An Unusually Persistent Triplet Carbene

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

Katsuyuki Hirai 1,*, Kana Bessho 2, Kosaku Tsujita 2 and Toshikazu Kitagawa 2,*

1 Community-University Research Cooperation Center, Mie University, Tsu, Mie 514-8507, Japan 2 Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan; [email protected] (K.B.); [email protected] (K.T.) * Correspondences: [email protected] (K.H.); [email protected] (T.K.); Tel.: +81-59-231-9417 (K.H.); +81-59-231-9416 (T.K.)

Academic Editor: Shunichi Fukuzumi Received: 20 October 2016; Accepted: 11 November 2016; Published: 15 November 2016

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 3 room temperature. The UV-vis spectrum of 1a 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 −3 −1 constant (2k/εl) of 5.5 × 10 ·s . 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 5 −1 −1 −1 −1 either oxygen (kO2 = 6.5 × 10 M ·s ) or 1,4-cyclohexadiene (kCHD = 1.5 M ·s ) 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.

Keywords: triplet carbene; diazo compound; photolysis; steric hindrance; substituent effect

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 [3–5] 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 [6–9]. 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

Molecules 2016, 21, 1545; doi:10.3390/molecules21111545 www.mdpi.com/journal/molecules Molecules 2016, 21, 1545 2 of 15

Molecules 2016, 21, 1545 2 of 15 diphenylcarbene overhang the carbene carbon more effectively. Moreover, halides involving bromine bromineatom are atom not reactive are not withreactive the with triplet the state triplet [12 stat–14e]. [12–14]. For example, For example, we previously we previously showed showed that triplet that tripletbis(2,6-dibromo-4- bis(2,6-dibromo-4-tert-butylphenyl)carbenetert-butylphenyl)carbene (31c) had (31c a) half-lifehad a half-life of 16 s of in 16 a benzenes in a benzene solution solution at room at roomtemperature temperature [15,16 ].[15,16]. The iodine iodine atom atom has has been been regarded regarded as asa better a better kine kinetictic protector protector of carbene of carbene because because its van its der van Waals der Waalsradius radius(198 pm) (198 is much pm) islarger much than larger that thanof the that bromine of the atom bromine (185 pm) atom [11]. (185 In pm) addition, [11]. according In addition, to theoreticalaccording tocalculations theoretical [17,18], calculations the length [17,18 of], the the C-I length bond of (210 the C-Ipm) bondof iodobenzene (210 pm) of is iodobenzenegreater than thatis greater of the thanC-Br bond that of(189 the pm) C-Br in bromobenzene. bond (189 pm) Recently, in bromobenzene. we generated Recently, and characterized we generated a triplet and diphenylcarbenecharacterized a triplet bearing diphenylcarbene two iodine and bearingtwo bromine two iodine groups and at the two ortho bromine positions, groups (2,6-dibromo- at the ortho positions,4-tert-butylphenyl)(4- (2,6-dibromo-4-tert-butyl-2,6-diiodophenyl)carbenetert-butylphenyl)(4-tert-butyl-2,6-diiodophenyl)carbene 31b [19] (Scheme 1). This3 1bspecies[19] (Schemewas found1). Thisto bespecies a persistent was found triplet to carbene, be a persistent but contrary triplet to carbene, our expectations, but contrary its to half-life our expectations, (t1/2 = 24 s) its was half-life only 3 3 3 (1.5t1/2 times= 24 longer s) was than only that 1.5 times of 1c longer. The bimolecular than that of rate1c. constants The bimolecular of the decay rate constants of 1b by oftriplet the decaycarbene of quenchers31b by triplet were carbene also only quenchers moderately were smaller also only than moderately those of 31c smaller. Although than thosean iodine of 31c group. Although shielded an theiodine carbene group center shielded more the effectivel carbene centery than morea bromine effectively group, than the a brominehalf-occupation group, theof an half-occupation iodine atom ofat thean iodinefour ortho atom positions at the four of aortho diphenylcarbenepositions of a was diphenylcarbene insufficient to was protect insufﬁcient the carbene to protect center. the One carbene may expectcenter. Onethat mayan extreme expect thatstabilization an extreme would stabilization be achieved would by be thorough achieved bysubstitution thorough substitution of the four ofortho the positionsfour ortho positionswith iodo with groups. iodo groups.In the Inpresent the present study, study, we were we were successful successful in inpreparing preparing a aprecursor precursor diphenyldiazomethane with four ortho-iodo groups, bis(4- tert-butyl-2,6-diiodophenyl)diazomethane 3 (1a-N2).). Here Here we we report that triplet diphenylcarbene 1a1a generated by the photolysis of 1a-N2,, is an unusually persistent triplet diphenylcarbene with an extended half-life of 18.4 min.min.

I N2 I I I hν

–N2 I I I I

1a-N2 1a

Br N2 I Br I hν

–N2 I I Br Br

1b-N2 1b

Br N2 Br Br Br

hν

Br –N2 Br Br Br 1c-N2 1c Scheme 1. Generation of diarylcarbenes 1.

2. Results Results and and Discussion Discussion

2.1. Preparation of Precursor Diazomethane Most diphenyldiazomethanes that that bear four kinetic protectors at the ortho positions have been prepared from the corresponding diphenylmethanolsdiphenylmethanols via via the carbamate derivatives derivatives [10,15,16,19]. [10,15,16,19]. For the synthesis of 1a-N2 (Scheme(Scheme 22),), thethe startingstarting alcoholalcohol waswas expectedexpected toto bebe obtainedobtained byby aa couplingcoupling reaction of of a a 2,6-diiodobenzaldehyde 2,6-diiodobenzaldehyde with with a 2,6-d a 2,6-diiodophenyllithium.iiodophenyllithium. According According to the to synthesis the synthesis plan, plan,we attempted we attempted the lithiation the lithiation of 5- oftert 5-tert-butyl-1,2,3-triiodobenzene-butyl-1,2,3-triiodobenzene (3), ( 3of), ofwhich which the the terttert-butyl-butyl group affords a para-substituent in 1a.A. A para-tert-butyl group is known to serve serve as as a a good good protecting protecting group in diphenylcarbenesdiphenylcarbenes [15,16]. [15,16]. Fortun Fortunately,ately, the the lithiation proceeded proceeded exclusively exclusively at 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,

Molecules 2016, 21, 1545 3 of 15 triiodobenzene to selectively afford the corresponding diphenylmethanol 4 by the subsequent reaction withMolecules 4-tert 2016-butyl-2,6-diiodobenzaldehyde, 21, 1545 [19]. The success of the synthesis of alcohol 4 opened the3 wayof 15 to diazomethane 1a-N2. Alcohol 4 was chlorinated, substituted with ethyl carbamate, nitrosated, and andtreated treated with potassiumwith potassiumtert-butoxide tert-butoxide to give theto give crude the diazomethane. crude diazomethane. The puriﬁcation The purification by preparative by preparativethin layer chromatography thin layer chromatography (alumina, hexane (alumina, as eluent) hexane afforded as eluent) pure afforded1a-N2 as pure yellow 1a-N crystals2 as yellow in a crystals33% yield. in Diazoa 33% compoundyield. Diazo1a -Ncompound2 was very 1a stable-N2 was for very a diazomethane stable for a anddiazomethane did not decompose and did innot a decomposerefrigerator in (at a− refrigerator20 ◦C) over (at an − extended20 °C) over period an extended of time. period of time.

I I I OH I Cl I I I I I2 1. BuLi SOCl2 O H F-TEDA-BF4 2. I I MeCN I I I I 3 4 5 60% 45% >99%

H CO Et ON CO Et I N 2 I N 2 I I I N2 I t H2NCO2Et N2O4 BuOK AgBF AcONa THF 4 I I I 1,4-dioxane I CCl4 I I

671a-N2 50% >99% 33%

Scheme 2. Synthesis of bis(2,6-diiodo-4- tert-butylphenyl)diazomethane ((1a-N-N22).

2.2. Spectroscopic Studies

2.2.1. ESR Studies in Rigid Matrix at Low Temperature Most sterically hindered diphenylcarbenes are are kn knownown to give rise to stable triplet ESR signals by irradiation of of the the correspond correspondinging diphenyldiazomethanes diphenyldiazomethanes in in 2-me 2-methyltetrahydrofuranthyltetrahydrofuran (2-MTHF) (2-MTHF) at 77at K 77 [10,15,16,20–24]. K [10,15,16,20– 24On]. the On other the hand, other the hand, signals the of signals triplet ofcarbene triplet 31b carbene with two31b iodinewith atoms two iodine were invisibleatoms were under invisible the same under conditions the same [19]. Similar conditions invisi [19bilities]. Similar were observed invisibilities in the were ESR measurements observed in the of 2-iodophenylnitreneESR measurements of and 2-iodophenylnitrene (2-iodophenyl)(phenyl)carbene, and (2-iodophenyl)(phenyl)carbene, and were explained by andextreme were broadening explained byof theextreme signal broadening due to orbital ofthe degeneracy signal due and to orbitalstrong spin-o degeneracyrbit coupling and strong in the spin-orbit iodine atoms coupling [25]. inAs the expected iodine fromatoms these [25]. facts, As expected photolysis from (λ > these350 nm) facts, of photolysisdiazomethane (λ >1a 350-N2 nm)in a oflow diazomethane temperature 2-MTHF1a-N2 in matrix a low affordedtemperature no triplet 2-MTHF signals matrix due afforded to carbene no triplet 1a (Figure signals 1a). due The to carbene influence1a of(Figure the iodine1a). The atoms inﬂuence on ESR of linethe iodinebroadening atoms may on ESR be larger line broadening than that in may 1b owing be larger to the than presence that in 1b of owingfour iodo to thegroups. presence of four iodoWe groups. previously reported that the ESR signals of carbene 1c with four ortho-bromo groups were observedWe previously in a 2-MTHF reported matrix that at 77 the K ESRand signalsdisappear ofed carbene irreversibly1c with by four warmingortho-bromo the matrix groups to 170 were K [26].observed On the in other a 2-MTHF hand, matrix triplet at carbene 77 K and 31b disappeared was invisible irreversibly under the same by warming conditions, the matrix and a new to 170 doublet K [26]. signalOn the at other 327 mT hand, due triplet to carbon carbene radical31b specieswas invisible appeared under by thewarming same conditions,up to 160 K, and which a new implies doublet the decompositionsignal at 327 mT of due31b to[19]. carbon This means radical that species the triplet appeared carbene by warming 31b abstracts up to a 160hydrogen K, which from implies 2-MTHF the ofdecomposition the matrix to ofform31b the[19 correspondin]. This meansg thatdiphenylmethyl the triplet carbene radical.31b If carbeneabstracts 1a a decays hydrogen by a from similar 2-MTHF form of thedegradation, matrix to forma doublet the corresponding signal of the corresponding diphenylmethyl radical radical. species If carbene would1a appeardecays when by a the similar matrix form is warmedof degradation, up. To a confirm doublet signalthis assumption, of the corresponding ESR measurements radical species were would carried appear out after when warming the matrix the is warmedmatrix involving up. To conﬁrm the photoproduct this assumption,s and ESRthen measurements recooling to 77 were K to carried compensate out after for warming the weakening the matrix of signalsinvolving due the to photoproductsthe Curie law [27–29]. and then Although recooling gradual to 77 K warming to compensate of the for photolyzed the weakening sample of up signals to 190 due K gaveto the no Curie ESR law signals, [27– 29a new]. Although doublet gradual signal appeared warming at of 327 the mT photolyzed at 200 K sample(Figure up1b). to The 190 signal K gave intensity no ESR increasedsignals, a newgradually doublet as signalthe sample appeared was atwarmed 327 mT up at 200to 230 K (Figure K, and1 b).decreased The signal only intensity slightly increased at 300 K (Figuregradually 1c–e). asthe The sample signal wasdisappeared warmed when up to 230the K,2-MT andHF decreased solution onlywas exposed slightly atto 300air (Figure K (Figure 1f).1c–e). The Theappearance signal disappearedof the doublet when signal the can 2-MTHF be attributed solution to the was formation exposed of to bis(4- air (Figuretert-butyl-2,6-diiodophenyl)1f). The appearance methyl radical via hydrogen abstraction. We used the temperature (Td) 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 Td, 31a (Td = 200 K) is considered much more resistant to degradation than 1b and 1c (Td = 170 and 160 K, respectively). We previously reported that diphenylcarbenes protected by two bromine and two trifluoromethyl groups, 1d–f, are known to be

Molecules 2016, 21, 1545 4 of 15 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 (Td) at which signals due to a triplet carbene disappear as a measure of the thermal stability of triplet carbenes [9]. If 31a is assumed 3 to abstract a hydrogen from the solvent molecule at Td, 1a (Td = 200 K) is considered much more resistant to degradation than 1b and 1c (Td = 170 and 160 K, respectively). We previously reported that Molecules 2016, 21, 1545 4 of 15 diphenylcarbenesMolecules 2016, 21, protected1545 by two bromine and two triﬂuoromethyl groups, 1d–f, are known4 of 15 to be the unusuallythe unusually persistent persistent diphenylcarbenes diphenylcarbenes with with TTddss of of 210–260 210–260 K (Scheme K (Scheme 3) [20,21].3)[ 20 The,21 ].present The present result result the unusually persistent diphenylcarbenes with Tds of 210–260 K (Scheme 3) [20,21]. The present result indicatesindicates that carbene that carbene1a is 1a also is also an an unusually unusually persistentpersistent diphenylcarbene diphenylcarbene with with a stability a stability comparable comparable indicates that carbene 1a is also an unusually persistent diphenylcarbene with a stability comparable to 1e. to 1e. to 1e.

(a) (a) (b) (b) (c) (c)

(d) (d)

(e) (e) (f)

(f) 0 200 400 600 800 Magnetic Field / mT 0 200 400 600 800 Figure 1. (a) ESR spectrum obtained by irradiation (λ > 350 nm) of diazo compound 1a-N2 in Figure 1. (a) ESR spectrum obtainedMagnetic by irradiation Field / (mTλ > 350 nm) of diazo compound 1a-N2 in 2-methyltetrahydrofuran at 77 K; (b–e) The same sample measured at 77 K after thawing to 200 K (b), 2-methyltetrahydrofuran at 77 K; (b–e) The same sample measured at 77 K after thawing to 200 K (b), Figure220 K 1.(c ),( a230) ESR K (d spectrum), and 300 Kobtained (e); (f) The by same irradiation sample measured(λ > 350 atnm) 77 Kof after diazo exposure compound to air. 1a-N2 in 220 K2-methyltetrahydrofuran (c), 230 K (d), and 300 at K 77 (e );K; ( f()b– Thee) The same same sample sample measured measured atat 77 K K after after thawing exposure to 200 to air.K (b), CF3 CF3 CF3 220 K (c), 230 K (d), and 300 K CF(e);3 (f) The same sampleBr measured at 77 K after exposureBr to air.

CF3 CF CF3 Br CF3 Br Ph Br 3 Ph Br Ph Br Ph Br CF3 CF3 1d 1e 1f Ph Br Ph Br Ph Br Ph Br CF3 CF3 Td (K) 260 210 240 1d 1e 1f t1/2 (min) 40 16 11 SchemeT 3.d (K)Diphenylcarbenes260 1d–f protected by two 210 ortho-trifluoromethyl and two 240 ortho-bromo groups. t1/2 (min) 40 16 11 2.2.2. Magnetic-Susceptibility Measurements Scheme 3. Diphenylcarbenes 1d–f protected by two ortho-trifluoromethyl and two ortho-bromo groups. SchemeTo 3. obtainDiphenylcarbenes evidence of the1d –groundf protected triplet by spin two stateortho of-triﬂuoromethyl carbene 1a, the and magnetic two ortho susceptibility-bromo groups. of 2.2.2.the Magnetic-Susceptibilityphotoproduct of diazomethane Measurements 1a-N2 was measured. A 2-MTHF solution of 1a-N2 (2.0 mM) was 2.2.2. Magnetic-Susceptibilityplaced inside the sample compartment Measurements of a superconducting quantum interference device (SQUID) magnet/susceptometerTo obtain evidence andof the was ground irradiated triplet at 10 spin K wi stateth a laserof carbene light at 1a442, the nm. magnetic The development susceptibility upon of Tothemagnetization obtainphotoproduct evidence was of measureddiazomethane of the ground at 5 K 1a in triplet-N a constant2 was spin measured. field state of 5000 ofA carbene2-MTHFOe. The magnetization 1asolution, the magnetic of 1a values-N2 (2.0 susceptibilitygradually mM) was of the photoproductplacedincreased inside with the of the diazomethanesample irradiation compartment time1a and-N2 ofreachedwas a supe measured. a rconductingplateau after A 2-MTHF 12quantum h. After solutioninterference a plateau of was1a device -Nestablished,2 (2.0 (SQUID) mM) was placedmagnet/susceptometer insidethe magnetization the sample values, compartmentand Mwasa, were irradiated measured of aat superconducting 10 at K5.0 wi Kth in aa laserrange light of quantum 0–50,000 at 442 Oe.nm. interference The The net development magnetization device upon (SQUID) magnet/susceptometermagnetizationgenerated by was the measured irradiation and was at 5of irradiated K 1a in-N a 2constant was at estimated 10 field Kwith of 5000from a laser Oe.the The difference light magnetization at 442between nm. values M Thea and gradually development the uponincreased magnetizationcorresponding with the magnetization was irradiation measured time values, atand 5M reached Kb, inobtained a constant a plateau before ﬁeldirradiation.after of12 5000h. AfterThe Oe. plot a plateau The of the magnetization magnetizationwas established, values (M) versus the magnetic field (H) was analyzed in terms of the Brillouin function as follows [30–35]: graduallythe magnetization increased with values, the M irradiationa, were measured time at and 5.0 K reached in a range aplateau of 0–50,000 after Oe. 12 The h. net After magnetization a plateau was generated by the irradiation of 1aM-N = M2 awas − M bestimated = FNgJμBB (fromχ) the difference between Ma and(1) the established, the magnetization values, Ma, were measured at 5.0 K in a range of 0–50,000 Oe. The net corresponding magnetization values, Mb, obtained before irradiation. The plot of the magnetization where F is the generation factor for carbene estimated from saturated magnetization, N is the number of magnetization(M) versus the generated magnetic by field the (H irradiation) was analyzed of in1a terms-N2 was of the estimated Brillouin function from the as differencefollows [30–35]: between the molecule, g is the Landé g-factor, J is the quantum number for the total angular momentum, and M = Ma − Mb = FNgJμBB(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

Molecules 2016, 21, 1545 5 of 15

Ma andMolecules the corresponding 2016, 21, 1545 magnetization values, Mb, obtained before irradiation. The5 of plot 15 of the magnetization (M) versus the magnetic ﬁeld (H) was analyzed in terms of the Brillouin function as followsμ [B30 is– the35 ]: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 M = Ma − M = FNgJµ B(χ) (1) plot is shown in Figure S1. The observed data wasb best fitted toB Equation (1) with S = 0.93 (F = 0.35) at where F5.0is K. the Although generation this S factorvalue is for somewhat carbene smaller estimated than the from theoretical saturated value magnetization, (S = 1) for a tripletN carbene,is the number of the molecule,this result gclearlyis the demonstrates Landé g-factor, the generationJ is the quantum of a triplet number species, fori.e., thecarbene total 1a angular, by the photolysis momentum, and of 1a-N2. µB is the Bohr magneton. Since diphenylcarbene is composed of light elements, the orbital angular momentum2.2.3. UV-Vis should Studies be negligible, in Rigid Matrix and J canat Low be Temperature 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 Photolysis (λ > 350 nm) of 1a-N2 in a 2-MTHF matrix at 77 K afforded new absorption bands, 5.0 K. Althoughstrong maxima this atS value350 and is 369 somewhat nm and broad smaller and weak than bands the theoretical at 462, 480 and value 506 ( Snm,= 1)at the for expense a triplet of carbene, this resultthe precursor clearly demonstrates (Figure 2A(a,b)). the Most generation of the sterica of ally triplet hindered species, triplet i.e.,diphenylcarbenes carbene 1a, in by 2-MTHF the photolysis of 1a-Nmatrix2. 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 2.2.3. UV-Vismatrix was Studies warmed in Rigidup to room Matrix temperature. at Low 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]. PhotolysisThe thermal (λ > 350 stability nm) ofof triplet1a-N2 carbenein a 2-MTHF 31a was then matrix estimated at 77 Kby afforded warming newthe matrix. absorption When bands, strong maximawarmed to at 100 350 K, and the absorption 369 nm and maxima broad of and the bands weak due bands to 31a at shifted 462, 480 from and 350 506 and nm, 369 atnm the to 354 expense of the precursorand 371 (Figurenm, respectively2A(a,b)). (Figure Most 2A(c)). of the The sterically spectral hinderedchanges can triplet be attributed diphenylcarbenes to a decrease in in the 2-MTHF matrix areviscosity known of the to show2-MTHF strong matrix, absorption because similar bands chan in theges in UV other region sterically (320–360 hindered nm) diarylcarbenes and broad and weak are interpreted in terms of geometrical changes in the molecules due to a sudden decrease in the bands in the visible region (430–510 nm) [9]. The absorption bands disappeared irreversibly when the viscosity of 2-MTHF from 107 to 103 P [36]. The bands of 31a started to decrease slowly at 180 K and matrix wasdisappeared warmed irreversibly up to room at 240 temperature. K (Figure 2B). We The assigned temperature the at species which havingthe bands these of 31a absorption started to bands to tripletdecrease carbene was3 1aroughlyon the in basisaccordance of similarity with the emergence in the spectrum of the ESR and signal in thermal of the radical. behavior of 31b [19].

A B 369 350 371 0.4 354 0.4

(a) Before irradiation 100 K (b) After irradiation 180 (c) 100 K 200 240 260 300 0.2 0.2 Absorbance Absorbance

0.0 0.0 300 400 500 600 300 400 500 600 Wavelength / nm Wavelength / nm

Figure 2. UV-vis spectra obtained by irradiation (λ > 350 nm) of diazo compound 1a-N2 in Figure 2. UV-vis spectra obtained by irradiation (λ > 350 nm) of diazo compound 1a-N2 in 2-methyltetrahydrofuran at 77 K: (A) (a) Spectrum of 1a-N2, (b) The same sample after irradiation, (c) 2-methyltetrahydrofuran at 77 K: (A) (a) Spectrum of 1a-N2, (b) The same sample after irradiation, The same sample after thawing to 100 K; (B) UV-vis spectral change measured upon thawing the (c) The same sample after thawing to 100 K; (B) UV-vis spectral change measured upon thawing the same sample from 100 to 300 K. same sample from 100 to 300 K. Diphenylcarbenes with no substituents at the ortho positions in a 2-MTHF matrix disappear 3 Thebetween thermal 100 and stability 105 K [7]. of Carbene triplet 1b carbene is protected31a bywas two bromo then and estimated two iodo bygroups, warming but it started the matrix. to decay at 150 K in 2-MTHF [19]. The even higher decay temperature of 1a clearly demonstrates that it When warmed to 100 K, the absorption maxima of the bands due to 31a shifted from 350 and 369 nm to is much more stable than 31b. 354 and 371 nm, respectively (Figure2A(c)). The spectral changes can be attributed to a decrease in the viscosity2.2.4. of theLaser 2-MTHF Flash Photolysis matrix, Studies because in similarSolution changes at Room inTemperature other sterically hindered diarylcarbenes are interpretedFor in termsquantitative of geometrical evaluation changesof the stability in the of moleculescarbene 1a, the due lifetime to a sudden measurement decrease was in carried the viscosity 7 3 3 of 2-MTHFout in from degassed 10 to benzene 10 P[ at36 25]. °C, The which bands we of ordinari1a startedly use tofor decreasethe lifetime slowly measurement at 180 K of and sterically disappeared irreversibly at 240 K (Figure2B). 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 Molecules 2016, 21, 1545 6 of 15 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 [6–9]. The lifetime of 1a was too long to monitor by the laser ﬂash photolysis

(LFP) apparatusMolecules 2016 that, 21, 1545 we routinely use. Thus, the lifetime was estimated using a conventional6 of 15 UV-vis spectroscopic method [20,21,37,38]. congested diarylcarbenes [6–9]. The lifetime of 1a was too long to monitor by the laser◦ flash photolysis Laser pulse photolysis at 308 nm of 1a-N2 in degassed benzene at 25 C produced a transient (LFP) apparatus that we routinely use. Thus, the lifetime was estimated using a conventional UV-vis species showing maxima at 366, 480, and 506 nm, which appeared coincidentally with the laser pulse spectroscopic method [20,21,37,38]. (Figure3). These absorption bands were essentially similar to those observed in the photolysis of Laser pulse photolysis at 308 nm of 1a-N2 in degassed benzene at 25 °C produced a transient 1a-N2 inspecies 2-MTHF showing at 77maxima K, although at 366, 480, the and maxima 506 nm, which were appeared slightly coincidentally shifted. The with transient the laser pulse bands were assigned(Figure to triplet 3). These31a, absorption since the bandsbands were were essentially quickly quenchedsimilar to those by oxygen observed to in give the photolysis the corresponding of diphenyl1a-N ketone2 in 2-MTHF oxide at ( vide77 K, infraalthough). The the absorptionmaxima were bands slightly decayed shifted. The very transient slowly bands but were did assigned not disappear 3 completelyto triplet even 1a after, since 1.5 the h. bands The decay were quickly was found quenched to be by a second-orderoxygen to give processthe corresponding (2k/εl = 5.5diphenyl× 10− 3·s−1) ketone oxide (vide infra). The absorption bands decayed very slowly but did not disappear completely with a half-life of 18.4 min (Figure3 inset, Table1). The kinetic data indicates that 31a is extremely 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 3 −1 3 −1 persistenthalf-life compared of 18.4 min with (Figure that 3 ofinset,1b Table(2k/ 1).εl The= 0.17 kinetic s data, t1/2 indicates= 24 that s) [ 1931a] is and extremely1c (2 persistentk/εl = 0.35 s , t1/2 = 16compared s) [15,16 with]. that of 31b (2k/εl = 0.17 s−1, t1/2 = 24 s) [19] and 31c (2k/εl = 0.35 s−1, t1/2 = 16 s) [15,16].

0.40

0.4 0.35

0.30 Absorbance 0.3 0.25 0 20 40 60 80 0 min Time / min 2 5

Absorbance 0.2 10 30 50 90 0.1

350 400 450 500 550 Wavelength / nm

Figure 3. Absorption of transient products formed during laser irradiation (λ = 308 nm) of 1a-N2 in Figure 3. Absorption of transient products formed during laser irradiation (λ = 308 nm) of 1a-N2 in degassed benzene at 25 °C recorded from immediately after irradiation to 90 min after irradiation. degassed benzene at 25 ◦C recorded from immediately after irradiation to 90 min after irradiation. The inset shows the time course of the absorption monitored at 366 nm. The inset shows the time course of the absorption monitored at 366 nm. Table 1. Kinetic Data of Carbenes 1.

−1 Table 1. Kinetic Data of Carbenes 1. Carbenes 2k/εl (s ) 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 −1 −1 −1 −1 −1 1 −2 −1 Carbenes 1b2k/ εl (s ) 0.17 t1/2 (s) 2.4 × 10 kO2 (M 7.3 ×·s 106 ) kCHD 3.9 ×(M 10 ·s ) Kk 0.30MeOH (M ·s ) 1a 0.00551c 0.33 min−0.351 1.1 × 103 18.4 1.6 min × 10 6.5 × 2.110 × 5107 5.3 × 101.52 0.42 0.033 6 1b 0.17 2.4 × 10 1 K = k7.3TS/k×ST. 10 3.9 × 10 0.30 1c 0.35 1.6 × 10 2.1 × 107 5.3 × 102 0.42 Since triplet carbenes are easily trapped with oxygen or 1,4-cyclohexadiene (CHD) [39], these 1 K k k trapping reactions can be employed as a quantitative= TS/ scaleST. 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 Sincewas tripletclearly different carbenes from are the easily band trappedof 1a (Figure with S2A(a)). oxygen In addition, or 1,4-cyclohexadiene the solution after the (CHD) LFP was [39 ], these trappingfound reactions to contain can the be corresponding employed as benzophenone a quantitative 1a scale-O as ofa major the reactivities product. It is of well the documented triplet carbenes. that the oxygen trapping of diphenylcarbenes generates the corresponding◦ benzophenone oxides with The LFP (λ = 308 nm) of 1a-N2 in O2 saturated benzene at 25 C resulted in a rapid decay of a broad absorption band at around 390–450 nm to afford the ketone derivative (Scheme 4) [40–47]. carbene 31a and a concurrent appearance of a broad absorption band at around 415 nm (Figure S2A(b)), Thus, our observations suggested that carbene 31a was rapidly trapped with oxygen to form the which was clearly different from the band of 1a (Figure S2A(a)). In addition, the solution after the LFP ketone oxide 1a-O2 like most of diphenylcarbenes. The rate constant (kO2) for the trapping of 31a with was foundoxygen to contain was estimated the corresponding to be 6.5 × 105 M benzophenone−1·s−1 based on a plot1a-O ofas the a observed major product. growth rate It is of well 1a-O documented2 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 (kO2 = 7.3 × 106 M−1·s−1) [19] and 30 times smaller than that of 31c (kO2 = 2.1 × 107 M−1·s−1) [16].

Molecules 2016, 21, 1545 7 of 15 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 (Scheme4)[ 40–47]. Thus, our observations suggested that carbene 31a was rapidly trapped with oxygen to form the ketone 3 oxide 1a-O2 like most of diphenylcarbenes. The rate constant (kO2) for the trapping of 1a with oxygen 5 −1 −1 was estimated to be 6.5 × 10 M ·s based on a plot of the observed growth rate of 1a-O2 as a function of the oxygen concentration (Figure S2B, Table1). The rate constant is one order of magnitude 3 6 −1 −1 smaller than that observed with 1b (kO2 = 7.3 × 10 M ·s )[19] and 30 times smaller than that of 3 7 −1 −1 1c (kO2Molecules= 2.1 × 201610, 21,M 1545 ·s )[16]. 7 of 15

O N2 O O Ar Ar' Ar Ar' Ar Ar'

1-N2 1-O2 1-O ν h –N2 kO2 O2

kST Ar Ar' Ar Ar' (K = kTS/kST) kTS 11 31

kMeOH MeOH kCHD

OMe

Ar Ar' Ar Ar' Ar Ar' 1-HOMe 1-H 1-H 2

Scheme 4. Photolysis of diazo compounds 1-N2 in the presence of carbene quenchers. Scheme 4. Photolysis of diazo compounds 1-N2 in the presence of carbene quenchers.

The LFP (λ = 308 nm) of 1a-N2 in degassed benzene containing 1,4-cyclohexadiene gave a new Thespecies LFP (withλ = 308an absorption nm) of 1a band-N2 inat degassed380 nm. The benzene diphenylmethyl containing radicals, 1,4-cyclohexadiene formed as a result gaveof a a new specieshydrogen with an abstraction absorption of bandthe carbenes at 380 1b nm.,c from The 1,4-cyclohexadiene, diphenylmethyl showed radicals, absorption formed maxima as a result at of a hydrogenwavelengths abstraction 20–30 of nm the longer carbenes than that1b ,ofc fromthe precursor 1,4-cyclohexadiene, carbene bands (ca. showed 350 nm) absorption[16,19]. Thus, maximathis at species, generated from 1a-N2, was also assigned to the corresponding diphenylmethyl radical (1a-H) wavelengths 20–30 nm longer than that of the precursor carbene bands (ca. 350 nm) [16,19]. Thus, (Scheme 4). The spent solution contained the diphenylmethane 1a-H2 as the main product. Owing to this species,the overlap generated between from the absorption1a-N2, was band also of the assigned radical and to the the correspondingtail of the carbene, diphenylmethyl the growth curves radical (1a-H) (Schemeof the radical4). were The not spent observed solution clearly contained (Figure S3A). the However, diphenylmethane a linear plot was1a obtained-H2 as thebetween main the product. Owing toobserved the overlap pseudo-first-order between the rate absorption constants of bandthe decay of the of the radical carbene and monitored the tail ofat 350 the nm carbene, and [CHD] the growth curves of(Figure the radicalS3B), and were its slope not gave observed the rate constant clearly (k (FigureCHD) for the S3A). reaction However, of 31a with a linearthe diene, plot 1.5 wasM−1·s− obtained1, which is 1 and 2 orders of magnitude smaller than those observed with triplet diiododibromo 31b between the observed pseudo-ﬁrst-order rate constants of the decay of the carbene monitored at (39 M−1·s−1) [19] and tetrabromo 31c (5.3 × 102 M−1·s−1) [15,16], respectively (Table 1). 350 nm and [CHD] (Figure S3B), and its slope gave the rate constant (k ) for the reaction of 31a with The LFP of 1a-N2 in a degassed benzene–methanol mixed solvent alsoCHD formed triplet carbene 31a. −1 −1 the diene,The 1.5 observed M · sdecay, which rate constant is 1 and (kobs 2) of orders 31a increased of magnitude as the fraction smaller of methanol than those increased. observed However, with triplet 3 −1 −1 3 2 −1 −1 diiododibromothe plot of k1bobs vs(39 methanol M ·s concentration)[19] and was tetrabromo not linear in1c the(5.3 concentration× 10 M range·s of)[ 2.4715,16 to], 9.88 respectively M. (Table1).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 3 The LFP of 1a-N2 in a degassed benzene–methanol mixed solvent also formed triplet carbene 1a. 2 [19,48]. Carbene 1a also showed a more3 linear plot when kobs was plotted against [MeOH] , and the The observed decay rate constant (kobs) of 1a increased as the fraction of methanol increased. However, trapping rate constant (KkMeOH) was 0.033 M−2·s−1 as estimated from the slope of this plot (Figure S4, the plotTable of kobs 1). vsThis methanol is one order concentration of magnitude wassmaller not than linear the inrate the constants concentration observed range with 31b of 2.47and 3 to1c 9.88 M. The trapping(0.30 and of 0.42 some M−2 sterically·s−1, respectively hindered [19]). diarylcarbenes with methanol have been reported to show better linearitiesThe insertion when the of plotcarbenes was to performed the O-H bond against of alco thehols square to form of ethers the methanol is usually concentration considered to [19,48]. occur from the singlet states of these species (Scheme 4). The decrease in KkMeOH may be 2caused either Carbene 1a also showed a more linear plot when kobs was plotted against [MeOH] , and the trapping by an increased energy gap between−2 −the1 ground triplet and singlet states or by the deceleration of rate constant (KkMeOH) was 0.033 M ·s as estimated from the slope of this plot (Figure S4, Table1). the O-H insertion. Since theoretical studies have suggested that the energy gap for 1a is only slightly This is one order of magnitude smaller than the rate constants observed with 31b and 31c (0.30 and larger than those for 1b and 1c (vide infra), the difference in the rate constants (KkMeOH) is mainly −2 −1 0.42 M controlled·s , respectively by the rate (k [MeOH19]).) of the insertion reaction of singlet carbenes to methanol, rather than by the Thesinglet-triplet insertion ofequilibrium carbenes constant to the (K O-H). Thus, bond the ofortho alcohols-iodine groups to form have ethers a significant is usually rate-retarding considered to 3 occur fromeffect theon the singlet insertion states reaction of these of the species singlet (Schemecarbenes. Thus,4). The carbene decrease 1a showed in Kk aMeOH highermay stability be caused either byestimated an increased in degassed energy benzene gap and between in degassed the ground 2-MTHF triplet and lower and reac singlettivities states toward or triplet by the carbene deceleration quenchers (oxygen and 1,4-cyclohexadiene) and a singlet carbene quencher (methanol) than the bromine-substituted carbenes 31b and 31c.

Molecules 2016, 21, 1545 8 of 15 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 (KkMeOH) is mainly controlled by the rate (kMeOH) 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

AlthoughMolecules 2016 steric, 21, 1545 protection is an efﬁcient method for stabilization of the triplet diphenylcarbenes,8 of 15 complete protection of triplet carbenes has not been achieved, because even such carbenes decay mainly2.3. Product by Analysis dimerization. Studies When long-lived diphenylcarbenes are generated in degassed benzene inAlthough the absence steric ofprotection carbene is an quenchers efficient method at room for stabilization temperature, of the most tripletare diphenylcarbenes, known to dimerize at thecomplete carbenic protection center of to triplet form carbenes the corresponding has not been achieved, tetraphenylethenes because even such as carbenes the main decay product. For example,mainly dimesitylcarbeneby dimerization. When and long-lived bis(2,4,6-trichlorophenyl)carbene diphenylcarbenes are generated produced in degassed tetramesitylethene benzene in the [10] absence of carbene quenchers at room temperature, most are known to dimerize at the carbenic center and tetrakis(2,4,6-trichlorophenyl)ethene [22] in high yields, respectively. On the other hand, to form the corresponding tetraphenylethenes as the main product. For example, dimesitylcarbene and the dimerizationbis(2,4,6-trichlorophenyl)carbene product from bis(2,6-dibromophenyl)carbenes produced tetramesitylethene [10] and underwent tetrakis(2,4,6-trichlorophenyl) photocyclization to give the correspondingethene [22] in phenanthrene high yields, respectively. derivatives On [15 the,16 ].other By hand, the same the dimerization token, carbene product2b affordedfrom the correspondingbis(2,6-dibromophenyl)carbenes phenanthrene derivative underwent with photocycliz the concomitantation to give elimination the corresponding of an iodine phenanthrene molecule [19]. derivatives [15,16]. By the same token, carbene 2b afforded the corresponding phenanthrene derivative Photolysis (λ > 350 nm) of 1a-N2 (5.0 mg) in degassed benzene-d6 (0.5 mL) in a sealed NMR tube at 25 ◦Cwith was the monitored concomitant byelimination1H-NMR of an spectroscopy. iodine molecule After [19]. 50 min irradiation, very weak 1H-NMR Photolysis (λ > 350 nm) of 1a-N2 (5.0 mg) in degassed benzene-d6 (0.5 mL) in a sealed NMR tube at signals at 9.03, 8.82, 7.84, 1.23, and 1.09 ppm, along with weak and broad peaks were observed at 25 °C was monitored by 1H-NMR spectroscopy. After 50 min irradiation, very weak 1H-NMR signals 1 the expenseat 9.03, of 8.82, the 7.84, signals 1.23, ofand1a 1.09-N 2ppm,. The alongH-NMR with weak spectrum and broad of peaks a fraction were observed separated at the by expense a recycled gel permeationof the chromatographysignals of 1a-N2. The (GPC) 1H-NMR of thespectrum resultant of a fraction mixture separated showed by the a recycled phenanthrene gel permeation derivative 2a (a brownchromatography oil in a 31% (GPC) yield) of (Scheme the resultant5). Duringmixture showed the photolysis, the phenanthrene it is reasonable derivative 2a that (a brown2a was oil formed as a resultin a of31% dimerization yield) (Scheme at 5). the During carbene the photolysis, center followed it is reasonable by photocyclization that 2a was formed and as deiodination. a result of The dimerization at the carbene center followed by photocyclization and deiodination. The 1H-NMR 1H-NMR spectra of other GPC fractions showed no detectable peaks. The yield of 2a was higher than spectra of other GPC fractions showed no detectable peaks. The yield of 2a was higher than that of that of 2b2b (18%) but but lower lower than than that that of 2c of (48%).2c (48%). The Thecarbene carbene dimer dimerintermediate intermediate may have may difficulty have in difﬁculty in formingforming from from1a 1acompared compared to to fromfrom 1c1c duedue to to the the steric steric repulsion repulsion between between the bulky the iodo bulky groups. iodo groups.

hν 1a 1a-N2 1a benzene –N2

I I I I I I

hν hν I I I I I I I I I I –I2

I I I I I I 2a 31%

Scheme 5. Photolysis of diazo compound 1a-N2 in degassed benzene-d6 at 25 °C. ◦ Scheme 5. Photolysis of diazo compound 1a-N2 in degassed benzene-d6 at 25 C.

Photolysis (λ > 350 nm) of 1a-N2 in an O2-saturated benzene solution afforded Photolysistetraiodobenzophenone (λ > 350 1a nm)-O as the of sole1a -Nproduct2 in in ana 60% O yield.2-saturated On the other benzene hand, irradiation solution (λ > afforded tetraiodobenzophenone350 nm) of 1a-N2 in degassed1a-O as benzene the sole in productthe presence in aof 60%1,4-cyclohexadiene yield. On thegave other diphenylmethane hand, irradiation 1a-H2 in a 54% yield. Similar irradiation in degassed benzene–methanol afforded methyl ether (λ > 350 nm) of 1a-N2 in degassed benzene in the presence of 1,4-cyclohexadiene gave diphenylmethane 1a-HOMe (52%) (Scheme 6). We previously showed that the irradiation of 1b-N2 and 1c-N2 in the 1a-H in a 54% yield. Similar irradiation in degassed benzene–methanol afforded methyl ether 2 presence of the carbene quenchers afforded ketones 1-O, diphenylmethanes 1-H2, and methyl ethers 1a-HOMe1-HOMe (52%) in (Schemegood yields6). [16,19]. We previously Thus, the lifetimes showed of carbenes that the 1 irradiationhardly affected of the1b product-N2 and yields.1c-N 2 in the

Molecules 2016, 21, 1545 9 of 15

presence of the carbene quenchers afforded ketones 1-O, diphenylmethanes 1-H2, and methyl ethers

1-HOMeMolecules in good 2016, yields 21, 1545 [16,19]. Thus, the lifetimes of carbenes 1 hardly affected the product9 of 15 yields.

X O Y O2 Molecules 2016, 21, 1545 9 of 15 benzene Y X X 1O-O Y O a: X = Y=2 I 60% b: Xbenzene = Br, Y = Ia 77% b Y c: X = Y = Br 66%X X 1-O Y X a: X = Y= I 60% N2 Y hν b: X = Br, Y = Ia 77% c: X = Y = Brb 66% –N2 benzene Y Y X X Y X X 1-H N2 Y 2 1-N2 ν h a: X = Y= I 54% b: X = Br, Y = Ia –N2 benzene 75%Y Y c: X = Y = Brb X59% X 1-H2 1-N2 a: X = Y= I X 54%OMe Y b: X = Br, Y = Ia 75% MeOH c: X = Y = Brb 59% benzene Y X XOMe 1-HOMeY a: XMeOH = Y= I 52% b: X = Br, Y = Ia 48% benzene Y b X48% a b c: X = Y = Br ref. [19]. ref. [16]. 1-HOMe a: X = Y= I 52% 2 Scheme 6. Photoproducts obtained by irradiation of adiazo compound 1-N at 25 °C. ◦ Scheme 6. Photoproducts obtained by irradiationb: X = Br, Y = ofI diazo compound48% 1-N2 at 25 C. c: X = Y = Brb 48% 2.4. DFT Calculationsa ref. [19]. b ref. [16]. 2.4. DFT Calculations To obtainScheme direct 6. insightPhotoproducts into the obtained differe bynces irradiation among ofthe diazo structures compound of carbenes1-N2 at 25 °C.1a, 1b, and 1c, Totheir obtain singlet direct and triplet insight states into were the optimized differences by DFT among calculations the structures [18]. The optimized of carbenes geometries1a, 1b at, and 1c, 2.4. DFT Calculations their singletthe M05/6–31G* and triplet level states of theory were optimized(3–21G level byfor DFTiodine calculations atom) indicated [18]. that The the optimized carbene-center geometries angle/dihedral angle between two phenyl rings of singlet 11a are 130.2°/80.1°, but those for triplet 31a at the M05/6–31G*To obtain leveldirect ofinsight theory into (3–21G the differe levelnces for among iodine the atom)structures indicated of carbenes that 1a the, 1b, carbene-center and 1c, are 179.9°/89.9° (Figure 4), indicating that triplet 31a has a linear structure with two phenyl rings their singlet and triplet states were optimized by DFT calculations1 [18]. The◦ optimized◦ geometries at 3 angle/dihedralmore orthogonal angle betweento each other two compared phenyl to rings those of in singlet11a. On the1a otherare 130.2hand, the/80.1 dihedral, but angles those infor triplet triplet 1a the◦ M05/6–31G*◦ level of theory (3–21G level for3 iodine atom) indicated that the carbene-center are 179.93 /89.93 (Figure4), indicating that triplet 1a has a linear structure with two phenyl rings more angle/dihedral1b and 1c were angle significantly between twosmaller phenyl than rings that ofin singlet1a, enabling 11a are the 130.2°/80.1°, optimal shielding but those of for the triplet carbene 31a 1 3 orthogonalcenterare 179.9°/89.9° to in each 1a. The other (FigureC-C compareddistances 4), indicating between to those that the triplet inaromatic1a .31a On an hasd the carbene a linear other carbons structure hand, are the with slightly dihedral two shorter phenyl angles for rings 1a in triplet 3 3 3 3 1b and(1.374more1c wereorthogonal Å) than signiﬁcantly for to either each other1b (1.375 smaller compared and than 1.376 to those thatÅ) or in 1c 11a (1.376. ,On enabling theÅ). otherThe spin the hand, optimaldensity the dihedral on shielding the carbeneangles in of carbon triplet the carbene (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 center in31b1a and. The 31c were C-C significantly distances betweensmaller than the that aromatic in 1a, enabling and carbene the optimal carbons shielding are of slightly the carbene shorter for below the singlet state. The energy gap between the singlet and triplet states is slightly larger than 31a (1.374center Å) in than 1a. The for eitherC-C distances31b (1.375 between and the 1.376 aromatic Å) or an3d1c carbene(1.376 carbons Å). The are spin slightly density shorter on for the 31a carbene that(1.374 for Å) either than forcarbene either 1b 31b (Δ (1.375EST = 15.4and kcal/mol)1.376 Å) or or 31c 1c (1.376 (ΔEST Å).= 14.2 The kcal/mol). spin density on the carbene carbon 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 (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 1.374 1.376 1.375 1.376 kcal/molbelow below the thesinglet singlet state. state. The energy The energy gap between gap between the singlet the and singlet triplet and states triplet is slightly states larger is slightly than larger than thatthat for for either either carbenecarbene 1b1b (Δ(E∆STE =ST 15.4=15.4 kcal/mol) kcal/mol) or 1c (Δ orEST1c = 14.2(∆E kcal/mol).ST = 14.2 kcal/mol).

1.374 1.376 1.375 1.376 179.9 169.9 169.3 3 3 3 1a (dihedral angle 89.9°) 1b (dihedral angle 84.2°) 1c (dihedral angle 64.8°) Figure 4. Calculated (M05/3–21G for I atom and 6–31G* for other atoms) structures and selected 3 3 3 bond distances179.9 (in Å) and angles (in degrees) of169.9 triplet carbenes 1a, 1b, and 1c. 169.3 3 3 3 1a (dihedral angle 89.9°) 1b (dihedral angle 84.2°) 1c (dihedral angle 64.8°)

Figure 4.FigureCalculated 4. Calculated (M05/3–21G (M05/3–21G for Ifor atom I atom and and 6–31G* 6–31G* for fo otherr other atoms) atoms) structuresstructures and and selected selected bond bond distances (in Å) and angles (in degrees) of triplet carbenes 31a, 31b, and 31c. distances (in Å) and angles (in degrees) of triplet carbenes 31a, 31b, and 31c.

Molecules 2016, 21, 1545 10 of 15

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 13 spectrometer (JEOL, Tokyo, Japan) in CDCl3 with Me4Si as an internal reference. C-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 1 give 5-tert-butyl-1,2,3-triiodobenzene (3) as yellow crystals (1.03 g, 60%): H-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; 1 H-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); 13 + C-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 1 purification: H-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 Molecules 2016, 21, 1545 11 of 15

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%): ◦ 1 m.p. 105.2–107.1 C; H-NMR (CDCl3, ppm) δ 7.92 (s, 4H), 6.23 (d, J = 8.5 Hz, 1H), 5.18 (d, J = 8.5 Hz, 13 1H), 4.24 (q, J = 7.2 Hz, 2H), 1.31–1.27 (m, 21H); C-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) 1 as a yellow semisolid (195 mg); H-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. ◦ 1 13 187.6–189.2 C (dec.); H-NMR (CDCl3, ppm) δ 7.94 (s, 4H), 1.29 (s, 18H); C-NMR (CDCl3, ppm) δ –1 153.8, 139.1, 131.9, 100.8, 77.7, 34.3, 31.0; IR (KBr disk, cm ) 2059 (νC=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 1 oil (1.3 mg, 31% after recycle GPC separation) ; H-NMR (CDCl3, ppm) δ 8.81 (d, J = 2.0 Hz, 2H), 8.53 13 (d, J = 2.0 Hz, 2H), 7.66 (s, 4H), 1.50 (s, 18H), 1.30 (s, 18H); C-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 ◦ 1 13 GPC separation); m.p. 219.5–220.4 C; H-NMR (CDCl3, ppm) δ 7.94 (s, 4H), 1.30 (s, 18H); C-NMR –1 (CDCl3, ppm) δ 195.9, 157.0, 139.4, 138.9, 97.1, 34.6, 30.9; IR (KBr, cm ) 1669 (νCO); 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 ◦ 1 colorless crystals (2.6 mg, 54% after recycle GPC separation); m.p. 190.6–191.4 C; H-NMR (CDCl3, 13 ppm) δ 7.84 (s, 4H), 4.70 (s, 2H), 1.27 (s, 18H); C-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, 13 CDCl3, ppm) δ 7.97 (s, 4H), 5.64 (s, 1H), 3.46 (s, 3H), 1.29 (s, 18H); C-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. Molecules 2016, 21, 1545 12 of 15

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. Molecules 2016, 21, 1545 13 of 15

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.

Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/21/ 11/1545/s1. Acknowledgments: We 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. 21550044). Author Contributions: Katsuyuki Hirai conceived and designed the study and analyzed the data. Katsuyuki Hirai, Kana Bessho and Kosaku Tsujita performed the experiments. Katsuyuki Hirai and Toshikazu Kitagawa wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References

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Sample Availability: Not available.

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