Synthesis and Reactivity of [2]Ferrocenophanes with C−Ge and C−Sn Bridges

The synthesis, characterization, and reactivity of novel carbogerma[2]ferrocenophanes,
(η-C5Me4)Fe(η-C5H4)CH2GeR2, 11 (a, R = Ph; b, R = Me), and carbostanna[2]ferrocenophanes,
(η-C5Me4)Fe(η-C5H4)CH2SnR2, 12 (a, R = tBu; b, R = Mes; c, R = Me), are reported.
Compounds 11a,b and 12a−c were isolated as orange crystals from the reaction of (η-C5Me4)Fe(η-C5H4Li)CH2Li·xTMEDA with Cl2ER2 (E = Ge, R = Ph, Me or E = Sn, R = tBu,
Mes, Me) and were characterized by 1H, 13C, and 119Sn NMR, UV/vis, MS, and in representative cases elemental analysis. X-ray diffraction analysis of these species revealed
structures possessing moderate ring-tilt with values in the range α = 10.99(2)−11.81(5)°
for 11a,b and α = 6.64(2)−7.5(1)° for 12a−c. The presence of a small or moderate degree of
ring-strain was further substantiated through the observation of only a small (ca. 25−30
nm) bathochromic shift in the UV/vis absorption maxima when compared to pentamethylferrocene. Attempts to induce thermal ring-opening polymerization (ROP) of 11b and 12a−c
proved to be unsuccessful and led only to decomposition at temperatures above 300 °C.
Treatment of 11b and 12a−c with amines such as pyridine and 4-(dimethylamino)pyridine
did not result in ROP of these compounds. In addition, treatment of 11b and 12a−c with
Pt(cod)2, Pt(PEt3)3, PtCl2, Karstedt's catalyst (a Pt(0)-divinyltetramethyldisiloxane complex),
PtCl2(COD), and nBuLi for 11b and 12c and 2,2‘-azobisisobutyronitrile (AIBN) in the
presence of Bu3SnH for 12a,b failed to induce any of the insertion or ring-opening reactions
that are known for the more highly strained [1]ferrocenophane analogues. However, reactions
of 11b and 12a with strong acids such as HCl and 12a with triflic acid (CF3SO3H) resulted
in the formation of the corresponding ring-opened species, (η-C5H5)Fe(η-C5Me4CH2GeMe2Cl), 14, (η-C5H5)Fe(η-C5Me4CH2Sn(tBu)2Cl), 15, and (η-C5H5)Fe(η-C5Me4CH2Sn(tBu)2OSO2CF3), 16, via selective ipso-Cp−Ge or ipso-Cp−Sn bond cleavage. Treatment of 12b with
HCl resulted in selective Sn−Mes bond cleavage, affording (η-C5H4)Fe(η-C5Me4)CH2SnMesCl,
17.