Zwitterionic and Cationic Bis(phosphine) Platinum(II) Complexes:  Structural, Electronic, and Mechanistic Comparisons Relevant to Ligand Exchange and Benzene C−H Activation Processes

Abstract

Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph_(2)B(CH_(2)PPh_(2))_(2)], ([Ph_(2)BP_(2)], [1]), Ph_(2)Si(CH_(2)PPh_(2))_(2), (Ph_(2)SiP_(2), 2), and H_(2)C(CH_(2)PPh_(2))_(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph_(2)BP_(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph_(2)SiP_(2))Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, η3-binding mode of the [Ph_(2)BP_(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (15), each undergo a C−H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph_(2)BP_(2)]Pt(Ph)(THF) (16), [(Ph_(2)SiP_(2))Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (18). Examination of the kinetics of each C−H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C6H6)/k(C6D6):  13 = 1.26; 14 = 6.52; 15 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [{(Ph_(2)SiP_(2))Pt}2(μ-η3:η3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(μ-η^(3):η^(3)-biphenyl)][B(C_(6)F_(5))_(4)]_(2) (20). Extended thermolysis of neutral [Ph_(2)BP_(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt {[Ph_(2)BP_(2)]PtPh_(2)}-{[Ph_(2)BP_(2)]Pt(THF)_(2)}+. The bulky phosphine adducts [Ph_(2)BP_(2)]Pt(Me){P(C_(6)F_(5))_(3)} (25) and [(Ph_(2)SiP_(2))Pt(Me){P(C_(6)F_(5))_(3)}][B(C_(6)F_(5))_(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13−15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d6 shows only CH_(4) and CH3D. Whereas CH_(3)D is the dominant byproduct for 14, 15, 25, and 29, CH_(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative [Ph_(2)BP_(2)]Pt(^(13)CH_(3))(THF) (13-^(13)CH_(3)), and its deuterium-labeled derivative [Ph_(2)B(CH_(2)P(C_(6)D5)_(2))_(2)]Pt(Me)(THF) (13-d20), establish that reversible [Ph_(2)BP_(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d_(20)-labeled 13-d_(20) in benzene-d_(6) affords k_(13)/k_(13-d20) ~ 3. The NMR data obtained for 13, 13-^(13)CH_3, and 13-d_20 suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph_(2)BP_(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences.

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