Kinetics and conformation in the reversible insertion of an alkene into a platinum-carbon bond in a chelated (pentenyl)platinum complex

Abstract

The synthesis and characterization, including detailed 'H, 2H, 31P, and 13C NMR analysis of [ (l,l-dideuteri0-2,2-dimethyl-4-penten-l-yl-K3C~~~*~) (dmpe)PtnlBF4, 1-adz, and its 3,3-dideuterio isotopomer, 1 -7 4 (dmpe = 1,2-bis(dimethylphosphino)ethane), are described. Thermolysis of each 1-a-dz and 1-y-dz was carried out a t 125 "C in several solvents to yield a 1/1 mixture of 1-a-dz and 1-ydz. The rate of rearrangement is cleanly first order. Thermolysis of 1-a-d2 between 81 and 125 "C in methylene chloride led to determinations of activation parameters, E, = 31.8(0.6) kcal/moland AS* = -0.5(1.6) eu (100 "C). AnX-ray crystalstructuredetermination was carried out on the compound. It crystallizes in the monoclinic space group P21/n (No. 14), with a = 19.337(11) A, b = 6.948(4) A, c = 17.260(9) A, B = 122.34(6)", V = 1959.3 A3, and 2 = 4. The structure was solved with heavy atom techniques and refined to a final R factor of 4.18 % . In the solid state, the coordinated vinyl group is slipped slightly out the plane of the molecule and is canted a t a dihedral angle of 52" from the molecular plane. The chelate ring shows only very slight distortions from a presumed strain-free shape. 2D NOESY spectra suggest that the in-plane conformation of the pentenyl chelate is at least partly populated in solution. (dmpe)Pt(CH2-c-C4H,)Cl, 11, was prepared. Thermolysis of 11 a t 110 "C yielded intractable metal-containing materials along with methylenecyclobutane and methylcyclobutane together in ca. 60 7% yield as the only identifiable organic products. Treatment of 11 with silver ion a t low temperatures in acetone or methylene chloride yielded two products in a 1:7 (NMR) ratio, methylenecyclobutane and a material stable only below 0 "C that is assigned the structure [ (4-penten-l-y1-~3C~~~~~)(dmpe)PtII]BF4 on the basis of its NMR spectrum.

A Neutral Three-Coordinate Alkylrhodium(I) Complex: Stabilization of a 14-Electron Species by γ-C−H Agostic Interactions with a Saturated Hydrocarbon Group

“…[16] Thus, rhodium compound 2 might resemble a structure on the pathway of b-carbon elimination at d 8 -ML 2 templates. [17] Owing to the averaging out of all nine methyl protons of the Np group by tBu and Me rotations, lowered C ± H streching frequencies, upfield NMR shifts, or reduced 13 C ± 1 H coupling constants for the agostic hydrogen atoms are not observed for 2. [15] However, agostic interactions are unequivocally established for the solid-state structure and by all NMR data of 2 in solution.…”

Reactivity and mechanism in the ring-opening of cyclopropylmethylplatinum(IV) complexes

“…3).While many cycloalkylmethyl complexes of transition metals undergo easy ring opening, complexes 2 and 3 were not decomposed over one hour in refluxing methanol solution. The ring-opening may require a vacant coordination site at platinum [7,18,32], so the complexes were treated first with silver trifluoroacetate to give the complexes [PtMe 2 (CH 2 R)(O 2 CCF 3 )(bipy)], 7, R=c-C 3 H 5 ; 8, R=c-C 4 H 7 ; 9, R=CH 2 CH 2 CH=CH 2 ; 10, R=CH 2-CH=CHMe, which contain the more weakly bound trifluoroacetate ligand. These complexes were all thermally stable in solution at room Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e temperature, and the cyclobutyl derivative 8 was also stable in refluxing methanol but the cyclopropyl derivative 7 did undergo ringopening to give 9 (as a mixture of isomers 9a and 9b, Scheme 2) under these conditions.…”

“…4. The cyclopropyl ring strain provides the driving force for the ring opening reaction (it can be considered as a β-carbon elimination [16][17][18]) to convert 2 to the 3-butenyl isomer 4a, but there is no easy pathway for the 18-electron complex to react. The E-2-butenyl complex 5 is expected to be slightly more stable than 4a and is the most stable of the isomeric complexes with linear C 4 H 7 groups.…”

“…The isomeric 3-butenyl complex 13 is more stable but it cannot be formed directly from 11. This explains in a qualitative way why the ring opening is more difficult in the octahedral organoplatinum(IV) complex 2 than in square planar complexes which are more commonly used in catalysis [1,2,[7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…The transition metal may play a direct role in the ring-opening step, but it is also well known that cyclopropylmethyl to butenyl isomerisation may occur within the organic free radicals, cations or anions [3][4][5][6]. Some insights into the mechanisms have been gained by study of stoichiometric reactions, but there are still surprisingly few known examples of thermally stable transition metal complexes with cyclopropylmethyl or cyclobutylmethyl ligands to act as model compounds [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Cyclopropylmethyl complexes are often generated by insertion of methylenecyclopropane [1,2,[7][8][9][10][11][12][13] while cyclobutylmethyl complexes have usually been prepared by transmetalation [16][17][18].…”