Bul

The reaction of scandium triflate with Li[C(PMe2)2SiMe3] gives a six-coordinate complex with a trigonal prismatic environment (12). There is an unusual ScP2C four-membered ring in this compound.158

4.1.8 OXYGEN DONOR LIGANDS 4.1.8.1 Phosphine and Arsine Oxide

CCC (1987) contains a section on sulfoxides, amide, amine oxides, and related ligands that have sections on phosphine oxides and arsine oxides.159 Phosphine and arsine oxide complexes of yttrium and the lanthanides were highlighted in CCC (1987).1 Scandium and yttrium phosphine oxide complexes have been reinvestigated and characterized by multinuclear NMR and X-ray

crystallography.160 Three phosphine oxides have been used, Ph3PO, Ph2MePO, and Me3PO and four types of compounds of both scandium and yttrium prepared: M(R3PO)2(NO3)3, M(R3PO)3(NO3)3, [M(R3PO)4(NO3)2]NO3, and M(R3PO)2(EtOH)(NO3)3. Scandium also forms a fifth complex [M(Me3PO)6](NO3)3. 45Sc, 89Y, and 31P NMR have been used to characterize species in solution and these show a range of complexes having variations in M and R3PO. The crystal structures of [Sc(Ph3PO)2(NO3)3] and [Sc(Ph?MePO)4(NO3)2]NO3 show that the scandium is eight-coordinate with bidentate nitrates. The yttrium complexes Y(Ph3PO)2(EtOH)(NO3)3 and Y(R3PO)3(NO3)3 (R3PO = Ph3PO, Ph2MePO, and Me3PO) all have nine-coordinate yttrium.

Arsine oxide complexes of scandium and yttrium nitrate have been synthesized and characterized by NMR and X-ray crystallography.5 Me3AsO produces one type of complex [M(Me3-AsO)6](NO3)3 and the crystal structure shows six coordination at the metal. 45Sc and 89Y NMR indicates that the major species in solution is [M(Me3PO)6]3+ but there are also the species [M(Me3AsO)6_n(NO3)n](3"n^+. Ph3AsO forms [Sc^AsOMNO^], [Sc^AsO^NO^^, [Y(Ph3AsO)4(NO3)2]NO3, and Y(Ph3AsO)2(EtOH)(NO3)3. The crystal structure of [Sc^AsO^-(NO3)2]NO3 shows the metal to be seven-coordinate and the crystal structure of [Y(Ph3AsO)4-(NO3)2]NO3 shows the metal to be eight-coordinate.

4.1.8.2 Alkoxides

The syntheses, physical properties, and molecular structures of alkoxides and aryloxides have been discussed in CCC (1987).161 The alkoxides of scandium and yttrium were reviewed in CCC (1987).1 There have been more recent developments in this area and the impetus for this chemistry has been the developments in materials research. Metal alkoxides and ^-diketonates can be used as precursors for oxide and nonoxide thin films.162 The stable M—O bond and the volatility of the metal alkoxides are important features of this area of chemistry. This has lead to more research in this area particularly in synthesis, NMR, and X-ray crystallography.

The three major preparative methods used to obtain the alkoxides of Sc and Y are:

(i) the reaction of the anhydrous metal halide with an alkalis metal alkoxide,

(ii) alcoholysis of a scandium or yttrium alkoxide, and

(iii) the reaction of the metal dialkylamides, principally the hexamethyldisilylamino compounds M[N(SiMe3)2]3.123

The simple alkoxides of scandium Sc(OR)3 (R = Me, Et, or Bun) prepared by alcoholysis of Sc5O(OPri)13 or anodic oxidation of scandium in alcohols are polymeric and do not contain the oxo ligand.163 Attempts to prepare the simple iso-propoxides of Sc or Y always produce the oxoalkoxides [M5(^5-O)(^3-OPri)4(^2-OPri)4(OPri)5] that are volatile crystalline solids.80'164'165 The stabilty of the M5O(OR)8 core is shown by the partial alcoholysis of Sc5O(OPri)13 by alcohols to produce Sc5O(OPri)8(OR)5 compounds.16 Metal oxoalkoxides have been reviewed166 and included in the review are some scandium and yttrium alkoxides (such as the iso-propoxides) and mixed metal alkoxides (such as the yttrium barium oxoalkoxides).167 Sc5O(OR)13 compounds have been prepared using an electrochemical synthesis.163 In a variation of the normal reaction of the metal halide with sodium iso-propoxide, Y5O(OPri)13 may be prepared using anhydrous (NH4)3Y(NO3)6.168 Two 89Y solution NMR studies of Y5O(OPri)n have been undertaken.79'80

220.0 215.0 210.0

Figure 8 89Y NMR of Y5O(OPri)13 showing the presence of two inequivalent yttrium atoms in the molecule (reproduced by permission of the American Chemical Society from Inorg. Chem., 1992, 31, 1262-1267).

220.0 215.0 210.0

Figure 8 89Y NMR of Y5O(OPri)13 showing the presence of two inequivalent yttrium atoms in the molecule (reproduced by permission of the American Chemical Society from Inorg. Chem., 1992, 31, 1262-1267).

The 89Y solution spectrum (Figure 8) of Y5O(OPr')13 shows that there are two distinct yttrium sites, which is consistent with the solid-state structure. Figure 9 shows these sites are very similar, with the yttrium atoms being coordinated to six oxygens, showing the sensitivity of 89Y chemical shifts to slight structural variations.79

Trimeric alkoxides of yttrium are formed where the R group is larger than Pr1, e.g., R = But or CH2C(CH3)3 giving compounds such as [Y3(OBut)9(ButOH)2] and with even larger alkyl groups such as CEt3 dimers are obtained.81

Well characterized "simple" alkoxides of scandium and yttrium M(OR)3 are rare. Three coordination about the metal requires a sterically large ligand such as the 2,6-di-tert-butylphen-oxide, which does form both a scandium and yttrium monomeric alkoxide.54'169 Y(OC6H4But2)3 has been used as an initiator in the polymerization of L-lactide.170 The 2,6-diphenylphenoxide also forms a monomeric yttrium alkoxide46 although thf adducts have been prepared for substituted 2,6-diphenylphenoxides of scandium and yttrium.171 The sterically smaller 2,6-dimethylphenoxide does not form three-coordinate yttrium species and allows coordination of donor ligands such as thf in the yttrium compound Y(OC6H3Me2)3(thf)3.172

Many other alkoxides show a tendency to polymerize, thus a toluene solution of Y(OC6H3-Me2)3(thf)3 produces a dimeric compound [Y(OC6H3Me2)3(thf)2]2 which when dissolved in thf reverts to the monomer Y(OC6H3Me2)3(thf)3. This shows the facile interconversion of the two alkoxides.172 The interconversion of yttrium alkoxides is also shown in the tert-butoxide compound Y3(OR)7Cl2(thf)2 which polymerizes in toluene solution to give the Y14 complex [Y7(OR)14Cl5O(thf)2]2.173 Substituted alcohols with potentially coordinating groups such as HOCR2CH2OMe react with the metal or Y[N(SiMe3)2]3. A decameric yttrium alkoxide [Y(OR)3]10 is formed by the reaction of 2-methoxyethanol with yttrium metal.174 The compound has a cyclic structure where the alkoxy groups act as bridging bidentate ligands. More highly substituted alkoxy groups such as OCEt2CH2OMe form dimeric compounds.175

Figure 9 Structure of Y5O(OPri)13 showing only the yttrium atoms (black) and the oxygen atoms. (open circles) The structure shows the similarity of the yttrium atom environments (reproduced by permission of Wiley from Mag. Res. Chem, 1991, 29, 1191-1195).

4.1.8.3 Fluorinated Alkoxides

The reactions of alcohols with Sc[N(SiMe3)2]3 or Y[N(SiMe3)2]3, may be used to prepare alkoxides but if excess alcohol is present the amine reacts to produce ammonia, which can act as a neutral ligand with the metal alkoxide:

The scandium compounds [Sc{OCMe(CF3)2}3fe and [Sc{OCH(CF3)2}3(NH3)2]2 have been prepared using the above method.49 A more extensive series of yttrium compounds having fluorinated alkoxy ligands have been prepared. The compounds [Y(ORf)3]n (where ORf = OCMe(CF3)2 or OCMe2CF3) appear to be trimers. Monomeric compounds Y(ORf)3L3 are formed in the presence of neutral ligands such as thf, NH3, diglyme, or ButOH. [Y{OCH(CF3)2}3(NH3)2]2 has been prepared by the reaction of YCl3 with hexafluoroisopropanol in the presence of anhydrous ammonia.176 The alcoholysis of Y5O(OPri)13 with hexafluoroisopropanol has been used to prepare [Y{OCH(CF3)2}3L3] where L = thf or PriOH.177

4.1.8.4 Mixed Metal Alkoxides

The stability of the M5O(OR)13 structure is shown by the nonreactivity of the scandium compound with Al(OR)3 whereas Sc(OBun)3 reacts with Al(OBun)3 to form Sc[Al(OBun)4]3. Sc[Al(O-Pri)4]3 may be prepared by the reaction of ScCl3 with KAl(OPri)4.163 Alkalis metalate complexes M0xY(OR)yLz (where M0 = Na or K and L = neutral ligand, generally having an oxygen donor atom) are formed where R = aryl or fluorinated group.178- 80 Examples of these complexes include [(dme)Li]2[Y(OAr)5]178 shown in Figure 10, [(thf)3K][Y(OAr)4(thf)2] (where Ar = 2,6-Me2C6H3),178 [NaY(OAr0^Cl] and [NaY2(OAr00^Cl] (where Ar0 = C6H2 (CH2NMe2)2Me, Ar00 = C6H4(CH2NMe2).179 The sterically large ligands afford compounds with one yttrium atom but the smaller alkoxide OC6H4(CH2NMe2) with only one ortho substituent allows self-assembly of a dimer structure. The fluorinated alkoxides Na3Y[(OCH(CF3)2]6(thf)3 and Na2Y[(OCH(CF3)2]5(thf)3 have six-coordinate yttrium atoms and are sublimable with loss of the thf ligand.180

Interest in barium yttrium alkoxides stems from metal organic vapor deposition experiments for the preparation of superconducting thin films. The compound [Y4Ba2O(OEt)8(dmp)6] (where dmp = ButC(O)CHC(O)But) has been prepared by the reaction of Y5O(OPri)13 with Ba(OEt)2 (PriOH)y in the presence of dmpH.167 The structure contains seven-coordinate yttrium atoms with

an unusual six-coordinate oxygen atom (coordinated to 4 Y and 2 Ba atoms) at the center of the molecule. The reaction of barium chips with (CF3)2CHOH in thf and YCl3 produced a volatile heterometallic species YBa[OCH(CF3)2]7(thf)3.181'182 The reaction of YBa[OCH(CF3)2]7(thf)3 with Y(dpm)3 produced a compound Y2Ba[OCH(CF3)2]4(dmp)4. The latter compound was purified by sublimation to produce the structure shown in Figure 11.

The reaction of anhydrous YCl3 with KTi2(OPri)9 in molar ratios 1:1 and 1:2 gave two compounds, Cl2YTi2(OPri)9 and ClY[Ti2(OPri)9]2, respectively.183 The molecular structure of Cl2YTi2(OPri)9 consisted of a six-coordinate yttrium bound to two chlorines and four oxygen atoms from the Ti2(OPri)9 ligand. Other mixed metal alkoxides are known including the yttrium copper compound with the alkoxide from 2-hydroxypyridine (Opy), [Y2Cu8(Opy)12(Cl)2(O)2 (NO3)4(H2O)2 • 2H2O]184 and the yttrium aluminum compound Y(OBu)3(AlMe3)2thf.185 Other yttrium copper alkoxides are known including the siloxy bridged compound [(Ph3SiO)2Y(^-OSiPh3)2CuPMe2Ph]55 and the ^-diketonate alkoxides YCu2(thd)2(OR)4 and YCu(thd)3(OR)2 (where thd = 2,2,6,6-tetramethylheptane-3,5-dionate and OR = OC2H4NMe2).186

4.1.8.5 Siloxides

The synthesis, structure, and applications to catalysis of metal-siloxide complexes has been reviewed.187 The preparation and NMR properties of [Sc(OSiBut3)3NH3]3NH3188 and Y(OSiPh3)3,189 was included. Structural details were also presented for the thf and the Bun3PO adducts of Y(OSiPh3)3,79 as well as the structure of a four-coordinate yttrium siloxide [Y(OSi-ButAr2){N(SiMe3)2}2] where Ar = OC6H4(CH2NMe2).190 One of the nitrogens attached to the aryl group coordinates to the yttrium making the siloxide a bidentate ligand. Further coordination of the second nitrogen is inhibited by the steric hindrance caused by the disilylamino ligands.

Figure 11 The molecular structure of Y2Ba[OCH(CF3)2]4(dmp)4 (where dmp = ButC(O)CHC(O)But) (reproduced by permission of The Royal Society of Chemistry from J. Chem. Soc., Chem. Commun., 1993,

1556-1558).

Figure 11 The molecular structure of Y2Ba[OCH(CF3)2]4(dmp)4 (where dmp = ButC(O)CHC(O)But) (reproduced by permission of The Royal Society of Chemistry from J. Chem. Soc., Chem. Commun., 1993,

1556-1558).

The reaction of a cyclopentadienyl scandium acac complex [(C5Me5)Sc(acac)3] with tetra-phenyldisiloxanediol (Ph2SiOH)2O results in displacement of the cyclopentaidiene group and the formation of a siloxane complex, [{(Ph2SiO)2OSc(acac)2}2Sc(acac)].19

A combination of 89Y and 29Si has been used to observe the fluxionality of some yttrium alkoxides, siloxides, and adducts. Large 89Y chemical shift changes have been observed on changing the coordination number of the yttrium.79

4.1.8.6 Triflate

One of the major developments in scandium chemistry has been the use of scandium salts, particularly scandium triflate, in organic synthesis. Kobayashi ' found that scandium triflate was stable in water and acted in aqueous solution as a Lewis acid, which could activate carbonyl and related compounds. This activity in water contrasts with most Lewis acids and scandium triflate is a reusable catalyst. In general Sc(OTf)3 acts as a better catalyst than Y(OTf)3 or the lanthanide triflates. A brief review of the Lewis acid catalytic activity of scandium triflate194 has appeared. Sc(OTf)3 has now been used as a catalyst in numerous synthetic reactions including aldol reactions,19 Meerwein-Ponndorf-Verley reductions,196 glycosidation,197 Friedel-Crafts,198 and Michael additions.199 Sc(OTf)3

in CD3OD is a very active catalyst for deuterium exchange of the aromatic hydrogens in 1,3,5-trimethoxybenzene.200 An arylscandium species is postulated as an intermediate in the reaction. The use of scandium triflate in organic synthesis is now extensive and has been reviewed.193 One more recent development has been the preparation of polymer supported scandium triflate catalysts.201-203 This development of supported catalysts aids easy recovery, and is quantitative in many cases. A supported catalyst has been tested on the reaction of carbonyl compounds with tetraallyltin202 and was shown to be most active with water as solvent.

Other scandium perfluoroalkanesulfonates such as the pentafluoroethanesulfonate and nona-fluorobutanesulfonate have also been shown to be good catalysts.204 Scandium salts such as the perchlorate and alkylsulfates205 are also active catalysts.

The crystal structure of the hydrated scandium triflate has been determined. Nine water molecules surround the scandium ion and the triflate anions are hydrogen bonded to the water molecules.206 A structural analysis of anhydrous scandium triflate, using X-ray powder diffraction and IR spectroscopy, indicates that the triflate acts as a bidentate ligand with scandium in an octahedral environment.207

Scandium triflate has been used as a cross-linking agent for dendrimers producing a heterogeneous catalyst with Lewis acid properties.208

A related compound scandium tris(trifluoromethylsulphonyl)methanide, Sc[(F3CSO2)3C]3 (scandium triflide) has been prepared and used as a catalyst in the nitration of o-nitrotoluene.209 Scandium triflide has proved to be a better catalyst than scandium triflate for the aromatic nitration of the electron deficient o-nitrotoluene.

4.1.8.7 Bidentate Oxygen Ligands

The reaction of a methanide ion [(Ph2PO)3C]— with the chlorides or nitrates of scandium or yttrium produce stable six-coordinate metal species (13) where the metal is coordinated to six oxygens in the form of three bidentate ligands.56

Was this article helpful?

0 0

Post a comment