Regiospecific formation and unusual optical properties of 2,5-bis(arylethynyl)rhodacyclopentadienes: A new class of luminescent organometallics

Article abstract of DOI:10.1002/chem.201304068

A series of 2,5-bis(arylethynyl)rhodacyclopentadienes has been prepared by a rare example of regiospecific reductive coupling of 1,4-(p-R-phenyl)-1,3- butadiynes (R=H, Me, OMe, SMe, NMe₂, CF₃, CO ₂Me, CN, NO₂, -

Full text of DOI:10.1002/chem.201304068

DOI: 10.1002/chem.201304068
Full Paper
&
Fluorescence
Regiospecific Formation and Unusual Optical Properties of
2,5-Bis(arylethynyl)rhodacyclopentadienes: A New Class of
Luminescent Organometallics
Andreas Steffen,[a, b] Richard M. Ward,[b] Meng Guan Tay,[c] Robert M. Edkins,[a] Fabian Seeler,[b]
Magda van Leeuwen,[b] Lars-Olof Pꢀlsson,[b] Andrew Beeby,[b] Andrei S. Batsanov,[b]
Judith A. K. Howard,[b] and Todd B. Marder*[a, b]
Abstract: A series of 2,5-bis(arylethynyl)rhodacyclopenta-
dienes has been prepared by a rare example of regiospecific
reductive coupling of 1,4-(p-R-phenyl)-1,3-butadiynes (R=H,
even stable to air and moisture in solution. The photophysi-
cal properties of the rhodacyclopentadienes are highly un-
usual in that they exhibit, exclusively, fluorescence between
Me, OMe, SMe, NMe2, CF3, CO2Me, CN, NO2, ꢀCꢁC-(p-C6H4ꢀ 500–800 nm from the S1 state, with quantum yields of F=
NHex2), ꢀCꢁCꢀ(p-C6H4ꢀCO2Oct)) at [RhX(PMe3)4] (1) (X=
ꢀCꢁCꢀSiMe3 (a), ꢀCꢁC-(p-C6H4ꢀNMe2) (b), ꢀCꢁCꢀCꢁCꢀ(p-
C6H4ꢀNPh2) (c) or ꢀCꢁCꢀ{p-C6H4-CꢁCꢀ(p-C6H4-N(C6H13)2)} (d)
or Me (e)), giving the 2,5-bis(arylethynyl) isomer exclusively.
The rhodacyclopentadienes bearing a methyl ligand in the
equatorial plane (compound 1e) have been converted into
their chloro analogues by reaction with HCl etherate. The
rhodacycles thus obtained are stable to air and moisture in
the solid state and the acceptor-substituted compounds are
0.01–0.18 and short lifetimes (t=0.45–8.20 ns). The triplet
state formation (FISC =0.57 for 2a) is exceptionally slow, oc-
curring on the nanosecond timescale. This is unexpected,
because the Rh atom should normally facilitate intersystem
crossing within femto- to picoseconds, leading to phosphor-
escence from the T1 state. This work therefore highlights
that in some transition-metal complexes, the heavy atom
can play a more subtle role in controlling the photophysical
behavior than is commonly appreciated.
Introduction
regard was given to metal complexes of type B bearing N^N-
bonded 2,2’-bipyridine (bpy) and related ligands. Rhenium(I)
and ruthenium(II) complexes, in particular, show great promise
for electrochemical and photochemical applications, such as
catalysts for the electrochemical reduction of CO2 or in dye-
sensitized solar cells as photosensitizers.[2] Remarkable progress
has been achieved, with metallacyclopentadienes featuring
(N^N)-bonded bpy- and C^N-bonded 2-phenylpyridyl (ppy) li-
gands, in the area of luminescent materials for organic light-
emitting diodes (OLEDs). Second- and third-row transition-
metal complexes of types B and C (Figure 1) are well-known as
efficient phosphorescent triplet-state emitters.[2n,3] The proto-
typical compounds [Ru(bpy)3]2+ and [Ir(ppy)3] and their deriva-
tives have been extensively studied to understand the excited-
state behavior and photophysical properties of organo-transi-
tion-metal compounds.[2e,4] In general, the strong spin-orbit
coupling (SOC) of the metal atom facilitates intersystem-cross-
ing (ISC) between the excited singlet and triplet states very ef-
ficiently. The ISC can be ultrafast, that is, <20 fs, as has been
measured for [M(bpy)3]2+ (M=Fe, Ru).[2d,5] In addition, the spin-
forbidden nature of the radiative transition between the triplet
state and the ground state is removed, and thus high quantum
yields of up to unity for phosphorescence are observed.[3b]
In contrast to their types A–C congeners, MC4 metallacyclo-
pentadienes of types D and E have been largely neglected for
photophysical applications. Investigations have concentrated
An immense variety of applications of metallacyclopentadienes
has arisen from the investigation of their structure–property re-
lationships. Diimine-based metallacyclopentadienes of type A
(Figure 1), for example, have been found to be highly active
catalysts for alkene polymerization, and systems based on
nickel subsequently became well-known as Brookhart’s cata-
lysts.[1] The redox-activity of metallacyclopentadienes with di-
imine ligands has been explored, but more attention in this
[a] Dr. A. Steffen, Dr. R. M. Edkins, Prof. Dr. T. B. Marder
Institut fꢀr Anorganische Chemie
Julius Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: todd.marder@uni-wuerzburg.de
[b] Dr. A. Steffen, Dr. R. M. Ward, Dr. F. Seeler, M. van Leeuwen,
Dr. L.-O. Pꢂlsson, Prof. Dr. A. Beeby, Dr. A. S. Batsanov,
Prof. Dr. J. A. K. Howard, Prof. Dr. T. B. Marder
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
[c] Dr. M. G. Tay
Faculty of Resource Science and Technology
Universiti Malaysia Sarawak
94300 Kota Samarahan, Sarawak (Malaysia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304068 (including characterization of
compounds, UV–visible absorption, excitation and emission spectra, and
data obtained from X-ray crystallography).
Chem. Eur. J. 2014, 20, 3652 – 3666
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CSiMe3)(PMe3)4].[20] Subsequently, Hill and co-workers
observed the same regiospecificity at ruthenium,
when [Ru(CO)2(PPh3)3] was treated with excess di-
phenyl-1,3-butadiyne at elevated temperature to give
the 2,5-dialkynyl isomer exclusively.[21] Reductive cou-
pling of 2,4-hexadiyne at osmium was also observed
to produce the 2,5-diethynyl metallacyle. Recently,
we also succeeded in preparing more rigid conjugat-
ed 2,5-bis(p-R-arylethynyl)rhodacyclopentadienes by
treatment of [Rh(CꢁCSiMe3)(PMe3)4] or [Rh(k2-S,S’-
S2CNEt2)(PMe3)2] with 1,12-diaryldodeca-1,3,9,11-tet-
raynes.[22] The rhodacyclopentadienes thus obtained
exhibit exceptional fluorescence properties, although
phosphorescence would have been expected due to
the heavy atom effect. Apart from our previous two
Figure 1. A) Metal–diimine complex; B) metal–bipyridine complex; C) metal-2-phenylpyr-
idyl complex; D) metal-2,2’-biphenylene complex; E) metallacyclopentadiene.
on the catalytic activity of 2,2’-biphenylene (bph) complexes in
CꢀC bond-forming reactions or the reduction of biphenylene.[6]
Photophysical studies of only a limited number of bph com-
plexes of Ir, Pd, and Pt have been reported; these compounds
exhibit phosphorescence with quantum yields of up to F=
0.16 and lifetimes of t=3–14 ms.[7] Despite these promising re-
sults, limited synthetic access to bph transition-metal com-
plexes is presumably the reason for the lack of photophysical
data in the literature. Metallacyclopentadienes of type E are
Figure 2. Reductive coupling of butadiynes at [CoCp(PPh3)], leading to the
formation of three regioisomers.[17]
well-established intermediates in catalytic cyclotrimerization re-
actions of alkynes to benzenes and alkyne/nitrile mixtures to
pyridines,[6b,8] but very little is known about their photophysical
and excited-state properties.[6a] This is astonishing, because
structurally related main group EC4 systems such as boroles,[9]
siloles,[10] phospholes,[11] and thiophenes[12] have attracted sig-
nificant attention due to their optical and electron-transporting
properties. In particular, 2,5-conjugated ring systems,[6a] such as
preliminary studies, none of the other reports address the opti-
cal properties of conjugated 2,5-bis(p-R-arylethynyl)metallacy-
clopentadienes.
As part of our program of research on metal acetylides and
their optical properties,[23] we examined the reactions of rho-
dium(I) acetylides with 1,3-diynes. We now demonstrate the
applicability of our general synthetic approach to access rho-
dacyclopentadienes by reductive coupling of 1,4-bis(p-R-aryl-
ethynyl)-1,3-butadiynes, with a variety of substituents R of dif-
fering electronic donor or acceptor strength and different
ligand combinations at the Rh center. The reactions occur re-
giospecifically, yielding the 2,5-isomer exclusively. Detailed
photophysical investigations of the metallacyclopentadienes
have been carried out, which show highly unusual excited-
state behavior in comparison with other luminescent organo-
metallic complexes.
2,5-bis(arylethynyl)thiophenes,[13]
2,5-bis(arylethynyl)phos-
pholes[14] and 2,5-bis(arylethynyl)siloles,[10a] are of interest. Simi-
lar to p-bis(phenylethynyl)benzenes (BPEBs)[15] and anthracenes
(BPEAs),[15f,r,t,16] the extended conjugation gives rise to interest-
ing linear and non-linear optical behavior.
Synthetic access to conjugated 2,5-bis(R-ethynyl)metallacy-
clopentadienes should, in principle, be possible by reductive
coupling of 1,3-butadiynes at a suitable transition-metal pre-
cursor complex.[6a] However, examples of reductive coupling of
1,3-butadiynes are scarce, and most of the reactions do not
occur with the desired regioselectivity. Nishihara and co-work-
ers found that in the reaction of [CoCp(PPh3)2] (Cp=cyclopen-
tadienyl) with diphenylbuta-1,3-diyne, an insoluble polymer
was the major product, with three different regioisomeric com- Results
plexes being formed only at elevated temperature (Figure 2).[17]
Synthesis and characterization
The problem of regioselectivity has also been observed in
the reaction of [TiCl2Cp2] with bis(trimethylsilyl)-1,3-butadiyne
in the presence of Mg as reducing agent, which forms the 2,4-
isomer in 65% yield.[18] Bruce and co-workers were able to iso-
late, although in very low yields of 4%, a mononuclear 2,5-di-
alkynyl ruthenacyclopentadiene from the reaction of diphenyl-
butadiyne with [Ru3(CO)10(NCMe)2].[19] We were able to demon-
strate the regiospecific synthesis of a luminescent rhodacyclo-
Treatment of [RhMe(PMe3)4] with terminal alkynes HCꢁCR (R=
SiMe3, p-C6H4ꢀNMe2, CꢁC-(p-C6H4ꢀNPh2),[24] p-C6H4-[CꢁC-{p-
C6H4ꢀN(C6H13)2}][25]
) gives the rhodium(I) compounds 1a–d
through CꢀH activation with loss of methane (Scheme 1). Sub-
sequent reductive coupling of two equivalents of 1,4-bis(p-R’-
phenyl)-1,3-butadiynes (R’=H, Me, OMe, SMe, CꢁCSiMe3, NMe2,
CF3, CO2Me, CN, NO2) at room temperature in the coordination
pentadiene from 1,4-di(p-tolyl)-1,3-butadiyne and [Rh(Cꢁ sphere of rhodium yields the rhodacyclopentadienes 2(a–d)–
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sidual [Rh(PMe3)4][Cl], the pre-
cursor to [RhMe(PMe3)4]. Howev-
er, the amount of the impurities
in the bulk material of 3e and
6e must be very small, that is,
less than 1%, because they were
not detected by NMR spectros-
copy or elemental analysis, indi-
cating analytically pure com-
pounds. The relatively larger
amount of 3 f and 6 f in the
single crystals is most likely due
Scheme 1. Regiospecific formation of 2,5-bis(arylethynyl)rhodacyclopentadienes 2–10.
10(a–d). It is striking that the reactions occur quantitatively
and regiospecifically in all cases, forming the 2,5-bis(p-R’-
phenyl)-ethynyl isomer exclusively as confirmed by in situ
31P{1H} NMR spectroscopy. We observed that p-R’-phenyl-1,3-
butadiynes with donor substituents react slower than the ac-
ceptor-substituted diynes, which may be due to diminished p-
backbonding from the electron-rich RhI center in the precursor
bis(diyne) complexes. We also prepared compounds 11–13
with longer conjugated chains in the para position of the
phenyl rings for exploration of liquid-crystal behavior and the
influence of conjugation on the photophysics (Scheme 2).
However, no evidence for liquid-crystal phase behavior was ob-
served using transmitted polarized light microscopy.
to their lower solubility than 3e and 6e, which leads to enrich-
ment during the crystallization process. The intentional reac-
tion with [Rh(PMe3)4][Cl] as precursor was only successful when
(p-CF3ꢀC6H4)-CꢁC-CꢁC-(p-C6H4ꢀCF3) was employed in a fourfold
excess with prolonged heating at 808C for four days, leading
to moderate yields of the desired conjugated 2,5-isomer 7 f
containing an equatorial chloride ligand. The low reactivity of
[Rh(PMe3)4][Cl] also indicates that the content of 3 f and 6 f
must be very small in the samples we prepared of 3e and 6e.
Attempts to accelerate the reaction in a microwave reactor
failed due to the fact that three different isomers were formed
according to 31P{1H} NMR spectroscopy and mass spectrometry.
However, addition of HCl·Et2O to 2e and 4e–8e converts the
Rh-methyl-substituted rhodacy-
clopentadienes to their corre-
sponding
chloro
analogues
quantitatively (Scheme 3). The
chemical stability of the rhoda-
cyclopentadienes is underlined
by the fact that selective remov-
al of the four SiMe3 groups at
the peripheral C6H4ꢀCꢁCꢀSiMe3
positions of 11 is possible with
n-Bu4NF in THF, whereas the
ꢀCꢁCꢀSiMe3 ligand on the Rh
center remains intact, yielding
14 (Scheme 4). Compounds 2a/
Scheme 2. Synthesis of 2,5-bis(arylethynyl)rhodacyclopentadienes 11–13 with conjugated groups at the para-posi-
tion of the phenyl substituents.
[RhMe(PMe3)4] also reacts di-
rectly with two equivalents of
1,4-bis(p-R’-phenyl)-1,3-buta-
diynes (R’=H, Me, OMe, SMe,
CF3, CO2Me) to give the corre-
sponding rhodacyclopentadienes
2e–8e bearing a methyl ligand
in the equatorial plane, although
higher reaction temperatures of
808C for 3–12 h are required for
full conversion (Scheme 3). We
note that for compounds 3e and
6e, small amounts of 3 f (13%)
and 6 f (15%), respectively, are
found in the single crystals used
for X-ray diffraction (see below),
presumably a result of the reac-
Scheme 3. Synthesis of methyl- (2e–8e) and chloro- (2 f, 4 f, 5 f, 7 f, and 8 f) substituted 2,5-bis(arylethynyl)rhoda-
tion of the butadiynes with re- cyclopentadienes.
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b–10a/b are exceedingly robust
and insensitive to air and mois-
ture in the solid state, whereas
the acceptor-substituted rhoda-
cylopentadienes 7a/b–10a/b are
stable to air and moisture for
months in THF or toluene solu-
tion. However, the complexes
Scheme 4. Deprotection of 11 with nBu4NF to give 14.
with longer conjugated substitu-
ents, either as R or R’ (2c/d–
10c/d and 11–14), are rather sensitive to air and moisture, and
6d and 14 also appear to be photochemically unstable.
tion is distorted octahedral. The metallacycle (i) is planar, with
bond length alternation consistent with a rhodacyclopenta-
diene structure. In the s-ethynyl derivatives, the endocyclic
RhꢀC(2) and RhꢀC(5) bond lengths are similar, indicating com-
parable trans-influence of the phosphine and s-ethynyl li-
gands. Replacement of the s-ethynyl with a s-methyl ligand
causes lengthening of the trans RhꢀC(2) bond, which, in 8e, is
0.05 ꢂ longer than RhꢀC(5). In contrast, replacement of s-eth-
ynyl with a chloro ligand (in 5 f, 7 f, 8 f) makes the RhꢀC(2)
bond significantly shorter than RhꢀC(5), by 0.04 ꢂ or more.
The trans-influences of the s-ligands thus agree with their elec-
tronegativities, that is, 3.0 (Cl), 3.1 (C sp), 2.4 (Me), and s-donat-
ing properties. The RhꢀC(2) bond lengths in 3e/f and 6e/f are
much longer (ca. 0.06 ꢂ) than in pure 5 f, 7 f, or 8 f, and they
are similar to those bearing strong s-donors, which is consis-
tent with only a very small amount of the methyl ligand being
replaced by the chloro ligand. The degree of p-electron delo-
calization in the rhodacyclopentadiene can be estimated by
the ratio D=2d1/(d2+d’2) (see Figure 3 and Table 2) between
the length of the nominally single (d1) and nominally double
bonds (d2, d’2). In the present complexes, D ranges from 1.055
to 1.072 ꢂ, compared to 1.058 in previously reported 3a[20] and
1.047–1.059 in four cyclohexano-rhodacyclopentadiene ana-
logues.[22] This indicates greater delocalization than in related
2,5-bis(arylethynyl)siloles (mean D=1.122(16) ꢂ for seven
structures)[10a] or in carbocyclic cyclopentadienes (mean D=
1.094(9) ꢂ for the 23 most accurate low-temperature structures
in the CSD[26]).
Crystal and molecular structure
Representations of the general atom and ring numbering for
the following discussion are shown in Figures 3 and 4 using
2a and 3d as examples. Selected bond lengths and angles are
given in Tables 1 and 2. More details and pictorial presenta-
tions are given in the Supporting Information (Figures S13–
S25, the Supporting Information). In all cases, the Rh coordina-
Figure 3. Molecular structure of 2a in the solid state, showing atom and
ring numbering. Hydrogen atoms omitted for clarity.
Arene rings iii and iv are inclined to the rhodacyclopenta-
diene plane substantially (by 43 to 648). The corresponding
angle for ring ii varies widely, notwithstanding the same intra-
molecular environment, and must be attributed to the effect
of crystal packing. This, in turn, suggests that variable orienta-
tion of ring v may also be due to intermolecular interactions,
rather than steric repulsion between this moiety and the s-
ligand. Indeed, it is noteworthy that ring v is almost always
tilted towards the latter, through bending of the ethynyl
group, whereas for the 2-ethynylarene substituent, such bend-
ing is much smaller and usually occurs perpendicularly to the
metallacycle plane, rather than within this plane.
Arene ring vi in 2b, 3b, and 3d (both independent mole-
cules), 4d and 7d forms a large dihedral angle with ring i. The
twist between rings vi and vii is substantial in 3d and 7d. In
4d, this twist is small, but the ligand rod is substantially bent;
whereas the RhꢀC(1) bond is tilted out of plane i by 48, for the
C(55)ꢀN bond this angle increases to 558. For comparison, the
corresponding angles equal only 2.0 and 7.48 in molecule I, or
Figure 4. Molecular structure of 3d in the solid state, showing atom and
ring numbering. Hydrogen atoms omitted for clarity.
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Table 1. Selected bond lengths [ꢂ].
RhꢀP(1)
RhꢀP(2)
RhꢀP(3)
RhꢀCl(1)
RhꢀC(1)
RhꢀC(2)
RhꢀC(5)
C(2)ꢀC(3)
C(3)ꢀC(4)
C(4)ꢀC(5)
D[a]
2a
2b
3b
2.3230(5)
2.3149(7)
2.3113(12)
2.320(3)
2.3164(7)
2.3316(13)
2.325(4)
2.3301(8)
2.3071(4)
2.3088(13)
2.3223(4)
2.3188(14)
2.3225(8)
2.3108(9)
2.3255(6)
2.3249(8)
2.3180(5)
2.3044(6)
2.3136(13)
2.308(3)
2.3011(6)
2.3079(14)
2.317(6)
2.3302(8)
2.3113(4)
2.3049(13)
2.3248(4)
2.3244(14)
2.3440(7)
2.3107(8)
2.3431(6)
2.3199(7)
2.3763(6)
2.3632(6)
2.3550(7)
2.364(2)
2.3550(7)
2.3798(13)
2.369(3)
2.4137(7)
2.3765(4)
2.3544(13)
2.3729(4)
2.3634(14)
2.3962(9)
2.347(1)
2.041(2)
2.042(2)
2.042(4)
2.028(9)
2.095(2)
2.085(2)
2.091(4)
2.067(7)
2.103(2)
2.082(4)
2.09(1)
2.030(2)
2.0976(15)
2.087(4)
2.093(2)
2.073(4)
2.038(3)
2.128(3)
2.034(2)
2.098(2)
2.100(2)
2.091(2)
2.072(4)
2.093(8)
2.083(2)
2.099(4)
2.08(1)
2.071(3)
2.0874(14)
2.078(4)
2.083(2)
2.092(5)
2.072(3)
2.077(3)
2.079(2)
2.072(3)
1.377(2)
1.381(3)
1.373(5)
1.362(11)
1.379(3)
1.385(6)
1.37(3)
1.375(3)
1.376(2)
1.381(6)
1.380(2)
1.384(7)
1.362(4)
1.374(4)
1.379(3)
1.380(3)
1.463(2)
1.462(3)
1.462(6)
1.464(12)
1.464(3)
1.456(6)
1.45(2)
1.456(3)
1.466(2)
1.461(5)
1.462(2)
1.452(7)
1.458(3)
1.470(4)
1.458(3)
1.455(3)
1.374(2)
1.368(3)
1.363(6)
1.368(11)
1.377(3)
1.370(6)
1.37(2)
1.372(3)
1.374(2)
1.374(6)
1.372(2)
1.372(2)
1.369(4)
1.370(4)
1.369(3)
1.372(3)
1.064(2)
1.064(3)
1.069(6)
1.072(12)
1.062(3)
1.057(6)
1.06(2)
1.060(3)
1.066(2)
1.061(6)
1.063(2)
1.055(7)
1.068(4)
1.071(4)
1.061(3)
1.057(3)
3d[c]
3e/3 f
4d
2.255(2)[b]
2.052(5)
2.05(2)
5a[c]
5 f
2.4802(7)
2.201(4)[b]
6a
6e/6 f
7a
2.0451(16)
2.037(2)
2.039(5)
7d
7 f
2.4670(7)
2.4490(5)
8e[c]
8 f
2.165(3)
2.036(2)
2.4015(6)
2.3660(7)
9a
[a] D=2dC(3)ꢀC(4)/(dC(2)ꢀC(3)+dC(4)ꢀC(5)). [b] Positions of C(1) and Cl(1) unresolved. [c] Average for two (3d, 8e) or eight (5a) independent molecules.
troduction,[6b,8] motivating us to
investigate the reactivity of our
compounds with respect to their
use as catalyst precursors. Addi-
tion of excess bis(p-R’-phenyl)-
1,3-butadiyne (R’=H, CO2Me,
CF3) to the respective rhodacy-
clopentadienes with ꢀCꢁCꢀ
SiMe3 (2a and 7a) or ꢀCꢁCꢀ(p-
C6H4ꢀNMe2; 8b) ligands gave
the 1,2,4-tris(p-R’-phenylethynyl)-
3,5,6-tris(p-R’-phenyl)benzene
derivatives 15–17 as the sole
products, but only under forcing
conditions, that is, T=808C for
three weeks and with low turn-
over numbers of three to seven
(Scheme 5). Single crystals of 15
suitable for X-ray diffraction ex-
periments were obtained, and
the molecular structure is shown
in Figure 5. The regiospecificity
of the cyclotrimerization reac-
tion, giving rare examples of
1,2,4-tris(arylethynyl)benzenes,
can be explained by the catalytic
cycle, which involves the regio-
specific regeneration of the key
Table 2. Angles [8] between ring planes and vectors.
i/ii
i/iii
i/iv
i/v
i/vi
vi/vii
f1
9.8
5.9
2.6
14.7
10.4
8.4
f2
2a
2b
3b
3d[a]
11.0
12.0
5.2
16.8
21.0
19.8
4.9
64.0
50.5
56.8
60.5
60.6
53.6
58.2
57.4
58.6
53.4
61.4
47.0
48.3
49.5
53.4
45.9
58.3
15.3
22.4
38.0
41.5
51.1
7.0
28.7
11.7
19.1
22.4
15.0
25.4
32.0
11.2
83.7
49.3
81.1
88.9
40.5
38.4
3e/3 f
4d
5a[a]
A
49, 40[b]
12, 5[b]
1.0
5.3,
4.1,
7.6[b]
9.5
10.6[b]
10.9
10.3, 6.0[b]
11.2, 7.0
9.5
B
C
D
E
F
G
H
2.2
62.6
58.6
58.4
64.7
62.3
62.1
62.9
56.4
55.0
52.4
53.1
63.3
55.7
45.1
44.3
51.1
49.3
47.0
46.7
45.6
48.5
46.1
49.0
49.7
50.6
43.3
44.1
46.9
51.7
50.0
54.7
55.9
52.6
56.6
20.1
12.8
8.0
15.9
6.2
10.9
15.4
11.9
14.5
27.5
65.1
28.4
10.4, 9.2[b]
39.6
17.8
6.7
11.3
16.8
9.8
10.9
15.9
29.2
22.7
15.4
18.6
26.8
20.3, 12.2[b]
5.0
1.0, 8.0[b]
3.0, 7.0[b]
3.1
3.3
5.6
2.9
19.6
18.5
39.8
18.3
16.5
24.0
72.1
71.6
19.9
6.4
10.1
11.7
8.9
9.3
11.8
1.3
5 f
6a
6e/6 f
7a
7d
7 f
8e[a]
2.2
73.4
35.9
11.0
13.6
16.7
16.7
5.4
21.9
41.1
14, 24, 38[b]
2.4
26.1
20, 20, 16[b]
8 f
9a
2.5
[a] Symmetrically independent molecules. [b] Alternative positions of disordered ligands.
2.0 and 10.38 in molecule II of 3d, and 2.3 and 12.68 in 7d (al-
though the angles between the RhꢀC(1) and C(55)ꢀN bonds
are larger, being 15.5, 15.4, and 22.08) in which the dihedral
angle i/vi is close to 908.
2,5-bis(arylethynyl)rhodacyclopentadiene intermediate. The low
catalytic activity may well be related to the limited phosphine
(PMe3) dissociation from the RhIII rhodacyclopentadienes,
a factor which contributes to their stability.
Cyclotrimerization reactions
Photophysical studies
Metallacyclopentadienes are well-known as intermediates in
cyclotrimerization reactions of alkynes as discussed in the in-
The optical properties of compounds 2a–10a and 11–14 are
summarized in Table 3, whereas absorption and emission spec-
Chem. Eur. J. 2014, 20, 3652 – 3666
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Figure 5. Molecular structure of 15 in the solid state, with hydrogen atoms
omitted for clarity, and ellipsoids drawn at 50% probability. Dihedral angles:
i/ii 62.4, i/iii 66.5, i/iv 62.2, i/v 75.5, i/vi 5.9, and i/vii 61.98.
Scheme 5. Synthesis of 1,2,4-tris(arylethynyl)-3,5,6-tri(aryl)benzenes 15–17 by
using 2a, 7a or 8b as a catalyst (top) and proposed catalytic cycle (bottom).
tra of selected compounds are presented in Figure 6 (see also
Figures S1–S4, the Supporting Information). Intense bands in
the UV–visible absorption spectra with maxima at labs =453–
517 nm
and
extinction
coefficients
of
e=21000–
35000 dm3 molꢀ1 cmꢀ1 are observed for the rhodacyclopenta-
dienes bearing a ꢀCꢁCꢀSiMe3 ligand in the equatorial plane.
All compounds exhibit emission in the visible region (lem
=
496–590 nm) with small Stokes shifts of about 1870–2390 cmꢀ1
and modest photoluminescence quantum yields of FPL =0.01–
0.18. We note that compound 10a has the highest quantum
yield, although in organic molecules the n!p* transition of
Figure 6. Normalized absorption (Abs) and emission (Em) spectra of 2a, 5a,
8a, and 10a recorded in degassed toluene at room temperature with excita-
tion at their respective absorption maxima.
Table 3. Selected photophysical data of 2,5-bis(arylethynyl)rhodacyclopentadienes bearing a ꢀCꢁCꢀSiMe3 ligand in the equatorial plane (a). All data were
recorded in degassed toluene at RT.
[a]
Compound
lmax (abs)
[nm]
e
lmax (em)
[nm]
Stokes shift
FPL
tf
FD
[molꢀ1 cmꢀ1 dm3]
[cmꢀ1
]
[ns]
2a (R=H)
3a (R=Me)
453
454
454
468
466
464
485
482
517
484
494
26000
25000
23000
35000
496
496
497
515
516
510
536
533
590
534
549
1910
1870
1910
1950
2080
1940
1960
1990
2390
1930
2030
0.15
0.12
0.09
0.10
0.87
0.66
0.45
0.55
0.57
4a (R=OMe)
5a (R=SMe)
6a (R=NMe2)
7a (R=CF3)
8a (R=CO2Me)
9a (R=CN)
10a (R=NO2)
11 (R=CCTMS)
12 (R=CC(C6H4)NHex2)
0.50
0.45
23000
21000
30000
22000
33000
48000
0.08
0.16
0.08
0.18
0.03
0.01
0.56
0.98
0.54
1.21
0.06 (27%)
1.64 (73%)
0.10 (47%)
0.86 (53%)
13 (R=CC(C6H4)CO2Oct)
500
478
40000
556
526
2010
1910
0.02
14 (R=CCH)
1
[a] Quantum yield for O2 formation in O2-saturated solutions.
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the nitro group often tends to quench luminescence. Both the
absorption and the emission occur with a vibrational progres-
sion of about 1300 cmꢀ1. In addition, the absorption and emis-
sion maxima shift bathochromically with the Hammett con-
stants of the para substituents R’ on the aryl rings (sp), a behav-
ior that is also found in structurally related 2,5-bis(phenylethy-
nyl)thiophenes (BPETs).[13] Electron-withdrawing para substitu-
ents have a more pronounced effect than electron-donating
ones on the energy gap between the ground state S0 and the
emissive excited state.
reason, rhodacyclopentadiene 3c with ꢀCꢁCꢀCꢁCꢀ(p-C6H4ꢀ
NPh2)[24] as a ligand was synthesized, in which the two alkynyl
groups were initially anticipated to ensure co-planarity of the
aryl rings. However, despite the additional alkyne unit, the ab-
sorption and emission spectra of 3c are similar to 3a/b and
3d. A molecular modeling study showed that steric hindrance
between the p-phenylene ortho-hydrogen atoms of the ꢀCꢁ
CꢀCꢁCꢀ(p-C6H4ꢀNPh2) ligand and the adjacent aryl ring on the
diethynylmetallacycle again prevents the desired co-planarity.
A more pronounced effect was expected by replacing the ace-
tylide with the more strongly s-donating methyl ligand in 2e–
8e, which should increase electron density at the metal center
and thus increase the metal-to-ligand charge transfer (MLCT)
character of the emitting excited state, consequently leading
to higher ISC rates. Indeed, the maxima of the absorption and
emission of compounds 2e, 4e, 6e, and 8e show a very small
bathochromic shift of about 350 cmꢀ1 compared with 2a, 4a,
6a, and 8a and the fluorescence quantum yield is reduced to
below 0.01 (Table 4). However, the singlet oxygen sensitization
During the photophysical investigation of this class of lumi-
nescent metallacyclopentadienes, we observed that the emis-
sion occurs with a lifetime of t=0.45–1.21 ns (Table 3), leading
us to conclude that the observed luminescence is fluorescence
resulting from the S1 state, similar to the related, more rigid
rhodacycles that we previously discussed in a preliminary
report.[22] This is highly unusual for octahedral, late organo-
transition-metal complexes, as the spin-orbit coupling (SOC) of
the metal atom would be expected to facilitate very fast inter-
system crossing (ISC) to the triplet state T1 on the timescale of
femto- to picoseconds (10ꢀ14–10ꢀ10 s).[2d,3b,5] Therefore, the ex-
cited singlet state S1 is normally too short lived to observe
fluorescence, and either non-radiative decay or phosphores-
cence from the T1 state with emission lifetimes of micro- to
milliseconds would be expected. Singlet oxygen sensitization
experiments were carried out for 2a, 5a, and 8a and quantum
yields for the formation of singlet oxygen (FD) were found to
be in the range 0.45–0.61, thus suggesting similar quantum
yields for the ISC (Table 3).
Table 4. Selected photophysical data of 2,5-bis(arylethynyl)rhodacyclo-
pentadienes bearing a Me ligand (e) or a chloride ligand (f) in the equato-
rial plane. All data were recorded in degassed toluene at RT.
[a]
Compound lmax
e
lmax
FPL
tf
FD
(abs) [molꢀ1 cmꢀ1 dm3] (em)
[ns]
[nm]
460
[nm]
505
2e
(R=H)
4e
(R=OMe)
5e
(R=SMe)
6e
(R=NMe2)[b]
8e
(R=CO2Me)
2 f
(R=H)
4 f
24000
17000
<0.01
<0.01
<0.01
1.47 (9%) 0.64
<1.0 (91%)
8.20 (86%)
2.05 (14%)
2.30 (83%)
458
474
474
492
463
461
471
488
505
527
523
545
505
508
522
543
The ISC from the S1 to the T1 state in compounds 2a–10a
occurs several orders of magnitude slower (10 ns) than one
would expect, leading to unusually long-lived emitting S1
states. Introducing longer conjugated substituents at the para
position of the phenyl rings (see compounds 11–14) leads to
a bathochromic shift in absorption as well as in emission, and
also drastically increases the values of the extinction coeffi-
cients. The short nanosecond emission lifetimes suggest that
the general photophysical excited-state behavior is very similar
to that of 2a–10a, although the low quantum yields of 0.01–
0.03 show that effective radiationless decay occurs, presumably
due to additional vibrational modes.
<1.0 (17%)
40000
22000
20000
27000
<0.01
<0.01
<0.01
1.00 (36%) 0.38
<1.0 (64%)
2.80 (9%) 0.65
<1.0 (91%)
3.38 (36%) 0.62
!1.0 (64%)
1.82 (28%) 0.21
<1.0 (72%)
(R=OMe)
5 f
(R=SMe)
8 f
26000
0.22
1.20
0.55
(R=CO2Me)
To understand the role of the s-donor ligand in the metalla-
cyclopentadiene plane and of the rhodium atom in the photo-
physical behavior of the excited states, the ꢀCꢁCꢀSiMe3
moiety was substituted by several ligands with different conju-
gation lengths and varying s/p-donor strengths. Compounds
2b/d–4b/d, 5d, 7b/d–8b/d, and 9d bearing the conjugated
[a] Quantum yield for 1O2 formation in O2-saturated solutions. [b] Analyti-
cally pure bulk material (see above).
experiments indicate a quantum yield for the ISC of only 0.38–
0.64, identical within the experimental error to the ꢀCꢁCꢀ
donor ligands ꢀCꢁCꢀ(p-C6H4ꢀNMe2) (b) and ꢀCꢁCꢀ(p-C6H4)ꢀ SiMe3-substituted compounds 2a, 5a, and 8a. The lower pho-
CꢁCꢀ(p-C6H4ꢀN(C6H13)2) (d) show almost identical absorption
and emission spectra with very similar extinction coefficients
and fluorescence lifetimes to 2a–10a and 11–14 (Table 3 and
the Supporting Information). We attribute the negligible influ-
ence of these ligands on the photophysical properties of the
compounds to the fact that the aryl rings are significantly ro-
tated out of co-planarity with the metallacycle (see Table 2)
and, thus, conjugation between the rhodium atom and the
substituent on the acetylide ligands is not increased. For this
toluminescence quantum yield presumably results from non-
radiative decay through CꢀH vibrational and/or RhꢀCH3 rota-
tional modes. No isotope effect on the quantum yield was
found by replacing CH3 with CD3 (d3–2a), suggesting very effi-
cient non-radiative decay pathways in compounds 2e–8e. We
also investigated the influence of the chloride ligand with
weak s/p-donor properties, but surprisingly, the general pho-
tophysical behavior of compounds 2 f, 4 f, 5 f, and 8 f was
rather similar to that of their ꢀCꢁCꢀSiMe3 analogues. Small
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redshifts of about 350–480 cmꢀ1 (ca. 6–10 nm) of the absorp-
tion and emission maxima were observed, and the fluores-
cence quantum yields strongly depend on the aryl substitu-
ents, ranging from below 0.01 for R’=H to 0.22 for R’=CO2Me
(Table 4). However, the ISC process occurs on the same time-
scale as observed for all other compounds (10ꢀ8 s), according
to our singlet oxygen sensitization experiments. The emission
of the rhodacyclopentadienes is not very sensitive to solvent
effects, as compounds 2 f, 4a, and 4 f exhibit a slight hypso-
chromic shift in MeCN, whereas the acceptor-substituted com-
pounds 8e/f display an emission shift in MeCN to lower ener-
gies by about 14 and 9 nm, respectively (see the Supporting
Information). This suggests some CT character of the
emission.
In our preliminary study on the absorption and emission
properties of 3a, we erroneously assigned the high-energy
bands at 366 and 384 nm to arise from emission from the rho-
dium complex.[20] We would like to correct this assignment, be-
cause further studies showed that the high-energy emission
arose from an organic trace impurity in the starting material.
Cyclovoltammetric measurements
The one-electron oxidation potentials for compounds 2a/b
and 2e/f, bearing different s-donor ligands at the rhodium
center, as well as for 5a and 8a with ꢀSMe and ꢀCO2Me as
electron-donating and electron-accepting para substituents at
the aryl rings, respectively, have been measured and are listed
in Table 6. The one-electron reduction potentials were outside
To ensure that the observed fluorescence does not result
from traces of organic products resulting from diyne cyclotri-
merization reactions, we recorded the absorption, emission,
and excitation spectra of the highly substituted benzene deriv-
atives 15–17. Their absorption maxima all lie in the UV region
(Table 5 and Figure 7) and intense blue fluorescence (Ff =
Table 6. Cyclic voltammetric and spectroscopic data for 2a/b, 2e/f, 5a,
and 8a.
Compound RhꢀX
R
E1/2 lmax (abs) lmax (em) HOMO
[V][a] [nm]
[nm]
[eV][b]
2a
2b
2e
2 f
5a
8a
CCꢀSiMe3
H
H
H
H
0.53 453
0.49 455
0.44 460
0.58 463
0.48 468
496
500
505
505
515
536
ꢀ4.74
ꢀ4.70
ꢀ4.65
ꢀ4.79
ꢀ4.69
ꢀ4.92
Table 5. Photophysical data of 1,2,4-tris(phenylethynyl)-3,5,6-trisphenyl-
benzene derivatives 15–17. All data were recorded in degassed toluene
at RT.
CCꢀPhꢀNMe2
Me
Cl
CCꢀSiMe3
SMe
Compound
lmax (abs)
lmax (em)
FPL
CCꢀSiMe3
CO2Me 0.71 485
[nm]
[nm]
[a] All potentials are reported versus [FeCp*2]/[FeCp*2]+ =0.00 V, from
0.1m nBu4NPF6/MeCN solutions at ambient temperature at a carbon
working electrode. [b] EHOMO =ꢀe[(EoxꢀE1/2(Fc))+4.8 V].
15 (R=H)
16 (R=CO2Me)
314, 342, 366
330, 354, 381
393
395
414
384
0.37
0.61
17 (R=CF3)
315, 342, 366
0.50
the measuring window and are thus not available. Using 2a as
a benchmark for comparison, the following general trends are
observed: substitution of CꢁCꢀSiMe3 for stronger s-donor li-
gands at the rhodium atom, such as CꢁCꢀC6H4ꢀNMe2 (2b) or
Me (2e), leads to a lower oxidation potential, whereas the
weaker s-donor Cl lowers the HOMO energy and increases the
oxidation potential slightly. Interestingly, the electron-donating
ꢀSMe group in the para position of the aryl rings (5a) has
a very similar effect as the CꢁCꢀC6H4ꢀNMe2 ligand at the rho-
dium atom in 2b. The most pronounced effect is observed
with the electron-accepting group ꢀCO2Me in the para posi-
tion of the aryl rings (8a), greatly increasing the oxidation po-
tential by about 0.17 V. Our results lead us to conclude that
the highest occupied molecular orbital (HOMO), which is pre-
dominantly responsible for the oxidation potential, has some
metal contribution, but it is mainly located at the p-system of
the organic ligand. Thus, one would expect that the photo-
physical properties are also dominated by the ligand p-system,
in agreement with our experimental results (see above).
Figure 7. Absorption (Abs) and emission (Em) spectra of 15–17 recorded in
degassed toluene at room temperature with excitation at their respective
absorption maximum.
0.37–0.61) was observed in all cases. As the observed photo-
physical properties of these potential side products differ sub- Discussion
stantially from those of 2–14, we can be sure that the ob-
served photoluminescence originates from the organometallic
Synthesis
complexes and not from an organic impurity such as free
diyne or cyclotrimerized products.
The formation of the 2,5-bis(arylethynyl)rhodacyclopentadienes
reported herein occurs regiospecifically in all cases, yielding
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the 2,5-bis(arylethynyl) isomer exclusively. Other metallacyclo-
pentadienes constructed by the reductive coupling of buta-
diynes at titanocenes or cobaltocenes appear to obey Wakat-
suki’s rule, according to which the carbon atoms with the steri-
cally most demanding substituents at the alkyne form the s-
bond with the transition-metal center.[27] As can be seen from
Scheme 6, the reason lies in the nature of the possible transi-
plane of the metallacycle. We are currently investigating the
reaction mechanism, which will be reported in due course.
Photophysics
Most studies of cyclometallated d6-rhodium compounds have
focused on homoleptic and heteroleptic bipyridine and phe-
nylpyridine complexes of the type [Rh(bpy)3ꢀn
-
(ppy)n](3ꢀn)+ (n=0–3) and derivatives thereof.[28] Few
of them exhibit significant luminescence at room
temperature in solution; thus, most rhodium com-
plexes have been characterized photophysically in
glassy matrices at 77 K or in their crystalline form.
Under those conditions, these previously investigated
cyclometallated rhodium complexes exclusively show
weak phosphorescence with emission lifetimes in the
range of t=3–500 ms. Their emitting excited states
3
have been assigned to be of IL(p–p*) nature, with
very little 3MLCT(d-p*) or 3MC(d–d*) admixtures to
enable a facile intersystem crossing from S1!T1.
These admixtures also enhance the radiative decay
rate constant of the spin-forbidden phosphorescence
(T1!S0) relative to the free ligands, which usually flu-
oresce from their S1 states.[3b] Although the phos-
phorescence of such cyclometallated rhodium(III)
complexes is weak in comparison with other heavy
transition-metal complexes of ruthenium, iridium, or
Scheme 6. Transition states a–c according to Wakatsuki for the formation of metallacy-
clopentadienes by reductive coupling of alkynes.[27]
tion states a–c, with the highest steric hindrance in c, which is
thus the least favored transition state. Whether a parallel (a) or
antiparallel arrangement (b) of the alkynes is preferred, re-
mains unclear. However, Rosenthal et al. concluded for their
five-membered 2,4-alkynyl-titanacyclopentadiene, produced by
reacting [TiCl2Cp2] with bis(trimethylsilyl)-1,3-butadiyne in the
presence of Mg as reducing agent, that the explanation for the
observed regiochemistry is the reduced steric interference re-
sulting from the SiMe3 group and the alkynyl group in the b-
positions of the metallacyclopentadiene ring, resulting in
a transition state similar to (a).[18a] Thus, it is not necessary for
the SiMe3 substituents to take advantage of the free lateral
sectors in the central plane of the bent metallocene unit.
Apart from our initial results with these Rh systems,[20] the
only comparable example for the synthesis of octahedral met-
allacyclopentadienes is that of the related ruthenacyclopenta-
diene [Ru{k2-C,C’-CR=CPhCPh=CR}(CO)2(PPh3)2] (R=CꢁCPh) re-
ported by Hill et al.[21] They proposed that the regioselectivity
resulting in the observed formation of the 2,5-bis(phenylethy-
nyl)isomer is electronic in origin, because the transition state
leading to the final product is sterically unfavorable. In contrast
to the Ru complexes containing CO ligands as strong p-accep-
tors, our rhodium systems involve rather strong s-donors in-
cluding PMe3 ligands, which are only weak p-acceptors. Thus,
we believe the reductive coupling of diynes at [RhMe(PMe3)4]
or [Rh(CꢁCR)(PMe3)4] (1) (R=ꢀSiMe3 (a), ꢀ(p-C6H4ꢀNMe2) (b),
ꢀCꢁCꢀ(p-C6H4ꢀNPh2) (c) or ꢀ{p-C6H4ꢀCꢁCꢀ(p-C6H4ꢀN(C6H13)2)}
(d)), giving the 2,5-isomers exclusively, to occur for steric rea-
sons. The larger aryl (vs. arylethynyl) groups avoid the 2,5-posi-
tions due to interaction with the s-donors in the developing
platinum,[28m] the SOC of rhodium is still large enough to
ensure efficient intersystem crossing (FISC =1) from the singlet
to the triplet excited states, occurring with rate constants of
the order of 1011 sꢀ1 as found, for example, in cis-[RhX2(bpy)2]
[X] (X=Cl, Br).[29] It should be noted that fluorescence has
been observed in late transition-metal complexes of rhenium,
1
osmium, iridium, and platinum resulting from a pure IL state
due to electronic isolation of the chromophore from the metal
center by a spacer unit, inhibiting the spin-orbit coupling
effect of the metal atom.[30–33] Only a few examples of Rh com-
plexes with 1IL fluorescence as well as 3IL phosphorescence
have been described on the basis of luminescence spectra, but
no further details, such as lifetimes and quantum yields, were
given.[34] Thus, it is difficult to judge the kinetics of the photo-
physical excited-state behavior and the efficiency of the inter-
system crossing process in those compounds. However, we
have investigated the kinetics of the excited-state conversions
in rigid conjugated 2,5-bis(p-R-arylethynyl)rhodacyclopenta-
dienes, which have been prepared by reaction of [Rh(Cꢁ
CSiMe3)(PMe3)4] or [Rh(k2-S,S’-S2CNEt2)(PMe3)2] with 1,12-diary-
ldodeca-1,3,9,11-tetraynes. The rhodacycles thus obtained
showed fluorescence quantum yields of up to Ff =0.69 and
ISC on the timescale of a few nanoseconds, that is, several
orders of magnitude slower than expected.[22]
With this in mind, it is remarkable that the rhodacyclopenta-
dienes presented in this work behave more like related
organo-main group cyclopentadienes such as boroles,[9] py-
roles, siloles,[10] phospholes,[11] and thiophenes,[12] which, in
general, show fluorescence in the visible region of the electro-
magnetic spectrum.[35] The kinetics of the general excited-state
Chem. Eur. J. 2014, 20, 3652 – 3666
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processes seem to be largely independent of the substituents
at the aryl rings or at the rhodium atom. A comparison with
main group 2,5-bis(arylethynyl)cyclopentadienes is of particular
interest, and highlights the unexpected photophysical behav-
ior of the rhodacyclopentadienes.[6a] Pagenkopf and co-workers
have prepared luminescent 2,5-bis(arylethynyl)siloles by adap-
tion of a synthetic route discovered by Tamao et al.[10c] The
compounds exhibit fluorescence with low to moderate quan-
tum yields of Ff =0.004–0.09, depending on the aryl substitu-
ents. We have reported similar behavior for a series of 2,5-bis-
(arylethynyl)thiophenes.[13] Their absorption and emission spec-
tra look very similar to the ones recorded for the compounds
in this work, although the thiophene compounds are blueshift-
ed compared to the rhodacyclopentadienes (labs =345–
387 nm, lem =382–435 nm), and the extinction coefficients (e=
33000–52000) and fluorescence quantum yields (Ff =0.19–
0.33) are both higher. The hypsochromic shift of the S0–S1
energy gap might arise from the bent structure of the 2,5-bis-
(arylethynyl)thiophenes, whereas the larger rhodium atom, in
comparison with sulfur, leads to a change in the bond angles
of the MC4 ring, influencing the orbital energies of the carbon
p-system. Intriguingly, oxygen sensitization with a quantum
yield of about 0.60 for 2,5-bis(phenylethynyl)thiophene and
the 210–400 ps emission lifetimes indicate an intersystem
crossing process on the timescale of 10ꢀ9–10ꢀ8 s mediated by
the spin-orbit coupling of the sulfur (x=100 cmꢀ1). It is difficult
to rationalize why the rhodium (x=1200 cmꢀ1) containing
compounds 2–14 exhibit an ISC that is about one order of
magnitude slower than for the thiophenes, and even several
orders of magnitude slower than in other octahedral organo-
transition-metal complexes.[2e,3b,c,4a,7b,28m] The singlet oxygen
sensitization experiments indicate the formation of the T1
state, but it is presumably very low in energy, undergoing fast
non-radiative decay, as we could not observe any phosphores-
cence. Large energy differences between the S1 and T1 state
are normally found in organic systems, whereas organo-transi-
tion-metal complexes often exhibit lower energy gaps due to
low-lying 1/3MLCT states. However, the small changes in the
photophysical properties upon substitution of the ligands at
the rhodium in the plane of the metallacycle indicate a certain
influence of the transition-metal center on the excited-state
coupling of two equivalents of 1,4-bis(p-R-aryl)-1,3-butadiyne
with RhI precursors. The reactions occur quantitatively and re-
giospecifically, forming the 2,5-isomer only. The methyl-substi-
tuted rhodacyclopentadienes 2e, 4e, 5e, 7e, and 8e were
converted into their respective chloro analogues by treatment
with HCl etherate. The compounds are generally stable to air
and moisture in the solid state, and the acceptor-substituted
rhodacyclopentadienes are exceedingly robust, being stable
even in aerated solution. Our photophysical investigations
show the highly unusual excited-state behavior of our rhoda-
cyclopentadienes with fluorescence originating from the S1
state in low to moderate quantum yields of Ff =0.01–0.22, but
no observable phosphorescence, which normally would have
been expected for octahedral late organo-transition metal
complexes. Nevertheless, singlet oxygen sensitization experi-
ments indicate relatively slow formation of a T1 state with
quantum yields of FD =0.38–0.65. The reason for the largely
inhibited spin-orbit coupling of the rhodium atom, which
should lead to efficient intersystem crossing to the triplet
states, remains unclear. The organic p-chromophore ligand ap-
parently neglects the presence of the heavy rhodium atom to
a large extent, so that the emitting state can be assigned as
1
a nearly unperturbed IL(p–p*) state. We are currently under-
taking detailed theoretical studies to clarify the details of the
photophysical processes in the rhodacyclopentadienes, as they
raise questions with respect to the traditional picture of optical
excited-state behavior of organo-transition-metal complexes,
according to which the decay occurs by steps well separated
in time and energy. Thus, intersystem crossing is expected to
be orders of magnitude faster (10ꢀ14–10ꢀ10 s) than the radiative
decay S1!S0, in contrast to what we observe in our rhodium
compounds, which show fluorescence and ISC on the time-
scale of nanoseconds (10ꢀ8 s). Our preliminary studies show
that this new class of luminescent materials and related organ-
ometallic systems deserve further attention. They have been
largely neglected with respect to their photophysical proper-
ties due to the fact that synthetic access was limited until now,
and because of the concentration on bpy- and ppy-based
metallacyclopentadienes.
1
properties, which leads us to assign the excited state as an IL
1
state with very little MLCT admixture. This assignment would
Experimental Section
be in agreement with the results of our cyclovoltammetric
measurements, indicating that the HOMO is mainly located at
the organic p-ligand system, and with the dominant influence
of the p-ligand system on the photophysical properties. Spin-
orbit coupling, leading to efficient triplet state formation,
seems to be largely inhibited. The origin of the unusual excit-
ed-state behavior of the rhodacyclopentadienes reported in
this paper is currently the subject of extensive theoretical and
experimental studies in our group.
General considerations
Unless otherwise stated, all manipulations were performed using
standard Schlenk or glovebox (Innovative Technology Inc.) tech-
niques under an atmosphere of dry nitrogen (BOC). Reagent grade
solvents (Fisher Scientific and J.T. Baker) were nitrogen-saturated
and were dried and deoxygenated using an Innovative Technology
Inc. Pure-Solv 400 Solvent Purification System, and further deoxy-
genated by using the freeze-pump-thaw method. C6D6 and CDCl3
were purchased from Cambridge Isotope Laboratories and were
dried over sodium/benzophenone or CaH2, respectively, deoxygen-
ated using the freeze-pump-thaw method and vacuum transferred
into sealed vessels. The precursor 1,4-bis(p-R-phenylethynyl)-1,3-
butadiynes[36] and [RhMe(PMe3)4][37] were prepared according to lit-
erature procedures. All other reagents were purchased from Alfa-
Conclusion
We have developed a versatile synthetic route to a series of
2,5-bis(p-R-arylethynyl)rhodacyclopentadienes by reductive
Chem. Eur. J. 2014, 20, 3652 – 3666
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Aesar, Acros, Aldrich or Lancaster, and were checked for purity by
GCMS and/or H NMR spectroscopy and used as received.
yields were corrected for refractive index differences of the
solvents.
1
The quantum yields of singlet-oxygen formation were determined
relative to perinaphthanone in toluene (FD =1.0) using a method
described by Nonell.[44] The samples and the reference compounds
were analyzed in the same solvent because of strong dependence
of the radiative and non-radiative rate constants for deactivation of
the triplet states on the solvent. The singlet-oxygen emission was
detected at 1269 nm from solutions in a 1 cm path length quartz
cuvette after being excited at 355 nm by a frequency-tripled Q-
switched Nd:YAG laser (Spectra Physics, Quanta Ray GCR-150–10)
with a 10 Hz repetition rate. The emission was collected at 908 to
the excitation beam by a liquid nitrogen cooled germanium photo-
diode (North Coast E0–817P) after passing through an interference
filter centered at 1270 nm. The photodiode output was amplified
and AC coupled to a digital oscilloscope, which digitized and aver-
aged the transients. The averaged data were then analyzed using
the Microsoft Excel package.
Flash chromatography was carried out in air using silica gel (Silica-
gel LC60A 40–63 mm) obtained from Fluorochem or basic alumina
(aluminum oxide, activated, basic, 58 ꢂ) from Alfa-Aesar. Commer-
cially available, precoated TLC plates (Polygram Sil G/UV254) were
purchased from Machery-Nagel. The removal of solvent was per-
formed on a rotary evaporator in vacuo at a maximum tempera-
ture of 408C.
All NMR spectra were recorded at ambient temperature using
200 MHz Varian Mercury (1H: 200 MHz), 400 MHz Varian Mercury
(1H: 400 MHz, 13C{1H}: 100 MHz, 19F: 376 MHz, 31P{1H}: 162 MHz),
400 MHz Bruker Avance 400 (1H: 400 MHz, 13C{1H}: 100 MHz), Varian
Inova 500 (1H: 500 MHz, 31P{1H}: 202 MHz) or 700 MHz Varian
1
VNMRS-700 spectrometers (1H: 700 MHz, 31P{1H}: 284 MHz). H NMR
chemical shifts are reported relative to TMS and were referenced
through residual proton resonances of the corresponding deuterat-
ed solvent (CDCl3: d=7.24, C6D6: 7.16 ppm) whereas 13C NMR spec-
tra are reported relative to TMS using the carbon signals of the
deuterated solvent (CDCl3: d=77.23, C6D6: 128.39 ppm). 19F NMR
chemical shifts are reported relative to external CFCl3 and 31P NMR
spectra were referenced to 85% H3PO4. Elemental analyses were
obtained using an Exeter Analytical Inc. CE-440 elemental analyzer.
Unit mass spectrometric determinations were obtained either by EI
using a Finnigan MAT 95 XP spectrometer at the EPSRC National
Mass Spectrometry Service Centre in Swansea, by ES using
a Thermo-Finnigan LTQ FT spectrometer operating in positive ion
mode or by using a MALDI-TOF Applied Biosystems Voyager-DE
STR mass spectrometer.
The fluorescence lifetimes were measured by using time correlated
single-photon counting (TCSPC), using either a 396 nm pulsed
laser diode or the 3rd harmonic of a cavity-dumped, mode-locked
Ti-sapphire laser (Coherent MIRA) at 300 nm. The fluorescence
emission was collected at right angles to the excitation source
with the emission wavelength selected by using a monochromator
and detected by a single-photon avalanche diode (SPAD). The in-
strument response function (IRF) was measured using a dilute
LUDOX suspension as the scattering sample, setting the mono-
chromator at the emission wavelength of the laser, giving an IRF of
200 or 100 ps at 396 or 300 nm, respectively. The resulting intensity
decay is a convolution of the fluorescence decay with the IRF and
iterative reconvolution of the IRF with a decay function and non-
linear least-squares analysis was used to analyze the convoluted
data.
Cyclic voltammograms were recorded (v=100 mVsꢀ1) from 0.1m
n-Bu4PF6/MeCN solutions about 1ꢃ10ꢀ4 m in analyte by using
a three-electrode cell equipped with a glassy carbon working elec-
trode, a Pt-wire counter electrode, and a Pt-wire pseudo reference
electrode. All redox potentials are reported with reference to an in-
ternal standard of the decamethylferrocene/decamethylferroceni-
um couple ([FeCp*2]/[FeCp*2]+ =0.00 V).
Raman spectra were recorded on solid samples using a LabRamHR
Raman microscope with a He:Ne laser (633 nm). IR spectra were re-
corded as KBr discs using either a PerkinElmer 1600 series or Spec-
trum 100 series FT-IR spectrometer.
X-ray diffraction experiments were carried out on Bruker 3-circle
UV–visible absorption and emission spectra, lifetime and quantum
yield measurements were all recorded in degassed toluene. UV–
visible absorption spectra and extinction coefficients were ob-
tained on a Hewlett–Packard 8453 diode array spectrophotometer
using standard 1 cm path length quartz cells. Fluorescence spectra
and quantum yield measurements of dilute solutions with absorb-
ance maxima of less than 0.2 were recorded on a Horiba Jobin–
Yvon Fluoromax-3 spectrophotometer using conventional 908 ge-
ometry. The emission spectra were fully corrected using the manu-
facturer’s correction curves for the spectral response of the emis-
sion optical components. The quantum yield of each compound
was measured either by comparison with standards of known
quantum yield, or by use of an integrating sphere with a HORIBA
Jobin–Yvon Fluorolog 3–22 Tau-3, following a method described in
the literature.[38] The absorbance of the samples was kept below
0.12 to avoid inner filter effects and all measurements were carried
out at room temperature. The fluorescence quantum yields of
compounds 2a–4a and 2b–4b were measured using 9,10-bis(phe-
nylethynyl)anthracene (BPEA) in CHCl3 (Ff =0.95)[16c] and fluores-
cein in 0.1m NaOH (Ff =0.79),[39] whereas those of 5a, 7a–9a, and
11 were measured using acridine orange in EtOH (Ff =0.46)[40] and
fluorescein in 0.1m NaOH (Ff =0.79),[39] that of 10a was measured
using cresyl violet in methanol (Ff =0.54)[41] and rhodamine 101 in
ethanol (Ff =1.00)[42] and that of compounds 15–17 were mea-
sured using quinine sulphate in 0.1m H2SO4 (Ff =0.54)[43] and an-
thracene in EtOH (Ff =0.27)[43b] as reference compounds. Quantum
diffractometers with CCD area detectors SMART 1000 (for 2a, 3e,
ꢀ
5 f, 8e), ProteumM APEX (for 7a) or SMART 6000, using MoK (l=
0.71073 ꢂ) radiation from a sealed X-ray tube with graphite mono-
chromator or (for 7a) a 60 W microfocus Bede Microsource with
glass polycapillary optics. The crystals were maintained at T=120 K
using Cryostream open-flow N2 gas cryostats. All structures were
solved by direct methods and refined by full-matrix least-squares
against F2 of all data, using SHELXTL[45] and OLEX2[46] software.
General procedure for the synthesis of rhodacyclopenta-
dienes with alkynyl ligands (2a–d–10a–d and 11–13)
The respective terminal alkyne HCCR (0.047 mmol) was dissolved in
THF (1 mL) and added drop-wise to a stirred solution of [RhMe-
(PMe3)4] (20 mg, 0.047 mmol) in THF (1 mL) at room temperature.
Then, 1,4-bis(p-R’-arylethynyl)-1,3-butadiyne (0.094 mmol) in THF
(1 mL) was added and the solution was stirred for an additional
minute before the volatiles were removed in vacuo. The residue
was re-dissolved in THF (2 mL) and the solution was stirred for
5 min. Again, the volatiles were removed in vacuo. This process
was repeated once more. In the case of donor-substituted 1,4-
bis(p-R’-arylethynyl)-1,3-butadiynes, the residue had to be dissolved
in toluene (5 mL) and heated at 808C, overnight, to achieve full
conversion. The product was recrystallized from THF and hexane
to give an orange/red solid.
Chem. Eur. J. 2014, 20, 3652 – 3666
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mer,cis-[Tris(trimethylphosphine)trimethylsilylethynyl-2,5-bis-
(phenylethynyl)-3,4-bis(phenyl)rhodacyclopenta-2,4-diene] (2a):
Orange solid; yield 33 mg (85%); H NMR (400 MHz, C6D6): d=7.72
Synthesis of cyclotrimers (15–17)
1,2,4-Tris(phenylethynyl)-3,5,6-triphenylbenzene (15): Compound
2a (20 mg, 0.027 mmol) was dissolved in toluene (2 mL) and
1
(d, 3J(H,H)=8 Hz, 2H; CHarom), 7.40 (d, 3J(H,H)=7 Hz, 2H; CHarom),
7.31 (d, 3J(H,H)=8 Hz, 2H; CHarom), 7.23 (d, 3J(H,H)=8 Hz, 2H;
CHarom), 7.06 (m, 12H; CHarom), 1.38 (d, 2J(H,P)=8 Hz, 9H; PMe3),
1.35 (vt, J=3 Hz, 18H; PMe3), 0.37 ppm (s, 9H; SiMe3); 31P{1H} NMR
added to
a solution of 1,4-diphenylbuta-1,3-diyne (114 mg,
0.559 mmol) in toluene (2 mL). The reaction mixture was heated at
reflux under N2 for 3 weeks. The volatiles were removed in vacuo.
The residual solid was applied to the top of a silica gel column,
which was first eluted with hexanes to remove unreacted diyne
starting material and then with 50:50 CH2Cl2/hexane as eluent. The
solvent was removed in vacuo and the product was obtained as
a light brown solid which was further purified by sublimation
under high vacuum. Slow evaporation of a CH2Cl2/hexane solution
of 15 gave white crystals that were suitable for X-ray diffraction
1
2
(121 MHz, C6D6): d=ꢀ9.32 (dd, J(P,Rh)=98 Hz, J(P,P)=31 Hz, 2P;
PMe3), ꢀ23.10 ppm (dt, 1J(P,Rh)=82 Hz, 2J(P,P)=31 Hz, 1P; PMe3);
IR (KBr): n˜ =2024, 2126, 2156 cmꢀ1 (all CꢁC); Raman (solid): n˜ =
1418, 1444, 1594 (all aryl), 2141 cmꢀ1 (CꢁC); elemental analysis
calcd (%) for C46H56RhP3Si: C 66.34, H 6.78, N 0.00; found: C 66.36,
H 6.94, N 0.00.
1
analysis. Yield 37 mg (33%); H NMR (400 MHz, C6D6): d=7.72 (m,
4H; CHarom), 7.54 (m, 8H; CHarom), 7.20 (m, 14H; CHarom), 6.74 ppm
(m, 4H; CHarom); IR (KBr): n˜ =1442, 1491, 1596 (all aryl), 2210 cmꢀ1
(CꢁC); MALDI: m/z: 606 [M+]; elemental analysis calcd (%) for
C48H30: C 95.02, H, 4.98, N 0.00; found: C 94.12, H 4.85, N 0.00
General procedure for the synthesis of rhodacyclopenta-
dienes with a methyl ligand (2e–8e)
[RhMe(PMe3)4] (50 mg, 0.118 mmol) and the respective 1,4-bis(p-R’-
phenyl)-1,3-butadiyne (0.237 mmol) were added to THF (5 mL) and
the resulting solution was stirred for 5 min. The volatiles were re-
moved in vacuo and then THF (2 mL) was added and the reaction
mixture was stirred for an additional 5 min before the volatiles
were again removed in vacuo. After further addition of THF (2 mL),
the solution was heated to 808C for 3 h. The solvent was removed
in vacuo to give the crude products as brown solids, which were
recrystallized from THF/hexane.
Acknowledgements
We thank J. Magee for performing the elemental analyses and
Prof. Dr. M. R. Bryce (Durham University) for a sample of p-(di-
phenylamino)phenylbutadiyne. A.S. thanks the EU (Marie-Curie
FP7) and the DAAD for postdoctoral fellowships. Generous fi-
nancial support by the Bavarian State Ministry of Science, Re-
search, and the Arts for the Collaborative Research Network
“Solar Technologies go Hybrid” is gratefully acknowledged.
mer,cis-[Tris(trimethylphosphine)methyl-2,5-bis(phenylethynyl)-
3,4-bis(phenyl)rhodacyclopenta-2,4-diene] (2e): Pale-brown solid;
1
3
yield 71 mg (81%); H NMR (500 MHz, C6D6): d=7.61 (d, J(H,H)=
8 Hz, 2H; CHarom), 7.48 (d, 3J(H,H)=8 Hz, 2H; CHarom), 7.41 (d,
3J(H,H)=8 Hz, 2H; CHarom), 7.31 (d, J(H,H)=8 Hz, 2H; CHarom), 7.21
(m, 4H; CHarom), 7.08 (t, J(H,H)=8 Hz, 2H; CHarom), 7.03 (t, J(H,H)=
3
3
3
Keywords: fluorescence · metallacycles · rhodium · spin-orbit
coupling · transition metals
2
8 Hz, 4H; CHarom), 6.93 (m, 2H; CHarom), 1.32 (d, J(H,P)=7 Hz, 9H;
PMe3), 1.16 (vt, J=4 Hz, 18H; PMe3), 0.09 ppm (quint, J=7 Hz, 3H;
RhMe); 31P{1H} NMR (121 MHz, C6D6): d=ꢀ6.35 (dd, 1J(P,Rh)=
[1] a) J. Y. Dong, Y. L. Hu, Coord. Chem. Rev. 2006, 250, 47–65; b) C. Bianchi-
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2
1
105 Hz, J(P,P)=34 Hz, 2P; PMe3), ꢀ18.47 ppm (dt, J(P,Rh)=89 Hz,
2J(P,P)=34 Hz, 1P; PMe3); IR (KBr): n˜ =2125 cmꢀ1 (CꢁC); elemental
analysis calcd (%) for C42H50RhP3: C 67.20, H 6.71, N 0.00; found: C
67.58, H 6.95, N 0.00.
General procedure for the synthesis of rhodacyclopenta-
dienes with a chloro ligand (2f, 4f, 5f, 7f, 8f)
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rhodium (compounds 2e, 4e, 5e, 7e, 8 f; 0.125 mmol) was dis-
solved in THF (4 mL) and a solution of HCl etherate (1m, 0.13 mL,
0.13 mmol) was added drop-wise while stirring at room tempera-
ture. After a further 5 min of stirring, the volatiles were removed in
vacuo. The crude products were recrystallized either from CH2Cl2/
hexane or from CH2Cl2/toluene.
ˇ
ˇ
d) A. Cannizzo, A. M. Blanco-Rodriguez, A. El Nahhas, J. Sebera, S. Zꢅlis,
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Dixon, K. Domen, D. L. DuBois, J. Eckert, E. Fujita, D. H. Gibson, W. A.
Goddard, D. W. Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons,
L. E. Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R. Periana, L. Que,
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mer,cis-[Tris(trimethylphosphine)chloro-2,5-bis(phenylethynyl)-
3,4-bis(phenyl)rhodacyclopenta-2,4-diene] (2f): Dark-red solid,
1
3
yield: 86 mg (89%); H NMR (400 MHz, CDCl3): d=7.43 (d, J(H,H)=
2
8 Hz, 2H; CHarom), 7.05–7.22 (m, 18H; CHarom), 1.73 (d, J(H,P)=7 Hz,
9H; PMe3), 1.52 ppm (vt, J=3 Hz, 18H; PMe3); 31P{1H} NMR
(162 MHz, CDCl3): d=ꢀ8.95 (dd, 1J(P,Rh)=102 Hz, 2J(P,P)=32 Hz,
2P; PMe3), ꢀ23.38 ppm (dt, 1J(P,Rh)=86 Hz, 2J(P,P)=32 Hz, 1P;
PMe3); elemental analysis calcd (%) for C41H47RhClP3: C 63.86, H
6.14, N 0.00; found: C 64.03, H 6.39, N 0.00.
Chem. Eur. J. 2014, 20, 3652 – 3666
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