Heats of reaction of RMo(CO)3C5H5 (R = H, Me, Et) with phosphines and phosphites: thermodynamic study of the carbonyl-insertion reaction

Abstract

The heats of reaction of HMo(CO)3C5H5 with a series of phosphines and phosphites, producing HMo(CO)2(PR3)C5H5, have been measured by solution calorimetry. The order of relative Mo-PR3 bond strengths, PMe3 = P-h-Bu3 > P(OMe)3 > PMe2Ph > PMePh2, closely resembles that determined earlier for the complexes/ac-(PR3)3Mo(CO)3. The heats of reaction of RMo(CO)3C5H5 (R = CH3, C2H5), producing the phosphine-substituted acyl complexes RC(0)Mo(CO)2(PR3)C5Hs, were also studied thermochemically. This reaction can be viewed as involving both phosphine substitution and carbonyl insertion. With use of the enthalpy of phosphine substitution of HMo(CO)3C5H5 as a model, the enthalpy of carbonyl insertion is calculated. For both R = CH3 and R = C2FI5 the carbonyl-insertion reaction is favored for more basic phosphines with a difference of about 2 kcal/mol between PMe3 (most favored) and P(OMe)3 (least favored). Carbonyl insertion into the Mo-Et bond is about 3 kcal/mol more favorable than into the Mo-CH3 bond. Attempts to prepare the unsubstituted acyls RC(0)Mo(CO)3C5H5 by carbonylation of the alkyls in THF led to facile reductive elimination of RC(0)C5H5 (35 °C, 1 atm of CO), producing Mo(CO)6. Migration of the acyl group to the coordinated cyclopentadienyl ring in the presence of CO pressure and thermodynamic instability in the absence of CO pressure explain the lack of success in preparing RC(0)Mo(CO)3C5H5 for simple alkyl groups.

Reactivity of Phosphorus Donors

“…For R = Et, x = 0.21, % fit = 3. Data from ref . k Activation enthalpy for the second-order substitution of CO by phosphines for Co(NO)(CO) 3 in toluene.…”

Steric effects of bulky phosphines on 95Mo NMR shieldings

“…Taking into account the recording error of v,,'s and the ambiguous assignments of the IR bands, one can see again that IR parameters are not as reliable as NMR chemical shift data for evaluation of ligand effects. Although most common phosphites are thought to be strong n-ligands, 9 5~o NMR spectrometry shows, consistent with the weaker M-P(OPh), bonding (4,16,19), that P(OPh), has some special electronic property different from other phosphites. Unfortunately, there previously was no structural information on a higher displaced phosphite-molybdenum compound available in the literature.…”

“…ber of experimental observations such as stable M-PMe, bond strengths (16) are not easily explained. An answer to this anomaly is more likely related to the T ability of PMe, (6,13,26).…”

“…(a) Strong IT-ligands: PF,, ETPB, P(OMe),, and P(OPh), (b) Strong a -and (or) moderate IT-ligands: PEt,, P(n-Bu),, and PMe,, (c) Extra bulky ligands: P(OSiMe,),, PCy,, P(2-Me-Ph),, and P(2,6-Me,-OPh), (a) Strong m-ligands: PF3(104"), ETPB(lOlO), P(OMe)3(1070), and P(OPh),(128") These four ligands are well known as good IT-acceptors, based on the high stretching frequencies of trans CO's in transition metal complexes (6,13), shorter M-P bond distances (14), high stability of the M-P link (15,16), high reactivity toward displacement of CO in metal carbonyl moieties (1,3,4), and other electrochemical (12J 17) and spectroscopic studies (6, 8a, 18). In agreement with these observations and the expectation that the AE term in the u p term is enhanced by good ITacceptor ability, the phosphites indeed make the 9 5~o nuclei in their molybdenum carbonyl derivatives quite shielded compared with most other ligands, whether in m l or f3 derivatives (Fig.…”

“…This suggests that if the steric parameters for P(OPh), and P(0-i-Pr), were not underestimated and overestimated, respectively, the weaker bonding ability or the extra deshielding effect of P(OPh), could only be attributed to a weaker a component (pK, = -2.0). It must be pointed out that there have already been many observations indicating the weak bonding ability (4,16,19) of the P(OPh), ligand even though the short Cr-P distance in Cr(CO)s(P(OPh)3) is usually interpreted in terms of strong n-acceptor character (20) of this common ligand. It was also noted experimentally in this work that the substitution of pyridines from fac-Mo(CO),(Py), by P(OPh), is much more difficult than by P(OMe), or ETPB.…”