The underappreciated influence of ancillary halide on metal–ligand proton tautomerism

Syntheses of Vaska-type complexes [IrP2X(CO)] (P = phosphine, X = halide) with all four common halides (fluoride, chloride, bromide, and iodide) was attempted using a protic and hemilabile imidazolyl di-tert-butyl phosphine ligand. In the solid-state, all four complexes were found to be ionic with the halides in the outer-sphere, and the fourth coordination site of the square plane occupied by the imidazole arm of the ligand. In solution, however, the chloride complex was found to be in equilibrium with an octahedral IrIII–H species at room temperature. For the bromide and iodide analogs, the corresponding IrIII–H species were also observed but only after heating the solutions. The neutral IrI Vaska's analogs for X = Cl, Br, and I were obtained upon addition of excess halide salt, albeit heating was required for X = Br and I. The IrIII–H species are proposed to originate from tautomerization of minor amounts of the electron rich neutral Vaska analog (halide inner-sphere and phosphines monodentate) that are in equilibrium with the ionic species. Heating is required for the larger anions of bromide and iodide to overcome a kinetic barrier associated with their movement to an inner-sphere position prior to tautormerization. For the fluoride analog, the IrIII–H was not observed, attributable to strong hydrogen bonding interactions of the imidazolyl proton with the fluoride anion.


General Methods
All experiments and manipulations were performed using standard Schlenk techniques under an argon or nitrogen atmosphere, or in a nitrogen filled glovebox (unless stated otherwise).
Deuterated solvents were purchased from Cambridge Isotope Laboratories. Acetone-d 6 was degassed by three successive freeze-pump-thaw cycles and stored over molecular sieves inside of an inert glovebox. DCM-d 2 was refluxed overnight with CaH 2 , vacuum transferred, and stored in a glovebox. Acetone was degassed by sparging using nitrogen for 30 minutes, followed by storage over molecular sieves in a glovebox. Diethyl ether, dichloromethane, and acetonitrile were dried by passage through activated alumina and molecular sieves columns under a stream of argon (a Grubb's type solvent purification system by JC Meyer Solvent Systems). [Ir(coe) 2 Cl] 2 (coe = cyclooctene), 1 2-(di-tert-butylphosphaneyl)-1H-imidazole (LH), 2 and [Ir(coe) 2 (acetone) 2 ][PF 6 ], 3 were synthesized following literature procedures. NEt 4 Cl, NEt 4 Br, NEt 4 I, NMe 4 F, NBu 4 Br, NBu 4 I, (NBu 4 )(PF 6 ), and 1,3,5-trimethoxybenzene were dried under vacuum prior to their use. NEt 3 was refluxed overnight with CaH 2 , distilled, and stored over molecular sieves inside a glovebox. All other chemicals were used as received from commercial sources without further purification.
The NMR spectra were collected on a Bruker 400 or 500 MHz spectrometer at ambient probe temperatures. NMR tubes fitted with a J. Young style Teflon valve were used to obtain NMR data in inert atmosphere. 1 H, 1 H{ 31 P}, 1 H{ 19 F} NMR spectral data are reported in ppm and were referenced to residual protons in the deuterated solvents: DCM-d 2 (5.32 ppm) and acetone-d 6 (2.05 ppm). 4 13 C{ 1 H} NMR shifts were referenced to the solvent's signal: DCM-d 2 (53.84 ppm) and acetone-d 6 (29.84 ppm). 4 31 P{ 1 H} NMR shifts were referenced to an 85% D 3 PO 4 (in D 2 O) external standard (0 ppm). 19 F and 19 F { 1 H} NMR shifts are reported relative to a fluorobenzene external standard (-113.15 ppm). IR spectra were taken on a PerkinElmer Spectrum 2000 FT-IR Spectrometer using a PerkinElmer IR liquid cell (NaCl). Accurate mass measurements were obtained on a Waters LCT Premier XE, time-of-flight, LCMS with electrospray ionization (ESI).
The signals were mass measured against an internal lock mass reference of leucine enkephalin.
Waters software calibrates the instruments, and reports measurements, by use of neutral atomic masses. Compositions of the samples for microanalysis were determined with a PerkinElmer 2400 Series II Analyzer at the CENTC Elemental Analysis Facility (University of Rochester). The samples were handled in inert conditions and combusted in a tin capsule that was crimp-sealed with a die apparatus.

Method 2:
Alternatively, [ LH Ir][PF 6 ] was also prepared in similar yield by exposing an acetone solution of Ir-1 to CO for 5 minutes and working up as described above.
CAUTION: Experiments involving CO gas must be performed in a well-ventilated and working fume hood with utmost care to avoid any inhalation of the toxic gas.

Synthesis and Characterization of [ LH Ir][I]
In the dark,   Thereafter, a total of 10 molar equivalents of (NBu 4 )(PF 6 ) salt was added to the solution of the in situ generated [ LH Ir-Cl] portion-wise ( 1 H NMR was collected after cumulative additions of 2, 5, and 10 molar equivalents; Figure S28). It can be noted that a profound increase in the ionic strength of the solution by addition of excess ionic salt has almost no impact on the signals; the N-H peak, for example, shifts downfield by a mere 0.03 ppm.        In the 19 F NMR spectrum ( Figure S35), signals corresponding to CD 2 F 2 , 5 HF 2 -, 6 and CD 2 ClF 6 are observed. CD 2 F 2 and CD 2 ClF are formed due to the reaction of the fluoride anion with CD 2 Cl 2 (with concomitant formation of stoichiometric chloride anion). 6      -169. 5 -169.0 -149.0 -148.5 -143.5 -143.0 -142.5 ppm

Calculation of K eq for the Overall Metal-Ligand Proton Tautomeric Equilibria of [ LH Ir][X] Complexes (X = Cl, Br, I) in DCM-d 2
As detailed in the manuscript (Scheme 1, A) [X], the above expression in eq. (S1) can be rewritten as, To calculate K eq , 1,3,5-trimethoxybenzene (TMB) was used as an internal standard (δ 6.07, 3.75 in DCM-d 2 ). The values for "conc.
It was hypothesized that these two positions were occupied by a Fion in half of the asymmetric units and by oxygen atoms of water molecules in the other half. Thus, F2 and F3 in Figure S42 are actually 50/50 mixtures of F and O. Refinement was by full-matrix lea squares based on F 2 using SHELXL-2018. 10 All reflections were used during refinement. The weighting scheme used was w