Rare examples of structurally characterized five-coordinate organocobalt complexes. Novel dynamic NMR evidence for synergistic enhancement of cis and trans effects in B12 models

Abstract

A major factor influencing Co-C bond stability both in B12 compounds and organocobalt BI2 models appears to be the coordination number. Five-coordinate complexes are unusually stable to Co-C bond thermolysis, but structurally characterized examples of five-coordinate organocobalt complexes are rare. None are known for cases where R is a bulky group, such as i-C3H7. We report two definitive examples of five-coordinate complexes of the type Co(saloph)R, where saloph = dianion of disalicylidene-o-phenylenediamine. The structure of one species (R = i-C,H7) permits the first assessment of the effects of R bulk on the structure and also provides insight into the structural basis for the thermal stability of the species of reduced coordination number. We have also characterized the third example of a six-coordinate organocobalt saloph compound.Specifically, the crystal and molecular structures of Co(saloph)-i-C,H7-1.5H20 (I), Co(saloph)CH,.0.5H20 (11), and pyCo(saloph)CH2CF3 (py = pyridine) (111) are described. Compounds I and I11 are pentacoordinate and hexacoordinate, respectively, while crystals of I1 are comprised of both pentacoordinate and dimeric molecules. Crystallographic details follow. I: C23H21C~N202-1.5H20, space group Pbcn, a = 26.075 ( 6 ) A, b = 9.617 (3) A, c = 16.109 (3) A, Dcalcd = 1.46 g cm-,, Z = 8, R = 0.057 for 5437 measured reflections. 11: C21H17CoN202~0.5H20, space group P i , a = 12.100 (8) A, b = 20.392 (9) A, c = 11.606 (8) A, LY = 53.3 (l)', p = 127.6 (I)', y = 121.7 (l)', Dcald = 1.50 g ~m -~, Z = 4, R = 0.041 for 7360 measured reflections. 111: C27H21CoN302F3-0.5H20.0.5C5H5N, space group P21/c, a = 14.255 (8) A, b = 19.318 (8) A, c = 19.434 (8) A, 0 = 93.7 (I)', Dcalcd = 1.45 g ~m -~, Z = 8, R = 0.055 for 13588 measured reflections. Several interestingfeatures emerge from these structures. First, the Co atom in I and I1 is displaced toward R. Second, the Co-C bond is shorter than in hexacoordinate compounds, but steric strain remains for i-C,H7 even in the pentacoordinate species. Third, in the unusual example of both a dimeric and monomeric molecule in the same crystal in compound 11, the monomeric form seems to be stabilized by the water of crystallization. This water is hydrogen-bonded to the saloph ligand and not coordinated to Co. Dynamic IH NMR measurements of 3,5-lut exchange in 3,5-lutCo(saloph)R (where 3,5-lut = 3,5-lutidine, R = CH2CN, CH2CF,, or CH,) reveal that the rate of exchange increases more rapidly with increasing electron donation by R in the series CH2CN to CH2CF, to CH3 than in other classes of BI2 compounds. The saloph ligand has recently been shown to exhibit a large structural cis influence and a large kinetic cis effect compared to the other classes of models. It was also recently recognized that the trans and cis influences are synergistic. Thus, the dynamic NMR measurements provide evidence that the cis and trans kinetic effects are also synergistic. When R = CH,, for example, exchange of the neutral axial ligand in saloph complexes is approximately 8 X 1O' O ti...

Binding of Cob(II)alamin to the Adenosylcobalamin-dependent Ribonucleotide Reductase fromLactobacillus leichmannii

“…Model studies have indicated that a weaker nitrogen donor ligand decreases the dissociation energy of the C-Co bond (33)(34)(35). Interest in this second hypothesis has been stimulated by extended x-ray absorption fine structure spectroscopy and x-ray crystallography studies on MMCoA mutase, which indicate displacement of the DMB lower axial ligand of 1 by a protein histidine in cob(II)alamin-bound MMCoA mutase, with a long Co-N bond (3,4,36).…”

Stable Five‐ and Six‐Coordinate Cobalt(III) Complexes with a Pentadentate Bispidine Ligand

“…Most of these are systems with electron-rich, often anionic ligands, and many are derived from macrocyclic ligand systems, [7][8][9][10][11] largely as a result of extensive modeling studies of vitamin B 12 . [12,13] Para-as well as diamagnetic square-pyramidal [14] and trigonal-bipyramidal [8] systems have been reported, and, in the course of the search for the elusive Co = O functional group, [15] a tetracoordinate amido-Co III complex was also structurally characterized. [11] All these complexes have interesting reactivities that are not observed in classical Co III systems, such as the efficient production of alkyl radicals [16] and CÀH activation.…”

Preparation and Characterization of Vitamin B₁₂ Model Complexes

“…Recently the chemistry of vitamin B 12 and its model complexes have been attracted the biochemists, inorganic chemists and organometallic chemists [1]. During the past few decades the research works are primarily on the preparation of model complexes of vitamin B 12 and investigation of their chemical behavior [4][5][6][7][8][9][10][11][12][13][14][15]. The conclusions reached from model complexes are then compared with vitamin B 12 with a view to explore its reaction and functions.…”