LIGAND CONTROL OF ELECTRONIC STABILITY OF CPCR(NO)(LIGAND)(2) COMPLEXES

Treatment of [CpCr(NO)I](2) with an excess of a Lewis base, L, in CH2C l2 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 an d formation of the neutral complexes, CpCr(NO)(NH(2)R)I (2, R = H; 4, R = CH2CH=CH2), respectively. In contrast, reaction of [CpCr(NO)I](2) with the bulkier NH(2)CMe(3) affords the neutral CpCr(NO)(NH(2)CMe(3)) 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 = NH(2)CMe(3); [8](+)[PF6](-), L = P(OMe)( 3)). All these species exhibit room-temperature ESR spectra and magnet ic moments consistent with their possessing 17-valence-electron config urations. 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 CNCMe(3) affords the previousl y unknown CpCr(NO)(CNCMe(3))(2) (9). The solid-state molecular structu res of [1](+)[BPh(4)](-). NCMe, 4, 8, and [8](+)[BPh(4)](-) have been established by single-crystal X-ray crystallographic analyses which af forded the following data. [CpCr(NO)(NH3)(2)][BPh(4)]. NCMe ([1](+)[BP h(4)](-). NCMe): monoclinic, space group P2(1)/n; Z = 4; a 9.478(3) An gstrom; b = 19.288(7) Angstrom; c = 15.427(6) Angstrom; beta = 91.99(3 )degrees; V = 2818.5 Angstrom(3); T = 200 K; R(F) = 0.038 for 2185 dat a (I-o greater than or equal to 2.5o(I-o)) and 310 variables. CpCr(NO) (NH2C3H5)(I) (4): triclinic, space group P (1) over bar; Z = 2; a = 8. 0497(8) Angstrom; b = 8.3273(17) Angstrom; c = 9.3284(9) Angstrom; alp ha = 108.182(12)degrees; beta = 92.370(8)degrees; gamma = 94.759(12)de grees; V = 590.54 Angstrom(3); T = 295 K; R(F) = 0.024 for 1756 data ( I-o greater than or equal to 2.5 sigma(I-o)) and 123 variables. CpCr(N O)(P{OMe}(3))(2) (8): monoclinic, space group P2(1)/a; Z = 8;(1)a = 18 .080(4) Angstrom; b = 9.320(4) Angstrom; c = 21.068(3) Angstrom; beta = 93.02(2)degrees; V = 3545.1 Angstrom(3); T = 205 K; R(F) 0.040 for 3 356 data (I-o greater than or equal to 2.5 sigma(I-o)) and 411 variabl es. [CpCr(NO)(P{OMe}(3))(2)][BP4] ([8](+)[BPh(4)](-)): monoclinic, spa ce group P2(1)/n; Z = 4; a 10.086(2) Angstrom; b = 22.253(3) Angstrom; c 16.150(4) Angstrom; beta 90.42(2)degrees; V = 3624.7 Angstrom(3); T = 195 K; R(F) = 0.044 for 3334 data (I-o greater than or equal to 2.5 sigma(I-o)) and 449 variables. Despite its 17-electron configuration, [1](+) does not undergo ligand substitution, nor does it effect H-ato m 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 [Cp(2)Fe](+)[PF6](- ) in acetonitrile produces [CpCr(NO)(NCMe)(2)](+)[PF6](-) a salt which contains a 17-electron cation similar to [1](+). These experimental o bservations lead to the conclusion that for CpCr(NO)L(2) complexes, si gma-base ligands stabilize the 17-electron configurations of cations w hereas pi-acid ligands stabilize the 18-electron configurations of the neutral congeners. Intermediate ligands (e.g. L = P(OMe)(3)) yield co mplexes which are capable of existing in both forms. This trend can be rationalized by the results of an Extended Huckel analysis of the CpC r(NO) fragment and the interaction of its frontier orbitals with those of various ligands, L.