Reactivity Studies of Iridium(III) - American Chemical Society

Organometallics 2010, 29, 1343–1354 DOI: 10.1021/om9008668

1343

Reactivity Studies of Iridium(III) Porphyrins with Methanol in Alkaline Media Chi Wai Cheung, Hong Sang Fung, Siu Yin Lee, Ying Ying Qian, Yun Wai Chan, and Kin Shing Chan* Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China Received October 6, 2009

Ir(ttp)Cl(CO) (1a; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) was found to cleave the C-O bond of CH3OH at 200 °C to give Ir(ttp)CH3 (3a). Addition of KOH promoted the reaction rate and gave a higher yield of Ir(ttp)CH3 in 70% yield in 1 day. Mechanistic studies suggest that, in the absence of KOH, Ir(ttp)Cl(CO) reacts with CH3OH initially to give Ir(ttp)OCH3, which then undergoes β-hydride elimination to produce Ir(ttp)H (4a). Ir(ttp)H further reacts slowly to cleave the C-O bond of CH3OH, likely via σ-bond metathesis, to give Ir(ttp)CH3. In the presence of KOH, Ir(ttp)Cl(CO) initially reacts with KOH more rapidly to give Ir(ttp)OH, which then cleaves the O-H bond of CH3OH by metathesis to give Ir(ttp)OCH3. Ir(ttp)OCH3 further isomerizes via β-hydride elimination/reinsertion to give Ir(ttp)CH2OH and concurrently undergoes base-assisted β-proton elimination to give Ir(ttp)-Kþ (5a). Ir(ttp)CH2OH subsequently condenses with CH3OH to form Ir(ttp)CH2OCH3 (2). Finally, Ir(ttp)-Kþ cleaves the C-O bond in CH3OH, most probably via nucleophilic substitution, to give Ir(ttp)CH3. Ir(ttp)CH2OCH3 also serves as the precursor of Ir(ttp)-Kþ as it undergoes nucleophilic substitution by KOH to give Ir(ttp)-Kþ.

Introduction Alcohols can in principle react with group 9 transitionmetal complexes with C-H, O-H, and C-O bond cleavages. The catalytic C-H and O-H bond cleavage reactions find many applications in organic syntheses.1 High-valent nonmacrocyclic iridium(III) complexes are active catalysts in the oxidations of primary and secondary alcohols to produce aldehydes and ketones,2 N-alkylation of amines with alcohols,3 and transfer hydrogenation of unsaturated organic compounds.4 Alkoxy iridium(III) complexes are proposed to be the intermediates.2 Inorganic bases have been shown to further promote the oxidation of alcohols.2b-d,4a,b,5 Group 9 macrocyclic complexes, such as cobalt(II) Schiff

base complexes6 and porphyrins,7 have also been reported to catalyze the aerobic oxidations of secondary alcohols to generate the corresponding ketones. KOH can promote the oxidation of alcohols to ketones when cobalt(II) phthalocyanine catalyst is used.8 One unique alcohol being investigated is methanol, as intensive research efforts have been devoted to its catalytic synthesis from methane.9 The stoichiometric C-H and O-H bond cleavages of methanol have been reported with macrocyclic rhodium(II) tetrakis(mesityl)porphyrin radical (Rh(tmp)).10 Rh(tmp) is shown to cleave the C-H bond of methanol to give Rh(tmp)CH2OH and Rh(tmp)H in benzene or the O-H bond to give Rh(tmp)OCH3 and Rh(tmp)H in a higher concentration of methanol in benzene. Despite the reported examples of group 9 transition-metalmediated C-O bond cleavages of ethers,11 esters,12 acetals,13

*To whom correspondence should be addressed. E-mail: ksc@ cuhk.edu.hk. (1) Fujita, K.; Yamaguchi, R. Synlett 2005, 4, 560–571. (2) (a) Fujita, K.; Furukawa, S.; Yamaguchi, R. J. Organomet. Chem. 2002, 649, 289–292. (b) Fujita, K.; Yamamoto, K.; Yamaguchi, R. Org. Lett. 2002, 4, 2691–2694. (c) Hanasaka, F.; Fujita, K.; Yamaguchi, R. Organometallics 2004, 23, 1490–1492. (d) Prades, A.; Corberan, R.; Poyatos, M.; Peris, E. Chem. Eur. J. 2008, 14, 11474–11479. (3) Fujita, K.; Li, Z.; Ozeki, N.; Yamaguchi, R. Tetrahedron Lett. 2003, 44, 2687–2690. (4) (a) Sakaguchi, S.; Yamaga, T.; Ishii, Y. J. Org. Chem. 2001, 66, 4710–4712. (b) Fujita, K.; Kitatsuji, C.; Furukawa, S.; Yamaguchi, R. Tetrahedron Lett. 2004, 45, 3215–3217. (c) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–1308. (5) (a) Murata, K.; Ikariya, T.; Noyori, R. J. Org. Chem. 1999, 64, 2186–2187. (b) Phillip, J.; Black, P. J.; Edwards, M. G.; Williams, J. M. J. Eur. J. Org. Chem. 2006, 4367–4378. (6) Yamada, T.; Higano, S.; Yano, T.; Yamashita, Y. Chem. Lett. 2009, 38, 40–41. (7) Kumar Mandal, A.; Iqbal, J. Tetrahedron 1997, 53, 7641–7648.

(8) Sharma, V. B.; Jain, S. L.; Sain, B. Tetrahedron Lett. 2003, 44, 383–386. (9) For examples: (a) Groves, J. T.; Nemo, T. E.; Myers, R. S. J. Am. Chem. Soc. 1979, 101, 1032–1033. (b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560–564. (c) Lersch, M.; Tilset, M. Chem. Rev. 2005, 6, 2471–2526. (d) Jones, W. D. Inorg. Chem. 2005, 44, 4475–4484. (10) Li, S.; Cui, W.; Wayland, B. B. Chem. Commun. 2007, 4024– 4025. (11) (a) Komiya, S.; Srivastava, R. S.; Yamamoto, A.; Yamamoto, T. Organometallics 1985, 4, 1504–1508. (b) Kawamoto, T.; Fujimura, Y.; Konno, T. Chem. Lett. 2003, 32, 1058–1059. (c) Wender, P. A.; Deschamps, N. M.; Sun, R. Can. J. Chem. 2005, 83, 838–842. (12) (a) Hayashi, Y.; Yamamoto, T.; Yamamoto, A.; Komiya, S.; Yoshihiko, K. J. Am. Chem. Soc. 1986, 108, 385–391. (b) Sjvall, S.; Andersson, C.; Wendt, O. F. Organometallics 2001, 20, 4919–4926. (13) Ohta, T.; Michibata, T.; Yamada, K.; Omori, R.; Furukawa, I. Chem. Commun. 2003, 1192–1193.

r 2010 American Chemical Society

Published on Web 02/15/2010

pubs.acs.org/Organometallics

1344

Organometallics, Vol. 29, No. 6, 2010

and alkyl triflates,14 the C-O bond cleavage of alcohols by group 9 transition-metal complexes is less commonly reported. To the best of our knowledge, the only example is the iridium-catalyzed homocoupling and cross-coupling of alcohols to give ethers with the iridium acting as the Lewis acid.2d Transition-metal-mediated C-O bond cleavage of methanol is an uncommon reactivity mode. A formal example of C-O bond cleavage of methanol is the industrial production of acetic acid by rhodium- and iridium-catalyzed carbonylation of methanol in the Monsanto and Cativa Processes, respectively,15 but with the prior activation of methanol with hydroiodic acid to give methyl iodide. In view of the successful C-H,16 Si-H,17 and C-C18 bond activations using group 9 rhodium(III) and iridium(III) porphyrin complexes and the recently reported C-O bond cleavage of methanol using rhodium(III) porphyrin complexes,19 we wish to explore the chemistry in the C-O bond cleavage with iridium(III) porphyrins. We have identified the successful base-promoted C-O bond cleavage of methanol by high-valent iridium(III) porphyrin chloride (IrIII(ttp)Cl(CO), ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion)20 to yield iridium(III) porphyrin methyl (IrIII(ttp)CH3).20a We now report the results and the mechanistic studies.

Result and Discussion Base Effect. Initially, Ir(ttp)Cl(CO) (1a) reacted with CH3OH at 200 °C slowly in 5 days to give Ir(ttp)CH3 (3a) in 55% yield with the cleavage of the C-O bond of CH3OH (Table 1, eq 1, entry 1). In view of the base-promoted bond activations with metalloporphyrins16b,c,e,17b,18c,19 and other metal complexes,21 various base additives (Table 1, entries 2-9) were examined in the reaction of Ir(ttp)Cl(CO) with CH3OH at 200 °C. Metal acetates, carbonates, phosphate, and NaOH promoted the rates of reactions but had only a slight effect on the yields (Table 1, entries 2-8). NaOAc, (14) N€ uckel, S.; Burger, P. Organometallics 2001, 20, 4345–4359. (15) Thomas, C. M.; S€ uss-Fink, G. Coord. Chem. Rev. 2003, 243, 125–142. (16) Aldehydic C-H bond activation of aromatic aldehydes with Rh(ttp)Cl (ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion): (a) Chan, K. S.; Lau, C. M.; Yeung, S. K.; Lai, T. H. Organometallics 2007, 26, 1981–1985. Benzylic C-H bond activation of toluenes with Rh(ttp)Cl: (b) Chan, K. S.; Chiu, P. F.; Choi, K. S. Organometallics 2007, 26, 1117–1119. Alkane C-H bond activation with Rh(ttp)Cl: (c) Chan, Y. W.; Chan, K. S. Organometallics 2008, 27, 4625–4635. Aldehydic C-H bond activation of aromatic aldehydes with Ir(ttp)Cl(CO): (d) Song, X.; Chan, K. S. Organometallics 2007, 26, 965–970. Benzylic C-H bond activation of toluenes with Ir(ttp)Cl(CO): (e) Cheung, C. W.; Chan, K. S. Organometallics 2008, 27, 3043–3055. (17) By Rh(ttp)Cl: (a) Zhang, L.; Chan, K. S. Organometallics 2006, 25, 4822–4829. By Ir(ttp)Cl(CO): (b) Li, B.; Chan, K. S. Organometallics 2008, 27, 4034–4042. (18) C-CN bond activation of nitriles with Rh(tmp) radical (tmp = 5,10,15,20-tetrakis(mesityl)porphyrinato dianion): (a) Chan, K. S.; Li, X. Z.; Fung, C. F.; Zhang, L. Organometallics 2007, 26, 20–21. (b) Chan, K. S.; Li, X. Z.; Zhang, L.; Fung, C. F. Organometallics 2007, 26, 2679– 2687. C-C bond activation of ethers with Rh(tmp)Cl: (c) Lai, T. H.; Chan, K. S. Organometallics 2009, 28, 6845–6846. (19) Fung, H. S.; Chan, Y. W.; Cheung, C. W.; Choi, K. S.; Lee, S. Y.; Qing, Y. Y.; Chan, K. S. Organometallics 2009, 28, 3981–3989. (20) (a) Yeung, S. K.; Chan, K. S. Organometallics 2005, 24, 6426– 6430. (b) Ogoshi, H.; Setsune, J.-I.; Yoshida, Z.-I. J. Organomet. Chem. 1978, 159, 317–328. (21) Harkins, S. B.; Peters, J. C. Organometallics 2002, 21, 1753–1755. (22) The formation of Ir(ttp)CH2OCH3 was further confirmed by the similar chemical shifts and splitting patterns in 1H NMR in both Ir(ttp)CH2OCH3 and Rh(ttp)CH2OCH3.19 See the Experimental Section for the 1H NMR spectroscopic data of both compounds.

Cheung et al. Table 1. Base Effects on Reaction of Ir(ttp)Cl(CO) with CH3OH base ð20 equivÞ

IrðttpÞClðCOÞ þ CH3 OH s 200 °C, time, N2 1a IrðttpÞCH2 OCH3 þ IrðttpÞCH3 2

entry

base

time/daysa

1 2 3 4 5 6 7 8 9

none NaOAc KOAc Na2CO3 K2CO3 Cs2CO3 K3PO4 NaOH KOH

5c 1 1 1 1 1 1 1 1

ð1Þ

3a

yield of 2/%b nil trace trace nil trace nil trace trace nil

yield of 3a/%b 55 57 46 62 55 43 51 57 70

a All Ir(ttp)Cl(CO) was consumed. b Isolated yield. c In 1 day, Ir(ttp)Cl(CO) reacted with CH3OH at 200 °C to give Ir(ttp)CH3 and Ir(ttp)H in 20% and 9% yields, respectively, with Ir(ttp)Cl(CO) recovered in 37% yield.

KOAc, K2CO3, K3PO4, and NaOH also produced a trace amount of Ir(ttp)CH2OCH3 (2)22 (Table 1, entries 2, 3, 5, 7, and 8). The X-ray structure of Ir(ttp)CH2OCH3 is shown in Figure 1. KOH was found to be the optimal base, as both the reaction rate and product yield of Ir(ttp)CH3 were enhanced (Table 1, entry 9). The optimal amount of KOH in the C-O bond cleavage was then examined. KOH at 5 and 10 equiv resulted in lower yields of Ir(ttp)CH3 (Table 2, eq 2, entries 1 and 2), whereas a higher amount of KOH at 30 equiv did not give a higher product yield (Table 2, entry 4). Thus, 20 equiv of KOH (Table 2, entry 3) was used in further studies. Temperature Effect. The KOH-promoted reactions between Ir(ttp)Cl(CO) and CH3OH were studied at 150 and 200 °C. At 150 °C, Ir(ttp)Cl(CO) reacted with CH3OH and KOH in 36 h to give Ir(ttp)CH2OCH3 in 4% yield and Ir(ttp)CH3 in 31% yield (Table 3, eq 3, entry 1). When the reaction temperature was increased to 200 °C, the selective formation of Ir(ttp)CH3 in a high yield of 70% was obtained (Table 3, entry 2). The optimal temperature was thus found to be 200 °C. Porphyrin Effect. The porphyrin ligand effect was also investigated. In comparison with Ir(ttp)Cl(CO) (Table 4, eq 4, entry 1), the more electron-rich Ir(btpp)Cl(CO) (1b, btpp = 5,10,15,20-tetrakis(p-tert-butylphenyl)porphyrinato dianion) (Table 4, entry 2) and the sterically more hindered Ir(tmp)Cl(CO)23 (1c, tmp = 5,10,15,20-tetrakis(mesityl)porphyrinato dianion) (Table 4, entry 3) reacted with CH3OH and KOH at 200 °C in 1 day to give Ir(btpp)CH3 (3b) and Ir(tmp)CH324 (3c) in 72% and 66% yields, respectively. No significant porphyrin ligand effect on product yield was observed. This base-promoted reaction of Ir(por)Cl(CO) (por = porphyrinato dianion) with CH3OH provides a convenient synthesis of Ir(por)CH3, in contrast with the more tedious reductive alkylation of Ir(por)Cl(CO) using NaBH4 and MeI.20 The replacement of more toxic MeI by CH3OH as the methylating agent also provides a green (23) Collman, J. P.; Chng, L. L.; Tyvoll, D. A. Inorg. Chem. 1995, 34, 1311–1324. (24) Zhai, H.; Bunn, A.; Wayland, B. B. Chem. Commun. 2001, 1294– 1295.

Article


1345

Figure 1. ORTEP presentation of the molecular structure with numbering scheme for Ir(ttp)CH2OCH3 (2) (30% probability displacement ellipsoids).

CD3OD (eq 5).

Table 2. Optimization of KOH Loading KOH ðequivÞ

IrðttpÞClðCOÞ þ CH3 OH s IrðttpÞCH3 200 °C, 1 day, N2 1a 3a

KOH ð20 equivÞ

IrðttpÞClðCOÞ þ CD3 OD s IrðttpÞCD3 ð5Þ 200 °C, 1 day, N2 1a 3a-d 3 70%

ð2Þ

a

entry

amt of KOH/equiv

yield of 3a/%a

1 2 3 4

5 10 20 30

26 59b 70 66b

Isolated yield. b A trace amount of Ir(ttp)CH2OCH3 was isolated.

Table 3. Temperature Effects KOH ð20 equivÞ

IrðttpÞClðCOÞ þ CH3 OH s temp, time, N2 1a IrðttpÞCH2 OCH3 þ IrðttpÞCH3 2

ð3Þ

3a

entry

temp/°C

time/h

yield of 2/%a

yield of 3a/%a

total yield/%a

1 2

150 200

36 24

4 0

31 70

35 70

a

Isolated yield.

Table 4. Porphyrin Effects on the Base-Promoted Reaction of Ir(por)Cl(CO) with CH3OH KOH ð20 equivÞ

IrðporÞClðCOÞ þ CH3 OH s IrðporÞCH3 200 °C, 1 day, N2 1 3 ð4Þ

a

entry

por

yield of 3/%a

1 2 3

ttp (1a) btpp (1b) tmp (1c)

70 (3a) 72 (3b) 66 (3c)

Isolated yield.

synthetic route of Ir(por)CH3. Furthermore, Ir(ttp)CD3 in 70% yield could also be prepared at 200 °C in 1 day from Ir(ttp)Cl(CO) and the much more easily accessible

Reaction Profile. In order to gain a mechanistic understanding of the bond cleavage steps, the progress of the reaction of CD3OD with Ir(ttp)Cl(CO) in a sealed NMR tube was monitored by 1H NMR spectroscopy in both the absence and presence of KOH. Since Ir(ttp)Cl(CO) was nearly insoluble in CD3OD, and the suspected reaction intermediate, Ir(ttp)D (4a-d), as well as the products, Ir(ttp)CD2OCD3 (2-d5) and Ir(ttp)CD3 (3a-d3), were only sparingly soluble at room temperature, the NMR yields of iridium porphyrin species could not be estimated accurately. Instead, only the reaction sequences and the isolated yield of Ir(ttp)CD3 could be determined. It should be noted that the reactions conducted in sealed NMR tubes were slower than those in Teflon screw-capped tubes at 200 °C (Table 1), likely due to the lack of stirring in the NMR tube experiments. (i). Without KOH. In the reaction of Ir(ttp)Cl(CO) with CD3OD (Scheme S1, Supporting Information), Ir(ttp)Cl(CO) did not react with CD3OD at room temperature in 1 day. The reaction temperature was then increased to 200 °C. Ir(ttp)Cl(CO) reacted slowly with CD3OD in 4 days to give Ir(ttp)D25 (δ(pyrrole) 8.50 ppm), with most of the reddish purple solid, which was likely Ir(ttp)Cl(CO), remaining unreacted. After a prolonged heating of 23 days, Ir(ttp)Cl(CO) and Ir(ttp)D were completely consumed, as indicated by the absence of characteristic signals of Ir(ttp)Cl(CO) and Ir(ttp)D by 1H NMR spectroscopy. Ir(ttp)CD325 (δ(pyrrole) 8.46 ppm) was finally formed as a deep brown precipitate and was isolated in 100% yield by column chromatography (eq 6). Therefore, Ir(ttp)D is a viable intermediate for Ir(ttp)CD3 formation. 200 °C IrðttpÞClðCOÞ þ CD3 OD s IrðttpÞCD3 ð6Þ 1a

23 days

3a-d 3 100%

(25) The iridium porphyrin species formed in the sealed NMR tubes were identified by the authentic samples in CD3OD using 1H NMR spectroscopy.

1346


Cheung et al.

(ii). With KOH. In the KOH-promoted reaction of Ir(ttp)Cl(CO) with CD3OD (Scheme S2 and Figure S1, Supporting Information), most of the Ir(ttp)Cl(CO) remained unreacted and a small amount of Ir(ttp)D25 (δ(pyrrole) 8.50 ppm) and Ir(ttp)CD2OCD325 (δ(pyrrole) 8.54 ppm) was formed at room temperature after 1 day. In order to increase the reaction rate, the reaction mixture was heated up to 200 °C. After 30 min, Ir(ttp)-Kþ25 (5a) (δ(o-phenyl) 7.91 ppm)26 and more Ir(ttp)CD2OCD3 appeared. After 90 min, a small amount of Ir(ttp)CD325 (δ(pyrrole) 8.46 ppm) was also formed. Upon further heating for 4 days, Ir(ttp)-Kþ and Ir(ttp)CD2OCD3 were consumed completely and Ir(ttp)CD3 was isolated in 81% yield by column chromatography (eq 7). Thus, Ir(ttp)-Kþ and Ir(ttp)CD2OCD3 were formed faster than Ir(ttp)CD3, and these findings suggest that Ir(ttp)-Kþ and Ir(ttp)CD2OCD3 are likely intermediates for Ir(ttp)CD3 formation.

Table 5. Effect of Time on Product Selectivity KOH ð20 equivÞ

IrðttpÞClðCOÞ þ CH3 OH s 200 °C, time, N2 1a IrðttpÞCH2 OCH3 þ IrðttpÞCH3 2

ð8Þ

3a

entry

time/h

yield of 2/%a

yield of 3a/%a

1 2 3

0.5 3 24

55 55 0

10 23 70

a

Isolated yield.

Scheme 1. Reaction Mechanism of Ir(ttp)Cl(CO) with CH3OH

66% yield (eq 9).

KOH ð20 equivÞ

IrðttpÞ ClðCOÞ þ CD3 OD s IrðttpÞCD3 ð7Þ 200 °C, 4 days 1a 3a-d 3 81%

As Ir(ttp)CD2OCD3 and Ir(ttp)CD3 did not dissolve very well in CD3OD for accurate yield determination by 1H NMR spectroscopy, they were quantified by isolation by separate experiments. When Ir(ttp)Cl(CO) was heated with KOH (20 equiv) in CH3OH for 30 min at 200 °C, Ir(ttp)CH2OCH3 and Ir(ttp)CH3 were produced in 55% and 10% yields, respectively, confirming the more rapid formation of Ir(ttp)CH2OCH3 (Table 5, eq 8, entry 1). In 3 h, the yield of Ir(ttp)CH2OCH3 remained at 55%, with a concomitant increase in the amount of Ir(ttp)CH3 to 23% yield (Table 5, entry 2). Likely, Ir(ttp)-Kþ remains in the alkaline reaction mixture and slowly reacts to give more Ir(ttp)CH3. After 1 day, Ir(ttp)CH2OCH3 was consumed and Ir(ttp)CH3 in 70% yield was obtained (Table 5, entry 3). Mechanism (No Base). On the basis of the above findings, Scheme 1 illustrates the proposed mechanism of the reaction of Ir(ttp)Cl(CO) with CH3OH. Ir(ttp)Cl(CO) initially undergoes CO ligand dissociation to give Ir(ttp)Cl,16e which then reacts with CH3OH via ligand substitution to yield Ir(ttp)OCH327 and HCl28 (Scheme 1, pathway i). Ir(ttp)OCH3 then converts to the observed Ir(ttp)H (Table 1, entry 1) and HCHO via β-hydride elimination29 (Scheme 1, pathway ii). Finally, Ir(ttp)H cleaves the C-O bond in CH3OH slowly, likely via σ-bond metathesis, to yield Ir(ttp)CH3 (Scheme 1, pathway iii). Indeed, Ir(ttp)H was found to independently react with CH3OH at 200 °C in 3 days to produce Ir(ttp)CH3 selectively in (26) The characteristic upfield pyrrole proton signal of Ir(ttp)-Kþ (δ(pyrrole) 8.11 ppm) was shielded by the upfield o-phenyl proton signal of the porphyrin’s tolyl groups in Ir(ttp)CD2OCD3 (δ(pyrrole) 8.10 ppm). Thus, the upfield o-phenyl proton signal of Ir(ttp)-Kþ (δ(o-phenyl) 7.91 ppm) was selected as the alternative characteristic peak. (27) (a) Tani, K.; Iseki, A.; Yamagata, T. Angew. Chem., Int. Ed. 1998, 37, 3381–3383. (b) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2006, 25, 3007–3011. (28) Jiang, B.; Feng, Y.; Ison, E. A. J. Am. Chem. Soc. 2008, 130, 14462–14464. (29) Ofer Blum, O.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 4582– 4594.

200 °C

IrðttpÞH þ CH3 OH s IrðttpÞCH3 3 days, N2 66%

ð9Þ

In fact, Ir(ttp)OCH3 can isomerize30 via a β-hydride elimination19 to give Ir(ttp)H and HCHO, followed by reinsertion19 of HCHO to Ir(ttp)-H to give Ir(ttp)CH2OH (eq 10). Indeed, the β-hydride elimination of Rh(tpp)OCH3 (tpp = 5,10,15,20-tetrakis(phenyl)porphyrinato dianion) to give Rh(tpp)H and HCHO31 and the insertion of HCHO to Rh(tpp)-H to give Rh(tpp)CH2OH32 have been reported. Ir(ttp)CH2OH then further condenses with CH3OH to yield Ir(ttp)CH2OCH319,33 (eq 11). Moreover, Ir(ttp)H reacted very quickly with paraformaldehyde (17 equiv of HCHO) in CD3OD at 200 °C in 1 day to give Ir(ttp)CH2OCD3 in 74% yield and Ir(ttp)CD3 in 5% yield (eq 12), which was formed by the slower reaction of Ir(ttp)H with CD3OD.

IrðttpÞOCH3 h IrðttpÞCH2 OH

ð10Þ

IrðttpÞCH2 OH þ CH3 OHs IrðttpÞCH2 OCH3 þ H2 O ð11Þ CD3 OD

IrðttpÞH þ HCHO s IrðttpÞCH2 OCD3 þ IrðttpÞCD3 17 equiv 200 °C, 1 day 74% 5%

ð12Þ However, such an isomerization does not occur, as Ir(ttp)CH2OCH3 was not formed in the reaction (Table 1, entry 1). Probably, the HCl formed (Scheme 1, pathway i) catalyzes the much faster reaction of HCHO with CH3OH to give dimethoxymethane,34 inhibiting the reinsertion of (30) The isomerization of Rh(tmp)OCH3 to Rh(tmp)CH2OH was reported in a rhodium porphyrin analogue.10 (31) Collman, J. P.; Boulatov, R. Inorg. Chem. 2001, 40, 560–563. (32) Wayland, B. B.; Vanvoorhees, S. L.; Wilker, C. Inorg. Chem. 1986, 25, 4039–4042. (33) Rh(tpp)CH2OH (tpp = 5,10,15,20-tetrakis(phenyl)porphyrinato dianion) has been reported to undergo condensation to give [Rh(tpp)CH2]2O. It is likely that Ir(ttp)CH2OH prefers to undergo condensation with excess CH3OH solvent to yield Ir(ttp)CH2OCH3, rather than a more sterically unfavorable condensation to yield [Ir(ttp)CH2]2O.32 (34) Lewis acid can catalyze the reaction of formaldehyde with methanol to give dimethoxymethane. See: Danov, S. M.; Kolesnikov, V. A.; Logutov, I. V. Russ. J. Appl. Chem. 2004, 77, 1994–1996.

Article


Scheme 2. Base-Promoted Reaction Mechanism of Ir(ttp)Cl(CO) with CH3OH

HCHO with Ir(ttp)H. Since Ir(ttp)H also did not react with CH3OH at 200 °C in 1 day to yield any Ir(ttp)CH2OCH3 (eq 9), it appears that Ir(ttp)H is not a reactive intermediate to cleave the C-H bond of CH3OH to give Ir(ttp)CH2OH. Ir(ttp)H only cleaves the C-O bond of CH3OH to yield Ir(ttp)CH3. Mechanism (Base Promoted). Initially, Ir(ttp)Cl(CO) reacts with KOH via facile ligand substitution to give Ir(ttp)OH16e,35 (Scheme 2, pathway i), which then exchanges rapidly with CH3OH by metathesis to afford Ir(ttp)OCH3 (Scheme 2, pathway ii). Alternatively, the deprotonation of coordinated CH3OH10 by KOH to give Ir(ttp)OCH3 also operates. Indeed, Ir(ttp)Cl(CO) reacted with KOH and isopropyl alcohol at room temperature in 10 min to give Ir(ttp)OiPr in ∼85% yield (eq 13), supporting the facile basepromoted substitution of Ir(ttp)Cl(CO) by CH3OH to yield Ir(ttp)OCH3. Facile isomerization30 of Ir(ttp)OCH3 via βhydride elimination19,31/reinsertion19,32 likely occurs to afford Ir(ttp)CH2OH (eq 10), which further condenses with CH3OH to give Ir(ttp)CH2OCH333 (eq 11; Scheme 2, pathway iii). Concurrently, the base-assisted β-proton elimination31 of Ir(ttp)OCH3 takes place to yield Ir(ttp)-Kþ, HCHO, and H2O (Scheme 2, pathway iv). Finally, both Ir(ttp)CH2OCH3 and Ir(ttp)-Kþ further react with CH3OH to give Ir(ttp)CH3 (to be discussed in the following section). KOH ð25 equivÞ

IrðttpÞClðCOÞþ i PrOH s IrðttpÞOi Pr ð13Þ 10 min, N2 ∼85% Since Ir(ttp)-Kþ can exist in equilibrium with CH3OH to give Ir(ttp)H via protonation with CH3OH,36 and Ir(ttp)H is also known to undergo fast base-promoted dehydrogenative dimerization to give [Ir(ttp)]2,16e therefore Ir(ttp)-Kþ, Ir(ttp)H, and [Ir(ttp)]2 are all possible intermediates existing in an alkaline medium (Scheme 3, pathways i and ii). The reactivities of these three possible intermediates toward the reaction with CH3OH to give Ir(ttp)CH2OCH3 and Ir(ttp)CH3 were also examined in sealed NMR tubes (Table 6, eq 14). (i). [Ir(ttp)]2. Among the three possible intermediates, only [Ir(ttp)]2 (δ(pyrrole) 8.71 ppm) reacted with CD3OD at 200 °C in 16 h to give Ir(ttp)CD2OCD3 in 44% yield accompanied by the formation of Ir(ttp)CD3 in 7% yield and a trace amount of Ir(ttp)D37 (Table 6, entry 1). It is likely that [Ir(ttp)]2 undergoes disproportionation with excess CD3OD (35) Substitution of Ir-Cl by OH- anion to give Ir-OH has been reported. See: (a) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35, 44–56. (b) Ir(ttp)Cl(CO) was reported to react with NaOPh in benzene to give Ir(ttp)OPh.16e Presumably, the more nucleophilic KOH reacts much more quickly with Ir(ttp)Cl(CO) to give Ir(ttp)OH. (36) Rh(TSPP)H (TSPP=tetrakis(p-sulfonatophenyl)porphyrinato) has been reported to exist in equilibrium with Rh(TSPP)- in water. The less acidic Ir(ttp)H is likely to be in equilibrium with Ir(ttp)- in basic medium. See: (a) Fu, X.; Basickes, L.; Wayland, B. B. Chem. Commun. 2003, 520–521. (b) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623–2631. (37) Ir(ttp)H can undergo fast proton exchange with CD3OD (50 equiv) in benzene-d6 at room temperature in 6 h to give Ir(ttp)D in 77% yield.

1347

Scheme 3. Formation of Ir(ttp)CH3 from Possible Intermediates

to give Ir(ttp)- and [Ir(ttp)(CD3OD)2]þ (Scheme 4, pathway i), which further reacts to yield Ir(ttp)D and Ir(ttp)OCD310 (Scheme 4, pathway ii). Ir(ttp)OCD3 proceeds to undergo β-hydride elimination19,31/reinsertion19,32 to give Ir(ttp)CD2OD (Scheme 4, pathway iii). Ir(ttp)CD2OD further condenses with CD3OD to give Ir(ttp)CD2OCD333 (eq 11), whereas Ir(ttp)D reacts more slowly with CD3OD to give Ir(ttp)CD3 (eq 9). However, in the presence of KOH, [Ir(ttp)]2 was observed to form Ir(ttp)-Kþ (δ(pyrrole) 8.11 ppm) in ∼90% yield upon heating in CD3OD at 200 °C in 5 min (Table 6, entry 2). This likely results from the disproportionation of [Ir(ttp)]2 with CD3OD to give Ir(ttp)- and [Ir(ttp)(CD3OD)2]þ 10 (Scheme 4, pathway i), which then undergoes base-promoted deprotonation (Scheme 4, pathway iv) and subsequent βproton elimination19,32 to give another Ir(ttp)- (Scheme 4, pathway v). Prolonged heating of the reaction mixture at 200 °C in 4 days resulted in the gradual consumption of Ir(ttp)-Kþ to give Ir(ttp)CD3 in 59% yield without any Ir(ttp)CD2OCD3 formed (Table 6, entry 2). [Ir(ttp)]2 is therefore not the direct intermediate in the basic medium to give Ir(ttp)CH2OCH3 and Ir(ttp)CH3. (ii). Ir(ttp)H. Ir(ttp)H (δ(pyrrole) 8.50 ppm) reacted very slowly with CD3OD in 23 days to give Ir(ttp)CD3 in 39% yield with some unreacted Ir(ttp)D37 (Table 6, entry 3). No Ir(ttp)CD2OCD3 was formed in the course of reaction. In the presence of KOH, Ir(ttp)H was rapidly deprotonated20b to give Ir(ttp)-Kþ (δ(pyrrole) 8.11 ppm) in ∼90% yield at 200 °C in 5 min. Prolonged heating of the reaction mixture at 200 °C over 4 days gave Ir(ttp)CD3 in 57% yield without Ir(ttp)CD2OCD3 formation (Table 6, entry 4). The rapid formation of Ir(ttp)CH3 from Ir(ttp)H/KOH rather than from Ir(ttp)H suggests that Ir(ttp)-Kþ is a key and direct intermediate to cleave the C-O bond of CH3OH to give Ir(ttp)CH3. (iii). Ir(ttp)-Kþ. Ir(ttp)-Kþ (δ(pyrrole) 8.11 ppm) was thus independently prepared in the form of Ir(ttp)-[K(18crown-6)]þ,38 which did react with CD3OD at 200 °C in 4 days to give only Ir(ttp)CD3 in 85% yield without the formation of Ir(ttp)CD2OCD3 (Table 6, entry 5). Therefore, Ir(ttp)-Kþ is also not the intermediate to give Ir(ttp)CH2OCH3 but the intermediate to yield Ir(ttp)CH3. Formation of Ir(ttp)CH3. (i). From Ir(ttp)CH2OCH3. Ir(ttp)CH2OCH3 is thermally stable, since it was recovered quantitatively upon heating in C6D6 at 200 °C in 14 days (Table 7, eq 15, entry 1). When Ir(ttp)CH2OCH3 reacted with CD3OD in the presence of KOH at 200 °C in 1 day, small amounts of Ir(ttp)-Kþ and Ir(ttp)CD3 were formed. Ir(ttp)CH2OCH3 also exchanged with CD3OD to yield Ir(ttp)CH2OCD3. In 4 days, Ir(ttp)CD3 was isolated in (38) Ir(ttp)-[K(18-crown-6)]þ was prepared using the synthetic method to prepare Rh(ttp)-[K(18-crown-6)]þ.32

1348


Cheung et al.

Table 6. Relative Reactivities of Iridium Porphyrin Species KOH ðequivÞ

IrðttpÞX þ CD3 OD s IrðttpÞCD2 OCD3 þ IrðttpÞCD3 200 °C, time 2-d 5 3a-d 3

ð14Þ

entry

Ir(ttp)X

amt of KOH/equiv

time

Ir(ttp)CD2OCD3 yield/%a

Ir(ttp)CD3 yield/%a

1 2 3 4 5

[Ir(ttp)]2 [Ir(ttp)]2c Ir(ttp)Hd Ir(ttp)Hc Ir(ttp)-Kþe

0 20 0 20 20

16 h 4 days 23 days 4 days 4 days

44 0 0 0 0

7b 59 39 57 85

a Isolated yield. b A trace amount of of Ir(ttp)D was also observed by 1H NMR spectroscopy. c Ir(ttp)-Kþ was formed in ∼90% yield upon heating at 200 °C in 5 min. d Ir(ttp)H converted to Ir(ttp)D quantitatively instantaneously, and some Ir(ttp)D remained unreacted after heating for 23 days. e As Ir(ttp)-[K(18-crown-6)]þ.

Scheme 4. Conversion of [Ir(ttp)]2 to Ir(ttp)OCD3, Ir(ttp)CD2OD, and Ir(ttp)-

62% yield with the unreacted Ir(ttp)CH2OCH3 and Ir(ttp)CH2OCD3 recovered in 4% and 24% yields, respectively (Table 7, entry 2). Probably, nucelophilic substitution of the R-carbon19,39 in Ir(ttp)CH2OCH3 by KOH takes place to afford Ir(ttp)-Kþ (Scheme 5), which further reacts with CD3OD to give Ir(ttp)CD3 (Scheme 3, pathway iii). The formation of Ir(ttp)CH3 from Ir(ttp)CH2OCH3 is slower than that from Ir(ttp)- (Table 6, entry 5) and is likely due to the poor leaving group of Ir(ttp)-. (ii). From Ir(ttp)-Kþ. The puzzling suggestion that Ir(ttp)-Kþ reacts directly with CH3OH, likely via bimolecular nucleophilic substitution (SN2), to yield Ir(ttp)CH3 and OHanion (Scheme 6, pathway ii), has been tested further experimentally. At room temperature, Ir(ttp)CH3(PPh3) (3d) was readily observed in the reaction of Ir(ttp)- with PPh3 (1 equiv) and CH3OH in 5 min (Table 8, eq 16, entry 1), but no Ir(ttp)CH3 was formed in the absence of PPh3 (Table 8, entry 2). When the reaction temperature was increased to 100 °C, Ir(ttp)- added with PPh3 reacted with CH3OH in 36 h to give Ir(ttp)CH3(PPh3) in 17% yield (Table 8, entry 1), whereas Ir(ttp)- alone gave Ir(ttp)CH3 in 9% yield in 72 h (Table 8, entry 2). The rate-promoting effect of PPh3 on the reaction rate of Ir(ttp)- with CH3OH was also observed at 200 °C. Ir(ttp)-/PPh3 (1 equiv) reacted with CH3OH in 12 h at 200 °C to yield Ir(ttp)CH3(PPh3) in 52% yield (Table 8, entry 3). Ir(ttp)-/PPh3 reacted with MeOH to give Ir(ttp)CH3(PPh3) in 77% yield upon heating for 1 day (Table 8, entry 4). Ir(ttp)- alone reacted more slowly with CH3OH in 12 h to yield Ir(ttp)CH3 in 28% yield (Table 8, entry 5) and completely in 1 day to produce Ir(ttp)CH3 in 87% yield (Table 8, entry 6). Thus, Ir(ttp)-/PPh3 reacts more quickly than Ir(ttp)- at 200 °C. Likely, PPh3 coordinates to Ir(ttp)- to generate more electron-rich and (39) Sanford, M. S.; Groves, J. T. Angew. Chem., Int. Ed. 2004, 43, 588–590.

more nucleophilic Ir(ttp)(PPh3)- 40 to react more quickly with CH3OH via an SN2 mechanism. To ensure that Ir(ttp)H(PPh3), possibly formed in the above reaction by protonation of Ir(ttp)(PPh3)-, did not affect the above conclusion, the reaction of Ir(ttp)H/PPh3 (1 equiv) with CH3OH was also examined (Table 8, entry 7). As expected, extremely slow reaction occurred in 3 days to give Ir(ttp)CH3(PPh3) in a low yield of 24%. Therefore, Ir(ttp)H(PPh3) is much less reactive than Ir(ttp)(PPh3)(Table 8, entry 4) and Ir(ttp)H as well (eq 9), presumably due to the lack of a vacant coordination site for the σ-bond metathesis. To rationalize why the nucleophilic substitution of Ir(ttp)- with CH3OH to give Ir(ttp)CH3 is competitive with the protonation to yield Ir(ttp)H, we reason that the protonation does occur by a rapid equilibrium (Scheme 6, pathway i). The reaction of Ir(ttp)H with CH3OH to give Ir(ttp)CH3 is shown to be very slow at 200 °C (eq 9). Therefore, Ir(ttp)- is a viable direct intermediate (Scheme 6, pathway ii) which reacts much more quickly with CH3OH to give Ir(ttp)CH3 at 200 °C (Table 8, entry 6). It has been reported that the weak base and nucleophile NH3 underwent acyl substitution with CH3CO2H to give CH3CONH2 instead of CH3CO2-NH4þ, which was also formed but dissociated rapidly.41 In the base-promoted reaction of Ir(ttp)Cl(CO) with CH3OH, the rate of formation of Ir(ttp)CH3 was slower than that of Ir(ttp)CH2OCH3 (Scheme S2 (Supporting Information); Table 5). This shows that the C-O bond cleavage of CH3OH by Ir(ttp)- (Scheme 3, pathway iii) appears to be the rate-limiting bond cleavage step rather than the O-H and C-H bond cleavages of CH3OH. Methyl Group Exchange of Ir(ttp)CH3. Ir(ttp)CH3 did not undergo any exchange with CD3OD in 1 day to yield Ir(ttp)CD3, with Ir(ttp)CH3 recovered in 80% yield (eq 17a). However, extensive methyl group exchange42 of Ir(ttp)CH3 occurred with CD3OD in the presence of KOH (20 equiv) in 1 day to give Ir(ttp)CD3 in 18% yield with Ir(ttp)CH3 recovered in 65% yield (eq 17b). These results further suggest that the nucleophilic substitution of Ir(ttp)CH3 by KOH occurs to give Ir(ttp)-Kþ (Scheme 7, pathway i). Ir(ttp)-Kþ (40) The 1H NMR spectrum of pure Ir(ttp)(PPh3)- cannot be obtained by adding 1 equiv of PPh3 to Ir(ttp)-K(18-crown-6)þ in CD3OD, as the reaction was very fast at room temperature to give a deep purple precipitate, which was likely Ir(ttp)CH3(PPh3). (41) Mitchell, J. A.; Reid, E. E. J. Am. Chem. Soc. 1931, 53, 1879– 1883. (42) Iridium porphyrin benzyl (Ir(ttp)Bn) have been shown to undergo base-promoted benzyl group exchange with toluene-d8.16e

Article


1349

Table 7. Conversion of Ir(ttp)CH2OCH3 to Ir(ttp)CH3 solvent, KOH ðequivÞ IrðttpÞCH2 OCH3 s IrðttpÞCH2 OCH3 þ IrðttpÞCH2 OCD3 þ IrðttpÞCD3 200 °C, time 2 2 3a 2-d 3

entry 1 2

solvent

amt of KOH/equiv

C6D6 CD3OD

time/days

0 20

14 4d

yield of 2/%

ð15Þ

yield of 2-d3c/%

yield of 3a-d3/%

0 24a

0 62b

a

100 4a

a NMR yield. b Isolated yield. c The CH3 group in Ir(ttp)CH2OCH3 exchanged with CD3OD to give Ir(ttp)CH2OCD3. d Ir(ttp)-Kþ and Ir(ttp)CD3 were formed in 1 day.

Table 8. Effect of PPh3 on Reactions of Ir(ttp)H and Ir(ttp)- with CH3OH PPh3 ðequivÞ

IrðttpÞX þ CH3 OH s IrðttpÞCH3 = IrðttpÞCH3 ðPPh3 Þ temp, time, N2 3a 3d

ð16Þ

entry

Ir(ttp)X

amt of PPh3/equiv

temp/°C

time/h

yield of 3a or 3d/%a

1 2 3 4 5 6 7

Ir(ttp)-Kþ b Ir(ttp)-Kþ b Ir(ttp)-Kþ b Ir(ttp)-Kþ b Ir(ttp)-Kþ b Ir(ttp)-Kþ b Ir(ttp)H

1 0 1 1 0 0 1

100 100 200 200 200 200 200

36 72 12 24 12 24 72

17 (3d)c 9 (3a)d 52 (3d)e 77 (3d) 28 (3a) 87 (3a) 24 (3d)f

a Isolated yield. b As Ir(ttp)-[K(18-crown-6)]þ. c At room temperature in 5 min, Ir(ttp)CH3(PPh3) was observed by thin-layer chromatography (TLC). At room temperature in 5 min, no Ir(ttp)CH3 was observed by TLC. e In 1 h, Ir(ttp)CH3(PPh3) was isolated in 29% yield. f In 1.5 h, no Ir(ttp)CH3(PPh3) was observed by TLC. d

Scheme 5. Nucleophilic Substitution of Ir(ttp)CH2OCH3 by KOH

then reacts with CD3OD to yield Ir(ttp)CD3 via an SN2 mechanism (Scheme 7, pathway ii). CD3 OD, 1 day, N2 IrðttpÞCH3 s

ðaÞ no base ðbÞ KOH ð20 equivÞ

IrðttpÞCH3 þ IrðttpÞCD3 80% 65%

0% 18%

ð17Þ Reactivity Comparison in Group 9 Metalloporphyrins. Our group has reported the base-promoted reaction of CH3OH with Rh(ttp)Cl at 150 °C, and Rh(ttp)H is proposed to be the intermediate to cleave the C-O bond of CH3OH to give Rh(ttp)CH3.19 In the iridium porphyrin system, Ir(ttp)-Kþ (Scheme 3, pathway iii) is shown to react with CH3OH much more quickly than Ir(ttp)H (Scheme 3, pathway iv) to yield Ir(ttp)CH3 (Table 6, entries 3 and 5), suggesting that Ir(ttp)-Kþ rather than Ir(ttp)H acts as the reacting species in a basic medium. The difference in reactivities between rhodium and iridium porphyrins is mainly due to (1) the lower nucleophilicity of Rh(ttp)- as compared to that of Ir(ttp)- and (2) the higher reactivitiy of Rh(ttp)H as compared to that of Ir(ttp)H. (1) It is reported that Rh(oep)H (oep = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion) dissolved readily in basic alcoholic solution with 1 M NaOH added to give Rh(ttp)-, whereas Ir(oep)H remained in solid form and did not form Ir(ttp)- under the same reaction conditions.20b Indeed, Rh(ttp)H was readily deprotonated in the presence of weak base of K2CO3 at 150 °C to give Rh(ttp)-.19 This shows that the

Scheme 6. Nucleophilic Substitution of CH3OH by Ir(ttp)-Kþ

pKa of Rh(ttp)-H is lower than that of Ir(ttp)-H43 and suggests a lower nucleophilicity of Rh(ttp)- as compared to that of Ir(ttp)-.44 (2) As the Rh(ttp)-H bond (∼61 kcal/ mol)45 is weaker than the Ir(ttp)-H bond (∼70 kcal/mol),46 the reaction of Rh(ttp)-H with CH3-OH is likely kinetically more favorable than that of Ir(ttp)-H. The CH3-OH cleavage by Rh(ttp)H is also estimated to be thermodynamically more favorable than that of Ir(ttp)H.47 Thus, (43) The pKa values of the M-H bonds in group 9 transition-metal complexes increase down the groups, and the pKa of Ir-H is reported to be higher than that of Rh-H by around 10. Ir(ttp)H is likely to be a weaker acid than Rh(ttp)H. See: Qi, X.-J.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics 2006, 25, 5879–5886. (44) The basicity of a conjugate base is related to its nucleophilicity. The higher the pKa of an acid, the more nucleophilic its conjugate base. See: (a) Bordwell, F. G.; Hughes, D. L. J. Org. Chem. 1983, 48, 2206– 2215. (b) Bordwell, F. G.; Hughes, D. L. J. Am. Chem. Soc. 1984, 106, 3234–3240. (45) Cui, W.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 8266–8274. (46) Bond dissociation energies (BDEs) of Ir(ttp)-H and Ir(ttp)-CH3 are about 70 and 62 kcal/mol, respectively: Wayland, B. B. Personal communication, University of Pennsylvania, Philadelphia, PA, 2007. (47) The exothermicities (ΔH) of the reactions of CH3-OH cleavage with Rh(ttp)H and Ir(ttp)H are estimated using the bond dissociation energies (BDEs) of Rh-H, Rh-CH3, Ir-H, Ir-CH3, CH3-OH, and H-OH. In the Rh(ttp)H system, ΔH = BDE[Rh-H] (61) þ BDE[CH3-OH] (92) - BDE[Rh-CH3] (57) - BDE[H-OH] (119) = -23 kcal/mol. In the Ir(ttp)H system, ΔH = BDE[Ir-H] (70) þ BDE[CH3-OH] (92) - BDE[Ir-CH3] (62) - BDE[H-OH] (119) = -19 kcal/mol. BDE of Rh-H: (a) Reference 45. BDE of Rh-CH3: (b) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305–5311. BDEs of Ir-H and Ir-CH3: (c) Reference 46. BDEs of CH3-OH and H-OH: (d) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic Compounds; CRC Press: Boca Raton, FL, 2002.

1350


Scheme 7. Base-Promoted Conversion of Ir(ttp)CH3 to Ir(ttp)CD3

Rh(ttp)H reacts more quickly than Rh(ttp)-, and Ir(ttp)H conversely reacts more slowly than Ir(ttp)-. Rh(ttp)H and Ir(ttp)- are the reactive intermediates in the C-O bond cleavage of CH3OH.

Conclusion Ir(ttp)Cl(CO) successfully reacted with methanol via C-O bond cleavage at 200 °C to give Ir(ttp)CH3. Addition of KOH promoted the reaction rate to give a higher yield of Ir(ttp)CH3. Mechanistic studies suggest that KOH promotes the rate of formation of Ir(ttp)CH3 by (i) promoting the ligand substitution of Ir(ttp)Cl(CO) with CH3OH to afford the more reactive Ir(ttp)OCH3, (ii) promoting the β-proton elimination of Ir(ttp)OCH3 to give Ir(ttp)-, and (iii) promoting the substitution of Ir(ttp)CH2OCH3 to give Ir(ttp)-. Ir(ttp)H and Ir(ttp)- are found to be the active intermediates to cleave the C-O bond in CH3OH in the absence and presence of KOH, respectively. Ir(ttp)H reacts with CH3OH, likely via σ-bond metathesis, whereas Ir(ttp)-Kþ reacts with CH3OH, probably via a bimolecular nucleophilic substitution, to give Ir(ttp)CH3.

Experimental Section Unless otherwise noted, all reagents were purchased from commercial suppliers and directly used without further purification. Hexane was distilled from anhydrous calcium chloride. Benzene was distilled from sodium. Benzene-d6 and methanol-d4 were dried in preheated 4 A˚ molecular sieves and were stored in a Teflon-capped tube under nitrogen prior to use. Thin-layer chromatography was performed on precoated silica gel 60 F254 plates. H2(ttp),48 H2(btpp),49 H2(tmp),50 Ir(ttp)Cl(CO) (1a),20a Ir(tmp)Cl(CO) (1c),23 Ir(ttp)CH3 (3a),20a Ir(tmp)CH3 (3c),24 Ir(ttp)H (4a),16e and [Ir(ttp)]2 dimer16e (6) had been characterized, and they were prepared according to the literature procedures. Silica gel (Merck, 70-230 mesh) was used in column chromatography to isolate Ir(por)Cl(CO) (por = ttp (1a), btpp (1b), tmp (1c)). Neutral alumina (Merck, 70-230 mesh)/H2O (∼10:1 v/v) was used in column chromatography to isolate Ir(ttp)CH2OCH3 (2) and Ir(por)CH3 (por = ttp (3a), btpp (3b), tmp (3c)). 1 H NMR and 13C NMR spectra were recorded on a Bruker DPX-300 instrument at 300 and 75 MHz, respectively, or a Bruker AV-400 instrument at 400 and 100 MHz, respectively. Chemical shifts were referenced with the residual solvent protons in CDCl3 (δ 7.26 ppm), C6D6 (δ 7.15 ppm), CD3OD (δ(CH3) 3.38 ppm), THF-d8 (δ(β-CH2) 1.85 ppm), or tetramethylsilane (δ 0.00 ppm) as the internal standard in 1H NMR spectra and CDCl3 (δ 77.16 ppm), C6D6 (δ 128.06 ppm), or THF-d8 (δ(β-CH2) 25.62 ppm) as the internal standard in 13C NMR spectra. Chemical shifts (δ) are reported as parts per million (ppm) in δ scale downfield from TMS. Coupling constants (J) are reported in hertz (Hz). High-resolution mass spectra (HRMS) were measured on a ThermoFinnigan MAT 95 XL mass spectrometer in fast atom bombardment (FAB) (48) Alder, A. D.; Logon, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakof, L. J. Org. Chem. 1967, 32, 476. (49) Chan, K. S.; Mak, K. W.; Tse, M. K.; Yeung, S. K.; Li, B. Z.; Chan, Y. W. J. Organomet. Chem. 2008, 693, 399–407. (50) Wagner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett. 1987, 28, 3069–3070.

Cheung et al. mode using 3-nitrobenzyl alcohol (NBA) matrix and CH2Cl2 as solvent and in the electrospray ionization (ESI) mode using MeOH:CH2Cl2 (1:1) as solvent. Preparation of Chlorocarbonyl(5,10,15,20-tetrakis(p-tertbutylphenyl)porphyrinato)iridium(III), [Ir(btpp)Cl(CO)] (1b). Ir(btpp)Cl(CO) (1b) was synthesized according to the procedures in the synthesis of Ir(ttp)Cl(CO).20a [Ir(COD)Cl]2 (437 mg, 0.65 mmol) and H2(btpp)49 (545 mg, 0.65 mmol) were heated under reflux in p-xylene (200 mL) for 3 days. The reaction mixture changed from deep brown to reddish brown, and it was dried under vacuum. The product was purified by column chromatography using CH2Cl2/hexane (1:2) as an eluent to remove the initial brown fraction and purple fraction. The bright red product fraction was then isolated using CH2Cl2/hexane (3:1) as the eluent. The red fraction was dried and further purified by crystallization using CH2Cl2 and methanol. Red crystalline Ir(btpp)Cl(CO) (483 mg, 0.44 mmol, 68%) was collected. 1H NMR (CDCl3, 300 MHz): δ 1.62 (s, 36 H), 7.77 (d, 8 H, J = 8.1 Hz), 8.13 (d, 4 H, J = 6.9 Hz), 8.20 (d, 4 H, J = 7.2 Hz), 8.95 (s, 8 H). 13C NMR (CDCl3, 75 MHz): 31.8, 35.1, 122.3, 123.7, 124.0, 132.0, 134.3, 134.5, 135.2, 138.4, 141.3, 151.0. HRMS (FABMS): calcd for [C61H61N4OClIr]þ ([M þ H]þ) m/z 1093.4158, found m/z 1093.4166. Preparation of Methoxymethyl(5,10,15,20-tetrakis(p-tolyl)porphyrinato)iridium(III), [Ir(ttp)CH2OCH3] (2). Ir(ttp)H (4a) (10.6 mg, 0.012 mmol), paraformaldehyde (3.7 mg, 0.12 mmol HCHO), and methanol (1.0 mL) were degassed for three freezethaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C for 2 h. The reaction mixture was then dried under vacuum, and it was purified by column chromatography using alumina with a solvent mixture of CH2Cl2 and hexane (1:4) as eluent to yield Ir(ttp)CH2OMe (2; 4.8 mg, 0.005 mmol, 43%). 1H NMR (CDCl3, 300 MHz): δ -2.93 (s, 2 H), -0.60 (s, 3 H), 2.68 (s, 12 H), 7.50 (d, 4 H, J = 6.0 Hz), 7.52 (d, 4 H, J = 6.0 Hz), 7.96 (d, 4 H, J = 6.7 Hz), 8.03 (d, 4 H, J = 7.2 Hz), 8.53 (s, 8 H). 1 H NMR (CD3OD, 300 MHz): δ -3.20 (s, 2 H), -0.46 (s, 3 H), 2.74 (s, 12 H), 7.62 (d, 8 H, J = 8.1 Hz), 8.03 (d, 4 H, J = 7.2 Hz), 8.08 (d, 4 H, J = 6.9 Hz), 8.54 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ 21.7, 33.9, 55.9, 124.3, 127.6, 131.4, 133.6, 133.9, 137.3, 138.8, 143.5. HRMS (FABMS): calcd for [C50H42N4OIr]þ ([M þ H]þ) m/z 907.2982, found m/z 907.2958. Preparation of Methyl(triphenylphosphine)(5,10,15,20-tetrakis(p-tolyl)porphyrinato)iridium(III), [Ir(ttp)CH3(PPh3)] (3d). Ir(ttp)CH3 (3a; 24.4 mg, 0.028 mmol), PPh3 (68.6 mg, 0.26 mmol), and benzene (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 60 °C under N2 for 2 h. The reaction mixture was then dried under vacuum, and it was purified by column chromatography using alumina with a solvent mixture of CH2Cl2 and hexane (1:4) as eluent to isolate the reddish brown fraction. The product was further crystallized using CH2Cl2/ CH3OH to obtain Ir(ttp)CH3(PPh3) (3d; 27.3 mg, 0.024 mmol, 85%). 1H NMR (CDCl3, 300 MHz): δ -7.36 (d, 3 H, 3JPH = 7.5 Hz), 2.66 (s, 12 H), 4.11 (t, 6 H, J = 8.1 Hz), 6.54 (t, 6 H, J = 7.6 Hz), 6.84 (t, 3 H, J = 7.4 Hz), 7.44 (d, 4 H, J = 7.3 Hz), 7.46 (d, 4 H, J = 7.6 Hz), 7.68 (d, 4 H, J = 7.4 Hz), 7.87 (d, 4 H, J = 7.7 Hz), 8.38 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ -18.7 (2JPC = 118.8 Hz), 21.6, 122.5, 126.89, 126.91 (d, 3JPC = 9.4 Hz), 127.4, 127.9, 129.2 (d, 1JPC = 23.6 Hz), 130.9 (d, 2JPC = 11.2 Hz), 131.3, 133.9, 134.3, 136.8, 139.4, 142.4. HRMS (FABMS): calcd for [C67H55N4PIr]þ ([M þ H]þ) m/z 1139.3788, found m/z 1139.3797. Preparation of Hydrido(triphenylphosphine)(5,10,15,20-tetrakis(p-tolyl)porphyrinato)iridium(III), [Ir(ttp)H(PPh3)] (4b). Ir(ttp)H(PPh3)3 was prepared according to the procedure in the synthesis of Ir(oep)H(PPh3) (oep = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion).51 Ir(ttp)H (4a; 4.7 mg, 0.005 mmol), (51) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc. 1986, 108, 3659–3663.

Article PPh3 (1.4 mg, 0.005 mmol), and benzene-d6 (0.5 mL) were added to the Teflon screw-capped NMR tube under N2, The reaction mixture was degassed for three freeze-thaw-pump cycles and then flame-sealed under vacuum. The reaction mixture changed to reddish brown instantaneously at room temperature to give Ir(ttp)H(PPh3) (4b) quantitatively, as no other iridium porphyrin species were observed in 1H NMR. 1H NMR (C6D6, 300 MHz): δ -32.57 (d, 1 H, 2JPH = 258.6 Hz), 2.38 (s, 12 H), 4.51 (br, 6 H), 6.48 (br, 6 H), 6.64 (br, 3 H), 7.31 (d, 4 H, J = 7.7 Hz), 7.81 (d, 4 H, J = 7.7 Hz), 7.90 (d, 4 H, J = 8.0 Hz), 8.75 (s, 8 H). One set of mphenyl H signals was shielded by the C6H6 signal. 13C NMR (C6D6, 75 MHz): δ 21.5, 123.6, 127.3, 131.5 (d, 2JPC = 10.1 Hz), 132.0, 134.3, 134.7, 136.8, 139.9, 143.9; ipso-, m-, and p-phenyl C signals of coordinated PPh3 and one m-tolyl group’s C signal of poprhyrin were shielded by the C6H6 signal. HRMS (FABMS): calcd for [C66H52N4IrP]þ ([M]þ) m/z 1124.3553, found m/z 1124.3520. Preparation of Potassium (18-crown-6)(5,10,15,20-tetrakis(ptolyl)porphyrinato)iridate(I), [Ir(ttp)-][K(18-crown-6)þ] (5b). Ir(ttp)-K(18-crown-6)þ (5b) was synthesized according to the procedure in the synthesis of Rh(ttp)-K(18-crown-6)þ.32 Ir(ttp)H (4a; 5.0 mg, 0.006 mmol), KOH (3.3 mg, 0.058 mmol), 18crown-6 ether (4.6 mg, 0.017 mmol), and THF-d8 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screwcapped NMR tube and then flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 150 °C in an oil bath in 45 min to yield Ir(ttp)-K(18-crown-6)þ (5b) quantitatively using the residual THF’s β-protons as the internal standard. The THF-d8 solvent in the tube could be removed under vacuum before flame-sealing, and degassed benzene-d6 was added to obtain stable Ir(ttp)-K(18-crown-6)þ (5b). 1H NMR (THF-d8, 300 MHz): δ 2.62 (s, 12 H). 7.45 (d, 8 H, J = 7.7 Hz), 7.89 (d, 8 H, J = 7.7 Hz), 8.14 (s, 8 H). 1H NMR (C6D6, 300 MHz): δ 2.38 (s, 12 H). 7.28 (d, 8 H, J = 7.7 Hz), 7.95 (d, 8 H, J = 7.9 Hz), 8.58 (s, 8 H). Preparation of Isopropoxy(5,10,15,20-Tetrakis(p-tolyl)porphyrinato)iridium(III), [Ir(ttp)OiPr] (7). Ir(ttp)Cl(CO) (1a; 6.6 mg, 0.0071 mmol), KOH (9.9 mg, 0.18 mmol), and isopropyl alcohol (0.50 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube. The reaction mixture was reacted at room temperature under N2 for 10 min. The reaction mixture was then dried at room temperature to remove most of the solvent under vacuum. No heating was used during the removal of solvent to avoid any decomposition of the product. C6D6 (0.5 mL) was added into the tube under N2, and the reaction mixture was further degassed for three freeze-thaw-pump cycles and then flame-sealed under vacuum. The NMR yield of Ir(ttp)OiPr (7) was estimated to be ∼85% using the ratios of pyrrole proton’s integrations of Ir(ttp)OiPr (7) and unknown iridium porphyrin species (8.00:1.42). 1H NMR (C6D6, 400 MHz): δ -3.25 (hep, 1 H, J = 6.0 Hz), -1.94 (d, 6 H, J = 5.6 Hz), 2.42 (s, 12 H), 7.28 (d, 4 H, J = 8.8 Hz), 7.30 (d, 4 H, J = 8.8 Hz), 8.06 (d, 4 H, J = 7.6 Hz), 8.10 (d, 4 H, J = 7.2 Hz), 8.51 (s, 8 H). 13C NMR and MS cannot be obtained, due to the decomposition of 7 upon standing under N2 over a short time. Reaction of Ir(ttp)Cl(CO) (1a) with Methanol. (i). In 1 Day. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol) and methanol (2.0 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent. The major orange fraction was collected to give Ir(ttp)CH3 (3a; 2.8 mg, 0.003 mmol, 20%). Unreacted Ir(ttp)Cl(CO) (1a) in 37% yield and Ir(ttp)H (4a) in 9% yield were observed in the crude reaction mixture by 1H NMR spectroscopy, but they cannot be isolated over alumina, and their yields were estimated using the ratios of pyrrole proton signals of Ir(ttp)CH3 (3a), Ir(ttp)Cl(CO) (1a), and Ir(ttp)H (4a) in the crude reaction mixture by 1H NMR spectroscopy and the isolated yield of Ir(ttp)CH3 (3a).


1351

(ii). In 5 Days. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol) and methanol (2.0 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 5 days. All Ir(ttp)Cl(CO) (1a) was consumed. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent. The major orange fraction was collected to give Ir(ttp)CH3 (3a; 7.8 mg, 0.009 mmol, 55%). General Procedures for Reactions of Ir(ttp)Cl(CO) (1a) with Methanol and Various Bases. Addition of 5 Equiv of KOH. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (4.6 mg, 0.081 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 3.7 mg, 0.004 mmol, 26%). Addition of 10 Equiv of KOH. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (9.1 mg, 0.16 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 8.4 mg, 0.010 mmol, 59%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of KOH in 30 min. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (18.2 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 30 min to give Ir(ttp)CH2OMe (2; 8.1 mg, 0.009 mmol, 55%) with CH2Cl2/hexane (1:4) as eluent and Ir(ttp)CH3 (3a; 1.4 mg, 0.002 mmol, 10%) with CH2Cl2/hexane (1:2) as eluent. Addition of 20 Equiv of KOH in 3 h. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (18.2 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 3 h to give Ir(ttp)CH3 (3a; 3.3 mg, 0.004 mmol, 23%) and Ir(ttp)CH2OMe (2; 8.1 mg, 0.009 mmol, 55%). Addition of 20 Equiv of KOH in 1 day. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (18.2 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 10.0 mg, 0.011 mmol, 70%). Addition of 20 Equiv of KOH at 150 °C. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (18.2 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1.5 days to give Ir(ttp)CH2OMe (2; 0.6 mg, 0.0007 mmol, 4%) and Ir(ttp)CH3 (3a; 4.4 mg, 0.005 mmol, 31%). Addition of 30 Equiv of KOH. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (27 mg, 0.49 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 9.4 mg, 0.011 mmol, 66%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of NaOH. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), NaOH (13.0 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 8.1 mg, 0.009 mmol, 57%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of K3PO4. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), K3PO4 (69 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon

1352


screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 7.2 mg, 0.008 mmol, 51%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of Na2CO3. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), Na2CO3 (34 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 8.8 mg, 0.010 mmol, 62%). Addition of 20 Equiv of K2CO3. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), K2CO3 (45 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 7.8 mg, 0.009 mmol, 55%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of Cs2CO3. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), Cs2CO3 (106 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 6.1 mg, 0.007 mmol, 43%). Addition of 20 Equiv of NaOAc. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), NaOAc (27 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 8.1 mg, 0.009 mmol, 57%) and a trace amount of Ir(ttp)CH2OMe (2). Addition of 20 Equiv of KOAc. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOAc (31 mg, 0.32 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CH3 (3a; 6.5 mg, 0.007 mmol, 46%) and a trace amount of Ir(ttp)CH2OMe (2). Reaction of Ir(btpp)Cl(CO) with Methanol and KOH: Preparation of Methyl(5,10,15,20-tetrakis(p-tert-butylphenyl)porphyrinato)iridium(III) (3b). Ir(btpp)Cl(CO) (1b; 15.0 mg, 0.014 mmol), KOH (15.4 mg, 0.27 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(btpp)CH3 (3b; 10.3 mg, 0.010 mmol, 72%). 1H NMR (THF-d8, 300 MHz): δ -6.86 (s, 3 H), 1.71 (s, 36 H), 7.88 (d, 4 H, J = 7.2 Hz), 7.91 (d, 4 H, J = 7.8 Hz), 8.15 (d, 4 H, J = 6.6 Hz), 8.17 (d, 4 H, J = 7.8 Hz), 8.54 (s, 8 H). 13C NMR (THF-d8, 100 MHz): δ -43.1, 32.3, 35.8, 124.5, 124.6, 124.7, 132.0, 134.89, 134.93, 140.5, 144.5, 151.2. HRMS (FABMS): calcd for [C61H63N4Ir]þ m/z 1044.4676, found m/z 1044.4646. Reaction of Ir(tmp)Cl(CO)23 (1c) with Methanol and KOH. Ir(tmp)Cl(CO) (1c; 15.0 mg, 0.014 mmol), KOH (16.2 mg, 0.29 mmol), and methanol (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(tmp)CH3 (3c;24 9.4 mg, 0.010 mmol, 66%). Reaction of Ir(ttp)Cl(CO) (1a) with Methanol-d4 and KOH. Ir(ttp)Cl(CO) (1a; 15.0 mg, 0.016 mmol), KOH (18.2 mg, 0.32 mmol), and methanol-d4 (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day to give Ir(ttp)CD3 (3a-d3; 10.0 mg, 0.011 mmol, 70%). Reaction of Ir(ttp)Cl(CO) (1a) with Methanol-d4 in a Sealed NMR Tube. (i). Without KOH. Ir(ttp)Cl(CO) (1a; 5.8 mg, 0.006 mmol) and methanol-d4 (0.5 mL) were degassed for three freezethaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The course of the reaction was monitored by 1H NMR spectroscopy. The reaction mixture was covered

Cheung et al. by aluminum foil, and no reaction occurred at room temperature in 1 day. The reaction mixture was then heated at 200 °C in an oil bath. In 4 days, Ir(ttp)D (4a-d) was observed, and most of the undissolved Ir(ttp)Cl(CO) remained unreacted as a reddish purple solid. After 23 days, Ir(ttp)Cl(CO) and Ir(ttp)D were consumed, and Ir(ttp)CD3 (3a-d3; 5.5 mg, 0.006 mmol, 100%) was isolated by column chromatography. The formation of Ir(ttp)D (4a-d) and Ir(ttp)CD3 (3ad3) was confirmed by the authentic samples of Ir(ttp)H (4a) and Ir(ttp)CH3 (3a) in CD3OD. 1H NMR of Ir(ttp)H (4a) (CD3OD, 300 MHz): δ 2.74 (s, 12 H). 7.61 (d, 8 H, J = 7.5 Hz), 8.04 (d, 4 H, J = 7.8 Hz), 8.06 (d, 4 H, J = 7.8 Hz), 8.50 (s, 8 H). No hydride signal of Ir(ttp)H was observed due to proton exchange between Ir(ttp)-H and CD3O-D to give Ir(ttp)D.37 1H NMR of Ir(ttp)CH3 (3a) (CD3OD, 300 MHz): δ -7.07 (s, 3 H), 2.74 (s, 12 H). 7.61 (d, 8 H, J = 7.5 Hz), 8.03 (d, 4 H, J = 9.0 Hz), 8.06 (d, 4 H, J = 8.4 Hz), 8.46 (s, 8 H). (ii). With KOH. Ir(ttp)Cl(CO) (1a; 5.2 mg, 0.006 mmol), KOH (6.3 mg, 0.11 mmol), and methanol-d4 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The course of the reaction was monitored by 1H NMR spectroscopy. The reaction mixture was covered by aluminum foil, and reaction occurred at room temperature in 1 day to give a small amount of Ir(ttp)D (4a-d) and Ir(ttp)CD2OCD3 (2-d5). The reaction mixture was then heated at 200 °C in an oil bath. In 30 min, Ir(ttp)Cl(CO) was completely consumed, and Ir(ttp)-Kþ (5a) and Ir(ttp)CD2OCD3 (2-d5) were observed. Ir(ttp)CD3 (3a-d3) was also formed in 90 min. In 4 days, all Ir(ttp)CD2OCD3 and Ir(ttp)-Kþ were consumed, and only Ir(ttp)CD3 (3a-d3) (4 mg, 0.005 mmol, 81%) was isolated using column chromatography with CH2Cl2/hexane (1:2) as the eluent. 1H NMR of Ir(ttp)-Kþ (5a) (CD3OD, 300 MHz): δ 2.67 (s, 12 H), 7.51 (d, 8 H, J = 7.5 Hz), 7.90 (d, 8 H, J = 6.3 Hz), 8.11 (s, 8 H). Reaction of Ir(ttp)H (4a) with Methanol. Ir(ttp)H (1a; 10.3 mg, 0.012 mmol) and methanol (1.0 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2. In 1 day, some brown Ir(ttp)H remained unreacted. In 3 days, a dark brown solid was formed. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to give Ir(ttp)CH3 (3a; 6.9 mg, 0.008 mmol, 66%). Reactivities of Iridium Porphyrin Intermediates with Methanol-d4 in Sealed NMR Tubes. (i). Ir(ttp)H (4a). Ir(ttp)H (4a; 5.0 mg, 0.006 mmol) and methanol-d4 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The reaction mixture was heated at 200 °C, and Ir(ttp)H turned to Ir(ttp)D (4a-d) instantaneously.37 Upon heating for 23 days, some Ir(ttp)D (4a-d) remained unreacted, as was observed by 1H NMR spsectroscopy, and a deep brown solid was formed. The solvent was then removed under vacuum. The deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to obtain Ir(ttp)CD3 (3a-d3; 2.0 mg, 0.002 mmol, 39%). Unreacted Ir(ttp)D (4a-d) cannot be isolated by column chromatography. (ii). Ir(ttp)H (4a) with KOH. Ir(ttp)H (4a; 5.0 mg, 0.006 mmol), KOH (6.5 mg, 0.12 mmol), and methanol-d4 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 200 °C for 5 min to give pale brown Ir(ttp)-Kþ (5a) in ∼90% yield and Ir(ttp)D in ∼10% yield. Ir(ttp)-Kþ (5a) and Ir(ttp)D (4a-d) were consumed at 200 °C in 4 days, and a deep brown solid was formed. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to obtain Ir(ttp)CD3 (3a-d3; 2.9 mg, 0.003 mmol, 57%).

Article (iii). [Ir(ttp)]2 Dimer16e (6). Ir(ttp)H (4a; 5.0 mg, 0.006 mmol) and benzene (0.5 mL) were degassed for three freezethaw-pump cycles in a Teflon screw-capped NMR tube. 2,2,6,6-Tetramethylpiperidinooxy (TEMPO; 1.0 mg, 0.006 mmol) was added to the reaction mixture under N2. The reaction mixture was then shaken for 1 min to undergo dehydrogenative dimerization of Ir(ttp)H (4a) to yield [Ir(ttp)]2 dimer (6) quantitatively. The reaction mixture was dried under vacuum in warm water overnight. Degassed methanol-d4 (0.5 mL) was added to the reaction mixture under N2, and the reaction mixture was further degassed for three freeze-thaw-pump cycles and then flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 200 °C for 16 h, and Ir(ttp)CD2OCD3 (2-d5), Ir(ttp)CD3 (3a-d3), and a trace amount of Ir(ttp)D (4a-d) were observed by 1H NMR spectroscopy. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to obtain Ir(ttp)CD2OCD3 (2-d5; 2.4 mg, 0.003 mmol, 44%) and Ir(ttp)CD3 (3a-d3; 0.4 mg, 0.0004 mmol, 7%). Unreacted Ir(ttp)D (4a-d) cannot be isolated by column chromatography. 1H NMR of [Ir(ttp)]2 (6) (CD3OD, 300 MHz): δ 2.77 (s, 12 H). 7.66 (d, 8 H, J = 7.5 Hz), 8.14 (d, 8 H, J = 7.8 Hz), 8.71 (s, 8 H). (iv). [Ir(ttp)]2 Dimer16e (6) with KOH. Ir(ttp)H (4a; 5.0 mg, 0.006 mmol) and benzene (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube. 2,2,6,6-Tetramethylpiperidinooxy (TEMPO; 1.0 mg, 0.006 mmol) was added to the reaction mixture under N2. The reaction mixture was then shaken for 1 min to undergo dehydrogenative dimerization of Ir(ttp)H (4a) to yield [Ir(ttp)]2 dimer (6) quantitatively. The reaction mixture was dried under vacuum in warm water overnight. Degassed methanol-d4 (0.5 mL) and KOH (6.5 mg, 0.12 mmol) were added to the reaction mixture under N2, and the reaction mixture was further degassed for three freeze-thaw-pump cycles and flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 200 °C for 5 min to give pale brown Ir(ttp)-Kþ (5a) in ∼90% yield with Ir(ttp)D in ∼10% yield (4a-d). Ir(ttp)-Kþ (5a) and Ir(ttp)D (4a-d) were consumed at 200 °C in 4 days, and a deep brown solid was formed. The solvent was then removed under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to obtain Ir(ttp)CD3 (3a-d3; 3.0 mg, 0.003 mmol, 59%). (v). Ir(ttp)-[K(18-crown-6)]þ (5b). Ir(ttp)H (4a; 5.2 mg, 0.006 mmol), KOH (6.8 mg, 0.12 mmol), 18-crown-6 ether (4.8 mg, 0.018 mmol), and THF (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C in an oil bath for 45 min to yield Ir(ttp)-[K(18-crown6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. Methanol-d4 (0.5 mL) was added to the tube under N2, and the tube was further degassed for three freeze-thaw-pump cycles and then flame-sealed under vacuum. The reaction mixture was heated at 200 °C, and pale brown Ir(ttp)-[K(18-crown-6)]þ (5b) was consumed in 4 days to give a deep brown solid. The reaction mixture was dried under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent to give Ir(ttp)CD3 (3a; 4.5 mg, 0.005 mmol, 85%). 1H NMR of pure Ir(ttp)-[K(18-crown-6)]þ (5b) in CD3OD cannot be obtained, as some of it reacted instantaneously with CD3OD at room temperature to yield Ir(ttp)CD3 (3a-d4). Reactivity of Ir(ttp)H with Paraformaldehyde and Methanold4. Ir(ttp)H (4a; 4.7 mg, 0.005 mmol), paraformaldehyde (2.8 mg, 0.09 mmol HCHO), and methanol-d4 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The reaction mixture


1353

was covered by aluminum foil and heated at 200 °C in an oil bath for 1 day to yield Ir(ttp)CH2OCD3 (2-d3; 3.7 mg, 0.004 mmol, 74%) and Ir(ttp)CD3 (3a-d3; 0.2 mg, 0.0003 mmol, 5%) after column chromatography using alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent. Thermal Stability of Ir(ttp)CH2OMe (2) in C6D6. Ir(ttp)CH2OMe (2; 1.9 mg, 0.002 mmol) and benzene-d6 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 200 °C for 14 days. Ir(ttp)CH2OMe (2) did not react, and it was recovered quantitatively using residual benzene’s protons as an internal standard. Reaction of Ir(ttp)CH2OMe (2) with Methanol-d4 and KOH. Ir(ttp)CH2OMe (2; 4.3 mg, 0.005 mmol), methanol-d4 (0.4 mL), and KOH (5.3 mg, 0.095 mmol) were degassed for three freezethaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. The reaction mixture was covered by aluminum foil and heated at 200 °C in an oil bath. In 1 day, Ir(ttp)-Kþ (5a) and Ir(ttp)CD3 (3a-d3) were formed, and some of the Ir(ttp)CH2OMe exchanged with CD3OD to yield Ir(ttp)CH2OCD3 (2-d3). In 4 days, Ir(ttp)CD3 (3a-d3; 2.6 mg, 0.003 mmol, 62%) was isolated by column chromatography using alumina with a solvent mixture of CH2Cl2 and hexane (1:2) as eluent. Recovered Ir(ttp)CH2OMe (2; 4%, NMR yield) and Ir(ttp)CH2OCD3 (2-d3; 24%, NMR yield) were observed in the crude reaction mixture by 1H NMR spectroscopy but cannot be isolated, and their yields were estimated using the integrations of o-phenyl protons of total iridium porphyrin products and the methylene and methyl protons of Ir(ttp)CH2OCH3/ Ir(ttp)CH2OCD3 in the crude reaction mixture by 1H NMR spectroscopy. Effect of PPh3 on the Reactions of Iridium Porphyrins and Methanol. (i). Ir(ttp)-Kþ with PPh3 at 100 °C. Ir(ttp)H (4a; 8.1 mg, 0.009 mmol), KOH (10.5 mg, 0.19 mmol), 18-crown-6 ether (7.4 mg, 0.028 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C for 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. PPh3 (2.5 mg, 0.009 mmol) and degassed methanol (2 mL) were added to the tube under N2, and the tube was further degassed for three freeze-thaw-pump cycles. Ir(ttp)CH3(PPh3) (3d) was observed by thin-layer chromatography when the reaction mixture was stirred at room temperature for 5 min. The reaction mixture was further heated at 100 °C for 36 h to give a deep purple precipitate. The reaction mixture was dried under vacuum, and the deep purple residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:4) as eluent to give Ir(ttp)CH3(PPh3) (3d; 1.8 mg, 0.002 mmol, 17%). (ii). Ir(ttp)-Kþ at 100 °C. Ir(ttp)H (4a; 8.4 mg, 0.010 mmol), KOH (10.9 mg, 0.19 mmol), 18-crown-6 ether (7.7 mg, 0.029 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C for 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. Degassed methanol (2 mL) was added to the tube under N2, and the tube was further degassed for three freeze-thaw-pump cycles. Ir(ttp)CH3 (3a) was not observed by thin-layer chromatography when the reaction mixture was stirred at room temperature for 5 min. The reaction mixture was further heated at 100 °C for 72 h. The reaction mixture was dried under vacuum, and the deep brown residue was purified by column chromatography over alumina with a solvent mixture of CH2Cl2 and hexane (1:3) as eluent to give Ir(ttp)CH3 (3a; 0.8 mg, 0.001 mmol, 9%).

1354


(iii). Ir(ttp)-Kþ with PPh3 at 200 °C. (a). 1 h. Ir(ttp)H (4; 9.8 mg, 0.011 mmol), KOH (6.4 mg, 0.11 mmol), 18-crown-6 ether (9.0 mg, 0.034 mmol), and THF (2 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C in 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. PPh3 (3.0 mg, 0.011 mmol) and degassed methanol (1 mL) were added to the tube under N2, and the tube was further degassed for three freeze-thaw-pump cycles. The reaction mixture was heated at 200 °C for 1 h to yield Ir(ttp)CH3(PPh3) (3d; 3.8 mg, 0.003 mmol, 29%). (b). 12 h. Ir(ttp)H (4a; 10.3 mg, 0.012 mmol), KOH (13.4 mg, 0.24 mmol), 18-crown-6 ether (9.5 mg, 0.036 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C for 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. PPh3 (3.1 mg, 0.012 mmol) and degassed methanol (1 mL) were added to the tube under N2, and the tube was further degassed for three freezethaw-pump cycles. The reaction was heated at 200 °C for 12 h to yield Ir(ttp)CH3(PPh3) (3d; 7.0 mg, 0.006 mmol, 52%). (c). 24 h. Ir(ttp)H (4a; 10.4 mg, 0.012 mmol), KOH (13.5 mg, 0.24 mmol), 18-crown-6 ether (9.6 mg, 0.036 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C for 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. PPh3 (3.2 mg, 0.012 mmol) and degassed methanol (1 mL) were added to the tube under N2, and the tube was further degassed for three freezethaw-pump cycles. The reaction was heated at 200 °C for 1 day to yield Ir(ttp)CH3(PPh3) (3d; 10.6 mg, 0.009 mmol, 77%). (iv). Ir(ttp)-Kþ at 200 °C. (a). 12 h. Ir(ttp)H (4a; 9.7 mg, 0.011 mmol), KOH (12.6 mg, 0.23 mmol), 18-crown-6 ether (8.9 mg, 0.034 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C for 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. Degassed methanol (1 mL) was added to the tube under N2, and the tube was further degassed for three freeze-thaw-pump cycles. The reaction mixture was heated at 200 °C for 12 h to give Ir(ttp)CH3 (3a; 2.8 mg, 0.003 mmol, 28%). (b). 24 h. Ir(ttp)H (4a; 10.2 mg, 0.012 mmol), KOH (13.3 mg, 0.24 mmol), 18-crown-6 ether (9.4 mg, 0.035 mmol), and THF (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 150 °C in 45 min to yield Ir(ttp)-[K(18-crown-6)]þ (5b) quantitatively. The solvent in the tube was then removed under vacuum, and the reaction mixture was further dried in warm water for a few hours. Degassed methanol (1 mL) was added to the tube under N2,

Cheung et al. and the tube was further degassed for three freeze-thaw-pump cycles. The reaction mixture was heated at 200 °C for 1 day to give Ir(ttp)CH3 (3a; 9.0 mg, 0.010 mmol, 87%). (v). Ir(ttp)H with PPh3 at 200 °C. Ir(ttp)H (4a; 10.0 mg, 0.012 mmol), PPh3 (3.0 mg, 0.012 mmol), and methanol (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture changed to reddish brown instantaneously due to the formation of Ir(ttp)H(PPh3) (4b).51 The reaction mixture was covered by aluminum foil and heated at 200 °C. No Ir(ttp)CH3(PPh3) (3d) was observed in 1.5 h by thin-layer chromatography. The reaction mixture was further heated for 3 days to yield Ir(ttp)CH3(PPh3) (3d; 3.2 mg, 0.003 mmol, 24%). Study of Methyl Group Exchange of Ir(ttp)CH3 (3a) with Methanol-d4. (i). Without Base. Ir(ttp)CH3 (3a) (11.0 mg, 0.013 mmol) and methanol-d4 (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day. The solvent was then removed under vacuum and the deep brown residue was purified by column chromatography over alumina eluting with a solvent mixture of CH2Cl2 and hexane (1:2). The major orange fraction was collected to give only unreacted Ir(ttp)CH3 (3a) (8.8 mg, 0.010 mmol, 80%) without Ir(ttp)CD3 (3a-d3), since the intergration signals of pyrrole: methyl = 8.00: 3.00 in the isolated product by 1 H NMR spectroscopy. (ii). With Base. Ir(ttp)CH3 (3a) (8.1 mg, 0.009 mmol), KOH (10.5 mg, 0.19 mmol), and methanol-d4 (1 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped tube. The reaction mixture was covered by aluminum foil and heated at 200 °C under N2 for 1 day. The solvent was then removed under vacuum and the deep brown residue was purified by column chromatography over alumina eluting with a solvent mixture of CH2Cl2 and hexane (1:2). The major orange fraction was collected to give Ir(ttp)CH3 (3a) (5.3 mg, 0.006 mmol, 65%) and Ir(ttp)CD3 (3a-d3) (1.5 mg, 0.002 mmol, 18%) using the ratio of 3a: 3a-d3 in the isolated product [3.6: 1.0 (integration of methyl’s proton was set as 3.00; integration of pyrrole’s proton of Ir(ttp)CH3 and Ir(ttp)CD3 = 10.23] by 1H NMR spectroscopy. Proton Exchange of Ir(ttp)H (4a) with 50 equiv of CD3OD. Ir(ttp)H (4a; 1.9 mg, 0.002 mmol), methanol-d4 (4.0 mg, 4.5 μL, 0.11 mmol), and benzene-d6 (0.5 mL) were degassed for three freeze-thaw-pump cycles in a Teflon screw-capped NMR tube and then flame-sealed under vacuum. After 6 h at room temperature, Ir(ttp)D (4a-d) in 77% NMR yield was formed and some Ir(ttp)H (4a) remained unreacted in 23% NMR yield using residual benzene as the internal standard.

Acknowledgment. We thank the Research Grants Council of the HKSAR, People’s Republic of China (CUHK 400308), for financial support. Supporting Information Available: Table and figure of crystallographic data for complex 2 (CIF and PDF), sequence of reactions monitored by 1H NMR, and 1H and 13C NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

Reactivity Studies of Iridium(III) - American Chemical Society

Recommend Documents