Diphenylcarbene Protected by Four ortho-Iodine Groups: An Unusually Persistent Triplet Carbene

Abstract

Diphenyldiazomethane with four iodine groups at the ortho positions and two tert-butyl groups at the para positions, i.e., bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N₂), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N₂ in solution effectively generated the corresponding triplet diphenylcarbene ³1a, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at room temperature. The UV-vis spectrum of ³1a was obtained by irradiating 1a-N₂ in a 2-methyltetrahydrofuran matrix at 77 K. The ESR spectrum showed no triplet carbene signals, while a radical species was observed at the anticipated temperature of the decomposition of triplet carbene ³1a. Transient absorption bands ascribable to ³1a were observed by laser flash photolysis of 1a-N₂ in a degassed benzene solution and decayed very slowly with a second-order rate constant (2k/εl) of 5.5 × 10⁻³·s⁻¹. Steady-state irradiation of 1a-N₂ in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a 31% yield. Triplet carbene ³1a was also trapped by either oxygen (k_O2 = 6.5 × 10⁵ M⁻¹·s⁻¹) or 1,4-cyclohexadiene (k_CHD = 1.5 M⁻¹·s⁻¹) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H₂. The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two ortho-iodo and two ortho-bromo groups, ³1b.

1. Introduction

Although stable N-heterocyclic carbenes (NHCs) have been frequently used as effective ligands for transition-metal catalysts [1,2], the instability of their triplet counterparts has hampered their use as materials. Thus, the stabilization and isolation of a triplet carbene has emerged as the next challenging target. Recent growing interest in triplet carbenes as potential organic ferromagnets [3,4,5] has added a more practical aspect to the project. Steric protection is an ideal method to achieve sufficient stabilization of a triplet carbene with its electronic integrity (one centered diradical) intact. Based on this concept, we generated many diphenylcarbenes that have various substituents at the ortho positions [6,7,8,9]. The strategy, however, encounters limitations when alkyl or aryl groups are employed as protecting groups. The central carbon atom of carbenes abstracts hydrogen even from very poor sources of electrons such as C-H bonds. Thus, the tert-butyl group, which has been successfully used to protect many reactive centers, is almost powerless to stabilize a triplet carbene center [10].

On the other hand, the van der Waals radius of bromine atom (185 pm) is larger than that of methyl group (175 pm) [11], and furthermore, the C-Br bond length (181 pm) of bromobenzene is longer than that of the C-CH₃ (177 pm) in toluene [11]. Thus, the bromine groups at ortho positions on diphenylcarbene overhang the carbene carbon more effectively. Moreover, halides involving bromine atom are not reactive with the triplet state [12,13,14]. For example, we previously showed that triplet bis(2,6-dibromo-4-tert-butylphenyl)carbene (³1c) had a half-life of 16 s in a benzene solution at room temperature [15,16].

The iodine atom has been regarded as a better kinetic protector of carbene because its van der Waals radius (198 pm) is much larger than that of the bromine atom (185 pm) [11]. In addition, according to theoretical calculations [17,18], the length of the C-I bond (210 pm) of iodobenzene is greater than that of the C-Br bond (189 pm) in bromobenzene. Recently, we generated and characterized a triplet diphenylcarbene bearing two iodine and two bromine groups at the ortho positions, (2,6-dibromo-4-tert-butylphenyl)(4-tert-butyl-2,6-diiodophenyl)carbene ³1b [19] (Scheme 1). This species was found to be a persistent triplet carbene, but contrary to our expectations, its half-life (t_1/2 = 24 s) was only 1.5 times longer than that of ³1c. The bimolecular rate constants of the decay of ³1b by triplet carbene quenchers were also only moderately smaller than those of ³1c. Although an iodine group shielded the carbene center more effectively than a bromine group, the half-occupation of an iodine atom at the four ortho positions of a diphenylcarbene was insufficient to protect the carbene center. One may expect that an extreme stabilization would be achieved by thorough substitution of the four ortho positions with iodo groups. In the present study, we were successful in preparing a precursor diphenyldiazomethane with four ortho-iodo groups, bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N₂). Here we report that triplet diphenylcarbene ³1a generated by the photolysis of 1a-N₂, is an unusually persistent triplet diphenylcarbene with an extended half-life of 18.4 min.

2. Results and Discussion

2.1. Preparation of Precursor Diazomethane

Most diphenyldiazomethanes that bear four kinetic protectors at the ortho positions have been prepared from the corresponding diphenylmethanols via the carbamate derivatives [10,15,16,19]. For the synthesis of 1a-N₂ (Scheme 2), the starting alcohol was expected to be obtained by a coupling reaction of a 2,6-diiodobenzaldehyde with a 2,6-diiodophenyllithium. According to the synthesis plan, we attempted the lithiation of 5-tert-butyl-1,2,3-triiodobenzene (3), of which the tert-butyl group affords a para-substituent in 1a. A para-tert-butyl group is known to serve as a good protecting group in diphenylcarbenes [15,16]. Fortunately, the lithiation proceeded exclusively at the 2-position of triiodobenzene to selectively afford the corresponding diphenylmethanol 4 by the subsequent reaction with 4-tert-butyl-2,6-diiodobenzaldehyde [19]. The success of the synthesis of alcohol 4 opened the way to diazomethane 1a-N₂. Alcohol 4 was chlorinated, substituted with ethyl carbamate, nitrosated, and treated with potassium tert-butoxide to give the crude diazomethane. The purification by preparative thin layer chromatography (alumina, hexane as eluent) afforded pure 1a-N₂ as yellow crystals in a 33% yield. Diazo compound 1a-N₂ was very stable for a diazomethane and did not decompose in a refrigerator (at −20 °C) over an extended period of time.

2.2. Spectroscopic Studies

2.2.1. ESR Studies in Rigid Matrix at Low Temperature

Most sterically hindered diphenylcarbenes are known to give rise to stable triplet ESR signals by irradiation of the corresponding diphenyldiazomethanes in 2-methyltetrahydrofuran (2-MTHF) at 77 K [10,15,16,20,21,22,23,24]. On the other hand, the signals of triplet carbene ³1b with two iodine atoms were invisible under the same conditions [19]. Similar invisibilities were observed in the ESR measurements of 2-iodophenylnitrene and (2-iodophenyl)(phenyl)carbene, and were explained by extreme broadening of the signal due to orbital degeneracy and strong spin-orbit coupling in the iodine atoms [25]. As expected from these facts, photolysis (λ > 350 nm) of diazomethane 1a-N₂ in a low temperature 2-MTHF matrix afforded no triplet signals due to carbene 1a (Figure 1a). The influence of the iodine atoms on ESR line broadening may be larger than that in 1b owing to the presence of four iodo groups.

We previously reported that the ESR signals of carbene 1c with four ortho-bromo groups were observed in a 2-MTHF matrix at 77 K and disappeared irreversibly by warming the matrix to 170 K [26]. On the other hand, triplet carbene ³1b was invisible under the same conditions, and a new doublet signal at 327 mT due to carbon radical species appeared by warming up to 160 K, which implies the decomposition of ³1b [19]. This means that the triplet carbene ³1b abstracts a hydrogen from 2-MTHF of the matrix to form the corresponding diphenylmethyl radical. If carbene 1a decays by a similar form of degradation, a doublet signal of the corresponding radical species would appear when the matrix is warmed up. To confirm this assumption, ESR measurements were carried out after warming the matrix involving the photoproducts and then recooling to 77 K to compensate for the weakening of signals due to the Curie law [27,28,29]. Although gradual warming of the photolyzed sample up to 190 K gave no ESR signals, a new doublet signal appeared at 327 mT at 200 K (Figure 1b). The signal intensity increased gradually as the sample was warmed up to 230 K, and decreased only slightly at 300 K (Figure 1c–e). The signal disappeared when the 2-MTHF solution was exposed to air (Figure 1f). The appearance of the doublet signal can be attributed to the formation of bis(4-tert-butyl-2,6-diiodophenyl)methyl radical via hydrogen abstraction. We used the temperature (T_d) at which signals due to a triplet carbene disappear as a measure of the thermal stability of triplet carbenes [9]. If ³1a is assumed to abstract a hydrogen from the solvent molecule at T_d, ³1a (T_d = 200 K) is considered much more resistant to degradation than 1b and 1c (T_d = 170 and 160 K, respectively). We previously reported that diphenylcarbenes protected by two bromine and two trifluoromethyl groups, 1d–f, are known to be the unusually persistent diphenylcarbenes with T_ds of 210–260 K (Scheme 3) [20,21]. The present result indicates that carbene 1a is also an unusually persistent diphenylcarbene with a stability comparable to 1e.

2.2.2. Magnetic-Susceptibility Measurements

To obtain evidence of the ground triplet spin state of carbene 1a, the magnetic susceptibility of the photoproduct of diazomethane 1a-N₂ was measured. A 2-MTHF solution of 1a-N₂ (2.0 mM) was placed inside the sample compartment of a superconducting quantum interference device (SQUID) magnet/susceptometer and was irradiated at 10 K with a laser light at 442 nm. The development upon magnetization was measured at 5 K in a constant field of 5000 Oe. The magnetization values gradually increased with the irradiation time and reached a plateau after 12 h. After a plateau was established, the magnetization values, M_a, were measured at 5.0 K in a range of 0–50,000 Oe. The net magnetization generated by the irradiation of 1a-N₂ was estimated from the difference between M_a and the corresponding magnetization values, M_b, obtained before irradiation. The plot of the magnetization (M) versus the magnetic field (H) was analyzed in terms of the Brillouin function as follows [30,31,32,33,34,35]:

M = M_a − M_b = FNgJμ_BB(χ)

(1)

where F is the generation factor for carbene estimated from saturated magnetization, N is the number of the molecule, g is the Landé g-factor, J is the quantum number for the total angular momentum, and μ_B is the Bohr magneton. Since diphenylcarbene is composed of light elements, the orbital angular momentum should be negligible, and J can be replaced with the spin quantum number S. The M vs. H plot is shown in Figure S1. The observed data was best fitted to Equation (1) with S = 0.93 (F = 0.35) at 5.0 K. Although this S value is somewhat smaller than the theoretical value (S = 1) for a triplet carbene, this result clearly demonstrates the generation of a triplet species, i.e., carbene 1a, by the photolysis of 1a-N₂.

2.2.3. UV-Vis Studies in Rigid Matrix at Low Temperature

Photolysis (λ > 350 nm) of 1a-N₂ in a 2-MTHF matrix at 77 K afforded new absorption bands, strong maxima at 350 and 369 nm and broad and weak bands at 462, 480 and 506 nm, at the expense of the precursor (Figure 2A(a,b)). Most of the sterically hindered triplet diphenylcarbenes in 2-MTHF matrix are known to show strong absorption bands in the UV region (320–360 nm) and broad and weak bands in the visible region (430–510 nm) [9]. The absorption bands disappeared irreversibly when the matrix was warmed up to room temperature. We assigned the species having these absorption bands to triplet carbene ³1a on the basis of similarity in the spectrum and in thermal behavior of ³1b [19].

The thermal stability of triplet carbene ³1a was then estimated by warming the matrix. When warmed to 100 K, the absorption maxima of the bands due to ³1a shifted from 350 and 369 nm to 354 and 371 nm, respectively (Figure 2A(c)). The spectral changes can be attributed to a decrease in the viscosity of the 2-MTHF matrix, because similar changes in other sterically hindered diarylcarbenes are interpreted in terms of geometrical changes in the molecules due to a sudden decrease in the viscosity of 2-MTHF from 10⁷ to 10³ P [36]. The bands of ³1a started to decrease slowly at 180 K and disappeared irreversibly at 240 K (Figure 2B). The temperature at which the bands of ³1a started to decrease was roughly in accordance with the emergence of the ESR signal of the radical.

Diphenylcarbenes with no substituents at the ortho positions in a 2-MTHF matrix disappear between 100 and 105 K [7]. Carbene ³1b is protected by two bromo and two iodo groups, but it started to decay at 150 K in 2-MTHF [19]. The even higher decay temperature of 1a clearly demonstrates that it is much more stable than ³1b.

2.2.4. Laser Flash Photolysis Studies in Solution at Room Temperature

For quantitative evaluation of the stability of carbene 1a, the lifetime measurement was carried out in degassed benzene at 25 °C, which we ordinarily use for the lifetime measurement of sterically congested diarylcarbenes [6,7,8,9]. The lifetime of 1a was too long to monitor by the laser flash photolysis (LFP) apparatus that we routinely use. Thus, the lifetime was estimated using a conventional UV-vis spectroscopic method [20,21,37,38].

Laser pulse photolysis at 308 nm of 1a-N₂ in degassed benzene at 25 °C produced a transient species showing maxima at 366, 480, and 506 nm, which appeared coincidentally with the laser pulse (Figure 3). These absorption bands were essentially similar to those observed in the photolysis of 1a-N₂ in 2-MTHF at 77 K, although the maxima were slightly shifted. The transient bands were assigned to triplet ³1a, since the bands were quickly quenched by oxygen to give the corresponding diphenyl ketone oxide (vide infra). The absorption bands decayed very slowly but did not disappear completely even after 1.5 h. The decay was found to be a second-order process (2k/εl = 5.5 × 10⁻³·s⁻¹) with a half-life of 18.4 min (Figure 3 inset,

Table 1. Kinetic Data of Carbenes 1.

Carbenes	2k/εl (s−1)	t1/2 (s)	kO2 (M−1·s−1)	kCHD (M−1·s−1)	KkMeOH 1 (M−2·s−1)
1a	0.0055 0.33 min−1	1.1 × 103 18.4 min	6.5 × 105	1.5	0.033
1b	0.17	2.4 × 10	7.3 × 106	3.9 × 10	0.30
1c	0.35	1.6 × 10	2.1 × 107	5.3 × 102	0.42

¹ K = k_TS/k_ST.

). The kinetic data indicates that ³1a is extremely persistent compared with that of ³1b (2k/εl = 0.17 s⁻¹, t_1/2 = 24 s) [19] and ³1c (2k/εl = 0.35 s⁻¹, t_1/2 = 16 s) [15,16].

Since triplet carbenes are easily trapped with oxygen or 1,4-cyclohexadiene (CHD) [39], these trapping reactions can be employed as a quantitative scale of the reactivities of the triplet carbenes.

The LFP (λ = 308 nm) of 1a-N₂ in O₂ saturated benzene at 25 °C resulted in a rapid decay of carbene ³1a and a concurrent appearance of a broad absorption band at around 415 nm (Figure S2A(b)), which was clearly different from the band of 1a (Figure S2A(a)). In addition, the solution after the LFP was found to contain the corresponding benzophenone 1a-O as a major product. It is well documented that the oxygen trapping of diphenylcarbenes generates the corresponding benzophenone oxides with a broad absorption band at around 390–450 nm to afford the ketone derivative (Scheme 4) [40,41,42,43,44,45,46,47]. Thus, our observations suggested that carbene ³1a was rapidly trapped with oxygen to form the ketone oxide 1a-O₂ like most of diphenylcarbenes. The rate constant (k_O2) for the trapping of ³1a with oxygen was estimated to be 6.5 × 10⁵ M⁻¹·s⁻¹ based on a plot of the observed growth rate of 1a-O₂ as a function of the oxygen concentration (Figure S2B, Table 1). The rate constant is one order of magnitude smaller than that observed with ³1b (k_O2 = 7.3 × 10⁶ M⁻¹·s⁻¹) [19] and 30 times smaller than that of ³1c (k_O2 = 2.1 × 10⁷ M⁻¹·s⁻¹) [16].

The LFP (λ = 308 nm) of 1a-N₂ in degassed benzene containing 1,4-cyclohexadiene gave a new species with an absorption band at 380 nm. The diphenylmethyl radicals, formed as a result of a hydrogen abstraction of the carbenes 1b,c from 1,4-cyclohexadiene, showed absorption maxima at wavelengths 20–30 nm longer than that of the precursor carbene bands (ca. 350 nm) [16,19]. Thus, this species, generated from 1a-N₂, was also assigned to the corresponding diphenylmethyl radical (1a-H) (Scheme 4). The spent solution contained the diphenylmethane 1a-H₂ as the main product. Owing to the overlap between the absorption band of the radical and the tail of the carbene, the growth curves of the radical were not observed clearly (Figure S3A). However, a linear plot was obtained between the observed pseudo-first-order rate constants of the decay of the carbene monitored at 350 nm and [CHD] (Figure S3B), and its slope gave the rate constant (k_CHD) for the reaction of ³1a with the diene, 1.5 M⁻¹·s⁻¹, which is 1 and 2 orders of magnitude smaller than those observed with triplet diiododibromo ³1b (39 M⁻¹·s⁻¹) [19] and tetrabromo ³1c (5.3 × 10² M⁻¹·s⁻¹) [15,16], respectively (Table 1).

The LFP of 1a-N₂ in a degassed benzene–methanol mixed solvent also formed triplet carbene ³1a. The observed decay rate constant (k_obs) of ³1a increased as the fraction of methanol increased. However, the plot of k_obs vs methanol concentration was not linear in the concentration range of 2.47 to 9.88 M. The trapping of some sterically hindered diarylcarbenes with methanol have been reported to show better linearities when the plot was performed against the square of the methanol concentration [19,48]. Carbene 1a also showed a more linear plot when k_obs was plotted against [MeOH]², and the trapping rate constant (Kk_MeOH) was 0.033 M⁻²·s⁻¹ as estimated from the slope of this plot (Figure S4, Table 1). This is one order of magnitude smaller than the rate constants observed with ³1b and ³1c (0.30 and 0.42 M⁻²·s⁻¹, respectively [19]).

The insertion of carbenes to the O-H bond of alcohols to form ethers is usually considered to occur from the singlet states of these species (Scheme 4). The decrease in Kk_MeOH may be caused either by an increased energy gap between the ground triplet and singlet states or by the deceleration of the O-H insertion. Since theoretical studies have suggested that the energy gap for 1a is only slightly larger than those for 1b and 1c (vide infra), the difference in the rate constants (Kk_MeOH) is mainly controlled by the rate (k_MeOH) of the insertion reaction of singlet carbenes to methanol, rather than by the singlet-triplet equilibrium constant (K). Thus, the ortho-iodine groups have a significant rate-retarding effect on the insertion reaction of the singlet carbenes. Thus, carbene ³1a showed a higher stability estimated in degassed benzene and in degassed 2-MTHF and lower reactivities toward triplet carbene quenchers (oxygen and 1,4-cyclohexadiene) and a singlet carbene quencher (methanol) than the bromine-substituted carbenes ³1b and ³1c.

2.3. Product Analysis Studies

Although steric protection is an efficient method for stabilization of the triplet diphenylcarbenes, complete protection of triplet carbenes has not been achieved, because even such carbenes decay mainly by dimerization. When long-lived diphenylcarbenes are generated in degassed benzene in the absence of carbene quenchers at room temperature, most are known to dimerize at the carbenic center to form the corresponding tetraphenylethenes as the main product. For example, dimesitylcarbene and bis(2,4,6-trichlorophenyl)carbene produced tetramesitylethene [10] and tetrakis(2,4,6-trichlorophenyl)ethene [22] in high yields, respectively. On the other hand, the dimerization product from bis(2,6-dibromophenyl)carbenes underwent photocyclization to give the corresponding phenanthrene derivatives [15,16]. By the same token, carbene 2b afforded the corresponding phenanthrene derivative with the concomitant elimination of an iodine molecule [19].

Photolysis (λ > 350 nm) of 1a-N₂ (5.0 mg) in degassed benzene-d₆ (0.5 mL) in a sealed NMR tube at 25 °C was monitored by ¹H-NMR spectroscopy. After 50 min irradiation, very weak ¹H-NMR signals at 9.03, 8.82, 7.84, 1.23, and 1.09 ppm, along with weak and broad peaks were observed at the expense of the signals of 1a-N₂. The ¹H-NMR spectrum of a fraction separated by a recycled gel permeation chromatography (GPC) of the resultant mixture showed the phenanthrene derivative 2a (a brown oil in a 31% yield) (Scheme 5). During the photolysis, it is reasonable that 2a was formed as a result of dimerization at the carbene center followed by photocyclization and deiodination. The ¹H-NMR spectra of other GPC fractions showed no detectable peaks. The yield of 2a was higher than that of 2b (18%) but lower than that of 2c (48%). The carbene dimer intermediate may have difficulty in forming from 1a compared to from 1c due to the steric repulsion between the bulky iodo groups.

Photolysis (λ > 350 nm) of 1a-N₂ in an O₂-saturated benzene solution afforded tetraiodobenzophenone 1a-O as the sole product in a 60% yield. On the other hand, irradiation (λ > 350 nm) of 1a-N₂ in degassed benzene in the presence of 1,4-cyclohexadiene gave diphenylmethane 1a-H₂ in a 54% yield. Similar irradiation in degassed benzene–methanol afforded methyl ether 1a-HOMe (52%) (Scheme 6). We previously showed that the irradiation of 1b-N₂ and 1c-N₂ in the presence of the carbene quenchers afforded ketones 1-O, diphenylmethanes 1-H₂, and methyl ethers 1-HOMe in good yields [16,19]. Thus, the lifetimes of carbenes 1 hardly affected the product yields.

2.4. DFT Calculations

To obtain direct insight into the differences among the structures of carbenes 1a, 1b, and 1c, their singlet and triplet states were optimized by DFT calculations [18]. The optimized geometries at the M05/6–31G* level of theory (3–21G level for iodine atom) indicated that the carbene-center angle/dihedral angle between two phenyl rings of singlet ¹1a are 130.2°/80.1°, but those for triplet ³1a are 179.9°/89.9° (Figure 4), indicating that triplet ³1a has a linear structure with two phenyl rings more orthogonal to each other compared to those in ¹1a. On the other hand, the dihedral angles in triplet ³1b and ³1c were significantly smaller than that in 1a, enabling the optimal shielding of the carbene center in 1a. The C-C distances between the aromatic and carbene carbons are slightly shorter for ³1a (1.374 Å) than for either ³1b (1.375 and 1.376 Å) or ³1c (1.376 Å). The spin density on the carbene carbon (1.48 for ³1a) are similar to those of ³1b and ³1c (1.48 and 1.51). For 1a, the triplet state lies 16.6 kcal/mol below the singlet state. The energy gap between the singlet and triplet states is slightly larger than that for either carbene 1b (ΔE_ST = 15.4 kcal/mol) or 1c (ΔE_ST = 14.2 kcal/mol).

3. Materials and Methods

3.1. General

¹H- (300 MHz) and ¹³C-NMR (75.5 MHz) spectra were determined with a JNM-AL300 FT/NMR spectrometer (JEOL, Tokyo, Japan) in CDCl₃ with Me₄Si as an internal reference. ¹³C-NMR (100 MHz) spectra were obtained using a JNM-ECX400 FT/NMR spectrometer (JEOL, Tokyo, Japan). The NMR spectra were shown in Supplementary materials. IR spectra were measured either with films on a NaCl plate or with KBr pellets using a FT/IR-410 spectrometer (JASCO, Tokyo, Japan). UV-vis spectra were measured with a JASCO V-560 or MaltiSpec-1500 spectrometer (Shimadzu, Kyoto, Japan). The mass spectra were recorded on a JEOL JMS-600H mass spectrometer or a Voyager DE-Pro MALDI-TOF mass spectrometer (Applied Biosystems, Foster City, CA, USA). Thin layer chromatography for diazo compounds was performed using aluminum oxide 60 PF₂₅₄ on a glass plate (Type E) (Merck, Tokyo, Japan). Column chromatography was performed using silica gel (63–210 μm) or neutral alumina (Act. I, inactivated with 5% water) for diazo compound. Recycle GPC was undertaken with a JASCO PU-2086 chromatograph with a UV-2070 UV-vis detector using a GPC H-2001 (20 mm × 50 cm) column (Shodex, Tokyo, Japan).

3.2. Synthesis of Bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N₂)

5-tert-Butyl-1,2,3-triiodobenzene (3): To a solution of 4-tert-butyl-1-iodobenzene (2.50 g, 9.61 mmol) in dry MeCN (100 mL) was added I₂ (8.50 g, 33.5 mmol) and 90% F-TEDA-BF₄ (6.72 g, 33.5 mmol), and the reaction mixture was stirred at 65 °C for 14 h. The solvent was removed under reduced pressure and the crude mixture was dissolved in CH₂Cl₂ (90 mL). After the insoluble material was filtered off, the solution was washed with saturated aqueous Na₂S₂O₃·5H₂O and brine, and dried over anhydrous Na₂SO₄. The solvent was evaporated, and the starting iodobenzene and by-product, 4-tert-butyl-1,2-diiodobenzene were removed using a glass tube oven (160 °C/3.0 Torr). The residue was chromatographed through a column of silica gel using hexane as an eluent to give 5-tert-butyl-1,2,3-triiodobenzene (3) as yellow crystals (1.03 g, 60%): ¹H-NMR (CDCl₃, ppm) δ 7.82 (s, 2H), 1.25 (s, 9H) [49].

Bis(4-tert-butyl-2,6-diiodophenyl)methanol (4): To a solution of 5-tert-butyl-1,2,3-triiodobenzene (3) (1.01 g, 2.0 mmol) in dry ether (40 mL) was added BuLi (2.64 M in hexane, 0.80 mL, 2.1 mmol) at –78 °C. The solution was stirred for 6 h, and then a solution of 4-tert-butyl-2,6-diiodobenzaldehyde (0.86 g, 2.1 mmol) in dry ether (7 mL) was added dropwise over 3 min at −78 °C. The mixture was stirred at room temperature for 20 h and was then quenched by saturated aqueous ammonium chloride (25 mL). The mixture was extracted with ether, and the organic layer was washed with water and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the residue was chromatographed through a column of silica gel using hexane–chloroform (2:1) as an eluent to give bis(4-tert-butyl-2,6-diiodophenyl)methanol (4) as colorless crystals (0.71 g, 45%): m.p. 77.7–80.6 °C; ¹H-NMR (CDCl₃, ppm) δ 7.94 (s, 4H), 6.19 (d, J = 8.8 Hz, 1H), 2.76 (d, J = 8.8 Hz, 1H), 1.28 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 154.0, 139.6, 137.7, 99.5, 86.9, 34.1, 31.0; HRMS calcd. for C₂₁H₂₃I₄O [M – H]⁺ 798.7923, found m/z 798.7922.

Ethyl N-[bis(4-tert-butyl-2,6-diiodophenyl)methyl]carbamate (6): A solution of alcohol 4 (472 mg, 590 μmol) and thionyl chloride (9 mL, 0.12 mol) was stirred at room temperature for 6 h, and the mixture was evaporated under reduced pressure to give bis(4-tert-butyl-2,6-diiodophenyl)chloromethane (5) as orange crystals (480 mg). This compound was used in the next experiment without further purification: ¹H-NMR (CDCl₃, ppm) δ 7.98 (s, 4H), 6.64 (s, 1H), 1.28 (s, 18H). To a mixture of silver tetrafluoroborate (110 mg, 567 μmol) and ethyl carbamate (2.50 g, 28.1 mmol), heated at 60 °C, was added a solution of chloromethane 5 (250 mg, 313 μmol) in 1,4-dioxane (3 mL), and the resultant mixture was heated at 100 °C for 20 h. After cooling at room temperature, the mixture was filtered, and CHCl₃ (20 mL) was added to the filtrate. The organic layer was washed well with water, dried over anhydrous Na₂SO₄, and the solvent was removed under reduced pressure. The residue was chromatographed through a column of silica gel using hexane–dichloromethane (1:5) as an eluent to give ethyl N-[bis(4-tert-butyl-2,6-diiodophenyl)methyl]carbamate (6) as yellow crystals (143 mg, 50%): m.p. 105.2–107.1 °C; ¹H-NMR (CDCl₃, ppm) δ 7.92 (s, 4H), 6.23 (d, J = 8.5 Hz, 1H), 5.18 (d, J = 8.5 Hz, 1H), 4.24 (q, J = 7.2 Hz, 2H), 1.31–1.27 (m, 21H); ¹³C-NMR (CDCl₃, ppm) δ 154.2, 153.8, 139.9, 136.0, 100.3, 71.5, 61.4, 34.0, 30.9, 15.0; HRMS calcd. for C₂₄H₃₀I₄NO₂ [M + H]⁺ 871.8450, found m/z 871.8410.

Bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N₂): Into a stirred solution of carbamate 6 (189 mg, 217 μmol) in anhydrous CCl₄ (15 mL) at −10 °C was bubbled dinitrogen tetroxide gas (3.00 g, 31.0 mmol), and sodium acetate (1.27 g, 15.5 mmol) was added. The mixture was stirred at room temperature for 23 h after the addition. The mixture was poured into a cold saturated aqueous sodium carbonate solution and extracted with CCl₄. The organic layer was washed with 5% aqueous Na₂CO₃ solution and brine, dried over anhydrous Na₂SO₄, and the solvent was removed under reduced pressure to give ethyl N-nitroso-N-[bis(4-tert-butyl-2,6-diiodophenyl)methyl]carbamate (7) as a yellow semisolid (195 mg); ¹H-NMR (CDCl₃, ppm) δ 7.88 (s, 4H), 6.57 (s, 1H) 4.63 (q, J = 7.0 Hz, 2H), 1.49 (t, J = 7.0 Hz, 3H), 1.26 (s, 18H). To a solution of nitroso compound 7 in anhydrous THF (13 mL), cooled at –15 °C, was added potassium tert-butoxide (183 mg, 1.63 mmol) at once under an Ar atmosphere. After stirring at room temperature for 42 h, the mixture was poured into water (50 mL) and extracted with ether (20 mL × 3). The organic layer was washed with water and brine, dried over anhydrous Na₂SO₄, and evaporated. The residue was purified by short column chromatography (neutral alumina, hexane) and preparative TLC (aluminum oxide, hexane) to give bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N₂) as yellow crystals (58 mg, 33%): m.p. 187.6–189.2 °C (dec.); ¹H-NMR (CDCl₃, ppm) δ 7.94 (s, 4H), 1.29 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 153.8, 139.1, 131.9, 100.8, 77.7, 34.3, 31.0; IR (KBr disk, cm^–1) 2059 (ν_C=N2).

3.3. Product Analysis

All benzene solutions of diazo compounds were photolyzed through an UV-35 glass filter (AGC TECHNO GLASS, Shizuoka, Japan) with a 500 W Xe lamp (Wacom, Saitama, Japan) at 25 °C. Photolysis (λ > 350 nm) of 1a-N₂ (5.0 mg, 6.2 mmol) in degassed benzene-d₆ (0.5 mL) for 50 min gave 3,6-di-tert-butyl-9,10-bis(4-tert-butyl-2,6-diiodophenyl)-1,8-diiodo-phenanthrene (2a): a brown oil (1.3 mg, 31% after recycle GPC separation) ; ¹H-NMR (CDCl₃, ppm) δ 8.81 (d, J = 2.0 Hz, 2H), 8.53 (d, J = 2.0 Hz, 2H), 7.66 (s, 4H), 1.50 (s, 18H), 1.30 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 154.3, 150.1, 143.9, 140.0, 136.7, 132.8, 128.1, 120.7, 108.2, 99.8, 92.9, 34.8, 34.2, 31.4, 31.2; MALDI-TOF-mass calcd. for C₄₂H₄₄I₆ (M⁺) 1309.7706, found m/z 1309.7829.

Photolysis (λ > 350 nm) of 1a-N₂ (5.0 mg, 6.2 mmol) in O₂-saturated benzene (1.0 mL) for 20 min gave bis(4-tert-butyl-2,6-diiodophenyl) ketone (1a-O) as colorless crystals (3.0 mg, 60% after recycle GPC separation); m.p. 219.5–220.4 °C; ¹H-NMR (CDCl₃, ppm) δ 7.94 (s, 4H), 1.30 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 195.9, 157.0, 139.4, 138.9, 97.1, 34.6, 30.9; IR (KBr, cm^–1) 1669 (ν_CO); MALDI-TOF-mass calcd. for C₂₁H₂₂I₄O (M⁺) 797.7844, found m/z 797.7792.

Photolysis (λ > 350 nm) of 1a-N₂ (5.0 mg, 6.2 mmol) in degassed benzene (0.9 mL) in the presence of 1,4-cyclohexadiene (0.1 mL) for 20 min gave bis(4-tert-butyl-2,6-diiodophenyl)methane (1a-H₂) as colorless crystals (2.6 mg, 54% after recycle GPC separation); m.p. 190.6–191.4 °C; ¹H-NMR (CDCl₃, ppm) δ 7.84 (s, 4H), 4.70 (s, 2H), 1.27 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 153.2, 139.4, 138.4, 101.7, 60.5, 34.1, 31.1; MALDI-TOF-mass calcd for C₂₁H₂₃I₄ [M − H]⁺ 782.7973, found m/z 782.7956.

Photolysis (λ > 350 nm) of 1a-N₂ (5.0 mg, 6.2 mmol) in degassed benzene (0.5 mL) in the presence of methanol (0.5 mL) for 20 min gave bis(4-tert-butyl-2,6-diiodophenyl)methyl methyl ether (1a-HOMe) as colorless crystals (2.6 mg, 52% after recycle GPC separation); m.p. 226.4–227.3 °C; ¹H-NMR (300 MHz, CDCl₃, ppm) δ 7.97 (s, 4H), 5.64 (s, 1H), 3.46 (s, 3H), 1.29 (s, 18H); ¹³C-NMR (CDCl₃, ppm) δ 154.4, 139.8, 135.2 100.4, 95.4, 59.2, 34.2, 31.1; MALDI-TOF-mass calcd. for C₂₂H₂₅ I₄O [M − H]⁺ 812.8079, found m/z 812.8140.

3.4. ESR Measurements

The diazo compound was dissolved in 2-MTHF (1.5 × 10^–2 M) and the solution was degassed in a quartz sample tube by five freeze-degas-thaw cycles. The sample was cooled in an optical transmission ESR cavity at 77 K and irradiated with a Wacom 500 W Xe lamp using a filter (λ > 350 nm). ESR spectra were measured on a JEOL JES TE 200 spectrometer (X-band microwave unit). The temperature was controlled by a 9650 microprocessor-based digital temperature controller (control ability within ±0.2 K).

3.5. SQUID Measurements

Magnetic susceptibility data were obtained on a MPMS-2A SQUID magnetometer/susceptometer (Quantum Design, Tokyo, Japan). Irradiation with light from a He-Cd laser (442 nm, 2074-M A03, Melles Griot, Saitama, Japan) through a flexible optical fiber 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 end was attached to a coupler for the laser. The bottom portion of the capsule (6 mm × 10 mm) was used as a sample cell. A quantity of 50 μL of the sample solution (1.0 mM) in 2-MTHF was placed in the cell that was held by a plastic straw. The irradiation was carried out until there was no further change of magnetization monitored at 5 K in 5000 Oe. The magnetization before and after irradiation was measured at 5 K in a field range of 0–50,000 Oe. The plot of the magnetization versus the magnetic field was analyzed in terms of the Brillouin function.

3.6. Low-Temperature UV-Vis Measurements

Low-temperature UV-vis spectra were obtained by using a variable-temperature liquid-nitrogen cryostat (DN 1704, Oxford, Oxfordshire, UK) equipped with quartz windows. The diazo compound was dissolved in dry 2-MTHF (1.5 × 10⁻³ M), placed in a long-necked quartz cuvette of 1-mm path length, and degassed thoroughly by five 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 min in the spectrometer with a Wacom 500 W Xe lamp using a filter (λ > 350 nm), 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).

3.7. Laser Flash Photolysis

Laser flash photolysis measurements were made on a TSP-601 flash spectrometer (Unisoku, Osaka, Japan). The excitation source for the laser flash photolysis was a LEXTRA XeCl excimer laser (Lambda Physik, Göttingen, Germany, output energy 100 mJ, wavelength 308 nm, pulse duration 10 ns). A 150-W xenon short arc lamp (L2195, Hamamatsu Photonics, Shizuoka, Japan) was used as the probe source, and the monitoring beam, guided using 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 linear image sensor. Timing of the laser excitation pulse, the probe beam, and the detection system was achieved through a model DS-8631 digital synchroscope (Iwatsu, Tokyo, Japan) which was interfaced to a 9821 RA266 computer (NEC, Tokyo, Japan).

A sample was placed in a long-necked Pyrex tube that had a sidearm connected to a quartz cuvette and was degassed by five freeze-degas-thaw cycles at a pressure near 10⁻⁵ Torr immediately prior to being photolyzed. The sample system was sealed, and the solution was transferred to the quartz cuvette, which was placed in the sample chamber of the flash photolysis apparatus.

4. Conclusions

A new diphenyldiazomethane 1a-N₂ with four iodine groups at all ortho positions was synthesized, and the triplet carbene, ³1a, generated by its photolysis, was characterized. In low-temperature matrices, an unusual stability was revealed from the time course of the absorption bands. In benzene at room temperature, ³1a decayed very slowly according to a second-order process to afford the corresponding phenanthrene. The present results demonstrated that carbene ³1a is one of the unusually persistent triplet carbenes with a half-life of over 10 min, and the ortho-iodine group is a simple and effective kinetic protector for triplet diphenylcarbene. This strategy will pave a road to the spin source of organic magnetic materials.