New Niobocene Alkyne Complexes:  Synthesis and Characterization of Neutral and Cationic Niobium Complexes with Functionalized Alkynes. X-ray Crystal Structure of [Nb(η⁵-C₅H₄SiMe₃)₂(Cl)(η²(C,C)-R¹C⋮CR²)] (R¹= C⋮CPh, R²= Ph (2b); R¹= CH₂CHC(CH₃)₂, R²= Ph (3b))

Abstract

The diyne-containing niobocene complexes Nb(η5-C5H4SiMe3)2Cl(η2(C,C)-R1C⋮CR2) (R1 = C⋮CSiMe3, R2 = SiMe3 (2a); R1 = C⋮CPh, R2 = Ph (2b); R1 = CH2CH2C⋮CH, R2 = H (2c)) were prepared by the reaction of Nb(η5-C5H4SiMe3)2Cl (1) with the diyne reagents, R−C⋮C−C⋮C−R (R = SiMe3 or Ph) and 1,5-hexadiyne. The enyne-containing niobocene complexes Nb(η5-C5H4SiMe3)2Cl(η2(C,C)-R1C⋮CR2) (R1 = CH2CHCH2, R2 = Ph (3a); R1 = CH2CHC(CH3)2, R2 = Ph (3b); R1 = CH2C(CH3)CH2, R2 = Ph (3c); R1 = C(CH3)CH2, R2 = H (3d)) were also prepared in the same way, namely by the reaction of 1 with the appropriate enyne reagents. The paramagnetic niobocene complex Nb(η5-C5H4SiMe3)2(η2(C,C)-R1C⋮CR2) (R1 = C⋮CSiMe3, R2 = SiMe3 (4a)) was also synthesized from 2a and subsequently oxidized in the presence of different neutral ligands to give the stable cationic d2 derivatives [Nb(η5-C5H4SiMe3)2(η2(C,C)-R1C⋮CR2)(L)][BPh4] (L = CH3CN (5a); L = tBuNC (5a‘)). Some diyne- and enyne-containing chloroniobocenes were alkylated by reacting with the appropriate dialkylmagnesium reagent to give the corresponding alkyl−alkyne compounds Nb(η5-C5H4SiMe3)2(R3)(η2(C,C)-R1C⋮CR2) (R1 = C⋮CSiMe3, R2 = SiMe3, R3 = Me (6a); R1 = CH2CHCH2, R2 = Ph, R3 = CH2Ph (7a); R1 = CH2CHC(Me)2, R2 = Ph, R3 = Me (7b)). Furthermore, complex 2b undergoes clean reaction with Co2(CO)8 to give the complex Nb(η5-C5H4SiMe3)2Cl(η2(C,C)-R1C⋮CR2) (R1 = CCPh(Co(CO)3)2, R2 = Ph (8b)) through the coordination of both Co(CO)3 moieties to the uncoordinated triple bond. Finally, the enyne-containing niobocene complexes 3a and 3b can be hydrogenated at the CC bond using Pd/C as catalyst to give the alkyne complexes Nb(η5-C5H4SiMe3)2Cl(η2(C,C)-R1C⋮CR2) (R1 = CH2CH2CH3, R2 = Ph (9a); R1 = CH2CH2CH(CH3)2, R2 = Ph (9b)). The structures of 2b and 3b were determined by single-crystal diffractometry.

Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes

“…Otero and Royo also showed that, in the case of (Cp Si ) 2 NbCl 2 ( 27 ), the use of 1 equiv of reducing agent afforded the isolable Nb III monochloride (Cp Si ) 2 NbCl ( 28 ), as shown in Scheme . While the addition of 1 equiv of alkyne generated the desired alkyne complex (Cp Si ) 2 M­(RCCR)Cl ( 29 ), treating 28 with 1 equiv of a diyne, such as 1,4-diphenylbuta-1,3-diyne, afforded the related complex Cp′ 2 M­(RCCCCR)­Cl ( 30 ) …”

“…59 While the addition of 1 equiv of alkyne generated the desired alkyne complex (Cp Si ) 2 M(RCCR)Cl (29), treating 28 with 1 equiv of a diyne, such as 1,4-diphenylbuta-1,3-diyne, afforded the related complex Cp′ 2 M(RCCCCR)Cl (30). 60 Although the majority of work in this field has focused on cyclopentadienyl-containing systems such as the ones discussed above, alkyne moieties have been installed via a similar reduction protocol in Ta and Nb complexes featuring a variety of ancillary ligand sets. For example, McConville and co-workers 30 investigated the synthesis of Ta alkyne complexes featuring a pyridine-diamido pincer ligand, BDPP, as shown in Scheme 9.…”

“…86,87 Similar to the case for complexes 52−54, Schrock and co-workers reported 88 the synthesis of a niobium alkyne chloride complex that is also believed to proceed via a reductive elimination step (Scheme 15). The hydrogenolysis of Cp*NbMe 2 Cl 2 afforded the Nb III dimer [Cp*NbCl 2 ] 2 (59), likely via the loss of H 2 from the putative Nb V dihydride Cp*Nb(H) 2 Cl 2 (58); treating complex 59 with 3-hexyne generated the mononuclear Nb alkyne complex Cp*Nb(3-hexyne)Cl 2 (60). In contrast to the Schwartz−Herberich or Rothwell systems (52−54), where the low-valent M III intermediate is generated in situ, complex 59 is stable enough to be isolated, similar to the case for complexes 28 (Otero 59 ) and 33 (Wolczanski 61 ).…”

11 Vanadium, niobium and tantalum

“…76 Syntheses, characterisation and some reactions have been reported for [NbCl(Z 2 (C,C)-R 1 CCR 2 )(Z 5 -C 5 H 4 SiMe 3 ) 2 ] (R 1 CCSiMe 3 , R 2 SiMe 3 ; R 1 CCPh, R 2 Ph; R 1 CH 2 CH 2 CCH, R 2 H). 77 The reactions of [NbCl 4 (Z 5 -C 5 H 4 SiMe 3 )] and [TaCl 2 (Me) 2 Cp*] with NHRSiMe 3 and CNR have been studied for a range of R groups. 78 Syntheses of the N-alkoxy-b-ketoiminate complexes [Nb(Z 5 -C 5 H 4 R){CH 3 C(O)-CHC(NCH 2 CHR H O)CH 3 }Cl 2 ] (R H, Me; R H H, Me), have been described.…”

Niobium- and zirconium-catalyzed reactions of substituted 2 alkynylamines with Et2Zn

“…From analysis of the published data, it follows that Zn, 39 Mg, Al, [40][41][42][43] and Na/Hg amalgam [44][45][46][47][48][49] are the most popular reducing agents suitable for efficient generation of low-valent niobium and tantalum in the presence of alkyne molecules. Oshima and co-workers reported for the rst time that the reaction of nonfunctionalized disubstituted acetylenes with low-valent niobium, generated by reduction of NbC1 5 with NaAlH 4 , results in the selective formation of monodeuterated Z-olens.…”