1

Rhodium and Iridium Complexes of Bulky Tertiary

Phosphine Ligands. Searching for Isolable Cationic M(III)

Alkylidenes

Jesús Campos,† ‡ Ernesto Carmona†*

†Instituto de Investigaciones Químicas-Departamento de Química Inorgánica.

Universidad de Sevilla-Consejo Superior de Investigaciones Científicas. Avda. Américo

Vespucio 49, 41092 Sevilla (Spain).

‡Present address: Inorganic Chemistry Laboratory, Department of Chemistry,

University of Oxford, South Parks Road, Oxford OX1 3QR, UK.

*guzman@us.es

TOC Graphic

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Abstract

Cyclometalated chloride complexes of rhodium and iridium based on (5-

C5Me5)M(III) fragments that result from the metalation of the xylyl substituent of a

coordinated PR2(Xyl) phosphine (Xyl=2,6-Me2C6H3) have been prepared by reaction of

the appropriate metal precursor with the corresponding phosphine. For iridium, four

complexes derived from the phosphines PiPr2(Xyl), 1a; PCy2(Xyl), 1b; PMe2(Xyl), 1c

and PPh2(Xyl), 1d, have been prepared, whereas for rhodium only the complexes

derived from PiPr2(Xyl), 2a and PMe2(Xyl), 2d have been studied. Chloride abstraction

from compounds 1 and 2 by NaBArF (BArF= B(3,5-C6H3(CF3)2)4) leads to either

cationic dichloromethane adducts or to cationic hydride-alkylidene structures resulting

from -H elimination. The rhodium complexes investigated yield only dichloromethane

adducts. However, in the iridium system the less sterically demanding phosphines

PMe2(Xyl) and PPh2(Xyl) provide also dichloromethane adducts as the only observable

products, whereas for the bulkier PiPr2(Xyl) and PCy2(Xyl) ligands the hydride-

alkylidene formulation is the prevailing one. Nonetheless, variable temperature NMR

studies reveal that in solution each of these two structures exists in equilibrium with

undetectable concentrations of the other by means of facile reversible -H elimination

and migratory insertion reactions. Reactivity studies on the cationic hydride-alkylidene

complexes of iridium are reported too.

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Introduction

Transition metal alkyl and carbene complexes are renowned families of

organometallic complexes that have been intensively investigated in the past decades.

Metal alkyls1,2 are at the heart of organometallic chemistry, as their reactive M-C

bonds participate in many useful reactions. In turn, the chemistry of metal carbenes has

become one the most successful areas of research in organometallic catalysis due to the

rich reactivity of the M=C bond.3,4,5 Even though effort has focused on carbocyclic6 and

heteroatom-stabilized carbenes,7,8 encompassing N-heterocyclic carbenes,4a,9 metal

alkylidenes, that is M=CR2 complexes where R= hydrogen or hydrocarbyl fragment,

continue to attract widespread attention.

In general, alkylidene M=CR2 complexes of the late transition elements contain

the metal in a low oxidation state.10 With reference to iridium, many Ir(I) alkylidenes

have been reported11 comprising examples of the Ir=CH2 parent unit, which

interestingly, may exhibit either electrophilic or nucleophilic carbene reactivity.11b-11d In

contrast, their Ir(III) counterparts are somewhat elusive species that are often proposed

as reactive intermediates for many relevant transformations.12 Recently, the oxidative

addition of CH3F to a pincer-ligated Ir(I) complex has been proposed to involve the

intermediacy of an Ir(III)=CH2 species that results from C-H activation of the

fluorocarbon followed by -fluorine migration.13

Not unexpectedly, cationic Ir(III) alkylidenes are rather fleeting species,

although examples have been known for many years. A transient hydride-ethylidene

complex (A in Figure 1) was generated by our group at -100ºC by protonation of an Ir-

CH=CH2 unit at the carbon atom, but at -50ºC it rearranged irreversibly to the

thermodynamically more stable hydride-ethylene isomer.14a,b Incorporation of the Ir=C

functionality to a metalacyclic structure hindered -H elimination and allowed for the

isolation of a stable hydride-alkylidene and observation of reversible carbene migratory

insertion and -H elimination.14c However, analogous 1,2-C shifts are usually

irreversible processes. We have reported recently that the cationic bis(iridacycle) B

(Figure 1), that contains Ir-CH2R and Ir=CHR termini, participates actively in C-H

activation and C-C bond forming reactions that ultimately lead to unusual hydride-

phosphepine structures.15 Intermediate B was in fact generated in situ by -hydride

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M-P-CH2

elimination from the neutral dialkyl C (Figure 1) which derived formally from double

metalation of a bis(xylyl) phosphine ligand, PMe(Xyl)2, for Xyl = 2,6-Me2C6H3.15

Figure 1. Previous examples of cationic Ir(III) alkylidenes [IrIII=C(H)(R)]+ structures

(A and B) and the bimetalacycle C precursor for B.14a,15

In view of these results we planned the synthesis of isolable cationic alkylidene

complexes of Rh(III) and Ir(III) from metalacyclic structures akin to those

represented for iridium in Figure 1. To avoid the irreversible C-C coupling reaction that

prevented the isolation of B, mono(xylyl) phosphines PR2(Xyl), rather than bis(xylyl)

phosphines, PR(Xyl)2,15 were chosen for this work. We envisaged that cationic

cyclometalated16 rhodium and iridium solvento complexes of type D in Scheme 1 might

experience reversible -H elimination leading to the desired hydride-alkylidene

complexes (E, in Scheme 1).17 As the pioneer work of Schrock on the somewhat related

-H abstraction reaction, that converts a M(CH2R)2 fragment into M=C(H)R with

elimination of RCH3, demonstrated the importance of increased steric pressure at the

metal coordination sphere,18 four tertiary phosphines PR2(Xyl) of different steric

properties were assayed. As indicated also in Scheme 1, the R substituents range from

the small methyl group to the very bulky cyclohexyl, with phenyl and i-propyl having

intermediate, increasing size. In the following sections we provide details of this work

that has led to stable complexes of the two kinds depicted in Scheme 1, i.e. cationic M-

CH2Cl2 adducts (D) and hydride-alkylidene structures, E.

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Scheme 1. Presumed reversible 1,2-shifts in the cationic Rh(III) and Ir(III) complexes

investigated in this work.

Results and Discussion

Synthesis of Iridium and Rhodium Precursors

As stated implicitly, the four phoshine ligands selected are prone to

cyclometalation.16 They were prepared by conventional procedures (see Supporting

Information, SI, for details). The corresponding iridium complexes (1a - 1d, Scheme 2)

resulted from the straightforward reaction of the Ir(III) dimer [{(5-C5Me5)IrCl2}2] with

the appropriate phosphine in the presence of the non-coordinating base 2,2,6,6-

tetramethylpiperidine (TMPP in Scheme 2). Under the specified conditions, complexes

1a and 1b were obtained as the exclusive reaction products in yields around 90%.

Nevertheless, compounds 1c and 1d formed together with the non-metalated dichlorides

(5-C5Me5)Ir(Cl)2PR2(Xyl) in ca. 1:5 (PMe2(Xyl)) and 2:1 (PPh2(Xyl)) ratios. Mild

heating of dichloromethane solutions of the non-metalated compounds (45ºC) in a

sealed flask in the presence of TMPP resulted in their quantitative conversion to the

desired 1c and 1d complexes.

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Scheme 2. Synthesis of cyclometalated iridium chloride complexes 1a-1d. Lower-case letters a-

d will be used throughout this paper to denote complexes of the above phosphines as specified

in this scheme.

For rhodium, only the PMe2(Xyl) and PiPr2(Xyl) derivatives were investigated.

In contrast to the iridium analogs, complexes 2a and 2c could be obtained in yields

<50% only after heating in refluxing toluene for four days, and were accompanied by

other unidentified compounds. Accordingly, the synthetic route of Scheme 3, that takes

advantage of the capacity of Zn(C5Me5)2 to act as a mild Cp* transfer reagent,19 was

utilized. Once more, the PiPr2(Xyl) complex 2a was the exclusive product of this

transformation while for the PMe2(Xyl) reaction, 2c was the minor product as it was

accompanied by (5-C5Me5)Rh(Cl)2PMe2(Xyl) in ca. double molar quantities.

Conversion of the former into the latter required treatment with 1 equiv of LinBu at -

20ºC, and gave 2c in moderate yields (ca. 60%).

Scheme 3. Synthesis of rhodium chloride complexes 2a and 2c.

7

The new complexes 1 and 2 feature characteristic 31P{1H} resonances which for

the rhodium complexes appear as the expected doublet with 1JPRh values of ca. 160 Hz.

In the 1H NMR spectra, the C5Me5 ring gives a doublet (4JHP 1.5 - 2.5 Hz) at roughly

1.7 ppm while the diastereotopic M-CH2 protons resonate between 3 and 4 ppm and

exhibit a two-bond 1H-1H coupling constant of around 12-14 Hz. In the iridium

compounds, only one of these two proton signals exhibit coupling to the phosphorous

nucleus (3JHP of ca. 4-5 Hz). Corresponding 13C{1H} resonances appear at around 17

(Ir) and 30 (Rh) ppm (2JCP ≈ 3 (Ir), 8 (Rh) and 1JCRh ≈ 23 Hz). Slow diffusion of pentane

into concentrated dichloromethane solutions of compounds 1a, 1c and 2a provided

suitable crystals for X-ray studies (Figure 2 and Figure S5), that further confirmed the

cyclometalation of the xylyl-substituted phosphine. In the three structures, the M–C,

M–P and M–Cl bond distances are similar to those found for related compounds

previously reported by our group.15,20,21 Bond angles for the three-legged piano stool,

are close to the ideal 90º value.

Figure 2. ORTEP diagram for compound 1a and 2a. Thermal ellipsoids are drawn at the 50 % probability

and most hydrogen atoms have been omitted for clarity.

Chloride Abstraction from Rh and Ir Chloride Complexes.

As anticipated in the Introduction section, we foresaw that chloride abstraction from

complexes of types 1 and 2 would generate cationic cyclometalated dichloromethane

adducts that could further rearrange by -H elimination and formation of the desired

hydride-alkylidene derivatives. As summarized in Scheme 4, treatment of

dichloromethane solutions of complexes 1a and 1b of the bulky PiPr2(Xyl) and

PCy2(Xyl) ligands with NaBArF (BArF= B(3,5-C6H3(CF3)2)4) led to the targeted

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alkylidenes 3a+ and 3b+ that formed as the only observable reaction products. Instead,

the analogous chloride complexes of the less sterically demanding phosphines

PMe2(Xyl) and PPh2(Xyl) originated the corresponding cationic adducts, 4c·CH2Cl2+

and 4d·CH2Cl2+, in quantitative yields (by NMR). In a similar manner, the rhodium

chloride precursors 2a and 2c led also to the dichloromethane adducts 5a·CH2Cl2+ and

5c·CH2Cl2+, once more as the only observable products. These complexes are not

represented in Scheme 4 but possess structures alike that of the iridium analogs

4·CH2Cl2+. It is rather intriguing that NMR investigations on the new compounds

provided no indication for the existence of a structure in which the cyclometalated

R2P(Xyl) unit binds to the metal center in a 4-P,C,C',C" fashion, similarly to our

previous findings for iridium15 and rhodium21 compounds constructed around the

PMe(Xyl)2 ligand. It seems clear that two Xyl units are required to stabilize such an

unusual 4 binding mode.

Scheme 4. Generation of cationic hydride-alkylidene and CH2Cl2 adducts of iridium, 3+ and 4·CH2Cl2+,

respectively. For R = iPr and Cy the 3+ ↔ 4+ equilibria favor 3 (i.e. 3a+ or 3b+), while for R = Me, Ph

only compounds of type 4·CH2Cl2+ have been isolated ([4·CH2Cl2]BArF). The rhodium analogs of

compounds 1 (viz. 2a and 2c, Scheme 3) form exclusively dichloromethane adducts (5a·CH2Cl2+ and

5c·CH2Cl2+, respectively).

Variable temperature NMR studies for the cationic iridium complexes of

Scheme 4 evince their interesting solution dynamic behavior. Thus, the room-

temperature 1H NMR spectra of hydride-alkylidenes 3+ show equivalent iPr (3a+) and

9

Cy (3b+) phosphine substituents and no signals attributable to their key Ir–H and

Ir=CH functionalities. However, upon cooling to -60 ºC, 1H NMR resonances at ca.

15.5 and -15.2 ppm due to Ir=CH and Ir–H protons of 3a+ (Figure S1) and 3b+ become

discernible. The low-frequency peaks correspond to the hydride ligands and exhibit a

two-bond coupling to the phosphorus nucleus of around 25 Hz, whereas signals due to

the carbenic protons exhibit no resolvable nuclear coupling. At these low temperatures

(below -60 ºC), the phosphine iPr substituents of 3a+ become inequivalent and give rise

to four somewhat broad, albeit clearly distinct, resonances. In almost all probability this

solution dynamic behavior results from a rapid and reversible 1,2-H shift between Ir and

the alkylidene carbon (vide infra). We analyzed the exchange of these protons by 1D-

EXSY studies. Rate constants of 14.7 and 45.5 s-1 were measured at -80 ºC for

compounds 3a+ and 3b+, respectively. An Eyring analysis (see SI, Figure S3) in the

temperature interval from –70 to –90 ºC yielded values of the activation parameters ΔH‡

= 7.3 ± 0.6 kcal mol-1 and ΔS‡ = -12 ± 1 cal mol-1K-1, with ΔG‡298K

= 11 ± 1 kcal mol-1

for compound 3a+ (and similar values for compound 3b+: ΔH‡ = 9 ± 2 kcal mol-1 and

ΔS‡ = -6 ± 1 cal mol-1K-1, with ΔG‡298K

= 11 ± 2 kcal mol-1).

We thought it of interest to demonstrate further the existence of an alkylidene unit in

3a+ by converting its cis hydride ligand to bromide by action of N-bromosuccinimide

(Scheme 5). The new alkylidene, 6a+, features dark green color and carbenic NMR

resonances observable at room temperature at δ 16.1 (1H) and 262.2 (13C) ppm, the

latter with 1JCH = 152 Hz. As expected, complex 6a+ displays no dynamic behavior in

solution.

Scheme 5. Reaction of 3a+ with N-bromosuccinimide

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The molecular structures of the cationic alkylidenes 3a+ and 6a+ were confirmed by

X-ray diffraction studies (Figure 3). Characteristic Ir=C bond lengths of 1.896(5) (3a+)

and 1.907(2) (6a+) Å, which are appreciably shorter than the Ir–CH2 bond distances of

ca. 2.10 Å found in other related complexes discussed in this paper or elsewhere,15,20 are

in accordance with the proposed alkylidene formulation. Other geometrical parameters

are comparable to those in related structures already considered and need no further

discussion.

Figure 3. ORTEP diagrams for compounds 3a+ and 6a+. Thermal ellipsoids are drawn at the 50 %

probability and hydrogen atoms and counterion have been omitted for clarity.

Despite the lability of the coordinated molecule of CH2Cl2 in the cationic solvento

complexes of iridium 4c·CH2Cl2+ and 4d·CH2Cl2

+ (Scheme 4), α-H elimination was

not observed in any case. Evidently, for these complexes derived from the less bulky

PMe2(Xyl) and PPh2(Xyl) ligands, the dichloromethane-solvated Ir-CH2 metalacyclic

structure is preferred relative to the non-solvated hydride-alkylidene formulation (D and

E, respectively, in Scheme 1). However, the dynamic behavior exhibited by these

complexes in solution may be suggestive of a fast equilibration of 4c·CH2Cl2+ and

4d·CH2Cl2+ with undetectable concentrations of the corresponding hydride-alkylidene

structures. Thus, the two cations present broad 31P{1H} NMR signals centered at 8.3

(4c·CH2Cl2+) and 35.5 ppm (4d·CH2Cl2

+). Furthermore, the room temperature 1H

NMR spectrum of 4d·CH2Cl2+ contains signals similar to those of the neutral chloride

precursor 1d, except for resonances due to the Ir-CH2 protons which are missing.

Cooling at -20ºC results in the appearance of two broad doublets centered at 3.82 and

3.42 ppm, with a 2JHH coupling of ca. 16 Hz. For cation 4c·CH2Cl2+ with the least

11

bulky phosphine substituents, cooling at -90ºC is needed for the observation of two

broad signals with 3.52 and 3.20 ppm. At 20ºC, they convert into a broad peak shifted

to ca. 2.90 ppm. These observations may be indicative of the attainment in solution of

the equilibria shown in Scheme 6, implying fast reversible CH2Cl2 dissociation

accompanied by also reversible -H elimination.22

Scheme 6. Proposed dynamic behavior for 4c·CH2Cl2+ and 4d·CH2Cl2

+

As summarized in Scheme 7, adducts 4c·py+ and 4d·NCMe+ were instantly

generated upon addition of a slight excess of the Lewis base to solutions of the

appropriate 4·CH2Cl2+ cationic species. They display sharp 31P{1H} NMR resonances

at -0.5 (4c·py+) and 2.84 ppm (4d·NCMe+) with chemical shifts close to those of the

neutral chloride precursors (3.3 and 28.6 ppm for 1c and 1d, respectively). In addition,

the superior binding properties of C5H5N and NCMe in comparison with CH2Cl2, result

in sharp IrCH2 resonances. The molecular structures of the two complexes were

ascertained by X-ray crystallography (see SI, Figure S7).

Scheme 7. Reaction of cationic adducts 4c·CH2Cl2+ and 4d·CH2Cl2

+ with Lewis Bases.

At variance with the iridium cations 4·CH2Cl2+, the analogous rhodium

complexes, 5a·CH2Cl2+ and 5c·CH2Cl2

+, exhibited no dynamic behavior in solution, in

agreement with their failure to undergo α-H elimination reactions. For instance, for

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cation 5a·CH2Cl2+, the 31P{1H} NMR spectrum consists of a doublet with 77.3 ppm

and 1JRhP= 158 Hz. The rhodium benzylic linkage, Rh-CH2-, gives rise to a proton

multiplet centered at 3.49 ppm and to a 13C{1H} doublet with 33.9 ppm and one-bond

13C -103Rh coupling of 23 Hz. It is worth remarking that all Rh and Ir dichlorometane

adducts studied in this work feature good thermal stability at room temperature and

remain unaltered after prolonged periods of time when kept under an inert atmosphere.

This is in contrast with reports on related complexes.15,23 We therefore performed X-ray

studies on [5a·CH2Cl2] BArF, with the results shown in Figure 4. Only a few rhodium

complexes with coordinated CH2Cl2 have been authenticated by X-ray

crystallography,23b,c,24 of which two are closely related to 5a·CH2Cl2+, namely [(η5-

C5Me5)Rh(PMe3)Me(CH2Cl2)]+ and [(η5-C5Me5)Rh (PMe3)Ph(CH2Cl2)]

+. The two are

air sensitive, thermally unstable solids that readily decompose at room temperature even

under inert atmosphere. In contrast, 5a-CH2Cl2+ is stable in solution for many days,

when stored under argon, and remained unaltered after several months in the solid state.

We propose that the existence of an intramolecular CH–π interaction between the

coordinated molecule of the dichloromethane and the metalated xylyl ring (Figure 4)

might account for this remarkable stability. This weak interaction is characterized by

CH(25b)···C(12) and CH(25b)···C(13) bond distances of ca. 2.64 and 2.73 Å, whereas

the C–H(25b)···Centr. angle has a value of ca. 157.6º (Centr. = metalated xylyl ring

centroid).

Figure 4. (a) ORTEP diagram for the complex 5a·CH2Cl2+. Thermal ellipsoids are drawn at the 50 %

probability and hydrogen atoms and counterion have been omitted for clarity. (b) Intramolecular CH–π

contacts in 5a·CH2Cl2+ (Centr. = xylyl ring centroid).

13

Reactivity Studies of Hydride-Alkylidene Complex 3a+

The solution dynamic behavior of hydride-alkylidene complexes 3+ (Scheme 4) that

was ascribed to very facile migratory insertion of the hydride ligand into the cationic

Ir=CH- functionality, was a clear sign of the otherwise expected electrophilicity of these

complexes. To gain additional insight into this chemical functioning, we performed

supplementary reactivity studies employing 3a+ as a representative complex. As shown

in Scheme 8, its reaction with PMe3 resulted in an immediate color change, due to the

generation of the phosphonium ylid 7a+ as the major reaction product (85 %). The

cationic adduct 4a·PMe3+ was formed too, albeit in low proportions (ca. 15 %).

Compound 4a·PMe3+ exhibits a 31P{1H} NMR signal at 48.1 ppm (very close to

corresponding signals due to 1a, at 49.6 ppm, and 4a·C2H4+, at 42.7 ppm; vide infra),

which appears as a doublet (2Jpp= 21 Hz) due to coupling to the PMe3 phosphorus

nucleus. The latter also features a doublet at -47.3 ppm. In contrast, the 31P{1H} NMR

signal due to the metalated phosphine of compound 7a+ is found at 59.3 ppm, while its

phosphonium PMe3 group yields a resonance at 33.1 ppm, highly deshielded with

respect to the corresponding resonance in compound 4a·PMe3+ (-47.3 ppm), and

without observable coupling to the phosphorus center of the cyclometalated phosphine.

A characteristic doublet due to the Ir–H unit appears in the 1H NMR spectrum at -17.7

ppm (dd, 2JHP = 33.3, 3JHP = 10.6 Hz). In the 13C{1H} NMR spectrum, the signal due to

the IrCHPMe3 is observed at 6.3 ppm (d, 1JCP = 32 Hz), widely shifted with respect to

the resonance due to the carbenic carbon of 3a+ (263.8 ppm). Compound 4a·PMe3+ was

further characterized by X-ray diffraction studies, whose details can be found in the

Supporting Information (Figure S6).

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Scheme 8. Reaction of 3a+ with PMe3.

Once more, formation of the phosphonium ylide 7a+ in the above reaction was an

obvious sign of the electrophilicity of the carbene linkage of complex 3a+. However,

7a+ was the kinetic product of the reaction as heating a mixture of 4a·PMe3+ and 7a+ at

40 ºC for 16 hours caused quantitative conversion of the ylide species into the

phosphine adduct (Scheme 8). Monitoring the reaction by 31P{1H} NMR spectroscopy

provided a first-order kinetic rate constant of 3.4·10-5 s-1, which corresponds to a ∆Gǂ of

24.8 ± 0.3 kcal·mol-1 at 40 ºC (see SI, Figure S4). The latter compound is thermally

stable and does not revert to 7a+, even after heating its solution in ClCH2CH2Cl over 80

ºC for several hours. It seems probable that whereas the ylid complex 7a+ could result

from direct nucleophilic attack by PMe3 at the carbene carbon, adduct 4a·PMe3+ must

have been formed by PMe3 coordination to an unsaturated intermediate alike F in

Scheme 6.7d

Treatment of a dichloromethane solution of 3a+ with C2H4 caused an immediate

color change, from the characteristic intense red of the alkylidene to pale yellow, due to

formation of the cationic ethylene adduct 4a·C2H4+ (Scheme 9). The reaction with

LiMe led to the formation of the neutral methyl complex 9a as the major product (ca. 70

% yield), along with other minor unidentified species. Use of the milder alkylating

reagent ZnMe2, led cleanly and quantitatively to 9a. The molecular structure of this

complex was confirmed by X-ray diffraction studies (see Figure S6 for details). We also

studied the reaction of 3a+ with H2, which resulted in quantitative formation of the

cationic alkyl-dihydride species 8a+. This reaction might proceed either by direct

addition of H2 to the Ir=CH unit, or more likely by dihydrogen-induced migratory

insertion chemistry, that would involve coordination of H2 to a species analogous to F

in Scheme 6. Interestingly, reaction of 3a+ with D2 led to the formation of [D5]-8a-D2+,

in which all benzylic positions of the xylyl ring became deuterated. We recently

15

reported an analogous C-H/D scrambling for the parent compound bearing the

PMe(Xyl)2 ligand.15

Scheme 9. Additional reactivity studies performed with complex 3a+

Compounds 4a-C2H4+ and 9a feature characteristic 1H NMR signals due to

diastereotopic Ir–CH2 protons at 3.16 (d, 2JHH = 14.2 Hz) and 2.05 (dd, 2JHH = 14.2, 3JHP

= 3.4 Hz) ppm (9a), and 3.67 (d, 2JHH = 12.5 Hz) and 3.39 (dd, 2JHH = 12.5, 3JHP = 3.3

Hz) ppm (4a-C2H4+). The Ir-Me unit of 9a leads to a shielded doublet at -0.20 ppm,

with a three-bond scalar coupling to phosphorus of 3.4 Hz. The ethylene ligand of 4a-

C2H4+ provides two broad resonances (2H each) at 2.77 and 2.29 ppm, with

corresponding 13C{1H} NMR resonances (also broad) at 46.8 and 42.5 ppm. The

cationic dihydride 8a+ exhibits a remarkable dynamic behavior in solution (Figure 5),

consisting in the exchange of the two hydride ligands and the two methylene protons.

This exchange probably proceeds by coupling of one of the hydrides with the Ir–CH2

bond and formation of a monohydride agostic intermediate.15a,25 At room temperature,

the 1H NMR spectrum does not contain any observable resonances due to these four

protons but at -20 ºC two broad signals become hardly visible at 2.45 and 2.93 ppm (Ir–

CH2), along with a broad resonance at -13.62 due to the hydride ligands. Coalescence is

attained at ca. -10 ºC and further cooling to -40 ºC allowed identifying a two-bond

coupling constant of 13.0 Hz between the diastereotopic methylene protons. At -50 ºC

16

the signal for the two hydride ligands splits into two different peaks, one of them

displaying coupling to the phosphorous center (2JHP = 16.1 Hz). Heating the solution at

45 ºC resulted in the appearance of a broad signal attributable to four protons at -4.98

ppm (not shown in Figure 5). Lineshape analysis of the corresponding resonances at

various temperatures in conjunction with Eyring analysis of the observed rate constants

yields the following activation parameters for the overall process: ΔH≠ = 20.3 ± 0.5

kcal·mol-1, ΔS≠ = 29 ± 2 cal·mol-1·K-1 and ΔG≠300K = 12 ± 2 kcal·mol-1 (more details in

the SI).

Figure 5. Solution dynamic behavior of complex 8a+ by 1H NMR

Concluding Remarks

Cationic hydride alkylidene complexes of the (5-C5Me5)Ir(III) fragment in which

the alkylidene functionality is part of a five-membered iridacycle that contains also a

phosphine terminus (complexes 3a+ and 3b+) have been prepared and characterized

using bulky PR2(Xyl) phosphines which are prone to cyclometalation. Their pivotal Ir-

H and Ir=CH- units derive from a metalated benzylic Ir-CH2- linkage by reversible -H

elimination. In accordance with previous findings, increased steric pressure at the metal

coordination sphere favors -H elimination, thereby stabilizing the hydride-alkylidene

8a+

17

structure. Indeed, this structure has only been observed for the two bulkiest phosphines

employed, namely PiPr2(Xyl) and PCy2(Xyl). At variance with these observations, the

cyclometalated Ir-CH2- binding motif of complexes of the less sterically demanding

PMe2(Xyl) and PPh2(Xyl) ligands is favored over the Ir(H)(=CH-) structure, leading to

stable, cationic dichloromethane adducts (complexes 4c·CH2Cl2+ and 4d·CH2Cl2

+,

respectively). Only CH2Cl2 adducts analogous to the latter compounds can be isolated

for the similar (5-C5Me5)Rh(III)- PR2(Xyl) reaction system.

Experimental Section

General Synthesis of Iridium Chloride Compounds 1a-1d. [Cp*IrCl2]2 (0.30 g,

ca. 0.38 mmol) dissolved in dry CH2Cl2 (5 mL) and cooled at 0ºC was reacted with a

dichloromethane solution of the phosphine (0.76 mmol), in the presence of 2,2,6,6-

tetramethyl piperidine (TMPP, 130 µL, 0.76 mmol). The reaction mixture was allowed

to warm to room temperature and additionally stirred for 2 h (16 hours at 45 ºC in the

case of PMe2(Xyl) and 4 hours at this temperature when the phosphine was PPh2(Xyl)).

The solvent was removed under vacuum and the product extracted with toluene. The

solution was evaporated to dryness providing a bright yellow powder, which was

washed with pentane to yield the desired chloride complexes in yields ca. 90 %.

Variable amounts of the non-metalated (5-C5Me5)IrCl2(PR2(Xyl)) compounds were

identified when performing the reactions with PMe2(Xyl) and PPh2(Xyl) at room

temperature. Compound 1a. 1H NMR (500 MHz, CD2Cl2, 25 ºC) : 7.22 (d, 1 H, Ha),

7.06 (td, 1 H, 5JHP = 1.6 Hz, Hb), 6.85 (d, 1 H, Hc), 3,67 (d, 1 H, 2JHH = 13.9 Hz,

IrCHH), 3.24 (m, 1 H, CH(iPr)), 3.07 (dd, 1 H, 2JHH = 13.9, 3JHP = 4.4 Hz, IrCHH), 2,50

(s, 3 H, Meα), 2.27 (m, 1 H, CH(iPr)), 1.71 (d, 15 H, 4JHP = 1.3 Hz, C5Me5), 1.30 (dd, 3

H, 3JHP = 18.6, 3JHH = 6.8 Hz, Me(iPr)}), 1.24 (dd, 3 H, 3JHP = 12.8, 3JHH = 7.2 Hz,

Me(iPr)), 1.18 (dd, 3 H, 3JHP = 14.6, 3JHH = 7.1 Hz, Me(iPr)), 0.73 (dd, 3 H, 3JHP = 15.7,

3JHH = 7.1 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (125

MHz, CD2Cl2, 25 ºC) : 162.9 (d, 2JCP = 26 Hz, C1), 140.5 (C3), 133.6 (d, 1JCP = 48 Hz,

C2), 129.9 (CHb), 128.0 (d, 3JCP = 7 Hz, CHc), 127.5 (d, 3JCP = 12 Hz, CHa), 91.9

(C5Me5), 30.9 (d, 1JCP = 30 Hz, CH(iPr)), 27.4 (d, 1JCP = 29 Hz, CH(iPr)), 22.8 (Meα),

20.9 (d, 2JCP = 7 Hz, Me(iPr)), 20.2 Me(iPr)), 19.2 (d, 2JCP = 5 Hz, Me(iPr)), 18.5

(Me(iPr)), 16.9 (d, 2JCP = 3 Hz, IrCH2), 9.0 (C5Me5).31P{1H} NMR (202 MHz, CD2Cl2,

18

25 ºC) δ: 49.6. Anal. Calcd. for C24H37ClIrP: C, 49.34; H, 6.38. Found: C, 49.3; H, 6.6.

Corresponding data for the remaining compounds of this type can be found in the SI.

Figure 6. Labeling scheme used for 1H and 13C{1H} NMR assignments.

Synthesis of Rhodium Chloride Compounds 2a, 2c. A solution of the phosphine

(P(iPr)2(Xyl): 111 mg, 0.5 mmol; PMe2(Xyl): 83 mg, 0.5 mmol) in 2 mL of THF was

added, at -40 ºC, to a solution of [RhCl(C2H4)2]2 (100 mg, 0.25 mmol) in 3 mL of THF.

The reaction mixture was stirred for 3 h at this temperature. Then, a solution of ZnCp2*

(84 mg, 0.25 mmol) in 1 mL of THF was added and the mixture stirred for 5 h, allowing

to warm slowly to -25 ºC. The solvent was removed under vacuum, the residue was

extracted with diethyl ether and it was then evaporated to dryness. The solid was

dissolved in 5 mL of CHCl3 and stirred for 3 h at room temperature. The solvent was

removed under vacuum and the crude product washed with pentane to yield chloride

complexes 2a and 2c as orange solids, which were purified by column chromatography

from Et2O/pentane (2a, 175 mg, 70 %; 2c, 110 mg, 83 %). Reaction with PMe2(Xyl)

gave a mixture of metalated (2c) and (5-C5Me5)RhCl2(PMe3(Xyl) complexes in a ca.

ratio of 30:70. Both compounds were separated by column chromatography from

Et2O/pentane to give pure samples of 2c (42 mg, 19 %) and the non-metalated

dichloride (130 mg, 55 %) as crystalline orange solids. In order to increase the amount

of 2c, a THF solution of the latter product (50 mg, 0.105 mmol) was reacted with a

solution of LinBu (3 M in hexanes, 45 µL) at -20 ºC. After 30 min of stirring at this

temperature, the reaction was quenched with MeOH (10 µL) and the volatiles removed

19

under vacuum. The product was extracted with Et2O, the solvent evaporated to dryness

and the orange solid washed with pentane to give complex 2c in 62 % yield (29 mg).

Compound 2a. 1H NMR (500 MHz, CDCl3, 25 ºC) : 7.14 (d, 1 H, Ha), 7.01 (td, 1 H,

5JHP = 2.2 Hz, Hb), 6.81 (dd, 1 H, 4JHP = 2.4 Hz, Hc), 3.42 (br. d, 1 H, 2JHH = 12.1 Hz,

RhCHH), 3.24 (br. d, 1 H, 2JHH = 12.1 Hz, RhCHH), 2.95 (septet, 2 H, 3JHH = 7.0 Hz,

CH(iPr)), 2.42 (s, 3 H, Meα), 2.29 (m, 2 H, CH(iPr)), 1.63 (d, 15 H, 4JHP = 2.4 Hz,

C5Me5), 1.36 (dd, 6 H, 3JHP = 19.1, 3JHH = 6.8, Me(iPr)), 1.24 (dd, 6 H, 3JHP = 12.3, 3JHH

= 7.1, Me(iPr)), 1.19 (dd, 6 H, 3JHP = 14.5, 3JHH = 7.1, Me(iPr)), 0.80 (dd, 6 H, 3JHP =

15.5, 3JHH = 7.1, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (160

MHz, CDCl3, 25 ºC) : 160.7 (d, 2JCP = 28 Hz, C1), 139.5 (C3), 131.2 (d, 1JCP = 42 Hz,

C2), 129.6 (d, 4JCP = 2 Hz, CHb), 127.7 (d, 4JCP = 6 Hz, CHc), 126.6 (d, 4JCP = 15 Hz,

CHa), 98.1 (t, 1JCRh = 2JCP = 4 Hz, C5Me5), 30.5 (dd, 1JCRh = 23, 2JCP = 8 Hz, RhCH2),

30.1 (d, 1JCP = 21 Hz, CH(iPr)), 27.5 (d, 1JCP = 22 Hz, CH(iPr)), 22.8 (Meα), 20.7 (d,

2JCP = 8 Hz, Me(iPr)), 20.2 (Me(iPr)), 19.4 (d, 2JCP = 5 Hz, Me(iPr)), 19.1 (Me(iPr)), 9.6

(C5Me5).31P{1H} NMR (200 MHz, CDCl3, 25 ºC) δ: 83.0 (d, 1JRhP = 159 Hz). Anal.

Calcd. for C24H37ClPRh: C, 58.25; H, 7.54. Found: C, 58.4; H, 7.4. Corresponding data

for the remaining compounds of this type can be found in the SI.

Synthesis of cationic hydride-alkylidenes 3a+ and 3b+. To a solid mixture of 1a or

1b (0.08 mmol) and NaBArF (72 mg, 0.08 mmol) placed in a Schlenk flask was added 5

mL of CH2Cl2. The resulting solution with intense red color was stirred for 15 min at

room temperature, then filtered and the solvent removed under vacuum. The red solid

was washed with pentane to give the BArF- salts of alkylidenes 3a+ or 3b+ in ca. 95 %

yield. These complexes can be recrystallized from a 1:2 mixture of CH2Cl2:pentane.

Compound 3a+. 1H NMR (500 MHz, CD2Cl2, 25 ºC) : 7.69 (d, 1 H, Ha), 7.62 (dd, 4JHP

= 2.6 Hz, Hc), 7.34 (td, 5JHP = 2.2 Hz, 1 H, Hb), 2.73 (m, 2 H, 2 CH(iPr)), 2,69 (s, 3 H,

Me), 2.17 (s, 15 H, C5Me5), 1.06, (dd, 6 H, 3JHP = 17.2, 3JHH = 6.9 Hz, Me(iPr)), 0.89

(dd, 3 H, 3JHP = 18.5, 3JHH = 6.9 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz;

1H NMR (500 MHz, CD2Cl2, -80 ºC) δ: 15.51 (s, 1 H, IrCH), -15.21 (d, 1 H, 2JHP = 24.7

Hz, Ir−H). Hydride and carbene signals are only detectable at temperatures below -

50ºC. 13C{1H} NMR (125 MHz, CD2Cl2, 25 ºC) : 263.8 (Ir=CH), 166.4 (d, 2JCP = 27

Hz, C1), 144.2 (C3) 137.2 (d, 3JCP = 7 Hz, CHc), 135.2 (C2, overlapped with BArF),

134.1 (CHb), 128.7 (d, 3JCP = 12 Hz, CHa), 104.5 (C5Me5), 25.5 (d, 1JCP = 32 Hz,

CH(iPr)), 22.0 (Me), 18.7, 18.3 (Me(iPr)), 10.3 (C5Me5). 31P{1H} NMR (202 MHz,

20

CD2Cl2, 25 ºC) δ: 73.1. IR (Nujol): υ(IrH) 2165 cm-1. Anal. Calcd. for C56H48BF24IrP: C,

47.67 ; H, 3.43. Found: C, 47.4; H, 3.5. See SI for corresponding data for 3b+.

Synthesis of cationic bromide-alkylidene 6a+. Chloride complex 1a (100 mg, 0.171

mmol) and NaBArF (152 mg, 0.171 mmol) were placed in a Schlenk and dissolved in

CH2Cl2 (5 mL) under argon. After stirring at room temperature for 15 minutes the red

solution of the resulting of complex 3a+ was filtered over a Schlenk flask containing N-

bromosuccinimide (30 mg, 0.171 mmol), with a change in color to dark green. The

reaction mixture was stirred for 15 minutes and then the succinimide removed by

extraction with deoxygenated water. Compound [6a]BArF was obtained as dark green

crystals (215 mg, 84 %) by slow diffusion of pentane into a dichloromethane solution of

the alkylidene.1H NMR (500 MHz, CD2Cl2, 25 ºC) : 16.1 (s, 1 H, Ir=CH), 8.10 (d, 1 H,

Ha), 6.98 (dd, 1 H, 5JHP = 2.9 Hz, Hc), 7.56 (Hb, overlapped with NaBArF), 3.74

(dseptet, 1 H, 2JHP = 11.9, 3JHH = 6.8 Hz), 2.79 (s, 3 H, Meα), 1.98 (m, 1 H, CH(iPr)),

1.94 (t, 15 H, 4JHP = 1.1 Hz, C5Me5), 1.66 (dd, 3 H, 3JHP = 15.8, 3JHH = 6.9 Hz, Me(iPr)),

1.30 (m, 6 H, 2 Me(iPr)), 0.24 (dd, 3 H, 3JHP = 17.3, 3JHH = 7.0 Hz, Me(iPr)). All

aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (125 MHz, CD2Cl2, 25 ºC) : 262.2

(1JCH = 152 Hz, Ir=CH), 167.2 (d, 2JCP = 25 Hz, C1), 145.5 (C3), 139.9 (d, 3JCP = 8 Hz,

CHa), 139.1 (d, 1JCP = 47 Hz, C2), 134.2 (CHb), 131.1 (d, 3JCP = 12 Hz, CHc), 107.6

(C5Me5), 29.7 (Meα), 28.1 (d, 1JCP = 28 Hz, CH(iPr)), 27.2 (d, 1JCP = 31 Hz, CH(iPr),

22.4 (Me(iPr)), 19.5 (d, 2JCP = 4 Hz, Me(iPr)), 18.6 (d, 2JCP = 6 Hz, Me(iPr)), 18.5

(Me(iPr)), 9.4 (C5Me5). 31P{1H} NMR (200 MHz, CD2Cl2, 25 ºC) δ: 60.3. Anal. Calcd.

for C56H47BBrF24IrP: C, 45.15; H, 3.18. Found: C, 45.1; H, 3.2.

Reaction of 3a+ with PMe3. A solution of PMe3 in toluene (33 µL, 1.1 M, 0.033

mmol) was added under argon over a CH2Cl2 (1 mL) solution of alkylidene 3a+ (BArF-

salts 40 mg, 0.028 mmol) placed in a Schlenk flask. The solution rapidly cleared up and

then it was heated at 40 ºC for 16 hours. The volatiles were removed under vacuum and

the residue washed with pentane to give complex 4a·PMe3+ as a pale orange powder

(35 mg, 84 %). Crystals suitable for X-ray analysis were obtained by slow diffusion

from CH2Cl2/pentane. In turn, characterization of the related ylide 7a+ was achieved by

the following procedure. A screw-capped NMR tube was charged with 3a+ (BArF- salt;

30 mg, 0.021 mmol) and CD2Cl2 (0.6 mL). The tube was shaken, placed at -40 ºC and

PMe3 (2.5 µL, 0.026 mmol) was added at this temperature. 31P{1H} NMR monitoring of

the reaction showed immediate conversion of the alkylidene to a mixture of the ylide

21

7a+ and the cationic adduct 4a·PMe3+ in a ca. ratio of 97:3. Spectroscopic data were

obtained at -40 ºC without further purification in order to avoid isomerization to

4a·PMe3+, which is slow at this temperature. Compound 4a·PMe3

+. 1H NMR (400

MHz, CD2Cl2, 25 ºC) : 7.27 (d, 1 H, Ha), 7.20 (td, 1 H, 5JHP = 2.7 Hz, Hb), 6.98 (dd, 1

H, 5JHP = 3.1 Hz, Hc), 3.43 (m, 1 H, IrCHH), 3.29 (dd, 1 H, 2JHH = 7.6, 3JHP = 3.1 Hz,

IrCHH), 3.28 (dseptet, 1 H, 2JHP = 10.9, 3JHH = 7.1 Hz, CH(iPr)), 2.51 (s, 3 H, Meα),

2.33 (m, 1 H, CH(iPr)), 1.77 (t, 15 H, 4JHP = 1.9 Hz, C5Me5), 1.28 (dd, 3 H, 3JHP = 13.2,

3JHH = 7.2 Hz, Me(iPr)), 1.24 (d, 9 H, 2JHP = 10.3 Hz, PMe3), 1.15 (dd, 3 H, 3JHP = 18.9,

3JHH = 7.1 Hz, Me(iPr)), 1.03 (dd, 3 H, 3JHP = 16.5, 3JHH = 6.7 Hz, Me(iPr)), 0.59 (dd, 3

H, 3JHP = 16.3, 3JHH = 7.1 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz.

13C{1H} NMR (100 MHz, CD2Cl2, 25 ºC) : 158.5 (d, 2JCP = 24 Hz, C1), 140.1 (C3),

132.4 (d, 1JCP = 47 Hz, C2), 131.9 (d, 4JCP = 3 Hz, CHb), 130.4 (d, 3JCP = 7 Hz, CHc),

126.9 (d, 3JCP = 12 Hz, CHa), 98.5 (C5Me5), 31.2 (dd, 1JCP = 29, 3JCP = 2 Hz, CH(iPr)),

26.3 (d, 1JCP = 30 Hz, CH(iPr), 22.6 (Meα), 20.7 (Me(iPr)), 20.4 (d, 2JCP = 5 Hz,

Me(iPr)), 20.0 (d, 2JCP = 6 Hz, Me(iPr)), 18.6 (Me(iPr)), 18.4 (d, 1JCP = 39 Hz, PMe3),

9.8 (C5Me5), 6.4 (dd, 2JCP = 8, 2JCP = 3 Hz, IrCH2). 31P{1H} NMR (160 MHz, CD2Cl2,

25 ºC) δ: 48.1 (d, 2JPP = 21 Hz, P(iPr)2Xyl), -47.3 (d, 2JPP = 21 Hz, PMe3). Anal. Calcd.

for C59H58BF24IrP2: C, 47.62; H, 3.93. Found: C, 47.9; H, 3.8. Compound 7a+. 1H

NMR (500 MHz, CD2Cl2, 25 ºC) : 7.19 (m, 2 H, Ha, Hb), 7.10 (m, 1 H, Hc), 3.60 (d, 1

H, 2JHP = 11.0 Hz, IrCHPMe3), 2.94 (m, 1 H, CH(iPr)), 2.56 (s, 3 H, Meα), 2.31 (m, 1 H,

CH(iPr)), 1.90 (t, 15 H, 4JHP = 1.8 Hz, C5Me5), 1.33 (d, 9 H, 2JHP = 11.9 Hz, PMe3), 1.23

(dd, 3 H, 3JHP = 12.8, 3JHH = 7.4 Hz, Me(iPr)), 1.13 (dd, 3 H, 3JHP = 18.3, 3JHH = 6.8 Hz,

Me(iPr)), 1.03 (dd, 3 H, 3JHP = 12.0, 3JHH = 7.2 Hz, Me(iPr)), 0.16 (dd, 3 H, 3JHP = 16.0,

3JHH = 7.1 Hz, Me(iPr)), -17.7 (dd, 1 H, 2JHP = 33.3, 3JHP = 10.6 Hz, IrH). All aromatic

couplings are of ca. 7.5 Hz. 13C{1H} NMR (125 MHz, CD2Cl2, -40 ºC) : 150.6 (d, 2JCP

= 27 Hz, C1), 141.8 (C3), 133.4 (C2, overlapped with BArF), 130.2 (CHb), 129.3 (CHc),

125.4 (CHa), 92.1 (C5Me5), 27.6 (d, 1JCP = 28 Hz, CH(iPr)), 24.7 (d, 1JCP = 36 Hz,

CH(iPr), 22.0 (Meα), 18.5 (Me(iPr)), 18.3 (Me(iPr)), 17.8 (Me(iPr)), 16.5 (Me(iPr)), 10.6

(d, 1JCP = 56 Hz, PMe3), 9.8 (C5Me5), 6.3 (dd, 1JCP = 32 Hz, IrCHPMe3). 31P{1H} NMR

(200 MHz, CD2Cl2, 25 ºC) δ: 59.3 (P(iPr)2Xyl), 33.1 (PMe3).

Associated Content

22

Supporting Information: general experimental details, synthesis and characterization

of phosphine ligands and rhodium and iridium complexes, solution dynamic 1H NMR

spectroscopy studies, kinetic studies on the conversion of 7a+ into 4a·PMe3+ and X-ray

crystallographic studies. This material is available free of charge via the Internet at

http://pubs.acs.org.

Author Information

Corresponding Author: *E-mail: guzman@us.es

Notes: The authors declare no competing financial interest.

Acknowledgments

Financial support from the Spanish Ministry of Science and Innovation (Projects

CTQ2010-17476 and Consolider-Ingenio 2010 CSD2007-00006) and the Junta de

Andalucía (Projects FQM-119 and P09-FQM-4832) is gratefully acknowledged. J. C.

thanks the Spanish Ministry of Education for a research grant (AP-20080256).

Dedication

This paper is dedicated to the memory of Professor M. F. Lappert, in recognition of

an outstanding career and of his most valuable contributions to the development of

Inorganic and Organometallic Chemistry.

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22 It is worth mentioning in this regard that the color of solutions of both cationic dichloromethane

adducts is temperature dependent. At low temperatures (below -20 ºC) the solutions are yellow, as found

for 1a-d and other cationic adducts described in this work. However, warming above 10 ºC results in

darkening of the mixture to the characteristic intense orange color typical of hydride-alkylidenes 3+.

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