Zwitterionic and Cationic Bis(phosphine) Platinum(II) Complexes: Structural, Electronic, and Mechanistic Comparisons Relevant to Ligand Exchange and Benzene C-H Activation Processes
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- 作者：J. Christopher Thomas and Jonas C. Peters
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：July 23, 2003
- 年：2003
- 卷：125
- 期：29
- 页码：8870 - 8888
- 全文大小：382K
- 年卷期：v.125,no.29(July 23, 2003)
- ISSN：1520-5126
文摘
Structurally similar but charge-differentiated platinum complexes have been prepared using thebidentate phosphine ligands [Ph2B(CH2PPh2)2], ([Ph2BP2], [1]), Ph2Si(CH2PPh2)2, (Ph2SiP2, 2), andH2C(CH2PPh2)2, (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metalcenter's electron-richness has been examined using comparative molybdenum and platinum model carbonyland alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than thosesupported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchangebetween neutral, formally zwitterionic [Ph2BP2]Pt(Me)(THF) (13) and its cationic relative [(Ph2SiP2)Pt(Me)(THF)][B(C6F5)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 ofTHF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving ananchimeric, 
3-binding mode of the [Ph2BP2] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C6F5)4] (15), each undergo a C-H bond activation reaction with benzene that generatestheir corresponding phenyl solvento complexes [Ph2BP2]Pt(Ph)(THF) (16), [(Ph2SiP2)Pt(Ph)(THF)][B(C6F5)4](17), and [(dppp)Pt(Ph)(THF)][B(C6F5)4] (18). Examination of the kinetics of each C-H bond activationprocess shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of theprimary 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 allthree cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces thedicationic, biphenyl-bridged platinum dimers [{(Ph2SiP2)Pt}2(
-3:3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(-3:3-biphenyl)][B(C6F5)4]2 (20). Extended thermolysis of neutral [Ph2BP2]Pt(Ph)(THF) (16) resultsprimarily in a disproportionation into the complex molecular salt {[Ph2BP2]PtPh2}-{[Ph2BP2]Pt(THF)2}+.The bulky phosphine adducts [Ph2BP2]Pt(Me){P(C6F5)3} (25) and [(Ph2SiP2)Pt(Me){P(C6F5)3}][B(C6F5)4](29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a muchslower rate than for 13-15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and29 in benzene-d6 shows only CH4 and CH3D. Whereas CH3D is the dominant byproduct for 14, 15, 25, and29, CH4 is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative[Ph2BP2]Pt(13CH3)(THF) (13-13CH3), and its deuterium-labeled derivative [Ph2B(CH2P(C6D5)2)2]Pt(Me)(THF)(13-d20), establish that reversible [Ph2BP2]-metalation processes are operative in benzene solution.Comparison of the rate of first-order decay of 13 versus the decay of d20-labeled 13-d20 in benzene-d6affords k13/k13-d20 ~ 3. The NMR data obtained for 13, 13-13CH3, and 13-d20 suggest that ligand metalationprocesses involve both the diphenylborate and the arylphosphine positions of the [Ph2BP2] auxiliary. Theformer type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All ofthese data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and thecationic systems that highlights their key similarities and differences.
3-binding mode of the [Ph2BP2] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C6F5)4] (15), each undergo a C-H bond activation reaction with benzene that generatestheir corresponding phenyl solvento complexes [Ph2BP2]Pt(Ph)(THF) (16), [(Ph2SiP2)Pt(Ph)(THF)][B(C6F5)4](17), and [(dppp)Pt(Ph)(THF)][B(C6F5)4] (18). Examination of the kinetics of each C-H bond activationprocess shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of theprimary 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 allthree cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces thedicationic, biphenyl-bridged platinum dimers [{(Ph2SiP2)Pt}2(-3:3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(-3:3-biphenyl)][B(C6F5)4]2 (20). Extended thermolysis of neutral [Ph2BP2]Pt(Ph)(THF) (16) resultsprimarily in a disproportionation into the complex molecular salt {[Ph2BP2]PtPh2}-{[Ph2BP2]Pt(THF)2}+.The bulky phosphine adducts [Ph2BP2]Pt(Me){P(C6F5)3} (25) and [(Ph2SiP2)Pt(Me){P(C6F5)3}][B(C6F5)4](29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a muchslower rate than for 13-15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and29 in benzene-d6 shows only CH4 and CH3D. Whereas CH3D is the dominant byproduct for 14, 15, 25, and29, CH4 is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative[Ph2BP2]Pt(13CH3)(THF) (13-13CH3), and its deuterium-labeled derivative [Ph2B(CH2P(C6D5)2)2]Pt(Me)(THF)(13-d20), establish that reversible [Ph2BP2]-metalation processes are operative in benzene solution.Comparison of the rate of first-order decay of 13 versus the decay of d20-labeled 13-d20 in benzene-d6affords k13/k13-d20 ~ 3. The NMR data obtained for 13, 13-13CH3, and 13-d20 suggest that ligand metalationprocesses involve both the diphenylborate and the arylphosphine positions of the [Ph2BP2] auxiliary. Theformer type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All ofthese data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and thecationic systems that highlights their key similarities and differences.