Tetramethylcyclobutadiene(cyclopentadienyl)cobalt complexes Cb*Co(C 5 H 4 R) (Cb* ) η 4 -C 4 Me 4 ; R ) H (5a), Me (5b), SiMe 3 (5d), C(O)H (5f), and C(O)Me (5g)) were obtained by reaction of cyclopentadienide anions either with the (carbonyl)iodide complex Cb*Co(CO) 2 I (1) (method A) or with the more reactive acetonitrile complex [Cb*Co(MeCN) 3 ] + (2) (method B). Analogous compounds Cb*CoCp* (5c), Cb*Co(1,3-C 5 H 3 (SiMe 3 ) 2 ) (5e), and Cb*Co(η 5indenyl) (6) can be prepared only by method B. Treatment of 5f,g with NaBH 4 /AlCl 3 or LiAlH 4 affords the alkyl derivatives 5b and 5h (R ) Et) or the alcohols 5i (R ) CH 2 OH) and 5j (R ) CH(OH)Me), respectively. The reaction of 1 with fluorene/AlCl 3 yields complex [Cb*Co-(η 6 -fluorene)] + (8), which was deprotonated by KOBu t to give Cb*Co(η 6 -fluorenyl) (9). Visible light irradiation of 9 induces η 6 fη 5 haptotropic rearrangement to afford Cb*Co(η 5 -fluorenyl) (7). The pyrrolyl and phospholyl complexes Cb*Co(C 4 R 4 N) (R ) H (10a), Me (10c)) and Cb*Co-(C 4 R 4 P) (R ) H ( 11a), Me (11c); R 4 ) H 2 Me 2 (11b)) were obtained by reaction of 2 with the corresponding pyrrolide or phospholide anions. Improved procedures for the preparation of the starting materials 1 and 2 were developed. Using a one-pot procedure, the iodide 1 was obtained in high yield from 2-butyne and Co 2 (CO) 8 . Complex 2 was prepared by heating or irradiation of the toluene complex [Cb*Co(C 6 H 5 Me)] + (4b) in acetonitrile. Structures of 5g, 6, and 11c were investigated by X-ray diffraction. Electrochemistry and joint UV-visible and EPR spectroelectrochemistry of complexes prepared were studied. Scheme 3Scheme 4
The coordination behavior, the conformational flexibility, and the stability of the novel (pyrazol-1-yl)borate ligands [Ph(pz)B(mu-N(Me))(mu-pz)B(pz)Ph](-) ([L(1)](-)), [Ph(pz)B(mu-O)(mu-pz)B(pz)Ph](-) ([L(2)](-)), and [Ph(pz)B(mu-O)(mu-OB(Ph)O)B(pz)Ph](2-) ([L(3)](2-)) have been experimentally assessed by investigating the following compounds: [Li(thf)L(1)], [Li(thf)L(2)], [Mg(Cl)(thf)(x)L(1)], [Mg(Cl)(thf)(2)L(2)], [Mn(CO)(3)L(2)], K(thf)[Mn(CO)(3)L(3)], [CoCl(2)(HL(1))], [L(1)Co(mu-Cl)(2)CoL(1)], [Zn(Br)L(1)], [Cu(Cl)L(1)], [Cu(Cl)L(2)]. The L(1)-complexes were prepared from a mixture of HL(1) and the appropriate metal salt by addition of a base. HCl elimination from [CoCl(2)(HL(1))], which gives [L(1)Co(mu-Cl)(2)CoL(1)], does not necessarily require the assistance of a base, but happens spontaneously when a solution of the complex is stored at room temperature for several days. K(thf)[Mn(CO)(3)L(3)] was obtained via in situ hydrolysis of HL(1)/[Mn(CO)(5)Br] in the presence of K(2)CO(3). In some other cases, formation of the coordination compounds proceeded with decomposition of a part of the ligand molecules and yielded pyrazole (e.g., [Zn(Cl)(Hpz)L(2)]) or pyrazolide (e.g., [L(2)Co(mu-Cl)(mu-pz)CoL(2)]) complexes. As evidenced by the crystal structure analyses of [Zn(Br)L(1)]/[Mg(Cl)(thf)(2)L(2)]/[Mn(CO)(3)L(2)] on the one hand and [L(1)Mg(mu-Cl)(2)Mg(thf)L(1)]/[Cu(Cl)L(1)]/[Cu(Cl)L(2)](2) on the other, [L(1)](-) and [L(2)](-) are able to adopt both a facial and a meridional conformation. Moreover, while many of the established design principles of scorpionate chemistry are still valid for [L(1)](-), [L(2)](-), and [L(3)](2-), the bonding situation of the central donor moiety (N(Me) in [L(1)](-); O in [L(2)](-), [L(3)](2-)) is distinctly different from the way the pyrazolyl rings are attached to the molecule, so that donor scrambling is not an issue in these [N,N,N] and [N,O,N] mixed-donor ligands.
The reaction of the tridentate [N,O,N] (pyrazol-1-yl)borate ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph] -([L 1 ] -) with [Cp*RuCl] 4 and [(p-cym)RuCl 2 ] 2 gives the Ru II complexes [Cp*Ru(L 1 )] and [(p-cym)-Ru(L 1 )]Cl, respectively (pz = pyrazolyl, Cp* = pentamethylcyclopentadienyl, p-cym = p-cymene). In order to avoid degradation of the [(p-cym)Ru(L 1 )] þ complex in solution, its Clcounterion has been exchanged for PF 6-, [B(C 6 F 5 ) 4 ] -, tosylate, and triflate. When the reaction between [L 1 ]and [(p-cym)Ru-Cl 2 ] 2 is carried out in the presence of 4 equiv of TlPF 6 , the dinuclear pyrazolyl-bridged complex [(p-cym)-Ru(μ-Cl)(μ-pz) 2 Ru(p-cym)]PF 6 and the mononuclear speciesIn a targeted synthesis, the lithium salt of the novel ligand [L 2 ] 2was prepared from 2 equiv of Lipz and 2,4,6-Ph 3 B 3 O 3 and successfully transformed into [(p-cym)Ru(L 2 )].[Cp*Ru(L 1 )], [(p-cym)Ru(L 1 )]PF 6 , and [(p-cym)Ru(L 2 )] have been characterized by NMR spectroscopy, X-ray crystallography, and (spectro)electrochemistry. One-electron oxidation of [Cp*Ru(L 1 )] by electrochemical or chemical ([Cp 2 Fe]PF 6 ) means leads to the Ru III species [Cp*Ru(L 1 )]PF 6 , which has been isolated and fully characterized (E 1/2 (Ru II /Ru III ) = -0.39 V; CH 2 Cl 2 , vs FcH/FcH þ ). A comparison of the solid-state structures of [Cp*Ru(L 1 )] and [Cp*Ru(L 1 )]PF 6 reveals that oxidation of the ruthenium center results in a lengthening of the average Ru-Cp* distances and a shortening of all Ru-L 1 bond lengths. According to the X-ray data, the angle strain within [(p-cym)Ru(L 1 )]PF 6 is higher than in [(p-cym)Ru(L 2 )], which could account for the fact that [L 1 ]is apparently less stable than [L 2 ] 2-.
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