Synthesis and Study of the (Ph3P)2Pt Complexes of Three Members of a Series of Highly Pyramidalized Alkenes

Abstract

The (Ph3P)2Pt complexes (3) of three members of a homologous series of tricyclo[3.3.n.03,7]alk-3(7)-enes (2a-c) have been prepared by allowing these highly pyramidalized alkenes to displace ethylene from (Ph3P)2PtC2H4. The X-ray crystal structures of 3a-c show that pyramidalization angles at the carbons bonded to platinum increase from an average of ϕ= 48.3° in 3c to ϕ = 55.1° in 3b to 62.3° in 3a. In addition, as predicted computationally, the lengths of the bonds between the carbons attached to platinum increase and the C-Pt bond lengths decrease from 3c to 3a. The 1H, 13C, 31P, and 195Pt NMR spectra of 3a-c provide information about how pyramidalization at these carbons affects the electronic structures of the complexes. The changes in the 13C, 31P, and 195Pt chemical shifts and also the changes in the 13C-31P and 31P-31P coupling constants are consistent with the expected increase in back-bonding from the HOMO of (Ph3P)2Pt into the empty, π* LUMO of the alkene as pyramidalization increases from 3c to 3a. The increase in the one-bond 13C-195Pt coupling constant from 3c to 3a is found to be linearly related to the decrease in the 31P-195Pt coupling constant. This finding indicates that, with increasing pyramidalization, donation from the π HOMO of the alkene into the 6s orbital of platinum also increases but at the expense of donation into platinum 6s from the in-phase combination of phosphine lone pair orbitals. © 1995, American Chemical Society. All rights reserved.