2.12.7 References for 2.12

η2(π)-Coordination of a Bis-Phosphonio-Benzophospholide Cation: A Novel Complexation Mode for Unsaturated Phosphorus Heterocycles

“…The former value is typical for cobalt phosphine complexes, and attachment of the phosphorus atom to cobalt is further substantiated by splittings due to 2 J(P,C) and 3 J(P,H) couplings on the 1 H and 13 C NMR signals of the nuclei in the C 5 H 5 moiety. The up®eld shift of d 31 P for the ring phosphorus atom is much larger than in g 1 (P)-complexes of phosphonio-benzophospholides [5] but matches the large negative coordination shifts in pcomplexes of phosphinines or phospha-alkenes [2,3]. Appropriate coordination shifts were as well observed for the adjacent carbon atoms (d 13 C 59.5, 61.7; Dd coord » ±50) while the chemical shifts of the remaining nuclei in the condensed ring system are similar to those in 4 [6].…”

“…In the course of our studies of bis-phosphonio-benzophospholide complexes we found that the steric encumbrance of the two-coordinate phosphorus which had impeded a wide-spread use of these systems as ligands [5] is released if one of the R 3 P-moieties contains an additional donor-site which enables the cations to act as bidentate ligands [6]. Following this strategy, we succeeded to synthesize metal carbonyl complexes such as 5 a, b[X] (X = OTf, BPh 4 ) in which the cation 4 chelates a Ni(CO) 2 or Fe(CO) 3 -moiety, respectively (scheme 1) [6].…”

“…Starting from the description of the bonding in bisphosphonio-benzophospholides in terms of the resonance structures A±C (scheme 3) [5,10], the g 2 -complexation locks the coordinated cation in 6 in one of the structures A or B, respectively. The shortening of the exocyclic P2±C2 bond in 6 as compared to the free ligand in connection with the pronounced shielding of the corresponding 13 C NMR signal (d 13 C 61.7) suggest that the loss of resonance energy associated with this p-bond ªlocalizationº [11] is offset by increased ylidic character of this bond, which improves stabilization of the remaining formal carbanion fragment.…”

“…The shortening of the exocyclic P2±C2 bond in 6 as compared to the free ligand in connection with the pronounced shielding of the corresponding 13 C NMR signal (d 13 C 61.7) suggest that the loss of resonance energy associated with this p-bond ªlocalizationº [11] is offset by increased ylidic character of this bond, which improves stabilization of the remaining formal carbanion fragment. The remarkable preference of the benzophospholide moiety for g 2 (p)-over the g 1 (P)-coordination which was observed for other complexes with these ligands [5,6,8,9] and is also preferred for phospholides and phosphinines acting as 2e-donor ligands [1±3] owes presumably to a combination of steric and electronic factors. Steric stress interfering with the formation of a g 1 (P)complex could arise from the mutual interference between the bulky Cp and Ph 3 P-moieties which limits the conformational freedom in the seven membered chelate ring and contributes thus to the build-up of substantial ring-strain.…”

“…Thus, the neutral phosphoniobenzo[c]phospholide 2 displays a balanced coordination behaviour similar to that of phosphinines [3], forming complexes via both the p-electrons (g 5 (p)-coordination) or the phosphorus lone-pair (g 1 (P)-coordination) [4]. The cationic 1,3-bis-phosphonio-benzo[c]phospholide 3 forms mononuclear and binuclear complexes in which the ligand is found in a g 1 (P)-or l 2 (P)-coordination mode, respectively, and whose structural features indicate that the p-electrons are far less nucleophilic than the lone-pair at phosphorus and interact only weakly with the metal [5]. In this context the question arose if bis-phosphonio-benzophospholide cations might be capable at all to act as pure p-ligands.…”