Theoretical and experimental investigations on stability and chemistry of organoiridium(III) complexes

Abstract

Novel iridacyclononanes of the type {where L = PPh₃, PEt₃ and P(OMe)₃} have been synthesized from their bis(1-pentenyl)iridium(III) precursors through a ring-closing metathesis reaction using Grubbs' 1st generation catalyst. The products were characterized by NMR and mass spectroscopic techniques. The composition and purity of these organoiridium(III) complexes were confirmed by elemental analysis. Various factors, influencing the stability of these complexes, such as the nature of ligand, solvent, and temperature are discussed. The organic product distributions obtained by thermolysis are discussed and compared among the complexes synthesized. The NBO analysis has provided detailed insight into the type of hybridization, nature of bonding in various organoiridium(III) complexes and also the effect of nature of ligand on the stability of these complexes. The highest second order perturbation energies correspond to the maximum delocalization between the donor–acceptor orbitals. The nature of ligand plays an important role in the stability of these complexes due to the varied electronic properties in PEt₃, P(OMe)₃ and PPh₃ ligands, the thermal stability follows the trend: [Cp*IrCl₂]₂(dppe)] > [Cp*IrR₂(PEt₃)] > [Cp*IrR₂P(OMe)₃] > [Cp*IrR₂(PPh₃)].

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1. Introduction

A variety of organoiridium complexes are known with different oxidation states (Ir(I), Ir(III) and Ir(IV)) and coordination numbers (generally 4 and 6).¹ These have attracted much attention with their varied applications in a wide range of areas, especially as reagents,² catalysts,³ pharmaceuticals⁴ and active species in electrophosphorescent devices.⁵ Recently, a dinuclear Ir catalyst was used in reversible hydrogen storage using carbon dioxide in an aqueous solution.⁶ Some of the Ir-based catalysts like pincer-ligated Ir catalysts (for n-alkane metathesis)⁷ and [Ir(CO)₂I₂]⁻ (the Cativa process for large scale carbonylation reactions) have been used for industrial applications.⁸

As it is evident from the literature that organoiridium complexes are key intermediates in various transformations and also potentially proved as efficient catalysts for the production of hydrogen from water and the dehydrogenation of chemically inert paraffins, then these complexes might also function as anticancer agents or other pharmaceuticals. In this context, it is indeed very important to understand the role of substituents on stability and structural aspects of new organoiridium(III) complexes. These complexes were synthesized by alkene metathesis reaction using a Grubbs' catalyst, which is one of the tools to the elegant synthesis of metal-containing catenanes,⁹ interesting “gyroscopic” molecules¹⁰ and metallacycloalkenes.^11,12 Earlier we have carried out the ring-closing metathesis (RCM) reactions of a series of square-planar 16-electron platinum complexes, cis-[Pt(1-alkenyl)₂L₂] (1) to give high yields of seven to fifteen-membered platinacycles (2) shown in Scheme 1.^13–15 Moreover, these metallacycloalkane compounds are known to be key intermediates in ethylene oligomerization and polymerization reactions, as well as other catalytic processes.¹⁶ We have also explored the feasibility of analogous reaction sequences with other metal complexes like cis-Pd(1-alkenyl)₂(diphos).¹⁷ In this present work, we describe the synthesis and thermal stability of iridacyclononane compounds derived from their bis(1-alkenyl)Cp*Ir(III) precursors through RCM route.

Scheme 1

2. Materials and methods

All reactions and manipulations were carried out under an inert atmosphere of dry nitrogen using standard Schlenk and vacuum-line techniques. [(C₅Me₅)IrCl₂]₂, [(C₅Me₄H)IrCl₂]₂, (C₅Me₅)IrCl₂(PPh₃), (C₅Me₄H)IrCl₂(PPh₃), [(C₅Me₅)RhX₂]₂ (X = Cl, Br), (C₅Me₄H)Ir(PPh₃)Cl₂ (1) and (C₅Me₅)Ir(PPh₃)Cl₂ were prepared as previously described.¹⁸. (C₅Me₅)IrCl₂(PEt₃),¹⁹ (C₅Me₅)IrCl₂(P(OMe)₃).¹⁹ The Grignard reagents, BrMgCH₂CH₂CH=CH₂ and BrMgCH₂CH₂CH₂CH=CH₂ were prepared as per the literature procedures.²⁰ The solvents were commercially available and distilled from dark purple solutions of sodium/benzophenone ketyl before use. ¹H, and ³¹P NMR spectra were recorded on a Bruker DMX-400 spectrometer and all ¹H chemical shifts are reported relative to the residual proton resonance in the deuterated solvents. Microanalyses were conducted with a Thermo Flash 1112 Series CHNS–O Analyzer instrument. Infrared spectra were recorded on a Perkin-Elmer Spectrum One FT-IR Spectrometer. GC analyses were carried out using a Varian 3900 gas chromatograph equipped with an FID and a 30 m × 0.32 mm CP-Wax 52 CB column (0.25 μm film thickness). The carrier gas was helium at 5.0 psi. The oven was programmed to hold at 32 °C for 4 min and then to ramp to 200 °C at 10 deg min⁻¹ and hold 5 min. GC-MS analyses for peak identification were performed using an Agilent 5973 gas chromatograph equipped with MSD and a 60 m × 0.25 mm Rtx-1 column (0.5 μm film thickness). The carrier gas was helium at 0.9 mL min⁻¹. The oven was programmed to hold at 50 °C for 2 min and then ramp to 250 °C at 10 deg min⁻¹ and hold 8 min. Thermogravimetric analyses were carried out on a Perkin Elmer TGA-7 and Mettler Toledo SDTA851e from ambient to about 400 °C under air (50 mL min⁻¹) and at a heating rate of 10.0 °C min⁻¹. Approximately 3 mg of the samples was used in the TG experiments.

2.1. Synthesis of iridacycloalkanes and their precursors

Compounds 1, 2, 3, 4a, 4b, 4c and 6 were prepared and the characterization data was comparable with the literature reports.¹⁸

(C₅Me₅)Ir(III)[(CH₂)₃CH=CH₂]₂(PPh₃) [5]. Cp*Ir(PPh₃)Cl₂ (205 mg, 0.31 mmol) in diethylether (20 mL) was cooled to −78 °C and 1.5 mL of 1-butenyl Grignard reagent (0.85 M, 1.24 mmol) was added. The solution was brought to 0 °C and then stirred until the solution became clear. The excess Grignard reagent was removed by hydrolyzing the reaction mixture with 5 mL of saturated aqueous NH₄Cl at −78 °C. The aqueous layer was washed with 2 × 5 mL of diethylether and the organic layer was separated by a separating funnel. The solvent was removed under reduced pressure and the residue recrystallized from a diethylether/hexane mixture (2 : 5) at −10 °C for 48 h. The yellow oily mass was separated by decanting the mother liquor and dried under vacuum for several hours. For 5, yield = 172 mg, 89%; UV-Vis (CHCl₃): λ_max = 420 nm; IR (cm⁻¹): 3065 (m), 2990 (m), 1582 (m), 1475 (s), 1433 (s), 1088 (m), 1024 (m), 743 (s), 492 (s) ¹H NMR δ 6.93–8.10 (m, 15H, Ph); 5.63–5.94 (m, 2H, =CH); 4.88–5.13 (m, 4H, =CH₂); 0.76–2.12 (m, 8H, CH₂); 1.86 (s, 15H, Cp*-CH₃). ³¹P{¹H} 26.1 (s); anal. calcd for C₃₆H₄₄IrP: C, 61.78; H 6.34. Found: C, 62.28; H, 6.89. MS: (m/z) 700 [M]⁺; 645 [M − C₄H₇]⁺; 589 [M − C₈H₁₄]⁺.

The complexes 5–7 were prepared in a similar manner. All of these complexes were found to be yellow oils with moderate yields (ca. 50–65%). Microanalyses of some of these oils showed the presence of solvent molecules.

(C₅Me₅)Ir(III)[(CH₂)₃CH=CH₂]₂(P(OMe)₃) (6). UV-Vis (CHCl₃): λ_max = 419 nm; IR (cm⁻¹): 3053 (w), 2937 (m), 1639 (m), 1481 (m), 1445 (m), 1385 (s), 1252 (s), 1165 (s), 1099 (s), 1076 (s), 1043 (s), 790 (m), 549 (m); ¹H NMR δ 5.60–5.86 (m, 2H, =CH); 4.88–5.15 (m, 4H, =CH₂); 2.86 (s, 9H, P–OMe); 1.82 (s, 15H, Cp*-CH₃); 0.76–2.12 (m, 12H, CH₂); ³¹P{¹H}: δ 92.2 (s). Anal. calcd for C₂₃H₄₂IrO₃P: C, 46.84: H, 7.18. Found: C, 46.94: H, 7.28.

(C₅Me₅)Ir(III)[(CH₂)₃CH=CH₂]₂(PEt₃) (7). UV-Vis (CHCl₃): λ_max = 420 nm; IR (cm⁻¹): 3085 (w), 2916 (m), 1628 (w), 1464 (s), 1414 (s), 1145 (s), 851 (s), 822 (m), 777 (s), 505 (m). ¹H NMR δ 5.63–5.94 (m, 2H, =CH); 4.82–5.13 (m, 4H, =CH₂); 1.83 (s, 15H, Cp*-CH₃); 0.70–2.92 (m, 27H, CH₂ and P-Et); ³¹P{¹H}: δ −15.3 (s).

Iridacyclononane (L = PPh₃) (8). Grubbs' 1st generation catalyst (8 mg) was added to a solution of 5a (226 mg, 0.323 mmol) in toluene (30 mL). The mixture as refluxed with stirring at 50 °C for 18 h, then cooled to room temperature after completion of the reaction. The reaction was monitored by ¹H-NMR until the signals for the terminal olefin chemical shifts disappeared completely. The formation of new signal at 5.5 ppm was observed during the reaction. 10% Pd/C (20 mg) was added to the crude reaction mixture. The solution was stirred under 1 atm hydrogen gas for 40 h. The mixture was then filtered and solvent was removed under reduced pressure and the product was obtained as brownish yellow oil. Then the pure product was obtained after extracting the crude into 3 × 5 mL of diethyl ether (0.152 g, 67%); UV-Vis (CHCl₃): λ_max = 425 nm; IR (cm⁻¹): 2978 (m), 1599 (w), 1481 (m), 1442 (m), 1394 (m), 1198 (s), 1109 (s), 1031 (s), 954 (s), 815 (s), 530 (s); ¹H NMR δ 6.82–8.14 (m, 15H, Ph); 1.84 (s, 15H, Cp*-CH₃); 0.68–2.16 (m, 16H, CH₂); ³¹P{¹H} 25.9 (s). Anal. calcd for C₃₆H₄₆IrP: C, 61.60; H, 6.61; found: C, 62.23; H, 7.18. MS: (m/z) 700 [M]⁺; 588 [M − C₈H₁₆]⁺. Complexes 6b–8b were prepared in a similar manner. All of the complexes are obtained yellow brown solids with 60–70% yields after stirring the oily residues with n-hexane at −78 °C.

Iridacyclononane (L = P(OMe)₃) (9). UV-Vis (CHCl₃): λ_max = 419 nm; IR (cm⁻¹): 3055 (w), 2956 (w), 1637 (m), 1635 (w), 1436 (s), 1369 (s), 1263 (w), 1176 (m), 1018 (m), 1120 (m), 1018 (m), 818 (s), 742 (s), 646 (m), 607 (m), 526 (m); ¹H NMR: δ 2.48 (s, 9H, P-OMe); 1.83 (s, 1H, Cp*-CH₃); 0.66–2.10 (m, 16H, CH₂); ³¹P{¹H} δ 92.1 (s). Anal. calcd for C₂₁H₄₀IrO₃P: C, 44.74; H, 7.15. Found: C, 44.86: H, 7.54.

Iridacyclononane (L = PEt₃) (10). UV-Vis (CHCl₃): λ_max = 410 nm; IR (cm⁻¹): 2939 (m), 1665 (s), 1469 (m), 1421 (m), 1301 (s), 1219 (m), 1149 (s), 1028 (m), 898 (m), 707 (s), 653 (s); ¹H NMR: δ 6.99–8.10 (m 15H, Ph); 5.63–5.94 (m, 2H, =CH); 4.88–5.13 (m, 4H, =CH₂); 4.19 (s, 1H, Cp*-H); 0.76–2.12 (m, 12H, CH₂); ³¹P{¹H}: δ −15.1 (s). Anal. calcd for C₂₄H₄₆IrP: C, 51.68; H, 8.31; found: C, 52.04: H, 8.63.

(C₅Me₅)IrCl₂]₂(μ-dppe) (11). [Cp*IrCl₂]₂ (201 mg, 0.252 mmol) was added to a dichloromethane solution of dppe (101 mg, 0.253 mmol). The mixture was stirred for 24 h at room temperature and the solvent was removed by rotavap. The obtained yellow orange solid was dried under vacuum for several hours. For 9; mp > 300 °C; yield 90%; UV-Vis (CHCl₃): λ_max = 460 nm; IR (cm⁻¹): 3053 (w), 2934 (w), 1433 (m), 1367 (m), 1263 (s), 1018 (m), 894 (w), 821 (m), 733 (s), 509 (m); ¹H NMR δ 6.94–8.16 (m, 20H, Ph); 2.26–2.42 (m, 4H, P–CH₂); 1.71 (s, 15H, Cp*-CH₃); 0.76–2.12 (m, 12H, CH₂); ³¹P{¹H}: δ 5.2 (s). Anal. calcd for C₃₇H₄₆IrP: C, 46.23; H, 4.55. Found: C, 46.36; H, 4.68. Mass spectral data: M⁺ = 1194.2, M⁺ − Cl = 1159.2, M⁺ − 2Cl = 1124, M⁺ − 3Cl = 1089.3, M⁺ − 4Cl = 1152.3, [Cp*Ir(dppe)Cl₂]⁺ = 796.15.

[(C₅Me₅)Ir(III){(CH₂)₂CH=CH₂}Cl]₂(μ-dppe) (12). [(C₅Me₅)IrCl₂]₂(μ-dppe) (256 mg, 0.214 mmol) in diethylether (15 mL) was cooled to −78 °C and 1.25 mL of 1-butenyl Grignard reagent (0.85 M, 1.07 mmol) was added. For 12; yield 192 mg, 75%; UV-Vis (CHCl₃): λ_max = 425; ¹H NMR δ 6.82–8.02 (m, 30H, Ph); 5.52–5.84 (m, 2H, =CH); 4.68–5.02 (m, 4H, =CH₂); 1.83 (s, 15H, Cp*-CH₃); 0.58–2.98 (m, 12H, CH₂ and P-CH₂); ³¹P{¹H}: δ 32.4 (s). Anal. calcd for C₅₄H₆₈Cl₂Ir₂P₂: C, 52.54; H, 5.55; found: C, 52.95; H, 5.85. MS: (m/z) 1262.37 [M]⁺; 918 [M − Cp*(C₅H₇)₂Cl₂]⁺; 726 [M − IrCp*(C₅H₇)₂Cl₂]⁺, 782 [M − (Cp*(C₅H₇)Cl)₂]⁺; 796 [M − IrCp*(C₅H₇)₂]⁺.

2.2. General procedure for the thermal decomposition experiments

Thermolysis reactions were carried out in clean, dry, sealed evacuated vertical Schlenk tubes of 1 cm o.d. and 10 cm lengths. The iridium complex was dissolved in DCM and transferred into the tube; the solvent was removed under vacuum and dried at least for 6 hours before thermolysis. The samples were then immersed in a thermostated oil bath constant to 170 + 5 °C. The tubes were removed at intervals (2 h) and quenched by immersion in liquid nitrogen. Decomposition products were extracted by 0.5 mL pentane containing 20 μL of chlorobenzene as internal standard and analyzed by GC/GCMS. Products were identified by comparison of retention times to those of authentic samples. Product yields were determined by response relative to the internal standard (chlorobenzene). Response factors were obtained from authentic samples.

DFT calculations. Gaussian 09 package was exploited to carry out all the density functional theory (DFT) based calculations.²¹ Geometry optimizations were performed utilizing Becke's hybrid three-parameter exchange functional and the nonlocal correlation functional of Lee, Yang and Parr (B3LYP)²² and the standard 6-31G* basis set function^23,24 of DFT for C, H, P and O atoms. For Iridium (Ir) element standard LANL2DZ basis set^25–27 was used and Ir is described by effective core potential (ECP) of Wadt and Hay pseudopotential²⁸ with a double-ζ valance using the LANL2DZ. To confirm the identity of each stationary point found as an energy minimum, vibrational analysis was performed. The population analysis has been performed by the natural bond orbital (NBO) method²⁹ at B3LYP/631G* level of theory using NBO 3.1 version of Gaussian 09 program package.³⁰

3. Results & discussion

3.1. Synthesis and characterization of iridacycloalkanes

Bis(1-pentenyl) complexes of iridium(III) (5–7) were obtained by the transmetalation reaction of 1-alkenyl Grignard reagents with the corresponding dichloroiridium(III) precursors (4a–c) as shown in Scheme 2. The formed metal–alkenyl complexes (5–7) were then readily converted into the corresponding metallacycloalkenes using the ring closing metathesis (RCM) reaction with a Grubbs' 1st generation catalyst in toluene, and subsequent hydrogenation to produce the corresponding iridacycloalkanes (8–10) in moderate yields. The formation of iridacycloalkenes was followed by observation of the disappearance of the =CH₂ signal of the alkenyl complex in the ¹H NMR spectrum and the appearance of a new signal (=CH). It is significant to note that the Pt–alkenyl complexes are more stable than Ir–alkenyl complexes.³¹

Scheme 2

The RCM reaction and subsequent hydrogenation of 5–7 yielded the iridacycloalkane complexes (8, 9 and 10) in moderate yields. All the reactions were monitored by ¹H-NMR to know the completion of reactions. All these products were isolated as yellow oily substances and the composition was determined by elemental analyses and using other spectroscopic methods. These products were purified from diethyl ether/n-hexane. All these organoiridium(III) were found to be sensitive to the heat and light. On standing at room temperature for a week under light, it was observed that these compounds decompose to unknown brown solids. IR spectra of organoiridium(III) complexes showed the corresponding to aliphatic groups confirming the proposed structures.

The ³¹P-NMR spectra of the iridacycles (8–10) displayed a singlet, which is almost similar to the precursors (5–7). ³¹P{¹H}-NMR spectra were in agreement with the literature data on the organoiridium(III) compounds.^32,33 Table 1 shows the chemical shifts of isolated complexes of iridium–alkenyls as well as iridacyles for the comparison with the precursor compounds. The position of methyl protons in Cp* of various complexes were influenced by the Grignard reagent reactions significantly by showing the chemical shift in ¹H-NMR ranging from 1.48–1.76 ppm. PPh₃ containing complexes yielded mixture of products and they were difficult to isolate and characterize completely, particularly when iodide precursor is used. The reaction was proceeded well using chloride precursors. The presence of large excess of Grignard reagent enhanced the rate of reaction towards the formation of expected bis(1-alkenyl)iridium(III) complexes when PPh₃ was used as ligand. In contrast to the preparation of bis(1-alkenyl)platinum(II), the time required for the completion of reactions of iridium(III)halide precursors with Grignard reagents at room temperature was significantly high. The RCM reaction of 7 (L = PEt₃) were smoothly completed relative to 5 (L = PPh₃) and this could be due to the bulkiness of the latter ligand. The complexes with P(OMe)₃ were relatively unstable and the products were found to be having higher masses in the mass spectra.

Table 1 ³¹P data for various complexes

Ligand used	Dichloro-Ir(III), ppm	Bis(1-pentenyl)Ir(III), ppm	Iridacyclononane, ppm
PPh3, 4a	0.2	25.6	26.6
P(OMe)3, 4b	82.2	92.4	92.6
PEt3, 4c	−6.3	−15.2	−15.6
Dppe, 11	5.2	32.4	—

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Mass spectral analysis. Relatively little work has been done on the mass spectrometric analysis of organoiridium compounds. The coordinated Cp* ligand cannot be easily protonated and then the analysis of metal–Cp* complexes is dependent on other ionization pathways. FAB can be complicated by redox, fragmentation and clustering processes for mass spectrometric analysis of organoiridium compounds.^34,35 As it is known in FAB, the pseudomolecular ion [MH]⁺ derived from the deprotonation of the solvent is also observed. Subsequently, [M + Na]⁺, [M + K]⁺ etc. are formed with the adduction of alkali metal ions depending on the composition of the matrix. It is amenable to get the mass spectrometric data for uncomplicated analysis of ionic compounds. This could be due to the easy transfer of ionic compounds into the gas-phase.

Neutral organoiridium(III) molecules acquire a charge via protonation or cationisation. The primary ionization route for the iridium(III) complex [Cp*Ir(PPh₃)Cl₂] (4a) in the combination of 3-nitrobenzyl alcohol (3-nba) is via loss of chloride ligand to give a series of peaks due to [M − Cl]⁺ (m/z = 611.21) and [M − 2Cl]²⁺ (m/z = 575.30) ions and fragments thereof as shown in Fig. 1. One of the methyl groups of Cp* ligand is easily migrated to PPh₃ and the formation of [Ph₃PMe]⁺ (m/z = 277.11) is evident in the mass spectrum (Fig. 1). Loss of neutral PPh₃ is also a major fragmentation pathway. This is in accordance with the earlier reports.³⁴

Fig. 1 Mass spectrum of 4a.

Mass spectral studies indicated the molecular ion formation in all cases. But it is interesting to note that most of the mass spectra of iridacycloalkanes show the fragments higher than the molecular ion. This could be either due to the interaction of already formed mass fragments during the analysis. The other important mass fragments from the molecular ion were also identified.

The iridium(III) complex [Cp*Ir(PPh₃)(1-pentenyl)₂] (5) (m/z 728.31) shows the primary ionization route through loss of pentenyl chains to give peaks due to [M − pentenyl]⁺ (m/z = 659.11) and [M − 2pentenyl]⁺ (m/z = 588.8) ions and fragments thereof as shown in Fig. 2.

Fig. 2 Mass spectrum of 5.

The mass spectrum of iridacyclononane (10) (m/z 557.11) indicates the initial ionization via loss of organic moiety to give peaks due to [Cp*Ir(PEt₃)]²⁺ (m/z = 445.19) ion and the other fragments as shown in Fig. 3.

Fig. 3 Mass spectrum of 10.

The mass spectrum of iridium(III) complex, [(Cp*IrCl₂)₂(dppe)] (11) (m/z 1194.32) shows the primary ionization by the loss of chlorides to give a series of peaks due to [M − Cl]⁺ (m/z = 1159.3), [M − 2Cl]²⁺ (m/z = 1124.3), [Cp*Ir(dppe)]²⁺ and other fragments (Fig. 4).

Fig. 4 Mass spectrum of 11.

Similar to 5, the mass spectrum of iridium(III) complex, [(Cp*Ir(1-pentenyl)Cl)₂(dppe)] (12) (m/z 1262.37) shows the primary ionization by the loss of chlorides to give a series of peaks due to [M − Cl]⁺ (m/z = 1227.4), [M − 2Cl]²⁺ (m/z = 1192.4), [(Cp*Ir)₂(dppe)]²⁺ (1052.3) and other fragments (Fig. 5).

Fig. 5 Mass spectrum of 12.

The factors that affect the RCM reaction to make the corresponding metallacycloalkenes are solvent, ligand, concentration, length of alkenyl chains and temperature similar to the platinum analogues. The formation of a mixture of monomeric, dimeric species of the respective metallacycloalkanes and also some unknown species as minor products are observed in concentrated solutions of alkenyl complexes. The minor products could not be isolated. The nature of ligand had a significant effect on the RCM rate, thus the alkenyl complexes with sterically bulky PPh₃ ligand are relatively unstable under refluxing temperatures in solution, which is quite a contrast to the role of PPh₃ in RCM reactions of bis(1-alkenyl)platinum(II) complexes.^12–15

The formation of traces of free PPh₃ or its oxide form and other unidentified species are observed during the RCM reaction. However, the reaction proceeds smoothly with other ligands like P(OMe)₃ and PEt₃ without any such decompositions. The rate of RCM strongly depends on the temperature as there is no RCM observed at room temperature. Better results were obtained when the temperature was maintained between 40–50 °C. The complexes were found to be unstable at higher temperatures (>80 °C). The time required for the completion of RCM reaction increases with respect to the ligands as follows: PEt₃ > dppe > P(OMe)₃ > PPh₃.

The formation of 11 from 3 was observed in the presence of 1 mole of dppe ligand. Mass spectra and analysis confirmed the dimeric nature of the complex (Scheme 3). The reaction of 9 with Grignard reagents showed the formation of bis-alkenyl dinuclear complexes instead of tetra-alkenyl complexes. This was confirmed by the elemental analysis and mass spectral data.

Scheme 3

The synthesis of iridacyclohaptane and iridacyclononane compounds with different ligands was also attempted using the well-known di-Grignard route using BrMg(CH₂)_nMgBr, but, the problem with this method is the isolation of pure products from the corresponding reaction mixtures.¹⁰ The presence of chlorinated solvents favours the decomposition during the RCM reaction by breaking the metal–carbon bonds to form their respective metal halides including iridium-η3-allyl chloride complexes as reported earlier.¹⁸

4. Computational studies on stability of organoiridium(III) complexes

The part played by the intermolecular orbital interaction, in particular, the charge transfer is explained by NBO analysis. All possible interactions between filled donor and empty acceptor NBOs are taken care of and then the assessment of their energy importance by second-order perturbation theory. The stabilization energy E⁽²⁾ linked with electron delocalization between each donor NBO (i) and acceptor NBO (j) is calculated aswhere q_i is the donor orbital occupancy, ε_i, ε_j are diagonal elements (orbital energies) and F_i,j is the off-diagonal NBO Fock matrix element.

4.1. Natural bond orbital analysis

Besides providing information about intra and intermolecular bonding among bonds, NBO analysis also gives an appropriate basis for exploration of charge transfer or conjugative interactions in molecular systems.³⁶ The degree of conjugation of electrons of the system and the extent of interaction between the electron donor(s) and the acceptor(s) has a direct connection with the stabilization energy, E⁽²⁾. The delocalization of the electron density between the Lewis type (bond or lone pair) NBO orbitals and formally unoccupied (antibonding or Rydberg) non Lewis NBO orbitals correspond to a stabilizing donor–acceptor interaction. A detailed insight into the nature of electronic conjugation between the bonds in A–H has been provided by the NBO analysis (Schemes 4 and 5).

Scheme 4 DFT calculations on relative stability of various organoiridium(III) complexes.

Scheme 5 Iridacyclononanes with different phosphorus donor ligands considered for theoretical studies.

In A–H the different interaction orbitals unveil that intramolecular charge transfer (ICT) leads to the stabilization of the systems to a varying degree. The highest second order perturbation energies for A–E are presented in Table 2. Increase in the electron density (ED) in antibonding orbitals leads to destabilization of a system.³⁷ In A, C and D the maximum delocalization correction E⁽²⁾ values correspond to delocalization into the antibonding orbitals which make them less stable than B and E where only the bonding orbitals get involved for such interactions. The donor–acceptor interaction σ*(C2–Ir7) → LP*(Ir7) has 301.35 kcal mol⁻¹, σ(Ir1–C29) → LP*(C28) has 200.84 kcal mol⁻¹, σ(Ir1–C48) → σ*(Ir1–C57) has 76.69 kcal mol⁻¹ and σ*(Ir1–C48) → σ*(Ir1–C51) has 78.80 kcal mol⁻¹ as maximum delocalization corrections in A, C, D and F respectively contributing to their destabilization while as the interaction π(C5–C7) → σ(Ir1–C3) has 77.25 kcal mol⁻¹ and π(C5–C7) → σ(Ir1–C3) has 74.63 kcal mol⁻¹ maximum delocalization correction for B and E respectively providing strongest stabilization to the corresponding molecules. Thus on the basis of E⁽²⁾ values, the order of stability is B > E > D > F > C > A.

Table 2 Second order perturbation analysis of the Fock matrix in NBO basis in A–F

Molecule	Donor NBO (i)	Type	Acceptor NBO (j)	Type	E^(2)^a [kcal mol^−1]	E(j) − E(i)^b [a.u]	F(i,j)^c [a.u]
a E^(2) means energy of hyper conjugative interaction (stabilization energy).b Energy difference between donor and acceptor i and j NBO orbitals.c F(i,j) is the Fock matrix element between i and j NBO orbitals.
A	C2–Ir7	σ*	Ir7	LP*	301.35	0.01	0.069
B	C5–C7	π	Ir1–C3	σ	77.25	0.06	0.084
C	Ir1–C29	σ	C28	LP*	200.84	0.28	0.070
D	Ir1–C48	σ*	Ir1–C57	σ*	76.69	0.03	0.083
E	C5–C7	π	Ir1–C3	σ	74.63	0.05	0.082
F	Ir1–C48	σ*	Ir1–C51	σ*	78.80	0.03	0.084

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To appreciate the effect of nature of ligand on stability of iridium complexes F, G and H are taken into account. Table 3 lists the calculated occupancies of natural orbitals and natural hybrids on atoms in F–H (Scheme 5). It is clearly reflected that the σ(Ir1–C48) bond in F is formed from an sd^2.17 hybrid on iridium (which is the mixture of 28.92% s, 8.35% p and 62.74% d atomic orbitals) and σ(Ir1–C51) bond is formed from an sd^1.80 hybrid on iridium (which is the mixture of 31.45% s, 12.09% p and 56.47% d atomic orbitals). In G the σ(Ir1–C40) bond is formed from an sp^1.78d^2.09 hybrid on iridium (which is the mixture of 20.54% s, 36.62% p and 42.85% d atomic orbitals) and σ(Ir1–C43) bond is formed from an sd^3.00 hybrid on iridium (which is the mixture of 22.67% s, 9.32% p and 68.00% d atomic orbitals). In the case of H σ(Ir1–C28) bond is formed from an sd^1.83 hybrid on iridium (which is the mixture of 31.32% s, 11.41% p and 57.27% d atomic orbitals) and σ(Ir1–C31) bond is formed from an sd^2.09 hybrid on iridium (which is the mixture of 29.98% s, 7.44% p and 62.58% d atomic orbitals). Table 4 accommodates some selected values of the calculated second order interaction energy E⁽²⁾ between donor–acceptor orbitals involving Ir1–P2 bond between metal and ligand in F–H. The donor–acceptor interaction σ(P2–C28) → σ*(Ir1–P2) has 0.57 kcal mol⁻¹, LP (O65) → σ*(Ir1–P2) has 1.49 kcal mol⁻¹ and σ(P2–C74) → σ*(Ir1–P2) has 0.90 kcal mol⁻¹ as maximum second order perturbation delocalization correction for F, G and H respectively leading to their destabilization. Gain of occupancy in the antibonding acceptor orbital can be directly correlated with weakening of the bond associated with this orbital. As shown in Table 3, the σ(Ir1–P2) bonds in F, G and H have 53.18%, 37.49% and 52.46% p-character respectively and are occupied by 1.799, 1.849 and 1.807 electrons respectively (which is consistent with a delocalization of electron density from the idealized occupancy of 2.0e). The donation of electron density from the Ir1–P2 bonds to Ir1–C51, Ir1–C48, Ir1–C43, Ir1–C28 and Ir1–C31 bonds in F–H has a clear correspondence to a chemical picture of coordination bonds and their strength. On the basis of E⁽²⁾ values and the occupancies of the Ir1–P2, Ir1–C51, Ir1–C48, Ir1–C43, Ir1–C28 and Ir1–C31 bonds, the order of stability of F–H is F > H > G.

Table 3 Occupancy of some natural orbitals (NBOs) and hybrids of F–H calculated by the B3LYP functional with 6-31G* basis set for C, H, P and O atoms and LANL2DZ for Ir atom

Molecule	Lewis-type NBOs	Occupancy	Hybrid^a	AO^b (%)
a Hybrid on A atom in the A–B bond or otherwise, as indicated.b Percentage contribution of atomic orbitals in NBO hybrid.
F	σ(Ir1–P2)	1.799	sp^2.49d^1.19	s(21.38%) p(53.18%) d(25.44%)
	σ(Ir1–C48)	1.880	sd^2.17	s(28.92%) p(8.35%) d(62.74%)
	σ(Ir1–C51)	1.852	sd^1.80	s(31.45%) p(12.09%) d(56.47%)
G	σ(Ir1–P2)	1.848	sp^1.12d^0.86	s(33.61%) p(37.49%) d(28.90%)
	σ(Ir1–C40)	1.774	sp^1.78d^2.09	s(20.54%) p(36.62%) d(42.85%)
	σ(Ir1–C43)	1.899	sd^3.00	s(22.67%) p(9.32%) d(68.00%)
H	σ(Ir1–P2)	1.807	sp^2.35d^1.13	s(22.23%) p(52.46%) d(25.22%)
	σ(Ir1–C28)	1.849	sd^1.83	s(31.32%) p(11.41%) d(57.27%)
	σ(Ir1–C31)	1.873	sd^2.09	s(29.98%) p(7.44%) d(62.58%)

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Table 4 Second order perturbation analysis of the Fock matrix in NBO basis in F–H involving Ir1–P2 bond and the maximum second order perturbation delocalization correction E^(2)a

Molecule	Donor NBO (i)	Type	Acceptor NBO (j)	Type	E^(2)a [kcal mol^−1]	E(j) − E(i)^b [a.u]	F(i,j)^c [a.u]
a a, b and c have same meaning as in Table 1 above.
F	P2–C28	σ	Ir1–P2	σ*	0.57	0.82	0.020
	Ir1–C48	σ*	Ir1–C51	σ*	78.80	0.03	0.084
G	O65	LP	Ir1–P2	σ*	1.49	0.81	0.032
	O65	LP	P2	LP*	219.80	0.47	0.304
H	P2–C74	σ	Ir1–P2	σ*	0.90	0.82	0.025
	Ir1–C31	σ*	Ir1–C28	σ*	130.91	0.01	0.082

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The nature of ligand influences the stability of Ir(III)-bis(1-alkenyl) and iridacycloalkane compounds and this can be seen in the following order: [Cp*IrCl₂]₂(dppe) > Cp*IrR₂(PEt₃) > Cp*IrR₂[P(OMe)₃] > Cp*IrR₂(PPh₃). The presence of bulky ligands such as ^tBu₃P or Cy₃P made the reactions very complex and finally the starting material (Cp*Ir(L)Cl₂) was isolated from the reactions.

5. Thermal decomposition studies

Thermolysis of metal-alkenyls as well as metallacycloalkane compounds depends on various factors such as metal, mode of decomposition, ligand, temperature and nature of organic moiety.^38,39 Thermal decomposition of iridacycloalkane compounds in the solid state generated a mixture of products mainly with alkenes, alkanes through the β-hydride elimination of metallacyclic moiety. For example, iridacycloheptane yielded a mixture of products as shown in eqn (2).

Though the thermolysis of some small size metallacycloalkanes (where M = platinum, rhodium, iridium) have been studied by TGA and DSC techniques,³⁹ the present study was carried out by heating of the compounds in the solid state at 175 °C.

It is interesting to note that the ring size of the platinacycloalkanes on the distribution of organic products plays an important role in thermolysis reactions.⁴⁰ Recently we have shown that the bonding nature of alkyl groups to the metal centre also plays a significant role in the thermolysis reactions; the ethylallyl (C₅), methylallyl (C₄) complexes of rhodium(III) and iridium(III) also produced good yields of hydrocarbons with higher carbon moieties like n-hexane, 1-hexene and n-pentane and 1-pentene respectively.⁴¹ Thermal decomposition of bis(1-alkenyl)iridium(III) compounds yielded mainly their respective 1-alkenes, 2-alkenes and dienes (eqn (3)). It is interesting to note that the product distribution is dependent on the length of alkenyl chain as well as the nature phosphine ligand (see Table 5).

Table 5 Thermal decomposition of bis(1-pentenyl)iridium(III)

Complex	n-Pentane	1-Pentene	2-Pentene	1,4-Pentadiene	Cyclopentane
5	3	8	72	9	8
6	5	14	33	42	6
7	—	18	29	48	5
12	—	28	37	28	7

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The organic products obtained from solid state thermolysis were significantly different from the solution decomposition, as the formation of higher percentages of 1-alkenes, 2-alkenes and dienes were evident by GC-MS. Bis(1-butenyl) complexes did not give 1-butene on decomposition. Table 6 illustrates the thermal decomposition patterns of iridacycloalkane compounds, indicating the formation of significant amounts of 2-alkenes, 1-alkenes and n-alkanes as major products, when the ligands are PPh₃, PEt₃ and P(OMe)₃.

Table 6 Thermal decomposition of Iridacyclononane

Iridacycles	n-Octane	1-Octene	2-Octene	1,7-Octadiene	Cyclooctane
8	21	29	41	09	—
9	23	30	47	—	—
10	22	34	44	—	—

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5.1. Thermogravimetric analysis

A comparison among the bis(1-pentenyl)iridium(III) complexes (5, 6, 7 and 12) was made on the mass loss dynamics and extent of ligand removal occurring via a single step decomposition. Fig. 6 presents the TGA thermograms obtained using samples 5, 6, 7 and 12 in the range of 30–350 °C. The weight loss occurred between 120 to 210 °C for compound 5, 6 and 7. But compound 12 showed the weight loss until 270 °C. The higher weight loss is attributed to the volatilization of the organoiridium species formed at high temperatures as per the literature reports.⁴²

Fig. 6 TGA curves of 5, 6, 7 and 12.

6. Conclusion

Bis(1-alkenyl)iridium(III) complexes are proven to be novel precursors for the preparation of iridacycloalkanes through ring closing metathesis reactions using Grubb's catalyst. The stability of the complexes strongly depends on varied electronic properties of PEt₃, P(OMe)₃ and PPh₃ ligands. The nature of ligands affects the organic product distribution up on thermolysis of organoiridium(III) complexes. The NBO analysis has provided detailed insight into the type of hybridization, nature of bonding in various organoiridium(III) complexes and the effect of nature of ligand on the stability of the complexes. The highest second order perturbation energies correspond to the maximum delocalization between the donor–acceptor orbitals. The increase in the electron density in the antibonding acceptor orbitals leads to the destabilization of the systems.

Acknowledgements

Dr ASRK is highly grateful to Council of Scientific and Industrial Research India (CSIR-INDIA) for the financial support of the present work through research grants Ref. No. 01(2541)/11/EMR-II. Ms Chinduluri Sravani is grateful to CSIR for providing senior research fellowship. Dr Sadhana Venkatesh is grateful to CSIR for the junior research fellowship. Dr ASRK and his group members thank DST-VIT-FIST for NMR and SIF-VIT University for other facilities.

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Footnotes

† Dedicated to Prof. Dr Upadhyayula Muralikrishna (Retired Professor of Chemistry from Andhra University, Visakhapatnam 530003, Andhra Pradesh, India) on his birthday.

‡ Electronic supplementary information (ESI) available: Tables with optimized cartesian coordinates for A–H mentioned in the text. See DOI: 10.1039/c5ra27350a

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