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Ring‐Borylated 15‐Electron and 17‐Electron ansa ‐Chromocene Complexes, their Physical Properties and Molecular Structures
Author(s) -
Shapiro Pamela J.,
Sinnema PietJan,
Perrotin Philippe,
Budzelaar Peter H. M.,
Weihe Høgni,
Twamley Brendan,
Zehnder Ralph A.,
Nairn Justin J.
Publication year - 2007
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/chem.200601901
Subject(s) - toluene , chemistry , thermal decomposition , ring (chemistry) , isocyanide , crystallography , carbon monoxide , decomposition , electron , stereochemistry , medicinal chemistry , catalysis , physics , organic chemistry , quantum mechanics
A detailed study of the thermal decomposition of the zwitterionic, ring‐borylated ansa ‐chromocene hydrido carbonyl complex [Cr(CO)H{Me 4 C 2 (C 5 H 4 )[C 5 H 3 B(C 6 F 5 ) 3 ]}] ( 2 ) is described. This complex is formed in the reaction between [Cr(CO){Me 4 C 2 (C 5 H 4 ) 2 }] ( 1 ) and B(C 6 F 5 ) 3 in toluene at −78 °C. Above −25 °C, 2 decomposes to a 50:50 mixture of the low‐spin, 17 e Cr III complexes [Cr(CO){Me 4 C 2 (C 5 H 4 )[C 5 H 3 B(C 6 F 5 ) 3 ]}] ( 3 b ) and [Cr(CO){Me 4 C 2 (C 5 H 4 ) 2 }][HB(C 6 F 5 ) 3 ] ( 4 ). Carbon monoxide elimination from 3 b generates high‐spin, 15 e [Cr{Me 4 C 2 (C 5 H 4 )[C 5 H 3 B(C 6 F 5 ) 3 ]}] ( 3 a ), which coordinates two other electron‐donating ligands, such as xylyl isocyanide, PMe 3 , and PPh 2 Me to form the low‐spin, 17 e electron complexes 3 c , 3 d , and 3 e , respectively. High‐spin, 15 e [Cr{Me 4 C 2 (C 5 H 4 ) 2 }][HB(C 6 F 5 ) 3 ] ( 5 ) is generated by heating 3 b in toluene at 100 °C and periodically removing the evolved CO. Efforts to isolate more than a few X‐ray quality crystals of 5 were thwarted by its tendency to form an insoluble precipitate ( 6 ) with the same molecular formula. Heating the solution of 5 at 120 °C results in its partial conversion (ca. 28 %) to 3 a , thereby allowing the formation of 3 a in yields as high as 74 % from the reaction between 1 and B(C 6 F 5 ) 3 . The X‐ray crystal structures of 3 b – e and 5 are described. Cyclic voltammetry measurements on 3 a – e reveal a dramatic reduction in the redox potentials of the complexes relative to their non‐borylated analogues. DFT calculations show that this is due primarily to electrostatic stabilization of the oxidized species by the negatively charged borylate group. EPR and 19 F NMR spectroscopy allow 3 a to be distinguished from its Lewis base adducts 3 b – e and reveal the relative affinities of different Lewis bases for the chromium.