Structural Properties and Dissociative Fluxional Motion of 2,9-Dimethyl-1,10-Phenanthroline in Platinum(II) Complexes

A dynamic 1H NMR study has been carried out on the fluxional motion of the symmetric chelating ligand 2,9-dimethyl-1,10-phenanthroline (Me2-phen) between nonequivalent exchanging sites in a variety of square-planar
complexes of the type [Pt(Me)(Me2-phen)(PR3)]BArf, 1−14, (BArf = B[3,5-(CF3)2C6H3]4). In these compounds, the
P-donor ligands PR3 encompass a wide range of steric and electronic characteristics [PR3 = P(4-XC6H4)3, X = H
1, F, 2, Cl 3, CF3 4, MeO 5, Me 6; PR3 = PMe(C6H5)2 7, PMe2(C6H5) 8, PMe3 9, PEt3 10, P(i-Pr)3 11, PCy(C6H5)2
12, PCy2(C6H5) 13, PCy3 14]. All complexes have been synthesized and fully characterized through elemental
analysis, 1H and 31P{1H} NMR. X-ray crystal structures are reported for the compounds 8, 11, 14, and for [Pt(Me)(phen)(P(C6H5)3)]PF6 (15), all but the last showing loss of planarity and a significant rotation of the Me2-phen
moiety around the N1−N2 vector. Steric congestion brought about by the P-donor ligands is responsible for tetrahedral
distortion of the coordination plane and significant lengthening of the Pt−N2 (cis to phosphane) bond distances.
Application of standard quantitative analysis of ligand effects (QALE) methodology enabled a quantitative separation
of steric and electronic contributions of P-donor ligands to the values of the platinum−phosphorus 1JPtP coupling
constants and of the free activation energies ΔG⧧ of the fluxional motion of Me2-phen in 1−14. The steric profiles
for both 1JPtP and ΔG⧧ show the onset of steric thresholds (at cone angle values of 150° and 148°, respectively),
that are associated with an overload of steric congestion already evidenced by the crystal structures of 11 and 14.
The sharp increase of the fluxional rate of Me2-phen can be assumed as a perceptive kinetic tool for revealing
ground-state destabilization produced by the P-donor ligands. The mechanism involves initial breaking of a metal−nitrogen bond, fast interconversion between two 14-electron three-coordinate T-shaped intermediates containing
η1-coordinated Me2-phen, and final ring closure. By use of the results from QALE regression analysis, a free-energy surface has been constructed that represents the way in which any single P-donor ligand can affect the
energy of the transition state in the absence of aryl or π-acidity effects.