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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