IV. Luminescent Organometallic Polymetallic Systems and Coordination Polymers Containing Bridging Isocyanides
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72
Luminescent Organometallic Coordination Polymers Built
M possessing an isonitrile bridge between the metal centers are more scarce in
comparison with the homodinuclear and heterodinuclear compounds bearing
bridging carbonyls.37,41
Bent and linear μ-CNR bonding modes are the two forms in homobimetallic M(μ-CN-R)M systems. The first complexes, 52À57, of the bent type
in the Ni triad [XPd(μ-dppm)2(μ-C5N-R)PdX] (R 5 C6H11, C6H5, CH3, X 5
Br, Cl) were prepared by Balch and collaborators in 197770 and were investigated for their properties to form inclusion compounds.71À74 The platinum (58,
59)75 and nickel (60À63) analogues76,77 were also investigated a few years later
(Fig. 29). These homobimetallics complexes were obtained by insertion of one
isocyanide into the M-M bond of [XM(μ-dppm)2MX] (M 5 Pd, Pt) or by
reaction of [NiI2(CNMe)8][PF6]2 with bis(diphenylphosphinomethane) (dppm).
The photophysical data for the binuclear system [Ni2(μ-L)(CNMe)2
(μ-dppm)2] (L5C5N-R, C5N-(Me)(R)1, NO1) complexes were examined by
Kubiak and collaborators.78 The nature of the lowest energy electronic
absorption band for these isocyanide complexes was assessed by varying the
nature of the bridging ligand and the solvent. The electronic absorption spectra
of these species exhibit an intense, broad band centered between 350 and 450
nm. Hypsochromic shifts of the emission band with the solvent polarity (a
difference as large as 85 nm was noticed) and a bathochromic shift and increase
of the maxima with the nature of the aryl group led the authors to assign the
lowest excited states to a metal to μ-ligand charge transfer (M2-μ2LCT).
Before our work on the assembly of metallopolymers through bridging
diisocyanides, there was just one precedent in the literature on metal-organic
frameworks with bridging CNR ligand where one C atom bridges two metals.79
Upon treatment of the molecular precursor [Pd(CNMe)4](BF4)2 with 4,4-diisocyanobiphenyl, IR characterization of the precipitated organometallic
polymer revealed strong IR absorptions in the 2180 cm21 region, typical for
terminal bound CNR groups, along with bands at 1700 and 1600 cm21. These
R
Ph2P
N
R
R
PPh2
Ph2P
N
Ph2P
PPh2
PPh2
C
C
Pd
X
Ph2P
N
Pd
PPh2
X
52 X = Br, R = Cy
53 X = Cl, R = Cy
54 X = Br, R = Ph
55 X = Cl, R = Ph
56 X = Br, R = Me
57 X = Cl, R = Me
Pt
X
Ph2P
Pt
X
PPh2
58 X = I, R = p-tolyl
59 X = Cl, R = p-tolyl
Me
N
Ni
C
Ph2P
Ni
C
PPh2
N
60 R = Ph
61 R = p-tolyl
62 R = p-Cl-Ph
63 R = Me
FIGURE 29. Some examples of homobimetallic complexes with μ-CNR ligands.
Me
Luminescent Organometallic Polymetallic Systems
73
latter are readily attributed to a μ2-bonding mode. Based on this spectroscopic
information, the authors proposed a polymeric network (Fig. 30).
In the case of the heterobimetallic systems, the few examples known so
far are [ClPt(μ-dppm)2(μ-C5N-Me)Ni(CNMe)]Cl (64), reported by Kubiak
and collaborators, which is obtained via a transmetallation of [(CNMe)]Ni
(μ-dppm)2(μ-C5N-Me)Ni(CNMe)] with [Pt(dppm)Cl2].80 The other examples
are the mixed M-Pt systems 65À74 (M 5 W, Fe, Mo, Cr) containing a disphosphine backbone (dppm or dppa), as described by Knorr and collaborators81À84 (Fig. 31). These complexes were obtained via substitution of
the bridging carbonyl by an isocyanide ligand.
Uson and collaborators described an A-frame PdPt complex [C6F5Pd
(μ-C5N-p-tolyl)(μ-dppa)2PtC6F5] (75),85 and more recently, [XPd(μ-dppm)2
(μ-C5N-R)Pt(CN-R)]1 (76À78) and [ClPd(μ-dppm)2(μ-C5N-R)PtCl] (79À81)
were reported by Knorr and collaborators86À88 (Fig. 32). In the bis(isonitrile)
heterobimetallics 76À78, a site selectivity in the second CNR ligand coordination (Pd vs. Pt) was noticed. The additional isocyanide is systematically
coordinated on the Pt site.86,88 This distinct selectivity between Pd and Pt was
explained by the greater lability of the Cl2 ion on the Pt center as well as a
better stabilization of the positive charge on the electron-rich PtI center.88 The
question arises as to why the addition of ligand isocyanide sometimes produces
the A-frame compound whereas under similar conditions, a d9-d9 isocyanide
C
N
C
C
N
N
N
N
N
C
C
Pd
C
C
N
Pd
C
C
N
N
N
N
C
C
N
C
n
FIGURE 30. Proposed structure of the organometallic polymer prepared from Pd(II)
and 4,4-diisocyanobiphenyl.
74
Luminescent Organometallic Coordination Polymers Built
Me
Ph2P
N
Ph2P
PPh2
PPh2
(OC)3Fe
Pt
C
Ni
C
PPh2
Cl
Ph2P
Ph2P
Pt PPh3
(OC)4M
C
Pt
PPh2
N
Cl
Me
N
N
R
R
65 M = W, R = CF3
66 M = W, R = CH2SO2tolyl
67 M = W, R = CH2PPh3[PF6]
68 M = Cr, R = CH2SO2tolyl
69 M = Mo, R = CH2SO2tolyl
64
PPh3
C
70 R = 2,6 xylyl
71 R = o - or p-anisyl
72 R = benzyl
73 R = tosylmethyl
74 R = p-C6H4NH2
FIGURE 31. Heterobimetallic complexes with μ-CNR ligands.
H
N R
N
Ph2P
PPh2
C
Pd
Pt
C6F5
C6F5
Ph2P
PPh2
N
H
75 R = p-tolyl
R
R
Ph2P
N
Ph2P
PPh2
PPh2
C
C
Pd
X
Ph2P
N
Pt
C
PPh2
X
N
R
76 X = Cl, R = [2.2]paracyclophane
77 X = Cl, R = o-anisyl
78 X = I ,R = o-anisyl
Pd
Cl
Ph2P
Pt
Cl
PPh2
79 R = [2.2]paracyclophane
80 R = o-anisyl
81 R = CH2PPh3
FIGURE 32. Heterobimetallic PdPt complexes with μ-CNR ligands.
terminal salt could be obtained, as described above. Knorr and collaborators
recently reported that the isocyanide bonding mode is related to various subtle
parameters: (1) the π-acceptor propensity of isocyanide ligand, (2) the nature of
the M-X bond, (3) the polarity of the solvent, and (4) the nature of the metal
center.89 Bimetallic complexes of the type [ClM(μ-dppm)2MCl] (M 5 Pd, Pt)
contain two electron-donating bidentate dppm ligands. This donation in these
low-valent, electron-rich systems could be, at least, partially compensated by
back bonding in the μ-CNR ligand π*-orbitals. Based on the theoretical work
of Howell and empirical observations on homodinuclear and heterodinuclear
systems,81,84,89,90 it seems that the π-acceptor propensity of the CNR ligand is
decisive for the bonding mode and determinates the C5N-R angle. Based on
calculations,89 a C5N-R angle ,140 indicates a strong back bonding and,
consequently, a strong π-acceptor ability for the isocyanide.
The photophysical properties of the heterobimetallics A-frame compounds 77, 78, and 80 were investigated. The absorption spectrum of these
compounds exhibit low energy bands in the 350À450 nm range, which are better
resolved at 77 K. These broad bands are characteristic of an A-frame
Luminescent Organometallic Polymetallic Systems
75
environment about the metal centers. The full width at half maximum (FWHM)
was found to be relatively constant whatever the nature of the halide. The
B350 nm band, which is encountered at the same position as that of d9-d9
homo- and hetero-M-M bonded complexes (M 5 Pd, Pt and M 5 Pd, Pt) may
stem from the weak M?M interactions.91 With reference to previous studies on
d8-d8 binuclear complexes78,92 and molecular orbital analysis, the low energy
band at B 450 nm may be assigned to a charge transfer (CT) process from the
metal to μ-ligand (M2-μÀLCT). The photophysical data for these compounds
are presented in Table 13.
The emission spectra in frozen butyronitrile (Fig. 33) exhibit broad and
unstructured bands at λmax 5 710 nm for 77 and at λmax 5 715 nm for 78. These
values compare favorably with other A-frame d8-d8 systems based on iridium
and platinum metal centers.91À93 The λmax and the non-radiative deactivation
values of dinuclear complexes 77 and 78 are in the order 77 , 78, indicating
that the nature of the excited states is influenced by the nature of the halide.94
TABLE 13. Emission Data for Polymers 77À78 and the Model Compound 80, in Solid
State at 298 K
Compound
λabs / nm
FWHM /cm21
λemi /nm
τe / μs
φe
77
78
80
384, 449
396, 456
390
1650, 2100
1600, 2150
3100
710
715
584, 677
3.06
12.70
0.52
0.0079
0.0043
0.0027
a
b
λexc 5 350 nm.
λexc 5 433 nm.
1.2
Correct Intensity (cps)
1.0
0.8
0.6
0.4
0.2
0
500
600
700
800
Wavelength (nm)
FIGURE 33. Emission spectrum of [XPd(μ-dppm)2(μ-C 5 N-C6H4-2-OCH3)Pt(CNC6H4-2-OCH3)]X (77. X 5 Cl, 78. X 5 I). in butyronitrile at 77 K. λexc 5 450 nm.
76
Luminescent Organometallic Coordination Polymers Built
Several examples of oligomers built on bridging diisocyanide have been
reported. A dimer of diiron diisocyanide (79) was described by Fehlhammer
and collaborators95 (Fig. 34). This compound was prepared by the reaction of
dinuclear [FeCp2(CO)3NCMe]2 prepared in situ with half an equivalent of 1,2diisocyanobenzene. The μ2-CNR bridge was evidenced by the presence of a
very strong band at 1675 cm21 in IR. The two bent CNC arrays were used for
further functionalization. The two basic sp2-N atoms were protonated with
HBF4, leading to the dicationic salt 80; due to the conformation of the Fedimer, pentametal complexes 81 were obtained by chelating a binary metal(II)
halide.
Other examples of oligomers are the dimers of a trinuclear cluster 82 and
83 (Fig. 35) supported by dppms and two triply-bridging I2 ligands.96 The two
O
O
C
Fe
Fe
Fe
Fe
OC
C
OC
CO
O
C
OC
N
N
Fe
Fe
CO
CO
N
H
MX2
H
N
N
OC
CO
C
Fe
OC
C
Fe
Fe
N
OC
CO
Fe
O
O
79
CO
C
Fe
Fe
O
81 M ϭ Zn, Fe, Pd, Cd and X ϭ Cl, I
80
FIGURE 34. Compounds 79 to 81.
2
P
P
I
Ni
Ni
P
P
P
C
Ni
N
P
R
P
N
P
Ni
C
P
Ni
Ni
I
P
P^P ϭ dppm
P
P
82 R ϭ Ϫ(CH2)6Ϫ
83 R ϭ
FIGURE 35. Structure of trinuclear Ni clusters linked by diNC ligands.
Luminescent Organometallic Polymetallic Systems
77
fragments are assembled together by flexible and rigide diisocyanides. The
oligomers were unambiguously identified by plasma desorption (PD) and fastatom bombardment mass spectrometry (FAB-MS). This material was obtained
by the reaction of the cluster [Ni3(μ3-I)2(dppm)3] with 1,6-diisocyanohexane in
benzene. The polymerization stopped at the dimer level because the iodide is a
poor leaving group on Ni. Its extraction is very difficult under mild conditions
and without a strong Lewis acid.97
Puddephatt and collaborators also reported a series of oligomers (84, 85)
and polymers of clusters 8698,99 (Fig. 36). The syntheses consist of reacting the
known precursors M3(μ-dppm)3(CO)21 (M 5 Pd, Pt) with the 1,4-diisocyanotetramethylbenzene in the appropriate ratios in CH2Cl2. Their reliable
identification was performed by using model clusters containing monoisocyanides and spectroscopy. These polymers exhibit extensive dissociation in
solution and fluxion motions of the isocyanide ligand. Unfortunately, no
photophysical data were described for these clusters of the Ni triad.
Taking advantage of the recently discovered site selectivity in ligand
binding on the Pt atom of the heterobimetallic ClPd(μ-dppm)2PtCl and the
better understanding of the isocyanide bonding mode described earlier,86À89
Knorr, Harvey, and others recently designed the first A-frame-containing
organometallic polymer using the bridging diisocyanide ligand 1,2-bis(2-isocyanophenoxyethane (diNC). This diisocyanide was first prepared by
Angelici and collaborators.100,101 Due to the ideal position of the isocyanide
groups, which can coordinate with donor groups at 90 angles to each other as
this occurs in square-planar or octahedral complexes, diNC and tBudiNC were
initially used as chelating linkers in Pt, Mo, Cr, or W complexes.100 Such
chelates are anticipated to exhibit a larger stability (i.e., binding constant) than
2ϩ
2ϩ
P
P
P
P
Pd
Pd
C
Pd
P
P
N
C
N
P
P
P
P
Pd P
P
O C
Pd
Pd P
P
Pd
Pd
P
P
C
Pd
P
N
N
P
P
P
P
Pt
C
P
Pt
Pt
C O
P
P
84 P^P ϭ dppm
85 P^P ϭ dppm
P
P
Pd
P
Pd
P
Pd
C
N
P
N
C
n
P
86 P^P ϭ dppm
FIGURE 36. Structure of the dimers of clusters M3(dppm)321, 84 and 85, as model
compounds and polymer 86 (Pd3(dppm)3(diiso)21)n (diiso 5 1,4-diisocyanotetramethylbenzene).
78
Luminescent Organometallic Coordination Polymers Built
FIGURE 37. Molecular structure of the model compound[IPd(μ-dppm)2(μ-C5N-C6H42-OCH3)Pt(CN-C6H4-2-OCH3)]1.
the corresponding monodentate phenyl isocyanide and benzonitrile complexes.
The electronic spectra for these chelates (i.e., free ligands) exhibit the two
lowest energy bands, which are assigned as MLCT transitions of the type
dπ-π*CN. The position of the dπ-π*CN bands for the d6 compounds varies
in energy in the following order: Cr(0) , Mn(I) , Fe(II) BCo(III).102
Based on the results obtained for the model ligand o-anisylisocyanide and
on X-ray diffraction study (Fig. 37) where the first o-anisylisocyanide spanned the
two metal centers and the second is coordinated at the Pt site, the A-frame heterobimetallic polymer 87 (Fig. 38) based on Pd(μ-dppm)2Pt was prepared from
the direct reaction between the heterobimetallic ClPd(μ-dppm)2PtX and diNC.87
This novel class of materials containing a bridging and terminal isocyanide
was investigated in more detail (influence of the counterion, and comparison with
the homonuclear complexes ClM(μ-dppm)2MCl (M 5 Pd, Pt) as starting material) to address the nature of the excited states.103 These new orange species are
weakly soluble and precipitated readily, thus hampering characterization in
solution. The solid-state IR spectra confirm the A-frame geometry in which the
two distinct absorptions for the bridging and terminal coordinated isocyanides
are observed at the expected positions (B1620 and 2160 cm21, respectively).
Estimation of the average molecular weight in number (Mn) values by the spin
lattice relaxation time (T1) and NOE 31P NMR measurements indicates that
these species can be only small oligomers (i.e., dimer), not anything larger in
Luminescent Organometallic Polymetallic Systems
O
Ph2P
X
M
ϩ X
N
79
Ϫ
PPh2
O
M'
C
N
Ph2P
PPh2
n
87 M ϭ M' ϭ Pd, X ϭ Cl
88 M ϭ M' ϭ Pd, X ϭ I
89 M ϭ Pd, M' ϭ Pt, X ϭ Cl
90 M ϭ Pd, M' ϭ Pt, X ϭ I
91 M ϭ M' ϭ Pt, X ϭ Cl
92 M ϭ M' ϭ Pt, X ϭ I
FIGURE 38. Structure of A-Frame bimetallic containing polymers 87À92.
solution, which is completely consistent with the observed solubility. The TGA
traces indicate a good thermal stability for these new materials where decomposition occurs at temperatures exceeding 210 C.
The absorption spectra of these polymers exhibit two low energy bands in
the 350À450 nm range (Table 14). The spectral similarity with the model
compounds 77 and 78 is striking and unambiguously address the A-frame
environment about the metal atoms as being the same. Based on the comparison with model compounds 77 and 78, the nature of the electronic transition and the excited states are certainly of the same as those described for the
formers. The fact that these bands were not red shifted with respect to
the corresponding model compound unsurprisingly indicates that the electronic
coupling between the A-frame unit along the polymer is weak due to absence of
conjugation.
These A-frame-containing organometallic polymers, 87À92, are moderately luminescent at 77 K in butyronitrile but are not luminescent at room
temperature, both in the solid state and in solution. This feature may be
associated with an energy-wasting photo-induced M2-(μ-isocyanide) bond
scission or a ligand dissociation in the excited states. Moreover, this finding is
not surprising because recent studies on the electronic communication of the
isocyanide bridge through the A-frame structure indicates that the conjugated
C5N linker exhibits moderate electronic communication properties.104 For
example, the [2.2]paracyclophane (PCP)-containing isocyanide 93, which is
itself luminescent, exhibits electronic communication via the bridging isocyanide group in the A-frame complex 94 (Fig. 39).
80
Luminescent Organometallic Coordination Polymers Built
TABLE 14. Photophysical Parameters of 87À92 in Butyronitrile and in the Solid State
at 77 Ka
Polymer
λabs / nm
87
88
89
90
91
92
386,
351,
374,
374,
360,
364,
a
450
428
440
430
444
468
λemi / nm
τe / μs
φe
λemi (Solid) / nm
680
705
660
730
615
635
0.20
0.19
0.18
0.78
1.78
2.12
0.0048
0.0028
0.0056
0.0048
0.003
0.002
725
740
705
735
635
715
From Ref. 89.
R
C
N
Ph2P
Pd
Cl
Ph2P
93 PCP-NC
N
PPh2
Pd
Cl
PPh2
94 R ϭ PCP
FIGURE 39. Structure of the isocyanide functionalized [2.2]paracyclophane 93 and the
A-Frame complex 94.
At 77 K in butyronitrile, the PCP-NC ligand exhibits two broad
(FWHM 5 12000 and 45000 cm21, respectively) unstructured emission bands
at λmax around 370 and 480 nm (Fig. 40a). The emission bands do not shift
with different solvents. The first emission maxima is assigned to fluorescence
(fluorescence lifetime, τFB1.85 ns in PrCN). The second band is due to
phosphorescence, as deduced from the long lifetime (τPB3.37 s in PrCN) and
the large Stokes shift. After coordination of the isocyanide on the homobimetallic ClPd(μ-dppm)2PdCl complex, the resulting product 94 exhibits a
strong emission band (in 2-MeTHF) centered at 480 nm and a weak luminescence at B360 nm (Fig. 40b). These two features are assigned to the
phosphorescence and fluorescence of the PCP-NC unit, respectively. No
emission is detected in the 550À850 nm region. According to Kasha’s rule, the
upper excited states should deactivate to the lowest excited state before one sees
emission. However, the spectra exhibit clear evidence of luminescence arising
from upper excited states (i.e., from the PCP fragment). These upper energy
emissions are observable due to lack of efficient nonradiative relaxation (i.e.,
electronic communication) between the upper states localized in the PCP ligand
(IL ππ*) and the lowest energy excited states located in the A-frame ClPd
(μ-dppm)2(μ-C5N-PCP)PdCl complex. This result is consistent with the fact
that no coordination polymer containing an isocyanide linker has so far been
reported as conducting (where conductivity proceeds across the CN group).
Luminescent Organometallic Polymetallic Systems
Corrected Intensity (cps)
(a)
81
0.4
0.3
0.2
0.1
0
300
400
500
600
Wavelength (nm)
Corrected Intensity (cps)
(b)
0.8
0.6
0.4
0.2
0
300
400
500
Wavelength (nm)
FIGURE 40. (a) Emission spectrum of PCP-NC 93 in butyronitrile at 77 K.
(b) Emission spectrum of ClPd(μ-dppm)2(μ-C5N-PCP)PdCl 94. λexc 5 310 nm. (Modified from Ref. 104.)
Unstructured emission bands are observed with maxima (λemi) in the
550À750 nm range for the A-frame organometallic polymers 87À92. These
later values are in good agreement with those obtained for the heterobimetallic
complexes 77À78 containing a bridging isocyanide acting as model compounds. The band maxima for these dinuclear materials in solution or in the
solid state follow the order Cl , I, indicating that the nature of the excited
states is influenced by the nature of the halide.
The emission spectra of homobimetallic and heterobimetallic polymers 89
and 91 in PrCN at 77 K are shown in Figure 41 as examples. Details of the
photophysical parameters are presented in Table 14.
Based on the model compounds 77 and 78 described earlier, the emission
bands in these homobimetallics and heterobimetallic systems can be assigned to a
charge transfer going from the fragment M(CN-C6H4-OCH2) to M(μ-C5NC6H4-OCH2). These materials are also luminescent in the solid state at 77 K,
providing an emission band in the 650À750 nm range. The emission spectrum for
82
Luminescent Organometallic Coordination Polymers Built
0.8
Corrected Intensity (cps)
(a)
0.6
0.4
0.2
0
450
550
650
Wavelength (nm)
750
Corrected Intensity (cps)
(b)
0.3
0.2
0.1
0
450
550
650
750
Wavelength (nm)
FIGURE 41. Emission spectra in butyronitrile of 89 (a) and 91(b) at 77 K. λexc 5 450
nm. (Modified from Ref. 104.)
([ClPt(μ-dppm)2(μ-CN-C6H4-2-OCH2CH2O-2-C6H4-NC)Pt]Cl)n (91) in the solid
state at 77 K, is shown in Figure 42 as an example. Red shifts of the emission band
by about 20À50 nm were observed in the solid state compared to the solution one.
To explain this observation, a polymer (solid)-oligomer (solution) equilibrium is
proposed. Evidence for the oligomer is provided by T1 and NOE constant measurements. This behavior was previously described by Harvey and collaborators
for the Pd2(dmb)2Cl2 binuclear complexes40 and other d8 Pd(diphos)(isocyanide)containing polymers (diphos 5 Ph2P-(CH2)m-PPh2; m 5 2-6).37
The emission lifetimes for these materials are found in the μs time scale,
consistent with a phosphorescence process. The lifetime data for Pdcontaining materials are significantly shorter than those of the Pt analogues (by
about two orders of magnitude). Compared to the heterobimetallic model
compounds [ClPd(μ-dppm)2(μ-C5N-C6H4-2-OCH3)Pt(CN-C6H4-2-OCH3)]Cl
(77) (τe 5 3.06 μs, Φe 5 0.0079) and [ClPd(μ-dppm)2(μ-C5N-C6H4-2-OCH3)Pt
(CN-C6H4-2-OCH3)]I (78) (τe 5 12.7 μs, Φe 5 0.0043), shorter values were
obtained for the heterobimetallic-containing polymers 89 and 90. The other