Guanidinato complexes of iridium: Ligand-donor strength, O2 reactivity, and (alkene)peroxoiridium(III) intermediates

Article

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Guanidinato Complexes of Iridium: Ligand-Donor Strength, O₂

Reactivity, and (Alkene)peroxoiridium(III) Intermediates

Matthew R. Kelley and Jan-Uwe Rohde*

Department of Chemistry, The University of Iowa, Iowa City, Iowa 52242, United States

S

* Supporting Information

ABSTRACT: A series of seven [Ir{ArNC(NR₂)NAr}(cod)]

complexes (1a−1g; where R = Me or Et; Ar = Ph, 4-MeC₆H₄,

4-MeOC₆H₄, 2,6-Me₂C₆H₃, or 2,6-ⁱPr₂C₆H₃; and cod = 1,5-

cyclooctadiene) were synthesized by two diﬀerent methods

from the neutral guanidines, ArNC(NR₂)NHAr, using

either MeLi and [{Ir(cod)}₂(μ-Cl)₂] or [{Ir(cod)}₂(μ-

OMe)₂]. Reaction of 1a−1g with CO produced the

corresponding [Ir{ArNC(NR₂)NAr}(CO)₂] complexes (2a−

2g), which were characterized by NMR and solution- and

solid-state IR spectroscopy. Complexes 1b (R = Et, Ar = Ph),

1d (R = Et, Ar = 4-MeC₆H₄), 1f (R = Me, Ar = 2,6-Me₂C₆H₃),

and 2b (R = Et, Ar = Ph) were characterized by X-ray crystallography as mononuclear complexes with a guanidinato-κ²N,N′

ligand and a cod or two CO ligands coordinated to the Ir center in a distorted square-planar environment. On the basis of the

CO stretching frequencies of 2a−2g [avg. ν_CO(n-pentane) = 2016−2019 cm⁻¹] and the alkene ¹³C chemical shifts of 1a−1g

[δ(¹³C_CC) = 58.7−61.0 ppm], the donor strength of the guanidinato ligands was evaluated and compared to that of related

monoanionic ligands. Reaction of 1a−1g in solution with O₂at 20 °C aﬀorded (alkene)peroxoiridium(III) intermediates,

[Ir{ArNC(NR₂)NAr}(cod)(O₂)] (3). The steric properties of the supporting ligand play a decisive role in O₂binding in that

complexes without ortho substituents react largely irreversibly with O₂(1a−1e; where Ar = Ph, 4-MeC₆H₄or 4-MeOC₆H₄),

whereas complexes with ortho substituents exhibit fully reversible O₂binding (1f and 1g; where Ar = 2,6-Me₂C₆H₃or

2,6-ⁱPr₂C₆H₃). Complexes 3a−3f were characterized by ¹H NMR and IR spectroscopy (ν_OO= 857−872 cm⁻¹). Decay of the new

intermediates and subsequent reaction with cod produced 4-cycloocten-1-one and the respective Ir^Iprecursor.

cod = 1,5-cyclooctadiene), for which the presence of a peroxo

ligand was conﬁrmed by IR spectroscopy (ν_OO= 880 and 850

cm⁻¹).^21,22Three other examples were characterized by X-ray

crystallography, namely, a Rh^III(alkene)(O₂²⁻) complex, in

which the alkene group is part of a tridentate bis(phosphine)

ligand,²³and two Ir^III(C₂H₄)(O₂²⁻) complexes supported by

bis(pyridine)amine ligands.²⁴Although vibrational data for ν_OO

were not reported in these cases, the assignment of the O₂-

derived ligand as a peroxo ligand could be based on the O−O

distance. With one exception, the known (alkene)peroxo

complexes did not exhibit the expected reactivity, however.

Only for [IrCl(PPh₃)₂(C₂H₄)(O₂)] has oxidation of the ethene

ligand to acetaldehyde been reported, but under concomitant

oxidation of PPh₃to OPPh₃.²⁵No alkene oxygenation has been

observed for the other (alkene)peroxo complexes.

1. INTRODUCTION

Catalytic methods for the oxidation of organic compounds that

use O₂as oxidant oﬀer signiﬁcant economic and environmental

advantages. Yet, the development of selective methods remains

challenging.¹While heterogeneous methods are eﬀective for the

bulk oxidation of selected substrates, homogeneous oxidation

catalysis promises to be of broader synthetic utility.²⁻⁴In this

context, organometallic O₂reactivity is an area of growing

interest,^5,6because insights into this chemistry provide a basis

for the design and development of catalytic methods that use

O₂or air. Complexes of Rh and Ir were extensively investigated

in early studies on catalytic alkene oxidation.^7,8These metals

have received renewed attention in recent work focusing on

stoichiometric O₂reactions of well-deﬁned alkene com-

plexes.^9,10In particular, the latter studies have led to the

characterization of the products from C−O bond formation,

such as metallaoxetanes and metalladioxolanes.⁹⁻¹⁷

By contrast, little is known about the mechanism of O₂

activation and how these products might form. In light of the

abundance of peroxo complexes of Rh and Ir,^18,19the

intermediacy of (alkene)peroxo complexes en route to C−O

bond formation has frequently been proposed.^{7,10,11,14,20}A few

such complexes have in fact been reported. These include

[IrCl(PPh₃)₂(C₂H₄)(O₂)] and [Ir(phen)(cod)(O₂)]⁺(where

With the goal of advancing mechanisms of O₂activation, we

have investigated the O₂reactivity of Ir^I(alkene) complexes

supported by monoanionic dialkyldiarylguanidinato ligands,

{ArNC(NR₂)NAr}⁻(Chart 1).²⁶Substituted guanidinates have

been employed as ligands in a wide variety of transition metal

complexes and, owing to their electronic properties, are well-

Received: November 27, 2012

Published: February 19, 2013

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Physical Methods. NMR spectra were recorded on a Bruker Avance

Chart 1. Structures of Guanidines and Complexes 1−3

MicroBay 300, Avance DPX 300, Avance 400, or Avance 500

1

spectrometer at ambient temperature, unless noted otherwise. H and

¹³C chemical shifts are reported in parts per million (ppm) and were

referenced to residual solvent peaks. Self-diﬀusion coeﬃcients (D)

were determined from stimulated-echo experiments⁴³using bipolar

gradients acquired in two-dimensional mode (stebpgp1s). These

experiments were performed on a Bruker Avance 400 spectrometer at

25 °C. Prior to each experiment, the diﬀusion time (delay between the

midpoints of the gradients, Δ) and gradient length (δ) were optimized

using the one-dimensional version of the pulse sequence

(stebpgp1s1d). The data acquisition for each diﬀusion experiment

took approximately 30 min. Data processing and analysis to determine

D were carried out within the TopSpin 2.1 software package.

IR spectra were recorded on a Bruker Vertex 70 Fourier-transform

IR spectrometer using solutions of the compounds in n-pentane (5−

15 mM) or solid samples. Solid samples were prepared by grinding the

solid compound with KBr and pressing the mixture into a disk.

Electrospray ionization mass spectral (ESI MS) data were acquired on

a quadrupole ion trap ThermoFinnigan LCQ Deca mass spectrometer

or on a quadrupole time-of-ﬂight Waters Q-Tof Premier mass

spectrometer. Electron impact ionization mass spectral (EI MS) data

were acquired on a single quadrupole ThermoFinnigan Voyager mass

spectrometer (equipped with a solids probe). UV−Visible spectra were

recorded on an HP 8453A diode array spectrophotometer (Agilent

Technologies), which was equipped with a cryostat from Unisoku

Scientiﬁc Instruments, Japan, for measurements at low temperature.

2.2. Synthesis of Guanidines and Ir^IComplexes. N,N-Diethyl-

N′,N″-bis(4-methylphenyl)guanidine, (4-MeC₆H₄)NC(NEt₂)NH(4-

MeC₆H₄). A solution of 2.00 g (7.80 mmol) of N,N′-bis(4-

methylphenyl)thiourea and 4.90 mL (11.2 g, 78.7 mmol) of

iodomethane in 15 mL of methanol was heated under reﬂux for 2 h.

After the solution was cooled to room temperature, the volatiles,

including excess iodomethane, were removed under reduced pressure

to aﬀord S-methyl-N,N′-bis(4-methylphenyl)isothiourea hydroiodide

as a yellow solid. Yield: 3.00 g (97%). ¹H NMR (300 MHz, (CD₃)₂SO,

δ): 10.81 (br, 2H, NH), 7.28 (br, 8H, Ar H), 2.65 (s, 3H, SCH₃), 2.32

(s, 6H, C₆H₄CH₃). ESI(+)MS (CH₃CN) m/z: {M − I}⁺calcd for

C₁₆H₁₉IN₂S, 271.1; found, 271.2.

A solution of 3.00 g (7.53 mmol) of S-methyl-N,N′-bis(4-

methylphenyl)isothiourea hydroiodide and 3.10 mL (2.19 g, 30.0

mmol) of diethylamine in 15 mL of methanol was heated in a sealed

ﬂask for 7 h at 70 °C. (Caution! Because of the formation of

methanethiol, which is a gas under the reaction conditions, the pressure in

the flask may increase substantially. A shield should be used for

protection.) The solution was cooled to room temperature, and

methanethiol was carefully removed with a stream of N₂. (MeSH was

trapped by routing the gas stream through concentrated HNO₃.)

Upon evaporation of the solvents, N,N-diethyl-N′,N″-bis(4-

methylphenyl)guanidinium iodide was obtained as a yellow solid.

Yield: 3.05 g (96%). ¹H NMR (300 MHz, CDCl₃, δ): 7.12 (d, J = 8.6

Hz, 4H, Ar H), 7.02 (d, J = 8.4 Hz, 4H, Ar H), 5.49 (br, 2H, NH),

2.95 (q, J = 7.3 Hz, 4H, NCH₂CH₃), 2.32 (s, 3H, C₆H₄CH₃), 2.31 (s,

3H, C₆H₄CH₃), 1.40 (t, J = 7.3 Hz, 6H, NCH₂CH₃). ESI(+)MS

(CH₃CN) m/z: {M − I}⁺calcd for C₁₉H₂₆IN₃, 296.2; found, 296.2.

A solution of 3.05 g (7.20 mmol) of N,N-diethyl-N′,N″-bis(4-

methylphenyl)guanidinium iodide in 20 mL of diethyl ether was

treated with 50 mL of a saturated aqueous solution of Na₂CO₃. The

biphasic mixture was vigorously stirred for 1 h, after which the organic

phase was separated and dried over MgSO₄and the volatiles were

removed under reduced pressure. The crude product was puriﬁed by

column chromatography on silica gel using a solution of 3% NEt₃in

diethyl ether as the eluent aﬀording a colorless solid. Yield: 1.45 g

(68%). Anal. Calcd for C₁₉H₂₅N₃: C, 77.25; H, 8.53; N, 14.22. Found:

C, 77.45; H, 8.52; N, 13.95. ¹H NMR (300 MHz, CDCl₃, δ): 7.04 (d, J

= 8.0 Hz, 4H, Ar H), 6.80 (d, J = 7.5 Hz, 4H, Ar H), 5.35 (br, 1H,

NH), 3.33 (q, J = 7.1 Hz, 4H, NCH₂CH₃), 2.29 (s, 6H, C₆H₄CH₃),

1.16 (t, J = 7.1 Hz, 6H, NCH₂CH₃). ¹³C{¹H} NMR (75.5 MHz,

CDCl₃, δ): 150.8, 131.4, 129.9, 122.2, and 118.7 (Ar), 42.1

suited for the coordination of metals in mid and high oxidation

states.²⁷⁻³⁰Only recently, guanidinato complexes of low-valent

35

metal centers such as Ir^I,^26,31,32Rh^I,³³Co^I,³⁴and Ni^Ihave

emerged. In addition to these examples, guanidinato ligands

have given access to a Cr^I₂complex³⁶with a very short metal−

metal distance and multinuclear Cu^I, Ag^I, and Au^Ispecies³⁷

with short metal−metal contacts or interactions.

We found that [Ir{ArNC(NR₂)NAr}(cod)] complexes (1)

react with O₂under ambient conditions and reported the

formation of an (alkene)peroxoiridium(III) intermediate,

[Ir{PhNC(NMe₂)NPh}(cod)(O₂)] (3a), for one of these

reactions.³⁸Remarkably, this intermediate could be generated

in high yield and persisted in solution for several hours, so that

it could be characterized by spectroscopic techniques. Data for

its decay products were consistent with the presence of an

oxygenated cod ligand. Indeed, reactions with added alkenes

(i.e., 1,5-cyclooctadiene or cis-cyclooctene) produced 4-cyclo-

octen-1-one, thus providing evidence for C−O bond formation.

In following up on our initial ﬁndings, we have prepared an

extended series of Ir^Icomplexes to examine electronic and

steric substituent eﬀects on the formation and properties of the

new (alkene)peroxo intermediates (3). The results of this study

are described in the present report. Furthermore, reactivity

studies conﬁrm the release of oxygenated alkene. We also

report the synthesis and characterization of the dicarbonyl

complexes [Ir{ArNC(NR₂)NAr}(CO)₂] (2) which were

prepared for an evaluation of the donor strength of the

guanidinato ligands.

2. EXPERIMENTAL SECTION

2.1. Materials and Methods. Materials. All reagents and solvents

were purchased from commercial sources and were used as received,

unless noted otherwise. Diethyl ether and toluene were deoxygenated

by sparging with N₂and puriﬁed by passage through two packed

columns of molecular sieves under an N₂pressure (MBraun solvent

puriﬁcation system). n-Pentane was dried over Na and distilled under

N₂prior to use.³⁹Preparation and handling of air- and moisture-

sensitive materials were carried out under an inert gas atmosphere by

using either standard Schlenk and vacuum line techniques or a

glovebox. Dioxygen was dried by passage through a short column of

Drierite. The Ir complexes [{Ir(cod)}₂(μ-Cl)₂],⁴⁰[Ir{PhNC(NMe₂)-

NPh}(cod)] (1a), [Ir{PhNC(NEt₂)NPh}(cod)] (1b), [Ir{(4-

MeC₆H₄)NC(NMe₂)N(4-MeC₆H₄)}(cod)] (1c), and [Ir{(2,6-

Me₂C₆H₃)NC(NMe₂)N(2,6-Me₂C₆H₃)}(cod)] (1f) were synthesized

according to published procedures (cod = 1,5-cyclooctadiene).²⁶

[{Ir(cod)}₂(μ-OMe)₂]⁴¹was prepared from [{Ir(cod)}₂(μ-Cl)₂] and

sodium methoxide⁴²in dichloromethane, isolated by ﬁltration and

removal of the volatiles, and used without further puriﬁcation.

Elemental analyses were performed by Atlantic Microlab, Inc.,

Norcross, GA, U.S.A.

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(NCH₂CH₃), 20.8 (C₆H₄CH₃), 13.0 (NCH₂CH₃). EIMS (70 eV) m/

z: M^+•calcd for C₁₉H₂₅N₃, 295.2; found, 295.3.

In an N₂atmosphere, a solution of 0.300 mmol of lithium

guanidinate in 10 mL of diethyl ether was added to a suspension of

100 mg (0.149 mmol) of [{Ir(cod)}₂(μ-Cl)₂] in 5 mL of diethyl ether

under stirring at 20 °C. The color of the reaction mixture changed

from orange to yellow, accompanied by precipitation of a colorless

solid. After 3 h, the solution was ﬁltered, and the solvent was removed

under reduced pressure. The residue was recrystallized from n-pentane

by preparing a concentrated solution at 20 °C and storing it at −30

°C. Yellow single crystals of X-ray diﬀraction quality were typically

obtained within 2 days.

Method B. In an N₂atmosphere, a solution of 0.300 mmol of the

N,N-dialkyl-N′,N″-diarylguanidine in 15 mL of diethyl ether was

added to a suspension of 0.149 mmol of [{Ir(cod)}₂(μ-OMe)₂] in 5

mL of diethyl ether under stirring. The dark-orange mixture turned

into a yellow-orange solution. After 3 h, the volatiles were removed

under reduced pressure to aﬀord a yellow-orange oil. The residue was

recrystallized as described above (Method A).

[Ir{(4-MeC₆H₄)NC(NMe₂)N(4-MeC₆H₄)}(cod)] (1c). The synthesis

and characterization of this compound were described previously, but

the new UV−Vis data diﬀer slightly from those reported earlier.^26,46

UV−Vis (toluene) λ_max, nm (ε): 370 (sh), 418 (1700), 470 (300). All

other characterization data, which were obtained from the same batch

as used for UV−Vis spectroscopy, were indistinguishable from the

reported data.

[Ir{(4-MeC₆H₄)NC(NEt₂)N(4-MeC₆H₄)}(cod)] (1d). Yield (Method

A): 114 mg (64%). Anal. Calcd for C₂₇H₃₆IrN₃: C, 54.52; H, 6.10; N,

7.06. Found: C, 54.81; H, 6.01; N, 7.02. ¹H NMR (300 MHz, CDCl₃,

δ): 7.00 (d, J = 8.6 Hz, 4H, Ar H), 6.72 (d, J = 8.2 Hz, 4H, Ar H), 3.60

(br m, 4H, =CHCH₂−), 2.86 (q, J = 7.1 Hz, 4H, NCH₂CH₃), 2.29 (s,

6H, C₆H₄CH₃), 2.18 (br m, 4H, =CHCH₂−), 1.43 (m, 4H,

=CHCH₂−), 1.02 (t, J = 7.1 Hz, 6H, NCH₂CH₃). ¹³C{¹H} NMR

(75.5 MHz, CDCl₃, δ): 145.2, 131.1, 129.2, and 121.8 (Ar), 60.7

(=CHCH₂−), 41.0 (NCH₂CH₃), 31.8 (=CHCH₂−), 20.8 (C₆H₄CH₃),

12.0 (NCH₂CH₃). EIMS (70 eV) m/z: M^+•calcd for C₂₇H₃₆IrN₃,

595.3; found, 595.2. UV−Vis (toluene) λ_max, nm (ε): 370 (sh), 419

(1700), 471 (300). This complex was also synthesized using Method

B. Yield: 58%. The UV−Vis spectrum of this batch was identical to

that of the product obtained from Method A.

N,N′-Bis(4-methoxyphenyl)thiourea, (4-MeOC₆H₄NH)₂CS. This

compound⁴⁴was synthesized using conditions reported for the

synthesis of a closely related N,N′-diarylthiourea.⁴⁵A solution of

1.12 mL (1.34 g, 7.95 mmol) of 4-methoxyphenyl isothiocyanate

(98%) and 1.00 g (8.12 mmol) of 4-methoxyaniline in 10 mL of

toluene was prepared and heated under reﬂux for 24 h, during which a

purple solid was formed. The reaction mixture was cooled to room

temperature, and the solid was separated by ﬁltration, washed with

1

cold toluene (3 × 5 mL) and dried in vacuo. Yield: 1.94 g (85%). H

NMR (300 MHz, (CD₃)₂SO, δ): 9.41 (s, 2H, NH), 7.30 (d, J = 8.9

Hz, 4H, Ar H), 6.88 (d, J = 8.9 Hz, 4H, Ar H), 3.73 (s, 6H,

C₆H₄OCH₃). EIMS (70 eV) m/z: M^+•calcd for C₁₅H₁₆N₂O₂S, 288.1;

found, 288.2.

N,N-Dimethyl-N′,N″-bis(4-methoxyphenyl)guanidine, (4-

MeOC₆H₄)NC(NMe₂)NH(4-MeOC₆H₄). A solution of 0.60 g (2.08

mmol) of N,N′-bis(4-methoxyphenyl)thiourea and 1.28 mL (2.92 g,

20.56 mmol) of iodomethane in 15 mL of methanol was heated under

reﬂux for 2 h. After the solution was cooled to room temperature, the

volatiles, including excess iodomethane, were removed under reduced

pressure to aﬀord S-methyl-N,N′-bis(4-methoxyphenyl)isothiourea

1

hydroiodide as a yellow solid. Yield: 0.85 g (95%). H NMR (300

MHz, (CD₃)₂SO, δ): 10.68 (br, 2H, NH), 7.33 (br, 4H, Ar H), 7.03

(br, 4H, Ar H), 3.77 (s, 6H, C₆H₄OCH₃), 2.66 (s, 3H, SCH₃).

ESI(+)MS (CH₃CN) m/z: {M − I}⁺calcd for C₁₆H₁₉IN₂O₂S, 303.1;

found, 303.1.

A solution of 0.75 g (1.74 mmol) of S-methyl-N,N′-bis(4-

methoxyphenyl)isothiourea hydroiodide and 3.67 mL of a dimethyl-

amine solution (5.6 M in EtOH; 20.55 mmol) in 15 mL of methanol

was heated in a sealed ﬂask for 7 h at 70 °C. (Caution! Because of the

formation of methanethiol, which is a gas under the reaction conditions, the

pressure in the flask may increase substantially. A shield should be used for

protection.) The solution was cooled to room temperature, and

methanethiol was carefully removed with a stream of N₂. (MeSH was

trapped by routing the gas stream through concentrated HNO₃.)

Upon evaporation of the solvents, N,N-dimethyl-N′,N″-bis(4-

methoxyphenyl)guanidinium iodide was obtained as a yellow solid.

Yield: 0.68 g (91%). ¹H NMR (300 MHz, (CD₃)₂SO, δ): 9.28 (br, 2H,

NH), 7.02 (d, J = 8.9 Hz, 4H, Ar H), 6.87 (d, J = 9.0 Hz, 4H, Ar H),

3.70 (s, 6H, C₆H₄OCH₃), 2.99 (s, 6H, NCH₃). ESI(+)MS (CH₃CN)

m/z: {M − I}⁺calcd for C₁₇H₂₂IN₃O₂, 300.2; found, 300.2.

[Ir{(4-MeOC₆H₄)NC(NMe₂)N(4-MeOC₆H₄)}(cod)] (1e). Yield (Meth-

od B): 116 mg (65%). Anal. Calcd for C₂₅H₃₂IrN₃O₂: C, 50.15; H,

5.39; N, 7.02. Found: C, 50.40; H, 5.58; N, 6.94. ¹H NMR (300 MHz,

CDCl₃, δ): 6.79−6.71 (m, 8H, Ar H), 3.77 (s, 6H, C₆H₄OCH₃), 3.63

(br m, 4H, =CHCH₂−), 2.49 (s, 6H, NCH₃), 2.19 (br m, 4H,

=CHCH₂−), 1.44 (m, 4H, =CHCH₂−). ¹³C{¹H} NMR (75.5 MHz,

CDCl₃, δ): 174.5 (CN₃), 154.7, 140.8, 123.3, and 113.9 (Ar), 60.7

(=CHCH₂−), 55.4 (C₆H₄OCH₃), 38.9 (NCH₃), 31.8 (=CHCH₂−).

EIMS (70 eV) m/z: M^+•calcd for C₂₅H₃₂IrN₃O₂, 599.2; found, 599.2.

UV−Vis (toluene) λ_max, nm (ε): 370 (sh), 419 (1600), 471 (300).

[Ir{(2,6-ⁱPr₂C₆H₃)NC(NMe₂)N(2,6-ⁱPr₂C₆H₃)}(cod)] (1g). This com-

pound was synthesized from Li{(2,6-ⁱPr₂C₆H₃)NC(NMe₂)N-

(2,6-ⁱPr₂C₆H₃)} (prepared in situ as described elsewhere^36,47) and

[{Ir(cod)}₂(μ-Cl)₂] according to the general procedure described

above (Method A). Yield: 110 mg (52%). Anal. Calcd for C₃₅H₅₂IrN₃:

A solution of 0.45 g (1.05 mmol) of N,N-dimethyl-N′,N″-bis(4-

methoxyphenyl)guanidinium iodide in 20 mL of diethyl ether was

treated with 50 mL of a saturated aqueous solution of Na₂CO₃. The

biphasic mixture was vigorously stirred for 1 h, after which the organic

phase was separated and dried over MgSO₄and the volatiles were

removed under reduced pressure. The crude product was puriﬁed by

column chromatography on silica gel using a solution of 5% NEt₃in

diethyl ether as the eluent aﬀording an oﬀ-white solid. Yield: 0.15 g

(47%). Anal. Calcd for C₁₇H₂₁N₃O₂: C, 68.20; H, 7.07; N, 14.04.

1

Found: C, 67.73; H, 7.11; N, 13.75. H NMR (300 MHz, CDCl₃, δ):

1

6.84−6.76 (m, 8H, Ar H), 4.82 (br, 1H, NH), 3.74 (s, 6H,

C₆H₄OCH₃), 2.83 (s, 6H, NCH₃). ¹³C{¹H} NMR (75.5 MHz,

CDCl₃, δ): 155.2, 153.0, 122.1, and 114.8 (Ar), 55.6 (C₆H₄OCH₃),

38.3 (NCH₃). EIMS (70 eV) m/z: M^+•calcd for C₁₇H₂₁N₃O₂, 299.2;

found, 299.4.

General Procedure for the Synthesis of [Ir{ArNC(NR₂)NAr}(cod)]

Complexes 1a−1f. Method A. In a typical procedure, 1.20 mmol of

the N,N-dialkyl-N′,N″-diarylguanidine was dissolved in 15 mL of

degassed diethyl ether. The solution was cooled to −40 °C and 1.1

equiv of methyllithium (1.6 M in Et₂O) was added under stirring. The

resulting solution was allowed to warm to 20 °C and stirred for 1 h.

Upon removal of the volatiles under reduced pressure, the Li⁺salt of

the N,N-dialkyl-N′,N″-diarylguanidinate anion, Li{ArNC(NR₂)-

NAr}·Et₂O, was isolated as a colorless powder and stored under

rigorous exclusion of moisture. The powder could also be recrystal-

lized from a concentrated n-pentane solution at −30 °C to aﬀord a

microcrystalline solid.

C, 59.46; H, 7.41; N, 5.94. Found: C, 59.88; H, 7.30; N, 6.14. H

NMR (300 MHz, CDCl₃, δ): 7.09−6.98 (m, 6H, Ar H), 3.77 (sept, J =

6.8 Hz, 4H, Ar CH(CH₃)₂), 3.36 (br m, 4H, =CHCH₂−), 2.23 (s, 6H,

NCH₃), 2.04 (br m, 4H, =CHCH₂−), 1.37 (d, J = 6.8 Hz, 12H, Ar

CH(CH₃)₂), 1.32 (m, 4H, =CHCH₂−), 1.21 (d, J = 6.9 Hz, 12H, Ar

CH(CH₃)₂). ¹³C{¹H} NMR (75.5 MHz, CDCl₃, δ): 175.4 (CN₃),

143.9, 141.7, 124.0, and 123.1 (Ar), 58.7 (=CHCH₂−), 38.5 (NCH₃),

31.8 (=CHCH₂−), 27.1 (Ar CH(CH₃)₂), 25.3 and 23.4 (Ar

CH(CH₃)₂). EIMS (70 eV) m/z: M^+•calcd for C₃₅H₅₂IrN₃, 707.4;

found, 707.6. UV−Vis (toluene) λ_max, nm (ε): 370 (900), 423 (900),

476 (300).

[Ir{PhNC(NMe₂)NPh}(CO)₂] (2a). A steady stream of CO(g) was

purged through a solution of 100 mg (0.186 mmol) of [Ir{PhNC-

(NMe₂)NPh}(cod)] in 15 mL of diethyl ether for 15 min at 20 °C.

The yellow solution gradually turned purple over the ﬁrst 5 min.

Subsequently, the volatiles were removed under reduced pressure to

aﬀord a purple solid. Because of the low solubility of the solid in n-

2566

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Inorganic Chemistry

Article

pentane, the preparation of a saturated solution suitable for growing

single crystals required about a day. After separation of undissolved

solid, the solution was stored at −30 °C. Formation of yellow single

crystals typically occurred within 2 days. Yield: 61 mg (68%). Anal.

Calcd for C₁₇H₁₆IrN₃O₂: C, 41.97; H, 3.31; N, 8.64. Found: C, 42.09;

CH(CH₃)₂). EIMS (70 eV) m/z: M^+•calcd for C₂₉H₄₀IrN₃O₂, 655.3;

found, 655.5. IR (n-pentane, cm⁻¹): 2052 and 1980 (ν_CO). IR (KBr,

cm⁻¹): 2044 and 1966 (ν_CO). UV−Vis (toluene) λ_max, nm (ε): 353

(2300), 380 (sh), 410 (sh).

2.3. Generation and Reactivity of Ir^IIIComplexes. Reactions

of 1a−1g with O₂. A 2 mM solution of [Ir{ArNC(NR₂)NAr}(cod)]

(1a−1g, 0.002 mmol) in 1 mL of toluene was placed in a 0.5-cm UV−

Vis cuvette and precooled to 0 °C. The solution was purged with

O₂(g) for 40 s and kept under an O₂atmosphere. The half-lives of the

reactions were determined by UV−Vis spectroscopy, and the reported

values represent averages of three runs. The reaction of 1a with O₂was

also investigated using a single-wavelength spectrophotometer (λ =

419 nm) with minimal exposure time and under protection from

ambient light, showing that the half-life was not aﬀected by light from

the UV lamp of the instrument or by ambient light.

1

H, 3.20; N, 8.59. H NMR (300 MHz, C₆D₆, δ): 7.06 (t, J = 8.1 Hz,

4H, Ar H), 6.85 (d, J = 8.1 Hz, 4H, Ar H), 6.81 (t, J = 7.3 Hz, 2H, Ar

H), 1.82 (s, 6H, NCH₃). ¹³C{¹H} NMR (75.5 MHz, C₆D₆, δ): 175.6

(IrCO), 171.1 (CN₃), 148.2, 129.3, 123.3, and 123.0 (Ar), 38.0

(NCH₃). EIMS (70 eV) m/z: M^+•calcd for C₁₇H₁₆IrN₃O₂, 487.1;

found, 487.1. IR (n-pentane, cm⁻¹): 2055 and 1983 (ν_CO). IR (KBr,

cm⁻¹): 2042 and 1956 (ν_CO). UV−Vis (toluene) λ_max, nm: 355 (sh). It

should be noted that solutions of 2a in n-pentane, benzene-d₆, and

diethyl ether slowly turned pink upon standing for several hours. No

changes, however, were observed in the IR (n-C₅H₁₂, C₆D₆and Et₂O)

and ¹H NMR spectra (C₆D₆) of these solutions. Upon standing at 20

°C, 2a was recovered from the n-pentane solution as a yellow

microcrystalline solid.

Synthesis and Characterization of [Ir{ArNC(NR₂)NAr}(CO)₂]

Complexes 2b−2g. The iridium dicarbonyls 2b−2g were synthesized

in a manner analogous to 2a. The reaction solutions turned purple (2b

and 2c), yellow-orange (2d, 2f, and 2g) or green (2e) over a similar

time frame as observed for 2a. Between 50 and 100 mg of the Ir^I(cod)

complexes was used, and the corresponding Ir^I(CO)₂complexes were

obtained as yellow solids in yields of 65−80% (2b−2f, after

recrystallization) and 48% (2g).

To investigate reversibility, the reaction of 2 mM 1f (0.002 mmol)

in 1 mL of toluene with O₂at 20 °C was monitored by UV−Vis

spectroscopy until the decay of 1f leveled oﬀ (ca. 2 h). The solution

was then purged with Ar for 1 min to remove O₂, after which the

regeneration of 1f was monitored.

Generation and Characterization of [Ir{ArNC(NR₂)NAr}(η⁴-cod)-

(η²-O₂)] Complexes 3b−3f. Complexes 3b−3f were generated as

recently described for [Ir{PhNC(NMe₂)NPh}(η⁴-cod)(η²-O₂)]

(3a).³⁸In a typical experiment, a 15−20 mM solution of 1b−1f

(0.0075−0.010 mmol) in 0.5 mL of C₆D₆was placed in an NMR tube,

purged with O₂(g) at 20 °C for 40 s, and kept under an O₂atmosphere

for the remainder of the reaction. The progress of the reaction was

monitored by ¹H NMR spectroscopy. Under these conditions, 1b−1e

(bright yellow solutions) converted into the corresponding

intermediates, 3b−3e (pale yellow solutions), in about 2−3 h, and

the intermediates were typically decayed (dark green solutions) within

20 h after the addition of O₂to 1b−1e. Reversible formation of 3f

reached its maximum within half a day (ca. 60% relative to 1,2-

dichloroethane added as a standard). For IR spectroscopy, 0.15 mL of

a solution of 3b−3f was mixed with KBr. The mixture was evaporated

to dryness (20 °C, in vacuo) and pressed into a disk.

[Ir{PhNC(NEt₂)NPh}(CO)₂] (2b). Anal. Calcd for C₁₉H₂₀IrN₃O₂: C,

1

44.35; H, 3.92; N, 8.17. Found: C, 44.64; H, 3.87; N, 8.20. H NMR

(300 MHz, C₆D₆, δ): 7.11−7.01 (m, 8H, Ar H), 6.82 (t, J = 7.1 Hz,

2H, Ar H), 2.45 (q, J = 7.2 Hz, 4H, NCH₂CH₃), 0.43 (t, J = 7.2 Hz,

6H, NCH₂CH₃). EIMS (70 eV) m/z: M^+•calcd for C₁₉H₂₀IrN₃O₂,

515.1; found, 515.2. IR (n-pentane, cm⁻¹): 2055 and 1982 (ν_CO). IR

(KBr, cm⁻¹): 2051 and 1963 (ν_CO). UV−Vis (toluene) λ_max, nm: 355

(sh).

[Ir{(4-MeC₆H₄)NC(NMe₂)N(4-MeC₆H₄)}(CO)₂] (2c). Anal. Calcd for

C₁₉H₂₀IrN₃O₂: C, 44.35; H, 3.92; N, 8.17. Found: C, 44.28; H, 3.87;

N, 8.08. ¹H NMR (300 MHz, C₆D₆, δ): 6.90 (d, J = 8.1 Hz, 4H, Ar H),

6.82 (d, J = 8.5 Hz, 4H, Ar H), 2.11 (s, 6H, C₆H₄CH₃), 1.89 (s, 6H,

NCH₃). EIMS (70 eV) m/z: M^+•calcd for C₁₉H₂₀IrN₃O₂, 515.1;

found, 515.1. IR (n-pentane, cm⁻¹): 2054 and 1981 (ν_CO). IR (KBr,

cm⁻¹): 2044 and 1970 (ν_CO). UV−Vis (toluene) λ_max, nm: 355 (sh).

[Ir{(4-MeC₆H₄)NC(NEt₂)N(4-MeC₆H₄)}(CO)₂] (2d). Anal. Calcd for

C₂₁H₂₄IrN₃O₂: C, 46.48; H, 4.46; N, 7.74. Found: C, 46.48; H, 4.52;

N, 7.69. ¹H NMR (300 MHz, C₆D₆, δ): 6.98 (d, J = 8.4 Hz, 4H, Ar H),

6.89 (d, J = 8.0 Hz, 4H, Ar H), 2.52 (q, J = 7.1 Hz, 4H, NCH₂CH₃),

2.10 (s, 6H, C₆H₄CH₃), 0.49 (t, J = 7.1 Hz, 6H, NCH₂CH₃). EIMS

(70 eV) m/z: M^+•calcd for C₂₁H₂₄IrN₃O₂, 543.2; found, 543.3. IR (n-

pentane, cm⁻¹): 2053 and 1980 (ν_CO). IR (KBr, cm⁻¹): 2053 and 1958

(ν_CO). UV−Vis (toluene) λ_max, nm: 355 (sh).

[Ir{(4-MeOC₆H₄)NC(NMe₂)N(4-MeOC₆H₄)}(CO)₂] (2e). Anal. Calcd

for C₂₀H_22.5IrN₃O_4.25(2e·0.25Et₂O): C, 42.51; H, 4.01; N, 7.44.

Found: C, 42.31; H, 3.73; N, 7.54. ¹H NMR (300 MHz, C₆D₆, δ): 6.82

(d, J = 9.0 Hz, 4H, Ar H), 6.69 (d, J = 9.0 Hz, 4H, Ar H), 3.32 (s, 6H,

C₆H₄OCH₃), 1.93 (s, 6H, NCH₃). EIMS (70 eV) m/z: M^+•calcd for

C₁₉H₂₀IrN₃O₄, 547.1; found, 547.3. IR (n-pentane, cm⁻¹): 2053 and

1980 (ν_CO). IR (KBr, cm⁻¹): 2041 and 1962 (ν_CO). UV−Vis (toluene)

λ_max, nm: 355 (sh).

[Ir{PhNC(NEt₂)NPh}(η⁴-cod)(η²-O₂)] (3b). ¹H NMR (300 MHz,

C₆D₆, δ): 7.44 (d, J = 7.3 Hz, 4H, Ar H), 7.18−7.09 (m, Ar H; these

signals partially overlap with the residual solvent peak), 6.96 (t, J = 7.4

Hz, 1H, Ar H), 6.89 (t, J = 7.3 Hz, 1H, Ar H), 4.76−4.67 (br m, 1H,

=CHCH₂−), 4.55−4.46 (br m, 1H, =CHCH₂−), 4.18−4.00 (br m,

2H, =CHCH₂−), 2.83 (m, 2H, NCH₂CH₃), 2.39−1.86 (br m, 6H,

=CHCH₂−), 2.33 (m, 2H, NCH₂CH₃), 1.71−1.55 (br m, 2H,

=CHCH₂−), 0.62 (dd, J = 7.1 Hz, J = 7.1 Hz, 6H, NCH₂CH₃). IR

(KBr, cm⁻¹): 865 (ν_OO), 576 and 459 (ν_IrO).

[Ir{(4-MeC₆H₄)NC(NMe₂)N(4-MeC₆H₄)}(η⁴-cod)(η²-O₂)] (3c). ¹H

NMR (300 MHz, C₆D₆, δ): 7.30 (d, J = 7.9 Hz, 2H, Ar H), 7.25

(d, J = 8.3 Hz, 2H, Ar H), 6.98 (d, J = 7.9 Hz, 2H, Ar H), 6.93 (d, J =

8.0 Hz, 2H, Ar H), 4.80−4.70 (br m, 1H, =CHCH₂−), 4.64−4.53 (br

m, 1H, =CHCH₂−), 4.20−4.10 (br m, 1H, =CHCH₂−), 4.10−4.00

(br m, 1H, =CHCH₂−), 2.42−1.95 (br m, 6H, =CHCH₂−), 2.12 (s,

3H, C₆H₄CH₃), 2.11 (s, 3H, C₆H₄CH₃), 2.09 (s, 6H, NCH₃), 1.77−

1.58 (br m, 2H, =CHCH₂−). IR (KBr, cm⁻¹): 862 (ν_OO), 575 and 459

(ν_IrO).

[Ir{(4-MeC₆H₄)NC(NEt₂)N(4-MeC₆H₄)}(η⁴-cod)(η²-O₂)] (3d). ¹H

NMR (300 MHz, C₆D₆, δ): 7.38 (d, J = 8.3 Hz, 4H, Ar H), 6.99

(d, J = 8.4 Hz, 2H, Ar H), 6.95 (d, J = 8.1 Hz, 2H, Ar H), 4.78−4.72

(br m, 1H, =CHCH₂−), 4.61−4.53 (br m, 1H, =CHCH₂−), 4.20−

4.06 (br m, 2H, =CHCH₂−), 2.91 (m, 2H, NCH₂CH₃), 2.38 (m, 2H,

NCH₂CH₃), 2.30−1.96 (br m, 6H, =CHCH₂−), 2.11 (s, 3H,

C₆H₄CH₃), 2.10 (s, 3H, C₆H₄CH₃), 1.76−1.61 (br m, 2H,

=CHCH₂−), 0.67 (dd, J = 7.1 Hz, J = 7.1 Hz, 6H, NCH₂CH₃). IR

(KBr, cm⁻¹): 862 (ν_OO), 575 and 458 (ν_IrO).

[Ir{(2,6-Me₂C₆H₃)NC(NMe₂)N(2,6-Me₂C₆H₃)}(CO)₂] (2f). Anal. Calcd

for C₂₁H₂₄IrN₃O₂: C, 46.48; H, 4.46; N, 7.74. Found: C, 46.63; H,

4.40; N, 7.68. ¹H NMR (300 MHz, C₆D₆, δ): 6.93 (d, J = 7.8 Hz, 4H,

Ar H), 6.85 and 6.83 (dd, J = 6.7 Hz, J = 8.5 Hz, 2H, Ar H), 2.40 (s,

12H, C₆H₃(CH₃)₂), 1.69 (s, 6H, NCH₃). EIMS (70 eV) m/z: M^+•

calcd for C₂₁H₂₄IrN₃O₂, 543.2; found, 543.3. IR (n-pentane, cm⁻¹):

2053 and 1980 (ν_CO). IR (KBr, cm⁻¹): 2044 and 1977 (ν_CO). UV−Vis

(toluene) λ_max, nm (ε): 353 (2400), 380 (sh), 410 (sh).

1

[Ir{(4-MeOC₆H₄)NC(NMe₂)N(4-MeOC₆H₄)}(η⁴-cod)(η²-O₂)] (3e). H

NMR (300 MHz, C₆D₆, δ): 7.32 (d, J = 8.8 Hz, 2H, Ar H), 7.26 (d, J =

8.6 Hz, 2H, Ar H), 6.78 (d, J = 8.9 Hz, 2H, Ar H), 6.73 (d, J = 8.7 Hz,

2H, Ar H), 4.78−4.67 (br m, 1H, =CHCH₂−), 4.66−4.58 (br m, 1H,

=CHCH₂−), 4.22−4.13 (br m, 1H, =CHCH₂−), 4.08−3.99 (br m,

1H, =CHCH₂−), 3.34 (s, 3H, C₆H₄OCH₃), 3.32 (s, 3H, C₆H₄OCH₃),

2.40−2.15 (br m, 4H, =CHCH₂−), 2.15−1.97 (br m, 2H,

[Ir{(2,6-ⁱPr₂C₆H₃)NC(NMe₂)N(2,6-ⁱPr₂C₆H₃)}(CO)₂] (2g). Anal. Calcd

for C₂₉H₄₀IrN₃O₂: C, 53.19; H, 6.16; N, 6.42. Found: C, 53.39; H,

1

6.27; N, 6.41. H NMR (300 MHz, C₆D₆, δ): 7.06−7.03 (m, 6H, Ar

H), 3.85 (sept, J = 6.8 Hz, 4H, Ar CH(CH₃)₂), 1.88 (s, 6H, NCH₃),

1.46 (d, J = 6.8 Hz, 12H, Ar CH(CH₃)₂), 1.21 (d, J = 6.9 Hz, 12H, Ar

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a

Table 1. Crystallographic Data and Structure Reﬁnement Parameters for 1b, 1d, 1f, and 2b

1b

C₂₅H₃₂IrN₃

1d

C₂₇H₃₆IrN₃

1f

C₂₇H₃₆IrN₃

2b

empirical formula

formula weight

temperature, T

wavelength, λ

C₁₉H₂₀IrN₃O₂

514.58

566.74

594.79

209(2) K

210(2) K

0.71073 Å

triclinic

190(2) K

0.71073 Å

monoclinic

P2₁/c

190(2) K

0.71073 Å

tetragonal

0.71073 Å

crystal system

monoclinic

space group

P2₁/n

P1

̅

P4n2

̅

unit cell dimensions

a = 10.2528(11) Å

b = 17.8077(19) Å

c = 12.6997(14) Å

a = 9.5336(11) Å

b = 9.6634(11) Å

c = 13.6109(15) Å

α = 87.444(5)°

β = 89.343(5)°

γ = 74.719(5)°

1208.4(2) Å³

2

a = 15.1915(16) Å

b = 7.6661(9) Å

c = 20.783(3) Å

a = 17.3359(18) Å

c = 6.1770(7) Å

β = 107.034(5)°

β = 94.776(5)°

volume, V

2217.0(4) Å³

4

2412.0(5) Å³

4

1856.4(3) Å³

4

Z

calculated density

absorption coeﬃcient, μ

reﬂections collected

1.698 Mg·m⁻³

6.038 mm⁻¹

19485

1.635 Mg·m⁻³

5.543 mm⁻¹

10816

1.638 Mg·m⁻³

5.554 mm⁻¹

19363

1.841 Mg·m⁻³

7.209 mm⁻¹

11591

b

independent reﬂections

reﬂections with I > 2σ(I)

data/restraints/parameters

5287 (R_int= 0.0388)

4310

5740 (R_int= 0.0166)

5298

5510 (R_int= 0.0369)

4630

2215 (R_int= 0.0253)

2052

5287/4/276

1.035

5740/4/296

1.118

5510/4/298

1.029

2215/0/117

1.210

c

goodness-of-ﬁt on F²

de

,

ﬁnal R indices [I > 2σ(I)]

R1 = 0.0246,

wR2 = 0.0472

R1 = 0.0371,

wR2 = 0.0505

R1 = 0.0225,

wR2 = 0.0516

R1 = 0.0264,

wR2 = 0.0531

R1 = 0.0224,

wR2 = 0.0437

R1 = 0.0332,

wR2 = 0.0468

R1 = 0.0275,

wR2 = 0.0857

R1 = 0.0308,

wR2 = 0.0873

0.08(2)

de

,

R indices (all data)

absolute structure parameter

largest diﬀ. peak and hole

0.922 and −0.707 e·A⁻³

1.108 and −1.175 e·A⁻³

1.386 and −0.775 e·A⁻³

c

1.281 and −0.571 e·A⁻³

a

b

d

Data for 1b and 1f taken from structures reported in ref 26. R_int= ∑|F_o− ⟨F_o⟩|/∑[F_o]. GOF = S = {∑[w(F_o²− F_c²)²]/(n − p)}^1/2. R1 =

2

e

∑||F_o|− |F_c||/∑|F_o|. wR2 = {∑[w(F_o− F_c²)²]/∑[w(F_o)²]}^1/2, where w = 1/[σ²(F_o) + (aP)²+ bP] and P = (F_o+ 2F_c²)/3.

2

=CHCH₂−), 2.12 (s, 6H, NCH₃), 1.82−1.57 (br m, 2H, =CHCH₂−).

IR (KBr, cm⁻¹): 857 (ν_OO), 573 and 457 (ν_IrO).

The biphasic mixture was vigorously stirred for 1 h. The aqueous

phase was separated and combined with 5 mL of diethyl ether and 15

mL of a saturated Na₂CO₃solution. The biphasic mixture was

vigorously stirred for 1 h, after which the organic phase was separated

and dried over MgSO₄and the volatiles were removed under reduced

pressure. Yield: 2.9 mg (67%). The ¹H NMR and ESI(+)MS data were

indistinguishable from those of the guanidine ligand precursor²⁶and

[Ir{(2,6-Me₂C₆H₃)NC(NMe₂)N(2,6-Me₂C₆H₃)}(η⁴-cod)(η²-O₂)] (3f).

¹H NMR (300 MHz, C₆D₆, δ): 7.04−6.83 (Ar H, 3f and 1f), 5.25−

5.17 (br m, 1H, =CHCH₂−, 3f), 4.79−4.69 (br m, 2H, =CHCH₂−,

3f), 4.08−3.98 (br m, 1H, =CHCH₂−, 3f), 3.59 (br m, 4H,

=CHCH₂−, 1f), 2.90 (s, 3H, C₆H₃(CH₃)₂, 3f), 2.64 (s, 3H,

C₆H₃(CH₃)₂, 3f), 2.57 (s, 12H, C₆H₃(CH₃)₂, 1f), 2.5−1.0

(=CHCH₂−, 3f and 1f), 2.22 (s, 3H, C₆H₃(CH₃)₂, 3f), 2.18 (s, 3H,

C₆H₃(CH₃)₂, 3f), 1.89 (s, 6H, NCH₃, 3f), 1.83 (s, 6H, NCH₃, 1f). IR

(KBr, cm⁻¹): 872 (ν_OO), 572 and 459 (ν_IrO).

1

did not reveal any other products. H NMR (300 MHz, CDCl₃, δ):

7.20 (t, J = 6.8 Hz, 4H, Ar H), 7.01 (t, J = 7.4 Hz, 2H, Ar H), 6.92 (d, J

= 7.6 Hz, 4H, Ar H), 2.86 (s, 6H, NCH₃). ESI(+)MS (CH₃CN) m/z:

{M + H}⁺calcd for C₁₅H₁₇N₃, 240.2; found, 240.2.

2.4. X-ray Crystallographic Analyses. A single crystal of each

compound was coated with Paratone N oil and mounted on a glass

capillary for data collection at 210(2) or 190(2) K on a Nonius

KappaCCD diﬀractometer using Mo Kα radiation (graphite

monochromator). The temperature was controlled by an Oxford

Cryostream Cooler (700 series, N₂gas). Data collection, data

reduction, and absorption correction were carried out following

standard CCD techniques using the software packages Collect and

HKL-2000.^48,49Final cell constants were calculated from 5525 (1d) or

8035 (2b) reﬂections from the complete data set. The space groups P1

Reaction of 3a with CO. A solution of 3a in C₆D₆was prepared as

previously described (ca. 3 h for the formation of 3a),³⁸then purged

with CO(g) for 10 min and kept under a CO atmosphere at 20 °C for

1 d. Complex 2a (IR, ¹H NMR), free cod (¹H NMR), and

[Ir{PhNC(NMe₂)NPh}(cod)(CO₃)] (4a, ESI MS, IR) were identiﬁed

as products. Quantiﬁcation of 2a and cod was carried out against 1,2-

dichloroethane (δ, 2.90 ppm) as a standard, using the NMe₂(2a) and

1

alkene (cod) proton signals. H NMR (300 MHz, C₆D₆, δ): 7.07 (m,

Ar H, 2a), 6.89−6.78 (m, Ar H, 2a), 5.57 (br m, 4H, =CHCH₂−, free

cod), 2.20 (br m, 8H, =CHCH₂−, free cod), 1.79 (s, 6H, NCH₃, 2a).

ESI(+)MS (m/z): {4a + H}⁺calcd for C₂₄H₂₈IrN₃O₃, 600.2; found,

538.2 ({4a − HCO₃}⁺), 554.2 ({3a − OH}⁺), 570.2 ({3a − H}⁺),

̅

(1d) and P4n2 (2b) were determined based on systematic absences

̅

(or lack thereof) and intensity statistics. The structures were solved by

direct methods and reﬁned by full-matrix least-squares minimization

and diﬀerence Fourier methods (SHELXTL v.6.12).^50,51All non-

hydrogen atoms were reﬁned with anisotropic displacement

parameters. All hydrogen atoms were placed in ideal positions and

reﬁned as riding atoms with relative isotropic displacement parameters,

with the exception of the alkene hydrogen atoms (=CH) in 1d, which

were located in the diﬀerence Fourier map and reﬁned with a

restrained C−H distance (1.00 Å) and relative isotropic displacement

parameters (1.2 times the equivalent isotropic value of the bonded

carbon atom). For 1d, the ﬁnal full-matrix least-squares reﬁnement

converged to R1 = 0.0264 and wR2 = 0.0531 (F², all data). The

600.2 ({4a + H}⁺), 622.2 ({4a + Na}⁺). IR (KBr, cm⁻¹): 2046 (ν_CO

,

2a), 2020 (ν_CO), 1969 (ν_CO, 2a), 1690 (ν_CO, 4a). For 3a-¹⁸O₂+ CO,

ESI(+)MS (m/z): 538.2 ({4a − HCO₃}⁺), 556.2 ({3a − OH}⁺),

574.2 ({3a − H}⁺), 604.2 ({4a + H}⁺), 626.2 ({4a + Na}⁺). IR (KBr,

cm⁻¹): 2046 (ν_CO, 2a), 2014 (ν_CO), 1969 (ν_CO, 2a), 1690 (ν_CO, 4a),

1667 (ν_CO, 4a).

Recovery of N,N-Dimethyl-N′,N″-diphenylguanidine, PhNC-

(NMe₂)NHPh, from the Decay Products of 3a. A solution of 9.8

mg (0.018 mmol) of 1a in 3 mL of toluene was purged with O₂(40 s)

at 20 °C. After 16 h, 3.0 μL (3.6 mg, 0.036 mmol) of concentrated

HCl and 3 mL of water were added to the resulting green solution.

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a

Scheme 1. Synthesis of N,N-Dialkyl-N′,N″-diarylguanidines

a

(i) NHR₂, thf, 0 °C, N(CH₂Ph)Et₃MnO₄; (ii) MeI, MeOH, reﬂux, 2 h; (iii) NHR₂, MeOH, 70 °C, 7 h; (iv) Na₂CO₃, H₂O−Et₂O.

structure of 2b was inverted during reﬁnement and subsequently

diarylcarbodiimides and N,N-dimethylamine [i.e., ArNC-

(NR₂)NHAr, where R = Me and Ar = Ph, 4-MeC₆H₄or 4-

MeOC₆H₄].^56,57In various other cases, the preparation of

tetrasubstituted lithium guanidinates from carbodiimides and

lithium amides has been the method of choice.^28h,36,47,58But,

considering the limited availability of carbodiimides, the routes

via thioureas appeared to be more straightforward and generally

applicable for the targeted range of substitution patterns. The

six guanidines described here were characterized by elemental

analysis, ¹H and ¹³C NMR spectroscopy, and EI mass

spectrometry (cf. the Experimental Section and ref 26).

reﬁned as an inversion twin, yielding a Flack x parameter of 0.08(2).

The ﬁnal full-matrix least-squares reﬁnement converged to R1 =

0.0308 and wR2 = 0.0873 (F², all data). Table 1 and Supporting

Information, Table S2 contain crystallographic data and parameters of

the data collection and structure reﬁnement. Selected distances and

angles are summarized in Supporting Information, Tables S3−S5.

3. RESULTS

3.1. Synthesis and Characterization. In this section, the

synthesis and characterization of the N,N-dialkyl-N′,N″-diaryl-

guanidines, used as ligand precursors for the complexes in this

study, and two series of Ir^Icomplexes of the anionic

guanidinates are described (Chart 1). The ligands carry either

methyl or ethyl groups on the distal amine nitrogen atom and a

range of aryl groups (i.e., Ph, 4-MeC₆H₄, 4-MeOC₆H₄, 2,6-

Me₂C₆H₃, or 2,6-ⁱPr₂C₆H₃) on the coordinating amidinate

nitrogen atoms.

3.1.2. (Cyclooctadiene)iridium(I) Complexes. Two diﬀerent

methods were employed for the synthesis of the air-sensitive

Ir^I(cod) complexes of the anionic guanidinates, [Ir{ArNC-

(NR₂)NAr}(cod)], 1a−1g (Scheme 2). In one method, the

a

Scheme 2. Synthesis of Alkene Complexes 1a−1g

3.1.1. N,N-Dialkyl-N′,N″-diarylguanidines. The six neutral

guanidines, ArNC(NR₂)NHAr, were obtained from N,N′-

diarylthiourea and N,N-dialkylamine building blocks (Scheme

1). While the thioureas ArHNC(S)NHAr with Ar = Ph and 4-

MeC₆H₄are commercially available, those with Ar = 4-

44

52

MeOC₆H₄and 2,6-Me₂C₆H₃were synthesized by addition

of the appropriate aniline, ArNH₂, to the corresponding

arylisothiocyanate, ArNCS, under conditions reported for the

closely related N,N′-bis(2,4,6-trimethylphenyl)thiourea.⁴⁵The

diphenyl-substituted guanidines PhNC(NMe₂)NHPh²⁶and

PhNC(NEt₂)NHPh^26,53were prepared in a convenient one-

pot procedure by oxidation of N,N′-diphenylthiourea with

benzyltriethylammonium permanganate in the presence of a 2-

fold excess of N,N-dialkylamine and were obtained in yields of

70−80% after recrystallization [Scheme 1, step (i)]. Other

procedures reported for the desulfurization of N,N′-diphenylth-

iourea to produce guanidines involved reaction with CuSO₄,

SiO₂, and NEt₃or oxidation with NaClO₂or NaIO₄.⁵⁴

For guanidines with substituted aryl groups [i.e., ArN

C(NR₂)NHAr, where R = Me, Ar = 4-MeC₆H₄;²⁶R = Et, Ar =

4-MeC₆H₄; R = Me, Ar = 4-MeOC₆H₄; and R = Me, Ar = 2,6-

Me₂C₆H₃²⁶], a diﬀerent method⁵⁵has proven more successful.

As shown in Scheme 1, steps (ii)−(iv), the appropriate thiourea

was converted into the S-methyl-N,N′-diarylisothiourea hydro-

iodide prior to reaction with an excess of amine to aﬀord the

guanidinium iodide. Deprotonation of the latter with Na₂CO₃

gave the guanidine in good overall yield (40−75%, after column

chromatography or recrystallization). It should be noted that

some of the guanidines in this study are also accessible from

a

(i) 1.1 equiv of MeLi, Et₂O, −40 → 20 °C; (ii) 0.5 equiv of

[Ir(cod)Cl]₂, Et₂O; (iii) 0.5 equiv of [Ir(cod)(OMe)]₂, Et₂O.

guanidine was deprotonated with MeLi at low temperature,

followed by reaction of the resulting lithium salt with

[{Ir(cod)}₂(μ-Cl)₂] to yield the yellow Ir^Icomplex of the

guanidinato(1−) ligand. Alternatively, [{Ir(cod)}₂(μ-OMe)₂]

reacted cleanly with the guanidine to give the desired complex,

with the bridging methoxido ligands acting as an internal base

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for deprotonation of the guanidine. While the deprotonation−

transmetalation route allowed the preparation of the diphenyl,

bis(4-methylphenyl) and bis(2,6-dimethylphenyl) derivatives

1a−1d and 1f, it only gave an impure product in poor yield in

case of the bis(4-methoxyphenyl) derivative 1e. This

compound was instead directly synthesized from the guanidine

using [{Ir(cod)}₂(μ-OMe)₂]. When we tested this method on

selected other examples (e.g., 1a and 1d), we found it to be

more reliable and produce good yields more consistently than

the initially used deprotonation−transmetalation method.

Complex 1g (where R = Me and Ar = 2,6-ⁱPr₂C₆H₃) was

prepared from the corresponding lithium guanidinate.³⁶The 7

guanidinato complexes are well soluble in nonpolar organic

solvents, can be recrystallized from n-pentane, and were

obtained as analytically pure solids.

Scheme 3. Synthesis of Dicarbonyl Complexes 2a−2g

Although the substitution reactions were accompanied by

conspicuous color changes (cf. the Experimental Section), the

isolated dicarbonyls appear yellow to yellow-orange after

recrystallization. The ¹H NMR spectra of 2a−2g and the

solid-state structure of 2b (cf. Section 3.2) conﬁrm that the

guanidinato(1−) ligands coordinate the Ir center in a

symmetric N,N′-chelating binding mode.

All complexes were characterized by ¹H and ¹³C NMR

spectroscopy, mass spectrometry, and UV−Vis spectroscopy,

and three complexes were also characterized by X-ray

Consistent with the presence of two cis-coordinated CO

ligands, two intense bands were observed in the CO stretching

region (ν_CO) of the IR spectra of 2a−2g. In n-pentane solution,

the energies of these bands fall in the ranges of 1980−1983 and

2052−2055 cm⁻¹(A and B modes), and these data are further

discussed in Section 3.4 (Table 3). Bands between 1400 and

1600 cm⁻¹, where CN stretching vibrations of the guanidinato

CN₃core are expected, could be observed in the IR spectra of

solid samples. The spectra of the N,N-dimethyl derivatives 2a,

2c, and 2e (R = Me and Ar = Ph, 4-MeC₆H₄or 4-MeOC₆H₄)

agree strikingly well and each exhibit three principal features at

about 1420, 1500, and 1580 cm⁻¹(Supporting Information,

Figure S2). Two of the three guanidinato ν_CNmodes may be

expected to fall into this region. For comparison, the CN

stretching vibrations of the unsubstituted guanidinium cation

were found around 1000 and 1650 cm⁻¹(A₁′ and E′ modes in

D_3h).⁵⁹In addition, arene CC stretching (likely at 1500 and

1580 cm⁻¹) and alkyl CH deformation vibrations (<1480

cm⁻¹) may give rise to absorptions in this region. Similarities in

the 1400−1600 cm⁻¹region were also observed for the N,N-

diethyl derivatives 2b and 2d (R = Et and Ar = Ph or 4-

MeC₆H₄) as well as the N,N′-bis(2,6-dialkylphenyl) derivatives

2f and 2g (R = Me and Ar = 2,6-Me₂C₆H₃or 2,6-ⁱPr₂C₆H₃;

Supporting Information, Figure S2).

1

crystallography (cf. Section 3.2). Each of the H NMR spectra

of 1a−1f exhibits only four or ﬁve resonance signals attributable

to the guanidinato-κ²N,N′ ligand and three to the η⁴-cod ligand.

Coordination of the guanidinato ligand to the Ir center (1a−

1g) is most clearly discerned from the diagnostic peak of the N-

methyl or N-methylene protons, which undergoes an upﬁeld

shift of 0.3−0.6 ppm relative to the neutral guanidine. As

expected for an alkene complex, the δ(¹H_CCH) and δ(¹³C_CC

)

values for the cod ligands in 1a−1g are signiﬁcantly lower than

those for the free alkene (cf. Section 3.4). While the molecules

1b, 1d, and 1f have nearly C₂symmetry in the solid state, the

NMR spectra indicate ﬂuxional behavior in solution with only

one set of resonance signals for the ortho (or ortho methyl

protons in 1f), meta and alkene protons as well as two signals

for the cod CH₂protons. No signiﬁcant changes were observed

in variable-temperature ¹H NMR measurements for 1a and the

bulkier 1f between 20 and −80 °C.

The UV−Vis spectra of the guanidinato complexes exhibit

absorption features near 370, 420, and 470 nm (Figure 1).

3.2. Crystal Structures. The structures of the cod

complexes 1b and 1f were brieﬂy reported in a previous

publication.²⁶Crystallographic analyses were carried out also

on single crystals of 1d and the dicarbonyl complex 2b.

Relevant structural data of these four compounds are

summarized in Table 2, and the molecular structures of 1d

and 2b are shown in Figures 2 and 3.

Complex 1d crystallized in the triclinic space group P1. The

̅

Ir−N distances are 2.078(2) and 2.087(2) Å, the Ir−C

distances are in the range of 2.104(3)−2.115(3) Å, and the

N−Ir−N angle is 63.76(9)° (Table 2 and Supporting

Information). These parameters are similar to those of 1b

and 1f, and the Ir−N and Ir−C distances fall into ranges found

for Ir^I(cod) complexes of other monoanionic nitrogen-donor

ligands.⁶⁰⁻⁶⁴The parameters of the guanidinato ligand are

consistent with signiﬁcant lone-pair donation from the NR₂

nitrogen atom into the CN₃core. In particular, the three C1−N

distances of 1.347(4)−1.359(4) Å are essentially equal,

indicating eﬀective π-electron delocalization over the three

C−N bonds. Also, the bond angle sum for the N3 atom of

359.9(5)° indicates trigonal-planar geometry in agreement with

sp²hybridization.

Figure 1. Electronic absorption spectra of 2 mM 1a−1g in toluene

(path length, 0.5 cm). Color key: 1a, solid black line; 1b, dashed black

line; 1c, solid red line; 1d, dashed red line; 1e, solid blue line; 1f, solid

green line; 1g, solid brown line.

According to the appearance of the bands below 450 nm, the

complexes can be divided into two groups. Complexes 1a−1e

each display one absorption maximum and a shoulder, whereas

the 2,6-dialkylphenyl-substituted complexes 1f and 1g show

two maxima, each with about half the extinction coeﬃcient

observed for 1a−1e.

3.1.3. Dicarbonyliridium(I) Complexes. As shown in

Scheme 3, reaction of 1a−1g with an excess of CO produced

the dicarbonyls [Ir{ArNC(NR₂)NAr}(CO)₂], 2a−2g.

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a b

,

Table 2. Selected Structural Data for 1b, 1d, 1f, and 2b

1b

1d

1f

2b

Interatomic Distances (Å)

2.083(2)

Ir−N1/N2

Ir−C

C1−N1/N2

C1−N3

2.077(2)

2.112(3)

1.355(4)

1.346(4)

1.408(5)

2.087(2)

2.111(3)

1.343(4)

1.354(4)

1.409(5)

2.069(6)

1.832(9)

1.359(8)

1.338(11)

2.109(3)

1.349(4)

1.359(4)

C−C (CC)

1.418(5)

C−O (CO)

1.171(11)

Angles (deg)

63.76(9)

N1−Ir−N2

N1−C1−N2

N3−C1−N1/N2

63.68(9)

108.0(3)

126.1(3)

63.72(9)

110.2(2)

124.9(3)

63.8(3)

109.3(2)

107.2(8)

126.4(4)

125.4(3)

Bond Angle Sums (deg)

352.4(3)

∑ (X−N1−X)

∑ (X−N2−X)

∑ (X−N3−X)

349.9(4)

352.4(4)

359.9(5)

346.0(3)

352.5(3)

359.8(4)

354.2(9)

360.0(9)

352.5(3)

359.9(5)

Dihedral Angles (deg)

1.6(3)

N1−C1−N2/N1−Ir−N2

1.3(2)

3.5(3)

0

c

C−N3−C/N1−C1−N2

25.6(1)

54.5(1)

51.7(1)

33.9(2)

18.7(3)

78.6(1)

74.1(1)

39.6(6)

44.7(3)

d

CN₃/Ar C₆

48.0(1)

46.1(1)

a

Numbering scheme for the guanidinate CN₃atoms as shown for 1d in Figure 2. Average distances and angles are given for chemically equivalent

b

c

groups. Data for 1b and 1f taken from structures reported in ref 26. Angle between the C−N3−C plane of the NR₂group and the N1−C1−N2

plane of the C(NAr)₂fragment. Angle between the least-squares planes of the guanidinate CN₃and aryl ring C atoms.

d

Figure 3. Molecular structure of 2b, showing the atomic numbering

scheme. Displacement ellipsoids are drawn at the 50% probability

level; hydrogen atoms have been omitted for clarity. Color key: pink =

Ir, blue = N, gray = C, red = O.

Figure 2. Molecular structure of 1d, showing the atomic numbering

scheme. Displacement ellipsoids are drawn at the 50% probability

level; hydrogen atoms have been omitted for clarity. Color key: pink =

groups. Thus, the twist angle of the C−N3−C and N1−C1−

Ir, blue = N, gray = C.

N2 planes increases with decreasing angles between arene ring

and CN₃planes (Table 2).

The dicarbonyl complex 2b crystallized in the tetragonal

Overall, the structural data for the three cod complexes are

very similar and do not show any signiﬁcant electronic

inﬂuence of the guanidinato substituents (Table 2). Although

the diﬀerence between the C1−NR₂(C1−N3) distance and

the average C1−NAr (C1−N1/N2) distance seems to vary

somewhat for 1b, 1d, and 1f, it is insigniﬁcant relative to the

uncertainty of the measurement for each complex. The steric

inﬂuences of the ortho methyl substituents in 1f, however, force

the aryl groups into an orientation nearly perpendicular to the

guanidinato CN₃plane [78.6(1) and 74.1(1)°]. Furthermore, a

comparison of dihedral angles in the three molecules reveals

steric interactions between the NR₂alkyl and the NAr aryl

space group P4n2. The Ir, C1, and N2 atoms are located on a

̅

crystallographic C₂axis (Figure 3). The Ir−N and Ir−C

distances are 2.069(6) and 1.832(9) Å, respectively, and the

N−Ir−N angle is 63.8(3)° (Table 2 and Supporting

Information). The Ir−C and C−O distances are comparable

to those of related Ir^Idicarbonyls,^62,65while almost all

parameters involving guanidinato ligand atoms are nearly

identical to those of the corresponding cod complex 1b. The

only notable diﬀerences are the dihedral angles involving the

NR₂alkyl and NAr aryl groups. The twist angle of the C8−

N2−C8#1 and N1−C1−N1#1 planes in 2b is greater whereas

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the angles between arene ring and CN₃planes are smaller than

the corresponding angles in 1b (Table 2). These diﬀerences

likely are a consequence of the diﬀerent sizes of the CO and

cod ligands, where the smaller CO ligands in 2b allow the aryl

substituents more rotational freedom.

Inspection of the packing of 2b revealed short intermolecular

contacts between the CO ligands and CH₃groups of adjacent

complexes with O···(H)C9 and C10···(H)C9 distances of

2.357(16) and 2.912(15) Å, respectively. These interactions

link four complexes that are related by the 4-fold inversion axis

parallel to the c axis (Figure 4). Metallophilic interactions,

3.3. Nuclearity in Solution. Both mononuclear^61,62,64,67

and dinuclear^68,69examples of Ir^I(cod) and Ir^I(CO)₂complexes

with bidentate nitrogen-donor ligands are well documented in

the literature. (Additional examples can be found in Section

3.4.) Having established the mononuclear solid-state structures

of the Ir^Icomplexes described here, we turned to an

1

examination of the solution nuclearity. The H and ¹³C NMR

spectra of 1a−1g and 2a−2g are consistent with monomeric

complexes but do not rule out symmetric or ﬂuxional dimers.

For the carbonyls 2a−2g, the observation of two bands in the

ν_COregion of the solution IR spectra supports a mononuclear

structure, whereas three bands may be expected for dinuclear

tetracarbonyls.⁶⁹To determine the solution nuclearity of the

Ir^I(cod) complexes, we measured the diﬀusion coeﬃcients (D)

of a pair of cod and dicarbonyl complexes, which allowed us to

compare their molecular sizes. The diﬀusion coeﬃcient can be

obtained from a stimulated-echo NMR experiment (eq 1,

where γ = gyromagnetic ratio, G = gradient strength, δ = length

of the gradient pulse, and Δ = delay between the midpoints of

the gradients).^43,70The Stokes−Einstein equation relates D

with the hydrodynamic radius (r_H; eq 2, where k = Boltzmann

constant, T = absolute temperature, and η = ﬂuid viscosity).

Under the assumption of spherical shape, the ratio of the

hydrodynamic volumes of two molecules is given by V₁/V₂=

(D₂/D₁)³.^70,71

ln(I/I₀) = −γ²δ²G²(Δ − δ/3)D

(1)

D = kT/6πηr_H

(2)

Figure 4. View of the unit cell of 2b along the crystallographic c axis,

highlighting intermolecular contacts involving the CO ligands. Color

key: pink = Ir, blue = N, gray = C, red = O, light gray = H. Short

intermolecular distances (Å): O···(H)C9#2, 2.357(16); C10···(H)-

C9#2, 2.912(15); C6···(H)C6#2, 3.756(16). Symmetry operation: #2,

2−y, x, 2−z.

Here, diﬀusion NMR experiments were carried out on 1a

and 2a as representative complexes having the same

guanidinato ligand. The values of D from measurements in

benzene-d₆are (8.4 0.1) × 10⁻¹⁰and (9.6 0.2) × 10⁻¹⁰

m²·s⁻¹for 1a³⁸and 2a, respectively (cf. the Supporting

Information). The ratio of the molecular volumes of 1a and 2a

in solution is then estimated to be V_1a/V_2a= 1.5. This result

agrees reasonably well with the ratio of the crystallographically

determined volumes of 1b and 2b (Table 1; V_1b/V_2b= 1.19). In

contrast, if the cod complex were dinuclear, its molecular size

relative to the corresponding dicarbonyl complex would be

signiﬁcantly greater (2V_1b/V_2b= 2.39). Complex 1a is thus

assigned a mononuclear structure in solution. By extension, the

heavier members of the cod series are expected to exhibit the

same nuclearity, because they were shown by their spectro-

scopic data to be structurally analogous to 1a.

which were observed for some Ir^Idicarbonyls,⁶⁶are not

present. In the crystal structures of the cod complexes 1b and

1d, close contacts were found between the Ir center and aryl

groups of neighboring molecules [for 1b: Ir···(H)C12, 3.917(4)

Å; Ir···(H)C6, 4.021(4) Å; for 1d: Ir···(H)C11, 3.788(3) Å;

Ir···(H)C4, 3.831(3) Å]. Such contacts do not exist in the

structure of 1f; apparently, close approach of an adjacent

molecule to Ir is prevented by the methyl groups in ortho

positions of the benzene rings (Figure 5).

To probe whether 1a and 2a might convert into dinuclear

species at higher concentrations, we recorded NMR and IR

spectra at diﬀerent concentrations. No changes were observed,

however, in ¹H NMR spectra of 1a at concentrations between 5

and 75 mM in CDCl₃and in IR and ¹H NMR spectra of 2a up

to 40 mM in C₆D₆. Thus, the Ir^I(cod) and Ir^I(CO)₂complexes

of the smallest guanidinato ligand remain mononuclear at least

in these concentration ranges.

3.4. Evaluation of Ligand-Donor Strength. The average

CO stretching frequencies of [IrCl(L)(CO)₂] complexes have

extensively been employed to compare the donor properties of

neutral monodentate ligands (L). This parameter has proven

particularly useful for ranking a wide range of phosphine⁷²and

N-heterocyclic carbene⁷³ligands, and correlations with related

parameters, such as the CO stretching frequencies of

corresponding Rh complexes and the Tolman Electronic

Parameter, have frequently been made. Studies with alkene

complexes, including [IrCl(L)(cod)], [Ir(PPh₃)(L)(cod)]⁺,

Figure 5. Ball-and-stick representation of 1f with distances illustrating

the steric inﬂuences of the ortho methyl substituents. Color key: pink =

Ir, blue = N, gray = C, light gray = H. Selected distances (Å):

Ir···(H)C8, 3.672(3); Ir···(H)C9, 4.316(3); Ir···(H)C16, 4.138(3);

Ir···(H)C17, 4.178(4); C8···C17, 4.682(5); C9···C16, 4.943(5).

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and [M(Tp^R)(cod)] complexes [M = Rh, Ir, and Tp =

hydrotris(1-pyrazolyl)borate(1−)], have shown that the ¹³C

chemical shifts of the alkene carbon atoms, δ(¹³C_CC), also

reﬂect the donor strength of other ligands coordinated to the

metal center.^12,60,74

To evaluate the electronic properties of the N,N-dialkyl-

N′,N″-diarylguanidinato(1−) ligands, we utilized both the

δ(¹³C_CC) values of the cod complexes 1a−1g and the average

ν_COvalues of the dicarbonyl complexes 2a−2g. These data are

summarized in Table 3. The δ(¹³C_CC) values of 1a−1g in

CDCl₃were found in the range of 58.7−61.0 ppm and the

average ν_COvalues of 2a−2g in n-pentane in the range of

2016−2019 cm⁻¹. Both parameters decrease with more

electron-donating substituents on the guanidinato ligands (1a

→ 1g; 2a → 2g). These trends reﬂect increasing π back-

donation from Ir into the π* orbitals of the alkene or CO

ligands, which in turn results from increasing electron density at

the metal center. Data for 2a in diﬀerent solvents clearly show

that ν_COis aﬀected by the solvent (Table 3). Even greater

variations were observed in the solid state (Supporting

Information, Table S1). These may be ascribed to intermo-

lecular interactions, as observed in the crystal structure of 2b

(cf. Section 3.2), or to interactions with the matrix (KBr).

Therefore, the solid-state data are not suitable for comparing

ligand-donor properties.

Literature data for complexes of some monoanionic ligands

are compiled in Table 4. Where available, ν_COdata for

complexes in alkane or chlorinated solvents were selected.

Some variation with the solvent may also be expected for the

δ(¹³C_CC) values of the [Ir(L)(cod)] complexes, but the data

should provide a reasonable basis for a comparison. Thus, the

N,N-dialkyl-N′,N″-diarylguanidinato ligands are more strongly

donating than bis(1-pyrazolyl)borato, β-diketiminato, and 2-(2-

pyridyl)pyrrolido ligands. Also included in Table 4 are poly(1-

pyrazolyl)borato and tris(2-oxazolinyl)phenylborato ligands as

well as the Cp ligand. Where the borato ligands function as κ²-

binding ligands, they are less donating than the guanidinato

Table 3. Alkene ¹³C Chemical Shifts of 1a−1g and CO

a

Stretching Frequencies of 2a−2g

complex δ(¹³C_CC) (ppm) complex

ν_CO(cm⁻¹

)

avg. ν_CO(cm⁻¹

)

1a

61.0

2a

2055, 1983

2052, 1978

2051, 1976

2046, 1970

2055, 1982

2054, 1981

2053, 1980

2052, 1980

2019

b

c

b

2015

c

2014

d

2008

1b

1c

1d

1e

1f

60.8

60.7

58.8

58.7

2b

2c

2d

2e

2f

2019

2018

2017

1g

2g

2016

a

b

1a−1g in CDCl₃. 2a−2g in n-C₅H₁₂, unless noted otherwise. In

c

d

Et₂O. In C₆H₆. In Me₂SO.

Table 4. Alkene ¹³C Chemical Shifts of [Ir(L)(cod)] Complexes and CO Stretching Frequencies of [Ir(L)(CO)₂] Complexes

[Ir(L)(cod)] complexes

solvent

δ(¹³C_CC) (ppm)

[Ir(L)(CO)₂] complexes

ν_CO(cm⁻¹

a

L

solvent

)

avg. ν_CO(cm⁻¹

)

refs

75

Monoanionic, Bidentate Ligands

H₂B(pz)₂

H₂B(pz^Me2

BDI^Dipp,CF3

BDI^Xyl,Me

BDI^Dipp,Me

PyInd

PyPyr^Ph2

guan^Ph,Me

guan^Xyl,Me

guan^iPr,iPr

F₂B(I^tBu)₂

H₂B(I^tBu)₂

CH₂Cl₂

2070, 2000

2035

)

CD₂Cl₂

C₆D₆

64.1

63

61

76

64

26

31

65

2

67.6

pentane

2069, 2005

2054, 1986

2053, 1985

2037

2020

2019

C₆D₆

63.2

C₆D₆

62.4

CD₂Cl₂

C₆D₆

63.0 (avg.)

61.0

Nujol

2050, 1986

2055, 1983

2053, 1980

2018

2019

2017

b

CDCl₃

pentane

58.8

58.9

hexane

2052, 1985

2048, 1979

2019

2014

Monoanionic B(pz)₄, Tp^R, To^R, and Cp

B(pz)₄-κ²

Tp^Me2-κ²

Tp^iPr2-κ²

CD₂Cl₂

CDCl₃

C₆D₆

65.1

63.4

63.6

CH₂Cl₂

2080, 2010

2045

63, 75

60

c

To^P-κ²/κ³

60.8 (avg.)

61.3 (avg.)

CH₂Cl₂

heptane

hexane

2065, 1991

2066, 1989

2029, 1934

2074, 2009

2049, 1974

2039, 1960

2033, 1961

2028

1982

2042

2012

2000

1997

77

c

d

To^M-κ²

thf-d₈

78

c

To^M-κ³

78

c

Tp-κ²

79

c

Tp-κ³

CDCl₃

54.1

45.5

60, 79

80

Tp^Me2-κ³

Cp

CH₂Cl₂

81

a

Ligand acronyms (see ref 82 for a complete list): H₂B(pz)₂, dihydrobis(1-pyrazolyl)borate(1−); BDI^Ar,Me, N,N′-diarylpentane-2,4-diiminate(1−);

PyInd, 2-(2-pyridyl)indolide(1−); PyPyr^Ph2, 3,5-diphenyl-2-(2-pyridyl)pyrrolide(1−); guan^Ar,R, N,N-dialkyl-N′,N″-diarylguanidinate(1−); H₂B-

(I^tBu)₂, bis(3-tert-butylimidazolin-1-yl-2-ylidene)dihydroborate(1−); Tp, hydrotris(1-pyrazolyl)borate(1−); To, tris(2-oxazolinyl)-

b

c

phenylborate(1−). This work. Solutions of [Ir(To^P)(cod)], [Ir(To^M)(CO)₂], and [Ir(Tp)(CO)₂] were reported to contain mixtures of

d

complexes with κ²- and κ³-binding ligands. Data for [Ir(To^M)(cod)]·LiCl.

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ligands in this study, whereas κ³-binding borates are more

donating.

Not included in the table but brieﬂy compared in the

following are several more elaborate hybrid ligands that also

possess a monoanionic nitrogen-donor set. For example,

bis(pyridyl)dihydroisoindolido-κ²ligands [avg. δ(¹³C_CC) =

65.1 and 65.4 ppm],⁸³one pyridine−amine−pyrrolido-κ²ligand

[avg. δ(¹³C_CC) = 61.9 ppm],⁸⁴and one imine−indolido ligand

[avg. δ(¹³C_CC) = 61.2 ppm]⁶²appear less electron-releasing in

their [Ir(L)(cod)] complexes than the guanidinato ligands,

while pyridazine−amido ligands seem to cover a wider range of

donor properties [avg. δ(¹³C_CC) = 56.2−66.8 ppm].⁸⁵Lastly,

cationic carbonyl and cod complexes of neutral, bidentate

nitrogen-donor ligands, such as phenanthroline derivatives [ν_CO

(MeCN) = 2055−2059 cm⁻¹]⁸⁶and bis(1-pyrazolyl)methane

and bis(2-imidazolyl)methane derivatives [δ(¹³C_CC) = 66.7−

70.3 ppm],⁸⁷exhibit greater ν_COand δ(¹³C_CC) values. These

data may be a consequence not only of weaker donor strength

but also of the positive charge of the complex.⁸⁸

3.5. Reaction of (Alkene)iridium(I) Complexes with O₂

and Characterization of (Alkene)peroxoiridium(III) In-

termediates. The solution reactivity of the Ir^I(cod) complexes

with diﬀerent guanidinato substituents toward O₂was

investigated by UV−Vis spectroscopy. The absorption bands

shown in Figure 1 decayed, giving spectra that are featureless

above 400 nm, before a broad absorption increase was observed

in a subsequent slower phase. Concomitant with these spectral

changes, the color of the solutions turned from bright yellow to

pale yellow to dark green. The intermediate in the reaction of

1a with O₂was recently characterized as the mononuclear

(alkene)peroxo complex [Ir{PhNC(NMe₂)NPh}(cod)(O₂)],

3a, on the basis of one- and two-dimensional NMR techniques

and IR spectroscopy (Scheme 4).³⁸

Figure 6. Time courses of the reactions of 2 mM 1a−1f in toluene

with O₂at 0 °C as monitored by electronic absorption spectroscopy

(path length, 0.5 cm). Color key: 1a, solid black line (λ = 417 nm); 1b,

dashed black line (λ = 418 nm); 1c, solid red line (λ = 418 nm); 1d,

dashed red line (λ = 419 nm); 1e, solid blue line (λ = 419 nm); 1f,

solid green line (λ = 423 nm).

graphically determined structures of 1b, 1d, and 1f (Figures 2

and 5 and ref 26).

More dramatic diﬀerences were discovered at higher

temperature. In contrast to 1a−1e, 1f decayed only partially

when treated with O₂at 20 °C, suggesting that the reaction is

reversible and that O₂binding is favored at lower temperature.

Further investigation revealed that 1f can be regenerated upon

removal of O₂. First, reaction of 1f in toluene with O₂at 20 °C

resulted in only about 70% of the absorbance change expected

for complete decay (Figure 7). When the solution was

Scheme 4. Generation of (Alkene)peroxo Intermediates 3a−

3e

Figure 7. Electronic absorption spectra of 2 mM 1f in toluene at 20 °C

(solid black line), the solution at maximum decay of 1f in the reaction

with O₂(dashed black line; t = 2.5 h), and 1f regenerated upon

removal of O₂(solid red line; t = 10 h; path length, 0.5 cm). The

spectrum showing full decay of 1f was obtained from the reaction of 2

mM 1f in toluene with O₂at 0 °C after 7.5 h (solid blue line). Inset:

Time course of the reaction of 1f in toluene with O₂at 20 °C and

regeneration of 1f after removal of O₂(λ = 423 nm).

Time courses of the O₂reactions of 1a−1f in toluene at 0 °C

to form the corresponding intermediates 3a−3f are shown in

Figure 6. Under these conditions, the half-lives range from

about 10 min to 1 h [t_1/2≈ 1300 s (1a), 1100 s (1b), 1100 s

(1c),⁴⁶850 s (1d), 700 s (1e), and 1 h (1f)]. The data are

consistent with expected electronic and steric substituent

eﬀects. Speciﬁcally, the half-lives decrease with more electron-

donating substituents in the para position of the benzene rings

(1a > 1c > 1e) and in the NR₂group (1a > 1b; 1c > 1d) due to

increasing destabilization of the Ir^Ioxidation state (or

stabilization of the Ir^IIIstate). Interestingly, substitution in

either one of the two ligand positions brings about the same

rate-accelerating eﬀect (1b ≈ 1c), which may reﬂect the

conjugated nature of the guanidinato core. On the other hand,

substituents in the ortho positions of the benzene rings have a

decelerating eﬀect (1f). This eﬀect is likely due to steric

hindrance, as an increased level of steric protection of the Ir

center in 1f is evident from a comparison of the crystallo-

subsequently purged with Ar to remove O₂, the absorption

bands of 1f began to regain intensity and were eventually fully

recovered. Thus, binding of O₂to 1f is fully reversible at 20 °C

(Scheme 5). An analogous experiment with 1a showed no

signiﬁcant reformation. Instead, intermediate 3a, formed from

reaction of 1a with O₂, decayed under Ar in nearly the same

way as under O₂, with a weak shoulder at 420 nm indicating the

presence of a minimal amount of 1a (Supporting Information,

Figure S4). Complex 1g with bulkier diisopropyl substituents

reacted with O₂even more slowly than 1f, and only partial

decay was observed even at −25 °C. It thus appears that the

equilibrium between the Ir^I(cod) complex and its O₂adduct is

shifted further to the left for 1g than for 1f.

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3e fall into the range of 800−900 cm⁻¹determined for other

(η²-peroxo)iridium(III) complexes, which include complexes

with phosphine,^{18,19,21b,89,90}PCP pincer,⁹¹N-heterocyclic

carbene,⁹²dithiocarbamato,⁹³and diimine⁹⁴ligands. For

example, values of 855−858 cm⁻¹were reported for the “O₂

adduct” of Vaska’s complex, [IrCl(PPh₃)₂(CO)(O₂)],^19,21b,89

while energies of 880 and 850 cm⁻¹were found for the

(alkene)peroxo complexes [IrCl(PPh₃)₂(C₂H₄)(O₂)] and [Ir-

(phen)(cod)(O₂)]⁺, respectively.^21,22Data for ν_IrOhave only

occasionally been mentioned in the literature (ca. 500−650

cm⁻¹).^94,95Comparison of the ν_OOdata of 3a−3e shows that

the band shifts to lower energies in the order 3a, 3b > 3c, 3d >

3e. Given the similar structures of these intermediates (vide

supra), this trend may reﬂect increasing population of peroxide

π* orbitals (i.e., a more reduced peroxo ligand) with more

electron-donating guanidinato ligands.

Scheme 5. Reversible Formation of (Alkene)peroxo

Intermediates 3f and 3g

For characterization purposes, the intermediates were

generated at a higher concentration. When 3a was generated

from 20 mM 1a in benzene-d₆at 20 °C, it accumulated in a

yield of 75%. The corresponding intermediates in the reactions

of 1b−1e with O₂also can be generated in high yield (Scheme

4). Under similar conditions, 3b−3e accumulated in about 2−3

h and decayed within 20 h of initiation of the reaction. The

shortest formation time and longest decay time were observed

for 3e, showing that electron-donating substituents prolong the

lifetime of the (alkene)peroxo intermediate. Like for 3a, the ¹H

NMR spectra of 3b−3e indicate desymmetrization, compared

to their respective Ir^Iprecursors, and the presence of

unmodiﬁed guanidinato and cod ligands (cf. the Experimental

Section). Speciﬁcally, each of the spectra exhibits two sets of

resonance signals attributable to two inequivalent guanidinato

aryl groups, four distinct cod alkene signals [δ(¹H) = 4.0−4.8

ppm], and several multiplets accounting for four CH₂groups.

Only one singlet is observed for the N-methyl protons in 3a, 3c,

and 3e, suggesting that rotation about the guanidinato C−

NMe₂bond is facile. Similarly, two signals are found for the

We also attempted to generate the (alkene)peroxo

intermediate of the bis(2,6-dimethylphenyl)-substituted ligand,

3f, at a higher concentration that is more suitable for

characterization (15−20 mM 1f). Because of the reversibility

of this reaction, the maximum accumulation of 3f was lower

than for the other intermediates (ca. 60% after about half a

day), and 1f was still present at that time (Supporting

Information, Figure S11). Both 1f and 3f eventually decayed

over the course of 2 weeks. Relevant IR data of 3f are included

in Table 5 (Supporting Information, Figure S10). At ﬁrst

glance, the high value of 872 cm⁻¹for ν_OOmay seem surprising,

because based on ligand-donor strength and the trend observed

for 3a−3e one might expect a lower value for 3f. Instead, the

higher wavenumber may be a consequence of steric pressure

exerted by the ortho methyl substituents on the peroxo ligand in

3f, which may cause structural changes and perhaps weakens

1

diastereotopic NCH₂protons in 3b and 3d. Overall, the H

NMR data of 3a−3e show that these intermediates adopt very

similar structures.

1

Ir−peroxo binding (i.e., a less reduced peroxo ligand). The H

Coordination of the peroxo ligand to Ir was conﬁrmed by IR

spectroscopy on solid samples. The samples were prepared by

solvent removal, and the integrity of the intermediates was

NMR data of 3f are consistent with greater steric constraints,

showing a wider shift range for the cod alkene resonance signals

(compared to 3a−3e) and four distinct singlets for the ortho

methyl groups (cf. the Experimental Section).

1

veriﬁed by H NMR spectroscopy on redissolved samples.

Attempts to grow single crystals of the intermediates have been

hampered by their limited lifetime in solution. For each

intermediate, one medium strong band was observed in the OO

stretching region (ν_OO) near 860 cm⁻¹(Table 5 and

3.6. Reactivity of (Alkene)peroxoiridium(III) Inter-

mediates. To gain insight into the reactivity of the

(alkene)peroxoiridium(III) intermediates, we investigated

reactions with PPh₃and CO as well as their self-decay leading

to cod oxygenation.

a

Table 5. OO and IrO Stretching Frequencies of 3a−3f

Peroxo complexes of late transition metals frequently oxidize

phosphines to phosphine oxides.^18,91,96In agreement with this

pattern, intermediate 3a reacted in deoxygenated solution with

PPh₃to yield OPPh₃.³⁸The same intermediate also reacted

with CO, producing a carbonato complex, 4a, as well as the

dicarbonyl complex 2a and free cod (eq 3). The carbonato

complex was identiﬁed by ESI mass spectrometry and IR

spectroscopy and is tentatively assigned as [Ir{PhNC(NMe₂)-

NPh}(cod)(CO₃)]. The masses and isotope distribution

patterns of peaks at m/z = 600 and 622 match the formulations

{4a + H}⁺and {4a + Na}⁺, respectively, while the presence of a

chelating carbonato-κ²O,O′ ligand⁹⁷⁻⁹⁹was inferred from a

band in the solid-state IR spectrum at 1690 cm⁻¹(ν_CO).¹⁰⁰

These assignments were conﬁrmed by an ¹⁸O-labeling experi-

ment using 3a-¹⁸O₂and CO (m/z = 604 and 626). The

appearance of a new IR absorption at 1667 cm⁻¹indicated

some scrambling of the labeled oxygen atoms into the terminal

position of the carbonato ligand.⁹⁹Also identiﬁed in the IR

spectra was complex 2a by its characteristic absorption bands in

the 1900−2100 cm⁻¹region. Both 2a and free cod could be

complex

ν_OO(cm⁻¹

)

ν_IrO(cm⁻¹

)

b

3a

865

813

816

865

862

857

872

575, 459

550, 442

545, 435

576, 459

575, 459

575, 458

573, 457

572, 459

b

3a-¹⁸O₂

b

calcd

3b

3c

3d

3e

3f

a

b

Solid state (KBr disk). From ref 38.

Supporting Information, Figures S5−S9). In addition, two

weak bands that can be assigned to IrO stretching modes (ν_IrO

)

were located at about 575 and 460 cm⁻¹. These three bands

were not observed in the spectra of the decay products of 3b−

3e (Supporting Information, Figures S5−S9). For 3a, the

assignments were previously corroborated by an ¹⁸O-isotope-

labeling study, with good agreement between observed and

calculated isotope shifts (Table 5). The energies of ν_OOin 3a−

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quantiﬁed by ¹H NMR spectroscopy, and the yields were about

30% for 2a and 40% for cod (with respect to Ir).

ion. Mechanisms for alkene oxygenation via (alkene)peroxo

intermediates usually invoke conversion into 3-metalla-1,2-

dioxolanes, if the reaction takes place at a single metal

center.^7,10Examples of 3-rhoda- and 3-irida-1,2-dioxolanes have

been reported,^16,17,20and rearrangement into β-oxoalkyl

complexes has been described in several cases.^17,102On the

basis of these precedents, we propose that formation of an

(oxo-η²-cyclooctenyl) complex (B) proceeds from 3 by alkene

insertion into the Ir−O bond to aﬀord an iridadioxolane,

[Ir(L)(η²-C₈H₁₂O₂-κC,κO)] (A), which in turn undergoes O−

O bond cleavage and hydrogen migration (Scheme 6). The

[Ir^III(L)(cod)(O₂)] + excess CO

→ [Ir^III(L)(cod)(CO₃)] + [Ir^I(L)(CO)₂] + cod

(3)

Formation of carbonato complexes from reaction of peroxo

complexes with CO has been described.^98,99It is conceivable

that the carbonato complex 4a formed here reacts further by

substitution of CO for the cod ligand and reduction to aﬀord

2a. Another possible pathway to 2a involves reaction of CO

with the Ir^I(cod) complex 1a, because a small amount of this

complex could be regenerated from 3a when O₂is absent

(Section 3.5) and conversion of 1a into 2a is facile (Section

3.1.3). In either case, the formation of 2a diﬀers somewhat from

the quantitative displacement of O₂from a Rh(O₂²⁻) complex

by a stoichiometric amount of CO.¹⁰¹Irrespective of the

mechanism, the production of nearly equimolar amounts of 2a

and free cod conﬁrms the presence of the Ir(L)(cod) entity in

intermediate 3a.

a

Scheme 6. Proposed Mechanism for the Decay of 3

a

L = {ArNC(NR₂)NAr}⁻.

In the absence of added substrates, intermediates 3a−3f

decayed to dark green products. In principle, self-decay of the

(alkene)peroxo intermediates may be initiated in several

diﬀerent ways, including oxidation of the guanidinato or alkene

ligands or dissociation of superoxide. Attack by the peroxo

ligand may be more facile at the alkene (cod) than at the

guanidinato aryl or alkyl groups. Unfortunately, the identity of

the decay products was not accessible from NMR spectroscopy,

because the spectra exhibit unresolved, broad resonances in the

ranges of 6.5−7.5 and 1−3 ppm. Yet, we were able to extract

the guanidinato ligand from the decay products of 3a and

recover it in a substantial yield, demonstrating that it remained

largely intact throughout the decay of the (alkene)peroxo

intermediate (cf. the Experimental Section). Furthermore, the

decay under N₂of intermediates without ortho substituents

gave similar results as under O₂, with the only diﬀerence being

the presence of a small amount of 1 (e.g., 10−15% of 1a, 1d, or

1e), which is consistent with a minor degree of reversibility in

the formation of 3a (cf. Section 3.5).

The ESI mass spectra of the decay products of 3a−3f

indicate that oxygen was retained upon decay. The assignments

of the peaks listed in Table 6 to {3 − OH}⁺and {3 − H}⁺are

supported by their isotope distribution patterns, the mass shifts

observed for complexes with diﬀerent guanidinato ligands, and

an ¹⁸O-labeling experiment for 3a. The monooxygenated

products are consistent with (oxo-η²-cyclooctenyl) complexes,

[Ir(L)(η²-C₈H₁₁O-κC)(OH)], upon dissociation of hydroxide

dioxygenated species observed in the mass spectra may be

related to the iridadioxolanes. Also observed were products that

do not contain oxygen, that is, {3 − O₂H}⁺(Table 6), and are

consistent with (η⁵-cycloocta-2,5-dien-1-yl) complexes, [Ir(L)-

(η⁵-C₈H₁₁)]⁺(C). These products might hint at the existence

of an unproductive decay channel by dissociation of O₂^•−from

3 to give Ir^II(cod) complexes (eq 4).¹⁰³Such species tend to

undergo allylic hydrogen atom transfer that aﬀords Ir^III(η²-ene-

η³-enyl) species (C),¹⁰⁴as indicated in eq 5.

[Ir^III(L)(η⁴‐C₈H₁₂)(η²‐O₂)] → [Ir^II(L)(η⁴‐C₈H₁₂)]⁺

•−

+ O₂

(4)

2[Ir^II(L)(η⁴‐C₈H₁₂)]⁺→ [Ir^III(L)(η⁵‐C₈H₁₁)]⁺

+ [Ir^III(L)(η⁴‐C₈H₁₂)H]⁺

(5)

Taken together, these results suggest that the decay of

intermediates 3a−3f, at least in part, led to oxygenation of the

cod ligand. In addition to the presence of multiple products

(A−C), it is likely that geometrical isomers of A and B and

possibly hydroxo- or oxo-bridged dimers of B were formed, and

a mixture of several such products might have led to the

convoluted NMR data. The integrity of the guanidinato ligand,

however, was established by its recovery from the product

mixture.

We therefore sought for ways to extrude the oxygenated cod

fragment from the decay products of 3. While heating of the

solution did not cause any changes (1 d at 70 °C), reaction with

cod under the same conditions aﬀorded 4-cycloocten-1-one and

1 (Scheme 7). For example, the yields from the reaction of the

decay products of 3a with 2 equiv of cod were 90 and 30%

Table 6. Mass-to-Charge Ratios (m/z) from the ESI Mass

Spectra of the Decay Products of 3a−3g

a

complex

{LH + H}⁺

{M − O₂H}⁺

{M − OH}⁺

{M − H}⁺

b

3a

240.2

268.1

296.2

300.2

296.2

408.4

538.3

538.2

538.4

566.2

594.2

598.3

594.3

706.5

554.2

556.3

582.1

610.1

614.2

610.2

722.3

570.1

570.2

574.3

598.1

626.1

630.1

calcd

3a-¹⁸O₂

b

a

Scheme 7. Oxygenation of 1,5-Cyclooctadiene from 3a−3f

3b

3c

3d

3e

3f

3g

a

b

a

LH = ArNC(NR₂)NHAr; M = 3a−3g. From ref 38.

(i) 1. C₆D₆, 20 °C, 1 d; 2. Two equiv of cod, C₆D₆, 70 °C, 1 d.

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(based on Ir) for 4-cycloocten-1-one and 1a, respectively.³⁸

Similar results were obtained for reactions employing the decay

products of 3b−3e with yields of 75−90% for 4-cycloocten-1-

one and 25−35% for 1. Electron-donating substituents on the

guanidinato ligand thus do not exhibit an appreciable eﬀect in

this reaction. In contrast, with the sterically more demanding

bis(2,6-dimethylphenyl)-substituted guanidinato ligand, the

yields were roughly halved (45 and 13%). Overall, the

signiﬁcantly diﬀerent yields of 4-cycloocten-1-one and 1

common to all of the cod reactions suggest that these products

are formed in two diﬀerent steps. A mechanism accounting for

both products could involve cod C−H bond activation by the

(oxo-η²-cyclooctenyl) complex (B) to give 4-cycloocten-1-one

and an (η⁵-cycloocta-2,5-dien-1-yl) complex, [Ir(L)(η⁵-C₈H₁₁)-

(OH)], followed by reaction with a second equivalent of cod by

alkene insertion into the Ir−OH bond¹⁰⁵and β-hydrogen

migration to yield 4-cycloocten-1-one and an (η⁵-cycloocta-2,5-

dien-1-yl)hydrido complex, [Ir(L)(η⁵-C₈H₁₁)H], which then

could regenerate 1.

remarkable in light of the limited reactivity of known

(alkene)peroxo complexes. C−O bond formation has only

been reported for [IrCl(PPh₃)₂(C₂H₄)(O₂)].²⁵Because of

PPh₃co-oxidation, the mechanism of this reaction must

necessarily diﬀer from that of cod oxygenation in 3a−3f. On

the other hand, the Rh^III(alkene)(O₂²⁻) complex even reverted

back to its Rh^I(alkene) precursor under O₂loss when heated

above 60 °C.²³The lack of reactivity between the alkene and

peroxo ligands was ascribed to their unique relative disposition

2−

at the metal center. In all three crystal structures, the O₂

ligand and the CC group are nearly coplanar and bound to

the metal center in a butterﬂy fashion.^23,24It was speculated

that this geometry may be unfavorable for C−O bond

formation.^10,23In contrast, at least one of the two CC

groups in 3a−3f must be positioned out of plane relative to the

2−

O₂²⁻ligand (while the other one may be in plane with the O₂

ligand). It would thus appear that such an orthogonal

arrangement is more favorable for an interaction between

these ligands that can lead to C−O bond formation.

Stereochemical factors may then indeed be critical for alkene

oxygenation at Rh and Ir.

4. DISCUSSION

An (oxo-η²-cyclooctenyl) complex of Ir was previously

reported, but that was obtained by oxidation of an Ir^I(cod)

complex with H₂O₂(in the presence of acid), which necessarily

involves a diﬀerent mechanism.¹⁰⁹Similar 2-oxoethyl com-

plexes of both Rh and Ir were obtained from reactions of

M^I(C₂H₄) complexes in the solid state with gaseous O₂via 3-

metalla-1,2-dioxolanes.^17,102It is possible that the formation of

the 3-metalla-1,2-dioxolanes proceeded through (alkene)peroxo

intermediates akin to those described here, although radical

pathways^9,110have also been considered. Cleavage of the

oxygenated organic fragment from the metal by heating or

protonation has not been achieved for any of these species.

More successful has been the elimination of organic products

from 2-metallaoxetanes,¹⁵which have been obtained by

oxidation of M^I(alkene) complexes with H₂O₂(M = Rh, Ir),⁹

by reaction of M₂(μ-O)₂complexes with alkenes (M = Pt,

Au),¹¹¹or from Pt(alkene)(OH) complexes.^5,112In a rare case

of a 2-rhodaoxetane prepared using O₂, 4-cycloocten-1-one

could be eliminated by reaction with PMe₃or CO.¹¹³The O₂

activation step in the generation of the 2-rhodaoxetane¹⁴was

proposed to involve an (alkene)peroxo species similar to 3,

before intermolecular trapping eventually leads to C−O bond

formation.

We have synthesized [Ir(L)(cod)] and [Ir(L)(CO)₂] com-

plexes supported by seven diﬀerent N,N-dialkyl-N′,N″-diaryl-

guanidinato ligands and investigated the formation of [Ir(L)-

(cod)(O₂)] intermediates, along with the ensuing alkene

oxygenation.

Ligand-Donor Strength. The electron-donating ability of

the guanidinato ligands was evaluated using the CO stretching

frequencies of the dicarbonyl complexes and the alkene ¹³C

chemical shifts of the cod complexes. Based on these

parameters and a comparison with literature data for complexes

of other monoanionic ligands, the donor strength of these

ligands increases in the order B(pz)₄-κ²< Tp^R2-κ²< BDI^Ar,Me

≈

PyPyr^Ph2< guan^Ar,R< Tp^R2-κ³< Cp. This ranking is in good

agreement with results from a recent study using [Rh(L)-

(CO)₂] complexes to compare the PyPyr^R2system with various

other monoanionic ligands.¹⁰⁶In that study, the ν_COvalues

given for complexes of relevant nitrogen donors decrease in the

order Tp^R2-κ²> PyPyr^R2≈ BDI^Ar,R> PyPyr^tBu2> amidinate^Cy,Fc

(where R = CF₃). Data for guanidinato ligands are not available,

but they usually function as stronger donors than amidinato

ligands.²⁷

Reversible and Irreversible Formation of (Alkene)-

peroxoiridium(III) Intermediates. The trends observed for

the reactivity of 1a−1g toward O₂conform with electronic and

steric factors (Section 3.5). The ligand substituents also aﬀect

the reversibility of the O₂reaction of 1. For complexes without

ortho substituents (1a−1e), the reaction is virtually irreversible

and allows the generation of 3a−3e in high yield. In contrast,

O₂binding is reversible for the bis(2,6-dialkylphenyl)

derivatives 1f and 1g. From the structures of the Ir^Icomplexes

and the spectroscopic diﬀerences between 3f and 3a−3e, it is

clear that the ortho substituents create steric pressure that likely

weakens O₂binding to Ir. Others have shown that O₂binding

can be tuned by diﬀerences in ligand polarizability.¹⁰⁷Steric

eﬀects on O₂reactivity at Ir have been described in early work

by Vaska, but there ortho methyl substituents on the

triarylphosphine ligands block the reactivity entirely.¹⁰⁸

Alkene Oxygenation. Notably, the new intermediates 3a−

3f decayed to oxygenate the alkene ligand. (Oxo-η²-cyclo-

octenyl) complexes were identiﬁed from the decay products,

and subsequent reaction with added cod aﬀorded signiﬁcant

yields of 4-cycloocten-1-one and 1a−1f. These results are

5. CONCLUSION

The modular synthesis of the N,N-dialkyl-N′,N″-diarylguani-

dine ligand precursors enabled us to produce a series of

[Ir{ArNC(NR₂)NAr}(cod)] complexes (1a−1g) for a system-

atic investigation of their reactivity. Activation of O₂by these

complexes proceeds with high selectivity yielding the

corresponding (alkene)peroxoiridium(III) intermediates (3a−

3e). Intriguingly, the introduction of sterically encumbering

substituents (1f and 1g) causes a marked switch from

irreversible to reversible formation. In addition to the

spectroscopic characterization of 3a−3f, each group in

intermediate 3a was probed by its chemical behavior. In

particular, the peroxo ligand oxygenated PPh₃to OPPh₃, the

Ir(L)(cod) entity could be converted by reaction with CO into

[Ir(L)(CO)₂] and free cod, and the free guanidine was

recovered from the decay products of 3a. The new

intermediates are quite unique, because only a small number

of (alkene)peroxo complexes has been described in the

2577

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Inorganic Chemistry

Article

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ASSOCIATED CONTENT

* Supporting Information

■

S

Additional spectroscopic data for 2a−2g and 3a−3f (Figures

S1−S11 and Tables S1 and S6, PDF), details of the crystal

structure determinations (Tables S2−S5, PDF), and crystallo-

graphic information ﬁles of 1d and 2b (CIF). This material is

available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: jan-uwe-rohde@uiowa.edu.

■

Notes

The authors declare no competing ﬁnancial interest.

(26) Rohde, J.-U.; Kelley, M. R.; Lee, W.-T. Inorg. Chem. 2008, 47,

ACKNOWLEDGMENTS

11461.

■

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This research was supported by the American Chemical Society

Petroleum Research Fund (46587-G3) and the National

Science Foundation (CAREER Award CHE-1057163). We

thank Dr. Dale C. Swenson for the collection of X-ray

diﬀraction data.

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Guanidinato complexes of iridium: Ligand-donor strength, O2 reactivity, and (alkene)peroxoiridium(III) intermediates

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