Thermally or Photochemically Induced Reductive Cleavage of Metal−Metal Bonds of Metal Carbonyl Dimers by a Titanocene(III) tert-Butoxide:  Novel Reversible Access to Heterobimetallic Complexes

Abstract

Novel synthetic methods for Ti−Mo, Ti−W, and Ti−Ru heterobimetallic complexes are established by the reaction of a certain titanium(III) alkoxide and the corresponding metal carbonyl dimer, [CpM(CO) n ]2 [M = Mo, W (n = 3); M = Ru (n = 2)]. Thus, a novel monomeric titanium complex, Cp2Ti(OtBu) (1), which is synthesized from Cp2TiCl and KOtBu and characterized by spectroscopy and crystallography, smoothly reacts with [CpM(CO)3]2 (M = Mo (2a), W (2b)) to give heterobimetallic complexes, Cp2Ti(OtBu)(μ-OC)M(CO)2Cp (M = Mo (3a) and W (3b)), of which two metallic moieties are linked by the isocarbonyl bridge. Similar reaction of 1 with [CpRu(CO)2]2 (4) does not occur thermally, but is accomplished under photoirradiation to afford Cp2(OtBu)Ti−Ru(CO)2Cp (5), of which two metallic moieties are linked by a direct metal−metal bond. Use of 1 is particularly important for these reactions; other titanium alkoxides such as [Cp2Ti(OMe)]2 and Cp2Ti[O(2,6-tBu2-4-Me)C6H2] do not react with the metal carbonyl dimers. An interesting feature of these new heterobimetallic complexes, 3a, 3b, and 5, is the existence of the thermal fragmentation process to regenerate the starting materials: 3a or 3b is actually in equilibrium with a mixture of 1 and 2a or 1 and 2b, respectively [ΔG 0 298K = −4.1 ± 0.2 kcal mol-1 (3a), −4.3 ± 0.2 kcal mol-1 (3b)]. The Ti−Ru compound 5 thermally undergoes fragmentation to regenerate 1 and 4. The formation of these heterobimetallic complexes is formally considered as the metal−metal bond cleavage of metal carbonyl dimers by a Ti(III) reducing reagent; possible mechanisms are discussed.

The effect of titanium alkoxides in the synthesis of heterobimetallic complexes by titanocene(III) alkoxide-induced metal–metal bond cleavage of metal carbonyl dimers

“…As reported previously, 1a and 2a reacted with [CpMo(CO) 3 ] 2 or [CpW(CO) 3 ] 2 to form the corresponding heterobimetallic complex, [15]. Among the Cp-substituted titanocene alkoxides, 1b and 1d did not react with [CpM(CO) 3 ] 2 , but the reaction with 1c reversibly formed the corresponding heterobimetallic complexes, 3c-Mo and 3c-W; a toluened 8 solution of a 1:1 mixture of 1c and [CpM(CO) 3 ] 2 gave a 40:60 equilibrium mixture of the heterobimetallic product and the starting materials.…”

“…and 2f, formed the Ti-Co heterobimetallic products. The reason why Ti-Co 3 clusters formed in the reactions of the Cp-substituted titanocene alkoxides can be attributed to the fact that the Ti-Co bimetallic complexes are generally not very stable unless the bulky and electrondonating Cp* ligands stabilize them, as discussed in our previous paper [15]. Among the monomeric titanocene alkoxides, 1a, 1c, 2a, 2e, and 2f, 1c reacted Co 2 (CO) 8 cleavage.…”

“…In typical examples, their catalysis has been investigated in hydroformylation [2][3][4][5], carbonylation [6], hydrogenation/isomerization [7], asymmetric synthesis [8][9][10][11], olefin methathesis [12], enol ester formation [13], and olefin polymerization [14]. In our previous papers, we have reported unique access to heterobimetallic complexes by reactions of titanocene t-butoxides, Cp 2 Ti(O t Bu) (Cp = g 5 -C 5 H 5 ; 1a) and Cp à 2 TiðO t BuÞ (Cp* = g 5 -C 5 Me 5 ; 2a), with certain metal carbonyl dimers, [CpM(CO) 3 ] 2 [M = Mo, W], [CpRu (CO) 2 ] 2 , and Co 2 (CO) 8 [15]. 3 (6a) and Cp à 2 TiðO t BuÞðl-OCÞCoðCOÞ 3 ð7aÞ were obtained by treatment of 1a or 1b with Co 2 (CO) 8 ; 6a readily reacted with Co 2 (CO) 8 A unique feature of these reactions is that the titanocene(III) t-butoxides are good reagents to cleave the metal-metal bond of metal carbonyl dimers, presumably via electron transfer from Ti(III) to the metal dimer, and the metal-metal bond cleavage leads to change of the oxidation state of titanium from III to IV and formation of heterobimetallic complexes.…”

Cationic Ti(iv) and neutral Ti(iii) titanocene–phosphinoaryloxide frustrated Lewis pairs: hydrogen activation and catalytic amine-borane dehydrogenation

“…However, the complete loss of the Cp resonances and broadening of the aromatic signals in the 1 H NMR spectrum for this new species suggest it is a paramagnetic Ti(III) complex, observations which are also consistent with the genuine Ti(III) species 6 (vide infra) and other related compounds. 22,23 A weak signal in the 31 P{ 1 H} NMR spectrum at δ 22.5 ppm is indicative of a P-H centre. The product was crystallized and unambiguously determined by X-ray crystallography to be 5 (Fig.…”

Reactions of early–late heterobimetallics with oxiranes: New examples for cooperative reactivity

“…The heterobimetallic product isolated in the case of 5b, (Cp * ) 2 Ti(O t Bu)(l-OC)Co(CO) 3 (6), contains a titanium atom connected to cobalt by an isocarbonyl bridge. On the other hand, a metal-metal-bonded species, (Cp) 2 (O t Bu)Ti-Ru(CO) 2 (Cp) (7) has been obtained from the photochemically induced ''radical-radical'' reaction of 5a with [CpRu(CO) 2 ] 2 [18]. While the reversible dissociation of Co 2 (CO) 8 to two Co(CO) 4 radicals takes place even at ambient temperature (see, e.g.…”