Cymantrene Radical Cation Family: Spectral and Structural Characterization of the Half-Sandwich Analogues of Ferrocenium Ion

Abstract

The anodic one-electron oxidation of three members of the half-sandwich family of piano-stool compounds MnCp (gamma)(CO) 3, where Cp (gamma) is a generic cyclopentadienyl ligand, has been studied in a CH 2Cl 2/[NBu 4][TFAB] electrolyte (TFAB = [B(C6F5) 4] (-)). The long-sought 17 e (-) radical cation of the parent complex MnCp(CO) 3 (cymantrene, 1, E 1/2 = 0.92 V vs ferrocene) has been shown to be persistent in solutions that use weakly coordinating anions in place of more nucleophilic traditional electrolyte anions. Spectroscopically characterized for the first time, 1 (+) was shown to absorb in the visible (530 nm), near-IR (2066 nm), and IR (2118, 1934 cm (-1)) regions. It was ESR-active at low temperatures (g parallel = 2.213, g perpendicular = 2.079, A parallel (Mn) = 79.2 G, A perpendicular (Mn) = 50 G) and NMR active at room temperature (delta = 22.4 vs TMS). The radical cations of the Cp-functionalized analogues, Mn(eta (5)-C5H 4NH2)(CO) 3, 2, E 1/2 = 0.62 V, and MnCp*(CO) 3 (Cp*= eta (5)-C 5Me 5, 3), E 1/2 = 0.64 V, were generated electrochemically as well by the chemical oxidant [ReCp(CO) 3] (+). The structures of 2 (+) and 3 (+) were determined by X-ray crystallographic studies of their TFAB salts. Compared to the structures of the corresponding neutral compounds, the cations showed elongated Mn-C(O) bonds and shortened C-O bonds, displaying the effect of diminished metal-to-CO backbonding. The bond-length changes in the Mn(CO) 3 moiety were much larger in 3 (+) (avg changes, Mn-C(O) = + 0.142 A, C-O = -0.063 A) than in 2 (+) (avg changes, Mn-C(O) = + 0.006 A, C-O = -0.003 A). Although there were only minor changes in the metal-to-center ring distances upon oxidation of either 2 or 3, there was decidedly less bending of the C(N) atom out of the cyclopentadienyl plane in 2 (+) compared to 2. The optical, vibrational, and magnetic resonance spectra of radicals 2 (+) and 3 (+) were also observed. The spectral data argue for the SOMOs of the 17-electron species being largely located on the Mn(CO) 3 moiety, having 40-50% Mn d-orbital character, with the ground states of the radicals, most likely (2)A'', lying close in energy (within about 6000 cm (-1)) to excited states that are responsible for their rapid electronic relaxations. The cymantrenyl moiety is proposed as an anodic redox tag (or label) having physical and chemical properties that are significantly different from those of its ferrocenyl analogue.

Redox Behaviour of Cymantrene Fischer Carbene Complexes in Designing Organometallic Multi‐tags

“…Chemical oxidation of the Mn(I) centre of 3a was achieved by the use of AgPF 6 [17] (Scheme 2) to yield the radical Mn(II) species [3a][PF 6 ]. All neutral compounds were purified by column chromatography to give products as dark yellow to brown (Th, Fu) or dark red to maroon (Fc) solids.…”

“…Thus, the average shift of 116 cm -1 from that of neutral 3a compares favourably with that previously reported for cationic cymantrene complexes (~115 cm -1 ). [6] [a] C carbene ( 13 C NMR, ppm) [a] IR (cm -1 ) [ The DFT-calculated IR spectra for 3a and 3a •+ (Figure 1) reproduced the key carbonyl band shifts upon oxidation of 3a in the experimental spectrum to within 11 and 16 cm -1 for the antisymmetric and symmetric stretching modes, respectively. Moreover, the calculated peak separation between the two CO modes for the Mn(I) complex (68 cm -1 , 3a) and Mn(II) complex (70 cm -1 , 3a •+ ) fell within 6 wavenumbers of the experimental peak separation.…”

“…The DFT-calculated Mn(I/II) coordination geometries ( Figure 1) are in excellent agreement with the X-ray data reported for the neutral and oxidized forms of the cymantrene derivative Mn(η 5 -C 5 H 4 NH 2 )(CO) 3 . [6] For example, the Mn-CO distances average 1.80(1) Å in the experimental structure (the data do not allow a distinction between Mn I and Mn II to be made). In the simulated structures of 3a and 3a •+ , the Mn-CO bonds contract from 1.83(1) Å to 1.77(1) Å upon oxidation of the metal.…”

“…The assignment of the thienyl and furyl ring proton chemical shifts is based on assignments following predicted shifts for ester derivatives. [19] No usable NMR spectra could be obtained for the paramagnetic [3a][PF 6 ]. The expected band pattern [20] associated with the carbonyl stretches of a [CpMn(CO) 2 L] system could be observed in the infrared spectra of all complexes, with very similar stretching frequencies ( Table 1).…”

“…[4] The electron-withdrawing properties and IR-active carbonyl groups of a cymantrenyl moiety, Mn(η 5 -C 5 H 4 )(CO) 3 , [5] make this organometallic compound an interesting and significantly different alternative as a molecular tag, although its redox chemistry has been underutilized. [6] Despite the use of cymantrenyl cathodic chemistry for immunoassays, [7] applications of cymantrenyl anodic chemistry are hitherto lacking. This dearth can be attributed not only to the high oxidation potential of the cymantrene unit (ca.…”