Different coordination modes of an aryl-substituted hydrotris(pyrazolyl) borate ligand in rhodium and iridium complexes

1 Different Coordination Modes of an Aryl-substituted Hydrotris(pyrazolyl)borate Ligand in Rhodium and Iridium Complexes Salvador Conejero, Ana Cristina Esqueda, José E. V. Valpuesta, Eleuterio Álvarez, Celia Maya, Ernesto Carmona* Instituto de Investigaciones Químicas, Departamento de Química Inorgánica, CSIC and Universidad de Sevilla, Avda. Américo Vespucio 49, 41092 Sevilla, Spain Keywords: Rhodium-Iridium-tris(pyrazolyl)borate-C–H activation “This paper is dedicated to Professor Robert G. Bergman, in recognition of his outstanding contributions to Organometallic Chemistry” Abstract Complexes TptolRh(C2H4)2 (1a) and TptolRh(CH2=C(Me)C(Me)=CH2) (1b) have been prepared by reaction of KTptol with the appropriate [RhCl(olefin)2]2 dimer (Tptol means hydrotris(3-p-tolylpyrazol-1-yl)borate). The two complexes show a dynamic behaviour that involves exchange between 2 and 3 coordination modes of the Tptol ligand. The iridium analogue, TptolIr(CH2=C(Me)CH=CH2) (2) has also been synthesized, and has been converted into the Ir(III) dinitrogen complex [(  4-N,N’,N’’,C-Tptol)Ir(Ph)(N2) (3) by irradiation with UV light under a dinitrogen atmosphere. Compound 3 constitutes a rare example of Ir(III)-N2 complex structurally characterized by X-ray crystallography. Its N2 ligand can be easily substituted by acetonitrile or ethylene upon heating and denticity changes in the Tptol ligand, from  4-N,N’,N’’,C (monometallated Tptol, from now on represented as Tptol’) to  5-N,N’,N’’,C,C’ (dimetallated Tptol ligand, represented as Tptol’’) have been observed. When complex 3 is heated in the presence of acetylene, dimerization of the alkyne takes place to yield the enyne complex [(  5-N,N’,N’’,C,C’- Tptol)Ir(CH2=CHC≡CH), 7¸ in which the unsaturated organic moiety is bonded to iridium through the carbon-carbon double bond. 2 1. Introduction Hydrotris(pyrazolyl)borate ligands are among the most widely used anionic ligands in different areas of coordination and organometallic chemistry [1]. Since they were first reported by Trofimenko more than 40 years ago [2], the number of structurally modified hydrotris(pyrazolyl)borates that have been synthesized is enormous [3]. However, very often either the unsubstituted Tp (Tp stands for the parent hydrotris(pyrazolyl)borate), or TpMe2 (TpMe2 ≡ hydrotris(3,5-dimethylpyrazolyl)borate) have been the ligands of choice when searching for new metal complexes and reactivity modes. These two ligands usually coordinate to metal centres in a 2 or 3 fashion, although “0” and 1 coordination modes have been also reported [4]. Less attention has been paid to other 3- substituted Tp’ ligands that can undergo metallation to act as 4 or 5 ligands [5]. We have previously reported that TpPh (hydrotris(3-phenylpyrazolyl)borate) may adopt unusual k5-coordination through double metallation of two phenyl rings at the pyrazolyl fragments [5i,j]. More recently, we have observed a similar behaviour for the Tptol system (Tptol ≡ hydrotris(p-tolylpyrazolyl)borate), which under certain reaction conditions experiences mono or double metallation of the tolyl groups [6]. Some of the complexes that these metallated Tp’ ligands form with group 9 transition metals are particularly active in C–H bond activation processes [7]. For instance, in recent years, we have shown that metallated Tp’ moieties may participate in the tautomerization of pyridines to N-heterocyclic carbenes [8]. Moreover, different reactivity patterns have been disclosed when TpMe2Ir fragments are compared with related iridium complexes containing aryl substituted Tp ligands such TpMs (TpMs ≡ hydrotris(3- mesitylpyrazolyl)borate) and TpPh. As a way of an example, a TpPhIr fragment promotes double C–H activation of diethyl ether to yield the corresponding Fischer-type carbene [Ir]=C(CH3)(OEt), while a related TpMe2Ir unit gives a complex reaction mixture when heated in the presence of diethyl ether [7i,j]. Another interesting case that exemplifies the effects of substituents at the Tp ligand is the already mentioned tautomerization of pyridines into N-heterocyclic carbenes promoted by Tp’Ir complexes. Thus, whereas a doubly metallated TpMsIr fragment promotes the conversion of pyridine into an N- heterocyclic carbene (in competition with formation of isomeric N-adduct), the related TpMe2Ir derivatives yield exclusively N-adducts [8,9]. 3 As a natural extension of this work we have investigated the coordination properties of Tptol ligands towards rhodium and iridium. We have also explored the reactivity of the iridium counterparts, and we report herein the synthesis of a dinitrogen complex stabilized by a monometallated Tptol’ ligand, including its X-ray structure and reactivity (Tptol’ stands for Tptol metallated in an ortho position of a p-tolyl substituent). 2. Experimental 2.1 General considerations. All syntheses were performed under an argon atmosphere using standard Schlenk techniques, employing dry solvents and glassware. Spectra were referenced to external SiMe4 ( 0 ppm) using the residual protio solvent peaks as internal standards (1H NMR experiments) or the characteristic resonances of the solvent nuclei (13C NMR experiments). 11B{1H} NMR has been referenced to BF3·OEt2. Spectral assignments were made by routine one- and two-dimensional NMR experiments where appropriate. The crystal structures were determined in a Bruker-AXSX8Kappa diffractometer. Compounds [RhCl(C2H4)2]2 [10], [IrCl(coe)2]2 [11], and KTptol [12] were prepared as previously described. 2.3 Synthesis of TptolRh(CH2=CH2)2 (1a). KTptol (184 mg, 0.38 mmol) is added to a solution of [RhCl(C2H4)2]2 (75 mg, 0.19 mmol) in THF (20 mL). After stirring for a period of 3 h the solvent is removed under vacuum and complex 1a is extracted with a 2:1 mixture of CH2Cl2-Et2O. The solution is filtered through Celite and the solvent is then evaporated under reduced pressure to obtain an orange solid (Yield, 92%). 5 6 7 4Rh N N 1 2 3N N N N B H 7 6 5 4 N N1 2 3 Rh N N B H 7' 6' 5' 4' N N 1' 2' 3' HhHgHf He Ha Hb Hc Hd Ha Hb Hc Hd Mea Meb  4  2-Tptol: IR (Nujol): (B-H) 2390 cm-1. 11B{1H} NMR (96 MHz, C6D 6, 25 ºC) δ: -1.9. 1H NMR (400 MHz, CD2Cl2, 25º C) : 8.15 (d, 1 H, He), 8.07 (d, 4 H, Hc), 7.88 (d, 2 H, Ha), 7.48 (d, 4 H, Hd), 7.47 (d, 2 H, Hg), 7.26 (d, 2 H, Hh), 6.79 (d, 1 H, Hf), 6.35 (d, 2 H, Hb), 4.80 (br, 1 H, BH), 2.70 (m br, 8 H, ethylene), 2.55 (s, 6 H, 2 Mea), 2.42 (s, 3 H, Meb). 13C{1H} NMR (100 MHz, CD2Cl2, 25º C) δ: 154.3 (C3’), 153.9 (C3), 139.3 (CH1’), 139.1 (C7), 137.4 (C7’), 136.0 (CH5’), 132.4 (C4’), 130.5 (C4), 129.5 (CH6’), 129.4 (CH6), 128.8 (CH5), 126.0 (CH1), 105.3 (CH2), 102.3 (CH2’), 65.4 (d, 1JCRh = 12 Hz, =CH2), 21.5 (Mea), 21.3 (Meb). Anal. calcd. for C36H38BN6Rh: C, 64.7; H, 5.7; N, 12.6. Found: C, 65.7; H, 5.4; N, 12.5.  3-Tptol: IR (Nujol): (B-H) 2420 cm-1. 1H NMR (400 MHz, CD2Cl2, 25 ºC) : 7.99 (d, 6 H, Hd), 7.90 (d, 3 H, Ha), 7.37 (d, 6 H, Hc), 6.57 (d, 3 H, Hb), 2.47 (s, 9 H, 3 Me), 2.78 (br, 8 H, =CH2). 13C{1H} NMR (100 MHz, CD2Cl2, 25º C) δ: 154.5 (C3), 138.4 (C7), 137.7 (CH1), 131.4 (C4), 129.5 (CH6), 127.8 (CH5), 104.8 (CH2), 66.4 (d, 1JCRh = 12 Hz, =CH2), 21.4 (Me). 2.3 Synthesis of TptolRh(CH2=C(Me)C(Me)=CH2) (1b). 2,3-dimethylbutadiene (200 l) is added, at 0 ºC, to a solution of [RhCl(C2H4)2]2 (75 mg, 0.19 mmol) in THF (20 mL). The mixture is stirred at this temperature for 15 min and for a further period of 1.5 h at 20 °C. After this time, a solution of KTptol (184 mg, 0.38 mmol) in THF (20 mL) is added and the mixture stirred for a period of 3 h. The solvent is removed under vacuum and complex 1b is extracted with a 2:1 mixture of CH2Cl2-Et2O. The solution is filtered through Celite and the solvent evaporated under reduced pressure to obtain an orange solid (Yield, 92%). 5 5 6 7 4Rh N N 1 2 3N N N N B H 7 6 5 4 N N1 2 3 Rh N N B H 7' 6' 5' 4' N N 1' 2' 3' HhHgHf He Ha Hb Hc Hd Ha Hb Hc Hd MeaMea Meb   2-Tptol: IR (Nujol): (B-H) 2395 cm-1. 11B{1H} NMR (96 MHz, C6D 6, 25 ºC) δ: -1.9. 1H NMR (400 MHz, CD2Cl2, 25º C) : 8.13 (d, 4 H, Hc), 7.96 (d, 1 H, He), 7.85 (d, 2 H, Hg), 7.42 (d, 4 H, Hd), 7.26 (d, 2 H, Ha), 7.24 (d, 2 H, Hh), 6.74 (d, 1 H, Hf), 6.45 (d, 2 H, Hb), 4.80 (br, 1 H, BH), 2.51 (s, 6 H, 2 Mea), 2.41 (s, 3 H, Meb), 2.51 (s, 6 H, 2 =CMe), 1.65, 0.57 (s, 2 H each, =CH2). 13C{1H} NMR (100 MHz, CD2Cl2, 25 ºC) δ: 154.9 (C3), 154.2 (C3’), 139.0 (CH1’), 138.8 (C7), 137.4 (C7’), 135.5 (CH1), 137.3 (C4’), 132.0 (C4), 129.5 (CH6’), 129.0 (CH6), 128.8 (CH5), 125.9 (CH5’), 104.1 (CH2), 102.1 (CH2’), 98.4 (d, 1JCRh= 10 Hz, C=CH2), 47.6 (d 1JCRh = 13 Hz, =CMe), 21.5 (Mea), 21.3 (Meb), 19.5 (=CMe). Anal. calcd. for C34H36BN6Rh.Et2O: C, 63.7; H, 6.5; N, 11.7. Found: C, 64.4; H, 6.4; N, 11.9.  3-Tptol: IR (Nujol): (B-H) 2415 cm-1. 1H NMR (400 MHz, CD2Cl2, 25 ºC) : 7.93 (d, 6 H, Hc), 7.62 (d, 3 H, Ha), 7.33 (d, 6 H, Hd), 6.58 (d, 3 H, Hb), 4.80 (br, 1 H, BH), 2.47 (s, 9 H, 3 Mea), 1.50, 0.32 (s, 2 H each, CH2), 1.12 (s, 6 H, 2 Meb). 13C{1H} NMR (100 MHz, CD2Cl2, 25 ºC) δ: 155.2 (C3), 138.2 (C7), 137.3 (CH1), 132.6 (C4), 129.1 (CH6), 128.1 (CH5), 104.4 (CH2), 98.0 (d, 1JCRh = 9 Hz, =CMe), 47.0 (d, 1JCRh = 12 Hz, =CH2), 21.4 (Mea), 18.5 (=CMe). 2.4 Synthesis of complex TptolIr(CH2=C(Me)CH=CH2) (2). Isoprene (4 mL) is added at 20 °C to a suspension of [IrCl(coe)2]2 (0.92 g, 1.03 mmol) in CH2Cl2 (80 mL). The resulting mixture is stirred for 30 min and after this time KTptol (1.08 g, 2.06 mmol) is added. After stirring for a period of 24 h the solution is 6 centrifuged and the supernatant separated and evaporated to dryness. The solid residue is washed with hexane (3 x 20 mL) and dried under vacuum to obtain complex 2 (Yield, 80 %). IR (Nujol): (B-H) 2500 cm-1. 1H NMR (500 MHz, CDCl3, 25 ºC) : 7.76 (s, 3 H, Ha), 7.20 (d, 6 H, Hd), 7.10 (d, 6 H, Hc), 6.10 (s, 3 H, Hb), 3.02 (m, =CH), 2.33 (s, 9 H, Me), 2.24 (m, 1 H, =CH2), 1.32 (m, 1 H, =CH2), 0.73 (s, 3 H, =CMe), -1.28 (m, 1 H, =CH2), - 1.33 (m, 1 H, =CH2). 13C{1H} NMR (125 MHz, CDCl3, 25 ºC) δ: 156.2 (3 C, C3), 137.2 (3 C, C4), 134.7 (3 C, CH1), 123.3 (3 C, C7), 130.7 ( 6 C, CH6), 127.9 (6 C, CH5), 107.7 (3 C, CH2), 84.9 (=CMe), 75.0 (=CH), 21.4 (3 Me), 18.2 (=CMe), 10.2 (=CH2)1, 7.6 (=CH2). Anal. calcd. for C34H36BN6Ir: C, 56.52; H, 4.88; N, 11.30. Found: C, 57.0; H, 4.9; N, 11.6. 2.5 Synthesis of complex Tptol’Ir(Ph)(N2) (3). Complex 2 (0.20g, 0.27 mmol) is dissolved in 6 mL of C6H6 under a dinitrogen atmosphere and irradiated with UV light for 5 h. The solvent is then removed under vacuum to yield complex 3 quantitatively. IR (Nujol): (B-H) 2480 cm-1, (N≡N) 2205 cm-1. 1H NMR (400 MHz, C6D6, 25 ºC) : 7.62, 7.39, 7.32 (d, 1 H each, 3JHH ~ 2 Hz, 2 Ha, Ha’), 7.39 (s, He’), 7.38 (d, 2 H, Hc), 7.07 (d, 2 H, Hd), 7.03 (d, 1 H, Hc’), 6.83 (d, 2 H, HPh), 6.71 (m, 6 H, 3 HPh, 2 Hd, Hd’), 6.39 (d, 2 H, Hc), 6.03, 5.95, 5.91 (d, 1 H each, 3JHH ~ 2 Hz, 2 Hb, Hb’), 2.36, 2.15, 2.00 7 (s, 3 H each, Me). 13C{1H} NMR (100 MHz, C6D6, 25 ºC) δ: 164.2, 157.6, 155.9 (2 C3, C3’), 145.8 (C9’), 138.5, 137.9 (2 C7), 137.8, 136.7, 136.5 (2 CH1, CH1’), 137.1 (3C, 2 CHPh, C7’), 135.4 (CH8’), 130.6 (2 CH5), 130.4 (2 CH5),130.3 (4 C, CH5, C4), 129.0 (2 CH6), 128.4 (3 C, CH6, C4’), 127.5 (Ir-CqPh), 126.9 (2 CHPh), 123.5 (CH6’), 123.0 (CHPh), 122.7 (CH5’), 108.3, 105.8, 103.1 (2 CH2, CH2’), 22.2, 21.4, 21.2 (Me). Anal. calcd. for C36H32BN8Ir: C, 55.45; H, 4.14; N, 14.37. Found: C, 55.5; H, 4.3; N, 13.6. 2.6 Synthesis of complex Tptol’Ir(Ph)(NCMe) (4). Acetonitrile (18 L) is added to a solution of complex 3 in 6 mL of C6H6. The solution is then heated at 60 oC for 2 and after this time the solvent is removed under vacuum. The solid residue is washed with diethyl ether (2 x 5 mL) and dried under vacuum to obtain analytically pure complex 4 (Yield, 60 %). IR (Nujol): (B-H) 2470 cm-1, (C≡N) 2290 cm-1. 1H NMR (500 MHz, CD2Cl2, 25 ºC) : 7.92, 7.89, 7.61 (br, 1 H each, 2 Ha, Ha’), 7.35 (d, 2 H, Hc), 7.32 (s, 1 H, He’), 7.18 (d, 1 H, Hc’), 7.14 (d, 2 H, Hd), 6.74 (d, 2 H, Hd), 7.74 (d, 1 H, Hd’), 6.67 (t, 1 H, HPh), 6.60 (m, 4 H, HPh), 6.23 (d, 2 H, Hc), 6.45, 6.14 (br, 3 H, 2 Hb, Hb’), 2.51, 2.39, 2.22 (s, 3 H each, Me), 1.42 (s, 3 H, NCMe). 13C{1H} NMR (125 MHz, CD2Cl2, 25 ºC) δ: 163.4, 156.2, 154.6 (2 C3, C3’), 148.3 (Ir-C9’), 137.8 (C4’), 137.7 (C7) 137.5, 125.7 (4 CHPh), 137.5, 136.3, 136.2 (2 CH1, CH1’), 137.0 (C7), 136.1 (C7’), 131.1 (C4), 131.0 (C4), 130.9 (Ir-CPh), 130.1 (2 C, CH5), 130.0 (2 C, CH5), 128.3 (2 C, CH6), 127.6 (2 C, CH6), 121.8 (CH6’), 121.2 (2 C, CH5’, CHPh), 114.7 (NCMe), 107.7, 105.1, 102.5 (2 CH2, CH2’), 22.0, 21.2, 21.0 (Me), 2.45 (NCMe). Anal. calcd. for C38H35BN7Ir: C, 58.53; H, 4.65; N, 10.75. Found: C, 58.6; H, 4.8; N, 10.4. 2.7 Synthesis of complex Tptol’Ir(Ph)(CH2=CH2) (5). 8 Complex 3 (0.2 g, 0.26 mmol) is dissolved in C6H6 (~ 6 mL) in a thick-wall glass vessel, charged with 2 atm of ethylene and heated at 60 ºC for 2 h. The solvent is then removed under vacuum to obtain a mixture of Tptol’Ir(Ph)(CH2=CH2), 5, and known Tptol’’Ir(CH2=CH2), 6, in a 1 : 0.4 ratio. Complex 5 can be obtained in a pure form by column chromatography, using a mixture of diethyl ether:hexane (10:1) as eluant. Crystals of complex 5 suitable for X-ray studies can be isolated from diethyl ether: hexane mixtures (Yield 40%). IR (Nujol): (B-H) 2525 cm-1. 1H NMR (400 MHz, C6D6, 25 ºC) : 7.73, 7.44, 7.35 (d, 1H each, 3JHH ~ 2 Hz, 2 Ha, Ha’), 7.06 (d, 1H, Hc’), 6.89 (d, 4 H, Hd), 6.84 (d, 2 H, 2 Hc), 6.77 (d, 1H, Hd’), 6.71 (m, 3 H, HPh), 6.57 (4 H, 2 Hd, 2 Hc), 6.54 (m, 2 H, HPh), 6.49 (s, 1 H, He’), 6.02, 5.88, 5.87 (d, 1 H each, 3JHH ~ 2 Hz, 2 Hb, Hb’), 3.65, 3.27 (m, AA’XX’ system, =CH2), 2.30, 2.13, 1.88 (s, 3 H each, Me). 13C{1H} NMR (100 MHz, C6D6, 25 ºC) δ: 162.7, 157.4, 157.1 (2 C3, C3’), 142.2 (Ir-C9’), 138.5 (C7), 138.4 (C4), 138.5 (C7), 138.0, 136.9, 136.2 (2 CH1, CH1’), 137.6 (2 CHPh), 136.0 (C7’), 132.4 (CH8’), 131.6 (C4), 131.1 (C4’), 130.7 (2 CH5), 130.0 (2 CH5), 128.7 (2 CH6), 128.4 (2 CH6), 126.1 (2 CHPh), 123.9 (Ir-CqPh), 123.6 (CH6’), 122.8 (CHPh), 122.5 (CH5’), 109.7, 107.2, 102.8 (2 CH2, CH2’), 65.3 (2 C, =CH2), 22.2, 21.4, 21.1 (Me). Anal. calcd. for C38H36BN6Ir: C, 58.53; H, 4.65; N, 10.75. Found: C, 58.6; H, 4.8; N, 10.4. 2.8 Synthesis of complex Tptol’’Ir(CH2=CHC≡CH) (7). Complex Tptol’Ir(Ph)(N2) (3) (0.1 g, 0.128 mmol) is dissolved in 6 mL of C6H6 in a thick-wall glass vessel, charged with 2 atm of C2H2 and heated at 60 °C for 1.5 h. The solution is centrifuged, filtered off and evaporated to dryness. The solid residue is dissolved in hexane and crystallized at -23 oC (Yield 15%). 9 IR (Nujol): (B-H) 2480 cm-1, (C≡C) 2020 cm-1. 1H NMR (400 MHz, C6D6, 25 ºC) : 7.78, 7.48, 7.14 (d, 1 H each, 3JHH ~ 2.4 Hz, 2 Ha’, Ha), 7.72 (br, He’), 7.31 (d, 1 H, Hc’), 7.18 (d, 1 H, Hc’), 7.05 (br d, Hd’), 6.90 (d, 2 H, Hd), 6.89 (br, 1 H, He’), 6.85 (d, 2 H, Hc), 6.59 (br d, Hd’), 6.29, 5.89, 5.86 (d, 1 H each, 3JHH ~ 2.4 Hz, 2 Hb’, Hb), 3.50 (d, 1 H, 3JHH ~ 10.5 Hz, H), 3.42 (d, 1 H, 3JHH ~ 9 Hz, H), 3.30 (m, H), 2.55, 2.17, 1.92 (s, 3 H each, Me), 1.62 (d, 1 H, 3JHH ~ 2 Hz, H). 13C{1H} NMR (100 MHz, C6D6, 25 ºC) δ: 164.5, 161.8, 155.8 (2 C3’, C3), 145.9 (Ir-C9’), 141.1, 137.9, 137.4 (2 CH1’, CH1), 138.5 (CH8’), 137.9 (C7), 137.8 (C4’), 137.5 (C7’), 137.4 (C4’), 137.0 (C7’), 136.4 (CH8’), 135.4 (Ir-C9’), 131.1 (2 CH5), 130.6 (C4), 128.7 (2 CH6), 124.6, 124.5 (2 CH6’), 122.9, 122.7 (2 CH5’), 107.2, 104.6, 103.3 (2 CH2’, CH2’), 84.2 (C≡CH), 71.1 (C≡CH), 60.1 (=CH2), 54.9 (=CH), 23.2, 21.8, 21.4 (Me). HRMS m/z calcd. for C34H30BN6Ir (M + Na)+ : requires 749.2152, found 749.2155. Crystalline samples of this compound obtained from different preparations give incorrect microanalysis even if they appear to be pure by NMR spectroscopy and provide very accurate mass spectral data. The X-ray analysis herein reported has been performed with this crystalline material. 3. Results and discussion 3.1 Synthesis of TptolRh(I) diene complexes Encouraged by the remarkable reactivity of rhodium(I) complexes of the auxiliary TpMe2 ligand [3c,7a,c-h,13], we have prepared TptolRh(I) compounds stabilized by coordination of olefins. Reaction of KTptol with [RhCl(C2H4)2]2, in THF at 0 °C, results in the high yield formation (92%) of the orange complex TptolRh(C2H4)2, 1a (Scheme 1). 1H NMR analysis reveals the existence of two species that are in slow equilibrium in 16 reactions of compound 3 and TpMe2Ir(C6H5)2(N2) with acetonitrile reveal that N2 displacement in the two complexes occurs with comparable rates. Ethylene is also able to substitute the N2 ligand in 3 with formation of the corresponding adduct, 5 (Scheme 5). However, in this case the known ethylene adduct Tptol’’Ir(C2H4), 6 [6], that contains a Tptol ligand metallated in two p-tolyl substituents forms also, with an approximate 5 : 6 ratio of 1 : 0.4. Prolonged heating of this mixture (ca. 24 h) does not change the ratio of the products. Moreover, the two compounds can be separated taking advantage of their different solubilities in diethyl ether and when pure 5 is heated under the above conditions (60 °C, 2h) conversion to 6 is not detected. Nevertheless, under more forcing conditions (80-100 °C, 24 h) the second metallation of the Tptol ligand occurs and 6 becomes the only reaction product. The different outcomes of the reactions of 3 with NCMe and C2H4 can be ascribed to the capacity of the Lewis base to trap the five-coordinate intermediate [Tptol’Ir(C6H5)] that results from N2 dissociation from complex 3. NCMe reacts very quickly with this intermediate and forms adduct 4 as the only reaction product. In contrast with this, in the ethylene case [Tptol’Ir(C6H5)] has a sufficiently long life to undergo intramolecular C–H activation of an ortho C–H bond in one of the two tolyl rings that are still free. A molecule of C6H6 is extruded and the resulting [Tptol’’Ir] species reacts then with C2H4 to form 6. H B N NN NN N Ir Ph 3 C6H6/ 60 oC CH2=CH2 - N2 5 H B N NN NN N Ir + 6 Scheme 5. X-ray diffraction analysis of 6 has already been performed [6]. Concentrated solutions of 5 in diethyl ether cooled at −10 °C provide adequate crystals for crystallographic studies. Figure 4 shows an ORTEP view of a molecule of this complex. It features an Ir−C bond length to the metallated ring of 2.038(2) Å, which is somewhat shorter than 17 the Ir−C6H5 bong length of 2.079(2) Å, and compares well with Ir–C separations found in other structurally characterized iridium complexes of metallated Tptol ligands [6]. Figure 4. ORTEP view for complex 5. We have recently shown that addition of acetylene to dinitrogen complex TpMs’’Ir(N2) (where TpMs’’ represents the doubly metallated TpMs ligand) at 60 oC leads to a vinylacetylene complex TpMs’’Ir(H2C=CH-C≡CH), in which the enyne is coordinated to iridium through the carbon-carbon double bond [8b]. To assess whether dimerization of acetylene by Tp’Ir(III) complexes is a general process and if coordination of the generated vinylacetylene through the double bond is preferred thermodynamically, complex 3 has been heated (60 oC) under an acetylene atmosphere (2 atm). A new complex 7 containing a coordinated molecule of vinylacetylene forms as a single product, with no indication being inferred regarding formation of a -acetylene compound similar to complex 6 (Scheme 6). 1H and 13C{1H} NMR data hint at coordination of the unsaturated hydrocarbon through the olefinic moiety. The =CH2 protons of the alkene resonate as doublets at 3.50 (H) and 3.42 (H) 18 3 C6H6/ 60 oC C2H2 - N2, - C6H6 H B N NN NN N Ir 7 Scheme 6. ppm whereas the =CH resonance appears as a multiplet at 3.30 (H) ppm. These chemical shift values are up-field (~ 2 ppm) in comparison with those of free vinylacetylene [20], as expected for olefinic protons bonded to transition metals. The C≡C–H proton also appears shifted up-field with respect to free terminal alkynes (~ 1 ppm). The 13C{1H} NMR spectrum supports also coordination of the double bond to the iridium centre. Two signals at  84.2 and 71.1 are recorded for the internal and terminal carbon nuclei of the triple bond, respectively, while the olefinic carbon atoms resonate at 60.1 (=CH2) and 54.9 (=CH) ppm. Another interesting piece of information that can be drawn from the NMR analysis is that the Tptol ligand has undergone metallation of a second tolyl fragment during reaction, and therefore behaves as 5. The structure of complex 7 has been unequivocally established by X-ray diffraction studies (Figure 5). The Ir–C separations to the double bond have values of 2.147(2) and 2.169(2) Å for the terminal (C31) and internal (C32) carbon atoms, respectively. The C31–C32, C32–C33 and C33–C34 bond distances are 1.396(3), 1.435(3) and 1.186(4) Å. All these values are identical, within experimental error, to those found in the analogous complex TpMs’’Ir(H2C=CHC≡CH) [8b]. 19 Figure 5. ORTEP view for complex 7. Formation of complex 7 implies C–H activation and C–C coupling reactions between two molecules of acetylene but, as in the case of TpMs’’Ir(H2C=CHC≡CH), no intermediates where detected in the course of the reaction. In view of the complexity of the latter system, no mechanistic studies on the reaction of 3 with C2H2 have been executed. 4. Conclusions Complexes TptolRh(olefin)2 are readily accessible by the straightforward reaction of KTptol with the corresponding [RhCl(olefin)2]2 starting material. Spectroscopic data for these complexes indicate that an equilibrium between 2 and 3 coordination modes is occurring in solution. The analogous iridium complex TptolIr(CH2=C(Me)CH=CH2) has also been prepared in a similar way and shown to be an excellent precursor for the synthesis of the Ir(III) dinitrogen complex Tptol’Ir(C6H5)(N2), in which the Tptol is monometallated, acting therefore as a 4 ligand. This compound is one of the few Ir(III) dinitrogen complexes that have been isolated and crystallographicaly characterized. Its reactivity towards acetonitrile, ethylene and acetylene has been explored and has revealed that N2 dissociation to yield transient, unsaturated 16-electron species [Tptol’Ir(C6H5)] is an easy process. 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