Dossier: Special Issue in Tribute to Yves Chauvin
Open Access
Issue
Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles
Volume 71, Number 2, March–April 2016
Dossier: Special Issue in Tribute to Yves Chauvin
Article Number 24
Number of page(s) 14
DOI https://doi.org/10.2516/ogst/2015047
Published online 05 April 2016

© A.A. Danopoulos and P. Braunstein, published by IFP Energies nouvelles, 2016

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction

The synthesis, coordination chemistry and catalytic applications of functional phosphine-type ligands have represented a very fast growing field of research during the past 30 years and the diversity of donor groups that can thus be associated with the phosphorus donor opens unlimited possibilities. The chemically different nature of the functions available for coordination to metal centres immediately leads to consider the intrinsic properties of the resulting bonds and their consequences on the nature of the interactions between the metal(s) and other ligands present in the coordination sphere. This is illustrated in Scheme 1 with the simple case of a square-planar metal complex. Even if the ligands A and B are chemically the same (e.g. two chlorido or two methyl ligands), their reactivity will not be identical because of the stereochemically-different environment generated by the chelating functional phosphine ligand. This would not be the case in the presence of a symmetrical P,P or a Z,Z chelate with identical donor groups. Hybrid ligands thus represent a straightforward stereoelectronic differentiation tool within the metal coordination sphere, allowing selective reactivity and fine-tuning of the catalytic properties of metal complexes, as exemplified in the case of ethylene oligomerization [1, 2].

Ethylene, propylene and butenes are in increasing demand [313]. Ethylene alone is at the origin of ca. 30% of the products originating from the petrochemical industry. Among the α-olefins in the range C4-C20, those with shorter chain lengths (C4-C8) enjoy a rapidly expanding market owing to their applications as comonomers for the production of high density polyethylene and linear low density polyethylene and as intermediates in the production of oxo alcohols, hexyl- and octyl-mercaptans, amines, etc. Other linear α-olefins find applications e.g. for the fabrication of lubricants (C10), biodegradable detergents (C10, C12, C14), surfactants and lubricant additives (C12-C20) and many other useful chemicals. These features explain the continuing interest in both academia and industry for this research area [1419].

The first part of this contribution will illustrate some of the work we carried out with metal complexes containing P,O-, P,N-, and N,O-type ligands relevant to ethylene oligomerization. This topic was selected because it corresponds to the starting points of our collaboration with Yves Chauvin and his group in the mid-90s. The second part deals with very recent developments concerning the stereoelectronic stabilization of ‘underligated’ metal complexes, a concept that illustrates a change of paradigm with respect to the classical 18 valence electron rule. Underligated complexes attract a rapidly increasing interest because of their often unusual features and reactivity and relevance to catalysis owing to the electronic unsaturation of their coordination sphere.

1 Metal Complexes with Functional Chelating Ligands

1.1 Metal Complexes with P,O-Type Ligands

In our first publication on transition metal complexes with functional phosphines, we described the coordination of the phosphino-ester ligand Ph2PCH2C(O)OEt to various transition metal centres [20, 21]. In the case of Rh(III) and Ir(III), octahedral trihalide complexes containing two such ligands were characterized, in which one of them is terminally P-bound to the metal whereas the other acts as a P,O chelate. It was found by 1H and 31P{1H} variable-temperature NMR spectroscopy that a dynamic behaviour was taking place in the Rh(III) complex, but not in the Ir(III) complex, that led the two ligands to exchange their role above coalescence temperature. This dynamic behaviour, which involves opening and closing of the metal-oxygen bond of the P,O chelate, became shortly after known as hemilabile behaviour [22, 23]. Related observations were made with Ru(II) complexes and applied to the reversible binding of carbon monoxide under ambient conditions (Scheme 2) [24].

Similarly to the phosphino-ester ligand Ph2PCH2C(O)OEt, the phosphino-ketone Ph2PCH2C(O)Ph was found to chelate transition metals and its easy and modular synthesis prompted numerous studies in coordination chemistry [25]. It was rapidly found that when this ligand is coordinated to a transition metal, either κ1-P or κ2-P,O, its deprotonation readily affords complexes containing a P,O-chelating phosphino-enolate ligand [Ph2PCHC(O)Ph], weak bases being sufficient as a result of the enhanced acidity of the PCH2 protons upon coordination [26]. This observation appeared very promising, in particular in view of the remarkable properties of nickel complexes containing such phosphino-enolate and related ligands in catalytic ethylene oligomerization, which form the basis of the industrial Shell Higher Olefin Process (SHOP) [2749]. This process does not require any cocatalyst and leads to linear α-olefins with remarkable selectivity. Furthermore, access to a diversity of transition metal complexes containing phosphino-enolates and related ligands opens interesting perspectives since the reaction pathway leading to the SHOP-type model catalyst (1), involving oxidative-addition of a P-Ph bond of the corresponding β-keto phosphorus ylid Ph3PCH=C(O)Ph across Ni(0), only works with Ni(0) (Scheme 3). These considerations formed the basis of stimulating discussions with Yves Chauvin at the Institut Français du Pétrole, Rueil-Malmaison, which initiated a fruitful collaboration, still very active more than 20 years later. In our first joint paper, we examined the influence of a OH or NHR ortho-substituent in the aryl group Ar of Ni(II) complexes of type 2 (Scheme 4) on their catalytic properties toward ethylene oligomerization [47]. It was found that H-bonding interactions between this substituent and the enolate O atom resulted in a remarkable shift of the molecular mass distribution towards the highly desirable shorter chains oligomers.

These very encouraging results on the influence of aryl substituents on catalytically relevant intramolecular H-bonding interactions prompted further studies. Thus, when the ortho-OH group on the ring Ar was replaced with a methoxy substituent in:

it was found by X-ray diffraction that the oxygen atom of the ether group is involved in a non-classical interaction with the hydrogen atom Ha of the C-H group α to the phosphorus atom (Scheme 5). Furthermore, the hydrogen atom Hb in the o’- position in Ar is involved in another non-classical intramolecular hydrogen bonding interaction with the enolate oxygen atom (2.34 Å) and in a C-H-π interaction with one of the phenyl rings of the PPh3 ligand (2.74 Å from its centroid) [48]. The contrasting orientations of the o-C6H4XH and o-C6H4OMe aromatic cycles in 2 and 3, respectively, and the associated H-bonding behaviour of the P,O ligand, which functions as acceptor in the former and both a donor and an acceptor in the latter, are noteworthy and emphasize the need for detailed structural investigations to identify the occurrence and consequences of subtle substituent effects.

These results triggered a theoretical structure-reactivity investigation of ethylene insertion into nickel-alkyl bonds [49]. The activation energy of the olefin insertion into nickel-alkyl bonds (i.e. alkyl migration) in a series of square-planar Ni(II) complexes of formula [Ni(X^Y)(ethyl)(ethylene)], where X^Y is a anionic bidentate ligand, was calculated by DFT methods. Where X ≠ Y, the reactions of the two possible isomers have been compared and it was found that when one of the coordinating groups exerts a Weak Trans Influence (WTI) and the other a Strong Trans Influence (STI), as in the case of a phosphino-enolate P^O ligand, one of the two isomers has an activation energy for olefin insertion significantly lower than that of the reaction of complexes with symmetrical bidentate ligands, either “WTI^WTI” or “STI^STI” [49].

The coordinated anionic P,O-ligands [Ph2PCHC(O)Ph] and [Ph2PCHC(O)OEt] were shown to carry significant electronic density at the α-PCH carbon and this was evidenced by reactions with a number of organic and inorganic electrophiles [26]. A remarkably facile reversible binding reaction of CO2 by a Pd(II) complex under ambient conditions was discovered and its application to the catalytic telomerization of butadiene with CO2 [50, 51], and carbon-carbon bond formation by insertion of organic isocyanates RN=C=O into the α-PC-H bond [52] were some of the outcome of these investigations. Extensions to isoelectronic or closely related metalloligands demonstrated the potential of these often air-stable but reactive systems [26, 53]. Furthermore, reactions with metal electrophiles provided novel rational entries into homo- and hetero-dinuclear chemistry [26].

1.2 Metal Complexes with P,N-Type Ligands

In the course of our studies – of which many were performed in collaboration with IFP Energies nouvelles – on the coordination properties of heterobidentate P,N-type ligands in relation to the catalytic properties of their nickel(II) complexes [2], the structures of numerous diamagnetic and paramagnetic complexes have been established by X-ray diffraction, which were often unpredictable in terms of metal coordination geometry and nuclearity (Scheme 6).

These investigations led i.a. to the finding that in the solid state, a microcrystalline sample of the green tetranuclear complex 4 could be transformed under pressure into its red mononuclear component 5 (Scheme 7) [54].

In other cases, different crystallisation temperatures could result in the formation of either mono- (6) or di-nuclear (7) complexes (Scheme 8) [55, 56]. The influence of the chelating ligands on activity, selectivity and catalyst lifetime was investigated while keeping the amount of cocatalysts to the minimum required. Complex 6 has been evaluated as precatalyst in the oligomerization of ethylene, with AlEtCl2 or MAO as cocatalyst, or propylene, with MAO as cocatalyst. With only 2 equiv. of AlEtCl2, it was highly selective for the formation of ethylene dimers (up to 96%) and with propylene, the selectivity for C6 products was higher than 98.5% [55].

These examples serve again to illustrate the importance of a full structural characterization of pre-catalysts in order to be in a position to draw meaningful structure-properties relationships.

1.3 Metal Complexes with N,O-Type Ligands

Extending our studies of Ni(II) complexes with P,O- or P,N- to N,O-type heterofunctional chelates also provided interesting findings. We focused on oxazolinyl- and pyridinyl-alcohols and – alcoholates as ligands and a range of polynuclear complexes with unexpected nuclearities and structures could be characterized (Scheme 9). The ability of the alcoholate oxygen to bridge between two or three metal centres plays a critical role and accounts for the isolation of a number of coordination clusters.

In particular, complexes with cubane-type structures could be structurally characterized, as well as higher nuclearity complexes constituted of assembled (incomplete) cubane units such as the octanuclear complex 8 (Fig. 1) [5761].

thumbnail Figure 1

The Ni8 complex 8 containing cubane-type structural subunits [61].

Many of these polynuclear complexes turned out to have interesting magnetic properties and some behave as single molecule magnets [5761].

Polynuclear metal architectures held together by coordination bonds or by direct metal-metal bonds may be considered as models to heterogeneous catalytic sites but can also serve as molecular precursors to nanosized heterogeneous particles. This is particularly useful in the case of heterometallic systems, where cooperativity and synergistic effects may arise [62]. Such compounds may also serve as reservoirs of lower nuclearity, or even underligated (see below), mononuclear, catalytically active fragments. These fundamental questions clearly deserve further investigations.

In conclusion to this section, it is clear that the diversity of heterobidentate ligands available offers an unlimited playground for transition metal coordination chemistry. In the course of systematic and rational approaches to fine-tune the coordination sphere of a transition metal ion and thus the chemical and/or physical properties of its complexes, unexpected results may occur which open the way to new ideas and unanticipated developments; the role of serendipity should not be underestimated [63].

2 Underligated Metal Complexes

The renewed interest in the catalytic applications of ‘base metals’ (e.g. Co, Fe, Ni), which fulfil to a certain extent green environmental criteria, drives the re-investigation of their coordination chemistry and the catalytic potential of well defined reactive complexes. In this respect, the paradigm that accompanied and guided organometallic chemistry with strong-field σ-donor and π-acceptor ligands since the Wilkinson years, viz. the 18 electron rule, predicting high stability in complexes with 18 valence electrons has to be abandoned to reach the realm of coordinative and electronic unsaturation in ‘underligated’, reactive complexes. With the expectation that a number of ligands and thus a valence electron count lower than usual will be associated with enhanced reactivity and catalytic properties, we have investigated, since 2010, the synthesis and properties of ‘underligated’ metal complexes of Fe, Co and Cr stabilized by N-Heterocyclic Carbene ligands (NHC).

The approach was based on the use of bulky NHC as a means of supporting homoleptic and heteroleptic underligated structures. The NHC have versatile and tuneable electronic properties (strong σ-donors, but occasionally accepting or donating π-electron density from or to the metal). Hence, compared to the strongly σ- and π-donating amido-, alkoxo- and nacnac- (anionic bidentate ligands derived from 1,3-diketimines) co-ligands, NHC should lead to ‘genuine’ electronically unsaturated metal centres, the stability, bonding behaviour and reactivity of which under these conditions are not predictable. Compared to other bulky ligands (amido, terphenyl etc.), NHC offer easier and diverse steric tuning options leading to concave metal nesting ‘pockets’ (Fig. 2).

thumbnail Figure 2

The NHC ‘cavity’ targeted to support ‘underligation’.

thumbnail Figure 3

The structure of the FeII complex 12.

thumbnail Figure 4

The structure of the FeII complex 13. H-atoms (except the one at the anilido N) are omitted.

thumbnail Figure 5

The structure of the FeII complex 14. One of the four molecules found in the asymmetric unit is shown. All H atoms are omitted.

thumbnail Figure 6

The structure of the FeII complex 17. All H atoms are omitted.

thumbnail Figure 7

The dinuclear FeII complex 18. All H atoms are omitted.

thumbnail Figure 8

The structure of the FeII complex 19. One of the two molecules found in the asymmetric unit is shown.

thumbnail Figure 9

The structure of the FeII complex 21. All H atoms are omitted.

thumbnail Figure 10

The structure of the FeII complex 22. All H atoms are omitted.

thumbnail Figure 11

The structure of the FeII complex 24. All H atoms are omitted.

thumbnail Figure 12

The structure of the bis(benzyl) FeII complex 25. All H atoms are omitted.

thumbnail Figure 13

The structure of the FeII complex 27. All H atoms are omitted.

thumbnail Figure 14

The structure of the FeII complex 28. All H atoms are omitted.

thumbnail Figure 15

The structure of the bis(benzyl) FeII complex 29. All H atoms are omitted.

thumbnail Figure 16

The structure of the dinuclear Mg complex 30. All H atoms are omitted.

thumbnail Figure 17

Polarization-induced metal-arene interactions operating in the 3-coordinate complex [Cr(IPr)(CH2Ph)2] (32). Reprinted from [71]. Copyright American Chemical Society.

thumbnail Scheme 1

Stereochemical differentiation in a metal coordination sphere resulting from chelation of a functional phosphine.

thumbnail Scheme 2

Reversible binding of CO resulting from the opening and closing of the metal-oxygen dative bond involving the P,O chelate [24].

thumbnail Scheme 3

Typical synthesis of a SHOP-type ethylene oligomerization pre-catalyst.

thumbnail Scheme 4

Intramolecular hydrogen bonding between the ortho-substituent in the aryl group and the oxygen atom of the P,O-chelate.

thumbnail Scheme 5

Non-classical intramolecular hydrogen bonding and C-H-π interactions involving the P,O-chelate.

thumbnail Scheme 6

Various oxazoline- and pyridine-based P,N-type Ni(II) complexes, with chelate ring sizes ranging from 5 to 7, applied to catalytic ethylene oligomerization [2].

thumbnail Scheme 7

Effect of pressure on complex nuclearity: conversion of a microcrystalline sample of the Ni(II) complex 4 into 5 under pressure in the solid state [54].

thumbnail Scheme 8

Influence of the crystallization temperature on the nuclearity of the complex [55, 56].

thumbnail Scheme 9

Examples of pyridinyl-alcohols with different spacers between the heterocycle and the alcohol function investigated as ligands in Ni(II) complexes.

In this respect, the bulky ligands IPr (diisopropylphenyl substituents on the imidazole N atoms) and IMes (with mesityl substituents at N), and the sterically relevant SIPr and SIMes (with a saturated heterocycle backbone), provide similar bulk and space organization to the bulky terphenyls developed by Ni and Power [64], although the latter are anionic and the NHC are neutral ligands.

Furthermore, underligated 3d metal complexes are expected to show weak ligand fields with the following implications:

  • open-shell energetically close-lying electronic states;

  • non-covalent interactions frequently becoming functional in the stabilization of structural motifs;

  • common occurrence of unusual oxidation states;

  • control of the stoichiometric or catalytic reactivity by metal-centred or -generated paramagnetic intermediates;

  • stabilization of metal-ligand multiple bonds with 3d n 2 < n < 10 electron counts;

  • spin crossovers during chemical reactions;

  • unusual magnetic properties related to spin-orbit coupling and anisotropy.

All these characteristics set the scene for novel structures and reactivities in functional complexes.

A convenient entry into the synthetically useful 3-coordinate complexes of FeII and CoII with high potential for mild and selective derivatization was the discovery of the aminolysis of [M{N(SiMe3)2}2]2 M = Fe, Co, with bulky imidazol(in)ium halides [(NHC)H]Cl, i.e. [(S)IPrH]Cl, [(S)IMesH]Cl (S for saturated) and the isolation of [M(NHC){N(SiMe3)2}Cl] complexes (Scheme 10) [65, 66]. The method was recently extended to [M{N(SiMe2Ph)2}2]2, M = Fe, Co [67]. However, a coordinating counteranion i.e. halide, should accompany the imidazole(in)um salt.

thumbnail Scheme 10

Aminolysis of [Fe{N(SiMe3)2}2]2 by imidazol(in)ium chlorides to give three-coordinate complexes; [Co{N(SiMe3)2}2]2 reacts analogously.

The structure of 12 is shown in Figure 3.

The presence of three types of ligands that can be replaced using different reaction types (e.g. chloride by salt metathesis, bis(silylamide) by further aminolysis and NHC by substitution with neutral 2 electron donors) adds to the versatility and value of the three-coordinate complexes in accessing new 3- or higher-coordinate species of Fe and Co under mild conditions. Selected aminolysis possibilities for the iron complexes are shown in Scheme 11 [64]; the structures of complexes 13, 14, 17-19 are given in Figures 4-8, respectively; analogous reactivity has been described for Co [65, 67].

thumbnail Scheme 11

Representative aminolysis reactions of 3-coordinate [Fe(NHC){N(SiMe3)2}Cl] complexes. Reagents and conditions: (i) 1 equiv. DiPPNH2 in octane, reflux for 5 min; (ii) 1 equiv. DiPPNH2 in toluene, room temp., 5 h; (iii) 1 or 2 equiv. DiPPNHLi in ether, −78°C to room temp., 5 h or 2 equiv. DiPPNH2 in ether, room temp., 12 h; (iv) 1 equiv. mesNH2 in toluene; (v) N,N’-di-cyclohexyl-acetamidine, toluene 100°C, 1 h; (vi) 1 equiv. Ph3SiOH in toluene, room temp.; (vii) 2 equiv. Ph3SiOTl in toluene, room temp., 24 h; (viii) 2 equiv. Ph3SiOH in toluene 100°C, 1 h.

It can be concluded that the nature of the products obtained is dependent on the bulk of the ligands at Fe. In this respect, the sterics of the bis(trimethylsilylamide) ligand promote mononuclear 3-coordinate structures (9-12). Mesitylanilido- and triphenylsilyloxo-chlorido species 16 and 18, respectively, dimerize via chlorido bridge formation, while the 3-coordinate DiPPanilido-chlorido derivative 13, that was prepared under carefully controlled conditions in octane, easily conproportionates in polar solvents by ligand redistribution to form the thermodynamically more stable mononuclear 3-coordinate bis(anilido) species 14 and 15. Conproportionation has been observed in the reaction of 10 with thallium triphenylsiloxide. It may be that conproportionation is favoured with the combination on the metal of the bulkiest co-ligands; LiN(SiMe3)2 and TlN(SiMe3)2 may be leaving groups in these cases.

The isoelectronic relationship between amido and alkyl ligands guided efforts to prepare 3-coordinate di-alkyls and chlorido-alkyls with the bulky (S)IPr and (S)IMes as co-ligands. For Fe, simple salt metathesis reactions using FeCl2 and MgR2(THF)2 (R = CH2SiMe3, CH2CMe2Ph, CH2Ph, mesityl), in the presence of IPr or SIPr in THF/dioxane was the method of choice. The alkylation was sequential (except for R = Mes); thus, adjustment of the stoichiometry led to either chlorido-alkyls or di-alkyls [68]. Remarkably, the chlorido-alkyls are mononuclear species (Scheme 12). The underligated complexes 20, 23, 26, 31 and 32 have been previously reported by us [6668]; all other complexes shown in Scheme 12 are new. The complex [Fe(NHC)Mes2], where NHC = N,N’-diisopropyl-imidazol-2-ylidene, was reported simultaneously with our work [69]. The thermally stable 12 valence electron dialkyl complexes undergo IPr or (S)IPr substitution by (S)IMes to give the analogous 3-coordinate complexes (one example, 29, is shown in Scheme 12). The mechanism of the substitution (associative versus dissociative) has not yet been clarified.

thumbnail Scheme 12

Synthetic transformations leading to 3-coordinate, underligated chlorido-alkyl and di-alkyl iron, chlorido-alkyl magnesium, alkyl cobalt and dibenzyl chromium complexes.

An insight into the speciation and the nature of the Mg alkylating agent was gained by carrying out the alkylation of FeCl2 with 2 equiv. of Mg(CH2SiMe3)2(THF)2 and 2 equiv. IPr, and fractionally crystallizing the reaction products. After 24, the magnesium complex 30 was isolated as colourless crystals from pentane containing a few drops of dioxane. It is plausible that 31 could be accessible from the reaction of the corresponding Grignard reagent with IPr in the presence of dioxane. The structures of the underligated complexes 21, 22, 24, 25, 27-30 are shown in Figures 9-16, respectively.

On attempting the preparation of the Co analogue of 26, i.e. [Co(IPr)(CH2Ph)2], an unprecedented C-H activation/coupling reaction occurred between the two benzyl ligands at the coordination sphere of the postulated intermediate [Co(IPr)(CH2Ph)2] to give eventually 31 that features the new aryl-substituted, anionic, benzyl 8e donor ligand (arene 6 + benzyl 2) [66]. In contrast, the introduction of CH2SiMe3 occurred smoothly, giving Co dialkyl complexes analogous to 24 [70].

The activation of C-H bonds can proceed by mechanistically distinct pathways with metals from across the periodic table. Low oxidation state ‘underligated’ species may participate in C-H activation via oxidative-addition, in higher oxidation state centres via σ-bond metathesis and in multiple-bonded species via 1,2-concerted additions or hydrogen atom transfers. One distinctive aspect of the ‘underligated’ 3d-complexes is the presence of energetically close, usually open-shell electronic structure(s) with different spin states that may be variable during reactions, potentially promoting diverse C-H activation mechanisms, some of them of bioinorganic relevance.

The synthesis of 3-coordinate Cr organometallics e.g. 32 (Scheme 12) may provide models for organometallic species involved in Cr-catalyzed C-C bond formation reactions [71]. The remarkable structural feature of the 10 valence electron complex 32 is the non-sterically induced severe angular distorsion away form the usual sp3 angle at the benzyl carbon atoms (97.9(2)° and 76.6(2)°), which led to theoretical investigations and the conclusion that “Polarization-Induced Metal−Arene” interactions of a new type were at work (Fig. 17). The energy of this interaction has been calculated to be ca. 50 kJ/mol [71].

The observation of structural effects attributable to weak energetic interactions may be possible due to the sterically open coordination sphere and the weak 3-coordinate ligand field. With respect to ethylene and alkene oligomerization, organometallic species like [Cr(IPr)(CH2Ph)2] raise interest for the following reasons:

  • they can be considered as models of precursors to catalytically active species;

  • they can show potential for activator-free oligomerization (since both Cr-C carbon bonds and coordinative unsaturation are already present features);

  • they may provide useful reagents for the Cr immobilization and creation of novel organometallics of catalytic relevance on supports.

The study of the organometallic chemistry of these simple systems in solution under catalytic conditions by XAS techniques should be particularly interesting. The strategy of using bulky NHC for the stabilization of ‘underligated’ 3d metal centres has been further developed recently by using the strongly σ-donating and π-accepting cAAC ligands [72, 73].

In conclusion, research on ‘underligated’ transition metal complexes opens new dimensions in molecular inorganic chemistry, leading:

  • to fundamental advances in the understanding of bonding and reactivity theory,

  • to applications in homogeneous and heterogeneous catalysis,

  • to the exploitation of electronic and physical properties for the manufacture of useful devices [74].

It is expected to further broaden the scope and applications of ‘base metals’ [75].

Acknowledgments

We are grateful to the coworkers and collaborators cited in the references and we thank Dr N. Stylianides and Dr R. Pattacini for assistance in solving the crystal structures of 12, 18 and 13, 24, respectively, and Dr. L. Karmazin and Miss C. Bailly (Service de Radiocristallographie, UdS) for the determination of the crystal structures of 17, 21, 24, 25, 29 and 30. The USIAS, CNRS, Université de Strasbourg, Région Alsace and Communauté Urbaine de Strasbourg are acknowledged for the award of fellowships and a Gutenberg Excellence Chair (2010–11) to AAD. We gratefully thank the CNRS, the MESR (Paris) and IFP Energies nouvelles for support.

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  • Electronic Supplementary Information (ESI) available: X-ray crystallographic data in CIF format for CCDC reference numbers: 1413910, 12; 1413905, 13; 1413906, 14; 1413907, 17; 1413908, 18; 1413909, 19; 1413835, 21; 1413836, 22; 1413837, 24; 1413838, 25; 1413839, 27; 1413840, 28; 1413841, 29; 1413842, 30. (In the text)

Cite this article as: A.A. Danopoulos and P. Braunstein (2016). Ligand Control of the Metal Coordination Sphere: Structures, Reactivity and Catalysis, Oil Gas Sci. Technol 71, 24.

All Figures

thumbnail Figure 1

The Ni8 complex 8 containing cubane-type structural subunits [61].

In the text
thumbnail Figure 2

The NHC ‘cavity’ targeted to support ‘underligation’.

In the text
thumbnail Figure 3

The structure of the FeII complex 12.

In the text
thumbnail Figure 4

The structure of the FeII complex 13. H-atoms (except the one at the anilido N) are omitted.

In the text
thumbnail Figure 5

The structure of the FeII complex 14. One of the four molecules found in the asymmetric unit is shown. All H atoms are omitted.

In the text
thumbnail Figure 6

The structure of the FeII complex 17. All H atoms are omitted.

In the text
thumbnail Figure 7

The dinuclear FeII complex 18. All H atoms are omitted.

In the text
thumbnail Figure 8

The structure of the FeII complex 19. One of the two molecules found in the asymmetric unit is shown.

In the text
thumbnail Figure 9

The structure of the FeII complex 21. All H atoms are omitted.

In the text
thumbnail Figure 10

The structure of the FeII complex 22. All H atoms are omitted.

In the text
thumbnail Figure 11

The structure of the FeII complex 24. All H atoms are omitted.

In the text
thumbnail Figure 12

The structure of the bis(benzyl) FeII complex 25. All H atoms are omitted.

In the text
thumbnail Figure 13

The structure of the FeII complex 27. All H atoms are omitted.

In the text
thumbnail Figure 14

The structure of the FeII complex 28. All H atoms are omitted.

In the text
thumbnail Figure 15

The structure of the bis(benzyl) FeII complex 29. All H atoms are omitted.

In the text
thumbnail Figure 16

The structure of the dinuclear Mg complex 30. All H atoms are omitted.

In the text
thumbnail Figure 17

Polarization-induced metal-arene interactions operating in the 3-coordinate complex [Cr(IPr)(CH2Ph)2] (32). Reprinted from [71]. Copyright American Chemical Society.

In the text
thumbnail Scheme 1

Stereochemical differentiation in a metal coordination sphere resulting from chelation of a functional phosphine.

In the text
thumbnail Scheme 2

Reversible binding of CO resulting from the opening and closing of the metal-oxygen dative bond involving the P,O chelate [24].

In the text
thumbnail Scheme 3

Typical synthesis of a SHOP-type ethylene oligomerization pre-catalyst.

In the text
thumbnail Scheme 4

Intramolecular hydrogen bonding between the ortho-substituent in the aryl group and the oxygen atom of the P,O-chelate.

In the text
thumbnail Scheme 5

Non-classical intramolecular hydrogen bonding and C-H-π interactions involving the P,O-chelate.

In the text
thumbnail Scheme 6

Various oxazoline- and pyridine-based P,N-type Ni(II) complexes, with chelate ring sizes ranging from 5 to 7, applied to catalytic ethylene oligomerization [2].

In the text
thumbnail Scheme 7

Effect of pressure on complex nuclearity: conversion of a microcrystalline sample of the Ni(II) complex 4 into 5 under pressure in the solid state [54].

In the text
thumbnail Scheme 8

Influence of the crystallization temperature on the nuclearity of the complex [55, 56].

In the text
thumbnail Scheme 9

Examples of pyridinyl-alcohols with different spacers between the heterocycle and the alcohol function investigated as ligands in Ni(II) complexes.

In the text
thumbnail Scheme 10

Aminolysis of [Fe{N(SiMe3)2}2]2 by imidazol(in)ium chlorides to give three-coordinate complexes; [Co{N(SiMe3)2}2]2 reacts analogously.

In the text
thumbnail Scheme 11

Representative aminolysis reactions of 3-coordinate [Fe(NHC){N(SiMe3)2}Cl] complexes. Reagents and conditions: (i) 1 equiv. DiPPNH2 in octane, reflux for 5 min; (ii) 1 equiv. DiPPNH2 in toluene, room temp., 5 h; (iii) 1 or 2 equiv. DiPPNHLi in ether, −78°C to room temp., 5 h or 2 equiv. DiPPNH2 in ether, room temp., 12 h; (iv) 1 equiv. mesNH2 in toluene; (v) N,N’-di-cyclohexyl-acetamidine, toluene 100°C, 1 h; (vi) 1 equiv. Ph3SiOH in toluene, room temp.; (vii) 2 equiv. Ph3SiOTl in toluene, room temp., 24 h; (viii) 2 equiv. Ph3SiOH in toluene 100°C, 1 h.

In the text
thumbnail Scheme 12

Synthetic transformations leading to 3-coordinate, underligated chlorido-alkyl and di-alkyl iron, chlorido-alkyl magnesium, alkyl cobalt and dibenzyl chromium complexes.

In the text

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