Photochemistry of (η⁵-C₅H₅)(η⁵-C₄H₄N)Fe and (η⁵-C₅H₅)(η¹-N-C₄H₄N)Fe(CO)₂ in Low-Temperature Matrixes and Room-Temperature Solution. Evidence for a Photoinduced Haptotropic Shift of the π-Coordinated Pyrrolyl Ligand

Abstract

Azaferrocene, (η 5 -C 5 H 5 )(η 5 -C 4 H 4 N)Fe, undergoes a η 5 f η 1 haptotropic shift of the pyrrolyl ligand upon long-wavelength photolysis (λ exc > 495 nm) both in alkane solvents at room temperature and in frozen matrixes at 12 K. Room-temperature photolysis (λ exc > 495 nm) in CO-saturated cyclohexane solution generated (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)Fe(CO) 2 . Irradiation with λ exc ) 532 nm also produced an allyl monocarbonyl species, exo-(η 5 -C 5 H 5 )(η 3 -C-C 4 H 4 N)-Fe(CO), identified by IR spectroscopy. In CO-doped matrixes at 12 K both (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)Fe(CO) and (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)Fe(CO) 2 are formed following broad-band irradiation (λ exc > 495 nm) of (η 5 -C 5 H 5 )(η 5 -C 4 H 4 N)Fe, in a ratio dependent on the concentration of CO in the matrix. Initial irradiation with λ exc ) 538 nm followed by broad-band photolysis (λ exc > 495 nm) in CO-doped matrixes formed additional monocarbonyl species, exo-(η 5 -C 5 H 5 )-(η 3 -C-C 4 H 4 N)Fe(CO), and a species absorbing at 1962 cm -1 , which is either the appropriate endo-isomer or aza-allyl species. Laser flash photolysis experiments of (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)-Fe(CO) 2 in either CO-saturated cyclohexane or toluene produced (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)Fe-(CO), which reacted with CO with rate constants measured at 298 K of (3.0 ( 0.3) × 10 8 and (3.3 ( 0.3) ×10 8 M -1 s -1 , respectively, regenerating (η 5 -C 5 H 5 )(η 1 -N-C 4 H 4 N)Fe(CO) 2 .

The synthesis and electrochemistry of 2,5-dimethylazaferrocenes with heteroaryl bridges

“…There remains the question whether this inclination reflects the known tendency of the pyrrole ligand to undergo a haptotropic g 5 ? g 1 shift [28].…”

Hartree–Fock and density functional theory study of remote substituent effects on heterolytic Fe–N bond energies of p‐G‐C6H4NHFe(CO)2(η5‐C5H5) and p‐G‐C6H4N(COMe)Fe(CO)2(η5‐C5H5)

“…For instance, evidence for a photoinduced haptotropic shift of the π‐coordinated pyrrolyl ligand has been found. Azaferrocene, (η 5 ‐C 5 H 5 )(η 5 ‐C 4 H 4 N)Fe, undergoes a η 5 → η 1 haptotropic shift of the pyrrolyl ligand upon long‐wavelength photolysis ( λ exc 495 nm) both in alkane solvents at room temperature and in frozen matrixes at 12 K. Room‐temperature photolysis ( λ exc 495 nm) in CO‐saturated cyclohexane solution generated (η 5 ‐C 5 H 5 )(η 1 ‐N‐C 4 H 4 N)Fe(CO) 2 . Transition metallo‐carbonyl complexes are useful bioprobes to study molecular recognition processes such as ligand–receptor interactions, owing to their intense absorption bands in the mid‐IR spectral range, which enable their detection at the picomole level with FT‐IR spectrometers.…”

Synthesis of Coordinatively Unsaturated Tetravalent Actinide Complexes with η5 Coordination of Pyrrole

“…While pyrrole is widely used in coordination complexes incorporating virtually all metals of the transition block, most of these show pyrrole to be bound in an η 1 fashion, directly forming a nitrogen–metal bond. − Interestingly, there are still many examples of η 5 -bound pyrrole in transition metal chemistry; however, it is often seen that such ligation is vulnerable to substitution reaction or haptotropic shifts. − The selected examples in which these side reactions are absent are mainly seen in early transition metal chemistry, and studies of such complexes elucidate the highly electrophilic nature of the metal and steric environment that are required to yield the pentahapto pyrrolyl-metal moiety. , Although studies of actinide-pyrrolyl complexes date as far back as 1974 with Marks’ preparation of tetrakis­(2,5-dimethylpyrrolyl)­uranium­(IV), it is only in the past decade that pyrrolyl ligands have been reintroduced into actinide chemistry and the focus of continued investigation . In the first of these studies, Boncella et al revealed a U VI center coordinated to a bridged dipyrrolylmethane (dpm) ligand.…”