8910 Chem. Commun., 2012, 48, 8910–8912 This journal is c The Royal Society of Chemistry 2012

Cite this: Chem. Commun., 2012, 48, 8910–8912

Low-coordinate cobalt(II) terphenyl complexes: precursors to sterically

encumbered ketonesw

Benjamin M. Gridley, Alexander J. Blake, Adrienne L. Davis, William Lewis,

Graeme J. Moxey and Deborah L. Kays*

Received 9th April 2012, Accepted 19th July 2012

DOI: 10.1039/c2cc34525k

Cobalt(II) diaryl complexes react with CO to afford Co2(CO)8 and

sterically encumbered ketones whose structure varies depending on

the nature of the aryl ligands.

One of the most important classes of reactions in organometallic

chemistry is the insertion of small molecules into the metal–carbon

bonds of transition metal–alkyl or –aryl bonds to form 1,1- or

1,2-addition products, and industrially, insertion reactions are

fundamental steps in a variety of important stoichiometric and

catalytic processes.1,2 The reaction of low-coordinate complexes

with CO provides an attractive route to the formation of

substituted ketones such as benzophenones or fluorenones which

have a wide variety of uses as building blocks in organic

chemistry.3 Benzophenone and related ketone derivatives have

a wide variety of uses,4,5 exhibiting a range of biological and

pharmacological activities6 and finding utility to treat conditions

such as Parkinson’s disease7 and some cancers.8 The search for

novel benzophenones, fluorenones and related ketones with new

properties is therefore an area of considerable interest.

Herein we report the reactivity of the cobalt(II) complexes

(2,6-Naph2C6H3)2Co(OEt2) (1; Naph = 1-C10H7) and (2,6-

Mes2C6H3)2Co (2; Mes = 2,4,6-Me3C6H2) towards carbon mon-

oxide, affording 3, a rare example of a tetra ortho-substituted

benzophenone,9 and 4, a terphenyl-substituted keto-fluorenone

(Scheme 1). 3 is the first structurally authenticated example of a

benzophenone featuring aryl groups in the meta-positions, thereby

providing considerable steric encumbrance around the carbonyl

moiety. These reactions represent the first example of the formation

of benzophenones and related compounds from m-terphenyl com-

plexes, a class of compound ideally suited as precursors to sterically

encumbered products due to the requirement of using sterically

inhibiting ligands in forming such low-coordinate complexes.10

The reaction between two equivalents of 2,6-Naph2C6H3Li

and CoBr2(DME) (DME = 1,2-dimethoxyethane) in

diethyl ether at low temperature affords the diaryl complex

(2,6-Naph2C6H3)2Co(OEt2) (1) in good yield (83%). (2,6-Mes2-

C6H3)2Co (2) was prepared according to our previously reported

procedure.12 Complex 1 has been identified unambiguously by

spectroscopic techniques and single crystal X-ray diffraction

measurements. The solid-state structure of 1 (Fig. 1) shows a

monomeric complex containing a three-coordinate cobalt(II)

centre.13 The metal centre occupies a distorted trigonal planar

environment, and is bound to two terphenyl ligands with a

C(1)�Co(1)�C(27) angle of 137.49(15)1. The coordination

environment around the metal is completed by a molecule

of diethyl ether, the Co–O distance being similar to other

cobalt–ether linkages.14 The Co�C distances of 2.030(4) and

2.024(3) A are slightly longer than those in two-coordinate

(2,6-Mes2C6H3)2Co 2 [Co�C = 1.999(2)–2.003(3) A]12 and

(2,6-Dipp2C6H3)2Co [2.014(2) A]15 (Dipp = 2,6-iPr2C6H3).

Scheme 1 Reaction of diaryls 1 and 2 with carbon monoxide to form

3 and 4. Reaction conditions: (i) x/s CO, Et2O, room temp., 16 h,

–Co2(CO)8; (ii) x/s CO, hexane, room temp., 16 h, –Co2(CO)8.

Fig. 1 Crystal structure of 1 with displacement ellipsoids set at 35%

probability. Hydrogen atoms except H(8), H(9), H(34) andH(35) omitted

for clarity. Selected bond lengths (A) and angles (1): Co(1)–C(1) 2.030(4),

Co(1)–C(27) 2.024(3), Co(1)–O(1a) 2.014(8), Co(1)� � �C(8) 3.085(4),

Co(1)� � �C(34) 3.076(4), Ar–Ar dihedral angle11 76.46(12).

School of Chemistry, University of Nottingham, University Park,Nottingham, UK. E-mail: Deborah.Kays@nottingham.ac.uk;Fax: +44 (0)115 9513563; Tel: +44 (0)115 9513491w Electronic supplementary information (ESI) available: Synthesis,characterising data, refinement details, crystal data and CIF files for1, 3�2OEt2 and 4�0.75C6H14. CCDC 861639, 861640 and 888882. ForESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/c2cc34525k

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 8910–8912 8911

The 2,6-Naph2C6H3� ligands in 1 are in a syn conformation16

which, together with the bent C�Co�C moiety brings two of the

flanking naphthyl rings towards the metal centre, leading to two

intramolecular Co� � �C distances [Co(1)� � �C(8) = 3.085(4) A,

Co(1)� � �C(34) = 3.076(4) A] which are less than the sum of the

van der Waals radii for these elements.17 Since these distances

are significantly longer than the corresponding distances in

(2,6-Dipp2C6H3)2Co (shortest Co� � �C = 2.878 A)15 the

Co� � �C interactions in 1 can be considered as very weak.

The ligand orientation in 1 allows the formation of intramolecular

CH� � �p interactions, within the recognised range for such inter-

actions,18 between the naphthyl protons in the 2- and 3-positions in

the C(7) and C(33) rings and the centroids of the C(17) and C(43)

aryl rings, respectively (Fig. 1). This particular type of intra-

molecular CH� � �p interaction, which is not observed in complexes

such as (2,6-Naph2C6H3)2Ge or (2,6-Naph2C6H3)2Pb,19 so far

appears to be unique to our naphthyl terphenyl complexes.

Given the interesting chemistry exhibited by two-coordinate

open-shell transition metal complexes of m-terphenyl ligands,20,21

we were keen to probe the reactivity of 1 and 2. Thus, exposure of a

green diethyl ether solution of 1 to dry CO at room temperature

leads to an immediate colour change to red-orange (Scheme 1).

After stirring the reaction mixture for 16 h, controlled cooling of a

saturated diethyl ether solution to �30 1C affords crystals of the

benzophenone (2,6-Naph2C6H3)2CO (3�2OEt2) (Fig. 2), which

features sterically demanding 2,6-di(1-naphthyl)phenyl substituents.

The concomitant metal-containing product in this reaction is

Co2(CO)8, as identified by 13C{1H} NMR and IR spectroscopy.

There is a small additional peak in the 13C{1H} NMR spectrum of

the reactionmixture at 223 ppm, corresponding to aminor reaction

component or intermediate. We were unable to identify this

compound unequivocally, but the chemical shift indicates that it

is likely to be a cobalt-containing acyl and/or carbonyl complex.22

Aromatic ketones bearing sterically demanding substituents

are of interest as they can adopt conformations in which the

planes of the aromatic rings and that containing the carbonyl

moiety are not coplanar23 and would be expected to exhibit

new reactivity. In the solid state structure of 3 (Fig. 2) the two

aryls form dihedral angles of 46.39(16)1 [C(2) ring] and

47.75(16)1 [C(28) ring] with the plane of the CC(QO)C moiety

which are larger than that for benzophenone, presumably due to

the steric demands of the flanking naphthyl groups.24 The angle

formed between the C(2) and C(28) rings is 65.59(15)1. The

naphthyl substituents adopt a syn configuration, and are oriented

to each other at angles of 51.01 [C(8) and C(18) rings] and 52.91

[C(34) and C(44) rings]. There appears to be some steric strain in

the molecule; the flanking naphthyl groups are oriented at an angle

greater than the ideal sp2 value of 1201 [122.7(4)–124.2(4)1].

The orientation of the naphthyl groups may also be due to intra-

molecular CH� � �p interactions; H(19)� � �centroid = 2.572(2) A,

C(19)–H(19)� � �centroid = 154.9(3)1; H(45)� � �centroid =

2.779(2) A, C(45)–H(45)� � �centroid = 177.2(3)1.

The 1H and 13C{1H} NMR spectra of 3 are consistent with

a conformation where the two 2,6-Naph2C6H3 groups are

symmetrically twisted with respect to the carbonyl plane,

leading to inequivalent ortho and meta positions on the central

phenyl rings and consequently to inequivalent meta-naphthyls.

The NMR spectra of 3 do not show syn and anti naphthyl

conformations on the terphenyl ligands, presumably as a result

of free rotation around the C–C bonds between the naphthyl

substituents and the central aryl rings. Somewhat gratifyingly,

the intramolecular CH� � �p interaction observed in the solid

state structure of 3 (Fig. 2) can also be seen in the NMR

spectra of this molecule. In particular, the 1H NMR spectrum

of 3 exhibits a resonance at 4.34 ppm corresponding to one of

the protons in the 2-position of the flanking naphthyl moieties

(the associated 13C{1H}NMR resonance of this CH unit appearing

at 123.7 ppm), the strong upfield shift of this value compared to

that observed for the other naphthyls being due to the magnetic

anisotropy induced by the aromatic ring of the naphthyl.25

In order to investigate the influence of the aryl ligand

substituents a dark red hexane solution of the related complex

(2,6-Mes2C6H3)2Co (2)12 was exposed to dry CO at room

temperature, leading to an immediate colour change to deep

orange (Scheme 1). Stirring at room temperature for 16 h,

followed by controlled cooling of a saturated hexane solution to

�30 1C, affords colourless crystals of diketone 4�0.75C6H14 and

concomitant formation of Co2(CO)8. 4�0.75C6H14 crystallises in

space group P%1, revealing the diketone structure (Fig. 3). Dearo-

matisation of one of the flanking Mes groups sees the generation

of two stereocentres at the 2- and 5-positions [C(46) and C(43),

respectively], which form the R,R and S,S enantiomers, but not

themeso. A five-membered cyclisation occurs viamigration of the

C(46) methyl group to the C(26) bound carbonyl, forming the

central cyclopentenone motif of a dearomatised fluorenone

analogue.26 Accordingly, C(26), C(31) and C(41) are distorted

significantly from sp2 geometry to accommodate the five-

membered ring [C(50)–C(26)–C(31) = 109.24(18)1; C(26)–

C(31)–C(41) = 109.18(18)1; C(31)–C(41)–C(46) = 106.70(17)1].

Activation at the 5-position of the C(41) ring leads to incor-

poration of 2,6-Mes2C6H3CO in the meta position, orientated

cis with respect to the C(49) methyl substituent. Containing

two mutually para sp3 carbon atoms, the C(41)–C(46) ring is a

twisted cyclohexadiene, with two CQC bonds [C(41)–C(42) =

1.331(3) A; C(44)–C(45) = 1.328(3) A], and four C–C bonds

[C–C = 1.492(3)–1.524(3) A].

Although the presence of paramagnetic intermediates

precludes determination of the mechanism for the formation

of 3 and Co2(CO)8 by NMR spectroscopy, it likely proceeds

Fig. 2 Crystal structure of 3�2OEt2 with displacement ellipsoids set at

40% probability. Solvent of crystallisation and hydrogen atoms except

for H(19) and H(45) are omitted for clarity. Selected bond lengths (A)

and angles (1): C(1)–O(1) 1.231(5), C(1)–C(2) 1.542(6), C(1)–C(28)

1.546(6), C(2)–C(3)–C(8) 122.7(4), Ar–Ar [C(2)–C(28) ring] 65.59(15).

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8912 Chem. Commun., 2012, 48, 8910–8912 This journal is c The Royal Society of Chemistry 2012

via the coordination of carbon monoxide to the cobalt(II)

centre in 1, facilitating migration of the Co–C bond to CO.

Subsequent reductive elimination of the benzophenone 3 in the

presence of an excess of CO then generates Co2(CO)8 as

the metal-containing product. The reactivity exhibited by 1

differs from that reported by Power for (2,6-Dipp2C6H3)2Fe,

which yields the 18-electron bis(Z2-acyl) complex (Z2-2,6-

Dipp2C6H3CQO)2Fe(CO)221 and terphenyl germylenes; the

latter yield a-germyloxy ketones via C–C bond cleavage and

formation.27 The role of the coordinated Et2O in the reaction

of 1 with CO is unclear, but it should be noted that 2 reacts

with CO, forming 4, despite the reaction being carried out in

hexane. This suggests that the presence of a coordinating

solvent is not necessary for these type of reactions. Direct

comparison of reactions in diethyl ether or hexane is not possible

for either 1 or 2 due to the solubilities of the compounds. As

with 3, the reaction mechanism for the formation of 4 likely

proceeds via insertion of CO into the Co–Cipso s-bond, followedby reductive elimination of the organic product and concomitant

formation of Co2(CO)8 in the presence of an excess of CO.

Indeed, the sequence by which the double-activation of the

flanking mesityl moiety occurs clearly involves two terphenyl

carbonyl species, perhaps suggesting the insertion of two CO

molecules into the Co–Cipso bonds in 2 prior to intramolecular

attack of the nearby mesityl ortho-C–Me bond. Dearomatisation

of the mesityl group would consequently activate the meta-C–H,

prompting formation of the new C(43)–C(25) bond.

In summary, we have reported the reactions of two

cobalt(II) terphenyl complexes with CO to yield the first

structurally authenticated benzophenone to feature terphenyl

moieties and anm-terphenyl containing diketone. The reactivity

of 1 and 2 towards CO differs greatly from that of other M(II)

terphenyl complexes21,27 and indicates that Co(II) derivatives

provide new routes to highly hindered ketones and related

compounds.

We gratefully acknowledge the support of EPSRC and the

University of Nottingham. We thank Mr Stephen Boyer

(London Metropolitan University) for elemental analyses

and the National Mass Spectrometry Service Centre, Swansea

for mass spectrometry.

Notes and references

1 Applied Homogeneous Catalysis with Organometallic Compounds,ed. B. Cornils and W. A. Herrmann, Wiley-VCH, Weinheim,Germany, 2nd edn, 2002, vol. 1.

2 C. Elschenbroich,Organometallics: A Concise Introduction, Wiley-VCH,Weinheim, Germany, 2006.

3 Gardner’s Commercially Important Chemicals: Synonyms, TradeNames and Properties, ed. G. W. A. Milne, Wiley-Interscience,Hoboken, N. J., 2005.

4 See, for example: B. R. Nayak and L. J. Mathias, J. Polym. Sci.Part A: Polym. Sci., 2005, 43, 5661; C. E. Hoyle, K. Viswanathan,S. C. Clark, C. W. Miller, C. Nguyen, S. Jonsson and L. Shao,Macromolecules, 1999, 32, 2793.

5 See, for example: C. G. J. Hayden, M. S. Roberts and H. A. E.Benson, Lancet, 1997, 350, 863; H. Gonzalez, A. Farbrot,O. Larko and A.-M. Wennberg, Br. J. Dermatol., 2006, 154, 337.

6 T. Tzanova, M. Gerova, O. Petrov, M. Karaivanova andD. Bagrel, Eur. J. Med. Chem., 2009, 44, 2724; S. A. Khanum,B. A. Begum, V. Girish and N. F. Khanum, Int. J. Biomed. Sci.,2010, 6, 60; L. Monzote, O. Cuesta-Rubio, A. Matheeussen,T. Van Assche, L. Maes and P. Cos, Phytother. Res., 2011, 25, 458.

7 See, for example: C. Olanow, M. D.Warren and P. B.Watkins, Clin.Neuropharmacol., 2007, 30, 287; K. Hassio, Int. Rev. Neurobiol.,2010, 95, 163.

8 J. Lee, S. J. Kim, H. Choi, Y. H. Kim, I. T. Lim, H. Yang,C. S. Lee, H. R. Kang, S. K. Ahn, S. K. Moon, D.-H. Kim, S. Lee,N. S. Choi and K. J. Lee, J. Med. Chem., 2010, 53, 6337; J. Lee,S. Bae, S. Lee, H. Choi, Y. H. Kim, S. J. Kim, G. T. Park,S. K. Moon, D.-H. Kim, S. Lee, S. K. Ahn, N. S. Choi andK. J. Lee, Bioorg. Med. Chem. Lett., 2010, 20, 6327.

9 F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, B58, 380.10 D. L. Kays, Dalton Trans., 2011, 40, 769.11 Defined as the angle between the best mean planes of the two

metal-substituted aryl rings.12 D. L. Kays (nee Coombs) and A. R. Cowley, Chem. Commun., 2007,

1053.13 The presence of a Co–Co bond in MesCo(m-Mes)2CoMes leads to

formally four-coordinate cobalt centre: K. H. Theopold, J. Silvestre,E. K. Byrne and D. S. Richeson, Organometallics, 1989, 8, 2001.

14 C. C. H. Atienza, C. Milsmann, E. Lobkovsky and P. J. Chirik,Angew. Chem., Int. Ed., 2011, 50, 8143; T. R. Dugan, X. Sun,E. V. Rybak-Akimova, O. Olatunji-Ojo, T. R. Cundari andP. L. Holland, J. Am. Chem. Soc., 2011, 133, 12418.

15 C. Ni, T. A. Stich and P. P. Power, Chem. Commun., 2010, 46, 4466.16 C.-T. Chen, R. Chadha, J. S. Siegel and K. Hardcastle, Tetrahedron

Lett., 1995, 36, 8403.17 S. S. Batsanov, Inorg. Mater., 2001, 37, 871.18 M. Nishio, CrystEngComm, 2004, 6, 130.19 G. L. Wegner, R. J. F. Berger, A. Schier and H. Schmidbauer,

Organometallics, 2001, 20, 418; X.-J. Yang, Y. Wang, P. Wei,B. Quillian and G. H. Robinson, Chem. Commun., 2006, 403.

20 B. Zhou, M. S. Denning, D. L. Kays and J. M. Goicoechea, J. Am.Chem. Soc., 2009, 131, 2802; C. Ni, H. Lei and P. P. Power,Organometallics, 2010, 29, 1988.

21 C. Ni and P. P. Power, Chem. Commun., 2009, 5543.22 I. Nagy-Gergely, G. Szalontai, F. Ungvary, L. Marko, M. Moret,

A. Sironi, C. Zucchi, A. Sisak, C. M. Tschoerner, A. Martinelli,A. Sorkau and G. Palyi, Organometallics, 1997, 16, 2740.

23 S. Grilli, L. Lunazzi, A. Mazzanti, D. Casarini and C. Femoni,J. Org. Chem., 2001, 66, 488.

24 H. Kutzke, H. Klapper, R. B. Hammond and K. J. Roberts, ActaCrystallogr., Sect. B: Struct. Sci., 2000, 56, 486.

25 M. Nishio, M. Hirota and Y. Umezawa, The CH–p Interaction:Evidence, Nature and Consequences, Wiley-VCH, New York, 1998,ch. 3, p. 61.

26 E. Grovenstein Jr., J. Singh, B. B. Patil and D. VanDerveer,Tetrahedron, 1994, 50, 5971.

27 X. Wang, Z. Zhu, Y. Peng, H. Lei, J. C. Fettinger and P. P. Power,J. Am. Chem. Soc., 2009, 131, 6912.

Fig. 3 Crystal structure of 4-R,R with displacement ellipsoids set at

the 40% probability level. Hydrogen atoms, except H(43), omitted for

clarity. Selected bond lengths (A) and angles (1): C(25)–O(1) 1.212(2),

C(41)–C(42) 1.331(3), C(41)–C(46) 1.523(3), C(44)–C(45) 1.328(3),

C(1)–C(25)–C(43) 118.52(17), C(50)–O(2) 1.206(3), C(31)–C(41)–

C(46) 106.70(17), C(26)–C(31)–C(41) 109.18(18).

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