Photochemistry of (η⁶-arene)Mo(CO)₃ and the Role of Alkane Solvents in Modifying the Reactions of Coordinatively Unsaturated Metal Carbonyl Fragments

Abstract

The reactions of (η 6 -arene)Mo(CO) 2 (Sol) and M(CO) 5 (Sol) with CO have been studied in a range of alkane solvents (Sol), and the kinetic and activation parameters have been determined (M ) Cr, Mo, or W). For M ) Cr the ∆H q is constant (22 ( 2 kJ mol -1 ), while the ∆S q term becomes less negative as the alkane chain length increases. For the larger metals the variation in kinetic and activation parameters is less significant. Solvent displacement by CO involves an interchange mechanism for the Cr system, while for Mo or W complexes the mechanism is more associative in character. The photochemistry of (η 6 -arene)Mo(CO) 3 (arene ) benzene, mesitylene, p-xylene, or hexamethylbenzene) compounds was investigated by laser flash photolysis, supported by matrix isolation and time-resolved infrared spectroscopy (TRIR). In contrast to the behavior to the analogous (η 6 -arene)Cr-(CO) 3 , it is found that the efficiency for photochemical expulsion of CO from (η 6 -mesitylene)-Mo(CO) 3 is markedly wavelength dependent (Φ CO ) 0.587, 0.120, and 0.053 at 266, 313, and 334 nm, respectively).

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

“…The observed rate constant for the decay of 2 depends linearly on the CO concentration, affording the second-order rate constant for the reaction of 2 with CO in scCH 4 (k CO ϭ 7.8 Ϯ 0.8 ϫ 10 6 dm 3 ⅐mol Ϫ1 ⅐s Ϫ1 , 298 K). W(CO) 5 (CH 4 ) is significantly more reactive (3-15 times) than other previously reported W(CO) 5 (alkane) complexes (12). However, the rate of the alkane-CO substitution still is significantly below the diffusion-controlled rate in scCH 4 (Ϸ8 ϫ 10 11 dm 3 ⅐mol Ϫ1 ⅐s Ϫ1 , estimated from the Stokes-Einstein equations).…”

“…Time-resolved infrared (TRIR) spectroscopy has proved to be a powerful tool for the study of metal carbonyl alkane complexes. Their reactivity decreases on going both across and down groups 5,6, and 7 (10)(11)(12), and these observations led to the identification of a very long-lived alkane complex, Re(Cp)(CO) 2 (nheptane) (Cp ϭ 5 OC 5 H 5 ), which has a lifetime of Ϸ25 ms at room temperature (13). The relative stability of Re(Cp)(CO) 2 (alkane) complexes allowed Re(Cp)(CO) 2 (C 5 H 10 ) to be observed at 180 K by NMR spectroscopy (14), and subsequent NMR studies have been carried out to determine the binding modes of a series of related alkanes to the Re(CpЈ)(CO) 2 moiety (15,16).…”

“…The lifetime of the alkane complexes depends strongly on the nature of the alkane, with cyclic alkanes tending to form longerlived complexes than the corresponding linear alkanes do (12). Metal-alkane binding enthalpies have been determined by photoacoustic calorimetry (PAC) and TRIR spectroscopy for a range of group 6 [M(CO) 5 (heptane)] and group 7 [M(Cp)(CO) 2 (heptane)] complexes in solution, and these are found to range between 40 and 57 kJ⅐mol Ϫ1 (10,(20)(21)(22)(23)(24).…”

Formation and reactivity of organometallic alkane complexes

“…The second order rate constants for the reaction of a series of n-heptane complexes with CO (k CO ) are shown in Table 1. The reactivity of the organometallic alkane complexes studied has been found to decrease on going both across and down Groups V-VII [49,51,56,57].…”

“…[59]. allowed for systematic investigations into the electronic and steric factors affecting the stability of transition metal alkane complexes [49][50][51][52]. The reactivity of the organometallic alkane complex to ligand substitution is measured and the second order rate constant (k 2 ) for this reaction, determined by measuring the pseudo-first order rate as a function of CO concentration, can be used as a measure of the relative stability of the alkane complex, Scheme 2 [53,54].…”