Structure-Reactivity Correlations for the Dissociative Uncatalyzed Isomerization of Monoalkylbis(phosphine)platinum(II) Solvento Complexes

Hydrogen Bonding in Platinum Indolylphosphine Polyfluorido and Fluorido Complexes

The reaction of the Pt complexes cis-[Pt(CH₃ )(Ar){Ph₂ P(Ind)}₂ ] (Ind=2-(3-methyl)indolyl, Ar=4-tBuC₆ H₄ (1 a)_, 4-CH₃ C₆ H₄ (1 b), Ph (1 c), 4-FC₆ H₄ (1 d), 4-ClC₆ H₄ (1 e), 4-CF₃ C₆ H₄ (1 f)) with HF afforded the polyfluorido complexes trans-[Pt(F(HF)₂ )(Ar){Ph₂ P(Ind)}₂ ] 2 a-f, which can be converted into the fluoride derivatives trans-[Pt(F)(Ar){Ph₂ P(Ind)}₂ ] (3 a-f) by treatment with CsF. The compounds 2 a-f and 3 a-f were characterised thoroughly by multinuclear NMR spectroscopy. The data reveal hydrogen bonding of the fluorido ligand with HF molecules and the indolylphosphine ligand. Polyfluorido complexes 2 a-f show larger |¹ J(F,Pt)|, but lower ¹ J(H,F) coupling constants when compared to the fluorido complexes 3 a-f. Decreasing ¹ J(P,Pt) coupling constants in 2 a-f and 3 a-f suggest a cis influence of the aryl ligands in the following order: 4-tBuC₆ H₄ (a) ≈4-CH₃ C₆ H₄ (b)<Ph (c)≪4-FC₆ H₄ (d)<4-ClC₆ H₄ (e)<4-CF₃ C₆ H₄ (f). In addition, the larger cis influence of aryl ligands bearing electron-withdrawing groups in the para position correlates with decreasing magnitudes of |¹ J(F,Pt)| coupling constants. The interpretation of the experimental data was supported by quantum-chemical DFT calculations.

Platinum‐Catalyzed Hydrofluorination of Alkynes: Hydrogen Bonding to Indolylphosphine Ligands to Provide Fluoride Reactivity

The reaction of the Pt complexes cis‐[Pt(CH₃)₂{R₂P(Ind)}₂] (Ind=2‐(3‐methyl)indolyl, R=Ph (1 a), 4‐FC₆H₄ (1 b), 4‐CF₃C₆H₄ (1 c)) with HF afforded the fluorido complexes trans‐[Pt(F(HF)₂)(CH₃){R₂P(Ind)}₂] 2 a–c, which can be converted into trans‐[Pt(F)(CH₃){R₂P(Ind)}₂] (3 a–c) by treatment with CsF. Addition of 3‐hexyne to 2 a–c gave alkyne complexes trans‐[Pt(C,C‐η²‐C₂H₅C≡CC₂H₅)(CH₃){R₂P(Ind)}₂{F(HF)₂}] (4 a–c) at which a fluoride is stabilised as polyfluoride in the coordination sphere by hydrogen bonding to the indolyl‐substituted phosphine ligands. Subsequent heating of a solution of 4 a in the presence of PVPHF led to fluoroalkene formation. Selective catalytic hydrofluorination of alkynes to yield (Z)‐fluoroalkenes were developed. The ability of hydrogen bonding to polyfluoride favours the fluorination step as demonstrated by studies with complexes bearing no indolyl groups at the phosphine ligands.

Generalization of the Tolman electronic parameter: the metal–ligand electronic parameter and the intrinsic strength of the metal–ligand bond

The MLEP is a new, generally applicable measure of the metal–ligand bond strength based on vibrational spectroscopy, replacing the TEP.

True and masked three-coordinate T-shaped platinum(II) intermediates

Although four-coordinate square-planar geometries, with a formally 16-electron counting, are absolutely dominant in isolated Pt(II) complexes, three-coordinate, 14-electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Although very few authenticated three-coordinate Pt(II) complexes have been characterized, a much larger number of complexes can be described as operationally three-coordinate in a kinetic sense. In these compounds, which we have called masked T-shaped complexes, the fourth position is occupied by a very weak ligand (agostic bond, solvent molecule or counteranion), which can be easily displaced. This review summarizes the structural features of the true and masked T-shaped Pt(II) complexes reported so far and describes synthetic strategies employed for their formation. Moreover, recent experimental and theoretical reports are analyzed, which suggest the involvement of such intermediates in reaction mechanisms, particularly C-H bond-activation processes.

Solution behavior of amidine complexes: an unexpected cis/trans isomerization and formation of di- and trinuclear platinum(III) and platinum(II) species

Platinum bis-amidine complexes (both the cis and trans isomers) are stable in acetone and chlorinated solvents but are unstable in protic solvents such as methanol or water. In the latter solvents an initial cis/trans isomerization leads to formation of an equilibrium mixture with a cis/trans ratio of about 1:4; subsequently a dinuclear platinum(III) complex (1) is formed under aerobic conditions while, under anaerobic conditions, a trinuclear platinum(II) compound (2) is obtained. We hypothesize that the process of isomerization and formation of polynuclear compounds (1 and 2) have a common precursor: a dinuclear platinum(II) species supported by two bridging amidinato ligands (3), formed in small yield, which can either dissociate back to monomers of cis/trans configuration or evolve in two different polynuclear species depending upon the aerobic/anaerobic conditions. In aerobic conditions, oxidation of platinum(II) to platinum(III) together with formation of two additional amidinato bridges across the two platinum centers takes place leading to compound 1. In contrast, in anaerobic conditions, oxidation of platinum is prevented and the dinuclear platinum(II) precursor remains in solution until it reacts with an extra molecule of the starting mononuclear complex which loses its two amidine ligands and cross-links the two bridging amidinato ligands of 3 to yield compound 2. This latter features two triply bridging amidinato ligands linking the three platinum units to form a pocket. Complexes 1 and 2 have been characterized by means of IR and NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography.

Coordinating Anions: (Phosphino)tetraphenylborate Ligands as New Reagents for Synthesis

Anionic, electron-releasing phosphines that incorporate a borate counteranion within the ligand framework are promising reagents for organometallic catalysis. This report describes the synthesis of a new class of monodentate tertiary phosphines built upon the commonly employed tetraphenylborate anion. These new phosphines are highly stable and strongly electron-releasing and readily coordinate transition metals. Moreover, they are promising reagents for catalysis, as demonstrated by their ability to promote the Suzuki cross-coupling of aryl chloride substrates.

Comments on the Electrostatic Minimum Potential (Vmin) as a Measure of the Electron-Donating Power of PZ3

In a recent publication, Suresh and Koga suggested that the molecular electrostatic potential minimum (V(min)) could be used as a quantitative measure of the "sigma donating power" of substituted phosphorus(III) ligands. In support of this assertion, they showed linear correlations between a number of physicochemical properties and V(min). Our examination of the analyses of the sets of experimental data used to support this assertion shows unequivocally that the variations in the properties cannot be described by a single parameter. Therefore, the utility of V(min) as a measure of the "sigma donating power" of substituted phosphorus(III) ligands remains unsubstantiated.

Fast drug stability determination by LC variable-parameter kinetic experiments

Variable-parameter kinetic experiments were carried out using HPLC as analytical instrument. The hydrolysis of aspirin was followed both at variable-temperature and at variable-pH conditions. The peak areas relative to salicylic acid were processed by direct fit to a mathematical model and/or by differential method obtaining, by single experiments, the values of the apparent rate constant in the whole range of temperature and pH studied. The results, although the discontinuity of this kind of analysis, are in agreement with those obtained by constant-parameter kinetics but saving experimental time.

Variable-ionic strength kinetic experiments in drug stability studies

The dependence of the pseudo-first-order rate constant on the ionic strength for the alkaline hydrolysis of indomethacin has been obtained, for the first time, in a single kinetic experiment carried out by varying with time the salt concentration inside the reaction vessel. The kinetic profile obtained was processed using as a mathematical model the variable-parameter kinetic equation containing the Bronsted-Bjerrum equation as the dependence function. The results are in good agreement with those obtained by the traditional method but the experimental time is reduced to about one-tenth.

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).

Temperature-rate profiles by polarimetric variable-temperature kinetic experiments to study racemization reactions

The racemization of (-)-adrenaline was followed by polarimetric variable-temperature kinetic experiments obtaining activation parameters and k(obs)(T) profile in one tenth of the time usually spent for traditional kinetic runs. A polarimeter connected to a computer for the acquisition and processing of the analytical data was used. The kinetic profiles were processed by both an integral method and a differential method. The results are in good agreement with each other and with those obtained by constant-temperature kinetics.

Fluxional behavior of the dinitrogen ligand 2,9-dimethyl-1,10-phenanthroline in cationic methyl platinum(II) complexes

The ionic methylplatinum(II) complexes [Pt(Me)(L)(dmphen)]X (dmphen = 2,9-dimethyl-1,10-phenanthroline, L = Me(2)SO, X = PF(6)(-) 1a, BF(4)(-) 1b, CF(3)SO(3)(-) 1c, ClO(4)(-) 1d, B(C(6)H(5))(4)(-) 1e, [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) 1f; L = n-Bu(2)SO, X = CF(3)SO(3)(-) 1g; L = PPh(3), X = PF(6)(-) 2a, BF(4)(-) 2b, CF(3)SO(3)(-) 2c, ClO(4)(-) 2d, B(C(6)H(5))(4)(-) 2e, [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) 2f; X = CF(3)SO(3)(-), L = CyNH(2) 3a, i-PrNH(2) 3b, 2,6-Me(2)py 3c, EtNH(2) 3d, AsPh(3) 3e, dimethylthiourea (Me(2)th) 3f and the uncharged [Pt(Me)(X)(dmphen)] (X = SCN(-) 4a, SeCN(-) 4b) complexes have been synthesized and fully characterized. In chloroform, as well as in acetone or methanol, complexes 1a-1g, 2a-2h (X = Cl(-) g, NO(2)(-) h, formed "in situ"), and 3e show dynamic behavior due to the oscillation of the symmetric chelating ligand dmphen between nonequivalent bidentate modes. All the other compounds feature a static structure in solution. The crystal structure of 2a shows a tetrahedral distortion of the square planar coordination geometry, a loss of planarity of the dmphen ligand, and, most notably, a rotation of the dmphen moiety, around the N1-N2 vector, to form a dihedral angle of 42.64(8) degrees with the mean coordination plane. The hexafluorophosphate ion lies on the side of the phenanthroline ligand. The interionic structures of 2a, 2b, and 2f were investigated in CDCl(3) at low temperature by (1)H-NOESY and (19)F[(1)H]-HOESY NMR spectroscopies. Whereas PF(6)(-) (2a) and BF(4)(-) (2b) show strong contacts with the cation [Pt(Me)(PPh(3))(dmphen)](+), being located preferentially on the side of the phenanthroline ligand, the [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) (2f) ion does not form a tight ion pair. The dynamic process was studied by variable-temperature NMR spectroscopy for 1a-1f and 2a-2h in CDCl(3). The activation energies DeltaG(298) for the sulfoxide complexes 1a-1f are lower than those of the corresponding phosphine complexes 2a-2f by approximately 10 kJ mol(-)(1). The nature of the counteranion exerts a tangible influence on the fluxionality of dmphen in both series of complexes 1 and 2. The sequence of energies observed for 2a-2h encompasses an overall difference of about 16 kJ mol(-)(1), increasing in the order Cl(-) approximately NO(2)(-) << CF(3)SO(3)(-) < ClO(4)(-) < B(C(6)H(5))(4)(-) < BF(4)(-) approximately PF(6)(-) < B(3,5-(CF(3))(2)C(6)H(3))(4)(-). Acetone and methanol have an accelerating effect on the flipping. Concentration-dependent measurements, carried out in CDCl(3) for 2a with n-Bu(4)NPF(6) and the ligands dmphen, n-Bu(2)SO, sec-Bu(2)SO, and sec-Bu(2)S showed that the rate of the fluxional motion is unaffected by added n-Bu(4)NPF(6), whereas in the other cases this increases linearly with increasing ligand concentration, according to a pattern of behavior typical of substitution reactions. Dissociative and associative mechanisms can be envisaged for the observed process of flipping. Dissociation can be prevalent within the ion pair formed by a "noncoordinating" anion with the metallic cationic complex in chloroform. Among the possible associative mechanisms, promoted by polar solvents or by relatively strong nucleophiles, a consecutive displacement mechanism is preferred to intramolecular rearrangements of five-coordinate intermediates.

Variable pH kinetics: an easy determination of pH-rate profile

The complete pH-rate profile of the hydrolysis reaction of aspirin, requiring many experiments in the past, was obtained by a single variable-parameter kinetic experiment carried out by varying the hydrogen ion concentration. The experiment was carried out spectrophotometrically, reading automatically absorbance, pH, and temperature inside the reaction vessel. The results obtained by the fast processing of the kinetic profile are identical to those obtained under the same conditions but using traditional constant-pH kinetic runs. This method provides the possibility of reducing the amount of time and chemicals usually spent in collecting kinetic data in such mechanistic studies by nearly two orders of magnitude.

Role of cyclometalation in controlling the rates of ligand substitution at platinum(II) complexes

The rates of chloride for triphenylphosphine substitution have been measured in dichloromethane for a series of cyclometalated [Pt(N-N-C)Cl] complexes containing a number of terdentate N-N-C anionic ligands, derived from deprotonated alkyl-, phenyl-, and benzyl-6-substituted 2,2'-bipyridines. These rates have been compared with those of the corresponding [Pt(N-N)(C)Cl] (N-N = 2,2'-bipyridine; C = CH3 or C6H5) complexes having the same set of donor atoms but less constrained arrangements of the ligands. The reactions of the cyclometalated compounds occur as a single-stage conversion from the substrate to the ionic pair [Pt(N-N-C)(PPh3)]Cl products. There is no evidence by 1H and 31P(1H) NMR spectroscopy for the formation of other Pt(II) species or of concurrent ring-opening processes. In contrast, in the monoalkyl- or monoaryl-2,2'-bipyridine complexes, chloride substitution is followed by subsequent slower processes which involve the detachment of one arm of the chelated 2,2'-bipyridine, fast cis to trans isomerization of the cis-[Pt(PPh3)2(eta 1-bipy)(R)]+ transient intermediate, and, eventually, the release of free bipy, yielding trans-[Pt(PPh3)2(R)Cl] (R = Me or Ph). All reactions are first-order with respect to complex and phosphine concentration, obeying the simple rate law kobsd = k2[PPh3]. The values of the second-order rate constant k2 do not seem particularly sensitive to the nature of the bonded organic moiety (alkyl or aryl), to its structure (cyclometalated or not), to the size of the ring, or to the number of alkyl substituents on it. The effects are those foreseen on the basis of an associative mode of activation. The only exception to this pattern of behavior is constituted by the complex [Pt(bipy phi-H)Cl] (bipy phi = 6-phenyl-2,2'-bipyridine), which features a significant rate enhancement with respect to the analogue [Pt(bipy)(Ph)Cl] complex. The results of this work, together with those of a previous paper, suggest that there is not a specific role of cyclometalation in controlling the reactivity, unless an in-plane aryl ring becomes part of the pi-acceptor system of the chelated 2,2-bipyridine, behaving as a cyclometalated analogue of the nitrogen terdentate 2,2':6',2"-terpyridine.

To what extent can cyclometalation promote associative or dissociative ligand substitution at platinum(II) complexes? A combined kinetic and theoretical approach

The ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 --> [Pt(bph)(*SR2)2] + 2SR2 (bph = 2,2'-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 --> cis-[PtPh2(*SMe2)2] + 2SMe2 have been determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labeling and magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration and the kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of the thioethers from [Pt(bph)(SR2)2] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) to yield [Pt(bph)(N-N)], carried out in the presence of sufficient excess of thioether and N-N to ensure pseudo-first-order conditions, follow a nonlinear rate law k(obsd) = a[N-N]/(b[SR2] + [N-N]). The general pattern of behavior indicates that the rate-determining step for substitution is the dissociation of a thioether ligand and the formation of a three-coordinated [Pt(bph)(SR2)] intermediate. The value of the parameter a, which measures the rate of ligand dissociation, is constant and independent of the nature of N-N, and it is in reasonable agreement with the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations were performed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon the loss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HF approximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). The activation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ moll compared to the experimental values in CDCl3 solution, 80 +/- 1 and 93 +/- 1 kJ mol(-1) for [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], respectively. The electrostatic potential maps of both parent compounds show a remarkable concentration of negative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. On the other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of the T-shaped intermediate in the singlet ground state may also rationalize the preference for the dissociative mechanism. All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation from the complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single aryl ligands. Electron back-donation from filled d orbitals of the metal to empty pi* of the in-plane cyclometalated rings is weak or absent and is not operative in promoting an associative mode of activation.