A new generation of metal string complexes: structure, magnetism, spectroscopy, theoretical analysis, and single molecular conductance of an unusual mixed-valence linear [Ni5]8+ complex.
Two new linear pentanickel complexes [Ni5(bna)4(Cl)2][PF6]2 (1) and [Ni5(bna)4(Cl)2][PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, [Ni5(bna)4(NCS)2][NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with [Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard [Ni5]10+ core, isoelectronic with that of [Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.
Liu
Isiah Po-Chun
IP
Department of Chemistry, National Taiwan University, 1, Roosevelt Rd., Sec. 4, Taipei 106, Taiwan.
Bénard
Marc
M
Hasanov
Hasan
H
Chen
I-Wen Peter
IW
Tseng
Wei-Hsiang
WH
Fu
Ming-Dung
MD
Rohmer
Marie-Madeleine
MM
Chen
Chun-hsien
CH
Lee
Gene-Hsiang
GH
Peng
Shie-Ming
SM
D018360:Crystallography, X-Ray; D004563:Electrochemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards; D008280:Magnetics; D008958:Models, Molecular; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D011789:Quantum Theory; D012015:Reference Standards; D019265:Spectroscopy, Near-Infrared / Q000379:methods; D013237:Stereoisomerism; D013696:Temperature
D009942:Organometallic Compounds; D009532:Nickel