A wacker-type strategy for the synthesis of unsymmetrical POCsp3E-nickel pincer complexes

POC E ‑Nickel Pincer Complexes

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A Wacker-Type Strategy for the Synthesis of Unsymmetrical

Lo ï c P. Mangin, Guillaume Michaud, and Davit Zargarian*

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00590

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sı Supporting Information

ABSTRACT: ECE-type pincer complexes have evolved into a

diverse family of compounds possessing interesting structural/

bonding features, reactivities, and practical applications. An

important factor promoting the growth of pincer chemistry is the

availability of versatile synthetic pathways that give access to ever-

diverse pincer complexes. This report describes the synthesis of

pincer−Ni complexes possessing the following features: a central

Ni−C linkage, two diﬀerent peripheral donor moieties, and two

diﬀerently sized metallacycles. The synthetic methodology

reported herein is based on the reactivity of a phosphinite derived

from 2-vinylphenol. Stirring the substrate 2-CH CHC H -

OP(i-Pr) , 1, with the Ni precursor {(i-PrCN)NiBr } and Et N

C P

at room temperature gave the 6,5-POC PO-type pincer complex {κ ,κ ,κ -2-[(i-Pr) P(O)CH CH]-C H -OP(i-Pr) }NiBr, 2.

Conducting this reaction in the presence of an excess of 1 hinders the formation of 2, giving instead the nickelacyclopropane

complex {κ ,κ,κ -2-[(2-CH CHC H O)P(i-Pr) CHCH]-C H -OP(i-Pr) }NiBr, 3, whereas introducing a second, stronger

nucleophile into the reaction mixture leads to the formation of pincer complexes featuring rare 4-membered metallacycles. For

C P

instance, using HNR(R′) as nucleophile gave the 6,4-POC N-type pincer complexes {κ ,κ ,κ -2-[R(R′)NCH CH]-C H -OP(i-

Pr) }NiBr (NR(R′) = N-morpholyl, 4; NPh(Et), 5; NH(i-Pr), 6; NH(Ph), 7; NH(Cy), 8; NH(t-Bu), 9), whereas using HPR as

C P′

nucleophile led to the 6,4-POC P-type pincer complexes {κ ,κ ,κ -2-[R PCH CH]-C H -OP(i-Pr) }NiBr (R = i-Pr, 10; iPh, 11).

Single crystal diﬀraction studies have established the solid-state structures of complexes 2−11. All the pincer complexes reported

here feature 6-membered metallacycles deﬁned by the phosphinite moiety, while the phosphine-oxide moiety in 2 deﬁnes a 5-

membered metallacycle and 4-membered metallacycles form by the coordination of the amine moieties in 4−9 and the phosphines

in 10 and 11. Cyclic voltammetry measurements on complexes 2, 4−6, 10, and 11 have shown that these pincer−Ni complexes

undergo facile one-electron oxidation.

INTRODUCTION

bonds in EC(H)E-type proligands and (b) the oxidative

addition or transmetalation of C−X bonds in EC(X)E-type

proligands. These classical strategies are exempliﬁed by

■

1,2

Since its inception more than four decades ago, the ﬁeld of

pincer chemistry has evolved rapidly to make major

contributions in diverse areas of application, including

Shaw’s and van Koten’s landmark studies on the synthesis

of (PCP)ML and (NCN)ML , respectively (Scheme 1).

catalysis and preparation of functional materials. A key

element in the rapid development of pincer chemistry has been

the successful elaboration of eﬃcient synthetic strategies that

have paved the way to a diverse array of ligand platforms based

on aromatic or aliphatic backbones. These synthetic method-

ologies have thus provided access to a variety of classical or

novel pincer complexes featuring symmetrical and unsym-

metrical architectures, diﬀerent peripheral and central donor

moieties, and (less frequently) diﬀerent metallacycle sizes. This

diversity has, in turn, led to the discovery of unique reactivities

and properties that are the general hallmarks of pincer

The success of synthetic routes based on activation/

metalation of C−H bonds depends on many factors, including

the nature of the donor moieties, the ligand backbone, and the

type of metal precursor used. For instance, PCP-type pincer

complexes of nickel are accessible via relatively facile C−H

nickelation reactions with m-phenylene type proligands

mentioned above. In contrast, analogous proligands featuring

Received: September 7, 2020

complexes today.

By and large, the most commonly used synthetic strategies

for the preparation of ECE-type tridentate, monoanionic

pincer ligands rely on (a) the activation/metalation of C−H

XXXX American Chemical Society

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Scheme 1. Classical Synthetic Strategies for Preparation of

ECE-Type Pincer Complexes

H nickelation would occur on the alkenyl substituent or the

aryl ring. As will be described herein, studying the reactivities

of the vinyl-substituted phenyl phosphinite 2-CHCH −C H −

OP(i-Pr) , 1, led to the development of a new synthetic route

that allows facile formation of novel families of unsymmetrical

pincer complexes without going through a C−H activation/

metalation step.

RESULTS AND DISCUSSION

■

Initial Exploration of the Reactivity of 2-Vinyl-

Phenylphosphinite. This study began by preparation of

the target substrate 1 and examination of its reactivity under

the conditions used for our previous nickelation studies.

Thus, 1 was treated in acetonitrile with {(i-PrCN)NiBr } and

13,14

Et N (molar ratio of 1.0:1.2:1.2), and the reaction progress was

monitored by NMR spectroscopy. P NMR spectral analysis

of the greenish mixture obtained after 2 h of stirring at room

two amine moieties are much less amenable to pincer

formation via C−H nickelation.

C−H nickelation is also more diﬃcult with aliphatic

proligands, even when they feature phosphine- or phosphin-

ite-type donor groups. For instance, the aliphatic proligands

temperature showed that the initial signal for 1 had been

replaced almost completely by two new P resonances at 103

and 202 ppm. The chemical shift of the latter signal was close

to that observed for the π-allyl complex shown in Scheme 2 (B,

201 ppm), thus hinting that nickelation might have occurred at

the vinyl C−H moiety. However, the 1:1 intensity ratio of the

new peaks suggested the formation of a complex featuring two

diﬀerent P-bearing moieties.

,3-(R PO) -(CH ) and 1,5-(R P) -(CH ) require more

2 3

2 5

forcing conditions to undergo C −H metalation in compar-

ison to their counterparts based on m-phenylene backbones.

Although these limitations have not hindered the advance of

pincer chemistry, it would be advantageous to develop a new

synthetic strategy that sidesteps the (often-signiﬁcant) barriers

In order to isolate the reaction product and characterize it

unambiguously, we worked-up the reaction mixture by

evaporating the solvent and extracting the residues with

to C −H activation/metalation steps.

This report describes one such strategy based on net

addition of nucleophiles to an oleﬁn moiety coordinated to a

toluene. The resulting orange solution displayed two P peaks

divalent nickel precursor. This new Wacker-type synthetic

methodology was discovered during our investigations on the

C−H nickelation of phenyl and naphthyl phosphinites. Earlier

studies had shown that in most cases nickelation of aryl

phosphinites occurs regioselectivity at the unsubstituted C−H

at 94 and 205 ppm in a 1:1 ratio; this mixture also contained

5−20% of the unreacted bis-phosphinite complex

2 2 2

(

1) NiBr . Evaporation and recrystallization from Et O

aﬀorded deep red crystals that displayed two very weakly

coupled P doublets at 94 and 206 ppm (CDCl : J ≈ 3 Hz).

PP′

moiety ortho to the OPR position, as illustrated in Scheme 2

XRD analysis of a single crystal revealed the unsymmetrical

ECE′-type pincer complex 2 shown in Scheme 3.

Scheme 2. Cyclonickelation of Phosphinites Derived from

Phenols and Naphthols (R = i-Pr)

Scheme 3. Unanticipated Formation of (POCPO)NiBr

Pincer Complex 2 and Its Molecular Diagram

(

reaction A). In other words, ring substituents were found to

be largely inert to nickelation. The only exception to this

regioselectivity was observed with 2-allyl-1-naphthyl-OP(i-Pr)

(

Scheme 2, reaction B). This observation was quite

The main structural parameters for the unexpected product

2 (and all the other solid-state structures presented herein) will

be presented and discussed fully in the last section of this

report. For now, it is worth noting that this complex features

two diﬀerent metallacycles, a 6-membered ring formed by the

coordination of the original phosphinite moiety in 1, and a 5-

membered ring deﬁned by the chelation of the in situ formed

phosphine oxide moiety. The latter originates from a second

equivalent of 1 via a rearrangement that results in the expulsion

of its aryl fragment. The overall transformation of 1 into 2 also

unexpected as the normally less facile nickelation of a

substituent C −H moiety proceeded in preference over

reactivity at the ring C−H. Indeed, the α C−H moiety in

the allyl substituent of this substrate was nickelated at room

temperature, which is in stark contrast to the requirement of

high temperatures (80−160 °C) for the nickelation of the

aromatic C−H moieties in most aryl phosphinites.

This discovery prompted us to examine the reactivity of

alkenyl-substituted aryl phosphinites to determine whether C−

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converts the sp -hybridized α-C of the vinyl substituent in 1

Another aspect of the mechanism proposed in Scheme 4 was

puzzling at ﬁrst: although the mechanism requires 2.0 equiv of

into the sp -hybridized central carbon moiety of the new 6,4-

POC P(O)-type pincer complex. Thus, no net C−H

nickelation takes place during the formation of the unsym-

metrical pincer complex 2.

The above considerations allowed us to envisage the

following speculative scenario for the formation of 2. First,

π-coordination of the vinyl moiety in 1 to the Ni center would

substrate 1 relative to the metal precursor and Et N, tests

showed that using more than 1.0 equiv of substrate slows down

the formation of 2. For instance, monitoring a reaction mixture

containing 2.0 equiv of 1 (acetonitrile, room temperature) by

P NMR spectroscopy showed a much slower conversion of 1

to 2. Similarly, a much slower reaction was observed when the

give an intermediate complex featuring a chelating phosphin-

ite/vinyl ligand, as shown in Scheme 4 (step A). The

reaction of 1 with the nickel precursor and Et N was

conducted in THF instead of acetonitrile, either with 1.0 or

.0 equiv of substrate.

Interestingly, allowing the above-noted 2:1:1 (1: Ni: Et N)

Scheme 4. Proposed Mechanism for the Formation of

Complex 2

reactions to proceed over extended periods led to the

formation of a new species that over time (>48 h) became

the major product of the reaction besides complex 2. That this

new species is structurally similar to complex 2 was indicated

by the emergence of a second set of P resonances fairly close

to those of 2 (i.e., around 97 and 210 ppm vs 103 and 202

ppm for 2 in MeCN). Workup of the reaction mixture in Et O

followed by cooling gave two sets of crystals: deep-red crystals

that were identiﬁed as the expected product 2, and bright

orange crystals that turned out to be a new zwitterionic

complex 3 cocrystallized with 2 (Scheme 5).

Scheme 5. Proposed Mechanism Leading to Complex 3

coordinated vinyl moiety would thus become susceptible to

an outer-sphere nucleophilic attack, and then a second

equivalent of 1 would add to the β-C of the vinyl moiety to

generate a new P−C bond and a new Ni−C bond (step B in

Scheme 4). A subsequent SN -type nucleophilic attack by a

bromide anion on the ipso carbon of the aryl ring in the newly

generated phosphonium moiety would break the Ar−O bond

to generate the PO moiety (step C in Scheme 4).

Although the sequence of steps illustrated in Scheme 4

seemed feasible at ﬁrst, subsequent tests generated a number of

observations that cast doubt on the validity of step C in this

transformation. First, the proposed SN -type nucleophilic

−

attack by Br resulting in the PO moiety should have led to

the concomitant formation of 2-Br-styrene, but GC-MS

analysis of the ﬁnal reaction mixture showed no trace of this

byproduct. Second, the proposed mechanism envisages no role

The structural discussion presented later on in this report

will address the contributions of the three canonical

representations of complex 3 illustrated in Scheme 5. The

resonance structure γ featuring an ylide moiety is in theory

possible, but the distance between the phosphorus and the

metal center of >3 Å in 3 rules out any P−Ni interaction. We

conclude, therefore, that the zwitterionic species α and β give

for Et N, and yet a few experiments helped establish that a

stoichiometric amount of Et N is necessary for the formation

of 2. For instance, treating 1 with 1.2 equiv of {(i-

PrCN)NiBr } in the absence of Et N failed to give any

conversion to 2, even after 20 h, whereas the analogous

reaction in the presence of 0.10 equiv of base led to partial

conversion.

more accurate descriptions of this complex. The H and

NMR spectra of 3 (Figures S29 −S41 ) indicate that the α form

shown in Scheme 5 and featuring a nickelacyclopropane

moiety is the prevalent resonance structure of compound 3 in

solution. Thus, the two C−H moieties bound to the metal

One more observation regarding the role of the base was

made in the aftermath of the reaction of 1 with {(i-

PrCN)NiBr } and Et N (1:1.2:1) in acetonitrile. ESI-MS

analysis of the crude reaction mixture after 2 h indicated the

center gave signals in the aliphatic region: δ H at 2.04 and

formation of the ammonium species [(2-vinylphenyl)NEt ]Br

2.95 ppm; δ C at 18.63 and 37.17 ppm.

at M and [(2-vinyl-phenyl)NEt ] Br at [M − 1] (see Figures

S1 and S2 ). Evidently, a stoichiometric amount of base is

required for the reaction under discussion. We believe that the

We propose that both complexes 2 and 3 form via the

intermediate species arising from the nucleophilic attack by 1

on the complex bearing the chelating ligand (shown in Scheme

4 and Scheme 5). The pathway that gives 3 instead of 2

involves a subsequent deprotonation of the proton adjacent to

the phosphonium site. It is worth noting that 2 is the main

role of Et N is to promote the cleavage of the Ar−O bond

required for the formation of the PO moiety (step D in

Scheme 4).

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product during the early stages of the reaction of substrate 1

ring metallacycle incorporating the amine moiety. It also has a

6-membered metallacycle incorporating the phosphinite

moiety, similarly to what was observed in 2.

Having established the competence of morpholine to

intercept the formation of 2, we moved next to test this new

reactivity with a less nucleophilic secondary amine in order to

determine the lower threshold of nucleophilicity required for

hindering the formation of 2. Substrate 1 was thus treated at

with the Ni precursor and Et N, whereas complex 3 forms

more gradually but becomes the predominant product over

time (Figures S126 −S127 ). For this reason, preparation of

reasonably pure samples of 3 required long reaction times (>3

days).

Nucleophilic Amines Lead to 6,4-POCN-NiBr Pincers.

The previous section established that coordination of the vinyl

moiety in substrate 1 to the Ni center renders it susceptible to

room temperature with 2.0 equiv of N-ethylaniline and 1.2

attack by even a weak nucleophile such as the phosphinite

moiety in 1. This attack generates a phosphonium-bearing

intermediate that undergoes subsequent transformations to

give either the 5,6-POCPO-type pincer complex 2 (without

going through a C−H metalation step) or the zwitterionic

nickelacyclopropane 3. On the basis of these observations, we

equiv each of Et N and the Ni precursor, in THF or

acetonitrile. As before, the reaction proceeded more slowly in

THF (16 h), but gave a somewhat cleaner conversion (Figure

S129 ). P NMR spectral analysis of these crystals showed two

singlets at 207 and 204 ppm in a 7:1 ratio (Figure S74),

suggesting the existence of two isomers of the target complex.

The nucleophilic moiety N(Et)(Ph) being nonsymmetrical, we

infer that these isomers could arise from diﬀerent orientations

of the N-substituents relative to the aromatic cycle of the

pincer ligand backbone (Scheme 8).

surmised that conducting the reaction of 1 with the Ni

precursor in the presence of a protic nucleophile might lead

to new pincer complexes featuring a 4-membered nickelacycle,

as illustrated in Scheme 6.

Scheme 6. Proposed Wacker-Type Strategy Leading to (6,4-

Scheme 8. Synthesis of Complex 5

POC E)NiBr Pincer Complexes

Morpholine was selected as nucleophile of choice for testing

the above hypothesis, because our previous experience with the

synthesis of 5,5-POCN-type pincer complexes of nickel had

shown this amine to be a viable donor for preparing stable

Workup of the above reaction mixture followed by

recrystallization gave single crystals of the target complex 5.

XRD analysis of a single crystal revealed one isomer of

complex 5 featuring the Ph moiety pointing away from the

aromatic cycle of the backbone (Scheme 8). Full NMR spectral

analysis revealed that this isomer is the major product in

solution in a 7:1 ratio. An NOE analysis also showed

magnetization transfer cross peaks resulting from dynamic

exchange between the two isomers, indicating that both

isomers are in equilibrium at room temperature.

pincer complexes. Thus, treating 1 in acetonitrile for 1 h with

.2 equiv of {(i-PrCN)NiBr } and 2.0 equiv of morpholine (as

both nucleophile and base) led to complete disappearance of

the P signal for (1) NiBr and emergence of a minor signal

for complex 2 and a new signal at 204 ppm. Conducting this

reaction in THF with 1.2 equiv of Et N slowed the rate (16 h

at room temperature), but led to a cleaner conversion, with

barely any byproducts being detected (Figure S128). The

orange powder isolated from this mixture showed NMR

spectra consistent with the formation of the expected (6,4-

Next, we tested the reactivity of primary amines to see if the

presence of a second N−H can hinder the formation of a 6,4-

POCN pincer complex. Treating our usual Ni precursor with

morph

POCN

)NiBr complex 4 (Scheme 7). For example, the H

Et N and i-PrNH as nucleophile showed the formation of the

iPr,H

target 6,4-POCN

complex as two diastereoisomers 6 and 6′

Scheme 7. Synthesis of the 6,4-POCN Pincer Complex

Arising from Morpholine (4) and Its Molecular Diagram

(

P δ: 200.6 and 200.9 ppm), as well as <5% of by-products.

Recrystallization aﬀorded single crystals of 6, the isomer with

the i-Pr substituent of the amine moiety pointing away from

the aromatic cycle of the backbone (Scheme 9).

As mentioned in the case of complex 5, magnetization

transfer cross peaks in the NOE spectrum indicated that

complex 6 was involved in a dynamic exchange process in

Scheme 9. Synthesis of Complex 6 and Observation of

Diastereoisomers of Similar Energy

NMR spectrum displayed signals at 1.59 for Ni−CH and at

.42 and 3.68 ppm for Ni−CH−CH , with the corresponding

C signals at −1.12 and 64.72 ppm, respectively.

Recrystallization of the orange powder and XRD analysis of

the obtained single crystals allowed us to conﬁrm the identity

of the product as the monomeric complex 4. Inspection of the

molecular diagram shown in Scheme 7 reveals that this

compound features the aforementioned unusual 4-membered

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solution, at room temperature. This conclusion was corrobo-

rated by the following experiment. Analysis of a batch of

crystals by powder XRD (Figure S133) conﬁrmed the presence

of only the one diastereoisomer of 6 previously identiﬁed by

single crystal XRD analysis. Redissolution of the powder in

Scheme 11. Competitive Formation of 6,4-POCN Pincer

Complexes 5 and 6

C D and analysis by P NMR spectroscopy showed again the

presence of two isomers in a close to 1:1 ratio. This ﬁnding

establishes that the isomerization process occurs in solution at

room temperature, likely owing to the hemilability of the

amine arm. That the dissociation of the amine moiety is

accessible at room temperature is presumably because it

releases the strain in the 4-membered nickelacycle.

more readily to the Ni center. Thus, a higher proportion of

the less nucleophilic amine would be expected to remain

uncoordinated in the reaction mixture. For this reason, and at

least in the early stages of the reaction, the less nucleophilic

amine would be more likely to participate in the outer-sphere

attack on the Ni-bound vinyl moiety group to give the

anticipated product. Indeed, when the above competition

experiment was conducted over longer reaction times, the ratio

of 5:6 reversed to 47:53 after 7 h and then 33:67 after 24 h

Repeating the reaction shown in Scheme 9 with the less

nucleophilic PhNH led to a less clean reaction as evidenced

by the appearance of several peaks in the 198−206 ppm region,

none being dominant (Figure S131). Nevertheless, since the

starting material had completely disappeared, we undertook

the workup as before and obtained some crystals of the target

Ph,H

complex (6,4-POCN )NiBr (7 in Scheme 10). XRD analysis

(

Scheme 11). This indicates that the formation of 6 is

thermodynamically more favored than that of 5, and that 6 can

be converted to 5 in the reaction medium.

R ,H

Scheme 10. Formation of (6,4-POCN ′ )NiBr Pincer

Complexes 7−9 from Primary Amines

,4-POCP-Type Pincer Complexes from P-Based

Nucleophiles. Having developed a relatively facile protocol

for the synthesis of the above-discussed rare 6,4-POCN-type

pincer complexes of Ni prompted us to test this approach for

the synthesis of the corresponding compounds based on 6,4-

POCP ligands that are also quite rare. As a proof of concept

for this idea, we stirred 1 in THF at room temperature with a

.2:1.2:1.0 ratio of the Ni precursor, Et N and an Et O

3 2

solution of i-Pr PH.

of one of these crystals showed it to be the isomer with the Ph

substituent of the amine moiety pointing away from the

aromatic ring of the pincer backbone (see the Supporting

Information for the structural diagram).

Analysis of the above reaction mixture (after 4 h) by ³¹P

NMR spectroscopy showed a major set of two AB doublets at

92 and −57 ppm. The large coupling constants of this set of

signals (J = 310 Hz) is characteristic of trans phosphines,

allowing us to conclude that they represent the expected

complex 10 (Scheme 12). The remaining minor resonances

Similar reactions conducted with CyNH and t-BuNH

proved not as clean as the reaction with i-PrNH , but still

much cleaner than that with PhNH . Thus, the P NMR

Scheme 12. Synthesis of the (6,4-POCP)NiBr Complexes

10 and 11 Arising from R′

intensities of these peaks were in a nearly 1:1 ratio, suggesting

the coexistence of two isomers close in energy.

The reaction conditions for the synthesis of these complexes

and the workup processes have not been optimized, but XRD

analyses were performed on crystals obtained from these

reactions for the sole purpose of structural comparison. These

analyses showed once again the formation of (6,4-POCN)NiBr

complexes (8 and 9, Scheme 10); the isomers obtained were

those wherein the N-alkyl substituent was pointing away from

the arene ring of the pincer backbone.

included the signals for complex 2, the known complex (i-

Pr PH) NiBr , and other unidentiﬁed resonances. Orange

To gain some insight into the mechanism of the reactions

with amine nucleophiles, we carried out a competition

experiment between the two nucleophiles PhN(Et)H and i-

crystals obtained from workup of this reaction mixture were

iPr2

identiﬁed as the desired (6,4-POCP )NiBr complex 10 by

NMR spectroscopy and XRD analysis (Scheme 12).

PrNH . P NMR spectral monitoring of the reaction of 1 with

To limit the formation of byproduct 2 in the above

approach, we changed the source of nucleophile from free i-

Pr PH to the complex (i-Pr PH) NiBr . Thus, treating 1 with

.2 equiv each of the Ni precursor, i-PrNH , PhN(Et)H and

Et N gave a surprising result. After 1 h of reaction in THF at

room temperature, the ratio of the two complexes 5:6 was

found to be 57:43, indicating that the less nucleophilic

PhN(Et)H produces the (6,4-POCN)NiBr product slightly

faster (Scheme 11).

1.2 equiv of Et N, 0.7 equiv of the usual Ni precursor, and 0.5

equiv of (i-Pr PH) NiBr gave complex 10 in 76% yield, along

with minor amounts of 2; the formation of the latter side-

product could not be suppressed completely.

The above seemingly counterintuitive observation can be

rationalized if we recall that the more nucleophilic amine binds

The formation of even minor quantities of complex 2 in the

above reaction with i-Pr PH was somewhat surprising, because

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to comment on the impact of various structural features on

Article

this side-product was known to form much more slowly (16 h

in THF). It occurred to us that in this case 2 might well arise

Our long-standing interest in the synthesis and reactivities of

10a,25

high-valent Ni complexes

voltammetry (CV) to measure the one-electron oxidation

potentials of the above-mentioned POC_s_pN and POC_s_pP

prompted us to use cyclic

from the reaction with small amounts of i-Pr P(O)H

impurities generated during the synthesis of i-Pr PH. To

validate this hypothesis, we prepared i-Pr P(O)H and used it

complexes. The main objective of this exercise was to study the

variations in redox potentials as a function of diﬀerent

structural features, and use any insight gleaned from such

studies to prepare thermally stable high-valent derivatives.

Although solid samples of the new complexes proved stable

in air over a few hours, exposing their solutions to air led to

quick decomposition as evidenced by the quick disappearance

of the yellow/orange colors of the complexes. For this reason,

the samples for CV measurements were prepared in the

glovebox. The CV measurements began with CH Cl solutions

in a reaction with 1.0 equiv of 1 and 1.2 equiv of both Et N

and the Ni precursor in THF. This mixture showed complete

disappearance of the starting material after 3 h and we detected

a sharp resonance at 205 ppm and a broad one at 96 ppm

attributed to the (6,5-POCPO)NiBr complex 2 (Scheme 13).

Scheme 13. Direct Preparation of Complex 2 from i-

Pr P(O)H

of 4 using [n-Bu N]PF as supporting electrolyte. These

showed a distinct but irreversible oxidation wave attributed to

III

Ni /Ni (Figure S3). That this one-electron oxidation is

irreversible implies that the in situ generated 15-electron

morph

III

species [(6,4-POCN

)Ni Br] is unstable. Thus, we

switched to using acetonitrile solution to see if the greater

binding ability of this solvent might stabilize the in situ formed

trivalent cationic species and favor reversibility of the redox

events.

The CV trace obtained at a scan rate of 100 mV/s in

acetonitrile was much cleaner, but the diﬀerence between the

anodic and cathodic peak potentials in this trace was 270 mV,

It is noteworthy that this reaction was much cleaner than the

synthetic approach described earlier with 1 only (Scheme 3).

Applying this new synthetic route allowed us to isolate 2 in

high yields and uncontaminated by the side-product 3.

which is much greater than the ideal value of 59 mV for |E −

p,a

Finally, we studied the reaction of 1 with Ph PH as shown in

E | of a fully reversible single electron redox process. This

p,c

Scheme 12. P NMR spectral analysis of the crude reaction

mixture (deep red) obtained after 4 h conﬁrmed the formation

of the target complex: the resonance for the starting material

had been replaced by AB doublets at 197 and −78 ppm with a

larger-than-ideal potential diﬀerence is attributed to the

likelihood that the redox process in question involves diﬀerent

species; in other words, the oxidation occurs with complex 4,

whereas the reduction likely occurs with the in situ generated

PP′

of 320 Hz. Workup of the reaction mixture followed by

N C P

penta- or hexacoordinate cationic MeCN adducts [{κ ,κ ,κ -2-

crystallization of the resulting brown solids from hot

[

(

(morphol-N-yl)CH CH]-C H −OP(i-Pr) }NiBr(NCMe) ]

acetonitrile aﬀorded deep red crystals of the target product

n = 1 or 2). For this reason, we have opted to base our

(

6,4-POCP )NiBr, 11, in only 35% yields. Unfortunately,

analysis of the CV data on the anodic peak potentials E . The

p,a

conducting the reaction in acetonitrile did not result in

improved yields. XRD analyses and full NMR spectroscopic

characterization conﬁrmed its identity.

It is noteworthy that the above reaction also generated a

mass of brick-red solid that remained insoluble in either polar

values for all the compounds investigated here have been

referenced to the E₁_/₂for ferrocene (Fc/Fc ), which is used as

the standard complex for calibrating our measurements.

Analogous CV measurements were also conducted on

acetonitrile samples of 6,4-POCN complexes 5 and 6, and

representative CV traces for 4−6 are shown in Figure 1.

Examination of the CV data presented in this ﬁgure allows us

or nonpolar solvents (acetonitrile, THF, Et O, or toluene).

One clue for this side reaction is the observation of minor

resonances attributed to complex 10 in the crude reaction

mixture, which suggests an exchange reaction between the

nucleophile used, Ph PH, and substrate 1, as shown in eq 1.

Such an exchange equilibrium would also rationalize the poor

isolated yield for complex 11, if Ni complexes generated from

the competitive Ph P−OAr ligand are less soluble.

i‐Pr P−OAr + Ph P−H V i‐Pr P−H + Ph P−OAr

(1)

Electrochemical Analysis. To the best of our knowledge,

the pincer-type compounds described in the above sections

represent new families of pincer−Ni complexes. Indeed,

Sciﬁnder and CCCD searches have revealed no previous

example of POCN-Ni complexes featuring an aliphatic

backbone, nor pincer-type Ni complexes based on POCP

ligands (phosphinite/phosphine donor moieties) or POCP(O)

ligands (phosphinite/phosphine oxide donor moieties) with

either aliphatic or aromatic linking groups. The access to these

unprecedented pincer-type Ni complexes led us, naturally, to

investigate their structures and other properties relevant to

reactivities.

Figure 1. Cyclic voltammograms of complexes 4−6 (1 mM) in the

presence of [n-Bu N]PF (100 mM) in MeCN at a glassy carbon

electrode. Potentials are referenced against the E₁_/₂of the Fc/Fc⁺

redox couple.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Curiously, the CV traces (Figure 2 ) show anodic potentials

Article

electrochemical oxidation potentials. For instance, the E_p_,_a

values for 4 (−13 mV), 5 (+89 mV), and 6 (+57 mV)

conﬁrm that the morpholine moiety bestows a greater electron

density to Ni relative to N(i-Pr)H and N(Ph)Et. It is also

worth noting here that the CV traces for these complexes were

all quasi-reversible; nevertheless, decomposition of the Ni

species is evident, implying that the presence/absence of an

oxidize than its CH PPh counterpart. This said, the diﬀerence

is not large.

The last complex studied by CV was the 6,5-POCP(O)

compound 2. The analysis of the data is complicated in this

case owing to the ﬂatness of the anodic wave: an oxidation

event is evident, but a clear anodic peak is not revealed (i.e.,

dI/dE never reaches 0 around the oxidation event). Thus, we

used the ﬁrst derivative dI/dE = f(E) to approximate the E_p_,_a

closer to the inﬂection point, which turned out to be between

III

N−H moiety has no major inﬂuence on this phenomenon.

It is also instructive to compare the above data for the 6,4-

POC N complexes 4−6 to the E values we have obtained

p,a

+25 and +50 mV vs Fc/Fc . This uncertainty in the precise

for the 6,4-POC P pincer complexes 10 and 11 that feature a

value of E_p_,_anotwithstanding, we conclude that the P(O)

phosphine donor moiety instead of an amine moiety.

donor moiety does not diminish the electrochemical oxidation

prospects for the Ni center relative to the 6,4-POC N

complex bearing the strong σ-donor NH(i-Pr). This is

presumably due to a strongly polarized phosphine oxide

−

moiety in which the R P -O resonance form is prevalent.

Overall, the CV measurements presented above allow us to

conclude that the new pincer complexes studied herein are

much easier to oxidize relative to the classical pincer-type

29a,b

complexes (5,5-POC N)NiBr and (5,5-POC OP)NiBr

that we have studied previously (see Figure S5). It appears,

therefore, that a C −Ni moiety bestows a great deal of

electron density to the Ni center. On the other hand, van

Koten’s (NC N)NiBr system is much easier to oxidize (−390

mV in acetone) in spite of its aromatic backbone, establishing

the superior oxidation prospects for pincer systems with two

Figure 2. Cyclic voltammograms of complexes 9, 10, and 2 (1 mM)

in the presence of [n-Bu N]PF (100 mM) in MeCN at a glassy

amine donor moieties.

Structural Characterization of the New Complexes.

Single crystal diﬀraction analyses have been performed for all

the new complexes prepared in this study. This section

presents the most pertinent parameters for all structures

(Table 1) along with molecular diagrams and brief descriptions

for the structures of complexes 2−6, 10, and 11. Complete

listings of the crystal data as well as the molecular diagrams for

complexes 7−9 are presented in the Supporting Information.

All the complexes under discussion display a square planar

geometry, the Ni center being located on or near the mean

coordination plane deﬁned by the four atoms bound to it. The

6-membered nickelacycle forces the aromatic ring originating

from ligand 1 to lift oﬀ the mean coordination plane by about

carbon electrode. Potentials are referenced against the E₁_/₂of the Fc/

Fc redox couple.

that are quite comparable to the 6,4-POC N-type complex 4:

−

8 mV (10) and +14 mV (11) vs −13 mV (4). These ﬁndings

were somewhat surprising, because we had anticipated that

replacing a “hard” amine moiety in 4 by a “soft” phosphine

moiety should be unfavorable to oxidation. We were also

surprised that the E_p_,_avalues for these 6,4-POC P-type

complexes did not seem to follow the anticipated order:

complex 10, the one bearing the presumed better σ-donor

CH P(i-Pr) moiety, was found slightly more diﬃcult to

Table 1. Selected Bonds (Å) and Angles (deg) for Complexes 2−11

Ni−C7

Ni−P1

Ni−Br

Ni−E

P−Ni−C

E−Ni−C

P−Ni−E

C−Ni−Br

1.9979(17)

2.1104(5)

2.1106(4)

2.1103(5)

2.1058(4)

2.1095(5)

2.0967(7)

2.1141(4)

2.1065(5)

2.1154(6)

2.1130(6)

2.1175(7)

2.1214(6)

2.1501(8)

2.1509(8)

2.1541(8)

2.3743(3)

2.3785(3)

2.3479(3)

2.3524(3)

3.3533(4)

2.3734(3)

2.3608(3)

2.3727(4)

2.3864(4)

2.3689(4)

2.3753(4)

2.3562(5)

2.3604(5)

2.3472(6)

1.9477(12)

1.9317(10)

2.0103(16)

2.0148(14)

2.0250(14)

2.026(2)

1.9884(13)

1.9884(15)

1.9828(18)

1.9836(18)

2.004(2)

90.41(5)

88.39(5)

92.46(5)

90.47(4)

90.22(5)

90.06(7)

90.70(5)

89.14(6)

89.54(7)

89.69(6)

88.17(7)

89.19(7)

89.79(8)

90.37(8)

90.30(8)

90.74(6)

91.11(6)

43.18(6)

43.33(6)

72.95(6)

72.36(9)

73.91(6)

74.07(7)

73.83(8)

73.69(8)

73.68(9)

73.53(9)

73.07(8)

73.44(8)

71.65(8)

165.08(4)

177.47(4)

135.02(5)

133.61(4)

158.13(4)

156.80(6)

164.39(4)

162.41(5)

163.98(6)

162.78(6)

159.91(6)

161.45(6)

161.47(3)

163.59(3)

155.44(4)

171.10(6)

171.85(5)

157.10(5)

155.99(4)

172.07(5)

168.82(7)

175.64(5)

173.25(5)

175.25(7)

174.41(6)

173.95(8)

175.77(7)

174.89(8)

173.42(8)

174.19(8)

.9978(15)

1.9101(16)

.9051(14)

1.9436(16)

1.943(2)

1.9433(15)

1.938(2)

1.945(2)

.951(2)

1.939(2)

.942(2)

1.991(3)

.998(3)

2.001(3)

2.008(2)

2.1762(8)

2.1650(8)

2.1897(8)

E refers to O2 in complex 2, C8 in complex 3, N1 in complexes 4−9, and P2 in complexes 10−11. The ﬁrst row corresponds to value found in

the structure of 2 only, and the second row to values found in cocrystals of 2 and 3. The ﬁrst row corresponds to value found in the structure of 3

only, and the second row to values found in cocrystals of 2 and 3.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Me substituents of P(i -Pr) moieties are omitted for clarity.

a,28

Article

9b,29

51−66° (Figures 3−5). Perhaps the most noteworthy feature

POCOP,

5,5-PCP,

and PCN complexes does not

of the 6,4-POCN complexes 4 -9 and 6,4-POCP complexes 10

reveal signiﬁcant diﬀerences: 1.983(2)−2.026(2) vs 1.97−2.05

Å for Ni−N distances; 2.17−2.19 vs 2.16−2.21 Å for Ni−P

distances.

One remarkable feature of the 6,4- and 6,5-pincer complexes

described here is the relative insensitivity of the 6-membered

metallacycle to the size of the adjoining metallacycle, as well as

to the identity of the donor moiety contained in it. This is

reﬂected in the P1−Ni1−C7 angles that are in the narrow

range of 88−91° over complexes 2 and 4−11. In contrast, the

P−Ni−C cis angles of typical 5,5-pincer-Ni complexes are

more acute (80−85°), whereas (5,6-POCOP)NiBr displays a

21b

wider cis angle of 93° for its 5-membered nickelacycle.

Figure 3. Side views of the molecular diagrams for complexes 2 (left)

and 4 (right). Thermal ellipsoids are shown at the 50% probability

level, and hydrogen atoms reﬁned via the riding model and terminal

The trans C7−Ni1−Br1 angles are greater than 169° in all

complexes except in the more distorted compound 3 wherein

it is much smaller (156°). Among the 6,4-POCN complexes

4−9, those featuring a N−H amine moiety show C7−Ni1−Br1

angles that are fairly close to the ideal value of 180°, whereas

the corresponding angles in the analogous complexes with

tertiary amine are somewhat narrower (174−176° vs 169−

172°).

Other Ni-ligand bond distances were also found to be nearly

constant, with only minor variations noted in certain cases.

Thus, the Ni−Br distances range from 2.345 to 2.388 Å, which

is in the same range as observed for the analogous POCOP,

PCP, POCN, and PIMCOP complexes bearing P(i-Pr)₂

phosphinites. Among the 6,4-POCN complexes, those

featuring a primary amine moiety display signiﬁcantly longer

Ni−Br distances (2.361−2.388 Å vs 2.352−2.353). Only small

variations were observed among the Ni1−C7 bond distances in

the 6,4-POCN complexes 4−9, (1.938(2) to 1.951(2) Å). This

is similar to the range of Ni−C distances reported for aliphatic

Figure 4. Molecular diagrams for complexes 5 (left) and 6 (right)

that crystallized as R,R and S,S isomers. Thermal ellipsoids are shown

at the 50% probability level; hydrogen atoms reﬁned via the riding

model are omitted for clarity.

,5-POCOP (1.94−1.96 Å); in contrast, signiﬁcantly larger

values were obtained for complexes 2, 10, and 11 (ranging

.991(3)−2.001(3) Å). For compound 3, the Ni1−C7 bond

distance is signiﬁcantly shorter with a value of 1.91 Å.

The Ni1−P1 bond distances found in complexes 2-9 are also

in the same range as in the analogous 5,5-POCN-Ni complexes

reported previously (2.11−2.12 vs 2.10−2.11 Å). The

somewhat longer Ni1−P1 bond distances of 2.150(1)−

Figure 5. Molecular diagrams for complexes 10 (left) and 11 (right).

Thermal ellipsoids are shown at the 50% probability level; hydrogen

atoms reﬁned via the riding model are omitted for clarity.

2.154(1) Å observed in complexes 10 and 11 are likely due

to the greater trans inﬂuence of the phosphine moieties.

Consistent with this assertion, similar Ni−P bond distances

(

2.14−2.16 Å) have been reported in the analogous aliphatic

i‑Pr

and 11 is the 4-membered nickelacycle. The acute cis angles

C7−Ni1−N1 and C7−Ni1−P2 deﬁning these 4- membered

rings range from 71° to 74°, whereas the C7−Ni1−P1 angles

deﬁning the 6-membered nickelacycles are closer to the ideal

right angles (88°−91°). By comparison, the C7−Ni1−O2

angle of 91° deﬁning the 5-membered nickelacycle in complex

and aromatic 5,5-POCOP complexes reported previously,

whereas even longer Ni−P bond distances were found in the 6-

i‑Pr

membered nickelacycle of the 6,5-POCOP (>2.17 Å).

The Ni1−O2 bond distances of 1.932(1)−1.948(1) Å and

P2−O2 of 1.523(1) Å in our 6,5-POCPO complex 2 are

similar to those observed in PCP(O) pincer−Ni complexes

is also close to the ideal value expected for a square planar

featuring a R(i-Pr)

PO → Ni interaction (1.93−1.96 Å for

geometry, whereas the corresponding cis angles in the

Ni−O and 1.50−1.53 Å for P−O). It is also worth noting

that the PO bond distance displayed in 2 is clearly longer

than in reported free i-Pr P(O)−CH −R molecules (with P−

analogous 5,5-aromatic-pincer-Ni complexes reported previ-

ously range from 83° to 85°.

As was alluded earlier (see for example Scheme 10), the

solution isomerization displayed by our POCN complexes 5−9

likely results from the hemilabile N→Ni interactions present in

the 4-membered metallacycles of these complexes. Interest-

ingly, the solid state data do not show any indication that the

acute cis angles inside the 4-membered metallacycles cause

unusually long Ni−N bond distances. Indeed, comparison of

the Ni−N and Ni−P bond distances in 4−11 to the

O bonds ranging 1.48−1.50 Å), indicating a bond order close

−

to unity and thus strong polarization of the R P −O bond,

which gives this arm an almost X-ligand character.

Additional structural analysis is required for complex 3

(Figure 6), in particular regarding what the structural

parameters of this compound might reveal regarding the

interactions between the Ni center and the carbons C7 and C8

arising from the vinyl moiety of ligand 1. We had argued earlier

on the basis of NMR spectral data that the α and β forms of

18,27

corresponding values in the analogous 5,5-POCN,

5,5-

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Article

; chelation of an electrophilic Ni center by this substrate

activates the oleﬁn moiety toward the requisite nucleophilic

attack. This approach works with readily available precursors

and proceeds at room temperature to give products that would

be otherwise diﬃcult to access without more elaborate starting

materials such as zerovalent Ni precursors and proligands

bearing one or more C-halide moieties. Thus, the new

synthetic strategy under discussion obviates the need for C−

H or C−X addition/metalation, frequently diﬃcult steps that

can constitute important obstacles for the synthesis of ECE′-

type pincer complexes.

An interesting and somewhat unexpected feature of the

pincer−Ni complexes synthesized in the current study is the

observed thermal stability of the 4-membered metallacycles.

The observed solution isomerization that these complexes

undergo underlines the hemilability of the N→Ni coordination

bond; this hemilability bodes well for potential reactivities and

catalytic applications of these complexes.

Figure 6. Molecular diagram for complex 3. Thermal ellipsoids are

shown at the 50% probability level; hydrogen atoms reﬁned via the

riding model and cocrystallized complex 2 are omitted for clarity.

compound 3 shown in Scheme 5 and featuring a nickel-

acyclopropane moiety are the prevalent resonance structures.

Comparison of solid-state parameters to previously reported

analogous complexes supports this argument. A CCDC search

of complexes regarded as Ni(0) species featuring one alkene or

trans-stillbene revealed that their C−C bond distances span the

range of 1.395(4)−1.428(4) Å in spite of the presence of

Another interesting aspect of the pincer complexes described

in this report touches on the ligand/complex features that

render the Ni center prone to relatively facile electrochemical

oxidation. The CV measurements we have reported demon-

strate the important impact of a Csp −Ni moiety for facilitating

single electron oxidation of the Ni centers in these complexes.

Future eﬀorts will be directed at exploiting the insights gleaned

from these studies to prepare thermally stable high-valent

analogues of these pincer complexes. We will also pursue the

possibility of extending this synthetic strategy for the

preparation of analogous pincer complexes with other metals,

as well as to other ligand platforms such as phosphinites

derived from homoallylic alcohols.

strong σ donors such as phosphines or NHC ligands.

Surprisingly, in our 6,3-POCC complex 3 that bears only

moderate σ donors (phosphinite, bromide), the C7−C8 bond

distance is signiﬁcantly longer, 1.446(2)−1.451(2) Å. This C−

C distance is consistent with a metalacyclopropane It is not

clear to us on what basis you write metalacyclopropane with a

single "l" but metallacycle is written with double"l". moiety or

an oleﬁn moiety bearing an electron-withdrawing substituent

(

phosphonium) and coordinated to a formally anionic Ni(0)

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00590.

■

center. Closely analogous nickel complexes reported by

Carmona’s and Li’s groups and bearing phosphonium

substituents show very similar C−C bond lengths (1.44(2)

35,36

and 1.458(5) Å).

Another important consideration pertains to the Ni−C bond

distances in complex 3. In mono-oleﬁn-Ni(0) complexes, the

Ni−C distances span the range of 1.96−2.01 Å, whereas much

shorter distances of 1.88−1.91 Å are reported for the highly π-

acidic Ni-polyﬂuoroalkenes, which are regarded as essentially

Synthetic procedures, NMR spectra, crystal structure

data, and additional ﬁgures (PDF)

Accession Codes

CCDC 2006809−2006820 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

II 37

nickelacyclopropanes bound to Ni . Given that the average

Ni1−C7 bond length of 1.91 Å found in complex 3 is much

shorter than C−Ni bonds in Ni−oleﬁn complexes, the

metalacyclopropane description for this moiety seems more

reasonable. Indeed, the Ni1−C7 distance is even shorter than

the corresponding distances in Ni−C

complexes (1.95−

benzyl

.02 Å), in the 6,4 pincer compounds 4−11 reported here

AUTHOR INFORMATION

■

(

1.93−2.01 Å), and also in previously reported aromatic/

9,29

Corresponding Author

aliphatic pincer−NiX complexes (1.94−1.96 Å).

It should

be acknowledged, however, that the second Ni−C distance in

the putative metalacyclopropane moiety of complex 3 is much

longer (Ni1−C8 = 2.010(2)−2.015(2) Å); we believe that the

greater steric bulk of the phosphonium substituent on C8

might be at least partially responsible for this lengthening.

Davit Zargarian − Département de Chimie, Université de

Montréal, Montréal, Québec H3C 3J7, Canada;

orcid.org/0000-0002-0207-7007 ;

Email: zargarian.davit@umontreal.ca

Authors

CONCLUSION

Loïc P. Mangin − Département de Chimie, Université de

Montréal, Montréal, Québec H3C 3J7, Canada

Guillaume Michaud − Département de Chimie, Université de

Montréal, Montréal, Québec H3C 3J7, Canada

■

The study presented herein has led to the development of a

new synthetic strategy for the preparation of unsymmetrical

ECE′-Ni pincer complexes that are unprecedented. The key

step in this synthetic route likely involves an outer-sphere

nucleophilic addition onto the oleﬁn moiety of the phosphinite

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00590

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Dehydrogenative Coupling of Primary Alcohols with Water,

Article

Notes

The authors declare no competing ﬁnancial interest.

Methanol, or Amines. Angew. Chem., Int. Ed. 2009, 48 (3), 559−563.

(4) (a) Batema, G. D.; Lutz, M.; Spek, A. L.; van Walree, C. A.;

Donega,

J.; Clays, K.; Bu

M.; Koten, G. v. Substituted 4,4 ′-Stilbenoid NCN-Pincer Platinum-

II) Complexes. Luminescence and Tuning of the Electronic and

C. d. M.; Meijerink, A.; Havenith, R. W. A.; Per

ez-Moreno,

ACKNOWLEDGMENTS

■

chel, M.; Dijken, A. v.; Bryce, D. L.; van Klink, G. P.

The authors gratefully acknowledge ﬁnancial support provided

by NSERC of Canada (Discovery grants to D.Z.); Centre in

Green Chemistry and Catalysis (CGCC/CCVC, summer

research stipends and travel awards); Universite

graduate scholarships to L.P.M.). We also thank our

colleagues for their help with specialized experiments in

NMR (Dr. P. Aguiar) and ESI-MS (Dr. T. Auvray).

Colon, J. L.; Grove, L.; Connick, W. B. Room-Temperature Emission

NLO Properties and the Application in an OLED. Organometallics

2008, 27 (8), 1690 −1701. (b) Albrecht, M.; Lutz, M.; Spek, A. L.;

van Koten, G. Organoplatinum crystals for gas-triggered switches.

Nature 2000, 406 (6799), 970−974. (c) Rivera, E. J.; Figueroa, C.;

from Platinum(II) Complexes Intercalated into Zirconium Phos-

de Montreal

́ ́

(

phate-Layered Materials. Inorg. Chem. 2007 , 46 (21), 8569 −8576.

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(17) One of the reviewers of this manuscript has pointed out that an

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(

756−4761.

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[n-Bu N][PF ] as the internal standard conﬁrmed that complex 5 was

(

11) Addition of nucleophiles on unsaturated hydrocarbons

(

including ethylene and other oleﬁns) coordinated to transition

not simply decomposing in the medium.

metal-based electrophiles is well-precedented and has a long history.

In the case of oleﬁns, the reversal of polarity (Umpolung) resulting

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(

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H O quenching. See Supporting Information for details.

(

synthesis of i-Pr PH showed about 8% i-Pr P(OH). See Supporting

23) Indeed, NMR analysis of the reaction mixture from the

Information.

24) This was prepared by the reaction i -Pr PCl with H O, followed

(

by the addition of the base. See Supporting Information for details.

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(

861−3874.

26) Note that by comparison, the Fc/Fc+ couple used under the

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−

E | value of 68−72 mV, which is close to the ideal values.

p,c

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