Rhenium(i) and platinum(ii) complexes with diimine ligands bearing acidic phenol substituents: Hydrogen-bonding, acid-base chemistry and optical properties

Article abstract of DOI:10.1039/c0dt00393j

Tricarbonylchloro-rhenium(i) (1-4) and catecholato-platinum(ii) complexes (6, 7) of diimine ligands bearing phenol and O-protected phenol substituents have been prepared and fully characterised including single crystal structure analyses of 1, 4 and 7. The redox behaviour of the catecholato platinum(ii) complexes 6 and 7 has been probed by cyclic voltammetry, preparative oxidation and EPR spectroscopy (6⁺, 7⁺). Reversible deprotonation of the hydroxy substituted complexes 1, 3 and 6 to 1^-, 3^- and 6^- resulted in significant changes in their electronic spectra. The luminescence properties of the diamagnetic complexes have been investigated using emission spectroscopy. DFT and TD-DFT calculations were invoked to shed some light onto the geometric and electronic structures as well as the experimental spectroscopic properties of the neutral complexes 1-7, the radical cations 6⁺ and 7⁺ and the conjugate bases 1^-, 3^- and 6^-.

Full text of DOI:10.1039/c0dt00393j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Rhenium(I) and platinum(II) complexes with diimine ligands bearing acidic
phenol substituents: hydrogen-bonding, acid–base chemistry and optical
properties†
Wei Liu and Katja Heinze*
Received 29th April 2010, Accepted 16th July 2010
DOI: 10.1039/c0dt00393j
Tricarbonylchloro-rhenium(I) (1–4) and catecholato-platinum(II) complexes (6, 7) of diimine ligands
bearing phenol and O-protected phenol substituents have been prepared and fully characterised
including single crystal structure analyses of 1, 4 and 7. The redox behaviour of the catecholato
platinum(II) complexes 6 and 7 has been probed by cyclic voltammetry, preparative oxidation and EPR
spectroscopy (6∑+, 7∑+). Reversible deprotonation of the hydroxy substituted complexes 1, 3 and 6 to 1-,
3- and 6- resulted in significant changes in their electronic spectra. The luminescence properties of the
diamagnetic complexes have been investigated using emission spectroscopy. DFT and TD-DFT
calculations were invoked to shed some light onto the geometric and electronic structures as well as the
experimental spectroscopic properties of the neutral complexes 1–7, the radical cations 6∑+ and 7∑+ and
the conjugate bases 1-, 3- and 6-.
Introduction
The chemistry of d6 and d8 transition metal complexes including
RuII, ReI, or PtII central atoms has attracted considerable attention
in the past two decades,1 because of their interesting photophysical
properties and their possible applications in NLO materials,2
in solar energy conversion processes,3 as well as in chemical or
biological sensors.4,5
Platinum(II) and rhenium(I) complexes incorporating redox-
active and/or photoactive “non-innocent” ligands are particularly
intriguing. For potential applications in molecular devices, photo-
catalysis, energy conversion and information storage6 their energy
and electron transfer properties are actively studied. In this context
the diimine type ligand is one of the most widely used bidentate
chelating moieties.7–9
We have recently discovered that combination of a diimine
ligand bearing a remote acidic phenol group with a platinum(II)
coordinated catecholato ligand (Scheme 1, top) resulted in the
unexpected observation of room temperature luminescence after
deprononation. The emission, probably originating from a ligand
excited triplet state, appeared to be associated with the planarisa-
tion of the phenolate-substituted diimine ligand.10
In this respect the anionic complex resembles tetradentate Schiff
base platinum(II) complexes. These planar and rigid systems have
Scheme
1
Deprotonation of a phenol-substituted diimine (cate-
Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128,
Mainz, Germany. E-mail: katja.heinze@uni-mainz.de; Fax: + 49-6131-
3927277
cholato)platinum(II) complex10 (top) and planar platinum(II) complexes
with salen-derived N2O2 ligands9 (bottom).
† Electronic supplementary information (ESI) available: Details con-
cerning crystal structure analyses and TD-DFT calculations, Cartesian
coordinates of DFT optimised structures, graphical representation of
molecular orbitals, UV/Vis/NIR spectra of 1, 2, 3, 4, 6, 7 and Gaussian
deconvolution, UV/Vis spectra of 1- and 3- upon titration with acid,
emission spectrum of 4 at 77 K, EPR spectrum of 6∑+ and simulation],
FD mass spectrum of 1 after the deprotonation/reprotonation sequence.
CCDC reference numbers 775540–775542. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt00393j
been sucessfully employed as phosphorescent materials in organic
light-emitting devices (OLEDs) (Scheme 1, bottom).9
To further investigate this observation and to possibly extract
some generalisable rules we replace the unsymmetrical phenol sub-
stituted ligand 4-[(pyridine-2-ylmethylene)amino]phenol L1 by the
doubly phenol substituted bis(4-hydroxyphenyl)ethylenediimine
9554 | Dalton Trans., 2010, 39, 9554–9564
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ligand L3 and the Pt(3,6-dbcat) fragment (3,6-dbcat = 3,6-di-
tert-butyl-1,2-catecholato) by a ReCl(CO)3 moiety. Rhenium and
platinum complexes of the O-protected analogues L2 and L4 are
also investigated for structural and spectroscopic comparison.
The geometric and electronic properties of the rhenium(I) and
platinum(II) complexes of L1–L4 are investigated in the following
using experimental and theoretical methods.
washing with diethyl ether and drying in vacuo 2 was obtained as an
orange red solid. Yield: 241 mg (0.42 mmol; 47%). IR (CsI, cm-1):
2022 (s, CO), 1917 (s, CO), 1910 (s, CO). IR (CH2Cl2, cm-1): 2026
(s, CO), 1926 (s, CO), 1903 (s, CO). FD-MS: m/z = 575.7; calcd
for C18H18ClN2O4ReSi 576.0. 1H NMR (400 MHz, CDCl3, ppm):
d = 9.03 (d, 3JHH = 5.1 Hz, 1H, H12), 8.95 (s, 1H, H7), 8.21 (pt, 1H,
H10), 8.15 (d, 3JHH = 7.2 Hz, 1H, H9), 7.69 (pt, 1H, H11), 7.49 (d,
3
3JHH = 6.5 Hz, 2H, H2,6), 7.02 (d, JHH = 6.7 Hz, 2H, H3,5), 0.30
(s, 9H, CH3). Calcd for C18H18ClN2O4ReSi (576.10): C, 37.53; H,
3.15; N, 4.86. Found: C, 36.45; H, 2.83; N, 4.61.
Experimental
General considerations
ReCl(CO)3(L1) [tricarbonylchloro[4-[(pyridine-2
ylmethylene)amino]phenol]rhenium(I), 1]
Standard Schlenk techniques were used in carrying out manip-
ulations under an Ar atmosphere. All solvents were dried with
standard methods and freshly distilled under Ar prior to use.
Reagents were purchased from commercial sources and used as
received. IR spectra were recorded with a BioRad Excalibur
FTS 3100 spectrometer using KBr cells in CH2Cl2 and as CsI
disks. Absorption spectra were measured on a Varian Cary
5000 UV/Vis/NIR spectrophotometer. Emission spectra were
recorded on a Varian Cary Eclipse spectrometer. NMR spectra
were measured on a Bruker AM 400 spectrometer at 400.13 MHz
(1H) and 100.03 MHz (13C). Chemical shifts (d (ppm)) were
reported with respect to residual solvent peaks as internal standard
(1H: chloroform, d = 7.26 ppm; d6-dmso, d = 2.50 ppm; 13C:
d6-dmso, d = 39.7 ppm). EPR spectra were acquired on a
Magnettech MiniScope MS300 benchtop cw EPR spectrometer
(X-band, ~9.4 GHz microwave frequency). EPR simulations
were performed using EasySpin 3.1.1.11 Cyclic voltammetry and
square wave voltammetry were performed on a BioLogic SP-
50 electrochemical analytical instrument, with platinum wires
as working and counter electrodes, Ag/AgNO3 as reference
electrode, and 0.1 M nBu4NPF6 as supporting electrolyte. All
potentials are reported relative to Ag/AgNO3. FD mass spectra
were recorded on a FD Finnigan MAT90 instrument. Elemental
analyses were performed by the analytical laboratory of the
Chemistry Department, University of Mainz, Germany.
Method 1: A mixture of ReCl(CO)5 (217 mg, 0.6 mmol) and
L1 (99 mg, 0.5 mmol) was refluxed in toluene (10 mL) for 2 h.
After solvent evaporation in vacuo, washing with diethyl ether and
drying 1 was obtained as an orange coloured solid. Yield: 195 mg
(0.39 mmol, 77%).
Method 2: To an orange red solution of 2 (127 mg, 0.22 mmol)
in CH2Cl2 (20 mL) was added tetra-n-butylammoniumfluoride
(TBAF, 85 mg, 0.27 mmol). The mixture was stirred for 2 h at room
temperature. Acetic acid (1 mL) was added. The resulting clear
orange coloured solution was stirred for 1 h at room temperature.
The solvent was evaporated and the residue was filtered and
washed with CH2Cl2 and Et2O. After drying under vacuum an
orange coloured powder was obtained. Yield: 99 mg (0.20 mmol,
89%). IR (CsI, cm-1): 3259 (m, OH), 2025 (s, CO), 1912 (s, CO),
1894 (s, CO). IR (CH2Cl2, cm-1): 2041 (s, CO), 1872 (br, CO).
FD-MS: m/z = 503.8; calcd for C15H10ClN2O4Re: 504.0. 1H NMR
(400 MHz, d6-dmso, ppm): d = 10.04 (s, 1H, OH), 9.24 (s, 1H,
3
H7), 9.04 (d, JHH = 5.6 Hz, 1H, H12), 7.83–8.36 (m, 2H, H9,10),
7.79–7.82 (m, 1H, H11), 7.45 (d, 3JHH = 9.0 Hz, 2H, H2,6), 6.92 (d,
3JHH = 9.0 Hz, 2H, H3,5). Calcd for C15H10ClN2O4Re (503.92): C,
35.75; H, 2.00; N, 5.56. Found: C, 35.57; H, 2.09; N, 5.55.
ReCl(CO)3(L4) [tricarbonylchloro[N,N¢-bis(4-
methoxyphenyl)ethylenediimine]rhenium(I), 4]
Synthesis
A mixture of ReCl(CO)5 (362 mg, 1 mmol) and L4 (268 mg,
1 mmol) was refluxed in toluene (15 mL) for 1 h. The solvent
was removed in vacuo to obtain the crude product. An orange
red crystalline solid of 4 was obtained after chromatography
on alumina using CH2Cl2 as eluent. Yield: 498 mg (0.87 mmol,
87%). Red crystals were obtained by slow diffusion of Et2O into
a solution of 4 in CH2Cl2 at room temperature. IR (CsI, cm-1):
2022 (s, CO), 1943 (s, CO), 1914 (s, CO). IR (CH2Cl2, cm-1): 2036
(s, CO), 1929 (s, CO), 1903 (s, CO). FD-MS: m/z = 573.6; calcd
The ligands 4-[(pyridine-2-ylmethylene)amino]phenol (L1),
pyridine-2-ylmethylene-(4-trimethylsilanyloxyphenyl)amine (L2),
N,N¢-bis(4-hydroxyphenyl)ethylenediimine (L3), N,N¢-bis(4-
methoxyphenyl)ethylenediimine (L4), and 3,6-di-tert-butyl-1,2-
benzoquinone (3,6-dbq) were synthesised as described in the
literature.12–14
ethylenediimine]rhenium(I) (3) was prepared according to
reported procedure.15 The precursor complex Pt(nbe)3
Tricarbonylchloro[N,N¢-bis(4-hydroxyphenyl)-
a
1
(nbe = norbornene, bicyclo[2,2,1]heptene) and the platinum(II)
complex with ligand L1 (5) were prepared by following known
procedures.10,16
for C19H16ClN2O5Re: 574.0. H NMR (400 MHz, CDCl3, ppm):
3
,6
d = 8.58 (s, 2H, H7,7¢), 7.51 (d, JHH = 8.8 Hz, 4H, H2,6,2¢ ¢), 7.00
(d, JHH = 9.2 Hz, 4H, H3,5,3¢ ¢), 3.89 (s, 6H, OCH3). Calcd for
C19H16ClN2O5Re (574.01): C, 39.76; H, 2.81; N, 4.88. Found: C,
39.75; H, 2.95; N, 4.95.
3
,5
ReCl(CO)3(L2) [tricarbonylchloro(pyridine-2-ylmethylene-(4-
trimethylsilanyloxyphenyl)amine)rhenium(I), 2]
Pt(dbcat)(L3) [(3,6-di-tert-butyl-catecholato)[N,N¢-bis(4-
hydroxyphenyl)ethylenediimine]platinum(II), 6]
All glassware was passivated with (CH3)3SiCl before use. A mixture
of ReCl(CO)5 (322 mg, 1.06 mmol) and L2 (241 mg, 0.89 mmol)
was refluxed in toluene (10 mL) for 1.5 h. The clear orange red
solution was allowed to cool to room temperature. The solvent was
removed under reduced pressure to afford the crude product. After
A mixture of L3 (120 mg, 0.5 mmol), nbe (80 mg, 0.84 mmol)
and Pt(nbe)3 (300 mg, 0.62 mmol) was stirred in THF (18 mL)
at room temperature for 3 h. 3,6-Di-tert-butyl-1,2-benzoquinone
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(120 mg, 0.54 mmol) was added as a solid. The mixture was stirred
for 20 h at room temperature. The solvent was removed in vacuo to
afford a dark green solid. The solid was washed with Et2O, THF,
and CH2Cl2 and dried under vacuum. Yield: 157 mg (0.24 mmol,
48%). IR (CsI, cm-1): 3421 (br, OH), 2922 (s, CH), 1601 (s), 1514
(s), 1458 (s). FD-MS: m/z = 655.2; calcd for C28H32N2O4Pt: 655.2.
1H NMR (400 MHz, d6-dmso, ppm): d = 10.10 (s, 2H, OH), 9.12
Results and discussions
Synthesis and characterisation
The unsymmetrical and symmetrical OR-substituted diimine
ligands L1, L3 and L4 (R = H, CH3) employed in this study
were prepared by previously reported routes using Schiff base
condensation methods in high yields (Scheme 2). The trimethylsilyl
protected ligand L2 was obtainedby silylation of the hydroxo group
of L1 with trimethylsilyl chloride and triethylamine as base.12
3
,6
(s, 2H, H7,7¢), 7.92 (d, JHH = 9.4 Hz, 4H, H2,6,2¢ ¢), 6.88–6.91 (m,
,5
4H, H3,5,3¢ ¢), 6.15 (s, 2H, H10,10¢), 1.29 (s, 18H, C(CH3)3). Calcd
for C28H32N2O4Pt (655.66)¥2THF¥CH2Cl2: C, 50.23; H, 5.70; N,
3.17. Found: C, 50.18; H, 6.01; N, 3.04.
Pt(dbcat)(L4) [(3,6-di-tert-butyl-catecholato)
[N,N¢-bis(4-methoxyphenyl)ethylenediimine]platinum(II), 7]
Complex 7 was prepared by a procedure similar to the one for 6
using L4 instead of L3. Yield: 48%. Dark green crystals suitable for
X-ray diffraction were obtained by slow evaporation of a solution
of 7 in a CH2Cl2–THF–CH3CN mixture. IR (CsI, cm-1): 2959 (s,
CH), 2867 (m, CH), 1605 (s), 1506 (s), 1453 (s), 1266 (s). FD-MS:
m/z = 683.8; calcd for C30H36N2O4Pt: 683.2. 1H NMR (400 MHz,
d6-dmso, ppm): d = 9.21 (s, 2H, H7,7¢), 8.02 (d, 3JHH = 8.9 Hz, 4H,
,6
,5
H2,6,2¢ ¢), 7.09 (d, 3JHH = 8.9 Hz, 4H, H3,5,3¢ ¢), 6.16 (s, 2H, H10,10¢),
3.88 (s, 6H, OCH3), 1.29 (s, 18H, C(CH3)3). 13C NMR (100 MHz,
d6-dmso, ppm) 161.3 (s), 161.2 (s), 160.4 (s), 141.8 (s), 134.0 (s),
126.6 (s), 113.9 (s), 113.1 (s), 55.6 (s), 33.6 (s), 29.4 (s). Calcd
1
for C30H36N2O4Pt (683.72)¥2 CH2Cl2: C, 50.45; H, 5.14; N, 3.86.
Found: C, 50.00; H, 5.41; N, 3.76.
Crystal structure determinations
The data were collected on a Bruker Smart Apex CCD diffrac-
tometer equipped with graphite-monochromated Mo-Ka (l =
˚
0.71073 A) radiation using a w-2q scan mode at 173 K. The
highly redundant datasets were reduced using SAINT17 and
corrected for Lorentz and polarization effects. Absorption cor-
rections were applied using MULABS.18 The structures were
solved by direct methods and refined by full-matrix least-
squares methods on F2 using SHELXTL-97.19 All non-hydrogen
atoms were found in alternating difference Fourier syntheses
and least-squares refinement cycles and, during the final cycles,
refined anisotropically. Hydrogen atoms were placed in calculated
positions and refined as riding atoms with a uniform value
Scheme 2 Syntheses of diimine rhenium(I) and platinum(II) complexes
and atom numbering for NMR assignments.
of Uiso
.
Coordination of ReCl(CO)5 to L1–L4 in refluxing toluene
under CO substitution gave the orange coloured ReI complexes
1–4 (Scheme 2). The hydroxo-substituted complex 1 can also be
obtained from the trimethylsilyl derivative 2 by desilylation with
fluoride ions in high yield. Although the synthesis of complex 3 has
been reported previously,15 its acid/base chemistry and electronic
structure properties have not yet been analysed and discussed.
Thus, 3 is additionally included for comparative studies.
The catecholato diimine platinum(II) complexes 6 and 7 were
prepared by coordination of the diimine ligand L3 or L4 to
Pt(nbe)3 (nbe = norbonene). Subsequent oxidative addition of
the C2v-symmetrical 3,6-di-tert-butyl-1,2-benzoquinone (3,6-dbq)
gave the symmetrical green complexes 6 and 7 similar to a reported
Computational details
All calculations were carried out with Gaussian03 programs.20
Density functional theory (DFT) and time-dependent DFT (TD-
DFT) were employed with the B3LYP hybrid functional without
symmetry constraints imposed on the molecules.21 The calcula-
tions were carried out using a 6-31G* basis set for C, H, N, O
and Cl atoms and effective core potentials (LANL2DZ) for Re
and Pt atoms. All geometries were characterised as minima by
frequency analysis (Nimag = 0). For solvent modeling the integral
equation formalism polarisable continuum model (IEFPCM)
was employed. All computational results are summarised in the
Supporting Information.†
9556 | Dalton Trans., 2010, 39, 9554–9564
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˚
Table 1 Crystallographic data for 1, 4 and 7
Table 2 Selected experimental and calculated bond lengths/A and
angles/◦ for 1, 4, and 7 together with DFT calculated metrical data for 1,
4, 7 and anions 1-, 3- and 6-
1
4
7
X-ray 1
DFT 1
DFT 1-
Empirical
formula
Mr
C15H10ClN2O4Re C19H16ClN2O5Re C30H36N2O4Pt
503.90
573.99
Triclinic
¯
P1
683.70
Monoclinic
C2/c
Re1–N1
Re1–N2
Re1–Cl1
Re1–C1
Re1–C2
Re1–C3
N1–C10
C13–O4
2.182(3)
2.172(3)
2.4887(10)
1.962(5)
1.932(4)
1.917(3)
1.438(4)
1.362(5)
74.62(10)
-50.2(6)
2.213
2.212
2.519
1.922
1.932
1.928
1.426
1.362
74.3
2.285
2.202
2.530
1.915
1.931
1.911
1.375
1.248
75.2
Crystal system Monoclinic
Space group
P21/n
˚
a/A
12.4274(8)
9.1175(6)
14.0330(9)
90.00
100.057(4)
90.00
7.08950(10)
11.2851(2)
12.3164(2)
97.4140(10)
97.2240(10)
94.2410(10)
965.31(3)
2
23.3474(5)
9.2838(2)
26.4446(5)
90.00
107.1440(10)
90.00
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
N1–Re1–N2
C9–N1–C10–C15
3
˚
50.7
22.9
V/A
1565.60(18)
4
5477.2(2)
8
Z
rc/g cm-3
F(000)
2.138
952
1.975
552
1.658
2720
X-ray 4
DFT 4
DFT 3-
m(Mo-Ka)/
7.952
6.466
5.161
Re1–N1
Re1–N2
Re1–Cl1
Re1–C1
Re1–C2
Re1–C3
N1–C13
N2–C4
C16–O5
C7–O4
N1–Re1–N2
C12–N1–C13–C18
C11–N2–C4–C9
2.1689(18)
2.1911(18)
2.4785(5)
1.906(2)
1.919(2)
1.929(2)
1.437(3)
1.434(3)
1.367(3)
1.365(3)
74.35(7)
49.1(5)
2.200
2.202
2.519
1.923
1.936
1.936
1.422
1.419
1.358
1.358
74.4
2.302
2.204
2.537
1.913
1.929
1.914
1.365
1.413
1.246
1.375
75.3
mm-1
Index ranges
-15 £ h £ 15
-11 £ k £ 11
-17 £ l £ 17
1.044
-9 £ h £ 9
-14 £ k £ 14
-16 £ l £ 16
1.057
-30 £ h £ 30
-12 £ k £ 12
-34 £ l £ 34
0.946
GOF (F2)
b
R1,a wR2
0.0236, 0.0664
0.0150, 0.0346
0.0206, 0.0441
(I > 2s(I))
R1,a wR2
0.0253, 0.0675
0.0166, 0.0351
0.0288, 0.0458
b
(all data)
a R1 = RꢀFo| - |Fcꢀ/RFo|. b wR2 = [Rw(Fo - Fc2)2/Rw(Fo2)]1/2
.
2
49.2
44.1
19.6
46.0
37.1(4)
procedure using L1 as ligand giving unsymmetrical 5 (Scheme 2).10
The acid/base chemistry of 5 has been reported previously by us10
and the results are also included here for comparison.
X-ray 7
DFT 7
DFT 6-
Pt1–N1
Pt1–N2
Pt1–O1
Pt1–O2
O1–C1
O2–C6
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
N1–C19
N2–C24
C27–O3
C16–O4
N1–Pt1–N2
O1–P1–O2
C22–N1–C19–C20
C23–N2–C24–C25
1.992(2)
1.990(2)
1.9748(17)
1.9741(16)
1.359(3)
1.359(3)
1.415(3)
1.396(3)
1.399(4)
1.390(3)
1.415(3)
1.409(3)
1.427(3)
1.428(3)
1.369(3)
1.371(3)
78.80(9)
82.07(7)
40.5(7)
2.025
2.023
1.997
1.996
1.343
1.343
1.420
1.393
1.405
1.393
1.420
1.422
1.416
1.417
1.360
1.361
79.1
2.095
2.046
2.025
2.008
1.353
1.351
1.415
1.401
1.346
1.401
1.416
1.417
1.411
1.368
1.247
1.375
79.8
The new complexes were characterised by IR and NMR spec-
troscopy, mass spectrometry and elemental analysis. All complexes
feature in their 1H NMR spectra sharp singlet resonances in
the range of d = 8.58–9.24 ppm for the imine proton H7 of
the coordinated diimine ligands. The resonances of the OH
protons of 1, 3 and 6 are observed around 10 ppm. Compared
with the respective uncoordinated ligands L1/L2 the proton
resonances of the pyridyl rings (H9–H12) of 1 and 2 are shifted
to higher frequency due to coordination of the metal. The proton
resonances of the coordinated dioxolene ligand in 6 and 7 appear
at expected positions and with correct intensities [d = 6.15 (2H),
1.29 (18H) ppm].10 Mass spectra and elemental analyses confirm
the proposed compositions of the new complexes 1, 2, 4, 6 and 7.
80.7
-41.3
-42.6
80.9
-16.2
-48.9
Crystal structure descriptions
-37.7(2)
The molecular structures of 1, 4, and 7 in the crystal were
determined by single crystal X-ray diffraction. Crystallographic
and data collection parameters are given in Table 1, selected bond
lengths and angles are listed in Table 2.
reported metal complexes with this ligand.22–25 As expected the
five-membered chelate ring is almost coplanar with the pyridine
ring, while a torsional angle C9–N1–C10–C15 a = -50.2(6)◦ is
formed with the phenol substituent (Fig. 1).
Orange red coloured crystals of the phenol-substituted rhe-
nium(I) complex 1 were obtained by slow diffusion of diethyl
ether into a solution of the trimethyl silyl-protected complex 2
in CH2Cl2 under a non-inert atmosphere. Under these conditions
the trimethyl silylether is slowly hydrolysed to the hydroxyl group.
The rhenium(I) atom of complex 1 adopts a distorted octahedral
coordination geometry with three carbonyl ligands occupying
facial positions (Fig. 1). A five-membered ring is formed by
the bidentate diimine ligand and the rhenium centre. The N1–
Re1–N2 angle amounts to 74.62(10)◦ similar to other previously
As shown in Fig. 2 and Table S1† close intermolecular hydrogen
contacts are formed between the hydroxy group O4–H4A as
hydrogen atom donor and the coordinated chloride Cl1 of an
adjacent molecule as hydrogen atom acceptor (O4–H4A ◊ ◊ ◊ Cl1
◦
˚
102 ; O4 ◊ ◊ ◊ Cl1 3.129(3) A). These hydrogen contacts connect
the molecules along the crystallographic b axis to give rise to
infinite one-dimensional chains. The same basic motif has been
previously observed in allyl chloro dicarbonyl molybdenum(II)
complexes of ligand L1.24b These OH ◊ ◊ ◊ Cl linked chains are
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Fig. 1 ORTEP view of 1 and atom numbering scheme. Thermal ellipsoids
are drawn at the 30% probability level.
further supported by OH ◊ ◊ ◊ OC contacts (O4–H4A ◊ ◊ ◊ O2 C2;
◦
˚
O4–H4A ◊ ◊ ◊ O2 142 ; O4 ◊ ◊ ◊ O2, 3.240(5) A) giving bifurcated
hydrogen contacts from H4A to Cl1 and to O2 (Fig. 2). Further
contacts from CH groups of the diimine ligand to equatorial
carbonyl groups of neighbouring molecules (C5–H5A ◊ ◊ ◊ O2 C2
and C9–H9A ◊ ◊ ◊ O3 C3) further cross-link the chains. Together
with the bifurcated OH ◊ ◊ ◊ Cl/O hydrogen contact a three-
dimensional architecture is generated (Fig. 2, Fig. 3).
Fig. 3 Crystal packing diagram of complex 1; view onto the ac-plane.
CH ◊ ◊ ◊ O contacts are indicated by dashed lines.
Fig. 4 ORTEP view of complex 4 and atom numbering scheme. Thermal
ellipsoids are drawn at the 30% probability level.
plane with torsional angles C22–N1–C19–C20 and C23–N2–C24–
C25 of a = 40.5(7)◦ and -37.7(2)◦, respectively. The coordinated
dioxolene ligand is well described as dianionic catecholate with
balanced C–C bond lengths and C–O single bonds (Table 2 and
Table S2, Supporting Information†).
Fig. 2 Crystal packing of complex 1; view onto the ab plane; O–H ◊ ◊ ◊ X
hydrogen contacts are indicated by dashed lines.
The molecular structure of 4 is basically similar to that of 1
(Fig. 4). The N1–Re1–N2 angle amounts to 74.35(7)◦ and the
torsion angles C12–N1–C13–C18 and C11–N2–C4–C9 between
the five-membered chelate ring and the two phenol substituents
are a = 49.1(5)◦ and 37.1(4)◦, respectively. Complex 4 features
intermolecular C–H ◊ ◊ ◊ X contacts to the chloro ligand and to
the equatorial carbonyl ligands in the crystal (C12–H12A ◊ ◊ ◊ Cl1,
C18–H18A ◊ ◊ ◊ O2 C2, C10–H10A ◊ ◊ ◊ O3 C3). Again, a three-
dimensional network is formed (Fig. S1 and Table S1, Supporting
Information†).
The central platinum(II) atom in complex 7 adopts a nearly
square planar coordination geometry (Fig. 5). The N1–Pt1–N2
angle amounts to 78.80(9)◦ and the O1–Pt1–O2 angle to 82.07(7)◦
while the N1–Pt1–O1 and N2–Pt1–O2 angles are correspondingly
larger with 99.2(4)◦ and 99.9(1)◦ resulting in an angular sum
around the metal centre of 360◦. The two p-anisyl substituents
of the diimine ligand are tilted relative to the N–Pt–N-chelate
Fig. 5 ORTEP view of complex 7 and atom-numbering scheme. Thermal
ellipsoids are drawn at the 30% probability level.
Infrared studies
Hydrogen bonding in the solid state is conveniently probed by
infrared spectroscopy. Undisturbed OH groups absorb above
3500 cm-1 while strong OH ◊ ◊ ◊ X hydrogen bonds are indicated
by vibrational energies well below 3350 cm-1. The hydroxy
substituted complexes 1, 3 and 6 display signals due to OH
stretching vibrations below 3500 cm-1 which indicates some
OH ◊ ◊ ◊ X interaction in the solid state in all cases. For 1 nOH
=
3259 cm-1 is observed confirming the strong OH ◊ ◊ ◊ Cl/OH ◊ ◊ ◊ O
hydrogen bonds in the solid state (vide supra). For 3 weaker
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OH ◊ ◊ ◊ X hydrogen bonds are indicated (nOH = 3354 cm-1) while
for 6 only very weak OH ◊ ◊ ◊ X contacts are observed (nOH
=
3421 cm-1). Similar observations (OH ◊ ◊ ◊ Ocarbonyl and OH ◊ ◊ ◊ Cl
hydrogen bonds) have been described previously with carbonyl
23,24
molybdenum(0) and molybdenum(II) complexes of ligand L1
as well as with water hydrogen-bonded to the OH group of
PtCl2(L1) (nOH = 3109 cm-1).22
All tricarbonyl complexes 1–4 display three CO absorp-
tions in their IR spectra as expected for fac-M(CO)3 frag-
ments lacking threefold symmetry (Experimental Section). In
MoII(allyl)(Cl)(CO)2(L1) complexes hydrogen bonds to the axial
chloro ligand result in a bathochromic shift of the asymmetrical
cis-Mo(CO)2 stretching vibration of DnCO ~10 cm-1 (solution →
solid).24 Similarly, in the chloro rhenium(I) complex 1 the hydrogen
bond to the axial chloro ligand shifts the “A1” fac-Re(CO)3
stretching vibration by DnCO = 16 cm-1 to lower energy (solution
→ solid).
Fig. 7 Experimental (bottom) and simulated (top) EPR spectra of 7∑+
(10-3 M) in a mixture of THF–CH3CN (v/v, 9/1) at room temperature,
1
9.421862 GHz, g = 1.9975, Aiso
(
195Pt) = 24 G, Aiso(2¥ H) = 3.7 G.
showing signals with g-values around 2.0 and superhyperfine
∑
∑
1
coupling to two protons with Aiso(2¥ H) = 3.3, 3.7 G (6 +, 7 +).
1
Platinum satellite subspectra (195Pt; I = ; 33.8%) with Aiso(195Pt) =
2
23, 24 G (6∑+, 7∑+) indicate that the unpaired electron is largely
localised on the semiquinonato ligand and only delocalised onto
the platinum diimine moiety to a minor extend. This is similar to
the electron spin distribution in the previously reported radical
5∑+.10
Electrochemical properties
The redox properties of complexes 1–7 were studied in CH2Cl2 by
cyclic voltammetry (CV) and square wave voltammetry (SWV).
Unfortunately, due to the very poor solubility of complexes 3 and
6 (and also of 510) no interpretable redox waves could be observed.
For 1, 2 and 4 an irreversible ReI/ReII oxidation is observed in
the cyclic voltammograms at Eox = 0.97, 0.99, 1.01 V, respectively,
in a typical range for complexes of this type.26,27 The catecholato
platinum(II) complex 7 displays a reversible redox couple at
E1/2 = 0.26 V. This redox process is ascribed to the oxidation of
the catecholato moiety 3,6-dbcat to the respective coordinated
semiquinone 3,6-dbsq (Fig. 6).10,28
Optical properties
The diimine rhenium complexes 1 and 2 display a complex
structured absorption band around 370–380 nm in THF (Fig.
S2–S3, Supporting Information†). For complexes 3 and 4 this
band is found bathochromically shifted around 440 nm (Fig. S4–
S5, Supporting Information†). These bands can be deconvoluted
to (at least) three bands [328, 370, 424, 430 nm (1); 338, 368,
412 nm (2); 413, 435, 448 nm (3); 419, 442, 461 nm (4), Supporting
Information†]. All these absorptions are tentatively attributed
to dp(Re) → p*(diimine) and to p(C6H4–OR) → p*(diimine)
transitions.
For the catecholato platinum(II) complexes 6–7 intense absorp-
tion bands are found at the low and high energy side of the visible
spectral region with maxima at 898, 639, 442 nm (6) and 898, 820
(sh), 650 (sh), 439 nm (7) (Fig. 8, Fig. S6, Supporting Information).
The solvatochromic low-energy bands [898 nm (THF), 892 nm
(CH2Cl2), 802 nm (CH3CN) (7), Fig. 8] are assigned to p(3,6-dbcat)
→ p*(diimine) LLCT absorptions10 while the 440 nm band might
arise from dp(Pt) → p*(diimine) and p(C6H4–OR) → p*(diimine)
transitions, in analogy to the Re(I) complexes 1–4. These data
Fig. 6 Square wave voltammogram (top) and cyclic voltammogram
(bottom) of 7 (5 ¥ 10-4 M) in CH2Cl2 (E vs. Ag/Ag+).
To substantiate the assumption of ligand centered oxidations
6 and 7 were treated with silver(I) triflate (AgOTf) as oxidant in
THF. The resulting red-brown species 6∑+ and 7∑+ have a reasonable
lifetime at room temperature which enables to characterise the
radical cations 6∑+ and 7∑+ by EPR spectroscopy (Fig. 7, Fig. S10,
Supporting Information†). Isotropic EPR spectra are obtained
Fig. 8 UV/Vis/NIR absorption spectra of 7 in CH3CN, CH2Cl2 and
THF.
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are fully compatible with those reported for the unsymmetrical
platinum complex 5.10
remote OH group to give 1-. Especially the totally symmetric “A1”
stretch is bathochromically shifted from 2041 cm-1 to 2019 cm-1 in
THF indicating stronger p-backbonding of rhenium(I) to the CO
ligands.
Deprotonation of 1 with P1-tBu29 (P1-tBu = tert-butylimino-
tris(dimethylamino)phosphorane) results in the decline of the
band at 377 nm and in the appearance of a new band at 589 nm
(e = 8900 M-1 cm-1) responsible for the pale blue colour (Fig. 9;
cuvette c). Two isosbestic points at 328 and 454 nm indicate a
simple equilibrium between complex 1 and its conjugate base 1-.
Addition of acetic acid practically reverses the optical changes
indicating a reversible behaviour. Only the original 377 nm band
is now observed at lower energy (419 nm; Fig. S7, Supporting
Information†) possibly due to the now different salt concentration,
polarity and availability of hydrogen donors (cf. Fig. 2; hydrogen
contacts to Cl and CO ligands). In fact, addition of a premixed
equimolar mixture of P1-tBu/HCl corresponding to the salt [P1-
tBu-H]Cl) to a solution of 1 results in a similar bathochromic shift
(Fig. S11, Supporting Information†). Furthermore, the FD mass
spectrum of 1 after the deprotonation/reprotonation sequence
reveals a [1+H]+ peak at m/z = 505 instead of the original [1]+
peak at m/z = 504 (Fig. S12, Supporting Information†). This
proves the stability of 1 and 1- and additionally suggests hydrogen
bonding interactions with acidic reagents. The new absorption
band of 1- at 589 nm might arise from an intraligand p–p* charge
transfer from the phenolate moiety to the tricarbonyl-rhenium(I)
coordinated diimine unit. Deprotonation of L1 coordinated to an
electron-rich dichloro platinum(II) fragment PtCl2(L1) has resulted
only in unremarkable changes in the optical spectra.22 It is thus
tempting to assume that the intraligand CT only occurs if an
electron-accepting MLn fragment is coordinated to deprotonated
L1. The electron accepting power of Re(CO)3 is seen by changes
of the CO stretching vibrations of 1 upon deprotonation of the
More dramatically, orange yellow coloured 3 is deprotonated
to the turquoise anion 3- (Fig. 10; cuvette b). The 436 band
decreases in intensity and an intense “blue band” at 673 nm (e =
32730 M-1 cm-1) appears. The latter band is much more intense
than the corresponding band of 1- and accordingly the visible
colour change from 3 to 3- is much more pronounced (Fig. 10;
cuvettes a and b). Isosbestic points for the 3/3- pair occur at
346 and 528 nm. No further changes are observed upon addition
of a second equivalent of base. Thus, the doubly charged 32- is
not accessible under these conditions. Re-protonation of 3- with
acetic acid qualitatively restores the original electronic spectrum.
However, the original 436 nm band is now found at 489 nm due
to the now different salt concentration, polarity and presence of
hydrogen donors analogous to the 1/1- case (Fig. S8, Supporting
Information†) resulting in a red solution (Fig. 10; cuvette d).
Similar to the 1/1- acid/base pair the IR band for the symmetric
“A1” CO stretching vibration of 3 is shifted from 2022 cm-1 to
2013 cm-1 in THF upon deprotonation to 3-.
Fig. 10 UV/Vis/NIR absorption spectra of 3 in THF upon addition of
phosphazene base P1-t-Bu (top); titration plot observed at 673 nm and
solution of 1 in THF (4 ¥ 10-5 M) upon addition of P1-t-Bu (a→b) and
re-acidification with acetic acid (b→c→d) (bottom).
The absorption spectra of the Pt(3,6-dbcat)(L3) complex 6 upon
titration with P1-t-Bu are depicted in Fig. 11. The solution remains
dark green so no changes are observed with the naked eye.
However, the electronic spectra reveal significant changes with
an increasing 639 nm band (final maximum at 659 nm). This
observation is in qualitative agreement with the results previously
obtained for the 5/5- pair showing a decrease of a 733 nm (5) and
a development of a 570 nm band (5-).10 Beginning precipitation
Fig. 9 UV/Vis/NIR absorption spectra of 1 in THF upon addition of
phosphazene base P1-t-Bu (top); titration plot observed at 589 nm and
solution of 1 in THF (4 ¥ 10-5 M) upon addition of P1-t-Bu (a→c) and
re-acidification with acetic acid (c→d→e) (bottom).
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The platinum(II) complexes 5–7 are also non-luminescent both
at room temperature and at 77 K. For the anion 5- a room-
temperature emission at 512 nm (CH2Cl2) has been reported.10
This emission has been assigned to a 3(p–p*) state. This emissive
state is not completely depopulated by radiationless deactivation
through the phenolate torsion around the C–N bond as the de-
protonation has restricted this intramolecular rotation. However,
for anion 6-, with the monodeprotonated ligand L3 coordinated
to platinum, no emission is observed at room temperature. This
could be due to the presence of an OH oscillator or the unrestricted
torsional motion of the remaining phenol moiety. In order to get
a deeper understanding of these spectroscopic observations DFT
calculations have been performed on 1–7, the cations 6∑+ and 7∑+
and the anions 1-, 3- and 6-.
Computational studies
The geometries of all complexes mentioned were optimised using
DFT methods (B3LYP functional; 6-31G* basis set for C, H, N,
O, and Cl atoms; effective core potentials (LANL2DZ) for Re and
Pt atoms; gas phase).
The DFT optimised bond lengths and angles of 1, 4 and 7 agree
sufficiently well with the experimentally observed ones (Table 2).
Even the absolute values of the torsional angles |a| with a
rather shallow energy potential are reproduced reasonably well.
The practically non-alternating calculated C–C bond lengths of
the catecholato ligand in 7 are also in good agreement with the
experiment.
The CO stretching vibrations of 2 and 4 are calculated
(scaled by 0.961431) and compared with the experimental ones
in solution (absence of hydrogen bonding). The symmetric “A1”
stretching mode is reproduced very well [2/4: calcd 2025/2029
cm-1; exp. 2026/2036 cm-1]. The two vibrations involving mainly
the equatorial CO ligands are calculated at slightly higher energy
by some 30 cm-1 [2/4: calcd 1956,1934/1966,1937 cm-1; exp.
1926,1903/1929,1903 cm-1] but are still in reasonably good agree-
ment with the experiment. Deprotonation of 1 to 1- and of 3 to 3-
results in a calculated, scaled shift of the CO stretching vibrations
from 2025, 1958, 1935 to 2006, 1927, 1909 cm-1 (1/1-) and from
2030, 1967, 1939 to 2005, 1928, 1910 cm-1 (3/3-), respectively, in
accordance with the experimentally observed trend (vide supra).
Thus, the calculations sufficiently reproduce the experimental
geometries and vibrational frequencies.
Fig. 11 UV/Vis/NIR absorption spectra of 6 in THF upon addition
of phosphazene base P1-t-Bu (top); titration plot observed at 639 nm
(bottom).
of charged 6- results in less well-defined isosbestic points and
hampers clean generation of doubly deprotonated 62-.
The rhenium complexes 1–4 are practically non-emissive at
room temperature in fluid solution. In a frozen THF matrix
(77 K) weak emission bands with vibrational fine structure are
observed for the O-protected complexes 2 and 4 (Fig. 12, Fig. S9,
Supporting Information†) while the OH-substituted complexes
1 and 3 are non-luminescent even at 77 K. Probably the OH
oscillator and hydrogen bonding interactions (vide supra) provide
radiationless deactivation pathways. The weak emissions of 2
1
and 4 (437 nm, Uem ꢁ 10-3) are attributed to ligand based (p–
p*) emissions, following assignments of ReCl(CO)3 complexes of
analogous 1-pyridylimidazo[1,5-a]pyridine ligands (428–456 nm,
respectively).30
UV/Vis/NIR spectra of 1–4 were calculated using time-
dependent density functional theory (TD-DFT). Exemplarily,
the calculated electronic absorption spectrum of 1 and the
dominantly involved orbitals (#86, #87 → #90) are depicted in
Fig. 13 (for spectra of 2–4 see Supporting Information, Table
S3). The structured absorption band at 377/371 nm (1/2) and
440 nm (3/4) is composed of several transitions with significant
oscillator strength (details are summarised in the Supporting
Information, Table S3). The centres of the bands (with Lorentzian
line broadening, FWHM 100 cm-1) are estimated at 406 nm (f =
0.25), 422 nm (f = 0.27), 459 nm (f = 0.40) and 469 nm (f =
0.43) for 1–4 in reasonable agreement with the corresponding
experimental values of 377 nm (e = 7860 M-1 cm-1), 371 nm (e =
12590 M-1 cm-1), 436 nm (e = 12760 M-1 cm-1) and 442 nm (e =
17530 M-1 cm-1). The calculated most intense transitions for 1
and 2 correspond to HOMO–2/HOMO–3 → LUMO electron
Fig. 12 Emission spectrum of 2 (THF matrix, 77 K, lexc = 388 nm, 10-7 M).
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Fig. 14 Experimental (top) and calculated (bottom) optical spectrum of
6 and relevant orbitals involved in the most intense transitions (511 nm,
f = 0.2159; 809 nm, f = 0.2216) (contour value 0.06 a.u.).
Fig. 13 Experimental (top) and calculated (bottom) optical spectrum of
1 and relevant molecular orbitals involved in the most intense transition
(417 nm, f = 0.1712) (contour value 0.06 a.u.).
excitations. The involved occupied orbitals are composed of the
combination of phenol-p and Re(COeq)2 (dxy) contributions
while the LUMO possesses largely pyridine/diimine p* character.
Basically the same holds for complexes 3 and 4. Thus, these
absorptions are correctly attributed to mixed p(Re-dxy/CO) →
p*(diimine) and p(C6H4–OR) → p*(diimine) transitions.
The two most intense absorptions of the catecholato plat-
inum(II) complexes at around 640 and 890 nm are reasonably
well reproduced by the TD-DFT calculations. Exemplarily, the
calculated UV/Vis/NIR absorption spectrum of 6 is shown
in Fig. 14 together with the experimental one. The two main
absorptions are assigned to quite pure p(C6H4–OR) → p*(diimine)
and p(catecholato) → p*(diimine) transitions, respectively.10 The
transition calculated at 511 nm is mainly due to the p(C6H4–OR,
#130) → p*(diimine, #133) electronic excitation and the 809 nm
band is due to the dbcat-HOMO (#132) → p*(diimine, #133)
excitation. The solvatochromic behaviour of these absorption
bands has been probed experimentally with complex 7 (Fig. 8).
Gratifyingly, the experimental trend for the low-energy band
(802 nm, 892 nm, 898 nm in CH3CN, CH2Cl2, THF) is reproduced
by TD-DFT/IEFPCM calculations (815 nm, 829 nm, 830 nm;
Table S3, Supporting Information†).
Oxidation of 5–7 with silver triflate leads to EPR active radical
cations (vide supra and ref. 10 for 5∑+). In accordance with the EPR
spectra and the calculated largely dbcat centered HOMOs of the
catecholato complexes 5–7 the spin density of the radical cations
5∑+–7∑+ is calculated to reside essentially on the semiquinonato
ligand with small contributions from the platinum centre and the
diimine (see Fig. 15 for a spin density map of 7∑+). Thus, the
spectroscopic properties of 1–7 and 5∑+–7∑+ are well reproduced by
the calculations.
Fig. 15 Calculated spin density map of 7∑+ (contour value 0.001 a.u.).
Deprotonating the hydroxy substituted complexes 1, 3, 5 and
6 results in anionic complexes 1-, 3-, 5- 10 and 6-. For anion 5-
the DFT calculations have predicted an iminoquinoid structure
of the phenolate substituent (C O 1.286; C C 1.380; C–C
10
˚
1.437/1.460; C N 1.389 A). The phenolate substituent is
oriented coplanar to the diimine moiety (torsion angle a = 0◦)
and a substantial rotation barrier has been calculated for 5-10
.
The corresponding rhenium(I) complex 1- also shows imino-
quinoid character for the substituent (C C 1.366/1.368; C–
˚
C 1.459/1.463/1.434/1.429 A; and Table 2). The phenolate
torsion angle a = 22.9◦ is smaller than that calculated for the
corresponding acid 1 (a = 50.7◦), but significantly distinct from
zero. Due to the small bite angle of the dbcat chelate the O1–Pt1–
O2 angles in the platinum complexes (~80◦) are smaller than the
C2–Re1–C3 angles in the rhenium complexes (~90◦). Obviously,
steric interaction of the equatorial carbonyl ligand C2–O2 with the
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CH group prevents full planarization of the ligand in the carbonyl
rhenium complex 1- (Scheme 3).
from the three highest occupied molecular orbitals to the p*
LUMO and LUMO+1 of the chelate ligand. The highest occupied
molecular orbitals are p-orbitals of the chelate ligand and the
Re(CO)3 fragment. A second high-intensity band is calculated
at 441 nm also involving molecular orbitals of p-symmetry
of the chelate and the MLn fragment (Table S3, Supporting
Information†). The calculations are in qualitative agreement with
the experiment (377/589 nm, Fig. 9). Analogous transitions are
calculated for 3- at 481 and 571 nm (exp. 449/673 nm, Fig. 10).
For 6- intense p–p* transitions are calculated at 506 and 859 nm
(exp. 639/898 nm). Gratifyingly, the calculated absorption band
positions do not change much for the 6/6- pair, while the intensity
of the former one increases from 6 to 6- as found experimentally
(Fig. 11; Table S3, Supporting Information†)).
Thus, the theoretical analyses coincide nicely with the experi-
mental data and provide confidence for our interpretations.
Conclusions
Tricarbonylrhenium(I) (1–4) and catecholato-platinum(II) com-
plexes (6, 7) of diimine ligands bearing phenol and O-protected
phenol substituents have been prepared and fully characterised
including single crystal structure analyses of 1, 4 and 7. The
catecholato platinum complexes 6 and 7 can be oxidised to
the semiquinonato complexes 6∑+ and 7∑+ as shown by EPR
spectroscopy and DFT calculations. The hydroxy substituted
complexes 1, 3 and 6 feature intermolecular OH ◊ ◊ ◊ X hydro-
gen bonds/contacts in the solid state. Deprotonation of these
complexes to 1-, 3- and 6- results in significant changes in the
optical spectra which are reproduced by TD-DFT calculations.
The ligand moieties of the deprotonated complexes 1-, 3- and
6- acquire some iminoquinoid character as found for 5- but are
not fully coplanar to the imine unit (as found for 5-) (Scheme 3).
In the carbonyl rhenium complexes 1- and 3- steric hindrance
prevents planarisation (Scheme 3; CHortho ◊ ◊ ◊ Ccarbonyl contacts;
green). The slight bending of the dbcat relative to the diimine in the
coordination plane (5-: N–Pt–O 93◦/105◦; 6-: N–Pt–O 97◦/103◦)
seems to be hampered due to the presence of the other phenol
substituent which inhibits full planarisation of 6- (Scheme 3).
As a planar rigid structure appears to be a premise for
room temperature ligand triplet emission 1-, 3- and 6- are non-
luminescent due to the non-planar substituted diimine ligand. We
are currently investigating sterically less demanding MLn frag-
ments in combination with diimine ligands bearing only a single
phenol/phenolate unit like L1. This could allow base-induced
planarisation of the diimine ligands with phenol/phenolate sub-
stituents and could eventually support acid/base sensitive, i.e.
switchable room temperature ligand based triplet emission similar
to 5- and covalently permanently rigidified complexes (Scheme 1).
Scheme 3 Calculated metrical data of anions 1-, 3-, 5- and 6- relevant to
planarisation.
The same arguments apply to the carbonyl rhenium complex
3- (Scheme 3). The anionic moiety displays some iminoquinoid
˚
character (C C 1.363/1.365; C–C 1.465/1.460/1.440/1.433 A;
and Table 2), but is twisted relative to the diimine (a = 19.6◦).
The phenol part of 3- remains essentially undisturbed (3/3-: a =
43.9/46.0◦). Thus, the non-rigid diimine ligand (together with the
presence of an OH oscillator in 3-) might impede the observation of
luminescence for 1- and 3- due to efficient deactivation pathways.
Similar to 3- the platinum complex 6- possesses a substituent
with some iminoquinoid character (C C 1.364/1.366; C–C
Acknowledgements
˚
Regine Jung-Pothmann is acknowledged for collection of the
X-ray data.
1.439/1.431/1.465/1.459 A) and phenol/phenolate substituents
with a = -41.9/-16.2◦, respectively (Scheme 3). Thus, the non-
planarity, the non-rigidity and the presence of an OH oscillator in
6- could be responsible for the absence of emission.
Notes and references
Finally, the optical spectra of the anions 1-, 3- and 6- were
calculated using TD-DFT. For 1- a low-energy band is calculated
at 539 nm which is essentially composed of electron excitations
1 (a) H. Tsubaki, A. Sekine, Y. Ohashi, K. Koike, H. Takeda and O.
Ishitani, J. Am. Chem. Soc., 2005, 127, 15544; (b) S. Y. Reece and D. G.
Nocera, J. Am. Chem. Soc., 2005, 127, 9448 and references therein.
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2 J. Zyss, Ed. Molecular Nonlinear OpticsAcademic Press Boston, MA,
1993.
17 SAINT-Plus, Bruker Analytical X-ray System, Madison, WI,
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3 (a) M. S. Wrighton and G. L. Geoffroy, Organometallic Photochemistry,
Academic Press, New York, 1979; Chapter 2; (b) M. K. Nazeeruddin,
C. Klein, P. Liska and M. Gra¨tzel, Coord. Chem. Rev., 2005, 249, 1460;
(c) A. S. Polo, M. K. Itokazu, K. M. Frin, A. O. de Toledo Patroc´ınio
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