Oxo-alkoxides

A recent addition to the many examples of octahedral coordination of O (Table 14.3) is the unusual volatile, hydrocarbon-soluble, crystalline oxo-alkoxide of barium [H4Ba6(/i(i-0)(0CH2-CH2,OMe)i4l, which forms rapidly when Ba granules are reacted with Me0CH2CH20H in toluene suspension. ... 

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

One of the most widely studied initiators is commercially available yttrium iso-propoxide, (291), which exists as an oxo-alkoxide cluster of the formula Yf/is 0)(//2 0 Pr)4(/r3-01Pr)4(01Pr)5, usually abbreviated to Yfm 0)(0 Pr) 3.873 The ROP of rac-LA using several Lndiis 0)(0 Pr)13 complexes (Ln = La, Sm, Y and Yb) has been reported 874 in all cases polymerization occurs at room temperature with rates La Sm>Y Yb. The polymerizations... 

In order to impart abetter control of the polymerization, less reactive and thus more selective organometaUic derivatives of metals with r/-orbitals of favorable energy were substimted for anionic initiators [17]. The first works were carried out by Teyssie and coworkers using bimetallic p-oxo-alkoxides [18, 19]. 

The electro-attractive alkoxo groups render metal alkoxides very sensitive to nucleophilic attack.6 They are extremely susceptible to hydrolysis, which under controlled conditions yields oxo alkoxides they require strictly anhydrous conditions of handling. 

The controlled hydrolysis of MC15 or [Ta(OR)5]2 (R = Et, SiMe3) led to polymeric oxo alkoxides. On the other hand the hydrolysis of [NbCl2(OR)3]2 in the presence of bipyridyl led to [NbOCl2(OR)(bipy)] (R = Et, Pr1) adducts. 13 >364>412 An X-ray structure for R = Et showed a distorted coordination octahedron, with the two chlorine atoms trans to each other (Nb=Or 1.71(3) A). The Nb—OR bond is only 0.16 A longer than the Nb=0 oxo bond, and was presumed to have considerable double bond character, which is also consistent with the very open NbOC bond angle of 149°. 

The reactions of some simple as well as chelating alcohols (e.g., 2-alkoxy-alkanols) with metals, resulting in the formation of oxo-alkoxides (in place of simple binary alkoxides) will be dealt with in Sections IV and V. 

Although the observation of Bradley (1) that a remarkable feature of metal alkoxides is that they form very few coordination compounds with donor molecules remains valid for homometal alkoxides (except for halogenoalkoxide derivatives cf. Section IV.B), the newer species of oxo-alkoxides tend to form adducts with donor molecules such as thf in consonance with their geometrical... 

X-ray crystallography has played a prominent role in establishing the inclusion of heteroinorganic ions. More specifically, in the case of oxo-alkoxides (Section V), where the presence of the 02 ion is undetectable analytically. 

By contrast, reaction of barium granules in toluene with 2-methoxyethanol follows a different course, yielding an oxo-alkoxide (Eq. 138) (372) ... 

The last reaction involves the conversion of an oxo-alkoxide Y50(0-i-Pr)13 into a homoleptic yttrium alkoxide Y(OC2H4OMe)3, which is probably due to the higher stability of the latter. 

Y5 Al(0-z -Pr)4 3 on condensation with 3 mol of Al(0-i-Pr)3, whereas the mother liquor of the reaction mixture could yield the expected Y Al(0-/-Pr)4 3 in fairly high (— 70%) yield. Although similar comments were made initially by Mehrotra (479), many of these oxo-alkoxides were formed by a variety of conventional routes, which were expected to yield the normal alkoxides. This number has been increasing so fast in spite of the most stringent anhydrous conditions employed by the investigators, that concerted efforts are being made to suggest novel pathways by which such oxo-alkoxide species could result under nonhydrolytic conditions as well. 

As mentioned in the above introduction, the formation of a number of hy-droxo- and oxo-alkoxide products of different metals was reported (till 1978) (1,6) during the extensive hydrolytic studies carried out on metal alkoxides and even crystal structures of a few oxide-alkoxides of Ti (475a), Nb (476a), Zr (477a), and Sn(II) (477b). 

Since 1989, however, various oxo-alkoxides (both homo as well as hetero) were isolated by a variety of methods that could be expected to yield the normal alkoxides under nonhydrolytic conditions. These are summarized in Section V.B.2 by a few illustrative equations. This is followed by Section V.B.3, which indicates a few special types of reactions that yield oxo-alkoxide derivatives. Finally, salient crystal data of some oxo-alkoxides are summarized in Table VI. 

Although most of the preparations appear to have been so far unplanned, the following equations represent the isolation and characterization of oxo-alkoxide species by conventional routes (Sections II.A. 1, II.A.2, II.C.l, II.C.2, and II.F.4), which are expected to yield the normal homo- or heterometal alkoxides. 

Crystal Data for Some Oxo-Alkoxides of Metals and Bridging Modes of Oxo Ligands... 

Note The formation of a diolate moiety (0CEt2CH20) in the oxo-alkoxide produced in Eq. 204 appears to arise in a side reaction with the solvent THF, as the same is not formed in the absence of THF.)... 

Note On standing, the above conversion of the initial product in toluene was suggested as an indication of the manner in which alkoxides could change into oxo-alkoxides, even during a sol-gel process.)... 

In addition to the formation of unexpected oxo-alkoxides by routine procedures, the following equations represent the synthesis of U02(0-/-Bu)2(Ph3P0)2... 

In addition to these routine procedures, the following equations illustrate rather unusual (novel) routes by which the formation of oxo-alkoxides of metals were reported. 

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