Premium
High‐Resolution EPR Spectroscopic Investigations of a Homologous Set of d 9 ‐Cobalt(0), d 9 ‐Rhodium(0), and d 9 ‐Iridium(0) Complexes
Author(s) -
Deblon Stephan,
Liesum Lorenz,
Harmer Jeffrey,
Schönberg Hartmut,
Schweiger Arthur,
Grützmacher Hansjörg
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020201)8:3<601::aid-chem601>3.0.co;2-c
Subject(s) - chemistry , iridium , rhodium , electron paramagnetic resonance , steric effects , intramolecular force , crystallography , cobalt , cis–trans isomerism , stereochemistry , inorganic chemistry , nuclear magnetic resonance , catalysis , biochemistry , physics
The 17‐electron complexes [M(tropp ph ) 2 ] (M=Co 0 , Rh 0 , Ir 0 ) were prepared and isolated (tropp= trop ylidene p hosphane). A structural analysis of [Co(tropp ph ) 2 ] revealed this complex to be almost tetrahedral, while the heavier homologues have more planar structures. Partially deuterated tropp complexes [D 6 ][M(tropp ph ) 2 ] were synthesised for M=Rh and Ir in order to enhance the resolution in the EPR spectra. This synthesis involves a four‐fold intramolecular C−H activation reaction, whereby alkyl groups are transformed into olefins. Dihydrides were observed as intermediates for M=Ir. The electronic and geometric structures of all complexes [M(tropp ph ) 2 ] (M=Co, Rh, Ir) and [D 6 ][M(tropp ph ) 2 ] (M=Rh, Ir) were investigated by continuous wave (CW) and echo‐detected EPR in combination with pulse ENDOR and ESEEM techniques. In accord with their planar structures, cis and trans isomers were detected for [M(tropp ph ) 2 ] (M=Rh 0 , Ir 0 ) for which a dynamic equilibrium was established. The thermodynamic data show that the cis isomer is slightly preferred by Δ H °=−4.7±0.3 kJ mol −1 (M=Rh) and Δ H °=−5.1±0.5 kJ mol −1 ; (M=Ir). The entropies for the process trans ‐[M(tropp ph ) 2 ]⇌ cis ‐[M(tropp ph ) 2 ] are also negative [Δ S °=−5±1.5 J mol −1 (M=Rh); Δ S °=−17±3.7 J mol −1 (M=Rh)], indicating higher steric congestion in the cis isomers. The cobalt(0) and irdium(0) complexes show rather large g anisotropies, while that of the rhodium(0) complex is small (Co: g ∥ =2.320, g ⊥ =2.080; cis ‐Rh: g ∥ =2.030, g ⊥ =2.0135; trans ‐Rh: g ∥ =2.050, g ⊥ =2.030; cis ‐Ir: g ∥ =2.030, g ⊥ =2.060; trans ‐Ir: g ∥ =1.980, g ⊥ =2.150). The g matrices of [M(tropp ph ) 2 ] (M=Co, Rh) are axially symmetric with g ∥ > g ⊥ , indicating either a distorted square planar structure (SOMO essentially d x 2 − y 2) or a compressed tetrahedron (SOMO essentially d xy ). Interestingly, for [Ir(tropp ph ) 2 ] the inverse ordering, g ⊥ > g ∥ , is found; this cannot be explained by simple ligand field arguments and must await a more sophisticated analysis. The hyperfine interactions of the unpaired electron with the metal nuclei, phosphorus nuclei, protons, deuterons and carbon nuclei were determined. By comparison with atomic constants, the spin densities on these centres were estimated and found to be small. However, the good agreement of the distance between the olefinic protons and the metal centres determined from the dipolar coupling parameter indicates that the unpaired electron is primarily located at the metal centre.