Metal Interactions with Boron Clusters

Abstract

An X-ray diffraction study of the purple paramagnetic title compound established it as a dimetallic complex having one iron atom sandwiched between two (CH3)2C2B4H42~ ligands (as in the previously characterized red diamagnetic monoiron compound [(CH3)2C2B4H4]2FeH2) with the second iron in a wedging location, coordinated to the complex via four Fe-B interactions. The "outer" iron atom is also bound to a 1,2-dimethoxyethane solvent ligand. The Fe-Fe distance of 2.414 (4) Á is well within bonding range, but the existence of a direct iron-iron bond is in doubt. From magnetic susceptibility and Móssbauer measurements, both iron atoms are assigned 2+ oxidation states with one metal in a low-spin (diamagnetic) configuration and the other high-spin with four unpaired electrons. Comparison with the red monoiron species, from which the diiron complex is prepared by insertion of Fe2+, suggests that the paramagnetism arises from the outer iron atom. The bis(tetrahydrofuran) analogue of the title compound, [(CH3)2C2B4H4]2Fe2(THF)2, is an intermediate in the conversion of [(CH3)2C2B4H4]2FeH2 to (CH3)4C4B8H8 via oxidative fusion, as described in the preceding article. Metal ions can bind to borane and carborane ligands in a remarkable variety of ways.3 While the earliest metallacarboranes were structural analogues of metallocenes,4 subsequent work has produced a wealth of structurally diverse species that have few counterparts outside of boron cluster chemistry. One such class consists of the so-called "wedged" complexes, in which the wedging atom occupies a crevice between two carborane ligands (Figure 1). Prior to this work the only structurally established example was the iron-cobalt species5 in Figure la; two other members of this group, based on NMR characterization,6 are [(CH3)2C2B4-H4]2FeGe and its tin analogue (Figure lb). This paper describes a detailed study of a new wedge-type complex that exhibits some structurally and electronically unusual features and in addition plays a synthetically significant role: it is an intermediate in the conversion of the monoiron complex [(CH3)2C2B4H4]2FeH2 (Figure 2) to (CH3)4C4B8H8 via oxidative ligand fusion.7 Results and Discussion The deep red diamagnetic dicarboranyliron(II) complex8 [2,3-(CH3)2C2B4H4] FeH2 (1), on standing in THF solution slowly converts to a deep purple paramagnetic complex as described elsewhere;7 the same purple compound forms rapidly in the presence of traces of FeCl3. Evaporation of the solvent yields an almost black crystalline material (2A), which is extremely air-sensitive but can be purified by recrystallization from THF/hexane under a rigorously Orfree atmosphere. Although 2A is obtainable in good yield and in gram quantities, its characterization proved difficult owing to its paramagnetism (which rendered NMR spectra useless) and its decomposition in the mass spectrometer, which precluded observation of a parent molecular ion (only the (1) Table I. Positional Parameters and Their Estimated Standard Deviations for [(CH3)2C2B4H4]2Fe2(OCH3)2C2H4 atom y Z Fe(l) 0.1100 (6) 0.1349 (3) 0.1760 (5) Fe(2) 0.2315 (6) 0.0802 (3) 0.3844 (5) 0(1) 0.400 (2) 0.085 (1) 0.593 (2) 0(2) 0.232 (2)-0.003 (1) 0.465 (2) C(2) 0.272 (4) 0.187 (2) 0.118 (4) C(3) 0.182 (4) 0.152 (2)-0.011 (4) C(4) 0.180 (4)-0.056 (2) 0.391 (4) C(5) 0.349 (4)-0.018 (2) 0.597 (4) C(6) 0.362 (4) 0.039 (2) 0.675 (3) C(7) 0.444 (4) 0.142 (2) 0.652 (3) C(2')-0.107 (4) 0.114(1) 0.136 (3) C(3')-0.089 (3) 0.175 (1) 0.131 (3) C(M2) 0.311 (4) 0.245 (2) 0.116 (4) C(M3) 0.118 (4) 0.189 (2)-0.153 (3) C(M2')-0.215 (3) 0.075 (2) 0.036 (3) C(M3')-0.192 (4) 0.214 (2) 0.014 (4) B(4) 0.173 (4) 0.090 (2) 0.005 (4) B(5) 0.282 (5) 0.077 (2) 0.167 (4) B(6) 0.351 (4) 0.138 (2) 0.231 (4) B(7) 0.341 (5) 0.123 (2) 0.056 (4) B(4') 0.012 (4) 0.203 (2) 0.271 (4) B(5') 0.054 (5) 0.148 (2) 0.369 (4) B(6)-0.033 (4) 0.092 (2) 0.294 (4) B(7')-0.142 (4) 0.152 (2) 0.290 (4) H(4) 0.08 (0) 0.06 (0)-0.05 (0) H(5) 0.33 (3) 0.03 (1) 0.23 (3) H(6) 0.46 (3) 0.16 (1) 0.32 (3) H(7) 0.44 (3) 0.12 (1) 0.01 (3)