The effect of -CN linkage isomerism and ancillary ligand set on directional metal-metal charge-transfer in cyanide-bridged dimanganese complexes

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PAPER

www.rsc.org/dalton | Dalton Transactions

The effect of l-CN linkage isomerism and ancillary ligand set on directional

metal–metal charge-transfer in cyanide-bridged dimanganese complexes†‡

Christopher J. Adams, Kirsty M. Anderson, Neil G. Connelly,* Estefania Llamas-Rey, A. Guy Orpen and

Rowena L. Paul

Received 20th April 2007, Accepted 19th June 2007

First published as an Advance Article on the web 5th July 2007

DOI: 10.1039/b705975b

5

The reaction of [Mn(CN)L^ꢀ(NO)(g -C₅R₄Me)] with cis- or trans-[MnBrL(CO)₂(dppm)], in

the presence of Tl[PF₆], gives homobinuclear cyanomanganese(I) complexes cis- or

5

trans-[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺, linkage isomers of which, cis- or

5

trans-[(dppm)(CO)₂LMn(l-CN)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺, are synthesised by reacting cis- or

5

trans-[Mn(CN)L(CO)₂(dppm)] with [MnIL^ꢀ(NO)(g -C₅R₄Me)] in the presence of Tl[PF₆]. X-Ray

structural studies on the isomers trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)-

5

(g -C₅H₄Me)]⁺and trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn(CNBu^t)(NO)(g -C₅H₄Me)]⁺show

nearly identical molecular structures whereas cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn-

{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺and cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-CN)Mn-

5

{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺differ, effectively in the N- and C-coordination respectively of two

5

different optical isomers of the pseudo-tetrahedral units (NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me) and

5

(CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me) to the octahedral manganese centre. Electrochemical and

spectroscopic studies on [(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺show that systematic

5

variation of the ligands L and L^ꢀ, of the cyclopentadienyl ring substituents R, and of the l-CN

orientation (XY = CN or NC) allows control of the order of oxidation of the two metal centres and

hence the direction and energy of metal–metal charge-transfer (MMCT) through the cyanide

bridge in the mixed-valence dications. Chemical one-electron oxidation of cis- or

5

trans-[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺with [NO][PF₆] gives the mixed-valence

5

dications trans-[(dppm)(CO)₂LMn^II(l-NC)Mn^IL^ꢀ(NO)(g -C₅R₄Me)]²⁺which show solvatochromic

absorptions in the electronic spectrum, assigned to optically induced Mn(I)-to-Mn(II) electron transfer

via the cyanide bridge.

Introduction

In our studies of the construction and properties of cyanide-

bridged complexes containing redox-active units such as A–C

(Scheme 1) we have previously investigated two series of complexes

in which linking two of the same or very similar mononuclear units

(i.e. two of the octahedral units, A and/or B, or two of the tetra-

hedral units, C) gives linkage isomers (due to the asymmetry of

Scheme 1 P–P = Ph₂PCH₂PPh₂= dppm.

the cyanide bridge), i.e. [(L-L)LMn(l-CN)MnL^ꢀ(NO)(L^ꢀ-L^ꢀ)][PF₆]

{L or L^ꢀ= P(OPh)₃, P(OEt)₃or PEt₃; L-L or L^ꢀ-L^ꢀ= dppm

or NC) in which one octahedral unit (based on A or B) and one

pseudo-tetrahedral unit (based on C) are linked. The order of one-

electron oxidation of the two metal centres in the monocations,

and hence the direction and energy of metal–metal charge-transfer

(MMCT) in the corresponding mixed-valence dications, can be

controlled by the systematic variation of the ligands at either

manganese atom, the cis or trans arrangement of the Mn(CO)₂

1,2

5

or dppe}

and [(g -C₅H₄Me)(NO)LMn(l-CN)MnL^ꢀ(NO)(g -

C₅H₄Me)][PF₆] (L or L^ꢀ= PPh₃, CNBu^tor CNXyl).³Varying the

ancillary ligands at each metal centre then allows ﬁne tuning of the

energy of intramolecular metal–metal charge-transfer (MMCT)

and thence electronic properties such as solvatochromism.

We now give details⁴of

a third series of complexes,

5

[(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺(XY = CN

5

fragment, the substitution pattern in the g -C₅R₄Me ring, and the

orientation of the cyanide bridge.

School of Chemistry, University of Bristol, Bristol, UK, BS8 1TS. E-mail:

neil.connelly@bristol.ac.uk; Fax: +44 (0)117 929 0509; Tel: +44 (0)117

928 8162

Results and discussion

† CCDC reference numbers 166635, 166636, 644601–644604. For

crystallographic data in CIF or other electronic format see DOI:

10.1039/b705975b

‡ Electronic supplementary information (ESI) available: Metal-to-metal

charge-transfer data. See DOI: 10.1039/b705975b

The synthesis and spectroscopic characterisation of linkage isomers

The room temperature reaction of the pseudo-tetrahedral

5

cyanomanganese

complexes

[Mn(CN)L^ꢀ(NO)(g -C₅R₄Me)]

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{R = H, L^ꢀ= PPh₃, P(OPh)₃, CNBu^tor CNXyl; R = Me, L =

CNBu^t} with octahedral cis- or trans-[MnBrL(CO)₂(dppm)]

{L = P(OPh)₃or P(OEt)₃} in the presence of Tl[PF₆] in

CH₂Cl₂gave orange or red solutions from which cis- or

weak second band observed at higher energy for the symmetry

forbidden A-mode). This, and the observation of redox-induced

cis–trans isomerisation on oxidation,⁵provides a useful diagnostic

tool for identifying the order of oxidation of the two redox-active

centres in 1⁺–20⁺.

5

trans-[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -C₅R₄Me)][PF₆]

1⁺[PF₆]⁻–10⁺[PF₆]⁻(Scheme 2) were isolated as yellow to red

microcrystalline solids. In some cases, the attempted synthesis of

trans-Mn(CO)₂complexes gave inseparable mixtures of cis- and

trans-Mn(CO)₂isomers, necessitating an alternative preparative

route. Thus, for example, trans-7⁺was synthesised by one-electron

oxidation of cis-7⁺with [NO][PF₆] and in situ reduction with

N₂H₄·xH₂O of the dication, trans-7²⁺, formed by redox-induced

cis–trans isomerisation.⁵

The IR spectra show not only that linkage isomers such as

1⁺and 11⁺are distinguishable but also that they are stable

to isomerisation (cf. the more ready isomerisation of species

such as [(Ph₃P)Au(l-CN)RhCl₂(PMe₂Ph)₃]⁺and [(OC)₅W(l-

NC)Cu(PPh₃)₃] to [(Ph₃P)Au(l-NC)RhCl₂(PMe₂Ph)₃]^{+ 8}and

[(OC)₅W(l-CN)Cu(PPh₃)₃]⁹respectively). In general, the m(NO)

band appears at approximately 10 cm⁻¹higher energy when the

pseudo-tetrahedral Mn centre is bound to the better p-accepting

C-end of the bridging cyanide {compare m(NO) = 1742 cm⁻¹for

cis-1⁺with m(NO) = 1734 cm⁻¹for the linkage isomer cis-11⁺}.

Conversely, the m(CO) bands appear at lower energy when the

octahedral centre is bound to the better r-donating N-end of

the bridge. {Compare m(CO) = 1971, 1905 cm⁻¹for cis-1⁺with

m(CO) = 1974, 1920 cm⁻¹for cis-11⁺.}

The X-ray structures of cis-2⁺, trans-7⁺, cis-8⁺, cis-9⁺, cis-12⁺and

trans-17⁺

As far as we are aware, only two pairs of stable dinuclear

complexes showing cyanide–isocyanide linkage isomerism

have been previously structurally characterised, namely

[(H₃N)₅Co(l-XY)Co(CN)₅] (XY

=

CN and NC)¹⁰and

5

[(OC)₅Cr(l-XY)Fe(dppe)(g -C₅H₅)] (XY = CN and NC).¹¹

Structural analyses of two pairs of the new linkage isomers,

trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-XY)Mn(CNBu^t)-

Scheme 2 Complexes cis- and trans-1⁺–20⁺(cis or trans refers to the

relative arrangement of the two carbonyl ligands at the octahedral

manganese centre).

namely

5

(NO)(g -C₅H₄Me)]⁺(XY = NC, trans-7⁺; XY = CN, trans-

17⁺), and cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-XY)Mn{P(OPh)₃}-

(NO)(g -C₅H₄Me)]⁺(XY = NC, cis-2⁺; XY = CN, cis-12⁺) were

5

The linkage isomers of 1⁺–10⁺, i.e. cis- and trans-[(dppm)(CO)₂-

therefore carried out in order to investigate possible changes

arising from l-CN isomerism.

LMn(l-CN)MnL^ꢀ(NO)(g -C₅R₄Me)][PF₆] 11⁺[PF₆]⁻–20⁺[PF₆]⁻

5

(Scheme 2), were synthesised by the room temperature reaction of

the octahedral cyanomanganese ligands cis- or trans-[Mn(CN)-

L(CO)₂(dppm)] {L = P(OPh)₃or P(OEt)₃} with the pseudo-

The structures of the cations trans-17⁺(also representative of

trans-7⁺, see below), cis-2⁺and cis-12⁺are shown in Fig. 1–3

respectively; selected bond lengths and angles are given in

Table 2 (for trans-7⁺and trans-17⁺) and Table 3 (for cis-2⁺and

for the two crystallographically independent units of cis-12⁺).

Dinuclear complexes containing the cyanomanganese ligands

5

tetrahedral iodide complexes [MnIL^ꢀ(NO)(g -C₅R₄Me)] {R =

H, L^ꢀ= PPh₃, P(OPh)₃, CNBu^tor CNXyl; R = Me, L^ꢀ= CNBu^t}

in the presence of Tl[PF₆] in CH₂Cl₂. Alternatively, cis-15⁺

and cis- and trans-[(dppm)(CO)₂{(RO)₃P}Mn(l-CN)Mn-

5

[Mn(CN)(CNXyl)(NO)(g -C₅H₄Me)] and [Mn(CN)(CNBu^t)-

5

(CNBu^t)(NO)(g -C₅H₄Me)]⁺(R = Ph 19⁺or Et 20⁺) were synthe-

(NO)(g -C₅Me₅)], namely cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

sised by the reaction of cis-[Mn(CN){P(OR)₃}(CO)₂(dppm)] with

NC)Mn(CNXyl)(NO)(g -C₅H₄Me)]⁺cis-8⁺(Fig. 4) and cis-

5

[Mn(CO)L (NO)(g -C₅H₄Me)][PF₆] (L = PPh₃or CNBu^t) in the

[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺

presence of Me₃NO. Analytical data for cis- and trans-1⁺[PF₆]⁻–

20⁺[PF₆]⁻are given in Table 1.

cis-9⁺(Fig. 5), have also been structurally characterised. Selected

bond lengths and angles are given in Table 4.

In CH₂Cl₂the IR spectra (Table 1) of 1⁺–20⁺show in-

creases in m(CN) of 20–30 cm⁻¹compared with the mononuclear

Each of the cations trans-7⁺, trans-17⁺, cis-2⁺, cis-12⁺, cis-8⁺and

cis-9⁺contains two manganese centres linked by a nearly linear

Mn–CN–Mn bridge, with Mn–C–N and C–N–Mn angles ranging

from 171.6 to 179.1^◦. The geometries of the [MnXL(CO)₂(dppm)]

5

cyanomanganese precursors [Mn(CN)L^ꢀ(NO)(g -C₅R₄Me)] and

[Mn(CN)L(CO)₂(dppm)]. Though such shifts to higher energy

have been usually attributed to the kinematic effect of constraining

the CN motion by N-coordination to a second metal fragment,⁶

a recent theoretical study⁷has shown this factor to be rather less

important than others such as r-bonding and cation charge.

The number of carbonyl stretching bands observed for

5

and [MnYL^ꢀ(NO)(g -C₅R₄Me)] fragments are octahedral and

pseudo-tetrahedral respectively (from now on designated as Mn_oct

and Mn_tet).

For a given pair of linkage isomers, the Mn–CN bond length

˚

appears shorter than Mn–NC {cf. Mn_tet–CN = 1.951(3) A in

5

+

[(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺1⁺–20⁺de-

trans-7 vs. Mn_tet–NC = 1.968(3) A in trans-17 , and CN–Mn_oct

=

˚

+

˚

pends on the geometry of the Mn(CO)₂unit; two are observed

for cis-Mn(CO)₂and one for trans-Mn(CO)₂species (with a very

2.010(3) A in trans-7 vs. NC–Mn_oct= 1.955(3) A in trans-17 },

indicating the better p-accepting ability of a C-donor fragment.

3610 | Dalton Trans., 2007, 3609–3622

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Table 1 Analytical and IR spectroscopic data for cis- and trans-[(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -C₅R₄Me)][PF₆]

Analysis (%)^b

C

IR^c/cm⁻¹

Complex^aXY

L

L^ꢀ

R

Colour Yield (%)

H

N

m(CNR) m(CN) m(CO)

m(NO)

cis-1⁺

NC P(OPh)₃PPh₃

NC P(OPh)₃P(OPh)₃

NC P(OPh)₃CNBu^t

NC P(OPh)₃CNXyl

NC P(OEt)₃PPh₃

NC P(OEt)₃P(OPh)₃

NC P(OEt)₃CNBu^t

NC P(OEt)₃CNXyl

NC P(OPh)₃CNBu^t

NC P(OEt)₃CNBu^t

CN P(OPh)₃PPh₃

CN P(OPh)₃P(OPh)₃

CN P(OPh)₃CNBu^t

CN P(OPh)₃CNXyl

CN P(OEt)₃PPh₃

CN P(OEt)₃P(OPh)₃

CN P(OEt)₃CNBu^t

CN P(OEt)₃CNXyl

CN P(OPh)₃CNBu^t

CN P(OEt)₃CNBu^t

H

Orange 63

Orange 68

Orange 62

Orange 59

Orange 71

Orange 45

Orange 75

Orange 54

Orange 66

Orange 59

Orange 70

Orange 55

59.7 (59.9)

59.6 (59.9)

58.0 (58.0)

56.4 (55.9)

55.6 (55.9)

57.9 (57.6)

55.1 (55.3)

55.5 (55.3)

53.2 (53.3)

53.8 (53.3)

50.2 (50.1)

50.4 (50.1)

51.8 (52.2)

51.7 (52.2)

4.4 (4.2) 2.0 (2.0)

4.1 (4.2) 2.2 (2.0)

3.6 (4.1) 2.2 (1.9)

4.2 (4.1) 2.2 (1.9)

4.7 (4.4) 3.6 (3.4) 2168m

4.4 (4.4) 3.6 (3.4) 2167m

4.3 (4.2) 3.3 (3.3) 2147m

4.3 (4.2) 3.2 (3.3) 2146m

4.7 (4.7) 2.5 (2.2)

4.6 (4.7) 2.3 (2.2)

4.3 (4.6) 2.1 (2.1)

4.6 (4.6) 2.2 (2.1)

—

2109w 1971, 1905

2116w 2018w, 1930 1741

1742

trans-1⁺

cis-2⁺

2127w 1971, 1905

2127w 2018w, 1930 1763

2130w 1971, 1908 1764

2132w 2020w, 1932 1762

2130w 1970, 1908 1766

2132w 2019w, 1932 1765

2102w 1959, 1894 1743

2113w 2010w, 1915 1742

2127w 1958, 1894 1763

2124w 2010w, 1915 1764

2133w 1958, 1894 1762

2132w 2010w, 1916 1763

2132w 1958, 1894 1766

2131w 2010w, 1916 1766

2121w 1970, 1905 1742

2123w 2021w, 1931 1740

2118w 1958, 1894 1741

2120w 2009w, 1914 1740

2104w 1974, 1920 1734

2081w 2016w, 1936 1735

2121w 1973, 1920 1756

2103w 2015w, 1935 1756

2128w 1975, 1922 1758

2111w 2015w, 1936 1757

2132sh 1974, 1922 1761

2111w 2017w, 1936 1760

2099w 1963, 1908 1736

2088w 2006w, 1921 1734

2113w 1962, 1906 1758

2096w 2006w, 1922 1757

2123w 1962, 1907 1758

2106w 2007w, 1921 1757

2128sh 1962, 1907 1761

2105w 2006w, 1921 1760

2114w 1969, 1903 1735

2113w 2015w, 1928 1733

2113w 1961, 1906 1734

2111w 2004w, 1920 1733

1763

trans-2⁺

cis-3⁺

trans-3⁺

cis-4⁺

trans-4⁺

cis-5⁺

—

trans-5⁺

cis-6⁺

trans-6⁺

cis-7⁺

Red

Yellow 73

Red 67

Orange 54

38

4.7 (5.0) 3.7 (3.9) 2168m

4.8 (5.0) 3.9 (3.9) 2168m

4.7 (4.7) 3.3 (3.7) 2145m

4.6 (4.7) 3.5 (3.7) 2146m

trans-7⁺

cis-8⁺

trans-8⁺

cis-9⁺

Me Orange 72

Me Orange 48

Me Red

54.8 (54.6)^d4.8 (4.7) 3.3 (3.1) 2152m

trans-9⁺

cis-10⁺

trans-10⁺

cis-11⁺

trans-11⁺

cis-12⁺

trans-12⁺

cis-13⁺

trans-13⁺

cis-14⁺

trans-14⁺

cis-15⁺

trans-15⁺

cis-16⁺

trans-16⁺

cis-17⁺

trans-17⁺

cis-18⁺

trans-18⁺

cis-19⁺

trans-19⁺

cis-20⁺

trans-20⁺

56.8 (57.2)

51.5 (51.8)

52.1 (51.8)

59.7 (59.9)

59.8 (59.9)

57.9 (58.0)

58.0 (58.0)

55.7 (55.9)

56.0 (55.9)

57.6 (57.6)

57.5 (57.6)

55.1 (55.3)

54.8 (55.3)

53.2 (53.3)

53.0 (53.3)

49.6 (50.1)

50.4 (50.1)

51.7 (52.2)

57.0 (57.2)

57.4 (57.2)

51.2 (51.8)

51.4 (51.8)

4.7 (4.8) 2.9 (3.3) 2154m

5.6 (5.4) 3.6 (3.7) 2154m

5.5 (5.4) 4.1 (3.7) 2153m

4.4 (4.2) 2.3 (2.0)

4.1 (4.2) 1.9 (2.0)

3.6 (4.1) 2.4 (1.9)

4.1 (4.1) 2.2 (1.9)

4.5 (4.4) 3.4 (3.4) 2171m

4.4 (4.4) 3.4 (3.4) 2172m

4.4 (4.2) 3.3 (3.3) 2147m

4.1 (4.2) 3.2 (3.3) 2147m

4.9 (4.7) 2.2 (2.2)

4.4 (4.7) 2.2 (2.2)

4.3 (4.6) 2.1 (2.1)

4.6 (4.6) 2.3 (2.1)

5.1 (5.0) 3.8 (3.9) 2172m

5.0 (5.0) 3.8 (3.9) 2172m

4.6 (4.7) 3.5 (3.7) 2145m

4.7 (4.7) 3.3 (3.7) 2146m

4.8 (4.8) 3.0 (3.3) 2158m

4.7 (4.8) 3.1 (3.3) 2157m

5.6 (5.4) 4.1 (3.7) 2158m

5.5 (5.4) 3.9 (3.7) 2157m

55

Me Orange 30

H

Green

Pink

47

31

56

—

Brown 45

Pink

81

74

62

53

44

28

44

Green

Purple

Black

Green

Red

—

Brown 40

Pink

Green

Pink

70

66

40

34

55

Green

Me Green

Me Brown 56

Me Pink 68

Me Brown 45

^aIsolated as [PF₆]⁻salts. ^bCalculated values in parentheses. ^cStrong absorptions in CH₂Cl₂unless stated otherwise; sh = shoulder, m = medium, w =

weak. ^dAnalysed as a 1 : 1 CH₂Cl₂solvate.

◦

t

5

+

Table 2 Selected bond lengths (A) and angles ( ) for trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu )(NO)(g -C₅H₄Me)] trans-7⁺and trans-

˚

[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-17⁺

5

trans-7⁺

trans-17⁺

trans-7⁺

trans-17⁺

Mn(1)–C(1)

Mn(1)–N(1)

Mn(2)–N(1)

Mn(2)–C(1)

C(1)–N(1)

Mn(1)–N(2)

Mn(1)–C(8)

N(2)–O(1)

1.951(3)

—

2.010(3)

—

1.968(3)

—

Mn(1)–C(1)–N(1)

Mn(1)–N(1)–C(1)

Mn(2)–N(1)–C(1)

Mn(2)–C(1)–N(1)

N(1)–Mn(1)–C(1)

N(1)–Mn(1)–N(2)

C(1)–Mn(1)–C(8)

C(8)–Mn(1)–N(1)

Mn(1)–N(2)–O(1)

C(8)–Mn(1)–N(2)

P(1)–Mn(2)–P(2)

P(2)–Mn(2)–P(3)

P(1)–Mn(2)–P(3)

N(1)–Mn(2)–P(2)

C(1)–Mn(2)–P(2)

C(14)–Mn(2)–C(13)

Mn(2)–C(14)–O(3)

Mn(2)–C(13)–O(2)

176.9(3)

—

171.6(2)

—

96.3(2)

—

88.6(1)

—

175.9(3)

96.1(2)

72.5(1)

100.4(1)

172.8(1)

165.3(1)

—

178.7(1)

177.3(3)

179.1(3)

—

173.6(2)

—

175.0(2)

—

98.3(1)

—

89.4(1)

174.4(3)

95.8(1)

72.2 (3)

100.8(3)

172.9(4)

—

164.8(8)

176.5(1)

176.6(3)

179.7(3)

1.955(3)

1.151(4)

1.655(3)

1.922(3)

1.176(3)

1.151(4)

2.283(9)

2.290(9)

2.205(10)

1.829(3)

1.827(3)

1.147(3)

1.149(3)

1.153(4)

1.656(3)

1.916(4)

1.171(4)

1.150(4)

2.296(1)

2.275(1)

2.211(1)

1.839(3)

1.834(3)

1.146(4)

1.147(4)

C(8)–N(3)

Mn(2)–P(1)

Mn(2)–P(2)

Mn(2)–P(3)

Mn(2)–C(14)

Mn(2)–C(13)

C(14)–O(3)

C(13)–O(2)

Mn(1) · · · Mn(2)

5.096

5.058

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◦

5

+

Table 3 Selected bond lengths (A) and angles ( ) for cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)] cis-2⁺and cis-[(dppm)-

˚

(CO)₂{(PhO)₃P}Mn(l-CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-12⁺

5

a

cis-2⁺

cis-12⁺

cis-2⁺

cis-12⁺

Mn(1)–C(1)

Mn(1)^ꢀ–C(1)^ꢀ

Mn(1)–N(1)

Mn(2)–N(1)

Mn(2)^ꢀ–N(1)^ꢀ

Mn(2)–C(1)

C(1)–N(1)

—

2.033(14)

—

1.912(17)

1.184(16)

—

1.637(17)

—

2.198(6)

—

1.218(17)

—

2.285(6)

—

2.340(6)

—

2.207(6)

—

1.753(19)

—

1.727(17)

—

1.197(18)

—

1.199(16)

—

1.960(8)

1.994(7)

—

1.927(6)

1.935(6)

—

Mn(1)–C(1)–N(1)

Mn(1)^ꢀ–C(1)^ꢀ–N(1)^ꢀ

Mn(1)–N(1)–C(1)

Mn(2)–N(1)–C(1)

Mn(2)^ꢀ–N(1)^ꢀ–C(1)^ꢀ

Mn(2)–C(1)–N(1)

P(4)–Mn(2)–N(1)

P(4)^ꢀ–Mn(2)^ꢀ–N(1)^ꢀ

P(4)–Mn(2)–C(1)

—

175.8(13)

—

171.1(15)

—

178.7(6)

179.0(6)

—

173.4(6)

176.7(6)

—

86.06(17)

86.16(15)

—

1.180(7)

1.147(7)

1.612(8)

1.631(7)

2.218(2)

2.204(2)

1.210(8)

1.192(7)

2.324(2)

2.348(2)

2.293(2)

2.285(2)

2.197(2)

2.208(2)

1.795(8)

1.814(7)

1.798(7)

1.785(7)

1.133(7)

1.170(7)

1.154(7)

1.161(7)

C(1)^ꢀ–N(1)^ꢀ

Mn(2)–N(2)

Mn(2)^ꢀ–N(2)^ꢀ

Mn(2)–P(4)

Mn(2)^ꢀ–P(4)^ꢀ

N(2)–O(1)

87.9(5)

N(2)–Mn(2)–N(1)

N(2)^ꢀ–Mn(2)^ꢀ–N(1)^ꢀ

N(2)–Mn(2)–C(1)

—

96.6(7)

98.9(3)

100.6(3)

—

N(2)^ꢀ–O(1)^ꢀ

Mn(1)–P(1)

Mn(1)^ꢀ–P(1)^ꢀ

Mn(1)–P(2)

Mn(1)^ꢀ–P(2)^ꢀ

Mn(1)–P(3)

Mn(1)^ꢀ–P(3)^ꢀ

Mn(1)–C(2)

Mn(1)^ꢀ–C(2)^ꢀ

Mn(1)–C(3)

Mn(1)^ꢀ–C(3)^ꢀ

C(2)–O(2)

C(1)–Mn(1)–P(2)

C(1)^ꢀ–Mn(1)^ꢀ–P(2)^ꢀ

N(1)–Mn(1)–P(2)

—

88.2(4)

86.38(19)

87.54(18)

—

Mn(2)–N(2)–O(1)

Mn(2)^ꢀ–N(2)^ꢀ–O(1)^ꢀ

P(4)–Mn(2)–N(2)

P(4)^ꢀ–Mn(2)^ꢀ–N(2)^ꢀ

P(1)–Mn(1)–P(2)

P(1)^ꢀ–Mn(1)^ꢀ–P(2)^ꢀ

P(2)–Mn(1)–P(3)

P(2)^ꢀ–Mn(1)^ꢀ–P(3)^ꢀ

P(1)–Mn(1)–P(3)

P(1)^ꢀ–Mn(1)^ꢀ–P(3)^ꢀ

C(2)–Mn(1)–C(3)

C(2)^ꢀ–Mn(1)^ꢀ–C(3)^ꢀ

Mn(1)–C(2)–O(2)

Mn(1)^ꢀ–C(2)^ꢀ–O(2)^ꢀ

Mn(1)–C(3)–O(3)

Mn(1)^ꢀ–C(3)^ꢀ–O(3)^ꢀ

172.7(14)

—

94.2(5)

—

72.9(2)

—

96.1(2)

—

168.7(2)

—

86.8(8)

—

175.3(17)

—

175.5(14)

—

171.1(6)

173.7(6)

98.6(2)

95.0(2)

72.35(7)

72.41(7)

167.05(8)

171.48(8)

94.94(8)

99.42(8)

89.7(3)

C(2)^ꢀ–O(2)^ꢀ

C(3)–O(3)

C(3)^ꢀ–O(3)^ꢀ

88.3(3)

Mn(1). . .Mn(2)

Mn(1)^ꢀ. . .Mn(2)^ꢀ

5.109

—

5.059

5.073

177.9(7)

174.2(6)

175.6(5)

178.1(6)

^aThere are two independent molecules in the asymmetric unit. Equivalent atoms for the second are distinguished from the ﬁrst by primes.

Fig.

2

The structure of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn-

5

{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-2⁺. Hydrogen atoms have been omitted

for clarity.

Fig. 1 The structure of trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn-

5

(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-17⁺. The hydrogen atoms have been

As found in the other structurally characterised linkage isomers

noted above,^10,11the molecular structures of trans-7⁺and trans-

17⁺are very similar. (They are crystallographically isostructural.)

By contrast, those of cis-2⁺and cis-12⁺are rather different, at

least in the solid state. Thus, the torsion angle, s, formed by

the two Mn–P(OPh)₃vectors (i.e. P–Mn–XY–Mn–P with the

omitted for clarity.

˚

The l-CN bond lengths are in the range 1.15–1.18 A, similar to

those in other cyanide-bridged complexes.¹²

3612 | Dalton Trans., 2007, 3609–3622

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◦

5

+

Table 4 Selected bond lengths (A) and angles ( ) for cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNXyl)(NO)(g -C₅H₄Me)] cis-8⁺and cis-[(dppm)-

˚

(CO)₂{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺cis-9⁺

5

cis-8⁺

cis-9⁺

cis-8⁺

cis-9⁺

Mn(1)–C(1)

Mn(2)–N(1)

C(1)–N(1)

Mn(1)–N(2)

Mn(1)–C(42)

N(2)–O(1)

C(42)–N(3)

Mn(2)–P(1)

Mn(2)–P(2)

Mn(2)–P(3)

Mn(2)–C(2)

Mn(2)–C(3)

C(2)–O(2)

1.951(5)

2.012(4)

1.157(5)

1.653(5)

1.892(6)

1.182(5)

1.158(6)

2.298(2)

2.358(2)

2.224(2)

1.806(8)

1.777(5)

1.159(7)

1.162(5)

5.114

1.938(9)

1.996(7)

1.161(9)

1.629(10)

1.883(10)

1.179(12)

1.150(10)

2.303(2)

2.352(2)

2.202(3)

1.791(9)

1.742(9)

1.153(9)

1.164(10)

5.086

Mn(1)–C(1)–N(1)

Mn(2)–N(1)–C(1)

Mn(1)–N(2)–O(2)

C(42)–Mn(1)–N(2)

N(2)–Mn(1)–C(1)

C(42)–Mn(1)–C(1)

P(1)–Mn(2)–P(2)

P(2)–Mn(2)–P(3)

P(3)–Mn(2)–P(1)

N(1)–Mn(2)–P(2)

C(2)–Mn(2)–C(3)

Mn(2)–C(2)–O(2)

Mn(2)–C(3)–O(3)

177.0(5)

176.3(4)

176.5(4)

96.2(2)

91.5(2)

72.3(1)

98.9(1)

170.9(1)

84.8(1)

89.8(2)

177.8(5)

178.8(5)

176.6(7)

176.0(6)

173.8(10)

97.2(5)

99.7(4)

90.5(3)

70.7(1)

97.7(1)

168.0(1)

84.8(2)

88.7(4)

174.9(7)

176.2(1)

C(3)–O(3)

Mn(1). . .Mn(2)

Fig.

5

The structure of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn-

Fig.

3

The structure of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-CN)Mn-

5

(CNBu^t)(NO)(g -C₅Me₅)]⁺cis-9⁺. Hydrogen atoms and all but the ipso

5

{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-12⁺. Hydrogen atoms have been omit-

carbons of the phenyl rings of dppm have been omitted for clarity.

ted for clarity.

5

mers of the pseudo-tetrahedral units (NC)Mn{P(OPh)₃}(NO)(g -

5

C₅H₄Me) and (CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me) to the oc-

tahedral manganese centre, i.e. cis-2⁺and cis-12⁺are different

diastereoisomers. (Each of cis-1⁺to cis-20⁺can exist as di-

astereoisomers although in no case has evidence been found for

the co-existence of two such isomers, e.g. by IR spectroscopy.)

The structural analysis of cis-9⁺is the ﬁrst of

a

complex containing the N-donor cyanomanganese ligand

5

[Mn(CN)(CNBu^t)(NO)(g -C₅Me₅)] the estimated cone angle of

◦

5

which, h, (106 ) is larger than that of [Mn(CN)(CNBu^t)(NO)(g -

C₅H₄Me)] (h = 57^◦) due to the increased methylation on the

cyclopentadienyl ring. Thus, the structure of cis-9⁺differs from

those of the other cis complexes, cis-2⁺, cis-8⁺and cis-12⁺, in the

relative arrangement of the metal fragments. In the less sterically

hindered complexes, such as cis-8⁺, the g -C₅H₄Me ring and the

5

dppm ligand are syn relative to each other (Fig. 4). However,

Fig.

4

The structure of cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn-

5

in complex cis-9⁺the bulky g -C₅Me₅and dppm ligands are

5

(CNXyl)(NO)(g -C₅H₄Me)]⁺cis-8⁺. Hydrogen atoms have been omitted

positioned anti to each other (Fig. 5).

for clarity.

Mn–XY–Mn backbone assumed to be linear) is 13.2^◦for cis-

2⁺and 86.1 and 76.2^◦for the two independent units of cis-

12⁺respectively. Formally, this appears to be the result of N-

and C-coordination respectively of two different optical iso-

Electrochemical studies

The cyclic voltammograms (CVs) of 1⁺–20⁺each show two

oxidation waves (Table 5) in CH₂Cl₂at a platinum electrode,

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5

Table 5 Electrochemical data^afor cis- and trans-[(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -C₅R₄Me)][PF₆]

Process (E^◦^ꢀ/V)^b

DE^◦^c/V

ꢀ

Complex

L

L^ꢀ

R

XY

+ → 2+

2+ → 3+

cis-1⁺

P(OPh)₃

P(OEt)₃

P(OPh)₃

P(OEt)₃

P(OPh)₃

P(OEt)₃

P(OPh)₃

P(OEt)₃

PPh₃

H

Me

H

NC

CN

1.03

0.68

1.24(I) (0.73)

0.73

1.20(I) (0.73)

0.72

1.26(I) (0.72)

0.74

1.02(I) (0.54)

0.53

1.09(I) (0.55)

0.57

1.06(I) (0.54)

0.55

1.11(I) (0.56)

0.55

0.99

0.67

0.99(I) (0.50)

0.51

0.84

0.76

1.08

0.90

1.01

0.87

1.07

0.92

0.79

0.69

1.06(I) (0.73)

0.77

0.98 (0.72)

0.72

1.05(I) (0.74)

0.75

0.88

0.78

1.54(I) (0.70, 1.36)^d

1.35

(1.60)

1.61

(1.43)

1.45

(1.51)

1.47

(1.34)

—

0.67

—

0.88

—

0.73

—

0.73

—

0.78

—

0.98

—

0.81

—

0.88

—

0.61

—

0.81

—

0.55

—

0.52

—

0.41

—

0.41

—

0.50

—

trans-1⁺

cis-2⁺

P(OPh)₃

CNBu^t

CNXyl

PPh₃

trans-2⁺

cis-3⁺

trans-3⁺

cis-4⁺

trans-4⁺

cis-5⁺

trans-5⁺

cis-6⁺

PPh₃

1.31

(1.56)

1.55

(1.36)

1.36

(1.44)

1.43

P(OPh)₃

CNBu^t

CNXyl

CNBu^t

PPh₃

trans-6⁺

cis-7⁺

trans-7⁺

cis-8⁺

trans-8⁺

cis-9⁺

1.51(I) (0.64, 1.25)^d

1.28

trans-9⁺

cis-10⁺

trans-10⁺

cis-11⁺

trans-11⁺

cis-12⁺

trans-12⁺

cis-13⁺

trans-13⁺

cis-14⁺

trans-14⁺

cis-15⁺

trans-15⁺

cis-16⁺

trans-16⁺

cis-17⁺

trans-17⁺

cis-18⁺

trans-18⁺

cis-19⁺

trans-19⁺

cis-20⁺

trans-20⁺

(1.30)

1.32

1.65(I) (1.28)

1.31

1.65(I) (1.40)

1.42

1.58(I) (1.39)

1.28

1.56(I) (1.39)

1.33

1.44(I) (1.22)

1.19

1.42

1.36

1.39(I) (1.21)

1.21

1.31

1.57(I) (1.34)

1.20

1.41(I) (1.21)

1.18

PPh₃

P(OPh)₃

CNBu^t

CNXyl

PPh₃

P(OPh)₃

CNBu^t

CNXyl

CNBu^t

0.59

—

0.49

—

0.56

—

0.32

—

H

Me

0.70

0.48

^aAt a Pt electrode in CH₂Cl₂, with potentials relative to the saturated calomel electrode, calibrated vs. the [Fe(g-C₅Me₅)₂]⁺/[Fe(g-C₅Me₅)₂] couple (at

−0.08 V). ^bFor an irreversible (I) process, the oxidation peak potential, (E_p)_ox, is given at a scan rate of 200 mV s⁻¹. Potentials for associated reversible

◦

ꢀ

product waves are given in parentheses. ^cDE is the difference between the potentials for the two reversible oxidation waves (of the trans⁺isomers only).

^dThe two product waves, observed only after scanning the second, irreversible, oxidation wave, are due to the trans-dication/trans-monocation and

trans-trication/trans-dication couples respectively.

corresponding to the sequential oxidation of the two metal centres.

Generally, the waves are reversible on the cyclic voltammetry

timescale when the coordination geometry of the manganese cen-

tre is pseudo-tetrahedral or trans-dicarbonyl octahedral. However,

they are irreversible when a cis-dicarbonyl unit is oxidised due

to the redox-induced cis–trans isomerisation of the Mn^II(CO)₂

fragment.⁵In these cases, the irreversible oxidation wave is coupled

to a reversible product wave, at a more negative potential.

C₅Me₅, lowers the oxidation potential by 210 mV; better donors L

and L^ꢀshift oxidation waves to less positive potentials, e.g. the

5

oxidation potential of [Mn(CN)L^ꢀ(NO)(g -C₅H₄Me)] decreases

from 1.11 V (L^ꢀ= CNXyl) to 0.85 V (L^ꢀ= PPh₃) and that of trans-

[Mn(CN)L(CO)₂(dppm)] decreases from 0.64 V {L = P(OPh)₃}

to 0.50 V {L = P(OEt)₃}.

The variation of the orientation of the cyanide bridge, of the

5

ligands L and L^ꢀ, of the extent of ring methylation in the g -

The occurrence of an irreversible oxidation wave for cis-

C₅R₄Me group, and the cis or trans orientation of the Mn(CO)₂

fragment all affect the order in which the metal centres in cis-

5

[(dppm)(CO)₂LMn(l-XY)Mn(NO)L^ꢀ(g -C₅R₄Me)]⁺therefore al-

5

lows the identiﬁcation of which manganese centre is oxidised ﬁrst

(aided by comparing the individual redox potentials with those

of the corresponding mononuclear units and noting the effects of

altering the ligands L and L^ꢀor the extent of ring methylation

[(dppm)(CO)₂LMn(l-XY)Mn(NO)L^ꢀ(g -C₅R₄Me)]⁺are oxidised.

This, in turn, leads to control of the direction and energy of metal–

metal charge-transfer in the corresponding dications (see below).

The CV of cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn-

5

in g -C₅R₄Me). As noted previously,³increased ring methylation

{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-6⁺[Fig. 6(a)] shows one

5

in [Mn(CN)(CNBu^t)(NO)(g -C₅R₄Me)], from g -C₅H₄Me to g -

irreversible oxidation wave at 1.09 V followed by a reversible

3614 | Dalton Trans., 2007, 3609–3622

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oxidative isomerisation. However, the CV of its linkage isomer

5

cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-CN)Mn{P(OPh)₃}(NO)(g -

C₅H₄Me)]⁺cis-12⁺[Fig. 7(b)] shows the ﬁrst process to be

reversible and therefore related to the oxidation of Mn_tet. The

second oxidation process is irreversible with associated product

waves identical to those of trans-12⁺. Changing the orientation of

the cyanide bridge from Mn_oct–NC–Mn_tet(in cis-2⁺) to Mn_oct–CN–

Mn_tetin (cis-12⁺) therefore reverses the order in which the metal

centres oxidise. The overall electrochemical behaviour of cis-2⁺

and cis-12⁺is summarised in Schemes 3 and 4 respectively.

Fig. 6 The CVs in CH₂Cl₂of (a) cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺, cis-6⁺, from 0.30 to 1.80 to 0.40 to

0.85 V, and (b) trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn{P(OPh)₃}-

5

(NO)(g -C₅H₄Me)]⁺, trans-6⁺, from 0.0 to 1.80 V at a scan rate of

200 mV s⁻¹

.

product oxidation wave at 1.56 V, and a product reduction

wave, at 0.52 V, which a subsequent scan shows to be reversible

and centred at 0.55 V. Thus, the ﬁrst step is oxidation at the

cis-Mn(CO)₂site; the two product waves appear at potentials

identical to those of trans-6⁺[Fig. 6(b)], conﬁrming that after

oxidation at the N-bound cis-Mn(CO)₂unit, cis-6²⁺rapidly

isomerises to trans-6²⁺.

The CV of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn{P-

Scheme

3

The oxidation of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)-

5

MnL(NO)(g -C₅H₄Me)]⁺{L = P(OPh)₃, cis-2⁺or CNBu^t, cis-3⁺}.

5

(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-2⁺[Fig. 7(a)] is very similar

to that of cis-6⁺, showing one irreversible process at 1.24 V

{oxidation at the cis-Mn(CO)₂site}, and two reversible prod-

uct waves at 1.60 and 0.73 V corresponding to the trans-

2³⁺/trans-2²⁺and trans-2²⁺/trans-2⁺couples for the product of

Scheme

4

The oxidation of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-XY)-

5

MnL(NO)(g -C₅R₄Me)]⁺{XY = NC, L = CNBu^t, R = Me, cis-9⁺; XY =

CN, L = P(OPh)₃, R = H, cis-12⁺}.

Complexes [(dppm)(CO)₂{(PhO)₃P}Mn(l-CN)Mn{P(OPh)₃}-

5

(NO)(g -C₅H₄Me)]⁺cis-12⁺and cis-[(dppm)(CO)₂{(EtO)₃P}Mn-

5

(l-CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-16⁺provide an ex-

ample of the effect of varying L, the ligand at cis-Mn_oct. Although

they differ only in the replacement of L = P(OPh)₃in cis-12⁺by L =

P(OEt)₃in cis-16⁺their electrochemistry is very different. The CV

of cis-16⁺(Fig. 8) shows an irreversible oxidation wave at 1.06 V,

corresponding to the oxidation of the octahedral metal centre, and

Fig.

7

The CVs in CH₂Cl₂of cis-[(dppm)(CO)₂{(PhO)₃P}-

5

Mn(l-XY)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺(a) XY

=

NC, cis-2⁺,

from 0.50 to 1.80 to 0.50 to 0.90 V, and (b) XY = CN, cis-12⁺, from 0.0 to

1.80 V, multiple scan, at a scan rate of 200 mV s⁻¹

.

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of cis-Mn_oct; the two reversible product waves correspond to

the trans-3³⁺/trans-3²⁺and trans-3²⁺/trans-3⁺couples. However,

5

the increased ring methylation in the g -C₅Me₅complex cis-9⁺

increases the electron density at Mn_tetwhich is then the ﬁrst

oxidation site; the ﬁrst oxidation wave is reversible and the

second irreversible (oxidation at cis-Mn_oct). The electrochemical

behaviour of cis-3⁺is described by Scheme 3 and that of cis-9⁺by

Scheme 4.

In the absence of redox-induced isomerisation, the order of

oxidation of the two metal centres in trans-1⁺–20⁺can only

be assigned by comparing the potentials for the two reversible

oxidation waves with those of mononuclear precursors (and

related complexes) and by observing the shifts in each oxidation

wave as the ligands on each centre are altered.

Fig.

8

The CV in CH₂Cl₂of cis-[(dppm)(CO)₂{(EtO)₃P}Mn-

5

(l-CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-16⁺from 0.0 to 1.65 to 0.0

to 0.90 V at a scan rate of 200 mV s⁻¹

.

The ﬁrst oxidation of trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

NC)Mn(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-7⁺occurs at 0.55 V.

two product waves at 1.42 and 0.73 V, identical to those of trans-

16⁺. Thus, changing the phosphite, L, to a better donor leads to

the site of ﬁrst oxidation changing from Mn_tet(ﬁrst oxidation wave

reversible) to cis-Mn_oct(ﬁrst oxidation wave irreversible).

Since the oxidation potential of the mononuclear cyanomanganese

5

ligand [Mn(CN)(CNBu^t)(NO)(g -C₅H₄Me)] occurs at ca. 1.0 V,

and the potential will increase when this ligand is N-coordinated

to a second metal, the process at 0.55 V must be related to the

oxidation of trans-Mn_octin trans-7⁺. The second wave, associated

with the oxidation of Mn_tet, is centred at 1.36 V which compares

By contrast, the complexes cis-[(dppm)(CO)₂{(PhO)₃P}Mn-

5

(l-NC)Mn(PPh₃)(NO)(g -C₅H₄Me)]⁺cis-1⁺and cis-[(dppm)-

5

(CO)₂{(PhO)₃P}Mn(l-NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-

well with that of the mononuclear complex [Mn(CNBu^t)₂(NO)(g -

◦

2⁺provide an example of how the ligand L^ꢀat Mn_tetinﬂuences

the order of oxidation. In the CV of cis-1⁺[Fig. 9(a)] the ﬁrst

wave is reversible and therefore associated with oxidation at Mn_tet.

However, on changing PPh₃(in cis-1⁺) for the better p-acceptor

P(OPh)₃(in cis-2⁺) the ﬁrst wave becomes irreversible [Fig. 9(b)]

C₅H₄Me)]⁺(E = 1.45 V).³(In this case, the assignment of the

order of oxidation for trans-7⁺has been conﬁrmed chemically and

spectroscopically—see below.)

ꢀ

By contrast, the ﬁrst oxidation wave of the linkage iso-

mer of trans-7⁺, i.e. trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn-

5

indicating oxidation at cis-Mn_oct

.

(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-17⁺, occurs at 0.72 V, which

compares well with the oxidation potential of mononuclear trans-

ꢀ

[Mn(CN){P(OEt)₃}(CO)₂(dppm)] (E^◦= 0.50 V),¹indicating

oxidation at Mn_oct. The potential for the second process for trans-

5

17⁺(1.21 V) compares with that of [Mn(CNBu^t)(NCMe)(NO)(g -

ꢀ

C₅H₄Me)]⁺(E^◦= 1.23 V).³

Thus, although in each case Mn_octis oxidised ﬁrst, reversal of the

cyanide bridge leads to a different route for MMCT from Mn(I)

to Mn(II) in the resulting dication, i.e. via C then N in trans-7²⁺

but from N to C in trans-17²⁺.

A

comparison of trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

CN)MnL^ꢀ(NO)(g -C₅H₄Me)]⁺{L = PPh₃trans-15⁺, P(OPh)₃

trans-16⁺, CNBu^ttrans-17⁺or CNXyl trans-18⁺} shows that

the ﬁrst oxidation potential (Table 5) is relatively unchanged

while the second decreases in the order P(OPh)₃> CNXyl >

CNBu^t> PPh₃. Thus, the ﬁrst oxidation occurs at the C-bound

octahedral centre. Indeed, further comparisons indicate that the

ﬁrst oxidation for all of trans-1⁺–20⁺occurs at Mn_oct, i.e. the order

of oxidation of the two metal centres is independent of L, L^ꢀand

R as well as the l-CN orientation (in marked contrast to the cis

isomers described above).

5

Fig.

9

The CVs in CH₂Cl₂of cis-[(dppm)(CO)₂{(PhO)₃P}Mn-

This therefore shows that the cis or trans arrangement of

the Mn(CO)₂fragment can also alter the oxidation order, as

shown above for cis- and trans-[(dppm)(CO)₂{(PhO)₃P}Mn(l-

5

(l-NC)MnL^ꢀ(NO)(g -C₅H₄Me)]⁺(a) L^ꢀ= PPh₃, cis-1⁺, and (b) L^ꢀ

=

P(OPh)₃, cis-2⁺from 0.0 to 1.8 V.

5

NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺, cis-9⁺and trans-9⁺, and

Finally, cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)-

cis- and trans-[(dppm)(CO)₂{(PhO)₃P}Mn(l-CN)Mn{P(OPh)₃}-

(NO)(g -C₅H₄Me)]⁺cis-3⁺and cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-

(NO)(g -C₅H₄Me)]⁺, cis-12⁺and trans-12⁺. To underline this point,

5

NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺cis-9⁺show how the extent

the order of oxidation of the metal centres for both 9⁺and

12⁺is described by Scheme 4. The ﬁrst oxidation in the cis

complexes occurs at Mn_tetand the second at Mn_oct. However, after

isomerisation the ﬁrst site to be reduced is Mn_tet, i.e. the order

5

of methylation of the g -C₅R₄Me group inﬂuences the order in

which the metal centres are oxidised. The ﬁrst wave in the CV of

cis-3⁺is irreversible and therefore associated with the oxidation

3616 | Dalton Trans., 2007, 3609–3622

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5

Table 6 Analytical data for trans-[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -C₅R₄Me)][PF₆]₂

Analysis (%)^b

IR^c/cm⁻¹

Complex^a

L

L^ꢀ

R

Yield (%)

C

H

N

m(CNR)

m(CN)

m(CO)

m(NO)

trans-1²⁺

trans-2²⁺

trans-3²⁺

trans-4²⁺

trans-5²⁺

trans-6²⁺

trans-7²⁺

trans-8²⁺

trans-9²⁺

trans-10²⁺

P(OPh)₃

P(OEt)₃

P(OPh)₃

P(OEt)₃

PPh₃

H

67

56

75

70

35

77

79

62

54

48

50.1 (50.3)

51.7 (51.8)^d

48.8 (50.0)

51.2 (51.7)

49.9 (49.6)

47.7 (48.0)

44.5 (44.1)

44.2 (44.4)

45.7 (46.0)^e

51.4 (51.4)

3.8 (3.7) 1.7 (1.8)

3.7 (3.7) 1.7 (1.7)

3.5 (3.9) 3.3 (3.1)

4.0 (3.8) 2.5 (3.0)

3.9 (4.2) 2.2 (2.0)

4.2 (4.1) 1.8 (1.9)

4.4 (4.4) 2.9 (3.4)

4.3 (4.1) 2.7 (3.1)

4.5 (4.8) 3.2 (3.3)

4.5 (4.3) 2.9 (3.0)

—

2177

2154

—

2052ms

2077m

2081 m

2085m

2054ms

2079m

2084m

2088m

2058

2068 w.sh, 1999

2067m, 2002

2069 m, 2005

2072m.sh, 2006

2065 m.sh, 1992

2065sh, 1995

2069sh, 1997

2071w.sh, 1996

2000

1752

1772

1774

1777

1752

1773

1774

1776

1755

P(OPh)₃

CNBu^t

CNXyl

PPh₃

P(OPh)₃

CNBu^t

CNXyl

CNBu^t

—

2176

2153

2162

2163

Me

2060m

2072m, 1994

^aIsolated as [PF₆]⁻salts. ^bCalculated values in parentheses. ^cStrong absorptions in CH₂Cl₂, unless otherwise stated; w = weak, m = medium, sh =

shoulder. ^dAnalysed as a 2 : 1 CH₂Cl₂solvate. ^eAnalysed as a 1 : 1 CH₂Cl₂solvate.

of oxidation of the monocation is not the reverse of the order of

reduction of the trication.

solutions. Nevertheless, trans-1²⁺–10²⁺2[PF₆]⁻were characterised

by elemental analysis (Table 6), cyclic voltammetry (each dication

shows one oxidation and one reduction wave at potentials

identical to those of the two oxidation waves of the corresponding

monocation trans-1⁺–10⁺) and IR spectroscopy (Table 6).

Finally, the different redox behaviour for linkage isomers reﬂects

the r-donor and p-acceptor properties of the cyanide ligand. Thus,

for a pair of linkage isomers in trans-1⁺–20⁺, that with the more

electron-rich fragment bound to the N atom is approximately 0.2 V

Each dication, trans-1²⁺–10²⁺, shows two IR carbonyl bands

of different intensity. The stronger absorption is shifted to

higher wavenumber with respect to that of the corresponding

monocation by ca. 80 cm⁻¹; the second absorption, assigned

to the symmetric stretch of the trans-Mn(CO)₂fragment, is

also shifted (by ca. 60 cm⁻¹) but, more signiﬁcantly, it is more

intense when compared to that of the monocations. Similar

behaviour has been found on one-electron oxidation of species

such as [(H₃N)₅Ru^II(l-NC)Mn^I(CO)₂{P(OPh)₃}(dppm)-trans]²⁺,

with the increased intensity of the symmetric stretch resulting

from coupling with the cyanide stretching vibration.¹³

The absorptions corresponding to the nitrosyl and, when

present, isocyanide ligands are also shifted to higher wavenumber

with respect to the monocations, but by only ca. 10 cm⁻¹. Thus, the

larger energy shift in the carbonyl bands conﬁrms that oxidation

of the trans monocations occurs at Mn_octwhich also leads to a

lower energy bridging cyanide band (decreased by ca. 40 cm⁻¹

with respect to that of the monocations).

ꢀ

◦

easier to oxidise than that bound to C (cf. E₁= 0.55 V for trans-

7⁺and 0.72 V for trans-17⁺). When the second process is detectable

by cyclic voltammetry, the difference in oxidation potentials, DE^◦

ꢀ

(DE^◦= E₂− E₁^ꢀ) also varies for linkage isomers, being 0.81 and

0.49 V for trans-7⁺and trans-17⁺respectively as a representative

example. The consequences for the electronic spectra of the mixed

◦

ꢀ

5

valence dications trans-[(dppm)(CO)₂LMn(l-XY)MnL^ꢀ(NO)(g -

C₅R₄Me)]²⁺are noted below.

The synthesis and characterisation of the mixed valence dications

1²⁺–20²⁺

With the exceptions of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-

5

NC)Mn(PPh₃)(NO)(g -C₅H₄Me)]⁺cis-1⁺and cis-[(dppm)(CO)₂-

5

{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺cis-9⁺, all

of the cis and trans isomers of [(dppm)(CO)₂LMn(l-

5

NC)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺1⁺–10⁺react with the one-electron

Addition of one equivalent of [NO][PF₆] to

a

so-

oxidant [NO][PF₆] to form the corresponding dications trans-

1²⁺–10²⁺. (The oxidation of cis-1⁺and cis-9⁺, at C-bound

Mn_tet, gave dications which are not stable at room tem-

perature although spectroelectrochemistry at −30 ^◦C pro-

vides evidence for cis-9²⁺at lower temperatures—see below.)

By contrast, chemical one-electron oxidation of the link-

lution of the dication trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]²⁺trans-10²⁺led to the forma-

tion of a complex showing m(CNR) and m(NO) shifted by ca.

70 cm⁻¹to higher wavenumbers and m(CO) shifted to higher

energy by 10 cm⁻¹, suggesting oxidation at C-bound Mn_tetand

some stability for trans-10³⁺. However, this trication could not be

isolated.

5

age isomers [(dppm)(CO)₂LMn(l-CN)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺

5

11⁺–20⁺with [Fe(g -C₅H₅)(g -C₅H₄COMe)][PF₆], [NO][PF₆] or

[N(C₆H₄Br-p)₃][SbCl₆] led to decomposition even at low temper-

ature (although, again, low temperature spectroelectrochemistry

suggests some stability for trans-17²⁺).

Although only trans-1²⁺–10²⁺proved isolable, other less sta-

ble dications (and one trication) were characterised in solu-

tion by low temperature IR spectroelectrochemistry, i.e. on

trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-XY)Mn(CNBu^t)(NO)(g -

5

The most convenient route to the purple mixed-valence

C₅H₄Me)]⁺(XY

=

NC, trans-7⁺; XY

=

CN, trans-17⁺)

salts trans-1²⁺–10²⁺2[PF₆]⁻involves treatment of trans-

and cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -

5

[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺trans-1⁺–10⁺

C₅Me₅)]⁺cis-9⁺.

in CH₂Cl₂with one equivalent of [NO][PF₆]. The solid products

decompose signiﬁcantly within 4 weeks, even when stored

under argon at −20 ^◦C, to a mixture containing the cis and

trans monocations; they are soluble in polar solvents such as

CH₂Cl₂, CHCl₃, thf, acetone and dmf, giving very air-sensitive

Electrolysis of trans-7⁺at an applied potential, E_app, of 0.6 V

(vs. a Ag wire electrode) at −30 ^◦C in CH₂Cl₂, resulted in the

m(CO) band at 1914 cm⁻¹collapsing and a new band forming at

1998 cm⁻¹. Increases of ca. 13 cm⁻¹and 8 cm⁻¹were observed

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Table 7 Electronic spectroscopic data for trans-[(dppm)(CO)₂LMn(l-

for m(NO) and m(CNR) respectively; the m(CN) band decreased in

energy by 48 cm⁻¹and became more intense. Thus, the shifts in all

of the bands are consistent with oxidation at the N-bound trans-

Mn(CO)₂centre, generating trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

NC)MnL^ꢀ(NO)(g -C₅R₄Me)]²⁺in CH₂Cl₂

k_max/nm

Complex

L

L^ꢀ

R

(e/dm³mol⁻¹cm⁻¹

)

5

NC)Mn(CNBu^t)(NO)(g -C₅H₄Me)]²⁺trans-7²⁺(cf. the isolated

trans-1²⁺

trans-2²⁺

trans-3²⁺

trans-4²⁺

trans-5²⁺

trans-6²⁺

trans-7²⁺

trans-8²⁺

trans-9²⁺

trans-10²⁺

P(OPh)₃

P(OEt)₃

P(OPh)₃

P(OEt)₃

PPh₃

H

501 (990), 1100 (3350)

503 (—^a), 852 (2930)

504 (1061), 972 (2633)

504 (1078), 948 (2096)

500 (1135), 1004 (2523)

464 (—^a), 811 (2909)

488 (1490), 877 (2685)

500 (1148), 856 (1409)

504 (1236), 1180 (2955)

499 (1310), 1021 (3460)

salt trans-7²⁺2[PF₆]⁻, see above).

P(OPh)₃

CNBu^t

CNXyl

PPh₃

P(OPh)₃

CNBu^t

CNXyl

CNBu^t

Increasing E_appto 1.45 V resulted in increases in energy for

m(CO), m(NO) and m(CNR) of 16, 101 and 62 cm⁻¹respectively,

this time consistent with oxidation at the C-bound manganese

centre and the generation of trans-7³⁺. Lowering E_appto 0.6 V and

then to 0.0 V regenerated trans-7²⁺and trans-7⁺respectively, with

the intensities of the corresponding IR bands greater than 85% of

those observed in the forward process. Thus, the trication trans-7³⁺

and the dication trans-7²⁺appear to be stable in CH₂Cl₂at −30 ^◦C.

Electrolysis at 0.72 V of trans-17⁺, the linkage isomer of

trans-7⁺, at −30 ^◦C in CH₂Cl₂, resulted in a large increase

in the energy of the carbonyl bands, while m(NO) and m(CNR)

move to higher energy by only 8 and 12 cm⁻¹. Thus, oxida-

tion at Mn_oct, as proposed on the basis of cyclic voltamme-

try, gave trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn(CNBu^t)-

Me

^aNot determined due to ill-deﬁned bands.

5

(NO)(g -C₅H₄Me)]²⁺trans-17²⁺. Lowering E_appto 0.0 V regener-

ated trans-17⁺, conﬁrming that the dication trans-17²⁺is stable at

−30 ^◦C (although not isolable at room temperature). By contrast,

increasing E_appto 1.21 V to generate trans-17³⁺resulted in sample

decomposition.

Cyclic voltammetry of cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-

5

NC)Mn(PPh₃)(NO)(g -C₅H₄Me)]⁺cis-1⁺and cis-[(dppm)-

(CO)₂{(PhO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]⁺cis-9⁺

5

indicated reversible one-electron oxidation at Mn_tetbut chemical

oxidation at room temperature led to sample decomposition.

Fig. 10 The electronic spectra of (a) trans-[(dppm)(CO)₂{(EtO)₃P}Mn-

5

(l-NC)Mn(CNBu^t)(NO)(g -C₅H₄Me)]²⁺trans-7²⁺and (b) trans-[(dppm)-

However, at −30 ^◦C IR spectroelectrochemistry of cis-9⁺, at E_app

=

5

(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅Me₅)]²⁺trans-10²⁺in

1.0 V, led to an increase in energy for m(NO) and m(CNR) of 94

and 69 cm⁻¹, but only a small increase for one of the m(CO) bands.

Thus, the formation of cis-9²⁺again conﬁrmed the assignment

made on the basis of cyclic voltammetry. Lowering E_appto 0.0 V

regenerated cis-9⁺, with the intensities of the bands greater than

90% of those observed in the forward process, indicating stability

of the dication cis-9²⁺in CH₂Cl₂at −30 ^◦C.

CH₂Cl₂.

value of k_max(1021 nm) for trans-10²⁺indicates relatively facile

Mn(I) to Mn(II) MMCT compared with trans-7²⁺(877 nm).

The electronic spectra of the air-sensitive dications trans-

7²⁺and trans-17²⁺were further studied by in situ spec-

troelectrochemical oxidation of the corresponding monoca-

tions trans-7⁺and trans-17⁺in CH₂Cl₂at −30 ^◦C. In addi-

tion, this technique was used to monitor the second oxida-

tion step, to the trications trans-[(dppm)(CO)₂{(EtO)₃P}Mn^II(l-

The electronic spectra of the mixed valence dications trans-1²⁺–10²⁺

The electronic spectra of trans-1⁺–10⁺and the mononuclear

5

XY)Mn^II(CNBu^t)(NO)(g -C₅H₄Me)]³⁺(XY = NC, trans-7³⁺;

XY = CN, trans-17³⁺). As E_appis increased from 0.0 to 0.6 V (vs. a

Ag/AgCl electrode) the dication trans-7²⁺is generated from trans-

7⁺. The octahedral Mn(I) site is oxidised to Mn(II) and the weak

absorption at 370 nm loses intensity. More obviously, a MMCT

band appears at ca. 890 nm, as observed for the isolated dication

trans-7²⁺. A second, less intense band appears at 493 nm. Isosbestic

points at 317 and 412 nm suggest that trans-7⁺and trans-7²⁺are

the only absorbing species in solution.

5

cyanomanganese(I)precursors[Mn(CN)L^ꢀ(NO)(g -C₅R₄Me)]and

trans-[MnBrL(CO)₂(dppm)] show no absorptions above 400 nm.

However, the dications trans-1²⁺–10²⁺show two distinct absorp-

tions in CH₂Cl₂, one in the visible or near-IR region and a

second weaker band at higher energy (Table 7), e.g. trans-3²⁺

shows absorptions at 504 nm (e = 1061 dm³mol⁻¹cm⁻¹) and

972 nm (e = 2633 dm³mol⁻¹cm⁻¹). The lower energy bands

are assigned to optically induced metal–metal charge-transfer

(MMCT) from Mn(I) to Mn(II) centre via the cyanide bridge as

they are solvatochromic (see below); the higher energy bands are

essentially unaffected by the solvent.

Increasing E_apppast the E^◦value for the second oxidation step

ꢀ

(1.36 V) causes the MMCT band to collapse as the intermetallic

interaction is switched off. Lowering E_appto 0.6 V regenerates

trans-7²⁺with the intensity of the corresponding MMCT band

greater than 90% of that observed in the forward process. As shown

by IR spectroelectrochemistry, trans-7³⁺and trans-7²⁺appear to be

stable in CH₂Cl₂at −30 ^◦C.

A comparison of the electronic spectral data for trans-1²⁺–

10²⁺shows that the energy of the MMCT band varies with the

ligands at the two manganese centres. Fig. 10 shows the spectra

5

of trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -

The linkage isomer of trans-7⁺, i.e. trans-17⁺, shows no signif-

icant absorptions above 220 nm. However, as E_appis increased

C₅R₄Me)]²⁺(R = H, trans-7²⁺; R = Me, trans-10²⁺). The higher

3618 | Dalton Trans., 2007, 3609–3622

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from 0.0 to 0.7 V, and the dication trans-17²⁺is generated, Mn_oct

is oxidised and a broad MMCT band appears at 1354 nm (with a

shoulder towards lower wavelength). A second band, signiﬁcantly

weaker, is observed at 537 nm; two isosbestic points are observed

at 305 and 408 nm. The lower energy of the MMCT band for

trans-17²⁺, cf. that of trans-7²⁺, indicates that electron transfer

from Mn(I) to Mn(II) in Mn^II_oct–CN–Mn^I_tet(in trans-17²⁺) is easier

than from Mn(I) to Mn(II) in Mn^II_oct–NC–Mn^I_tet(of trans-7²⁺).

Although the effect of the bridging ligand on the extent of

intermetallic interaction in mixed-valence complexes has been

frequently studied, no such study has been made with respect

to cyanide–isocyanide isomerism in dinuclear complexes. Indirect

effects of the cyanide orientation have been recorded during

spectroelectrochemistry) (Fig. 11) is approximately linear (R²=

0.86); as DE^◦decreases, the Mn(I) to Mn(II) electron transfer

ꢀ

becomes generally more favourable and thus the MMCT band

energy decreases. Such a relationship has also been found for the

5

trinuclear complexes [(pc)Fe{(l-XY)Ru(PPh₃)₂(g -C₅H₅)}₂] and

5

[(pc)Fe{(l-XY)Fe(dppe)(g -C₅H₅)}₂] (XY = CN or NC).¹⁴

Although the MMCT band for trans-[(dppm)(CO)₂LMn(l-

5

NC)MnL^ꢀ(NO)(g -C₅R₄Me)]²⁺trans-1²⁺–10²⁺is solvatochromic,

it is rather weakly so. (Data for trans-2²⁺in four solvents are

given in Table 8; those for trans-4²⁺, trans-6²⁺–8²⁺and trans-

10²⁺are given as ESI‡.) This behaviour contrasts markedly

with the pronounced solvatochromism exhibited by [X₃Fe^III-

(l-NC)Mn^I(CO)₂L(dppm)-trans)]¹⁶and [(H₃N)₅Ru^III(l-NC)Mn^I-

5

the spontaneous isomerisation of [(pc)Fe{(l-CN)Fe(dppe)(g -

(CO)₂L(dppm)-trans)]³⁺.¹³

For

example,

trans-[(dppm)-

5

C₅H₅)}₂]⁺(pc = phthalocyaninate) to [(pc)Fe{(l-NC)Fe(dppe)(g -

(CO)₂{(PhO)₃P}Mn^II(l-NC)Mn^I{P(OPh)₃}(NO)(g -C₅H₄Me)]²⁺

trans-2²⁺shows MMCT bands at 848, 852 and 870 nm in

MeCN, CH₂Cl₂and thf respectively, an overall shift of only ca.

20 nm. By contrast, the MMCT band of trans-[(H₃N)₅Ru^III(l-

NC)Mn^I(CO)₂{P(OPh)₃}(dppm)-trans)]³⁺occurs at 963, 1134 and

946 nm in the same solvents, the overall difference of ca. 190 nm

resulting from variable hydrogen bonding of the solvent with the

Ru^III(NH₃)₅group. Such hydrogen bonding is likely to be much

C₅H₅)}₂]⁺,¹⁴in which the near-IR spectrum shows a change in the

wavelength of the MMCT band from 1293 to 2150 nm. Therefore,

the effect of cyanide orientation on the energy of the MMCT

band has been studied in trans-[(dppm)(CO)₂{(EtO)₃P}Mn^II(l-

5

XY)Mn^I(CNBu^t)(NO)(g -C₅H₄Me)]²⁺(XY = NC, trans-7²⁺;

XY = CN, trans-17²⁺) as a representative example.

As shown by CV and IR spectroelectrochemistry, the ﬁrst

oxidation of trans-7⁺and trans-17⁺occurs at Mn_oct. There-

fore, MMCT in the mixed-valence species trans-[(dppm)-

less signiﬁcant for trans-[(dppm)(CO)₂LMn(l-NC)MnL^ꢀ(NO)(g -

5

C₅R₄Me)]²⁺.

5

(CO)₂{(EtO)₃P}Mn^II(l-XY)Mn^I(CNBu^t)(NO)(g -C₅H₄Me)]²⁺

will take place in the same direction, from Mn_tetto Mn_oct, but

across a bridging cyanide in two different orientations, i.e. from

Mn(I) to CN to Mn(II) in trans-7²⁺and from Mn(I) to NC to Mn(II)

in trans-17²⁺(Scheme 5).

Scheme 5 The direction of MMCT in the mixed-valence complexes (a)

5

trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -C₅H₄Me)]²⁺

trans-7²⁺and (b) trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn(CNBu^t)-

5

(NO)(g -C₅H₄Me)]²⁺trans-17²⁺

.

Fig. 11 Plot of DE^◦vs. E_opfor trans-1²⁺–10²⁺and trans-17²⁺

.

ꢀ

Hush theory¹⁵relates the energy of the MMCT band, E_op, to the

ground state free energy difference, DG^◦, and the reorganisation

For mixed-valence species the magnitude of the intermetallic

interaction can be expressed in terms of the coupling parameter J,

in units of cm⁻¹, or the degree of electron delocalisation, a²[eqn

(2) and (3)].

◦

energy, v, according to eqn (1). In addition, DG^◦= −nFDE ,

ꢀ

where DE^◦is the difference between the potentials for the two

ꢀ

redox processes.

E_op= DG^◦+ v = −nFDE^◦

(1)

ꢀ

J = 2.05 × 10⁻²[e × m_1/2× E]^1/2× r⁻¹(cm⁻¹)

(2)

(3)

A plot of the energy of the MMCT band, E_opin cm⁻¹, against

DE^◦for trans-1²⁺–10²⁺and trans-17²⁺(obtained by electronic

a²= 4.24 × 10⁻⁴[(e × m_1/2)/(E × r²)] (%)

ꢀ

5

Table 8 Metal–metal charge-transfer data for trans-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]²⁺trans-2²⁺

Solvent

DN ^a

k_max/nm

E/cm⁻¹

e/dm³mol⁻¹cm⁻¹

m_1/2/cm⁻¹

J/cm⁻¹

a²(%)

CH₂Cl₂

MeCN

acetone

thf

1.0

14.1

17.0

20.0

852

848

855

870

11737

11792

11692

11494

2930

2550

2834

2725

3530

3501

3650

3642

1403

1302

1395

1355

1.43

1.22

1.42

1.39

^aDN = Gutmann donor number.

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5

Here, e is the extinction coefﬁcient for the MMCT band in

dm³mol⁻¹cm⁻¹, m_1/2is the width of the band at half-height in cm⁻¹,

E is the energy of the band maximum in cm⁻¹and r is the metal–

(CNBu^t)(NO)(g -C₅H₄Me)]²⁺trans-7²⁺occurs at higher energy

than through the Mn^II(l-CN)Mn^Iin the linkage isomer

trans-[(dppm)(CO)₂{(EtO)₃P}Mn^II(l-CN)Mn^I(CNBu^t)(NO)(g -

5

C₅H₄Me)]²⁺trans-17²⁺.

˚

metal separation in A.

Values of J and a²for trans-2²⁺are given in Table 8, and

for trans-4²⁺, trans-6²⁺-8²⁺and trans-10²⁺in the ESI.‡ (The

Experimental

˚

value of r = 5.110 A, the metal–metal separation along the

The preparation, puriﬁcation and reactions of the complexes de-

scribed were carried out under an atmosphere of dry nitrogen using

dried and deoxygenated solvents puriﬁed either by distillation or

by using Anhydrous Engineering double alumina or alumina–

copper catalyst drying columns. Reactions were monitored by IR

spectroscopy where necessary. Unless stated otherwise, complexes

were puriﬁed using a mixture of two solvents. The impure solid

was dissolved in the more polar solvent, the resulting solution

was ﬁltered and then treated with the second solvent, and the

mixture was reduced in volume in vacuo to induce precipitation.

Unless otherwise stated, complexes are stable under nitrogen and

dissolve in polar solvents such as CH₂Cl₂, thf and acetone to give

moderately air-stable solutions.

Mn–C–N–Mn linkage, was calculated from the X-ray structure

5

of trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNBu^t)(NO)(g -

C₅H₄Me)]⁺trans-7⁺. The through space distance is insigniﬁcantly

˚

different, at 5.096 A.)

The values of J for trans-1²⁺–10²⁺in CH₂Cl₂vary only modestly,

ranging from 1513 cm⁻¹for trans-6²⁺to 967 cm⁻¹for trans-8²⁺,

and are generally smaller than those of other mixed-valence com-

plexes containing cyanomanganese ligands, such as [(H₃N)₅Ru(l-

5

13

NC)MnL^ꢀ(NO)(g -C₅H₄Me)]³⁺{L^ꢀ= PPh₃or P(OPh)₃} or

[X₃Fe(l-NC)MnL_x] {L_x= cis- or trans-(CO)₂{P(OR)₃}(dppm);

R = OPh or OEt}.¹⁶The computed values of a², the degree of

delocalisation, are consistent with the assignment of trans-1²⁺–

10²⁺as Class II mixed-valence complexes.

The compounds trans- and cis-[Mn(CN)(CO)₂L(dppm)]

1

17

{L = P(OEt)₃and P(OPh)₃} and [MnBr(CO)₂L(dppm)]

Conclusions

1

5

{L

=

P(OEt)₃and P(OPh)₃},¹⁸[Mn(CO)(PPh₃)(NO)(g -

C₅H₄Me)][PF₆],¹⁹[Mn(CN)(PR₃)(NO)(g -C₅H₄Me)] (R = Ph

5

The reaction of cyanomanganese complexes [Mn(CN)L^ꢀ(NO)(g -

5

and OPh),^20,21[Fe(g -C₅H₅)₂][PF₆],²²and [Fe(g -C₅H₅)(g -

C₅H₄COMe)][PF₆]²²were prepared by published methods. The

compounds [NO][PF₆], CNXyl and CNBu^twere purchased from

Aldrich and Tl[PF₆] was purchased from Strem Chemicals.

IR spectra were recorded on a Nicolet 5ZDX FT spectrometer.

Electrochemical studies were carried out using an EG&G Model

273A potentiostat linked to a computer using EG&G Model 270

Research Electrochemistry software in conjunction with a three-

electrode cell. The auxiliary electrode was a platinum wire and

the working electrode a platinum disc (1.6 mm diameter). The

reference was an aqueous saturated calomel electrode separated

from the test solution by a ﬁne porosity frit and an agar

bridge saturated with KCl. Solutions were 5.0 × 10⁻⁴or 1.0 ×

10⁻³mol dm⁻³in the test compound and 0.1 mol dm⁻³in

C₅R₄Me)] with cis- or trans-[MnBrL(CO)₂(dppm)] in the pres-

ence of Tl[PF₆] gives homobinuclear cyanomanganese(I) species

of the general formula cis- or trans-[(dppm)(CO)₂LMn(l-

5

NC)MnL^ꢀ(NO)(g -C₅R₄Me)]⁺1⁺–10⁺. The linkage isomers

of 1⁺–10⁺, namely cis- or trans-[(dppm)(CO)₂LMn(l-CN)-

5

MnL^ꢀ(NO)(g -C₅R₄Me)]⁺11⁺–20⁺, were synthesised by reac-

ting the cyanomanganese ligands cis- or trans-[Mn(CN)L-

5

(CO)₂(dppm)] with [MnIL^ꢀ(NO)(g -C₅R₄Me)] in the presence of

Tl[PF₆].

The linkage isomers trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-

5

NC)Mn(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-7⁺and trans-[(dppm)-

5

(CO)₂{(EtO)₃P}Mn(l-CN)Mn(CNBu^t)(NO)(g -C₅H₄Me)]⁺trans-

17⁺are isostructural but cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-

5

NC)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-2⁺and cis-[(dppm)-

[NBuⁿ][PF₆] as the supporting electrolyte with CH₂Cl₂as the

5

(CO)₂{(PhO)₃P}Mn(l-CN)Mn{P(OPh)₃}(NO)(g -C₅H₄Me)]⁺cis-

4

solvent. Under these conditions, E^◦^ꢀfor the one-electron oxidation

12⁺differ by coordination of two different optical isomers of the

pseudo-tetrahedral unit.

5

of [Fe(g -C₅Me₅)₂], added to the test solutions as an internal

calibrant, is −0.08 V. UV-visible spectra were recorded on a Perkin

Elmer Lambda 2 spectrometer using 1.0 cm path length quartz

cells. UV-visible-near-IR spectra were recorded on a Perkin Elmer

Lambda 19 spectrometer using 0.1 cm path length quartz cells. UV-

visible-near-IR spectroelectrochemical measurements were made

using an EG&G model 273A potentiostat in conjunction with

a three-electrode system ﬁtted to the Perkin Elmer Lambda 19

spectrometer. IR spectroelectrochemical measurements were made

in conjunction with a three-electrode system ﬁtted to a Bruker

IFS25 Spectrometer.

The cations cis- and trans-1⁺–20⁺undergo sequential oxidation

of the two Mn(I) centres to Mn(II); the redox-induced isomeri-

sation observed for units containing the cis-Mn(CO)₂fragment

allows assignment of the order of oxidation of the cis isomers. This

order may be controlled by systematic variation of the ligands L

and L^ꢀ, the substituents on the cyclopentadienyl ring and the l-CN

orientation.

One-electron oxidation of trans-[(dppm)(CO)₂LMn^I(l-

5

NC)Mn^IL^ꢀ(NO)(g -C₅R₄Me)]⁺trans-1⁺–10⁺with [NO][PF₆] in

CH₂Cl₂forms the dications trans-1²⁺–10²⁺which are also obtained

Microanalyses were carried out by the staff of the Microanalysis

Service of the School of Chemistry, University of Bristol.

5

by

oxidising

cis-[(dppm)(CO)₂LMn^I(l-NC)Mn^IL^ꢀ(NO)(g -

C₅R₄Me)]⁺cis-1⁺–10⁺by cis–trans isomerisation at the octahedral

Mn(CO)₂site.

Syntheses

The dications show weakly solvatochromic MMCT bands

consistent with Class II mixed-valency. The energy of these bands

is affected by the orientation of the cyanide bridge such that

electron transfer from Mn(I) to Mn(II) through the unit Mn^II(l-

cis-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn(PPh₃)(NO)(g⁵-C₅H₄-

Me)][PF₆] cis-1⁺[PF₆]⁻. To a stirred solution of [Mn(CN)-

5

(PPh₃)(NO)(g -C₅H₄Me)] (67 mg, 0.15 mmol) and cis-

NC)Mn^I

in

trans-[(dppm)(CO)₂{(EtO)₃P}Mn^II(l-NC)Mn^I-

[MnBr(CO)₂{P(OPh)₃)}(dppm)] (130 mg, 0.15 mmol) in CH₂Cl₂

3620 | Dalton Trans., 2007, 3609–3622

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(20 cm³) was added Tl[PF₆] (54 mg, 0.15 mmol). After 24 h the

orange solution was ﬁltered through Celite and evaporated to

dryness in vacuo. After washing the product with toluene (2 ×

10 cm³), the orange residue was puriﬁed using CH₂Cl₂–n-hexane

to give an orange crystalline solid, yield 129 mg (63%).

The complexes cis-2⁺[PF₆]⁻–cis-8⁺[PF₆]⁻were prepared simi-

larly, although the reaction times varied from 24 h to 72 h.

Complexes trans-1⁺[PF₆]⁻–trans-6⁺[PF₆]⁻, trans-9⁺[PF₆]⁻and

trans-10⁺[PF₆]⁻were prepared similarly by using trans-

[MnBr(CO)₂{P(OR)₃}(dppm)].

cis - [ (dppm)(CO)₂{(PhO)₃P }Mn(l - NC)Mn(CNBu^t)(NO)(g⁵-

C₅Me₅)][PF₆] cis-9⁺[PF₆]⁻. To

a

stirred solution of

[Mn(CN)(CNBu^t)(NO)(g -C₅Me₅)] (26 mg, 0.08 mmol) and

cis-[MnBr(CO)₂{P(OPh)₃}(dppm)] (70 mg, 0.08 mmol) in CH₂Cl₂

(15 cm³) was added Tl[PF₆] (31 mg, 0.09 mmol). After 24 h the

orange solution was ﬁltered through Celite and the volume of the

solution reduced to ca. 5 cm³. After adding n-hexane (5 cm³) the

volume of the solution was reduced in vacuo to leave an orange

solid, yield 72 mg (72%).

5

The

complex

cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn-

5

(CNBu^t)(NO)(g -C₅Me₅)][PF₆] cis-10⁺[PF₆]⁻was prepared

similarly as a red solid.

trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNXyl)(NO)(g⁵-

C₅H₄Me)][PF₆] trans-8⁺[PF₆]⁻. To a stirred solution of cis-

5

[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn(CNXyl)(NO)(g -C₅H₄-

Me)][PF₆] cis-8⁺[PF₆]⁻(100 mg, 0.09 mmol) in CH₂Cl₂(15 cm³)

was added [NO][PF₆] (17 mg, 0.10 mmol). After 15 min,

N₂H₄·xH₂O (0.3 cm³) was added to the purple solution. The

resulting orange solution was stirred for 15 min in the presence

of MgSO₄and then ﬁltered through Celite. Addition of n-hexane

(15 cm³) induced precipitation of a solid which was puriﬁed using

CH₂Cl₂–n-hexane to give a crystalline orange solid, yield 54 mg

(54%).

The complex trans-[(dppm)(CO)₂{(EtO)₃P}Mn(l-NC)Mn-

5

(CNBu^t)(NO)(g -C₅H₄Me)][PF₆] trans-7⁺[PF₆]⁻was prepared

similarly as a yellow solid.

cis - [ (dppm)(CO)₂{ (EtO)₃P }Mn(l - CN)Mn(CNBu^t)(NO)(g⁵-

C₅H₄Me)][PF₆] cis-17⁺[PF₆]⁻. To a stirred solution of cis-

[Mn(CN)(CO)₂{P(OEt)₃}(dppm)] (200 mg, 0.29 mmol) and

5

[MnI(CNBu^t)(NO)(g -C₅H₄Me)] (109 mg, 0.29 mmol) in CH₂Cl₂

(15 cm³) was added Tl[PF₆] (112 mg, 0.32 mmol). After 50 min

the red solution was ﬁltered through Celite and the volume of

the ﬁltrate reduced to ca. 5 cm³. After adding n-hexane (5 cm³) the

volume of the solution was reduced in vacuo to induce precipitation

of a pink solid which was thoroughly washed with diethyl ether

(3 × 10 cm³), yield 220 mg (70%).

The complexes cis- or trans-11⁺[PF₆]⁻–18⁺[PF₆]⁻, except cis-

15⁺[PF₆]⁻, were prepared similarly with the following mod-

iﬁcations: trans-17⁺[PF₆]⁻and trans-18⁺[PF₆]⁻were puriﬁed

using CH₂Cl₂–n-hexane; the reaction time for cis-11⁺[PF₆]⁻

was 2 h.

cis-[(dppm)(CO)₂{(EtO)₃P}Mn(l-CN)Mn(PPh₃)(NO)(g⁵-

C₅H₄Me)][PF₆] cis-15⁺[PF₆]⁻. To a stirred solution of cis-

[Mn(CN)(CO)₂{P(OEt)₃}(dppm)] (150 mg, 0.22 mmol) and

5

[Mn(CO)(PPh₃)(NO)(g -C₅H₄Me)] (130 mg, 0.22 mmol) in

CH₂Cl₂(25 cm³) was added Me₃NO (16 mg, 0.22 mmol). After

45 min the green solution was ﬁltered through Celite and the

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volume reduced in vacuo to ca. 15 cm³. After adding toluene

(30 cm³) the volume of the solution was reduced in vacuo to induce

precipitation of a green solid which was thoroughly washed with

n-hexane (3 × 10 cm³), yield 122 mg (44%).

Many of the details of the structure analyses are listed in Table 9.

CCDC reference numbers 166635, 166636, 644601–644604.

For crystallographic data in CIF or other electronic format see

DOI: 10.1039/b705975b

The complexes cis-19⁺[PF₆]⁻, trans-19⁺[PF₆]⁻, cis-20⁺[PF₆]⁻and

trans-20⁺[PF₆]⁻were prepared similarly.

Acknowledgements

trans - [ (dppm)(CO)₂{(PhO)₃P}Mn(l - NC)Mn(PPh₃)(NO)(g⁵-

We thank the University of Bristol for Postgraduate Scholarships

(K.M.A., E.L.-R.), the Leverhulme Foundation for a Postdoctoral

Fellowship (R.L.P.) and Professor M.D. Ward for providing

facilities for the spectroelectrochemical studies.

C₅H₄Me)][PF₆]₂trans-1²⁺2[PF₆]⁻. To

a

stirred solution

5

of trans-[(dppm)(CO)₂{(PhO)₃P}Mn(l-NC)Mn(PPh₃)(NO)(g -

C₅H₄Me)][PF₆] trans-1⁺[PF₆]⁻(150 mg, 0.133 mmol) in CH₂Cl₂

(20 cm³) was added [NO][PF₆] (26 mg, 0.146 mmol). After 50 min

the resulting purple solution was ﬁltered through Celite, n-Hexane

(20 cm³) was added and the volume of the solution was reduced in

vacuo to induce precipitation of a purple solid which was washed

with n-hexane–Et₂O (1 : 1) (20 cm³) and dried in vacuo, yield

93 mg (55%).

References

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2005.

The complexes trans-2²⁺2[PF₆]⁻–10²⁺2[PF₆]⁻were prepared sim-

ilarly as purple solids.

Crystal structure determinations of cis-2⁺[PF₆]⁻,

trans-7⁺[PF₆]⁻·Me₂CO, cis-8⁺[PF₆]⁻·1.5CH₂Cl₂,

cis-9⁺[PF₆]⁻·CH₂Cl₂, cis-12⁺[PF₆]⁻and trans-17⁺[PF₆]⁻·Me₂CO

Crystals suitable for X-ray diffraction study were grown as follows:

orange-red crystals of cis-2⁺[PF₆]⁻, cis-8⁺[PF₆]⁻·1.5CH₂Cl₂, cis-

9⁺[PF₆]⁻·CH₂Cl₂and cis-12⁺[PF₆]⁻, slow diffusion of n-hexane

◦

into a concentrated CH₂Cl₂solution of the complex at −20 C;

orange-red crystals of trans-7⁺[PF₆]⁻·Me₂CO and dark green

crystals of trans-17⁺[PF₆]⁻·Me₂CO, slow diffusion of diethyl ether

into an acetone solution of the salt at −20 ^◦C.

Crystals of trans-7⁺[PF₆]⁻and trans-17⁺[PF₆]⁻both contain

one molecule of acetone per asymmetric unit, whilst that of cis-

8⁺[PF₆]⁻contains 1.5 equivalents of CH₂Cl₂per asymmetric unit

and that of cis-9⁺[PF₆]⁻contains one molecule of CH₂Cl₂per

asymmetric unit. The Bu^tgroup of trans-7⁺is rotationally disor-

dered over two positions in an approximately 3 : 1 ratio, and two of

the ethyl groups of the P(OEt)₃ligand of cis-8⁺are each disordered

over two positions in a 1 : 1 ratio. The structure of cis-9⁺contains

two disordered phenyl rings belonging to the dppm ligand, and

two disordered OPh groups on the P(OPh)₃ligand. The orientation

of the cyanide bridges in all structures was assigned on the basis

of the best agreement for the thermal parameters and occupancy

factors for the C and N atoms after reﬁnement in two alternative

models in which the C and N atoms were exchanged.

For cis-12⁺, the crystal also contained disordered solvent

molecules which could not be resolved. This was modelled using

the programme SQUEEZE,²³which found the unit cell to contain

3

˚

four voids of 243 A , each containing 52 electrons, and four voids

3

˚

of 263 A each containing 48 electrons. This is believed to be due to

the presence of highly disordered CH₂Cl₂molecules located within

these voids.

3622 | Dalton Trans., 2007, 3609–3622

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The Royal Society of Chemistry 2007

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Article Doi

The effect of -CN linkage isomerism and ancillary ligand set on directional metal-metal charge-transfer in cyanide-bridged dimanganese complexes

DOI: 10.1039/b705975b

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Authors:

Article abstract of DOI:10.1039/b705975b

Full text of DOI:10.1039/b705975b

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