Chasing the agostic interaction in ligand assisted cyclometallation reactions of palladium(ii).

A 500 MHz NMR study of the reaction between 1-tetralone oxime and PdCl42- in CD3OD shows resonances attributable to a potential agostic intermediate prior to the formation of the insoluble cyclopalladated product which itself was characterised by X-ray crystallography. Calculated structural, spectroscopic, QTAIM, NBO and NCI analysis results obtained from density functional theory (DFT) calculations give a full description of the putative agostic intermediate [PdCl2(1-tetralone oxime)] (1) which is shown to include a previously unrecognised π-electron density donation from the aromatic ring to the metal in close proximity to the agostic carbon atom. Changing the (N)-OH donor to (N)-OMe does not effect the magnitude of these interactions. (N)-OH and (N)-OMe acetophenone imines in which the aromatic ring has the potential to rotate show similar agostic and π-electron donation to the alicyclic ring counterparts. 1-Tetralone which coordinates to the metal by a Pd-O bond that is much weaker than the Pd-N complexes has a slightly stronger agostic component and slightly weaker π-electron donation than the oxime counterpart.

New complexity for aromatic ring agostic interactions.

Density functional theory (DFT) calculations reveal that for ligand directed aromatic ring C-H bond activation, the agostic donation can share the same antibonding acceptor orbitals as a previously unrecognised π-donation from the aromatic ring of the ligand. The recognition of carbon based orbitals assisting the agostic interaction has significant implication for C-H bond activation chemistry.

4-tert-Butyl-pyridinium chloride-4,4'-(propane-2,2-di-yl)bis-(2,6-di-methyl-phenol)-toluene (2/2/1).

In the title solvated salt, C9H14N(+)·Cl(-)·C19H24O2·0.5C7H7, two mol-ecules of 4,4'-(propane-2,2-di-yl)bis-(2,6-di-methyl-phenol) are linked via O-H⋯Cl hydrogen bonds to two chloride ions, each of which is also engaged in N-H⋯Cl hydrogen bonding to a 4-tert-butyl-pyridinium cation, giving a cyclic hydrogen-bonded entity centred at 1/2, 1/2, 1/2. The toluene solvent mol-ecule resides in the lattice and resides on an inversion centre; the disorder of the methyl group requires it to have a site-occupancy factor of 0.5. No crystal packing channels are observed.

Di-µ-methanolato-bis[(2-tert-butyl-6-methylphenolato-κO)methyl-titanium(IV)].

The molecule of the title compound, [Ti2(CH3)2(CH3O)2(C11H15O)4] or {[Ti(Me)(µ-OCH3)(OC6H3CMe3-2-Me-6)]2}, has a centrosymmetric, dimeric structure with a distorted square pyramidal array about each titanium atom. The methoxide ligands form an asymmetric bridge between the two Ti(IV) atoms [Ti-O bond lengths of 1.9794 (12) and 2.0603 (12) Å] with the two phenolato ligands occupying the remaining basal sites [Ti-O 1.8218 (11) and 1.8135 (11) Å]. The Ti-O-C phenolato bond angles are similar at 161.24 (10) and 160.66 (11)°. The methyl ligand attached to the metal atom has a Ti-C bond length of 2.0878 (17) Å.

Identification of non-classical C-H···M interactions in early and late transition metal complexes containing the CH(ArO)3 ligand.

The fully optimised DFT structure of the d(0) complex [{CH(ArO)3}Ti(NEt2)] (2) at the B3LYP level compares well with the distorted tetrahedral geometry shown by the X-ray crystal structure. QTAIM analysis of the electron density associated with the C-H···Ti interaction shows a well defined bond critical point, a bond path between the hydrogen and titanium centres and a negative value for the energy density indicative of covalency. A natural bond orbital (NBO) picture of the interaction shows that the C-H σ bond electron density donates to a d hybrid orbital on the metal in a linear fashion. Calculated IR and NMR data for the components of the interaction are consistent with experiment. The computed structures for [{CH(ArO)3}Ti(OPh)] (3), [{CH(ArO)3}Zr(NEt2)] (4), [{CH(ArO)3}Hf(NEt2)] (5), show tetrahedral geometries and QTAIM and NBO properties similar to (2). [{CH(ArO)3}Mo(NEt2)] (6) shows distortion of the tripodal ligand and a reduced C-H···M bond angle with properties more consistent with a C-H···M side-on donor interaction. In [{CH(ArO)3}Fe(NEt2)] (7) the C-H···M bond angle is linear and involves a donor interaction. An energy minimised structure maintaining the three fold coordination to the tripodal ligand was not obtained for [{CH(ArO)3}Ni(NEt2)](2-) but changing from a diethyl amide ligand to phenolato gave energy minimised [{CH(ArO)3}Ni(OPh)](2-) (8). This structure shows a distorted square planar geometry with a substantially bent phenoxo ligand and a near linear C-H···M covalent interaction with donor and back bonding properties. The work shows that linear C-H···M interactions can have both agostic and weak hydrogen bond-like covalency.

[6,6'-Bis(1,1-di-methyl-eth-yl)-4,4'-dimethyl-2,2'-methyl-enediphenolato-κ(2) O,O']di-chlorido-(9H-fluoren-9-ol-κO)titanium(IV)-fluorene-diethyl ether (1/0.5/1).

The title adduct, [TiCl2(C23H30O2)(C13H10O]·0.5C13H10·C4H10O, is a monomer with a trigonal-bypyramidal coordination sphere of the Ti(IV) atom in which the ligand O atoms of the bidentate diphenolate anion are located in both apical and equatorial positions. Chloride ligands occupy the remaining two equatorial sites of the trigonal bypyramid with the fluoren-9-ol O atom occupying the other apical site. The hy-droxy group H atom of this latter group is hydrogen bonded to an O atom of a non-coordinating diethyl ether mol-ecule. The title compound also contains a further fluorene solvent mol-ecule, which lies across a centre of symmetry and which is equally disordered over an inversion centre.

Bis(µ-2-hy-droxy-methyl-2-methyl-propane-1,3-diolato)bis-[di-chlorido-titanium(IV)] diethyl ether disolvate.

The title complex, [Ti2Cl4{CH3C(CH2O)2(CH2OH)}2], lies across a centre of symmetry with a diethyl ether solvent mol-ecule hydrogen bonded to the -CH2OH groups on either side of it. The Ti(IV) atom is coordinated in a distorted octa-hedral geometry by a tripodal ligand and two terminal chloride atoms. There are three coordination modes for the tripodal ligand distinguishable on the basis of their very different Ti-O bond lengths. For the terminal alkoxo ligand, the Ti-O distance is 1.760 (1) Å, the asymmetric bridge system has Ti-O bond lengths of 1.911 (1) and 2.048 (1) Å. The Ti-O bond length for the alcohol O atom is the longest at 2.148 (1) Å.

Analysis of M···H-Si interactions in [{M(CpSiMe2H)Cl3}2], (M = Zr, Hf, Ti and Mo) complexes.

For the d(0) complex [{Zr(CpSiMe(2)H)Cl(3)}(2)] which contains a linear Si-H···Zr interaction across the dimer, DFT calculations are in good agreement with X-ray structures. The BP86 functional shows a slightly stronger interaction than B3LYP but for qualitative purposes either functional is sufficient. QTAIM analysis shows a bond critical point (bcp) for the interaction, a small negative value for the total energy density [H((r))] and the H atomic basin decreases in energy, E(H), and atomic volume compared to the free ligand. NBO analysis showed E(2) for Si-H σ to Zr(dz(2)) donation at 42.8 kcal mol(-1) and a 34% spatial overlap for the interaction consistent with an inverse hydrogen bond. The Wiberg bond index for the interaction is 0.1735 (0.7205 for the Si-H bond), ν((Si-H)) and (1)J((Si-H)) at 2060 cm(-1) and 145.4 Hz compared to 2183 cm(-1) and 172.1 Hz in the free ligand. Using a "synthesis by computation" approach to forming like complexes, similar features were found for [{Hf(CpSiMe(2)H)Cl(3)}(2)]. The titanium complex [{Ti(CpSiMe(2)H)Cl(3)}(2)] does not contain any Si-H···Ti interaction as rotation about the C-Si bond of the ligand occurs to place the Si-H bond hydrogen closer to a terminal chloro ligand across the dimer. An increase in electron density on the metal in the d(2) complex [{Mo(CpSiMe(2)H)Cl(3)}(2)] results in a stronger interaction with a distinct QTAIM analysis bcp [ρ((r)) 0.0448 a.u.], a small negative value for H((r)) and a much reduced H atomic volume. NBO analysis shows E(2) for Si-H σ to Mo(dz(2)) donation at 143.1 kcal mol(-1) and a 29% spatial overlap. Mo(dz(2)) to Si-H σ* donation (back donation) is minimal [E(2) 1.3 kcal mol(-1), ~1% spatial overlap]. The Wiberg bond index is 0.3114 (0.5667 for the Si-H bond), ν((Si-H)) 2015 cm(-1) and (1)J((Si-H)) 120.6 Hz.

(2,2'-Bipyridine-κN,N')[2-tert-butyl-anilinato(2-)]dichloridooxido-molybdenum(VI) dichloro-methane hemisolvate.

The Mo(VI) atom in the title structure, [Mo(C(10)H(13)N)Cl(2)O(C(10)H(8)N(2))]·0.5CH(2)Cl(2), has a distorted octa-hedral coord-ination sphere with cis-orientated oxide and imide ligands, trans-chloride ligands and the 2,2'-bipyridine (bipy) ligand N atoms lying trans to the oxide and imide ligands. An imide-ligand tert-butyl-methyl-group H atom makes a close approach with the oxide ligand (distance = 2.53 Å) and the imide-ligand N atom (distance = 2.41 Å). Another imide-ligand tert-butyl-methyl-group H atom makes a close approach to a chloride ligand (distance = 2.82 Å). One bipy-ligand α-H atom makes a close approach to the oxide ligand (distance = 2.4 Å) and the other α-H atom makes a close approach to the imide-ligand phenyl-ring ortho-H atom (distance = 2.52 Å). These close approaches suggest the presence of weak intra-molecular hydrogen bonds. The solvent molecule has been modelled under consideration of half-occupancy.

A case for linear agostic interactions: identification by DFT calculation in a complex of Ta containing a uniquely caged triphenylmethyl C-H hydrogen.

Reaction of one equivalent of tris(3,5-di-tert-butyl-2-hydroxy)methane with TaCl5 in CH2Cl2 along with Et3N gave a solid which on prolonged crystallisation led to a small quantity of crystalline material. An X-ray crystal structure determination showed one crystal was [TaCl3[[OC6H2(CMe3)2-2,4]3CH]]- Et2NH2+.3C6H6.1.5H2O with the anion consisting of three chloro ligands and three phenoxides of the tripodal ligand about the tantalum centre. The triphenylmethyl group proton was located and refined and was found to be enclosed in a cage making contacts of 2.09(8), 2.09(8) and 1.89(12)A with the phenoxide ligand oxygens consistent with weak C-H bond hydrogen bonding. The hydrogen atom points at the tantalum atom at a distance of 2.14(11) A from it, the TaH-C angle is 166 degrees and the C-H bond distance is 1.04(12) A. DFT calculations at the B3LYP level indicate that where a hydrogen atom is attached to the triphenylmethyl carbon on the inside of the cage, there is good agreement with the crystal structure. The C-H bond points directly at the tantalum centre and an NBO analysis indicates there is significant overlap of the triphenylmethyl C-H bond electron density in a linear sense with an "unfilled" metal d orbital. Based on the NBO analysis, the C-HTa overlap would appear to be an example of a linear agostic interaction under the definition of agostic bonding.

A zirconium zwitterion containing a caged amine H atom.

The crystallographically centrosymmetric zwitterion bis[tris(3,5-dimethyl-2-oxidobenzyl-kappaO)ammonium]zirconium(IV) crystallizes as the chloroform disolvate, [Zr(C(27)H(31)NO(3))(2)].2CHCl(3), with the two molecules of chloroform closely associated with two of the aromatic rings. The Zr atom has a distorted octahedral geometry with three phenoxy O atoms from each of the two ligands coordinated to it. Charge balance is maintained by protonation of each N atom, which then forms intramolecular hydrogen-bonding interactions to all three adjacent O atoms.