76136-83-9Relevant academic research and scientific papers
Discontinuum between a thiolate and a thiol ligand
McGuire, Danny G.,Khan, Masood A.,Ashby, Michael T.
, p. 2202 - 2208 (2002)
The effect of H-bond donation to the thiolate ligand of (η5-C5H5)Fe(CO)2SR (1) to give H-bond adducts (1·HX) and eventually protonation to give [(η5-C5H5 )Fe(CO)2(HSR)]+ (1H+) has been investigated experimentally and computationally. The electronic structures of 1(R = Me), several derivatives of 1(R = Me)·HX, and 1(R = Me)H+ have been investigated using DFT (density functional theory) computational methods. As previously suggested, these calculations indicate the HOMO of 1 is FedπSpπ antibonding and largely sulfur in character. The calculations indicate the electronic structure of 1 is not altered markedly by H-bond donation to the S center, but protonation results in a reorganization of the electronic structure of 1H+ and a HOMO that is largely metal in character. The reduction of Fe-S distances upon protonation of 1(R = Ph) to give 1(R = Ph)H+·BF4- (2.282(2) and 2.258(2) A, respectively), as determined by single-crystal X-ray crystallography, also indicates diminished Fedπ-Spπ antibonding. Using the carbonyl stretching frequencies as a gauge of the donor ability of the thiolate ligand, we conclude that H-bonding has a continuous effect on the donor properties of the thiolate ligand of 1 (i.e., is a function of the pKa of the H-bond donor). A discontinuous effect results when the pKb of 1 is reached and the complex is protonated. For our study of 1, the maximal effect of H-bonding is about 30% of protonation. Because the position of acid-base equilibrium depends on the relative basicities of the thiolate ligand and the conjugate base of the H-bond donor (and the relative heats of solvation of the acids and their conjugate bases), a true continuum of effects can be anticipated only for systems that are pK-matched in their given environments. Thus, when the conjugate base of the H-bond donor is a stronger base than the thiolate ligand (as in the present case), H-bond donation has a relatively small effect, but protonation triggers a large, discontinuous effect on the electronic structure of 1.
