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Koop Lammertsma

Publications and source records attributed to Koop Lammertsma.

34 records · Page 2Linked to original sources
PubMed2004

Reaction of a terminal phosphinidene complex with azulenes: eta1-complexes, C--H bond insertions, and 1,4-adducts.

Reaction of an in situ generated phosphinidene complex [PhPW(CO)(5)] with the aromatic azulene and guaiazulene leads to unexpected 1,4-adducts of the seven-membered ring and to C--H bond insertion of the five-membered ring. A DFT analysis suggests that the reaction is initiated by formation of a eta(1)-complex between the phosphinidene and the five-membered ring of the aromatic substrate. Four conformations of this complex were identified. Two convert without barrier to the slightly more stable syn- and anti-1,2-adducts. These undergo pericyclic 1,7-sigmatropic rearrangements with remarkably low barriers to give 1,4-adducts, with an inverted configuration at the phosphorus center. An X-ray crystal structure is presented for one of the 1,4-adducts of guaiazulene. The other two eta(1)-complexes insert with modest barriers into a C--H bond of the five-membered ring.

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PubMed2004

Branched phospha[7]triangulanes.

A highly strained, thermally stable (up to 150 degrees C) branched phospha[7]triangulane was synthesized from the second-generation bicyclopropylidene and transient phosphinidene [Ph-P=W(CO)5], followed by demetalation in refluxing xylene. Bulkier transient CuCl-alkene-complexed phosphinidene gave 2-phosphabicyclo-[3.2.0]hept-1(5)-ene as an additional product. The "outer sphere" spirocyclopropanes provide a stabilizing factor for both of these novel compounds.

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PubMed2003

Nonplanarity at tri-coordinated aluminum and gallium: cyclic structures for X3Hn(m) (X = B, Al, Ga).

Structures and energies of X3H3(2-), X3H4-, X3H5, and X3H6+ (X = B, Al and Ga) were investigated theoretically at B3LYP/6-311G(d) level. The global minimum structures of B are not found to be global minima for Al and Ga. The hydrides of the heavier elements Al and Ga have shown a total of seven, six and eight minima for X3H3(2-), X3H(4-), and X3H5, respectively. However, X3H(6+) has three and four minima for Al and Ga, respectively. The nonplanar arrangements of hydrogens with respect to X3 ring is found to be very common for Al and Ga species. Similarly, species with lone pairs on heavy atoms dominate the potential energy surfaces of Al and Ga three-ring systems. The first example of a structure with tri-coordinate pyramidal arrangement at Al and Ga is found in X3H(4-) (2g), contrary to the conventional wisdom of C3H3+, B3H3, etc. The influence of pi-delocalization in stabilizing the structures decreases from X3H3(2-) to X3H6+ for heavier elements Al and Ga. In general, minimum energy structures of X3H4-, X3H5, and X3H6+ may be arrived at by protonating the minimum energy structures sequentially starting from X3H3(2-). The resonance stabilization energy (RSE) for the global minimum structures (or nearest structures to global minimum which contains pi-delocalization) is computed using isodesmic equations.

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PubMed2003

Copper(I) chloride initiated decomposition of 7-phosphanorbornadiene. Evidence for a solvent-assisted catalytic mechanism.

Density functional theory is used to explore the mechanism of the copper(I)-chloride-catalyzed decomposition of W(CO)(5)-complexed 7-phosphanorbornadiene and the subsequent olefin trapping of the terminal phosphinidene complex. CuCl lowers the activation barrier by interacting directly with the breaking P-C bond. Contrary to the prevailing notion that a free terminal phosphinidene complex (W(CO)(5)=PR) is generated in the CuCl-catalyzed cheletropic elimination of the 7-phosphanorbornadiene-W(CO)(5) complex, the present mechanism suggests that CuCl is attached to the terminal phosphinidene. Furthermore, a "chloride shuttle" takes place where the chloride first migrates to the phosphorus center and then is returned back to the copper center by the incoming olefin in an S(N)2 reaction step. When the substituent on phosphorus is a phenyl group (R = Ph), the uncatalyzed reaction has an activation barrier of 17.9 kcal/mol, which is reduced by 10.9 kcal/mol on including the CuCl catalyst. The CuCl-catalyzed decomposition of 7-phosphanorbornadiene followed by olefin trapping of the terminal phosphinidene complex has a close parallel with the Cu(I)-catalyzed cyclopropanation reaction of diazoalkane. In both catalyzed reactions, copper(I) is coordinated to the phosphinidene/carbene as a Lewis acid, while a Lewis base is displaced from the phosphorus/carbon center as the olefin is added.

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PubMed2003

N-Heterocyclic carbene functionalized iridium phosphinidene complex [Cp*(NHC)Ir=PMes*]: comparison of phosphinidene, imido, and carbene complexes.

The novel phosphinidene complex [Cp*(NHC)Ir=PMes*] (3; NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) was prepared in high yield from [Cp*(NHC)IrCl(2)] (2) and [LiPHMes*].3 THF. It represents the first example of an NHC ligated transition metal phosphinidene complex. The X-ray crystal structure for 3 is also reported. DFT calculations on the N-heterocyclic carbene containing parent complexes [Cp(NHC)Ir=E] (E=PH, NH, CH(2)) show that the NHC ligand acts as good sigma-donor/weak pi-acceptor ligand and forms strong Ir-C(NHC) single bonds. The Ir=E double bonds result from strong triplet-triplet interactions between [Cp(NHC)Ir] and E.

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PubMed2003

Synthesis and reactions of terminal osmium and ruthenium complexed phosphinidenes [(eta6-Ar)(L)Md=PMes*].

Novel, very stable ruthenium and osmium containing terminal phosphinidene complexes [(eta(6)-Ar)(L)M=Mes*] (Ar=benzene, p-cymene; L=PR(3), CO, and RNC) have been prepared by dehydrohalogenation of novel [(eta(6)-Ar)MX(2)(PH(2)Mes*)] complexes in the presence of a stabilizing ligand. Xray crystal structures are reported for [(eta(6)-C(6)H(6))(PPh(3))Rud=PMes*] (9) and [(eta(6)-pCy)(PPh(3))Os=PMes*] (4). Dehydrohalogenation in the absence of a stabilizing ligand resulted in the new P-spiroannulated Ru(2)P(2)-ring structure 16. Dehydrohalogenation in the presence of but-2-yne gave a novel phosphaallyl complex [(eta(6)-Ar)Ru(eta(3)-R(2)PC(Me)CHMe)] 26, for which an X-ray crystal structure is reported. The mechanism by which 16 and 26 are obtained is presumed to involve the intermediate formation of the 16-electron (eta(6)-benzene)Rud=PMes* phosphinidene complex.

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PubMed2003

Photodissociation of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)L and Fe(CO)(4)L: a theoretical study.

The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)(6) and Fe(CO)(5). The lowest excited states of Cr(CO)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH(3) ligand. Analysis of their ground-state Cr-P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.

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PubMed2002

Vinylphosphirane-phospholene rearrangements: pericyclic [1,3]-sigmatropic shifts or not?

The conversion of "free" and Cr(CO)(5)-complexed 2-vinylphosphiranes into 3-phospholenes via [1,3]-sigmatropic shifts was studied with density functional theory and compared with the corresponding hydrocarbon system, that is, the vinylcyclopropane-cyclopentene rearrangement. All three systems behave similarly with subtle but important differences. No intermediate was found on any of the potential energy surfaces. 2-Vinylphosphiranes have smaller rearrangement barriers than vinylcyclopropane, and those carrying the Cr(CO)(5) group have still smaller ones. The rearrangement of both anti- and syn-2-vinylphosphiranes occurs in a concerted pericyclic manner with inversion of configuration at the migrating phosphorus, requiring, respectively, 29.3 and 36.7 kcal/mol, much in contrast to the 44.6 kcal/mol demanding diradical-like process for the hydrocarbon analogue. Epimerization at the phosphorus center (syn right arrow over left arrow anti) requires approximately 32.0 kcal/mol and occurs in a single step, reflecting a diradical-like ring opening-ring closure process that can occur in both a clockwise and counterclockwise fashion. Complexation of the phosphorus center by Cr(CO)(5) results in the substantial stabilization of reagents and products and further reduces the barriers for rearrangement. The anti isomer has the lowest barrier for the [1,3]-shift (DeltaE = 20.5 kcal/mol), which is slightly less than that needed for P-epimerization and for conversion of the syn isomer, both of which are nonpericyclic processes. When a P-phenyl group is introduced, the diradical-like conversion of the syn isomer is favored over the anti isomer, in agreement with experimental reports. The influence of torquoselectivity is discussed for the rearrangements of these structures with their heavy substituents. The origin of the stabilization rendered by the Cr(CO)(5) group and its influence on the [1,3]-conversion are also analyzed. The DFT activation energies for the diradical-like [1,3]-sigmatropic shifts were verified with a multireference method.

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PubMed2002

Synthesis, structures, and strain energies of dispirophosphiranes. Comparisons with dispirocyclopropanes.

Six novel dispirophosphirane complexes have been synthesized from the reaction of bicycloalkylidenes with the electrophilic phosphinidene complex PhPW(CO)(5). They contain a central phosphirane ring, which is spirofused on one side to a cyclopropane or cyclobutane ring and on the other side with a three-, four-, five-, or six-membered ring. Their crystal structures and MP2/6-31G-computed geometries for simplified parent systems suggest that spirofusion with small rings results in a tightening of the central three-membered phosphaheterocycle, while spirofusion with larger rings results in a relaxation of the phosphirane geometry. Similar theoretical predictions are made for the corresponding annulated hydrocarbons. Strain energies for both the hydrocarbon and phosphorus series of structures have been calculated at G3(MP2). Whereas the [3]triangulane hydrocarbon and phospha[3]triangulane have a significant excess strain of 8.1 and 5.2 kcal/mol per spiroatom, respectively, the excess strain for systems spirofused with larger rings are negligible for the hydrocarbons and even negative for the phosphorus-containing species because of hyperconjugative stabilization.

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PubMed2002

Nucleophilic or electrophilic phosphinidene complexes ML(n)=PH; what makes the difference?

Density functional studies, based on the local density approximation including nonlocal corrections for correlation and exchange self-consistently, have been carried out for the equilibrium structures of the phosphinidene transition metal complexes ML(n)=PH, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir and L = CO, PH(3), Cp. The chemical reactivity of the transition metal-stabilized phosphinidene P-R is influenced by its spectator ligands L. Ligands with strong sigma-donor capabilities on the metal increase the electron density on the phosphorus atom, raise the pi-orbital energy, and enhance its nucleophilicity. Spectator ligands with strong pi-acceptor capabilities lower the charge concentration on P and stabilize the pi-orbital, which results in a higher affinity for electron-rich species. The ML(n)=PH bond is investigated using a bond energy analysis in terms of electrostatic interaction, Pauli repulsion, and orbital interaction. A symmetry decomposition scheme affords a quantitative estimate of the sigma- and pi-bond strengths. It is shown that the investigated phosphinidenes are strong pi-acceptors and even stronger sigma-donors. The metal-phosphinidene interaction increases on going from the first to the second- and third-row transition metals.

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PubMed2002

Metal-template-directed synthesis of diphosphorus compounds through intramolecular phosphinidene additions.

Heating the nonchelating cis-bis-7-phosphanorbornadiene-[Mo(CO)4] complex (13) results in the thermal decomposition of one of the 7-phosphanorbornadiene groups. The phosphinidene thus generated adds intramolecularly to a C=C bond of the other ligand to give the novel diphosphorus complex 14. This reaction constitutes a metal-template-directed synthesis. Likewise, the intramolecular phosphinidene addition to the C=C bond of a Mo-phospholene ligand affords the diphos complex 18. Its crystal structure exhibits an extremely small P-Mo-P bite-angle for a five-membered chelate ring. The similar intramolecular 1,2-addition to a C=C bond of a phosphole ligand gives a highly strained, unstable intermediate product. Scission of its P-Mo bond generates a free coordination site, which is then occupied by either CO or a phosphole to yield complexes 22 and 23, respectively. The analogous intermolecular addition of [PhPW(CO)5] to a [phosphole-W(CO)5] complex gives the di-[W(CO)5] complexed adduct 28. The directing effect of the metal on the intra- and intermolecular additions is discussed.

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PubMed2001

Norbornadiene-Quadricyclane Valence Isomerism for a Tetraphosphorus Derivative The Netherlands Organization for Scientific Research (NOW/CW) is acknowledged for partial support (A.L.S., M.L., K.L.), as are the NSERC of Canada for a scholarship (S.B.C.), and EPSRC for funding phosphaorganometallic chemistry at Sussex University (J.F.N.). Dr. H. Zappey is thanked for the HR-MS spectra and J. C. Slootweg for his initiative.

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