Heterometallic alkoxides structures

Figure 6.23 The molecular (left) and crystal (right) structures of [Pr(mmp)3LiCl]2. For clarity, the ligand methyl groups are omitted from the molecular structure [49]. (Redrawn from J.M. Gaskell et al., Deposition of Pr- and Nd-almninate by liquid injection MOCVD and ALD using single-som-ce heterometallic alkoxide precursors, Chemistry of Materials, 19, 4796 803, 2007.)...

Caulton, K.G and Hubert-Pfalzgraf, L.G. (1990) Synthesis structural principles and reactivity of heterometallic alkoxides. Chemical Reviews, 90, 969-995. 

The close relationships in the structural features of homo- and heterometallic alkoxides can be exemplified by those of Al Al(0-i-Pr)4 3 (150) and Ln Al(0-/-Pr)4 3 (18). The molecular weights of the latter in benzene correspond to their empirical formulas. All of these compounds can be distilled in the range of 200-180°C/0.1 mm, with a lowering of the boiling point as was expected from increasing the covalent character which results from lanthanide contraction in the series. The tetrameric aluminum isopropoxide A1 A1(0-/-Pr)4 3 or A1(0-/-Pr)3 4, however, disproportionates and distills as a dimeric vapor around... 

A description of the various physicochemical properties (e.g., volatility, dipole moments, refractivity, density, viscosity, magnetic susceptibilities, IR, ESR spectra, mass spectra, and NMR spectra) for elucidation of the structural features of homo- and heterometallic alkoxides was already presented in a number of publications [e.g., (3, 4, 6, 19, 34)]. 

Observed magnetic susceptibilities of homo- and heterometallic alkoxides of latter 3d metals (Cr, Mn, Fe, Co, Ni, or Cu) are in consonance with their structures, which were derived on the basis of electronic spectra (7, 19). For example, chromium(III) alkoxides as well as their substituted derivatives, Cr(OEt)(acac)2 and Cr(OEt)2(acac), all depict a magnetic susceptibility of 3.8/xB, indicative of an octahedral geometry for chromium with three unpaired electrons. 

As mentioned earlier, fairly successful conjectures (147, 148) about the structural features of homo- as well as heterometallic alkoxides were made earlier on the basis of physicochemical studies in solution. Even as late as 1989, Bradley (72) pointed out the special difficulties in the X-ray structural elucidation of metal alkoxide complexes. Such difficulties were reemphasized by Caul-ton and Hubert-Pfalzgraf (4), particularly the heterometallic alkoxides, which in many cases are obtained in a highly plastic (deformable) state even when crystallizable, which renders them unsuitable for single-crystal X-ray analysis. Therefore, the process of X-ray crystallographic structural characterization was gathering considerable momentum (4, 23, 28, 38, 39) and even with the availability of other more sophisticated techniques, publications were being limited mainly to derivatives characterized crystallographically. 

Pr)2Ba(/i-0-i-Pr)2]2. This observation is intriguing in heterometallic alkoxide chemistry. The analytical as well as other physicochemical data could not have distinguished between the expected and the rather unusual actual formulation, which was established by X-ray crystallography (Fig. 37). This structure is consistent with the preferential characteristics of the two metals involved (barium and cadmium) for attaining coordination numbers 6 and 4, respectively. 

Recent successes [cf. (4a, 23, 28, 38, 39, 42, 199)] in X-ray structural elucidation of an increasing number of homo- and heterometallic alkoxides revealed novel features in addition to general quantification of their basic framework, which was postulated on the basis of spectroscopic (mainly NMR) studies described in Section III.A.4.a. 

The earlier account of the structural features of di- and polynuclear homo-and heterometallic alkoxides have already been comprehensively reviewed (4a, 23, 28, 38, 39, 42, 199, 202). However, with the current resurgence of interest in the structural characteristics of associated homometal alkoxides and monomeric heterobimetallic alkoxides, we present current status of the solid state structures (as determined by X-ray crystallographic studies) of such derivatives. 

In addition to what was already discussed in the preceding sections, great diversification in the structural features of homo- and heterometallic alkoxide complexes were revealed by X-ray crystallography for many poly- (octa-,... 

A new dimension has been opened during the last decade in the field of heterometal coordination chemistry because of the synthesis of a variety of thermally stable ter and higher heterometallic alkoxides. This area is expected to receive increasing attention because of the first X-ray structural elucidation done in 1996 (39a) of a termetallic isopropoxide of Ba, Zr, and Cd. This derivative also revealed an even more fascinating behavior of the shifting of the alkoxo-metallate ligands to suit the nature of the three metals involved in the species. 

Transition-metal-based heterometallic alkoxide complexes have been prepared. For example, the heterometallic complex Fe(Al(OBut)4)(Ti2(OPr1)9) was reported but not structurally characterized.494 The reaction of (Rh(/i-OH)(COD))2 with TiMe(OBu )3 at low temperatures afforded (Ti(OBu )2)2-(/i-03)(Rh(C0D))4 (50)495 In the same vein, the Ti/Rh complex (Ti(OPr)gRh(COD))2 was obtained.496... 

This chapter deals with the synthesis and general properties of homo- and heterometallic alkoxides that are or might be used in chemical routes to electrooptical ceramic materials. Some considerations concerning structure, reactivity, and tailoring of their properties are also given. Emphasis is given to the most recent results. 

Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic. Partial hydrolysis of [LiNb(OEt)6]< leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in which both the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1 1 molar ratio) offers Ba4Tii30i8(0C2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron of BaOs units surimposed on a Ti06(Ti03)i2 core this Tii3042 core is related to that of the aluminum salt [A1i304(0H)24(H20)]2+ [83]. The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur. 

V.C. Kessler, Molecular structure design and synthetic approaches to the heterometallic alkoxide complexes (soft chemistry approach to inorganic materials by the eyes of a crystallographer), Chem. Commun., vol. 11, pp. 1213-1222, 2003. 

In addition to the homoleptic bimetallic derivatives like [M Al(OPr )4 j] hetero-leptic tri- and tetrametalUc derivatives with the general formula, [M Al(OIV)4 a Nb(OPr )6 6 Zr2(OPr )9 cX c a c] (where X is a ligand of the type Cl, OR, acac, etc.) have also been synthesized in the laboratories of Mehrotra " since 1985. Many of these have been shown to volatilize unchanged under reduced pressure. These stable derivatives have thus added a novel dimension to heterometallic coordination systems. The prominent effect of steric factors on the stability and structural features of some bimetallic species has been observed in a comparison of similar pairs (i) [NaZr2(OPr )9] and [ NaZr(OBu )s 2] (ii) [Ni (/4-OMe)3Al(OMe) 2] and [Ni (/4-OPr )(/i-OBu )Al(OBu )2 2]." Some other novel types of heterometallic alkoxide/glycolate/aminoalkoxide derivatives have also been synthesized." ... 

The chemistry of heterometallic alkoxides involving a common chelating ligand, Al(OPr )4 finds a close parallelism in an early suggestion by Bradley that tetrameric aluminium isopropoxide Al(OPr )3 4 may be represented by Al (/r-OPr )2Al(OPr )2 3 in which the central aluminium is hexacoordinated by being ligated with three bidentate Al(OPr )4 figands. This unusual structure of... 

As illustrated by a number of examples in Section 1, the X-ray structural elucidation of a rapidly increasing number of heterometallic alkoxides (Chapter 5) has in general confirmed their coordination models ° with chelating ligands like M(OR) +i (M = Al, Ga, Nb, Ta) and M2(OR)2n+i (M = Zr, Hf, Sn(iv)) of metals with valency n, as suggested since 1971. 

The role of steric effects in determining the stoichiometry and structure of a heterometallic alkoxide can be further illustrated by the comparative stability of M M2(0R)9 and M M(OR)5 systems. 

Although no oxo-alkoxide structures involving cobalt or nickel appear to have been reported there is an interesting octanuclear nickel antimony heterometallic compound [Ni5Sb3(/u.4-0)2(/u,3-OEt)3(/r-OEt)9(OEt)3(EtOH)4] in which the Ni(n) atoms are octa-hedraUy coordinated, one Sb is four-coordinated (lone pair in the vacant site of a... 

Caulton K.G., Hubert-Pfalzgraf L.G. Synthesis, Structural principles, and reactivity of heterometallic alkoxides. Chem. Rev. 1990 90 969... 

K.G. Caulton, L.G. Hubert-Pfalzgraf - Synthesis, Structural Principles and Reactivity of Heterometallic Alkoxides, Chem. Rev. 90, 969, 1990. 

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