-workers (Scheme 2).4 This represents an important milestone in the historical development of catalytic applications of pyridine-based pincer complexes5 and such a unique MLC involving ligand aromatizationdearomatization processes enables facile activation of various chemical bonds and environmentally benign catalysis.5 Soon enough, this concept was further applied in several extraordinary reactions by the Milstein group: the dehydrogenative acylation of amines with alcohols,6 water splitting,7 hydrogenation of carbonates,8 etc. (Scheme 3).

N Ru

Et2N H2

N M

PPh2

tBu2P

M

PtBu2

Ph2P

X X X M = Ni, Fe or Co M = Ni, Pd, Pt or Rh X = Cl, Br, I, NCS or ClO4 X = Cl, Br, I, H, CN or CO Nelson 1971 Shaw 1976

Scheme 1. PNP and PCP pincer complexes.

N M

CO2Me

PtBu2

H COH

Scheme 2. The discovery of a dearomatized pincer Ru complex as an effective catalyst.

R1

OH + R2NH2

O

catalyst

NHR2 tBu2 P

R1

N Ru

NEt2

H CO

N + H 2O H NEt2 Ru

tBu2 P

CO H2O + 0.5 O2

N H tBu2P Ru NEt2 HO CO H 2O

H2 H 2O 2

tBu2P

N Ru

MeO

OMe

NEt2

hv

+ H2

catalyst

N CO tBu2P Ru NEt2 HO OH

CH3OH tBu2 P

N Ru

NEt2

H CO

M = Pd or Pt Sacco 1988

RCO2CH2R2

N Ru

Et2N

O

PPh2

RCH2OH

Milstein 2005

CO

Ph2P

PtBu2

H CO

Scheme 3. Selected examples of MLC catalysis developed by the Milstein group.

ACS Paragon Plus Environment

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Table 1. Differences in bond-dissociation energies (BDEs) between C–H and N–H bonds.

BDE (kcal/mol):

BDE (kcal/mol):

H2N-H

PhNH-H

107

92

87.5

H3C-H

PhCH2-H

Ph2CH-H

105

88

82

N N NH R2 P PR2 "dearomatized" PN3P*

N R''

complexes have been intensively studied for 24 years,13 the catalytic applications involving deprotonation/reprotonation of the NH arm and dearomatization/rearomatization of the central pyridine ring were unknown until our discovery in 2011 (Scheme 6). 9,14

R'

O

N

HN R2 P

Ph2N-H

HN R2 P

R'

N

N N

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O R

1

R

2

N NEt2

HN R2 P

HN R2 P

N N

R

N

N M

NH PPh2

XX M = Ni, Pd, or Pt X = Cl

N NH HN PPh2 M Ph2P CO OC CO

Haupt 1987

M = Cr, Mo, or W Ph

HN R 2P

N M

NH PR2

Ln M = Mo, Fe, Ru, Ni, Pd or Pt

N HN R 2P

N N M Ln

OH

catalyst R

1

R2

+

O

N tBu2 P

NH PR2

1

R

2

+

OH

OH

catalyst R

1

R

2

Kirchner 2006

M =Mo, Fe or Pd

Scheme 5. Earlier studies on the coordination chemistry of PN3P-pincer complexes. Inspired by the Milstein mode of substrate activation and also recognizing the distinct properties of N-H bonds as they are more acidic and yet stronger than C-H ones (Table 1), we started the investigation on the potential MLC reactivities by replacing the CH2 arms with one or two NH groups, with the idea that the more acidic N-H bond can facilitate the deprotonation (and thus dearomatization) process while the reprotonation/rearomatization process may allow different bond activation and formation events due to the stronger N-H bonds. Accordingly, we have designed and synthesized a series of symmetrical and unsymmetrical pincer ligands and the corresponding metal complexes (Scheme 4).9 The term “PN3(P)” was proposed by us in an invited review to denote this class of ligands since “N” can be either a spacer or a coordinating atom.9,10 In fact, the synthesis of symmetric PN3P pincer complexes based on 2,6-diaminopyridine were first reported by Haupt and co-workers in 1987,11 and a simple and modular synthetic strategy for various PN3P pincer ligands was also developed by Kirchner and co-workers in 2006 (Scheme 5).12 While the coordination chemistry and catalytic applications of these symmetric PN3P pincer

N Ru

NH PtBu2

Huang 2011

H CO

+

O

N tBu2P

N Ru

O N

Huang 2012

R H CO R R = Me or H

N

Scheme 4. The PN3(P)-pincer ligand platform designed by the Huang group.

HN Ph2P

OH

R' O

HN R2 P

+

Scheme 6. Catalytic studies on the PN3(P)-pincer complexes involving MLC. During the course of the exploring the effect of NH spacers in the backbone of PN3(P)-pincer complexes, we have immediately recognized that this seemingly small change from CH2 to NH in the spacer of the ligand resulted in remarkably different kinetic and thermodynamic properties that in some cases enhanced or in more cases completely altered the catalytic activities due to the inherent features of this unique ligand platform.9b, 14b, 15, 16 Based on the findings from us and others, the steady growth of PN3(P)-pincer chemistry have been seen.9b, 15d, 17 Unfortunately, the deprotonation/reprotonation and dearomatization/rearomatization processes involving the NH arm in PN3(P)-pincer complexes as crucial steps have been largely neglected and overlooked in the mechanistic discussion and catalyst design in recent research articles17a, 17b, 17d-g, 17j-l and reviews.2d, 13b, 18 The aim of this Viewpoint is thus to familiarize the readers with the historical development, progress, and more importantly the distinct potential of the PN3(P)-pincer chemistry in the following three aspects: (i) reactivities of the PN3(P)-pincer complexes with special attention to the MLC via dearomatization/rearomatization; (ii) rationale for the kinetic and thermodynamic differences caused by the replacement of the CH2 spacers with NH groups in the PN3(P)-pincer platform; and (iii) concluding remarks for the future potential of the PN3(P)-pincer chemistry. 2. THE DISCOVERY CHEMISTRY

OF

THE

PN3(P)-PINCER

Owing to the great success of the Milstein system,5a, 5c, 5d, 19 we designed and prepared a series of PN3(P)-Ru complexes for comparison and demonstrated that the “dearomatization” could be achieved through the deprotonation of the N-H group.14 Very interestingly, while Milstein’s PNN complex showed unprecedented activity towards water splitting,7 ours were stable in the presence of water even under heating at above 80 °C overnight.14 Significant thermodynamic differences between the two systems were evidenced. In fact, our early preliminary calculations also


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ACS Catalysis H

H2 separation

H

Pd-Ag membrane CH3CH2OH

catalyst Pd-Ag membrane

O

+ O

H2

100%

O N N tBu2P Ru N H CO

Figure 1. Quantitative conversion of ethanol to ethyl acetate and hydrogen. revealed that the addition of water or H2 molecules to our dearomatized species were thermodynamically uphill. However, the energy differences between those of the computed complexes after the addition reaction and those of dearomatized species were small (2-5 kcal/mol),9b suggesting that these potentially catalytically active intermediates were still thermodynamically accessible. Indeed, at slightly more elevated temperatures (160 °C), our dearomatized PN3(P)*-Ru complexes were active in hydrogen transfer reactions, dehydrogenation and hydrogenation reactions.14b, 15a, 15b, 15d, 15f, 15g, 15j, 15p, 15r, 15s, 15u, 16 More importantly, such an enhanced thermostability (thus a higher reaction temperature is needed) became advantageous for us to integrate with the design of a palladium-based membrane reactor that can only work efficiently and effectively at high temperatures. Accordingly, in collaboration with the Lai group, the first demonstration of quantitative conversion of biorenewable ethanol to the industrially important solvent and starting material, ethyl acetate, was achieved with H2 as a valuable byproduct (Figure 1).15b NH2

N tBu2 P

N NH H PtBu2 Ru

Ru catalyst

N

N tBu2P

CO 90%

CO 93%

N tBu2 P

N H Ru NEt2 CO 71%

O N H Ru N

tBu2 P

complexes were more selective for the dehydrogenation of amines. In contrast, Milstein’s catalyst exhibited a lower reactivity in the same reaction (Figure 2). A protocol for the coupling of amines to imines was thus developed.15a This was the time we realized that this seemingly small change from CH2 to NH in the spacer of the ligand could lead to dramatic influences on the reactivities of the resulting transition metal complexes. We had then dedicated ourselves to the mechanistic understanding to enable the future development of new reactions with this distinct ligand platform. Ph OH

R3

H 2N

R

KOtBu

HO

R2

R

R2

N

N

N

NH

iPr2 P

Ir

PiPr2

Scheme 7. Dehydrogenation reactions catalyzed by PN3PIr complexes. During the course of our mechanistic investigations, Kempe and co-workers have reported the dehydrogenative coupling of secondary alcohols and β-amino alcohols to synthesize pyrroles catalyzed by PN3P-Ir complexes.17a In collaboration with the Wang group, our DFT calculations supported that the dearomatized Ir complex was most likely to be the active catalytic species in this reaction and the proton shuttle process played an important role (Scheme 7).15d Along this line, when compared to the Milstein’s PNN-Ru system, we further demonstrated that two protonic molecules, e.g. water or alcohol, were needed for H2 activation in our PN3*-Ru system in order to connect the reactive sites of the N atom of the imine arm with the Ru metal center (Figure 3).16a Remarkably, the PN3*-Ru complex can effectively catalyze the ester hydrogenation even in the presence of water. These observations not only indicate that the dearomatization/rearomatization process via deprotonation/reprotonation of the CH2 or N-H spacer is an essential step in both PNP/PNN and PN3(P) catalytic systems, but also illustrate their distinct kinetic properties. H N Ru

(tBu)2P

N

HC (tBu)2P

H CO

Figure 2. Coupling of amines to imines under oxidant-free conditions.

H N (tBu)2P

N Ru

N

N

H

H CO

O R

O

R2 + H2

N (tBu)2P

PN3-Ru catalyst KOtBu

N

Ru

H CO H O

H

HO

Wang & Huang, 2014

1

+ +

H

Milstein, 2010

The dehydrogenative coupling of alcohols and amines to amides is one of the most impressive reactions invented with the MLC concept by Milstein.6 Extremely interestingly, the dearomatized PN3(P)*-Ru complexes gave very poor reactivities towards the amide formation although Hong and co-workers have shown that the same reactions could be achieved by simply mixing RuCl3 salt and a carbene ligand precursor.20 After careful analysis of the resulting products, it was found that the PN3(P)*-Ru

N

3

COD

N H Ru NEt2 CO 49%

Ir catalyst

+

R1

H N

1

+ +

H N Ru N H CO

R1

2 OH + R OH

Figure 3. Comparison between the systems with CH2 and HN arms.


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The enhanced stability in the presence of water is particularly important for the selective decomposition of formic acid to H2 and CO2 as a strategy to utilize formic acid as a hydrogen energy carrier.21 In this regard, the PN3P*-Ru complex showed excellent activity with an unprecedented long life.15g, 15u Notably, no decarbonylation reaction was detected, suggesting that a practical application is now possible. The conformation in solution of the transient formate intermediate in this process was recently elucidated by 2D-IR spectroscopy reiterating the important role of the N-H arm(s).15u Moreover, by replacing the t-butyl with phenyl groups, directly knitting the PN3P-Ru complex into a porous polymer network (featured with rigid methylene bridges using dimethoxymethane as an external crosslinker to combine the phenyl groups in the PN3P-Ru complex and benzene) resulted in a new porous heterogeneous ruthenium catalyst with single-atomic sites that offered a reusable catalyst with increased activity for the selective decomposition of formic acid (Scheme 8).15p Further studies on the reverse reaction revealed a good activity in the hydrogenation of CO2 to formate. Remarkably, effective transformation of atmospheric CO2 into formate was achieved in the presence of amines (Scheme 8).15r

Page 4 of 11

deprotonation/reprotonation process. Similar observations were also reported by the Sortais, Kirchner, and Kempe groups respectively in their Mn or Co analogs in various hydrogenative and dehydrogenative reactions (Scheme 9).17f-l, 22

HN

N

iPr2 P

Fe Cl

CO

NH

Zn

PiPr2

CO

R1

H

HCOOH

KOtBu

HCOOH

N

PiPr2

HN

N

iPr2 P

Fe H

CO

HN

N

iPr2P

Fe

R2

Fe catalyst

OH + PhNH2

Ph

O R1

CO

H

Br

ref. 17d

NH PiPr2

ref. 17d

CO

R1

+

NH

+

R

1

iPr2 P

H

Mn catalyst

R1

N

NH CO Mn PiPr2 N

H

ref. 17k

CO

R2

ref. 17f Ph

OH

R

Mn catalyst

R3

R2

OH

+ R2

N

KOtBu

Na2CO3

N

HN iPr2 P

R2

N Ph

Mn catalyst

OH

3

R1

KOtBu

Mn H

NH ref. 17g CO PiPr2

CO

N

R1

ref. 17g R2 Ph

R3

H 2N

Mn catalyst

+

R1

N H PPh2 Ph2P H N N

Het

R1

R

+ R2

OH

H N

1

R

N

3

N N

HN

KOtBu

HO

R2

O

HCOONa

ref. 17e

ref. 17i

OH

Mn catalyst

OH

N

NH N HN Br iPr2P Fe PiPr2 Br CO

HN

2 OH + R NH2 Ph

R1

N

Ph

N H

R2

iPr2P

Mn H

CO N

Het

Mn catalyst

R1

iPr2 P

R1

N N H NH Ph2P Ru PPh2 Ph3P CO

R2

+ H2 NH2

R1

Mn H

KOtBu

Br

R2

R1

H N

Mn catalyst R1

KOtBu

NH ref. 22c CO PiPr2

CO

OH

Mn catalyst

+ CH3OH

N N

HN

R2

N

KOtBu

ref. 17l

NH CO PiPr2

the direct knitting in a porous organic polymer catalyst

ref. 17b

PiPr2

DBU-H HCOO

DBU

+ H2

H

Ph

KOtBu

Mn catalyst

CO2 + H2

N

H2 + CO2

NH

NMe2

H 2N

PPh2 Ph2 P N N N Ph2 P H H PPh2 OC Ru HN NH RuCO

H2 + CO2

OH R1

OH

catalyst

R1

catalyst

R2

Ph

H2 + CO2

H2

NH

CO

+ H2

O R1

CO

OH

NH2

N N NH tBu2P Ru PtBu2 H CO

N Fe

O

R1

catalyst

N iPr2 P

NH CO Mn PiPr2

ref. 17h

N

HN

iPr2 P OC CO

ref. 17i

NH-C3H5

H2

N R 2P

HN N H NH R2P Ru PR2 H CO

N H NH Ru PR2 CO

HCOOH

Ph

CO2

OH + PhNH2

Co catalyst KOtBu

Ph

N H

Ph

N HN iPr2P Cl

N ref. 22a

N

NH

Co

PiPr2 Cl C6H4-4-CF3

HN N H NH R2P Ru PR2 OHCO CO

Scheme 8. Formic acid decomposition and formation processes catalyzed by PN3P-Ru complexes via MLC. The crucial role of the dearomatization/rearomatization via deprotonation/reprotonation of the NH arm in the PN3(P) pincer catalysis appears to be general. For example, Kirchner and co-workers described the heterolytic H2 cleavage via the dearomatized PN3P-iron intermediate via MLC (Scheme 9).17b,17d Catalytic alkylation of amines with alcohols was achieved but when NH groups were methylated to N-Me, the reactivity was shut down,17e suggesting the importance of the NH

OH Ph

OH +

PhCH2OH

Co catalyst KHMDS

Ph

N Ph

HN iPr2P Cl

N N

NH

Co

PiPr2

ref. 22b

Cl

Scheme 9. MLC-triggered hydrogenation and dehydrogenation reactions catalyzed by PN3P-pincer complexes.

3. UNIQUE REACTIVITY MODES OF PN3(P)-PINCER COMPLEXES Since the discovery of the catalytic applications of the dearomatized PNx(P)* metal complexes, many groups join


Page 5 of 11 A) Van Koten and Milstein(31a-c)

C) Our (pseudo)-aromaticity concept

- length of arm; X is CH2, NH or O - (Chiral) substituents - donor groups can be alike or different

X Y

X M

- substituent R - nano-sized object - type of ring

R

labile arm

Ln

Y and Y' sterics and electronics

Z Y reactive sites

double bond migration

Type of Cipso sp3 or sp2

B) Morales-Morales(31d)

R

D) The effects of the pincer arm on the hydrogenation of the Milstein and Huang systems Hu an +H2 g sy G st em kcal/mol HN

introduction of chiral centers and modulation of steric effects

Y X

soft tuning and anchoring site

- choice of 2e donor atom - substituents on donor atoms - Y can be part of a heterocyclic group

X Y

Z' Y'

X-M bond changes from covalent to dative

- choice of the metal - site for counter ions or ancillary ligands

C MLn Rx

spectator spacer

gain/loss of the aromaticity

Cavity size control

C MLn

hardness and softness modulation

0.0

Potential anchoring site and fine tuning of electronic effects

X tBu2P

+2.7

tem

R

tBu2P

ys ns

Y

ei lst Mi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

N N Ru H CO

H H C

-7.7

tBu2P

N H N Ru H CO

N H N Ru H CO

Figure 4. Reactivity view of the PNx(P)* pincer complexes.

product

pseudo-dearomatized ring

catalytic

new active catalytic sides

R-Z Y

Z-R

X

Ln M N

Y X

+Z-R

R

Y HX

Ln

Ln M N

Y X

Y HX

M N

Ln A

Y X

+A B

resonance

R

dearomatized

Metal-Substrate Cooperation (MSC)

Ln Q' M Y X N R

Metal Centered Reactivity (MCR)

M N

R

Y X

B

R

aromatic zwitterionic form

+Q

Y HX

Y HX

Metal-Ligand Cooperation via Aromatization/Dearomatization (MLC-ADA)

+E Ln Y HX

M N

Y X

E'

R Ligand Centered Reactivity (LCR)

L - ligand X - CH or N Y - P, N Q, E, Z-R - substrate Q' and E' - activated substrate/product

Figure 5. Pictorial view of the key catalytic activation modes of the PNx(P)* dearomatized pincer complexes. their efforts to understand and rationalize the reactivities of such systems for the small molecule activation,23 dehydrogenative coupling,15d, 24 and hydrogenation reactions;25 under active debate. Li and Hall recently reviewed the reactions involving the pincer complexes and noted that aromatization/dearomatization step is not always the lowest energy path.26 The Li and co-workers in

2009 suggested the importance of the hydrogen tunneling for the activation of the (PNP)IrPh complex,23b while Iron and co-workers proposed that the hydrogen transfer from ligand to metal was driven by aromatization in a heterolytic manner.23a Very recently, the study by Lei, Liu, Schaefer and co-workers introduced the metal-substrate mechanism rather than metal-ligand cooperation as a low


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energy alternative pathway.27 It was suggested that the proton shuttle mechanism is not involved in the lower energy pathway, which strongly contradicts with studies on other systems.23a, 23b, 28 For some activation modes, the metal-ligand cooperation may occur without undergoing the aromatization/dearomatization step.29 Collectively, the activation modes may be system sensitive, and molecular design can lead to unique chemical applications. The core of such diversity of reactivity modes lies on the cooperative properties and non-innocent character of the pyridine-based ligands.5, 9b, 30 As mentioned above, from the distinct frontier molecular orbitals of the imine and olefin arms in the dearomatized PNx(P)* structures, we were able to rationalize the kinetic differences.15d, 16a In addition to the reactivity views suggested on the PCP ligands by van Koten, Milstein and Morales-Morales (Figures 4A and 4B)2d, 31 and based on the our interest in the aromaticity,32 we proposed the system based on the (pseudo)dearomatized platform (Figure 4C) and consequently elucidated the thermodynamic discrepancies between the Milstein and the Huang systems through different electron delocalization and aromatic stabilization energy (ASE) (Figure 4D).15k The “dearomatized” PN3(P)* complexes indeed carry certain aromatic properties as the enhancement of ligand aromaticity through complexation becomes more pronounced compared to those of analogous dearomatized PNP* or PNN* based complexes, suggested by our nucleusindependent chemical shift (NICSzz), anisotropy of the current (induced) density (ACID), isochemical shielding surfaces (ICSSzz), and natural bond order (NBO) calculations.15k This work also represents the first example of theoretic studies on aromaticity in catalytic systems involving ligand dearomatization/rearomatization processes. More importantly, the view of the (pseudo)aromaticity has led to a novel paradigm of strategy for the catalyst design. The dearomatized pyridine-based PN3(P)* pincer complexes must be understood by two leading resonance forms: neutral and zwitterionic structures (Figure 5).3, 15k The contribution of the aromatic form is much higher in the PN3(P)* system than those in the PNP and PNN counterparts, partially from the difference of the atom electronegativity. In such flexible frame of pseudo-dearomatized frame, the metal center can act as an acid side and unsaturated arm can act as a basic side. As a result, the system expresses a few activation modes leading to unique reactivities incorporated in one system (Figure 5). The metal-centered reactivity (MCR) is conventional for the majority of metal complexes via ligand exchange, oxidative addition, reductive elimination, insertion, bond metathesis, etc.33 The metal-substrate cooperation (MSC) was also suggested as the key step for some pincer complexes with labile arms.27 In those two modes, the ligands do not participate in the catalytic reactions. The metal-ligand cooperation via aromatization/dearomatization (MLC-ADA) process requires unique rearrangement of the bonding patterns. In this transformation, two new sigma bonds are formed and the existing ones are rearranged to deliver the aromatic ring.15d, 15k Although MLC-ADA generally involves the metal

Page 6 of 11

center and an unsaturated arm, it can also be promoted by metal and a reactive donor group.34 The forth mode of the reactivity, ligand-centered reactivity (LCR), is highly intriguing and can be triggered by the non-innocent character of the ligand and/or enabled by the zwitterionic resonance characters.15t, 35 [Ni-H] catalyst

CO2 + Ph2SiH2 1

R NH + CO2 + Ph2SiH2 R

2

O H

R

+ PhSiH3

NaOH/H2O R1 N R2

[Ni-H] catalyst By tuning the conditions

[Ni-H] catalyst

CH3OH or

R1 N R2

O H

OH

NaOH R O

O R R1 NH R

2

[Ni-H] catalyst

+ CO2

O

O

OH

catalytic amount of [Ru(p-cymene)Cl2]2

N

N

NH

tBu2P

Ni

PtBu2

H

R [Ni-H] (as a ligand)

+ Ph

NHCs-like catalytic reactive center

O Ph

N R 1R 2

Scheme 10. A pseudo-dearomatized PN3P*Ni-H complex as a ligand and σ-nucleophilic catalyst. For the PNP system, the basicity of the arm was studied experimentally by Milstein in 2010.36 It was demonstrated that at ambient temperature the neutral form is entropically favored while the cationic form is favored in low temperature due to the effect of charge separation. In the PN3P* platform, the LCR offers unprecedented reactivities.15q The σ-nucleophilicity of the N atom of the iminic arm can be significantly enhanced such that it becomes catalytically active to reach N-heterocyclic carbene-like nucleophilicity in the hydrosilylation of CO2 to methanol, selective reductive methylation and formylation of amines with CO2, hydrosilylation of aldehydes to alcohols, and cycloaddition of CO2 to epoxides, and serves as a ligand in the Ru-catalyzed dehydrogenative acylation of amines with alcohols (Scheme 10).15q, 15t 4. COORDINATION CHEMISTRY COOPERATIVE CATALYSIS

BEYOND

THE

The structural modification in the pincer system can offer significantly different reactivities. Within the LCR concept, our initial progress in creating new ligand structures is encouraging.15l-o We have developed a post-modification strategy taking advantage of the enhanced ligand reactivity to prepare a new class of diimine-amido anionic PN3Ppincer complexes, 2nd generation PN3P, that could not be easily synthesize by conventional synthetic organic methods (Scheme 11). Upon treatment of dearomatized PN3P-pincer complexes with excess amounts of a base (e.g. KOtBu) and an alkyl halide (e.g. EtI), the backbone of the ligands can be alkylated. Unique activities were discovered. For example, the 2nd generation PN3P-Ni azide complex can be photo-activated to react with isocyanides to afford monosubstituted carbodiimides, presumably via a transient nitrido intermediate.15m The 2nd generation PN3PNi moiety allowed the isolation of a terminal hydroxide complex and enabled the investigation of the diverse


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ACS Catalysis

reactivities of the hydroxide group with respect to its nucleophilic

N

N

NH

tBu2P

M X

PtBu2

N

N

N

N

NH

tBu2P

M X

PtBu2

KOtBu EtI

N

N

N

tBu2P

M X

PtBu2

N

KOtBu EtI

M = Ni, Pd, or Pt X = hal or N3

N

N

N

N

tBu2P

Ni X

PtBu2

X = Cl or I

N PtBu2 N tBu2P

N tBu2P

N tBu2P

RCN

N

N

Ni N3

PtBu2

N

N

Ni OH

PtBu2

N

N

Ni OH

PtBu2

Irradiation

N C NR R = tBu or 2,6Me2C6H3

N PtBu2 tBu2 N P O

CS2

E

N

N

tBu2P

Ni X

O

catalyst THF/H2O

N Ni

R-NC

R

N NH2

tBu2P

tBu2 P N

N Ni S

S Ni N

N P tBu2

P N tBu2

E = CO, CO2, ArNCO, ArNCS, or RCN X = COOH, OCOOH, OCONHAr, SCONHAr, PtBu2 or NHCOR

N

N

N

Ni OH

PtBu2

5. CONCLUDING REMARKS PN3(P)-pincer complexes are not only incredibly effective catalysts for the hydrogenative and dehydrogenative reactions, but they also possess distinct kinetic and thermodynamic properties compared to their CH2 analogs (ie. the Milstein mode). We have elucidated the role of MLC and rationalized the influences of the aromaticity as crucial factors in the catalytic processes. We have further demonstrated the LCR concept and utilized the unique pseudo-aromaticity of our PN3(P) platform to enhance the ligand reactivity. A pseudo-dearomatized PN3P*Ni-H complex was synthesized as a new NHC-like ligand and σnucleophilic catalyst. Last but not least, a new class of 2nd generation of PN3P-pincer complexes was established (Figure 6). We anticipate that our discussion herein can provide better understanding of the PN3(P)-pincer ligand platform we established. Other unique reactivities of various transition metal complexes are expected to be discovered in the near future. The novel LCR that the imine arm reaches unprecedented carbene-like reactivity may open a new direction for the catalyst design. The properties and potentials of 2nd generation of PN3P complexes are awaiting further exploration.

Scheme 11. Formation of a new PN3P-pincer platform through ligand post-modification. and basic characters (Scheme 11).15n This monomeric NiOH complex was also identified to catalyze the nitrile hydration to various amides efficiently.

Figure 6. Historical Development of the PN3(P)-pincer chemistry.


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AUTHOR INFORMATION

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Gunanathan, C.; Ben-David, Y.; Milstein, D. Di Amides from Alcohols and Amines with Liberation 2007, of H 2. Science, 317, 790-792. Corresponding Author (7) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konst *E-mail: [email protected] Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstei Thermal H Light-Induced O from Water Promoted by a 2 and 2 Evolution Metal Complex. Science, 2009, 324, 74-77. (8) Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, D. Efficient Hydrogenation of Organic Carbonates, C Formates Indicates Alternative Routes 2to and Methanol CO. Bas Nat. Chem., 2011, 3, 609. ORCID (9) (a) Huang, K.-W.; Chen, T.; He, L.; Gong, D.; Jia, W. Kuo-Wei Huang: 0000-0003-1900-2658 amino Pincer-Type Ligands and Catalytic Metal Complex Patents, US 8,598,351 B2: 2013. (b) Li, H.; Zheng, B.; 3-Pincer New Class of PN Ligands for Metal–Ligand Cooperative Notes Coord. Chem. Rev., 2015, 293–294, 116-138 and references therein. K.-W.H. declares the following competing financial (10) In order to avoid confusion with the 1,3,5-triaza-7 interest(s): Huang is one of the inventors of US Patent# "PN3" ligand and to indicate that N can be a donor atom or a s 3(P). 8,598,351 defined the new ligand platform as PN (11) Schirmer, W.; Flörke, U.; Haupt, H.-J. Darstellung “Phospho-amino pincer-type ligands and catalytic metal Molekülstrukturen von Komplexen des Versteiften complexes thereof” assigned to KAUST. Chelatliganden N, &&&&&&&d&&&&&&&&&&&&&&&&&&2& 6-di # These authors contributed equally to this work. - und M0-Übergangsmetallen [MII = Ni, Pd, Pt; M0 = Cr, Anorg. Allg. Chem., 1987, 545, 83-97. ACKNOWLEDGMENT (12) Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Ac Transition Metal Complexes with Modularly Designed T Financial support and the service of Ibex, Shaheen 2 High Pincer-Type Ligands Based on N-Heterocyclic Diamines. Performance Computing Facilities were provided by King 2006, 25, 1900-1913. Abdul(13) (a) Benito-Garagorri, D.; Kirchner, K. Modularly Des lah University of Science and Technology (KAUST). Metal PNP and PCP Pincer Complexes based on Aminopho Synthesis and Catalytic Applications. Acc. Chem. Res., 2008, 4 (b) Gorgas, N.; Kirchner, K. Isoelectronic Mangane REFERENCES Hydrogenation/Dehydrogenation Catalysts: Similariti (1) (a) Van Koten, G.; Milstein, D. Organometallic Pincer Acc. Chemistry; 2013, Chem. Res., 2018, 51, 1558-1569. Springer: Heidelberg: 2013. (b) Szabó, K. J.; Wendt, O. F. Pincer and (14) (a) He, L.-P.; Chen, T.; Xue, D.-X.; Eddaoudi, M. Pincer-Type Complexes: Applications in Organic Synthesis and Catalysis; Efficient Transfer Hydrogenation Reaction Catalyzed b John Wiley & Sons: 2014. (c) Van Koten, G.; Gossage, A. The privileged PN3PR. Ruthenium Pincer Complex under Base-Free Condit pincer-metal platform: Coordination chemistry & applications; Springer:Organomet. 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ACS Catalysis

SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).

Y HX

Ln

Ln M N

Y X

Y HX

M N

Y X

resonance

R

R

Ln Y HX

M N

Pseudo-Aromatized Zwitterionic form

Pseudo-dearomatized

Y X

E'

R

Evolution of concepts N

N

X

N

N

N

tBu2P

M X

L

tBu2P

Ni X

PtBu2


PN3(P)-Pincer Complexes: Cooperative Catalysis and Beyond | ACS

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