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The proposal that paramagnetic transition metal complexes could be used as qubits for quantum information processing (QIP) requires that the molecules retain the spin information for a sufficient length of time to allow computation and error correction. Therefore, understanding how the electron spin-lattice relaxation time ( T 1 ) and phase memory time ( T m ) relate to structure is important. Previous studies have focused on the ligand shell surrounding the paramagnetic centre, seeking to increase rigidity or remove elements with nuclear spins or both. Here we have studied a family of early 3d or 4f metals in the +2 oxidation states where the ground state is effectively a 2 S state. This leads to a highly isotropic spin and hence makes the putative qubit insensitive to its environment. We have studied how this influences T 1 and T m and show unusually long relaxation times given that the ligand shell is rich in nuclear spins and non-rigid.

The first crystallographically characterizable complex of Sc , [Sc(NR ) ] (R=SiMe ), has been obtained by LnA /M reactions (Ln=rare earth metal; A=anionic ligand; M=alkali metal) involving reduction of Sc(NR ) with K in the presence of 2.2.2-cryptand (crypt) and 18-crown-6 (18-c-6) and with Cs in the presence of crypt. Dark maroon [K(crypt)] , [K(18-c-6)] , and [Cs(crypt)] salts of the [Sc(NR ) ] anion are formed, respectively. The formation of this oxidation state of Sc is also indicated by the eight-line EPR spectra arising from the I=7/2 Sc nucleus. The Sc(NR ) reduction differs from Ln(NR ) reactions (Ln=Y and lanthanides) in that it occurs under N without formation of isolable reduced dinitrogen species. [K(18-c-6)][Sc(NR ) ] reacts with CO to produce an oxalate complex, {K (18-c-6) }{[(R N) Sc] (μ-C O -κ O:κ O'')}, and a CO radical anion complex, [(R N) Sc(μ-OCO-κ O:κ O')K(18-c-6)] .

Although previous studies of the stabilization of Ln­(II) ions across the lanthanide series have relied on Me3Si-substituted cyclopentadienyl ligands, we now find surprisingly that these ions can also exist surrounded by three tetramethylcyclopentadienyl ligands. Reduction of the 4f n Ln­(III) complexes, Cptet 3Ln (Cptet = C5Me4H) using potassium graphite in the presence of 2.2.2-cryptand (crypt) produces the Ln­(II) complexes, [K­(crypt)]­[Cptet 3Ln] for Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, and Dy, all of which were characterized by X-ray crystallography. These complexes display intense absorptions in the UV–visible–near IR region that are red-shifted compared to those of previously characterized (Cp′3Ln)1– complexes (Cp′ = C5H4SiMe3). The thermal stability of these new Ln­(II) complexes decreases with the size of the metal.

The tris(cyclopentadienyl) yttrium complexes Cp 3 Y-(THF), Cp Me 3 Y(THF), Cp″ 3 Y, Cp″ 2 YCp, and Cp″ 2 YCp Me [Cp = C 5 H 5 , Cp Me = C 5 H 4 Me, Cp″ = C 5 H 3 (SiMe 3 ) 2 ] have been treated with potassium graphite in the presence of 2.2.2-cryptand to search for more stable examples of complexes featuring the recently discovered Y 2+ ion first isolated in [K(18-crown-6)][Cp′ 3 Y] and [K(2.2.2-cryptand)][Cp′ 3 Y], 1-Y (Cp′ = C 5 H 4 SiMe 3 ). Reduction of the tris(cyclopentadienyl) complexes generates dark solutions like that of 1-Y, and the EPR spectra contain doublets with g values between 1.990 and 1.991 and hyperfine coupling constants of 34−47 gauss that are consistent with the presence of Y 2+ . [K(2.2.2-cryptand)][Cp″ 2 YCp], 2-Y, was characterizable by X-ray crystallography. Reduction of the Cp″ 3 Gd, Cp″ 2 GdCp, and Cp″ 2 GdCp Me complexes containing the larger metal gadolinium were also examined. In each case, dark solutions and EPR spectra like that of [K(2.2.2-cryptand)][Cp′ 3 Gd], 1-Gd, were obtained, and [K(2.2.2-cryptand)][Cp″ 2GdCp], 2-Gd, was crystallographically characterizable. None of the new yttrium and gadolinium complexes displayed greater stability than 1-Y and 1-Gd. Exploration of this reduction chemistry with indenyl ligands did not give evidence for +2 complexes. The only definitive information obtained from reductions of the Cp In 3 Ln (Cp In = C 9 H 7 , Ln = Y, Ho, Dy) complexes was the X-ray crystal structure of {K(2.2.2-cryptand)} 2 {[(C 9 H 7 ) 2 Dy(μ−η 5 :η 1 -C 9 H 6 )] 2 }, a complex containing the first example of the indenyl dianion, (C 9 H 6 ) 2− , derived from C−H bond activation of the (C 9 H 7 ) 1− monoanion. Density functional theory analysis of these results provides an explanation for the observed hyperfine coupling constants in the yttrium complexes and for the C−H bond activation observed for the indenyl complex. ■ INTRODUCTIONRecent studies of the reduction chemistry of yttrium and the f elements have shown that the +2 ions are available for yttrium, 1 all the lanthanides 2−4 (except promethium, which was not studied due to its radioactivity), uranium, 5 and thorium. 6 These new oxidation states have been obtained by reduction of the tris(cyclopentadienyl) complexes, Cp′ 3 M and Cp″ 3 M [Cp′ = C 5 H 4 SiMe 3 , M = Y, lanthanide, U; Cp″ = C 5 H 3 (SiMe 3 ) 2 , M = La, Ce, Th] to form (Cp′ 3 M) 1− and (Cp″ 3 M) 1− complexes, Schemes 1 and 2.Structural, spectroscopic, and density functional theory analyses suggest that these new ions could be accessed for the first time because the (Cp′ 3 ) 3− and (Cp″ 3 ) 3− ligand sets allow the d z 2 orbital to be populated such that the new ions have 4f n 5d 1 electron configurations for the lanthanides, 5f 3 6d 1 for uranium, 6d 2 for thorium, and 4d 1 for yttrium. This is consistent with numerous theoretical analyses of the f elements in trigonal tris(cyclopentadienyl) coordination environments. 8−13 Whereas reduction of a 4f n Ln 3+ ion to a 4f n+1 Ln 2+ product would be difficult due to the highly negative calculated generic r...