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1 C(sp3)-Metalated Pincer Complexes Based on All-Aliphatic Scaffolds

1 C(sp3)-Metalated Pincer Complexes Based on All-Aliphatic Scaffolds

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292



D. Gelman and R. Romm

t



H



t



Bu2

P

H

TM Cl



Bu2

P



H



t



TM Cl

PtBu2

3 TM = Ni

4 TM = Pd

5 TM = Pt



P Bu 2

1 TM = Rh

2 TM = Ir



Fig. 4 Prototypical aliphatic pincer complexes



For example, iridium [52], rhodium [53], osmium [54], ruthenium [54, 55], platinum [56], palladium [56], and nickel [57] prototypes were synthesized employing

1,5-bis(di-t-butylphosphino)pentane and appropriate metal precursors (Fig. 4). Formation of the C(sp3)–metal bond may proceed either via electrophilic activation or

via oxidative addition of low-valent metals across the C–H bond. However, in both

of the cases, yield of the metalated compounds is poor to moderate (depending on

the bulk of the phosphine donors) on account of the formation of large-ring chelates

or linear oligomeric by-products.



P Bu2

H

H



TMX n



Bu2

P



Bu2

P



t



P Bu 2



H



t



t



t



t



TM



X



P tBu2



TM



Bu2

P



ð1Þ



+

P TM

Bu2



t



P

Bu2



t



Initial comparative IR studies confirmed the expected trans influence of the sp3metalated carbon. For example, n (M–Cl) vibrations in 2–5 were found 10–30 cmÀ1

lower than in the analogous aromatic sp2-metalated compounds, suggesting a more

electron-rich character of the metal centers and, consequently, a more reactive

carbon–metal bond [24].

Indeed, its high reactivity has been demonstrated by Zhao, Goldman, and

Hartwig by an unprecedented oxidative addition/reductive elimination equilibrium

displayed in Scheme 1, where the sp3-based Ir(I) complex 6 oxidatively adds across

the N–H bond of ammonia to form the corresponding Ir(III) amido hydride 7 at

room temperature and excellent yield [58]. Remarkably, analogous aromatic pincer

complexes fail to establish the equilibrium favoring the oxidized product. This clear

ligand effect may be attributed to a stronger donating ability of the aliphatic

backbone that enhances electron density at the metal center needed to achieve

activation of otherwise inert ammonia. On the other hand, according to the X-ray

data interpretation, the lone pair of the amido nitrogen is oriented to allow

p-donation into the empty d-orbital that lies in the plane of the iridium(III) centers

containing the ipso carbon. This stabilization is, apparently, less pronounced in the

presence of the conjugated p system of the aromatic PCP ligand, which may explain

the favorable thermodynamics of the reaction. Computational studies have given

support to these findings [59].



PC(sp3)P Transition Metal Pincer Complexes. . .



293



P t Bu2

Ir



Pt Bu2

C3 H7

+



NH3



K=9



Ir NH2 +

H

Pt Bu2

7



Pt Bu2

6



C3 H7



Scheme 1 N–H activation in ammonia



2



Ph

Ph



N



t



BuP



H

NH



Ir



H2N



P t Bu2



N



Ph



Ph



t

2 BuP



8



P tBu2

Ir Ph

H

Pt Bu2

10



N



NH2



-Ph-H



N



C3 H7



NH2



Pt Bu2

H

N

NH



Ir



Ir



Pt Bu2

6



P tBu2

9



Pt Bu2

Ir



H

N

NH



Pt Bu2

11



Pt Bu2

Ir

-H 2



H

N

N



Pt Bu2

12



Scheme 2 N–H activation in hydrazines and hydrozones



Very recent studies by the same group demonstrated similar N–H activation in

hydrazines and hydrozones by 6 (Scheme 2) [60]. In this work, however, the

transformation was not limited to the aliphatic pincer complexes and both 6 and

its aromatic analog 10 were shown to react in a similar fashion, although resulted in

the formation of products of different stability. For example, while 9 is stable at

room temperature, 11 undergoes the second N–H activation at room temperature

over several days to form aminoniterene complex 12 and one equivalent of molecular hydrogen. Here again, structural characterization of the aliphatic complexes

8 and 9 clearly pointed out on the presence of nitrogen-metal p-stabilizing interaction which can rationalize relative stability of the sp3-metalated complexes over the

sp2-based ones.

Carbon dioxide reduction by rhodium [61] and iridium [62] complexes has an

utmost catalytic potential. As was demonstrated, nonclassical dihydrogen complexes

13 and 14 form corresponding formate compounds in the presence of gaseous CO2

(Scheme 3, left). The following sequence of the mechanistic events has been

suggested (1) carbon dioxide coordination to the Z2–H2 binding PCP–Rh center

gives rise to the formation of the dihydrogen complex with a weakly bound

Z1–CO2 complex. This type of Z1 interaction can be viewed as the donation of the

HOMO of the Rh–H2 complex, a dz2 orbital, to the p* orbital of the CO2 molecule;

(2) the dihydride species can undergo the hydride transfer from the Rh center to the C

center of CO2; (3) after the hydride insertion event, the kinetic products formed are

hydridorhodium Z1–formate complexes, which should transform into the most stable

Z2–formate coordination compounds [63].



294



D. Gelman and R. Romm

Pt Bu2

M



H

H



Pt Bu2

CO 2



P t Bu2

13 M = Ir

14 M = Rh



H

M



O

O



P t Bu2

H



H



H2



M CO 2



Pt Bu2

15 M = Ir

16 M = Rh



Pt Bu2

17 M = Rh



Scheme 3 Hydrogenation of CO2



Pt Bu2

Rh



H

Cl



Pt Bu2

18



NaH



Hb



Pt Bu2



H2C



Rh N 2



Pt Bu2

N2



H2C



P t Bu2

19



Rh H

P t Bu2

20



Scheme 4 Coordination flexibility in all aliphatic rhodium pincer complexes



Interestingly, the rhodium formate complex 16 can be also prepared by the

reaction of fully characterized 17 with molecular hydrogen (Scheme 3, right).

The transformation, apparently, proceeds via predissociation of CO2, as a small

amount of 14 is co-produced. However, preparation of 17 is interesting by itself

because carbon dioxide rarely coordinates transition metals and agostic stabilization was suggested to explain the stability of 17.

Remarkably also, the stability of the formate complex 16 is significantly higher

in comparison to the analogous compounds bearing sp2-based PCP ligand.

Tendency of transition metal complexes bearing all-aliphatic ligands to reversible a- and b-hydride elimination to form isomeric carbene [52] or olefin chelate

[55, 64] complexes is another evidence for high reactivity of the C(sp3)–metal

bond. Thus, Vigalok et al. reported that reduction of the Rh(III) PC(sp3)P complex

18 leads to the formation of the dinitrogen Rh(I) complex 19 and the b–H

eliminated 20. It was found that under nitrogen atmosphere these complexes exist

in the temperature-dependent equilibrium (Scheme 4) [64]. Remarkably, unlike

classical migratory insertion process requiring cisoid coordination of the

participating ligands, this reaction proceeds via direct trans insertion. Similar

transformation has been reported for ruthenium complexes [55].

Gusev et al. performed an interesting study on the osmium and ruthenium PC

(sp3)P complexes possessing both a- and b-hydrogens [54]. As shown, osmium

carbometalated compound 21 demonstrates a clear agostic interaction between the

osmium center and the central methine hydrogen that facilitates the extrusion

hydrogen to form the thermodynamically stable carbene complex 22. In contrast,

the assumed Ru-based PC(sp3)P pincer compound disproportionates via competitive a- and b-hydride elimination pathways to form the 1.5:98.5 mixture of the

carbene (23) and olefin (24) isomers (Scheme 5).

Zhou and Hartwig utilized this coordination flexibility for the design of very

efficient catalyst for H/D exchange at vinyl groups that operates under very mild



PC(sp3)P Transition Metal Pincer Complexes. . .

P t Bu2

H

Os H

Cl

Pt Bu 2

21



Pt Bu2



295

Pt Bu2

H

Os Cl



-H2

H2



Pt Bu2

22



Pt Bu2



[RuCl2(cumene)]2



P t Bu2



P tBu2



-H 2

Ru Cl

H

P tBu2

23



Ru Cl

H

Pt Bu2

24



H2



Scheme 5 Coordination flexibility in all aliphatic ruthenium and osmium complexes



R

D0

t



7 (5 mol%)

C6D6, RT



CH2



R



D0

D 97



Bu



H

C



Me3Si



D0

(RT, 1 min)



D

C



CD2



D 94

D 97



H2N



D 97

(RT, 1 min)



D 97

D 97

(RT, 20 h)



CD2 94

(RT, 40 h)

CH3



D

NC



D 17 D 94



D 0 D 13

D 97 H3C



D 97

(50 °C, 48 h)



O



D 96



D 94

D0



D 44



(RT, 1.5 h)



H3C



D 42

D 96

(RT, 60 h)



Fig. 5 H/D exchange catalyzed by 7



conditions and without isomerization of the double bonds [65]. As was demonstrated,

Ir(III) amido hydride complex 7 induces very fast (minutes to hours) room temperature deuteration of internal and terminal double bonds with absolute regioselectivity

(Fig. 5).

In addition to absolute regioselectivity, the method is highlighted by excellent

functional group compatibility. For example, nitriles, primary amines, alcohols, esters,

and ketones can be present, although acidic a-hydrogens to the electron-withdrawing

groups may suffer from partial exchange. Aliphatic hydrogens do not react under

the developed reaction conditions. This regio- and chemoselectivity allowed efficient isotopic labeling of some biologically active molecules and natural products

(Fig. 6).

Aromatic pincer complexes were found practically inactive under the described

conditions and, therefore, it is suggested the reaction is operated by a mechanism in

which the methine position of the backbone acts as a shuttle. If so, the following

mechanistic scheme may be suggested: after dissociation of olefin, the iridium(I)



296



D. Gelman and R. Romm

D

99



O

OH

O



H



D



OH

4

D

D



2

CH(D)3



H



D

50



66

D 91



H



O

O

OH

forksolin

(RT, 5 min)



O

altrenogest

(RT, 20 h)



Fig. 6 Labeling of the functionalized targets



P tBu2



Pt Bu2



R



Pt Bu2

R C6 D6



Ir NH 2

NH 3

H

t

P Bu2

7



Ir



Ir



R



P t Bu2

6



C 6D5



D



D



H



P t Bu2

25



Pt Bu2



Pt Bu2



Ir C 6D 5 or



H

Ir C 6D 5



P t Bu2

26



D



P t Bu2

27



Scheme 6 Proposed mechanism for the Ir-catalyzed H/D exchange



Pi Pr2

Ni Br

Br

P i Pr2

31



Pi Pr2

FeBr3



Ni Br

Pi Pr2

28

O Pi Pr2

Ni Br



O Pi Pr2

Ni Br



O Pi Pr2

CuBr2



O Pi Pr2

29



Ni Br

Br

O P i Pr2

30



no

oxidation



O Pi Pr2

32



Scheme 7 Formation of the paramagnetic 17 electron Nickel pincer complexes



fragment (6) (generated by reductive elimination of NH3 from 7) undergoes oxidative addition of the aryl deuterium bond to form 25. Subsequent reversible C–D

reductive elimination involving the methine carbon center on the ligand could

generate aryl iridium(I) complex 26. Alternatively, the complex resulting from

the oxidative addition of the arene (25) could undergo reversible a-hydrogen

elimination from the ligand backbone to give iridium(III) carbene species 27

(Scheme 6). A parallel process with the vinylic C–H bonds would lead to

incorporation of deuterium into the olefinic substrates.

C(sp3)-based group 10 transition metal complexes have been also reported as

catalytically relevant. For example, Zargarian and coworkers synthesized a series of

unprecedented Ni compounds bearing 1,-5-diphosphinopentane [57] or 1,3diphosphinitopropane (Scheme 7) [66]. The diamagnetic PC(sp3)P 28 and POC



PC(sp3)P Transition Metal Pincer Complexes. . .



297



(sp3)OP 29 are electrochemically active and undergo facile one electron oxidation

to Ni(III) species. Indeed, chemical oxidation led to the formation of the paramagnetic 17-electron complexes 30 and 31 (Scheme 7) [67, 68]. Interestingly, attempts

to oxidize the aromatic POC(sp2)OP 32 failed despite that cyclic voltammetry

shows quasi-reversible single-electron oxidation wave (E1/2 ¼ 1.17) [66].

Ni(II) PC(sp3)P 28 (0.5–2 mol%) was found active in catalyzing Kumada

coupling of chlorobenzene with MeMgCl and BuMgCl demonstrating maximal

TON of 84 (Eq. 2) [68]. The same complex serves as a nucleophilic catalyst in

regioselective hydroamination of acrylonitrile with aniline (Eq. 3) [69].

Cl

+



MeMgCl



Me



28 (2 mol%)

THF, reflux

20 h



+



ð2Þ



67%

NH2

+



CN



2%



28 (1 mol%)

C6H5CH3, 60 °C

18 h



CN



ð3Þ



74%

Cl



+



CCl4



29 (0.1 mol%)

ACN, 85 °C

24 h



CCl3



ð4Þ



100%



The same group demonstrated the use of Ni(III) POC(sp3)OP 29 as a promoter in

Kharasch addition of CCl4 to olefins (Eq. 4). The reaction gives the 1:1 antiMarkovnikov addition product exclusively. The catalyst shows up to 100 turnovers

and works equally well with styrene and 4-methylstyrene, while somewhat less

efficiently with acrolein, methyl acrylate, and acrylonitrile, and not at all with

a-methylstyrene, 1-hexyne, and 3-hexyne.

2.1.2



Diphosphinocycloalkanes



The first C(sp3)-metalated compound bearing cyclohexyl-based pincer ligand was

synthesized by Mayer and coworkers who studied coordination chemistry of a

potentially tripodal cis,cis-1,3,5-tris[(diphenylphosphino)methyl]cyclohexane (33).

They found that treatment of 33 with [Rh(COD)Cl]2 [70] or Vaska complex [43] in

hot toluene gives carbometalated 34 and 35 almost quantitatively (Scheme 8). In spite

of the high flexibility of 33, the third phosphine group is either non-coordinated (e.g.,

35) or loosely bound (e.g., 34) and may be easy displaced in solution. Therefore, 34

exists in solution as oligomeric species.



298



D. Gelman and R. Romm



Cl

PPh2

Rh

Ph2P

PPh2

H



PPh2

[Rh(COD)Cl]2



PPh2



PPh2



Vaska

Ph2 P



PPh2



34



33



Cl

Ir

OC H

35



PPh2



Scheme 8 Prototypical cyclohexane-based pincer complexes

Y Pt Bu2



P tBu2



PtBu2



PtBu2



M Cl



M Cl

H

Pt Bu2

39: M = Rh

40: M = Ir



Pd Cl



Rh Cl

H

Pt Bu2

42



Y P tBu2

36: M = Pd, Y = CH2

37: M = Pt,Y = CH2

38: M = Pt, Y = O



Pt Bu2

41



36



Fig. 7 Recently synthesized cyclohexane-based pincer complexes



Treatment of 34 with D2 shows H/D exchange at both the hydride ligand and the

methine hydrogen which suggests the lability of the metal–carbon bond (Eq. 5) [61].



34



D2



Ph2P



P Cl

Rh

D



D



PPh2

PPh2

34'



Ph2 P

n



Cl

Ir

P H



PPh2



ð5Þ



PPh2

34''



n



More recently, Wendt [71] and Gusev [72] independently reported on the

employment of cis- and trans-1,3-bis(di-t-butylphosphinomethyl)cyclohexane, as

well as cis-1,3-bis(di-t-butylphosphinito) cyclohexane, as platforms for the construction of palladium, rhodium, platinum, and iridium pincer complexes (Fig. 7).

The chelate systems bearing cis-substituted cyclohexane derivatives adopt a

double-bent conformation with a bisecting perpendicular pseudo-mirror plane

(Fig. 7). For example, five of the atoms in 36 (P1, P2, C2, C7, and C8) are almost

situated in the plane of the complex with the displaced methine groups C1 and C3

pointing away in the same direction. This arrangement results in magnetically



PC(sp3)P Transition Metal Pincer Complexes. . .



O Pt Bu2



O P t Bu2



[IrCl(COD)]

toluene

180 °C



299

O Pt Bu2



O P t Bu2

Ir



Cl

H

O Pt Bu 2

44



+



3 H2



Ir



Cl

H

O P tBu2

43

was not

observed



Scheme 9 Acceptorless dehydrogenation of the ligand scaffold



different environments above and below the square plane, which is also manifested

in solution with dual resonances in the 13C NMR spectrum of 36 for the tert-butyl

substituents on the phosphorus atoms.

The cyclohexane ring adopts the chair conformation and is aligned with the

plane of the complex, with the equatorial C2–H bond being activated. There is

almost no built-in strain in the cyclohexane ring, which is reflected by its

endocyclic bond angles that do not deviate substantially from free cyclohexane

[40, 73, 74].

Despite that DFT calculations predicted more strain built in complexes bearing

trans-substituted cyclohexane-based ligands (by 9–11 kcal/mol), comparison

between the structural data of 39 and 42 revealed that cyclohexane rings are

practically identical and both adopt almost ideal chair conformation [72].

In contrast to a,o-diphosphinoalkane-based pincer complexes that often experience temperature induced a- and b-hydride elimination to form isomeric carbene or

olefin-type complexes, cyclohexyl-based PC(sp3)P complexes are thermally stable.

For example, according to TGA, 40 displays a first insignificant weight loss at

220 C in both air and nitrogen, while substantial thermal decomposition starts only

at 250 C in air and 300 C in nitrogen [74]. The thermal stability can be rationalized

by a lesser flexibility of the cyclic ligand frame.

However, interesting aromatization of the cis-1,3-bis(di-t-butylphosphinito)

cyclohexane ligand accompanied by extrusion of three equivalents of dihydrogen

was discovered upon attempted synthesis of the corresponding Ir PC(sp3)P complex

43 (Scheme 9) [74].

Arguably, this unusual transformation may proceed either via iridium-catalyzed

acceptorless dehydrogenation of cyclohexane ligand into the known aromatic

POCOP followed by its facile metalation, or, alternatively, via the formation of

the hypothetical POC(sp3)OP Ir(III) hydride species (43) that undergoes a-elimination of hydrogen, followed by a,b-hydride shift and double bond isomerization

(Scheme 10). Thus, existence of a- and b-hydride elimination processes cannot be

completely ruled out in the less flexible cyclohexyl-based systems.

Complex 36 was employed as a catalytic promoter in the Suzuki–Miyaura crosscoupling [75] and Mizoroki–Heck olefination of aryl halides [38, 71]. Although

both these reactions have been shown to proceed under very harsh conditions

(140–160 C), TONs achieved in, e.g., Heck reaction of aryl iodides with activated

substrates were impressive (up to 5.34 Â 105).



300



D. Gelman and R. Romm

O P t Bu2

H

Ir Cl

H

O Pt Bu2

43



-H 2



O P t Bu2



O P t Bu2



Ir



Ir



Cl



O Pt Bu2



Cl

H

O Pt Bu2

double bond

isomerization



a,b-shift



O P t Bu2

H

Ir Cl

H

O Pt Bu2



44

- 2H 2



Scheme 10 Proposed mecanism for the acceptorless dehydrogenation of the ligand scaffold



Frech and coworkers reported on the employment of the adamantanyl scaffold

for the design of PC(sp3)P pincer ligands [44]. Unlike other all-aliphatic scaffolds

mentioned in this section, the suggested ligand is very rigid and lacking labile

b-hydrogens which ensures thermal stability of the resulting transition metal

complexes.

Studying reactivity of the palladium hydride 45 revealed that in contrast to the

aromatic pincer complexes [24], it reacts reversibly with water resulting in the

formation of the palladium hydroxide 46 and molecular hydrogen (Eq. 6). This

difference in reactivity was attributed to a greater “hydridic character” of the

hydride ligand due to a stronger trans-effect exerted by the C(sp3) center.

PCy2

Pd H

45



PCy2



PCy2

H2O



Pd OH



H2

46



ð6Þ



PCy2



Yet, catalytic tests proved that palladium chloride bearing the new PC(sp3)P (47)

is a robust catalyst for Suzuki–Miyaura [76] coupling of aryl bromides. This

catalyst operates under much milder reaction conditions and enables quantitative

coupling of a wide variety of electronically activated, deactivated, and/or sterically

hindered, highly functionalized aryl bromides with phenylboronic acid in pure

water with NaOH as base within very short reaction times under low catalyst

loading and without the need for exclusion of air. Hydrophobic substrates, which

lead to inefficient conversions in aqueous medium, were efficiently and quantitatively coupled in toluene with K3PO4 as base. Similar reactivity was exhibited by

the same catalyst in Negishi cross-coupling (Scheme 11) [77].

Very often, activity of pincer complexes in cross-coupling chemistry is

contested due to the fact that these compounds cannot participate in the widely

accepted Pd(0)/Pd(II) catalytic cycle and they are only regarded as a source for

palladium nanoparticles that virtually catalyze the reactions [78]. However, based

on the results of the mercury test performed independently by the groups of Frech

[76] and Wendt [75] for the reactions catalyzed by 36 and 47, it was concluded that

the reactions operate either via Pd(II)/Pd(IV) catalytic cycle or via s-bond metathesis

(Scheme 12).



PC(sp3)P Transition Metal Pincer Complexes. . .



301

PCy2

Pd Cl



Br



Zn



46 (0.01 mol%)

NMP, 100 °C

R' 2

30 min



R



O



47



EtO



PCy2



R'

R100 examples

95% yield



EtO

OEt



OEt

S



S



OMe

95%



95%



95%



95%



Scheme 11 Negishi cross coupling reaction catalyzed by 47

PCy2

Br

Pd Ph



47



PCy2



PCy2



Pd Cl PhB(OH)2



Pd Ph Ar-Br



PCy2



PCy2



Ph

PCy2

PCy2

Ph

Pd

Br

PCy2



Ar



Scheme 12 Proposed mechanism for the suzuki cross coupling of aryl bromides catalyzed by 47



2.2



2.2.1



C(sp3)-Metalated Pincer Complexes Based

on Non-aliphatic Scaffolds

Cycloheptatriene-Based Scaffold



A seminal design of the PC(sp3)P pincer ligands taking advantage of cycloheptatriene platform was introduced by Kaska, Mayer, and coworkers [45]. Significance of the tropylium resonance form leading to the lability the a-methine

hydrogen is the most striking difference between the all-aliphatic and the

cycloheptatriene-based sp3-metalated pincer ligands because the corresponding

transition metal pincer complexes, if prepared, must be regarded as organometallic

species of a reversed polarity (Fig. 8).

The synthesis of the first representative of this family of compounds (48) was

realized via the reaction of the corresponding ligand with tricarbonyl iridium

chloride (Scheme 13). Complex 48 is a highly stable, coordinatively as well as



302



D. Gelman and R. Romm

PR2

Ha

M L



PR2



PR2



M



M



PR2



PR2



PR2



Fig. 8 Proposed reversed polarity in the cycloheptatziene based pincer complexes



H

H



P t Bu2

Cl

Ir CO



TMS-OTf

H



P t Bu2



48



P t Bu2

DBU



H



P tBu2 OTf

Cl

Ir CO

P tBu2

49



P t Bu2



Ir CO



Ir CO

H



t



P Bu2



P t Bu2



50



P t Bu2

Ir CO

P t Bu2

51



H

52



Pt Bu 2

H

Ir CO



Pt Bu 2

Ir CO



Pt Bu 2



Pt Bu 2

53



Scheme 13 Attempted synthesis of 50



electronically saturated compound. As the carbonyl in it is located trans to the

metalated carbon atom, the hydride and the chloride ligands are forced into a

mutual trans orientation. The C–H group in the free backbone is strongly bent

out of the plane which encompasses the three double bonds. Consequently, the

residual hydrogen atom at the metal-bound C(sp3) and the chloride ligand are synperiplanar at the iridium center.

Indeed, treatment of 48 with one equivalent of TMS-OTf leads to the formation

of a stable tropylium derivative 49 (Scheme 13) that is expectedly characterized by

a longer n (Ir–CO) vibration (2,030 cmÀ1 in 49 vs. 2,000 cmÀ1 in 48) [79].

However, attempted treatment of 48 with DBU to form carbene–tropylium

product 50 led to the formation of the sp2-carbometalated 51–53 [80]. As was

suggested, HCl is removed from the cycloheptatrienyl PCP pincer 48 across the C

(sp3)–Ir bond forming a carbene structure as outlined in Scheme 13. The proposed

carbene, however, was not observed due to the fast rearrangement affording 52,

which, in due turn, may rearrange into 51 or to expel H2 to form 53. Indeed, DFT

calculations predict the 50 as the least stable species being destabilized by 10.7 kcal/

mol in comparison to 52 and by 26.6 kcal/mol to 53. Considerably higher stability of

the C(sp2)–Ir bond is, apparently, a driving force for the isomerization processes.



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