Octahedral metal carbonyls. 76. Ligand-exchange studies of cis-(pip)(L)Cr(CO)4 complexes (pip = piperidine; L = Lewis base)

Article abstract of DOI:10.1021/om00052a031

A series of cis-(pip)(L)Cr(CO)₄ complexes (pip = piperidine; L = P(OMe)₃, P(OEt)₃, P(O-i-Pr)₃, P(OPh)₃, P(OCH₂)₃CCH₃, PPh₃, P(CH₂)₃N₃C₃H₆), potential precursors for the creation of [(L)Cr(CO)₄] intermediates via flash photolysis, have been synthesized. The complexes react thermally in chlorobenzene (=CB) with L′ (=P(OMe)₃, P(OEt)₃, P(O-i-Pr)₃, P(OPh)₃, P(OCH₂)₃CCH₃, PPh₃, P(n-Bu)₃) to afford cis- and trans-(L)(L′)Cr(CO)₄ products according to a mechanism that kinetics data indicate to involve reversible dissociation of pip from cis-(pip)(L)Cr(CO)₄ and competition between pip and L′ for the cis-[(L)Cr(CO)₄] intermediates thus produced. Intramolecular isomerization follows, affording largely trans-(L′)(L)Cr(CO)₄, where both L and L′ are phosphines or are phosphites, but also substantial quantities of the cis isomer for mixed complexes containing one phosphine and one phosphite ligand. This observation is attributed to intramolecular hydrogen bonding (P-O··H-C-P) in the mixed complexes. Where the L ligands are phosphites, rates of dissociation of pip from cis-(pip)(L)Cr(CO)₄ complexes increase with increasing steric bulk of L; the [(L)Cr(CO)₅] intermediates produced upon Cr-pip bond breaking show little discriminating ability among incoming L′. The rate of pip dissociation from cis-(pip)(P(CH₂)₃N₃C₃H ₆)Cr(CO)₄ at 31.1 °C is 137 (2) times faster than that for cis-(pip)(P(OCH₂)₃CCH₃)Cr(CO)₄, in which L is sterically similar. This rate enhancement arises from a lack of intramolecular (N-H··O-P) hydrogen bonding between pip and L in the latter complex, which is present in the former. Activation energies suggest the strength of the hydrogen bond to be ca. 5 kcal/mol. These data suggest that the enthalpy of activation for Cr-pip bond dissociation closely approximates and represents an upper limit for the Cr-pip bond dissociation energy in this complex.