Comparative Studies with Zwitterionic Platinum(II) Bis(pyrazolyl)borate and 2,2‘-Bipyridylborate Complexes

A comparison between the mononuclear platinum complexes of three structurally different
monoanionic borato ligands is presented:  [Ph2B(pyrazolyl)2]- ([Ph2B(pz)2], 1), [4-Ph3B(2,2‘-bipyridine)]- ([(4-BPh3)bpy], 2), and [Ph2B(CH2PPh2)2]- ([Ph2BP2], 3). The new bipyridylborate
ligand 2 is introduced in this study. The relative trans influence of these ligands has been
assessed by comparison of the structural and spectroscopic (NMR) data of the platinum
dimethyl complexes [[Ph2B(pz)2]Pt(Me)2][NBu4] (4), [[(4-BPh3)bpy]Pt(Me)2][NBu4] (5), and
[[Ph2BP2]Pt(Me)2][ASN] (6). The neutral complexes [Ph2B(pz)2]Pt(Me)(NCCH3) (7), [Ph2B(pz)2]Pt(Me)(CO) (8), [Ph2B(pz)2]Pt(Me)(P(C6F5)3) (9), [(4-BPh3)bpy]Pt(Me)(NCCH3) (10),
[(4-BPh3)bpy]Pt(Me)(CO) (11), and [(bpy)Pt(Me)(CO)][BPh4] (12) were prepared, and the
carbonyl complexes 8, 11, and 12 provide information pertaining to the relative electron-releasing character of each ligand type. The CO stretching frequencies suggest that the
charged borate moiety renders the borato ligands more electron-donating than their neutral
analogues. Of the neutral platinum methyl solvento complexes supported by ligands 1, 2,
and 3, only those of 1 display very different C−H activation propensities. Upon protonation
or methide abstraction in benzene at room temperature, complex 4 rapidly activates two
molecules of benzene to generate [[Ph2B(pz)2]Pt(Ph)2][NBu4] (13). Isotopic scrambling of
deuterium into methane in C6D6 solvent suggests the intermediacy of a methane σ-adduct
in this reaction. The double C−H activation reaction can be halted by addition of acetonitrile
to trap the intermediate [Ph2B(pz)2]Pt(Ph)(NCCH3) (14). Complex 3 also displays reactivity
toward the benzylic C−H bonds of mesitylene at room temperature to form [Ph2B(pz)2]Pt(pzH)(CH2C6H3(CH3)2) (15) in modest yield.