Twelve-electron organochromium species: Synthesis and characterization of high-spin square-planar chromium(II) alkyls and aryls

Article abstract of DOI:10.1021/om00101a018

Alkylation of the chromium phosphine complex [CrCl₂(dippe)]₂ (dippe = 1,2-bis(diisopropyl-phosphino)ethane) with Grignard or dialkylmagnesium reagents gives the unusual 12-electron chromium(II) alkyls cis-CrR₂(dippe) (R = CH₂CMe₃, CH₂SiMe₃, C₆H₂Me₃). The unidentate phosphine complex trans-Cr(C₆H₂Me₃)₂(PMe ₃)₂ may be prepared similarly by treatment of CrCl₂(THF) with dimesitylmagnesium in the presence of PMe₃. All of these four-coordinate alkyls possess magnetic moments characteristic of high-spin species, μ = 4.9 μ_B, and exhibit broadened ¹H NMR resonances consistent with the paramagnetism. X-ray crystal structures of Cr(CH₂CMe₃)₂(dippe) and Cr(CH₂SiMe₃)₂(dippe) reveal tetrahedrally distorted square-planar structures, where the dihedral angle between the C-Cr-C and P-Cr-P planes is larger in the neopentyl complex, 28.4° vs 15.8°, due to steric effects. For similar reasons, the Cr-C and Cr-P distances in the neopentyl compound of 2.149 (8) and 2.556 (2) A? are slightly longer than those of 2.128 (4) and 2.517 (1) A? in the (trimethylsilyl)methyl derivative, and the C-Cr-C angle is also larger in the neopentyl complex at 101.9 (3)° vs 95.7 (2)°. In contrast to these results, the mesityl complex Cr(C₆H₂Me₃)₂(PMe₃) ₂ is essentially flat: the ipso carbon atoms and the phosphorus atoms are +0.9° and -0.9° out of the mean CrC₂P₂ plane, respectively. The Cr-C and Cr-P distances in this molecule are 2.130 (6) and 2.462 (2) A?, while all the C-Cr-P angles fall between 89.3 (2)° and 90.8 (2)°. Crystal data: for Cr(CH₂CMe₃)₂(dippe), space group C2/c, a = 15.867 (5) A?, b = 10.401 (4) A?, c = 18.154 (6) A?, β = 106.67 (3)°, V = 2870 (2) A?³, Z = 4, R_F = 5.9%, R_wF = 7.7% on 126 variables and 1205 data; for Cr(CH₂SiMe₃)₂(dippe), space group C2/c, a = 15.666 (4) A?, b = 10.983 (3) A?, c = 18.099 (6) A?, β = 102.43 (2)°, V = 3041 (1) A?³, Z = 4, R_F = 4.1% R_wF = 4.6% on 128 variables and 1472 data; for Cr(C₆H₂Me₃)₂(PMe₃) ₂, space group P2₁/n, a = 9.710 (2) A?, b = 14.121 (3) A?, c = 19.446 (3) A?, β = 98.36 (1)°, V = 2638.0 (9) A?³, Z = 4, R_F = 5.1%, R_wF = 6.1% on 260 variables and 2199 data. The chromium alkyls are able to polymerize ethylene slowly at 25°C and 10 atm.