2 Homometallic Alkoxides

320 Alkoxo and Aryloxo Derivatives of Metals
2.12 Alkoxides of Manganese, Technetium, and Rhenium
Only a few structures have been determined of alkoxo compounds of manganese and
rhenium.
2.12.1 Manganese and Technetium
The bulky triphenylmethoxo ligand gives rise to the mononuclear Mn(II) complex
[Mn⊲OCPh3⊳2⊲py⊳2] in which the manganese is in an irregular four-coordinated configuration
(O �MnO, 140.4 Ž ;N�MnN, 95.6 Ž ). 377
An octahedrally coordinated Mn(IV) complex has been obtained using the tridentate
ligand 3-(5-chlorosalicylidene amino)propanolate (5-Cl-salahp) [Mn(5-Cl-salahp) 2 ].
The chelating ligands adopt the meridional configuration. 378 The mononuclear
Mn(IV) cation [MnL] C (L D N ,N 0 ,N 00 -tris((2R)-2-olato-3-methylbutyl)-1,4,7triazacyclononane)
was obtained as the PF6 salt using the hexadentate pendant-arm
macrocyclic ligand L. The steric hindrance of this bulky ligand gave rise to a distorted
octahedral configuration. 379
In the tetranuclear complexes [Mn4⊲ 3-OMe⊳4⊲MeOH⊳4L4] (LD 2,6-tetramethylheptane-3,5-dionate;
1,3-diphenyl-propane-1,3-dionate) the Mn4⊲ 3-OMe⊳4 core has a
cubane-like configuration (Fig. 4.6) with each metal having one chelating diketonate
and a coordinated methanol to give an octahedral configuration. 380
Although some oxo alkoxides of technetium have been characterized there are no
homoleptic alkoxide structures available.
2.12.2 Rhenium
Several rhenium alkoxo structures have recently been reported. The homoleptic
pentamethoxide has the edge-bridged dimeric structure [Re2⊲ -OMe⊳2⊲OMe⊳8]
(ReDRe, 2.532 ˚A) with a rhenium–rhenium double bond (Table 4.12, p. 322). 306
In the [Re2⊲ -Cl⊳2⊲ -dppm⊳2Cl3⊲OEt⊳] (dppm D diphenylphosphinomethane) (Re–Re,
2.667 ˚A) the ethoxo group is in a terminal position. 381
Some dinuclear organorhenium alkoxo complexes have also been characterized. The
nonbridged dimer [Re2⊲OBu t ⊳4⊲CBu t ⊳2] (ReDRe, 2.396 ˚A) has the ethane-like configuration
with the alkylidyne ligands trans across the metal–metal double bond. A similar
structure is present in [Re2fOCMe⊲CF3⊳2g4⊲CBu t ⊳2] (Table 4.12). 382 In the interesting
alkylidyne alkylidene alkoxo complex syn-[RefOCMe⊲CF3⊳2g2⊲CBu t ⊳⊲CHBu t ⊳⊲thf⊳]
the five-coordinated Re(VII) is in a face-capped tetrahedral or distorted TBP configuration,
owing to the weakly bound THF ligand being trans to the alkylidyne. 382
In anti-[RefOCMe⊲CF3⊳2g2⊲CBu t ⊳⊲CHC5H4FeC5H5⊳] the Re atom is in a distorted
tetrahedral coordination. 383
The trinuclear cluster [Re3⊲ -Cl⊳3⊲OBu t ⊳6] (ReDRe, 2.438 ˚A) has an interesting
structure with the triangular plane of Re3⊲ -Cl⊳3 and the terminal tert-butoxo groups
placed three above and three below the plane. There are three short Re–O bonds (av.
1.82 ˚A) on one side of the plane and three long bonds (av. 1.99 ˚A) on the other side.
The inner triangular Re3 core is held together by metal–metal double bonds. 385 In
[Re3⊲ -Cl⊳⊲ -mentholate⊳2⊲mentholate⊳5Cl] (ReDRe, 2.389 ˚A) the isosceles triangle
of Re3 atoms is bridged by one chloride and two alkoxo ligands. The terminal
chloride is bonded to the Re atom which is bridged by alkoxo ligands. 385 The