Monomeric phosphinoboranes

Coordination Chemistry Reviews 297–298 (2015) 77–90

Contents lists available at ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Monomeric phosphinoboranes Jonathan A. Bailey, Paul G. Pringle ∗ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structures of phosphinoboranes and borylphosphines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Borylphosphine H2 P BH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Computational studies of H2 P BH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Coordination chemistry of H2 P BH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Chalcogenation of H2 P BH2 NMe3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Substituted phosphinoboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Substituent effects on the reactivity of phosphinoboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Synthesis of phosphinoboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Chemistry of phosphinoboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Lewis acid/base properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Pericyclic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Chalcogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Phosphinoboranes as ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. Group 6 complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Rhodium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3. Platinum complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 10 December 2014 Accepted 1 February 2015 Available online 11 February 2015 Keywords: Phosphorus Boron Phosphinoboranes Borylphosphines Ligand synthesis Homogeneous catalysis

a b s t r a c t The key developments in the chemistry of R2 PBR2 over the last 20 years are reviewed. The categorization of phosphinoboranes as R2 P BR2 species and borylphosphines as R2 P BR2 species is suggested based on an analysis of the literature X-ray crystal structures. The chemistry of derivatives of the simplest borylphosphine H2 P BH2 is discussed. Six methods of preparation of R2 PBR2 are presented along with the reactions of the products with H2 , amine–boranes, elemental chalcogens, Me3 NO and compounds featuring C O, C C and C N functionalities. The two modes of coordination of R2 PBR2 to transition metals, ␬1 (P) and ␩2 (P B), are covered, along with the applications of borylphosphine complexes in homogeneous catalysis. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Phosphinoboranes (R2 PBR2 ) contain trivalent and tricoordinate P and B atoms bonded together. They are valence isoelectronic

∗ Corresponding author. Tel.: +44 117 928 8114. E-mail address: [email protected] (P.G. Pringle). URL: http://pringle.blogs.ilrt.org/ (P.G. Pringle). http://dx.doi.org/10.1016/j.ccr.2015.02.001 0010-8545/© 2015 Elsevier B.V. All rights reserved.

77 78 80 80 80 81 81 81 82 82 82 83 84 86 86 86 88 89 89

with aminoboranes and alkenes and can be formulated containing a B P double bond with a planar P as depicted in the Lewis structures for A shown in Fig. 1. The term borylphosphine has been used interchangeably with phosphinoborane but is more appropriate for the alternative structure B shown in Fig. 1 containing a B P single bond with a pyramidal P. X-ray crystallography can be used to categorise the structures as phosphinoboranes (A-type) or borylphosphines (B-type) although there are structures which sit between these extremes. The stereoelectronic properties of the

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J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

R'

R P R

R

B R'

R'

R' P

R

B R'

R R

A

P

B R' B

Fig. 1. The two structural extremes of R2 PBR2 compounds.

Fig. 3. Optimised computed structure of H2 P BH2 .

substituents R and R determine the P B bond order (as evidenced by the P B bond lengths and geometry at P) and the chemistry of the P B bond (see below). The name phosphinoboranes is used in this review to denote A-type structures as well as the generic term for R2 PBR2 and borylphosphine is reserved for B-type structures. The chemistry of phosphinoboranes can be compared with that of their lighter congeners, the aminoboranes, which, along with the related amine–boranes R3 N · BR3 have attracted much recent attention due to their potential applications in hydrogen-storage [1–4] and as precursors for BN-oligomers, polymers [5,6] and materials such as BN-nanotubes [7–9] and graphene analogues [9–11]. Since the comprehensive 1995 review by Paine and Noeth [12] there have been no reviews of R2 PBR2 compounds per se but they have featured in the reviews by Power et al. on ␲-bonding between p-block elements [13,14] and the reviews by Manners et al. [6,15,16] and Weller et al. [17] which deal with phosphine–borane dehydrocoupling. This review will primarily concern the literature in the period 1995–2014, highlighting the recent developments in monomeric R2 PBR2 compounds; oligomeric and polyphosphinoboranes are outside of the scope of this review.

Calculations carried out in the early 1990s [20,21] suggested that the ␲-component of the P B in structures with planar phosphorus centres are similar in strengths to N B bonds. It was suggested that the reason the P B is less important is because of the strong preference for a pyramidalised phosphorus environment, as evidenced by the high barrier to inversion at P in tertiary phosphines [22]; calculations showed that the boryl group reduces this inversion barrier from 34.2 kcal/mol for PH3 to 5.9 kcal/mol for H2 BPH2 [20]. The conclusions from more recent calculations suggest that the ␲and ␴-bonding in the optimised H2 BPH2 structure are significantly weaker than in H2 BNH2 (see below, Section 3.1). Fu et al. [23] made a direct comparison of N B with P B ␲bonding in the structures of the boratobenzenes [C5 H5 B EPh2 ]− (E = P, N). In the diphenylamido compound, the N is planar (˙(angles at N) = 360◦ ) and the N B bond length is short ˚ whereas in the diphenylphosphido compound the P (1.510(10) A) is pyramidal (˙(angles at P) = 310◦ ) and the P B bond length is ˚ By tuning of the substituents at P and B, R2 PBR2 long (1.968(7) A). compounds with varying degrees of P B ␲-bond character have been obtained. Similar considerations have been expressed in discussing the bonding in silenes (R2 Si CR2 ) which are isoelectronic with phosphinoboranes (R2 P BR2 ) [24–26]. The P B ␲-bonding will be maximised by planar geometry at P and therefore the closer the sum of the angles at P is to 360◦ , the greater the potential P B bond order. As a result, it might be expected that the P B bond length and the sum of angles at P would be negatively correlated. The X-ray crystal structures of over 35 R2 PBR2 compounds have been determined and Table 1 contains structures reported post-1995; Noeth reviewed earlier crystal structures [12]. A plot of all of the R2 PBR2 structures is given in Fig. 2

2. Structures of phosphinoboranes and borylphosphines It is of interest to compare the P B bonding in R2 PBR2 with the N B bonding in R2 NBR2 . The greater covalent radius of P than N naturally leads to P B bonds being longer than their N B counterparts. Thus, the P B bond length of 1.762(4) A˚ in Cy2 P B(C6 F5 )2 [18], which is the shortest recorded P B bond in a phosphinoborane, is much longer than the 1.372(2) A˚ in Me2 N B(C6 F5 )2 [19]. A defining characteristic of aminoboranes is the strong N B double bond.

26b AC

360

23a

23b

D 19f

B 26c

350

Sum of angles around P / °

E

26d 19c 19e G 26a 19b

340

19d

‘Phosphinoborane’

330

F

24a

320

38 19a

310

290 280

22a 36a H 3

A Mes2BP(Mes)Li(Et2O)2 B Mes2BP(1-Ad)Li(Et2O)2 C Mes2BP(Cy)Li(Et2O)2 D Mes2BP(SiMe3)Li(THF)3 E Trip2BP(tBu)Li(Et2O)2 F (Mes2B)2PPh G (Mes(Cl)B)2PMes H Tmp(Cl)BP(H)Mes

300

37 34

33b

‘Borylphosphine’

35b 35a

33a

270 1.74

1.76

1.78

1.80

1.82

1.84

1.86

1.88

1.90

1.92

1.94

1.96

P-B bond length / Å ˚ Grey horizontal bars represent ± (3× esd). The geometrical data for structures A–H Fig. 2. Plot of the sum of the angles around phosphorus (◦ ) against the P B bond length (A). and 19a–f were taken from reference [12]. The data for all other complexes can be found in Table 1. Mes = 2,4,6-Me3 C6 H2 , 1-Ad = 1-adamantyl, Trip = 2,4,6-[(CH3 )2 CH]3 C6 H2 , Tmp = 2,2,6,6-tetramethylpiperidino.

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Table 1 Crystallographic data for phosphinoboranes. Mes = 2,4,6-Me3 C6 H2 , DMP = 2,6-dimethylphenyl, Dipp = 2,6-diisopropylphenyl. Compound

No.

P B/A˚

Sum of angles at P/◦

Ref.

No.

P B/A˚

Sum of angles at P/◦

Ref.

22a

1.9483(9)

314.9

[47]

33a

1.923(2)

284

[60]

23a

1.762(4)

359.8

[18]

33b

1.931(3)

299

[60]

23b

1.786(4)

359.1

[54]

34

1.9303(15)

317.3

[60]

24a

1.889(3)

328.3

[53]

35a

1.951(2)

292.1

[60]

26a

1.835(2)

350.4

[55]

35b

1.953(2)

293.8

[60]

26b

1.833(2)

357.6

[55]

36a

1.9448(18)

307.6

[63]

26c

1.836(2)

356.8

[55]

37

1.917(4), 1.927(3)

316.7

[63]

26d

1.857(2)

359.9

[55]

38

1.9080(19)

322.2

[63]

Structure

which shows that there is indeed a negative correlation between ˚ and the angles at P (spanP B bond distances (which span ∼0.2 A) ning ∼80◦ ). It is no surprise that the correlation is fallible given that the plotted parameters will be governed by steric effects and crystal packing in addition to P B ␲-bonding character. The plot has been divided into quadrants by lines drawn arbitrarily at 1.88 A˚ and 330◦ and it is clear that the data points then fall into the upper left and lower right quadrants. These regions have been denoted phosphinoborane and borylphosphine with the appreciation that these descriptions are the extremes of a continuum.

The strength of the P B ␲-interaction in R2 PBR2 is a function of the Lewis basicity of the P and the Lewis acidity of the B which are in turn controlled by the R and R substituents. When the B-substituents are NR2 , the strong ␲-interactions between B and N (due to good orbital size matching) essentially nullifies the ␲-component of the B P bond, making such species borylphosphine-like. In contradistinction, the P B ␲-bonding will be maximised when the B-substituents are electron-withdrawing alkyl or aryl groups and the P-substituents are electron-releasing alkyl groups, making such species phosphinoborane-like.

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Scheme 1. Synthesis and reactivity of 1.

with Cs symmetry was lower in energy than the P-planar geometry with C2v symmetry by 5.9 kcal/mol. More recent computational work by Scheer et al. at the DFT-B3LYP level [30] gave an optimised structure with a P B bond length of 1.873 A˚ and a B P H angle of 105.1◦ (Fig. 3), i.e. a pyramidalized-P having a localised lone pair, consistent with the ‘borylphosphine’ denomination (Btype structure in Fig. 1). This contrasts with the structure of gaseous H2 BNH2 which has a planar-N structure (analogous to the A-type structure in Fig. 1) with a B N bond length (calculated from its ˚ consistent with a B N double microwave spectrum) of 1.391 A, bond [31]. The calculated ␴- and ␲-bond energies for H2 BPH2 (77.6 and 9.2 kcal/mol) are significantly lower than the values for H2 BNH2 (109.8 and 29.9 kcal/mol) [32].

Scheme 2. Main group Lewis acid/base stabilisation of H2 BPH2 .

3. Borylphosphine H2 P BH2 3.2. Coordination chemistry of H2 P BH2

The parent phosphinoborane, H2 BPH2 , has eluded experimental isolation due to its ready polymerisation to [H2 BPH2 ]n but some of its derivatives have been isolated. It has been the subject of computational studies that have shed light on the differences between H2 BPH2 and H2 BNH2 .

Scheer et al. [30] reported the synthesis of [(OC)5 W(PH2 BH2 NMe3 )] (1), a Lewis acid/base-stabilised derivative of H2 BPH2 by the salt elimination reaction shown in Scheme 1. The crystal ˚ as expected structure of 1 revealed a P B bond length of 1.955(4) A, for the P B single bond present. The analogous chromium complex [(OC)5 Cr(PH2 BH2 NMe3 )] was subsequently reported [33]. Complex 1 undergoes dehydrogenation/decarbonylation upon photolysis to give 2, P H oxidative addition to Pt(0) to give 3 and 4 [34] and oxidation by elemental bromine to give W(II) species 5 and 6 [35].

3.1. Computational studies of H2 P BH2 In the early 1990s, the energies and conformations of the ground states and transition states for P B rotation of H2 BPH2 were calculated [20,27–29]. It was found that the P-pyramidal geometry

H2 P BH2(NMe3 ) W(CO)5

+

hν

P(OMe)3

H2 P BH2 (NMe3)

– W{(P(OMe)3}n(CO)6-n

1

8

H (Me3N)BH2 P

Me3 Si

PH TiCp2

HB

8

Fe(CO)4

PH2 BH2NMe3

10

Cp2 Ti H P

H2 B H P (Me3N)BH2 HP 14

H2P BH2 (NMe3)

SiMe 3

+

Cp2 Ti

Cp2Ti

CuI

Cp2Ti

BH PH

13

Fe2 (CO)9

Cp2Ti(btmsa)

Cp2 Ti

BH H

P H

H

TiCp2

BH PH Ti Cp2

PH B H2

L BH2(NMe3 )

Cu Cu L

I

L

I Cu L I

+

I 11

Scheme 3. Synthesis and coordination chemistry of 8.

(L = 8)

L

L

Cu

Cu

Cu

Cu I I 12

I

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81

Fig. 4. Crystal structure of H2 PBH2 NMe3 (8). C H hydrogen atoms are omitted for ˚ clarity. P B 1.976(2) A.

H2 P BH2

AlCl3 , ClBH2 NMe3

Me3N

or IBH2NMe3 8

NMe3

B H2

H2 P

B H2

NMe3

[X]-

15 X = AlCl4 or I 0.5 IBH2SMe2 or [VCl3(thf)3 ]

Me3 N

B H2

H2 P

B H2

H2 P

B H2

Fig. 6. Crystal structure of 38. All hydrogen atoms are omitted for clarity. P B ˚ ˙(angles at P) = 322.2◦ . 1.9080(19) A,

NMe3

[X]-

16 X = I or VCl4 (thf)2 Scheme 4. Off-metal oligomerisation of 8.

The phosphonic acid 18a was obtained upon reaction of 8 with an excess of O2 and was crystallographically characterised. The sulphur and selenium analogues 18b,c were proposed to be the products (on the basis of their 31 P NMR spectra) of the reactions of an excess of chalcogen with 8. Similar computational and experimental work to that described in Sections 3.1–3.3 has been reported with the related H2 P AlH2 and H2 As BH2 species [33,42,43].

4. Substituted phosphinoboranes 4.1. Substituent effects on the reactivity of phosphinoboranes Scheme 5. Chalcogenation reactions of 8.

Borylphosphine is also stabilised cooperatively by main group Lewis acid/base pairs such as 7 and 9 prepared by the routes shown in Scheme 2 [36,37]. The amine-stabilized species PH2 BH2 NMe3 (8) was first isolated in 2006 by displacement from 1 by P(OMe)3 [38] and its crystal structure determined (Fig. 4). It was then coordinated to Fe [38], Cu [39], or Ti [40] to produce the complexes 10–14 shown in Scheme 3. The formation of the Ti complexes 13 and 14 involves an on-metal oligomerisation (Scheme 3), providing experimental support for mechanisms proposed for early transition metal-mediated polymerisation of phosphinoboranes [40]. Off-metal oligomerisation to give the first examples of mixed Group 13/15 cationic chains (15 and 16) was achieved upon reaction of PH2 BH2 NMe3 with monohaloboranes (Scheme 4) [41]. 3.3. Chalcogenation of H2 P BH2 NMe3 The reactions of H2 PBH2 NMe3 (8), with chalcogens are summarised in Scheme 5. When 8 was reacted with 1 equivalent of elemental sulphur or selenium, the P(V) species 17b,c formed quantitatively [38]. It was suggested, on the basis of 31 P NMR spectroscopy, that the oxygen analogue 17a was formed in the reaction of H2 BPH2 NMe3 with Me3 NO (TMAO).

The juxtaposition of the Lewis basic and Lewis acidic sites explains the high reactivity of R2 PBR2 (see below) but this can be controlled by manipulation of the steric and electronic environment at the P and B via the R and R substituents. Power et al. [44] used bulky groups on P and B to protect the phosphinoborane from reactions such as head to tail dimerization (see below); they have prepared the series of phosphinoboranes 19a–f and have structurally characterised all of them (Fig. 5). The Lewis acidity of the B site can be suppressed by using substituents that can delocalize electron density into the vacant p-orbital on the B. This has been achieved in several ways: (i) NR2 substituents on B as exemplified by 20 [45], (ii) incorporating the B into a polyaromatic hydrocarbon as in 21a [46], and (iii) incorporating the B into a polyhedral carborane as in 22a [47]. A combination of steric and electronic substituent effects can explain the stability of Cy2 P B(C6 F5 ) (23a) [18]. The Cy group is electron-donating and C6 F5 is electron-withdrawing and both groups are bulky. Thus these substituents enhance the P B double bond character which partly quenches the Lewis acid/base properties as well as provide steric protection. The rigidity of a 1,8-naphthalene backbone has been exploited as a way of stabilising a wide range of bonds between main group elements by holding them in close proximity [48–52]. Thus phosphinoboranes 24a in which the P and B are in the 1,8-positions have been shown to be kinetically stable [53].

Fig. 5. Examples of the different ways of stabilising monomeric phosphinoboranes. 1-Ad = 1-adamantyl, Mes = 2,4,6-Me3 C6 H2 .

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bromoborane or from the nucleophilic boryllithium species [61,62] as shown in Scheme 9. (iii) Me3 SiCl elimination Borylphosphines 21 and 36 featuring aromatic BN-containing heterocycles have been prepared by the reactions of chloroboranes with silylphosphines (Scheme 10) [46,63]. The efficiency of this route under mild conditions has been associated with the aromaticity of the boryl fragment. The route has been extended to the synthesis of the diborylphosphine 37 and the first structurally characterised triborylphosphine 38 (Fig. 6). (iv) Pd-catalysed cross-coupling The series of borylphosphines 22a–e in which the B is part of another aromatic system, meta-carborane, has been prepared by a Pd-catalysed P B coupling reaction as shown in Scheme 11 [47]. (v) Reductive 1,2-aryl migration A route specific for the preparation of borylphosphines such as 24, based on a 1,8-naphthalene framework, is shown in Scheme 12. This features a reductive aryl migration from B to P [53]. 4.3. Chemistry of phosphinoboranes

Scheme 6. Different synthetic methods for the preparation of monomeric phosphinoboranes.

4.2. Synthesis of phosphinoboranes The methods used for the synthesis of phosphinoboranes are summarised in Scheme 6 and are reviewed in sequence below. Route (i) is the original and most commonly used method for the synthesis of phosphinoboranes (and borylphosphines) [12]. Routes (ii)–(v) are less general methods that have been developed over the last 20 years for the preparation of borylphosphines. Dehydrocoupling of phosphine–borane adducts (Scheme 7) is an important route to oligo- and polyphosphinoboranes [6,15–17] but to date, has not been applied to the preparation of monomeric phosphinoboranes. (i) P-nucleophile The salt elimination reaction between a lithium phosphide and haloborane is by far the most widely used method for the preparation of R2 PBR2 compounds (see Scheme 8). It has been applied to the synthesis of the bulky phosphinoboranes 23a–c [18,54], 25 [26] and 26a–d [55] whose properties and reactivity have been extensively studied (see Section 4.3) as well as the borylphosphines 27 [56], 28 [57], 30 [58] and 31a–c and 32 [59] which have been used as ligands (see Section 4.4). (ii) B-nucleophile Gudat et al. [60] showed that borylphosphines 33–35 which are diazaborole derivatives can be made from the electrophilic

-H2

H3B

R2 P

H3B•PR1R2H -H2

H2 B R2

B H2 R1 P H

PR2H

4.3.1. Lewis acid/base properties The intrinsic Lewis acid/base properties of phosphinoboranes are evident from their chemistry, such as their well-known sensitivity to hydrolysis. For example, the 1,8-naphthalene phosphinoborane 24 adds H OH across the P B bond to give 39 (Scheme 13) [53]. Head-to-tail dimerization is a common reaction of less bulky phosphinoboranes and is also a manifestation of their acid/base properties. For example, when the substituents on phosphorus in phosphinoboranes 23 are small (Et, Ph), dimerisation occurs (Scheme 14) [18]. The planar species Cy2 P B(C6 F5 )2 (23a) reacts with Lewis acids and bases (Scheme 15) [18]. Thus treatment of 23a with the 4tert-butylpyridine afforded the acid–base adduct 40 in which the quaternary boron results in the P centre losing its planarity and any P B ␲-overlap. Reaction of 23a with BCl3 afforded the dimer 41 with loss of ClB(C6 F5 )2 presumably via an initially formed Lewis adduct between the BCl3 and the lone pair on the P in 23a. Since the seminal report by Stephan [64] on the reversible activation of dihydrogen using a Lewis acid/base pair with remote P and B centres, the topic of ‘Frustrated Lewis Pairs’ (FLPs) has blossomed. FLP chemistry has been the subject of several reviews [65–67] as well as a two-volume book [68,69]. The first reported use of phosphinoboranes of the type R2 PB(C6 F5 )2 in FLP-like small molecule activation came in 2008 [54]. As mentioned previously, with small substituents on phosphorus (Et, Ph), the phosphinoboranes dimerize to [R2 PB(C6 F5 )2 ]2 (23d,e) and these are inactive for H2 activation [18]. Their bulkier analogues (R = Cy, t Bu) are monomeric R2 PB(C6 F5 )2 (23a,b) and they irreversibly and heterolytically split H2 at 60 ◦ C over 48 h to give phosphine–borane

+

H2B R2P

R2 P

B H2

BH2 PR2

+

n

=H

Scheme 7. Dehydrocoupling of phosphine–borane adducts.

R2P H2B

BH2 PR2

R2P H2B

BH2 PR2

J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

83

Scheme 8. Synthesis of phosphinoboranes by salt elimination of a lithium phosphide LiPR2 and haloborane XBR2 .

adducts 42 (Scheme 16) [54]. The same phosphinoboranes 23 dehydrogenate the amine–boranes R2 NH·BH3 (R = H or Me) to form 42 and the dehydrocoupling products [H2 N BH2 ]n or [Me2 N BH2 ]2 [18]. The phosphine–borane adducts exhibit much longer P B

Dipp N B Br N Dipp MPR1R2

LiC10H8

ClP(NMe2)2

Dipp N B PR1R2 N Dipp 33a R1 = R2 = H 33b R1 = H; R2 = Ph 33c R1 = R2 = Ph

Dipp NMe2 N B P N NMe2 Dipp 34

Dipp N B Li(THF)2 N Dipp R N Cl P N R Dipp R N N B P N N Dipp R 35a R = tBu 35b R = DMP

Scheme 9. Synthesis of phosphinoboranes 33–35 by salt elimination of a lithium boryl species LiBR2 and chlorophosphines XPR2 . DMP = 2,6-dimethylphenyl, Dipp = 2,6-diisopropylphenyl.

˚ and highly pyramidalised phosbond lengths (42b: P B 1.966(9) A) phorus centres compared to the phosphinoboranes (23b: P B ˚ ˙(angles at P) = 359.1◦ , see Fig. 7). Other computational 1.786(4) A, work [70] has suggested that phosphinoboranes should dehydrogenate alcohols to ketones and aldehydes. The antiaromatic nature of dibenzoborole [71–74] led to the prediction of high reactivity for the phosphinoborane 25 and indeed it reacts rapidly with 1 atm H2 to give phosphine–borane 43 (Scheme 17) [26]. 4.3.2. Pericyclic reactions In 2007, a computational study suggested that phosphinoboranes of the type t Bu2 P B(CF3 )2 should undergo pericyclic reactions with alkenes ([2 + 2] cycloadditions) and dienes ([4 + 2] cycloadditions) [75]. In 2013, it was shown that the phosphinoborane 25, featuring the dibenzoborole group, undergoes cycloaddition reactions with molecules containing C O, C C or C N bonds (Scheme 18) [26]. With benzophenone, product 44 was obtained whose crystal structure and NMR data confirmed the P C O B linkage in the [2 + 2] cycloaddition product. With 2,3-dimethylbutadiene, [4 + 2] cycloaddition of 25 produced the heterocycle 45 which was structurally characterised. Addition of 25 to acetonitrile gave the addition product 46.

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Scheme 10. Synthesis of borylphosphines 21 and 36–38 via Me3 SiCl elimination.

Ar2B

Ar

PCl2 Mg

B

P

Ar

24a Ar = Mes 24b Ar = DMP Scheme 12. Synthesis of phosphinoboranes 24a,b by reductive 1,8-aryl migration. Mes = 2,4,6-Me3 C6 H2 , DMP = 2,6-dimethylphenyl.

Scheme 13. Hydrolysis of 24a to form 39.

˚ Fig. 7. Crystal structures of 23b (top) and 42b (bottom). 23b: P B 1.786(4) A, ˚ no ˙(angles at P) is given due to ˙(angles at P) = 359.07◦ . 42b: P B 1.966(9) A, the P centre being 4-coordinate and pseudotetrahedral.

HC

CH

I2/AlCl 3 HC

CH B I Pd/L, HPR2 Cs2CO3

Scheme 14. Synthesis of monomeric (23a–c) and dimeric (23d–e) phosphinoboranes.

R HC

CH B PR2 22

a Ph b Cy c Cyp d iPr e iBu

Scheme 11. Synthesis of phosphinoboranes 22a–e by Pd-catalysed cross-coupling.

4.3.3. Chalcogenation (i) Sulfuration and selenation Nöth showed that phosphinoborane 47 reacts with elemental sulfur to give 48 which contains two inserted S atoms (Scheme 19) [76]. Treatment of the 1,8-substituted naphthalene 24a [53] with Se or S8 gave the heterocycles 49 and 50 in which a

J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

C6F5 Cy B P Cy C6F5

BCl3

Cl2B PCy2 Cy2P BCl2

23a

B P

41

O Ph

N

t

85

t

Bu

t

Bu

25

Ph

C N

Bu t

t

Bu

Bu t P Bu

B

t

Bu2P

O C

Ph Ph

N (C6F5)2B PCy2

44

N

t

Bu t B P Bu

B

B N

40 t

Scheme 15. Reactivity of 23a with 4-tert-butylpyridine and BCl3 .

45

Bu2P 46

Scheme 18. Pericyclic reactions of 25 with benzophenone, 2,3-dimethylbutadiene and acetonitrile.

Scheme 16. Hydrogenation of 23a,b by H2 , H3 NBH3 and Me2 NBH3 . Scheme 19. Sulfurisation of 47. tBu

B P

H2

H B

H P

t Bu Bu

t

Bu

25

t

43

Scheme 17. H2 activation by 25.

single chalcogen atom has been inserted into the P B bond (Scheme 20) [77]. Reaction with another equivalent of chalcogen gave the species 51 and 53 featuring P(V) centres. A mixed S/Se product 52 was also obtained upon sequential treatment of 24 with S followed by Se. The diselenium product 51 shows fluxionality in solution involving intramolecular interchange of terminal and bridging Se atoms.

Scheme 20. Chalcogenation reactions of 24a.

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J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90 Table 2 IR data for complexes 57–59 and calculated Tolman electronic parameters.

Scheme 21. Oxygenation of 54.

L

(CO) (A1 )/cm−1 cis-[MoL2 (CO)4 ]

(CO) (A1 )/cm−1 [NiL(CO)3 ] (calc)

PPh3 36a 37 38

2022 [80] 2018.1 (57) 2015.8 (58) 2012.8 (59)

2068.9 [81] 2067.7 2066.4 2064.6

Scheme 22. Oxygen insertion into 36b to form 56. Scheme 24. Synthesis of 60.

(ii) Oxygenation Nöth suggested, based on IR spectroscopy, that treatment of Et2 P B(NMe2 )2 (54) with O2 gave the B O P O species 55 [78] (Scheme 21). The insertion of a single O into P B bonds was achieved to give 56 upon treatment of borylphosphine 36b with one equivalent of TMAO (Scheme 22) [79]. The large change in 31 P NMR chemical shift upon oxygenation of 36b (from −48 ppm for 36b to 135 ppm for 56) is consistent with the structure of 56 and this was confirmed by the crystal structures of its rhodium(I) complexes (see Section 4.4.2). 4.4. Phosphinoboranes as ligands If the bonding in the phosphinoborane approximates to the borylphosphine structure (B in Fig. 1) then the electropositive boron should result in an electron-rich, strongly ␴-donating Pligand. If however the bonding approximates to structure A in Fig. 1, then side-on bonding with the P B would be predicted. Both types of coordination have been observed as discussed below. The complexes formed by H2 PBH2 and H2 PBH2 (NMe3 ) were discussed in Section 3.2. 4.4.1. Group 6 complexes The early phosphinoborane coordination chemistry with transition metals was mainly with Group 6 metal carbonyls complexes and has been previously reviewed [12]. The complexes cis-[Mo(L)2 (CO)4 ] (57–59) were prepared according to Scheme 23 in order to probe the donor properties of the borylphosphine analogues of triphenylphosphine [63]. The IR spectra showed that upon sequential replacement of phenyl rings with the aromatic 1,8-diaminonaphthylboronamide heterocycle, the electron-donating ability of the ligands increased. Using Crabtree’s formula [80] for converting the A1 bands in cis-[MoL2 (CO)4 ] to those for [NiL(CO)3 ] (in order to compare with the Tolman electronic parameters for related ligands), it was found that 38 has electronic properties similar to PMe3 (see Table 2). The phosphinoborane 30 displaces NMe3 from [Cr(CO)5 (NMe3 )] to give the pentacarbonyl complex 60 (Scheme 24) whose crystal

Scheme 23. Synthesis of complexes 57–59.

structure has been determined. The IR spectrum of 60 (CO 2058, 1975, 1935 cm−1 ) suggests that ligand 30 is a poor ␲-acceptor and modest ␴-donor [58]. The diphosphinoborane 27 featuring two primary phosphine groups forms the bimetallic species 61 upon reaction with two equivalents of [Cr(CO)5 (NMe3 )] (Scheme 25) [56]. A combination of X-ray crystallographic and IR data suggest the ligand acts as a poor ␲-acceptor and modest ␴-donor. The bidentate borylphosphines 31a and 32 form the 4- or 5membered chelates 62–65 (Scheme 26) [58]. An unusual bimetallic complex 67 is formed by the sequence of reactions shown in Scheme 27 where the product can be considered to be a molybdenum chelate of a bidentate phosphinoborane 66 which has a zirconocene backbone [57]. 4.4.2. Rhodium complexes The trans-[RhCl(CO)(L)2 ] (L = 21a,b or 68a,b), which are the azaborinyl complexes 69a,b and their organophosphorus analogues 70a,b (Scheme 28) were found to be isostructural in the solid state [46]. Electronically however, the Rh CO

Scheme 25. Synthesis of 61.

Scheme 26. Synthesis of 62–65.

J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

87

Scheme 27. Synthesis of 66 and 67.

stretching frequencies showed that the P B N ligands 21a,b ((CO) = 1951 cm−1 (21a) and 1946 cm−1 (21b)) are more electrondonating than their P C C counterparts 68a,b ((CO) = 1966 cm−1 (68a) and 1951 cm−1 (68b)). This was attributed to the BN unit being more electron releasing than the CC unit. The cationic complexes [Rh(nbd)(L)2 ][BF4 ] (71a,b and 72a,b) were tested for the catalytic hydrogenation of cyclohexene. It was noted that (i) complexes 71a,b featuring the P B N ligands are more efficient catalysts than their P C C counterparts 72a,b; (ii) the complexes 71b and 72b featuring the cyclic phosphines are more efficient than their acyclic counterparts 71a and 72a. These were the first reports of borylphosphines being used as ligands for homogeneous catalysis. Oxygen insertion into the P B bond of ligand 36b to give 56 was discussed above (Scheme 22); it has been shown that the rhodium(I) complexes 73 and 75 also react with TMAO to give complexes 74 and 76 respectively, which were made independently from 56 (Scheme 29) [79]. The ease of this on-metal oxygenation was a surprise given that (i) no oxidation of the metal centre was observed and (ii) no decarbonylation of 73 occurred. Complexes 75, 76 and the phosphinite analogue [RhCl(i Pr2 POPh)(1,5-cod)] were tested in the catalytic hydrosilylation and hydroboration of 4-methoxystyrene using HSiPh3 and pinacolborane respectively. The P O B complex 76 was found to be much more active than its P B and P O C counterparts 75 and [RhCl(i Pr2 POPh)(1,5-cod)] in hydrolsilylation. In hydroboration, similar activities were observed

Scheme 28. Synthesis of 69–72.

for 76 and [RhCl(i Pr2 POPh)(1,5-cod)]; complex 76 catalysed the reaction to the linear product and [RhCl(i Pr2 POPh)(1,5-cod)] the branched product. Ligands with carborane substituents are known for a variety of donor atoms [82,83]. The bonding properties of these carboranyl ligands are strongly dependent on the vertex to which the donor atom is bonded [84,85]. The borylphosphines 22a–e are the first examples of B-bound carboranyl phosphines and the (CO) stretching frequencies measured for their complexes 77a–e (Scheme 30) are lower than reported for any other phosphine ligand (Table 3) [47]. Ligand 22b is the most electron-donating phosphine ligand reported to date. Borylphosphine 22a is much more electrondonating than the analogous carbon-bound carboranylphosphine 78; there is a 25 cm−1 difference in the CO stretching frequencies of their complexes 77a (1956 cm−1 ) and 79 (1981 cm−1 ).

Cl i

0.25 [RhCl(CO)2]2

Pr2P

HN

B

PiPr2

Rh NH

Cl Pr2P Rh PiPr2 CO O O B B HN NH HN NH i

CO

HN

B

NH

ONMe3

0.25 [RhCl(CO)2]2 PiPr2 O B HN NH

PiPr2 HN

B

73

74

NH

Rh

Rh

36b

i

0.5 [RhCl(cod)]2

Pr2P

HN

B

i

Cl NH

Pr2P

ONMe3 HN

B

Cl

56 0.5 [RhCl(cod)]2

O NH

cod = cyclooctadiene

75

76

Scheme 29. On-metal oxygen insertion into 73 and 75 to form 74 and 76.

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J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

Ph

Ph

P B

HC

Ph Ph P B

0.5 PtCl2(cod) CH

HC

22a Ph

Cl Pt Cl

CH

Ph P Ph B HC

80

CH

+ cis product

Ph P C

0.5 PtCl2(cod)

no reaction

HC 78 Scheme 31. Synthesis of 78. Scheme 30. Synthesis of 77a–e and 79.

Table 3 IR data for trans-[RhCl(CO)L2 ] complexes. L

R Ph R Cy R Cyp (cyclopentyl) R i Pr R i Bu

Ligand number

Complex number

(CO)/cm−1

21a

69a

1951

21b

69b

1946

68a

70a

1966

68b

70b

1951

36b

73

1956

56

74

1977

22a 22b 22c 22d 22e

77a 77b 77c 77d 77e

1956 1935 1936 1939 1941

78

79

1981

Fig. 8. Crystal structure of 80, showing the end-on bonding of borylphosphine 22a ˚ ˙(angles at to Pt(II). All hydrogen atoms are omitted for clarity. P B1 1.954(4) A, P) = 319.2.

4.4.3. Platinum complexes End-on coordination of borylphosphine 22a to Pt to give 80 was achieved upon reaction with [PtCl2 (cod)] (Scheme 31) [47]. X-ray crystallography showed that there was no significant difference between the P B bond lengths in 80 and 22a (Fig. 8). Surprisingly, ligand 78, the P C counterpart of 22a, does not react with [PtCl2 (cod)] and this was attributed to electronic rather than steric factors (Scheme 31). The coordination chemistry of phosphinoboranes with P B double bonds has undergone much less investigation compared to that of their singly-bonded (borylphosphine) counterparts. In view of their similarity to alkenes, it was proposed that side-on bonding via the ␲-electrons to transition metals would be possible. This was realised in 2012 by Bourissou et al., who reported the side-on coordination of the phosphinoboranes R2 P B(C6 F5 )2 23a,b (R = Cy, t Bu) to Pt(0) in complexes 81a,b (Scheme 32) [86]. The crystal structures revealed a lengthening of the P B bond upon coordination of ca. 0.15 A˚ and a sharp decrease in 1 JP B coupling (42 Hz for the 81a and 150 Hz for the free phosphinoborane

C6F5

PPh 3

R

B P + R C6F5

Pt PPh 3

RR PPh3 P Pt B PPh3 F5C6 C F 6 5

23a R = Cy 23b R = t Bu

81a R = Cy 81b R = tBu Scheme 32. Synthesis of 81a,b.

J.A. Bailey, P.G. Pringle / Coordination Chemistry Reviews 297–298 (2015) 77–90

Fig. 9. Crystal structure of 81a, showing the side-on bonding of phosphinoborane ˚ ˙(angles 23a to Pt(0). All hydrogen atoms are omitted for clarity. P1 B 1.917(3) A, at P) = 351.2◦ .

RR P

PPh3

Pt PPh3 B F5C6 C F 6 5 a

RR P PPh3 Pt B PPh3 F5 C6 C6 F5 b

Fig. 10. Bonding modes of 23 to Pt(0) in 81.

23a) consistent with a weakening of the P B bond (Fig. 9). A computational analysis of complexes 81a,b suggested that the bonding is best represented as a combination of the two forms shown in Fig. 10. 5. Conclusion The chemistry of R2 PBR2 has expanded significantly over the last 20 years, particularly their coordination chemistry and FLP-like reactivity. As ligands, the observation of the end-on and side on coordination of R2 PBR2 amply demonstrates the phosphinoborane–borylphosphine dichotomy. Other aspects of the chemistry of R2 PBR2 compounds such as their reactions with chalcogens and cycloaddition reactions can similarly be rationalised in terms of the P B/P B formalism. Analysis of X-ray crystal structures of R2 PBR2 shows that they can indeed be broadly categorised as P B or P B bonded while recognising that these descriptions represent the extremes of a continuum. The recent discovery that R2 PBR2 compounds can be effective ligands for homogeneous catalysis may usher in an important future direction for this field. References [1] F.H. Stephens, V. Pons, R.T. Baker, Dalton Trans. 20 (2007) 2613. [2] A. Staubitz, A.P.M. Robertson, I. Manners, Chem. Rev. 110 (2010) 4079. [3] C.W. Hamilton, R.T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 38 (2009) 279. [4] T. Hügle, M. Hartl, D. Lentz, Chem. Eur. J. 17 (2011) 10184. [5] A. Staubitz, M.E. Sloan, A.P.M. Robertson, A. Friedrich, S. Schneider, P.J. Gates, J.S. auf der Günne, I. Manners, J. Am. Chem. Soc. 132 (2010) 13332. [6] T.J. Clark, K. Lee, I. Manners, Chem. Eur. J. 12 (2006) 8634. [7] C. Zhi, Y. Bando, C. Tang, D. Golberg, Mater. Sci. Eng. R: Rep. 70 (2010) 92. [8] D. Golberg, Y. Bando, C.C. Tang, C.Y. Zhi, Adv. Mater. 19 (2007) 2413. [9] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, ACS Nano 4 (2010) 2979. [10] R.T. Paine, C.K. Narula, Chem. Rev. 90 (1990) 73. [11] M. Xu, T. Liang, M. Shi, H. Chen, Chem. Rev. 113 (2013) 3766. [12] R.T. Paine, H. Noeth, Chem. Rev. 95 (1995) 343. [13] P.P. Power, Chem. Rev. 99 (1999) 3463.

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