Rh(II) and Rh(I) Two-Legged Piano-Stool Complexes:  Structure, Reactivity, and Electronic Properties

The ligand 1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetramethylbenzene, 3, was used to synthesize a mononuclear Rh(II) complex [(η1:η6:η1-1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetramethylbenzene)Rh][PF6]2, 6 +, in a two-legged piano-stool geometry. The structural and electronic properties of this novel complex including a single-crystal EPR analysis are reported. The complex can be cleanly interconverted with its Rh(I) form, allowing for a comparison of the structural properties and reactivity of both oxidation states. The Rh(I) form 6 reacts with CO, tert-butyl isocyanide, and acetonitrile to form a series of 15-membered mononuclear cyclophanes [(η1:η1-1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetramethylbenzene)Rh(CO)3][PF6] (8), [(η1:η1-1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetramethylbenzene)Rh(CNC(CH3)3)2][PF6] (10), and [(η1:η1-1,4-bis[4-(diphenylphosphino)butyl]-2,3,5,6-tetramethylbenzene)Rh(CO)(CH3CN)][PF6] (11). The Rh(II) complex 6 + reacts with the same small molecules, but over shorter periods of time, to form the same Rh(I) products. In addition, a model two-legged piano-stool complex [(η1:η6:η1-1,4-bis[3-(diphenylphosphino)propoxy]-2,3,5,6-tetramethylbenzene)Rh][B(C6F5)4], 5, has been synthesized and characterized for comparison purposes. The solid-state structures of complexes 5, 6, 6 + , and 11 are reported. Structure data for 5: triclinic; P1̄; a = 10.1587(7) Å; b = 11.5228(8) Å; c = 17.2381(12) Å; α = 96.4379(13)°; β = 91.1870(12)°; γ = 106.1470(13)°; Z = 2. 6: triclinic; P1̄; a = 11.1934(5) Å; b = 12.4807(6) Å; c = 16.1771(7) Å; α = 81.935(7)°; β = 89.943(1)°; γ = 78.292(1)°; Z = 2. 6 + : monoclinic; P2(1)/n; a = 11.9371(18) Å; b = 32.401(5) Å; c = 12.782(2) Å; β = 102.890(3)°; Z = 4. 11: triclinic; P1̄; a = 13.5476(7) Å; b = 13.8306(7) Å; c = 14.9948(8) Å; α = 74.551(1)°; β = 73.895(1)°; γ = 66.046(1)°; Z = 2.