Reaction of the Transient Species W(CO)₅(Cyclohexane) with Pyrrolidine and with Pyrrole

Abstract

Time-resolved infrared spectroscopy is used to examine the reactions of the transient intermediate W(CO) 5 (CyH) (CyH ) cyclohexane), formed by photolysis of a cyclohexane solution of W(CO) 6 , with L ) pyrrole and pyrrolidine. Time-and temperature-dependent rate constants for the ligand substitution reaction to form W(CO) 5 (L) are determined. These reactions appear to go through an associative interchange mechanism. Eyring activation parameters for the ligand exchange are derived. For L ) pyrrolidine, ∆H q ) 1.8 ( 0.1 kcal mol -1 and ∆S q ) -18.9 ( 0.4 eu; for L ) pyrrole, ∆H q ) 5.1 ( 0.2 kcal mol -1 and ∆S q ) -11.6 ( 1.2 eu. Most unusually, the observed pseudo-first-order rate constant for reaction with pyrrole is not a linear function of pyrrole concentration. This deviation from simple first-order behavior is explained in terms of association of pyrrole in solution to form dimers and higher clusters that lower the ligand's effective concentration. The enthalpy of dimerization of pyrrole is estimated from the kinetic data to be approximately 4 kcal mol -1 . The results of reaction of W(CO) 5 (CyH) with pyrrole and pyrrolidine are discussed in terms of the behavior of this intermediate with other ligands.

Formation and reactivity of organometallic alkane complexes

“…Complexes of the type M(CO) 5 (L) (M = Cr, Mo, W; L = alkane) have been observed following the excitation of M(CO) 6 by UV photolysis [24][25][26] and by pulse radiolysis [27] in both solution and in the gas phase at room temperature. Ultrafast transient absorption experiments have demonstrated that Cr(CO) 5 (cyclohexane) is formed within 1 ps, following the photolysis of Cr(CO) 6 in a room temperature cyclohexane solution [28].…”

“…From these kinetic experiments an estimate of the W-alkane bond energy of 16 kJ mol −1 was reported [29]. A series of TRIR experiments have also examined the activation parameters for the reaction of M(CO) 5 (alkane) complexes with competing ligands [24][25][26]. Metal alkane binding energies have previously been determined by photoacoustic calorimetry (PAC) for a range of Group VI metal alkane complexes including: Cr(CO) 5 (n-heptane) (46 (±4) kJ mol −1 ) [30], Cr(benzene)(CO) 2 (n-heptane) (ca.…”

Ligand substitution reactions of the CpMn(CO)2(cyclohexane) intermediate studied by time-resolved infrared absorption spectroscopyElectronic supplementary information (ESI) available: Complete tables of pseudo-first order rate constants kobs as a function of [L] and of temperature. See http://www.rsc.org/suppdata/dt/b3/b302023a/

“…For an associative reaction, we would expect that the enthalpic barrier would be lower for a more electron-donating ligand which could better stabilize the electron-poor transition state. § This kind of behavior is observed, for example, in the ligand substitution reactions of W(CO) 5 (CyH), [16][17][18][19]30 for which the activation enthalpy for reaction with furan is some 20 kJ mol Ϫ1 higher than that for reaction with pyrrolidine. 17, 19 The contrasting behavior of the Mn and W systems is shown graphically in a Derived as the slopes to linear fits to k obs as a function of [L], weighted by the uncertainties in k obs ; see the text.…”

“…§ This kind of behavior is observed, for example, in the ligand substitution reactions of W(CO) 5 (CyH), [16][17][18][19]30 for which the activation enthalpy for reaction with furan is some 20 kJ mol Ϫ1 higher than that for reaction with pyrrolidine. 17, 19 The contrasting behavior of the Mn and W systems is shown graphically in a Derived as the slopes to linear fits to k obs as a function of [L], weighted by the uncertainties in k obs ; see the text. 1σ uncertainties in k a are given in parentheses.…”

“…11 Our interest in reaction (1) comes from a slightly different point of view. We have recently performed a series of studies [16][17][18][19] on the ligand substitution reactions of the W(CO) 5 -(CyH) (CyH = cyclohexane) complex in which we used TRIR to investigate how the electronic and steric properties of the incoming ligand affect the course of the reaction. We undertook the present study for similar reasons.…”