Helical Structures Featuring Thiolato Donors - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 158

SH SH Ph SH SH

NC SH

SH SH Ph SH SH NC SH

H 2 - E

H 2 - A H 2 - B H 2 - C H 2 - D

Figure 5.1 1,2-Dithiol ligands used for the preparation of trigonal-prismatic complexes.

TP coordination geometry was initially only known in the solid state for compounds such
as MoS 2 [6], WS 2 [7] and NiAs [8]. Later, reports appeared on molecular compounds like
[Re(S 2 C 2 Ph 2 ) 3 ] [9], [Mo(S 2 C 2 H 2 ) 3 ] [10] and [V(S 2 C 2 Ph 2 )3] [11], featuring a metal with
TP coordination geometry. In addition, TP coordination geometry has been observed for a
few permethylated complexes such as [W(CH 3 ) 6 ] [12,13], [Re(CH 3 ) 6 ] [13], [Mo(CH 3 ) 6 ]
[14], [Li(tmed)] 2 [Zr(CH 3 ) 6 ] [15] and [Li(OEt 2 ) 3 ][Ta(CH 3 ) 6 ] [13] and for some complexes
with macrobicyclic tris(catecholylamide) ligands [16]. These observations led to an
ongoing debate regarding the reasons responsible for the preference of TP over OC coor-
dination geometry. Most TP complexes, however, have been observed with unsaturated o -
dithiolato ligands obtained after deprotonation of o -dithiols like H 2 - A -H 2 - E (Figure 5.1).
Molybdenum or tungsten complexes with three benzene- o -dithiolato ligands A 2 have
been shown to change their coordination geometry from TP to OC and vice versa depend-
ing on the oxidation state of the metal center. A series of mononuclear complexes of type
[M(bdt) 3 ] n (M 0, 1, 2) have been

prepared and characterized by X-ray diffraction. Figure 5.2 shows the molecular struc-
tures of [W(bdt) 3 ] n ( n

¼ Mo [17], W [18], bdt ¼ benzene- o -dithiolate, n ¼

2, 1, 0). Complex [W VI (bdt) 3 ] shows a crystallographically
imposed almost perfect TP geometry around the metal center (Figure 5.2, right). The lig-
and bending at the sulfur atoms is thought to be a consequence of a second-order Jahn-
Teller distortion [19]. The complex anion [W V (bdt) 3 ] , obtained after a one-electron
reduction of [W VI (bdt) 3 ], exhibits a distorted octahedral geometry (Figure 5.2, middle).
The dianionic complex [W IV (bdt) 3 ] 2 exhibits a more or less distorted TP coordination
geometry. This facile change of the coordination geometry indicates that the energy
required to modify the twist angle w is small and factors different from the d-electron
count at the metal center may influence the coordination geometry. A study on the influ-
ence of the counter cations on the coordination geometry of 3,6-dichlorobenzene-1,2-
dithiolato complexes of tungsten and molybdenum corroborated this assumption [20].

¼

S

S S

S

[W IV (bdt) 3 ] 2- [W V (bdt) 3 ]

-

[W VI (bdt) 3 ]

n

Figure 5.2 Molecular structures of the complexes [W(bdt) 3 ] ( n ¼2, 1, 0).