Investigation of the Thermal Decomposition of [biphen(QPiPr)Pt(alk)2]: An Entry to C–C Single Bond Activation?

Abstract

AbstractThe reaction of [(COD)PtCl2] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) yields cis‐[{biphen(QPiPr)(Q′PiPr)}PtCl2]. Treatment of trans‐[PtCl2(ethene)(py)] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) gives trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] for Q or Q′ ≠ O. No trans/cis isomerization can be observed for these compounds after 24 h at 105 °C in [D8]toluene. In the case of Q = Q′ = O a trans/cis isomerization takes place at room temperature. X‐ray structures of a molecule in the solid state were determined for the compounds cis/trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH2O the closer of the two bridging C atoms in the biphenyl fragment of the ligand is only 3.42 Å away from the Pt center but does not show a coupling to it in the 13C NMR spectrum. Treatment of both cis‐ and trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with two equivalents of ethyl‐ or n‐propylmagnesium chloride gives cis‐[{biphen(QPiPr)(Q′PiPr)}PtAlk2]; the X‐ray structures of cis‐[{biphen(CH2OPiPr)2}PtEt2] and cis‐[{biphen(OPiPr)2}Pt(nPr)2] are discussed. These complexes give the related alkene complexes as a result of β‐H elimination/reductive elimination if heated to temperatures above 90 °C in [D8]toluene, but none of the complexes with Q or Q′ ≠ O is stable at 95 °C. The thus‐formed Pt(alkene) complex loses the alkene (ethene or propene) in a consecutive reaction. For [{biphen(CH2OPiPr)2}PtEt2], selective deuteration of the CH2O group and the Pt‐ethyl group proves that scrambling of deuterium into the iPr methyl groups occurs (by a CH activation/insertion/β‐H elimination/reductive elimination sequence) after the β‐H elimination and before the loss of ethene, which at 95 °C is almost as fast as the loss of ethene. In the final alkene‐free 14 e complex a fast dynamic C–H activation of the iPr CH3 protons by the Pt center (one H at a time) can be detected [an Isotopic Perturbation of Resonance (IPR) is found for the partially deuterated compound], which most probably prevents a further reaction of the complex (e.g. activation of the bridging C–C single bond in the biphenyl backbone or permanent coordination of PPh3). H2, however, is activated by the final complex, and the use of D2 proves that deuterium is incorporated into the iPr groups of the ligand. The reaction of [(COD)PtCl2] with o‐Br‐phen(QPiPr) gives cis‐[PtCl2{o‐Br‐phen(QPiPr)}2] (X‐ray structure reported), which forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. The reaction of trans‐[PtCl2(ethene)(py)] with o‐I‐phen(QPiPr) gives trans‐[PtCl2{o‐I‐phen(QPiPr)}2] (X‐ray structure reported), which also forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. cis‐[Pt{phen(CH2OPiPr)}2] (X‐ray structure reported) slowly isomerizes quantitatively at 95 °C in toluene to the trans complex. No product of reductive elimination can be detected up to 150 °C in mesitylene. cis‐[Pt{phen(OPiPr)}2] (X‐ray structure reported) forms two products ― the trans isomer (X‐ray structure reported) and most likely a dimeric trans‐configured complex ― upon heating to 95 °C. Again, no reductive elimination is observed regardless of whether the reaction is performed under argon or ethene atmosphere up to 150 °C in mesitylene. All materials were characterized by means of 1H, 2H, 13C, 31P, and 195Pt NMR spectroscopy, FAB mass spectrometry, FT‐IR spectroscopy, elemental analysis, and X‐ray crystallography. DFT calculations (B3PW91/LANL2DZ) are also included for the most important compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)