SYNTHESIS AND STRUCTURAL-BONDING ANALYSIS OF (ETA-5-C5H4ME)4FE4(CO)6P8 AND (ETA-5-C5H4ME)4FE6(CO)13P8 - 2 UNPRECEDENTED TRANSITION-METAL COMPLEXES CONTAINING THE CAGE-LIKE P8 SUBUNIT OF HITTORF MONOCLINIC PHOSPHORUS ALLOTROPE

As part of our investigation into the photochemical reactions of P4 with transition-metal carbonyl and organometallic precursors, we generated Cp'4Fe4(CO)6P8 (1) (where Cp' denotes eta-5-C5H4Me) (11% yield) by cophotolysis of [Cp'Fe(CO)2]2 and P4. In addition, we have synthesized Cp'4Fe6(CO)13P8 (2) (40-50% yield) via a nonphotolytic reaction of 1 with sources of the electron-deficient Fe(CO)4 fragment. Both compounds were characterized by single-crystal X-ray diffraction, laser desorption FT mass spectrometry, and spectroscopic (H-1, P-31 NMR; IR) and electrochemical measurements. 1 and 2 are the first examples of transition-metal polyphosphide complexes containing a realgar-type alpha-P8 core. This P8 cage, with four nonbonding planar phosphorus atoms coordinated to two bonding pairs of tetrahedrally related bridgehead phosphorus atoms above and below the plane, is one component of Hittorf's elemental phosphorus (3). While 3 has a complicated tube-like cross-linked polymeric structure consisting of alternating P8 and P9 cage subunits linked by mu-2-P2 bridges, the molecular compounds 1 and 2 each contain a P8 cage with two mu-2-FeCp'(CO) single-atom bridging ligands which chelate two opposite pairs of the planar phosphorus atoms. Two additional nonbridging FeCp'(CO)2 substituents in 1 are coordinated to two diagonally related planar phosphorus atoms, resulting in a pseudo-C2 molecular geometry. Formation of 2 from 1 involves the coordination of two Lewis acid Fe(CO)4 adducts to the other two planar phosphorus atoms, followed by loss of a CO from one adduct and concomitant electron-pair Fe-Fe bonding with the adjacent terminal FeCp'(CO)2 group. The alpha-P8 cages of 1 and 2 are primarily stabilized by two mu-2-Fe chelating atoms; evidence of this alpha-P8 cage stabilization is given by LD/FTMS data in which all P8-containing ion fragments invariably include at least two FeCp' groups. An indication of additional stabilization by the electron-deficient iron substituents in 1 and 2 is derived from a comparative analysis of geometrical differences in their P8 cages with those in Hittorf's violet phosphorus (3) and P8(P2iPr2)2 (4) (where iPr denotes isopropyl) and with the electronically equivalent alpha-P4S4 molecule (5); the significantly shorter bridgehead P-P single-bond distances in 1 and 2, relative to those in 3, 4, and 5, are presumed to be a consequence of markedly reduced repulsions between the nonbonding electron pairs on these bridgehead phosphorus atoms. Crystal data for (eta-5-C5H4CH3)4Fe4(CO)6P8, 1 are as follows: fw = 955.73 g/mol; monoclinic, P2(1)/c; a = 15.305 (6) angstrom, b = 9.650 (2) angstrom, c = 24.532 (4) angstrom, beta = 97.71 (2)-degrees, V = 3590 (2) angstrom-3 at -100-degrees-C; d(calcd) = 1.74 g/cm3 for Z = 4; least-squares refinement converged at R1(F) = 0.040, R2(F) = 0.052. Crystal data for (eta-5-C5H4CH3)4Fe6(CO)13P8, 2, are as follows: fw = 1263.49 g/mol; monoclinic, P2(1)/c; a = 7.587 (2) angstrom, b = 27.073 (8) angstrom, c = 22.021 (7) angstrom, beta = 99.12 (2)-degrees, V = 4466 (2) angstrom-3 at -100-degrees-C; d(calcd) = 1.88 g/cm3 for Z = 4; R1(F) = 0.066, R2(F) = 0.071.