Studies on Chemical Reactivity and Electrocatalysis of Two Acylmethyl(hydroxymethyl)pyridine Ligand-Containing [Fe]-Hydrogenase Models (2-COCH₂-6-HOCH₂C₅H₃N)Fe(CO)₂L (L = η¹-SCOMe, η¹-2-SC₅H₄N)

Abstract

On the basis of preparation and characterization of [Fe]-Hase models (2-COCH-6-HOCHCHN)Fe(CO)L (A, L = η-SCOMe; B, L = η-2-SCHN), the chemical reactivities of A and B with various electrophilic and nucleophilic reagents have been investigated, systematically. Thus, when A reacted with 1 equiv of MeCOCl in the presence of EtN in MeCN to give the η-SCOMe-coordinated acylation product (2-COCH-6-MeCOCHCHN)Fe(CO)(η-SCOMe) (1), treatment of A with excess HBF·EtO in MeCN gave the cationic MeCN-coordinated complex [(2-COCH-6-HOCHCHN)Fe(CO)(MeCN)](BF) (2). In addition, when 2 was treated with 1 equiv of 2,6-(p-4-MeCH)CHSK or PPh in CHCl to give the thiophenolato- and PPh-substituted derivatives (2-COCH-6-HOCHCHN)Fe(CO)[2,6-(p-MeCH)CHS] (3) and [(2-COCH-6-HOCHCHN)Fe(CO)(PPh)](BF) (4), treatment of B with 1 equiv of PMe or P(OMe) in THF afforded the phosphine- and phosphite-substituted complexes (2-COCH-6-HOCHCHN)(η-2-SCHN)Fe(CO)L (5, L = PMe; 6, L = P(OMe)). Interestingly, in contrast to A, when B reacted with excess HBF·EtO in MeCN to afford the BF adduct [2-COCH-6-HO(BF)CHCHN]Fe(CO)(η-2-SCHN) (7), reaction of B with 1 equiv of p-MeCHCOCl in the presence of EtN in MeCN gave not only the expected 2-acylmethyl-6-p-toluoyloxomethylpyridine-containing complex (2-COCH-6-p-MeCHCOCHCHN)Fe(CO)(η-2-SCHN) (8), but also gave the unexpected 2-toluoyloxovinyl-6-toluoyloxomethylpyridine-containing complex (2-p-MeCHCOCH-6-p-MeCHCOCHCHN)Fe(CO)(η-2-SCHN) (9). While the possible pathways for the novel reactions leading to complexes 1, 2, and 7-9 are suggested, the structures of complexes B, 1-4, and 6-9 were unambiguously confirmed by X-ray crystallography. In addition, model complexes A and B have been found to be catalysts for proton reduction to H from TFA under CV conditions.

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

“…Furthermore, deprotonation of the methylene-acyl moiety has been observed in another model compound by Song and coworkers through a suggested keto–enol tautomerization and acylation mechanism, although the analogous intermediate was not identified in that case. 27 These observations suggest that this acyl moiety is rather reactive, and must be fully understood in structural and functional synthetic mimics of this enzyme. Indeed, exogenous base has been noted to decompose previous non-scaffolded acyl-containing model compounds, 16 perhaps related to this process.…”

Synthetic and Structural Studies on a New Type of [Fe]‐Hydrogenase Mimics Each Containing One Hantzsch Ester Moiety

“…In the 13 C{ 1 H} NMR spectra of L 1 and L 2 each displayed one singlet at 168.1 ppm for their ester carbonyl C atoms, whereas 1 exhibited two singlets at 166.2 and 174.9 ppm for its ester carbonyl C atoms, two singlets at 211.7 and 213.3 ppm for its terminal carbonyl C atoms, and one singlet at 259 ppm for its acyl carbonyl C atoms attached to its Fe(II) center. [21,36] In addition, the IR spectra of L 1 and L 2 showed one strong absorption band at 1684 and 1697 cm -1 respectively for their ester carbonyls, while 1 exhibited two strong absorption bands at 1709 and 1610 cm -1 for its ester carbonyls, two strong bands at 2032 and 1968 cm -1 for its terminal carbonyls, and one strong band at 1646 cm -1 for its acyl carbonyls. [21,36] Interestingly, the molecular structures of precursor L 2 and model 1 were further confirmed by X-ray crystallography.…”

“…[21,36] In addition, the IR spectra of L 1 and L 2 showed one strong absorption band at 1684 and 1697 cm -1 respectively for their ester carbonyls, while 1 exhibited two strong absorption bands at 1709 and 1610 cm -1 for its ester carbonyls, two strong bands at 2032 and 1968 cm -1 for its terminal carbonyls, and one strong band at 1646 cm -1 for its acyl carbonyls. [21,36] Interestingly, the molecular structures of precursor L 2 and model 1 were further confirmed by X-ray crystallography. While Figure 1 and Figure 2 show their molecular structures, Table 1 lists their selected bond lengths and angles.…”

“…In the 1 H NMR spectrum, precursor L 3 displayed one singlet at 10.12 ppm for its CHO group and at 4.95 ppm for its OCH 2 group, precursors L 4 -L 6 exhibited one singlet in the range 2.26-2.34 ppm for the two CH 3 groups attached to their Hantzsch ester rings and one singlet in the range 4.70-5.20 ppm for the OCH 2 group attached to their pyridine rings, and model 2 displayed two singlets at 2.33/2.35 ppm for the two CH 3 groups attached to its Hantzsch ester ring and the typical coupling pattern of an AB spin system at 4.53/ 4.93 ppm for the acylmethyl COCH 2 group attached to its pyridine ring, respectively. In the 13 C{ 1 H} NMR spectra, while precursor L 3 displayed one singlet at 192.3 ppm for its formyl C atom, model 2 exhibited one singlet at 168.4 ppm for its ester carbonyl C atoms, three singlets in the region 195-211 ppm for its three terminal carbonyl C atoms, and one singlet at 257.2 ppm for its acyl carbonyl C atom, [21,36] respectively. In addition, the IR spectrum of L 3 showed one strong absorption band at 1702 cm -1 for its CHO group, while model 2 displayed one strong absorption band at 1700 cm -1 for its ester carbonyl groups, three strong bands in the range 2079-1974 cm -1 for its terminal carbonyls, and one strong band at 1636 cm -1 for its acyl carbonyl group, [21,36] respectively.…”

“…Although synthetic chemists in bioinorganic community have so far prepared a variety of [Fe]-H 2 ase models, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] of them contains an hydride donor such as the well-known Hantzsch ester HEH (2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate) moiety. [29] Previously, we and others reported a type of [FeFe]-H 2 ase models, in which a simple [FeFe]-H 2 ase model skeleton is attached to a photosensitizer moiety such as terpyridyl Ru(II) complex, [30] porphyrin, [31] metalloporphyrin, [32] and fullerene C 60 .…”