No abstract

Oxidative addition of SeCN(-) to tertiary phosphine ligands has been investigated in methanol at 298 K by use of UV-Vis stopped-flow and conventional spectrophotometry. In most cases k(obs) vs. [SeCN(-)] plots were linear with zero intercepts corresponding to a rate expression of k(obs) = k(1)[SeCN(-)]. Reactions rates are dependent on the electron density of the phosphorus centre with k(1) varying by five orders of magnitude from 1.34 +/- 0.02 x 10(-3) to 51 +/- 3 mol(-1) dm(3) s(-1) for P(2-OMe-C(6)H(4))(3) to PCy(3) respectively. Activation parameters range from 27 +/- 1 to 49.0 +/- 1.3 kJ mol(-1) for DeltaH(double dagger) and -112 +/- 9 to -140 +/- 3 J K(-1) mol(-1) for DeltaS(double dagger) supporting a S(N)2 mechanism in which the initial nucleophilic attack of P on Se is rate determining. Reaction rates are promoted by more polar solvents supporting the mechanistic assignment. Reasonable linear correlations were observed between log k(1)vs. pK(a), (1)J(P-Se) and chi(d) values of the phosphines. The reaction rates are remarkably sensitive to the steric bulk of the substituents, and substitution of phenyl rings in the 2 position resulted in a decrease in the reaction rate. The crystal structures of SePPh(2)Cy and SePPhCy(2) have been determined displaying Se-P bond distances of 2.111(2) and 2.1260(8) A respectively.

Water-soluble fac-[Re(CO)(3)(L,L'-Bid)(X)] (L,L'-Bid = tropolonato, X = H(2)O, methanol) complexes have been synthesized, and the aqua and methanol substitution reactions were investigated in water (pH range 6.3-10.0) and methanol, respectively, and compared. Thiocyanate ions were used as monodentate entering ligand. The complexes were characterized by UV-vis, IR, and NMR spectroscopy. The crystal structures of the complexes [NEt(4)] fac-[Re(Trop)(CO)(3)(H(2)O)].NO(3).H(2)O (reactant) and fac-[Re(CO)(3)(Trop)(Py)], a substitution product, are reported. Overall it was found that the aqua substitution of fac-[Re(CO)(3)(Trop)(H(2)O)] is about 10 times faster than the methanol substitution reaction for fac-[Re(CO)(3)(Trop)(MeOH)], with forward and reverse rate and stability constants [k(1) (M(-1) s(-1)), k(-1) (s(-1)), K(1), (M(-1))] for thiocyanate as monodentate entering ligand as follows: fac-[Re(CO)(3)(Trop)(H(2)O)] = 2.54 ± 0.03, 0.0077 ± 0.0005, 330 ± 22/207 ± 14 and fac-[Re(CO)(3)(Trop)(MeOH)] = 0.268 ± 0.002, 0.0044 ± 0.0002, (61 ± 3)/(52 ± 4). The activation parameters [ΔH(‡)(k1) (kJ mol(-1)), ΔS(‡)(k1) (J K(-1) mol(-1))] for the aqua and methanol complex respectively are 56.1 ± 0.7, -49 ± 2 and 64 ± 1, -43 ± 5.

A range of fac-[Re(CO)(3)(L,L'-Bid)(H(2)O)](n) (L,L'-Bid = neutral or monoanionic bidentate ligands with varied L,L' donor atoms, N,N', N,O, or O,O': 1,10-phenanthroline, 2,2'-bipydine, 2-picolinate, 2-quinolinate, 2,4-dipicolinate, 2,4-diquinolinate, tribromotropolonate, and hydroxyflavonate; n = 0, +1) has been synthesized and the aqua/methanol substitution has been investigated. The complexes were characterized by UV-vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Re(CO)(3)(Phen)(H(2)O)]NO(3)·0.5Phen, fac-[Re(CO)(3)(2,4-dQuinH)(H(2)O)]·H(2)O, fac-[Re(CO)(3)(2,4-dQuinH)Py]Py, and fac-[Re(CO)(3)(Flav)(CH(3)OH)]·CH(3)OH are reported. A four order-of-magnitude of activation for the methanol substitution is induced as manifested by the second order rate constants with (N,N'-Bid) < (N,O-Bid) < (O,O'-Bid). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k(1), M(-1) s(-1); k(-1), s(-1); K(1), M(-1)) for bromide anions as entering nucleophile are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) (50 ± 3) × 10(-3), (5.9 ± 0.3) × 10(-4), 84 ± 7; fac-[Re(CO)(3)(2,4-dPicoH)(MeOH)] (15.7 ± 0.2) × 10(-3), (6.3 ± 0.8) × 10(-4), 25 ± 3; fac-[Re(CO)(3)(TropBr(3))(MeOH)] (7.06 ± 0.04) × 10(-2), (4 ± 1) × 10(-3), 18 ± 4; fac-[Re(CO)(3)(Flav)(MeOH)] 7.2 ± 0.3, 3.17 ± 0.09, 2.5 ± 2. Activation parameters (ΔH(k1)(++), kJmol(-1); ΔS(k1)(), J K(-1) mol(-1)) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) iodide 70 ± 1, -35 ± 3; fac-[Re(CO)(3)(2,4-dPico)(MeOH)] bromide 80.8 ± 6, -8 ± 2; fac-[Re(CO)(3)(Flav)(MeOH)] bromide 52 ± 5, -52 ± 15. A dissociative interchange mechanism is proposed.

The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis [(di-tert-butylphosphino)methyl]phenyl) with CO 2 is first-order in palladium and first-order in CO 2 with a rate constant k s = 8.9 ( 0.8 M -1 s -1 at 353 K. Activation parameters are ΔH q = 73 ( 7 kJ/mol and ΔS q = -118 ( 19 J/K mol. Based on this and theoretical calculations we propose an S E 2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd-crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO 2 to give the complex PCPPd-O(CO)CH(CH 3 )CHCH 2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an S E 2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η 2 -bonded olefin complex as an intermediate. The rearrangement of this intermediate to the O-bonded product is concluded to be rate determining. The crystal structure of PCPPd-O(CO)C(CH 3 ) 2 CHCH 2 is reported and as well as the solubility of carbon dioxide in benzene-d 6 at different pressures and temperatures.

Rhodium(i) acetylacetonato complexes of the formula [Rh(acac)(CO)(PR(3))] [acac = acetylacetonate, PR(3) = PPh(3) 1, PCyPh(2) 2, PCy(2)Ph 3, PCy(3) 4] were synthesized and the iodomethane oxidative addition to these complexes were studied. Spectroscopic and low temperature (100 K) single crystal X-ray crystallographic data of the rhodium complexes (1-4) indicate a systematic increase in both steric and electronic parameters of the phosphine ligands as phenyl groups on the tertiary phosphine are progressively replaced by cyclohexyl groups in the series. Second order rate constants for the alkyl formation in the oxidative addition of iodomethane in dichloromethane at 25 degrees C vary with approximately one order-of-magnitude from 6.98(6) x 10(-3) M(-1)s(-1) (PCyPh(2)) to 55(1) x 10(-3) M(-1) s(-1) (PCy(2)Ph 3) and do not follow the expected electronic pattern from to 1-4, which indicates a flexibility of the cyclohexyl group, significantly influencing the reactivity. Activation parameters for the reactions range from 35(3) to 44(1) kJ mol(-1) for DeltaH( not equal) and -140(5) to -154(9) J K(-1) mol(-1) for DeltaS( not equal), and are supporting evidence for an associative activation for the oxidative addition step.

The dinuclear complex [Co 2 (CO) 6 {P(OPh) 3 } 2 ] (2) has been synthesised and was fully characterised. The solid state structure revealed a trans diaxial geometry, no bridging carbonyls, and Co-Co and Co-P bond lengths of 2.6722(4) and 2.1224(4) Å, respectively. Catalysed hydroformylation of 1-pentene with 2 was attempted at temperatures in the range 120 to 210 °C and pressures between 34 and 80 bar. High pressure spectroscopy (HP-IR and HP-NMR) was used to detect hydride intermediates. High pressure infrared (HP-IR) studies revealed the formation of [HCo(CO) 3 P(OPh) 3 ] (4) at ca. 110 °C, but at higher temperatures absorption bands corresponding to [HCo(CO) 4 ] (3) were observed. The hydride intermediate 4 has also been synthesised and characterised. Upon increased ligand concentration, HP-IR studies showed the formation of new carbonyl absorption bands due to a higher substituted cobalt carbonyl complex-[HCo(CO) 2 {P(OPh) 3 } 2 ] (5), which is believed to be catalytically less active. Complex 5 has been synthesised independently and was fully characterised. A low temperature crystal structural study of 5 revealed a trigonal bipyramidal structure with a trans H-Co-CO arrangement and two equatorial phosphite ligands, the Co-P bond lengths being 2.1093(8) and 2.1076(8) Å, respectively.

Complex equilibria in methanol/chloroform/dichloromethane solutions containing Zeise's anion, [PtCl(3)(C(2)H(4))](-) (1), the solvento species, trans-[PtCl(2)(C(2)H(4))(MeOH)] (2), and the dinuclear complex, trans-[PtCl(2)(C(2)H(4))](2) (3), have been studied by UV-vis, (1)H, and (195)Pt NMR spectroscopy, giving average values of K(Cl) = (1.6 +/- 0.2)10(3) M(-)(1) and K(S) = (0.16 +/- 0.02) M(-)(1) for the equilibrium constants between 2 and 1 and 3 and 2, respectively. The bridged complex 3 is completely split into monomeric solvento complexes 2 in methanol and in chloroform or dichloromethane solutions with [MeOH] > 0.5 M. Ethene exchange at the mononuclear complexes 1 and 2 was studied by (1)H NMR line-broadening experiments in methanol-d(4). Observed overall exchange rate constants decrease with an increase in free chloride concentration due to the displacement of the rapid equilibrium between 1 and 2 toward the more slowly exchanging parent chloro complex 1. Ethene exchange rate constants at 298 K for complexes 1 and 2 are k(ex1) = (2.1 +/- 0.1)10(3) M(-)(1) s(-)(1)and k(ex2) = (5.0 +/- 0.2)10(5) M(-)(1) s(-)(1), respectively, with corresponding activation parameters DeltaH(1)() = 19.1 +/- 0.3 kJ mol(-)(1), DeltaS(1)() = -117 +/- 1 J K(-)(1) mol(-)(1), DeltaH(2)() = 10.2 +/- 0.4 kJ mol(-)(1), and DeltaS(2)() = -102 +/- 2 J K(-)(1) mol(-)(1). The activation process is largely entropy controlled; the enthalpy contributions only amounting to approximately 30% of the free energy of activation. Ethene exchange takes place via associative attack by the entering olefin at the labile site trans to the coordinated ethene, which is either occupied by a chloride or a methanol molecule in the ground state. The intimate mechanism might involve a two-step process via trans-[PtCl(2)(C(2)H(4))(2)] in steady state or a concerted process via a pentacoordinated transition state with two ethene molecules bound to the platinum(II).