Synthesis, Characterization, and Solution Redox Properties of (trimpsi)M(CO)₂(NO) Complexes [M = V, Nb, Ta; trimpsi = ^tBuSi(CH₂PMe₂)₃]

Abstract

Treatment of [Et(4)N][M(CO)(6)] (M = Nb, Ta) with I(2) in DME at -78 degrees C produces solutions of the bimetallic anions [M(2micro-I)(3)(CO)(8)](-). Addition of the tripodal phosphine (t)BuSi(CH(2)PMe(2))(3) (trimpsi) followed by refluxing affords (trimpsi)M(CO)(3)I [M = Nb (1), Ta (2)], which are isolable in good yields as air-stable, orange-red microcrystalline solids. Reduction of these complexes with 2 equiv of Na/Hg, followed by treatment with Diazald in THF, results in the formation of (trimpsi)M(CO)(2)(NO) [M = Nb (3), Ta (4)] in high isolated yields. The congeneric vanadium complex, (trimpsi)V(CO)(2)(NO) (5), can be prepared by reacting [Et(4)N][V(CO)(6)] with [NO][BF(4)] in CH(2)Cl(2) to form V(CO)(5)(NO). These solutions are treated with 1 equiv of trimpsi to obtain (eta(2)-trimpsi)V(CO)(3)(NO). Refluxing orange THF solutions of this material affords 5 in moderate yields. Reaction of (trimpsi)VCl(3)(THF) (6) with 4 equiv of sodium naphthalenide in THF in the presence of excess CO provides [Et(4)N][(trimpsi)V(CO)(3)] (7), (trimpsi)V(CO)(3)H, and [(trimpsi)V(micro-Cl)(3)V(trimpsi)][(eta(2)-trimpsi)V(CO)(4)].3THF ([8][9].3THF). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2.(1)/(2)THF, 3-5, and [8][9].3THF have been established by X-ray diffraction analyses. The solution redox properties of 3-5 have also been investigated by cyclic voltammetry. Cyclic voltammograms of 3 and 4 both exhibit an irreversible oxidation feature in CH(2)Cl(2) (E(p,a) = -0.71 V at 0.5 V/s for 3, while E(p,a) = -0.55 V at 0.5 V/s for 4), while cyclic voltammograms of 5 in CH(2)Cl(2) show a reversible oxidation feature (E(1/2) = -0.74 V) followed by an irreversible feature (0.61 V at 0.5 V/s). The reversible feature corresponds to the formation of the 17e cation [(trimpsi)V(CO)(2)(NO)](+) ([5](+)()), and the irreversible feature likely involves the oxidation of [5](+)() to an unstable 16e dication. Treatment of 5 with [Cp(2)Fe][BF(4)] in CH(2)Cl(2) generates [5][BF(4)], which slowly decomposes once formed. Nevertheless, [5][BF(4)] has been characterized by IR and ESR spectroscopies.

A journey into the electrochemistry of vanadium compounds

“…Exploration of nitrosyl cation as ligand for group 15 metals lead to the isolation, among alia, of the neutral 18 electrons species V(trimpsi)(CO) 2 (NO), where trimpsi is tris(dimethylphosphanomethyl)-tert-butylsilane [233]. Its cyclic voltammogram in DCM (Bu 4 NPF 6 as the supporting electrolyte) showed a reversible oxidation feature to the 17e cation [V(trimpsi)(CO) 2 (NO)] + , at −0.74 V (vs FcH + /FcH) followed by an irreversible feature (0.61 V vs FcH + /FcH) to an unstable 16e dication.…”

Laying the Groundwork for a Tantalum Dearomatization Agent

“…One species, Ta(trimpsi) (CO)2(NO) did have an electrochemical measurement reported (Ep,a=0.01 V vs. NHE). 14 This is similar to the hypothetical TaTpm(CO)(NO)(" 2 -benzene) in that both contained the metal fragment {Ta(tridentate ligand)(CO)(NO)}. {TaTpm(CO)(NO)} was chosen as our hypothetical metal fragment because it matched the characteristics for a dearomatization agent discussed in Chapter 1.…”

“…The II/I E1/2 of the TpRe(CO)(MeIm)(" 2 -benzene) complex was measured to be -0.16 V vs. NHE, supporting the hypothesis that electrochemical matching should be a useful tool in determining the proper ligand set for a dearomatization agent. 14 Furthermore, as evidenced by the ability of the bound benzene to undergo a cycloaddition with N-methylmaleimide when the benzene ligand of Os(NH3)5(" 2 -benzene) would not, the rhenium species was found to be more activating than the osmium system. 11 However, like the osmium metal fragment, the rhenium fragments had limitations that encouraged us to search for yet another dearomatization agent.…”

“…Legzdins and coworkers published the following synthesis of (trimpsi)Ta(CO)2(NO) (Scheme 3b.1). 14 Scheme 3b.1: Synthesis of (trimpsi)Ta(CO)2(NO). 14 Not only does the synthesis in Scheme 3b.1 demonstrate that the reduction and nitrosylation can be performed, but it also uses a tridentate phosphine ligand which can be seen as analogous to the desired Tp and Tpm ligands.…”

“…14 Scheme 3b.1: Synthesis of (trimpsi)Ta(CO)2(NO). 14 Not only does the synthesis in Scheme 3b.1 demonstrate that the reduction and nitrosylation can be performed, but it also uses a tridentate phosphine ligand which can be seen as analogous to the desired Tp and Tpm ligands. This left only the final step of the proposed synthesis (Scheme 2.6) undemonstrated; there were no reports of photolysis on a dicarbonyl nitrosyl tantalum species to give a different product.…”