Complex Kinetics of “Simple” Substitution Reactions of Os₃(CO)₉(μ-C₄Ph₄) with Smaller P-Donor Nucleophiles

Abstract

The osmacyclopentadiene ring in the triangular Os3 cluster Os3(CO)9(μ-C4Ph4) bridges two of the Os atoms and is found to activate associative attack at the third Os atom (which is in an Os(CO)4 moiety) by a factor of ∼109 compared with reactions of the parent cluster Os3(CO)12. The overall reactions in heptane of 17 P-donor nucleophiles with Tolman cone angles θ ≤ 143° lead to substitution at the Os(CO)4 center in three rapid but kinetically observable stages: (i) reversible attack by a nucleophile to form an adduct in which an Os−Os bond in the cluster has been broken, (ii) loss of CO and formation of a new closed Os3 cluster, and, usually, (iii) isomerization of the initially formed cluster to form the final substituted product. The dependence of the rates on the σ-basicity and size of the nucleophiles shows that the standard reactivity toward adduct formation is very high, only high nuclearity clusters that contain encapsulated C atoms being known to react faster. Nucleophiles with θ ≤ 120 ± 4° react at rates that are independent of their size and that increase substantially with their σ-basicity. When the cone angles are larger, steric retardation is observed. Equilibrium constants for adduct formation by 14 of the nucleophiles were obtained from the rates of adduct formation and loss of L from the Os3(CO)9L(μ-C4Ph4) adducts and by two other methods. Loss of CO or L from the adducts becomes slower the greater the net donicity of the ligands but is essentially independent of ligand size. Rates of isomerization of the initial product are not very precise and are not appreciably sensitive to the nature of the ligands. Activation parameters were obtained for reactions involving five of the nucleophiles, and these, together with the magnitudes of the electronic and steric effects, can lead to estimates of the contributions of bond making and bond breaking in the transition states.

Electronic and steric effects in fragmentation reactions of Os3(CO)9(μ-C4Ph4)

“…Although there is no spectroscopic or kinetic evidence for intermediates in the reactions of the LNs, the results can best be discussed in terms of Scheme 1. This includes the formation of kinetically observed intermediate adducts (II), proposed for reactions with the SNs, 14 but indicates that LNs form quite different adducts (II ). These are capable of undergoing spontaneous fragmentation by two different paths to form the observed pairs of mixtures of mono-and di-nuclear products.…”

“…20 M −1 while values in excess of 10 3 M −1 are even observed, particularly for the two largest of the SNs. 14 The kinetic results can be expressed in terms of a 'free energy surface' as shown in Fig. 3.…”

“…† The activation parameters were used to calculate values of k 2 at 13.6 • C (also given in Table 1) for comparison with kinetic parameters for reactions with the SNs (h ≤ 143 • ) which were obtained at that temperature. 14…”

“…(1). 14 The fit was originally carried out by using an equation in which the electronic effect was simply probed by the NMR parameter, d, 21 alone, instead of pK a and pK a (p) since values for the latter were not available. This was done in the belief that it would allow for some contributions from p-acidity effects.…”

“…Analysis of the kinetics of the reactions in terms of the QALE equation has been undertaken to see if the derived sensitivities of log k to the electronic and steric properties of the P-donors could quantify, and perhaps explain, the differentiation. Thus the kinetics of substitution reactions by the group of 17 smaller nucleophiles have been described, 14 and show that reactions occur via very rapid reversible attack by the nucleophiles to form nido adducts, which then undergo CO dissociation to form the closo substituted product clusters. Remarkably, the initial step is ∼10 9 times faster than the corresponding reactions of Os 3 (CO) 12 .…”