Comparing the Acidity of (R 3 P) 2 BH-Based Donor Groups in Iridium Pincer Complexes

: In the current manuscript, we describe the reactivity of a series of iridium(III) pincer complexes with the general formulae [(PEP)IrCl(CO)(H)] n ( n = +1, +2) towards base, where PEP is a pincer-type ligand with different central donor groups, and E is the ligating atom of this group (E = B, C, N). The donor groups encompass a secondary amine, a phosphine-stabilised borylene and a protonated carbodiphosphorane. As all ligating atoms E exhibit an E–H bond, we addressed the question of wether the coordinated donor group can be deprotonated in competition to the reductive elimination of HCl from the iridium(III) centre. Based on experimental and quantum chemical investigations, it is shown that the ability for deprotonation of the coordinated ligand decreases in the order of (R 3 P) 2 CH + > R 2 NH > (R 3 P) 2 BH. The initial product of the reductive elimination of HCl from [(PBP)IrCl(CO)(H)] n ( 1c ), the square planar iridium(I) complex, [(PBP)Ir(CO)] + ( 3c ), was found to be unstable and further reacts to [(PBP)Ir(CO) 2 ] + ( 5c ). Comparing the C–O stretching vibrations of the latter with those of related complexes, it is demonstrated that neutral ligands based on tricoordinate boron are very strong donors. reactivity of the of boron


Introduction
Tricoordinate boron compounds, BR 3 , are typically Lewis acids and stabilise their electron deficiency by π-donating substituents, hyperconjugation or dimerisation and formation of two-electron three-centre bonds.In consequence, they can accept electron donation from electron rich metal centres and serve as Z-type ligands [1,2].More recently, several groups demonstrated that the introduction of π-accepting substituents allows to stabilise an occupied p z -orbital and therewith of a trigonal planar Lewis-base with the general formulae L 2 BR (III) [3][4][5][6][7][8][9].Consequently, such compounds are able to serve as electron-donating or L-type ligands, but the coordination chemistry of such nucleophilic boron compounds is rather unexplored [8][9][10].
In particular, the similarity to related carbon compounds of the type L 2 CH + (II) and secondary amines (I) caught our attention.Pseudo-tetrahedral, secondary amines (I) can serve as cooperative ligands in homogeneous catalysts (Figure 1), by providing a proton in concerted proton hydride transfers or simply by pre-coordination of the substrate via hydrogen bridge bonds (e.g., in Figure 1, cycle A) [11].Protonated carbodiphosphoranes of the type (R 3 P) 2 CH + (II) can be deprotonated by strong bases and easily form their deprotonated analogues when coordinated to a metal centre [12].For the corresponding boron compounds, (R 3 P) 2 BH (III), previous studies indicated that the boron-bound hydrogen atom in such ligands is not hydridic [13,14].Due to the π-accepting nature of the cyanido substituents in compounds like [HB(CN) 3 ] − , they can be deprotonated [15], which stands in contrast to the reactivity of the majority of hydrogen-containing boron compounds.Motivated by these observations, we began to study a series of isotypical iridium complexes in their reactivity towards base.Herein, we demonstrate that among this series I-III the carbon-based ligand II is the most acidic ligand, while for the other ligands a competitive reductive elimination is observed.In case of the boron-based ligand, this leads to an unique iridium(I) complex.The comparison with related iridium dicarbonyl complexes reveals strong electron donating properties of donor groups akin to III.

Results and Discussion
As the starting point for our study, we choose the isotypical iridium(III) pincer complexes 1a-1c to investigate.

Deprotonation vs. Reductive Elimination
The reaction of the cationic complex 1a with one equivalent of LiN(SiMe 3 ) 2 results in the formation of a new complex 2a (Figure 3), as judged by the observation of a single resonance at 55.5 ppm in the 31 P{ 1 H} NMR spectrum of the reaction mixture.The resonance at −16.12 ppm in the 1 H NMR spectrum, corresponding to the hydrido ligand in 1a disappears and the absence of a resonance in this region (0 to −40 ppm) suggests that no hydrido ligand is present in the newly formed 2a (Supplementary Materials).By comparison of NMR spectroscopic data with analogues isopropyl-substituted iridium pincer complexes [17], we concluded that the reductive elimination of HCl is the preferred reaction pathway.Addition of a second equivalent of LiN(SiMe 3 ) 2 resulted in the formation of a mixture of complexes and the 31 P{ 1 H} NMR spectrum displayed several new singlet resonances as well as a new AB spin system (Supplementary Materials).The latter finding either indicates a conformational change to a facially coordinated ligand with different ligands in trans-position, but this seems to be unlikely for a square pyramidal iridium(I) complex that is already formed with the first equivalent of base.A second possibility involves a β-hydride elimination from the amide ligands and subsequent tautomerisation, as previously observed for different noble metal complexes with this type of ligand [18].The NMR spectra of the iridium(III) complex 1b at ambient temperature show the presence of the cisand the trans-isomers (ca.1:1) as well as small quantities of 3b (ca.1%) [16].The 1 H NOESY NMR spectrum of 1b at ambient temperature displays exchange correlations between the hydride resonances of cisand trans-1b, as well as between the resonances of trans-1b and 3b (Figure 4a).These findings suggests the presence of an equilibrium between the two isomers of 1b (Figure 4b).Furthermore one of the isomers (trans-1b) seems to be in an equilibrium with the deprotonated species 1b, even though no additional base is present in the mixture.A broad resonance at 3.51 ppm in the 1 H NMR spectrum is assigned to HCl [19], which provides further support for reversible (de)protonation equilibrium.To get further insights about the solution behaviour of 1b, we acquired 1 H and 1 H{ 31 P} NMR spectra at different temperatures.The ratio of integrals for the hydride resonances enables to estimate the equilibrium constant K cis/trans at different temperatures.The corresponding Van't Hoff plot (Figure 4c) displays two regions of linearity between 300 and 270 K (R 2 = 0.995) as well as between 260 and 230 K (R 2 = 0.937), which might be related to the presence of a second equilibrium or solubility issues at low temperatures.However, a reliable quantification of 3b turned out to be difficult, due to the low concentration at ambient temperature, which decreases even further at lower temperatures.The corresponding exchange rates were accessed by line-shape-analysis of the hydride resonances in the 1 H{ 31 P} NMR spectra at different temperatures.An Eyring analysis (Figure 4d) revealed an Gibbs enthalpy of activation ∆G = | 298 = 69.23 kJ•mol −1 for the cis-/trans-isomerisation process.In view of the primary question, these observations suggest that 1b gets selectively deprotonated at the coordinated donor group.The iridium(III) complex 3b is indeed observed by NMR spectroscopy in reactions with base.As complex 1b, in contrast to 1a and 1c, is dicationic, one would expect a higher acidity of the coordinated donor group, but the acidity of hydrido ligands was previously demonstrated to be increased by several orders of magnitude with an increasing charge of the complex [20].
Addition of an excess base (DBU) to 1b results in the formation of the iridium(I) complex 4b as major product according to the 31 P{ 1 H} NMR spectrum of the reaction mixture (Figure 5), which displayed new triplet resonances at 23.4 ppm ( 2 J P,P = 48.5 Hz) and 38.3 ppm ( 2 J P,P = 49.3Hz).A broad multiplet resonance at 4.01-4.12ppm with an integral of four in combination with multiplet resonances between 6.9 and 7.8 ppm with an overall integral of 40 protons are observed in the 1 H NMR spectrum (Supplementary Materials), while the absence of resonances corresponding to a hydrido ligand or a protonated CDP moiety indicate that a deprotonated pincer ligand is coordinated in 4b.The observation of one band at 1925 cm −1 for the C-O stretching vibration of a carbonyl ligand is in line with an electron-rich mono-carbonyl complex.The composition of the cationic complex [{(dppm) 2 C}Ir(CO)] + Cl -in 4b was further confirmed by high resolution ESI-MS.A similar observation to the reaction of 1a is made for the boron-based iridium pincer complex (1c).Treatment of complex 1c with one equivalent LiN(SiMe 3 ) 2 leads to the formation of two species according to the 31 P{ 1 H} NMR spectrum of the reaction mixture, broadened resonance at −5.6 ppm, as well as a broad resonance at 24.9 and a multiplet at 2.9 ppm, both assignable to the newly formed complex 5c (Figure 6).After removal of all volatiles and washing of the residue with n-hexane, complex 5c is obtained in analytically pure form.The 1 H NMR spectrum of 5c shows a complete set of resonances for the dppm arms of the coordinated ligand (Supplementary Materials), while resonances corresponding to a boron-bound hydrogen atom and potential hydrido ligands are absent (Figure 7b).Upon 11 B-decoupling a triplet resonance at 3.20 ppm ( 2 J P,H = 23.2Hz) is observed in the 1 H{ 11 B} NMR spectrum, assignable to a boron-bound hydrogen atom, clearly indicating that a reductive elimination is favoured over of the ligand deprotonation.The 11 B{ 1 H} NMR spectrum of 5c gives rise to a broadened resonance at −35.4 ppm, which is in agreement with previously reported boron-based donor ligands [8][9][10]13,21].The identity of 5c was finally confirmed by single crystal X-Ray diffraction experiments (Figure 7a), which revealed a cationic iridium(I) complex with a trigonal bipyramidal environment (τ 5 = 0.70) [22].In addition to the facially coordinated PBP-ligand, two carbonyl ligands are observed, one occupying an equatorial and one an axial coordination site.The Ir-B bond in 5c is with 2.276 Å slightly shorter than in the octahedral iridium(III) complex 1c (d Ir−B = 2.285 Å) [16].
As the yield of the dicarbonyl complex 5c was below 50% and no other potential source of carbon monoxide was present in the reaction mixture, we assumed that the formation of 5c proceeds via a square planar iridium(I) intermediate 2c that subsequently reacts in carbonyl transfer step to 5c and unidentified decomposition products (Figure 6).This hypothesis is further verified by an increased yield of 59% in the deprotonation reaction in the presence of carbon monoxide.

Proton Affinities and Deprotonation Pathways
Quantum chemical investigations using density functional theory (DFT) were performed to get further insights about the reactivity of the reported iridium complexes towards bases.First we confirmed that deprotonation of the coordinated donor group results in an energetic minimum (3a-3c) according to the frequency calculation (no imaginary modes) and calculated the proton affinities (PAs) for 3a-3c (Table 1 and Figure 8).In agreement with the experimental results, complex 3b exhibits the lowest proton affinity (PA, represents the energy difference between complexes calculated without solvation and counter ions; the energy of free proton is not considered) with 864 kJ•mol −1 , while the PAs of the neutral complexes 3a (1129 kJ•mol −1 ) and 3c (1257 kJ•mol −1 ) are significantly higher.
The low PA of the CDP-group in the coordinated pincer-type ligand indicates that it might be less efficient as internal base in a potential catalyst, but in turn it suggests that protonated CDPs might be potential cooperative groups that facilitate an efficient proton-hydride-transfer from or to the catalyst.
In comparison, the value of 1257 kJ•mol −1 is too high to expect metal-ligand-cooperativity via proton-hydride-transfer, but it clearly suggests that deprotonation of coordinated (R 3 P) 2 BH groups should be facile with strong bases in the absence of more acidic sites, which would yield an unprecedented phosphine-stabilized boride.To elucidate the reductive elimination pathway, we removed a proton from the metal-coordinated hydrido ligand in 1a-1c in a gedankenexperiment and performed geometry optimisations.The resulting complexes (6a-6c) exhibit elongated iridium chloride distances (Figure 8), but were confirmed as energetic minima by frequency calculations.Although the Ir-Cl distances in 6a-6c are in range between a weak bond (2.737 Å) and non-bonding (4.181 Å), the resulting proton affinities may be used as estimate in comparison to 3a-3c.It becomes evident that in case of the amine-based ligand product of ligand-(3a) and metal-deprotonation (6a) exhibit very similar proton affinities (∆PA = 3 kJ•mol −1 ), which suggests that both pathways are in principle favourable.The experimentally observed selectivity for the reductive elimination might be kinetically favoured.In case of the protonated CDP-based ligand in 1b the ligand deprotonation is favoured 36 kJ•mol −1 over the deprotonation at the metal site, which again is in line with the experimental observations.Notably, both PAs, of 3b and 6b, are rather low.For the boron-based pincer-type ligand in 1c the deprotonation at the metal centre is clearly favoured.

Comparison with Related Iridium(I) Dicarbonyl Complexes
In comparison to related trigonal bipyramidal iridium(I) dicarbonyl complexes, 5c exhibits very similar structural features (Table 2).All complexes with two Ph 2 RP-groups and one carbonyl ligand in the equatorial plane differ in the ligand or donor group in the apical position, trans to the second carbonyl ligand [23][24][25].With τ 5 -parameters between 0.58 and 0.75, four of the five complexes are best described as trigonal bipyramidal complexes.In the IR spectrum, two bands for the C-O-stretching frequency are observed for each complex, which in principle allow to estimate the net electron donor ability of the specified donor group in comparison.Like for other dicarbonyl-based ligand parameters [26][27][28], averaging of cisand trans-influences on symmetric and asymmetric C-O-stretching modes can provide a rough picture of the net donor strength.For the neutral complexes, all values, respectively, indicate an increasing donor ability in the order R 3 SiCH 2 -> Cl -> Br -.The cationic complex with a Ph 2 RP-group in the apical position gives rise to an increased value of νCO (av) = 1996 cm −1 , confirming that anionic ligands exhibit stronger donor abilities.An unexpected finding in this context is the low value measured for complex 5c ( νCO (av) = 1958 cm −1 ), which is significantly lower than those of the anionic donor groups.Despite the fact that donor groups based on (Ph 3 P) 2 BH are overall neutral, this observation suggests that they are stronger donors than alkyl-ligands, which are known as one of the strongest donors in coordination and organometallic chemistry.

Materials and Methods
All experiments were carried out under an atmosphere of purified argon or nitrogen in the MBraun glove boxes LABmaster 130 and UNIlab or using standard Schlenk techniques.THF and diethyl ether were dried over Na/K alloy, n-hexane was dried over LiAlH 4 , toluene was dried over sodium, dichloromethane was dried over CaH 2 , methanol was dried over magnesium and ethyl acetate was dried over potassium carbonate.After drying, solvents were stored over appropriate molecular sieves.Deuterated solvents were degassed with freeze-pump-thaw cycles and stored over appropriate molecular sieves under argon atmosphere.Complexes 1a-1c synthesised according to previously reported procedures [16].
1 H, 13 C, 11 B and 31 P NMR spectra were recorded using Bruker BioSpin GmbH (Rheinstetten, Germany) Avance HD 250, 300 A, DRX 400, DRX 500 and Avance 500 NMR spectrometers at 300 K. 1 H and 13 C{ 1 H}, 13 C-APT (attached proton test) NMR chemical shifts are reported in ppm downfield from tetramethylsilane.The resonance of the residual protons in the deuterated solvent was used as internal standard for 1 H NMR spectra.The solvent peak of the deuterated solvent was used as internal standard for 13 C NMR spectra.The assignment of resonances in 1 H and 13 C NMR spectra was further supported by 1 H COSY, 1 H NOESY, 1 H, 13 C HMQC and 1 H, 13 C HMBC NMR spectra.NH)IrCl(CO)(H)]Cl (1a, 27.3 µmol, 1.0 eq.) and 4.6 mg LiHMDS (27.5 µmol, 1.0 eq.) were suspended in 0.6 mL THF-d 8 .After stirring for 16 h, the resulting light orange suspension was filtered and, after addition of 0.2 ml THF-d 8 , the first NMR spectra were recorded.
[({Ph 2 PCH 2 CH 2 } 2 NH)Ir(CO)]Cl (2a) was identified as the main product, while small amounts of 1a remained unreacted.Further 4.7 mg of LiHMDS (27.5 µmol, 1.0 eq.) were added, upon which the color changed to a dark orange, and the second set of NMR spectra were recorded.

Figure 1 .
Figure 1.(a) Secondary amines (I), protonated carbodiphosphoranes (II) and phosphine-stabilized borylenes (III) in comparison; (b) Secondary amine ligands and their role in cooperative catalysis in comparison to the analogous metal complexes with II and III as ligands.

Figure 4 .Figure 5 .
Figure 4. (a) Hydride region in the 1 H NOESY NMR spectrum at ambient temperature, showing chemical exchange correlations; (b) Equilibrium of the complexes in solution; (c) Van't Hoff plot for the cis-/trans-isomerisation of 1b; (d) Eyring plot for the cis-/trans-isomerisation of 1b.

Table 2 .
Comparison of Iridium(I) dicarbonyl complexes from literature with the new complex 5c.
11B NMR chemical shifts are reported in ppm downfield from BF 3 • Et 2 O and referenced to an external solution of BF 3 • Et 2 O in CDCl 3 . 31P NMR chemical shifts are reported in ppm downfield from H 3 PO 4 and referenced to an external 85 % solution of phosphoric acid in D 2 O.The following abbreviations are used for the description of NMR data: br (broad), s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), m (multiplet).FT-IR spectra were recorded by attenuated total reflection of the solid samples on a Bruker Tensor IF37 spectrometer.The intensity of the absorption band is indicated as w (weak), m (medium), s (strong), vs (very strong) and br (broad).HR-ESI mass spectra were acquired with a LTQ-FT mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).The resolution was set to 100,000.