Unusual Redox Chemistry of Ytterbium Carbazole–Bis(oxazoline) Compounds: Oxidative Coupling of Primary Phosphines by an Ytterbium Carbazole–Bis(oxazoline) Dialkyl

The
1,8-bis­(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-tert-butylcarbazole anion (Czx) forms monomeric, six-coordinate
halide complexes of Yb­(II), (Czx)­Yb­(X)­(THF)2 (X = I (2), Cl (3)), by metathesis of YbX2 with NaCzx (1) or Na/Hg reduction of (Czx)­Yb­(Cl)2(THF). The crystal structure of 1 reveals a polymeric
chain structure in which the oxazoline ring bridges to the Na+ of an adjacent unit. The iodo complex 2 serves
as a precursor to divalent silylamide, alkyl, and phosphide complexes,
(Czx)­Yb­(X)­(THF)n (4, X =
N­(SiMe3)2, n = 1; 5, X = CH­(SiMe3)2, n = 1; 7a, X = 2,4,6-Me3C6H2PH, n = 2; 7b, X = 2,4,6-Pri3C6H2PH, n = 2). The X-ray
structure of 4 reveals a distorted-trigonal-bipyramidal
geometry with the Czx ligand occupying two axial sites and one equatorial
site in a pseudo-mer coordination mode. In contrast
to the typical metathesis chemistry observed with LiCH­(SiMe3)2, an unusual oxidation occurs when 2 or 3 is treated with LiCH2SiMe3 to generate the previously isolated trivalent alkyl (Czx)­Yb­(CH2SiMe3)2. Trivalent Yb complexes with
the Czx ligand also display unusual redox chemistry: rapid reduction to the Yb­(II) phosphides 7a,b is observed on treatment of mer,cis-(Czx)­Yb­(Cl)2(THF) with ArPH– Na+ (6a,b) or, equivalently, on treatment of (Czx)­Yb­(CH2SiMe3)2 with ArPH2. In both
cases, oxidative coupling of the phosphide or phosphine was observed
to form meso- and rac-biphosphines,
ArPH–PHAr (Ar = 2,4,6-Me3C6H2 (9a), 2,4,6-Pri3C6H2 (9b)).