Influence of the Cone Angles and the π-Acceptor Properties of Phosphorus-Containing Ligands in the Chemistry of Dihydrogen Complexes of Ruthenium

Monofluorophos-Metal Complexes: Ripe for Future Discoveries in Homogeneous Catalysis

The discovery that cyclic (ArO)2PF can support Rh-catalysts for hydroformylation with significant advantages in tuning regioselectivity transformed the study of metal complexes of monofluorophos ligands from one of primarily academic interest to one with potentially important applications in catalysis. In this review, the syntheses of monofluorophosphites, (RO)2PF, and monofluorophosphines, R2PF, are discussed and the factors that control the kinetic stability of these ligands to hydrolysis and disproportionation are set out. A survey of the coordination chemistry of these two classes of monofluorophos ligands with d-block metals is presented, emphasising the bonding of the fluorophos to d-block metals, predominantly in low oxidation states. The application of monofluorophos ligands in homogeneous catalysis (especially hydroformylation and hydrocyanation) is discussed, and it is argued that there is great potential for monofluorophos complexes in future catalytic applications.

Imidazol-2-ylidene stabilized tetrahedral cobalt carbonyl complexes: A computational and structural database study

A combined crystallographic database and computational chemistry analyses of tetrahedral cobalt (Co) carbonyl (CO) complexes bearing substituted imidazol-2-ylidene (Im) ligands were studied. These group of compounds are potential catalysts in the hydroformylation of alkenes as well as effective precursors for both chemical vapour deposition and atomic layer deposition in integrated circuit manufacture. A substructure search revealed seventeen (17) crystal structures in the Cambridge Structural Database (CSD). Generally, there was a positive correlation between the Im-Co bond length and the Co-CO bond length implying that a strong Im-CO bonding interaction could give rise to a more stable carbonyl complex. The gas phase structures of these complexes were further studied computationally at the BP86 level of the density functional theory. The def2-TZVPP basis set was used for cobalt with 6-31G (d,p) for all other atoms. It was observed that the Co-CO bond length increased with increase in bulkiness of alkyl substituents at the ortho positions of the Im ligands. Subsequent natural bond orbital (NBO) and chemical reactivity parameter analyses revealed that the chemical softness, electronic chemical potential and the ease of ionization also increased with increase in bulkiness of the alkyl substituents. This property has useful applications in catalytic systems and for the design of precursors in integrated circuit manufacture.

Synthesis, characterization and reactivity studies of electrophilic ruthenium(II) complexes: a study of H₂ activation and labilization

Reaction of 2,2'-bipyridine (bpy) with dinuclear complexes [RuCl(dfppe)(μ-Cl)3Ru(dmso-S)3] (dfppe = 1,2-bis(dipentafluorophenyl phosphino)ethane (C6F5)2PCH2CH2P(C6F5)2; dmso = dimethyl sulfoxide) (1) or [RuCl(dfppe)(μ-Cl)3RuCl(dfppe)] (2) affords the mononuclear species trans-[RuCl2(bpy)(dfppe)] (3). Using this precursor complex (3), a series of new cationic Ru(II) electrophilic complexes [RuCl(L)(bpy)(dfppe)][Z] (L = P(OMe)3 (5), PMe3 (6), CH3CN (7), CO (8), H2O (9); Z = OTf (5, 6, 7, 8), BAr(F)4 (9) have been synthesized via abstraction of chloride by AgOTf or NaBAr(F)4 in the presence of L. Complexes 5 and 6 were converted into the corresponding isomeric hydride derivatives [RuH(PMe3)(bpy)(dfppe)][OTf] (10a, 10b) and [RuH(P(OMe)3)(bpy)(dfppe)][OTf] (11a, 11b) respectively, when treated with NaBH4. Protonation of the cationic monohydride complex (11a) with HOTf at low temperatures resulted in H2 evolution accompanied by the formation of either solvent or triflate bound six coordinated species [Ru(S)(P(OMe)3)(bpy)(dfppe)][OTf]n [(S = solvent (n = 2), triflate (n = 1)] (13a/13b); these species have not been isolated and could not be established with certainty. They (13a/13b) were not isolated, instead the six-coordinated isomeric aqua complexes cis-[Ru(bpy)(dfppe)(OH2)(P(OMe)3)][OTf]2 (14a/14b) were isolated. Reaction of the aqua complexes (14a/14b) with 1 atm of H2 at room temperature in acetone-d6 solvent resulted in heterolytic cleavage of the H-H bond. Results of the studies on H2 lability and heterolytic activation using these complexes are discussed. The complexes 3, 5, 11a, and 14a have been structurally characterized.

Heterolytic Activation of H−X (X = H, Si, B, and C) Bonds: An Experimental and Theoretical Investigation

The highly electrophilic, coordinatively unsaturated, 16-electron [Ru(P(OH)3)(dppe)2][OTf]2 (dppe = Ph2PCH2CH2PPh2) complex 1 activates the H-H, the Si-H, and the B-H bonds, in H2(g), EtMe2SiH and Et3SiH, and H3B.L (L = PMe3, PPh3), respectively, in a heterolytic fashion. The heterolysis of H2 involves an eta2-H2 complex (observable at low temperatures), whereas the computations indicate that those of the Si-H and the B-H bonds proceed through unobserved eta1-species. The common ruthenium-containing product in these reactions is trans-[Ru(H)(P(OH)3)(dppe)2][OTf], 2. The [Ru(P(OH)3)(dppe)2][OTf]2 complex is unique with regard to activating the H-H, the Si-H, and the B-H bonds in a heterolytic manner. These reactions and the heterolytic activation of the C-H bond in methane by the model complex [Ru(POH)3)(H2PCH2CH2PH2)2][Cl][OTf], 4, have been investigated using computational methods as well, at the B3LYP/LANL2DZ level. While the model complex activates the H-H, the Si-H, and the B-H bonds in H2, SiH4, and H3B.L (L = PMe3, PPh3), respectively, with a low barrier, activation of the C-H bond in CH4 involves a transition state of 57.5 kcal/mol high in energy. The inability of the ruthenium complex to activate CH4 is due to the undue stretching of the C-H bond needed at the transition state, in comparison to the other substrates.

Solution chemistry of a water-soluble eta2-H2 ruthenium complex: evidence for coordinated H2 acting as a hydrogen bond donor

The ability of an eta2-H2 ligand to participate in intermolecular hydrogen bonding in solution has long been an unresolved issue. Such species are proposed to be key intermediates in numerous important reactions such as the proton-transfer pathway of H2 production by hydrogenase enzymes. We present the synthesis of several new water-soluble ruthenium coordination complexes including an eta2-H2 complex that is surprisingly inert to substitution by water. The existence of dihydrogen hydrogen bonding (DHHB) was experimentally probed by monitoring the chemical shift of H-bonded Ru-(H2) complexes using NMR spectroscopy, by UV-visible spectroscopy, and by monitoring the rotational dynamics of a hydrogen-bonding probe molecule. The results provide strong evidence that coordinated H2 can indeed participate in intermolecular hydrogen bonding to bulk solvent and other H-bond acceptors.

16-Electron Elongated Dihydrogen Complex Stabilized by Agostic Interaction

A 16-electron dicationic dihydrogen complex [Ru(eta2-H...H)(PP)2][OTf]2 [4; PP = (C6H5CH2)2PCH2CH2P(CH2C6H5)2] has been prepared and characterized by protonating the precursor hydride complex [Ru(H)(PP)2)][OTf] (2) using HOTf. The hydride and dihydrogen complexes are stabilized via agostic interaction of the ortho C-H fragment of the phenyl ring on the benzyl group. The intact nature of the H-H bond in this derivative was established from the short spin-lattice relaxation time and the observation of a substantial J(H,D) of 22.0 Hz for the HD isotopomer. The H-H bond distance calculated from J(H,D) is 1.05 A, which falls under the category of elongated dihydrogen ligands.

Highly Electrophilic, 16-Electron [Ru(P(OMe)(OH)2)(dppe)2]2+ Complex Turns H2(g) into a Strong Acid and Splits a Si−H Bond Heterolytically. Synthesis and Structure of the Novel Phosphorous Acid Complex [Ru(P(OH)3)(dppe)2]2+

The highly electrophilic, 16-electron, coordinatively unsaturated [Ru(P(OMe)(OH)(2))(dppe)(2)][OTf](2) complex brings about the heterolytic activation of H(2)(g) and spontaneously generates HOTf. In addition, trans-[Ru(H)(P(OMe)(OH)(2))(dppe)(2)](+) and an unprecedented example of a phosphorous acid complex, [Ru(P(OH)(3))(dppe)(2)](2+), are formed. The [Ru(P(OMe)(OH)(2))(dppe)(2)][OTf](2) complex also cleaves the Si-H bond in EtMe(2)SiH in a heterolytic fashion, resulting in the trans-[Ru(H)(P(OMe)(OH)(2))(dppe)(2)](+) derivative.

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.

Trans --> cis isomerization of trans-[(dppm)2Ru(H)(L)][BF4] (L = P(OR)3) complexes: preparation of cis-[(dppm)2Ru(eta2-H2)(L)][BF4]2

A series of new dicationic dihydrogen complexes of ruthenium of the type cis-[(dppm)(2)Ru(eta(2)-H(2))(L)][BF(4)](2) (dppm = Ph(2)PCH(2)PPh(2); L = P(OMe)(3), P(OEt)(3), PF(O(i)Pr)(2)) have been prepared by protonating the precursor hydride complexes cis-[(dppm)(2)Ru(H)(L)][BF(4)] (L = P(OMe)(3), P(OEt)(3), P(O(i)Pr)(3)) using HBF(4).Et(2)O. The cis-[(dppm)(2)Ru(H)(L)][BF(4)] complexes were obtained from the trans hydrides via an isomerization reaction that is acid-accelerated. This isomerization reaction gives mixtures of cis and trans hydride complexes, the ratios of which depend on the cone angles of the phosphite ligands: the greater the cone angle, the greater is the amount of the cis isomer. The eta(2)-H(2) ligand in the dihydrogen complexes is labile, and the loss of H(2) was found to be reversible. The protonation reactions of the starting hydrides with trans PMe(3) or PMe(2)Ph yield mixtures of the cis and the trans hydride complexes; further addition of the acid, however, give trans-[(dppm)(2)Ru(BF(4))Cl]. The roles of the bite angles of the dppm ligand as well as the steric and the electronic properties of the monodentate phosphorus ligands in this series of complexes are discussed. X-ray crystal structures of trans-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], cis-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], and cis-[(dppm)(2)Ru(H)(P(O(i)Pr)(3))][BF(4)] complexes have been determined.

Phosphine and phosphonite complexes of a ruthenium(II) porphyrin. 1. Synthesis, structure, and solution state studies

We have investigated the effect of complexation of different phosphorus ligands on the stability, solid state structure, and spectroscopic properties (NMR, IR, UV-vis) of a 5,15-diphenyl-substituted ruthenium porphyrin, (MeOH)Ru(II)(CO)(DPP) 2 [DPP = 5,15-bis(3',5'-di-tert-butyl)phenyl-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin]. The ligands used are PPh(3), diphenyl(phenylacetenyl)phosphine (DPAP), bis(diphenylphosphino)acetylene (DPPA), tris(phenylacetenyl)phosphine [(PA)(3)P], and diethyl (phenylacetenyl)phosphonite [PAP(OEt)(2)]. The mono-phosphine complexes (PR(3))Ru(II)(CO)(DPP) are readily formed in solution in quantitative yields. The complexes display association constants ranging from 1.2 x 10(4) M(-1) for PPh(3) to 4.8 x 10(6) M(-1) for PAP(OEt)(2). The weak association of PPh(3) does not correlate with its pK(a), delta((31)P), or cone angle value and is attributed to steric effects. Due to their kinetic lability, which is shown by 2D NMR spectroscopy, and the weakening of the carbonyl ligand via a trans effect, the mono-phosphine complexes could not be isolated. IR spectroscopy gives the relative order of pi-acceptor strength as PPh(3) < DPAP, DPPA < (PA)(3)P < PAP(OEt)(2), whereas the relative order of the sigma-donor strength is PPh(3) < (PA)(3)P < DPAP, DPPA < PAP(OEt)(2), based on the calculated pK(a) values and on the (31)P((1)H) NMR chemical shifts of the ligands. The chemical shift differences in the (31)P9(1)H)) NMR spectra upon ligand binding display a linear correlation with the calculated pK(a) values of the protonated ligands HPR(3)(+); we propose that the pK(a), and probably other electronic properties, of a specific phosphorus ligand can be estimated on the basis of the chemical shift difference Deltadelta((31)P) upon complexation to a metalloporphyrin. The bis-phosphine complexes can be isolated in pure form by crystallization from CHCl(3)-MeOH solutions using excess ligand. Association of the second ligand is in the same order of magnitude as the first binding for the phosphines, but the second phosphonite binding is decreased by a factor of about 100. The solid state structures show only marginal differences in the geometrical parameters. The calculated and the crystallographic cone angles of the ligands generally do not match, apart from the values obtained for PAP(OEt)(2).