The chemistry of phosphanyl-ferrocenecarboxylic ligands

MICROREVIEW

The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

[a]

ˇ

Petr Stepnicka*

Keywords: Ferrocene / Phosphanyl-carboxylic ligands / Coordination compounds / Catalysis / Phosphanes / Structure

elucidation

The chemistry of ferrocene attracts considerable research

interest even more than 50 years after its discovery, being

stimulated mainly by applications of ferrocene ligands in ca-

systems. This review provides a systematic survey of the

chemistry of phosphanyl-ferrocenecarboxylic ligands as a

specific class of hybrid ferrocene ligands that emerged about

talysis. To the present day, the design of new ferrocene com- ten years ago, illustrating their synthesis, coordination be-

pounds ensued−sometimes by serendipity−the preparation of

unprecedented coordination compounds and organometallic

materials and, in the case of ferrocene compounds containing

appropriate donor sets, led to numerous successful catalytic

haviour, and catalytic applications. Attention is also paid to

the structural chemistry of these functional ferrocene ligands.

Germany, 2005)

the chemical properties of both heteroatom donor groups,

phosphanyl-carboxylic donors possess great structural vari-

ability: the properties of the phosphane part can be varied

considerably by changing the substituents at the phospho-

rus atom whereas the carboxy group can be converted into

the corresponding carboxylate (pH-dependent coordination

behaviour possible) or to numerous functional derivatives

(typically esters and amides). This provides a relatively easy

access to the families of related compounds as well as to

entirely new types of ligands. Combination of the hard (car-

boxy) and soft (phosphane) donor groups^[2]classify car-

boxy-phosphanes as typical examples of the so-called hy-

brid ligands.

The presence of the very different donor sites and chemi-

cal variability of the carboxy group enables phosphanyl-car-

boxylic ligands to coordinate to almost any metal ion and

in diverse coordination modes. Soft metals commonly used

in catalysis bind phosphanyl-carboxylic ligands predomi-

nantly as P-donors; coordination of the O-donor group re-

sults in relatively weaker dative bonds and may result in the

Introduction

It has long been recognised that the introduction of a

functional group into a ligand molecule changes its donor

properties. Tuning ligand properties by means of the at-

tached substituents proved exceptionally successful in the

case of tertiary phosphanes. For instance, the application of

carefully “tailored”, functionalised phosphane ligands al-

lows for the synthesis of coordination compounds with spe-

cific properties and helps in improving the efficacy and se-

lectivity of transition metal mediated reactions.

Among the vast number of modified phosphane ligands

reported to date, phosphanyl-carboxylic acids constitute a

distinct class of ligands with unique properties.^[1]Owing to

[a] Department of Inorganic Chemistry, Faculty of Science,

Charles University,

Hlavova 2030, 12840 Prague 2, Czech Republic

E-mail: stepnic@natur.cuni.cz

ˇ

Petr Steˇpnicˇka was born in Liberec, Czech Republic in 1972. He studied inorganic chemistry at

the Faculty of Science, Charles University in Prague (1990–1995), where he also received his

Ph.D. for the work on phosphanyl-ferrocenecarboxylic acids with Professor Jaroslav Podlaha in

1998. After a postdoctoral fellowship at the Catalysis Research Center, Sapporo University in

Japan with Professor Tamotsu Takahashi (1999), he returned to his alma mater and completed

his habilitation in 2005, working in the field of phosphanyl-ferrocenecarboxylic ligands. His main

research interests include the synthesis and studies on structural, coordination and catalytic prop-

erties of ferrocene compounds. He has authored more than 70 papers in reviewed journals.

MICROREVIEWS: This feature introduces the readers to the authors’ research through a concise overview of the

selected topic. Reference to important work from others in the field is included.

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DOI: 10.1002/ejic.200500485

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formation of hemilabile chelates.^[1,3]This has a particular of mixed-donor ferrocene ligands is still strongly dominated

impact on the catalytic performance of these ligands since by donors of N/P and P/PЈ types that are accessible in a

the metal–oxygen dative bonds can be cleaved by the sub- stereodefined form through diastereoselective metallation/

strate and formed again after the product of the catalytic functionalization of C-chiral ferrocene compounds {e.g.,

process is expelled from the coordination sphere of the me- [1-(dimethylamino)ethyl]ferrocene, ferrocenyl-dihydrooxa-

tal ion. Intermediate chelation makes the catalytic species zoles, and -dioxanes} and subsequent functional group

more robust towards inhibitors and leads to stabilization of transformations.^[9,14]Even though several potential ferro-

reaction intermediates, thus increasing the selectivity and cene-containing O,P-donors have been reported in the lit-

stability of the catalytic system. Furthermore, the presence erature, little attention has been paid to their coordination

of carboxy groups increases the solubility of carboxy-phos- chemistry and catalytic applications.^[14e]

phanes and their complexes in polar solvents, allowing us

to perform some catalysed reactions even in water.^[4]Repre-

sentative examples of successful applications of phos-

phanyl-carboxylic ligands in catalysis include nickel-cata-

lysed ethene oligomerisation (SHOP process),^[5]palladium-

catalysed co-dimerisation of ethene with styrene and alter-

nating ethene/CO co-polymerization,^[6]and rhodium-cata-

lysed arene hydrogenation.^[7]In addition, several chiral

phosphanyl-carboxylic acids have been tested in palladium-

catalysed enantioselective allylic alkylation.^[8]

Considering the effort devoted separately to the synthe-

sis, coordination, and catalytic chemistry of phosphanyl-

carboxylic ligands with purely organic backbones as well as

to ferrocene ligands, it is somewhat surprising that ferro-

cene-containing phosphanyl-carboxylic acids had not been

reported until 1996. Since then, a number of phosphanyl-

ferrocenecarboxylic derivatives have emerged in the

literature−however, as this review focuses exclusively on the

synthesis, coordination chemistry, and catalytic and syn-

thetic utility of phosphanyl-carboxylic acids derived from

ferrocene, only compounds which have been studied as li-

gands and catalyst components are included. These com-

prise mainly−in chronological order−1Ј-(diphenylphos-

phanyl)ferrocenecarboxylic acid (Hdpf), its planarly chiral

More recently, progress in organometallic chemistry initi-

ated research on functionalised organometallic derivatives,

particularly ferrocene compounds. During the studies in-

volving ferrocene derivatives^[9]it had been soon recognised

that ferrocene donors differ in many respects from their “or-

ganic” counterparts. First, any ferrocene ligand is a coordi-

nation compound by itself and its coordination gives rise

to multinuclear complexes. Second, the ferrocene molecule

represents a conjugated, redox-active system.^[9a,10]Monitor-

ing the redox properties of the ferrocene unit within elec-

tronically communicating molecules may provide an insight

into the nature of chemical interactions between the molecu-

lar parts, particularly changes accompanying the coordina-

tion of ferrocene donors. Besides, switching the oxidation

state of the ferrocene unit may influence reactivity at the

coordinated metal centre by changing donor properties of

ferrocene ligands. Oxidation of the strongly electron-donat-

ing ferrocene moiety creates cationic ferricenium (and vice

versa) whose electron-withdrawing character will be re-

flected by the considerably reduced donor ability of the li-

gand. Third, the ferrocene skeleton has a well-defined cylin-

drical shape. It is quite rigid against tilting of the cyclopen-

tadienyl rings, the exception being some 1,1Ј-bridged deriva-

tives (ferrocenophanes).^[11]On the other hand, the energy

barrier for the rotation of the rings along the molecular axis

is usually very low.^[12]This conformational flexibility is par-

ticularly important for 1,1Ј-disubstituted compounds, en-

abling a favourable arrangement of the donor groups.^[13]Fi-

nally, ferrocene exhibits an exceptionally high chemical sta-

positional

isomer,

(S_p)-2-(diphenylphosphanyl)ferro-

cenecarboxylic acid [(S_p)-HL¹], and homologues of the lat-

ter: rac-[2-(diphenylphosphanyl)ferrocenyl]acetic acid (rac-

HL²) and rac-2-[(diphenylphosphanyl)methyl]ferrocenecar-

boxylic acid (rac-HL³; Scheme 1).

Scheme 1.

Compounds whose coordination and catalytic properties

bility among organometallic compounds allowing many have not been studied systematically (albeit the compounds

synthetic transformations to be performed. The basic syn- were often deemed “potentially useful ligands”) and those

thetic methodology in ferrocene chemistry is well documen- serving just as reaction intermediates do not meet the scope

ted including many efficient routes to chiral compounds.^[9]

of this review. Examples of such compounds (1–4) are

The number of ferrocenylphosphanes bearing further shown in Scheme 2.^[15]The same applies also for carboxy

functional groups has grown enormously over the past se- derivatives of 1,1Ј-diphosphaferrocene, e.g. to 3,3Ј,4,4Ј-tet-

veral decades and investigations into this area has led to ramethyl-1,1Ј-diphosphaferrocene-2-carboxylic acid (5),^[16]

the discovery of many useful ligands, some of them having representing an alternative but still rather uncommon ap-

practical importance. However, despite this effort, the field proach to phosphanyl-ferrocenecarboxylic donors.

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Scheme 2.

Scheme 3. Synthesis of Hpdf and related compounds.

stepwise metalation/functionalization of 1,1Ј-dibromoferro-

cene has been reported by Butler et al.^[22]

1Ј-(Diphenylphosphanyl)ferrocenecarboxylic Acid

(Hdpf)

In addition to the standard characterization, Hdpf and

some related compounds were studied in detail by mass

spectrometry^[23]and by electrochemical methods.^[17]In cy-

clic voltammograms in acetonitrile, Hdpf shows the ex-

pected one-electron oxidation to the corresponding fer-

ricenium at E°Ј = +0.32 V vs. ferrocene/ferricenium refer-

ence, the shift to higher potential vs. the reference couple

being in agreement with the electron-withdrawing nature of

both substituents at the ferrocene unit (process A in

Scheme 4). However, this electrochemical oxidation is fol-

lowed by chemical complications, resulting in a pseudore-

versible behaviour upon back-scan. The product of the fol-

lowing chemical steps was identified as the phosphane ox-

ide HdpfO, an apparent product of the reactions between

the electrogenerated ferricenium species and traces of oxy-

gen and/or water. The presence of HdpfO is clearly reflected

Preparation and Further Synthetic Utilization

In the original synthesis of Hdpf (Scheme 3)^[17]we made

use of the ring opening in the strained ferrocenophane 6^[18]

with phenyllithium to give 1Ј-(diphenylphosphanyl)lithi-

oferrocene (7).^[18b,19]In situ carboxylation of 7 with excess

solid carbon dioxide and a subsequent acidification of the

water-soluble part of the reaction mixture followed by

recrystallization of the crude product afforded analytically

pure Hdpf as an air-stable, rusty red-brown crystalline solid

in typical yields around 80%. A similar methodology has

been utilised for the synthesis^[20]of 1Ј-(diphenylphosphanyl)

ferrocenecarboxaldehyde (8).^[21,22]

The acid was further converted into the corresponding

phosphane oxide (HdpfO), methyl ester (Medpf), and re-

duced to alcohol 9 (Scheme 3).^[20]A synthesis of Hdpf by

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by an additional standard reversible ferrocene/ferricenium cribed to several intermediates with different conformation

wave at more anodic potentials (C in Scheme 4). The overall at the ferrocene unit participating in the catalysed reac-

behaviour of Hdpf in the anodic region can be formulated tion.^[14c,27,28]This assumption is supported by spectral evi-

as an ECE process, though with some peculiar features: as dence and also by the fact, that the conformational equilib-

the formation of HdpfO definitely involves an attack at the rium can be markedly influenced by introducing a ste-

phosphorus atom, one may consider an electron transfer reogenic phosphorus atom.^[29]

from the phosphorus to the electron-poor iron atom yield-

A different example of the synthetic usefulness of Hdpf

ing a ferrocenyl-phosphonium cation-radical or its reso- represents the preparation of group-6 Fischer-type carbenes

nance form (see B in Scheme 4), which are more reactive bearing the P-chelating 1Ј-(diphenylphosphanyl)ferrocenyl

at the phosphorus atom than Hdpf.^[24]Similar ECE redox group.^[30]In this case, Hdpf was first treated in the presence

behaviour has been observed for 1,1Ј-bis(diphenylphos- of peptide-coupling agents {1-hydroxybenztriazole/N-[3-(di-

phanyl)ferrocene.^[25]In contrast, nondissociating complexes methylamino)propyl]-NЈ-ethylcarbodiimide} with second-

with P-coordinated Hdpf exhibit normal reversible one- ary amines to give tertiary amides 12. The reaction of

electron oxidations at the ferrocene unit (see below).

amides 12 with Na₂[Cr(CO)₅] and Me₃SiCl^[31]afforded mix-

tures of the respective P-chelated carbenes 13 and phos-

phane complexes 14 (Scheme 6), which were easily sepa-

rated by chromatography and fully characterised.

Scheme 4. Schematic depiction of the ECE electrochemical behav-

iour observed for Hdpf.

Scheme 6. Preparation of (carbene)chromium(0) compounds [NR₂

= NEt₂(a) and morpholin-4-yl (b)].

In the following work, Hdpf has been used^[26]as a pre-

cursor for an alternative synthesis of chiral phosphanylox-

azoline ligands (Scheme 5).^[27]Due to an instability of the

A similar reaction between 12a and Na₂[W(CO)₅] under

phosphanyl group under the reaction conditions, Hdpf was identical conditions (Scheme 7) is more complex, yielding a

first converted to HdpfO. Then, the carboxy-phosphane ox- mixture of nonchelated carbene 15, trinuclear “W₂Fe” com-

ide was treated with oxalyl chloride yielding the respective plex 16 and the expected side-product 17. Removal of 17

acid chloride, which was immediately treated with chiral β- by chromatography and heating of the 15/16 mixture in tol-

amino alcohols to give amides 10. Subsequently, the stan- uene (80 °C, 4 h) afforded another mixture, whose compo-

dard oxazoline ring closure reaction with 10 and deprotec- nents were separated by column chromatography and char-

tion of the phosphorus group gave [1Ј-(diphenylphos- acterised as unreacted 16, the P-chelated carbene 18, and

phanyl)ferrocenyl]oxazolines 11 (Scheme 5).

phosphanyl-carbaldehyde complex 19 resulting from de-

composition of the carbenes.

The reaction course as observed for tungsten complexes

provides strong evidence for a stepwise formation of the

P-chelated carbenes. Somewhat surprisingly, an analogous

reaction with [Fe(CO)₄]^2–salts affords−in sharp contrast

with the behaviour of organic amides^[32]−exclusively the

amidophosphane complex [Fe(CO)₄(12a-κP)] (20).

A comparison of the crystal structures for 13a, 13b, 18,

and the nonchelated complex 16 (see Figure 1), which is

void of possible structural constraints imposed by the chela-

tion, clearly indicates that the M(CO)₄units (M = Cr and

W) perfectly fit into the pocket of the chelating ferrocene

ligand and that the coordination causes no deformation

within either part. An electrochemical study performed

with 12a–b, 13a–b, and 14a–b showed that the precursor

Scheme 5. Synthesis of ferrocenyl-oxazolines [R = (S)-iPr, (R)-iPr,

(S)-tBu, (R)-Ph].

Oxazolines 11 were tested in palladium-catalysed allylic amides 12 and the phosphane complexes 14a–b behave as

substitution, giving nearly quantitative yields of the substi- standard one (Fe^II/III) and two localised redox systems (Fe^II/

III

tution products, though with a lower enantioselectivity than

and Cr^0/I), respectively, whereas the carbene complexes

their analogues bearing both functional substituents on one 13a–b represent electronically coupled redox systems, where

cyclopentadienyl ring. The lowered selectivity can be as- any redox change involves the entire molecule.

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Scheme 7. Carbene-forming reactions with 12a and pentacarbonyltungstate.

Figure 1. Views of the molecular structures of complexes 18 (left) and 16 (right).

Coordination Chemistry and Catalytic Applications

vates 21a·2CH₃CO₂H and 22·2CH₃CO₂H obtained by

recrystallisation from acetic acid are isostructural, forming

molecular crystals built up of molecular triplexes formed by

one complex molecule and two molecules of solvating acetic

acid, bonded by double hydrogen bonds to the uncoordi-

nated carboxy groups of the ligands (O···O 2.62 Å; Fig-

ure 2). The same type of intermolecular interaction is re-

sponsible for intermolecular aggregation in 22 and 23·(2+x)

CH₂Cl₂but with a different outcome. The absence of polar

solvate molecules in 22 leads to the formation of infinite

hydrogen-bonded zig-zag chains (O···O 2.60–2.63 Å)

whereas the molecules of 23 assemble by means of double

hydrogen bonds into macrocyclic dimers (O···O 2.64 Å; Fig-

ure 2), whose conformation is further stabilised by graphite-

like π···π stacking of the phenyl groups in cis-disposed li-

gands at a ring-centroid distance of 3.64 Å. The formation

of closed supramolecular assembly [23]₂in the solvate

23·(2+x)CH₂Cl₂is very likely related to much lower po-

larity of the crystallization solvent.^[35]

As Hdpf was the first reported carboxy-ferrocenyl-phos-

phane, we turned our attention to studying its coordination

behaviour towards selected metal ions in order to establish

its coordination preferences, which could be expected to dif-

fer from the chemistry of either of the related donor-uni-

form ligands, (diphenylphosphanyl)ferrocene^[33]and ferro-

cenecarboxylic acid.^[34]

With soft metals, Hdpf coordinates as a simple tertiary

phosphane. For instance, K₂[PdCl₄] and [Pd(cod)Cl₂] (cod

= η²:η²-cycloocta-1,5-diene) reacted smoothly with 2 equiv.

of Hdpf to give the bis(phosphane) complex trans-

[PdCl₂(Hdpf-κP)₂] (21a). A similar reaction with [Pd(cod)-

Br₂] gave trans-[PdBr₂(Hdpf-κP)₂] (21b). For the case of

kinetically inert platinum(), the isomers trans-[PtCl₂(Hdpf-

κP)₂] (22) and cis-[PtCl₂(Hdpf-κP)₂] (23) were isolated de-

pending on the precursor used.^[35]According to the spectro-

scopic data, all compounds possess the expected square-

planar arrangement, which was unambiguously corrobo-

rated by the structure determination for 22 and the adducts

21a·2CH₃CO₂H, 22·2CH₃CO₂H, and 23·(2+x)CH₂Cl₂. Sol-

In contrast to the very stable palladium() and plati-

num() complexes, their nickel() analogues are prone to

dissociative decomposition in polar solvents. In the series

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of the electrochemical behaviour observed for a solution of

the complexes in acetonitrile/N,N-dimethylformamide, ex-

plainable as a superposition of the redox response of the

dissociated free ligand (see above) and (at least partly) sol-

volysed mercury complexes.^[37]

The crystal structures of the bromide complexes, 26b and

the solvate 27b·4CH₃CO₂H (Figure 3), revealed a tetrahe-

dral coordination environment around the mercury ion,

though with significant deviations from the regular geome-

try resulting from steric demands of the bulky carboxy-phos-

phane ligand (note: the overall structures are highly sym-

metric due to the imposed crystallographic symmetry). In

both cases, the ligand carboxy groups do not participate in

the coordination but instead take part in hydrogen bonding

to either the carboxy group in a neighbouring ligand moiety

resulting in infinite twisted chains (26b) or to the molecules

of solvating acetic acid (27b·4CH₃CO₂H). The crystal pack-

ing of 27b·4CH₃CO₂H, unlike the polymeric 26b, is essen-

tially molecular since two solvate molecules saturate the li-

gand carboxy groups while the remaining two form hydro-

gen-bonded dimers (CH₃CO₂H)₂located in structural voids

(Figure 3).^[37]

Yet another example of carboxy groups of the P-coordi-

nated Hdpf associating with solvating acetic acid molecules

is the octanuclear heterocubane complex [(µ₃-I)₄{Cu(Hdpf-

κP)}₄]·2CH₃CO₂H (28·2CH₃CO₂H), which is obtained

upon refluxing CuI/Hdpf mixtures in glacial acetic acid.

Notably, the reaction proceeds with the formation of the

well-defined, stable solvate in a relatively broad range of the

metal/ligand stoichiometry (Scheme 8).^[38]

Figure 2. Views of the repeating units in the crystals of

22·2CH₃CO₂H (top) and 23·(2+x)CH₂Cl₂(bottom). The O···O hy-

drogen bonds are shown as thin dashed lines and the π···π stacking

The structure of 28·2CH₃CO₂H determined by X-ray

interactions of the phenyl rings are indicated by double arrows. crystallography revealed that the {Cu₄I₄} core is quite regu-

Hydrogen atoms and solvating dichloromethane (for the latter

compound) are omitted for clarity.

lar as far as bond lengths and angles are concerned but

possesses no higher symmetry. In the crystal, the molecules

of solvating acetic acid associate with the uncoordinated

[NiX₂(Hdpf-κP)₂] [24, X = Cl (a), Br (b), I (c), and SCN- carboxy groups of ligands bonded at one face of the hetero-

κN (d)], the halide complexes 24a–c are paramagnetic and cubane core (O···O 2.62–2.65 Å). Likewise, the remaining

tetrahedral whereas the thiocyanate 24d is trans-square- ligand carboxy groups form the usual double hydrogen

planar and diamagnetic. Another nickel() complex, bridges but with the ligand carboxy groups from the neigh-

[Ni(dpf)₂] (25), was obtained by salt metathesis of nickel() bouring molecules (O···O 2.59 and 2.63 Å), thus linking the

sulfate with Nadpf. Compound 25 is an amorphous solid, individual molecules in necklace-like zig-zag chains parallel

practically insoluble in organic solvents and water. Hence, to the crystallographic c axis (Scheme 8).

its formulation as a coordination polymer involving dif-

In further work we turned to mixed-donor carbonyl com-

ferent nickel() sites and multidentate dpf^–anions was plexes involving group-6 metals [M(CO)₅(Hdpf-κP)], where

based mainly on magnetic measurements and spectroscopic M = Cr (29), Mo (30), and W (31).^[39]Complexes 29 and

data (IR and photoelectron spectra).^[36]

31 have been synthesized in good yields by replacement of

A similar situation has been observed for (Hdpf)mer- the weakly coordinated solvent in photogenerated interme-

cury() complexes. The reaction of Hdpf with mercury() diates [M(CO)₅(THF)] (THF = tetrahydrofuran), while 30

halides in acetic acid (bromide and chloride) or dichloro- was obtained from the direct, thermally induced substitu-

methane (iodide) gave two types of products depending on tion of one carbonyl ligand with Hdpf in [Mo(CO)₆]. The

the stoichiometry of the starting materials: the trinuclear compounds are sufficiently stable to be isolated in pure

compounds [HgX₂(Hdpf-κP)₂] (26a–c) and the dimeric, tet- form albeit they slowly decompose in air and under day-

ranuclear complexes [{Hg(µ-X)X(Hdpf-κP)₂}₂] [27a–c; X = light; their decomposition seems to be facilitated by the

Cl (a), Br (b), and I (c)]. The complexes are insoluble in presence of the acidic carboxy function.

common solvents and decompose in strongly donating

The formulation of compounds 29–31 is based mainly on

ones. The solvolytic equilibria are reflected, for example, spectroscopic data and further corroborated by the struc-

by extensive broadening of the NMR resonances (spectra ture determination for 29. An electrochemical study of the

recorded for [D₆]DMSO solutions) and by the complexity whole series by cyclic voltammetry showed that, upon rais-

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Figure 3. Views of the molecular structures of 26b (top) and 27b·4CH₃CO₂H (bottom). The O···O hydrogen bonds are shown as dashed

lines and the propagation of the chains in the structure of 26b is indicated with arrows. The dimers (CH₃CO₂H)₂present in the structure

of the solvate are not shown for clarity.

romethane solution, 0.1 Vs^–1scan rate), which contrasts

with the redox behaviour of free Hdpf and solvolytically

unstable mercury() complexes (see above).

Furthermore, Hdpf together with other phosphanes have

been utilised as ligands in the synthesis of an extensive

series of (alkoxycarbene)(η⁶-hexamethylbenzene)rutheni-

um() compounds (Scheme 9).^[40]The starting dichloro-

(phosphane) complexes 32a–d were prepared by a bridge-

splitting reaction from [{Ru(µ-Cl)Cl(η⁶-C₆Me₆)}₂] and then

converted into methoxycarbenes 33. It is assumed that car-

bene formation starts with a removal of one chloride ligand

with NaPF₆. The formed cationic, coordinatively unsatu-

rated species coordinates one molecule of terminal alkyne

giving an intermediate [L_nRu(η²-RCϵCH)]⁺, which isomer-

ises to a cationic vinylidene complex [L_nRu=C=CHR]⁺.

Then, the carbene formation is completed by nucleophilic

addition of methanol onto the electron-deficient vinylidene

C-α atom. For carbenes with RЈ = H, (trimethylsilyl)ethyne

is used as the alkyne component because the C–Si bond is

Scheme 8. Preparation and structure of the solvated heterocubane

28·2CH₃CO₂H. Dashed arrows indicate where propagation of the

infinite, hydrogen-bonded chains occurs.

readily solvolysed under the reaction conditions.^[41]

All compounds were characterised by standard spectral

methods and the solid-state structure of the solvate

33da·CH₂Cl₂was determined by single-crystal X-ray dif-

ing the external potential, the complexes first undergo a fraction. Even in this case double hydrogen bonds between

ligand centred, reversible one-electron oxidation (ferrocene/ carboxy groups of the P-coordinated ligand dominate the

ferricenium) followed by a group-6 metal centred redox solid-state assembly, connecting into centrosymmetric di-

process at more positive potentials. The position of the first mers the enantiomeric cations (R_Ru)- and (S_Ru)-[(η⁶-

wave is independent of the group-6 metal and shifted by C₆Me₆)RuCl(Hdpf-κP){=C(OMe)CH₂Fc}]⁺(Fc = ferro-

ca. +180 mV with respect to uncoordinated Hdpf due to cenyl) which are equally abundant in the racemic crystal

electron-density transfer from the ligand to the ligated me- (O···O 2.58 Å). In addition, the carboxy OH groups interact

tal ion. The reversibility of the first redox process indicates with the solvating dichloromethane, thus fixing the solvate

that the complexes are stable in both the usual and the oxi- in the structure by means of weak O–H···Cl hydrogen

dised forms under the conditions of measurement (dichlo- bonds, the O···Cl distance being 3.44 Å.

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Scheme 9. Synthetic route to cationic (methoxycarbene)ruthenium compounds 33.

The entire series of phosphane complexes 32 and carb-

enes 33 was studied by cyclic voltammetry in the anodic

region. Analysis of the obtained data allowed for a com-

parison in both groups as well as across them; the observed

trends undoubtedly deserve comment (Figure 4). As for the

phosphane complexes, the compounds involving simple

phosphanes, 32a and 32b, exhibit single redox waves attrib-

utable to the reversible oxidation at the central atom (Ru^II

Ǟ Ru^III). The potential of this process varies with the do-

nor properties of the phosphane ligand: PPh₃as a poorer

σ-donor and better π-acceptor makes this oxidation more

difficult [E°Ј = 0.39 (32a), 0.48 V (32b); the potentials are

given relative to that of the internal ferrocene/ferricenium

reference].

The presence of a ferrocenyl-phosphane in 32c and 32d

is clearly reflected in the voltammograms by an additional

one-electron ferrocene/ferricenium wave at lower potentials

[E°Ј = 0.03 (32c), 0.28 V (32d)], an anodic shift for the latter

compound being in agreement with the presence of an elec-

tron-withdrawing carboxy group (Figure 4). However, these

Figure 4. Chart diagram of the anodic redox potentials observed

for 32 and 33 in dichloromethane solutions. The redox potentials

were determined as an average of the anodic and cathodic peak

potentials and given relative to the ferrocene/ferricenium reference.

redox potentials are slightly lower than those for free li- The values are distinguished as follows: the symbol Ru indicates a

ruthenium-centred oxidation, while C and P denote the ferrocene/

gands, which is in a sharp contrast with the anodic shift

ferricenium potentials for carbene and phosphanyl-ferrocenyl

typically accompanying coordination of these donors (cf.

groups, respectively. For compound labels see Scheme 9.

the data for 29–31^[39]and for the related FcPPh₂com-

plexes^[42]). Nonetheless, this rather unexpected phenome-

non can be explained by an efficient compensation of the ligand oxidation converting the strongly electron-donating

ligand-to-metal σ-donation by π-back Ru Ǟ P bonding ferrocene into an electron-poor, cationic ferricenium. This

from the electron-rich ruthenium atom. At higher poten- change is naturally reflected by the donor properties of the

tials, the ferrocene/ferricenium wave of the ligands is fol- whole phosphanyl-ferrocene ligand and is relayed onto the

lowed by the Ru^IIǞ Ru^IIIoxidation−however, at much ruthenium centre (though perhaps in synergism with other

higher potentials than for 32a and 32b [E°Ј = 0.66 (32c), factors).

0.68 V (32d)]. The origin of this anodic shift, which does

The redox behaviour of the carbenes is similar. However,

not correspond with the strong electron-donating character the redox processes occur at higher potentials due to the

of the ferrocene unit, can be accounted for by the preceding cationic nature of these compounds (by ca. 0.5 V for the

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

ruthenium-centred oxidations, Figure 4). Another differ- boxylate groups (ν_C=O= 1602–1606 cm^–1; cf. 1666 cm^–1for

ence can be seen in the position of the ferrocene/ferricenium Hdpf^[17]) and the terminal carbonyl ligands (ν_CϵO= 1961–

waves due to the phosphane ligands, which are shifted an- 1965 cm^–1). For all compounds, the NMR spectra indicate

odically by 0.24 V as compared to free ligands because the trans-P–P arrangement around the rhodium atom, as was

electron density lowering at the ferrocene unit following the proven for 36a by X-ray diffraction analysis (Figure 5).

coordination cannot be effectively compensated now by

back-donation from the electron-poor ruthenium centres.

In summary, the data for compounds bearing the ferro-

cenyl-phosphanes (32c, 32d, and the respective carbenes)

clearly indicate electronic coupling between the ruthenium

atom and the ferrocene ligand. On the contrary, the ferro-

cene unit located in the carbene part (33aa, 33ba, 33ca and

33da) remains virtually unaffected by changes occurring in

the rest of the molecule, which corresponds with the pres-

ence of a nonconjugated linker between the Ru atom and

the (carbene)ferrocenyl group. A similar observation has

been made also for (arene)ruthenium() complexes bearing

Ru-coordinated ferrocenyl-phosphanes and ferrocenoyl

groups at the terminus of a nonconjugated chain attached

to the arene ligand.^[43]

Interaction of rhodium() compounds with Hdpf leads

to two types of complexes differing in coordination of the

Figure 5. View of the molecular structure of 36a. Hydrogen atoms

carboxy-phosphane ligand. The reaction of the dimeric

are omitted.

complexes [{Rh(µ-X)(CO)₂}₂] with 4 equiv. of Hdpf gave

the expected square-planar complexes with P-monodentate

Replacing

35

with

the

dicarbonyl

complex

carboxy-phosphane, trans-[RhX(CO)(Hdpf-κP)₂] [34, X = [Rh(acac)(CO)₂] in the reaction with Hdpf results in an im-

Cl (a) and Br (b)]. On the other hand, the reactions of Hdpf mediate and exclusive formation of complex 37

with rhodium() acetylacetonate complexes [Rh(acac)- (Scheme 10). The reaction proceeds with elimination of CO

(CO)(L)] [35; L = PCy₃(a), PPh₃(b), and FcPPh₂(c), acac and acetylacetone, requiring 2 equiv. of Hdpf for 1 equiv. of

= pentane-2,4,-dionate(1–)] afforded the corresponding the rhodium complex. For other molar ratios, the reaction

square-planar complexes [Rh(CO)(L)(dpf-κ²O,P)] (36) in stops when one of the starting materials is consumed: for

which the phosphanyl-carboxylate anion dpf^–coordinates instance, mixing of Hdpf and [Rh(acac)(CO)₂] in a 1:1 mo-

as an O,P-chelating, bidentate ligand (Scheme 10). The for- lar ratio affords an equimolar mixture of 37 and unreacted

mation of 36 from 35 and Hdpf represents an acid-base [Rh(acac)(CO)₂]. The IR spectra of 37 display bands as-

reaction with Hdpf acting as the proton donor to the leav- signable to the protonated (1703 cm^–1) and deprotonated

ing pentane-2,4-dione.^[44]

(1551 cm^–1) carboxy group and the band of the terminal

carbonyl ligand (1962 cm^–1). In contrast, ¹H and ¹³C NMR

show only one set of resonances due to “Hdpf” while the

³¹P NMR spectrum features only one rhodium-coupled sig-

nal at δ_P= 20.2 (¹J_RhP= 132 Hz) ppm, which is roughly

half-way between the values observed for Hdpf in 34a–b (δ_P

= 22.2 ppm) and the dpf^–anion in 35a–c (δ_P= 18.4–

18.7 ppm). Such a peculiar discrepancy was attributed to

hemilabile coordination of the Hdpf/dpf^–ligand pair where

the two forms of ligand rapidly exchange the carboxy pro-

ton (Scheme 11). The exchange rate is sufficient to cause an

averaging on the NMR time scale but definitely not in IR

spectra, where both forms of the ligand are clearly observ-

able.

Testing complexes 36a, 36b and 37 (pure or with various

co-catalysts) and the related metal precursor/ligand mix-

tures [Rh(acac)(CO)₂]/xHdpf, 35a/Hdpf, 35b/Hdpf, and

Scheme 10. Preparation of rhodium() complexes with O,P-chelat-

ing phosphanyl-carboxylate dpf^–[PR₃= PCy₃(a), PPh₃(b), and

FcPPh₂(c); Cy = cyclohexyl].

Compounds 36a–c are air-stable crystalline solids show-

ing in their IR spectra the characteristic bands due to car- Scheme 11.

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MICROREVIEW

[RhH(CO)(PPh₃)₃]/Hdpf in hydroformylation of 1-hexene tated as exemplified by the dihedral angle subtended by the

has shown that most of these pre-catalysts are active, pro- TiOOЈ plane and the plane of the carboxy-substituted fer-

ducing aldehyde mixtures in very good yields and with rocene cyclopentadienyl ring of ca. 25°.

favourable n/iso ratios; halogen complexes 34a and 34b were

In order to elucidate the coordination properties of the

methyl ester Medpf, we turned to palladium() complexes

practically inactive.^[45]

Thus, complex 37 as such or generated in situ from with chelate C,N-ligands (Scheme 12). The reaction of or-

[Rh(acac)(CO)₂] and Hdpf represents an efficient hydrofor- tho-palladated dimeric complex 39 with Medpf gave the ex-

mylation catalyst precursor, not requiring any further co- pected bridge-cleavage product 40. However, the following

catalyst; additional components may, however, be used to attempts to synthesize O,P-chelate complexes failed. Re-

increase the reaction selectivity. For example, at a catalyst/ moval of the halide ligand from 40 as well as the direct

1-hexene ratio of ca. 3.8×10^–5under 10 atm of CO/H₂(1:1 reaction of bis(acetonitrile) complexes 41 yielded only the

mixture), pre-catalyst 37 gave isomeric heptanals in ca. 80% phosphane complexes 42 with the acetonitrile molecule

yield and n/iso Ϸ 2 at 80 °C after 3 h, while the best n/iso completing the coordination sphere around the palladium

ratio of 4.6 (without diminished yield) was achieved with ion.^[48]

the 37/P(OPh)₃system. Besides, complex 37 can be used

Compounds 40, 42b, and 21a as well as some palladi-

repeatedly, cf. 84, 89, 83, 69, 48, 13% yields of aldehydes in um() acetate/ligand mixtures (ligand = Hdpf, Medpf, and

six consecutive runs when the liquid part of the reaction FcPPh₂) were tested as catalyst precursors for Suzuki–Mi-

mixture has been simply decanted from the catalyst residue yaura cross-coupling of 4-bromotoluene and phenylboronic

(n/iso ratio ranged from 2.1 to 2.7). Spectral data indicate acid.^[48]In all cases, the reaction proceeded with excellent

that 37 itself is not the catalytically active species but it isolated yields of 4-methylbiphenyl (Ն 80%). The most

is converted under the reaction conditions to the hydride active catalysts proved to be the one generated in situ by

[RhH(CO)₂(Hdpf-κP)₂].^[45]

mixing palladium() acetate with 2 equiv. of Hdpf or

In our attempts to synthesize compounds featuring a Medpf, which afforded the product in 96% isolated yield.

form of Hdpf as an O-donor, we pursued the preparation The least active in the series tested proved to be the

of simple salts. Actually, the reaction of Hdpf with inor- [Pd(cod)Cl₂]/2Hdpf mixture, a formal analogue to 21a,

ganic bases proceeded under the formation of carboxylate which gave only 44% yield.

salts as evidenced by changes in the carbonyl stretching re-

Recently, Hdpf was tested as an anchoring agent of a

gion of the IR spectra, but the products were ill-defined transition metal catalyst to a solid support. Treatment of

amorphous materials, tending to retain varying amounts of mesoporous molecular sieves MCM-41 with Hdpf and,

solvents.^[46]Hence, whenever a dpf^–salt is required for syn- subsequently, with [{Ru(µ-Cl)Cl(η⁶-p-cymene)}₂] gave an

thesis, it is advantageous to generate it only in situ by treat- Ru/Hdpf-modified sieve. Because the analytical data for Ru/

ing Hdpf with an appropriate base.

Hdpf-MCM-41 implied an independent immobilization of

The only example of exclusive carboxylate coordination the modifiers rather than a surface reaction, a similar mate-

for Hdpf is the paramagnetic titanocene carboxylate [(η⁵- rial but without Hdpf was prepared from the dimeric ruthe-

C₅HMe₄)₂Ti(dpf-κ²O,OЈ)] (38) obtained by allowing equi- nium complex and the sieve. All these materials along with

molar amounts of “titanocene equivalent” [(η⁵-C₅HMe₄)₂- their

molecular

analogue

[{RuCl₂(Hdpf-κP)(η⁶-p-

Ti(η²-Me₃SiCϵSiMe₃)] and Hdpf to react (Figure 6).^[47]cymene)}₂] (43) were tested as catalysts in the reaction of

According to structural data, the dpf^–anion coordinates benzoic acid with propargyl alcohol to give 2-oxopropyl

to the titanium ion as a simple carboxylate without any benzoate. The supported catalysts Ru-MCM-41 and Hdpf/

participation of the phosphane group: the carboxy plane Ru-MCM-41 proved less active than 43, promoting a side

nearly ideally bisects the titanocene fragment, the Ti–O reaction of propargyl alcohol, which further lowered the

bond lengths being 2.158 and 2.166 Å. Both metallocene yield of the ester. In spite of some leaching of the catalyti-

units possess the expected geometry and are mutually ro- cally active species from the solid support, the catalysts

could be re-used without any apparent loss of catalytic ac-

tivity.^[49]

(S_p)-2-(Diphenylphosphanyl)ferrocenecarboxylic

Acid

(S_p)-2-(Diphenylphosphanyl)ferrocenecarboxylic

acid,

(S_p)-HL¹, has been reported independently by three groups.

The first mention dates back to the year 2000 when this

compound was utilised as a chiral synthon for the prepara-

tion of chiral diamide ligands,^[50]which were further tested

in palladium-catalysed alkylation reactions^[50a–50d]and the

silver()-mediated formation of pyrroles.^[50c]Notably, the

acid itself proved to be a relatively poor ligand for the for-

Figure 6. Molecular structure of bis(metallocene) complex 38.

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Scheme 12.

mer reaction.^[51]More recently, (S_p)-HL¹has been em-

ployed as a chiral auxiliary (catalyst-directing group)^[52]and

its ester, (S_p)-MeL¹, utilised for the synthesis of ferrocenyl-

oxazolines.^[53]

The synthetic route leading to (S_p)-HL¹as reported by

all three groups was identical,^{[50a–50c,54]}based the on a chi-

ral protecting/ortho-directing group approach (Scheme 13).

It starts with chiral ferrocenyl-oxazoline (S,S_p)-44, which is

accessible in three steps (amidation, oxazoline ring closure,

and diastereoselective lithiation/phosphanylation) from fer-

rocenecarboxylic acid in good yields and stereodefined

form. The oxazoline was hydrolysed by the standard proto-

col: the oxazoline ring was first cleaved by action of aque-

ous trifluoroacetic acid to give an ammonium salt interme-

diate, which was directly converted into the more stable

amido ester (S,S_p)-45. The following saponification of the

ester amide with sodium hydroxide, acidification of the re-

action mixture and chromatographic purification afforded

(S_p)-HL¹in 60–65% yields vs. (S,S_p)-44.

Scheme 13.

It should be noted that a similar methodology with the

appropriate oxazolines furnished also 1Ј-(diphenylphos- neutralization of (S_p)-HL¹with the equivalent amount of

phanyl)-2-(trimethylsilyl)ferrocenecarboxylic acid,^[50c]and KOtBu. The reaction yields a kinetic 1:1 mixture of diaste-

esters of C₂-symmetric 2,2Ј-bis(diphenylphosphanyl)ferro- reoisomeric chelate complexes differing in configuration at

cene-1,1Ј-dicarboxylic acid.^[55]These compounds and the stereogenic ruthenium atom, (R_Ru,S_p)-46 and (S_Ru,S_p)-

amides relating to the later esters^[56]were tested as ligands 46. However, this mixture undergoes a spontaneous epi-

for palladium-catalysed allylic substitution.

merization in favour of the thermodynamically preferred

Compared to the numerous catalytic applications of (S_p)- isomer (R_Ru,S_p)-46, which was isolated in 82% yield by

HL¹, the coordination chemistry of this carboxy-phosphane crystallization over several days and structurally character-

remains still unexplored. An exception are (arene)rutheni- ised (Scheme 14, Figure 7).^[54]

um() complexes with phosphanyl-carboxylate [(S_p)-L¹]^–,

Apparently, the thermodynamic driving force for this

resulting from the reaction of [{Ru(µ-Cl)Cl(η⁶-p-cymene)}₂] spontaneous equilibration is the destabilization of one of

with 2 mol-equiv. of salt K[(S_p)-L¹] generated in situ by the diastereoisomers, (S_Ru,S_p)-46, due to steric interactions

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MICROREVIEW

phanyl)ferrocenyl]methyl}dimethylamine (47) (Scheme 15).

The amine was converted by alkylation with benzyl bro-

mide to ammonium salt 48, which upon heating with aque-

ous sodium cyanide gave nitrile 49. Hydrolysis of the nitrile

followed by acidification of the reaction mixture and

recrystallisation of the crude acid from aqueous acetic acid

yielded rac-HL²as a yellow, air-stable solid (Scheme 16; ca.

63% yield from 47 over two steps; the salt does not need to

be isolated). The acid was further converted into the respec-

tive phosphane oxide (50), phosphane sulfide (51), and

methyl ester. The coordination ability of rac-HL²and rac-

MeL²has been assessed in palladium() complexes.^[57]

Scheme 14.

Scheme 15. Preparation of rac-HL²and its derivatives.

First, we studied palladium() complexes containing the

2-[(dimethylamino)methyl]phenyl-κ²C¹,N spectator ligand

(Scheme 16). The reaction of rac-MeL²with dipalladium

precursor 39 gave the expected bridge-cleavage product 52.

Attempts to isolate a similar complex with rac-HL²failed

due to consecutive reactions of the primary product analo-

gous to 52, most likely an acidolysis of the Pd–C bonds.

The removal of the chloride ligand with silver() perchlorate

from 52 gave cationic chelate complex 53, which results also

from a replacement of acetonitrile ligands in the cationic

precursor 42a (note: the same reaction but with Medpf pro-

ceeds with substitution of only one MeCN ligand, see

above). The related complex containing the O,P-chelating

[rac-L²]^–anion was obtained by treating dimer 39 with the

stoichiometric amount of salt K[rac-L²]^–generated in situ

from the acid and KOtBu. The number, types and sequence

of atoms within the “L²-chelate” rings in 53 and 54 are

Figure 7. View of the molecular structure of (R_Ru,S_p)-46 as ob-

tained for the solvate (R_Ru,S_p)-46·0.7CH₂Cl₂. Molecules of solvat-

ing dichloromethane are not shown.

between the rigid ferrocene unit and the bulky η⁶-arene li-

gand. The epimerization occurs most likely via Ru–O bond

opening, followed by a rotation along of the ligand moiety

into the sterically preferred position and the repetitive clo-

sure of the chelate ring. The hemilabile nature of the [(S_p)-

L¹]^–ligand evidently facilitates the isomerisation reaction.

rac-[2-(Diphenylphosphanyl)ferrocenyl]acetic

Acid

rac-[2-(Diphenylphosphanyl)ferrocenyl]acetic acid, rac- identical and, hence the rings are structurally very similar,

HL², a formal homologue of (S_p)-HL¹, has been synthe- which is manifested also by practically identical ligand bite

sized from the known precursor, rac-{[2-(diphenylphos- angles O–Pd–C of ca. 98°.^[57]

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Yet another coordination mode for the [rac-L²]^–anion

exploiting all (classical) donor atoms has been observed for

carboxylate-bridged dipalladium() complex 56 resulting

from a reaction of dipalladium() complex 57 with K[rac-

L²] (Scheme 17). The formation of 56 likely involves a sub-

stitution of chloride ligands with the carboxylate and che-

late-assisted replacement of monodentate phosphanes PR₃

with the phosphanyl group of the carboxylate ligand. Simi-

larly to 55, complex 56 results in a single diastereoisomer

(R_p,S_p)-56 (i.e., meso-form), combining both enantiomers

of the racemic ligand in one complex molecule.^[60]

rac-[2-(Diphenylphosphanyl)

methyl]ferrocenecarboxylic Acid

The ligand rac-HL³or rac-[2-(diphenylphosphanyl)

methyl]ferrocenecarboxylic acid is an isomer of the pre-

viously discussed compound rac-HL², differing in the posi-

tion of the methylene spacer. It has been synthesized from

rac-[(2-bromoferrocenyl)methyl]dimethylamine (58), which

was converted to acetate 59 and then to bromo alcohol 60

(Scheme 18). Since the direct phosphanylation^[61]com-

monly operative in ferrocene chemistry failed with com-

pounds 58–60, we sought for alternative methods. Fortu-

nately, the key phosphanyl bromide 61 was obtained in ex-

cellent yield by treating alcohol 60 in acetonitrile with Me₃-

SiCl/NaI and then with Ph₂PH, as recently reported for

some related compounds.^[62]Metallation of 61 followed by

carboxylation with carbon dioxide and chromatographic

purification gave a good yield of rac-HL³. For the sake of

comparison with the rac-HL²family, the acid was further

converted into the respective phosphane oxide 62, phos-

phane sulfide 62, and methyl ester rac-MeL³.^[63]

Scheme 16.

Synthesis

of

{[(dimethylamino)methyl]phenyl-

κ²C¹,N}palladium() complexes with “L²family” ligands.

The reaction of 53 with sodium hydride or potassium

tert-butoxide in THF provided bis(chelate) complex 55. A

formal dehydrohalogenation 53 Ǟ 55 can be rationalised

by deprotonation at the activated ligand methylene group

and a subsequent intramolecular substitution of the chlo-

ride ligand under preservation of the trans-P–N donor ar-

rangement around the palladium ion. The reaction dia-

stereoselectively creates a new chiral centre at the deproton-

ated carbon atom, producing the single diastereoisomer,

(S,R_p)/(R,S_p), from the racemic ester.^[58]Compared to the

related complexes with chelating [R₂PCHCO₂Et]^–

anions,^[59]compound 55 is chemically very robust, inert to

air, CO, SO₂, and some alkynes.

Scheme 18.

Scheme 17. Formation of dipalladium() complex 56 featuring a

bridging tridentate [rac-L²]^–anion [PR₃= PMe₃(a), PPh₃(b), and

PFur₃, where Fur = fur-2-yl (c)].

The coordination properties of rac-HL³and the respec-

tive ester have been studied up to the present only for rho-

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MICROREVIEW

dium complexes (Scheme 19). (Cyclopentadienyl)rhodi- rangement of complexes featuring Hdpf and the related

um() complexes with P-monodentate ligands (64 and 65) forms of this ligand. The number of structurally character-

were prepared by bridge cleavage from [{Rh(µ-Cl)Cl(η⁵- ised compounds (Table 1) enables us to draw some general

C₅Me₅)}₂] and the respective ligands. Attempts to synthe- conclusions about the ligand geometry. First, the ferrocene

size O,P-chelated complexes by treating 64 with bases or moieties remain quite regular, showing no deformation or

from 65 by halide removal failed but, finally, the O,P-chelat- tilting of the cyclopentadienyl rings: the maximum dihedral

ing coordination of the [rac-L³]^–anion was attained with a angle of the cyclopentadienyl planes of ca. 7.5° was ob-

rhodium() complex 66 obtained from rac-HL³and 35a (see served for 29 and 23·(2+x)CH₂Cl₂. Second, the rotation of

also above).^[63]

the carboxy plane from the plane of its parent cyclopen-

tadienyl ring (twist angle θ) does not usually exceed ca. 10°,

the exceptions being 36a (θ = 26°), 38 (θ = 19°) and one of

the four ligand moieties in the structure of 28·2CH₃CO₂H

(θ = 16°). Whereas the carboxy rotation in 36a apparently

results from a compromise between the O,P-chelating coor-

dination of the dpf^–anion and the steric demands of the

central atom, twisting of the carboxy group in 38 and

28·2CH₃CO₂H is probably caused by crystal packing ef-

fects. However, in view of DFT calculations for ferrocene-

carboxamide,^[64]indicating that rotation of the amide moi-

ety from the plane of its bonded cyclopentadienyl ring by

as much as 30° reduces orbital interaction of their π-sys-

tems by approx. 14%, even such rotations of the carboxy

plane do not destabilise the mentioned structures by pre-

cluding the carboxy-ferrocene conjugation.

Scheme 19.

Structural Aspects

Hdpf, Medpf, and dpf^–Complexes

The third but most varying parameter is the torsion an-

The specific steric properties of the ferrocene moiety as gle τ (Table 1), which relates to the conformation of the

mentioned above are obviously reflected in the spatial ar- 1,1Ј-disubstituted ferrocene core (see also ref.^[13]). Of the

27 structurally characterised (H)dpf ligand moieties, twenty

have τ in the range of 135–160°, i.e. around anti-eclipsed

conformations [Scheme 20, (a); τ(AB) = 144°]. Such an ar-

rangement apparently minimizes the steric interactions of

the substituents at the ferrocene unit by bringing their

centres of gravity (not the bonds to the substituents which

would lead to anti-staggered conformations with τ = 180°)

into a mutually opposite position [see side and top views in

Scheme 20, (b)]. The minimum τ value of 60° was found for

the chelate complex 36a, where the proximity of both donor

groups is enforced by the O,P-chelating coordination.

Table 1. Conformation parameter τ for ferrocene units in the struc-

turally characterised Hdpf derivatives and complexes.^[a]

Compound

|τ| [°]

Hdpf and its complexes

Hdpf^[17]

162, 162

146

HdpfO^[17]

21a·2CH₃CO₂H^[35]

22^[35]

83

151, 141, 145

22·2CH₃CO₂H^[35]

83

148, 89

144

79

[35]

23·(2+x)CH₂Cl₂

[36]

24d·2CHCl₃

26b^[37]

27b·4CH₃CO₂H^[37]

28·2CH₃CO₂H^[38]

29^[39]

135

158, 148, 158, 118

68

148

143, 143

162

[40]

33da·CH₂Cl₂

[49]

43·CH₂Cl₂

[PdCl{η³-CH₂C(CH₃)CH₂}(Hdpf-κP)]^[66]

dpf^–complexes

36a^[44]

60

163

38^[47]

Medpf and its complexes

Scheme 20.

Medpf^[17]

162

145

146

42a^[48]

42b^[48]

Energy changes associated with conformational alter-

ations may be compensated for by an energy gain upon the

formation of intermolecular bonds and a favourable crystal

assembly.^[65]This can be exemplified by six entries with τ =

68–118°, all featuring P-monodentate Hdpf, whose carboxy

group takes part in the usual double hydrogen bridges to

another ligand carboxy group or to solvating acetic acid.

[a]

Parameter τ is defined as the torsion angle (C^P–Cg^P–Cg^C–C^C),

where Cg^Pand Cg^Cdenote the centres of gravity of phosphanyl-

(P) and carboxy-substituted (C) cyclopentadienyl rings, and C^Pand

C^Cring carbon atoms bearing the substituents. Absolute values for

τ are given to eliminate chirality of the ligand moieties in the solid-

state (see ref.^[13]). Entries for all crystallographically independent

ligand molecules are given where applicable.

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The Chemistry of Phosphanyl-ferrocenecarboxylic Ligands

MICROREVIEW

Intermolecular Interactions

ditional hydrogen bonds to the solvate molecules [P=O···H–

CCl₃: O···O 2.986(2) Å; Scheme 21, D].

The supramolecular architectures of crystalline organo-

metallic solids are currently gaining increasing attention

due to their relation to the rapidly growing research area of

organometallic crystal engineering.^[67]In this regard, ferro-

cene chemistry holds great potential for future develop-

ments because ferrocene molecules are usually stable, struc-

turally well defined and, most importantly, capable of bear-

ing various polar groups to be used as the structure-as-

sembling tool.

The molecules of ferrocene phosphanyl-carboxylic li-

gands combine hard polar carboxy moieties with a nonpo-

lar ferrocene skeleton and substituents at the phosphorus

atom. Interactions between the polar parts are usually

clearly detectable from the structural data and virtually

dominate the crystal assembly. However, they operate to-

gether with the relatively weaker but important intermo-

lecular forces such as, for example, C–H···X and C–H···π

hydrogen bonds (X stands for a heteroatom), and π···π

stacking interactions of the aromatic rings.^[68]

Scheme 21. Hydrogen bonding patterns observed for phosphorus-

substituted ferrocenecarboxylic acids.

A situation similar to the free ligand has been observed

also for a number of complexes with P-monodentate Hdpf:

the ligand carboxy group typically forms characteristic

double hydrogen bonds to an adjacent ligand moiety [22,

26b, 23·(2+x)CH₂Cl₂, 24d·2CHCl₃, 28·2CH₃CO₂H (see

above), 29, 33da·CH₂Cl₂, 43·CH₂Cl₂] or to solvating acetic

acid [21a·2CH₃CO₂H, 22·2CH₃CO₂H, 27b·4CH₃CO₂H,

28·2CH₃CO₂H (see above)]. In this regard, complex

[PdCl{η³-CH₂C(CH₃)CH₂}(Hdpf-κP)] is a notable excep-

tion as it forms dimers through Pd–Cl···H–OC(=O) bonds

[Cl···O 3.082(2) Å].^[66]

The summary of the principal intermolecular interac-

tions observed for the discussed ligands and their corre-

sponding phosphane oxides and sulfides is given in Table 2.

The phosphane derivatives typically assemble via double

hydrogen bridges characteristic for carboxylic acids

[Scheme 21, interaction A; notation according to graph set

theory: R²(8)^[69]]. A similar assembly is usually encoun-

2

tered also for the respective phosphane sulfides whose thio-

phosphoryl sulfur atom cannot effectively compete for the

hydrogen-bond donor with the carboxy oxygen atoms. On

the other hand, the conversion of phosphanes into phos-

phane oxides is associated with an introduction of a strong

hydrogen-bond acceptor, which further increases the struc-

tural variability. Among the ferrocene phosphanoyl-carbox-

ylic acids which have been structurally characterised,

HdpfO and (S_p)-67 (see footnotes to Table 2) form one-di-

mensional zig-zag chains propagating by means of

P=O···H–OC(=O) hydrogen bonds (Scheme 21, B) whilst 50

forms centrosymmetric cyclic dimers assembled via a pair

of the similar P=O···H–OC(O) interactions (Scheme 21, B).

In contrast, the solvated phosphane oxide 62·CHCl₃holds

the structure of the parent carboxy dimer, forming ad-

Conclusions

The introduction of phosphanyl-ferrocenecarboxylic li-

gands about ten years ago brought some new aspects into

the chemistry of mixed-donor ferrocene ligands. The par-

ticular combination of the functional groups and the ferro-

cene backbone makes these compounds chemically and

structurally markedly diverse. Consequently, phosphanyl-

ferrocenecarboxylic ligands represent very flexible donors,

which typically offer several accessible, sometimes mutually

interconvertible, coordination modes, and are capable of

hemilabile coordination. From a practical viewpoint, phos-

phanyl-ferrocenecarboxylic derivatives hold much potential

as perspective ligands for catalytic applications and as valu-

able synthons for the design of supramolecular organome-

tallic materials. The results already obtained are promising

and open up new areas for future research, ranging from

the development of new ligands and complexes on one side

to their applications in catalysis and material science on the

other.

Table 2. Hydrogen bond lengths for the carboxylic acids.^[17,54,57]

Compound

Interaction^[a]

d(O···O) [Å]

Hdpf

A

B

A

C

A

D

A

2.650(6), 2.653(6)^{[b] [c]}

HdpfO

(S_p)-67^[e]

rac-HL²

50

2.588(4)^[b]

2.556(4)^[d]

2.646(2)^[d]

2.596(3)^[d]

51

2.649(2)^[d]

rac-HL³

62·CHCl₃

63·½CH₂Cl₂

2.629(3)^[d]

2.646(2)^[d]

2.663(2)^[d]

Acknowledgments

[a] See text and Scheme 21 for definitions. [b] Determined at room

temperature. [c] Entries for two crystallographically independent

molecules. [d] Determined at 150 K. [e] (S_p)-2-(diphenylphos-

phanoyl)ferrocenecarboxylic acid, (S_p)-[HL¹O].

I am indebted to all my past and present co-workers who have

contributed to the research presented here for their enthusiastic

Eur. J. Inorg. Chem. 2005, 3787–3803

www.eurjic.org

3801

ˇ

P. S teˇpnicˇka

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Received: May 30, 2005

Published Online: August 29, 2005

Eur. J. Inorg. Chem. 2005, 3787–3803

www.eurjic.org

3803

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The chemistry of phosphanyl-ferrocenecarboxylic ligands

DOI: 10.1002/ejic.200500485

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Article abstract of DOI:10.1002/ejic.200500485

Full text of DOI:10.1002/ejic.200500485

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