Sterically Demanding Aryloxides as Supporting Ligands in Organoactinide Chemistry. Synthesis, Structural Characterization, and Reactivity of Th(O-2,6-t-Bu₂C₆H₃)₂(CH₂SiMe₃)₂ and Formation of the Trimeric Thorium Hydride Th₃H₆(O-2,6-t-Bu₂C₆H₃)₆

Abstract

Reaction of ThBr 4 (THF) 4 with 2 equiv of KOAr (Ar ) 2,6-t-Bu 2 C 6 H 3 ) produces the bis-(aryloxide) complex ThBr 2 (OAr) 2 (THF) 2 (1) in 67% yield. Alkylation of 1 with 2 equiv of Me 3 SiCH 2 MgCl allows the isolation of Th(OAr) 2 (CH 2 SiMe 3 ) 2 (2) in 61% yield. Thermolysis of 2 (benzene, 60 °C, 36 h) in the presence of NEt 3 results in the formation of the cyclometalated ligand redistribution product Th(OC 6 H 3 -t-BuCMe 2 CH 2 )(OAr) 2 (3). Reaction of 2 with 1 equiv of 2,6-dimethylphenyl isocyanate leads to insertion into the Th-C bond to yield (ArO) 2 Th[OC(dNR)CH 2 SiMe 3 ](CH 2 SiMe 3 ) (4; R ) 2,6-Me 2 C 6 H 3 ). Aminolysis of 2 with 2 equiv of 2,6-diisopropylaniline allows the isolation of the bis(amido) species Th(OAr) 2 (NH-2,6-i-Pr 2 C 6 H 3 ) 2 (5) in 92% yield. 2 reacts with dihydrogen (1.5 atm) over a period of 7 days to form the trimeric dihydride complex Th 3 (µ 3 -H) 2 (µ 2 -H) 4 (OAr) 6 (6). In the presence of 1 equiv of [HNMe 2 Ph][B(C 6 F 5 ) 4 ], 2 catalyzes the hydrogenation of 1-hexene (N t ) 1 h -1 ), while 6 is found to be a single-component catalyst for the analogous process (N t ) 3 h -1 ). Complexes 1-6 have been characterized by 1 H NMR and IR spectroscopy, microanalysis, and, in the case of 2 and 6, by single-crystal X-ray diffraction studies. 2 comprises a pseudotetrahedral thorium metal center bearing two aryloxide and two alkyl ligands. The O-Th-O angle between the aryloxide ligands (119.2(4)°) is significantly larger than the C-Th-C angle (101.4(6)°). Th-O and Th-C distances average 2.137(11) and 2.462(18) Å, respectively, while Th-C-Si angles are 125.9(8) and 122.8(8)°. The alkyl groups of 2 also display a reduced C-H coupling constant (J CH ) 98 Hz), suggestive of R-agostic interaction between the methylene group and the Th metal center. 6 exhibits a triangular arrangement of three thorium metal centers, each bearing two terminal aryloxide ligands. Two sides of the trimetallic core (Th-Th distances 3.781(1) and 3.818(1) Å) are bridged by single µ 2 -hydride ligands, while the third side is bridged by two µ 2 -hydride ligands (Th-Th distance 3.588(1) Å). Each face of the trimer is capped by a µ 3 -hydride ligand to produce a structurally unique M 3 (µ 3 -X) 2 (µ 2 -X) 4 X 6 geometry. Th-(µ 2 -H) and Th-(µ 3 -H) distances lie in the ranges 2.0(1)-2.3(1) and 2.3(1)-2.6(1) Å, respectively, while Th-O distances range from 2.126(7) to 2.164-(7) Å.

Molecular Thorium Trihydrido Clusters Stabilized by Cyclopentadienyl Ligands

“…The ThÀH distances range from 2. 16(2) to 2.59(2) , which are comparable to the reported range for (C 5 Me 4 H) 4 [m-h 5 -C 5 Me 3 H(CH 2 )-kC] 2 Th 4 H 8 (2.11 (5) to 2.56(6) ), [13] [(2,6-t Bu 2 -C 6 H 3 O) 2 Th(m-H) 2 ] 3 (2.0(1) to 2.3-(1) ) [5] and [(Cp * ) 2 ThH 2 ] 2 (terminal: 2.03 (3) ; bridging: 2.29(3) ). [2] As shown in Figure 3, the seven thorium centers in complex 8 adopt a distorted pentagonal bipyramidal geometry, with Th3, Th4, Th5, Th4 i , Th5 i defining the pentagonal plane with a mean deviation of 0.248 .…”

“…[4] Clark and co-workers disclosed that the sterically demanding 2,6-di-tert-butylphenoxide group can serve as a suitable non-Cp ancillary ligand for trinuclear thorium hexahydrido complexes [(2,6-t Bu-C 6 H 3 O) 2 Th(m-H) 2 ] 3 . [5] Compared with the rich chemistry of actinide dihydrido complexes of the general type Cp 2 AnH 2 , [2][3][4]6] which are supported by two cyclopentadienyl (Cp) ligands per metal, other types of Cp ligated actinide hydride complexes are to date, limited. The first example of actinide monohydrido complex [(Cp # ) 3 UH] (Cp # = C 5 H 4 SiMe 3 ) ( Figure 1, left) was reported by Ephritikhine and co-workers.…”

“…[11] In contrast to the mild conditions (1-5 atm H 2 at room temperature, [2] or PhSiH 3 at 50 8C [12] ) for the synthesis of actinide dihydrido complexes [(Cp * ) 2 AnH 2 ] 2 (An = Th, U), moderate pressure (10-60 atm H 2 ) and high temperature (in most cases, 60 8C) are required to convert the thorium tribenzyl complexes to the corresponding thorium trihydrido complexes. Hydrogenolysis of mono- (19.25 ppm), [2a] [Me 2 Si(C 5 Me 4 ) 2 Th(m-H) 2 ] 2 (18.36 ppm), [6a] and [(2,6-t Bu 2 -C 6 H 3 O) 2 Th(m-H) 2 ] 3 (20.54 ppm), [5] and thorium monohydrido complex [(Cp * ) 3 ThH] (15.40 ppm) and [(Cp Me4 ) 3 ThH] (15.34 ppm). [4] The polynuclear structures of complexes 7-9 were corroborated by single-crystal X-ray diffraction.…”

“…(3.632(2) ), [6a] [(2,6-t Bu 2 -C 6 H 3 O) 2 Th(m-H) 2 ] 3 (3.588(1) to 3.818(1) ), [5] and (C 5 Me 4 H) 4 [m-h 5 -C 5 Me 3 H(CH 2 )-kC] 2 Th 4 H 8 (4.1822(3) to 3.6960(3) ). [13] Each square face is occupied by seven bridging hydrides, two m 3 -H hydrides (H44, H44 i and H21, H21 i ), and five m 2 -H hydrides (H4, H34, H4 i , H34 i , H3 and H10, H12, H10 i , H12 i , H1), with H1/H3 located on twofold rotational axis.…”

“…The other six hydrides are located on six of the eight faces of the octahedron. Thus, each Th center is bonded to a h 5 (12) in 72 % and 85 % yields, respectively (Scheme 2). Since the steric bulk of Cp ttt and Cp''' ligands is very similar, the different products might be attributed to the electronic effects.…”