Three-Fold Intramolecular Ring Closing Alkene Metatheses of Square Planar Complexes with cis Phosphorus Donor Ligands P(X(CH2) mCH═CH2)3 (X = -, m = 5-10; X = O, m = 3-5): Syntheses, Structures, and Thermal Properties of Macrocyclic Dibridgehead Diphosphorus Complexes

Abstract

Reactions of cis-PtCl₂(P((CH₂) _mCH═CH₂)₃)₂ and Grubbs' first generation catalyst and then hydrogenations afford cis- PtCl₂(P((CH₂) _n)₃ P) ( cis-2; n = 2 m + 2 = 12 (b), 14 (c), 16 (d), 18 (e), 20 (f), 22 (g); 6-40%), derived from 3-fold interligand metatheses. The phosphite complexes cis-PtCl₂(P(O(CH₂) _m*CH═CH₂)₃)₂ are similarly converted to cis- PtCl₂(P(O(CH₂) _n*O)₃ P) ( cis-5; n* = 8 (a), 10 (b), 12 (c), 10-20%). The substitution products cis- PtPh₂(P((CH₂) _n)₃ P) ( cis-6c,d) and cis- PtI₂(P(O(CH₂)₁₀O)₃ P) are prepared using Ph₂Zn and NaI, respectively. Crystal structures of cis-2c,d,f, cis-5a,b, and cis-6c show one methylene bridge that roughly lies in the platinum coordination plane and two that are perpendicular. The thermal behavior of the complexes is examined. When the bridges are sufficiently long, they rapidly exchange via an unusual "triple jump rope" motion over the PtX₂ moieties. NMR data establish Δ H^⧧, Δ S^⧧, and Δ G_298K^⧧/Δ G_393K^⧧ values of 7.8 kcal/mol, -27.9 eu, and 16.1/18.8 kcal/mol for cis-2d, and a Δ G_393K^⧧ of ≥19.6 kcal/mol for the shorter bridged cis-2c. While cis-2c,g gradually convert to trans-2c,g at 150-185 °C in haloarenes, trans-2c,g give little reaction under analogous conditions, establishing the stability order trans > cis. Similar metathesis/hydrogenation sequences with octahedral complexes containing two cis phosphine ligands, fac-ReX(CO)₃(P((CH₂)₆CH═CH₂)₃)₂ (X = Cl, Br), give fac- ReX(CO)₃( P(CH₂)₁₃ CH₂)((CH₂)₁₄)( P(CH₂)₁₃ CH₂) (19-50%), which are derived from a combination of interligand and intraligand metathesis. The relative stabilities of cis/ trans and other types of isomers are probed by combinations of molecular dynamics and DFT calculations.