Trans-dihydride Complex Research Articles

Reactivity of four coordinate iridium complex towards hydrogen: An experimental and computational study

Reaction of a four coordinate, 16-electron Ir complex, [Ir(iPr)4(POCOP)(PPh3)] (4) ((iPr)4(POCOP = 2,6-bis(di-isopropyl phosphinito)benzene, κ3-C6H3-1,3-[OP(iPr)2]2), with H2 resulted in an oxidative addition product, cis-dihydride complex, cis-[Ir(H)2(iPr)4(POCOP)(PPh3)] (cis-5) presumably via the intermediacy of a sigma complex, [Ir(η2-H2)(iPr)4(POCOP)(PPh3)]. The cis-dihydride complex completely isomerizes to the trans-dihydride complex trans-[Ir(H)2(iPr)4(POCOP)(PPh3)] (trans-5) under ambient conditions in about 3 h. It was found that the steric and electronic features on the iridium center have significant influence on the approach of H2 onto the metal center followed by oxidative addition and isomerization. The isomerization process was studied in detail and all the mechanistic aspects have been elucidated using a combination of both experimental work and computation. The cis-dihydride complex isomerizes to the trans-dihydride by compensating the trans influence of the strongly trans-directing hydride ligand. A mechanism involving the exchange of the position of PPh3 with a hydride ligand cis to itself via PPh3 dissociation and re-coordination thereby resulting in the formation of the trans-dihydride complex, has been proposed for the isomerization. The cis-dihydride was found to be a highly active catalyst for hydrogenation of ethylene. A competing reactivity study of cis-dihydride between isomerization versus insertion of C2H4 into the Ir—H bond, was studied experimentally and computationally.

Calculations on the non-classical β-hydride elimination observed in trans-(H)(OMe)-Ir(Ph)(PMe3)3: possible production and reaction of methyl formate

The octahedral trans hydrido-alkoxide complex trans-(H)(OMe)-Ir(Ph)(PMe3)3 (2-OCH3) was prepared by Milstein and coworkers by addition of methanol to Ir(Ph)(PMe3)3 (1). 2-OCH3 was discovered to undergo a methanol catalyzed outer-sphere carbonyl de-insertion in which a vacant coordination site is not required. The reaction yields the octahedral trans dihydride complex trans-(H)2-Ir(Ph)(PMe3)3 (2-H) as a kinetic product along with formaldehyde derivatives reported as [CH2=O]x. We investigate the mechanism and products of this reaction using density functional theory. The de-insertion transition state has an ion-pair character leading to a high barrier in benzene continuum: ΔG ‡ = 27.9 kcal/mol. Adding one methanol molecule by H-bonding to the alkoxide of 2-OCH3 lowers the barrier to 22.7 kcal/mol. When the calculations are conducted in a methanol continuum, the barrier drops to 8.8 kcal/mol. However, the thermodynamics of de-insertion are endergonic by near 5 kcal/mol in both benzene and methanol. The calculations identify a low energy outer-sphere H/OMe metathesis pathway that transforms the formaldehyde and another 2-OCH3 molecule directly into a second 2-H complex and methyl formate. Likewise, a second H/OCH3 metathesis reaction interconverting methyl formate and 2-OCH3 into 2-H and dimethyl carbonate is computed to be exergonic and kinetically facile. These results imply that the production of methyl formate and dimethyl carbonate from 2-OCH3 is plausible in this system. The net transformation from the square planar 1 and methanol to 2-H and either methyl formate or dimethyl carbonate would represent a unique stoichiometric dehydrogenative coupling reaction taking place at room temperature by an outer-sphere mechanism.

Metal Hydride Vibrations: The Trans Effect of the Hydride

trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)2, trans-RuH2(PPh(OEt)2)4, and mer-IrH3(PiPr2CH2pyCH2PiPr2) provide M-H force constants and H-M-H interaction force constants that increase as FeII < RuII < IrIII. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsymH-M-H and antisymmetric νasymH-M-H vibrational modes of hydrides that are mutually trans in d6 octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm-1, while the 3d metals have only 90 cm-1. This has been verified experimentally for 3d, 4d, and 5d transition-metal trans-dihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh(OEt)2)4 (from the literature) and trans-FeH2(PPh2CH2PPh2)2 and mer-IrH3(PiPr2CH2pyCH2PiPr2) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Δνt -235 cm-1) allows us to improve the simple equation reported previously, which allows a better estimate of νM-H.

A Dihydride Mechanism Can Explain the Intriguing Substrate Selectivity of Iron-PNP-Mediated Hydrogenation

Iron-PNP pincer complexes are efficient catalysts for the hydrogenation of aldehydes and ketones. A variety of hydrogenation mechanisms have been proposed for these systems, but there appears to be no clear consensus on a preferred pathway. We have employed high-level quantum chemical calculations to evaluate various mechanistic possibilities for iron-PNP catalysts containing either CH2, NCH3, or NH in the PNP linker. For all three catalyst types, we propose that the active species is a trans-dihydride complex. For CH2- and NH-containing complexes, we predict a dihydride mechanism involving a dearomatization of the backbone. The proposed mechanism proceeds through a metal-bound alkoxide intermediate, in excellent agreement with experimental observations. Interestingly, the relative stability of the iron-alkoxide can explain why complexes with NCH3 in the PNP linker are chemoselective for aldehydes, whereas those with CH2 or NH in the linker do not show a clear substrate preference. As a general concept in...

Hydrogen Addition to (pincer)IrI(CO) Complexes: The Importance of Steric and Electronic Factors

(POCOP)IrI(CO) [POCOP = κ3-C6H3-2,6-(OPR2)2 for R = tBu, iPr] and (PCP)IrI(CO) [PCP = κ3-C6H3-2,6-(CH2PR2)2 for R = tBu and iPr] complexes can add hydrogen via two distinct pathways. When R = tBu, (POCOP)Ir(CO) and (PCP)Ir(CO) complexes only add hydrogen via a proton-catalyzed pathway due to steric effects, yielding trans-dihydride complexes. For R = iPr, both systems oxidatively add hydrogen to give cis-dihydride complexes which thermally isomerize to more thermodynamically favorable trans-dihydride species, consistent with previous reports. Proton-catalyzed hydrogen addition pathways are also observed for both iPr-substituted (pincer)Ir(CO) complexes. (PCP)Ir(CO) complexes add hydrogen under milder conditions than the analogous POCOP species. Intermediate hydrido-pyridine Ir(III) carbonyl complexes from the proton-catalyzed pathway have been synthesized and characterized. This is the first report of a series of complexes shown to add hydrogen via concerted oxidative addition or a proton-catalyzed pathwa...

Mechanisms for dehydrogenation and hydrogenation of N-heterocycles using PNP-pincer-supported iron catalysts: a density functional study.

The catalytic dehydrogenation and hydrogenation of N-heterocycles have potential applications in organic hydrogen storage. Recently, Fe(HPNP)(CO)(H)(HBH3) (cp1) and Fe(HPNP)(CO)(H)(Br) (cp2), the iron(ii) complexes supported by bis(phosphino)amine pincer (Fe-PNP) (PNP = N(CH2CH2P(i)Pr2)2), have been reported to be the starting complexes which can catalyze the dehydrogenation and hydrogenation of N-heterocycles. The active species were proposed to be the trans-dihydride complexes, Fe(HPNP)(CO)(H)2 (cp4) and Fe(PNP)(CO)(H) (cp3), which can be interconverted. Here, our density functional study revealed that the N-heterocyclic substrate plays a role in the formation of cp4 from cp1, while the tert-butoxide base assists with the formation of cp3 from cp2. The mechanism for cp3 catalyzed dehydrogenation of a 1,2,3,4-tetrahydroquinoline (THQ) substrate to quinoline (Q) involves two main steps: (i) dehydrogenation of THQ to 3,4-dihydroquinoline (34DHQ) and (ii) dehydrogenation of 34DHQ to Q. In each dehydrogenation step, the proton is transferred from the substrate to the N of the PNP ligand of cp3. An ion-pair complex between Fe-PNP and the deprotonated substrate is then formed before the hydride at the adjacent C is transferred to Fe. Notably, the isomerization of 34DHQ to 14DHQ or 12DHQ is not necessary, as the bifunctionality of Fe-PNP in cp3 can stabilize the ion-pair complex and facilitate direct dehydrogenation of the C3-C4 bond in 34DHQ. On the other hand, the mechanism for hydrogenation of Q involves the initial formation of 14DHQ, which can easily isomerize to 34DHQ with the assistance of a tert-butoxide base. Finally, 34DHQ is dehydrogenated to THQ. As the overall energy barriers for cp3 catalyzed dehydrogenation of THQ (+27.6 kcal mol(-1)) and cp4 catalyzed hydrogenation of Q (+23.8 kcal mol(-1)) are only slightly different, reaction conditions can be conveniently adjusted to favor either the dehydrogenation or hydrogenation process. Insights into the role of metal-ligand cooperativity in Fe-PNP complexes in promoting the dehydrogenation and the hydrogenation of N-heterocycles should benefit the development of efficient catalysts for organic hydrogen storage.

Computational Mechanistic Study of Fe-Catalyzed Hydrogenation of Esters to Alcohols: Improving Catalysis by Accelerating Precatalyst Activation with a Lewis Base

DFT calculations have been performed to gain mechanistic insight into ester hydrogenation to alcohols (exemplified by PhCO2CH3 +2H2 → PhCH2OH + CH3OH), catalyzed by a well-defined Fe-PNP pincer hydridoborohydride complex (1). The entire catalytic process includes precatalyst activation to an active species trans-dihydride complex 2, 2-catalyzed transformation of PhCO2CH3 + H2 → PhCHO + CH3OH, hydrogenation of PhCHO to PhCH2OH, and catalyst regeneration via H2 addition to the dehydrogenated 2 (i.e., complex 5). The transformation, PhCO2CH3 + H2 → PhCHO + CH3OH, proceeds via hydrogenation of PhCO2CH3 to a hemiacetal PhCH(OH)(OCH3), followed by decomposition of the hemiacetal to methanol and benzaldehyde. The Fe-complex 5 was found to be capable of facilitating the decomposition of the hemiacetal. The ineffectiveness of the catalytic system in hydrogenating methyl salicylate is attributed to the intrinsically lower reactivity of the ester toward C═O reduction and a very facile side-reaction, which is adding the phenol OH group of the hemiacetal intermediate stemmed from methyl salicylate to the Fe–N active site of 5. Computations of various catalyst initiation pathways show that the initiation without the aid of an additive is very unfavorable, thus we suggest the use of a Lewis base such as NR3 (R = Me and Et) and PR3 (R = nBu and tBu) to accelerate precatalyst (1) activation, because a Lewis base could form a stable adduct (BH3–NR3/PR3) with the BH3 moiety of 1. In agreement with greatly enhanced kinetics and thermodynamics of the initiation process suggested by the DFT calculations, experimental study shows that the addition of a catalytic amount of NEt3 doubled the yield of benzyl alcohol from the hydrogenation of methyl benzoate, when compared to the case without using any additive. The trans effect of the hydride on the reactivity of 2 and steric effect of the pincer substituents on the stability of 2 are also discussed.

Unravelling the Mechanism of the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)] Catalysts

The mechanism of catalytic hydrogenation of acetophenone by the chiral complex trans-[RuCl2{(S)-binap}{(S,S)-dpen}] and KO-t-C4H9 in propan-2-ol is revised on the basis of DFT computations carried out in dielectric continuum and the most recent experimental observations. The results of these collective studies suggest that neither a six-membered pericyclic transition state nor any multibond concerted transition states are involved. Instead, a hydride moiety is transferred in an outer-sphere manner to afford an ion-pair, and the corresponding transition state is both enantio- and rate-determining. Heterolytic dihydrogen cleavage proceeds neither by a (two-bond) concerted, four-membered transition state, nor by a (three-bond) concerted, six-membered transition state mediated by a solvent molecule. Instead, cleavage of the H-H bond is achieved via deprotonation of the η(2)-H2 ligand within a cationic Ru complex by the chiral conjugate base of (R)-1-phenylethanol. Thus, protonation of the generated (R)-1-phenylethoxide anion originates from the η(2)-H2 ligand of the cationic Ru complex and not from NH protons of a neutral Ru trans-dihydride complex, as initially suggested within the framework of a metal-ligand bifunctional mechanism. Detailed computational analysis reveals that the 16e(-) Ru amido complex [RuH{(S)-binap}{(S,S)-HN(CHPh)2NH2}] and the 18e(-) Ru alkoxo complex trans-[RuH{OCH(CH3)(R)}{(S)-binap}{(S,S)-dpen}] (R = CH3 or C6H5) are not intermediates within the catalytic cycle, but rather are off-loop species. The accelerative effect of KO-t-C4H9 is explained by the reversible formation of the potassium amidato complexes trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH2}] or trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH(K)}]. The three-dimensional (3D) cavity observed within these molecules results in a chiral pocket stabilized via several different noncovalent interactions, including neutral and ionic hydrogen bonding, cation-π interactions, and π-π stacking interactions. Cooperatively, these interactions modify the catalyst structure, in turn lowering the relative activation barrier of hydride transfer by ~1-2 kcal mol(-1) and the following H-H bond cleavage by ~10 kcal mol(-1), respectively. A combined computational study and analysis of recent experimental data of the reaction pool results in new mechanistic insight into the catalytic cycle for hydrogenation of acetophenone by Noyori's catalyst, in the presence or absence of KO-t-C4H9.

PNS-Type Ruthenium Pincer Complexes

The PNS pincer-type ligand 1 and the novel Ru(PNS) complexes 2–8 were synthesized and characterized. The (PNS)RuH(Cl)CO complex 2 was prepared by reaction of ligand 1 with RuH(Cl)CO(PPh3)3. 2 reacted with KHMDS (potassium bis(trimethylsilyl)amide) to form the symmetrical dimeric complex 4 via the intermediacy of the dearomatized complex (PNS*)Ru(H)CO 3, in which deprotonation of the benzylic-S “arm” took place. Reaction of 2 with excess NaH gave the dimeric 4, by a formal intermolecular attack of the benzylic “arm” on a second ruthenium center. Complex 4 underwent spontaneous transformation in solution to the dinuclear complex 5 via C–S bond cleavage, resulting in the loss of a S-bound tBu group. Treatment of 2 with KHMDS in the presence of PEt3 resulted in the trapping of intermediate 3 in the form of the dearomatized complex 8. Reaction of 2 with LiHBEt3 gave the trans-dihydride complex 6, which reacted with CO2 to give the formato complex 7, in which the formato ligand is located trans to the hydride. Complexes 2, 4, and 5 were also investigated as catalysts for the dehydrogenative coupling of alcohols with amines.

Formation of Stabletrans-Dihydride Ruthenium(II) and 16-Electron Ruthenium(0) Complexes Based on Phosphinite PONOP Pincer Ligands. Reactivity toward Water and Electrophiles

The synthesis of a series of new ruthenium complexes based on the new PONOP ligands 1 and 10 ((C5H3N-1,3-(OPR2)2: 1, R = iPr; 10, R = tBu) is presented, including the stable trans-dihydride complexes (iPr-PONOP)Ru(H)2(PPh3) (4) and (tBu-PONOP)Ru(H)2(CO) (12) and the stable Ru(0) complexes (R-PONOP)Ru(CO)2 (6, R = iPr; 15, R = tBu). A surprisingly stable 16-electron Ru(0) complex (13) was formed by deprotonation of 12 with KOtBu. Complex 13 reacts with H2 to afford the cis-dihydride complex 12a, which isomerized to the trans-dihydride 12. Complex 13 reacted with CO to afford the saturated Ru(0) complex 15. Reaction of complex 12 with water led to hydrolysis of the phosphinite PONOP ligand and rearrangement to a dimeric product (14). Reaction of the trans-dihydride complex 4 with the electrophiles PhCOCl, MeI, and MeOTf led to abstraction of one of the hydride ligands, forming the monohydride complexes (iPr-PONOP)Ru(H)(PPh3)(X) (X = Cl (2), I (8a), OTf (8b)) together with benzaldehyde in the case of 2. Simi...

Kinetic Hydrogen/Deuterium Effects in the Direct Hydrogenation of Ketones Catalyzed by a Well-Defined Ruthenium Diphosphine Diamine Complex

The trans-dihydride complex trans-RuH(2)(NH(2)CMe(2)CMe(2)NH(2))((R)-binap) (1) is an active catalyst for the homogeneous hydrogenation of ketones in benzene under pressure of H(2) gas. The mechanism of the catalysis is proposed to occur through a rapid transfer of a hydride from the ruthenium and a proton from the amine on 1 to the carbonyl of the ketone to give the product alcohol and a hydrido-amido intermediate RuH(NHCMe(2)CMe(2)NH(2))((R)-binap) (2). The dihydride is then regenerated by the turnover-limiting heterolytic splitting of dihydrogen by complex 2. In this work the kinetic isotope effect (KIE) is measured to be 2.0 (+/-0.1) for the reduction of acetophenone to 1-phenylethanol catalyzed by 1 using 8.0 atm of H(2) versus D(2) gas. DFT calculations predict a KIE of 2.1 for the described mechanism using the simplified catalyst RuH(NHCH(2)CH(2)NH(2))(PH(3))(2) with H(2) or the catalyst RuD(NDCH(2)CH(2)ND(2))(PH(3))(2) with D(2). This supports the observation that deuterium scrambles into the catalyst when a pressure of D(2) is used. We discuss the significance of these results relative to the KIE of 2 that was reported by Sandoval et al. (J. Am. Chem. Soc. 2003 125, 13490) for the hydrogenation/deuteriation of acetophenone catalyzed by trans-RuH(eta(1)-BH(4))((S)-tolbinap)((S,S)-dpen) in basic isopropanol/isopropanol-d(8).

Chemistry of Ruthenium(II) Monohydride and Dihydride Complexes Containing Pyridyl Donor Ligands Including Catalytic Ketone H2-Hydrogenation1

In this study we determine the changes to the properties of dihydride catalysts for ketone H2-hydrogenation by successively replacing the amine donors in the known dach complex RuH2(PPh3)2(dach) (2a), dach = 1,2-(R,R)-diaminocyclohexane, with one pyridyl group in the corresponding 2-(aminomethyl)pyridine (ampy) complexes RuH2(PPh3)2(ampy) (2b) and with two pyridyl groups in the complexes RuH2(PPh3)2(bipy) (2c) and RuH2(PPh3)2(phen) (2d). The ruthenium monohydride complex, (OC-6-54)-RuHCl(PPh3)2(ampy), (1b with Cl trans to H) was prepared by the addition of 1 equiv of ampy to RuHCl(PPh3)3 in THF. Treatment of the monohydride complex with K[BH(sec-Bu)3] in THF or KOtBu/H2 in toluene resulted in the formation of a mixture of at least two isomers of the highly reactive, air-sensitive ruthenium dihydride complex 2b. One is the cis dihydride (OC-6-14)-2b or more simply c,t-2b with trans PPh3 groups and another is the cis dihydride c,c-2b (OC-6-42) that has PPh3 trans to H and PPh3 trans to N(pyridyl). The isomer c,c-2b slowly converts to c,t-2b in solution. The reaction of 1b with KOtBu under Ar results in the formation of a mixture that includes a complex with an imino ligand HN=CH-2-py while the same reaction under H2 leads to c,c-2b and then c,t-2b. The dach complex c,t-2a, reacts with ampy, 2,2'-bipyridine (bipy), and 1,10-phenanthroline (phen) in refluxing THF to form the substituted cis-dihydride complexes c,t-2b, (OC-6-13)-RuH2(PPh3)2(bipy) (c,t-2c with trans PPh3 groups) and (OC-6-13)-RuH2(PPh3)2(phen), c,t-2d, respectively. The dihydrides containing amino groups and cis-PPh3 groups, i.e., c,c-2a or c,c-2b, are active precatalysts for the H2-hydrogenation of acetophenone (neat or in benzene) under mild reaction conditions, whereas those with trans-PPh3 groups, c,t-2a and c,t-2b are much less active. The combination of ampy complex 1b and KOtBu also provides a catalyst in benzene that is more active than the corresponding dach system. The complexes without amino groups c,t-2c and c,t-2d are air-stable and inactive as hydrogenation catalysts under comparable conditions. The mechanism of hydrogenation of ketones catalyzed by isomers of 2a,b is thought to be similar and to proceed via a trans-dihydride complex, t,c-2a or t,c-2b, and an amido complex, neither of which are directly observed for the ampy complexes. The dihydride complex c,t-2b reacts with formic acid to give (OC-6-45)-RuH(OCHO)(PPh3)2(ampy), 3b, with formate trans to hydride. The structures of 1b, c,t-2b, c,t-2c, and 3b have been determined by single-crystal X-ray diffraction.

Monocyclopentadienylhydride derivatives of ruthenium: stereoselective proton transfer and proton-hydride exchange in an extremely short dihydrogen bond.

Diastereomerically pure complexes of formula CpRuCl(PP) and CpRuH(PP) with chiral ferrocenyl diphosphines were prepared and the selectivity of proton-transfer processes over the monohydride compounds with different acids was studied. With 1 equiv of HBF(4) the cis-dihydrogen and trans-dihydride complexes were formed while with 3 equiv of CF(3)CO(2)H the trans-dihydride derivative was the only product. However, the use of 1 equiv of CF(3)CO(2)H led to a dihydrogen bonded complex with an extremely short RuH...HO(2)CF(3) interaction that exhibits proton-hydride exchange. Using the labeled acid CF(3)CO(2)D, a stereoselective transference of the deuteron was demonstrated that implies the previous epimerization of the monohydride and the subsequent attack of the acid in the position previously occupied by the hydride.

Hydrogen Bonding in Transition Metal Complexes: Synthesis, Dynamics, and Reactivity of Platinum Hydride Bifluoride Complexes

Platinum hydride bifluoride (FHF) complexes trans-[Pt(PR3)2H(FHF)] (R = Cy, iPr) were prepared from the reaction of the corresponding trans-dihydride complexes with NEt3·3(HF) in THF. They were also formed in C−F activation reactions of the same precursors with C6F6 in the presence of [Me4N]F. The low-temperature NMR spectra exhibit a complete network of coupling between the spin 1/2 nuclei, 1H, 31P, 19F, and 195Pt. At higher temperatures, fluxional behavior is observed which is principally associated with intermolecular exchange of HF between platinum centers. However, satisfactory simulation of the spectra also requires inclusion of intermolecular exchange of the distal fluoride. Addition of [Bu4N]FHF results in coalescence of the bifluoride proton resonance of the complex and the added bifluoride, demonstrating that the bifluoride ligand can exchange with free bifluoride, FHF-. The IR spectrum of trans-[Pt(PCy3)2H(FHF)] shows two broad bands at 2604 and 1832 cm-1 assigned to the H−F stretching modes and a sharp band at 2272 cm-1 for the Pt−H stretching mode. Bifluoride is a weakly coordinated ligand which can be replaced to give trans-[Pt(PCy3)2(H)X] complexes where X = N3, OTf. Hydrogen fluoride may be removed from trans-[Pt(PCy3)2H(FHF)] by treatment with CsOH in the presence of [NMe4]F, yielding trans-[Pt(PCy3)2(H)F]. In addition, these bifluoride complexes fluorinate organic compounds, such as CH3I, CH3COCl, and C6H5COCl, to give CH3F, CH3COF, and C6H5COF together with trans-[Pt(PCy3)2(H)X] (X = Cl, I). Reactions with PPh3 and pyridine yield trans-[Pt(PCy3)2(PPh3)H]FHF and trans-[Pt(PCy3)2(C5H5N)H]FHF.

Synthesis and structural characterization of platinum(II) bis(chelate) complexes derived from the ligand system meso/rac-Ph(O)HPCH2CH2PH(O)Ph

The ligand (R,S)-Ph(O)HPCH2CH2PH(O)Ph reacted with [Pt(PPh3)4] to give the complex syn-[Pt{(R,S)-Ph(O)P(CH2)2P(OH)Ph}2]3. In this reaction a long-lived intermediate was observed and spectroscopically characterized as the six-co-ordinate platinum(IV)trans-dihydride complex syn-[PtH2{(R,S)-Ph(O)P(CH2)2P(OH)Ph}2]4. Complex 4 exhibits fluxional behaviour in CD2Cl2 solution at 20 °C as a result of a rapid proton exchange between the OH protons and the hydride ligand that is syn with respect to these groups. Mechanistic proposals were made to account for the OH proton–hydride exchange process and for the decomposition of the dihydride complex 4 to complex 3. Treatment of 3 with dry HCl afforded the monoprotonated complex syn-[Pt{(R,S)Ph(O)P(CH2)2P(OH)Ph}{(R,S)-Ph(OH)P(CH2)2P(OH)Ph}]Cl 12. The diprotonated complex anti[Pt{(R,S)-Ph(OH)P(CH2)2P(OH)Ph}2]Cl29 was obtained by the reaction of [{PtCl2(C2H4)}2] with (R,S)-Ph(O)HPCH2CH2PH(O)Ph. Reaction of BF3·Et2O with 3 gave the macrocyclic complex syn-[Pt{[(R,S)-Ph(O)P(CH2)2P(O)Ph]BF2}2]·CH2Cl213. Treatment of [Pt(PPh3)4] with the racemic ligand (RR,SS)-Ph(O)HPCH2CH2PH(O)Ph afforded a white insoluble solid composed of the polymeric complexes meso-[{Pt[(R,R)-Ph(O)P(CH2)2P(OH)Ph][(S,S)-Ph(O)P(CH2)2P(OH)Ph]}n]15 and rac-[{Pt[(RR,SS)-Ph(O)P(CH2)2P(OH)Ph]2}n]16. Addition of HCl to this solid in CH2Cl2 led to the isolation of a mixture of the dicationic complexes meso-[Pt{(R,R)-Ph(OH)P(CH2)2P(OH)Ph}{(S,S)-Ph(OH)P(CH2)2P(OH)Ph}]Cl217 and rac-[Pt{(RR,SS)-Ph(OH)P(CH2)2P(OH)Ph}2]Cl218. The structures of complexes 3, 9, 12, 13, 15, 17 and 18 have been determined by single-crystal X-ray diffraction studies.