Cu(I)–phosphorus pincer chemistry

Chapter 6

Cu(I)
phosphorus pincer chemistry Jarl Ivar van der Vlugt Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Amsterdam, The Netherlands

Pincer chemistry with the coinage metals Cu, Ag, and Au is heavily underdeveloped compared to the chemistry of the Group 10 neighbors Ni
Pt and even more so when taking Group 8 and 9 metals into account. This caveat may originate from a preconceived notion that tetrahedral, trigonal planar or linear coordination geometries for Cu(I), Ag(I), and Au(I) do not combine very well with pincer donor sets that induce a meridional orientation and consequently either T-shaped trigonal or square planar geometries or even higher coordination numbers. Furthermore, Au(III)-pincer chemistry has only emerged in the last 5 years, despite the preferred square planar geometry of this d8 metal center. Notably, the pincer chemistry that is known with copper is largely dominated by tridentate nitrogen-based donor ligands, such as bis(dimethylamino) methylpyridine, bis(oxazolinyl)-pyridine, -phenyl, and -pyrrole, and derivatives thereof. These relatively hard donors preferentially bind the more Lewis acidic Cu(II), which has also led to a range of catalytic applications of these Cu
pincer complexes, including enantioselective transformations. Notwithstanding these developments, it is of academic and potentially applied interest to also explore the coordination chemistry of phosphinebased pincer ligands with copper. Given the soft donor characteristics of this main-group element, Cu(I) is likely the preferred oxidation state. This chapter will provide an overview of the recent literature on this topic and address the question whether phosphine-based pincer ligands can accommodate the specific preferred geometries of Cu(I). This review is not chronological in nature but instead organized around the various types of phosphine pincer

Copper(I) Chemistry of Phosphines, Functionalized Phosphines and Phosphorus Heterocycles. DOI: https://doi.org/10.1016/B978-0-12-815052-8.00006-3 © 2019 Elsevier Inc. All rights reserved.

145

146

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

ligand classes known to coordinate copper. Perhaps not surprisingly, copper complexes with tridentate coordination of the archetypical anionic carbonpivot, that is, the PCP-type ligands, are not known to date, likely because of the mismatch between the soft phosphine side arms and the need for cyclometalation at the carbon-pivot, which likely enforces Cu(II) coordination. The group of van der Vlugt has investigated the coordination chemistry of a potentially proton-responsive lutidine-derived PNP pincer ligand with Cu(I) in a series of publications. Initially, coordination of CuBr with 2,6-bis [(di-tert-butylphosphino)methyl]pyridine (PNPtBu) resulted in formation of yellow trigonal planar complex X1 with no evidence for any Cu
NPy interaction by either infra-red (IR) spectroscopy or X-ray diffraction (Scheme 6.1; Fig. 6.1) [1]. Strikingly, use of the diphenylphosphine derivative PNPPh provided yellow four-coordinated Cu(I) complex X2 with complete tridentate coordination of the PNP-pincer. Hence, steric hindrance likely inhibits formation of the four-coordinate analog of X1. Upon bromide abstraction using a Ag1-salt, the pyridine fragment enters the coordination sphere of Cu(I), resulting in the formation of light green T-shaped Cu(I) complexes X3 and X4. Both species seem to resist reaction or decomposition by air. Addition of PMe3 or tBuNC to X3 also did not yield any appreciable four-coordinate derivative, whereas this was readily achieved for X4, as evidenced inter alia by the molecular structure of the complex Cu(PNPPh)(PMe3) and the observation of a strong band at 2177 cm21 for the tBuNC-adduct. The T-shaped cationic center in both X3 and X4 is susceptible to reaction with strongly donating anionic ligands such as KSCN, resulting in the κ1-N-thiocyanato complex.

PPh 2 N

Cu

P tBu 2

PR 2 Br

CuBr R = Ph

PPh 2

CuBr N

R = tBu PR 2 R

X2

P NP

N

C u Br P t Bu 2 X1

AgSbF6

PPh 2 N Cu PPh 2 X4

SbF6

AgSbF6

P tBu 2 N Cu P tBu 2 X3

SCHEME 6.1 Synthesis of complexes X1
X4 using lutidine-based PNPR ligand.

SbF6

Cu(I)
phosphorus pincer chemistry Chapter | 6

147

FIGURE 6.1 Crystal structures of complexes X1 (top) and X3 (bottom).

Both the Cu-bromide X1 as well as the T-shaped complex X3 are susceptible to reaction with strong Brønsted bases such as KOtBu or NaN(SiMe3)2 to produce the very sensitive bright orange derivative X5, featuring a deprotonated methine-fragment in one of the side arms and a dearomatized pyridine ring, as suggested from nuclear magnetic resonance (NMR) and IR spectroscopic data (Scheme 6.2) [2]. The 31P NMR spectrum displays two inequivalent phosphines (JP
P 5 80 Hz). Although no structural information could be gathered for this compound, due to its high sensitivity coupled to high solubility in both polar and apolar solvent, including pentane, this species is proposed to be a mononuclear T-shaped species Cu(PNP ), with PNP denoting the deprotonated ligand scaffold. The highest occupied molecular orbital of this species is localized at the methine carbon, as revealed by DFT calculations. Rapid reprotonation of the ligand backbone occurs with acetic acid, resulting in the formation of κ1-O-acetato complex X6 (Scheme 6.3). This was confirmed by singlecrystal X-ray diffraction as well as by using monodeuterated acetic acid.

148

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles HOMO P tBu 2 N

Cu

Br

NaN(SiMe 3) 2 or KO tBu THF

P t Bu 2 N M P t Bu 2

P tBu 2 X1

X5

SCHEME 6.2 Generation of complex X5 with a dearomatized lutidine-based PNPR ligand and the density functional theory (DFT) calculated HOMO of this species. HOMO, Highest occupied molecular orbital.

+ OTf P tBu 2 N

Cu OAc P tBu 2

(D)H

X6

P tBu 2

P tBu 2 H(D)OAc Et 2O

N Cu P tBu 2 X5

MeOTf Et 2O

N Cu P tBu 2 X7

SCHEME 6.3 Synthesis of complexes X6 and X7 from dearomatized complex X5.

More interestingly, reaction with methyl triflate (MeOTf) as carbon electrophile led to selective mono-methylation at the methine position of the PNP fragment to form X7. This crystallographically characterized species (Fig. 6.2) is the first example of irreversible C
C bond formation on such a dearomatized ligand platform, which also inspired follow-up research on reversible C
C bond formation with CO2 on Re and Ru complexes by various groups [3]. This dearomatization reactivity to generate X5 also afforded entry into additive-free Cu(I)-based click catalysis using the, proposedly mononuclear, acetylide complex X8, which was also independently synthesized from free PNP ligand and commercially available [Cu(I)(CCPh)]. Complex X8 delivers 4-benzyl-1-phenyltriazole upon reaction with benzyl azide with cogeneration of an orange solution, supposedly containing X5 (Scheme 6.4). Hence, the catalytic applicability of X5 in the click-reaction was demonstrated at 1 mol% for both tridentate Cu(PNP ) and related bidentate Cu(PN) systems [4]. The latter were also exploited to investigate metal-ligand cooperative N
H bond activation [5]. Lastly, the reactivity of X5 with thiols led to an unexpected change of coordination mode of the tridentate PNP scaffold, with formation of dinuclear Cu(I) species X10 featuring a bridging thiolato ligand (either
SPh or
SCH2Ph) and a terminal bromido fragment (Scheme 6.5) [6]. An alternative precursor to

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149

FIGURE 6.2 Crystal structure of complex X7 with a monomethylated ligand backbone.

H

P tBu 2

N

Cu C

CPh

P tBu 2

PhC CH

N3

X8 P tBu 2

H

N Cu P tBu

N 2

P t Bu 2 Cu

Ph N N N

P t Bu 2 Ph

X5 H Ph

N N N

Ph

SCHEME 6.4 Preparation of complex X8 from X5 and proposed catalytic cycle for the 2 1 3polar cycloaddition of alkynes and azides using this species.

form this complex in a more targeted and stoichiometrically controlled fashion turned out to be the one-dimensional polymer X9 formed from the reaction between CuBr and PNPtBu in a 2:1 ratio (Fig. 6.3). In complex X10, copper center 1 has a P,N,S donor set, while copper center 2 has a P,S,Br donor set. This coordination behavior is highly unusual for any type of pincer platform. The bromido co-ligand in X10 can be substituted for an N-bound thioisocyanate unit via halide abstraction and subsequent reaction with KSCN to yield X11 (Scheme 6.6). Furthermore, ligand-centered deprotonation-dearomatization

150

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

Cu

PNP tBu

2 eq CuBr(SMe2) THF

P t Bu 2 N

t Bu

Cu P

t Bu

X9

2

Br Br

1) NaN(SiMe 3) 2 2) HSR THF-Et2O - HN(SiMe 3) 2 - NaBr

P

N

2

Br

Cu

Cu

SR

P

tBu

2

X10

SCHEME 6.5 Formation of 1D-polymer X9 and its conversion to dinuclear species X10 wherein the binding of the PNPR ligand deviates from traditional pincer coordination.

FIGURE 6.3 Crystal structures of 1D-polymer X9 (top) and two derivatives of complex X10 (with SR 5 SPh, bottom left; and R 5 SCH2Ph, bottom right).

151

Cu(I)
phosphorus pincer chemistry Chapter | 6 tBu

P

N

2

NCS

Cu

Cu

SR

P

tBu

2

tBu

1) AgSbF6 2) NaNCS THF - AgBr - NaSbF 6

P

N

2

Br

Cu

Cu P

tBu

2

SCH 2Ph

tBu

1) NaN(SiMe 3) 2 2) HSPh THF-Et2O - HN(SiMe 3) 2 - NaBr

N

2

SPh

Cu

Cu

SR

P

tBu

2

X12

X10

X11

P

SCHEME 6.6 Reactivity of dinuclear species X10 toward two-step substitution of the terminal bromido fragment.

N PPh 2

N PPh 2 CuBr

Br

N

N Cu

N PPh 2

N PPh 2

PN 3PPh

X13

SCHEME 6.7 Coordination of Cu(I) to the pincer ligand PN P cies X13.

3 Ph

to give four-coordinated spe-

followed by addition of one equivalent of thiophenol also initiates halide substitution to form di-thiolato complex X12. Strikingly, all attempts to furnish complexes of PNPtBu with Cu(II) resulted in ligand oxidation concomitant with reduction at copper. This may be symptomatic for the mismatch between hard cupric salts and the Lewis acidic Cu(II) ion and the soft donor set of many phosphine-based pincer ligands, as none of the other reports presented herein show bona fide pincertype coordination to Cu(II) with phosphine(s) as part of the donor set. Related to this work on lutidine-based PNP pincers, the group of Richeson has reported on the coordination of coinage metals to a diphosphine-pyridine pincer ligand based on 2,6-diaminopyridine, coined PN3PPh (Scheme 6.7), wherein both nitrogen linkers have been methylated (and thus can no longer act as potential proton-responsive entities [7]) [8]. Both phosphine donors are decorated with phenyl groups, which correlates with the ease of formation of four-coordinated Cu(I) complexes X13 with either bromide or triflate as coordinating co-ligand, similarly as observed for X2. Kirchner et al. have investigated the catalytic application of the very related air-stable complex CuBr(PN3PiPr) X14, that is, bearing isopropyl groups on phosphorus rather than phenyl rings, in palladium-free Cu-catalyzed C
C and C
N cross-coupling reactions (Scheme 6.8) [9]. The former included cross-couplings between aryl halides and alkyl triflate with phenyl and alkynyl Grignard reagents as well as alkynes, whilst the use of anilines was probed for C
N cross-coupling with aryl halides. Good to excellent

152

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

R 2-MgX N P iPr 2

N Cu

R2 R1 R2

Br X

N P iPr 2

H 2N

X14

R2

R1

R1 R2

NH R1 R2

SCHEME 6.8 Application of CuBr complex X14, featuring PN P as pincer ligand, in the cross-coupling of aryl halides with Grignard reagents, alkynes, and anilines. 3 iPr

H N PPh 2 CuX N

X = I, ClO 4

N PPh 2 H P NHNNHPP h

PPh 2

PPh 2

N

N

N H

N

H H

Cu

X

N N P H Ph 2

P Ph 2 X15X

SCHEME 6.9 Nonpincer coordination of ligand PNHNNHPPh to CuX (X 5 I or ClO4) to give bis(ligated) complexes X15X, featuring dangling phosphine arms.

yields toward the desired products were obtained at moderate to elevated temperatures (50 C
130 C), depending on the specifics of the reaction. In stark contrast to the observations made for the N-methylated PN3P ligand, the protonated analog PNHNNHPPh selectively forms a dinuclear Cu-iodide complex wherein both Cu(I) centers are four-coordinated with a tetrahedral geometry, as reported by Roesky et al. [10]. Changing the stoichiometry from 1:1 to 2:1 ligand-to-copper, the ionic homoleptic species X15I [Cu(κ2-P;N-PNP)2]I with two dangling aminophosphine arms was obtained quantitatively (Scheme 6.9). Both complexes were crystallographically characterized. The origin of this difference in coordination behaviors between the
NH and
NMe variants is not understood to date, but it may relate to the slight steric effect of the methyl groups, which pushes the two phosphine donors closer together. This work was later followed up by Ferraudi, Lemus, et al., who not only reported on the perchlorate analog X15ClO4, but also explored inner-sphere substitution by triphenylphosphine, providing complex X16, with tridentate

153

Cu(I)
phosphorus pincer chemistry Chapter | 6 H

ClO 4

N PPh 2 PPh 3

Cu

N

N PPh 2 H

PPh 2

PPh 2

N

N

N H

N

H H

Cu

ClO 4

PPh 2

ClO 4

N

H N

N

N N P H Ph 2

P Ph 2

X16

H

N

Cu N

P Ph 2 X17

X15ClO 4

ClO 4

N

N Cu PPh 2

Ph 2P N

N H

N

P Ph 2

H H

Cu

X18

N

N N P H Ph 2

SCHEME 6.10 Versatile bi- and tridentate coordination behavior of ligand PNHNNHPPh in complexes X16
X18 depending on the co-ligand present at Cu(I).

PNP binding, as well as by various phenanthroline bidentate ligands (Scheme 6.10) [11]. The latter led to either a mononuclear heteroleptic complex X17, with one free-dangling phosphine arm, or dinuclear coordination to form a helicate X18, depending on the substitution pattern at the phenanthroline ligand. The latter structure features a bis(P,N) binding mode to one Cu(I), with the other Cu(I) bound to two phosphine side arms and the phenanthroline ligand. The fluxionality in these systems was explored by variable temperature (VT) NMR spectroscopy and the electronic absorption and emission spectra of the mixed-ligand helicate Cu(I) complex (which does not contain true pincer ligation) were also detailed, both in solution and solid state. Lastly, cyclic voltammetry data revealed that reversible Cu(I/II) redox chemistry appears possible [12], particularly for the dinuclear species, which display rather flexible coordination geometry around Cu, enabling switching between tetrahedral and square planar coordination.

154

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

PPh 2 P Ph

N N

PPh 2 PPhNPhP

N

NPPhN

CuI

CuI

Ph P

N Ph 2P

Cu

N Cu

PPh 2

N I

I X20

X19 SCHEME 6.11 Coordination behavior of ligands PN NN P give X19 and X20. H

H Ph

and NPPhN toward Cu(I) to

As part of a larger study on the photochemistry of Cu complexes with P, N-ligand motifs, the group of Wang recently reported on a PPhNPhP and a NPPhN ligand platform, both of which are able to coordinate to Cu(I) iodide, providing access to complexes X19 and X20, as confirmed by X-ray crystallography (Scheme 6.11) [13]. Similar to observations made for the 2,6-lutidine-derived PNP ligand (see above), the bis(phosphine)pyridine ligand PPhNPhP displays no significant Cu
NPy σ interaction, whereas the NPPhN ligand does behave as a tridentate ligand with tetrahedral coordination to Cu(I). The backbones of both compounds are significantly distorted, preventing conjugation, which likely correlates with the low-emission quantum efficiencies measured during phosphorescence studies. Besides the use of common diaryl- or dialkylphosphines as side groups for pincer ligand designs, low-coordinate phosphorus donors have also been investigated for Cu coordination. Le Floch, Mezailles, et al. [14] investigated the coordination of a phosphinine-containing SPS ligand to amongst others Cu(I). The SPS ligand is obtained by sulfurization of the corresponding 2,6-bis(diphenylphosphino)-λ3-phosphinine platform [15]. This potential pincer ligand easily undergoes addition of strong nucleophiles such as BuLi because of the electrophilic character of the phosphinine phosphorus. This creates a rather unique anionic SPRS2 motif that undergoes coordination to simple Cu(I) salts. Initially, formation of an orange precipitate prevented

Cu(I)
phosphorus pincer chemistry Chapter | 6

155

structural information, but this compound did allow reaction with additional neutral donor ligands such as isocyanides, phosphines, phosphites, and pyridine to afford soluble mononuclear tetrahedral Cu(I) complexes X21L (Scheme 6.12). Crystallographic characterization confirmed the tridentate donation from the SPS platform. The bonding situation of this unusual platform was proposed to be intermediate between the two resonance forms possible, that is, an anionic ylide with a λ4-phosphinine or a P(III) phosphine with a delocalized anionic charge on its entire periphery (Scheme 6.13). The polymeric orange precipitate and the well-defined mononuclear pyridine adduct X21Py (pyridine being the weakest donor in the X21L series) were tested in the cyclopropanation of styrene with ethyl diazoacetate (EDA). No reaction with excess styrene was observed for either complex, but both species reacted readily when two equivalents of EDA were added to afford the crystallographically characterized copper-free λ5-phosphinine species X22. Mu¨ller et al. reported a four-coordinate Cu(I) complex X23 bearing a di (phosphininyl)pyridine ligand PNPine (Scheme 6.14) [16]. This new neutral ligand contains two strongly π-accepting flanking P donors and can be considered an analog of the archetypical terpyridine. The metal is not localized on the ideal axis of the phosphorus lone pairs, which results in a butterflytype overall geometry. No further reactivity of this species has been reported to date. Nakajima et al. investigated the binding of a bis(phosphaalkenyl-pyridine) ligand PNPMes, an interesting analog to PNPR, to CuBr, yielding complex X24 (Scheme 6.15) [17]. Reduced steric hindrance compared to PNPtBu

1/ 4

Ph 2P

P

S8

Li

BuLi

PPh 2

Ph 2P

P

PPh 2

S

Ph 2P

S Bu

S

PPh 2

PH

S

SPRSLi Ph 2P S

Bu

P

1/ 4

S8

EDA

L Ph 2P

PPh 2

S

S

Bu

P

PPh 2

Cu

S

SPRS-

Ph 2P n

Ph 2P

PPh 2 P S Bu CH 2 S

PPh 2

P

S Bu Cu

S

EtO 2C

L

X22

X21L

SCHEME 6.12 Generation of anionic ligand SPRS2 and follow-up coordination to Cu(I).

Ph 2P

S Me

P

PPh 2 S

Ph 2P

S Me

P

PPh 2 S

SCHEME 6.13 Ylide form (left) and delocalized form (right) of anionic ligand SPRS2.

156

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

P

P CuBr N

N

Cu

Br

P

P

PNPine

X23

SCHEME 6.14 Formation of CuBr complex X23, featuring bis(phosphinine)pyridine ligand PNPine.

Ph

Ph

PMes*

PMes* CuBr N

N

PNPMes

Cu

Br

PMes*

PMes* Ph

Ph

N Cu

AgSbF6

Ph

Ph

X24

benzene

AgPF6

FSbF 5

N Cu

Me 3SiN3 Ph

X25

Ph

Ph

SbF6

*MesP

N N N

PMes*

PMes* Ph

Ph PMes*

PMes*

Cu N

*MesP X27

Ph

PF 6

PMes*

toluene N Cu

*MesP F4 F P F Cu N

PMes* Ph

*MesP X26

Ph

SCHEME 6.15 Reactivity of CuBr complex X24, featuring phosphaalkene PNPMes, with “noncoordinating anions” acting as inner-sphere ligand to generate X25 or X26, depending on solvent and anion, and conversion into azido-species X27.

allowed for inner-sphere coordination of a bromide, similar to that observed for diphenylphosphine-appended PNP systems (see above). Abstraction of the bromide with AgSbF6 or AgPF6 in polar solvents led to the corresponding ionic complexes, which hence readily underwent reaction with acetonitrile, carbon monoxide, or tert-butyl isocyanide (not shown). The CO adduct produced a band at 2132 cm21, which is close to that of free gaseous CO. Also, the corresponding tBuNC adduct (ν 2198 cm21) bears testimony to the highly electron-deficient nature of this particular PNP platform. Strikingly, the noncoordinating PF6 ion (in CH2Cl2) was found to turn into an inner-sphere ligand in benzene, as determined from the change in 31P NMR chemical shift and from X-ray diffraction studies, which revealed formation of [Cu(PNP)(SbF6)] X25 with a short Cu
F bond. Even more surprisingly, crystallization from toluene led to a cationic PF6-bridged dimer X26 with four Cu
F interactions in total, based on X-ray data. Furthermore, reaction of the mononuclear cationic complexes with trimethylsilyl azide in a ration 1:1.5 led to a 1,3-azido-bridged dinuclear species X27.

Cu(I)
phosphorus pincer chemistry Chapter | 6 R

O N

Ph PMes

Ph

Mes P

CuOTf N

O PNNR

R

Cu

PMes 2L

R

N

Ph

OTf

N O

Ph N

TfO

P Cu Mes

N

N

157

Cu

L

N O

X28

OTf

R

X29L

SCHEME 6.16 Reactivity of hybrid ligand PNN to CuOTf to give complex X28 and subsequent reaction with strong donor ligands L to produce mononuclear species X29L. R

The same group later reported on the isolation of the ionic T-shaped analog, using ion-exchange with NaBArF wherein the BArF anion is too bulky to allow inner-sphere coordination [18]. Furthermore, exchange of the innersphere coordinated ions PF6 or SbF6 with trimethylsilylcyanide led to the corresponding cyanide adduct and fluoride abstraction from the fluorinated anion, with the generated XF6 fragment coordinating as Lewis acid toward the cyanide nitrogen. Gates et al. recently reported a hybrid PNNR ligand comprising a phosphaalkene, a pyridine, and an oxazoline fragment [19]. Initial coordination to copper(I) triflate resulted in a dinuclear complex X28 with κ1-P; κ2-N,N-binding of the ligand to accommodate two four-coordinated tetrahedral copper ions, with bound triflate (Scheme 6.16). This anion can be readily displaced using strong neutral donor ligands (phosphines, phosphites, nitrogen-based ligands) to afford the corresponding mononuclear ionic complexes X29L. Combined ultraviolet-visible (UV
vis) spectroscopy, temperature dependent (TD)-DFT calculations, and crystallographic data demonstrated that the π-accepting character of the phosphaalkene fragment can tune the Cu
P binding, depending on the electronic character of the co-ligand involved. Pincer ligands with aliphatic backbones have been known since the early days of the reports by Shaw et al. but have not received nearly the same extent of attention as aryl-based counterparts. This has changed in the last decade, with several discoveries that such systems, particularly when featuring a nitrogen as pivot, can facilitate very interesting catalytic applications [20]. In this context, Arnold et al. reported on the tetrahedral Cu(I) complex X30 with a flexible aliphatic neutral PNHPiPr ligand (Scheme 6.17), with the N
H and Cu
Br fragments oriented in a mutual syn-fashion [21]. No further exploration of the chemistry of this compound has been reported to date. The group of Thomas prepared a PPXP pincer ligand bearing a central chlorido- or iodidophosphine fragment that readily formed complexes X31Y in CH2Cl2 with CuCl or CuI with an intact P
X bond, as determined by

158

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles PiPr 2

PiPr 2 CuBr

H

NH

N Cu

Br

PiPr 2

PiPr 2 PNHPPr

X30

SCHEME 6.17 Formation of CuBr complex X30, featuring the aliphatic PNHPiPr ligand with a protonated nitrogen pivot.

N P Ph 2

P X

N

CuY

N

PP XP X = Cl or I

CuI, THF

THF

50oC

50oC

N

P

P Cu Ph 2

N

X P Cu P Ph 2 Ph 2 Y X31Y

CH 2Cl 2

P Ph 2

P

X = Y = Cl X = Cl, Y = I X= Y=I

N OR P Ph 2 I

X31O R = CH 2CH 2CH 2CH 2I SCHEME 6.18 Formation of Cu(I) complexes X31Y from the chlorophosphine-diphosphine PPXP and follow-up reactivity of X31I with THF to give X31O.

NMR spectroscopy and X-ray crystallography (Scheme 6.18) [22]. The analogous complexation reaction for PPIP in THF under mild heating or reaction of X31I with THF at 50 C resulted in ring opening of THF and formation of a new P
O bond (as well as a C
I bond) to give X31O. Formation of the phosphenium by abstraction of the halide using NaBPh4 to give PPP1 prior to reaction with Cu(I) in dichloromethane (DCM) at either room temperature or 50 C led to aryl transfer from the borate to the phosphenium phosphorus atom as well as coordination to Cu(I), giving rise to complex X31Ph (Scheme 6.19). Reaction of PPClP with Cu(Mes) led to redistribution of the halide (transfer to Cu) concomitant with alkylation of the phosphenium P atom, providing X31Mes. Based on related structural data available from the application of SNS pincer-type ligands [23], the group of Peters investigated the potentially tridentate interaction of both a PNHPiBu ligand based on diphenylamine as

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Cu(I)
phosphorus pincer chemistry Chapter | 6 BPh 4 N P Ph 2

P X

N P Ph 2

NaBPh 4

N

CH 2Cl 2

P

N P Ph 2

P Ph 2

PP X P

N

CuI

PPP +

N P Ph 2

P X

N

CuMes CH 2Cl 2

P Ph 2

PP XP

N

P

P

N

+ BPh 3

Ph P Cu P Ph 2 Ph 2 I X31Ph

CH 2Cl 2

N

Mes P Cu P Ph 2 Ph 2 Cl X31Mes

SCHEME 6.19 Formation of Cu(I) complexes X31Ar (Ar 5 Ph or Mes) via two methods.

PiBu 2

N H

PiBu 2

P

1) BuLi 2) CuBr(SMe 2)

PNHPiBu

P P [Cu(Mes)] 5

PiPr 2

P H

P iPr 2

PNHPiPr

Cu Cu

Cu

P

(BF 4) 2

N

E P

NOBF 4

P

for E = N

N

N P

X32E

Cu

P

X33

SCHEME 6.20 Formation of dinuclear Cu complexes X32E using potential pincer ligands PNHPiBu and PPHPiPr that act as bridging scaffolds and follow-up ligand-centered oxidation for X32N to give X33 featuring true pincer-type PNP coordination.

backbone [24] and the all-phosphorus PPHPiPr analog with copper [25]. In the case of PNHP, deprotonation of the ligand to generate the monoanionic fragment preceded coordination with CuBr(SMe2) to form X32N, whereas the PPHP analog was directly reacted with [Cu(Mes)]5 to give X32P (Scheme 6.20). These systems do not lead to tridentate pincer ligation but rather form well-defined dinuclear Cu(I)2 species wherein the pivotal anionic nitrogen or phosphorus acts as bridging ligand to both Cu(I) sites, forming diamond core complexes that are highly emissive. For the Cu-dimer with PPHPiPr ligands, the mixed-valent Cu(I)
Cu(II) as well as the all-cupric Cu (II)2 were easily accessible, both in solution and in solid state. Strikingly, oxidation of the Cu2 species bearing PNHPiBu ligands with NOBF4 led to ligand-centered oxidation to give X33, with a transition to true pincer-type PNP coordination, as deduced from inter alia preliminary X-ray data. Use of tert-butyl rather than iso-butyl as substituents at P allowed the isolation of the diamond core structures in three oxidation states [26]. Follow-up work focused on the electronic structure determination of these particular diamond core complexes [27]. Coordination of tripodal tetradentate PP3 ligands based on 3-methylindole and with either a P(N)3 or P(C)3 pivot [28] to CuCl led to pseudopincer ligation of two of the peripheral phosphines and the central P donor,

160

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

SCHEME 6.21 Formation of CuBr complexes X34 and X35, featuring tripodal tetradentate PP3 ligands. OTf

I

Cu

Cu PiPr 2

i 2 PrP

PiPr 2

P iPr 2

i 2 PrP

PiPr 2

CuI

AgOTf

X36OTf

X36I

P(C=C)P

AgPF6

i 2 PrP

Cu

PiPr 2

PF 6

X37

SCHEME 6.22 Formation of Cu complexes X36X and the cationic derivative X37 which features a P(CQC)P pincer donor set in the solid state.

leaving one phosphine side arm uncoordinated in complexes X34 and X35 (Scheme 6.21) [29]. These systems were shown to be competent catalysts for the cyclopropanation of styrene and EDA. The group of Iluc investigated the coordination chemistry of a potentially hemilabile pincer-like diphosphine ligand P(CQC)P featuring an olefinic backbone to inter alia Cu(I) [30]. Similarly to 2,6-lutidine-based PNP pincers, the olefin is not interacting when a coordinating anion, that is, iodide in the case of complex X36I, was bound to copper (Scheme 6.22). Abstraction of the halide using AgOTf led to triflate species X36OTf preferentially over interaction with the CQC bond. Upon reaction of X36I with AgPF6, the crystallographically characterized [Cu{P(CQC)P}] complex X37 was obtained. In solution, however, the olefin remains unbound, as deduced from NMR spectroscopic data and supported by DFT calculations. There are various examples where phosphorus-based fragments are present as a peripheral part of a pincer ligand but (1) not directly involved in binding to Cu or (2) binding to copper in a mode that parallels pincer binding to a different metal. These situations will not be discussed in detail, but representative examples have been reported by Auffrant et al. [31] and the

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group of Balakrishna [32], respectively. The former involved the nitrogenbased coordination of a bis(iminophosphorane)pyridine ligand to one or more Cu centers [either Cu(I) or Cu(II)], the latter the coordination of bis (cyclodiphosphazane)benzene to Pd with flanking phosphazane P donors bound to Cu(I).

Conclusions This short overview of the chemistry of copper with phosphine-containing pincer-type ligands demonstrates that (1) a variety of platforms are capable of accommodating nonplanar geometries that allow for Cu(I) binding, (2) Cu (II) binding appears to be virtually incompatible thus far with soft phosphine-based donors, although some exceptions have been established, albeit without true pincer-type binding, and (3) reactivity studies and catalytic applications of Cu complexes with most platforms are still lacking but deemed interesting, particularly also in the context of the growing field of metal-ligand bifunctional bond activation strategies.

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