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

The reaction of [(COD)PtCl 2 ] with biphen(QP i Pr)(Q′P i Pr) (Q, Q′ = O, CH 2 O) yields cis ‐[{biphen(QP i Pr)(Q′P i Pr)}PtCl 2 ]. Treatment of trans ‐[PtCl 2 (ethene)(py)] with biphen(QP i Pr)(Q′P i Pr) (Q, Q′ = O, CH 2 O) gives trans ‐[{biphen(QP i Pr)(Q′P i Pr)}PtCl 2 ] for Q or Q′ ≠ O. No trans / cis isomerization can be observed for these compounds after 24 h at 105 °C in [D 8 ]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(QP i Pr)(Q′P i Pr)}PtCl 2 ] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH 2 O 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 13 C NMR spectrum. Treatment of both cis ‐ and trans ‐[{biphen(QP i Pr)(Q′P i Pr)}PtCl 2 ] with two equivalents of ethyl‐ or n ‐propylmagnesium chloride gives cis ‐[{biphen(QP i Pr)(Q′P i Pr)}PtAlk 2 ]; the X‐ray structures of cis ‐[{biphen(CH 2 OP i Pr) 2 }PtEt 2 ] and cis ‐[{biphen(OP i Pr) 2 }Pt( n Pr) 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 [D 8 ]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(CH 2 OP i Pr) 2 }PtEt 2 ], selective deuteration of the CH 2 O group and the Pt‐ethyl group proves that scrambling of deuterium into the i Pr 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 i Pr CH 3 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 PPh 3 ). H 2 , however, is activated by the final complex, and the use of D 2 proves that deuterium is incorporated into the i Pr groups of the ligand. The reaction of [(COD)PtCl 2 ] with o ‐Br‐phen(QP i Pr) gives cis ‐[PtCl 2 { o ‐Br‐phen(QP i Pr)} 2 ] (X‐ray structure reported), which forms cis ‐[Pt{phen(QP i Pr)} 2 ] upon treatment with s BuLi. The reaction of trans ‐[PtCl 2 (ethene)(py)] with o ‐I‐phen(QP i Pr) gives trans ‐[PtCl 2 { o ‐I‐phen(QP i Pr)} 2 ] (X‐ray structure reported), which also forms cis ‐[Pt{phen(QP i Pr)} 2 ] upon treatment with s BuLi. cis ‐[Pt{phen(CH 2 OP i Pr)} 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(OP i Pr)} 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 1 H, 2 H, 13 C, 31 P, and 195 Pt 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)