Ligand control of electronic stability of CpCr(NO)(ligand)2 complexes

Article abstract of DOI:10.1021/ja00147a013

Treatment of [CpCr(NO)I]₂ with an excess of a Lewis base, L, in CH₂Cl₂ leads to the formation of the complex salts [CpCr(NO)(L)₂]⁺[I]^- ([1]⁺I^-, L = NH₃; [3]⁺I^-, L = NH₂CH₂CH=CH₂; [7]⁺I^-, L = 1/2en). Heating of salts [1]⁺I^- and [3]⁺I^- results in loss of L and formation of the neutral complexes, CpCr(NO)(NH²R)I (2, R = H; 4, R = CH₂CH=CH₂), respectively. In contrast, reaction of [CpCr(NO)I]₂ with the bulkier NH₂CMe₃ affords the neutral CpCr(NO)(NH₂CMe₃)I (6) directly. Sequential reaction of 6 or CpCr(NO)(P{OMe}₃)I with AgPF₆ and further L affords respectively the salts [CpCr(NO)(L)₂]⁺[PF₆]^- ([5]⁺[PF₆]^-, L = NH₂CMe₃; [8]⁺[PF₆]^-, L = P(OMe)₃). All these species exhibit room-temperature ESR spectra and magnetic moments consistent with their possessing 17-valence-electron configurations. Zinc reduction of [CpCr(NO)I]₂ in the presence of P(OMe)₃ leads to the improved synthesis of the known complex CpCr(NO)(P{OMe}₃)₂ (8), and a similar reduction with CNCMe₃ affords the previously unknown CpCr(NO)(CNCMe₃)₂ (9). The solid-state molecular structures of [1]⁺[BPh₄]^-·NCMe, 4, 8, and [8]⁺[BPh₄]^- have been established by single-crystal X-ray crystallographic analyses which afforded the following data [CpCr(NO)(NH₃)₂][BPh₄]·NCMe ([1]⁺[BPh₄]^-·NCMe): monoclinic, space group P2₁/n; Z = 4; a = 9.478(3) A?; b = 19.288(7) A?; c = 15.427(6) A?; β = 91.99(3)°; V = 2818.5 A°³; T = 200 K; R_F = 0.038 for 2185 data (I_o ≥ 2.5σ(I_o)) and 310 variables. CpCr(NO)(NH₂C₃H₅)(I) (4): triclinic, space group P1; Z = 2; a = 8.0497(8) A?; b = 8.3273(17) A?; c = 9.3284(9) A?; α = 108.182(12)°; β = 92.370(8)°; γ = 94.759(12)°; V = 590.54 A?³; T = 295 K; R_F = 0.024 for 1756 data (I_o ≥ 2.5σ(I_o)) and 123 variables. CpCr(NO)(P{OMe}₃)₂ (8): monoclinic, space group P2₁/a; Z = 8; a = 18.080(4) A?; b = 9.320(4) A?; c = 21.068(3) A?; β = 93.02(2)°; V = 3545.1 A?³; T = 205 K; R_F = 0.040 for 3356 data (I_o ≥ 2.5σ(I_o)) and 411 variables. [CpCr(NO)(P{OMe}₃)₂][BPh₄] ([8]⁺[BPh₄]^-): monoclinic, space group P2₁/n; Z = 4; a = 10.086(2) A?; b = 22.253(3) A?; c = 16.150(4) A?; β = 90.42(2)°; V = 3624.7 A?³; T = 195 K; R_F = 0.044 for 3334 data (I_o ≥ 2.5σ(I_o)) and 449 variables. Despite its 17-electron configuration, [1]⁺ does not undergo ligand substitution, nor does it effect H-atom abstraction from HSn(n-Bu)₃. However, it exhibits an irreversible reduction at E_p,c = -1.3 V vs SCE in THF, and zinc reduction of [1]⁺ (as its [PF₆]^- salt) in the presence of CO (1 atm) affords CpCr(NO)(CO)₂. In a reverse manner, oxidation of 2 by [Cp₂Fe]⁺[PF₆]^- in acetonitrile produces [CpCr(NO)(NCMe)₂]⁺[PF₆]^-, a salt which contains a 17-electron cation similar to [1]⁺. These experimental observations lead to the conclusion that for CpCr(NO)L₂ complexes, σ-base ligands stabilize the 17-electron configurations of cations whereas π-acid ligands stabilize the 18-electron configurations of the neutral congeners. Intermediate ligands (e.g. L = P(OMe)₃) yield complexes which are capable of existing in both forms. This trend can be rationalized by the results of an Extended Hu?ckel analysis of the CpCr(NO) fragment and the interaction of its frontier orbitals with those of various ligands, L.