Models of the iron-only hydrogenase: Structural studies of chelating diphosphine complexes [Fe₂(CO)₄(µ-pdt)(κ²P,P′-diphosphine)]

Abstract

Six chelating diphosphine complexes, [Fe2(CO)4(micro-pdt)(kappa2P,P'-diphosphine)], have been crystallographically characterised allowing differences between basal-apical and dibasal conformations to be analysed.

Models of the iron-only hydrogenase: Synthesis and protonation of bridge and chelate complexes [Fe2(CO)4{Ph2P(CH2)nPPh2}(μ-pdt)] (n = 2–4) – evidence for a terminal hydride intermediate

“…71e75 . For example, the diphosphine bite-angle in the dibasal complex [Fe 2 (CO) 4 (k 2 -dppm)(m-pdt)] is 74.55(4) [3,4]. Thus, exchange between apicalebasal and dibasal isomers requires a significant change in the bite-angle of the diphosphine and may account for the slow interconversion of the different isomers in solution.…”

“…5:1 in 3a. Crystal structures of related dibasal complexes [3] reveal much smaller bite-angles of ca. 71e75 .…”

“…2, the sixmembered chelate ring adopts an approximate chair conformation. In an earlier communication we compared the structure of 3b to five other crystallographically characterised complexes of the type [Fe 2 (CO) 4 [3]. The structure of 3b is very similar to that of 3a (Table 1) although the bite-angle [P(1)eFe(1)eP(2)] of 94.96(2) found in 3b is significantly larger than that of 87.7(4) observed in the dppe analogue.…”

“…The molecular structure closely resembles that of 2a characterised by Song. The diphosphine links the two diiron centres by binding to each in an apical manner as defined by the Fe(1)eFe(2)eP(1) angle of 151.81 (3) . Tetranuclear 2d was the only product isolated from the reaction of 1 and trans-Ph 2 PCH] CHPPh 2 under all reaction conditions employed in this study.…”

“…The irone iron bond at 2.6246(3) Å is significantly longer than those found in other bridged diphosphine complexes of this type and is more akin to those found in the unsymmetrically substituted dibasal complexes. For example, the longest ironeiron bond we have previously encountered during our work in this area is 2.6236(5) Å in [Fe 2 (CO) 4 {k 2 -Ph 2 PN(Pr i )PPh 2 }(mpdt)] [3]. The FeeFeeP bond angles of 122.64(1) and 118.09(1) also vary considerably from those in the analogous dppm and dppe complexes, being much larger.…”