Regioselective Mono-, Di-, and Trifunctionalization of Iridabenzofurans

Organometallics 2011, 30, 129–138 DOI: 10.1021/om100888z

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Regioselective Mono-, Di-, and Trifunctionalization of Iridabenzofurans through Electrophilic Substitution Reactions George R. Clark, Paul M. Johns, Warren R. Roper, Tilo S€ ohnel, and L. James Wright* Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand. Received September 14, 2010

Reaction between the cationic iridacyclopentadiene complex [Ir(C4H4)(NCMe)(CO)(PPh3)2][CF3SO3] (1) and methylpropiolate produces the cationic iridabenzofuran [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) in high yield. On treatment of 2 with chloride, the carbonyl ligand is displaced and the corresponding neutral iridabenzofuran Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) is formed. The fused metallacyclic rings of the iridabenzofurans 2 and 3 bear only one substituent (OMe), and therefore these compounds are well suited for studies of electrophilic aromatic substitution reactions. Bromination of cationic 2 with pyridinium tribromide proceeds to give the monobrominated iridabenzofuran [Ir(C7H5O{OMe-7}{Br-6})(CO)(PPh3)2][CF3SO3] (4) exclusively. Bromination of neutral 3 with the same reagent gives the dibrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-2})Br(PPh3)2 (5) exclusively. Treatment of compound 3 with mercury(II) trifluoroacetate followed by excess bromide (to displace coordinated trifluoroacetate) produces the trimercurated iridabenzofuran Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6). The three Hg-C bonds in 6 are readily cleaved on addition of pyridinium tribromide, and the resulting product is the tribrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7). These regioselective mono-, di-, and trifunctionalization reactions of iridabenzofurans have been studied by DFT calculations, and the derived condensed Fukui functions have been used to rationalize the preferred sites for electrophilic attack. The crystal structures of 2-7 have been obtained.

*To whom correspondence should be addressed. E-mail: lj.wright@ auckland.ac.nz. (1) Bleeke, J. R. Chem. Rev. 2001, 101, 1205–1227. (2) Landorf, C., W.; Haley, M. M. Angew. Chem., Int. Ed. 2006, 45, 3914–3936. (3) Wright, L. J. Dalton Trans. 2006, 1821–1827. (4) He, G.; Xia, H.; Jia, G. Chin. Sci. Bull. 2004, 49, 1543–1553. (5) Fernandez, I.; Frenking, G. Chem.;Eur. J. 2007, 13, 5873–5884. (6) Iron, M. A.; Lucassen, A. C. B.; Cohen, H.; van der Boom, M. E.; Martin, J. M. L. J. Am. Chem. Soc. 2004, 126, 11699–11710. (7) Rickard, C. E. F.; Roper, W. R.; Woodgate, S. D.; Wright, L. J. Angew. Chem., Int. Ed. 2000, 39, 750–752. (8) Clark, G. R.; Johns, P. M.; Roper, W. R.; Wright, L. J. Organometallics 2008, 27, 451–454. (9) Clark, G. R.; Johns, P. M.; Roper, W. R.; Wright, L. J. Organometallics 2006, 25, 1771–1777. (10) Clark, G. R.; O’Neale, T. R.; Roper, W. R.; Tonei, D. M.; Wright, L. J. Organometallics 2009, 28, 567–572. (11) Clark, G. R.; Lu, G.-L.; Roper, W. R.; Wright, L. J. Organometallics 2007, 26, 2167–2177. (12) Dalebrook, A. F.; Wright, L. J. Organometallics 2009, 28, 5536– 5540. (13) Wang, T.; Li, S.; Zhang, H.; Lin, R.; Han, F.; Lin, Y.; Wen, T. B.; Xia, H. Angew. Chem., Int. Ed. 2009, 48, 6453–6456.

metallabenzothiazole,13 and metallabenzoxazole13 cores. In all cases the metallacyclic rings are heavily substituted. The chemistry of these fused ring metallabenzenes remains very underdeveloped, and many key questions remain unanswered. Particularly relevant to this study is the issue of how closely the reactivity of these compounds parallels that of the parent organic benzenoids. With respect to the two reported metallabenzofurans, preliminary studies indicate that electrophilic attack occurs preferentially at the unsubstituted C6 position (the numbering scheme and isomeric form of all the metallabenzofurans discussed in this paper are given in Chart 1). Thus, treatment of the osmabenzofuran Os(C7H2O{OMe-7}{CO2Me-4}{Ph-2}{Ph-1})(CS)(PPh3)2 with acid results in protonation at C6 and formation of the cationic tethered osmabenzene [Os(C5H{CH2CO2Me-5}{CO2Me-4}{Ph-2}{Ph-1})(CS)(PPh3)2]þ, while bromination of the same osmabenzofuran results in substitution of the hydrogen at C6.9 The ruthenabenzofuran Ru(C7H3O{OMe-7}{CO2Me-4}{CO2Me-2})(CO)(PPh3)2 also undergoes protonation at C6 to form the corresponding cationic tethered ruthenabenzene, [Ru[C5H2(CO2Me-2)(CO2Me-4)(CH2CO2Me-5)](CO)(PPh3)2]þ.10 However, more comprehensive studies of the reactions between these compounds and electrophiles are not feasible because in both cases the metallabenzofuran rings are extensively substituted. In this paper we now report (i) the syntheses in high yield of the cationic iridabenzofuran [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) and the derived neutral iridabenzofuran Ir(C7H5O{OMe-7})Cl(PPh3)2 (3),

r 2010 American Chemical Society

Published on Web 12/10/2010

Introduction In contrast to the situation with simple metallabenzenes, where there is now a considerable amount of relevant synthetic, structural, spectral, computational, and reactivity data available,1-8 examples of fused heterocyclic ring metallabenzenes are much rarer. Among this class of metallabenzenoids are very limited numbers of compounds with metallabenzofuran,9,10 metallabenzothiophene,11 metallabenzothiazolium,12

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Chart 1. Valence Bond Representations of the Delocalized Bonding in Metallabenzofurans

Clark et al. Scheme 1. Regioselective Electrophilic Aromatic Substitution Reactions of Iridabenzofurans 2 and 3 to Give Mono-, Di-, or Trisubstituted Products

both of which bear only one fused ring substituent (OMe at C7); (ii) bromination of cationic 2 to form exclusively the monobrominated (at C6) iridabenzofuran [Ir(C7H5O{OMe-7}{Br-6})(CO)(PPh3)2][CF3SO3] (4); (iii) bromination of neutral 3 to form exclusively the dibrominated (at C6 and C2) iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-2})Br(PPh3)2 (5); (iv) mercuration of 3 to form the trimercurated (at C6, C4, and C2) iridabenzofuran Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6); (v) cleavage of the mercurycarbon bonds in 6 with bromine to give the tribrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7); (vi) the application of DFT calculations to rationalize the preferred positions for electrophilic attack in these regioselective mono-, di-, and trifunctionalization reactions; and (vii) the X-ray crystal structure determinations of 2-7.

Results and Discussion It has been demonstrated that the cationic iridacyclopentadiene complex [Ir(C4H4)(NCMe)(CO)(PPh3)2][CF3SO3] (1) can be obtained in high yield from the reaction between [Ir(NCMe)(CO)(PPh3)2][CF3SO3] and ethyne.14,15 Furthermore, the σ-acetylide complex Ir(C4H4)(CtCPh)(CO)(PPh3)2 derived from 1 is transformed into the iridabenzene [Ir(C5H4{CH2Ph-5})(NCMe)2(PPh3)2][BF4]2 on treatment with HBF4 and acetonitrile.16 It was proposed that a key intermediate in this reaction is a vinylidene complex that is formed through protonation of the σ-acetylide ligand.17 Since methyl propiolate has a demonstrated propensity to take part in metallcyclization reactions,9,11,18,19 even in the absence of added acid, we reasoned that treatment of 1 with methylpropiolate could give an iridabenzofuran directly. Indeed this was found to be the case, and the cationic iridabenzofuran [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) can be obtained in 88% yield when 1 and methylpropiolate are heated together under reflux in 1,2-dichloroethane (Scheme 1). It is possible that the vinylidene complex depicted in Scheme 1 is an intermediate (14) Chin, C. S.; Park, Y.; Kim, J.; Lee, B. J. Chem. Soc., Chem. Commun. 1995, 1495–1496. (15) Chin, C. S.; Kim, M.; Lee, H.; Noh, S.; Ok, K. M. Organometallics 2002, 21, 4785–4793. (16) Chin, C. S.; Lee, H. Chem.;Eur. J. 2004, 10, 4518–4522. (17) Chin, C. S.; Lee, H.; Eum, M.-S. Organometallics 2005, 24, 4849– 4852. (18) Grossmann, U.; Hund, H.-U.; Bosch, H. W.; Schmalle, H.; Berke, H. J. Organomet. Chem. 1991, 408, 203–218. (19) Yamazaki, H.; Aoki, K. J. Organomet. Chem. 1976, 122, C54–C58.

in this reaction although we have no direct evidence to support this. Characterizing data for 2 and all other new compounds are detailed in the Experimental Section. The numbering scheme used for the iridabenzofuran ring system is indicated in both Chart 1 and Scheme 1 and follows the normal numbering system used for metallabenzenes.1,2,20 The carbonyl ligand in 2 is relatively labile, and the corresponding neutral iridabenzofuran Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) is readily formed on treatment of 2 with chloride (Scheme 1). The only other reported metallabenzofurans, the ruthenabenzofurans Ru(C7H5O{OMe-7}{CO2Me-4}{CO2Me-2})L(PPh3)2 (L = CO, CN-p-tolyl)10,19 and the osmabenzofuran Os(C7 H5 O{OMe-7}{CO 2Me-4}{Ph-2}{Ph-1})(CS)(PPh3)29 were also formed by metallacyclization reactions involving methylpropiolate. In these cases the cyclization reactions probably also proceed through vinylidene intermediates.9 Whereas these ruthena- and osmabenzofurans are highly substituted, an important feature of the iridabenzofurans 2 and 3 reported here is that they bear only one substituent on the iridabenzofuran ring systems (OMe at C7). The spectroscopic and structural data for 2 and 3 (see discussion below) indicate there is delocalization within the π-bonding system of the fused metallacyclic rings but that this is less extensive than in related iridabenzenes.1,8,11 A distinctive feature of metallabenzenes is that the metal-bound carbon atoms and the attached protons show characteristically (20) Bleeke, J. R.; Ortwerth, M. F.; Chiang, M. Y. Organometallics 1993, 12, 985–987.

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downfield shifted signals in the 13C and 1H NMR spectra, respectively, while the resonances of the remaining ring atoms appear in the aromatic region. The downfield shifts of the atoms closely associated with the metal are consistent with significant π-bonding between the metal and the ring carbon atoms.1-3 In the 1H NMR spectra of 2 and 3 the chemical shifts for H1 (7.70 and 9.36 ppm, respectively) are intermediate between those typically observed for H1 or H5 in iridabenzenes (10-14 ppm)1 and those observed for the H1/4

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in the iridacyclopentadiene 1 (6.75 and 7.30 ppm). The remaining protons within the six-membered iridacyclic rings of 2 and 3 are shifted slightly upfield compared to the corresponding signals in the related iridabenzene Ir(C5H4{SMe-1})Cl2(PPh3)28 (6.25 (H2), 6.99 (H3), 6.38 (H4) ppm). The resonances for H6 in the fused iridafuran rings of 2 and 3 (5.55 and 4.91 ppm, respectively) appear in the region normally observed for iridafurans.21 In the 13C NMR spectra of 2 and 3 the two metal-bound carbon atoms (C1 and C5) appear at relatively low-field values. The average value of the chemical shifts of C1 and C5 in 2 (163 ppm) is intermediate between the average shifts for the metal-bound carbon atoms in the iridacyclopentadiene 1 (142 ppm) and the iridabenzene Ir(C5H4{SMe-1})Cl2(PPh3)28 (214 ppm). The corresponding average for 3 (171 ppm) is also intermediate in value. The resonances of the remaining ring carbon atoms (C2, C3, C4, and C6) of 2 and 3 all fall within the normal aromatic range. With the exception of C4 and C7, all the fused ring carbon atoms (C1-C7) of 2 and 3 appear as triplets due to coupling to phosphorus. This extensive long-range coupling to phosphorus is consistent with electron delocalization throughout the fused ring systems in 2 and 3. The crystal structure of 2 has been determined, and the molecular structure is shown in Figure 1. The crystal data and refinement details for 2 and for all the other crystal structures reported in this paper are available in the Supporting Information. In addition, an X-ray structure summary table for 2 and all the other crystal structures reported in this paper is provided in Table 1. The overall geometry is approximately octahedral with the two triphenylphosphine ligands arranged mutually trans. The fused five- and sixmembered metallacyclic rings form a planar bicyclic ring system with iridium occupying a ring junction position. Although the Ir-C(1) (2.049(3) A˚) and Ir-C(5) (2.073(2) A˚) distances are at or just beyond the upper limit of the range of TM-C distances normally observed for iridabenzenes (ca. 1.93-2.05 A˚),1,2 they are slightly shorter than the three

Figure 1. ORTEP diagram of the cation of 2 showing 50% probability displacement ellipsoids for non-hydrogen atoms and hydrogen atoms as arbitrary spheres. Hydrogen atoms on the phenyl groups have been removed for clarity. Selected distances [A˚]: Ir-C(1) 2.049(3), Ir-C(5) 2.073(2), Ir-O(1) 2.2152(19), C(1)-C(2) 1.358(4), C(2)-C(3) 1.455(4), C(3)-C(4) 1.353(4), C(4)-C(5) 1.441(3), C(5)-C(6) 1.382(3), C(6)-C(7) 1.435(3), C(7)-O(1) 1.265(3).

Table 1. X-ray Structure Summary for Compounds 2-7 2

4

)

C46H38F3IrO6 C44H38ClIr C46H37BrF3IrO6 P2S P2S O2P2 mol wt 1029.96 888.33 1108.87 cryst syst monoclinic monoclinic triclinic P1 space group Cc P21/c a, A˚ 10.3509(1) 9.641(5) 11.6419(1) b, A˚ 24.1537(2) 18.457(5) 14.2429(1) c, A˚ 17.6886(1) 20.325(5) 14.4475(2) R, deg 90 90 86.026(1) β, deg 105.935(1) 93.287(5) 70.497(1) γ, deg 90 90 69.747(1) 4252.44(6) 3611(2) 2115.64(4) V, A˚3 T, K 85(2) 90(2) 84(2) Z 4 4 2 1.609 1.634 1.741 D(calcd), g cm-3 F(000) 2048 1768 1092 3.326 3.899 4.287 μ, mm-1 cryst size, mm 0.32 0.30 0.31 0.08 0.36 0.26 0.04 0.30 0.26 2θ (min., max.), deg 2.07, 26.33 2.12, 28.69 1.50, 26.40 no. of rflns collected 37 530 25 426 20 683 no. of indep rflns 8342 8905 8604 T (min., max.) 0.3654, 0.4931 0.630, 0.856 0.3333, 0.4122 0.676 0.927 1.036 goodness of fit on F2 0.0147, 0.0367 0.0405, 0.0712 0.0254, 0.0620 R1, wR2 (obsd data)a R1, wR2 (all data) 0.0155, 0.0374 0.0781, 0.0799 0.0264, 0.0628 P P P a 2 2 2 P R1 = Fo| - |Fc / |Fo|; wR2 = { [w(Fo - Fc ) ]/ [w(Fo2)2]}1/2. )

formula

3

5 3 CH2Cl2

6 3 2(CH2Cl2)

7

C44H36Br3IrO2P2 3 CH2Cl2 1175.52 monoclinic P21/c 18.0293(3) 9.6157(1) 25.6486(2) 90 109.346(1) 90 4195.48(9) 84(2) 4 1.861 2280 6.281 0.32 0.20 0.10 1.68, 26.37 19 587 8518 0.2976, 0.4212 1.108 0.0375, 0.0775 0.0478, 0.0830

C44H35Br4Hg3IrO2P2 3 2(CH2Cl2) 1941.12 orthorhombic Pbca 16.3082(11) 24.8178(17) 25.2409(15) 90 90 90 10216(1) 90(2) 8 2.524 7088 15.024 0.20 0.05 0.02 1.64, 25.50 94 685 9501 0.039, 0.861 0.842 0.0766, 0.1732 0.2152, 0.2199

C44H35Br4Ir O2P2 1169.50 triclinic P1 10.5215(1) 13.5293(1) 14.1961(1) 99.110(1) 93.698(1) 97.116(1) 1972.63(3) 86(2) 2 1.969 1124 7.555 0.30 0.22 0.10 1.46, 25.00 6922 6922 0.1966, 0.3199 1.040 0.0468, 0.1264 0.0552, 0.1307

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Figure 2. ORTEP diagram of 3 showing 50% probability displacement ellipsoids for non-hydrogen atoms and hydrogen atoms as arbitrary spheres. Hydrogen atoms on the phenyl and methyl groups have been removed for clarity. Selected distances [A˚]: Ir-C(1) 1.998(5), Ir-C(5) 1.987(6), Ir-O(1) 2.256(3), C(1)-C(2) 1.337(7), C(2)-C(3) 1.425(9), C(3)-C(4) 1.360(9), C(4)-C(5) 1.444(8), C(5)-C(6) 1.385(8), C(6)-C(7) 1.403(8), C(7)-O(1) 1.267(6).

Ir-C(sp2) distances reported for the related cationic iridacyclopentadiene complex [Ir(C4H4)(CHdCHNEt3)(CO)(PPh3)2]ClO4 (2.114(6), 2.094(6), 2.149(5) A˚).22 The difference between the two Ir-C bonds in 2 is very small, but significant. There is considerable C-C bond length alternation within the fused ring system (C(1)-C(2) 1.358(4), C(2)C(3) 1.455(4), C(3)-C(4) 1.353(4), C(4)-C(5) 1.441(3), C(5)C(6) 1.382(3), C(6)-C(7) 1.435(3) A˚). Nevertheless, these distances all fall within the range reported for metallabenzenes.1,2 The structure of 3 has also been determined, and the molecular structure is shown in Figure 2. The overall geometry about iridium is very similar to that observed for 2 except that chloride replaces the CO ligand. The two Ir-C bonds in 3 (Ir-C(1) 1.998(5), Ir-C(5) 1.987(6) A˚) are both shorter than the corresponding bonds in 2, and there is no significant difference between their lengths. Within the planar fused bicyclic ring system of 3 there is still some C-C bond length alternation within the six-membered iridacyclic ring (C(1)C(2) 1.337(7), C(2)-C(3) 1.425(9), C(3)-C(4) 1.360(9), C(4)C(5) 1.444(8) A˚). However, unlike the situation in 2, there is no significant difference between the two C-C bond lengths within the five-membered iridacyclic ring in 3 (C(5)-C(6) 1.385(8), C(6)-C(7) 1.403(8) A˚), and both are close to normal aromatic distances. This suggests that in 3 the delocalization within the five-membered ring is greater than in 2. The delocalized bonding within the fused bicyclic ring systems of 2 and 3 can be considered in terms of the three valence bond structures depicted in Chart 1.9,10 The NMR (21) Bierstedt, A.; Clark, G. R.; Roper, W. R.; Wright, L. J. J. Organomet. Chem. 2006, 691, 3846–3852. (22) Chin, C. S.; Lee, H.; Oh, M. Organometallics 1997, 16, 816–818.

Clark et al.

and structural data discussed above suggest that for 2 the major contribution is from valence structure III, with smaller but significant contributions from I and II. For 3 the most important contribution probably also comes from valence structure III. However, in this case the equal Ir-C(1) and Ir-C(5) distances, the indistinguishable C(5)-C(6) and C(6) C(7) distances, and the pronounced downfield shifts for H1 and C1 in the 1H and 13C NMR spectra all suggest the contributions from I and II are relatively larger than in 2. A possible reason for this could be that the cationic charge and the π-accepting CO ligand in 2 both serve to reduce slightly the π-bond delocalization compared to 3, although confirmation of this speculation would have to await the results of a reliable theoretical study. Although the relative contributions of the valence structures I, II, and III may differ somewhat for 2 and 3, the data indicate that in both cases the bonding is delocalized. Accordingly 2, 3, and the derived products depicted in Scheme 1 are all represented with delocalized bonding appropriate for iridabenzofurans. Since the fused ring metallabenzenes 2 and 3 exhibit spectral and structural data that are consistent with an aromatic description for these compounds and recent computational studies have established that metallabenzenes in general are aromatic,5 the electrophilic substitution reactions involving 2 and 3 described below are all referred to as electrophilic aromatic substitutions. The absence of substituents on C1-C6 of the iridabenzofuran rings in 2 and 3 is a unique feature of these metallabenzenoids that makes them ideally suited to investigations involving electrophilic aromatic substitution reactions. On treatment of 2 with one equivalent of pyridinium tribromide, bromination occurs at C6 in the five-membered ring, and the monobrominated iridabenzofuran [Ir(C7H5O{OMe-7}{Br-6})(CO)(PPh3)2][CF3SO3] (4) is formed in good yield (see Scheme 1). The signals in the 1H and 13C NMR spectra of 4 are similar to the corresponding signals observed for 2 with the exception that in the 13C NMR spectrum the resonance for C6 is observed at 104.36 ppm (cf. 121.57 ppm in 2). Upfield shifts of this type are normally expected on substitution with bromide. A single-crystal X-ray structure determination of 4 has been carried out, and the molecular structure is shown in Figure 3. The geometry about iridium in 4 is very similar to that in 2, and monobromination at C6 is confirmed. Within the iridafuran fused ring system the only bond lengths that are significantly different from those in 2 are Ir-C(1) (2.019(3) in 4 vs 2.049(3) A˚ in 2) and C(5)-C(6) (1.353(5) in 4 vs 1.382(3) A˚ in 2). Halogenation of parent organic heterocyclic compound benzo[b]furan gives primarily the corresponding R,β-addition products,23-25 whereas bromination of R-methylbenzo[b]furan with N-bromosuccinimide or bromine results in the substitution product β-bromo-R-methylbenzo[b]furan.26,27 The iridabenzofuran 2, which has the R-position of the iridafuran ring blocked by a methoxy substituent, therefore (23) Joule, J. A.; Mills, K. Heterocyclic Chemistry; Blackwell Science: Oxford, 2000. (24) Okuyama, T.; Kunugiza, K.; Fueno, T. Bull. Chem. Soc. Jpn. 1974, 47, 1267–1270. (25) Clementi, S.; Linda, P.; Marino, G. J. Chem. Soc. (B) 1971, 79–82. (26) Yamaguchi, T.; Irie, M. J. Org. Chem. 2005, 70, 10323–10328. (27) Baciocchi, E.; Clementi, S.; Sebastiani, G. V. J. Chem. Soc., Perkin Trans. 2 1976, 266–271.

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Figure 3. ORTEP diagram of the cation of 4 showing 50% probability displacement ellipsoids for non-hydrogen atoms and hydrogen atoms as arbitrary spheres. Hydrogen atoms on the phenyl groups have been removed for clarity. Selected distances [A˚]: Ir-C(1) 2.019(3), Ir-C(5) 2.080(3), Ir-O(1) 2.188(2), C(1)-C(2) 1.350(5), C(2)-C(3) 1.451(5), C(3)-C(4) 1.350(5), C(4)-C(5) 1.443(4), C(5)-C(6) 1.353(5), C(6)-C(7) 1.445(4), C(7)-O(1) 1.261(4).

Figure 4. ORTEP diagram of 5 showing 50% probability displacement ellipsoids for non-hydrogen atoms and hydrogen atoms as arbitrary spheres. Hydrogen atoms on the phenyl groups have been removed for clarity. Selected distances [A˚]: Ir-C(1) 2.023(5), Ir-C(5) 2.017(5), Ir-O(1) 2.221(4), C(1)-C(2) 1.331(8), C(2)-C(3) 1.439(8), C(3)-C(4) 1.340(8), C(4)-C(5) 1.451(7), C(5)-C(6) 1.378(7), C(6)-C(7) 1.437(8), C(7)-O(1) 1.262(7).

behaves in the same way as R-methylbenzo[b]furan toward brominating agents. In related studies it has been noted that bromination of the R-substituted metallafuran complexes [Ir(OC{Me}CHC{Me})H(PEt3)3]PF6 and Mn(OC{Me}CHC{Ph})(CO)4 results in the β-brominated metallafurans [Ir(OC{CH3 }C{Br}C{CH3})Br2(PEt3 )3 28 and Mn(OC{Me}C{Br}C{Ph})(CO)4,29 respectively. On treatment of 2 with a large excess of pyridinium tribromide for extended periods at ambient temperature, products resulting from further bromination of the fused ring system were not detected. Under these conditions some decomposition occurred and only 4 in reduced yields could be isolated. Attempted bromination reactions of 2 at higher temperatures resulted in the formation of unidentified decomposition products. In contrast to these results, treatment of 3 with slightly more than two equivalents of pyridinium tribromide, followed by treatment with excess bromide to exchange the chloride ligand on iridium, the dibrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-2})Br(PPh3)2 (5) is formed in good yield (see Scheme 1). As expected, in the 1H NMR spectrum of 5 resonances for only three iridabenzofuran protons are evident (9.80 (d, H1), 5.36 (dd, H3), and 4.92 (d, H4) ppm). The multiplicity and coupling constants of these signals confirm that substitution occurs at C2 in the iridabenzene ring and at C6 in the fused iridafuran ring. The signals for 5 in the 13C NMR spectrum are observed at similar positions to those for 3 with the exception of the

carbon atoms that bear the bromide substituents, i.e., C2 (99.95 in 5 vs 121.12 ppm in 3) and C6 (97.35 in 5 vs 114.82 in 3). The crystal structure of 5 has been determined and the molecular structure is shown in Figure 4. The overall geometry about iridium is essentially the same as that for 3, and the structure verifies that substitution with bromide occurs at both C2 and C6. The iridabenzofuran ring is planar, and the bond distances within this fused ring system do not show any striking differences from those determined for 3. Treatment of 3 with excess pyridinium tribromide for extended periods of time at ambient temperature did not result in further bromination of the iridabenzofuran ring, and the yields of 5 obtained under these conditions were not significantly different from those obtained using two equivalents of this reagent. Attempts to monobrominate 3 by the addition of one equivalent of pyridinium tribromide resulted only in a mixture of 5 and unreacted 3. Formation of the dibrominated product 5 through reaction between 3 and pyridinium tribromide led us to consider whether other ring-substituted products might be accessible through electrophilic substitution reactions involving 3. Mercuration was a particularly interesting possibility because a mercurated product could serve as a useful precursor to other substituted iridabenzofurans through routine mercurycarbon bond cleavage with appropriate electrophilic reagents. As mercury trifluoroacetate is an active mercurating agent that is soluble in many organic solvents and is convenient to use,30 our initial investigations focused on this reagent. Treatment of 3 with seven equivalents of mercury trifluoroacetate in dichloromethane at ambient temperature produced a

(28) Bleeke, J. R.; New, P. R.; Blanchard, J. M. B.; Haile, T.; Rohde, A. M. Organometallics 1995, 14, 5127–5137. (29) DeShong, P.; Sidler, D. R.; Rybczynski, P. J.; Slough, G. A.; Rheingold, A. L. J. Am. Chem. Soc. 1988, 110, 2575–2585.

(30) Sokolov, V. I.; Bashilov, V. V.; Reutov, O. A. J. Organomet. Chem. 1978, 162, 271–282.

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crude product from which a pure compound could not be obtained directly. However, after the separation of a red material from this crude product by chromatography and subsequent treatment of this with excess lithium bromide, the pure trimercurated iridabenzofuran Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6) was isolated in 41% overall yield. The possibility that other mercurated iridabenzofurans were formed during this reaction cannot be ruled out, but no products of this type were isolated. On treatment of 3 with reduced amounts of mercury trifluoroacetate only 6, in reduced yields, could be isolated and no mono- or dimercurated products were obtained. These results can be compared with the mercuration of benzo[b]furan, which occurs preferentially in the R-position of the furan ring.31 If both the R- and β-carbon atoms of the furan ring are substituted, monomercuration of the benzene ring can occur when more vigorous conditions are employed.32 As far as we are aware, no multimercurated benzo[b]furans have been reported. The trimercuration of 3 under relatively mild conditions suggests that the iridabenzofuran core in this compound is considerably activated toward electrophilic substitution compared to benzo[b]furan. In the 1H NMR spectrum of 6 H1 is observed as a doublet of triplets at 10.10 ppm due to coupling with the two equivalent phosphorus atoms and H3. The resonance for H3 appears as a doublet at 5.90 ppm. Compound 6 was too insoluble to obtain satisfactory 13C NMR spectra, but the crystal structure of 6 has been determined, and the molecular structure is shown in Figure 5. The structure determination is not as precise as the other structures reported in this paper, but it clearly shows that 6 has essentially the same overall geometry about iridium as 3 and it provides confirmation that mercuration occurs at C2, C4, and C6. The iridabenzofuran fused ring system is essentially planar, with the largest deviation from the mean plane through Ir, C1-C7, and O1 occurring for C4 (0.043 A˚). The geometric constraints imposed by the fused ring system bring Hg(1) and Hg(3) into close proximity and the distance Hg(1) 3 3 3 Hg(3) is only 3.154 A˚. This is considerably shorter than the sum of the van der Waals radii for two mercury(II) atoms (3.50 A˚)33 and is at the short end of the range of Hg 3 3 3 Hg separations reported for compounds with intermolecular mercuriophilic interactions.34,35 An intramolecular Hg 3 3 3 Hg separation of 3.102 A˚ has been reported for the planar molecule 1,8-naphthalenediylbis(mercury chloride), where a similar geometric constraint to that present in 6 is imposed on the two mercury atoms.36 In 6 Hg(3) and Hg(1) are displaced slightly to either side of the mean plane through Ir, C1-C7, and O1 (by 0.170 and 0.128 A˚, respectively) and the torsion angle Br(3)Hg(3)-Hg(1)-Br(1) is 9.3°. However, Hg(3) makes a close intermolecular approach (3.185 A˚) to the metal-bound bromide (Br(4)) in an adjacent molecule. The weak repulsive force associated with this close approach may be a significant (31) Izumi, T.; Takeda, T.; Kasahara, A. Yamagata Daigaku Kiyo, Kogaku 1974, 13, 193–203. (32) Skulski, L.; Wroczynski, P. Molecules 2001, 6, 927–958. (33) Pyykk€ o, P.; Straka, M. Phys. Chem. Chem. Phys. 2000, 2, 2489– 2493. (34) Haneline, M. R.; Gabbaı¨ , F. P. Angew. Chem., Int. Ed. 2004, 43, 5471–5474. (35) Bravo, J.; Casas, J. S.; Mascarenhas, Y. P.; Sanchez, A.; Santos, C. O. P.; Sordoa, J. J. Chem. Soc., Chem. Commun. 1986, 1100–1101. € (36) Schmidbaur, H.; Oller, H.-J.; Wilkinson, D. L.; Huber, B.; M€ uller, G. Chem. Ber. 1989, 122, 31–36.

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factor contributing to the small out-of-plane displacement of Hg(3) in the direction away from Br(4). Compound 6 is the first example of a metallo-substituted metallabenzene or metallabenzenoid. In principle 6 could act as a precursor to numerous other functionalized iridabenzofurans through cleavage of the C-Hg bonds with suitable electrophiles. An example of this possibility is provided by the reaction between 6 and pyridinium tribromide. In this case selective cleavage of the Hg-C bonds results in formation of the tribromo-functionalized iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7) in high yield (see Scheme 1). It is noteworthy that this transformation bears some relationship to the reported reaction between a bis(trimethylsilyl)-substituted osmabenzyne and excess bromine to give the corresponding dibromo-substituted osmabenzyne.37 In the 1H NMR spectrum of compound 7 H1 is observed at 9.88 ppm as a doublet of triplets (3JHH = 2.3 Hz, 3JHP = 2.3 Hz) and H3 is observed at 6.10 ppm, also as a doublet of triplets (3JHH =2.3 Hz, 5JHP = 0.7 Hz). Compound 7 is more soluble than 6, and the 13C NMR spectrum shows the iridabenzofuran carbon atom resonances at similar positions to those observed for 5. As for 5, all the iridabenzofuran carbon resonances were observed as triplets except for C3, C4, and C7. The crystal structure of 7 has been determined, and the molecular diagram is shown in Figure 6. The presence of bromide substituents at C(2), C(4), and C(6) is confirmed, and the bond distances within this fused ring system do not show any striking differences from those determined for 5. As with the other structures, the iridabenzofuran skeleton is essentially planar (largest deviation from (37) Wen, T. B.; Ng, S. M.; Hung, W. Y.; Zhou, Z. Y.; Lo, M. F.; Shek, L.-Y.; Williams, I. D.; Lin, Z.; Jia, G. J. Am. Chem. Soc. 2003, 125, 884–885.

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Table 2. Condensed Fukui Functions (FF, fk-) for 2-4, A, and B (L = PPh3, TFA = CF3CO2-)a


mean plane through Ir, C(1)-C(7), O(1) occurs for C(2) (0.058 A˚)). Although the separation between Br(1) and Br(3) is only 3.305 A˚ (cf. sum of the van der Waals radii for two bromine atoms is 3.84 A˚),38 these two atoms are essentially coplanar with the iridabanzofuran plane, and the angles C(5)-C(6)-Br(1) (129.39°) and C(5)-C(4)-Br(3) (124.26°) are very similar to the corresponding angles in 6 (129.27° and 120.34°, respectively). By way of comparison, a slightly closer distance (3.206 A˚) between the two bromine substituents in the nearly planar molecule dimethyl 1,8-dibromonaphthalene-2,7-dicarboxylate has been reported.39 The remaining bromide substituent in 7 (Br(2)) deviates from the mean iridabenzofuran plane toward P1 by 0.398 A˚. There are no short contacts involving Br2 that would account for this relatively small displacement. In an attempt to rationalize the regioselectivity of the electrophilic aromatic substitution reactions of 2 and 3 that resulted in mono-, di-, and trifunctionalized iridabenzofurans, the condensed Fukui functions (fk-) for each of the atoms in these and some closely related compounds were computed. The condensed Fukui function derived from DFT has previously been used as a reactivity index to successfully predict the most nucleophilic site (i.e., the atom with largest fk- value) and the most electrophilic site (the atom (38) Bondi, A. J. Phys. Chem. 1964, 68, 441–451. (39) Thirsk, C.; Hawkes, G. E.; Kroemer, R. T.; Liedl, K. R.; Loerting, T.; Nasser, R.; Pritchard, R. G.; Steele, M.; Warren, J. E.; Whiting, A. J. Chem. Soc., Perkin Trans. 2 2002, 1510–1519. (40) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, USA, 1989. (41) Otero, N.; Mandado, M.; Mosqueraa, R. A. J. Chem. Phys. 2007, 126, 234108/1–234108/6. (42) Martı´ nez, A.; Vazquez, M.-V.; Carre on-Macedo, J. L.; Sansores, L. E.; Salcedo, R. Tetrahedron Lett. 2003, 59, 6415–6422.

a HOMO molecular orbital pictures (at isosurface level 0.04) for these compounds with PPh3 ligands (as well as TFA in B) removed for clarity of viewing.

with largest fkþ value) in a range of molecules including aromatic heterocycles.40-42 The computed fkvalues for the iridabenzofuran ring carbon atoms of 2 and 3 are collected in Table 2 (see the Experimental Section for details of these and all other calculations). The complete molecules were used in the calculations because we found that replacing PPh3 with the model ligand PH3 gave significantly different results that did not match as closely the observed reactivity. For compound 2, C(1), C(2), C(4), and C(6) all have relatively large values for fk-, indicating that attack by electrophiles at each of these atoms is electronically favorable. However, the atom with the largest fk- value (0.16) is C(6), and this is consistent with the observation that bromination occurs exclusively at this sterically accessible site to give 4 (see Scheme 1). As might be expected, visual inspection of the HOMO for 2 (see Table 2) shows that the orbital lobes are largest in extent at the atoms C(1), C(2), C(4), and C(6), and indeed this is the case for all the iridabenzofurans in Table 2. The fkvalues for C(3) and

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C(5) are very small, indicating electrophilic attack at these atoms is not favorable. Further bromination of 4 under the conditions studied was not observed. Inspection of the computed fkvalues for C(1) (0.11), C(2) (0.08), and C(4) (0.12) in 4 (Table 2) indicate that reaction with electrophiles at these positions is still electronically feasible. However, in all cases these fkvalues are significantly smaller than they are in the unsubstituted parent compound 2. In the case of C(1) and C(4) it is possible that steric constraints (which are not accounted for in this approach) are important factors that contribute to the reluctance of these atoms to undergo bromination. In the case of C(2), which is sterically accessible, the failure of this atom to undergo bromination under the conditions employed presumably has its origin in electronic factors; that is, the fk- value for C(2) (0.08) is too small. For compound 3, C(1), C(2), C(4), and C(6) all have relatively large values for fk-, and the value for C(6) (0.17) is again clearly the largest. In contrast with the situation for 2, 3 undergoes dibromination to give the 2,6-dibromo product 5 (see Scheme 1). As we have no information about the mechanism of this dibromination reaction, any analysis must remain speculative. However, if it is assumed that the bromination occurs sequentially and the first substitution occurs at the atom with the largest fk- value (C(6)), the intermediate product formed would be Ir(C7H5O{OMe-7}{Br-6})Cl(PPh3)2 (A, Table 2). The computed fkvalues for the putative intermediate A are presented in Table 2 and indicate the effect of bromination of 3 at C(6) is to reduce somewhat the favorability of attack by electrophiles at C(1) (0.08), C(2) (0.10), and C(4) (0.12). Further bromination of intermediate A at C2 would then give the observed product (5), even though the fk- value for this atom (0.10) is less than that for C(4) (0.12). Again a possible explanation for this discrimination is that bromination at C4 is sterically less favorable than it is at C2. In contrast to these results, treatment of 3 with the alternative electrophile Hg(O2CCF3)2 resulted directly in the trimercurated product 6 (Scheme 1). The three ring atoms of 3 that undergo mercuration are those with the largest fkvalues, C(2) (0.12), C(4) (0.13), and C(6) (0.17). Unlike the situation for bromination of 3 at C(6), initial monomercuration at C(6) (to give the hypothetical intermediate compound B, Table 2) results in an increase in the fk- value for C(4) (0.14), while the value for C(2) (0.12) remains unchanged. One can speculate that if trimercuration proceeded sequentially via the intermediate B, then this increase in reactivity index experienced at C(4) could more than compensate for any steric congestion that might be associated with subsequent reaction at this atom. Concluding Remarks. The cationic iridabenzofuran [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) and the related neutral iridabenzofuran Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) can be prepared in high-yielding reactions. The absence of substituents on C(1)-C(6) of the iridabenzofuran rings of these compounds makes them attractive substrates for studying multiple electrophilic aromatic substitution reactions. The cationic iridabenzofuran 2 undergoes monobromination only (selectively at C(6)) and in this sense behaves similarly to R-methylbenzo[b]furan. In contrast, the neutral iridabenzofuran 3 is more active toward electrophilic aromatic substitution and undergoes dibromination (selectively at C(6) and C(2)) and trimercuration (selectively at C(6), C(4), and C(2)).

Clark et al.

The degree of substitution that occurs in these regioselective reactions of 2 and 3 is therefore determined by the ancillary ligands of the iridabenzofuran and the nature of the electrophile. The multiple substitution reactions of 3 provide the first examples of reactions of this type for either a metallabenzene or metallabenzenoid, and the trimercurated iridabenzofuran Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6) is the first metallo-substituted metallabenzenoid to be reported. Metallo-substituted metallabenzenes are still unknown. The reaction of 6 with bromine to form Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7) through cleavage of the Hg-C bonds indicates that differently substituted iridabenzofurans could be accessed through treatment of 6 with other electrophiles, and this possibility is currently being explored. We have demonstrated that the computed condensed Fukui functions (fk-) for the atoms in the substrate iridabenzofurans provide a useful reactivity index that can be used to rationalize the observed regioselectivity of the electrophilic substitution reactions. Although the Fukui approach successfully identifies the ring atoms that are the most susceptible to electrophilic substitution, the predictive value of this method has some limitations in situations where a distinction has to be made between sites with similarly favorable fkvalues. It is proposed that steric congestion, which is not accounted for in this approach, could be an important secondary factor in determining the outcome in these cases.

Experimental Section General Comments. Standard laboratory procedures were followed, as have been described previously.43 Infrared spectra (4000-400 cm-1) of solid samples were recorded on a PerkinElmer Spectrum 400 spectrometer as Nujol mulls. NMR spectra were obtained on a Bruker Avance 300 at 25 °C. 1H, 13C, 31P, and 19F NMR spectra were obtained operating at 300.13 (1H), 75.48 (13C), 121.50 (31P), and 282.4 (19F) MHz, respectively. Resonances are quoted in ppm, and 1H NMR spectra referenced to either tetramethylsilane (0.00 ppm) or the proteo-impurity in the solvent (7.25 ppm for CHCl3). 13C NMR spectra were referenced to CDCl3 (77.00 ppm), 31P NMR spectra to 85% orthophosphoric acid (0.00 ppm) as an external standard, and 19 F NMR spectra to CFCl3 (0.00 ppm) as an external standard. High-resolution mass spectra were recorded using the fast atom bombardment technique with a Varian VG 70-SE mass spectrometer or electrospray ionization using a Bruker Daltronics MicrOTOF instrument. Elemental analyses were obtained from the Microanalytical Laboratory, University of Otago. Synthesis of [Ir(C4H4)(NCMe)(CO)(PPh3)2][CF3SO3] (1) (refs 14, 15). Ethyne was slowly bubbled through a stirred solution of [Ir(CO)(MeCN)(PPh3)2][CF3SO3] (2.454 g, 2.560 mmol) in dichloromethane (100 mL) for 3 h. Approximately half of the dichloromethane was removed under vacuum; then cyclohexane (70 mL) and ethanol (0.05 mL) were added. Further dichloromethane was removed under vacuum until a colorless product started to precipitate. The solution was then left to stand until precipitation was complete. The colorless solid was collected by filtration and washed with cyclohexane followed by hexanes to give pure 1 (2.368 g, 2.399 mmol, 92%). MS (m/z, FABþ): calcd for C43H37193IrNOP2 [M - (CF3SO3)-]þ 838.19798, found 838.19731. Anal. Calcd for C44H37O4F3IrNP2S 3 1/2CH2Cl2: C, 51.92; H, 3.72; N, 1.36. Found: 51.73; H, 3.83; N, 1.32 (1H NMR spectrum showed the presence of ca. 0.5 molar equiv of CH2Cl2 in the analytical sample). IR (cm-1): 2040s ν(CO), (43) Maddock, S. M.; Rickard, C. E. F.; Roper, W. R.; Wright, L. J. Organometallics 1996, 15, 1793–1803.

Article 1956w ν(CN). 1H NMR (CDCl3, δ): 1.86 (s, CH3CN, 3H), 5.65 (m, H2 or H3, 1H), 5.94 (m, H2 or H3, 1H), 6.75 (broad d, 3 JHH = 6.5 Hz, H1 or H4, 1H), 7.30 (m, H1 or H4, 1H), 7.38-7.47 (m, PPh3, 30H). 13C NMR (CDCl3, δ): 2.79 (s, CH3CN), 123.20 (s, CH3CN), 127.24 (t0 43, 1,3JCP = 59.1 Hz, i-PPh3), 128.17 (t0 , 2,4JCP = 10.7 Hz, o-PPh3), 131.42 (s, p-PPh3), 132.00 (t, 2JCP = 6.6 Hz, C1 or C4), 134.44 (t0 , 3,5JCP = 10.7 Hz, m-PPh3), 143.12 (s, C2 or C3), 143.45 (t, 3JCP = 3.1 Hz, C2 or C3), 151.84 (t, 2JCP = 10.6 Hz, C1 or C4), 172.94 (t, 2JCP = 7.6 Hz, CO), SO3CF3 not observed. 31P NMR (CDCl3, δ): 0.93. 19 F NMR (CDCl3, δ): -79.07. Synthesis of [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2). A solution of [Ir(C4H4)(NCMe)(CO)(PPh3)2][CF3SO3] (1) (0.568 g, 576 μmol) and methyl propiolate (0.512 mL, 5.755 mmol) in 1,2-dichloroethane (30 mL) was heated under reflux for 2.5 h. The dichloroethane was removed under vacuum, and the crude product recrystallized from dichloromethane/n-hexane to give pure 2 (522 mg, 507 μmol, 88%) as orange crystals. MS (m/z, FABþ): calcd for C45H38O3193IrP2 [M - (CF3SO3)-]þ 881.19256, found 881.19101. Anal. Calcd for C46H38O6F3IrP2: C, 53.64; H, 3.72. Found: C, 53.66; H, 3.73. IR (cm-1): 2051s ν(CO), 1552 m, 1353s, 1263vs, 1152 m, 1031s. 1H NMR (CDCl3, δ): 3.48 (s, 3H, OCH3 on C7), 5.55 (t, 4JHP = 3.1 Hz, 1H, H6), 5.58 (d, 3JHH = 10.0 Hz, 1H, H4), 5.64 (m, 1H, H3), 5.92 (m, 1H, H2), 7.37-7.41 (m, 12H, PPh3), 7.46-7.49 (m, 12H, PPh3), 7.53-7.56 (m, 6H, PPh3), 7.70 (d, 3JHH = 8.5 Hz, 1H, H1). 13 C NMR (CDCl3, δ): 54.37 (s, OCH3), 121.57 (t, 3JCP = 4.7 Hz, C6), 123.72 (t, 3JCP = 3.6 Hz, C2), 125.79 (s, C4), 125.88 (t0 , 1,3JCP = 58.8 Hz, i-PPh3), 128.20 (t, 2JCP = 8.9 Hz, C1), 128.70 (t0 , 2,4JCP = 10.9 Hz, o-PPh3), 132.12 (s, p-PPh3), 134.26 (t0 , 3,5JCP = 10.8 Hz, m-PPh3), 140.97 (t, 4JCP = 2.5 Hz, C3), 174.25 (t, 2JCP = 8.9 Hz, CO), 184.87 (s, C7), 197.60 (t, 2JCP = 10.1 Hz, C5), SO3CF3 not observed. 31P NMR (CDCl3, δ): 1.50. 19 F NMR (CDCl3, δ): -79.05. Synthesis of Ir(C7H5O{OMe-7})Cl(PPh3)2 (3). A solution of [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) (202 mg, 196 μmol) and LiCl (416 mg, 9.814 mmol) in n-propanol (12 mL) was heated under reflux for 2.5 h. The solvent was removed from the reaction mixture under vacuum, and the residue dissolved in dichloromethane and purified by column chromatography using silica gel as the support and dichloromethane as eluant. The red band was collected, ethanol was added, and on slow removal of the dichloromethane under vacuum pure 3 was obtained as red crystals (140 mg, 158 μmol, 80%). MS (m/z, FABþ): calcd for C44H3835Cl193IrO2P2 [M]þ 888.16650, found 888.16431. Anal. Calcd for C44H38O2ClIrP2: C, 59.49; H, 4.31. Found: C, 59.29; H, 4.29. IR (cm-1): 1551s, 1515m, 1340s, 1167s. 1H NMR (CDCl3, δ): 3.35 (s, 3H, OCH3), 4.68 (d, 3JHH = 9.7 Hz, 1H, H4), 4.91 (t, 4JHP = 2.1 Hz, 1H, H6), 5.52 (dd, 3JHH = 9.5, 3JHH = 6.6 Hz, 1H, H3), 5.93 (ddt, 3JHH = 8.5, 3JHH = 6.6, 4JHP = 1.9 Hz, 1H, H2), 7.22-7.34 (m, PPh3, 18H), 7.65-7.72 (m, PPh3, 12H), 9.36 (ddt, 3JHH = 8.5, 4JHH = 2.0, 3JHP = 1.9 Hz, 1H, H1). 13C NMR (CDCl3, δ): 52.20 (s, OCH3), 114.82 (t, 3JCP = 2.9 Hz, C6), 121.08 (s, C4), 121.12 (t, 3JCP = 3.4 Hz, C2), 127.12 (t0 , 2,4JCP = 10.1 Hz, o-PPh3), 129.66 (s, p-PPh3), 129.69 (t0 , 1,3JCP = 53.4 Hz, i-PPh3), 135.17 (t0 , 3,5JCP = 10.4 Hz, m-PPh3), 141.05 (t, 4JCP = 2.3 Hz, C3), 148.61 (t, 2JCP = 9.6 Hz, C1), 182.95 (s, C7), 193.71 (t, 2JCP = 6.8 Hz, C5). 31P NMR (CDCl3, δ): 1.63. Synthesis of [Ir(C7H5O{OMe-7}{Br-6})(CO)(PPh3)2][CF3SO3] (4). Pyridinium tribromide (35 mg, 109 μmol) was added to a solution of [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) [(100 mg, 97 μmol) in methanol (6 mL), and the solution stirred at room temperature for 2 h. The solvent was removed from the reaction mixture under vacuum, and the residue dissolved in dichloromethane and purified by column chromatography using silica gel as the support. The column was first eluted with dichloromethane and then 2% ethanol in dichloromethane. The red band was collected and recrystallized from dichloromethane/hexanes to give pure 4 as red crystals (75 mg, 68 μmol,


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70%). MS (m/z, FABþ): calcd for C45H3779Br193IrO3P2 [M]þ 959.10258, found 959.10308. Anal. Calcd for C46H37O6BrF3IrP2S: C, 49.82; H 3.36. Found: C, 49.77; H, 3.38. IR (cm-1): 2056s ν(CO), 1549m, 1345s, 1273s, 1150m, 1031m. 1H NMR (CDCl3, δ): 3.48 (s, 3H, OCH3), 5.75 (dd, 3JHH = 10.0 Hz, 3JHH = 6.3 Hz, 1H, H3), 5.79 (d, 3JHH = 10.2 Hz, 1H, H4), 5.98 (dd, 3JHH = 8.3 Hz, 3JHH = 6.4 Hz, 1H, H2), 7.32-7.36 (m, 12H, PPh3), 7.49-7.52 (m, 12H, PPh3), 7.55-7.57 (m, 6H, PPh3), 7.89 (d, 3JHH = 8.8 Hz, 1H, H1). 13 C NMR (CDCl3, δ): 56.11 (s, OCH3), 104.36 (t, 3JCP = 5.9 Hz, C6), 124.49 (t, 3JCP = 3.4 Hz, C2), 124.55 (s, C4), 125.30 (t0 , 1,3JCP = 59.2 Hz, i-PPh3), 129.02 (t0 , 2,4JCP = 10.9 Hz, o-PPh3), 131.44 (t, 2JCP = 8.5 Hz, C1), 132.29 (s, p-PPh3), 134.14 (t0 , 3,5JCP = 10.8 Hz, m-PPh3), 142.89 (t, 4JCP = 2.2 Hz, C3), 172.64 (t, 2JCP = 8.8 Hz, CO), 179.48 (s, C7), 189.31 (t, 2JCP = 10.1 Hz, C5), SO3CF3 not observed. 31P NMR (CDCl3, δ): 1.71. 19 F NMR (CDCl3, δ): -79.05. Synthesis of Ir(C7H5O{OMe-7}{Br-6}{Br-2})Br(PPh3)2 (5). A solution of pyridinium tribromide (54 mg, 169 μmol) in methanol (1 mL) was added dropwise to a solution of Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) (55.7 mg, 63 μmol) in dichloromethane (5 mL), and the mixture stirred at room temperature for 3 h. Crystallization was effected by addition of methanol (10 mL) and reduction of the solvent volume under vacuum. The crystalline material formed was collected by filtration, then dissolved in 1,2-dichloroethane (8 mL). A solution of LiBr (272 mg, 3.136 mmol) dissolved in ethanol (2 mL) was added, and the mixture was heated under reflux for 6 h. The solvent was removed under vacuum, and the residue was then dissolved in dichloromethane and purified by column chromatography using silica gel as the support and dichloromethane/hexanes (9:1) as eluent. The red band was collected, the solvent removed under vacuum, and the product recrystallized from dichloromethane/methanol to give pure 5 as red crystals (49.5 mg, 45 μmol, 72%). MS (m/z, FABþ): calcd for C44H3679Br281Br193IrO2P2 [M]þ 1089.93496, found 1089.93510. Anal. Calcd for C46H37O6BrF3IrP2S CH2Cl2: C, 45.98; H 3.26. Found: C, 46.21; H, 3.30 (1H NMR spectrum and X-ray structure show the presence of 1 molar equiv of CH2Cl2). IR (cm-1): 1549m, 1332s, 1056mw, 933mw, 841m. 1H NMR (CDCl3, δ): 3.47 (s, 3H, OCH3), 4.92 (d, 3JHH = 10.3 Hz, 1H, H4), 5.36 (dd, 3JHH = 10.3 Hz, 4JHH = 2.6 Hz, 1H, H3), 7.29-7.40 (m, 18H, PPh3), 7.56-7.63 (m, 12H, PPh3), 9.80 (dt, 4JHH = 2.3 Hz, 3JHP = 2.3 Hz, 1H, H1). 13C NMR (CDCl3, δ): 54.18 (s, OCH3), 97.35 (t, 3JCP = 3.9 Hz, C6), 99.95 (t, 3JCP = 4.2 Hz, C2), 124.11 (s, C4), 127.65 (t0 , 2,4JCP = 10.0 Hz, o-PPh3), 128.79 (t0 , 1,3JCP = 54.2 Hz, i-PPh3), 130.17 (s, p-PPh3), 135.03 (t0 , 3,5JCP = 11.1 Hz, m-PPh3), 142.83 (t, 2JCP = 9.8 Hz, C1), 145.23 (s, C3), 179.00 (s, C7), 182.82 (t, 2JCP = 6.8 Hz, C5). 31P NMR (CDCl3, δ): -2.33. Synthesis of Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr2})Br(PPh3)2 (6). Hg(CO2CF3)2 (194 mg, 455 μmol) was added to a solution of Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) (57.6 mg, 65 μmol) in dichloromethane (5 mL), and the mixture stirred for 3 h. The solvent was removed under vacuum, and the resulting blue residue dissolved in dichloromethane and purified by column chromatography using silica gel as the support and dichloromethane/ethanol (94:6) as eluant. The red band was collected, the dichloromethane and ethanol were removed under vacuum, chloroform (10 mL) was added to the residue, and the mixture was then left to stand for 2 h. The pink solid that formed was collected from the red solution by filtration, dissolved in dichloromethane (10 mL), and stirred with a solution of LiBr (140 mg, 3.30 mmol) in methanol (2.5 mL) for 17 h. Further methanol (9 mL) was added, and the dichloromethane removed under vacuum to give pure 6 as pink crystals (47.3 mg, 27 μmol, 41%). MS (m/z, ESI): calcd for C44H3579Br3202Hg3193IrO2P2 [M þ H, -Br]þ 1692.8380, found 1692.8405. Anal. Calcd for C44H35O2Br4Hg3IrP2: C, 29.84; H, 1.99. Found: C, 30.08; H, 2.01. IR (cm-1): 1548mw, 1295s, 857mw. 1H NMR (CD2Cl2, δ): 3.61 (s, 3H, OCH3), 5.90 (broad d, 4JHH = 1.0 Hz, 1H, H3),

138


7.30-7.50 (m, 18H, PPh3), 7.63-7.70 (m, 12H, PPh3), 10.10 (dt, 3 JHP = 2.2, 4JHH = 1.3 Hz, 1H, H1). 31P NMR (CD2Cl3, δ): -0.66. 13C NMR: very low solubility prevented collection of a useful spectrum. Synthesis of Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7). Pyridinium tribromide (35 mg, 109 μmol) in methanol (2 mL) was added dropwise to a solution of Ir(C7H5O{OMe7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6) (40 mg, 23 μmol) in dichloromethane (10 mL). The mixture was stirred at room temperature for 30 min, methanol (8 mL) was then added, and the dichloromethane was removed under vacuum to give pure 7 as burgundy-colored crystals (23 mg, 20 μmol, 87%). MS (m/z, FABþ): calcd for C44H3579Br281Br2193IrO2P2 [M]þ 1169.84343, found 1169.84387. Anal. Calcd for C44H35O2Br4IrP2 3 2H2O: C, 43.84; H, 3.26. Found: C, 43.89; H, 2.96 (1H NMR spectrum shows the presence of ca. 2 molar equiv of H2O in the analytical sample). IR (cm-1): 1578, 1316, 933, 840. 1H NMR (CDCl3, δ): 3.53 (s, 3H, OCH3), 6.10 (dt, 4JHH = 2.3 Hz, 5J HP = 0.7 Hz, 1H, H3), 7.33-7.43 (m, 18H, PPh3), 7.54-7.62 (m, 12H, PPh3), 9.88 (dt, 4JHH= 2.3 Hz, 3JHP= 2.3 Hz, 1H, H1). 13C NMR (CDCl3, δ): 54.18 (s, OCH3), 98.33 (t, 3JCP = 4.4 Hz, C6), 101.74 (t, 3JCP = 3.8 Hz, C2), 112.90 (s, C4), 127.92 (t0 , 2,4JCP = 9.8 Hz, o-PPh3), 128.40 (t0 , 1,3JCP = 54.5 Hz, i-PPh3), 130.39 (s, p-PPh3), 134.77 (t0 , 3,5JCP = 9.8 Hz, m-PPh3), 150.09 (t, 2JCP = 9.6 Hz, C1), 154.54 (s, C3), 170.94 (t, 2JCP = 7.2 Hz, C5), 181.07 (s, C7). 31P NMR (CDCl3, δ): -3.47. Computational Details. Using the Gaussian 03 program,44 full geometry optimizations of the N-electron iridabenzofuran complexes 2, 3, 4, A, and B were carried out at the B3LYP level using the LANL2DZ basis sets for the atom Ir and 6-31(d) basis sets (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Lyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03 (Revision E.01); Gaussian Inc.: Wallingford, CT. 2004.

Clark et al. for all other atoms. NBO atomic charges were obtained for the N and N - 1 complexes (with the calculations for both species being performed at the optimized geometry of the N-electron complex) from single-point calculations at the B3LYP level using the LANL2DZ basis sets for the atom Ir and 6-311þ (2d,p) basis sets for all other atoms. The condensed Fukui functions were obtained according to eq 1:40,41,45

fk - ¼ qk ðN Þ - qk ðN - 1Þ

ð1Þ

where qk are the atomic charges at the kth atomic site and qk(N) and qk(N - 1) are the electron populations on atom k for the N and N - 1 electron species. NBO atomic charges (and for comparison Mullikan charges) were used in the calculation of fk-. Much closer agreement with experimental findings was obtained using the NBO charges as opposed to the Mulliken charges. The values derived from Mulliken charges are included in the Supporting Information. The whole molecules were used in the calculations because in a complementary study we found that replacing PPh3 with the model ligand PH3 in compounds of this type gave significantly different results that did not match the observed reactivity as closely.

Acknowledgment. We thank the Marsden Fund, administered by the Royal Society of New Zealand, for granting a scholarship to P. M. Johns. We also thank The University of Auckland for partial support of this work through grants-in-aid and for access to the Computational Chemistry Group facilities. Supporting Information Available: For compounds 2, 3, 4, A, and B coordinates of optimized geometries and Fukui functions for all atoms. Crystallographic data for 2-7 including experimental details for the X-ray structure determinations (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org. The crystallographic data for 2-7 are also available from the Cambridge Crystallographic Data Centre (fax: þ44-1223-336-033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk) as supplementary publication nos. CCDC 780433-780438, respectively. (45) Lewars, E. G. Computational Chemistry. Introduction to the Theory and Applications of Molecular and Quantum Mechanics; Kluwer Academic Publishers: Secaucus, NJ, USA, 2003.

Regioselective Mono-, Di-, and Trifunctionalization of Iridabenzofurans

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