Construction of Titanasiloxanes by Incorporation of Silanols to the Metal Oxide Model [{Ti(η⁵‐C₅Me₅)(μ‐O)}₃(μ₃‐CR)]: DFT Elucidation of the Reaction Mechanism

Abstract

A family of novel titanasiloxanes containing the structural unit ([Ti(eta 5-C5Me5)O]3) were synthesized by hydron-transfer processes involving reactions with equimolecular amounts of mu3-alkylidyne derivatives [(Ti(eta 5-C5Me5)(mu-O))(3)(mu3-CR)] (R=H (1), Me (2)) and monosilanols, R3'Si(OH), silanediols, R2'Si(OH)2, and the silanetriol tBuSi(OH)3. Treatment of 1 and 2 with triorganosilanols (R'=Ph, iPr) in hexane affords the new metallasiloxane derivatives [(Ti(eta 5-C5Me5)(mu-O))(3)(mu-CHR)(OSiR3')] (R=H, R'=Ph (3), iPr (4); R=Me, R'=Ph (5), iPr (6)). Analogous reactions with silanediols, (R'=Ph, iPr), give the cyclic titanasiloxanes [(Ti(eta 5-C5Me5)(mu-O))(3)(mu-O2SiR'2)(R)] (R=Me, R'=Ph (7), iPr (8); R=Et, R'=Ph (9), iPr (10)). Utilization of tBuSi(OH)3 with 1 or 2 at room temperature produces the intermediate complexes [(Ti(eta 5-C5Me5) (mu-O))(3)(mu-O2Si(OH)tBu)(R)] (R=Me (11), Et(12)). Further heating of solutions of 11 or 12 affords the same compound with an adamantanoid structure, [(Ti(eta 5-C5Me5)(mu-O))3(mu-O3SitBu)] (13) and methane or ethane elimination, respectively. The X-ray crystal structures of 3, 4, 6, 8, 10, 12, and 13 have been determined. To gain an insight into the mechanism of these reactions, DFT calculations have been performed on the incorporation of monosilanols to the model complex [(Ti(eta 5-C5H5)(mu-O))3(mu3-CMe)] (2 H). The proposed mechanism consists of three steps: 1) hydron transfer from the silanol to one of the oxygen atoms of the Ti3O3 ring, forming a titanasiloxane; 2) intramolecular hydron migration to the alkylidyne moiety; and 3) a mu-alkylidene ligand rotation to give the final product.

Hydron‐Transfer Processes Involving an Organotitanium Oxide and Alcohols

“…One of the key features of the 13 C NMR spectra is the very strong shielding [∆δ = 192-195 (3-6) and 203-210 ppm (7)(8)(9)(10)(11)(12)] observed for the resonances of the µ-alkylidene carbon atoms (µ-CHR) with respect to the starting µ 3 -alkylidyne group [δ = 383.5 (1), 401.7 (2)]. This fact is in agreement with the trend followed by other µ-methylene [4b,5,8,9] and µ-ethylidene [5,6] titanium systems reported in the literature. The parent ion was observed in the EI mass spectra for 10 and 11 and related fragments such as [ The structural situation of complexes 3-12 shown in Scheme 1, where the alkylidene group lies above the Ti 3 O 3 unit and the alkoxide ligand below, was confirmed by the X-ray diffraction studies of 3, 5, 10 and 11.…”

“…The results are qualitatively similar to those previously found in the analogous process involving silanols and 2H. [6] However, the differences between alcohols and silanols, and between the methylidyne complex 1H and the ethylidyne complex 2H, merit a more detailed discussion. Table 2 collects the calculated reaction and activation energies for these different processes.…”

“…[6] This mechanism involves the following steps (see Figure 4): 1) hydron transfer from the substrate to one of the oxygen atoms of the Ti 3 O 3 ring; 2) intramolecular hydron migration to the alkylidyne moiety; and, in the case of ethylidyne complex 2H, 3) µ-alkylidene ligand rotation to give the observed product. Interestingly, Corma and Ujaque have recently reported that alcohol solvent molecules play an active role in heterolytic H 2 cleavage by Au III complexes involving hydron migration.…”

“…Treatment of the µ 3 -alkylidyne species [{Ti(η 5 -C 5 Me 5 )(µ-O)} 3 (µ 3 -CR)] [R = H (1), Me (2)] with alcohols (Ph 3 COH, Ph 2 CHOH, PhCH 2 OH, Ph 2 CMeOH, tBuOH, Me 2 CHOH and MeCH 2 OH) [7] in hexane at room temperature or under slight heating gives the new oxo derivatives [{Ti(η 5 -C 5 Me 5 )(µ-O)} 3 (µ-CHR)(ORЈ)] [R = H, RЈ = Ph 3 C (3), Ph 2 CH (4), Ph 2 CMe (5), tBu (6); R = Me, RЈ = Ph 3 C (7), Ph 2 CH (8), Ph 2 CMe (9), tBu (10), Me 2 CH (11), MeCH 2 (12)] as reddish orange (3)(4)(5)(6) or violet microcrystalline products (7)(8)(9)(10)(11)(12) in good yields (Scheme 1). The analytical, mass and NMR spectroscopic data of these compounds are consistent with partial hydronation of the alkylidyne moiety supported on the organometallic oxide moiety [Ti 3 The NMR spectra of these complexes display two types of η 5 -C 5 Me 5 ligands in a 2:1 ratio, which is consistent with a C s symmetry and in contrast with the C 3v symmetry of the starting materials 1 and 2, in addition to signals for the alkoxide substituents.…”

“…[4] to act as real hydrogen acceptor substrates in their reactions with different reagents. The overall results obtained in the treatment of these species with a large variety of amines [5] and silanols [6] can be considered as sequential hydron-transfer processes of alkylidyne groups on an organometallic oxide under mild conditions. Additionally, the different steps of this process (alkylidene, alkyl and alkane) have been identified.…”