Jaap P. Smit

Kinetics of the substitution reaction between aquaoxotetracyanomolybdate(IV) and cyanide/hydrogen cyanide

Abstract The kinetics of the substitution reaction between aquaoxotetracyanomolybdate (IV) ions and CN− and HCN, as an important intermediate step in the formation of the [Mo(CN)8]4− complex, was studied by stopped-flow spectrometry. The aqua ligand in the [MoO(H2O)(CN)4]2− complex is substituted by both CN− ions and hydrogen cyanide according to the reaction: At 15.0°C and μ = 1.0 M (KNO3) the equilibrium, forward and reverse rate constants of the reaction with CN− ions were determined as 92(5) M−1 s−1 and 0.87(3) s−1, and for HCN as 3(1) M−1, 458(5) M−1 s−1 and 140(30) s−1, respectively. Coordination of the HCN to the metal centre results in a decrease in the pKa value of hydrogen cyanide by approximately 1.5 pH units to ca 7.6(2).

The kinetics and mechanism of ligand exchange in photogenerated (η1-bromobenzene)Cr(CO)5/alkene systems

The kinetics and mechanism of the reaction of photogenerated (η1-bromobenzene)Cr(CO)5 (bromobenzene = PhBr), which coordinates weakly (= Lw) to Cr through a ‘dative’ Br → Cr bond, with more strongly bonding alkene ‘traps’ (Ls: 1-hexene: 1-decene: 2,4,4-trimethyl-pentene), has been studied by employing pulsed laser flash photolysis with visible and infrared detection. The kinetics data are (η1-PhBr)Cr(CO)5 → (η2-Ls)Cr(CO)5 consistent with a single mechanism, involving dissociation of PhBr from (η1-PhBr)Cr(CO)5, followed by competitive reaction of the [Cr(CO)5] intermediate thus produced with PhBr or L5. Data taken ‘neat’ to very high [Ls], and in dilute solution in the ‘inert’ solvents (S: fluorobenzene, n-heptane) agree closely, indicating that solvent effects are relatively unimportant. Relative rates of attack at [Cr(CO)6] increase in the order 1-hexene < 1-decene < 2,2,4-trimethylpentene. Carbonyl stretching spectra for (C6H5X)Cr(CO)5 complexes (X = H,F, Cl, Br) are quite similar, and indicate that these Lw all are weak σ and π bonders, but do not enable one to distinguish between ‘edge-on’ binding of the ring to the metal (H, F) from dative X → Cr bonding. However, activation parameters for complexes exhibiting the two bonding modes are diagnostic for each linkage.

Bonding modes of arenes in photogenerated (arene)Cr(CO)5 complexes

Abstract Detailed kinetics studies of arene displacement by 1-hexene from the ten (arene)Cr(CO) 5 complexes (arene=C 6 H 6− n (CH 3 ) n ; n =2−5) generated by pulsed laser flash photolysis have been conducted. The data indicate that all reactions obey a reversible dissociation-competition for the [Cr(CO) 5 ] intermediate mechanism. The systematic variations in rate constants for unimolecular dissociation of arenes in the presence of 1-hexene ‘trap’ strongly support ‘edge-on’ bonding of the arene to Cr for the methyl-substituted benzene compounds in which there is a CC ‘ring-edge’ unsubstituted by methyls. Where no such ring-edge is present, i.e., for mesitylene, 1,2,3,5-tetramethylbenzene, 1,2,4,5-tetramethylbenzene and pentamethylbenze, the kinetics data suggest that a rapid equilibrium is e stablished two binding modes, the second possibly through the ring center.

Molecular structure of tetraphenylphosphonium tetracyanooxo(hydrogen cyanide-κN)molybdate(IV) pentahydrate: novel coordination of HCN to a MoIV centre

The crystal structure of (PPh4)2[MoO(NCH-κN)(CN)4]·5H2O, displaying the first example of hydrogen cyanide coordinated to MoIVviathe nitrogen atom, is described.

BONDING OF HALOGENATED ARENES IN PHOTOGENERATED (ARENE)M(CO)5 COMPLEXES (M= CR, MO, W)

Abstract Detailed kinetics studies of arene displacement by the trapping nucleophile, 1-hexene, from (haloarene)Cr(CO) 5 complexes (haloarene = X-arene; X = F, Cl, Br) and selected Mo and W analogues generated by pulsed laser flash photolysis have been conducted. All reactions obey a ‘reversible dissociation-competition for the [M(CO) 5 ] intermediate’ mechanism. The systematic variations in rate constants and activation parameters for unimolecular dissociation of haloarenes in the presence of the trap strongly support coordinate covalent bonding of Cl and Br to Cr, where Cr-Br Bonds are stronger than are Cr- Cl bonds. In contrast, evidence indicates that fluorobenzene coordinates more weakly, bound through an arene ring edge. Carbonyl stretching frequencies for the complexes are diagnostic for the type of bonding, and enthalpies of activation decrease in the order W > Mo > Cr. The failure to observe more than one reaction pathway where more than one complex is generated upon photolysis may suggest very rapid linkage isomerization to afford the thermodynamically most stable (X-arene)Cr(CO) 5 complex.

Kinetics and mechanism of the reaction of photogenerated (η1-(tetrachloromethane) pentacarbonylmetal(O) complexes, (η1-(CCl4)M(CO)5; M = Mo, W) with 1-hexene: The initial steps in the creation of olefin methathesis catalysts

Abstract The initial steps in the formation of the heterogeneous olefin methathesis catalysts produced by photolysis of M(CO) 6 /CCl 4 solutions (M = Mo, W) have been studied by pulsed laser flash photolysis with infrared and visible detection. In both systems, (Cl 3 CCl)M(CO) 5 (datively bonded to M through a single Cl) is produced immediately after the flash. In the presence of the trapping nucleophile, 1-hexene (hex), this species affords (η 2 -hex)M(CO) 5 . For M = W, there is also a competitive unimolecular decay pathway which affords the solid catalyst. Infrared and rate data for (Cl 3 CCl)W(CO) 5 and related complexes suggest that CCl 4 bonds weakly to W and is a poor net donor of charge to the metal atom, which could promote fast oxidative addition of CCl 4 to [M(CO) 5 ]. The rate-determining step for the unimolecular pathway is envisioned to involve slow decomposition via CO loss of the possible oxidative addition product to afford the heterogeneous catalyst.

Fast non-dissociative cis-to-trans isomerization in the cis-(P(O-i-Pr)3)2Cr(CO)4 intermediate

Abstract Pulsed laser flash photolysis of cis-(pip)(L)Cr(CO)4/CB/L solutions (pip=piperidine, L=tri(isopropyl phosphite, CB=chlorobenzene) produces cis- and trans-(CB)(L)Cr(CO)4 transients, which decay via three distinct reaction pathways: cis-(CB)(L)Cr(CO)4+L→cis-(L)2Cr(CO)4; trans-(CB)(L)Cr(CO)4+L→trans-(L)2Cr(CO)4; cis-(L)2Cr(CO)4→trans-(L)2Cr(CO)4. The third process, which takes place on the sub-ms time-scale, involves non-dissociative isomerization, likely via a trigonal prismatic transition state, and occurs on a time-scale>six orders of magnitude faster than had been observed previously. The independent existence of the two solvates indicates that cis-to-trans isomerization does not take place in the respective five-coordinate intermediates. These observations have general applicability to other systems.