Preparation of a Copper Ion Complex of Sterically Congested

Oct 18, 2005 - Chemistry, Aichi Institute of Technology, Toyota, Aichi 470-0392, Japan. ReceiVed: July 1, 2005; In Final Form: September 19, 2005. To ...
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J. Phys. Chem. B 2005, 109, 20763-20772

20763

Preparation of a Copper Ion Complex of Sterically Congested Diphenyldiazomethanes Having a Pyridine Ligand and Characterization of Their Photoproducts Tetsuji Itoh,*,§ Masayoshi Matsuno,§ Shuhei Ozaki,§ Katsuyuki Hirai,‡ and Hideo Tomioka*,§,† Chemistry Department for Materials, Faculty of Engineering, Instrumental Analysis Facilities, Life Science Research Center, Mie UniVersity, Tsu, Mie 514-8507, Japan, and Department of Applied Chemistry, Aichi Institute of Technology, Toyota, Aichi 470-0392, Japan ReceiVed: July 1, 2005; In Final Form: September 19, 2005

To show that persistent high-spin polycarbenes can be realized by utilizing hetero spin systems, two diphenyldiazomethanes having pyridyl groups, i.e., bis{4-(4-pyridyl)-2,6-dimethylphenyl}diazomethane (4,4′DPy-1-N2) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2,4-DPy1-N2), were prepared. Triplet carbenes, 4,4′-DPy-1 and 2,4-DPy-1, generated by photolysis of the corresponding diazomethanes were characterized by spectroscopic means (ESR and UV/vis in matrix at low temperatures and laser flash photolysis in solution at room temperature). The results showed that they were fairly persistent. Magnetic properties of the photoproducts from a 1:1 complex between DPy-1-N2 and Cu(hfac)2 (hfac ) hexafluoroacetylacetonate) were characterized by ESR and a superconducting quantum interference device (SQUID) magneto/susceptometer. The field dependences of magnetization for the complexes, expressed by using M versus H/T plots, were analyzed in terms of the Brillouin function to be S ) 6.80 (F ) 0.60) for the 1:1 complex of 4,4′-DPy-1 and Cu(hfac)2 and S ) 3.71 (F ) 0.73) for the 1:1 complex of 2,4-DPy-1 and Cu(hfac)2 at 2.0 K. Thus, it has been demonstrated that a high-spin species is actually generated in the photoproducts and that the complexed carbenes showed significant stability.

1. Introduction In typical magnetic materials, the sources of electron spins are the d and f electrons on metals or metal ions. On the other hands, there has been an ever-increasing interest in molecular magnetism, in which the 2p electron spins in π-orbitals of light atoms, such as carbon, nitrogen, and oxygen, are mainly responsible.1,2 In the past decade, the first organic ferro- and ferrimagnets have been prepared.3 They are mainly based on crystals of small radicals or charge-transfer salts. An alternative approach to organic magnets is to prepare conjugated polymers containing electron spins. In this approach exchange couplings between unpaired electron spin are mediated through a π-conjugated system.4,5 Such approaches have several problems, which prevent the formation of useful systems. For instance, a “bottom-up” synthetic approach to obtaining molecular magnets is faced with synthetic limitations, especially when the orders of the alignment of spins become higher.6 On the other hand, efforts to increase the number of aligned spins are hampered by the development of antiferromagnetic intrachain and/or interchain interactions between the radical centers.7 The third approach utilizing heterospin systems comprised of 2p spins of organic radicals and 3d spins of magnetic metal ions has been proposed.8,9 The strategy is based on the supramolecular chemistry exhibited by pyridine- and polypyridine-metal ions.10 For instance, magnetic interaction between radical centers and metal ions can be realized through a pyridyl * Address correspondence to this author. Phone: +81-59-231-9418. Fax: +81-59-231-9418. E-mail: [email protected]. § Chemistry Department for Materials, Faculty of Engineering. ‡ Life Science Research Center, Mie University. † Aichi Institute of Technology.

ligand to generate a high spin unit.8,9 The strategy has several advantages. One of them, and probably the most important one from synthetic viewpoints, is that precursors, e.g., pyridyldiazo compounds, produce polymer chains by ligation with coordinatively unsaturated metal ions by themselves. This allows us to extend the dimension of the spin network from one (1D) to two (2D) and three (3D) by simple self-assembly between pyridyl groups and metal ions.8,9 This suggests that if a pyridyl group is introduced on a phenyl ring of a sterically congested diaryldiazomethane, a precursor for a persistent triplet diarylcarbene, a persistent high-spin species should be attainable by irradiation of the polymeric diazo compound chain formed as a result of coordination with metal ions. We have confirmed that a sterically congested diphenyldiazomethane having a 4-pyridyl group as a ring substituent is able to form a complex copper ion and also that 2p spins of triplet carbene generated by photolysis of the complex containing diazo units can actually interact with the 3d spin of metal ions through the pyridyl group located remote from the carbene center to generate high spin species.11 So the next step is to apply this method to prepare a longer chain by introducing two pyridyl units on diaryldiazomethanes and to confirm whether actually high-spin and persistent polycarbene can be generated by photolysis of the complex. To test this idea, we prepared dianthryldiazomethanes having two pyridyl groups and characterized magnetic properties of the photoproducts from their complex with metal ions. Although it has been demonstrated that high spin species are generated in the photoproducts, the spin quantum number is not very high (S ∼ 3).12 Among several factors that affect the overall magnetic properties, delocalization of spin is part of the reason for the smaller S value in this case since significantly small D values are observed for dianthrylcarbenes and spin densities are low.13

10.1021/jp053622l CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005

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To avoid this drawback, we decided to use diphenylcarbene units as a spin unit to construct hetero-spin high-spin species. Thus we prepared diphenyldiazomethanes (DPy-1-N2) having two pyridyl groups, i.e., bis{4-(4-pyridyl)-2,6-dimethylphenyl}diazomethane (4,4′-DPy-1-N2) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2,4DPy-1-N2), and characterized magnetic properties of photoproducts from a complex between DPy-1-N2 and Cu(hfac)2 (hfac ) hexafluoroacetylacetonate). Both diazo compounds are expected to form a chain as a result of complexation with metal ions (to form -DPy-1-N2M-), and the chain is expected, upon irradiation, to generate a polycarbene chain (-DPy-1-M-) in which the spins on the chain start to interact ferromagnetically. However, there is a potential difference between the two in the connectivity of the ferromagnetic coupling unit with respect to carbene centers (Scheme 1). In the -4,4′-DPy-1-M- chain, for instance, carbene centers are involved in coupling units in the chain (Class 1 in Scheme 2), while in the -2,4-DPy-1-M- chain, carbenes are attached as pendants to a coupling unit of the chain (Class 2 in Scheme 2). It has been pointed out that, in the linear connectivity (Class 1), there is a fundamental problem associated with the presence of only one exchange pathway (through the π-system) between any two remote radical or carbene sites.14 Therefore, a failure to generate a carbene in the interior of the chain may interrupt the exchange pathway. One way to avoid this problem is to introduce a carbene unit as a pendant to a coupling unit of polymer backbone (Class 2). In other words, failure to generate a carbene (a chemical defect) in the interior of a polycarbene chain may interrupt the exchange pathway in the former, while this chemical defect may be circumvented in the latter.4,5 2. Experimental Section Materials. The preparations15,16 of the desired diazo precursors, bis{2,6-dimethyl-4-(4-pyridyl)phenyl}diazomethane (4,4′DPy-1-N2) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl4-tert-butylphenyl)diazomethane (2,4-DPy-1-N2), are described in the Supporting Information. EPR Measurements. The diazo compound was dissolved in 2-methyltetrahydrofuran (10-3 M) and the solution was SCHEME 2

Itoh et al. degassed in a quartz cell by three freeze-degas-thaw cycles. The sample was cooled in an optical transmission EPR cavity at 77 K and irradiated with a Wacom 500 W Xe lamp, using a Pyrex filter. EPR spectra were measured on a JEOL JES TE 200 spectrometer (X-band microwave unit, 100 kHz field modulation). The signal positions were read with a gaussmeter. The temperature was controlled by a 9650 Microprocessor-based Digital Temperature Indicator/Controller, providing the measurements accuracy within (0.1 K and the control ability within (0.2 K. Errors in the measurements of component amplitudes did not exceed 5%, and the accuracy of the resonance field determination was within (0.5 mT. Low-Temperature UV/Vis Spectra. Low-temperature spectra at 77 K were obtained by using an Oxford variabletemperature liquid-nitrogen cryostat (DN 1704) equipped with a quartz outer window and a sapphire inner window. The sample was dissolved in dry 2-MTHF, placed in a long-necked quartz cuvette of 1-mm path length, and degassed thoroughly by repeated freeze-degas-thaw cycles at a pressure near 10-5 Torr. The cuvette was flame-sealed, under reduced pressure, placed in the cryostat, and cooled to 77 K. The sample was irradiated for several minutes in the spectrometer with a Halos 300-W high-pressure mercury lamp with a Pyrex filter, and the spectral changes were recorded at appropriate time intervals. The spectral changes upon thawing were also monitored by carefully controlling the matrix temperature with an Oxford Instrument Intelligent Temperature Controller (ITC 4). Flash Photolysis. All flash measurements were made on a Unisoku TSP-601 flash spectrometer. Three excitation light sources were used depending on the precursor absorption bands and lifetime of the transient species. They were (i) a cylindrical 150-W Xe flash lamp (100 J/flash with 10-µs pulse duration), (ii) a Quanta-Ray GCR-11 Nd:YAG laser (355 nm pulses of up to 40 mJ/pulse and 5-6 ns duration; 266 nm pulses of up to 30 mJ/pulse and 4-5 ns duration), and (iii) a Lamda Physik LEXTRA XeCl excimer laser (308 nm pulses of up to 200 mJ/ pulse and 17 ns duration). The beam shape and size were controlled by a focal length cylindrical lens. A Hamamatsu 150 W xenon short arc lamp (L 2195) was used as the probe source, and the monitoring beam, guided by an optical fiber scope, was arranged in an orientation perpendicular to the excitation source. The probe beam was monitored with a Hamamatsu R2949 photomultiplier tube through a Hamamatsu S3701-512Q MOS linear image sensor (512 photodiodes used). Timing of the excitation pulse, the probe beam, and the detection system was achieved through an Iwatsu Model DS-8631 digital synchro scope, which was interfaced to a NEC 9801 RX2 computer. This allowed for rapid processing and storage of the data and provided printed graphic capabilities. Each trace was also displayed on a NEC CRT N5913U monitor. A sample was placed in a long-necked Pyrex tube that had a

Preparation of Diphenyldiazomethanes

Figure 1. ESR spectra obtained by irradiation of diazo compound 4,4′DPy-1-N2 (a) and 2,4-DPy-1-N2 (b) in 2-methyltetrahydrofuran at 77 K.

sidearm connected to a quartz fluorescence cuvette and degassed with a minimum of 4 freeze-degas-thaw cycles at a pressure near 10-5 Torr immediately prior to being flashed. The sample system was flame-sealed under reduced pressure, and the solution was transferred to the quartz cuvette, which was placed in the sample chamber of the flash spectrometer. A cell holder block of the sample chamber was equipped with a thermostat and allowed to come to thermal equilibrium. The concentration of the sample was adjusted so that it absorbed a significant portion of the excitation light. SQUID Measurements. Magnetic susceptibility data were obtained on a Quantum Design MPMS-2A Superconducting Quantum Interference Device (SQUID) magnetometer/susceptometer. Irradiation with light from an argon ion laser (488 nm, Omnichrome 543-150BS) through a flexible optical fiber that passes through the inside of the SQUID sample holder was performed inside the sample room of the SQUID apparatus at 5-11 K. One end of the optical fiber was located 40 mm above the sample cell (capsule) and the other was attached to a coupler for the laser. The bottom part of the capsule (6 mm × 10 mm) without a cap was used as a sample cell. A 80 µL sample of the solution (1.0 mM) in 2-MTHF was placed in the cell, which was held by a straw. The irradiation was carried out until there was no further change of magnetization monitored at 5 K in a constant field of 5 kOe. The magnetization, Mb and Ma, before and after irradiation was measured at 2, 3, and 5 K in a field range of 0-50 kOe. The plots of the magnetization [M ) (Ma - Mb)] versus the magnetic field were analyzed in terms of Brillouin function. 3. Results and Discussion Spectroscopic Studies of Metal-Free Diazomethane. Before examining the magnetic properties of the species generated by photolysis of a complex between the pyridylated diazo compound DPy-1-N2 and copper ion, we first characterized metalfree carbenes from DPy-1-N2. (a) Electron Spin Resonance (ESR). Irradiation of 4,4′-DPy1-N2 (5.0 × 10-3 M) in 2-methyltetrahydrofuran (2-MTHF) at 77 K gave ESR signals of randomly oriented triplet molecules (Figure 1a). The signals were analyzed in terms of zero-field splitting (ZFS) parameter with |D| ) 0.259 cm-1 and |E| ) 0.00139 cm-1 (E/D ) 0.0054). The values showed unequivocally that the triplet signals are due to triplet bis{4-(4-pyridyl)-

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Figure 2. UV/vis spectra obtained by irradiation of diazo compound 4,4′-DPy-1-N2. (a) Spectra of 4,4′-DPy-1-N2 in 2-methyltetrahydrofuran at 77 K. (b) The same sample after irradiation (λ > 350 nm). (c-e) The same sample after thawing to 90 (c), 120 (d), and 130 K (e).

2,6-dimethylphenyl}carbene (4,4′-DPy-1) generated by photodissociation of 4,4′-DPy-1-N2. The structure of triplet carbenes is characterized by ESR ZFS parameters D and E.17 The D value is related to the separation between the unpaired electrons. The E value, on the other hand, when weighed by D, is a measure of the deviation from axial symmetry. For diarylcarbenes, this value will thus depend on the magnitude of the central C-C-C angle. Since the E value depends on the magnitude of the central angle, the reduction in E indicates that the carbene adopts a structure with an expanded C-C-C angle upon annealing. This interpretation is supported by the observation that the substantial reduction of E is usually accompanied by a significant reduction in D, indicating that the electrons are becoming more delocalized. The ZFS parameters of 4,4′-DPy-1, especially D values, are apparently smaller than those of bis(2,4,6-trimethylphenyl)carbene (|D| ) 0.381 cm-1 and |E| ) 0.0121 cm-1, E/D ) 0.0318).18 This suggests that the delocalization of unpaired electrons into two pyridyl groups at para positions is significant. The ESR signals were stable at this temperature but disappeared irreversibly when the matrix was thawed to room temperature. The thermal stability of the triplet carbenes could be estimated by thawing the matrix containing triplet carbenes gradually and recooling again to 77 K to measure the signal. This procedure can compensate for the weakening of signals due to Currie law.19 The characteristic bands due to 4,4′-DPy31 started to decay slowly at 100 K and decayed rather sharply at 120 K. Similar irradiation of 2,4-DPy-1-N2 (2.0 × 10-3 M) in 2-MTHF at 77 K gave ESR signals due to 2,4-DPy-31 with ZFS parameters of |D| ) 0.346 cm-1 and |E| ) 0.0067 cm-1, E/D ) 0.01194 (Figure 1b). The values are again smaller compared with those for triplet carbene before pyridination, i.e., (2,4,6tribromophenyl)(2,6-dimethyl-4-tert-butylphenyl)carbene (|D| ) 0.423 cm-1 and |E| ) 0.0326 cm-1, E/D ) 0.0771),16 thereby suggesting delocalization of unpaired electrons to pyridyl groups. The signals were observable up to 120 K. (b) UV/Visible Spectroscopy (UV/Vis). Optical spectroscopical monitoring in the frozen medium gave analogous results. Irradiation of 4,4′-DPy-1-N2 (2.5 × 10-4 M) in 2-MTHF at 77 K resulted in the appearance of new absorption bands at the expense of the original absorption due to 4,4′-DPy-1-N2 (Figure 2b). The new spectrum consists of two identifiable features, an

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Figure 3. UV/vis spectra obtained by irradiation of diazo compound 2,4-DPy-1-N2. (a) Spectra of 2,4-DPy-1-N2 in 2-methyltetrahydrofuran at 77 K. (b) The same sample after irradiation (λ > 350 nm). (c and d) the same sample after thawing to 90 (c) and 100 K (d).

intense UV band with an apparent maximum at 360 nm and weak, broad bands with apparent maxima around 496 and 530 nm. These features are usually present in the spectrum of triplet diarylcarbenes.17 Since ESR signals due to triplet carbene were observed under identical conditions, we can safely assign the absorption spectrum to triplet carbene 4,4′-DPy-31. Similar irradiation of 2,4-DPy-1-N2 (5.0 × 10-4 M) under the identical conditions also gave rather strong absorption bands at 342 nm and a shoulder band extending from 370 to 550 nm with apparent maxima at 380 nm which are ascribable to 2,4DPy-31 (Figure 3b). These bands are to be compared with those observed for carbene before pyridination, i.e., bis(4-bromo-2,6-dimethylphenyl)carbene (341 and 432-466 nm)19 and (4-tert-butyl-2,6dimethylphenyl)(2,4,6-tribromophenyl)carbene (337 and 349 nm)16 and also with the mono-pyridinated one, i.e., (4-tert-butyl-2,6-dimethylphenyl){2,6-dibromo-4-(4-pyridyl)phenyl}carbene (337 nm and broad bands extending from 410 to 550 nm).11 Rather strong bands at the longer wavelength region exhibited by 4,4′-DPy-31 and 2,4-DPy-31 again showed the role of two pyridyl groups at para and ortho positions. The absorption bands were stable for hours when kept at this low temperature, but upon thawing the matrix, the bands due to 4,4′-DPy-31 and 2,4-DPy-31 started to disappear rather sharply at around 120 and 90 K and disappeared completely at around 130 (Figure 2c-e) and 100 K (Figure 3c,d), respectively. The absorption bands due to bis(2,4,6-trimethylphenyl)carbene18 and (4-tert-butyl-2,6-dimethylphenyl){2,6-dibromo-4-(4-pyridyl)phenyl}carbene16 disappeared at 105 and 110 K, respectively. Thus thermal stability of triplet DPC is slightly increased by pyridyl substituents when introduced at para positions but decreased when introduced at the ortho position (vide infra). (c) Laser Flash Photolysis (LFP). Laser flash photolysis of 4,4′-DPy-1-N2 (2.0 × 10-4 M) in a degassed benzene at room temperature produced a transient species showing a strong absorption band at 385 nm (Figure 4). Due to overlap of the absorption maxima of the diazo precursor 4,4′-DPy-1-N2, the sample was not sufficiently transparent for adequate monitoring in the 300-370 nm region. Therefore the direct comparison of the transient bands with that observed in the matrix at low temperature was not possible. It is likely that we are observing the tailing part of the absorption band due to 4,4′-DPy-31. This assignment was supported by trapping experiments (vide infra).

Itoh et al.

Figure 4. Absorption spectrum of transient products formed during irradiation of diazo compound 4,4′-DPy-1-N2 in degassed benzene at room temperature recorded 10 µs after excitation. The inset shows the time course of the absorption at 385 nm (oscillogram trace).

SCHEME 3

All absorption bands decayed in a similar manner. The inset in Figure 4 shows the decay of 4,4′-DPy-31 in the absence of trapping reagents, which is found to be of second order (2k/l ) 3.6 s-1). The spent solution showed the presence of carbene dimer (2) as the main product, indicating that triplet carbene 4,4′-DPy-31 decayed by undergoing dimerization. The rough lifetime of 4,4′-DPy-31 is estimated in the form of half-life, t1/2, to be 0.27 s, which is slightly larger than that of triplet bis(2,4,6-trimethylphenyl)carbene (2k/l ) 3.6 s-1, t1/2 ) 0.16 s).18 When LFP was carried out on a nondegassed benzene solution of 4,4′-DPy-1-N2, the half-life of 4,4′-DPy-31 decreased dramatically, and a broad absorption band with a maximum at 410 nm appeared at the expense of the absorption due to 4,4′-DPy31 (Figure S1, Supporting Information). The spent solution was found to contain bis{4-(4-pyridyl)-2,6-dimethylphenyl}ketone (4,4′-DPy-1-O) as the main product. It is well-documented20,21 that the diarylcarbenes with triplet ground states are readily trapped by oxygen to generate the corresponding diaryl ketone oxides, which show a broad absorption band centered at 396450 nm. The oxides eventually formed the corresponding ketones (Scheme 3). Thus, the observations can be interpreted as indicating that 4,4′-DPy-31 is trapped by oxygen to form the carbonyl oxide (4,4′-DPy-1-O2), which confirms that the transient absorption quenched by oxygen is due to 4,4′-DPy31.

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Figure 5. Absorption spectrum of transient products formed during irradiation of diazo compound 2,4-DPy-1-N2 in degassed benzene at room temperature recorded 10 µs after excitation. The inset shows the time course of the absorption at 385 nm (oscillogram trace).

The apparent built-up rate constant, kobs, of the carbonyl oxide (4,4′-DPy-1-O2) is expressed as given in eq 1

kobs ) k0 + kO2[O2]

(1)

where k0 represents the rate of decay of 4,4′-DPy-31 in the absence of oxygen and kO2 is the quenching rate constant of 4,4′-DPy-31 by oxygen. A plot of the observed pseudo-firstorder rate constant of the formation of the oxide against [O2] is linear (Figure S2, Supporting Information). From the slope of this plot, kO2 was determined to be 5.02 × 107 M-1 s-1, which is approximately 2 orders of magnitude smaller than that observed with the “parent” 3DPC (kO2 ) 5.0 × 109 M-1s-1)21 and even smaller than that observed for bis(2,4,6-trimethylphenyl)carbene (kO2 ) 2.8 × 108 M-1s-1).18 LFP of 2,4-DPy-1-N2 gave somewhat different results. Thus LFP of 2,4-DPy-1-N2 (1.5 × 10-4 M) in benzene produced a transient species showing a rather broad band with apparent maximum at 430 nm extending up to 550 nm, which appeared coincident with the pulse (Figure 5). Again due to the absorption maxima of the diazo precursor, the sample is not transparent for adequate monitoring in the 300-370 nm region. The inset in Figure 5 shows the decay of the transient bands in the absence of trapping reagents, which is found to be first order with the rate constant of k ) 1.7 × 105 s-1, the lifetime being 5.9 µs. The transient species are significantly different from that observed for 2,4-DPy-31 in matrix at low temperature (Figure 3b) and hence are not assignable to 2,4-DPy-31. Analysis of the spent solution showed the presence of 2-azafluorene compound (3) as a main product obviously formed as a result of formal insertion of diphenylcarbene into the C-H bond of the o-pyridyl group. The observation is very similar with the photochemistry of o-arylated diphenyldiazomethanes leading to fluorenes as a result of formal insertion of diphenylcarbene into the C-H bond of the o-aryl group.22,23 The transient bands detected in LFP of o-arylated diphenyldiazomethanes are shown to be triplet fluorenes. Spectroscopic features (broad bands centered around 400-420 nm) and decay kinetics (the first order with 7-9 × 105 s-1) observed for triplet fluorenes are in good agreement with those observed in the present system. Actually LFP of 2-azafluorene (3) gave a transient absorption very similar to that observed in LFP of 2,4DPy-1-N2 (Figure S3). These observations suggest that 2,4-DPy-11 (or its precursor excited state of 2,4-DPy-1-N2) decayed very quickly by interact-

ing with the pyridyl group at the ortho position at least in solution at room temperature. However, in rigid matrix at low temperature, 2,4-DPy-31 was observable up to 120 K while most unprotected triplet diphenylcarbenes decomposed at around 90 K in MTHF matrix. This is interpreted by assuming the effect of the matrix on the reaction pathway leading to 2-azafluorene 3. If o-pyridyl groups are not coplanar with the carbenic phenyl group, cyclization of the carbenes will require molecular motions. Such motion can be blocked in rigid matrix to some extent and hence the decay reaction in the singlet state leading to azafluorene is retarded.22 Thus the triplet state becomes observable as the singlet state has enough lifetime to intersystemcross to the triplet under these conditions. Spectroscopic Studies of Species Obtained by Photolysis of a 1:1 Complex between DPy-1-N2 and Cu(hfac)2. All of above observations suggest that we have now a fairly persistent triplet carbene unit that has a pyridine ligand bridgeable with metal ions. So the next step is to characterize the magnetic properties of the photoproducts from a complex between DPy1-N2 and metal ions. It has been demonstrated that the sign and magnitude of the exchange coupling between metal ions and an organic radical depend not only on the periodicity of the ligand π-orbitals, but also on the orbitals occupied by the unpaired d electrons of the metal ions.9a For instance, ferromagnetic interaction is observed between the carbenes and the copper ions in the [Cu(hfac)2{4DPy(C:)}] (hfac ) hexafluoroacetylacetonate) system, while antiferromagnetic interaction is observed between the carbenes and the manganese ions in the [Mn(hfac)2{4-DPy(C:)}] system. This is explained in terms of the difference in the overlapping mode of the magnetic orbitals in the metal ions and the nitrogen atom on the pyridine ligands. In the Cu(II) complexes, the magnetic orbital is dx2-y2 and orthogonal to the pπ orbital at the pyridyl nitrogen, in which the spin is polarized due to the presence of the triplet carbene. Hund’s rule would predict the coupling to be ferromagnetic. On the other hand, in the Mn(II) complexes, the magnetic orbital that interacts with the carbene is dxz and hence the interaction with the 2pπ orbital at the pyridine nitrogen is a π-type magnetic one. In this case, the unpaired electrons are expected to couple antiferromagnetically. Thus, if Cu(II) is used as a metal ion, a ferromagnetic interaction would be expected in the complex with 4,4′- and 2,4DPy-31. We thus chose Cu(hfac)2 as a metal ion for this work. (a) ESR. A solution of 4,4′-DPy-1-N2 and Cu(hfac)2 (hfac ) hexafluoroacetylacetonate) mixed in a 1:1 molar ratio (3.3 × 10-3 M) in 2-MTHF was allowed to stand overnight and was used as the sample. The solution showed ESR signals at 269, 285, 301, and 322 mT due to Cu(II) ion in Cu(hfac)224 at 77 K before irradiation (Figure 6Aa). When the solution was irradiated at 77 K, rather strong and broad signals gradually appeared at 312 and 324 mT at the expense of the signals due to isolated Cu(II) ion in the Cu(hfac)2 unit and replaced the signal due to Cu(II) ion after 80 min of irradiation (Figure 6Ab,c). The small signal spacing observed here is consistent with the tendency that as the spin multiplicity became higher, the D values became smaller. The observation that no significant signals due to isolated triplet carbene 4,4′-DPy-31 were observed suggested that the pyridine moiety binds with Cu(hfac)2 essentially in a quantitative manner under these cryogenic conditions.9a,25 To estimate the thermal stability of the signals, the sample was warmed to a desired temperature, allowed to stand at this temperature for 5 min, and recooled to 77 K to measure ESR signals. The new signals started to disappear at 130 K but were

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Figure 6. ESR spectra obtained by irradiation of 4,4′-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar ratio. (A, a-c) ESR spectrum before (a) and after irradiation (λ > 350 nm) at 77 K for 10 (b) and 80 min (c). (B, e and f) ESR spectra observed at 77 K in 2-MTHF after warming the matrix to (e) 130 and (f) 170 K.

Figure 7. ESR spectra obtained by irradiation of 2,4-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar ratio. (A, a-c) ESR spectrum before (a) and after irradiation (λ > 350 nm) for 10 (b) and 30 min (c) at 77 K. (B, e and f) ESR spectra observed at 77 K in 2-MTHF after warming the matrix to (e) 100 and (f) 120 K.

observable up to 150 K. The signals were replaced by those due to Cu(II) ion at around 170 K (Figure 6Be,f). Similar irradiation of a 1:1 complex of 2,4-DPy-1-N2 and Cu(hfac)2 (2.4 × 10-3 M) gave somewhat different results. The solution before irradiation showed only broad ESR signals at 310 mT without showing characteristics of the Cu(II) ion of Cu(hfac)2 (Figure 7Aa). This is most probably because 3d spins on Cu(II) ions in the complex can already interact with each other through pyridyl aromatic π-networks since they are involved in the ferromagnetic chains. Upon irradiation, the signals started to decrease with concomitant shift of the signal maximum until the maximum stopped growing and reached 330 mT after irradiation for 0.5 h (Figure 7Ab,c). Again the fact

that no significant signals due to isolated triplet carbene 2,4DPy-31 were observed suggested that the pyridine moiety binds with Cu(hfac)2 essentially in a nearly quantitative manner under these cryogenic conditions. Upon warming the matrix containing the sample, the new signals gradually disappeared and the original signals before irradiation recovered at 120 K (Figure 7Be,f). It is to be noted here that the signals of bis(4-pyridyl)carbene-Cu(hfac)2 disappeared irreversibly at temperatures higher than 60 K.25d Thus remarkable thermal stability of the DPy-1-Cu complex is noted. (b) UV/Vis. Irradiation of a 1:1 mixture of 4,4′-DPy-1-N2 and Cu(hfac)2 (1.9 × 10-4 M) in 2-MTHF at 77 K gave rise to new

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Figure 8. UV/vis spectra obtained by irradiation of 4,4′-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar. (a and b) UV/vis spectra before (a) and after (b) irradiation at 77 K. (c-e) The same sample after thawing to 90 (c), 120 (d), and 150 K (e).

Figure 10. (A) Plot of magnetization (M in emu) as a function of irradiation time observed in the photolysis of 4,4′-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar ratio measured at 5.0 K and 5 kOe. (B) Field dependence of the magnetization of the photoproduct from a 1:1 complex of 4,4′-DPy-1-N2 with Cu(hfac)2 in 0.3 mM 2-MTHF matrix measured at 2.0, 3.0, and 5.0 K. Mb and Ma refer to the magnetization value before and after irradiation, respectively.

Figure 9. UV/vis spectra obtained by irradiation of 2,4-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar. (a and b) UV/vis spectra before (a) and after (b) irradiation at 77 K. (c-e) The same sample after thawing to 90 (c), 100 (d), and 120 K (e).

absorption bands at the expense of the original absorption due to the complex (Figure 8b). The new spectrum again consists of two identifiable features, an intense UV band with an apparent maximum at 378 nm and weak, broad bands with apparent maxima around 508 and 539 nm. Those bands are to be compared with that of metal-free triplet carbene 4,4′-DPy-31 (360, 496, and 530 nm). Since ESR signals most likely ascribable to the carbene-Cu complex are observed under identical conditions, we can assign the absorption spectrum to the carbene-metal complex. A similar slight red shift of the characteristic absorption due to carbene after complexation with Cu(hfac)2 also has been noted in bis(4-pyridyl)carbene-Cu(hfac)2 systems.25 The glassy solution did not exhibit any changes for hours at this temperature, but all the bands disappeared irreversibly when the temperature was raised to 150 K (Figure 8c-e). Similar irradiation of a 1:1 mixture of 2,4-DPy-1-N2 and Cu(hfac)2 (2.5 × 10-4 M) in 2-MTHF at 77 K gave similar results (Figure 9). The new spectrum after irradiation again showed an intense UV band with apparent maxima at 313, 338, and 353 nm and weak, broad bands with apparent maxima around 508 and 560 nm, which are slightly red-shifted compared with

that of metal-free triplet carbene 2,4-DPy-31 (342 and 370 to 550 nm). Since ESR signals most likely ascribable to a carbeneCu complex are observed under identical conditions, we can assign the absorption spectrum to the carbene-metal complex. All the bands disappeared irreversibly when the temperature was raised to 120 K (Figure 9c-e). (c) Superconducting Quantum Interference DeVice (SQUID) Measurements. To determine the spin quantum numbers of the polycarbene bridged by copper ions, the magnetic properties before and after photolysis of a 1:1 mixture of 4,4′-DPy-1-N2 and Cu(hfac)2 in 2-MTHF solutions were studied by means of a SQUID magneto/susceptometer. The 2-MTHF solution (80 µL) of the 4,4′-DPy-1-N2-copper (1:1) complex (1.0 × 10-3 M)26 was placed inside the sample compartment of a SQUID magnet/susceptometer and was irradiated at 5-10 K with light (λ ) 488 nm) from an Ar ion laser through an optical fiber. The development on magnetization (M/emu) at 5 K in a constant field of 5 kOe with the irradiation time for the sample was measured in situ and is shown in Figure 10A. As the irradiation time increased, the M values gradually increased and reached a plateau after 3 h. After the M values reached a plateau, the magnetization values after irradiation, Ma, were measured at 2.0, 3.0, and 5.0 K in a field range of 0-50 kOe. The magnetization values of the sample before irradiation, Mb, were also measured under the same conditions (Figure 10B).

20770 J. Phys. Chem. B, Vol. 109, No. 44, 2005

Itoh et al.

Figure 11. Plot of M vs H/T of the photoproduct from a 1:1 complex of 4,4′-DPy-1-N2 with Cu(hfac)2 measured at 2.0 (O), 3.0 (4), and 5.0 (0) K. The solid line shows the curve fitted with eq 2.

Figure 12. Temperature dependence of the observed χmolT for the photoproduct from a 1:1 mixture (1.0 × 10-3 M) of 4,4′-DPy-1-N2 and Cu(hfac)2 obtained at a field of 5 kOe.

The magnetization generated by irradiation was then obtained by subtracting the corresponding values obtained before and after irradiation (M ) Ma - Mb). Thus, any magnetization from diamagnetic and paramagnetic impurity could be canceled by this treatment. The field dependence of magnetization for the complex expressed by M versus H/T plots after irradiation for 400 min was shown in Figure 11. The plots were analyzed in terms of the Brillouin function as follows:1c,25,27

M ) Ma - Mb ) Mcomplex - MCu ) NgµB{SB(χ) - B(χ′)/2}

(2)

where χ ) gSµBH/(kBT), χ′ ) g′µBH/(2kBT), and the other symbols have their usual meaning. The g′ value for the complex was 2.166, which was determined from EPR spectra before irradiation under similar condition. The experimental data were fitted with the theoretical eq 2 to give spin quantum number S ) 6.80 and F (generation factor for carbene obtained from fitting parameters) ) 0.60 at 2.0 K. The values were found to be somewhat temperature dependent, S/F being 6.43/0.58 and 5.90/0.46 at 3.0 and 5.0 K, respectively. Temperature dependence of the molar paramagnetic susceptibility before and after irradiation, (χmolb and χmola, respectively) in the range of 2-70 K was measured at constant field at 5 kOe. χmol () χmola - χmolb)T versus T plots (χmol ) χmola χmolb) are shown in Figure 12. The value of χmolT increased with decreasing temperature from 70 to 4 K. This indicates that

Figure 13. (A) Plot of magnetization (M in emu) as a function of irradiation time observed in the photolysis of 2,4-DPy-1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 1:1 molar ratio measured at 5.0 K and 5 kOe. (B) Field dependence of the magnetization of the photoproduct from a 1:1 complex of 2,4-DPy-1-N2 with Cu(hfac)2 in a 0.3 mM 2-MTHF matrix measured at 2.0, 3.0, and 5.0 K. Mb and Ma refer to the magnetization value before and after irradiation, respectively.

J/k is small but positive, suggesting the presence of intramolecular ferromagnetic interaction.28 The data obtained for photoproducts from similar irradiation of the 2-MTHF solution of a 1:1 2,4-DPy-1-N2-copper complex (1.0 mM) were also analyzed by the Brillouin function (Figure 13).29 The experimental data (M vs H/T) for the 2,4-DPy-1N2-copper complex after irradiation were analyzed in terms of eq 2 with fitting curve with S ) 3.71 and F ) 0.73 (Figure 14). The values were again found to be somewhat temperature dependent, S/F being 3.31/0.72 and 2.85/0.72 at 3.0 and 5.0 K, respectively. χmolT versus T plots (Figure 15) showed that the value of χmolT increased with decreasing temperature from 70 to 4 K, suggesting again the presence of intramolecular ferromagnetic interaction. 4. Conclusions The present results demonstrate that a high spin species is actually generated in the photoproduct from a copper ion complex with sterically congested diaryldiazomethanes having a 4-pyridyl group as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group located remote from the carbene center. The complexed carbene showed significant stability surviving up to 120-150 K in 2-MTHF, while the analogous copper ion complex with unprotected pyridylcarbene decays at temperatures higher than 60 K.25d The observations

Preparation of Diphenyldiazomethanes

J. Phys. Chem. B, Vol. 109, No. 44, 2005 20771 support from the Mitsubishi Foundation and the Nagase Science and Technology Foundation is also appreciated. Supporting Information Available: General methods, preparation of diazo compounds, analytical and spectroscopic data of photoproducts from diazo compounds, LFP of 4,4′-DPy1-N2 in the presence of oxygen, plot of the growth rate of 4,4′DPy-1-O2 as a function of oxygen concentration, and LFP of 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 14. Plot of M vs H/T of the photoproduct from a 1:1 complex of 2,4-DPy-1-N2 with Cu(hfac)2 measured at 2.0 (O), 3.0 (4), and 5.0 (0) K. The solid line shows the curve fitted with eq 2.

Figure 15. Temperature dependence of the observed χmolT for the photoproduct from a 1:1 mixture (1.0 × 10-3 M) of 2,4-DPy-1-N2 and Cu(hfac)2 obtained at a field of 5 kOe.

suggest that it is potentially possible to prepare a persistent highspin polycarbene by extending this method. The value for the corresponding complex with the 2,4-DPy-1 is, however, significantly smaller (S ) 3.7) although the D value of this carbene is larger than 4,4′-DPy-1. Incomplete photolysis of diazo groups is often pointed out as a reason for rather small S and F values. However, almost complete photolysis of the sample was confirmed by taking the difference of absorption at 2040-2050 cm-1 due to the diazo moieties before and after SQUID measurements. It may be then that there is a defect in the ligation of pyridine groups in a 2,4-DPy-1-N2-copper complex. If there are partially uncoordinated copper ions in the complex, the magnetic contribution of the uncoordinated copper ions will be deleted in the equation of Mcomplex - MCu and this will result in small S and F values. However, in the 2,4-DPy-1 system, the chemical defect can be avoided since carbene units are attached as pendants to a coupling unit of the chain involving copper ions and aromatic π-networks. It may be then that the ligation to copper ions through pyridyl groups at the 2 and 4 positions on the same phenyl ring suffers hindrance more than that those on the 4 and 4′ positions on different rings. Acknowledgment. The authors are grateful to the Ministry of Education, Culture, Sports, Science and Technology of Japan for support of this work through a Grant-in-Aid for Scientific Research for Specially Promoted Research (No. 12002007). The

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Itoh et al. (24) Sugiura, Y. Inorg. Chem. 1978, 17, 2178. (25) (a) Sano, Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.; Rabu, P.; Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246. (b) Karasawa, S.; Koga, N. Polyhedron 2001, 20, 1387. (c) Karasawa, S.; Sano, Y.; Akita, T.; Koga, N.; Itoh, T.; Iwamura, H.; Rabu, P.; Drillon, M. J. Am. Chem. Soc. 1998, 120, 10080. (d) Karasawa, S.; Kumada, H.; Koga, N.; Iwamura, H. J. Am. Chem. Soc. 2001, 123, 9685. (e) Morikawa, H.; Imamura, F.; Tsurukami, Y.; Itoh, T.; Kumada, H.; Karasawa, S.; Koga, N.; Iwamura, H. J. Mater. Chem. 2001, 11, 493. (f) Karasawa, S.; Koga, N. Polyhedron 2003, 22, 1877. (26) The measurements were carried out at this low concentration not only to avoid intermolecular magnetic interaction of sample but also to increase photolysis efficiency. The latter is important especially because we generate magnetic species (carbene-Cu complexes) in situ by photolysis of precursory diazo-Cu complexes: carbene-Cu complexes show broad absorption bands at 450-550 nm and hence, at higher concentration, the bands due to carbene-Cu complexes grow and overlap with the bands due to the starting complex (see Figures 8 and 9). (27) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Germany, 1986. (28) In the present study, magnetic measurements were made in a frozen solution where the chain may be more flexible than in a solid state. Hence, it is still unsettled whether the complexes are formed by a one-dimensional chain having alternating S ) 1/2 and 1 spins and hence we cannot estimate unequivocally the magnitude of the exchange coupling between Cu 3d spin and carbene 2p spin. However, our results apparently indicate that the pyridyl group ligates with Cu ion to form 1:1 complexes and the spin quantum numbers of the complexes increase to some extent in this system. (29) Before irradiation, the magnetic interaction between Cu ions through pyridyl π-aromatic networks in 2,4-DPy-1-N2 can be presumed to be very much smaller than that between Cu ion and the carbenic center for photoproducts. Hence, we did not take into consideration this interaction to magnetic analysis for the photoproducts.