σ-Pt-BODIPY Complexes with Platinum Attachment to Carbon Atoms C2 or C3: Spectroscopic, Structural, and (Spectro)Electrochemical Studies and Photocatalysis

Article

Cite This: Organometallics XXXX, XXX, XXX −XXX

σ‑Pt-BODIPY Complexes with Platinum Attachment to Carbon Atoms

C2 or C3: Spectroscopic, Structural, and (Spectro)Electrochemical

Studies and Photocatalysis

Peter Irmler and Rainer F. Winter*

Fachbereich Chemie, Universitat Konstanz, Universitatsstraße 10, D-78457 Konstanz, Germany

̈

S

* Supporting Information

ABSTRACT: In this work we discuss ﬁve new complexes with the

general formula trans-Pt(bodipy)I(PEt₃)₂, where diﬀerently substituted

bodipy dyes attach to the coordination center via a direct Pt−C σ-bond to

the pyrrolic carbon atom C2 or C3. We also report an isolable

intermediate of the oxidative addition step where the bodipy is η²-bonded

to the cis-Pt(0)(PEt₃)₂moiety. Comparison between the new complexes,

meso-platinated analogue 8-Pt, and the parent dyes reveals that the site of

platinum attachment inﬂuences the spectroscopic, photophysical, electro-

chemical, and electronic properties. In contrast to 8-Pt, absorption and

emission bands are red-shifted with respect to the parent dyes. 2-

Platinated bodipy dyes 2-Pt-6H, 2-Pt-6I, 2-Pt-Mes-6I, and 2-Pt-6Et

exhibit dual ﬂuorescence and NIR phosphorescence emissions, with low

quantum yields, whereas 3-Pt emits solely by ﬂuorescence (Φ_Fl= 52.7%). The complexes are modestly eﬃcient sensitizers for

1

photochemical O₂production but outperform methylene blue. They also undergo one reversible one-electron reduction and

oxidation as indicated by cyclic voltammetry. Half-wave potentials are cathodically shifted by 340−510 mV with respect to the

parent dyes. The one-electron reduced and some of the one-electron oxidized forms were generated and investigated by UV/vis/

NIR and EPR spectroscopy as well as TD-DFT calculations. The similarity of their spectra to those of the one-electron reduced

or oxidized forms of other bodipy dyes as well as the richly structured EPR spectra and g-values close to g_eattest to a dominant

bodipy character of the relevant frontier MOs.

them with the capabilities of energy or charge transfer between

their individual constituents on photoexcitation^2,19−21or makes

them amenable to bioconjugate chemistry for medical

purposes.²²⁻²⁴Eﬀective ISC to the excited triplet state of

bodipys has been known since 2005,²⁵when it was observed

that the ﬂuorescence quantum yield (QY) of 2,6-diiodo-bodipy

is strongly reduced with respect to the nonhalogenated dye due

to the competitiveness of ISC with ﬂuorescence emission.

Further progress was made by linking two bodipys to rigid

orthogonal arrays²⁶⁻³⁰or bringing bodipy dyes into the

coordination sphere of a heavy metal ion such as ruthenium,

rhodium, iridium, or platinum, usually by covalently attaching

them to a polypyridine ligand motif or by introducing them as

acetylide ligands.³¹⁻⁴⁰Representative examples of the latter

kind of compounds are mono- and dinuclear Pt(II) bipyridine

or diphosphane bis(acetylide) complexes with two identical or

nonidentical bodipy-acetylide ligands.^32−35,40Most of these

complexes exhibit dual ﬂuorescence and phosphorescence

emissions with, however, usually rather modest phosphor-

escence QYs at room temperature (r.t.). Upon lowering the

temperature or embedding the compounds into solid ﬁlms, the

phosphorescence QY may be signiﬁcantly increased.^32,34,41

INTRODUCTION

■

Color has always fascinated mankind. The fabrication of dyes

has been key to the development of synthetic organic chemistry

and the shaping of modern chemical industry. Nowadays,

myriads of dyes allow for generating any desired hue of color.

More recently, dyes have gained importance as sensitizers in

organic or organometallic light-emitting diodes (OLEDs)^1,2or

for bioimaging purposes.^3,4Further applications of modern

dyes rely on their capability to undergo eﬃcient intersystem

crossing (ISC) to their excited triplet states T₁and to

radiatively relax to the electronic ground S₀state by

phosphorescence emission. The signiﬁcantly longer phosphor-

escence lifetime (usually by three or more orders of magnitude)

compared to ﬂuorescence from the excited singlet state S₁

opens further avenues for such photosensitizers like their

utilization in dye-sensitized solar cells (DSSCs),⁵photo-

catalysis,⁶⁻⁸photodynamic therapy,⁹⁻¹²or triplet−triplet

annihilation (TTA) upconversion.^13,14

The family of 4,4-diﬂuoro-4-bora-3a,4a-diaza-s-indacen-8-yl

(bodipy) ﬂuorophores holds a particularly prominent place in

these respects. Their chemical robustness and easy derivatiza-

tion allow for precise adjustment of the energy levels of the

excited singlet and triplet states of bodipy dyes, to decorate

bodipys with a huge variety of substituents, and to incorporate

them into a vast number of functional arrays.¹⁵⁻¹⁸This endows

Received: November 6, 2017

A

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Scheme 1. Overview of the Complexes and Their Syntheses

Experimental and quantum chemical studies show that the

frontier molecular orbitals (FMOs) of the latter platinum

complexes are dominated by the acetylide ligands with only

modest contributions from the platinum ion(s). The platinum

ion then acts as a remote heavy atom. In this scenario, the

eﬃcacy of ISC scales with d⁻⁶and therefore rapidly decreases

with increasing spatial separation d of the center of the π

chromophore from the heavy metal ion.⁴²⁻⁴⁴It is therefore

advantageous to bring the platinum ion as close to the

chromophore as possible, in particular by attaching the dye to

the heavy metal ion via a direct metal−carbon σ-bond. That

approach was ﬁrst applied to anthracenes,⁴⁵perylenes,⁴⁶

corannulenes,^47,48or dibenz[a,c]anthracenes.⁴⁹The latter

chromophores were introduced into the coordination sphere

of a platinum ion via oxidative addition of the corresponding

haloarene to a bis(phosphane) platinum(0) source such as

Pt(PR₃)₄or Pt(PR₃)₂(η²-oleﬁn). The resulting σ-aryl Pt(II)

complexes, however, were mostly only ﬂuorescent, again as a

result of the large distance between the heavy metal ion and the

center of the extended π-chromophore.

We have recently observed that platinum complexes cis/

trans-Pt(PR₃)₂X (R = Ph, Et; X = monoanionic ligand) with a

σ-bonded thioxanthonyl or bodipy ligand show dual

ﬂuorescence and phosphorescence emissions with phosphor-

escence QYs of up to 41% in ﬂuid solution at r.t., even though

the free ligands are not or only very weakly phosphorescent

under these conditions. So far these studies only involved

complexes where the platinum ion attaches to the bodipy dye

via its meso position C8. As an expansion of our previous work,

we here report on ﬁve new complexes where the metal ion

binds to the pyrrolic carbon atoms C2 or C3 of a bodipy dye

(see Scheme 1). Monoplatination of diiodo-derivatives of the

bodipys also allows us to compare analogous complexes with or

without additional iodo atoms at the bodipy dye and to probe

for the additional heavy atom eﬀect of the remaining iodo

substituent.

In this context one should note that the HOMO of bodipys

has a nodal plane along the B−C8 vector.^50,51This severely

limits platinum contribution to the HOMO, thus reducing the

impact of the Pt(PR₃)₂X fragment to that of a potent electron

donor with a remote heavy atom. In contrast, positions C2 and

C3 have large contributions to the FMOs. Platinum attachment

to any of these positions is therefore expected to cause major

perturbations of the bodipy π-system which should become

manifest in their optical, photophysical, and electrochemical

properties.^{8,33,41,52−56}Quantum chemical calculations have also

been performed and are augmented by UV/vis/NIR and EPR

spectroelectrochemical studies to directly probe for the identity

of the respective redox sites and the platinum contribution to

the SOMO of the reduced and oxidized forms of these

complexes.

RESULTS AND DISCUSSION

■

Synthesis and Characterization. All new complexes of

this study and their syntheses are summarized in Scheme 1.

The platinum fragment was introduced by oxidative addition of

the corresponding 2- or 3-halogenated bodipy dye to Pt(η²-

C₂H₄)(PEt₃)₂, which itself is conveniently prepared from the

diethyl complex by thermally induced reductive elimination of

ethane.⁵⁷Those reactions inevitably aﬀorded mixtures of the cis

and trans isomers, which were conveniently converted to the

pure trans complexes by successive treatment with AgOTf and

NaI. Quite interestingly, further reaction of complexes 2-Pt-

Mes-6I and 2-Pt-6I with an additional equivalent of Pt(η²-

C₂H₄)(PEt₃)₂or direct treatment of the 2,6-diiodinated dye

with 2 equiv of the platinum precursor failed in our hands to

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Table 1. Selected Bond Lengths [Å] and Angles [deg] for the Complexes

a

bond parameter

2-Pt-6H

2-Pt-6I

2-Pt-Mes-6I

2-Pt-6Et

3-Pt

Pt−C_bodipy

2.027(4)

2.023(4)

2.6943(4)

2.6872(4)

2.3119(14)

2.3022(14)

2.3142(14)

2.3064(15)

1.404(7)

1.393(7)

89.73(14)

90.56(15)

91.46(14)

91.10(15)

89.37(3)

89.20(3)

89.48(3)

89.22(4)

178.48(5)

176.02(5)

177.11(15)

178.63(14)

85.7

2.018(4)

1.991(11)

2.016(7)

2.020(7)

1.977(7)

2.6849(5)

2.326(2)

2.325(2)

1.392(11)

89.7(3)

Pt1−I

2.6852(3)

2.2992(11)

2.3181(10)

1.427(5)

92.82(11)

88.16(11)

88.77(3)

90.27(3)

177.31(3)

178.38(10)

85.4

2.6812(9)

2.299(3)

2.304(4)

1.425(17)

90.3(4)

90.2(4)

90.56(9)

88.95(8)

177.15(16)

177.30(11)

88.28

2.6766(6)

2.6841(7)

2.305(2)

2.301(2)

2.306(2)

2.303(2)

1.421(11)

1.400(11)

90.6(3)

90.1(2)

91.4(3)

89.6(2)

89.28(5)

90.08(6)

89.45(6)

90.24(5)

172.68(8)

178.56(9)

174.4(2)

179.2(3)

88.8

Pt1−P1

Pt1−P2

C8−C9

C−Pt1−P1

C−Pt1−P2

I1−Pt1−P1

I1−Pt1−P2

P1−Pt1−P2

C−Pt1−I1

90.6(3)

91.24(7)

89.29(7)

175.18(9)

170.0(2)

89.6

b

[B]−[Pt]

85.5

88.5

c

[B]−[Ar]

76.9

80.3

86.86

79.8

83.7

88.9

d

[Pyr]−[Pyr]

5.2

3.4

7.0

2.0

6.5

3.2

6.4

a

b

Data for the two independent molecules of the unit cell. Interplanar angle between the best plane through the inner six-membered ring of the

c

bodipy ligand and the coordination plane at the platinum ion. Interplanar angle between the best plane through the inner six-membered ring of the

bodipy ligand and the meso-aryl substituent. Interplanar angle between the two pyrrolic subunits of the bodipy ligand.

d

provide the dinuclear complexes, even under forcing con-

ditions. This is possibly due to electronic deactivation on the

introduction of the ﬁrst platinum fragment. The stronger C−Cl

bond of the 3-chloro-5,7-dimethylbodipy increases the energy

barrier for oxidative addition such that the intermediate

substitution product 8,9-Pt, where the ethylene ligand of the

precursor is replaced by the η²-coordinated bodipy, can be

isolated. Owing to the poor solubility of that complex in

benzene, crystals suitable for X-ray molecular structure

determination were directly deposited from the reaction

solution. Above 40 °C in THF-d₈, 8,9-Pt slowly converts to

the products of oxidative addition. Full conversion required 72

h at 60 °C as depicted in Figure S1 and aﬀorded a mixture of

trans- and cis-3-Cl-Pt with the cis isomer as the kinetic and the

trans isomer as the thermodynamically preferred product. Van

der Boom and co-workers have reported on a similar

intermediate with η²-coordination of a d¹⁰Pt(0) center to the

CC double bond of a brominated stilbazole. After primary

coordination to a remote double bond, the low valent platinum

center migrates along the conjugated π-system until it reaches

the halogenated double bond.⁵⁸

Hz). The ¹⁹⁵Pt NMR resonance signal of every complex is

consequently split into a triplet with a chemical shift of −4784

to −4802 ppm for the complexes where platinum(II) ion is

bound to C2 and of −4599 ppm for complex 3-Pt. The

presence of a direct Pt−C σ-bond was also conﬁrmed by

platinum satellites of the corresponding adjacent nuclei in the

¹H and ¹³C NMR spectra of the complexes. Owing to the

orthogonal arrangement of the platinum coordination plane to

the bodipy ligand (vide infra) and a hindered rotation around

the Pt−C axis the methylene protons of the PEt₃ligands are

diastereotopic. For complexes with the platinum ion attached

to carbon atom C2, shift diﬀerences are rather minor such that

only slight broadening of the respective resonance signals is

observed. 3-Pt, however, shows two separate resonances for the

1

methylene protons in its H NMR spectrum (Figure S27).

Platinum coordination to the C8C9 double bond of 3-

chloro-5,7-dimethylbodipy in the complex 8,9-Pt splits the

3

resonance signal for H8 into a doublet of doublets with J_PH

couplings of 9.1 and 4.3 Hz to the two inequivalent phosphane

2

ligands and additional platinum satellites with a J_PtHcoupling

constant of 49.2 Hz. As the result of η²-coordination, all

corresponding resonance signals appear at higher ﬁeld as

compared to the parent dye. This is most dramatically seen for

H8 and C8, whose resonances are shifted from 6.15 to 3.83

ppm (compare Figures S21 and S23) or from 123.5 to 42.1

ppm (compare Figures S22 and S26, J_PtC= 43.6 Hz). The ¹⁹⁵Pt

NMR spectrum of 8,9-Pt shows the expected doublet of

doublets at δ = −4950 ppm; the coupling constants J_PtPof 4800

All complexes were readily identiﬁed by multinuclear (¹H,

¹³C, ³¹P, and ¹⁹⁵Pt) NMR spectroscopy and their purities

conﬁrmed (see Figures S2 −S30). The ³¹P NMR spectra of the

trans-Pt(bodipy)I(PEt₃)₂complexes feature just one sharp

singlet which is located at 7.69 ppm for 3-Pt or in between 9.03

and 9.75 ppm for the 2-platinated bodipy complexes and is

symmetrically ﬂanked by ¹⁹⁵Pt satellites (J_PtP= 2540−2560

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and 3320 Hz match with the ¹⁹⁵Pt satellites in the ³¹P NMR

spectrum.

C−C bond length of 1.431(13) Å is still in the usual range of

1.275(3)−1.454(6) Å.^58,60,61

Single-Crystal X-ray Diﬀraction Analysis. Single crystals

of complexes 2-Pt-6H, 2-Pt-6I, 2-Pt-Mes-6I, and 2-Pt-6Et as

well as of complexes 3-Pt and 8,9-Pt were grown by the

procedures given in the Experimental Section and were

successfully subjected to X-ray diﬀraction analyses. Details of

the data collection and structure reﬁnement are collected in

Table S1. The most important metric parameters derived from

the structure solutions are provided in Tables 1 and 2. The unit

Not unexpectedly, the Pt−C bond to the sterically less

hindered carbon atom C8 of 2.127(9) Å is signiﬁcantly shorter

than that of 2.226(9) Å to the pyrrolic carbon atom C9. The

coordination plane at the platinum atom as deﬁned by the

atoms Pt, P1, P2, C8, and C9 forms an interplanar angle of

81.5° with the plane of the bodipy ligand. η²-Coordination to

the C8C9 bond and concomitant rehybridization of the meso

carbon atom causes a larger deviation of the pyrrole rings from

coplanarity of 17.0° (Table 2) as compared to that of 2.0−7.0°

for the other complexes (Table 1). The distortion nevertheless

remains small and is thus not expected to severely impact on π-

conjugation within the bodipy ligand.

Table 2. Selected Bond Lengths [Å] and Angles [deg] for

8,9-Pt

bond parameters

8,9-Pt

Packing diagrams of the various crystal structures are

provided as Figures S31 −S36. Short intermolecular H···I

contacts between pyrrolic (3-Pt, 2-Pt-6H) or the meso H

atom (3-Pt, d(H···I = 3.005 Å) and the iodide ligand as well as

short intermolecular H···F contacts between the BF₂group and

methyl (8,9-Pt, d(H···F) = 2.612 Å) and/or methylene protons

of the PEt₃ligands (8,9-Pt, d(H···F) = 2.579 Å; 2-Pt-6I, d(H···

F) = 2.561 Å, 2-Pt-6H, and d(H···F) = 2.490 Å) or to

cocrystallized benzene (2-Pt-6I, d(H···F) = 2.537, 2.571, and

2.658 Å) or CH₂Cl₂molecules (2-Pt-6Et, d(H···F) = 2.480 Å)

are found.

C8−Pt1

C9−Pt1

Pt1−P1

Pt1−P2

C8−C9

2.127(9)

2.226(9)

2.301(2)

2.229(3)

1.431(13)

C8−Pt1−C9

C8−Pt1−P2

C9−Pt1−P1

C8−Pt1−P1

38.3(4)

97.5(3)

120.0(3)

157.9(3)

Electronic Absorption Spectroscopy. Figures 2 and S37

display the electronic absorption spectra of the parent dyes and

their platinum complexes, while Table 3 lists all relevant data

including band assignments and calculated transition energies

derived from time-dependent density functional theory (TD-

DFT; for comparisons of experimental and calculated spectra

see Figures S38 −S40). Remarkably, η²-coordination of the 3-

chloro-5,7-dimethylbodipy dye to the Pt(PEt₃)₂moiety does

not aﬀect the position and intensity of the characteristic bodipy

π → π* band. Complex 8,9-Pt and the parent dye have their

absorption maximum at 508 nm, and, on platinum coordina-

tion, the extinction coeﬃcient is reduced by about the same

factor as the molecular mass increases. Geometry optimization

of the molecular structure on the pbe1pbe/6-31g(d) level with

the CH₂Cl₂solvent accounted for by the PCM formalism

overestimates the structural perturbation due to η²-coordina-

tion somewhat. Thus, the two pyrrole rings are more bent with

a calculated interplanar angle of 24.5° as opposed to the

experimental value of 17.0° (vide supra). The calculated

stronger structural distortion might also cause some diﬀerences

between the TD-DFT calculated and experimental spectra in

terms of the energetic ordering of the individual transitions as

indicated by Figure S40.

According to our calculations all relevant MOs of 8,9-Pt

receive strong contributions from the Pt(PEt₃)₂fragment (40

and 37%, respectively as inferred from Mulliken decomposition

analysis, see Table S2). An intense absorption arising from the

typical π → π* excitation of the π-coordinated bodipy dye is

calculated as the HOMO−1 → LUMO transition and hence at

higher energy than the weaker, but still rather strong, HOMO

→ LUMO band (see Figure S41). The latter transition is

predicted to have appreciable intraligand charge-transfer (CT)

character with a shifting of electron density from the chloro-

substituted to the dimethyl-substituted pyrrole ring. Both, the

relative intensities and the solvatochromism of these bands

rather point to an opposite ordering of the HOMO and

HOMO−1 levels. Thus, the prominent band at ca. 500 nm is

only slightly solvatochromic with a blue shift of 280 cm⁻¹

C9−Pt1−P2

P1−Pt−P2

135.8(3)

103.92(9)

81.6

a

[B]−[Pt]

[Pyr]−[Pyr]

b

17.0

a

Interplanar angle between the best plane through the inner six-

membered ring of the bodipy ligand and the coordination plane at the

b

platinum ion. Interplanar angle between the planes of the pyrrolic

subunits.

cell of 8,9-Pt contains both diﬀerent enantiomers that result

from platinum coordination to either the Re or Si face of the

prochiral oleﬁn. The crystal of 2-Pt-6I contained highly

disordered molecules of n-pentane. All attempts to reﬁne the

solvent molecules were unsuccessful and the corresponding

reﬂexes were removed using the solvent mask tool in OLEX2.

The molecular structures are displayed in Figure 1. Every σ-

bodipy platinum complex exhibits an almost ideal square-planar

coordination geometry at the coordination center with the cis

bond angles at the platinum ion close to 90°. The P−Pt−P

angle is slightly bent at 172.68−178.56°. The coordination

plane at the platinum atom as deﬁned by the atoms Pt, P1, P2,

I, and C2 or C3 is in a close to orthogonal orientation

(interplanar angles 85.4−89.6°) with respect to the plane

through the inner six-membered ring of the attached bodipy

ligand. The lengths of the Pt−C2 σ-bonds fall within a narrow

range of 1.991(11)−2.027(4) Å and are nearly identical within

the limits of the standard deviations, while the bond length Pt−

C3 of 1.977(7) Å is slightly shorter. Previously reported 8-Pt,

where the Pt ion attaches to the meso position of the bodipy,

has a similar Pt−C bond length of 1.994(10) Å.⁵⁹With

interplanar angles of 76.9−88.9°, the meso aryl substituents are

also in a close to orthogonal orientation with the bodipy plane.

The C8−Pt−C9 bond angle of 38.3(4)° and the angle P1−Pt−

P2 of 103.92(9)° (Table 2) closely resemble those of other

diphosphane platinum oleﬁn complexes in the literature.^58,60,61

The Pt−C bond lengths are longer than those of

2.031(3)−2.114(4) Å found for related compounds, while the

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Figure 1. ORTEP⁶²diagrams of (a) 2-Pt-6H, (b) 2-Pt-6I, (c) 2-Pt-Mes-6I, (d) 2-Pt-6Et, (e) 3-Pt, and (f) 8,9-Pt. The ellipsoids are drawn at a 50%

probability level. Solvent molecules and hydrogen atoms are omitted for reasons of clarity. For 2-Pt-6H, 2-Pt-6Et, and 8,9-Pt only one of the two

independent molecules of the unit cell are shown.

Figure 2. (a) Electronic absorption spectra of the halogenated dye precursors in ca. 1 × 10⁻⁵M toluene solutions. (b) UV−vis spectra of the bodipy

platinum complexes in ca. 1 × 10⁻⁵M toluene solutions.

E

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Table 3. Absorption Data of the Complexes in ca. 1 × 10⁻⁵M Solutions in THF, Toluene, or CH₂Cl₂and TD-DFT-Calculated

a

(PBE1PBE/6-31G(d) PCM (CH₂Cl₂)) Energies and Band Assignments

absorption data

λ_max/nm (ε/L mol⁻¹cm⁻¹

TD-DFT calculations

major contributions

a

compound

solvent

)

λ (nm)

f

Assigment

b

2-Pt-6H

toluene 324 (3100), 399 (7500), 425 (10300), 532 (40700),

546 (42400)

n.c.

CH₂Cl₂321 (4200), 400 (8000), 425 (10700), 531 (36100)

THF

321 (4500), 400 (8200), 424 (10700), 537 (39300)

2-Pt-6I

toluene 393 (7600), 433 (11000), 534 (41200), 554 (44700)

CH₂Cl₂393 (7600), 433 (11000), 535 (36000), 552 (35900)

THF

393 (7600), 431 (11100), 535 (36000), 555 (34200)

2-Pt-Mes-6I

toluene 362 (7300), 384 (8900), 430 (10900), 530 (38300),

553 (44400)

465

392

0.66

0.26

HOMO → LUMO (94%)

π → π*

ML→L′CT

a

HOMO−3 → LUMO (23%),

HOMO−1 → LUMO (70%)

HOMO−5 → LUMO (76%),

HOMO−3 → LUMO (11%)

HOMO−7 → LUMO (97%)

HOMO−13 → LUMO (91%)

a

CH₂Cl₂381 (7400), 418 (10700), 430 (11300), 528 (38800),

553 (40900)

352

0.032

ML→L′CT, ILCT

a

THF

378 (6200), 415 (8600), 430 (9110), 529 (39300),

554 (40800)

335

261

0.083

0.082

ILCT

a

ML→L′CT

b

2-Pt-6Et

toluene 377 (6900), 420 (11000), 541 (47400), 560 (59300)

CH₂Cl₂378 (8000), 420 (11600), 535 (40700), 556 (49000)

n.c.

THF

toluene 320 (4800), 508 (38900)

THF 349 (4800), 501 (37900)

384 (7700), 421 (10700), 539 (44000), 556 (51500)

a

c

8,9-Pt

3-Pt

409

334

439

334

248

392

309

295

0.19

0.36

0.77

0.070

0.064

0.39

0.07

0.17

HOMO → LUMO (96%)

HOMO−1 → LUMO (86%)

HOMO → LUMO (99%)

HOMO−4 → LUMO (97%)

HOMO−10 → LUMO (91%)

HOMO → LUMO (97%)

HOMO−5 → LUMO (81%)

ILCT with mc

c

π → π* with mc

π → π*

toluene 314 (7900), 359 (8300), 531 (125000)

CH₂Cl₂312 (4400), 355 (4400), 528 (101100)

a

ILCT

a

THF

312 (4400), 355 (4400), 527 (109300)

ML→L′CT

π → π*

ML→L′CT

π → π*

8-Pt

CH₂Cl₂323 (12500), 344 (6500), 471 (51600)

a

HOMO−6 → LUMO (90%)

a

b

c

f = Oscillator strength, ML → L′CT = metal/ligand to ligand′ charge transfer, ILCT = intraligand charge transfer. Not calculated. mc = Metal

contribution.

Figure 3. Graphical representation of the relevant MOs, transitions, and TD-DFT energies of 2-Pt-Mes-6I. Solid arrows symbolize the main

contributor(s) to the respective transition.

F

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Figure 4. Graphical representation of the relevant MOs, transitions, and TD-DFT energies of 3-Pt. Solid arrows symbolize the main contributor to

the respective transition.

Table 4. Photophysical Data of the σ-Bodipy Platinum Complexes in Degassed CH₂Cl₂, Toluene, or 2-Me-THF Solutions at

Concentrations of ca. 10⁻⁶M at r. t.

λ_max,Fl/nm

(Stokes shift/ (Stokes shift/

cm⁻¹cm⁻¹

λ_max,Ph/nm

k_ISC,S1→T1

k_ISC,T1→S0

f

c

f

)

Φ_Fl

Φ_Ph

Φ_Δ

τ_Fl(ns)

τ_Ph(μs)

(s⁻¹

)

(s⁻¹

)

a

8-Pt

481 (441)

637 (5669)

804 (5877)

0.002

0.364

0.95 0.484

297

2

2.1 × 10⁹

2.2 × 10⁸

2.0 × 10³

1.3 × 10⁴

b

2-Pt-6H

580 (1074)

0.0008

0.0009

0.51 4.5 0.2

80 4,

d

765 8,

d

475

9

d

2-Pt-6I

590 (1036)

588 (1109)

587 (853)

804 (5548)

797 (5569)

815 (5619)

0.0003

0.0009

0.0006

0.0004

0.0014

0.0003

0.53 3.7 0.2, 30

1

67 3,

1.5 × 10⁴

6.7 × 10³

4.1 × 10³

d

433 11,

d

247

5

e

2-Pt-Mes-6I

2-Pt-6Et

0.48 n.d.

150 10,

843 64,

d

269

9

0.34 4.12 0.0.09,

243 4,

683 18

2.4 × 10⁸

d

0.0459 0.008,

4.23 0.09

d

e

8,9-Pt

516 (305)

535 (141)

501 (532)

0.486

0.527

0.73

n.d.

5.33 0.02,

6.28 0.04

0.96 × 10⁸

2.06 × 10⁸

g

3-Pt

0.19 2.30 0.01,

2.52 0.01

d

h

3-Cl-5,7-Me₂-bodipy

5.60 0.01

a

b

c

Measured in CH₂Cl₂solution at r.t. Determined using 2,6-I₂-8-Ph-Bodipy (Φ = 3.13%) as a standard. ¹O₂generation quantum yield corrected

d

e

f

using the value of methylene blue (MB, Φ_Δ= 0.57) as a reference. Measured in 2-Me-THF glass at 77 K. Not determined. k_ISCis the rate constant

for intersystem crossing and was estimated from eq 2. Measured in toluene glass at 77 K. According to ref 64.

g

h

between toluene and THF, while the second, less intense band

exhibits a sizable red shift of 2600 cm⁻¹, indicating a more polar

excited state (see Figure S37f).

the HOMO of a bodipy dye has a nodal plane along the C8−B

vector, whereas the LUMO has not. Hence, the electron-

donating Pt(PR₃)₂I fragment destabilizes the LUMO more than

the HOMO, thereby increasing the HOMO−LUMO gap. In

contrast, when bound to carbon atoms C2 or C3, the Pt(PR₃)₂I

moiety contributes more to the HOMO than to the LUMO,

which decreases the HOMO−LUMO gap by 500 or 1100 cm⁻¹

with respect to the free halogenated dye (Figures 3 and 4).

Similar to 8-Pt, the π → π* band of 3-Pt is sharp, whereas it

is broadened signiﬁcantly in all of the 2-platinated bodipy

complexes. Such broadening suggests a stronger perturbation of

the bodipy π-system by the attached Pt(PEt₃)₂I fragment and

additional underlying transitions resulting in richer absorption

All complexes exhibit the characteristic sharp and intense π

→ π* absorption band of a bodipy dye. Platinum coordination

to the 2- or 3-position shifts the main absorption band

bathochromic, e.g., from 508 nm for 3-chloro-5,7-dimethylbo-

dipy to 531 nm (Δυ

̃

= 850 cm⁻¹) for 3-Pt or from 516 nm for

2-iodo-8-phenylbodipy to 546 nm (Δυ

̃

= 1070 cm⁻¹) in 2-Pt-

6H. This is in striking contrast to the hypsochromic shift of

1600 cm⁻¹observed for the previously reported complex 8-Pt,

where the platinum ion attaches to the meso position C8.⁶³

(TD)-DFT calculations explain this disparate behavior. Thus,

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Figure 5. Left: Luminescence spectra of 3-Pt (green) and 8,9-Pt (cyan) in ca. 10⁻⁶M toluene solution; right: luminescence spectra of 2-Pt-6H

(orange), 2-Pt-6I (red), 2-Pt-Mes-6I (blue), and 2-Pt-6Et (magenta).

features. This is also borne out by our TD-DFT calculations on

model complexes 3-Pt and 2-Pt-Mes-6I. Calculated and

experimental spectra are compared in Figures S38 and S39,

while Figures 3 and 4 display the crucial MOs and the

computed transitions. Electron density diﬀerence maps

(EDDMs) of the calculated transitions are also included as

Figures S43 and S44; fragment contributions to the

corresponding MOs according to Mulliken decomposition

analysis are provided as Tables S3 and S4. Our calculations

provide only a qualitative level of agreement with the

experimental data (see Table 3) because the energies of the

electronic transitions are overestimated by DFT methods. This

is a known problem for bodipys and related cyanine dyes.^33,34

Nevertheless, TD-DFT calculations agree with our experimen-

tal observations in predicting additional, rather intense

HOMO−1/HOMO−3 → LUMO transitions with PtI →

bodipy CT character for 2-Pt-Mes-6I at rather close energies to

the dominant π → π* transition (see Table 3 and Figure 3).

The latter have no equivalent in 3-Pt (Figure 4) or 8-Pt.⁵⁹The

predicted ML → L′CT admixture to the high-energy part of

this band still fails to cause any notable solvatochromism (see

Figure S37a −d). A similar broadening of the HOMO →

LUMO absorption band of a Pt(bipy)(2-ethynylbodipy)₂

complex was recently observed by Zhong et al. and likewise

attributed to MLCT and intraligand charge-transfer (ILCT)

admixtures to the main π → π* excitation.⁴¹The results of our

TD-DFT calculations for 3-Pt provide an excellent match with

the experimental spectrum in predicting a single, intense

bodipy-based π → π* band and additional ML → L′CT and

ILCT bands of only minor intensity in the UV (see Figures 4

and S39).

very weakly ﬂuorescent but phosphoresces at 641 nm with a

quantum yield of up to 41%.⁵⁹Complexes 3-Pt and 8,9-Pt are

strongly ﬂuorescent in solution at r.t. (Φ_Flca. 50%) with an

emission peak at 535 or 516 nm, respectively, but do not

phosphoresce, even in a 2-MeTHF glass matrix at 77 K

(Figures S45 and S46). Like in the absorption spectrum,

platinum binding to the CC double bond does not cause any

appreciable shift of the emission wavelength with respect to 3-

chloro-5,7-dimethyl-bodipy but decreases the quantum yield

from 76 to 53%.⁶⁴

In contrast, the C2-plantinated complexes exhibit dual

luminescence with a ﬂuorescence peak at 580−590 nm and

phosphorescence in the near-infrared (NIR), at 797−815 nm.

NIR phosphorescence of a bodipy dye at r.t. in ﬂuid solution is

a rather rare asset.^{26,30,41,65−67}Comparison of the excitation

spectra recorded for the ﬂuorescence and the phosphorescence

emissions with the absorption spectra in Figure S47 proves that

both emissions emanate from the same complex and that none

of them is due to an impurity. As an unfortunate consequence

3

of the small energy gap between the ππ* and the S₀states of

the 2-Pt-bodipy complexes and the concomitantly higher

probability of radiationless decay to the ground state as

expressed by the energy gap law,^68,69phosphorescence

quantum yields are signiﬁcantly smaller than those for 8-Pt.

k_ISC= (1 − Φ_F1)/τ_F1− k_IC

(1)

k_ISC≤ (1 − Φ)/τ

(2)

ISC rate constants k_ISCwere estimated from eq 2.⁷⁰By

assuming k_IC≈ 0 in eq 1 for all complexes, an upper limit for

k

_ISCis obtained.⁷⁰Overall, the rate of ISC from the S₁to the T₁

Luminescence Spectroscopy. As just discussed, changing

the site of platinum attachment to the bodipy chromophore

from meso/C8 to C2 or C3 gauges platinum contribution to the

relevant frontier MOs and, in case of the 2-complexes, also

mixes some PtI → bodipy CT character into the main π → π*

absorption. One therefore expects that the positioning of the

Pt(PEt₃)₂I entity inﬂuences the ISC rates and emission energies

as well as the relative intensities of the ﬂuorescence and

phosphorescence emissions. This is indeed the case.

Table 4 and Figure 5 summarize the luminescence properties

of the new complexes as measured in gastight cuvettes under

rigorous exclusion of air, along with those of the previously

reported 8-Pt as a point of comparison. The latter complex is

state follows the trend 8-Pt > 2-Pt-6H,Et,I > 3-Pt (Table 4).

Comparison of the luminescence properties of the complexes

2-Pt-6H, 2-Pt-6Et, 2-Pt-6I, and 2-Pt-Mes-6I reveals that the

substituent in 6-position at the second pyrrole ring, and, in

particular, the presence of the iodine heavy atom or blocking

the rotation of the meso aryl substituent, have only minor eﬀects

on the ISC rates, emission lifetimes, and, consequently, the

relative intensities of the ﬂuorescence and phosphorescence

emissions. It was observed on other occasions that replacing the

meso phenyl by a mesityl substiuent serves to close channels for

radiationless decay by rotation of the aryl substituent around

the C8-aryl bond.^71,72In the case of complexes 2-Pt-6I and 2-

Pt-Mes-6I, this modiﬁcation also increases the QY of the NIR

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DOI: 10.1021/acs.organomet.7b00806

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Figure 6. Spin density surfaces of the T₁state of 2-Pt-Mes-6I (left), 3-Pt (middle), and 8-Pt (right).⁷³

phosphorescence emission, but it still remains very modest at

0.14%. Phosphorescence lifetimes of the 2-Pt-bodipy complexes

range from 67 and 243 μs at r.t. and increase signiﬁcantly upon

cooling to 77 K in a rigid 2-MeTHF matrix (Table 4). DFT

calculations indicate that the ﬂuorescence and phosphorescence

emissions emanate from the ¹ππ* and ³ππ* excited states of the

bodipy ligand. Figure 6 shows that the spin density of the triplet

state T₁of complexes 8-Pt, 2-Pt-Mes-6I, and 3-Pt concentrates

on the bodipy ligand with only minor contributions from the

Pt(PEt₃)₂I fragment (see also Table S5 for fragment

contributions according to Mulliken).

UV/vis spectra at regular intervals during photoirradiation and

calculating the pseudo ﬁrst-order kinetics for DHN con-

sumption (for details see the Experimental Section and Figures

S48 −S52). Slight deviations of the slopes when plotting ln(C_t/

C₀) as a function of time t after 60 min mark the onset of

photodecomposition.

Table 5 summarizes the results of these experiments.

Complexes of the 2-Pt series reach only modest quantum

yields of 53 and 34%. Still, DHN oxidation to Juglone occurs

with higher turnover frequencies (TOFs) than for 8-Pt as a

result of a better overlap of their absorption envelopes with the

radiation of the light source as expressed by the higher value of

I (Table 5). Although 3-Pt is nonphosphorescent, its excited

triplet state is still populated to an appreciable extent. This

follows from the reduction of the ﬂuorescence lifetime τ_Fl

compared to 3-chloro-5,7-dimethyl-bodipy (5.46 0.01 ns)⁶⁴

An important asset of bodipy dyes is their potency to form

3

¹O₂from O₂via TTA. This makes bodipys highly interesting

sensitizers for applications in photocatalysis^31,74−77and photo-

dynamic therapy (PDT).^{10,25,26,78,79}8-Pt and its closely related

trans-Pt(bodipy)Cl(PEt₃)₂analogue have shown up to almost

1

unitarian quantum yields for photochemical O₂generation

and from the O₂quantum yield of 19% using 3-Pt as the

with no signs of decomposition over 3 h. The model

photooxidation of 1,5-dihydroxynaphthalene (DHN) to

Juglone pursuant to Scheme 2 was monitored by recording

sensitizer. We suspect that the substantially lower quantum

yields of the present complexes for ¹O₂generation when

compared to 8-Pt are also rooted in the comparatively low

energies of their T₁states, which renders energy transfer to ³O₂

less exergonic while making decay by other pathways more

eﬃcient. In summary, we note that the QYs for ¹O₂generation

mirror the trend for the ISC rates and decrease from 8-Pt to the

2-platinated complexes and to 3-Pt, paralleling the trends

obtained from luminescence spectroscopy (vide supra).

1

Scheme 2. Photooxidation of DHN to Juglone with O₂in

the Presence of a Photosensitizer

In order to probe whether DHN oxidation occurs preferably

via the direct reductive quenching of the excited state of the

complexes with DHN as the electron donor or by energy

3

transfer to O₂and the subsequent reaction of DHN with

1

photogenerated O₂we exemplarily monitored the phosphor-

escence quenching of 2-Pt-Mes-6I with oxygen or DHN as the

quencher. Stern−Volmer plots and stack plots showing the

emission intensities and decays without or with the respective

quencher at various concentration levels are provided as Figures

Table 5. Pseudo First-Order Kinetics, ¹O₂Generation Quantum Yields, and Turnover Frequencies for the DHN Photooxidation

Using the Complexes 8-Pt,⁵⁹2-Pt-6H, 2-Pt-6I, 2-Pt-Mes-6I, 2-Pt-6Et, 3-Pt, and Methylene Blue (MB) as Sensitizers

a

b

c

d

e

f

λ_ex(nm)

k_obs(min⁻¹

)

ν_i(× 10⁻⁶M min⁻¹

)

I

Φ_Δ

yield (%)

TOF (s⁻¹

)

8-Pt

460

550

530

655

5

0.00145

0.00377

0.00413

0.00449

0.00344

0.00337

0.00102

0.1757

0.4649

0.5093

0.5130

0.4242

0.4155

0.1296

0.810

3.988

4.221

4.665

5.462

9.557

1

0.95

0.51

0.53

0.48

0.34

0.19

0.57

25

35

24

26

23

26

5

0.0014

0.0037

0.0025

0.0029

0.0024

0.0027

0.0003

2-Pt-6H

2-Pt-6I

2-Pt-Mes-6I

2-Pt-6Et

3-Pt

MB

a

b

c

Pseudo-ﬁrst-order rate constant for DHN consumption. Initial rate of DHN consumption. Relative number of photons absorbed by the sensitizer

(I = 1 for the standard sensitizer MB). Corrected ¹O₂generation quantum yields using the value of MB (Φ_Δ= 0.57)⁸¹⁻⁸³as a reference. Yield of

Juglone after a reaction time of 60 min. Turnover frequency.

d

e

f

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S53 −S58. With a Stern−Volmer quenching constant K_SVof

the bodipy carbon atom C2 or C3 slightly surpasses that of

340−510 mV for the 0/− process (Table 6). This is consistent

with the higher contribution of the Pt(PEt₃)₂I fragment to the

HOMO (c. f. Figures 3 and 4 and Tables S3 and S4).

2150

200 bar⁻¹or 670

61 μM⁻¹, the phosphorescence

emission of 2-Pt-Mes-6I was found to be almost as sensitive to

oxygen as that of 8-Pt (K_SV= 2580

71 bar⁻¹).⁵⁹The

quenching constant for DHN as the quencher is smaller by a

factor of 22 (K_SV= 31

2 μM⁻¹), which identiﬁes energy

The oxidation (0/+) or reduction (0/−) half-wave potentials

also trace the degree of the electronic interaction of the

Pt(PR₃)₂I-fragment with the bodipy chromophore. Attachment

to the meso-position C8 causes a smaller potential diﬀerence

ΔE_1/2of 240 mV with respect to the parent dye than platinum

attachment to carbon atom C2 (ΔE_1/2= 340 mV) or C3

(ΔE_1/2= 510 mV; note, however, that the presence of a chloro

substituent in 3-Cl-bodipy instead of the iodo substituent in

the other dyes likely contributes to the increased potential shift

of 3-Pt). Exchanging the substituent in 6-position of the 2-

platinated dyes has a similar eﬀect on both half-wave potentials

and causes a cathodic shift with increasing electron donating

capability in the ordering Et > H > I.

All our previous data indicate that the HOMO and LUMO of

the platinum σ-bodipy complexes are dominated by the bodipy

ligand. Monitoring the changes in the UV/vis/NIR spectra on

one-electron reduction or oxidation provides a direct means to

experimentally probe for the identity of the corresponding

redox site. Some accounts of spectroelectrochemical inves-

tigations on bodipy dyes have already appeared in the literature.

The results of these studies can serve as reference points for our

present results.^{84,85,87,89,91,92,94,95}Like for the other studies, the

radical cations or anions were generated inside a transparent

UV/vis/NIR cell⁹⁶or inside an EPR tube under the in situ

conditions of spectroelectrochemistry. The results of these

studies are displayed in Figures 9 and S61 −S63, while Table 7

summarizes the relevant data and the TD-DFT calculated band

energies and assignments. Plots of the pertinent MOs and

transitions along with the corresponding electron density

diﬀerence maps (EDDMs) of the oxidized and reduced forms

of representative complexes as well as fragment contributions

according to Mulliken analysis can be retrieved from Figures

S64 −S71 and Tables S7 −S10.

As it was previously observed for other bodipy dyes, the

prominent π → π* band partially bleaches on reduction while a

new, weaker absorption band grows in. The latter is by ca. 40−

50 nm to the red from the former HOMO → LUMO band

(see Figures 9a−c, S61a, and S62a). According to our TD-DFT

results, the new low-energy band of the radical anions is

assigned as a bodipy → Pt(PEt₃)₂CT transition (see Table 7,

Figures S64, S65, and S68 −S71). As shown in Figure S62a, the

reduction of 8-mesityl-2,6-diiodo-bodipy produces nearly

identical spectroscopic changes. All reduction processes are

highly reversible as shown by the multiple isosbestic points and

the nearly complete recovery of the original spectra after a full

reduction/reoxidation cycle (c.f. Figures S62b and S63).

On the time scale of spectroelectrochemical experiments,

only 2-Pt-Mes-6I (Figures 9d and S63e) and 2-Pt-6Et (Figure

S61b and S63f) can be reversibly oxidized. Concomitant with

the bleaching of the characteristic π → π* transition, a weak,

broad NIR band enveloping the entire regime between 750 and

1750 nm is observed. This again agrees with previous studies

on other bodipys.^87,88,94,95TD-DFT calculations suggest that

the NIR absorption of the 2-Pt-Mes-6I radical cation originates

from a mixed ILCT (Mes → bodipy) and PtI → bodipy ML →

L′CT transition at 1162 nm and at 805 nm with CT from the

iodo and mesityl substituents to the bodipy π system (see

Figures S66 and S67 and Table 7).

transfer as the dominant pathway. In agreement with that,

quenching by electron transfer is calculated to be endergonic by

1.01 eV (97.5 kJ/mol) based on the Rehm−Weller equation

(see also Figure S59 for the peak potential of electrochemical

DHN oxidation).⁸⁰Similar values were derived for the other

complexes so that this statement should apply for these as well.

Electrochemistry and UV/Vis/NIR and EPR Spectro-

electrochemistry. Bodipys are generally electroactive and can

be oxidized and reduced by one electron each.^41,53,54,56Their

associated radical cations or anions are sometimes suﬃciently

persistent to allow for their spectroscopic characterization by

UV/vis/NIR⁸⁴⁻⁹⁰or EPR spectroscopy.⁹¹Such investigations

are particularly relevant for the spectroscopic identiﬁcation of

bodipy radical species in the photoexcited, charge-separated

states. Examples are dyads or triads, where a bodipy is linked to

a peripheral 2,4,5-trimethoxybenzene,⁸⁵phenylanthracene,⁸⁶

triarylamine⁸⁷or N-(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]pyrrole

(DTP)⁹²electron donor or a fullerene⁸⁹or carborane

acceptor.⁹⁰The latter are of great interest with respect to

light-to-energy conversion schemes or mimicking natural

enzymes.^{85,87,90,92,93}

The results of our voltammetric studies on the σ-platinum

bodipy complexes are summarized in Table 6. In their cyclic

Table 6. Electrochemical Data for all Complexes and

a

Representative Halogenated Bodipys

E_1/2

complex

0/+

0/−

8-Br-bodipy

8-Mes-2,6-I₂-bodipy

3-Cl-bodipy

2-Pt-6H

−1530

−1520

−1420

−1920

−1810

−1860

−1960

−1930

−1770

890

950

450

540

370

410

2-Pt-6I

2-Pt-Mes-6I

2-Pt-6Et

3-Pt

8-Pt

a

All potentials are given in mV relative to the Cp₂Fe^0/+couple (E_1/2

=

0.000 V) and were measured in CH₂Cl₂at 293 K with NBu₄PF₆as the

supporting electrolyte.

voltammograms, all complexes except for 8-Pt show a fully or

nearly reversible one-electron oxidation and a reversible to

partially reversible reduction within the electrochemical

window of the CH₂Cl₂/NBu₄PF₆supporting electrolyte (see

Figures 7 and 8). Figure S60 illustrates the voltammetric traces

of the 8-mesityl-2,6-diiodobodipy, 3-chloro-5,7-dimethyl-

bodipy, and 8-bromo-bodipy precursors under the same

conditions as points of comparison. For the latter two

compounds, higher sweep rates were required in order to

outrun chemical follow processes following oxidation or

reduction. Similar to related bis(phosphane) platinum alkynyl

complexes containing 2- or 8-ethynylated bodipy ligands,^33,35

attachment of the electron-rich platinum entity causes sizable

cathodic shifts of both redox waves. The cathodic shift of 450−

540 mV for the 0/+ redox couple on platinum attachment to

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Figure 7. Cyclic voltammograms (v = 100 mV/s) recorded for anodic (top) and cathodic (bottom) sweeps of (a, b) 2-Pt-6H, (c, d) 2-Pt-6I, (e, f) 2-

Pt-Mes-6I, and (g, h) 2-Pt-6Et in CH₂Cl₂at T = 293 K with NBu₄PF₆(0.1 M) as the supporting electrolyte.

EPR spectra of the radical cations of the Pt complexes are

broader but exhibit clearly resolved hfs to the platinum nucleus

of the ¹⁹⁵Pt-containing isotopomer. At 100.9 G, the value of the

2-platinated complex 2-Pt-Mes-6I is larger than that of 81.7 G

for 3-Pt. As a result of the minor metal contribution to the

SOMO (for calculated spin densities, see Figures 13 and 14,

and for fragment analysis, see Tables S7 −S10), the g-values are

only moderately increased with respect to the Lande factor g_eof

́

2.002319 of the free electron and that of the 8-mesityl-2,6-

diiodo-bodipy radical cation.

Quite remarkably, the EPR spectrum of the 8-Pt radical

anion is entirely dominated by hyperﬁne interactions with ³¹P

and the ¹⁹⁵Pt nuclei of the Pt(PEt₃)₂fragment (see Figure 10,

Table 8). Calculated spin densities are displayed in Figure S73,

while a fragment analysis is provided in Table S11. The EPR

spectra of the radical anions of free dyes and associated

platinum complexes 2-Pt-Mes-6I and 3-Pt oﬀer an even richer

structuring due to resolved hfs to the ¹⁴N, ^10/11B, and ¹⁹F nuclei

(see Figures 13 and 14 for calculated spin density

distributions). Comparison of the spectra of the 8-mesityl-

2,6-diiodo-bodipy radical anion (see Figure S72 and Table S11

for calculated spin density distributions) with those of reduced

2-Pt-Mes-6I and 3-Pt evidence the presence of hfs interactions

with the ¹⁹⁵Pt nucleus in the latter. Spectra simulated with the

hfs constants compiled in Table 8 are also displayed in Figures

10−12 and S72. Despite the overall good agreement between

experimental and simulated spectra, the multitude of possible

hfs interactions render our simulations and assignments of hfs

constants only tentative. Again, the g-values of the reduced

complexes (1.9917−1.9993) are close to that of 1.9935 for the

8-mesityl-2,6-diiodo-bodipy radical anion. This again conﬁrms

the organic parentage of these paramagnetic species.

Figure 8. Cyclic voltammograms recorded for anodic (top) and

cathodic (bottom) sweeps of (a) 8-Pt (v = 100 mV/s) and (b, c) 3-Pt

(v = 200 mV/s) in CH₂Cl₂solution at T = 293 K with NBu₄PF₆(0.1

M) as the supporting electrolyte.

Owing to the presence of multiple nuclei with a nuclear spin

I ≠ 0, and the fundamental diﬀerences between ligand- and

metal-centered spin, EPR spectroscopy provides another potent

tool to probe for the identity of the respective redox site. In

particular, the hyperﬁne splittings (hfs) to the abundant

(33.8%) ¹⁹⁵Pt I = 1/2 nucleus provides a means for probing

the platinum contribution to the relevant SOMO. We note that

EPR spectroscopic investigations on bodipy-derived radical

cations or anions are extremely rare.⁹¹The radical forms of all

compounds were prepared by electrochemical oxidation or

reduction inside an EPR tube using a three-electrode

conﬁguration. EPR spectra of the one-electron reduced forms

of the complexes 2-Pt-Mes-6I, 3-Pt, and 8-Pt as well as those

of oxidized 2-Pt-Mes-6I and 3-Pt are displayed in Figures

10−12. Comparison with the EPR spectra of the oxidized and

reduced forms of the 8-mesityl-2,6-diiodo-bodipy precursor

(see Figure S72) aids in the identiﬁcation of the ¹⁹⁵Pt and ³¹P

hfs.

In summary, the close resemblance of the UV/vis/NIR and

EPR spectra of the oxidized and reduced forms of 8-mesityl-2,6-

diiodo-bodipy and related bodipy dyes to those of the

electrogenerated radical cations and anions of the σ-bodipy

platinum complexes fully agrees with our conclusions derived

from their optical and photophysical properties and reconﬁrms

that their HOMOs and LUMOs are largely based on the

bodipy ligands.

The radical cation of 8-mesityl-2,6-diiodo-bodipy gives a

partially resolved EPR spectrum at a g-value of 2.0042. The

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Figure 9. Changes of the UV/vis/NIR spectra (1,2-C₂H₄Cl₂, NBu₄PF₆, T = 293 K) on (a) reduction of 8-Pt, (b) reduction of 2-Pt-Mes-6I, (c)

reduction of 3-Pt, and (d) oxidation of 2-Pt-Mes-6I.

vis/NIR and EPR spectroelectrochemistry allowed for spectro-

scopic characterization of the persistent radical anions and of

some suﬃciently stable radical cations of the complexes and

provides direct experimental evidence for ligand-centered

frontier MOs. Thus, their UV/vis/NIR spectroscopic patterns

are very similar to those of the one-electron oxidized or

reduced forms of their bodipy precursors. Moreover, richly

structured EPR spectra with resolved hfs to heteronuclei (¹⁹⁵Pt,

³¹P, ¹⁴N, and ¹⁹F) with g values close to g_ehave been recorded

and attest to a dominantly organic spin, again in full agreement

with DFT calculated spin density distributions.

SUMMARY AND CONCLUSIONS

■

Five new platinum bodipy complexes, where a trans-Pt(PEt₃)₂I

moiety attaches to the pyrrolic carbon atom C2 or C3 of a

bodipy dye via a direct Pt−bodipy σ-bond have been prepared.

They complement previous series of complexes where a trans-

Pt(PR₃)₂X (R = Ph, X = Br; R = Et, X = Cl, Br, I, NCS, NO₂,

and CH₃) entity binds to the meso bodipy position C8.^59,63The

higher activation barrier for oxidative addition of the Pt(PEt₃)₂

fragment to a C_aryl−Cl bond as compared to a C_aryl−I bond has

also allowed us to isolate complex 8,9-Pt as a direct precursor

to complexes cis/trans 3-Cl-Pt, Pt−Cl analog of the iodo

complex 3-Pt. Thermal conversion of 8,9-Pt to 3-Cl-Pt and the

gradual (yet incomplete) cis → trans isomerization of 3-Cl-Pt

were followed by ³¹P NMR spectroscopy. The spectroscopic,

structural, photophysical, and electrochemical properties of all

complexes were studied and compared to those of the meso-

substituted complex 8-Pt and the corresponding bodipy

precursors. Contrary to 8-Pt, platinum attachment to either

position C2 or C3 puts the Pt(PEt₃)₂I entity within the

conjugated π-system and thus induces a sizable red shift of the

characteristic bodipy absorption band along with a broadening

for the 2-isomers. Its dominant bodipy π → π* character is

retained as inferred from the DFT-calculated frontier MOs and

the TD-DFT based band assignments.

Like 8-Pt, all complexes where the Pt ion is bonded to

carbon atom C2 dually emit through ﬂuorescence and

phosphorescence with phosphorescence emission well in the

NIR, at 797−815 nm. However, quantum yields are severely

reduced from 41% in 8-Pt to meager values of 0.14% or less.

Nevertheless, all complexes are active sensitizers for photo-

1

chemical O₂generation and outperform methylene blue as a

catalyst for the photooxidation of 1,5-dihydroxynaphthalene to

Juglone. Our present results show that attachment to either the

2- or 3-positions is no viable route to Pt−bodipy dyes with

intense NIR phosphorescence emission. It remains to be

investigated whether such compounds are accessible from

appropriately substituted 8-bromo (meso) bodipys with

extended π-conjugation at the pyrrole rings. Work along

these lines is presently being pursued in our laboratories.

All complexes can be oxidized or reduced by one electron

each as probed by cyclic voltammetry. Introduction of the

electron-donating Pt(PEt₃)₂I entity causes strong cathodic

shifts of 340−510 mV for both oxidation and reduction. UV/

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X-Area software package delivered with the diﬀractometer. A

semiempirical absorption correction was performed.¹⁰²All structures

were solved by the heavy-atom methods (SHELXS-97,¹⁰³SHELXS-

2013,¹⁰⁴SHELXS-2014,¹⁰⁵or OLEX2¹⁰⁶). Structure solutions were

completed with diﬀerence Fourier syntheses and full-matrix least-

squares reﬁnements using SHELXL-97,¹⁰³SHELXS-2013,¹⁰⁴

SHELXS-2014,¹⁰⁵or OLEX2¹⁰⁶minimizing ω(F₀− F_c²)². The

2

weighted R factor (wR²) and the goodness of the ﬁt GOF are based on

F². All non-hydrogen atoms were reﬁned with anisotropic displace-

ment parameters, while hydrogen atoms were treated in a riding

model. Molecular structures in this work are plotted with PLATON¹⁰⁷

or Mercury.¹⁰⁸CIF ﬁles of the complexes have been deposited at the

Cambridge Structure Data Base as CCDC 1583642 (2-Pt-6H),

CCDC 1583615 (2-Pt-6I), CCDC 1583616 (2-Pt-Mes-6I), CCDC

1583637 (2-Pt-6Et), CCDC 1583640 (8,9-Pt), and CCDC 1583638

(3-Pt) and can be obtained free of charge via www.ccdc.cam.ac.uk/

conts/retrieving.html or from the Cambridge Crystallographic Data

Center, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax (+44)1223−

336−033, or at deposit@ccdc.cam.ac.uk. Luminescence spectra and

lifetimes as well as quantum yields were measured on thoroughly

deaerated solutions (three freeze−pump−thaw cycles in a quartz

cuvette equipped with an angle valve from Normag) of the complexes

in CH₂Cl₂, 2-MeTHF, or toluene solutions on a PicoQuant FluoTime

300 spectrometer. Unless stated otherwise absolute quantum yields

were determined with an integrating sphere. UV/vis/NIR spectra were

recorded on a TIDAS ﬁber optic diode array spectrometer (combined

MCS UV/NIR and PGS NIR instrumentation) from J&M in

HELLMA quartz cuvettes with 0.1 cm optical path lengths. Electron

paramagnetic resonance (EPR) experiments were performed on a

MiniScope MS400 table-top X-band spectrometer from Magnettech.

Simulation of the experimental EPR spectra was performed with the

MATLAB EasySpin program.¹⁰⁹All electrochemical experiments were

executed in a home-built cylindrical vacuum-tight one-compartment

cell. A spiral shaped Pt wire and an Ag wire as the counter and

pseudoreference electrodes are sealed into glass capillaries and ﬁxed by

quickﬁt screws via standard joints. A platinum electrode is introduced

as the working electrode through the top port via a Teﬂon screw cap

with a suitable ﬁtting. It is polished ﬁrst with 1 μm diamond paste and

then 0.25 μm diamond paste before measurements. The cell may be

attached to a conventional Schlenk line via a side arm equipped with a

Teﬂon screw valve, allowing experiments to be performed under an

argon atmosphere with approximately 5−7 mL of analyte solution.

NBu₄PF₆(0.1 mM) was used as the supporting electrolyte.

Referencing was done with addition of an appropriate amount of

decamethylferrocene (Cp*₂Fe) as an internal standard to the analyte

solution after all data of interest had been acquired. Representative sets

of scans were repeated with the added standard. Electrochemical data

were acquired with a computer-controlled BASi CV50 potentiostat.

The optically transparent thin-layer electrochemical (OTTLE) cell was

lab-built and followed the design of Hartl and co-workers.⁹⁶It

Figure 10. Experimental (top curve) and simulated (bottom curve)

EPR spectra of the radical anion of 8-Pt in CH₂Cl₂/NBu₄⁺PF₆⁻at 248

K.

EXPERIMENTAL SECTION

■

Materials and Methods. The oxidative addition reactions were

performed under a N₂atmosphere using standard Schlenk techniques

or inside a glovebox. The cis/trans isomerization reactions and workup

were conducted in air. C₆D₆, CD₂Cl₂, CDCl₃and THF-d₈were

supplied from Eurisotop. Compounds 6-H-bodipy,⁹⁷6-I-bodipy,⁹⁷2,6-

I₂-bodipy,⁹⁷2,6-I₂-8-Mes-bodipy,^53,986-Et-bodipy,⁹⁹3-Cl-bodi-

py,^100,101and Pt(Et)₂(PEt₃)₂⁵⁷were synthesized according to literature

protocols. NMR experiments were carried out on a Varian Unity Inova

400, a Bruker Avance III DRX 400, or a Bruker Avance DRX 600

spectrometer. ¹H and ¹³C spectra were referenced to the solvent

signal, while ³¹P and ¹⁹⁵Pt spectra were referenced to external

standards (85% H₃PO₄or saturated K₂[PtCl₆] in D₂O, respectively).

NMR data are given as follows: chemical shift (δ in ppm), multiplicity

(br, broad; d, doublet; dd, doublet of doublets; m, multiplet; s, singlet;

t, triplet, vt virtual triplet), integration, and coupling constant (Hz).

Unequivocal signal assignments were achieved by 2D NMR experi-

ments. The numbering of the nuclei follows that of the chemical

structures in Figure 1. Combustion analysis was conducted with an

Elementar vario MICRO cube CHN-analyzer from Heraeus.

X-ray diﬀraction analysis of single crystals was performed at 100 K

on a STOE IPDS-II diﬀractometer equipped with a graphite-

monochromated radiation source (λ = 0.71073 Å) and an image

plate detection system. A crystal mounted on a ﬁne glass ﬁber with

silicon grease was employed. If not indicated otherwise, then the

selection, integration, and averaging procedure of the measured

reﬂection intensities, the determination of the unit cell dimensions and

a least-squares ﬁt of the 2θ values as well as data reduction, LP-

correction and space group determination were performed using the

Figure 11. Experimental (top curve) and simulated (bottom curve) EPR spectra of the radical cation (left) and the radical anion (right) of 2-Pt-

Mes-6I in CH₂Cl₂/ NBu₄PF₆at 248 K.

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Figure 12. Experimental (top curve) and simulated (bottom curve) EPR spectra of the radical cation (left) and the radical anion (right) of complex

3-Pt in CH₂Cl₂/NBu₄PF₆at 233 K.

Gaussian 09¹¹⁰program package. Quantum-chemical studies were

performed without any symmetry constraints. Open-shell systems

were calculated by the unrestricted Kohn−Sham approach.¹¹¹

Geometry optimizations followed by vibrational analysis were

performed either in a vacuum or in solvent media. The quasi-

relativistic Wood−Boring small-core pseudopotentials (MWB),^112,113

the corresponding optimized set of basis functions¹¹⁴for platinum and

the halogen atoms, and the 6-31G(d)-polarized double-ζ basis set¹¹⁵

for the remaining atoms were employed together with the Perdew−

Burke−Ernzerhof exchange and correlation functional (PBE0).^116,117

Figure 13. Calculated spin densities of the radical cation (left) and the

Solvent eﬀects were accounted for by the polarizable conductor

continuum model (PCM)^118−121with standard parameters for

dichloromethane. Absorption spectra and orbital energies were

calculated using time-dependent DFT (TD-DFT)¹²²with the same

functional/basis set combinations as those mentioned above. For an

easier comparison with the experiment, the obtained absorption and

emission energies were converted into wavelengths and broadened by

a Gaussian distribution (full width at half-maximum = 3000 cm⁻¹)

using the program GaussSum.¹²³Atomic coordinates of the calculated

structures are provided as a separate ﬁle in the Supporting

Information.

radical anion (right) of 2-Pt-Mes-6I (PBE1PBE/6-31G(d) PCM

(CH₂Cl₂)).

¹O₂Generation from Sensitizers. For the photoreactions

involving ¹O₂generation, a CH₂Cl₂/MeOH (9/1) solution containing

DHN (1.2 × 10⁻⁴M) and a sensitizer (1.7 mol % vs DHN) was

irradiated in a quartz cell of 1 cm path length using the xenon lamp of

a PicoQuant FluoTime 300 spectrometer (λ_exc(3-Pt) = 530 5 nm,

I_f(530 5 nm) = 2.40 mW; (λ_exc(2-Pt-6H, 2-Pt-6I, 2-Pt-Mes-6I, 2-

Pt-6Et) = 550 5 nm, I_f(550 5 nm) = 2.37 mW; λ_exc(MB) = 655

5 nm, I_f(655 5 nm) = 580 μW). UV−vis absorption spectra were

recorded at intervals of 5−20 min on a Varian Cary 50 spectrometer.

The consumption of DHN was monitored by the decrease in

absorption at 301 nm (ε = 7664 M⁻¹cm⁻¹),¹²⁴while Juglone

production was monitored by an increase of the absorption at 427 nm

(ε = 3811 M⁻¹cm⁻¹).¹²⁴The yield of Juglone was calculated from the

Figure 14. Calculated spin densities of the mono-cationic (left) and

mono-anionic (right) form of 3-Pt (PBE1PBE/6-31G(d) PCM

(CH₂Cl₂)).

comprised a Pt working and counter electrode and a thin silver wire as

a pseudoreference electrode sandwiched in between two CaF₂

windows of a conventional liquid IR cell with the working electrode

positioned in the center of the spectrometer beam.

Computational Details. The ground-state electronic structures

were calculated by density functional theory (DFT) methods using the

a

Table 8. EPR Data of 8-Pt, 8-Mes-2,6-I₂-bodipy, 2-Pt-Mes-6I, and 3-Pt

b

bc

,

b

q

g-value

A(¹⁹⁵Pt)

A(³¹P)

A(¹⁴N)

A(^10/11B)

A(¹⁹F)

A(¹²⁷I)

8-Pt

−

+

1.9993

2.0042

1.9935

2.0194

1.9955

2.0075

1.9917

153.8 (1)

21.6 (2)

9.6 (2)

12.4 (2)

11.1 (1)

9.6 (2)

6.2 (2)

8-Mes-2,6-I₂-bodipy

−

+

100.9 (1)

c

2-Pt-Mes-6I

−

+

12.0 (2)

5.8 (1), 3.4 (1)

7.2 (1), 5.3 (1)

8.2 (2)

81.7 (1)

37.5 (1)

c

3-Pt

−

5.8 (1)

11.1 (2)

5.8 (1)

a

b

c

In CH₂Cl₂solution; all hyperﬁne coupling constants are given in G. The number of interacting nuclei is given in parentheses. Unsymmetrically

substituted complexes give two diﬀerent hyperﬁne couplings for the two ¹⁴N nuclei.

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concentration of Juglone and the initial concentration of DHN. The

singlet oxygen quantum yield (Φ_Δ) was determined using eq 3:^124,125

was washed with 2 mL of n-pentane once and dried in vacuo. Yield:

58%. Single crystals were obtained by cooling a saturated C₆H₆/n-

pentane solution to −25 °C. ¹H NMR (399.79 MHz, C₆D₆, 304 K): δ

7.03 (m, 3H, H17/H18/H19), 6.88 (m, 2H, H16/H20), 3.00 (s, 3H,

H12), 2.84 (s, 3H, H13), 1.70 (m, 12 H, P−CH₂CH₃), 1.53 (s with

Φ = Φ_Δ,std(v_iI_std/v_i,stdI)

(3)

Δ

where Φ_Δ,stdis the singlet oxygen quantum yield of a standard

sensitizer (Φ_Δ= 0.57 for methylene blue, MB),⁸¹⁻⁸³ν_iis the initial

rate of DHN consumption, and I and I_stdare the number of photons

absorbed by the sensitizer and the standard, respectively.

shoulders, 3H, H11), 1.35 (s, 3H, H14), 0.80 (dt, ³J_HH= 7.7 Hz, ³J_PH

=

16.3 Hz, 18H, P−CH₂CH₃). ³¹P NMR (161.84 MHz, C₆D₆, 305 K): δ

9.42 (s with satellites, J_PtP= 2540 Hz). ¹⁹⁵Pt NMR (85.55 MHz, C₆D₆,

300 K): δ −4779 (t, J_PtP= 2540 Hz). ¹³C NMR (100.53 MHz, CDCl₃,

2

300 K): δ 164.8 (s with satellites J_PtC= 85.3 Hz, C3), 149.4 (s, C5),

I = I (λ)(1 − 10^{−ε(λ)c l}) dλ

s

∫

f

3

(4)

144.7 (s with satellites J_PtC= 45.8 Hz, C10), 139.1 (s, C8), 136.6 (s,

2

C9), 135.9 (s, C7), 135.4 (t, J_PC= 9.4 Hz, with satellites J_PtC= 1090

I was estimated from eq 4 using the λ interval 525−535 nm for 3-Pt,

545−555 nm for the 2-Pt-bodipy sensitizers, and 650−660 nm for

MB, where I_f(λ) is the wavelength dependence of the intensity of the

incident light evaluated with a photometer (for values vide supra),

ε(λ) is the extinction coeﬃcient of the respective sensitizer recorded in

CH₂Cl₂/MeOH (9/1), c_sis the concentration of the sensitizer, and l is

the length of the cell. Stern−Volmer plots for emission quenching by

oxygen or DHN were obtained by admitting appropriate amounts of

air or of a DHN stock solution into the cuvette with enough time for

equilibration between solvent and gas phase. As for the oxygen

quenching experiment, partial pressures were also transformed into

concentrations with the literature-known oxygen solubilities in

toluene.¹²⁶

2

Hz, C2), 134.9 (s with satellites J_PtC= 37.2 Hz, C1), 129.5 (s, C15),

129.3 (s, C17/C19), 128.9 (s, C18), 128.3 (s, C16/C20), 81.7 (s, C6),

4

3

18.4 (t, J_CF= 2.4 Hz, C12), 18.3 (s with satellites J_PtC= 60.9 Hz,

C11), 16.2 (s, C14), 16.0 (vquint, J_PC= ³J_PC= 17.6 Hz, with satellites

²J_PtC= 35.8 Hz, P−CH₂CH₃), 15.5 (t, ⁴J_CF= 2.5 Hz, C13), 8.4 (s, with

3

satellites J_PtC= 24.2 Hz, P−CH₂CH₃). Anal. Calcd for

C₃₁H₄₇BF₂I₂N₂P₂Pt: C, 36.96; H, 4.70; N, 2.78. Found: C, 36.21, H,

4.34, N, 2.73.

trans-Iodo-(6-idodo-1,3,5,7-tetramethyl-8-mesityl-4,4-diﬂuoro-4-

bora-3a,4a-diaza-s-indacene-2-yl)-bis(triethylphosphane)-

platinum(II) (2-Pt-Mes-6I). Oxidative addition was conducted at 60

°C for 7 h. The ﬁnal crude product was puriﬁed by column

chromatography (Al₂O₃, petrol ether/CH₂Cl₂6:1). The product

fraction was recrystallized form a saturated n-pentane/CH₂Cl₂mixture

forming single crystals at −25 °C. Yield: 42%. ¹H NMR (399.78 MHz,

CDCl₃, 300 K): δ 6.95 (s, 2H, H17/H19), 2.68 (s, 3H, H12), 2.58 (s,

Synthesis and Characterization. Except for 8,9-Pt and 3-Pt, all

other synthesis followed the representative synthesis protocol provided

below. Individual parameters, such as reaction time and temperature,

eluent mixtures for chromatography, and crystallization parameters,

are given for each compound separately.

Typically, a Young tube was ﬁlled with 150 μmol (1 equiv) of

Pt(Et)₂(PEt₃)₂and 0.7 mL of C₆D₆. The solution was frozen,

evacuated, and heated to 110 °C for 90 min. Inside a glovebox, 1.09

equiv of the respective bodipy dye was added. The mixture was

allowed to react for the given time at the stated temperature. Prior to

the next step all volatiles were removed, and 1.4 equiv of AgOTf was

added. The mixture was reﬂuxed in CH₂Cl₂for 15 min. The

precipitate was ﬁltered oﬀ and the resulting solution was added to a

methanolic solution of 2 equiv of NaI. After stirring for 20 min the

solvents were removed and the crude product was puriﬁed as detailed

for every compound separately.

3H, H13), 2.34 (s, 3H, H22), 2.06 (s, 6H, H21/H23), 1.85 (m, 12H,

3

P−CH₂CH₃), 1.34 (s, 3H, H11), 1.33 (s, 3H, H14), 1.01 (dt, J_HH

=

7.7 Hz, J_PH= 16.3 Hz, 18H, P−CH₂CH₃). ³¹P NMR (161.83 MHz,

CDCl₃, 300 K): δ 5.14 (s with satellites, J_PtP= 2539 Hz). ¹⁹⁵Pt NMR

(85.55 MHz, CDCl₃, 300 K): δ −4802 (t, J_PtP= 2539 Hz). ¹³C NMR

3

2

(100.53 MHz, CDCl₃, 300 K): δ 164.1 (s with satellites, J_PtC= 82.1

Hz, C3), 149.4 (s, C5), 144.0 (s with satellites, ³J_PtC= 43.2 Hz, C10),

138.6 (s, C16/C20), 138.3 (s, C7), 136.2 (s, C9), 134.9 (s, C8), 134.7

2

(t, J_PC= 9.7 Hz, C2), 134.2 (s with satellites, J_PtC= 72.1 Hz, C1),

132.0 (s, C15), 129.2 (s, C17/C19), 128.4 (s, C18), 81.3 (s, C6), 21.4

(s, C22), 19.5 (s, C21/C23), 18.4 (t, ⁴J_CF= 2.5 Hz, with satellites, ³J_PtC

3

= 39.5 Hz, C12), 17.3 (s with satellites, J_PtC= 60.5 Hz, C11), 16.2

(vquint, J_PC= J_PC= 17.5 Hz, with satellites J_PtC= 35.9 Hz, P−

CH₂CH₃), 15.6 (t, J_CF= 2.5 Hz, C13), 15.1 (s, C14), 8.4 (s with

satellites, J_PtC= 24.3 Hz, P−CH₂CH₃). Anal. Calcd for

C₃₄H₅₃BF₂I₂N₂P₂Pt: C, 38.91; H, 5.09; N, 2.61. Found: C, 39.44; H,

5.22; N, 2.91.

3

2

trans-Iodo-(1,3,5,7-tetramethyl-8-phenyl-4,4-diﬂuoro-4-bora-

3a,4a-diaza-s-indacene-2-yl)-bis(triethylphosphane)platinum(II) (2-

Pt-6H). The oxidative addition was performed at 45 °C for 8 h. The

ﬁnal product was puriﬁed by column chromatography (silica, CH₂Cl₂/

petrol ether 1:2), washed with 4 × 2 mL of n-pentane, and dried in

vacuo. Yield: 32%. Single crystals for X-ray structure analysis were

obtained cooling a saturated C₆H₆/n-pentane solution of the complex

4

3

trans-Iodo-(6-ethyl-1,3,5,7-tetramethyl-8-phenyl-4,4-diﬂuoro-4-

bora-3a,4a-diaza-s-indacene-2-yl)-bis(triethylphosphane)-

platinum(II) (2-Pt-6Et). For the oxidative addition step, the reaction

mixture was heated to 50 °C for 72 h. The ﬁnal product was puriﬁed

on a short silica column (petrol ether/ethyl acetate 40:1). Crystals

suitable for X-ray structure analysis were obtained from diﬀusion of n-

pentane into a concentrated CH₂Cl₂solution of the product. Yield

1

to −25 °C. H NMR (399.79 MHz, C₆D₆, 304 K): δ 7.07 (m, 3H,

H17/H18/H19), 7.00 (m, 2H, H16/H20), 5.75 (s, 1H, H6), 3.02 (s,

3H, H12), 2.70 (s, 3H, H13), 1.72 (m, 12H, P−CH₂CH₃), 1.57 (s

with shoulders, 3H, H11), 1.32 (s, 3H, H14), 0.81 (dt, ³J_HH= 7.7 Hz,

³J_PH= 16.4 Hz, 18H, P−CH₂CH₃). ³¹P NMR (161.84 MHz, C₆D₆,

304 K): δ 9.56 (s with satellites, J_PtP= 2556 Hz). ¹⁹⁵Pt NMR (85.56

MHz, C₆D₆, 304 K): δ −4784 (t, J_PtP= 2556 Hz). ¹³C NMR (100.53

1

26%. H NMR (399.78 MHz, C₆D₆, 300 K): δ 7.09 (m, 3H, H19/

H20/H21), 7.02 (m, 2H, H18/H22), 3.04 (s, 3H, H12), 2.66 (s, 3H,

H13), 2.11 (q, ³J_HH= 7.5 Hz, 2H, H15), 1.73 (m, 12H, P−CH₂CH₃),

1.59 (s with shoulders, 3H, H11), 1.27 (s, 3H, H14), 0.82 (m, 21 H,

H16/P−CH₂CH₃). ³¹P NMR (161.83 MHz, C₆D₆, 300 K): δ 9.75 (s

with satellites, J_PtP= 2561 Hz). ¹⁹⁵Pt NMR (85.55 MHz, C₆D₆, 300

K): δ −4788 (t, J_PtP= 2561 Hz). ¹³C NMR (100.53 MHz, CDCl₃, 300

2

MHz, CDCl₃, 300 K): δ 162.3 (s with satellites, J_PtC= 82.8 Hz, C3),

3

150.0 (s, C5), 143.6 (s with satellites, J_PtC= 45.8 Hz, C10), 139.2 (s,

C9), 137.3 (s with shoulders, C8), 136.2 (s, C7), 134.5 (s with

2

satellites, J_PtC= 71.1 Hz, C1), 133.6 (t, J_PC= 9.4 Hz, C2), 129.6 (s,

C15), 129.1 (s, C16/C20), 128.6 (s, C18), 128.4 (s, C17/C19), 119.3

2

4

3

K): δ 160.8 (s with satellites, J_PtC= 83 Hz, C3), 149.1 (s, C5), 143.0

(s, C6), 18.2 (t, J_CF= 2.6 Hz, with satellites J_PtC= 37.2 Hz, C12),

3

18.1 (s with satellites, ³J_PtC= 60.2 Hz, C11), 16.0 (vquint, J_PC= ³J_PC

=

(s with satellites, J_PtC= 46 Hz, C10), 137.0 (s, C8), 136.5 (s, C9),

136.3 (s, C7), 134.1 (s with satellites, ²J_PtC= 71 Hz, C1), 132.7 (t, ²J_PC

= 9.3 Hz, C2), 131.4 (s, C6), 129.3 (s, C17), 129.1 (s, C19/C21),

128.6 (s, C20), 128.5 (s, C18/C22), 18.16 (m with satellites, ³J_PtC= 38

2

17.9 Hz, with satellites J_PtC= 36.1 Hz, P−CH₂CH₃), 14.3 (m, C13),

3

14.1 (s, C14), 8.4 (s with satellites, J_PtC= 24.4 Hz, P−CH₂CH₃).

Anal. Calcd for C₃₁H₄₈BF₂IN₂P₂Pt: C, 42.24; H, 5.49; N, 3.18. Found:

C, 42.82; H, 5.67; N, 3.39.

3

Hz, C12), 18.15 (s with satellites, J_PtC= 60 Hz, C11), 17.2 (s, C15),

3

2

16.0 (vquint, J_PC= J_PC= 17.5 Hz, with satellites J_PtC= 36.1 Hz, P−

trans-Iodo-(6-idodo-1,3,5,7-tetramethyl-8-phenyl-4,4-diﬂuoro-4-

bora-3a,4a-diaza-s-indacene-2-yl)-bis(triethylphosphane)-

platinum(II) (2-Pt-6I). The oxidative addition was conducted at 45 °C

for 4h. The ﬁnal product was puriﬁed by column chromatography

(silica, CH₂Cl₂/petrol ether 60:40 → 50:50), the vacuum-dried residue

4

CH₂CH₃), 15.0 (s, C16), 12.3 (t, J_CF= 2.5 Hz, C13), 11.5 (s, C14),

3

8.4 (s with satellites, J_PtC= 24.6 Hz, P−CH₂CH₃). Anal. Calcd for

C₃₃H₅₂BF₂IN₂P₂Pt: C, 43.58; H, 5.76; N, 3.08. Found: C, 43.50; H,

5.73; N, 3.28.

P

DOI: 10.1021/acs.organomet.7b00806

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

cis-(8,9-η²-3-Chloro-5,7-dimethyl-4,4-diﬂuoro-4-bora-3a,4a-

diaza-s-indacene)-bis(triethylphosphane)platinum(II) (8,9-Pt). A

Young tube was ﬁlled with 216 μmol (1 equiv) of Pt(Et)₂(PEt₃)₂

and 0.7 mL of C₆D₆. The solution was frozen, evacuated, and heated

to 110 °C for 90 min. Inside a glovebox, 1.09 equiv of 3-Cl-bodipy was

added. The complex crystallized directly from the reaction mixture

forming single crystals. After removing the solvent the compound was

dried under reduced pressure. Yield: 94%. The following signal

assignment follows the atom numbering in Figure 1. ¹H NMR (399.78

MHz, THF-d₈, 300 K): δ 5.85 (s, 1H, H6), 5.80 (br s, 1H, H1), 5.75

Spin density distributions and atomic coordinates of the

DFT-optimized structures (XYZ)

Accession Codes

CCDC 1583615−1583616, 1583637−1583638, 1583640, and

1583642 contain the supplementary crystallographic data for

this paper. These data can be obtained free of charge via

www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_

request@ccdc.cam.ac.uk, or by contacting The Cambridge

Crystallographic Data Centre, 12 Union Road, Cambridge CB2

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

3

(d, J_HH= 2.54 Hz, 1H, H2), 3.83 (dd, J_PH= 9.10 Hz, 4.34 Hz, with

satellites ²J_PtH= 49.2 Hz, 1H, H8), 2.40 (d with shoulders, ⁶J_PH= 5.90

Hz, 3H, H11), 1.82 (m, 3H, H12), 1.80−1.48 (m, 12H, P−CH₂CH₃),

0.96 (m, 18H, P−CH₂CH₃). ³¹P NMR (161.83 MHz, THF-d₈, 300

K): δ 19.10 (d, ²J_PP= 9.70 Hz, with satellites J_PtP= 4801 Hz, P2), 6.89

AUTHOR INFORMATION

Corresponding Author

*E-mail: rainer.winter@uni-konstanz.de.

ORCID

■

2

(d, J_PP= 9.70 Hz, with satellites J_PtP= 3321 Hz, P1). ¹⁹⁵Pt (85.51

MHz, THF-d₈, 300 K): δ −4950 (dd, J_PtP= 4801 Hz, J_PtP= 3321 Hz).

¹³C NMR (150.97 MHz, THF-d₈, 290 K): δ 152.5 (s, C5), 132.8 (s,

C10), 127.5 (s, C9), 119.1 (s, C3), 112.8 (s, C6), 106.9 (s, C2), 106.2

(s, C1), 103.3 (s, C7), 42.1 (d, ²J_PC= 43.6 Hz, C8), 17.7 (d, J_PC= 30.7

Rainer F. Winter: 0000-0001-8381-0647

Notes

2

Hz, with satellites J_PtC= 80.5 Hz, P2-CH₂CH₃), 15.6 (d with

The authors declare no competing ﬁnancial interest.

shoulders, J_PC= 25.7 Hz, P1-CH₂CH₃), 14.8 (s, C11), 14.3 (s, C12),

3

8.6 (s with satellites, J_PtC= 35.6 Hz, P2-CH₂CH₃), 8.5 (s with

ACKNOWLEDGMENTS

■

shoulders, P1-CH₂CH₃). Anal. Calcd for C₂₃H₄₀BF₂ClN₂P₂Pt: C,

40.28; H, 5.88; N, 4.08. Found: C, 40.47; H, 6.14; N, 4.51.

Financial support of this work by Deutsche Forschungsge-

meinschaft through grant Wi1262/10-2 and the Fachbereich

Chemie of the University of Konstanz are gratefully acknowl-

edged. We also thank Bernhard Weibert for data collection and

structure reﬁnement in X-ray crystallography as well as Nils

Rotthowe and Steﬀen Oßwald for their help with the EPR

spectroscopy.

trans-Iodo-(5,7-dimethyl-4,4-diﬂuoro-4-bora-3a,4a-diaza-s-inda-

cene-3-yl)-bis(triethylphosphane)platinum(II) (3-Pt). First, 100 μmol

of 8,9-Pt was loaded into a Young tube and heated in THF-d₈to 60

°C for 72 h. The resulting isomeric mixture was treated as explained in

the general protocol (vide supra). Puriﬁcation by column chromatog-

raphy (silica, ethyl acetate/pentane 10:1) and recrystallization from

1

CH₂Cl₂yielded orange needles. Yield: 54%. H NMR (399.78 MHz,

C₆D₆, 300 K): δ 6.60 (d with shoulders, ³J_HH= 4.1 Hz, 1H, H1), 6.49

(s with satellites, ⁵J_PtH= 13.3 Hz, 1H, H8), 6.40 (d, ³J_HH= 4.1 Hz, with

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DOI: 10.1021/acs.organomet.7b00806

Organometallics XXXX, XXX, XXX−XXX

Article Doi

σ-Pt-BODIPY Complexes with Platinum Attachment to Carbon Atoms C2 or C3: Spectroscopic, Structural, and (Spectro)Electrochemical Studies and Photocatalysis

DOI: 10.1021/acs.organomet.7b00806

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Full text of DOI:10.1021/acs.organomet.7b00806

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