Iridium Oxidation States in Catalytic Hydrogenation Intermediates

Os conhecidos hidretos de irídio(III) [IrH2(NCMe)3(PiPr3)]BF4, [IrH(h-C3H5)(NCMe)2(PiPr3)] BF4, [IrH(E-CH=CHPh)(NCMe)3(PiPr3)]BF4 e [IrH{C(Ph)=CH2}(NCMe)3(PiPr3)]BF4, derivados do precursor catalítico tipo Crabtree [Ir(cod)(NCMe)(PiPr3)]BF4, foram investigados em reações com substratos tipicamente envolvidos na catálise homogênea de hidrogenação. Novos complexos como as espécies tris-etileno irídio(I) [Ir(NCMe)(h-C2H4)3(PiPr3)]BF4, os produtos de inserção de difenilacetileno [IrH{Z-C(Ph)=CHPh}(NCMe)3(PiPr3)]BF4 e [Ir(h-C3H5){Z-C(Ph)=CHPh} (NCMe)2(PiPr3)]BF4, e os derivados de [Ir(kO-acac)(h-C3H5){Z-C(Ph)=CHPh}(PiPr3)] e [Ir{kC-C6H4-2-E-(CH=CPh)}(NCMe)3(PiPr3)]BF4, foram caracterizados. O conjunto de observações experimentais sugere que espécies irídio(I), embora acessíveis, são improváveis como intermediários de hidrogenação. Baseados em experimentos de deuteração, uma nova tautomerização do hidreto de alquenil a carbeno foi proposta.


Introduction
Iridium homogeneous catalysts have proved suitable for large scale enantioselective hydrogenations of C=N bonds 1 and could soon become industrial also for unfunctionalized olefin substrates. 2 To this end, cationic Crabtree-type catalysts, in particular, are currently under intense investigation to unravel mechanistic details useful for the optimization of ligands and catalysts. 3Such an investigation is mainly based on theoretical calculations, 4 with just a few experimental contributions. 5So far, the studies left no doubt that the olefin hydrogenation mechanism involves iridium(III) dihydrides, which are usually observable and often isolable, 5,6 but are less conclusive about whether the catalytic cycle closes via iridium(I) or iridium(V) intermediates.
Fifteen years ago, we reported the cationic iridium(III) dihydride complex [IrH 2 (NCMe) 3 (PiPr 3 )]BF 4 (1), which was prepared from the Crabtree-type catalyst precursor [Ir(cod)(NCMe)(PiPr 3 )]BF 4 under conditions typical of homogeneous hydrogenation. 7This compound turned out to be an adequate precursor for the nuclear magnetic resonance (NMR) observation of organometallic species potentially involved as intermediates in olefin hydrogenation cycles.In fact, we presented possible catalytic cycles for ethylene and propylene hydrogenation totally based on observed intermediates and reaction steps.Since then, our frequent use of this compound as starting material in synthetic work has provided new observations that may contribute to this renewed mechanistic discussion.These observations, presented and discussed in the following pages, suggest that Ir(I) species, though easily accessible, are unlikely hydrogenation intermediates.Vol. 25, No. 12, 2014
This type of insertion products retain from precursor 1 three easily replaceable acetonitrile ligands, which guarantee further reactivity, and in some cases are stable towards reductive elimination.This is not the case for the labile hydride-alkyl complexes derived from ethylene or propylene insertions, which were reported to readily evolve alkene or alkane via facile H β-eliminations or C-H reductive eliminations, respectively. 7The hydridealkenyl complex 2 proved to be more stable than the alkyl derivatives, 9 although it was observed to slowly decompose at room temperature forming Z-stilbene, even in the solid state.The reported phenylacetylene analogues of 2, isomers [IrH(E-CH=CHPh)(NCMe) 3 (PiPr 3 )]BF 4 (3-trans) and [IrH{C(Ph)=CH 2 }(NCMe) 3 (PiPr 3 )]BF 4 (3-gem) 8 (Scheme 2), were found to be rather robust and their study constitutes the first part of this work.In addition, the hydride-h 3 -allyl derivative [IrH(h 3 -C 3 H 5 )(NCMe) 2 (PiPr 3 )] BF 4 (4), 7 closely related to the hydride-alkyls but less labile due to the additional coordination of the olefin moiety, was also interrogated in the search for possible hydrogenation pathways other than reductive elimination.
As shown in Scheme 2, the treatment of a 1:1 mixture of the hydride-alkenyl isomers 3 8 with hydrogen (ca. 1 bar) at room temperature readily produced styrene and complex 1.At low temperature, however, the NMR monitoring of the reaction between these isomers and D 2 revealed deuterium incorporation into the hydride ligand prior to styrene formation.This could be conveniently followed by 31 P{ 1 H} NMR, given that hydride ligand deuteration produced isotopic shifts of about +0.072 ppm in the phosphorous signal of each isomer 3.This shift is just slightly smaller than those observed for similar isotopic substitutions in the precursor complex 1 (0.12 ppm). 7Although such a process scrambles the deuterium label and therefore impede further conclusions about the C-H bond forming step leading to styrene, it clearly indicates that complexes 3 do not need to undergo reductive elimination to cleave the H 2 molecule, in line with the mechanistic alternatives that postulate Ir(V) hydrogenation intermediates. 3terestingly, whereas the scrambling of deuterium between D 2 and 3-trans exclusively involved the hydride ligand, the isomer 3-gem was observed to further incorporate deuterium into the two geminal positions of the alkenyl ligand.The 1 H and 31 P{ 1 H} NMR signals of Figure 2, which correspond to a mixture of isotopomers 3-gem after a few minutes of exposition to D 2 at 253 K, show that the extent of deuterium incorporation into each of the three positions is similar.This could indicate that isomer 3-gem is in equilibrium with a putative carbene intermediate (Scheme 3), in which methyl rotation statistically distributes the deuterium label into the three positions where it is eventually observed.Given that the process only affects to the gem isomer of 3, a bulky substituent at the alkenyl  α carbon might be important to achieve this rare hydridealkenyl to carbene tautomerization.In fact, whereas the protonation of alkenyl complexes is a known synthetic route to carbene derivatives, 10 to the best of our knowledge, an intramolecular version of such reaction has never been reported.Other more conventional explanations for the observed deuteration pattern, in particular those involving styrene formation, can be ruled out because styrene does not react at all with 1 under the conditions of this experiment.
In view of the behavior of well-characterized analogues, 11 the proposed intermediate of Scheme 3 should be considered an Ir(I) carbene complex even though it contains a typical Schrock type alkylidene ligand.Unconventional species aside, the experiment of Scheme 2 suggests that other Ir(I) intermediates resulting from reductive eliminations of the hydrogenation products are unlikely in the presence of hydrogen.This conclusion is also consistent with the reported behavior of the hydride-allyl complex 4, whose reaction with D 2 at room temperature produced propene and the hydride-deuteride isotopomer of 1. 7 4 was also tested in reactions with common hydrogen acceptors such as ethylene (Scheme 4).
The quick exposition to ethylene of a concentrated solution of 4 in chlorinated solvents, followed by a rapid precipitation in diethyl ether, led to the complex [IrH(h 3 -C 3 H 5 )(NCMe)(h 2 -C 2 H 4 )(PiPr 3 )]BF 4 (5, Scheme 4), in which ethylene was incorporated as a ligand instead of one acetonitrile.Note that unlike the reactions with alkynes (see later) and despite the presence of free acetonitrile, the favored product is the hydride-alkene complex instead of an alkyl derivative.Most signals of the NMR spectra of 5 are similar to those of its precursor 4, 7 including a doublet at d −29.25 (J HP = 15.0Hz) in the 1 H spectrum corresponding to a hydride ligand cis to phosphorous (and trans to acetonitrile) and a doublet at d 56.61 (J CP = 16.2Hz) in the 13 C{ 1 H} spectrum, consistent with an allyllic carbon trans to phosphorous.The new ethylene ligand displays a singlet at d 44.08 in the 13 C{ 1 H} NMR spectrum and an AA'BB' spin system at d 3.23 (J AA' = J BB' = 9.6 Hz, J AB = J A'B' = 8.7 Hz) in the 1 H NMR spectrum, in agreement with the fast rotation of the coordinated ethylene and the lack of symmetry elements in the complex.
More prolonged reactions with ethylene led to solutions whose color changed depending on the amount of dissolved ethylene; being red under vacuum and colorless under ethylene excess.The NMR spectra of these solutions were rather meaningless at any temperature because of the presence of broad signals.Nevertheless, we were able to obtain crystals of what we believe is the major species at low temperature under ethylene excess: the Ir(I) trisethylene complex [Ir(NCMe)(h 2 -C 2 H 4 ) 3 (PiPr 3 )]BF 4 (6).The X-ray structure of this highly symmetric compound is shown in Figure 3, relevant distances and angles are listed in Table 1.The compound displays a regular trigonal bipyramidal structure in which the metal atom is surrounded by six carbons in a planar arrangement, with Ir-C distances in the range 2.20 to 2.24 Å.
The formation of 6 indicates that the excess ethylene is indeed capable of stabilizing the oxidation state Ir(I).The mild reaction conditions leading to this compound also suggest that the replacement of the hard acetonitrile ligands of the Ir(III) precursor by ethylene may facilitate the reductive elimination of propene required to access the Ir(I) intermediates.Yet, the versatility of this type of complexes may enable pathways for propene elimination not necessarily involving Ir(I) intermediates, as suggested by the sequence of reactions in Scheme 5.
Similarly to precursor 1, complex 4 can undergo insertion of diphenylacetylene into the Ir-H bond to form an alkenyl derivative, [Ir(h 3 -C 3 H 5 ){Z-C(Ph)=CHPh} (NCMe) 2 (PiPr 3 )]BF 4 (7).Although we were not capable of obtaining crystals suitable for a diffraction experiment, the likely structural features of 7 can be inferred in the X-ray structure of its neutral derivative [Ir(k 2 O-acac)(h 3 -C 3 H 5 ) {Z-C(Ph)=CHPh}(PiPr 3 )] (8) (Figure 4 left and Table 1), which was obtained after the replacement of the two labile acetonitrile ligands of 7 by acetylacetonate.The NMR spectra of 8 are consistent with the solid state structure and resemble those of 7, thus supporting the structural proposal shown in Scheme 5.Both alkenyl-allyl derivatives display characteristic doublets in the 13  While derivative 8 is highly stable, its precursor 7 was observed to readily transform at room temperature into the complex [Ir{k 2 C-C 6 H 4 -2-E-(CH=CPh)}(NCMe) 3 (PiPr 3 )] BF 4 (9), with simultaneous evolution of propene.In view of the structure found for 9 (Figure 4 right and Table 1), the mildness of this transformation is surprising, as it should involve various C-H bond cleavages and formations, not only to enable propene elimination but also to isomerize the former Z alkenyl ligand into the E form.Such alkenyl ligand isomerizations have been previously observed 12 and seem to play a role in certain catalytic transformations. 13ypically, they have been attributed to the formation of carbene-like intermediates (zwitterionic or h 2 -vinyl), 14 although recent work has disclosed new mechanistic alternatives in iridium complexes. 15In any case, regardless of the specific mechanism, the transformation of 7 into 9 evidences that these complexes can carry out elaborate transformations of organic molecules without resorting to the oxidation state Ir(I).

Conclusions
The set of reactions and new compounds described in this study confirms previous observations indicating that Ir(III) dihydride complexes derived from Crabtreetype catalyst precursors can readily react with hydrogen acceptors (alkenes and alkynes) according to coordination and insertion sequences, although they are less likely to undergo reductive eliminations of the hydrogenated products to form Ir(I) intermediates.As an alternative, the Ir(III) insertion products have proved their ability to cleave H-H and C-H bonds, thus leading to the hydrogenation products without resorting to the oxidation state Ir(I).Nevertheless, the experiments have also allowed observing and proposing Ir(I) intermediates potentially accessible under hydrogenation conditions.One of them, a carbene complex, might result from the rare tautomerization of an Ir(III) hydride-alkenyl derivative.

General
All manipulations were carried out with exclusion of air by using standard Schlenk techniques or in an argon-filled drybox (MBraun).Solvents were obtained from a solvent purification system (MBraun).Deuterated solvents were dried with appropriate drying agents and degassed with argon prior to use.C, H and N analyses were carried out in a Perkin-Elmer 2400 CHNS/O analyzer.Mass spectrometry (MS) data were recorded on a VG Autospec double-focusing mass spectrometer operating in the positive mode; ions were produced with the Cs + gun at ca. 30 kV, and 3-nitrobenzyl alcohol (NBA) was used as the matrix.Infrared spectra were recorded as Nujol mulls on polyethylene sheets or in KBr using the spectrometers Bruker Equinox 55 or Perkin-Elmer Spectrum One.Conductivities were measured in ca. 3 × 10 −4 mol L −1 solutions using a Philips PW 9501/01 conductometer.NMR spectra were recorded on Bruker Avance 300 MHz spectrometer. 1 H (300.13 MHz) and 13 C (75.5 MHz) NMR chemical shifts were measured relative to partially deuterated solvent peaks but are reported in ppm relative to tetramethylsilane (TMS). 31P (121.5 MHz) chemical shifts were measured relative to H 3 PO 4 (85%).Coupling constants, J, are given in Hertz.In general, NMR spectral assignments were achieved through 1 H COSY, 1 H NOESY, 1 H{ 31 P}, 13 C APT, and 1 H/ 13 C HSQC experiments.Unless otherwise indicated, the NMR data are given at room temperature.

Synthesis and characterization of the complexes
The complexes 1 and 4, 7 and the 1:1 mixture of isomers 3-trans and 3-gem 8 were prepared following published procedures.All other reagents were commercial and were used as received.The new complexes described below are air-sensitive in solution and solid state.

Preparation of [Ir
Preparation of [Ir(k 2 O-acac)(h 3 -C 3 H 5 ){Z-C(Ph)=CHPh} (PiPr 3 )] (8): A solution of Na(acac) (34 mg, 0.28 mmol) in MeOH (1 mL) was added at 273 K to a freshly prepared solution of 7 (100 mg, 0.13 mmol) in MeOH (2 mL).The resulting solution was allowed to reach room temperature and stirred for 30 min.The microcrystalline yellow solid obtained was separated by decantation, washed with MeOH and dried in vacuo.Yield 79 mg (90 %); IR ν/cm −1 1570, 1590 (C=O); 1  X-ray data were collected at 100.0(2) K on Bruker SMART APEX area detector diffractometer equipped with a normal focus, 2.4 kW, sealed tube source (molybdenum radiation, λ 0.71073 Å) operating at 50 kV and 30 mA.In all cases, single crystals were mounted on a fiber and covered with protective perfluoropolyether.Each frame covered 0.3° in ω.Data were corrected for absorption by using a multi-scan method applied with the SADABS program. 16he structures were solved by the Patterson method and refined by full-matrix least squares on F 2 using the Bruker SHELXTL program package, 17 including isotropic and subsequently anisotropic displacement parameters for all non-hydrogen non-disordered atoms.Weighted R factors (R w ) and goodness of fit (S) are based on F 2 , and conventional R factors are based on F. Hydrogen atoms were included in calculated positions and refined riding on the corresponding carbon atoms, or in observed positions and refined freely.For most of the structures the highest electronic residuals were observed in the proximity of the Ir center and make no chemical sense.

Figure 1 .
Figure 1.Crystal structure of the cation of complex 2 at the 50% probability level.Hydrogen atoms, except the hydride ligand, are omitted for clarity.

Figure 2 .
Figure 2. Selected NMR signals of a mixture of isotopomers of 3-gem in CDCl 3 at 253 K: 1 H signals corresponding to the two geminal alkenyl protons (left) and 31 P{ 1 H} NMR signals (right).

Figure 3 .
Figure 3. Crystal structure of the cation of complex 6 at the 50% probability level.Hydrogen atoms are omitted for clarity.
C{ 1 H} NMR spectrum corresponding to alkenyl α carbons cis to phosphororus, at d 124.10 (J CP = 7.2 Hz) for 7 and at d 129.85 (J CP = 7.5 Hz) for 8.In addition, as mentioned for 5, doublets at d 64.47 (J CP = 20.5 Hz) for 7 and d 59.22 (J CP = 31.8Hz) for 8 are diagnostic of the relative trans disposition of one of the allyllic carbons and the phosphine.

Figure 4 .
Figure 4. Crystal structures of complex 8 (left) and the cation of complex 9 (right) at the 50% probability level.Hydrogen atoms are omitted for clarity.