Carbon Ligands

Organometallic compounds are not specifically discussed in this section.

No carbonyl chemistry of scandium and yttrium has been reported yet and there is also no cyanide chemistry of these two elements although thiocyanato complexes of scandium [Sc(NCS)6]3~ (bonded through nitrogen) are known. 8 The important developments involving scandium or yttrium with carbon have involved the fullerene derivatives of these elements. There have been some scandium carbide systems prepared but these will be highlighted in the chemistry of the halides.

4.1.5.1 Endohedral Fullerene Compounds of Sc and Y

The fullerenes were first discovered by laser ablation cluster ion formation in a mass spectrometer.89 Soon after the identification of C60 as a ''magic number cluster,'' [LaC60]+ was also identified.90 Later, macroscopic quantities of fullerenes were produced by the carbon arc method91 soon followed by the macroscopic production of the lanthanum endohedral fullerenes.92 Fullerene chemistry has developed rapidly since the preparation of macroscopic quantities of these compounds. The development of the metallofullerenes has been hampered by the low yield of these compounds in conventional syntheses. Newer and better synthetic routes to metalloful-lerenes will probably be found in the near future speeding up the investigation of the properties, particularly of the scandium and yttrium metallofullerenes. Laser vaporization and the arc method of metal-doped carbon have been used to produce endohedral Sc and Y compounds and a recent review has details of these methods.93 The initial studies of metallofullerenes involved gas phase laser ablation, used to observe the ions by mass spectrometry.94 Macroscopic quantities of scandium metallofullerenes are prepared by the arc method and although laser desorption of the soots formed in the preparation show the presence of [ScC60]+ and [ScC70]+ the extractable compounds typically give only one monoscandium fullerene ScC82.95'96 Di-, tri- and tetrascan-dium fullerenes such as Sc2C84, Sc3C82, and Sc4C82, are extractable. The extractable yttrium fullerenes are less extensive than the scandium counterparts and only YC82 and Y2C82 have been recovered in small quantities.97'98

The endohedral (metal trapped inside the carbon cage) nature of the metallofullerenes has been questioned, but gas phase photofragmentation experiments of ions99'100 and solid-state measurements indicate that the metals are endohedral not exohedral.101 Exohedral metallofullerenes have been prepared in the gas phase but they have different properties to the endohedral metallofuller-enes prepared by the arc or laser methods.102-104 The symbol @ has been used to denote a species inside the fullerene cage and this symbol will be now used to denote endohedral metallofullerenes.

The solids produced by the arc method are often complex mixtures of fullerenes and metallo-fullerenes and an important part of this chemistry has involved the purification of the individual metallofullerenes and even isomers of a particular species. In general a two-stage HPLC method has been used to separate the metallofullerenes from the empty fullerenes and purify individual metallofullerenes.105'106

[email protected] and [email protected] have both been investigated by synchrotron X-ray diffraction107 and in each case the metal is inside the carbon cage; not in the center but close to one of the six-membered rings of the cage. The scandium atom in [email protected] is shown to have 18.8 electrons and

so the ion may be formulated as Sc + C82 —; this agrees with UPS measurements. The Y atom in [email protected] is also close to one of the six-membered rings of the cage and at room temperature the Y is stationary with respect to the cage. This stationary state for the metals Sc and Y differs from lanthanum where the metal atom appears to be in motion. ESR and XPS measurements suggest that the formulation of [email protected] is best described as Y3+ C823—.97,98

The discandium endohedral fullerenes [email protected], [email protected] and [email protected] have been isolated and three isomers of [email protected] have been separated. 45Sc NMR of [email protected] isomers indicates that the scandium atoms are separated and that motion of these atoms is restricted.109 This is supported by theoretical and structural studies.110 Synchrotron powder X-ray studies indicate that [email protected] is best represented as 2(Sc2+ )C844—. The Raman scattering and infrared absorptions of [email protected] are consistent with a simple ionic picture of [email protected] The results for [email protected] differ from [email protected] that has since been characterized.112 In this endohedral metallofullerene the two scandium atoms appear to form a dimer. The scandium atoms sit close to two pairs of fused five-membered rings in the C66 cage. This indicates that the size and structure of the fullerene influences the trapped species. The compound may be regarded as (Sc2)2+ C662—.

The triscandium endohedral fullerene [email protected] has been isolated and studied by ESR.96 Synchrotron X-ray studies indicate that the three scandium atoms form a triangle and this trimer has a formal charge of three allowing formulation of the compound as (Sc3)3+ C823—.113 The temperature-dependent metal cluster position inside the C82 cage has been studied.114

One area of development of metallofullerene chemistry has been the synthesis of adducts by addition to the carbon cage.115 Another advance in the endohedral metallofullerene area has been the identification of metal compounds inside the carbon cages. Introducing nitrogen (pressure, 1-3 mbar) into the arc process using a scandium-doped graphite rod results in the isolation of [email protected] Structural characterization of this endohedral fullerene has been accomplished by X-ray crystallography of a crystal formed when the endohedral fullerene co-crystallized with cobalt octaethylporphyrin.116 A similar method has been used to crystallize [email protected] The latter compound studied by synchrotron X-ray crystallographic analysis shows the Sc3N as a planar group with nitrogen at the center bonded to three scandium atoms. There has also been an EPR study of [email protected] radical anions.118 Recently [email protected] has been prepared and this compound has been studied by 13C and 45Sc NMR.119 It is postulated that the Sc3N group is planar and is inside a C66 cage of D3 symmetry. The structure of a scandium carbide endohedral metallofullerene: [email protected] has been investigated by 13C NMR and synchrotron X-ray diffraction on the powder sample.120 The carbon atoms in Sc2C2 unit are shown to be equivalent by 13C NMR. The diffraction study indicates that the ScC2 unit has a C—C bond intermediate in bond length

Figure 6 Schematic representation of [email protected] showing the planar Sc2C2 ring. The scandiums are shown in black and the carbons of the C2 unit are stippled.

between a single and double bond. The scandium atoms are on opposite sides of the C—C unit and the Sc2C2 ring is planar. The structure is shown in Figure 6 along with the planar Sc2C2 ring.

4.1.6 NITROGEN DONOR LIGANDS 4.1.6.1 Amido Complexes

The simplest amide of yttrium Y(NH3)2 has been prepared by the reaction of Y(SCN)3 with KNH2 in liquid ammonia.121 The amidometallates K3Y(NH2)6 and Rb3Y(NH2)6 may be prepared by dissolving the metals in supercritical ammonia.122

The volatile metal amido compounds M(NR2)x are potential precursors for solid-state materials such as metal nitrides. The metal amido compounds have also become very important synthetic starting materials for many types of compounds. The low coordination of these compounds often allows addition of molecules with suitable donor ligands. The relatively weak M—N bond allows the NR2 group to be replaced by other groups and in the case of scandium and yttrium the M—O bond is much stronger than the M—N bond.

As previously reported1 the three-coordinate Sc[N(SiMe3)2]3 has been known since 1972 and the similar yttrium compound Y[N(SiMe3)2]3 was reported in 1980.123 The scandium compound was reported to have ScN3 with pyramidal geometry in the solid state, but the gas phase structure determined by electron diffraction42 has a ScN3 with planar structure. There is no significant change in the Sc—N bond length in the solid and gas phase and the Sc—N bond length is shorter than most other Sc—N bond lengths. The difference in configuration is attributed to the crystal packing forces in the crystalline state that are absent in the gas phase. The M[N(SiMe3)2]3 compounds of scandium and yttrium have proved to be good starting materials in the preparation of coordination compounds of the two metals. Similar to all transition metal dialkylamino compounds, they may be used in alcoholysis or aminolysis reactions to form alkoxides or other Sc—N systems.

The reaction of ScCl3(thf)3 with Sc[N(SiMe3)2]3 gives the dichloro complex [ScCl2{N(Si-Me3)2}(thf)2] which when heated to 400 °C is converted to scandium nitride ScN, with loss of thf and ClSiMe3.124 Sc[N(SiMe3)2]3 undergoes deprotonation (and loss of HN(SiMe3)2) when reacted with NaN(SiMe3)2 in thf,125 forming the complex [Na(thf)3Sc(CH2SiMe2NSiMe3)-{N (SiMe3)2}2]. The reaction of M[N(SiHMe2)2]3 (M = Sc or Y) with a substituted bis-benzenesulfo-namidocyclohexane (1) results in the replacement of two N(SiHMe2)2 groups.126 The yttrium compound appears to have a coordinated thf molecule that is absent in the scandium compound. The H and 13C NMR are complex and a monomer-dimer equilibrium is suggested, complicated by the presence of cis- and trans- isomers. The yttrium compound is desolvated under vacuum to yield a less soluble compound that may be crystallized. The crystal structure reveals a dimer structure with unusual bridging S=O groups.

Y[N(SiHMe2)2]3 has been used to attach a YNR2 group to the surface of the mesoporous silica MCM-41. The NR2 group may be replaced by other ligands. The catalytic reactivity of these modified surfaces in the Diels-Alder reaction has been investigated.127

The reaction of YCl3 with KNPPh3 and Baysilon paste, (OSiMe2)n, gives a dimer with a Y2O2 bridge in the compound [Y(NPPh3)2(OSiMe2NPPh3)]2 (2).128

Me Me

Ph3PI^ I NPPh3

Ph3PN I >NPPh3

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