Synthetic Endeavors toward Titanium Based Frustrated Lewis Pairs with Controlled Electronic and Steric Properties

Abstract

A new family of cationic Ti complexes 4′ with a pendant phosphine of general formula [CpCpPTiOAr]­[BPh4] (Cp = η5-C5H5; CpP = η5-C5H4(CMe2)­PR2) has been prepared in four steps from 6,6-dimethylfulvene. These complexes were designed to behave as Ti based frustrated Lewis pairs (FLPs). The key synthetic step is a reduction–oxidation sequence from [CpCpPTiClOAr] complexes 3 using lithium phosphide salts as the reductants and ferricinium tetraphenylborate as the oxidant. Four complexes have been structurally characterized by X-ray diffraction and show elongated Ti–P bonds, above 2.60 Å. One complex (4b′: OAr = 2,6-Me2C6H3; PR2 = PCy2) reacted with benzaldehyde to form a typical FLP activation product. Complex 4b′ also reacted with 2 equiv of trans-chalcone to form a 10-membered Ti phosphonium macrocycle (6b′) by extrusion of 6,6-dimethylfulvene.

The Taming of Redox‐Labile Phosphidotitanocene Cations

“…The synthesis of the target phosphidotitanocene cations can be effected in two steps from previously described [Cp P CpTiCl 2 ] complexes 1 (Cp = cyclopentadienyl). [13] Ti III precursors were first prepared on 1-2 mmols cale by reacting 2equivalents of lithium phosphide( PPh 2 Li or PCy 2 Li)w ith 1 (Scheme 4). [14] Compounds 2a-d were obtaineda sg reen (2a, 2b, 2d)o r brown( 2c)s olids in moderate yield.…”

“…We next turned our attention to the synthesis of the target phosphidotitanocene cations 3 , following the oxidation route described above (Scheme ). Thus, Ti III complexes 2 a – d were oxidized with [Cp 2 Fe][BPh 4 ] in C 6 H 5 Br, following a modification of a previously reported procedure …”

Insertion Reactions of Neutral Phosphidozirconocene Complexes as a Convenient Entry into Frustrated Lewis Pair Territory

“…However, further spectroscopic evidence was consistent with a racemic mixture ( S , S / R , R ) of 1,4‐addition product. Evidence for the formation of a phosphonium enolate comes from the 13 C{ 1 H} NMR spectrum, in which the signals of the α‐ and β‐carbon atoms of chalcone were observed at δ =95.9 ( 2 J PC =3.4 Hz) and 36.6 ppm ( 1 J PC =33.9 Hz) . In addition, positive NOEs were observed by 1D NOESY spectroscopy between 1) the Cp ring at δ =5.97 ppm and the benzaldehyde hydrogen ( δ =6.33 ppm; 2 J PH =2.7 Hz), on one hand, and 2) the Cp ring at δ =6.66 ppm and the hydrogen attached to the β carbon of chalcone ( δ =5.43 ppm; 2 J PH =16.9 Hz, 3 J HH =10.5 Hz) on the other hand.…”

“…Evidence for the formation of ap hosphoniume nolate comes from the 13 C{ 1 H} NMR spectrum, in which the signals of the a-a nd b-carbon atoms of chalcone wereo bserved at d = 95.9 ( 2 J PC = 3.4 Hz) and 36.6 ppm ( 1 J PC = 33.9 Hz). [24] In addition, positive NOEs were observed by 1D NOESY spectroscopy be-Scheme6.Synthesis of 12a (X À = MeB(C 6 F 5 ) 3 À ).…”

“…Material prepared according to this procedure contained < 5% of (Cp 2 ZrMe) 2 O, as evidenced by 1 HNMR spectroscopy.S ingle crystals suitable for XRD were obtained by cooling as aturated solution of 5a in cyclopentane at À35 8C. [29] Elemental analysis calcd (%) for C 24 Compound 6a:I naglove box with an Ar atmosphere, complex 1a (434 mg, 1.0 mmol) and paraformaldehyde (30 mg, 1.0 mmol) were mixed in of a1:1mixture of pentane/THF (3 mL). The reaction mixture was stirred for 3h under Ar outside the glove box at room temperature, after which time it was clarified with ab orosilicate filter in the glove box and evaporated to almost dryness.…”