Synthesis, Structure and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines

A Journal of

Accepted Article

Title: Synthesis, Structure and Nickel Carbonyl Complexes of

Dialkylterphenyl Phosphines

Authors: Ernesto Carmona, Mario Marín, Juan J. Moreno, Carlos

Navarro-Gilabert, Celia Maya, Riccardo Peloso, M. Carmen

Nicasio, and Eleuterio Álvarez

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Synthesis, Structure and Nickel Carbonyl Complexes of

Dialkylterphenyl Phosphines

Mario Marín,^[b]Juan J. Moreno,^[a]Carlos Navarro-Gilabert,^[a]Eleuterio Álvarez,^[a]Celia Maya,^[a]Riccardo

Peloso,^[a]M. Carmen Nicasio,*^[b]Ernesto Carmona*^[a]

Dedicated to Professor William D. Jones on the occasion of his 65th birthday

Abstract: We describe the experimental and computational

characterization of a series of dialkylterphenyl phosphines, PR₂Ar’

phosphines,⁶along with other bulky phosphines,^7-9have become

indispensable ligands in organometallic chemistry and catalysis,

and within the recent past many tertiary phosphines of assorted

(Figure 2). Molecules of composition PR₂Ar^Dtbp, for R = Me, Et,

2

ⁱPr, c-C₅H₉and c-C₆H₁₁, are accompanied by five PR₂Ar’

electronic and steric characteristics have been made available.^10-

i

15

phosphines containing the bulky alkyl groups Pr, c-C₅H₉or c-

Xyl’₂

Ph₂

C₆H₁₁, in combination with Ar’ = Ar^Xyl, Ar , or Ar (L1-L10).

Steric and electronic parameters have been determined

computationally and from IR and X-ray data obtained for the

phosphines and for some derivatives, including tricarbonyl and

As of late, remarkable success in unveiling subtle phosphine

ligand effects has been witnessed in nickel, palladium and gold

catalysis.¹⁶Besides, for tri(1-adamantyl)phosphine, a molecule

for which steric and electronic properties beyond conventional

limits were disclosed, unique catalytic features, seemingly

influenced by van der Waals dispersion forces,¹⁷were

uncovered.^12aFurthermore, widely used parameters such as

2

dicarbonyl

nickel

complexes,

Ni(CO)₃(PR₂Ar’)

and

Ni(CO)₂(PR₂Ar’). In the solid state, molecules of PR₂Ar’ adopt one

of the three possible structures represented in Figure 3, formally

related by rotation around the C_ipso-P bond. Information on their

relative energies and on the influence of the free phosphine

structure on its coordination chemistry towards Ni(CO)_n(n = 2, 3)

fragments has been gained by experimental and computational

methods.

19

Tolman’s cone angle ¹⁸and the buried volume %V_burmay be

envisaged as accounting, respectively, for remote and proximal

steric effects, the former potentially giving rise to significant

dispersion forces.^16a

The impact of dialkylbiaryl phosphines in organotransition

metal chemistry and catalysis prompted us to explore the m-

terphenyl analogues, PR₂Ar’, for Ar’ = terphenyl radical. The

conspicuous features of sterically demanding terphenyl fragments,

either as bulky organometallic aryl ligands²⁰or as substituents of

various types of Lewis bases^21,22have been investigated. At the

outset of our work, however, just a few terphenyl phosphine

ligands, and a consequently small number of their complexes with

late transition metals, had been described.^23-25Our first reports

centred Rh, Ir, Pt and Au of PMe₂Ar’ phosphines (Ar’ = C₆H₃-2,6-

(C₆H₃-2,6-R₂)₂for R = Me and ⁱPr, in shorthand notation

Introduction

Tertiary phosphines, PR₃, and related molecules containing a

three-coordinate phosphorus atom, PX₃(X = OR, NR₂and others),

form an invaluable family of ligands, with an influence in

organometallic chemistry and homogeneous catalysis that cannot

be overstated.^1-3By and large, during the second half of the past

century, studies on transition metal complexes focused on alkyl

and aryl phosphines such as PMe₃, PⁱPr₃, P(c-C₆H₁₁)₃, PPh₃and

others. Notwithstanding the versatility of these ligands, relatively

little progress was made on the synthesis of monodentate

phosphines until fairly recently.⁴Some twenty years ago,

Buchwald and coworkers demonstrated that when bound to

palladium, dialkylbiaryl phosphines were able to catalyse with

high proficiency C-C and C-N coupling reactions of aryl halides,

including unactivated aryl chlorides.⁵Lately, Buchwald’s

PMe₂Ar^Xyland PMe₂Ar^Dipp, respectively), and evinced their

aptitude to stabilize low-coordinate structures, as well as their

potential to adopt different coordination modes (Figure 1), where

P-bonding is complemented by relatively weak M···C_arene

2

interactions with a flanking aryl ring of the terphenyl substituent.^23-

28

[a]

J. J. Moreno, C. Navarro-Gilabert, Dr. Eleuterio Álvarez, Dr. Celia

Maya, Dr. R. Peloso, Prof. Dr. E. Carmona

Instituto de Investigaciones Químicas (IIQ), Departamento de

Química Inorgánica and Centro de Innovación en Química

Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones

Científicas (CSIC) and Universidad de Sevilla

Avda. Américo Vespucio 49, 41092 Sevilla, Spain

E-mail: guzman@us.es

[b]

M. Marín, Prof. Dr. M. C. Nicasio

Departamento de Química Inorgánica, Universidad de Sevilla

Aptdo 1203, 41071 Sevilla (Spain)

E-mail: mnicasio@us.es

Figure 1. Different coordination modes found for terphenyl phosphines.^23-28

Supporting information for this article is given via a link at the end of

the document.

The objective of the present work was the description of a

series of PR₂Ar’ molecules selected in accordance with the

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Figure 2. Structural formulae, abbreviated names and numbering scheme for the terphenyl phosphines described in this work (upper part) and others already in

the literature (bottom row). Crystallographic data for phosphines with underlined numbers available. ^[a]See reference 24a.

following criteria. Firstly, besides some PMe₂-containing

employed by Tolman to ascertain phosphine electronic

properties,³⁵but also some

Ni(CO)₂(PR₂Ar’) derivatives,

phosphines,^28,29we set out to prepare PR₂Ar’ phosphines of the

i

branched and cyclic alkyl groups Pr and c-C₆H_11,respectively,

2·PR₂Ar’, for which an uncommon oblique trigonal pyramidal

geometry, with a relatively weak Ni-η²-C_areneinteraction has been

unveiled. Although many Ni(0) tricoordinate complexes are

familiar, analogous phosphine dicarbonyl Ni(0) compounds are

unknown. Furthermore, we were surprised to learn that nearly fifty

years after the generation by Tolman of Ni(CO)₃(PR₃) complexes

in CH₂Cl₂solutions,³⁵there is an astonishing dearth of

crystallographic information on compounds of this type.³⁶

which are phosphine substituents extensively employed in

organometallic chemistry and catalysis. The new terphenyl

phosphines find applications in efficient Pd-catalyzed amination

reactions and Cu-promoted cycloaddition of azides and alkynes.

These and other catalytic processes will be described separately.

We also considered of interest the c-pentyl phosphine

analogues,^30-35despite known drawbacks for P(c-C₅H₉)₃as, for

instance, its facile oxidation and the higher reactivity of c-C₅H₉

rings relative to c-C₆H₁₁ones, due mainly to steric strains.³⁴For

the second group of phosphines, we chose a terphenyl moiety

containing two 3,5-bis(t-butyl)phenyl substituents, 3,5-C₆H₃-

(CMe₃)₂, at the 2- and 6-positions of the central aryl ring (in short

Results and Discussion

Synthesis and spectroscopic characterization of PR₂Ar’

ligands

Ar^Dtbp2). As presented in Figure 2, a total of five PR₂Ar^Dtbpligands

2

have been prepared for R = Me, Et, ⁱPr, c-C₅H₉and c-C₆H₁₁, (L1-

L5). Attempts to prepare their tert-butyl analogues, P^tBu₂Ar’,

proved unsuccessful.

Our recent synthesis of terphenyl phosphines, PR₂Ar’,²⁸

followed a slightly modified earlier procedure^23aand consisted in

the stepwise reaction of a terphenyl Grignard reagent, Mg(Ar’)X,

with PCl₃, followed by alkylation using, once more, the appropriate

magnesium agent, Mg(R)Br, as shown in Scheme 1a. Whereas

this method yields satisfactory results for linear hydrocarbyl R

groups and it was thus applicable to the synthesis of the new

phosphines L1 and L2, attempts to extend it to the branched or

cyclic i-propyl, c-pentyl and c-hexyl phosphines led either to

unreacted starting materials or to complex mixtures of products

that could not be characterized (Scheme 1b). Emulating the

synthesis of biaryl phophines,³⁷the alkylation of PX₂Ar’ was

Here, we focus attention on ligand synthesis and structural

characterization by X-ray diffraction methods. We show that all

dialkylterphenyl phosphines that have been authenticated by X-

ray crystallography, exhibit

a

solid-state structure that

corresponds to one of the three conformations depicted in Figure

3, for which different coordination properties towards unsaturated

metal fragments, ML_n, can be foreseen. Besides, we consider

closely Ni(0)-CO-PR₂Ar’ complexes, not only tricarbonyl-

phosphine species, Ni(CO)₃(PR₂Ar’), 1·PR₂Ar’, similar to those

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effected in the presence of CuCl. After careful monitoring of

reaction conditions (temperature, concentration of reagents and

copper salt), best results were obtained performing the reaction

at room temperature and using PX₂Ar’:Mg(R)Br molar ratios of ca.

1:4, in the presence of overstoichiometric quantities of CuCl

(approximately 1.5 equiv. relative to PX₂Ar’). The formation of the

target phosphines was always accompanied by small amounts of

Cu(X)(PR₂Ar’) complexes. Work-up of reaction mixtures was

complicated by the generation of metallic copper and of sticky,

insoluble materials that easily clogged filtration apparatus.^37bThe

synthesis of the new phosphines is summarized in Scheme 2,

particular details regarding their purification are presented in the

Experimental Section and the Supporting Information. As stated

earlier, all attemps to prepare P^tBu₂Ar’ phosphines were fruitless.

phosphines but L16 and L9 can be produced on gram-scale runs

in 50-60% yields, hence comparable to those given originally for

dialkylbiaryl analogues³⁷and for the recently prepared PMe₂Ar’

(Methyl-JohnPhos; ca. 52% three-step overall yield).^29bWith the

exception of PEt₂Ar^Xyl, L16, reported to form in unexpected low

2

yields (about 40%),²⁸it can be noticed that sterically demanding

R groups provoke a significant decrease in isolated yields, which

i

remain in the 35-60% range for Pr, c-C₅H₉and c-C₆H₁₁, in

contrast with the 70-80% values of the PMe₂Ar’ analogues. This

may explain failure to isolate P^tBu₂Ar’ phosphines. It appears

plausible that steric hindrance triggers side reactions involving,

among others, formal hydride transfer from Mg(R)Br to PX(R)Ar’

intermediates, thereby giving rise to alkene and secondary

phosphine P(H)R(Ar’) products.³⁸In support of this hypothesis,

³¹P NMR resonances around −40 ppm, that could be due to

P(H)R(Ar’) molecules,³⁹were frequently detected in the reaction

crudes. Furthermore, reactions aimed at the synthesis of

i

PR₂Ar^Dipp, for R = Pr, c-C₅H₉and c-C₆H₁₁, did not afford the

2

desired products, and for R = c-C₅H₉the secondary phosphine

31

PH(c-C₅H₉)Ar^Dipp, with  ( P) = −43.8 ppm, was isolated and

2

characterized by X-ray crystallography (see the SI, Figure S9).

31

The similar phosphine PH(ⁱPr)Ar^Xyl, with ( P) = −37.7 ppm, was

2

also isolated and characterized (see the SI).

Table 1. Yields (based on P), ³¹P{¹H} NMR chemical shifts and  C-P-C angles

for the ligands listed in Figure 2.

Ligand

Yield (%)

³¹P{¹H}

 C-P-C (º)

NMR (ppm)

L1, PMe₂Ar^Dtbp

71

65

49

37

51

−36.6

−12.8

12.6

0.9

298.53

2

L2, PEt₂Ar^Dtbp

2

L3, PⁱPr₂Ar^Dtbp

L4, P(c-C₅H₉)₂Ar^Dtbp

310,72

309,44

2

L5, P(c-C₆H₁₁)₂Ar^Dtbp

1.6

2

L6, PⁱPr₂Ar^Xyl

57

56

53

58

80

60

65

54

40

16.2

4.6

315,84

313,84

316,59

2

Scheme 1. General synthesis of m-terphenyl dialkyl phosphines with linear

hydrocarbyl groups.

L7, P(c-C₅H₉)₂Ar^Xyl

2

L8, P(c-C₆H₁₁)₂Ar^Xyl

10.1

15.3

16.3

−35.8

−40.4

−36.9

2

L9, PⁱPr₂Ar^Xyl’

2

L10, PⁱPr₂Ar^Ph

2

L11, PMe₂Ar^Ph

298,63

309,76

309,02

308,45

[a]

[b]

2

L12, PMe₂Ar^Xyl

2

L13, PMe₂Ar^Mes

[c]

2

L14, PMe₂Ar^Dipp

[d]

[a]

2

41.3

−40.7

−8.0

L15, PMe₂Ar^Tripp

2

L16, PEt₂Ar^Xyl

[a]

2

[a] See ref. 28. [b] See ref. 26a. [c] See ref. 23a. [d] See ref. 26b.

The new terphenyl phosphines were obtained as analytically

pure white solids that could be stored under air for extended

periods of time. This is in agreement with the behaviour unveiled

previously for dialkylbiaryl phosphines.^29,40aSome of the PR₂Ar’

phosphines exhibit solution dynamic behaviour, as discussed

Scheme 2. Optimized synthesis of m-terphenyl dialkyl phosphines PR₂Ar’ with

branched or cyclic substituents.

later for PⁱPr₂Ar^Dtbp

, L3. In general, however, the room

2

temperature ¹H and ¹³C{¹H} NMR spectra recorded for L1-L10 are

simple and indicative of a high degree of apparent symmetry

introduced by rotation around the P-C_ipsobond. ³¹P{¹H} chemical

shifts span a range of over 50 ppm (Table 1), from −36.6 (L1) to

16.3 ppm (L10). As expected, the major influence in the  value

comes from the R groups.^10,39Given that, as already pointed out,

Table 1 collects relevant information on the new phosphines

L1-L10. Corresponding data reported originally for L11-L16 are

also included. Before discussing pertinent characterization data,

trends in isolated yields deserve some brief comments. All

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Dipp₂

P(C≡CH)₂Ar^Mes, P(CH₂CH=CH₂)₂Ar

and the Buchwald-type

2

the syntheses of bulky phosphines often give rise to kinetically

competitive, undesirable side reactions, predicting the ³¹P NMR

chemical shifts of target molecules can be helpful. With the aid of

the ³¹P NMR contributions compiled for the R groups of L1-L10,¹⁰

biaryl phosphine Methyl-JohnPhos.^29a

the  values listed in Table 1 lead to group contributions for Ar^Xyl

2

and Ar^Dtbpof approximately +4 and +1 ppm, respectively. A

2

comparable value of +2 ppm can be estimated for Ar^Dipp, yet the

2

latter is based on only two PR₂Ar^Dipp2phosphines (Table 1). It

should be remarked that for tertiary phosphines, deviations

between estimated and experimental values are usually ≤3 ppm,

exceeding rarely 6 ppm.¹⁰

Solution, X-ray and Gas–Phase Molecular Structure of

Dialkylterphenyl Phosphines, PR₂Ar’

The solid-state structures of the molecules of the newly

reported terphenyl phosphines L1, L3, L4 and L6-L8 were

determined by X-ray crystallography. To have on hand a

sufficiently large number of structures, the molecular geometries

of PMe₂Ar^Ph(L11), PMe₂Ar^Xyl(L12) and PMe₂Ar^Dipp(L14) were

also ascertained.²⁸Adding these X-ray data to other already

reported,^24a,28a structural database of more than a dozen X-ray

structures becomes currently available, allowing to discriminate

among three distinct phosphine conformations which formally

interconvert by rotation around the C_ipso-P bond (Figure 3).

Supplementary to X-ray analysis, solution and computational

studies on selected phosphines were developed.

2

Figure 3. Different conformations adopted by dialkylterphenyl phosphines in the

solid state.

The three solid-state structural types are represented

schematically in Figure 3 with A, B and C labels. Notice, however,

that in Figure 3: (i) The R’ substitution pattern at the side rings is

3,5 in A but 2,6 in B. (ii) PMe₂Ar’ phosphines exhibit structure of

type A or B, while C is characteristic of the bulkier PR₂Ar’

phosphines presented in Figure 2 (R = ⁱPr, c-C₅H₉or c-C₆H₁₁). (iii)

To mitigate steric hindrance between the P-bound alkyl groups

and the neighbouring flanking ring, the PR₂moiety in structures B

and C bends away from the ring in close proximity, such that one

of the P-C_ipso-C_orthoangles widens (>120°) at the expense of the

other (<120°).

The third type of structure found in the solid-state for the

PR₂Ar’ investigated becomes the preferred one for the bulky,

i

branched or cyclic alkyl groups Pr, c-C₅H₉and c-C₆H₁₁(L3, L4,

L6-L8 in Figure 2). Now, steric hindrance among the R groups

and the substituents at the flanking aryl rings forces the bulky P-

bonded alkyl units to place themselves above and below the plane

of the central aryl ring, i.e. frontwards and backwards in the

perspective shown for conformation C in Figure 3. Likewise, and

similar to structure B, to relieve steric tension between the R

groups and the proximal ring, the PR₂moiety bends away from

the ring, enlarging the corresponding P-C_ipso-C_orthobond angle

(see below for details). Interestingly, along with the bulky PR₂Ar’

phosphines studied in this work, all the sterically demanding

dialkylbiaryl phosphines analysed by the group of Buchwald,^40aor

by others,^40b,cfeature also a structure of type C.

On the basis of the experimental and computational results

discussed in this work, we propose that a certain phosphine

PR₂Ar’ adopts in the solid state one of the three A, B or C

structures, depending on steric repulsions among the phosphorus

substituents and electron-electron repulsion between the

phosphorus atom lone pair and the -system of the adjacent ring.

On these grounds, structure A is the preferred geometry for the

least sterically demanding phosphines studied, namely

To gain insights into the solution behaviour of terphenyl

phosphines, variable temperature NMR and complementary

computational studies were additionally carried out.

A

comparative analysis of PMe₂Ar^Dtbp(L1, structure A) and

2

PMe₂Ar^Dtbp(L1) and PMe₂Ar^Ph(L11), the two featuring P-Me

2

PⁱPr₂Ar^Dtbp(L3, structure C), appears appropriate. Computational

2

bonds and no ring substitution at positions 2 and 6. Besides L1

calculations revealed that for L1, rotation around the P-C_ipsobond

and L11, A is also the structural type found for P(H)₂Ar^Mesand

2

is

a very facile process, with transition states between

PMe₂Ar^Xyl’(Mes = 2,4,6-C₆H₂Me₃; Xyl’ = 3,5-C₆H₃Me₂), reported

2

conformations of the order of 3-3.5 kcal·mol^-1relative to the

ground state structure (Figure S1). This is in accordance with

solution NMR data down to −80 ºC, where both flanking rings

remain equivalent (Figure S3). In contrast, the ¹H-NMR spectrum

of L3 reveals inequivalence of the two rings at −80 ºC, as denoted,

for instance, by the observation of two signals for the t-Bu groups

with  1.29 and 1.28 ppm, or by the appearance of two distinct

signals (7.17 and 7.04 ppm) for the ortho H atoms of the Dtbp

rings (Figure S4). Upon warming, exchange of the flanking rings

becomes faster, averaging the exchanging sites. Thus, at −50 ºC

the two t-Bu resonances merge and originate a broad signal

centred at 1.33 ppm. From this and other data of the interchanging

H-positions, an energy barrier ΔG^‡≈ 11.7 kcal·mol^-1can be

by Wehmschulte and coworkers.^24a

Concerning PMe₂Ar’ phosphines bearing 2,6-disubstituted

flanking rings, the enlarged steric impediments brought in by

these substituents results in an observable structural change from

conformation A to B. Combination of a ca. 50° rotation of the –

PMe₂half around the P-C_ipsobond to make one of the P-Me bonds

almost coplanar with the Ar’ central ring, and opening of the P-

C_ipso-C_orthobond angle of the PMe₂unit and the closer side ring,

partially relieves steric tension making conformation B somewhat

more favourable than A. Experimentally, this is the molecular

Dipp₂

geometry ascertained for PMe₂Ar^Xyl(L12) and PMe₂Ar

(L14),

2

as well as for the previously reported^24a,28PMe₂Ar^Mes(L13),

2

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estimated, using Eyring equation and the value of the rate

constant determined at the coalescence temperature (−50 ºC). In

reasonable agreement with these experimental results, the

computational analysis of terphenyl rotation around the P-C_ipso

bond reveals that ring exchange in the ground state structure C

needs surmounting an energy barrier of ca. 14 kcal·mol^-1(see SI,

p. 16 for details).

With this information in mind, some comments are pertinent

regarding the coordination capabilities of the different terphenyl

phosphine structural types. Regardless of the nature of Ar’,

structure A is readily accessible for dimethylterphenyl phosphines

PMe₂Ar’. It is of note that for this rather symmetric conformation,

the phosphorus lone-pair points to a void region of space with little

steric interference from the terphenyl substituents. Classical P-

coordination is therefore expected, with little or no front-strain (F-

strain),³⁴depending upon the nature of the unsaturated transition-

metal Lewis acid centre. As conformations B and C can also be

readily accessed by these phosphines, other coordination modes

involving the -system of the proximate aryl ring, i.e. -P,ⁿ-C_arene

coordination (see Figure 1), can be foreseen, contingent on metal

needs. Contrary to this situation, for bulky phosphines with

structure C the electron density of the phosphorus lone pair faces

the nearby aryl ring such that considerable F-strain might arise

when bonding to crowded, or relatively crowded, transition-metal

fragments. At the same time, this directionality of the lone pair

facilitates the formation of complementary M···C_arenebonds, that

is, once again -P,ⁿ-C_arenebinding. As discussed later, these

considerations are in excellent agreement with the reactivity

found for PR₂Ar’ phosphines and Ni(CO)₄as a source of

unsaturated “Ni(CO)_n” fragments (n = 2, 3).

Figure 4. Two views of the molecular structure of PMe₂Ar^Dtbp, L1. Selected

2

bond lengths (Å) and angles (º): P-C(1), 1.864(3); P-C(35), 1.848(3); P-C(36)

1.851(3); C(36)-P-C(35), 95.2(2); C(35)-P-C(1), 101.4(1); C(36)-P-C(1),

101.9(1); P-C(1)-C(2), 121.3(2); P-C(1)-C(6), 121.4(2).

The molecular structures of PMe₂Ar^Dipp(L14, structure of type

B) and PⁱPr₂Ar^Xyl(L6, structure C) are illustrated in Figure 5. X-

ray data for other terphenyl phosphines studied in this work that

display structure B or C can be found in the Supporting

Information (Figures S10-S11). For PMe₂Ar^Dipp(Figure 5), one of

2

Figure 4 contains two views of the molecular structure of

PMe₂Ar^Dtbp, (L1), emphasizing the regular distribution of the

2

−PMe₂half relative to the terphenyl group. Besides other metrics,

this is nicely evinced by two almost identical P-C_ipso-C_orthobond

angles of 121.3(2) and 121.4(2)º. A conspicuous structural feature

clearly perceivable in Figure 4a is the deviation of the flanking aryl

ring ipso carbon atoms, C7 and C21, by ca. 0.32 and 0.33 Å from

the plane of the central aryl ring, with the result of two Me groups

of t-Bu substituents in opposite rings approaching to a distance of

about 4.77 Å, only ca. 20% longer than twice the van der Waals

radius of a Me group (2.0 Å).⁴¹While it is tempting to attribute this

distortion to London dispersion forces,¹⁷dispersion-corrected

DFT-D3 calculations do not support this assumption. Moreover,

the P-Me bonds namely P-C(2), is near to coplanar with the

central aryl ring, with a C(Me)−P−C_ipso−C_orthotorsion angle of 19.2º.

In addition, to attenuate steric repulsions between the P-bonded

methyl groups and the adjacent aryl ring, the pertinent P-C_ipso

-

C_orthobond angle distends to 128.64(9)º with an accompanying

decrease of the other to 113.30(9)º. Molecules of PⁱPr₂Ar^Xyl

2

(Figure 5) and of other i-propyl-, c-pentyl- and c-hexyl-terphenyl

phosphines investigated, present also two distinct P-C_ipso-C_ortho

bond angles, the wider between ca. 127.5 and 130.6º, and the

smaller in the interval 111.6-115.1º. But the prominent structural

feature in these bulky phosphines is doubtless the placing of the

two R groups in opposite regions of space relative to the plane of

the terphenyl central aryl ring (above and below in the perspective

shown in Figure 5). Some steric properties of the terphenyl

phosphines in their Ni(0)-CO-PR₂Ar’ complexes, in particular their

angular symmetric deformation coordinate S4’ parameters,⁴²will

be analysed in the coming section.

Gas-phase, energy-minimized structures of selected

terphenyl phosphines were obtained with the aid of DFT

calculations. Fair agreement between experimental and

calculated geometries was found. Relevant bond distances and

angles are collected in Table S1.

the unsubstituted terphenyl analogue, PMe₂Ar^Ph, (L11), exhibits

2

a similar deformation, with deviations of the C_ipsoatoms of about

0.28 Å (Figure S8). It must also be noted that due to the absence

of substituents in the ortho positions of the lateral rings, in

conformation A the latter are rotated around the C_ortho-C_ipsobond

from the almost perpendicular arrangement with respect to the

central ring observed in the other two conformations. This results

in an angle between the planes containing the lateral rings and

the plane of the central ring of 48° in the structure of PMe₂Ar^Ph

2

(vs. an average of ca. 85° in PMe₂Ar^Xyl).

2

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IR data were not given. Yet, Ni(CO)₃(PR₃) complexes of PⁱPr₃,

PPh₃, P(c-C₆H₁₁)₃and other common phosphine derivatives are

conspicuously absent, with the relevant exception of

Ni(CO)₃(P^tBu₃).^44cMoreover, attempts to synthesize Ni(CO)₃(P(c-

C₆H₁₁)₂Ar) complexes of Buchwald-type biaryl (Ar) phosphines

proved unsuccessful, because the ligands appeared to be too

sterically demanding to stabilize such complexes.⁴⁶Recently, the

steric and electronic properties of another series of dialkylbiaryl

phosphines, PR₂Ar^Ph, were investigated using IR and X-ray data

collected for a diversity of Cr(0), Pd(0) and Pd(II) compounds.^29a

We found that the target Ni(0)-CO-PR₂Ar’ complexes could be

accessed with comparable reaction outcomes either by treatment

of Ni(CO)₄with PR₂Ar’ or by carbonylation of 1:1 mixtures of

Ni(cod)₂and PR₂Ar’ (cod = 1,5-cyclooctadiene). For convenience,

the latter procedure was optimized and employed in all reactions

investigated.⁴⁷The interaction of equimolar mixtures of Ni(CO)₄

and PR₂Ar’ in THF solution could be readily monitored by IR and

³¹P{¹H} NMR spectroscopy, showing that depending upon the

nature of the phosphine, two types of complexes could form,

namely, tricarbonyls Ni(CO)₃(PR₂Ar’) (1) or dicarbonyl species

Ni(CO)₂(PR₂Ar’) (2) (Scheme 3). The two kinds of compound give

rise to the expected carbonyl stretching bands, that were

registered in the vicinity of 2065 (A₁) and 1980 cm^-1(E modes) for

tricarbonyls 1, and around 1995 (_sym) and 1920 (_asym) for the

dicarbonyl complexes 2. Similarly, though ³¹P NMR chemical

shifts for PR₂Ar’ depend markedly on the nature of the R group

(Table 1), for compounds 1·PR₂Ar’ a ∆ shift of ca. 30-40 ppm to

higher frequencies relative to the free phosphine was recorded,

and of about 40-50 ppm, i.e., an extra ~6-10 ppm shift, for the

dicarbonyls Ni(CO)₂(PR₂Ar’). Comparison with NMR data already

in the literature for other transition metal terphenyl phosphine

complexes^27,28suggests classical P-coordination of the

phosphine in 1·PR₂Ar’ and bidentate binding in 2·PR₂Ar’,

resulting from additional Ni···C_areneelectronic interactions with the

terphenyl substituent. This supposition is supported by X-ray

studies and will be discussed later in sufficient detail.

i

Xyl₂

Figure 5. Molecular structures of PMe₂Ar^Dipp(L14) (above) and PPr₂Ar (L6)

(below). Selected bond lengths (Å) and angles (º): L14: P-C(1), 1.858(1); P-

C(31), 1.834(2); P-C(32) 1.839(2); C(31)-P-C(32), 99.29(9); C(31)-P-C(1),

109.54(6); C(32)-P-C(1), 99.61(6); P-C(1)-C(2), 128.64(9); P-C(1)-C(6),

113.30(9) . L6: P-C(6), 1.872(2); P-C(16), 1.882(2); P-C(19), 1.866(2); C(16)-P-

C(19), 104.16(7); C(16)-P-C(6), 107.99(6); C(19)-P-C(6), 103.69(7); P-C(6)-

C(5), 129.1(1); P-C(6)-C(7), 113.6(1) .

2

Dialkylterphenyl Phosphine Nickel Carbonyl Complexes

We considered of interest ascertaining the electronic

properties of dialkylterphenyl phosphines analysing their

coordination chemistry towards Ni(CO)₄, and studying the

resulting Ni(0) carbonyl-phosphine complexes, Ni(CO)_n(PR₂Ar’).

The bulkiness of PR₂Ar’, as well as the relatively small nickel

covalent radius of 1.34 Å,⁴³anticipated coordination of only one

phosphine per nickel atom. In his pioneering studies, Tolman

generated Ni(CO)₃(L) complexes for a variety of three-coordinate

P-donor ligands by the room temperature reaction of Ni(CO)₄and

L in CH₂Cl₂, and determined the widely employed Tolman

electronic parameter for the ligand L,  or TEP, as the frequency

of the symmetric A₁carbonyl stretching mode in the

corresponding Ni(CO)₃(L) species.^18,35aDespite the marked, long-

standing impact of this work in organometallic chemistry and

catalysis, it is all the more surprising that, as briefly cited, a CSD

search (Cambridge Structural Database) revealed³⁶an

astonishing scarcity of X-ray data on Ni(CO)₃(PR₃) complexes of

commonly utilized, commercially available alkyl and aryl

Scheme 3. Synthesis of complexes 1·PR₂Ar’ and 2·PR₂Ar’.

As represented in Scheme 3a, only the dimethyl and diethyl

terphenyl phosphines, PMe₂Ar’ and PEt₂Ar’, originated the sought

Ni(CO)₃(PR₂Ar’) complexes, 1·PR₂Ar’, in reactions that took

place readily at room temperature and a CO pressure of 1 bar.

phosphines.

Thus,

some

ferrocenylphosphine^44a

and

fluoroalkylphosphine^44bNi(CO)₃(PR₃) compounds have been

characterized, among others, by X-ray crystallography. In addition,

carbonylation of a binuclear Ni^I-Ni^Icomplex of the p-terphenyl

diphosphine 1,4-bis(2-(diisopropylphosphino)phenyl) benzene,

afforded a crystalline material shown by X-ray diffraction⁴⁵to be a

mixture of nickel(0) carbonyls, in which 80% of the phosphines

are bound to Ni(CO)₂and the remaining phosphines to Ni(CO)₃.

i

Instead, ligands containing the more sterically demanding Pr, c-

C₅H₉and c-C₆H₁₁alkyl groups afforded either the dicarbonyl

derivatives 2·PR₂Ar’ of Scheme 3b, or non-isolable complexes.

In this instance, however, reaction mixtures had to be stirred

under vacuum, in many cases at high temperatures (around

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80 °C). A fair number of Ni(CO)_n(PR₂Ar’) (n = 2, 3) complexes

were fully characterized by microanalysis, IR and NMR

spectroscopy, and some of them were additionally authenticated

by X-ray crystallography. Others were, however, generated in

solution for IR spectroscopy studies (Tables 2 and 3). The Tolman

electronic parameter, TEP, of commercial P(c-C₅H₉)₃was

additionally measured, and for the sake of completeness complex

Ni(CO)₃(PPh₃)⁴⁸was also prepared and crystallographically

characterized. Likewise, two dialkylbiaryl phosphines, specifically

P(c-C₆H₁₁)₂Ar^Tripp(XPhos) and P^tBu₂Ar^Tripp(^tBuXPhos) were

examined, and, whereas in accordance with previous studies the

latter originated no isolable products,⁴⁶we were able to

characterize a stable Ni(CO)₃(XPhos) complex. Within the series

of dicarbonyl derivatives (Scheme 3b), X-ray studies provided

precise details of the unusual nickel coordination environment

Nolan et al.⁴⁶by linear correlation from ν(CO)_averagein

IrCl(CO)₂(XPhos).⁴⁹We note further that the TEPs obtained for

P(c-C₅H₉)₂Ar^Dtbp(L5), and P(c-C₆H₁₁)₂Ar^Tripp(i.e. XPhos) of 2060

2

and 2059 cm^-1, respectively, are only a few cm^-1higher than for

P(c-C₆H₁₁)₃(2056.4 cm^-1) and comparable to the 2060.6 cm^-1

figure that can be calculated for P(c-C₆H₁₁)₂Ph employing Tolman

substituent contributions.¹⁸The TEP measured in this work for

P(c-C₅H₉)₃is 2059 cm^-1.

The symmetric and antisymmetric ν(CO) stretchings found for

the formally three-coordinate dicarbonyl complexes 2·PR₂Ar’ can

be found in Table S1. Although non-carbonyl-containing, three-

coordinate, 16 valence-electron Ni(0) complexes have long been

known,⁵⁰analogous CO complexes are very rare and seem to be

limited to a few examples encompassing derivatives incorporating

strongly nucleophilic carbene ligands (see also reference 45). To

the best of our knowledge, truly three-coordinate, 16 valence-

electron Ni(CO)₂(PR₃) compounds are unknown, and those

reported herein constitute no exception, because X-ray studies to

be described next unambiguously demonstrate the existence in

compounds 2·PR₂Ar’ of weak Ni···C_areneinteractions implicating

one of the terphenyl flanking aromatic rings. This structural

peculiarity limits comparison of IR (CO) data with four-coordinate

Ni(CO)₂(PR₃)₂complexes, and also with the few known examples

of three-coordinate Ni(CO)₂(L) derivatives of carbene ligands.⁵¹

Notwithstanding these shortcomings, it is appropriate remarking

that for the 2·PR₂Ar’ complexes collected in Table S1, _sym

centres in the proximity of 1995 cm^-1and _asymat about 1923 cm^-

¹, values comparable to the 1990 and 1926 wavenumbers

characteristic of the four-coordinate, bis-PMe₃complex

Ni(CO)₂(PMe₃)₂. With due caution, given the different nature of

the compounds, the IR properties of complexes 2·PR₂Ar’ reflect

the high metal basicity of their [Ni(-P,²-C_arene-PR₂Ar’)] metal

fragment.

We have studied the solution dynamic behaviour of

2·PⁱPr₂Ar^Dtbp2by variable temperature NMR spectroscopy. Two

intramolecular rearrangements can be envisioned. First,

interchange of the two degenerate ²-structures, each involving

the ipso and one of the ortho carbon atoms of the Dtbp ring

engaged in nickel bonding, through a -P,¹-C_ipsotransition state

(see Scheme 4), is expected to be fast (only 1.7 kcal·mol^-1energy

barrier according to DFT calculations), generating an effective

plane of symmetry containing the terphenyl central ring and the

Ni-P bond. Second, since a relatively weak Ni-²-C_arenebonding

interaction is foreseeable (vide infra), temporary cleavage of the

Xyl₂

existing in 2·PⁱPr₂Ar^Dtbpand 2·P(c-C₅H₉)₂Ar

.

2

Table 2. IR Wavenumbers (cm^-1) for the Carbonyl Stretching Vibrations in

Ni(CO)₃(PR₃) complexes in CH₂Cl₂solution.

Ligand

L12

Complex



_CO(A₁)

_CO(E)

Ni(CO)₃(PMe₂Ar^Xyl2

)

2063.8

2063

1987

1988

1986

1985

1980^[b]

1980

1973

1971

1990

L13

Ni(CO)₃(PMe₂Ar^Mes2

Ni(CO)₃(PMe₂Ar^Dtbp2

Ni(CO)₃(PMe₂Ar^Dipp2

Ni(CO)₃(PMe₂Ar^Tripp2

Ni(CO)₃(PEt₂Ar^Dtbp2

Ni(CO)₃(P(c-C₆H₁₁)₂Ar^Dtbp2

)

L1

)

2063

L14

)

2062.9

2062

L15

)

L2

)

2061

L5

)

2060

XPhos

P(c-C₅H₉)₃

Ni(CO)₃(P(c-C₆H₁₁)₂Ar^Tripp

Ni(CO)₃(P(c-C₅H₉)₃)

Ni(CO)₃(P(c-C₆H₁₁)₃)^[a]

Ni(CO)₃(P^tBu₃)^[a]

)

2059

P(c-C₆H₁₁

)

3

2056.4

2056.1

2068.9

P^tBu₃

PPh₃

Ni(CO)₃(PPh₃)^[a]

[a] From ref. 18. [b] Partially obscured band.

Tolman evinced that the symmetric A₁mode of the carbonyl

ligands in Ni(CO)₃(PR₃) spanned the range 2056.1 (P^tBu₃) to

2011 cm^-1(PF₃), while the degenerate E modes were recorded

between 1971 (P^tBu₃) and 2016 cm^-1(PCl₂Ph) (the figure for PF₃

was not given).^35aAs represented in Table 2, the new

Ni-²-Dtbp linkage to form

a

truly three-coordinate

Ni(CO)₂(PⁱPr₂Ar^Dtbp) intermediate or transition state, followed by

rotation around the C_ipso-P bond would result in the shuffle of the

two Dtbp units.

2

Ni(CO)₃(PR₂Ar’)

complexes

featuring

a

coordinated

In the room temperature ¹H NMR spectrum of complex

dimethylterphenyl phosphine ligand are characterized by A₁

stretching vibrations in the narrow range 2062 to 2064 cm^-1, i.e. a

few wavenumbers below PMe₂Ph (2065.3 cm^-1) and even slightly

down the PMe₃value (2064.1 cm^-1). These data confirm the

electron-rich nature of PMe₂Ar’ ligands, i.e. their overall electron-

donor capacity. Similarly, for the Ni(CO)₃(PR₂Ar’) derivatives of

2·PⁱPr₂Ar^Dtbpthe two isopropyl groups of the phosphine ligand

2

are equivalent and originate a multiplet centred at 1.94 ppm (2 H,

2

3

doublet of septets, J_HP= 13.3, J_HH= 6.6 Hz) accompanied by

another, also well-defined multiplet spanning across the 0.92-0.84

3

ppm interval (6 H + 6 H; J_HP= 10.4, 9.4 Hz). In like manner, a

doublet ¹³C NMR resonance arises at 198.9 ppm (²J_CP= 10 Hz)

due to two equivalent carbonyl ligands. It is therefore evident that

the first of the aforementioned dynamic processes, namely,

interconversion of the two equivalent -P,²-C_arenestructures,

occurs in a swift manner under ambient conditions. In contrast,

the ^tBu protons of the two lateral rings remain inequivalent at room

temperature and are seen in the form of a broad resonance at

Dtbp₂

PEt₂Ar^Dtbp(L2) and P(c-C₆H₁₁)₂Ar

(L5), the A₁were registered

2

at 2061 for L2, and 2060 for L5. The above figures for L2 should

be compared with the 2063.7 and 1982 cm^-1stretchings found for

PEt₂Ph.¹⁸Regarding P(c-C₆H₁₁)₂Ar’ phosphines, it is noteworthy

that the TEP obtained in our work for Ni(CO)₃(P(c-C₆H₁₁)₂Ar^Tripp

)

of 2059 cm^-1compares well with that given above for L5, but it is

somewhat higher than the 2054 cm^-1value that was estimated by

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Scheme 4. Solution dynamic behaviour of 2·PiPr₂Ar^Dtbp²exchanging degenerate Ni-²-C_arenestructures.

around 1.39 ppm. Nevertheless, this signal resolves in two

singlets at 1.36 and 1.39 ppm (18 H, 18 H) on cooling at −30 °C.

As this change is not accompanied by noticeable variations of the

¹H NMR resonances of the –PⁱPr₂moiety, in all probability it can

be proposed to be due to significantly slowed down commuting of

the Dtbp rings, and hence to attainment at low temperatures of a

static structure in which the phosphine coordination mode is akin

to that found in the solid state. From the value of the rate constant

for exchange determined at the coalescence temperature of ca.

15 °C, an energy barrier ∆G^‡≈ 15 kcal·mol^-1can be estimated⁵²

for the flipping of the terphenyl flanking rings. DFT calculations on

this process are in accordance with this value (see SI).

only PMe₂Ar’ adopts a conformation alike A, inasmuch as

Dipp₂

PMe₂Ar^Xyland PMe₂Ar

prefer a structure of type B. In view of

2

the meagre differences in energy between conformations A and

B in the free phosphines, it seems plausible that to attenuate F-

strain between the B phosphine conformation and the Ni(CO)₃

fragment in the Ni(CO)₃(PMe₂Ar’) complex, conformation A

becomes favoured. As mentioned earlier, in conformation A the

phosphorus lone pair points towards an unoccupied region of

space, whereas B-type complexation would place the Ni(CO)₃

moiety relatively close to one of the flanking aryl rings of the

terphenyl substituent.

To attain unambiguous, definitive structural information on the

nickel carbonyl terphenyl phosphine complexes, 1·PR₂Ar’ and

2·PR₂Ar’, single crystals suitable for X-ray studies were grown for

some of these compounds. Concretely, the tricarbonyl derivatives

Dipp₂

of PMe₂Ar^Xyl, PMe₂Ar

and PMe₂Ar^Dtbp, as well as the

2

dicarbonyls Ni(CO)₂(PⁱPr₂Ar^Dtbp) and Ni(CO)₂(P(c-C₅H₉)₂Ar ),

were crystallographically characterized. For comparative

purposes, the structure of the known Ni(CO)₃(PPh₃),⁴⁸was

likewise determined (Figure S12).

Xyl₂

2

Figure 6. Molecular structure of tricarbonyl Ni(CO)₃(PMe₂Ar^Dtbp2

) (1·L1).

Selected bond lengths (Å) and angles (º): Ni-P, 2.2401(7); Ni-C(21), 1.797(3);

Ni-C(20), 1.782(3); P-C(1), 1.861(3); P-C(19), 1.832(2); C(19)-P-C(19), 97.2(2);

C(19)-P-C(1), 105.38(9).

As illustrated in Figures 6 and S12, the three tricarbonyls

investigated feature the expected, somewhat distorted tetrahedral

geometry around the metal centre. A conspicuous structural

feature is the common conformation of type A (Figure 3) adopted

by the phosphine in these complexes. As can be seen, not only

the two P-Me bonds occupy the same region of space relative to

the plane of the terphenyl central aryl ring, but moreover the –

PMe₂moiety is symmetrically disposed with respect to this ring

such that the two P-C_ipso-C_orthoangles are practically identical (ca.

121° in the three complexes). As free, non-coordinated molecules,

Figure 7. Molecular structures of dicarbonyls (Ni(CO)₂(PⁱPr₂Ar^Dtbp2) (2·L3) and

Ni(CO)₂(P(c-C₅H₉)₂Ar^Xyl2) (2·L7). Selected bond lengths (Å) and angles (º): 2·L3:

Ni-P, 2.2056(6); Ni-C(41), 1.763(3); Ni-C(42), 1.776(3); Ni-C(7), 2.449(2); Ni-

C(8), 2.332(2); P-C(1), 1.849(2); P-C(35), 1.863(3); P-C(38), 1.848(3); C(35)-P-

C(38), 106.1(2); C(35)-P-C(1), 104.0(1); C(38)-P-C(1), 104.5(1). 2·L7: Ni-P,

2.2037(8); Ni-C(33), 1.767(4); Ni-C(34), 1.770(4); Ni-C(7), 2.438(3); Ni-C(8),

2.414(3); P-C(1), 1.850(2); P-C(23), 1.845(3); P-C(28), 1.845(3); C(23)-P-C(28),

104.2(1); C(23)-P-C(1), 107.7(1); C(28)-P-C(1), 104.3(1).

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Dtbp₂

in Table 3. The Ni-P distances in 1·PMe₂Ar^Xyland 1·PMe₂Ar

2

A comparison of selected bond distances and angles for

compounds 1·PR₂Ar’ and related Ni(CO)₃(L) structures is given

are very similar (2.248(1) and 2.240(1) Å) and comparable to that

Table 3. Selected Structural Data for Ni(CO)₃(L) and Ni(CO)₂(L) complexes of tertiary phosphine and nucleophilic carbene ligands.

Ligand

L1

Complexes

Ni‒P (Å)

2.240

2.248

2.294

2.224

2.289

1.979

1.963

2.206

2.204

1.96^[d]

1.99^[d]

Ni‒CO (average, Å)

1.792

∑C‒P‒C (°)

308.0

S4’ (°)

38.0

31.5

24.4

37.5

12.9

Ni(CO)₃(PMe₂Ar^Dtbp2

)

L12

Ni(CO)₃(PMe₂Ar^Xyl2

)

1.780

311.1

L14

Ni(CO)₃(PMe₂Ar^Dipp2

Ni(CO)₃(PPh₃)

Ni(CO)₃(P^tBu₃)

Ni(CO)₃(IPr)

)

1.776

315.7

PPh₃

1.800

308.7

[a]

P^tBu₃

1.721

323.4

IPr^[b]

1.792

[c]

CAAC^Methyl

L3

Ni(CO)₃(CAAC^Methyl

)

1.800

Ni(CO)₂(PⁱPr₂Ar^Dtbp2

)

1.769

314.6

316.2

26.0

23.4

L7

Ni(CO)₂(P(c-C₅H₉)ⁱPr₂Ar^Xyl2

)

1.770

IAd^[a]

I^tBu^[b]

Ni(CO)₂(IAd)

1.760

Ni(CO)₂(I^tBu)

1.751

[e]

C(PPh₃)₂

Ni(CO)₂(C(PPh₃)₂)

1.746

[a] Ref. 44c. [b] Ref. 51b. [cb] Ref. 51c. [d] d(Ni-C_Carbene). [e] Ref. 51a

in Ni(CO)₃(PPh₃), the latter being equal to 2.235(1) Å. The most

13.0 for P^tBu₃. Clearly, the steric demands of our terphenyl

phosphines are significantly smaller than for P^tBu₃, and in

sterically demanding of the three phosphines, PMe₂Ar^Dipp, forms

2

comparison with PPh₃, PMe₂Ar^Dtbpis equivalent and PMe₂Ar^Xyl

2

the longest Ni-P bond in this series at 2.294(2) Å. Referring to Ni-

CO bond distances, which for the three complexes studied cluster

around 1.78 Å, it is hardly surprising that they are shorter than in

Ni(CO)₄(ca. 1.82 Å) and comparable to those in Ni(CO)₃(L)

complexes containing PPh₃, N-heterocyclic carbenes (NHC) or

cyclic (alkyl)(amino)carbenes (CAAC) ligands (Table 4).⁵¹

Probably, the length of the Ni-CO bonds in these complexes is

more a reflection of the sterics than of the electron density at the

Ni(0) centre. The C-O distances, with values of approximately

1.14 Å, are also unexceptional and of scant structural utility,

because, as stated by Cotton and Wilkinson,⁵³in the 2-3 bond

order range concerned, the CO bond length is relatively

insensitive to bond order.

and PMe₂Ar^Dippincreasingly larger.

2

As already noted, PR₂Ar’ phosphines containing the sterically

i

demanding Pr, c-C₅H₉and c-C₆H₁₁alkyl groups did not provide

isolable Ni(CO)₃(PR₂Ar’) complexes, although for P(c-

C₆H₁₁)₂Ar^Dtbpminor amounts of the tricarbonyl derivative

2

accompanied the major dicarbonyl reaction product. Once again,

it should be recalled that the very bulky PR₂Ar’ phosphines adopt

a structure of type C (Figure 3) with the phosphorus lone pair

facing the  system of one of the side aromatic rings, as found

earlier for dialkylbiaryl phosphines.⁴⁰Under these circumstances,

it is reasonable to surmise that F-strain between the phosphine

and Ni(CO)₃fragment destabilizes the purported Ni(CO)₃(PR₂Ar’)

complexes and promotes formation of dicarbonyls 2·PR₂Ar’ (see

For the three 1·PMe₂Ar’ complexes investigated, the sum of

the C-P-C bond angles is approximately 10° larger than for the

Supporting Information, p. 18).

free ligands and amounts to 315.7(3)° (PMe₂Ar^Dipp), 311.1(3)°

X-ray studies on complexes Ni(CO)₂(PⁱPr₂Ar^Dtbp

)

and

2

Dtbp₂

(PMe₂Ar^Xyl) and 308.0(2)° (PMe₂Ar

). The values of the

Ni(CO)₂(P(c-C₅H₉)₂Ar^Xyl

)

demonstrate that their seemingly

2

angular symmetric deformation coordinate S4’ for the phosphine

ligands in complexes 1·PR₂Ar’ are also included in Table 3.⁴²It is

worth recalling that PMe₃, PPh₃and P^tBu₃possess mean S4’

parameters of 46.5, 27.6 and 2.6°, respectively, and that S4’

values for a given phosphine may span a considerable wide

range.⁴²Consequently, it is best to compare series of related

unsaturated, three-coordinate structure corresponds in fact to

four-coordinated nickel centres, as in each complex there is a

relatively weak Ni-η²-C_areneinteraction with one of the Dtbp or Xyl

substituents (Figure 7). The molecules of these complexes have

in the solid state a very uncommon oblique triangular pyramidal

geometry in which one of the C_ipso-C_orthobonds of the proximal

lateral ring occupies the apex. The electronic interaction with the

flanking ring is undoubtedly weak,⁵⁴as it is characterized by fairly

complexes.

A

recent study based on trans-PdCl₂(PR₃)₂

complexes provided S4’ values for Buchwald-type dialkylbiaryl

phosphines, PR₂Ar^Ph, of ca. 28° (R = Me), 29.3° (Et), 29.8° (Ph)

long Ni···C_arenedistances. For example, in the Xyl-substituted

2

and 16.9 (c-C₆H₁₁).^29aCorresponding parameters for PMe₃,

PMe₂Ph and P^tBu₂Ph in these complexes were 35.3, 32.8 and

18.4°, respectively. As shown in Table 3, X-ray data for the

Ni(CO)₃(PR₃) complexes discussed in this work lead to S4’ values

Ni(CO)₂(P(c-C₅H₉)₂Ar^Xyl

)

complex, where the nickel atom

2

interacts with one of the C_ipso=C(Me) bonds of a lateral ring, the

two Ni-C bond distances are equal to ca. 2.44 (to C7) and 2.41 Å

(to C8), both standing well above the 1.97 Å value of the sum of

Xyl₂

Dipp₂

of 38.0° (PMe₂Ar^Dtbp), 31.5° (PMe₂Ar ) and 24.4° (PMe₂Ar

)

the covalent radii of C_sp(0.73 Å) and Ni (1.24 Å).⁴³Complex

2

Ni(CO)₂(PⁱPr₂Ar^Dtbp) possesses no substituents at the ortho

2

in what refers to terphenyl phosphines, and of 37.5° for PPh₃and

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carbon atoms of the Dtbp ring, which causes the Ni···C_orthoConclusions

distance (to C8, Figure 7) to be somewhat shorter at 2.332(2) Å,

while the other to C_ipso(C7) remains long at 2.449(2) Å. Yet, the

Ni···C_arenedistances in complexes 2·PR₂Ar’ are significantly

longer than in known Ni(0)-olefin complexes. For example,

Ni(cod)₂exhibits Ni-C bond lengths in the range 2.11-2.13 Å,^55a

and in Ni(C₂H₄)₂(PPh₃) the Ni-C distances are close to 2.0 Å.^55b

The weakness of the Ni-²-C_arenebond denoted by these metrics

is congruent with the facile solution exchange of the coordinated

and free Dtbp rings measured, as discussed earlier, by variable

The experiments and theoretical calculations described in

foregoing sections lead to the conclusion that dialkylterphenyl

phosphines, PR₂Ar’, adopt in the solid state one of the three

structures A, B or C, represented in Figure 3. For a given

phosphine, the structural choice is contingent upon the

importance of: (i) steric repulsions among the two R and the Ar’

phosphorus substituents, with a strong influence of alkyl groups

i

occupying the 2,6-positions of the lateral rings (Me or Pr in this

temperature ¹H NMR spectroscopy for Ni(CO)₂(PⁱPr₂Ar^Dtbp).

2

work), and (ii) electron-electron repulsions between the

phosphorus lone pair and the -system of the nearby ring. All the

PMe₂Ar’ phosphines presently known feature structures of type A

or B, for which energy differences appear to be small.

Conformation C is also readily accessible. On the contrary, the

bulkier PⁱPr₂Ar’, P(c-C₅H₉)₂Ar’ and P(c-C₆H₁₁)₂Ar’ phosphines

prefer a structure of type C, which is also the structure adopted

by the related, Buchwald-type, dialkylbiaryl phosphines.

Recall that the ∆G^‡value for exchange calculated from the rate

constant at the coalescence temperature of 15 °C is of ca. 15

kcal·mol^-1. It is thus clear that complexes 2·PR₂Ar’ may be viewed

as a source of unsaturated, three-coordinate, sixteen valence

electron species.

To complete our study devoted to nickel carbonyls containing

terphenyl phosphine ligands, we essayed their capacity to

undergo oxidative addition reactions.⁵¹Room temperature

The described analysis of the steric and electronic properties of

dialkylterphenyl phosphines leads to the extra conclusion of their

accentuated basicity, that is, high overall electron-donor capacity,

manifested, for instance, in the elevated metal basicity evinced for

Ni(0)-PR₂Ar’ units in the two phosphine coordination modes

present in Ni(CO)₃(PR₂Ar’) and Ni(CO)₂(PR₂Ar’) complexes.

In perspective, P-binding of PMe₂Ar’ ligands to transition metal

centres is expected on steric grounds, as no F-strain is

foreseeable to develop for their most favourable A-type

conformation. Nevertheless, when interacting with low-coordinate,

highly unsaturated ML_ngroups, phosphine polydentate,

hemilabile, -P,ⁿ-C_arenecoordination could readily be attained,

given that, as noted above, conformation C is easily accessible.

Conversely, for the bulkier PⁱPr₂Ar’, P(c-C₅H₉)₂Ar’ and P(c-

C₆H₁₁)₂Ar’, monodentate P-coordination can be predicted to

originate considerable F-strain, as a consequence of the close

proximity of ML_nto one of the terphenyl flanking rings, such that

this bonding mode is expected only in complexes of ML_n

fragments of reduced steric hindrance and favourable geometry,

e.g. M-L and ML₂, or planar ML₃. Included in the above are,

naturally, M(PR₂Ar’)⁺(M = Cu, Ag, Au) and M(PR₂Ar’)_n(M = Ni,

Pd; n = 1, 2) fragments, of well-known high catalytic relevance.

The anterior hypothesis finds support in the observation that even

addition

of

4-bromotoluene

)

to

Ni(CO)₃(PMe₂Ar^Dtbp),

2

Ni(CO)₃(PMe₂Ar^Dipp

and Ni(CO)₂(PⁱPr₂Ar^Dtbp

)

gave no

2

observable chemical changes, though upon heating at

temperatures around 70°C decomposition occurred with

formation of metallic nickel. Attempts to oxidatively add MeI to the

above dicarbonyls were also fruitless. Despite these failures, the

Dtbp₂

two tricarbonyls Ni(CO)₃(PMe₂Ar^Dtbp) and Ni(CO)₃(PEt₂Ar

)

2

experienced smooth room temperature reactions with 3-bromo-1-

propene (Scheme 5). These occurred with displacement of the

carbonyl ligands and formation of the Ni(II) ³-allyl compounds

Ni(η³-C₃H₅)Br(PR₂Ar^Dtbp), as the only organometallic products

2

(Scheme 5a). Characterization of the nickel allyls by elemental

analysis and NMR studies fully supports the proposed formulation

(see the Supporting Information). For instance, ³¹P{¹H} NMR

singlets appear with chemical shifts −2.5 (PMe₂Ar^Dtbp) and 23.2

2

ppm (PEt₂Ar^Dtbp), therefore with  values of ca. 34 and 36 ppm,

2

respectively, relative to the free phosphine ligands. The two

complexes were isolated as orange solids, soluble in C₆H₆and

other aromatic hydrocarbons. In contrast, the analogous reaction

of Ni(CO)₂(PⁱPr₂Ar^Dtbp) with allyl bromide resulted in the formation

2

of a bluish green solid, insoluble in benzene, but soluble in the

more polar acetone or acetonitrile solvents. These and other

properties suggest the salt-like formulation presented in Scheme

the relatively small Ni(CO)₃fragment, with local C₃symmetry

v

+

5b, based on the allyl phosphonium cation [P(C₃H₅)ⁱPr₂(Ar^Dtbp)]

2

when bonded to PR₂Ar’, cannot form the expected

Ni(CO)₃(PR₂Ar’) complexes. Instead, harsher reaction conditions

are necessary to force the dissociation of another carbon

monoxide ligand, to yield complexes Ni(CO)₂(PR₂Ar’). Further

work in support of these hypotheses is presently under way, along

with related research on the catalytic applications of G10 M(0) and

G11 M(I) complexes of dialkylterphenyl phosphines.



and a tribromonickelate anion, NiBr₃. This proposal finds support

in elemental analysis and ESMS, as well as in NMR spectra

recorded for the phosphonium cation. We did not consider



necessary the definitive structural characterization of the NiBr₃

anion.⁵⁶

Experimental Section

All preparations and manipulations were carried out under oxygen-free

nitrogen, using conventional Schlenk techniques and, when specified, at

low temperature. Solvents were rigorously dried and degassed before use.

Mg(Ar’)Br were prepared by following the synthesis reported by Power⁵⁷

for the related Ar^Xylsubstituted compounds without adding I₂in the last

2

58

step of the preparation. Ligands L11-L16²⁸and Ni(cod)₂

were

synthesized by following previously reported procedures. PCl₃was distilled

prior to use and kept under a nitrogen atmosphere. Other chemicals were

commercially available and used as received. Solution NMR spectra were

Scheme 5. Reactivity of the nickel carbonyl complexes towards oxidative

addition of allyl bromide.

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recorded on Bruker Avance DPX-300, Avance DRX-400, Avance DRX-

500, and 400 Ascend/R spectrometers. The ¹H and ¹³C resonances of the

solvent were used as the internal standard and the chemical shifts are

reported relative to TMS. Complete synthetic procedures and

characterization data for new compounds are provided in the Supporting

Information. A selection of representative syntheses of ligands and Ni(0)

complexes are reported below.

Acknowledgements

We thank MINECO (CTQ2013-42501-P, CTQ2016-75193-P

(AEI/FEDER, UE) and CTQ2017-82893-C2-2-R) for financial

support. MMG and JJM thank MINECO and the Universidad de

Sevilla (V Plan Propio de Investigación), respectively, for

research fellowships. We are grateful to María F. Espada and

Raquel J. Rama and for assistance with X-ray diffraction studies

of phosphines L7 (MFE), L8, L11 and L12 (RJR).

Dtbp₂

Synthesis of PMe₂Ar^Dtbp, L1. A freshly prepared solution Mg(Ar

)Br

2

(3.6 mmol) in THF (ca. 20 mL) was added dropwise to a solution of an

equimolar amount of PCl₃in THF (0.30 mL, 3.6 mmol) at 80 ºC. The

reaction mixture was allowed to reach slowly the room temperature and

stirred overnight. All volatiles were removed by evaporation under reduced

pressure and the solid residue was extracted three times with pentane (3

× 10 mL). The combined organic fractions were dried under vacuum giving

Keywords: phosphines • terphenyl • nickel carbonyls • Tolman

parameters

Dtbp₂

a mixture of the three dihalophosphines PCl2Ar^Dtbp, PBr2Ar

, and

2

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For some early reviews see for instance: a) Transition Metal Complexes

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2

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Synthesis of P(c-C₅H₉)₂Ar^Xyl, L6. A 0.50 M solution of Mg(c-C₅H₉)Br in

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from Et₂O/EtOH (ca. 1:2) at −32 °C. Yield: 1.4 g (58%).

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R groups control the coordination

mode of terphenyl phosphines in

Ni(CO)_n(PR₂Ar’) complexes. Steric

and electronic parameters of a series

of dialkylterphenyl phosphines, PR₂Ar’

have been determined

Mario Marín,^[b]Juan J. Moreno,^[a]Carlos

Navarro-Gilabert,^[a]Eleuterio Álvarez,^[a]

Celia Maya,^[a]Riccardo Peloso,^[a]M.

Carmen Nicasio,*^[b]Ernesto Carmona*^[a]

Page No. – Page No.

computationally and experimentally.

PR₂Ar’ units shows two different

coordination modes in

Ni(CO)₃(PR₂Ar’) and Ni(CO)₂(PR₂Ar’)

complexes that depend on the size of

the R groups.

Synthesis, Structure and Nickel

Carbonyl Complexes of

Dialkylterphenyl Phosphines

14

Article Doi

Synthesis, Structure and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines

DOI: 10.1002/chem.201803598

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