Photoluminescent properties and molecular structures of [NaphAu(PPh 3)] and [μ-Naph {Au(PPh3)}2] ClO4 (Naph = 2-naphthyl)

Article abstract of DOI:10.1039/b714609d

The complexes [NaphAu(PPh₃)], 1 and [μ-Naph{Au(PPh ₃)}₂]ClO₄, 2 having the Au-C (aromatic) bond have been synthesized and characterized. The unique structure of 2 with two gold atoms bridged by a naphthyl group has been determined by X-ray crystallography. The intramolecular Au-Au separation in 2 is 2.7731(4) A. Upon excitation at 266 nm, both complexes display intraligand phosphorescence at room temperature in solution and in solid state. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/b714609d

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Photoluminescent properties and molecular structures of [NaphAu(PPh3)] and
[l-Naph {Au(PPh3)}2] ClO4 (Naph = 2-naphthyl)†
Masahisa Osawa,* Mikio Hoshino and Daisuke Hashizume
Received 21st September 2007, Accepted 30th January 2008
First published as an Advance Article on the web 10th March 2008
DOI: 10.1039/b714609d
The complexes [NaphAu(PPh3)], 1 and [l-Naph{Au(PPh3)}2]ClO4, 2 having the Au–C (aromatic) bond
have been synthesized and characterized. The unique structure of 2 with two gold atoms bridged by a
naphthyl group has been determined by X-ray crystallography. The intramolecular Au–Au separation
˚
in 2 is 2.7731(4) A. Upon excitation at 266 nm, both complexes display intraligand phosphorescence at
room temperature in solution and in solid state.
of the naphthyl moiety and one or two triphenylphosphine
gold(I) units.
Introduction
There is growing interest in the synthesis of phosphorescent
materials because of their potential application as luminescent
dopants in OLED’s and optsensing chemicals in biological sys-
tems, and various optelectronic devices.1 According to a number
of reports on the spectroscopic and photochemical studies of
such materials, heavy atom effects2 are often utilized for realizing
room-temperature phosphorescence of aromatic molecules. The
effects cause an increase in the rate constant of spin-forbidden
processes, S→T and T→S, due to spin–orbit coupling induced by
the internal3 and/or external4 heavy atoms. The merit to use heavy
atom effects is an easy color determination by making choice of
an organic chromophore with the desirable local triplet level.4c
Recently, we have reported the Au(I) complex which ex-
hibits room-temperature phosphorescence from the locally excited
triplet state of its polyaromatic ligand,3LE (I in Scheme 1).5c The
phosphorescence from this complex was clearly explained by the
heavy atom effect of Au(I) attached to the polyaromatic unit
through Au–C bond. During the course of this study,5 an intro-
duction of a triphenylphosphine gold(I) unit into an aromatic,
making the metal–carbon bond, has a dramatic effect on the
triplet state properties. The complex (I) was found to exhibit
intense blue room temperature phosphorescence (Up = 0.24 and
sT = 117 ls) originated from the locally excited triplet of the
biphenyl moiety (3LE) in a degassed 2-methyltetrahydrofuran
solution. On the assumption that UST = 1.0 for I, the radiative
rate constant (kr) in the triplet state is calculated to be 2.06 ×
103 s−1.5c We envisaged that if two Au(I) units directly attach to
the one carbon of the polyaromatic ligand, the more internal
heavy atom effect may be attained, resulting in the higher
yield of phosphorescence from the polyaromatic moiety at room
temperature. The present paper reports the photophysics and
photochemistry of the two type complexes6 [2-naphthylAu(PPh3)]
and [l-2-naphthyl(AuPPh3)2]ClO4 (1, 2 in Scheme 1) composed
Scheme 1
Experimental
General information
All reactions were carried out under the atmosphere of Ar,
unless otherwise indicated. All solvents used for spectroscopic
measurement, 2-methyltetrahydrofuran (2-MeTHF), tetrahydro-
furan (THF), and dichloromethane were distilled prior to use.
1H NMR, and 31P NMR ware recorded using a JEOL EX-400
and a JEOL EX-500, respectively. ESI-MS spectra were recorded
on a ThermoQuest Finnigan LCQ Duo mass spectrometer.
Steady-state absorption and emission spectra were recorded by
a Shimadzu UV-3100 and a RF-5300, respectively. The light-
intensity distribution of a Xenon lamp has been corrected with
the use of Rodamine B in ethyleneglycole and the wavelength-
dependent characteristics of photomultiplier have been calibrated
by BaSO4 powder in the 300–600 nm. UV-vis absorption spectra of
solid sample were measured on a SIS-50 optical waveguide spec-
trometer (System Instruments). For emission studies, dissolved
dioxygen was removed by repeated freeze-pump-thaw cycles. The
RIKEN (The Institute of Physical and Chemical Research), Wako-shi,
Saitama, 351-0198, Japan. E-mail: osawa@postman.riken.jp; Fax: +81 48
467 9389; Tel: +81 48 462 8351
† CCDC reference numbers 661723–661724. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b714609d
2248 | Dalton Trans., 2008, 2248–2252
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Table 1 Crystallographic data for 1 and 2
fluorescence quantum yields were determined by standard meth-
ods with quinine sulfate in 1 N H2SO4 (UF = 0.546) as references.7
The O.D. of measured samples were adjusted to 0.06 (1; 3.7 × 10−6
M) at the excited wavelength, 266 nm. Solid-state phosphorescence
quantum yields were determined with an absolute PL quantum
yield mesurement system, C-9920-02 (HAMAMATSU). Laser
photolysis studies were carried out with the use of a Nd:YAG
laser (Sure Light 400 from Hoya Continuam Ltd.) equipped with
second, third, and forth harmonic generators. Excitation light for
lifetime measurements of phosphorescence was forth harmonics
(266 nm): the duration and the energy of the laser pulse are 5 ns and
30 mJ/pulse, respectively. The monitoring system for the decay
of phosphorescence as well as transient absorption spectra has
already been reported elsewhere.8
1
2
Formula
Formula weight
Cryst syst
C28H22AuP
586.42
Monoclinic
P21/c
16.523(2)
10.0926(15)
14.4157(18)
114.104(4)
2194.4(5)
4
C46H37Au2ClO4P2
1145.1
Monoclinic
P21/c
14.5352(14)
13.7991(13)
20.974(2)
103.2925(15)
4094.2(7)
4
Space group
˚
a/A
˚
b/A
˚
c/A
b/deg
3
˚
V/A
Z
d
calcd/g cm−3
1.775
213
1.858
90.0
T/K
˚
˚
Radiation
Mo Ka (k = 0.71069 A) Mo Ka (k = 0.71073 A)
l/cm−1
6.813
7.370
Diffractometer
Max 2h/deg
Reflns collcd
Indep reflns
No. of param refined 272
R1, wR2 (I > 2r I)
S
Rigaku RAXIS IV
55
16914
4973 (Rint = 0.043)
Rigaku AFC-8
60
82832
Preparations
11963 (Rint = 0.057)
[2-naphthylAu(PPh3)], 1. The synthetic method for 1 is analo-
gous to that previously reported.5c Au(PPh3)Cl was synthesized
according to literature.9 To a solution of 2-bromonaphthalene
(100 mg, 0.48 mmol) in THF (5 ml) cooled below −70 ◦C, n-BuLi
(0.31 ml, 1.63 M in hexane) was added via micro-syringe. After
10 min, a solution of Au(PPh3)Cl (201 mg, 0.41 mmol) in THF
(12 ml) was added slowly. After stirring for 30 min at −70 ◦C, the
reaction mixture was allowed to warm to room temperature. Then,
the solvent was removed under vacuum. Recrystallization of the
crude product from the diethylether solution afforded analytical
pure complex 1 as colorless crystals.
497
0.0281, 0.0721
1.095
0.0543, 0.1360
1.144
Results and discussion
Synthesis and crystallographic results
The structures of 1 and 2 determined by X-ray crystallography
are shown in Fig. 1. Crystal and data collection parameters
are given in Table 1, and representative structural parameters
are summarized in Table 2. The Au atom in the molecule 1
adopts almost linear coordination geometry due to d10 electron
configuration, the corresponding bond angle is 177.90(10)◦. The
bond distances of Au–P and Au–C are typical values for aryl
1. Yield: 168 mg, 70%; 1H NMR (400 MHz, CDCl3): d 8.053
(d, J(P, H) = 5.87 Hz, 1H, H1), 7.755–7.744 (m, 4H, H3,4,5,8), 7.668–
7.597 (m, 6H, Hph), 7.536–7.439 (m, 9H, Hph), 7.402–7.297 (m, 2H,
1
H6,7); 31P{ H} NMR (160 MHz): d43.77(s) in CD2Cl2, d43.80(s) in
d8-THF; Anal. Calcd for C28H22Au1P1: C, 57.35; H, 3.78; P, 5.28.
Found C, 57.51; H, 3.87; P, 5.11.
[l-2-naphthyl(AuPPh3)2]ClO4, 2. The synthetic methodology
for 2 is analogous to the reference.6c To a solution of the complex
1 (150 mg, 0.25mmol) in ether (30 ml) at −50 ◦C, neat HClO4
(176 ll) was added via micro-syringe. After 10 min stirring the
white precipitate was filtered off and washed with cold ether.
Recrystallization of the crude product from THF solution gave
pure complex 2 as light yellow crystals.
Fig. 1 The X-ray structures of 1 and 2; (elliposoids at 50%, H atoms and
counter anion, ClO4 in 2 are excluded for clarity).
−
2. Yields: 95 mg, 66%; ESI-MS: m/z 1045.8 [M-ClO4]+;
˚
Table 2 Selected bond distances (A) and angles (deg) for 1 and 2
1
1P{ H} NMR (160 MHz): d 36.41 (s) in CD2Cl2, d 45.13(s) and
35.99 (s) in d8-THF; Anal. Calcd for C46H37Au2Cl1O4P2: C,48.25;
H, 3.26; P, 5.41. Found C, 48.61; H, 3.48; P, 5.32.
1
2
˚
Bond distances (A)
Au1–P1
Au1–C2
Au2–P2
Au2–C2
Au1–Au2
2.2975(8)
2.040(3)
—
—
—
2.271(2)
2.126(5)
2.272(2)
2.140(5)
2.7731(4)
Crystal structure determinations
The crystallographic data and the results of the structure re-
finements are summarized in Table 1. In the reduction of data,
Lorentz and polarization corrections and empirical absorption
corrections were made.10 The structures were solved by the direct
method (SIR2004).11 All non-hydrogen atoms were refined with
anisotoropic thermal parameters. Hydrogen atoms were fixed at
calculated positions.
Bond angles (deg)
C2–Au1–P1
C2–Au1–P2
Au1–C2–Au2
C1–C2–C3
177.90(10)
—
—
168.8(2)
169.4(2)
81.1(2)
115.8(3)
118.0(5)
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12
phosphine gold(I) complexes,3b,5c
,
the corresponding distances
˚
are 2.2975(8) and 2.040(3) A for Au–P and Au–C, respectively, in
1. The naphthyl ring of the present compounds displays a slightly
distorted angle of 115.8(3)◦ at the C2 carbon. Such distortions are
a common feature of aryl phosphine gold(I) complexes (Fig. 1
and Table 2).3b,5c,12 The long Au · · · Au distance in the crystal
lattices of 1 suggests that the intermolecular Au · · · Au interaction
is negligibly small. In the molecule 2, the Au1–Au2 distance
˚
˚
(2.7731(4) A) is shorter than that in metallic gold (2.88 A) because
of gold-gold bonding. In addition, the distorted P–Au–Cipso
angles from the linear configuration also support the existence
of the Au–Au bond, the angles are 168.8(2)◦ and 169.4(2)◦ for P1–
Au1–C2 and P2–Au2–C2, respectively. Two Au atoms are bonded
to C2 almost equivalently and the plane of the C2–Au1–Au2
triangle is approximately perpendicular to that of naphthyl unit,
the angle is 83.2(3)◦. The bond style in the triangle C2–Au1–
Au2 is considered to be a three-centre two-electron (3c-2e) bond
on the basis of the following aspects: (i) the angle, 81.1(2)◦, at
the bridging carbon (Au1–C2–Au2) is acute,13 (ii) the Au1–C2
Fig. 2 Electronic absorption and emission spectra of 1 (full) and 2 (blue)
in solid state at 298 K (kex = 266 nm). Intensities of emission spectra
were adjusted arbitrarily for clarity. The inset pictures are shown for the
emissions of 1 and 2 in microcrystalline solid at ambient temperature under
UV lamp (kex = 254 nm).
and [dr* → pr] transition band17 based on polynuclear d10–d10
system can been seen in the wavelength region 240–400 nm. Upon
excitation at 266 nm, 1 and 2, in both solution and solid state
at room temperature, afford emission spectra very similar to the
phosphorescence spectrum of naphthalene (Fig. 2 and Table 3).
Though the complex 1 shows weak green emission, the complex 2
displays more intense yellow green structured phosphorescence.‡
The quantum yield of 2 (0.19) is twice larger than that of 1 (0.08)
(Table 3). These observations clearly suggest that the triangle
structure, Cipso-Au1-Au2 is a strong heavy atom perturber for
realizing room-temperature phosphorescence in solid state.
˚
and Au2–C2 bonds (2.126(5) and 2.140(5) A) are longer than the
5c,12
˚
normal 2c–2e bonds (2.04–2.08 A) between Au and aryl groups.
This binuclear structure in 2 resembles the known structurally
characterized complexes with two gold atoms bridged by an aryl
group.14 The formations of such triangle fragments have been often
found in the multimetallic supramolecular systems.15
The structure of 2 in solutions is, however, complicated. 31P
NMR spectrum of 2 in CD2Cl2 taken at 297 K exhibits only one
signal (d 36.41 ppm with H3PO4 as external standard), indicating
that 2 adopts the symmetrical Cipso-Au1-Au2 triangle structure.
On the other hand, in d8-THF solution, two signals of the same
intensity due to the two non-equivalent phosphorous atoms (d
45.13 ppm and d 35.99 ppm) are observed. The former signal is
near to that of 1 (d 43.8 ppm) and the latter is close to that of
cationic Au-phosphine complex,16 [AuPPh3]+ (d 33.8 ppm) in d8-
THF. The structure of 2 in d8-THF might be written as Scheme 2.
Photophysical properties of 1 and 2 in solutions
The absorption spectra of 2-bromonaphthalene, 1, and 2 in
2-MeTHF are shown in Fig. 2. 2-Bromonaphthalene displays
the structured band at 260–300 nm. Complexes 1 and 2 show
almost same spectra of which the first absorption band at ca.
280–310 nm are slightly red-shifted in comparison to that of 2-
bromonaphthalene. The moderately strong absorption of 2 at
300–350 nm in the solid-state (blue in Fig. 2) disappears in the
2-MeTHF solution.
As shown in Fig. 3, the Au(I) complex (1) shows intense green
emission at 479, 514, 555 nm in degassed 2-MeTHF at 298 K. This
emission is assigned to room-temperature phosphorescence from
naphthalene chromophore (3LE) of 1 on the basis of the following
observations: (1) the lifetime of the emission is as long as 124 ls
(Table 3), (2) the emission is substantially quenched in the presence
of oxygen, and (3) the emission spectrum with the vibronic
structure agrees well with that of phosphorescence spectrum
of 2-bromonaphthalene at 77 K (Fig. 3). It is noteworthy that
fluorescence from the naphthalene moiety is very weak for 1 and
2.
Scheme 2
The proposed structure in Scheme 2 can be regarded as a
intermediate in the another synthetic route of the type 2 complex;
[AuPPh3]ClO4 + NaphAuPPh3 (1) → l-Naph(AuPPh3)2ClO4 (2).6c
THF molecules might contribute to stabilize the cationic structure,
[AuPPh3]+ by coordination.
These observations suggest that the strong spin–orbit interac-
tion caused by the Au atom (Z = 79) in 1 leads to (1) an efficient
intersystem crossing from the singlet to the triplet excited state
and (2) the large kr value of the naphthyl chromophore at T1 by
Photophysical properties of 1 and 2 in solid state
The UV/Vis and emission data of 1 and 2 in solid state are
summarized in Fig. 2 and Table 3. The structured absorption
band (280–310 nm) of 1 is originated from the naphthalene moiety
as shown in Fig. 2. The absorption band of 2 (300–350 nm)
is slightly red-shifted compared to that of 1. No MLCT band
‡ When the microcrystalline sample of 2 is pounded in a mortar, it exhibits
intense structureless yellow phosphorescence. This observation indicates
the polymorphs of 2.17b
2250 | Dalton Trans., 2008, 2248–2252
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Table 3 Photophysical data for 1 and 2
Complex
Medium (T/K)
kabs /nm (e × 10−3/M−1cm−1
)
kph/nma (s0/ls)
Up
0.08
e
[2-naphthylAu(PPh3)] 1
Microcrystalline solid (297)
2MeTHF (297)
2MeTHF (77)
—
480 (27, 1.9)b,c
479 (124)
476
242(55), 290(sh), 300(sh)
—
242(58), 290(sh), 300(sh)
0.20
—
CH2Cl2 (297)
481 (92)d
0.27d
[l-2-naphthyl(AuPPh3)2]ClO4 2
Microcrystalline solid (297)
2MeTHF (297)
2MeTHF (77)
266, 312(sh), 353(sh)
243(56), 268(sh), 275(sh), 287(sh), 300(sh)
—
507 (38, 2.8)b
479 (56)
476
0.19
0.10
—
CH2Cl2 (297)
261(50), 300(sh), 312(10), 350(sh)
507 (12)d
>0.03
a k(0,0). b Two-component. c Weak. d Decomposed during measurement. e Very broad.
partial removal of the spin-forbidden nature of the T1-S0 radiative
transition.
The radiative rate constant, kr, at T1 is formulated as eqn (1)
kr = UP/(sTUST
)
(1)
The Au(I) complex (1) affords, Up = 0.20 and sT = 124 ls in
2-MeTHF at 298 K. Since the fluorescence yield of 1 is very small
(Uf < 10−3), we assumed UST = 1.0 for 1. With the use of eqn (1)
and UST = 1.0 for 1, kr is calculated to be 1.61 × 103 s−1. This value
is ca. 2 order of magnitude larger than the radiative rate constant
of the triplet 2-bromonaphthalene (56.8 s−1: kex = 307 nm).18 Thus,
the coordinated Au(I) atom is concluded to give a markedly large
heavy-atom effects on kr of the naphthalene chromophore in 1.
The absorption and phosphorescence spectra of the binuclear
gold complex (2) closely resembles those of 1 (Fig. 3 and
Table 3), suggesting 2 takes a mononuclear structure in 2-MeTHF
solution at room temperature (Scheme 2). This observation is well
consistent with 31P NMR results in d8-THF. The experimental
phosphorescence quantum yield (0.10) of 2 (completely divided
into 1 and [Au(PPh3)]+) in 2-MeTHF is close to the calculated
value (0.12) based on Up of 1 in 2-MeTHF and e values of 1 and
2 at 266 nm (Table 3).
Fig. 4 Emission spectra of 1 (full) and 2 (blue) in 2-MeTHF at 77 K (kex
=
266 nm) and the difference spectrum (dashed red) between 1 and 2 under
the condition that two spectra were normalized at 476 nm. Intensities of
emission spectra were adjusted arbitrarily for clarity.
composed of two spectra: one is the spectrum of 1 at 77 K
and another, an additional spectrum. The additional spectrum
is obtained by subtracting the phosphorescence spectrum of 1
from that of 2 at 77 K. Here the spectra of 1 and 2 at 77 K are
normalized at 476 nm before subtraction. The difference spectrum
(dashed red), thus obtained, is very similar to the phosphorescence
spectrum of 2 in solid state at 297 K (blue in Fig. 2), indicating both
binuclear and mononuclear structures of 2 exists in 2-MeTHF at
77 K (Scheme 3).
Fig. 4 shows emission spectra of 1 and 2 in degassed 2-MeTHF
at 77 K. The vibrational structures of the phosphorescence
spectrum of 1 (full) are more sharp than those at 298 K. The
phosphorescence spectrum of 2 (blue) at 77 K is found to be
Scheme 3
In CH2Cl2 solutions, the absorption spectra of the Au com-
plexes, 1 and 2, are very different each other (Fig. 5). In comparison
with 1, the absorption band at 300–350 nm of 2 is markedly red-
shifted. The features of each absorption spectra in CH2Cl2 bear a
strong resemblance to those in the solid state.
The Au complex (1) shows intense green emission in degassed
CH2Cl2 at 298 K as seen in 2-MeTHF solution. The phospho-
rescence quantum yield of 1 in CH2Cl2 is larger than that in
Fig. 3 Electronic absorption spectra of 2-bromonaphthalene (dashed), 1
(full) and 2 (blue) in 2-MeTHF at 298 K. Emission spectra of 1 (full) and
2 (blue) in degassed 2-MeTHF at 298 K (kex = 266 nm). The luminescence
spectrum of 2-bromonaphthalene in 2-MeTHF at 77 K (kex = 266 nm).
Intensities of emission spectra were adjusted arbitrarily for clarity. The
inset picture displays phosphorescence of degassed 2-MeTHF solution of
1 at 298 K.
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Fig. 5 Electronic absorption and emission spectra of 1 (full) and 2 (blue)
in degassed CH2Cl2 at 298 K (kex = 266 nm). Intensities of emission spectra
were adjusted arbitrarily for clarity.
2-MeTHF (Table 3). The binuclear gold complex (2) shows yellow
green emission at 507, 541 nm, slightly red-shifted compared with
1 (Fig. 5). This red shift is probably caused by the substituent effect
of the second gold(I) phosphine unit. The green emission of 1 and
yellow green emission of 2 can be assigned to the phosphorescence
from the naphthalene moiety. Both complexes 1 and 2 are unstable
in CH2Cl2 solution. They gradually decomposed during the life
time measurement of phosphorescence.§
Conclusion
The two type complexes [2-naphthylAu(PPh3)] and [l-2-
naphthyl(AuPPh3)2]ClO4 (1, 2) have been synthesized and char-
acterized by X-ray crystallography. Two complexes show intrali-
gand phosphorescence from the naphthyl unit (3LE) at room-
temperature in both solutions and solid state. In the solid state,
the dinuclear complex 2 exhibits more intense phosphorescence
than the mononuclear complex 1, suggesting the triangle structure,
Cipso–Au1–Au2 can be regarded as an effective heavy atom
perturber for realizing room-temperature phosphorescence. In 2-
MeTHF solution, the mononuclear complex (1) shows intense
green room temperature phosphorescence (Up = 0.20 and sT
=
124 ls) from the locally excited triplet of the naphthalene moiety
(3LE). Since the complex 2 exists as an equilibrium mixture of
dinuclear and mononuclear structures in 2-MeTHF at 77 K,
it exhibits the mixed phosphorescence spectrum from the two
structures.
14 (a) S. Gambarotta, C. Floriani, A. Chiesi-Villa and C. Guastini,
J. Chem. Soc., Chem. Commun., 1983, 1304–1306; (b) R. Uson, A.
Laguna, E. J. Fernandez, A. Mendia and P. G. Jones, J. Organomet.
Chem., 1988, 350, 129–138.
15 (a) E. Cerrada, M. Contel, A. D. Valencia, M. Laguna, T. Gelbrich
and M. B. Hursthouse, Angew. Chem., Int. Ed., 2000, 39, 2353–2356;
(b) E. J. Fernandez, A. langna, J. M. Lopez-de-Luzuriaga, M. Montiel,
M. E. Olmos, J. Perez and R. C. Puelles, Organometalics, 2006, 25,
4307–4315.
16 N. C. Payne, R. Ravindranath, G. Schoettel, J. J. Vittal and R. J.
Puddephatt, Inorg. Chem., 1991, 30, 4048–4053.
17 (a) For selected reviews, see: V. W.-W. Yam and K. K.-W. Lo, Chem.
Soc. Rev., 1999, 28, 323–334; (b) A. L. Balch, in Structure & Bonding
vol. 123, ed. V. W.-W. Yam, Springer, Berlin - Heidelberg, 2007, pp.
1–40.
Acknowledgements
We thank Dr Akira Tsuboyama (Canon Inc.) for his help with
absolute quantum yield determinations in solid state.
§ Complex 1 decomposes to naphthalene and Au(PPh3)Cl via the ho-
molytic cleavage of the Au–C bond under irradiation in CH2Cl2.5c Since
this photo reaction occurs even under the aerobic condition, we consider
that the excited singlet state is responsible for this dissociation reaction. The
quantum yield of this dissociation reaction is 0.03. Though the complex 2
decomposes with purple deposits in CH2Cl2 in short period, the decompo-
sition is more slowly in 2-MeTHF. With decomposition, phosphorescence
intensity decreases and fluorescence intensity of naphthalens (at 310–
400 nm) increases.
18 J. C. Miller, J. S. Meek and S. J. Strickler, J. Am. Chem. Soc., 1977, 99,
8175–8179.
2252 | Dalton Trans., 2008, 2248–2252
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