Organometallic Isocyanocyclopentadienides:  A Combined Synthetic, Spectroscopic, Structural, Electrochemical, and Theoretical Investigation

Abstract

This article reports on the chemistry of two organometallic isocyanocyclopentadienides, which represent an emerging new class of aromatic isocyanides incorporating nonbenzenoid π-systems. Interaction of aminoferrocene with a mixture of phenyl formate/phenol followed by subsequent dehydration of the resulting ferrocenylformamide with POCl 3 produced air and thermally stable isocyanoferrocene (CNFc, Fc ) ferrocenyl) in a high yield. Treating lithiocymantrene, LiCm (Cm ) (η 5 -C 5 H 4 )Mn(CO) 3 ), with tosyl azide afforded thermally sensitive cymantrenyl azide. Without isolation, CmN 3 was reduced by NaBH 4 to form aminocymantrene, which was converted into air stable but thermally and light sensitive isocyanocymantrene, CNCm. Combining 6 equiv of CNR (R ) Fc, Cm) with bis(naphthalene)chromium(0) afforded Cr(CNR) 6 . Successive one-electron oxidations of Cr(CNR) 6 with Ag + produced the corresponding paramagnetic [Cr(CNR) 6 ] + and [Cr(CNR) 6 ] 2+ . The compounds [Cr(CNR) 6 ] 0,1+,2+ (R ) Fc, Cm) are remarkable due to the incorporation of seven transition metal atoms within relatively compact ML 6 motifs. The physical, chemical, electrochemical, and spectroscopic properties of the structurally characterized series [Cr(CNFc) 6 ] 0,1+,2+ indicate that the electronic influence of the ferrocenyl moiety, often compared to an alkyl group, is in fact more similar to that of aryl substituents. Electrochemical properties of [Cr(CNR) 6 ] 0,1+,2+ (R ) Fc, Cm) are consistent with isocyanocymantrene being a substantially stronger π-acid than isocyanoferrocene. This conclusion was unambiguously corroborated by a DFT analysis of the Frontier molecular orbitals of CNFc and CNCm. Unpaired spin delocalization within odd-atom, nonbenzenoid aromatic π-systems of [Cr(CNR) 6 ] 1+,2+ (R ) Fc, Cm) was studied by multinuclear paramagnetic NMR and contrasted with patterns observed for similar complexes incorporating benzenoid and even-atom, nonbenzenoid aromatic moieties.

Application of Lever’s EL Parameter Scale toward Fe(II)/Fe(III) versus Pc(2-)/Pc(1-) Oxidation Process Crossover Point in Axially Coordinated Iron(II) Phthalocyanine Complexes

“… a Ref . b Estimated on the basis of constant difference in Tz/Tz – and BzIm/BzIm – pairs from ref . c Ref . d Assumed to be equal to the E L value of DMF (ref ). e Estimated based on the redox properties in the series of chromium compounds from refs and − . f Assumed to be equal to the E L value in P­(OMe) 3 (ref ). …”

Tuning π-Acceptor/σ-Donor Ratio of the 2-Isocyanoazulene Ligand: Non-Fluorinated Rival of Pentafluorophenyl Isocyanide and Trifluorovinyl Isocyanide Discovered

“…The pertinent metric data for the above new [(OC) 5 Cr(CNR)] species, along with those for [(OC) 5 Cr(CN t Bu)] [ 32 ], [(OC) 5 Cr(CNC 2 F 3 )] [ 10 ], and [(OC) 5 Cr(CN p -FAr DArF2 )] [ 6 ], are collated in Table 2 . For 6 , 7 , 8 , and 10 , the Cr–CNR bond length, which reflects the π-acceptor/σ-donor ability of the CNR ligand (particularly, the extent of π back-bonding interaction between the chromium(0) center and the isocyanide) [ 33 ], is significantly shorter than that in [(OC) 5 Cr(CN t Bu)]. Moreover, the Cr–CNR bond distance of 1.937(2) Å in 10 is 0.03 Å shorter than the corresponding Cr–C bond length in [(OC) 5 Cr(CN p -FAr DArF2 )] and even slightly shorter (albeit at the edge of the 3σ criterion margin) than the Cr–CNR bond distance of 1.942(2) Å documented for perfluorinated [(OC) 5 Cr(CNC 2 F 3 )] ( Table 2 ).…”

Cymantrene–Triazole “Click” Products: Structural Characterization and Electrochemical Properties

“…In our approach to synthesizing various triazole-derivatized cymantrene complexes, we identified two potential retrosynthetic starting points: either derivatizing the cyclopentadiene ring with a pendant alkyne group or derivatizing the cyclopentadiene ring with an azide group. We note that Geiger has recently introduced pendant alkyne groups into cymantrene complexes and used these as linkers for surface attachment, 18 while Holovics 19 and Sunkel 20 have both previously reported the synthesis of cymantrenyl azide 3 in good yields, and in each case, handled the product as a crude mixture to be used in subsequent synthetic steps. Given that our ultimate aim is to form organometallic triazoles for electrode surface modification, we opted for the latter approach, starting from the cymantrene azide 2, for several reasons.…”

“…Unless stated otherwise, all reagents were purchased from Sigma-Aldrich (Gillingham, U.K.) and used as supplied, with the exception of cymantrene (Strem Chemicals, Cambridge, U.K.) and 4-ethynylaniline and 3-ethynylaniline (Fluorochem, Derbyshire, U.K.). p -Toluenesulfonyl azide and cymantrene azide 2 , were prepared according to literature procedures. All nonaqueous solvents were degassed to remove dissolved oxygen and rigorously dried prior to use by distillation under N 2 over the appropriate drying agent: tetrahydrofuran was distilled over sodium benzophenone ketyl; toluene was distilled over sodium wire; dichloromethane and petroleum ether (40/60) were distilled over calcium hydride.…”

“…We note that Geiger has recently introduced pendant alkyne groups into cymantrene complexes and used these as linkers for surface attachment 18 whilst Holovics 19 and Sünkel 20 have both previously reported the synthesis of cymantrenyl azide 3 in good yields, and in each case, handled the product as a crude mixture to be used in subsequent synthetic steps. Given that our ultimate aim is to form organometallictriazoles for electrode surface modification, we opted for the latter approach, starting from the cymantrene azide, 2, for several reasons.…”