Broadening the Scope of Ancillary Phosphane‐Type Ligands: A Systematic Comparison of PR₃, PR₂BH₃^–, and SiR₃^– and Their Chalcogen Derivatives

Abstract

This work describes a systematic experimental and theoretical study of the properties of two series of isoelectronic and largely isosteric ligands, namely PPh2Me, PPh2BH3–, and SiPh2Me– and SPtBu3, SPtBu2BH3–, and SSitBu3–. In addition, we have also investigated the oxo derivatives OPPh2BH3– and OSiPh2Me–. Based on X‐ray crystal structure determinations (Fe–CO and C–O bond lengths) as well as NMR [e.g. δ(13CO)] and IR [ν(CO)] spectroscopic investigations of the corresponding [CpFe(CO)2]+ complexes, we can conclude that, with respect to electron donor strength, phosphanyl borohydrides occupy an intermediate position between phosphanes (weakest donors) and silyl ligands (strongest donors). The same is true for the thio derivatives, although the differences are smaller. In the reaction with [CpFe(CO)2]+, the oxo derivative OPPh2BH3– transfers a hydride ion rather than forming a stable [CpFe(CO)2(OPPh2BH3)] complex. The tendency to undergo hydride‐transfer reactions was studied by density functional calculations for the series PtBu2BH3–, OPtBu2BH3–, and SPtBu2BH3–. The results reveal that OPtBu2BH3– is the strongest and SPtBu2BH3– the weakest hydride donor, in accordance with the experimental observations. Theoretical analysis indicates that the three derivatives PPh2Me, PPh2BH3–, and SiPh2Me– are truly isolobal species despite variations in their charge distributions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The first bidentate phosphanylborohydride: Synthesis, structure, and reactivity towards [CpFe(CO)2I]

“…(dppe) [18][19][20][21][22][23][24][25][26][27]. A special feature of our ligand 3 that could be responsible for the fact that 3 does not readily adopt a chelating coordination mode, is its reducing power [3]. [28].…”

“…The chemistry of phosphanylborohydrides B, in contrast, is still largely undeveloped [1][2][3][10][11][12][13][14][15].…”

“…Possible decomposition pathways include one-electron reduction processes ([SiR 3 ] À ) or hydride transfer from boron to the metal atom ([P(BH 3 )R 2 ] À ) [3]. The unwanted cleavage of the metal-silicon bond in silanide complexes has been suppressed by the introduction of a chelating ether into the ligand molecule (e.g.…”

“…One such toolbox with highly promising potential for development consists of the series of isoelectronic molecules A-C ( Fig. 1) [1][2][3]. In moving from the generic methyldiorganyl phosphane A to the phosphanylborohydride B, a BH 3 moiety is substituted for the phosphane methyl group.…”

“…Our group has recently assessed the relative Lewis basicities of [P(BH 3 )Ph 2 ] À and P(CH 3 )Ph 2 with respect to the main group Lewis acids H + , CH þ 3 and BH 3 . Based on NMR spectroscopy, X-ray crystal structure analyses and displacement experiments, we came to the conclusion that the phosphanylborohydride [P(BH 3 )Ph 2 ] À forms dative bonds of higher p character, is thus better suited to direct its electron lone pair towards the acceptor orbital of the coordinated Lewis acid and consequently forms more stable r adducts than its neutral counterpart P(CH 3 )Ph 2 [1].…”