Novel Chemistry of Carbon Bound to Cobalt in Organocobalt Complexes Related to B12

Abstract

A rare Co−N−C three-membered ring has been formed by a novel pathway involving a facile interligand nucleophilic addition of an equatorial nitrogen donor to a ligated axial carbon in some organocobalt complexes. The highly distorted structure of a typical resulting complex provides clear experimental evidence useful in assessing hypotheses and computations concerning Co−C bond activation in B12 chemistry. Treatment of R = −CH2X (X = halogen) derivatives of imine/oxime B12 models of the type [LCo((DO)(DOH)pn)R]+ with base afforded major products with striking NMR spectral features in common, e.g., two one-proton olefinic doublets (J ≈ 3 Hz) and two one-proton singlets. The X-ray structure for the pyridine (py) derivative [pyCo(N-CH2-CHEL)]+ (1) reveals two unexpected features: a three-membered metallocycle containing an η2-aminomethylene group, and an enamine (N−CCH2) in place of one imine moiety (NC−CH3). The C−Co−N ring angle is acute (43.7°) with the distortion of the coordination sphere concentrated in the Co−C and the Co−N ring linkages, which move away from the normal pseudooctahedral positions. Studies of the formation of the aqua analog of 1, [H2OCo(N-CH2-CHEL)]+ (3), in aqueous solution suggest that the initial intermediate formed is deprotonated at one imine methyl. Ring closure in the short-lived deprotonated intermediate is facile. Published calculations predict that such ring closure could occur and that, of two processes which could facilitate Co−C bond cleavage (θ-bending of the N−Co−C angle and φ-bending of the Co−C−C angle), the latter factor should be more significant in weakening the Co−C bond. There is large θ-bending in 1; thus, the new metallocyclic B12 analogs afford the first experimental test of these calculations. Our findings that the Co−C bond in 1 is short and that the bond did not readily cleave support the predictions of the calculations.

Decarboxylation of an α-amino acid coordinated to cobalt(iii): kinetic stabilisation and molecular structure of a Co–C–N three-membered ring incorporated into a cobalt(iii) macrocyclic ligand complex†‡

“…Only two other structurally characterised examples of a Co-CH 2 -NR 2 ring have been reported, both incorporated into a polydentate ligand, although they were prepared by quite different routes to that reported here. 10,11 In one of these examples, the ring was formed by C-H activation of an N-CH 3 group incorporated in a tetraaza macrocycle. 11 We observe the complementary reaction to this, wherein [Co(L 1 )]BPh 4 undergoes Co-CH 2 bond cleavage upon heating in aqueous dilute HCl.…”

“…8 Both hydrogen atoms on C(13) were located crystallographically. The C(1)-N(1)-C (10) and H(13A)-C(13)-H(13B) angles of 110.4(2)° and 108(3)°, respectively, indicate sp 3 hybridization at N(1) and C (13).…”

Mechanistic behaviour of alkylcobaloximes and imino-oxime complexes related to vitamin B12

“…13 In the attempt to reproduce the same reaction for trans-[(CH 2 X)Co((DO)(DOH)pn)L] + (X = Cl, Br, I) derivatives, quite a different type of compound was isolated (in almost quantitative yield), which is characterized by the presence of a three-membered Co-N-C ring resulting from an interligand nucleophilic addition of an equatorial N donor to the ligated axial carbon. 14 In addition, the cobalt complexes of the type trans-[(R) 2 Co((DO)(DOH)pn)] or trans-[(R¢)(R¢¢)Co((DO)(DOH)pn)] 10,15-17 are well known. However, the synthesis of the corresponding trans-dialkylcobaloximes has not been reported so far.…”

Coenzyme B12 Model Studies − Kinetics of Axial Ligation of Alkylcobalt Complexes Containing a Tridentate Amino‐Oximate Ligand

“…The reaction of the complex [11] trans-[(CH 2 )Co(LNHpy)(HLNpy)] ϩ (4) with CN Ϫ was also studied. Complex 4 was characterized to involve a ϪCH 2 Ϫ group that forms a three-membered metallacycle, [34,35] in which the other two members are the Co atom and the equatorial nitrogen donor. Complexes containing this type of metallacycle are quite rare [34,35] and scarcely studied.…”

“…Complex 4 was characterized to involve a ϪCH 2 Ϫ group that forms a three-membered metallacycle, [34,35] in which the other two members are the Co atom and the equatorial nitrogen donor. Complexes containing this type of metallacycle are quite rare [34,35] and scarcely studied. In view of the unusual geometry of this complex, it seemed to be of interest to examine its kinetic properties with reference to the reactivity of the trans position with respect to the bridging ϪCH 2 Ϫ fragment.…”