A time-resolved IR study of the gas-phase reactions of 1,3-and 1,4-pentadiene with Cr(CO)₄

The reactions of 1,3- and 1,4-pentadiene (PD) with the coordinatively unsaturated species Cr(CO)₄ were probed with time-resolved infrared spectroscopy in an effort to compare the reactivities of conjugated and nonconjugated dienes toward metal carbonyls. Both spectroscopic and kinetic evidence that indicates the addition of pentadiene to Cr(CO)₄ follows a branched reaction mechanism. Formation of the initial product, highly excited [(η⁴-PD)Cr(CO)₄]*, occurs with rate constants that are near gas kinetic: (6 ± 2) × 10¹⁴ cm³ mol^-1 s^-1 for 1,3-pentadiene and (4 ± 2) × 10¹⁴ cm³ mol^-1 s^-1 for 1,4-pentadiene. This excited complex can rearrange via bond scission lo form (η²-PD)Cr(CO)₄ or, at high buffer gas pressures, collisionally relax to form (η⁴-PD)Cr(CO)₄, In the 1,3-pentadiene system, (η²-1,3-PD)Cr(CO)₄ forms stable (η⁴-1,3-PD)Cr(CO)₄ in an unactivated but slow [(2.2 ±0.9) × 10³ s^-1] rearrangement. In contrast, (η²-1,4-PD)Cr(CO)₄ is unreactive on a millisecond time scale, while the (η⁴-1,4-PD)Cr(CO)₄ formed by collisional relaxation of the initial complex rearranges to a more stable isomeric species. It is likely that this more stable isomer is (η^2:CH-1,4-PD)Cr(CO)₄, where η^2:CH represents an agostic Cr-H-C bond. This latter reaction is activated [(11.4 ± 0.4) kcal/mol] and has a first-order rate constant of (4.5 ± 1.3) × 10⁴ s^-1 at room temperature. The factors that lead to the differences in the reaction mechanism for 1,3- versus 1,4-pentadiene with Cr(CO)₄ are discussed.