Gerd von Frantzius

The strongest bond in the universe? Accurate calculation of compliance matrices for the ions N2H+, HCO+, and HOC+

Compliance matrices of protonated CO and N2 are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 A/mdyn) to CO–H+ the C–O bond is weakened (0.062 A/mdyn), while the C–O bond in H–CO+ is getting stronger (0.045 A/mdyn). After protonation, the N–N bond strength is getting stronger (from 0.043 to 0.042 A/mdyn), too. The invariance of compliance matrix elements Cij under completion of (xi,xj) to a complete set (…,xi,…,xj,…) of internal coordinates is demonstrated.

Novel access to azaphosphiridine complexes and first applications using Brønsted acid-induced ring expansion reactions

Synthesis of azaphosphiridine complexes 3a-e was achieved via thermal group transfer reaction using 2H-azaphosphirene complex 1 and N-methyl C-aryl imines 2a-e (i) or via reaction of transient Li/Cl phosphinidenoid complex 5 (prepared from dichloro(organo)phosphane complex 4) using 2a-c (ii), respectively. Reaction of complexes 3a,d and trifluoromethane sulfonic acid in the presence of dimethyl cyanamide led to a highly bond- and regioselective ring expansion yielding 1,3,4sigma3lambda3-diazaphosphol-2-ene complexes 8a,d after deprotonation with NEt3. 31P NMR reaction monitoring revealed that protonation of complex 3a yields the azaphosphiridinium complex 6a, unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR and UV/Vis (for 3a,d, 6a, 8a,d), MS and single-crystal X-ray crystallography in the cases of complexes 3b-d, 8a and 8d. DFT studies on the reaction mechanism and compliance constants of the model complex of 6a are presented.

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

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(η2-CO)), the dimer (HPCO)2 as well as PCH, HPCH2 and H2P–CH3 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 (in A mdyn−1) reveal a clear trend that shows a weakening of the P–CO bond strength from 1a to mono-ligation as in [(OC)5W{P(CO)Me}] (1c) (0.301), in H3BP(CO)Me (1b) (0.322), to bis-ligation as in [{(OC)5W}2P(CO)R] (1f) (0.488) to (H3BP)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 π–back-donation, bis complexes are better described as weak CO (C→P) adducts to phosphorus. In complexes [(OC)5W{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 H2P–CH3 (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 (PP)→(CO) complex, in contrast to its aza-analogue 2aN. A drastic weakening of the P–CO bond strength is observed from P2CO (2a) (0.502) to the C2-symmetrical (H3BP)2CO (2b) (0.913); the latter represents an extreme case of a weakly bound CO. Furthermore, calculated 31P NMR shifts and scalar 1J(P,E) couplings were correlated with P–CO and PC–O compliance constants as a tool for experimentalists.

Deoxygenation of carbon dioxide by electrophilic terminal phosphinidene complexes

Deoxygenation of carbon dioxide was achieved using transient terminal phosphinidene chromium and tungsten complexes 2a,b. The overall reaction is exothermic according to DFT calculations on the model terminal P-methyl phosphinidene complex Me-2b; this was also supported by the calculated thermodynamic oxygen-transfer potential. The oxaphosphiran-3-one complex intermediates 3a,b possess an unprecedented bonding situation as some characteristics of a side-on bound carbon dioxide to the (formally) low-coordinated phosphorus centre come to the fore. This is expressed by equidistant P–C and P–O bonds and unusual bond strength relationship, i.e. P–C > P–O, as revealed by the relaxed force constants and other related parameters. The decomposition of 3a,bvia CO extrusion yields terminal phosphinidene oxide complexes 4a,b which dimerise to the final products, the 1,3-dioxa-2,4-diphosphetane complexes 5a,b–8a,b. Additional experimental evidence for the transient formation of phosphinidene oxide complexes 4a,b was obtained by a cross dimerisation experiment using transient chromium and tungsten complexes 2a,b. First comparative investigations on the reaction of Li–Cl phosphinidenoid complex 10 and CO2 at low temperature revealed the formation of the carbamoyl–phosphane complex 11.