Alkynyl(phenyl) iodonium triflates as precursors to iridium(III) σ –acetylide complexes

The reaction of Vaska’s complex 2 with alkynyl(phenyl)iodonium triflate salt 5 has been used to generate the Ir(III) σ –acetylide complex 6 . The labile triflate ligand is easily displaced from 6 , and this property has been used to explore the ability of a nitrile ligand to coordinate to the iridium center in exchange reactions, giving complexes 7a–d in excellent yields. All complexes are stable under ambient conditions. The solid–state structure of alkynyl iodonium salt 5 , as well as complexes 7a and 7d have been investigated by single crystal X–ray crystallography.


Introduction
Alkynyl(aryl)iodonium salts are a versatile class of compounds that can function as precursors for the formation of a wide range of acetylenic derivatives. 1 They also serve well as a source of alkylidene carbene intermediates, which can be strategically trapped via intramolecular C-H bond insertion toward the formation of complex molecules. 2,3Several reports have also shown that alkynyl(aryl)iodonium triflates (1) can be effectively transformed into σ-acetylide complexes (3) through reaction with Vaska's complex (2) or its rhodium analogue. 4While a number of Ir(III) acetylide complexes have been successfully formed using this general methodology, the potential for elaboration of the products 3 has, surprisingly, gone unexplored.
We expected that the triflate ligand of complexes such as 3 should be easily displaced.Therefore, these complexes, and those derived from bis(alkynyl)iodonium salts such as 4, 4a could thus function as conjugated building blocks for the construction of coordination polymers and networks with potentially interesting structural, electronic, and optical properties. 5As the first step in this study, we report herein the successful derivatization of iridium triflate complexes such as 3 with alkyl and aryl nitriles.

Results and Discussion
Alkynyl iodonium salt 5 6 was reacted at room temperature in toluene with Vaska's complex 2 to give acetylide complex 6 in 92% yield as a stable white solid (Scheme 1).Dissolving complex 6 in CH 2 Cl 2 , followed by the addition of CH 3 CN results in the complete displacement of the triflate moiety by CH 3 CN, and the resulting complex 7a can be precipitated from this solution by the addition of hexanes.Using a similar protocol, addition of the appropriate arylnitrile (in excess) to a CH 2 Cl 2 solution of 6 resulted in the precipitation of the desired complexes 7b-d over the period of approximately one hour.Hexanes were then added to ensure complete precipitation from the reaction mixture, and the complexes were isolated in good yields by filtration.
All of the complexes 7 are off-white to pale yellow solids, are thermally stable to nearly 200 o C, and can be handled under ambient conditions with no noticeable decomposition.Several aspects of the IR and NMR spectra are particularly useful for structural characterization (Table 1).The IR spectrum for 7a shows an absorbance for the C≡N stretch at 2336 cm -1 , and this absorbance for aryl nitriles of 7b-d falls in the range of 2290-2300 cm -1 .The intense C≡C and C≡O stretches appear at approximately 2110 and 2070 cm -1 , respectively.The 13 C spectra show that the resonances of the C≡C-Ir carbons are shifted upfield, falling in the range of 72-73 ppm, with 2 J CP ~ 12 Hz.The resonances of the C≡C-Ir carbons are found between 115-117 ppm, with 3 J CP = 2 Hz.The signals for the carbon monoxide ligand are seen as a triplet in the range of 152-155 ppm, with 2 J CP = 6 Hz, while the N≡C carbon resonates near 120 ppm.Somewhat surprisingly, there appears to be little variance in the NMR spectral data as a function of the electronic nature of the ancillary nitrile ligand.A weak trend in the 31 P NMR data shows that the phosphorus resonance does shift to higher field as the nitrile's electron donating ability is increased in the series of 7d → 7b → 7c.
Electrospray mass spectrometric analysis of complexes 6 and 7a-d is particularly useful for confirming their composition and the association in solution of the nitrile ligand.These MS analyses show that the iridium complexes tend to form dimeric clusters under the analysis conditions used.Thus, the highest mass species detected in each case consists of [2M -TfO -] + .The main spectrometric pattern in the lower m/z region for each complex includes a weak signal indicating [M -TfO -] + , a slightly stronger signal m/z 877 ([M -RCN -TfO -] + ), and a base peak at m/z 849 ([M -RCN -CO -TfO -] + ).The [M -TfO -] + signal observed for each compound clearly displays an isotope pattern consistent with the proposed composition.The highly crystalline nature of 5 led us to investigate the solid-state characteristics of this alkynyl(iodonium) triflate salt. 7Single crystals of 5 were grown by the slow evaporation of a concentrated CDCl 3 solution at 4 o C. As is often observed in the solid state for iodonium triflate salts, secondary bonding interactions between the triflate anion and the I(III) center afford a dimeric structure, which is in the current case centrosymmetric.The overall core structure is nearly planar (excluding the phenyl group), with C(1), C( 6    Single crystals of σ−acetylide complexes 7a and 7d were each grown by diffusion of hexanes into a CHCl 3 solution, and each complex co-crystallizes with a half-equivalent of n-hexane.Although the refined structure of 7a is of slightly lower quality than that of 7d, structural comparisons are still possible (Table 2).The geometry about iridium in both cases is approximately octahedral, with the two triphenylphosphine groups occupying trans-positions.There is little significant variance in bond lengths between comparable bonds of the two structures, and the observed lengths are within expected ranges 8 .The Ir-C(2) distances, at 1.94(2) and 1.965(11), are somewhat shorter than other Ir(III) acetylide complexes that have been reported, but not remarkably (i.e., ≤ 0.1 Å). 8 The coordination of the acetylene to iridium is essentially linear, as is the orientation of the nitrile moiety, in both cases.The most significant difference between the two structures is found in the geometry of the C≡O ligand, with Ir-C(1)-O(1) angles at 166(2) o for 7a and 173.6(10) o for 7d.

Conclusions
Using an alkynyl(phenyl)iodonium triflate salt, 5, the Ir(III) σ-acetylide complex 6 is formed via reaction with Vaska's complex.The triflate moiety of this complex is easily displaced with an alkyl-or arylnitriles and affords complexes 7a-d, which are stable microcrystalline solids.Spectroscopic and solid state characterization demonstrate that the electronic influence of the nitrile ligand on the remainder of the molecule is minimal.

Figure 2 .
Figure 2. X-Ray crystal structure of 7a with the co-crystallized hexane molecule and the triflate anion removed for clarity (20% probability level).

Figure 3 .
Figure 3. X-Ray crystal structure of 7d with co-crystallized hexane molecule and the triflate anion removed for clarity (20% probability level).

Table 2 .
Selected bond lengths and angles for compounds 7a and 7d