Steric influences of phosphorus-donor ligands on the structure and ligand-exchange rates of cobaloximes

Abstract

The crystal structures of two cobaloximes, [CoMe(Hdmg),(PMe,)](1 ) and [ C O M ~( H ~~~) ~{ P ( C , H , , ) , ) ] (2) (H,dmg = dimethylglyoxime), are reported and discussed. Compound (1) crystallizes in the space group PT with cell parameters a = 15.830(8), b = 12.279(7), c = 12.257 A, a = 94.85(7), /3 = 84.49(8), y = 130.07(9)', and Z = 4; (2) crystallizes in the space group P2,2,2, with a = 18.50(1), b = 16.83(1), c = 9.943(8) A, and Z = 4. Both structures have been solved by Patterson and Fourier methods and refined by block-diagonal anisotropic least-squares methods to final R values of 0.037 (1 ) and 0.038 (2), using 4 81 2(1) and 2 710(2) independent reflections. The Me-Co-P fragment of (1) is characterized by a C-Co-P angle of 178.9(2)' (mean) and Co-P and Co-C bond lengths of 2.293(1) (mean) and 2.015(3) A (mean); the corresponding figures for (2) are 179.1 (2)",2.463(1),and 2.016(5) Arespectively. Thevalue of 2.463(1) Ais the largestso far reportedfora CoI'I-P bond length. The two nearly planar Hdmg units make a mean interplanar angle of 4" in (1) and are bent towards the axial methyl group. A similar but more pronounced bending is observed in (2). The trend in Co-P distances in these and other similar complexes [CoX(Hdmg),(PR,)] (X = CI or Me; R = OMe, Me, Bu9, Ph, or C6H11) is discussed and compared with 3lP and l H chemical shifts. Deformations observed in the geometry of these octahedral cobalt(ll1) complexes are interpreted in terms of ' steric ' and ' electronic ' influences. Finally, kinetic measurements of the conversion of [CoMe(Hdmg),{P(C,H,,),}] to [CoMe(Hdmg),(py)] (py = pyridine) give no indication of a significant contribution from steric effects on the rate of thefive-to six-co-ordinate step. However, the relatively high rate of displacement of the P(C8HI1), ligand compared to other phosphines of smaller cone angle almost certainly arises from the steric interaction of the C,H11 rings with the Co(Hdmg), moiety.WE are attempting to evaluate steric effects in vitamin B,, models in order to probe the feasibility of the mechanochemical mechanism of Co-C bond cleavage during substrate to product catalysis by the B,, coenzyme-enzyme complex.1-5 Such a study also has some bearing on steric effects of P-donor ligands since such ligands are used to generate steric distortions in model compounds.6-8 These distortions are suggested to occur in the coenzyme, caused by enzyme side chains.lb5 In addition to ground-state structural effects (referred to here as ' influences '), steric factors play some role in modifying reaction rates (referred to as ' effects '). 9910 The cobaloximes (cobalt complexes containing two ' coplanar ' dioxime ligands) have proved to be useful in evaluating the influence of bulky groups on ground-state structural parameters. We recently demonstrated that the Co-C bond length in the compound [CoPr' (Hdmg),-(py)] (where Hdmg = monoanion of dimethylglyoxime [1/690

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

“…Experimental tests for φ-bending exist in the structures of neopentyl derivatives, 4, [27][28][29] and the bond has been shown to be weak. 30 The compounds presented here provide the first opportunity to gain experimental insight into the likely consequences of θ-bending on the Co-C bond.…”

Molecular Mechanics Modeling of the Cobaloximes and Reevaluation of the Parameters for Modeling of the Cobalt Corrins

“…9,20,32 There is structural evidence for and against these bond shortening effects. In addition to [Co(DH) 2 (i-Pr)(OH 2 )], the C 1 -C 2 bond is also short in [Co(DH) 2 (Et)(PPh 3 )] (1.317 Å), 20 marginally shorter in [Co(DH) 2 (CH(Me)Et))(PPh 3 )] (1.449 Å for C-Et; 1.506 Å for C-Me), 20 normal in [Co(DH) 2 (CH 2 C(Me) 3 )] (1.555 Å), 19 and shorter and longer in [Co(DH) 2 (i-C 3 H 7 ) (1.58 and 1.49 Å). 22 In the alkylcobalamins the structural evidence is also inconclusive.…”

“…The parameters previously used 2 for the Co−C−H angle were used unmodified. The structures used were R = Me, L = PMe 3 ; R = Me, L = P(C 6 H 11 ) 3 ; R = CH 2 C(Me) 3 , L = PPh 3 ; R = C 2 H 5 , L = PPh 3 ; R = CH(Me)(Et), L = PPh 3 ; R = Me, L = P(OMe) 2 Ph; R = Me, L = P(OMe)Ph 2 ; R = i -C 3 H 7 , L = PPh 3 ; R = Me, L = PPh 3 ; R = Me, L = py; R = CH 2 C(Me) 3 , L = py; and R = i -C 3 H 7 , L = py …”

Structures of azidobis(dimethylglyoximato)(triphenylphosphine)cobalt(III), C26H29CoN7O4P, and [4,5-bis(methoxycarbonyl)-1,2,3-triazolato]bis(dimethylglyoximato)(triphenylphosphine)cobalt(III), C32H35CoN7O8P

“…A lengthening of the Co-C1 distance has already been observed (Marzilli, Toscano, Ramsden, Randaccio & Bresciani-Pahor, 1982) in PR3Co(DH)2CI analogues as compared with NH3Co(DH)2C1 [from 2.235(3) to 2.261 (4)A] and has been attributed to the electronic trans influencing ability of P donor ligands greater than that of N donor ligands. However, a contribution to the observed lengthening due to the 'steric' trans influence of PPh 3, which is greater than that of pyridine, may not be ruled out, in view of the larger deformation induced in the equatorial ligand by PPh 3 which may provoke a lengthening of the trans Co-N bond (Bresciani-Pahor, Randaccio, Toscano, Sandercock & Marzilli, 1982). Whatever the origin, the observed Co-N lengthening should influence the rate constants for the cycloaddition reaction of the coordinated azide group having different trans ligands.…”