Unusual NMR spectral properties of the 1,2-dicyanoethyl alkyl ligand in organocobaloximes. Structure of the organocobalt complex trans-bis(dimethylglyoximato)(1,2-dicyanoethyl)(1,5,6-trimethylbenzimidazole)cobalt(III)
“…There are a number of mechanism which are possible alternatives to the one presented here; we considered only radical-type, single-step processes not involving the ligands. The possibility of H−D exchange through protic intermediates was recognized even in the very earliest work on hydridocobalt(III) chelate reactions. , The work described here demonstrates that olefinic impurities can lead to very rapid aprotic exchange which must also be considered in labeling experiments. These alternative mechanisms are considered in more detail in the Supporting Information.…”
The process of cobalt porphyrin catalyzed chain transfer in methacrylate free radical polymerizations shows a deuterium kinetic isotope effect greater than 3, indicating that hydrogen atom transfer occurs in the rate-limiting step of the catalytic cycle. This step involves formation of a transient hydridocobalt(III) complex by a reversible hydrogen atom abstraction from an incoming radical; the other product is an olefin. In some cases, the resulting radical can be trapped as the alkylcobalt(III) product. The use of labeled cyclic olefins indicates that the formation of the alkylcobalt(III) species can be at least partially stereoselective in some cases, and when it is, the addition is cis. Deuterated vinylidene olefins such as methyl methacrylate-d 8 were found to exchange the hydrogen of a cobalt porphyrin hydride to deuteride under free radical conditions. This exchange reaction can be used for deuterium labeling experimentssthe transient cobalt deuterides are similarly trapped by addition of protioolefins. Thermal isomerization of the labeled product with time results in gradual loss of stereoselectivity, but observation of the initial stereoselectivity of this reaction indicates an underlying limit to the reaction selectivity. The ready availability of radical homolysis products will always limit the selectivity of the products. Similar experiments with acetylenes usually result in trans-vinyl products except when steric effects dominate. This shift from cis products with olefins to trans products for acetylenes is unexplained but may be attributed to radical intermediates or may indicate a role for a second cobalt complex in the transition state.
“…There are a number of mechanism which are possible alternatives to the one presented here; we considered only radical-type, single-step processes not involving the ligands. The possibility of H−D exchange through protic intermediates was recognized even in the very earliest work on hydridocobalt(III) chelate reactions. , The work described here demonstrates that olefinic impurities can lead to very rapid aprotic exchange which must also be considered in labeling experiments. These alternative mechanisms are considered in more detail in the Supporting Information.…”
The process of cobalt porphyrin catalyzed chain transfer in methacrylate free radical polymerizations shows a deuterium kinetic isotope effect greater than 3, indicating that hydrogen atom transfer occurs in the rate-limiting step of the catalytic cycle. This step involves formation of a transient hydridocobalt(III) complex by a reversible hydrogen atom abstraction from an incoming radical; the other product is an olefin. In some cases, the resulting radical can be trapped as the alkylcobalt(III) product. The use of labeled cyclic olefins indicates that the formation of the alkylcobalt(III) species can be at least partially stereoselective in some cases, and when it is, the addition is cis. Deuterated vinylidene olefins such as methyl methacrylate-d 8 were found to exchange the hydrogen of a cobalt porphyrin hydride to deuteride under free radical conditions. This exchange reaction can be used for deuterium labeling experimentssthe transient cobalt deuterides are similarly trapped by addition of protioolefins. Thermal isomerization of the labeled product with time results in gradual loss of stereoselectivity, but observation of the initial stereoselectivity of this reaction indicates an underlying limit to the reaction selectivity. The ready availability of radical homolysis products will always limit the selectivity of the products. Similar experiments with acetylenes usually result in trans-vinyl products except when steric effects dominate. This shift from cis products with olefins to trans products for acetylenes is unexplained but may be attributed to radical intermediates or may indicate a role for a second cobalt complex in the transition state.
“…All four Re−N−B2 bond angles (supporting information) are consistently more acute than the four Re−N−B9 bond angles. Unsymmetrical angles are common in Co III complexes with Me 3 Bzm: Co−N−B2 angles range 121.1(3)−123.1(2)°; Co−N−B9 angles range 132.3(3)−133.7(2)°. ,− …”
In examining NMR methods to assess orientation and fluxional motions of planar N-donor heterocyclic coordinated ligands (L's) in solution, we introduce the use of exchange-NOE NMR data as a powerful method for defining (i) the extent of L rotation about the metal-N bond, (ii) the direction of L rotation, and (iii) even the halves of C 2 -symmetrical L's that interchange during dynamic processes. The full value of the approach depends on a strategy in which the complexes studied are chiral and similar except that one has a lopsided L (Me 3 Bzm ) 1,5,6trimethylbenzimidazole in [Re 2 O 3 Cl 4 (Me 3 Bzm) 4 ] ( 1)) and the other has a C 2 -symmetrical L (3,5-lut ) 3,5-lutidine in [Re 2 O 3 Cl 4 (3,5-lut) 4 ] (2)). Each Re is part of a nearly linear OdRe-O-RedO grouping and has a "terminal" L (L t ) and a stacked L (L s ). The fluxional inversion of the two chiral dimers involves rotations of ∼180°about the Re-O-Re bonds and of ∼90°about all four Re-L bonds. The exchange-NOE data for 2 show that the half of L t away from the dimer center interchanges with the half of L s close to the center, with the L plane rotating past the OdRe-O bonds, not the N-Re-Cl bonds. Thus, the exchange-NOE data help to establish the direction of L rotation. In 1, Me 3 Bzm t has the rare head-to-head (HH) orientation with respect to the partner. This partner Me 3 -Bzm s stacks with the Me 3 Bzm s from the other Re in the common head-to-tail (HT) orientation. Compelling evidence that the predominant solution conformer of 1 has the HH,HT,HH structure includes unusual chemical shift dispersions and a strong interligand NOE cross-peak. This is the only case in which cis,bis imidazole-ring-ligated untethered ligands have been found to be predominantly HH in solution. This predominance can be attributed to the electrostatic attraction of the δ + N 2 C proton for the negative core of the molecule (bridging O, cis Cl on same Re, and two cis Cl's on the other Re).
“…The planes of the two N ligands form a dihedral angle of 49.2°. Interestingly, the Ru−N2−C19 bond angle is 131.4(1)° (Table ); normally an ∼133° angle is found for M−BN3−B9. − 1 Perspective drawing and labeling scheme for the non-hydrogen atoms of cis,cis,cis -RuCl 2 (DMSO) 2 (py)(Me 3 Bzm) ( 1 ).…”
The solution and solid state structures of two octahedral Ru(II) complexes, cis,cis,cis-RuCl(2)(Me(2)SO)(2)(py)(Me(3)Bzm) (Me(3)Bzm = 1,5,6-trimethylbenzimidazole, py = pyridine) (1) and cis,cis,cis-RuCl(2)(Me(2)SO)(2)(Me(3)Bzm)(2) (2), were compared. 2, the subject of a preliminary report, is described in more detail here. 1 has two possible geometric isomers with py trans to Cl in one (position "a") and trans to Me(2)SO in the other (position "b"), Me(3)Bzm occupying the other position in each isomer. The X-ray structure of 1 revealed that py is at "a". Since Me(3)Bzm is lopsided, each Me(3)Bzm has two possible orientations related by a rotation of approximately 180 degrees about the Ru-N3 bond; there are two possible atropisomers for each geometric isomer of 1 and four for 2. For 1, the solid state structure shows that Me(3)Bzm adopts the orientation with H2 (H on C between the two N's) pointing between the two cis Cl ligands, the same disposition as Me(3)Bzm "b" in 2 in the solid. For 1, the py signals (two broad py alpha and beta signals, a sharp gamma signal) in CDCl(3) show that py "a" is rotating on the NMR time scale and that only one atropisomer is present. This interpretation was supported by ROESY and EXSY (1)H NMR spectra. The (1)H NMR shift pattern and the NOE data can be understood best if Me(3)Bzm "b" remains primarily in the orientation found in the solid. The solution data for 1, with the nonlopsided and sterically less demanding py ligand, provide insight into the more complicated properties of 2. For 2, there is a marked dispersion of (1)H NMR signals of Me(3)Bzm "a" between the two atropisomers, which have nearly equal stability. One atropisomer is a head-to-head (HH) and the other a head-to-tail (HT) species. Me(3)Bzm "a" flips between the two species. Thus, ligand "a" is fluxional in both complexes. The dispersion of Me(3)Bzm "a" signals is due to the effect of Me(3)Bzm "b" anisotropy. For 1 and both atropisomers of 2, Me(3)Bzm "b" prefers one orientation, which appears to be the most hindered orientation. We postulate that the H2 of Me(3)Bzm "b" is electrostatically attracted to the two cis halides, accounting for this surprising result. Crystallographic details for 1 are as follows: C(19)H(29)Cl(2)N(3)O(2)RuS(2), P2(1)/c, a = 10.947(1) Å, b = 9.046(1) Å, c = 24.221(2) Å, D(calcd) = 1.580 g cm(-)(3), Z = 4, R = 0.026 for 4627 independent reflections.
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