Coordination of CO to low-valent phosphorus centres and other related P–C bonding situations. A theoretical case study

Abstract

The multi-faceted bonding of CO in molecular phosphorus compounds is described using calculated P-C bond strengths as a criterion. Full compliance matrices at coupled cluster level of HPCO (1a), singlet oxaphosphirane-3-ylidene HP(h 2 -CO)), the dimer (HPC]O) 2 as well as P^CH, HP]CH 2 and H 2 P-CH 3 were calculated to obtain quantifiable data and enable comparison. The quest for CO coordination and activation was examined for phosphaketenes 1a-f: the P-C compliance constants (inÅ mdyn À1 ) reveal a clear trend that shows a weakening of the P-CO bond strength from 1a to mono-ligation as in [(OC) 5 W {P(CO)Me}] (1c) (0.301), in H 3 BP(CO)Me (1b) (0.322), to bis-ligation as in [{(OC) 5 W} 2 P(CO)R] (1f) (0.488)to (H 3 BP) 2 (CO)Me (1d) (0.649). Availability of p-type electron density at phosphorus drastically strengthens the P-CO bond and weakens the C-O bond via p-back-donation, bis complexes are better described as weak CO (C/P) adducts to phosphorus. In complexes [(OC) 5 W{P(CO)R}] the CO activation by phosphorus equals that of CO activation through tungsten in pentacarbonyltungsten complexes. A comparative study of various CO bonding motifs in molecular compounds indicates that acyclic (2) or cyclic diphospha-urea derivatives (2-5) or isomers (6) display P-CO bond strengths (compliance constants range 0.502-0.640) well below that of the P-C bond of H 2 P-CH 3 (0.364), thus providing insight into the bonding and the ease of CO extrusion, experimentally known for some cases. A highly unusual adduct of CO was obtained in silico through two-fold P-ligation in diphosphiren-3-ones 2a-d, the parent compound of which was found to be properly described as a side-on (P]P)/(C]O) complex, in contrast to its aza-analogue 2a N . A drastic weakening of the P-CO bond strength is observed from P 2 CO (2a) (0.502) to the C 2 -symmetrical (H 3 BP) 2 CO (2b) (0.913); the latter represents an extreme case of a weakly bound CO. Furthermore, calculated 31 P NMR shifts and scalar 1 J(P,E) couplings were correlated with P-CO and PC-O compliance constants as a tool for experimentalists.In the area of dinuclear electrophilic phosphinidene complexes and their higher homologues [{L n M} 2 PnR] (L n M ¼ 16e transition-metal complex, pnictogen (Pn), Pn ¼ P, As, Sb, Bi, R ¼ e.g. alkyl, aryl etc.) it has long been known that the central Pn atom can add Lewis bases (e.g. thf, acetone, NH 3 , NEt 3 , TMEDA, pyridine) 13,14thus representing an early landmark in the coordination-to-phosphorus chemistry. 15 In this case the mostly reversible and strongly temperature dependent adduct formation is accompanied by drastic changes in colour caused by the disturbance of the M-P-M p-system. In a recent study, the P-acetonitrile adduct [{W(CO) 5 } 2 P(N^CMe)Cp*] was deduced based on 31 P NMR spectroscopy. 16 In a broader context and although dissociation has not been reported as yet, "adducts" of N-heterocyclic carbenes to pnictinidenes, formally described by the formula NHC/PnR (Pn ¼ P, As), should be mentioned. 17 Upon borane complexation of the phospho...

Coordination of N2and Other Small Molecules to the Phosphorus Centre of RPW(CO)5: A Theoretical Study on the Janus Facets of the Stabilization/Activation Problem

“…For the sake of comparison with N‐donor derivatives previously reported, adducts 1 v – z containing C‐donor ligands have been included in this study. In all cases the P atom has pyramidal geometry (Σ<P=305.6–316.1°; Table ).…”

“…In contrast, carbodiimides were successfully transformed into 3‐imino‐azaphosphiridine complexes, despite their thermal lability and exchange tendencies . Maybe the most striking example for this paralleled transition‐metal coordination chemistry was the outcome of a bonding analysis of neutral complexes having CO κC‐bound to a phosphorus centre . Concerning the fixation of CO by non‐metal main group element group 14 centres, related results were reported for a CAAC derivative, yielding a bent ketene, and for silylene H 2 Si forming a Lewis acid base complex with CO ]12] as matrix isolation studies have shown.…”

“…Other stable ligand‐phosphinidene complex end ‐on adducts III were isolated having donors such as phosphanes, NHCs and isocyanides . In the same vein, several transformations were explained on the basis of intermediates having ligand‐to‐P phosphinidene complex structures with ligands such as water, imines, dimethylcyanamide, carbodiimides, carbon dioxide, carbon monoxide, ketones and thiourea . In case of ligands having polar E‐H functionalities of group 13 to 16 elements, recently studied by DFT methods, the stepwise oxidative addition process to the phosphorus centre was described to proceed through the initial coordination of the ligand‐to‐P …”

“…[9] Maybe the most strikingexample for this paralleled transition-metal coordination chemistry was the outcome of ab ondinga nalysis of neutral complexes having CO kC-bound to ap hosphorus centre. [5,10] Concerningt he fixation of CO by non-metal main group element group 14 centres,r elated results were reported for aC AAC derivative, yieldingabent ketene, [11] and for silylene H 2 Si formingaLewisa cid base complex with CO ]12] as matrix isolation studies have shown.I nformationo nr eactions of similarc omplexes of divalent carbon and silicon species with N 2 is even more scarce, [12] despite the fact that diazoalkanes [13] have ar ich chemistry and H 2 CN 2 has been known since 1894. [14] From ad ifferentp erspective, there was al ongstanding targetinthe chemistry of neutralterminal phosphinidenecomplexes ID the stabilization of such fleeting neutral intermediates.…”

Unprecedented Ring–Ring Interconversion of N,P,C‐Cage Ligands

“…As previously proposed (Scheme ), complex 4 Cr f is also the intermediate resulting from nucleophilic attack of the nitrogen atom of the corresponding model imine ArCHNH ( 3′ ) on the model electrophilic phosphinidene complex 2 Cr . According to a recent report on the reactions of phosphinidene complexes with carbon monoxide,19 the two reagents approach very exergonically20 and in a barrier‐less fashion as no van der Waals complex could be located at the current level of theory. The privileged conformation in 4 Cr f undergoes a kinetically and thermodynamically favoured transformation into the phosphinidene complex 10 Cr f by means of an azaphospha‐Cope [3,3] shift (Scheme ) 21…”